Unusual molecular material formed through irreversible transformation and revealed by 4D electron microscopy

R.M. Van der Veen, A. Tissot, A. Hauser and A.H. Zewail

Physical Chemistry Chemical Physics, 15 (20) , 2013 , p7831

Abstract:
Four-dimensional (4D) electron microscopy (EM) uniquely combines the high spatial resolution to pinpoint individual nano-objects, with the high temporal resolution necessary to address the dynamics of their laser-induced transformation. Here, using 4D-EM, we demonstrate the in situirreversible transformation of individual nanoparticles of the molecular framework Fe(pyrazine)Pt(CN)₄. The newly formed material exhibits an unusually large negative thermal expansion (i.e. contraction), which is revealed by time-resolved imaging and diffraction. Negative thermal expansion is a unique property exhibited by only few materials. Here we show that the increased flexibility of the metal–cyanide framework after the removal of the bridging pyrazine ligands is responsible for the negative thermal expansion behavior of the new material. This in situ visualization of single nanostructures during reactions should be extendable to other classes of reactive systems.

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Single-nanoparticle phase transitions visualized by four-dimensional electron microscopy

R.M. Van der Veen, O.-H. Kwon, A. Tissot, A. Hauser and A.H. Zewail

Nature Chemistry, 5 (5) , 2013 , p395 -402

Abstract:
The advancement of techniques that can probe the behaviour of individual nanoscopic objects is of paramount importance in various disciplines, including photonics and electronics. As it provides images with a spatiotemporal resolution, four-dimensional electron microscopy, in principle, should enable the visualization of single-nanoparticle structural dynamics in real and reciprocal space. Here, we demonstrate the selectivity and sensitivity of the technique by visualizing the spin crossover dynamics of single, isolated metal–organic framework nanocrystals. By introducing a small aperture in the microscope, it was possible to follow the phase transition and the associated structural dynamics within a single particle. Its behaviour was observed to be distinct from that imaged by averaging over ensembles of heterogeneous nanoparticles. The approach reported here has potential applications in other nanosystems and those that undergo (bio)chemical transformations.

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Electron Transfer between Hydrogen-Bonded Pyridylphenols and a Photoexcited Rhenium(I) Complex

W. Herzog, C. Bronner, S. Löffler, B. He, D. Kratzert, D. Stalke, A. Hauser and O.S. Wenger

ChemPhysChem, 14 (6) , 2013 , p1168 -1176

Abstract:
Two pyridylphenols with intramolecular hydrogen bonds between the phenol and pyridine units have been synthesized, characterized crystallographically, and investigated by cyclic voltammetry and UV/Vis spectroscopy. Reductive quenching of the triplet metal-to-ligand charge-transfer excited state of the [Re(CO)₃(phen)(py)]⁺ complex (phen=1,10-phenanthroline, py=pyridine) by the two pyridylphenols and two reference phenol molecules is investigated by steady-state and time-resolved luminescence spectroscopy, as well as by transient absorption spectroscopy. Stern–Volmer analysis of the luminescence quenching data provides rate constants for the bimolecular excited-state quenching reactions. H/D kinetic isotope effects for the pyridylphenols are on the order of 2.0, and the bimolecular quenching reactions are up to 100 times faster with the pyridylphenols than with the reference phenols. This observation is attributed to the markedly less positive oxidation potentials of the pyridylphenols with respect to the reference phenols (≈0.5 V), which in turn is caused by proton coupling of the phenol oxidation process. Transient absorption spectroscopy provides unambiguous evidence for the photogeneration of phenoxyl radicals, that is, the overall photoreaction is clearly a proton-coupled electron-transfer process.

Keywords: electron transfer;luminescence;photochemistry;proton transfer;transient absorption

Links: unige:27484 Abstract Article PDF DOI (Digital Object Identifier)

Tetrathiafulvalene-s-tetrazine: versatile platform for donor-acceptor systems and multifunctional ligands

F. Pop, J. Ding, L.M. Lawson Daku, A. Hauser and N. Avarvari

RSC Advances, 3 , 2013 , p3218 -3221

Abstract:
The structurally characterized tetrathiafulvalene-1,2,4,5-tetrazine donor–acceptor system shows redox tuneable intramolecular charge transfer, solvatochromic and electrochromic behaviour. Attachment of a dipicolyl-amine chelating unit affords a multifunctional ligand, which allows the preparation of the ZnCl₂ complex in which an anion-π interaction is seen.

