Synthesis and Structural, Spectroscopic, and Electrochemical Characterization of New Ruthenium Dimethyl Sulfoxide Nitrosyls

The reactivity of ruthenium(II)− and ruthenium(III)−chloride−dimethyl sulfoxide precursors and of the antimetasatic
drug [ImH][trans-RuCl4(dmso-S)(Im)] (NAMI-A, Im = imidazole, dmso = dimethyl sulfoxide) toward NO was
investigated. Treatment of [(dmso)2H][trans-RuCl4(dmso-S)2] and mer-RuCl3(dmso)3 with gaseous NO yielded
[(dmso)2H][trans-RuCl4(dmso-O)(NO)] (1) and mer,cis-RuCl3(dmso-O)2(NO) (2), respectively. Thus, coordination of
the strong π-acceptor NO induces a S to O linkage isomerization of the dmso trans to it to avoid competition for
π-electrons. In light-protected nitromethane solutions, complex 2 equilibrates slowly with the two isomers mer-RuCl3(dmso-S)(dmso-O)(NO) (3), with NO trans to Cl, and mer-RuCl3(dmso-S)(dmso-O)(NO) (4), with NO trans to
dmso-O; the equilibrium mixture consists of ca. 64% 2, 3% 3, and 33% 4. Treatment of the Ru(II) precursor
trans-RuCl2(dmso-S)4 with gaseous NO in CH2Cl2 solution yielded the nitrosyl−nitro derivative trans,cis,cis-RuCl2(dmso-O)2(NO)(NO2) (5). Finally, [(Im)2H][trans-RuCl4(Im)(NO)] (6) was prepared by treatment of [ImH][trans-RuCl4(dmso-O)(NO)] (1Im) with an excess of imidazole in refluxing acetone. The spectroscopic features are consistent
with the {Ru(NO)}6 formulation for all complexes, that is, a diamagnetic RuII nucleus bound to NO+. Compounds
1, 2, 5, and 6 were characterized also by X-ray crystallography; they all show a linear nitrosyl group, with short
Ru−NO bond distances consistent with a strong dπ → π* NO back-bonding. An unusual inertness of O-bonded
dmso was observed in compound 1. Complexes 1, 2, 3, 5, and 6 are all redox active in DMF solutions showing
irreversible reductions whose peak potentials depend on the other ligands attached to the Ru metal center. The
site of reduction is the NO+ moiety. The reduced complexes are not stable and release a Cl- or NO2- ligand
followed by the NO• radical. The chemical reactions following electron transfer are all fast (rate constant >100 s-1
at 293 K). The Ru product species are not redox active within the DMF window.