Dialkyl Complexes Research Articles

Iron- and Cobalt-Catalyzed Alkene Hydrogenation: Catalysis with Both Redox-Active and Strong Field Ligands.

The hydrogenation of alkenes is one of the most impactful reactions catalyzed by homogeneous transition metal complexes finding application in the pharmaceutical, agrochemical, and commodity chemical industries. For decades, catalyst technology has relied on precious metal catalysts supported by strong field ligands to enable highly predictable two-electron redox chemistry that constitutes key bond breaking and forming steps during turnover. Alternative catalysts based on earth abundant transition metals such as iron and cobalt not only offer potential environmental and economic advantages but also provide an opportunity to explore catalysis in a new chemical space. The kinetically and thermodynamically accessible oxidation and spin states may enable new mechanistic pathways, unique substrate scope, or altogether new reactivity. This Account describes my group's efforts over the past decade to develop iron and cobalt catalysts for alkene hydrogenation. Particular emphasis is devoted to the interplay of the electronic structure of the base metal compounds and their catalytic performance. First generation, aryl-substituted pyridine(diimine) iron dinitrogen catalysts exhibited high turnover frequencies at low catalyst loadings and hydrogen pressures for the hydrogenation of unactivated terminal and disubstituted alkenes. Exploration of structure-reactivity relationships established smaller aryl substituents and more electron donating ligands resulted in improved performance. Second generation iron and cobalt catalysts where the imine donors were replaced by N-heterocyclic carbenes resulted in dramatically improved activity and enabled hydrogenation of more challenging unactivated, tri- and tetrasubstituted alkenes. Optimized cobalt catalysts have been discovered that are among the most active homogeneous hydrogenation catalysts known. Synthesis of enantiopure, C1 symmetric pyridine(diimine) cobalt complexes have enabled rare examples of highly enantioselective hydrogenation of a family of substituted styrene derivatives. Because improved hydrogenation performance was observed with more electron rich supporting ligands, phosphine cobalt(II) dialkyl complexes were synthesized and found to be active for the diastereoselective hydrogenation of various substituted alkenes. Notably, this class of catalysts was activated by hydroxyl functionality, representing a significant advance in the functional group tolerance of base metal hydrogenation catalysts. Through collaboration with Merck, enantioselective variants of these catalysts were discovered by high throughput experimentation. Catalysts for the hydrogenation of functionalized and essentially unfunctionalized alkenes have been discovered using this approach. Development of reliable, readily accessible cobalt precursors facilitated catalyst discovery and may, along with lessons learned from electronic structure studies, provide fundamental design principles for catalysis with earth abundant transition metals beyond alkene hydrogenation.

Synthesis and Characterization of Organo-Rare-Earth Metal Monoalkyl Complexes Supported by Carbon σ-Bonded Indolyl Ligands: High Specific Isoprene 1,4-Cis Polymerization Catalysts

A series of N-protected 3-imino-functionalized indolyl ligands 1-R-3-(R'N═CH)C8H5N [R = Bn, R' = 2,6-(i)Pr2C6H3 (HL(1)); R = CH3, R' = 2,6-(i)Pr2C6H3 (HL(2)); R = Bn, R' = (t)Bu (HL(3))] and 1-CH3-2-(2,6-(i)Pr2C6H3N═CH)C8H5N (HL(4)) was prepared via reactions of N-protected indolyl aldehydes with corresponding amines. The C-H σ-bond metathesis followed by alkane elimination reactions between RE(CH2SiMe3)3(thf)2 and HL(1)-HL(3) afforded the carbon σ-bonded indolyl-ligated rare-earth metal monoalkyl complexes. Reactions of RE(CH2SiMe3)3(thf)2 with 2 equiv of HL(1) or HL(2) gave the carbon σ-bonded indolyl-ligated rare-earth metal monoalkyl complexes L(1)2RECH2SiMe3 (RE = Y(1), Er(2), Dy(3)) and L(2)2RECH2SiMe3 (RE = Y(5), Er(6), Dy(7), Yb(8)), while reaction of Yb(CH2SiMe3)3(thf)2 with 2 equiv of HL(1) afforded the ytterbium dialkyl complex L(1)Yb(CH2SiMe3)2(thf)2 (4). Reactions of RE(CH2SiMe3)3(thf)2 with HL(3) gave the tris(heteroaryl) rare-earth metal complexes L(3)3RE (RE = Y(9), Er(10)). In the presence of cocatalysts, the rare-earth metal monoalkyl complexes initiated isoprene polymerization with a high activity (90% conversion of 1000 equiv of isoprene in 25 min) producing polymers with high regio- and stereoselectivity (1,4-cis polymers up to 99%).

