Synthesis, structural studies and solubility properties of zinc(II), nickel(II) and copper(II) complexes of bulky tris(triazolyl)borate ligands Jessica L. Bongiovanni a , Brittany W. Rowe b , Patrick T. Fadden b , Michael T. Taylor b , Kirsten R. Wells b , Mukesh Kumar a , Elizabeth T. Papish a, * , Glenn P.A. Yap c , Matthias Zeller d a Department of Chemistry, Drexel University, 3141 Chestnut St., Philadelphia, PA 19104, United States b Salisbury University, 1101 Camden Ave., Salisbury, MD 21801, United States c University of Delaware, Newark, DE 19716, United States d Youngstown State University, 1 University Plaza, Youngstown, OH 44555, United States article info Article history: Received 24 October 2009 Received in revised form 25 February 2010 Accepted 3 March 2010 Available online 7 March 2010 Keywords: Water-soluble ligands Triazole Tris(triazolyl)borate Bioinorganic models Transition metals abstract Tris(triazolyl)borate (Ttz) ligands are sterically similar to tris(pyrazolyl)borate (Tp) but complexes of Ttz show improved solubility in water and alcohols due to their propensity for forming hydrogen bonds. Recently developed bulky tris(triazolyl)borate ligands can produce four and five coordinate transition metal complexes and serve as models for enzyme active sites in an aqueous environment. Herein we report the synthesis of such complexes, i.e. (Ttz tBu,Me )ZnCl, (Ttz tBu,Me )ZnBr, (Ttz tBu,Me )NiCl, and (Ttz tBu,Me )CuCl, which were analyzed by X-ray crystallographic and spectroscopic methods [Ttz tBu,Me = tris(3-t-butyl-5-methyl-1,2,4-triazolyl)borate]. (Ttz tBu,Me )ZnCl crystallizes as two different polymorphs with cubic and monoclinic symmetry. Both polymorphs of (Ttz tBu,Me )ZnCl and (Ttz tBu,Me )ZnBr have tetra- hedral zinc atoms whereas the geometries at the metal in (Ttz tBu,Me )NiCl and (Ttz tBu,Me )CuCl are distorted tetrahedral. All complexes are methanol soluble and they also dissolve in methanol/water mixtures with up to 60% water. Ó 2010 Published by Elsevier B.V. 1. Introduction Tris(pyrazolyl)borate (Tp) ligands are an extremely versatile class of facially coordinating N,N,N donors due to their ease of ste- ric and electronic modification [1–6]. Tp ligands form complexes with nearly every metal on the periodic table, and these complexes have been used for diverse coordination chemistry goals including enzyme modeling and catalysis [3]. However, their poor water- solubility limits their usage for modeling hydrophilic enzyme active sites and impedes green chemistry applications in water and alcoholic solvents. Although charged groups, such as sulfonate, can be used to enhance water-solubility, anionic groups are often good ligands and can compete with the pyrazole donors and preferentially bind to certain metals, thereby changing the coordination geometry [7–12]. We therefore sought to use tris(triazolyl)borate (Ttz) ligands since the third nitrogen in each triazole ring would be available to accept hydrogen bonds and enhance the water-solubility while still allowing for the formation of N,N,N metal complexes [13,14]. Like the Tp ligands, the Ttz ligands (Chart 1) are sterically and electronically tunable by modification of the R and R 0 positions. Prior to our work, only hydrogen atoms and methyl groups had been used in these positions since bulkier triazoles are not commercially available and can be difficult to make. Late transition metal complexes of the Ttz H,H and Ttz Me,Me ligands generally lead to octahedrally coordinated complexes of the form ðTtz R;R 0 Þ 2 M [13,15–18], and are thus not well suited for modeling four and five coordinate enzyme active sites. In the Tp literature, methyl groups in the fifth position (R 0 ) help reduce the likelihood of hydride trans- fer from the B-H group [19]. Thus, in a prior communication [20], we reported the first synthesis of 3-t-butyl-5-methyl-1,2,4-triazole and of the bulky Ttz ligand K(Ttz tBu,Me ). We have since improved the synthesis of 3-t-butyl-5-methyl-1,2,4-triazole and we report here a simpler procedure that includes fewer steps and is higher yielding. This allows us to further explore the steric and solubility properties of complexes with bulky Ttz ligands as compared to the more traditional Tp complexes. Initial results show that the potassium salt of the ligand (KTtz tBu,Me ) is very water-soluble (99 mg/mL) [20] and Ttz transi- tion metal complexes made thus far also exhibit significant solubil- ity in water and/or alcohols [21]. Tp salts and complexes are generally not water-soluble, and the improved solubility of Ttz R;R 0 and its complexes can thus be attributed to the third nitrogen in each triazole ring that confers water-solubility without altering the coordination geometry [13]. The increased water-solubility of 0020-1693/$ - see front matter Ó 2010 Published by Elsevier B.V. doi:10.1016/j.ica.2010.03.010 * Corresponding author. Tel.: +1 215 895 2666. E-mail address: elizabeth.papish@drexel.edu (E.T. Papish). Inorganica Chimica Acta 363 (2010) 2163–2170 Contents lists available at ScienceDirect Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica