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

The solution reaction of Ru(QL1)(PPh3)2(CO)Cl (3) and Os(QL1)(PPh3)2(CO)Br (4) with carbon monoxide at one atmosphere pressure has respectively afforded the orange acylruthenium system Ru(QL2)(PPh3)2(CO)Cl (5) and the yellow arylosmium dicarbonyl system Os(QL3)(PPh3)2(CO)2Br (6) in excellent yields. (QL1 is C6H2O-2-CHNHC6H4Q(p)-3-Me-5, QL2 is C6H2(CO-1)O-2-CHNHC6H4Q(p)-3-Me-5 and QL3 is C6H2OH-2-CHNC6H4Q(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 RuC2P2OCl 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 Cdouble bond; length as m-dashN stretch in 5 (∼1630 cm−1) is significantly higher than that in 6 (∼1600 cm−1) which displays two strong Ctriple bond; length of mdashO 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(PPh3)2 configuration). The voltammetric reduction potentials of the MIII/MII 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.