Polydentate Amidinato-Silylenes, -Germylenes and -Stannylenes

This review article focuses on amidinatotetrylenes that potentially can (or have already shown to) behave as bi- or tridentate ligands because they contain at least one amidinatotetrylene moiety (silylene, germylene or stannylene) and one (or more) additional coordinable fragment(s). Currently, they are being widely used as ligands in coordination chemistry, small molecule activation and catalysis. This review classifies those that have been isolated as transition metal-free compounds into five families that differ in the position(s) of the donor group(s) (D) on the amidinatotetrylene moiety, namely: ED{R1NC(R2)NR1}, EX{DNC(R2)NR1}, EX{R1NC(D)NR1}, EX{DNC(R2)ND} and E{R1NC(R2)ND}2 (E=Si, Ge or Sn). Those that do not exist as transition metal-free compounds but have been observed as ligands in transition metal complexes are cyclometallated and ring-opened amidinatotetrylene ligands. This article presents schematic descriptions of their structures, the approaches used for their syntheses and a quick overview of their involvement (as ligands) in transition metal-catalysed reactions. The literature is covered up to the end of 2023.

Synthetic, structural and reactivity studies of a boryl-ethynyl Silylene

The salt metathesis of a boryl-ethynyl lithium salt {[(HCDipN)2]B-CC-Li} with a monochlorosilylene [LSi(:)Cl; L = PhC(NtBu)2] produced an isolable boryl-ethynyl silylene {1; [(HCDipN)2]B-CC-Si(L)}. The Si(II) center in 1 possesses a nonbonding lone pair and forms a covalent bond with the ethynyl group. The characterization of 1 was carried out by multinuclear NMR spectroscopy, single-crystal X-ray structure analysis and DFT calculations. Additionally, a reactivity study of 1 was conducted towards oxygen-containing and aryl C-F substrates.

Stabilization of NH- Group Adjacent to Naked Silicon(II) Atom in Base Stabilized Aminosilylenes

Aminosilylene, comprising reactive NH- and Si(II) sites next to each other, is an intriguing class of compounds due to its ability to show diverse reactivity. However, stabilizing the reactive NH- group next to the free Si(II) atom is challenging and has not yet been achieved. Herein, we report the first examples of base stabilized free aminosilylenes Ar*NHSi(PhC(Nt Bu)2 ) (1 a) and Mes*NHSi(PhC(Nt Bu)2 ) (1 b) (Ar*=2,6-dibenzhydryl-4-methylphenyl and Mes*=2,4,6-tri-tert-butylphenyl), tolerating a NH- group next to the naked Si(II) atom. Remarkably, 1 a and 1 b exhibited interesting differences in their reactivity upon heating. With 1 a, an intramolecular C(sp3 )-H activation of one of the benzhydryl methine hydrogen atoms to the Si(II) atom produced the five-membered cyclic silazane 2. However, with 1 b, a rare 1,2-hydrogen shift to the Si(II) atom afforded a silanimine 3, with a hydride ligand attached to an unsaturated silicon atom. Further, the coordination capabilities of 1 a were also tested with Ru(II) and Fe(0) precursors. Treatments of 1 a with [Ru(η6 -p-cymene)Cl2 ]2 led to the isolation of a η6 -arene tethered complex [RuCl2 {Ar*NHSi(PhC(t BuN)2 )-κ1 -Si-η6 -arene}] (4), whereas with the Fe(CO)5 precursor a Fe(0) complex [Fe(CO)4 {Ar*NHSi(PhC(t BuN)2 )-κ1 -Si}] (5) was obtained. Density functional theory (DFT) calculations were conducted to shed light on the structural, bonding, and energetic aspects in 1-5.

Amidinatotetrylenes Donor Functionalized on Both N Atoms: Structures and Coordination Chemistry

