Singlet Diradical Complexes of Chromium, Molybdenum, and Tungsten with Azo Anion Radical Ligands from M(CO)6 Precursors
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Inorganic Chemistry 46 (21): 8584--8593 (2007)

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)phenylazopyridine (HL = NH4C5N:NC6H4N(H)C6H4(H) (HLa) or NH4C5N:NC6H4N(H)C6H4Me (HLb)) in refluxing n-octane. In the case of M = Mo, dinuclear Mo(L)(pap)2(m-O) (3a,b) (pap = 2-(phenylazo)pyridine), were obtained as second products in moist solvent. X-ray diffraction anal. for Cr(Lb)2 (1b), Mo(La)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 N atoms. Whereas the Nazo-M-Nazo angle is larger than 170°, the other two trans angles are smaller, at $\sim$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 \textgreater0.08 \AA than those to the anilido or azo N atoms, reflecting neg. charge on the latter. The N-N bond distances vary between 1.339(2) \AA for 1b and 1.373(3) \AA for 4a, clearly indicating the azo radical anion oxidn. state. Considering the addnl. neg. charge on anilido-N, the mononuclear complexes are thus formulated as MIV(L.bul.2-)2. The diamagnetism of the complexes as shown by magnetic susceptibility and 1H NMR expts. is believed to result from spin-spin coupling between the trans-positioned azo radical functions, resulting in a singlet diradical situation. The exptl. structures are well reproduced by d. functional theory calcns., which also support the overall electronic structure indicated. The dinuclear 3a with N-N distances of 1.348(10) \AA for La and 1.340(9) \AA for pap is also formulated as an azo anion radical-contg. Mo(IV) species, i.e., MoIV(L.bul.2-)(pap.bul.-)2(m-O). All compds. can be reversibly reduced; the Cr complexes 1a,b are also reversibly oxidized in two steps. ESR spectroscopy indicates metal-centered spin for 1a+ and 1a- and g $\approx$ 2 signals for 2a-, 3a+, 3a-, and 4a-. Spectroelectrochem. in the UV-visible-NIR region showed small changes for the redn. of 2a, 3a, and 4a but extensive spectral changes for the redn. and oxidn. of 1a. on SciFinder(R)
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