EPR Insensitivity of the Metal-Nitrosyl Spin-Bearing Moiety in Complexes [LnRuII-NO·]k

Nitrosylruthenium Complexes as Polymerization Catalysts for Acrylonitrile in DMF

Nitrosylruthenium complexes bearing two 2,2'-bipyridine (bpy) or 2-pyridinecarboxylate (pyc) ligands, [Ru(NO)X(bpy)₂ ]³⁺ (X=CH₃ CN, CH₂ =CHCN, H₂ O, Cl, ONO₂ ) and [Ru(NO)(OH₂ )(pyc)₂ ]⁺ , were used as catalysts for the polymerization of acrylonitrile in N,N-dimethylformamide (DMF) under air without initiators to obtain polyacrylonitrile (PAN) with a high molecular weight and a narrow molecular weight distribution.

Rare and Nonexistent Nitrosyls: Periodic Trends and Relativistic Effects in Ruthenium and Osmium Porphyrin-Based {MNO}7 Complexes

Relativistic and nonrelativistic density functional theory calculations were used to investigate rare or nonexistent ruthenium and osmium analogues of nitrosylhemes. Strong ligand field effects and, to a lesser degree, relativistic effects were found to destabilize {RuNO}7 porphyrins relative to their {FeNO}7 analogues. Substantially stronger relativistic effects account for the even greater instability and/or nonexistence of {OsNO}7 porphyrin derivatives.

Pushing the photodelivery of nitric oxide to the visible: are {FeNO}7 complexes good candidates?

Photodelivery of NO requires stable compounds which can be made reactive by irradiation with (visible) light. Traditional {MNO}⁶ complexes require a substantial ligand design to shift their absorption spectra to the appropriate region of the electromagnetic spectrum. [Fe((CH₂Py₂)₂Me[9]aneN₃)(NO)](BF₄)₂ is a new {FeNO}⁷ octahedral coordination compound, which is thermally and air-stable in solution. Illumination with a 450 nm light source induces significant photodetachment of the coordinated NO (ϕ_NO = 0.52 mol einstein^-1), suggesting that {FeNO}⁷ compounds can be in fact suitable compounds for therapeutic NO-photorelease.

The effect of one valence electron: contrasting (PNP)Ni(CO) with (PNP)Ni(NO) to understand the half-bent NiNO unit

Reaction of a (PNP)Ni radical with NO finishes in the time of mixing to form a 1:1 adduct with a NO stretching frequency of 1654 cm (-1). NMR data of this diamagnetic product indicate C 2 v symmetry, which is contradicted by the X-ray structure, which shows it to be nonplanar at Ni, with a geometry intermediate between planar and tetrahedral; the planar geometry is thus the transition state for fluxionality giving time-averaged C 2 v symmetry. The X-ray structure, together with DFT calculations, reveals that the "half-bent" NiNO unit and the intermediate coordination geometry result from a Ni --> NO charge transfer, which has a nonintegral value, resulting in a continuum between NO (+) (hence Ni (0)) and NO (-) (hence Ni (II)). This is related to the nonaxially symmetric character of the Ni --> NO back-donation caused by the (PNP) environment on Ni. Steric effects of ( t )Bu and even chelate constraints are ruled out as the cause of the unusual electronic and structural features.

Spectroelectrochemistry and DFT Analysis of a New {RuNO}n Redox System with Multifrequency EPR Suggesting Conformational Isomerism in the {RuNO}7 State

