Department of Chemistry, Texas A&M University, College Station, TX 77843, USA.
Chemistry. 2010 Mar 8;16(10):3083-9. doi: 10.1002/chem.200902684.
Pentacoordinate iron dicarbonyls, (NS)Fe(CO)(2)P (NS=2-amidothiophenylate, P=PCy(3) (4), PPh(3), (5), and P(OEt)(3) (6)) were prepared as potential biomimetics of the active site of the mono-iron hydrogenase, [Fe]-H(2)ase. Full characterization including X-ray diffraction, density functional theory (DFT) computations, and Mössbauer studies for complexes 5 and 6 find that, despite similar infrared v(CO) pattern and absorption frequencies as the active site of the [Fe]-H(2)ase, the geometrical distortions towards trigonal bipyramidal, the negative isomer shift parameters, and the differences in CO-uptake reactivity are due to the "non-innocence" of the NS ligand. Ligand-based protonation with a strong acid, HBF(4).Et(2)O, interrupted the extensive pi-delocalization over Fe and NS ligand of complex 4 and switched on CO uptake (1 bar) to form a CO adduct, mer-(H-NS)Fe(CO)(3)(PCy(3)) or 4(CO)-H(+). The extrinsic CO is reversibly removed on deprotonation with Et(3)N to regenerate complex 4. In a (13)CO atmosphere, concomitant CO uptake by 4-H(+) and exchange with intrinsic CO groups provide a facile route to (13)C-labeled 4(CO)-H(+) and, upon deprotonation, (13)C-labeled complex 4. DFT calculations show substantial Fe character in the LUMO of 4-H(+) typical of the d(6) Fe(II) in a regular square-pyramidal geometry. Thus, the Lewis acidity of 4-H(+) makes it amenable for CO binding, whereas the dianionic NS ligand renders the iron center of 4 insufficiently electrophilic and largely of Fe(I) character.
五配位二羰基铁配合物[(NS)Fe(CO)2P]（NS=2-氨硫基苯基，P=PCy3（4），PPh3（5），P（OEt）3（6））被制备为单铁氢化酶活性中心的潜在模拟物。对配合物 5 和 6 的全面表征包括 X 射线衍射、密度泛函理论（DFT）计算和 Mössbauer 研究表明，尽管与 [Fe]-H2ase 活性中心的红外 v（CO）模式和吸收频率相似，但由于 NS 配体的“非中性”，几何扭曲为三角双锥，负的各向异性位移参数以及 CO 吸收反应性的差异。基于配体的质子化作用与强酸 HBF4·Et2O 结合，打断了配合物 4 中 Fe 和 NS 配体的广泛π离域，并开启了 CO 的吸收（1 巴），形成 CO 加合物，mer-[(H-NS)Fe(CO)3(PCy3)]+或 4（CO）-H+。通过用 Et3N 去质子化，可将外加的 CO 可逆地除去，从而再生配合物 4。在（13）CO 气氛中，同时通过 4-H+的 CO 吸收和与内在 CO 基团的交换，提供了一种简便的途径来获得（13）C 标记的 4（CO）-H+，并在去质子化后，获得（13）C 标记的配合物 4。DFT 计算表明，4-H+的 LUMO 中具有大量的 Fe 特征，这是典型的规则四方锥几何结构中的 d6 Fe（II）。因此，4-H+的路易斯酸度使其易于与 CO 结合，而二价阴离子 NS 配体使 4 中的铁中心的亲电性不足，且主要具有 Fe（I）特征。
