Highly Selective Substitution and Insertion Reactions of Silylenes in a Metal-Coordinated Polyphosphide

Applications of low-valent compounds with heavy group-14 elements

Over the last two decades, the low-valent compounds of group-14 elements have received significant attention in several fields of chemistry owing to their unique electronic properties. The low-valent group-14 species include tetrylenes, tetryliumylidene, tetrylones, dimetallenes and dimetallynes. These low-valent group-14 species have shown applications in various areas such as organic transformations (hydroboration, cyanosilylation, N-functionalisation of amines, and hydroamination), small molecule activation (e.g. P4, As4, CO2, CO, H2, alkene, and alkyne) and materials. This review presents an in-depth discussion on low-valent group-14 species-catalyzed reactions, including polymerization of rac-lactide, L-lactide, DL-lactide, and caprolactone, followed by their photophysical properties (phosphorescence and fluorescence), thin film deposition (atomic layer deposition and vapor phase deposition), and medicinal applications. This review concisely summarizes current developments of low-valent heavier group-14 compounds, covering synthetic methodologies, structural aspects, and their applications in various fields of chemistry. Finally, their opportunities and challenges are examined and emphasized.

Reactivity of [Cp*Fe(η5 -As5 )] towards Carbenes, Silylenes and Germylenes

The reaction behavior of [Cp*Fe(η5 -As5 )] (I) (Cp*=C5 Me5 ) towards carbenes and their heavier analogs was investigated. The reaction of I with NHCs (NHCs=N-heterocyclic carbenes) results in the first substitution products of polyarsenic ligand complexes by NHCs [Cp*Fe(η4 -As5 NHC)] (1 a: NHC=IMe=1,3,4,5-tetramethylimidazolin-2-ylidene, 1 b: NHC=IMes=1,3-bis(2,4,6-trimethylphenyl)-imidazolin-2-ylidene). In contrast, the reaction of I with Et CAAC (Et CAAC=2,6-diisopropylphenyl)-4,4-diethyl-2,2-dimethyl-pyrrolidin-5-ylidene) leads to a fragmentation and the formation of an unprecedented As6 -sawhorse-type compound [As2 (AsEt CAAC)4 ] (2). The reaction of (LE)2 (L=PhC(Nt Bu)2 ; E=Si, Ge) with I resulted in a rearrangement and an insertion of LE fragments, forming unique silicon- (4: [Cp*Fe(η4 -As4 SiL)], 5 a: [Cp*Fe(η4 -As6 SiL)) and germanium-containing (5 b: [Cp*Fe(η4 -As6 GeL)) cyclic polyarsenic ligand complexes.

Synthesis and Reactivity of N-Heterocyclic Carbene Coordinated Formal Germanimidoyl-Phosphinidenes

Treatment of N-heterocyclic carbene (NHC) ligated germylidenylphosphinidene MsFluidtBu-GeP(NHCiPr) (where MsFluidtBu is a bulky hydrindacene substituent, and NHCiPr is 1,3-diisopropyl-4,5-dimethyl-imidazolin-2-ylidene) with mesityl azide and 4-tertbutylphenyl azide afforded NHC coordinated formal germanimidoyl-phosphinidenes, which represent the first compounds bearing both Ge═N double bond and phosphinidene functionalities. Studies of the chemical properties revealed that the reactions preferred to occur at the Ge═N double bond, which underwent [2 + 2] cycloadditions with CO2 and ethyl isocyanate, and coordinated with coinage metals through the nitrogen atom.