The compound [Ru(NO)(bpym)(terpy)](PF6)3, bpym = 2,2'-bipyrimidine and terpy = 2,2':6',2"-terpyridine, with a {RuNO}6 configuration (angle Ru-N-O 175.2(4) degrees ) was obtained from the structurally characterized precursor [Ru(NO2)(bpym)(terpy)](PF6), which shows bpym-centered reduction and metal-centered oxidation, as evident from EPR spectroscopy. The relatively labile [Ru(NO)(bpym)(terpy)](3+), which forms a structurally characterized acetonitrile substitution product [Ru(CH3CN)(bpym)(terpy)](PF6)2 upon treatment with CH3OH/CH3CN, is electrochemically reduced in three one-electron steps of which the third, leading to neutral [Ru(NO)(bpym)(terpy)], involves electrode adsorption. The first-two reduction processes cause shifts of nu(NO) from 1957 via 1665 to 1388 cm(-1), implying a predominantly NO-centered electron addition. UV-vis-NIR Spectroscopy shows long-wavelength ligand-to-ligand charge transfer absorptions for [Ru(II)(NO(-I))(bpym)(terpy)]+ in the visible region, whereas the paramagnetic intermediate [Ru(NO)(bpym)(terpy)](2+) exhibits no distinct absorption maximum above 309 nm. EPR spectroscopy of the latter at 9.5, 95, and 190 GHz shows the typical invariant pattern of the {RuNO}7 configuration; however, the high-frequency measurements at 4 and 10 K reveal a splitting of the g1 and g2 components, which is tentatively attributed to conformers resulting from the bending of RuNO. DFT calculations support the assignments of oxidation states and the general interpretation of the electronic structure.

Complex series [Ru(tpy)(dpk)(X)]n+ (tpy = 2,2':6',2''-terpyridine; dpk = 2,2'-dipyridyl ketone; X = Cl-, CH3CN, NO2(-), NO+, NO*, NO-): substitution and electron transfer, structure, and spectroscopy

The complex framework [Ru(tpy)(dpk)]2+ has been used to study the generation and reactivity of the nitrosyl complex [Ru(tpy)(dpk)(NO)]3+ ([4]3+). Stepwise conversion of the chloro complex [Ru(tpy)(dpk)(Cl)]+ ([1]+) via [Ru(tpy)(dpk)(CH3CN)]2+ ([2]2+) and the nitro compound [Ru(tpy)(dpk)(NO2)]+ ([3]+) yielded [4]3+; all four complexes were structurally characterized as perchlorates. Electrochemical oxidation and reduction was investigated as a function of the monodentate ligand as was the IR and UV-vis spectroscopic response (absorption/emission). The kinetics of the conversion [4]3+/[3]+ in aqueous environment were also studied. Two-step reduction of [4]3+ was monitored via EPR, UV-vis, and IR (nu(NO), nu(CO)) spectroelectrochemistry to confirm the {RuNO}7 configuration of [4]2+ and to exhibit a relatively intense band at 505 nm for [4]+, attributed to a ligand-to-ligand transition originating from bound NO-.

The Redox Series [M(bpy)2(Q)]n+, M = Ru or Os, Q = 3,5-Di-tert-butyl-N-phenyl-1,2-benzoquinonemonoimine. Isolation and a Complete X and W Band EPR Study of the Semiquinone States (n = 1)

The complexes [M(bpy)(2)(Q)](PF(6)) (bpy = 2,2'-bipyridyl; M = Ru, Os; Q = 3,5-di-tert-butyl-N-phenyl-1,2-benzoquinonemonoimine) were isolated and studied by X and W band EPR in a dichloromethane solution at ambient temperatures and at 4 K. For M = Ru, the (14)N hyperfine splitting confirms the Ru(II)/semiquinone formulation, although at a > 1 mT, the (99,101)Ru satellite coupling is unusually high. W band EPR allowed us to determine the relatively small g anisotropy Delta g = g(1) - g(3) = 0.0665 for the ruthenium complex. The osmium analogue exhibits a much higher difference Delta g = 0.370, which is attributed not only to the larger spin-orbit coupling constant of Os versus that of Ru but also to a higher extent of metal contribution to the singly occupied molecular orbital. The difference Delta E between the oxidation and reduction potentials of the radical complexes is larger for the ruthenium compound (Delta E = 0.87 V) than for the osmium analogue (Delta E = 0.72), confirming the difference in metal/ligand interaction. The electrochemically generated states [M(bpy)(2)(Q)](n+), n = 0, 1, 2, and 3, were also characterized using UV-vis-near-infrared spectroelectrochemistry.