Chemistry of tricarbonylrhenium(I) chelates of azopyrimidines, azoimidazoles and anion radicals thereof

The azopyrimidine and azoimidazole ligands (general abbreviations, RL) used in the present work are 2-(p-R-C6H4NN)C4H3N2, RLpm (R=H, Cl) and 2-(p-R-C6H4NN)-1-(Me)C3H2N2, RLim (R=Me, Cl), respectively. The reaction of Re(CO)5Cl with a slight excess of RL in boiling benzene has furnished blue–violet complexes of type Re(CO)3Cl(RL) which have been spectrally characterized. In Re(CO)3Cl(HLpm) and Re(CO)3Cl(ClLim) the Re–Nh, Re–Na distances are 2.173(6), 2.136(6) Å and 2.150(5), 2.166(5) Å, respectively (Nh and Na are heterocyclic and azo N atoms, respectively). Their N–N lengths (1.271(8), 1.281(7) Å) implicate relatively weak Re-azo(π*) back-bonding. In the lattice of Re(CO)3Cl(HLpm), pair-wise C–H⋯O hydrogen bonding between symmetry related molecules is present (C⋯O; 3.264(9) Å, H⋯O; 2.460(10) Å; C–H⋯O; 130.6(5)°). The lattice of Re(CO)3Cl(ClLim) also consists of centrosymmetric dimers held by aromatic π–π stacking between parallely placed pendant aryl rings (centroid⋯centroid distance, 3.781(9) Å). Extended Hückel calculations reveal that the LUMO of Re(CO)3Cl(RL) is ∼60% azo in character. One-electron quasireversible electrochemical reduction occurs near −0.1 and −0.4 V vs. SCE in the cases of Re(CO)3Cl(RLpm) and Re(CO)3Cl(RLim), respectively. The redox orbital is believed to be to the above noted LUMO. Electrogenerated Re(CO)3Cl(RLradical dot−) underwent spontaneous solvolytic chloride displacement in MeCN furnishing Re(CO)3(MeCN)(RLradical dot−) which has been isolated. The latter in turn reacted with imidazole and triphenyl phosphine furnishing Re(CO)3(C3H4N2)(RLradical dot−) and Re(CO)3(PPh3)(RLradical dot−), respectively. The pattern of carbonyl stretching frequencies of these radical anion complexes is similar to that of Re(CO)3Cl(RL) but for shifts to lower frequencies by 10–40 cm−1. All the three radical anion systems are one-electron paramagnets (1.7–1.8 μB). The unpaired electron is primarily localized in a predominantly azo-π* orbital. A small metal contribution (185, 187Re, I=5/2) is present and both Re(CO)3(MeCN)(RLradical dot−) and Re(CO)3(C3H4N2)(RLradical dot−) display six-line EPR spectra (A∼28 G). The line shapes and intensities are characteristic of the presence of g-strain. In the case of Re(CO)3(PPh3)(RLradical dot−) seven nearly equispaced lines are observed due to virtually equal coupling with metal and 31P (I=1/2) nuclei. The g values of the radical species span the range 2.0033–2.0066.