Benchmark Study on the Calculation of 207Pb NMR Chemical Shifts

A benchmark set for the computation of 207Pb nuclear magnetic resonance (NMR) chemical shifts is presented. The PbS50 set includes conformer ensembles of 50 lead-containing molecular compounds and their experimentally measured 207Pb NMR chemical shifts. Various bonding motifs at the Pb center with up to seven bonding partners are included. Six different solvents were used in the measurements. The respective shifts lie in the range between +10745 and -5030 ppm. Several calculation settings are assessed by evaluating computed 207Pb NMR shifts for the use with different density functional approximations (DFAs), relativistic approaches, treatment of the conformational space, and levels for geometry optimization. Relativistic effects were included explicitly with the zeroth order regular approximation (ZORA), for which only the spin-orbit variant was able to yield reliable results. In total, seven GGAs and three hybrid DFAs were tested. Hybrid DFAs significantly outperform GGAs. The most accurate DFAs are mPW1PW with a mean absolute deviation (MAD) of 429 ppm and PBE0 with an MAD of 446 ppm. Conformational influences are small as most compounds are rigid, but more flexible structures still benefit from Boltzmann averaging. Including explicit relativistic treatments such as SO-ZORA in the geometry optimization does not show any significant improvement over the use of effective core potentials (ECPs).

Ligand-free supermolecules: [Pd2@Ge18]4- and [Pd2@Sn18]4- as multiple-bonded Zintl-ion clusters based on Pd@Ge9 and Pd@Sn9 assembled units

Understanding intercluster bonding interactions is important in the rational synthesis of building blocks for molecular materials. Such characteristics have been developed for coinage metal clusters resembling single-, double-, and triple-bonded species, coined as supermolecules. Herein, we extend such an approach for understanding main-group clusters, thus evaluating [Pd2@E18]4- clusters (E = Ge, Sn) involving the fusion of parent spherical aromatic [Pd@E12]2- building units. Our results indicate intercluster bonding provided by contribution from 2P and 1G shells centered at each building motif, leading to an overall bond order of 2.70 and 2.31 for [Pd2@Ge18]4- and [Pd2@Sn18]4-, respectively. In addition, 119Sn-NMR patterns were evaluated to complement the experimental characterization of a single peak owing to the insolution fluxional behavior of [Pd2@Sn18]4- as three peaks owing to the three sets of unique Sn atoms within the structure. Magnetic response properties revealed that spherical aromatic characteristics from parent [Pd@E12]2- building units are retained in the overall [Pd2@E18]4- oblate cluster as two spherical aromatic units. Hence, the notion of superatomic molecules is extended to Zintl-ion clusters, favoring further rationalization for the fabrication of cluster-assembled solids.

Exploration of the potential energy surface in mixed Zintl clusters applying an automatic Johnson polyhedra generator: the case of arachno E6M24- (E = Si, Ge, Sn; M = Sb, Bi)

A new algorithm called Automatic Johnson Cluster Generator (AJCG) is presented, which, as its name indicates, allows the definition of the desired Johnson polyhedron to subsequently carry out all the possible permutations between the atoms that form this polyhedron. This new algorithm allows the exhaustive study of the structures' potential energy surface (PES). In addition, the AJCG algorithm is helpful for the study of three-dimensional compounds such as boranes or Zintl clusters and their structural derivatives with two or more different atoms. The automatic filling of vertices is particularly useful in mixed compounds because of the possibility of taking into account all possible configurations in the structure. As a test system, we investigated the arachno-type E6M24- (E = Si, Ge, Sn; M = Sb, Bi) structure which has eight vertices and complies with Wade-Mingos rules. Initially, we defined a bipyramidal structure (10 vertices), and filled the vertices with the atoms in all possible configurations. Since the selected system has eight atoms, the two remaining vertices were filled with pseudo atoms to complete the structure. After re-optimizing the initial population generated with AJCG, a large number of isomers with energy below 10 kcal mol-1 are identified. These results show that the most stable isomers possess homonuclear M-M bonds, except Sn6Bi24-. Although the overall putative minima differ at the PBE0-D3 and DLPNO-CCSD(T) levels, they are always competitive minima. In addition to using high-precision methodologies to correctly study relative energies, applying solvent effects in highly charged systems becomes mandatory. The aromatic character of these studied systems was demonstrated qualitatively with two- and three-dimensional mapping and quantitatively by calculating the value of the z-component of the induced magnetic field at the cage center, including scalar and spin-orbit correction for relativistic effects. The compounds studied have a high degree of aromaticity, which allows us to establish that despite structural modifications (i.e., from closo to arachno), the aromaticity is preserved.

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.

