Guanidinium and Imidazolium Borates Containing the First Examples of an Isolated Nonaborate Oxoanion:  [B₉O₁₂(OH)₆]^3-

Abstract

Borate compounds in which boron is bound only to oxygen have considerable mineralogical and industrial importance. These contain anionic components composed of BO 3 and BO 4 groups that may link together by sharing oxygen atoms to form isolated rings and cages or polymerize into infinite chains, sheets, and networks. Many examples of isolated boron oxoanions containing one to six borons exist in mineral and synthetic borates. However, isolated boron oxoanions having more than six borons are rare and no previous examples have nine borons. 2 This communication describes the synthesis and structural characterization of crystalline guanidinium and imidazolium borates, [C(NH 2 ) 3 ] 3 [B 9 O 12 (OH) 6 ] (I) and [C 3 H 5 N 2 ] 3 [B 9 O 12 (OH) 6 ] (II), containing the first examples of the isolated nonaborate anion, [B 9 O 12 (OH) 6 ] 3-. These borates have resolved oxide formulas [C(NH 2 ) 3 ] 2 O‚3B 2 O 3 ‚2H 2 O and [C 3 H 5 N 2 ] 2 O‚3B 2 O 3 ‚2H 2 O. Isolated boron oxoanions are found in the commercially significant metaborates ([B(OH) 4 ] -), tetraborates ([B 4 O 5 (OH) 4 ] 2-), and pentaborates ([B 5 O 6 (OH) 4 ] -), as well as in triborates ([B 3 O 3 (OH) 4 ] -and [B 3 O 3 (OH) 5 ] 2-). 3 Also, some mineral and synthetic borates contain the isolated hexaborate anion, [B 6 O 7 (OH) 6 ] 2-. 2 The mineral ammonioborite contains an unusual example of a large isolated anion, [B 15 O 20 (OH) 8 ] 3-. 2 Polyborate anions can be regarded as products of varying degrees of neutralization of orthoboric acid, B(OH) 3 , with a strong base in relatively concentrated aqueous solution. The [B 3 O 3 (OH) 4 ] -and [B 6 O 7 (OH) 6 ] 2-anions, as well as the [B 9 O 12 (OH) 6 ] 3-anion described herein, can all be viewed as products of a 1 / 3 neutralization with strong base of boric acid to [B(OH) 4 ] -. At least three crystalline binary guanidinium borates exist. Guanidinium tetraborate, [C(NH 2 ) 3 ] 2 [B 4 O 5 (OH) 4 ]‚2H 2 O (III), was reported as early as 1921, 4 and brief references are made to guanidinium pentaborates, including [C(NH 2 ) 3 ][B 5 O 6 (OH) 4 ]‚2H 2 O (IV). 5 We have recently found that the novel guanidinium nonaborate I crystallizes in the [C(NH 2 ) 3 ] 2 O-B 2 O 3 aqueous system at temperatures above ca. 45°C at B 2 O 3 /[C(NH 2 ) 3 ] 2 O mole ratios (defined as q) greater than ca. 2.5 over a range of concentrations. Tetraborate III crystallizes at lower temperatures or q values. At q > 5 either boric acid or guanidinium pentaborate IV crystallizes, depending on concentrations. Nonaborate I can be prepared by aqueous stoichiometric or near-stoichiometric reaction of guanidinium carbonate with boric acid: 6Alternatively, I can be prepared by reaction of guanidinium salts with borax and boric acid under appropriate conditions. 7 Nonaborate I exhibits temperature-dependent stability in water contact. At 20°C, an aqueous slurry of I converts to tetraborate III and boric acid in about 1 day. At elevated temperatures, I is stable in aqueous slurry and can be recrystallized from water. Nonaborate I can also be pre...

Synthesis and X-ray structural studies of pentaborate(1−) salts containing substituted imidazolium cations

“…DSC traces indicated that the decomposition steps below 250 o C were endothermic and those above 250 o C were exothermic. This is consistent with previous observations [15,26,27], which have shown the thermal decomposition generally occur via a series of steps involving dehydration (endothermic) and oxidation (exothermic), as shown in Eqns 3 and 4.…”

“…Characterization data reported for 1 and 2 are limited to single-crystal XRD studies and elemental analyses [15,16]. We successfully re-synthesised these two compounds by the literature method [15], with bulk samples of satisfactory elemental analysis data.…”

“…We successfully re-synthesised these two compounds by the literature method [15], with bulk samples of satisfactory elemental analysis data.…”

“…Both systems may be prepared from aqueous solution although more forcing conditions are often required for the condensed systems [4]. Within the isolated systems, polyborate anions containing three [5,6], four [7,8], five [9], seven [10][11][12][13], eight [14], nine [15,16], fourteen [17] and fifteen [18] class. It is becoming increasingly apparent that crystalline polyborate products which are formed are more strongly influenced by H-bonding interactions and crystal packing forces than by reaction conditions/reactant stoichiometry since many polyborate species co -exist in aqueous solution as a consequence of rapidly attained equilibria [19,20].…”

“…Literature reports of nonaborate (3-) cations, respectively [15,16]. As part of a continuing study relating to the thermal properties of NMC polyborates [6,13,21] we were interested in preparing more examples of these rare non-metal cation salts with nonaborate anions (Figure 1) and have investigated the synthesis of polyborate salts derived from related imidazolium cations.…”