The chemical behavior of cyclopropyl-substituted alkynes has been probed using the reaction conditions of ruthenium-catalyzed oxidative C-H/O-H and C-H/N-H bond functionalizations. The oxidative ...annulations proceeded with complete conservation of all cyclopropane fragments and allowed for the one-step preparation of synthetically useful cyclopropyl-substituted isocoumarins and isoquinolones with high regioselectivities and chemical yields. The connectivities of the key heterocyclic products were unambiguously established by X-ray diffraction analysis.
The balance between strain relief and aromatic stabilization dictates the form and function of non-planar π-aromatics. Overcrowded systems are known to undergo geometric deformations, but the ...energetically favourable π-electron delocalization of their aromatic ring(s) is typically preserved. In this study we incremented the strain energy of an aromatic system beyond its aromatic stabilization energy, causing it to rearrange and its aromaticity to be ruptured. We noted that increasing the steric bulk around the periphery of π-extended tropylium rings leads them to deviate from planarity to form contorted conformations in which aromatic stabilization and strain are close in energy. Under increasing strain, the aromatic π-electron delocalization of the system is broken, leading to the formation of a non-aromatic, bicyclic analogue referred to as 'Dewar tropylium'. The aromatic and non-aromatic isomers have been found to exist in rapid equilibrium with one another. This investigation demarcates the extent of steric deformation tolerated by an aromatic carbocycle and thus provides direct experimental insights into the fundamental nature of aromaticity.
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A new design of a setup for in situ laser irradiation of single crystals during an X‐ray diffraction experiment is presented. The system is designed for use with a Bruker three‐circle goniometer in ...combination with a Helix ultra‐low‐temperature cryostat and consists of a laser mount and a set of three adjustable mirrors. The main advantages of the presented system include a stationary laser mount, the ability to irradiate a sample inside the Be nozzle and no impediments to the goniometer movements.
A new design of a setup for in situ laser irradiation of single crystals during an X‐ray diffraction experiment is reported.
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Platinum(II) complexes of NNC-cyclometalating ligands based on 6-phenyl-2,2′-bipyridine (HL1) have been widely investigated for their luminescence properties. We describe how PtL1Cl and five ...analogues with differently substituted aryl rings, PtL2–6Cl, can be oxidized with chlorine and/or iodobenzene dichloride to generate Pt(IV) compounds of the form Pt(NNC-L n )Cl3 (n = 1–6). The molecular structures of several of them have been determined by X-ray diffraction. These PtL n Cl3 compounds react with 2-arylpyridines to give a new class of Pt(IV) complex of the form Pt(NNC)(NC)Cl+. Elevated temperatures are required, and the reaction is accompanied by competitive reduction processes and generation of side-products; however, four examples of such complexes have been isolated and their molecular structures determined. Reaction of PtL1Cl3 with HL1 similarly generates Pt(NNC-L1)22+, which we believe to be the first example of a bis-tridentate Pt(IV) complex. The lowest-energy bands in the UV–vis absorption spectra of all the PtL n Cl3 compounds are displaced to higher energy relative to the Pt(II) precursors, but they red-shift with the electron richness of the aryl ring, consistent with predominantly 1πAr → π*NN character to the pertinent excited state. A similar trend is observed for the Pt(NNC)(NC)Cl+ complexes. They display phosphorescence in solution at room temperature, centered around 500 nm for PtL1(ppy)Cl+ and Pt(L1)22+, and 550 nm for methoxy-substituted derivatives. The lifetimes are in the microsecond range, rising to hundreds of microseconds at 77 K, consistent with triplet excited states of primarily 3πAr → π*NN character with relatively little participation of the metal.
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The hydrothermal synthesis of the novel NaLnSiO4 (Ln = Gd, Eu, Tb) disordered orthorhombic system is reported. At 100 K, and above, these materials are best described in the centrosymmetric ...orthorhombic Pnma space group. At lower temperatures (structure solved at 30 K) the unit cell changes to body-centered with Imma symmetry. The materials exhibit unique photophysical properties, arising from both, this phase transformation, and the disorder of the Ln3+ ions, located at a site with D 2d point symmetry. Na(Gd0.8Eu0.1Tb0.1)SiO4 is an unprecedented case of a luminescent ratiometric thermometer based on a very stable silicate matrix. Moreover, it is the first example of an optical thermometer whose performance (viz., excellent sensitivity at cryogenic temperatures <100 K) is determined mainly by a structural transition, opening up new opportunities for designing such devices.
