Synthesis, Structural Analysis, and Functional Group Interconversion in the [ closo ‐B 10 H 8 ‐1,10‐X 2 ] 2 – (X = CN, [OCRNMe 2 ] + , OCOR, and [OH 2 ] + ) Derivatives

[closo-B10H8-1-CN-10-Azinium]- Anions: Photoactive Heteroditopic Ligands for Metal Complexes

A new class of rigid, photoactive heteroditopic anionic ligands based on the 1,10-disubstituted [closo-B10H10]2- anion was designed and six of these compounds were obtained from [closo-B10H10]2- in three steps with yields in the range of 25-30%. The design includes two apical substituents, a metal coordinating cyano group and an azinium (4-cyanopyridinium, 4,4'-bipyridinium, pyrazinium, pyrimidinium, and pyridazinium), which provides a secondary binding site. The azinium substituent is involved in an efficient intramolecular charge transfer process and compensates one of the two delocalized negative charges of the boron cluster. The compounds exhibit intramolecular CT bands with maxima in the range of 340-410 nm (MeCN) and two are weakly fluorescent with significant Stokes shifts. A highly colored bis-zwitterionic byproduct with two clusters, [closo-1-CN-B10H8-10-(pyrazinium-1,4-diyl)-10'-B10H8-1'-CN-closo]2-, was also obtained in a low yield. The ligands were structurally characterized (XRD) and their geometrical and photophysical properties were compared to those of the analogues lacking the CN group and the parent pyridinium derivative. A comparative analysis of experimental data was augmented with DFT and TD-DFT results. Two of the ligands (with the 4,4'-bipyridinium and pyrazinium) were converted to (η5-Cp)(dppe)Fe complexes and one of them was used to obtain a heterodinuclear complex with Cu(pdc)(H2O)3 to demonstrate the ditopic function of the ligand. All three iron complexes were characterized by the XRD method.

[closo-B10H8-10-PhI-1-COOH]- Anion: An Intermediate for Functional Anionic Carboxylate Ligands

A potentially general intermediate, [closo-B10H8-10-PhI-1-COOH]-, for a class of functional anionic carboxylic acids, [closo-B10H8-10-X-1-COOH]2-, was obtained in four steps and 26% overall yield from [closo-B10H10]2-. It was converted to the pyridinium derivative (X = C5H5N+) and subsequently to coordination complexes with (phen)2Cu2+ and (phen)2Zn2+ ions. Both the acid and Zn(II) complex exhibit a cage-to-pyridine charge-transfer band. The availability of such acids opens access to functional metal-ion complexes with compensated charges.

Effect of Nature of Substituents on Coordination Properties of Mono- and Disubstituted Derivatives of Boron Cluster Anions [BnHn]2– (n = 10, 12) and Carboranes with exo-Polyhedral B–X Bonds (X = N, O, S, Hal)

This review systematizes data on the coordination ability of mono- and disubstituted derivatives of boron cluster anions and carboranes in complexation with transition metals. Boron clusters anions [BnHn]2–, monocarborane anions [CBnHn–1]–, and dicarboranes [C2BnHn–2] (with non-functionalized carbon atoms) (n = 10, 12) containing the B–X exo-polyhedral bonds (X = N, O, S, Hal) are discussed. Synthesis and structural features of complexes known to date are described. The effect of complexing metal and substituent attached to the boron cage on the composition and structures of the final complexes is analyzed. It has been established that substituted derivatives of boron cluster anions and carboranes can act as both ligands and counterions. A complexing agent can coordinate substituted derivatives of the boron cluster anions due to three-center two-electron 3c2e MHB bonds, by the substituent functional groups, or a mixed type of coordination can be realized, through the BH groups of the boron cage and the substituent. As for B-substituted carboranes, complexes with coordinated substituents or salts with non-coordinated carborane derivatives have been isolated; compounds with MHB bonding are not characteristic of carboranes.

[closo-B10H8-1,10-(COOH)2]2-: a building block for functional materials?

[closo-B₁₀H₈-1,10-(COOH)₂]^2- was obtained in five steps and 40% overall yield from [closo-B₁₀H₁₀]^2-. It can be converted to [closo-B₁₀H₈-1,10-(CO)₂] and subsequently to carbonium ylides [closo-B₁₀H₈-1-COOH-10-(C(NRCH₂)₂)]. The diacid, its derivatives, and di-ylide [closo-B₁₀H₈-1,10-(C(NHMe)₂)₂] are characterized by spectroscopic and single crystal XRD methods augmented with DFT results.

Photonic materials derived from the [closo-B10H10]2− anion: tuning photophysical properties in [closo-B10H8-1-X-10-(4-Y-NC5H5)]−

Substitution of the parent [closo-B₁₀H₉-1-NC₅H₅]⁻ either at the B(10) position or at the pyridine ring tunes absorption and emission energy.

C(1)-Phenethyl Derivatives of [closo-1-CB11 H12 ]- and [closo-1-CB9 H10 ]- Anions: Difunctional Building Blocks for Molecular Materials

C(1)-vinylation of [closo-1-CB₉ H₁₀ ]^- (A) and [closo-1-CB₁₁ H₁₂ ]^- (B) with 4-benzyloxystyryl iodide followed by hydrogenation of the double bond and reductive deprotection of the phenol functionality led to C(1)-(4-hydroxyphenethyl) derivatives. The phenol functionality was protected as the acetate. The esters were then treated with PhI(OAc)₂ and the resulting isomers were separated kinetically (for derivatives of anion A) or by chromatography (for derivatives of anion B) giving the difunctionalized building blocks in overall yields of 9 % and 50 %, respectively. A similar series of reactions was performed starting with anions A and B and 4-methoxystyryl bromide and iodide. Significant differences in the reactivity of derivatives of the two carborane anions were rationalized with DFT computational results. Application of the difunctionalized carboranes as building blocks was demonstrated through preparation of two ionic liquid crystals. The extensive synthetic work is accompanied by single crystal XRD analysis of six derivatives.