Bis(Dicarbollide) Complexes of Transition Metals: How Substituents in Dicarbollide Ligands Affect the Geometry and Properties of the Complexes.

The interaction between different types of substituents in dicarbollide ligands and their influence on the stabilization of various rotational conformers (rotamers) of transition metal bis(dicarbollide) complexes [3,3'-M(1,2-CBH)] are considered. It has been shown that the formation of intramolecular CH···X hydrogen bonds between dicarbollide ligands is determined by the size of the proton acceptor atom X rather than its electronegativity. Due to the stabilization of rotamers with different dipole moments, intramolecular hydrogen bonds between ligands in transition metal bis(dicarbollide) complexes can have a significant impact on the biological properties of their derivatives.

Charge-compensated -carborane derivatives in the synthesis of iron(II) bis(dicarbollide) complexes.

A series of stable iron(II) bis(dicarbollide) derivatives [8,8'-(RNHC(Et)HN)-3,3'-Fe(1,2-CBH)] (R = Pr, R = Ph, (CH)OH, (CH)OH, (CH)NMe) was prepared starting from FeCl or [FeCl(dppe)] and the corresponding -carboranyl amidines [10-RNHC(Et)HN-7,8-CBH]. In a similar way, the reactions of the oxonium derivatives of -carborane with FeCl in tetrahydrofuran in the presence of -BuOK lead to the corresponding stable oxonium derivatives iron(II) bis(dicarbollide) [8,8'-(RR'O)-3,3'-Fe(1,2-CBH)] (RR' = (CH), (CH)O(CH), (CH); R = R' = Et), which can be alternatively prepared by the reaction of the parent iron(II) bis(dicarbollide) with tetrahydrofuran or 1,4-dioxane in the presence of MeSO. The cyclic voltammetry studies of the synthesized iron(II) bis(dicarbollide) derivatives revealed that the introduction of amidinium and oxonium substituents leads to a significant increase in the Fe/Fe redox potential relative to the parent iron(II) bis(dicarbollide).

Development of a Gadolinium-Boron-Conjugated Albumin for MRI-Guided Neutron Capture Therapy.

Noninvasive monitoring of boron agent biodistribution is required in advance of neutron capture therapy. In this study, we developed a gadolinium-boron-conjugated albumin (Gd-MID-BSA) for MRI-guided neutron capture therapy. Gd-MID-BSA was prepared by labeling bovine serum albumin with a maleimide-functionalized gadolinium complex and a maleimide-functionalized -dodecaborate orthogonally.

Nickel(II) and Palladium(II) Complexes with η:κ()-Coordinated Dicarbollide Ligands Containing Pendant Pyridine Group.

A series of - and -substituted -carborane derivatives with a pendant pyridyl group was prepared. The synthesized compounds were used as ligands in the complexation reactions with bis(triphenylphosphine)nickel(II) and palladium(II) chlorides to give six new metallacomplexes with unusual η:κ()-coordination of the metal center. The single crystal structures of 1-(NCH-2'-S)-1,2-CBH, 1-(NCH-2'-CHS)-1,2-CBH, Cs [7-(NCH-2'-CHS)-7,8-CBH] - and -carboranes and 3-PhP-3-(4(7)-NCH-2'-S)--3,1,2-NiCBH and 3-PhP-3-(4(7)-NCH-2'-CHS)--3,1,2-NiCBH metallacarboranes were determined using single crystal X-ray diffraction.

New Boron Containing Acridines: Synthesis and Preliminary Biological Study.

The synthesis of the first conjugates of acridine with cobalt bis(dicarbollide) are reported. A novel 9-azido derivative of acridine was prepared through the reaction of 9-methoxyacridine with NCHCHNH, and its solid-state molecular structure was determined via single-crystal X-ray diffraction. The azidoacridine was used in a copper (I)-catalyzed azide-alkyne cycloaddition reaction with cobalt bis(dicarbollide)-based terminal alkynes to give the target 1,2,3-triazoles.

