Twinned versus linked organometallics - bimetallic “half-baguette” pentalenide complexes of Rh(I)

Abstract

The application of Mg[Ph₄Pn] and Li·K[Ph₄Pn] in transmetalation reactions to a range of Rh(I) precursors led to the formation of “half-baguette” anti-[Rh^I(L)_n]₂[μ:η⁵:η⁵Ph₄Pn] (L = 1,5-cyclooctadiene, norbornadiene, ethylene; n = 1, 2) and syn-[Rh^I(CO)₂]₂[μ:η⁵:η⁵Ph₄Pn] complexes as well as the related iridium complex anti-[Ir^I(COD)]₂[μ:η⁵:η⁵Ph₄Pn]. With CO exclusive syn metalation was obtained even when using mono-nuclear Rh(I) precursors, indicating an electronic preference for syn metalation. DFT analysis showed this to be the result of π overlap between the adjacent M(CO)₂ units which overcompensates for d_z² repulsion of the metals, an effect which can be overridden by steric clash of the auxiliary ligands to yield anti-configuration as seen in the larger olefin complexes. syn-[Rh^I(CO)₂]₂[μ:η⁵:η⁵Ph₄Pn] is a rare example of a twinned organometallic where the two metals are held flexibly in close proximity, but the two d⁸ Rh(I) centres did not show signs of M–M bonding interactions or exhibit Lewis basic behaviour as in some related mono-nuclear Cp complexes due to the acceptor properties of the ligands. The ligand substitution chemistry of syn-[Rh^I(CO)₂]₂[μ:η⁵:η⁵Ph₄Pn] was investigated with a series of electronically and sterically diverse donor ligands (P(OPh)₃, P(OMe)₃, PPh₃, PMe₃, dppe) yielding new mono- and bis-substituted complexes, with E-syn-[Rh^I(CO)(P{OR})₃]₂[μ:η⁵:η⁵Ph₄Pn] (R = Me, Ph) characterised by XRD.

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Introduction

The past decade has witnessed renewed interest in bimetallic complexes and the opportunities they offer in terms cooperative effects that derive from attractive or repulsive metal–metal interactions¹ within molecularly defined homometallic and heterobimetallic complexes of the s-, d- and f-block.^2–4 Bimetallics featuring Rh(I) are a particularly useful test for intermetallic effects due to its diamagnetic nature, useful ¹⁰³Rh coupling information, wide range of known mono-nuclear complexes and rich reactivity (stoichiometric as well as catalytic). The majority of known bimetallic Rh(I) complexes consist of a ligand with two distinct binding sites connected by a linker moiety, where the size and nature of the linker determines the proximity and the extent of electronic communication between the two Rh(I) centres.^5–19 With a view on cooperative effects it is thus useful to distinguish between linked bimetallics (two monometallic complexes tethered together) and twinned bimetallics where the two metals are held closely enough for direct metal–metal interactions to exist.^20,21 This distinction is not purely a function of atomic distances but depends on orbital overlap and is thus a sliding scale akin to the Robin–Day classification,^22,23 where the extremes of a truly twinned bimetallic would be class III and a distantly linked bimetallic class I. A particularly interesting ligand platform to explore bimetallic effects is pentalenide (Pn²⁻, C₈H₆²⁻), the bicyclic analogue of cyclopentadienide (Cp⁻, C₅H₅⁻), itself one of the most widely used ligands in organometallic chemistry.²⁴ The bicyclic nature of Pn²⁻ offers unique advantages over its monocyclic relative cyclooctatetraenide (COT²⁻, C₈H₈²⁻), including the formation of mono- and di-nuclear complexes paired with the ability to engage in a wide range of hapticities that allow for flexibility in the metal binding.²⁵ Monometallic Pn²⁻ complexes often show folding around the central bridgehead carbon–carbon bond to engage in η⁸ bonding, a mode that thus far is exclusive to early d- and f-block metals with a d⁰ electron count.^26,27 Bimetallic Pn²⁻ complexes can be classified as either syn- or anti-based on the relative orientation of the two metals, and can exhibit a range of coordination modes to Pn²⁻ with η⁵/η⁵, η⁵/η³, η³/η³ and η⁵/η¹ known to exist.²⁷ The η¹/η¹ mode is also known for Si and Sn.^28–30Syn-Pn²⁻ complexes are most common for the homobimetallic double-sandwich (or perhaps more apposite, baguette) complexes of the type M₂Pn₂ which are known for Ti–Ni^31–35 as well as Rh and Pd.³⁶ Although intriguing compounds with regards to their structure, bonding and (photo)electrochemistry, in terms of chemical reactivity and potential use in bimetallic catalysis^1–4 the corresponding syn-bimetallic “half-baguette” complexes (analogous to the widely used mononuclear “half-sandwich” Cp complexes) would be very interesting to access and study. Several examples of such homobimetallic half-baguette complexes (M₂L_xPn) of Mn,^37,38 Fe,^39–43 Co,⁴³ Ni,⁴⁴ Ru,^45,46 Re^37,38 Rh and Ir⁴⁷ have been reported bearing a range of differently substituted pentalenide frameworks, including Pn^R (R = Ph, Me, NMe₂, H), 1,3-(SiMe₃)₂Pn²⁻, 1,3,5-(SiMe₃)₃Pn²⁻ and 1,2,3,4,5,6-Me₆Pn²⁻ (Pn*). However, the factors that lead to syn/anti selectivity are not yet clear, nor have any of these complexes been used in ligand exchange reactions or catalysis. Intriguingly, the close proximity of two metals in syn bimetallic Pn²⁻ complexes can result in various and flexible degrees of metal–metal bonding. This can range from a double bond as in the homobimetallic baguette complex [Ti(Pn^†)]₂ (Pn^† = 1,4-(ⁱPr₃Si)₂Pn) which can undergo a wide range of reactivity including CO₂,^48,49 H₂ and CH activation,^50,51 to cases where the metal centres repel each other, such as syn-[M(CO)₂]₂[Pn*] (M = Co, Rh, Ir) which were found to have M–M bond orders of −0.11, −0.06 and 0.16 for Co, Rh and Ir, respectively.^27,47 Further modulation of the reactivity within pentalenide complexes can be achieved by ligand exchange reactions. O'Hare and co-workers have derivatised their monometallic group 4 Pn*²⁻ dichloride complexes to introduce a variety of alkyl (Me, allyl, CH₂Ph), phenyl and amide (NMe₂, NPh₂) ligands to study their effects on stoichiometric CO₂ activation,^52–54 and similar modulation has allowed for the synthesis of group 4 mixed sandwich complexes.^55–57 However, auxiliary ligand exchange reactions of late d-block pentalenide complexes are unknown save for Manriquez and co-workers who reported that [Cp*Ru^II(μ:η⁵:η³Pn)Rh^I(COD)] will liberate COD to form [Cp*Ru^II(μ:η⁵:η³Pn)Rh^I(CO)₂] when exposed to a CO atmosphere.⁵⁸

We have recently described the facile and modular syntheses of tetra-substituted dihydropentalenes⁵⁹ and introduced the first tetra-arylated pentalenide, M₂[Ph₄Pn] (M = Li, Na, K)⁶⁰ and the first alkaline earth pentalenide, Mg[Ph₄Pn].^61,62 The increased solubility and ease of separation of MgX₂ transmetalation by-products (through induced precipitation by addition of 1,4-dioxane^63,64) are key advantages of Mg[Ph₄Pn] over M₂[Ph₄Pn] which make it a prime candidate to explore the coordination chemistry of Ph₄Pn²⁻ by way of salt metathesis. The majority of known pentalenide complexes contain pentalenide scaffolds containing electron-donating groups (such as alkyl and trialkylsilyl substituents), and the electron-withdrawing nature of the phenyl substituents should make Ph₄Pn²⁻ significantly less reducing and thus more suited to bind to electron-rich, late d-block metals. Indeed, recent DFT calculations have shown that the presence of the four phenyl groups reduces the HOMO–LUMO gap of Ph₄Pn²⁻ by 0.26 eV relative to Pn²⁻ alongside a lowering in the absolute energy levels of both the HOMO and the LUMO (by 0.20 eV and 0.46 eV respectively).⁶⁵ The phenyl groups of Ph₄Pn²⁻ may allow for additional metal-arene interactions^66–68 that have not been seen with known (non-arylated) pentalenide complexes. In this work, we report the synthesis and characterisation of a series of bimetallic Rh(I) half-baguette pentalenide complexes and compare the outcome of transmetalation with Mg[Ph₄Pn] to that of Li·K[Ph₄Pn]. Furthermore, we explore the reactivity of a dirhodium tetracarbonyl complex (syn-[Rh^I(CO)₂]₂[μ:η⁵:η⁵Ph₄Pn]) in ligand substitution reactions to evaluate the tolerance of the Ph₄Pn²⁻ scaffold to a range of donor and acceptor ligands.

