Unusual 9-->10 rearrangement of the substituted cage carbon in the ferratricarbollide series. Synthesis of the isomeric complexes [2-eta 5-(C5H5)-10-X-closo-2,1,7,10-FeC3B8H10] (where X = H2N, MeHN, Me2N, and ButHN)

Focus on Chemistry of the 10-Dioxane-nido-7,8-dicarba-undecahydrido Undecaborate Zwitterion; Exceptionally Easy Abstraction of Hydrogen Bridge and Double-Action Pathways Observed in Ring Cleavage Reactions with OH- as Nucleophile

Ring cleavage of cyclic ether substituents attached to a boron cage via an oxonium oxygen atom are amongst the most versatile methods for conjoining boron closo-cages with organic functional groups. Here we focus on much less tackled chemistry of the 11-vertex zwitterionic compound [10-(O-(CH₂-CH₂)₂O)-nido-7,8-C₂B₉H₁₁] (1), which is the only known representative of cyclic ether substitution at nido-cages, and explore the scope for the use of this zwitterion 1 in reactions with various types of nucleophiles including bifunctional ones. Most of the nitrogen, oxygen, halogen, and sulphur nucleophiles studied react via nucleophilic substitution at the C1 atom of the dioxane ring, followed by its cleavage that produces six atom chain between the cage and the respective organic moiety. We also report the differences in reactivity of this nido-cage system with the simplest oxygen nucleophile, i.e., OH^-. With compound 1, reaction proceeds in two possible directions, either via typical ring cleavage, or by replacement of the whole dioxane ring with -OH at higher temperatures. Furthermore, an easy deprotonation of the hydrogen bridge in 1 was observed that proceeds even in diluted aqueous KOH. We believe this knowledge can be further applied in the design of functional molecules, materials, and drugs.

Carbon insertion into arachno-6,9-C2B8H14 via acyl chlorides. skeletal alkylcarbonation (SAC) reactions: a new route for tricarbollides

Reactions between arachno-6,9-C2B8H14 (1) and selected acyl chlorides, RCOCl, in the presence of PS (PS = "proton sponge", 1,8-dimethylamino naphthalene) in CH2Cl2 for 24 h at reflux, followed by in situ acidification with concentrated H2SO4 at 0 °C, generate a series of neutral alkyl and aryl tricarbollides 8-R-nido-7,8,9-C3B8H11 (2) (where R = CH3, 2a; C2H5, 2b; n-C4H9, 2c; C6H5, 2d; 4-Cl-C6H4, 2e; 4-Br-C6H4, 2f; 4-I-C6H4, 2g; 1-C10H7, 2h; and 2-C10H7, 2i). The best yields were achieved for aryl derivatives (80-95%) while the yields of the corresponding alkyl substituted compounds are lower (60-70%). These skeletal alkylcarbonation (SAC) reactions are consistent with an aldol-type condensation between the RCO group and open-face hydrogen atoms on the dicarbaborane 1, which is associated with the insertion of the carbonyl carbon atom into the structure of arachno-6,9-C2B8H14 (1) under elimination of three extra hydrogen atoms as H2O and HCl. The reactions thus result in an effective R-tricarbaborane cross-coupling. Individual compounds of structure 2 have been purified by chromatography on a silica gel support, using hexane as the mobile phase (R(F) = ∼0.3). Deprotonation agents, such as NEt3, NaOH, NaH, etc., convert tricarbaboranes 2 into the corresponding conjugated anions [8-R-nido-7,8,9-C3B8H10](-) (2(-)) which were isolated as salts with suitable countercations (for example, Et3NH(+), Tl(+), NEt4(+), etc.). The compounds have been characterized by multinuclear ((11)B, (1)H, and (13)C) NMR spectroscopy, mass spectrometry, and elemental analyses. The structures of anions [8-R-nido-7,8,9-C3B8H10]¯ (where R = C6H5, 4-I-C6H4 and 1-C10H7; 2a(-), 2g(-), and 2h(-)) and that of the neutral 8-(1-C10H7)-nido-7,8,9-C3B8H11 (2h) have been established by X-ray diffraction analyses.

