Closo Carborane Research Articles

The VCD method – a simple and reliable way to distinguish cage C and B atoms in (hetero)carborane structures determined crystallographically

Replacing a boron vertex in a [B(n)H(n)](2-) cluster with a smaller atom, e.g. carbon, results in the distance from that atom to the polyhedral centroid being shorter. This forms the basis of a simple but very effective method of distinguishing between B and C atoms in (hetero)carboranes that have been studied crystallographically, the Vertex-to-Centroid Distance (VCD) method. Examples of well-characterised icosahedral and sub-icosahedral closo carboranes, nido carboranes, icosahedral metallacarboranes and supraicosahedral metallacarboranes are used to illustrate the generality of the VCD method. In this process a number of structures are identified in which the method suggests B/C disorder not previously recognised and some structures in which it appears that a cage C atom has been wrongly assigned. The largest sub-group of heterocarboranes is the family of 3,1,2-MC(2)B(9) compounds, and for these species consideration of Exopolyhedral Ligand Orientation (ELO) can sometimes be used to quickly suggest when a literature structure is suspect in terms of cage C atom positioning. The crystal structure of one such compound, 3,3-NO(3)-3-PPh(3)-3,1,2-closo-RhC(2)B(9)H(11), has been redetermined and the correct location of the cage C atoms established.

Influence of the group 11 metal on the emissive properties of complexes [M{(PR2)2C2B9H10}L

The metal atoms in group 11 complexes [M{(PR2)2C2B9H10}L] [R = Ph, (i)Pr; L = tertiary phosphane; M = Au, Ag, Cu] play an important role in the emissive properties of these compounds. The influence of the metal follows the order Au ≫ Ag ≥ Cu. The three-coordinated complexes are obtained from the reaction of [AuClL], [Ag(OTf)L], or [Cu(NO3)(PPh3)2] with the closo carborane diphosphane in refluxing ethanol. For L = PPh2NHPy, cleavage of the P-N bond and the formation of the monophosphane PPh2OEt are observed, depending on the metal and the nido carborane diphosphane substituent ((i)Pr or Ph).

Uncommon Coordination Behaviour of P(S) and P(Se) Units when Bonded to Carboranyl Clusters: Experimental and Computational Studies on the Oxidation of Carboranyl Phosphine Ligands

Oxidation of closo-carboranyl diphosphines 1,2-(PR(2))(2)-1,2-closo-C(2)B(10)H(10) (R=Ph, iPr) and closo-carboranyl monophosphines 1-PR(2)-2-R'-1,2-closo-C(2)B(10)H(10) (R=Ph, iPr, Cy; R'=Me, Ph) with hydrogen peroxide, sulfur and elemental black selenium evidences the unique capacity of the closo-carborane cluster to produce uncommon or unprecedented P/P(E) (E=S, Se) and P=O/P=S chelating ligands. When H(2)O(2) reacts with 1,2-(PR(2))(2)-1,2-closo-C(2)B(10)H(10) (R=Ph, iPr), they are oxidized to 1,2-(OPR(2))(2)-1,2-closo-C(2)B(10)H(10) (R=Ph, iPr). However, when S and Se are used, different reactivity is found for 1,2-(PPh(2))(2)-1,2-closo-C(2)B(10)H(10) and 1,2-(PiPr(2))(2)-1,2-closo-C(2)B(10)H(10). The reaction with sulfur produces mono- and dioxidation products for R=Ph, whereas Se produces the mono-oxidation product only. For R=iPr, only monooxidation takes place with S, and the second C(c)-PiPr(2) bond breaks to yield 1-SPiPr(2)-1,2-closo-C(2)B(10)H(11). When Se is used, only 1-SePiPr(2)-1,2-closo-C(2)B(10)H(11) is formed. The potential of the mono-chalcogenide carboranyl diphosphines 1-EPPh(2)-2-PPh(2)-1,2-closo-C(2)B(10)H(10) (E=S, 9; Se, 15) to behave as unsymmetric chelating bidentate ligands was studied for different metal complexes, different solvents and in the solid state. Dechalcogenation takes place in each case. Computational studies provided information on the P=E (E=S, Se) bonds. Steric effects block the bonding ability of the P=E group due to interactions between the chalcogen and the neighbouring hydrogen atoms (three from the phenyl rings and one from the carborane cluster). The electronic effects originate from the strongly electron-withdrawing character of the closo carborane cluster, which polarizes the P=E (E=S, Se) bond towards the phosphorus atom. As a consequence, the E atom is the electron-poor site and the P atom the electron-rich site in the P=E bond.

