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]

✍ Scribed by Anna M. Trzeciak; Beata Borak; Zbigniew Ciunik; Józef J. Ziółkowski; M. Fátima C. Guedes da Silva; Armando J. L. Pombeiro

• Publisher: John Wiley and Sons
• Year: 2004
• Tongue: English
• Weight: 178 KB
• Volume: 2004
• Category: Article
• ISSN: 1434-1948
• DOI: 10.1002/ejic.200300517

✦ Synopsis

Abstract

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)