Composites and Materials Prepared from Boron Cluster Anions and Carboranes.

Here, we present composites and materials that can be prepared starting with boron hydride cluster compounds (decaborane, decahydro--decaborate and dodecahydro--dodecaborate anions and carboranes). Recent examples of their utilization as boron protective coatings including using them to synthesize boron carbide, boron nitride, metal borides, metal-containing composites, and neutron shielding materials are discussed. The data are generalized demonstrate the versatile application of materials based on boron cluster anions and carboranes in various fields.

Decaborane: From Alfred Stock and Rocket Fuel Projects to Nowadays.

The review covers more than a century of decaborane chemistry from the first synthesis by Alfred Stock to the present day. The main attention is paid to the reactions of the substitution of hydrogen atoms by various atoms and groups with the formation of -polyhedral boron-halogen, boron-oxygen, boron-sulfur, boron-nitrogen, boron-phosphorus, and boron-carbon bonds. Particular attention is paid to the chemistry of -borane -[BH], whose structure is formed by two decaborane moieties with a common edge, the chemistry of which has been intensively developed in the last decade.

How the Position of Substitution Affects Intermolecular Bonding in Halogen Derivatives of Carboranes: Crystal Structures of 1,2,3- and 8,9,12-Triiodo- and 8,9,12-Tribromo -Carboranes.

The crystal structures of two isomeric triiodo derivatives of -carborane containing substituents in the three most electron-withdrawing positions of the carborane cage, 1,2,3-I-1,2-CBH, and the three most electron-donating positions, 8,9,12-I-1,2-CBH, as well as the crystal structure of 8,9,12-Br-1,2-CBH, were determined by single-crystal X-ray diffraction. In the structure of 1,2,3-I-1,2-CBH, an iodine atom attached to the boron atom (position 3) donates its lone pairs simultaneously to the σ-holes of both iodine atoms attached to the carbon atoms (positions 1 and 2) with the I⋯I distance of 3.554(2) Å and the C-I⋯I and B-I⋯I angles of 169.

New Azido Coumarins as Potential Agents for Fluorescent Labeling and Their "Click" Chemistry Reactions for the Conjugation with -Dodecaborate Anion.

Novel fluorescent 7-methoxy- and 7-(diethylamino)-coumarins modified with azido-group on the side chain have been synthesized. Their photophysical properties and single crystals structure characteristics have been studied. In order to demonstrate the possibilities of fluorescent labeling, obtained coumarins have been tested with -dodecaborate derivative bearing terminal alkynyl group.

New approaches to the functionalization of the 1-carba--decaborate anion.

Two new approaches to the functionalization of the 1-carba--decaborate anion [1-CBH] at boron atoms the ring-opening of its 1,4-dioxane derivative with various nucleophiles and Pd-catalysed cross-coupling of its iodo derivative with aromatic amines and heteroaromatics were developed.

Nucleophilic addition reactions to nitrilium derivatives [BHNCCH] and [BHNCCHCH]. Synthesis and structures of -dodecaborate-based iminols, amides and amidines.

The synthesis of the acetonitrilium and propionitrilium derivatives of -dodecaborate [BH] was discussed. The nucleophilic addition reactions of water, alcohols and secondary amines to the activated triple bond of the nitrilium derivatives yielded the corresponding iminols, imidates and amidines. The obtained results were supported by X-ray diffraction studies and DFT calculations.

Synthesis of 3-Aryl--carboranes with Sensitive Functional Groups.

A simple and efficient method was developed for the one-pot synthesis of 3-aryl derivatives of -carborane with sensitive functional groups using 3-iodo--carborane and aryl zinc bromides that were generated in situ. A series of 3-aryl--carboranes, including those containing nitrile and ester groups, 3-RCH-1,2-CBH (R = -Me, -NMe, -OCHOMe, -OMe, -CN, -CN, -COOEt, -COOEt, -COOEt) was synthesized using this approach. The solid-state structures of 3-RCH-1,2-CBH (R = -OMe, -CN, and -CN) were determined by single crystal X-ray diffraction.

