Direct observation of a nonchelated metal-alkyl-alkene complex and measurement of the rate of alkyl migration to a coordinated alkene

Conformationally dynamic titanium and zirconium cationic complexes of bis(naphthoxy)pyridine ligands: structure, "oscillation" and olefin polymerization catalysis

Discrete ionic complexes {ONO^SiMe₂tBu}M(CH₂Ph)((η⁶-Ph)CH₂B(C₆F₅)₃) (M = Ti, 2-Ti; Zr, 2-Zr) have been prepared from the parent neutral dibenzyl precursors and B(C₆F₅)₃. Also, a neutral dichloro complex {ONO^SiMe₂tBu}ZrCl₂(Me₂NH) (3) was synthesized by the reaction of a proligand {ONO^SiMe₂tBu}H₂ (1-H2) with (Me₂N)₂ZrCl₂(DME). The compounds were characterized by NMR spectroscopy and X-ray crystallography (for 2-Zr and 3). In the solid state, both 2-Zr and 3 feature meso-like binding of the ligand. VT NMR studies of 2-Ti and 2-Zr revealed that the former species dissociates in solution to form an outer-sphere ion pair (OSIP) that features a complicated dynamic behavior, while the latter compound retains an inner-sphere ion pair (ISIP) structure that interconverts between C_s-symmetric (meso-like) and C₁-symmetric (rac-like) conformations. The mechanism of this interconversion was assessed by DFT calculations and the corresponding barrier for the straightforward interconversion was calculated ().

Alkenes as Chelating Groups in Diastereoselective Additions of Organometallics to Ketones

Alkenes have been discovered to be chelating groups to Zn(II), enforcing highly stereoselective additions of organozincs to β,γ-unsaturated ketones. ¹H NMR studies and DFT calculations provide support for this surprising chelation mode. The results expand the range of coordinating groups for chelation-controlled carbonyl additions from heteroatom Lewis bases to simple C–C double bonds, broadening the 60 year old paradigm.

Unprecedented alkene complex of zinc(II): structures and bonding of divinylzinc complexes

This report describes the solid-state structures of a series of divinylzinc complexes, one of which represents the only structurally characterized zinc(II) pi-complex. Vinylzinc reagents, Zn[C(Me)=CH2]2 (1) and Zn[C(H)=CMe2]2 (2), have been synthesized and isolated as white crystalline solids in 66 and 72% yield, respectively. Each compound exhibits an infinite polymeric architecture in the solid state via a series of zinc-pi (1) and zinc-sigma-bonded (2) bridging interactions. Addition of chelating ligands to these divinylzinc compounds allowed isolation of the monomeric adducts (bipy)Zn[C(Me)=CH2]2 (1.bipy), (tmeda)Zn[C(Me)=CH2]2 (1*tmeda), (bipy)Zn[C(H)=CMe2]2 (2*bipy), and (tmeda)Zn[C(H)=CMe2]2 (2*tmeda), of which 1*bipy, 2*bipy, and 2*tmeda have been characterized crystallographically.

Nonchelated Alkene and Alkyne Complexes of d0 Zirconocene Pentafluorophenyl Cations

This paper describes the generation and properties of nonchelated d(0) zirconocene-aryl-alkene and alkyne adducts that are stabilized by the presence of beta-SiMe(3) substituents on the substrates and the weak nucleophilicity of the -C(6)F(5) ligand. The cationic complexes [(C(5)H(4)R)(2)Zr(C(6)F(5))][B(C(6)F(5))(4)] (4a: R = H, 4b: R = Me) were generated by methide abstraction from (C(5)H(4)R)(2)Zr(C(6)F(5))Me by Ph(3)C(+). NMR studies show that 4a,b contain an o-CF...Zr dative interaction and probably coordinate a PhCl molecule in PhCl solution. Addition of allyltrimethylsilane (ATMS) to 4a,b in C(6)D(5)Cl solution at low temperature produces an equilibrium mixture of (C(5)H(4)R)(2)Zr(C(6)F(5))(H(2)C=CHCH(2)SiMe(3))(+) (7a,b), 4a,b, and free ATMS. Similarly, addition of propargyltrimethylsilane (PTMS) to 4a produces an equilibrium mixture of Cp(2)Zr(C(6)F(5))(HCCCH(2)SiMe(3))(+) (8a), 4a, and free PTMS. The NMR data for 7a,b,and 8a are consistent with highly unsymmetrical substrate coordination and substantial polarization of the substrate multiple bond with significant positive charge buildup at C(int) and negative charge buildup at C(term). PTMS binds to 4a more strongly than ATMS does. The ATMS adducts undergo nondissociative alkene face exchange ("alkene flipping"), i.e., exchange of the (C(5)H(4)R)(2)Zr(C(6)F(5))(+) unit between the two alkene enantiofaces without decomplexation of the alkene, on the NMR time scale.

