Brad C. Bailey

A Tungsten(VI) Nitride Having a W2(μ-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.

Snapshots of an oxidatively induced α-hydrogen abstraction reaction to prepare a terminal and four-coordinate titanium imide

One electron oxidation of the bis-anilido titanium(III) complex (Nacnac)Ti(NHAr)2 (Nacnac− = ArNC(CH3)CHC(CH3)NAr, Ar = 2,6-(CHMe2)2C6H3) with AgOTf affords the cation [(Nacnac)Ti(NHAr)2][OTf] which is isolated and shown to gradually transform, by α-hydrogen abstraction, to the terminal and four-coordinate titanium imide (Nacnac)TiNAr(OTf).

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/β-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/β-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/β-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.

Understanding the competitive dehydroalkoxylation and dehydrogenation of ethers with Ti–C multiple bonds

The divergent reactivity of a transient titanium neopentylidyne, (PNP)TiCtBu (A) (PNP = N[2-PiPr2-4-methylphenyl]2−), that exhibits competing dehydrogenation and dehydroalkoxylation reaction pathways in the presence of acyclic ethers (Et2O, nPr2O, nBu2O, tBuOMe, tBuOEt, iPr2O) is presented. Although dehydrogenation takes place also in long-chain linear ethers, dehydroalkoxylation is disfavoured and takes place preferentially or even exclusively in the case of branched ethers. In all cases, dehydrogenation occurs at the terminal position of the aliphatic chain. Kinetics analyses performed using the alkylidene-alkyl precursor, (PNP)TiCHtBu(CH2tBu), show pseudo first-order decay rates on titanium (kavg = 6.2 ± 0.3 × 10−5 s−1, at 29.5 ± 0.1 °C, overall), regardless of the substrate or reaction pathway that ensues. Also, no significant kinetic isotope effect (kH/kD ∼ 1.1) was found between the activations of Et2O and Et2O-d10, in accord with dehydrogenation (C–H activation and abstraction) not being the slowest steps, but also consistent with formation of the transient alkylidyne A being rate-determining. An overall decay rate of (PNP)TiCHtBu(CH2tBu) with a t1/2 = 3.2 ± 0.4 h, across all ethers, confirms formation of A being a common intermediate. Isolated alkylidene-alkoxides, (PNP)TiCHtBu(OR) (R = Me, Et, nPr, nBu, iPr, tBu) formed from dehydroalkoxylation reactions were also independently prepared by salt metatheses, and extensive NMR characterization of these products is provided. Finally, combining theory and experiment we discuss how each reaction pathway can be altered and how the binding event of ethers plays a critical role in the outcome of the reaction.

Latent low-coordinate titanium imides supported by a sterically encumbering β-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.