Reversible Cleavage/Formation of the Chromium–Chromium Quintuple Bond in the Highly Regioselective Alkyne Cyclotrimerization

Herein we report the employment of the quintuply bonded dichromium amidinates [Cr{κ²‐HC(N‐2,6‐ⁱPr₂C₆H₃)(N‐2,6‐R₂C₆H₃)}]₂ (R=iPr ( 1 ), Me ( 7 )) as catalysts to mediate the [2+2+2] cyclotrimerization of terminal alkynes giving 1,3,5‐trisubstituted benzenes. During the catalysis, the ultrashort Cr−Cr quintuple bond underwent reversible cleavage/formation, corroborated by the characterization of two inverted arene sandwich dichromium complexes (μ‐η⁶:η⁶‐1,3,5‐(Me₃Si)₃C₆H₃)[Cr{κ²‐HC(N ‐2,6‐ⁱPr₂C₆H₃)(N ‐2,6‐R₂C₆H₃)}]₂ (R=ⁱPr ( 5 ), Me ( 8 )). In the presence of σ donors, such as THF and 2,4,6‐Me₃C₆H₂CN, the bridging arene 1,3,5‐(Me₃Si)₃C₆H₃ in 5 and 8 was extruded and 1 and 7 were regenerated. Theoretical calculations were employed to disclose the reaction pathways of these highly regioselective [2+2+2] cylcotrimerization reactions of terminal alkynes.     

Homoleptic Two‐Coordinate Silylamido Complexes of Chromium(I), Manganese(I), and Cobalt(I)

Anionic two‐coordinate complexes of first‐row transition‐metal(I) centres are rare molecules that are expected to reveal new magnetic properties and reactivity. Recently, we demonstrated that a N(SiMe₃)₂^? ligand set, which is unable to prevent dimerisation or extraneous ligand coordination at the +2 oxidation state of iron, was nonetheless able to stabilise anionic two‐coordinate Fe^I complexes even in the presence of a Lewis base. We now report analogous Cr^I and Co^I complexes with exclusively this amido ligand and the isolation of a [Mn^I{N(SiMe₃)₂}₂]₂^2? dimer that features a Mn?Mn bond. Additionally, by increasing the steric hindrance of the ligand set, the two‐coordinate complex [Mn^I{N(Dipp)(SiMe₃)}₂]^? was isolated (Dipp=2,6‐iPr₂‐C₆H₃). Characterisation of these compounds by using X‐ray crystallography, NMR spectroscopy, and magnetic susceptibility measurements is provided along with ligand‐field analysis based on CASSCF/NEVPT2 ab initio calculations.     

Direct Synthesis of Titanium Complexes with Chelating cis‐9,10‐Dihydrophenanthrenediamide Ligands through Sequential C?C Bond‐Forming Reactions from o‐Metalated Arylimines

A series of new titanium(IV) complexes with o‐metalated arylimine and/or cis‐9,10‐dihydrophenanthrenediamide ligands, [o‐C₆H₄(CH?NR)TiCl₃] (R=2,6‐iPr₂C₆H₃ ( 3 a ), 2,6‐Me₂C₆H₃ ( 3 b ), tBu ( 3 c )), [cis‐9,10‐PhenH₂(NR)₂TiCl₂] (PhenH₂=9,10‐dihydrophenanthrene; R=2,6‐iPr₂C₆H₃ ( 4 a ), 2,6‐Me₂C₆H₃ ( 4 b ), tBu ( 4 c )), [{cis‐9,10‐PhenH₂(NR)₂}{o‐C₆H₄(HC?NR)}TiCl] (R=2,6‐iPr₂C₆H₃ ( 5 a ), 2,6‐Me₂C₆H₃ ( 5 b ), tBu ( 5 c )), have been synthesised from the reactions of TiCl₄ with o‐C₆H₄(CH?NR)Li (R=2,6‐iPr₂C₆H₃, 2,6‐Me₂C₆H₃, tBu). Complexes 4 and 5 were formed unexpectedly from the reactions of TiCl₄ with two or three equivalents of the corresponding o‐C₆H₄(CH?NR)Li followed by sequential intramolecular C? C bond‐forming reductive elimination and oxidative coupling reactions. Attempts to isolate the intermediates, [{o‐C₆H₄(CH?NR)}₂TiCl₂] ( 2 ), were unsuccessful. All complexes were characterised by ¹H and ¹³C NMR spectroscopy, and the molecular structures of 3 a , 4 a – c , 5 a , and 5 c were determined by X‐ray crystallography.     

