Mechanochemical halogenation of unsymmetrically substituted azobenzenes.

The direct and selective mechanochemical halogenation of C-H bonds in unsymmetrically substituted azobenzenes using N-halosuccinimides as the halogen source under neat grinding or liquid-assisted grinding conditions in a ball mill has been described. Depending on the azobenzene substrate used, halogenation of the C-H bonds occurs in the absence or only in the presence of PdII catalysts. Insight into the reaction dynamics and characterization of the products was achieved by in situ Raman and ex situ NMR spectroscopy and PXRD analysis. A strong influence of the different 4,4'-substituents of azobenzene on the halogenation time and mechanism was found.

A Detailed Kinetico-Mechanistic Investigation on the Palladium C-H Bond Activation in Azobenzenes and Their Monopalladated Derivatives.

Palladium C-H bond activation in azobenzenes with R1 and R2 at para positions of the phenyl rings (R1 = NMe2, R2 = H (L1); R1 = NMe2, R2 = Cl (L2); R1 = NMe2, R2 = I (L3); R1 = NMe2, R2 = NO2 (L4); R1 = H, R2 = H (L5)) and their monopalladated derivatives, using cis-[PdCl2(DMF)2], has been studied in detail by in situ 1H NMR spectroscopy in N,N-dimethylformamide-d7 (DMF-d7) at room temperature; the same processes have been monitored in parallel via time-resolved UV-vis spectroscopy in DMF at different temperatures and pressures. The final goal was to achieve, from a kinetico-mechanistic perspective, a complete insight into previously reported reactivity results. The results suggest the operation of an electrophilic concerted metalation-deprotonation mechanism for both the mono- and dipalladation reactions, occurring from the coordination compound and the monopalladated intermediates, respectively. The process involves deprotonation of the C-H bond assisted by the presence of a coordinated DMF molecule, which acts as a base. For the first time, NMR monitoring provides a direct evidence of all the intermediate stages: that is, (i) coordination of the azo ligand to the PdII center, (ii) formation of the monopalladated species, and (iii) coordination of the monopalladated species to another PdII unit, which finally result in the (iv) formation of the dipalladated product. All of these species have been identified as intermediates in the dipalladation of azobenzenes, evidenced also by UV-vis spectroscopy time-resolved monitoring. The data also confirm that the cyclopalladation of asymmetrically substituted azobenzenes occurs by two concurrent reaction paths. In order to identify the species observed by NMR and by UV-vis spectroscopy, the final products, intermediates, and the PdII precursor have been prepared and characterized by X-ray diffraction and IR and NMR spectroscopy. DFT calculations have also been used in order to explain the isomerism observed for the isolated complexes, as well to assign their NMR and IR spectra.

Mechanism of Mechanochemical C-H Bond Activation in an Azobenzene Substrate by PdII Catalysts.

Mechanism of C-H bond activation by various PdII catalysts under milling conditions has been studied by in situ Raman spectroscopy. Common PdII precursors, that is PdCl2 , [Pd(OAc)2 ]3 , PdCl2 (MeCN)2 and [Pd(MeCN)4 ][BF4 ]2 , have been employed for the activation of one or two C-H bonds in an unsymmetrical azobenzene substrate. The C-H activation was achieved by all used PdII precursors and their reactivity increases in the order [Pd(OAc)2 ]3

Reversible Gas-Solid Ammonia N-H Bond Activation Mediated by an Organopalladium Complex.

N-H bond activation of gaseous ammonia is achieved at room temperature in a reversible solvent-free reaction using a solid dicyclopalladated azobenzene complex. Monitoring of the gas-solid reaction in real-time by in situ solid-state Raman spectroscopy enabled a detailed insight into the stepwise activation pathway proceeding to the final amido complex via a stable diammine intermediate. Gas-solid synthesis allowed for isolation and subsequent structural characterization of the intermediate and the final amido product, which presents the first dipalladated complex with the PdII-(µ-NH2)-PdII bridge. Gas-solid reaction is readily followed via color changes associated with conformational switching of the palladated azobenzene backbone. The reaction proceeds analogously in solution and was characterized by UV-vis and NMR spectroscopies showing the same stepwise route to the amido complex. Combining the experimental data with density functional theory calculations we propose a stepwise mechanism of this heterolytic N-H bond activation assisted by exogenous ammonia.

