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-C 6H 4NN)C 4H 3N 2, RL pm (R=H, Cl) and 2-( p-R-C 6H 4NN)-1-(Me)C 3H 2N 2, 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–N h, Re–N a 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–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(ClL im) 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(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(C 3H 4N 2)(RL − ) and Re(CO) 3(PPh 3)(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(C 3H 4N 2)(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(PPh 3)(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.