A new polyborate anion, [B 7 O 9 (OH) 6 ] 3− : Self assembly, XRD and thermal properties of s-fac- [Co(dien) 2 ][B 7 O 9 (OH) 6 ]·9H 2 O

Abstract

The title compound, s-fac-[Co(dien)2][B7O9(OH)6].9H2O(1) (dien = HN(CH2CH2NH2)2), has been prepared as a crystalline solid in moderate yield (35%) from the reaction of B(OH)3 with [Co(dien)2][OH]3 in aqueous solution (10:1 ratio). The structure contains a novel polyborate anion [B7O9(OH)6] 3-which is structurally based on the known 'ribbon' isomer of [B7O9(OH)5] 2-, with an additional [OH] -group coordinated to a B atom in one of the outer boroxole rings. Compound 1 is formed by a self-assembly process in which the cation and anion mutually template themselves from equilibrium mixtures under reaction conditions. The [B7O9(OH)6] 3-, anions are H-bonded to each other in layers with 'cavities' suitable for the [Co(dien]2] 3+ complex. Three [B7O9(OH)6] 3-anions are in the secondary coordination sphere (via H-bonds) of each cation, with each anion H-bonded to three cations. ___________________________________________________________________________Polyborate anions are conveniently classified as either 'condensed' (polymeric chains, sheets or networks) or 'isolated' with discrete anionic moieties. Salts containing polyborate anions have attracted recent attention due to their possible applications as luminescent, second harmonic generation, ferroelectric, flame retardant and non-linear optical materials [1]. Isolated polyborate anions are readily synthesised by solvothermic methods [2] or by the addition of B(OH)3 to a basic aqueous solution containing a potential templating cation [3]. Self-assembled products arise since B(OH)3 in basic aqueous solution exists as a dynamic combinatorial library of a number of polyborate anions [4]. In general, pentaborate(1-) salts (containing the [B5O6(OH)4] -anion) are formed because these salts have a strong H-bonded anionic lattice and this lattice is sufficiently flexible to accommodate many small to medium sized cations [5]. Salts containing polyborate anions other than pentaborate(1-) are relatively rare [6]. We are interested in the synthesis of structurally novel polyborate anions and have adopted a strategy of templating such species by the use of sterically demanding and/or highly charged cations [7]. Herein, we report the self-assembly of a salt containing a novel isolated heptaborate(3-) anion, partnered with cationic transition-metal coordination complex. This salt's thermal properties, and its structure as determined by single-crystal XRD are also reported. [8]. Compound 1 was initially characterized by elemental analysis, IR and NMR ( 1 H, 13 C, and 11 B) spectroscopy, and by thermal (TGA/DSC) analysis (air). The thermal and spectroscopic data indicated that 1 was not a pentaborate(1-) salt. In particular, the compound displayed several BO bands in the IR spectra but the diagnostic pentaborate(1-) peak (926 cm -1 ) [9] was absent. At 'infinite' dilution its 11 B NMR spectrum gave a single peak at +12.3 ppm, rather than at +16.1 [5] which would be indicative of pentaborate(1-). This higher field 11 B signal would be more consistent with a lower B/charge ra...

Hexaborate(2−) and Dodecaborate(6−) Anions as Ligands to Zinc(II) Centres: Self-Assembly and Single-Crystal XRD Characterization of [Zn{κ3O-B6O7(OH)6}(κ3N-dien)]·0.5H2O (dien = NH(CH2–CH2NH2)2), (NH4)2[Zn{κ2O-B6O7(OH)6}2 (H2O)2]·2H2O and (1,3-pnH2)3[(κ1N-H3N{CH2}3NH2) Zn{κ3O-B12O18(OH)6}]2·14H2O (1,3-pn = 1,3-diaminopropane)

“…Compound 2 had a TGA trace consistent with loss of initial interstitial water (<110 °C), loss of ammonia (110-250 °C), and final condensation of hexaborate(2−) anions (250-500 °C). Similar thermal behaviour has been observed in other metal polyborate species [12,13,24,[32][33][34][35] [15]. Magnetic susceptibility χm data for 1-3 were ~ −200 × 10 −6 cm 3 •mol −1 and typical for diamagnetic zinc(II) complexes.…”

“…Bond lengths (B-O) and OBO and BOB bond angles associated with the hexaborate(2−) ligands in both 1 and 2 were very similar. For example, bond lengths to the central pyramidal O + (1.5154(18)-1.5231(18) Å, 1; 1.5053(15)-1.5247(16) Å, 2) > other bond lengths to four coordinate borons (1.4407(19)-1.4791(19)Å, 1; 1.4413(18)-1.4889(15) Å, 2) > bond-lengths to three coordinate borons (1.362(2)-1.418(4) Å, 1; 1.3570(17)-1.3793(17) Å, 2) and consistent with distances and angles previously reported specifically for hexaborate(2−) complexes[15,32,38,39] and related polyborate systems[8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][32][33][34][35][36][38][39][40].…”

“…We have developed a strategy to overcome pentaborate(1−) salt formation by utilizing more highly charged (> (+1)) metal complex cations with ligands having the potential to form multiple H-bond interactions to template crystallization from aqueous solution of polyborate salts of unusual structures. In this context we have isolated two novel polyborate anions: [B 7 O 9 (OH) 6 ] 3− [12] and [B 8 O 10 (OH) 6 ] 2− [13]. We have also recently started to investigate Zn(II)/polyborate chemistry and have been able to isolate an insular bi-Zn(II) complex containing a rare dodecaborate(6−) anion [14] and two polymeric 1-D coordination chains with hexaborate(2−) ligands bridging Zn(II) centres [15].…”

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

“…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.…”

Two 1-D Coordination Polymers Containing Zinc(II) Hexaborates: [Zn(en){B6O7(OH)6}]·2H2O (en = 1,2-diaminoethane) and [Zn(pn){B6O7(OH)6}]·1.5H2O (pn = 1,2-diaminopropane)

“…In addition, cations that specifically could form many H-bond donor interactions with polyborate anions were chosen. This strategy was successful and we recently reported two such novel isolated polyborate anions: heptaborate (3−) [14] and octaborate (2−) [15].…”