Terminal dysprosium and holmium organoimides

Abstract

Terminal rare-earth-metal imide complexes Tp^tBu,MeLn(NC₆H₃iPr₂-2,6)(dmap) of the mid-late rare-earth elements dysprosium and holmium were synthesized via double methane elimination of Lewis acid stabilized dialkyl precursors Tp^tBu,MeLnMe(GaMe₄) with primary aniline derivative H₂NC₆H₃iPr₂-2,6 (H₂NAr^iPr). Exploiting the weaker Ln–CH₃⋯[GaMe₃] interaction compared to the aluminium congener, addition of the aniline derivative leads to the mixed methyl/anilido species Tp^tBu,MeLnMe(HNAr^iPr) which readily eliminate methane after being exposed to the Lewis base DMAP (=N,N-dimethyl-4-aminopyridine). Under the same conditions, [AlMe₃]-stabilized dimethyl rare-earth-metal complexes transform immediately to Lewis acid bridged imides Tp^tBu,MeLn(μ₂-NC₆H₃Me₂-2,6)(μ₂-Me)AlMe₂ (Ln = Dy, Ho). DMAP/THF donor exchange is accomplished by treatment of Tp^tBu,MeLn(NC₆H₃iPr₂-2,6)(dmap) with 9-BBN in THF while the terminal imides readily insert carbon dioxide to afford carbamate complexes.

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Introduction

Recent years have witnessed major progress in the field of rare-earth-metal (Ln) complexes with multiply bonded (dianionic) main-group ligands, most notably (organo)imide chemistry.^1–3 Upon closer inspection this is not all that surprising, since [Ln=NR] moieties display a favourable Pearson hard/hard match and enhanced steric/electronic variability through the imido substituent R,⁴ compared to other fragments such as [Ln=O],⁵ [Ln=PR],⁶ or [Ln=CR₂].⁷ Of particular interest have been the synthesis of terminal rare-earth-metal imides^8–15 and a fundamental understanding of the Ln–imido bonding as well as reactivity,¹⁶ and in particular small-molecule-activation scenarios.¹⁷ Closely based on Chen's synthesis protocol of the first terminal scandium imide complex L¹Sc(NAr^iPr)(dmap) (Ar^iPr = C₆H₃iPr₂-2,6; L¹ = [Ar^iPrNC(Me)CHC(Me)N(CH₂)₂NMe₂]),⁸ a donor-promoted intramolecular methane/tetramethylsilane elimination from mixed methyl(neosilyl)/primary amido precursors emerged as the most efficient approach for accessing terminal Ln(III) imides (Scheme 1/A).^8–13 Meanwhile, another three strategies have been successfully pursued for the early larger rare-earth metals. While anionic terminal ceric imides could be obtained by the deprotonation of a neutral primary amide complex with alkali-metal silylamides (Scheme 1/B),¹⁴ a [Ln=CH₂(GaMe₃)₂] → [Ln=NR(thf)₂] transformation proved feasible for generating the open-shell terminal imides Tp^tBu,MeLn(NAr^iPr)(thf)₂ (Ln = Ce, Nd, Sm) (Scheme 1/C).¹⁵ More recently, an anionic cerium(IV) terminal imide was accessed by a two-electron oxidation of a “Ce(II)” complex supported by a tripodal tris(amido)arene ligand using azide N₃Ar^CF₃ (Ar^CF₃ = C₆H₃(CF₃)₂-3,5) (Scheme 1/D).¹⁸ Less surprising, terminal imides of the extremely large divalent rare-earth-metal centres have remained elusive.¹⁹

Scheme 1 Synthesis strategies for terminal rare-earth-metal imides.

Common features of all terminal rare-earth-metal imides, reported so far, are that the imido ligand is derived from a substituted aniline, H₂NAr^R (R = iPr, Me, CF₃), and an indispensable kinetic stabilization by use of sterically demanding ancillary co-ligands. Aliphatic amines, benzylic amines, and silylamines engage in imido ligand formation as well but have been detected only as metal-bridging and Lewis acid stabilized versions.³ Successfully applied ancillaries include β-diketiminato (nacnac),^8,9,11 phosphazene,¹⁰ TriNox,¹⁴ and multidentate pyrazolato ligands.^12,13,15 We found that especially the bulky monoanionic scorpionato ligand hydrotris(3-tert-butyl-5-methylpyrazolyl)borato (Tp^tBu,Me) provides a useful scaffold for stabilizing terminal imides as well as phosphinidenes.^6c,12,15 However, like for all terminal Ln(III) imides, stabilization of the highly polarized Ln=N bond, which predominantly consists of non-directional ionic interactions, is still challenging as it readily reacts with solvent molecules or the ancillary ligand of the aspired complexes.^2,3 DFT calculations performed on the yttrium compound Tp^tBu,MeY(NC₆H₃Me₂-2,6)(dmap) confirmed a large ionic character of the Y–N(imido) bond but also a significant covalent bonding pattern with one σ-type and two π-type interactions.^6c

