Doubly Pyrazolate-Bridged Dinuclear Complexes of a Highly Constrained Bis-terdentate Ligand: Observation of a [High Spin-Low Spin] State for [FeII 2(PMAP)2][SbF6]2 3 2.25(C3H8O) (PMAP = 3,5-bis{[N-(2-pyridylmethyl)amino]- methyl}-1H-pyrazolate)

The bis-terdentate pyrazole-based ligand 3,5-bis{[N-(2-pyridylmethyl)amino]methyl}-1H-pyrazole (PMAPH) was synthesized from 3,5-(1H)-pyrazoledicarbaldehyde and 2 equiv of 2-(aminomethyl)pyridine, using sodium borohydride to reduce the imine intermediate. A family of dinuclear complexes [MII/III2(PMAP)2](X)2/4 was prepared by 2:2:2 reactions of MX2/PMAPH/base, where M = ZnII and X = BF4 -; M =CuII and X = ClO4-, BF4-, OAc-, NO3-; M =NiII, FeIII and X = ClO4-, BF4-; M = FeII and X = SbF6-. Single crystal X-ray structure determinations on four complexes: [FeIII 2(PMAP)2]-(BF4)4 3 2MeCN, [NiII2(PMAP)2](ClO4)2 3 2MeCN, [CuII2(PMAP)2](BF4)2 3 2MeCN, and [ZnII2(PMAP)2](BF4)2 3 2MeCN confirmed a dinuclear doubly pyrazolate-bridged structure for each. The two metal centers in these complexes have similar N6 distorted octahedral coordination spheres, with all donors provided by the two deprotonated PMAP- ligands. Magnetic measurements reveal intra-dinuclear antiferromagnetic interactions for both the M = CuII and NiII [M2(PMAP)2](BF4)4 complexes, with J/kB = -252(2) K and J/kB = -24.7(2) K (H = -2JSMSM), respectively. Interestingly magnetic measurements show that the complex [Fe2 II(PMAP)2](SbF6)2 3 2.25(C3H8O) is in a mixed high spin (HS)-low spin (LS) spin state, [HS-LS], from 300 to 1.8 K, with no sign of spin crossover to a fully low spin form [LS-LS] even at 1.8 K