Search

Reaction of the thallium salt Tl[7,8-Me2-10-SMe2-7,8-nido-C2B9H8] with [(cod)RhCl]2 in THF furnishes new rhodacarborane 1,2-Me2-3,3-(cod)-8-SMe2-3,1,2-closo-RhC2B9H8 (1). The latter undergoes 1,2 → 1,7 carbon atom isomerization upon reflux in o-xylene for 5 h producing complex 1,8-Me2-2,2-(cod)-11-SMe2-2,1,8-closo-RhC2B9H8 (2). Reaction of complex 2 with 1 equiv of I2 in benzene results in a loss of the cod ligand and formation of product 3, consisting of cationic [(1,8-Me2-11-SMe2-closo-2,1,8-RhC2B9H8)2I3]+ (3a+) and anionic [(1,8-Me2-11-SMe-closo-2,1,8-RhC2B9H8)2I3]− (3b–) iodide species. In contrast, unisomerized analog 3,3-(cod)-8-SMe2-closo-3,1,2-RhC2B9H10 reacts with I2 affording dimeric iodide [3,3-I2-8-SMe2-closo-3,1,2-RhC2B9H10]2 (5). The structures of 2 and 5 were determined by a single crystal X-ray diffraction.

The reactions of thallium salt Tl[7-SMe2-7,8-C2B9H10] with [(cod)RhCl]2 and [Cp*RuCl]4 are accompanied by room temperature polyhedral rearrangement giving rhoda- and ruthenacarboranes 1-cod-8-SMe2-1,2,8-RhC2B9H10 (2) and 1-Cp*-8-SMe2-1,2,8-RuC2B9H10 (3). According to DFT-calculations, the rearrangement could be attained to the Triangular Face Rotation (TFR) mechanism. The reaction of iridium derivative 1-cod-8-SMe2-1,2,8-IrC2B9H10 (1) with anhydrous hydrohalic acids HX (X = Cl, Br, I) results in the dimeric halide iridacarboranes [1,1-X2-8-SMe2-1,2,8-IrC2B9H10]2 (4a-c, X = Cl (a), Br (b), I (c)). Bromide 4b reacts with Tl[Tl(η-7,8-C2B9H11)] and TlCp giving iridacarboranes 8'-SMe2-1,1'-Ir(2,3-C2B9H11)(2',8'-C2B9H10) (5) and 1-Cp-8-SMe-1,2,8-IrC2B9H10 (6). The formation of latter compound is accompanied by demethylation of the SMe2 substituent. Structures of 2, 3, 5 and 6 were determined by X-ray diffraction. Iridacarboranes 4b and [1,1-Br2-4-SMe2-1,2,3-IrC2B9H10]2 (7) catalyze the dimerization of diphenylacetylene giving 1,2,3-triphenylnaphthalene in 20-24% yields. 4b and 7 also effectively catalyze the reductive amination reaction between aldehydes (or ketones) and primary (or secondary) amines in the presence of carbon monoxide giving the corresponding secondary and tertiary amines in high yields (60-85%).

Metallacarborane [1,8-Me2-2,2-(C2H4)2-7-SMe2-2,1,8-IrC2B9H8] (1) was obtained by an interaction of [(C2H4)2IrCl]2 with Tl[9-SMe2-7,8-Me2-7,8-C2B9H8]. Reaction proceeds with a low temperature “1,2 → 1,7” polyhedral rearrangement. Ethene complex 1 reacts with iodine in toluene with the formation of ionic compound 3, consisting of cationic [(1,8-Me2-7-SMe2-2,1,8-IrC2B9H8)2I3]+ (3a+) and anionic [(1,8-Me2-7-SMe-2,1,8-IrC2B9H8)2I3]− (3b–) iodide species. The structures of 1 and 3 were determined by X-ray diffraction.

The review covers the synthesis, structural features and catalytic activity of rhoda- and iridacarborane halide complexes containing monoanionic dicarbollide [L-C2B9H10]− (L = SMe2, NMe3) and tricarbollide [tBuHN-C3B8H10]− ligands. These complexes are structural analogs of the well-known cyclopentadienyl complexes [Cp*MCl2]2, which are widely used as catalysts for various C–H activation processes. Although the search for applications of halide metallacarboranes started not a long time ago, they have already proved to be efficient catalysts for oxidative coupling of benzoic acids with internal alkynes and reductive amination of carbonyl compounds without external hydrogen source. The influence of substituents in the carborane cage on the structure and catalytic activity is also discussed.

Room temperature reactions of the rhodium and iridium bromides (η-9-SMe2-7,8-Me2-7,8-C2B9H8)MBr2 (M = Rh (1a), Ir (1b)) with CpTl give the dicarbollide thioether complexes (η-9-SMe-7,8-Me2-7,8-C2B9H8)MCp (2a,b), as a result of demethylation of the SMe2 substituent. These reactions are accompanied by transformation of cage structure from pseudocloso to closo. Refluxing of the sulfonium derivatives [(η-9-SMe2-7,8-Me2-7,8-C2B9H8)MCp]+ (3a,b) with [Bu4N]Br also affords 2a,b. In the same manner ruthenacarboranes [(η-9-SMe2-7,8-C2B9H10)Ru(η-C6H6)]+ (4a) and [(η-10-SMe2-7,8-C2B9H10)Ru(η-C6H6)]+ (4b) react with halide ions giving the thioether complexes (η-9-SMe-7,8-C2B9H10)Ru(η-C6H6) (5a) and (η-10-SMe-7,8-C2B9H10)Ru(η-C6H6) (5b). Dependence of the demethylation rate on the nature of halide ion and solvent was studied both experimentally and theoretically. The structures of 2a and 3aPF6 were determined by X-ray diffraction.