Given the feasibility of terminal imides of the early open-shell cations Ce(III) (f¹), Nd(III) (f³), and Sm(III) (f⁵),¹⁵ we herein envisaged the synthesis of those of the mid-late open-shell cations Dy(III) (f⁹) and Ho(III) (f¹⁰). These metal centres exhibit ionic radii similar to Y(III) but would (if at all) contribute to a distinct covalent bonding. On the other hand, their much higher molar mass might promote the crystallization behaviour and hence, the single-crystal X-ray structure diffraction (SCXRD) analysis of the targeted complexes. We also examined the reactivity of the first terminal, trivalent dysprosium and holmium imide complexes.

Results and discussion

Selection of precursors

Our original approach toward the terminal yttrium imide Tp^tBu,MeY(NC₆H₃Me₂-2,6)(dmap) involved the mixed methyl/tetramethylgallato complex Tp^tBu,MeYMe(GaMe₄) as a suitable precursor.¹² There, the ease of GaMe₃ displacement proved to be crucial for the successful synthesis. Consequently, we chose the trimethylgallium-stabilized dialkyl complexes Tp^tBu,MeLnMe(GaMe₄) (1-Ln^Ga; Ln = Y,¹² Dy,^6c Ho) as precursors for the present study. Like their aluminium congeners,²⁰ complexes 1-Ln^Ga are available in moderate yield via protonolysis of the homoleptic gallates Ln(GaMe₄)₃ (ref. 21) with H[Tp^tBu,Me] (ref. 22) and precipitation from toluene or n-hexane solution (Scheme 2; for detailed metrics of Ln(GaMe₄)₃ (Dy, Ho) and 1-Ho^Ga; see the ESI, Fig. S1–S3†). The solid-state structure of the bimetallic compounds Tp^tBu,MeLnMe(μ₂-MeEMe₃) (1-Ln^E, E = Al, Ga), depicting one terminal methyl group and an almost linear Ln–Me–E linkage, is not reflected in the solution NMR spectra, which reveal highly fluxional methyl groups at ambient temperature, an even higher mobility in case of the gallium derivatives. The isostructural 1-Ln^Ga show Ln–C(Me) (Dy: 2.389(3) Å,^6c Y: 2.385(3) Å,¹² Ho: 2.356(5) Å) and Ln–C(Me_Ga) distances (Dy: 2.736(2) Å,^6c Y: 2.688(2) Å,¹² Ho: 2.652(4) Å) in accordance with the distinct Ln(III) radii.²³ Unexpectedly, the solid-state structures of the formally five-coordinate 1-Ln^Ga of the similar sized yttrium and holmium differ in the hapticity of the Tp^tBu,Me ligand and in the Ln1–C26–Ga1 angle, which is more linear for the holmium derivative (174.9(2) vs. 163.3(1)°). Packing effects of co-crystallizing toluene in 1-Ho^Ga have presumably a major impact on the coordination of the GaMe₄ moiety and might also cause the bending of one pyrazolyl moiety toward the rare-earth metal centre. The pyrazolyl nitrogen atoms exhibit Ho–N interatomic distances ranging from 2.334(3) to 2.376(3) Å with an additional close contact to the tilted pyrazolato ligand (Ho⋯N5, 2.859(3) Å, see Fig. S3†). The proposed mechanism for the formation of 1-Ln^Ga includes the preformation of [Tp^tBu,MeLn(GaMe₄)₂] under release of methane and trimethylgallium, and the elimination of a second molecule GaMe₃ sterically induced by the bulky Tp^tBu,Me ligand.

Scheme 2 Formation of Tp^tBu,MeLn(NAr^iPr)(dmap) (4-Ln: Ln = Y, Dy, Ho) via reaction of Tp^tBu,MeLnMe(GaMe₄)] (1-Ln^Ga) with primary aniline H₂NAr^iPr to afford mixed methyl/amido complexes Tp^tBu,MeLnMe(HNAr^iPr) (2-Ln), and subsequent addition of DMAP. Use of 1-adamantylamine led to bis(amido) complex [Tp^tBu,MeHo(HNAd)₂] (3-Ho). Nucleophilic attack of acetonitrile by Dy–CH₃ affords 7-Dy.

