Synthesis of Molybdenum and Tungsten Alkylidene Complexes that Contain a <i>tert</i>-Butylimido Ligand

A variety of molybdenum or tungsten complexes that contain a <i>tert</i>-butylimido ligand have been prepared. For example, the <i>o</i>-methoxybenzylidene complex W­(N-<i>t</i>-Bu)­(CH-<i>o</i>-MeOC₆H₄)­(Cl)₂(py) was prepared through addition of pyridinium chloride to W­(N-<i>t</i>-Bu)₂­(CH₂-<i>o</i>-MeOC₆H₄)₂, while Mo­(N-<i>t</i>-Bu)­(CH-<i>o</i>-MeOC₆H₄)­(OR_F)₂(<i>t</i>-BuNH₂) complexes (OR_F = OC₆F₅ or OC­(CF₃)₃) were prepared through addition of two equivalents of R_FOH to Mo­(N-<i>t</i>-Bu)₂­(CH₂-<i>o</i>-MeOC₆H₄)₂. An X-ray crystallographic study of Mo­(N-<i>t</i>-Bu)­(CH-<i>o</i>-MeOC₆H₄)­[OC­(CF₃)₃]₂­(<i>t</i>-BuNH₂) showed that the methoxy oxygen is bound to the metal and that two protons on the <i>tert</i>-butylamine ligand are only a short distance away from one of the CF₃ groups on one of the perfluoro-<i>tert</i>-butoxide ligands (H···F = 2.456(17) and 2.467(17) Å). Other synthesized tungsten <i>tert</i>-butylimido complexes include W­(N-<i>t</i>-Bu)­(CH-<i>o</i>-MeOC₆H₄)­(pyr)₂(2,2′-bipyridine) (pyr = pyrrolide), W­(N-<i>t</i>-Bu)­(CH-<i>o</i>-MeOC₆H₄)­(pyr)­(OHMT) (OHMT = O-2,6-(mesityl)₂C₆H₃), W­(N-<i>t</i>-Bu)­(CH-<i>t</i>-Bu)­(OHMT)­(Cl)­(py) (py = pyridine), W­(N-<i>t</i>-Bu)­(CH-<i>t</i>-Bu)­(OHMT)­(Cl), W­(N-<i>t</i>-Bu)­(CH-<i>t</i>-Bu)­(pyr)­(ODFT)­(py), W­(N-<i>t</i>-Bu)­(CH-<i>t</i>-Bu)­(OHMT)₂, and W­(N-<i>t</i>-Bu)­(CH-<i>t</i>-Bu)­(ODFT)₂ (ODFT = O-2,6-(C₆F₅)₂C₆H₃). Interestingly, W­(N-<i>t</i>-Bu)­(CH-<i>t</i>-Bu)­(OHMT)₂ does not react with ethylene or 2,3-dicarbomethoxynorbornadiene. Removal of pyridine from W­(N-<i>t</i>-Bu)­(CH-<i>t</i>-Bu)­(Biphen_CF3)­(pyridine) (Biphen_CF3 = 3,3′-di-<i>tert</i>-butyl-5,5′-bistrifluoromethyl-6,6′-dimethyl-1,1′-biphenyl-2,2′-diolate) with B­(C₆F₅)₃ led to formation of a five-coordinate 14<i>e</i> neopentyl complex as a consequence of CH activation in one of the methyl groups in one <i>tert</i>-butyl group of the Biphen_CF3 ligand, as was proven in an X-ray study. An attempted synthesis of W­(N-<i>t</i>-Bu)­(CH-<i>t</i>-Bu)­(Biphen_Me) (Biphen_Me = 3,3′-di-<i>tert</i>-butyl-5,5′,6,6′-tetramethyl-1,1′-biphenyl-2,2′-diolate) led to formation of a 1:1 mixture of W­(N-<i>t</i>-Bu)­(CH-<i>t</i>-Bu)­(Biphen_Me) and a neopentyl complex analogous to the one characterized through an X-ray study. The metallacyclobutane complexes W­(N-<i>t</i>-Bu)­(C₃H₆)­(pyrrolide)­(ODFT) and W­(N-<i>t</i>-Bu)­(C₃H₆)­(ODFT)₂ were prepared in reactions involving W­(N-<i>t</i>-Bu)­(CH-<i>t</i>-Bu)­(pyr)₂(bipy), ZnCl₂(dioxane), and one or two equivalents of DFTOH, respectively, under 1 atm of ethylene

Formation of Alternating <i>trans</i>-<b>A</b>-<i>alt</i>-<b>B</b> Copolymers through Ring-Opening Metathesis Polymerization Initiated by Molybdenum Imido Alkylidene Complexes

