Allyl complexes of molybdenum with Schiff base ligands. The crystal structures of [MoCl(CO)2{N(C6H4-2-OMe)C(Me)C5H4N}(η-C3H5)] and [MoCl(CO)2{N(Me)C(Ph)C5H4N}(η-C3H5)] are described

Treatment of the molybdenum tetracarbonyl complexes of [Mo(CO)₄L₂] (L₂=pyridyl amine Schiff base ligands) with allyl chloride in refluxing THF afforded η³-allyl complexes [MoCl(CO)₂L₂(η³-allyl)] (1-9). These complexes have been characterised by various techniques including ¹H-NMR, IR and FABMS spectroscopies and the single crystal X-ray structure determinations of the complexes [MoCl(CO)₂{N(C₆H₄-2-OMe)C(Me)C₅H₄N}(η³-C₃H₅)] (3) and [MoCl(CO)₂{N(Me)C(Ph)C₅H₄N}(η³-C₃H₅)] (4).     

HMB and Cp* ruthenium(II) complexes containing bis- and tris-(mercaptomethimazolyl)borate ligands: Synthetic, X-ray structural and electrochemical studies (HMB = η-C6Me6, Cp* = η-C5Me5)

The reactions of [(HMB)RuCl₂]₂ with K[HB(mt)₃] and Na[H₂B(mt)₂] (mt = N-methyl-2-mercaptoimidazol-1-yl) led to the isolation of [(HMB)Ru{HB(mt)₃}]Cl (1) (ca. 66% yield) and [(HMB)Ru{H₂B(mt)₂}]Cl (2) (ca. 70% yield), respectively. The reaction of [Cp*RuOMe]₂ with Na[H₂B(mt)₂] yielded Cp*Ru[H₂B(mt)₂] (3) (ca. 85% yield). Single crystal X-ray diffraction analyses were carried out on all three complexes, together with cyclic voltammetric measurements.     

Atom-transfer radical reactions catalyzed by a coordinatively unsaturated diruthenium amidinate, [(η-C5Me5)Ru(μ2-i-PrNC(Me)Ni-Pr)Ru(η-C5Me5)]

Atom-transfer radical cyclization (ATRC) and addition (ATRA) catalyzed by a coordinatively unsaturated diruthenium amidinate complex 4, [(η⁵-C₅Me₅)Ru(μ₂-i-PrNC(Me)Ni-Pr)Ru(η⁵-C₅Me₅)]⁺, are investigated, and their features are compared with those of atom-transfer radical polymerization (ATRP). As an example of ATRC, a cationic diruthenium amidinate 4 is found to exhibit excellent catalytic reactivity for the cyclization of N-allyl α-halogenated acetamides including an alkaloid skeleton at ambient temperature. A catalytic species generated in situ from a halide complex, (η⁵-C₅Me₅)Ru(μ₂-i-PrNC(Me)Ni-Pr)Ru(η⁵-C₅Me₅)(X) [X=Cl, Br] and sodium salts of weakly coordinating anions such as NaPF₆ and NaBPh₄ also shows high catalytic activity; this actually provides a solution for a problematic instability of 4 as the practical catalyst. The in situ-generated catalyst species 4 is also active towards the intermolecular ATRA of α,α,γ-trichlorinated γ-lactam with alkenes at rt to afford the corresponding α-alkylated γ-lactams in moderate yields. Examination of ATRP of methyl methacrylate (MMA) showed that both the isolated 4 [Y=PF₆] and in situ-generated 4 [Y=PF₆] are effective for the polymerization of MMA in the presence of 2-bromoisobutylate as the initiator. Use of the isolated catalyst results in controlled polymerization at initial stage of the reaction; in contrast, the polymerization with in situ-generated catalyst produces poly(MMA) with wide molecular weight distribution. The isolated catalyst 4 is powerful for the activation of a C-Br bond of macromolecule initiators; BrCMe₂CO₂[O(CH₂)₄]_n-n-Bu (M_n=3800; M_w/M_n=1.2) initiated ATRP of MMA even at 25 °C to afford the poly(THF)-poly(MMA) block copolymer of M_n=26,000 and M_w/M_n=1.2 with the aid of 4. The roles of the coordinatively unsaturated ruthenium species for these reactions are discussed.     

