Preparation of a 4-(Phosphonato) Pyridine Ligand That Can Efficiently

1. P31 NMR Figure 2. 1H and 31P NMR of 4-(diethylphosphonato) pyridine www.rwu.edu Preparation of a 4-(Phosphonato) Pyridine Ligand That Can Efficiently Bind Metal Complexes to Transparent Semiconductive Substrates Aimée L. Fay Cliff J. Timpson, Daniel D. Von Riesen and Michael P. Balogh Roger Williams University, One Old Ferry Road, Bristol, RI 02809-2921 Abstract Three different synthetic approaches to produce the compound 4-(diethylphosphonato) pyridine were attempted. The first two approaches involved attempts to couple diethyl phosphite with 4-bromopyridine via palladium (0) catalyzed coupling reactions. The third approach involved direct substitution of an activated pyridine intermediate with diethyl phosphide anion. In general, the palladium catalyzed reactions resulted in fair yields (~30%) but required a significant amount of chromatograpy to remove unwanted oxidation products. Direct substitution of pyridine with diethyl phosphide anion ultimately yielded ~25% of the desired product in greater than 99% purity by vacuum distillation. Conversion of 4-diethylphosphonato pyridine into the diacid by de-esterification of the diethylphosphonate ester was easily accomplished in near quantitative yield. Preliminary studies reveal the compound can act as a ligand to efficiently bind metal metal complexes to hydroxylated glass and/or semiconductive substrates. Discussion Attempts to produce 4-(diethylphosphonato) pyridine by modifying palladium (0) catalyzed coupling procedures developed by Grätzel et al.5 for the synthesis of 4,4-bis (diethylphosphonato) bipyridine and by Lin et al.6 for the diethylphosphonated pyridine generally resulted in low yields of the desired product. LC-MS analyses of fractions obtained by column chromatography revealed that the desired product was in fact formed in fair yield in our synthetic attempts but it also indicated that it was quite difficult to isolate reasonably pure product. Compounds identified by LC-MS to be present in even our best chromatographic fractions included, triphenyl phosphine, triphenyl phosphine oxide, diethyl phosphite starting material and minor products resulting from the reaction of diethyl phosphite with acetone. We have found that generation of the carbenium-pyridinium salt followed by reaction with the sodium salt of diethyl phosphide as described by Redmore7 leads to reasonable yields (~25%) of the desired product in high purity (>99%) following simple vacuum distillation of the crude product. Once the 4-(diethylphosphonato) pyridine was obtained, we have been able to quantitatively convert a fraction of the product into the corresponding compound 4-(di-acid phosphonato) pyridine by reacting the phosphonate diester with Me3SiBr. Furthermore, we have found it possible to produce metal complexes of the type trans-[Cl(py)4Ru(py-PO3H2)]2+ by first attaching the 4-(diethylphosphonato)pyridine ligand to the metal complex followed by subsequent reaction of the metal complex with Me3SiBr to yield the diacid phosphonato complex. In addition, we have demonstrated that it is possible to attach trans-[Cl(py)4Ru(py-PO3H2)]2+ to hydroxylated ITO electrodes via the –PO3H2 functionality. Attempts to attach trans-[Cl(py)4Ru(py-PO3Et2)]2+ to ITO electrodes did not yield any bound complex. Comparison studies of metal complexes bound to ITO electrodes via py-PO3H2 versus commercially available py-COOH (isonicotinic acid) are currently underway. Introduction The synthesis, characterization and study of photo/redox active molecules attached to oxide surfaces has been the focus of a number of studies over the last decade.1 In many of these studies, photo/redox active transition metal complexes have been covalently bound to oxide surfaces via ligands that possess the R-PO3H2 phosphonic acid functionality.2 We are specifically interested in producing metal complexes of the type trans-[ClRu(pyridine)4(py-PO3H2)]+ where py-PO3H2 is 4-phosphonatopyridine. We are generally interested in compounds of this type for two reasons: First, the phosphonic acid functionality should allow us to bind metal complexes to glass and semiconductor substrates thus allowing direct comparisons of the binding affinity of the py-PO3H2 versus the readily obtained py-COOH isonicotinic acid derivative.3 And