Synthesis and investigation of the boron cluster anion [7-(2'-pyridyl)-7,8-nido-dicarbaundecaborate] and its protonated form

The structurally chiral [7-(2'-pyridyl)-7,8-nido-C(2)B(9)H(11)](-), [](-), anion was prepared by a partial degradation reaction of 1-(2'-pyridyl)-1,2-closo-C(2)B(10)H(11). From this anion a protonated specie, H[7-(2'-pyridyl)-7,8-nido-C(2)B(9)H(11)] , and a tetramethylammonium salt, [NMe(4)][7-(2'-pyridyl)-7,8-nido-C(2)B(9)H(11)], [NMe(4)][] can be obtained. The (1)H{(11)B} DNMR study on in the temperature range from 298 to 203 K identified the weakly basic nitrogen atom in the pyridine ring as the proton accepting site in solid state and low temperature and revealed pronounced weakening of the nitrogen-proton interaction while the temperature increases. Capillary electrophoresis and X-ray diffraction confirmed the pyridine nitrogen atom as the proton binding site. Separation of the electrophoretically pure racemic [7-(2'-pyridyl)-7,8-nido-C(2)B(9)H(11)](-) ion into two peaks by the chiral selector beta-cyclodextrine has been achieved.     

Unexpectedly facile isomerisation of [7,10-Ph2-7,10-nido-C2B10H10]2- to [7,9-Ph2-7,9-nido-C2B10H10]2-

The 2e-reduction of 1,12-Ph2-1,12-closo-C(2)B(10)H(10) followed by oxidation or metallation gives products that arise from [7,9-Ph2-7,9-nido-C(2)B(10)H(10)](2-), formed by unexpectedly facile isomerisation of the kinetic 7,10-isomer: the 4,1,6-MC(2)B(10) compounds which result are progressively isomerised to 4,1,8- and 4,1,12-isomers for M = {CpCo} but to an equilibrium mixture of 4,1,8- and 4,1,12-isomers for M = {(arene)Ru}.     

Electrophilic substitution of the nido-dicarbaborate anion 7,8-nido-C2B9H12- with sulfenyl chlorides

Sulfenyl chlorides RSCl (R = p-C(6)H(4)OMe, Ph, p-C(6)H(4)NO(2), CN or 2-C(5)H(4)N) react with 7,8-nido-C(2)B(9)H(12)(-) with asymmetric substitution on the pentagonal C(2)B(3) face to give 9-RS-7,8-nido-C(2)B(9)H(11)(-) (R = p-C(6)H(4)OMe (3), Ph (4), p-C(6)H(4)NO(2) (5), CN (6)) and the zwitterion 9-(S-2-C(5)H(4)NH)-7,8-nido-C(2)B(9)H(11) (7), respectively, in high yield, while tBuSCl did not react and S(2)Cl(2) led to decomposition. Further reaction of 5-7 with iodine gave the corresponding iodo derivatives NMe(4) [9-I-11-RS-7,8-nido-C(2)B(9)H(10)] (R = p-C(6)H(4)NO(2) (8), CN (9)) and the zwitterion 9-I-11-(S-2-C(5)H(4)NH)-7,8-nido-C(2)B(9)H(11) (10), respectively. Compounds 3-10 were fully characterised by (1)H, (11)B, (11)B{(1)H}, (13)C{(1)H} spectroscopy, IR spectroscopy, mass spectrometry and elemental analysis, 3-7 also by (11)B-(11)B{(1)H} COSY NMR spectroscopy and 8-10 by X-ray structure determination.     

Synthesis, reactivity and structural studies of carboranyl thioethers and disulfides

The equimolar reaction of 1-SH-2-R-1,2-closo-C2B10H10(R=Me, H, Ph) with KOH in ethanol produces the thiolate species [1-S-2-R-1,2-closo-C2B10H10]-. These react with iodine to give the disulfide bridged dicluster (1-S-2-R-1,2-closo-C2B10H10)2(R=H, Me, Ph) compounds as analytically pure, white and air-stable solids in high yield. Synthesis of monothioether bridged species is synthetically more difficult. In fact three procedures have been tested to obtain the thioether bridged dicluster compounds (2-R-1,2-closo-C2B10H10)2S (R=Me, H, Ph) but only (2-Me-1,2-closo-C2B10H10)2S was successfully synthesized and characterized. Attempts to produce mixed compounds (1-R-1,2-closo-C2B10H10)S(1-R'-1,2-closo-C2B10H10), R not=R', were unsuccessful. Deboronation reaction of this dicarboranylthioether lead, depending on the reaction conditions, to monoanionic [(2-Me-1,2-closo-C2B10H10)S(8-Me-7,8-nido-C2B9H10)]- or dianionic [(8-Me-7,8-nido-C2B9H10)2S]2- sulfur bridge anions. Deboronation of carboranyl disulfides gave the corresponding dianionic [(7-S-8-R-7,8-nido-C2B9H10)2]2-(R=H, Me, Ph) species. This reaction was very dependent, however, on the reaction conditions. With slight variation of the reaction conditions, splitting of the S-S bond leading to the thiolate species with retention of the closo cluster was also found. Carboranyl disulfides (1-S-2-R-1,2-closo-C2B10H10)2(R=H, Me, Ph) do not lead to thiosulfinates R-S(O)-S-R' by oxidation with H2O2 or I2 as organic disulfides do. This behaviour is attributed to the presence of the sulfur atom directly bonded to the carbon cluster that produces electronic transfer from the filled orbitals on the sulfur atom into the cage LUMO (largely located on the cage Cc-Cc bond). This causes a depletion of electron density on the sulfur, thence impairing sulfur oxidation, and facilitating S-S breaking. Crystal structures of monothioethers (2-Me-1,2-closo-C2B10H10)2S, [NMe4][(2-Me-1,2-closo-C2B10H10)S(8-Me-7,8-nido-C2B9H10)](the first example reported in the literature of a two cluster compound incorporating the closo C2B10 and the nido[C2B9]- moieties linked by a one member spacer) and disulfides (1-S-1,2-closo-C2B10H11)2, (1-S-2-Me-1,2-closo-C2B10H10)2, (1-S-2-Ph-1,2-closo-C2B10H10)2 are reported which support the behaviour of these species.     

