Dedicated to Professor David N. Reinhoudt on the occasion of his 65th birthdayThe properties of transition-metal catalysts are largely dominated by the direct environment around the metal center, and traditionally the steric environment has been adjusted by ligands that are coordinated to the metal center. Enzymes create a suitable cavity around an active site to provide the required steric and electronic environment. This has inspired scientists to create nanosized capsules to facilitate stochiometric and catalytic transformations in synthetic cavities, resulting in unique conversions. [1,2] We have been particularly interested in the encapsulation of transition-metal complexes and the effect on the catalytic performance, and we therefore developed a template-ligandassisted approach as a new strategy to make such complexes generally accessible. [3][4][5][6][7] For example, trispyridylphosphine templates and rhodium complexes thereof can be encapsulated by three zinc(II) porphyrin units through nitrogen-zinc coordination. Rhodium encapsulation results in a remarkable change in catalyst selectivity; in the hydroformylation of 1-octene, [3,4] mainly branched aldehydes are formed; for internal octenes, such as 3-octene, regioselective reactions were also accomplished.[7] The approach was extended to templated ligand assemblies based on zinc(II) salphen complexes (salphen = N,N'-bis(salicylidene)-o-phenylenediamine dianion), which are more accessible building blocks and easier to vary structurally.[5] Scheme 1 shows an example of the efficient ligand encapsulation by zinc(II) salphen complexes and also displays the solid-state structure of the three-to-one assembly that was also shown to exist in solution. [6] So far, templated encapsulation has been based on the complementary use of donor atoms; zinc has a low affinity for phosphine ligands, and the monopyridyl ligands have lower affinity for the catalytically active metals used to date. Herein, we demonstrate that similar assemblies can be based on template ligands that only have nitrogen donor atoms, in which case selective coordination is based on steric differences. The palladium complexes thus obtained are active in the CO/4-tert-butylstyrene copolymerization, and the activity and selectivity strongly depend on the building blocks used for the ligand assembly.With the 3-pyridyl-bian ligand (mPy-bian (1), see Scheme 2), encapsulation [8] takes place through the pyridyl groups, whereas in the presence of zinc(II) salphen building blocks, palladium complexes are coordinated to the bis-imine Scheme 1. Formation of a three-to-one assembly as a typical example of the template-ligand approach to encapsulation. The structure depicts the three-to-one assembly; P orange, Zn light blue, O red, N blue, Cl yellow, C green, H gray.Scheme 2. Formation of an encapsulated neutral palladium complex and the PM3-TM optimized structure of the assembly with salphen C from Scheme 4; Pd, Zn, Cl all purple, O red, N blue, C green, H gray.