Preparation and Evaluation of Carborane Analogues of Tamoxifen

A stereoselective synthesis of closo carborane analogues of tamoxifen was developed where the products represent a new approach to developing metabolically robust SERMs. The A-ring found in the backbone of tamoxifen was replaced with an ortho carborane cluster; the product was determined to be the desired Z isomer, which showed superior chemical stability to tamoxifen both in solution and in the solid state. By use of microwave heating, it was possible to convert some of the Z carborane tamoxifen analogue to the corresponding E isomer. Cell growth assays using both isomers and a carborane that is known to target the ER were conducted using estrogen receptor (ER) positive and ER negative human breast cancer cells with and without the presence of estradiol (E2). The Z carborane isomer was able to inhibit cell proliferation better than tamoxifen in an E2 free environment, while the E isomer inhibited cell growth better than tamoxifen when E2 was present.

Supraicosahedral indenyl cobaltacarboranes

13-vertex indenyl cobaltacarboranes with 4,1,6-, 4,1,10- and 4,1,2-CoC(2)B(10) architectures have been synthesised by reduction of the corresponding closo carborane and metallation with an {(eta-C(9)H(7))Co} fragment. Variants of the 4,1,6-isomer were prepared with no, one and two methyl groups on cage C atoms, whilst 4,1,2-species were obtained both with two methyl groups and a trimethylene tether on the cage C atoms. Thermolysis of the 4,1,6-isomers yielded the corresponding 4,1,8-isomers, which in turn were converted to 4,1,12-isomers by thermolysis at higher temperatures. Alternatively relatively mild heating of the 4,1,10-isomer led to the 4,1,12-isomer directly. Products were characterised by mass spectrometry, (1)H and (11)B NMR spectroscopies and, in most cases, elemental analysis, and nine compounds were studied crystallographically. The 4,1,6-, 4,1,8-, 4,1,10- and 4,1,12- species have docosahedral cages whilst the 4,1,2-species are henicosahedral. In the structural studies attention focused on the orientation of the indenyl ligand with respect to the carborane ligand since this affords experimental information on the metal-cage bonding through the structural indenyl effect. There is a general tendency for the indenyl ligand to adopt orientations in which the ring junction C atoms lie trans to cage B atoms. In cases where the orientation is not compromised by the presence of a non-H substituent on the face of the carborane there is generally good agreement between the experimental orientation and that computed by DFT calculations for the related naphthalene ferracarboranes (eta-C(10)H(8))FeC(2)B(10)H(12). The presence of C-methyl substituents in the indenyl cobaltacarboranes tends to override this preference except in the case of 1,6-Me(2)-4-(eta-C(9)H(7))-4,1,6-closo-CoC(2)B(10)H(10) where the indenyl ligand instead is forced to incline away from the cage methyl groups. In DCM solution the 4,1,6-, 4,1,8-, 4,1,10- and 4,1,12- isomers of (eta-C(9)H(7))CoC(2)B(10)H(12) exhibit two, stepwise, 1-electron reductions assigned to Co(III)/Co(II)/Co(I) couples at less negative potentials than those of the corresponding Cp compounds. Moreover these reductions are easier for those isomers (4,1,6- and 4,1,10-) in which there are two cage C atoms in the carborane face to which the metal atom is bound. By spectroelectrochemical and EPR measurements it is concluded that the reductions of these indenyl cobaltacarboranes are largely metal-based.