Geometrically Compelled Silicon(II)/Silicon(IV) Donor-Acceptor Interaction Enables the Enamination of Nitriles

Discovering new bonding scenarios and subsequently exploring the reactivity contribute substantially to advance the main group element chemistry. Herein, we report on the isolation and characterization of an intriguing class of the hydrido-benzosiloles 2-4. These compounds exhibit a side arm of the amidinatosilylenyl group, featuring unidirectional silicon(II)/silicon(IV) donor-acceptor interaction on account of the geometric constraint. Furthermore, the reactions involving 2-4 with nitriles yield the tricyclic compounds that edge-fused of the Si-heteroimidazolidine-CN2 Si2 , silole-C4 Si, and phenyl-C6 -rings (5-13). These compounds are manifesting a unique reaction that the silicon(II)/silicon(IV) interaction enables the enamination of the α-H-bearing nitriles. The reaction mechanism involved in H-shift under oxidative addition at silylene followed by hydrosilylation of a ketenimine intermediate was revealed by density function theory (DFT) calculations.

Snapshots of sequential polyphosphide rearrangement upon metallatetrylene addition

Insertion and functionalization of gallasilylenes [LPhSi-Ga(Cl)LBDI] (LPh = PhC(NtBu)2; LBDI = [{2,6-iPr2C6H3NCMe}2CH]) into the cyclo-E5 rings of [Cp*Fe(η5-E5)] (Cp* = η5-C5Me5; E = P, As) are reported. Reactions of [Cp*Fe(η5-E5)] with gallasilylene result in E-E/Si-Ga bond cleavage and the insertion of the silylene in the cyclo-E5 rings. [(LPhSi-Ga(Cl)LBDI){(η4-P5)FeCp*}], in which the Si atom binds to the bent cyclo-P5 ring, was identified as a reaction intermediate. The ring-expansion products are stable at room temperature, while isomerization occurred at higher temperature, and the silylene moiety further migrates to the Fe atom, forming the corresponding ring-construction isomers. Furthermore, reaction of [Cp*Fe(η5-As5)] with the heavier gallagermylene [LPhGe-Ga(Cl)LBDI] was also investigated. All the isolated complexes represent rare examples of mixed group 13/14 iron polypnictogenides, which could only be synthesized by taking advantage of the cooperativity of the gallatetrylenes featuring low-valent Si(ii) or Ge(ii) and Lewis acidic Ga(iii) units/entities.

Gradual Donor Stabilization of a Transient Ferrocene Bridged Bisphosphanyl Phosphenium Cation

A transient phosphenium cation embedded into a [3]ferrocenophane scaffold was formed via chloride abstraction. The cation has been trapped with phosphane, carbene, and silylene donors resulting in stable adducts or bond activation of the ferrocenophane bridge. In the absence of donors, dimerization of the phosphenium cation to the corresponding dication is observed or P-C bond activation with migration of a substituent leading to a putative phosphoniodiphosphene. Using 1,3-di-tert-butylimidazol-2-silylene as the donor, further reaction of the initially formed chlorosilane leads to activation of a P-P bond of the ferrocenophane scaffold with ring expansion of the ansa-bridge. The donor formation and bonding situation are investigated by density functional theory calculations as well as experimental methods (e.g., NMR spectroscopy and X-ray crystallography).

Stimuli Responsive Silylene: Electromerism Induced Reversible Switching Between Mono- and Bis-Silylene

Electromerism is a very well-known phenomenon in transition metal chemistry. In main group chemistry, this concept has only started getting attention recently. We report stimuli responsive low-valent silicon compounds exhibiting electromerism. A mixed-valent silaiminyl-silylene 1, [LSi-Si(NDipp)L] (L=PhC(N^t Bu)₂ ), was synthesized in a single step from amidinate-chlorosilylene. Compound 1 has two interconnected Si atoms in formally +I and +III oxidation states. Upon treatment with Lewis acidic Cu^I X (X=mesityl, Cl, Br, I), electron redistribution occurs resulting in the formation of [{LSi(NDipp)Si(L)}-CuX], in which both silicon atoms are in the +II formal oxidation state. Removal of the copper center from [{LSi(NDipp)Si(L)}-CuX] by using a Lewis basic carbene led to reformation of the precursor [LSi-Si(NDipp)L]. Thus, the process is fully reversible. This showcases the first example of Lewis acid/base-induced reversible electromerism in silicon chemistry.

