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1.
Chemistry ; 27(67): 16690-16700, 2021 Dec 01.
Artigo em Inglês | MEDLINE | ID: mdl-34634149

RESUMO

Organic materials with multiple emissions tunable by external stimuli represent a great challenge. TTPyr, crystallizing in different polymorphs, shows a very rich photophyisics comprising excitation-dependent fluorescence and phosphorescence at ambient conditions, and mechanochromic and thermochromic behavior. Transformation among the different species has been followed by thermal and X-ray diffraction analyses and the emissive features interpreted through structural results and DFT/TDDFT calculations. Particularly intriguing is the polymorph TTPyr(HT), serendipitously obtained at high temperature but stable also at room temperature, whose non-centrosymmetric structure guarantees an SHG efficiency 10 times higher than that of standard urea. Its crystal packing, where only the TT units are strongly rigidified by π-π stacking interactions while the Pyr moieties possess partial conformational freedom, is responsible for the observed dual fluorescence. The potentialities of TTPyr for bioimaging have been successfully established.

2.
Inorg Chem ; 59(22): 16421-16429, 2020 Nov 16.
Artigo em Inglês | MEDLINE | ID: mdl-33119298

RESUMO

Anion recognition by neutral hosts that function in aqueous solution is an emerging area of interest in supramolecular chemistry. The design of neutral architectures for anion recognition still remains a challenge. Among neutral anion receptor systems, urea and its derivatives are considered as "privileged groups" in supramolecular anion recognition, since they have two proximate polarized N-H bonds exploitable for anion recognition. Despite promising advancements in urea-based structures, the strong hydrogen bond drives detrimental self-association. Therefore, immobilizing urea fragments onto the rigid structures of a metal-organic framework (MOF) would prevent this self-association and promote hydrogen-bond-accepting substrate recognition. With this aim, we have synthesized two new urea-containing metal-organic frameworks, namely [Zn(bpdc)(L2)]n·nDMF (TMU-67) and [Zn2(bdc)2(L2)2]n·2nDMF (TMU-68) (bpdc = biphenyl-4,4'-dicarboxylate; bdc = terephthalate; L2 = 1,3-bis(pyridin-4-yl)urea), and we have assessed their recognition ability toward different anions in water. The two MOFs show good water stability and anion affinity, with a particular selectivity toward dihydrogen arsenate for TMU-67 and toward fluoride for TMU-68. Crystal structure characterizations reveal 3-fold and 2-fold interpenetrated 3D networks for TMU-67 and TMU-68, respectively, where all single interpenetrated networks are hydrogen bonded to each other in both cases. Despite the absence of self-quenching, the N-H urea bonds are tightly hydrogen bonded to the oxygen atoms of the dicarboxylate ligands and cannot be directly involved in the recognition process. The good performance in anion sensing and selectivity of the two MOFs can be ascribed to the network interpenetration that, shaping the void, creates monodimensional channels, decorated by exposed oxygen atom sites selective for arsenate sensing in TMU-67 and isolated cavities, covered by phenyl groups selective for fluoride recognition in TMU-68.

3.
Dalton Trans ; 49(10): 3102-3111, 2020 Mar 14.
Artigo em Inglês | MEDLINE | ID: mdl-32065186

RESUMO

Two novel rhenium(i) tricarbonyl complexes of general formula fac-[Re(N^C:)(CO)3X] are herein presented, where N^C: is the pyridoannelated N-heterocyclic carbene (NHC) arising from 2-(2-pyridinyl)imidazo[1,5-a]pyridinium hexafluorophosphate proligand, namely [pyipy]PF6, and X being Cl and Br. The synthetic pathway is a one-pot reaction that starts from the azolium salt as the NHC source and [Re(CO)5X] to yield the desired charge-neutral fac-[Re(pyipy)(CO)3X] complexes (1-2). Both complexes were thoroughly characterized by spectroscopic, electrochemical, theoretical investigation as well as X-ray diffraction analysis. They display a rather similar electronic absorption spectrum in dilute CH2Cl2 solution, which is characterized by a broad profile extending into the blue region. This lowest-lying absorption band is attributed to a transition with admixed metal-to-ligand charge transfer and intraligand charge transfer (1MLCT/1ILCT) character. Degassed samples of the complexes display moderate (Φ≈ 1.5%) and long-lived (τ = 12.8-13.4 µs) red photoluminescence with highly structured profile independent of the nature of the ancillary halogen ligand and little sensitivity to the solvent polarity, highlighting the markedly different nature of the emitting excited state in comparison with the lowest-lying absorption. Indeed, photoluminescence is ascribed to a long-lived excited state with metal-perturbed triplet ligand-centred (3LC) character as supported by both experimental and density functional theory (DFT) investigations.

