Showing papers by "Jonas C. Peters published in 2004"

A Tetrahedrally Coordinated L3Fe−Nx Platform that Accommodates Terminal Nitride (FeIV⋮N) and Dinitrogen (FeI−N2−FeI) Ligands

Abstract: A tetrahedrally coordinated L3Fe−Nx platform that accommodates both terminal nitride (L3FeIV⋮N) and dinitrogen (L3FeI−N2−FeIL3) functionalities is described. The diamagnetic L3FeIV⋮N species featured has been characterized in solution under ambient conditions by multinuclear NMR (1H, 31P, and 15N) and infrared spectroscopy. The electronic structure of the title complex has also been explored using DFT. The terminal nitride complex oxidatively couples to generate the previously reported L3FeI−N2−FeIL3 species. This reaction constitutes a six-electron transformation mediated by two iron centers. Reductive protonation of the nitride complex releases NH3 as a significant reaction product.

Considering Fe(II/IV) redox processes as mechanistically relevant to the catalytic hydrogenation of olefins by [PhBP iPr 3]Fe-H x species.

Abstract: Several coordinatively unsaturated pseudotetrahedral iron(II) precursors, [PhBP^(iPr)_3]Fe−R ([PhBP^(iPr)_3] = [PhB(CH2P^(iPr)_2)_3]^-; R = Me (2), R = CH_2Ph (3), R = CH_2CMe_3 (4)) have been prepared from [PhBP^(iPr)_3]FeCl (1) that serve as precatalysts for the room-temperature hydrogenation of unsaturated hydrocarbons (e.g., ethylene, styrene, 2-pentyne) under atmospheric H_2 pressure. The solid-state crystal structures of 2 and 3 are presented. To gain mechanistic insight into the nature of these hydrogenation reactions, a number of [PhBP^(iPr)_3]-supported iron hydrides were prepared and studied. Room-temperature hydrogenation of alkyls 2−4 in the presence of a trapping phosphine ligand affords the iron(IV) trihydride species [PhBP^(iPr)_3]Fe(H)_3(PR_3) (PR_3 = PMe_3 (5); PR_3 = PEt_3 (6); PR3 = PMePh_2 (7)). These spectroscopically well-defined trihydrides undergo hydrogen loss to varying degrees in solution, and for the case of 7, this process leads to the structurally identified Fe(II) hydride product [PhBP^(iPr)_3]Fe(H)(PMePh_2) (9). Attempts to prepare 9 by addition of LiEt_3BH to 1 instead lead to the Fe(I) reduction product [PhBP^(iPr)_3]Fe(PMePh_2) (10). The independent preparations of the Fe(II) monohydride complex [PhBP^(iPr)_3]FeII(H)(PMe_3) (11) and the Fe(I) phosphine adduct [PhBP^(iPr)_3]Fe(PMe_3) (8) are described. The solid-state crystal structures of trihydride 5, monohydride 11, and 8 are compared and demonstrate relatively little structural reorganization with respect to the P_3Fe−P‘ core motif as a function of the iron center's formal oxidation state. Although paramagnetic 11 (S = 1) is quantitatively converted to the diamagnetic trihydride 5 under H_2, the Fe(I) complex 8 (S = 3/2) is inert toward atmospheric H_2. Complex 10 is likewise inert toward H_2. Trihydrides 5 and 6 also serve as hydrogenation precatalysts, albeit at slower rates than that for the benzyl complex 3 because of a rate-contributing phosphine dependence. That these hydrogenations appear to proceed via well-defined olefin insertion steps into an Fe−H linkage is indicated by the reaction between trihydride 5 and ethylene, which cleanly produces the ethyl complex [PhBP^(iPr)_3]Fe(CH_2CH_3) (13) and an equivalent of ethane. Mechanistic issues concerning the overall reaction are described.

The Coordination Chemistry of “[BP_3]NiX” Platforms: Targeting Low-Valent Nickel Sources as Promising Candidates to L_3Ni=E and L_3Ni≡E Linkages

