N-Heterocyclic silylene

An N-heterocyclic silylene (NHSi) is a neutral heterocyclic chemical compound consisting of a divalent silicon atom bonded to two nitrogen atoms. The isolation of the first stable NHSi, also the first stable dicoordinate silicon compound, was reported in 1994 by Michael Denk and Robert West three years after Anthony Arduengo first isolated an N-heterocyclic carbene, the lighter congener of NHSis. Since their first isolation, NHSis have been synthesized and studied with both saturated and unsaturated central rings ranging in size from 4 to 6 atoms. 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. Though not used outside of academic settings, complexes containing NHSis are known to be competent catalysts for industrially important reactions. This article focuses on the properties and reactivity of five-membered NHSis.

Synthesis and stability

Five-membered N-heterocyclic silylenes are synthesized from two-carbon diimine or diamine precursors. Reaction with metallic lithium yields a lithiated diimide or diamine that when stirred with tetravalent SiCl4 yields a tetravalent dichlorosilicocycle. The compound then is reduced with an alkali metal, or potassium graphite to reveal a divalent silicon center. Care must be taken to not over reduce the silicon; often triethylamine is added to prevent over reduction. The yield for this reduction can be as high as 80%. N-Heterocyclic silylenes containing Silicon(II) with four-membered rings (1 carbon) and six-membered rings (3 carbons) are well known and derived from the reactions of SiX4 with amidinate and NacNac ligands respectively. The silylenes synthesized prior to West and Denk's first NHSi decomposed at temperatures below 77K. 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. 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.

Structure and aromaticity

Structure Five-membered NHSis can be classified by the bonding in the two-carbon backbone of their rings. Saturated NHSis have two methylene units, the unsaturated have doubly-bonded methine carbons, and the benzo-fused NHSis share their carbon backbone with a fused aromatic ring. Though other N substituents such as aryl groups are also well known, in general, studies are conducted with tert-butyl groups bonded the nitrogen of the saturated and unsaturated NHSis, while the benzo-fused have neopentyl groups. X-ray crystallography and electron diffraction show significant differences between the saturated and unsaturated structures. The N-Si-N angle for the saturated tert-butyl substituted NHSi is 92°, slightly puckering C2 symmetric ring. The N-Si-N angle of the unsaturated analog is 90.5° (gas phase XRD) achieving a planar C2v ring. Upon saturation of the carbon backbone, the N-Si bonds shorten from 175.3pm to 171.9pm, shorter distances than would be expected for a single bond between nitrogen and divalent silicon. The longer bond in the unsaturated molecule is due to the nitrogens' lone pair electrons delocalization through the carbon π-bond, resulting in lower Lewis basicity of the nitrogen lone pairs toward the silicon, weakening and lengthening the N-Si bond. 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.

Aromaticity The structural differences, distinct reactivity, and increased stability of the unsaturated ring is attributed the olefinic C=C bond which creates a 6π aromatic ring with the two nitrogen lone pairs. The Si-N bond length discrepancy mentioned above is a strong indication of π delocalization. In proton NMR the vinylic protons resonate at 6.75ppm for [tBuN−CH=CH−tBuN]Si: versus 5.73ppm for the tetravalent dichloride starting material or 6.00ppm for the tetravalent NHSi-H2 dihydride. In the tetravalent species, the lack of the silicon's empty 3p orbital breaks the π ring, causing an upfield shift as the deshielding due to aromatic ring current is lost. 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. The four-coordinate silanes exhibit C=C stretches at 1620 cm−1. Conversely [tBuN−CH=CH−tBuN]Si:'s C=C resonance is found bifurcated at 1566 cm−1 and 1573 cm−1 at a much greater intensity. In the divalent species, electron density is shifted out of the C=C bond into the ring and empty Si 3p orbital, weakening the C=C bond and causing the observed energy drop and shape change in the C=C resonance.

Several computational studies suggest aromatic delocalization over the unsaturated NHSi rings. DFT calculations of NHSi-phosphonyl radical adducts show far greater radical electron delocalization over the rings of unsaturated NHSis (14% Si, 65% ring) than the saturated NHSi (58% Si, 19% ring). Aromaticity is further supported by calculated heats of hydrogenation. At the MP4/6-31G*//6-31G* level, hydrogenation at the carbon backbone of an unsaturated divalent silicon NHSi is 13 kcal/mole more exothermic than the corresponding hydrogenation of the tetracoordinate-silane analog. The stabilization of the divalent silicon ring, with an empty 3p orbital and in-ring lone pair, versus the tetravalent silane which lacks an empty Si 3p orbital indicates significant electronic communication between the silicon and the olefinic carbons. 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. In reference to the perfectly aromatic cyclopentadienyl anion's shift at -14.3ppm, NICS data supports the aromaticity of NHSis.

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