Tacconi Norma R de, Chitakunye Rungano, Macdonnell Frederick M, Lezna Reynaldo O
Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, Texas 76019-0065, USA.
J Phys Chem A. 2008 Jan 24;112(3):497-507. doi: 10.1021/jp0770099. Epub 2008 Jan 3.
A combination of electrochemistry, spectroelectrochemistry, and 1H NMR has been used to study the reduction and solution speciation in acetonitrile of two mononuclear Ru complexes containing the redox-active 9,11,20,22-tetraazatetrapyrido [3,2-a:2',3'-c:3' ',2' '-l:2' '',3' ''-n]pentacene (tatpp) ligand. These complexes, [(bpy)2Ru(tatpp)][PF6]2 (1[PF6]2), and [(phen)2Ru(tatpp)][PF6]2 (2[PF6]2) (where bpy is 2,2'-bipyridine and phen is 1,10-phenanthroline), form pi-pi stacked dimers (e.g., pi-{1}24+ and pi-{2}24+) in solution as determined by 1H NMR studies in an extended concentration range (90 - 5000 microM) as well as via simulation of the electrochemical data. The dimerization constant for 12+ in acetonitrile is 2 x 10(4) M(-1) as determined from the NMR data. Slightly higher dimerization constants (8 x 10(4) M(-1)) were obtained via simulation of the electrochemical data and are attributed to the presence of the supporting eletrolyte. Electrochemical and spectroelectrochemical data show that the pi-pi stacked dimers are electroreduced in two consecutive steps at -0.31 and -0.47 V vs Ag/AgCl, which is assigned to the uptake of one electron by each tatpp ligand in pi-{1}24+ to give first pi-{1}23+and then pi-{1}22+. At potentials negative of -0.6 V, the electrochemical data reveal two different reaction pathways depending on the complex concentration in solution. At low concentrations (< or =20 microM), the next electroreduction occurs on a monomeric species (e.g., [(bpy)2Ru(tatpp)]+/0) showing that the doubly reduced pi-pi dimer (pi-{1}22+ and pi-{2}22+) dissociates into monomers. At high concentrations (> or =100 microM), reduction of pi-{1}22+ or pi-{2}22+ induces another dimerization reaction, which we attribute to the formation of a sigma-bond between the radical tatpp ligands and is accompanied by the appearance of a new peak in the absorption spectrum at 535 nm. This new sigma-dimer can undergo one additional tatpp based reduction to form sigma-{1}20 or sigma-{2}20, in which the tatpp-bridged assembly is the site of all four reductions. Finally, potentials negative of -1.2 V result in the electroreduction of the bpy or phen ligands for complexes 12+ or 22+, respectively. For the latter complex 22+, this process is accompanied by the formation of an electrode adsorbed species.
采用电化学、光谱电化学和1H NMR相结合的方法，研究了两种含氧化还原活性9,11,20,22 - 四氮杂四吡啶并[3,2 - a:2',3'-c:3' ',2' '-l:2' '',3' ''-n]并五苯（tatpp）配体的单核钌配合物在乙腈中的还原及溶液形态。这些配合物，[(bpy)2Ru(tatpp)][PF6]2（1[PF6]2）和[(phen)2Ru(tatpp)][PF6]2（2[PF6]2）（其中bpy为2,2'-联吡啶，phen为1,10 - 菲咯啉），通过在较宽浓度范围（90 - 5000 microM）内的1H NMR研究以及电化学数据模拟，确定它们在溶液中形成π - π堆积二聚体（如π-{1}24+和π-{2}24+）。根据NMR数据，12+在乙腈中的二聚常数为2×10(4) M(-1)。通过电化学数据模拟得到稍高的二聚常数（8×10(4) M(-1)），这归因于支持电解质的存在。电化学和光谱电化学数据表明，π - π堆积二聚体在相对于Ag/AgCl为 - 0.31和 - 0.47 V时经历两个连续的电还原步骤，这归因于π-{1}24+中每个tatpp配体摄取一个电子，首先生成π-{1}23+，然后生成π-{1}22+。在负于 - 0.6 V的电位下，电化学数据显示根据溶液中配合物浓度有两种不同的反应途径。在低浓度（≤20 microM）时，下一个电还原发生在单体物种上（如[(bpy)2Ru(tatpp)]+/0），表明双还原的π - π二聚体（π-{1}22+和π-{2}22+）解离成单体。在高浓度（≥100 microM）时，π-{1}22+或π-{2}22+的还原引发另一个二聚反应，我们将其归因于自由基tatpp配体之间形成σ键，并伴随着在535 nm处吸收光谱中出现一个新峰。这个新的σ二聚体可以再进行一次基于tatpp的还原，形成σ-{1}2^0或σ-{2}2^0，其中tatpp桥连的组装体是所有四次还原的位点。最后，负于 - 1.2 V的电位分别导致配合物12+或22+的bpy或phen配体发生电还原。对于后一种配合物22+，这个过程伴随着形成一种电极吸附物种。