C–H Bond Activations by Monoanionic, PNP-Supported Scandium Dialkyl Complexes

A series of scandium dialkyl complexes, (PNP)ScR2 (R = neopentyl, trimethylsilylmethyl), supported by the monoanionic, chelating PNP ligand (2,5-bis(dialkylphosphinomethyl)pyrrolide; alkyl = cyclohexyl, tert-butyl) was synthesized, and the reactivities of these complexes toward simple hydrocarbons was investigated. The scandium–carbon bonds undergo σ-bond metathesis reactions with hydrogen, and these complexes are catalysts for the hydrogenation of alkenes. Reactions with primary amines led to formation of amido complexes that undergo cyclometalation via σ-bond metathesis, without involvement of an imido complex intermediate. A variety of carbon–hydrogen bonds are also activated, including sp-, sp2-, and sp3-C–H bonds (intramolecularly in the latter case).

Phosphinimino-amino supported complex: Synthesis, polymerization of ethylene and dearomatisation of pyridine

Alkane elimination reaction of phosphinimino-amine ligand HL (L = (2,6-iPr2-C6H3)NC(Me)CHP(Cy2)N(2,6-Me2-C6H3)) with MgnBu2 and Sc(CH2SiMe3)3(THF)2, respectively, can afford the corresponding phosphinimino-amine supported magnesium monoalkyl complex LMgnBu(THF) (1) and scandium dialkyl complex LSc(CH2SiMe3)2 (2). 2 can initiate the polymerization of ethylene efficiently under the activation of [Ph3C][B(C6F5)4]. Magnesium hydride complex [LMgH]2 (3) can be easily obtained by the reaction of 1 and phenylsilane. The hydride species in 3 can conduct the dearomatisation of pyridine to give dearomatisation product 1,2-dihydropyridide (4).

1,4-Selective Polymerization of 1,3-Cyclohexadiene and Copolymerization with Styrene by Cationic Half-Sandwich Fluorenyl Rare Earth Metal Alkyl Catalysts

The regioselective coordination–insertion polymerization of 1,3-cyclohexadiene (CHD) and copolymerization with styrene (S) could be achieved by cationic half-sandwich fluorenyl rare earth metal alkyl catalysts generated by treating half-sandwich fluorenyl rare earth metal dialkyl complexes Flu′Ln(CH2SiMe3)2(THF)n (1–10) with an activator (such as [Ph3C][B(C6F5)4] (A), [PhNHMe2][B(C6F5)4] (B), or B(C6F5)3 (C)) and AliBu3. The homopolymerization of CHD afforded poly(CHD)s with complete 1,4 selectivity (1,4 selectivity up to 100%). The copolymerization of CHD with styrene gave new random CHD–S copolymers with CHD content ranging from 22 to 74 mol % containing 1,4-linked CHD–CHD, alternating CHD–S, and syndiotactic S–S sequences unavailable previously. The activity of the copolymerization and the comonomer compositions and sequences of the resulting CHD–S copolymers could be easily controlled by changing the substituted fluorenyl ligand, the metal center, the activator, the temperature, and the molar ratio of...