Influence of the solvent and R groups on the structure of (carboranyl)R2PI2 compounds in solution. Crystal structure of the first iodophosphonium salt incorporating the anion [7,8-nido-C2B9H10]-

The influence of the electron-donor or electron-acceptor capacity of the R groups (R = (i)Pr, Ph, Et) and the solvent on the molecular geometry in solution of adducts of carboranylphosphanes [(carboranyl)(i)Pr2P, (carboranyl)Ph2P and (carboranyl)Et2P] with I2 in 1 : 1 ratios, has been studied in detail by 31P{1H} and 11B{1H} NMR spectroscopies. The more electron-accepting Ph groups make the (carboranyl)Ph2P less nucleophilic, thus stabilizing the I2 encapsulated neutral biscarboranylphosphane-diiodine adducts in solution, such as (carboranyl)Ph2PI-IPPh2(carboranyl), generating P---I-I---P motifs. Even in a polar solvent, such as EtOH, the arrangement is preserved. The expected basicity of these carboranylphosphanes is: (carboranyl)(i)Pr2P > (carboranyl)Et2P > (carboranyl)Ph2P. Thus, the comparatively higher basicity of (carboranyl)Et2P vs. (carboranyl)Ph(2)P facilitates a higher transfer of electron density to the I2 sigma*, generating the ionic species, [(carboranyl)Et2PI]+I-, even in low polar solvents, such as CH2Cl2 and toluene, with no degradation of the cluster. However, in EtOH, the formation of the anionic [7,8-nido-C2B(9)H10]- cluster takes place by removal of one boron atom from the closo cluster. The basicity of (carboranyl)(i)Pr(2)P should be the highest, superior to (carboranyl)Et2P. This is observed in the reaction of these carboranylphosphanes with I(2) in EtOH. Whereas the formation of P four-coordinated molecular "spoke" charge-transfer complexes, (carboranyl)(i)Pr2P-I-I, are suggested for (carboranyl)(i)Pr2P in low polarity solvents, ionic species are formed in ethanolic solutions, which deboronate in a few days, faster than (carboranyl)Et2P, to yield the zwitterionic species. This is attributed to the higher basicity of (carboranyl)(i)Pr2P vs. (carboranyl)Et2P. The X-ray crystal structure of [7-PI(i)Pr2-8-Ph-7,8-nido-C2B9H10], 2c, obtained from the reaction of 1-P(i)Pr2-2-Ph-closo-1,2-closo-C2B10H10 with I(2) in EtOH, confirms the formation of the zwitterion. These results prove that minor changes in the nature of the R substituents on the P atom in carboranylphosphanes, along with the solvent in which the reaction takes place, produce major alterations in the geometry of the (carboranyl)R2P-I-I species in solution, and in their possible further reactivity.     

Untethered 4,1,2-MC(2)B(10) supraicosahedral metallacarboranes, their C,C'-dimethyl 4,1,6-, 4,1,8- and 4,1,12-MC(2)B(10) analogues, and DFT study of the (4,)1,2- to (4,)1,6-isomerisations of C(2)B(11) carboranes and MC(2)B(10) metallacarboranes

Reduction of the tethered carborane 1,2-μ-(CH(2)SiMe(2)CH(2))-1,2-closo-C(2)B(10)H(10) followed by metallation with {CpCo} or {(p-cymene)Ru} fragments affords both C,C'-dimethyl 4,1,2-MC(2)B(10) and 4,1,6-MC(2)B(10) species. DFT calculations indicate that the barriers to isomerisation of both 4-Cp-4,1,2-closo-CoC(2)B(10)H(12) and 4-(η-C(6)H(6))-4,1,2-closo-RuC(2)B(10)H(12) to their respective 4,1,6-isomers are too high for this to be the origin of the unexpected formation of 4,1,6-MC(2)B(10) products (in marked contrast to the related isomerisation of 1,2-closo-C(2)B(11)H(13) to 1,6-closo-C(2)B(11)H(13)), and, indeed, the 4,1,2-species are recovered unchanged from refluxing toluene. Equally, the DFT-calculated profile for the isomerisation of [7,8-nido-C(2)B(10)H(12)](2-) to [7,9-nido-C(2)B(10)H(12)](2-) suggests that the unexpected formation of 4,1,6-metallacarboranes is unlikely to result from isomerisation of a reduced (nido) carborane following desilylation. Instead, the source of the 4,1,6-MC(2)B(10) compounds is traced to desilylation of 1,2-μ-(CH(2)SiMe(2)CH(2))-1,2-closo-C(2)B(10)H(10) by Li or Na prior to reduction. The supraicosahedral metallacarboranes 1,8-Me(2)-4-Cp-4,1,8-closo-CoC(2)B(10)H(10), 1,12-Me(2)-4-Cp-4,1,12-closo-CoC(2)B(10)H(10) and 1,12-Me(2)-4-(p-cymene)-4,1,12-closo-RuC(2)B(10)H(10) are also reported with all new species characterised both spectroscopically and crystallographically.     