Synthesis of Boronated Amidines by Addition of Amines to Nitrilium Derivative of Cobalt Bis(Dicarbollide).

A series of novel cobalt bis(dicarbollide) based amidines were synthesized by the nucleophilic addition of primary and secondary amines to highly activated B-N≡C-R triple bond of the propionitrilium derivative [8-EtC≡N-3,3'-Co(1,2-CBH)(1',2'-CBH)]. The reactions with primary amines result in the formation of mixtures of and isomers of amidines, whereas the reactions with secondary amines lead selectively to the -isomers. The crystal molecular structures of -[8-EtC(NMe)=HN-3,3'-Co(1,2-CBH)(1',2'-CBH)], -[8-EtC(NEt)=HN-3,3'-Co(1,2- CBH)(1',2'-CBH)] and -[8-EtC(NCH)=HN-3,3'-Co(1,2-CBH)(1',2'-CBH)] were determined by single crystal X-ray diffraction.

Synthesis and crystal structures of nickel(ii) and palladium(ii) complexes with o-carboranyl amidine ligands.

A number of new nido-carboranyl amidines 10-R(CH)NHC(Et)[double bond, length as m-dash]HN-7,8-CBH (n = 2, R = OH, OMe, and NMe; n = 3, R = OH) were synthesized by the nucleophilic addition of amino alcohols and N,N-dimethylethylenediamine to the highly activated -C[triple bond, length as m-dash]N- triple bond of the 10-propionitrilium derivative of nido-carborane. A similar reaction of 10-EtC[triple bond, length as m-dash]N-7,8-CBH with ethylenediamine unexpectedly resulted in the cleavage of the C[triple bond, length as m-dash]N bond to form the ammonium derivative 10-HN-7,8-CBH. The complexation of the synthesized carboranyl amidines 10-MeO(CH)NHC(Et)[double bond, length as m-dash]HN-7,8-CBH and 10-MeN(CH)NHC(Et)[double bond, length as m-dash]HN-7,8-CBH with nickel and palladium phosphine complexes [(PhP)MCl] (M = Ni, Pd) was studied.

Synthesis of Bis(Carboranyl)amides 1,1'-μ-(CHNH(O)C(CH)-1,2-CBH) ( = 0, 1) and Attempt of Synthesis of Gadolinium Bis(Dicarbollide).

Bis(carboranyl)amides 1,1'-μ-(CHNH(O)C(CH)-1,2-CBH) ( = 0, 1) were prepared by the reactions of the corresponding carboranyl acyl chlorides with ethylenediamine. Crystal molecular structure of 1,1'-μ-(CHNH(O)C-1,2-CBH) was determined by single crystal X-ray diffraction. Treatment of bis(carboranyl)amides 1,1'-μ-(CHNH(O)C(CH)-1,2-CBH) with ammonium or cesium fluoride results in partial deboronation of the -carborane cages to the -carborane ones with formation of [7,7'(8')-μ-(CHNH(O)C(CH)-7,8-CBH)].

The unexpected reactivity of 9-iodo-nido-carborane: from nucleophilic substitution reactions to the synthesis of tricobalt tris(dicarbollide) Na[4,4',4''-(MeOCHCHO)-3,3',3''-Co(μ-O)(μ-S)(1,2-CBH)].

