Trigonal bi- and monopyramidal cobalt(II) complexes of a novel guanidine-based tripodal ligand.

The novel ligand DIG(3)tren has three N',N''-diisopropylguanidinyl (DIG) moieties. We report on the structures of two cobalt complexes that show how an isopropylamino group from each DIG acts as a flap that can either close over the metal or rotate away from the metal to open up a site for auxiliary ligand binding. Two of the -NH(iPr) flaps are open in pink [Co(DIG(3)tren)(OAc)]OAc (1), and each of these flaps provides a hydrogen bond to stabilize acetate binding to trigonal bipyrimidal cobalt. The flaps are closed in blue [Co(DIG(3)tren)][BPh(4)](2) (2), yielding a rare example of a trigonal (mono)pyramidal [ML](2+) ion.

Development of porous materials for heterogeneous catalysis: kinetic resolution of epoxides.

Template copolymerization methods have been utilized to prepare porous materials with immobilized cobalt complexes that catalyze the hydrolytic kinetic resolution of epoxides.

How does cyanide inhibit superoxide reductase? Insight from synthetic FeIIIN4S model complexes.

Superoxide reductases (SORs) are nonheme iron-containing enzymes that reduce HO(2) to H(2)O(2). Exogenous substrates such as N(3)(-) and CN(-) have been shown to bind to the catalytic iron site of SOR, and cyanide acts as an inhibitor. To understand how these exogenous ligands alter the physical and reactivity properties of the SOR iron site, acetate-, azide-, and cyanide-ligated synthetic models of SOR have been prepared. The x-ray crystal structures of azide-ligated [Fe(III)(S(Me2)N(4)(tren))(N(3))](+) (3), dimeric cyanide-bridged ([Fe(III)(S(Me2)N(4)(tren))](2)-mu-CN)(3+) (5), and acetate-ligated [Fe(III)(S(Me2)N(4)(tren))(OAc)](+) (6) are described, in addition to x-ray absorption spectrum-derived and preliminary crystallographic structures of cyanide-ligated [Fe(III)(S(Me2)N(4)(tren))(CN)](+) (4). Cyanide coordination to our model (4) causes the redox potential to shift anodically by 470 mV relative to acetate-ligated 6 and 395 mV relative to azide-ligated 3. If cyanide coordination were to cause a similar shift in redox potential with SOR, then the reduction potential of the catalytically active Fe(3+) center would fall well below that of its biological reductants. These results suggest therefore that cyanide inhibits SOR activity by making the Fe(2+) state inaccessible and thus preventing the enzyme from turning over. Cyanide inhibits activity in the metalloenzyme superoxide dismutase via a similar mechanism. The reduced five-coordinate precursor to 3, 4, and 6 [Fe(II)(S(Me2)N(4)(tren))](+) (1) was previously shown by us to react with superoxide to afford H(2)O(2) via an [Fe(III)(S(Me2)N(4)(tren))(OOH)](+) intermediate. Cyanide and azide do not bind to 1 and do not prevent 1 from reducing superoxide.

A dihydroxo-bridged Fe(II)-Fe(III) complex: a new member of the diiron diamond core family.

The first structurally characterized Fe(II)-Fe(III) complex containing a M2(mu-OH)2 diamond core is a Robin and Day class II mixed-valence complex.

Synthetic models for the cysteinate-ligated non-heme iron enzyme superoxide reductase: observation and structural characterization by XAS of an Fe(III)-OOH intermediate.

