Pierluigi Mercandelli

Highly Emitting Concomitant Polymorphic Crystals of a Dinuclear Rhenium Complex

The dinuclear complex [Re(2)(μ-Cl)(2)(CO)(6)(μ-4,5-(Me(3)Si)(2)pyridazine)] gives in the solid state two polymorphs (yellow, 1Y, and orange, 1O), which can be either concomitantly or separately obtained on varying the crystallization rate. Both crystal phases exhibit intense photoluminescence from the lowest lying triplet metal-to-ligand charge transfer state, much stronger than in solution (quantum yields 0.56 and 0.52, for 1O and 1Y respectively, vs 0.06 in toluene), likely due to the restricted rotation of the Me(3)Si groups in the solid state. A clean, irreversible 1O → 1Y single-crystal-to-single-crystal phase transition occurs at 443 K, as revealed by variable temperature X-ray diffraction analysis. In spite of the absence of any strong intermolecular interactions in both forms, 1O and 1Y show very different absorption and emission maxima (λ(abs) 370 and 393 nm, λ(em) 534 and 570 nm, for 1Y and 1O, respectively). This behavior highlights the importance of the local organization of molecular dipoles in perturbing the photophysical properties of the molecule in the crystal.

Luminescent neutral platinum complexes bearing an asymmetric N(^) N(^) N ligand for high-performance solution-processed OLEDs.

The synthesis and full characterization of new platinum complexes bearing a bulky asymmetric dianionic tridentate ligand is reported. The hindrance of the ligand prevents detrimental intermolecular interactions yielding to highly emitting species in both crystalline state and thin-film. Such properties prompted their successful use in solution-processed OLEDs, showing remarkable external quantum efficiency up to 5.6%.

A new class of luminescent tricarbonyl rhenium(I) complexes containing bridging 1,2-diazine ligands: electrochemical, photophysical, and computational characterization.

A novel class of luminescent tricarbonyl rhenium(I) complexes of general formula [Re2(mu-X)2(CO)6(mu-diaz)] (X=halogen and diaz=1,2-diazine) was prepared by reacting [ReX(CO)5] with 0.5 equiv of diazine (seven different ligands were used). The bridging coordination of the diazine in these dinuclear complexes was confirmed by single-crystal X-ray analysis. Cyclic voltammetry in acetonitrile showed for all the complexes (but the phthalazine derivative) a chemically and electrochemically reversible ligand-centered reduction, as well as a reversible metal-centered bielectronic oxidation. With respect to the prototypical luminescent [ReCl(CO)3(bpy)] complex, the oxidation is more difficult and the reduction easier (about +0.3 V), so that a similar highest occupied molecular orbital-lowest unoccupied molecular orbital gap is observed. All of the complexes exhibit photoluminescence at room temperature in solution, with broad unstructured emission from metal-to-ligand charge-transfer states, at lambda in the range 579-620 nm. Lifetimes (tau=20-2200 ns) and quantum yields (Phi up to 0.12) dramatically change upon varying the bridging ligand X and the diazine substituents: in particular, quantum yields decrease in the series Cl, Br, and I and in the presence of substituents at the alpha positions of the pyridazine ring. A combined density functional and time-dependent density functional study of the geometry, relative stability, electronic structure, and photophysical properties of all the pyridazine derivatives was performed. The nature of the excited states involved in the electronic absorption spectra was ascertained, and trends in the energy of the highest occupied and lowest unoccupied molecular orbitals upon changing the pyridazine substituents and the bridging halogen ligands were discussed. The observed emission properties of these complexes were shown to be related to a combination of steric and electronic factors affecting their ground-state geometry and their stability.

Tricarbonyl rhenium(I) complexes containing a bridging 2,5-diphenyl-1,3,4-oxadiazole ligand: structural, spectroscopic, electrochemical, and computational characterization.