Results and discussion

Synthesis and structure of syn- and anti-bimetallic Rh(I) and Ir(I) complexes

Synthesis of [Rh^I(COD)]₂[Ph₄Pn] (1) and [Ir^I(COD)]₂[Ph₄Pn] (2). Previous half-baguette Rh(I) and Ir(I) pentalenide complexes have required the use of stannylated pentalenides and low reaction temperatures to avoid unwanted redox chemistry between the electron-rich (reducing) Pn*²⁻ and the late d-block precusors.⁴⁷ In the case of the more π-accepting Ph₄Pn²⁻ this should be supressed sufficiently to allow room temperature reactions without the need for “softer” stannylated derivatives which are highly toxic. To test this idea the reaction of Mg[Ph₄Pn] and [Rh^I(COD)(μ-Cl)]₂ was carried out at room temperature in THF which gave an immediate colour change from orange to dark red indicative of a transmetalation reaction. The ¹H NMR spectrum of the reaction mixture revealed full consumption of Mg[Ph₄Pn] and showed the presence of a new singlet at 6.17 ppm (FWHM = 3.7 Hz) assigned to the two equivalent pentalenide wingtip protons (H_w) in a symmetrical transmetalation product. In THF-h₈ the COD signals were obscured by the solvent, however, redissolution into C₆D₆ showed three signals at 3.92 ppm, 2.08 ppm and 1.77 ppm, all shifted from those in the starting material [Rh^I(COD)(μ-Cl)]₂. In C₆D₆H_w was observed at 6.10 ppm (FWHM = 4.6 Hz) with a corresponding carbon (C_w) signal resolving as a doublet at 99.3 ppm with ¹J_RhC = 4.8 Hz, all indicative of the clean formation of [Rh^I(COD)]₂[μ:η⁵:η⁵Ph₄Pn] (1).

XRD analysis of single crystals of 1 grown from standing of a THF solution at −35 °C revealed the two rhodium(I) centres to be bound to Ph₄Pn²⁻ in an anti-geometry, with rhodium–C₅–centroid (Rh–C_t) distances of 1.9391(11) Å and 1.9502(11) Å (Fig. 1). The C_w–C_t–Rh angles of 86.4° and 87.4° suggested the Rh(I) centres to sit closer to the wingtip carbons than the bridgehead carbons (C_B), as also reflected in the corresponding Rh–C distances where the Rh–C_w distances were on average 0.15 Å shorter than the Rh–C_B distances. The heterobimetallic Pn²⁻ complex anti-[Cp*Ru^II(μ:η⁵:η³Pn)Rh^I(COD)] is the only other known pentalenide complex to contain a Rh^I(COD) fragment, exhibiting Rh–C_t distances (1.944 Å) and Rh–C_t–C_w angles (86.6°) similar to that of 1 in the solid state but without any reported Rh–C_w coupling in solution.⁵⁸

Fig. 1 Synthesis of anti-[Rh^I(COD)]₂[μ:η⁵:η⁵Ph₄Pn] (1) and anti-[Ir^I(COD)]₂[μ:η⁵:η⁵Ph₄Pn] (2) (top) and corresponding X-ray crystal structures with thermal ellipsoids at the 50% probability level (bottom; hydrogen atoms omitted for clarity).

Using the same procedure as described for 1, the analogous reaction of [Ir^I(COD)(μ-Cl)]₂ with Mg[Ph₄Pn] resulted in the formation of the isostructural and isoelectronic complex anti-[Ir^I(COD)]₂[μ:η⁵:η⁵Ph₄Pn] (2). The two equivalent H_w protons of 2 were found at the same chemical shift as in 1 (6.10 ppm in C₆D₆) but a 7 ppm upfield shift of C_w was noticed in the Ir complex (92.3 ppm vs. 99.1 ppm for Rh). XRD analysis of single crystals of 2 confirmed the anti-geometry of the two iridium(I) centres as in 1. The M–C_t distances in 2 (1.9241(19) Å, 1.9290(17) Å) were up to 0.026 Å shorter than in the rhodium complex 1 but otherwise the two compounds were iso-structural. The only previously reported iridium Pn²⁻ complex syn-[Ir^I(CO)₂]₂[μ:η⁵:η⁵Pn*]⁴⁷ had an Ir–C_t distance of 2.167 Å, significantly longer than in 2. However, this difference is due to a combination of differences in the auxiliary ligands, Pn²⁻ frameworks, and geometry. 2 also exhibited some deviation towards C_w, as reflected in the shorter Ir–C_w distances than the Ir–C_B distances (by 0.17 Å) and the more acute C_w–C_t–Ir angles (86.3° and 86.9°), greater than what was noted for 1.

Rh(I) and Ir(I) d⁸ complexes typically prefer 16 e⁻ configuration so a slight deviation from η⁵ to η³ may be expected, as η⁵ coordination would yield a formal 18 e⁻ count for each metal. The hapticity of the L₃X₂-type pentalenide ligand can be quantified with the displacement parameter Δ defined as the difference in the mean M–C_B and M–C_NB distances (C_NB = non-bridgehead carbons).^47,69,70 Relatively small ring slippage values of Δ = 0.13 and 0.15 can thus be calculated for 1 and 2, respectively, which suggest a formal η⁵ coordination of both Rh(I) and Ir(I) to Ph₄Pn²⁻. The slightly larger value for 2 compared to 1 may be attributed to increased atomic size of Ir versus Rh.⁴⁷ In contrast, the Rh centre in anti-[Cp*Ru^II(μ:η⁵:η³Pn)Rh^I(COD)] was reported as bound η³ to Pn²⁻, suggesting a formal 16 e⁻ count. However, based on the XRD data reported for this complex, a value of Δ = 0.20 can be calculated which is markedly less than what was reported for syn-[Rh^I(CO)₂]₂[Pn*] and that found for complex 5 (vide infra) which are both considered η⁵ and thus 18 e⁻ per Rh(I). Complexes 1 and 2 also displayed close structural similarities to their mononuclear analogues [CpM^I(COD)] (M = Rh,^71,72 Ir⁷³), with the COD ligand adopting the expected distorted boat conformation in all cases.

When the transmetalations of [Rh^I(COD)(μ-Cl)]₂ and [Ir^I(COD)(μ-Cl)]₂ were performed using Li·K[Ph₄Pn]⁶⁰ instead of Mg[Ph₄Pn], the same colour changes (dark red for Rh, dark yellow for Ir) were observed and analysis by ¹H and ¹³C{¹H} NMR spectroscopy confirmed the formation of 1 and 2, respectively (Fig. S4, S5, S9 and S10†). This observation showed that the nature of the counter cation(s) of Ph₄Pn²⁻ does not influence the outcome of transmetalation reactions in donor solvents such as THF where these salts exist as solvent-separated ion pairs, despite the formal presence of a LiCp-like unit⁶⁰ which was found to be too reducing in the case of reacting Li₂[Pn*] with Rh(I) and Ir(I) precursors.⁴⁷ This difference in reactivity likely reflects the electronic nature of the pentalenide used, in line with recent calculations showing Ph₄Pn²⁻ to have significantly lower frontier orbital energies than Pn²⁻ and its alkylated derivatives,⁶⁵ making it a less reducing π base (donor) and a better π acid (acceptor).

Synthesis of [Rh^I(NBD)]₂[Ph₄Pn] (3) and [Rh^I(C₂H₄)₂]₂[Ph₄Pn] (4). With the COD precursors yielding anti-isomers, it was interesting to extend the transmetalation to other olefinic precursors that possess a smaller steric profile than COD to see if the selectivity of the transmetalation was dictated by sterics. Therefore, the reaction of [Rh^I(NBD)(μ-Cl)]₂ with Mg[Ph₄Pn] was carried out which led to the formation of anti-[Rh^I(NBD)]₂[μ:η⁵:η⁵Ph₄Pn] (3) (Fig. 2). Compared to 1 and 2, a slower formation of complex 3 was noticed with the colour change from orange to bright yellow occurring over the course of several of minutes at room temperature. The ¹H NMR spectrum of the solution showed a new Ph₄Pn²⁻ environment with the emergence of a new H_w signal resolved as a doublet at 6.22 ppm (²J_RhH = 0.8 Hz). The associated carbon shift, C_w, was also resolved as a doublet in the ¹³C{¹H} NMR spectrum at 97.4 ppm (¹J_RhC = 5.8 Hz). XRD analysis of single crystals of 3 showed the two Rh(I) centres to be located anti to each other as in 1 bearing COD instead of NBD (Fig. 2). The Rh–C_t distance of 1.938(3) Å in 3 was near identical to that of 1, and the Rh–C_t–C_w angle of 86.1° was marginally smaller than in 1. The ring slippage value of 0.15 was the same as in 1 and the rhodium thus best described as bonded η⁵ to Ph₄Pn²⁻.

Fig. 2 Synthesis of anti-[Rh^I(NBD)]₂[μ:η⁵:η⁵Ph₄Pn] (3) (top) and its X-ray crystal structure with thermal ellipsoids at the 50% probability level (bottom; hydrogen atoms omitted for clarity).

