Metal Complexes of 1,3-Bis(Pyridin-2-yl)pyrazole: Spectral, Magnetic and Structural Studies

Abstract

Bis(ligand)iron(II) and nickel(II) complexes of the asymmetric tridentate ligand 1,3-bis(pyridin-2-yl) pyrazole , L, have been prepared. The iron(II) complex, [FeL2] [PF6]2, is high-spin in the solid state over the temperature range 304-102 K, with a magnetic moment of 5.27 BM at room temperature. The crystal structure of bis (1,3-bis(pyridin-2-yl) pyrazole )iron(II) bis (hexafluorophosphate ) has been determined by single-crystal X-ray diffractometry. The compound crystallized as yellow prisms, with the structure being disordered in the tetragonal space group P421c with Z = 2. Crystal data a = b = 8.785(1) Ǻ, c = 19.804(6) Ǻ. The iron(II) centre is in an N6 environment, where the six donor nitrogen atoms are provided by the two tridentate heterocyclic ligands. The complex cation has an approximately octahedral structure exhibiting tetragonal compression. The observed Fe-N(pyridine) and Fe-N( pyrazole ) distances are 2.308(4) and 2.019(7) Ǻ respectively, with the Fe-N(pyridine) distance being the longest observed to date.

Copper complexes of 2,6-bis(iminomethyl)pyridine derivatives and of 1,3-bis(pyridin-2-yl)pyrazole. Effects of ligand bulk and conformational strain on the ground state of a six-co-ordinate copper(II) ion

“…Complexation of Cu(ClO 4 ) 2 ؒ6H 2 O by 2 mol of L 3 in MeNO 2 yields a bright green solution, from which green microcrystals analysing as [Cu(L 3 ) 2 ][ClO 4 ] 2 8 were obtained following concentration of the solution and addition of diethyl ether. Importantly, the IR spectrum of this compound was indistinguishable from those of its iron() and nickel() congeners, 29 which is consistent with the [Cu(L 3 ) 2 ] 2ϩ dication adopting a 6-coordinate structure in the solid, rather than an equally plausible 4-co-ordinate geometry with each ligand binding bidentately. 30 This proposal was confirmed by a crystal structure analysis.…”

“…Therefore, we tentatively propose that the solution EPR spectrum of 8 reflects partial isomerisation of the dissolved complex, to a [4 ϩ 2] co-ordination geometry analogous to that shown by [Cu(L 5 ) 2 ][BF 4 ] 2 (L 5 = 3-{2,5-dimethoxyphenyl}-1-{pyridin-2-yl}pyrazole). 30 We note that the 1 H NMR spectrum of [Ni(L 3 ) 2 ][BF 4 ] 2 29 is much more complex than would be expected from its solid state structure, and demonstrates the presence of several paramagnetic species in solution. 27 It is likely that the solution structure of 8 is similarly complicated.…”

“…We noted that the crystal structure of [Fe(L 3 ) 2 ][PF 6 ] 2 had been published previously, which showed that L 3 enforces extremely long Fe-N{pyridine} bonds in this complex. 29 This is a result of the inclusion of a 5-membered ring at the centre of the tridentate L 3 framework, rather than a 6-membered ring as in L 1 R. We therefore suggested that conformational strain may induce correspondingly long Cu-N{pyridine bonds} in [Cu(L 3 ) 2 ] 2ϩ , and that this would result in a {d z 2} 1 ground state for this complex.…”

“…The resolution of the structure was too low to distinguish between the pyrazole N1 and C3 sites on the L 3 ligands, which could in any case be disordered. 29 Hence these sites in the structure, namely C(3), C(6), C (20) and C( 23), were all refined as C atoms.…”

Interpreting and controlling the structures of six-coordinate copper(ii) centres – When is a compression really a compression?

“…53 This is a reflection of the narrower internal angles of the central pyrazole ring in L 4 , compared to the pyridine ring in L 2 R, which prevent the pyridine donors from approaching the Cu ion more closely. 116 So, the {d z 2} 1 configuration in [Cu(L 4 ) 2 ] 2ϩ is induced by conformational strain within the ligand backbone, rather that by inter-ligand steric repulsion. Finally, the hydrazone and oxime ligand complexes…”