Remarkable Changes of the Acidity of Bound Nitroxyl (HNO) in the [Ru(Me3[9]aneN3)(L2)(NO)] n+ Family ( n = 1-3). Systematic Structural and Chemical Exploration and Bioinorganic Chemistry Implications.

This work demonstrates that the acidity of nitroxyl (HNO) coordinated to a metal core is significantly influenced by its coordination environment. The possibility that NO- complexes may be the predominant species in physiological environments has implications in bioinorganic chemistry and biochemistry. This (apparently simple) result pushed us to delve into the basic aspects of HNO coordination chemistry. A series of three closely related {RuNO}6,7 complexes have been prepared and structurally characterized, namely [Ru(Me3[9]aneN3)(L2)(NO)]3+/2+, with L2 = 2,2'-bipyridine, 4,4'-dimethoxy-2,2'-bipyridine, and 2,2'-bipyrimidine. These species have also been thoroughly studied in solution, allowing for a systematic exploration of their electrochemical properties in a wide pH range, thus granting access and characterization of the elusive {RuNO}8 systems. Modulation of the electronic density in the {RuNO} fragment introduced by changing the bidentate coligand L2 produced only subtle structural modifications but affected dramatically other properties, most noticeably the redox potentials of the {RuNO}6,7 couples and the acidity of bound HNO, which spans over a range of almost three pH units. Controlling the acidity of coordinated HNO by the rational design of coordination compounds is of fundamental relevancy in the field of inorganic chemistry and also fuels the growing interest of the community in understanding the role that different HNO-derived species can play in biological systems.

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 (ϕNO = 0.52 mol einstein-1), suggesting that {FeNO}7 compounds can be in fact suitable compounds for therapeutic NO-photorelease.

Structural, Spectroscopic, and Photochemical Investigation of an Octahedral NO-Releasing {RuNO}(7) Species.

[Ru(Me3[9]aneN3)(bpy)(NO)](BF4)2 ([1](BF4)2) was explored by single-crystal X-ray diffractometry, leading to the first crystal structure of an octahedral {RuNO}(7) complex. The metal resides on the center of a distorted octahedron, with dN-O and ∠Ru-N-O at 1.177(3) Å and 141.6(2)°, respectively. [1](BF4)2 can be stored indefinitely under argon. Solutions of [1](2+) show no signs of decomposition when protected from air and light. The electron paramagnetic resonance X-band spectrum at 85 K in vitrified acetonitrile (MeCN) shows signals consistent with an S = (1)/2 spin state, better described as Ru(II)NO(•) (g = [2.030, 1.993, 1.880] and A = [11.0, 30.4, 3.9]/10(-4) cm(-1)). In water, the {RuNO}(7) species reacts with O2 in a 1:4 stoichiometry. The reaction is first-order in both reactants with k = (1.9 ± 0.2) M(-1) s(-1) at 25 °C (ΔH(⧧) = 11.5 ± 0.3 kJ mol(-1); ΔS(⧧) = -189 ± 1 J K(-1) mol(-1)). Solutions of [1](2+) evolve NO when irradiated a 365 nm with ϕNO = 0.024 and 0.090 mol einstein(-1) in H2O and MeCN, respectively.

Nitrosyl-centered redox and acid-base interconversions in [Ru(Me3[9]aneN3)(bpy)(NO)](3,2,1+). The pKa of HNO for its nitroxyl derivative in aqueous solution.

This work reports the preparation of a new 6-coordinated nitrosyl compound and its use as a model to explore the redox and acid-base properties of the three redox states of bound nitrosyl (formally NO(+), NO(•), NO(-)/HNO) in {RuNO}(6,7,8) species. We prepared the octahedral {RuNO}(6) complex [Ru(Me3[9]aneN3)(bpy)(NO)](3+) (Me3[9]aneN3: 1,4,7-trimethyl-1,4,7-triazacyclononane; bpy = 2,2'-bipyridine), and the related [Ru(Me3[9]aneN3)(bpy)(NO2)](+) nitro derivative. The compounds were characterized by chemical analysis, X-ray diffraction, NMR, IR, and UV-vis spectroscopies, cyclic voltammetry (CV), UV-vis/IR spectroelectrochemistry, and theoretical calculations (DFT, (TD)DFT). The reaction kinetics between the {RuNO}(6) complex and the nucleophile OH(-) is also presented. The incorporation of tridentate and bidentate ligands in the coordination sphere prevents labilization issues associated with the trans effect when attaining the reduced states of the nitrosyl group. This allows for a consistent interpretation of the changes in the main geometrical parameters: Ru-N and N-O distances, Ru-N-O angle, and the νNO frequency and electronic transitions. We explore the redox properties in acetonitrile and aqueous solutions, and provide a potential (E1/2) - pH (Pourbaix) diagram for the three diatomic nitrosyl-bound species, as well as for HNO and NO2(-), including the report of the pKa of the [Ru(Me3[9]aneN3)(bpy)(HNO)](2+) ion, 9.78 ± 0.15 at 25.0 °C.

