The reaction of Cpx 2ZrCl2 (Cpx = Cp, Cp*) with ammonia borane in presence of n-butyllithium yielded Cp2Zr(Cl)NH2BH3 and Cpx 2Zr(H)NH2BH3. These derivatives are isoelectronic with the ethyl zirconocene chloride and hydride, respectively, and feature a chelating amidoborane ligand coordinating through a Zr−N bond and a Zr−H−B bridge. In solution, each of the complexes consists of an equilibrium mixture of two isomers differing in the orientation of the amidoborane ligand with respect to the Zr−X bond (X = H, Cl), while in the solid state, only one isomer was observed. Such isomers have not been characterized for any metal complexes containing the isoelectronic β-agostic ethyl ligand or any other agostic alkyl group.

(2010). The influence of electron delocalization upon the stability and structure of potential N-heterocyclic carbene precursors with 1,3-diaryl-imidazolidine-4,5-dione skeletons. New Journal of Chemistry, 34 (7), 1295-1308. doi:10.1039 Aiming to develop synthetic protocols for the preparation of N-heterocyclic carbenes (NHCs) with increased π-acceptor character featuring N-pentafluorophenyl substituents, the molecular 2-chloro-1,3-bis(pentafluorophenyl)imidazolidine-4,5-dione (1a) was isolated from the condensation reaction of the corresponding formamidine with o xalyl chloride, instead of the expected, ionic 1,3-10 bis(pentafluorophenyl)-4,5-dioxo-4,5-dihydro-3H-imidazolium chloride. The nature of the extraannular N-substituents does not appear to exercise a significant influence on the structure of the products of this reaction, as evidenced by the similar structures observed with 2,6-dimethylphenyl (1b) and 2,6-diisopropylphenyl (1c) N-substituted analogues. These formal adducts of NHCs with hydrogen chloride demonstrate reactivity akin to that expected of alkyl halides. For instance, 1,3,1',3'-tetrakis(2,6-dimethylphenyl)-[2,2']diimidazolidinyl-4,5,4',5'-tetraone (2b) is formed via the reductive coupling of two units of 1b, while 1,3-15 bis(2,6-diisopropylphenyl)-4,5-dioxoimidazolidin-2-yl acetate (3c) is formed as the result of a metathesis reaction with mercury acetate. Chloride abstraction results in the formation of unstable imidazolium-4,5-dione salts (4a-c) that decompose rapidly, except in the case of the kinetically-stabilized 1,3-bis(2,6-diisopropylphenyl)imidazolium-4,5-dione hexafluorophosphate 4c, which could be observed spectroscopically. All imidazolium-4,5-dione hexafluorophosphate salts decompose to neutral 2-fluoro-1,3-bis(aryl)imidazolidine-4,5-diones (5a-c) via fluoride abstraction, demonstrating a clear preference for a four-coordinate 20 geometry about the diaminocarbon center in these compounds. 2-Methoxy-1,3-di(aryl)imidazolidine-4,5-diones (6a-c) were also prepared and they proved to be highly stable, failing to undergo thermolysis and yield the free NHCs. Computational analyses revealed that the instability of NHCs with an oxalamide skeleton, as well as that of their precursors, imidazolium-4,5-diones, results from a π-framework which extends over both carbonyl moieties and gives rise to a very low energy LUMO, rendering the compounds in question highly electrophilic.25

NB! The anionic ligand 2 was synthesized through deprotonation of a planar, formally zwitterionic diazadiborine precursor, isolated as a lithium salt, and structurally characterized. According to experimental evidence and theoretical calculations, 2 can be considered as an intermediate between two classical classes of ligands: N‐heterocyclic carbenes 1 and terphenyls 3.

The gas-phase reaction chemistry of using 1-methylsilacyclobutane (MSCB) in the hot-wire chemical vapor deposition (CVD) process has been investigated by studying the decomposition of MSCB on a heated tungsten filament and subsequent gas-phase reactions in a reactor. Three pathways exist to decompose MSCB on the filament to form ethene/methylsilene, propene/methylsilylene, and methyl radicals. The activation energies for forming propene and methyl radical, respectively, are determined to be 68.7 ± 1.3 and 46.7 ± 2.5 kJ·mol(-1), which demonstrates the catalytic nature of the decomposition. The secondary gas-phase reactions in the hot-wire CVD reactor are characterized by the competition between a free radical chain reaction and the cycloaddition of silene reactive species produced either from the primary decomposition of MSCB on the filament or the isomerization of silylene species. At lower filament temperatures of 1000-1100 °C and short reaction time (t ≤ 15 min), the free radical chain reaction is equally important as the silene chemistry. With increasing filament temperature and reaction time, silene chemistry predominates.