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) (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) (M=Rh, Ir) by reacting the cationic precursor complexes simply with zinc powder. The M((H)tropdad) 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): A(iso)((14)N)=11.9 MHz, A(iso)((1)H)=14.3 MHz, A(iso)((103)Rh)= -5.3 MHz; Ir((H)tropdad): 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) are best described as [M(+1)((H)tropdad)(.-)] with a [16+1] electron configuration.