[Ni(Et₂PCH₂NMeCH₂PEt₂)₂]²⁺ as a Functional Model for Hydrogenases

The reaction of Et2PCH2N(Me)CH2PEt2 (PNP) with [Ni(CH3CN)6](BF4)2 results in the formation of [Ni(PNP)2](BF4)2, which possesses both hydride- and proton-acceptor sites. This complex is an electrocatalyst for the oxidation of hydrogen to protons, and stoichiometric reaction with hydrogen forms [HNi(PNP)(PNHP)](BF4)2, in which a hydride ligand is bound to Ni and a proton is bound to a pendant N atom of one PNP ligand. The free energy associated with this reaction has been calculated to be −5 kcal/mol using a thermodynamic cycle. The hydride ligand and the NH proton undergo rapid intramolecular exchange with each other and intermolecular exchange with protons in solution. [HNi(PNP)(PNHP)](BF4)2 undergoes reversible deprotonation to form [HNi(PNP)2](BF4) in acetonitrile solutions (pKa = 10.6). A convenient synthetic route to the PF6- salt of this hydride involves the reaction of PNP with Ni(COD)2 to form Ni(PNP)2, followed by protonation with NH4PF6. A pKa of value of 22.2 was measured for this hydride. This value, together with the half-wave potentials of [Ni(PNP)2](BF4)2, was used to calculate homolytic and heterolytic Ni−H bond dissociation free energies of 55 and 66 kcal/mol, respectively, for [HNi(PNP)2](PF6). Oxidation of [HNi(PNP)2](PF6) has been studied by cyclic voltammetry, and the results are consistent with a rapid migration of the proton from the Ni atom of the resulting [HNi(PNP)2]2+ cation to the N atom to form [Ni(PNP)(PNHP)]2+. Estimates of the pKa values of the NiH and NH protons of these two isomers indicate that proton migration from Ni to N should be favorable by 1−2 pKa units. Cyclic voltammetry and proton exchange studies of [HNi(depp)2](PF6) (where depp is Et2PCH2CH2CH2PEt2) are also presented as control experiments that support the important role of the bridging N atom of the PNP ligand in the proton exchange reactions observed for the various Ni complexes containing the PNP ligand. Similarly, structural studies of [Ni(PNBuP)2](BF4)2 and [Ni(PNP)(dmpm)](BF4)2 (where PNBuP is Et2PCH2N(Bu)CH2PEt2 and dmpm is Me2PCH2PMe2) illustrate the importance of tetrahedral distortions about Ni in determining the hydride acceptor ability of Ni(II) complexes.二乙基膦甲基-N-甲基-亚甲基-二乙基膦（Et2PCH2N(Me)CH2PEt2，简称PNP）与[六(乙腈)合镍(II)]四氟硼酸盐([Ni(CH3CN)6](BF4)2)反应，生成二(PNP)合镍(II)四氟硼酸盐([Ni(PNP)2](BF4)2)，该配合物同时具备氢负离子受体与质子受体位点。该配合物可作为电催化剂，将氢气氧化为质子；其与氢气发生计量比反应时，生成[HNi(PNP)(PNHP)](BF4)2，其中氢负离子配体与Ni中心结合，而质子则结合于一个PNP配体的悬挂N原子上。通过热力学循环计算，该反应的自由能变为-5 kcal/mol。该配合物中的氢负离子配体与NH质子可发生快速分子内交换，同时也能与溶液中的质子发生分子间交换。在乙腈溶剂中，[HNi(PNP)(PNHP)](BF4)2可发生可逆脱质子反应，生成[HNi(PNP)2](BF4)，其酸解离常数(pKa)为10.6。制备该氢负离子配合物的六氟磷酸根(PF6-)盐的便捷合成路线为：先使PNP与双(1,5-环辛二烯)合镍(Ni(COD)2)反应生成Ni(PNP)2，再用六氟磷酸铵(NH4PF6)进行质子化。该氢负离子配合物的pKa经测定为22.2。结合[Ni(PNP)2](BF4)2的半波电位，可计算得到[HNi(PNP)2](PF6)中Ni-H键的均裂键解离自由能与异裂键解离自由能分别为55 kcal/mol与66 kcal/mol。通过循环伏安法对[HNi(PNP)2](PF6)的氧化过程进行了研究，实验结果与质子从生成的[HNi(PNP)2]2+阳离子的Ni中心迁移至N原子、进而形成[Ni(PNP)(PNHP)]2+的过程相符。对这两种异构体的Ni-H与NH质子的pKa值进行估算后可知，质子从Ni迁移至N的过程在热力学上有利，其pKa差值约为1~2个单位。本文还报道了[HNi(depp)2](PF6)（其中depp为Et2PCH2CH2CH2PEt2）的循环伏安法与质子交换研究结果，作为对照实验，证实了PNP配体的桥联N原子在含PNP配体的各类Ni配合物的质子交换反应中发挥的关键作用。类似地，对[Ni(PNBuP)2](BF4)2与[Ni(PNP)(dmpm)](BF4)2（其中PNBuP为Et2PCH2N(Bu)CH2PEt2，dmpm为Me2PCH2PMe2）的结构研究表明，Ni中心的四面体畸变程度是决定Ni(II)配合物氢负离子受体能力的关键因素。