Synthesis and Characterization of an Eclipsed Digermylene as a Building Block to Construct a Cyclic Octagermylene

The preparation of an unprecedented Ge^I‐Ge^I bonded digermylene [K₂{Ge₂(μ‐κ²:η²:η⁴‐2,6‐(2,6‐ⁱPr₂C₆H₃‐N)₂‐4‐CH₃C₅H₂N)₂}] in an eclipsed conformation stabilized by two bridging diamidopyridyl ligands is presented. Although it exhibits an eclipsed conformation, the Ge−Ge bond length is 2.5168(6) Å, which is shorter than those in the trans ‐bent and gauche digermylenes. In combination with two pendant amido groups, the Ge^I₂ motif is employed as a building block to assemble the first example of octagermylene [Ge₄(μ‐κ²:κ¹‐2,6‐(2,6‐ⁱPr₂C₆H₃‐N)₂‐4‐CH₃C₅H₂N)₂]₂ showing a cyclic configuration and containing three distinct types of Ge^I−Ge^I bonds.     

A Family of Multiply Bonded Dimolybdenum Boraamidinates with the Formal Mo−Mo Bond Orders of 3, 4, 4.5, and 5

A boraamidinato ligand [PhB(N‐2,6‐ⁱPr₂C₆H₃)₂]^2? was employed to stabilize a new family of multiply bonded dimolybdenum complexes [MoCl(μ‐κ²‐PhB(N‐2,6‐ⁱPr₂C₆H₃)₂)]₂ ( 4 ) and [Mo(μ‐κ²‐PhB(N‐2,6‐ⁱPr₂C₆H₃)₂)]₂^n? (n=0 ( 5 ), 1 ( 6 ), 2 ( 7 )), with the respective formal Mo?Mo bond orders of 3, 4, 4.5, and 5. Each metal center in 5 – 7 is two‐coordinate with respect to the ligands. Of particular interest is the quadruply bonded dimolybdenum complex 5 , featuring an unprecedented angular conformation. The bent Mo₂N₄ core of 5 distorts toward planarity upon reduction. As a result, compound 7 features a planar Mo₂N₄ core, while that of 6 is still bent but less significantly than that of 5 . Additionally, the Mo?Mo bond lengths of 4 – 7 systematically decrease as the valency of the central Mo₂ units decreases. Complex 7 features the shortest Mo?Mo bond length (2.0106(5) Å) yet reported.     

