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Zinc complexes of TtzR,Me with O and S donors reveal differences between Tp and Ttz ligands: Acid stability and binding to H or an additional metal (TtzR,Me = tris(3-R-5-methyl-1,2,4-triazolyl)borate; R = Ph, tBu)

Alkylzinc complexes, (TtzR,Me)ZnR′ (R = tBu, Ph; R′ = Me, Et), show interesting reactivity with acids, bases and water. With acids (e.g. fluorinated alcohols, phenols, thiophenol, acetylacetone, acetic acid, HCl and triflic acid) zinc complexes of the conjugate base (CB), (Ttz R,Me)ZnCB, are generated. Thus the B-N bonds in Ttz ligands are acid stable. (TtzR,Me)ZnCB complexes were characterized by 1H, 13C-NMR, IR, MS, elemental analysis, and, in most cases, single crystal X-ray diffraction. The four coordinate crystal structures included (TtzR,Me)Zn(CB) [where R = Ph, CB (conjugate base) = OCH 2CF3 (2), OPh (6), SPh (8), p-OC6H 4(NO2) (10); R = tBu, CB = OCH(CF3)2 (3), OPh (5), SPh (7)*, p-OC6H4(NO2) (9) (* indicates a rearranged Ttz ligand)]. The use of bidentate ligands resulted in structures [(TtzPh,Me)Zn(CB) (CB = acac (12), OAc (14))] in which the coordination geometries are five, and intermediate between four and five, respectively. Interestingly, three forms of (TtzPh,Me)Zn(p- OC6H4(NO2)) (10) were analyzed crystallographically including a Zn coordinated water molecule in 10 H2O, a coordination polymer in 10CP, and a p-nitrophenol molecule hydrogen bonded to a triazole ring in 10Nit. Ttz ligands are flexible since they are capable of providing κ3 or κ2 metal binding and intermolecular interactions with either a metal center or H through the four position nitrogen (e.g. in 10 CP and HTtztBu,Me·H2O, respectively). Preliminary kinetic studies on the protonolysis of LZnEt (L = Ttz tBu,Me, TptBu,Me) with p-nitrophenol in toluene at 95 °C show that these reactions are zero order in acid and first order in the LZnEt. The Royal Society of Chemistry 2011.

Factors influencing the thermodynamics of zinc alkoxide formation by alcoholysis of the terminal hydroxide complex, [TpBut,Me]ZnOH: An experimental and theoretical study relevant to the mechanism of action of liver alcohol dehydrogenase

The factors that influence the formation of a tetrahedral alkoxide complex related to a critical intermediate of the catalytic cycle of liver alcohol dehydrogenase have been probed by a combined experimental and computational investigation of the reactions of the tris(pyrazolyl)hydroborato zinc hydroxide complexes [TpRR′]ZnOH with alcohols. The study demonstrates that zinc alkoxide formation is electronically favored by incorporation of electron-withdrawing substituents in the alcohol but is sterically disfavored for bulky alkoxides. A computational analysis indicates that these trends are a result of homolytic Zn-OR and Zn-OAr BDEs being more sensitive to the nature of R and Ar than are the corresponding H-OR and H-OAr BDEs. Thus, electron-withdrawing substituents increase Zn-OAr bond energies to a greater extent than H-OAr bond energies, while bulky substituents decrease Zn-OR bond energies to a greater extent than H-OR bond energies. With the exception of derivatives of acidic alcohols (e.g., nitrophenol), the zinc alkoxide complexes [TpRR′]ZnOR are very unstable toward hydrolysis. This hydrolytic instability of simple zinc alkoxide complexes suggests that the active site environment of LADH plays an important role in stabilizing the alkoxide intermediate, possibly via hydrogen-bonding interactions.