Bis Phenolate Ligand Research Articles

Ring‐opening polymerization of trimethylenecarbonate by the bridged diphenoxo ytterbium (II) complex

AbstractA novel ytterbium (II) complex 2 supporting by a bridged bisphenolate ligand H2LOC4H7 (L = C4H7OCH2N(CH2‐2‐OC6H2‐3,5‐Bu)2) with a tetrahydrofuran donor on side‐arm was synthesized in high yield and characterized by elementary analysis, IR, 1H‐NMR, and 13C‐NMR. Ring‐opening polymerization of 1,3‐trimethylenecarbonate (TMC) was carried out using complex 2 and a known complex [Me2NCH2CH2N(CH2‐2‐O‐3,5‐C6H2(But)2)2]Yb 1 as the initiators, respectively. It was found that both complexes 1 and 2 can alone initiate the ring‐opening polymerization of TMC, and complex 1 showed higher activity than complex 2. The activity of both complexes 1 and 2 was found to be higher than that of monodentate phenoxo ytterbium (II) complex (2,6‐Bu‐C6H3O)2Yb(THF)3. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Metal complexes of an amine bisphenol with a thiophene side-arm

Dioxomolybdenum(VI) and oxotungsten(VI) complexes with a new amine bisphenol ligand (H2L) are reported. The ligand which carries a neutral nitrogen atom, two phenolic oxygen atoms and a thiophene side-arm was synthesized by a simple one-pot Mannich reaction. Further reaction with [MoO2(acac)2] yielded a monomeric molybdenum complex [MoO2(L)(MeOH)] (2a) or a dimeric complex [Mo2O2(μ-O)2(L)2] (2b), depending on the reaction conditions. The reaction with a tungsten trisglycolate [W(eg)3] led to the formation of a monomeric compound [WO(eg)(L)] (3). In these complexes, the potentially tetradentate amine bisphenolate dianion coordinates as a tridentate O,N,O donor while the sulfur side-arm donor remains intact. The solid-state structure of 2a was investigated by X-ray crystallography.

Synthesis and Structural Studies of Heterobimetallic Alkoxide Complexes Supported by Bis(phenolate) Ligands: Efficient Catalysts for Ring-Opening Polymerization of l-Lactide

A series of heterobimetallic titanium(IV) complexes [LTi(O(i)Pr)(mu-O(i)Pr)(2)Li(THF)(2)], [LTi(O(i)Pr)(mu-O(i)Pr)(2)Na(THF)(2)], [LTi(mu-O(i)Pr)(2)Zn(O(i)Pr)(2)], and [LTi(mu-O(i)Pr)(2)Mg(O(i)Pr)(2)] (where L = bidentate bisphenol ligands) have been synthesized and characterized including a structural determination of [L(1)Ti(mu(2)-O(i)Pr)(2)(O(i)Pr)Li(THF)(2)] (1a). These complexes were investigated for their utility in the ring-opening polymerization (ROP) of l-lactide (LA). Polymerization activities have been shown to correlate with the electronic properties of the substituent within the bisphenol ligand. In contrast to monometallic titanium initiator 1e, all the heterobimetallic titanium initiators (Ti-Li, Ti-Na, Ti-Zn, and Ti-Mg) show enhanced catalytic activity toward ring-opening polymerization (ROP) of l-LA. In addition, the use of electron-donating methoxy or methylphenylsulfonyl functional ligands reveals the highest activity. The bisphenol bimetallic complexes give rise to controlled ring-opening polymerization, as shown by the linear relationship between the percentage conversion and the number-average molecular weight. The polymerization kinetics using 2c as an initiator were also studied, and the experimental results indicate that the reaction rate is first-order with respect to both monomer and catalyst concentration with a polymerization rate constant, k = 81.64 M(-1) min(-1).

Synthesis of Early Transition Metal Bisphenolate Complexes and Their Use as Olefin Polymerization Catalysts

Bisphenolate ligands with pyridine- and benzene-diyl linkers have been synthesized and metalated with group 4 and 5 transition metals. The solid-state structures of some of the group 4 complexes have been solved. The titanium, zirconium, hafnium, and vanadium complexes were tested for propylene polymerization and ethylene/1-octene copolymerization activities with methylaluminoxane as cocatalyst. The vanadium(III) precatalyst is the most active for propylene polymerization and shows the highest 1-octene incorporation for ethylene/1-octene copolymerization. The zirconium(IV) precatalyst was the most active for propylene polymerization of the group 4 precatalysts. Titanium(IV) and zirconium(IV) precatalysts with pyridine-diyl linkers provided mixtures of isotactic and atactic polypropylene while titanium(IV) precatalysts with benzene-diyl linkers gave atactic polypropylene only. The hafnium(IV) precatalyst with a pyridine-diyl linker generated moderately isotactic polypropylene.

