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

Abstract

Two heterocyclic cyclopentadienyl analogs with a CB2N2 skeleton, 4-methyl-1,2,3,5-tetraphenyl-1,2-diaza-3,5-diborolidine and 4-methyl-3,5-dimethylamino-1,2-diphenyl-1,2-diaza-3,5-diborolidine were prepared through cyclocondensation of the corresponding 1,1-bis(organochloroboryl)ethane with 1,2-diphenylhydrazine. The former diazadiborolidine featured a cyclopentadiene-like structure with short B-N bonds and a planar ring framework, while in the latter the B-N bonds were noticeably longer and the ring framework was considerably folded as a result of the interaction between boron and the electron donating NMe2 groups. The dimethylamino substituted diazadiborolidine could not be deprotonated due to the reduced acidity of the ring proton, however, the B-phenylated analog was easily deprotonated and the lithium, sodium and potassium 1,2-diaza-3,5-diborolyls were isolated and structurally characterized. The solid state structures of the lithium and sodium salts were similar, with an eta(1)-coordinated pi ligand and three THF molecules completing the coordination sphere of the metal. The potassium salt featured a highly unusual mono-dimensional polymeric structure with the metal pi-coordinated by the CB2N2 ligand and two of the phenyl groups on boron and nitrogen, and sigma-coordinated by one THF molecule.

Construction of Boron-Containing Aromatic Heterocycles

“…In order to gain insight into the electronic structures of the CB 2 N 4 skeleton, the Roesler group developed two different precursors 156 and 157 (Scheme 47). 84 Compound 156 was readily deprotonated by various bases, and the solid state structures of the salts 158 suggest that the π-electrons are delocalized over the N-B-C-B-N skeleton without the involvement of N-N π-bonding interaction. This is because of the electron-withdrawing nature of phenyl groups substituted at the N atoms.…”

Alkali and alkaline-earth metals

“…The solid state structures of the lithium and sodium salts were similar, with an Z 1 -coordinated p ligand and three THF molecules completing the coordination sphere of the metal. 50 Reduction of dibenzopentalene with excess lithium gave dilithium dibenzopentalenide while reaction with methyllithium gave lithium 5-methyldibenzopehtalenide, the formation of which was evidenced by some trapping experiments and X-ray crystallographic analysis. 51 The multidentate ligand (MeOMe 2 Si) 3 Si À has been employed in the synthesis of a variety of derivatives including a lithium silanide which is formally composed of zwitterionic bicyclooctane units, (Si(SiO) 3 Li), 5, which are associated in a head-to-tail fashion to give a linear infinite chain.…”

Boron

“…The lithium, sodium and potassium 1,2-diaza-3,5-diborolyls have been synthesised, isolated and structurally characterized. 114 The synthesis, reactions and computational analysis of a series of 1,2-diboracyclopentanes have been described. 115 o-(Hydroxydimethylsilyl)(dimesitylboryl)benzene has been shown to undergo intramolecular cyclisation to yield the Si-O-B heterocycle, 5,5-dimesityl-2,2-dimethyl-3,4-benzo-1,2,5-oxasilaboracyclopentene which was characterized by X-ray crystallographic analysis and HF calculations.…”