Synthesis and structure of osmium(II) organometallics incorporating a four-membered salicylideneiminium metallacycle and Os–η¹-NO₂ binding

The reaction of (3-alkyliminio-5-methyl-2-oxidophenyl-C1,O)carbonylbromobis(triphenylphosphine)osmium(II), [Os(η2-LR)(PPh3)2(CO)Br] 1 (R = Me or Et) with NaNO2 furnished [Os(η2-LR)(PPh3)2(CO)(η1-NO2)]·H2O 2 (R = Me or Et) in excellent yields. Structure determination of the R = Et complex revealed that the LR ligand is C,O-chelated and the nitrite is N-bonded with two unequal N–O distances. The oxygen atom of the longer bond is involved in hydrogen bonding with the water molecule. The presence of the hydrogen-bonded iminium phenolato motif in LEt is consistent with 1H NMR and IR data. The Os–C bond lying trans to Os–NO2 in 2 (R = Et) is lengthened. In going from 1 to 2 a metal-to-ligand charge-transfer band and ν(C· · ·O) are shifted to higher frequencies and the OsIII–OsII reduction potential is shifted anodically. In contrast to 2, ruthenium affords [Ru(η1-LR)(PPh3)2(CO)(η2-O2N)] in which the nitrite ligand is O,O′-chelated. The origin of this differentiation is scrutinised