Revisiting the 1,2,4-Triaza-3,5-diborolyl Ligand:  σ and π Coordination Modes in the Alkali Metal and Rhodium Complexes of a Planar, 6-π-Electron B₂N₃^- Ring

Abstract

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.

Tuning the electronic properties of cyclopentadienyl analogs with CB2N2 frameworks: 1,2-diphenyl-1,2-diaza-3,5-diborolyl ligands and their alkali metal salts

“…The 1 H and 13 C NMR spectra of 1 and 2 featured the expected signals. The distinctive doublet-quartet pattern of the Me-CHB 2 moiety was observed in the 1 H NMR spectra of both substances, and only one signal was observed for the methyl protons of the Scheme 1 Synthesis of ligand precursors 1 and 2. dimethylamino groups in 2, indicative of a low rotational barrier around the exocyclic B-N bond.…”

“…The disappearance of the ring proton and the resulting collapse of the doublet signal corresponding to the methyl group were easily noticeable in the 1 H NMR spectra, which displayed all the expected resonances. The most remarkable change in the 13 C NMR spectra upon deprotonation was the shift of the broad signal corresponding to the ring carbon from 23.0 ppm in the neutral precursor to 93.6-98.1 ppm in the alkali metal salts, indicative of the change in hybridization and electron density of this atom. Such a dramatic shift is typical for cyclopentadienyl derivatives with highly ionic character and has been observed for C 1 and C 2 .…”

Construction of Boron-Containing Aromatic Heterocycles

“…90 In 2007, the Roesler group reinvestigated the chemical behavior of a 1,4,2,3,5-triazadiborole (Scheme 51). 91 Selfassembly of bis(dimethylamino)phenylborane 168…”

Alkali and alkaline earth metals

“…59 Sodium and potassium salts of 1-methyl-3,5-diphenyl-4-methylamino-1,2,4-triaza-3,5-diborolyl have been prepared by deprotonation of the ring nitrogen in the neutral precursor. 60 The ring closure of 1,2-diisopropylhydrazine and 1,1-bis(phenylchloroboryl)ethane produced a heterocyclic compound with a CB 2 N 2 framework. Deprotonation of this precursor yielded 1,2-diisopropyl-4methyl-3,5-diphenyl-1,2-diaza-3,5-diborolyl, a cyclopentadienyl analogue containing only one carbon atom in the ring skeleton.…”