Synthesis and Characterization of Nitrosyl Diruthenium Complexes. Interaction between NO and CO across the Metal−Metal Bond

Two neutral diruthenium complexes and one anionic diruthenium complex, Ru2(dpf)4(NO), Ru2(dpf)4(NO)2, and
[Ru2(dpf)4(NO)]-, where dpf is diphenylformamidinate anion, were synthesized and characterized as to their
electrochemical and spectroscopic properties. Two of the compounds, Ru2(dpf)4(NO) and Ru2(dpf)4(NO)2, were
also structurally characterized. Ru2(dpf)4(NO) undergoes reversible one-electron reductions under N2 at E1/2 =
0.06 and −1.24 V in CH2Cl2, 0.1 M TBABr. These processes are shifted to E1/2 = 0.18 and −0.78 V under CO due
to the trans-coordination of a CO molecule which stabilizes the singly and doubly reduced forms of the metal−metal bonded complexes, thus leading to easier reductions. CO does not coordinate to Ru2(dpf)4(NO), but it does
bind to the singly reduced species to generate [Ru2(dpf)4(NO)(CO)]- under a CO atmosphere in solution; characteristic
NO and CO bands are seen for this compound at νNO = 1674 cm-1 and νCO = 1954 cm-1. Ru2(dpf)4(NO)2
displays a reversible one-electron reduction at E1/2 = −1.24 V versus SCE and an irreversible reduction at Epc =
−1.96 V in CH2Cl2, 0.1 M TBAP under N2. There are also two reversible one-electron oxidations at E1/2 = 0.24 and
1.15 V. Spectroelectrochemical monitoring of the Ru2(dpf)4(NO)2 oxidation processes in a thin-layer cell shows only
a single NO vibration for each electrogenerated product and νNO is located at 1726 (neutral), 1788 (singly oxidized),
or 1834 (doubly oxidized) cm-1. Finally, a labile CO complex, [Ru2(dpf)4(NO)(CO)]-, could be generated by passing
CO into a solution of [Ru2(dpf)4(NO)]-. Formation of the mixed CO/NO adduct was confirmed by electrochemistry
and infrared spectroscopy. Analysis of the NO and CO stretching vibration frequencies for [Ru2(dpf)4(NO)(CO)]- by
in-situ FTIR spectroelectrochemistry and comparisons with data for Ru2(dpf)4(NO) and Ru2(dpf)4(CO) reveal the
presence of a strong interaction between NO and CO across the Ru−Ru bond.