New ferrocenylmethylphosphines - Preparation, characterisation and coordination chemistry of PH(CH2Fc)2, P(CH2Fc)3 [Fc = Fe(η-C5H5)(η-C5H4)] and their derivatives

The new ferrocenylmethylphosphines PH(CH₂Fc)₂ (1) [Fc = Fe(η⁵-C₅H₅)(η⁵-C₅H₄)] and P(CH₂Fc)₃ (2) and the phosphonium salt [P(CH₂Fc)₃(CH₂OH)]I (3) were synthesised from P(CH₂OH)₃ and [FcCH₂NMe₃]I. [P(CH₂Fc)(CH₂OH)₃]Cl (4) was obtained from P(CH₂Fc)(CH₂OH)₂, CH₂O and HCl. The new phosphines and phosphonium salts were fully characterised by NMR and IR spectroscopy and MS. [Mo(CO)₆] reacts with 1 to give [Mo(CO)₅{PH(CH₂Fc)₂}] (5) in high yield, but attempts to employ 2 as a ligand failed. The reaction of [P(CH₂Fc)₃(CH₂OH)]I (3) and [PH(CH₂Fc)₃]I (obtained in situ from 3 and Na₂S₂O₅) with [WI₂(CO)₃(NCMe)₂] gave the complex salts [P(CH₂Fc)₃(CH₂OH)][WI₃(CO)₄] (6) and [PH(CH₂Fc)₃][WI₃(CO)₄] (7), respectively. [P(CH₂Fc)₄]I (8) was synthesized from PH₂CH₂Fc and [FcCH₂NMe₃]I. Crystal structures were obtained for 1, 3-8.     

Protonation of the lithio derivatives of the 1,2,4-tri-phosphaferrocenes, [LiFe(η-P2C2Bu2PBu)(η-C5R5)] (R = H, Me): Crystal and molecular structure of [Fe(η-P3C2Bu2BuH)(η-C5Me5)]

A new synthetic route is described to generate the 4-centre-5 electron donor ring system (P₃C₂^tBu₂BuH), via protonation of the lithium salts [LiFe(η⁴-P₂C₂^tBu₂PBu)(η⁵-C₅R₅)] (R = H, Me). The molecular structure of [Fe(η⁴-P₃C₂^tBu₂BuH)(η⁵-C₅R₅)] (R = Me) has been determined by a single crystal X-ray study.     

Synthesis and reactivity of asymmetrically substituted ansa-bridged zirconocene complexes: X-ray crystal structures of [Zr{R(H)C(η-C5Me4)(η-C5H4)}Cl2] (R = Bu, Bu) and [Zr{Bu(H)C(η-C5Me4)(η-C5H4)}(CH2Ph)2]

The dialkyl complexes, (R = Prⁱ, R′ = Me (2a), CH₂Ph (3a); R = Buⁿ, R′ = Me (2b), CH₂Ph (3b); R = Bu^t, R′ = Me (2c), CH₂Ph (3c); R = Ph, R′ = Me (2d), CH₂Ph (3d)), have been synthesized by the reaction of the ansa-metallocene dichloride complex, [Zr{R(H)C(η⁵-C₅Me₄)(η⁵-C₅H₄)}Cl₂] (R = Prⁱ (1a), Buⁿ (1b), Bu^t (1c), Ph (1d)), and two molar equivalents of the alkyl Gringard reagent. The insertion reaction of the isocyanide reagent, CNC₆H₃Me₂-2,6, into the zirconium-carbon σ-bond of 2 gave the corresponding η²-iminoacyl derivatives, [Zr{R(H)C(η⁵-C₅Me₄)(η⁵-C₅H₄)}{η²-MeCNC₆H₃Me₂-2,6}Me] (R = Prⁱ (4a), Buⁿ (4b), Bu^t (4c), Ph (4d)). The molecular structures of 1b, 1c and 3b have been determined by single-crystal X-ray diffraction studies.     