Synthesis of Alkyl and Alkylidene Complexes of Tungsten Bearing Imido and Redox-Active α-Diimine or o-Iminoquinone Ligands and Their Application as Catalysts for Ring-Opening Metathesis Polymerization of Norbornene

We report a new strategy to synthesize tungsten imido complexes bearing bidentate redox-active ligands through reduction of high-valent tungsten imido complexes by 1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene (abbreviated MBTCD) without forming any metal salt waste. Reaction of W(═NC6H3-2,6-iPr2)Cl4 and MBTCD in the presence of redox-active ligands, such as α-diimine and o-iminoquinone, produced tungsten imido complexes with the corresponding redox-active ligands, (α-diimine)W(═NC6H3-2,6-iPr2)Cl2 (1), [(o-iminoquinone)W(═NC6H3-2,6-iPr2)Cl]2(μ-Cl)2 (3), and (o-iminoquinone)W(═NC6H3-2,6-iPr2)Cl2(THF) (4), along with Me3SiCl and toluene as whole byproducts. Reaction of the brown complex 1 with [nBu4N][Cl] afforded intensely green single crystals of [nBu4N][(α-diimine)W(═NC6H3-2,6-iPr2)Cl3] (2). The versatile coordination modes of the α-diimine and o-iminoquinone ligands were clarified by spectroscopic methods and X-ray diffraction studies. Treatment of complex 1 with 1 equiv of Mg(CH2Ph)2·Et2O resulted in the formation of (α-diimine)W(═NC6H3-2,6-iPr2)(CH2Ph)2 (5), and thermolysis of 5 in the presence of PMe2Ph at 80 °C afforded the alkylidene complex (α-diimine)W(═NC6H3-2,6-iPr2)(═CHPh)(PMe2Ph) (6). On the other hand, thermolysis of 5 in the presence of CCl4 afforded the dissymmetric benzylidene complex (Cl3C-amido-imino)W(═NC6H3-2,6-iPr2)(═CHPh)Cl (7) via reductive cleavage of the C–Cl bond of CCl4. Isolated alkylidene complexes 6 and 7 served as catalysts for ring-opening metathesis polymerization of norbornene with 1 mol% of catalyst loading in toluene at 80 °C. Treatment of o-iminoquinone complex 4 with 2 equiv of LiCH2CMe2Ph afforded the dialkyl complex (o-iminoquinone)W(═NC6H3-2,6-iPr2)(CH2CMe2Ph)2 (8). Dialkyl complexes 5 and 8 at 80 °C served as catalysts to give poly(norbornene) with rather broad polydispersity.

Isoselective 3,4-(co)polymerization of bio-renewable myrcene using NSN-ligated rare-earth metal precursor: an approach to a new elastomer.

The polymerization of bio-renewable myrcene (MY), using a cationic β-diimidosulfonate lutetium catalyst in high activity, afforded an unprecedented isotactic 3,4-polymyrcene (PMY) with a low Tg (-42 °C). Based on this result, the random and block copolymerization of myrcene and isoprene (IP) resulted in novel elastomers that comprise the amorphous iso-3,4-PMY and iso-3,4-PIP sequences.

Mono(boratabenzene) rare-earth metal dialkyl complexes: synthesis, structure and catalytic behaviors for styrene polymerization.

Four mono(boratabenzene) rare-earth metal dialkyl complexes, [(3,5-Me2-C5H3BR)Ln(CH2SiMe3)2(THF)] (1: R = NEt2, Ln = Sc; 2: R = NEt2, Ln = Lu; 3: R = Ph, Ln = Sc; 4: R = Ph, Ln = Lu), were synthesized efficiently via a one-pot strategy with Li[3,5-Me2-C5H3BR] (R = NEt2, Ph), LnCl3(THF)x (Ln = Sc, x = 3; Ln = Lu, x = 0), and LiCH2SiMe3. The solid-state structures of 1 and 2 were determined by single-crystal X-ray diffraction. Variable-temperature NMR studies indicated that the energy barrier for the rotation of aminoboratabenzene in 1 (ΔG‡ ≈ 71 kJ mol−1) is higher than that of phenylboratabenzene in 3 (ΔG‡ ≈ 59 kJ mol−1). These mono(boratabenzene) rare-earth metal dialkyl complexes’ catalytic behaviors for styrene polymerization were investigated, and found that mono(boratabenzene) scandium dialkyl complexes show high catalytic activities for syndiotactic polymerization upon activation with cocatalysts.

