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 = NH4C5N=NC6H4N(H)C 6H4(H) (HLa) or NH4C 5N=NC6H4N(H)C6H4(CH 3) (HLb)) in refluxing n-octane. In the case of M = Mo, the dinuclear compounds [Mo(L)(pap)]2(μ-O) (3a,b) (pap = 2-(phenylazo)pyridine), were obtained as second products in moist solvent. X-ray diffraction analysis for Cr(Lb)2 (1b), Mo(L a)2 (2a), and W(La)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-Nazo angle is larger than 170°, the other two trans angles are smaller, at about 155° (M = Cr, 1b) or 146° (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 Å 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) Å for 1b and 1.373(3) Å 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 MIV(L •2-)2. The diamagnetism of the complexes as shown by magnetic susceptibility and 1H 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) Å for La and 1.340(9) Å for pap is also formulated as an azo anion radical-containing molybdenum(IV) species, i.e., [MoIV(L•2-)(pap•-)] 2(μ-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 ≈ 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. © 2007 American Chemical Society.