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]

Rhodium 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 E-L 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 E-L 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 Rh-1 complexes is also obtained and adjusted values for the Lever S-M and I-M parameters for the Rh-I/Rh-II 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 Angstrom 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.