4.
Inorg Chem ; 53(20): 11242-51, 2014 Oct 20.
Artigo em Inglês | MEDLINE | ID: mdl-25285663

RESUMO

The novel [Re2(µ-H)(µ-OOC-CPDT)(CO)6(µ-3-Me-pydz)] complex (1; OOC-CPDT = 4H-cyclopenta[2,1-b:3,4-b']dithiophene-4-carboxylate, 3-Me-pydz = 3-methylpyridazine) has been prepared and characterized by single-crystal X-ray diffraction, density functional theory (DFT), and time-dependent DFT computations, UV-vis absorption and emission spectroscopy, and cyclic voltammetry (CV). The measured properties indicate the lack of electronic communication in the ground state between the CPDT and the rhenium diazine moieties. Oxidative electropolymerization of 1, achieved by repeated potential cycling (-0.4 to +1.0 V vs Fc(+)/Fc, in acetonitrile) with different supporting electrolytes, on different electrodes, afforded an electroactive and stable metallopolymer (poly-1). In situ measurements of the mass of the growing film (on a gold electrode, with an electrochemical quartz microbalance) confirmed the regularity of the polymerization process. The polymer exhibits two reversible oxidation peaks of the thiophene chain and a broad irreversible reduction peak (-1.4 V, quite close to that observed for the reduction of monomer 1), associated with a remarkably delayed sharp return peak, of comparable associated charge, appearing in close proximity (+0.3 V) to the first oxidation peak of the neutral polythiophene chain. This charge-trapping effect can be observed upon repeated cycles of p and n doping, and the negative charge is maintained even if the charged electrode is removed from the solution for many hours. Electrochemical impedance spectroscopy showed that the main CV oxidation peak corresponds to facile charge transfer, combined with very fast diffusion of both electrons and ions within the polymer. In summary, poly-1 provides a new example of a metallopolymer, in which the conductive properties of the π-conjugated system are added to the redox behavior of the pendant-isolated complexes.

5.
Adv Mater ; 25(3): 437-42, 2013 Jan 18.
Artigo em Inglês | MEDLINE | ID: mdl-22972396

RESUMO

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%.

