Lipoxygenases are mononuclear non-heme metalloenzymes that regio- and stereospecifically convert 1,4-pentadiene subunit-containing fatty acids into alkyl peroxides. The rate-determining step is generally accepted to be hydrogen atom abstraction from the pentadiene subunit of the substrate by an active metal(III)-hydroxide species to give a metal(II)-water species and an organic radical. All known plant and animal lipoxygenases contain iron as the active metal; recently, however, manganese was found to be the active metal in a fungal lipoxygenase. Reported here are the synthesis and characterization of a mononuclear Mn(III) complex, [Mn(III)(PY5)(OH)](CF(3)SO(3))(2) (PY5 = 2,6-bis(bis(2-pyridyl)methoxymethane)pyridine), that reacts with hydrocarbon substrates in a manner most consistent with hydrogen atom abstraction and provides chemical precedence for the proposed reaction mechanism. The neutral penta-pyridyl ligation of PY5 endows a strong Lewis acidic character to the metal center allowing the Mn(III) compound to perform this oxidation chemistry. Thermodynamic analysis of [Mn(III)(PY5)(OH)](2+) and the reduced product, [Mn(II)(PY5)(H(2)O)](2+), estimates the strength of the O-H bond in the metal-bound water in the Mn(II) complex to be 82 (+/-2) kcal mol(-)(1), slightly less than that of the O-H bond in the related reduced iron complex, [Fe(II)(PY5)(MeOH)](2+). [Mn(III)(PY5)(OH)](2+) reacts with hydrocarbon substrates at rates comparable to those of the analogous [Fe(III)(PY5)(OMe)](2+) at 323 K. The crystal structure of [Mn(III)(PY5)(OH)](2+) displays Jahn-Teller distortions that are absent in [Mn(II)(PY5)(H(2)O)](2+), notably a compression along the Mn(III)-OH axis. Consequently, a large internal structural reorganization is anticipated for hydrogen atom transfer, which may be correlated to the lessened dependence of the rate of substrate oxidation on the substrate bond dissociation energy as compared to other metal complexes. The results presented here suggest that manganese is a viable metal for lipoxygenase activity and that, with similar coordination spheres, iron and manganese can oxidize substrates through a similar mechanism.

A series of ferrous complexes with the pentadentate ligand 2,6-(bis-(bis-2-pyridyl)methoxymethane)pyridine (PY5) was prepared and examined. PY5 binds ferrous iron in a square-pyramidal geometry, leaving a single coordination site accessible for complexation of a wide range of monodentate exogenous ligands: [Fe(II)(PY5)(X)](n+), X = MeOH, H(2)O, MeCN, pyridine, Cl-, OBz-, N(3)-, MeO-, PhO-, and CN-. The spin-states of these ferrous complexes are extremely sensitive to the nature of the single exogenous ligand; the spectroscopic and structural properties correlate with their high-spin (hs) or low-spin (ls) electronic ground state. Systematic metrical trends within six crystallographic structures clearly indicate a preferred conformational binding mode of the PY5 ligand. The relative binding affinities of the exogenous ligands in MeOH indicate that exogenous ligand charge is the primary determinant of the binding affinity; the [Fe(II)(PY5)](2+) unit preferentially binds anionic ligands over neutral ligands. At parity of charge, strong-field ligands are preferentially bound over weak-field ligands. In MeOH, the pK(a) of the exogenously ligated MeOH in [Fe(PY5)(MeOH)](2+) (9.1) limits the scope of exogenous ligands, as strongly basic ligands preferentially deprotonate [Fe(PY5)(MeOH)](2+) to yield [Fe(PY5)(OMe)](1+) rather than ligate to the ferrous center. Exogenous ligation by a strongly basic ligand, however, can be achieved in polar aprotic solvents.

New unbridged mixed-ligand zirconocenes were synthesized, characterized, and studied in the copolymerizations of ethylene-propylene (EP) and ethylene-1-hexene (EH). The MAO-activated metallocene [1-benzyl-2-(3′,5′-di-tert-butyl)phenylindenyl][2-phenylindenyl]zirconium dichloride (8) catalyzed EP and EH copolymerizations with products of reactivity ratios greater than one (r erp ) 2.4, rerh ) 2.2) to give rubbery, semicrystalline, high-melting polymers. Crystallinity was detected by differential scanning calorimetry (DSC) in both EP and EH copolymers containing only 50 mol % ethylene. Solvent fractionation of high ethylene content EH copolymers revealed that these copolymers were composed of a small ether-soluble fraction and heptane-soluble and heptane-insoluble fractions whose compositions and sequence distributions matched closely with those of the unfractionated polymer. Collectively, the reactivity ratios, DSC, and fractionation results gave evidence that these copolymers contained long crystallizable ethylene sequences. A comparison of the melting point behavior of a series of ethylene-1-hexene copolymers with 80 mol % ethylene revealed a sensitive dependence of the melting transition to the comonomer sequence distribution.

A novel anionic N-heterocyclic carbene was prepared and ligated to [Pd(allyl)Cl] 2 to yield the complex (NHC-enolate)Pd(allyl). At elevated temperatures in acetonitrile, the allyl ligand undergoes a stereoselective η 3 -η 1 -η 3 isomerization in an associative process that is first order in CD 3 CN with ∆H q ) 13.5 ( 0.7 kcal mol -1 and ∆S q ) -17 ( 2 eu. This complex reacts with electrophiles (HCl, CH 3 I) at the enolate position.

Mono-Cp titanium coordination compounds bearing monoanionic ligands derived from stable nitroxyl radicals have been synthesized by two methods:  (i) trapping of CpTi(III) species with the stable nitroxyl radical TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl) to provide Cp‘TiCl2(TEMPO) (Cp‘ = Cp (1) and Cp* (2)) and (ii) salt metathesis of Ti(IV) halides with a nitroxide anion generated by the in situ methylation of tert-butyl-α-phenylnitrone. Alkylation of these complexes with MeLi or MeMgBr furnishes Cp*TiMe2(TEMPO) (3) and Cp‘TiMe2(ON( t Bu)(CHMePh)) (Cp‘ = Cp(4) and Cp* (5)). The molecular structure of 2 has been determined by X-ray crystallography to reveal a monoanionic η1-TEMPO ligated to titanium. Complexes 3 and 4 activated with iprAFPB (2,6,-diisopropyl-N,N-dimethylanilinium tetrakis(pentafluorophenyl)borate) efficiently copolymerize ethylene and 1-hexene to provide copolymers having higher 1-hexene contents and higher productivities than the related Cp*Ti(CH2Ph)3 under identical conditions. Comparison of structural and electronic features as well as the ethylene/1-hexene copolymerization behavior of 3 and 4 with the constrained geometry catalyst [MeSi2(η5-Me4Cp)(η1-N-tBu)]TiMe2 (6) provides insights into factors governing high comonomer incorporation by mono-Cp titanium complexes.