Publications

We report a broad theoretical study on [(PMe₃)₃MER]⁺ complexes, with M = Ni, Pd, Pt, E = C, Si, Ge, Sn, Pb, and R = Ar^Mes, Tbb, (Ar^Mes = 2,6-dimesitylphenyl; Tbb = C₆H₂-2,6-[CH(SiMe₃)₂]₂-4-^tBu). A few years ago, our group succeeded in obtaining heavier homologues of cationic group 10 carbyne complexes via halide abstraction of the tetrylidene complexes [(PMe₃)₃M=E(X)R] (X = Cl, Br) using a halide scavenger. The electronic structure and the M-E bonds of the [(PMe₃)₃MER]⁺ complexes were analyzed utilizing quantum-chemical tools, such as the Pipek-Mezey orbital localization method, the energy decomposition analysis (EDA), and the extended-transition state method with natural orbitals of chemical valence (ETS-NOCV). The carbyne, silylidyne complexes, and the germylidyne complex [(PMe₃)₃NiGeAr^Mes]⁺ are suggested to be tetrylidyne complexes featuring donor-acceptor metal tetrel triple bonds, which are composed of two strong π(M→E) and one weaker σ(E→M) interaction. In comparison, the complexes with M = Pd, Pt; E = Sn, Pb; and R = Ar^Mes are best described as metallotetrylenes and exhibit considerable M-E-C bending, a strong σ(M→E) bond, weakened M-E π-components, and lone pair density at the tetrel atoms. Furthermore, bond cleavage energy (BCE) and bond dissociation energy (BDE) reveal preferred splitting into [M(PMe₃)₃]⁺ and [ER] fragments for most complex cations in the range of 293.3-618.3 kJ·mol⁻¹ and 230.4-461.6 kJ·mol⁻¹, respectively. Finally, an extensive study of the potential energy hypersurface varying the M-E-C angle indicates the presence of isomers with M-E-C bond angles of around 95°. Interestingly, these isomers are energetically favored for M = Pd, Pt; E = Sn, Pb; and R = Ar^Mes over the less-bent structures by 13-29 kJ·mol⁻¹.

L. R. Maurer, J. Rump, A. C. Filippou, Inorganics, 2023, 11, 3, 129.

Experimental and theoretical studies are reported of the first two-coordinated Si⁰-isocyanide compound (SIDipp)Si═C═N–Ar^Mes (1: SIDipp (NHC) = C[N(Dipp)CH₂]₂, Ar^Mes = 2,6-dimesitylphenyl), supported by an N-heterocyclic carbene (NHC). A Si atom economic two-step synthesis of 1 involves a 2e reduction of the isocyanide-stabilized silyliumylidene salt [SiBr(CNAr^Mes)(SIDipp)][B(Ar^F)₄] (2[B(Ar^F)₄], Ar^F = B(C₆H₃-3,5-(CF₃)₂)₄) with KC₈. 2[B(Ar^F)₄] was obtained from SiBr₂(SIDipp) after bromide abstraction with an equimolar mixture of Na[B(Ar^F)₄] and Ar^MesNC. Exact adherence to the stoichiometry is crucial in the latter reaction, since 2[B(Ar^F)₄] reacts with SiBr₂(SIDipp) via isocyanide exchange to afford the disilicon(II) salt [Si₂Br₃(SIDipp)₂)][B(Ar^F)₄] (3[B(Ar^F)₄]), the reaction leading to an equilibrium that favors 3[B(Ar^F)₄] (K_eq(298 K) = 10.6, ΔH° = −10.6 kJ mol^–1; ΔS° = −16.0 J mol^–1 K^–1). 3[B(Ar^F)₄] was obtained selectively from the 2:1 reaction of SiBr₂(SIDipp) with Na[B(Ar^F)₄] and fully characterized. Detailed studies of 1 reveal an intriguing structure featuring a planar C_NHC–Si–C–N skeleton with a V-shaped geometry at the dicoordinated Si⁰ center, a slightly bent Si═C═N core, a C_NHC–Si–C_CNR 3c-2e out of plane π-bond (HOMO), and an anticlinal conformation of the SIDipp and Ar^Mes substituents leading to axial chirality and the presence of two enantiomers, (R_a)-1 and (S_a)-1. Compound 1 displays structural dynamics in solution, rapidly interconverting the enantiomers. The silacumulene 1 is a potent Si(SIDipp) transfer agent as demonstrated by the synthesis and full characterization of the NHC-supported germasilyne (Z)-(SIDipp)(Cl)Si═GeAr^Mes (4) from 1 and Ge(Ar^Mes)Cl.

