TROPDAD: A New Ligand for the Synthesis of Water-Stable Paramagnetic [16+1]-Electron Rhodium and Iridium Complexes
Résumé
The new tetradentate ligand 1,4-bis(5H-dibenzo[a,d]cyclohepten-5-yl)-1,4-diazabuta-1,3-diene (Htropdad) allows the syntheses of the 16-electron cationic rhodium complexes [M(Htropdad)](O3SCF3) (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(Htropdad)0] with short C=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(Htropdad)]+ and [Ir(Htropdad)]+ are reduced at very low potentials (E1(1/2)=-0.56 V and E1(1/2)=-0.35 V, respectively) which allowed the quantitative synthesis of the neutral paramagnetic complexes [M(Htropdad)]0 (M = Rh, Ir) by reacting the cationic precursor complexes simply with zinc powder. The [M(Htropdad)]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(Htropdad)]0 : Aiso(14N)=11.9 MHz, Aiso(1H)=14.3 MHz, Aiso(103Rh)= -5.3 MHz; [Ir(Htropdad)]0 : Aiso(14N)=11.9 MHz, Aiso(1H)=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(Htropdad)]0 are best described as [M+1(Htropdad).-] with a [16+ 1] electron configuration.