Better understanding of the species with the shortest Re26+ bonds and related Re27+ species with tetraguanidinate paddlewheel structures

A series of compounds has been made containing quadruply bonded Re-2(hpp)(4)X-2 species (hpp = the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2a]pyrimidine), where X is CF3SO3 (1), CF3CO2 (2), and F (3). The distances of 2.1562(7), 2.1711(5), and 2.1959(4) angstrom for 1-3 show significant effects of the sigma and pi electron donating ability of the axial ligands on the metal-metal distance. With the weakly coordinating triflate ligand the Re-Re distance is the shortest for any quadruple bonded species known. In addition to examining the effects of axial ligands on the Re-2(hpp)(4)(2+) core, our study of the Re-2(hpp)(4)(3+) core is being extended beyond the preliminary results previously reported in only one compound [Re-2(hpp)(4)Cl-2]PF6 (Dalton Trans. 2003, 1218). We now report the structural characterization by both X-ray and neutron diffraction of the compound [Re-2(hpp)(4)F](TFPB)(2), 4 (TFPB = the anion tetrakis[3,5-bis(trifluoromethyl)phenyl]borate), and a detailed study by EPR spectroscopy of [Re-2(hpp)(4)Cl-2]PF6 at 9.5, 34.5, and 95 GHz frequencies, using dilute fluid solutions, frozen glass, and neat powder, show that the unpaired electron in the [Re-2(hpp)(4)Cl-2](+) ion is in an MO of predominant metal character with little mixing from the guanidinate ligands.