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

Abstract

The reaction of (3-alkyliminio-5-methyl-2-oxidophenyl-C¹,O)carbonylbromobis(triphenylphosphine)osmium(II), [Os(η²-L^R)(PPh₃)₂(CO)Br] 1 (R = Me or Et) with NaNO₂ furnished [Os(η²-L^R)(PPh₃)₂(CO)(η¹-NO₂)]·H₂O 2 (R = Me or Et) in excellent yields. Structure determination of the R = Et complex revealed that the L^R 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 L^Et 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 Os^III–Os^II reduction potential is shifted anodically. In contrast to 2, ruthenium affords [Ru(η¹-L^R)(PPh₃)₂(CO)(η²-O₂N)] in which the nitrite ligand is O,O′-chelated. The origin of this differentiation is scrutinised.