Oxidoborates Templated by Cationic Nickel(II) Complexes and Self-Assembled from B(OH) 3

Iraq. Abstract: Several oxidoborates, self-assembled from B(OH) 3 and templated by cationic Ni (II) coordination compounds, were synthesized by crystallization from aqueous solution. These include the ionic compounds trans -[Ni(NH 3 ) 4 (H 2 O) 2 ][B 4 O 5 (OH) 4 ] . H 2 O ( 1 ), s-[Ni(dien) 2 ][B 5 O 6 (OH) 4 ] 2 (dien = N -(2-aminoethyl)-1,2-ethanediamine ( 2 ), trans -[Ni(dmen) 2 (H 2 O) 2 ] [B 5 O 6 (OH) 4 ] 2. 2H 2 O (dmen = N,N -dimethyl-1,2-diaminoethane) ( 3 ), [Ni(HEen) 2 ][B 5 O 6 (OH) 4 ] 2 (HEen = N -(2-hydroxyethyl)-1,2-diaminoethane) ( 4 ), [Ni(AEN)][B 5 O 6 (OH) 4 ] . H 2 O (AEN = 1-(3-azapropyl) -2,4-dimethyl-1,5,8-triazaocta-2,4-dienato(1-)) ( 5 ), trans -[Ni(dach) 2 (H 2 O) 2 ][Ni(dach) 2 ] [B 7 O 9 (OH) 5 ] 2. 4H 2 O (dach = 1,2-diaminocyclohexane) ( 6 ), and the neutral species trans - [Ni(en)(H 2 O) 2 {B 6 O 7 (OH) 6 }] . H 2 O ( 7 ) (en = 1,2-diaminoethane), and [Ni(dmen)(H 2 O){B 6 O 7 (OH) 6 }] . 5H 2 O ( 8 ). Compounds 1–8 were characterized by single-crystal XRD studies and by IR spectroscopy and 2 , 4–7 were also characterized by thermal (TGA/DSC) methods and powder XDR studies. The solid-state structures of all compounds show extensive stabilizing H-bond interactions, important for their formation, and also display a range of gross structural features: 1 has an insular tetraborate(2-) anion, 2–5 have insular pentaborate(1-) anions, 6 has an insular heptaborate(2-) anion (‘O + ’ isomer), whilst 7 and 8 have hexaborate(2-) anions directly coordinated to their Ni (II) centers, as bidentate or tridentate ligands, respectively. The Ni (II) centers are either octahedral ( 1–4 , 7 , 8 ) or square-planar ( 5 ), and compound 6 has both octahedral and square-planar metal geometries present within the structure as a double salt. Magnetic susceptibility measurements were undertaken on all compounds.

Structurally, borates salts consist of metallic or non-metallic cationic centers and hydroxyoxidoborate units, with variable degrees of condensation, as either insular anions or as anionic 1-D chains, 2-D layers or 3-D networks [2][3][4][5]. Oxidoborate materials can be synthesized from aqueous solution or from solid-state or solvothermal methods and the latter non-aqueous methods often lead to the formation of the more highly condensed oxidoborate structures [5]. In aqueous solution, oxidoborate speciation is pH and boron-concentration dependent [19,20], and a dynamic combinatorial library (DCL) [21] of oxidoborate anions co-exist in rapidly attained aqueous equilibria.
We are interested in expanding the structural diversity of isolated and coordinated oxidoborate chemistry and have developed a strategy of incorporating relatively highly charged (>+1) and/or cations, with the potential of forming multiple H-bond interactions, into the aqueous DCL of borate anions so that they can template and crystal engineer novel structures [29,30]. Using labile cationic transition-metal complexes [31] introduces a further DCL of potential cations into reaction medium and leads to the possibility of oxidoborate anions entering the primary coordination shell of the metal, as O-donor ligands.
Ni (II) complexes have a d 8 electronic configuration and should also be relatively labile [31]. In this manuscript we report on the use coordination complexes of Ni (II) containing N-donor ligands NH 3 , en, dien, dmen, HEen, AEN, dach (see Figure 1 for ligand structures and abbreviations) to synthesize eight new oxidoborate compounds. Their single-crystal structures and their spectroscopic and physical properties are reported.

