The subtle influence of binary versus homoatomic zintl ions: the phenyl-ligated trimetallic cage [Sn(2)Sb(5)(ZnPh)(2)](3-)

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.

Between Elemental Match and Mismatch: From K12 Ge3.5 Sb6 to Salts of (Ge2 Sb2 )2- , (Ge4 Sb12 )4- , and (Ge4 Sb14 )4

The solid mixture "K2 GeSb" was shown to comprise single-crystalline K12 Ge3.5 Sb6 (1), a double salt of K5 [GeSb3 ] with carbonate-like [GeSb3 ]5- anions, and the metallic Zintl phase K2 Ge1.5 . Extraction of 1 with ethane-1,2-diamine in the presence of crypt-222 afforded [K(crypt-222)]+ salts of several novel binary Zintl anions: (Ge2 Sb2 )2- (in 2), (Ge4 Sb12 )4- (in 3), and in the presence of [AuMePPh3 ] also (Ge4 Sb14 )4- (in 4). The anion in 2 represents a predicted, yet heretofore missing pseudo-tetrahedral anion. 4 comprises a cluster analogous to (Ge4 Bi14 )4- and (Ga2 Bi16 )4- , and thus one of the most Sb-rich binary p-block anions. The unprecedented cluster topology in 3 can be viewed as a defect-version of the one in 4 upon following a "dead end" of cluster growth. The findings indicate that Ge and Sb atoms are at the border of a well-matching and a mismatch elemental combination. We discuss the syntheses, the geometric structures, and the electronic structures of the new compounds.

Electronic structure and bonding in endohedral Zintl clusters

Endohedral Zintl clusters-multi-metallic anionic molecules in which a d-block or f-block metal atom is enclosed by p-block (semi)metal atoms-are very topical in contemporary inorganic chemistry. Not only do they provide insight into the embryonic states of intermetallic compounds and show promise in catalytic applications, they also shed light on the nature of chemical bonding between metal atoms. Over the past two decades, a plethora of endohedral Zintl clusters have been synthesized, revealing a fascinating diversity of molecular architectures. Many different perspectives on the bonding in them have emerged in the literature, sometimes complementary and sometimes conflicting, and there has been no concerted effort to classify the entire family based on a small number of unifying principles. A closer look, however, reveals distinct patterns in structure and bonding that reflect the extent to which valence electrons are shared between the endohedral atom and the cluster shell. We show that there is a much more uniform relationship between the total valence electron count and the structure and bonding patterns of these clusters than previously anticipated. All of the p-block (semi)metal shells can be placed on a ladder of total valence electron count that ranges between 4n+2 (closo deltahedra), 5n (closed, three-bonded polyhedra) and 6n (crown-like structures). Although some structural isomerism can occur for a given electron count, the presence of a central metal cation imposes a preference for rather regular and approximately spherical structures which maximise electrostatic interactions between the metal and the shell. In cases where the endohedral metal has relatively accessible valence electrons (from the d or f shells), it can also contribute its valence electrons to the total electron count of the cluster shell, raising the effective electron count and often altering the structural preferences. The electronic situation in any given cluster is considered from different perspectives, some more physical and some more chemical, in a way that highlights the important point that, in the end, they explain the same situation. This article provides a unifying perspective of bonding that captures the structural diversity across this diverse family of multimetallic clusters.

Di-tert-butyldiphosphatetrahedrane as a Source of 1,2-Diphosphacyclobutadiene Ligands

Reactions of di‐tert‐butyldiphosphatetrahedrane (1) with cycloocta‐1,5‐diene‐ or anthracene‐stabilised metalate anions of iron and cobalt consistently afford complexes of the rarely encountered 1,2‐diphosphacyclobutadiene ligand, which have previously been very challenging synthetic targets. The subsequent reactivity of 1,2‐diphosphacyclobutadiene cobaltates toward various electrophiles has also been investigated and is compared to reactions of related 1,3‐diphosphacyclobutadiene complexes. The results highlight the distinct reactivity of such isomeric species, showing that the 1,2‐isomers can act as precursors for previously unknown triphospholium ligands. The electronic structures of the new complexes were investigated by several methods, including NMR, EPR and Mößbauer spectroscopies as well as quantum chemical calculations.