An unusual reactivity of 9-iodo-nido-carborane [9-I-7,8-CBH] towards nucleophiles under strong basic conditions was revealed. The nucleophilic substitution of iodine with O- and N-nucleophiles results in [9-RO-7,8-CBH] (R = H, CHCHOMe) and [9-L-7,8-CBH] (L = Py, NEt, MeNCHCHNMe), respectively. Reaction of [9-I-7,8-CBH] with CoCl in 1,2-dimethoxyethane in the presence of t-BuOK, depending on the order of addition of the reagents, leads either to a diastereomeric mixture of diiodo derivatives cobalt bis(dicarbollide) rac-[4,4'-I-3,3'-Co(1,2-CBH)] and meso-[4,7'-I-3,3'-Co(1,2-CBH)] or to the corresponding mixture of 2-methoxyethoxy derivatives rac-[4,4'-(MeOCHCHO)-3,3'-Co(1,2-CBH)] and meso-[4,7'-(MeOCHCHO)-3,3'-Co(1,2-CBH)].

The First Nickelacarborane with - Structure.

The first nickelacarborane with structure [10',11'-(Py)-3,9'-Ni(1,2-CBH)(7',8'-CBH)] was isolated from the reaction of nickel(IV) bis(dicarbollide) with pyridine. The molecular structure of this complex was determined by single crystal X-ray diffraction. The nickel atom is a common vertex for the -NiCB cluster and the -NCB cluster where it is located together with carbon atoms in the open NiCB pentagonal face.

Effects of Linkers on the Development of Liposomal Formulation of Cholesterol Conjugated Cobalt Bis(dicarbollides).

Boron neutron capture therapy (BNCT) remains an important treatment arm for cancer patients with locally invasive malignant tumors. This therapy needs a significant amount of boron to deposit in cancer tissues selectively, sparing other healthy organs. Most of the liposomes contain water-soluble polyhedral boron salts stay in the core of the liposomes and have low encapsulation efficiency.

Bis(dicarbollide) Complexes of Transition Metals as a Platform for Molecular Switches. Study of Complexation of 8,8'-Bis(methylsulfanyl) Derivatives of Cobalt and Iron Bis(dicarbollides).

Complexation of the 8,8'-bis(methylsulfanyl) derivatives of cobalt and iron bis(dicarbollides) [8,8'-(MeS)-3,3'-M(1,2-CBH)] (M = Co, Fe) with copper, silver, palladium and rhodium leads to the formation of the corresponding chelate complexes, which is accompanied by a transition from the to the conformation of the bis(dicarbollide) complex. This transition is reversible and can be used in design of coordination-driven molecular switches based on transition metal bis(dicarbollide) complexes. The solid-state structures of {(PhP)ClPd[8,8'- (MeS)-3,3'-Co(1,2-CBH)-κ-S,S']} and {(COD)Rh[8,8'-(MeS)-3,3'-Co(1,2-CBH)-κ-S,S']} were determined by single crystal X-ray diffraction.

Boron-Containing Lipids and Liposomes: New Conjugates of Cholesterol with Polyhedral Boron Hydrides.

A series of boron-containing lipids were prepared by reactions of cyclic oxonium derivatives of polyhedron boranes and metallacarboranes (closo-dodecaborate anion, cobalt and iron bis(dicarbollides)) with amine and carboxylic acids which are derived from cholesterol. Stable liposomal formulations, on the basis of synthesized boron-containing lipids, hydrogenated soybean l-α-phosphatidylcholine and (HSPC) 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-2000] (DSPE-PEG) as excipients, were prepared and then characterized by dynamic light scattering (DLS) that revealed the formation of particles to be smaller than 200 nm in diameter. The resulting liposomal formulations showed moderate to excellent loading and entrapment efficiency, thus justifying the design of the compounds to fit in the lipid bilayer and ensuring ease of in vivo use for future application.

Conjugate of chlorin with iron bis(dicarbollide) nanocluster: synthesis and biological properties.