Links: unige:27369 Abstract Article HTML Article PDF DOI (Digital Object Identifier)

Vibrational spectra and structure of borohydrides

V. D'Anna, L.M. Lawson Daku and H. Hagemann

Journal of Alloys and Compounds, in press , 2013

Abstract:
A comparison of the vibrational spectra of many inorganic borohydrides allows us to distinguish compounds with isolated BH₄^- ions and compounds containing complex ions such as Sc(BH₄)₄^-. The characteristic spectral features of both types of compounds are identified, showing that the B–H bonding is quite different in both cases. A detailed analysis of the vibrations of the isolated BH₄^- ions provides new information about their local structure. Angular deformations of individual borohydride ion are analyzed quantitatively. It appears that the compounds containing isolated BH₄^- ions belong to those with the most electropositive cations and the highest decomposition temperature, while the complex borohydrides show significantly lower decomposition temperatures and possible diborane formation.

Keywords: hydrogen storage; vibrational spectra; borohydrides; FG method

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Synthesis and Redox and Photophysical Properties of Benzodifuran–Spiropyran Ensembles

H. Li, J. Ding, S. Chen, C. Beyer, S.-X. Liu, H.-A. Wagenknecht, A. Hauser and S. Decurtins

Chemistry - A European Journal, in press , 2013

Abstract:
Two benzodifuran (BDF)-coupled spiropyran (SP) systems and their BDF reference compounds were obtained in good yields through Huisgen–Meldal–Sharpless “click” chemistry and then subjected to investigation of their electrochemical and photophysical properties. In both SP and merocyanine (MC) forms of the coupled molecules, the BDF-based emission is quenched to around 1 % of the quantum yield of emission from the BDF reference compounds. Based on electrochemical data, this quenching is attributed to oxidative electron-transfer quenching. Irradiation at 366 nm results in ring opening to the MC forms of the BDF-coupled SP compounds and the SP reference compound with a quantum efficiency of about 50 %. The rate constants for the thermal ring closing are approximately 3.4×10⁻³ s⁻¹. However, in the photostationary states the MC fractions of the coupled molecules are substantially lower than that of the reference SP compound, attributed to the observed acceleration of the ring-closing reaction upon irradiation. As irradiation at 366 nm invariably also excites higher-energy transitions of the BDF units in the coupled compounds, the ring-opening reaction is accelerated relative to the SP reference, which results in lower MC fractions in the photostationary state. Reversible photochromism of these BDF-coupled SP compounds renders them promising in the field of molecular switches.

Keywords: asymmetric synthesis;click chemistry;electron transfer;photochromism;redox chemistry;spiro compounds

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Analysis of the Experimental Data for Pure and Diluted [Fe_xZn_1–x(bbtr)₃](ClO₄)₂ Spin-Crossover Solids in the Framework of a Mechanoelastic Model

P. Chakraborty, C. Enachescu and A. Hauser

European Journal of Inorganic Chemistry, 2013 (5-6) , 2013 , p770 -780

Abstract:
The mechanoelastic model is applied to reproduce the experimental relaxation and thermal transition curves as determined for crystals of pure and diluted {[Fe_xZn_1–x(bbtr)₃](ClO₄)₂}_∞ [bbtr = 1,4-di(1,2,3-triazol-1-yl)butane] spin-crossover systems. In the mechanoelastic model, the spin-crossover complexes are situated in a hexagonal planar lattice, which is similar to the 2D coordination polymer with (3,6) network topology of [Fe(bbtr)₃](ClO₄)₂. These complexes are linked by springs, which simulate the elastic interactions between them. Owing to the change in volume of the complexes during the spin transition, an elastic force accompanies the switch of every complex. This force propagates through the entire lattice and causes a shift of all molecules in the system and thus results in a new nuclear configuration. First, the ability of the model to reproduce various shapes of thermal transition and relaxation curves in pure compounds is analyzed; these range from gradual to very steep and include hysteresis behavior for the former and from single exponential to sigmoidal or with several steps for the latter. A structural phase transition can also be accounted for by changing the shape of the sample at a fixed temperature from a regular to an elongated hexagon. Furthermore, the effect of adding Zn as a dopant in a mixed crystal series is discussed. The role of dopants on the cluster evolution is also analyzed directly and by using the correlation factor.