6.
Inorg Chem ; 51(5): 2966-75, 2012 Mar 05.
Artigo em Inglês | MEDLINE | ID: mdl-22360569

RESUMO

A series of [Re(2)(µ-ER)(2)(CO)(6)(µ-pydz)] complexes have been synthesized (E = S, R = C(6)H(5), 2; E = O, R = C(6)F(5), 3; C(6)H(5), 4; CH(3), and 5; H, 6), starting either from [Re(CO)(5)O(3)SCF(3)] (for 2 and 4), [Re(2)(µ-OR)(3)(CO)(6)](-) (for 3 and 5), or [Re(4)(µ(3)-OH)(4)(CO)(12)] (for 6). Single-crystal diffractometric analysis showed that the two µ-phenolato derivatives (3 and 4) possess an idealized C(2) symmetry, while the µ-benzenethiolato derivative (2) is asymmetrical, because of the different conformation adopted by the phenyl groups. A combined density functional and time-dependent density functional study of the geometry and electronic structure of the complexes showed that the lowest unoccupied molecular orbital (LUMO) and LUMO+1 are the two lowest-lying π* orbitals of pyridazine, whereas the highest occupied molecular orbitals (HOMOs) are mainly constituted by the "t(2g)" set of the Re atoms, with a strong Re-(µ-E) π* character. The absorption spectra have been satisfactorily simulated, by computing the lowest singlet excitation energies. All the complexes exhibit one reversible monoelectronic reduction centered on the pyridazine ligand (ranging from -1.35 V to -1.53 V vs Fc(+)|Fc). The benzenethiolato derivative 2 exhibits one reversible two-electron oxidation (at 0.47 V), whereas the OR derivatives show two close monoelectronic oxidation peaks (ranging from 0.85 V to 1.35 V for the first peak). The thioderivative 2 exhibits a very small electrochemical energy gap (1.9 eV, vs 2.38-2.70 eV for the OR derivatives), and it does not show any photoluminescence. The complexes containing OR ligands show from moderate to poor photoluminescence, in the range of 608-708 nm, with quantum yields decreasing (ranging from 5.5% to 0.07%) and lifetimes decreasing (ranging from 550 ns to 9 ns) (3 > 4 > 6 ≈ 5) with increasing emission wavelength. The best emitting properties, which are closely comparable to those of the dichloro complex (1), are exhibited by the pentafluorophenolato derivative (3).


Assuntos
Complexos de Coordenação/química , Substâncias Luminescentes/química , Piridazinas/química , Rênio/química , Ânions/química , Complexos de Coordenação/síntese química , Técnicas Eletroquímicas , Luminescência , Substâncias Luminescentes/síntese química , Modelos Moleculares , Conformação Molecular , Piridazinas/síntese química
7.
J Am Chem Soc ; 132(41): 14397-9, 2010 Oct 20.
Artigo em Inglês | MEDLINE | ID: mdl-20866093

RESUMO

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.

9.
Inorg Chem ; 47(23): 11154-65, 2008 Dec 01.
Artigo em Inglês | MEDLINE | ID: mdl-18986135

RESUMO

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.

10.
Inorg Chem ; 47(10): 4243-55, 2008 May 19.
Artigo em Inglês | MEDLINE | ID: mdl-18422309

RESUMO

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.

11.
Inorg Chem ; 45(26): 10909-21, 2006 Dec 25.
Artigo em Inglês | MEDLINE | ID: mdl-17173450

RESUMO

The reaction of the electronically unsaturated (56 valence electrons, ve) tetrahedral cluster [Re4(mu3-H)4(CO)12] (1) with pyridazine (pydz) gives as the main initial product the tetranuclear cluster [Re4(mu-H)4(mu-pydz)(pydz)2(CO)12] (2a), with 64 ve and four hydrogen-bridged metal-metal interactions, with a spiked-triangle geometry. One of the three pydz ligands bridges, in a cis configuration, the cluster edge opposite to the vertex bearing the spike, as indicated by the X-ray single-crystal analysis. This species slowly decomposes, affording the dinuclear unsaturated (32 ve) complex [Re2(mu-H)2(mu-pydz)(CO)6] (3a) and two isomers of the tetranuclear cluster [Re4(mu-H)4(mu-pydz)2(CO)12] (64 ve), sharing an unusual square cluster geometry and differing in the trans (major, 85%, 4a) or cis (4a') configuration of the bridging pydz ligands. The structures of 3a and 4a have been ascertained by X-ray analysis, while the characterization of 4a' was hampered by its instability (slowly transforming into 3a in THF solution). Both the dimer and the square cluster 4a are also formed directly (and quickly) from 1, being present in solution since the beginning of the reaction. Cluster 4a is the main final reaction product. The reaction with phthalazine follows a similar course, with some differences in the relative amount of the final products 3b and 4b. Most of the novel complexes are able to emit light in solution at room temperature, and photophysical measurements were performed in CH2Cl2 solution on the main stable reaction products (i.e., the dinuclear species 3a and 3b and the trans square clusters 4a and 4b). The emission was in the range of 580-645 nm, from MLCT excited states, with lifetimes on the order of a hundred nanoseconds (50-473 ns). The quantum yields were 1 order of magnitude higher for the squares (1.7 and 1.3% for 4a and 4b, respectively, in CH2Cl2) than for the dinuclear complexes ( approximately 0.1%). In the case of 4a, a blue shift and an increase of the emission intensity were observed upon decreasing the solvent polarity.

