A new pathway of activation of C–H bonds of alkyl- and arylnitriles by a cooperative action of TaCl5 and PPh3 under mild conditions is reported. Coordination of nitriles to the highly Lewis acidic Ta(V) center resulted in an activation of their aliphatic and aromatic C–H bonds, allowing nucleophilic attack and deprotonation by the relatively weak base PPh3. The propensity of Ta(V) to form multiple bonds to nitrogen-containing ligands is an important driving force of the reaction as it led to a sequence of bond rearrangements and the emergence of, in the case of benzonitrile, a zwitterionic enediimido complex of Ta(V) through C═C double bond formation between two activated nitrile fragments. These transformations highlight the special role of the high-valent transition metal halide in substrate activation and distinguish the reactivity of the TaCl5–PPh3 system from both non-metal- and late transition metal-based frustrated Lewis pairs.

•B-carboranyl pincer complex of nickel(II) with a strained exohedral BNi bond was synthesized.•Oxidation of the complex with iodine resulted in the selective nickel-mediated iodination of the vicinal BH bond.•Further iodination of the complex with a large excess of iodine led to iodination of the second boron vertex of the cluster.The (POBOP)NiCl pincer complex (POBOP = 1,7-OP(i-Pr)2-m-carboranyl) was synthesized by the selective BH activation of the chelating carboranyl ligand and structurally and spectroscopically characterized. The reaction of (POBOP)NiCl and 1 equiv. of I2 at room temperature led to the selective iodination of the vicinal BH bond in the complex. The reaction of (POBOP)NiCl and 7 equiv. of I2 at room temperature led to further iodination of the ligand and dissociation of the metal from the complex.Download high-res image (144KB)Download full-size image

In this work, we introduce a novel concept of a borane group vicinal to a metal boryl bond acting as a supporting hemilabile ligand in exohedrally metalated three-dimensional carborane clusters. The (POBOP)Ru(Cl)(PPh3) pincer complex (POBOP = 1,7-OP(i-Pr)2-m-2-carboranyl) features extreme distortion of the two-center-two-electron Ru–B bond due to the presence of a strong three-center-two-electron B–H⋯Ru vicinal interaction. Replacement of the chloride ligand with a hydride afforded the (POBOP)Ru(H)(PPh3) pincer complex, which possesses B–Ru, B–H⋯Ru, and Ru–H bonds. This complex was found to exhibit a rapid exchange between hydrogen atoms of the borane and the terminal hydride through metal center shuttling between two boron atoms of the carborane cage. This exchange process, which involves sequential cleavage and formation of strong covalent metal–boron and metal–hydrogen bonds, is unexpectedly facile at temperatures above −50 °C corresponding to an activation barrier of 12.2 kcal mol−1. Theoretical calculations suggested two equally probable pathways for the exchange process through formally Ru(0) or Ru(IV) intermediates, respectively. The presence of this hemilabile vicinal B–H⋯Ru interaction in (POBOP)Ru(H)(PPh3) was found to stabilize a latent coordination site at the metal center promoting efficient catalytic transfer dehydrogenation of cyclooctane under nitrogen and air at 170 °C.

The reaction of Os3(CO)10(NCMe)2 with closo-o-(1-SCH3)C2B10H11 has yielded the complex Os3(CO)9[μ3-η3-C2B10H9(SCH3)](μ-H)2, 1, by the loss of the two NCMe ligands and one CO ligand from the Os3 cluster and the coordination of the sulfur atom and the activation of two B–H bonds with transfer of the hydrogen atoms to the cluster. Reaction of 1 with a second equivalent of Os3(CO)10(NCMe)2 yielded the complex Os3(CO)9(μ-H)[(μ3-η3-1,4,5-μ3-η3-6,10,11-C2B10H8S(CH3)]Os3(CO)9(μ-H)2, 2, that contains two triosmium triangles attached to the same carborane cage. The carborane cage was opened by cleavage of two B–C bonds and one B–B bond. The B–H group that was pulled out of the cage became a triply bridging group on one of the Os3 triangles but remains bonded to the cage by two B–B bonds. When heated to 150 °C, 2 was transformed into the complex Os3(CO)9(μ-H)[(μ3-η3-μ3-η3-C2B10H7S(CH3)]Os3(CO)9(μ-H), 3, by the loss of two hydrogen atoms and a rearrangement that led to further opening of the carborane cage. Reaction of 1 with a second equivalent of closo-o-(1-SCH3)C2B10H11 has yielded the complex Os3(CO)6)(μ3-η3-C2B10H9-R-SCH3) (μ3-η3-C2B10H10-S-SCH3)(μ-H)3, 4a, containing two carborane cages coordinated to one Os3 cluster. Compound 4a was isomerized to the compound Os3(CO)6(μ3-η3-C2B10H9-R-SCH3)(μ3-η3-C2B10H10-R-SCH3)(μ-H)3, 4b, by an inversion of stereochemistry at one of the sulfur atoms by heating to 174 °C.