Keywords: spin crossover;mechanoelastic model;doping;iron;zinc

Links: unige:26499 Abstract Article PDF DOI (Digital Object Identifier)

Towards accurate estimates of the spin-state energetics of spin-crossover complexes within density functional theory: a comparative case study of cobalt(ii) complexes

A. Vargas, I. Krivokapic, A. Hauser and L.M. Lawson Daku

Physical Chemistry Chemical Physics, 15 (11) , 2013 , p3752 -3763

Abstract:
We report a detailed DFT study of the energetic and structural properties of the spin-crossover Co(II) complex [Co(tpy)₂]²⁺ (tpy = 2,2′:6′,2′′-terpyridine) in the low-spin (LS) and the high-spin (HS) states, using several generalized gradient approximation and hybrid functionals. In either spin-state, the results obtained with the functionals are consistent with one another and in good agreement with available experimental data. Although the different functionals correctly predict the LS state as the electronic ground state of [Co(tpy)₂]²⁺, they give estimates of the HS–LS zero-point energy difference ΔE⁰_HL (tpy)  which strongly depend on the functional used. This dependency on the functional was also reported for the DFT estimates of the zero-point energy difference ΔE⁰_HL (bpy)  in the HS complex [Co(bpy)₃]²⁺ (bpy = 2,2′-bipyridine) [A. Vargas, A. Hauser and L. M. Lawson Daku, J. Chem. Theory Comput., 2009, 5, 97]. The comparison of the ΔE⁰_HL (tpy)  and ΔE⁰_HL (bpy)  estimates showed that all functionals correctly predict an increase of the zero-point energy difference upon the bpy → tpy ligand substitution, which furthermore weakly depends on the functionals, amounting to (ΔE⁰_HL)_bpy->tpy  ≈ +2670 cm^-1 . From these results and basic thermodynamic considerations, we establish that, despite their limitations, current DFT methods can be applied to the accurate determination of the spin-state energetics of complexes of a transition metal ion, or of these complexes in different environments, provided that the spin-state energetics is accurately known in one case. Thus, making use of the availability of a highly accurate ab initio estimate of the HS–LS energy difference in the complex [Co(NCH)₆]²⁺ [L. M. Lawson Daku, F. Aquilante, T. W. Robinson and A. Hauser, J. Chem. Theory Comput., 2012, 8, 4216], we obtain for [Co(tpy)₂]²⁺ and [Co(bpy)₃]²⁺best estimates of ΔE⁰_HL (bpy) ≈ -2800 cm^-1  and ΔE⁰_HL (tpy) ≈ 0 cm^-1 , in good agreement with the known magnetic behaviour of the two complexes.

Links: unige:26498 Abstract Article HTML Article PDF DOI (Digital Object Identifier)

Tetrathiafulvalene-Benzothiadiazoles as Redox-Tunable Donor-Acceptor Systems: Synthesis and Photophysical Study

F. Pop, A. Amacher, N. Avarvari, J. Ding, L.M. Lawson Daku, A. Hauser, M. Koch, J. Hauser, S.-X. Liu and S. Decurtins

Chemistry - A European Journal, 19 (7) , 2013 , p2504 -2514

Abstract:
Electrochemical and photophysical analysis of new donor–acceptor systems 2 and 3, in which a benzothiadiazole (BTD) unit is covalently linked to a tetrathiafulvalene (TTF) core, have verified that the lowest excited state can be ascribed to an intramolecular-charge-transfer (ICT) π(TTF)→π*(benzothiadiazole) transition. Owing to better overlap of the HOMO and LUMO in the fused scaffold of compound 3, the intensity of the ¹ICT band is substantially higher compared to that in compound 2. The corresponding CT fluorescence is also observed in both cases. The radical cation TTF^+. is easily observed through chemical and electrochemical oxidation by performing steady-state absorption experiments. Interestingly, compound 2 is photo-oxidized under aerobic conditions.