S. Karwasara, L. R. Maurer, B. Peerless, G. Schnakenburg, U. Das, A. C. Filippou, J. Am. Chem. Soc. 2021, 143, 36, 14780–14794.

Highlight: Chemistry Views, 2021

While a variety of compounds containing planar tetracoordinated carbon (ptC), the so-called anti-van’t Hoff/Le Bel carbon, are known experimentally, stable systems containing planar tetracoordinated silicon (ptSi) are barely known. As part of our studies on the application of stereoelectronically well-defined transition-metal fragments to stabilize silicon in unprecedented bonding modes, we report herein the synthesis and full characterization of a series of thermally stable complexes of the general formula [Tp′(CO)₂MSiC(R¹)C(R²)M(CO)₂Tp′] (M = Mo, W; R¹ = R² = Me or R¹ = H, R² = SiMe₃, Ph; Tp′ = κ³-N,N′,N″-hydridotris(3,5-dimethylpyrazolyl)borate), which incorporate a ptSi atom in addition to two ptC atoms. The complexes were obtained by reacting the metallasilylidyne complexes [Tp′(CO)₂M≡Si–M(CO)₂(PMe₃)Tp′] with alkynes R¹C≡CR² and were comprehensively analyzed by experimental studies and quantum chemical calculations. The analyses revealed that the ptSi atom is embedded in a tricyclic trapezoidal core featuring one internal SiC₂ and two outer M–Si–C three-membered rings, which are fused via two Si–C bonds. The structural peculiarities evoked by the presence of an anti-van’t Hoff/Le Bel ptSi center, such as the short M–Si bonds, a nearly linear M–Si–M spine, long M–C bonds, and the presence of two planar tetracoordinated carbon atoms were elucidated by a detailed analysis of the electronic structure, suggesting that one factor for the stabilization of the ptSi geometry is the aromaticity of the central SiC₂ ring having two delocalized π electrons. Remarkably, the results further indicate the existence of both anti-van’t Hoff/Le Bel carbon and silicon centers next to each other in the isolated complexes.

P. Ghana, J. Rump, G. Schnakenburg, M. I. Arz, A. C. Filippou, J. Am. Chem. Soc. 2021, 143, 1, 420–432.

Highlight: Universität Bonn 2020.

Highlight: Chemistry World 2021.