We report a new CH3CN activation mode where an imido group is directly formed by deprotonation of the nitrile coordinated to the highly Lewis acidic TaV center. The unexpected deprotonation of TaCl5(CH3CN) by NEt3 resulted in isolation of the triethylammonium vinylimido complex [HNEt3][Ta(NC(CH2)NEt3)Cl5]. The reaction is proposed to proceed through rearrangement of the initial nucleophilic carbanion to the electrophilic azaallene/carbocation intermediate. The use of more sterically hindered (i-Pr)CN and weakly nucleophilic N(i-Pr)2Et resulted in the isolation of a vinylimido group formed upon dimerization of deprotonated nitriles, suggesting deprotonation as the first step of the transformation.

We report the synthesis of the chelating phosphinite-arm carboranyl POBOP-H (POBOP = 1,7-OP(i-Pr)2-m-carboranyl) ligand precursor, preparation of its rhodium complexes, and their reactivity in oxidative addition/reductive elimination reactions. The oxidative addition of iodobenzene to the low-valent (POBOP)Rh(PPh3) resulted in the selective formation of the 16-electron complex (POBOP)Rh(Ph)(I), featuring a highly strained exohedral rhodium–boron bond. The complex (POBOP)Rh(Ph)(I) is the first example of a B-carboranyl aryl metal complex, which is a proposed intermediate in metal-promoted B–C coupling reactions. The complex (POBOP)Rh(Ph)(I) was selectively and directly converted, in the presence of acetonitrile, to (POB(BPh)OP)Rh(H)(I)(CH3CN) (POB(BPh)OP = 1,7-OP(i-Pr)2-2-Ph-m-carboranyl) through unprecedented cascade reductive elimination of the phenyl-B-carboranyl and the oxidative addition of a vicinal B–H bond of the boron cluster to the metal center, exhibiting both metal- and cluster-centered reactivity.

In this work, we introduce a novel concept of a borane group vicinal to a metal boryl bond acting as a supporting hemilabile ligand in exohedrally metalated three-dimensional carborane clusters. The (POBOP)Ru(Cl)(PPh3) pincer complex (POBOP = 1,7-OP(i-Pr)2-m-2-carboranyl) features extreme distortion of the two-center-two-electron Ru–B bond due to the presence of a strong three-center-two-electron B–H⋯Ru vicinal interaction. Replacement of the chloride ligand with a hydride afforded the (POBOP)Ru(H)(PPh3) pincer complex, which possesses B–Ru, B–H⋯Ru, and Ru–H bonds. This complex was found to exhibit a rapid exchange between hydrogen atoms of the borane and the terminal hydride through metal center shuttling between two boron atoms of the carborane cage. This exchange process, which involves sequential cleavage and formation of strong covalent metal–boron and metal–hydrogen bonds, is unexpectedly facile at temperatures above −50 °C corresponding to an activation barrier of 12.2 kcal mol−1. Theoretical calculations suggested two equally probable pathways for the exchange process through formally Ru(0) or Ru(IV) intermediates, respectively. The presence of this hemilabile vicinal B–H⋯Ru interaction in (POBOP)Ru(H)(PPh3) was found to stabilize a latent coordination site at the metal center promoting efficient catalytic transfer dehydrogenation of cyclooctane under nitrogen and air at 170 °C.

The activation of O–H bonds of water at room temperature driven by the cage-opening rearrangement of a biscarborane-based cluster is reported. The reaction of the 12-vertex-closo-12-vertex-nido biscarborane cluster with water led to the quantitative and selective two-vertex decapitation of a carborane cluster and formation of the pendent boronic acid hydride B(H)(OH) group. Remarkably, this transformation can be quantitatively reversed with the release of water and re-formation of the starting biscarborane cage. The flexibility of cage decapitation/expansion and its influence on the reactivity of an exohedral substituent represent a new approach to cluster-induced organic transformations.