From organotin hydrides to heteronuclear main group metal compounds: isolation of the first neutral bismuth/tin clusters

From conversions of Ar₂SnH₂ (Ar = Tripp, Dipp; Tripp = 2,4,6-Triisopropylphenyl, Dipp = 2,6-Diisopropylphenyl), and a bismuth(III) amide, Bi[N(SiMe₃)₂]₃, we isolated the first representatives of mixed, uncharged Bi/Sn clusters, Bi₈Sn₃Ar₆. Along with unprecedented bicyclo[2.2.0]hexanes, (HAr₂Sn)₂Sn₂Bi₄, these have been characterized by single crystal X-Ray diffraction, heteronuclear NMR, vibrational and UV-Vis spectroscopy. Quantum-chemical calculations were carried out in order to understand bonding within the isolated polyhedral compounds.

Ternary aromatic and anti-aromatic clusters derived from the hypho species [Sn2Sb5]3

Heterometallic clusters have attracted broad interests in the synthetic chemistry due to their various coordination modes and potential applications in heterogeneous catalysis. Here we report the synthesis, experimental, and theoretical characterizations of four ternary clusters ([M₂(CO)₆Sn₂Sb₅]^3- (M = Cr, Mo), and [(MSn₂Sb₅)₂]^4-, (M = Cu, Ag)) in the process of capturing the hypho- [Sn₂Sb₅]^3- in ethylenediamine (en) solution. We show that the coordination of the binary anion to transition-metal ions or fragments provides additional stabilization due to the formation of locally σ-aromatic units, producing a spherical aromatic shielding region in the cages. While in the case of [Mo₂(CO)₆Sn₂Sb₅]^3- stabilization arises from locally σ-aromatic three-centre and five-centre two-electron bonds, aromatic islands in [(AgSn₂Sb₅)₂]^4- and [(CuSn₂Sb₅)₂]^4- render them globally antiaromatic. This work describes the coordination chemistry of the versatile building block [Sn₂Sb₅]^3-, thus providing conceptual advances in the field of metal-metal bonding in clusters.

Systematic DFT Studies on Binary Pseudo-tetrahedral Zintl Anions: Relative Stabilities and Reactivities towards Protons, Trimethylsilyl Groups, and Iron Complex Fragments

Binary pseudo-tetrahedral Zintl anions composed of (semi)metal atoms of the p-block elements have proven to be excellent starting materials for the synthesis of a variety of heterometallic and intermetalloid transition metal-main group metal cluster anions. However, only ten of the theoretically possible 48 anions have been experimentally accessed to date as isolable salts. This brings up the question whether the other species are generally not achievable, or whether synthetic chemists just have not succeeded in their preparation so far. To contribute to a possible answer to this question, global minimum structures were calculated for all anions of the type (TrTt3 )5- , (TrPn3 )2- , and (Tt2 Pn2 )2- , comprising elements of periods 3 to 6 (Tr: triel, Al⋅⋅⋅Tl; Tt: tetrel, Si⋅⋅⋅Pb; Pn: pnictogen, P⋅⋅⋅Bi). By analyzing the computational results, a concept was developed to predict which of the yet missing anions should be synthesizable and why. Additionally, the results of an electrophilic attack by protons or trimethylsilyl groups or a nucleophilic attack by transition metal complex fragments are described. The latter yields butterfly-like structures that can be viewed as a new form of adaptable tridentate chelating ligands.

First experimental evidence for the elusive tetrahedral cations [EP3]+ (E = S, Se, Te) in the condensed phase

Condensed phase access to the unprecedented tetrahedral cations [EP3]+ (E = S, Se, Te) was achieved through the reaction of ECl3[WCA] with white phosphorus ([WCA]- = [Al(ORF)4]- and [F(Al(ORF)3)2]-; -RF = -C(CF3)3). Previously, [EP3]+ was only known from gas phase MS investigations. By contrast, the reaction of ECl3[A] with the known P3 3- synthon Na[Nb(ODipp)3(P3)] (enabling AsP3 synthesis), led to formation of P4. The cations [EP3]+ were characterized by multinuclear NMR spectroscopy in combination with high-level quantum chemical calculations. Their bonding situation is described with several approaches including Atoms in Molecules and Natural Bond Orbital analysis. The first series of well-soluble salts ECl3[WCA] was synthesized and fully characterized as starting materials for the studies on this elusive class of [EP3]+ cations. Yet, with high [ECl3]+ fluoride ion affinity values between 775 (S), 803 (Se) and 844 (Te) kJ mol-1, well exceeding typical phosphenium ions, these well-soluble ECl3[WCA] salts could be relevant in view of the renewed interest in strong (also cationic) Lewis acids.