A detailed experimental and theoretical analysis is presented of unprecedented molybdenum complexes featuring a linearly coordinated, multiply bonded silicon atom. Reaction of SiBr₂(SIdipp) (SIdipp = C[N(C₆H₃-2,6-iPr₂)CH₂]₂) with Na[Tp′Mo(CO)₂(PMe₃)] (Na-1) in the ratio 1:2 afforded the reddish-brown metallasilylidyne complex [Tp′(CO)₂Mo≡Si—Mo(CO)₂(PMe₃)Tp′] (Tp′ = κ³-N,N′,N″-hydridotris(3,5-dimethylpyrazolyl)borate) (2), in which an almost linearly coordinated silicon atom (∠(Mo1–Si–Mo2) = 162.93(7)°) is bridging the 15VE metal fragment Tp′Mo(CO)₂ with the 17VE metal fragment Tp′Mo(CO)₂(PMe₃) via a short Mo1–Si bond (2.287(2) Å) and a considerably longer Mo2–Si bond (2.438(2) Å), respectively. The reddish-orange silylidyne complex [Tp′(CO)₂Mo≡Si—Tbb] (3) was also prepared from Na-1 and the 1,2-dibromodisilene (E)-Tbb(Br)Si═Si(Br)Tbb (Tbb = C₆H₂-2,6-[CH(SiMe₃)₂]₂-4-tBu) and contains as 2 a short Mo–Si bond (2.2614(9) Å) to an almost linearly coordinated Si atom (∠(Mo–Si–C_Tbb) = 160.8(1)°). Cyclic voltammetric studies of 2 in diglyme revealed an irreversible reduction of 2 at −1.907 V vs the [Fe(η⁵-C₅Me₅)₂]^+/0 redox couple. Two-electron reduction of 2 with potassium graphite yielded selectively the 1,3-dimetalla-2-silaallene dianion [Tp′(CO)₂Mo═Si═Mo(CO)₂Tp′]^2– (4^2–), which was isolated as the bright yellow dipotassium salt [K(diglyme)]₂-4. Single crystal X-ray diffraction analysis revealed a centrosymmetric structure of 4^2–. The Mo–Si bond length of 4^2– (2.3494(2) Å) compares well with those of Mo–Si double bonds and lies in-between the Mo1–Si triple bond and Mo2–Si single bond length of 2. Compounds 2, 3 and [K(diglyme)₂]-4 were characterized by elemental analyses, IR and multinuclear NMR spectroscopy. Comparative ELF (electron localization function), NBO (natural bond orbital) and NRT (natural resonance theory) analyses of 2, 3 and 4^2– shed light into the electronic structures of these compounds. 

P. Ghana, M. I. Arz, U. Chakraborty, G. Schnakenburg, A. C. Filippou, J. Am. Chem. Soc. 2018, 140, 7187.

A novel method to prepare SiCp*₂ (1) (Cp* = C₅Me₅) is presented involving the reaction of N-heterocyclic carbene (NHC)-stabilized Si^II halides SiX₂(NHC) (X = Cl, Br) with 2 equiv of KCp*. This route is a superior alternative to the laborious, multistep synthesis of 1 presently known and provides via the selective protonolysis of 1 with [H(Et₂O)₂][B(C₆F₅)₄] facile access to [Si(η⁵-Cp*)][B(C₆F₅)₄] (2) in multigram scale and high yield. Reaction of 2 with the tailored metallate Na[Tp^MeMo(CO)₂(PMe₃)] afforded the unprecedented silylidyne complex [Tp^Me(CO)₂MoSi(η³-Cp*)] (3; Tp^Me = κ³-N,N′,N″-hydridotris(3,5-dimethyl-1-pyrazolyl)borate). Single crystal X-ray diffraction and spectroscopic studies in combination with quantum chemical calculations shed light on the unique bonding situation of 3 featuring a delocalized Mo–Si bond with partial triple bond character and a Cp* substituent, which is η³-coordinated to silicon.

