Reactivity of a Unique Si(I)–Si(I)-Based η2-Bis(silylene) Iron Complex

In this paper, we report the synthesis of a unique silicon(I)-based metalla-disilirane and report on its reactivity toward TMS-azide and benzophenone. Metal complexes containing disilylenes ((bis)silylenes with a Si–Si bond) are known, but direct ligation of the Si(I) centers to transition metals always generated dinuclear species. To overcome this problem, we targeted the formation of a mononuclear iron(0)–silicon(I)-based disilylene complex via templated synthesis, starting with ligation of two Si(II) centers to iron(II), followed by a two-step reduction. The DFT structure of the resulting η2-disilylene-iron complex reveals metal-to-silicon π-back donation and a delocalized three-center–two-electron (3c–2e) aromatic system. The Si(I)–Si(I) bond displays unusual but well-defined reactivity. With TMS-azide, both the initial azide adduct and the follow-up four-membered nitrene complex could be isolated. Reaction with benzophenone led to selective 1,4-addition into the Si–Si bond. This work reveals that selective reactions of Si(I)–Si(I) bonds are made possible by metal ligation.


■ INTRODUCTION
Transition metal silylene complexes (Scheme 1a) have attracted significant interest as they have shown interesting (electronic) structures and reactivity, giving rise to synthetic and catalytic applications that differ significantly from transition metal carbene complexes. 1 Several variants of transition metal silylene complexes have been reported, varying in the oxidation state of the silicon atom and the types of substituents, and among those variants, N-heterocyclic silylenes have been studied most extensively. 2 Two types of the general class of bis(silylene) compounds have been reported: (1) "bis(silylenes) with a direct Si−Si bond", wherein the two divalent silicon atoms are adjacent to each other and are connected by a central Si−Si bond (like others in the field, 3 we term these "disilylenes"); (2) "spacerseparated bis(silylenes)", with the two divalent silicon atoms separated by a spacer. 4 Driess and co-workers recently reported a ferrocene-separated bis(silylene) acting as a bidentate ligand in catalytically active mono-and dicobalt complexes (Scheme 1b). 4e We consider "disilylenes" to be particularly interesting due to the potential reactivity associated with their Si−Si bond. Compound I, with each Si center bearing an amidinate ligand (Scheme 2), was synthesized by Roesky and co-workers, 5 while Jones et al. reported a bulkier derivative thereof. 6 Additionally, "disilylenes" that are stabilized by an N-heterocyclic carbene (NHC) or an intramolecular phosphine are also reported. 4a,7 Targeting the synthesis of η 2 -coordinated Si(I)-based "disilylene" complexes seems particularly useful because the altered electronic structure and the lability of the Si−Si bond induced by η 2 -coordination are expected to give rise to unique reactivity.
Thus far, the reported reactivity of these platforms predominantly involves the Si-centered lone pairs rather than the Si−Si σ-bond. For example, So et al. reported the formation of the bimetallic disilylene Rh and Ir complexes II and III (Scheme 2a). 8 Also, the dinuclear iron(II)bromide complex IV is formed cleanly, but the Si−Si bond is cleaved by cobalt(II)bromide to form dinuclear cobalt(bromosilylene)-(silyl) complex V. 9 To the best of our knowledge, no mononuclear metal adducts of ligand I are reported to date.
To achieve η 2 -coordination of I to a mononuclear transition metal complex, we decided to explore the reductive coupling of two Si(II)-chlorides in the coordination sphere of a metal center (Scheme 2b). We herein describe iron complex 3 as the first η 2 -disilylene complex prepared by this protocol (Scheme 3). This species has an unusual electronic structure, with the Si−Si bond acting as a π-acceptor moiety, while the threemembered Fe−Si−Si ring shows 2π-aromaticity. It also displays selective Si−Si bond-centered reactivity toward azide activation and ketone addition, giving access to new "spacerseparated bis(silylenes)". ■ RESULTS AND DISCUSSION Synthesis of 3. The amidinate-stabilized silylene ironhalide precursor 1, synthesized from Fe{N(SiMe 3 ) 2 } 2 using a literature procedure, 10 was characterized with zero-field 57 Fe Mossbauer spectroscopy (Figure 1). The isomer shift δ (0.61 m/s) and the quadrupole splitting ΔE Q (2.81 mm/s) are in agreement with a four-coordinated Fe(II) center. 11 Iron-centered reduction of this well-defined Fe(II) species with a slight excess of KC 8 in a benzene-THF mixture (1:1 v/v%) generated bis-silylene compound 2, featuring an η 6 -benzene fragment bound to Fe(0).
