Mono- and bidentate imidates of five-coordinate nickel(ii) with macrocyclic ligands: spectroscopic and photophysical properties

Abstract

The hydroxo-complexes [Ni₂(mcN₃)₂(μ-OH)]₂(PF₆)₂] [mcN₃ = 2,4,4-trimethyl-1,5,9-triazacyclododec-1-ene (Me₃-mcN₃) or 2,4,4,9-tetramethyl-1,5,9-triazacyclododec-1-ene (Me₄-mcN₃)] react with imides (Him) in 1 : 2 (succinimide, glutarimide, saccharine and 1,8-naphthalimide) or 1 : 1 (pyromellitic diimide) molar ratio, leading to the formation of the imidate complexes: [Ni(mcN₃)(H₂O)(Im)]⁺ (Im = succi, gluti, sac), [Ni(mcN₃)(naphthi)]⁺ and [Ni(mcN₃)(H₂O)]₂(μ-pyrdi)]²⁺. The single crystal structures of [Ni(Me₃-mcN₃)(H₂O)(sac)](PF₆) (3a) and [Ni(Me₄-mcN₃)(H₂O)(sac)](sac) (3b) show that, in 3a, sac exhibits a N-monodentate coordination mode, while in 3b the coordination is through oxygen. X-Ray structure of [Ni(Me₄-mcN₃)(naphthi)](PF₆) (4b) shows N,O-bonded 1,8-naphthalimidate ligand. The VT ¹H NMR study, carried out on this complex, suggests that the rotation around the Ni–N bond is hindered by ca. 11.6 kcal mol⁻¹. These species exhibit blue luminescence which arises from a mixed MLCT and LC excited state, and these properties may be useful for structural predictions in the solid state of complexes for which crystal structures have not been established.