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[M1; M2; R], have been prepared,
where metal ion M1 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−M2 complexes 1[Cu; VO; R] and 1[Cu; Cu; R], 2 × 102 ≤ |J(Cu−VO)/J(Cu−Cu)|;
for the ClMn−M2 complexes 1[ClMn; Cu; Pr] and 1[ClMn; VO; Pr], J(ClMn−VO)/J(ClMn−Cu) ≈ 1/3, an
inverse ratio from that of the Cu−M2 complexes, but with lesser discrimination. This coupling pattern is
explained in terms of a novel orientation relative to the M1−M2 direction:  the “square-planar” Schiff base
ligand set of M2 is rotated in-plane by 45° relative to the effectively coplanar pz ligand set of M1.