A cross-bridged cyclam ligand bearing two N-carboxymethyl pendant arms (1) has been found to form a copper(II) complex that exhibits significantly improved biological behavior in recent research towards (64)Cu-based radiopharmaceuticals. Both the kinetic inertness and resistance to reduction of Cu-1 are believed to be relevant to its enhanced performance. To explore the influence of pendant arm length on these properties, new cross-bridged cyclam and cyclen ligands with longer N-carboxyethyl pendant arms, 2 and 4, and their respective copper(II) complexes have been synthesized. Both mono- as well as di-O-protonated forms of Cu-2 have also been isolated and structurally characterized. The spectral and structural properties of Cu-2 and Cu-4, their kinetic inertness in 5 M HCl, and electrochemical behavior have been obtained and compared to those of their N-carboxymethyl-armed homologs, Cu-1 and Cu-3. Only the cyclam-based Cu-1 and Cu-2 showed unusually high kinetic inertness towards acid decomplexation. While both of these complexes also exhibited quasi-reversible Cu(II)/Cu(I) reductions, Cu-2 is easier to reduce by a substantial margin of +400 mV, bringing it within the realm of physiological reductants. Similarly, of the cyclen-based complexes, Cu-4 is also easier to reduce than Cu-3 though both reductions are irreversible. Biodistribution studies of (64)Cu-labeled 2 and 4 were performed in Sprague Dawley rats. Despite comparable acid inertness to their shorter-armed congeners, both longer-armed ligand complexes have poorer bio-clearance properties. This inferior in vivo behavior may be a consequence of their higher reduction potentials.

A highly active enantiopure bimetallic cobalt complex was explored for the enantioselective polymerization of a variety of monosubstituted epoxides. The polymerizations were optimized for high rates and stereoselectivity, with s-factors (k(fast)/k(slow)) for most epoxides exceeding 50 and some exceeding 300, well above the threshold for preparative utility of enantiopure epoxides and isotactic polyethers. Values for mm triads of the resulting polymers are typically greater than 95%, with some even surpassing 98%. In addition, the use of a racemic catalyst allowed the preparation of isotactic polyethers in quantitative yields. The thermal properties of these isotactic polyethers are presented, with many polymers exhibiting high T(m) values. This is the first report of the rapid synthesis of a broad range of highly isotactic polyethers via the enantioselective polymerization of racemic epoxides.

The enantioselective polymerization of propylene oxide (PO) using biaryl-linked bimetallic salen Co catalysts was investigated experimentally and theoretically. Five key aspects of this catalytic system were examined: (1) the structural features of the catalyst, (2) the regio- and stereoselectivity of the chain-growth step, (3) the probable oxidation and electronic state of Co during the polymerization, (4) the role of the cocatalyst, and (5) the mechanism of monomer enchainment. Several important insights were revealed. First, density functional theory (DFT) calculations provided detailed structural information regarding the regio- and stereoselective chain-growth step. Specifically, the absolute stereochemistry of the binaphthol linker determines the enantiomer preference in the polymerization, and the interaction between the salen ligand and the growing polymer chain is a fundamental aspect of enantioselectivity. Second, a new bimetallic catalyst with a conformationally flexible biphenol linker was synthesized and found to enantioselectively polymerize PO, though with lower enantioselectivity than the binaphthol linked catalysts. Third, DFT calculations revealed that the active form of the catalyst has two active exo anionic ligands (chloride or carboxylate) and an endo polymer alkoxide which can ring-open an adjacent cobalt-coordinated epoxide. Fourth, calculations showed that initiation is favored by an endo chloride ligand, while propagation is favored by the presence of two exo carboxylate ligands.

