Superbasic Amidine Monodentate Ligands in <i>fac</i>-[Re(CO)₃(5,5′-Me₂bipy)(Amidine)]BF₄ Complexes: Dependence of Amidine Configuration on the Remote Nitrogen Substituents

Addition of various RNH2 to fac-[Re(CO)3(5,5′-Me2bipy)(CH3CN)]BF4 (1) converts the acetonitrile ligand to the amidine ligand (a superbase) in fac-[Re(CO)3(5,5′-Me2bipy)(HNC(CH3)NHR)]BF4 products. Each complex has four conceivable isomers (E, E′, Z, and Z′) because the amidine CN bonds have double-bond character, and the two remote NHR group substituents are different. The reaction of 1 in acetonitrile is complete in 6 to 96 h (25 °C) and forms fac-[Re(CO)3(5,5′-Me2bipy)(HNC(CH3)NHR)]BF4 E′ and Z isomers. Only the E′ isomer formed crystals (R = methyl, isopropyl, isobutyl, tert-butyl, and benzyl). Upon dissolution of such crystals in acetonitrile-d3, NMR spectra with highly dominant E′ signals gradually changed (∼15 min at room temperature) to spectra with signals for an equilibrium mixture of E′ and Z isomers. Such slow E′-to-Z isomer interchange is also indicated by 2D ROESY NMR data used primarily to assign solution structure. Equilibrium ratios (E′:Z) of ∼65:35 for R = methyl, isopropyl, and isobutyl and 83:17 for R = tert-butyl demonstrate that increasing the remote NHR group steric bulk above a threshold size favors the E′ isomer. Consistent with this trend, fac-[Re(CO)3(5,5′-Me2bipy)(HNC(CH3)NH2)]BF4, with a remote NH2 (low bulk) group, favors the Z isomer. In contrast, although the remote NH(benzyl) group in fac-[Re(CO)3(5,5′-Me2bipy)(HNC(CH3)NH(CH2C6H5)]BF4 has only moderate bulk, the E′ isomer has high abundance as a result of favorable 5,5′-Me2bipy/benzyl stacking, evidence for which is present in both solid and solution states. The fac-[Re(CO)3(5,5′-Me2bipy)(HNC(CH3)NHR)]BF4 E isomer can be detected in solvents of low polarity. However, the Z′ isomer was not observed, undoubtedly because unfavorable remote-group clashes with the equatorial ligands destabilize this isomer. Challenge studies with a 5-fold excess of 4-dimethylaminopyridine in acetonitrile-d3 establish that fac-[Re(CO)3(5,5′-Me2bipy)(HNC(CH3)NHCH(CH3)2)]BF4 is robust because the isopropylamidine ligand was not displaced, consistent with the superbase character of amidine ligands.