Utilising the ethylene complex [Rh^I(C₂H₄)₂(μ-Cl)]₂ featuring a minimally substituted, monodentate olefin ligand in a transmetalation with Mg[Ph₄Pn] in THF resulted in an immediate colour change from orange to dark yellow. The ¹H NMR spectrum of the mixture revealed the appearance of a new H_w signal at 6.21 ppm, indicating successful transmetalation. After 10 minutes a black solid started to form, presumed to be elemental rhodium from decomposition, indicating the product to be unstable at room temperature. Lower temperatures and lower polarity solvents did not fully prevent this decomposition but slowed it sufficiently for analysis. In C₆D₆H_w of the product appeared as a doublet at 5.98 ppm (²J_RhH = 0.7 Hz), 0.1–0.2 ppm upfield relative to the COD and NBD analogues. In the ¹³C{¹H} NMR spectrum C_w was resolved as a doublet at 98.6 ppm (¹J_RhC = 5.2 Hz), suggesting the electronics of the COD, NBD and ethylene complexes to be very similar. Crystals of 4 could be grown from the reaction mixture at −35 °C after MgCl₂ had been removed via addition of dioxane and filtration. Complex 4 was found to be isostructural to 1 and 3 by XRD (Fig. 3), with an anti-configuration and Rh–C_t distances of 1.9408(9) Å and 1.9383(9) Å, and Rh–C_tC_w angles of 86.6° and 87.8°.

Fig. 3 Synthesis of anti-[Rh^I(C₂H₄)₂]₂[μ:η⁵:η⁵Ph₄Pn] (4) (top) and its X-ray crystal structure with thermal ellipsoids at the 50% probability level (bottom; hydrogen atoms omitted for clarity).

Synthesis of [Rh^I(CO)₂]₂[Ph₄Pn] (5). The formation of the linked anti-bimetallics 1–4 suggested that to access a twinned syn-Rh(I) bimetallic pentalenide complex a different auxiliary ligand would be needed. [Rh^I(CO)₂(μ-Cl)]₂ has previously been used to give the syn-dirhodium complex [Rh^I(CO)₂]₂[Pn*]⁴⁷ and as such the Ph₄Pn²⁻ analogue was targeted. Addition of a THF solution of [Rh^I(CO)₂(μ-Cl)]₂ to either Mg[Ph₄Pn] or Li·K[Ph₄Pn] gave an immediate colour change to dark yellow (Fig. 4) and the corresponding ¹H NMR spectrum revealed a new Ph₄Pn²⁻ environment with a slight shift in H_w to a doublet at 6.91 ppm (²J_RhH = 1.4 Hz) from the singlet at 6.80 ppm in Mg[Ph₄Pn]. In the ¹³C{¹H} NMR spectrum the associated C_w was found to be a doublet at 90.1 ppm (¹J_RhC = 7.0 Hz) and was shifted slightly downfield relative to complexes 1–4, likely a result of the increased acceptor ability of CO compared with COD, NBD and ethylene. The monometallic analogue of 5, [Rh^I(CO)₂(Ph₂Cp)] has been reported to have a ¹H NMR signal at 5.99 ppm with a coupling constant of ²J_RhH = 0.5 Hz (C₆D₆) for the Cp proton between the two phenyl rings,⁷⁴ suggesting Ph₄Pn²⁻ to be a better acceptor ligand that binds more strongly to Rh(I) than Ph₂Cp⁻. The ¹³C{¹H} NMR spectrum of 5 showed a doublet at 194.5 ppm (¹J_RhC = 83.8 Hz) for the carbonyl groups, consistent with [(Rh^I(CO)₂(Ph₂Cp)] and [Rh^I(CO)₂]₂[Pn*], though in the latter a much smaller coupling constant of ¹J_RhC = 49.1 Hz was reported. However, the IR data suggests a stronger Rh–CO bond in [Rh^I(CO)₂]₂[Pn*] than in 5 consistent with increased donor ability of Pn*²⁻ compared to Ph₄Pn²⁻ (Table 1).

Fig. 4 Synthesis of syn-[Rh^I(CO)₂]₂[μ:η⁵:η⁵Ph₄Pn] (5, top) and its X-ray crystal structure with thermal ellipsoids at the 50% probability level (bottom; hydrogen atoms omitted for clarity).

Table 1 Key NMR and IR data of complex 5 and related literature compounds

	δ Hw/ppm	δ Cw/ppm	δ CO/ppm	ν CO /cm^−1
a Calculated from reported NMR data. b IR data recorded using a KBr pellet.
[Rh(CO) 2 ] 2 [Ph 4 Pn] (5)	6.91 (^2JRhH = 0.45 Hz)	90.1 (^1JRhC = 6.96 Hz)	194.5 (^1JRhC = 83.8 Hz)	2064, 2046, 2025,
[Rh(CO) 2 (Ph 2 Cp)] ^74	5.99 (^2JRhH = 0.45 Hz)	83.7 (^1JRhC = 3.17 Hz) and 85.4 (^1JRhC = 3.64 Hz)	191.3 (^1JRhC = 84.0 Hz)	2040, 1962
[Rh(CO) 2 ] 2 [Pn*] ^47	n/a	71.0	196.3 (^1JRhC = 49.1 Hz)	2034, 1999, 1965, 1946^b
[Rh(CO) 2 ][Cp*] ^75	n/a	n/a	195.1 (^1JRhC = 75.6 Hz)^a	2016, 1948

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The X-ray crystal structure of 5, depicted in Fig. 4, confirmed a syn-dirhodium arrangement. A comparison of the key structural features with syn-[Rh^I(CO)₂]₂[Pn*] as well as complexes 1–4 can be found in Table 2. As in the Pn*²⁻ analogue, the two Rh^I(CO)₂ fragments were located next to each other on the Ph₄Pn²⁻ with an intermetallic distance of 2.9130(4) Å, too long to ascribe any direct Rh–Rh interaction^36,47 as also reflected in a formal shortness ratio (FSR) of 1.16 using a R₁ value of 1.247.^76,77 O'Hare related this increased distance to the C₅ rings of Pn*²⁻ being partially bent away at a hinge angle of 11.4° (defined as the angle between the plane of the C_w and non-wingtip carbons (C_NW) and the plane of C_NW and C_B),⁴⁷ with DFT analysis showing the HOMO of [Rh^I(CO)₂]₂[Pn*] to contain a Rh–Pn* antibonding component that is offset by the Rh–CO π-backdonation.^27,475 exhibited a smaller hinge angle of 10.2° due to the increased π-accepting ability of the phenyl substituents in Ph₄Pn²⁻. However, despite the changes in electronics of the pentalenide ligand an identical ring slippage value of Δ = 0.27 was found. This is noticeably larger than for the anti-complexes 1–4 signifying a greater deviation towards η³ in the syn-systems, further evidenced by the more acute Rh–C_t–C_w angles of 82.4° and 82.6°, suggesting some electronic repulsion between the Rh^I(CO)₂ fragments. While complex 4 had a L–Rh–L versus pentalenide dihedral angle of 86.1° (close to the ideal 90° geometry, Fig. S61†), in 5 the two L–Rh–L units were angled at 77.8° relative to the pentalenide. This outward bending together with the greater Δ value towards η³ and 10.2° pentalenide hinging in 5 all contribute to an enlarged Rh–Rh distance signifying intermetallic repulsion. Previous DFT analysis of [Rh^I(CO)₂]₂[Pn^*] had revealed an out of phase interaction of the metal d_z² orbitals in the HOMO−1 leading to a formal negative bond order of −0.06.^27,47

Table 2 Key NMR and structural parameters of complexes 1–5 and related literature compounds

	δ Hw/ppm	δ Cw/ppm	M-M/Å	M-Ct/Å	M-Ct-Cw/ °	Ring slippage Δ
anti-[Rh ^I (COD)] 2 [Ph 4 Pn] (1)	6.17	99.3 (^1JRhC = 5 Hz)	4.5305(5)	1.9391(11), 1.9502(11)	86.4, 87.4	0.11, 0.15
anti-[Ir ^I (COD)] 2 [Ph 4 Pn] (2)	6.28	93.6	4.5411(5)	1.9241(19), 1.9290(17)	86.3, 86.9	0.15
anti-[Rh ^I (NBD)] 2 [Ph 4 Pn] (3)	6.22 (^2JRhH = 0.8 Hz)	97.4 (^1JRhC = 6 Hz)	4.5462(9)	1.938(3)	86.1	0.15
anti-[Rh ^I (C 2 H 4 ) 2 ] 2 [Ph 4 Pn] (4)	5.98 (^2JRhH = 0.7 Hz)	98.6 (^1JRhC = 5 Hz)	4.4862(3)	1.9383(9), 1.9408(9)	86.6, 87.8	0.14, 0.10
anti-[Cp*Ru ^II (μ:η ^5 :η ^3 Pn)Rh ^I (COD)] ^58	5.84	n/a	4.4166(4)	1.944	86.6	0.20
syn-[Rh ^I (CO) 2 ] 2 [Ph 4 Pn] (5)	6.50 (^2JRhH = 1.7 Hz)	89.8 (^1JRhC = 7 Hz)	2.913(6)	1.9917(9), 1.9940(9)	82.4, 82.6	0.27
syn-[Rh ^I (CO) 2 ] 2 [Pn*] ^47	n/a	n/a	2.9130(4)	1.976	83.0, 83.1	0.27

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The average Rh–CO bond length found in 5 was 1.876(1) Å, 0.01 Å longer than in syn-[Rh^I(CO)₂]₂[Pn*] suggesting less Rh–CO backbonding in 5. This was further supported by IR spectroscopy where 5 exhibited four carbonyl bands at 1992, 2025, 2046 and 2064 cm⁻¹, similar to what was reported for [Rh^I(CO)₂(Ph₂Cp)] (1962 and 2040 cm⁻¹),⁷⁴ and shifted to higher wavenumbers compared to syn-[Rh^I(CO)₂]₂[Pn*]⁴⁷ indicative of a stronger CO bond and weaker Rh–CO backbonding as a reflection of the increased π-accepting ability of Ph₄Pn²⁻ over Pn*²⁻.