Reactivity of Sodium Pentaphospholide Na[ cyclo ‐P 5 ] towards C≡E (E=C, N, P) Triple Bonds

A diglyme solution of Na[cyclo-P₅ ] (1) reacts with alkynes and isolobal nitriles and phosphaalkynes to afford the otherwise elusive (aza)phospholide anions 2 a-c, 4 a,b, and 6. The reaction of Na[cyclo-P₅ ] with alkynes and nitriles was studied by means of DFT methods, which suggested a concerted mechanism for the formation of 2 a and 4 b. The anions 2 a-c, 4 a,b, and 6 coordinate in an η⁵ -fashion towards Fe^II to give the sandwich (aza)phosphametallocenes 3 a-c, 5 a,b and 7 in moderate to good yields. The new compounds were characterized by means of multinuclear NMR spectroscopy, single-crystal X-ray diffraction and cyclic voltammetry.

Pn Ring Size Dependence on NHC-Induced Ring Contraction Reactions of [CoCp‴(η4-P4)], [Ta(CO)2Cp″(η4-P4)], and [FeCp*(η5-P5)]: A DFT Study

The facile ring contraction of [CoCp‴(η⁴-P₄)] and [Ta(CO)₂Cp″(η⁴-P₄)] to [CoCp‴(η³-P₃)][(^MeNHC)₂P] and [Ta(CO)₂Cp″(η³-P₃)] [(^MeNHC)₂P] induced by ^MeNHC and the absence of the ring contraction of [FeCp*(η⁵-P₅)] under the same conditions are studied by density functional theory (DFT) computations. The latter is estimated to be thermodynamically the least favorable reaction and also has a very high energy barrier. The similar strain energies of P₃ and P₄ rings and the lower strain energy of the P₅ ring play a decisive role in the ring contraction capability of these [TM-cyclo-P_n] complexes. Theoretical approaches involving NBO and IBO analysis have been employed to provide a qualitative picture of the overall reactions. The role of substituents and the nature of transition metals in determining the energetics of these reactions has also been studied and an isolobal perspective on these systems affords a simplified picture.

Triple-decker complexes incorporating three distinct deck architectures

The reactivity of the dilithioplumbole ([Li₂(thf)₂(μ,η⁵-L^Pb)], L^Pb = 1,4-bis-tert-butyl-dimethylsilyl-2,3-bis-phenyl-plumbolyl) towards the reactive pnictogen precursors P₄, pentaphosphaferrocene, and pentaarsaferrocene ([Cp*Fe(η⁵-E₅)] (Cp* = η⁵-C₅Me₅, E = P, As)) is reported. The reaction with P₄ afforded a phospholyl lithium complex, via lead-phosphorus exchange, while the reactions with [Cp*Fe(η⁵-E₅)] yielded the first examples of Pb-Fe-Li heterotrimetallic triple-decker polypnictogenides with three different deck motifs.

Substituted aromatic pentaphosphole ligands - a journey across the p-block

The functionalization of pentaphosphaferrocene [Cp*Fe(η5-P5)] (1) with cationic group 13-17 electrophiles is shown to be a general synthetic strategy towards P-E bond formation of unprecedented diversity. The products of these reactions are dinuclear [{Cp*Fe}2{μ,η5:5-(P5)2EX2}][TEF] (EX2 = BBr2 (2), GaI2 (3), [TEF]- = [Al{OC(CF3)3}4]-) or mononuclear [Cp*Fe(η5-P5E)][X] (E = CH2Ph (4), CHPh2 (5), SiHPh2 (6), AsCy2 (7), SePh (9), TeMes (10), Cl (11), Br (12), I (13)) complexes of hetero-bis-pentaphosphole ((cyclo-P5)2R) or hetero-pentaphosphole ligands (cyclo-P5R), the aromatic all-phosphorus analogs of prototypical cyclopentadienes. Further, modifying the steric and electronic properties of the electrophile has a drastic impact on its reactivity and leads to the formation of [Cp*Fe(μ,η5:2-P5)SbICp'''][TEF] (8) which possesses a triple-decker-like structure. X-ray crystallographic characterization reveals the slightly twisted conformation of the cyclo-P5R ligands in these compounds and multinuclear NMR spectroscopy confirms their integrity in solution. DFT calculations shed light on the bonding situation of these compounds and confirm the aromatic character of the pentaphosphole ligands on a journey across the p-block.