A synthetic, structural and reactivity study of [(η-C4R4)Co(η-C5H4X)] complexes, R = Me or Et; X = CHO, CHCHFc, CHCH(η-C5H4)Co(η-C4Ph4), CHC(CN)2: Unexpected formation of [(η-cyclopentadienyl)(η-3,4,5,6-tetraethyl-α-pyrone)cobalt]

The tetraethyl- and tetramethyl-cyclobutadiene complexes [(η⁴-C₄R₄)Co(η⁵-C₅H₄CHO)] R = Et, 5, R = Me, 7, and [(η⁴-C₄R₄)Co(η⁵-C₅H₄CO₂Me)] R = Et, 6, R = Me, 8, are conveniently prepared by photolysis of the corresponding isocobaltocenium cations [(η⁴-C₄R₄)Co(η⁶-C₆H₅Me)]⁺ in acetonitrile, and subsequent treatment with Na[C₅H₄CHO] or Na[C₅H₄CO₂Me]. The aldehydes 5 and 7 undergo Wittig and Knoevenagel reactions with [FcCH₂PPh₃]I and CH₂(CN)₂, to form [(η⁴-C₄R₄)Co(η⁵-C₅H₄CH=CHFc)] and [(η⁴-C₄R₄)Co(η⁵-C₅H₄CH=C(CN)₂], 11 and 15, respectively. The Horner-Wittig reaction of [(η⁴-C₄R₄)Co(η⁵-C₅H₄CH₂P(O)(OEt)₂] with [(η⁴-C₄Ph₄)Co(η⁵-C₅H₄CHO)] yields [(η⁴-C₄R₄)Co(η⁵:η⁵-C₅H₄CHCH-C₅H₄)Co(η⁴-C₄Ph₄)], 12 and 13. [(η⁴-C₄Me₄)Co(η⁵-C₅H₄CHO)] also reacts with t-BuLi and FcLi to furnish the corresponding secondary alcohols, 16 and 17, respectively. Surprisingly, the attempted direct synthesis of 5 by reaction of Na[C₅H₅] and ethyl formate with [(η⁴-C₄Et₄)Co(CO)₂I], 1, instead yielded [(η⁵-C₅H₅)Co(η⁴-3,4,5,6-tetraethyl-α-pyrone)], 18, and a mechanistic proposal is advanced. The X-ray crystal structures of 1, 7, 8, 11(Z), 15 and 18, and also the isocobaltocenium salts [(η⁴-C₄Et₄)Co(η⁶-C₆H₅Me)][PF₆], 2, and [(η⁴-C₄Et₄)Co(η⁶-1,3,5-C₆H₃Me₃)][PF₆], 4, are reported.     

Cyclopentadienyl chromium and tungsten complexes with halide, methyl and σ-phenylethynyl ligands: Structures of (η-C5H5)Cr(NO)2(-CC-C6H5), (η-C5H5)Cr(NO)2I and [(η-C5H4)-COOCH3]W(CO)3Cl

With copper(I) iodide as catalyst, σ-alkynyls, compounds (η⁵-C₅H₅)Cr(NO)₂(CC-C₆H₅) (5), [(η⁵-C₅H₄)-COOCH₃]Cr(NO)₂(CC-C₆H₅) (10), and [(η⁵-C₅H₄)-COOCH₃]W(CO)₃(CC-C₆H₅) (13), were prepared from their corresponding metal chloride 1, 6 and 12. Structures of compound 3, 5 and 12 have been solved by X-ray diffraction studies. In the case of 5, there is an internal mirror plane passing through the phenylethynyl ligand and bisecting the Cp ring. The phenyl group is oriented perpendicularly to the Cp with an eclipsed conformation. The twist angle is 0° and 118.4° for -CC-Ph and two NO ligands, respectively. The orientation is rationalized in terms of orbital overlap between ψ₃ of Cp, dπ of Cr atom, and π^∗ of alkynyl ligand, and complemented by molecular orbital calculation. The opposite correlation was observed on the chemical shift assignments of C(2)-C(5) on Cp ring in compounds 6 and 12, using HetCOR NMR spectroscopy. The electron density distribution in the cyclopentadienyl ring is discussed on the basis of ¹³C NMR data and compared with the calculations via density functional B3LYP correlation-exchange method.     