Versatile Reactivity of β-Diketiminato-Supported Yttrium Dialkyl Complex toward Aromatic N-Heterocycles

The reactions of β-diketiminatoyttrium dialkyl complex LY(CH2Ph)2(THF) (1, L = [{N(2,6-iPr2C6H3)C(Me)}2CH]−) with a series of aromatic N-heterocycles such as 2-phenylpyridine, benzothiazole, and benzoxazole were studied and displayed discrete reactivity including C–H activation, C–C coupling, ring-opening/insertion, and dearomatization. The reaction of 1 with 2-phenylpyridine in 1:2 molar ratio in THF at 30 °C for 14 days afforded a structurally characterized metal complex, LY(η2-N,C-C5H4NC6H4-2)[C5H4N(CH2Ph-4)Ph] (2), in 73% isolated yield, indicating the occurrence of phenyl ring C(sp2)–H activation and pyridine ring 1,4-addition/dearomatization. However, when this reaction was done at 5 °C for 7 days, it gave the pyridine ring 1,2-addition product LY(η2-N,C-C5H4NC6H4-2)[C5H4N(CH2Ph-2)Ph] (3) in 54% isolated yield. Further investigations revealed that complex 2 is the thermodynamic controlled product and complex 3 is the kinetically controlled product; 3 converted slowly into 2, as confirmed by 1H NMR spectroscopy. The equimolar reaction of 1 with benzothiazole or benzoxazole produced two C–C coupling/ring-opening/insertion products, LY[η2-S,N-SC6H4NCH(CH2Ph)2](THF) (4) and {LY[μ-η2:η1-O,N-OC6H4NCH(CH2Ph)2]}2 (5), in 84% and 78% isolated yields, respectively.

Synthesis of (Imido)vanadium(V) Alkyl and Alkylidene Complexes Containing Imidazolidin-2-iminato Ligands: Effect of Imido Ligand on ROMP and 1,2-C–H Bond Activation of Benzene

A series of (imido)vanadium(V) alkylidene complexes containing an imidazolidin-2-iminato ligand of the type V(CHSiMe3)(NR)(X)(PMe3) (R = 2,6-Me2C6H3 (Ar, 3a), 1-adamantyl (Ad, 3b), C6H5 (3c); X = 1,3-Ar′2(CH2N)2C═N; Ar′ = 2,6-iPr2C6H3) have been prepared in n-hexane in the presence of PMe3 from the corresponding dialkyl complexes V(NR)(CH2SiMe3)2(X) (2a–c). These alkylidene complexes (3a–c) exhibit catalytic activities for ring-opening metathesis polymerization (ROMP) of norbornene: the phenylimido analogue (3c) exhibits remarkable activity at 80 °C (e.g. a TOF value of 84800 h–1 (7070 turnovers after 5 min)), affording high-molecular-weight polymers with uniform molecular weight distributions. The reaction of the (arylimido)vanadium(V) dialkyl complex 2a with C6H6 afforded the phenyl complex V(NAr)(CH2SiMe3)(C6H5)(X) (4a) by 1,2-C–H activation via an alkylidene intermediate and the diphenyl complex V(NAr)(C6H5)2(X) as the final product. The activity by 3a–c for ROMP, the reactivity of 2a–c for formation ...

DFT study of the reaction of a two-coordinate iron(II) dialkyl complex with molecular oxygen.

DFT studies are reported of a monomeric iron dialkyl for which oxygen atom insertion into metal-methyl bonds occurs with O2: FeMe2 + O2 → Fe(OMe)2. Computation of the reaction coordinate implicates the intermediacy of Fe(III)-peroxo, Fe(VI)-dioxo, and Fe(IV)-oxo intermediates, connected by O2 oxidative addition and two methyl migration steps. Analysis of the reaction of O2 with d(6)-Fe(Me)2 indicates that oxy-insertion for this iron complex occurs with lower free energy barriers than competing homolytic/radical pathways, exploiting "spin-flip" processes via minimum energy crossing points (MECPs).