Exchange coupling across the cyanide bridge: structural and DFT interpretation of the magnetic properties of a binuclear chromium(III) complex.

The reaction of [Cr(CN)6]3- with a mixture of trans-[Cr(cyclam)(OH)2]Cl, [Cr(cyclam)(OH)Cl]Cl and [Cr(cyclam)Cl2]Cl affords the cyanide bridged dimer, trans-[HO-Cr(cyclam)-NC-Cr(CN)5]-. The tetraphenylphosphonium salt of the anion crystallizes in space group P2(1)/n and shows a bent arrangement of the Cr1-CN-Cr2 unit with the Cr1-CN bond angle at 166.9 degrees and CN-Cr2 at 160.32 degrees . The Cr2-O bond, trans to the hexacyanide fragment, is very short at 1.902 A. Two dimers are held together by two hydrogen bonds connecting the Cr2-OH group of each dimer with one of the NH groups of the cyclam ligand of an adjacent molecule, leading to an almost linear configuration. These dimers of dimers get packed parallel to each other, generating layers separated by the tetraphenylphosphonium cations. Four of the cyanide groups of the anion are engaged in H-bonds with the four water molecules present in the structure or with a NH group of the macrocycle of an adjacent molecule. From magnetic susceptibility measurements, the dimer was found to exhibit antiferromagnetic interaction between the Cr(III) centers with J=-16 cm(-1)(H=-2JS(A)S(B)). Structural and magnetic parameters have been calculated by density functional theoretical methods at the B3LYP level. The exchange coupling constant, J, calculated for the dimer at the X-ray geometry is -23.2 cm(-1) which is in excellent agreement with the experimental value.

Fine tuning of the electronic coupling between metal centers in cyano-bridged mixed-valent trinuclear complexes.

We report the synthesis, characterization, and spectroscopic properties of a family of trinuclear cyano-bridged mixed-valent compounds, trans-[Ru(II)L(4)[NCFe(III)(CN)(5)](2)](4-), trans-[Ru(II)L(4)[CNFe(III)(CN)(5)](2)](4-), and cis-[Ru(II)(bpy)(2)[NCFe(III)(CN)(5)](2)](4-) (L = pyridine, 4-tert-butylpyridine, and 4-methoxypyridine). Tetraphenylphosphonium salts of complexes trans-[Ru(II)L(4)[NCFe(III)(CN)(5)](2)](4-) (L = pyridine and 4-tert-butylpyridine) crystallize in the space groups C2 and P2(1)/c, respectively, and show a linear arrangement of the metal units and an almost completely eclipsed configuration of the equatorial ligands. An intense band (epsilon approximately 2000-9000 M(-1) cm(-1)) is observed for all of the compounds in the NIR region of the spectrum, not present in the separated building blocks, and strongly solvent dependent. We assign it as a metal-to-metal charge transfer (MMCT) from the Ru(II) to the terminal Fe(III) moieties in the context of a simplified three-center model. The electrochemistry measurements reveal a splitting of the redox waves for the reduction of the iron centers for some of the complexes with a trans configuration between the metal units, ranging from 100 to 260 mV, depending on the substituting pyridine ligand and the solvent, suggesting long-range metal-metal interactions. These interactions are rationalized in terms of the energy matching between the pi-symmetry orbitals of the metals and the cyanide bridge. The one- and two-electron reduced species derived from compounds trans-[Ru(II)L(4)[NCFe(III)(CN)(5)](2)](4-,5-,6-) were characterized in methanolic solution. The mixed-valent Fe(II)-Ru(II)-Fe(III) system exhibits an intense red shifted band in the NIR region of the spectrum, arising from the superposition of MMCT bands from the central Ru(II) to the terminal Fe(III) fragments and from the 1 nm distant Fe(II) to Fe(III) centers.