Retention of the Zn−Zn bond in [Ge9 Zn−ZnGe9 ]6− and Formation of [(Ge9 Zn)−(Ge9 )−(ZnGe9 )]8− and Polymeric 1∞ [−(Ge9 Zn)2− −]1

Reactive Solubilization of Heterometallic Clusters by Treatment of (TrBi3 )2- Anions (Tr=Ga, In, Tl) with [Mn{N(SiMe3 )2 }2 ]

Lowering the charge of Zintl anions by (element-)organic substituents allows their use as sources of (semi)metal nanostructures in common organic solvents, as realized for group 15 anions or Ge9 4- and Sn9 4- . We developed a new strategy for other anions, using low-coordinate 3d metal complexes as electrophiles. [K(crypt-222)]+ salts of (TrBi3 )2- anions dissolved in situ in Et2 O and/or THF when reacted with [Mn(hmds)2 ]. Work-up afforded soluble [K(crypt-222)]+ salts of [{(hmds)2 Mn}2 (TlBi3 )]2- (in 1), [{(hmds)2 Mn}2 (Bi2 )]2- (in 2), and [{(hmds)Mn}4 (Bi2 )2 ]2- (in 3) (crypt-222=4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane; Tr=Ga, In, Tl; hmds=N(SiMe3 )2 ), representing rare cases of Zintl clusters with open-shell metal atoms. 1 comprises the first coordination compound of the (TlBi3 )2- anion, 2 features a diamond-shaped {Pn2 M2 } unit, and 3 is a mixed-valent MnI /MnII compound. The uncommon electronic structures in 1-3 and magnetic coupling were studied by comprehensive DFT calculations.

Missing Link in the Growth of Lead-Based Zintl Clusters: Isolation of the Dimeric Plumbaspherene [Cu4Pb22]4

We report here the structure of an endohedral plumbaspherene, [Cu₄Pb₂₂]^4–, the gold analogue of which was previously postulated to be a “missing link” in the growth of larger clusters containing three and four icosahedral subunits. The cluster contains two [Cu₂Pb₁₁]^2– subunits linked through a Cu₂Pb₄ trigonal antiprism. Density functional theory reveals that the striking ability of mixed Pb/coinage metal Zintl clusters to oligomerize and, in the case of Au, to act as a site of nucleation for additional metal atoms, is a direct consequence of their nd¹⁰(n + 1)s⁰ configuration, which generates both a low-lying (n + 1)s-based LUMO and also a high-lying Pb-centered HOMO. Cluster growth and nucleation is then driven by this amphoteric character, allowing the clusters to form donor–acceptor interactions between adjacent icosahedral units or to additional metal atoms.

[Co2@(Ge17Ni)]4-: the first edge-sharing double-cage endohedral germanide

We report here a new double-cage endohedral Ge cluster, [Co₂@(Ge₁₇Ni)]^4-, fused through two [Co@(Ge₉Ni)] moieties with a shared Ni-Ge edge. This ternary Co-Ge-Ni species not only represents the first double-cage example of an 18-vertex Zintl cluster, but also fills in the missing link of the edge fusion model in the double-cage systems.

Chemi-Inspired Silicon Allotropes-Experimentally Accessible Si9 Cages as Proposed Building Block for 1D Polymers, 2D Sheets, Single-Walled Nanotubes, and Nanoparticles

Numerous studies on silicon allotropes with three-dimensional networks or as materials of lower dimensionality have been carried out in the past. Herein, allotropes of silicon, which are based on structures of experimentally accessible [Si9]4- clusters known as stable anionic molecular species in neat solids and in solution, are predicted. Hypothetical oxidative coupling under the formation of covalent Si-Si bonds between the clusters leads to uncharged two-, one- and zero-dimensional silicon nanomaterials not suffering from dangling bonds. A large variety of structures are derived and investigated by quantum chemical calculations. Their relative energies are in the same range as experimentally known silicene, and some structures are even energetically more favorable than silicene. Significantly smaller relative energies are reached by the insertion of linkers in form of tetrahedrally connected Si atoms. A chessboard pattern built of Si9 clusters bridged by tetrahedrally connected Si atoms represents a two-dimensional silicon species with remarkably lower relative energy in comparison with silicene. We discuss the structural and electronic properties of the predicted silicon materials and their building block nido-[Si9]4- based on density functional calculations. All considered structures are semiconductors. The band structures exclusively show bands of low dispersion, as is typical for covalent polymers.

