Transfer Reagent for Bonding Isomers of Iron Complexes

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''2 Zr(η1:1 -E4 )] (E=P, As) towards Nucleophiles

The functionalization of the polypnictogen ligand complexes [Cp''₂ Zr(η^1:1 -E₄ )] (E=P (1 a), As (1 b); Cp''=1,3-di-tertbutyl-cyclopentadienyl) is focused to modify the features of the polypnictogen unit to explore new synthetic pathways for further transformations. The reaction behavior of 1 towards main group nucleophiles is investigated. The reaction of 1 a with ^t BuLi leads to the ionic product Li[Cp''₂ Zr(η^1:1 -P₄ ^t Bu)] (2) where an organic group is attached to a bridgehead phosphorus atom of the butterfly unit. Further reactions of 2 with quenching electrophilic reagents enable the introduction of other substituents. Moreover, a condensation of 2 to [(Cp''₂ Zr)₂ (μ,η^1:1:1:1 -P₈ ^t Bu₂ )] (3), containing a novel P₈ -unit, has been observed. The reaction of 1 with LiNMe₂ and LiCH₂ SiMe₃ leads to a partial fragmentation of the E₄ unit and the compounds [Cp''₂ Zr(η² -E₃ Nu)] (Nu=NMe₂ : E=P (6 a), As (6 b); Nu=CH₂ SiMe₃ : E=P (7 a), As (7 b)) are formed.

Synthesis and Characterization of [Fe3 (As3 )3 (As4 )]3- , a Binary Fe/As Zintl Cluster With an Fe3 Core

We report here the synthesis and structural characterization of the first binary iron arsenide cluster anion, [Fe₃ (As₃ )₃ (As₄ )]^3- , present in both [K([2.2.2]crypt)]₃ [Fe₃ (As₃ )₃ (As₄ )] (1) and [K(18-crown-6)]₃ [Fe₃ (As₃ )₃ (As₄ )]⋅en (2). The cluster contains an Fe₃ triangle with three short Fe-Fe bond lengths (2.494(1) Å, 2.459(1) Å and 2.668(2) Å for 1, 2.471(1) Å, 2.473(1) Å and 2.660(1) Å for 2), bridged by a 2-butene-like As₄ unit. An analysis of the electronic structure using DFT reveals a triplet ground state with direct Fe-Fe bonds stabilizing the Fe₃ core.

Binding, Release and Functionalization of Intact Pnictogen Tetrahedra Coordinated to Dicopper Complexes

The bridging MeCN ligand in the dicopper(I) complexes [(DPFN)Cu2 (μ,η1  : η1 -MeCN)][X]2 (X=weakly coordinating anion, NTf2 (1 a), FAl[OC6 F10 (C6 F5 )]3 (1 b), Al[OC(CF3 )3 ]4 (1 c)) was replaced by white phosphorus (P4 ) or yellow arsenic (As4 ) to yield [(DPFN)Cu2 (μ,η2  : η2 -E4 )][X]2 (E=P (2 a-c), As (3 a-c)). The molecular structures in the solid state reveal novel coordination modes for E4 tetrahedra bonded to coinage metal ions. Experimental data and quantum chemical computations provide information concerning perturbations to the bonding in coordinated E4 tetrahedra. Reactions with N-heterocyclic carbenes (NHCs) led to replacement of the E4 tetrahedra with release of P4 or As4 and formation of [(DPFN)Cu2 (μ,η1  : η1 -Me NHC)][X]2 (4 a,b) or to an opening of one E-E bond leading to an unusual E4 butterfly structural motif in [(DPFN)Cu2 (μ,η1  : η1 -E4 Dipp NHC)][X]2 (E=P (5 a,b), E=As (6)). With a cyclic alkyl amino carbene (Et CAAC), cleavage of two As-As bonds was observed to give two isomers of [(DPFN)Cu2 (μ,η2  : η2 -As4 Et CAAC)][X]2 (7 a,b) with an unusual As4 -triangle+1 unit.

