Synthesis and characterization of a new class of chelating bis(amidinate) ligands

A series of diamidines based on trans-1,2-diaminocyclohexane have been prepared. Reactions of trans-1,2-diaminocyclohexane with a variety of aryl acid chlorides yields the corresponding diamides in very high (91-95%) yield. Conversion of the diamides to the diimine chloride is carried out by treatment with PCl5 in CH2Cl2. Reaction of the diimine chloride with aniline in CH2Cl2 finally yields the diamidine in good yields on multi-gram scales. Alternatively, a one-pot reaction between the diamide, PCl5 and aniline gives the products, although yields are generally lower. The solid-state structure of the unsubstituted diamidine shows localized C-N single and double bonds in the amidine moiety; intra- and inter-molecular hydrogen bonding is also observed between amidine groups. Alkali metal derivatives (M = Li, Na, K) were prepared by reaction of the diamidines with either (BuLi)-Bu-n or MN(SiMe3)(2). Isolated yields of these alkali metal derivatives, which crystallize as thf adducts, are moderate (34-64%). X-Ray crystallography shows that in all cases the alkali metal atoms bridge the two amidinate groups within the same molecule, forming a C-2 symmetric eight-membered ring. Nonetheless, there are marked differences in coordination geometries of the series.