Metrical Oxidation States of 2-Amidophenoxide and Catecholate Ligands: Structural Signatures of Metal–Ligand π Bonding in Potentially Noninnocent Ligands

Catecholates and 2-amidophenoxides are prototypical “noninnocent” ligands which can form metal complexes where the ligands are best described as being in the monoanionic (imino)­semiquinone or neutral (imino)­quinone oxidation state instead of their closed-shell dianionic form. Through a comprehensive analysis of structural data available for compounds with these ligands in unambiguous oxidation states (109 amidophenolates, 259 catecholates), the well-known structural changes in the ligands with oxidation state can be quantified. Using these correlations, an empirical “metrical oxidation state” (MOS) which gives a continuous measure of the apparent oxidation state of the ligand can be determined based on least-squares fitting of its C–C, C–O, and C–N bond lengths to this single parameter (a simple procedure for doing so is provided via a spreadsheet in the Supporting Information). High-valent d0 metal complexes, particularly those of vanadium­(V) and molybdenum­(VI), have ligands with unexpectedly positive, and generally nonintegral, MOS values. The structural effects in these complexes are attributed not to electron transfer, but rather to amidophenoxide- or catecholate-to-metal π bonding, an interpretation supported by the systematic variation of the MOS values as a function of the degree of competition with the other π-donating groups in the structures.儿茶酚盐（catecholates）与2-酰胺基酚盐（2-amidophenoxides）是典型的非无辜配体（noninnocent ligands），可与金属形成配合物，此类配体的氧化态更适合描述为单阴离子（亚胺基）半醌或中性（亚胺基）醌，而非其闭壳层二阴离子形式。通过对明确氧化态下含此类配体的化合物（109个酰胺基酚盐配合物、259个儿茶酚盐配合物）的结构数据进行全面分析，可量化配体随氧化态变化的已知结构演变。基于上述关联规律，可通过将配体的C-C、C-O及C-N键长对该单一参数进行最小二乘拟合，得到可连续表征配体表观氧化态的经验性“度量氧化态（metrical oxidation state, MOS）”（具体操作的简易流程见支持信息中的电子表格）。高氧化态d⁰金属配合物，尤其是钒(V)与钼(VI)的配合物，其配体展现出出乎意料的正且通常为非整数的MOS值。此类配合物中的结构效应并非源于电子转移，而是源自酰胺基酚盐或儿茶酚盐向金属的π键作用；该结论得到了以下现象的支撑：MOS值随结构中其他π给体基团的竞争程度呈系统性变化。