E(hmds)(bqfam) (E = Ge (1a), Sn (1b); hmds = N(SiMe3)2, bqfam = N,N'-bis(quinol-8-yl)formamidinate), which are amidinatotetrylenes equipped with quinol-8-yl fragments on the amidinate N atoms, have been synthesized from the formamidine Hbqfam and Ge(hmds)2 or SnCl(hmds). Both 1a and 1b are fluxional in solution at room temperature, as the E atom oscillates from being attached to the two amidinate N atoms to being chelated by an amidinate N atom and its closest quinolyl N atom (both situations are similarly stable according to density functional theory calculations). The hmds group of 1a and 1b is still reactive and the deprotonation of another equivalent of Hbqfam can be achieved, allowing the formation of the homoleptic derivatives E(bqfam)2 (E = Ge, Sn). The reactions of 1a and 1b with [AuCl(tht)] (tht = tetrahydrothiophene), [PdCl2(MeCN)2], [PtCl2(cod)] (cod = cycloocta-1,5-diene), [Ru3(CO)12] and [Co2(CO)8] have been investigated. The gold(I) complexes [AuCl{κE-E(hmds)(bqfam)}] (E = Ge, Sn) have a monodentate κE-tetrylene ligand and display fluxional behavior in solution the same as that of 1a and 1b. However, the palladium(II) and platinum(II) complexes [MCl{κ3E,N,N'-ECl(hmds)(bqfam)}] (M = Pd, Pt; E = Ge, Sn) contain a κ3E,N,N'-chloridotetryl ligand that arises from the insertion of the tetrylene E atom into an M-Cl bond and the coordination of an amidinate N atom and its closest quinolyl N atom to the metal center. Finally, the binuclear ruthenium(0) and cobalt(0) complexes [Ru2{μE-κ3E,N,N'-E(hmds)(bqfam)}(CO)6] and [Co2{μE-κ3E,N,N'-E(hmds)(bqfam)}(μ-CO)(CO)4] (E = Ge, Sn) have a related κ3E,N,N'-tetrylene ligand that bridges two metal atoms through the E atom. For the κ3E,N,N'-metal complexes, the quinolyl fragment not attached to the metal is pendant in all the germanium compounds but, for the tin derivatives, is attached to (in the Pd and Pt complexes) or may interact with (in the Ru2 and Co2 complexes) the tin atom.

Planar three-coordinate iron(II) complexes supported by sterically demanding -Si(SiMe3)2(SiMe2tBu) ligands

Sterically demanding organosilyl ligands support the formation of coordinatively unsaturated complexes. In this study, we found that using the ligand -Si(SiMe3)2(SiMe2tBu) affords exclusively planar three-coordinate iron bis(silyl) complexes that show good catalytic performance in the hydrosilylation of acetophenone.

Synthesis of Bis(silylene) Iron Chlorides and Their Catalytic Activity for Dinitrogen Silylation

In this study, three tetracoordinated bis(silylene) iron(II) chlorides, namely, [SiCHRSi]FeCl2 (1) (R = H), (2) (R = CH3), and (3) (R = Ph), were synthesized through the reactions of the three different bis(silylene) ligands [LSiCHRSiL] (L = PhC(NtBu)2, L1 (R = H), L2 (R = CH3), L3 (R = Ph)) with FeCl2·(THF)1.5 in THF. The bis(silylene) Fe complexes 1-3 could be used as effective catalysts for dinitrogen silylation, with complex 3 demonstrating the highest turnover number (TON) of 746 equiv among the three complexes. The catalytic mechanism was explored, revealing the involvement of the pentacoordinated bis(dinitrogen) iron(0) complexes [SiCHRSi]Fe(N2)2(THF), (4)-(6), as the active catalysts in the dinitrogen silylation reaction. Additionally, the cyclic silylene compound 10 was obtained from the reaction of L1 with KC8. Single-crystal X-ray diffraction analyses confirmed the molecular structures of complexes 1-3 and 10 in the solid state.

Effects of silylene ligands on the performance of carbonyl hydrosilylation catalyzed by cobalt phosphine complexes

In order to study the effects of silylene ligands on the catalytic activity of carbonyl hydrosilylation catalyzed by cobalt phosphine complexes, readily available model catalysts are required. In this contribution, a comparative study of the hydrosilylation of aldehydes and ketones catalyzed by tris(trimethylphosphine) cobalt chloride, CoCl(PMe₃)₃ (1), and bis(silylene) cobalt chloride, Co(LSi:)₂(PMe₃)₂Cl (2, LSi: = {PhC(N^tBu)₂}SiCl), is presented. It was found that both complexes 1 and 2 are good catalysts for the hydrosilylation of aldehydes and ketones under mild conditions. This catalytic system has a broad substrate scope and selectivity for multi-functional substrates. Silylene complex 2 shows higher activity than complex 1, bearing phosphine ligands, for aldehydes, but conversely, for ketones, the activity of complex 1 is higher than that of complex 2. It is worth noting that in the process of mechanistic studies the intermediates (PMe₃)₃Co(H)(Cl)(PhH₂Si) (3) and (LSi:)₂(PMe₃)Co(H)(Cl)(PhH₂Si) (4) were isolated from the stoichiometric reactions of 1 and 2 with phenylsilane, respectively. Further experiments confirmed that complex 3 is a real intermediate. A possible catalytic mechanism for the hydrosilylation of carbonyl compounds catalyzed by 1 was proposed based on the experimental investigation and literature reports, and this mechanism was further supported by DFT studies. The bis(silylene) complex 4 showed complicated behavior in solution. A series of experiments were designed to study the catalytic mechanism for the hydrosilylation of carbonyl compounds catalyzed by complex 2. According to the experimental results, the hydrosilylation of aldehydes catalyzed by 1 proceeds via a different mechanism than that of the analogous reaction with complex 2 as the catalyst. In the case of ketones, complex 4 is a real intermediate, indicating that both 1 and 2 catalyze the reaction by the same mechanism. The molecular structures of 3 and 4 were determined by single crystal X-ray diffraction analysis.

