Judith C. Gallucci

Lactide polymerization by well-defined calcium coordination complexes: comparisons with related magnesium and zinc chemistry.

Amide and alkoxide coordination complexes of calcium supported by beta-diiminato and bulky trispyrazolylborate complexes are reported together with their activity in lactide ring-opening polymerization; some are amongst the most active systems discovered to date.

Enantioselective desymmetrization of meso-aziridines with TMSN3 or TMSCN catalyzed by discrete yttrium complexes.

using a variety ofnucleophiles have been the subject of extensive research. Theless developed ring-opening reactions, those of meso-aziri-dines by carbon and nitrogen nucleophiles, give direct accessto enantiopure b-amino acids and 1,2-diamines—two classesof compounds which have broad chemical and pharmaceut-ical relevance. Li, Fernndez, and Jacobsen first reportedenantioselective ring-opening of meso-aziridines withTMSN

Electron delocalization in the S1 and T1 metal-to-ligand charge transfer states of trans-substituted metal quadruply bonded complexes

The singlet S1 and triplet T1 photoexcited states of the compounds containing MM quadruple bonds trans-M2(TiPB)2(O2CC6H4-4-CN)2, where TiPB = 2,4,6-triisopropylbenzoate and M = Mo (I) or M = W (I′), and trans-M2(O2CMe)2((N[i Pr ])2CC ≡ CC6H5)2, where M = Mo (II) and M = W (II′), have been investigated by a variety of spectroscopic techniques including femtosecond time-resolved infrared spectroscopy. The singlet states are shown to be delocalized metal-to-ligand charge transfer (MLCT) states for I and I′ but localized for II and II′ involving the cyanobenzoate or amidinate ligands, respectively. The triplet states are MoMoδδ* for both I and II but delocalized 3MLCT for I′ and localized 3MLCT for II′. These differences arise from consideration of the relative orbital energies of the M2δ or M2δ* and the ligand π∗ as well as the magnitudes of orbital overlap.

Chemistry of magnesium alkyls supported by 1,5,9-trimesityldipyrromethene and 2-[(2,6-diisopropylphenyl)amino]-4-[(2,6-diisopropylphenyl)imino]pent-2-ene. A comparative study

The compounds LMgBun(THF) and L′MgBun(THF) where L = 1,5,9-trimesityldipyrromethene and L′ = 2-[(2,6-diisopropylphenyl)amino]-4-[(2,6-diisopropylphenyl)imino]pent-2-ene have been prepared from reactions between MgBun2 and the protonated ligands LH and L′H, respectively. Single crystal X-ray crystallographic studies reveal that in each compound the Mg2+ ion is in a distorted tetrahedral environment and further that the ligand L is more sterically demanding with respect to access to the Mg–Bun group. Related alkyl complexes (R = Me, Et, Prn, Pri, n-hexyl and CH2CH2Ph) were prepared from reactions involving LLi(THF) or L′Li(THF) and the appropriate RMgX (X = Cl or Br) and characterized by 1H NMR spectroscopy. Those where R = CH2CH2X (X = alkyl or phenyl) have characteristic α-CH2 proton resonances which are the part of an AA′XX′ pattern. Reactions with alcohols ROH (1 equiv.) give the kinetic products LMg(OR)(THF) and L′Mg(OR)(THF) which are isolable and kinetically persistent when R = a bulky group such as But and the structure of LMg(OBut)(THF) is reported and compared with that of the known compound L′Mg(OBut)(THF). With less bulky groups the compounds are labile toward ligand scrambling and the compound L′MgBun(THF) reacts with alcohols (2 equiv.) to give L′H and Mg(OR)2. Reactions with amines and carbon dioxide are described which indicate the greater reactivity of the Mg–Bun group relative to both the L′ and L ligand. With PhCHO, Ph2CO and cyclohexanone both LMgBun(THF) and L′MgBun(THF) react via β-hydrogen atom transfer to generate the appropriate alkoxide with the elimination of 1-butene. Similarly L-lactide reacts by β-H transfer to give poly-L-lactide (P-L-LA) and 1-butene while rac-lactide yields atactic polylactide in toluene–dichloromethane solutions but in the presence of ≥10 equiv. of THF, heterotactic PLA is formed. The ring-opening polymerization of e-caprolactone also is initiated by β-H transfer and is about ten times faster than for lactide; kp(LA) = 10.7 M−1 s−1vs. kp(CL) = 110 M−1 s−1. Interestingly, the presence of lactide completely suppresses the polymerization of e-caprolactone.

