Tropdad: 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 (^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 Å) and a long C-C bond (1.46 Å) 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 (E ¹_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: A_iso( ¹⁴N) = 11.9 MHz, A_iso(¹H) = 14.3 MHz, A _iso(¹⁰³Rh) = -5.3 MHz; [Ir(^Htropdad)] ⁰: A_iso(¹⁴N) = 11.9 MHz, A_iso( 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.