Unusual Reduction of Ammonium Heptamolybdate to Novel Molybdenum(IV)-Stabilized Azo Anion Radical Complexes

In an unusual reaction of the polyacid, ammoniumheptamolybdate tetrahydrate ((NH4)6[Mo7O24]·4H2O), and the
ligand, 2-[(arylamino)phenylazo]pyridine (general abbreviation HL), in the presence of PPh3 afforded the brown oxo
free molybdenum complexes of type [Mo(L)2] in high yields (ca. 80%). The reaction occurs smoothly in ethanol. It
is slow on a steam bath (25 h) but is complete in about an hour in a microwave oven. X-ray structures of two
representatives are reported. In these complexes the ligand acted as a tridentate ligand using its pyridyl(N), azo(N),
and the deprotonated amine(N), respectively. The geometry is meridional, and the relative orientations within the
coordinated pairs of nitrogens are cis, trans, and cis, respectively. Bond length data of the coordinated ligands are
consistent with a MoIV[L•]2 ([L•]2- = azo dianion radical formed by one electron reduction of the deprotonated
anionic ligand, [L]-) description. For example, the N−N lengths (1.349(5)−1.357(2) Å) in these complexes are
appreciably longer than that (1.246(3) Å) in the uncoordinated and protonated salt of a representative ligand,
[H2Ld]ClO4. The N−N lengths, however, correspond well with metal complexes of the ligand containing azo ion
radical. The complexes are diamagnetic and showed highly resolved 1H NMR and 13C NMR spectra. The two
coordinated ligands in these are magnetically equivalent, and resonances for only one ligand were observed in
their spectra. Diamagnetism in the present molybdenum complexes is attributed to strong antiferromagnetic coupling
between MoIV(4d2) and the two planar radical [L•]2- ligands. The complexes display multiple redox responses. The
ESR spectrum of electrogenerated [1a]- showed a characteristic spectrum for Mo(III) with weak hyperfine lines
due to the presence of molybdenum isotopes having nonzero nuclear spin. Visible range multiple charge transfer
transitions in these complexes are ascribed to ligand-to-metal transitions.

在一种不寻常的多酸反应中，七水合铵钼酸铵((NH4)6[Mo7O24]·4H2O)与配体，2-[(芳氨基)苯偶氮]吡啶（通用缩写为HL），在PPh3的存在下，生成了高产率（约80%）的棕色氧化物自由钼配合物[Mo(L)2]。该反应在乙醇中平稳进行。在蒸汽浴中反应较慢（25小时），但在微波炉中大约一小时即可完成。报道了两个代表性化合物的X射线结构。在这些配合物中，配体通过其吡啶基(N)、偶氮(N)和去质子化的胺(N)分别作为三齿配体。几何构型为经向型，氮配位对内的相对取向分别为顺式、反式和顺式。配位配体的键长数据与MoIV[L•]2([L•]2- = 由去质子化的阴离子配体[L]-通过一个电子还原形成的偶氮二阴离子自由基)描述一致。例如，这些配合物中的N−N长度(1.349(5)−1.357(2) Å)明显长于代表性配体的未配位和质子化盐的N−N长度(1.246(3) Å)。然而，这些N−N长度与含偶氮离子自由基的配体金属配合物相吻合。这些配合物为抗磁性，并表现出高度分辨的1H NMR和13C NMR谱。这些配合物中的两个配位配体在磁性上是等价的，其谱图中仅观察到单个配体的共振。本实验中的钼配合物的抗磁性归因于MoIV(4d2)与两个平面的自由基配体[L•]2-之间的强反铁磁性耦合。这些配合物表现出多种氧化还原响应。电生成的[1a]的ESR光谱显示了具有弱超精细线的Mo(III)的特征光谱，这是由于存在具有非零核自旋的钼同位素。这些配合物在可见光范围内的多电荷转移跃迁归因于配体到金属的转移。