Michael W. Day

A Synthetic Model of the Mn3Ca Subsite of the Oxygen-Evolving Complex in Photosystem II

A model compound sheds light on the puzzling role of calcium in the metal cluster that oxidizes water during photosynthesis. Within photosynthetic organisms, the oxygen-evolving complex (OEC) of photosystem II generates dioxygen from water using a catalytic Mn4CaOn cluster (n varies with the mechanism and nature of the intermediate). We report here the rational synthesis of a [Mn3CaO4]6+ cubane that structurally models the trimanganese-calcium–cubane subsite of the OEC. Structural and electrochemical comparison between Mn3CaO4 and a related Mn4O4 cubane alongside characterization of an intermediate calcium-manganese multinuclear complex reveals potential roles of calcium in facilitating high oxidation states at manganese and in the assembly of the biological cluster.

Nickel-Mediated Hydrogenolysis of C–O Bonds of Aryl Ethers: What Is the Source of the Hydrogen?

Mechanistic studies of the hydrogenolysis of aryl ethers by nickel were undertaken with (diphosphine)aryl methyl ethers. A Ni(0) complex containing Ni-arene interactions adjacent to the aryl-O bond was isolated. Heating led to aryl-O bond activation and generation of a nickel aryl methoxide complex. Formal β-H elimination from this species produced a nickel aryl hydride which can undergo reductive elimination in the presence of formaldehyde to generate a carbon monoxide adduct of Ni(0). The reported complexes map out a plausible mechanism of aryl ether hydrogenolysis catalyzed by nickel. Investigations of a previously reported catalytic system using isotopically labeled substrates are consistent with the mechanism proposed in the stoichiometric system, involving β-H elimination from a nickel alkoxide rather than cleavage of the Ni-O bond by H(2).

Conformations of N-heterocyclic carbene ligands in ruthenium complexes relevant to olefin metathesis.

The structure of ruthenium-based olefin metathesis catalyst 3 and model pi-complex 5 in solution and in the solid state are reported. The N-tolyl ligands, due to their lower symmetry than the traditional N-mesityl substituents, complicate this analysis, but ultimately provide explanation for the enhanced reactivity of 3 relative to standard catalyst 2. The tilt of the N-tolyl ring provides additional space near the ruthenium center, which is consistent with the enhanced reactivity of 3 toward sterically demanding substrates. Due to this tilt, the more sterically accessible face bears the two methyl substituents of the N-aryl rings. These experimental studies are supported by computational studies of these complexes by DFT. The experimental data provides a means to validate the accuracy of the B3LYP and M06 functionals. B3LYP provides geometries that match X-ray crystal structural data more closely, though it leads to slightly less (approximately 0.5 kcal mol-1) accuracy than M06 most likely because it underestimates attractive noncovalent interactions.

Decomposition Pathways of Z-Selective Ruthenium Metathesis Catalysts

The decomposition of a Z-selective ruthenium metathesis catalyst and structurally similar analogues has been investigated utilizing X-ray crystallography and density functional theory. Isolated X-ray crystal structures suggest that recently reported C-H activated catalysts undergo decomposition via insertion of the alkylidene moiety into the chelating ruthenium-carbon bond followed by hydride elimination, which is supported by theoretical calculations. The resulting ruthenium hydride intermediates have been implicated in previously observed olefin migration, and thus lead to unwanted byproducts in cross metathesis reactions. Preventing these decomposition modes will be essential in the design of more active and selective Z-selective catalysts.

Bimetallic effects on ethylene polymerization in the presence of amines: inhibition of the deactivation by Lewis bases.

Dinickel complexes supported by terphenyl ligands appended with phenoxy and imine donors were synthesized. Full substitution of the central arene blocks rotation around the aryl-aryl bond and allows for the isolation of atropisomers. The reported complexes perform ethylene polymerization in the presence of amines. The inhibiting effect of polar additives is up to 250 times lower for the syn isomer than the anti isomer. Comparisons with mononuclear systems indicate that the proximity of the metal centers leads to the observed inhibitory effect on the deactivation of the catalysts.