Nonchelated d0 Zirconium−Alkoxide−Alkene Complexes

The reaction of Cp'2Zr(O(t)Bu)Me (Cp' = C5H4Me) and [Ph3C][B(C6F5)4] yields the base-free complex [Cp'2Zr(O(t)Bu)][B(C6F5)4] (6), which exists as Cp'2Zr(O(t)Bu)(ClR)+ halocarbon adducts in CD2Cl2 or C6D5Cl solution. Addition of alkenes to 6 in CD2Cl2 solution at low temperature gives equilibrium mixtures of Cp'2Zr(O(t)Bu)(alkene)+ (12a-l), 6, and free alkene. The NMR data for 12a-l are consistent with unsymmetrical alkene bonding and polarization of the alkene C=C bond with positive charge buildup at C(int) and negative charge buildup at C(term). These features arise due to the lack of d-pi* back-bonding. Equilibrium constants for alkene coordination to 6 in CD2Cl2 at -89 degrees C, K(eq) = [12][6](-1)[alkene](-1), vary in the order: vinylferrocene (4800 M(-1)) >> ethylene (7.0) approximately alpha-olefins > cis-2-butene (2.2) > trans-2-butene (<0.1). Alkene coordination is inhibited by sterically bulky substituents on the alkene but is greatly enhanced by electron-donating groups and the beta-Si effect. Compounds 12a-l undergo two dynamic processes: reversible alkene decomplexation via associative substitution of a CD2Cl2 molecule, and rapid rotation of the alkene around the metal-(alkene centroid) axis.

Yttrium complexes incorporating the chelating diamides {ArN(CH2)xNAr}2−(Ar = C6H3-2,6-iPr2, x = 2, 3) and their unusual reaction with phenylsilane

Novel yttrium chelating diamide complexes [(Y[ArN(CH(2))(x)NAr](Z)(THF)(n))(y)] (Z = I, CH(SiMe(3))(2), CH(2)Ph, H, N(SiMe(3))(2), OC(6)H(3)-2,6-(t)Bu(2)-4-Me; x = 2, 3; n = 1 or 2; y = 1 or 2) were made via salt metathesis of the potassium diamides (x = 3 (3), x = 2 (4)) and yttrium triiodide in THF (5,10), followed by salt metathesis with the appropriate potassium salt (6-9, 11-13, 15) and further reaction with molecular hydrogen (14). 6 and 11(Z = CH(SiMe(3))(2), x = 2, 3) underwent unprecedented exchange of yttrium for silicon on reaction with phenylsilane to yield (Si[ArN(CH(2))(x)NAr]PhH) (x = 2 (16), 3) and (Si[CH(SiMe(3))(2)]PhH(2)).

Determining the Scope of the Organolanthanide-Catalyzed, Sequential Intramolecular Amination/Cyclization Reaction: Formation of Substituted Quinolizidines, Indolizidines, and Pyrrolizidines

The scope of the lanthanide-mediated, intramolecular amination/cyclization reaction was determined for the formation of substituted quinolizidines, indolizidines, and pyrrolizidines. A methyl group was installed at diverse positions in the substrates to determine the sense and magnitude of diastereoselection. High diastereoselectivity (>20:1) was achieved for the formation of some quinolizidines and indolizidines. The sense of relative asymmetric induction was contrary to previously studied systems, and although some questions remain, a rationalization for these results is put forward.