12.
J Phys Chem B ; 110(51): 25565-7, 2006 Dec 28.
Artigo em Inglês | MEDLINE | ID: mdl-17181185

RESUMO

One-dimensional metal-organic compounds with cis, trans symmetry-controlled counter anions were synthesized (cis compound {[Cu(azpy)(H2O)2(OTs)2]*2H2O*(acetone)} (1) and trans compound {[Cu(H2O)4Cu(azpy)2(OTs)2(H2O)2]*2(OTs)*2H2O*2EtOH} (2)). Only 2, having trans conformation, exhibited a complete structure-restoration effect with a mechanism involving layering of molecular "bricks" of water and solvent molecules.

13.
Inorg Chem ; 45(4): 1683-92, 2006 Feb 20.
Artigo em Inglês | MEDLINE | ID: mdl-16471981

RESUMO

Reaction between 7-azaindole and B(C6F5)3 quantitatively yields 7-(C6F5)3B-7-azaindole (4), in which B(C6F5)3 coordinates to the pyridine nitrogen of 7-azaindole, leaving the pyrrole ring unreacted even in the presence of a second equivalent of B(C6F5)3. Reaction of 7-azaindole with H2O-B(C6F5)3 initially produces [7-azaindolium]+[HOB(C6F5)3]- (5) which slowly converts to 4 releasing a H2O molecule. Pyridine removes the borane from the known complexes (C6F5)3B-pyrrole (1) and (C6F5)3B-indole (2), with formation of free pyrrole or indole, giving the more stable adduct (C6F5)3B-pyridine (3). The competition between pyridine and 7-azaindole for the coordination with B(C6F5)3 again yields 3. The molecular structures of compounds 4 and 5 have been determined both in the solid state and in solution and compared to the structures of other (C6F5)3B-N-heterocycle complexes. Two dynamic processes have been found in compound 4. Their activation parameters (DeltaH = 66 (3) kJ/mol, DeltaS = -18 (10) J/mol K and DeltaH = 76 (5) kJ/mol, DeltaS = -5 (18) J/mol K) are comparable with those of other (C6F5)3B-based adducts. The nature of the intramolecular interactions that result in such energetic barriers is discussed.

14.
Inorg Chem ; 44(14): 5030-41, 2005 Jul 11.
Artigo em Inglês | MEDLINE | ID: mdl-15998031

RESUMO

The previously known anion [(C6F5)3B(mu-OH)B(C6F5)3]- (2) has been prepared by a two-step procedure, involving deprotonation of (C6F5)3BOH2 to give [B(C6F5)3OH]- (1), followed by addition of B(C6F5)3. The solution structure and the dynamics of 2 have been investigated by 1H and 19F NMR spectroscopy. The reaction of [NHEt3]2 with NEt3 resulted in the formation of [NHEt3]+ [(C6F5)3BOH]-, [NHEt3]+ [(C6F5)3BH]-, and (C6F5)3B- (CH2CH=N+ Et2). This indicates that in the presence of a nucleophile anion 2 can dissociate to B(C6F5)3 and 1. The reaction of [HDMAN]2 with 1,8-bis(dimethylamino)naphthalene (DMAN) confirmed this trend. In the presence of water, 2 transformed into the adduct [(C6F5)3BO(H)H...O(H)B(C6F5)3]- (3), containing the borate 1 hydrogen-bonded to a water molecule coordinated to B(C6F5)3. The same compound is formed by treating (C6F5)3BOH2 with 0.5 equiv of a base. A competition study established that for 1 the Lewis acid-base interaction with B(C6F5)3 is about 5 times preferred over H-bonding to (C6F5)3BOH2. The X-ray single-crystal analysis of [2-methyl-3H-indolium]3 provided the first experimental observation of an asymmetric H-bond in the [H3O2]- moiety, the measured O-H and H...O bond distances being significantly different [1.14(2) vs 1.26(2) A]. The reaction of NEt3 with an equimolar mixture of B(C6F5)3 and bis(pentafluorophenyl)borinic acid, (C6F5)2BOH, afforded the novel borinatoborate salt [NHEt3]+ [(C6F5)3BOB(C6F5)2]- ([NHEt3]4). X-ray diffraction showed that the B-O bond distances are significantly shorter than in [(C6F5)3B(mu-OH)B(C6F5)3]-. Variable-temperature 19F NMR revealed high mobility of the five aryl rings, at variance with the more crowded anion 2. 2D NMR correlation experiments showed that in CD2Cl2 the two anions [(C6F5)3BOH]- and [(C6F5)3BH]- form tight ion pairs with [NHEt3]+, in which the NH proton establishes a conventional (BO...HN) or an unconventional (BH...HN), respectively, hydrogen bond with the anion. The diborate anions 2-4, on the contrary, gave loose ion pairs with the ammonium cation, due both to the delocalized anionic charge and to the more sterically encumbered position of the oxygen atoms that should act as H-bond acceptors.

