Formation of Ir?Rh and Ir?Mo Bonds by Using an Ancillaryortho-Carborane-1,2-diselenolato Ligand

Controlled functionalization of o-carborane via transition metal catalyzed B–H activation

This review summarizes recent advances in transition metal catalyzed vertex-specific BH functionalization of o-carborane for controlled synthesis of its derivatives.

Self-assembly of manganese(i) based thiolato bridged dinuclear metallacycles: synthesis, characterization, cytotoxicity evaluation and CO-releasing studies

Manganese(i) based thiolato bridged dinuclear metallacycles were assessed as anticancer agents along with myoglobin assay for CO-releasing studies.

Palladium-promoted sulfur atom migration on carboranes: facile B(4)−S bond formation from mononuclear Pd-B(4) complexes

For the first time, carborane complexes containing a B(4)–S bond were obtained directly by heating mononuclear Pd-B(4)-bound carborane complexes. A possible mechanism involved in sulfur atom migration is presented in which the leaving group, pyridine, benzyl isocyanide or PPh3, is demonstrated to be the trigger of the reaction process. In this work, efficient routes are developed through one-pot reactions to prepare B(4)-S carborane derivatives.

Transition-Metal-Catalyzed Selective Cage B-H Functionalization of o-Carboranes

Carboranes are a class of carbon-boron molecular clusters with unusual thermal and chemical stabilities. They have been proved as very useful building blocks in supramolecular design, optoelectronics, nanomaterials, boron neutron capture therapy agents and organometallic/coordination chemistry. Thus, the functionalization of o-carboranes has received growing interests. Over the past decades, most of the works in this area have been focused on cage carbon functionalization as the weakly acidic cage C-H proton can be readily deprotonated by strong bases. In sharp contrast, selective cage B-H activation/functionalization among chemically very similar ten B-H vertices is very challenging. Considering the differences in electron density of ten cage B-H bonds in o-carborane and the nature of transition metal complexes, we have tackled this selectivity issue by means of organometallic chemistry. Our strategy is as follows: using electron-rich transition metal catalysts for the functionalization of the most electron-deficient B(3,6)-H vertices (bonded to both cage CH vertices); using electron-deficient transition-metal catalysts for the functionalization of relatively electron-rich B(8,9,10,12)-H vertices (with no bonding to both cage CH vertices); and using the combination of directing groups and electrophilic transition metal catalysts for the functionalization of B(4,5,7,11)-H vertices (bonded to only one cage CH vertex). Successful applications of such a strategy result in the preparation of a large variety of cage B-functionalized carboranes in a regioselective and catalytic manner, which are inaccessible by other means. It is believed that as this field progresses, other cage B-functionalized carboranes are expected to be synthesized, and the results detailed in this concept article will further these efforts.

Heterometallic boride clusters: synthesis and characterization of butterfly and square pyramidal boride clusters*

A number of heterometallic boride clusters have been synthesized and structurally characterized using various spectroscopic and crystallographic analyses. Thermolysis of [Ru3(CO)12] with [Cp*WH3(B4H8)] (1) yielded [{Cp*W(CO)2}2(μ 4-B){Ru(CO)3}2(μ-H)] (2), [{Cp*W(CO)2}2(μ 5-B){Ru(CO)3}2{Ru(CO)2}(μ-H)] (3), [{Cp*W(CO)2}(μ 5-B){Ru(CO)3}4] (4) and a ditungstaborane cluster [(Cp*W)2B4H8Ru(CO)3] (5) (Cp*=η 5-C5Me5). Compound 2 contains 62 cluster valence-electrons, in which the boron atom occupies the semi-interstitial position of a M4-butterfly core, composed of two tungsten and two ruthenium atoms. Compounds 3 and 4 can be described as hetero-metallic boride clusters that contain 74-cluster valence electrons (cve), in which the boron atom is at the basal position of the M5-square pyramidal geometry. Cluster 5 is analogous to known [(Cp*W)2B5H9] where one of the BH vertices has been replaced by isolobal {Ru(CO)3} fragment. Computational studies with density functional theory (DFT) methods at the B3LYP level have been used to analyze the bonding of the synthesized molecules. The optimized geometries and computed 11B NMR chemical shifts satisfactorily corroborate with the experimental data. All the compounds have been characterized by mass spectrometry, IR, 1H, 11B and 13C NMR spectroscopy, and the structural architectures were unequivocally established by crystallographic analyses of clusters 2–5.

