HRh(dppb)₂, a Powerful Hydride Donor

Abstract

The Rh(I) and Rh(III) hydrides HRh(dppb)2 and [HRh(dppb)2(NCCH3)](BF4)2 (where dppb is 1,2-(bis(diphenylphosphino)benzene) have been prepared, and a structural study of [HRh(dppb)2(NCCH3)](BF4)2 has been completed. The latter complex is an octahedral complex with a trans arrangement of the hydride and acetonitrile ligands. A pK a value of 9.4 was measured for this complex by equilibration of [Rh(dppb)2](BF4) with 4-bromoanilinium tetrafluoroborate in acetonitrile. [Rh(dppb)2](BF4) reacts with H2 in the presence of Pt(dmpp)2, which acts as a base, to form HRh(dppb)2 and [HPt(dmpp)2](BF4) (where dmpp = 1,2-bis(dimethylphosphino)propane). An equilibrium constant of 0.42 ± 0.2 was measured for this reaction. Using this equilibrium measurement and a thermodynamic cycle, the hydride donor ability (ΔG°H-) of HRh(dppb)2 was determined to be 34 kcal/mol. This value indicates that HRh(diphosphine)2 complexes are powerful hydride donors. Similarly the pK a value of HRh(dppb)2 was calculated to be 35 from a thermodynamic cycle that included the potential of the Rh(I/−I) couple (E 1/2 = −2.02 V vs ferrocene). These results combined with results from the literature suggest the following order of hydricity for five-coordinate, 18-electron hydrides: second row > third row > first row. Similarly an acidity order of second row ≥ first row > third row is deduced.

Phenoxaphosphine-Based Diphosphine Ligands. Synthesis and Application in the Hydroformylation Reaction

“…The synthesis of bischelate complexes B has previously been reported for several ligands, 27 including ligand 4a. 28 James and co-workers showed that structure B containing either 1,2-bis(diphenylphosphino)ethane or 1,3-bis(diphenylphosphino)propane ligands is quantitatively converted to dimer C under hydroformylation conditions, presumably via compound A. 29 Also for ligand 4a, we observed the slow conversion of B to dimer C in the highpressure NMR experiments.…”

Calculation of thermodynamic hydricities and the design of hydride donors for CO 2 reduction

“…DuBois and coworkers (11)(12)(13)(14) investigated the thermodynamic hydride-donor abilities (i.e., hydricities) of transition-metal hydrides, formyl complexes, and NADPH-model complexes, such as 1-benzyl-1,4-dihydronicotinamide, in order to develop CO 2 -reduction catalysts in CH 3 CN. The thermodynamic hydricity of a species AH − (or BH) is defined as the standard free energy change for the reaction:…”

Behaviour of [PdH(dppe)2]X (X=CF3SO3−, SbF6−, BF4−) as Proton or Hydride Donor: Relevance to Catalysis

“…The stability in solution decreases in the order 1'' > 1' > 1, and the lifetime depends on the temperature and solvent: typically, at 300 K in CH 2 Cl 2 it is a few hours for 1'' and roughly one hour for 1. Therefore, [PdH(dppe) 2 ] + BF 4 À was further characterized by 13 C and 19 F NMR spectroscopy at low temperature, as in the time needed for the recording the spectrum no decomposition took place. Conversely, 1 and 1' under the same conditions start to decompose, so that it is not possible to obtain a clean spectrum, and signal assignment is difficult.…”