Ring-opening metathesis polymerization (ROMP) is used to prepare <i>trans</i>-poly­(<b>A</b>-<i>alt</i>-<b>B</b>) polymers from a 1:1 mixture of <b>A</b> and <b>B</b> where <b>A</b> is a cyclic olefin such as cyclooctene (<b>A</b>_<b>1</b>) or cycloheptene (<b>A</b>_<b>2</b>) and <b>B</b> is a large norbornadiene or norbornene derivative such as 2,3-dicarbomethoxy-7-isopropylidene­norbornadiene (<b>B</b>_<b>1</b>) or dimethyl­spirobicyclo[2.2.1]­hepta-2,5-diene-2,3-dicarboxylate-7,1′-cyclopropane (<b>B</b>_<b>2</b>). The most successful initiators that were examined are of the type Mo­(NR)­(CHCMe₂Ph)­[OCMe­(CF₃)₂]₂ (R = 2,6-Me₂C₆H₃ (<b>1</b>) or 2,6-<i>i-</i>Pr₂C₆H₃ (<b>2</b>)). The <i>trans</i> configuration of the <b>AB</b> linkages is proposed to result from the steric demand of <b>B</b>. Both <i>anti</i>-<b>MB</b> and <i>syn</i>-<b>MB</b> alkylidenes are observed during the copolymerization, where <b>B</b> was last inserted into a MoC bond, although <i>anti</i>-<b>MB</b> dominates as the reaction proceeds. <i>anti</i>-<b>MB</b> is <i>lower</i> in energy than <i>syn</i>-<b>MB</b>, does not react readily with <i>either</i> <b>A</b> or <b>B</b>, and interconverts slowly with <i>syn</i>-<b>MB</b> through rotation about the MoC bond. <i>Syn</i>-<b>MB</b> does not readily react with <b>B</b>, but it does react slowly with <b>A</b> (rate constant ∼1 M^–1 s^–1) to give <i>anti</i>-<b>MA</b> and one <i>trans</i>-<b>AB</b> linkage. <i>anti</i>-<b>MA</b> then reacts with <b>B</b> (rate constant ∼300 M^–1 s^–1 or larger) to give <i>syn</i>-<b>MB</b> and the second <i>trans</i>-<b>AB</b> linkage. The reaction has been modeled using experimental data in order to obtain the estimated rate constants above. The reaction between <i>anti</i>-<b>MA</b> and <b>A</b> is proposed to give rise to <b>AA</b> linkages, but <b>AA</b> dyads can amount to <5%. Several other <b>A</b> and <b>B</b> monomers, initiators, and conditions were explored

Regioselective Termination Reagents for Ring-Opening Alkyne Metathesis Polymerization

Alkyne cross-metathesis of molybdenum carbyne complex [TolCMo­(OCCH₃(CF₃)₂)₃]·DME with 2 equiv of functional ynamines or ynamides yields the primary cross-metathesis product with high regioselectivity (>98%) along with a molybdenum metallacyclobutadiene complex. NMR and X-ray crystal structure analysis reveals that ynamides derived from 1-(phenylethynyl)­pyrrolidin-2-one selectively cleave the propagating molybdenum species in the ring-opening alkyne metathesis polymerization (ROAMP) of ring-strained 3,8-dihexyloxy-5,6-dihydro-11,12-didehydrodibenzo­[<i>a</i>,<i>e</i>]­[8]­annulene and irreversibly deactivate the diamagnetic molybdenum metallacyclobutadiene complex through a multidentate chelate binding mode. The chain termination of living ROAMP with substituted ethynylpyrrolidin-2-ones selectively transfers a functional end-group to the polymer chain, giving access to telechelic polymers. This regioselective carbyne transfer strategy gives access to amphiphilic block copolymers through synthetic cascades of ROAMP followed by ring-opening polymerization of strained ε-caprolactone

Syntheses of Tungsten <i>tert</i>-Butylimido and Adamantylimido Alkylidene Complexes Employing Pyridinium Chloride As the Acid

Routes to new tungsten alkylidene complexes that contain <i>tert</i>-butylimido or adamantylimido ligands have been devised that begin with a reaction between WCl₆ and 4 equivalents of HNR­(TMS) to give [W­(NR)₂Cl­(μ-Cl)­(RNH₂)]₂ (R = t-Bu or 1-adamantyl). Alkylation leads to W­(NR)₂(CH₂R′)₂ (R′ = t-Bu or CMe₂Ph), which upon treatment with pyridinium chloride yields W­(NR)­(CHR′)­Cl₂(py)₂ complexes, from which W­(NR)­(CHR′)­(pyrrolide)₂ and two W­(N-t-Bu)­(CHR′)­(pyrrolide)­(OAr) complexes (OAr = hexamethyl- or hexaisopropylterphenoxide) have been prepared

<i>Z</i>‑Selective Ring-Opening Metathesis Polymerization of 3‑Substituted Cyclooctenes by Monoaryloxide Pyrrolide Imido Alkylidene (MAP) Catalysts of Molybdenum and Tungsten

