Dihydrogen Splitting by Intramolecular Borane-Phosphane Frustrated Lewis Pairs: A Comprehensive Characterization Strategy Using Solid State NMR and DFT Calculations.

Four hydrogenated intramolecular phosphane-borane frustrated Lewis pair (B/P FLP) compounds bearing unsaturated cyclic or aromatic carbon backbones have been synthesized and structurally characterized using 11 B, 31 P, 1 H and 2 H solid-state NMR spectroscopy. A comparison of the spectra with those of the corresponding free B/P FLPs shows that both 11 B isotropic chemical shifts as well as nuclear electric quadrupolar coupling constants decrease significantly upon FLP hydrogenation, revealing the breakage of the partial B-P bond present in the starting materials. Likewise, the 31 P isotropic chemical shift, the chemical shift anisotropy, and the asymmetry parameter decrease significantly upon FLP hydrogenation, reflecting the formation of a more symmetric, C3v -like local environment. 11 B{31 P} rotational echo double resonance (REDOR) experiments can be used to measure the B-P internuclear distance (about 3.2 Å) of these compounds. Observation of the hydrogen atoms bound to the Lewis centers is best accomplished via 31 P{1 H} and 11 B{1 H} cross-polarization-heteronuclear correlation experiments or by direct observation of the 2 H MAS NMR signals on especially prepared FLP-D2 adducts. For accurately measuring the phosphorus-deuterium distance via 31 P{2 H} rotational echo adiabatic passage double resonance (REAPDOR), it is essential to take the secondary dipolar coupling of 31 P with the boron-bonded 2 H nuclei explicitly into consideration, by simulating a 2 HP -31 P-2 HB three-spin system based on structural input. All of the experimental NMR interaction parameters are found in excellent agreement with values calculated by DFT methods, using the geometries obtained either by energy optimization or from single-crystal structures.

A Frustrated Phosphane-Borane Lewis Pair and Hydrogen: A Kinetics Study.

The energy profile of a frustrated Lewis pair (FLP) dihydrogen splitting system was determined by a combined experimental kinetic and DFT study. A trimethylene-bridged phosphane-borane FLP was converted into its endothermic H2 -cleavage product by sequential H(+) /H(-) addition. The system could be handled at low temperature, and the kinetics of the H2 elimination were determined to give a rate constant of kHH,exp (299 K)=(2.87±0.1)×10(-4)  s(-1) in solution. The primary kinetic isotope effects were determined; for example, (kHH /kDD )exp =3.19. The system was accurately analyzed by DFT calculations.

Why Does the Intramolecular Trimethylene-Bridged Frustrated Lewis Pair Mes2 PCH2 CH2 CH2 B(C6 F5 )2 Not Activate Dihydrogen?

The methyl labelled C3 -bridged frustrated phosphane borane Lewis pair (P/B FLP) 2 b was prepared by treatment of Mes2 PCl with a methallyl Grignard reagent followed by anti-Markovnikov hydroboration with Piers' borane [HB(C6 F5 )2 )]. The FLP 2 b is inactive toward dihydrogen under typical ambient conditions, in contrast to the C2 - and C4 -bridged FLP analogues. Dynamic NMR spectroscopy showed that this was not due to kinetically hindered P⋅⋅⋅B dissociation of 2 b. DFT calculations showed that the hydrogen-splitting reaction of the parent compound 2 a is markedly endergonic. The PH(+) /BH(-) H2 -splitting product of 2 b was indirectly synthesized by a sequence of H(+) /H(-) addition. It lost H2 at ambient conditions and confirmed the result of the DFT analysis.