Synthesis and reactivity of heteroleptic zinc(I) complexes toward heteroallenes

Heteroleptic zinc(I) complexes L^1,2Zn-ZnCp* (L¹ = HC[C(CF₃)NC₆F₅]₂1; L² = HC[C(Me)NDipp]₂; Dipp = 2,6-i-Pr₂C₆H₃2) are synthesized by reactions of Cp*₂Zn₂ with L¹H and L²ZnH. 2 reacts with t-BuNCO to give unprecedented carbamate complex (4), while reactions with RN₃ gave bis-hexazene, triazenide, and trimeric azide complexes (5-7).

Contrasting Bonding Extremes in Two [Ge6]n- Complexes: A Wadian Polyhedron (n = 2) versus a Hydrocarbon-like 2c-2e Polygermanide (n = 12)

[Nb(η⁶-C₆H₃Me₃)₂] reacts with ethylenediamine (en) solutions of K₄Ge₉ in the presence of 18-crown-6 to give [(η⁶-C₆H₃Me₃)NbHGe₆]^2- (1) and [(η⁶-C₆H₃Me₃)NbGe₆Nb(η⁶-C₆H₃Me₃)]^2- (2) as their corresponding [K(18-crown-6)]⁺ salts. The crystalline solids are dark brown, air-sensitive, and sparingly soluble or insoluble in most solvents. The [K(18-crown-6)]⁺ salts of cluster ions 1 and 2 have been characterized by energy-dispersive X-ray (EDX) analysis, NMR studies, single-crystal X-ray diffraction, and electrospray ionization time-of-flight (ESI-TOF) mass spectrometry studies. Cluster ions 1 and 2 have markedly different [Ge₆] moieties: an electron-deficient carborane-like subunit in 1 and a two-center, two-electron cyclohexane-like subunit in 2.

Intermetallic phases meet intermetalloid clusters

In this article intermetalloid clusters of Cu-Zn, Cu-AI, Cu-Sn, and Cu-Pb are discussed. Intermetallic compounds based on these metal combinations are of the Hume-Rothery type with well-defined structures related to the valence electron count of the involved metals. Many Zintl-type and molecular clusters with these metals are known with remarkable structural parallels to the respective solid-state phases. On several examples, this article discusses intermetalloid clusters in terms of their metal core structures and relates them to structural principles in intermetallic solid-state phases. Also the syntheses of such clusters are addressed. Zintl-type and molecular clusters are most generally accessible from organometallic precursor complexes with redox processes between the different metals as an underlying synthesis concept.

[Sn8 ]6- -Bridged Mixed-Valence ZnI /ZnII in {[K2 ZnSn8 (ZnMes)]2 }4- Inverse Sandwich-Type Cluster Supported by a ZnI -ZnI Bond

Since [Sn₈ ]^6- was discovered from the solid-state phase in 2000, its solution chemistry has been elusive due to the high charges and chemical activity. Herein, we report the synthesis and characterization of an inverse sandwich-type cluster dimer {[K₂ ZnSn₈ (ZnMes)]₂ }^4- (1 a), in which the highly charged [Sn₈ ]^6- is captured by mixed-valence Zn^I /Zn^II to form the dimer {closo-[Zn₂ Sn₈ ]}₂ moieties bridged by a Zn-Zn bond. Such Zn-Sn cluster not only exhibits a novel example of mixed-valence Zn^I /Zn^II for stabilizing highly active anion species, but also indicates the [Sn₈ ]^6- cluster can act as a novel bridging ligand, like arene, with a η⁴ :η⁴ -fashion. Theoretical calculations indicate that a significant delocalization of electrons over Zn atoms plays a vital role in the stabilization of the [Sn₈ ]^6- species. The AdNDP and magnetic response analyses clearly showed the presence of local σ-aromaticity in three cluster fragments: two ZnSn₄ caps and Sn₈ square antiprism.

