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)(4)][Cu(4-Hmpz)(2)(µ(3)-ox-κ(2)O(1),O(2):κO(2′):κO(1′))(ClO(4))(2)]}(n) (1) and {[Cu(3,4,5-Htmpz)(4)](2)[Cu(3,4,5-Htmpz)(2)(µ(3)-ox-κ(2)O(1),O(2):κO(2′):κO(1′))(H(2)O)(ClO(4))](2)[Cu(2)(3,4,5-Htmpz)(4)(µ-ox-κ(2)O(1),O(2):κ(2)O(2′),O(1′))]}(ClO(4))(4)·6H(2)O (2) have been obtained by using 4-methyl-1H-pyrazole (4-Hmpz) and 3,4,5-trimethyl-1H-pyrazole (3,4,5-Htmpz) as terminal ligands and oxalate (ox) as the polyatomic inverse coordination center. The crystal structure of 1 consists of perchlorate counteranions and cationic copper(II) chains with alternating bis(pyrazole)(µ(3)-κ(2)O(1),O(2):κO(2′):κO(1′)-oxalato)copper(II) and tetrakis(pyrazole)copper(II) fragments. The crystal structure of 2 is made up of perchlorate counteranions and cationic centrosymmetric hexanuclear complexes where an inner tetrakis(pyrazole)(µ-κ(2)O(1),O(2):κ(2)O(2′),O(1′)-oxalato)dicopper(II) entity and two outer mononuclear tetrakis(pyrazole)copper(II) units are linked through two mononuclear aquabis(pyrazole)(µ(3)-κ(2)O(1),O(2):κO(2′):κO(1′)-oxalato)copper(II) units. The magnetic properties of 1 and 2 were investigated in the temperature range 2.0–300 K. Very weak intrachain antiferromagnetic interactions between the copper(II) ions through the µ(3)-ox-κ(2)O(1),O(2):κO(2′):κO(1′) center occur in 1 [J = −0.42(1) cm(−1), the spin Hamiltonian being defined as H = −J∑S(1,i) · S(2,i+1)], whereas very weak intramolecular ferromagnetic [J = +0.28(2) cm(−1)] and strong antiferromagnetic [J’ = −348(2) cm(−1)] couplings coexist in 2 which are mediated by the µ(3)-ox-κ(2)O(1),O(2):κO(2′):κO(1′) and µ-ox-κ(2)O(1),O(2):κ(2)O(2′),O(1′) 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.