Preparation of Site-Differentiated Mixed Ligand and Mixed Ligand/Mixed Metal Metallacrowns

Assembly reactions that can prepare reliably regioselective metallamacrocyclic complexes have been a target in
the development of metallacrowns. To this end, a series of mixed ligand and mixed ligand/mixed metal
metallacrowns have been synthesized in high yield and structurally characterized. Two distinct connectivities
have been observed in these types of metallacrowns. The monomeric, vacant metallacrown with mixed ligand
composition [12-MCNi(II)N(Hshi)2(pko)2-4] (1a) shows the connectivity pattern [-O−Ni−O−N−Ni−N-]2 while the
other Ni metallacrowns, [12-MCNi(II)N(shi)2(pko)2-4] (2a) and the coupled [12-MCNi(II)N(shi)2(pko)2-4][12-MCNi(II)N(shi)3(pko)-4] (3a) fused metallacrowns as well as the mixed metal Mn−Ni metallacrown [12-MCNi(II)Mn(III)N(shi)2(pko)2-4] (4a), follow the pattern [-Ni−O−N-]4. Also, three distinct arrangements of the chelate
rings around the metal ions have been observed. The syntheses are completely general, allowing for the substitution
of different ligands into the metallacrown core. Compounds 1 and 4 show the 6−5−6−5−6−5−6−5 arrangement,
compounds 2 and 3(1) the 6−6−5−5−6−6−5−5, and the 3(2) component the 6−6−5−5−6−5−6−5. The
obtained structures can be rationalized by balancing the charge at each metal site in the metallacrown. Variable
temperature magnetic susceptibility measurements show that exchange interactions for all the compounds are
weak and dominantly antiferromagnetic (e.g., 2a gives an exchange coupling of J = −1.2 cm-1 with g = 2.2).
In solution, the metallacrowns are shown to be stable both to decomposition and ligand exchange.