Carbonylation of four-membered ruthenium and osmium metallacycles incorporating an orthometallated phenolic function: New acylruthenium and arylosmium complexes

Abstract

The solution reaction of Ru(QL 1)(PPh 3) 2(CO)Cl ( 3) and Os(QL 1)(PPh 3) 2(CO)Br ( 4) with carbon monoxide at one atmosphere pressure has respectively afforded the orange acylruthenium system Ru(QL 2)(PPh 3) 2(CO)Cl ( 5) and the yellow arylosmium dicarbonyl system Os(QL 3)(PPh 3) 2(CO) 2Br ( 6) in excellent yields. (QL 1 is C 6H 2O-2-CHNHC 6H 4Q( p)-3-Me-5, QL 2 is C 6H 2(CO-1)O-2-CHNHC 6H 4Q( p)-3-Me-5 and QL 3 is C 6H 2OH-2-CHNC 6H 4Q( p)-3-Me-5 and Q is Me, OMe and Cl.) It is proposed that in the case of 3 a dicarbonyl complex similar to 6 is formed as an intermediate which rapidly undergoes aryl migration with concomitant phenolato coordination furnishing 5. The stability of 6 is consistent with the greatly diminished ability of osmium in promotion of migratory reactions. In the reaction 4 → 6 the Os–O(phenolato) bond is cleaved and the Schiff base moiety undergoes iminium-phenolato → imine-phenol tautomerization. The observed cis geometry of 6 may arise by a concerted route involving edge displacement of the halide ligand. The crystal and molecular structure of 5(Q = Cl) has revealed the presence of a distorted octahedral RuC 2P 2OCl coordination sphere and a highly planar acyl chelate ring characterized by a Ru–C distance of 2.013(4) Å. In the hydrogen bonded zwitterionic iminium-phenolato ring the N ⋯ O distance is 2.561(6) Å. The acyl complexes of type 5 display an MLCT band near 500 nm which is absent in 6. The Schiff base C N stretch in 5 (∼1630 cm −1) is significantly higher than that in 6 (∼1600 cm −1) which displays two strong C O stretches near 2020 and 1940 cm −1 ( cis-Os(CO) 2 configuration). A single 31P NMR signal occurs in both 5 and 6 near 37 and −6 ppm, respectively ( trans-M(PPh 3) 2 configuration). The voltammetric reduction potentials of the M III/M II couple is observed near 1.0 and 0.8 V vs. SCE in 5 and 6, respectively. Both are significantly higher than those in parent complexes ( 3 and 4) due to stabilization of the bivalent state upon carbonylation.