Five-coordinate Species Research Articles

Electrophilic Platinum Complexes: Methyl Transfer Reactions and Catalytic Reductive Elimination of Ethane from a Tetramethylplatinum(IV) Complex

Methyl ligand transfer reactions occur easily between tetramethylplatinum(IV) complexes [PtMe4(LL)] [LL = 4,4‘-di-tert-butyl-2,2‘-bipyridine (bu2bpy) (1), N,N,N‘,N‘-tetramethylethylenediamine (tmeda) (2), bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene (Ar2NN) (3), 1,2-bis(diphenylphosphino)ethane (dppe) (4)] and electrophilic platinum complexes fac-[PtMe3(SO3CF3)(LL)] [LL = bu2bpy (6a), tmeda (7a), Ar2NN (8a)] or [PtMe(SO3CF3)(dppe)] (10a). It is proposed that the complexes 6a, 7a, 8a, or 10a undergo initial dissociation of the triflate ligand to give an electrophilic platinum cation which attacks one of the mutually trans methylplatinum ligands of [PtMe4(LL)]. The reaction of [PtMe4(LL)] [LL = bu2bpy (1), tmeda (2), Ar2NN (3)] with fac-[PtMe3(SO3CF3)(L‘L‘)] [L‘L‘ = bu2bpy (6a), tmeda (7a), Ar2NN (8a)] gives an equilibrium mixture between the reactants and fac-[PtMe3(SO3CF3)(LL)] and [PtMe4(L‘L‘)], in which [PtMe4(LL)] is favored in the same order as the π-accepting properties of the LL ligand (Ar2NN ≫ bu2bpy > tmeda). The reaction of [PtMe4(dppe)] (4) with 6a, 7a, 8a, or 10a results in rapid room-temperature catalytic, C−C bond reductive elimination to give C2H6 and [PtMe2(dppe)] (5). It is proposed that the actual reductive elimination step occurs from the five-coordinate species [PtMe3(dppe)]+, formed by methyl ligand transfer from 4 to the electrophilic platinum complex (6a, 7a, 8a or 10a). The triflato complexes 6a−10a are in rapid equilibrium with the cationic aqua complexes fac-[PtMe3(OH2)(LL)]SO3CF3 [LL = bu2bpy (6b), tmeda (7b), Ar2NN (8b), dppe (9b)] and [PtMe(OH2)(dppe)]SO3CF3 (10b), respectively, thus confirming lability of the triflate ligands.

Influence of coordination number and ligand size on the dissociation mechanisms of transition metal-monosaccharide complexes

Several features of metal-carbohydrate complexes, in the form of deprotonated metal/ N-glycoside ions, were varied in order to determine which types of complexes would best enable mass spectrometric differentiation of the stereochemical features of the coordinated monosaccharides. Metal complexes were generated by using transition metals such as nickel, copper, and zinc. Additionally, the size and coordination number of ligands in the complex, as well as the number of these ligands coordinated to the metal, were varied. By using a quadrupole ion trap mass spectrometer, multistage mass spectrometric experiments were performed on the electrospray-generated metal N-glycoside complexes. Several tricoordinate Ni N-glycoside systems were capable of differentiating the stereochemistry about the C-2 center in the monosaccharide ring, whereas the tricoordinate Cu/ en system allowed for the differentiation of both the C-2 and C-4 stereocenters. No stereochemical differentiation was possible from four- or five-coordinate species with the exception of the four-coordinate Zn/ dien complexes. Changes in the metal center or size of the N-glycoside ligand generated the greatest changes in the product ion spectra of the tricoordinate complexes. Such alterations in four- or five-coordinate complexes often did not result in greatly differing product ion spectra. Whereas the product ion spectra of most metal/ N-glycoside complexes could be easily categorized according to structural features of the precursor ion, exceptional species such as the four coordinate [Zn( dien/monosaccharide)–H] + precursor ion, also capable of differentiating the stereochemical features about C-2 and C-4, can give unexpected stereochemical information.