Solution-Based Group 14 Zintl Anions: New Frontiers and Discoveries

ConspectusGroup 14 Zintl anions [E_x]^q- (E = Si-Pb, x = 4, 5, 9, 10) are synthetically accessible, and their diverse chemical reactivity makes them valuable synthons in the construction of larger nanoclusters with remarkable structures, intriguing patterns of chemical bonding, and tunable physical and chemical properties. A plethora of novel cluster anions have now been isolated from the reactions of polyanionic [E_x]^q- precursors with low-valent d-/f-block metal complexes, main-group organometallics, or organics in polar aprotic solvents. The range of products includes intermetalloid clusters with transition metal atom(s) embedded in main-group element cages, organometallic Zintl anions in which [E_x]^q- acts as a ligand, intermetallic Zintl anions where [E_x]^q- is bridged by ligand-free transition metal atom(s), organo-Zintl anions where [E_x]^q- is functionalized with organic-group(s), and oligomers formed through oxidative coupling reactions. The synthesis and characterization of these unconventional complexes, where important contributions to stability come from ionic, covalent, and metal-metal bonds as well as weaker aurophilic and van der Waals interactions, extend the boundaries of coordination chemistry and solid-state chemistry. Substantial progress has been made in this field over the past two decades, but there are still many mysteries to unravel related to the cluster growth mechanism and the controllable synthesis of targeted clusters, along with the remarkable and diverse patterns of chemical bonding that present a substantial challenge to theory. In this Account, we hope to shed some light on the relationship between structure, electronic properties, and cluster growth by highlighting selected examples from our recent work on homoatomic deltahedral [E_x]^q- anions, including (1) germanium-based Zintl clusters, such as the supertetrahedral intermetallic clusters [M₆Ge₁₆]^4- (M = Zn, Cd) and the sandwich cluster {(Ge₉)₂[η⁶-Ge(PdPPh₃)₃]}^4- with a heterometallic Ge@Pd₃ interlayer; (2) tin-based intermetalloid clusters [M_x@Sn_y]^q- and the application of [Co@Sn9]^4- in bottom-up synthesis; and (3) lead clusters with precious metal cores, including the largest Zintl anion [Au₁₂Pb₄₄]^8-. In addition to their intrinsic appeal from a structural and electronic perspective, these new cluster anions also show promise as precursors for the development of new materials with applications in heterogeneous catalysis, where we have recently reported the selective reduction of CO₂.

Cluster Expansion versus Complex Formation: Coinage Metal Coordination to Silylated [Ge9] Cages

Deltahedral nine-atom tetrel element Zintl clusters are promising building blocks for the straightforward solution-based synthesis of intermetalloid clusters through the reaction with organometallic compounds. Herein we report on novel coordination sites of metal-N-heterocyclic carbene (NHC) complexes to [Ge₉] clusters and unexpected cluster isomerization. We present the synthesis of a series of coinage metal-NHC complexes of silylated [Ge₉] clusters [NHC^iPrCu(η⁴-Ge₉{Si(TMS)₃}₃)] (1; TMS = trimethylsilyl) and [NHC^RM(η⁴-Ge₉{Si(TMS)₃}₂)]^- (2a, M = Cu, R = iPr; 3a, M = Cu, R = Mes; 4a, M = Cu, R = Dipp; 5a, M = Ag, R = Dipp; 6a, M = Au, R = Dipp), in which the coinage metals coordinate to open rectangular cluster faces and act as additional cluster vertex atoms. Besides representing promising intermediates on the way to larger intermetalloid clusters, the formation of compound 1 shows that Cu-NHC fragments also coordinate to the open-square Ge faces of the tris-silylated [Ge₉] clusters, contrasting the typical interactions with triangular faces of tris-silylated [Ge₉] clusters. In compounds 3a and 4a bearing bulky NHC moieties, an unusual silyl group substitution pattern is observed in contrast to 2a, which corresponds to the silyl group arrangement of other metal complexes of bis-silylated [Ge₉] clusters. In this context, potential silyl group migration mechanisms are discussed.

