Spectroscopic, Magnetic, and Computational Investigations on a Series of Rhenium(III) Cyclopentadienide β‑diketiminate Halide and Pseudohalide Complexes

The
low-valent rhenium­(I) salt Na­[Re­(η5-Cp)­(BDI)]
(BDI = N,N′-bis­(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate)
was shown to react with various halide and chalcogenolate reagents,
leading to the isolation of a series of rhenium­(III)–halide
and −pseudohalide complexes: Re­(X)­(η5-Cp)­(BDI)
(1-X, X = F, Cl, Br, I; 2, X = OTf) and
Re­(ER)­(η5-Cp)­(BDI) (3-ER, ER = SBn,
SeBn, TePh). The 1H NMR spectra of these complexes displayed
sharp resonances shifted several ppm from typical diamagnetic regions,
as well as distinct chemical shift trends down both the halide (with
the exception of F) and chalcogenolate series, with both Cp and BDI
backbone peaks moving downfield with increasing atomic number. Subsequent
magnetic susceptibility measurements of the rhenium­(III) halides 1-Cl, 1-Br, and 1-I indicated that
these complexes display temperature-independent paramagnetism (TIP),
with χTIP values of 7.41(44) × 10–4 to 1.50(51) × 10–3 cm3 mol–1. Multireference complete active space self-consistent
field (CASSCF) computations incorporating spin–orbit state
mixing revealed small energy separations (1.7–3.0 kcal/mol)
between thermally isolated ground states and the first excited spin–orbit
states for the rhenium halides, confirming that low-lying excited
states are responsible for the observed TIP behavior.