Sterically-constrained tripodal phosphorus-bridged tris-pyridyl ligands

Abstract

Introducing substituents into the 6-position of the 2-pyridyl rings of neutral tris-pyridyl phosphanes of the type P(2-py′)₃ (where 2-py′ is a substituted 2-pyridyl ring), has a marked impact on their coordination of transition metal ions, as revealed in the current study. Whereas the unsubstituted phosphorus-bridged tris-pyridyl ligand P(2-py)₃ (1) forms the sandwich cation [{P(py)₃}₂Fe]²⁺ (4) with iron(II), coordinating via all of the donor nitrogen atoms, the reaction of the methyl-substituted counterpart P(6-Me-2-py)₃ (2) and FeCl₂ results in the half-sandwich arrangement [{P(6-Me-2-py)₃}FeCl₂]·toluene (5·toluene), in which only two N-atoms of the ligand coordinate to the metal. A similar half-sandwich type complex, [{P(6-Me-2-py)₃}FeCl(OTf)]·2THF (6·2THF), is obtained from reaction of 2 with Fe(OTf)₂ in the presence of LiCl, only now with all three of the N-atoms of the ligand coordinated to Fe^II. The formation of a half-rather than full-sandwich complex with 2 suggests that steric clashing of the Me groups prevents the formation of sandwich-type arrangements. The reaction of [Cu(MeCN)₄]PF₆ with P(6-Me-2-py) (2) gives the complex [(MeCN)₃Cu{P(6-Me-2-py)₃}Cu(MeCN)](PF₆)₂ (7), in which two Cu^I atoms are coordinated by the bridgehead P-atom and by the three N-atoms of the tris-pyridyl ligand (a unique coordination mode in this area). Overall, the results indicate that 6-Me substitution results in a promising 6-electron capping ligand for organometallic synthesis and catalysis.