[(1,2,5,6-η)-1,5-Cyclo­octa­diene](1-isopropyl-3-methyl­imidazolin-2-yl­idene)(triphenyl­phosphine)iridium(I) tetra­fluorido­borate dichloro­methane solvate

In the title compound, [Ir(C8H12)(C7H12N2)(C18H15P)]BF4·CH2Cl2, the Ir(I) atom has a square-planar conformation with normal bond lengths. One of the phenyl rings, and the solvent dichloro­methane mol­ecule, were refined using separate two part disorder models, each in an approximately 1:1 ratio

Towards controlling the solid state valence tautomeric interconversion character by solvation

Crystals of [Co<i>(diox)</i>₂­(4-NO₂-py)₂] (<b>1</b>) and [Co<i>(diox)</i>₂­(4-CN-py)₂] (<b>2</b>) where <i>diox</i> are the <i>o</i>-dioxolene 3,5-di-<i>t</i>-butylsemiquinonate (SQ^•–) and/or 3,5-di-<i>t</i>-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 <i>ls</i>-[M⁺³(SQ^•–)­(Cat^2–)] ↔ <i>hs</i>-[M²⁺(SQ^•–)₂] valence tautomeric interconversion (VTI). In <b>2</b>, the thermal-induced VTI is cooperative, characterizing an abrupt conversion, and in <b>1</b> it is noncooperative. In this work, crystals of [Co<i>(diox)</i>₂­(4-NO₂-py)₂]­·benzene (<b>1BZ</b>), [Co<i>(diox)</i>₂­(4-NO₂-py)₂]­·toluene (<b>1TL</b>), [Co<i>(diox)</i>₂­(4-CN-py)₂]­·benzene (<b>2BZ</b>), and [Co<i>(diox)</i>₂­(4-CN-py)₂]­·toluene (<b>2TL</b>) 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 Δ<i>H</i>° and Δ<i>S</i>° VTI thermodynamic parameters. The solvate crystals, like the nonsolvated ones, present essentially reversible thermally induced VTI. The <b>1TL</b> crystal presents the same monoclinic symmetry and the same intermolecular hydrogen-bonded network of <b>1</b>, and both present a noncooperative thermal-induced VTI. The <b>1BZ</b> crystal has triclinic symmetry and presents a cooperative VTI with a thermal hysteresis of ∼30 K. In contrast to <b>2</b>, thermally induced VTI in <b>2BZ</b> and <b>2TL</b> is noncooperative despite the fact that <b>2</b>, <b>2BZ</b>, and <b>2TL</b> crystals exhibit the same monoclinic symmetry and the same intermolecular hydrogen-bonded network. In <b>2BZ</b> and <b>2TL</b> benzene and toluene molecules as well as the <i>t</i>-butyl groups of the <i>o</i>-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<i>(diox)</i>₂­(4-X-py)₂], X = CN and NO₂, molecules in all solvate crystals is ∼15 Å, whereas in <b>2</b>, which presents cooperative VTI, it is ∼12 Å. An order–disorder component might account for the stabilization of the metastable <i>hs</i>-Co²⁺ state in <b>2BZ</b> and in <b>2TL</b>, but no disorder was found in the <b>1TL</b> 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 <b>1BZ</b> crystal is likely to be related with the unique molecular bond scheme network that connects neighboring active [Co<i>(diox)</i>₂­(4-NO₂-py)₂] molecules through the <i>o</i>-dioxolene oxygen atoms bonded directly to the Co ion

Towards controlling the solid state valence tautomeric interconversion character by solvation