Keywords: charge transfer;donor-acceptor systems;fluorescence;photophysics;redox chemistry

Links: unige:26401 Abstract Article PDF DOI (Digital Object Identifier)

A Donor–Acceptor Tetrathiafulvalene Ligand Complexed to Iron(II): Synthesis, Electrochemistry, and Spectroscopy of [Fe(phen)₂(TTF-dppz)](PF₆)₂

N. Dupont, Y.-F. Ran, S.-X. Liu, J. Grilj, E. Vauthey, S. Decurtins and A. Hauser

Inorganic Chemistry, 52 (1) , 2013 , p306 -312

Abstract:
The synthesis and photophysical properties of the complex [Fe(phen)₂(TTF-dppz)]²⁺ (TTF-dppz = 4′,5′-bis-(propylthio)tetrathiafulvenyl[i]dipyrido[3,2-a:2′,3′-c]phenazine, phen = 1,10-phenanthroline) are described. In this complex, excitation into the metal–ligand charge transfer bands results in the population of a high-spin state of iron(II), with a decay lifetime of approximately 1.5 ns, in dichloromethane, at room temperature. An intraligand charge transfer state can also be obtained and has a lifetime of 38 ps. A mechanism for the different states reached is proposed based on transient absorption spectroscopy.

Links: unige:25113 Abstract Article HTML Article PDF DOI (Digital Object Identifier)

Chromium(III)-trisoxalate, a versatile building block for luminescent materials

M. Milos and A. Hauser

Journal of Luminescence, 133 , 2013 , p15 -20

Abstract:
Chromium(III)-trisoxalate,[Cr(ox)₃]^3- (ox = C₂O₄^2-), incorporated into polymeric networks of composition [NaCr(ox)₃][M^II(bpy)₃] and [NaCr(ox)₃][M^III(bpy)₃]ClO₄ (bpy= 2,2'-bipyridine, M^{II =} Zn, Fe, Ru; M^{III =} Rh, Cr), results in interesting features ranging from phonon-assisted and resonant energy migration within the R1 line the ²E state to persistent spectral side-hole burning via the latter, and manifestations of specific nearest-neighbour π–π interactions between bipyridine and oxalate.

Keywords: chromium(III)-tris-oxalate; energy migration; fluorescence line narrowing; spectral diffusion; persistent spectral hole burning

Links: unige:24060 Article HTML Article PDF DOI (Digital Object Identifier)

Effect of Pressure on the Free Ion and Crystal Field Parameters of Sm²⁺ in BaFBr and SrFBr Hosts

P. Pal, T. Penhouët, V. D'Anna and H. Hagemann

Journal of Luminescence, 134 , 2013 , p678 -685

Abstract:
The emission spectra of Sm²⁺ doped in BaFBr and SrFBr hosts were measured at 10 K from ambient pressure to 8 GPa. The crystal field energy levels determined from the emission spectra were used to extract the free ion parameters (F_k and ζ ) and crystal field parameters (B_q^k). The variation of F_k and ζ as a function of pressure was studied systematically and was discussed in relation to the central field and symmetry restricted covalency models. The change of the spin orbit coupling parameter (ζ) with pressure for SrFBr:Sm²⁺ showed very different behavior than in other matlockite hosts. Moreover the variation of B_q^k under pressure was studied. The pressure dependence of the B_q^k was described quantitatively using the Superposition Model (SM) with the help of structural parameters as a function of pressure, obtained from periodic DFT calculations. The validity of the SM was tested for Sm²⁺ in BaFBr and SrFBr. It is shown that this model does not apply to SrFBr, in contrast to other matlockite host materials.

Keywords: Sm2+; high pressure luminescence; crystal field; BaFBr; SrFBr

Links: unige:24061 Abstract Article HTML Article PDF DOI (Digital Object Identifier)

Heteroleptic FeII Complexes of 2,2′-Biimidazole and Its Alkylated Derivatives: Spin-Crossover and Photomagnetic Behavior

H.V. Phan, P. Chakraborty, M. Chen, Y.M. Calm, K. Kovnir, L.K. Keniley, J.M. Hoyt, E.S. Knowles, C. Besnard, M.W. Meisel, A. Hauser, C. Achim and M. Shatruk

