Stabilization of a Silaaldehyde by its η2 Coordination to Tungsten

Heavier tetrylene- and tetrylyne-transition metal chemistry: it's no carbon copy

Since the late 19th century, heavier tetrylene- and tetrylyne-transition metal chemistry has formed an important cornerstone in both main-group and organometallic chemistry alike. Driven by the success of carbene systems, significant efforts have gone towards the thorough understanding of the heavier group 14 derivatives, with examples now known from across the d-block. This now leads towards applications in cooperative bond activation, and moves ultimately towards well-defined catalytic systems. This review aims to summarise this vast field, from initial discoveries of tetrylene and tetrylyne complexes, to the most recent developments in reactivity and catalysis, as a platform to the future of this exciting, blossoming field.

Heavier group 14-transition metal π-complex congeners

Since the dawn of organometallic chemistry, transition metal complexes of unsaturated organic molecules, namely π-complexes, have remained a central focus: our thorough understanding of the electronic nature of such species, and their importance in countless reactive processes continues to drive research in their synthesis and utilisation. Since the late 1900s, research regarding the related chemistry for the heavier group 14 elements has become increasingly more fervent. Today, heavier congeners of a vast array of classical π-complexes have been realised, from alkene to arene systems, involving Si, Ge, Sn, and Pb. This has given deeper insights into the bonding observed for these heavier elements, which typically involves a lessened degree of π-bonding and an increased polarisation. This review aims to summarise this field, identifying these disparities, and highlighting areas which we believe may be exciting for future exploration.

Reactions of (mesityl)n(methyl)2-nsilylene complexes with pyridine-N-oxide (n = 1 and 0): formation of silanone complexes and a disiloxanyloxy complex

Silanones (OSiR2), a heavier congener of ketones (R2CO), are highly reactive species that are readily converted to oligomeric siloxane (O-SiR2)n. Coordination of silanones to the transition-metal fragments to afford silanone-coordinated complexes is a reliable silanone stabilization method. Recently, our group reported the synthesis, structures, and reactivity of dimesityl-substituted silanone complexes Cp*(OC)2M{OSiMes2(L)}(SiMe3) (M = W, Mo, L: Lewis base, Cp*: η5-C5Me5, Mes: 2,4,6-Me3C6H2). Herein, to investigate the effect of substituents on the silicon atom during the formation of a silanone complex, we demonstrated the use of Mes and smaller Me groups. As a result, the formation of Mes(Me)-substituted silanone molybdenum complex Cp*(OC)2Mo{OSiMes(Me)(py)}(SiMe3) (5b, py: pyridine) was suggested, the silanone tungsten complex Cp*(OC)2W{OSiMes(Me)(DMAP)}(SiMe3) (4a, DMAP: 4-(dimethylamino)pyridine) was obtained, and a dimethyl-substituted disiloxanyloxy(dioxo) complex Cp*(O)2W(OSiMe2OSiMe3) (9) was formed. The reaction of 4a with PMe3 proceeded via the elimination of DMAP and migration of the SiMe3 group to the oxygen atom of the silanone ligand to afford Cp*(OC)2W(SiMes(Me)OSiMe3)(PMe3) (11a). The Mo complex Cp*(OC)2Mo(SiMes(Me)OSiMe3)(PMe3) (11b) was produced by the reaction of Cp*(OC)2Mo{SiMes(Me)}(SiMe3) (7b) with pyridine-N-oxide in the presence of PMe3.

Stabilization and One Electron Reduction of a Silicon Analogue of a Carboxylic Acid Anhydride

Reaction of the 1,2-disilylenes {(DipAr Am)Si}2 (DipAr Am=[(NDip)2 CAr]- , Dip=2,6-diisopropylphenyl, Ar=4-C6 H4 But (Ar') 1 a or Ph 1 b) and two abnormal N-heterocyclic silylenes, (DipAr Am)SiOCSi{(NDip)2 CAr} (Ar=Ar' 3 a or Ph 3 b) with N2 O led to formation of unprecedented examples of uncoordinated silicon analogues of carboxylic acid anhydrides, (DipAr Am)(O=)SiOSi(=O)(DipAr Am) (Ar=Ar' 2 a or Ph 2 b). Both compounds have been fully characterized, and the mechanism of formation of one explored using DFT calculations. Reduction of sila-acid anhydride 2 a with a dimagnesium(I) compound, [{(Mes Nacnac)Mg}2 ] (Mes Nacnac=[(MesNCMe)2 CH]- , Mes=mesityl), led to the one-electron reduction of the anhydride and formation of a magnesium complex of a sila-acid anhydride radical anion [(Mes Nacnac)Mg{(OSi(DipAr' Am)}2 O] 5. A combination of EPR spectroscopic studies and DFT calculations reveal the unpaired electron to largely reside on one of the amidinate ligands of the complex.