Chromium to chromium quintuple bonds in a trigonal lantern configuration

A new family of the quintuply bonded dichromium complexes [Cr₂{μ‐κ²‐HC(N‐2,6‐R₂C₆H₃)₂}₂(μ‐κ²‐HC[NAr]₂)] (R = ⁱPr, Ar = 4‐MeC₆H₄ ( 5 ), Ar = 3,5‐Me₂C₆H₃ ( 6 ), and Ar = 2,6‐Me₂C₆H₃ ( 7 ); R = Et, Ar = 4‐MeC₆H₄ ( 8 ), Ar = 3,5‐Me₂C₆H₃ [ 9 ], and Ar = 2,6‐Et₂C₆H₃ ( 10 )) with a heteroleptic lantern configuration was obtained upon the addition of one equivalent of amidinate to the quintuply bonded dichromium amidinates [Cr{μ‐κ²‐HC(N‐2,6‐R₂C₆H₃)₂}]₂ (R = ⁱPr, Et). Additionally, the same approach was applied to the preparation of the acetate derivative [Cr₂{μ‐κ²‐HC(N‐2,6‐ ⁱPr₂C₆H₃)₂}₂(μ‐κ²‐CH₃CO₂)] ( 11 ), which represents the first example that the quintuply bonded dinuclear complex contains an oxygen‐containing ligand. Of particular interest is that the Cr‐Cr bond lengths in these new trigonal paddlewheel quintuple Cr‐Cr bond species are comparable with those in their precursor compounds. They show ultrashort Cr‐Cr bond lengths in a narrow range of 1.740–1.755 å on the basis of single‐crystal X‐ray crystallography. The small Mayer bond orders of the long Cr‐N bonds as well as divergent, C_2v and D_3h, structural conformations in 5 – 11 suggest that the metal–ligand interactions possess minor covalent character and the electrostatic interactions play a dominant role. As a result, these extremely short Cr‐Cr quintuple bonds are caused by the overlap between five pairs of d orbitals that do not involve much in metal–ligand bonding. Additionally, anionic lantern dichromium trisamidinates 5 – 10 can be chemically oxidized by one electron, supported by electrochemistry, and their ease to undergo oxidation is presumably associated with their neutral lantern dichromium trisamindinate products, whose structures inherently display a Jahn‐Teller distortion, exemplified by the structure of the homoleptic dichromium complex [Cr₂{μ‐κ²‐HC(N‐2,6‐Et₂C₆H₃)₂}₃] [ 12 ] determined by X‐ray crystallography. These results unambiguously support the Cr‐Cr quintuple bonding in these novel anionic lantern dichromium complexes.     

Structural and Magnetic Insights into the Trinuclear Ferrocenophane and Unexpected Hydrido Inverse Crown Products of Alkali‐Metal‐Mediated Manganation(II) of Ferrocene

With the aim of introducing the diisopropylamide [NiPr₂] ^? ligand to alkali‐metal‐mediated manganation (AMMMn) chemistry, the temperature‐dependent reactions of a 1:1:3 mixture of butylsodium, bis(trimethylsilylmethyl)manganese(II), and diisopropylamine with ferrocene in hexane/toluene have been investigated. Performed at reflux temperature, the reaction affords the surprising, ferrocene‐free, hydrido product [Na₂Mn₂ (μ‐H)₂{N(iPr)₂}₄]?2 toluene ( 1 ), the first Mn hydrido inverse crown complex. Repeating the reaction rationally, excluding ferrocene, produces 1 in an isolated crystalline yield of 62 %. At lower temperatures, the same bimetallic amide mixture leads to the manganation of ferrocene to generate the first trimanganese, trinuclear ferrocenophane, [{Fe(C₅H₄)₂}₃{Mn₃Na₂(NiPr₂)₂ (HNiPr₂)₂}] ( 2 ) in an isolated crystalline yield of 81 %. Both 1 and 2 have been characterised by X‐ray crystallographic studies. The magnetic properties of paramagnetic 1 and 2 have also been examined by variable‐temperature magnetisation measurements on powdered samples. For 1 , the room‐temperature value for χT is 3.45 cm³ K mol^?1, and on lowering the temperature a strong antiferromagnetic coupling between the two Mn ions is observed. For 2 , the room‐temperature value for χT is 4.06 cm³ K mol^?1, which is significantly lower than the expected value for three isolated paramagnetic Mn^II ions.     

