A change in the sign of the ground state electron spin polarization (ESP) is reported in complexes where an organic radical (nitronylnitroxide, NN) is covalently attached to a donor–acceptor chromophoreviatwo differentmeta-phenylene bridges.

The torsional dependence of the ground state magnetic exchange coupling (J) and the corresponding electronic coupling matrix element (H-DA) for eight transition metal complexes possessing donor-acceptor (D-A) biradical ligands is presented. These biradical ligands are composed of an S = 1/2 metal semiquinone (SQ) donor and an S = 1/2 nitronylnitroxide (NN) acceptor, which are coupled to each other via paraphenylene, methyl- substituted para- phenylenes, or a bicyclo[2.2.2]octane ring. The observed trends in electronic absorption and resonance Raman spectral features are in accord with a reduction in electronic and magnetic coupling between D and A units within the framework of our valence bond configuration interaction model. Moreover, our spectroscopic results highlight different orbital mechanisms that modulate coupling in these complexes, which is not manifest in the ferromagnetic J(SQ-B-NN) values. The work provides new detailed insight into the effects of torsional rotations which contribute to inhomogeneities in experimentally determined exchange couplings, electron transfer rates, and electron transport conductance measurements.

We report here the Zn(II)-catalyzed convergence of two metastable and isostructural polyesters to an isomeric polymer having a hyperbranched architecture. Ring-opening transesterification polymerization (ROTEP) of 4-carbomethoxyvalerolactone (CMVL) under Brønsted catalysis is known to give the linear polyester PCMVL. We show here that this can be isomerized to the equilibrated (and highly branched) polyester EQ-PCMVL. Analysis of the fragments obtained from eliminative degradation of EQ-PCMVL were critical in the formulation of its structure. The isomerization of PCMVL to EQ-PCMVL is a direct consequence of the presence of the second ester functional group in the CMVL ester-lactone, a rarely studied class of monomer. Zn(II)-catalysis of the ROTEP of the isomeric β-lactone, 2-(2-carbomethoxyethyl)propiolactone (isoCMVL), as well as isomerization of the isostructural linear homopolymer derived from that isomeric monomer, led to the same EQ-PCMVL. These results suggest a new strategy for the introduction of branching into various polyesters.

The ring-opening transesterification polymerization (ROTEP) of rac-lactide (rac-LA) using LXZn catalysts (LX = ligand having phenolate, amine, and pyridine donors with variable para substituents X on the bound phenolate donor; X = NO2, Br, t-Bu, OMe) was evaluated through kinetics experiments and density functional theory, with the aim of determining how electronic modulation of the ligand framework influences polymerization rate, selectivity, and control. After determination that zinc-ethyl precatalysts required 24 h of reaction with benzyl alcohol to convert to active alkoxide complexes, the subsequently formed species proved to be active and fairly selective, polymerizing up to 300 equiv of rac-LA in 6-10 min while yielding isotactic (Pm = 0.72-0.78) polylactide (PLA) with low dispersities: Đ = 1.06-1.17. In contrast to previous work with aluminum catalysts for which electronic effects of ligand substituents were significant (Hammett ρ = +1.2-1.4), the LXZn systems exhibited much less of an effect (ρ = +0.3). Density functional calculations revealed details of the initiation and propagation steps, enabling insights into the high isotacticity and the insensitivity of the rate on the identity of X.