Ring-opening metathesis polymerization of a series of 3-substituted cyclooctenes (3-MeCOE, 3-HexCOE, and 3-PhCOE) initiated by various Mo and W MAP complexes leads to <i>cis</i>,HT-poly­(3-RCOE) polymers. The apparent rate of polymerization of 3-HexCOE by W­(N-t-Bu)­(CH-t-Bu)­(Pyr)­(OHMT) (<b>1c</b>; Pyr = pyrrolide; OHMT = O-2,6-Mesityl₂C₆H₃) is greater than the rate of polymerization by Mo­(N-t-Bu)­(CH-t-Bu)­(Pyr)­(OHMT) (<b>1b</b>), but both gave the same <i>cis</i>,HT polymer structures. Formation of HT-poly­(3-RCOE) employing <b>1c</b> takes place via propagating species in which the R group (methyl, hexyl, or phenyl) is on C2 of the propagating alkylidene chain, a type of intermediate that has been modeled through the preparation of W­(N-t-Bu)­(CHCHMeEt)­(Pyr)­(OHMT). The rate of ROMP is exceedingly sensitive to steric factors: e.g., W­(N-t-Bu)­(CH-t-Bu)­(Me₂Pyr)­(OHMT), the dimethylpyrrolide analogue of <b>1c</b>, essentially did not polymerize 3-HexCOE at 22 °C. When a sample of W­(N-t-Bu)­(CHCHMeEt)­(Pyr)­(OHMT) and 3-methyl-1-pentene in CDCl₃ is cooled to −20 °C, the alkylidene resonances for W­(N-t-Bu)­(CHCHMeEt)­(Pyr)­(OHMT) disappear and resonances that can be ascribed to protons in a <i>syn</i>_α<i>/syn</i>_α′ disubstituted trigonal bipyramidal metallacyclobutane complex appear. 3-Methyl-1-pentene is readily lost from this metallacycle on the NMR time scale at room temperature

Stereospecific Ring-Opening Metathesis Polymerization (ROMP) of <i>endo</i>-Dicyclopentadiene by Molybdenum and Tungsten Catalysts

We report an examination of the ring-opening metathesis polymerization (ROMP) of <i>endo-</i>dicyclopentadiene (DCPD) by 10 well-defined molybdenum-based and 16 tungsten-based alkylidene initiators. Five tungsten-based MAP (monoaryloxide pyrrolide) initiators with the general formula W­(X)­(CHCMe₂Ph)­(Me₂Pyr)­(OAr) (X = arylimido, alkylimido, or oxo; Me₂Pyr =2,5-dimethylpyrrolide; OAr = an aryloxide) were found to yield >98% <i>cis</i>, >98% <i>syndiotactic</i> poly­(DCPD); they are W­(N-<i>t</i>-Bu)­(CHCMe₃)­(pyr)­(OHMT) (<b>2</b>, OHMT = O-2,6-(2,4,6-Me₃C₆H₂)₂C₆H₃, pyr = pyrrolide), W­(N-2,6-<i>i</i>-Pr₂C₆H₃)­(CHCMe₂Ph)­(pyr)­(OHMT) (<b>3</b>), W­(O)­(CHCMe₂Ph)­(Me₂Pyr)­(OHMT)­(PPh₂Me) (<b>7</b>, Me₂Pyr =2,5-dimethylpyrrolide), W­(O)­(CHCMe₂Ph)­(Me₂Pyr)­(ODFT)­(PPh₂Me) (<b>9</b>, ODFT = O-2,6-(C₆F₅)₂C₆H₃), and W­(O)­(CHCMe₂Ph)­(Me₂Pyr)­(OTPP)­(PMePh₂) (<b>10</b>, OTPP = O-2,3,5,6-Ph₄C₆H). Two biphenolate alkylidene complexes, Mo­(N-2,6-Me₂C₆H₃)­(CHCMe₂Ph)­(<i>rac</i>-biphen) (<b>17</b>) and W­(N-2,6-Me₂C₆H₃)­(CHCMe₂Ph)­(<i>rac</i>-biphen) (<b>22</b>, biphen =3,3′-(<i>t-</i>Bu)₂-5,5′-6,6′-(CH₃)₄-1,1′-biphenyl-2,2′-diolate), were found to yield >98% <i>cis</i>, >98% <i>isotactic</i> poly­(DCPD). <i>Cis</i>, <i>syndiotactic</i> or <i>cis</i>, <i>isotactic</i> poly­(DCPD)­s (made with 50–1000 equiv of DCPD) are accessible within seconds to minutes in dichloromethane at room temperature. No isomerization or cross-linking reactions are observed, and addition of a chain transfer reagent (1-hexene) or the use of THF as a solvent does not decrease the stereospecificity of the polymerizations. <i>Cis</i>, <i>syndiotactic</i> and <i>cis</i>, <i>isotactic</i> poly­(DCPD)­s can be distinguished readily from each other by ¹³C NMR spectroscopy. Hydrogenation of each stereoregular poly­(DCPD) produces H-poly­(DCPD)­s that have melting points near 270 °C (<i>syndiotactic</i>) or 290 °C (<i>isotactic</i>) and high crystallinities (<i>w</i>_c = 0.83 for <i>syndiotactic</i> and <i>w</i>_c = 0.74 for <i>isotactic</i>)