Versatility in the mode of coordination {(N), (N,O), (C,N) or (C,N,O)} of [(η-C5H5)Fe{(η-C5H4)-CHN-(C6H4-2OH)}] to palladium(II)

The study of the reactivity of the ferrocenyliminoalcohol [(η⁵-C₅H₅)Fe{(η⁵-C₅H₄)-CHN-(C₆H₄-2OH)}] (1b) with Na₂[PdCl₄] or Pd(OAc)₂ has allowed the isolation and characterization of the heterotrimetallic complexes: trans-[Pd{(η⁵-C₅H₅)Fe[(η⁵-C₅H₄)-CHN-(C₆H₄-2OH)]}₂Cl₂] (2b), [Pd{[(η⁵-C₅H₃)-CHN-(C₆H₄-2O)]Fe(η⁵-C₅H₅)}{(η⁵-C₅H₅)Fe[(η⁵-C₅H₄)-CHN-(C₆H₄-2OH)]}] (3b) and trans-[Pd{(η⁵-C₅H₅)Fe[(η⁵-C₅H₄)-CHN-(C₆H₄-2O)]}₂] (4b). Ligand 1b acts as a (N) (in 2b) or a (N,O)⁻ (in 4b) ligand; while in 3b the two units of the iminoalcohol exhibit simultaneously different modes of binding {(N) and [C(sp², ferrocene),N,O]²⁻}. The crystal structures of 2b · 3H₂O and 3b · 1/2CHCl₃ are also reported and confirm the mode of binding of the ligand in these compounds. The relative importance of the factors affecting the preferential formation of products (2b-4b) is also discussed. The study of the reactivity of 3b with PPh₃ has enabled the obtention of the cyclopalladated complexes [Pd{[(η⁵-C₅H₃)-CHN-(C₆H₄-2O)]Fe(η⁵- C₅H₅)}(PPh₃)] (6b) and [Pd{[(η⁵-C₅H₃)-CHN-(C₆H₄-2OH)]Fe(η⁵-C₅H₅)}Cl(PPh₃)] (7b), in which 1b behaves as a [C(sp², ferrocene),N,O]²⁻ (in 6b) or [C(sp², ferrocene),N]⁻ (in 7b) ligand. Treatment of 3b with MeO₂C-CC-CO₂Me produces [Pd{[(MeO₂C-CC-CO₂Me)₂(η⁵-C₅H₃)-CHN-(C₆H₄-2O)]Fe(η⁵-C₅H₅)}] (8b), that arises from the bis(insertion) of the alkyne into the σ[Pd-C(sp², ferrocene)] bond. The comparison of the results obtained for 1b and [C₆H₅-CHN-(C₆H₄-2OH)] (1a) has allowed to establish the influence of the substituents on the imine carbon on their reactivity in front of palladium(II) as well as on the lability of the Pd-ligands bond. ⁵⁷Fe Mössbauer studies of 2b-4b and 6b provide conclusive evidence of the effect induced by the mode of binding of 1b on the environment of the iron(II).     

Syntheses, structures, and dynamic properties of M(CO)2(η-C3H5)(en)(X) (M = Mo, W; X = Br, N3, CN) and [(en)(η-C3H5)(CO)2M(μ-CN)M(CO)2(η-C3H5)(en)]Br (M = Mo, W)

Mononuclear compounds M(CO)₂(η³-C₃H₅)(en)(X) (X = Br, M = Mo(1), W(2); X = N₃, M = Mo(3), W(4); X = CN, M = Mo(5), W(6)) and cyanide-bridged bimetallic compounds [(en)(η³-C₃H₅)(CO)₂M(μ-CN)M(CO)₂(η³-C₃H₅)(en)]Br (M = Mo (7), W(8)) were prepared and characterized. These compounds are fluxional and display broad unresolved proton NMR signals at room temperature. Compounds 1-6 were characterized by NMR spectroscopy at −60 °C, which revealed isomers in solution. The major isomers of 1-4 adopt an asymmetric endo-conformation, while those of 5 and 6 were both found to possess a symmetric endo-conformation. The single crystal X-ray structures of 1-6 are consistent with the structures of the major isomer in solution at low temperature. In contrast to mononuclear terminal cyanide compounds 5 and 6, cyanide-bridged compounds 7 and 8 were found to adopt the asymmetric endo-conformation in the solid state.     