Synthesis of Yttrium Complexes that Contain the [NON]2-Ligand, [(t-Bu-d6-N-o-C6H4)2O]2-

Li2[NON] ([(t-Bu-d6−N-o-C6H4)2O]2- = [NON]2-) reacts with YCl3 in THF to give [NON]YCl(THF)2, from which [NON]Y[CH(SiMe3)2](THF) (2a) and [NON]Y(CH2SiMe3)(THF)2 (2b) could be prepared straightforwardly. An X-ray structure of 2a showed it to be a distorted five-coordinate species in which the nitrogen atoms of the [NON]2- ligand occupy (very approximately) apical sites in a trigonal bipyramid. Both 2a and 2b proved to be relatively unreactive toward molecular hydrogen or ethylene.

Synthesis and characterization of aluminum ethyl-(S)-lactate complexes and VT NMR studies of [Bu 2iAl((S)-(−)- μ2-OC(H)(Me)C(O)OEt))] 2

The reactions of ethyl-( S)-(−)-lactate with R 3Al in diethyl ether yield the pentacoordinated complexes [R 2Al(( S)-(−)- μ 2-OC(H)(Me)C(O)OEt))] 2 (1: R=Me; 2: R=Et; 3: R=Bu i ). These compounds have been characterized by spectroscopic studies as wells as crystallographic method. These structures show dimeric features containing unusual 5.4.5-fused rings with an Al 2O 2 core and the ester group bonded to Al through the CO. VT NMR studies of compound 3 suggest that the weak (CO)→M dative bond may undergo dissociation in solution, giving rise to the equilibrium with a four-coordinate species among two five-coordinate species at ambient temperature.

Fluxional Behavior of η1,η2-Cyclooctenylpalladium Acetylacetonate and Related Complexes

The cyclooctenylpalladium complexes [Pd({eta}{sup 1},{eta}{sup 2}-C{sub 8}H{sub 12}{center_dot}R)(acac)] exhibit fluxional behavior in solution that equilibrates the acac methyl environments. Free energies of activation for the process have been determined by variable-temperature NMR spectroscopy. These appear to show no dependence on the nature of exo substituents, and they are higher for bulky endo groups. Lower {Delta}G{sup {double_dagger}} values are obtained in more polar solvents or with added phosphines, suggesting the operation of an associative mechanism. In contrast, the platinum complex [Pt({eta}{sup 1},{eta}{sup 2}-C{sub 8}H{sub 12}{center_dot}OMe)(acac)] is static up to 380 K, as is the C,N-bonded palladium species [Pd(C{sub 6}H{sub 4}N{double_bond}NC{sub 6}H{sub 5}-2)(acac)]. These are fluxional, however, in the presence of a tertiary phosphine. The hexafluoroacetylacetonate (hfac) complex [Pd({eta}{sup 1},{eta}{sup 2}-C{sub 8}H{sub 12}{center_dot}OMe)-(hfac)] exhibits a lower value of {Delta}G{sup {double_dagger}} than its acad analogue, with or without added PEt{sub 3}. When a phosphine is added to one of the metal complexes, low-temperature {sup 31}P NMR studies reveal equilibria involving a phosphine-ligated species, the starting acac complex, and free phosphine. The position of equilibrium depends on the nature of the R group and on the phosphine. The rearrangement is proposed to take place from a five-coordinate species, either by a series of pseudorotationsmore » or by way of a turnstile mechanism. Although it is not possible to distinguish between these two, the simplicity of the latter in this case is particularly attractive.« less

Five-coordinate platinum (II) alkyne complexes: synthesis, ab initio calculations and crystal and molecular structure of [PtI 2(Me 2phen) η2PhC CPh]·CHCl 3

