N-Heterocyclic carbenes (NHC) are nowadays ubiquitous and indispensable in many research fields, and it is not possible to imagine modern transition metal and main group element chemistry without t...

N-Heterocyclic Carbene Adducts of Main Group Elements and Their Use as Ligands in Transition Metal Chemistry.

Hydroxyl radical generated from the reaction of Cu(+) with hydrogen peroxide results in oxidative DNA damage, and this damage is implicated in aging, cancer, and many other diseases. Selenium- and sulfur-containing compounds can act as antioxidants, and coordination of selenium and sulfur to copper is one explanation for this antioxidant activity. To determine how copper coordination results in antioxidant activity, biologically relevant tris(pyrazolyl)methane and borate Cu(+) complexes of the formulas Tp*Cu(L) and [Tpm(R)Cu(L)](+), where (L = N, N'-dimethylimidazole selone, dmise; N, N'-dimethylimidazole thione, dmit; Tp* = hydrotris(3,5-dimethylpyrazolyl)borate; Tpm(R) = tris(pyrazolyl)methane, R = H; Tpm, R = Me; Tpm*, R = iPr; Tpm(iPr)), have been synthesized and characterized. The structures of complexes Tp*Cu(Dmit), Tp*Cu(dmise), [Tpm(R)Cu(dmise)][BF(4)], and [Tpm(R)Cu(Dmit)][BF(4)] (where R = H; Tpm, R = Me; Tpm*, R = iPr; Tpm(iPr)) were determined by X-ray crystallography. All the Cu(+) centers adopt distorted tetrahedral coordination geometry, and Cu-Se and Cu-S distances for all the complexes are approximately 2.30 A, and 2.20 A, respectively. The effects of counterion and steric bulk at the 3 and 5 positions of the pyrazolyl ring on the structural and spectroscopic properties are discussed. Selone or thione coordination to copper significantly alters the Cu(+/2+) redox potential: Cu-selone complexes have Cu(2+/+) potentials from -283 to -390 mV, whereas those of Cu-thione complexes range from 70 to -232 mV versus NHE. The Cu-selone complexes have Cu(2+/+) potentials near or below that of the cellular reductant NADH (-324 mV). Thus, selenium and sulfur coordination to copper in biological systems may prevent the Cu(2+) reduction by NADH required for the catalytic formation of damaging hydroxyl radical.

Probing the antioxidant action of selenium and sulfur using Cu(I)-chalcogenone tris(pyrazolyl)methane and -borate complexes.

Coordination of Cu(I) halides with N,N′-dimethylimidazole selone (dmise) and thione (dmit) ligands was examined by treating CuX (X = Cl, Br, I) with one or two equivalents of dmise or dmit. The reaction of CuI and CuBr with one molar equivalent of dmise results in unusual selenium-bridged tetrameric Cu4(μ-dmise)4(μ-X)2X2copper complexes with average Cu–Se bond lengths of 2.42 A and a Cu2(μ-X)2 core (X = I (1) or Br (6)) that’s in a rhomboidal structure. The reaction of CuX (X = Cl, Br, and I) with two equivalents of dmit or dmise results in trigonal planar Cu(I) complexes of two different conformations with the formula Cu(dmit)2X (3a, 3b, 4, and 7) or Cu(dmise)2X (2, 5, and 8) with average Cu–S and Cu–Se bond lengths of 2.23 A and 2.34 A, respectively. The coordination geometry around the copper center in complexes 1 to 8 is determined by the type of halide and chalcogenone ligand used, intramolecular π–π interactions, and short contact interactions between X–H (X = I, Br, Cl, Se or S). The theoretical DFT calculations are in good agreement with experimental X-ray structural data and indicate that dmise ligands are required for formation of the tetrameric complexes 1 and 6. Electrochemical studies show that the trigonal copper selone complexes have more negative potentials relative to analogous copper thione complexes by an average of 108 mV.

Synthesis, characterization, and DFT studies of thione and selone Cu(I) complexes with variable coordination geometries†

Thione- and selone-containing compounds and their metal complexes show promise as antioxidants, as antithyroid drugs, and for applications in lasers and blue light-emitting diodes. Although Cu(I/II), Co(II), Ag(I), and Zn(II) coordination to thione and selone ligands has been broadly studied and Fe(II) plays an important role in oxidative damage, very few iron–thione complexes and no iron–selone complexes are reported. Novel Fe(II)-containing thione and selone complexes of the formulae FeL2Cl2, [FeL2(CH3CN)2]2+, and [FeL4]2+, and {FeL′Cl2}n, (L = N,N′-dimethylimidazole selone (dmise), and thione (dmit); L′ = bis(thioimidazolyl)ethane (ebit) and bis(selenoimidazolyl)ethane (ebis)) have been synthesized and characterized. Structures of Fe(dmise)2Cl2, Fe(dmit)2Cl2, [Fe(dmit)4][BF4]2, [Fe(dmise)4][BF4]2, and {Fe(ebit)Cl2}n were determined by X-ray crystallography. All Fe(II) centers adopt a distorted tetrahedral coordination geometry with Fe–S distances ranging from 2.339(1) to 2.397(1) A and F–Se distances ranging from 2.453(1) to 2.514(1) A. Density functional theory optimized structures of FeL2Cl2, [FeL2(CH3CN)2]2+, and [FeL4]2+ are consistent with experimental results and suggest that thiones and selones are π-donor ligands that coordinate through their zwitterionic resonance structures. Thione and selone coordination to Fe(II) lowers the Fe(II/III) reduction potential, with a greater decrease for Fe(II)-bound dmise than Fe(II)-bound dmit. Dmit and dmise ligand-based oxidation potentials also significantly increase upon Fe(II) binding compared, indicating that bound thione and selone ligands will undergo oxidation prior to Fe(II). The synthesis of these complexes suggests that iron coordination by thione and selone ligands may occur in vivo and may contribute to the protective antioxidant properties of sulfur and selenium.

Synthesis, characterization, DFT calculations, and electrochemical comparison of novel iron(II) complexes with thione and selone ligands

Templated Synthesis and Intact Coordination of a Diorganotriselenane: [RuCl2(PPh3){κ3-Se,N,N‘-Se(mtSe)2}] (mtSe = selenomethimazolyl)

The crystal structure of the title compound, [CoCl2(C5H8N2Se)2], shows a discrete molecular structure with tetrahedral geometry around the cobalt ion. The mean Co—Cl and Co—Se distances are 2.277 (3) and 2.456 (3) A, respectively. Bond angles around the Co atom range from 101.1 (1) to 120.0 (1)°. Ring distances and angles in the ligand compare favorably with the literature values of analogous compounds, and show significant increases from values observed in the uncoordinated selone. However, the mean C—Se bond distance of 1.88 (2) A shows no significant difference from the free ligand. Other corresponding bond distances and angles show no significant differences between the free and coordinated ligands.

/pdf/dichlorobis-1-3-dimethylimidazole-2-3h-selone-se-cobalt-ii-3o96o142yx.pdf

Edward D. Rice

Papers

Dichlorobis[1,3-dimethylimidazole-2(3H)selone-Se]cobalt(II)