second, we hope to ultimately activate the coordination site trans- to the 4-phosphonatopyridine ligand in order to build oligomers of metal complexes attached to surfaces with fixed geometries that may lead to interesting photo/redox properties.4 Figure 1. Synthesis of 4-(diethylphosphonato) pyridine • Conclusions • 4-(diethylphosphonato) pyridine can be synthesized either by palladium catalyzed coupling of 4-bromopyridine • and diethylphosphite or by direct reaction of activated pyridine with the diethylphosphide anion. • 4-(diethylphosphonato) pyridine can be quantitatively converted to 4-(phosphonic acid) pyridine by • conventional de-esterification with Me3SiBr. • 4-(diethylphosphonato) pyridine can be easily reacted with the metal complex trans-[Cl(py)4Ru(NO)]2+ to • produce trans-[Cl(py)4Ru(4-diethylphosphonato-pyridine)]2+ in good yield. • The trans-[Cl(py)4Ru(4-(diethylphosphonato) pyridine)]2+ complex can be converted to • trans-[Cl(py)4Ru(4-phosphonic acid pyridine)]2+ with Me3SiBr. • Direct reaction of trans-[Cl(py)4Ru(NO)]2+ with 4-phosphonic acid pyridine in acetone was significantly • hindered by the insolubility of the acid. • Preliminary electrochemical studies suggest the trans-[Cl(py)4Ru(4-phosphonic acid pyridine)]2+ can be • attached to hydroxylated surfaces such as glass or ITO substrates with high efficiency in contrast to • the related diester complex which does not show any evidence of binding to the substrate. Methods and Materials Spectroscopic grade solvents (Aldrich, Fisher) and reagents (Aldrich) were obtained commercially. Solvents were either freshly distilled and/or dried over molecular sieves prior to use. All reactions were conducted under an argon atmosphere. Column chromatography was carried out using silica gel 60 (70-230 mesh) (Aldrich) with either dichloromethane:methanol (5% to 15% MeOH) or chloroform:acetone (2:1 V/V) as the eluent.  All chromatography solvents were HPLC grade and were used as received. LC-MS data was collected using a Waters 2690 Separations Module equipped with a Waters 996 Photodiode Array detector and a Waters Micromass ZQ mass spectrometer.  UV-Vis data were collected on a Hewlett-Packard HP-8453 Diode Array spectrophotometer.  Infrared data was collected as either thin films on NaCl plates or as KBr pellets on a nitrogen-purged Perkin-Elmer 1600 series FT-IR. Cyclic voltammetric measurements were obtained in 0.1M TBAH/CH3CN using a Bio-Analytical Systems (BAS) CV-50W.  All E1/2 values are reported versus a Ag+/AgCl reference electrode. Transparent semiconductive electrodes, In2O3 doped Tin Oxide (ITO) on float glass, were obtained commercially from Delta Technologies (Stillwater, MN) and were used as supplied. Acknowledgments Financial support for travel and supplies from RWU Provost-Sponsored Student Scholarship Research Award Financial support for chemicals and lab supplies from RWU Committee for Undergraduate Research Dr. Paul Lefebre from Waters Corporation (Milford, MA) for mass directed chromatography Mr. Randy Petrichko (Astra-Zeneca, Worchester, MA) for the NMR data Mr. David Futoma (RWU adjunct faculty) for his endless patience and help in the lab Mr. Jay Hill (RWU ‘05) for all the good natured assistance in the lab RWU Facilities and Stockroom Personnel Figure 3. Synthesis of [Cl(py)4Ru(py-PO3H2)]2+ References 1. Roundhill, D. M. Photochemistry and Photophysics of Coordination Compounds, Wiley, New York, 1994. 2. Kalyanasundaram, K. Photochemistry of Polypyridine & Porphyrin Complexes, Academic Press, 1992. 3. Juris, A; Campagna, S.; Balzani, V.; Belser, P.; von Zelewsky, A. Coord. Chem. Rev.,1988, 84, 85. 4. Solar Energy Materials and Solar Cells, Lampert, C. M., 1994, Vol 32, No. 3. 5. Kalyanasundaram, K., Grätzel, M., et al. Inorg. Chem., 1997, 36, 5937. 6. Ayyappan, P.; Evans, O.; Foxman B.; Wheeler, K.; Warren, T.; Lin, W., Inorg. Chem., 2001, 40, 5954-5961. 7. Redmore, D. United States Patent 4,187,378. Feb. 5, 1980. 8. Coe, B.; Meyer, T. J.; White, P.S. Inorg. Chem., 1995, 34, 593. [Cl(py)4Ru(py-PO3H2)]2+ [Cl(py)4Ru(py-PO3Et2)]2+ E1/2 = 842.5 mV E peak = 53 mV CH3CN / 0.1M TBAH Ag+/AgCl reference Scan rate = 200 mV/s CH3CN / 0.1M TBAH Ag+/AgCl reference Scan rate = 200 mV/s Figure 4. Cyclic voltammograms of ITO electrodes exposed to mMolar solutions of [Cl(py)4Ru(py-PO3H2)]2+ and [Cl(py)4Ru(py-PO3Et2)]2+