Crystals of [Co(diox)2(4-NO2-py)2] (1) and [Co(diox)2(4-CN-py)2](2) where diox are the o-Dioxolene 3,5-di-tbutylsemiquinonate (SQ)and/or 3,5-di-t-butylcatecholate (Cat)ions, 4-NO2-py is 4-nitro-pyridine, 4-CN-py is 4-cyano-pyridine, are among the few known crystals presenting both thermal-induced and photoinduced ls-[M(SQ)(Cat)] hs-[M(SQ )2]valence tautomerism interconversion (VTI). In 2 the thermal-induced VTI is cooperative, characterizing an abrupt conversion and in 1 it is non-cooperative. In this work, crystals of [Co(diox)2(4-NO2-py)2]·benzene (1BZ), [Co(diox)2(4-NO2-py)2]·toluene (1TL), [Co(diox)2(4-CN-py)2]·benzene (2BZ) and [Co(diox)2(4-CN-py)2]·toluene (2TL) have been prepared and analyzed by single crystal X-ray diffraction in order to investigate how solvation modulates thermal-induced VTI. The solvate crystals, like the nonsolvated ones, present essentially reversible thermal-induced and photoinduced VTI. 1TL crystal presents the same monoclinic symmetry and the same intermolecular hydrogen-bonded network of 1 and both present a non-cooperative thermal-induced VTI. 1BZ crystal has triclinic symmetry and present a cooperative and abrupt VTI with a thermal hysteresis of ~30 K. In contrast to 2, thermal-induced VTI in 2BZ and 2TL is non-cooperative despite the fact that 2, 2BZ and 2TL crystals exhibit the same monoclinic symmetry and the same intermolecular hydrogen-bonded network. In 2BZ and 2TLbenzene and toluene molecules as well as the tbutyl groups of the o-dioxolene molecules convert gradually from being dynamically disordered at about 300 K to a static disorder state below 150 K. The layer separation distance of interacting [Co(diox)2(4-X-py)2], X=CN and NO2, molecules in all solvate crystals is ~15 Å whereas in the 2, which presents cooperative VTI, it is ~12 Å. An order-disorder component might account to the stabilization of the metastable hs-Co state in 2BZ and in 2TL but no disorder was found in the 1TL crystals. Therefore, the lack of cooperativeness in the thermal-induced VTI in these crystals seems to be due to the large distance between the layers of interacting molecules. Cooperativeness in the VTI of 1BZ crystal is likely to be related with the unique molecular bond scheme network that connects neighboring active [Co(diox)2(4-NO2-py)2] molecules through the o-Dioxolene oxygen atoms bonded directly to the Co ion.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Crystals of [Co(diox)(2)(4-NO2-py)(2)] (1) and [Co(diox)(2)(4-CN-py)(2)] (2) where diox are the o-dioxolene 3,S-di-t-butylsemiquinonate (SQ(center dot-)) and/or 3,5-di-t-butylcatecholate (Cat(2-)) ions, 4-NO2-py is 4-nitro-pyridine, 4-CN-py is 4-cyano-pyridine, are among the few known crystals presenting both thermally induced and photoinduced ls-[M+3(SQ(center dot-))(Cat(2-))] <-> hs-[M2+(SQ(center dot-))(2)] valence tautomeric interconversion (VTI). In 2, the thermal-induced VTI is cooperative, characterizing an abrupt conversion, and in 1 it is noncooperative. In this work, crystals of [Co(diox)(2)(4-NO2-py)(2)]center dot benzene (1BZ), [Co(diox)(2)(4-NO2-py)(2)]center dot toluene (1TL), [Co(diox)(2)(4-CN-py)(2)]center dot benzene (2BZ), and [Co(diox)(2)(4-CN-py)(2)]center dot toluene (2TL) have been prepared and analyzed by single crystal X-ray diffraction in order to investigate how solvation modulates thermally induced VTI. Crystallographic data were also successfully used together with the two-state equilibrium equation to estimate Delta H degrees and Delta S degrees VTI thermodynamic parameters. The solvate crystals, like the nonsolvated ones, present essentially reversible thermally induced VTI. The 1TL crystal presents the same monoclinic symmetry and the same intermolecular hydrogen-bonded network of 1, and both present a noncooperative thermal-induced VTI. The 1BZ crystal has triclinic symmetry and presents a cooperative VTI with a thermal hysteresis of similar to 30 K. In contrast to 2, thermally induced VTI in 2BZ and 2TL is noncooperative despite the fact that 2, 2BZ, and 2TL crystals exhibit the same monoclinic symmetry and the same intermolecular hydrogen-bonded network. In 2BZ and 2TL benzene and toluene molecules as well as the t-butyl groups of the o-dioxolene molecules convert gradually from being dynamically disordered at about 300 K to a static disorder state below 150 K. The layer separation distance of interacting [Co(diox)(2)(4-X-py)(2)], X = CN and NO2, molecules in all solvate crystals is similar to 15 angstrom, whereas in 2, which presents cooperative VTI, it is similar to 12 angstrom. An order-disorder component might account for the stabilization of the metastable hs-Co2+ state in 2BZ and in 2TL, but no disorder was found in the 1TL crystals. Therefore, the lack of cooperativity in the thermally induced VTI in these crystals seems to be due to the large distance between the layers of interacting molecules. Cooperativity in the VTI of 1BZ crystal is likely to be related with the unique molecular bond scheme network that connects neighboring active [Co(diox)(2)(4-NO2-py)(2)] molecules through the o-dioxolene oxygen atoms bonded directly to the Co ion.

Polymerization of ε-caprolactone (CL) using an aluminum alkoxide catalyst (1) designed to prevent unproductive trans binding was monitored at 110 °C in toluene-d8 by 1H NMR and the concentration versus time data fit to a first-order rate expression. A comparison of t1/2 for 1 to values for many other aluminum alkyl and alkoxide complexes shows much lower activity of 1 toward polymerization of CL. Density functional theory calculations were used to understand the basis for the slow kinetics. The optimized geometry of the ligand framework of 1 was found indeed to make CL trans binding difficult: no trans-bound intermediate could be identified as a local minimum. Nor were local minima for cis-bound precomplexes found, suggesting a concerted coordination-insertion for polymer initiation and propagation. The sluggish performance of 1 is attributed to a high-framework distortion energy required to deform the "resting" ligand geometry to that providing optimal catalysis in the corresponding transition-state structure geometry, thus suggesting a need to incorporate ligand flexibility in the design of efficient polymerization catalysts.

The torsional dependence of donor-bridge-acceptor (D-B-A) electronic coupling matrix elements (H(DA), determined from the magnetic exchange coupling, J) involving a spin SD = 1/2 metal semiquinone (Zn-SQ) donor and a spin S(A) = 1/2 nitronylnitroxide (NN) acceptor mediated by the σ/π-systems of para-phenylene and methyl-substituted para-phenylene bridges and by the σ-system of a bicyclo[2.2.2]octane (BCO) bridge are presented and discussed. The positions of methyl group(s) on the phenylene bridge allow for an experimentally determined evaluation of conformationally dependent (π) and conformationally independent (σ) contributions to the electronic and magnetic exchange couplings in these D-B-A biradicals at parity of D and A. The trend in the experimental magnetic exchange couplings are well described by CASSCF calculations. The torsional dependence of the pairwise exchange interactions are further illuminated in three-dimensional, "Ramachandran-type" plots that relate D-B and B-A torsions to both electronic and exchange couplings. Analysis of the magnetic data shows large variations in magnetic exchange (J ≈ 1-175 cm(-1)) and electronic coupling (H(DA) ≈ 450-6000 cm(-1)) as a function of bridge conformation relative to the donor and acceptor. This has allowed for an experimental determination of both the σ- and π-orbital contributions to the exchange and electronic couplings.