Recent advances in the chemistry of heavier group 14 analogues of carbonyls

Heavier analogues of carbonyls, in the form of "R2E=O" (E = Si, Ge, Sn, Pb), feature a high polar E=O double bond. In contrast to carbonyl compounds, heavier analogues are extremely unstable and prone to proceed head-to-tail oligomerization. Thus, the isolation of such species under ambient conditions is a challenging synthetic target in main group chemistry. In recent years, much progress has been achieved in the synthesis and isolation of a variety of Lewis base/acid, Lewis base-stabilized and even Lewis acid/base free heavier analogues. These compounds exhibit interesting reactivities, such as small molecule activation and metathesis reactions, indicating the potential of heavier analogues in synthetic chemistry. This review summarizes the recent achievements in the chemistry of Lewis base and/or acid stabilized heavier analogues of carbonyls, including synthetic approaches, structural parameters and reactivity of these isolable compounds.

An NHC-Mediated Metal-Free Approach towards an NHC-Coordinated Endocyclic Disilene

A convenient metal-free approach towards an N-heterocyclic carbene (NHC)-coordinated disilene 2 is described. Compound 2, featuring the disilene incorporated in cyclopolysilane framework, was obtained in good yield and characterized using NMR spectroscopy and X-ray crystallography. Density functional theory (DFT) calculations of the reaction mechanism provide a rationale for the observed reactivity and give detailed information on the bonding situation of the base-stabilized disilene. Compound 2 undergoes thermal or light- induced (λ=456 nm) NHC loss, and a dimerization process to give a corresponding dimer with a Si10 skeleton. In order to shed light on the dimerization mechanism, DFT calculations were performed. Moreover, the reactivity of 2 was examined with selected examples of transition metal carbonyl compounds.

Synthesis and structure of a pyridine-stabilized silanone molybdenum complex and its reactions with PMe3 and acetone

The synthesis, structure and reactivity of a pyridine-stabilized silanone molybdenum complex are described.

Molecular Main Group Metal Hydrides

This review serves to document advances in the synthesis, versatile bonding, and reactivity of molecular main group metal hydrides within Groups 1, 2, and 12-16. Particular attention will be given to the emerging use of said hydrides in the rapidly expanding field of Main Group element-mediated catalysis. While this review is comprehensive in nature, focus will be given to research appearing in the open literature since 2001.

Saccharified Uranyl Ions: Self-Assembly of UO2 2+ into Trinuclear Anionic Complexes by the Coordination of Glucosamine-Derived Schiff Bases

The reaction of UO2 (OAc)2  ⋅ 2H2 O with the biologically inspired ligand 2-salicylidene glucosamine (H2 L1 ) results in the formation of the anionic trinuclear uranyl complex [(UO2 )3 (μ3 -O)(L1 )3 ]2- (12- ), which was isolated in good yield as its Cs-salt, [Cs]2 1. Recrystallization of [Cs]2 1 in the presence of 18-crown-6 led to formation of a neutral ion pair of type [M(18-crown-6)]2 1, which was also obtained for the alkali metal ions Rb+ and K+ (M=Cs, Rb, K). The related ligand, 2-(2-hydroxy-1-naphthylidene) glucosamine (H2 L2 ) in a similar procedure with Cs+ gave the corresponding complex [Cs(18-crown-6)]2 [(UO2 )3 (μ3 -O)(L2 )3 ([Cs(18-crown-6)]2 2). From X-ray investigations, the [(UO2 )3 O(Ln )3 ]2- anion (n=1, 2) in each complex is a discrete trinuclear uranyl species that coordinates to the alkali metal ion via three uranyl oxygen atoms. The coordination behavior of H2 L1 and H2 L2 towards UO2 2+ was investigated by NMR, UV/Vis spectroscopy and mass spectrometry, revealing the in situ formation of the 12- and 22- dianions in solution.

