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-C¹,O]bis(triphenylphosphine)rhodium(III), Rh(XL_sb)(PPh₃)₂Cl₂ (5; X = H, Me, OMe, Cl). Organometallics of the type (carboxylato)[4-methyl-6-formylphenolato-C¹,O]bis(triphenylphosphine)rhodium(III), Rh(L_al)(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(L_al)(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(L_al)(PPh₃)₂Cl (8a), from which the N-bonded nitrite Rh(L_al)(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)(PPh)₃₂Cl₂ (5b; bis(dichloromethane) adduct), Rh(L_al)(PPh₃)₂(MeCO₂) (6b), Rh(L_al)(PPh₃)₂(NO₃) (7), and Rh(L_al)(PPh₃)₂(NO₂) (8b) have been determined. Among these, 5b, 6b, and 7 are pseudooctahedralthe 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.