Kneebone JL, Fleischauer VE, Daifuku SL, Shaps AA, Bailey JM, Iannuzzi TE, Neidig ML. Electronic Structure and Bonding in Iron(II) and Iron(I) Complexes Bearing Bisphosphine Ligands of Relevance to Iron-Catalyzed C-C Cross-Coupling.
Inorg Chem 2015;
55:272-82. [PMID:
26654097 PMCID:
PMC4887941 DOI:
10.1021/acs.inorgchem.5b02263]
[Citation(s) in RCA: 28] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/17/2023]
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.
Use of bisphosphines as supporting ligands in iron-catalyzed C−C
cross-coupling has led to numerous successful reaction methodologies;
herein, spectroscopic and density functional theory investigations
provide fundamental insight into consequences of bisphosphine ligand
structure on electronic structure and bonding within iron(II) and
iron(I) bearing catalytically relevant ligand scaffolds. On the basis
of these studies, potential contributions of electronic effects and
peripheral steric effects in iron-catalyzed cross-coupling reactions
are discussed.
Collapse