ABSTRACT: 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 (Cat2-) 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•-)(Cat2-)]↔hs-[M2+(SQ•-)2] valencetautomeric interconversion (VTI). In 2, the thermal-induced VTI is cooperative, characterizing an abrupt conversion, and in 1it 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 crystalX-ray diffraction in order to investigate how solvation modulates thermally-induced VTI. Crystallographic data was also successfullyused together with two-state equilibrium equation to estimate ΔH° and ΔS° VTI thermodynamic parameters. The solvate crystals,like the non-solvated ones, present essentially reversible thermally-induced VTI. 1TL crystal presents the same monoclinicsymmetry 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 VTI with a thermal hysteresis of ~30 K. In contrast to 2, thermallyinducedVTI in 2BZ and 2TL is non-cooperative despite the fact that 2, 2BZ and 2TL crystals exhibit the same monoclinic symmetryand the same intermolecular hydrogen-bonded network. In 2BZ and 2TL benzene and toluene molecules as well as thet-butyl groups of the o-dioxolene molecules convert gradually from being dynamically disordered at about 300 K to a static disorderstate below 150 K. The layer separation distance of interacting [Co(diox)2(4-X-py)2], X=CN and NO2, molecules in all solvate crystalsis ~15 Å whereas in the 2, which presents cooperative VTI, it is ~12 Å. An order-disorder component might account to the stabilizationof the metastable hs-Co2+ state in 2BZ and in 2TL but no disorder was found in the 1TL crystals. Therefore, the lack ofcooperativity in the thermally-induced VTI in these crystals seems to be due to the large distance between the layers of interactingmolecules. Cooperativity in the VTI of 1BZ crystal is likely to be related with the unique molecular bond scheme network that connectsneighboring active [Co(diox)2(4-NO2-py)2] molecules through the o-dioxolene oxygen atoms bonded directly to the Co ion

Electronic and Exchange Coupling in a Cross-Conjugated D–B–A Biradical: Mechanistic Implications for Quantum Interference Effects

A combination of variable-temperature EPR spectroscopy, electronic absorption spectroscopy, and magnetic susceptibility measurements have been performed on Tp^Cum,MeZn­(SQ-<i>m-</i>Ph-NN) (<b>1-meta</b>) a donor–bridge–acceptor (D–B–A) biradical that possesses a cross-conjugated <i>meta</i>-phenylene (<i>m-</i>Ph) bridge and a spin singlet ground state. The experimental results have been interpreted in the context of detailed bonding and excited-state computations in order to understand the excited-state electronic structure of <b>1-meta</b>. The results reveal important excited-state contributions to the ground-state singlet–triplet splitting in this cross-conjugated D–B–A biradical that contribute to our understanding of electronic coupling in cross-conjugated molecules and specifically to quantum interference effects. In contrast to the conjugated isomer, which is a D–B–A biradical possessing a <i>para</i>-phenylene bridge, admixture of a single low-lying singly excited D → A type configuration into the cross-conjugated D–B–A biradical ground state makes a negligible contribution to the ground-state magnetic exchange interaction. Instead, an excited state formed by a Ph-NN (HOMO) → Ph-NN (LUMO) one-electron promotion configurationally mixes into the ground state of the <i>m-</i>Ph bridged D–A biradical. This results in a double (dynamic) spin polarization mechanism as the dominant contributor to ground-state antiferromagnetic exchange coupling between the SQ and NN spins. Thus, the dominant exchange mechanism is one that activates the bridge moiety via the spin polarization of a doubly occupied orbital with phenylene bridge character. This mechanism is important, as it enhances the electronic and magnetic communication in cross-conjugated D–B–A molecules where, in the case of <b>1-meta</b>, the magnetic exchange in the active electron approximation is expected to be <i>J</i> ∼ 0 cm^–1. We hypothesize that similar superexchange mechanisms are common to all cross-conjugated D–B–A triads. Our results are compared to quantum interference effects on electron transfer/transport when cross-conjugated molecules are employed as the bridge or molecular wire component and suggest a mechanism by which electronic coupling (and therefore electron transfer/transport) can be modulated

Why So Slow? Mechanistic Insights from Studies of a Poor Catalyst for Polymerization of ε‑Caprolactone

Polymerization of ε-caprolactone (CL) using an aluminum alkoxide catalyst (<b>1</b>) designed to prevent unproductive trans binding was monitored at 110 °C in toluene-<i>d</i>₈ by ¹H NMR and the concentration versus time data fit to a first-order rate expression. A comparison of <i>t</i>_1/2 for <b>1</b> to values for many other aluminum alkyl and alkoxide complexes shows much lower activity of <b>1</b> 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 <b>1</b> 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 <b>1</b> 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