P. Ghana, M. I. Arz, G. Schnakenburg, M. Straßmann, A. C. Filippou, Organometallics, 2018, 37, 772.

A systematic, efficient approach to first complexes containing a triple bond between niobium and the elements silicon, germanium or tin is reported. The approach involves a metathetical exchange of the niobium-centered nucleophile (NMe₄)[Nb(CO)₄(κ²-tmps)] (1) (tmps = MeSi(CH₂PMe₂)₃) with a suitable organotetrel(II)halide. Compound 1 was obtained from (NMe₄)[Nb(CO)₆] and the triphosphane tmps by photodecarbonylation. Reaction of 1 with the disilene E-Tbb(Br)Si=Si(Br)Tbb in the presence of 4-dimethylaminopyridine afforded selectively the red-brown silylidyne complex [(κ³-tmps)(CO)₂Nb≡Si–Tbb] (2-Si, Tbb = 4-tert-butyl-2,6-bis(bis(trimethylsilyl)methyl)phenyl). Similarly, treatment of 1 with E(Ar^Mes)Cl (E = Ge, Sn; Ar^Mes = 2,6-mesitylphenyl) afforded after elimination of (NMe₄)Cl and two CO ligands the deep magenta colored germylidyne complex [(κ³-tmps)(CO)₂Nb≡Ge–Ar^Mes] (3-Ge), and the deep violet, light-sensitive stannylidyne complex [(κ³-tmps)(CO)₂Nb≡Sn–Ar^Mes] (3-Sn), respectively. Formation of 3-Sn proceeds via the niobiastannylene [(κ³-tmps)(CO)₃Nb–SnAr^Mes] (4-Sn), which was detected by IR and NMR spectroscopy. The niobium tetrylidyne complexes 2-Si, 3-Ge and 3-Sn were fully characterized and their solid-state structures determined by single-crystal X-ray diffraction studies. All complexes feature an almost linear tetrel coordination and the shortest Nb–E bond lengths (d(Nb–Si) = 232.7(2) pm; d(Nb–Ge) = 235.79(4) pm; d(Nb–Sn) = 253.3(1) pm) reported to date. Reaction of 3-Ge with a large excess of H₂O afforded upon cleavage of the Nb–Ge triple bond the hydridogermanediol Ge(Ar^Mes)H(OH)₂. Photodecarbonylation of [CpNb(CO)₄] (Cp = η⁵-C₅H₅) in the presence of Ge(Ar^Mes)Cl afforded the red-orange chlorogermylidene complex [Cp(CO)₃Nb=Ge(Ar^Mes)Cl] (5-Ge). The molecular structure of 5-Ge features an upright conformation of the germylidene ligand, a trigonal–planar coordinated Ge atom, and a Nb–Ge double bond length of 251.78(6) pm, which lies in-between the Nb–Ge triple bond length of 3-Ge (235.79(4) pm) and a Nb–Ge single bond length (267.3 pm). Cyclic voltammetric studies of 2-Si, 3-Ge, and 3-Sn reveal several electron-transfer steps. One-electron oxidation and reduction of the germylidyne complex of 3-Ge in THF are electrochemically reversible suggesting that both the radical cation and radical anion of 3-Ge are accessible species in solution.

A. C. Filippou, D. Hoffmann, G. Schnakenburg, Chem. Sci. 2017, 8, 6290.

The eminent role of metallacyclobutadienes as catalytic intermediates in organic synthesis and polymer chemistry is widely acknowledged. In contrast, their photochemistry is as yet entirely unexplored. Herein, the photo-induced primary processes of a ferracyclobutadiene tricarbonyl complex in solution are revealed by femtosecond mid-infrared spectroscopy. The time-resolved vibrational spectra expose an ultrafast substitution of a basal CO ligand by a solvent molecule in a consecutive dissociation–association mechanism. Following optical excitation, the system relaxes non-radiatively to the triplet ground state from which a CO is expelled. Since the triplet state is bound with respect to Fe−CO cleavage, the dissociation can only occur from vibrationally excited states. The excitation energy, vibrational relaxation, and intersystem crossing to the singlet ground state control the primary quantum yield for formation of the ferracyclic dicarbonyl–solvent product complex.

