FULL PAPER DOI: 10.1002/ejic.200800804 cis, trans – or Both: Steric Bulk Determines Coordination Mode of Dimeric Palladium Complexes with Bridging Pyridine-Phosphane Ligands Jitte Flapper, [a,b] Philip Wormald, [c] Martin Lutz, [d] Anthony L. Spek, [d] Piet W. N. M. van Leeuwen, [a] Cornelis J. Elsevier, [a] and Paul C. J. Kamer* [a,c] Keywords: Bimetallic complexes / Coordination modes / N,P ligands / Palladium / Solid-state NMR The coordination mode in a metal complex is critically de- pendent on the ligands surrounding the metal, on the precur- sors used, and on the conditions during synthesis. Our goal was to obtain palladium compounds with pyridine-phos- phane ligands for catalysis. Therefore, we synthesized li- gands 1a–d, which differ in the bulky aryl substituent at the pyridyl moiety. The palladium complexes 6a–d of these li- gands are insoluble and have been characterized by various techniques, including solid-state NMR and (for 6a, b, and d) single-crystal X-ray diffraction, showing that bimetallic com- plexes are formed in which two ligands span two palladium centers. The configuration around these centers is deter- mined by the steric bulk of the ligand. In complexes 6c,d, Introduction The catalytic performance of palladium complexes is strongly dependent on the ligands coordinated to the metal. Properties such as activity and selectivity can be steered by electronic and steric factors of the ligand. The way in which a ligand coordinates is important as well. Subtle changes in coordination mode, like chelate bite angles, can have large effects on catalysis. [1] Likewise, a change of coordination geometry can induce a significant effect in catalysis; palla- dium complexes that contain chelating ligands in a cis con- figuration can show totally different behavior from that of trans-coordinated species. For example, the competition be- tween alkoxycarbonylation or CO/ethene copolymerization activity of palladium catalysts seems to be dependent on the coordination modes of the ligand. [2] [a] van ’t Hoff Institute for Molecular Sciences, University of Am- sterdam, Nieuwe Achtergracht 166, 1018WV Amsterdam, The Nether- lands [b] Dutch Polymer Institute P. O. Box 902, 5600 AX Eindhoven, The Netherlands [c] School of Chemistry, University of St. Andrews, St. Andrews, Fife, Scotland KY16 9ST, UK Fax: +44-1334-463-808 E-mail: pcjk@st-andrews.ac.uk [d] Bijvoet Center for Biomolecular Research, Crystal and Struc- tural Chemistry, Utrecht Univerity, Padualaan 8, 3584 CH Utrecht, The Netherlands © 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Inorg. Chem. 2008, 4968–4976 4968 with the ligands bearing the largest steric groups, both cen- ters have a trans configuration of the methyl and chloride anions. With intermediately sized pyridyl substituents, com- plex 6b is formed, having one cis surrounded and one trans- surrounded palladium center in the molecule. This kind of complexation has not been observed before. With the least bulky ligand, complex 6a is formed. Depending on the syn- thetic conditions employed, the methyl and the chloride in this complex can be in a cis configuration at both palladium atoms, or show the unique cis-trans coordination. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) Different donor atoms of a chelating ligand induce asym- metry and invoke different trans-influence and -effect on their responsive trans-ligands. By choosing these features of the ligating groups, selective binding and reactivity of coordinated substrates can be achieved. This is one of the reasons why P,N ligands (chelating ligands with one nitro- gen and one phosphorus donor atom) are extensively studied in catalysis. [3,4] In our previous work concerning palladium complexes of these bidentates, we have studied the effects on carbonylation [5] and allylic alkylation. [6] In our current research, we were particularly interested in pal- ladium complexes of pyridine-phosphane ligands. [4] Recent applications in catalysis of this class of complexes include ethene oligomerization and polymerization, [7] alkene/CO copolymerization, [8] allylic substitution, [9] carbonylation, [10] and Suzuki coupling. [11a] Usually, the 1:1 complexes of pyridine-phosphane li- gands with palladium are monometallic with a cis configu- ration around the metal, but trans-coordinated monometal- lic species [11] and bimetallic complexes in which two ligands span two palladium centers [11a,11c,12] have been reported. The latter type of coordination can be of particular interest for bimetallic catalysis. In this type of catalysis, ideally the two metal centers have a cooperative effect, thus enhancing activity and selectivity. [13] This phenomenon is the reason behind the extreme efficiency of many enzymatic systems [14] and it’s value has also been shown in chemical catalysis. [15]