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Versatile coordination of a reactive P,N-ligand toward boron, aluminum and gallium and interconversion reactivity

The synthesis and reactivity of the first Group 13 complexes bearing a dearomatized phosphino-amido ligand are reported, i.e. alane AlEt2(L) , gallane GaCl2(L) and borane B(Cl)(Ph)(L) . The three complexes react very differently with Group 13 trihalogenides, providing access to zwitterionic anti-·GaCl3 and the unique bis(metalloid) ·BCl2, with the boron center part of a highly unusual anionic four-membered ring (charge on C) and Ga bound to P. The coordination chemistry and the various transformations are supported by DFT calculations, X-ray crystallography and multinuclear NMR spectroscopic data.


Introduction
The synthetic chemistry and reactivity of Group 13 complexes has recently attracted much attention, both with respect to the preparation of unsaturated derivatives 1 and for application in bond activation processes and homogeneous catalysis, 2 often in combination with a Lewis base as a "Frustrated Lewis Pair" (FLP). 3 At the same time, the bioinspired strategy to combine proton-responsive ('reactive') ligands and (transition) metal centers has become prominent, resulting in novel coordination chemistry with applications in small molecule activation and metal-ligand bifunctional catalysis. 4 One particular strategy that has come to the forefront in this context is the use of lutidine-and picoline-based P,N donor ligands that can undergo reversible dearomatization. 5 Many transition metals coordinate strongly to both phosphorus and nitrogen in these ligands, 6 and deprotonation of the ligand backbone merely generates a nucleophilic carbon center that does not engage in any direct interaction with the metal coordination sphere. No examples of transmetallation on complexes bearing lutidine-or picoline-derived P,N-ligands are known, to the best of our knowledge. 7 The coordination chemistry of Group 13 metals and metalloids such as boron and aluminum is unexplored with this type of reactive ligand scaffold. Hence, the fundamental reac-tivity of complexes bearing a dearomatized P,N ligand is currently unknown but it could be envisioned that other ligand coordination modes may be accessible with these elements.
To address both challenges, we decided to exploit the reversible dearomatization chemistry of P,N-ligands to generate the first examples of Group 13 species with a dearomatized pyridine ligand (see Scheme 1). This approach has allowed access to highly reactive aluminum, gallium and boron compounds stabilized by an anionic P,N-scaffold and to study their versatile reactivity with G13 trihalogenides.

Results and discussion
Reaction of the known bidentate P,N-ligand 2-(diphenylphosphinomethyl)-6-methyl-pyridine L H with one molar equivalent of n-BuLi followed by electrophilic trapping with chlorodiethylaluminum (Scheme 2) resulted in the high yield synthesis of complex AlEt 2 (L) 1 as a red oil, which was characterized by multinuclear NMR spectroscopy. Contrary to what was previously observed for the anion of lutidine, 8 this reaction selectively generates an N-Group 13 element bond, with dearomatization of the N-heterocycle, rather than a C-Group 13 bond. The 1 H NMR spectrum contained three different resonances in the olefinic region (δ 5.28, 6.07 and 6.42) and a broad signal at δ 3.46 attributed to the methine proton of the vCH-PPh 2 arm. Only one resonance was observed for the two tert-butyl groups connected to phosphorus and both Et-groups at aluminum are also equivalent, confirming the C s symmetry of the molecule. This compound features a clear P → Al interaction, as evidenced by the broad 31 P NMR signal at δ −0.9 (Δδ −36.2 relative to L H ) and the 27 Al NMR resonance at δ 162, which is characteristic of a four-coordinated amido-alane. 9 Strikingly, the analogous reaction of L H with AlCl 3 failed to generate any well-defined product.
To get insight into the geometric and electronic features of 1, we optimized its structure using DFT calculations (Fig. 1).
The ligand framework adopts an almost planar configuration, with the phosphine and alane fragments slightly out of the N-heterocycle plane (∠P2-C4-C6-C7 177.8°, ∠Al1-N1-C6-C7 174.6°). The N-heterocycle shows clear alternation of C-C and CvC bonds. The HOMO is mainly located on the C1 carbon (64.1%), suggesting that this alane complex with an anionic P,N ligand would likely react with electrophiles at the methine site, as previously established for e.g. MeOTf, CO 2 and nitrile electrophiles with either Cu and Ru complexes bearing dearomatized P,N pincers. 10 However, common inorganic Lewis acids have not been employed as electrophiles, to the best of our knowledge.
