title: Better understanding of the species with the shortest Re2(6+) bonds and related Re2)(7+) species with tetraguanidinate paddlewheel structures.
abstract: A series of compounds has been made containing quadruply bonded Re2(hpp)4X2 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) A 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 Re2(hpp)42+ core, our study of the Re2(hpp)43+ core is being extended beyond the preliminary results previously reported in only one compound [Re2(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.
authors.affiliation: Department of Chemistry and Laboratory of Molecular Structure and Bonding, P.O. Box 30012, Texas A&M University, College Station, Texas 77842-3012, USA. cotton@tamu.edu
authors.forename: F Albert
authors.initials: FA
authors.lastname: Cotton
authors.forename: Naresh S
authors.initials: NS
authors.lastname: Dalal
authors.forename: Penglin
authors.initials: P
authors.lastname: Huang
authors.forename: Sergey A
authors.initials: SA
authors.lastname: Ibragimov
authors.forename: Carlos A
authors.initials: CA
authors.lastname: Murillo
authors.forename: Paula M B
authors.initials: PM
authors.lastname: Piccoli
authors.forename: Chris M
authors.initials: CM
authors.lastname: Ramsey
authors.forename: Arthur J
authors.initials: AJ
authors.lastname: Schultz
authors.forename: Xiaoping
authors.initials: X
authors.lastname: Wang
authors.forename: Qinliang
authors.initials: Q
authors.lastname: Zhao