The three complexes [Re2(mu-X1)(mu-X2)(CO)6(mu-ppd-kappaN3:kappaN4)] (X1, X2 ) H, 1; X1 ) H, X2 ) Cl, 2; X1, X2 ) Cl, 3; ppd) 2,5-diphenyl-1,3,4-oxadiazole) have been synthesized by different routes, involving the reaction of [Re4(mu3-H)4(CO)12]with ppd for 1, the reaction of 1 with HCl for 2, and the reaction of [ReCl(CO)5] with ppd for 3. The three complexes possess a different number of valence electrons, so the formal Re-Re bond order varies from 2 to 1 to 0 in complexes 1, 2, and 3, respectively. This is reflected in the Re-Re bond distance (277.9, 297.9, and 358.5 pm in the same series)and in the stability of the complexes in the coordinating solvent acetonitrile (t1/2 for ppd displacement 13.6, 4.5, and 3.7 h,for 1, 2, and 3, respectively). Both experimental and calculated structures indicates that coordination induces a distortion from planarity of the diphenyloxadiazole moiety due to the interaction of the equatorial carbonyls with the bridging ppd,which increases on going from 1 to 2 to 3 (dihedral angle between the oxadiazole and the phenyl rings 18.4 degrees, 23.3 degrees, and 45.0 degrees, respectively). The UV spectra show pi-pi* transitions of the oxadiazole ligand (which shift to higher energy on increasing the distortion from the planarity, from 252 to 267 nm) and metal-to-ligand charge transfer absorptions (from 300 to 362 nm). Upon irradiation between 340 and 380 nm, complex 2 only features a weak broad emission at 527 nm(phi)0.02%), whereas upon excitation at 300 nm, the emission typical of free ppd is observed, suggesting photodissociation.Cyclic voltammetry investigations in acetonitrile showed that the three complexes exhibit ligand-centered irreversible reduction peaks (from -1.83 to -1.93 V vs Fc+|Fc), shifted to more positive values with respect to free ppd (-2.50 V). The shift however is smaller than in the analogous derivatives containing 1,2-diazines, suggesting a smaller electron depletion of the heterocycle ligand upon coordination. The complexes also show a metal-centered, bi-electronic, irreversible oxidation peak (from 1.05 to 1.37 V vs Fc+/Fc). A combined density functional and time-dependent density functional (TD DFT)study allowed us to understand the factors affecting the stability of the three complexes and to rationalize their electrochemical and photophysical properties in terms of their electronic structure.

An Intramolecular NH⋅⋅⋅(-H)Re2 Dihydrogen Bond and a Novel 3-2 Coordination Mode of the Pyrazolate Anion on a Triangular Cluster Face

The quantitative addition of pyrazole (Hpz) to the 44 valence-electron, triangular cluster anion [Re3(μ3-H)(μ-H)3(CO)9]− gives the novel unsaturated anion [Re3(μ-H)4(CO)9(Hpz)]− (1, 46 valence electrons), which contains a pyrazole molecule that is terminally coordinated on a cluster vertex. Solid-state X-ray and IR analyses reveal a rather weak hydrogen-bonding interaction between the NH proton and one of the hydrides bridging the opposite triangular cluster edge (ΔH°=−3.1 kcal mol−1 from the Iogansen equation). Both IR and NMR data indicate that such a proton–hydride interaction is maintained in the major conformer present in CD2Cl2, but also provide evidence of the presence of minor conformers of 1 in which the NH proton is involved in an intermolecular hydrogen bond with the solvent. The μ-H⋅⋅⋅HN bond length evaluated in solution through the T1 minimum value (2.07 A) and that determined in the solid state by X-ray diffraction (2.05 A) are in good agreement. NMR experiments show that, in acetone, intermolecular NH⋅⋅⋅solvent interactions replace the intramolecular dihydrogen bond. At room temperature in CH2Cl2, the pyrazole ligand in 1 is labile and 1 slowly “disproportionates” to [Re3(μ3-H)(μ-H)3(CO)9]− and [Re3(μ-H)3(CO)9(μ-η2-pz)(Hpz)]−, with H2 evolution. Slow H2 evolution also leads to the formation of the anion [Re3(μ-H)3(CO)9(pz)]− (5), in which the pyrazolate anion adopts a novel μ3-η2-coordination mode, as revealed by a single-crystal X-ray analysis. The analysis of the bond lengths indicates that the pyrazolate anion in 5 acts as a six-electron donor, with loss of the aromaticity. The formation of 5 from 1 is much faster in solvents with a high dielectric constant, such as acetone or DMF. Anion 5 was also obtained from the reaction of pyrazole with [Re3(μ-H)3(CO)9(μ3-CH3)]− through the intermediate formation of two isomeric addition derivatives and following CH4 evolution.

Solid state and solution structure of the hetero-dimetallic complexes [Cp(CO)2Re{μ-C(OMe)(Ph)}Pt(COD)] and [Cp(CO)2Re{μ-CPh}Pt(COD)]+, containing bridging carbene and carbyne ligands