Interested in whether the formation of the syn-isomer was partially influenced by the use of a dimeric rhodium precursor, the reaction was repeated with the monomeric [Rh^I(acac)(CO)₂], with the formation of 5 confirmed by NMR and IR spectroscopy. Moreover, the influence of the cation was found to be negligible as the reaction of either [Rh^I(acac)(CO)₂] or [Rh^I(CO)₂(μ-Cl)]₂ with Li·K[Ph₄Pn] also resulted in clean formation of 5 (Scheme 1).

Scheme 1 Synthetic pathways to [Rh^I(CO)₂]₂[Ph₄Pn].

Electronic structure and reactivity

To gain deeper insight into the origin of syn versus anti formation on 4 and 5, comparative Density Functional Theory (DFT) calculations were carried out on syn-4, anti-4, syn-5 and anti-5 (see ESI† for details). The optimised geometries of anti-4 and syn-5 showed good agreement with the crystallographic data, and anti-4 was found to be 1.2 kcal mol⁻¹ lower in energy than syn-4 whereas syn-5 was 4.6 kcal mol⁻¹ more stable than anti-5. These results are fully in line with our experimental findings and show syn/anti selectivity in the transmetalation of solvent-separated group 1 and group 2 pentalenide salts with Rh(I) and Ir(I) precursors to be thermodynamically controlled. NBO calculations on syn-4 and syn-5 returned Wiberg Bond Index (WBI) values of 0.06 and 0.05 between the two Rh(I) centres, respectively, indicating no significant M–M interactions. As reported for the Pn*²⁻ system,⁴⁷ the HOMO−1 of syn-5 contained an out-of-phase (repulsive) interaction between the Rh d_z² orbitals, whilst an in-phase (attractive) interaction between the two co-planar Rh(CO)₂ moieties was found in the HOMO of syn-5 (Fig. 5). Interestingly, the HOMO of syn-4 contained a similar positive interaction between the two adjacent Rh(C₂H₄)₂ moieties (Fig. 6), suggesting that there is a general energetic preference for syn-metallation in pentalenide complexes of Rh(I) and Ir(I) with π acceptor ligands despite a repulsive M–M interaction. As the switch in isomer preference from syn to anti when going from CO to C₂H₄ shows, steric clashing between the ancillary ligands easily overrides the net electronic preference for syn to yield anti-geometry in a bimetallic more akin to two linked (or fused) Cp complexes.

Fig. 5 HOMO−1, HOMO and LUMO of syn-[Rh(CO)₂]₂[Ph₄Pn] (syn-5) at the BP86/BS2//BP86/BS1 level of theory. Hydrogen atoms omitted for clarity.

Fig. 6 HOMO−1, HOMO and LUMO of syn-[Rh(C₂H₄)₂]₂[Ph₄Pn] (syn-4) at the BP86/BS2//BP86/BS1 level of theory. Hydrogen atoms of the pentalenide omitted for clarity.

To investigate whether the orientation of the metals resulted in any significant changes in their electronic structure the UV-vis spectra of 1, 5 and the monomeric derivative of 5, [Rh^I(CO)₂(Ph₂Cp)] were recorded in THF (Fig. S25†). All three displayed peaks attributable to π–π* transitions of the π-ligands (320 nm for 1 and 5, 350 nm for [Rh(CO)₂(Ph₂Cp)]), with a slight red shift for the monometallic complex also observed in the group 1 salts of Ph₄Pn²⁻ and Ph₂Cp⁻,⁶⁰ suggesting the orientation of the metals to have negligible influence on the main electronic transitions.

Given the repulsive d_z² interactions found in the HOMO−1 of the twinned syn bimetallics, we tested the influence of the metal orientation on the Lewis basicity of anti-3 and syn-5. Related mononuclear four-coordinate d⁸ complexes are known to display nucleophilic behaviour by virtue of their filled, non-bonding d_z² orbital which may act as a σ-donor to a range of Lewis acids.⁷⁸ This effect has even been seen (and exploited for intermetallic reactivity in metal-based Lewis pairs^79–82) in formally five-coordinate 18 electron η⁵-CpRh(I) complexes that have an empty, non-bonding d_z² orbital,⁷⁹ where their nucleophilic behaviour may either arise from an electron-rich Rh–L bond or a temporary η⁵–η³ slip of the Cp ligand. However, addition of AlCl₃ to anti-3 and syn-5 in THF showed no changes in their ¹H NMR spectra even after one week at room temperature. Control reactions with 1 and [Rh^I(CO)₂(Ph₂Cp)] also showed no reactivity with AlCl₃, highlighting the low nucleophilicity of these complexes which may be ascribed to the phenyl groups lowering the HOMO and LUMO of the Pn²⁻ and increase the π-acceptor character of the ligand.⁶⁵ This is in contrast to the electron-rich Rh(I) complexes furnished with strongly σ-donating phosphines (e.g. PMe₃) and electron rich π-ligands (e.g.Cp*) which serve to raise the energy of the HOMO and thus increase their nucleophilicity.^83–85

Ligand substitution chemistry of [Rh^I(CO)₂]₂[Ph₄Pn] (5)

N-donor ligands. Interested in modulating the reactivity and electronic structure of the twinned bimetallic 5 by ligand exchange (either symmetrically or asymmetrically) we explored the introduction of N-based donor ligands. CpRh^I(bipy) complexes are well-known compounds with interesting redox properties such as facile switching between Rh(I) and Rh(III) which has been utilised in a wide range of applications.^86–92 However, the addition of two equivalents of bipy to a THF solution of 5 led to no change in the NMR spectra even after adding an excess and heating the mixture to 50 °C for one week (Scheme 2, and Fig. S26†). Performing the analogous reaction with pyridine yielded the same result, and even photolytic conditions⁹³ (exposure to 150 W white light for one hour) did not allow displacement of the carbonyls of 5 with N-donor ligands.

Scheme 2 Attempted introduction of pyridyl ligands to syn-[Rh^I(CO)₂]₂[Ph₄Pn].

This surprising inertness of 5 either reflects either a kinetic barrier due to steric shielding by the phenyl substituents of the pentalenide or a thermodynamic preference of the complex for strong acceptor ligands like CO. To probe this question further we tried to introduce a range of P-donor ligands with varying degrees of donor/acceptor abilities.

P-donor ligands.
Phosphines. The addition of one equivalent of PPh₃ to 5 in THF led to no discernible colour change over several days at room temperature, however, the ¹H NMR spectrum showed a desymmetrisation of the pentalenide ligand consistent with monosubstitution to form [Rh^I(CO)₂;Rh^I(CO)(PPh₃)][μ:η⁵:η⁵Ph₄Pn] (6) (Scheme 3). Two singlets at 6.23 ppm and 6.71 ppm in the ¹H NMR spectrum could be assigned to two new wingtips H_wa and H_wb respectively (as confirmed by ¹H-³¹P HMBC), indicating an electronically asymmetric Ph₄Pn²⁻ environment with one C₅-ring bound to a more electron-rich Rh^I(CO)(PPh₃) fragment and one C₅-ring bound to a more electron-deficient Rh^I(CO)₂ fragment. This was reflected with the chemical shift of H_wb being closer to that of H_w in 5, with the slight upfield shift of 0.2 ppm reflective of the delocalised electronic nature of the Ph₄Pn²⁻ anion. The ³¹P{¹H} NMR spectrum of 6 revealed a doublet centered at 48.1 ppm (¹J_RhP = 202 Hz), similar to that reported for [CpRh^I(CO)(PPh₃)] (52.7 ppm, ¹J_RhP = 199.5 Hz).⁹⁴ The UV-vis spectrum of 6 displayed similar π–π* transitions of the Ph₄Pn²⁻ in the UV region around 300 nm to that of 5, however, with an additional broad band centred at 838 nm (ε = 7383 M⁻¹ cm⁻¹) which was absent for the symmetrical complexes 5 and 11 (Fig. S56 and 57†) and indicated intermetallic charge transfer between the electronically inequivalent Rh(I) centres in 6.^22,23

Scheme 3 Synthesis of mono and bis-triphenylphosphine derivatives of syn-[Rh^I(CO)₂]₂[Ph₄Pn] (5).