A Structural Diversity of Molecular Alkaline-Earth-Metal Polyphosphides: From Supramolecular Wheel to Zintl Ion

A series of molecular group 2 polyphosphides has been synthesized by using air-stable [Cp*Fe(η5 -P5 )] (Cp*=C5 Me5 ) or white phosphorus as polyphosphorus precursors. Different types of group 2 reagents such as organo-magnesium, mono-valent magnesium, and molecular calcium hydride complexes have been investigated to activate these polyphosphorus sources. The organo-magnesium complex [(Dipp BDI-Mg(CH3 ))2 ] (Dipp BDI={[2,6-i Pr2 C6 H3 NCMe]2 CH}- ) reacts with [Cp*Fe(η5 -P5 )] to give an unprecedented Mg/Fe-supramolecular wheel. Kinetically controlled activation of [Cp*Fe(η5 -P5 )] by different mono-valent magnesium complexes allowed the isolation of Mg-coordinated formally mono- and di-reduced products of [Cp*Fe(η5 -P5 )]. To obtain the first examples of molecular calcium-polyphosphides, a molecular calcium hydride complex was used to reduce the aromatic cyclo-P5 ring of [Cp*Fe(η5 -P5 )]. The Ca-Fe-polyphosphide is also characterized by quantum chemical calculations and compared with the corresponding Mg complex. Moreover, a calcium coordinated Zintl ion (P7 )3- was obtained by molecular calcium hydride mediated P4 reduction.

Versatile Coordination of AgI and CuI Ions towards cyclo-As5 Ligands

The reactions of the cyclo‐As₅ complex [Cp*Fe(η⁵‐As₅)] (B) with the Ag^I and Cu^I salts of the weakly coordinating anion (WCA) [FAl{OC₆F₁₀(C₆F₅)}₃]⁻ ([FAl]⁻) are studied. These reactions allow the synthesis of the mononuclear complexes [M(η⁵ : η²‐B)₂][FAl] (M=Ag (1), Cu (2)) when a ratio of B/M(FAl) 2 : 1 is used. Compound 1 shows an unusual disorder of the central Ag^I cation between two π‐coordinating cyclo‐As₅ ligands, which is absent in 2 pointing to a weak interaction of the Ag center towards the cyclo‐As₅ ligands in B. When the ratio of B/Ag(FAl) is changed to 3 : 1 or 1 : 1, the respective coordination compounds [Ag(η²‐B)₃][FAl] (3) and [Ag₂(η² : η²‐B)₂][FAl]₂ (4) are accessible. The coordination modes of the cyclo‐As₅ units in 1, 3 and 4 are all different, reflecting the adaptive coordination behavior of B towards Ag^I ions. The optimized geometries in the gas phase of 1–4 are determined by DFT calculations to support the bonding situation observed in their solid‐state structures.

