Dibenzotropylidene phosphanes (TROPPs): synthesis and coinage metal complexes

Abstract

Dibenzocycloheptatrienyl phosphanes (dibenzotropylidene phosphanes=TROPP^R) 11a–c may be easily prepared from dibenzocycloheptatrienyl chloride 8 and the secondary phosphanes R₂PH [9a: R=Ph; 9b: R=4-Me-C₆H₄; 9c: R=cyclohexyl (Cyc)] in good yields. Alternatively, the di(tert-butyl)phosphanyl substituted TROPP^But derivative 4 was obtained along with the phosphane 5 by a mechanistically still unknown rearrangement of a strained phosphorus ylide I. The conformations of these new phosphanes were established by X-ray analyses performed for the compounds TROPP^But4 and TROPP^Ph11a. The R₂P moiety is bonded to an axial position of the central seven-membered ring, which adopts a boat conformation. Thereby a rigid concave binding site containing a phosphane and an olefin function is formed, which should allow the synthesis of a wide range of transition metal complexes. In order to test how far the particular shape of the TROPP-type ligands enforces metal–olefin interactions, the coinage metal complexes [(TROPP^Ph)Cu(µ₂-Cl)]₂, 13, [(TROPP^Ph)Ag(µ₂-O₂SOCF₃)]₂, 16, [(TROPP^Ph)₂Ag][O₃SCF₃], 17 and [(TROPP^Ph)AuCl], 19 were prepared. These were completely characterized by multinuclear NMR experiments in solution and the solid state, as well as by X-ray analyses. The structural and NMR data show that the interaction between the metal center M and the olefin moiety of the TROPP ligand is weak and decreases in the order Cu>Ag>Au. Indeed, for 19 (M=Au) no interaction could be observed. In the silver complex 17, coupling between an Ag nucleus and a proton of a bonded olefin was determined for the first time [J(^109/107AgH)=0.4 Hz]. In solution the complexes derived from TROPP-type ligands seem to have an enhanced (kinetic) stability.