Direct Evidence from Electron Paramagnetic Resonance for Additional Configurations in Uncommon Paddlewheel Re27+ Units Surrounded by an Unsymmetrical Bicyclic Guanidinate

Three rare compounds have been synthesized and structurally characterized; these species have paddlewheel structures and Re27+ cores surrounded by four bicyclic guanidinates and two axial ligands along the Re–Re axis. Each possesses a formal bond order of 3.5 and a σ2π4δ1 electronic configuration that entails the presence of one unpaired electron for each compound. The guanidinate ligands characterized by having CH2 entities and a central C­(N)3 unit that joins two cyclic unitsone having two fused 6-membered rings (hpp) and the other having a 5- and a 6-membered ring fused together (tbn)allowed the isolation of [Re2(tbn)4Cl2]­PF6, 1, [Re2(tbn)4Cl2]­Cl, 2, and [Re2(hpp)4(O3SCF3)2]­(O3SCF3), 3. Because of the larger bite angle of the tbn relative to the hpp ligand, the Re–Re bond distances in 1 and 2 (2.2691(14) and 2.2589(14) Å, respectively) are much longer than that in 3 (2.1804(8) Å). Importantly, electron paramagnetic resonance (EPR) studies at both X-band (∼9.4 GHz) and W-band (112 GHz) in the solid and in frozen solution show unusually low g-values (∼1.75) and the absence of zero-field splitting, providing direct evidence for the presence of one metal-based unpaired electron for both 1 and 3. These spectroscopic data suggest that the unsymmetrical 5-/6-membered ligand leads to the formation of isomers, as shown by significantly broader EPR signals for 1 than for 3, even though both compounds possess what appears to be similar ideal crystallographic axial symmetry on the X-ray time scale.