Bis(β‐diketiminato) lanthanide amides: synthesis,structure and catalysis for the polymerization of l‐lactide and ε‐caprolactone

Treatment of the chlorides (L^2,6‐iPr2_Ph)₂LnCl (L^2,6‐iPr2_Ph = [(2,6‐ⁱPr₂C₆H₃)NC(Me)CHC(Me)N(C₆H₅)]^?) with 1 equiv. of NaNH(2,6‐ⁱPr₂C₆H₃) afforded the monoamides (L^2,6‐iPr2_Ph)₂LnNH(2,6‐ⁱPr₂C₆H₃) (Ln = Y ( 1 ), Yb ( 2 )) in good yields. Anhydrous LnCl₃ reacted with 2 equiv. of NaL^2,6‐iPr2_Ph in THF, followed by treatment with 1 equiv. of NaNH(2,6‐ⁱPr₂C₆H₃), giving the analogues (L^2,6‐iPr2_Ph)₂LnNH(2,6‐ⁱPr₂C₆H₃) (Ln = Sm ( 3 ), Nd ( 4 )). Two monoamido complexes stabilized by two L^2‐Me ligands, (L^2‐Me)₂LnNH(2,6‐ⁱPr₂C₆H₃) (L^2‐Me = [N(2‐MeC₆H₄)C(Me)]₂CH)^?; Ln = Y ( 5 ), Yb ( 6 )), were also synthesized by the latter route. Complexes 1 , 2 , 3 , 4 , 5 , 6 were fully characterized, including X‐ray crystal structure analyses. Complexes 1 , 2 , 3 , 4 , 5 , 6 are isostructural. The central metal in each complex is ligated by two β‐diketiminato ligands and one amido group in a distorted trigonal bipyramid. All the complexes were found to be highly active in the ring‐opening polymerization of L‐lactide (L‐LA) and ε‐caprolactone (ε‐CL) to give polymers with relatively narrow molar mass distributions. The activity depends on both the central metal and the ligand (Yb < Y < Sm ≈ Nd and L^2‐Me < L^2,6‐iPr2_Ph). Remarkably, the binary 3/benzyl alcohol (BnOH) system exhibited a striking ‘immortal’ nature and proved able to quantitatively convert 5000 equiv. of L‐LA with up to 100 equiv. of BnOH per metal initiator. All the resulting PLAs showed monomodal, narrow distributions (M_w/M_n = 1.06 ? 1.08), with molar mass (M_n) decreasing proportionally with an increasing amount of BnOH. The binary 4/BnOH system also exhibited an ‘immortal’ nature in the polymerization of ε‐CL in toluene. Copyright © 2014 John Wiley & Sons, Ltd.     

Synthesis and Characterization of the Monomeric Sterically Encumbered Diaryls E{C6H3‐2,6‐(C6H3‐2,6‐Pri2)2}2 (E = Ge,Sn, or Pb)

The reaction of two equivalents of LiC₆H₃‐2,6‐(C₆H₃‐2,6‐Prⁱ₂)₂ with GeCl₂·dioxane, SnCl₂ or PbBr₂ in a diethyl ether solution resulted in the isolation of the monomeric σ‐bonded diaryl tetrylene series E{C₆H₃‐2,6‐(C₆H₃‐2,6‐Prⁱ₂)₂}₂ (E = Ge ( 1 ), Sn ( 2 ), or Pb( 3 )). Compounds 1 ‐ 3 are highly sterically congested blue crystalline solids, which possess V‐shaped structures and wide interligand bond angles. The solid state structures of 1 ‐ 3 were determined by single‐crystal X‐ray methods while their solution structures were investigated by UV spectroscopy and in the cases of 2 and 3 , respectively, by ¹¹⁹Sn and ²⁰⁷Pb NMR spectroscopy. The series 1 ‐ 3 constitutes the most sterically crowded examples of σ‐bonded diorgano group 14 derivatives yet isolated and, in contrast to previously reported: ER₂ species, the C‐E‐C angles increase with increasing atomic number.     

