Structural (XRD) Characterization and an Analysis of H-Bonding Motifs in Some Tetrahydroxidohexaoxidopentaborate(1-) Salts of N -Substituted Guanidinium Cations.

The synthesis and characterization of six new substituted guanidium tetrahydroxidohexaoxidopentaborate(1-) salts are reported: [C(NH 2 ) 2 (NHMe)][B 5 O 6 (OH) 4 ]·H 2 O ( 1 ), [C(NH 2 ) 2 (NH{NH 2 })][B 5 O 6 (OH) 4 ] ( 2 ), [C(NH 2 ) 2 (NMe 2 )][B 5 O 6 (OH) 4 ] ( 3 ), [C(NH 2 )(NMe 2 ) 2 ][B 5 O 6 (OH) 4 ] ( 4 ), [C(NHMe)(NMe 2 ) 2 ][B 5 O 6 (OH) 4 ]·B(OH) 3 ( 5 ), and [TBDH][B 5 O 6 (OH) 4 ] ( 6 ) (TBD = 1,5,7-triazabicyclo [4.4.0]dec-5-ene). Compounds 1 - 6 were prepared as crystalline salts from basic aqueous solution via self-assembly processes from B(OH) 3 and the appropriate substituted cation. Compounds 1 - 6 were characterized by spectroscopic (NMR and IR) and by single-crystal XRD studies. A thermal (TGA) analysis on compounds 1 - 3 and 6 demonstrated that they thermally decomposed via a multistage process to B 2 O 3 at >650 °C. The low temperature stage (<250 °C) was endothermic and corresponded to a loss of H 2 O. Reactant stoichiometry, solid-state packing, and H-bonding interactions are all important in assembling these structures. An analysis of H-bonding motifs in known unsubstituted guanidinium salts [C(NH 2 ) 3 ] 2 [B 4 O 5 (OH) 4 ]·2H 2 O, [C(NH 2 ) 3 ][B 5 O 6 (OH) 4 ]·H 2 O, and [C(NH 2 ) 3 ] 3 [B 9 O 12 (OH) 6 ] and in compounds 1 - 6 revealed that two important H-bonding R 2 2 (8) motifs competed to stabilize the observed structures. The guanidinium cation formed charge-assisted pincer cation-anion H-bonded rings as a major motif in [C(NH 2 ) 3 ] 2 [B 4 O 5 (OH) 4 ]·2H 2 O and [C(NH 2 ) 3 ] 3 [B 9 O 12 (OH) 6 ], whereas the anion-anion ring motif was dominant in [C(NH 2 ) 3 ][B 5 O 6 (OH) 4 ]·H 2 O and in compounds 1 - 6 . This behaviour was consistent with the stoichiometry of the salt and packing effects also strongly influencing their solid-state structures.