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; M²; 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[CIMn; Cu; Pr], and 1[CIMn; VO; Pr] is discussed, the crystal structures of 1[Cu; Cu; TMP] and 1[CIMn; 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-M² complexes 1[Cu; VO; R] and 1[Cu; Cu; R], 2 × 10² ≤