B. Wezisla, J. Lindner, U.Das, A. C. Filippou, P. Vöhringer, Angew. Chem. Int. Ed. 2017, 56, 6901.

The synthesis and full characterization of the NHC-stabilized tungstenochlorostannylene [Cp*(CO)₃W–SnCl(Idipp)] (1Sn), the NHC-stabilized chlorogermylidyne complex [Cp*(CO)₂W═GeCl(Idipp)] (2), the tungsten germylidyne complex salt [Cp*(CO)₂W≡Ge(Idipp)][B(C₆H₃-3,5-(CF₃)₂)₄] (3), and the cationic metallostannylene [Cp*(CO)₃W–Sn(Idipp)][Al(OC(CF₃)₃)₄] (4Sn) are reported (Idipp = 2,3-dihydro-1,3-bis(2,6-diisopropylphenyl)-1H-imidazol-2-ylidene, Cp* = η⁵-C₅Me₅). Metathetical exchange of SnCl₂(Idipp) with Li[Cp*W(CO)₃] afforded selectively 1Sn. Photolytic decarbonylation of the Ge analogue [Cp*(CO)₃W–GeCl(Idipp)] (1Ge) afforded the NHC-stabilized chlorogermylidyne complex (2), featuring a trigonal-planar coordinated germanium center and a W–Ge double bond (W–Ge 2.3496(5) Å). Chloride abstraction from 2 with Na[B(C₆H₃-3,5-(CF₃)₂)₄] yielded the germylidyne complex salt 3, which contains an almost linear W–Ge–C1 linkage (angle at Ge = 168.7(1)°) and a W–Ge triple bond (2.2813(4) Å). Chloride elimination from 1Ge afforded the tungstenogermylene salt [Cp*(CO)₃W–Ge(Idipp)][B(C₆H₃-3,5-(CF₃)₂)₄] (4Ge), which in contrast to 1Ge could not be decarbonylated to form 3 despite the less strongly bound carbonyl ligands. The tin compounds 1Sn and 4Sn did not afford products bearing multiple W–Sn bonds. Treatment of 4Ge with Me₂NC≡CNMe₂ yielded unexpectedly the neutral germyl complex 5 containing a pendant 1-germabicyclo-[3,2,0]-hepta-2,5-diene ligand instead of the anticipated [2 + 1]-cycloaddition product at the Ge-center.

Y. N. Lebedev, U. Das, G. Schnakenburg, A. C. Filippou, Organometallics 2017, 36, 1530.

Detailed studies of the reactivity of the NHC-stabilized (NHC = N-heterocyclic carbene) silicon(II) halides SiI₂(Idipp) (1) and SiClR(IMe₄) (2-Trip: R = Ar^Trip; 2-Mes: R = Ar^Mes) towards diazoalkanes and azides are presented (Idipp = C[N(C₆H₃–2,6-iPr₂)CH]₂, IMe4 = C[N(Me)CMe]₂, Ar^Trip = C₆H₃-2,6-Trip₂, Ar^Mes = C₆H₃-2,6-Mes₂, Trip = C₆H₂-2,4,6-iPr₃, Mes = C₆H₂-2,4,6-Me₃). Treatment of 1, 2-Trip, and 2-Mes with the diazoalkanes (p-Tol)₂CN₂ (p-Tol = C₆H₄-4-Me) and Ar^MesCHN₂ afforded the NHC-stabilized silazines SiI₂{N₂C(p-Tol)₂}(Idipp) (3) and SiClR{N₂CH(Ar^Mes)}(IMe₄) (4-Trip: R = Ar^Trip, 4-Mes: R = Ar^Mes) as orange to yellow solids, respectively. No N₂ elimination from the diazoalkanes was observed in these reactions. In comparison, the reactions of 1 or 2-Trip with the covalent azides MesN₃ or Me₃SiN₃, yielded after N₂ elimination the yellow to colorless NHC-stabilized silaimines SiI₂(NMes)(Idipp) (5), Si(Ar^Trip)Cl(NMes)(IMe₄) (6) and Si(Ar^Trip)(N₃)(NSiMe₃)(IMe₄) (7), respectively. All compounds were characterized by elemental analyses, FT-IR and multinuclear magnetic resonance spectroscopy. Moreover, the molecular structures of 4-Mes, 5, 6, and 7 were determined by single-crystal X-ray diffraction analyses.

