LAUSR

Abstract In exploring the conformational behavior of cyclic tungsten bis-alkyne complexes, two dialkynylamides (14a and 14c) and two dialkynylesters (14b and 14d) derived from 1,1’-ferrocenedicarboxylic acid were prepared. They were subsequently reacted with W(CO)3(dmtc)2 to yield the desired cyclic tungsten bis-alkyne complexes 8–11. In the cyclization of 14a to yield 8 a dimeric macrocyclic complex, 15, featuring two tungsten bis-alkyne complexes in the ring, also was isolated. The conformational behavior of these complexes was assessed by analysis of the 1H NMR resonances for the alkyne hydrogens, which appear around 11 ppm. The spectra for complexes 10, 11 and 15 show multiple singlets of varying integrations for these protons, while the spectra for complexes 8 and 9 show only two resonances of equal integration for the alkyne hydrogens. The spectra for 8 and 9 changed very little when examined at higher temperatures, indicating that the solution conformation is robust. A ROESY spectrum was obtained for 8. It did not show any crosspeaks between the two alkyne hydrogens. The NMR data shows that the alkyne ligands in 10, 11 and 15 are able to rotate about the tungsten-alkyne bond; these complexes adopted several different solution conformations relating to syn and anti arrangements of the alkyne ligands. In contrast, complexes 8 and 9 adopt only one solution conformation, and the alkyne ligands in these species do not rotate about the tungsten-alkyne bond. The NMR spectra for 8 and 9 also show that these complexes are asymmetric. The 1H NMR spectra for 8 and 9 show that each hydrogen atom has its own unique resonance in the 1H NMR spectrum. There are 8 resonances for the 8 Cp protons, 4 resonances for the methylene protons, 2 resonances for the alkyne protons, and in the case of 8, 2 resonances for the NH protons. The two NH protons on complex 8 were found to have widely different chemical shifts. A DMSO titration was performed and it showed that one of the two NH protons in 8 is involved in an intramolecular hydrogen bond. Given that the diester 9 adopts a similar conformation as the diamide 8, this intramolecular hydrogen bond appears to result from the conformation imposed by cyclization of the ring system. Overall, the data show that the ring system for 8 and 9 provides a unique, rigid, robust, and air stable cyclic molecule where the alkyne ligands are limited to one orientation, presumably the syn orientation. The lack of mobility for the alkyne ligands limits the cyclic molecule to only one solution conformation. Complexes 8 and 9 are the first reported examples of cyclic tungsten bis-alkyne complexes that only adopt a single, robust conformation in solution.