Catalyst Editing via Post-Synthetic Functionalization by Phosphonium Generation and Anion Exchange for Nickel-Catalyzed Ethylene/Acrylate Copolymerization.

Rapid, efficient development of homogeneous catalysts featuring desired performance is critical to numerous catalytic transformations but remains a key challenge. Typically, this task relies heavily on ligand design that is often based on trial and error. Herein, we demonstrate a "catalyst editing" strategy in Ni-catalyzed ethylene/acrylate copolymerization. Specifically, alkylation of a pendant phosphine followed by anion exchange provides a high yield strategy for a large number of cationic Ni phosphonium catalysts with varying electronic and steric profiles. These catalysts are highly active in ethylene/acrylate copolymerization, and their behaviors are correlated with the electrophile and the anion used in late-stage functionalization.

Acrylate-Induced ß-H Elimination in Coordination Insertion Copolymerizaton Catalyzed by Nickel.

Polar monomer-induced ß-H elimination is a key elementary step in polar polyolefin synthesis by coordination polymerization but remains underexplored. Herein, we show that a bulky neutral Ni catalyst, 1Ph, is not only a high-performance catalyst in ethylene/acrylate copolymerization (activity up to ∼37,000 kg/(mol·h) at 130 °C in a batch reactor, mol % tBA ∼ 0.3) but also a suitable platform for investigation of acrylate-induced ß-H elimination. 4Ph-tBu, a novel Ni alkyl complex generated after acrylate-induced ß-H elimination and subsequent acrylate insertion, was identified and characterized by crystallography. A combination of catalysis and mechanistic studies reveals effects of the acrylate monomer, bidentate ligand, and the labile ligand (e.g., pyridine) on the kinetics of ß-H elimination, the role of ß-H elimination in copolymerization catalysis as a chain-termination pathway, and its potential in controlling the polymer microstructure in polar polyolefin synthesis.

Highly Active and Thermally Robust Nickel Enolate Catalysts for the Synthesis of Ethylene-Acrylate Copolymers.

The insertion copolymerization of polar olefins and ethylene remains a significant challenge in part due to catalysts' low activity and poor thermal stability. Herein we demonstrate a strategy toward addressing these obstacles through ligand design. Neutral nickel phosphine enolate catalysts with large phosphine substituents reaching the axial positions of Ni achieve activity of up to 7.7×103  kg mol-1 h-1 (efficiency >35×103  g copolymer/g Ni) at 110 °C, notable for ethylene/acrylate copolymerization. NMR analysis of resulting copolymers reveals highly linear microstructures with main-chain ester functionality. Structure-performance studies indicate a strong correlation between axial steric hindrance and catalyst performance.

Understanding intermolecular C-F bond activation by a transient titanium neopentylidyne: experimental and theoretical studies on the competition between 1,2-CF bond addition and [2 + 2]-cycloaddition/ß-fluoride elimination.

Complex (PNP)Ti=CH(t)Bu(CH(2)(t)Bu) (PNP(-) = N[2-P(CHMe(2))(2)-4-methylphenyl](2)) eliminates H(3)C(t)Bu to form transient (PNP)Ti≡C(t)Bu, which activates the C-F bond of ortho-difluoropyridine and ortho-fluoropyridine to form the alkylidene-fluoride complexes, (PNP)Ti=C[(t)Bu(NC(5)H(3)F)](F) (1) and (PNP)Ti=C[(t)Bu(NC(5)H(4))](F) (2), respectively. When (PNP)Ti=CH(t)Bu(CH(2)(t)Bu) is treated with meta-fluoropyridine, the ring-opened product (PNP)Ti(C((t)Bu)CC(4)H(3)-3-FNH) (3) is the only recognizable titanium metal complex formed. Theoretical studies reveal that pyridine binding disfavors 1,2-CF bond addition across the alkylidyne ligand in the case of ortho-fluoride pyridines, while sequential [2 + 2]-cycloaddition/ß-fluoride elimination is a lower energy pathway. In the case of meta-fluoropyridine, [2 + 2]-cycloaddition and subsequent ring-opening metathesis is favored as opposed to C-H bond addition or sequential [2 + 2]-cycloaddition/ß-hydride elimination. In all cases, C-H bond addition of ortho-fluoropyridines or meta-fluoropyridine is discouraged because such substrate must bind to titanium via its C-H bond, which is rather weak compared to the titanium-pyridine binding.

A tungstenVI nitride having a W2(mu-N)2 core.

