Han-Ying
Liu
,
Samuel E.
Neale
,
Michael S.
Hill
*,
Mary F.
Mahon
and
Claire L.
McMullin
*
Department of Chemistry, University of Bath, Claverton Down, Bath, BA2 7AY, UK. E-mail: msh27@bath.ac.uk; cm2025@bath.ac.uk
First published on 15th February 2023
The copper(I) alumanyl derivative, [{SiNDipp}Al–Cu(NHCiPr)] (SiNDipp = {CH2SiMe2NDipp}2; Dipp = 2,6-di-isopropylphenyl; NHCiPr = N,N′-di-isopropyl-4,5-dimethyl-2-ylidene), reacts in a stepwise fashion with up to three equivalents of various terminal alkynes. This reactivity results in the sequential formation of cuprous (hydrido)(alkynyl)aluminate, (alkenyl)(alkynyl)aluminate and bis(alkynyl)aluminate derivatives, examples of which have been fully characterised. The process of alkene liberation resulting from the latter reaction step constitutes a unique case of alkyne transfer semi-hydrogenation in which the C–H acidic alkyne itself acts as a source of proton, with the Cu–Al bond providing the requisite electrons to effect reduction. This reaction sequence is validated by DFT calculations, which rationalise the variable stability of the initially formed heterobimetallic hydrides.
In related recent, but stoichiometric, observations of group 11 reactivity, Yamashita and co-workers have described the behaviour of a gold–boryl complex, [(IPr)AuBAr2] (IPr = 1,3-bis(2,6-diisopropylphenyl-imidazole-2-ylidene); Ar = o-tolyl), towards internal alkynes (Fig. 1a).11 Although this reactivity displayed some dependence on alkyne identity, a transiently formed syn insertion product was observed to isomerise to an alternative syn-disubstituted borylalkenylgold complex via an interchange of an initial alkyne R group and an aryl substituent of the BAr2 unit. Density functional theory (DFT) calculations suggested that a common intermediate in this reaction was a gold alkynyl-borate, which can facilitate a 1,2-shift of the organic substituents. In a similar vein, Aldridge and co-workers have very recently demonstrated that the phosphine-stabilised copper–alumanyl complex, [t-Bu3PCu–Al(NON)] (NON = 4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl-9,9-dimethylxanthene) undergoes addition across internal alkynes to provide a vinylcopper compound featuring a syn configuration of the two metal centres (Fig. 1b).12a This initially formed species then further rearranges on heating via C–C bond cleavage and an inferred aluminate intermediate, before providing the corresponding trans-vinylcopper isomers as the thermodynamically favoured products.
The results summarised in Fig. 1a and b pertain to reactivity between unsupported Au–B and Cu–Al bonds and internal alkynes. Neither of these studies, however, addressed variations arising from the introduction of terminal alkynes to these systems. We have recently described a family of molecules bearing unsupported group 11-aluminium bonds, in which the coinage metal centres are stabilised by various neutral donors, and the aluminium atom is supported by a six-membered diamide chelate.13 In this contribution, therefore, we describe the reactivity arising from the copper alumanyl species, [{SiNDipp}Al–Cu(NHCiPr)] (SiNDipp = {CH2SiMe2NDipp}2; Dipp = 2,6-di-isopropylphenyl; NHCiPr = N,N′-di-isopropyl-4,5-dimethyl-2-ylidene), (1) and a variety of terminal alkynes. This chemistry provides an unusual alkyne-to-alkene transformation that may be viewed as an alkyne transfer semi-hydrogenation in which reduction is facilitated by the Cu–Al bond and in which the requisite protons are provided by the terminal alkyne itself (Fig. 1c).
