Electronic Structure and Bonding in Iron(II) and Iron(I)Complexes Bearing Bisphosphine Ligands of Relevance to Iron-CatalyzedC–C Cross-Coupling

摘要

Chelating phosphines are effective additives and supporting ligands for a wide array of iron-catalyzed cross-coupling reactions. While recent studies have begun to unravel the nature of the in situ-formed iron species in several of these reactions, including the identification of the active iron species, insight into the origin of the differential effectiveness of bisphosphine ligands in catalysis as a function of their backbone and peripheral steric structures remains elusive. Herein, we report a spectroscopic and computational investigation of well-defined FeCl2(bisphosphine) complexes (bisphosphine = SciOPP, dpbz, ^tBudppe, or Xantphos) and known iron(I) variants to systematically discern the relative effects of bisphosphine backbone character and steric substitution on the overall electronic structure and bonding within their iron complexes across oxidation states implicated to be relevant in catalysis. Magnetic circular dichroism (MCD) and density functional theory (DFT) studies demonstrate that common o-phenylene and saturated ethyl backbone motifs result in small but non-negligible perturbations to 10Dq(Td) and iron–bisphosphine bonding character at the iron(II) level within isostructural tetrahedra as well as in five-coordinate iron(I) complexes FeCl(dpbz)2 and FeCl(dppe)2. Notably, coordination of Xantphos toFeCl2 results in a ligand field significantly reduced relativeto those of its iron(II) partners, where a large bite angle and consequentreduced iron–phosphorus Mayer bond orders (MBOs) could playa role in fostering the unique ability of Xantphos to be an effectiveadditive in Kumada and Suzuki–Miyaura alkyl–alkyl cross-couplings.Furthermore, it has been found that the peripheral steric bulk ofthe SciOPP ligand does little to perturb the electronic structureof FeCl2(SciOPP) relative to that of the analogous FeCl2(dpbz) complex, potentially suggesting that differences inthe steric properties of these ligands might be more important indetermining in situ iron speciation and reactivity.