The tungsten nitrido species, [W(mu-N)(CH2-t-Bu)(OAr)2]2 (Ar = 2,6-diisopropylphenyl), has been prepared in a reaction between the alkylidyne species, W(C-t-Bu)(CH2-t-Bu)(OAr)2, and organonitriles. The dimeric nature of the nitride was established in the solid state through an X-ray study and in solution through a combination of 15N NMR spectroscopy and vibrational spectroscopy. Reaction of the nitride with trimethylsilyl trifluoromethanesulfonate afforded the monomeric trimethylsilyl imido species, W(NSiMe3)(CH2-t-Bu)(OAr)2(OSO2CF3), which was also characterized crystallographically. The W2N2 core can be reduced by one electron electrochemically or in bulk with metallocenes to afford the radical anion, {n-Bu4N}{[W(mu-N)(CH2-t-Bu)(OAr)2]2}. Density functional theory calculations suggest that the lowest-energy allowable transition in [W(mu-N)(CH2-t-Bu)(OAr)2]2 is from a highest occupied molecular orbital consisting largely of ligand-based lone pairs into what is largely a metal-based lowest unoccupied molecular orbital.

Intermolecular C-H bond activation reactions promoted by transient titanium alkylidynes. Synthesis, reactivity, kinetic, and theoretical studies of the Ti[triple bond]C linkage.

The neopentylidene-neopentyl complex (PNP)Ti=CH(t)Bu(CH2(t)Bu) (2; PNP(-) = N[2-P(CHMe2)(2-)4-methylphenyl]2), prepared from the precursor (PNP)Ti[triple bond]CH(t)Bu(OTf) (1) and LiCH2(t)Bu, extrudes neopentane in neat benzene under mild conditions (25 degrees C) to generate the transient titanium alkylidyne, (PNP)Ti[triple bond]C(t)Bu (A), which subsequently undergoes 1,2-CH bond addition of benzene across the Ti[triple bond]C linkage to generate (PNP)Ti=CH(t)Bu(C6H5) (3). Kinetic, mechanistic, and theoretical studies suggest the C-H activation process to obey pseudo-first-order in titanium, the alpha-hydrogen abstraction to be the rate-determining step (KIE for 2/2-d(3) conversion to 3/3-d(3) = 3.9(5) at 40 degrees C) with activation parameters DeltaH = 24(7) kcal/mol and DeltaS = -2(3) cal/mol.K, and the post-rate-determining step to be C-H bond activation of benzene (primary KIE = 1.03(7) at 25 degrees C for the intermolecular C-H activation reaction in C6H6 vs C6D6). A KIE of 1.33(3) at 25 degrees C arose when the intramolecular C-H activation reaction was monitored with 1,3,5-C6H3D3. For the activation of aromatic C-H bonds, however, the formation of the sigma-complex becomes rate-determining via a hypothetical intermediate (PNP)Ti[triple bond]C(t)Bu(C6H5), and C-H bond rupture is promoted in a heterolytic fashion by applying standard Lewis acid/base chemistry. Thermolysis of 3 in C6D6 at 95 degrees C over 48 h generates 3-d(6), thereby implying that 3 can slowly equilibrate with A under elevated temperatures with k = 1.2(2) x 10-5 s(-1), and with activation parameters DeltaH = 31(16) kcal/mol and DeltaS = 3(9) cal/mol x K. At 95 degrees C for one week, the EIE for the 2 --> 3 reaction in 1,3,5-C6H3D3 was found to be 1.36(7). When 1 is alkylated with LiCH2SiMe3 and KCH2Ph, the complexes (PNP)Ti=CHtBu(CH2SiMe3) (4) and (PNP)Ti=CHtBu(CH2Ph) (6) are formed, respectively, along with their corresponding tautomers (PNP)Ti=CHSiMe3(CH2tBu) (5) and (PNP)Ti=CHPh(CH2tBu) (7). By means of similar alkylations of (PNP)Ti=CHSiMe3(OTf) (8), the degenerate complex (PNP)Ti=CHSiMe3(CH2SiMe3) (9) or the non-degenerate alkylidene-alkyl complex (PNP)Ti=CHPh(CH2SiMe3) (11) can also be obtained, the latter of which results from a tautomerization process. Compounds 4/5 and 9, or 6/7 and 11, also activate benzene to afford (PNP)Ti=CHR(C6H5) (R = SiMe3 (10), Ph (12)). Substrates such as FC6H5, 1,2-F2C6H4, and 1,4-F2C6H4 react at the aryl C-H bond with intermediate A, in some cases regioselectively, to form the neopentylidene-aryl derivatives (PNP)Ti=CHtBu(aryl). Intermediate A can also perform stepwise alkylidene-alkyl metatheses with 1,3,5-Me3C6H3, SiMe4, 1,2-bis(trimethylsilyl)alkyne, and bis(trimethylsilyl)ether to afford the titanium alkylidene-alkyls (PNP)Ti=CHR(R') (R = 3,5-Me2C6H2, R' = CH2-3,5-Me2C6H2; R = SiMe3, R' = CH2SiMe3; R = SiMe2CCSiMe3, R' = CH2SiMe2CCSiMe3; R = SiMe2OSiMe3, R' = CH2SiMe2OSiMe3).

Room temperature ring-opening metathesis of pyridines by a transient Ti[triple bond]C linkage.

