Nickel(II) complex of a biradical: Structure, magnetic properties, high NMR temperature sensitivity and moderately fast molecular dynamics

Abstract

A novel nickel(II) complex of a tridentate iminonitroxyl biradical, [Ni(bisimpy)MeOH(ReO4)2], (1) (bisimpy=2,6-bis(4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-2-yl-1-oxyl)pyridine) was structurally and magnetically characterized. Due to an effective separation of the molecules in solid the magnetic behavior of 1 differs considerably from that of a related complex studied earlier, the spin carriers in the former having been strongly ferromagnetically coupled even at room temperature. Dynamic behavior of the paramagnetic cation [Ni(bisimpy)(CD3OD)3] in CD3OD solution was studied by 1H NMR. A temperature dependence of the spectrum of 1 has been analyzed using a dynamic NMR approach. It was shown that an interconversion of two conformers is liable for a fast dynamic process occurring in solution of 1 at elevated temperatures. An estimated value of ΔG‡(298K) for this process was found to be of 59±3kJ/mol. It was confirmed that a considerable extension of the measurable rate constants range compared to that for diamagnetic compounds is a result of the paramagnetic chemical shifts in the Ni complex. Application of paramagnetic d-metal complexes as probes for studying of the free-energy changes in chemical exchange processes, as well as the advantages of this method compared to diamagnetic species are considered. The investigated complex represents an example of the new type of thermometric NMR sensors being to our knowledge is the most temperature sensitive reagent among the d-metal paramagnetic coordination compounds.