Synthesis and structures of anionic rhenium polyhydride complexes of boron–hydride ligands and their application in catalysis†‡

The rhenium complex, [K(DME)(18-c-6)][ReH4(Bpin)(η2-HBpin)(κ2-H2Bpin)] 1, comprising hydride and boron ligands only, has been synthesized by exhaustive deoxygenation of the commercially available perrhenate anion (ReO4−) with pinacol borane (HBpin). The structure of 1 was analysed by X-ray crystallography, NMR spectroscopy, and DFT calculations. While no hydrides were located in the X-ray crystal structure, it revealed a trigonal arrangement of pinacol boron ligands. Variable-temperature NMR spectroscopy supported the presence of seven hydride ligands but further insight was hindered by the fluxionality of both hydride and boron ligands at low temperature. Further evaluation of the structure by Ab Initio Random Structure Searching (AIRSS) identified the presence of hydride, boryl, σ-borane, and dihydroborate ligands. This complex, either isolated or prepared in situ, is a catalyst for the 1,4-hydroboration of N-heteroaromatic substrates under simple operating procedures. It also acts as a reagent for the stoichiometric C–H borylation of toluene, displaying high meta regioselectivity in the borylated products. Reaction of 1 with 9-BBN resulted in HBpin substitution to form the new anionic tetra(dihydroborate) complex [K(DME)(18-c-6)][Re(κ2-H-9-BBN)4] 4 for which the hydride positions were clearly identified by X-ray crystallography. The method used to generate these isolable yet reactive boron–hydride complexes is direct and straightforward and has potential utility for the exploitation of other metal oxo compounds in operationally simple catalytic reactions.


S3
by using a riding model, with the exception of the hydrides in compound 4 which were located in the difference Fourier map and were isotropically refined with restraints as appropriate. Crystallographic data have been deposited with CCDC (1979241 and 1979242).

Synthesis of [K(DME)(18-c-6)][Re(κ2-H2BBN)4] (4)
Solid 9-BBN dimer (101 mg, 0.412 mmol) was added to a solution of 1 (100 mg, 0.103 mmol) in toluene (3 mL) with stirring at room temperature. After 1 hour, a red precipitate and a light red solution formed. The mixture was filtered and the solids were dried under vacuum after which they were dissolved in DME (1 mL) and hexane was layered on to the solution.
After 48 hours, large red crystals formed and these were collected by filtration and dried under vacuum to give the desired product 4 as a red solid (90. 6  volatility of the DME ligand bound to potassium the integration for the DME signals in the 1 H NMR spectrum is low and the elemental analysis is low in C.

Addition of DABCO to solution of 4
Solid DABCO (4.26 mg, 0.038 mmol) was added to a solution of 4 (20 mg, 0.019 mmol) in THF-d8 in a Youngs' tap NMR tube. The mixture was analysed by NMR spectroscopy and compared with the NMR spectra prior to DABCO addition. Within 10 minutes, 4 was consumed and the single hydride species, 5, was observed as well as 1 equivalent of

Addition of 9-BBN dimer to solution of 4
Solid 9-BBN dimer (4.6 mg, 0.019 mmol) dimer was added incrementally in two equal portions to a solution of 4 (20 mg, 0.019 mmol) in THF-d8 in a Youngs' tap NMR tube. The reaction was analysed by NMR spectroscopy between additions and compared with the NMR spectra prior to DABCO addition. After the first addition of 0.5 equivalents of 9-BBN dimer, 4 was almost fully consumed and a single hydride species, 4, was observed as well as free 9-

C-H borylation of toluene using 1
A solution of 1 (100 mg, 0.103 mmol) in toluene (1 mL) was stirred at room temperature for 3 days, during which a dark brown precipitate formed. The solvents was evaporated under vacuum and to the residue was added C6D6 with trimethoxybenzene (15.7 mg, 0.093 mmol) as an internal standard. NMR analysis of the crude reaction mixture showed that the mono-S5 borylated product was formed as mixture of meta and para regioisomers 3a/3b (41 % NMR yield, 80:20 (m/p)).

