Joseph W. Ziller

Self-Healing Supramolecular Block Copolymers†

Polymer, heal thyself! Supramolecular ABA triblock copolymers formed by dimerization of 2-ureido-4-pyrimidinone (UPy) end-functionalized polystyrene-b-poly(n-butyl acrylate) (PS-b-PBA) AB diblock copolymers have been synthesized, resulting in a self-healing material that combines the advantageous mechanical properties of thermoplastic elastomers and the dynamic self-healing features of supramolecular materials.

Formation, Structure, and EPR Detection of a High Spin FeIV—Oxo Species Derived from Either an FeIII—Oxo or FeIII—OH Complex

High spin oxoiron(IV) complexes have been proposed to be a key intermediate in numerous nonheme metalloenzymes. The successful detection of similar complexes has been reported for only two synthetic systems. A new synthetic high spin oxoiron(IV) complex is now reported that can be prepared from a well-characterized oxoiron(III) species. This new oxoiron(IV) complex can also be prepared from a hydroxoiron(III) species via a proton-coupled electron transfer process--a first in synthetic chemistry. The oxoiron(IV) complex has been characterized with a variety of spectroscopic methods: FTIR studies showed a feature associated with the Fe-O bond at nu(Fe(16)O) = 798 cm(-1) that shifted to 765 cm(-1) in the (18)O complex; Mossbauer experiments show a signal with an delta = 0.02 mm/s and |DeltaE(Q)| = 0.43 mm/s, electronic parameters consistent with an Fe(IV) center, and optical spectra had visible bands at lambda(max) = 440 (epsilon(M) = 3100), 550 (epsilon(M) = 1900), and 808 (epsilon(M) = 280) nm. In addition, the oxoiron(IV) complex gave the first observable EPR features in the parallel-mode EPR spectrum with g-values at 8.19 and 4.06. A simulation for an S = 2 species with D = 4.0(5) cm(-1), E/D = 0.03, sigma(E/D) = 0.014, and g(z) = 2.04 generates a fit that accurately predicted the intensity, line shape, and position of the observed signals. These results showed that EPR spectroscopy can be a useful method for determining the properties of high spin oxoiron(IV) complexes. The oxoiron(IV) complex was crystallized at -35 degrees C, and its structure was determined by X-ray diffraction methods. The complex has a trigonal bipyramidal coordination geometry with the Fe-O unit positioned within a hydrogen bonding cavity. The Fe(IV)-O unit bond length is 1.680(1) A, which is the longest distance yet reported for a monomeric oxoiron(IV) complex.

The effects of redox-inactive metal ions on the activation of dioxygen: isolation and characterization of a heterobimetallic complex containing a Mn(III)-(μ-OH)-Ca(II) core.

Rate enhancements for the reduction of dioxygen by a Mn(II) complex were observed in the presence of redox-inactive group 2 metal ions. The rate changes were correlated with an increase in the Lewis acidity of the group 2 metal ions. These studies led to the isolation of heterobimetallic complexes containing Mn(III)-(μ-OH)-M(II) cores (M(II) = Ca(II), Ba(II)) in which the hydroxo oxygen atom is derived from O(2). This type of core structure has relevance to the oxygen-evolving complex within photosystem II.

Formation of SrBi 2 Ta 2 O 9 : Part I. Synthesis and characterization of a novel “sol-gel” solution for production of ferroelectric SrBi 2 Ta 2 O 9 thin films

We have developed a simple and rapid method for the synthesis of a precursor solution used in the production of SBT powders and thin films of the layered-perovskite phase SrBi{sub 2}Ta{sub 2}O{sub 9} (SBT). Precursor solution preparation takes less than 30 min and involves the generation of two solutions: (a) Bi(O{sub 2}CMe){sub 3} dissolved in pyridine and (b) Ta(OCH{sub 2}Me){sub 5} added to Sr(O{sub 2}CMe){sub 2} and then solubilized by HO{sub 2}CMe. After stirring separately for 10 min, these solutions are combined, stirred for an additional 10 min, and used without any further modifications. The individual solutions and ternary mixture were studied using a variety of analytical techniques. Films of the layered-perovskite phase were formed at temperatures as low as 700{degree}C. Ferroelectric testing of SBT films, fired at 750{degree}C, reveals standard hysteresis loops with no fatigure for up to 4{times}10{sup 9} cycles. {copyright} {ital 1996 Materials Research Society.}