Contribution of the nido-[7,8-C2B9H10]- anion to the chemical stability, basicity, and 31P NMR chemical shift in nido-o-carboranylmonophosphines

The icosahedral dicarboranes and their decapitated anion, 1-R'-1,2-C(2)B(10)H(10) (closo) and [7-R'-7,8-C(2)B(9)H(10)](-) (nido), exert a distict influence at the alpha position of substituents attached to the cage carbon atom. The closo fragment is electron-withdrawing while the nido anion is electron-releasing. These effects are studied by (31)P NMR, phosphorus oxidation, and phosphorus protonation in [7-PR(2)-8-R'-7,8-C(2)B(9)H(10)](-) species. The (31)P NMR chemical shift dependence is related to the R alkyl or aryl nature of [7-PR(2)-8-R'-7,8-C(2)B(9)H(10)](-). No direct relationship to the nature of the R substituent on the nido-carboranylmonphosphine toward oxidation has been found. The basicity of the nido-alkylcarboranylmonophosphines is the highest while the lowest corresponds to the nido-arylcarboranylmonophosphines. Interpretation can be carried out qualitatively by considering the electronic properties of the cluster and the nature of the R groups. The influence of R' is less relevant. Confirmation of the molecular structure of the oxidated and protonated nido-carboranylmonophosphine compounds was obtained by X-ray diffraction analysis of [NBu(4)][7-P(O)Ph(2)-8-Ph-7,8-C(2)B(9)H(10)] and [7-PH((i)Pr)(2)-8-Me-7,8-C(2)B(9)H(10)].     

Synthesis, reactivity and complexation studies of N,S exo-heterodisubstituted o-carborane ligands. Carborane as a platform to produce the uncommon bidentate chelating (pyridine)N-C-C-C-s(H) motif

The synthesis of N,S-heterodisubstituted 1-(2'-pyridyl)-2-SR-1,2-closo-C2B10H10 compounds (R = Et, 2; R = (i)Pr, 3) has been accomplished starting from 1-(2'-pyridyl)-l,2-closo-C2B10H11 (1), and their partial deboronation reaction leading to the structurally chiral [7-(2'-pyridyl)-8-SR-7,8-nido-C2B9H10]-derivatives (R = Et, [4]-; R = (i)Pr, [5]-) has been studied. Capillary electrophoresis combined with the chiral selector alpha-cyclodextrin has permitted the separation of the electrophoretically pure racemic [7-(2'-pyridyl)-8-SR-7,8-nido-C2B9H11]- ions into two peaks each one corresponding to the interaction of one enantiomer with the alpha-cyclodextrin. The N,S-heterodisubstituted o-carborane containing a mercapto group, 1-(2'-pyridyl)-2-SH-1,2-closo-C2B10H10, 1, is one of the two examples of a rigid bidentate chelating (pyridine)N-C-C-C-S(H) motif having been structurally fully characterized. To study the potential of such a binding site, 1 has been tested as a ligand with metal ions requiring different coordination numbers, two (Au(+)) and four (Pd2+ and Rh+). The crystal structures of the Pd(II) and Au(I) complexes are reported. For the Pd(II) complex, 1 acts as a bidentate ligand whereas for Au(I), 1 acts as a monodentate ligand through the thiolate.     

Synthesis and structural characterization of symmetrical closo-4,7-I2-1,2-C2B10H10 and [(CH3)3NH][nido-2,4-I2-7,8-C2B9H10

The boron-atom insertion reaction of nido-9,11-I(2)-7,8-C(2)B(9)H(9)(2-), with the HBCl(2):SMe(2) complex yields closo-4,7-I(2)-1,2-C(2)B(10)H(10), 1, in excellent yield. Although the two boron atoms (B3 and B6) nearest to the carbon atoms in 1 are equally available for attack by nucleophiles, the boron-degradation reaction of 1 with alkoxide ion occurs only at the B6 vertex, yielding regioselectively [(CH(3))(3)NH][nido-2,4-I(2)-7,8-C(2)B(9)H(10)], 2. The molecular structures of 1 and 2 have been determined by X-ray diffraction studies. Crystallographic data are as follows. For 1, monoclinic, space group P2(1)/n, a = 6.9199(19) Angstroms, b = 23.9560(7) Angstroms, c = 7.2870(2) Angstroms, beta = 94.081(4) degrees, V = 1204.9(6) Angstroms(3), Z = 4, rho(calcd) = 2.18 g cm(-3), R = 0.020, R(w) = 0.0610; for 2, orthorhombic, space group Pca2(1), a = 14.1141(7) Angstroms, b = 7.0276(4) Angstroms, c = 16.4602(9) Angstroms, V = 1632.7(15) Angstroms(3), Z = 4, rho(calcd) = 1.81 gcm(-3), R = 0.022, R(w) = 0.0623.     

