Chun-Hsing Chen

Hydrodefluorination and Other Hydrodehalogenation of Aliphatic Carbon−Halogen Bonds Using Silylium Catalysis

Trialkylsilylium cation equivalents partnered with halogenated carborane anions (such as Et(3)Si[HCB(11)H(5)Cl(6)]) function as efficient and long-lived catalysts for hydrodehalogenation of C-F, C-Cl, and C-Br bonds with trialkylsilanes as stoichiometric reagents. Only C(sp(3))-halogen bonds undergo this reaction. The range of C-F bond-containing substrates that participate in this reaction is quite broad and includes simple alkyl fluorides, benzotrifluorides, and compounds with perfluoroalkyl groups attached to an aliphatic chain. However, CF(4) has proven immune to this reaction. Hydrodechlorination was carried out with a series of alkyl chlorides and benzotrichlorides, and hydrodebromination was studied only with primary alkyl bromide substrates. Competitive experiments established a pronounced kinetic preference of the catalytic system for activation of a carbon-halogen bond of a lighter halide in primary alkyl halides. On the contrary, hydrodechlorination of C(6)F(5)CCl(3) proceeded much faster than hydrodefluorination of C(6)F(5)CF(3) in one-pot experiments. A solid-state structure of Et(3)Si[HCB(11)H(5)Cl(6)] was determined by X-ray diffraction methods.

Multielectron Redox Activity Facilitated by Metal−Metal Interactions in Early/Late Heterobimetallics: Co/Zr Complexes Supported by Phosphinoamide Ligands

To assess the effect of dative M-->M interactions on redox properties in early/late heterobimetallic complexes, a series of Co/Zr complexes supported by phosphinoamide ligands have been synthesized and characterized. Treatment of the Zr metalloligands (Ph(2)PN(i)Pr)(3)ZrCl (1), ((i)Pr(2)PNMes)(3)ZrCl (2), and ((i)Pr(2)PN(i)Pr)(3)ZrCl (3) with CoI(2) leads to reduction from Co(II) to Co(I) and isolation of the heterobimetallic complexes ICo(Ph(2)PN(i)Pr)(3)ZrCl (4), ICo((i)Pr(2)PNMes)(3)ZrCl (5), and ICo((i)Pr(2)PN(i)Pr)(3)ZrCl (6), respectively. Interestingly, treatment of CoI(2) with the phosphinoamine Ph(2)PNH(i)Pr in the absence of a bound Zr center leads to the disubstituted Co(II) complex (Ph(2)PNH(i)Pr)(2)CoI(2) (7). The tris(phosphinoamine) Co(I) complex (Ph(2)PNH(i)Pr)(3)CoI (8) can only be generated in the presence of an added reductant such as Zn(0), indicating that the reduction of Co(II) to Co(I) only occurs in the presence of Zr in the formation of complexes 4-6. Structural characterization of 4-6 reveals a Zr-Co interaction, with interatomic distances of 2.7315(5) A, 2.6280(5) A, and 2.6309(5) A, respectively. This distance appears to decrease as the phosphine donors on Co become more electron-releasing, strengthening the dative Co-->Zr interaction. Cyclic voltammetry of 4-6 shows that all three compounds can be further reduced by two electrons at relatively mild reduction potentials (-1.65 V to -2.07 V vs Fc/Fc(+)). The potentials at which these reductions occur in each of these complexes are largely governed by the extent to which electron-density is donated to Zr, as well as the electron-donating ability of the phosphine substituents. Moreover, cyclic voltammetry of complex 8 reveals that in the absence of Zr, the Co center is significantly more electron rich, and thus more difficult to reduce. Chemical reduction of 5 leads to the isolation of the two-electron reduced dinitrogen complex [N(2)Co((i)Pr(2)PNMes)(3)ZrX][Na(THF)(5)] (9). X-ray crystallography of 9 reveals that two-electron reduction is accompanied by a significant contraction of the Co-Zr interatomic distance from 2.6280(5) A to 2.4112(3) A. These heterobimetallic complexes have been studied computationally using density functional theory to examine the nature of the metal-metal interactions in these species.

Metal-metal multiple bonds in early/late heterobimetallics support unusual trigonal monopyramidal geometries at both Zr and Co.