Four-Electron Oxidative Formation of Aryl Diazenes Using a Tantalum Redox-Active Ligand Complex†

Transition-metal complexes capable of mediating multielectron transformations are critical components for a variety of small-molecule transformations. For example, the oxidation of C H bonds and the reduction of protons to H2 are both two-electron transformations. The oxidation of water to O2 is a four-electron process and the reduction of nitrogen to ammonia is an overall six-electron process. The design of metal complexes to promote or catalyze these multielectron reactions usually relies on one or more transition-metal ions capable of two-electron changes in a formal oxidation state. An alternative strategy is to incorporate redox-active ligands into the metal coordination sphere to supply reducing or oxidizing equivalents during a multielectron transformation. Herein, we report the use of a tridentate redox-active ligand, N,N-bis(3,5-di-tert-butyl-2-phenoxide)amide ([ONO] ), coordinated to tantalum, to effect the fourelectron oxidative formation of aryl diazenes. In its reduced form, [ONO] is a planar, tridentate ligand that coordinates to transition metals in a meridional geometry. The organometallic synthon TaMe3Cl2 [7] has been used to prepare [ONO]TaMe2 (1), which was then converted into the bridging imido dimer {[ONO]Ta[m-N(p-tolyl)]L}2 (2a, L= NH2(p-tolyl); 2b, L= pyridine (py); Scheme 1). Oxidation of 2b resulted in the quantitative elimination of (p-tolyl)N=N(ptolyl). To the best of our knowledge, this is the first example of N=N double bond formation and organic diazene elimination from a tantalum(V) bridging imido dimer. Oxidation studies of the related complex [ONO]TaCl2 (4) with PhICl2 suggest that the redox-active ligand plays the pivotal role of collecting oxidizing equivalents within the tantalum coordination sphere. The work presented herein highlights a new strategy for the design of metal complexes capable of multielectron oxidation reactions. The bridging imido complexes 2a and 2b were prepared via dimethyl complex 1 (Scheme 1). Double deprotonation of H3[ONO ] with nBuLi (2 equiv) followed by treatment with TaCl2Me3 afforded 1 in 49% yield following recrystallization from pentane. The H and C NMR spectra of 1 showed diagnostic resonances for the [ONO] ligand. The methyl ligands of 1 resonated at d = 0.77 and 59.7 ppm in the H and C NMR spectra, respectively. The methyl ligands of 1 are susceptible to protonolysis by anilines, which results in the formation of bimetallic complexes with two bridging imido ligands. As shown in Scheme 1, benzene solutions of 1 heated to reflux with two equivalents of NH2(p-tolyl) resulted in the formation of

Synthesis of Heteroleptic Uranium (μ−η6:η6-C6H6)2− Sandwich Complexes via Facile Displacement of (η5-C5Me5)1− by Ligands of Lower Hapticity and Their Conversion to Heteroleptic Bis(imido) Compounds

