Variable-temperature nuclear magnetic resonance spectroscopy allows direct observation of carboxylate shift in zinc carboxylate complexes

Tetranuclear complexes [Zn-4(bdmap)(2)(OOCR)(6)] 1 (R = Me) and 2 (R = Et), where Hbdmap = 1,3-bis(dimethylamino)-2-propanol, were prepared from zinc carboxylates and Hbdmap in tetrahydrofuran (THF). The solid-state structures of isomers 1a and 2a consist of two pairs of zinc atoms, each bridged by two mu-1,2 and one mu-1,1 carboxylate ligands. Two pairs are connected by two tridentate bdmap ligands with oxygen acting as a bridging donating atom. The complexes retain the tetranuclear structure in solution and two dynamic processes are observed from variable-temperature H-1 and C-13 NMR spectra. A low-temperature process (LT dynamics) observed already below 200 K is a coalescence of the mu-1,2 and the mu-1,1 resonances to a single resonance, An additional dynamic process (HT dynamics) is observed above 247 K (1) and 263 K (2), leading to a coalescence of two dimethylamino resonances. Both dynamic processes are rationalized by a mechanism involving changes in the carboxylate coordination mode termed as carboxylate shift. The LT dynamics is ascribed to interconversions of a single mu-1,2 and a single mu-1,1 carboxylate ligation by rotations of 60degrees. The interconversions involve all carboxylate ligands in 1 and 2. The HT dynamics is ascribed to the exchange of the coordinating geometries of two carboxylate-bridged zinc atoms. We propose a mechanism that starts with a cleavage of the Zn-N coordination bond. The resulting coordinatively unsaturated zinc atom acquires an additional oxygen donor atom by carboxylate shift of mu-1,2 carboxylate to mu-1,1 mode. The activation parameters (DeltaHdouble dagger values in kilocalories per mole, DeltaSdouble dagger values in calories per mole per kelvin) were determined by line-shape analysis of VT NMR spectra: for 1 in THF-d(8), DeltaHdouble dagger(LT) = 8.1(3), DeltaSdouble dagger(LT) = -12(2), DeltaHdouble dagger(HT) = 17.9(2), DeltaSdouble dagger(HT) = 14(1); for 1 in CDCl3, DeltaHdouble dagger(HT) = 13.6(5), DeltaSdouble dagger(HT) = 3(3); for 1 in CD2Cl2, DeltaHdouble dagger(HT) = 9.9(3), DeltaSdouble dagger(HT) = -8(2); for 2 in THF-d(8), DeltaHdouble dagger(LT) = 11 (1), DeltaSdouble dagger(LT) = -5(3), DeltaHdouble dagger(HT) = 19,6(5), DeltaSdouble dagger(HT) = 18(3). Polymeric [Zn-4(bdmap)(2)(OOCMe)(6)](n) 1-catena crystallizes from a dichloromethane solution of 1. In 1 -catena, the zinc atoms are linked into a chain through mu-1,2 and mu-1,1 acetate alternated by mu-1,2 acetate and bdmap.