Ein 69‐gliedriger Ring (siehe Bild) entsteht bei einer Metathesereaktion, die ein metalliertes Tris(pinzetten)‐substituiertes Benzol als Gerüst verwendet. Das makrocyclische Produkt kann durch Zugabe von Cl− vom Templat abgespalten und das Templat quantitativ zurückgewonnen werden.

Dehydrogenation of alcohols into aldehydes and ketones by Ru 3 (CO) 12 /PPh 3 based homogeneous catalysis has been investigated as an alternative for the classical Oppenauer oxidation. Several catalytic systems have been screened in the Oppenauer-like oxidation of alcohols. A systematic study of various combinations of Ru 3 (CO) 12 , mono-and bidentate ligands and hydride acceptors was performed to enable dehydrogenation of primary alcohols to stop at the aldehyde stage. Among many H-acceptors screened, diphenylacetylene (tolane) proved the most suitable judged from its smooth reduction. Electron rich and deficient analogues of tolane have been synthesized and, based on competition experiments between these H-acceptors, a tentative catalytic cycle for the Ru 3 (CO) 12 /PPh 3 -catalyzed dehydrogenations has been proposed.

Pincer‐palladium(II) and ‐platinum(II) cations, YCY‐M (YCY=[2,6‐(YCH2)2C6H3]; Y=NMe2, SPh; M=PdII, PtII), bound to diolefin‐substituted pyridines (3,5‐ or 2,6‐substitution) were successfully synthesized, and subsequently used in olefin metathesis (RCM) as a model study for template‐directed synthesis of macrocycles. Especially a 3,5‐disubstituted pyridine bound to a NCN‐PtII‐center (5a) gave a fast metathesis reaction, while the same reaction with the PdII analogue (4a) was much slower and less selective (isomerization products were formed). Furthermore, it was found that 2,6‐diolefin‐substituted pyridines (4b, 5b, 5c) gave slow metathesis reactions, which is mainly ascribed to steric hindrance during the ring‐closing step. In all cases where prolonged reaction times were required an isomerization process, most likely assisted by cationic pincer‐MII species, was observed as a competing reaction. 1H NMR spectroscopy experiments revealed that pyridines are stronger bound to a cationic NCN‐PtII‐center than to its PdII‐analogue. This aspect is of crucial importance when these pincer‐pyridine complexes are applied in metathesis, since free pyridine in solution deactivates the Ru‐metathesis catalyst. For the templated construction of macrocycles, a strong M‐N(py) bond is also important since it determines the selectivity for the desired product. In addition, these results open a new research field in which organometallic (pincer) complexes are used as protecting groups for strong Lewis‐basic groups in catalysis. From failed attempts to prepare macrocycles using hexakis[SCS‐PdII‐(1a)] complex 14, and from the results obtained with the monometallic pincer complexes in RCM, it can be concluded that the most suitable candidate for constructing macrocycles should comprise 2,6‐diolefin‐substituted pyridines bound to a multi‐(NCN‐PtII)‐template. In such a system, intrapyridine metathesis (steric hindrance) as well as isomerization reactions (strong M‐N(py) bond) are suppressed.

The incompletely condensed silsesquioxane disilanol (c-C 5 H 9 ) 7 Si 7 O 9 (OH) 2 OSiMePh 2 has been used as a backbone for the synthesis of the diphosphinite ligand (c-C 5 H 9 ) 7 Si 7 O 9 (OPPh 2 ) 2 -OSiMePh 2 (1), based on a silsesquioxane framework. By reaction with black selenium, the corresponding selenide (2) was obtained, showing a J Se-P value of 815 Hz in the 31 P NMR spectrum. DFT calculations established a good insight into the electron density of the P atoms present in the two model compounds CH 3 OPPh 2 (3) and Ph 2 P(HSiOH) 2 OPH 2 (4). The Mulliken charge distributions show a clear electron-withdrawing effect of the siloxy group, which is also present in diphosphinite ligand 1. By reaction of compound 1 with PdCl 2 -(C 6 H 5 CN) 2 , the palladium complex [PdCl 2 R(OPPh 2 ) 2 ] (5) was obtained (R ) (c-C 5 H 9 ) 7 Si 7 O 9 -OSiMePh 2 ). From a similar reaction of 1 with PtCl 2 (cod), the platinum analogue [PtCl 2 R(OPPh 2 ) 2 ] (6) could be isolated. The equimolar reaction of Mo(CO) 4 (pip) 2 (pip ) piperidine) with 1 yielded the molybdenum complex [Mo(CO) 4 R(OPPh 2 ) 2 ] (7). Ligand 1 showed a clear tendency to coordinate in a cis fashion for all complexes 5-7, as was determined by NMR spectroscopy and X-ray crystallography. However, the analogous reaction of 1 with RhCl(CO) 2 dimer yielded the mononuclear trans-[RhCl(CO)(1)] (8). Complexes 5-8 have been structurally characterized as the first examples of transition-metal complexes with a silsesquioxane-based bidentate phosphinite ligand.