Synthesis and Structures of Nickel Complexes with a PN-Chelate Phosphaalkene Ligand

Phosphaalkenes with a P═C bond possess extremely strong π-accepting ability, often providing transition metal complexes with interesting structures and properties. This paper describes unique structures of nickel complexes coordinated with a PN-chelate phosphaalkene ligand (PEP = 2-[1-phenyl-2-(2,4,6-tri-tert-butylphenyl)-2-phosphaethenyl]pyridine). The PEP ligand combines with [NiBr2(dme)] (dme = 1,2-dimethoxyethane) in toluene to afford [Ni(Br)(μ-Br)(PEP)]2 (1), which reacts with R2Mg(thf)2 in Et2O or THF to form three types of nickel complexes depending on the R groups and reaction conditions. The reaction with Ph2Mg(thf)2 produces a Ni(I) dimer bridged with two μ-Br ligands, [Ni(μ-Br)(PEP)]2 (2). Treatment of 1 with R2Mg(thf)2 (R = Me, Me3SiCH2) at −35 °C leads to dialkyl complexes [NiR2(PEP)] (3 and 4), with a significantly distorted square planar configuration. DFT calculations support the occurrence of effective π-back-bonding between Ni to PEP to cause the structural distortion. On the other hand, the reaction of 1 with R2Mg(thf)2 (R = Me3SiCH2) conducted at a low temperature of −78 °C forms an aryl bromide complex of the formula [Ni(Mes*)(Br)(PEP*)] (5; Mes* = 2,4,6-tBu3C6H2), in which the Mes* group originally bonded to the phosphorus atom of PEP is shifted to nickel; instead, the phosphorus atom is substituted with the R group to form the PEP* ligand. The temperature-dependent formation of 4 or 5 is rationalized by considering a common five-coordinate intermediate.

Bis(phosphine)cobalt dialkyl complexes for directed catalytic alkene hydrogenation.

Planar, low-spin cobalt(II) dialkyl complexes bearing bidentate phosphine ligands, (P-P)Co(CH2SiMe3)2, are active for the hydrogenation of geminal and 1,2-disubstituted alkenes. Hydrogenation of more hindered internal and endocyclic trisubstituted alkenes was achieved through hydroxyl group activation, an approach that also enables directed hydrogenations to yield contrasteric isomers of cyclic alkanes.

3,4-Polymerization of Isoprene by Using NSN- and NPN-Ligated Rare Earth Metal Precursors: Switching of Stereo Selectivity and Mechanism

The rare-earth metal complexes bearing NSN-bidentate β-diimidosulfonate ligands (RNSNdipp)Ln(CH2SiMe3)2(THF)n (R = Ph, Ln = Lu (1a), n = 1, Y (1b), n = 2, Sc (1c), n = 1; R = PhNMe2, Ln = Lu (1d), n = 1) were synthesized by treatment of the ion-pairs [Ln(CH2SiMe3)2(THF)x][BPh4] with equimolar amount of the ligand lithium salts (RNSNdipp)Li(THF)2 (NSNdipp = S(NC6H4iPr2-2,6)2). Addition reaction between lutetium tris(alkyl)s, Ln(Z)3(THF)n and NSNdipp gave the corresponding dialkyl complexes (ZNSNdipp)Lu(Z)2(THF)n (Z = CH2SiMe3, n = 1 (1e); Z = o-CH2C6H4NMe2, n = 0 (1f)). Deprotonation of β-imidophosphonamido ligands H–NPNdipp and H–NPNEt (NPNdipp = Ph2P(NC6H3iPr2-2,6)2, NPNEt = PPh2(NC6H3iPr2-2,6)(NC6H4-Et-2)) with Lu(CH2SiMe3)3(THF)2 yielded the corresponding dialkyl complexes (NPNdipp)Lu(CH2SiMe3)2(THF) (2) and (NPNEt)Lu(CH2SiMe3)2(THF) (3). All the complexes had been structurally well-defined, and 1a, 1b, 1e, 2, and 3 were further characterized by X-ray diffraction analysis where the almost planar NSN ra...