Symmetry collapse due to the presence of multiple local aromaticity in Ge244

Understanding the structural changes taking place during the assembly of single atoms leading to the formation of atomic clusters and bulk materials remains challenging. The isolation and theoretical characterization of medium-sized clusters can shed light on the processes that occur during the transition to a solid-state structure. In this work, we synthesize and isolate a continuous 24-atom cluster Ge₂₄⁴⁻, which is characterized by X-ray diffraction analysis and Energy-dispersive X-ray spectroscopy, showing an elongated structural characteristic. Theoretical analysis reveals that electron delocalization plays a vital role in the formation and stabilization of the prolate cluster. In contrast with carbon atoms, 4 s orbitals of Ge-atoms do not easily hybridize with 4p orbitals and s-type lone-pairs can be localized with high occupancy. Thus, there are not enough electrons to form a stable symmetrical fullerene-like structure such as C₂₄ fullerene. Three aromatic units with two [Ge₉] and one [Ge₆] species, connected by classical 2c-2e Ge-Ge σ-bonds, are aligned together forming three independent shielding cones and eventually causing a collapse of the global symmetry of the Ge₂₄⁴⁻ cluster.

Gaining insight on the structural transformations from atomic clusters to bulk materials is challenging. Here the authors synthesize a continuous cluster of germanium Ge₂₄⁴⁻, which can be viewed as two terminal Ge₉ units bridged via a Ge₆ central fragment, and characterize it by several techniques including X-ray diffraction; theoretical analysis indicates the presence of three aligned independent aromatic fragments.

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.

Unprecedented icosahedral clusters built of polyantimony: from single [Ni0.5@{Sb6Ni6(CO)8}]4− and [Ni@{Sb7Ni5(CO)6}]3− to the Sb84−-linked dimer [(Sb8){Sb7Ni5(CO)4}2]6−

First icosahedral clusters built of polyantimony were prepared from the reactions of K2ZnSb and Ni(CO)2(PPh3)2. Sb84−-linked dimer provides a new way of thinking for the synthesis of hybrid clusters datively coordinated by polydentate Zintl anions.

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.

A family of lead clusters with precious metal cores

Gold nanoparticles have been used for centuries, both for decoration and in medical applications. More recently, many of the major advances in cluster chemistry have involved well-defined clusters containing tens or hundreds of atoms, either with or without a ligand shell. In this paper we report the synthesis of two gold/lead clusters, [Au₈Pb₃₃]⁶⁻ and [Au₁₂Pb₄₄]⁸⁻, both of which contain nido [Au@Pb₁₁]³⁻ icosahedra surrounding a core of Au atoms. Analogues of these large clusters are not found in the corresponding Ag chemistry: instead, the Ag-centered nido icosahedron, [Ag@Pb₁₁]³⁻, is the only isolated product. The structural chemistry, along with the mass spectrometry which shows the existence of [Au₂Pb₁₁]²⁻ but not [Ag₂Pb₁₁]²⁻, leads us to propose that the former species is the key intermediate in the growth of the larger clusters. Density functional theory indicates that secondary π-type interactions between the [Au@Pb₁₁]³⁻ ligands and the gold core play a significant part in stabilizing the larger clusters.

Many Zintl ions with a single endohedrally encapsulated transition metal ion are known, but relatively few where clusters of two or more metals are present. Here, the authors report the synthesis and characterization of two clusters, [Au₈Pb₃₃]⁶⁻ and [Au₁₂Pb₄₄]¹²⁻, which contain Au₈ and Au₁₂ cores surrounded by Pb shells.

[Cp*RuPb11]3− and [Cu@Cp*RuPb11]2−: centered and non-centered transition-metal substituted zintl icosahedra

Cluster anions [Cp*RuPb₁₁]³⁻ (1) and [Cu@Cp*RuPb₁₁]²⁻ (2) represent the first vertex-substituted zintl icosahedra and 1 is the first non-centered zintl icosahedron isolated in the condensed phase.

Th@C86, Th@C82, Th@C80, and Th@C76: role of thorium encapsulation in determining spherical aromatic and bonding properties on medium-sized endohedral metallofullerenes

Thorium encapsulated metallofullerenes (Th-EMFs) with external C76, C80, C82, and C86 cages have been synthesized, with the 13C-NMR spectrum recorded for Th@C82. Here, we explore computationally the chemical bonding, NMR and spherical aromaticity of Th@C82 and related thorium-encapsulated metallofullerenes. Our results show that these Th-EMFs are new examples of spherical aromatic structures, representing interesting low-symmetry exceptions to the Hirsch 2(N + 1)2 rule of spherical aromaticity. Their electronic structures are based on π-electron counts of 80, 84, 86, and 90, respectively, with a shell structure ranging from S2P6D10F14G18H22I8 to S2P6D10F14G18H22I18, where the partially filled I-shell remains as a frontier orbital. Their behavior is comparable to that of the spherical aromatic alkali-C606- phases, which in addition to the favorable endohedral Th-fullerene bonding account for their particular abundance exhibiting the ability to sustain a long-range shielding cone as a result of the favorable metal-cage bonding. This rationalization of such species as neutral spherical aromatic EMFs suggests the possibility of an extensive series of aromatic fullerenes with nuclearity larger than C60 buckminsterfullerene as stable building blocks towards nanostructured metal-organic materials.