Cooperative N–H bond activation by amido-Ge(II) cations†

N-heterocyclic carbene (NHC) and tertiary phosphine-stabilized germylium-ylidene cations, [R(L)Ge:], featuring tethered amido substituents at R have been synthesized via halide abstraction. Characterization in the solid state by X-ray crystallography shows these systems to be monomeric, featuring a two-coordinate C,Nor P,N-ligated germanium atom. The presence of the strongly Lewis acidic cationic germanium centre and proximal amide function allows for facile cleavage of N–H bonds in 1,2-fashion: the products resulting from reactions with carbazole feature a tethered secondary amine donor bound to a threecoordinate carbazolyl-Ge centre. In each case, addition of the components of the N–H bond occurs to the same face of the germanium amide function, consistent with a coordination/proton migration mechanism. Such as sequence is compatible with the idea that substrate coordination via the pπ orbital at germanium reduces the extent of N-to-Ge π donation from the amide, thereby enhancing the basicity of the proximal N-group.


Introduction
The metal-mediated activation of N-H bonds ( particularly those in ammonia) is a challenging fundamental chemical step with potential significance to a number of important transformations of industrial relevance. 1 The scarcity of transition metal systems capable of effecting N-H cleavage via oxidative addition, in a manner familiar for a plethora of other E-H bonds, reflects the competing tendency of ammonia to form classical Werner complexes at unsaturated metal centres. 2 Within p-block chemistry, a number of systems have been reported in the last 15 years which will cleave ammonia to give a derivative containing the E(H)(NH 2 ) function, 3,4 including several carbene and related heavier group 14 species in the +2 oxidation state. 3 The presence of a low-lying formally vacant pπ orbital in such systems allows for simple coordination of amines (akin to d-block metal complexes); facile N-to-E proton transfer, however, has been proposed to offer a route to generate an amido hydride species without the need for amine dissociation. 3e,h, 5,6 Moreover, in addition to single site N-H oxidative addition, heavier group 14 analogues of carbenes have also been shown to offer a number of alternative (cooperative) pathways for N-H cleavage involving H-atom transfer to a ligand site (Scheme 1). The relative propensity for different modes of activation has been shown to reflect the identity of the group 14 element/supporting ligand set (and the associated E II /E IV redox potential). In the case of germylene systems, for example, both single site (1,1 addition) and ligand-assisted 1,4 activation modes have been reported, depending not only the basicity of ligand backbone sites, but also the ability of the donor set to promote formation of the Ge IV oxidation state. 3b-f While charge-neutral tetrelenes of the type EX 2 have been investigated in some depth, 3 isoelectronic cations of the type [R(L)E] + have been less extensively studied, [7][8][9][10][11][12] despite the fact that the net positive charge should promote initial coordination of ammonia/amines, enhance the acidity of the NH bond and thereby promote N-to-E proton migration. We have recently examined the chemistry of N-nacnac supported tetrylium-ylidene cations towards N-H containing substrates, with the isolation of products derived from oxidative addition or simple amine adduct formation being found to be dependent on the group 14 element (Scheme 2). 13 Given the lack of productivity in N-H activation exhibited by germylium-ylidene systems stabilized by these β-diketiminate (amido/imine) systems we were interested in (i) exploring the possibilities for the synthesis of two-coordinate amidogermylium-ylidene species stabilized by alternative (strong) donor sets (e.g. carbenes 14 and phosphines) which might promote the formation of Ge IV products; and (ii) exploring the mode(s) of reactivity of such systems towards N-H bonds. These studies are reported in this manuscript.

