Paula L. Diaconescu

Redox control of a ring-opening polymerization catalyst.

The activity of an yttrium alkoxide complex supported by a ferrocene-based ligand was controlled using redox reagents during the ring-opening polymerization of L-lactide. The oxidized complex was characterized by X-ray crystallography and (1)H NMR, XANES, and Mössbauer spectroscopy. Switching in situ between the oxidized and reduced yttrium complexes resulted in a change in the rate of polymerization of L-lactide. Synthesized polymers were analyzed by gel permeation chromatography. Polymerization of trimethylene carbonate was also performed with the reduced and oxidized forms of an indium alkoxide complex. The indium system showed the opposite behavior to that of yttrium, revealing a metal-based dependency on the rate of polymerization.

Redox control of group 4 metal ring-opening polymerization activity toward l -lactide and ε-caprolactone

The activity of several group 4 metal alkoxide complexes supported by ferrocene-based ligands was controlled using redox reagents during the ring-opening polymerization of l-lactide and ε-caprolactone. Switching in situ between the oxidized and reduced forms of a metal complex resulted in a change in the corresponding rate of polymerization. Opposite behavior was observed for each monomer used. One-pot copolymerization of the two monomers to give block copolymers was also achieved.

Cerium(IV) catalysts for the ring-opening polymerization of lactide.

A rare example of a cerium(IV) alkoxide catalyst for lactide polymerization is reported. The lactide polymerization activity of the new cerium(IV) complex supported by a ferrocene Schiff base ligand, salfen, is compared to the activity of the yttrium analogue and to that of Ce(O(t)Bu)(4)(THF)(2). The complex Ce(salfen)(O(t)Bu)(2) is less active than Ce(O(t)Bu)(4)(THF)(2) and the corresponding yttrium(III) alkoxide, Y(salfen)(O(t)Bu)(THF). The different activity was correlated with the Mulliken charges calculated by density functional theory for the two complexes.

Synthesis and characterization of cerium and yttrium alkoxide complexes supported by ferrocene-based chelating ligands.

Two series of Schiff base metal complexes were investigated, where each series was supported by an ancillary ligand incorporating a ferrocene backbone and different N=X functionalities. One ligand is based on an imine, while the other is based on an iminophosphorane group. Cerium(IV), cerium(III), and yttrium(III) alkoxide complexes supported by the two ligands were synthesized. All metal complexes were characterized by cyclic voltammetry. Additionally, NMR, Mössbauer, X-ray absorption near-edge structure (XANES), and absorption spectroscopies were used. The experimental data indicate that iron remains in the +2 oxidation state and that cerium(IV) does not engage in a redox behavior with the ancillary ligand.

Scandium arene inverted-sandwich complexes supported by a ferrocene diamide ligand.

The synthesis and characterization of the first scandium arene inverted-sandwich complexes supported by a ferrocene diamide ligand (NN(fc)) are reported. Through the use of (NN(fc))ScI(THF)(2) as a precursor and potassium graphite (KC(8)) as a reducing agent, the naphthalene and anthracene complexes [(NN(fc))Sc](2)(μ-C(10)H(8)) and [(NN(fc))Sc](2)(μ-C(14)H(10)), respectively, were synthesized and isolated in moderate to high yields. Both molecular structures feature an inverted-sandwich geometry and exhibit short Fe-Sc distances. DFT calculations were employed to gain understanding of the electronic structures of these new scandium arene complexes. A variable-temperature NMR spectroscopic study of [(NN(fc))Sc](2)(μ-C(14)H(10)) indicated that two different structures are accessible in solution. Reactivity studies showed that the naphthalene complex [(NN(fc))Sc](2)(μ-C(10)H(8)) can be converted to the corresponding anthracene species [(NN(fc))Sc](2)(μ-C(14)H(10)) and that [(NN(fc))Sc](2)(μ-C(10)H(8)) can act as either a reductant or a proton acceptor. The reaction of [(NN(fc))Sc](2)(μ-C(10)H(8)) with excess pyridine led to a rare example of C-C bond formation between two pyridine rings at the para position.