E4 Transfer (E=P, As) to Ni Complexes

The use of [Cp''2 Zr(η1:1 -E4 )] (E=P (1 a), As (1 b), Cp''=1,3-di-tert-butyl-cyclopentadienyl) as phosphorus or arsenic source, respectively, gives access to novel stable polypnictogen transition metal complexes at ambient temperatures. The reaction of 1 a/1 b with [CpR NiBr]2 (CpR =CpBn (1,2,3,4,5-pentabenzyl-cyclopentadienyl), Cp''' (1,2,4-tri-tert-butyl-cyclopentadienyl)) was studied, to yield novel complexes depending on steric effects and stoichiometric ratios. Besides the transfer of the complete En unit, a degradation as well as aggregation can be observed. Thus, the prismane derivatives [(Cp'''Ni)2 (μ,η3:3 -E4 )] (2 a (E=P); 2 b (E=As)) or the arsenic containing cubane [(Cp'''Ni)3 (μ3 -As)(As4 )] (5) are formed. Furthermore, the bromine bridged cubanes of the type [(CpR Ni)3 {Ni(μ-Br)}(μ3 -E)4 ]2 (CpR =Cp''': 6 a (E=P), 6 b (E=As), CpR =CpBn : 8 a (E=P), 8 b (E=As)) can be isolated. Here, a stepwise transfer of En units is possible, with a cyclo-E4 2- ligand being introduced and unprecedented triple-decker compounds of the type [{(CpR Ni)3 Ni(μ3 -E)4 }2 (μ,η4:4 -E'4 )] (CpR =CpBn , Cp'''; E/E'=P, As) are obtained.

Transfer of polyantimony units

Transfer reagents are useful tools in chemistry to access metastable compounds. The reaction of [Cp′′₂ZrCl₂] with KSb(SiMe₃)₂ leads to the formation of the novel polyantimony triple decker complex [(Cp′′Zr)₂(μ,η^1:1:1:1:1:1-Sb₆)] (1, Cp′′ = 1,3-di-tertbutyl-cyclopentadienyl), containing a chair-like Sb₆⁶⁻ ligand. Compound 1 represents a valuable transfer reagent to form novel antimony ligand complexes. Thus, the reaction of 1 with Cp^R-substituted transition metal halides of nickel, cobalt and iron leads to the formation of a variety of novel Sb_n ligand complexes, such as the cubane-like compounds [(Cp′′′Ni)₄(μ₃-Sb)₄] (2) and [(Cp′′′Co)₄(μ₃-Sb₄)] (3a) or the complexes [(Cp^BnCo)₃(μ₃-Sb)₂] (4) and [(Cp′′′Fe)₃(μ₃-Sb)₂] (5), representing a trigonal bipyramidal structure. Moreover, beside the transfer of Sb₁ units, also the complete entity can be transferred as seen in the iron complex [(Cp′′′Fe)₃(μ_3,η^4:4:4-Sb₆)] (6). DFT calculations shed light on the bonding situation of the products.

The synthesis and characterization of the unique polyantimony complex [(Cp′′Zr)₂(μ,η^1:1:1:1:1:1-Sb₆)] (1) is described. Compound 1 was used as antimony source to transfer Sb_n units to late transition metal fragments [Cp^RM] (M = Fe, Co, Ni).

Redox Chemistry of Heterobimetallic Polypnictogen Triple-Decker Complexes - Rearrangement, Fragmentation and Transfer

