Biguanidinate supported thioxo- and selenoxoboranes: structural insights and reactivity studies.

Herein, we report the monoanionic N,N'-chelated biguanidinate-supported thioxo- and selenoxoboranes XylLBS (1) and XylLBSe (2) (L = {(ArHN)(ArN)-CN-C(NAr)(NHAr)}; Ar = Xyl = 2,6-Me2-C6H3). These compounds are synthesized by treating the tetra-N-aryl substituted biguanide ligand (XylLH) with BH3·THF to generate an in situ boron dihydride via deprotonation, followed by chalcogenation with elemental sulfur or selenium. Single-crystal X-ray diffraction analyses unequivocally confirm the presence of BS and BSe multiple bonds. Protonation of 1 and 2 with [PhNMe2H][B(C6F5)4] affords the cationic complexes {[XylLBEH]+[B(C6F5)4]-} [E = S (3), Se (4)], via selective protonation at the chalcogen atom. Complexes 3 and 4 represent rare, structurally authenticated examples of cationic S/Se-protonated thioxoborane and selenoxoborane species. Furthermore, treatment of 1 with B(C6F5)3, HF, and H2O furnishes the biguanidinate-supported tricoordinate hydroxyborenium ion, (XylLB-OH⋯F-B(C6F5)3) (5), whereas a reaction with BF3·OEt2 produces the boron difluoride complex XylLBF2 (6). Moreover, XylLBF2 (6) undergoes hydride/fluoride exchange with zinc and magnesium hydride complexes to provide the corresponding soluble zinc and magnesium fluoride complexes, demonstrating its potential as a fluoride-transfer reagent. Collectively, these findings establish the biguanidinate ligand as an effective platform for stabilizing thioxoborane and selenoxoborane species, which serve as versatile precursors to access elusive cationic boron-chalcogen, hydroxyborenium, and soluble metal fluoride species.