'Bench-top' pyridine Hydroboration (in air, using 'wet' solvents and reagents)
[       The anion deviates substantially from C2 symmetry. If it were present in this crystal structure with the same geometry, disorder would be expected to be substantial and obvious in a the difference synthesis and refined with free uiso. Bond lengths between C65, C64, and C65A were restrained to be equal within the standard uncertainty of 0.02 through the use of SADI command. Data collected at 220 K due to suspected phase transition upon cooling.

Geometry optimisation of the [ReH7(Bpin)3] anion
The seven hydrogen atoms around the Re centre in [ReH7(Bpin)3] − were located in a nonbiased way through a series of AIRSS/CASTEP geometry optimisation calculations. 11,12 The model comprised the anion located inside a 15 Å 3 cubic box alongside a non-coordinating K + to maintain charge neutrality of the periodic boundary condition box. AIRSS constructed the initial models by positioning the hydrogen atoms randomly on a sphere of radius 1.6-1.85 Å centred on the Re coordinate. Two further constraints were imposed on the initial hydrogen atom placement, to ensure a minimum separation of 1.1 Å between the hydrogen atoms (to prevent possible formation of H2) and 1.5 Å between the hydrogen atoms and the potassium centre (to prevent loss of hydrogen around the Re centre through the formation of K-H bonds). Over 30 random input structures were generated in this way, and optimised using CASTEP (version 16) for full atom optimisation (basis set energy cut-off: 750 eV; functional: PBE; dispersion correction: TS; pseudopotentials: on-the-fly; geometry optimisation criteria: energy tolerance: 5 × 10 −4 eV, maximum force tolerance: 5 × 10 −2 eV Å −1 atomic displacement tolerance: 5 × 10 −2 Å). The relative energies of the resulting optimised structures are arranged, from lowest to highest, in Figure S36 below, alongside images of the minima obtained. The first point to observe is that all (bar six) of the optimised structures obtained conform to one structure type, where the Re is coordinated by one bridging dihydroborate ligand and one S46 σ-borane ligand, leaving four Re-H (three equatorial, one axial) terminal interactions, and thereby creating a Re(V) centre. The repetition of this basic structural motif, which falls over an energy range of no more than 6 kJ mol −1 , present variations in the three B−B−B angles (which typically present as ca. 110, 115 and 130  5°). The next most stable minimum, which was found three times (some 5-10 kJ mol −1 above the lowest energy structure) comprises one bridging dihydroborate ligand and two σ-borane ligands, leaving three (equatorial) Re-H bonds; the biggest variations between these structures again rests with the B−B−B angles, which adopt similar ranges to that noted above. The three remaining highest energy structures each have a multiplicity of one. The first (at +14 kJ mol −1 ) is very similar to the lowest energy motif, except that the four terminal Re-H bonds now adopt a two equatorial, two axial formation; the second (at +15 kJ mol −1 ) comprises three bridging dihydroborate ligands and one axial Re-H bond. The final highest energy structure (at +38 kJ mol −1 ) has just one bridging dihydroborate and five terminal Re-H (three equatorial, two axial) bonds, and thereby represents a Re(VII) centre.
The most likely reason for the variation in energy and geometry for the multiple instances of the lowest energy motif is the relatively low quality convergence criteria used in the AIRSS process. Taking three of the minima obtained from the above run, and re-optimising to more stringent conditions (energy tolerance: 2 × 10 −6 eV, maximum force tolerance: 1 × 10 −2 eV Å −1 atomic displacement tolerance: 5 × 10 −4 Å) has clearly resulted in the same minimum being obtained, as witnessed by the structural overlay diagram shown in Figure S37. The three B−B−B angles have now converged to 108.6, 116.9 and 133.0°. The energy range of ca. 5 kJ mol −1 still remains, however. In all likelihood this is due to some small variation in interaction energy from the K + counterion; variations in its location are apparent from the overlay diagram shown below.
S47 Figure S39. Overlay of the optimised structures obtained for three of the lowest energy structures in the AIRSS run, using tighter optimisation criteria.