Di‐ and Tetranuclear Complexes of Bis‐Tetradentate Pyrimidine‐Based Ligands with All‐Methylene‐ Versus Mixed Methylene/Ethylene‐Linked Arms

Abstract

Eleven dinuclear and six tetranuclear complexes of two bis-tetradentate ligands, 4,6-bis[N,N-bis(2???-pyridylmethyl)aminomethyl]-2-phenylpyrimidine (LMe) and 4,6-[(2???-pyridylmethyl)(2???-pyridylethyl)aminomethyl]-2-phenylpyrimidine (LMix), have been prepared through reactions of ligand and metal salt in a ratio of 1:2 in MeCN or MeOH: FeII2LMe(H2O)4(MeCN)2(BF4)4 (1), FeII2LMix(H2O)4(BF4)4 (2), [CoII2LMe(H2O)(MeCN)](BF4)4??3/2H2O (3), [CoII2LMix(H2O)(MeCN)3](BF4)4??2H2O (4), [NiII2LMe(H2O)4](BF4)4??H2O (5), [NiII2LMix(H2O)3(MeCN)](BF4)4 (6), CuII2LMe(H2O)(MeCN)(BF4)4 (7), [CuII2Mix(MeCN)2(BF4)4] (8), [ZnII2LMe(H2O)(MeCN)3(BF4)4] (9), [ZnII2LMix(MeCN)3(H2O)(BF4)4] (10), [ZnII2LMe(H2O)2L(MeCN)2](ClO4)4 (11), FeII2FeIII2(LMe)2F4(BF4)6??1/2H2O (12), FeII4(LMix)2F4(BF4)4??5H2O (13), CoII2CoIII2(LMe)2F4(BF4)6 (14), [CoII4(LMix)2F4](BF4)4??3H2O (15), [NiII4(LMe)2F4](BF4)4 (16) and NiII4(LMix)2F4(BF4)4??8H2O (17). Dinuclear complexes were obtained from the reactions conducted in MeCN, whereas tetranuclear complexes resulted from reactions performed in MeOH. Five dimetallic (3??????6??? and 9???) and one tetrametallic complex (15???) were structurally characterized: the ligands are bis-tetradentate and the metal centres are bridged through the pyrimidine ring. In the tetrametallic 15???, two such dimetallic fragments are bridged by two pairs of fluoride anions. Variable-temperature magnetic susceptibility measurements and Heisenberg fitting of dinuclear 1???8 and tetranuclear 13???15 in all cases revealed weak intramolecular anti-ferromagnetic exchange coupling between the high-spin metal centres. A rigorous spin???orbit and exchange analysis was also performed on the dicobalt(II) complex 4. The dicopper(II) complexes 7 and 8 underwent a two-electron quasi-reversible reduction (CuII2 ??? CuI2), in both cases at a more negative potential (7: Em = ???0.18 V; 8: Em = ???0.19 V vs. 0.01 M AgNO3/Ag) than the analogous LEt (all-ethylene-armed ligand) complex (Em = +0.11 V vs. 0.01 M AgNO3/Ag), consistent with LMe and LMix being better able to stabilize copper(II) than LEt.