Terminal N2 Dissociation in [(PNN)Fe(N2 )]2 (μ-N2 ) Leads to Local Spin-State Changes and Augmented Bridging N2 Activation

Nitrogen fixation at iron centres is a fundamental catalytic step for N₂ utilisation, relevant to biological (nitrogenase) and industrial (Haber-Bosch) processes. This step is coupled with important electronic structure changes which are currently poorly understood. We show here for the first time that terminal dinitrogen dissociation from iron complexes that coordinate N₂ in a terminal and bridging fashion leaves the Fe-N₂ -Fe unit intact but significantly enhances the degree of N₂ activation (Δν≈180 cm^-1 , Raman spectroscopy) through charge redistribution. The transformation proceeds with local spin state change at the iron centre (S= 1 / 2 ${{ 1/2 }}$ →S=³ /₂ ). Further dissociation of the bridging N₂ can be induced under thermolytic conditions, triggering a disproportionation reaction, from which the tetrahedral (PNN)₂ Fe could be isolated. This work shows that dinitrogen activation can be induced in the absence of external chemical stimuli such as reducing agents or Lewis acids.

[N,N′-Di-tert-butyl-P,P-diphenylphosphinimidic Amidato-κN,κN′]chlorosilicon-κSi-tetracarbonyliron

The title complex {[Ph2P(tBuN)2](Cl)Si:->Fe(CO)4} (2) was synthesized via the reaction of chlorosilylene [Ph2P(tBuN)2]SiCl (1), supported by an iminophosphonamide ligand with Fe(CO)5 in THF. The molecular structure of 2 was fully characterized by NMR (1H, 13C, 29Si, and 31P) and IR spectroscopies, as well as single-crystal X-ray diffraction (SCXRD) analysis. In the SCXRD analysis of 2, the silylene ligand was located in the axial positions of the coordination sphere of the central iron atom and other sites were occupied by carbonyl ligands.

Synthesis and Reactivity of Cobalt-Dinitrogen Complexes Bearing Anionic PCP-Type Pincer Ligands toward Catalytic Silylamine Formation from Dinitrogen

A series of cobalt(I)-dinitrogen complexes bearing anionic 4-substituted benzene-based PCP-type pincer ligands are synthesized and characterized. These complexes work as highly efficient catalysts for the formation of silylamine from dinitrogen under ambient reaction conditions to produce up to 371 equiv of silylamine based on the cobalt atom of the catalyst.

Dinitrogen activation by a phosphido-bridged binuclear cobalt complex

The reduction of PNPCoBr under a N2 atmosphere yielded a binuclear cobalt dinitrogen anion complex via the C–P bond cleavage of the PNP ligand.

Synthesis of silyl iron dinitrogen complexes for activation of dihydrogen and catalytic silylation of dinitrogen

