Ruthenium Nitrosyl Complexes with N-Heterocyclic Ligands

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Abstract
A new route was developed for preparing a series of trans nitrosyl complexes of general formula trans-[Ru(NH3)4L(NO)](BF4)3, where L = imidazole, l-histidine, pyridine, or nicotinamide. The complexes have been characterized by elemental analysis, molar conductance measurements, UV−visible, infrared, proton nuclear magnetic, and electron paramagnetic resonance spectroscopies, and electrochemical techniques. The compounds possess relatively high ν(NO) stretching frequencies indicating that a high degree of positive charge resides on the coordinated nitrosyl group. The nitrosyl complexes react with OH- according to the equation trans-[Ru(NH3)4L(NO)]3+ + 2OH- ⇄ trans-[Ru(NH3)4L(NO2)]+ + H2O, with a Keq (at 25.0 °C in 1.0 mol/L NaCl) of 2.2 × 105, 5.9 × 107, 9.7 × 1010, and 4.6 × 1013 L2 mol-2 for the py, nic, imN, and l-hist complexes, respectively. Only one redox process attributed to the reaction [RuII(NH3)4L(NO+)]3+ + e- ⇄ trans-[RuII(NH3)4L(NO0)]2+ was observed in the range −0.45 to 1.20 V for all the nitrosyl complexes. Linear correlations are observed in plots of ν(NO) versus E1/2 and of E1/2 versus ∑EL showing that the oxidizing strength of the coordinated NO increases with increase in L π-acidity. The crystal structure analysis of trans-[Ru(NH3)4nicNO]2(SiF6)3 shows that the mean Ru−N−O angle is very close to 180° (177 ± 1°) and the mean N−O distance is 1.17 ± 0.02 Å, thus confirming the presence of the RuII−NO+ moiety in the nitrosyl complexes studied.