The primary aniline H₂NAr^iPr (Ar^iPr = C₆H₃iPr₂-2,6) has proven a privileged imido ligand precursor in rare-earth-metal chemistry.^1–3 The molecule not only features the right balance of adequately acidic protons, but also a sufficient steric protection with bulky substituents in the positions 2 and 6 at the aryl group. The reaction of bis(alkyl) 1-Ln^Ga with H₂NAr^iPr to form the mixed methyl/primary amido complexes Tp^tBu,MeLnMe(HNAr^iPr) 2-Ln (Ln = Y, Dy, Ho) is clearly visible by the elimination of methane and displacement of trimethylgallium (Scheme 2).

Complexes 2-Dy and 2-Ho are isostructural but crystallize in different space groups (2-Dy: P2₁/c; 2-Ho: P1̄; for the crystal structures and detailed metrics, see the ESI, Fig. S4/S5†). Both compounds are insoluble in n-hexane, but dissolve in aromatic solvents such as toluene. The ancillary ligand coordinates in a κ³ fashion (N, N′, N′′) with considerably varying Ln–N_pz distances (2-Dy: 2.3752(17)–2.5163(18) Å; 2-Ho: 2.369(2)–2.536(2) Å). The Ln–N_amido (2-Dy: 2.212(2) Å; 2-Ho: 2.222(2) Å) and the Ln–CH₃ (2-Dy: 2.436(2) Å; 2-Ho: 2.427(3) Å) interatomic distances lie in the range of the similar mixed methyl/amido complexes Tp^tBu,MeLuMe(HNAr^R) (Ar^R = Ar^Me₂ = C₆H₃Me₂-2,6: Lu–N_amido 2.189(2) Å, Lu–CH₃ 2.369(2) Å; Ar^R = Ar^CF₃ = C₆H₃(CF₃)₂-3,5: Lu–N_amido 2.215(1) Å, Lu–CH₃ 2.360(1) Å).¹² The Ln–N_amido–C_ipso angle spans a wide range from 142.5(1)° (Ln = Lu, Ar^R = Ar^CF3),¹² 153.8(1)° (Ln = Lu, Ar^R = Ar^Me2)¹² over 155.84(16)° (Ln = Dy, Ar^R = Ar^iPr) to 160.28(19)° (Ln = Ho, Ar^R = Ar^iPr), dependent on the Ln(III) centre and the substitution pattern of the amido ligand. As the paramagnetic nature of dysprosium and holmium impedes conclusive interpretations of their NMR spectra, the yttrium congener 2-Y was accessed from 1-Y^Ga and H₂NAr^iPr. The ¹H NMR spectrum of 2-Y shows a sharp singlet at 0.46 ppm for the methyl group and a broader singlet at 4.87 ppm for the proton of the amido ligand.

Noteworthy, the reaction of 1-Ho^Ga with 1-adamantylamine in a 0.9 : 1 ratio gave the bis(amido) holmium complex Tp^tBu,MeHo(HNAd)₂ (3-Ho, Ad = adamantyl), even though the primary amine was added in deficit. Apparently, the single deprotonation is favored over the second deprotonation, as the less Brønsted acidic second proton is less prone to be abstracted than the first proton of a second primary aniline. According to the present synthesis protocol, so far only sufficiently acidic aniline derivatives lead to the successful isolation of terminal rare-earth-metal imides, as other, less bulky and less electronically advantageous amines only form bis(amido) complexes,^3,6c or in case 1-Ln^Al are employed result in trimethylaluminium-stabilized imide species.^20a Like its precursors, complex 3-Ho is insoluble in aliphatic solvents, but readily dissolves in toluene and THF. The crystal structure of 3-Ho (triclinic P1̄ space group) shows the expected κ³ fashion (N, N′, N′′) of the ancillary ligand with Ho–N_pz distances in the range of 2.393(3)–2.610(3) Å (Fig. 1).

Fig. 1 Crystal structure of 3-Ho. All atoms are represented by atomic displacement ellipsoids set at 50% probability. Solvent molecules and hydrogen atoms except for those of B–H and N–H are omitted for clarity. Selected interatomic distances (Å) and angles (°): Ho1–N7 2.172(3), Ho1–N8 2.170(3); Ho1–N7–C35 144.0(3), Ho1–N8–C25 148.5(3) (for further metrics, see ESI†).

The Ln–N_amido distances (ø 2.171 Å) are longer compared to the Ln–N_imido interactions in Tp^tBu,MeHo(μ₂-NAd)AlMe₃ (2.087(2) Å),^20a but slightly shorter compared to the mixed methyl/amido complexes 2-Ln (Ln = Dy, Ho) described beforehand. This reflects a better electron donation of two nitrogen atoms to the metal centre, despite increased steric bulk. However, the angles Ho1–N7–C35 (144.0(3)°) and Ho1–N8–C25 (148.5(3)°) are significantly more bent compared to the methyl/anilido complex 2-Ho (160.28(19)°).

