4d vs. 5d - Reactivity and Fate of Terminal Nitrido Complexes of Rhodium and Iridium

The stability of (pyridinediimine)rhodium- and -iridium-azido complexes was studied by a combination of thermoanalytical methods (DTG/MS and DSC) and DFT calculations. On a preparative scale, the isolation and X-ray crystallographic characterization of the thermolysis products confirmed intramolecular C-H activation processes with concomitant reorganisation of C-C, C-N, N-H and Ir-N bonds to yield tuck-in complexes with a different constitution of the ligand framework for the Rh and Ir products. The tentatively formed (Rh) or initially present (Ir) nitrido unit was converted into either an amine (Rh) or amido (Ir) moiety. Furthermore, the dimerization of the nitrido complexes to the corresponding dinitrogen compounds, i.e. 2 LnM equivalent to N -> L-n M-N-2-MLn, was investigated. Experimental evidence for the relevance of this step was provided by the isolation and X-ray crystallographic characterization of a related dinuclear N-2-bridged (pyridinediimine)dirhodium complex. DFT calculations revealed that the formation of dinitrogen complexes is thermodynamically strongly favourable and evidenced that the previous isolation of a terminal iridium-nitrido complex was possible due to a high barrier for the dimerization process and a sizeable barrier for the intramolecular C-H activation step.