Silver as an exopolyhedral auxiliary to the rhenacarborane anion [Re(CO)3(eta5-7,8-C2B9H11)]-

The rhenacarborane salt Cs[Re(CO)3(eta5-7,8-C2B9H11)] (1) has been used to synthesize the tetranuclear metal complex [[ReAg(mu-10-H-eta5-7,8-C2B9H10)(CO)3]2[mu-Ph2P(CH2)2PPh2]] (3) where two [ReAg(mu-10-H-eta5-7,8-C2B9H10)(CO)3] fragments have been shown by X-ray crystallography to be bridged by a single 1,2-bis(diphenylphosphino)ethane ligand. Reaction of 1 with Ag[BF4] in the presence of the ligands bis- or tris(pyrazol-1-yl)methane yields the complexes [ReAg(mu-10-H-eta5-7,8-C2B9H10)(CO)3[kappa2-CH2(C3H3N2-1)2]] (4) or [[ReAg(mu-10-H-eta5-7,8-C2B9H10)(CO)3]2[mu-kappa1,kappa2-CH(C3H3N2-1)3]] (5), respectively. From X-ray studies, the former comprises a Re-Ag bond bridged by the carborane cage and with the bis(pyrazol-1-yl)methane coordinating the silver(I) center in an asymmetric kappa(2) mode. Complex 5 was unexpectedly found to contain a tris(pyrazol-1-yl)methane bridging two [ReAg(mu-10-H-eta5-7,8-C2B9H10)(CO)3] fragments in a kappa1,kappa2 manner. Treatment of 1 with Ag[BF4] in the presence of 2,2'-dipyridyl and 2,2':6',2' '-terpyridyl yields [ReAg(mu-10-H-eta5-7,8-C2B9H10)(CO)3[kappa2-(C5H4N-2)(2)]] (6) and [ReAg(mu-10-H-eta5-7,8-C2B9H10)(CO)3[kappa3-C5H3N(C5H4N-2)2-2,6]] (7). The X-ray structure determination of 7 revealed an unusual pentacoordinated silver(I) center, asymmetrically ligated by a kappa3-2,2':6',2' '-terpyridyl molecule. The same synthetic procedure using N,N,N',N'-tetramethylethylenediamine gave a tetranuclear metal complex [[ReAg(mu-10-H-eta5-7,8-C2B9H10)(CO)3]2[mu-Me2N(CH2)2NMe2]2] (8) which is believed, in the solid state, to be bridged between the silver atoms by two of the diamine molecules. The salt 1 with Ag[BF4] in the absence of any added ligand gave the tetrameric cluster [ReAg[mu-5,6,10-(H)3-eta5-7,8-C2B9H8](CO)3]4 (9) where, in the solid state, four [ReAg(mu-10-H-eta5-7,8-C2B9H10)(CO)3] units are held together by long interunit B-H right harpoon-up Ag bonds.     

Mild mono and double demethylation in dimethylsulfonium substituted ruthenacarborane complexes. A regioselective reaction

Reaction of [RuCl(2)(eta(6)-C(6)H(6))](2) with [10-(CH(3))(2)S-7,8-nido-C(2)B(9)H(10)](-) or [9-(CH(3))(2)S-7,8-nido-C(2)B(9)H(10)](-) afforded the expected cationic complexes [Ru(eta(5)-n-(CH(3))(2)S-7,8-C(2)B(9)H(10))(eta(6)-C(6)H(6))](+)(n= 10, (1); 9, (3)), but also the unexpected neutral Ru(eta(5)-10-HS-7,8-C(2)B(9)H(10))(eta(6)-C(6)H(6))(2) or Ru(eta(5)-9-(CH(3))S-7,8-C(2)B(9)H(10))(eta(6)-C(6)H(6))(4) by double and mono demethylation of the (CH(3))(2)S moiety, respectively.     

Gold(I) Complexes with the nido-Diphosphino Ligand [7,8-(Ph(2)P)(2)-7,8-C(2)B(9)H(10)](-). Preparation of the First Metallocarborane Complex of this Ligand. Crystal Structures of [Au{(PPh(2))(2)C(2)B(9)H(10)}(PPh(3))].CH(2)Cl(2) and [Au(2){(PPh(2))(2)C(2)B(9)H(10)}(2){(PPh(2))(2)(CH(2))(3)}].3Me(2)CO

