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)
Please log in
to take part in the discussion (add own reviews or comments).