Stabilizing a metalloid {Zn12} unit within a polymetallide environment in [K2Zn20Bi16]6

The access to molecules comprising direct Zn–Zn bonds has become very topical in recent years for various reasons. Low-valent organozinc compounds show remarkable reactivities, and larger Zn–Zn-bonded gas-phase species exhibit a very unusual coexistence of insulating and metallic properties. However, as Zn atoms do not show a high tendency to form clusters in condensed phases, synthetic approaches for generating purely inorganic metalloid Zn_x units under ambient conditions have been lacking so far. Here we show that the reaction of a highly reductive solid with the nominal composition K₅Ga₂Bi₄ with ZnPh₂ at room temperature yields the heterometallic cluster anion [K₂Zn₂₀Bi₁₆]^6–. A 24-atom polymetallide ring embeds a metalloid {Zn₁₂} unit. Density functional theory calculations reveal multicenter bonding, an essentially zero-valent situation in the cluster center, and weak aromaticity. The heterometallic character, the notable electron-delocalization, and the uncommon nano-architecture points at a high potential for nano-heterocatalysis.

Low-valent zinc clusters, though exceedingly rare, are appealing synthetic targets because there is evidence that they may show unconventional chemical and physical behavior. Here, the authors obtain a large heterometallic zinc-bismuth cluster anion and discover that it bears a metalloid {Zn₁₂} core with four-center bonding and essentially zero-valent character.

Intermediate snapshot on the insertion reaction of isocyanates into the Zn-Cp* bond of dizincocene Cp*2Zn2

Heteroleptic Zn(i) complexes Cp*Zn-Zn(N(R)C(Cp*)O) (R = Dipp = 2,6-i-Pr2-C6H32, t-Bu 3) with unsymmetrically η4-coordinated Cp* substituents represent snapshots of the insertion reaction of RNCO into the Zn-Cp* bond of Cp*2Zn21. The bonding situation in 2 and 3, which represent the first Zn(i) olefin complexes, was evaluated by computational calculation and further compared to other Zn(i) complexes.

Intermediates and products of the reaction of Zn(ii) organyls with tetrel element Zintl ions: cluster extension versus complexation

Upon reactions of Zintl ions with Zn(ii) organyls various Zn-Zintl clusters as well as Zn-amide intermediates were isolated.

Filling the void: controlled donor-acceptor interaction facilitates the formation of an M-M single bond in the zero oxidation state of M (M = Zn, Cd, Hg)

The intriguing question of whether it is possible to form a genuine M0-M0 single bond for the M2 species (M = Zn, Cd, Hg) is addressed here. So far, all the bonds reported in the literature are exclusively MI-MI. Herein, we present viable M2(NHBMe)2 (M = Zn, Cd, Hg; NHBMe = (HCNMe)2B) complexes in which the controlled donor-acceptor interaction leads to an M0-M0 single bond. In these complexes, M2 in the 1∑g ground state with the (nσg+)2(nσu+)2 (n = 7, 10 and 14 for M = Zn, Cd and Hg, respectively) valence electron configuration forms donor-acceptor bonding with singlet 2NHBMe ligands where a combined effect of dominant (+,-) σ-backdonation from the antibonding (nσu+)2 orbital of M2 to the 2NHBMe ligands and a somewhat weaker (+,+) σ-donation from the 2NHBMe ligands to the bonding (n + 1)σg+ orbital leads to the unorthodox bonding situation of forming an M-M single bond in the zero oxidation state by eventually nullifying one effect by another. This is an unprecedented situation in the sense that the NHBMe ligand acts as a strong σ-acceptor and a weaker σ-donor. A comparison with the experimentally reported M2(PhDipp)2 complexes reveals the uniqueness of the NHBMe ligand in exhibiting such a bonding scenario. The M2(NHBMe)2 complex is thermochemically viable with respect to possible dissociation channels at room temperature, except for metal extrusion processes, M2(NHBMe)2 → M + M(NHBMe)2 and M2(NHBMe)2 → M2 + (NHBMe)2. Although the latter two processes are exergonic, they are kinetically protected by a high free energy barrier of 26.5-39.5 kcal mol-1. The experimental characterization of M2(PhDipp)2 despite similar exergonic channels reveals such kinetic stability to be enough for the viability of the M2(NHBMe)2 complexes. Furthermore, the ligand exchange reaction considering M2(PhMe)2 as the starting material also turned out to be feasible. Therefore, the M2(NHBMe)2 complexes are the first cases that feature a neutral M2 moiety with a single M0-M0 covalent bond, where M is a Group 12 metal.