[PtX 2(Me 2phen)] complexes ( 1)Me 2phen = 2,9-dimethyl-1,10-phenanthroline; X = Br, Iy; I, Iz), having a highly distorted square-planar geometry, undergo alkynes uptake in chlorinated solvents to give the five-coordinate trigonal-bipyramidal species [PtX 2(Me 2phen)-(alkyne)] ( 2) (alkyne = ethyne, a; propyne, b; 2-butyne, c; 1-pentyne, d; phenylacetylene, e; methylphenylacetylene, f; diphenylacetylene, g). The equilibrium constants for formation ( K 1) decrease in the order (1>Br) already observed for the analogous reaction with alkenes to give the [PtX 2(Me 2phen)(alkene)] complexes ( 3) (1>Br>Cl) although the differences in K 1 values are much greater in the case of alkynes. In contrast, the number and bulk of substituents at the metal-coordinated carbons have practically no effect on the K 1 of the alkyne complexes, while they greatly reduce the stability of the five-coordinate olefin compounds. Finally, the Δ G of activation for rotation around the Ptalkyne bond calculated by variable temperature 1H NMR spectroscopy (71.5 ± 0.8 kJ mol −1 for 2zb) is considerably smaller than the value found for the corresponding five-coordinate propene complex (86.2 ± 0.8 kJ mol −1). The crystal and molecular structure of the chloroform solvate of [PtI 2(Me 2phen( η 3-PhCCPh)] ( 2zg·CHCl 3) has been determined by X-ray diffraction methods. In the crystals two crystallographically independent complexes and chloroform molecules are present: space group Cmc2, a = 14.367(3), b = 15.529(3), c = 26.959(5)Å, Z = 8, R = 0.0444. The average bending back of the phenyl groups with respect to the CC triple bond axis is 25(1)°. Spectroscopic and crystal data show that the conformation of the coordinated alkynes in these five-coordinate complexes is intermediate between those found for the trigonal Pt(0) and the square-planar Pt(II) alkyne complexes. Ab initio Hartfree-Fock calculations and constrained space orbital variation analyses have shown that in the five-coordinate species ethylene exhibits better σ-donating and π-accepting properties than acetylene.

Photoinduced Solvent Ligation to Nickel(II) Octaethylporphyrin Probed by Picosecond Time-Resolved Resonance Raman Spectroscopy

Pump/probe picosecond time-resolved resonance Raman spectra of nickel(II) octaethylporphyrin (NiOEP) in pyridine were observed in the time range −5 to +1000 ps. The spectra demonstrate generation of the vibrationally and electronically excited (d,d) state (B1g) immediately after the excitation to the ππ* state of the macrocycle, subsequent vibrational relaxation, and the formation of six-coordinate exciplex, B1g(L)2 (L = pyridine). Singular value decomposition analysis was applied to a series of picosecond time-resolved Raman spectra to investigate whether a five-coordinate complex was generated prior to the formation of the six-coordinate species. The results indicate insignificant population of the five-coordinate species preceding the formation of B1g(L)2 and suggest concerted coordination of two solvent molecules to axial positions of the (d,d) excited NiOEP. The formation process of the B1g(L)2 species is discussed.

Stability of mesoporous aluminosilicate MCM-41 under vapor treatment, acidic and basic conditions

The coordination state of aluminum in relation to the stability of mesoporous aluminosilicate MCM-41 following various treatments (vapor treatment and treatment in solution at different pH) is characterized using XRD, MAS NMR and BET techniques. As a result of the treatment with water vapor at 723 K, five-coordinated aluminum is observed in unusually large proportion in mesoporous Al-MCM-41. This coordination is stable under dehydration conditions. The amount of five-coordinated aluminum species can be controlled by varying the vapor treatment time without essentially damaging the MCM-41 structure. In contrast, upon treatment in solution, the state of aluminum coordination as well as the stability depend strongly on pH and treatment time. Moreover, amorphous aluminosilicate containing simultaneously four- and five-coordinated aluminum species can also be prepared from Al-MCM-41 without the formation of octahedral aluminum species.

Two-step concerted mechanism for alkane hydroxylation on the ferryl active site of methane monooxygenase

A two-step concerted mechanism for the conversion of methane to methanol catalyzed by soluble methane monooxygenase (sMMO) is discussed. We propose that the enzymatic reaction mechanism is essentially the same as that of the gas-phase methane-methanol conversion by the bare FeO+ complex. In the initial stage of our mechanism, the ferryl (Fe—O) "iron" active site of intermediate Q and substrate methane come into contact to form the initial Q (CH4) complex with an OFe—CH4 bond. The C—H bonds of methane are significantly weakened by the formation of a five-coordinate carbon species, through orbital interactions between a C 3v - or D 2d -distorted methane and the Fe—O active site. The important transition state for an H atom abstraction exhibits a four-centered structure. The generated intermediate involves an HO—Fe—CH3 moiety, and it is then converted into the final product complex including methanol as a ligand through a methyl migration that occurs via a three-centered transition state. The two-step concerted mechanism is consistent with recent experiments on regioselectivity of enzyme-catalyzed alkane hydroxylations.