M. I. Arz, D. Hoffmann, G. Schnakenburg, A. C. Filippou Z. Anorg. Allg. Chem. 2016, 642, 1287.

Cyclic voltammetric studies of the hydridodisilicon(0,II) borate [(Idipp)(H)Si^II=Si⁰(Idipp)][B(Ar^F)₄] (1H[B(Ar^F)₄], Idipp = C[N(C₆H₃-2,6-iPr₂)CH]₂, Ar^F = C₆H₃-3,5-(CF₃)₂) reveal a reversible one-electron reduction at a low redox potential (E_1/2 = −2.15 V vs. Fc⁺/Fc). Chemical reduction of 1H[B(Ar^F)₄] with KC₈ affords selectively the green, room-temperature stable mixed-valent disilicon(0,I) hydride Si₂(H)(Idipp)₂ (1H), in which the highly reactive Si₂H molecule is trapped between two N-heterocyclic carbenes (NHCs). The molecular and electronic structure of 1H was investigated by a combination of experimental and theoretical methods and reveals the presence of a π-type radical featuring a terminal bonded H atom at a flattened trigonal pyramidal coordinated Si center, that is connected via a Si–Si bond to a bent two-coordinated Si center carrying a lone pair of electrons. The unpaired electron occupies the Si=Si π* orbital leading to a formal Si–Si bond order of 1.5. Extensive delocalization of the spin density occurs via conjugation with the coplanar arranged NHC rings with the higher spin density lying on the site of the two-coordinated silicon atom.

M. I. Arz, G. Schnakenburg, A. Meyer, O. Schiemann, A. C. Filippou Chem. Sci. 2016, 7, 4973.

Protonation and alkylation of (Idipp)Si═Si(Idipp) (1) afforded the mixed-valent disilicon(I)-borates [(Idipp)(R)Si^II═Si⁰(Idipp)][B(Ar^F)₄] (1R[B(Ar^F)₄]; R = H, Me, Et; Ar^F = C₆H₃-3,5-(CF₃)₂; Idipp = C[N(C₆H₃-2,6-iPr₂)CH]₂) as red to orange colored, highly air-sensitive solids, which were characterized by single-crystal X-ray diffraction, IR spectroscopy and multinuclear NMR spectroscopy. Dynamic NMR studies in solution revealed a degenerate isomerization (topomerization) of the “σ-bonded” tautomers of 1H[B(Ar^F)₄], which proceeds according to quantum chemical calculations via a NHC-stabilized (NHC = N-heterocyclic carbene) disilahydronium ion (“π-bonded” isomer) and is reminiscent of the degenerate rearrangement of carbenium ions formed upon protonation of olefins. The topomerization of 1H[B(Ar^F)₄] provides the first example of a reversible 1,2-H migration along a Si═Si bond observed in a molecular system. In contrast, 1Me[B(Ar^F)₄] adopts a “rigid” structure in solution due to the higher energy required for the interconversion of the “σ-bonded” isomer into a putative NHC-stabilized disilamethonium ion. Addition of alkali metal borates to 1 afforded the alkali metal disilicon(0) borates 1M[BAr₄] (M = Li, Ar = C₆F₅; M = Na, Ar = Ar^F) as brown, air-sensitive solids. Single-crystal X-ray diffraction analyses and NMR spectroscopic studies of 1M[BAr₄] suggest in concert with quantum chemical calculations that encapsulation of the alkali metal cations in the cavity of 1 predominantly occurs via electrostatic cation−π interactions with the Si═Si π-bond and the peripheral NHC aryl rings. Displacement of the [Si(NHC)] fragments by the isolobal fragments [PR] and [SiR]⁻ interrelates the cations [(NHC)(R)Si═Si(NHC)]⁺ to a series of familiar, multiply bonded Si and P compounds as verified by analyses of their electronic structures.

M. I. Arz, M. Straßmann, D. Geiß, G. Schnakenburg, A. C. Filippou J. Am. Chem. Soc. 2016, 138, 4589.

Flash photolysis combined with step-scan and rapid-scan Fourier-transform infrared spectroscopy was carried out to explore the photochemistry of a puckered, quasi-square pyramidal ferracyclobutadiene, [Fe{κ²-C₃(NEt₂)₃}(CO)₃]BF₄ ([1]BF₄), that features three additional carbonyl ligands in the metal coordination sphere. In liquid solution at room temperature, an excitation with λ=355 nm light resulted in the loss of one CO ligand, which is cleaved from a basal metal-coordination site. Within the time resolution of the experiment, a solvent molecule promptly refills the resultant vacancy at the coordinatively unsaturated metal center. In the weakly interacting liquid, dichloromethane, the counterion of the complex is subsequently able to substitute the solvent in the coordination sphere of the iron center, thereby forming as a stable product a neutral dicarbonyl tetrafluoroborato iron(0) species containing a four-membered ferracycle.