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Intermolecular hydroarylation reactions of highly strained methylenecyclopropanes 2‐phenylmethylenecyclopropane (1), 2,2‐diphenylmethylenecyclopropane (2), methylenespiropentane (3), ...bicyclopropylidene (4), (dicyclopropylmethylene)cyclopropane (5), and benzhydrylidenecyclopropane (6) through CH bond functionalization of 2‐phenylpyridine (7 a) and other arenes with directing groups were studied. The reaction was very sensitive to the substitution on the methylenecyclopropanes. Although these transformations involved (cyclopropylcarbinyl)–metal intermediates, substrates 1 and 4 furnished anti‐Markovnikov hydroarylation products with complete conservation of all cyclopropane rings in 11–93 % yield, whereas starting materials 3 and 5 were inert toward hydroarylation. Methylenecyclopropane 6 formed the products of formal hydroarylation reactions of the longest distal CC bond in the methylenecyclopropane moiety in high yield, and hydrocarbon 2 afforded mixtures of hydroarylated products in low yields with a predominance of compounds that retained the cyclopropane unit. As byproducts, Diels–Alder cycloadducts and self‐reorganization products were obtained in several cases from substrates 1–3 and 5. The structures of the most important new products have been unambiguously determined by X‐ray diffraction analyses. On the basis of the results of hydroarylation experiments with isotopically labeled 7 a‐D5, a plausible mechanistic rationale and a catalytic cycle for these unusual ruthenium‐catalyzed hydroarylation reactions have been proposed. Arene‐tethered ruthenium–phosphane complex 53, either isolated from the reaction mixture or independently prepared, did not show any catalytic activity.
A highly chemo‐ and site‐selective ruthenium catalyst has achieved intermolecular hydroarylation reactions of methylenecyclopropanes (see scheme) through chelation assistance. The CH bond functionalization of 2‐phenylpyridine and other arenes with directing groups has been studied. Furthermore, a plausible mechanistic rationale and a catalytic cycle for these unusual ruthenium‐catalyzed hydroarylation reactions are proposed.
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The iron(
ii
) salt Fe(bpp)
2
(isonicNO)
2
·HisonicNO·5H
2
O (
1
) (bpp = 2,6-bis(pyrazol-3-yl)pyridine; isonicNO = isonicotinate N-oxide anion) undergoes a partial spin crossover (SCO) with symmetry ...breaking at
T
1
= 167 K to a mixed-spin phase (50% high-spin (HS), 50% low-spin (LS)) that is metastable below
T
2
= 116 K. Annealing the compound at lower temperatures results in a 100% LS phase that differs from the initial HS phase in the formation of a hydrogen bond (HB) between two water molecules (O4W and O5W) of crystallisation. Neutron crystallography experiments have also evidenced a proton displacement inside a short strong hydrogen bond (SSHB) between two isonicNO anions. Both phenomena can also be detected in the mixed-spin phase.
1
undergoes a light-induced excited-state spin trapping (LIESST) of the 100% HS phase, with breaking of the O4W O5W HB and the onset of proton static disorder in the SSHB, indicating the presence of a light-induced activation energy barrier for proton motion. This excited state shows a stepped relaxation at
T
1
(LIESST) = 68 K and
T
2
(LIESST) = 76 K. Photocrystallography measurements after the first relaxation step reveal a single Fe site with an intermediate geometry, resulting from the random distribution of the HS and LS sites throughout the lattice.
A proton migration across a short strong hydrogen bond can be triggered by spin crossover of a remote Fe
2+
cation, with the onset of a photoinduced activation energy barrier for proton motion at low temperatures.
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The spin crossover salt Fe(bpp)2(isonicNO)2⋅ 2.4 H2O (1⋅2.4 H2O) (bpp=2,6‐bis(pyrazol‐3‐yl)pyridine; isonicNO=isonicotinate N‐oxide anion) exhibits a very abrupt spin crossover at T1/2=274.4 K. This ...triggers a supramolecular linkage (H‐bond) isomerization that responds reversibly towards light irradiation or temperature change. Isotopic effects in the thermomagnetic behavior reveal the importance of hydrogen bonds in defining the magnetic state. Further, the title compound can be reversibly dehydrated to afford 1, a material that also exhibits spin crossover coupled to H‐bond isomerization, leading to strong kinetic effects in the thermomagnetic properties.