Ln(III) imide synthesis

Treatment of the mixed methyl/anilido complexes 2-Ln with the Lewis base DMAP led to the isolation of the targeted terminal lanthanide imide complexes Tp^tBu,MeLn(NAr^iPr)(dmap) (4-Ln: Ln = Y, Dy, Ho). DMAP was used previously for the synthesis of terminal rare-earth-metal imide complexes,^8,10–12 exploiting its strong donor capacity (versus e.g., THF) to induce the elimination of methane via inner-sphere deprotonation of the amido species. We assume, that the preceding coordination of DMAP to the metal centre affects the geometry of its coordination sphere in a way that the methyl group and the amido proton come into close proximity and finally evolve methane, being a very potent leaving group. The ¹H NMR spectrum of 4-Y evidences the deprotonation of the amido functionality via the methyl ligand since both the N–H and methyl signal disappeared (see Fig. S21†). Complexes 4-Ln are insoluble in non-polar solvents (e.g. n-hexane), but easily dissolve in aromatic or polar solvents like toluene and THF.

While the yttrium complex 4-Y could not be obtained in very pure form and gave only poor crystal quality (connectivity structure only, see ESI Fig. S7†), the dysprosium and holmium congeners displayed good crystallization and diffraction behaviours (isotypic, monoclinic space group C2/c, Fig. 2 and S8/9†). The metal centres are pentacoordinate with the ancillary Tp^tBu,Me ligand coordinating in the familiar κ³ fashion (N, N′, N′′). The Ln–N_pz distances typical of scorpinate ligands show two shorter bonds (4-Dy: 2.450(3)/2.452(3) Å, 4-Ho: 2.426(4)/2.436(3) Å) and one longer (4-Dy: 2.517(3) Å, 4-Ho: 2.492(4) Å). The Ln–N_imido distances are in line with those of other terminal imides, considering the changes in the ionic radii (Table 1). Except for the anionic ceric complex [(TriNOx)Ce(NAr^CF₃)[Cs(2.2.2-cryptand)] (157.3(4)°),^14a the Ln–N_imido–C_ipso angles of all trivalent terminal imides are larger than 165° and almost linear (4-Dy: 166.0(2)°; 4-Ho: 166.7(3)°; Table 1). Exchange of the donor ligand dmap for thf seems to entail a shortening of the Ln–N_imido bond but doesn't appear to affect the Ln–N_imido–C_ipso angle.

Fig. 2 Left: Crystal structure of 4-Ho. All atoms are represented by atomic displacement ellipsoids set at 50% probability. Solvent molecules, and hydrogen atoms except for that of B–H are omitted for clarity. For selected interatomic distances and angles, see Table 1 and ESI.† Middle: Crystal structure of 7-Dy. All atoms are represented by atomic displacement ellipsoids set at 50% probability. Solvent molecules and hydrogen atoms except for those of B–H and N–H are omitted for clarity. Selected interatomic distances (Å) and angles (°): Dy1–N7 2.219(3), Dy1–N8 2.148(3); Dy1–N7–C25 156.3(3), Dy1–N8–C44 165.3(3) (for further metrics, see ESI†). Right: Crystal structure of 8-Dy. All atoms are represented by atomic displacement ellipsoids set at 50% probability. Only one molecule of the asymmetric unit is shown. Solvent molecules, and hydrogen atoms except for that of B–H are omitted for clarity. For selected interatomic distances and angles, see Table 1 and ESI.†