We thus explored its reactivity toward gallium trichloride as a prototypical Lewis acid (Scheme 2). After addition of GaCl 3 to a solution of 1 in toluene at room temperature and work-up, the 31 P NMR spectrum contained only a very broad resonance at δ −3.1, while the 1 H NMR spectrum still showed the characteristic pattern for a dearomatized P,N backbone, but was lacking any signals for an AlEt 2 fragment. These spectroscopic data indicate a formal transmetallation from Al to Ga to generate GaCl 2 (L) 2, which was detected by HR-MS data (FD + , m/z = 390.04338), and AlClEt 2 . The structure of this gallane complex 2 was confirmed by X-ray structure determination on air-sensitive orange crystals of GaCl 2 (L), obtained from a diethyl ethertoluene solution at r.t. (Fig. 2). In the crystal structure, the geometry around gallium is strongly pyramidal due to the interaction with the phosphine donor (sum of angles 331.9°). The N-heterocycle shows alternation of single and double C-C bonds, while the external C-C double bond lies in-between (C1-C2 = 1.394(2) Å) and the C1 carbon is clearly sp 2 hybridized.
The intriguing conversion of 1 into 2 may be envisioned to involve initial formation of a bis-metalloid intermediate 1·GaCl 3 with a C-Ga bond and a formally cationic Al-center, but this proposed intermediate could not be detected using 1, despite various attempts. We hypothesized that such a bis-Scheme 1 Dearomatized Group 13 complexes of ( phosphinomethyl)pyridine ligands described in this work, their reactivity with Group 13 trihalogenides and follow-up transformations.
Scheme 2 Preparation of the phosphorus-stabilized amidoalane 1 and subsequent transmetallation to form the parent gallane 2. metalloid structure could be more readily accessible with B instead of Al, as boron cations are known to be stable in tetracoordinated environments (commonly termed boroniums). 11 The boron analogue of 1 could be accessed via deprotonation of L H and subsequent addition of Cl 2 BPh (Scheme 3). Complex B(Cl)(Ph)(L) 3 was obtained in 53% yield as a very air-sensitive red oil that was completely characterized by multinuclear NMR and HR-MS (CSI [M + H] + , m/z 374.1976). The 31 P NMR spectrum of 3 shows a broad resonance at δ 24.5 (Δδ = −10.8 ppm) and the doublet in 11 B NMR is found in the typical region for tetracoordinated boron atoms (δ 5.1, 1 J BP = 106 Hz), confirming its connectivity to phosphorus. The 1 H NMR spectrum of 3 showed a doublet at δ 3.46 ( 2 J HP = 5.1 Hz) attributed to the proton of the vCH-PPh 2 arm.
Addition of 1 eq. of GaCl 3 to 3 instantaneously yielded a yellow precipitate, which was characterized by a broad signal at δ 34.0 in the 31 P NMR spectrum (Δδ = 9.5 ppm) and a slightly upfield shifted 11 B NMR signal (δ 5.6); (Δδ = 0.5 ppm). The presence of a doublet at δ 3.67 in the 1 H NMR spectrum with a relatively large 2 J HP coupling constant of 13.4 Hz (ΔJ = 8.3 Hz compared to 3) indicates rearomatization of the N-heterocycle and supports addition of GaCl 3 to the methylene carbon spacer. Single crystal X-ray structure determination confirmed the diastereoselective formation of the formally zwitterionic anti-3·GaCl 3 (anti refers to the relative positions of the chloride at boron and the GaCl 3 fragment with respect to the G13-pyridine plane, Fig. 3). The new C1-Ga1 bond is in the range of previously reported carbogallates. 12 The boronium fragment is strongly pyramidalized (sum of angles 333.5°). DFT calculations (Gaussian 09, B3PW91, 6-31G**) show that the syn-3·GaCl 3 diastereoisomer is 3.7 kcal mol −1 higher in energy, which is within the upper limit of the precision of the methods. Heating a solution of anti-3·GaCl 3 for 10 h at 100°C led to partial isomerization to syn-3·GaCl 3 , as evidenced by multinuclear NMR spectroscopy.
Although there is some precedent for nucleophilic attack of dearomatized pincer transition metal complexes on a Group 13 Lewis acid, 13 this reactivity is unprecedented for main group elements or metalloids. Moreover, given the propensity of GaCl 3 to act as halide scavenger toward main group halides, including boron for the generation of boron cations, 14 the observed preference to act as electrophile in a carbon-gallium bond formation step is deemed highly remarkable. This bond is indeed reactive, as evidenced by the reaction of anti-3·GaCl 3 with HCl, which led to protonation at the methylene carbon with the formation of tetrachlorogallate salt 4 (Scheme 4), which was fully characterized, including by single crystal X-ray structure determination (Fig. 4).  The electronegativity of Ga lies in-between that of B and Al. We decided to explore the reaction of gallane 2 with boron trichloride as Lewis acid in order to probe whether the resulting reactivity of these Group 13 elements bound to a dearomatized P,N ligand follows the same trend. Upon addition of BCl 3 to 2, the bright orange reaction mixture turned colorless, which is diagnostic of re-aromatization (Scheme 5).
