|We are quite used to a trigonal pyramidal molecular geometry for trivalent nitrogen and a tetrahedral one for tetravalant carbon. It does not have to be that way. For example in the compound class of the fenestranes the central carbon atom is flattened and so is the central nitrogen atom in triisopropylamine. Two recent publications describe more flattening: of nitrogen and of carbon.|
Livant et al. synthesised tris(1,3-dihydroxy-2-propyl)amine (DOI) and determined its X-ray structure. The degree of planarity for this molecule can be measured as the height of the nitrogen atom above the carbon plane (0.082 angstrom) and by the sum of CNC angles (359°). In an ordinary amine these values are 0.4 A and 321°. The molecule has two internal hydrogen atoms and every oxygen atom is both a hydrogen-bond donor and an acceptor. The nitrogen center is flattened for two reasons: a steric contribution is crowding of three hydroxymethyl groups when the nitrogen atom would move away from planarity. The hydrogen-bond network may also play a role. The electronic contribution is a favorable interaction between the nitrogen lone pair and the antibonding C-O sigma orbitals. The shortening of the C-N bonds - 1.454 angstrom vs 1.47 angstrom - is taken as evidence. The basicity is very low - pKa 3.08, compare triethylamine pKa = 10 - and the protonated molecule is flattened as well more resembling trigonal planar than tetrahedral.
Liddle et al. present (DOI) a distorted trans-planar 4 coordinate carbon atom in a complex molecule constructed from a diphosphazene CH2(P(Ph)2=N(dipp))2, buLi and TMEDA. It is basically a methyl group with two phosphazene groups and twice deprotonated and replaced by lithium. The Angewandte helpfully added the header planar 4-coordinate carbon to the article but what exactly is the geometry? The phrase The root-mean square deviation from the mean plane of C1, P1, P2, Li1, and Li2 is only 0.34 angstrom, and C1 deviates from this plane by 0.007(2) angstrom is not at all help full. The atoms making up the trans-planar geometry are not identified. The P-C-P angle of 132° and the Li-C-Li angle of 161° certainly do not imply overall planarity. The longest Li-C bond length is 2.53 angstrom but as that same lithium atom also connects to one of the dipp isopropyl carbon atoms by 2.66 angstrom one must wonder if that Li-C bond is a true bond.
Jie, Y., Livant, P., Li, H., Yang, M., Zhu, W., Cammarata, V., Almond, P., Sullens, T., Qin, Y., & Bakker, E. (2010). An Acyclic Trialkylamine Virtually Planar at Nitrogen. Some Chemical Consequences of Nitrogen Planarity The Journal of Organic Chemistry, 75 (13), 4472-4479 DOI: 10.1021/jo100628v
Cooper, O., Wooles, A., McMaster, J., Lewis, W., Blake, A., & Liddle, S. (2010). A Monomeric Dilithio Methandiide with a Distorted trans-Planar Four-Coordinate Carbon Angewandte Chemie International Edition DOI: 10.1002/anie.201002483