Asymmetric functionalisation of aza macrocycles. Syntheses, crystal structures and electrochemistry of [Ni(Bz[9]aneN3)2][PF6]2 and [Pd(Bz[9]aneN3)2][PF6]2·2MeCN (Bz[9]aneN3= 1-benzyl-1,4,7-triazacyclononane)

Abstract

1-Benzyl-1,4,7-triazacyclononane (Bz[9]aneN₃) has been prepared and its co-ordination chemistry with Ni^II and Pd^II investigated. The crystal structure of the complex [Ni(Bz[9]aneN₃)₂][PF₆]₂ revealed six independent Ni–N distances, affording a distorted-octahedral co-ordination geometry. Four lie in the range 2.089(4)–2.123(4)Å, while the distances to the benzyl amine N-donors are significantly longer at 2.262(4) and 2.251(4)Å. The benzyl amine N-donors occupy adjacent sites at the metal centre affording a syn geometry. Cyclic voltammetry of [Ni(Bz[9]aneN₃)₂][PF₆]₂ in MeCN at a platinum electrode showed a reversible Ni^II–Ni^III oxidation at E_½=+0.76 V vs. ferrocene–ferrocenium at 235 K. This process was irreversible at higher temperatures. The ESR spectrum of [Ni(Bz[9]aneN₃)₂]³⁺ exhibits an axial signal, with g_‖= 2.03 and g_⊥= 2.15, consistent with a tetragonally elongated octahedral co-ordination geometry at Ni^III. The crystal structure of [Pd(Bz[9]aneN₃)₂][PF₆]₂·2MeCN revealed the Pd atom on an inversion centre with the ligands bound via two N-donors from each, Pd–N 2.0588(19) and 2.0605(19)Å. The benzyl amine N-donors do not interact with the metal centre, Pd ⋯ N 3.402(4)Å. Cyclic voltammetry of [Pd(Bz[9]aneN₃)₂][PF₆]₂ in MeCN showed a reversible Pd^II–Pd^III couple at E_½=+0.36 V at 298 K. The ESR spectrum of [Pd(Bz[9]aneN₃)₂]³⁺ exhibits an axial signal, with g_‖= 2.007 and g_⊥= 2.117. Superhyperfine coupling (A_‖= 26.9 G) is observed to two N-donors in the axial component consistent with a tetragonally elongated octahedral co-ordination geometry at the Pd^III.