Fission processes following core level excitation in closo ‐1,2‐orthocarborane

AbstractTime‐of‐flight mass analysis with multi‐stop coincidence detection was used to study the multi‐cation ionic fragmentation of the closo carborane cage molecule closo ‐1,2‐orthocarborane (C2B10H12) following inner‐shell excitation in or above the B 1s regime. Electron ion coincidence spectra reveal the cationic products which are formed after core level excitation. Distinct changes in fragmentation pattern are observed as a function of excitation energy. Photoelectron–photoion–photoion coincidence (PEPIPICO) spectroscopy was used to study the dominant fission routes in the core level excitation regime. Series of ion pairs are identified, where asymmetric fission dominates, leading to ion pairs of different mass. Suitable fission and fragmentation mechanisms are discussed. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Self-assembled 2D supramolecular networks of copper(i) carborane complexes through C–H⋯H–B dihydrogen bonding interactions

Three dinuclear copper(I) complexes with the formula [Cu2(mu-X)2(1,2-(PiPr2)2-1,2-C2B10H10)2] (X = Cl (1), Br (2), I (3)) containing the closo carborane diphosphine ligand 1,2-(PiPr2)2-1,2-C2B10H10 have been prepared and characterized by elemental analysis, FT-IR and X-ray structure determination. The X-ray structure analyses revealed that the three complexes were isostructural and crystallized in the monoclinic system and space group C2/m. The carborane cage ligand was coordinated bidentately to the Cu(I) center through its two phosphorus atoms, and the coordination geometry around each copper atom was distorted tetrahedral. Two halogen atoms bridged the metal centers forming a dimer structure [Cu2(mu-X)2(1,2-(PiPr2)2-1,2-C2B10H10)2], which were linked into 2D supramolecular networks through novel C-H...H-B dihydrogen bonding interactions

Synthesis and characterization of two types of skeleton heterobimetallic trinuclear Mo(W)–Cu–S clusters containing 1,2-bis(diphenylphosphino)-1,2-dicarba- closo-dodecaborane

Reactions of (NH 4) 2MS 4 or (NH 4)MOS 3 (M = Mo, W) with CuSCN and the closo carborane diphosphine 1,2-(PPh 2) 2-1,2-C 2B 10H 10 in CH 2Cl 2 yielded five heterobimetallic trinuclear Mo(W)–Cu–S clusters with the formula Cu 2MS 4L 2 (M = Mo( 1), W( 3), L = 1,2-(PPh 2) 2-1,2-C 2B 10H 10), Cu 2MoS 4L 2 · CH 2Cl 2 ( 2) and Cu 2MOS 3L 2 (M = Mo( 4),W( 5)). All the clusters have been characterized by elemental analysis, FT-IR, UV/Visible, 1H and 13C NMR spectroscopy and X-ray structure determination. X-ray crystal structure analysis showed that the metal skeleton of these clusters could be classified into two types. With (NH 4) 2MS 4 (M = Mo, W), the three metal atoms (two Cu atoms and one M atom (M = Mo, W)) are almost in a linear conformation, while with (NH 4) 2MOS 3 the conformation of the heterobimetallic trinuclear cluster core was a butterfly-shaped (or referenced as defective cubane-like with two corners missing). The coordination sphere of the metal atoms in all the clusters, either for Cu or M, should be described as a distorted tetrahedron. For each cluster, the closo carborane diphosphine ligand 1,2-(PPh 2) 2-1,2-C 2B 10H 10 was introduced into the Cu 2MS 4 or Cu 2MOS 3 cluster cores and coordinated bidentately through the P atoms to Cu(I), and this resulted in a stable five-member chelating ring between the bis-diphosphine ligand and the metal.

