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

Abstract

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 to FeCl2 results in a ligand field significantly reduced relative to those of its iron(II) partners, where a large bite angle and consequent reduced iron–phosphorus Mayer bond orders (MBOs) could play a role in fostering the unique ability of Xantphos to be an effective additive in Kumada and Suzuki–Miyaura alkyl–alkyl cross-couplings. Furthermore, it has been found that the peripheral steric bulk of the SciOPP ligand does little to perturb the electronic structure of FeCl2(SciOPP) relative to that of the analogous FeCl2(dpbz) complex, potentially suggesting that differences in the steric properties of these ligands might be more important in determining in situ iron speciation and reactivity.

Intermediates and Reactivity in Iron-Catalyzed Cross-Couplings of Alkynyl Grignards with Alkyl Halides

“…49 The dominance of the d–d transitions of 1 in the NIR MCD spectrum of this mixture is consistent with large magnitudes of Δ ε observed for monoarylated iron(II)-SciOPP complexes compared to their dihalide and bis-arylated analogues. 55 Saturation magnetization data collected at 6060 cm −1 are well-fit by a negative zero-field split (−ZFS) S = 2 ground state doublet model with δ = 1.4 ± 0.2 cm −1 and g || = 8.6 ± 0.3 cm −1 , corresponding to D = −8 ± 1 cm −1 and | E / D | = 0.25 ± 0.03 (Figure 6C). The reaction conditions employed to achieve this distribution combined with the frozen solution Mössbauer and MCD characterization of 1 enabled its assignment as the distorted tetrahedral high-spin iron(II) species Fe(CC-TIPS)-Br(SciOPP).…”

Towards Iron-Catalyzed Sonogashira Cross-Coupling Reactions

“… Bidentate phosphine ligands and their bite angles in known Pt and Fe‐complexes applied within this study …”

“…To gain some insight on possible reasons why the iron complexes of 5–9 showed no catalytic activity whereas complex 3 did, we compared the metal‐diphosphine bite angles. The P−M−P (M=Pt or Fe) bite angles of complexes 5–9 show significantly altered bite angles compared to that of complex 3 . Figure shows the comparison of known bite angles for Pt and Fe‐complexes with the (“P 2 ”)MCl 2 binding motifs.…”

A Physical‐Inorganic Approach for the Elucidation of Active Iron Species and Mechanism in Iron‐Catalyzed Cross‐Coupling

“…(B) Example Mössbauer spectra indicating the isomer shift (δ) and quadrupole splitting (ΔE Q ) parameters (top) and a mixture in which two iron species are present (bottom). Spectra adapted with permission from [75] and [84]. Copyright 2014 and 2016, American Chemical Society.…”

“…(B) Ligand-field transitions in the near-infrared MCD spectrum of FeCl 2 (dpbz) collected at T=5 K, H=7 T. (C) Saturation magnetization data of FeCl 2 (dpbz) collected at 6536 cm −1 fit to a –ZFS S =2 ground-state model with D= −9±2 cm −1 and |E/D| =0.29±0.02. Spectra adapted with permission from [75]. Copyright 2016, American Chemical Society.…”