NEW LIGAND MOTIFS FOR PLATINUM-BASED `SHILOV CHEMISTRY' AND DETOURS INTO BASIC ORGANOMETALLIC RESEARCH

The C-H activation reaction at cationic platinum centers utilizing chelating aromatic N-type ligands has been widely studied in TFE (trifluoroethanol): a weakly coordinating solvent. In our laboratory, recent studies involving a modified dipyridine methane ligand revealed that benzene C-H activation in water, methanol and the activation of alkane substrates in TFE is possible. Anionic Pt(II) centers created via an anionic dipyridyl borate ligand present a new and promising direction towards realizing selective oxidation of alkanes. Rapid CH activation of alkanes and arenes is possible in biphasic water/hydrocarbon solvent mixtutes. In the course of CH activation studies with [dpbPtII(Me)2]- (dpb = di-2pyridyl-dimethyl-borate), the complex was found to yield olefin hydrides upon alkane activation. The yield of olefin hydride complexes with the dpb ligand proved low (30-40%). A lipophilic ligand (dtBupb = di-t-butylpyridyl-dimethyl-borate) activated various cyclic and linear olefins with near quantitative yields. The resultant olefin hydride complexes proved to be catalysts for transfer dehydrogenation of cyclic alkanes (TONs up to 13). We found that in the presence of a hydroxylic solvent, a very rapid oxidation of [dpbPtII(Me)2]- complex towards a PtIV species was observed. The proposed reaction mechanism includes rapid coordination of O2 by the highly electron-rich metal complex with subsequent nucleophiilic substitution reaction at boron and a methyl group transfer from the boron atom to the PtIV center. Oxidation with methyl iodide to give penta-coordinate dpbPtIVMe3 and its subsequent reaction with a hydroxylic solvent furnished the same product as under aerobic oxidation conditions. This proved that oxidation had to occur prior to methyl group transfer. Since in this case, our system can be considered as a mechanistic probe for Suzuki coupling, the insight into the nature of alkyl transfer provides a clear model of one the key steps of this widely-utilized transformation. Eventually, we were able to observe a reversible alkyl group transfer between PtIV and B in DMSO solutions. To probe the transfer of an aryl group between PtIV and B, a dpbPtIVMePh2 complex and a PtIVMe3 complex supported by (dpydphb = dipyridyl-diphenyl-borate) were synthesized. While phenyl transfer from PtIV to B was facile already in THF, the reverse, B-to-PtIV phenyl transfer was not observed due to the greater stabilization conferred to the complex by a B-Ph---PtIV moiety. The feasibility of a B-to-PtIV phenyl transfer was demonstrated when [dpydphbPtIIMe2] was oxidized by O2 in isopropanol

Stable Nickel(I) Complexes with Electron-Rich, Sterically-Hindered, Innocent PNP Pincer Ligands

The electronic and steric properties of a new class of electron-rich and sterically hindered tertamethylated PNP pincer ligands (Me4PNPR = 2,6-bisRdialkylphosphino)propylipyridine with R = Pr-i, Bu-t) are discussed. Introducing the methyl groups on the pincer arm prevents dearomatization of the pincer framework and increases the bulkiness and electron-donating capacity of the ligand. Highly reactive Ni-I species are thus prevented from dimerizing and can be analyzed by a wide variety of spectroscopic methods. X-ray diffraction study shows that steric bulk has an important influence on the resulting geometric and spectroscopic properties of the Ni-I complexes. Complexes S and 6, which contain Pr-i groups on the phosphorus atoms, show a very rare seesaw geometry around the metal center, while Bu-t complexes 7 and 8 show a distorted square-planar geometry. Computational analysis reveals that the SOMO for all complexes has a d(x)(-y)(2)(2) character with the spin density mostly residing on the nickel