15.
Chemistry ; 11(2): 650-61, 2005 Jan 07.
Artigo em Inglês | MEDLINE | ID: mdl-15565722

RESUMO

The two ion pairs [(4,7-Me(2)indenyl)(2)ZrMe](+)[MeB(C(6)F(5))(3)](-) (1 b) and [(indenyl)(2)ZrMe](+) [MeB(C(6)F(5))(3)](-) (2 b) have been generated in situ by reaction of stoichiometric B(C(6)F(5))(3) with the corresponding dimethyl zirconocenes. It has been shown that molecular mechanics computations, guided by experimental (1)H/(1)H NOE correlations, can provide information on the conformers present in solution. The dynamics of the ion pairs has also been investigated, showing the occurrence of both the processes previously characterized for this class of compounds, namely the B(C(6)F(5))(3) migration between the two methyl groups and dissociation-recombination of the whole [MeB(C(6)F(5))(3)](-) anion, the latter process being much faster than the first one (about three order of magnitude). Moreover, it has been shown that in certain conditions intermolecular processes can occur, which mimic the above-mentioned dissociative exchanges. In particular, the presence of species containing loosely bound [MeB(C(6)F(5))(3)](-) anion fastens the exchange of this anion, while the presence of free B(C(6)F(5))(3) accelerates its exchange between the two methyl sites.


Assuntos
Compostos de Boro/química , Zircônio/química , Algoritmos , Simulação por Computador , Concentração de Íons de Hidrogênio , Espectroscopia de Ressonância Magnética , Conformação Molecular , Soluções , Estereoisomerismo , Relação Estrutura-Atividade
16.
J Org Chem ; 69(10): 3574-7, 2004 May 14.
Artigo em Inglês | MEDLINE | ID: mdl-15132576

RESUMO

The (13)C NMR CP-MAS spectrum of 2-naphthylphenylsulfoxide in the solid state displays line broadening effects due to the restricted rotation about the Ph-S bond. Line shape simulation of the temperature-dependent traces allowed the corresponding barrier to be determined in the solids (14.7 kcal mol(-1)). By making use of the information obtained from single-crystal X-ray diffraction, this barrier could be satisfactorily reproduced by theoretical calculations (14.5 kcal mol(-1)) that take into account the correlated phenyl motion involving a large set of molecules in the crystalline state

17.
J Org Chem ; 68(14): 5445-65, 2003 Jul 11.
Artigo em Inglês | MEDLINE | ID: mdl-12839436

RESUMO

The reaction of pyrroles and indoles with B(C(6)F(5))(3) and BCl(3) produces 1:1 B-N complexes containing highly acidic sp(3) carbons, for example, N-[tris(pentafluorophenyl)borane]-5H-pyrrole (1) and N-[tris(pentafluorophenyl)borane]-3H-indole (2), that are formed by a new formal N-to-C hydrogen shift, the mechanism of which is discussed. With some derivatives, restricted rotation around the B-N bond and/or the B-C bonds was observed by NMR techniques, and some rotational barriers were calculated from experimental data. The acidity of the sp(3) carbons in these complexes is shown by their ability to protonate NEt(3), with formation of pyrrolyl- and indolyl-borate ammonium salts. The driving force for this reaction is given by the restoration of the aromaticity of the heterocycle.

