{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⁺)](ClO₄)₃ ([4](ClO₄)₃) and [Ru^II(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: [Ru^II(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, 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 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)Ru^II 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⁺ ({Ru^II–NO⁺}) and 5²⁺ ({Ru^II–NO^.}) undergo photolytic Ru–NO bond cleavage process but with a widely varying k_NO, s^-1 (t_½ , s) of 1.56 × 10^-1(4.4) and 0.011 × 10^-1(630), respectively.