Iron-Catalyzed H/D Exchange of Primary Silanes, Secondary Silanes, and Tertiary Siloxanes

A synthetic study into the catalytic hydrogen/deuterium (H/D) exchange of 1° silanes, 2° silanes, and 3° siloxanes is presented, facilitated by iron-β-diketiminato complexes (1a and 1b). Near-complete H/D exchange is observed for a variety of aryl- and alkyl-containing hydrosilanes and hydrosiloxanes. The reaction tolerates alternative hydride source pinacolborane (HBpin), with quantitative H/D exchange. A synthetic and density functional theory (DFT) investigation suggests that a monomeric iron-deuteride is responsible for the H/D exchange.

Title compound was synthesised according to literature procedure. 7 To a solution of 1a (200 mg, 0.36 mmol, 1.0 equiv.) in toluene (1 mL), TMP.BH 3 (167 mg, 1.08 mmol, 3.0 equiv.) was added. The vessel was sealed and stirred at 80°C for 16 hours. The mixture was concentrated under reduced pressure and pentane (5 mL and Fe( dmp BDK) 2 were isolated. The relevant crystallographic data are reported in the X-ray crystallography section. Thus far, a direct and selective synthesis of these complexes has not been achieved. The asymmetric unit in 1b comprises one quarter of a molecule, in which atoms Fe1, Si1, C10 and C13 are coincident with crystallographic mirror plane. The proximity of the iron centre to a space-group inversion centre serves to complete the dimer. C12 was seen to be disordered either side of the aforementioned mirror plane. The hydrogen atoms attached to this carbon were located and refined with the inclusion of some distance restraints, to assist convergence.  The asymmetric unit in 3a comprises half of one molecule. Atoms Fe1, C3, B1, C18 and C20 are coincident with a crystallographic mirror plane which serves to generate the remainder of the complex. H1 and H2 (attached to B1, and also seated on the mirror, were located and refined without restraints).
2,6-dipp BDKFe-(µ-H) 2 -9-BBN, 3b The boron bound hydrogens were located in the structure of compound 3b and refined without restraints. In the structure of [ dmp BDKFeH]2, the dipp group based on C11 was modelled to take account of 50:50 disorder about the crystallographic mirror plane on which atoms Fe1, Fe2, N1, N2, C1-C6, C9 and C21 are located. This mirror plane serves to generate the remaining half of the dimer from the motif which constitutes the asymmetric unit. H1 was located and refined without restraints.
Fe( dmp BDK) 2 Methylphenyl(silane-d 2 (98%)) was prepared according to general procedure for deuteration of silanes. To a flamedried 60 mL Teflon-sealed J-young ampoule containing pre-catalyst 1a (5 mol%), a solution of methylphenyl (silaned 2 (98%)) in C 6 D 6 (500 µL) was added. The vessel was sealed, removed from the glovebox and subjected to freeze-pump-thaw cycles until a continuous vacuum was achieved. The ampoule was cooled in liquid nitrogen and backfilled with H 2 gas. The mixture was warmed to room temperature and stirred for 16 hours. The solution was transferred to a H-distillation tube and separated from 1a by vacuum distillation. 1,3,5-Trimethoxybenzene (0.25 mmol) was added as a stock solution and spectroscopic yield and D-incorporation were determined by 1 H NMR spectroscopy.

Method for Deuterium Labelling of Propylbenzene
Title compound was synthesised by modified literature procedure. 9 To a flame-dried 60 mL Teflon-sealed J-young ampoule containing pre-catalyst 1a (5 mol%), C 6 D 6 (500 µL) and diphenylsilane (46.3 µL, 0.25 mmol, 1.0 equiv.) were added. The vessel was sealed, removed from the glovebox and free-pump-thawed until a continuous vacuum was achieved. The ampoule was cooled in liquid nitrogen and backfilled with D 2 gas. The mixture was warmed to room temperature and stirred for 16 hours. The vessel was depressurised and returned to the glovebox. Allyl benzene (33.1 µL, 0.25 mmol, 1.0 equiv.) and aniline (22.8 µL, 0.25 mmol, 1.0 equiv.) were added. The tube was sealed and the reaction was allowed to proceed for 16 hours at room temperature. The solution was transferred to a H-distillation tube and separated from 1a by vacuum distillation. 1,3,5-Trimethoxybenzene (0.25 mmol) was added as a stock solution and spectroscopic yield and D-incorporation were determined by 1 H and 2 H NMR spectroscopy.

General Method
To a high-pressure J-young NMR tube containing [Fe] (5 mol%) and 1,3,5-trimethoxybenzene (0.25 mmol), C 6 D 6 (500 µL) and silane (0.25 mmol) were added. The tube was sealed, removed from the glovebox and subjected to freeze-pump-thaw cycles until a continuous vacuum was achieved. The tube was cooled in liquid nitrogen and backfilled with D 2 gas. The mixture was warmed to room temperature and the reaction was monitored for 16 hours.
D-incorporation was determined by 1 H NMR spectroscopy.

Dimethyl(4-vinylphenyl)silane
Title compound was synthesised by modified literature procedure. 10 To a solution of 4-bromostyrene (719 mg, 3.93 mmol, 1 equiv.) in THF (10 mL) at -78°C, n butyllithium (2.5M solution in hexanes, 1.89 mL, 1.2 equiv.) was added dropwise over 30 minutes. The mixture was stirred at this temperature for 2 hours before the dropwise addition of chlorodimethylsilane (873 µL, 7.86 mmol, 2.0 equiv.). The solution was warmed to room temperature and stirred for 16 hours. The reaction was quenched with saturated aqueous NH 4 Cl solution (10 mL) and extracted with diethylether (3 × 10 mL). The organic extracts were combined, washed with water (20 mL) and brine (20 mL), dried over MgSO 4 , filtered and concentrated under reduced pressure. The crude product was purified by FCC (SiO 2 , pentane) yielding dimethyl(4-vinylphenyl)silane as a colourless oil (495 mg, 3.05 mmol, 78%). 1 16 This method follows that employed by Webster and co-workers, following successful benchmarking against an experimental β-hydrogen transfer with an analogous iron β-diketiminate system. 17 Free-energy profiles are valued in kcalmol -1 at the B3PW91-D3BJ/Def2-TZVP/IEF-PCM(C 6 H 6 )//BP86/BS1 theory level described above. 18 Following Webster and co-workers study, 17  All iron containing structures for catalyst activation were optimised in both the quintet (denoted 5 X) and triplet (denoted 3 X) spin states to identify whether any spin-crossover mechanism is operable, again following preceding work by Webster and co-workers. 17 TSs for the H/D exchange on the triplet energy surface did not converge. Given the triplet state is consistently higher in energy than the quintet, these structures were omitted from the mechanistic investigation.