A series of crystalline nonmetal borate compounds was prepared by reactions of linear α,ω-diaminoalkanes, H2N(CH2) n NH2 (n = 5−12), with excess boric acid in aqueous solution. These simple reagents self-assemble in water to form complex ordered structures. The products were characterized by a combination of elemental, thermal, and X-ray diffraction analyses as well as single-crystal X-ray diffraction studies. In each case except for n = 7 and 12, a bis(pentaborate) product of general formula [H3N(CH2) n NH3][B5O6(OH)4]2 (1, 2, 4−7) was obtained in good yield. When n = 12, the product, [H3N(CH2)12NH3][B5O6(OH)4]2·4H2O (8), contained interstitial water. When n = 7, a product of composition [H3N(CH2) 7 NH3][B8O10(OH)6]·2B(OH)3 (3) was obtained, containing an unusual octaborate anion and interstitial boric acid. The borate anions form hydrogen-bonded supramolecular anionic frameworks that host alkyldiammonium dications. These cations contribute to hydrogen bonding and arrange either extended linearly or folded to varying degrees across channels. This study reveals an interplay between the anionic borate host and the cationic guest in which the nature of the cation controls the borate framework structure, which in turn influences cation conformation.

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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...