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.

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.

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.

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.

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.