Synthesis and electrochemistry of some ferrocenyl substituted dicarba- and tricarbaborane compounds

Treatment of arachno-4-Me(2)S-B(9)H(13) (1) with ethynylferrocene, FcC[triple bond]CH (Fc = ferrocenyl), in boiling benzene afforded an inseparable 2 : 1 mixture of 6-Fc-nido-5,6-C(2)B(8)H(11) (2a) and 5-Fc-nido-5,6-C(2)B(8)H(11) (2b) in a 10% yield. However, compound 2b was converted quantitatively to the pure, violet 2a by the action of proton sponge (PS), followed by acidification. The structure of 2a was determined by X-ray diffraction analysis. Heating of 2a at 400 degrees C led to the dehydrogenation and isolation of the orange 1-Fc-closo-1,10-C(2)B(8)H(9) (3) in almost quantitative yield, while treatment of 2a with t-BuNC and PS (proton sponge = 1,8-bis(dimethylamino)naphthalene) in CH(2)Cl(2) generated the neutral absolute tautomer 7-t-BuNH-8-Fc-7,8,9-C(3)B(8)H(10) (N4) (yield 50%), which could be converted quantitatively into the zwitterionic tautomer 7-t-BuNH(2)-8-Fc-7,8,9-C(3)B(8)H(9) (Z4) via dissolution in MeCN. Tautomers N4 and Z4 sharply differ in NMR properties and both gave rise to a pair of cage-isomeric closo ferratricarbollides, [1-Cp-2-Fc-12-t-BuNH-1,2,4,12-FeC(3)B(8)H(9)] (5a) (Cp = eta(5)-C(5)H(5)) (yield 48%) and [1-Cp-2-Fc-10-t-BuNH-1,2,4,10-FeC(3)B(8)H(9)] (5b) (yield 32%), upon heating with [CpFe(CO)(2)](2) in mesitylene at reflux. Electrochemical studies revealed that the compounds display one (compounds 2a and 3) or two (compounds 5a and 5b) well-defined, reversible one-electron Fe(II)/Fe(III) redox changes.

Synthesis and Rearrangements of Aminosubstituted Ferra- and Ruthenatricarbaboranes

A room-temperature reaction between the [7-tBuNH-nido-7,8,9-C3B8H10]- anion (1a) and [Cp*RuCl]4 leads to the ruthenatricarbollide [1-Cp*-12-tBuNH-1,2,4,12-RuC3B8H10] (2) (yield 85%). Analogously, the room-temperature photochemical reaction of 1a with [CpFe(C6H6)]PF6 gives the previously reported iron complex [1-Cp-12-tBuNH-1,2,4,12-FeC3B8H10] (3) (yield 82%). Both reactions are associated with extensive polyhedral rearrangement, which occurs under very mild conditions and brings the carbon atoms to positions of maximum separation within the framework. Compounds 2 and 3 were also surprisingly obtained via complexation of the isomeric [8-tBuNH-nido-7,8,9-C3B8H10]- (1b) anion. Complex 2 rearranges further to [1-Cp*-10-tBuNH-1,2,4,10-RuC3B8H10] (4) upon refluxing in xylene (145 degrees C). Density functional theory calculations at the B3LYP/SDD level were used to estimate relative stabilities of these metallacarborane isomers. Compounds 2 and 4, along with the 11-vertex closo compounds [1-Cp*-1,2,3,10-RuC3B7H10] (5) and [1-Cp*-10-tBuNH-1,2,3,10-RuC3B7H9] (6), were also isolated from the reaction between [Cp*RuCl2]2 and 1a in boiling xylene. The structure of 2 was established by an X-ray diffraction study, and the constitution of all compounds was determined unambiguously by multinuclear NMR spectroscopy, mass spectrometry, and elemental analyses.

Diphosphacarborane analogues of ferrocene: The synthesis of two isomeric twelve-vertex closo-[(η5-C5H5)FeP2CB8H9] complexes

The reaction of the Tl+ salt of the [nido-7,8,9-P2CB8H9]- anion (1-) with [CpFe(CO)2I](Cp =eta(5)-C5H5) in refluxing mesitylene for 12 h gives mixed-sandwich [1-Cp-closo-1,2,3,4-FeP2CB8H9] (2) (yield 63%). Reaction of the PPh4+ salt of the isomeric [nido-7,8,10-P2CB8H9]- anion 3- with [CpFe(CO)2I] in refluxing mesitylene gives [1-Cp-closo-1,2,3,5-FeP2CB8H9]4 (yield 56%), isomeric with 2. Compound 4 also results (yield 92%) from the sublimation of 2 under argon at ca. 350 degrees C. The constitution of all compounds is established by mass spectrometry, IR spectroscopy and multinuclear NMR spectroscopy (1H, 11B, 31P, and 13C; two-dimensional [11B-11B]-COSY, and 1H- 11B(selective)), further confirmed in the case of 4 by a single-crystal X-ray diffraction analysis.

Neutral nido-heteroboranes with non ionisable hydrogen as arenes in coordination

Designed ligands have been synthesised to produce the first arene-like metallacarborane. For arene-like coordination the number of electronegative elements on the coordinating site must be kept to a minimum. Choosing ligands with bulky substituents on the heteroatom allows easy rearrangement and arene-like coordination. This is more hampered the higher the number of hetereoatoms to be re-located.