Reactivity studies on the sterically crowded [(C(5)Me(5))(2)U](2)(mu-eta(6):eta(6)-C(6)H(6)), 1, have revealed that eta(1)-ligands can displace one of the normally inert (eta(5)-C(5)Me(5))(1-) ligands in each metallocene unit to form a series of heteroleptic bimetallic sandwich complexes of nonplanar (C(6)H(6))(2-), namely, [(C(5)Me(5))(X)U](2)(mu-eta(6):eta(6)-C(6)H(6)), where X = N(SiMe(3))(2), OC(6)H(2)(CMe(3))(2)-2,6-Me-4, and CH(SiMe(3))(2). Displacement by an amidinate is also possible, that is, X = (i)PrNC(Me)N(i)Pr. This allows the multielectron reactivity of the (mu-eta(6):eta(6)-C(6)H(6))(2-) sandwich complexes to be studied as a function of ancillary ligands. Specifically, the reaction of 1 with K[N(SiMe(3))(2)], previously found to form {(C(5)Me(5))[(Me(3)Si)(2)N]U}(2)(C(6)H(6)), 2, also occurs with K[OC(6)H(2)(CMe(3))(2)-2,6-Me-4], Li[CH(SiMe(3))(2)], and Li[(i)PrNC(Me)N(i)Pr] to form {(C(5)Me(5))[4-Me-2,6-(Me(3)C)(2)C(6)H(2)O]U}(2)(C(6)H(6)), 3, {(C(5)Me(5))[(Me(3)Si)(2)CH]U}(2)(C(6)H(6)), 4, and {(C(5)Me(5))[(i)PrNC(Me)N(i)Pr]U}(2)(C(6)H(6)), 5, respectively. The reactivity of 2-5 vis-a-vis 1 has been compared with the substrates 1,3,5,7-cyclooctatetraene (C(8)H(8)) and 1-azidoadamantane (AdN(3)). Complex 1 acts as a six electron reductant to convert three equiv of C(8)H(8) to [(C(5)Me(5))(C(8)H(8))U](2)(mu-eta(3)-eta(3)-C(8)H(8)), whereas the sterically less crowded 2-5 provide only four electrons to reduce two equiv of C(8)H(8) generating U(4+) products of formula (C(5)Me(5))(X)U(C(8)H(8)). With AdN(3), complexes 1, 2, and 5 react similarly to form bis(imido) U(6+) complexes, (C(5)Me(5))(X)U(=NAd)(2). Complexes 2 and 5 also form the ligand redistribution product, (C(5)Me(5))(2)U(=NAd)(2). The reaction of 4 with AdN(3) generates at least three imido complexes: (C(5)Me(5))(2)U(=NAd)(2) from reduction and ligand redistribution, (C(5)Me(5))[AdN(3)CH(SiMe(3))(2)-kappa(2)N(1,2)]U(=NAd)(2), from reduction and insertion, and (C(5)Me(5))(eta(5):kappaNu-C(5)Me(4)CH(2)NAd)U(=NAd), from reduction, ligand redistribution, metalation, and insertion.

Axial donating ligands: a new strategy for late transition metal olefin polymerization catalysis.

An alpha-diimine ligand (1) containing an axial donating pyridine group is developed for late metal polymerization catalysis. Despite having no substitution on the bottom face of the ligand, the nickel and palladium complexes of 1 are highly active for ethylene polymerization, producing linear high molecular weight polymers. For example, 1-NiBr2 (3) forms PE with a Mn of up to 109 224 g/mol with 1.4 branches/1000 Cs. Similarly, 1-PdMeCl (5) forms PE with a Mn of up to 880 379 g/mol with 5.1 branches/1000 Cs. In sharp contrast, catalysts containing the control ligand (2) consisting of a noncoordinating phenyl group gave only low molecular weight branched oligomers. It is observed that AlMe2Cl plays a specific role in generating the active species for the pyridine-based complexes. Presumably, the pyridine group may interact with AlMe2Cl to form a bimetallic species which suppresses the beta-hydride elimination process, hence resulting in reduced chain transfer and more linear structure.