Facile synthesis and X-ray structures of (η-C5Me5)Ti(OAr)3 (OAr = OC6F5, OCH2C6F5, and OCH2C6F2H3)

New half-sandwich titanocene complexes (η⁵-C₅Me₅)Ti(OC₆F₅)₃ (1), (η⁵-C₅Me₅)Ti(OCH₂C₆F₅)₃ (2), and (η⁵-C₅Me₅)Ti(OCH₂C₆F₂H₃)₃ (3) were synthesized via the displacement of methoxide ligands in (η⁵-C₅Me₅)Ti(OMe)₃ by the corresponding aryloxy or benzyloxy ligands. These compounds have been fully characterized by various spectroscopic methods including X-ray crystallography. Compound 1 has a distorted three-legged piano stool structure. However, complexes 2 and 3 have the chariot-like structure, where chariot means a two-wheeled horse-drawn vehicle. The π electron donation of oxygen atom to Ti center in complexes 1-3 is considerable.     

Reactions of diferrocenyl dichalcogenides with [W(CO)5(THF)]: X-ray crystal structures of Fc2Te2 and [W2(μ-SeFc)2(CO)8] (Fc = [Fe(η-C5H5)(η-C5H4)])

Treatment of [W(CO)₅THF] with diferrocenyl diselenide, Fc₂Se₂, yielded the novel metal-metal bonded tungsten(I) complex, [W₂(μ-SeFc)₂(CO)₈] (1: Fc = ferrocenyl, [Fe(η⁵-C₅H₅)(η⁵-C₅H₄)]), which was characterised by NMR and IR spectroscopy, mass spectrometry, and X-ray crystallography. The corresponding tellurium derivative could not be prepared by an analogous route. The X-ray crystal structure of Fc₂Te₂ has also been determined.     

Hydrogenation of cyclohexene catalyzed by ruthenium nitrosyl complexes: Crystal structures of catalyst precursors [CpRu(μ2-NO)2RuCp] and [CpRu(NO)(η-C6H10)] (Cp = η-C5(CH3)5)

Hydrogenation of cyclohexene with 0.1 mol% of the (nitrosyl)ruthenium catalyst [Cp^∗Ru(NO)(C₆H₅)₂] (1; Cp^∗ = η⁵-C₅(CH₃)₅) under 1.0 MPa of H₂ in water at 90 °C for 13 h afforded cyclohexane in 94% yield. The nitrosyl-bridged dinuclear complex [Cp^∗Ru(μ₂-NO)₂RuCp^∗] (2) and the mononuclear cyclohexene complex [Cp^∗Ru(NO)(η²-C₆H₁₀)] (3), which also serve as catalyst precursors, have been obtained from the reaction mixture. X-ray crystallographic analyses of 2 and 3 have revealed that the bridging nitrosyl ligands in 2 form an almost planar Ru₂N₂ four-membered ring with the Ru–Ru distance of 2.5366(5) Å, whereas the nitrosyl ligand in 3 is linear. On the other hand, a ruthenium complex without a nitrosyl ligand [Cp^∗Ru(CH₃CN)₃][OSO₂CF₃] proved to be less effective for this hydrogenation.     

Synthesis and structure of the monometallic cationic complex [Cp(CO)2Fe{η-(CH2CHCH2CH3)}]PF6 (Cp = η-C5H5)

The bimetallic carbocation complex [{Cp(CO)₂Fe}₂(μ-C₄H₇)]PF₆ reacted with trifluoroacetic acid to give the mononuclear cationic complex [Cp(CO)₂Fe{η²-(CH₂CHCH₂CH₃)}]PF₆, which formed yellow orthorhombic crystals in the space group P2₁2₁2₁ with a = 7.652(4), b = 13.422(7), c = 14.037(7); α = β = γ = 90.00 and Z = 4. The carbocation is coordinated to the metal in a η²-fashion forming a chiral metallacyclopropane type structure. The β-CH carbon (C9) is disordered over two positions (C9A and C9B), each having about 50% occupancy. This is attributed to there being both the R and S enantioface isomers in equal amounts in the crystal sample. NMR data indicate that the metallacyclopropane structure observed in the solid state is preserved in solution.     