The remarkable influence of M2δ to thienyl π conjugation in oligothiophenes incorporating MM quadruple bonds

Oligothiophenes incorporating MM quadruple bonds have been prepared from the reactions between Mo2(TiPB)4 (TiPB = 2,4,6-triisopropyl benzoate) and 3′,4′-dihexyl-2,2′-:5′,2″-terthiophene-5,5″-dicarboxylic acid. The oligomers of empirical formula Mo2(TiPB)2(O2C(Th)-C4(n-hexyl)2S-(Th)CO2) are soluble in THF and form thin films with spin-coating (Th = thiophene). The reactions between Mo2(TiPB)4 and 2-thienylcarboxylic acid (Th-H), 2,2′-bithiophene-5-carboxylic acid (BTh-H), and (2,2′:5′,2″-terthiophene)-5-carboxylic acid (TTh-H) yield compounds of formula trans-Mo2(TiPB)2L2, where L = Th, BTh, and TTh (the corresponding thienylcarboxylate), and these compounds are considered as models for the aforementioned oligomers. In all cases, the thienyl groups are substituted or coupled at the 2,5 positions. Based on the x-ray analysis, the molecular structure of trans-Mo2(TiPB)2(BTh)2 reveals an extended Lπ-M2δ-Lπ conjugation. Calculations of the electronic structures on model compounds, in which the TiPB are substituted by formate ligands, reveal that the HOMO is mainly attributed to the M2δ orbital, which is stabilized by back-bonding to one of the thienylcarboxylate π* combinations, and the LUMO is an in-phase combination of the thienylcarboxylate π* orbitals. The compounds and the oligomers are intensely colored due to M2δ–thienyl carboxylate π* charge transfer transitions that fall in the visible region of the spectrum. For the molybdenum complexes and their oligomers, the photophysical properties have been studied by steady-state absorption spectroscopy and emission spectroscopy, together with time-resolved emission and transient absorption for the determination of relaxation dynamics. Remarkably, THF solutions the molybdenum complexes show room-temperature dual emission, fluorescence and phosphorescence, originating mainly from 1MLCT and 3MM(δδ*) states, respectively. With increasing number of thienyl rings from 1 to 3, the observed lifetimes of the 1MLCT state increase from 4 to 12 ps, while the phosphorescence lifetimes are ≈80 μs. The oligomers show similar photophysical properties as the corresponding monomers in THF but have notably longer-lived triplet states, ≈200 μs in thin films. These results, when compared with metallated oligothiophenes of the later transition elements, reveal that M2δ–thienyl π conjugation leads to a very small energy gap between the 1MLCT and 3MLCT states of <0.6 eV.

Assembly of Amphiphilic Baskets into Stimuli-Responsive Vesicles. Developing a Strategy for the Detection of Organophosphorus Chemical Nerve Agents

We designed basket 1 to comprise a C3-symmetric hydrophobic cage (477 Å(3)) at its southern edge and three polar ammonium caps at the northern edge. This amphiphilic molecule was observed to assemble into large unilamellar vesicles (350 nm, TEM) in water and thereby entrap dimethyl phenylphosphonate (184 Å(3)) in its cavity (K(app) = (1.97 ± 0.02) × 10(3) M(-1)). The entrapment of the organophosphonate, akin to soman in size (186 Å(3)), triggers the transformation of the vesicular material into nanoparticles (100 nm, TEM). Stimuli-responsive vesicles, containing baskets of type 1 in their bilayer membrane, are unique assemblies and important for obtaining novel sensing devices.

