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41899-B3
The Synthesis and Characterization of Chiral Platinum(II) Extended Linear Chain Materials and Their Potential Application as Gas Sensing Transducers
The development of new gas sensing technologies is predicated on the discovery of new materials that respond selectively to volatile organic compounds (VOC). This research project involves the synthesis and characterization of new platinum(II) extended linear chain (ELC) materials, composed of chiral isonitrile ligands and cyanide ligands, that have the potential of being enantiomerically selective for chiral VOC. The platinum(II) ELC materials studied are composed of stacks of alternating square planar platinum(II) dications and tetracyanoplatinate(II) dianions commonly known as "double salts." The stacked alternating dications and dianions are linked together by weak Pt-Pt bonds. Care is taken to ensure a mismatch in size between the dication and dianion leading to void spaces in the resulting crystalline solids. This allows for the facile penetration of VOC deep into the lattice. The combination of a porous solid-state structure and the formation of Pt-Pt bonds creates a material that is "vapochromic."
Research during the summer of 2007 focused on the synthesis and characterization of additional new platinum(II) ELC materials containing tert-butyl substituted polypyridyl ligands in addition to chiral isonitrile ligands (note that tert-butyl substituted polypyridines were used to maximize the size of the dication). The goal was to investigate whether the chiral selectivity being observed in tetrakis double salts such as enantiomerically pure [Pt(b-methylphenethylisonitrile)4][Pt(II)(CN)4] could be observed in bis- and mono-substituted isonitrile double salts if the other sites on the platinum dication were occupied by tert-butyl substituted bipyridine or terpyridine, respectively. In addition we wanted to investigate whether more hindered chiral isonitriles that will not form tetrakis double salts might form bis or mono isonitrile double salts. Finally, we investigated the vapochromic characteristics of these new materials to see if any of them displayed chiral selectivity using 2-butanol as a test vapor.
Enatiomerically pure isonitrile ligands (CNR) were synthesized using established techniques.1,2 Over the past year chiral isonitrile ligands were synthesized from the corresponding R and S amine enantiomers: a-methylbenzylamine (1), b-methylphenethylamine (2), 1,2,3,4-tetrahydro-1-naphthylamine (3), and 1-(1-naphthyl)ethylamine (4). Previous research had shown that tetrakis double salts could not be obtained for chiral isonitriles of CN1, CN3, and CN4. The tetrakis double salt of CN2 is moderately stable and shows evidence of enantiomeric selectivity using 2-butanol as a test vapor. Mono isonitrile double salts containing enantiomerically pure CN1, CN2, CN3, and CN4 and 4,4',4"tri-tert-butyl-2,2':6',2"terpyridine (tBu3-trpy) were synthesized. The starting material [Pt(tBu3-trpy)CH3CN](CF3SO3)2 was synthesized from equimolar amounts of [Pt(CH3CN)4](CF3SO3)23 and tBu3-trpy that were refluxed in acetonitrile for 48 hours.4 The mono isonitrile double salt was then made by mixing equimolar amounts of enantiomerically pure isonitrle and [Pt(tBu3-trpy)CH3CN](CF3SO3)2 in acetonitrile followed by addition of (TBA)2[Pt(CN)4]5 to affect formation of the double salt. Enantiomerically pure mono isonitrile double salts containing chiral isonitriles of CN1, CN2, CN3, and CN4 were successfully obtained. Initial synthetic experiments were also performed to make bis isonitrile double salts containing 4,4'-di-tert-butyl-2,2'-bipyridyl (tBu2-bpy). First, Pt(tBu2-bpy)Cl2 was synthesized using a literature preparation.6 [Pt(tBu2-bpy)(CH3CN)2](CF3SO3)2 was obtained by reacting Pt(tBu2-bpy)Cl2 with a four-fold excess of AgCF3SO3 in refluxing acetonitrile for 24 hours.7 Two enantiomerically pure double salt presumed to be [Pt(tBu2-bpy)(R-CN2)2][Pt(CN)4] and [Pt(tBu2-bpy)(S-CN2)2][Pt(CN)4] have been obtained by reacting [Pt(tBu2-bpy)(CH3CN)2](CF3SO3)2, CN2, and (TBA)2[Pt(CN)4] in acetonitrile.
Enantiomerically pure [Pt(tBu3-trpy)CN1][Pt(CN)4], [Pt(tBu3-trpy)CN2][Pt(CN)4], [Pt(tBu3-trpy)CN3][Pt(CN)4], [Pt(tBu3-trpy)CN4][Pt(CN)4], and [Pt(tBu2-bpy)(CN2)2][Pt(CN)4] are red to purple colored fluorescent solids with CN stretches characteristic of platinum double salt materials. The tBu3-trpy containing mono isonitrile double salts are quite stable and melt well above 100¼C. A survey of the vapochromic response of the tBu3-trpy containing mono isonitrile double salt materials indicted that they were vapochromic, but did not show any evidence of enantiomeric selectivity between R- and S-2-butanol. Vapochromic studies of [Pt(tBu2-bpy)(CN2)2][Pt(CN)4] are planned for the near future.
References
1. C. A. Daws, C. L. Exstrom, J. R. Sowa Jr., K. R. Mann, "Vapochromic compounds as environmental sensors. 2. Synthesis and near-infrared and infrared spectroscopy studies of [Pt(arylisocyanide)4][Pt(CN)4] upon exposure to volatile organic compound vapors," Chem. Mater. 1997, 9, 363-368.
2. Ivar Ugi, Rudolf Meyr, "o-Tolyl isocyanide," Org. Synth. 1961, 41, 101-104.
3. Ola F. Wendt, Nils-Fredrik K. Kaiser, Lars I. Elding, "Acetonitrile and propionitrile exchange at palladium(II) and platinum(II)," J. Chem. Soc., Dalton Trans. 1997, 4733-4737.
4. J. R. Burney, PhD Thesis, University of Minnesota, "Synthesis and Characterization of Platinum (II) Complexes for Use as Environmental Sensors," 2006, 112.
5. W. R. Mason, H. B. Gray, "Electronic structures of square-planar complexes," J. Am. Chem. Soc. 1968, 90, 5721-5729.
6. T. J. Egan, K. R. Koch, P. L. Swan, C. Clarkson, D. A. Van Schalkwyk, P. J. Smith, "In Vivo Antimalarial Activity of a Series of Cationic 2,2'-Bipyridyl- and 1,10-Penanthrolineplatinum(II) Benzoylthiourea Complexes," J. Med. Chem. 2004, 47, 2926-2934.
7. J. S. Field, R. J. Haines, G. C. Summerton, "Synthesis and Crystal Structure Determination of the Triflate Salt of Diacetonitrile(2,2'-bipyridine)platinum(II)," J. Coord. Chem. 2003, 56, 1149-1155.
