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

Abstract

The azopyrimidine and azoimidazole ligands (general abbreviations, RL) used in the present work are 2-(p-R-C₆H₄NN)C₄H₃N₂, RL_pm (R=H, Cl) and 2-(p-R-C₆H₄NN)-1-(Me)C₃H₂N₂, RL_im (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(HL_pm) and Re(CO)₃Cl(ClL_im) the Re-Nh, Re-Na distances are 2.173(6), 2.136(6) Å and 2.150(5), 2.166(5) Å, respectively (N^h and N^a 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(HL_pm), pair-wise C-HO hydrogen bonding between symmetry related molecules is present (CO; 3.264(9) Å, HO; 2.460(10) Å; C-HO; 130.6(5)°). The lattice of Re(CO)₃Cl(ClL_im) also consists of centrosymmetric dimers held by aromatic π-π stacking between parallely placed pendant aryl rings (centroidcentroid distance, 3.781(9) A). Extended Huckel 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(RL_pm) and Re(CO)₃Cl(RL_im), respectively. The redox orbital is believed to be to the above noted LUMO. Electrogenerated Re(CO)₃Cl(RL^∗-) underwent spontaneous solvolytic chloride displacement in MeCN furnishing Re(CO)₃(MeCN)(RL^∗-) which has been isolated. The latter in turn reacted with imidazole and triphenyl phosphine furnishing Re(CO)₃(C₃H₄N₂)(RL^-) and Re(CO)₃(PPh₃)(RL^-), 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 μ_B). 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)(RL^∗-) and Re(CO)₃(C₃H₄N₂)(RL^∗-) 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)(RL^∗-) 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. Reaction of Re(CO)₅Cl and 2-(arylazo)pyrimidine/2-(arylazo)-1-methylimidazole, RL, has afforded Re(CO)₃Cl(RL) (such as A) which has been structurally characterized. Electroreduction in presence of D has furnished the EPR-active azo anion radical system (S=1/2) Re(CO)₃(D)(RL^∗-) (D=MeCN, C₃H₄N₂ and PPh₃) as in B.