When two equivalents of PPh₃ were added to 5, initially only the monosubstituted complex 6 was observed in the ¹H and ³¹P{¹H} NMR spectra of the solution (Fig. S31†), but over the course of 12 hours the emergence of a new doublet at 51.4 ppm (¹J_RhP = 198 Hz) along with the diminishing of the signals assigned to 6 was observed in the ³¹P{¹H} NMR spectrum. The new signal is ascribed to the symmetrically disubstituted product 7, but full characterisation was impeded by the transformation not going to completion and all attempts at isolating 7 from the dynamic mixture being unsuccessful. The electronic similarity of 7 to 6 and to [CpRh^I(CO)(PPh₃)]⁹⁴ reflected in their NMR data strongly suggested each Rh^I(CO)₂ centre underwent mono-substitution instead of one Rh^I(CO)₂ centre undergoing double substitution to yield a species akin to [CpRh^I(PPh₃)₂] (³¹P = 57.3 ppm, ¹J_RhP = 222 Hz).⁹⁵ Using more than two equivalents of PPh₃ per 5 led to complex mixtures that contained 6 and 7 but also a multitude of other products (Fig. S31†), showing the introduction of phosphines to be more easily achievable than pyridines albeit with some barrier likely arising from steric hindrance (PPh₃ being much larger than CO).

When the smaller and more σ-donating trimethylphosphine was employed in place of PPh₃ a colour change from the dark yellow of 5 to red was noticed over the course of five minutes at room temperature. The ³¹P{¹H} NMR spectrum of this solution showed a signal centred at −24.0 ppm with second order coupling, indicative of a rhodium-phosphorous dimer (Fig. S34†).⁹⁶ Two broad peaks at −16.2 ppm and −34.4 ppm were also observed suggesting some fluxionality within the product(s) formed. Although all attempts at isolating a clean sample lead to decomposition, standing of a 1 : 1 THF/pentane solution of the crude reaction mixture at −35 °C overnight led to the formation of crystals of the ion pair [Rh^II(CO)(PMe₃)₄][Ph₄Pn] (8, Fig. 7) where four PMe₃ ligands had detached the Rh-carbonyl centre from the pentalenide. To the best of our knowledge this is the first crystallographically characterised uncomplexed pentalenide and exhibits a perfectly co-planar core with the phenyl rings twisted by 21.2°–26.8° to minimise steric repulsion between adjacent rings and allow some conjugation of the charge into the phenyl substituents. This rotation is less than what was found for Li₂[Ph₄Pn]⁶⁰ (29.3°–35.8°) but within the range seen for Mg[Ph₄Pn] (17.1°–35.2)°.⁶¹

Fig. 7 Synthesis of [Rh^II(PMe₃)₄(CO)][Ph₄Pn] (8) (top) and its X-ray crystal structure with thermal ellipsoids at the 50% probability level (bottom; hydrogen atoms omitted for clarity).

The cationic fragment of 8 consisted of a Rh(II) centre in a distorted trigonal bipyramidal geometry with PMe₃ groups occupying both axial and two equatorial positions with one carbonyl group occupying the third equatorial site. Exchange between the equatorial and axial positions is likely responsible for the observation of two inequivalent, broad signals in the ³¹P{¹H} NMR spectra, although variable temperature NMR data were inconclusive (Fig. S35†) due to the paramagnetic nature of Rh(II) which was independently confirmed by the Evans method (Fig. S36†). In the solid state, the Rh–P_eq bonds were 0.056 Å longer than Rh–P_ax bonds (2.3227(4)–2.3859(7) Å), with the axial PMe₃ groups bent away from the equatorial PMe₃ to give a P_ax–Rh–P_ax angle of 165.6° to relive steric clash between the methyl groups. The related cation [Rh^I(CO)(PMe₃)₄]⁺ has previously been reported,⁹⁷ however, with no spectroscopic or crystallographic data provided. Mononuclear Rh(II) complexes are uncommon and often adopt square planar or octahedral geometries in the presence of bulky chelating ligands with 15 e⁻ or 19 e⁻ counts, respectively,^98–104 whereas the cation of 8 is a rare example of a five-coordinate 17 electron complex with monodentate ligands only. The formation of a dicationic Rh(II) complex from the two adjacent Rh(I) centres in 5 is likely the result of a disproportionation reaction induced by the introduction of the strongly σ-donating PMe₃ ligand, producing dimeric Rh(0) carbonyl species responsible for the signal at −24.0 ppm in the ³¹P{¹H} NMR spectrum of the product mixture.

To see if a bidentate ligand would span across the two rhodium centres (μ²) or chelate around a single metal (κ²) in 5 the introduction of bis(diphenylphosphino)ethane (dppe) was tested. Whereas the ¹H NMR spectrum showed no significant change in the position of H_w after adding dppe to 5 in THF, the ³¹P{¹H} NMR spectrum showed a new doublet at 57.6 ppm (¹J_RhP = 128.3 Hz) with C_w still exhibiting rhodium-carbon coupling. Crystals suitable for XRD, grown from standing of the reaction mixture at −35 °C, revealed the formation of the non-contact ion pair [Rh^I(dppe)₂][Rh^I(CO)₂(η⁵Ph₄Pn)] (9, Fig. 8) where one Rh(I) centre was bound to Ph₄Pn²⁻ in an η⁵ coordination fashion with a Rh-C_t distance of 1.959(2) Å, 0.35 Å shorter than the equivalent distance in the di-Rh(I) complex 5. This significant shortening likely is a consequence of the monometallic coordination of Rh^I(CO)₂ to Ph₄Pn²⁻ leading to increased interaction of the cationic d⁸ centre with the dianionic 10π system. There was also less pronounced deviation to η³ in 9 compared to 5, as evidenced by the slight increase in the Rh–C_t–C_w angle (84.1° vs. an average 82.5° in 5) and a decrease in the ring slippage value Δ = 0.23 (cf. 0.27 in 5). Moreover, a smaller hinge angle of 8.4° was found for 9, presumably due to a relaxation of Ph₄Pn² and the loss of steric strain from syn-dimetallation, whilst the uncomplexed C₅-ring exhibited a hinge angle of 2°. The Rh–CO distances in 9 were also 0.016 Å shorter than in 5 suggesting a greater degree of back-bonding in the former, which was further supported by the IR spectrum of 9 exhibiting two CO bands at 1980 cm⁻¹ and 1918 cm⁻¹ (Fig. S41†). The [Rh^I(CO)₂(η⁵Ph₄Pn)] anion could also be crystallised with a [Mg₂Cl₃(THF)₆] cation when MgCl₂ was not removed by addition of dioxane prior to dppe addition (Fig. S42†). The cationic [Rh^I(dppe)₂]⁺ resulting from mono-demetallation of 5 by dppe has previously been reported with a ³¹P NMR signal at 57 ppm (¹J_RhP = 134.2 Hz),¹⁰⁵ consistent with the NMR signature of 9 in the crude reaction mixture. The fact that one signal for H_w and C_w respectively was observed in the NMR spectra of 9 implies rapid ring slippage of the Rh^I(CO)₂ fragment across the pentalenide core in solution.

Fig. 8 Synthesis of [Rh^I(dppe)₂][Rh^I(CO)₂(η⁵Ph₄Pn)] (9) (top) and its X-ray crystal structure with thermal ellipsoids at the 50% probability level (bottom; hydrogen atoms omitted for clarity).

The above results show that although 5 does not react with mono- or bidentate pyridines, chelating phosphines readily bind to the Rh^I(CO)₂ centre to displace both carbonyls and lead to demetallation of the pentalenide. With monodentate phosphines progressive carbonyl substitution occurs on both Rh^I(CO)₂ centres which in the case of strong σ-donors can lead to a disproportionating demetallation reaction. Thus, ligand exchange reactions using less basic and more π-accepting phosphites¹⁰⁶ with lower steric demand in comparison to phosphines were tried next.