2a | 3 | 7 | 8 | 10 | |
---|---|---|---|---|---|
a Al1–C22. b Cu1–C16. c Cu1–C22. d Cu1–C23. e N11–Al1–N1. f C22–Al1–C221. g N1–Al1–C22. | |||||
Al1–N1 | 1.8745(11) | 1.836(3) | 1.8429(12) | 1.8449(19) | 1.8523(11) |
Al1–N2 | 1.8799(11) | 1.845(3) | 1.8507(12) | 1.8537(19) | — |
Al1–C42 | 1.9959(14) | 1.990(3) | 1.9972(14) | 1.983(2) | 1.9852(15)a |
Al1–C44 | 1.9876(14) | 1.979(3) | 1.9980(14) | 1.975(3) | — |
C42–C43 | 1.203(2) | ||||
C44–C45 | 1.327(2) | ||||
Cu1–C31 | — | 1.960(3) | 1.9614(13) | 1.957(2) | 1.980(2)b |
Cu1–C42 | — | 2.135(3) | 2.1595(13) | 2.163(2) | 2.1846(13)c |
Cu1–C43 | — | 2.239(3) | 2.1641(14) | 2.209(3) | 2.3534(14)d |
Cu1–C44 | — | 2.158(3) | 2.1698(13) | 2.131(2) | — |
Cu1–C45 | — | 2.245(3) | 2.1763(14) | 2.200(3) | — |
N1–Al1–N2 | 112.07(5) | 115.80(12) | 113.13(5) | 114.61(9) | 113.23(7)e |
C44–Al1–C42 | 103.76(6) | 92.39(12) | 92.19(10) | 91.70(8)f | |
N1–Al1–C42 | 105.57(5) | 113.40(12) | 116.88(9) | 105.04(5)g | |
N2–Al1–C44 | 106.21(5) | 117.32(12) | 117.71(9) | — |
Continued spectroscopic monitoring of reaction mixtures containing 2 over the course of several days at room temperature also revealed the presence of trace amounts of styrene and minor quantities of a further new species, compound 3. A solution of compound 2 prepared in situ was, therefore, treated with a further equivalent of phenylacetylene and heated at 60 °C for 12 hours. This process initiated complete conversion to compound 3 and the liberation of an equimolar quantity of styrene, which was readily identifiable in the resultant 1H NMR spectrum and was deduced to have arisen from the apparent protonolysis of the E-phenylvinylide unit of 2 by the acidic proton (pKaca. 26)15 of the additional phenylacetylene equivalent (Scheme 1). The 1H NMR signals arising from the NHCiPr and SiNDipp ligand environments associated with compound 3 were consistent with a C2v-symmetric species, an inference subsequently confirmed by single-crystal X-ray diffraction analysis (Fig. 2b). This solid-state characterisation identified 3 as a molecular heterobimetallic complex, comprising a NHCiPr-coordinated copper centre that is further ligated by twofold dihapto binding to a SiNDipp-supported bis(phenylethynyl)aluminate anion. The similarity of the Al–Csp distances in 3 [Al1–C42 = 1.990(3) Å, Al1–C34 = 1.979(3) Å] and the comparable Al–C bond of the unperturbed anion of compound 2a [Al1–C42 1.9959(14) Å], lend credence to the continued attribution of a formally anionic nature to the aluminium centre in 3.
