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

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)(mu-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)(mu-2-butyl-3-metllylpyrazine)(2)], which can be regarded as the central moiety of the pentanuclear one. We also prepared a dinuclear complex [{Cu(hfaC)(2)}(2)(mu-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)(mu-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 sigma-type orbital overlap between the Cu and N atoms is essential for superexchange interactions. (C) 2008 Elsevier Ltd. All rights reserved.