supplementary materials

Recently, the coordination chemistry of di-Schiff bases derived from 2-pyridyl ketones or aldehydes has generated a great deal of interest (Hamblin et al., 2002; Gourbatsis et al., 1998; Szklarzewicz & Samotus, 2002; Mentes et al., 2007). These studies have been mostly stimulated by an interest in modelling the enzyme, copper-zinc superoxide dismutase (SOD) (Luo et al., 1993) and also for the synthesis of metal containing polymers with interesting optical, magnetic and electrical properties (Hanack et al., 1988; Marks, 1990). It has also been found that such tetradentate Schiff base ligands may form complexes with different nuclearity according to the coordination preferences of the metal centre (Fielden et al., 2006).

Our interest in the ligand, L, (Scheme 2) was stimulated by the analogy between its donor set and that of the pyridylmethylketazine (L¹) and 2-pyridinealdazine (L²) system which form triple-stranded helical complexes with the formula [M₂(L)₃]⁴⁺ (M = Co, Fe and Ni). The helical complexes were shown to undergo exchange reactions on standing to form mono-nuclear complexes [M(L)₂]²⁺ in which the ligand twists to coordinate as tridentate with non-coordinated imine residue (Hamblin et al., 2002). Mononuclear species are favoured by coordination to octahedral metal centres whose equatorial sites are occupied by N4 donor set of the bis(axial) ligand, and their axial sites being occuppied by solvent molecules or counterions. In addition, dinuclear metal complexes are favoured by the four-coordinate tetrahedral metal centres whose ca 90° twist angle provides good geometric match for the bis(equatorial) ligand (Fielden et al., 2006).

Recently, the single-crystal X-ray analysis of L was reported (Mentes et al., 2007). The molecule adopts a centrosymmetric trans geometry and forms a dinuclear complex by reacting with Mo(CO)₆. The reaction of L with ZnX₂ (X = Cl or Br) in tetrahydrofuran yielded an octahedral complex [ZnX₂(L)] (Gourbatsis et al., 1999), whereas by reacting with a silver(I) cation the double-stranded helical complex [Ag₂(L)₂][BF₄]₂ (Bowyer et al., 1998) is formed. The synthesis of copper(II) complex by using Cu(NO₃)₂.3H₂O resulted in the tetragonally distorted octahedral complex, [Cu(L)(ONO₂)(OH₂)][NO₃] (Gourbatsis et al., 1998).

Herein we present the synthesis and structure of the complex [Cu(L)(OTf)₂], (where OTf = trifluoromethanesulfonate) with a molecular structure as illustrated in Scheme 1 and Figure 1. The crystal structure is composed of discrete neutral [Cu(L)(OTf)₂] units. The copper ion exhibits an elongated tetragonal octahedral CuN₄O₂ cromophore with four nitrogen atoms from the ligand occupying the equatorial plane and two axial oxygen atoms from the trans-coordinated unidentate trifluoromethanesulfonate anions. The four equatorial Cu–N distances [Cu1–N1 2.008 (3) Å, Cu1–N4 2.011 (3) Å, Cu1–N2 1.950 (2) Å, and Cu1–N3 1.944 (2) Å] are normal for this class of compounds and also very similar to those of Cu1–Npyridine 2.002 (4) and 2.022 (4) Å, and Cu1–Nimine 1.943 (4) and 1.938 (4) Å as found in [Cu(L)(ONO₂)(OH₂)][NO₃] (Gourbatsis et al., 1998). The bond lengths to the imine N atoms are slightly shorter than those to the pyridine N atoms (Table 1), which is presumably a consequence of the more effective σ donation or π back donation (Hamblin et al., 2002). The coordination is of the 4 + 2 type and thus belongs to the myriad of examples of such Jahn-Teller elongated pseudo-octahedral structures (Şengül & Büyükgüngör, 2005).

The structure contains unidentate trifluoromethanesulfonate anions which are semi-coordinated to the copper ion [Cu1–O2 2.568 (3) Å and Cu1–O4 2.476 (4) Å] in the axial positions with the angle of O2–Cu1–O4 177.9 (4)°. The bonding parameters for the trifluoromethanesulfonate anions are similar to those found for [Cu(pyridine)₄(CF₃SO₃)₂] (Haynes et al., 1988). For example, the trifluoromethanesulfonate anions adopt a staggered-ethane configuration about the S–C bond and the O–S–O angles [O3–S1–O1 116.01 (15)°, O3–S1–O2 113.97 (14)°, O1–S1–O2 114.93 (15)°] are greater than the C–S–O angles [C17–S1–O3 103.25 (15)°, C17–S1–O1 103.00 (15)°, and C17–S1–O2 103.25 (15)°]. The S–O bond lengths are also very similar to those found in [Cu(pyridine)₄(CF₃SO₃)₂], the S1–O2 1.450 (2) and S2–O4 1.446 (2) Å bonds involving the O atoms bound to copper being longer than those involving the terminally bound oxygen atoms [S1–O1 1.442 (2), S1–O3 1.440 (2) Å and S2–O5 1.441 (2), S2–O6 1.440 (2) Å].

The bite angles around the copper ion [N2–Cu1–N3 83.2 (2), N1–Cu1–N2 81.9 (7), N3–Cu1–N4 81.6 (7)°] are very similar to those found in [Cu(L)(ONO₂)(OH₂)][NO₃] with the corresponding angles of 83.2 (2), 80.6 (2) and 81.4 (2)°, respectively.

In the free ligand the pyridylimine units adopt a transoid configuration to minimize unfavourable electronic interactions between the lone pairs of pyridine nitrogen and imine nitrogen atoms. However, in the presence of a metal ion, the pyridine rings rotate by 180° with respect to the Aryl–C bond, positioning the two nitrogen atoms of each pyridylimine moiety on the same side of the ligand. Otherwise the geometric parameters in the free ligand are very similar to those of the coordinated moiety.

The pyridylimine units are not ideally planar due to a combined effect of the ring to the metal centre and a twist induced by the ethylene bridge [Cu1, N1, N2, C1>C6 and Cu1, N3, N4, C10, C12>C16 have devaitions from the mean plane of 0.088 (6) Å and 0.106 (6) Å for N2 and N3 respectively]. From puckering analysis (Cremer & Pople, 1975) the ring formed by the metal centre, the imine N atoms and the ethylene bridge has a Q value of 0.283 (3) Å and forms a twisted envelope conformation about the C8—C9 bond. This effect has the result of pushing the methyl groups out of the ring unit plane, with C7 deviating by 0.149 (5) Å and C11 by 0.167 (6) Å and accordingly the pyridylimine units are not coplanar, with the angle formed between these planes being 12.98 (9)°.

The crystal structure does not exhibit any classical hydrogen bonds and is primarily comprised of stacked undulating sheets formed by close packing and C—H···O interactions between the SO₃ group and pyridylimine ring H atoms.