Low-temperature MCD studies of low-spin ferric complexes of tetramesitylporphyrinate: Evidence for the novel (d(xz),d(yz))(4)(d(xy))(1) ground state which models the spectroscopic properties of heme d
Low-temperature MCD studies of low-spin ferric complexes of tetramesitylporphyrinate: Evidence for the novel (d(xz),d(yz))(4)(d(xy))(1) ground state which models the spectroscopic properties of heme d
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DOI：
10.1021/ja960344i
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发表时间：
1996-08-07
影响因子：
15
通讯作者：
Walker, FA
中科院分区：
文献类型：
--
作者：
Cheesman, MR;Walker, FA
记录了四均三甲苯基卟啉铁的双4-（二甲氨基）吡啶（1）、双（1-甲基咪唑）（2）和双（4-氰基吡啶）（3）配合物的低温MCD谱。1和2的基态电子构型形式上为（d（xy））（2）（d（xz），d（yz））（3），但这两种配合物产生具有非常不同MCD强度的近红外卟啉（pi）->铁（d）电荷转移带（NIR-CT）。这些强度的差异与对称性施加在铁离子上的配体平面在这两个配合物中的不同的相对取向。1中的近垂直配体平面导致在三价铁离子处的有效4重对称性和强NIR-CT MCD，而2中的平行配体平面产生相对弱的NIR-CT MCD。配合物3具有垂直取向的配体，但显示出轴向EPR光谱，具有不寻常的g值和非常弱的MCD跃迁，不仅在NIR中，而且在紫外和可见波长范围内。有人认为，这些低强度是一个新的（d（xz），d（yz））（4）（d（xy））（1）基态的MR-CT跃迁是形式上对称禁止的结果。因此，络合物3提供了一个不寻常的EPR和MCD性能的低自旋铁形式的血红素d，它具有类似的EPR光谱，以配合物3的解释。得出的结论是，这些性质是铁的d-轨道的能量重排的轴向配体的卟啉配合物，或由大环的二氢卟酚配合物，在每种情况下导致一个主要的（d（xz），d（yz））（4）（d（xy））（1）基态的结果。
Low-temperature MCD spectra were recorded for the bis 4-(dimethylamino)pyridine (1), bis(1-methylimidazole) (2), and bis(4-cyanopyridine (3) complexes of ferric tetramesitylporphyrinate. The ground state electronic configuration is formally (d(xy))(2)(d(xz),d(yz))(3) for both 1 and 2, but these two complexes give rise to near-infrared porphyrin(pi)-->ferric(d) charge transfer bands (NIR-CT) with very different MCD intensities. These intensity differences are correlated with the symmetry imposed on the ferric ion by different relative orientations of the ligand planes in these two complexes. Near perpendicular ligand planes in 1 result in effective 4-fold symmetry at the ferric ion and intense NIR-CT MCD while parallel ligand planes in 2 give rise to relatively weak NIR-CT MCD. Complex 3 has perpendicularly orientated ligands but displays an axial EPR spectrum with unusual g-values and anomalously weak MCD transitions not only in the NIR but also across the ultraviolet and visible wavelengths. It is argued that these low intensities are the result of a novel (d(xz),d(yz))(4)(d(xy))(1) ground state for which the MR-CT transition is formally symmetry forbidden. Complex 3 thus provides an explanation for the unusual EPR and MCD properties of low-spin ferric forms of heme d, which have similar EPR spectra to complex 3. It is concluded that these properties are a consequence of a reordering of the energies of the ferric d-orbitals by the axial ligands for the porphyrinate complex, or by the macrocycle for chlorin complexes, in each case leading to a predominantly (d(xz),d(yz))(4)(d(xy))(1) ground state.