New Approaches to 12-Coordination: Structural Consequences of Steric Stress, Lanthanoid Contraction and Hydrogen Bonding



The anionic dinitrile ligand dicyanonitrosomethanide (dcnm), C(CN)2(NO), and the anion resulting from its addition product with water, carbamoylcyanonitrosomethanide (ccnm), C(CN)(CONH2)(NO), have been incorporated into lanthanoid complexes and display unusual η2(N,O) nitroso coordination modes. (Et4N)3[Ln(ccnm)6] (1Ln; 1Ln = 1La, 1Ce, 1Pr, 1Nd, 1Sm) and (Me4N)3[Ln(ccnm)6] (2Ln; 2Ln = 2La, 2Ce, 2Pr, 2Nd) are systems containing 12-coordinate homoleptic trianionic lanthanoidate complexes. The nitroso groups of the ccnm ligands form three-membered ring chelates with the lanthanoid metal centre, with the asymmetry of the nitroso η2 interactions dependent upon the intramolecular N–H···O=N hydrogen bonding. Additional intermolecular hydrogen bonding interactions exist between adjacent amide and nitrile groups giving rise to 3D α-Po and 6,8-connected (412.63)(420.68) networks in 1Ln and 2Ln, respectively. The compounds (Me4N)3[Ln(dcnm)6] (3Ln; 3Ln = 3La, 3Ce, 3Nd, 3Sm) also contain a 12-coordinate trianionic lanthanoidate complex with the nitroso group exhibiting a highly symmetrical η2 interaction. The sterically crowded environments of [Ln(18-crown-6)(dcnm)3] (4Ln; 4Ln = 4La, 4Ce, 4Pr, 4Nd) result in a shift towards a more asymmetric η2 bonding of the nitroso group with decrease in the Ln3+ radius. There is a corresponding increase of the Ln–O–N angle, and one ligand is η1(O) binding in 4Nd. The dcnm ligands in the discrete complexes [La(phen)3(dcnm)(3–x)Clx], x ≈ 0.25 (5) (phen = 1,10-phenanthroline), (Et4N)[Ce(phen)2(dcnm)4] (6a/b, 6c) and [Ce(phen)2(dcnm)Cl2H2O] (7) display a variety of coordination modes. Complex 5 has 1D chains formed by π–π stacking of adjacent phen co-ligands. Complexes 6 contain the monoanionic complex [Ce(phen)2(dcnm)4] with two geometric isomers present in the crystal structure of 6a/b. Complex 7 forms extended 1D chains via hydrogen bonding between coordinated water and chloride atoms and an extensive array of face-to-face π interactions.