Dimeric allylpalladium(II) complexes with pyrazolate bridges: Synthesis, characterization, structure and thermal behaviour |
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Authors: | A Singhal SK Kulshreshtha Paul V Bernhardt |
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Institution: | a Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai, Maharashtra 400 085, India b School of Molecular and Microbial Sciences, University of Queensland, Brisbane 4072, Qld, Australia c Department of Chemistry, The University of Texas at San Antonio, San Antonio, TX 78249-0698, USA |
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Abstract: | The synthesis, characterization and thermal behaviour of some new dimeric allylpalladium (II) complexes bridged by pyrazolate ligands are reported. The complexes ; R = H, R′ = C(CH3)3 (1b), R = H, R′ = CF3 (1c); R = CH3, R′ = CH(CH3)2 (2a); R = CH3, R′ = C(CH3)3 (2b); and R = CH3, R′ = CF3 (2c)] have been prepared by the room temperature reaction of Pd(η3-CH2C(R)CH2)(acac)](acac = acetylacetonate) with 3,5-disubstituted pyrazoles in acetonitrile solution. The complexes have been characterized by NMR (1H, 13C{1H}), FT-IR, and elemental analyses. The structure of a representative complex, viz. 2c, has been established by single-crystal X-ray diffraction. The dinuclear molecule features two formally square planar palladium centres which are bridged by two pyrazole ligands and the coordination of each metal centre is completed by allyl substituents. The molecule has non-crystallographic mirror symmetry. Thermogravimetric studies have been carried out to evaluate the thermal stability of these complexes. Most of the complexes thermally decompose in argon atmosphere to give nanocrystals of palladium, which have been characterized by XRD, SEM and TEM. However, complex 2c can be sublimed in vacuo at 2 mbar without decomposition. The equilibrium vapour pressure of 2c has been measured by the Knudsen effusion technique. The vapour pressure of the complex 2c could be expressed by the relation: ln (p/Pa)(±0.06) = −18047.3/T + 46.85. The enthalpy and entropy of vapourization are found to be 150.0 ± 3 kJ mol−1 and 389.5 ± 8 J K−1 mol−1, respectively. |
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Keywords: | η3-Allylpalladium complexes Pyrazolate ligands NMR spectroscopy X-ray crystal structure Transmission electron microscopy |
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