Synthesis, characterization and luminescence of the tetranuclear gold cluster: [Au4{7,8-(PPh2)2-7,8-C2B9H10}2(PPh3)2

A tetranuclear gold cluster has been synthesized by the reaction of [Au(PPh3)NO3] with the closo carborane diphosphine 1,2-(PPh2)2-1,2-C2B10H10 in THF, and characterized by elemental analysis, FT-IR, 1H and 13C NMR spectroscopy and X-ray structure determination. The cluster crystallizes in the triclinic Pī, a = 15.118(8) Å, b = 16.057(9) Å, c = 24.284(13) Å, α = 80.822(9)°, β = 79.624(8)°, γ = 81.938(8)°, Z = 2, R 1 = 0.0626, wR 2 = 0.1894. A single crystal structure determination showed that four gold atoms form a tetrahedral framework. Among these four gold atoms, two were chelated by two nido carborane diphosphine [7,8-(PPh2)2-7,8-C2B9H10]− anions coming from the degradation of the initial closo ligand 1,2-(PPh2)2-1,2-C2B10H10, while the other two were ligated to two PPh3 groups. The luminescence of this cluster was also investigated in dichloromethane solution at room temperature.

Synthesis, characterization and luminescence property of heterobimetallic trinuclear Mo(W)–Ag–S clusters containing 1,2-bis(diphenylphosphino)-1,2-dicarba- closo-dodecaborane

Reactions of (NH 4) 2MS 4 or (NH 4) 2MOS 3 (M = Mo, W) with AgSCN and closo carborane diphosphine ligand 1,2-(PPh 2) 2-1,2-C 2B 10H 10 (L) in CH 2Cl 2 yielded four heterobimetallic trinuclear Mo(W)–Ag–S clusters: [Ag 2MoS 4L 2] ( 1), [Ag 2WS 4L 2] ( 2), [Ag 2MoOS 3L 2] ( 3) and [Ag 2WS 4L 2] ( 4), respectively. All the new clusters have been characterized by elemental analysis, FT-IR, UV–Vis, 1H and 13C NMR spectroscopy and their molecular structures (except for 3) were further confirmed by single-crystal X-ray diffraction. X-ray crystal structure analysis showed that the closo carborane diphosphine ligand was coordinated bidentately to Ag(I) atom through its two phosphorus atoms, resulting in a stable five-member chelating ring between the diphosphine ligand and the metal. The coordination sphere of the central M atom, as well as all the Ag atoms, was tetrahedron. The skeletons of these clusters could be classified into two types: with (NH 4) 2MS 4, the three metal atoms (two Ag atoms and one M atom) are in a linear conformation, while with (NH 4) 2MOS 3, the conformation of the heterobimetallic trinuclear cluster is butterfly shaped. The luminescence properties of the clusters were investigated in CH 2Cl 2 solution at room temperature and for the first time the butterfly-shaped Ag–W–S cluster containing the Ag 2WS 4 core has been proved to show luminescence property.

Synthesis and characterization of three nickel(ii) carborane complexes containing nido- or closo-carborane diphosphine ligand

Three nickel(II) carborane complexes, [Ni 2(μ-Cl) 2{7,8-(PPh 2) 2-7,8-C 2B 9H 10} 2] ( 1), [Ni{7-(OPPh 2)-8-(PPh 2)-7,8-C 2B 9H 10}{7,8-(PPh 2) 2-7,8-C 2B 9H 10}] ( 2) and [NiBr 2{1,2-(PPh 2) 2-1,2-C 2B 10H 10}] · CH 2Cl 2 ( 3), have been synthesized by the reactions of 1,2-bis(diphenylphosphino)-1,2-dicarba- closo-dodecaborane with NiCl 2 · 6H 2O or NiBr 2 · 6H 2O in ethanol under different conditions, respectively. For complex 1, it could also be obtained under the solvothermal condition. All the three complexes were characterized by elemental analysis, FT-IR, 1H and 13C NMR spectroscopy and X-ray structure determination. Single crystal analysis shows that the molecular symmetry of complex 1 is centrosymmetric, containing two same structure units – Ni(7,8-(PPh 2) 2-7,8-C 2B 9H 10) linked by two bridged-Cl atoms. The central square plane formed by the [Ni 2Cl 2] unit is almost parallel to the two side NiPP planes. For complex 2, the coordination environment of the Ni atom is a seriously distorted square-planar, in which two positions come from the chelating diphosphine ligand [7,8-(PPh 2) 2-7,8-C 2B 9H 10] − degraded from the closo species, while the other two are occupied by an unsymmetrical chelating phosphine oxide ligand [7-(OPPh 2)-8-(PPh 2)-7,8-C 2B 9H 10] −. As for complex 3, the geometry at the Ni atom is a slightly distorted square-planar. The closo carborane diphosphine ligand 1,2-(PPh 2) 2-1,2-C 2B 10H 10 was coordinated bidentately to the metal ion through the two phosphorus atoms, and the two Br atoms are at cis position which can fulfill the four coordination mode of the metal.

