Hoi-Shan Chan

Synthesis, structure, and reactivity of zirconacyclopentene incorporating a carboranyl unit.

A zirconacyclopentene incorporating a carboranyl unit 1,2-[Cp2ZrC(Et)=C(Et)]-1,2-C2B10H10 (1), an analogue of well-known zirconacyclopentadienes, was prepared and fully characterized from the reaction of Cp2Zr(mu-Cl)(mu-C2B10H10)Li(OEt2)2 with EtC OCEt in reflux toluene. Complex 1 resembles zirconacyclopentadienes in some reactions, and on the other hand, it has unique properties of its own. Many carborane derivatives with interesting structural features such as 1-[CI(Et)=C(Et)]-1,2-C2B10H11, naphthalocarborane, and 1,2-[C(Et)=C(Et)]-1,2-C2B10H10 can be conveniently prepared from 1. This work shows that 1 provides a very valuable entry point to the synthesis of various kinds of carborane derivatives that can not be prepared by conventional methods.

Hydrogen-mediated metal-carbon to metal-boron bond conversion in metal-carboranyl complexes.

A hydrogen-mediated Ru-C to Ru-B bond conversion was observed experimentally and supported by the theoretical calculations. Treatment of [eta(5):sigma(C)-Me(2)C(C(5)H(4))(C(2)B(10)H(10))]Ru(COD) (1) bearing a Ru-C(cage) sigma bond with PR(3) in the presence of H(2) gave Ru-B(cage) bonded complexes [eta(5):sigma(B)-Me(2)C(C(5)H(4))(C(2)B(10)H(10))]RuH(2)(PR(3)) (R = Cy (2), Ph (3)) (sigma(C): Ru-C(cage) sigma bond; sigma(B): Ru-B(cage) sigma bond). Complex 3 was converted to [eta(5):sigma(B)-Me(2)C(C(5)H(4))(C(2)B(10)H(10))]Ru(L(2)) in the presence of L(2) (L(2) = dppe (4), PPh(3)/P(OEt)(3) (5), PPh(3)/pyridine (6)) via liberation of H(2) upon heating. These complexes were fully characterized by various spectroscopic techniques, elemental analyses, and single-crystal X-ray diffraction studies. DFT calculations show that this conversion process is both kinetically and thermodynamically favorable and requires involvement of a hydride ligand.

Reaction of 13‐Vertex Carboranes with Nucleophiles: Unprecedented Cage‐Carbon Extrusion and Formation of Monocarba‐closo‐dodecaborate Anions