Efficient synthesis of carborane azo derivatives and their reactivity

An iridium(iii)-catalyzed C–N-bond-forming direct synthesis of carborane azo derivatives was developed.

Palladium catalyzed regioselective B-C(sp) coupling via direct cage B-H activation: synthesis of B(4)-alkynylated o-carboranes

Pd-catalyzed carboxylic acid guided regioselective alkynylation of cage B(4)-H bonds in o-carboranes has been achieved for the first time using two different catalytic systems. In the presence of 5 mol% Pd(OAc)2 and 3 equiv. of AgOAc, the reaction of 1-COOH-2-R1-C2B10H10 with R3SiC[triple bond, length as m-dash]CBr in ClCH2CH2Cl gives 4-(R3SiC[triple bond, length as m-dash]C)-2-R1-o-C2B10H10 in moderate to high yields. This reaction is compatible with alkynes possessing sterically bulky silyl groups such as iPr3Si or t BuMe2Si. Meanwhile, another catalytic system of Pd(OAc)2/AgOAc/K2HPO4 can catalyze the direct B(4)-alkynylation of 1-COOH-2-R1-C2B10H10 with terminal alkynes R2C[triple bond, length as m-dash]CH in moderate to high yields. The latter has a broader substrate scope from bulky silyl to aromatic to carboranyl substituents. Desilylation of the resultant products affords carboranyl acetylene 4-(HC[triple bond, length as m-dash]C)-2-R1-o-C2B10H10 which can undergo further transformations such as Sonogashira coupling, dimerization and click reactions. It is suggested that the above two catalytic systems may proceed via Pd(ii)-Pd(iv)-Pd(ii) and Pd(ii)-Pd(0)-Pd(ii) catalytic cycles, respectively. In addition, the silver salt is found to promote the decarboxylation reaction and thereby controls the mono-selectivity.

Palladium-catalyzed regioselective intramolecular coupling of o-carborane with aromatics via direct cage B-H activation

Palladium-catalyzed intramolecular coupling of o-carborane with aromatics via direct cage B-H bond activation has been achieved, leading to the synthesis of a series of o-carborane-functionalized aromatics in high yields with excellent regioselectivity. In addition, the site selectivity can also be tuned by the substituents on cage carbon atom.

Ir(2-phenylpyridine)2(benzene-1,2-dithiolate) anion as a diastereoselective metalloligand and nucleophile: stereoelectronic effect, spectroscopy, and computational study of the methylated and aurated complexes and their oxygenation products

The anionic complex [Ir(2-phenylpyridine)2(benzene-1,2-dithiolate)](-) ([IrSS](-)) is a nucleophile and metalloligand that reacts with methyl iodide and AuPR3(+) (R = Ph or Et) to form S-methylated complexes (thiother-thiolate and dithiother complexes) and S-aurated complexes, respectively. The reactions are completely diastereselective, producing only the enantiomers ΛS and ΔR or ΛSS and ΔRR. The diastereoselectivity is stereoelectronically controlled by the orientation of the highest occupied molecular orbital (HOMO) of [IrSS](-) arising from filled dπ-pπ antibonding interactions, and the chirality of the iridium ion. Methylation or auration removes the high-energy lone pair of the thiolate S atom, leading to low-lying HOMOs composed mainly of the Ir d-orbital and the 2-phenylpyridine π (ppyπ) orbital. The methylated and aurated complexes can be oxidized by H2O2 or peracid to give sulfinate-thiother, disulfoxide, and sulfinate-sulfoxide complexes, and the oxygenation further stabilizes the HOMO. All the complexes are luminescent, and their electronic spectra are interpreted with the aid of time-dependent density functional theory calculations. The thiother-thiolate complex exhibits ligand(S)-to-ligand(π* of ppy)-charge-transfer/metal-to-ligand-charge-transfer absorption (LLCT/MLCT) and a relatively low-energy (3)LLCT/MLCT emission, while the other complexes display (3)ππ*/MLCT emissions.