Synthesis and characterization of three group 10 complexes containing nido-carborane diphosphine: [MCl(PPh 3){7,8-(PPh 2) 2-7,8-C 2B 9H 10}] (M = Ni, Pd, Pt)

Three group 10 complexes containing nido-carborane diphosphine, [NiCl(PPh 3){7,8-(PPh 2) 2-7,8-C 2B 9H 10}] ( 1), [PdCl(PPh 3){7,8-(PPh 2) 2-7,8-C 2B 9H 10}] · 1.25CH 2Cl 2 ( 2) and [PtCl(PPh 3){7,8-(PPh 2) 2-7,8-C 2B 9H 10}] · 2.5CH 2Cl 2 ( 3) have been synthesized by the reactions of [M(PPh 3) 2Cl 2] (M = Ni, Pd, Pt) with closo carborane diphosphine 1,2-(PPh 2) 2-1,2-C 2B 10H 10 in ethanol. For complex 3, it could also be obtained under solvothermal condition. All three complexes were characterized by elemental analysis, FT-IR, 1H and 13C NMR spectroscopy and X-ray structure determination. Single crystal structures show that their structures are similar to each other. In each complex, the nido [7,8-(PPh 2) 2-7,8-C 2B 9H 10] −, which resulted from the degradation of the initial closo ligand 1,2-(PPh 2) 2-1,2-C 2B 10H 10 during the reaction process, was coordinated bidentately through the P atoms to M(II) ion, and this resulted in a stable five-membered chelating ring between the bis-diphosphine ligand and the metal. The coordination mode of the metal can be described as a slightly distorted square-planar, in which the remaining two positions were occupied by one Cl − and one PPh 3 group.

Synthesis of ether- and carbon-linked polycarboranyl porphyrin dimers for cancer therapies

Porphyrin dimers bearing multiple carborane cages for potential use as sensitizers in boron neutron capture therapy (BNCT) and photodynamic therapy (PDT) were synthesized from protoporphyrin dimethyl ester and characterized. Diastereomeric ether-linked dimers bearing four closo carborane cages (40 boron atoms) were found to be unstable to the acidic conditions necessary for conversion into water-soluble salts. In contrast, the carboncarbon-linked dimers bearing six icosahedral carboranes (60 boron atoms) were stable to acid and could be isolated as water-soluble sodium salts. In vitro and in vivo studies of these novel molecules are currently under investigation.

Cage opening of 1,2‐(CH2OH)2‐closo‐1,2‐C2B10H10 with 2‐(aminomethyl)‐pyridine: synthesis and structure of a cobalt salt of functionalized nido‐C2B9H10 mono anion

AbstractThe reaction of 1,2‐(CH2OH)2‐closo‐1,2‐C2B10 H10 carborane with 2‐(aminomethyl)‐pyridine in methylene chloride at room temperature led to the partial cage degradation of the closo carborane and its conversion to the nido derivative. The addition of cobalt chloride hexahydrate in methanol to the mixture produced orange crystals 1. The crystal structure of the orange compound has been determined. Copyright © 2003 John Wiley & Sons, Ltd.