Selective Route to Stable Silicon-Boron Radicals and Their Corresponding Cations

Herein, we report on a facile and selective one-pot synthetic route to silicon-boron radicals. Reduction of Br₂BTip (Tip = 2,4,6-ⁱPrC₆H₂) with KC₈ in the presence of LSi-R affords LSi(^tBu)-B(Br)Tip (1) and LSi(N(TMS)₂)-B(Br)Tip (2) [L = PhC(N^tBu)₂]. These first examples of silicon-boron isolated radical species feature spin density on the silicon and boron atoms. 1 and 2 exhibit extraordinary stability to high temperatures under inert conditions in solution and air stability in the solid state. Both radicals have been isolated and fully characterized by electron paramagnetic resonance spectroscopy, SQUID magnetometry, mass spectrometry, cyclic voltammetry, single-crystal X-ray structure analysis, and density functional theory calculations. Moreover, compound 1 exhibits one-electron transfer when treated with 1 equiv of AgSO₃CF₃ and [Ph₃C]⁺[B(C₆F₅)₄]^-, respectively, resulting in the corresponding cations [LSi(^tBu)-B(Br)Tip]⁺[CF₃SO₃]^- (3) and [LSi(^tBu)-B(Br)Tip]⁺[B(C₆F₅)₄]^- (4). Compounds 3 and 4 have been characterized with multinuclear NMR and mass spectrometry.

Synthesis and Multiple Subsequent Reactivity of Anionic cyclo-E3 Ligand Complexes (E=P, As)

A synthetic pathway for the synthesis of novel anionic sandwich complexes with a cyclo-E3 (E=P, As) ligand as an end deck was developed giving [Cp'''Co(η3 -E3 )]- (Cp'''=1,2,4-tri-tert-butylcyclopentadienyl, E=P ([5]), As ([6])) in good yields suitable for further reactivity studies. In the reaction with the chlorophosphanes R2 PCl (R=Ph, Cy, t Bu), neutral complexes with a disubstituted cyclo-E3 P (E=P, As) ligand in [Cp'''Co(η3 -E3 PR2 )] (E=P (7 a-c), As (9 a-c)) were obtained. These compounds can be partially or completely converted into complexes with a cyclo-E3 (E=P, As) ligand with an exocyclic {PR2 } unit in [Cp'''Co(η2 :η1 -E3 PR2 )] (E=P (8 a-c), As (10 a-c)). Additionally, the insertion of the chlorosilylene [LSiCl] (L=(t BuN)2 CPh) into the cyclo-E3 ligand of [5] and [6] was achieved and the novel heteroatomic complexes [Cp'''Co(η3 -E3 SiL)] (E=P (11), As (12)) could be isolated. The reaction pathway was elucidated by DFT calculations.

Reactivity of Cu(I) Nacnac Complexes Toward Polypnictogen Compounds

The nacnac Cu(I) compound [LCu(MeCN)] (2) (L = [{N(C₆H₃Me₂-2,6)C(Me)}₂CH]^-) was reacted with complexes containing aromatic cyclo-E₅ ([Cp*Fe(η⁵-E₅)], E = P (1a), As (1b), Cp* = η⁵-C₅Me₅), cyclo-P₄ ([Cp‴Co(η⁴-P₄)] (3), Cp‴ = η⁵-C₅H₂^tBu₃) and cyclo-E₃ ligands ([Cp‴Ni(η³-E₃)], E = P (4a), As (4b)) yielding the heterometallic complexes [(Cp*Fe)(μ,η^5:2-E₅)(LCu)] (E = P (5a), As (5b)), [(Cp*Fe)(μ₃,η^5:2:1-E₅)(LCu)₂] (E = P (6a), As (6b)), [(Cp‴Co)(μ,η^4:2-P₄)(LCu)] (7), [(Cp‴Co)(μ₃,η^4:2:1-P₄)(LCu)₂] (8), and [(Cp‴Ni)(μ,η^3:2-E₃)(LCu)] (E = P (9a), As (9b)). These complexes are rare examples of the coordination of a group 11 metal to aromatic cyclo-E_n (E = P, As; n = 3-5) ligands. All products were comprehensively characterized by crystallographic and spectroscopic methods. Their dynamic behavior in solution was studied by VT (variable-temperature) NMR spectroscopy, and their electronic structures were elucidated by DFT calculations.

