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)C₆H₄(H) (HL^a) or NH₄C₅N=NC₆H₄N(H)C₆H₄(CH₃) (HL^b)) 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(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°, 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 M^IV(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 L^a and 1.340(9) Å for pap is also formulated as an azo anion radical-containing molybdenum(IV) species, i.e., [Mo^IV(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.