Decarbonylative metalation of diformylphenol Schiff bases: new osmium and ruthenium organ ometallics incorporating the iminium-phenolato zwitterionic motif

Abstract

The reaction of Schiff bases of 2,6-diformyl-4-methylphenol with M(PPh₃)₃X₂ in ethanol has afforded carbonylhalo[4-methyl-6-(N-R-iminiomethyl)phenolato-C²,O]bis(triphenylphosphine)metal(II) compounds, M^II(RL)(PPh₃)₂(CO)X (M = Os, X = Br; M = Ru, X = Cl; R = alkyl, aryl). The process is believed to proceed via oxidative addition of the aldehydic C(O)-H bond to the metal followed by CO extrusion and reductive proton elimination. The X-ray structure of Os(EtL)(PPh₃)₂(CO)Br·CH₂Cl₂ has revealed the presence of the distorted octahedral OsC₂P₂BrO coordination sphere. In the four-membered, Os(C,O) chelate ring, the metal atom is bonded to the carbon site to which an aldehyde group was attached in the parent ligand. The presence of the iminium-phenolato zwitterionic motif is consistent with the N···O length as well as with IR and ¹H NMR spectra. Bond parameters of the osmium and ruthenium systems are compared. Significant dπ-pπ back-bonding with the metalated aromatic ring occurs in the case of osmium. The complexes display a quasi-reversible one-electron cyclic voltammetric couple (E_1/2 = 0.50-0.70 V), but the oxidized congeners are too unstable to be isolated. Upon reaction of M(RL)(PPh₃)₂(CO)X with carboxylates, the latter gets chelated, the M-O(phenolato) linkage is cleaved, and the iminium-phenolato function tautomerizes to the imine-phenol function.