Ferro- and Antiferromagnetic Interactions in Oxalato-Centered Inverse Hexanuclear and Chain Copper(II) Complexes with Pyrazole Derivatives.

Two novel copper(II) complexes of formulas {[Cu(4-Hmpz)][Cu(4-Hmpz)(µ-ox-,::)(ClO)]} () and {[Cu(3,4,5-Htmpz)][Cu(3,4,5-Htmpz)(µ-ox-,::)(HO)(ClO)][Cu(3,4,5-Htmpz)(µ-ox-,:,)]}(ClO)·6HO () have been obtained by using 4-methyl-1-pyrazole (4-Hmpz) and 3,4,5-trimethyl-1-pyrazole (3,4,5-Htmpz) as terminal ligands and oxalate (ox) as the polyatomic inverse coordination center. The crystal structure of consists of perchlorate counteranions and cationic copper(II) chains with alternating bis(pyrazole)(µ-,::-oxalato)copper(II) and tetrakis(pyrazole)copper(II) fragments. The crystal structure of is made up of perchlorate counteranions and cationic centrosymmetric hexanuclear complexes where an inner tetrakis(pyrazole)(µ-,:,-oxalato)dicopper(II) entity and two outer mononuclear tetrakis(pyrazole)copper(II) units are linked through two mononuclear aquabis(pyrazole)(µ-,::-oxalato)copper(II) units. The magnetic properties of and were investigated in the temperature range 2.0-300 K. Very weak intrachain antiferromagnetic interactions between the copper(II) ions through the µ-ox-,:: center occur in [ = -0.42(1) cm, the spin Hamiltonian being defined as = - · ], whereas very weak intramolecular ferromagnetic [ = +0.28(2) cm] and strong antiferromagnetic [ = -348(2) cm] couplings coexist in which are mediated by the µ-ox-,:: and µ-ox-,:, centers, respectively. The variation in the nature and magnitude of the magnetic coupling for this pair of oxalato-centered inverse copper(II) complexes is discussed in the light of their different structural features, and a comparison with related oxalato-centered inverse copper(II)-pyrazole systems from the literature is carried out.