The reaction of [AuCl(PR(3))] with [1,2-(Ph(2)P)(2)-1,2-C(2)B(10)H(10)] in refluxing ethanol proceeds with partial degradation (removal of a boron atom adjacent to carbon) of the closo species to give [Au{(PPh(2))(2)C(2)B(9)H(10)}(PR(3))] [PR(3) = PPh(3) (1), PPh(2)Me (2), PPh(2)(4-Me-C(6)H(4)) (3), P(4-Me-C(6)H(4))(3) (4), P(4-OMe-C(6)H(4))(3) (5)]. Similarly, the treatment of [Au(2)Cl(2)(&mgr;-P-P)] with [1,2-(Ph(2)P)(2)-1,2-C(2)B(10)H(10)] under the same conditions leads to the complexes [Au(2){(PPh(2))(2)C(2)B(9)H(10)}(2)(&mgr;-P-P)] [P-P = dppe = 1,2-bis(diphenylphosphino)ethane (6), dppp = 1,3-bis(diphenylphosphino)propane (7)], where the dppe or dppp ligands bridge two gold nido-diphosphine units. The reaction of 1 with NaH leads to removal of one proton, and further reaction with [Au(PPh(3))(tht)]ClO(4) gives the novel metallocarborane compound [Au(2){(PPh(2))(2)C(2)B(9)H(9)}(PPh(3))(2)] (8). The structure of complexes 1 and 7 have been established by X-ray diffraction. [Au{(PPh(2))(2)C(2)B(9)H(10)}(PPh(3))] (1) (dichloromethane solvate) crystallizes in the monoclinic space group P2(1)/c, with a = 17.326(3) ?, b = 20.688(3) ?, c = 13.442(2) ?, beta = 104.710(12) degrees, Z = 4, and T = -100 degrees C. [Au(2){(PPh(2))(2)C(2)B(9)H(10)}(2)(&mgr;-dppp)] (7) (acetone solvate) is triclinic, space group P&onemacr;, a = 13.432(3) ?, b = 18.888(3) ?, c = 20.021(3) ?, alpha = 78.56(2) degrees, beta = 72.02(2) degrees, gamma = 73.31(2) degrees, Z = 2, and T = -100 degrees C. In both complexes the gold atom exhibits trigonal planar geometry with the 7,8-bis(diphenylphosphino)-7,8-dicarba-nido-undecaborate(1-) acting as a chelating ligand.     

Reaction of nido-7,8-C(2)B(9)H(13) with Dicobalt Octacarbonyl: Crystal Structures of the Complexes [Co(2)(CO)(2)(eta(5)-7,8-C(2)B(9)H(11))(2)], [Co(2)(CO)(PMe(2)Ph)(eta(5)-7,8-C(2)B(9)H(11))(2)], and [CoCl(PMe(2)Ph)(2)(eta(5)-7,8-C(2)B(9)H(11))

The compounds [Co(2)(CO)(8)] and nido-7,8-C(2)B(9)H(13) react in CH(2)Cl(2) to give a complex mixture of products consisting primarily of two isomers of the dicobalt species [Co(2)(CO)(2)(eta(5)-7,8-C(2)B(9)H(11))(2)] (1), together with small amounts of a mononuclear cobalt compound [Co(CO)(2)(eta(5)-10-CO-7,8-C(2)B(9)H(10))] (5) and a charge-compensated carborane nido-9-CO-7,8-C(2)B(9)H(11) (6). In solution, isomers 1a and 1b slowly equilibrate. However, column chromatography allows a clean separation of 1a from the mixture, and a single-crystal X-ray diffraction study revealed that each metal atom is ligated by a terminal CO molecule and in a pentahapto manner by a nido-C(2)B(9)H(11) cage framework. The two Co(CO)(eta(5)-7,8-C(2)B(9)H(11)) units are linked by a Co-Co bond [2.503(2) ?], which is supported by two three-center two-electron B-H right harpoon-up Co bonds. The latter employ B-H vertices in each cage which lie in alpha-sites with respect to the carbons in the CCBBB rings bonded to cobalt. Addition of PMe(2)Ph to a CH(2)Cl(2) solution of a mixture of the isomers 1, enriched in 1b, gave isomers of formulation [Co(2)(CO)(PMe(2)Ph)(eta(5)-7,8-C(2)B(9)H(11))(2)] (2). Crystals of one isomer were suitable for X-ray diffraction. The molecule 2a has a structure similar to that of 1a but differs in that whereas one B-H right harpoon-up Co bridge involves a boron atom in an alpha-site of a CCBBB ring coordinated to cobalt, the other uses a boron atom in the beta-site. Reaction between 1b and an excess of PMe(2)Ph in CH(2)Cl(2) gave the complex [CoCl(PMe(2)Ph)(2)(eta(5)-7,8-C(2)B(9)H(11))] (3), the structure of which was established by X-ray diffraction. Experiments indicated that 3 was formed through a paramagnetic Co(II) species of formulation [Co(PMe(2)Ph)(2)(eta(5)-7,8-C(2)B(9)H(11))]. Addition of 2 molar equiv of CNBu(t) to solutions of either 1a or 1b gave a mixture of two isomers of the complex [Co(2)(CNBu(t))(2)(eta(5)-7,8-C(2)B(9)H(11))(2)] (4). NMR data for the new compounds are reported and discussed.     