Rb{Pr6C2}I12, an Iodide with a 13‐Electron Isolated Octahedral {Pr6C2} Cluster

Rb{Pr₆(C)₂}I₁₂ was obtained from a mixture of RbI, PrI₃, Pr and C as black single crystals at elevated temperatures. The black crystals are triclinic, (no. 2), a = 960.1(2), b = 957.0(2), c = 1003.4(2) pm, α = 71.74(2), β = 70.69(2), γ = 72.38(2)°, V = 805.6(3) 10⁶ pm³, Z = 1; R₁ = 0.0868 for all 2749 measured independent reflections. Rb{Pr₆(C)₂}I₁₂ contains {Pr₆(C₂)} clusters isolated from each other, surrounded by twelve edge‐bridging and six terminal ligands. The [{Pr₆(C)₂}Iⁱ₁₂I^a₆]^? units are connected via i‐a/a‐i bridges according to {Pr₆C₂}Iⁱ_6/1I^i‐a_6/2I^a‐i_6/2 with rubidium ions occupying twelve‐coordinate interstices.     

Dinuclear neodymium and lanthanum bis(2,6‐diisopropylphenyl) phosphate complexes bearing a hydroxide ligand: catalytic activity of the Nd complex in 1,3‐diene polymerization

The reactions of K[(2,6‐ⁱPr₂C₆H₃‐O)₂POO] either with LaCl₃(H₂O)₇ or with Nd(NO₃)₃(H₂O)₆ in a 3:1 molar ratio, followed by vacuum drying and recrystallization from alkanes, have led to the formation of diaquapentakis[bis(2,6‐diisopropylphenyl) phosphato]‐μ‐hydroxido‐dilanthanum hexane disolvate, [La₂(C₂₄H₃₄O₄P)₅(OH)(H₂O)₂]·2C₆H₁₄, ( 1 )·2(hexane), and tetraaquatetrakis[bis(2,6‐diisopropylphenyl) phosphato]‐μ‐hydroxido‐dineodymium bis(2,6‐diisopropylphenyl) phosphate heptane disolvate, [Nd₂(C₂₄H₃₄O₄P)₄(OH)(H₂O)₄]·2C₆H₁₄, ( 2 )·2(heptane). The compounds crystalize in the P2₁/n and P space groups, respectively. The diaryl‐substituted organophosphate ligand exhibits three different coordination modes, viz. κ²O,O′‐terminal [in ( 1 ) and ( 2 )], κO‐terminal [in ( 1 )] and μ₂‐κ¹O:κ¹O′‐bridging [in ( 1 ) and ( 2 )]. Binuclear structures ( 1 ) and ( 2 ) are similar and have the same unique Ln₂(μ‐OH)(μ‐OPO)₂ core. The structure of ( 2 ) consists of an [Nd₂{(2,6‐ⁱPr₂C₆H₃‐O)₂POO}₄(OH)(H₂O)₄]⁺ cation and a [(2,6‐ⁱPr₂C₆H₃‐O)₂POO]⁻ anion, which are bound via four intermolecular O—H…O hydrogen bonds. The molecular structure of ( 1 ) displays two O—H…O hydrogen bonds between OH/H₂O ligands and a κ¹O‐terminal organophosphate ligand, which resembles, to some extent, the `free' [(2,6‐ⁱPr₂C₆H₃‐O)₂POO]⁻ anion in ( 2 ). NMR studies have shown that the formation of ( 1 ) undoubtedly occurs due to intramolecular hydrolysis during vacuum drying of the aqueous La tris(phosphate) complex. Catalytic experiments have demonstrated that the presence of the coordinated hydroxide anion and water molecules in precatalyst ( 2 ) substantially lowered the catalytic activity of the system prepared from ( 2 ) in butadiene and isoprene polymerization compared to the catalytic system based on the neodymium tris[bis(2,6‐diisopropylphenyl) phosphate] complex, which contains neither OH nor H₂O ligands.     