Tethered olefin studies of alkene versus tetraphenylborate coordination and lanthanide olefin interactions in metallocenes

The tethered olefin cyclopentadienyl ligand, [(C(5)Me(4))SiMe(2)(CH(2)CH=CH(2))](-), forms unsolvated metallocenes, [(C(5)Me(4))SiMe(2)(CH(2)CH=CH(2))](2)Ln (Ln = Sm, 1; Eu, 2; Yb, 3), from [(C(5)Me(4))SiMe(2)(CH(2)CH=CH(2))]K and LnI(2)(THF)(2) in good yield. Each complex in the solid state has both tethered olefins oriented toward the Ln metal center with the Ln-C(terminal alkene carbon) distances 0.2-0.3 A shorter than the Ln-C(internal alkene carbon) distances. The olefinic C-C bond distances in 2 and 3, 1.328(4) and 1.328(5) A, respectively, are normal. Like its permethyl analogue, (C(5)Me(5))(2)Sm(THF)(2), complex 1 reductively couples CO(2) to form the oxalate-bridged dimer [[(C(5)Me(4))SiMe(2)(CH(2)CH=CH(2))](2)Sm](2)(mu-eta(2):eta(2)-O(2)CCO(2)), 4, in which the tethered olefins are noninteracting substituents. Complex 1 reacts with AgBPh(4) to form an unsolvated cation that has the option of coordinating [BPh(4)](-) or a pendant olefin, a competition common in olefin polymerization catalysis. The structure of [[(C(5)Me(4))SiMe(2)(CH(2)CH=CH(2))](2)Sm][BPh(4)], 5, shows that both pendant olefins are located near samarium rather than the [BPh(4)](-) counterion.

Coordination of alkenes and alkynes to a cationic d(0) zirconocene alkoxide complex

This paper describes the synthesis of base-free (C5R5)2Zr(OtBu)+ cations, the direct observation of nonchelated alkene and alkyne adducts of these cations, and studies of the thermodynamic and dynamic properties of these novel species. Reaction of %@mt;sys@%Cp%@/xs;55;%lnwidth@%'%@/xs;63;(%lnwidth-x55)@%%@mh;-x63@%%@sb@%2%@sbx@%%@/hd@%ZrMe%@sb@%2%@sbx@%%@mx@% (Cp' = C5H4Me) with tert-butyl alcohol followed by [Ph3C][B(C6F5)4] in benzene yields [%@mt;sys@%Cp%@/xs;55;%lnwidth@%'%@/xs;63;(%lnwidth-x55)@%%@mh;-x63@%%@sb@%2%@sbx@%%@/hd@%Zr%@/hd@%%@fn;(;vis;full;auto@%O%@ital@%%@ex@%t%@rsf@%%@exx@%%@/hd@%Bu%@fnx;);vis;full@%%@/hd@%%@mx@% ][B(C6F5)4] (1), which exists as %@mt;sys@%Cp%@/xs;55;%lnwidth@%'%@/xs;63;(%lnwidth-x55)@%%@mh;-x63@%%@sb@%2%@sbx@%%@/hd@%Zr%@/hd@%%@fn;(;vis;full;auto@%O%@ital@%%@ex@%t%@rsf@%%@exx@%%@/hd@%Bu%@fnx;);vis;full@%%@/hd@%%@fn;(;vis;full;auto@%ClR%@fnx;);vis;full@%%@ex@%+%@exx@%%@mx@% solvent adducts in C6D5Cl and CD2Cl2 solutions. Addition of ligands L (L = ethylene, propylene, propyne, 2-butyne, CO, phenylacetylene, allene, 1-hexene, cis-2-butene) to 1 in CD2Cl2 at -89 degrees C results in reversible formation of %@mt;sys@%Cp%@/xs;55;%lnwidth@%'%@/xs;63;(%lnwidth-x55)@%%@mh;-x63@%%@sb@%2%@sbx@%%@/hd@%Zr%@/hd@%%@fn;(;vis;full;auto@%O%@ital@%%@ex@%t%@rsf@%%@exx@%%@/hd@%Bu%@fnx;);vis;full@%%@/hd@%%@fn;(;vis;full;auto@%L%@fnx;);vis;full@%%@ex@%+%@exx@%%@mx@% adducts. NMR data for %@mt;sys@%Cp%@/xs;55;%lnwidth@%'%@/xs;63;(%lnwidth-x55)@%%@mh;-x63@%%@sb@%2%@sbx@%%@/hd@%Zr%@/hd@%%@fn;(;vis;full;auto@%O%@ital@%%@ex@%t%@rsf@%%@exx@%%@/hd@%Bu%@fnx;);vis;full@%%@/hd@%%@fn;(;vis;full;auto@%H%@sb@%2%@sbx@%C=%@/bd@%CHMe%@fnx;);vis;full@%%@ex@%+%@exx@%%@mx@% (4) indicate that the propylene coordinates unsymmetrically and is polarized with positive charge buildup at Cint. Equilibrium constants, defined by Keq = [Zr-L][1]-1[L]-1, vary in the order CO > propyne > 2-butyne > phenylacetylene > ethylene > allene > propylene > 1-hexene > cis-2-butene > vinyl chloride. Loss of L from %@mt;sys@%Cp%@/xs;55;%lnwidth@%'%@/xs;63;(%lnwidth-x55)@%%@mh;-x63@%%@sb@%2%@sbx@%%@/hd@%Zr%@/hd@%%@fn;(;vis;full;auto@%O%@ital@%%@ex@%t%@rsf@%%@exx@%%@/hd@%Bu%@fnx;);vis;full@%%@/hd@%%@fn;(;vis;full;auto@%L%@fnx;);vis;full@%%@ex@%+%@exx@%%@mx@% to give 1 appears to proceed via associative displacement by CD2Cl2 in most cases.

