Lawrence Mark A W, Celestine Michael J, Artis Edward T, Joseph Lorne S, Esquivel Deisy L, Ledbetter Abram J, Cropek Donald M, Jarrett William L, Bayse Craig A, Brewer Matthew I, Holder Alvin A
Department of Chemistry and Biochemistry, Old Dominion University, Norfolk, VA 23529, USA.
University of the Virgin Islands, #2 John Brewers Bay, Charlotte Amalie, VI 00802, USA.
Dalton Trans. 2016 Jun 21;45(25):10326-42. doi: 10.1039/c6dt01583b.
[Co(dmgBF2)2(H2O)2] (where dmgBF2 = difluoroboryldimethylglyoximato) was used to synthesize [Co(dmgBF2)2(H2O)(py)]·0.5(CH3)2CO (where py = pyridine) in acetone. The formulation of complex was confirmed by elemental analysis, high resolution MS, and various spectroscopic techniques. The complex [Co(dmgBF2)2(solv)(py)] (where solv = solvent) was readily formed in situ upon the addition of pyridine to complex . A spectrophotometric titration involving complex and pyridine proved the formation of such a species, with formation constants, log K = 5.5, 5.1, 5.0, 4.4, and 3.1 in 2-butanone, dichloromethane, acetone, 1,2-difluorobenzene/acetone (4 : 1, v/v), and acetonitrile, respectively, at 20 °C. In strongly coordinating solvents, such as acetonitrile, the lower magnitude of K along with cyclic voltammetry, NMR, and UV-visible spectroscopic measurements indicated extensive dissociation of the axial pyridine. In strongly coordinating solvents, [Co(dmgBF2)2(solv)(py)] can only be distinguished from [Co(dmgBF2)2(solv)2] upon addition of an excess of pyridine, however, in weakly coordinating solvents the distinctions were apparent without the need for excess pyridine. The coordination of pyridine to the cobalt(ii) centre diminished the peak current at the Epc value of the Co(I/0) redox couple, which was indicative of the relative position of the reaction equilibrium. Herein we report the first experimental and theoretical (59)Co NMR spectroscopic data for the formation of Co(i) species of reduced cobaloximes in the presence and absence of py (and its derivatives) in CD3CN. From spectroelectrochemical studies, it was found that pyridine coordination to a cobalt(i) metal centre is more favourable than coordination to a cobalt(ii) metal centre as evident by the larger formation constant, log K = 4.6 versus 3.1, respectively, in acetonitrile at 20 °C. The electrosynthesis of hydrogen by complexes and in various solvents demonstrated the dramatic effects of the axial ligand and the solvent on the turnover number of the respective catalyst.
[Co(dmgBF2)2(H2O)2]（其中dmgBF2 = 二氟硼二甲基乙二肟）用于在丙酮中合成[Co(dmgBF2)2(H2O)(py)]·0.5(CH3)2CO（其中py = 吡啶）。配合物的组成通过元素分析、高分辨率质谱和各种光谱技术得以确认。向配合物中加入吡啶后，原位容易形成配合物[Co(dmgBF2)2(solv)(py)]（其中solv = 溶剂）。涉及该配合物和吡啶的分光光度滴定证明了此类物种的形成，在20℃下，在2-丁酮、二氯甲烷、丙酮、1,2-二氟苯/丙酮（4∶1，v/v）和乙腈中的形成常数log K分别为5.5、5.1、5.0、4.4和3.1。在强配位溶剂如乙腈中，K值较低，同时结合循环伏安法、核磁共振和紫外-可见光谱测量表明轴向吡啶发生了广泛解离。在强配位溶剂中，只有加入过量吡啶后才能将[Co(dmgBF2)2(solv)(py)]与[Co(dmgBF2)2(solv)2]区分开来，然而，在弱配位溶剂中，无需过量吡啶就能明显区分。吡啶与钴（II）中心的配位降低了Co（I/0）氧化还原对的Epc值处的峰电流，这表明了反应平衡的相对位置。在此，我们报告了在CD3CN中存在和不存在py（及其衍生物）时还原型钴肟形成Co（I）物种的首个实验和理论（59）Co NMR光谱数据。通过光谱电化学研究发现，吡啶与钴（I）金属中心的配位比与钴（II）金属中心的配位更有利，在20℃的乙腈中，形成常数log K分别为4.6和3.1，这一点很明显。配合物在各种溶剂中电合成氢气证明了轴向配体和溶剂对相应催化剂周转数的显著影响。
