FULL PAPER DOI: 10.1002/ejoc.200600847 Supramolecular Structures by Hydrogen Bonding: The Solid-State Structure of Tris[3-(3-dimethylamino-1-oxoprop-2-enyl)phenyl]phosphane Oxide Andreas Reis, [a] Yu Sun, [a] Gotthelf Wolmershäuser, [a] and Werner R. Thiel* [a] Keywords: Solid-state structures / Hydrogen bonds / Phosphane oxides Tris[3-(3-dimethylamino-1-oxoprop-2-enyl)phenyl]phosphane oxide, which can be synthesized quantitatively by oxidation of the corresponding phosphane, crystallizes in the trigonal space group R3 ¯ . Because of the four different strong proton accepting sites (1  P=O, 3  C=O), this molecule can form P=O···H–O–H···O=P and C=O···H–C hydrogen bonds in the Introduction Hydrogen bonds are of fundamental importance for the world we are living in. This fact is mainly related to the hydrogen bond strength (up to about 40 kJ/mol), which al- lows reversible bond formation and cleavage under ambient conditions. [1] The resulting dynamics, especially in protic and/or polar solvents like water, enables a hydrogen bond- ing system, in contrast to a purely covalently bound struc- ture, to adapt to the requirements of its environment. [2] This reveals the relevance of hydrogen bonding in molecular biology and biochemistry. The secondary structures of proteins, for example, are strongly determined by intramolecular hydrogen bonds mainly performed between the proton donating and ac- cepting sites of the amide units [-C(O)–NH-]. Here, the in- volvement of the N–H group as a proton donor in hydrogen bonding increases the hydrogen accepting feature of the car- bonyl moiety and vice versa. Removal of the proton of the N–H group against a carbon substituent leaves a strong hy- drogen acceptor, which manifests itself in polar compounds such as dmf, where the dimethylamino moiety enhances the electron density at the carbonyl oxygen atom. Introduction of a -CH=CH- fragment into the C–N bond results in vi- nylogous 3-dimethylaminoprop-2-en-1-ones. Such mole- cules, which are accessible in high yields by reacting an aryl methyl ketone with an activated dmf analogue have recently attracted our interest as precursors for the synthesis of pyr- azole- and pyrimidine-based ligands. [3] Additionally, 3-di- methylaminoprop-2-en-1-ones are powerful proton ac- cepting compounds, which we were able to prove spectro- scopically and by quantum chemical calculations as well as [a] Fachbereich Chemie, Technische Universität Kaiserslautern, Erwin-Schrödinger-Str., Geb. 54, 67663 Kaiserslautern, Germany Fax: +49-631-2054676 E-mail: thiel@chemie.uni-kl.de Eur. J. Org. Chem. 2007, 777–781 © 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 777 solid state, which results in the generation of two interpen- etrating networks. In the cavities of the solid-state structure, water and acetone are enclosed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) by the elucidation of several solid-state structures. [4] The proton accepting power of the electron rich carbonyl group of such compounds enables intermolecular C=O···H–C hy- drogen bonding with the protons of the –CH=CH– frag- ment or the NMe 2 groups of a neighbouring molecule. In the present manuscript, we will show that the specific intro- duction of multiple 3-dimethylaminoprop-2-en-1-one units allows to design the solid-state structure of such systems. The introduction of two 3-dimethylaminoprop-2-en-1- one side chains at one central unit leads to chain-type hy- drogen bound aggregates, as realized, for example, in the solid-state structure of 2,6-bis(3-dimethylamino-1-oxoprop- 2-enyl)pyridine. [4b] By adding a third 3-dimethylaminoprop- 2-en-1-one function to one planar central unit, as it is real- ized in 1,3,5-tris(3-dimethylamino-1-oxoprop-2-enyl)ben- zene, should thus make layer-type structures accessible. Crystallization from wet acetone gives the trihydrate of this compound wherein the water molecules act as proton do- nors and acceptors in the expected two-dimensional hydro- gen bonded network. However, cocrystallization with larger proton donors like hydroquinone or 4,4'-dihydroxybiphenyl gives linear structures wherein only two of the three 3-di- methylaminoprop-2-en-1-one side chains are engaged in hy- drogen bonding, presumably this is due to steric reasons. [4a] By following something like a crystal engineering strategy, the steric problems can be overcome by changing the core from a planar 1,3,5-trisubstituded benzene into a pyramidal arrangement like it is, for example, realized in triarylphos- phanes. A similar strategy is realized in supramolecular coordina- tion chemistry where the thermodynamically reversible co- ordination of transition metal building blocks to symmetri- cally substituted donor ligands is used to build up oligonu- clear aggregates of nanometer dimension. [5] M. Albrecht re- cently highlighted the supramolecular chemistry of C 3 -sym- metric ligands leading to C 3 -symmetric structures. [6]