18.
J Am Chem Soc ; 124(18): 5117-26, 2002 May 08.
Artigo em Inglês | MEDLINE | ID: mdl-11982377

RESUMO

Protonation of the anion [Re(2)H(CO)(9)](-) (1) with a strong acid at 193 K affords the neutral complex [Re(2)H(2)(CO)(9)] (2), that in THF above 253 K irreversibly loses H(2) to give [Re(2)(CO)(9)(THF)], previously obtained by room-temperature protonation of 1. Treatment of 2 with NEt(4)OH restores the starting anion 1. Variable temperature (1)H and (13)C NMR spectra as well as T(1) measurements agree with the formulation of 2 as a classical [Re(2)H(mu-H)(CO)(9)] complex, in which two dynamic processes takes place. The "windshield-wiper motion" observed in several related complexes equalizes the two carbonyls trans to the hydrides (E(a) = 44(1) kJ mol(-)(1)), while another much faster process equalizes bridging and terminal hydrides already at 172 K. The variable temperature behavior of the (1)H transverse relaxation times revealed also proton exchange between 2, water, and the parent anion 1 (due to the acidity of 2), but such a process is too slow to account for the fast hydrides exchange in 2. The nature of the latter process has been investigated both experimentally and theoretically. Kinetic data, obtained by the analysis of the variable temperature (1)H spectra (E(a) = 24.5(5) kJ mol(-1)), revealed a small normal kinetic isotope effect (ca. 1.5). The (2)H chemical shift of the fully deuterated isotopomer 2-d(2) was found isochronous with 2, thus ruling out the presence of a significant concentration of a nonclassical [Re(2)(eta(2)-H(2))(CO)(9)] tautomer, in fast exchange with the classical dihydride. Density functional theory (DFT) calculations, carried out at the B3LYP level, confirmed the formulation of [Re(2)H(2)(CO)(9)] as a classical complex. However, when DFT was used to obtain a detailed description of the dynamic behavior of 2 in solution, a new type of hydride fast exchange emerged, involving the nonclassical tautomer as a relatively high energy (12.7 kJ mol(-1)) intermediate. Isotopic perturbation of the equilibrium by partial deuteration of 2 indicated the preference of deuterium for the bridging sites, with Delta H degrees = -475(4) J mol(-1) and Delta S degrees = -0.80(2) J K(-1) mol(-1). The same preference was observed in the anion [Re(2)H(mu-H)Cl(CO)(8)](-).

19.
Chemistry ; 8(23): 5340-50, 2002 Dec 02.
Artigo em Inglês | MEDLINE | ID: mdl-12561305

RESUMO

The quantitative addition of pyrazole (Hpz) to the 44 valence-electron, triangular cluster anion [Re3(mu 3-H)-(mu-H)3(CO)9]- gives the novel unsaturated anion [Re3(mu-H)4(CO)9(Hpz)]- (1, 46 valence electrons), which contains a pyrazole molecule that is terminally coordinated on a cluster vertex. Solidstate 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 (delta H degree = -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 mu-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 N-H...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(mu 3-H)-(mu-H)3(CO)9]- and [Re3(mu-H)3(CO)9-(mu-eta 2-pz)(Hpz)]-, with H2 evolution. Slow H2 evolution also leads to the formation of the anion [Re3(mu-H)3-(CO)9(pz)]- (5), in which the pyrazolate anion adopts a novel mu 3-eta 2-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(mu-H)3(CO)9(mu 3-CH3)]- through the intermediate formation of two isomeric addition derivatives and following CH4 evolution.

20.
Angew Chem Int Ed Engl ; 37(15): 2128-2131, 1998 Aug 17.
Artigo em Inglês | MEDLINE | ID: mdl-29711064

RESUMO

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.

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