Metal‐Free Dihydrogen Oxidation by a Borenium Cation: A Combined Electrochemical/Frustrated Lewis Pair Approach

In order to use H₂ as a clean source of electricity, prohibitively rare and expensive precious metal electrocatalysts, such as Pt, are often used to overcome the large oxidative voltage required to convert H₂ into 2 H⁺ and 2 e^?. Herein, we report a metal‐free approach to catalyze the oxidation of H₂ by combining the ability of frustrated Lewis pairs (FLPs) to heterolytically cleave H₂ with the in situ electrochemical oxidation of the resulting borohydride. The use of the NHC‐stabilized borenium cation [(IiPr₂)(BC₈H₁₄)]⁺ (IiPr₂=C₃H₂(NiPr)₂, NHC=N‐heterocyclic carbene) as the Lewis acidic component of the FLP is shown to decrease the voltage required for H₂ oxidation by 910 mV at inexpensive carbon electrodes, a significant energy saving equivalent to 175.6 kJ mol^?1. The NHC–borenium Lewis acid also offers improved catalyst recyclability and chemical stability compared to B(C₆F₅)₃, the paradigm Lewis acid originally used to pioneer our combined electrochemical/frustrated Lewis pair approach.     

Synthesis and Structural Characterization of Magnesium‐Substituted Polystibides [(LMg)4Sb8]

Redox reactions of [(L^1,2Mg)₂] and Sb₂R₄ (R=Me, Et) yielded the first Mg‐substituted realgar‐type Sb₈ polystibides [(L^1,2Mg)₄(μ₄,η^2:2:2:2‐Sb₈)] (L¹=HC[C(Me)N(2,4,6‐Me₃C₆H₂)]₂, L²=HC[C(Me)N(2,6‐i‐Pr₂C₆H₃)]₂). Compounds [(L^1,2Mg)₂] serve both as reducing agents, initiating the cleavage of the Sb?C bonds, and as stabilizers for the resulting Sb₈ polyanion. The polystibides were characterized by NMR and IR spectroscopies, elemental analysis, and X‐ray structure analysis. In addition, results from quantum chemical calculations are presented.     

Transition‐Metal‐Mediated Cleavage of a SiSi Double Bond

Reaction of carbene‐stabilized disilicon ( 1 ) with Fe(CO)₅ gives the 1:1 adduct L:Si?Si[Fe(CO)₄]:L (L:=C{N(2,6‐Prⁱ₂C₆H₃)CH}₂) ( 2 ) at room temperature. At raised temperature, however, 2 may react with another equivalent of Fe(CO)₅ to give L:Si[μ‐Fe₂(CO)₆](μ‐CO)Si:L ( 3 ) through insertion of both CO and Fe₂(CO)₆ into the Si₂ core, which represents the first experimental realization of transition metal‐carbonyl‐mediated cleavage of a Si?Si double bond. The structures and bonding of both 2 and 3 have been investigated by spectroscopic, crystallographic, and computational methods.     

A Mixed‐Valence Tri‐Zinc Complex, [LZnZnZnL] (L=Bulky Amide), Bearing a Linear Chain of Two‐Coordinate Zinc Atoms

Reduction of a variety of extremely bulky amido Group 12 metal halide complexes, [LMX(THF)_0,1] (L=amide; M=Zn, Cd, or Hg; X=halide) with a magnesium(I) dimer gave a homologous series of two‐coordinate metal(I) dimers, [L′MML′] (L′=N(Ar^?)(SiMe₃), Ar^?=C₆H₂{C(H)Ph₂}₂Prⁱ‐2,6,4); and the formally zinc(0) complex, [L*ZnMg(^MesNacnac)] (L*=N(Ar*)(SiPrⁱ₃); Ar*=C₆H₂{C(H)Ph₂}₂Me‐2,6,4; ^MesNacnac=[(MesNCMe)₂CH]^?, Mes=mesityl), which contains the first unsupported Zn? Mg bond. Two equivalents of [L*ZnMg(^MesNacnac)] react with ZnBr₂ or ZnBr₂(tmeda) to give the mixed valence, two‐coordinate, linear tri‐zinc complex, [L*Zn^IZn⁰Zn^IL*], and the first zinc(I) halide complex, [L*ZnZnBr(tmeda)], respectively. The analogues [L*ZnMZnL*] (M=Cd or Hg), were also prepared, the Cd species contains the first Zn? Cd bond in a molecular compound. Metal–metal bonding was studied by DFT calculations.     

