Variable-Temperature Nuclear Magnetic Resonance Spectroscopy Allows Direct Observation of Carboxylate Shift in Zinc Carboxylate Complexes
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- 作者：Alojz Dem
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- 刊名：Journal of the American Chemical Society
- 出版年：2002
- 出版时间：April 17, 2002
- 年：2002
- 卷：124
- 期：15
- 页码：3951 - 3958
- 全文大小：148K
- 年卷期：v.124,no.15(April 17, 2002)
- ISSN：1520-5126
文摘
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 bytwo 
-1,2 and one -1,1 carboxylate ligands. Two pairs are connected by two tridentate bdmap ligandswith oxygen acting as a bridging donating atom. The complexes retain the tetranuclear structure in solutionand 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 -1,2 and the-1,1 resonances to a single resonance. An additional dynamic process (HT dynamics) is observed above247 K (1) and 263 K (2), leading to a coalescence of two dimethylamino resonances. Both dynamicprocesses are rationalized by a mechanism involving changes in the carboxylate coordination mode termedas carboxylate shift. The LT dynamics is ascribed to interconversions of a single -1,2 and a single -1,1carboxylate 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-bridgedzinc atoms. We propose a mechanism that starts with a cleavage of the Zn-N coordination bond. Theresulting coordinatively unsaturated zinc atom acquires an additional oxygen donor atom by carboxylateshift of -1,2 carboxylate to -1,1 mode. The activation parameters (
H
values in kilocalories per mole,S values in calories per mole per kelvin) were determined by line-shape analysis of VT NMR spectra:for 1 in THF-d8, HLT = 8.1(3), SLT = -12(2), HHT = 17.9(2), SHT = 14(1); for 1 in CDCl3, HHT =13.6(5), SHT = 3(3); for 1 in CD2Cl2, HHT = 9.9(3), SHT = -8(2); for 2 in THF-d8, HLT = 11(1),SLT = -5(3), HHT = 19.6(5), SHT = 18(3). Polymeric [Zn4(bdmap)2(OOCMe)6]n 1-catena crystallizesfrom a dichloromethane solution of 1. In 1-catena, the zinc atoms are linked into a chain through -1,2 and-1,1 acetate alternated by -1,2 acetate and bdmap.
-1,2 and one -1,1 carboxylate ligands. Two pairs are connected by two tridentate bdmap ligandswith oxygen acting as a bridging donating atom. The complexes retain the tetranuclear structure in solutionand 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 -1,2 and the-1,1 resonances to a single resonance. An additional dynamic process (HT dynamics) is observed above247 K (1) and 263 K (2), leading to a coalescence of two dimethylamino resonances. Both dynamicprocesses are rationalized by a mechanism involving changes in the carboxylate coordination mode termedas carboxylate shift. The LT dynamics is ascribed to interconversions of a single -1,2 and a single -1,1carboxylate 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-bridgedzinc atoms. We propose a mechanism that starts with a cleavage of the Zn-N coordination bond. Theresulting coordinatively unsaturated zinc atom acquires an additional oxygen donor atom by carboxylateshift of -1,2 carboxylate to -1,1 mode. The activation parameters (H values in kilocalories per mole,S values in calories per mole per kelvin) were determined by line-shape analysis of VT NMR spectra:for 1 in THF-d8, HLT = 8.1(3), SLT = -12(2), HHT = 17.9(2), SHT = 14(1); for 1 in CDCl3, HHT =13.6(5), SHT = 3(3); for 1 in CD2Cl2, HHT = 9.9(3), SHT = -8(2); for 2 in THF-d8, HLT = 11(1),SLT = -5(3), HHT = 19.6(5), SHT = 18(3). Polymeric [Zn4(bdmap)2(OOCMe)6]n 1-catena crystallizesfrom a dichloromethane solution of 1. In 1-catena, the zinc atoms are linked into a chain through -1,2 and-1,1 acetate alternated by -1,2 acetate and bdmap.