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

Abstract

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 (1)H and (13)C 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 60 degrees. 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 (DeltaH values in kilocalories per mole, DeltaS values in calories per mole per kelvin) were determined by line-shape analysis of VT NMR spectra: for 1 in THF-d(8), DeltaH(LT) = 8.1(3), DeltaS(LT) = -12(2), DeltaH(HT) = 17.9(2), DeltaS(HT) = 14(1); for 1 in CDCl(3), DeltaH(HT) = 13.6(5), DeltaS(HT) = 3(3); for 1 in CD(2)Cl(2), DeltaH(HT) = 9.9(3), DeltaS(HT) = -8(2); for 2 in THF-d(8), DeltaH(LT) = 11(1), DeltaS(LT) = -5(3), DeltaH(HT) = 19.6(5), DeltaS(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.

An Air‐ and Water‐Tolerant Zinc Hydride Cluster That Reacts Selectively With CO2

“…These variable‐temperature data agree with the crystallographic result, but point to fluxional coordination of the formate ligand at room temperature, probably as a result of a carboxylate shift through a μ‐1,3 bridging mode (see Scheme S4 in the Supporting Information). The calculated activation energy for this process is 10.0(8) kcal mol −1 for a coalescence temperature of 238 K and is similar to Δ G ≠ values for carboxylate shifts in Zn II acetates 16…”

Influence of the Counterion on the Synthesis of ZnO Mesocrystals under Solvothermal Conditions

“…[11] According to the existing literature about acetate complexes of Zn II we assume that at room temperature and in a base free solution this structure around ZnA C H T U N G T R E N N U N G (O 2 CCH 3 ) 2 ·2 H 2 O is retained. [18] Moreover, we suggest that the interactions identified above are retained also during the reaction, as for instance Tokumoto et al have indicated that the primary nuclei species for the formation of ZnO nanoparticles are Zn 4 (O)- 6 clusters and that this species undergoes further condensation to Zn 10 (O) 4 A C H T U N G T R E N N U N G (O 2 CCH 3 ) 12 .…”

Modelling Binuclear Metallobiosites: Insights from Pyrazole‐Supported Biomimetic and Bioinspired Complexes

“…The carboxylate shift has thus received significant attention in the literature, and has been experimentally observed in the gas phase86 and in solution87 in the case of zinc complexes. Iron model complexes have reproduced aspects of the carboxylate shift,6,77,88–90 but not until recently was it visualized directly in the solid state 91.…”