Synthesis and General Discussion
The Ni (II) complex oxidoborates 1-8 were prepared in acceptable yields as a crystalline solids from the reaction of a Ni (II) complex cation hydroxide salt with boric acid in a ratio of either 1:5 or 1:10 (Scheme 1). The Ni (II) complex cation hydroxides were obtained

Synthesis and General Discussion
The Ni (II) complex oxidoborates 1-8 were prepared in acceptable yields as a crystalline solids from the reaction of a Ni (II) complex cation hydroxide salt with boric acid in a ratio of either 1:5 or 1:10 (Scheme 1). The Ni (II) complex cation hydroxides were obtained in situ from the corresponding salts containing either chloride or sulfate anions. Anion exchange (from Cl − ) was used for 2, 4, 5, 6, and 8 and stoichiometric reactions with Ag 2 O (from Cl − ) was used for 1 and 3 and a stoichiometric reaction with Ba(OH) 2  The new compounds were identified by XRD single-crystal studies (Section 2.2), IR spectroscopic analysis and accompanied by thermal decomposition analysis (TGA and DSC) and powder XRD studies for 2, 4-7 (Section 2.3). Six of these eight compounds contain insular oxidoborate anions and two compounds contain coordinated hexaborate (2-) anions; the oxidoborate anions found in these new compounds are drawn out in Figure 2. The coordination geometry around the Ni (II) centers are either octahedral (1-4, 7, 8) or square-planar (5) and compound 6 has both octahedral and square-planar metal geometries present within the structure as a double salt. The new Ni (II) oxidoborate complexes are coloured with those with square-planar centres (5 and 6) are orange/red with the others having blueish hues. All compounds were stable in the solid state were insoluble in organic solvents and decomposed by aqueous solution (see 11 B NMR, Section 2.3). Since the products arise through self-assembly processes that we believe are associated with Hbonded structure directing effects induced by the cations present [21][22][23][24], these interactions are discussed in detail in Section 2.2.2. Scheme 1. Reagents and stoichiometry of reactants involved in self-assembly of Ni (II) oxidoborates from cationic Ni (II) complexes with B(OH) 3 in aqueous or methanolic aqueous solution.
The new compounds were identified by XRD single-crystal studies (Section 2.2), IR spectroscopic analysis and accompanied by thermal decomposition analysis (TGA and DSC) and powder XRD studies for 2, 4-7 (Section 2.3). Six of these eight compounds contain insular oxidoborate anions and two compounds contain coordinated hexaborate (2-) anions; the oxidoborate anions found in these new compounds are drawn out in Figure 2. The coordination geometry around the Ni (II) centers are either octahedral (1-4, 7, 8) or square-planar (5) and compound 6 has both octahedral and square-planar metal geometries present within the structure as a double salt. The new Ni (II) oxidoborate complexes are coloured with those with square-planar centres (5 and 6) are orange/red with the others having blueish hues. All compounds were stable in the solid state were insoluble in organic solvents and decomposed by aqueous solution (see 11 B NMR, Section 2.3). Since the products arise through self-assembly processes that we believe are associated with H-bonded structure directing effects induced by the cations present [21][22][23][24], these interactions are discussed in detail in Section 2.2.2.  respectively. Compounds 1, 3, 5 and 6 also contain one, two, one and four interstitial H2O molecules per formula unit, respectively. The dien ligand in 2, and the HEen ligand in 4 are both disordered over two positions. Compounds 7 and 8 are uncharged coordination compounds with [B6O7(OH)6] 2-coordinated as O-donor ligands to octahedral Ni (II) centres either as a bidentate or a tridentate ligand in 7 or 8, respectively. Compounds 7 and 8 have one and five interstitial H2O molecules, respectively, and are also partially aquated. There is some disorder of the interstitial H2O molecules in both 7 and 8 and some additional disorder in the en ligand and the dmen ligands of 7 and 8. Compound 6 is unusual and is best formulated as a double salt. It contains both octahedral trans-[Ni(dach)2(H2O)2] 2+ and square-planar [Ni(dach)2] 2+ cations partnered by two crystallographically equivalent [B7O9(OH)5] 2− anions and four interstitial H2O molecules. The dach ligands are also disordered in 6 but have chair conformations with their amino groups equatorial and with one dach ligand in each cation having R,R stereochemistry and the other S,S. Interestingly, two of these interstitial H2O molecules (O22) in 6 are in 'axial' positions in the squareplanar complex at 3.1517(13) Å (T = 0.61 [49]). In the octahedral complex, these two H2O ligands (O21) are at 2.1422(15) Å (T = 0.98). A structurally related (but d 9 ) Cu (II) compound, prepared by an identical method [37], contains a square-planar (also with very long axial H2O 'bonds', T = 0.70) and a Jahn-Teller distorted axially elongated octahedral complexes (T = 0.80). Structurally, the insular [B7O9(OH)5] 2-dianion is previously known to adopt either a 'chain' or 'O + ' isomeric forms [28,[50][51][52] and the 'O + ' isomer is present in 6. The new Ni (II) oxidoborates complex salts 1-6 all contain cations and anions that are known in other salts and the gross structures of the component ions, bond distances and internuclear angles for the component ions in 1-6 are as expected and need no further comment [5,[53][54][55]. The uncharged complexes 7 and 8 are structurally novel although similar compounds have been observed in Co (II) , Cu (II) and Zn (II) oxidoborate chemistry [34][35][36][37][38][56][57][58]. Compounds 7 and 8 contain isolated species [35,37,38] rather than polymer 1-D coordination chains [34,36,38] and these two structures will be discussed in more detail below.