Chemistry - A European Journal, 18 (49) , 2012 , p15805 -15815

Abstract:
Three iron(II) complexes, [Fe(TPMA)(BIM)](ClO₄)₂⋅0.5H₂O (1), [Fe(TPMA)(XBIM)](ClO₄)₂ (2), and [Fe(TPMA)(XBBIM)](ClO₄)₂ ⋅0.75CH₃OH (3), were prepared by reactions of Fe^II perchlorate and the corresponding ligands (TPMA=tris(2-pyridylmethyl)amine, BIM=2,2′-biimidazole, XBIM=1,1′-(α,α′-o-xylyl)-2,2′-biimidazole, XBBIM=1,1′-(α,α′-o-xylyl)-2,2′-bibenzimidazole). The compounds were investigated by a combination of X-ray crystallography, magnetic and photomagnetic measurements, and Mössbauer and optical absorption spectroscopy. Complex 1 exhibits a gradual spin crossover (SCO) with T_1/2=190 K, whereas 2 exhibits an abrupt SCO with approximately 7 K thermal hysteresis (T_1/2=196 K on cooling and 203 K on heating). Complex 3 is in the high-spin state in the 2–300 K range. The difference in the magnetic behavior was traced to differences between the inter- and intramolecular interactions in 1 and 2. The crystal packing of 2features a hierarchy of intermolecular interactions that result in increased cooperativity and abruptness of the spin transition. In 3, steric repulsion between H atoms of one of the pyridyl substituents of TPMA and one of the benzene rings of XBBIM results in a strong distortion of the Fe^II coordination environment, which stabilizes the high-spin state of the complex. Both 1 and 2 exhibit a photoinduced low-spin to high-spin transition (LIESST effect) at 5 K. The difference in the character of intermolecular interactions of 1 and 2 also manifests in the kinetics of the decay of the photoinduced high-spin state. For 1, the decay rate constant follows the single-exponential law, whereas for 2 it is a stretched exponential, reflecting the hierarchical nature of intermolecular contacts. The structural parameters of the photoinduced high-spin state at 50 K are similar to those determined for the high-spin state at 295 K. This study shows that N-alkylation of BIM has a negligible effect on the ligand field strength. Therefore, the combination of TPMA and BIM offers a promising ligand platform for the design of functionalized SCO complexes.

Keywords: iron complexes;LIESST effect;N ligands;photomagnetism;spin crossover

Links: unige:24449 Abstract Article PDF DOI (Digital Object Identifier)

Accurate Spin-State Energetics of Transition Metal Complexes: I. CCSD(T), CASPT2 and DFT Study of [M(NCH)₆]²⁺ (M = Fe, Co)

L.M. Lawson Daku, F. Aquilante, T.W. Robinson and A. Hauser

Journal of Chemical Theory and Computation, 8 (11) , 2012 , p4216 -4231

Abstract:
Highly accurate estimates of the high-spin/low-spin energy difference ΔE_HL^el in the high-spin complexes [Fe(NCH)₆]²⁺ and [Co(NCH)₆]²⁺ have been obtained from the results of CCSD(T) calculations extrapolated to the complete basis set limit. These estimates are shown to be strongly influenced by scalar relativistic effects. They have been used to assess the performances of the CASPT2 method and of 30 density functionals of the GGA, meta-GGA, global hybrid, RSH and double-hybrid types. For the CASPT2 method, the results of the assessment support the proposal [Kepenekian, M.; Robert, V.; Le Guennic, B. J. Chem. Phys.2009, 131, 114702] that the ionization potential–electron affinity (IPEA) shift defining the zeroth-order Hamiltonian be raised from its standard value of 0.25 au to 0.50–0.70 au for the determination of ΔE_HL^el in Fe(II) complexes with a [FeN₆] core. At the DFT level, some of the assessed functionals proved to perform within chemical accuracy (±350 cm^-1) for the spin-state energetics of [Fe(NCH)₆]²⁺, others for that of [Co(NCH)₆]²⁺, but none of them simultaneously for both complexes. As demonstrated through a reparametrization of the CAM-PBE0 range-separated hybrid, which led to a functional that performs within chemical accuracy for the spin-state energetics of both complexes, performing density functionals of broad applicability may be devised by including in their training sets highly accurate data like those reported here for [Fe(NCH)₆]²⁺ and [Co(NCH)₆]²⁺.

