An effective synthesis of alkyl β-cyano-α,γ-diketones using chlorosulfonylisocyanate and a representative Cu(II) complex
Synthesis and characterization of three ambidentate β-cyano-α,γ-diketones is accomplished using chlorosulfonylisocyanate and dimethylformamide. These ambidentate ligands are isolated in high yields and excellent purity. Furthermore, two Cu(II) complexes were isolated and fully characterized including the single crystal X-ray structure of bis(4-cyano-2,2,6,6-tetramethyl-3,5-heptanedionato)copper(II), in which the solid-state structure is dominated by symmetry-related intermolecular CuNC contacts averaging 2.563 Å.
Introduction
Controlling intermolecular forces is the crux of crystal engineering or ‘materials by design’. Currently, establishing criteria to predictably construct new materials using discrete organic and inorganic molecules is under intense investigation [1], [2], [3]. A theme often exploited in organic compounds is hydrogen bonding, a concept recognized by Etter [4]. However, identifying structural commonalities in inorganic systems remains challenging as weaker secondary interactions, maximized by larger available coordination numbers, often dominate solid-state packing motifs. This is unfortunate given the technological opportunities of metal-based optical, electronic, and magnetic materials, porous solids, and new heterogeneous catalysts.
Ligand design is the critical component in fabricating new extended inorganic structures. Successful chelates are ambidentate (i.e. support secondary interactions) and display appropriate shape and orbital topology. The latter is particularly relevant when either transport or magnetic properties are of interest [3]. Furthermore, inexpensive, air- and moisture-stable ligands whose syntheses and purification proceed in straightforward, high-yielding procedures also offer inherent advantages. Considering these factors metal diketonates, undoubtedly the most studied inorganic chelates [5], seem obvious candidates as molecular subunits in extended solids; however, examples that support dative bonds are rare. Recently, β-diazo [6], β-pyridyl [7], and β-halogen substituents [8], capable of supporting secondary interactions through exocyclic atoms have appeared, but reports of simple β-cyano diketonates [9], [10], [11], [12], [13], [14], [15] remain sparse (Fig. 1). We believe this is largely attributable to their formidable synthesis [16], [17], [18], [19] which traditionally involved toxic cyanogen gas [9], [20], and alternate methods [15] are actively being explored driven by the rich coordination chemistry [10], [11], [12], [13], [14] and interesting magnetic properties [11], [21]. Herein, we report a safe and general preparation of three β-cyano-α,γ-diketones and a representative Cu(II) complex derived from 4-cyano-2,2,6,6-tetramethyl-3,5-heptanedione.
Section snippets
Materials and methods
Dichloromethane (CH2Cl2) and N,N-dimethylformamide (DMF) were dried and distilled over P2O5 and CaH2, respectively using standard Schlenk techniques. The diketones: 2,4-pentanedione (acacH, Acros), 2,2,6,6-tetramethyl-3,5-heptanedione (dpmH, Aldrich), and 7,7-dimethyl-3,5-hexanedione [22] were all distilled from Na2CO3 immediately before use. Chlorosulfonylisocyanate (ClSO2NCO, Acros), Cu(NO3)2·2.5H2O (Aldrich), propylamine (C3H9N, Acros), ethanol (EtOH, Pharmco), and tetrahydrofuran (THF,
Ligand synthesis
The organic synthesis of compounds 1–3 is accomplished with ClSO2NCO, DMF, and the appropriate diketone. Previously, these conditions were established as an effective source for ‘electrophilic’ cyanide addition to enolizable ketones and aldehydes [23], [24]. In optimizing our reaction variables, distilling all diketones from Na2CO3 immediately prior to use greatly increased product yield, presumably by removing trace acidic impurities. This is also supported by the observation that any CF3
Summary
This report describes the straightforward synthesis, purification, and complete characterization of three β-cyano-α,γ-diketones using chlorosulfonylisocyanate and dimethylformamide. Previously, routes to these ambidentate ligands involved toxic cyanogen gas and yielded a variety of side products. Also included is the synthesis and X-ray crystal structure of the new homoleptic complex, bis(4-cyano-2,2,6,6-tetramethyl-3,5-heptanedionato)copper(II), and the complete characterization of
Supplementary material
Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Center, CCDC No. 146534 and 146535 for compounds 1 and 4, respectively. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44-1223-336-033; email: [email protected] or www: http://www.ccdc.cam.ac.uk).
Acknowledgements
The University of Delaware acknowledges the National Science Foundation for the purchase of a CCD-based diffractometer (CHE-9628768), C.M.S. acknowledges the University of Nebraska for a research fellowship, and J.A.B. acknowledges the UNL-Center for Materials Research and Analysis.
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