Superoxide reductases (SORs) belong to a new class of metalloenzymes that degrade superoxide by reducing it to hydrogen peroxide. These enzymes contain a catalytic iron site that cycles between the Fe(II) and Fe(III) states during catalysis. A key step in the reduction of superoxide has been suggested to involve HO(2) binding to Fe(II), followed by innersphere electron transfer to afford an Fe(III)-OO(H) intermediate. In this paper, the mechanism of the superoxide-induced oxidation of a synthetic ferrous SOR model ([Fe(II)(S(Me2)N(4)(tren))](+) (1)) to afford [Fe(III)(S(Me2)N(4)(tren)(solv))](2+) (2-solv) is reported. The XANES spectrum shows that 1 remains five-coordinate in methanolic solution. Upon reaction of 1 with KO(2) in MeOH at -90 degrees C, an intermediate (3) is formed, which is characterized by a LMCT band centered at 452(2780) nm, and a low-spin state (S = 1/2), based on its axial EPR spectrum (g(perpendicular) = 2.14; g(parallel) = 1.97). Hydrogen peroxide is detected in this reaction, using both (1)H NMR spectroscopy and a catalase assay. Intermediate 3 is photolabile, so, in lieu of a Raman spectrum, IR was used to obtain vibrational data for 3. At low temperatures, a nu(O-O) Fermi doublet is observed in the IR at 788(2) and 781(2) cm(-)(1), which collapses into a single peak at 784 cm(-1) upon the addition of D(2)O. This vibrational peak diminishes in intensity over time and essentially disappears after 140 s. When 3 is generated using an (18)O-labeled isotopic mixture of K(18)O(2)/K(16)O(2) (23.28%), the vibration centered at 784 cm(-1) shifts to 753 cm(-1). This new vibrational peak is close to that predicted (740 cm(-1)) for a diatomic (18)O-(18)O stretch. In addition, a nu(O-O) vibrational peak assigned to free hydrogen peroxide is also observed (nu(O-O) = 854 cm(-1)) throughout the course of the reaction between Fe(II)-1 and superoxide and is strongest after 100 s. XAS studies indicate that 3 possesses one sulfur scatterer at 2.33(2) A and four nitrogen scatterers at 2.01(1) A. Addition of two Fe-O shells, each containing one oxygen, one at 1.86(3) A and one at 2.78(3) A, improved the EXAFS fits, suggesting that 3 is an end-on peroxo or hydroperoxo complex, [Fe(III)(S(Me2)N(4)(tren))(OO(H))](+). Upon warming above -50 degrees C, 3 is converted to 2-MeOH. In methanol and methanol:THF (THF = tetrahydrofuran) solvent mixtures, 2-MeOH is characterized by a LMCT band at lambda(max) = 511(1765) nm, an intermediate spin-state (S = 3/2), and, on the basis of EXAFS, a relatively short Fe-O bond (assigned to a coordinated methanol or methoxide) at 1.94(10) A. Kinetic measurements in 9:1 THF:MeOH at 25 degrees C indicate that 3 is formed near the diffusion limit upon addition of HO(2) to 1 and converts to 2-MeOH at a rate of 65(1) s(-1), which is consistent with kinetic studies involving superoxide oxidation of the SOR iron site.

The first example of a nitrile hydratase model complex that reversibly binds nitriles.

Nitrile hydratase (NHase) is an iron-containing metalloenzyme that converts nitriles to amides. The mechanism by which this biochemical reaction occurs is unknown. One mechanism that has been proposed involves nucleophilic attack of an Fe-bound nitrile by water (or hydroxide). Reported herein is a five-coordinate model compound ([Fe(III)(S(2)(Me2)N(3)(Et,Pr))](+)) containing Fe(III) in an environment resembling that of NHase, which reversibly binds a variety of nitriles, alcohols, amines, and thiocyanate. XAS shows that five-coordinate [Fe(III)(S(2)(Me2)N(3)(Et,Pr))](+) reacts with both methanol and acetonitrile to afford a six-coordinate solvent-bound complex. Competitive binding studies demonstrate that MeCN preferentially binds over ROH, suggesting that nitriles would be capable of displacing the H(2)O coordinated to the iron site of NHase. Thermodynamic parameters were determined for acetonitrile (DeltaH = -6.2(+/-0.2) kcal/mol, DeltaS = -29.4(+/-0.8) eu), benzonitrile (-4.2(+/-0.6) kcal/mol, DeltaS = -18(+/-3) eu), and pyridine (DeltaH = -8(+/-1) kcal/mol, DeltaS = -41(+/-6) eu) binding to [Fe(III)(S(2)(Me2)N(3)(Et,Pr))](+) using variable-temperature electronic absorption spectroscopy. Ligand exchange kinetics were examined for acetonitrile, iso-propylnitrile, benzonitrile, and 4-tert-butylpyridine using (13)C NMR line-broadening analysis, at a variety of temperatures. Activation parameters for ligand exchange were determined to be DeltaH(+ +) = 7.1(+/-0.8) kcal/mol, DeltaS(+ +) = -10(+/-1) eu (acetonitrile), DeltaH(+ +) = 5.4(+/-0.6) kcal/mol, DeltaS(+ +) = -17(+/-2) eu (iso-propionitrile), DeltaH(+ +) = 4.9(+/-0.8) kcal/mol, DeltaS(+ +) = -20(+/-3) eu (benzonitrile), and DeltaH(+ +) = 4.7(+/-1.4) kcal/mol DeltaS(+ +) = -18(+/-2) eu (4-tert-butylpyridine). The thermodynamic parameters for pyridine binding to a related complex, [Fe(III)(S(2)(Me2)N(3)(Pr,Pr))](+) (DeltaH = -5.9(+/-0.8) kcal/mol, DeltaS = -24(+/-3) eu), are also reported, as well as kinetic parameters for 4-tert-butylpyridine exchange (DeltaH(+ +) = 3.1(+/-0.8) kcal/mol, DeltaS(+ +) = -25(+/-3) eu). These data show for the first time that, when it is contained in a ligand environment similar to that of NHase, Fe(III) is capable of forming a stable complex with nitriles. Also, the rates of ligand exchange demonstrate that low-spin Fe(III) in this ligand environment is more labile than expected. Furthermore, comparison of [Fe(III)(S(2)(Me2)N(3)(Et,Pr))](+) and [Fe(III)(S(2)(Me2)N(3)(Pr,Pr))](+) demonstrates how minor distortions induced by ligand constraints can dramatically alter the reactivity of a metal complex.