The redox chemistry of the heterobimetallic triple‐decker complexes [(Cp*Fe)(Cp′′′Co)(μ,η⁵:η⁴‐E₅)] (E=P (1), As (2), Cp*=1,2,3,4,5‐pentamethyl‐cyclopentadienyl, Cp′′′=1,2,4‐tri‐tertbutyl‐cyclopentadienyl) and [(Cp′′′Co)(Cp′′′Ni)(μ,η³:η³‐E₃)] (E=P (10), As (11)) was investigated. Compound 1 and 2 could be oxidized to the monocations 3 and 4 and further to the dications 5 and 6, while the initially folded cyclo‐E₅ ligand planarizes upon oxidation. The reduction leads to an opposite change in the geometry of the middle deck, which is now folded stronger into the direction of the other metal fragment (formation of monoanions 7 and 8). For the arsenic compound 8, a different behavior is found since a fragmentation into an As₆ (9) and As₃ ligand complex occurs. The Co and Ni triple‐decker complexes 10 and 11 can be oxidized initially to the heterometallic monocations 12 and 13, which are not stable in solution and convert selectively into the homometallic nickel complexes 14 and 15 and the cobalt complexes 16 and 17. This behavior was further proven by the oxidation of [(Cp′′′Co)(Cp′′Ni)(μ,η³:η²‐P₃)] (19, Cp′′=1,3‐di‐tertbutyl‐cyclopentadienyl) comprising two different Cp ligands. The transfer of {Cp^RM} fragments can be suppressed when a {W(CO)₅} unit is coordinated to the P₃ ligand (20) prior to the oxidation and the mixed cobalt and nickel cation 21 can be isolated. The reduction of 10 and 11 yields the heterometallic monoanions 22 and 23, where no transfer of the {Cp^RM} fragments is observed.

Coordination Behavior of [Cp″2Zr(µ1:1-As4)] towards Lewis Acids

The functionalization of the arsenic transfer reagent [Cp″2Zr(η1:1-As4)] (1) focuses on modifying its properties and enabling a broader scope of reactivity. The coordination behavior of 1 towards different Lewis-acidic transition metal complexes and main group compounds is investigated by experimental and computational studies. Depending on the steric requirements of the Lewis acids and the reaction temperature, a variety of new complexes with different coordination modes and coordination numbers could be synthesized. Depending on the Lewis acid (LA) used, a mono-substitution in [Cp″2Zr(µ,η1:1:1:1-As4)(LA)] (LA = Fe(CO)4 (4); B(C6F5)3 (7)) and [Cp″2Zr(µ,η3:1:1-As4)(Fe(CO)3)] (5) or a di-substitution [Cp″2Zr(µ3,η1:1:1:1-As4)(LA)2] (LA = W(CO)5 (2); CpMn(CO)2 (3); AlR3 (6, R = Me, Et, iBu)) are monitored. In contrast to other coordination products, 5 shows an η3 coordination in which the butterfly As4 ligand is rearranged to a cyclo-As4 ligand. The reported complexes are rationalized in terms of inverse coordination.

Aryl-substituted triarsiranes: synthesis and reactivity

Cyclotriarsanes are rare and described herein is a scalable synthetic protocol towards (AsAr)3, which allowed to study their reactivity towards [Cp2Ti(C2(SiMe3)2)], affording titanocene diarsene complexes, and towards N-heterocyclic carbenes (NHCs) to give straightforward access to a variety of NHC-arsinidene adducts.

Atom Exchange Versus Reconstruction: (Gex As4-x )x- (x=2, 3) as Building Blocks for the Supertetrahedral Zintl Cluster [Au6 (Ge3 As)(Ge2 As2 )3 ]3