The 1 H NMR spectrum of diamagnetic 2 in deuterated benzene exhibits a chemical shift for the η 6 -benzene fragment at δ 5.15 ppm, which is similar to that seen for the previously reported zero-valent iron complex [(SiFcSi)Fe-η 6 (C 6 H 6 )] bearing a ferrocene-bridged bis(silylene) ligand (δ 5.16 ppm). 12 The 29 Si NMR spectrum of 2 exhibits two sharp singlets at δ 45.12 and 42.45 ppm (suggesting chemically inequivalent Si centers), which are close to the reported chemical shift (δ 43.10 ppm) for an N-heterocyclic silylene iron(0) complex. 13 Species 2 was also examined using zerofield 57 Fe Mossbauer spectroscopy to confirm the oxidation state of Fe ( Figure 1). Indeed, both the isomer shift δ (0.38 m/ s) and quadrupole splitting ΔE Q (1.53 mm/s) support reduction to Fe(0). 14 Single crystals of 2 that were obtained from pentane at −30°C proved suitable for crystallographic analysis by X-ray diffraction at room temperature; loss of crystallinity was observed at 150 K. The molecular structure ( Figure 2) shows two different orientations for the Si−Cl bonds with respect to the Fe(η 6 -benzene) fragment (one "up" and one "down"), which explains the presence of two chemically inequivalent Si nuclei in the 29 Si NMR spectrum. The Si−Fe−Si angle is nearly 90°(91.99°), and the intramolecular Si1···Si2 distance is 3.100(2) Å. The Si−Fe1 bond lengths are 2.162(2) Å (Si1) and 2.148(1) Å (Si2), which are significantly shorter than the corresponding bond lengths in precursor 1 (∼2.445 Å) and slightly shorter than those found in the zero-valent iron silylene  complexes ([(NHSi)Fe(dmpe) 2 ] (2.184(2) Å) and Fe(0)-[SiNSi] (2.164(15) and 2.170(13) Å)). 15 Further reduction of this zero-valent Fe species 2 by an excess of KC 8 at room temperature yielded the threemembered metallacyclic complex 3, featuring a direct Si−Si bond (Scheme 3). This reaction was also monitored using 1 H NMR spectroscopy, revealing quantitative conversion (see also Figure S26). The complex was characterized by multinuclear NMR spectroscopy ( 1 H, 13 C, and 29 Si). The 1 H NMR signal of the Fe-coordinated η 6 -benzene shifts from δ 5.15 ppm in 2 to δ 5.34 ppm in 3. The 29 Si NMR spectrum of 3 contains only one singlet at δ 34.49 ppm, indicating formation of a symmetric molecule.
Dark red single crystals of 3 were collected after storing a pentane solution at −30°C for 1 month. Compound 3 crystallizes in the monoclinic space group C 2 /c, which is in line with the high degree of symmetry observed in the 1 H NMR spectrum. The Si(I)−Fe−Si(I) three-membered heterocycle forms an equilateral triangle with interatomic distances of 2.217(6) Å ( Figure 3); for the Si−Si bond, this value falls in the range observed for Si�Si double bonds (2.120−2.250 Å). 16 The isosceles Si−B−Si disilaborirane compound reported by Roesky and co-workers shows a Si−Si bond length of 2.188(5) Å, 17 while the equilateral triangular (di-tbutyl(methyl)silyl)bis(tri-t-butylsilyl)cyclotrisilenylium cation has an average Si−Si bond length of 2.217(3) Å. 16 Electronic Structure of 3. Given the unprecedented nature of the ferracyclic motif found in this species, the bonding of the Si(I)−Si(I) fragment to iron(0) was theoretically investigated at the TZ2P/OPBE 18 level of theory using energy decomposition analysis (EDA). 19 The lowest energy structure with C 2 symmetry is analogous to that derived from X-ray diffraction. To facilitate the EDA analysis (for symmetry reasons), the bonding of the disilylene moiety to iron has been analyzed using a C 2v optimized structure, which is only 4.4 kcal·mol −1 higher in energy. This small energy difference is consistent with the dynamic behavior observed in solution-state NMR spectroscopy, resulting in a single signal for the t Bu substituents at nitrogen.