Table 1 Selected metrical parameters of terminal rare-earth-metal imides

Compound	Ln=Nimido/Å	Ln–do/Å	Ln–Nimido–Cipso/deg	IR^d/Å (ref. 23)	CN	Ref.
a Two molecules in the asymmetric unit. b nacnac^R1 = [Ar^iPrNC(Me)CHC(Me)N(CH2)2NMe2]. c nacnac^R2 = [Ar^iPrNC(Me)CHC(Me)N(CH2)2N(CH2)2NMe2]. d Effective ionic radii.
Tp^tBu,MeLu(NAr^CF3)(dmap)	1.993(5)	2.377(5)	175.8(5)	0.861	5	12
Tp^tBu,MeY(NAr^Me2)(dmap)	2.024(4)	2.426(4)	173.6(4)	0.900	5	12
Tp^tBu,MeHo(NAr^iPr)(dmap) (4-Ho)	2.012(4)	2.429(4)	166.7(3)	0.901	5	This work
Tp^tBu,MeDy(NAr^iPr)(dmap) (4-Dy)	2.017(3)	2.450(3)	166.0(2)	0.912	5	This work
Tp^tBu,MeDy(NAr^iPr)(thf) (8-Dy)^a	2.008(3)/2.004(4)	2.397(3)/2.392(3)	166.9(3)/165.3(3)	0.912	5	This work
Tp^tBu,MeSm(NAr^iPr)(thf)2	2.067(5)	2.527(5)/2.560(4)	169.3(5)	0.958	6	15
Tp^tBu,MeNd(NAr^iPr)(thf)2	2.076(4)	2.557(4)/2.594(3)	169.2(4)	0.983	6	15
Tp^tBu,MeNd(NAr^iPr)(thf)^a	2.036(7)/2.047(7)	2.517(6)/2.511(6)	165.8(4)/163.2(6)	0.983	5	15
Tp^tBu,MeCe(NAr^iPr)(thf)2	2.101(5)	2.599(3)/2.628(3)	171.3(3)	1.01	6	15
(nacnac^R1)Sc(NAr^iPr)(dmap)^b	1.881(8)	2.271(5)	169.6(5)	0.745	5	8
(nacnac^R1)Sc(NAr^iPr)(thf)^b	1.852(4)	2.251(3)	168.6(3)	0.745	5	11
(nacnac^R2)Sc(NAr^iPr)(dmap)^c	1.8591(18)	2.369(2)	167.90(17)	0.745	5	9
[(PhN = Ph2P)2N]Sc(NAr^iPr)(dmap)2	1.853(3)	2.379(3)/2.326(3)	168.8(3)	0.745	6	10
(BPz2Py3)Sc(NAr^iPr)	1.877(3)	—	173.1(3)	0.745	6	13
[(TriNOx)Ce(NAr^CF3)][Cs(2.2.2-cryptand)]	2.077(3)	—	157.3(4)	0.87	8	14a
[(^AdTPBN3)Ce(NAr^CF3)][K(2.2.2-cryptand)]	2.0742(4)	—	176.3(4)	<0.87	4	18

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In contrast, and as pointed out previously, the reaction of the primary aniline H₂NAr^Me₂ (Ar^Me₂ = C₆H₃Me₂-2,6) with the aluminium congeners Tp^tBu,MeLnMe(AlMe₄) (1-Ln^Al) implies the formation of trimethylaluminium-stabilized imide complexes.¹² These complexes cannot be converted into unsupported terminal rare-earth-metal imide complexes by applying Lewis bases such as 1,4-dioxane, pyridine, DMAP, or TMEDA (N,N,N′,N′-tetramethylethylenediamine). Complex 1-Dy^Al is accessible from Dy(AlMe₄)₃ (ref. 24) and H[Tp^tBu,Me],²² in analogy to the yttrium and holmium complexes reported previously.²⁰ In order to probe the effect of the substituents on the aniline, compounds 1-Ln^Al (Ln = Dy, Ho) were reacted with H₂NAr^iPr, H₂NAr^Me2, and H₂NAr^Me3 (Ar^Me3 = C₆H₂Me₃-2,4,6) (Scheme 3).

Scheme 3 Synthesis of Lewis acid stabilized, bimetallic imides Tp^tBu,MeLn(NAr^Me2)(μ₂-MeAlMe₃) (5-Ln; Ln = Dy, Ho) and Tp^tBu,MeHo(NAr^Me3)(μ₂-MeAlMe₃).

Contrary to the reaction with the weaker coordinated trimethylgallium in complexes 1-Ln^Ga, the 1-Ln^Al/H₂NAr^iPr reaction was inconclusive. Like in the case of yttrium,¹² the sterically less demanding anilines gave the trimethylaluminium-stabilized imides Tp^tBu,MeLn(μ₂-NAr^Me2)AlMe₃ (5-Ln: Ln = Dy, Ho) and Tp^tBu,MeHo(μ₂-NAr^Me3)AlMe₃ (6-Ln). The coordinated trimethylaluminium is not removable, neither under vacuum, nor with Lewis bases (e.g., DMAP, THF). Presumably, after the first methane elimination and the coordination of the primary amido functionality to the metal centre, one of the methyl groups at the [μ₂-MeAlMe₃] unit abstracts the second amido proton via release of another molecule of methane, resulting in 5-Ln. Compounds 5-Ln are insoluble in aliphatic solvents, but dissolve in aromatic and polar solvents. The isostructural complexes crystallize in different space groups (5-Dy: monoclinic, P2₁/n; 5-Ho: triclinic, P1̄; 6-Ho: monoclinic, Cc; Fig. 3, S10 and S11†).

Fig. 3 Crystal structure of 5-Ho. All atoms are represented by atomic displacement ellipsoids set at 50% probability. Solvent molecules, and hydrogen atoms except for that of B–H are omitted for clarity. Selected interatomic distances (Å) and angles (°): Ho1–N7 2.116(4), Ho1–C25 2.549(5), Al1–C25 2.105(6); Ho1–N7–C28 146.7(4) (for isostructural 5-Dy and 6-Ho and further metrics, see ESI†).

