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. 2019 Nov 11;38(21):4224-4232.
doi: 10.1021/acs.organomet.9b00534. Epub 2019 Oct 10.

Masked Radicals: Iron Complexes of Trityl, Benzophenone, and Phenylacetylene

K Cory MacLeod  1 , Ida M DiMucci  2 , Edward P Zovinka  3 , Sean F McWilliams  1 , Brandon Q Mercado  1 , Kyle M Lancaster  2 , Patrick L Holland  1
Affiliations

Masked Radicals: Iron Complexes of Trityl, Benzophenone, and Phenylacetylene

K Cory MacLeod et al. Organometallics. .

Abstract

We report the first Fe─CPh3 complex, and show that the long Fe─C bond can be disrupted by neutral π-acceptor ligands (benzophenone and phenylacetylene) to release the triphenylmethyl radical. The products are formally iron(I) complexes, but X-ray absorption spectroscopy coupled with density functional and multireference ab initio calculations indicates that the best description of all the complexes is iron(II). In the formally iron(I) complexes, this does not imply that the π-acceptor ligand has radical character, because the iron(II) description arises from doubly-occupied frontier molecular orbitals that are shared equitably by the iron and the π-acceptor ligand, and the unpaired electrons lie on the metal. Despite the lack of substantial radical character on the ligands, alkyne and ketone fragments can couple to form a high-spin iron(III) complex with a cyclized metalladihydrofuran core.

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Figures

Figure 1.
Figure 1.
Molecular structure of 1 from X-ray crystallography. Thermal ellipsoids are displayed at the 50% probability level and hydrogens are omitted for clarity.
Figure 2.
Figure 2.
Molecular structure of 2, from X-ray diffraction studies. Thermal ellipsoids are displayed at the 50% probability level and hydrogens are omitted for clarity.
Figure 3.
Figure 3.
Fe K-edge absorption spectra of species 1-5 showing little to no pre-edge and rising-edge shifts between the formally iron(I) and formally iron(II) species.
Figure 4.
Figure 4.
Overlay of experimental and TDDFT calculated spectra of 1 showing acceptor molecular orbitals for pre-edge transitions. Orbitals are plotted at an isovalue of 0.03 au. TDDFT calculations were initiated from B3LYP single point calculations employing the CP(PPP) basis set on Fe and ZORA-def2-TZVP(-f) basis set on all other atoms.
Figure 5.
Figure 5.
Overlay of experimental and TDDFT calculated spectra of 2 showing acceptor molecular orbitals for pre-edge transitions. Orbitals are plotted at an isovalue of 0.03 au. TDDFT calculations were initiated from B3LYP single point calculations employing the CP(PPP) basis set on Fe and ZORA-def2-TZVP(-f) basis set on all other atoms.
Figure 6.
Figure 6.
Overlay of experimental and TDDFT calculated spectra of 3 showing acceptor molecular orbitals for pre-edge transitions. Orbitals are plotted at an isovalue of 0.03 au. TDDFT calculations were initiated from B3LYP single point calculations employing the CP(PPP) basis set on Fe and ZORA-def2-TZVP(-f) basis set on all other atoms.
Figure 7.
Figure 7.
Overlay of experimental and TDDFT calculated spectra of 4 showing acceptor molecular orbitals for pre-edge transitions. Orbitals are plotted at an isovalue of 0.03 au. TDDFT calculations were initiated from B3LYP single point calculations employing the CP(PPP) basis set on Fe and ZORA-def2-TZVP(-f) basis set on all other atoms.
Figure 8.
Figure 8.
Overlay of experimental and TDDFT calculated spectra of 5 showing acceptor molecular orbitals for pre-edge transitions. Orbitals are plotted at an isovalue of 0.03 au. TDDFT calculations were initiated from B3LYP single point calculations employing the CP(PPP) basis set on Fe and ZORA-def2-TZVP(-f) basis set on all other atoms.
Figure 9.
Figure 9.
Leading configurations of the quartet ground states produced via the SORCI procedure carried out using a CAS(15,11) reference on compound 2. Electrons lying in orbitals with nearly equal parentage from Fe and benzophenone (2) are boxed in red. SORCI calculations employed the ZORA-def2-TZVPP basis set on Fe and ZORA-def2-SVP basis set on all other atoms.
Figure 10.
Figure 10.
Leading configurations of the quartet ground states produced via the SORCI procedure carried out using a CAS(15,11) reference on compound 5. Electrons lying in orbitals with nearly equal parentage from Fe and alkyne (5) are boxed in red. SORCI calculations employed the ZORA-def2-TZVPP basis set on Fe and ZORA-def2-SVP basis set on all other atoms.
Figure 11.
Figure 11.
Molecular structure of 6. Thermal ellipsoids are displayed at the 50% probability level and hydrogens are omitted for clarity.
Chart 1.
Chart 1.
Iron species studied in this report.
Scheme 1.
Scheme 1.
Synthesis of β-Diketiminatoiron Complexes 1 and 2
Scheme 2.
Scheme 2.
Coupling of Benzophenone and Phenylacetylene
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