van Gastel Maurice, Shaw Jennifer L, Blake Alexander J, Flores Marco, Schröder Martin, McMaster Jonathan, Lubitz Wolfgang
Max-Planck-Institut für Bioanorganische Chemie, Stiftstrasse 34-36, D-45470 Mülheim an der Ruhr, Germany.
Inorg Chem. 2008 Dec 15;47(24):11688-97. doi: 10.1021/ic801383x.
The binuclear complex Ni(2)(L)(MeCN)(2) (L(2-) = compartmental macrocycle incorporating imine N and thiolate S donors) has a Ni(III) center bridged via two thiolate S-donors to a diamagnetic Ni(II) center. The ground-state has dominant 3d(z)(1)(2) character similar to that observed for [NiFe] hydrogenases in which Ni(III) is bridged via two thiolate donors to a diamagnetic center (Fe(II)). The system has been studied by X-ray crystallography and pulse EPR, ESEEM, and ENDOR spectroscopy in order to determine the extent of spin-delocalization onto the macrocycle L(2-). The hyperfine coupling constants of six nitrogen atoms have been identified and divided into three sets of two equivalent nitrogens. The most strongly coupled nitrogen atoms (a(iso) approximately 53 MHz) stem from axially bound solvent acetonitrile molecules. The two macrocycle nitrogens on the Ni(III) side have a coupling of a(iso) approximately 11 MHz, and those on the Ni(II) side have a coupling of a(iso) approximately 1-2 MHz. Density functional theory (DFT) calculations confirm this assignment, while comparison of the calculated and experimental (14)N hyperfine coupling constants yields a complete picture of the electron-spin density distribution. In total, 91% spin density is found at the Ni(III) of which 72% is in the 3d(z)(2) orbital and 16% in the 3d(xy) orbital. The Ni(II) contains -3.5% spin density, and 7.5% spin density is found at the axial MeCN ligands. In analogy to hydrogenases, it becomes apparent that binding of a substrate to Ni at the axial positions causes a redistribution of the electron charge and spin density, and this redistribution polarizes the chemical bonds of the axial ligand. For [NiFe] hydrogenases this implies that the H(2) bond becomes polarized upon binding of the substrate, which may facilitate its heterolytic splitting.
双核配合物[Ni₂(L)(MeCN)₂]³⁺（L²⁻ = 包含亚胺氮和硫醇盐硫供体的分隔大环）具有一个通过两个硫醇盐硫供体桥连到一个抗磁性Ni(II)中心的Ni(III)中心。基态具有与[NiFe]氢化酶中观察到的相似的占主导地位的3d(z)²特征，其中Ni(III)通过两个硫醇盐供体桥连到一个抗磁性中心（Fe(II)）。为了确定自旋离域到大环L²⁻上的程度，该体系已通过X射线晶体学以及脉冲EPR、ESEEM和ENDOR光谱进行了研究。已确定了六个氮原子的超精细耦合常数，并将其分为三组，每组两个等价氮原子。耦合最强的氮原子（a(iso)约为53 MHz）来自轴向结合的溶剂乙腈分子。Ni(III)侧的两个大环氮原子的耦合为a(iso)约11 MHz，而Ni(II)侧的那些氮原子的耦合为a(iso)约1 - 2 MHz。密度泛函理论（DFT）计算证实了这一归属，同时计算得到的和实验测得的¹⁴N超精细耦合常数的比较给出了电子自旋密度分布的完整图景。总共，91%的自旋密度位于Ni(III)上，其中72%在3d(z)²轨道，16%在3d(xy)轨道。Ni(II)含有 - 3.5%的自旋密度，并且在轴向MeCN配体上发现7.5%的自旋密度。与氢化酶类似，很明显底物在轴向位置与Ni的结合会导致电子电荷和自旋密度的重新分布，并且这种重新分布使轴向配体的化学键极化。对于[NiFe]氢化酶而言，这意味着底物结合时H₂键会极化，这可能有助于其异裂分解。