μ-η6,η6-Arene-bridged diuranium hexakisketimide complexes isolable in two states of charge.

Diuranium μ-η(6),η(6)-arene complexes supported by ketimide ligands were synthesized and characterized. Disodium or dipotassium salts of the formula M(2)(μ-η(6),η(6)-arene)[U(NC(t)BuMes)(3)](2) (M = Na or K, Mes = 2,4,6-C(6)H(2)Me(3)) and monopotassium salts of the formula K(μ-η(6),η(6)-arene)[U(NC(t)BuMes)(3)](2) (arene = naphthalene, biphenyl, trans-stilbene, or p-terphenyl) were both observed. Two different salts of the monoanionic, toluene-bridged complexes are also described. Density functional theory calculations have been employed to illuminate the electronic structure of the μ-η(6),η(6)-arene diuranium complexes and to facilitate the comparison with related transition-metal systems, in particular (μ-η(6),η(6)-C(6)H(6))[VCp](2). It was found that the μ-η(6),η(6)-arene diuranium complexes were isolobal with (μ-η(6),η(6)-C(6)H(6))[VCp](2) and that the principal arene-binding interaction was a pair of δ bonds (total of 4e) involving both metals and the arene lowest unoccupied molecular orbital. Reactivity studies have been carried out with the mono- and dianionic μ-η(6),η(6)-arene diuranium complexes, revealing contrasting modes of redox chemistry as a function of the systems state of charge.

Synthesis and structural studies of chiral indium(III) complexes supported by tridentate diaminophenol ligands.

Indium(III) dimethyl, dihalide, and alkoxy-bridged complexes bearing a chiral diaminophenoxy tridentate ligand [NN(H)O](-) were synthesized. The dimethyl complex (NN(H)O)InMe(2) (1) was unreactive toward ethanol and 2-propanol and only partially reactive toward the more acidic phenol. The dihalide complexes (NN(H)O)InX(2) (X = Cl (3), Br (4), I (5)) reacted with NaOEt to form robust alkoxy-bridged complexes with the formula {[(NN(H)O)InX](2)(mu-X)(mu-OEt)} (X = Cl (6), Br (7), I (8)). The reaction of the alkoxy-bridged complexes with water produced hydroxy-bridged dinuclear indium compounds. The hydroxy-bridged complex bearing a chloride ligand [(NN(H)O)InCl(mu-OH)](2) (9) was significantly more reactive toward dissociation and formation of a pyridine adduct than the iodo analogue [(NN(H)O)InI(mu-OH)](2) (10). All compounds were fully characterized in solution by NMR spectroscopy and in the solid state by single-crystal X-ray diffraction. In addition, DFT calculations were used to help explain the reactivity trends observed.

d0fN-METAL COMPLEXES SUPPORTED BY FERROCENE-BASED CHELATING LIGANDS

Ferrocene is widely incorporated in pharmaceutical candidates, materials, and redox agents. In addition, ligand scaffolds make use of ferrocene groups because of their steric, electronic, and redox properties. In some cases, ferrocene is involved directly in the reactivity of a metal center even when it is part of the supporting ligand. While metal-ligand cooperation plays an important role in enzymatic catalysis, it is less developed in organometallic chemistry. Our group has focused on ferrocene-based chelating ligands because they possess unique electronic characteristics that make them especially versatile in supporting a wide range of reactivity behaviors for the resulting metal complexes. The present review discusses the chemistry of metal complexes with two types of ferrocene-based chelating ligands: (1) Schiff base; and (2) diamide. The first class of ligands supports yttrium and cerium alkoxides, while the second class is used for group 3 metal (scandium, yttrium, lutetium, and lanthanum) alkyls. Two series of Schiff base metal complexes are presented. The two ancillary ligands differ by the type of the N=X functionality that they incorporate: one ligand is based on an imine group, whereas the other is based on an iminophosphorane group. Cerium(IV) bis(alkoxide) complexes were targeted in order to determine whether the presence of a strongly oxidizing metal center would give rise to a non-innocent redox behavior in the supporting ligands. The experimental data indicated that iron remained in the +2 oxidation state and that cerium(IV) did not engage any part of the ancillary ligand in redox behavior. The reactivity of group 3 metal complexes supported by 1,1′-ferrocenylene diamide ligands toward aromatic N-heterocycles is also discussed. These reactions are compared to analogous reactions studied with group 3 metal complexes supported by pincer-type pyridine diamides. That comparison showed that similar reactions were observed with 1-methylimidazole, 2-picoline, and isoquinoline, although other types of reactions and a larger substrate scope were identified for the ferrocene- than for the pyridine-based complexes. Based on the reactions discussed herein and on isolated examples drawn from the literature, it is concluded that the ferrocene diamides represent a versatile and privileged ligand framework. It is proposed that the privileged status of these organometallic ancillary ligands is a consequence of irons ability to accommodate changes in the electronic density at the metal center more readily than classical supporting ligands.

