Better understanding of the species with the shortest Re₂ ⁶⁺ bonds and related Re₂⁷⁺ species with tetraguanidinate paddlewheel structures

A series of compounds has been made containing quadruply bonded Re ₂(hpp)₄X₂ species (hpp = the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2a]pyrimidine), where X is CF ₃SO₃ (1), CF₃CO₂ (2), and F (3). The distances of 2.1562(7), 2.1711(5), and 2.1959(4) Å for 1-3 show significant effects of the σ and π 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)₄²⁺ core, our study of the Re ₂(hpp)₄³⁺ core is being extended beyond the preliminary results previously reported in only one compound [Re ₂(hpp)₄Cl₂]PF₆ (Dalton Trans. 2003, 1218). We now report the structural characterization by both X-ray and neutron diffraction of the compound [Re₂(hpp)₄F](TFPB) ₂, 4 (TFPB = the anion tetrakis[3,5-bis(trifluoromethyl)phenyl] borate), and a detailed study by EPR spectroscopy of [Re₂(hpp) ₄Cl₂]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 [Re₂(hpp)₄Cl₂] ⁺ ion is in an MO of predominant metal character with little mixing from the guanidinate ligands.