Experimental Evidence and DFT Studies of Next-Nearest-Neighbor Magnetic Interactions through Diamagnetic 3d and 4d Ions

The copper template effect allows the preparation of
tridentate
ligands that chelate copper ions, leaving unoccupied the fourth basal
coordination position and at least one axial position of the copper
coordination polyhedron. Two such cationic complexes, [LCu]+ and [L1Cu]+ (L– = 2-{(E)-[(2-aminoethyl)­imino]­methyl}­phenoxo] and L1– = 2-{(E)-[(2-aminopropyl)­imino]­methyl}­phenoxo),
react with diamagnetic polycyanometalate tectons such as Ni­(CN)42– or Ag­(CN)2– to yield different neutral 1D complexes. In {[(LCu)2Ni­(CN)4]}n (1) the four
cyano nitrogen atoms are involved in coordination with copper ions
in such a manner that each copper atom is pentacoordinated and linked
to two cyano functions that occupy axial and equatorial coordination
positions. Two L1Cu+ cationic entities are linked,
through their equatorial plane, to two trans cyano groups of the Ni­(CN)42– tecton in complex [(L1Cu)2Ni­(CN)4] (2), the two uncoordinated
cyano groups being involved in hydrogen bonds. 2 is a
racemate, a S stereoisomer being associated with
a R one in each [(L1Cu)2Ni­(CN)4] unit. Zigzag Cu–Ag chains are present in [(LCu)­Ag­(CN)2] (3), where the copper centers are pentacoordinated
and connected to the cyano groups in an alternate axial–equatorial
coordination scheme. A bidimensional structure is developed by interchain
argentophilic interactions. In complex 4, {(L1CuMeOH)­(L1Cu)­[Ag­(CN)2]2}, two L1Cu units are connected by a NC–Ag–CN bridge
in an equatorial position. These resulting units exhibit argentophilic
interactions with [Ag­(CN2)]− entities
that are monocoordinated in the equatorial position to the next unit,
ultimately leading to a chain. Weak Cu–Cu magnetic interactions
are detected in the four compounds, antiferromagnetic in the case
of equatorial–equatorial copper interactions, ferromagnetic
for orthogonal interacting copper orbitals (axial–equatorial
interactions), while axial–axial bridges are characterized
by an absence of interaction. The presence of weak ferromagnetic interactions
through large NC–Ni–CN or NC–Ag–CN bridges
(Cu···Cu distances larger than 10 Å) furnishes
experimental evidence for the existence of next-nearest-neighbor
interactions through diamagnetic centers. DFT calculations do confirm
the existence of these magnetic transmission pathways through the
diamagnetic metal bridge.