Pathways to Polynuclear Complexes with α‑S,N‑Substituted Alkynes as Bridging Ligands

Recently, alkyne complexes with terminal thiolate/amide substitution have been shown to act as S,N-chelate ligands in dinuclear complexes. In this study, the detailed synthesis and reactivity of W­(II) alkyne complexes bearing different amino groups in the α-position are reported. The preparative scheme starts with cationic alkyne complexes [Tp′W­(CO)2-η2-C,C′-C2Br­(SR)]­PF6 {Tp′ = hydridotris­(3,5-dimethylpyrazolyl)­borate, R = benzyl (Bn), C2H4SiMe3}, which are obtained by applying free bromo alkynes. Subsequent nucleophilic substitution of the bromine substituent led to unsymmetrical substituted alkyne complexes [Tp′W­(CO)2-η2-C,C′-C2(SR)­(NHBn)]­PF6 with S,N substitution in the α position of the coordinated alkyne. Depending on the base used, deprotonation of the secondary amine resulted in either neutral iminoketenyl complexes [Tp′W­(CO)2-η2-C,C′-C2(SR)­(NBn)] or a zwitterionic PF5 adduct. Reductive removal of the benzyl protective group was primarily observed at the imine substituent, causing a side-on/end-on rearrangement to the cyanide substituted carbene complex K­[Tp′W­(CO)2-η1-C­(CN)­(SBn)]. The reversibility of the rearrangement was proven with HBF4/Et2O, because double protonation led to [Tp′W­(CO)2-η2-C,C′-C2(SR)­(NH2)]­BF4 exhibiting an unprecedented primary amine substitution at the coordinated alkyne. The reaction sequence starting with the thiol SC2H4SiMe3 derivative leading to the desired [Tp′W­(CO)2-η2-C,C′-C2S­(NHBn)] with a terminal S atom as well as the bonding situation in those complexes is discussed based on full spectroscopic characterization including X-ray structure analyses. Finally, a trinuclear complex assembled by homoleptic coordination of Pd­(II) by two anionic S,N-chelates [Tp′W­(CO)2-η2-C,C′-C2S­(NBn)]− is presented.