Ion-pair complexes of oxidorhenate(V) with phenolate-oxazoline ligands: synthesis, solvatochromism and catalytic...

Ion-pair complexes, in which an organic cation exists entirely in the second coordination sphere of an anionic metal complex, represent a structurally and electronically distinct compound class, whose solution behavior is influenced by solvent polarity and hydrogen-bonding interactions. Their capacity to undergo ion pair charge transfer (IPCT) transitions makes them attractive model complexes for ion-pairing equilibria and potential active species in catalytic cycles. Herein we report the synthesis, structural characterization, spectroscopic investigation and catalytic evaluation of the oxidorhenate(V) outer-sphere ion pair complexes (H₂L1)[ReOCl₃(L1)] (1) and (H₂L2)[ReOCl₃(L2)] (2). Both complexes crystallize as contact ion pairs stabilized by N-H⋯Cl and O-H⋯Cl hydrogen bonds. The hydrogen-bond-accepting Re-Cl bonds are significantly elongated as shown by single crystal X-ray diffraction analysis, indicating enhanced lability at these sites. Both complexes display a striking solvatochromism, assigned by UV/Vis spectroscopy to an IPCT transition operative exclusively in apolar media (λ_max = 555 nm, ε = 2700 L mol^-1 cm^-1 for 1; λ_max = 577 nm, ε = 920 L mol^-1 cm^-1 for 2, solvent: CHCl₃). Additionally, DFT calculations across a series of solvents establish the thermodynamic basis for the observed solvent-dependent ion-pair equilibria. Cyclic voltammetry reveals quasi-reversible Re(V/VI) couples for 1 in both solvents and for 2 in MeCN (E_1/2 = +0.431 V for 1; +0.258 V for 2 in MeCN vs. Fc⁺/Fc). Both complexes catalyze perchlorate reduction with dimethyl sulfide, achieving 92% conversion (TOF 0.37 h^-1) for 1 and 55% (TOF 0.22 h^-1) for 2. ¹H NMR monitoring shows that 1 is not converted into the bis-chelate complex during catalysis.