Structure characterisation of the organoaluminium intermediate resulting from the alkylation of a chelated carbonyl group. Molecular structure of the trinuclear [MeAl][C 12H 20O 4][AlMe 2] 2 compound

The interaction of Me 3Al with Me 2Al(acac) results in the carbonyl alkylation of the chelating acetylacetonate ligand and formation of trinuclear complex [MeAl][C 12H 20O 4][AlMe 2] 2 ( 1). The title compound has been characterised by 1H-and 27Al-NMR spectroscopy. The 1H-NMR spectra are consistent with the presence of two distinct isomers in an equimolar ratio: cis- 1 and trans- 1. Both isomers contain two methylated acac units bridged by three organoaluminium moieties: central five-coordinated methyl aluminium species and two terminal four-coordinated dimethylaluminium species. The structure of cis- 1 has been confirmed by X-ray crystallography which revealed that the five-coordinated aluminium atom rises in almost ideal square pyramidal geometry. The role of the molar ratio of reactants is discussed.

Kinetic and Thermodynamic Studies of Iron(III) and Iron(IV) σ-Bonded Porphyrins. Formation and Reactivity of [(OEP)Fe(R)]n+, Where OEP Is the Dianion of Octaethylporphyrin (n = 0, 1, 2, 3) and R = C6H5, 3,4,5-C6F3H2, 2,4,6-C6F3H2, C6F4H, or C6F5

A series of high- and low-spin iron(III) phenyl and fluorophenyl octaethylporphyrin complexes are characterized by their electrochemical and spectroscopic properties in nonaqueous media. The investigated compounds are represented as (OEP)Fe(R), where R = C6H5, 3,4,5-C6F3H2, 2,4,6-C6F3H2, C6F4H, or C6F5 and OEP is the dianion of 2,3,7,8,12,13,17,18-octaethylporphyrin. The two C6F3H2 complexes are of special interest in that these isomers differ in the spin state of the iron(III). Electrochemical studies indicate that three one-electron oxidations are seen for all of the (OEP)Fe(R) derivatives which were investigated both at room and low temperature under conditions where migration of the σ-bonded ligand does not occur on the time scale of the experiment. The first one-electron oxidation of each compound leads to an Fe(IV) porphyrin, and this is followed by a migration of the axial group from the iron center to one of the four nitrogen atoms independent of the nature of the axial group or the iron(III) spin state. The kinetics were examined to evaluate the migration rate constants in the presence and absence of pyridine as a sixth axial ligand. The results of this study show that the stronger the electron donor ability of the R group, the faster the migration rate in the case of the five-coordinate species. However, an increase in charge density at the metal center by axial coordination of pyridine retards the migration rate and this result is interpreted in terms of a rate determining electron transfer step from R to Fe(IV) of the singly oxidized species prior to the migration. Our results also show that the spin state of the iron(III) octaethylporphyrin is not a key factor which governs the migration of the axial ligand of the electrooxidized species. For the first time, an overall mechanism is proposed to explain the migration reaction in the σ-bonded iron porphyrin complexes.

Nitridocyanometalates of CrV, MnV, and MnVI

Reaction of [CrV(N)(salen)]·CH3NO2 and [MnV(N)(salen)] in dimethylformamide or a methanol/water mixture with an aqueous solution of NaCN and CsCl or [N(CH3)4]Cl at elevated temperatures affords in good yields, respectively, yellow microcrystalline compounds of Cs2Na[CrV(N)(CN)5] (1); [N(CH3)4]2Na[CrV(N)(CN)5]·H2O (1a) and pink-violet crystals of Cs2Na[MnV(N)(CN)5] (4); and [N(CH3)4]2Na[MnV(N)(CN)5]·H2O (4a). These six-coordinate nitridopentacyanometalates contain a terminal M⋮N unit and a labile cyano group in trans position. Recrystallization from water and addition of [PPh4]Cl yields the five-coordinate species [PPh4]2[M(N)(CN)4]· 2H2O (M = CrV (2), MnV (5)). From pyridine solutions of 2 and 5 the six-coordinate species [PPh4]2[M(N)(CN)4(py)]·H2O·py (M = CrV (3), MnV (6)) crystallize. Complexes 1, 1a, 2, and 3 are paramagnetic (1.8 ± 0.1 μB at 298 K; d1) whereas 4, 4a, 5, and 6 have a low-spin d2 ground state. Electrochemically, 5 can be reversibly one-electron-oxidized in CH3CN with formation of a rela...