The bulky Tp^tBu,Me ligand coordinates again in the κ³ fashion (N, N′, N′′) with interatomic Ln–N distances in the range of 2.423(2)–2.492(2) Å (5-Dy), 2.407(4)–2.437(4) Å (5-Ho) and 2.394(3)–2.454(3) Å (6-Ho). The central metal ion is pentacoordinate and the Ln–N_imido–C_ipso angle is strongly bent (5-Dy: 146.68(17)°; 5-Ho: 146.7(4)°; 6-Ho: 151.0(2)°), which is due to the interaction with the Lewis acid trimethylaluminium. Hence, the electronic situation differs considerably compared to the terminal rare-earth-metal imide complexes 4-Ln (Ln = Dy, Ho) as electron density of the imido nitrogen is shifted to the empty p orbitals of the aluminium ion. This is also reflected in the Ln–N_imido bonds of 5-Ln and 6-Ho which are elongated by ca. 0.1 Å (5-Dy: 2.129(2) Å; 5-Ho: 2.116(4) Å; 6-Ho: 2.116(4) Å), matching those in Tp^tBu,MeY(μ₂-NAr^Me)AlHMe₂ (Y–N_imido: 2.133(2) Å; Y–N_imido–C_ipso: 145.1(2)°).²⁵ Further structural comparison with the similar holmium imides Tp^tBu,MeHo(μ₂-NR)AlMe₃ (R = tBu, adamantyl) clearly indicate a more pronounced Ln–N_imido interaction for the latter (Ho–N_imido: 2.083(2), 2.087(2) Å; Ho–N_imido–C_ipso: 140.4(2), 140.2(1)°).^20a Other Lewis acid supported monomeric rare-earth-metal imide complexes include Mindiola's (PNP)Sc(μ₂-NAr^iPr)(μ₂-Me)AlMe₂ [PNP=N(2-P-(CHMe₂)₂-4-methylphenyl)₂] (ref. 26) or Tp^tBu,MeLn(NAr^iPr)(MMe₃) (Ln = Ce, Nd, Sm; M = Al, Ga) from our group.^6c

Probing donors other than DMAP

N,N-Dimethyl-4-aminopyridine (DMAP) emerged as a most valuable donor for forcing the elimination of methane via inner-sphere deprotonation of the primary amido species in 2-Ln-type complexes. In order to assess the impact of the donor molecule on the Ln–N_imido bonding, the implementation of other donor ligands was examined. In general, donor ligands might be introduced according to Scheme 1 promoting methane elimination (route A) or via post-imide-synthesis exchange (Scheme 4). Since imido ligand formation according to route A could not be achieved with ethereal donors such as OEt₂ or THF, complex 2-Dy was treated with N-donors TMEDA (tetramethylethylenediamine), TMPDA (tetramethylpropane-1,3-diamine) and acetonitrile. While the potentially bidentate diamines did not undergo any reaction yielding only in the isolation of the starting compounds, acetonitrile formed the insertion complex Tp^tBu,MeDy[NC(Me)₂](HNAr^iPr) (7-Dy, Scheme 2). It was previously shown that acetonitrile does react with rare-earth-metal alkyls either via C–H-bond activation/deprotonation or insertion.^27–29 While the product of the deprotonation reaction (C₅Me₅)₂La[CH(SiMe₃)₂]/CH₃CN was structurally characterized as [(C₅Me₅)₂La(μ-CH₂CN)]₂,²⁷ the insertion product of the reaction (C₅Me₅)₂ScCH₃/CH₃CN was only spectroscopically analyzed as (C₅Me₅)₂Sc[NC(Me)₂].²⁸ The SCXRD study of five-coordinate 7-Dy features distinct bonding behaviour of the primary amido and the dimethyliminato ligand (Dy–N: 2.219(3) versus 2.148(3) Å; Dy–N–C: 156.3(3) versus 165.3(3)°) (Fig. 2). Thus, the dimethyliminato coordination compares to that of imidazolin-2-iminato complexes like LY(CH₂SiMe₃)₂(thf)₂ (L = 1,3-bis(2,6-diisopropylphenyl)imidazoline-2-iminato; Y–N, 2.1255(13) Å, Y–N–C, 176.85(12)° or LYCl₂(thf)₃ Y–N, 2.1278(18) Å, Y–N–C, 174.35(16)°) supposedly featuring very short Ln–N_iminato bonds.^30,31

Scheme 4 Reactivity of Tp^tBu,MeDy(NAr^iPr)(dmap) (4-Dy) (Ln = Y, Dy) towards Lewis acid 9-BBN in THF and toluene.

