Synthesis and crystal structure of the mixed lithium sodium guanidide [Li4Na2{NC(NMe2)2}6]: A stack of three (metal-nitrogen)2 dimeric rings with additional intramolecular (tertiary amine) nitrogen-lithium coordinations

The tetralithium disodium guanidide [Li4Na2{N=C(NMe2)2}6], crystallized from a 1 : 1 : 2 Bu(n)Li : Bu(n)Na : HN=C(NMe2)2 mixture, has been studied by X-ray crystallography and found to be related to the ketimide [Li4Na2{N=C(Ph)But}6], reported earlier as having a stacked-core of three (metal-nitrogen)2 ring dimers with Li+ and Na+ cations in end rings and the central ring, respectively. However, it also displays a unique feature: intramolecular donor-acceptor interactions which connect dimethylaminonitrogen atoms project from the central ring to Li+ cations in the end rings. Although these interactions are long range (average length 2.304 angstrom, cf. 2.139 angstrom for Li-N core bonds), they still significantly influence the core bonding as evidenced by an analysis of bond lengths and bond angles.