Germylium-ylidene synthesis
We initially targeted halo-germylene precursors featuring amido/NHC ligand [L 1 ] − or amido/phosphine ligand [L 2 ] − (Scheme 3). NHC-ligated bromo-germylene precursor 1 can be synthesized via one of two routes: (i) the reaction between protio-ligand [(L 1 )H 2 ]Br and one equivalent of the germanium (II) bis amide Ge{N(SiMe 3 ) 2 } 2 , 14,15 or (ii) in situ double deprotonation of [(L 1 )H 2 ]Br (e.g. with n BuLi) followed by metathesis with GeCl 2 ·dioxane. In our hands, route (i) is preferable, leading to yields of ca. 90%. By contrast, chloro-germylene complex 2 is most readily synthesized by deprotonation of (L 2 ) H, followed by reaction of the lithiated ligand with GeCl 2 ·dioxane. The overall yield for the two steps combined is typically in the region of 50%. Both 1 and 2 have been characterized by standard spectroscopic and analytical methods, and by X-ray crystallography (Fig. 1). The structures of both compounds in the solid state are in line with related complexes, featuring angles at the germanium centre which are close to 90°(e.g. 90.0(1)-98.9(1)°for 1) consistent with the expected (low) degree of ns/np mixing for n = 4. 16 From these precursors, two-coordinate NHC-or phosphinestabilized Ge II cations (germylium-ylidenes) can be synthesised by halide abstraction, most conveniently using Li[Al(OC (CF 3 ) 3 ) 4 ] as a weakly coordinating anion (WCA) source (Scheme 4). 17 Treatment of 1 or 2 with Li[Al(OC(CF 3 ) 3 ) 4 ] in bromobenzene at room temperature leads to the formation of the respective cationic species 3 and 4 in reasonable yields (30-40%). 18 In both cases, the 1 H and 13 C NMR spectra reveal distinct changes from the respective bromogermylene precursor: for 3 the N t Bu signal is shifted from δ H = 1.42 to 0.98 ppm, and the carbenic 13 C resonance is shifted upfield from δ C = 169.5 to 165.6 ppm. In the case of phosphine-ligated system 4, the 31 P resonance is shifted from δ P = −24.4 (for 2) to −2.2 ppm.
Both 3 and 4 could be obtained as single crystals suitable for X-ray diffraction, to allow for unambiguous confirmation of the monomeric two-coordinate structures in the solid state (Fig. 2). Cation formation is reflected in marked shortening of the Ge-N bonds compared to precursors 1 and 2, presumably due to enhanced possibilities for N-to-Ge π bonding in the two-coordinate systems (e.g. d(Ge-N) = 1.889(1), 1.811(3) Å for 2 and 4, respectively). Consistently, in both cases, the geometry  Scheme 2 N-H activation vs. amine coordination: reactions of N-nacnac stabilized silylium-, germylium-and stannylium-ylidenes with NH 3 and t BuNH 2 . 13 around the amido nitrogen is significantly more planar than in the halo-germylene precursor (e.g. 359.7°for 3 vs. 350.0°for 1). The distances from germanium to the neutral NHC or phosphine donor, on the other hand, are much less affected by halide abstraction (e.g. d(Ge-P) = 2.446(1), 2.449(1) Å for 2 and 4, respectively). In each cation, the angle subtended at germanium is relatively narrow (91.9(2) and 88.5(1)°for 3 and 4, respectively) reflecting the constraints of the six-membered chelate ring. 12 The effect of the differing strengths of the neutral donor (i.e. NHC vs. phosphine) on the Ge-N moiety appear not to be statistically significant: the Ge-N bond lengths for 3 and 4 are 1.829(4) and 1.811(3) Å, respectively.
These studies also reveal that the product obtained is strongly dependent on the conditions employed. In the case of 3, clean product formation requires the use of a haloarene solvent (fluoro-or bromobenzene), while the use of benzene leads to the formation of different products arising from incomplete halide abstraction. 14

Reactivity studiesactivation of N-H bonds
Mechanistically, the pathways for activation of E-H bonds by tetrelene and related systems are known to be dependent on the nature of E. For H 2 , mechanisms have been advanced for carbene and silylene systems which involve simultaneous interaction of the substrate with the C/Si centred lone pair and the orthogonal, formally vacant, pπ orbital. 3a, 19 The orientation of the H 2 molecule in the transition state then reflects the relative importance of the donor and acceptor capabilities of the tetrelene. Such mechanistic proposals emphasize the importance of the n-to-pπ energy gap (which is often equivalent to the HOMO-LUMO gap) in facilitating the activation of H 2 . 20 On the other hand, protic substrates, such as ammonia, have been shown to be activated by an alternative coordination/proton migration pathway. 3e,h,5,6,21 This sequence involves initial coordination of the NH 3 molecule, with the tetrelene acting as an electrophile. Subsequent N-to-E proton migration (facilitated, for example, by a second molecule of NH 3 ) then completes the formal N-H oxidative addition process. 3e,h, 5,6 In this case it is the energy of the vacant pπ orbital of the tetrelene, and its consequent ability to coordinate and activate the NH 3 substrate that is thought to be important in bond cleavage.

