Synthesis and magnetic properties comparison of M-Cu(II) and M-VO(II) Schiff base-porphyrazine complexes: What is the mechanism for spin-coupling?

Dimetallic Schiff base-porphyrazine (pz) compounds, denoted 1[M-1; M-2; R], have been prepared, where metal ion M-1 is incorporated into the pz core, and metal ion M2 is bound to a bis(5-tert-butylsalicylidenimine) chelate built onto two amino nitrogens attached to the pz periphery; R is a solubilizing group (either propyl (Pr) or 3,4,5-trimethoxyphenyl (TMP)) attached to the remaining carbons of the pz periphery. The synthesis of 1[Cu; Cu; R], 1[Cu; VO; R], 1[ClMn; Cu; Pr], and 1[ClMn; VO; Pr] is discussed, the crystal structures of 1 [Cu; Cu; TMP] and 1 [ClMn; VO; Pr] are presented, and the magnetic properties of these compounds are compared. The pattern of ligand-mediated exchange coupling in these complexes is startling: for the Cu-M-2 complexes 1[Cu; VO; R] and 1[Cu; Cu; R], 2 x 10(2) <= vertical bar J(Cu-VO)/J(Cu-Cu)vertical bar; for the ClMn-M-2 complexes 1[CIMn; Cu; Pr] and 1[ClMn; VO; Pr], J(ClMn-VO)J(ClMn-Cu) approximate to 1/3, an inverse ratio from that of the Cu-M-2 complexes, but with lesser discrimination. This coupling pattern is explained in terms of a novel orientation relative to the M-1-M-2 direction: the "square-planar" Schiff base ligand set of M-2 is rotated in-plane by 45 degrees relative to the effectively coplanar pz ligand set of M-1.