Porphyrins Bearing Arylphosphonic Acid Tethers for Attachment to Oxide Surfaces

Article abstract of DOI:10.1021/jo034945l

Synthetic molecules bearing phosphonic acid groups can be readily attached to oxide surfaces. As part of a program in molecular-based information storage, we have developed routes for the synthesis of diverse porphyrinic compounds bearing phenylphosphonic acid tethers. The routes enable (1) incorporation of masked phosphonic acid groups in precursors for use in the rational synthesis of porphyrinic compounds and (2) derivatization of porphyrins with masked phosphonic acid groups. The precursors include dipyrromethanes, monoacyldipyrromethanes, and diacyldipyrromethanes. The tert-butyl group has been used to mask the dihydroxyphosphoryl substituent. The di-tertbutyloxyphosphoryl unit is stable to the range of conditions employed in syntheses of porphyrins and multiporphyrin arrays yet can be deprotected under mild conditions (TMS-C1/TEA or TMS-Br/TEA in refluxing CHCl₃) that do not cause demetalation of zinc or magnesium porphyrins. The porphyrinic compounds that have been prepared include (1) A₃B-, trans-AB ₂C-, and ABCD-porphyrins that bear a single phenylphosphonic acid group, (2) a trans-A₂B₂-porphyrin bearing two phenylphosphonic acid groups, (3) a chlorin that bears a single phenylphosphonic acid group, and (4) a porphyrin dyad bearing a single phenylphosphonic acid group. For selected porphyrin-phosphonic acids, the electrochemical characteristics have been investigated for molecules tethered to SiO₂ surfaces grown on doped Si. The voltammetric behavior indicates that the porphyrin-phosphonic acids form robust, electrically well-behaved monolayers on the oxide surface.

Full text of DOI:10.1021/jo034945l

P or p h yr in s Bea r in g Ar ylp h osp h on ic Acid Teth er s for Atta ch m en t
to Oxid e Su r fa ces
Kannan Muthukumaran,† Robert S. Loewe,† Arounaguiry Ambroise,† Shun-ichi Tamaru,†
Qiliang Li,‡ Guru Mathur,‡ David F. Bocian,*,§ Veena Misra,*,‡ and J onathan S. Lindsey*,†
Departments of Chemistry and Electrical and Computer Engineering, North Carolina State University,
Raleigh, North Carolina 27695-8204, and Department of Chemistry, University of California,
Riverside, California 92521-0403
jlindsey@ncsu.edu; vmisra@eos.ncsu.edu; david.bocian@ucr.edu
Received J uly 1, 2003
Synthetic molecules bearing phosphonic acid groups can be readily attached to oxide surfaces. As
part of a program in molecular-based information storage, we have developed routes for the synthesis
of diverse porphyrinic compounds bearing phenylphosphonic acid tethers. The routes enable (1)
incorporation of masked phosphonic acid groups in precursors for use in the rational synthesis of
porphyrinic compounds and (2) derivatization of porphyrins with masked phosphonic acid groups.
The precursors include dipyrromethanes, monoacyldipyrromethanes, and diacyldipyrromethanes.
The tert-butyl group has been used to mask the dihydroxyphosphoryl substituent. The di-tert-
butyloxyphosphoryl unit is stable to the range of conditions employed in syntheses of porphyrins
and multiporphyrin arrays yet can be deprotected under mild conditions (TMS-Cl/TEA or TMS-
Br/TEA in refluxing CHCl3) that do not cause demetalation of zinc or magnesium porphyrins. The
porphyrinic compounds that have been prepared include (1) A3B-, trans-AB2C-, and ABCD-
porphyrins that bear a single phenylphosphonic acid group, (2) a trans-A2B2-porphyrin bearing
two phenylphosphonic acid groups, (3) a chlorin that bears a single phenylphosphonic acid group,
and (4) a porphyrin dyad bearing a single phenylphosphonic acid group. For selected porphyrin-
phosphonic acids, the electrochemical characteristics have been investigated for molecules tethered
to SiO2 surfaces grown on doped Si. The voltammetric behavior indicates that the porphyrin-
phosphonic acids form robust, electrically well-behaved monolayers on the oxide surface.
In tr od u ction
We recently described a new design for a molecular-
based memory device wherein a layer of redox-active
molecules is tethered to an ultrathin dielectric surface,
which in turn is deposited on a semiconductor.1 The
dielectric layer and the molecular tether (linker and
surface attachment group) both provide barriers to
electron transfer between the semiconductor and the
redox-active molecule. The design can be implemented
as the gate region of a field effect transistor, as shown
in Figure 1. In this type of molecular-based field effect
transistor, the charge stored in the molecules can change
the current level in the transistor, thereby affording a
nondestructive means by which the charge state of the
molecules can be detected.
F IGURE 1. Schematic of an electrode-molecule-oxide-elec-
trode device for potential memory applications. The molecule
is composed of a redox-active unit and a tether (linker and
surface attachment group).
Great latitude also exists in the design of the barriers
presented by both the tether and the dielectric layer. The
barrier presented by the tether can be tuned via synthetic
organic chemistry, and that of the dielectric can be tuned
by semiconductor-processing techniques. In particular,
the composition (and length) of the tether can be varied
from insulating aliphatic groups to more conducting
conjugating species. Likewise the composition of the
dielectric layer can be a commonly used SiO2 layer or a
metal oxide such as HfO2, ZrO2, etc.
Our preliminary studies of the design shown in Figure
1 employed ferrocenylmethylphosphonic acid as the
charge-storage molecule.1 The phosphonic acid group
anchors the charge-storage molecule to the oxide surface.
The initial success of this approach has prompted us to
In contrast with conventional semiconductor-based
devices, the use of charge-storage molecules exploits the
power of synthetic design to tailor molecules that operate
at low voltage and that provide multiple charged states.
† Department of Chemistry, North Carolina State University.
‡ Department of Electrical and Computer Engineering, North
Carolina State University.
§ University of California.
(1) Li, Q.; Surthi, S.; Mathur, G.; Gowda, S.; Sorenson, T. A.; Tenent,
R. C.; Kuhr, W. G.; Tamaru, S.-I.; Lindsey, J . S.; Liu, Z.; Bocian, D. F.;
Misra, V. Appl. Phys. Lett. 2003, 83, 198-200.
10.1021/jo034945l CCC: $27.50 © 2004 American Chemical Society
Published on Web 12/11/2003
1444
J . Org. Chem. 2004, 69, 1444-1452