Only in recent years has significant progress been made in the chemistry of supercarboranes (carboranes with more than 12 vertices). A number of 13and 14-vertex carboranes have been prepared and structurally characterized since 2003. They are readily reduced by Group 1 metals to give the corresponding nido-supercarborane dianions. The carbonatoms-adjacent (CAd) carborane 1,2-(CH2)3-1,2-C2B11H11 can even undergo single-electron reduction to generate a stable carborane radical anion with 2n + 3 framework electrons. It can also react with various electrophiles to afford hexasubstituted CAd 13-vertex carboranes 8,9,10,11,12,13-X6-1,2(CH2)3-1,2-C2B11H5 (X = Me, Br, I). [4] We are interested in the reaction of supercarboranes with nucleophiles. We now report that unprecedented products of cage-carbon extrusion are isolated instead of the expected deborated species after treatment of 13-vertex carboranes with nucleophiles. A solution of 1,2-(CH2)3-1,2-C2B11H11 (1) [3] in methanol was stirred at room temperature for one day to give, after addition of [Me3NH]Cl, Me3NH[1,2-(CH2)3CH(OMe)-1CB11H10] (2), which was isolated in 75% yield (Scheme 1). This reaction can be monitored by B NMR spectroscopy. Whereas 1 reacted with MeOH/NaOH to afford a mixture of inseparable products, its icosahedral cousin 1,2-(CH2)3-1,2C2B10H10 is stable in refluxing MeOH and is converted to the nido species (CH2)3C2B9H10 in refluxing MeOH/NaOH solution. When PPh3 was used as nucleophile, the zwitterionic compound [1,2-(CH2)3CH(PPh3)-1-CB11H10]·CH2Cl2 (3) was isolated in 80 % yield after recrystallization from CH2Cl2. Similarly, the carbon-atoms-apart (CAp) 13-vertex carborane 1,6-Me2-1,6-C2B11H11 (4) [7] reacted with MeOH to afford, on addition of [Me3NH]Cl, Me3NH[1-Me-2-CH(OMe)Me-1CB11H10] (5) in 55% yield. The B NMR spectra of 2 and 5 show similar 1:1:6:3 patterns, whereas that of 3 displays a 1:3:7 pattern. The signal of the substituted B2 atom in both 2 and 5 is clearly distinguished from others at d = 7.7 and 5.0 ppm as a singlet in the proton-coupled B NMR spectra. However, the resonance of B2 in 3 overlaps with other cage B peaks, and is hardly resolved. The a-C atom bonded to B2 is unambiguously identifiable, as it appears as a broad signal in the C NMR spectra due to coupling to a B nucleus, at d = 74.2 ppm in 2, d = 72.2 ppm in 5, and d = 17.2 ppm in 3. The P NMR spectrum of 3 exhibits one sharp peak at d = 32.7 ppm, supportive of a tertiary phosphonium salt. Single-crystal X-ray analyses confirm the molecular structures of 2, 3, and 5, as shown in Figures 1–3, respectively. The icosahedral cages in the three compounds have the same structural features of a monocarba-closo-dodecaborate anion. Cage-carbon extrusion from carborane clusters is very rare but not unknown. Two examples have been reported. A recent closo-to-closo example is the transformation of [1-H2N-closo-CB11F11] into [3-NC-closo-B11F10] 2 , which is limited to highly fluorinated boron clusters. The other is the conversion of [7-R-m-(9,10-HR’C)-7-nido-CB10H11] to [1-R6-CH2R’-1-closo-CB9H8] , in which cage-carbon extrusion is suggested to proceed after removal of one BH vertex. In this regard, a plausible pathway for formation of monocarbacloso-dodecaborate anions is proposed in Scheme 2. Attack of the nucleophile on one of the cage carbon atoms of the 13vertex carborane leads to cleavage of the Ccage Ccage bond and formation of new Ccage B bonds to preserve cluster integrity. Hydrogen migration then generates the final icosahedral product. This behavior is significantly different from that of o-carboranes, in which a cage boron atom is attacked by nucleophiles to give deboration products. Although the reasons are not yet clear, comparison of the C chemical shift of the cage carbon atoms of 136 ppm in 1 with Scheme 1. Reaction of CAd and CAp 13-vertex carboranes with nucleophiles.

Reaction of a 14-vertex carborane with nucleophiles: formation of nido-C(2)B(12), nido-C(2)B(11,) and closo-CB(11) carborane anions.

Nucleophilic reactions of a 14-vertex closo-carborane are reported. 2,3-(CH(2))(3)-2,3-C(2)B(12)H(12) (1) reacts with MeOH at 70 degrees C to give closo-CB(11) anions [1,2-(CH(2))(3)CH(OMe)-1-CB(11)H(10)](-) ([2a](-)), [1,2-(CH(2))(2)CH(OMe)CH(2)-1-CB(11)H(10)](-) ([2b](-)), and [1,2-(CH(2))(2)CH horizontal lineCH-1-CB(11)H(10)](-) ([2c](-)). It is suggested that [2c](-) is an intermediate for the isomerization from [2a](-) to [2b](-). Treatment of 1 with MeOH/Me(3)N, (t)BuOK or LiNMe(2) affords nido-C(2)B(12) species [8,9-(CH(2))(3)-mu-11,12-(Nu)BH-8,9-C(2)B(11)H(11)](-) (Nu = MeO ([3a](-)), (t)BuO ([3b](-)), and Me(2)N ([3c](-))). In the presence of acid such as HCl, anions [3](-) are converted to 1. However, [3](-) undergo deboration reaction, in the presence of bases, to generate a nido-C(2)B(11) anion [8,9-(CH(2))(3)-8,9-C(2)B(11)H(12)](-) ([4](-)) that can also be formed directly from the reaction of 1 with excess CsF or piperidine. Mechanistic studies show that [3a](-) is the first intermediate in the reaction of 1 with MeOH and [4](-) is unlikely an intermediate.