Links: unige:24068 Abstract Article PDF DOI (Digital Object Identifier)

Thermal and Light-Induced Spin Switching Dynamics in the 2D Coordination Network of {[Zn_1-xFe_x(bbtr)₃](ClO₄)₂}_∞: The Role of Cooperative Effects

P. Chakraborty, C. Enachescu, C. Walder, R. Bronisz and A. Hauser

Inorganic Chemistry, 51 (18) , 2012 , p9714 -9722

Abstract:
The thermal spin transition, the photoexcitation, and the subsequent spin relaxation in the mixed crystal series of the covalently linked two-dimensional network {[Zn_1-xFe_x(bbtr)₃](ClO₄)₂}_∞ (x = 0.02–1, bbtr =1,4-di(1,2,3-triazol-1-yl)-butane) are discussed. In the neat compound, the thermal spin transition with a hysteresis of 13 K is accompanied by a crystallographic phase transition (Kusz, J.; Bronisz, R.; Zubko, M.; Bednarek, H. Chem. Eur. J.2011, 17, 6807). In contrast, the diluted crystals with x ≤ 0.1 stay essentially in the high-spin state down to low temperatures and show typical first order relaxation kinetics upon photoexcitation, and the structural phase transition is well separated from the spin transition. With increasing Fe(II) concentration, steeper thermal transitions and sigmoidal relaxation curves indicate increasingly important cooperative effects. Already at x = 0.38, the spin relaxation is governed by cooperative interactions between Fe(II) centers, and the crystallographic phase transition begins to influence the spin transition. The kinetic behavior of the thermal spin transition is reproduced within the framework of a dynamic mean-field model.

Links: unige:22920 Abstract Article HTML Article PDF DOI (Digital Object Identifier)

Optimizing Millisecond Time Scale Near-Infrared Emission in Polynuclear Chrome(III)-Lanthanide(III) Complexes

L. Aboshyan-Sorgho, H. Nozary, A. Aebischer, J.-C.G. Bünzli, P.-Y. Morgantini, K.R. Kittilstved, A. Hauser, S.V. Eliseeva, S. Petoud and C. Piguet

Journal of the American Chemical Society, 134 (30) , 2012 , p12675 -12684

Abstract:
This work illustrates a simple approach for optimizing long-lived near-infrared lanthanide-centered luminescence using trivalent chromium chromophores as sensitizers. Reactions of the segmental ligand L2 with stoichiometric amounts of M(CF₃SO₃)₂ (M = Cr, Zn) and Ln(CF₃SO₃)₃ (Ln = Nd, Er, Yb) under aerobic conditions quantitatively yield the D₃-symmetrical trinuclear [MLnM(L2)₃](CF₃SO₃)_n complexes (M = Zn, n = 7; M = Cr, n = 9), in which the central lanthanide activator is sandwiched between the two transition metal cations. Visible or NIR irradiation of the peripheral Cr(III) chromophores in [CrLnCr(L2)₃]⁹⁺ induces rate-limiting intramolecular intermetallic Cr→Ln energy transfer processes (Ln = Nd, Er, Yb), which eventually produces lanthanide-centered near-infrared (NIR) or IR emission with apparent lifetimes within the millisecond range. As compared to the parent dinuclear complexes [CrLn(L1)₃]⁶⁺, the connection of a second strong-field [CrN₆] sensitizer in [CrLnCr(L2)₃]⁹⁺ significantly enhances the emission intensity without perturbing the kinetic regime. This work opens novel exciting photophysical perspectives via the buildup of non-negligible population densities for the long-lived doubly excited state [Cr*LnCr*(L2)₃]⁹⁺ under reasonable pumping powers.

Links: unige:22645 Abstract Article HTML Article PDF DOI (Digital Object Identifier)

A Synthetic and Mechanistic Investigation of the Chromium Tricarbonyl-Mediated Masamune–Bergman Cyclization. Direct Observation of a Ground-State Triplet p-Benzyne Biradical

K.E.O. Ylijoki, S. Lavy, A. Fretzen, E.P. Kündig, T. Berclaz, G. Bernardinelli and C. Besnard

