Singlet Diradical Complexes of Chromium, Molybdenum, and Tungsten with Azo Anion Radical Ligands from M(CO)₆ Precursors

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) (HLa) or NH4C5NNC6H4N(H)C6H4(CH3) (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(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 nitrogen atoms. Whereas the Nazo−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.将六羰基化合物M(CO)6（M=Cr、Mo、W）与三齿配体2-[(2-N-芳基氨基)苯基偶氮]吡啶（HL=NH4C5N=NC6H4N(H)C6H4(H)（HLa）或NH4C5N=NC6H4N(H)C6H4(CH3)（HLb））在正辛烷中回流反应，得到同配反磁性配合物M(mer-L)2（即1a、1b、2a、2b及4a、4b）。当M为Mo时，在潮湿溶剂中会副产双核化合物[Mo(L)(pap)]2(μ-O)（3a、3b，其中pap=2-(苯基偶氮)吡啶）。对Cr(Lb)2（1b）、Mo(La)2（2a）及W(La)2（4a）进行X射线衍射分析，结果显示其均为畸变八面体结构：偶氮氮原子处于反式位置，2-吡啶基氮与苯胺基氮原子处于顺式位置。其中Nazo−M−Nazo键角大于170°，而其余两个反式键角则更小，在M=Cr（1b）时约为155°，M=Mo、W（2a、4a）时约为146°，这源于面式三齿配体的固有咬角限制。金属与中性2-吡啶基氮原子的配位键显著长于与苯胺基或偶氮氮原子的配位键，差值超过0.08 Å，这反映出后两类氮原子带有负电荷。配合物的N−N键长介于1b的1.339(2) Å与4a的1.373(3) Å之间，清晰表明偶氮基团处于自由基阴离子氧化态。结合苯胺基-N上的额外负电荷，单核配合物的化学式可表示为MIV(L•2-)2。通过磁化率测试与氢核磁共振（1H NMR）实验证实的配合物反磁性，被认为源于反式位置的偶氮自由基官能团之间的自旋-自旋耦合，从而形成单重态双自由基结构。密度泛函理论计算很好地重现了实验测得的晶体结构，同时也佐证了上述整体电子结构。对于双核配合物3a，其La配体的N−N键长为1.348(10) Å，pap配体的N−N键长为1.340(9) Å，同样被归为含偶氮阴离子自由基的四价钼物种，即[MoIV(L•2-)(pap•-)]2(μ-O)。所有配合物均可发生可逆还原；Cr基配合物1a、1b还可分两步发生可逆氧化。电子顺磁共振（EPR）波谱显示，1a+与1a-的自旋定域于金属中心，而2a-、3a+、3a-及4a-则呈现g≈2的信号。紫外-可见-近红外（UV−vis−NIR）区的光谱电化学测试表明，2a、3a及4a的还原过程仅伴随微弱的光谱变化，而1a的还原与氧化过程则存在显著的光谱变化。