Magnetic Circular Dichroism and Density Functional Theory Studies of Iron(II)-Pincer Complexes: Insight into Electronic Structure and Bonding Effects of Pincer N-Heterocyclic Carbene Moieties

Abstract

Iron complexes containing pincer ligands that incorporate N-heterocyclic carbene (NHC) moieties are of significant interest in organometallic catalysis in order to generate more oxidatively robust complexes that may exhibit novel catalytic properties. In order to define the effect that introducing NHC moieties into pincer ligands has on electronic structure and bonding in iron­(II)-pincer complexes, MCD and DFT studies of (iPrCDA)­FeBr2, (iPrPDI)­FeBr2, and (iPrCNC)­FeBr2 were performed. These studies quantify the electronic structures and bonding interactions as a function of pincer ligand variation. They also demonstrate that the observed ligand fields (and, hence, spin states) directly correlate to the increased Fe–C bonding and pincer-donating abilities that result from introducing NHC moieties into the pincer ligand. However, the net donor abilities of the pincers and the strength of the Fe-pincer interaction do not directly correlate to the number of NHC moieties present, but instead are determined to be due to differences in Fe–C and overall Fe-pincer bonding as a result of the position of the NHC moieties in the pincer ligand and the overall geometric constraints of the pincer architecture.

NHC Effects on Reduction Dynamics in Iron‐Catalyzed Organic Transformations**

“…Similarly, the saturation magnetization behavior of 4 collected at 11905 cm −1 is well described by a S=1 positive zero‐field split (+ZFS) non‐Kramers doublet with ground‐state spin‐Hamiltonian parameters D =27±5 cm −1 , E/D=0.10±0.05 and g=2.25±0.05 (Figure 3H). While there are very few examples of saturation magnetization data for S=1 ground states of iron, the large D value observed here is consistent with those previously reported [46,47,51] …”

“…MCD spectra of Fe(0) complexes are rare, [51] likely due to the propensity for this oxidation state to be low‐spin and therefore diamagnetic. As such, TD‐DFT calculations using B3LYP/def2TZVP were performed and the resulting absorption spectra were used to assign the transitions observed in the experimental MCD spectra.…”

Insight into the Electronic Structure of Formal Lanthanide(II) Complexes using Magnetic Circular Dichroism Spectroscopy

“…While these NIR MCD studies provide the first direct observation of weak NIR transitions in one of these complexes, further corroborating their previously assigned ground-state electronic configurations, it is noteworthy that the assigned f–f transitions in [PrCp′ 3 ] − (4f 2 5d 1 ) are significantly broader than would be expected. Typically, lanthanide f–f transitions in the NIR region are very narrow (fwhm ∼50 cm –1 ), but the observed transitions in [PrCp′ 3 ] − exhibit broader line widths (fwhm ∼600 cm –1 ), intermediate between those typically observed for f–f transitions and d–d transitions in transition metals (fwhm ∼1000 cm –1 ). − This broadening suggests that the ground-state molecular orbitals containing the “f-electrons” are unlikely to be purely f-orbital in nature but, instead, contain additional d-orbital character. This 4f/5d mixing has previously been proposed for similar systems with the 4f n 5d 1 configuration and is believed to result from the reduced symmetry of the complexes in the tris-Cp′ ligand environment combined with the large number of near-degenerate metal-centered orbitals .…”

How to tame a palladium terminal oxo

“…Magnetic circular dichroism studies allowed to evaluate the influence of NHC ligands on the ligand field of iron( ii ) complexes. 118 , 119 It has been reported that an ordering of both carbene ligands and N-donor as well as P-donor ligands according to their ligand field is a challenging task due to side effects with the ancillary ligands. 119 Nevertheless, it can be argued that CAAC ( 10 ) ligands with their very strong σ-donor and π-acceptor capabilities should be extraordinary strong field ligands.…”