Hydro tris borate, zinc complexes

Some of the first catalytic model systems for the simulation of the function of methane monooxygenase comprise monomeric as well as dimeric iron-containing model complexes bearing hydro-tris(pyrazolyl)borate ligands [6]. These complexes, e.g. 3, catalyze the oxidation of aromatic and aliphatic carbon-hydrogen bonds in the presence of oxygen (1 atm), acetic acid and zinc powder at room temperature (Scheme 2). 

A mononuclear zinc hydroxide complex supported by a hydrophobic hydro-tris(pyrazolyl)borate ligand mediates the stoichiometric hydrolysis of an activated amide.94 As shown in Scheme 17, treatment of [(TpCum,Me)Zn-OH] with... 

Model studies for metallo- 3-lactamases have been performed using mononuclear zinc hydroxide complexes.99,129,130 The breadth of (3-lactam hydrolysis reactivity of hydro-tris(pyrazolyl)borate-ligated mononuclear zinc hydroxide complexes has been explored.129 Treatment of the mononuclear zinc hydroxide complex [(Tpph Mc)Zn OH] with simple 3-lactams ( 3-propiolactam, 4-phenyl-(3-propiolactam, Scheme 21) does not result in ring opening, but instead results in the formation of 3-lactamide complexes and water. Treatment of [(Tpph,Me)Zn-OH] with /V-alkyl or -aryl 3-lactam derivatives instead results in no reaction (Scheme 21). Use of natural derivatives of penicillin and cephalosporin (Scheme 22) did not yield 3-lactam hydrolysis, but instead coordination of the carboxylate moiety of the antibiotic derivatives to the mononuclear Zn(II) center and release of water. 

The previously unreported complex, Tp Zn(K -BH4) (Tp =hydro-tris(3,5-dimethylpyrazolyl)-borate anion), has been characterized by Des-rochers et al. by the use of IR and NMR spectroscopies including measurements of Vhb couplings which allowed the authors to compare it with analogous closed shell borohydride complexes. All measurements have confirmed the persistent but fluxional zinc-borohydride coordination in solution on the NMR timescale. The 1 1 1 1 quartet assigned to coordinated borohydride in the proton spectrum (Vhb = 83Hz) has confirmed the equivalence of all hydrogen atoms because of rapid interconversion of the protons coordinated and not coordinated to the metal. 

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