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 eta(2)(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 eta(2) interactions dependent upon the intramolecular N-H center dot center dot center dot O=N hydrogen bonding. Additional intermolecular hydrogen bonding interactions exist between adjacent amide and nitrile groups giving rise to 3D alpha-Po and 6,8-connected (4(12).6(3))(4(20).6(8)) 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 eta(2) 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 eta(2) bonding of the nitroso group with decrease in the Ln(3+) radius. There is a corresponding increase of the Ln-O-N angle, and one ligand is eta(1)(O) binding in 4Nd. The dcnm ligands in the discrete complexes [La(phen)(3)(dcnm)((3-x))Cl-x], x approximate to 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 pi-pi 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 pi interactions.