Controlled direct synthesis of C-mono- and C-disubstituted derivatives of [3,3'-Co(1,2-C2B9H11)2]- with organosilane groups: theoretical calculations compared with experimental results

Mono- and dilithium salts of [3,3'-Co(1,2-C(2)B(9)H(11))(2)](-), (1(-)), react with different chlorosilanes (Me(2)SiHCl, Me(2)SiCl(2), Me(3)SiCl and MeSiHCl(2)) with an accurate control of the temperature to give a set of novel C(c)-mono- (C(c) = C(cluster)) and C(c)-disubstituted cobaltabis(dicarbollide) derivatives with silyl functions: [1-SiMe(2)H-3,3'-Co(1,2-C(2)B(9)H(10))(1',2'-C(2)B(9)H(11))](-) (3(-)); [1,1'-mu-SiMe(2)-3,3'-Co(1,2-C(2)B(9)H(10))(2)](-) (4(-)); [1,1'-mu-SiMeH-3,3'-Co(1,2-C(2)B(9)H(10))(2)](-) (5(-)); [1-SiMe(3)-3,3'-Co(1,2-C(2)B(9)H(10))(1',2'-C(2)B(9)H(11))](-) (6(-)) and [1,1'-(SiMe(3))(2)-3,3'-Co(1,2-C(2)B(9)H(10))(2)](-) (7(-)). In a similar way, the [8,8'-mu-(1',2'-C(6)H(4))-1,1'-mu-SiMe(2)-3,3'-Co(1,2-C(2)B(9)H(9))(2)](-) (8(-)); [8,8'-mu-(1',2'-C(6)H(4))-1,1'-mu-SiMeH-3,3'-Co(1,2-C(2)B(9)H(9))(2)](-) (9(-)) and [8,8'-mu-(1',2'-C(6)H(4))-1-SiMe(3)-3,3'-Co(1,2-C(2)B(9)H(9))(1',2'-C(2)B(9)H(10))](-) (10(-)) ions have been prepared from [8,8'-mu-(1',2'-C(6)H(4))-3,3'-Co(1,2-C(2)B(9)H(10))(2)](-) (2(-)). Thus, depending on the chlorosilane, the temperature and the stoichiometry of nBuLi used, it has been possible to control the number of substituents on the C(c) atoms and the nature of the attached silyl function. All compounds were characterised by NMR and UV/Vis spectroscopy and MALDI-TOF mass spectrometry; [NMe(4)]-3, [NMe(4)]-4 and [NMe(4)]-7 were successfully isolated in crystalline forms suitable for X-ray diffraction analyses. The 4(-) and 8(-) ions, which contain one bridging -mu-SiMe(2) group between each of the dicarbollide clusters, were unexpectedly obtained from the reaction of the monolithium salts of 1(-) and 2(-), respectively, with Me(2)SiHCl at -78 degrees C in 1,2-dimethoxyethane. This suggests that an intramolecular reaction has taken place, in which the acidic C(c)-H proton reacts with the hydridic Si-H, with subsequent loss of H(2). Some aspects of this reaction have been studied by using DFT calculations and have been compared with experimental results. In addition, DFT theoretical studies at the B3 LYP/6-311G(d,p) level of theory were applied to optimise the geometries of ions 1(-)-10(-) and calculate their relative energies. Results indicate that the racemic mixtures, rac form, are more stable than the meso isomers. A good concordance between theoretical studies and experimental results has been achieved.     

Highly stable neutral and positively charged dicarbollide sandwich complexes

Novel sandwich metallacarboranes commo-[3,3'-Ni(8-SMe2-1,2-C2B9H10)2] (1), commo-[3,3'-Co(8-SMe2-1,2-C2B9H10)2]+ (2+), commo-[3,3'-Ru(8-SMe2-1,2-C2B9H10)2] (4) and commo-[3,3'-Fe(8-SMe2-1,2-C2B9H10)2] (5) have been prepared by reaction of [10-SMe2-7,8-nido-C2B9H10]- with NiCl2 x 6 H2O, CoCl2, [RuCl2(dmso)4] and [FeCl2(dppe)], respectively. Reduction of 2+ with metallic Zn leads to the neutral and isolable complex commo-[3,3'-Co(8-SMe2-1,2-C2B9H10)2] (3). Theoretical calculations using the ZINDO/1 semiempirical method show three energy minima for complexes 1-3 and 5 that agree with the presence of three different rotamers in solution at low temperature, while four relative energy minima have been found for 4. The calculated rotational energy barriers for complexes 1-5 have been found in the range 5.2+/-0.2 and 11.5+/-0.2 kcal mol(-1). These values are in agreement with the experimental data calculated for complexes 2+ and 5. Only one rotamer is found in the X-ray crystal structure of complexes 1-3, while two are observed for 4. Neutral complexes 1, 3 and 4 exhibit a gauche conformation, whereas a cisoid conformation is found for the 2+ ion. Rotamers evident from X-ray diffraction studies are in agreement with the global energy minimum calculated by the ZINDO/1 method. The electrochemical studies conducted on 1, 3, 4 and 5 support the proposal that the charge-compensated ligand [10-SMe2-7,8-nido-C2B9H10]- stabilises lower oxidation states in metals than the dianionic [7,8-nido-C2B9H11]2- and even the [C5H5]- ligands.     