Oxidomolybdenum(VI) complexes with atrane-type [O 3N] ligands

Dioxomolybdenum(VI) complex [MoO 2Cl 2(dmso) 2] reacts with a series of tetradentate O 3N-type aminoalcohol–bisphenol ligands to form oxomolybdenum(VI) complexes of type [MoOCl(L n )]. The reaction of H 3L 1 produces [MoOCl(L 1)] as two separable isomers, whereas the reaction of H 3L 2 or H 3L 3 yields a single product. The X-ray analyses of cis- and trans-[MoOCl(L 1)] reveal that the complexes are formed of monomeric molecules. The ligands have tetradentate coordination through three oxygen donors and one nitrogen donor, which is located trans to the terminal oxo group. The sixth coordination site is occupied by a chloro ligand.

A chiral diamine bis-phenolate complex of dioxomolybdenum(VI)

A new chiral tetradentate ligand reacts with [MoO 2 (acac) 2 ] to form a monomeric complex. The syntheses and X-ray crystal structures of the ligand and the complex are reported. A new dioxomolybdenum(VI) complex with a chiral tetradentate ligand is reported. The tripodal ligand containing two nitrogen atoms and two phenolic oxygen atoms was synthesized starting from a chiral diamine precursor. Further reaction with [MoO 2 (acac) 2 ] yielded a monomeric molybdenum complex as a bright yellow solid. The structures of the molybdenum complex and the free diamine bis-phenol ligand were determined by X-ray diffraction.

Vanadium-based imido-alkoxide pro-catalysts bearing bisphenolate ligands for ethylene and ε-caprolactone polymerisation

The pro-catalysts [V(NAr)(L)(OR)] (Ar = p-tolyl, p-ClC(6)H(4), p-(OMe)C(6)H(4), p-(CF(3))C(6)H(4); R = t-Bu, i-Pr, n-Pr, Et, C(CH(3))(CF(3))(2)) have been prepared in good yields from the reaction of [V(NAr)(OR)(3)] and the bisphenol 2,2'-CH(3)CH[4,6-(t-Bu)(2)C(6)H(2)OH](2) (LH(2)). X-Ray crystal structure determinations for the Ar = p-tolyl, R = t-Bu (1), R = C(CH(3))(CF(3))(2) (2) and Ar = p-ClC(6)H(4), R = t-Bu (3) derivatives revealed monomeric complexes, whereas use of R = i-Pr, n-Pr or Et led to alkoxide-bridged dimeric structures of the form [V(NAr)(L)(mu-OR)](2) (R = i-Pr, Ar = p-tolyl (4), p-ClC(6)H(4) (5), p-(CF(3))C(6)H(4) (6), p-(OMe)C(6)H(4) (7); R = n-Pr, Ar = p-tolyl (8), p-(CF(3))C(6)H(4) (9); R = Et, Ar = p-ClC(6)H(4) (10), p-tolyl (11)). Complexes 1-11 yield highly active ethylene polymerisation catalysts when treated with DMAC (dimethylaluminium chloride) in the presence of ETA (ethyltrichloroacetate), with activities in the range 38,800 to 75,200 g mmol(-1) h(-1) bar(-1). The molecular weights of the resultant polymers were in the range 37,000 to 411,000 g mol(-1), with molecular weight distribution 2.2 to 4.7. The effect of the nature of the para-arylimido substituent and the alkoxide group OR upon the catalytic activity has been investigated. For epsilon-caprolactone polymerisation, mononuclear 1-3 exhibit low conversion (< or = 25%; 0% for 2), whereas use of the dimeric species 4-11 led to higher conversions (41-78%).

