Mole ratio dependent formation of mononuclear versus pentanuclear zinc(II) pivalate complexes and the ‘carboxylate shift’ process

The reaction of Zn(O2CtBu)2 with 4-(dimethylamino)pyridine (DMAP) in 1:2 and 1:1 mol ratios afforded [Zn(O2CtBu)2(DMAP)2]·H2O (1·H2O) and [Zn5(μ3-OH)2(O2CtBu)2(μ2-κ1O:κ1O′-O2CtBu)6(DMAP)2] (2) in 95% and 98% yields, respectively. Complex 1·H2O upon crystallization from CHCl3/n-hexane mixture at ambient temperature afforded 1·CHCl3 in 77% yield. Complex 2 upon re-crystallization in a CH2Cl2/n-hexane mixture or CDCl3 at ambient condition afforded a unique pentanuclear complex, [Zn5(μ3-OH)2(O2CtBu)(κ2O,O′-O2CtBu)(μ2-κ1O:κ1O′-O2CtBu)6(DMAP)2] (3) in 98% yield. Complex 3 upon re-crystallization in methanol transformed back to 2. The new complexes were characterized by elemental analysis, TGA/DTA/DSC analysis, FT-IR, NMR (solution 1H and 13C, and solid state CP-MAS) spectroscopy, and selected complexes were characterized by single crystal X-ray diffraction. The molecular structures of 2 and 3 represent a crystallographic snapshot of a monodentate ↔ chelating ‘carboxylate shift’ process. Variable temperature 1H NMR measurements carried out on 2 in methanol-d indicated the presence of two species, possibly 2 and 3, in about a 2:1 ratio, respectively at 198 K suggesting the aforementioned ‘carboxylate shift’ process operating in solution. Various factors that are responsible for the ‘carboxylate shift’ process are discussed.

Graphical abstractMono and pentanuclear 4-(dimethylamino)pyridine coordinated zinc(II) pivalate complexes prepared. Nuclearity and carboxylate coordination modes depend on the ratio of reactants. CP-MAS 13C NMR is handy to distinguish the crystallographically different OC(O) carbon atoms. The ‘carboxylate shift’ in solution is investigated by variable temperature 1H NMR spectroscopy.Figure optionsDownload full-size imageDownload as PowerPoint slide