1,2-Diphenyl-o-carborane and Its Chromium Derivatives: Synthesis, Characterization, X-ray Structural Studies, and Biological Evaluations

The objective of this study is to design and synthesize substituted η⁶-chromium(0) tricarbonyl metal complexes carrying o-carborane units as potential boron neutron capture therapy (BNCT) agents. In this study, 1,2-diphenyl-o-carborane (1) units were used as starting materials to generate biologically active species. We investigated how the structural changes of 1 substituted with chromium(0) tricarbonyl affect the biological properties, and 1-(Phenyl-η⁶-chromium(0) tricarbonyl)-2-phenyl-o-carborane (2) and 1,2-bis(phenyl-η⁶-chromium(0) tricarbonyl)-o-carborane (3) species were produced in moderate yields. The molecular structures of compounds 1-3 were identified and established by infrared (IR); ¹H, ¹¹B, and ¹³C nuclear magnetic resonance (NMR) and X-ray crystallography analyses. Crystal structures of 1,2-diphenyl-o-carborane and the corresponding chromium complexes 1, 2, and 3 were obtained. In an in vitro study using B16 and CT26 cancer cells containing the triphenyl-o-carboranyl chromium(0) complexes Ph3C2BCr2 and Ph3C2BCr3, which we reported previously, compounds 2 and 3 accumulated at higher levels than compounds Ph3C2BCr2 and Ph3C2BCr3. However, the phenylated o-carboranyl chromium complexes have been found to be more cytotoxic than p-boronophenylalanine (BPA).

Synthesis and Structure of an o-Carboranyl-Substituted Three-Coordinate Borane Radical Anion

Bis(1-(4-tolyl)-carboran-2-yl)-(4-tolyl)-borane [(1-(4-MeC6 H4 )-closo-1,2-C2 B10 H10 -2-)2 (4-MeC6 H4 )B] (1), a new bis(o-carboranyl)-(R)-borane was synthesised by lithiation of the o-carboranyl precursor and subsequent salt metathesis reaction with (4-tolyl)BBr2 . Cyclic voltammetry experiments on 1 show multiple distinct reduction events with a one-electron first reduction. In a selective reduction experiment the corresponding paramagnetic radical anion 1.- was isolated and characterized. Single-crystal structure analyses allow an in-depth comparison of 1, 1.- , their calculated geometries, and the S1 excited state of 1. Photophysical studies of 1 show a charge transfer (CT) emission with low quantum yield in solution but a strong increase in the solid state. TD-DFT calculations were used to identify transition-relevant orbitals.

Experimental proof for emission annihilation through bond elongation at the carbon-carbon bond in o-carborane with fused biphenyl-substituted compounds

Because of their unique luminescence properties, such as aggregation-induced emission (AIE), intense solid-state luminescence and stimuli-responsive luminochromism, aryl-substituted o-carboranes have attracted attention as a platform for developing functional optoelectronic materials. However, there still remains one fundamental issue with the detailed mechanism of solution quenching in AIE behaviors. Aryl-modified o-carboranes with AIE properties exhibit intense emission not in solution but in the solid state. According to quantum calculations and many experimental results, the elongation at the carbon-carbon bond in o-carborane in the excited state, followed by nonradiative decay, has been proposed as a main path for emission annihilation in solution. However, intramolecular rotation would simultaneously occur, and there is a possibility that emission annihilation could be induced by the combination of both bond elongation and rotation. In this study, we designed two types of biphenyl-substituted o-carboranes having fused structures at the neighbor carbon and boron atoms for fixing molecular conformation. In these molecules, bond elongation is allowed, while rotation would be prohibited. From the series of optical measurements and theoretical investigations, we proved that emission annihilation can occur through bond elongation in the absence of rotation. Moreover, we show that bond elongation could be suppressed by introducing a bulky substituent at the adjacent carbon, and emission color tuning was achieved. This is the first example, to the best of our knowledge, to prove that excitation decay can proceed only through bond elongation without electronic perturbation caused by rotation.

