Structure, Electrochemistry and Hydroformylation Catalytic Activity of the Bis(pyrazolylborato)rhodium(I) Complexes [RhBp(CO)P] [P = P(NC4H4)3, PPh3, PCy3, P(C6H4OMe‐4)3

Abstract

AbstractRhodium complexes of formula [RhBp(CO)P] [Bp = bis(pyrazolylborate), P = P(NC4H4)3 1, PPh3 2, PCy3 3, P(C6H4OMe‐4)3 4] have been prepared by exchange of the acetylacetonate (acac−) ligand in [Rh(acac)(CO)P] complexes. The spectroscopic and electrochemical properties as well as X‐ray data of [Rh(acac)(CO)P] and [RhBp(CO)P] complexes have been compared with the aim to estimate the relative donor properties of both anionic ligands (acac− and Bp−). The cyclic voltammetric results indicate that the Bp− ligand behaves as a much stronger electron donor than acac− and a value of the Lever EL ligand parameter identical to that of the pyrazolate ligand (−0.24 V vs. NHE for each coordinating arm) is proposed for the bis‐ and tris(pyrazolyl)borate ligands, whereas P(C6H4OMe‐4)3 is also shown to have an identical EL value (0.69 V) to that of P(NC4H4)3. An improved linear relationship between the oxidation potential and the sum of the ligand EL values for square‐planar RhI complexes is also obtained and adjusted values for the Lever SM and IM parameters for the RhI/RhII redox couple are given. The trans influence of phosphanes was not observed in crystals of complexes 2 and 3, in contrast to analogous acetylacetonato complexes in which the Rh−O bonds differ by ca. 0.04−0.06 Å. Complexes 1−4 are very attractive precursors for hydroformylation catalysts and yields of aldehydes of 80−87% have been obtained with all complexes without extra phosphane as co‐catalyst. During the hydroformylation reaction, however, small amounts of a catalytically inactive [RhBp(CO)2] complex were formed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)