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.

(Multi-)Metallic Cluster Growth

This review article provides a survey of contemporary investigations on main group metal cluster formation, addressing homo- and heterometallic clusters (including small numbers of transition metal atoms), with or without an external ligand shell, thereby excluding clusters with non-metal atoms as bridging ligands. Most of the studies reflected herein represent insights into the formation of intermediates from the starting material, or the final cluster formation from established intermediates. In rare cases, the entire process was suggested as a result of comprehensive, multi-method elucidations. The article is to be understood as a state-of-the-art report, as the subject matter is currently a rising field of research, which is still in its infancy, despite some early activities that date back to the 1980s. At the same time, the article intends to point toward both the importance and the feasibility of according studies, in order to encourage researchers to gain even more knowledge in this field. Only deep understanding of cluster formation will allow for design, and ultimately control, of their syntheses, with the long-term goal of their optimization and purposeful application in catalysis or novel material synthesis.

(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]⁻.

Between Localization and Delocalization: Ru(cod)2+ Units in the Zintl Clusters [Bi9 {Ru(cod)}2 ]3- and [Tl2 Bi6 {Ru(cod)}]2

Reactions of [K(crypt-222)]₂ (TlBi₃ )⋅0.5 en (1 b) with [Ru(cod)(H₂ CC(Me)CH₂ )₂ ] (A) in 1,2-diaminoethane (en) led to the formation of two compounds with new bismuth-rich cluster anions, [K(crypt-222)]₃ [Bi₉ {Ru(cod)}₂ ]⋅1.5 en (2) and [K(crypt-222)]₂ [Tl₂ Bi₆ {Ru(cod)}]⋅2 tol (3), alongside the salt of a binary nido cluster, [K(crypt-222)]₃ (Tl₄ Bi₅ )⋅2 en (4). The anions in 2 and 3 are two further examples of rare heterometallic clusters containing Ru atoms. As one cod ligand is retained on each Ru atom in both clusters, the anions may be viewed as intermediates on the way towards larger, ligand-free intermetalloid clusters. Quantum-chemical studies provided insight into the bonding situation in these clusters. According to these studies, the anion of 2 features both electron-precise and electron-deficient parts. Electrospray ionization mass spectrometry analysis indicated that the clusters undergo stepwise fragmentation.

{[CuSn5 Sb3 ](2-) }2 : A Dimer of Inhomogeneous Superatoms

Reaction of the binary Zintl anion (Sn2 Sb2 )(2-) with the β-diketiminato complex [LCu(NCMe)] (L=nacnac=[(N(C6 H3 (i) Pr2 -2,6)C(Me))2 CH](-) ) in ethylenediamine or DMF affords the ternary cluster dimer {[CuSn5 Sb3 ](2-) }2 (1) as its [K(crypt-222)](+) salt. The chemical formulation of 1 is supported by energy-dispersive X-ray spectroscopy (EDX) and quantum chemical calculations. Each monomeric part of the dimer represents a trimetallic "[CuSn5 Sb3 ](2-) " cluster, with an architecture in between a tricapped trigonal prism and a capped square antiprism. As shown by quantum chemical investigations, the presence of Sb atoms and, in particular, of Cu atoms in the cluster skeleton makes the monomeric unit behave like an inhomogeneous superatom, which clearly prefers to dimerize, thereby producing a relatively short, yet virtually non-bonding Cu⋅⋅⋅Cu distance.

A Step in Between: [Sn3Bi3](5-) and Its Structural Relationship to [Sn3Bi5](3-) and [Sn4Bi4](4.)