Pincer‐Type Heck Catalysts and Mechanisms Based on PdIV Intermediates: A Computational Study

Pincer‐type palladium complexes are among the most active Heck catalysts. Due to their exceptionally high thermal stability and the fact that they contain Pd^II centers, controversial Pd^II/Pd^IV cycles have been often proposed as potential catalytic mechanisms. However, pincer‐type Pd^IV intermediates have never been experimentally observed, and computational studies to support the proposed Pd^II/Pd^IV mechanisms with pincer‐type catalysts have never been carried out. In this computational study the feasibility of potential catalytic cycles involving Pd^IV intermediates was explored. Density functional calculations were performed on experimentally applied aminophosphine‐, phosphine‐, and phosphite‐based pincer‐type Heck catalysts with styrene and phenyl bromide as substrates and (E)‐stilbene as coupling product. The potential‐energy surfaces were calculated in dimethylformamide (DMF) as solvent and demonstrate that Pd^II/Pd^IV mechanisms are thermally accessible and thus a true alternative to formation of palladium nanoparticles. Initial reaction steps of the lowest energy path of the catalytic cycle of the Heck reaction include dissociation of the chloride ligands from the neutral pincer complexes [{2,6‐C₆H₃(XPR₂)₂}Pd(Cl)] [X=NH, R=piperidinyl ( 1 a ); X=O, R=piperidinyl ( 1 b ); X=O, R=iPr ( 1 c ); X=CH₂, R=iPr ( 1 d )] to yield cationic, three‐coordinate, T‐shaped 14e^? palladium intermediates of type [{2,6‐C₆H₃(XPR₂)₂}Pd]⁺ ( 2 ). An alternative reaction path to generate complexes of type 2 (relevant for electron‐poor pincer complexes) includes initial coordination of styrene to 1 to yield styrene adducts [{2,6‐C₆H₃(XPR₂)₂}Pd(Cl)(CH₂?CHPh)] ( 4 ) and consecutive dissociation of the chloride ligand to yield cationic square‐planar styrene complexes [{2,6‐C₆H₃(XPR₂)₂}Pd(CH₂?CHPh)]⁺ ( 6 ) and styrene. Cationic styrene adducts of type 6 were additionally found to be the resting states of the catalytic reaction. However, oxidative addition of phenyl bromide to 2 result in pentacoordinate Pd^IV complexes of type [{2,6‐C₆H₃(XPR₂)₂}Pd(Br)(C₆H₅)]⁺ ( 11 ), which subsequently coordinate styrene (in trans position relative to the phenyl unit of the pincer cores) to yield hexacoordinate phenyl styrene complexes [{2,6‐C₆H₃(XPR₂)₂}Pd(Br)(C₆H₅)(CH₂?CHPh)]⁺ ( 12 ). Migration of the phenyl ligand to the olefinic bond gives cationic, pentacoordinate phenylethenyl complexes [{2,6‐C₆H₃(XPR₂)₂}Pd(Br)(CHPhCH₂Ph)]⁺ ( 13 ). Subsequent β‐hydride elimination induces direct HBr liberation to yield cationic, square‐planar (E)‐stilbene complexes with general formula [{2,6‐C₆H₃(XPR₂)₂}Pd(CHPh?CHPh)]⁺ ( 14 ). Subsequent liberation of (E)‐stilbene closes the catalytic cycle.     

Homo‐ and Copolymerization of Butadiene Catalyzed by an Bis(imino)pyridyl Vanadium Complex

Summary: The bis(imino)pyridyl vanadium(III ) complex [VCl₃{2,6‐bis[(2,6‐iPr₂C₆H₃)NC(Me)]₂(C₅H₃N)}] activated with different aluminium cocatalysts (AlEt₂Cl, Al₂Et₃Cl₃, MAO) promotes chemoselective 1,4‐polymerization of butadiene with activity values higher than classical vanadium‐chloride‐based catalysts. The polymer structure depends on the nature of the cocatalyst employed. The MAO‐activated complex was also found to be active in ethylene‐butadiene copolymerization, producing copolymers with up to 45 mol‐% of trans‐1,4‐butadiene. Crystalline polyethylene and trans‐1,4‐poly(butadiene) segments were detected in these copolymers by DSC and ¹³C NMR spectroscopy.