The broad 31 P NMR signal appeared at lower field (δ 31.1 ppm, Δδ −10.8 ppm) with no evidence for P-B coupling, indicative that the phosphine is still connected to gallium. In the 1 H NMR spectrum, the vCH-PPh 2 proton is deshielded (δ 3.93 ppm) and broadened due to scalar proximity of a 11 B nucleus (confirmed by 1 H{ 11 B} NMR analysis). Single crystal X-ray structure determination of 5·BCl 2 confirmed the incorporation of both Ga and B (Fig. 5). However, concomitant with exchange of Ga for B for the coordination to the heterocycle nitrogen atom, the latter has undergone an unprecedented change from a P,N to a C,N binding pocket.
The boron atom is incorporated in a rare four-membered ring that is fused to a rearomatized pyridine ring ( judging from the almost identical carbon-carbon bond lengths in the ring). The carbon atom binds to boron while the pendant phosphine binds the trichlorogallane. The C1-B1 bond length of 1.682(3) Å likely reflects the ring-strain in this structure. To the best of our knowledge, this is the first example wherein the methine spacer of a dearomatized lutidinylphosphine engages in direct coordination to a metal(loid). The only two structurally characterized fused heterobicyclic structures with boron and pyridine resulted either from (i) C-H activation of a lutidine-borenium adduct in the presence of an external Lewis base 15 or (ii) ring-closure of a bis( pentafluorophenyl)boryl derived FLP. 16 In both cases, extremely strong Lewis acids are involved.
Given its unique nature, we decided to get further insight into the bonding situation within 5·BCl 2 by DFT calculations. Natural bond order (NBO) analysis demonstrated that the C1-B1 and the N1-B1 bonds are very polarized toward C and N (70% and 80%, respectively). In line with this observation, the Natural Population Analysis (NPA) showed a positively charged boron atom (+0.4). Interestingly, the C1 carbon atom bears a significant negative charge (−0.9). As a result, 5·BCl 2 is best described as the combination of a boronium and an anionic C,N ligand. The carbanion might be partially stabilized by negative hyperconjugation into the σ*(P-C) orbitals as in reported α-monolithiated 17 and α-dilithiated 18 phosphineboranes, although second order NBO analysis does not provide clear evidence for this. The analogous 5·B(Cl)(Ph) was obtained as a mixture of diastereomers by reacting 2 with BCl 2 Ph.
Compounds 3·GaCl 3 and 5·B(Cl)(Ph) are linkage isomers (Scheme 6). The average DFT calculated energy differences of the linkage isomers 3·GaCl 3 and 5·B(Cl)(Ph) (syn and anti) are within the upper limit of the precision of the methods (3.6 kcal mol −1 in favor of 5·B(Cl)(Ph)). Given this small energy difference, we speculated that 5·B(Cl)(Ph) could conceivably still be of relevance in the formal transmetallation of 2 with BCl 2 Ph to provide 3·GaCl 3 under forcing conditions. To verify this hypothesis, a solution of 5·B(Cl)(Ph) in CD 2 Cl 2 was heated in a pressure tube to 100°C for 6 hours, which cleanly generated 3·GaCl 3 as a mixture of diastereoisomers. It is plausible that this structural rearrangement, involving both C-B and  P-Ga bond cleavage and C-Ga as well as P-B bond formation, proceeds via reconstitution of a mixture of 3 (with the dearomatized N-heterocycle) and GaCl 3 , which were found to react instantaneously at room temperature.
With the well-defined coordination chemistry for the heterodinuclear species in hand, we wondered if the reaction of 1 with boron trichloride would also lead to selective transmetallation and generation of product BCl 2 (L) (as observed with GaCl 3 ) or an analogous reaction as observed for 3 to 5 but with a phosphino-alane pendant arm. The addition of one equivalent of BCl 3 to a solution of alane 1 at room temperature led to only 50% conversion of starting material, but the reaction smoothly went to completion when two equivalents of BCl 3 were used (Scheme 7).