Synthesis and structure of a family of rhodium polystannide clusters [Rh@Sn10]3-, [Rh@Sn12]3-, [Rh2@Sn17]6- and the first triply-fused stannide, [Rh3@Sn24]5

Through relatively subtle changes in reaction conditions, we have been able to isolate four distinct Rh/Sn cluster compounds, [Rh@Sn10]3-, [Rh@Sn12]3-, [Rh2@Sn17]6- and [Rh3@Sn24]5-, from the reaction of K4Sn9 with [(COE)2Rh(μ-Cl)]2(COE = cyclooctene). The last of these has a hitherto unknown molecular topology, an edge-fused polyhedron containing three Rh@Sn10 subunits, and represents the largest endohedral Group 14 Zintl cluster yet to have been isolated from solution. DFT has been used to place these new species in the context of known cluster chemistry. ESI-MS experiments on the reaction mixtures reveal the ubiquitous presence of {RhSn8} fragments that may play a role in cluster growth.

Heterotetrametallic Re-Zn-Zn-Re Complex Generated by an Anionic Rhenium(I) β-Diketiminate

We report the synthesis of an anionic rhenium(I) compound, Na[Re(η⁵-Cp)(BDI)] (1; Cp = cyclopentadienide, BDI = N, N'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate) and initial investigations of its use as a strong chemical reductant and metalloligand. Chemical oxidation of 1 gives a rare example of a rhenium(II) compound Re(η⁵-Cp)(BDI) (2), while protonation of 1 yields the rhenium(III) hydride complex Re(H)(η⁵-Cp)(BDI) (3). The reaction of 1 with ZnCl₂ generated both 2 and the zinc(I) compound [ZnRe(η⁵-Cp)(BDI)]₂ (4), which features a linear, tetrametallic Re(I)-Zn(I)-Zn(I)-Re(I) core. Computational studies of 4 were performed to characterize the metal-metal bonding interactions; the results indicate a dative interaction from rhenium to zinc and covalent bonding between the two zinc centers. One-electron oxidation of 4 yielded both 2 and the triflate-bridged zinc(II) complex [(μ-OTf)ZnRe(η⁵-Cp)(BDI)]₂ (5, OTf = trifluoromethanesulfonate).

Zinc as a versatile connecting atom for zintl cluster oligomers

Homoatomic cluster aggregation is a versatile route to build up atom-precise nano structures. In this work, we present first structures representing building motifs of hypothetical ZnGe₉ networks.

Structural isomerism in the [(Ni@Sn9)In(Ni@Sn9)]5− Zintl ion

A new Zintl cluster, [(Ni@Sn₉)In(Ni@Sn₉)]⁵⁻, has been isolated in two distinct isomeric forms, one where both Ni@Sn₉ units are coordinated to the bridging indium atom in an η³- mode, the other where one is η³- and the other η⁴-.

Recent developments in Zintl cluster chemistry

Zintl anions have been known for more than a century and were studied systematically by Eduard Zintl in the 1930s. Since then, they have been investigated for their interesting structures, bonding, and physical properties - in solid Zintl phases, in solvate salts, and in solution. While their popularity remained limited for several decades, Zintl ion chemistry has recently experienced a renaissance as a result of breakthroughs regarding their modifications into multinary anions that include transition metal atoms, their organic derivatization, and their oxidative linkage. A plethora of reports from the past two decades - demonstrating the ever growing variety of Zintl ion chemistry - have been since summarized in several review articles. Herein, we intend to present the most recent developments, which also shed light on Zintl anions and clusters as useful precursors for materials development, as illustrated by one recent example.

