Copper(2+) Complexes of Hydroxyoxidoborates. Synthesis and Characterization of Two Clusters Containing the Hexaborate(2−) Ligand: [Cu(NH2CH2CH2NEt2){B6O7(OH)6}]·5H2O and [Cu(NH3)2{B6O7(OH)6}]·2H2O

[Cu(NH2CH2CH2NEt2){B6O7(OH)6}]·5H2O (1) and [Cu(NH3)2{B6O7(OH)6}]·2H2O (2) have been obtained as crystalline materials from aqueous solutions of Dynamic Combinatorial Libraries (DCLs) originating from B(OH)3 and appropriate Cu(II) amine complexes. These two Cu/O/B clusters are formed through self-assembly processes and contain tridentate hexaborate(2−)-κ3O ligands. Both compounds have been characterized by TGA/DSC, magnetic susceptibility measurements, IR spectroscopy, and single-crystal XRD studies. The intermolecular H-bond interactions between neighbouring hexaborate units are implicated in their templated synthesis. Compound 2 is a coordination polymer and stabilization is also gained through formation of an additional O–Cu coordinate bond. Steric congestion in 1 blocks formation of this bond resulting in insular complexes.


Introduction
Boron is always found in nature combined with oxygen and these compounds are generally known as 'borates' [1][2][3][4][5][6][7]. There are more than two hundred known borate minerals, more than a hundred of which have been structurally characterized, and additionally there are several hundred known synthetic borate salts [4,6]. Borate anions are structural diverse with insular anions through to anionic polymeric chains, layers, clusters, or networks, all well-represented and new structural types are always of interest. Two general synthetic approaches have been used to prepare new polyborate materials and these are either selfassembly (with templating cations) from aqueous solution or high temperature solvothermal methods [7]. Polyborate salts obtained from aqueous solution are usually comprised of discrete, insular anions, partnered by the templating cations, whereas solvothermal methods often lead to more condensed polymeric structures. These templated selfassembly processes are possible since it is well known that B(OH) 3 exists in aqueous solution as a Dynamic Combinatorial Library (DCL) [8,9] of rapidly interconverting B(OH) 3 , [B(OH) 4 ]and polyborate anions [10,11]. The templating cation interacts with this DCL of borate anions and crystallization of energetically favourable salts occur. Stoichiometry, pH, and energetics associated with crystal packing, steric congestion, hydrogen bonding, and coordination bond formation all have important roles to play in these self-assembly processes [12,13]. However, pentaborate(1-) salts are often formed in these systems since this anion is well suited to forming a wide variety of supramolecular crystalline lattices which are held together by strong H-bond interactions [14][15][16][17]. In our search for novel polyborate anions we have adopted a strategy of using transition-metal complexes as templating cations [18][19][20][21][22][23]. Transition-metal complexes have been chosen since they offer unique opportunities to affect the selfassembly processes in polyborate salt formation and in turn, this will lead to a better understanding of crystalengineering in general. Following this strategy we have recently reported the synthesis of several salts containing isolated polyborate anions partnered with transition-metal complexes and have described the synthesis and structures of two novel isolated polyborate anions containing the heptaborate(3-) [19] and octaborate(2-) [18] anions. Copper(II) borates [20,22,24,25] are an interesting subclass of metal polyborates and ammonium copper(II) borates have been used as fungicides [4,6] and have other potential commercial applications. Copper(II) complexes are generally labile [26] and in aqueous solution will generate a DCL of cations to interact with the DCL equilibrium mixture of polyborate anions adding a further dimension to self-assembly synthons present in the reactions solutions. We have also recently reported some novel species containing Cu(II) centers [20,22] and in this manuscript we describe the self-assembly of two new Cu(II) compounds containing hexaborate(2-) anions and their solid-state structures. The hexaborate(2-) borate anion observed in these Cu(II) compounds is drawn schematically in Fig. 1. (OH) 6 }]Á2H 2 O (2) have been prepared through self-assembly processes from aqueous solutions originally containing appropriate templating Cu(II) amine complexes and B(OH) 3 and Ba(OH) 2 (Scheme 1). The Ba(OH) 2 is added to the reaction mixture to convert the sulphate salt to the hydroxide salt by removal (precipitation) of BaSO 4. Products 1 and 2 crystallize from the solution, after several days at room temperature, in moderate yields. Schematic drawings of the coordination compounds in 1 and 2 are shown in Fig. 2. Compounds 1 and 2 were characterized by thermal studies (TGA/DSC), magnetic susceptibility measurements, elemental analyses, IR spectroscopy and singlecrystal XRD studies. Elemental analyses data were consistent with their single-crystal structures confirming that crystals chosen for XRD studies were representative of the bulk sample.

