N-heterocyclic silylene

The stability of NHSis, especially 6π aromatic unsaturated five-membered examples, make them useful systems to study the structure and reactivity of silylenes and low-valent main group elements in general.

Alternatively, the original West/Denk NHSi [tBuN−CH=CH−tBuN]Si: is exceptionally stable even after heating for 4 months at 150 °C dissolved in toluene in a sealed NMR tube.

It is also unreactive with Lewis bases and triethylsilane a silylene scavengers, but it does react with air and water (see below) and decomposes at its melting point 220 °C.

[1][3] On the other hand, the saturated NHSi with no backbone substitutions, [tBsN−CH2CH2−tBuN]Si:, is far more reactive as a Lewis acid and far less stable, decomposing at 25 °C.

The overall silylene character is supported by strong downfield shifting in 29Si NMR at +78.3ppm for the unsaturated at +119ppm for the NHSis compared to their tetravalent dichloride starting materials which resonate at -40.7ppm.

[2] Additionally there are significant discrepancies in the Raman spectroscopy signals between the unsaturated NHSi and tetravalent silicon analogues, such as NHSi-Cl2 and spiro-[tBuN−CH=CH−tBuN]2Si.

[5] Additionally, calculations of Nuclear Independent Chemical Shift of the ring-centered NMR resonances of [HN-CH=CH-NH]Si: at -10.2ppm indicate a high degree of delocalization about the unsaturated NHSi ring.

[8] Computational studies and photoelectron spectroscopy create a more nuanced picture of electronic structure and π-electron sharing in NHSis.

Natural Bond Orbital analysis of [HN-CH=CH-NH]Si: provides strong theoretical evidence of π electron delocalization.

Interestingly, for all three Group 11 metals, the NHSi in question is a better σ donor and π acceptor than its germylene and carbene analogs.

In concentrated solutions, the saturated unsubstituted silylene [tBuN−CH2CH2−tBuN]Si: reacts with itself to form a tetramer with a Si-Si double bond.

Additionally a double bond between tert-butyl substituted NHSis is calculated to not be stable, further corroborating the identity of the mixed valent dimer as the intermediate.

[12] Similarly, the unsubstituted NHSis will react with analogous N heterocyclic germylenes, ultimately forming a silicon-flanked germene.

[3] Reacting NHSis with oxygen strips the diimine backbone, and produces solid silicon dioxide, likely through a two NHSi dioxo-bridged intermediate.

[13] NHSis insert into σ bonds to form terminal silane-like products, NHSi-NHSi disilane bridges, or oxo-bridged NHSi siloxanes.

Upon heating, the disilanes either disproportionate to an NHSilane and NHSilylene, or form a one-carbon bridge between two NHSi-X fragments in a vein similar to water's reactivity.

[14][4] Generally, when reacting with unsaturated species an NHSi will add across a double or triple bond to form a spirocyclic compound, containing one or two equivalents of NHSi-derived silicon.

Alkenes and alkynes react with NHSis only if their C-C bond is highly polarized such as is the case with phenyltrimethylsilylacetylene, which produces a silicocyclopropene with one NHSi equivalent or a disilicocyclobutene with two.

With ketones two equivalents of benzo-fused NHSi react to form disilyloxetanes as well as further reaction products depending on the carbon substituents.

Dienes, as well as heterodienes including benzil or the common NHSi backbone N,N'-diterbutyl-1,4diazabutadiene ([tBuN-CH=CH-tBuN]) react in a [4+1] fashion to yield molecules with two five-membered rings sharing a single silicon atom.

With the Lewis acid tris(pentafluorophenyl)borane, the expected acid/base adduct forms, however, over long periods of time, the silicon will insert between a B-C bond, oxidizing the silyene to a silane.

Reaction of the N,N' dineopentyl beznofused NHSi with the analogous carbene, creates a long, highly polarized C-Si bond.

[2] Both the saturated and unsaturated five-membered NHSis are calculated to have substantial singlet-triplet energy gaps, at 74 kcal/mol and 69 kcal/mol respectively for the hydride N-substituted models.

The orbitals of the triethylphosphine substitution product are unable to form π interaction as exhibited by a 29Si NMR signal of 139.3ppm which indicates silicenium (Si+) character, while the molybdosilane has a resonance at 43.6ppm.

[22][4] Beyond being interesting models to study the structure and bonding of low valent silicon, NHSis transition metal complexes have been demonstrated as active catalysts in industrially important reactions.

With 1% catalyst loading at 140 °C the Pd(II) silylene complex catalyzes the coupling of p-bromoacetophenone and styrene in four hours in quantitative 99%+ yield.

Under the same conditions the corresponding NHC complex yields only 91% product, showing that there are instances where the electronics or stearics do make silylenes comparatively superior catalysts.