Synthesis of mono- and trinuclear palladium(II) complexes via oxidative addition of a bulky hexathioether containing a disulfide bond to palladium(0)

Abstract

The oxidative addition reactions of a bulky hexathioether containing a disulfide bond, TbtS( o-phen)S( o-phen)SS( o-phen)S( o-phen)STbt ( 1) (Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl, o-phen = o-phenylene), to a palladium(0) complex were studied. In the reaction of 1 with 3 molar amounts of [Pd(PPh 3) 4], a trinuclear palladium(II) complex, [Pd 3{S( o-phen)S} 2{( o-phen)STbt} 2(PPh 3) 2] ( 2), was formed via three-step palladium insertion reaction including unusual C(aryl)–S bond cleavages. On the other hand, the reaction of 1 with an equimolar amount of [Pd(PPh 3) 4] afforded mononuclear palladium(II) complex having a pseudo-octahedral structure, [Pd{S( o-phen)S( o-phen)STbt} 2] ( 3). The hexa-coordinated geometry for the palladium center in 3 was confirmed by the atoms in molecule (AIM) analysis, which revealed the presence of the bond critical points between the central Pd atom and the S atoms at the axial positions. In contrast to the bulky system, the reaction of Ph-substituted hexathioether, PhS( o-phen)S( o-phen)SS( o-phen)S( o-phen)SPh ( 4), with an equimolar amount of [Pd(PPh 3) 4] gave a palladium(II) complex having square-planar structure, [Pd{S( o-phen)S( o-phen)SPh} 2] ( 5). Theoretical calculations revealed that there is no remarkable difference among the energies of isomers of [Pd{S( o-phen)SPh} 2], 6a- syn , 6a- anti , 6b- syn , and 6b- anti . This result suggests that a reason for the preference of the trans-anti-conformation in 3 is the steric repulsion between the bulky Tbt groups, and that of the cis-syn-conformations in 5 and 6 is the intermolecular interactions.