Variable-Temperature Nuclear Magnetic Resonance Spectroscopy Allows Direct Observation of Carboxylate Shift in Zinc Carboxylate Complexes

Tetranuclear complexes [Zn4(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 μ-1,2 and one μ-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 1H and 13C NMR spectra. A low-temperature process (LT dynamics) observed already below 200 K is a coalescence of the μ-1,2 and the
μ-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 μ-1,2 and a single μ-1,1
carboxylate ligation by rotations of 60°. 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 μ-1,2 carboxylate to μ-1,1 mode. The activation parameters (ΔH⧧ values in kilocalories per mole,
ΔS⧧ values in calories per mole per kelvin) were determined by line-shape analysis of VT NMR spectra: 
for 1 in THF-d8, ΔH⧧LT = 8.1(3), ΔS⧧LT = −12(2), ΔH⧧HT = 17.9(2), ΔS⧧HT = 14(1); for 1 in CDCl3, ΔH⧧HT =
13.6(5), ΔS⧧HT = 3(3); for 1 in CD2Cl2, ΔH⧧HT = 9.9(3), ΔS⧧HT = −8(2); for 2 in THF-d8, ΔH⧧LT = 11(1),
ΔS⧧LT = −5(3), ΔH⧧HT = 19.6(5), ΔS⧧HT = 18(3). Polymeric [Zn4(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 μ-1,2 and
μ-1,1 acetate alternated by μ-1,2 acetate and bdmap.