Beyond CH Activation with Uranium: A Cascade of Reactions Mediated by a Uranium Dialkyl Complex†

The ring opening of aromatic N heterocycles has been restricted to a few examples involving transition metals, such as niobium, tantalum, titanium, scandium, yttrium, and rhenium; strong actinide–oxygen bonds could also drive the ring opening of pyridine N-oxides. Our interest in the reactions of electrophilic alkyl complexes 12] supported by a ferrocene (fc) diamide ligand with aromatic N heterocycles prompted us to investigate the reactivity of the uranium dibenzyl complex 1-(CH2Ph)2 ((NN )U(CH2Ph)2; NN = 1,1’-fc(NSitBuMe2)2) [11, 16] with imidazoles. It has been reported that actinide complexes are more reactive than analogous Group IV complexes towards aromatic N heterocycles, and that C H activation occurs from neutral dialkyl complexes as opposed to the cationic alkyl complexes of group IV metals. We reasoned that diamide ligands may enhance the reactivity of the actinide complexes, since such ligands are known to support metal centers that are more electrophilic than those in metallocenes. This characteristic becomes important when aromatic N heterocycles are involved, because these substrates tend to be strong Lewis bases and shut down further reactivity at the metal center. The uranium dibenzyl complex 1-(CH2Ph)2 features two alkyl ligands; therefore, we became interested in finding reactions that would involve two C H activation events. We anticipated that this complex would show different reactivity from that reported by Guram and Jordan for a related zirconium complex ([Cp2ZrMe(THF)] + did not engage in C H activation with the imidazole ring; only coordination was observed). The ability to undergo two C H activation reactions is unique to uranium. It has been reported that both alkyl ligands of a uranium dialkyl complex react with the C H bonds of terminal acetylenes; however, reactions that involve two sp-hybridized C H bonds are not known. One reason for this lack of reactivity may be the fact that the acidity of C(sp) H bonds is lower than that of C(sp) H bonds: the pKa value of phenyl acetylene is 23.2, [22] whereas the pKa value of 1-methylimidazole is 33.1 (experimental; [23] calculated: pKa 35.1; the pKa value of 1-methylbenzimidazole is 32.5). Herein, we report a novel double C H activation followed by the C C coupling, ring opening, and migratory insertion of imidazoles; these reactions are uniquely promoted by 1-(CH2Ph)2 and represent the first examples of the cleavage of aromatic N heterocycles by actinide complexes without the involvement of oxygen atoms or redox processes. The reaction between 1-(CH2Ph)2 and 2 equivalents of 1methylimidazole occurred at room temperature (Scheme 1). The H NMR spectrum was indicative of a symmetrical

Transmetalation reactions of a scandium complex supported by a ferrocene diamide ligand.

Efforts to transfer to aluminum the heterocyclic ligand of a ring-opened imidazole scandium complex, which was previously reported, are presented. A ring-opened imidazole aluminum compound was formed at 50 °C and characterized as a trialuminum complex. At high temperature (85 °C), the formation of an unusual scandium/aluminum methylidene was observed. The reaction products were characterized by standard spectroscopic techniques and X-ray crystallography. Density functional theory calculations were used to understand the electronic structure of the scandium/aluminum methylidene complex.