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 d_x²−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.