The Fe(η 6 -benzene) fragment has been computed in the singlet state (Fe(0)-d 8 ). It has a two-electron occupied d xz orbital (perpendicular to the Si−Fe−Si plane) and an empty d yz orbital. The electronic structure of the Si(I)−Si(I) fragment is best described as having a Si−Si single bond with a lone pair on each silicon atom and an empty π-orbital perpendicular to that plane ( Figure S31). The symmetryadapted linear combinations of the two lone pairs on silicon donate electron density toward the Fe(η 6 -benzene) fragment, forming two (delocalized) σ-bonds of −30.3 kcal·mol −1 (A1) and −74.4 kcal·mol −1 (B1). The iron itself donates electron density back from the occupied d xz orbital into an empty πorbital formed by the two Si p z orbitals (B2: −67.8 kcal· mol −1 ), giving rise to considerable π-backbonding ( Figure 4).
To probe the resulting orbital (which is delocalized over the Si−Fe−Si three-membered ring) for aromaticity, we employed the nucleus-independent chemical shift (NICS(0/1)) approach at the B3LYP/6-11+G(d,p) level (negative value of the isotropic magnetic shielding at the center of the Fe−Si−Si three-membered ring and 1 Å perpendicularly above and below, respectively). These are found to be −47.8 and −21.4 ppm, respectively, corroborating the magnetic aromaticity. For comparison, the NICS(0/1) values at the center of the benzene ring in 3 are calculated to be −42.8 and −18.4 ppm. Similar values have been reported for other metallacycles. 20 A more refined method by Stanger 21 following the out-of-plane component to the shielding tensor along a trajectory orthogonal to the plane of the ring (NICS zz ) has been used by Roesky et al. to assign 2π-aromaticity to their disilaborirane species. 17 However, instead of showing the typical off-center minimum for 2π-aromatic systems, the NICS scan of 3 shows a steady and continuous increase to less negative NICS zz values over a value of 3 Å (see Figure S33), which may be caused by anisotropy of the metal center at the ring and/or by σaromaticity. 17,19,22 Therefore, we resorted to the canonical molecular orbital (CMO) analysis of the NICS(0), which separates the total shieldings into contributions from canonical molecular orbitals. 23 Indeed, there is a sizable contribution of −13.8 ppm from the Fe d xy orbital (HOMO−5, see Figure S32), which is part of the σ-framework. More importantly, the major contribution of −16.6 ppm originates from the delocalized πorbital shown in Figure 4, substantiating the 2π-aromaticity of the Si 2 Fe three-membered ring in complex 3.
Reactivity of 3. The unligated Si(I)-based disilylene species I shows stoichiometry-dependent reactivity toward trimethylsilyl azide, forming either a silaazatriene or a silaimine

Inorganic Chemistry
pubs.acs.org/IC Article product. 24,25 Roesky and co-workers described ring opening of their disilaborirane species VI with TMSN 3 , forming a 1-aza-2,3-disila-4-boretidine derivative (Scheme 4). 17 We observed selective conversion of 3 with 1 equiv of TMSN 3 to give the unique azide adduct 4 after 1 h at 40°C in benzene solution. This species was obtained as a single crystalline material by recrystallization from pentane at −30°C. The molecular structure, as determined by X-ray diffraction, is displayed in Figure 5. It contains a planar four-membered Si1−Fe1−Si2− N5 heterocycle resulting from the insertion of the azide terminal N-atom in the Si−Si bond, with Fe−Si−N5 angles of roughly 100°. The two Si−Fe distances are slightly different, Si1/2−Fe1 (2.1687(6) and 2.1746(7) Å). The intramolecular Si···Si distance is long at 2.504(8) Å. Prolonged heating of a mixture of 3 with TMSN 3 or isolated 4 at 80°C generated the follow-up trimethylsilylnitrene complex 5 as the major product, together with small amounts of intermediate 4 (see Figure S17). Single crystals of 5, obtained by recrystallization from benzene at room temperature, were analyzed by X-ray diffraction ( Figure 5).
