Wilson Aaron D, Shoemaker R K, Miedaner A, Muckerman J T, DuBois Daniel L, DuBois M Rakowski
Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309, USA.
Proc Natl Acad Sci U S A. 2007 Apr 24;104(17):6951-6. doi: 10.1073/pnas.0608928104. Epub 2007 Mar 8.
Studies of the role of proton relays in molecular catalysts for the electrocatalytic production and oxidation of H(2) have been carried out. The electrochemical production of hydrogen from protonated DMF solutions catalyzed by Ni(P(2)(Ph)N(2)(Ph))(2)(CH(3)CN)(2), 3a (where P(2)(Ph)N(2)(Ph) is 1,3,5,7-tetraphenyl-1,5-diaza-3,7-diphosphacyclooctane), permits a limiting value of the H(2) production rate to be determined. The turnover frequency of 350 s(-1) establishes that the rate of H(2) production for the mononuclear nickel catalyst 3a is comparable to those observed for Ni-Fe hydrogenase enzymes. In the electrochemical oxidation of hydrogen catalyzed by Ni(P(2)(Cy)N(2)(Bz))(2)(2), 3b (where Cy is cyclohexyl and Bz is benzyl), the initial step is the reversible addition of hydrogen to 3b (K(eq) = 190 atm(-1) at 25 degrees C). The hydrogen addition product exists as three nearly isoenergetic isomers 4A-4C, which have been identified by a combination of one- and two-dimensional (1)H, (31)P, and (15)N NMR spectroscopies as Ni(0) complexes with a protonated amine in each cyclic ligand. The nature of the isomers, together with calculations, suggests a mode of hydrogen activation that involves a symmetrical interaction of a nickel dihydrogen ligand with two amine bases in the diphosphine ligands. Single deprotonation of 4 by an external base results in a rearrangement to HNi(P(2)(Cy)N(2)(Bz))(2), 5, and this reaction is reversed by the addition of a proton to the nickel hydride complex. The small energy differences associated with significantly different distributions in electron density and protons within these molecules may contribute to their high catalytic activity.
关于质子传递在用于电催化产生和氧化氢气的分子催化剂中的作用的研究已经开展。由Ni(P₂(Ph)N₂(Ph))₂(CH₃CN)₂（3a，其中P₂(Ph)N₂(Ph)是1,3,5,7 - 四苯基 - 1,5 - 二氮杂 - 3,7 - 二磷环辛烷）催化质子化的N,N - 二甲基甲酰胺（DMF）溶液进行电化学产氢，可确定氢气产生速率的极限值。350 s⁻¹的周转频率表明单核镍催化剂3a的产氢速率与镍铁氢化酶所观察到的产氢速率相当。在由Ni(P₂(Cy)N₂(Bz))₂₂（3b，其中Cy是环己基且Bz是苄基）催化的氢气电化学氧化过程中，初始步骤是氢气与3b的可逆加成（25℃时Kₑq = 190 atm⁻¹）。氢加成产物以三种能量几乎相等的异构体4A - 4C形式存在，它们已通过一维和二维¹H、³¹P和¹⁵N核磁共振光谱相结合的方法鉴定为每个环状配体中带有质子化胺的Ni(0)配合物。异构体的性质以及计算结果表明了一种氢活化模式，该模式涉及镍二氢配体与二膦配体中的两个胺碱的对称相互作用。用外部碱对4进行单去质子化会导致重排为HNi(P₂(Cy)N₂(Bz))₂（5），并且通过向镍氢配合物中添加质子可使该反应逆转。这些分子中电子密度和质子分布显著不同但能量差异较小，这可能有助于它们具有高催化活性。