J. Torres-Alacan, U. Das, B. Wezisla, M. Straßmann, A. C. Filippou, P. Vöhringer Chem. Eur. J. 2015, 21, 17184.

An efficient method for the synthesis of the NHC-stabilised Si(I) halides Si₂X₂(Idipp)₂ (2-X, X = Cl, Br, I; Idipp = C[N(C₆H₃-2,6-iPr₂)CH]₂) was developed, which involves the oxidation of Si₂(Idipp)₂ (1) with 1,2-dihaloethanes. Halogenation of 1 is a diastereoselective reaction leading exclusively to a racemic mixture of the RR and SS enantiomers of 2-X. Compounds 2-Br and 2-I were characterised by single-crystal X-ray crystallography and multinuclear NMR spectroscopy, and their electronic structures were analysed by quantum chemical methods. Dynamic NMR spectroscopy unraveled a fluxional process of 2-Br and 2-I in solution, which involved a hindered rotation of the NHC groups about the Si–C_NHC bonds. Iodide abstraction from 2-I by [Li(Et₂O)_2.5][B(C₆F₅)₄] selectively afforded the disilicon(I) salt [Si₂(I)(Idipp)₂][B(C₆F₅)₄] (3). X-ray crystallography and variable-temperature NMR spectroscopy of 3 in combination with quantum chemical calculations shed light on the ground-state geometric and electronic structure of the [Si₂(I)(Idipp)₂]⁺ ion, which features a Si=Si bond between a trigonal planar coordinated Si^II atom with a Si–I bond and a two-coordinate Si⁰ center carrying a lone pair of electrons. The dynamics of the [Si₂(I)(Idipp)₂]⁺ ion were studied in solution by variable-temperature NMR spectroscopy and they involve a topomerisation, which proceeds according to quantum theory via a disilaiodonium intermediate (“π-bonded” isomer) and exchanges the two heterotopic Si sites.

M. I. Arz, D. Geiß, M. Straßmann, G. Schnakenburg, A. Filippou Chem. Sci. 2015, 6, 6515.

One-electron oxidation of the disilicon(0) compound Si₂(Idipp)₂ (1, Idipp=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) with [Fe(C₅Me₅)₂][B(Ar^F)₄] (Ar^F=C₆H₃-3,5-(CF₃)₂) affords selectively the green radical salt [Si₂(Idipp)₂][B(Ar^F)₄] (1-[B(Ar^F)₄). Oxidation of the centrosymmetric 1 occurs reversibly at a low redox potential (E_1/2=−1.250 V vs. Fc⁺/Fc), and is accompanied by considerable structural changes as shown by single-crystal X-ray structural analysis of 1-B(Ar^F)₄. These include a shortening of the Si-Si bond, a widening of the Si-Si-C_NHC angles, and a lowering of the symmetry, leading to a quite different conformation of the NHC substituents at the two inequivalent Si sites in 1⁺. Comparative quantum chemical calculations of 1 and 1⁺ indicate that electron ejection occurs from the symmetric (n₊) combination of the Si lone pairs (HOMO). EPR studies of 1-B(Ar^F)₄ in frozen solution verified the inequivalency of the two Si sites observed in the solid-state, and point in agreement with the theoretical results to an almost equal distribution of the spin density over the two Si atoms, leading to quite similar ²⁹Si hyperfine coupling tensors in 1⁺. EPR studies of 1-B(Ar^F)₄ in liquid solution unraveled a topomerization with a low activation barrier that interconverts the two Si sites in 1⁺.