Three novel iron dinitrogen hydrides, [FeH(ⁱPr-PSi^MeP)(N₂)(PMe₃)] (1), [FeH(ⁱPr-PSi^PhP)(N₂)(PMe₃)] (2), and [FeH(ⁱPr-PSi^Ph)(N₂)(PMe₃)] (3), supported by a silyl ligand are synthesized for the first time by changing the electronic effect and steric hindrance of the ligands through the reaction of ligands L1-L3 with Fe(PMe₃)₄ in a nitrogen atmosphere. The ligands containing an electron-donating group with large steric hindrance on the phosphorus atom are beneficial for the formation of dinitrogen complexes. A penta-coordinate iron hydride [FeH(ⁱPr-PSi^Ph)(PMe₃)₂] (4) was formed through the reaction of ligand L3 with Fe(PMe₃)₄ in an argon atmosphere under the same conditions. The reactions between complexes 1-3 with an atmospheric pressure of dihydrogen gas resulted in Fe(II) dihydrides, [(ⁱPr-PSi^Me(μ-H)P)Fe(H)₂(PMe₃)] (5), [(ⁱPr-PSi^Ph(μ-H)P)Fe(H)₂(PMe₃)] (6) and [(iPr-PSi^Ph(μ-H))Fe(H)₂(PMe₃)₂] (7), with an η²-(Si-H) coordination. The isolation of dihydrides 5-7 demonstrates the ability of the dinitrogen complexes 1-3 to realize the activation of dihydrogen under ambient temperature and pressure. The molecular structures of complexes 1-7 were elucidated by single crystal X-ray diffraction analysis. The iron dinitrogen hydrides 1-3 are effective catalysts for the silylation of dinitrogen under ambient conditions and among them 3 is the best catalyst.

Ammonia Formation Catalyzed by a Dinitrogen-Bridged Dirhenium Complex Bearing PNP-Pincer Ligands under Mild Reaction Conditions*

A series of rhenium complexes bearing a pyridine-based PNP-type pincer ligand are synthesized from rhenium phosphine complexes as precursors. A dinitrogen-bridged dirhenium complex bearing the PNP-type pincer ligands catalytically converts dinitrogen into ammonia during the reaction with KC₈ as a reductant and [HPCy₃ ]BAr^F ₄ (Cy=cyclohexyl, Ar^F =3,5-(CF₃ )₂ C₆ H₃ ) as a proton source at -78 °C to afford 8.4 equiv of ammonia based on the rhenium atom of the catalyst. The rhenium-dinitrogen complex also catalyzes silylation of dinitrogen in the reaction with KC₈ as a reductant and Me₃ SiCl as a silylating reagent under ambient reaction conditions to afford 11.7 equiv of tris(trimethylsilyl)amine based on the rhenium atom of the catalyst. These results demonstrate the first successful example of catalytic nitrogen fixation under mild reaction conditions using rhenium-dinitrogen complexes as catalysts.

Progress in the preparation and characterization of silylene iron, cobalt and nickel complexes

The synthesis and characterization of Fe, Co and Ni complexes supported by silylene ligands in the past ten years are summarized. Due to the decrease of the electron cloud density on the Si atom after coordination, the downfield shift of the 29Si chemical shift is accompanied by the coordination between the free silylene ligand and metal. The strong electron-donating ability of silylene makes the metal center more electron-rich, which is conducive to the oxidative addition reaction in the metal center. In some cases, the coordination ability of silylene is stronger than those of phosphine and carbene ligands. Therefore, silylene transition metal complexes have better catalytic activity. The further challenges in this field are to develop new polydentate silylene ligands, synthesize chelate silylene-phosphine and silylene-carbene ligands, and design new silylene transition metal complexes for more catalytic research.

Comprehensive insights into synthetic nitrogen fixation assisted by molecular catalysts under ambient or mild conditions

N2 is fixed as NH3 industrially by the Haber-Bosch process under harsh conditions, whereas biological nitrogen fixation is achieved under ambient conditions, which has prompted development of alternative methods to fix N2 catalyzed by transition metal molecular complexes. Since the early 21st century, catalytic conversion of N2 into NH3 under ambient conditions has been achieved by using molecular catalysts, and now H2O has been utilized as a proton source with turnover frequencies reaching the values found for biological nitrogen fixation. In this review, recent advances in the development of molecular catalysts for synthetic N2 fixation under ambient or mild conditions are summarized, and potential directions for future research are also discussed.