Phosphites. As seen before with PPh₃, the addition of one equivalent P(OPh)₃ to a solution of 5 in THF resulted in the desymmetrisation of the Ph₄Pn²⁻ ligand with two new wingtip signals H_wa and H_wb appearing in the ¹H NMR at 6.22 ppm and 6.75 ppm, respectively, suggesting monosubstitution on one Rh centre of 5 to form [Rh^I(CO)₂;Rh^I(CO)(P{OPh}₃)][μ:η⁵:η⁵Ph₄Pn] (10). The two inequivalent wingtips could be distinguished by ¹H–³¹P-HMBC, with H_Wa at 6.22 ppm displaying a cross peak with the new ³¹P signal at 130.6 ppm. The difference in wingtip shift, Δw, of 0.53 ppm is similar to that observed with monosubstitution with PPh₃ in 6 (Δw = 0.48 ppm). Interestingly, while the magnitude of the Rh–P coupling in 10 was consistent with that reported for [CpRh^I(CO)(P{OPh}₃)] (¹J_RhP = 328.8 Hz), the ³¹P{H} shift of the asymmetrically twinned bimetallic was found 70 ppm downfield compared to the mononuclear complex (130.6 ppm versus 62.7 ppm).⁹⁴ When an excess of P(OPh)₃ was added to 5, the ¹H NMR spectrum showed an increase in the symmetry of the aromatic region again, with a single H_w signal emerging at 5.90 ppm (1 ppm upfield of that of 5) suggesting increased back-bonding from the metals to Ph₄Pn²⁻. These signatures were consistent with each Rh^I(CO)₂ fragment undergoing monosubstitution each to give the symmetrically di-substituted complex [Rh^I(CO)(P{OPh}₃)]₂[μ:η⁵:η⁵Ph₄Pn] (11) instead of forming inequivalent Rh^I(CO)₂ and Rh^I(P{OPh}₃)₂ moieties within the same complex. The ³¹P{¹H} NMR spectrum of 11 showed a major peak centred at 129.3 ppm (¹J_RhP = 342.7 Hz), close to the shift of 10 indicating an electronically similar environment consistent with two Rh^I(P{OPh}₃)(CO) moieties. When two equivalents of P(OPh)₃ were used a mixture of the mono- and bis-complexes 10 and 11 was observed, suggesting some (perhaps steric) barrier to forming the bis-product. With four or more equivalents of P(OPh)₃ clean formation of 11 was consistently observed, with no signs of demetallation unlike when using phosphines. X-ray crystallographic analysis of 11, the first heteroleptic half-baguette pentalenide complex, confirmed retention of the syn-geometry of metal fragments with the phosphite ligands arranged in an E-geometry (Fig. 9). The Rh–Rh distance of 2.960 Å was slightly longer in 11 than in 5 (2.913 Å), leading to a FSR of 1.19, likely as a consequence of the increased steric crowding around each Rh(I) centre. The Rh–C_t distances were comparable to that of 5 (1.9969(12) Å and 2.0013(11) versus 1.997 Å) and the ring slippage values Δ were marginally lower (0.25 versus 0.27) revealing a slightly smaller degree of deviation towards η³ than in 5. The hinge angle of 11 showed that the C₅-rings deviated from planarity by an average of 8.5°, 1.7° less than in 5 whilst the angle between the L–Rh–L plane and Pn plane was 76.7°. The Rh–CO bond lengths were on average 0.03 Å shorter than in 5 suggesting an increase in Rh–CO back-bonding which was further supported by blueshifted CO bands at 1982 and 1961 cm⁻¹ in the IR spectrum of 11 (cf. 1992, 2025, 2046 and 2064 cm⁻¹ in 5). The analogous reaction with P(OMe)₃ led to the formation of a variety of products, including a bis-substituted product, E-syn-[Rh^I(CO)(P{OMe}₃)]₂[μ:η⁵:η⁵Ph₄Pn] (12) which was found to be isostructural to 11 by single-crystal XRD exhibiting a ring slippage value of Δ = 0.27, a hinge angle of 9.5° and an angle of 78.2° between the L–Rh–L plane and the plane of Ph₄Pn²⁻.

Fig. 9 Synthesis of E-syn-[Rh^I(CO)(P{OPh}₃)]₂[μ:η⁵:η⁵Ph₄Pn] (11) and E-syn-[Rh^I(CO)(P{OMe}₃)]₂[μ:η⁵:η⁵Ph₄Pn] (12) (top) and their X-ray crystal structure with thermal ellipsoids at the 50% probability level (bottom; hydrogen atoms omitted for clarity).

Conclusion

We have reported the synthesis of a series of bimetallic Rh(I) half-baguette complexes from transmetalation of Ph₄Pn²⁻ salts with [Rh^IL₂X]₂ complexes. With either Mg[Ph₄Pn] or Li·K[Ph₄Pn] the same transmetalation product was obtained, consistent with the notion that in ethereal solution s-block pentalenides exist as solvent-separated ion pairs.^60,61 However, Mg[Ph₄Pn] provides practical advantages with cleaner formation of the transmetalation product and easy removal of MgCl₂ by induced precipitation of [MgCl₂(dioxane)]. [Rh^I(COD)]₂[Ph₄Pn] (1), [Ir^I(COD)]₂[Ph₄Pn] (2), [Rh^I(NBD)]₂[Ph₄Pn] (3) and [Rh^I(C₂H₄)₂]₂[Ph₄Pn] (4) all formed exclusively as linked anti-bimetallics, whilst the twinned syn-bimetallic [Rh^I(CO)₂]₂[Ph₄Pn] (5) was obtained regardless whether the monometallic [Rh^I(acac)(CO)₂] or bimetallic [Rh^I(CO)₂(μ-Cl)]₂ precursors were used. DFT calculations revealed a mix of repulsive M–M and attractive ML₂–ML₂ interactions which weakly favour syn arrangement in the absence of steric clash between the auxiliary ligands L. Neither anti-[Rh^I(NBD)]₂[Ph₄Pn] nor syn-[Rh^I(CO)₂]₂[Ph₄Pn] displayed Lewis-basic reactivity towards AlCl₃ due to being surrounded by π-acceptor ligands in a distorted trigonal–bipyramidal 18-electron configuration.

Attempted ligand exchange reactions of syn-[Rh^I(CO)₂]₂[Ph₄Pn] with N-donor ligands were unsuccessful, whilst the introduction of strongly σ-donating and/or chelating phosphines led to rhodium abstraction from Ph₄Pn²⁻. With PMe₃ this process was accompanied by disproportionation, forming Rh(0) dimers and the ion pair [Rh^II(PMe₃)₄(CO)][Ph₄Pn] (8). With dppe a single Rh(I) centre was abstracted to yield the salt [Rh^I(dppe)₂][Rh^I(CO)₂(η⁵Ph₄Pn)] (9) featuring an anionic, mono-nuclear pentalenide complex which could serve as a synthon to access heterobimetallic complexes in the future. PPh₃ and P(OPh)₃ reacted similarly towards 5 to successively yield the monosubstituted complexes [Rh^I(CO)₂;Rh^I(CO)(PPh₃)][Ph₄Pn] (6) and [Rh^I(CO)₂;Rh^I(CO)(P{OPh}₃)][Ph₄Pn] (10) respectively. The di-substituted complexes E-syn-[Rh^I(CO)(PPh₃)]₂[Ph₄Pn] (7) and E-syn-[Rh^I(CO)(P{OR}₃)]₂[Ph₄Pn] (R = Ph 11, R = Me 12) could be accessed using stoichiometric amounts of PPh₃ and an excess of P(OR)₃ respectively. The latter were characterised by XRD which showed each rhodium centre underwent monosubstitution, with overall retention of the syn-geometry and the phosphite ligands adopting E-conformation due to sterics. These results will be useful in guiding the targeted derivatisation of future transition metal pentalenide complexes to test and exploit intermetallic cooperation in linked and twinned bimetallics. Related monometallic Cp complexes have been employed in a range of chemistries including C–H and Si–H activation¹⁰⁷ and cycloadditions^108–113 as well as synthons for heterometallic clusters,^114,115 offering a wealth of opportunities for bimetallic half-baguette pentalenide complexes.

Experimental

General considerations

All reactions were conducted under argon using standard Schlenk techniques or a MBraun Unilab Plus glovebox unless stated otherwise. All commercially available materials were purchased from Sigma Aldrich, Fisher or Acros.

Solvents

Methanol was dried and distilled over magnesium. Toluene was dried and distilled over sodium. THF, hexane and pentane were dried and distilled over potassium. C₆D₆ was distilled over CaH₂ and stored over 4 Å molecular sieves. TMEDA, DME and 1,4-dioxane were distilled over CaH₂.

Reagents

1,3,4,6-Tetraphenyl-1,2-dihydropentalene (Ph₄PnH₂),⁶⁰ Lithium potassium 1,3,4,6-tetraphenylpentalenide (Li·K[Ph₄Pn]),⁶⁰ Magnesium 1,3,4,6-tetraphenylpentalenide (Mg[Ph₄Pn]),⁶¹ [Rh^I(COD)(μ-Cl)]₂,¹¹⁶ [Ir^I(COD)(μ-Cl)]₂¹¹⁷ and [Rh^I(NBD)(μ-Cl)]₂¹¹⁸ were prepared according to literature procedures. [Rh^I(CO)₂(μ-Cl)]₂ and [Rh^I(C₂H₄)₂(μ-Cl)]₂ were used as received.

Analysis

NMR spectra were obtained using either a 500 MHz Bruker Avance III, a 500 MHz Bruker Avance III HD with nitrogen cooled CryoProbe Prodigy (BBO) or a 400 MHz Bruker AvanceNeo with room temperature iProbe. Spectra were recorded at 25 °C unless stated otherwise. Chemical shifts (δ) are given in ppm and referenced to residual proton chemical shifts from the NMR solvent for ¹H and ¹³C{¹H} spectra. UV-vis, NIR and IR spectroscopy were performed inside a MBraun Unilab Plus glovebox. UV-vis and NIR spectra were obtained using a fibre-optic AvaSpec-2048L photospectrometer (UV-vis) and an AvaSpec-NIR256 (NIR) with an AvaLight-DH-S-BAL light source and 400 μm cables (Avantes), data was collected between 250 nm and 1000 nm with an integration time of 4 ms (UV-vis) and between 910 nm and 1700 nm with an integration time of 0.26 ms (NIR). IR spectra were obtained using a Bruker Alpha II FT-IR (recorded between 4000 and 400 cm⁻¹ with 24 scans at 2 cm⁻¹ resolution).

Single X-ray diffraction analysis was carried out at the Material and Chemical Characterisation (MC²) Facility at the University of Bath using a RIGAKU SuperNova Dual EoS2 single crystal diffractometer. Mass spectrometry was carried out at the Material and Chemical Characterisation Facility at the University of Bath using a Bruker MaXis HD ESI-QTOF with direct injection. Samples were prepared in THF inside an argon filled glovebox.