Solid-State H-bonding Interactions in 1-8
Compounds 1-8 are self-assembled and crystallized from the various oxidoborate and the Ni (II) amine complexes that are each in equilibrium in the reacting aqueous solution [19,20,31]. There is a strong preference for the oxidoborate anion to enter the primary coordination shell in related Cu (II) and Zn (II) chemistry with the formation of energetically favourable O-donor coordinate bonds [32,[34][35][36][37][38][39][40]58]. Interestingly, Ni (II) salts are described as labile, although in practice they are often considerably less labile than corresponding Cu (II) and Zn (II) complexes [31]. In accord with this, the Ni (II) cations used in this study generally remain more or less intact in salts 1-6 after prolonged crystallization from aqueous solution, with formation of oxidoborate O-donor bonds only observed in compounds 7 and 8. H-bond interactions also appear to be of paramount importance in driving this crystallization process and these interactions are described in detail for each compound in the following paragraphs.
A drawing of the components of the formula unit of 1, showing important atomic numbering, is shown in Figure 5. The cations, anions and interstitial H2O molecules have numerous H-bond donor and/or acceptor sites and all potential H-bond donor sites are used in the solid-state structure. The dotted red lines in Figure 5 illustrate H-bond interactions which link cation to anion (O11H11 … O6), cation to interstitial H2O (N11H11 … O21),

Solid-State H-Bonding Interactions in 1-8
Compounds 1-8 are self-assembled and crystallized from the various oxidoborate and the Ni (II) amine complexes that are each in equilibrium in the reacting aqueous solution [19,20,31]. There is a strong preference for the oxidoborate anion to enter the primary coordination shell in related Cu (II) and Zn (II) chemistry with the formation of energetically favourable O-donor coordinate bonds [32,[34][35][36][37][38][39][40]58]. Interestingly, Ni (II) salts are described as labile, although in practice they are often considerably less labile than corresponding Cu (II) and Zn (II) complexes [31]. In accord with this, the Ni (II) cations used in this study generally remain more or less intact in salts 1-6 after prolonged crystallization from aqueous solution, with formation of oxidoborate O-donor bonds only observed in compounds 7 and 8. H-bond interactions also appear to be of paramount importance in driving this crystallization process and these interactions are described in detail for each compound in the following paragraphs.