Expanding the Scope of Palladium-Catalyzed B - N Cross-Coupling Chemistry in Carboranes

Over the past several years, a number of strategies for the functionalization of dicarba-closo-dodecaboranes (carboranes) have emerged. Despite these developments, B - N bond formation on the carborane scaffold remains a challenge due to the propensity of strong nucleophiles to partially deboronate the parent closo-carborane cluster into the corresponding nido form. Here we show that azide, sulfonamide, cyanate, and phosphoramidate nucleophiles can be straightforwardly cross-coupled onto the B(9) vertices of the o- and m-carborane core from readily accessible precursors without significant deboronation by-products, laying the groundwork for further study into the utility and properties of these new B-aminated carborane species. We further showcase select reactivity of the installed functional groups highlighting some unique features stemming from the combination of the electron-donating B(9) position and the large steric profile of the B-connected carborane substituent.

Icosahedral carboranes as scaffolds for congested regioselective polyaryl compounds: the distinct distance tuning of C–C and its antipodal B–B

Four-fold dense precisely defined patterns of substitution in o-carborane have been demonstrated by experimentation and computed.

Nido-Carboranes: Donors for Thermally Activated Delayed Fluorescence

An approach to the design of nido-carborane-based luminescent compounds that can exhibit thermally activated delayed fluorescence (TADF) is proposed. 7,8-Dicarba-nido-undecaboranes (nido-carboranes) having various 8-R groups (R=H, Me, i-Pr, Ph) are appended to the meta or para position of the phenyl ring of the dimesitylphenylborane (PhBMes₂ ) acceptor, forming donor-acceptor compounds (nido-m1-m4 and nido-p1-p4). The bulky 8-R group and meta substitution of the nido-carborane are essential to attain a highly twisted arrangement between the donor and acceptor moieties, leading to a very small energy splitting between the singlet and triplet excited states (ΔE_ST <0.05 eV for nido-m2, -m3, and -p3). These compounds exhibit efficient TADF with microsecond-range lifetimes. In particular, nido-m2 and -m3 display aggregation-induced emission (AIE) with TADF properties.

Developing nitrosocarborane chemistry

New nitrosocarboranes or their hydroxylamine or Diels–Alder cycloadducts have been prepared and fully characterised.

Isomerisation of nido-[C2B10H12]2- dianions: unprecedented rearrangements and new structural motifs in carborane cluster chemistry

Dianionic nido-[C2B10]2- species are key intermediates in the polyhedral expansion from 12- to 13-vertex carboranes and metallacarboranes, and the isomer adopted by these nido intermediates dictates the isomeric form of the 13-vertex product. Upon reduction and metallation of para-carborane up to five MC2B10 metallacarboranes can be produced (Angew. Chem., Int. Ed., 2007, 46, 6706), the structures of which imply the intermediacy of 1,7-, 3,7-, 4,7-, 7,9- and 7,10-isomers of the nido-[C2B10]2- species. In this paper we use density functional theory (DFT) calculations to characterise the reduction of closo-C2B10H12 carboranes and the subsequent isomerisations of the nido-[C2B10H12]2- dianions. Upon reduction para-carborane initially opens to [1,7-nido-C2B10H12]2- (abbreviated to 1,7) and [4,7-nido-C2B10H12]2- (4,7) and isomerisation pathways connecting 1,7 to 7,9, 4,7 to 7,10 and 1,7 to 3,7 have been characterised. For ortho- and meta-carborane the experimental reduction produces 7,9 in both cases and computed pathways for both processes are also defined; with ortho-carborane rearrangement occurs via7,8, whereas with meta-carborane 7,9 is formed directly. The 7,9 isomer is the global minimum nido-structure. The characterisation of these isomerisation processes uncovers intermediates that adopt new structural motifs that we term basket and inverted nido. Basket intermediates feature a two-vertex basket handle bridging the remaining 10 vertices; inverted nido intermediates are related to known nido species, in that they have 5- and 6-membered belts, but where the latter, rather than the former, is capped, leaving a 5-membered open face. These new intermediates exhibit similar stability to the nido species, which is attributed to their relation to the 13-vertex docosahedron through the removal of 5-connected vertices. Isomerisation pathways starting from nido geometries are most often initiated by destabilisation of the cluster through a DSD process causing the 3-connected C7 vertex to move into a 4-connected site and a neighbouring B vertex to become 3-connected. The ensuing rearrangement of the cluster involves processes such as the pivoting of a 4-vertex diamond about its long diagonal, the pivoting of two 3-vertex triangles about a shared vertex and DSD processes. These processes are all ultimately driven by the preference for carbon to occupy low-connected vertices on the open 6-membered face of the resulting nido species.

