Copolymerization of CO 2 and epoxide catalyzed by metal complexes has drawn much attention recently.[1] In the initial report by Inoue and co-workers in 1969 the catalyst activity was poor (turnover number (TON): 6; turnover frequency (TOF): 0.12 h À1 ), [2] but by the early 2000s it had been significantly improved by using binary catalytic systems of [Co(salen)] [3] or [Cr(salen)] complexes [4] with an onium salt or base (1; salen = N,N'-bis(salicylidene)ethylenediamine,[5] It was proposed that the two components or the two zinc centers participated in the propagation reaction, and therefore binding of the two metal centers or the two components further improved catalysis to the maximum reported TON and TOF of 14 500 and 4600 h À1 , respectively. [6,7] These values, which were achieved with 2 in the copolymerization of CO 2 and propylene oxide, are still low enough to warrant further improvement, [7] because low activity means higher catalyst cost and higher levels of catalyst-derived metal residue in the resin.[8] This metal residue either colors the resin or causes toxicity. For the TON of 14 500 attained with 2, the residual cobalt level in the resin reached 40 ppm unless it was removed.[9] Herein, we report a highly active catalytic system for CO 2 /propylene oxide copolymerization and an efficient recovery strategy of the catalyst.We prepared various cobalt-salen complexes by varying either the ortho substituent or the number of attached quaternary ammonium groups (Scheme 1). Thus, the Friedel-Crafts acylation of ortho-substituted phenol using 4-chlorobutyryl chloride and subsequent hydrogenation attached a (CH 2 ) 4 Cl group in the para position. In a similar manner, attack by 1,7-dichloroheptan-4-one on lithium 2-alkyl-4-lithiophenolate yielded a tertiary benzylic alcohol, and subsequent hydrogenation attached CH[(CH 2 ) 3 Cl] 2 . After formylation, the chloro substituent, that was not susceptible to nucleophilic substitution with tributylamine, was transformed into a more reactive iodo substituent. Formation of the quaternary ammonium salt occurred in nearly quantitative yields after generation of the salen-type ligand. Because the halide ion interfered with the metalation reaction, we replaced it with inert BF 4 À . Following metalation by a routine method, an active 2,4-dinitrophenolate anion replaced this BF 4 À ion.[3a]We screened the newly prepared complexes for CO 2 / propylene oxide copolymerization under the following conditions: [propylene oxide (PO)]/[catalyst (cat.)] = 25 000, 80 8C, and CO 2 pressure 2.0-1.7 MPa (Table 1). The polymerization was terminated before 25 % conversion was reached, because after this time the solution became highly viscous, thus making stirring and diffusion difficult. The identity of the ortho substituent strongly influenced the activity. Contrary to other reports, [3] the bulky tert-butyl group was not the best choice in this study, and replacing it with a small methyl group significantly enhanced TOF. The TOF also increased dramat-

Abstract:In recent years, graphene and its derivatives have attracted much interest in various fields, including biomedical applications. In particular, increasing attention has been paid to the effects of reduced graphene oxide (rGO) on cellular behaviors. On the other hand, dicalcium phosphate (DCP) has been widely used in dental and pharmaceutical fields. In this study, DCP composites coated with rGO (DCP-rGO composites) were prepared at various concentration ratios (DCP to rGO concentration ratios of 5:2.5, 5:5, and 5:10 µg/mL, respectively), and their physicochemical properties were characterized. In addition, the effects of DCP-rGO hybrid composites on MC3T3-E1 preosteoblasts were investigated. It was found that the DCP-rGO composites had an irregular granule-like structure with a diameter in the range order of the micrometer, and were found to be partially covered and interconnected with a network of rGO. The zeta potential analysis showed that although both DCP microparticles and rGO sheets had negative surface charge, the DCP-rGO composites could be successfully formed by the unique structural properties of rGO. In addition, it was demonstrated that the DCP-rGO composites significantly increased alkaline phosphatase activity and extracellular calcium deposition, indicating that the DCP-rGO hybrid composites can accelerate the osteogenic differentiation by the synergistic effects of rGO and DCP. Therefore, in conclusion, it is suggested that the DCP-rGO hybrid composites can be potent factors in accelerating the bone tissue regeneration.

A novel Ta(V) t Bu-imido/aminoalkoxide complex, Ta(N t Bu)(dmamp) 2 Cl (1), was synthesized by metathesis reaction between Ta(N t Bu)Cl 3 (py) 2 and 2 equiv of Na-(dmamp), and subsequent reaction of 1 with 1 equiv of MeLi gave a new tantalum complex, Ta(N t Bu)(dmamp) 2 Me (2). Compounds 1 and 2 have been characterized by IR, 1 H and 13 C NMR spectroscopy, and microanalytical data. The molecular structure of 1, determined by X-ray single crystallography, revealed distorted octahedral geometry. The behavior of compound 1 in solution was studied by variable-temperature 1 H NMR spectra. Thermogravimetric analysis revealed superior thermal properties of 2 as compared to those of 1.