Intermolecular versus intramolecular C-H activation reaction in the thermolysis of [Ru(Me)Cp*(PMe2Ph)2] (Cp* = η-C5Me5): formation and crystallographic characterisation of [Ru(Ph)Cp*(PMe2Ph)2]

Thermolysis of the ruthenium complex [Ru(Me)Cp*(PMe₂Ph)₂] (1) (Cp* = η⁵-C₅Me₅) in benzene gives methane and [Ru(Ph)Cp*(PMe₂Ph)₂] (2), which is converted slowly to (3) through the loss of benzene. 2 was structurally characterised by single-crystal X-ray diffraction experiments. DFT calculations were performed in order to understand the behaviour of the ruthenium complex 1 towards inter- or intra-molecular C-H bond activation reactions.     

Transformation of C2 units on a bimetallic tetranuclear cluster.: Reactivity of [Ru3Mo(μ3-η-CC)(μ-CO)3(CO)2(η-C5H4R)3(η-C5H5)] (R = H, Me)

The reactivity of [Ru₃Mo(μ₄-η²-CC)(μ-CO)₃(CO)₂(η-C₅H₄R)₃(η-C₅H₅)] (R = H; Me) have been investigated, initially to elucidate the nature of the starting material, and, latterly, to define the reactivity of an interesting ethane-1,2-bis(ylidyne) species. While the mixed RuMo clusters were unreactive towards simple electrophiles and carbonyl substitution by phosphine ligands they did react with atmospheric oxygen or carbon monoxide to give substantially different products. In all instances oxygen was incorporated either at the metal centre or at the C₂ fragment. High-pressure carbonylations yielded [Ru₃(μ-CO)₃(η-C₅H₅)₃(μ₃-C-C(O)O{Ru(CO)₂(η-C₅H₅)})] and [{Ru₂(μ-CO)(CO)₂(η-C₅H₄Me)₂}(μ₄-η²-CC){Ru(CO)(η-C₅H₄Me)Mo(η-C₅H₅)(=O)(μ-O)}], an ethane-1,2-bis(ylidene) complex, this exemplifying a relatively rare raft geometry which further reacted with Cl₂CCCl₂ to give [Mo₃(μ₄-C₂(Ru(CO)₂(η-C₅H₄Me))(CO)(μ-CO)(η-C₅H₅)₃(Cl)₂] having a similar geometry and undergone halogenation. In order to extend the extant examples of these raft clusters we explored the reaction of [{Ru(CO)₂(η-C₅H₄R)₂}₂(μ-C₂)] with [{Ru(CO)₂(η-C₅H₅)₂}₂] to provide a rational synthetic pathway leading to very reactive [Ru(μ₄-η²-CC)(μ₂-CO)₂(CO)₄(η-C₅H₄Me)₂(η-C₅H₄R)₂] rafts.     

Synthesis and properties of carboxy-substituted half-sandwich ruthenium complexes with chelating bisphosphine ligands (η-C5H4CO2H)Ru(η-L)X (X = I, H)

Several Ru(II) complexes (η⁵-C₅H₄CO₂H)Ru(η²-L)I have been prepared by the hydrolysis of the ester linkage in (η⁵-C₅H₄CO₂t-Bu)Ru(η²-L)Cl with trimethylsilyl iodide. The hydrides (η⁵-C₅H₄CO₂H)Ru(η²-L)H may be prepared by reduction of the iodide complexes in KOH/MeOH solutions followed by acidification. Complexes with several chelating bisphosphine ligands have been prepared in this way. The carboxylate anions [(η⁵-C₅H₄CO₂)Ru(η²-L)H]⁻ are readily protonated by weak acids to give the carboxyCp complexes. The pK_a of the carboxy proton of (η⁵-C₅H₄CO₂H)Ru(dppe)H (dppe = 1,2-bis(diphenylphosphino)ethane) is 11.3 in DMSO. Protonation of the neutral hydride complex (η⁵-C₅H₄CO₂H)Ru(dppf)H gives the cationic dihydride (η⁵-C₅H₄CO₂H)Ru(dppf)H⁺₂; the dihydride structure has been confirmed by measuring the T₁ of its ¹H NMR hydride resonance over a range of temperatures. The oxidations of the halide complexes (η⁵-C₅H₄CO₂H)Ru(dppf)I and (η⁵-C₅H₄CO₂t-Bu)Ru(dppf)Cl (dppf = 1,1′-bis(diphenylphosphino)ferrocene) have been studied by cyclic voltammetry.     