Synthesis and General Characterization
The thermal TGA/DSC data obtained for 1 and 2 (see supplementary material) were also consistent with the structures determined by single-crystal X-ray diffractions studies (see below) and were interpreted in terms of threestep decomposition processes. For 1 this involved loss of interstitial water, further loss of water with cross-condensation of hexaborate(2-) ligands, and finally oxidation of the organic ligand. For 2 this involved loss of interstitial water, loss of coordinated ammonia, and finally further loss of water with cross-condensation of hexaborate(2-) ligands. The final residues for both compounds had a mass consistent with that expected from the stoichiometry for anhydrous CuB 6 O 10 (=CuOÁ3B 2 O 3 ). This behavior has been previously observed for other related Cu(II) complex polyborate salts [20,22,[27][28][29].
Room temperature magnetic susceptibility measurements were obtained for 1 and 2 and both compounds were paramagnetic with v m values slightly lower than those usually seen for d 9 Cu(II) centres and one unpaired electron. IR spectra can be used to help identify polyborate species since B-O stretches are generally characteristically strong and are observed in diagnostic areas [30]. In particular, 1 and 2 both show a bands at * 960(m) cm -1 and 808(s) cm -1 which have been tentatively assigned as diagnostic of the hexaborate(2-) anion [20,30]. Compounds 1 and 2 are insoluble in organic solvents and dissolve, with decomposition, in aqueous solution. NMR was attempted for these solutions but it was not possible to observe 1 H and 13 C spectra for the deen ligand in 1, possibly due to its continued coordination to the paramagnetic Cu(II) centre. 11 B spectra of 1 and 2 both showed just a single signal at ? 16.3 and ? 14.4 ppm, respectively. This signal for 2 is in accord with that calculated [16] for a boron/charge ratio of three for a hexaborate(2-) system, assuming fast B(OH) 3 /[B(OH) 4 ]exchange [11] and associated with the pH of the solution. The signal for 1 is slightly more downfield than expected.
All six hydroxyl hydrogen atoms of the hexaborate(2-) ligand are involved with H-bond donor interactions. The three coordinated hydroxyl hydrogen atoms link with two adjacent hexaborate(2-) units (H11 and H12) and H13 with a H 2 O molecule (O21). The three peripheral hydroxyl hydrogen atoms all link with neighbouring hexaborate(2-) units with hydrogens H9, H10 and H12 involved in familiar R 2 2 (8) interactions. O11H11 and O13H13 are linked in a unique R 3 3 (10) ring involving a neigbouring hexaborate(2-) unit (O10* acceptor) and H 2 O (O21 acceptor). O8H8 H-bonds to O13* on an adjacent hexaborate(2-) unit as part of a unique R 4 4 (10) ring that includes two hexaborate(2-) ligands and two H 2 O molecules. As noted above, multiple intermolecular R 2 2 (8) H-bond interactions have been shown to be influential in stabilizing solid-state structures of self-assembled polyborate salts from the DCL moieties present in the reaction solution, and the unique H-bond interactions present in 2 are likely to be strong and will contribute further to the stabilization of the structure. The structure of 2 is further stabilized by formation of an

Experimental
General All chemicals were commercially obtained. TGA and DSC analysis was performed between 10 and 800°C (in air) on an SDT Q600 V4.

Synthesis, Spectroscopic and Analytical Data for 2
A solution of ammonium hydroxide (2.2 mL, 20 mmol, 35%) was added to an aqueous solution of copper(II) sulphate pentahydrate (1.25 g, 5 mmol) in distilled water (5 mL). The reaction mixture was stirred at room temperature for 60 min, and then a solution of barium hydroxide octahydrate (1.58 g, 5 mmol) in water (10 mL) was added. The mixture was then stirred for a further 30 min and filtered. A solution of boric acid (3.09 g, 50 mmol) in water (10 mL) was added to the filtrate and the solution was stirred at room temperature for 3 h before its volume was reduced to 15

X-Ray Crystallography
Suitable crystals of 1 and 2 were selected and mounted on a MITIGEN holder in perfluoroether oil on a Rigaku FRE ? equipped with VHF Varimax confocal mirrors an AFC12 goniometer and HG Saturn 724 ? detector diffractometer. Crystals were kept at T = 100(2) K during data collection. Cell determination and data collection was carried out using CrystalClear [37] for 1 or CrysAlisPro [38] for 2, with data reduction, cell refinement and absorption correction carried out using CryAlisPro [38]. Using Olex2 [39], the structures were solved with the ShelXT [40] structure solution program, using the Intrinsic Phasing solution method. The models were refined with version 2014/7 of ShelXL [41] using Least Squares minimisation.
Crystal data. 1 Associated content CCDC1890710 (1) and CCDC1890711 (2) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif. Crystallographic data for 1 and 2 are also available as supplementary material together with 11 B NMR, IR, TGA data.