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

Eleven dinuclear and six tetranuclear complexes of two bis-tetradentate ligands, 4,6-bis[N,N-bis(2-pyridylmethyl)aminomethyl]-2-phenylpyrimidine (L-Me) and 4,6-[(2'-pyridylmethyl)(2'-pyridylethyl)aminomethyl]-2-phenylpyrimidine (L-Mix), have been prepared through reactions of ligand and metal salt in a ratio of 1:2 in MeCN or MeOH: (Fe2LMe)-L-II(H2O)(4)(MeCN)(2)(BF4)(4) (1), (Fe2LMix)-L-II(H2O)(4)(BF4)(4) (2), [(Co2LMe)-L-II(H2O)(MeCN)](BF4)(4)center dot(3)/2H2O (3), [(Co2LMix)-L-II(H2O)(MeCN)(3)](BF4)(4)center dot 2H(2)O (4), [(Ni2LMe)-L-II(H2O)(4)](BF4)(4)center dot H2O (5), [(Ni2LMix)-L-II(H2O)(3)(MeCN)](BF4)(4) (6), (Cu2LMe)-L-II(H2O)(MeCN)(BF4)(4) (7), [Cu-2(II)Mix(MeCN)(2)(BF4)(4)] (8), [(Zn2LMe)-L-II(H2O)(MeCN)(3)(BF4)(4)] (9), [(Zn2LMix)-L-II(MeCN)(3)(H2O)(BF4)(4)] (10), [(Zn2LMe)-L-II(H2O)(2)L(MeCN)(2)](ClO4)(4) (11), (Fe2Fe2III)-Fe-II(L-Me)(2)F-4(BF4)(6)center dot(1)/2H2O (12), Fe-4(II)(L-Mix)(2)F-4(BF4)(4)center dot 5H(2)O (13), (Co2Co2III)-Co-II(L-Me)(2)F-4(BF4)(6) (14), [Co-4(II)(L-Mix)(2)F-4](BF4)(4)center dot 3H(2)O (15), [Ni-4(II)(L-Me)(2)F-4](BF4)(4) (16) and Ni-4(II)(L-Mix)(2)F-4(BF4)(4)center dot 8H(2)O (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 ((Cu2Cu2I)-Cu-II), in both cases at a more negative potential (7: E-m = -0.18 V; 8: E-m = -0.19 V vs. 0.01 M AgNO3/Ag) than the analogous L-Et (all-ethylene-armed ligand) complex (E-m = +0.11 V vs. 0. 01 M AgNO3/Ag), consistent with L-Me and L-Mix being better able to stabilize copper(II) than L-Et.