Planar Four-Coordinate Carbon DOI: 10.1002/anie.201002483 A Monomeric Dilithio Methandiide with a Distorted trans-Planar Four-Coordinate Carbon** OliverJ. Cooper, AshleyJ. Wooles, Jonathan McMaster, William Lewis, Alexander J. Blake, and Stephen T. Liddle* The proposal of tetrahedral geometry for four-coordinate carbon (I) by vant Hoff in 1874 was a seminal advance in the understanding of the structural properties of molecules, and this theory subsequently became a cornerstone of organic chemistry. [1] This paradigm of tetrahedral four-coordinate carbon appeared absolute for many years. Nevertheless, chemists have long been fascinated with the concept that compounds exhibiting a planar four-coordinate carbon geometry might be found (II). [2] Nearly a hundred years after vant Hoffs proposal, planar four-coordinate carbon became more seri- ously considered after Hoffmann described a theoretical model for this geometry in which an sp 2 -hybridized carbon forms two two-electron-two-center (2e,2c) bonds and a two- electron-three-center (2e,3c) bond to the four substituents (III). [3] For the simplest example, methane, the tetrahedral geometry is more stable relative to the planar form by about 530 kJ mol À1 . [3, 4] Therefore, to experimentally realize planar four-coordinate carbon, various strategies to impose a planar four-coordinate geometry have been pursued, and of these, organometallic derivatives have delivered the most significant advances. [5] For genuine methanes, von Schleyer showed theoreti- cally [6] and experimentally [7] that the sequential replacement of hydrogen atoms with alkali metals results in an increase in the stabilization of planar four-coordinate methanes. Fur- thermore, for disubstituted methane derivatives, such as H 2 CLi 2 , the trans-planar form (IV) was found to be destabi- lized by about 125 kJ mol À1 relative to the cis-planar form (V). [6] Although the linear 2e,3c p orbital in the trans geometry is reminiscent of the allyl cation, the cis form is more stable because the 2e,3c p orbital can be considered homoaromatic and iso-conjugate to the cyclopropenium cation (VI). The greater stabilization of a cis- relative to trans-planar four-coordinate carbon suggests that cis-planar methanes should be more prevalent than their trans isomers. Gas-phase theoretical models of H 2 CMM’ (M = Li; M’ = Li–Cs) show dimers of typically D 2d symmetry that can be considered to be derived from cis-planar H 2 CMM’ units containing a formal four-coordinate carbon center. [8, 9] As the simplest prototype H 2 CLi 2 adopts a polymeric salt-like structure, [10] investiga- tions have targeted methandiides with bulky solubilizing groups. [11] The groups of Cavell and of Stephan simultane- ously reported the dimeric complex [{Li 2 (bipm N-TMS )} 2 ] (bipm N-TMS = C(PPh 2 NSiMe 3 ) 2 ). [12, 13] The groups of Hender- son and of Harder have subsequently reported the dimeric homo- and heterometallic heavier alkali metal congeners [{MM’(bipm N-TMS )} 2 ] (M = Li; M’ = Li, Na, K) [14] and [{M 2 - (bipm N-Ph )} 2 ] (M = K, Rb). [9] We have reported the dimeric dilithio complex [{Li 2 (bipm N-Mes )} 2 ] (Mes = 2,4,6-Me 3 C 6 H 2 ), [15] and Le Floch and MØzailles have reported dimeric [{Li 2 - (bipm N-R )} 2 ] (R = (S)-MeCHiPr) and [{Li 2 (C[PPh 2 S] 2 )} 2 ·n (OEt 2 )] (n = 2, 3). [16] In all cases, these dilithio methandiides can be considered to formally derive from the orthogonal dimerization of two planar [R 2 CMM’] units that contain a cis- planar four-coordinate carbon atom. Notable by their absence are the corresponding monomeric and trans-planar conge- ners. Herein, we describe the synthesis of a monomeric dilithio methandiide complex 3 that is notable for the fact it exhibits a distorted trans-planar four-coordinate carbon atom; this compound was obtained by the simple addition of the chelating diamine tetramethylethylenediamine (tmeda) to aid deprotonation (Scheme 1). As part of our investigations of f-block methandiides, [15, 17] we have utilized dilithio methandiides such as [{Li 2 - (bipm N-TMS )} 2 ] and [{Li 2 (bipm N-Mes )} 2 ] as ligand-transfer agents. However, to tune ligand substitution patterns, we investigated the use of the sterically demanding methane derivative H 2 (bipm N-Dipp )(1; Dipp = 2,6-diisopropylphenyl) [18] [*] O.J. Cooper, A.J. Wooles, Dr. J. McMaster, Dr. W. Lewis, Prof. A. J. Blake, Dr. S. T. Liddle School of Chemistry, University of Nottingham University Park, Nottingham, NG7 2RD (UK) Fax: (+ 44) 115-951-3563 E-mail: stephen.liddle@nottingham.ac.uk [**] We thank the Royal Society, the EPSRC, the European Research Council, and the University of Nottingham for generously support- ing this work. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201002483. Communications 5570  2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2010, 49, 5570 –5573