Reversible Activation and Transfer of White Phosphorus by Silyl-Stannylene

Use of a silyl supported stannylene (Mes TerSn(Sit Bu3 ) [Mes Ter=2,6-(2,4,6-Me3 C6 H2 )2 C6 H3 ] enables activation of white phosphorus under mild conditions, which is reversible under UV light. The reaction of a silylene chloride with the activated P4 complex results in facile P-atom transfer. The computational analysis rationalizes the electronic features and high reactivity of the heteroleptic silyl-substituted stannylene in contrast to the previously reported bis(aryl)stannylene.

Transition-Metal-Mediated Functionalization of White Phosphorus

Recently there has been great interest in the reactivity of transition-metal (TM) centers towards white phosphorus (P4 ). This has ultimately been motivated by a desire to find TM-mediated alternatives to the current industrial routes used to transform P4 into myriad useful P-containing products, which are typically indirect, wasteful, and highly hazardous. Such a TM-mediated process can be divided into two steps: activation of P4 to generate a polyphosphorus complex TM-Pn , and subsequent functionalization of this complex to release the desired phosphorus-containing product. The former step has by now become well established, allowing the isolation of many different TM-Pn products. In contrast, productive functionalization of these complexes has proven extremely challenging and has been achieved only in a relative handful of cases. In this review we provide a comprehensive summary of successful TM-Pn functionalization reactions, where TM-Pn must be accessible by reaction of a TM precursor with P4 . We hope that this will provide a useful resource for continuing efforts that are working towards this highly challenging goal of modern synthetic chemistry.

Synthesis and Reactivity of Bis(silylene)-Coordinated Calcium and Divalent Lanthanide Complexes

Divalent lanthanide complexes of Eu (1) and Yb (2) coordinated by a chelating pyridine-based bis(silylene) ligand were isolated and fully characterized. Compared to the EuII complex 1, the YbII complex 2 presents a lower thermal stability, resulting in the activation of one SiII -N bond and formation of an YbIII complex (3), which features a unique silylene-pyridyl-amido ligand. The different thermal stability of 1 and 2 points towards reduction-induced cleavage of one SiII -N bond of the bis(silylene) ligand. Successful isolation of the corresponding redox-inert bis(silylene) CaII complex (5) was achieved at low temperature and thermal decomposition into a CaII complex (4) bearing the same silylene-pyridyl-amido ligand was identified. In this case, the thermolysis reaction proceeds through another, non-redox induced, mechanism. An alternative higher yielding route to 4 was developed through an in situ generated silylene-pyridyl-amine proligand.

Regioselective Insertion of Aluminum(I) in the cyclo-P5 Ring of Pentaphosphaferrocene

A route to directly access mixed Al-Fe polyphosphide complexes was developed. The reactivity of pentaphosphaferrocene, [Cp*Fe(η5 -P5 )] (Cp*=C5 Me5 ), with two different low-valent aluminum compounds was investigated. The steric and electronic environment around the [AlI ] centre are found to be crucial for the formation of the resulting Al-Fe polyphosphides. Reaction with the sterically demanding [Dipp-BDIAlI ] (Dipp-BDI={[2,6-i Pr2 C6 H3 NCMe]2 CH}- ) resulted in the first Al-based neutral triple-decker type polyphosphide complex. For [(Cp*AlI )4 ], an unprecedented regioselective insertion of three [Cp*AlIII ]2+ moieties into two adjacent P-P bonds of the cyclo-P5 ring of [Cp*Fe(η5 -P5 )] was observed. The regioselectivity of the insertion reaction could be rationalized by isolating an analogue of the reaction intermediate stabilized by a strong σ-donor carbene.

A bis(germylene) functionalized metal-coordinated polyphosphide and its isomerization

Pentaphosphaferrocene, [Cp*Fe(η⁵-P₅)], was used as an air-stable source of polyphosphorus units to synthesize a novel bis(germylene)-functionalized transition metal polyphosphide complex, which further undergoes a unique constitutional isomerization.