Mono-, di- and trinuclear 2,3,5,6-tetrakis(2-pyridyl)pyrazine (tppz)-containing copper(II) complexes: syntheses, crystal structures and magnetic properties

摘要

Three new copper(II) complexes of formula [Cu(tppz) (NCO)(2)] . 0.4H(2)O(1), [Cu-2(tppz) Br-4] (2) and [Cu-3(tppz) (C5O5)(3)(H2O)(3)] . 7H(2)O(3) [tppz = 2,3,5,6-tetrakis(2-pyridyl) pyrazine; C5O52- = croconate, dianion of 4,5-dihydroxycyclopent-4-ene-1,2,3-trione] have been synthesised and structurally characterized by X-ray diffraction methods. The structure of complex 1 is made up of neutral [Cu(tppz) (NCO)(2)] mononuclear units and uncoordinated water molecules. The mononuclear units are grouped by pairs to give a rather short copper copper distance of 3.9244(4) Angstrom. The structure of complex 2 consists of neutral tppz-bridged [Cu-2(tppz)Br-4] dinuclear units, the copper copper separation across tppz being 6. 6198(1) Angstrom. The dinuclear units are further connected through weak, double out-of-plane Cu-Br ... Cu bridges [Br( 1) ... Cu(1a) 4.0028(17) Angstrom] creating tetranuclear entities, the copper copper separation through this interaction being 4.3299(21) Angstrom. The structure of complex 3 is built of neutral [Cu-3(tppz) (C5O5)(3)(H2O)(3)] trinuclear units and uncoordinated water molecules. Tppz and one of the croconate groups act as bridging ligands, the former exhibiting the bis-terdentate coordination mode and the latter adopting an unusual asymmetrical bis-bidentate bridging mode through three adjacent oxygen atoms. The other two croconate groups exhibit the bidentate coordination mode. The intramolecular copper copper separations are 6. 5417( 9) ( across tppz) and 4.3234(9) Angstrom (through bis-bidentate croconato). The magnetic properties of 2 and 3 have been investigated in the temperature range 1.9-300 K. The magnetic behaviour of complex 2 is that of an antiferromagnetically coupled copper(II) dimer (J = -40.9 cm(-1), the Hamiltonian being (H) over cap = -J (S) over cap (A).(S) over cap (B)). In the case of compound 3, the chi(M) T vs. T plot is typical of an overall antiferromagnetic coupling with a low-lying spin doublet being fully populated at T < 10 K. The values of the intramolecular antiferromagnetic interactions in 3 are -19.9 (across tppz) and -32.9 cm(-1) ( through bridging croconato). Density functional type calculations were performed on model dinuclear fragments of 3 in order to analyze the efficiency of the exchange pathways involved and also to substantiate the coupling parameters.