Terminal dysprosium and holmium organoimides

Terminal rare-earth-metal imide complexes TptBu,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 TptBu,MeLnMe(GaMe4) with primary aniline derivative H2NC6H3iPr2-2,6 (H2NAriPr). Exploiting the weaker Ln–CH3⋯[GaMe3] interaction compared to the aluminium congener, addition of the aniline derivative leads to the mixed methyl/anilido species TptBu,MeLnMe(HNAriPr) which readily eliminate methane after being exposed to the Lewis base DMAP ( 
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Created by potrace 1.16, written by Peter Selinger 2001-2019
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 N,N-dimethyl-4-aminopyridine). Under the same conditions, [AlMe3]-stabilized dimethyl rare-earth-metal complexes transform immediately to Lewis acid bridged imides TptBu,MeLn(μ2-NC6H3Me2-2,6)(μ2-Me)AlMe2 (Ln = Dy, Ho). DMAP/THF donor exchange is accomplished by treatment of TptBu,MeLn(NC6H3iPr2-2,6)(dmap) with 9-BBN in THF while the terminal imides readily insert carbon dioxide to afford carbamate complexes.


EDGE ARTICLE
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. 3Successfully applied ancillaries include b-diketiminato (nacnac), 8,9,11 phosphazene, 10 TriNox, 14 and multidentate pyrazolato ligands. 12,13,15We 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.Given the feasibility of terminal imides of the early openshell cations Ce(III) (f 1 ), Nd(III) (f 3 ), and Sm(III) (f 5 ), 15 we herein envisaged the synthesis of those of the mid-late open-shell cations Dy(III) (f 9 ) and Ho(III) (f 10 ).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 rst terminal, trivalent dysprosium and holmium imide complexes.

Selection of precursors
Our original approach toward the terminal yttrium imide Tp tBu,Me Y(NC 6 H 3 Me 2 -2,6)(dmap) involved the mixed methyl/ tetramethylgallato complex Tp tBu,Me YMe(GaMe 4 ) as a suitable precursor. 12There, the ease of GaMe 3 displacement proved to be crucial for the successful synthesis.Consequently, we chose the trimethylgallium-stabilized dialkyl complexes Tp tBu,Me LnMe(GaMe 4 ) (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(GaMe 4 ) 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(GaMe 4 ) 3 (Dy, Ho) and 1-Ho Ga ; see the ESI, Fig. S1-S3 †).The solid-state structure of the bimetallic compounds Tp tBu,Me LnMe(m 2 -MeEMe 3 ) (1-Ln E , E = Al, Ga), depicting one terminal methyl group and an almost linear Ln-Me-E linkage, is not reected in the solution NMR spectra, which reveal highly uxional 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. 23Unexpectedly, the solid-state structures of the formally ve-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 GaMe 4 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,Me Ln(GaMe 4 ) 2 ] under release of methane and trimethylgallium, and the elimination of a second molecule GaMe 3 sterically induced by the bulky Tp tBu,Me ligand.
2][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 H 2 NAr iPr to form the mixed methyl/primary amido complexes Tp tBu,Me LnMe(HNAr iPr Noteworthy, the reaction of 1-Ho Ga with 1-adamantylamine in a 0.9 : 1 ratio gave the bis(amido) holmium complex Tp tBu,Me Ho(HNAd) 2 (3-Ho, Ad = adamantyl), even though the primary amine was added in decit.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 rst 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 trimethylaluminiumstabilized 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 P 1 space group) shows the expected k 3 fashion (N, N 0 , N 00 ) of the ancillary ligand with Ho-N pz distances in the range of 2.393(3)-2.610(3)Å (Fig. 1).

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,Me Ln(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 nally evolve methane, being a very potent leaving group.The 1 H 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 nonpolar solvents (e.g.n-hexane), but easily dissolve in aromatic or polar solvents like toluene and THF.
acid 9-BBN displaces all of the coordinated DMAP, rendering a highly reactive donor-free terminal imide [Tp tBu,Me Dy(NAr iPr )] (Scheme 4, intermediate I 1 , lower trace).Subsequent 1,2-addition of a tBu methyl group (C-Hbond activation) across the highly reactive Dy-N imido bond of transient species I 1 reforms the primary amido ligand along with a 5-membered metallacycle in I 2 .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 [(Me 3 Si) 2 NC(NiPr) 2 ]-Y[m-H(m-Et) 2 BEt] 2 (thf) 2 (
2,3DFT calculations performed on the yttrium compound Tp tBu,Me Y(NC 6 H 3 -Me 2 -2,6)(dmap) conrmed a large ionic character of the Y-N(imido) bond but also a signicant covalent bonding pattern with one s-type and two p-type interactions.6c
15, 3562-3570 | 3567 Edge Article Chemical Science depends on two crucial factors: rst, 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 H 2 NAr iPr (Ar iPr = C 6 H 3 iPr 2 -2,6).With these key factors in mind, it was possible to generate a series of terminal rare-earth-metal imides Tp tBu,Me Ln(NAr iPr )(dmap) for mid-sized to small rareearth 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 CO 2 .These reactions revealed effective DMAP/THF donor exchange and CO 2 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,Me Ho(NAd) 2 (Ad = adamantyl) and dimethyliminate complex Tp tBu,Me Dy[NC(Me) 2 ](HNAr iPr ).