Silicon(II) Coordination Chemistry: N‐Heterocyclic Carbene Complexes of Si²⁺ and SiI⁺

Abstract

Si 2+ ions are highly reactive, two-valence-electron species that have been generated in laser-induced plasmas and studied by photoabsorption and microwave spectroscopy. [1] The ions have also been employed as implants for waveguide fabrication in lithium niobate crystals, which are of great interest for various photonic applications owing to their physical and electro-optical properties. [2] A possible method of trapping these highly electrophilic ions in the condensed phase, may involve complexation by three strongly basic, neutral ligands (L) to give dications of the general formula [SiL 3 ] 2+ , in which the silicon center attains a noble gas configuration. However, dicationic silicon(II) complexes are presently unknown, in marked contrast to the few germanium homologues, which were reported some years ago by K. M. Baines et al. [3] This difference can be explained by the anomalous low electronegativity of silicon versus germanium, [4] which makes the isolation of silicon(II)-centered dications a highly challenging goal. Even monocationic silicon(II) compounds are very rare, and only the nido-type cluster cation [(C 5 Me 5 )Si] + , [5] [6] and the Si II cation [{C 10 H 6a,a-NP(nBu) 3 }SiCl] + stabilized by a chelating bis(iminophosphorane) ligand [7] have been isolated thus far.Since the first report of the silicon(0) compound [Si 2 -(Idipp) 2 ] (Idipp = 1,3-bis(2,6-diisopropylphenyl)-imidazol-2ylidene) by G. Robinson et al. in 2008, [8] N-heterocyclic carbenes have been shown to be particularly effective ligands for the stabilization of silicon compounds in unusually low oxidation states. Remarkable examples include NHC adducts of the dihalosilylenes SiX 2 (X = Cl, Br), [9] which are very valuable precursors in Si II chemistry, [9c, 10] or NHC adducts of the organohalosilylenes RSiCl (R = m-terphenyl, (2,6-diisopropylphenyl)(trimethylsilyl)amino), [11] which paved the way for the preparation of the first complexes featuring metalsilicon triple bonds. [12] Herein, the exchange of NHC ligands at Si II centers is illustrated for the first time to provide access to unprecedented dicationic NHC complexes of silicon(II) and NHC adducts of the iodosilyliumylidene cation SiI + .The entry into this chemistry started with the triiodosilylimidazolium salt 1 (Scheme 1), which was obtained from the reaction of SiI 4 with Idipp in toluene and isolated as a yellow, thermally robust solid in 96 % yield. Reduction of 1 with potassium graphite (2.3 equiv) in benzene afforded the yellow NHC-diiodosilylene adduct 2-I in 81 % yield (Scheme 1). [13] Under rigorous exclusion of air, the silicon(II) compound 2-I is stable in benzene or toluene solution at ambient temperature for several days, and decomposes in the solid state upon heating above 160 8C.The solid-state structure of 1·3 (CHCl 3 ) was determined by single-crystal X-ray crystallography. [13] It resembles that of [SiBr 3 (Idipp)]Br·3 (CH 2 Cl 2 ) [9b] and reveals that the chloroform trissolvate of 1 is composed of well separated [SiI 3 -(Idipp)] + cations and i...

SiP Double Bonds: Experimental and Theoretical Study of an NHC‐Stabilized Phosphasilenylidene

“…Deconvolution of the observed bands, backed up by TDDFT calculations, allowed a full assignment of these bands. [36] Interestingly, the energy required for the SiÀC NHC bond dissociation (D8(0) = 120 kJ mol À1 ) to give IDipp and the phosphasilenylidene (Si= PMes*) (D, Figure 1) compares well with those of SiX 2 -(IDipp) (X = Cl, Br, I: D8(0) = 121-124 kJ mol À1 ) [13] suggesting that 1 might act as a phosphasilenylidene transfer reagent in the presence of a suitable IDipp trapping agent. LUMO(p*(Si=P)) transition.…”

“…The 31 P NMR signal of 1 appears at even lower field than the most deshielded 31 P NMR signal observed so far for a phosphasilene (d( 31 P) of (tBu 2 MeSi) 2 Si=PMes* in C 6 D 6 = 389.3 ppm, 1 J(P,Si) = 171.3 Hz), [28j, 30] but at higher field than that of the diphosphene P 2 Mes* 2 (d( 31 P) in C 6 D 6 = 494.2 ppm). [21] The chemical shift tensor components d ii derived from the solid-state MAS 31 P{ 1 H} spectrum of 1 reveal a large chemical shift anisotropy with a span Dd (d 11 Àd 33 ) of [42] Ellipsoids are set at 30 % probability; hydrogen atoms and the solvent are omitted for clarity. [21] The chemical shift tensor components d ii derived from the solid-state MAS 31 P{ 1 H} spectrum of 1 reveal a large chemical shift anisotropy with a span Dd (d 11 Àd 33 ) of [42] Ellipsoids are set at 30 % probability; hydrogen atoms and the solvent are omitted for clarity.…”

Synthetic chemistry with nitrous oxide

“…Due to the very inert character of N 2 O, only highly reactive compounds are able to react with N 2 O under mild conditions. 17 It is worth noting that a metallosilylene 18 and an osmium silylene complex 19 were also found to react with N 2 O. For example, it is possible to perform a solutionbased oxidation of cyclohexene and cyclopentene to the corresponding cyclic ketones, but temperatures above 200 1C and elevated pressures (425 bar) are needed to achieve good conversions.…”

Efficient Synthesis of a NHC‐Coordinated Trisilacyclopropylidene and Its Coordination Behavior

“…[2] The same NHC was also found to coordinate to SiX 2 (X = Cl, Br,I ) units by Roesky and Filippou providing access to dihalogeno-Si II precursors. [5] Driess reported on a cyclic bis-NHC stabilized silicon compound with only one silicon atom in the formalo xidations tate zero coordinated by the ligandt hat was named silylone. [5] Driess reported on a cyclic bis-NHC stabilized silicon compound with only one silicon atom in the formalo xidations tate zero coordinated by the ligandt hat was named silylone.…”