Hydroxy-telechelic poly(propylene oxide) (PPO) is widely used industrially as a midsegment in polyurethane synthesis. These atactic polymers are produced from racemic propylene oxide using chain shuttling agents and double-metal cyanide catalysts. Unlike atactic PPO, isotactic PPO is semicrystalline with a melting temperature of approximately 67 °C. Currently there is no practical route to hydroxy-telechelic isotactic PPO using racemic propylene oxide as the monomer. In this paper, hydroxy-telechelic isotactic PPO is synthesized from racemic propylene oxide with control of molecular weight using enantioselective and isoselective bimetallic catalysts in conjunction with chain shuttling agents. The discovery of an easily accessible bimetallic chromium catalyst is reported for this transformation. Diol, triol, and polymeric chain shuttling agents are used to give hydroxy-telechelic isotactic PPO of varying functionality and structure. Detailed quantum chemical studies are used to reveal the polymerization mechanism and origin of stereoselectivity.

A highly active bimetallic cobalt catalyst system is reported for the polymerization of racemic terminal epoxides to yield isotactic polyethers.Stereoregular polymers often display superior mechanical and thermal properties compared to their atactic analogues. 1 Polyethers are an important class of polymers with industrial and biological applications. 2 Much progress has been made in controlling the regiochemistry and molecular weight of polyethers, 3 but controlling the stereochemistry of polyethers synthesized from racemic epoxides remains a challenge. 4 Previous isoselective polymerization systems for racemic epoxides produced mixtures of atactic and isotactic materials, or generated only moderately isotactic polymers. 5 We previously reported a highly active and isoselective cobalt catalyst for the polymerization of racemic propylene oxide (rac-PO). 6 Using the information gained by studying the mechanism of this catalyst, 7 we have recently designed a highly active and enantioselective polymerization catalyst system that exhibits greater substrate scope. It consists of a bimetallic cobalt complex (1) and a bis(triphenylphosphine)iminium (PPN) acetate cocatalyst (4, Scheme 1). This system selectively polymerizes one enantiomer of racemic terminal epoxides to produce highly isotactic polyethers and enantiopure epoxides. 8 Unfortunately, this limits the theoretical maximum yield of polymer at ca. 50% unless the racemic form of 1 is used. Attempts to synthesize rac-1 (equimolar mixture of (R 4 S)-1 and (S 4 R)-1) from racemic starting materials produced inseparable diastereomers, which led us to develop a new catalyst that is active and isoselective while being derived from inexpensive racemic and/or achiral starting materials.Insight into the origin of enantioselectivity of 1 was gained through the reactivity of the diastereomeric cobalt complex 2, which possesses all S stereochemistry. When activated with PPN compound 4, complex 2 displayed lower activity than 1 for the polymerization of neat rac-PO (T rxn = 0 1C), but preferred the same enantiomer (S) with a k S /k R selectivity factor of 210. 9 This result confirmed that the stereochemistry of the binaphthol linker determines which enantiomer of monomer is enchained, possibly allowing simplification of 1 and 2 by removal of the chiral diamine linker.Having determined that the axial chirality of the binaphthol linker is responsible for the stereoselectivity of the polymerization, we synthesized complex 3, which incorporated ethylene diamine and a rac-binaphthol linker.w All attempts at crystallizing 3 from non-coordinating solvents were unsuccessful, however crystals were grown from pyridine and toluene giving black needles that were analyzed by single crystal X-ray diffraction. Fig. 1 shows the structure of 3 with hydrogens omitted and pyridine ligands truncated for clarity. Two pyridines are bound to each cobalt center displacing the chlorides giving a pseudo C 2 symmetric complex. 10 The Co-Co distance is 6.774 Å and the endo naphthol-naphthol dihedral...

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Flexible variable transmission films (VTFs) have been developed and demonstrated using charged, polarizable pigment particles. Unlike electrochromic VTF devices, these E Ink films have a neutral black color and are not current driven. And unlike conventional PDLCs, the films are truly bistable, requiring no power to maintain the transmission states. Applications include automotive sunroofs and windows as well as architectural skylights and windows where energy savings and improved lighting comfort can be realized.

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