Structural variability and dynamics in carboxylato- and carbamatomagnesium bromides. relationship to the carboxylate shift

Two new carbamatomagnesium bromide complexes 1 (=[Mg(02CN(Me)Ph)(THF)2Br]2) and 2 (=[Mg 3 (O 2 CN-Ph2)4(THF)5Br][(THF)MgBr 3 ]) were prepared and crystallographically characterized. Complex 1 consists of a dinudear core withtwo syn-syn-μ 1 , 3 bridging carbamato ligands. In solution, the 1 H and 1 3 C NMR spectra revealed a two-site fluxional behavior for the carbamato ligands between 0 °C and 25 °C that is consistent with a bridge-mode isomerization relating the syn-syn-μ 1 , 3 form and an alternative syn-anti-μ 1 , 1 form. At temperatures below 0 °C, a more complex NMR signal is observed that is ascribed to further resolution of C-N rotational isomers, which has the effect of increasing the number of molecular isomers that can be resolved on the NMR time scale. New temperature-dependent NMR spectra of the previously known diethylcarbamato- and benzoato-bridged complexes [Mg(02CNEt2)(THF)2Br] 2 , 3, and [Mg(02CPh)(THF)2Br]2, 5, revealed fluxional behavior that was also interpreted in terms of bridge-mode isomerization. Kinetic activation parameters for 1, 3, and 5 are consistent with an intramolecular motion relating the syn-syn-μ 1 , 3 and syn-anti-μ 1 , 1 isomers. These results provide new insight into bridge-mode isomerism that is closely related to the carboxylate shift concept. Complex 2 is compositionally related to 1 but exists as a salt consisting of a trinuclear cationic unit [Mg3(02CNPh2)4(THF)5Br]+ and a solvated tribromomagnesiate anion [(THF)MgBr 3 ] - . This shows that changing the hydrocarbon substitutents on the carbamato ligand changes the structure and dynamics of multinuclear carbamatomagnesium complexes in ways that limit our ability to predict structural topologies in this system.