We have previously described the impact on the reactivity of the Cu–Al bond induced through variation of the copper-coordinated carbene ligand and the differentiated behaviour observed across the metals of the group 11 triad.13 To investigate the generality of the transformations observed to provide compounds 2 and 3, therefore, phenylacetylene was reacted with further compounds comprising unsupported group 11–Al{SiNDipp} bonds. In stark contrast to the exclusive production of 2, treatment of [(Me2CAAC)CuAl{SiNDipp}] (4), containing the more basic Me2CAAC carbene (Me2CAAC = 1-(2,6-di-isopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene), with phenylacetylene provided a mixture of several products, even at room temperature and irrespective of the reaction stoichiometry. Although fractional crystallisation of these reaction mixtures yielded single crystals of two further species, compounds 5 and 6, and comprising the [(PhCC)2Al{SiNDipp}]− anion observed in compound 3, the introduction of the Me2CAAC ligand evidently results in more labile heterobimetallic species and no pure, bulk samples could be isolated (Scheme 2). Compound 5 was identified by a single crystal X-ray analysis as a further charge separated heterobimetallic species, in which charge balance with the aluminate is maintained by a linear [(Me2CAAC)2Cu]+ cation (Fig. S48†). Similarly, X-ray diffraction analysis identified compound 6 as a heterotrimetallic {Cu2Al} complex, which may be viewed as resulting from the formal insertion of a [(PhCC)Cu] unit into the copper-to-carbene bond of a Me2CAAC derivative, with an aluminate structure otherwise analogous to that of compound 3 (Fig. S49†). To a similar end, the reactivity of Ag–Al and Au–Al bonds towards phenylacetylene was explored by exploiting the previously reported heavier group-11 analogues of 1, [(NHCiPr)AgAl{SiNDipp}] and [(NHCiPr)AuAl{SiNDipp}].13b In neither case, however, was any reaction observed at ambient temperature, while the application of external heating invariably resulted in decomposition and the deposition of a black precipitate assumed to be the elemental group 11 metal.
Initial studies assessed the generality of the chemistry leading to compound 3. A series of reactions were performed between three equivalents of each alkyne and compound 1 in C6D6 solution. The progress of each reaction at 60 °C was then monitored through the acquisition of their 1H NMR spectra until the starting materials had been completely consumed. While the reaction of 1 with the less sterically demanding 1-hexyne was again complete within approximately 12 hours, the bulkier acetylenes required ca. 3 days to achieve complete conversion. In a comparable fashion to the reactivity observed between 1 and phenylacetylene, each of these reactions resulted in the generation of a single stoichiometric equivalent of the corresponding terminal alkene and the production of a series of complexes, [(NHCiPr)Cu{(RCC)2Al{SiNDipp}] (7–9) (Scheme 3). All three compounds displayed 1H and 13C{1H} spectra consistent with solution structures comparable to that identified for compound 3, an assignment that was confirmed by single crystal X-ray diffraction analysis for compounds 7 and 8 (Fig. 3a and b, Table 1). The molecular structures of both compounds exhibit similar features to that of 3, with the coordination sphere of a {SiNDipp}-supported aluminate completed by two σ-bonded acetylides, with each of these anions chelated to the metal centre of the {Cu(NHCiPr)} components of the molecules via a pair of η2–π-interactions. Although the resultant metal-carbon distances show some dependence on the steric bulk of the alkynyl substituent (Table 1), the similarity of the structures to that of compound 3 obviates further necessary comment.
The selective formation of compounds 7–9 indicates that the Al–Cu bond of 1 displays analogous reactivity toward terminal acetylenes irrespective of any electronic or steric variations. Attempted extension of this study to the reaction of 1 with three equivalents of 2,4,6-trimethylphenylacetylene at the same 60 °C temperature, however, resulted in a mixture of two compounds, 10 and 10a, which could be tentatively identified in an approximate 4:1 ratio by analysis of the 1H NMR spectrum provided by the crude reaction mixture. Although pure bulk samples of neither compound could be obtained, crystallisation of the reaction solution and mechanical separation of the resultant single crystals enabled the identification of 10 (Scheme 4a, Fig. 3c and Table 1) and 10a (Scheme 4a and Fig. S50†) as the respective molecular and charge separated analogues of compounds 3 and 5. Suspecting the reduced stability of the heterobimetallic species 10 towards extrusion of the copper cation to be consequence of the modified steric profile of the 2,4,6-trimethylphenylacetylide unit under the thermal conditions used in its synthesis, the reaction was repeated at room temperature. This procedure, however, resulted in the deposition of a further compound, 11, as a colourless crystalline solid. Although the subsequent insolubility of 11 precluded its further characterisation in solution, its identity as a further charge separated derivative, [(NHCiPr)Cu(NHCiPr)][(MesCHCH)(MesCC)Al{SiNDipp}], comprising a (mesitylethenyl)(E-mesitylethynyl)aluminate analogous to that characterised in the structure of 2a, but in this case with charge balance achieved by a bis-carbene ligated copper cation, was confirmed by a further X-ray diffraction analysis (Scheme 4a and Fig. S51†).
