Novel Boron Clusters

Tricarbollide clusters

Contact person: B. Štíbr

Development of the chemistry of carboranes containing three cage carbon atoms in the borane skeleton. These compounds were prepared for the first time in our Institute in 1995. Our attention is now focused on the development of new, original synthetic procedures based on the incorporation of one more carbon atom into dicarbaborane cluster structures.
Tricarbaboranes under studies are complete structural analogues of the cyclopentadienide anion that also exhibit similar behaviour, for example, extremely stable ferrocene analogues can be obtained via incorporation of the {CpFe} fragment. The potential of this chemistry is hidden in modifications of either exohedral organic substituent or carborane-cluster shape, which can be used for developing a new generation of biologically active compounds based on boron clusters.

Arene Complexes of Iron

Just recently we have been aiming at resurrection of the half-forgotten chemistry of arene-iron complexes, which comprises reactivity studies of the cationic [(arene)₂Fe]²⁺, [(arene)FeCp]⁺ and [(arene)Fe(chd)]⁺ (chd = cyclohexadienyl) complexes. Attention is paid to ligand-exchange reactions associated with the replacement of an arene ligand for a carborane or heteroborane ligand which is capable of forming pi-donor bonding to the central Fe atom. Apart from this, we also persuit nucleophile-addition reactions involving anions that tend to form a sigma-type bond to the aromatic ligand in the Fe-complex (carbanions and carborane anions derived by proton abstraction from the CH bond). Polymethylated benzenes  are usually employed as arene ligands and the complexes thus obtained exhibit variablities in the Fe^II/III redox potential which can be tuned by changing the number of methyl substituents on the arene ring..

• Holub J., Růžička A., Padělková Z., Štíbr B.: Alternative syntheses and X-ray diffraction analyses of the parent tricarbaborane compounds [nido-7,8,9-C₃B₈H₁₁](-), [nido-7,8,10-C₃B₈H₁₁]^- and [1-eta⁵-(C₅H₅)-closo-1,2,4,10-FeC₃B₈H₁₁], J. Organomet. Chem. 2011, 696, 2742-2745.
• Štíbr B., Bakardjiev M.; Holub J., Růžička A., Padělková Z., Štěpnička P.: Additive Character of Electron Donation by Methyl Substituents within a Complete Series of Polymethylated [1-(eta⁶-Me_nC₆H_6-n)-closo-1,2,3-FeC₂B₉H₁₁] Complexes. Linear Correlations of the NMR Parameters and FeII/III Redox Potentials with the Number of Arene Methyls, Inorg. Chem. 2011, 50, 3097-3102.
• Holub J., Štíbr B., Bakardjiev M., Růžička A., Padělková Z.: Thermal isomerization of eta⁶-arene ferradicarbolllides. Experimental proof for isolobal relation between (eta⁶-arene)Fe and (eta⁵-cyclopentadienyl)Co cluster units, Dalton Trans. 2011, 40, 6623-6625.
• Štíbr B., Bakardjiev M., Hájková Z., Holub J., Růžička A., Padělková Z., Kennedy J. D.:  .Polymethylated [Fe(eta⁶-arene)₂]²⁺ dications: methyl-group rearrangements and application of the EINS mechanism, Dalton Trans. 2011, 40, 5916-5920.
• Štíbr B., Bakardjiev M., Holub J., Růžička A., Padělková Z., Olejník R., Švec P.: Skeletal alkylcarbonation (SAC) reactions as a simple design for cluster-carbon insertion and cross coupling: High-yield access to substituted tricarbollides from 6,9-dicarba-arachno-decaborane(14), Chem. Eur. J. 2011, 17, 13156-13159 . 

Funding
Czech Science Foundation "Arene complexes of iron modified by carboranes" (P207/11/0705, 2011-2015)

1. The Synthesis of New Large Boron Hydride Clusters and their Properties

Furthermore...

By using the [B₁₉H₂₂]^- anion as an intermediate, we successfully effected the first isomerisation pathway between anti-B₁₈H₂₂ and syn-B₁₈H₂₂!  As the major synthesis of B₁₈H₂₂ results in the formation of both isomers, the isomerisation route we describe could be useful in individual isomer enrichment.

