Formation of Hypercoordinated Carbon inside a (μ-Hydrocarbyl)bis(group 4 metallocene) Framework by a Simple Protonation Route

Abstract

(Alkenyl)zirconocene chlorides Cp2Zr(Cl)(CHCHR) (4, R = H, phenyl, n-butyl, or cyclohexyl) react with “Cp2Zr” generated from various precursors to yield the neutral dinuclear alkenyl-bridged bis(metallocene) complexes 6 [Cp2Zr(μ-Cl)(μ-η1:η2-CHCHR)ZrCp2]. The complexes 6 are regioselectively protonated by treatment with (HNMe2Ph+)(BPh4 -) (8a) to yield the unusually structured products 9 [Cp2Zr(μ-Cl)(μ-η1:η2-C1H2C2HR)ZrCp2 +], which exhibit an unsymmetrically bridged hydrocarbyl ligand containing a novel type of a hypercoordinated carbon center (C1) inside the rigid organometallic framework. From the NMR analysis and an X-ray crystal structure determination of the example 10c [(MeCp)2Zr(μ-Cl)(μ-C1H2C2H-n-butyl)Zr(MeCp)2 +], it is evident that the hypercarbon atom C1 is coordinated to four close neighboring atoms (Zr1, Zr2, C2, and H1a) in a distorted square-planar arrangement with the remaining C1−H1b bond being oriented perpendicular to it. H1b thus marks the apex of a distorted square pyramid; the hypercarbon atom C1 is located in the center of the basal plane. This unusual structural coordination geometry around C1 is determined by the stereoelectronic features of the two adjacent group 4 bent metallocene units. From the dynamic features of complex 9a (R = H), a stabilization energy of ca. 10 kcal mol-1 is estimated for the uncommon coordination mode of C1, which is favored here, relative to a “normal” sp3-hybridized structure in a C 2 v -symmetric metallacyclic framework. Part of the pronounced thermodynamic stabilization of this unusual pentacoordinate carbon geometry originates from the strong α-agostic Zr···H1a−C1 interaction in the basal plane. From the monodeuterated derivative 9a-C1HD, an energy difference of 220 cal mol-1 between D and H favoring the bridging position was determined.

Isobutene Polymerization and Isobutene-Isoprene Copolymerization Catalyzed by Cationic Zirconocene Hydride Complexes

“…The proposal gains support by Erker's isolation and crystallographic identification of a related binuclear complex 7, synthesized by protonation of a chloro/ alkenyl-bridged precursor. 24 The bridging carbon atom C 1 has square pyramidal structure, and one hydrogen is agostically bound to Zr, while the other has two possible orientations perpendicular to the Zr 1 ,Zr 2 ,C 1 ,C 2 plane, leading to the possibility of two stereoisomers. The chemical shifts observed are remarkably similar to those of 6.…”

Preparation and Characterization of 1,3-Butadiene and Isoprene Complexes, W2(OCH2tBu)6(diene)(py), and Studies of the Selective Hydrogenation of 1,3-Dienes

“…The coordination of the μ,η 1 -carbon atom is “hypervalent” in that the carbon atom forms bonds to two hydrogen atoms, one carbon atom, and two metal atoms. Such coordination is typical of bridging alkyl ligands as in Al 2 (μ-CH 3 ) 2 (CH 3 ) 4 , but continues to attract attention in the current literature …”

Using bent metallocenes for stabilizing unusual coordination geometries at carbon

“…Protonation occurs exclusively at the alkenyl methine group with formation of the very unusually structured cationic hypercarbon complex 22 (Scheme 9). 24 The phenyl-substituted derivative 22b shows very characteristic NMR features of the m-h 1 +h 2 -C1H 2 C2HPh moiety, that is, bridging uniquely between the two metal centers of the Cp 2 Zr-Cl-ZrCp 2 unit. It features 1 H NMR signals at d 5.17 (C 2 H), 4.63 and 25.55 with coupling constants of 3 J = 15.6 and 6.6 Hz, 2 J = 5.4 Hz.…”

“…8). 24 The unique stereochemical properties of both zirconocenes 2 have been used as tools to construct and stabilize the very unusual coordination geometry of the pentacoordinated carbon (C1) in the complexes 22. This goes so far as to utilize even a C-H bond as a functional group to determine the shape of the coordination polyhedron at the hypercoordinated carbon atom 1a in the center of these cationic complexes.…”