Acid-Base Free Main Group Carbonyl Analogues

Main group carbonyl analogues (R2 E=O) derived from p-block elements (E=groups 13 to 15) have long been considered as elusive species. Previously, employment of chemical tricks such as acid- and base-stabilization protocols granted access to these transient species in their masked forms. However, electronic and steric effects inevitably perturb their chemical reactivity and distinguish them from classical carbonyl compounds. A new era was marked by the recent isolation of acid-base free main group carbonyl analogues, ranging from a lighter boracarbonyl to the heavier silacarbonyls, phosphacarbonyls and a germacarbonyl. Most importantly, their unperturbed nature elicits exciting new chemistry, spanning the vista from classical organic carbonyl-type reactions to transition metal-like oxide ion transfer chemistry. In this Review, we survey the strategies used for the isolation of such systems and document their emerging reactivity profiles, with a view to providing fundamental comparisons both with carbon and transition metal oxo species. This highlights the emerging opportunities for exciting "crossover" reactivity offered by these derivatives of the p-block elements.

Main Group Multiple Bonds for Bond Activations and Catalysis

Since the discovery that the so-called "double-bond" rule could be broken, the field of molecular main group multiple bonds has expanded rapidly. With the majority of homodiatomic double and triple bonds realised within the p-block, along with many heterodiatomic combinations, this Minireview examines the reactivity of these compounds with a particular emphasis on small molecule activation. Furthermore, whilst their ability to act as transition metal mimics has been explored, their catalytic behaviour is somewhat limited. This Minireview aims to highlight the potential of these complexes towards catalytic application and their role as synthons in further functionalisations making them a versatile tool for the modern synthetic chemist.

Planar Tetracoordinated Silicon (ptSi): Room-Temperature Stable Compounds Containing Anti-van't Hoff/Le Bel Silicon

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.

Synthesis, structure, and reactivity of a pyridine-stabilized silanonetungsten complex

A pyridine-stabilized silanonetungsten complex Cp*(OC)₂W{O[double bond, length as m-dash]SiMes₂(py)}(SiMe₃) (1b, Cp* = η⁵-C₅Me₅, Mes = 2,4,6-Me₃C₆H₂, py = C₅H₅N) was obtained by the reaction of a silyl(silylene) complex Cp*(OC)₂W([double bond, length as m-dash]SiMes₂)(SiMe₃) (3) with pyridine-N-oxide in pyridine. X-ray crystal structure determination revealed that complex 1b shows a similar geometry to that observed for a previously synthesized DMAP-stabilized analogue, Cp*(OC)₂W{O[double bond, length as m-dash]SiMes₂(DMAP)}(SiMe₃) (1a, DMAP = 4-NMe₂C₆H₄N). The Si[double bond, length as m-dash]O and W-O bond distances in 1b are comparable to those observed in 1a, but the nitrogen to silicon coordination bond of 1b is slightly longer (ca. 0.05 Å) than that of 1a, indicating the weaker coordination of pyridine than that of DMAP. The reaction of 1b with excess PMe₃ in C₆D₆ at r. t. proceeded via elimination of pyridine to afford a five-membered metallacyclic carbene complex, Cp*(OC)W([double bond, length as m-dash]C(SiMe₃)OSiMes₂O)(PMe₃) (5), but that of 1a with PMe₃ did not proceed at all. Complex 5 was further transformed in C₇D₈ at 100 °C for 4 h to give a four-membered W-O-Si-O metallacyclic complex with carbyne and PMe₃ ligands, Cp*W(OSiMes₂O)([triple bond, length as m-dash]CSiMe₃)(PMe₃) (7). The structural features of complexes 1b, 5, and 7 are comparable to those suggested theoretically as intermediates in the reaction of 3 with a sulfuration reagent to afford a six-membered metallacyclic carbene complex, Cp*W(S){[double bond, length as m-dash]C(SiMe₃)C([double bond, length as m-dash]O)OSiMes₂S} (6), indicating that complex 1b and the theoretically proposed silanethione complex are transformed via a similar reaction pathway.

An Air-Stable Heterobimetallic Si2 M2 Tetrahedral Cluster

Air- and moisture-stable heterobimetallic tetrahedral clusters [Cp(CO)2 MSiR]2 (M=Mo or W; R=SitBu3 ) were isolated from the reaction of N-heterocyclic carbene (NHC) stabilized silyl(silylidene) metal complexes Cp(CO)2 M=Si(SitBu3 )NHC with a mild Lewis acid (BPh3 ). Alternatively, treatment of the NHC-stabilized silylidene complex Cp(CO)2 W=Si(SitBu3 )NHC with stronger Lewis acids such as AlCl3 or B(C6 F5 )3 resulted in the reversible coordination of the Lewis acid to one of the carbonyl ligands. Computational investigations revealed that the dimerization of the intermediate metal silylidyne (M≡Si) complex to a tetrahedral cluster instead of a planar four-membered ring is due to steric bulk.