Neutral and Cationic Alkyl and Amido Group 3 Metal Complexes of Amidine‐Amidopyridinate Ligands: Synthesis, Structure, and Polymerization Catalytic Activity

AbstractAlkane and amine elimination reactions between the amidine‐aminopyridine proligands (NR1NNR1CMeNR2)H [R1 = R2 = Me (1a), R1 = H, R2 = Me (1b), R1 = Me, R2 = iPr (1c)] and the precursors [Ln(CH2SiMe3)3(thf)2] (Ln = Sc and Y) and [Ln{N(SiHMe2)2}3(thf)x] (Ln = Sc, Y, and La) have been investigated. Bulky 1c and [Ln(CH2SiMe3)3(thf)2] (Ln = Sc and Y) selectively afforded dialkyl complexes 2c‐Sc and 2c‐Y. Yttrium and lanthanum diamido complexes 3a‐Y, 3b‐Y, 3c‐Y, and 3c‐La were synthesized and isolated in high yields; however, similar aminolysis reactions to prepare the scandium analogues were unsuccessful. Detailed 29Si NMR spectroscopic studies combined with X‐ray crystallography revealed the presence of β‐Si–H···Ln agostic interactions in the diamido complexes. Cationic monoamido complex 4a‐Y was obtained from the reaction of 3a‐Y with [Et3NH]+[BPh4]–. Both the dialkyl and diamido complexes, activated with [Ph3C]+[B(C6F5)4]–/AliBu3 or [PhNMe2H]+[B(C6F5)4]–/AliBu3, were found to be moderately active at 60 °C in the trans‐1,4‐selective polymerization of isoprene, and the diamido complexes promoted the ring‐opening polymerization of rac‐lactide with moderate activities.

Cyclocopolymerization of 1,6-heptadiene with ethylene by half-sandwich scandium catalysts

The cyclocopolymerization of 1,6-heptadiene (HPD) with ethylene (E) by a series of half-sandwich scandium alkyl catalysts bearing various auxiliary ligands have been examined. Significant ligand influence on the copolymerization activity and selectivity was observed. In combination with one equivalent of [Ph3C][B(C6F5)4], the half-sandwich scandium dialkyl complexes bearing a Lewis base THF (1) or an NHC side arm (2) yielded the copolymer products together with cross-linked polymers in the copolymerization of HPD with ethylene. In contrast, the THF-free complexes Cp’Sc(CH2C6H4NMe2-o)2 (3: Cp’ = C5H5; 4: Cp’ = C5Me4SiMe3) and the phosphine oxide side-arm containing complex (C5Me4SiMe2CH2(O)PPh2)Sc(CH2SiMe3)2 (5) showed excellent activity and selectivity for the cyclocopolymerization reaction, without giving cross-linked products. The 1H and 13C NMR analyses revealed that the resulting copolymers consist of E-E sequences and six-membered ring methylene-1,3-cyclohexane (MCH) and five-membered ring ethylene-1,2-cyclopentane (ECP) units. The HPD content in the copolymer products could be easily controlled by changing the feeding amount of HPD under 1 atm of ethylene.

Synthesis and Hydrogenation Activity of Iron Dialkyl Complexes with Chiral Bidentate Phosphines

The activity of bis(phosphine) iron dialkyl complexes for the asymmetric hydrogenation of alkenes has been evaluated. High-throughput experimentation was used to identify suitable iron–phosphine combinations using the displacement of pyridine from py2Fe(CH2SiMe3)2 for precatalyst formation. Preparative-scale synthesis of a family of bis(phosphine) iron dialkyl complexes was also achieved using both ligand substitution and salt metathesis methods. Each of the isolated organometallic iron complexes was established as a tetrahedral and hence high-spin ferrous compound, as determined by Mossbauer spectroscopy, magnetic measurements, and, in many cases, X-ray diffraction. One example containing a Josiphos-type ligand, (SL-J212-1)Fe(CH2SiMe3)2, proved more active than other isolated iron dialkyl precatalysts. Filtration experiments and the lack of observed enantioselectivity support dissociation of the phosphine ligand upon activation with dihydrogen and formation of catalytically active heterogeneous iron. The...