Abstract The reaction of the carbene complex [Cp(CO)2ReC(OMe)(Ph)] with [Pt(COD)2] affords in high yield the novel complex [Cp(CO)2Re{μ-C(OMe)(Ph)}Pt(COD)] (2), containing a bridging carbene ligand. Reaction of 2 with (Me3O)BF4 results in removal of the carbene-bound methoxy group, with formation of the cation [Cp(CO)2Re{μ-CPh}Pt(COD)]+ (3), in which a bridging carbyne is present. Both the complexes contain only ligands coordinated through carbon atoms and represent the highest members, as far as the Pt:Re ratio is concerned, of the small family of phosphorous-free Pt–Re mixed metal complexes containing PtRe bonds. Their full 1H and 13C NMR characterization in solution and their single-crystal solid state diffractometric analysis are reported and discussed. The X-ray analysis evidenced the substantially symmetric nature of the carbene bridge in the dimetalla-cyclopropane ring of complex 2 [RePt 2.7188(3), Re-(μ-C) 2.166(4) and Pt-(μ-C) 2.055(4) A]. In complex 3 the bond length pattern within the three-membered ring is suggestive of a significant double-bond character for the Re-(μ-C) interaction [RePt 2.707(1), Re-(μ-C) 1.904(9) and Pt-(μ-C) 1.981(8) A]. Both complexes show a semi-bridging CO ligand, with Pt⋯C distances of 2.452(4) and 2.299(9) A, in 2 and 3, respectively. The 13C NMR characterization showed that the Pt⋯CO interaction is also maintained in solution in both complexes. In cation 3 a libration of the ReCp(CO)2 fragment around the PtRe axis, fast down to 193 K, interchanges the two carbonyl locations observed in the solid state, as well as the environments of couples of COD carbon atoms, leading to an apparent Cs symmetry. The correlation between spectroscopic data and structural features (concerning bridging carbenes or carbynes and semi-bridging carbonyls) in the class of heterodimetallic M–Pt complexes is discussed.

[Re5(μ‐H)4(CO)20]− and [Re5(μ‐H)5(CO)20], Two Isolobal Analogues of Cyclopentane

The first five-membered rings of metal atoms connected by M-M or M-H-M bonds only have been obtained by a Re2 +Re3 condensation in which a polyhydride acts as a bridging bidentate ligand toward a coordinatively unsaturated fragment (see scheme below). In spite of the octahedral coordination of the Re centers, the Re5 rings display conformations (twisted and envelope) comparable with those observed for organic five-membered rings of tetrahedral carbon atoms.

New dinuclear hydrido-carbonyl rhenium complexes designed as photosensitizers in dye-sensitized solar cells

The possible use of some dinuclear rhenium complexes as sensitizers for dye sensitized solar cells (DSSCs) has been investigated. They have general formula [Re2(μ-X)(μ-Y)(CO)6(μ-pyridazine-4-COOH)], with X = Y = Cl (1), X = H, Y = benzoato (2), and X = H, Y = 4-diphenylaminobenzoato (3). An original synthetic strategy has been set for preparing the hydrido-carboxylato derivatives 2 and 3. They have been indicated by DFT and TD-DFT computations as the most promising dyes, endowed with good light harvesting capability. The complexes have absorption maxima in the range of 405–443 nm, on TiO2 films, arising from metal-to-ligand-charge transfer transitions. Cyclic voltammetry experiments have been performed on the derivatives containing the methyl ester of the pyridazine-4-COOH acid, showing electrochemical band gaps in the range of 2.25–1.63 eV. The best DSSC results have been obtained using complex 3, with an overall solar-to-electric conversion efficiency of 1.0%. Noteworthy the presence of a hydrido ligand did not show any detrimental effect on the stability of the sensitizers under the operating conditions.

A ketone complex by alkylation of an acyl anion. Synthesis, crystal structure and spectroscopic characterization of [Cp(CO)2Re{OC(Me)Ph}]

Abstract Reaction of Li[Cp(CO) 2 Re(COPh)] ( 1 ) with CF 3 SO 3 Me afforded, besides the expected carbene complex [Cp(CO) 2 ReC(OMe)(Ph)] ( 2 ) and the alkyl–acyl derivative [Cp(CO) 2 Re(Me)(COPh)] ( 3 ), a third structural isomer [Cp(CO) 2 Re{OC(Me)Ph}] ( 4 ), which contains an acetophenone molecule coordinated to the metal center. The X-ray analysis showed that in solid acetophenone is bound to ‘CpRe(CO) 2 ’ exclusively through an oxygen σ-donor interaction, while in solution an equilibrium between σ-bound (η 1 ) and π-bound (η 2 ) forms occurs, as judged by IR data and 1 H and 13 C variable temperature NMR spectra (π/σ ratio 2.87 at 183 K and 1.16 at 263 K in CD 2 Cl 2 , Δ H °=−4.5 kJ mol −1 for the σ⇔π reaction, E a 58(1) kJ mol −1 ). In solvents different from Et 2 O ( n -hexane, THF, acetone) and with alkylating agent different from CF 3 SO 3 Me (MeI, Me 3 OBF 4 ) the formation of 4 was negligible. It has been demonstrated that 4 does not originate by acetophenone reductive elimination from 3 . No evidence of the involvement of radicals has been obtained.

[Re6(μ-H)5(CO)24]−: The First Carbonyl Cluster with a Cyclohexane-Like Structure

A chair conformation comparable to that observed for six-membered rings composed of tetrahedral carbon atoms is found for the cluster anion [Re6(μ-H)5(CO)24]− (see picture; black spheres: Re, white spheres: μ-H; CO ligands omitted for clarity) in spite of the octahedral coordination at the Re centers. This is the first example of a carbonyl cluster exhibiting a cyclohexane-like geometry of the metallic framework.