{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

Abstract

The ruthenium–nitrosyl complexes [Ru II(trpy)(tmp)(NO +)](ClO 4) 3 ([ 4](ClO 4) 3) and [Ru II(trpy)(tmp)(NO )](ClO 4) 2 ([ 5](ClO 4) 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: [Ru II(trpy)(tmp)(X)] n , X/ n = Cl/+ ( 1 +) → CH 3CN/2+ ( 2 2+) → NO 2/+ ( 3 +) → NO +/3+ ( 4 3+) → NO /2+ ( 5 2+). The single-crystal X-ray structures of two precursor complexes [ 1]ClO 4 and [ 3]ClO 4 have been determined. The DFT optimized structures of 4 3+ and 5 2+ suggest that the Ru–N–O geometries in the complexes are linear (177.9°) and bent (141.4°), respectively. The nitrosyl complexes with linear ( 4 3+) and bent ( 5 2+) geometries exhibit ν(NO) frequencies at 1935 cm −1 (DFT: 1993 cm −1) and 1635 cm −1 (DFT: 1684 cm −1), respectively. Complex 4 3+ undergoes two successive reductions at 0.25 V (reversible) and −0.48 V (irreversible) versus SCE involving the redox active NO function, Ru II–NO + ⇄ Ru II–NO and Ru II–NO → Ru II–NO −, respectively, besides the reductions of trpy and tmp at more negative potentials. The DFT calculations on the optimized 4 3+ 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)Ru II → π∗(NO +) back-bonding. The lowest energy transitions in 4 3+ and 5 2+ at 360 nm and 467 nm in CH 3CN (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 5 2+ exhibits an anisotropic EPR spectrum with g 1 = 2.018, g 2 = 1.994, g 3 = 1.880 (〈 g〉 = 1.965 and Δ g = 0.138) in CH 3CN, 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 5 2+ 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 4 3+ 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 CH 3CN–H 2O (10:1). On exposure to light (Xenon 350 W lamp) both the nitrosyl species, 4 3+ ({Ru II–NO +}) and 5 2+ ({Ru II–NO }) undergo photolytic Ru–NO bond cleavage process but with a widely varying k NO, s −1 ( t 1/2, s) of 1.56 × 10 −1(4.4) and 0.011 × 10 −1(630), respectively.