-5.00E-05

-4.00E-05

-3.00E-05

-2.00E-05

-1.00E-05

0.00E+00

1.00E-05

2.00E-05

3.00E-05

4.00E-05

5.00E-05

0200400600800100012001400

mV

A

Preparation of a 4-(Phosphonato) Pyridine Ligand That Can EfficientlyBind Metal Complexes to Transparent Semiconductive Substrates

Aimée L. Fay Aimée L. Fay Cliff J. Timpson, Daniel D. Von Riesen and Michael P. BaloghCliff J. Timpson, Daniel D. Von Riesen and Michael P. Balogh

Roger Williams University, One Old Ferry Road, Bristol, RI 02809-2921Roger Williams University, One Old Ferry Road, Bristol, RI 02809-2921

AbstractThree 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.

Methods and MaterialsSpectroscopic 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.

DiscussionAttempts 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.

AcknowledgmentsFinancial support for travel and supplies from RWU Provost-Sponsored Student Scholarship Research AwardFinancial support for travel and supplies from RWU Provost-Sponsored Student Scholarship Research AwardFinancial support for chemicals and lab supplies from RWU Committee for Undergraduate Research Financial support for chemicals and lab supplies from RWU Committee for Undergraduate Research Dr. Paul Lefebre from Waters Corporation (Milford, MA) for mass directed chromatographyDr. Paul Lefebre from Waters Corporation (Milford, MA) for mass directed chromatographyMr. Randy Petrichko (Astra-Zeneca, Worchester, MA) for the NMR dataMr. Randy Petrichko (Astra-Zeneca, Worchester, MA) for the NMR dataMr. David Futoma (RWU adjunct faculty) for his endless patience and help in the labMr. David Futoma (RWU adjunct faculty) for his endless patience and help in the labMr. Jay Hill (RWU ‘05) for all the good natured assistance in the labMr. Jay Hill (RWU ‘05) for all the good natured assistance in the labRWU Facilities and Stockroom PersonnelRWU Facilities and Stockroom Personnel

References1. 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.

www.rwu.edu

IntroductionThe 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

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.

P31 NMR

VerticalScaleFactor = 1

200 180 160 140 120 100 80 60 40 20 0 -20 -40 -60 -80 -100 -120 -140 -160 -180 -200Chemical Shift (ppm)

0

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

Nor

mal

ized

Inte

nsity

0.95

N P

O

OEt

OEt

Figure 2. 1H and 31P NMR of 4-(diethylphosphonato) pyridine

Figure 3. Synthesis of [Cl(py)4Ru(py-PO3H2)]2+

Figure 1. Synthesis of 4-(diethylphosphonato) pyridine

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+

-3.50E-05

-3.00E-05

-2.50E-05

-2.00E-05

-1.50E-05

-1.00E-05

-5.00E-06

0.00E+00

5.00E-06

1.00E-05

1.50E-05

0200400600800100012001400

mV

A

P

O

H

O

ONPO

O

O

N

Br

P

O

H

O

ONPO

O

O

N

Br

N

C

N C

P

O

H

O

O P

OO

O

N C

P

OO

O

Pd(PPh3)4

+

Toluene, Triethylamine, Ar0 reflux 16 h

+Pd(PPh3)4

Triethylamine, Ar0

reflux 5 h1a

1b

Na+Na0

1c

+

1e

1d

+

+

CH2Cl2

C6H6

+

NPO

O

O

[Cl(py)4Ru(py-PO3H2)]2+ [Cl(py)4Ru(py-PO3Et2)]2+

CH3CN / 0.1M TBAHAg+/AgCl referenceScan rate = 200 mV/s

CH3CN / 0.1M TBAHAg+/AgCl referenceScan rate = 200 mV/s

Ru NOCl

py py

py py

Ru NCl

py py

py py

P

O

OEt

OEt

Ru NCl

py py

py py

P

O

OH

OH

Ru NCl

py py

py py

P

O

OEt

OEt

2+

acetone stir 30 mat 25oC

2 Eq. Me3SiBr

NaN3

stir 1 h at 25oC

1 Eq. L

1. CH2Cl2, stir 16 h2. MeOH

E1/2 = 842.5 mVE peak = 53 mV