Oxidation and hydrolysis of tris-tert-butylgallium

Abstract The oxidation of GaBu3t with oxygen leads to the formation of [Bu3tGa(μ-OOBut)]2 (1). The thermolysis of 1 yields the alkoxide complex [Bu2tGa(μ-OBut)]2 (2), which may also be prepared directly from GaBu3t and ButOH. The reaction of [ButGaCl(μ-Cl)]2 (3) with oxygen does not result in its oxidation, but may be used in its purification due to the oxidation of the GaBu3t impurities. The hydrolysis of GaBu3t in thf solution, in which it exists as the solvated complex 4, results in the formation of the monomeric hydroxide complex Bu2tGa(OH)(thf) (5). In contrast, the use of non-coordinating solvents results in the trimeric hydroxide [Bu2tGa(μ-OH)]3 (6). Compound 6 is also isolated from the reaction of Bu2tGaCl(thf) (7) with KOH in refluxing thf. The solid state pyrolysis of 6 gives the polymeric oxide [ButGa(O)]x (8). All the compounds have been characterized by NMR, IR and mass spectroscopy, while the structures of 1, 2 and 3 have been confirmed by X-ray crystallography. Compound 1 crystallizes in the monoclinic space group C2/m with a = 16.375(2), b = 11.323(2), c = 8.895(2) A and β = 116.710(12)°, Z = 2, R = 0.042 and Rw, = 0.038. Compound 2 crystallizes in the orthorhombic space group Pbca with a = 15.0072(17), b = 9.8399(8), c = 18.3840(15) A, Z = 4, R = 0.037 and Rw = 0.041. Compound 3 crystallizes in the monoclinic space group P21/c with a = 6.816(4), b = 6.743(5), c = 17.062(10)A and β = 95.87(4)°, Z = 2, R = 0.042, Rw = 0.049.

Isolation of dysprosium and yttrium complexes of a three-electron reduction product in the activation of dinitrogen, the (N2)3- radical.

DyI(2) reacts with 2 equiv of KOAr (OAr = OC(6)H(3)(CMe(3))(2)-2,6) under nitrogen to form not only the (N(2))(2-) complex, [(ArO)(2)(THF)(2)Dy](2)(mu-eta(2):eta(2)-N(2)), 1, but also complexes of similar formula with an added potassium ion, [(ArO)(2)(THF)Dy](2)(mu-eta(2):eta(2)-N(2))[K(THF)(6)], 2, and [(ArO)(2)(THF)Dy](2)(mu(3)-eta(2):eta(2):eta(2)-N(2))K(THF), 3. The 1.396(7) and 1.402(7) A N-N bond distances in 2 and 3, respectively, are consistent with an (N(2))(3-) ligand, but the high magnetic moment of 4f(9) Dy(3+) precluded definitive identification. The Y[N(SiMe(3))(2)](3)/K reduction system was used to synthesize yttrium analogues of 2 and 3, {[(Me(3)Si)(2)N](2)(THF)Y}(2)(mu-eta(2):eta(2)-N(2))[K(THF)(6)] and {[(Me(3)Si)(2)N](2)(THF)Y}(2)(mu(3)-eta(2):eta(2):eta(2)-N(2))K, that had similar N-N distances and allowed full characterization. EPR, Raman, and DFT studies are all consistent with the presence of (N(2))(3-) in these complexes. (15)N analogues were also prepared to confirm the spectroscopic assignments. The DFT studies suggest that the unpaired electron is localized primarily in a dinitrogen pi orbital isolated spatially, energetically, and by symmetry from the metal orbitals.

Importance of Energy Level Matching for Bonding in Th3+-Am3+ Actinide Metallocene Amidinates, (C5Me5)2[iPrNC(Me)NiPr]An

The synthesis of a rare trivalent Th(3+) complex, (C(5)Me(5))(2)[(i)PrNC(Me)N(i)Pr]Th, initiated a density functional theory analysis on the electronic and molecular structures of trivalent actinide complexes of this type for An = Th, Pa, U, Np, Pu, and Am. While the 6d orbital is found to accommodate the unpaired spin in the Th(3+) species, the next member of the series, Pa, is characterized by an f(2) ground state, and later actinides successively fill the 5f shell. In this report, we principally examine the evolution of the bonding as one advances along the actinide row. We find that the early actinides (Pa-Np) are characterized by localized f orbitals and essentially ionic bonding, whereas the f orbitals in the later members of the series (Pu, Am) exhibit significant interaction and spin delocalization into the carbon- and nitrogen-based ligand orbitals. This is perhaps counter-intuitive since the f orbital radius and hence metal-ligand overlap decreases with increasing Z, but this trend is counter-acted by the fact that the actinide contraction also leads to a stabilization of the f orbital manifold that leads to a near degeneracy between the An 5f and cyclopentadienyl π-orbitals for Pu and Am, causing a significant orbital interaction.