Dibenzo[b,f]phosphepines: Novel Phosphane–Olefin Ligands for Transition Metals

New, stable heterobidentate phosphane–olefin ligands based on the dibenzo­[b,f]­phosphepine backbone are reported together with their redox properties and coordination chemistry to rhodium­(I). The X-ray crystal structures and DFT calculations show different conformations for the P-phenyl (6a) and P-mesityl (6b) derivatives. Cyclic voltammetry (vs Fc/Fc+) of 6a supported by UV–vis spectroelectrochemistry showed two cathodic waves, a reversible one at E1/2 = −2.62 V (If/Ib = 1.0) and a quasi-reversible (If/Ib ≈ 1.2) one at E1/2 = −3.03 V. Reduction with sodium afforded a mixture of the radical anion [6a•]−, characterized by EPR spectroscopy, and dianion [6a]2–, for which an X-ray crystal structure was obtained. Both 6a and 6b bind to RhI centers, giving rise to 3:1 (8a) and 2:1 (8b) ligand:Rh complexes, respectively. Two dibenzo­[b,f]­phosphepines in 8a and 8b act as heterobidentate ligands in which both the phosphorus atom and the olefinic double bond coordinate to rhodium, but the third ligand in 8a binds as a monodentate P-donor. The cyclic voltammogram of 8b showed two close-lying waves, a one-electron reversible wave at −1.45 V and a two-electron quasi-reversible wave at −1.80 V (If/Ib ≈ 1.3). 8a showed a reversible wave at −1.71 V and irreversible waves at lower potentials.