A pyrazine-bridged linear pentanuclear copper(II) complex and related tri- and dinuclear complexes showing various coordination structures and magnetic couplings

Abstract

Sterically bulky pyrazines have been successfully utilized for the preparation of discrete oligo-nuclear TBP (trigonal bipyramidal), SqP (square pyramidal), and Oh (octahedral) copper(II) complexes. We have synthesized a unique linear pentanuclear complex [{Cu(hfac) 2} 5(μ-2-butyl-3-methylpyrazine) 4]. The two terminal copper(II) ions have a SqP structure while the three inner ions have an Oh one. The solvent molecule was incorporated in the clearance of the lattice. From another reaction under harsh conditions, we separated [{Cu(hfac) 2} 3(μ-2-butyl-3-methylpyrazine) 2], which can be regarded as the central moiety of the pentanuclear one. We also prepared a dinuclear complex [{Cu(hfac) 2} 2(μ-tetramethylpyrazine)], in which the pyrazine nitrogen atoms were located at TBP equatorial positions. Single-crystal EPR measurements supported its compressed TBP structure. The exchange coupling was antiferromagnetic with J TBP–TBP/ k B = −3.6 K. The linear trinuclear [{Cu(hfac) 2} 3(μ-2,3,5-trimethylpyrazine) 2], having two TBP Cu ions with an intervening Oh Cu ion, showed very weak antiferromagnetic coupling. DFT calculations on these compounds indicated that the σ-type orbital overlap between the Cu and N atoms is essential for superexchange interactions.