Abstract: A series of divalent, monovalent, and zerovalent nickel complexes supported by the electron-releasing, monoanionic tris(phosphino)borate ligands [PhBP_3] and [PhBP^(iPr)_3] ([PhBP_3] = [PhB(CH_2PPh_2)_3]-, [PhBP^(iPr)_3] = [PhB(CH_2PiPr_2)_3]-) have been synthesized to explore fundamental aspects of their coordination chemistry. The pseudotetrahedral, divalent halide complexes [PhBP_3]NiCl (1), [PhBP_3]NiI (2), and [PhBP^(iPr)_3]NiCl (3) were prepared by the metalation of [PhBP_3]Tl or [PhBP^(iPr)_3]Tl with (Ph_3P)_2NiCl_2, NiI_2, and (DME)NiCl_2 (DME = 1,2-dimethoxyethane), respectively. Complex 1 is a versatile precursor to a series of complexes accessible via substitution reactions including [PhBP_3]Ni(N_3) (4), [PhBP_3]Ni(OSiPh_3) (5), [PhBP_3]Ni(O-p-tBu-Ph) (6), and [PhBP_3]Ni(S-p-tBu-Ph) (7). Complexes 2−5 and 7 have been characterized by X-ray diffraction (XRD) and are pseudotetrahedral monomers in the solid state. Complex 1 reacts readily with oxygen to form the four-electron-oxidation product, {[PhB(CH_2P(O)Ph_2)_2(CH_2PPh_2)]NiCl} (8A or 8B), which features a solid-state structure that is dependent on its method of crystallization. Chemical reduction of 1 using Na/Hg or other potential 1-electron reductants generates a product that arises from partial ligand degradation, [PhBP_3]Ni(η^2-CH_2PPh_2) (9). The more sterically hindered chloride 3 reacts with Li(dbabh) (Hdbabh = 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene) to provide the three-coordinate complex [κ^2-PhBP^(iPr)_3]Ni(dbabh) (11), also characterized by XRD. Chemical reduction of complex 1 in the presence of L-type donors produces the tetrahedral Ni(I) complexes [PhBP_3]Ni(PPh_3) (12) and [PhBP3]Ni(CNtBu) (13). Reduction of 3 following the addition of PMe_3 or tert-butyl isocyanide affords the Ni(I) complexes [PhBP^(iPr)_3]Ni(PMe_3) (14) and [PhBP^(iPr)_3]Ni(CN^tBu) (15), respectively. The reactivity of these [PhBP_3]Ni^IL and [PhBP^(iPr)_3]NiI^L complexes with respect to oxidative group transfer reactions from organic azides and diazoalkanes is discussed. The zerovalent nitrosyl complex [PhBP_3]Ni(NO) (16) is prepared by the reaction of 1 with excess NO or by treating 12 with stoichiometric NO. The anionic Ni(0) complexes [[κ^2-PhBP_3]Ni(CO)_2][^nBu_4N] (17) and [[κ^2-PhBP^(iPr)_3]Ni(CO)_2][ASN] (18) (ASN = 5-azoniaspiro[4.4]nonane) have been prepared by reacting [PhBP_3]Tl or [PhBP^(iPr)_3]Tl with (Ph_3P)_2Ni(CO)_2 in the presence of R_4NBr. The photolysis of 17 appears to generate a new species consistent with a zerovalent monocarbonyl complex which we tentatively assign as {[PhBP_3]Ni(CO)}{^nBu_4N}, although complete characterization of this complex has been difficult. Finally, theoretical DFT calculations are presented for the hypothetical low spin complexes [PhBP_3]Ni(N^tBu), [PhBP^(iPr)_3]Ni(N^tBu), [PhBP^(iPr)_3]Ni(NMe), and [PhBP^(iPr)_3]Ni(N) to consider what role electronic structure factors might play with respect to the relative stability of these species.

Hydrogenolysis of [PhBP3]Fe⋮N-p-tolyl: Probing the Reactivity of an Iron Imide with H2

Abstract: This paper describes the reductive hydrogenolysis of a low-spin (S = 1/2) iron(III) imide. Pseudotetrahedral [PhBP_3]Fe^(III)≡N-p-tolyl is reduced by hydrogen at ambient temperature and pressure in benzene solution. The reduction appears to proceed in a stepwise fashion. An intermediate S = 2 iron(II) anilide, [PhBP_3]Fe(N(H)-p-tolyl), is observed and has been independently generated and structurally characterized. Prolonged hydrogenolysis in benzene results in the complete hydrogenolysis of the Fe−N linkage to release H2N-p-tolyl. The major iron-containing product formed from this step is the diamagnetic cyclohexadienyl complex, [PhBP_3]Fe(η^5-cyclohexadienyl), which has also been independently prepared and structurally characterized. Evidence is presented to suggest that the final [PhBP_3]Fe(η^5-cyclohexadienyl) product is formed via benzene insertion into a reactive [PhBP_3]Fe^(II)-H intermediate.

Amido-bridged Cu2N2 diamond cores that minimize structural reorganization and facilitate reversible redox behavior between a Cu1Cu1 and a class III delocalized Cu1.5Cu1.5 species.