Dimethylsulfide-dicarbaborane chemistry. Isolation and characterisation of isomers [9-(SMe2)-nido-7,8-C2B9H10-X-Me] (where X = 1, 2, 3 and 4) and some related compounds. An unusual skeletal rearrangement

Oxidative coupling by FeCl(3) of the [nido-7,8-C(2)B(9)H(11)-9-Me](-) anion 1a with SMe(2) yields a mixture of four isomers of 9-(SMe(2))-nido-7,8-C(2)B(9)H(10)-X-Me, where X = 1, 2, 3 and 4 (compounds 2a, 2b, 2c and 2d respectively). On high dilution of the reaction mixture, the 9-(SMe(2))-nido-7,8-C(2)B(9)H(10)-10-Me 2e isomer is also isolated in a low yield. The isomers are separated by HPLC, and are identified and characterised by NMR spectroscopy and by single-crystal X-ray diffraction analyses of 2c and 2d. The formation of the products implies an unexpected cluster rearrangement, which is discussed in terms of dsd and vertex-flip reaction pathways. Two additional isomers, 9-(SMe(2))-nido-7,8-C(2)B(9)H(10)-5-Me 2f and 9-(SMe(2))-nido-7,8-C(2)B(9)H(10)-6-Me 2g occur when [nido-7,8-C(2)B(9)H(11)-5-Me](-) 1b is used as the starting substrate, in a reaction in which no cluster rerarrangement is observed. The corresponding bromide, [nido-7,8-C(2)B(9)H(11)-5-Br](-) 1c, behaves similarly, forming only 9-(SMe(2))-nido-7,8-C(2)B(9)H(10)-5-Br 2h and 9-(SMe(2))-nido-7,8-C(2)B(9)H(10)-6-Br 2i.     

Synthesis, characterization, and dynamic studies of 12-vertex eta5-ruthenium(II) closo-phosphine complexes with monoanionic [10-L-nido-7-R-7,8-C2B9H9]- ligands

Ruthenacarborane complexes of formula [3-H-3,3-(PPh3)2-8-L-closo-3,1,2-RuC2B9H10)] (L = SMe2 (2a), SEt2 (2b), S(CH2)4 (2c), SEtPh (2d)) and [1-Me-3-H-3,3-(PPh3)2-8-L-closo-3,1,2-RuC2B9H9)] (L = SMe2 (2e), SEt2 (2f)) were prepared by reaction of the respective monoanionic charge-compensated ligands [10-L-nido-7,8-C2B9H10]- and [7-Me-10-L-nido-7,8-C2B9H9]- with [RuCl2(PPh3)3]. Similary, complexes [3-H-3,3,8-(PPh3)3-closo-3,1,2-RuC2B9H10)] (4a) and [3-H-3,3-(PPh3)2-8-PPh2Me-closo-3,1,2-RuC2B9H10)] (4b) were prepared from the corresponding phosphonium ligands. The reaction is done in one pot by reacting the ligand with the Ru(II) complex in a 1.5:1 ratio. All compounds have been fully characterized by multinuclear NMR spectroscopy, and the molecular structures for 2a and 4a have been elucidated by single-crystal X-ray diffraction analysis. The Ru(II) atom in this complex is on the open face of the monoanionic charge-compensated ligand adopting a pseudooctahedral coordination. Formally, three positions are supplied by the C2B3 open face, two PPh3 groups occupy two other positions, and a hydride fulfills the remaining one. The hydride complexes were generated with no special reagent. They result from a dehalogenation in the presence of ethanol.     

Spectroscopic, structural, computational and (spectro)electrochemical studies of icosahedral carboranes bearing fluorinated aryl groups

The icosahedral carboranes 1-C(6)F(5)-2-Ph-1,2-closo-C(2)B(10)H(10) (1), 1-(4'-F(3)CC(6)H(4))-2-Ph-1,2-closo-C(2)B(10)H(10) (2), 1,2-(4'-F(3)CC(6)H(4))(2)-1,2-closo-C(2)B(10)H(10) (3), 1-(4'-H(3)CC(6)F(4))-2-Ph-1,2-closo-C(2)B(10)H(10) (4), 1-(4'-F(3)CC(6)F(4))-2-Ph-1,2-closo-C(2)B(10)H(10) (5), 1,2-(4'-F(3)CC(6)F(4))(2)-1,2-closo-C(2)B(10)H(10) (6), 1,7-(4'-F(3)CC(6)F(4))(2)-1,7-closo-C(2)B(10)H(10) (7) and 1,12-(4'-F(3)CC(6)F(4))(2)-1,12-closo-C(2)B(10)H(10) (8), with fluorinated aryl substituents on cage carbon atoms, have been prepared in good to high yields and characterised by microanalysis, (1)H, (11)B and (19)F NMR spectroscopies, mass spectrometry, single-crystal X-ray diffraction and (spectro)electrochemistry. By analysis of <δ(11)B>, the weighted average (11)B chemical shift, a ranking order for the ortho carboranes 1-6 is established based on the combined electron-withdrawing properties of the C-substituents, and is in perfect agreement with that established independently by electrochemical study. In a parallel computational study the effects of a wide range of different substituents on the redox properties of carboranes have been probed by comparison of ΔE values, where ΔE is the energy gap between the DFT-optimised [7,9-R(2)-7,9-nido-C(2)B(10)](2-) anion and its DFT-optimised basket-shaped first oxidation product. The overall conclusion from the NMR spectroscopic, electrochemical and computational studies is that strongly electron withdrawing substituents significantly stabilise [7,9-nido-C(2)B(10)](2-) dianions with respect to oxidation, and that the best practical substituent is 4-F(3)CC(6)F(4). Thus attention focussed on the reduction of 1,2-(4'-F(3)CC(6)F(4))(2)-1,2-closo-C(2)B(10)H(10), compound 6. The sequence 6/[6](-)/[6](2-) appears reversible on the cyclic voltammetric timescale but on the longer timescale of macroelectrolysis the radical anion is only partially stable. EPR study of the electrogenerated monoanions from the ortho-carboranes 1-6 confirms the cage-centred nature of the redox processes. In contrast, the reduction of the meta- and para-carboranes 7 and 8, respectively, appears to be centred on the aromatic substituents, a conclusion supported by the results of DFT calculation of the LUMOs of compounds 6-8. Bulk 2-electron reduction of 6 affords a dianion which is remarkably stable to reoxidation, surviving for several hours in the open laboratory in the absence of halogenated solvents.     