Unsaturation in binuclear cyclopentadienylchromium carbonyl thiocarbonyls: Viability of (η-C5H5)2Cr2(CS)2(CO)3 in contrast to (η-C5H5)2Cr2(CO)5

The cyclopentadienylchromium carbonyl thiocarbonyls Cp₂Cr₂(CS)₂(CO)_n (n = 4, 3, 2, 1) have been studied by density functional theory using the B3LYP and BP86 functionals. The lowest energy Cp₂Cr₂(CS)₂(CO)₄ structure can be derived from the experimentally characterized unbridged Cp₂Cr₂(CO)₆ structure by replacing the two terminal carbonyl groups furthest from the Cr-Cr bond with two terminal CS groups. The two lowest energy Cp₂Cr₂(CS)₂(CO)₃ structures have a single four-electron donor η²-μ-CS group and a formal Cr-Cr single bond of length ∼3.1 Å. In contrast to the carbonyl analogue Cp₂Cr₂(CO)₅ these Cp₂Cr₂(CS)₂(CO)₃ structures are viable with respect to disproportionation into Cp₂Cr₂(CS)₂(CO)₄ and Cp₂Cr₂(CS)₂(CO)₂ and thus are promising synthetic targets. The lowest energy Cp₂Cr₂(CS)₂(CO)₂ structures have all two-electron donor CO and CS groups and short CrCr distances around ∼2.3 Å suggesting the formal triple bonds required to give the chromium atoms the favored 18-electron configurations. These Cp₂Cr₂(CS)₂(CO)₂ structures are closely related to the known structure for Cp₂Cr₂(CO)₄. In addition, several doubly bridged structures with four-electron donor η²-μ-CS bridges are found for Cp₂Cr₂(CS)₂(CO)₂ at higher energies. The global minimum Cp₂Cr₂(CS)₂(CO) structure is a triply bridged triplet with a CrCr triple bond (2.299 Å by BP86). A higher energy singlet Cp₂Cr₂(CS)₂(CO) structure has a shorter Cr-Cr distance of 2.197 Å (BP86) suggesting the formal quadruple bond required to give each chromium atom the favored 18-electron configuration.     

Synthesis, characterization and catalytic behaviour of ansa-zirconocene complexes containing tetraphenylcyclopentadienyl rings: X-ray crystal structures of [Zr{Me2Si(η-C5Ph4)(η-C5H3R)}Cl2] (R = H, Bu)

The ansa-bis(cyclopentadiene) compounds, Me₂Si(C₅HPh₄)(C₅H₄R) (R = H (2); Bu^t (3)), have been prepared by the reaction of C₅HPh₄(SiMe₂Cl) (1) with Na(C₅H₅) or Li(C₅H₄Bu^t), respectively, and transformed to the di-lithium derivatives, Li₂{Me₂Si(C₅Ph₄)(C₅H₃R)} (R = H (4); Bu^t (5)), by the action of n-butyllithium. The ansa-zirconocene complexes, [Zr{Me₂Si(η⁵-C₅Ph₄)(η⁵-C₅H₃R)}Cl₂] (R = H (6); Bu^t (7)), were synthesized from the reaction of ZrCl₄ with 4 or 5, respectively. Compounds 6 and 7 have been tested in the polymerization of ethylene and compared with their methyl-substituted analogues, [Zr{Me₂Si(η⁵-C₅Me₄)(η⁵-C₅H₃R)}Cl₂] (R = H (8); Bu^t (9)). Whilst 8 and 9 are catalytically active, the tetraphenyl-substituted complexes 6 and 7 proved to be inactive in the polymerization of ethylene. This phenomenon has been explained by DFT calculations based on the reaction intermediates in the polymerization processes involving 6 and 7, which showed that the extraction of a methyl group from the zirconocene complex to form the cationic active specie is endothermic and therefore unfavourable.