Vapour-induced solid-state C-H bond activation for the clean synthesis of an organopalladium biothiol sensor.

Room-temperature accelerated aging in the solid state has been applied for atom- and energy-efficient activation of either one or two C-H bonds of azobenzene and methyl orange by palladium(ii) acetate. Organopalladium complexes are prepared in quantitative reactions without potentially harmful side products. Dicyclopalladated methyl orange is water-soluble and is a selective chromogenic biothiol sensor at physiologically-relevant micromolar concentrations in buffered aqueous media.

Mechanochemical reactions studied by in situ Raman spectroscopy: base catalysis in liquid-assisted grinding.

In situ Raman spectroscopy was employed to study the course of a mechanochemical nucleophilic substitution on a carbonyl group. We describe evidence of base catalysis, akin to catalysis in solution, achieved by liquid-assisted grinding.

Mechanochemical C-H bond activation: rapid and regioselective double cyclopalladation monitored by in situ Raman spectroscopy.

The first direct mechanochemical transition-metal-mediated activation of strong phenyl C-H bonds is reported. The mechanochemical procedure, resulting in cyclopalladated complexes, is quantitative and significantly faster than solution synthesis and allows highly regioselective activation of two C-H bonds by palladium(II) acetate in asymmetrically substituted azobenzene. Milling is monitored by in situ solid-state Raman spectroscopy which in combination with quantum-chemical calculations enabled characterization of involved reaction species, direct insight into the dynamics and reaction pathways, as well as the optimization of a milling process.

Dicyclopalladated complexes of asymmetrically substituted azobenzenes: synthesis, kinetics and mechanisms.

Two series of new dicyclopalladated complexes {(DMF)PdCl(µ-R(1)C6H3N═NC6H3R(2))PdCl(DMF)} of 4,4'-functionalized azobenzenes with substituents of varying electron-donating or electron-withdrawing strength (R(1) = H, NMe2; R(2) = H, Cl, Br, I, OMe, PhNH, CO2H, SO3Na, or NO2) have been synthesized and fully characterized. (1)H NMR spectroscopy along with the ESI mass spectrometry unambiguously identified the new complexes in the solution, and their solid-state structures were determined by X-ray crystallography. The presence of easily exchangeable solvent ligands was confirmed by (1)H NMR spectroscopy, X-ray experiments, and ESI mass spectrometry. The complexes were additionally characterized by UV-vis and fluorescence spectroscopies. The effect of different 4,4'-substituents on the formation rate of mono- and dicyclopalladated azobenzenes was studied by UV-vis spectroscopy. The experimental results are complemented by the quantum-chemical (DFT) calculations in order to rationalize the kinetic results as well as substituent effects on the reaction rates. It was found that the mono- and dicyclopalladation reactions of azobenzenes proceed in two consecutive processes, adduct formation and palladation steps. The rate-determining step in both palladations is the breaking of the ortho C-H bond, which has been confirmed as an electrophilic substitution process by Hammett correlations and DFT calculations.

New insight into solid-state molecular dynamics: mechanochemical synthesis of azobenzene/triphenylphosphine palladacycles.

Solid-state reactions of dicyclopalladated azobenzenes and triphenylphosphine lead to the thermodynamically favorable bridged complexes. It was demonstrated for the first time that very complex molecular dynamics involving a series of structural transformations is also feasible in the solid state.

Unusual azobenzene/bipyridine palladacycles: structural, dynamical, photophysical and theoretical studies.