Magnetic, Spectroscopic, Thermal and p-XRD Characterization of Compounds 1-8
Magnetic susceptibility measurements on 1-8 confirmed that all, excepting 5, are paramagnetic with χ m values ranging from 2500 × 10 −6 to 3600 × 10 −6 cm 3· mol −1 (µ eff of 2.6-3.0 BM) corresponding to two unpaired electrons per formula unit. Compound 5 is diamagnetic (χ m = −170 × 10 −6 cm 3 ·mol −1 ), and this value is in accord with the Ni (II) centre being square-planar. The χ m values of 1-4, 7 and 8 are typical of octahedral Ni (II) complexes [61] and have similar values to those obtained for the starting octahedral Ni (II) complexes. Compound 6 is unusual in that it has a µ eff corresponding to two unpaired electrons but there are two Ni (II) centres in the formula unit; the XRD study confirmed that one Ni (II) centre is square planar and the other is octahedral.
Compound 5, in D 2 O solvent, gave 1 H and 13 C NMR signals for the AEN ligand at similar intensities and chemical shifts to those obtained for [Ni(AEN)]Cl . H 2 O and other reported data [62]. 1 H and 13 C NMR signals for the organic ligands (in D 2 O) were not observable for the other Ni (II) oxidoborates despite their apparent solubility, due to their paramagnetic properties. Despite the possible lability of the organic ligands the complexes the organic ligands are not sufficiently labile to be remote from the Ni (II) centres during the lifetime of the NMR experiment. 11 B NMR spectra for all D 2 O solutions arising from the Ni (II) oxidoborates were observed and a probable explanation for this being two-fold: (i) compounds with insular oxidoborate anions (1)(2)(3)(4)(5)(6) are not coordinated to the Ni (II) centres and hence are not in close proximity and unaffected by the paramagnetic centres, and (ii) the coordinated hexaborate(2-) ligands are significantly more labile than the N-donor ligands in 7 and 8. The 11 B spectra were not particularly characteristic of the specific oxidopolyborate anions in 1-8 since it is known that dissolution of oxidopolyborate anions in aqueous solution yields solutions containing equilibrium mixtures of various oxidopolyborate anions whose concentrations are pH are boron concentration dependent [19,20]. However, pentaborate(1-) salts often afford spectra in a characteristic pattern with three signals at ca. +17, +13 and +1 ppm which have been assigned to B(OH) 3 4 ] − , at a chemical shift which is variable [19] but can be calculated assuming fast exchange of B(OH) 3 /[B(OH) 4 ] − according to their relative proportions present in the oxidopolyborate anion originally present [27]. Unsurprisingly, therefore, compounds 1 and 5-8 all gave single signals at +11.1, +16.3, +15.8, +17.7 and +15.8 ppm respectively. The observed chemical shifts for the tetraborate(2-) (1) and the pentaborate(-) (5) salts are as would be expected from this calculation (+11.0 and +16.1, respectively) but chemical shifts for the hexaborate(2-) (7 and 8) and the heptaborate(2-) (6) salts are ca. 2 ppm more downfield than expected (13.8 and 14.6 ppm calc., respectively), indicating that more B(OH) 3 might be present than expected. Similar behaviour has been observed before for hexaborate [34][35][36][37][38] and heptaborate [33] complexes.
Selected Ni (II) oxidoborates (2, 4-7) were thermally decomposed air and insights into these decompositions were obtained through TGA/DSC studies. The Ni (II) oxidoborates all yielded green glassy residues by 700 • C with masses consistent with anhydrous Ni (II) oxidoborates being produced with Ni:B stochiometries in the elemental ratios of their precursors. Thus, the pentaborates 2 and 4 were decomposed to NiB 10 O 16 whilst 5 gave Ni 2 B 10 O 17 , the heptaborate (6) gave Ni 2 B 14 O 23 and the hexaborate (7) yielded NiB 6 O 10 . These decomposition reactions are generally two-step processes with the first steps being a lower temperature (<280 • C) endothermic dehydration (loss of interstitial H 2 O and condensation reactions of the hydrated oxidoborates) followed by higher temperature (280-700 • C) exothermic oxidations of the organic ligands. Details for individual compounds (2, 4-7) are given in the experimental section. This type of decomposition behaviour has been observed previously for other hydrated Ni (II) oxidoborates [33,[42][43][44][45][46][47][48] and more generally applies to the thermolysis of many hydrated transition-metal oxidopolyborates [30,33,35,37]. Powder XRD data were also obtained for this selection of Ni (II) oxidoborates (viz. 2, 4-7). The data obtained were good matches with data calculated from the single-crystal XRD data indicating that these samples were crystalline and homogeneous.

General
The Ni ( [68] were prepared by modified literature procedures. FTIR spectra were obtained as KBr pellets on a Perkin-Elmer 100FTIR spectrometer (Perkin-Elmer, Seer Green, UK). 11 B NMR spectra were obtained on a Bruker Avance-400 spectrometer (Bruker, Coventry, UK) on samples dissolved in D 2 O at 128 MHz. TGA and DSC were performed on an SDT Q600 instrument (TA Instruments, New Castle DE, USA) using Al 2 O 3 crucibles with a ramp rate of 10 • C per minute (RT to 1000 • C in air). X-ray crystallography was performed at the EPSRC national crystallography service centre at Southampton University. Magnetic susceptibility measurements were performed on a Johnson-Matthey magnetic susceptibility balance. (Johnson-Matthey, UK). CHN analyses were obtained from OEA Laboratories (Callingham, Cornwall).

Conclusions
Eight new oxidoborate Ni (II) compounds were prepared by self-assembly crystallization processes from aqueous solution starting from B(OH) 3 and selected Ni (II) complexes. Six of these compounds were salts containing insular oxidoborate anions, but two products were complexes containing coordinated oxidoborate(2-) anions, with energetically favourable Ni-O bonds. The two products that contained coordinated hexaborate(2-) ligands were neutral monomeric complexes, rather than the 1-D coordination polymers that we have occasionally observed in related Cu (II) and Zn (II) chemistry. The new compounds were templated in all cases by numerous strong structure-directing inter/intramolecular H-bonding interactions involving the oxidoborate ligands and many novel H-bonding motifs are noted and described. This work further demonstrates that in general the inclusion of labile metal complexes into aqueous solutions containing B(OH) 3 can lead to the self-assembly of novel species including those with coordinated oxidoborate ligands and that this synthetic strategy could be applied successfully to other systems.