Cationic Closo Carboranes—Promising Weakly Coordinating Ions

AbstractThe unexplored carbon rich cationic closo carboranes, C3Bn−3Hn+1 (n=5, 6, 7, 10, 12) are investigated theoretically. The position isomers were calculated at the B3LYP/6‐31G* level, and the charge distribution in the cluster is estimated by NBO analysis. The criterion of ring‐cap orbital overlap compatibility along with the number of BC, CC, and BB bonds help in explaining the stability order in each category. The most stable isomer is the one with maximum ring‐cap orbital overlap and largest number of BC bonds. The order of relative stability among the trigonal bipyramid is 1c>1b>1a′, where the stability is proportional to the number of CH caps over the small three‐membered ring. The C3B3H6+ isomer with the one allyl C3 group (2b) is more favorable than the one with a cyclopropenyl group (2a). Among the C3B4H7+ isomers the stability order is 3e>3d>3c>3b>3a, which mostly depends on the ring‐cap orbital overlap. In the bicapped square antiprism (4) where there is large number of isomers, the order follows the rule of ring cap compatibility and the number of BC bonds. The order of 5e>5d>5c>5b>5a obtained from the calculations is in perfect agreement with the above sited rules. Equations (1)–(5) devised for estimating the stability of isomers of C3Bn−3Hn+ indicate an increase in stability with cage size. The mono‐positive charge of the isomers is distributed throughout the cage, making them suitable candidates as weakly electrophillic cations. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1542–1551, 2001

ChemInform Abstract: Relative Reactivities of the Small Closo Carboranes 1,6‐C2B4H6 and 2,4‐C2B5H7 and of closo‐1,10‐C2B8H10 Toward “Electrophilic” Reagents.

AbstractThe relative reactivities of the title carboranes (I)‐(III) and some of their derivatives toward electrophilic reagents of the type RX/AlCl3 (RX: MeCl, EtCl, Cl2, Br2) are reported from competition studies.

ChemInform Abstract: Reaction of the Small Closo Carboranes 1,6‐C2B4H6 and 2,4‐C2B5H7 with BX3 (X: Halogen, Phenyl): A Method of Preparing “3”‐Substituted 2,4‐C2B5H7 Derivatives and Preparation of 2‐Br2B‐closo‐1,6‐C2B4H5.

AbstractA new way of preparing 3‐substituted 2,4‐C2B5H7 derivatives (II) is the reaction of (I) with BX3 at elevated temperatures.

Relative reactivities of the small closo carboranes 1,6-C2B4H6 and 2,4-C2B5H7 and of closo-1,10-C2B8H10 toward electrophilic reagents

Relative reactivities of the small closo carboranes 1,6-C2B4H6 and 2,4-C2B5H7 and of closo-1,10-C2B8H10 toward electrophilic reagents

Reaction of the small closo carboranes 1,6-C2B4H6 and 2,4-C2B5H7 with BX3 (X = halo, phenyl): a method of preparing 3-substituted 2,4-C2B5H7 derivatives and preparation of 2-Br2B-closo-1,6-C2B4H5

Reaction of the small closo carboranes 1,6-C2B4H6 and 2,4-C2B5H7 with BX3 (X = halo, phenyl): a method of preparing 3-substituted 2,4-C2B5H7 derivatives and preparation of 2-Br2B-closo-1,6-C2B4H5

Antipodal hydrogen-hydrogen coupling in small cage carbon-boron compounds

Antipodal coupling (H, H) in the small closo carboranes is found to be surprisingly large and may indicate that there may be significant s-orbital bonding through the center of each of the cages. Typical J H,H values range from 6–14 Hz and were determined directly from white noise boron-decoupled proton spectra: H(1)…H(5) of 1,5-C 2B 3H 5, 11.0 Hz; H(1)…H(6) of 1,2-C 2B 4H 6, 11.1 Hz; H(3)…H(5) of I,2-C 2B 4H 6, 6.9 Hz; H(1)…H(6) of 1,6-C 2B 4H 6, 14.0 Hz; H(2)…H(4) of 1,6-C 2B 4H 6, 7.1 Hz; H(1)…H(6) of CB 5H 7, 12.4 Hz; and H(1)…H(7) of 2,4-C 2B 5H 7, 9.7 Hz. In contrast, nearest neighbor H…H coupling (e.g., H(2)… H(3) of C 2B 3H 5) ranges from 1–7 Hz with an average value of around 2–3 Hz. A comparison of known antipodal cage distances with corresponding J H,H reveals an inverse correlation with shorter antipodal skeletal atom distances corresponding to larger coupling values. This relationship appears to be valid regardless of the cage atoms (C or B) attached to the hydrogen.