Synthesis of Dinuclear Mo-Fe Hydride Complexes and Catalytic Silylation of N2

Two transition-metal atoms bridged by hydrides may represent a useful structural motif for N₂ activation by molecular complexes and the enzyme active site. In this study, dinuclear Mo^IV -Fe^II complexes with bridging hydrides, Cp^R Mo(PMe₃ )(H)(μ-H)₃ FeCp* (2 a; Cp^R =Cp*=C₅ Me₅ , 2 b; Cp^R =C₅ Me₄ H), were synthesized via deprotonation of Cp^R Mo(PMe₃ )H₅ (1 a; Cp^R =Cp*, 1 b; Cp^R =C₅ Me₄ H) by Cp*FeN(SiMe₃ )₂ , and they were characterized by spectroscopy and crystallography. These Mo-Fe complexes reveal the shortest Mo-Fe distances ever reported (2.4005(3) Å for 2 a and 2.3952(3) Å for 2 b), and the Mo-Fe interactions were analyzed by computational studies. Removal of the terminal Mo-H hydride in 2 a-2 b by [Ph₃ C]⁺ in THF led to the formation of cationic THF adducts [Cp^R Mo(PMe₃ )(THF)(μ-H)₃ FeCp*]⁺ (3 a; Cp^R =Cp*, 3 b; Cp^R =C₅ Me₄ H). Further reaction of 3 a with LiPPh₂ gave rise to a phosphido-bridged complex Cp*Mo(PMe₃ )(μ-H)(μ-PPh₂ )FeCp* (4). A series of Mo-Fe complexes were subjected to catalytic silylation of N₂ in the presence of Na and Me₃ SiCl, furnishing up to 129±20 equiv of N(SiMe₃ )₃ per molecule of 2 b. Mechanism of the catalytic cycle was analyzed by DFT calculations.

Synthesis and Reactivity of a Hypersilylsilylene

Stabilization of an amidinatosilylene with a bulky tris(trimethylsilyl)silyl substituent was realized with the preparation of PhC(NtBu)₂Si{Si(SiMe₃)₃} (1) from PhC(NtBu)₂SiHCl₂ with K{Si(SiMe₃)₃} in more than 90% yield. The highly deshielded ²⁹Si NMR resonance (δ = 76.91 ppm) can be attributed to the absence of a π-donating substituent. The molecular structure of 1 shows a trigonal-planar geometry around the Si^II center with a Si^II-Si^IV bond length of 2.4339(13) Å. A series of reactions of 1 with Me₃NO, S, Se, and Te were performed. While siloxane derivatives (2 and 3) are obtained from reactions with Me₃NO, silachalcogenones (4-6) are formed with other chalcogens. The presence of Si═E (E = S, Se, and Te) bonds in 4-6 have been confirmed by single-crystal X-ray studies. Silaoxirane (7) formation was observed when 1 was treated with acetone, demonstrating the importance of the tris(trimethylsilyl)silyl group to kinetically and thermodynamically protect the silaoxirane derivative with less bulky substituents on the C atom.

Conversion of dinitrogen to tris(trimethylsilyl)amine catalyzed by titanium triamido-amine complexes

By using a triaryl-Tren ligated titanium dinitrogen complex, K2[{(Xy-N3N)Ti}2(μ2-N2)] (3), prepared by two-electron reduction of [TiCl(Xy-N3N)] (1-Cl) under N2 atmosphere, catalytic fixation of molecular nitrogen to form tris(trimethylsilyl)amine was achieved under ambient conditions with a turnover number (TON) of up to 16.5 per titanium atom.

Synthesis of N-Heterocycle Substituted Silyl Ligands within the Coordination Sphere of Iron

N-Heterocycle-substituted silyl iron complexes have been synthesized by nucleophilic substitution at trichlorosilyl ligands bound to iron. The homoleptic (tripyrrolyl)- and tris(3-methylindolyl)silyl groups were accessed from (Cl₃Si)CpFe(CO)₂ (Cl₃SiFp) by substitution of chloride with pyrrolide or 3-methylindolide, respectively. Analogously, nucleophilic substitution of Cl with pyrrolide on the anionic Fe(0) synthon Cl₃SiFe(CO)₄ ^- generates the (tripyrrolyl)silyl ligand, bound to the iron tetracarbonyl fragment. The bulkier 2-mesitylpyrrolide substitutes a maximum of 2 chlorides on Cl₃SiFp under the same conditions. The tridentate, trianionic nucleophile tmim (tmimH₃ = tris(3-methylindol-2-yl)methane) proves reluctant to perform the substitution in a straightforward manner; instead, ring-opening and incorporation of THF occurs to form the tris-THF adduct tmim(C₄H₈O)₃SiFe(CO)₄ ^-. The bidentate, monoanionic nucleophile 2-(dipp-iminomethyl)pyrrolide (^DippIMP, dipp = 2,6-diisopropylphenyl) shows chloride displacement and addition of a second ^DippIMP moiety on the imine backbone. The heterocycle-based silyl ligands were shown to be sterically and electronically tunable, moderately electron-donating ligands. The presented approach to new silyl ligands avoids strongly reducing conditions and potentially reactive hydrosilane intermediates.