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)

Abstract

Alkylzinc complexes, (Ttz^R,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. (Ttz^R,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 (Ttz^R,Me)Zn(CB) [where R = Ph, CB (conjugate base) = OCH₂CF₃ (2), OPh (6), SPh (8), p-OC₆H₄(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 [(Ttz^Ph,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 (Ttz^Ph,Me)Zn(p-OC₆H₄(NO₂)) (10) were analyzed crystallographically including a Zn coordinated water molecule in 10_H2O, a coordination polymer in 10_CP, and a p-nitrophenol molecule hydrogen bonded to a triazole ring in 10_Nit. Ttz ligands are flexible since they are capable of providing κ³ or κ²metal binding and intermolecular interactions with either a metal center or H through the four position nitrogen (e.g. in 10_CP and HTtz^tBu,Me·H₂O, respectively). Preliminary kinetic studies on the protonolysis of LZnEt (L = Ttz^tBu,Me, Tp^tBu,Me) with p-nitrophenol in toluene at 95 °C show that these reactions are zero order in acid and first order in the LZnEt.