The influence of transannular interactions on the redox properties of the tricarbonylchromium complexes of ortho-, meta- and para-[2.2]cyclophane

Abstract

The redox behaviour of the [2.2]cyclophane complexes, [Cr(CO)₃(η⁶-C₁₆H₁₆)] [ortho (1,2), meta (1,3) and para (1,4) isomers], has been examined by cyclic voltammetry in 0.2 mol dm^–3 [NBu₄][PF₆]–CH₂Cl₂. The complexes display a one-electron oxidation in the region +0.7–+1.0 V (vs. Ag–AgCl), with the reversible half-wave potential varying in the order ortho > meta > para. The variation in reversible potential for the oxidation process indicates that the three cyclophane isomers impart different degrees of stabilisation on the {Cr(CO)₃} moiety, in accord with the anticipated variation in the inter-ring π–π interaction within ortho-, meta- and para-cyclophane. The IR spectra of the [Cr(CO)₃(η⁶-C₁₆H₁₆)]⁺˙ radical cations have been collected by in situ spectroelectrochemical electrogeneration. Despite the different reversible potentials for the oxidation of the three [Cr(CO)₃(η⁶-C₁₆H₁₆)]^z complexes, their IR spectra, both in the neutral and cationic states, are all quite similar, with the A₁ and E bands occurring in the region 1961–1956 and 1887–1876 cm^–1 (z = 0) and 2071–2068 and 2006–2003 cm^–1 (z = 1+) respectively. The stability of the one-electron oxidised products, as judged by cyclic voltammetry, IR and EPR spectroelectrochemistry, also varies with cyclophane isomer, in the order para > meta > ortho. The stability of the [Cr(CO)₃(η⁶-C₁₆H₁₆-1,4)]⁺˙ cation is such that it has also been characterised by EPR spectroscopy. The EPR spectrum consists of a rhombic pattern of three lines at g = 2.0860, 2.0374 and 1.9940 G corresponding to g₁, g₂ and g₃.