Ferromagnetic and antiferromagnetic spin coupling in Ni 4O 4 cubane-type clusters with 4-amino-3,5-bis(hydroxymethyl)-1,2,4-triazole as a ligand. The x-ray structure of a new dumbbell-like double cubane cluster

Abstract

The synthesis, spectroscopic and magnetic properties of two nickel clusters are described, Ni 8(NCS) 8(Hahmt) 6(H 2ahmt) 4(ahmt)(H 2O) 12 ( A) and Ni 4(Hahmt) 4(H 2ahmt) 2(NCS) 4(H 2O) 4 ( B) (H 2ahmt = 4-amino-3,5-bis(hydroxymethyl)-1,2,4-triazole). The X-ray structure of A has been determined. The compound crystallizes in the space group C2/ c, a = 25.458(2), b = 15.466(2), c = 26.959(3) Å, β = 90.648(5)°. The structure was refined to R = 0.108 for 4169 observed reflections. The structure consists of two Ni 4O 4 cubane-type clusters, each consisting of four nickel atoms, three singly deprotonated and one doubly deprotonated ligands. Hahmt coordinates as a bidentate chelating ligand through its triazole-N 1 and its 3-oxymethyl-part. The doubly deprotonated ligand chelates in a bis-bidentate manner to two Ni 4 clusters. In this way dumbbell-like pairs of ligand bridged cubanes are formed. Along two opposite diagonals of the Ni 4O 4 cubanes a neutral ligands, coordinating through its triazole-N 1,N 2 atoms, forms a bridge between two nickel ions. The NiN 3O 3 chromophore is completed by a monodentate N-coordinating thiocyanate anion. Compound B, for which only partial structure determination was possible, has the same Ni 4O 4 cubane-type cluster, however, without the bridging ligand between the cubanes. The two types of NiNi bridges result in two unequivalent superexchange pathways. In the compounds both ferromagnetic and antiferromagnetic interaction pathways are present, resulting in an overall antiferromagnetic behaviour. The nature of the interaction for the different pathways is related to the observed NiONi angles. The intercluster exchange is much larger through the triazole bridge in A than it is in B, where only hydrogen bridges keep the clusters together.