Synthesis and characterization of a family of binuclear non-heme iron monooxygenase model compounds: Evidence for a “phenolate/amide carbonyl (PAC) shift” upon oxidation

A series of binuclear iron compounds has been synthesized using diamide, bis-phenolate ligands in which the carbon-linker between the amide nitrogen atoms has been varied. Two diferrous compounds in the series, Fe 2 II ( H 2 Hbach ) 2 ( N-MeIM ) 2 and Fe 2 II ( H 2 Hbame ) 2 ( N-MeIM ) 2 along with their two-electron oxidized, di-μ-methoxy-bridged counterparts, Fe III ( H 2 Hbach ) 2 ( OMe ) 2 and Fe III ( H 2 Hbame ) 2 ( OMe ) 2 have been crystallographically characterized, as have the di-μ-methoxy compounds Fe 2 III ( H 2 Hbap ) 2 ( OMe ) 2 , Fe 2 III ( H 2 Hbabn ) 2 ( OMe ) 2 and Fe 2 III ( H 2 Hbapen ) 2 ( OMe ) 2 (H 2Hbab = 1,2-bis(2-hydroxybenzamido) benzene, H 2Hbach = trans-1,2-bis(2-hydroxybenzamido) cyclohexane, H 2Hbame = 1,2-bis(2-hydroxybenzamido) ethane, H 2Hbap = 1,3-bis(2-hydroxybenzamido) propane, H 2Hbabn = 1,4-bis(2-hydroxybenzamido) butane, H 2Hbapen = 1,5-bis(2-hydroxybenzamido) pentane, N-MeIM = N-methylimidazole and OMe = methoxide). Fe 2 II ( H 2 Hbach ) 2 ( N-MeIM ) 2 and Fe 2 II ( H 2 Hbame ) 2 ( N-MeIM ) 2 are structurally very similar to previously reported diferrous compounds of this family of ligands that have been shown to be active as oxygen atom transfer catalysts. Flexibility in the carbon-linker allows some variability in the orientation of the phenolate arms of the ligands in the diferric di-μ-methoxy compounds, but the Fe 2O 2 core remains largely unchanged across the series. Two-electron oxidation of the ferrous compounds in methanol shows a substantial ligand rearrangement that is consistent with other spectroscopic, electrochemical and kinetic investigations. The loss of both phenolate bridges upon oxidation is reminiscent of the “carboxylate shift” observed in binuclear non-heme enzymes and could provide insight into the driving force behind this family of compounds’ function as a catalyst.

Oxovanadium(V) complexes of bis(phenolate) ligands with acetylacetone as co-ligand: Synthesis, crystal structure, electrochemical and kinetics studies on the oxidation of ascorbic acid

Two vanadium(V) complexes, [VO(L 1)](acac)] ( 1) and [VO(L 2)(acac)] ( 2), where H 2L 1 = N, N-bis(2-hydroxy-3-5-di- tert-butylbenzyl)propylamine and H 2L 2 = 2,2′-selenobis(4,6-di- tert-butylphenol), have been synthesized and characterized by elemental analyses, IR, 51V NMR, both in the solid and in solution, and cyclic voltammetric studies. Single crystal X-ray studies reveal that in complex 1 the vanadium atom is octahedrally coordinated with an O 5N donor environment, where the oxygen atom of the V V O moiety and the N atom of the ONO ligand occupy the axial sites while two oxygen atoms (O1 and O2) from the bisphenolate ligand and two oxygen atoms (O3 and O4) from the acac ligand occupy the equatorial plane. A similar bonding pattern has also been encountered for 2 with the exception that a Se atom instead of N is involved in weak bonding to the metal center. Both complexes showed reversible cyclic voltammeric responses and E 1/2 appears at −0.18 and 0.10 V versus NHE for complexes 1 and 2, respectively. The kinetics of oxidation of ascorbic acid by complex 1 were carried out in 50% MeCN–50% H 2O (v/v) at 25 °C. The high formation constant value, Q = 63 ± 7 M −1, reveals that the reaction proceeds through the rapid formation of a H-bonded intermediate. The low k 2 Q 2/ k 1 Q 1 ratio (13.4) for 1 points out that there is extensive H-bonding between the oxygen atom of the V V O group and the OH group of ascorbic acid.

Sterically variable dizinc complexes bearing bis(iminopyridyl)phenolate ligands: synthesis, structures and reactivity studies