Bis(pyrazolyl)carbazole as a Versatile Ligand for Supporting Lutetium Alkyl and Hydride Complexes

Two derivatives of a new bis(pyrazolyl)carbazole pincer ligand H(CzPzR), R = iPr and Me, and their syntheses are reported. Lutetium dialkyl complexes of the ligand (CzPzR)Lu(CH2SiMe3)2, R = iPr and Me, have been prepared and found to exhibit high thermal stability in solution. These organolutetium compounds are Lewis base free, and the solid-state structure of (CzPziPr)Lu(CH2SiMe3)2 revealed that the complex is monomeric with a trigonal-bipyramidal geometry. Hydrogenolysis of (CzPziPr)Lu(CH2SiMe3)2 afforded a trimetallic lutetium hydride complex that possesses five bridging hydride ligands. Notably, intramolecular C–H bond activation of the CzPziPr ligand was found to occur during the formation of this latter hydride complex, with metalation of a pyrazole carbon atom.

Cationic Tropidinyl Scandium Catalyst: A Perfectly Acceptable Substitute for Cationic Half-Sandwich Scandium Catalysts in cis-1,4-Polymerization of Isoprene and Copolymerization with Norbornene

Different nonmetallocene rare earth metal alkyl complexes such as monotropidinyl (Trop) scandium dialkyl complex (Trop)Sc(CH2SiMe3)2(THF) (1), ditropidinyl yttrium alkyl complex (Trop)2Y(CH2SiMe3)(THF) (3) as well as binuclear lutetium alkyl complex bearing one tetradentate dianionic 6-N-methyl-1,4-cycloheptadienyl (NMCH) ligand [(NMCH)Lu(CH2SiMe3)(THF)]2 (2) have been synthesized in high yields via one-pot acid–base reaction by using of the tris(trimethylsilylmethyl) rare earth metal complexes with the readily available natural product tropidine. The polymerization experiments indicate that the monotropidinyl scandium dialkyl complex 1 displays reactivity akin to that of the analogous monocyclopentadienyl scandium dialkyl complexes. In the presence of activator and a small amount of AlMe3, complex 1 exhibits similar activities (up to 1.6 × 106 g molSc–1 h–1) but higher cis-1,4-selectivities (up to 100%) than (C5H5)Sc(CH2SiMe3)2(THF) (cis-1,4-selectivity as 95%) in the isoprene polymerization, yielding the pure cis-1,4-PIPs with moderate molecular weights (Mn = 0.5–11.2 × 104 g/mol) and bimodal molecular weight distributions (Mw/Mn = 1.48–6.07). Moreover, the complex 1/[Ph3C][B(C6F5)4/AliBu3 ternary system also shows good comonomer incorporation ability in the copolymerization of isoprene and norbornene similar to the [C5Me4(SiMe3)]Sc(η3-CH2CHCH2)2/activator binary system, affording the random isoprene/norbornene copolymers with a wide range of isoprene contents around 57–91 mol % containing cis-1,4 configuration up to 88%.

Sequential Reduction and Alkyl Exchange Reactions of Bis(imino)pyridine Dialkyliron(II) with Trimethylaluminum

The sequence of reactions ensuing when the 2,6-bis(imino)pyridine Fe(II) dialkyl complexes [Fe(CH2SiMe3)2(BIP)] react with trimethylaluminum is described in detail. The first step is the reduction of the dialkyl to the corresponding monoalkyl complex [Fe(CH2SiMe3)(BIP)]. In the presence of an excess of trimethylaluminum, these undergo exchange of the remaining CH2SiMe3 group for methyl, and ultimately the methyliron fragment is displaced from the complex, leading to previously known paramagnetic aluminum compounds. The diethylaluminum complex [AlEt2(iPrBIP)] was obtained when triethylaluminum was used instead of trimethylaluminum.