Reduction of Zr/Co heterobimetallic complexes ICo(MesNP(i)Pr(2))(3)ZrCl (1) and ICo((i)PrNP(i)Pr(2))(3)ZrCl (2) with excess Na/Hg under N(2), followed by subsequent benzene extraction to remove coordinated Na halide salts, leads to neutral two-electron reduced, dinitrogen-bound complexes (THF)Zr(MesNP(i)Pr(2))(3)Co-N(2) (4) and Zr((i)PrNP(i)Pr(2))(3)Co-N(2) (5). Upon halide loss, a THF solvent molecule coordinates to the axial site of the Zr center in 4, while this axial site remains unoccupied in 5. X-ray crystallography reveals short Co-Zr distances in 4 and 5, indicative of metal-metal multiple bonding, and an unprecedented trigonal monopyramidal geometry about the Zr center in 5. Reduction of 4 under an Ar atmosphere (in the absence of N(2)) results in another unusual structure type: an unoccupied axial Co coordination site and a trigonal monopyramidal Co center in (THF)Zr(MesNP(i)Pr(2))(3)Co (6). X-ray crystallography reveals that, in the absence of coordinated N(2), the Co-Zr bond can attain full triple bond character with a Co-Zr distance of 2.14 A, the shortest M-M distance in an early/late heterobimetallic complex reported to date. To further assess the electronic structure and bonding in 4, 5, and 6, calculations were performed on these molecules using DFT and the results of these theoretical investigations will be discussed.

Ligand Reactivity in Diarylamido/Bis(Phosphine) PNP Complexes of Mn(CO)3 and Re(CO)3

The syntheses of meridionally ligated tricarbonyl complexes (PNP)Mn(CO)(3) and (PNP)Re(CO)(3) are described (PNP = [2-P(CHMe(2))(2)-4-MeC(6)H(3)](2)N(-)). Cyclic voltammograms show reversible one-electron redox couples for both parent compounds (-0.34 V vs Cp(2)Fe(0/+) for (PNP)Mn(CO)(3), -0.25 V vs Cp(2)Fe(0/+) for (PNP)Re(CO)(3)), and chemical oxidation with AgOTf results in formation of the corresponding paramagnetic triflate salts [(PNP)Mn(CO)(3)]OTf and [(PNP)Re(CO)(3)]OTf. Electron paramagnetic resonance spectra and computational results indicate that this event is primarily ligand centered; allylation of the organic ligand moiety of [(PNP)Mn(CO)(3)]OTf with allyltributylstannane is consistent with this assignment. The oxidation (PNP)Mn(CO)(3) to [(PNP)Mn(CO)(3)]OTf results in a shift in average CO stretching frequency of 30 cm(-1); protonation of (PNP)Mn(CO)(3) with TfOH to form [(PNHP)Mn(CO)(3)]OTf results in a similar magnitude shift.

Room Temperature Dehydrogenation of Ethane to Ethylene

The transient titanium alkylidyne, (PNP)Ti≡C(t)Bu (PNP = N[2-P(i)Pr(2)-4-methylphenyl](2)(-)), activates a C-H bond of ethane at room temperature, and a β-hydrogen of the resulting ethyl ligand is subsequently transferred to the adjacent alkylidene ligand to form an ethylene adduct of titanium. Treatment of the ethylene complex with two-electron oxidants such as organic azides results in extrusion of ethene concomitant with formation of a mononuclear titanium imido complex.

A Planar Three-Coordinate Vanadium(II) Complex and the Study of Terminal Vanadium Nitrides from N2: A Kinetic or Thermodynamic Impediment to N–N Bond Cleavage?

We report the first mononuclear three-coordinate vanadium(II) complex [(nacnac)V(ODiiP)] and its activation of N2 to form an end-on bridging dinitrogen complex with a topologically linear V(III)N2V(III) core and where each vanadium center antiferromagnetically couples to give a ground state singlet with an accessible triplet state as inferred by HFEPR spectroscopy. In addition to investigating the conversion of N2 to the terminal nitride (as well as the microscopic reverse process), we discuss its similarities and contrasts to the isovalent d(3) system, [Mo(N[(t)Bu]Ar)3], and the S = 1 system [(Ar[(t)Bu]N)3Mo]2(μ2-η(1):η(1)-N2).

Reactivity of a Pd(I)−Pd(I) Dimer with O2: Monohapto Pd Superoxide and Dipalladium Peroxide in Equilibrium

The Pd(I)-Pd(I) dimer [((F)PNP)Pd-](2) reacts with O(2) upon exposure to light to produce either the superoxide ((F)PNP)PdO(2) or the peroxide [((F)PNP)PdO-](2), which exist in equilibrium with free O(2). Both complexes contain square-planar Pd(II) centers. The unpaired electron density in ((F)PNP)PdO(2) is localized on the superoxide ligand.

Fluorescence Switching of Imidazo[1,5-a]pyridinium Ions: pH-Sensors with Dual Emission Pathways

Imidazo[1,5-a]pyridinium ions are identified as highly emissive and water-soluble fluorophores accessed by an efficient three-component coupling reaction. Synthetic modifications of groups conjugated to the polyheterocyclic core are shown to profoundly impact the emission properties of these molecules. Notably, two structural isomers of functionalized imidazo[1,5-a]pyridinium ions were found to exhibit distinct de-excitation pathways, which are responsible for either a fluorescence turn-on or ratiometric response to pH change.