A series of chiral dizinc complexes of the type [(2,6-{ArN=C(Me)C5H3N}2C6H3O)Zn2(micro-Cl)Cl2] [Ar=2,6-i-Pr2C6H3 (), 2,6-Me2C6H3 (), 2,4,6-Me3-C6H2 (), 2,4-Me2C6H3 ()] can be conveniently prepared in good yield by the template reaction of 2,6-{O=C(Me)C5H3N}2C6H3OH with an excess of the corresponding aniline and two equivalents of zinc dichloride in n-BuOH at elevated temperature. Alternatively, the pro-ligands, 2,6-{(ArN=C(Me)C5H3N}2C6H3OH [Ar=2,6-i-Pr2C6H3 (L1-H), 2,6-Me2C6H3 (L2-H), 2,4,6-Me3C6H2 (L3-H), 2,4-Me2C6H3 (L4-H)], can be isolated and then treated with two equivalents of zinc dichloride to afford . Interaction of with two equivalents of NaOAc in the presence of TlBF4 gives the diacetate-bridged salt [(L1)Zn2(micro-OAc)2](BF4) () while with Nadbm (dbm=dibenzoylmethanato) the bis(dbm)-chelated salt [(L1)Zn2(dbm)2](BF4) () is obtained. Hydrolysis occurs on reaction of with TlOEt to furnish [(L1)Zn2(micro-OH)Cl2] () as the only isolable product. Conversely, reaction of with Tlhp (hp=2-pyridonate) affords the neutral bis(pyridonate)-bridged trimetallic complex [(L1)Zn3(micro-hp)2Cl3] () as the major product along with as the minor one. Complex and mixtures of / act as modest activators for the ring-opening polymerisation of epsilon-caprolactone. Single crystal X-ray diffraction studies have been performed on , , , , and reveal Zn...Zn separations in the range: 3.069(4)-4.649(6) A.

Lithium complexes supported by amine bis-phenolate ligands as efficient catalysts for ring-opening polymerization of l-lactide

Lithium complexes bearing dianionic amine bis(phenolate) ligands are described. Reactions of ligand precursors H(2)O(2)NN(Me), H(2)O(2)NN(Py) or H(2)O(2)NO(Me) [H(2)O(2)NN(Me)=Me(2)NCH(2)CH(2)N-(CH(2)-2-HO-3,5-C(6)H(2)((t)Bu)(2))(2); H(2)O(2)NN(Py)=(2-C(5)H(4)N)CH(2)N-(CH(2)-2-HO-3,5-C(6)H(2)((t)Bu)(2))(2); H(2)O(2)NO(Me)=MeOCH(2)CH(2)N-(CH(2)-2-HO-3,5-C(6)H(2)((t)Bu)(2))(2)] with 2.2 molar equivalents of (n)BuLi in diethylether afford (Li(2)O(2)NN(Me))(2) (1), (Li(2)O(2)NN(Py))(2) (2) and (Li(2)O(2)NO(Me))(2) (3) as tetra-nuclear lithium complexes. The crystalline solids of partially hydrolyzed product, (LiO(HO)NN(Py)) (4), were obtained from recrystallization of 2 in diethylether solution for three months. The synthesis of (LiO(HO)NO(Me))(2) (5) was carried out at ambient temperature by carefully layering a solution of water in hexane on top of a solution of 3 in Et(2)O. Crystalline solids of were obtained after two months. Molecular structures are reported for compounds 1, 3, 4 and 5. Compounds 1-3 show excellent catalytic activities toward the ring-opening polymerization of L-lactide in the presence of benzyl alcohol.

Iron(III) complexes of certain tetradentate phenolate ligands as functional models for catechol dioxygenases

Catechol 1,2-dioxygenase (CTD) and protocatechuate 3,4-dioxygenase (PCD) are bacterial non-heme iron enzymes, which catalyse the oxidative cleavage of catechols tocis, cis-muconic acids with the incorporation of molecular oxygen via a mechanism involving a high-spin ferric centre. The iron(III) complexes of tripodal phenolate ligands containing N3O and N2O2 donor sets represent the metal binding region of the iron proteins. In our laboratory iron(III) complexes of mono- and bisphenolate ligands have been studied successfully as structural and functional models for the intradiol-cleaving catechol dioxygenase enzymes. The single crystal X-ray crystal structures of four of the complexes have been determined. One of thebis-phenolato complexes contains a FeN2O2Cl chromophore with a novel trigonal bipyramidal coordination geometry. The Fe-O-C bond angle of 136.1‡ observed for one of the iron(III) complex of a monophenolate ligand is very similar to that in the enzymes. The importance of the nearby sterically demanding coordinated -NMe2 group has been established and implies similar stereochemical constraints from the other ligated amino acid moieties in the 3,4-PCD enzymes, the enzyme activity of which is traced to the difference in the equatorial and axial Fe-O(tyrosinate) bonds (Fe-O-C, 133, 148‡). The nature of heterocyclic rings of the ligands and the methyl substituents on them regulate the electronic spectral features, FeIII/FeII redox potentials and catechol cleavage activity of the complexes. Upon interacting with catecholate anions, two catecholate to iron(III) charge transfer bands appear and the low energy band is similar to that of catechol dioxygenase-substrate complex. Four of the complexes catalyze the oxidative cleavage of H2DBC by molecular oxygen to yield intradiol cleavage products. Remarkably, the more basic N-methylimidazole ring in one of the complexes facilitates the rate-determining productreleasing phase of the catalytic reaction. The present study provides support to the novel substrate activation mechanism proposed for the intradiol-cleavage enzymes.

