| Summary: | The excellent π-accepting azodicarboxylic esters adcOR (R = Et, <i>i</i>Pr, <i>t</i>Bu, Bn (CH<sub>2</sub>-C<sub>6</sub>H<sub>5</sub>) and Ph) and the piperidinyl amide derivative adcpip were used as bridging chelate ligands in dinuclear Re(CO)<sub>3</sub> complexes [{Re(CO)<sub>3</sub>Cl}<sub>2</sub>(µ-adcOR)] and [{Re(CO)<sub>3</sub>Cl}<sub>2</sub>(µ-adcpip)]. From the adcpip ligand the mononuclear derivatives [Re(CO)<sub>3</sub>Cl(adcpip)] and [Re(CO)<sub>3</sub>(PPh<sub>3</sub>)(µ-adcpip)]Cl were also obtained. Optimised geometries from density functional theory (DFT) calculations show <i>syn</i> and <i>anti</i> isomers for the dinuclear <i>fac</i>-Re(CO)<sub>3</sub> complexes at slightly different energies but they were not distinguishable from experimental IR or UV–Vis absorption spectroscopy. The electrochemistry of the adc complexes showed reduction potentials slightly below 0.0 V vs. the ferrocene/ferrocenium couple. Attempts to generate the radicals [{Re(CO)<sub>3</sub>Cl}<sub>2</sub>(µ-adcOR)]<sup>•−</sup> failed as they are inherently unstable, losing very probably first the Cl<sup>−</sup> coligand and then rapidly cleaving one [Re(CO)<sub>3</sub>] fragment. Consequently, we found signals in EPR very probably due to mononuclear radical complexes [Re(CO)<sub>3</sub>(solv)(adc)]<sup>•</sup>. The underlying Cl<sup>−</sup>→solvent exchange was modelled for the mononuclear [Re(CO)<sub>3</sub>Cl(adcpip)] using DFT calculations and showed a markedly enhanced Re-Cl labilisation for the reduced compared with the neutral complex. Both the easy reduction with potentials ranging roughly from −0.2 to −0.1 V for the adc ligands and the low-energy NIR absorptions in the 700 to 850 nm range place the adc ligands with their lowest-lying π* orbital being localised on the azo function, amongst comparable bridging chelate N^N coordinating ligands with low-lying π* orbitals of central azo, tetrazine or pyrazine functions. Comparative (TD)DFT-calculations on the Re(CO)<sub>3</sub>Cl complexes of the adcpip ligand using the quite established basis set and functionals M06-2X/def2TZVP/LANL2DZ/CPCM(THF) and the more advanced TPSSh/def2-TZVP(+def2-ECP for Re)/CPCMC(THF) for single-point calculations with BP86/def2-TZVP(+def2-ECP for Re)/CPCMC(THF) optimised geometries showed a markedly better agreement of the latter with the experimental XRD, IR and UV–Vis absorption data.
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