The Zintl anion (Ge2 As2 )2- represents an isostructural and isoelectronic binary counterpart of yellow arsenic, yet without being studied with the same intensity so far. Upon introducing [(PPh3 )AuMe] into the 1,2-diaminoethane (en) solution of (Ge2 As2 )2- , the heterometallic cluster anion [Au6 (Ge3 As)(Ge2 As2 )3 ]3- is obtained as its salt [K(crypt-222)]3 [Au6 (Ge3 As)(Ge2 As2 )3 ]⋅en⋅2 tol (1). The anion represents a rare example of a superpolyhedral Zintl cluster, and it comprises the largest number of Au atoms relative to main group (semi)metal atoms in such clusters. The overall supertetrahedral structure is based on a (non-bonding) octahedron of six Au atoms that is face-capped by four (Gex As4-x )x- (x=2, 3) units. The Au atoms bind to four main group atoms in a rectangular manner, and this way hold the four units together to form this unprecedented architecture. The presence of one (Ge3 As)3- unit besides three (Ge2 As2 )2- units as a consequence of an exchange reaction in solution was verified by detailed quantum chemical (DFT) calculations, which ruled out all other compositions besides [Au6 (Ge3 As)(Ge2 As2 )3 ]3- . Reactions of the heavier homologues (Tt2 Pn2 )2- (Tt=Sn, Pb; Pn=Sb, Bi) did not yield clusters corresponding to that in 1, but dimers of ternary nine-vertex clusters, {[AuTt5 Pn3 ]2 }4- (in 2-4; Tt/Pn=Sn/Sb, Sn/Bi, Pb/Sb), since the underlying pseudo-tetrahedral units comprising heavier atoms do not tend to undergo the said exchange reactions as readily as (Ge2 As2 )2- , according to the DFT calculations.

Iodination of cyclo-E5 -Complexes (E=P, As)

In a high‐yield one‐pot synthesis, the reactions of [Cp*M(η⁵‐P₅)] (M=Fe (1), Ru (2)) with I₂ resulted in the selective formation of [Cp*MP₆I₆]⁺ salts (3, 4). The products comprise unprecedented all‐cis tripodal triphosphino‐cyclotriphosphine ligands. The iodination of [Cp*Fe(η⁵‐As₅)] (6) gave, in addition to [Fe(CH₃CN)₆]²⁺ salts of the rare [As₆I₈]²⁻ (in 7) and [As₄I₁₄]²⁻ (in 8) anions, the first di‐cationic Fe‐As triple decker complex [(Cp*Fe)₂(μ,η^5:5‐As₅)][As₆I₈] (9). In contrast, the iodination of [Cp*Ru(η⁵‐As₅)] (10) did not result in the full cleavage of the M−As bonds. Instead, a number of dinuclear complexes were obtained: [(Cp*Ru)₂(μ,η^5:5‐As₅)][As₆I₈]_0.5 (11) represents the first Ru‐As₅ triple decker complex, thus completing the series of monocationic complexes [(Cp^RM)₂(μ,η^5:5‐E₅)]⁺ (M=Fe, Ru; E=P, As). [(Cp*Ru)₂As₈I₆] (12) crystallizes as a racemic mixture of both enantiomers, while [(Cp*Ru)₂As₄I₄] (13) crystallizes as a symmetric and an asymmetric isomer and features a unique tetramer of {AsI} arsinidene units as a middle deck.

Iron β-diiminate complexes with As2-, As4- and As8-ligands

Depending on the sterics of the flanking group at the β-diiminate ligand at Fe(i) a tetramerisation or dimerisation of As₄ units is observed; for the latter case the ligand conformation was influenced by the crystallization temperature.

Synthesis and structural characterization of arsinoamides - early transition metal (Zr and Hf) and main group metal (Al, In, Sn, and Pb) complexes

By reaction of MCl4 (M = Zr, Hf) with 2 equiv. of [(Mes2AsNPh){Li(OEt2)2}], the first group 4 metal arsinoamide complexes [(Mes2AsNPh)2MCl2(THF)] were synthesized. They feature two weak M-As interactions. After formally replacing the chloride atoms by the amido ligands [NMe2]-, a more diverse M-As interaction arises in [(Mes2AsNPh)2M(NMe2)2]: only one M-As contact can be observed with a substantially shorter distance. This type of interaction may originate from the steric effect of the substituents on the metal center. In addition, the coordination behavior of arsinoamide in main group metal chemistry was investigated. Thus, arsinoamide complexes of Al(iii), In(iii), Sn(ii) and Pb(ii) are reported. In contrast to the group 4 complexes, no M-As interaction can be detected in these four complexes.