Cyclic esters and cyclodepsipeptides derived from lactide and 2,5-morpholinediones

The reaction between BunLi in benzene and the solid polystyrene support PS-C6H4CH2NH2 leads to a lithiated species that can be represented as PS-C6H4CH2NHLi(LiBu)x, where x ∼ 4, which is active in the ring-opening of the cyclic esters L-lactide, rac-lactide, and 2,5-morpholinediones. With ≈10 eq of these monomeric six-membered rings and with heating, cyclic esters (MeCHC(O)O)n and [MeCHC(O)OCHRC(NH)O]n are reversibly released to the solution. These have been characterized by electrospray ionization MS, and some small rings have been separated by gel-permeation chromatography. Addition of NaBPh4 to a heated benzene solution containing these rings preferentially removes the 18-membered rings from solution. For lactide this is shown to form the basis for chemical amplification from a dynamic combinatorial library and lactide can be converted to (MeCHC(O)O)6 in >80% yield. Metallated supports derived from Me2Mg and Et2Zn are less reactive but do show some ability for lactide ring-enlarging. The 18-membered ring (R,R,R,S,S,S)- and meso-(R,S,R,S,R,S)-(MeCHC(O)O)6 and the 24-membered ring (MeCHC(O)OCHPriC(NH)O)4 have been characterized by single-crystal x-ray diffraction studies, together with the complex Na[η3-S,S,S,S,S,S-(MeCHC(O)O)6]2BPh4.

Bimetallic catalysis in the highly enantioselective ring–opening reactions of aziridines

Bimetallic yttrium- and lanthanide-salen complexes, readily prepared from commercially available metal isopropoxides, 2-dimethylaminoethanol, 1,1′-binaphthyl-2,2′-diamine and 2-hydroxy-3-methoxybenzaldehyde (3 steps) catalyze highly enantioselective ring opening (∼90–99% ee) reactions of meso-N-4-nitrobenzoyl aziridines by TMSCN and TMSN3. The TMSN3-mediated reactions give the highest enantioselectivities reported to date for several prototypical aziridines. Selectivity in a related ring opening by silyl isothiocyanates depends on the substituents on silicon, larger tBuPh2SiNCS giving the best selectivities, especially when an yttrium or ytterbium complex is used as the catalyst. The bimetallic yttrium complex also effects unprecedented regiodivergent parallel kinetic resolution of racemic monosubstitued aziridines upon reaction with TMSN3. In these reactions, each of the enantiomers undergoes nucleophilic addition of azide at different carbons giving two products in nearly enantiomerically pure form. To explain the dramatic differences in the selectivities between the mono- and bimetallic catalysts in these reactions, a mechanism that involves activation of both the electrophile (aziridine) and the nucleophile (azide or cyanide) at two different metals of the bimetallic complex is proposed. Molecular weight determination by vapor pressure osmometry, diffusion ordered NMR spectroscopy (DOSY) and kinetic studies, which suggest first order dependence on the concentration of the catalyst, provide strong support for this proposal.

Li2B12H12 · 7NH3: a new ammine complex for ammonia storage or indirect hydrogen storage

A new ammine complex, Li2B12H12·7NH3, that can reversibly release all the NH3 at below 200 °C and reabsorb NH3 at room temperature and 0.5 bar was synthesized and investigated for reversible ammonia storage or indirect hydrogen storage.

Quadruply Bonded Dimetal Units Supported by 2,4,6-Triisopropylbenzoates MM(TiPB)4 (MM ) Mo2, MoW, and W2): Preparation and Photophysical Properties

The preparation and characterization (elemental analysis, (1)H NMR, and cyclic voltammetry) of the new compounds MM(TiPB)(4), where MM = MoW and W(2) and TiPB = 2,4,6-triisopropylbenzoate, are reported. Together with Mo(2)(TiPB)(4), previously reported by Cotton et al. (Inorg. Chem. 2002, 41, 1639), the new compounds have been studied by electronic absorption, steady-state emission, and transient absorption spectroscopy (femtosecond and nanosecond). The compounds show strong absorptions in the visible region of the spectrum that are assigned to MMdelta to arylcarboxylate pi* transitions, (1)MLCT. Each compound also shows luminescence from two excited states, assigned as the (1)MLCT and (3)MMdeltadelta* states. The energy of the emission from the (1)MLCT state follows the energy ordering MM = Mo(2) > MoW > W(2), but the emission from the (3)MMdeltadelta* state follows the inverse order: MM = W(2) > MoW > Mo(2). Evidence is presented to support the view that the lower energy emission in each case arises from the (3)MMdeltadelta* state. Lifetimes of the (1)MLCT states in these systems are approximately 0.4-6 ps, whereas phosphorescence is dependent on the MM center: Mo(2) approximately 40 micros, MoW approximately 30 micros, and W(2) approximately 1 micros.