Dalton Transactions Paper
In the cases of cationic systems 3 and 4, the presence of strongly π-donor amido α-substituents would be expected to lead to significant elevation of the Ge-centred pπ orbital, and this, taken together with the relative narrow angle at germanium in each case (and the associated high degree of 4s character in the lone pair) would be expected to lead to a wide n-topπ energy separation. 3i, 22 Consistently, DFT calculations (PBE1PBE, Def2-TZVP level of theory), exemplified for 3 ( Fig. 4) reveal that this separation is >400 kJ mol −1 . The LUMO features significant Ge pπ character, with some delocalization onto the carbene carbon, and the expected anti-bonding phase relationship with the N pπ orbital (Fig. 4). The germaniumcentred lone pair is relatively low in energy, being associated with the HOMO−3.
Unsurprisingly then, we find that neither 3 and 4 shows any hint of reactivity towards H 2 , or the hydridic E-H bonds present in PhSiH 3 , Et 3 SiH or Me 3 N·BH 3 , for which more-orless concerted oxidative activation would be expected. On the other hand, the low-lying nature of the orbital manifold (and the implied high Lewis acidity) for both systems would appear to be better suited to the activation of polar bonds, such as N-H linkages. Accordingly, the cleavage of N-H bonds can be demonstrated explicitly through the reactions of 3 and 4 with carbazole (Scheme 6). The corresponding reactions with ammonia are much more difficult to control in terms of stoichiometry, 23 and invariably result in the presence of protonated ligand among the products formed. Carbazole, by contrast, can easily be added stoichiometrically and its planar structure proves to be critical in isolating the reaction product by crystallization.
In contrast to two-coordinate diaryl germylene and cationic β-diketiminate silylium-ylidene complexes (Schemes 1 and 2), for which single-site N-H activation processes result in net oxidative addition at the group 14 element, the mode of activation in the cases of 3 and 4 involves 1,2-addition across the amido Ge-N bond (Scheme 6). As such, products 6 and 7 are generated, in which the Ge II oxidation state is retained, the amido donor is protonated (to generate a secondary amine) and coordination of the anionic carbazolyl conjugate base increases the germanium coordination number from two to three.
N-H bond formation is signalled in each case by the appearance of an additional signal in the respective 1 H NMR spectrum, at δ H = 3.72 and 6.02 ppm (for 6 and 7, respectively). In the case of 7, the signal in question is a doublet with a 3 J HP coupling of ca. 11 Hz to the germanium-bound phosphine donor. In addition (notwithstanding the problems associated with the definitive location of hydrogen atoms by X-ray crystallography), both the pyramidalization of the heavy atom skeleton at N and the lengthening of the Ge-N bond [1.829(4) to 2.134(7) Å for 3/6 and 1.811(3) to 2.137(4) Å for 4/7] are also consistent with the conversion of an anionic amido donor to a charge neutral secondary amine ligand (Fig. 5). In addition, the location of the carbazolyl substituent and the H atom on the same face of the resulting cations 6 and 7 is consistent with a mechanistic hypothesis involving initial N-coordination   at the highly Lewis acidic germanium centre followed by proton migration to the proximal amido ligand. 3e,h,5,6 Precedent for the formation of an initial donor/acceptor adduct of this sort comes from a recently reported β-diketiminate supported germylium-ylidene cation, which can be isolated due to the presence of a less acidic N-H bond and a less basic amido ligand. 13 Subsequent proton transfer to a basic ligand site has previously been reported for Nacnacderived germanium and aluminium/gallium systems, 3d,g,24 and finds more general precedent in the pyridine-derived ligand systems pioneered by Milstein and co-workers. 25 Conclusions NHC-and phosphine-stabilized germylium-ylidene cations, featuring tethered amido substituents have been isolated for the first time and shown definitively to be two-coordinate in the solid state by X-ray crystallography. The presence of the strongly Lewis acidic cationic germanium centre and proximal amide function allows for facile cleavage of protic E-H bonds in cooperative (1,2-) fashion (exemplified by the N-H bond in carbazole), leading to the formation of a tethered secondary amine donor bound to a three-coordinate Ge II centre. By analogy with chemistry reported for neutral stannylene and germylene systems, 3e,h,5,6 and consistent with structural results which imply that addition of the components of the E-H bond happen at one face of the Ge-N linkage, we propose that this chemistry proceeds via coordination of the substrate at the highly electrophilic germanium centre, followed by proton migration (i.e. intramolecular deprotonation) involving the nearby amide group. Such a sequence is consistent with the idea that substrate coordination via the pπ orbital at germanium markedly reduces the extent of N-to-Ge π donation from the amide, thereby enhancing the basicity of the proximal N-group. As such, the presence of the highly Lewis acidic site in cations of this sort is key to cooperative activation of the substrate across the germanium-nitrogen bond. Differences in the regiochemistry of N-H addition compared to other Ge II systems (1,2-vs. 1,1-(single site) or 1,4-addition, for example), 3d,e can then be rationalized on the basis of the location of the most accessible basic site within an initially formed amine adduct. Consistent with these hypotheses, we find that the HOMO of the model adduct 3·NH 3 (at −9.18 eV/−886 kJ mol −1 ) is characterized as the amide N lone pair: this orbital is elevated significantly from its counterpart in the free cation 3 (the HOMO-2 at −10.22 eV/−986 kJ mol −1 ). The germanium-centred lone pair in the adduct 3·NH 3 is found in the HOMO−1 (at −9.44 eV/−911 kJ mol −1 ) (see ESI †).