S-alkylation and S-amination of methyl thioethers--derivatives of closo-[B(12)H(12)](2-). synthesis of a boronated phosphonate, gem-bisphosphonates, and dodecaborane-ortho-carborane oligomers

A variety of S-alkylated products was prepared by alkylation of methyl thioethers [MeSB(12)H(11)](2-) (5), [1-(MeS)-2(7,12)-(Me(2)S)B(12)H(10)](-) (6-8), and [1,2(7,12)-(MeS)(2)B(12)H(10)](2-) (9-11) with alkyl halides and tosylates in acetonitrile. Since these methyl thioethers can be prepared easily in B-10-enriched form on a large scale and due to their chemical versatility, they are potentially very attractive boron entities for the design and synthesis of therapeutics for boron neutron capture therapy of cancer. It was found that alkylation of 6-8 can be complicated by an equilibrium which establishes between, on the one hand, one of the former species and, on the other hand, 1,2(7,12)-(Me(2)S)(2)B(12)H(10) (2-4) and [1,2(7,12)-(MeS)(2)B(12)H(10)](2-) (9-11). A boronated phosphonate 1-(MeS(CH(2))(4)P(O)(OEt)(2))-7-(Me(2)S)B(12)H(10) (14g) and a gem-bisphosphonate 1-(MeS(CH(2))(3)CH[P(O)(OEt)(2)](2))-7-(Me(2)S)B(12)H(10) (14h) were prepared from thioether 7 and the corresponding iodide and tosylate, respectively, and subsequently converted to their sodium salts. The propargyl sulfonium salts obtained by alkylation of thioethers 7, 8, 10, and 11 with propargyl bromide have been further converted to two- and three-cage oligomers containing both ortho-carborane and dodecaborane moieties. Methyl thioethers derived from closo-[B(12)H(12)](2-) are excellent participants in Michael addition reactions in the presence of a strong acid. The sulfonium salts with tertiary alkyl and vinyl substituents have been prepared by this method. Methyl thioethers 5-11 react with hydroxylamine-O-sulfonate yielding the corresponding aminosulfonium salts, albeit in lower yields as compared to those in the alkylation reactions. Several derivatives of methyl thioethers 5-11 have been characterized by single-crystal X-ray diffraction.     

Monocarbaborane anion chemistry. [COOH], [CH2OH] and [CHO] units as functional groups on ten-vertex monocarbaborane anionic compounds

B(10)H(14) reacts with para-C(6)H(4)(CHO)(COOH) in aqueous KOH solution to give the [nido-6-CB(9)H(11)-6-(C(6)H(4)-para-COOH)](-) anion 1, which undergoes cage closure with iodine in alkaline solution to give the [closo-2-CB(9)H(9)-2-(C(6)H(4)-para-COOH)](-) anion 2. Upon heating, anion 2 rearranges to form the [closo-1-CB(9)H(9)-1-(C(6)H(4)-para-COOH)](-) anion 3. Similarly, B(10)H(14) with glyoxylic acid OHCCOOH in aqueous KOH gives the [arachno-6-CB(9)H(13)-6-(COOH)](-) anion 4, which undergoes cage closure with iodine in alkaline solution to give the [closo-2-CB(9)H(9)-2-(COOH)](-) anion 5. Upon heating, anion 5 rearranges to give the [closo-1-CB(9)H(9)-1-(COOH)](-) anion 6. Reduction of the [COOH] anions 3 and 6 with diisobutylaluminium hydride gives the [CH(2)OH] hydroxy anions [closo-1-CB(9)H(9)-1-(C(6)H(4)-para-CH(2)OH)](-) and [closo-1-CB(9)H(9)-1-(CH(2)OH)](-) 8 respectively. The [closo-1-CB(9)H(9)-1-(C(6)H(4)-para-CH(2)OH)](-) anion 7 can also be made via isomerisation of the [closo-2-CB(9)H(9)-2-(C(6)H(4)-para-CH(2)OH)](-) anion 9, in turn obtained from the [nido-6-CB(9)H(11)-6-(C(6)H(4)-para-CH(2)OH)](-) anion 10, which is obtained from the reaction of B(10)H(14) with terephthaldicarboxaldehyde, C(6)H(4)-para-(CHO)(2), in aqueous KOH solution. Oxidation of the hydroxy anions 7 and 8 with pyridinium dichromate gives the aldehydic [closo-1-CB(9)H(9)-1-(C(6)H(4)-para-CHO)](-) anion 11 and the aldehydic [closo-1-CB(9)H(9)-1-(CHO)](-) anion 12 respectively, characterised as their 2,4-dinitrophenylhydrazone derivatives, the [closo-1-CB(9)H(9)-1-C(6)H(4)-para-CH=N-NHC(6)H(3)(NO(2))(2)](-) anion 13 and the [closo-1-CB(9)H(9)-1-CH=N-NHC(6)H(3)(NO(2))(2)](-) anion respectively.