School of Chemical Sciences, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA.
J Am Chem Soc. 2010 Oct 27;132(42):14877-85. doi: 10.1021/ja105312p.
The [NiFe]-hydrogenase model complex NiFe(pdt)(dppe)(CO)(3) (1) (pdt = 1,3-propanedithiolate) has been efficiently synthesized and found to be robust. This neutral complex sustains protonation to give the first nickel-iron hydride [1H]BF(4). One CO ligand in [1H]BF(4) is readily substituted by organophosphorus ligands to afford the substituted derivatives [HNiFe(pdt)(dppe)(PR(3))(CO)(2)]BF(4), where PR(3) = P(OPh)(3) ([2H]BF(4)); PPh(3) ([3H]BF(4)); PPh(2)Py ([4H]BF(4), where Py = 2-pyridyl). Variable temperature NMR measurements show that the neutral and protonated derivatives are dynamic on the NMR time scale, which partially symmetrizes the phosphine complex. The proposed stereodynamics involve twisting of the Ni(dppe) center, not rotation at the Fe(CO)(2)(PR(3)) center. In MeCN solution, 3, which can be prepared by deprotonation of [3H]BF(4) with NaOMe, is about 10(4) stronger base than is 1. X-ray crystallographic analysis of [3H]BF(4) revealed a highly unsymmetrical bridging hydride, the Fe-H bond being 0.40 Å shorter than the Ni-H distance. Complexes [2H]BF(4), [3H]BF(4), and [4H]BF(4) undergo reductions near -1.46 V vs Fc(0/+). For [2H]BF(4), this reduction process is reversible, and we assign it as a one-electron process. In the presence of trifluoroacetic acid, proton reduction catalysis coincides with this reductive event. The dependence of i(c)/i(p) on the concentration of the acid indicates that H(2) evolution entails protonation of a reduced hydride. For 2H, 3H, and 4H, the acid-independent rate constants are 50-75 s(-1). For 2H and 3H, the overpotentials for H(2) evolution are estimated to be 430 mV, whereas the overpotential for the N-protonated pyridinium complex 4H(2) is estimated to be 260 mV. The mechanism of H(2) evolution is proposed to follow an ECEC sequence, where E and C correspond to one-electron reductions and protonations, respectively. On the basis of their values for its pK(a) and redox potentials, the room temperature values of ΔG(H•) and ΔG(H-) are estimated as respectively as 57 and 79 kcal/mol for 1H.
[NiFe]-氢化酶模型配合物 NiFe(pdt)(dppe)(CO)(3) (1)（pdt = 1,3-丙二硫醇）已被高效合成并被证明是稳定的。该中性配合物能够质子化生成第一个镍-铁氢化物[1H]BF(4)。[1H]BF(4)中的一个 CO 配体容易被有机磷配体取代，得到取代衍生物[HNiFe(pdt)(dppe)(PR(3))(CO)(2)]BF(4)，其中 PR(3) = P(OPh)(3) ([2H]BF(4))；PPh(3) ([3H]BF(4))；PPh(2)Py ([4H]BF(4)，其中 Py = 2-吡啶基)。变温 NMR 测量表明，中性和质子化衍生物在 NMR 时间尺度上是动态的，这部分使膦配合物对称化。所提出的立体动力学涉及 Ni(dppe)中心的扭曲，而不是 Fe(CO)(2)(PR(3))中心的旋转。在 MeCN 溶液中，[3H]BF(4)与 NaOMe 去质子化可制备 3，其碱性比 1 强约 10(4)倍。[3H]BF(4)的 X 射线晶体学分析揭示了一个高度非对称的桥接氢化物，Fe-H 键比 Ni-H 距离短 0.40 Å。配合物[2H]BF(4)、[3H]BF(4)和[4H]BF(4)在近-1.46 V vs Fc(0/+)下发生还原。对于[2H]BF(4)，该还原过程是可逆的，我们将其指定为单电子过程。在三氟乙酸存在下，质子还原催化与该还原事件同时发生。i(c)/i(p)对酸浓度的依赖性表明，H(2)的释放需要还原氢化物的质子化。对于2H、3H和4H，酸独立的速率常数为 50-75 s(-1)。对于2H和3H，H(2)释放的过电势估计为 430 mV，而 N-质子化吡啶鎓配合物4H(2)的过电势估计为 260 mV。H(2)释放的机制被提议遵循 ECEC 序列，其中 E 和 C 分别对应于单电子还原和质子化。根据其 pK(a)和氧化还原电位值，在室温下，1H的 ΔG(H•)和 ΔG(H-)分别估计为 57 和 79 kcal/mol。