Folding intermediates in cytochrome c.

Folding of cytochrome c from its low pH guanidine hydrochloride (Gdn-HCl) denatured state revealed a new intermediate, a five-coordinate high spin species with a water molecule coordinated to the heme. Incorporation of this five-coordinated intermediate into the previously reported ligand exchange model can quantitatively account for the observed folding kinetics. In this new model, unfolded cytochrome c is converted to its native structure through an obligatory folding intermediate, the histidine-water coordination state, whereas the five-coordinate state and a bis-histidine state are off-pathway intermediates. When the concentration of Gdn-HCl in the refolding solution was increased, an acceleration of the conversion from the bis-histidine coordinated state to the histidine-water coordinated state was observed, demonstrating that the reaction requires unfolding of the mis-organized polypeptide structure associated with the bis-histidine state.

Molecular rearrangements in the formation of five-coordinate platinum(II) olefin complexes. Structural and mechanistic aspects

The olefin addition reaction to square-planar complexes of the type [PtXMe(N-N)] (X = Cl, CH(CO2Me)2, N-N=bidentate nitrogen ligand), to form trigonal bipyramidal complexes has been shown to occur in two successive steps. First, a five-coordinate species (A) is formed in which the axial positions are occupied by the methyl group and by one nitrogen atom. In most cases this subsequently rearranges to the more stable isomer (B), in which both axial positions are occupied by the anionic ligands. The structure of the type A complex [PtMe{CH}CO2Me)2) (E-NCCH-CHCN) (dmphen)] has been determined by an X-ray diffraction study (triclinic, space group P1− (No. 2), a = 9.497(3), b = 10.884(4), c = 15.671(4) Å, α = 75.62(3), β = 78.47(2), γ = 66.52(3)°, Z = 2). For the first time the equatorial-axial coordination of the dmphen ligand has been unequivocally ascertained. The molecule reveals a significant tension due to the short contacts between the methyl groups. The electronic and steric factors affecting the relative stability of type A and B species are discussed, and a mechanisms for the overall process is proposed.

RuX(CO)(NO)L2 and Ru(CO)(NO)L2+: Ru(0) or Ru(II) or In Between?

The synthesis of RuCl(CO)(NO)L2 (L = PtBu2Me) and replacement of Cl- by BF4-, CO, CH3CN, H2O, F-, and H- are reported. NaBArF4 (ArF = 3,5-(CF3)2C6H3) removes halide to produce the four-coordinate 16-electron cation Ru(NO)(CO)L2+, shown by X-ray diffraction to have a nonplanar structure symptomatic of greater back bonding than isoelectronic Ru(CO)2L2. Variable-temperature 1H NMR studies of the tBu groups in [Ru(NO)(CO)L2](BArF4) show a ΔG⧧ (100 °C) for inversion through planar Ru of 19.1 kcal/mol. Ru(η1-BF4)(CO)(NO)L2 is shown by X-ray diffraction to have a square-pyramidal structure with apical bent NO. The IR frequencies of NO and CO are analyzed to conclude that all five-coordinate species except Ru(NO)(CO)2L2+ and RuH(NO)(CO)L2 have bent nitrosyls; these last two have linear NO. RuX(CO)(NO)(PH3)2+ (X = Cl-, BF4-, H-, no ligand, NCH, CO) was calculated with ab initio calculations at the Becke3LYP level. Depending on the nature of X, one minimum (square pyramid with bent NO) or two minima (square pyramid...