The Chemistry of Yellow Arsenic

The generation and handling of the light-sensitive and metastable yellow arsenic (As₄) is extremely challenging. In view of recent breakthroughs in synthesizing As₄ storage materials and transfer reagents, the more intensive use of yellow arsenic as a source for further reactions can be expected. Given these aspects, the current stage of knowledge of the direct use of As₄ is comprehensively summarized in the present review, which lists the activation of As₄ by main group elements as well as transition metal compounds (including the f-block elements). Moreover, it also partly compares the reaction outcomes in relation to the corresponding reactions of P₄. The possibility of using alternative sources for generating arsenic moieties and compounds is also discussed. The release of As₄ molecules from precursor compounds and the use of transfer reagents for polyarsenic entities open up new synthetic pathways to avoid the direct generation of yellow arsenic solutions and to ensure its smooth usage for subsequent reactions.

Modern applications of low-valent early transition metals in synthesis and catalysis

Low-valent early transition metals are often intrinsically highly reactive as a result of their strong propensity toward oxidation to more stable high-valent states. Harnessing these highly reducing complexes for productive reactivity is potentially powerful for C-C bond construction, organic reductions, small-molecule activation and many other reactions that offer orthogonal chemoselectivity and/or regioselectivity patterns to processes promoted by late transition metals. Recent years have seen many exciting new applications of low-valent metals through building new catalytic and/or multicomponent reaction manifolds out of classical reactivity patterns. In this Review, we survey new methods that employ early transition metals and invoke low-valent precursors or intermediates in order to identify common themes and strategies in synthesis and catalysis.

The Influence of β-diiminato Ligands on As4 Activation by Cobalt Complexes

In a systematic study of the activation of As₄, three [LCo(tol)] (L=β‐diiminato) complexes have revealed different steric and electronic influences. 2,6‐Diisopropylphenyl (Dipp) and 2,6‐dimethylphenyl (dmp) flanking groups were used, one of the ligands with H backbone substituents (β‐dialdiminate L⁰) and two with Me substituents (β‐diketiminates L³ and L¹). In the reaction with As₄, different dinuclear products [(LCo)₂As₄] (LM=L⁰ (1), L¹ (2), L³ (3)) were isolated, with all showing differently shaped [Co₂As₄] cores in the solid state: octahedral in 1, prismatic in 2, and asterane‐like in 3. Thermal treatment of 3 leads to the abstraction of one arsenic atom to yield [(L³Co)₂As₃] (4). All products were comprehensively characterized by single‐crystal X‐ray diffraction, FD‐MS, and ¹H NMR spectroscopy. A rational explanation for the different reactivity is also proposed and DFT calculations shed light on the nature of the highly flexible [Co₂As₄] cores.

Mono- and dinuclear tetraphosphabutadiene ferrate anions

Reduction of [Cp^ArFe(μ-Br)]₂ (1, Cp^Ar = C₅(C₆H₄-4-Et)₅) by potassium napthalenide, followed by the addition of white phosphorus, affords [K(18-c-6){Cp^ArFe(η⁴-P₄)}] (2, 18-c-6 = [18]crown-6), which features a planar cyclo-P₄^2- ligand. The related diiron complex [Na₂(THF)₅(Cp^ArFe)₂(μ,η^4:4-P₄)] (3) was obtained by reducing 1 with sodium amalgam in the presence of P₄. Protonation of 3 affords [Na(THF)₃][(Cp^ArFe)₂(μ,η^4:4-P₄)(H)] (4), while the reaction of 3 with trimethylchlorosilane gives the nortricyclane compound P₇(SiMe₃)₃ as the main product.

Unexpected differences in the reactivity between the phosphorus and arsenic derivatives [(CpBIGFe)2(μ,η4:4-E4)] (E = P and As)

Different reaction pathways were determined between thermolysis and photolysis reactions of the butterfly complex [{Cp^BIGFe(CO)₂}₂(μ,η^1:1-As₄)] (1b) in comparison to its P analogues. The thermolysis leads to novel arsenic-rich compounds, whereas the photolysis of [{Cp^BIGFe(CO)₂}₂(μ,η^1:1-E₄)] (1a: E = P; 1b: E = As) exclusively results in the decarbonylation products.