Novel inorganic heterocycles from dimetalated carboranylamidinates

Mono- and dianionic carboranylamidinates are readily available in one-pot reactions directly from o-carborane (1). In situ-monolithiation of 1 followed by treatment with N,N'-diisopropylcarbodiimide, (i)PrN=C=N(i)Pr, or N,N'-dicyclohexylcarbodiimide, CyN=C=NCy, provided the lithium carboranylamidinates (o-C2B10H10C(NH(i)Pr)(=N(i)Pr)-κ(2)C,N)Li(DME) (2a) and (o-C2B10H10C(NH(i)Cy)(=N(i)Cy)-κ(2)C,N)Li(THF)2 (2b). Controlled hydrolysis of 2a,b afforded the free carboranylamidines o-C2B10H11C(NH(i)R)(=N(i)R) (3a: R = (i)Pr, 3b: R = Cy). The first dimetalated carboranylamidinates, o-C2B10H10C(N(i)Pr)(=N(i)Pr)Li2(DME)2 (4a) (DME = 1,2-dimethoxyethane) and o-C2B10H10C(N(i)Pr)(=N(i)Pr)Li2(THF)4 (4b), were prepared in high yield (83% yield) directly from 1 using a simple one-pot synthetic protocol. Treatment of 4b with 2 equiv. of Me3SiCl afforded the disilylated derivative o-C2B10H10-κ(2)C,N-[C(N(i)PrSiMe3)(=N(i)Pr)]SiMe3 (5). Dianionic 4b also served as an excellent precursor for novel inorganic heterocycles incorporating the closo-1,2-C2B10H10 cage, including the unsymmetrical distannene [o-C2B10H10C(N(i)Pr)(=N(i)Pr)-κ(2)C,N]Sn=Sn[((i)PrN)2C(n)Bu]2 (6) and the azaphosphole derivative [o-C2B10H10C(N(i)Pr)(=N(i)Pr)-κ(2)C,N]PPh (7). Surprisingly, it was found that the synthesis of new inorganic ring systems from dianionic carboranylamidinates can also be achieved by employing only 1 equiv. of n-butyllithium in the generation of the anionic carboranylamidinate intermediates. Using this straightforward one-pot synthetic protocol, the Group 14 metallacycles [o-C2B10H10C(NCy)(=NCy)-κ(2)C,N]SiR2 (R = Cl (8), Me (9), Ph (10)) and [o-C2B10H10C(NCy)([=NCy)-κ(2)C,N]GeCl2 (11) have become accessible. The same synthetic strategy could be successfully adapted to prepare the corresponding Group 4 metallocene derivatives Cp2Ti[o-C2B10H10C(NCy)(=NCy)-κ(2)C,N] (12) and Cp2Zr[o-C2B10H10C(NCy)(=NCy)-κ(2)C,N] (13). The molecular structures of 2b, 3b, 4b, 5, 6, 7, 10, 12, and 13 were confirmed by single-crystal X-ray diffraction.

Dicarba-closo-dodecarborane-containing half-sandwich complexes of ruthenium, osmium, rhodium and iridium: biological relevance and synthetic strategies

This review describes how the incorporation of dicarba-closo-dodecarboranes into half-sandwich complexes of ruthenium, osmium, rhodium and iridium might lead to the development of a new class of compounds with applications in medicine. Such a combination not only has unexplored potential in traditional areas such as Boron Neutron Capture Therapy agents, but also as pharmacophores for the targeting of biologically important proteins and the development of targeted drugs. The synthetic pathways used for the syntheses of dicarba-closo-dodecarboranes-containing half-sandwich complexes of ruthenium, osmium, rhodium and iridium are also reviewed. Complexes with a wide variety of geometries and characteristics can be prepared. Examples of addition reactions on the metal centre, B-H activation, transmetalation reactions and/or direct formation of metal-metal bonds are discussed (103 references).