COMMUNICATIONThe first direct mechanochemical transition-metal-mediated activation of strong phenyl C-H bonds is reported. 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 solidstate Raman spectroscopy and in combination with quantumchemical calculations enabled characterization of involved reaction species, direct insight into the dynamics and reaction pathways, as well as the optimization of a milling process.Cyclometalation via the transition-metal-mediated activation of carbon-hydrogen (C-H) bond is the simplest and the most common method for the formation of a metal-carbon (M-C) σ bond. 1 After the first reports in the middle 1960s, 2 the C-H activation process remains the most straightforward method for preparation of cyclometalated compounds which are widely used in organic synthetic chemistry for the insertion of various functionalities into hydrocarbons. 3 Among them, cyclopalladated compounds have been studied most extensively not only for their wide application in organic synthesis and catalysis, but also due to their mesogenic, bioactive and photoluminescent properties. 1a,1c,4 The synthesis of all known metalacycles relies exclusively on solvent-based techniques which may require elevated temperatures, toxic solvents (benzene, toluene, chloroform) and are often time-consuming. 1a,1c,4 During the last decade mechanochemical reactions emerged as a viable and environmentally-friendly alternative to solvent-based techniques. 5 Apart from the processing of inorganic materials, 6 mechanosynthesis has been recognized as a rapid, selective and atom-and energy-efficient pathway to various classes of compounds Ruđer Bošković Institute, Bijenička 54, HR-10000 Zagreb, Croatia E-mail: curic@irb.hr †Electronic Supplementary Information (ESI) available: Experimental and computational details. CCDC 1005635-1005636 For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4cc04423a ‡ These authors contributed equally to this work.ranging from organic molecules 7 and cocrystals 8 up to metal-organic coordination compounds. 9 Despite its wide application in different areas of chemistry, solid-state synthesis of organometallic compounds by direct formation of a M-C σ bond has not been reported yet.Here we describe the first example of palladium-mediated solidstate C-H bond activation in asymmetrically substituted azobenzene 1, achieved by mechanochemical milling process (Scheme 1). The palladation of 1 proved to be regioselective, producing regioisomer 1A in higher yield and much faster than the analogous reaction in solution, with the reduction of solvent volume more than 10000 times. Further reaction of 1A with palladium(II) acetate (Pd(OAc) 2 ) leads to the second C-H bond activation forming in a quantitative yield the doubly palladated complex 1...

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.

Mechanism of C-H bond activation by various Pd catalysts under milling conditions has been studied by in situ Raman spectroscopy. Common Pd precursors, that is PdCl , [Pd(OAc) ] , PdCl (MeCN) and [Pd(MeCN) ][BF ] , 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 Pd precursors and their reactivity increases in the order [Pd(OAc) ]

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.