Intramolecular and intermolecular C-H activation at a cationic PtII center

Abstract Syntheses and C-H bond activation reactions of the novel electrophilic PtII complexes [(tmeda)Pt(CH3)(OEt2)][BAr1], [(tmeda)Pt(CH3)(THF)][BArf], and [(tmeda)Pt(CH3)(NC5F5)][BArf] are described {[BArf]− = [(3,5-C6H3(CF3)2)4B]−} (tmeda is N,N,N′,N′-tetramethylethylenediamine), [(tmeda)Pt(CH3)(OEt2)][BArf] and [(tmeda)Pt(CH3)(THF)][BArf] are unstable at room temperature, yielding methane and the Fischer carbene PtII hydrides, [(tmeda)Pt(=C(CH3)(OCH2CH3))(H)][BArf] and P t ( = C C H 2 C H 2 C H 2 ⎴ O ) ( H ) ] [ B A r 1 ] . The methane liberated from [(tmeda)Pt(CH3)(OEt2-d10)][BArf] consists of an isotopomeric mixture, (CH4, CH3D, CH2D2 and CHD3), indicating a multiple H/D exchange reaction following the C-D activation and prior to methane loss. [(tmeda)Pt(CH3)(THF-d8)][BAr] liberates CH4 and CH3D. Methane-13C, cyclohexane, toluene, and benzene react with [(tmeda)Pt(CH3)(NC5F5)][BArf] to yield methane and new organoplatinum complexes. Deuterated alkanes and arenes react with [(tmeda)Pt(CH3)(NC5F5] [BArf] to give a mixture of methane isotopomers. The relevance of these results to the oxidation of alkanes by aqueous platinum complexes is discussed.

Trinucleating Copper: Synthesis and Magnetostructural Characterization of Complexes Supported by a Hexapyridyl 1,3,5‐Triarylbenzene Ligand

Copper threesome: A hexapyridyl ligand based upon a 1,3,5-triphenylbenzene framework coordinates three metal centers in a constrained environment (see picture). The tricopper(I) complex reduces dioxygen to form a tricopper(II) cluster. The capping anions affect the magnetism and EPR spectra of these species and reveal a linear dependence between the antiferromagnetic exchange parameter and the Cu-O-Cu angles.

Nickel Hydrides Supported by a Non-Innocent Diphosphine Arene Pincer: Mechanistic Studies of Nickel−Arene H-Migration and Partial Arene Hydrogenation

Nickel hydrides supported by a terphenyl diphosphine were synthesized and found to undergo nickel-to-arene H-transfers. Some of the resulting complexes also undergo the reverse (C-to-Ni) H-migration, indicating the potential for storing H-equivalents in this type of pincer ligand. NMR spectroscopy, single crystal X-ray diffraction, and isotopic labeling studies investigating the mechanism of these processes are discussed.

Crystallization and X-ray structure determination of cytochrome c2 from Rhodobacter sphaeroides in three crystal forms.

Cytochrome c(2) serves as the secondary electron donor that reduces the photo-oxidized bacteriochlorophyll dimer in photosynthetic bacteria. Cytochrome c(2) from Rhodobacter sphaeroides has been crystallized in three different forms. At high ionic strength, crystals of a hexagonal space group (P6(1)22) were obtained, while at low ionic strength, triclinic (P1) and tetragonal (P4(1)2(1)2) crystals were formed. The three-dimensional structures of the cytochrome in all three crystal forms have been determined by X-ray diffraction at resolutions of 2.20 A (hexagonal), 1.95 A, (triclinic) and 1.53 A (tetragonal). The most significant difference observed was the binding of an imidazole molecule to the iron atom of the heme group in the hexagonal structure. This binding displaces the sulfur atom of Met l00, which forms the axial ligand in the triclinic and tetragonal structures.

A tetrahedral coordination compound for second-order nonlinear optics: synthesis, crystal structure and SHG of Zn(2-NH2py)2Cl2

A coordination compound with a tetrahedral molecular configuration, Zn(NH 2 py) 2 Cl 2 (2-NH 2 py=2-aminopyridine), was prepared. It is transparent in the visible region and shows second harmonic generation (SHG) effect 8.0 times as strong as that of KDP. X-ray single crystal structure analysis reveals that all Zn(NH 2 py) 2 Cl 2 molecules are aligned in a fully parallel direction. The advantages and disadvantages of tetrahedral zinc coordination compounds as nonlinear optical (NLO) materials are discussed. The results may represent a novel strategy for designing a new class of transparent NLO materials.