Revisiting the 1,2,4-triaza-3,5-diborolyl ligand: sigma and pi coordination modes in the alkali metal and rhodium complexes of a planar, 6-pi-electron B2N3- ring.

Lithium (2a), sodium (2b), and potassium (2c) salts of 1-methyl-3,5-diphenyl-4-methylamino-1,2,4-triaza-3,5-diborolyl were prepared by deprotonation of the ring nitrogen in neutral precursor 1. The alkali metal derivatives were characterized by multinuclear NMR, mass spectrometry, and single-crystal X-ray diffraction. The structural determinations revealed extended 2D structures for 2a and 2b and an extended 1D structure for 2c. All three solvent-free structures are dominated by sigma interactions, and pi interactions are also present for the potassium derivative. Addition of triphenylborane to 2a, 2b, and 2c produced the adducts 3a, 3b, and 3c, respectively, and these were characterized by multinuclear NMR and mass spectrometry. Structural determinations have been performed for the lithium and potassium salt, showing that Ph3B coordinates at the 2 position of the ring, whereas the alkali metal is coordinated by the pendant methylamino group. The lithium ion is additionally coordinated by three acetonitrile molecules in the monomeric structure of 3a, whereas the potassium ion is coordinated by three phenyl groups, forming the 1D polymeric structure of 3c. Reaction of 2a with [Rh(cod)Cl]2 yielded the dimeric 4, containing two 1,2,4-triaza-3,5-diborolyl rings bridging two Rh(cod) fragments through the substituent-free ring nitrogen atoms.