Kinetic study and crystal structure of tetrakis(1,4-thioxane)palladium(II). A comparison between platinum(II) and palladium(II) reactivity

The compound [Pd(1,4-thioxane) 4](BF 4) 2·4CH 3NO 2 crystallizes in the monoclinic space group P2 1/ c with a=9.474(2), b=9.529(5), c=21.945(9) Å, β=99.77(3)°, Z=2; final R=0.051 for 4325 observed reflections (Pd, C, S and O anisotropically refined). The thioxane ligands around Pd form a square-planar arrangement, with two nitromethane molecules at a distance of 3.115(2) Å occupying sites perpendicular to the square-plane formed by the sulfur-bonding thioxanes giving an overall tetragonal bipyramidal coordination around Pd. Intermolecular exchange of 1,4-thioxane on [Pd(1,4-thioxane) 4](BF 4) 2 in deuterated nitromethane and intramolecular interconversion in the bound ligand have been studied as a function of temperature and pressure by 1H NMR lineshape analysis. The first-order rate constant and activation parameters obtained for the intramolecular process, assigned to sulfur inversion, are as follows: k i 298=294±26 s −1, Δ H i ≠=61.9±1.9 kJ mol −1, Δ S i ≠=+10.2±6.5 J K −1 mol −1, Δ V i ≠=+2.7±0.2 cm 3 mol −1. The intermolecular process yielded the following second-order rate constant and activation parameters: k 2 298=920±40 m −1 s −1, Δ H 2 ≠=39.5±0.9 kJ mol −1, Δ S 2 ≠=−55.6±2.8 J K −1 mol −1, Δ V 2 ≠=−9.5±0.3 cm 3 mol −1. Second-order kinetics and negative values for the entropy and volume of activation indicate an associative Ia or A exchange mechanism. The reactivity of the analogous Pt 2+ compound is estimated using the proposed empirical linear correlation function Δ G ≠(Pt)=−32.1+1.92Δ G ≠(Pd). The calculated second-order rate constant and free energy of activation are 0.3 m −1 s −1 and 76 kJ mol −1, respectively. This relationship indicates also that the well established reactivity order Pd 2+≫Pt 2+ could be reversed for better π-accepting ligands than MeNC and that, in this case, five-coordinated species on Pt(II) are more stable than the Pd(II) analogue.

Novel Coordination Isomerization in Organotin(IV) Compounds. Synthesis, Molecular Structures, and NMR Studies of LSnPhX2 (X = Ph, Cl, Br, I, SPh), LCH2SnPhX2 (X = Ph, Cl, Br, I), and LSiPh3, Where LH Is (2-MeO-3-tBu-5-Me-C6H2)2CH2

The series of complexes LSnPhX2 (1, X = Ph; 3, X = Cl; 4, X= Br; 5, X = I; 6, X = SPh), LSiPh3 (2), LSnCl3 (7), LCH2SnPhX2 (9, X = Ph; 10, X = Cl; 11, X = Br; 12, X = I) and LCH2SnCl3 (13), where LH is bis(2-methoxy-3-tert-butyl-5-methylphenyl)methane (2-MeO-3tBu-5-Me-C6H2)2CH2), has been synthesised, and compounds 1−6 have been characterized in the solid state by X-ray crystallography. Distorted tetrahedral geometries are found in the structures of 1 and 2. By contrast, distorted trigonal bipyramidal geometries are found in the structures of 3−6, owing to the presence of significant intramolecular Sn···O interactions that lie in the range from 2.559(4) Å for 3 to 2.989(4) Å for 6; the magnitudes of the Sn···O interactions have been correlated with the Lewis acidity of the tin atoms in the PhSnX2 entities. NMR spectroscopy indicates a similar correlation in solution. Compounds 1, 2, 6, and 9 are essentially four coordinate in solution between 30 and −100 °C. In compound 4, intramolecular coordination of one of the methoxy groups results in the novel situation where both four- and five-coordinate tin species exist simultaneously at low temperature. At higher temperature, an average of these two coordination states is observed. The remaining compounds also participate in intramolecular exchange equilibria, with the position of the equilibrium being affected by both the Lewis acidity at tin and by the ring size; the six-membered rings in 10−13 are more labile than the five-membered rings in 3−5 and 7. Adduct formation between tributylphosphine oxide, Bu3PO, and compounds 3−5 is described.

Kinetics of nitric oxide dissociation from five- and six-coordinate nitrosyl hemes and heme proteins, including soluble guanylate cyclase.

