Structure, Electrochemistry and Hydroformylation Catalytic Activity of the Bis(pyrazolylborato)rhodium( I ) Complexes [RhBp(CO)P] [P = P(NC 4 H 4 ) 3 , PPh 3 , PCy 3 , P(C 6 H 4 OMe‐4) 3 ]

Rhodium complexes of formula [RhBp(CO)P] [Bp = bis(pyrazolylborate), P = P(NC 4 H 4 ) 3 1 , PPh 3 2 , PCy 3 3 , P(C 6 H 4 OMe‐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(C 6 H 4 OMe‐4) 3 is also shown to have an identical E L value (0.69 V) to that of P(NC 4 H 4 ) 3 . An improved linear relationship between the oxidation potential and the sum of the ligand E L values for square‐planar Rh I 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 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)