Abstract: A novel Cu_(2)N_(2) diamond core structure supported by an [SNS]^(-) ligand (1) ([SNS]^(-) = bis(2-tert-butylsulfanylphenyl)amido) has been prepared. This dicopper system exhibits a fully reversible one-electron redox process between a reduced Cu1Cu1 complex, {[SNS][Cu]}_(2) (2), and a class III delocalized Cu^(1.5)Cu^(1.5) state, [{[SNS][Cu]}_(2_][B(3,5-(CF_(3))_(2)C_(6)H_(3))_(4)] (3). Structural snapshots of both redox forms have been obtained to reveal remarkably little overall structural reorganization. The Cu···Cu bond distance nonetheless undergoes an appreciable compression (0.13 A) upon oxidation, providing a Cu···Cu distance of 2.4724(4) A in the mixed-valence state that is virtually identical to the Cu···Cu distance observed in the reduced form of the Cu_(A) site of thiolate-bridged cytochrome c oxidase. Despite the low structural reorganization evident between 2 and 3, the [SNS]^(-) ligand is quite flexible. For example, square-planar geometries can prevail for divalent copper ions supported by [SNS]^(-) as evident from the crystal structure of [SNS]CuCl (4). Physical characterization for the mixed valence complex 3 includes electrochemical, magnetic (SQUID), EPR, and optical data. The complex has also been examined by density functional methods. An attempt was made to measure the rate of electron self-exchange ks between the Cu^(1)Cu^(1) and the Cu^(1.5)Cu^(1.5) complexes 2 and 3 by NMR line-broadening analysis in dichloromethane solution. While the system is certainly in the fast-exchange regime, the exchange process is too fast to be accurately measured by this technique. The value for ks can be bracketed with a conservative lower boundary of ≥107 M^(-1) s^(-1), a value that appears to be larger than other low molecular weight copper model complexes for which similar data is available. The unusually large magnitude of ks likely reflects the minimal structural reorganization that accompanies Cu^(1)Cu^(1) ↔ Cu^(1.5)Cu^(1.5) interchange.

Synthetic, structural, and mechanistic aspects of an amine activation process mediated at a zwitterionic Pd(II) center.

Abstract: A zwitterionic palladium complex [[Ph_(2)BP_(2)]Pd(THF)_(2)][OTf] (1) (where [Ph_(2)BP_(2)] = [Ph_(2_B(CH_(2)PPh_(2))_(2)]-) reacts with trialkylamines to activate a C−H bond adjacent to the amine N atom, thereby producing iminium adduct complexes [Ph_(2)BP_(2)]Pd(N,C:η^(2)-NR_(2)CHR‘). In all cases examined the amine activation process is selective for the secondary C−H bond position adjacent to the N atom. These palladacycles undergo facile β-hydride elimination/olefin reinsertion processes as evident from deuterium scrambling studies and chemical trap studies. The kinetics of the amine activation process was explored, and β-hydride elimination appears to be the rate-limiting step. A large kinetic deuterium isotope effect for the amine activation process is evident. The reaction profile in less polar solvents such as benzene and toluene is different at room temperature and leads to dimeric {[Ph_(2)BP_(2)]Pd}_(2) (4) as the dominant palladium product. Low-temperature toluene-d8 experiments proceed more cleanly, and intermediates assigned as [Ph_(2)BP_(2)]Pd(NEt_(3))(OTf) and the iminium hydride species [[Ph_(2)BP_(2)]Pd(H)(Et_(2)NCHCH_(3))][OTf] are directly observed. The complex (Ph2SiP2)Pd(OTf)2 (14) was also studied for amine activation and generates dimeric [(Ph_(2)SiP_(2))Pd]_(2)[OTf]_(2) (16) as the dominant palladium product. These collective data are discussed with respect to the mechanism of the amine activation and, in particular, the influence that solvent polarity and charge have on the overall reaction profile.

Coordinating Anions: (Phosphino)tetraphenylborate Ligands as New Reagents for Synthesis

Abstract: Anionic, electron-releasing phosphines that incorporate a borate counteranion within the ligand framework are promising reagents for organometallic catalysis. This report describes the synthesis of a new class of monodentate tertiary phosphines built upon the commonly employed tetraphenylborate anion. These new phosphines are highly stable and strongly electron-releasing and readily coordinate transition metals. Moreover, they are promising reagents for catalysis, as demonstrated by their ability to promote the Suzuki cross-coupling of aryl chloride substrates.

Issues Relevant to C-H Activation at Platinum(II): Comparative Studies between Cationic, Zwitterionic, and Neutral Platinum(II) Compounds in Benzene Solution

Abstract: Cationic late metal systems are being highly scrutinized due to their propensity to mediate so-called electrophilic C-H activation reactions. This contribution compares the reactivity of highly reactive cationic platinum(II) systems with structurally related but neutral species. Our experimental design exploits isostructural neutral and cationic complexes supported by bis(phosphine) ligands amenable to mechanistic examination in benzene solution. The data presented herein collectively suggests that neutral platinum complexes can be equally if not more reactive towards benzene than their cationic counter-parts. Moreover, a number of unexpected mechanistic distinctions between the two systems arise that help to explain their respective reactivity.

Chemistry of tris(phosphino)borate supported iron compounds featuring metal-nitrogen multiple bonding

Abstract: Iron compounds featuring metal-ligand multiple bonds have been postulated in both biological and catalytic processes. Synthetically, however, isolated examples of such compounds have remained relatively elusive. To this extent, we have employed the anionic tris(phosphino)borate ligand [PhB(CH_2PPh_2)_3]- for the synthesis of iron compounds featuring metal-nitrogen multiple bonds; including monomeric terminal imides. The synthesis, properties, and reactivity of these compounds will be presented, highlighting both group transfer chemistry and the hydrogenolytic cleavage of an iron-nitrogen triple bond.