Decarbonylative Metalation of Diformylphenol Schiff Bases:  New Osmium and Ruthenium Organometallics 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-C 2,O]bis(triphenylphosphine)metal(II) compounds, MII(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.

Synthesis, characterization and photophysical properties of quinolin-8-olato chelated osmium(II) organometallics bearing a pendant imine-phenol motif and electrogeneration of trivalent analogue

“…Also the aldimine CH signal in 1 H NMR in 3 (Figure 1) occurs at lower field viz. 7.8 ppm as compared to ~7.5 ppm in 1 [17]. The O-H resonance in 3 is a relatively sharp signal near 12.5 ppm having halfheight width of ~30 Hz.…”

“…The compounds Os(PPh 3 ) 3 Br 2 [40] and Os(RL 1 )(PPh 3 ) 2 -(CO)Br [17] were prepared by reported methods. 8-hydroxyquinoline was obtained from Merck.…”

“…The O-H resonance in 3 is a relatively sharp signal near 12.5 ppm having halfheight width of ~30 Hz. In contrast, the iminium N-H resonance in 1 is broad (width, ~150 Hz) evidently due to the quadrupole moment of the nitrogen atom [17]. The prototropic transformation between 1 and 3 has certain similarities with the imine-iminium tautomerization in rhodopsins [42].…”

“…In 1 H NMR (Figure 1) the 3-H proton of the metallated ring occur as sharp singlets near 6.0 ppm, while the C(4)-Me protons resonate near 1.9 ppm. These protons are subject to shielding by phosphine phenyl rings [17,27,43]. The PPh 3 , Schiff base (C 6 H 4 R) aromatic protons form a complex multiplet in the region 6.60 -7.05 ppm and the quinolin-8-olate aromatic protons appear in the 6.00 -8.05 ppm region.…”

“…It was demonstrated that the decarbonylative metallation of 4-methyl-2,6-diformyl phenol by M(PPh 3 ) 3 X 2 in the presence of primary aromatic amines (RC 6 H 4 NH 2 ) is known to furnish four-membered ruthenium/osmium organometallics of the type M(RL 1 )(PPh 3 ) 2 (CO)X (1) juxtaposed to a hydrogen bonded iminium-phenolato function [17][18][19]. We are scrutinizing the reactivity of these compounds.…”