M. I. Arz, M. Straßmann, A. Meyer, G. Schnakenburg, O. Schiemann, A. Filippou Chem. Eur. J. 2015, 21, 12509.

An efficient two-step synthesis of the first NHC-stabilized disilavinylidene (Z)-(SIdipp)Si=Si(Br)Tbb (2; SIdipp=C[N(C₆H₃-2,6-iPr₂)CH₂]₂, Tbb=C₆H₂-2,6-[CH(SiMe₃)₂]₂-4-tBu, NHC=N-heterocyclic carbene) is reported. The first step of the procedure involved a 2:1 reaction of SiBr₂(SIdipp) with the 1,2-dibromodisilene (E)-Tbb(Br)Si=Si(Br)Tbb at 100 °C, which afforded selectively an unprecedented NHC-stabilized bromo(silyl)silylene, namely SiBr(SiBr₂Tbb)(SIdipp) (1). Alternatively, compound 1 could be obtained from the 2:1 reaction of SiBr₂(SIdipp) with LiTbb at low temperature. 1 was then selectively reduced with C₈K to give the NHC-stabilized disilavinylidene 2. Both low-valent silicon compounds were comprehensively characterized by X-ray diffraction analysis, multinuclear NMR spectroscopy, and elemental analyses. Additionally, the electronic structure of 2 was studied by various quantum-chemical methods.

P. Ghana, M. I. Arz, U. Das, G. Schnakenburg, A. C. Filippou Angew. Chem. Int. Ed. 2015, 54, 9980.

A novel method for the synthesis of 1H-siloles is presented. It involves a [2+2+1] cycloaddition of the ynediamines R₂N—C≡C—NR₂ (R=Me, Et) with SiI₂(Idip) (Idip=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) to afford the orange-colored, highly water-sensitive 1,1-diiodo-2,3,4,5-tetraamino-1H-siloles SiI₂{C₄(NR₂)₄} (1-I: R=Me; 2-I R=Et). Treatment of 2-I with an excess of SiBr₄ afforded after I/Br exchange the 1,1-dibromo-1H-silole SiBr₂{C₄(NEt₂)₄} (2-Br). The 1H-siloles 1-I, 2-I, and 2-Br were fully characterized and their molecular structures determined by single-crystal X-ray diffraction. The compounds feature a slightly twisted five-membered silacyclopenta-2,4-diene ring and a double/single C—C bond alternation in the diene fragment. Reaction of 2-I with the N-heterocyclic carbene IMe₄ (IMe₄=1,3,4,5-tetramethylimidazolin-2-ylidene) leads, after displacement of the iodide groups, to the unprecedented diiodide salt [Si(IMe₄)₂{C₄(NEt₂)₄}](I)₂ (3), containing a 1H-silole dication with a four-coordinate Si^IV center. The crystal structure of 3 reveals similar bonding characteristics for the dicationic 1H-silole to those of the neutral 1H-siloles 1-I–2-Br. Two-electron reduction of 3 with C₈K affords, after elimination of one IMe₄ group, the thermolabile, carbene-stabilized 1-silacyclopentadien-1-ylidene Si{C₄(NEt₂)₄}(IMe₄) (4), which was characterized by elemental analysis and ¹H, ¹³C{¹H}, and ²⁹Si{¹H} NMR spectroscopies.

Y. N. Lebedev, U. Das, O. Chernov, G. Schnakenburg, A. C. Filippou Chem. Eur. J, 2014, 20, 9280.

Si=O in a complex: The first silanone that is stable at room temperature (3) is reported. The two-step synthesis involves carbonylation of the silylidyne complex 1 to give the chromiosilylene 2, followed by oxidation of 2 with N₂O. Silanone 3 features a polar, short Si=O bond (1.526(3) Å) to a trigonal-planar-coordinated silicon center and reacts with water to give the dihydroxysilyl complex.

A. C. Filippou, B. Baars, O. Chernov, Y. N. Lebedev, G. Schnakenburg Angew. Chem. 2014, 126, 576.

S. S. Sen, Angew. Chem., 2014, 126, 8964.