Two types of Pd(ii) azobenzene/bipyridine complexes with unusual coordination mode of azobenzenes, PdCl{(mu-Cl)(mu-R(1)C(6)H(3)N=NC(6)H(3)R(2))}Pd(bpy) 1a-4a and [(bpy)PdCl(mu-NH(2)C(6)H(3)N=NC(6)H(4))Pd(bpy)]Cl 3b were formed by the reaction of dicyclopalladated azobenzenes (DMF)PdCl(mu-R(1)C(6)H(3)N=NC(6)H(3)R(2))PdCl(DMF) with excess of bpy, where bpy=2,2'-bipyridine; R(2)=H and R(1)=H (1), CH(3) (2), NH(2) (3) or R(1)=N(CH(3))(2) and R(2)=NO(2) (4). Neutral species 1a-4a were obtained in acetone, while in DMSO or MeOH the ionic complex 3b was produced. When dissolved, 3b decomposes to 3a and free bpy; however in DMSO upon addition of bpy 3b crystallizes again. X-ray structures of all complexes confirmed breaking of one Pd-N bond in the initial precursors, thus allowing rotation of one phenyl ring and positioning of both Pd atoms on the same side of the azobenzene ligand. Two Pd atoms are connected by the azobenzene ligand and in neutral complexes additionally by the Cl-bridge. In all complexes in the solid-state azobenzenes act simultaneously as monodentate C- and bidentate C,N-donors while bpy acts as bidentate donor. Variable-temperature (1)H NMR experiments established that structures of 1a-4a in DMF and DMSO at ambient temperature are not consistent with solid-state structures due to the fast exchange of one of the bpy nitrogen atoms bound to the Pd atom with solvent molecules. Theoretical studies confirmed the experimental structures as the most stable isomers. Photoabsorption and photoemission properties of the new complexes have been measured and photoabsorption is rationalized by time dependent DFT calculations. The presence of bpy significantly increases the intensity of fluorescence either in the solution (4a) or in the solid state (3a, 4a, 3b) at ambient temperature.

Synthesis and characterization of dicyclopalladated complexes of azobenzene derivatives by experimental and computational methods.

A series of doubly cyclopalladated complexes of azobenzene and its unsymmetrical substituted derivatives, namely, {LPdCl(mu-AZB)LPdCl}, where AZB is azobenzene, 4-methylazobenzene, 4-aminoazobenzene, or 4-(dimethylamino)-4'-nitroazobenzene, while L is N,N-dimethylformamide, dimethylsulfoxide, or pyridine, have been prepared. Their structural and spectroscopic properties were determined by X-ray diffraction analysis as well as by (1)H NMR, IR, UV-vis, and fluorimetric studies. Experimental results were rationalized by quantum chemical calculations. Crystal structures of several complexes have been resolved, and for the first time, it was demonstrated that the cyclopalladation may take place at the azobenzene aromatic ring having the strong electron-withdrawing substituent at the para position. In all cases, the metalated carbon and N,N-dimethylformamide or dimethylsulfoxide ligands are mutually trans, whereas the pyridine ligands are in the cis arrangement. cis/trans isomerism in the isolated compounds is explained by comparing the calculated energies of isomeric structures. All of the complexes absorb strongly in the visible region, and according to time-dependent density functional theory calculations, most of the absorptions can be attributed to intraligand pi --> pi* or metal-to-ligand charge-transfer transitions. The fluorescence emission was observed for the complexes with 4-aminoazobenzene or 4-(dimethylamino)-4'-nitroazobenzene. The aromaticity of palladacycles is evaluated by several aromaticity indices and related to relevant experimental findings.

Simple route to the doubly ortho-palladated azobenzenes: building blocks for organometallic polymers and metallomesogens.

A new class of doubly cyclopalladated complexes, {PdCl(dmf)}2(mu-azb) (1) and {PdCl(dmf)}2(mu-aazb) (2), has been prepared in dimethylformamide (dmf) by reaction of azobenzene (azb) and 4-aminoazobenzene (aazb), respectively, with an excess of PdCl2(CH3CN)2 complex. Recrystallization of 1 and 2 in dimethyl sulfoxide (dmso) yields complexes {PdCl(dmso)}2(mu-azb) (3) and {PdCl(dmso)}2(mu-aazb) (4), respectively. The crystal structures of 1 and 4 have been determined by X-ray diffraction. All complexes are characterized by 1H and 13C NMR spectra and elemental analysis. In both crystal structures, solvent molecules are bound to palladium through oxygen atoms and oriented trans to carbon. In view of greater preference of palladium to nitrogen and sulfur atoms, the experimental structures were rationalized by quantum-chemical calculations and confirmed as the most stable isomers.