Find out more: An Experimental Solution to the "Missing Hydrogens" Question Surrounding the Macropolyhedral 19-Vertex Boron Hydride Monoanion [B₁₉H₂₂]^-, a Simplification of Its Synthesis, and Its Use as an Intermediate in the First Example of syn-B₁₈H₂₂ to anti-B₁₈H₂₂ Isomer Conversion. Londesborough MGS, Bould J , Base T, Hnyk D, Bakardjiev M, Holub J, Cisarova I, Kennedy JD. Inorg. Chem., 2010, 49, 9, 4092-4098

We are also interested in the photophysical properties of B₁₈H₂₂. The anti-B₁₈H₂₂ isomers shows a beautiful blue fluorescence with a high quantum yield, ΦF = 0.97, which is very rare amongst inorganic compounds and unique amongst the binary boron hydrides. It also produces singlet oxygen O2(1Δg, ΦΔ 0.008), which opens the door to several interesting possible applications. Conversely, isomer syn-B₁₈H₂₂ shows no measurable fluorescence, instead displaying much faster, picosecond nonradiative decay of excited singlet states. Computed potential energy hypersurfaces (PEHs) for both isomers rationalize these observations, pointing to a deep S1 minimum for anti-B₁₈H₂₂ and a conical intersection (CI) between its S0 and S1 states that lies 0.51 eV higher in energy. Such an energy barrier to nonradiative relaxation is not present in the PEH of syn-B₁₈H₂₂, and the system therefore has sufficient initial energy on excitation to reach the (S0/S1) CI and to then decay to the ground state without fluorescence. The computational analysis of the geometries at stationary points along the PEH of both isomers shows that the determining factor for the dissimilar photophysics of anti- and syn-B₁₈H₂₂ is reasonably due to the significant differences in the geometrical rearrangements at their respective conical intersections.

Find out more: The Distinct Photophysics Of the Isomers of B₁₈H₂₂ Explained. M.G.S. Londesborough, D. Hnyk, J. Bould, L. Serrano-Andrés, V. Sauri, J. M. Oliva, P. Kubát, T. Polívka, and K. Lang. Inorg. Chem. 2012. DOI: 10.1021/ic201726k.

2. Metallaboranes and Their Properties

Contact person: M.G.S. Londesborough

The flexible bonding nature of the boron hydrides allows ligated metal fragments that are isolobal and isoelectronic with vertices to be accompanied into their cluster structures.  The result is the diverse and exotic field of the metallaboranes.  We are interested in exploring the addition of platinum, palladium, rhodium, iridium and cobalt insertions into borane clusters and experimenting with their chemical and physical properties.

Recent Results from the lab...

We recently discovered that the metallaborane system L₄M₂B₁₀H₁₀ (where M = Pt, Pd and L = phosphine or other ligand) can selectively and reversibly uptake various small molecules of gases (SO₂, CO, NOx, acetylene, ethylene, etc.). There are numerous potential ways to tune the properties of this system according to requirement. It is presently undergoing an extensive experimental screening for potential applications such as low-concentration gas sensors.

A dark purple solution of (PMe₂Ph)₄Pt₂B₁₀H₁₀ turns yellow after bubbling SO₂ (right) through and orange after exposure to air (left). The colour change indicates the SO₂ or O₂ gas uptake on the bimetallaborane clusters.

In the M₂B₁₀H₁₀ system there is potential for artificial photosynthetic use with attached photoantennas, for small molecules recognition and delivery, dioxygen, CO or other small molecule activation and detection. Advanced catalytic processes are one of the potential applications of this system. Immobilizations of these molecules on a solid support can be used for heterogeneous performance.

Find out more: Reversible Capture of Small Molecules On Bimetallaborane Clusters: Synthesis and Structural and Photophysical Characterisation. J. Bould, T. Baše, M.G.S. Londesborough, J. D. Kennedy, Luis A.Oro, Ramón Macías, P. Kubát, M. Fuciman, T Polívka and K. Lang, Inorg. Chem., 2011, 50, 7511-7523.