(Ge2P2)2−: a binary analogue of P4 as a precursor to the ternary cluster anion [Cd3(Ge3P)3]3−

The novel binary P₄ analogue (Ge₂P₂)²⁻ proved to be a suitable precursor for heteroatomic cluster synthesis. Upon reaction with CdPh₂, an unprecedented cluster anion is formed, [Cd₃(Ge₃P)₃]⁻.

[(μ2-H)(η2-Ge4)ZnPh2]3−, an edge-on protonated E4 cluster establishing the first three-center two-electron Ge–H–Ge bond

The first example of a protonated and a rare example of a metal complex of the tetrahedral tetrel cluster anion [Ge₄]⁴⁻ was obtained from a solution of K₆Rb₆Ge₁₇ in liquid ammonia in the presence of ZnPh₂ and [18]crown-6.

N-Heterocyclic Carbene Coinage Metal Complexes of the Germanium-Rich Metalloid Clusters [Ge₉R₃]- and [Ge₉RI₂]²- with R = Si(iPr)₃ and RI = Si(TMS)₃

We report on the synthesis of novel coinage metal NHC (N-heterocyclic carbene) compounds of the germanium-rich metalloid clusters [Ge₉R₃]^- and [Ge₉R^I₂]²^- with R = Si(ⁱPr)₃ and R^I = Si(TMS)₃. NHC^DippCu{η³Ge₉R₃} with R = Si(ⁱPr)₃ (1) represents a less bulky silyl group-substituted derivative of the known analogous compounds with R = Si(ⁱBu)₃ or Si(TMS)₃. The coordination of the [NHC^DippCu]⁺ moiety to the cluster unit occurs via one triangular face of the tri-capped trigonal prismatic [Ge₉] cluster. Furthermore, a series of novel Zintl cluster coinage metal NHC compounds of the type (NHCM)₂{η³Ge₉R^I₂} (R^I = Si(TMS)₃ M = Cu, Ag and Au; NHC = NHC^Dipp or NHC^Mes) is presented. These novel compounds represent a new class of neutral dinuclear Zintl cluster coinage metal NHC compounds, which are obtained either by the stepwise reaction of a suspension of K₁₂Ge₁₇ with Si(TMS)₃Cl and the coinage metal carbene complexes NHCMCl (M = Cu, Ag, Au), or via a homogenous reaction using the preformed bis-silylated cluster K₂[Ge₉(Si(TMS)₃)₂] and the corresponding NHCMCl (M = Cu, Ag, Au) complex. The molecular structures of NHC^DippCu{η³Ge₉(Si(ⁱPr)₃)₃} (1) and (NHC^DippCu)₂{η³-Ge₉(Si(TMS)₃)₂} (2) were determined by single crystal X-ray diffraction methods. In 2, the coordination of the [NHC^DippCu]⁺ moieties to the cluster unit takes place via both open triangular faces of the [Ge₉] entity. Furthermore, all compounds were characterized by means of NMR spectroscopy (¹H, ¹³C, ²⁹Si) and ESI-MS.

[Ge9{M(CO)5}3]4-: electrophilic addition of M(CO)5 and [E9]4- Zintl anions (M = Cr, Mo, W)

Nucleophilic substitution reactions of K₄Ge₉ and M(CO)₆ produced [Ge₉{M(CO)₅}₃]^4- (M = Cr, 1a; Mo, 2a; W, 3a). The electrophilic addition of M(CO)₅ and [Ge₉]^4- occurred at the three capping sites to form almost perfect tricapped trigonal prismatic 1a-3a with crystallographic C_3h symmetry.

Targeted attachment of functional groups at Ge9 clusters via silylation reactions

A metalloid Ge clusters receives reactive substituents.

Formation of the intermetalloid cluster [AgSn18]7− – the reactivity of coinage metal NHC compounds towards [Sn9]4−

A series of novel polyanionic coinage metal NHC Zintl clusters [NHC^DippM(η⁴-Sn₉)]³⁻ are obtained from the reaction of [Sn₉]⁴⁻ with NHC^DippMCl (M: Cu, Ag, Au; Dipp: diisopropylphenyl) in liquid ammonia. For M = Ag a larger intermetalloid Ag-bridged nonastannide dimer [(η⁴-Sn₉)Ag(η¹-Sn₉)]⁷⁻ forms.