Electronic Structures of Rhenium(II) β-Diketiminates Probed by EPR Spectroscopy: Direct Comparison of an Acceptor-Free Complex to Its Dinitrogen, Isocyanide, and Carbon Monoxide Adducts.

Electron paramagnetic resonance (EPR) studies of the rhenium(II) complex Re(η ⁵ -Cp)(BDI) ( 1 ; BDI = N , N '-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate) have revealed that this species reversibly binds N ₂ in solution: flash frozen toluene solutions of 1 disclose entirely different EPR spectra at 10 K when prepared under N ₂ versus Ar atmospheres. This observation was additionally verified by the synthesis of stable CO and 2,6-xylylisocyanide (XylNC) adducts of 1 , which display EPR features akin to those observed in the putative N ₂ complex. While we found that 1 displays an extremely large g _max value of 3.99, the binding of an additional ligand leads to substantial decreases in this value, displaying g _max values of ca. 2.4. Following the generation of isotopically enriched ¹⁵ N ₂ and ¹³ CO adducts of 1 , HYSCORE experiments allowed for the measurement of the corresponding hyperfine couplings associated with spin delocalization onto the electron-accepting ligands in these species, which proved to be small. A cumulative assessment of the EPR data, when combined with insights provided by near-infrared (NIR) spectroscopy and time-dependent density functional theory (TDDFT) calculations, indicated that while the binding of electron acceptors to 1 does lead to decreases in g _max in relative accord with the field strength (i.e., π-acidity) of the variable ligand, the magnitude of these decreases is primarily due to the changes in electronic structure at the Re center.