Tropad: a new ligand for the synthesis of water-stable paramagnetic [16+1]-electron rhodium and iridium complexes

The new tetradentate ligand 1,4-bis(5 H-dibenzo[a,d]cyclohepten-5-yl)-1,4-diazabuta-1,3-diene ((H)tropdad) allows the syntheses of the 16-electron cationic rhodium complexes [M((H)tropdad)](O(3)SCF(3)) (M=Rh, Ir). The structure of the rhodium complex was determined by X-ray analysis and points to a description of these as [M(+1)((H)tropdad)(0)] with short Cd-N bonds (av 1.285 A) and a long C-C bond (1.46 A) in the diazabutadiene (dad) moiety, that is the M-->dad charge-transfer is negligible. Both [Rh((H)tropdad)](+) and [Ir((H)tropdad)](+) are reduced at very low potentials (E(1) (1/2)= -0.56 V and E(1) (1/2)=-0.35 V, respectively) which allowed the quantitative synthesis of the neutral paramagnetic complexes [M((H)tropdad)](0) (M=Rh, Ir) by reacting the cationic precursor complexes simply with zinc powder. The [M((H)tropdad)](0) complexes are stable against protic reagents in organic solvents. Continuous wave and pulse EPR spectroscopy was used to characterize the paramagnetic species and the hyperfine coupling constants were determined: [Rh((H)tropdad)](0): A(iso)((14)N)=11.9 MHz, A(iso)((1)H)=14.3 MHz, A(iso)((103)Rh)= -5.3 MHz; [Ir((H)tropdad)](0): A(iso)((14)N)=11.9 MHz, A(iso)((1)H)=14.3 MHz. In combination with DFT calculations, the experimentally determined g and hyperfine matrices could be orientated within the molecular frame and the dominant spin density contributions were determined. These results clearly show that the complexes [M((H)tropdad)](0) are best described as [M(+1)((H)tropdad)(.-)] with a [16+1] electron configuration.