1,5-Diarylbiguanides and their nickel(ii) complexes

Abstract

1,5-Diarylbiguanides, where the aryl groups are phenyl (HL1), 3,5-dimethylphenyl (HL2), 3,5-dimethoxyphenyl (HL3), 4-t-butylphenyl (HL4) or 4-bromophenyl (HL5), have been prepared and characterised. HL3 and HL5 have been structurally characterised by X-ray crystallography, which shows them to adopt the expected tautomeric form for biguanides. They have extensive hydrogen-bonding interactions in the solid state, involving the biguanide NH groups supported by, in the case of HL3, the OCH₃ aryl substituents or, in the case of HL5, Br⋯Br interactions. Reactions of HL1–HL4 with Ni(BF₄)₂ gives complexes of the type [Ni(HL)₂](BF₄)₂, while reactions of HL1–HL4 with Ni(BF₄)₂ and triethylamine give neutral complexes of the type [Ni(L)₂], where the biguanide ligand has been deprotonated at the N_ring nitrogen. Both series of complexes were characterised in solution and the solid state. Cyclic voltammetry shows a largely irreversible Ni(II)/Ni(III) oxidation which becomes easier by ca. 70 mV upon ligand deprotonation, with more subtle variations resulting from the changes in aryl ring substituents. Infrared and ¹H NMR spectroscopies both provide evidence for ligand deprotonation leading to the chelate ring becoming increasingly aromatised. X-ray crystallographic analyses of five of the complexes also show changes in bond lengths and angles within the chelate ring, consistent with increased electron delocalisation. A variety of hydrogen bonding motifs involving the complex ions, counterions and solvent molecules are found. The results of DFT calculations on both cationic and neutral complexes provide calculated structures consistent with the experimental ones and these, along with the results of vibrational spectroscopic studies, provide further evidence for increased aromatisation upon deprotonation. The potential for the complexes to act as tectons for the rational assembly of hydrogen bonded metallosupramolecules is discussed and the X-ray structure of such an assembly, between [Ni(L3)₂] and 1,8-naphthalimide, is presented.