Copper(II) complexes of new potentially hexadentate N3S3- or N6-donor podand ligands based on the tris(pyrazolyl)borate or tris(pyrazolyl)methane core

The mononuclear copper(II) complexes of the following potentially hexadentate podand ligands have been prepared and crystallographically characterised: tris[3-{2-(methylsulfanyl)phenyl}pyrazol-1-yl]hydroborate [L-1](-), phenyltris[3-(2-pyridyl)pyrazol- 1-yl]methane (L-2), and tris[3- {(6-methyl)-pyrid-2-yl}pyrazol-1-yl]hydroborate [L-3](-) Of these, [L-1](-) [a potentially N3S3 donor with three pyrazolyl and three thioether groups, based on a tris(pyrazolyl)borate core] and L-2 [a potentially N-6 donor with three pyrazolyl and three pyridyl groups, based on a tris(pyrazolyl)methane core] have been prepared for the first time. In [Cu(L-1)][PF6] the Cu(II) centre is in a five-coordinate N3S2 coordination environment which is approximately square pyramidal; the pendant thioether group has a weak, long-range interaction with the sixth coordination site of Cu(II). [Cu(L-1)][PF6] undergoes a reversible Cu(I)-Cu(II) redox conversion. In [Cu(L-2)(MeOH)][PF6](2) the ligand is four-coordinate via two bidentate pyridyl/pyrazolyl arms, with the third arm pendant; an axial methanol ligand completes the square-pyramidal coordination. In [Cu(L-3)(H2O)][PF6], which is five coordinate and approximately trigonal bipyramidal, [L-3](-) acts as an N-4 donor via all three pyrazolyl groups but only one pyridyl group; the two pendant pyridyl groups are involved in O-H ... N hydrogen-bonding interactions with the coordinated water molecule. This 'second-sphere' stabilisation of a coordinated ligand is strongly reminiscent of the cooperative interactions by which substrates are bound to the active sites of metalloproteins. The EPR spectra of this complex are solvent-dependent, showing a change from a d(z2) ground state in non-donor solvents to a d(x2-y2) ground state in donor solvents.