Bis(fluoromesityl) Palladium Complexes, Archetypes of Steric Crowding and Axial Protection by ortho Effect − Evidence for Dissociative Substitution Processes − Observation of 19 F− 19 F Through‐Space Couplings

Bisarylated complexes trans ‐[Pd(Fmes) 2 (SR 2 ) 2 ] [Fmes = 2,4,6‐tris(trifluoromethyl)phenyl (fluoromesityl); SR 2 = SMe 2 , tht; tht = tetrahydrothiophene] are precursors for various bisarylated fluoromesityl palladium( II ) complexes by ligand‐substitution reactions. Boiling under reflux in acetonitrile gives the mixed complexes trans ‐[Pd(Fmes) 2 (NCMe)(SR 2 )], whereas boiling under reflux in toluene leads to trans ‐[PdCl 2 L 2 ] (L = PMe 3 , t BuNC, p Tol‐NC, 4‐MePy), in the presence of neutral monodentate ligands, or to (N n Bu 4 )[ trans ‐Pd(Fmes) 2 I(SR 2 )] when treated with (N n Bu 4 )I. trans ‐[Pd(Fmes) 2 (SMe 2 ) 2 ] reacts with bidentate ligands, also boiling under reflux in toluene, to give [Pd(Fmes) 2 (L−L)] [L−L = Me 2 bipy, 2,2′ ‐ biquinolyl, κ 2 N , N′ ‐OCPy 2 , dppm (Ph 2 PCH 2 PPh 2 ), dppe (Ph 2 PCH 2 CH 2 PPh 2 ), pte (PhSCH 2 CH 2 SPh), κ 2 S , N ‐SPPh 2 Py, κ 2 O , N ‐OPPhPy 2 ], or the bimetallic complex [Pd(Fmes) 2 (μ‐1κ N :1,2κ O :2κ N ‐Py 2 MeCO)Pd(Fmes)(SMe 2 )] (characterized by X‐ray diffractometry) when treated with (OH)(CH 3 )CPy 2 . The crowding associated with two Fmes groups produces several interesting features: (1) trans complexes are preferred over cis complexes, against the expected electronic preferences; (2) the low‐temperature NMR spectra of several complexes, or the X‐ray diffraction structure of [Pd(Fmes) 2 (2,2′ ‐ biquinolyl)], reveal significant structural distortions associated with steric crowding; (3) the need for boiling under reflux in the synthesis suggests a dissociative substitution mechanism, which is unknown so far for Pd; (4) some of the complexes show 19 F− 19 F through‐space couplings. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)