Fluorescence of new o-carborane compounds with different fluorophores: can it be tuned?

Two sets of o-carborane derivatives incorporating fluorene and anthracene fragments as fluorophore groups have been successfully synthesized and characterized, and their photophysical properties studied. The first set, comprising fluorene-containing carboranes 6-9, was prepared by catalyzed hydrosilylation reactions of ethynylfluorene with appropriate carboranylsilanes. The compound 1-[(9,9-dioctyl-fluorene-2-yl)ethynyl]carborane (11) was synthesized by the reaction of 9,9-dioctyl-2-ethynylfluorene and decaborane (B10H14). Furthermore, reactions of the lithium salt of 11 with 1 equivalent of 4-(chloromethyl)styrene or 9-(chloromethyl)anthracene yielded compounds 12 and 13. Members of the second set of derivatives, comprising anthracene-containing carboranes, were synthesized by reactions of monolithium or dilithium salts of 1-Me-1,2-C2B10H11, 1-Ph-1,2-C2B10H11, and 1,2-C2B10H12 with 1 or 2 equivalents of 9-(chloromethyl)anthracene, respectively, to produce compounds 14-16. In addition, 2 equivalents of the monolithium salts of 1-Me-1,2-C2B10H11 (Me-o-carborane) and 1-Ph-1,2-C2B10H11 (Ph-o-carborane) were reacted with 9,10-bis(chloromethyl)anthracene to produce compounds 17 and 18, respectively. Fluorene derivatives 6-9 exhibit moderate fluorescence quantum yields (32-44 %), whereas 11-13, in which the fluorophore is bonded to the Ccluster (Cc), show very low emission intensity (6 %) or complete fluorescence quenching. The anthracenyl derivatives containing the Me-o-carborane moiety exhibit notably high fluorescence emissions, with ϕF = 82 and 94 %, whereas their Ph-o-carborane analogues are not fluorescent at all. For these compounds, we have observed a correlation between the Cc-Cc bond length and the fluorescence intensity in CH2Cl2 solution, comparable to that observed for previously reported styrene-containing carboranes. Thus, our hypothesis is that for systems of this type the fluorescence may be tuned and even predicted by changing the substituent on the adjacent Cc.

Crystal structures of the carborane dianions [1,4-(PhCB₁₀H₁₀C)₂C₆H₄]²⁻ and [1,4-(PhCB₁₀H₁₀C)₂C₆F₄]²⁻ and the stabilizing role of the para-phenylene unit on 2n+3 skeletal electron clusters

While carboranes with 2 n+2 and 2 n+4 (n=number of skeletal atoms) skeletal electrons (SE) are widely known, little has been reported on carboranes with odd SE numbers. Electrochemical measurements on two-cage assemblies, where two C-phenyl-ortho-carboranyl groups are linked by a para-phenylene or a para-tetrafluorophenylene bridge, revealed two well separated and reversible two-electron reduction waves indicating formation of stable dianions and tetraanions. The salts of the dianions were isolated by reduction with sodium metal and their unusual structures were determined by X-ray crystallography. The diamagnetic dianions contain two 2 n+3 SE clusters where each cluster has a notably long carborane C-carborane C distance of ca 2.4 Å. The π conjugation within the phenylene bridge plays an important role in the stabilization of these carboranes with odd SE counts.

C,C'-bis(benzodiazaborolyl)dicarba-closo-dodecaboranes: synthesis, structures, photophysics and electrochemistry