     

Synthesis,Interionic Structure,and Reactivity toward CO and p‐Methylstyrene of Palladacyclic Compounds Bearing α‐Diimine Ligands

Palladacyclic compounds [Pd(C₆H₄(C₆H₅C?O)C?N? R)(N? N)] [X] (R = Et, ⁱPr, 2,6‐ⁱPr₂C₆H₃; N? N = bpy = 2,2′‐bipyridine, or 1,4‐(o,o′‐dialkylaryl)‐1,4‐diazabuta‐1,3‐dienes; [X]^? = [BF₄]^? or [PF₆]^?) were synthesized from the dimers [{Pd(C₆H₄(C₆H₅C?O)C?N? R)(μ‐Cl)}₂] and N? N ligands. Their interionic structure in CD₂Cl₂ was determined by means of ¹⁹F,¹H‐HOESY experiments and compared with that in the solid state derived from X‐ray single‐crystal studies. [Pd(C₆H₄(C₆H₅C?O)C?N? R)(N? N)] [X] complexes were found to copolymerize CO and p‐methylstyrene affording syndiotactic or isotactic copolymers when bpy or 1,4‐(o,o′‐dimethylaryl)‐1,4‐diazabuta‐1,3‐dienes were used, respectively. The reactions with CO and p‐methylstyrene of the bpy derivatives were investigated. Two intermediates derived from a single and a double insertion of CO into the Pd? C bonds were isolated and completely characterized in solution.     

Crystallographic report: Fluoro‐bis‐iso‐propyl‐(2,4,6‐tris‐iso‐propylphenyl)‐ silane,i‐Pr2(2,4,6‐i‐Pr3C6H2)SiF

Owing to steric congestion in i‐Pr₂(2,4,6‐i‐Pr₃C₆H₂)SiF, the geometry at the Si atom deviates slightly from ideal tetrahedral geometry with an increased C? Si? C angle of 119.02(9)° and elongated Si? C and Si? F bond distances. Copyright © 2005 John Wiley & Sons, Ltd.     

Molecular and Silica‐Supported Mo and W d0 Imido‐Methoxybenzylidene Complexes: Structure and Metathesis Activity

5‐Coordinated methoxybenzylidene complexes M(=NAr)(=CH?C₆H₄?o‐OMe)(O^tBu_F3)₂ (Ar=2,6‐ⁱPr₂C₆H₃; ^tBu_F3=CMe₂(CF₃)) of Mo ( 1m_Mo ) and W ( 1m_W ) were synthesized by cross‐metathesis from the corresponding neophylidene/neopentylidene precursors and o‐methoxystyrene. 1m_Mo and 1m_W were grafted onto the surface of silica partially dehydroxylated at 700 °C to give well‐defined silica‐supported alkylidenes (≡SiO)M(=NAr)(=CH?C₆H₄?o‐OMe)(O^tBu_F3) (M=Mo ( 1_Mo ), W ( 1_W )). Supported methoxybenzylidene complexes were tested in metathesis of cis‐4‐nonene, 1‐nonene, and ethyl oleate, and compared to their molecular precursors and supported classical analogs (≡SiO)M(=NAr)(=CHCMe₂R)(O^tBu_F3) (M=Mo, R=Ph ( 2_Mo ), M=W, R=Me ( 2_W )). Both grafted complexes 1_Mo and 1_W show significantly better performance as compared to their molecular precursors 1m_Mo and 1m_W but are less efficient than the classical 4‐coordinated alkylidenes 2_Mo and 2_W . Noteworthy, both 1_Mo and 1_W can reach equilibrium conversion in metathesis of cis‐4‐nonene at catalyst loadings as low as 50 ppm.