A new family of acylrhodium organometallics

Abstract

The Schiff mono bases of 2,6-diformyl-4-methylphenol, 1, react with RhCl3·3H2O and PPh3 in ethanol, affording dichloro[4-methyl-6-((arylimino)methyl)phenolato-C 1,O]bis(triphenylphosphine)rhodium(III), Rh(XLsb)(PPh3)2Cl2 (5; X = H, Me, OMe, Cl). Organometallics of the type (carboxylato)[4-methyl-6-formylphenolato-C 1,O]bis(triphenylphosphine]rhodium(III), Rh(Lal)(PPh3)2(RCO2) (6; R = H, Me, Et, Ph), have been synthesized by oxidative addition of 1 to RhCl(PPh3)3 in the presence of dilute RCO2H in ethanol. Replacement of RCO2H by dilute HNO3 has afforded the nitrate analogue Rh(Lal)(PPh3)2(NO3) (7). Species of type 5 are susceptible to aldiminium → aldehyde hydrolysis in a dichloromethane-acetone-water mixture with concomitant chloride dissociation, furnishing Rh(Lal)(PPh3)2Cl (8a), from which the N-bonded nitrite Rh(Lal)(PPh3)2(NO2) (8b) has been generated metathetically. The four types of species 5-8 are interconvertible, and a possible reaction pathway involving pentacoordinate intermediates is proposed. The X-ray structures of Rh(MeLsb)(PPh3)2Cl2 (5b; bis(dichloromethane) adduct), Rh(Lal)(PPh3)2(MeCO2) (6b), Rh(Lal)(PPh3)2(NO3) (7), and Rh-(Lal)(PPh3)2(NO2) (8b) have been determined. Among these, 5b, 6b, and 7 are pseudooctahedral - the bonds trans to the acyl function being longer by 0.2-0.4 Å compared to those trans to phenolato oxygen. Complex 8b is square pyramidal, there being no ligand trans to the acyl function. In 5b iminium-phenolato (N⋯O, 2.66(1) Å) and in the remaining species aldehyde-phenolato (C⋯O, 2.86(1) Å) hydrogen bonding is present. Internal charge balance is crucial for the stability of the present organometallics.