Structural transformations in boron clusters induced by metal doping

In the last decades, experimental techniques in conjunction with theoretical analyses have revealed the surprising structural diversity of boron clusters. Although the 2D to 3D transition thresholds are well-established, there is no certainty about the factors that determine the geometry adopted by these systems. The structural transformation induced by doping usually yields a minimum energy structure with a boron skeleton entirely different from that of the bare cluster. This review summarizes those clusters no larger than 40 boron atoms where one or two dopants show a radical transformation of the structure. Although the structures of these systems are not easy to predict, they often adopt familiar shapes such as umbrella-like, wheel, tubular, and cages in various cases.

The teetotum cluster Li2FeB14 and its possible use for constructing boron nanowires

Systematic density functional theory (DFT) calculations using the TPSSh functional and the def2-TZVP basis set were carried out to identify the global energy minimum structure of the Li2FeB14 cluster. Keeping the double ring tubular shape of FeB14, capping of two Li atoms leads to a teetotum form at a low spin state, in which the Fe atom is endohedrally covered by two B7 strings, and both Li atoms are attached to Fe along the C7 axis at both sides. Calculated results show that strong electrostatic interactions between 2Li+ and Fe2- arising from Li electron transfer upon doping particularly provide a key driving force for stabilizing this charge-transfer structure. The bonding pattern of the teetotum can be understood from the hollow cylinder model (HCM). TD-DFT calculations demonstrate that this cluster can also be regarded as a useful material for transparent optoelectronic devices. Furthermore, the Li2FeB14 superatom can be used as a building block for making boron-based nanowires with metallic character. Replacement of Li atoms by Mg atoms was also found to lead to nanowires.

Evolutionary search for (M©B16)Q (M = Sc-Ni; Q = 0/-1) clusters: bowl/boat vs. tubular shape

Herein, using universal structure predictor: evolutionary xtallography (USPEX) method, followed by density functional theory (DFT) calculations, we performed global searches for the most stable structures of (M©B₁₆)^Q (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni; Q = 0, -1) clusters. It was found that the obtained ground-state structures of (M©B₁₆)^Q clusters exhibited a distinct structural evolution as M changed from V to Ni: from bowl-shaped, to boat-shaped, to an M-centered tubular structure named wheel-shaped, to drum-shaped (the metal atom was adsorbed on top of the cross section of the B₁₆ species). Our analysis shows that hyper-coordination and the size of the metal atom are two competing factors determining the relative stability and topological properties of the (M©B₁₆)^0/-1 clusters, resulting in unprecedented structures for Sc, Ti, and Ni-doped clusters. The calculated binding energies for these new configurations are even larger than those of the previously synthesized B₁₆^-1, (Mn©B₁₆)^-1, and (Co©B₁₆)^-1 clusters, indicating their very good stability and possible experimental synthesis. A net charge transfer from the metal atom to the boron moiety occurs for all clusters, indicating that electrostatic interactions play an important role in the stability of these materials. Finally, the Sc©M₁₆ and Ti©B₁₆ clusters exhibit not only excellent thermal stability but also large first hyper-polarizability. Hence, they are expected to be potential innovative candidates for excellent electro-optical materials.

Formation of the M2B18q Teetotum Boron Clusters with 4d and 5d Transition Metals M = Rh, Pd, Ir, and Pt

In the present theoretical study using DFT computations, we successfully designed a series of boron clusters possessing a teetotum shape stabilized upon metal doping. The resulting M₂B₁₈ teetotum geometry emerges as a general structural motif for the B₁₈ boron cluster doped by either two 4d and 5d transition metal atoms. Doping of two different metal atoms also conserves the teetotum motif such as in the mixed RhPdB₁₈⁺ and IrPtB₁₈⁺ clusters. A bonding analysis points out that each dimeric metal unit enjoys several stabilizing orbital interactions with an idealized B₁₈ tubular moiety and thereby establish a bimetallic configuration of [σ(σ_5s-B₁₈)]² [π(π_4d-B₁₈)]⁴ [σ(σ_4d-B₁₈)]² [δ(δ_4d-B₁₈)]⁴ [σ*(σ_4d-B₁₈)]² [π*_4d]⁴ [σ(σ*_4d-B₁₈)]² [δ*_4d]⁴ for 4d metals and of [σ(σ_6s-B₁₈)]² [π(π_5d-B₁₈)]⁴ [σ(σ_5d-B₁₈)]² [δ(δ_5d-B₁₈)]⁴ [σ*(σ_5d-B₁₈)]² [π*_5d]⁴ [σ(σ*_5d-B₁₈)]² [δ*_5d]⁴ for 5d metal atoms for 24 electrons. All the M₂B₁₈^q teetotum structures considered behave as aromatic compounds whose character is indicated by the strongly diatropic current density.

Probing the structures and bonding of size-selected boron and doped-boron clusters

Because of their interesting structures and bonding and potentials as motifs for new nanomaterials, size-selected boron clusters have received tremendous interest in recent years. In particular, boron cluster anions (Bn-) have allowed systematic joint photoelectron spectroscopy and theoretical studies, revealing predominantly two-dimensional structures. The discovery of the planar B36 cluster with a central hexagonal vacancy provided the first experimental evidence of the viability of 2D borons, giving rise to the concept of borophene. The finding of the B40 cage cluster unveiled the existence of fullerene-like boron clusters (borospherenes). Metal-doping can significantly extend the structural and bonding repertoire of boron clusters. Main-group metals interact with boron through s/p orbitals, resulting in either half-sandwich-type structures or substitutional structures. Transition metals are more versatile in bonding with boron, forming a variety of structures including half-sandwich structures, metal-centered boron rings, and metal-centered boron drums. Transition metal atoms have also been found to be able to be doped into the plane of 2D boron clusters, suggesting the possibility of metalloborophenes. Early studies of di-metal-doped boron clusters focused on gold, revealing ladder-like boron structures with terminal gold atoms. Recent observations of highly symmetric Ta2B6- and Ln2Bn- (n = 7-9) clusters have established a family of inverse sandwich structures with monocyclic boron rings stabilized by two metal atoms. The study of size-selected boron and doped-boron clusters is a burgeoning field of research. Further investigations will continue to reveal more interesting structures and novel chemical bonding, paving the foundation for new boron-based chemical compounds and nanomaterials.

B10M2 (M = Rh, Ir): finally a stable boron-based icosahedral cluster

The putative global minimum of clusters with formula B₁₀M₂ (M = Rh, Ir) corresponds to icosahedral structures formed by two alternately stacked B₅ rings with the metals located at the top and bottom vertices.

The scandium doped boron cluster B27Sc2+: a fruit can-like structure

A systematic exploration of the potential energy surface through evolutionary search algorithms was carried out to identify the most stable B₂₇Sc₂⁺ structure.