Unusual Structural and Spectroscopic Features of Some PNP-Pincer Complexes of Iron

We report a structural, spectroscopic, and computational study of two tBuPNP (2,6-bis(di-tert-butyl-phosphinomethyl)pyridine) complexes of iron, (tBuPNP)FeCl2 (1) and (tBuPNP)Fe(CO)2 (3). Complex 1, (tBuPNP)FeCl2, also independently synthesized by Milstein, has unusually long iron−ligand bond distances. DFT calculations show that these are clearly attributable to its high-spin electronic structure, and in particular to occupancy of the strongly antibonding dx2−y2 orbital. The crystal structure of 3 reveals two unusual aspects. (1) The geometry around the iron atom in 3 is much closer to square pyramidal (SQP; apical CO) than to trigonal bipyramidal (TBP), although five-coordinate Fe(0) complexes are generally expected to be TBP; moreover, Chirik et al. have reported that (iPrPNP)Fe(CO)2 has essentially a perfect TBP structure (iPrPNP = 2,6-bis(di-isopropyl-phosphinomethyl)pyridine). (2) The apical carbonyl ligand in 3 deviates significantly from linearity (Fe−C−O = 171.9°). Additionally, complex 3 is intensely blue in color, which is unusual for an Fe(0) complex and also significantly different from the red color of Chirik’s (iPrPNP)Fe(CO)2 species. Results from DFT calculations reproduce and explain these structural and spectroscopic aspects as well as the contrast between 3 and its iPrPNP analogue.

我们报道了两种基于2,6-双(二叔丁基膦甲基)吡啶（tBuPNP）的铁配合物的结构、光谱与计算化学研究，分别为(tBuPNP)FeCl₂（化合物1）与(tBuPNP)Fe(CO)₂（化合物3）。化合物1，即(tBuPNP)FeCl₂，亦由Milstein独立合成，其铁-配体键长异乎寻常地长。密度泛函理论（DFT）计算表明，该异常键长可明确归因于其高自旋电子结构，尤其是强反键轨道dx²−y²的电子占据。化合物3的晶体结构展现出两处反常特征：(1) 尽管五配位Fe(0)配合物通常被认为应为三角双锥（TBP）构型，但化合物3中铁原子的配位几何更接近四方锥（SQP；顶端为CO配体）而非三角双锥；此外，Chirik等人报道的(2,6-双(二异丙基膦甲基)吡啶（iPrPNP）)Fe(CO)₂配合物具有近乎完美的三角双锥构型。(2) 化合物3中的顶端羰基配体显著偏离线性结构（Fe−C−O键角为171.9°）。此外，化合物3呈现出深邃的蓝色，这对于Fe(0)配合物而言颇为反常，且与Chirik课题组报道的(iPrPNP)Fe(CO)₂的红色外观存在显著差异。密度泛函理论（DFT）计算结果重现并解释了上述结构与光谱特征，以及化合物3与其iPrPNP衍生物之间的性质差异。