Voltage Imaging of Cortical Oscillations in Layer 1 with Two-Photon Microscopy

Membrane voltage oscillations in layer 1 (L1) of primary sensory cortices might be important indicators of cortical gain control, attentional focusing, and signal integration. However, electric field recordings are hampered by the low seal resistance of electrodes close to the brain surface. To study L1 membrane voltage oscillations, we synthesized a new voltage-sensitive dye, di1-ANNINE-6plus, that can diffuse into tissue. We applied it with a new surgery, leaving the dura intact but allowing injection of large quantities of staining solution, and imaged cortical membrane potential oscillations with two-photon microscopy depth-resolved (25 to 100 microm below dura) in anesthetized and awake mice. We found delta (0.5-4 Hz), theta (4-10 Hz), low beta (10-20 Hz), and low gamma (30-40 Hz) oscillations. All oscillations were stronger in awake animals. While the power of delta, theta, and low beta oscillations increased with depth, the power of low gamma was more constant throughout L1. These findings identify L1 as an important coordination hub for the dynamic binding process of neurons mediated by oscillations

Cobalt Complexes of Bulky PNP Ligand: H2 Activation and Catalytic Two-Electron Reactivity in Hydrogenation of Alkenes and Alkynes

The reactivity of cobalt pincer complexes supported by the bulky tetramethylated PNP ligands Me4PNPR(R = iPr, tBu) has been investigated. In these ligands, the undesired H atom loss reactivity observed earlier in some classical CH2-arm PNP cobalt complexes is blocked, allowing them to be utilized for promoting two-electron catalytic transformations at the cobalt center. Accordingly, reaction of the formally CoIMe complex 3 with H2 under ambient pressure and temperature afforded the CoIII trihydride 4-H, in a reaction cascade reasoned to proceed by two-electron oxidative addition and reductive eliminations. This mechanistic proposal, alongside the observance of alkene insertion and ethane production upon sequential exposure of 3 to ethylene and H2, prompted an exploration into 3 as a catalyst for hydrogenation. Complex 4-H, formed in situ from 3 under H2, was found to be active in the catalytic hydrogenation of alkenes and alkynes. The proposed two-electron mechanism is reminiscent of the platinum group metals and demonstrates the utility of the bulky redox-innocent Me4PNPR ligand in the avoidance of one-electron reactivity, a concept that may show broad applicability in expanding the scope of earth-abundant first-row transition-metal catalysis.journal articl

Oxygen transfer reactivity mediated by nickel perfluoroalkyl complexes using molecular oxygen as a terminal oxidant

Nickel perfluoroethyl and perfluoropropyl complexes supported by naphthyridine-type ligands show drastically different aerobic reactivity from their trifluoromethyl analogs resulting in facile oxygen transfer to perfluoroalkyl groups or oxygenation of external organic substrates (phosphines, sulfides, alkenes and alcohols) using O2 or air as a terminal oxidant. Such mild aerobic oxygenation occurs through the formation of spectroscopically detected transient high-valent NiIII and structurally characterized mixed-valent NiII–NiIV intermediates and radical intermediates, resembling O2 activation reported for some Pd dialkyl complexes. This reactivity is in contrast with the aerobic oxidation of naphthyridine-based Ni(CF3)2 complexes resulting in the formation of a stable NiIII product, which is attributed to the effect of greater steric congestion imposed by longer perfluoroalkyl chains.journal articl

Facile and reversible double dearomatization of pyridines in non-phosphine MnI complexes with N,S-donor pyridinophane ligand

Single and double dearomatization of pyridine rings was observed in Mn(I) complexes with an N2S2 pyridinophane ligand via deprotonation of one or two CH2 arms, respectively. In contrast to other N,S-donor pincer-like systems, the dearomatized (N2S2)Mn species were found to be stable, with the dearomatization being reversible

Nickel(II) Complexes with Electron-Rich, Sterically Hindered PNP Pincer Ligands Enable Uncommon Modes of Ligand Dearomatization