Synthesis of anti-[Rh^I(COD)]₂[μ:η⁵:η⁵Ph₄Pn] (1)

[Rh(COD)(μ-Cl)]₂ (16 mg, 0.03 mmol) was dissolved in 0.5 mL THF and added, by syringe, to a THF solution of Mg[Ph₄Pn] (20 mg, 0.03 mmol) in THF (0.5 ml). The reaction was stirred for one hour during which all solid dissolved and a colour change to red was observed. Then dioxane (0.3 mL) was added and the solution stirred for another hour, during which a creamy white precipitate formed. The supernatant was extracted via filtration and concentrated to 0.2 mL volume. To this, pentane (0.5 mL) was added, and the mixture stood overnight at −35 °C. The supernatant was removed, and the resultant red powder dried under vacuum for one hour to give the product (5 mg, 20%). Crystals suitable for XRD could be grown from standing of a THF solution at −35 °C.

¹ H NMR (500 MHz, C₆D₆) δ: 7.44 (d, ³J_HH = 7.2 Hz, 8H, ArH), 7.13–7.09 (m, 11H, ArH), 6.10 (s, 2H, H_w), 3.92 (bs, 8H, H_a), 2.08 (bs, 9H, H_c/H_b), 1.77 (d, ²J_HH = 7.9 Hz, 8H, H_c/H_b).

¹³ C{ ¹ H} NMR (126 MHz, THF-H₈) δ: 136.2, 129.7, 128.1, 126.8, 99.3 (d, ¹J_RhC = 4.8 Hz, C_w), 91.2 (d, ¹J_RhC = 4.3 Hz), 89.9, 72.9 (d, ¹J_RhC = 13.6 Hz, C_a), 32.3 (C_b).

HR APCI-MS (+) m/z expected for [M + H] = 829.1782, found 829.1722

Synthesis of anti-[Ir^I(COD)]₂[μ:η⁵:η⁵Ph₄Pn] (2)

[Ir(COD)(μ-Cl)]₂ (20 mg, 0.03 mmol) was dissolved in 0.5 mL THF and added, by syringe, to a THF solution of Mg[Ph₄Pn] (20 mg, 0.03 mmol) in THF (0.5 ml). The reaction was stirred for one hour during which all solid dissolved and a colour change to red was observed. Then dioxane (0.3 mL) was added and the solution stirred for another hour, during which a creamy white precipitate formed. The supernatant was extracted via filtration and concentrated to 0.2 mL volume. To this, pentane (0.5 mL) was added, and the solution stood overnight at −35 °C. The supernatant was removed, and the resultant yellow powder dried under vacuum for one hour to give the product (12 mg, 39%). Crystals suitable for XRD could be grown from standing of a THF solution at −35 °C.

¹ H NMR (500 MHz, C₆D₆) δ: 7.32–7.31 (m, 8H, ArH), 7.10–7.09 (m, 12H, ArH), 6.10 (s, 2H, H_w), 3.98 (bs, 8H, H_a), 2.00–1.98 (m, 9H, H_c/H_b), 1.74–1.72 (m, 8H, H_c/H_b)

¹³ C{ ¹ H} NMR (126 MHz, THF-H₈) δ: 134.9, 130.2, 128.3, 127.2, 92.3 (C_w), 86.9, 86.3, 56.1 (C_a), 33.4 (C_b)

HR APCI-MS (+) m/z expected for [M + H] = 1009.2931, found 1009.2829

Synthesis of anti-[Rh^I(NBD)]₂[μ:η⁵:η⁵Ph₄Pn] (3)

Mg[Ph₄Pn] (20 mg, 0.03 mmol) was dissolved in THF (0.5 mL) and to this solution [Rh(NBD)(μ-Cl)]₂ (14 mg, 0.03 mmol) in THF (0.5 mL) was added dropwise and stirred. Upon addition, complete dissolution of all solid was observed and a colour change to dark yellow was seen. The reaction was stirred for one hour during which a yellow precipitate formed. The precipitate was collected via filtration over a silica frit and washed with hexane (2 × 1 mL) and dried to afford the product as a yellow/orange powder (18 mg, 76%). Crystals suitable for XRD could be grown from standing of a THF solution at −35 °C.

¹ H NMR (500 MHz, C₆D₆) δ: 7.35–7.33 (m, 9H, ArH), 7.07–7.06 (m, 13H ArH), 6.22 (d, ²J_RhH = 0.8 Hz, 2H, H_w), 3.4–3.32 (m, 8H, H_a), 3.17 (bs, 5H, H_b), 0.81 (s, 4H, H_c).

¹³ C{ ¹ H} NMR: (126 MHz, C₆D₆) δ: 136.8, 129.5, 128.4, 126.3, 97.4 (d, ¹J_RhC = 5.8 Hz, C_w), 88.8 (d, ¹J_RhC = 4.3 Hz), 88.1, 58.1 (d, ³J_RhC = 7 Hz, C_c), 48.5 (C_b), 41.3 (d, ¹J_RhC = 10.3 Hz, C_a).

HR APCI-MS (+) m/z expected for [M + H] = 797.1156, found 797.1205.

Synthesis of [Rh^I(C₂H₄)₂]₂[μ:η⁵:η⁵Ph₄Pn] (4)

Mg[Ph₄Pn] (20 mg, 0.03 mmol) was dissolved in THF (0.5 mL) and to this solution [Rh(C₂H₄)₂(μ-Cl)]₂ (12 mg, 0.03 mmol) in THF (0.5 mL) was added. An immediate colour change from orange to dark green was observed. Full conversion as determined by ¹H NMR spectroscopy. Crystals suitable for XRD could be grown from standing of a THF solution at −35 °C, after treatment with dioxane to remove MgCl₂ by-product. Due to the instability of this complex, it could not be cleanly isolated to obtain a yield.

¹ H NMR (500 MHz, C₆D₆) δ: 7.41–7.39 (m, 8H, Ar–H), 7.10–7.09 (m, 12H, Ar–H), 5.98 (d, ²J_RhH = 0.7 Hz, 2H, H_w), 2.19 (d, ²J_RhH = 1.8 Hz, 16H, H_a).

¹³ C{ ¹ H} NMR: (126 MHz, C₆D₆) δ: 134.8, 130.8, 129.8, 127.2, 98.6 (d, ¹J_RhC = 5.2 Hz, C_w), 91.8 (d, ¹J_RhC = 4.3 Hz), 89.5, 50.4 (d, ¹J_RhC = 14.2 Hz, C_a).

Synthesis of syn-[Rh^I(CO)₂]₂[μ:η⁵:η⁵Ph₄Pn] (5)

Method A: [Rh(CO)₂(μ-Cl)]₂ (12 mg, 0.03 mmol) was dissolved in THF (1 mL) and added dropwise to a solution of Mg[Ph₄Pn] (20 mg, 0.03 mmol) in THF (0.5 mL) and stirred. A colour change from orange to dark yellow was observed along with complete dissolution of Mg[Ph₄Pn]. The reaction was allowed to stir for one hour, after dioxane (0.3 mL) was added and the reaction stirred for a further hour during which a brown precipitate formed. The supernatant was collected via filtration and concentrated to ∼0.3 mL. To this pentane (0.5 mL) was added and the solution stood overnight at −35 °C. The supernatant was decanted off and the powder dried under vacuum to give the product (7 mg, 32%). Crystals suitable for XRD could be grown from standing of a THF/pentane solution (2 : 1) at −35 °C.

Method B: [Rh(acac)(CO)₂] (8 mg, 0.031 mmol) was dissolved in THF (0.5 mL) and added dropwise to a solution of Mg[Ph₄Pn] (10 mg, 0.015 mmol) in THF (0.5 mL). A colour change from orange to dark yellow was observed alongside complete dissolution of Mg[Ph₄Pn]. Formation of the product was confirmed in situ by ¹H NMR.

Method C: Li·K[Ph₄Pn] (0.05 mmol) was generated in situ and to this a solution of [Rh(CO)₂(μ-Cl)]₂ (29 mg, 0.07 mmol) in THF (0.5 mL) was added dropwise. A colour change from red to dark yellow was observed and the product identified in situ by ¹H NMR.

Method D: Li·K[Ph₄Pn] (0.05 mmol) was generated in situ and to this a solution of [Rh(acac)(CO)₂] (51 mg, 0.2 mmol) in THF (0.5 mL) was added dropwise. A colour change from red to dark yellow was observed and the product identified in situ by ¹H NMR.

Method E: [MgⁿBu(THF)₂][Ph₄Pn] (10 mg, 0.012 mmol) was dissolved in C₆D₆ (0.5 mL) and to this a solution of [Rh(CO)₂(μ-Cl)]₂ (5 mg, 0.012 mmol) in C₆D₆ (0.5 mL) was added slowly. A colour change from bright yellow to dark yellow was observed and the product identified in situ by ¹H NMR.