Compound 5 also shows a planar four-membered Si1−Fe1− Si2−N5 heterocycle with Fe−Si−N5 angles of 103(1)°but with a different substitution pattern at N5, i.e., only a TMS group resulting from the insertion of trimethylsilylnitrene in the Si−Si bond. The two Si−Fe distances are nearly identical, Si1/2−Fe1 (2.174(2) and 2.177(2) Å), and so are the Si1/2− N5 (1.770(5) and 1.767(4) Å) bond lengths. The Si−Fe σbonds are on average 0.2547 Å longer than the Si−B distance in the N-insertion product obtained from disilaborirane, 18 while the Si−N5 bond length is similar. The intramolecular Si···Si distance (2.412(2) Å) in 5 is shortened relative to that in 4. Apart from the X-ray crystallographic analysis, reaction monitoring and product identification have also been achieved using NMR spectroscopy. The 1 H NMR signal of the Febound η 6 -benzene fragment shifts from δ 5.34 ppm (3) to δ 5.26 ppm (4) and 5.12 ppm (5), and the trimethylsilyl group in azide adduct 4 appears at δ 0.47 ppm (for 5, δ 0.39 ppm) compared to −0.08 ppm for TMSN 3 . In 29 Si NMR, the NSiN signals for both 4 and 5 are strongly upfield-shifted (Δδ ∼ 27 ppm) with respect to 3 (4: δ 7.05 ppm; 5: δ 7.42 ppm), in line with the rupture of the Si−Si bond, which also disrupts the 2πaromaticity and leads to loss of ring current. The Si(CH 3 ) 3 signal appears at δ 15.41 ppm for 4 and at −20.71 ppm for 5.
To understand the observed two-step reaction between 3 and trimethylsilylazide, we used DFT calculations to explore the reaction mechanism. The initial step involves the nucleophilic attack of the azide onto one of the Si centers, which results in induced nucleophilic character at the second Si center that subsequently attacks back onto the azide in TS1 ( Figure 6). As a result, this insertion can be considered to involve induced FLP reactivity. The energy barrier to afford Int1 (15.4 kcal·mol −1 ) is consistent with the experimentally determined barrier (15.1 kcal·mol −1 ) for formation of 4 (Arrhenius plot, Figure 5). After forming this symmetric azide adduct, the energy barrier for the subsequent dinitrogen release (29.4 kcal·mol −1 ) is high enough to rationalize the successful isolation of 4. Finally, a Staudinger pathway, involving fourmembered ring transition state TS3, releases dinitrogen from 4 to afford product 5 (−110 kcal·mol −1 ).
The ambivalent reactivity of the Si(I) centers in the Si−Si bond of 3, able to act either as nucleophile or electrophile, was also apparent during the conversion of species 3 conversion with benzophenone. Disilylene I was previously reported to react with benzophenone to furnish selective C−O cleavage with formation of a cyclodisiloxane (Scheme 5). 26 Strikingly

Inorganic Chemistry
pubs.acs.org/IC Article different reactivity was observed when complex 3, featuring a "protected" Si−Si bond, was exposed to benzophenone for 1 h at 80°C. Selective formation of seven-membered ring product 6 was obtained via a formal 1,4-addition of benzophenone, displaying strongly attenuated and controlled reactivity of the Si−Si fragment in complex 3. Complex 6 was characterized in the solid state using single-crystal X-ray diffraction (Figure 7).     29 and complex 1 (FeCl 2 {PhC-(N t Bu) 2 SiCl} 2 ) 10 were synthesized according to reported procedures. Other reagents were purchased from J&K Chemical and SCRC. Glassware was dried at 150°C overnight. Celite and molecular sieves were dried at 200°C under vacuum. Benzene, pentane, hexanes, and diethyl ether were degassed with nitrogen, dried over activated molecular sieves, and kept over 4 Å molecular sieves in a N 2 -filled glovebox. NMR data were recorded either on a Bruker 400 or a 500 MHz spectrometer and were internally referenced to residual proton solvent signals in C 6 D 6 (7.16 ppm). Data for 1 H NMR are reported as follows: chemical shift (δ ppm) and multiplicity (s = singlet, d = doublet, t = triplet, m = multiplet, br = broad). IR data were recorded on a Thermo Scientific Nicolet iS5 FTIR, and signal strength is represented as follows: VS = very strong, W = weak, S = strong, VW = very weak, m = middle, w = wide. The UV−Vis spectra were recorded using a StellarNet BLACK Comet C-SR diode array miniature spectrophotometer connected to deuterium and halogen lamp by optical fiber using 1 cm matched quartz cuvettes at room temperature. Elemental analysis was performed by the Analytical Laboratory of Shanghai Institute of Organic Chemistry (CAS).
