Michael K. Takase

Influence of the Ligand Field on Slow Magnetization Relaxation versus Spin Crossover in Mononuclear Cobalt Complexes

The electronic and magnetic properties of the complexes [Co(terpy)Cl2 ] (1), [Co(terpy)(NCS)2 ] (2), and [Co(terpy)2 ](NCS)2 (3) were investigated. The coordination environment around Co(II) in 1 and 2 leads to a high-spin complex at low temperature and single-molecule magnet properties with multiple relaxation pathways. Changing the ligand field and geometry with an additional terpy ligand leads to spin-crossover behavior in 3 with a gradual transition from high spin to low spin.

Evidence for in Situ Catalyst Modification during the Pd-Catalyzed Conversion of Aryl Triflates to Aryl Fluorides

A mechanistic investigation of the Pd-catalyzed conversion of aryl triflates to fluorides is presented. Studies reveal that C-F reductive elimination from a LPd(II)(aryl)F complex (L = t-BuBrettPhos or RockPhos) does not occur when the aryl group is electron rich. Evidence is presented that a modified phosphine, generated in situ, serves as the actual supporting ligand during catalysis with such substrates. A preliminary study of the reactivity of a LPd(II)(aryl)F complex based on this modified ligand is reported.

Z-Selective Olefin Metathesis Reactions Promoted by Tungsten Oxo Alkylidene Complexes

Addition of LiOHMT (OHMT = O-2,6-dimesitylphenoxide) to W(O)(CH-t-Bu)(PMe(2)Ph)(2)Cl(2) led to WO(CH-t-Bu)Cl(OHMT)(PMe(2)Ph) (4). Subsequent addition of Li(2,5-Me(2)C(4)H(2)N) to 4 yielded yellow W(O)(CH-t-Bu)(OHMT)(Me(2)Pyr)(PMe(2)Ph) (5). Compound 5 is a highly effective catalyst for the Z-selective coupling of selected terminal olefins (at 0.2% loading) to give product in >75% yield with >99% Z configuration. Addition of 2 equiv of B(C(6)F(5))(3) to 5 afforded a catalyst activated at the oxo ligand by B(C(6)F(5))(3). 5·B(C(6)F(5))(3) is a highly active catalyst that produces thermodynamic products (~20% Z).

Synthesis and Structure of Six-Coordinate Iron Borohydride Complexes Supported by PNP Ligands

The preparation of a number of iron complexes supported by ligands of the type HN{CH2CH2(PR2)}2 [R = isopropyl (((i)Pr)PNP) or cyclohexyl ((Cy)PNP)] is reported. This is the first time this important bifunctional ligand has been coordinated to iron. The iron(II) complexes (((i)Pr)PNP)FeCl2(CO) (1a) and ((Cy)PNP)FeCl2(CO) (1b) were synthesized through the reaction of the appropriate free ligand and FeCl2 in the presence of CO. The iron(0) complex (((i)Pr)PNP)Fe(CO)2 (2a) was prepared through the reaction of Fe(CO)5 with ((i)Pr)PNP, while irradiating with UV light. Compound 2a is unstable in CH2Cl2 and is oxidized to 1a via the intermediate iron(II) complex [(((i)Pr)PNP)FeCl(CO)2]Cl (3a). The reaction of 2a with HCl generated the related complex [(((i)Pr)PNP)FeH(CO)2]Cl (4a), while the neutral iron hydrides (((i)Pr)PNP)FeHCl(CO) (5a) and ((Cy)PNP)FeHCl(CO) (5b) were synthesized through the reaction of 1a or 1b with 1 equiv of (n)Bu4NBH4. The related reaction between 1a and excess NaBH4 generated the unusual η(1)-HBH3 complex (((i)Pr)PNP)FeH(η(1)-HBH3)(CO) (6a). This complex features a bifurcated intramolecular dihydrogen bond between two of the hydrogen atoms associated with the η(1)-HBH3 ligand and the N-H proton of the pincer ligand, as well as intermolecular dihydrogen bonding. The protonation of 6a with 2,6-lutidinium tetraphenylborate resulted in the formation of the dimeric complex [{(((i)Pr)PNP)FeH(CO)}2(μ2,η(1):η(1)-H2BH2)][BPh4] (7a), which features a rare example of a μ2,η(1):η(1)-H2BH2 ligand. Unlike all previous examples of complexes with a μ2,η(1):η(1)-H2BH2 ligand, there is no metal-metal bond and additional bridging ligand supporting the borohydride ligand in 7a; however, it is proposed that two dihydrogen-bonding interactions stabilize the complex. Complexes 1a, 2a, 3a, 4a, 5a, 6a, and 7a were characterized by X-ray crystallography.

