Debeer George Serena, Huang Kuo-Wei, Waymouth Robert M, Solomon Edward I
Stanford Synchrotron Radiation Laboratory, SLAC, Stanford University, Stanford, California 94309, USA.
Inorg Chem. 2006 May 29;45(11):4468-77. doi: 10.1021/ic060402t.
Ti-TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxyl) provides a means for generating Ti(III) complexes by homolysis of the Ti-O bond. It has been determined that bis-Cp-Ti-TEMPO complexes readily undergo homolytic cleavage while the mono-Cp-Ti-TEMPO complexes do not. Here Ti K- and Cl K-edge XAS are applied to directly determine the oxidation state of TiCl3TEMPO, TiCpCl2TEMPO, and TiCp2ClTEMPO, with reference to Ti(III) and Ti(IV) complexes of known oxidation state. The Ti K-edge data show that Ti(III) complexes exhibit a pre-edge feature approximately 1 eV lower than any of the Ti(IV) complexes; while the Cl K-edges show that Ti(III) complexes have a Cl K- pre-edge feature to approximately 1 eV higher energy than any of the Ti(IV) complexes. Taken together, the Ti and Cl K-edge data indicate that the Ti-TEMPO complexes are best described as Ti(IV)-TEMPO anions (rather than Ti(III)-nitroxyl radicals). In addition, the Cl K-edges indicate that replacement of Cl by Cp weakens the bonding with the remaining ligands, with the Cl 3p covalency decreasing from 25% to 21% to 17% on going from TiCl3TEMPO to TiCpCl2TEMPO to TiCp2ClTEMPO. DFT calculations also show that the electronic structures of the Ti-TEMPO complexes are modulated by the replacement of chloride by Cp. The effect of the Cp on the ancillary ligation is one factor that contributes to facile Ti-O bond homolysis in TiCp2ClTEMPO. However, the results indicate the primary contribution to the energetics of Ti-O bond homolysis in TiCp2ClTEMPO is stabilization of the three-coordinate product by Cp.
钛- TEMPO（TEMPO = 2,2,6,6-四甲基哌啶-N-氧基）提供了一种通过Ti - O键均裂生成Ti(III)配合物的方法。已确定双环戊二烯基钛- TEMPO配合物容易发生均裂，而单环戊二烯基钛- TEMPO配合物则不然。在这里，Ti K边和Cl K边X射线吸收光谱（XAS）被用于直接确定TiCl₃TEMPO、TiCpCl₂TEMPO和TiCp₂ClTEMPO的氧化态，并参考已知氧化态的Ti(III)和Ti(IV)配合物。Ti K边数据表明，Ti(III)配合物的前缘特征比任何Ti(IV)配合物低约1 eV；而Cl K边表明，Ti(III)配合物的Cl K前缘特征比任何Ti(IV)配合物的能量高约1 eV。综合来看，Ti和Cl K边数据表明，Ti - TEMPO配合物最好描述为Ti(IV) - TEMPO阴离子（而不是Ti(III) - 硝酰自由基）。此外，Cl K边表明，用环戊二烯基（Cp）取代Cl会削弱与其余配体的键合，从TiCl₃TEMPO到TiCpCl₂TEMPO再到TiCp₂ClTEMPO，Cl 3p共价性从25%降至21%再降至17%。密度泛函理论（DFT）计算还表明，Ti - TEMPO配合物的电子结构受Cp取代氯的影响。Cp对辅助配体的影响是导致TiCp₂ClTEMPO中Ti - O键容易均裂的一个因素。然而，结果表明，TiCp₂ClTEMPO中Ti - O键均裂能量的主要贡献是Cp对三配位产物的稳定作用。
