Synthesis and Characterization of Dimeric, Trimeric, and Tetrameric Gallophosphonates and Gallophosphates

THF/toluene solutions of phosphonic or phosphoric acids were reacted with (t)Bu(3)Ga at low temperature to yield the cyclic dimers [(t)Bu(2)GaO(2)P(OH)R](2) (R = Ph, Me, (t)Bu, H, OH; 1-5). Poor crystallinity and variable thermal stabilities of 1-5 necessitated derivatization with Me(3)SiNMe(2) to yield [(t)Bu(2)GaO(2)P(OSiMe(3))R](2) (R = Ph, Me, (t)Bu, H, OSiMe(3); 6-10), which were more amenable to purification and characterization. In solution, trans isomers were predominant for 6 and 7 at ambient temperature, whereas the cis isomer of 8 was predominant. NMR spectroscopy demonstrated cis-trans interconversion for 6-8 and crossover experiments showed interconversion to occur by, or be accompanied with, an intermolecular exchange process. Thermolysis of 3 in refluxing toluene yielded the cluster [((t)BuGa)(2)((t)Bu(2)Ga)(O(3)P(t)Bu)(2){O(2)P(OH)(t)Bu}] (11), which was converted to [((t)BuGa)(2)((t)Bu(2)Ga)(O(3)P(t)Bu)(2){O(2)P(OSiMe(3))(t)Bu}] (12) with Me(3)SiNMe(2). Thermolysis of 1-3 in refluxing diglyme, or solid-state pyrolysis at 250 degrees C in vacuo, yielded [(t)BuGaO(3)PR](4) (R = Ph, (t)Bu, Me; 13-15). The gallophosphate [(t)BuGaO(3)P(OSiMe(3))](4) (16) was similarly obtained by reaction of (t)Bu(3)Ga with H(3)PO(4) in refluxing diglyme, followed by trimethylsilylation with Me(3)SiNMe(2). Compounds 13-16 possess cuboidal Ga(4)P(4)O(12) cores analogous to double-four-ring secondary building units in the gallophosphates cloverite, gallophosphate-A, and ULM-5. The thermal, hydrolytic, and oxidative stabilities of 13-16 are discussed, as are observed intermolecular exchange processes. In addition to characterization of 1-16 by multinuclear ((1)H, (13)C, (31)P) NMR spectroscopy, infrared spectroscopy, mass spectrometry, and elemental analysis, molecular structures for compounds 6, 8, 10, 12, 14, 15, and 16 were determined by X-ray crystallography.