Flip the light on: The iron(II) salt Fe(bpp)2(isonicNO)2⋅2.4 H2O (bpp=2,6‐bis(pyrazol‐3‐yl)pyridine; isonicNO=isonicotinate N‐oxide anion) undergoes spin crossover near room temperature (T1/2↑=274.9 K; T1/2↓=273.8 K) in an exceptionally abrupt manner (the fraction of HS Fe2+ cations γHS changes from 1 to 0 in less than 3 K). This triggers a hydrogen‐bond isomerization in the solid state that can be reversed by temperature change or under light irradiation.
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Transfer hydrogenation (TH) is a powerful synthetic tool in the production of secondary alcohols from ketones by using a non-H
2
hydrogen source along with metal catalysts. Among homogeneous ...catalysts, Ru(
ii
) complexes are the most efficient catalysts. In our research, six novel ruthenium(
ii
) complexes bearing bipyridine-based ligands Ru(
L1
)Cl
2
(
1
), Ru(
L1
)(PPh
3
)ClCl (
2
) and Ru(
L2
)Cl
2
(
3
) and N-heterocyclic carbene-supported pyridine (NCN) ligands RuCp(
L3
)PF
6
(
4
), RuCp*(
L3
)PF
6
(
5
), and Ru(
p
-cymene)(
L3
)ClPF
6
(
6
) (where
L1
= 6,6′-bis(aminomethyl)-2,2′-bipyridine,
L2
= 6,6′-bis(dimethylaminomethyl)-2,2′-bipyridine and
L3
= 1,3-bis(2-methylpyridyl)imidazolium bromide) were synthesised and characterised by NMR spectroscopy, HRMS, and X-ray crystallography. The catalytic transfer hydrogenation of 28 ketones in 2-propanol at 80 °C in the presence of KO
t
Bu (5 mol%) was demonstrated and the effect of ligands is highlighted. The results show that catalyst
1
exhibits improved TH efficiency compared to the commercially available Milstein catalyst and displays higher catalytic activity than
2
due to the steric effect from PPh
3
. From a combination of kinetic data and Eyring analysis, a zero-order dependence on the acetophenone substrate is observed, implying a rate-limiting hydride transfer step, leading to the proposed inner-sphere hydride transfer mechanism.
A series of ruthenium complexes is presented. The complexes are designed as catalysts for transfer hydrogenation of ketones. The most active complexes show improvements upon commercially available catalysts. A mechanistic study is included.
The reaction of trans-RuCl(CCC6H4R1-4)(dppe)2 (2: R1 = Me (a), C5H11 (b), OMe (c), CO2Me (d), NO2 (e), CCSiMe3 (f), CCBut (g), NH2 (h)), prepared in situ from reactions of RuCl(dppe)2OTf ...(1OTf) with terminal alkynes in CH2Cl2 solutions containing 1,8-diazabicycloundec-7-ene (DBU) and TlBF4, provides a convenient and rapid route to bis(acetylide) complexes trans-Ru(CCC6H4R1-4)2(dppe)2 (3a–h) and trans-Ru(CCC6H4R1-4)(CCC6H4R2-4)(dppe)2 (4, R1 = CCSiMe3, R2 = NH2; 5, R1 = CO2Me, R2 = NH2; 6, R1 = CO2Me, R2 = OMe). However, even in the absence of the chloride abstracting reagent, more strongly electron donating substituents (e.g., R1 = OMe (2c), NH2 (2h)) promote sufficient ionization of the Ru–Cl bond in trans-RuCl(CCC6H4R1-4)(dppe)2 to lead to slow conversion to bis(alkynyl) complexes 3c,h in the presence of excess alkyne and DBU. Desilylation of 2f and 3f affords 2i and 3i (R1 = CCH), respectively. The molecular structures of 3a–d,f–i have been determined and are reported together with the structures of the monoalkynyl complexes 2f,g,i and compared with related compounds from the literature. Complexes 3a–i and 4–6 undergo one reversible electrochemical oxidation process, which can be attributed to depopulation of an orbital with significant alkynyl ligand character. The one-electron-oxidation products 3f•+, 3h•+, 4•+, and 5•+, chosen to serve as representative examples of this family of complexes, each exhibit a series of NIR absorptions between 15000 and 5000 cm–1 which on the basis of TDDFT calculations cannot be attributed to a single, static lowest energy molecular structure. Rather, the transitions that are responsible for the absorption band envelope have varying degrees of LMCT and inter-alkynyl ligand IVCT or MLCT character that depend not only on the nature of the Rn groups but also on the ensemble of thermally populated molecular conformers in solution with various relative orientations of the metal fragment and arylethynyl moieties.
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