Homopolymerization of cyclic esters initiated by lanthanide isopropoxides supported by 2,2′‐ethylene‐bis(4,6‐di‐tert‐butylphenolate) ligands

AbstractLanthanide isopropoxides supported by carbon‐bridged bisphenolate ligands of 2,2′‐ethylene‐bis(4,6‐di‐tert‐butylphenoxo) {[(EDBP)Ln(μ‐OPri)(THF)2]2, where Ln is Nd (1), Sm (2), or Yb (3) and THF is tetrahydrofuran} were synthesized by protic exchange reactions in high yields with Cp3Ln compounds as raw materials, and complex 1 was structurally characterized. Complexes 1–3 were shown to be efficient initiators for the ring‐opening polymerization of ε‐caprolactone (ε‐CL) and 2,2‐dimethyltrimethylene carbonate (DTC). Complexes 1–3 could initiate the controlled polymerization of ε‐CL, and the polymerization rate was first‐order with respect to the monomer. The influence of the reaction conditions on the monomer conversion, molecular weight, and molecular weight distribution of the resultant polymers was investigated. End‐group analyses of the oligomers of ε‐CL and DTC showed that the polymerization underwent a coordination–insertion mechanism. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4409–4419, 2006

3,3‘-Oxazolidinyl-Substituted 2,2‘-Biphenyldiols: Novel Tropos Ligands with a Large Induction on the Chiral Axis

A novel type of bisphenol ligand is presented which provides high modularity. Chiral information is introduced in the 3,3'-positions as N-arenesulfonyl-1,3-oxazolidines inducing an unexpectedly high diastereoselectivity of up to 98:2 onto the chiral axis. The impact of each part of the molecule on the diastereomeric ratio is evaluated with extended variable-temperature (VT) NMR studies. A first example of application in asymmetric catalysis is given. [structure: see text]

Aluminum bisphenoxides: Promising challengers for a catalyzed copolymerization of cyclohexene oxide with CO 2

Reacting triethyl aluminum and diethyl aluminum chloride with the chelating bisphenol ligands mdtbpH2 and mmcpH2 [mmcpH2: 2,2′-methylenebis(4-methyl-6-(1-methyl-cyclohexyl)phenol); mdtbpH2: 2,2′-methylenebis(4,6-di- tert-butyl-phenol)] afforded monomeric and dimeric aluminum bisphenoxides with reactive Al–Cl and Al–C 2H 5 moieties. We present here the X-ray structure determination of some of the isolated compounds as well as the preliminary catalytic tests in copolymerization reactions with cyclohexene oxide and carbon dioxide.

Ethylene Oxide Polymerization Catalyzed by Aluminum Complexes of Sulfur-Bridged Polyphenols

The products derived from the reaction of sterically hindered sulfur-bridged bisphenols with aluminum trialkyls enable the efficient polymerization of ethylene oxide. The activity of the catalyst is largely determined by the structure of the bisphenol ligand and the size of the aluminum alkyl groups. Bonds of sulfur to aluminum are a common feature of the solid-state structures of compounds examined to this point. In solution, precatalysts of the formula (3-t-Bu-5-Me-2-OAlEt2−C6H2)2S are fluxional, and two processes appear to contribute to the equilibration of all ethyl substituents. The addition of a hindered monomer, 1,2-epoxyhexane, causes the initial formation of solvation complexes with the aluminum precatalysts and effects the disproportionation of the initial dialuminum complex of the bisphenol. One of the polymer end groups appears to be easily replaced by a halide atom during acid workup.