Prompted by the significantly extended reaction times required in the synthesis of compounds 8 and 9, equimolar reactions between 1 and the more sterically encumbered trimethylsilylacetylene and 3,3-dimethylbut-1-yne were performed at room temperature. Assessment by in situ1H NMR spectroscopy evidenced the selective formation of single predominant new species in both reactions, which were characterised by a loss of the C2 symmetry associated with the chelated SiNDipp ligand of compound 1. Crystallisation of the reaction mixtures in both cases afforded single crystals suitable for X-ray crystallography, which revealed their identities to be the μ-hydride- and η2-Cu-κ1-acetylide-bridged copper species differing solely in their respective, trimethylsilyl-(12) and tert-butylalkynyl (13) substituents (Scheme 4b, Fig. 4). Although copper hydride species are by no means uncommon,16 molecular derivatives comprising comparable Al-μ-H-Cu bridging are limited to a single report of several compounds arising from the treatment of β-diketiminato Cu(I) complexes with similarly ligated aluminium dihydrides.17
In contrast to similar reactions performed with both phenylacetylene (Scheme 1) and 1-hexyne, no evidence for the generation of copper-(alkenyl)(alkynyl)-aluminate species analogous to 2/2a could be observed. Heating (60 °C), however, of both heterobimetallic hydrides 12 and 13 in the presence of additional equivalents of the relevant acetylenes provided the corresponding bis(alkynyl)aluminate derivatives, 8 and 9, respectively (Scheme 4b).
With the experimental results and the proposed stepwise mechanism in hand, the reactivity of compound 1 with both phenylacetylene (PhCCH) and 3,3-dimethylbut-1-yne (t-BuCCH) was assessed by DFT at the BP86-D3BJ, (PCM = C6H6)/BS2//BP86/BS1 level of theory (see the ESI† for full computational details and results). Initial calculations focussed on these two substrates owing to the evidently contrasting kinetic facility of their reactivity with 1 under ambient conditions. Although a common pathway may be assumed, the hydride-bridged species 13 could be characterised upon addition of two equivalents of t-BuCCH at room temperature (Scheme 4), with onwards addition of further acetylene equivalents only affording 10 upon heating. Conversely, no evidence of hydride formation could be observed during reactions of 1 with phenylacetylene, with the (E-phenylethenyl)(phenylethynyl)aluminate 2 as the first observable product prior to the formation of 3. Fig. 5 details the free energy pathway of formation of bridging hydride species for both terminal acetylenes, while Fig. 6 illustrates the onward reactivity to form 3 and 10, respectively for PhCCH and t-BuCCH. For ease of discrimination, compound 1 is relabelled as I in this computational study.