Six new C,C'-bis(benzodiazaborolyl)dicarba-closo-dodecaboranes, 1,A-R2-1,A-C2B10H10, where R represents the group 2-(1,3-Et2-1,3,2-N2BC6H4) or 2-(1,3-Ph2-1,3,2-N2BC6H4) and A is 2, 7 or 12, were synthesized from o-, m-, and p-dicarbadodecaboranes (carboranes) by lithiation and subsequent treatment with the respective 2-bromo-1,3,2-benzodiazaboroles. UV-visible and fluorescence spectra of all carboranes display low energy charge transfer emissions. While such emissions with Stokes shifts between 17,330 and 21,290 cm(-1) are typical for C,C'-bis(aryl)-ortho-carboranes, the observed low-energy emissions with Stokes shifts between 8320 and 15,170 cm(-1) for the meta- and para-isomers are unusual as high-energy emissions are typical for meta- and para-dicarbadodecaboranes. Fluorescence quantum yields (φF) for the novel 1,7- and 1,12-bis(benzodiazaborolyl)-carboranes depend on the substituents at the nitrogen atoms of the heterocycle. Thus, the para-carborane with N-ethyl substituents 1,12-(1',3'-Et2-1',3',2'-N2BC6H4)2-1,12-C2B10H10 has a φF value of 41% in cyclohexane solution and only of 9% in the solid state, whereas the analogous 1,12-(1',3'-Ph2-1',3',2'-N2BC6H4)2-1,12-C2B10H10 shows quantum yields of 3% in cyclohexane solution and 72% in the solid state. X-ray crystallographic, computational and cyclic voltammetry studies for these carboranes are also presented.

Luminescence properties of C-diazaborolyl-ortho-carboranes as donor-acceptor systems

Seven derivatives of 1,2-dicarbadodecaborane (ortho-carborane, 1,2-C(2)B(10)H(12)) with a 1,3-diethyl- or 1,3-diphenyl-1,3,2-benzodiazaborolyl group on one cage carbon atom were synthesized and structurally characterized. Six of these compounds showed remarkable low-energy fluorescence emissions with large Stokes shifts of 15100-20260 cm(-1) and quantum yields (Φ(F)) of up to 65% in the solid state. The low-energy fluorescence emission, which was assigned to a charge-transfer (CT) transition between the cage and the heterocyclic unit, depended on the orientation (torsion angle, ψ) of the diazaborolyl group with respect to the cage C-C bond. In cyclohexane, two compounds exhibited very weak dual fluorescence emissions with Stokes shifts of 15660-18090 cm(-1) for the CT bands and 1960-5540 cm(-1) for the high-energy bands, which were assigned to local transitions within the benzodiazaborole units (local excitation, LE), whereas four compounds showed only CT bands with Φ(F) values between 8-32%. Two distinct excited singlet-state (S(1)) geometries, denoted S(1)(LE) and S(1)(CT), were observed computationally for the benzodiazaborolyl-ortho-carboranes, the population of which depended on their orientation (ψ). TD-DFT calculations on these excited state geometries were in accord with their CT and LE emissions. These C-diazaborolyl-ortho-carboranes were viewed as donor-acceptor systems with the diazaborolyl group as the donor and the ortho-carboranyl group as the acceptor.

Carborane photochemistry triggered by aryl substitution: carborane-based dyads with phenyl carbazoles

A bright combination: a new type of donor-acceptor dyad, carbazolylaryl-substituted ortho-carboranes, which are conveniently prepared from the corresponding acetylenes and decaborane pathways, showed unique excited-state behavior associated with electron transfer unlike the meta- and para-counterparts.

2-[(Cyclo-penta-1,3-dien-2-yl)diphenyl-meth-yl]-1-methyl-1H-imidazole

The title compound, C(22)H(20)N(2), (Ib), forms along with 2-[(cyclo-penta-1,3-dien-1-yl)diphenyl-meth-yl]-1-methyl-1H-imid-azole, (Ia), which differs with respect to the position of the double-bonds in the C(5)H(5) ring, in an approximately 3:7 ratio (Ia:Ib; NMR spectroscopy data). However, in a single crystal, only compound (Ib) is present. H atoms of the CH(2) group (C(5)H(5) ring) were found from the difference Fourier synthesis and refined isotropically using the riding model. Hypothesis on possible presence of the (Ia) isomer in crystal lattice (model with a C(5)H(5) ring disordered between two positions) was especially checked and rejected due to its inconsistency. In the crystal structure, no significant hydrogen-bonding inter-actions between the CH(2) groups of the C(5)H(5) rings and nonsubstituted N-atoms of the imidazole rings were observed. Despite the fact that the chemically achiral compound (I) crystallizes in a chiral space group P2(1)2(1)2(1), neither the absolute structure determination nor assignment of the inversion twinning was possible in the absence of a heavy atom.