The post-synthesis exchange approach (route 2, Scheme 4) was probed with 4-Dy and the strong Lewis acid 9-borabicyclo[3.3.1]nonane (9-BBN) in THF.¹¹ Accordingly, the equimolar reaction led to the displacement of DMAP and coordination of one THF molecule to the dysprosium centre in Tp^tBu,MeDy(NAr^iPr)(thf) (8-Dy, Scheme 4). Like the dmap adduct, 8-Dy is soluble in toluene and THF, but insoluble in aliphatic solvents. It crystallized in the orthorhombic space group Pna2₁ and shows the same κ³ coordination of the Tp^tBu,Me ligand as complex 4-Dy before (Dy–N_pz: 2.465(4)–2.510(4)/2.450(3)–2.539(4) Å) (Fig. 2). As the coordination number did not change, the angle of the imido functionality stayed nearly the same (Dy–N_imido–C_ipso: 165.2(3)/166.9(3)°) as detected for 4-Dy. However, the Dy–N_imido distance of 2.004(4)/2.008(3) Å) in 8-Dy appears to be slightly shorter compared to 4-Dy (Table 1) which can be attributed to the weaker donor properties of the thf ligand.

The 9-BBN-promoted donor exchange was previously introduced by Chen, revealing that prior activation of L¹Sc(NAr^iPr)(dmap) (L¹ = [Ar^iPrNC(Me)CHC(Me)N(CH₂)₂NMe₂]) with 9-BBN led to abstraction of the donor molecule DMAP.¹¹ The emerging donor-free imide intermediate [L¹Sc(NAr^iPr)] could be trapped with THF to afford L¹Sc(NAr^iPr)(thf) featuring also a shorter Sc–N_imido bond than the DMAP adduct (Table 1: 1.852(4) versus 1.881(8) Å). It was also mentioned that the direct synthesis of the THF adduct L¹Sc(NAr^iPr)(thf) is not possible by thermolysis of the mixed methyl/amido scandium complex in THF, which is the same case for 8-Dy as well.² In contrast, terminal imides Tp^tBu,MeLn(NAr^iPr)(thf)₂ of the larger rare-earth metals can be obtained directly according to the donor(THF)-assisted methylidene → imido transformation (Scheme 1 and Table 1).^6c

Treatment of 4-Dy with excess of 9-BBN in toluene gave the mixed primary amido/hydroborato complex 9-Dy (Scheme 4 and Fig. 4). Again, this is in line with the observation made by Chen with the system L²Sc(NAr^iPr)/9-BBN (L² = [Ar^iPrNC(Me)CHC(Me)N(CH₂)₂N(CH₂)₂NMe₂]).³² Correspondingly, it can be hypothesized that initially the strong Lewis acid 9-BBN displaces all of the coordinated DMAP, rendering a highly reactive donor-free terminal imide [Tp^tBu,MeDy(NAr^iPr)] (Scheme 4, intermediate I¹, lower trace). Subsequent 1,2-addition of a tBu methyl group (C–H-bond activation) across the highly reactive Dy–N_imido bond of transient species I¹ reforms the primary amido ligand along with a 5-membered metallacycle in I². Then, the highly nucleophilic alkyl attached to the dysprosium attacks a second molecule of 9-BBN to afford the alkylhydroborato moiety. The resulting Dy–N_amido–C_ipso angle (143.4(2)°) and the Dy–N_amido distance (2.237(3) Å) of 9-Dy are comparable to the dysprosium amide complexes discussed beforehand. The Dy–B distance of 2.703(4) Å is in the range of the Y–B distances in [(Me₃Si)₂NC(NiPr)₂]Y[μ-H(μ-Et)₂BEt]₂(thf)₂ (2.658(4) and 2.671(4) Å)³³ and (C₅Me₅)₂YH(9-BBN) (2.767(6) Å),³⁴ but longer than those observed in (tBu₄Carb)Dy(BH₄)₂(thf) (2.473(2) and 2.487(2) Å).³⁵

Fig. 4 Crystal structure of 9-Dy. All atoms are represented by atomic displacement ellipsoids set at 50% probability. Solvent molecules and hydrogen atoms except for those of B–H and N–H are omitted for clarity. Selected bond distances (Å) and angels (°) for 9-Dy: Dy1–N7 2.237(3), Dy 1–N2 2.452(3), Dy 1–N4 2.405(3), Dy 1–N6 2.461(3), Dy 1–B2 2.703(4); Dy1–H1AA 2.17(3), B2–H1AA 1.23(3); Dy1–N7–C35 143.4(2), N7–Dy1–B2 117.06(11), N7–Dy1–N4 161.07(10), N7–Dy1–N2 93.18(10), N7–Dy1–N6 92.04(10).