Structural dependence of thermodynamics of alkene binding to yttrium alkyl complexes and of kinetics of alkyl migration to coordinated alkenes

Agostic interactions in yttrium alkyls are structure dependent. Primary alkyl yttrium complexes have beta-CH(2) agostic interactions at low temperature, but a shift toward alpha-agostic interactions occurs on warming. For the more crowded beta-disubstituted yttrium alkyls, an alpha-CH(2) agostic interaction is seen. The thermodynamics of alkene binding to the primary alkyl yttrium complex Cp(2)YCH(2)CH(2)CH(CH(3))(2) (2) depend strongly on the structure of the alkene. A single allylic substituent on the alkene has a small effect on alkene binding, but a second allylic substituent has a large destabilizing effect. Propene binding to yttrium alkyls is largely independent of the nature of the alkyl ligand. Equilibrium constants for propene binding to n-, gamma-substituted, beta-substituted, and secondary alkyl yttrium complexes are similar. The rate of migration of an alkyl group to a coordinated alkene depends strongly on the structure of the alkyl group: n-alkyl approximately gamma-substituted >> beta-substituted >> alpha-substituted. The approximately 200-fold slower insertion of propene into Cp(2)YCH(2)CH(CH(3))(2) (6) than that into Cp(2)YCH(2)CH(2)CH(CH(3))(2) (2) is therefore due to kinetically slow migration of the beta-disubstituted alkyl group of 6 and not to differences in the equilibrium binding of propene. Processes related to chain transfer and site epimerization at the metal center are also reported.

Chain epimerization during propylene polymerization with metallocene catalysts: mechanistic studies using a doubly labeled propylene

The mechanisms of chain epimerization during propylene polymerization with methylaluminoxane-activated rac-(EBTHI)ZrCl(2) and rac-(EBI)ZrCl(2) catalysts (EBTHI = ethylenebis(eta(5)-tetrahydroindenyl); EBI = ethylenebis(eta(5)-indenyl)) have been studied using specifically isotopically labeled propylene: CH(2)=CD(13)CH(3). These isospecific catalysts provide predominantly the expected [mmmm] pentads with [minus signCH(2)CD(13)CH(3)(-)] repeating units ((13)C NMR). Under relatively low propylene concentrations at 50 and 75 degreesC, where stereoerrors attributable to chain epimerization are prevalent, (13)C NMR spectra reveal (13)C-labeled methylene groups along the polymer main chain, together with [CD(13)CH(3)] units in [mmmr], [mmrr], and [mrrm] pentads and [CH(13)CH(3)] units in [mmmmmm] and [mmmmmr] heptads, as well as [mrrm] pentads. The isotopomeric regiomisplacements and stereoerrors are consistent with a mechanism involving beta-D elimination, olefin rotation and enantiofacial interconversions, and insertion to a tertiary alkyl intermediate [Zr-C(CH(2)D)((13)CH(3))P] (P = polymer chain), followed by the reverse steps to yield two stereoisomers of [Zr-CHDCH((13)CH(3))P] and [Zr-(13)CH(2)CH(CH(2)D)P], as well as unrearranged [Zr-CH(2)CD((13)CH(3))P]. The absence of observable [-CH(2)CH(13)CH(2)D-] in the [mrrm] pentad region of the (13)C NMR spectra provides evidence that an allyl/dihydrogen complex does not mediate chain epimerization.