Efficiency of both photodynamic and boron-neutron capture anticancer therapies (BNCT) depends on the properties of the used photo- and neutronsensitizer. We report on the synthesis and properties of the advanced photo- and neutronsensitizer designed as a conjugate of chlorin with iron bis(dicarbollide) nanocluster. The conjugate is shown to accumulate efficiently in rat glioblastoma C6 cells delivering >10 boron atoms per cell and thus meeting requirements for BNCT agents, to provide photoinduced 50% death of C6 cells at 35 ± 3 nM, to be not toxic for cells without activating stimulus.

Synthesis and study of -substituted methylthio derivatives of cobalt bis(dicarbollide).

The -methylthio derivatives of cobalt bis(dicarbollide) were synthesized by reaction of anhydrous CoCl with -carborane [7-MeS-7,8-CBH] and isolated as a mixture of -[1,1'-(MeS)-3,3'-Co(1,2-CBH)] and -[1,2'-(MeS)-3,3'-Co(1,2-CBH)] isomers. The structures of both isomers were studied using DFT quantum chemical calculations. The most preferable geometry of rotamers and the stabilization energy of -methylthio derivatives of cobalt bis(dicarbolide) were calculated.

Synthesis and Structure of Methylsulfanyl Derivatives of Nickel Bis(Dicarbollide).

Symmetrically and unsymmetrically substituted methylsulfanyl derivatives of nickel(III) bis(dicarbollide) (BuN)[8,8'-(MeS)-3,3'-Ni(1,2-CBH)], (BuN)[4,4'-(MeS)-3,3'-Ni(1,2-CBH)], and (BuN)[4,7'-(MeS)-3,3'-Ni(1,2-CBH)] were synthesized, starting from [Ni(acac)] and the corresponding methylsulfanyl derivatives of -carborane (BuN)[10-MeS-7,8-CBH] and (BuN)[10-MeS-7,8-CBH]. Structures of the synthesized metallacarboranes were studied by single-crystal X-ray diffraction and quantum chemical calculations. The symmetrically substituted 8,8'-isomer adopts conformation stabilized by two pairs of intramolecular C-H···S hydrogen bonds between the dicarbollide ligands.

Arene-Ruthenium Complexes of 1,1'-Bis(-carborane): Synthesis, Characterization, and Catalysis.

Deprotonation of 1,1'-bis(-carborane) with BuLi in THF followed by reaction with [RuCl(-cymene)] affords, in addition to the known compound [Ru(κ-2,2',3'-{1-(1'--1',2'-CBH)--1,2-CBH)}(-cymene)] (), a small amount of a new species, [Ru(κ-2,2',11'-{1-(7'--7',8'-CBH)--1,2-CBH)}(-cymene)] (), with two -agostic B-H⇀Ru bonds, making the bis(carborane) unit a --X()L ligand, a previously unreported bonding mode. Similar species were also formed with arene = benzene (), mesitylene (), and hexamethylbenzene (), although in the last two cases the metallacarborane-carborane species [1-(1'--1',2'-CBH)-3-(arene)--3,1,2-RuCBH)], and , were also isolated. With the bis(-carborane) transfer reagent [Mg(κ-2,2'-{1-(1'--1',2'-CBH)--1,2-CBH)}(DME)], the target compounds [Ru(κ-2,2',3'-{1-(1'--1',2'-CBH)--1,2-CBH)}(arene)], and , were prepared in reasonable-to-good yields, although for arene = benzene and mesitylene small amounts of the unique paramagnetic species [{Ru(arene)}(μ-Cl)(μ-κ-2,2',3,3'-{1-(1'--1',2'-CBH)--1,2-CBH})], and , were also formed.

Ferrocene and Transition Metal Bis(Dicarbollides) as Platform for Design of Rotatory Molecular Switches.

Design of rotatory molecular switches based on extremely stable sandwich organometallic complexes ferrocene and bis(dicarbollide) complexes of transition metals is reviewed. The "on"-"off" switching in these systems can be controlled by various external stimuli such as change of the solution pH, interactions with coordinating species or redox reactions involving the central atom or substituents in the ligands.