The transient titanium alkylidyne complex (PNP)TiCtBu (PNP = N[2-P(CHMe2)2-4-methylphenyl]2-), prepared from alpha-hydrogen abstraction of the corresponding alkylidene-alkyl species (PNP)Ti=CHtBu(CH2tBu), can readily cleave the C-N bond of N-heterocycles such as pyridine and 4-picoline at room temperature to afford azametallabicyclic systems. Experimental and theoretical studies strongly favor a ring-opening metathesis pathway where [2 + 2] cycloaddition of pyridine across the TiC linkage ultimately leads to C-N bond rupture.

Intermolecular C-H bond activation promoted by a titanium alkylidyne.

The transient titanium alkylidyne complex (PNP)TiCtBu (PNP = N-[2-P(CHMe2)2-4-methylphenyl]2-), prepared from alpha-hydrogen abstraction of the corresponding alkylidene-alkyl species (PNP)Ti=CHtBu(CH2tBu), can readily undergo intermolecular 1,2-addition of C-H bonds of benzene and SiMe4. Synthesis and reactivity, isotopic labeling, kinetics, and theoretical studies strongly favor an alkylidyne pathway and the alpha-H abstraction step to be the rate-determining step.

Latent low-coordinate titanium imides supported by a sterically encumbering beta-diketiminate ligand.

Addition of an equal molar quantity of R- (R = Me, SiMe3) to complex (Nacnac)Ti=NAr(OTf) (Nacnac- =[ArNC(tBu)]2CH, Ar = 2,6-iPr2C6H3) forms the imido alkyl (Nacnac)Ti=NAr(R), which can be readily protonated to afford [(Nacnac)Ti=NAr(L)]+ (L = THF, Et2O, eta1-C6H5NMe2), or treated with B(C6F5)3 to afford the zwitterion (Nacnac)Ti=NAr(micro-CH3)B(C6F5)3.

Terminal vanadium-neopentylidyne complexes and intramolecular cross-metathesis reactions to generate azametalacyclohexatrienes.

Four-coordinate vanadium complexes containing a terminal neopentylidyne functionality have been prepared by two consecutive alpha-hydrogen abstraction reactions both of which were induced by one-electron oxidations. Among these vanadium-alkylidyne complexes are the neutral and the cation (Nacnac)VCtBu(OTf) and [(Nacnac)VCtBu(THF)]+, respectively (Nacnac- = [Ar]NC(CH3)CHC(CH3)N[Ar], Ar = 2,6-(CHMe2)2C6H3). The vanadium-alkylidynes have been characterized by 1H, 13C, 51V NMR spectroscopy and single-crystal X-ray diffraction and are consistent with a short VC bond. These alkylidynes were found to transform to azametalacyclohexatriene systems via an intramolecular cross-metathesis reaction. Kinetic studies of the transformation of (Nacnac)VCtBu(OTf) in C7D8 reveal the formation of the azametalacyclohexatriene to be independent of solvent (toluene vs THF) and the reaction to be first order in vanadium (k = 3.30(5) x 10-5 s-1 at 80 degrees C, with activation parameters DeltaH= 25.4(3) kcal/mol, DeltaS = -6(3) cal/molK). High-level DFT calculations on the full model suggest an intramolecular mechanism invoking only one transition state. The overall thermodynamic driving force for the reaction (DeltaG) in solution phase was estimated to be -21.3 kcal/mol.

Terminal and four-coordinate vanadium(IV) phosphinidene complexes. A pseudo Jahn-Teller effect of second order stabilizing the V-P multiple bond.

Treatment of the four-coordinate vanadium neopentylidene (Nacnac)V=CHtBu(I) (Nacnac- = [Ar]NC(Me)CHC(Me)N[Ar], Ar = 2,6-iPr2C6H3) with a bulky primary lithium phosphide LiPHR (R = 2,4,6-iPr3C6H2, 2,4,6-tBu3C6H2) leads to alpha-hydrogen migration concomitant with the formation of a four-coordinate vanadium complex containing a terminal phosphinidene functionality (Nacnac)V=PR(CH2tBu). The crystal structures for the vanadium phosphinidene complexes prepared herein were determined by single-crystal X-ray diffraction methods. Solution EPR and magnetic measurements of the vanadium phosphinidenes are also in accordance with such systems containing a V(IV) metal center, and DFT calculations indicate the V=P bond to be stabilized through a pseudo Jahn-Teller effect of second order.

A terminal and four-coordinate titanium alkylidene prepared by oxidatively induced alpha-hydrogen abstraction.

One-electron oxidation of the beta-diketiminate titanium(III) bis-neopentyl complex (Nacnac)Ti(CH2tBu)2 (Nacnac = [Ar]NC(Me)CHC(Me)N[Ar], Ar = 2,6-(CHMe2)2C6H3) promotes alpha-abstraction to afford the rare and terminal four-coordinate neopentylidene (Nacnac)Ti=CHtBu(OTf), which was structurally characterized. Alkylidene (Nacnac)Ti=CHtBu(OTf) reacts cleanly with benzophenone and the imine functionality of the Nacnac ligand to afford the corresponding Wittig-type products.