title: Singlet diradical complexes of chromium, molybdenum, and tungsten with azo anion radical ligands from M(CO)6 precursors.
abstract: The homoleptic diamagnetic complexes M(mer-L)(2), M = Cr, Mo,W (1a,b, 2a,b, and 4a,b), were obtained by reacting the hexacarbonyls M(CO)(6) with the tridentate ligands 2-[(2-N-arylamino)phenylazo]pyridine (HL = NH(4)C(5)N=NC(6)H(4)N(H)C(6)H(4)(H) (HL(a)) or NH(4)C(5)N=NC(6)H(4)N(H)C(6)H(4)(CH(3)) (HL(b))) in refluxing n-octane. In the case of M = Mo, the dinuclear compounds [Mo(L)(pap)](2)(mu-O) (3a,b) (pap = 2-(phenylazo)pyridine), were obtained as second products in moist solvent. X-ray diffraction analysis for Cr(L(b))(2) (1b), Mo(L(a))(2) (2a), and W(L(a))(2) (4a) reveals considerably distorted-octahedral structures with trans-positioned azo-N atoms and cis-positioned 2-pyridyl-N and anilido nitrogen atoms. Whereas the N(azo)-M-N(azo) angle is larger than 170 degrees, the other two trans angles are smaller, at about 155 degrees (M = Cr, 1b) or 146 degrees (M = Mo, W; 2a, 4a), due to the overarching bite of the mer-tridentate ligands. The bonds from M to the neutral 2-pyridyl-N atoms are distinctly longer by more than 0.08 A than those to the anilido or azo nitrogen atoms, reflecting negative charge on the latter. The N-N bond distances vary between 1.339(2) A for 1b and 1.373(3) A for 4a, clearly indicating the azo radical anion oxidation state. Considering the additional negative charge on anilido-N, the mononuclear complexes are thus formulated as M(IV)(L*(2-))(2). The diamagnetism of the complexes as shown by magnetic susceptibility and (1)H NMR experiments is believed to result from spin-spin coupling between the trans-positioned azo radical functions, resulting in a singlet diradical situation. The experimental structures are well reproduced by density functional theory calculations, which also support the overall electronic structure indicated. The dinuclear 3a with N-N distances of 1.348(10) A for L(a) and 1.340(9) A for pap is also formulated as an azo anion radical-containing molybdenum(IV) species, i.e., [Mo(IV)(L*(2-))(pap*-)](2)(mu-O). All compounds can be reversibly reduced; the Cr complexes 1a,b are also reversibly oxidized in two steps. Electron paramagnetic resonance spectroscopy indicates metal-centered spin for 1a+ and 1a- and g approximately 2 signals for 2a-, 3a+, 3a-, and 4a-. Spectroelectrochemistry in the UV-vis-NIR region showed small changes for the reduction of 2a, 3a, and 4a but extensive spectral changes for the reduction and oxidation of 1a.
authors: Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Kolkata 700 032, India.
authors: Anasuya
authors: A
authors: Sanyal
authors: Sudipta
authors: S
authors: Chatterjee
authors: Alfonso
authors: A
authors: Castiñeiras
authors: Biprajit
authors: B
authors: Sarkar
authors: Priti
authors: P
authors: Singh
authors: Jan
authors: J
authors: Fiedler
authors: Stanislav
authors: S
authors: Zális
authors: Wolfgang
authors: W
authors: Kaim
authors: Sreebrata
authors: S
authors: Goswami
mesh_terms: D002857:Chromium / Q000032:analysis* / Q000737:chemistry; D004563:Electrochemistry / Q000379:methods; D004578:Electron Spin Resonance Spectroscopy; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D008280:Magnetics; D008956:Models, Chemical; D008968:Molecular Conformation; D008982:Molybdenum / Q000032:analysis* / Q000737:chemistry; D013053:Spectrophotometry / Q000379:methods; D013056:Spectrophotometry, Ultraviolet / Q000379:methods; D019265:Spectroscopy, Near-Infrared / Q000379:methods; D014414:Tungsten / Q000032:analysis*; D014961:X-Ray Diffraction
chemical_list: D008024:Ligands; D002857:Chromium; D008982:Molybdenum; D014414:Tungsten