{Ru–NO}6 and {Ru–NO}7 configurations in [Ru(trpy)(tmp)(NO)]n+ (trpy = 2,2′:6′,2′′-terpyridine, tmp = 3,4,7,8-tetramethyl-1,10-phenanthroline): An experimental and theoretical investigation

The ruthenium–nitrosyl complexes [RuII(trpy)(tmp)(NO+)](ClO4)3 ([4](ClO4)3) and [RuII(trpy)(tmp)(NO)](ClO4)2 ([5](ClO4)2) with {Ru–NO}6 and {Ru–NO}7 configurations, respectively (trpy = 2,2′:6′,2′′-terpyridine, tmp = 3,4,7,8-tetramethyl-1,10-phenanthroline) have been isotaled. The nitrosyl complexes [4]3+ and [5]2+ have been generated by following a stepwise synthetic procedure: [RuII(trpy)(tmp)(X)]n, X/n = Cl/+ (1+) → CH3CN/2+ (22+) → NO2/+ (3+) → NO+/3+ (43+) → NO/2+ (52+). The single-crystal X-ray structures of two precursor complexes [1]ClO4 and [3]ClO4 have been determined. The DFT optimized structures of 43+ and 52+ suggest that the Ru–N–O geometries in the complexes are linear (177.9°) and bent (141.4°), respectively. The nitrosyl complexes with linear (43+) and bent (52+) geometries exhibit ν(NO) frequencies at 1935 cm−1 (DFT: 1993 cm−1) and 1635 cm−1 (DFT: 1684 cm−1), respectively. Complex 43+ undergoes two successive reductions at 0.25 V (reversible) and −0.48 V (irreversible) versus SCE involving the redox active NO function, RuII–NO+ ⇄ RuII–NO and RuII–NO → RuII–NO−, respectively, besides the reductions of trpy and tmp at more negative potentials. The DFT calculations on the optimized 43+ suggest that LUMO and LUMO+1 are dominated by NO+ based orbitals of around 65% contribution along with partial metal contribution of ∼25% due to (dπ)RuII → π∗(NO+) back-bonding. The lowest energy transitions in 43+ and 52+ at 360 nm and 467 nm in CH3CN (TD-DFT: 364 and 459 nm) have been attributed to mixed MLLCT transitions of tmp(π) → NO+(π∗), Ru(dπ)/tmp(π) → NO+(π∗) and Ru(dπ)/NO(π) → trpy(π∗), respectively. The paramagnetic reduced species 52+ exhibits an anisotropic EPR spectrum with g1 = 2.018, g2 = 1.994, g3 = 1.880 (〈g〉 = 1.965 and Δg = 0.138) in CH3CN, along with 14N (I = 1) hyperfine coupling constant, A2 = 35 G at 110 K due to partial metal contribution in the singly occupied molecular orbital (DFT:SOMO:Ru (34%) and NO (53%)). Consequently, Mulliken spin distributions in 52+ are calculated as 0.115 for Ru and 0.855 for NO (N, 0.527; O, 0.328). The reaction of moderately electrophilic nitrosyl center in 43+ with the nucleophile, OH− yields the nitro precursor, 3+ with the second-order rate constant value of 1.7 × 10−1 M−1 s−1 at 298 K in CH3CN–H2O (10:1). On exposure to light (Xenon 350 W lamp) both the nitrosyl species, 43+ ({RuII–NO+}) and 52+ ({RuII–NO}) undergo photolytic Ru–NO bond cleavage process but with a widely varying kNO, s−1 (t1/2, s) of 1.56 × 10−1(4.4) and 0.011 × 10−1(630), respectively.