Room Temperature Dehydrogenation of Ethane, Propane, Linear Alkanes C4–C8, and Some Cyclic Alkanes by Titanium–Carbon Multiple Bonds

The transient titanium neopentylidyne, [(PNP)Ti≡C(t)Bu] (A; PNP(-)≡N[2-P(i)Pr2-4-methylphenyl]2(-)), dehydrogenates ethane to ethylene at room temperature over 24 h, by sequential 1,2-CH bond addition and β-hydrogen abstraction to afford [(PNP)Ti(η(2)-H2C═CH2)(CH2(t)Bu)] (1). Intermediate A can also dehydrogenate propane to propene, albeit not cleanly, as well as linear and volatile alkanes C4-C6 to form isolable α-olefin complexes of the type, [(PNP)Ti(η(2)-H2C═CHR)(CH2(t)Bu)] (R = CH3 (2), CH2CH3 (3), (n)Pr (4), and (n)Bu (5)). Complexes 1-5 can be independently prepared from [(PNP)Ti═CH(t)Bu(OTf)] and the corresponding alkylating reagents, LiCH2CHR (R = H, CH3(unstable), CH2CH3, (n)Pr, and (n)Bu). Olefin complexes 1 and 3-5 have all been characterized by a diverse array of multinuclear NMR spectroscopic experiments including (1)H-(31)P HOESY, and in the case of the α-olefin adducts 2-5, formation of mixtures of two diastereomers (each with their corresponding pair of enantiomers) has been unequivocally established. The latter has been spectroscopically elucidated by NMR via C-H coupled and decoupled (1)H-(13)C multiplicity edited gHSQC, (1)H-(31)P HMBC, and dqfCOSY experiments. Heavier linear alkanes (C7 and C8) are also dehydrogenated by A to form [(PNP)Ti(η(2)-H2C═CH(n)Pentyl)(CH2(t)Bu)] (6) and [(PNP)Ti(η(2)-H2C═CH(n)Hexyl)(CH2(t)Bu)] (7), respectively, but these species are unstable but can exchange with ethylene (1 atm) to form 1 and the free α-olefin. Complex 1 exchanges with D2C═CD2 with concomitant release of H2C═CH2. In addition, deuterium incorporation is observed in the neopentyl ligand as a result of this process. Cyclohexane and methylcyclohexane can be also dehydrogenated by transient A, and in the case of cyclohexane, ethylene (1 atm) can trap the [(PNP)Ti(CH2(t)Bu)] fragment to form 1. Dehydrogenation of the alkane is not rate-determining since pentane and pentane-d12 can be dehydrogenated to 4 and 4-d12 with comparable rates (KIE = 1.1(0) at ~29 °C). Computational studies have been applied to understand the formation and bonding pattern of the olefin complexes. Steric repulsion was shown to play an important role in determining the relative stability of several olefin adducts and their conformers. The olefin in 1 can be liberated by use of N2O, organic azides (N3R; R = 1-adamantyl or SiMe3), ketones (O═CPh2; 2 equiv) and the diazoalkane, N2CHtolyl2. For complexes 3-7, oxidation with N2O also liberates the α-olefin.

Net heterolytic cleavage of B-H and B-B bonds across the N-Pd bond in a cationic (PNP)Pd fragment.

The use of weakly coordinating anions BAr(F)(4) (where Ar(F) = 3,5-(CF(3))(2)C(6)H(3)) and CB(11)H(12) allows one to access clean reactions of the [(PNP)Pd](+) fragment (PNP = bis(2-(i)Pr(2)P4-Me-phenyl)amido) with the B-H bond in catecholborane (CatBH) and catecholdiboron (CatBBCat). In both cases, a net heterolytic cleavage of B-H or B-B takes place, with the nitrogen atom of PNP being a recipient of a boryl fragment. The resultant products [(PN(BCat)P)PdH](+) (2) and [(PN(BCat)P)PdBCat](+) (3) were isolated as either BAr(F)(4) or CB(11)H(12) salts and fully characterized. They are susceptible to hydrolysis, with the B-N bond hydrolyzing selectively and rapidly at RT to give [(PN(H)P)PdH](+) (1) and [(PN(H)P)PdBCat](+) (4). Notably, 4 and 2 are isomers, but they do not interconvert even under thermolysis at 90 °C. The Pd-B bond in 4 can be further hydrolyzed more slowly, to give 1. On the other hand, a Pd-B bond was formed from the Pd-H bond in 2 by reaction with excess CatBH (and evolution of H(2)), producing 3.