X-ray Crystallography. Crystals were coated with Paratone-N oil and mounted on a Bruker D8 Venture diffractometer equipped with an APEX-II CCD diffractometer. The crystal was kept at 150 K during data collection. Using Olex2, 30 the structure was solved with the ShelXT 31 structure solution program using Intrinsic Phasing and refined with the XL 32 refinement package using least squares minimization. CCDC 2157512−2157516 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/data_request/cif. Mossbauer Spectroscopy. Mossbauer spectra were recorded with a 57 Co source in a Rh matrix using an alternating constant acceleration Wissel Mossbauer spectrometer operated in transmission mode and equipped with a Janis closed-cycle helium cryostat. Isomer shifts are given relative to the iron metal at ambient temperature. Simulation of the experimental data was performed with the Mfit program (developed by Dr. E. Bill, Max-Planck Institute for Chemical Energy Conversion, Mulheim/Ruhr, Germany) using Lorentzian line doublets.
Computational Details. Geometries were fully optimized as minima or transition states using the Turbomole program package, 33 coupled to the PQS Baker optimizer 34 via the BOpt package. 35 We used unrestricted ri-DFT-D3 calculations at the B3LYP level, 36 in combination with the def2-TZVP basis set 37 and a small (m4) grid size. Grimme's dispersion corrections 38 (version 3, disp3, "zero damping") were used to include van der Waals interactions. The energy decomposition analysis (EDA) 19 was performed on the TZ2P/ OPBE 18 optimized geometry constrained to C 2v symmetry (+3.9 kcal· mol −1 ). The nucleus-independent chemical shift (NICS) 39 was used as a diagnostic probe for quantitative measure for aromaticity at the B3LYP/6-11+G(d,p) level. 40,41 For further details, see the Supporting Information.
{PhC (N t Bu) 2 SiCl} 2 Fe(C 6 H 6 ) (2). A solution of 1 (110 mg, 0.153 mmol) in benzene (5 mL) was added dropwise to a solution of KC 8 (53.7 mg, 0.398 mmol) in THF (5 mL) in a vial while stirring. The color of the reaction mixture turned from yellow to dark red-brown. After stirring for 12 h, volatile materials were removed under vacuum and compound 2 was extracted with pentane solution. The solid was crystallized in pentane solution in a −30°C freezer for 2 days, and only crystalline material was used for subsequent reactions (yield: 40 mg, 40%). 1 (N 3 SiMe 3 ) (4). Compound 4, which is also formed during the formation of species 5 (vide infra), can be obtained as an isolable species upon reaction of 3 with TMSN 3 (7.5 μL, 0.153 mmol) for 1 h at 40°C in benzene solution in a J-Young tube (quantitative conversion). After 40 min of reaction, the solution was evaporated to dryness under vacuum and redissolved in pentane. Xray quality crystals were grown from a pentane solution stored in a −30°C freezer. 1 (5). TMSN 3 (7.5 μL, 0.153 mmol) was added by a pipette to a solution of 3 (36 mg, 0.055 mmol) in a J-Young tube or Schlenk tube while stirring at 80°C. After heating the reaction mixture for 90 min, the product was isolated (yield: 20.3 mg, 50%) as a purple-red solid by removing all solvents and other volatile materials under vacuum and washing with cold pentane. At shorter reaction times, this product coexists with complex 4 according to NMR spectroscopy. X-ray quality crystals were collected by dissolving the obtained solid in benzene solution and slow evaporation of the solution at room temperature. 1 29 Si NMR (99 MHz, benzene-d 6 (6). Benzophenone (437 μL of a 100 mg/10 mL stock solution in benzene) was added dropwise to a solution of 3 (15.6 mg) in benzene (5 mL) in a J-Young tube or Schlenk tube at 80°C with stirring. Product 6 was isolated as a dark greenish-black solid by washing with cold pentane after removing all solvents and other volatile materials. Yield: 12 mg, 60%. X-ray quality crystals were collected by dissolving the solid in diethyl ether solution and then placing the sample in a −30°C freezer. 1  The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.2c01369. General information; synthetic procedures of new compounds, including NMR, IR, and UV−Vis spectra; reaction of 3 with TMS-azide followed by NMR; EDA and DFT calculations; single-crystal X-ray data diffraction (PDF) Computational details (ZIP)