Organometallics, 31 (15) , 2012 , p5396 -5404

Abstract:
A new room-temperature chromium tricarbonyl-mediated cycloaromatization of enediynes is reported. The reaction occurs with both cyclic and acyclic enediynes in the presence of [Cr(CO)₃(η⁶-naphthalene)] and both a coordinating solvent and a hydrogen atom source, providing chromium–arene complexes in reasonable yield and good diastereocontrol. The mechanism of the reaction has been probed through DFT computational and spectroscopic methods. These studies suggest that direct C1–C6 bond formation from an η⁶-enediyne complex is the lowest-energy path, forming a metal-bound p-benzyne biradical. NMR spectroscopy suggests that enediyne alkene coordination occurs in preference to alkyne coordination, forming a THF-stabilized olefin intermediate; subsequent alkyne coordination leads to cyclization. While biradical quenching occurs rapidly and primarily via the singlet biradical, the triplet state biradical is detectable by EPR spectroscopy, suggesting intersystem crossing to a triplet ground state.

Links: unige:22151 Abstract Article HTML Article PDF DOI (Digital Object Identifier)

Optical sensitization and upconversion in discrete polynuclear chromium–lanthanide complexes

L. Aboshyan-Sorgho, M. Cantuel, S. Petoud, A. Hauser and C. Piguet

Coordination Chemistry Reviews, 256 (15-16) , 2012 , p1644 -1663

Abstract:
Due to its extreme kinetic inertness, trivalent chromium, Cr(III), has been rarely combined with labile trivalent lanthanides, Ln(III), to give discrete self-assembled (supra)molecular polynuclear complexes. However, the plethora of accessible metal-centered excited states possessing variable lifetimes and emissive properties, combined with the design of efficient intramolecular Cr(III) ^↔ Ln(III) energy transfer processes open attractive perspectives for programming directional light-conversion within these heterometallic molecules. Efforts made to address this exciting challenge for both light-sensitization and light-upconversion are discussed in this article.

Links: unige:21642 Abstract Article PDF DOI (Digital Object Identifier)

Bimetallic Borohydrides in the System M(BH₄)₂–KBH₄ (M = Mg, Mn): On the Structural Diversity

P. Schouwink, V. D'Anna, M.B. Ley, L.M. Lawson Daku, B. Richter, T.R. Jensen, H. Hagemann and R. Cerný

The Journal of Physical Chemistry C, 116 (20) , 2012 , p10829 -10840

Abstract:
Four novel bimetallic borohydrides have been discovered, K₂M(BH₄)₄ (M = Mg or Mn), K₃Mg(BH₄)₅, and KMn(BH₄)₃, and are carefully investigated structurally as well as regarding their decomposition reaction mechanism by means of in situ synchrotron radiation powder X-ray diffraction (SR-PXD), vibrational spectroscopies (Raman and IR), thermal analysis (TGA and DTA), and ab initio density functional theory (DFT) calculations. Mechano-chemical synthesis (ball-milling) using the reactants KBH₄, α-Mg(BH₄)₂, and α-Mn(BH₄)₂ ensures chlorine-free reaction products. A detailed structural analysis reveals significant similarities as well as surprising differences among the two isomorphs K₂M(BH₄)₄, most importantly concerning the extent to which the complex anion [M(BH₄)₄]^2– is isolated in the structure. Anisotropic thermal expansion and an increase in symmetry at high temperatures in K₃Mg(BH₄)₅ is ascribed to the motion of BH₄ groups inducing hydrogen repulsive effects, and the dynamics of K₃Mg(BH₄)₅ are investigated. Decomposition in the manganese system proceeds via the formation of KMn(BH₄)₃, the first perovkite type borohydride reported to date.

Links: unige:21580 Abstract Article HTML Article PDF DOI (Digital Object Identifier)

Lanthanide-mediated triangular cationic assemblies: structural and physico-chemical properties

S. Zebret, N. Dupont, C. Besnard, G. Bernardinelli and J. Hamacek

Dalton Transactions, 41 (16) , 2012 , p4817 -4824

Abstract:
This contribution investigates Ln^III complexes formed with a small ditopic ligand, L1, and their structural, thermodynamic and photophysical properties. The spectrophotometric and NMR titrations evidence the triangular assemblies [Ln₃(L1-H)₃]⁶⁺ at stoichiometric conditions and their properties are discussed in relation to L2-containing analogues. In addition, the dinuclear species, [Ln₂(L1-H)]⁵⁺, is observed with an excess of metal.

Links: unige:20257 Abstract Article HTML Article PDF DOI (Digital Object Identifier)