Decarbonylative Metalation of Diformylphenol Schiff Bases:  New Osmium and Ruthenium Organometallics Incorporating the Iminium−Phenolato Zwitterionic Motif

The reaction of Schiff bases of 2,6-diformyl-4-methylphenol with M(PPh 3 ) 3 X 2 in ethanol has afforded carbonylhalo[4-methyl-6-( N -R-iminiomethyl)phenolato- C 2, O ]bis(triphenylphosphine)metal(II) compounds, M II (RL)(PPh 3 ) 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)(PPh 3 ) 2 (CO)Br·CH 2 Cl 2 has revealed the presence of the distorted octahedral OsC 2 P 2 BrO 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 1 H 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)(PPh 3 ) 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.