We report the reactivity and characterization of hydride, methyl, and bromo Ni-II complexes with a new class of electron-rich and sterically hindered PNP pincer ligands, Me₄PNPR (R = Pr-i, Bu-t), in which a classical metal-ligand cooperative mode of reactivity via CH2 arm deprotonation is blocked by methylation. This enables new, uncommon modes of PNP ligand dearomatization that involve reactivity in the para position of the pyridine ring. In particular, the reduction of [(Me₄PNPiPr) (NiMe)-Me-II]B(Ar-F)(4) with KC₈ leads to the formation of a new C-C bond via dimerization of two complexes through the para position. This reactivity stands in sharp contrast to the previously reported bromo or chloro complexes, where stable Ni-I halogen moieties are formed. Computational analysis showed a greater propensity for ligand-centered radical formation for the presumed intermediate one-electron-reduced species. UV-induced homolysis of the Ni-II-Me bond in [(Me₄PNPiPr) (NiMe)-Me-II]B(Ar-F)(4) leads to the formation of a Me radical detected by radical traps and Ni(I )intermediates that can be trapped in the presence of halide ions to give previously characterized, stable Ni-I halogen complexes. In addition, treatment of the bromo complexes [(Me₄PNPR)(NiBr)-Br-II]BPh₄ with a powerful hydride source, LiBEt₃H, leads to the reduction of the pyridine ring and the formation of Ni-II complexes with an anionic amide donor reduced pincer ligand, although aromatic Ni-II hydride complexes could also be obtained with a weaker hydride source. We have observed that steric bulk at the phosphine donors controls the reactivity of the resulting Ni(II)H( )complexes. While t-Bu-substituted [(Me₄PNPtBu) (NiH)-H-II]Y(Y=BPh₄, B(Ar-F)(4)) does not react with O-2, the less sterically hindered Pr-i-substituted [(Me₄PNPiPr)(NiH)-H-II]Y reacts instantaneously to give an unstable superoxide adduct that can be observed by EPR

Catalytic Sulfone Upgrading Reaction with Alcohols via Ru(II)

Sulfones and sulfonamides with an α-CH bond can be easily alkylated by aliphatic alcohols to add the carbon skeleton of the alcohol via a one-step, Ru(II)-catalyzed redox neutral reaction. The reaction requires a substoichiometric amount of base and produces only water as a byproduct. Several pharmaceutically relevant functional groups such as piperidine, morpholine, etc. are well-tolerated under the reaction conditions to give higher value-added products in one step from widely available substrates. The reaction proceeds through a sulfone carbanion addition to an in-situ-generated aldehyde formed via catalytic dehydrogenation and subsequent catalyst-mediated replacement of the secondary alcohol by hydrogen

The Milstein Bipyridyl PNN Pincer Complex of Ruthenium Becomes a Noyori-Type Catalyst under Reducing Conditions

Hydrogenation of the dearomatized PNN ligand of the Milstein bipyridyl complex RuH(CO)[PNN] (2) gives a square-pyramidal Ru(II) product RuH(CO)[pPNN] (5). The central ring of the pPNN ligand is a piperidine. A minor byproduct of the hydrogenation reaction is complex 6 which has a dimeric structure made of two Ru(II) fragments each possessing a partly hydrogenated PNN ligand. The structures of 5 and 6 have been elucidated by NMR spectroscopy and X-ray crystallography. The PNN ligand of 2 is also hydrogenated under the conditions of the catalytic dehydrogenative coupling of ethanol to ethyl acetate. No direct evidence of the aromatized dihydride RuH2(CO)[PNN] (4) was found in this study. However, treating RuHCl(CO)[PNN] with Li[HBEt3] or reacting 2 with H2 at low temperature resulted in a structurally characterized hydride-bridged dimer (7) bearing intact aromatized bipyridyl ligands. M06-L/def2-QZVP DFT calculations provided insights into the thermodynamics of the stoichiometric reactions of this work and into the nature of the intermediates of the catalytic ester hydrogenation facilitated by RuH2(CO)[pPN(H)N] (8) formed from 5 under H2