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.