¹ H NMR (500 MHz, THF-H₈) δ: 7.16 (d, ³J_HH = 7.1 Hz, 8H, ArH), 7.12 (d, ³J_HH = 7.3 Hz, 3H, ArH), 7.04 (t, ³J_HH = 7.7 Hz, 8H, ArH), 6.91 (d, ²J_RhH = 1.4 Hz, 2H, H_w).

¹³ C{ ¹ H} NMR (126 MHz, THF-H₈) δ: 194.5 (d, ¹J_RhC = 83.8 Hz, CO), 133.7, 130.5, 128.2, 128.2, 112.1, 90.1 (d, ¹J_RhC = 7.0 Hz, C_w), 81.9 (d, ¹J_RhC = 4.3 Hz).

HR APCI-MS (+) m/z expected for [M + H] = 724.9706, found = 724.9677.

IR (ν _CO , cm⁻¹): 2064, 2046, 2025, 1992.

General procedure for ligand substitution reactions

5 was generated in situ by the addition of [Rh(CO)₂(μ-Cl)]₂ (12 mg, 0.03 mmol in 0.3 mL THF) to Mg[Ph₄Pn] (20 mg, 0.03 mmol in 0.5 mL THF) with formation confirmed by ¹H NMR. An appropriate amount of ligand was then added (0.03 mmol, 0.06 mmol, or 0.12 mmol). Solid ligands were added as a THF solution (0.3 mL) and liquids were added directly to the solution of 5, before dioxane (0.2 mL) was added, and the solution left to stand for 10 minutes after which it was syringe filtered to remove the precipitated [MgCl₂(dioxane)]_∞. In the cases of 6, 7, 8 and 10 the products could not be cleanly isolated and were identified in situ using multi-nuclear 1D and 2D NMR techniques supported by mass spectrometry. The original NMR spectra can be found in Fig. S28–S47.†

syn-[Rh^I(CO)₂;Rh^I(CO)(PPh₃)][μ:η⁵:η⁵Ph₄Pn] (6)

¹ H NMR (500 MHz, THF-H₈) δ: 6.71 (s, 1H, H_wb), 6.23 (s, 1H, H_wa).

¹³ C{ ¹ H} NMR (126 MHz, THF-H₈) δ: 195.2 (dd, ¹J_RhC = 84.3 Hz, ²J_CP = 3.3 Hz, CO), 93.4 (C_wa), 88.7 (d, ¹J_RhC = 6.5 Hz, C_wb).

³¹ P{ ¹ H} NMR (202 MHz, THF-H₈) δ: 48.1 ppm (¹J_RhP = 202 Hz).

HR ESI-MS (+) m/z expected for [M − Rh(CO)₂] = 799.1632, found = 799.1622.

syn-[Rh^I(CO)(PPh₃)]₂[μ:η⁵:η⁵Ph₄Pn] (7)

³¹ P{ ¹ H} NMR (202 MHz, THF-H₈) δ: 51.4 (¹J_RhP = 198 Hz).

HR ESI-MS (+) m/z expected for [M] = 1192.1547, found = 1192.1459; m/z expected for [M − Rh(CO)(PPh₃)] = 799.1632, found = 799.1590.

[Rh^II(PMe₃)₄(CO)][Ph₄Pn] (8)

As per the general procedure, PMe₃ (0.12 mmol, 1M in THF) was added to a THF solution of 5. A colour change to dark red was observed. To this solution dioxane (0.2 mL) was added and subsequently filtered. Addition of pentane (1 mL, 1 : 1 ratio with THF) followed by standing of the solution at −35 °C yielded dark red crystals of [Rh^II(PMe₃)₄(CO)][Ph₄Pn] suitable for X-ray diffraction. Due to the instability of this complex, it could not be cleanly isolated to obtain a yield.

³¹ P{ ¹ H} NMR (202 MHz, THF-H₈) δ: −16.2 (bs), −34.4 (bs).

HR APCI-MS (−) m/z expected for [M − Rh(CO)(PMe₃)₄] = 406.1722, found = 406.1769.

[Rh^I(dppe)₂][Rh^I(CO)₂(η⁵Ph₄Pn)] (9)

The crude reaction mixture was concentrated to circa 0.3 mL, 1 mL hexane added and the solution stood at −35 °C for one hour. The supernatant was removed and the solid dried under vacuum to afford the product (20.3 mg, 45%).

¹ H NMR (500 MHz, THF-H₈) δ: 7.29 (t, ³J_HH = 7 Hz, 6H, ArH), 7.18–7.11 (m, 36H, ArH), 7.04 (t, ³J_HH = 7.6 Hz, 8H, ArH), 6.91 (d, ²J_RhH = 0.9 Hz, 2H, H_w), 2.11 (bs, CH₂).

¹³ C{ ¹ H} NMR (126 MHz, THF-H₈) δ: 194.4 (d, ¹J_RhH = 82.8 Hz, CO), 134.0, 133.6, 131.3, 130.5, 129.4, 129.0, 128.9, 128.5, 128.2, 128.1, 121.1, 90.1 (d, ¹J_RhC = 7.1 Hz, C_w), 81.9 (d, ¹J_RhC = 4.4 Hz), 42.1 Hz (CH₂).

³¹ P{ ¹ H} NMR (202 MHz, THF-H₈) δ: 57.6 (d, ¹J_RhP = 128 Hz).

HR ESI-MS (+) m/z expected for [M − Rh(CO)₂(Ph₄Pn)] = 899.1756, found = 899.1715.

IR (ν _CO , cm⁻¹): 1980, 1918.

syn-[Rh^I(CO)₂;Rh^I(CO)(P{OPh}₃)][μ:η⁵:η⁵Ph₄Pn] (10)

¹ H NMR (500 MHz, THF-H₈) δ: 6.75 (s, 1H, H_b), 6.22 (s, 1H, H_a).

¹³ C{ ¹ H} NMR (126 MHz, THF-H₈) δ: 88.6 (d, ¹J_RhC = 7.4 Hz, H_wb), 81.9 (d, ¹J_RhC = 4.5 Hz, H_wa).

³¹ P{ ¹ H} NMR (202 MHz, THF-H₈) δ: 130.6 (d, ¹J_RhP = 328 Hz).

E-syn-[Rh^I(CO)(P{OPh}₃)]₂[μ:η⁵:η⁵Ph₄Pn] (11)

Using 0.12 mmol P(OPh)₃. Hexane (1 mL) was added to the crude reaction mixture, and the solution stood overnight at −35 °C. The supernatant was removed, and the resultant brown powder dried under vacuum for 2 hours to give the product (16.4 mg, 41%).

¹ H NMR (500 MHz, THF-H₈) δ: 7.18–7.15 (m, 13H, ArH), 7.05–6.99 (m, 19H, ArH), 6.91–6.87 (m, 16H, ArH), 5.90 (s, 2H, H_w).

¹³ C{ ¹ H} NMR (126 MHz, THF-H₈) δ: 152.8 (d, ¹J_RhC = 10.5 Hz), 135.4, 130.9, 129.7, 127.6, 126.7, 124.7, 122.4 (¹J_RhC = 3.8 Hz), 109.6, 89.1 (m, C_w), 81.0 [CO not resolved].

³¹ P{ ¹ H} NMR (202 MHz, THF-H₈) δ: 128.5 (d, ¹J_RhP = 344 Hz).

HR ESI-MS (+) m/z expected for [M − Rh(CO)(P{OPh}₃)] = 847.1479, found = 847.1561.

IR (ν _CO , cm⁻¹): 1982, 1961.

E-syn-[Rh^I(CO)(P{OMe}₃)]₂[μ:η⁵:η⁵Ph₄Pn] (12)

Using 0.06 mmol P(OMe)₃. The solvent was removed under vacuum and the resultant brown solid washed with n-hexane (3 × 0.5 mL) to afford the product (21.4 mg, 74%).

¹ H NMR (500 MHz, THF-H₈) δ: 7.20 (d, ³J_HH = 7 Hz, 8H, ArH), 6.99–6.93 (m, 12H, ArH), 6.64 (s, 2H, H_w) [methyl groups obscured by solvent].

¹³ C{ ¹ H} NMR (126 MHz, THF-H₈) δ: 136.5, 130.7, 127.3, 126.2, 110.1, 88.3 (¹J_RhC = 7 Hz, C_w), 80.1 (dd, ¹J_RhC = 10 Hz, ²J_PC = 4 Hz), 52.2 (d, ²J_RhC = 3 Hz, CH₃).

³¹ P{ ¹ H} NMR (202 MHz, THF-H₈) δ: 146.2 (d, ¹J_RhP = 309 Hz).

HR ESI-MS (+) m/z expected for [M] = 916.0303, found = 916.0284.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

We thank the Royal Society (award UF160458) and the University of Bath for funding this work. Dr Mandeep Kaur is acknowledged for help and advice with the synthesis and isolation of several compounds, and Dr Kathryn Proctor and Dr Samantha Lau for assistance with conducting air-sensitive mass spectrometry measurements. This research made use of the Anatra High Performance Computing (HPC) Service at the University of Bath (doi.org/10.15125/b6cd-s854).

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Footnote

† Electronic supplementary information (ESI) available: Additional analytical data (NMR, NIR, XRD); Computational methods. CCDC 2309145–2309154. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3dt04325h

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