Nickel(I) Monomers and Dimers with Cyclopentadienyl and Indenyl Ligands

The reaction of (μ-Cl)2Ni2(NHC)2 (NHC = 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene (IPr) or 1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene (SIPr)) with either one equivalent of sodium cyclopentadienyl (NaCp) or lithium indenyl (LiInd) results in the formation of diamagnetic NHC supported Ni(I) dimers of the form (μ-Cp)(μ-Cl)Ni2(NHC)2 (NHC = IPr (1 a) or SIPr (1 b); Cp = C5H5) or (μ-Ind)(μ-Cl)Ni2(NHC)2 (NHC = IPr (2 a) or SIPr (2 b); Ind = C7H9), which contain bridging Cp and indenyl ligands. The corresponding reaction between two equivalents of NaCp or LiInd and (μ-Cl)2Ni2(NHC)2 (NHC = IPr or SIPr) generates unusual 17 valence electron Ni(I) monomers of the form (η(5)-Cp)Ni(NHC) (NHC = IPr (3 a) or SIPr (3 b)) or (η(5)-Ind)Ni(NHC) (NHC = IPr (4 a) or SIPr (4 b)), which have nonlinear geometries. A combination of DFT calculations and NBO analysis suggests that the Ni(I) monomers are more strongly stabilized by the Cp ligand than by the indenyl ligand, which is consistent with experimental results. These calculations also show that the monomers have a lone unpaired-single-electron in their valence shell, which is the reason for the nonlinear structures. At room temperature the Cp bridged dimer (μ-Cp)(μ-Cl)Ni2(NHC)2 undergoes homolytic cleavage of the Ni-Ni bond and is in equilibrium with (η(5)-Cp)Ni(NHC) and (μ-Cl)2Ni2(NHC)2. There is no evidence that this equilibrium occurs for (μ-Ind)(μ-Cl)Ni2(NHC)2. DFT calculations suggest that a thermally accessible triplet state facilitates the homolytic dissociation of the Cp bridged dimers, whereas for bridging indenyl species this excited triplet state is significantly higher in energy. In stoichiometric reactions, the Ni(I) monomers (η(5)-Cp)Ni(NHC) or (η(5)-Ind)Ni(NHC) undergo both oxidative and reductive processes with mild reagents. Furthermore, they are rare examples of active Ni(I) precatalysts for the Suzuki-Miyaura reaction. Complexes 1 a, 2 b, 3 a, 4 a and 4 b have been characterized by X-ray crystallography.

Toward models for the full oxygen-evolving complex of photosystem II by ligand coordination to lower the symmetry of the Mn3CaO4 cubane: demonstration that electronic effects facilitate binding of a fifth metal.

Synthetic model compounds have been targeted to benchmark and better understand the electronic structure, geometry, spectroscopy, and reactivity of the oxygen-evolving complex (OEC) of photosystem II, a low-symmetry Mn4CaOn cluster. Herein, low-symmetry MnIV3GdO4 and MnIV3CaO4 cubanes are synthesized in a rational, stepwise fashion through desymmetrization by ligand substitution, causing significant cubane distortions. As a result of increased electron richness and desymmetrization, a specific μ3-oxo moiety of the Mn3CaO4 unit becomes more basic allowing for selective protonation. Coordination of a fifth metal ion, Ag+, to the same site gives a Mn3CaAgO4 cluster that models the topology of the OEC by displaying both a cubane motif and a “dangler” transition metal. The present synthetic strategy provides a rational roadmap for accessing more accurate models of the biological catalyst.