The new species gives rise to a quadruplet in 31 P NMR at δ 22.8, which is characteristic of the direct coordination of the phosphine to a boron atom. In line with the required stoichiometry, the 11 B NMR shows two signals, a doublet at δ 4.7 with a 1 J BP = 139. 4 Hz and a broad singlet at δ 6.6. Single crystal X-ray structure determination of 6 confirmed the incorporation of two boron atoms in the molecule (Fig. 6). Complex 6 is structurally related to borane-gallane 5.BCl 2 , with the same strained four-membered ring including the boron atom, but featuring a trichloroborane-coordinated phosphine arm. It is likely that the interaction of AlClEt 2 with the pendant phosphine is relatively weak (perhaps partly due to steric hindrance), resulting in facile displacement by an additional equivalent of BCl 3 .
In summary, we demonstrate the first examples of Group 13 complexes (boron, gallium and aluminum) stabilized by a dearomatized P,N ligand. Depending on the nature of the metal(loid) involved, very different reactivity toward Group 13 halogenides is observed. The rich coordination chemistry of this reactive ligand scaffold with G13 elements may open up e.g. new avenues for non-transition metal based small molecule activation and cooperative catalysis. Research in this direction is currently ongoing in our laboratories.

General comments
All reactions and manipulations were carried out under an atmosphere of dry dinitrogen using standard Schlenk techniques or in a glove-box. All solvents were purged with dinitrogen and dried using an MBRAUN Solvent Purification System (SPS). 1 H, 13 C, 31 P, 11 B and 27 Al NMR spectra were recorded on Bruker AV 300, Bruker DRX 300 or Bruker AV 400 spectrometers. Chemical shifts were expressed in positive sign, in parts per million, calibrated to residual 1 H (5.32 ppm) and 13 C (53.84 ppm) solvent signals, external BF 3 ·Et 2 O, 85% H 3 PO 4 and Al(NO 3 ) 3 1 M in D 2 O (0 ppm) respectively. Mass spectra were recorded on an AccuTOF LC, JMS-T100LP or an AccuTOF GC v 4 g, JMS-T100GCV Mass spectrometer. Di-(tert-butylphosphino)methyl-6-methyl-pyridine L H was prepared according to a literature procedure. 19 Complex 1. A solution of nBuLi (2.5 M in hexanes, 0.25 mL, 0.63 mmol, 1 eq.) was added dropwise to an ice-cooled stirred solution of 2-(di-tert-butylphosphino)methyl-6-methyl-pyridine L H (157 mg, 0.63 mmol) in diethylether (6.5 mL) and the resulting orange solution was allowed to react for 1 h at the same temperature. Then, the reaction mixture was cooled down to −78°C and chlorodiethylaluminum (0.32 M in toluene, 1.98 mL, 1.01 eq.) was added dropwise. The reaction mixture was allowed to warm up to room temperature over 1 h, giving an orange solution and a white precipitate. After elimination of the salts by filtration and removal of the volatiles under reduced pressure, 1 was obtained as a red oil in 95% yield.   9 Hz, C quat. ). The highly air-sensitive nature of this oil (slow decomposition even in an N 2 -filled glovebox) interfered during our repetitive attempts to obtain high-resolution mass analysis and elemental analysis.
Complex 2. Gallium trichloride (350.3 mg, 1.99 mmol, 1 eq.) in toluene (5 mL) was added dropwise to an in situ prepared solution of 1 (667.3 mg, 1.99 mmol) in diethyl ether (25 mL) under stirring at room temperature and the resulting mixture was stirred for an additional 4.5 h. Then, volatiles were removed under vacuum leading to an orange residue. This residue was extracted with a mixture of dichloromethane/ pentane (0.3 mL/10 mL) and filtered. The resulting solution was concentrated to 2 mL, resulting in the crystallization of 2 at room temperature as sticky orange blocks in 96% yield. Crystals suitable for X-ray crystallography were obtained from a saturated solution of 2 in a toluene/diethylether mixture at room temperature. 1  390.04357; found 390.04338. Despite several attempts, no satisfactory combustion analysis data was obtained for this compound, likely as a result of its air-sensitive nature. Complex 3. A solution of nBuLi (2.5 M in hexanes, 0.21 mL, 0.52 mmol, 1 eq.) was added dropwise under stirring to an icecooled solution of 2-(di-tert-butylphosphino)methyl-6-methylpyridine (150 mg, 0.52 mmol) in diethylether (5 mL) and the resulting orange solution was allowed to react for 1 h at the same temperature. Then, the reaction mixture was cooled down to −78°C and dichlorophenylborane (77 μL, 0.52 mmol, 1 eq.) was added dropwise. The reaction mixture was allowed to warm up to room temperature over 30 minutes and was further stirred for 1 h at room temperature, giving an orange solution and a white precipitate. Only 3 was detected in the crude mixture by 31 P{ 1 H} NMR analysis. After removal of the volatiles, the residue was extracted with pentane (3 mL) and dried affording the expected compound in a mixture with the starting material (7.7/1) (yield = 53%) due to partial hydrolysis. 