In Chen's system [L²Sc(NAr^iPr)]/9-BBN the respective C(sp³)–H bond borylation took place at the [–CH₂NMe₂] side arm of the ancillary nacnac ligand (Sc–H_hydrido, 2.01(3) Å).³² Chen also reported on the reaction of L¹Sc(NAr^iPr)(dmap) with three equivalents of 9-BBN which led to mixed boroamido/hydroborato complex L¹Sc[NAr^iPr(9-BBN–H)](9-BBN + H) (Sc–H_hydrido, 1.89 Å and 1.20 Å).¹¹

Preliminary studies on the reactivity of terminal imide complex 4-Dy towards carbon dioxide (1 bar) in toluene at ambient temperature clearly indicate CO₂ insertion into the Dy–N_imido bond and, hence, carbamate formation in product 10-Dy. The DRIFT spectrum of 10-Dy revealed typical carbonyl vibrations at 1701 cm⁻¹ and 1641 cm⁻¹ assigned to asymmetric and symmetric C=O stretching vibrations (Fig. S17†). Moreover, the B–H vibration of 4-Dy at 2557 cm⁻¹, typical for the terminal B–H stretch of the Tp^tBu,Me ligand when coordinated in a tridentate fashion,³⁶ changed to 2431 cm⁻¹ in 10-Dy. Such a dramatic change of the position of the B–H band can be ascribed to a κ³ → κ² coordination switch of the Tp^tBu,Me ligand, likely involving a Dy⋯H–B interaction.³⁷ Unfortunately, crystals of 10-Dy suitable for SCXRD analysis could not be obtained, but 10-Dy can be tentatively assigned as the dicarboxylate species [Tp^tBu,MeDy{(O₂C)₂NAr^iPr}] on the basis of the double CO₂-insertion chemistry of the terminal scandium imides L²Sc(NAr^iPr) (A, Fig. 5)^17a and (BPz₂Py₃)Sc(NAr^iPr) ([small nu, Greek, tilde] = 1685 and 1632 cm⁻¹).¹³ For further comparison, mono insertion of carbon dioxide was observed for the Lu₃(μ₃-NPh) moiety of the trinuclear cluster [L³Lu₃(μ₂-Me)₃(μ₃-Me)(μ₃-NPh)] (L³ = PhC(NC₆H₃iPr₂-2,6; C, Fig. 5)^17d and the alkali-metal stabilized Ce(IV)–N_imido bond of [(TriNOx)Ce(NAr^CF₃)][Cs(2.2.2-cryptand)] (B, Fig. 5).³⁸

Fig. 5 Structurally authenticated CO₂-insertion products A,^17aB,³⁸ and C (ref. 17d) emerged from rare-earth-metal imides.

Conclusion

The successful application of the donor-assisted intramolecular primary amido deprotonation protocol towards terminal imides of the rare-earth metals yttrium, dysprosium, and holmium depends on two crucial factors: first, the kinetic stabilization of the targeted imide via appropriate steric shielding of the ancillary ligand, herein supplied by the bulky Tp^tBuMe scorpionate ligand; second, sufficient Brønsted acidity and steric demand of the employed primary amine, herein provided by H₂NAr^iPr (Ar^iPr = C₆H₃iPr₂-2,6). With these key factors in mind, it was possible to generate a series of terminal rare-earth-metal imides Tp^tBu,MeLn(NAr^iPr)(dmap) for mid-sized to small rare-earth metals. Their reactivity parallels that of terminal scandium imides, as revealed by treatment with the strong Lewis acid 9-borabicyclo[3.3.1]nonane (9-BBN) and the heteroallene CO₂. These reactions revealed effective DMAP/THF donor exchange and CO₂ insertion into the Ln–N_imido bond (carbamate formation). The reactivity studies also include the isolation and structural characterization of the bis(amido) species Tp^tBu,MeHo(NAd)₂ (Ad = adamantyl) and dimethyliminate complex Tp^tBu,MeDy[NC(Me)₂](HNAr^iPr).

Data availability

Experimental, spectroscopic and structural data supporting this article have been uploaded as part of the ESI. Crystallographic data for all compounds have been deposited at the CCDC under 2312212–2312226, and can be obtained from https://www.ccdc.cam.ac.uk/structures/.

Author contributions

TR, synthesis and characterization of compounds, writing original draft; DS, synthesis and characterization of compound 1-Ho^Ga, editing original draft; CM-M, crystallography, editing original draft; RA, conceptualization, supervision, writing and project administration, funding acquisition.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful to Felix Kracht for assistance with recording NMR and IR spectra.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2312212–2312226. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3sc06584g

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