Single-site achiral and chiral C 3 -symmetric complexes LMOR, where M ) Mg and Zn, L ) an η 3 -trispyrazolyl-or η 3 -trisindazolyl-borate ligand and R ) Et, t Bu, Ph, or SiMe 3 , have been synthesized and employed in the ring-opening polymerization of L-, rac-, and meso-lactide in CH 2 Cl 2 at 25 °C and below. The polymerization occurs by acyl cleavage and gives rise to polylactide, PLA, with PDI of 1.1-1.25 up to 90% conversion. Studies of the kinetics of polymerization reveal first order behavior in both lactide and metal catalyst. For L ) tris(3-tert-butylpyrazolyl)borate, ( t Bupz) 3 BH, polymerization of ∼500 equiv of L-lactide proceeds to 90% conversion within 1 h and 6 d for the magnesium and zinc catalysts, respectively. The zinc complexes are, however, more tolerant to air and moisture and solid samples where R ) SiMe 3 are persistent in air for several days. The rate of polymerization is also significantly influenced by the nature of the η 3 -L spectator ligand. Chiral C 3 -symmetric catalysts, where L ) tris(indazolyl)borates derived from camphor and menthone, show only slight enantioselectivity in their polymerization of rac-lactide but do show significant diastereoselectivity in their ability to preferentially polymerize meso-lactide from a mixture of rac-and mesolactide. The poly(mesolactide) shows a modest preference for syndiotactic junctions, RSRSRS. The molecular structures, deduced from single-crystal X-ray crystallography, are reported for η 3 -HB(3-Phpz) 3 MgEt(THF), [η 3 -HB((7R)-i Pr-(4R)-Me-4,5,6,7-tetrahydro-2H-indazolyl) 3 ]ZnMe and [η 3 -HB(3-t Bupz)(3,5-(CF 3 ) 2 pz) 2 ]-ZnOSiMe 3 and serve as structural models for activated and ground-state configurations of the metal ions during the polymerization reaction. The molecular structure of meso-lactide is also reported. These results are compared with polymerizations of lactide by other coordinate catalysts.

A label-free biosensor for Escherichia coli (E. coli) ORN 178 based on faradaic electrochemical impedance spectroscopy (EIS) was developed. α-Mannoside or β-galactoside was immobilized on a gold disk electrode using a self-assembled monolayer (SAM) via a spacer terminated in a thiol functionality. Impedance measurements (Nyquist plot) showed shifts due to the binding of E. coli ORN 178, which is specific for α-mannoside. No significant change in impedance was observed for E. coli ORN 208, which does not bind to α-mannoside. With increasing concentrations of E. coli ORN 178, electron-transfer resistance (R(et)) increases before the sensor is saturated. After the Nyquist plot of E. coli/mixed SAM/gold electrode was modeled, a linear relationship between normalized R(et) and the logarithmic value of E. coli concentrations was found in a range of bacterial concentration from 10(2) to 10(3) CFU/mL. The combination of robust carbohydrate ligands with EIS provides a label-free, sensitive, specific, user-friendly, robust, and portable biosensing system that could potentially be used in a point-of-care or continuous environmental monitoring setting.

From the reactions between 2,2′-ethylidenebis(4,6-di-tert-butylphenol) and 2,2′-methylidenebis(4-dimethyl-6-di-tert-butylphenol) and Et 2AlCl the biphenoxide complexes [(O∼∼CHMe∼∼O)AlCl]2, 1, and [(O∼∼CH2∼∼O)AlCl]2, 2, have been isolated and characterized. These dimers are broken up by donor ligands, and the molecular structure of ethylidenebis(4,6-di-tert-butylphenoxide)AlCl(THF), 3, has been structurally characterized. Racemic 5,5′-6,6′-tetramethyl-3,3′-di-tert-butyl-1,1′-biphen-2,2′-diol and Et2AlCl react in hexane to give [(O∼∼O)AlCl]2, compound 6, as a hydrocarbon insoluble white precipitate. In the donor solvent THF monomeric species are formed, and (O∼∼O)AlX(THF) has been crystallographically characterized, X ) 20% Cl and 80% Et occupancy. Refluxing in THF favors X ) Cl, compound 4. The reaction of Et2Al(OEt) with the biphenol gives (O∼∼O)AlEt(THF), 5, in the presence of THF by displacement of one ethyl and one ethoxide ligand. Compounds 1, 2, 3, 4, 5, 6, [(O∼∼CHMe∼∼O)Al-(O i Pr-d7)]2, and [Cp2Zr(OEt)(OEt2)] + [HB(C6F5)3]act as propylene oxide, PO, polymerization catalyst precursors. The polymers have been examined by MS techniques and NMR spectroscopy, and these results are compared with polypropylene oxide, PPO, formed by base catalysis and by porphyrin-and salen-AlCl catalyst precursors. The new Al compounds and the cationic zirconium alkoxide give close to 50:50 HH to TT junctions with end groups C-Cl, OH, and )CH 2 being identified by MS and NMR. Polymerizations employing [(O∼∼CHMe∼∼O)Al(O i Pr-d7)]2 give HO-(PO)n-O i Pr-d7 oligomers, in addition to vinylterminated species. Polymerization of S-PO and 50:50 mixtures of S-PO and rac-PO reveals that the stereoirregular polymer is formed by a stereoselective ring-opening step. An analysis of the HH and TT junctions at the triad level is made, extending the earlier assignments of Tonelli and Schilling. This analysis leads us to suggest that polymerization occurs by a cationic coordinate mechanism wherein ring opening occurs by backside attack on an activated PO molecule which leads to inversion at the methine carbon. The rac-biphenoxide-Al complexes show a preference for ii and i linkages in (HT)(HT)(HT) units. These results are compared to coordinate catalysis polymerizations of PO employing the Union Carbide calcium amide-alkoxide system and (porphyrin)AlCl and lead us to predict that a cis-migratory ringopening polymerization process is not likely to be developed for polymerization of PO. † Dedicated to Dr. Walter Reichle, Corporate Research Fellow, Union Carbide, on the occasion of his retirement. * Corresponding author. E-mail Chisholm@chemistry. ohio-state.edu.

The cationic complexes [Mo 2 (DPhF)(MeCN) 6 ] 3ϩ [BF 4 Ϫ ] 3 ؒMeCN 1a [throughout this paper the formamidinate anions, RNC(H)NR Ϫ , will be abbreviated as DRF Ϫ , with specific aryl groups represented by Ph for phenyl, and Ani forand [Mo 2 (DPhF) 3 (MeCN) 2 ] ϩ [BF 4 Ϫ ] 3, have been prepared from the reactions between Mo 2 (DArF) 4 and HBF 4 ؒEt 2 O or (Me 3 O)BF 4 in an appropriate stoichiometry in acetonitrile or CH 2 Cl 2 -MeCN mixtures. A better procedure for the preparation of 3 involves the reaction between 2a and Li ϩ (DPhF Ϫ ) in acetonitrile. Compound 2a undergoes a reaction in pyridine to give the compound trans-[Mo 2 (DPhF) 2 -(py) 4 ] 2ϩ [BF 4 Ϫ ] 2 4. The new compounds 1a, 1b, 2a, 2b and 4 have been structurally characterized and shown to contain Mo-Mo quadruple bonds.

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