Fig. 6 Computed free energy profile (BP86-D3BJ(PCM = C6H6)/BS2//BP86/BS1 level, energies quoted in kcal mol−1) of onwards formation of PPh (black) and PBu (blue) from VPh and VBu, respectively. |
Beginning with PhCCH (black, Fig. 5), alkyne coordination to the Cu centre of Ivia an η2 interaction takes place viaTS(I-II)Ph (+10.0 kcal mol−1) and results in the exergonic formation of IIPh (−5.0 kcal mol−1). Subsequently facile Al–C bond formation proceeds with simultaneous Cu–Al cleavage viaTS(II-III)Ph, (+0.9 kcal mol−1) to form the syn-addition product IIIPh (−15.2 kcal mol−1). C–H cleavage then takes place viaTS(III-IV)Ph (+7.6 kcal mol−1) with an activation barrier of 22.8 kcal mol−1, in which Al–H bond formation occurs concomitantly with slippage of the NHCiPr–Cu moiety across the alkynyl group to yield IVPh (−15.2 kcal mol−1). It must be emphasised that this mode of C–H activation differs to that of direct Ph–CC–H deprotonation at the alumanyl centre, with subsequent Al–C formation. Migration of the NHCiPr–Cu unit then occurs through rotation about the Ph–CC– vector viaTS(IV-V)Ph (−8.8 kcal mol−1) to afford the bridging hydridocopper species VPh (−28.5 kcal mol−1). Moving to the t-BuCCH profile (blue, Fig. 5), alkyne addition viaTS(I-II)Bu (+14.3 kcal mol−1) proceeds with an identical mode of η2-coordination to the Cu centre, but with a higher barrier than that of PhCCH. The resultant Z-alkenyl species IIIBu (−10.2 kcal mol−1) ultimately forms in an analogous manner to that of PhCCH, albeit less exergonically. Consistent with the observation of 12, however, the formation of the bridging hydride species VBu is also kinetically feasible at room temperature from IIIBuvia the energetically accessible TS(III-IV) (ΔG‡ = 20.8 kcal mol−1).18 The calculations, thus, validate the kinetic and thermodynamic viability of the bridging hydride intermediates for both alkynes. The contrasting stabilities of VPh and VBu evident from the synthetic study, however, may be attributed to their subsequent facility towards reactivity with further equivalents of alkyne, as depicted in Scheme 5 and Fig. 6.
From VPh, addition of a second equivalent of PhCCH (black, Fig. 6) affords VIPh (−18.1 kcal mol−1) via η2-coordination with Cu and concomitant cleavage of the Cu–H bridging interaction. Formation of the E-phenylvinylide adduct, EPh (−59.6 kcal mol−1), is then initiated by hydride transfer viaTS(E)Ph (−5.1 kcal mol−1) and an energetic span of 23.4 kcal mol−1 (relative to VPh).19 It should be noted that kinetically viable H-transfer routes were also identified from VIPh that afford 1,1-phenylvinylidenyl-(viaTS(1,1)Ph, −6.9 kcal mol−1) and Z-phenylvinylidyl- (viaTS(Z)Ph, −6.6 kcal mol−1) derivatives. While these processes have comparable activation barriers, the ultimate product, EPh, is thermodynamically favoured, suggesting that this product dominates in a post-H-transfer equilibration given that the calculations indicate H-transfer is irreversible (see the ESI† for further details). From EPh, uptake of a third equivalent of PhCCH affords INT(E-P)Ph (−50.3 kcal mol−1), whereupon subsequent proton transfer to the E-phenylvinylide moiety viaTS(E-P)Ph (−42.7 kcal mol−1) is facile to yield INT2(E-P)Ph (−49.9 kcal mol−1). Finally, styrene-dissociation from the Al centre, simultaneous with Cu to Al acetylide transfer, viaTS2(E-P)Ph (−44.4 kcal mol−1), releases styrene to form INT3(E-P)Ph (−55.0 kcal mol−1), and ultimately, PPh (−79.1 kcal mol−1, equivalent to 3 in the synthetic study) following Ph–CC transfer to the Al centre.
Characterisation of t-BuCCH addition to VBu (blue, Fig. 6) reveals an analogous η2-adduct, VIBu (−4.6 kcal mol−1), and subsequent H-transfer to form the E-alkyenyl adduct EBu (−47.7 kcal mol−1) proceeds viaTS(E)Bu (+5.4 kcal mol−1). The energetic span in this case, however, (from VBu) is 30.2 kcal mol−1, indicating that the observation of 12 (VBu) at room temperature is due to the kinetic disinclination of VBu towards uptake of an additional bulky t-BuCCH to initiate H-transfer at ambient conditions to form EBu.
Footnote |
† Electronic supplementary information (ESI) available. CCDC 2234315–2234325. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3sc00240c |
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