Kinetics of NO dissociation were characterized for three five-coordinate systems, heme-NO, HSA-heme-NO (human serum albumin), GC-NO (soluble guanylate cyclase), and for the six-coordinate system, Im-heme-NO. Nitrosyl myoglobin was redetermined for comparison. Previously known, six-coordinate R and T state nitrosyl hemoglobins are also included in the comparison. The data indicate that NO dissociates more than 1000 times faster from five-coordinate model heme than it does from the six-coordinate analog. Such a negative trans-effect between NO and a proximal base is in sharp contrast to carboxy heme derivatives, in which ligand dissociation rates are greatly slowed in when a trans base is present. As a result of opposite trans-effects, six-coordinate carboxy and nitrosyl derivatives have comparable dissociation rates, even though the five-coordinate species are very different. In proteins, five- and six-coordinate forms do not show a large difference in dissociation rates. Part of the reason may be due to different probabilities for geminate recombination in the different proteins, but this cannot explain all the facts. There must also be influences of the protein structure on bond-breaking rate constants themselves. With the exception of hemoglobin in the T state, nitrosyl guanylate cyclase shows the highest NO dissociation rate constant, k(obs) = 6 x 10(-4) s(-1). This would yield a half-life of about 2 min at 37 degrees C for dissociation of NO from GC-NO, a number that has implications for the mechanism of regulation of the activity of this key heme enzyme.

Magnetic Circular Dichroism Spectroscopy as a Probe of Geometric and Electronic Structure of Cobalt(II)-Substituted Proteins: Ground-State Zero-Field Splitting as a Coordination Number Indicator

Variable-temperature magnetic circular dichroism (VT MCD) is used as a probe of the ground-state electronic structure in Co(II)-substituted liver alcohol dehyrogenase, carbonic anhydrase, carboxypeptidase, substrate and inhibitor complexes of these enzymes, and four- and five-coordinate Co(II) model complexes. VT MCD was used to determine the magnitude of the ground-state zero-field splitting (ZFS) in these samples. The four-coordinate Co(II) species had ZFS's that ranged from 2.3 to 30 cm-1 and the five-coordinate species had ZFS's that ranged from 2.7 to 98 cm-1, values which fall outside of the ranges previously suggested for distinguishing 4-coordinate and 5-coordinate Co(II) (Makinen, M. W.; Kuo, L. C.; Yim, M. B.; Wells, G. B.; Fukuyama, J. M.; Kim, J. E. J. Am. Chem. Soc. 1985, 107, 5245−5255). The magnitude of the ZFS is not very sensitive to ligand heterogeneity but can be very sensitive to bond angles. Since protein active sites are in general highly angularly distorted from ideal four- or five-...

The oxidation of secondary alcohols by potassium tetraoxoferrate(VI)

The kinetics of the oxidation of 2-propanol, 1,1,1-trifluoro-2-propanol, 1,1,1,3,3,3-hexafluoro-2-propanol, 1-phenyl-2,2,2-trifluoroethanol, 1-(4-methylphenyl)-2,2,2-trifluoroethanol, 1-(3-bromophenyl)-2,2,2-trifluoroethanol, and 1-(3-nitrophenyl)-2,2,2-trifluoroethanol by potassium tetraoxoferrate(VI) have been studied under basic conditions. The products are ketones, formed in almost quantitative yields, iron(III) hydroxide, and dioxygen. The reactions are characterized by substantial enthalpies of activation (40–60 kJ/mol), very unfavorable entropies of activation, large primary deuterium isotope effects, and a positive Hammett ρ value. Both acid and base catalysis are observed. Acid catalysis is attributed to formation of a more reactive oxidant, HFeO4−, at low pH. Base catalysis is attributed partly to the conversion of the reductants to alkoxide ions at high pH, and partly to the reaction of hydroxide ion with tetraoxoferrate(VI) to give a five-coordinated species, HOFeO43−, that reacts rapidly with nucleophiles. A reaction mechanism involving formation of an intermediate ferrate ester is proposed. Keywords: oxidation, alcohols, potassium tetraoxoferrate(VI), ferrate esters, base catalysis, acid catalysis.