Nonoxovanadium(IV) and Oxovanadium(V) Complexes with Mixed O, X, O-Donor Ligands (X = S, Se, P, or PO)

Ligating properties of four potentially tridentate bisphenol ligands containing [O, X, O] donor atoms (X = S 1, Se 2, P 3, or P=O 4) toward the vanadium ions in +IV or +V oxidation states have been studied. Each ligand with different heterodonor atoms yields as expected nonoxovanadium(IV) complexes, V(IV)L(2), whose structures have been determined by X-ray diffraction methods as having six-coordinate V(IV), VO(4)X(2), core. Compounds 1-4 have also been studied with electrochemical methods, variable-temperature (2-295 K) magnetic susceptibility measurements, X-band electron paramagnetic resonance (EPR) (2-60 K) spectroscopy, and magnetic circular dichroism (MCD) (5 K) measurements. Electrochemical results suggest metal-centered oxidations to V(V) (i.e., no formation of phenoxyl radicals from the coordinated phenolates). A combination of density functional theory calculations and experimental EPR investigations indicates a dramatic effect of the heteroatoms on the electronic structure of 1-4 with consequent reordering of the energy levels; 1 and 3 possess a trigonal ground state (d(z)()(2))(1), but 4 with the phosphoryl oxygen as the heterodonor atom in contrast exhibits a tetragonal ground state, (d(xy)())(1). On the basis of the intense electronic transitions in absorption spectra, all electronic transitions observed for 4 have been assigned to ligand-to-metal charge-transfer transitions, which have been confirmed by preliminary resonance Raman measurements and C/D ratios obtained from low-temperature MCD spectroscopy. Moreover, diamagnetic complexes 5 and 6 containing mononuclear and dinuclear oxovanadium(V) units have also been synthesized and structurally and spectroscopically ((51)V NMR) characterized.

Aluminium metal complexes supported by amine bis-phenolate ligands as catalysts for ring-opening polymerization of ε-caprolactone

A family of aluminium complexes supported by dianionic amine bis(phenolate) ligands are described. Treatment of ligand precursors H2O2NNPy or H2O2NNMe [H2O2NNPy = (2-C5H4N)CH2N(CH2-2-HO-3,5-C6H2(tBu)2)2; H2O2NNMe = Me2NCH2CH2N(CH2-2-HO-3,5-C6H2(tBu)2)2] with 1.1 molar equivalent of AlMe3 in toluene affords MeAl(O2NNPy) (1) and MeAl(O2NNMe) (2) as five-coordinate aluminium methyl complexes, which further react with one equivalent of benzyl alcohol affording (C6H5CH2O)Al(O2NNPy) (3) and (C6H5CH2O)Al(O2NNMe) (4) as five-coordinate aluminium benzyloxy complexes. The molecular structures are reported for compounds 1 and 4. Compounds 3 and 4 show excellent catalytic activity toward the ring-opening polymerization of ε-caprolactone.

Manganese complexes of mixed O, X, O-donor ligands (X = S or Se): synthesis, characterization and catalytic reactivity

Complexes of manganese with two tridentate ligands, H2LSe and H2LS, containing [O, Se, O] or [O, S, O]-donor atoms, respectively, are described. The bisphenolate ligands yield complexes [MnIII2LSe2(μ-OCH3)2(CH3OH)2] 1, [MnIII2LS2(μ-OCH3)2(CH3OH)2] 2, Bu4N[MnIIILSe2] 3, Bu4N[MnIIILS2] 4, [MnIVLSe2] 5 and [MnIVLS2] 6. The ferrocene adduct of 5, i.e. [MnIVLSe2]·Fe(C5H5)25a is also described. The crystal structures of 1, 5a and 6 have been determined. The compounds have been thoroughly characterized by various physical methods, including IR, MS, UV-vis, EPR and magnetic susceptibility measurements. Low temperature electrochemical results at −25 °C strongly suggest that the electrochemical oxidations are not only ligand-centered, i.e. formation of phenoxyl radicals from the coordinated phenolates, but also metal-centered generating Mn(IV) species. The manganese(III) monomers 3 and 4 have been found to be oxidation catalysts with dihydrogen peroxide as the oxidant for epoxidation of alkenes, like styrene, stilbene and cyclohexene. The selenium containing ligand exhibits higher turnover numbers than the sulfur containing ligand.

Bisphenol ligands in the stereocontrol of transformations catalyzed by titanium and tungsten

Bisphenol ligands have a rich coordination chemistry in the early transition elements. 1,1′-Bi-2-naphthols are of particular interest because of their continued utility in controlling the stereochemistry of metal catalyzed processes. In several projects in our laboratory, we have endeavored to examine the potential scope and mechanism by which modifications of bisphenolate ligands can influence the stereoselectivity of electrocyclic reactions. In this paper we discuss two of these studies: an examination of stereochemically modified ROMP precatalysts and a study of well characterized binaphtholate titanium chloride Lewis acid catalysts.