{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 [Ru(II)(trpy)(tmp)(NO(+))](ClO(4))(3) ([4](ClO(4))(3)) and [Ru(II)(trpy)(tmp) (NO(center dot))](ClO(4))(2) ([5](ClO(4))(2)) with {Ru-NO}(6) and {Ru-NO}(7) configurations, respectively (trpy = 2,2':6',2"-ter-pyridine, 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(3)CN/2+ (2(2+))-> NO(2)/+ (3(+))-> NO(+)/3+ (4(3+))-> NO(center dot)/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 degrees) and bent (141.4 degrees), respectively. The nitrosyl complexes with linear (4(3+)) and bent (5(2+)) geometries exhibit nu(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(+)reversible arrow Ru(II)-NO(center dot) and Ru(II)-NO(center dot) -> 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 similar to 25% due to (d pi)Ru(II)->pi*(NO(+)) back-bonding. The lowest energy transitions in 4(3+) and 5(2+) at 360 nm and 467 nm in CH(3)CN (TD-DFT: 364 and 459 nm) have been attributed to mixed MLLCT transitions of tmp(pi)-> NO(+)(pi*), Ru(d pi)/tmp(pi)-> NO(+)(pi*) and Ru(d pi/NO(center dot)(pi)-> trpy(pi*), 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 ( = 1.965 and Delta g = 0.138) in CH(3)CN, along with (14)N (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(center dot) (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 x 10 (1) M (1) s (1) at 298 K in CH(3)CN-H(2)O (10:1). On exposure to light (Xenon 350W lamp) both the nitrosyl species, 4(3+) ({Ru(II)-NO(+)}) and 5(2+) ({Ru(II)-NO(center dot)}) undergo photolytic Ru-NO bond cleavage process but with a widely varying k(NO), s (1) (t(1/2), s) of 1.56 x 10 (1)(4.4) and 0.011 x 10 (1)(630), respectively. (C) 2010 Elevier B. V.