Unusual Ar-H/Rh-H J(HH) NMR coupling in complexes of rhodium(III): experimental evidence and theoretical support for an eta1-arene structure

The synthesis and structural properties of three new hydridorhodium(III) complexes are reported. Hydrogenolysis of the cyclometalated rhodium dichloride complexes [RhCl2{(S,S)-benbox(Me-2)}] (2a-c) leads to formation of the new complexes [RhCl2(H){(S,S)-ip-benbox(Me-2)H}] (3a-c) in 45% to 85% yield. Compounds 3a-c were found to have unusual features by NMR spectroscopy: in particular, downfield shifted aryl proton resonances (8.88-9.03 ppm) that were coupled to the rhodium hydride resonances. Using X-ray crystallographic studies, a variety of solid- and solution-state characterization techniques, and DFT calculations, these features were attributed to the presence of weak pi-type eta(1)-arene interactions in 3a-c.