1  Complex anti-3·GaCl 3 . A solution of 3 (445 mg, 1.19 mmol) in ether (15 mL) was added slowly to a solution of gallium trichloride (210 mg, 1.19 mmol, 1 eq.) in toluene (4 mL) at r. t. under stirring. The reaction mixture was then further stirred for 30 minutes leading to an orange solution and a yellowish precipitate. After removal of the mother liquor via cannula filtration, the precipitate was extracted with dichloromethane (20 mL), filtered and the limpid dichloromethane solution thus obtained was layered with pentane (21 mL), affording colorless crystals of anti-3·GaCl 3 in a yield of 40%. Crystals suitable for X-ray diffraction analysis were obtained under the same conditions. 1  for 10 h, leading to partial isomerization to its diastereoisomer syn-3·GaCl 3 in a 0.70/1.0 (anti/syn) ratio (as determined by integration of each vCHPPh 2 signal in the 1 H NMR spectrum, D1 = 10 s). The mixture of diastereoisomers was fully characterized by NMR spectroscopy: anti-3·GaCl 3  A solution of hydrochloric acid in diethylether (1 M, 86 μL, 1.1 eq.) was added dropwise to a suspension of anti-3·GaCl 3 (42 mg, 78.4 μmol) in dichloromethane (2 mL) at −78°C and the resulting suspension was allowed to warm up to room temperature overnight to give a colorless solution. The solution was filtered via cannula, the reaction flask was washed with dichloromethane (1 mL) and the organic layers were combined. The dichloromethane solution was layered with pentane (8 mL), affording colorless crystals of 4 with a yield of 89% at room temperature after one day. Crystals suitable for X-ray diffraction analysis were obtained by layering a dichloromethane solution of 4 with pentane at room temperature. 1  Complex 5·BCl 2 . Boron trichloride in toluene (1 M, 0.65 mL, 0.65 mmol, 1 eq.) was added dropwise at room temperature to a solution of 2 (255 mg, 0.65 mmol) in toluene (5 mL) under stirring. Upon addition, the solution changed from orange to colorless. Then, pentane (20 mL) was added and the reaction mixture was slowly concentrated until a white powder precipitated. After removal of the supernatant via cannula filtration, the solid material was dried under reduced pressure. The solid was then dissolved in dichloromethane (3 mL) and filtered again. Layering of this solution with pentane (8 mL) led to the crystallization of 5·BCl 2 at room temperature in 3 days as colorless crystals with a yield of 34%. Crystals suitable for X-ray diffraction analysis were obtained by slow diffusion of pentane into a saturated dichloromethane solution of 5·BCl 2 at room temperature. 1  Complex 5·B(Cl)(Ph). Dichlorophenylborane (49.8 μL, 0.38 mmol, 1 eq.) in toluene (0.4 mL) was added dropwise at room temperature to a solution of 2 (155 mg, 0.38 mmol) in toluene (3 mL) under stirring, leading to a colorless solution. Analysis of the crude mixture by 1 H NMR showed the formation of a mixture of diastereoisomers in a 1.9/1 ratio. After removal of the volatiles under reduced pressure, the residue was dissolved in dichloromethane (4 mL) and filtered in order to have a limpid solution. Layering of this solution with pentane (14 mL) afforded colorless crystals of 5·B(Cl)(Ph) in one day at room temperature with a yield of 51%. The mixture of diastereoisomers was fully characterized by NMR spectroscopy and the relative configuration of each isomer was attributed based on 2D NOESY NMR spectroscopy (in the case 33 460 reflections were measured up to a resolution of (sin θ/λ) max

DFT calculations
Calculations were carried out with the Gaussian09 software 23 at the DFT level, using the hybrid functional B3PW91. 24 Aluminum, gallium, chlorine and phosphorus atoms were treated with small-core pseudopotentials from the Stuttgart group, with additional polarization orbitals. 25 Nitrogen, carbon, boron and H atoms have been treated with the all-electron 6-31G** Pople basis set. The geometry optimizations were done without any symmetry constraints. Analytical calculations of the vibrational frequencies of the optimized geometries were performed to confirm that the structures obtained were minimums, and also obtained the thermal corrections over the energies. The Natural Bond Orbital analysis was done over the optimized structure, with the version of the NBO software included in Gaussian09. 26