An Anionic N-Donor Ligand Promotes Manganese-Catalyzed Water Oxidation

Four manganese complexes of pentadentate ligands have been studied for their ability to act as oxygen evolution catalysts in the presence of Oxone or hydrogen peroxide. The complexes [Mn(PaPy3)(NO3)](ClO4) (1) (PaPy3H = N,N-bis(2-pyridylmethyl)-amine-N-ethyl-2-pyridine-2-carboxamide) and [Mn(PaPy3)(μ-O)(PaPy3)Mn](ClO4)2 (2) feature an anionic carboxamido ligand trans to the labile sixth coordination site, while [Mn(N4Py)OTf](OTf) (3) (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) and [Mn(PY5)(OH2)](ClO4)2 (4) (PY5 = 2,6-bis(bis(2-pyridyl)methoxymethane)-pyridine) have neutral ligands of varying flexibility. 1 and 2 are shown to evolve oxygen in the presence of either Oxone or hydrogen peroxide, but 3 evolves oxygen only in the presence of hydrogen peroxide. 4 is inactive. The activity of 1 and 2 with Oxone suggests that the presence of an anionic N-donor ligand plays a role in stabilizing putative high-valent intermediates. Anionic N-donor ligands may be viewed as alternatives to μ-oxo ligands that are prone to protonation in low-valent Mn species formed during a catalytic cycle, resulting in loss of catalyst structure.

Proton donor acidity controls selectivity in nonaromatic nitrogen heterocycle synthesis.

Acid-Derived Diversity Compounds with nitrogen-bearing rings have proven rather promising in pharmaceutical research, spurring the need for improved synthetic methods to access structurally diverse variants of this motif. Duttwyler et al. (p. 678) show that applying acids of different strengths to a dihydropyridine intermediate leads to selective protonation at either of two sites, depending on whether the reaction proceeds under kinetic or thermodynamic (that is, equilibrated) control. The protonations in turn activate the rings for addition of various carbon nucleophiles to the periphery, thereby affording multiple different substitution patterns for use in screening studies. Acids of different strengths propel a common intermediate to a diverse array of compounds sought in pharmaceutical research. Piperidines are prevalent in natural products and pharmaceutical agents and are important synthetic targets for drug discovery and development. We report on a methodology that provides highly substituted piperidine derivatives with regiochemistry selectively tunable by varying the strength of acid used in the reaction. Readily available starting materials are first converted to dihydropyridines via a cascade reaction initiated by rhodium-catalyzed carbon-hydrogen bond activation. Subsequent divergent regio- and diastereoselective protonation of the dihydropyridines under either kinetic or thermodynamic control provides two distinct iminium ion intermediates that then undergo highly diastereoselective nucleophilic additions. X-ray structural characterization of both the kinetically and thermodynamically favored iminium ions along with density functional theory calculations provide a theoretical underpinning for the high selectivities achieved for the reaction sequences.

Proton-Assisted Reduction of CO2 by Cobalt Aminopyridine Macrocycles

We report here the efficient reduction of CO2 to CO by cobalt aminopyridine macrocycles. The effect of the pendant amines on catalysis was investigated. Several cobalt complexes based on the azacalix[4](2,6)pyridine framework with different substitutions on the pendant amine groups have been synthesized (R = H (1), Me (2), and allyl (3)), and their electrocatalytic properties were explored. Under an atmosphere of CO2 and in the presence of weak Brønsted acids, large catalytic currents are observed for 1, corresponding to the reduction of CO2 to CO with excellent Faradaic efficiency (98 ± 2%). In comparison, complexes 2 and 3 generate CO with TONs at least 300 times lower than 1, suggesting that the presence of the pendant NH moiety of the secondary amine is crucial for catalysis. Moreover, the presence of NH groups leads to a positive shift in the reduction potential of the Co(I/0) couple, therefore decreasing the overpotential for CO2 reduction.

Origins of Initiation Rate Differences in Ruthenium Olefin Metathesis Catalysts Containing Chelating Benzylidenes

A series of second-generation ruthenium olefin metathesis catalysts was investigated using a combination of reaction kinetics, X-ray crystallography, NMR spectroscopy, and DFT calculations in order to determine the relationship between the structure of the chelating o-alkoxybenzylidene and the observed initiation rate. Included in this series were previously reported catalysts containing a variety of benzylidene modifications as well as four new catalysts containing cyclopropoxy, neopentyloxy, 1-adamantyloxy, and 2-adamantyloxy groups. The initiation rates of this series of catalysts were determined using a UV/vis assay. All four new catalysts were observed to be faster-initiating than the corresponding isopropoxy control, and the 2-adamantyloxy catalyst was found to be among the fastest-initiating Hoveyda-type catalysts reported to date. Analysis of the X-ray crystal structures and computed energy-minimized structures of these catalysts revealed no correlation between the Ru-O bond length and Ru-O bond strength. On the other hand, the initiation rate was found to correlate strongly with the computed Ru-O bond strength. This latter finding enables both the rationalization and prediction of catalyst initiation through the calculation of a single thermodynamic parameter in which no assumptions about the mechanism of the initiation step are made.