Isolation of a NHC-stabilized heavier nitrile and its conversion into an isonitrile analogue

Nitriles (R-C≡N) have been investigated since the late eighteenth century and are ubiquitous encounters in organic and inorganic syntheses. In contrast, heavier nitriles, which contain the heavier analogues of carbon and nitrogen, are sparsely investigated species. Here we report the synthesis and isolation of a phosphino-silylene featuring an N-heterocyclic carbene-phosphinidene and a highly sterically demanding silyl group as substituents. Due to its unique structural motif, it can be regarded as a Lewis base-stabilized heavier nitrile. The Si-P bond displays multiple bond character and a bent R-Si-P geometry, the latter indicating fundamental differences between heavier and classical nitriles. In solution, a quantitative unusual rearrangement to a phosphasilenylidene occurs. This rearrangement is consistent with theoretical predictions of rearrangements from heavier nitriles to heavier isonitriles. Our preliminary reactivity studies revealed that both isomers exhibit highly nucleophilic silicon centres capable of oxidative addition and coordination to iron tetracarbonyl.

Carbene-Stabilized Phosphagermylenylidene: A Heavier Analog of Isonitrile

Phosphagermylenylidenes (R-P═Ge), as heavier analogs of isonitriles, whether in their free state or as complexes with a Lewis base, have not been previously identified as isolable entities. In this study, we report the synthesis of a stable monomeric phosphagermylenylidene within the coordination sphere of a Lewis base under ambient conditions. This species was synthesized by Lewis base-induced dedimerization of a cyclic phosphagermylenylidene dimer or via Me3SiCl elimination from a phosphinochlorogermylene framework. The deliberate integration of a bulky, electropositive N-heterocyclic boryl group at the phosphorus site, combined with coordination stabilization by a cyclic (alkyl)(amino)carbene at the low-valent germanium site, effectively mitigated its natural tendency toward oligomerization. Structural analyses and theoretical calculations have demonstrated that this unprecedented species features a P═Ge double bond, characterized by conventional electron-sharing π and σ bonds, complemented by lone pairs at both the phosphorus and germanium atoms. Preliminary reactivity studies show that this base-stabilized phosphagermylenylidene demonstrates facile release of ligands at the Ge atom, coordination to silver through the lone pair on P, and versatile reactivity including both (cyclo)addition and cleavage of the P═Ge double bond.

Phosphine-Stabilized Germylidenylpnictinidenes as Synthetic Equivalents of Heavier Nitrile and Isocyanide in Cycloaddition Reactions with Alkynes

The reactions of chlorogermylene M^sFluind^tBu-GeCl 1, supported by a sterically encumbered hydrindacene ligand M^sFluind^tBu, with NaPCO(dioxane)_2.5 and NaAsCO(18-c-6) in the presence of trimethylphosphine afforded trimethylphosphine-stabilized germylidenyl-phosphinidene 2 and -arsinidene 3, respectively. Structural and computational investigations reveal that the Ge-E' bond (E' = P and As) features a multiple-bond character. 2 and 3 exhibit diverse reactivity toward trimethylsilylacetylene and 4-tetrabutylphenylacetylene. Specifically, 2 underwent cycloadditions with both alkynes affording the first six-membered aromatic phosphagermabenzen-1-ylidenes 4 and 5, respectively, through the heavier isocyanide intermediate M^sFluind^tBu-PGe. In contrast, 3 could serve as a synthetic equivalent of heavier isocyanides and nitriles when treated with trimethylsilylacetylene and 4-tetrabutylphenylacetylene yielding arsagermene 6 and arsolylgermylene 7, respectively. The reaction mechanisms for the cycloadditions were investigated through density functional theory calculations. The reactivity studies highlight the potential of 2 and 3 in accessing heavy main-group element-containing heterocycles.

Bonding and stability of donor ligand-supported heavier analogues of cyanogen halides (L')PSi(X)(L)

Fluoro- and chloro-phosphasilynes [X-Si[triple bond, length as m-dash]P (X = F, Cl)] belong to a class of illusive chemical species which are expected to have Si[triple bond, length as m-dash]P multiple bonds. Theoretical investigations of the bonding and stability of the corresponding Lewis base-stabilized species (L')PSi(X)(L) [L' = cAACMe (cyclic alkyl(amino) carbene); L = cAACMe, NHCMe (N-heterocyclic carbene), PMe3, aAAC (acyclic alkyl(amino) carbene); X = Cl, F] have been studied using the energy decomposition analysis-natural orbitals for chemical valence (EDA-NOCV) method. The variation of the ligands (L) on the Si-atom leads to different bonding scenarios depending on their σ-donation and π-back acceptance properties. The ligands with higher lying HOMOs prefer profoundly different bonding scenarios than the ligands with lower lying HOMOs. The type of halogen (Cl or F) on the Si-atom was also found to have a significant influence on the overall bonding scenario. The reasonably higher value and endergonic nature of the dissociation energies along with the appreciable HOMO-LUMO energy gap may corroborate to the synthetic viability of the homo and heteroleptic ligand-stabilized elusive PSi(Cl/F) species in the laboratory.