Determining the Conformational Landscape of σ and π Coupling Using <i>para</i>-Phenylene and “Aviram–Ratner” Bridges

The torsional dependence of donor–bridge–acceptor (D–B–A) electronic coupling matrix elements (<i><b>H</b></i>_{<i><b>DA</b></i>}, determined from the magnetic exchange coupling, <i><b>J</b></i>) involving a spin S_D = 1/2 metal semiquinone (Zn-<b>SQ</b>) donor and a spin S_A = 1/2 nitronylnitroxide (<b>NN</b>) acceptor mediated by the σ/π-systems of <i>para</i>-phenylene and methyl-substituted <i>para</i>-phenylene bridges and by the σ-system of a bicyclo[2.2.2]­octane (<b>BCO</b>) 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 (<i><b>J</b></i> ≈ 1–175 cm^–1) and electronic coupling (<i><b>H</b></i>_{<i><b>DA</b></i>} ≈ 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

Superexchange Contributions to Distance Dependence of Electron Transfer/Transport: Exchange and Electronic Coupling in Oligo(<i>para</i>-Phenylene)- and Oligo(2,5-Thiophene)-Bridged Donor–Bridge–Acceptor Biradical Complexes

The preparation and characterization of three new donor–bridge–acceptor biradical complexes are described. Using variable-temperature magnetic susceptibility, EPR hyperfine coupling constants, and the results of X-ray crystal structures, we evaluate both exchange and electronic couplings as a function of bridge length for two quintessential molecular bridges: oligo­(<i>para</i>-phenylene), β = 0.39 Å^–1 and oligo­(2,5-thiophene), β = 0.22 Å^–1. This report represents the first direct comparison of exchange/electronic couplings and distance attenuation parameters (β) for these bridges. The work provides a direct measurement of superexchange contributions to β, with no contribution from incoherent hopping. The different β values determined for oligo­(<i>para</i>-phenylene) and oligo­(2,5-thiophene) are due primarily to the D–B energy gap, <b>Δ</b>, rather than bridge–bridge electronic couplings, <b>H</b>_<b>BB</b>. This is supported by the fact that the <b>H</b>_<b>BB</b> values extracted from the experimental data for oligo­(<i>para</i>-phenylene) (<b>H</b>_<b>BB</b> = 11 400 cm^–1) and oligo­(2,5-thiophene) (12 300 cm^–1) differ by <10%. The results presented here offer unique insight into the intrinsic molecular factors that govern <b>H</b>_<b>DA</b> and β, which are important for understanding the electronic origin of electron transfer and electron transport mediated by molecular bridges

Superexchange Contributions to Distance Dependence of Electron Transfer/Transport: Exchange and Electronic Coupling in Oligo(<i>para</i>-Phenylene)- and Oligo(2,5-Thiophene)-Bridged Donor–Bridge–Acceptor Biradical Complexes

The preparation and characterization of three new donor–bridge–acceptor biradical complexes are described. Using variable-temperature magnetic susceptibility, EPR hyperfine coupling constants, and the results of X-ray crystal structures, we evaluate both exchange and electronic couplings as a function of bridge length for two quintessential molecular bridges: oligo­(<i>para</i>-phenylene), β = 0.39 Å^–1 and oligo­(2,5-thiophene), β = 0.22 Å^–1. This report represents the first direct comparison of exchange/electronic couplings and distance attenuation parameters (β) for these bridges. The work provides a direct measurement of superexchange contributions to β, with no contribution from incoherent hopping. The different β values determined for oligo­(<i>para</i>-phenylene) and oligo­(2,5-thiophene) are due primarily to the D–B energy gap, <b>Δ</b>, rather than bridge–bridge electronic couplings, <b>H</b>_<b>BB</b>. This is supported by the fact that the <b>H</b>_<b>BB</b> values extracted from the experimental data for oligo­(<i>para</i>-phenylene) (<b>H</b>_<b>BB</b> = 11 400 cm^–1) and oligo­(2,5-thiophene) (12 300 cm^–1) differ by <10%. The results presented here offer unique insight into the intrinsic molecular factors that govern <b>H</b>_<b>DA</b> and β, which are important for understanding the electronic origin of electron transfer and electron transport mediated by molecular bridges