The efficient (turnover numbers attained 1200) oxidation of alkanes to the corresponding alkyl hydroperoxides by H2O2 in the presence of catalytic amounts of ferrocene proceeds in MeCN at 40–50 °C. Benzene is oxidized in the same system to phenol. An obligatory component of the catalytic system for both reactions is pyrazine-2-carboxylic acid (PCA) or trifluoroacetic acid (TFA). Kinetic study as well as selectivity parameters testified that the oxidation proceeds with the participation of hydroxyl radicals. In the case of PCA as a co-catalyst the initial rate W0 of the reaction with both cyclohexane and benzene depends quadratically on [Cp2Fe]0 whereas the two reactions in the presence of TFA are of half order in ferrocene. The ferrocene-catalyzed reaction of cyclohexane with H 2 O 2 16 in an atmosphere of labeled 18O2 gave after 2 h a mixture of labeled and unlabeled cyclohexyl hydroperoxide (total yield 20% based on starting cyclohexane) containing up to 69% of 18O (the analysis was after reduction of cyclohexyl hydroperoxide into cyclohexanol with PPh3).

Two new osmium(II) complexes containing π-coordinated p-cymene (cym) ligands, [(η6-p-cym)Os(bipy)(MeCN)](PF6)2 (1) and (η6-p-cym)Os(py)Cl2 (2) where bipy is 2,2′-bipyridine, have been synthesized. Both complexes efficiently catalyze oxygenation of alkanes (cyclohexane, n-octane, methylcyclohexane, cis- and trans-isomers of 1,2-dimethylcyclohexane), benzene and 1-phenylethanol with hydrogen peroxide in acetonitrile solution in air at 60 °C. Alkanes are oxidized predominantly to alkyl hydroperoxides which can be easily reduced by PPh3 to afford corresponding alcohols. In the cyclohexane oxygenation catalyzed by complex 2 used in a very low concentration (2 × 10−7 M) turnover number (TON) attained 83,000 after 24 h; initial turnover frequency (TOF) was 3400 h−1. The ESI-MS spectrum of the reaction mixture demonstrated the absence of the initial dication [(η6-p-cym)Os(bipy)(MeCN)]2+ which indicates that complex 1 is only a pre-catalyst. Benzene and 1-phenylethanol were transformed into phenol and acetophenone, respectively. The regio- and bond-selectivity parameters measured for the oxidation of linear and branched alkanes and a kinetic analysis of the dependence of initial cyclohexane oxidation rate on concentration of the alkane indicates that the reaction proceeds via attack of hydroxyl radicals at a substrate. The half order of the reaction rate in respect to osmium concentration [Os] can be explained by the assumption that an osmium compound Os introduced into the reaction mixture is rapidly dimerized Os + Os ⇌ D with the equilibrium constant K1. The catalytic activity of the system is due to a monomeric osmium species. The monomer Os forms an adduct with a pyridine molecule (constant K9): Os + py ⇌ Os·py. The interaction with H2O2 affords hydroxyl radicals with rate constant k10. The obtained kinetic data allowed us to estimate values K1/K92 = 520 M and k10 = 1 M−1 s−1.

Reaction of (η-9-SMe2-7,8-Me2-7,8-C2B9H8)M(cod) with hydrohalic acids affords the 16-valence-electron complexes (η-9-SMe2-7,8-Me2-7,8-C2B9H8)MX2 (M = Rh, X = Cl (3a), Br (3b), I (3c); M = Ir, X = Cl (4a), Br (4b), I (4c)). According to X-ray analysis, 3a, 3b, 4b, and 4c have monomeric pseudocloso-structures with a long cage C⋯C distance (2.122, 1.985, 2.238, and 2.186 A, respectively), in contrast to dimeric closo-structures of the cage-non-methylated analogs. The structural difference was explained using DFT calculations. 3b and 4b react with SMe2 and Tl[Tl(η-7,8-C2B9H11)] giving closo-metallacarboranes (η-9-SMe2-7,8-Me2-7,8-C2B9H8)M(SMe2)Br2 (M = Rh (5), Ir (6)) and (η-9-SMe2-7,8-Me2-7,8-C2B9H8)M(η-7,8-C2B9H11) (M = Rh (7), Ir (8)).

The benzene complexes [(η-9-L-7,8-C2B9R2H8)Fe(η-C6H6)]+ (3a: L = SMe2, R = H; 3b: L = SMe2, R = Me; 3c: L = NMe3, R = H) were prepared by photochemical reaction of [(η5-C6H7)Fe(η-C6H6)]+ with carbo...