Extraction of "RbSnBi" in liquid ammonia yielded the cluster anion [Sn3Bi5](3-), which could be crystallized in the compound [Rb@[2.2.2]crypt]3[Sn3Bi5]⋅8.87 NH3. This anion is found to be derived from the formerly reported [Sn4Bi4](4-) by the formal substitution of one tin atom by bismuth. In contrast, the extraction of "RbSn2/Rb3Bi2" in liquid ammonia yielded the anion [Sn3Bi3](5-) in the compound Rb6[Sn3Bi3][Sn4]1/4⋅6.75 NH3. The structural correlation of the two novel clusters indicates that [Sn3Bi3](5-) might be an intermediate of the reaction pathway to [Sn3Bi5](3-) and [Sn4Bi4](4-). Each cluster is investigated by means of the electron localization function and further characterization was performed by using ESI-MS.

Understanding of multimetallic cluster growth

The elucidation of formation mechanisms is mandatory for understanding and planning of synthetic routes. For (bio-)organic and organometallic compounds, this has long been realized even for very complicated molecules, whereas the formation of ligand-free inorganic molecules has widely remained a black box to date. This is due to poor structural relationships between reactants and products and the lack of structurally related intermediates—due to the comparably high coordination flexibility of involved atoms. Here we report on investigations of the stepwise formation of multimetallic clusters, based on a series of crystal structures and complementary quantum-chemical studies of (Ge₂As₂)²⁻, (Ge₇As₂)²⁻, [Ta@Ge₆As₄]³⁻, [Ta@Ge₈As₄]³⁻ and [Ta@Ge₈As₆]³⁻. The study makes use of efficient quantum-chemical tools, enabling the first detailed screening of the energy hypersurface along the formation of ligand-free inorganic species for a semi-quantitative picture. The results can be generalized for an entire family of multimetallic clusters.

Elucidation of formation mechanisms of inorganic cluster compounds is challenging due to the high coordination flexibility of the atoms involved. Here, the authors combine crystallographic and quantum-chemical studies to probe the energy hypersurface of a series of multimetallic clusters.

Origin and location of electrons and protons during the formation of intermetalloid clusters [Sm@Ga(3-x)H(3-2x)Bi(10+x)](3-) (x = 0, 1)

Reaction of [GaBi3](2-) with [Sm(C5Me4H)3] yielded the first protonated ternary intermetalloid clusters [Sm@Ga(3-x)H(3-2x)Bi(10+x)](3-) (1; x = 0,1). The presence of the Ga-H bonds and the transfer of electrons and protons during the formation of 1 were elucidated by a combination of experimental and quantum chemical methods, thereby rationalizing the role of the solvent ethane-1,2-diamine as a Brønsted acid. As an organic by-product, we observed the previously unknown octamethylfulvene (2) upon C-C coupling of (C5Me4H)(-).

Making practical use of the pseudo-element concept: an efficient way to ternary intermetalloid clusters by an isoelectronic Pb(-)-Bi combination

Syntheses of [K([2.2.2]crypt)](+) salts of binary Pb-Bi Zintl anions and their reaction with ZnPh(2) or Ni(cod)(2), yielding ternary intermetalloid Ni-Pb-Bi or Zn-Pb-Bi clusters, proved the use of binary precursors with fully isoelectronic atoms as an efficient and thus valuable synthetic approach to this class of clusters. (207)Pb NMR and ESI mass spectra provided insight into the solution behavior of the binary or ternary cages.

Subtle impact of atomic ratio, charge and lewis basicity on structure selection and stability: the Zintl anion [(La@In2Bi11)(μ-Bi)2(La@In2Bi11)]6-

Ternary intermetalloid cluster: the first intermetalloid M/13/15 Zintl anion [(La@In(2)Bi(11))(μ-Bi)(2)(La@In(2)Bi(11))](6-) was obtained upon reaction of [InBi(3)](2-) with [La(C(5)Me(4)H)(3)] in ethane-1,2-diamine. DFT calculations served to analyze and explain the Lewis acid/base interaction upon assignment of formal charges as "Bi(+) ←In(2-) ", which discriminates the anion from an isoelectronic La/Sn/Bi analogue that does not require bridging.