Enhanced Fe-Centered Redox Flexibility in Fe-Ti Heterobimetallic Complexes

Previously, we reported the synthesis of Ti[N(o-(NCH₂P(ⁱPr)₂)C₆H₄)₃] and the Fe–Ti complex, FeTi[N(o-(NCH₂P(ⁱPr)₂)C₆H₄)₃], abbreviated as TiL (1), and FeTiL (2), respectively. Herein, we describe the synthesis and characterization of the complete redox families of the monometallic Ti and Fe–Ti compounds. Cyclic voltammetry studies on FeTiL reveal both reduction and oxidation processes at −2.16 and −1.36 V (versus Fc/Fc⁺), respectively. Two isostructural redox members, [FeTiL]⁺ and [FeTiL]⁻ (2^ox and 2^red, respectively) were synthesized and characterized, along with BrFeTiL (2-Br) and the monometallic [TiL]⁺ complex (1^ox). The solid-state structures of the [FeTiL]^+/0/– series feature short metal–metal bonds, ranging from 1.94–2.38 Å, which are all shorter than the sum of the Ti and Fe single-bond metallic radii (cf. 2.49 Å). To elucidate the bonding and electronic structures, the complexes were characterized with a host of spectroscopic methods, including NMR, EPR, and ⁵⁷Fe Mössbauer, as well as Ti and Fe K-edge X-ray absorption spectroscopy (XAS). These studies, along with hybrid density functional theory (DFT) and time-dependent DFT calculations, suggest that the redox processes in the isostructural [FeTiL]^+,0,– series are primarily Fe-based and that the polarized Fe–Ti π-bonds play a role in delocalizing some of the additional electron density from Fe to Ti (net 13%).

An isostructural redox series of Fe≡Ti complexes was investigated using a combination of spectroscopic methods and density functional theory to elucidate their electronic structures and to understand their polarized metal−metal bonding. Overall, the results support that the redox changes occur primarily at the Fe site though some electron density is delocalized to Ti. Hence, the Ti plays an important role in enhancing the redox flexibility of the single Fe site.

Principles and applications of EPR spectroscopy in the chemical sciences

Electron spins permeate every aspect of science and influence numerous chemical processes: they underpin transition metal chemistry and biochemistry, mediate photosynthesis and photovoltaics and are paramount in the field of quantum information, to name but a few. Electron paramagnetic resonance (EPR) spectroscopy detects unpaired electrons and provides detailed information on structure and bonding of paramagnetic species. In this tutorial review, aimed at non-specialists, we provide a theoretical framework and examples to illustrate the vast scope of the technique in chemical research. Case studies were chosen to exemplify systematically the different interactions that characterize a paramagnetic centre and to illustrate how EPR spectroscopy may be used to derive chemical information.

Gd(III) complexes as paramagnetic tags: Evaluation of the spin delocalization over the nuclei of the ligand

Complexes of the Gd(III) ion are currently being established as spin labels for distance determination in biomolecules by pulse dipolar spectroscopy. Because Gd(III) is an f ion, one expects electron spin density to be localized on the Gd(III) ion - an important feature for the mentioned application. Most of the complex ligands have nitrogens as Gd(III) coordinating atoms. Therefore, measurement of the (14)N hyperfine coupling gives access to information on the localization of the electron spin on the Gd(III) ion. We carried out W-band, 1D and 2D (14)N and (1)H ENDOR measurements on the Gd(III) complexes Gd-DOTA, Gd-538, Gd-595, and Gd-PyMTA that serve as spin labels for Gd-Gd distance measurements. The obtained (14)N spectra are particularly well resolved, revealing both the hyperfine and nuclear quadrupole splittings, which were assigned using 2D Mims ENDOR experiments. Additionally, the spectral contributions of the two different types of nitrogen atoms of Gd-PyMTA, the aliphatic N atom and the pyridine N atom, were distinguishable. The (14)N hyperfine interaction was found to have a very small isotropic hyperfine component of -0.25 to -0.37MHz. Furthermore, the anisotropic hyperfine interactions with the (14)N nuclei and with the non-exchangeable protons of the ligands are well described by the point-dipole approximation using distances derived from the crystal structures. We therefore conclude that the spin density is fully localized on the Gd(III) ion and that the spin density distribution over the nuclei of the ligands is rightfully ignored when analyzing distance measurements.