Interconversion and reactivity of manganese silyl, silylene, and silene complexes

Interconversions between manganese silylene and silene complexes are reported, including those involving the first spectroscopically observed silene complexes with an SiH substituent, and their involvement in ethylene hydrosilylation is discussed.

Manganese disilyl hydride complexes [(dmpe)₂MnH(SiH₂R)₂] (4^Ph: R = Ph, 4^Bu: R = ⁿBu) reacted with ethylene to form silene hydride complexes [(dmpe)₂MnH(RHSi Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CHMe)] (6^Ph,H: R = Ph, 6^Bu,H: R = ⁿBu). Compounds 6^R,H reacted with a second equivalent of ethylene to generate [(dmpe)₂MnH(REtSi Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CHMe)] (6^Ph,Et: R = Ph, 6^Bu,Et: R = ⁿBu), resulting from apparent ethylene insertion into the silene Si–H bond. Furthermore, in the absence of ethylene, silene complex 6^Bu,H slowly isomerized to the silylene hydride complex [(dmpe)₂MnH( Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> SiEtⁿBu)] (3^Bu,Et). Reactions of 4^R with ethylene likely proceed via low-coordinate silyl {[(dmpe)₂Mn(SiH₂R)] (2^Ph: R = Ph, 2^Bu: R = ⁿBu)} or silylene hydride {[(dmpe)₂MnH( Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> SiHR)] (3^Ph,H: R = Ph, 3^Bu,H: R = ⁿBu)} intermediates accessed from 4^R by H₃SiR elimination. DFT calculations and high temperature NMR spectra support the accessibility of these intermediates, and reactions of 4^R with isonitriles or N-heterocyclic carbenes yielded the silyl isonitrile complexes [(dmpe)₂Mn(SiH₂R)(CNR′)] (7a–d: R = Ph or ⁿBu; R′ = o-xylyl or ^tBu), and NHC-stabilized silylene hydride complexes [(dmpe)₂MnH{ Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> SiHR(NHC)}] (8a–d: R = Ph or ⁿBu; NHC = 1,3-diisopropylimidazolin-2-ylidene or 1,3,4,5-tetramethyl-4-imidazolin-2-ylidene), respectively, all of which were crystallographically characterized. Silyl, silylene and silene complexes in this work were accessed via reactions of [(dmpe)₂MnH(C₂H₄)] (1) with hydrosilanes, in some cases followed by ethylene. Therefore, ethylene (C₂H₄ and C₂D₄) hydrosilylation was investigated using [(dmpe)₂MnH(C₂H₄)] (1) as a pre-catalyst, resulting in stepwise conversion of primary to secondary to tertiary hydrosilanes. Various catalytically active manganese-containing species were observed during catalysis, including silylene and silene complexes, and a catalytic cycle is proposed.

The Quest for Stable Silaaldehydes: Synthesis and Reactivity of a Masked Silacarbonyl

The first donor-acceptor complex of a silaaldehyde, with the general formula (NHC)(Ar)Si(H)OGaCl₃ (NHC=N-heterocyclic carbene), was synthesized using the reaction of silyliumylidene-NHC complex [(NHC)₂ (Ar)Si]Cl with water in the presence of GaCl₃ . Conversion of this complex to the corresponding silacarboxylate dimer [(NHC)(Ar)SiO₂ GaCl₂ ]₂ , free silaacetal ArSi(H)(OR)₂ , silaacyl chloride (NHC)(Ar)Si(Cl)OGaCl₃ , and phosphasilene-NHC adduct (NHC)(Ar)Si(H)PTMS unveil its true potential as a synthon in silacarbonyl chemistry.

Synthesis and DFT Study of a Diphenylsilanone-Bridged Dimolybdenum Complex

Reaction of the 30-electron benzylidyne complex [Mo2 Cp2 (μ-CPh)(μ-PCy2 )(μ-CO)] with excess Ph2 SiH2 under visible-UV irradiation yields the silylene-bridged complex [Mo2 Cp2 (μ-CPh)(μ-PCy2 )(μ-SiPh2 )]. This compound undergoes selective oxidation with O2 to give the unsaturated complex [Mo2 Cp2 (μ-CPh)(μ-PCy2 )(μ-κ(1) :κ(1) -OSiPh2 )], which contains an unprecedented bridging diphenylsilanone ligand, as confirmed by X-ray diffraction analysis and DFT calculations. The bonding within the central Mo2 SiO ring of this complex approaches the extreme description of a dimetallacyclosiloxane according to the relevant solid-state bond lengths and theoretical calculations.