Elsevier

Polyhedron

Volume 23, Issues 2–3, 22 January 2004, Pages 405-412
Polyhedron

Tris(imidazolyl) cadmium and zinc complexes: structural and spectroscopic characterization of M–OH2 motifs

Dedicated to Jerry Trofimenko, colleague and friend, for his contributions to tripodal coordination chemistry
https://doi.org/10.1016/j.poly.2003.11.006Get rights and content

Abstract

The tris(imidazolyl) cavitand ligand, 1,3,5-triethyl-2,4,6-tris[N-methyl-imidazol-2-yl-thiomethyl]benzene, TriMIm, is used to prepare monomeric aqua complexes of cadmium and zinc with the general formula, [(TriMIm)M(OH2)]X2 (M=Cd, X=BF4; M=Zn, X=BF4 or NO3). The molecular structures of each species are characterized by trigonal pyramidal metal ion coordination and hydrogen-bonding between the bound water molecule and the non-coordinating counter ions. The binuclear zinc hydroxo-bridged complex, [{(TriMIm)Zn}2(μ-OH)](BF4)3, has been characterized by X-ray diffraction methods.

Monomeric aqua complexes of Cd and Zn coordinated by a tris(imidazolyl) ligand are reported. The molecular structures are characterized by trigonal pyramidal metal ion coordination and hydrogen-bonding between the bound water molecule and the noncoordinating counter ions.

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Introduction

The tris(pyrazolyl)hydroborato ligand, [TpR,R], developed by Jerry Trofimenko more than 35 years ago [1], [2] has found great utility as a scaffold for modeling a number of important metallobiochemistry motifs [2], [3], [4]. The tripodal [N3] donor environment of the ligand mimics the nitrogen-rich ligation sphere found at the active site of numerous non-heme proteins. Leading examples include Kitajima’s dicopper peroxo complex [5], [(Tp2iPr)Cu]2(O2) and Parkin’s and Vahenkamp’s [TpR,R]Zn(OH) species [6]. The former was prepared and authenticated as a structural and spectroscopic precedent of the oxygenated form of the O2-binding protein hemocyanin in advance of evidence of such μ-η22-O2 coordination at the protein active site [7]. The mononuclear zinc hydroxide species provided the opportunity to assess the inherent nucleophilicity, and more recently, basicity [8] of a [N3]Zn(OH) species. Inherent in each of these examples is the utility of systematic variation of the pyrazole substituents in controlling structure and reactivity at the attendant metal complexes, clearly one of the strengths of the [Tp] ligand family.

Recently, we have prepared a new tris(imidazolyl)-based cavitand ligand and reported aspects of its coordination chemistry with copper(I) [9]. The ligand, TriMIm,1 is preorganized to ligate a single metal ion in a trigonal planar array of imidazoles. The enforced geometry is distinct from the [Tp] ligands where more acute N–M–N angles are observed. Herein, we report the synthesis and structural characterization of cadmium- and zinc-aqua complexes, [(TriMIm)M(OH2)]2+.

Section snippets

Materials

2-Mercapto-1-methylimidazole, NaH, MgSO4, Cd(BF4)2 · 6H2O, Cd(NO3)2 · 4H2O, Zn(BF4)2 · xH2O, and Zn(NO3)2 · 6H2O were used as supplied by commercial vendors. [Cu(MeCN)4]BF4 [10], and TriMIm [9] were prepared according to literature procedures. Pentanes, tetrahydrofuran, and diethyl ether were freshly distilled from sodium/benzophenone. Methylene chloride was freshly distilled from calcium hydride. Acetone was freshly distilled over MgSO4. Acetonitrile (UV Grade) was purchased from Burdick and Jackson

Synthesis of Cd and Zn aqua complexes

Reaction of equimolar mixtures of TriMIm and a hydrated metal salt in the appropriate solvent yielded the corresponding monomeric metal aqua complex, [(TriMIm)M(OH2)]X2 (M=Cd, X=BF4 (1); M=Zn, X=BF4 (4) and M=Zn, X=NO3 (5)), in moderate to good yields as a white, microcrystalline solid. The materials possess good solubility in acetone, acetonitrile, and methanol but, are only sparingly soluble in hydrocarbons and chlorinated solvents. Spectroscopic (1H, 13C NMR, FT-IR) and combustion analytical

Acknowledgements

We thank the National Institutes of Health (GM-59191) for support of this work.

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