General considerations
All manipulations were carried out using standard Schlenk line or dry-box techniques under an atmosphere of argon.
Toluene and hexane were degassed by sparging with argon and dried by passing through a column of the appropriate drying agent using a commercially available Braun SPS and stored over potassium; fluorobenzene and bromobenzene were dried by refluxing over CaH 2 and stored over molecular sieves. Benzened 6 was dried using a potassium mirror and bromobenzene-d 5 dried using CaH 2 and stored over molecular sieves. NMR samples were prepared under argon in 5 mm Wilmad 507-PP tubes fitted with J. Young Teflon valves. NMR spectra were measured on Bruker Avance III HD Nanobay or Bruker AVII spectrometers operating at 400 or 500 MHz, respectively (for 1 H measurements); 1 H and 13 C NMR spectra were referenced internally to residual protio-solvent ( 1 H) or solvent ( 13 C) resonances and are reported relative to tetramethylsilane. 19 F and 27 Al NMR spectra were referenced with respect to CFCl 3 and

DFT calculations
All computational work reported here was carried out using density functional theory (DFT) within the Gaussian16 (Revision C.01) program package. 27 Geometry optimizations were performed with the PBE1PBE exchange correlation functional, [28][29][30] using the Def2-TZVP basis set with an ultrafine integration grid and Grimme's empirical dispersion correction (GD3BJ). 31 The nature of stationary points found (minimum) was confirmed by full frequency calculations (no imaginary frequencies).

Crystallography
Single-crystal X-ray diffraction data for all compounds were collected using an Oxford Diffraction Supernova dual-source diffractometer equipped with a 135 mm Atlas CCD area detector. Crystals were selected under Paratone-N oil, mounted on MiTeGen Micromount loops and quench-cooled using an Oxford Cryosystems open flow N 2 cooling device. 32 Data were collected at 150 K using mirror monochromated Cu K α radiation (λ = 1.5418 Å; Oxford Diffraction Supernova). Data collected were processed using the CrysAlisPro package, including unit cell parameter refinement and inter-frame scaling (which was carried out using SCALE3 ABSPACK within CrysAlisPro). 33 Equivalent reflections were merged and diffraction patterns processed with the CrysAlisPro suite. 33 Structure were solved ab initio from the integrated intensities using SHELXT 34 and refined on F 2 using SHELXL 34 with the graphical interface Olex2 35 or X-Seed. 36 Full details are given in the supplementary deposited CIF files (CCDC 1952091-1952095, 1952097 and 2005217-2005219 †).

Dalton Transactions Paper
(15 mL) at −78°C. The reaction mixture was warmed to room temperature and then heated to 80°C for 2 d, over which time a colourless solution was formed. Volatiles were removed in vacuo, and 1 was isolated as a pale yellow powder. Single crystals suitable for X-ray crystallography were obtained from a concentrated solution in THF layered with hexane and stored at room temperature. Yield: 709 mg, 99%. 1 (2). To a mixture of Li(L 2 ) (750 mg, 1.64 mmol) and GeCl 2 ·dioxane (379 mg, 1.64 mmol) was added toluene (15 mL) at −78°C. The reaction mixture was warmed to room temperature and stirred for 12 h, over which time a light green suspension was formed. The reaction mixture was filtered, and the filtrate concentrated in vacuo. A small portion of hexane (1 mL) was added and the solution stored −30°C to give 2 as a pale yellow crystalline solid. Single crystals suitable for X-ray crystallography were obtained from a concentrated solution in toluene layered with hexane and stored at −30°C. Yield: 610 mg, 67%. 1 H NMR (400 MHz, benzene-d 6  The reaction mixture was stirred overnight, over which time a yellow solution and white precipitate were formed. Volatiles were removed in vacuo and compound 5 isolated as a yellow oil. Single crystals suitable for X-ray crystallography were obtained from a concentrated solution in dichloromethane layered with hexane and stored at −30°C. Yield: 276 mg, 34%. 1 H NMR (400 MHz, benzene-d 6 , 298 K): δ H 1.09 (9H, s, t Bu),