Porphyrins Bearing Arylphosphonic Acid Tethers
investigate the synthesis of a much wider variety of
charge-storage molecules, particularly porphyrinic mol-
ecules, which bear phosphonic acid terminated linkers.
Porphyrins bearing phosphonic acid tethers have been
synthesized and attached to oxide surfaces for a variety
of other applications including solar energy, oxidative
catalysis, sensing, and recognition of polysaccharides.2-14
The synthetic approaches that have been employed to
prepare porphyrins bearing phosphonic acid/phosphonate
units can be characterized by (1) whether the phospho-
nate unit is introduced into precursors to the porphyrin
or by derivatization of a preexisting porphyrin, (2)
whether statistical or rational routes are employed, (3)
the number and pattern of phosphonate groups at the
perimeter of the porphyrin, (4) the type of phosphonic
acid protecting group employed, (5) the nature of the
central metal, and (6) the method of cleavage of the
phosphonic acid protecting groups.
A4-Porphyrins bearing four arylphosphonic acids have
been prepared by condensation of a dialkoxyphosphoryl-
benzaldehyde with pyrrole followed by deprotection of the
free base porphyrin.2 Alternatively, the free base por-
phyrin can be metalated followed by deprotection.4,5 A4-
Porphyrins bearing four alkylphosphonic acids have been
prepared by derivatization of a reactive halo-substituted
porphyrin.5-7 A3B-Porphyrins bearing a single phospho-
nic acid have been prepared by a mixed-aldehyde con-
densation of a dialkoxyphosphorylbenzaldehyde, benzal-
dehyde, and pyrrole4 or by derivatization of a porphyrin
bearing a single reactive halo group.6,14 trans-A2B2-
Porphyrins bearing two phosphonic acid groups have
been prepared by condensation of a dialkoxyphosphoryl-
benzaldehyde and dipyrromethane.12 Chlorins bearing
two phosphonic acids have been prepared by derivatiza-
tion of a deuterochlorin-dibromide with tris(trimethylsi-
lyl)phosphite.9 In each case, the porphyrinic species were
employed as the free base or as a metal chelate that is
rather robust toward the acidic conditions for cleavage
of the dialkyl phosphonate. The metals include Mn,4,5 Fe,9
Co,9 Ni,9 Pd,6 and Os,7 which are all categorized in the
porphyrin field as class I or class II metals, affording
chelates that are exceptionally resilient toward acids.15
In general, phosphonic acids combine with metals to give
extended, often insoluble, metal phosphonates. A rare
case wherein metalation was performed in the presence
of a free phosphonic acid employed a porphyrin super-
structure containing a hindered phosphonic acid.14
In this paper, we first describe new strategies for the
synthesis of porphyrins bearing phenylphosphonic acid
tethers. This extends our prior work where diverse
porphyrinic compounds bearing alcohol, thiol, or selenol
tethers (in free or protected form) have been prepared
for attachment to metals or silicon.16 We employ the di-
tert-butyl group as a masking group for the phosphonic
acid, with predominant use of strategies wherein the di-
tert-butyl phosphonate is incorporated in precursors of
the porphyrin. In the course of this work, we have also
developed methods for unveiling the free phosphonic acid
that are compatible with zinc (class III) or magnesium
(class IV) chelates of porphyrins. We then describe the
electrochemical characteristics of selected porphyrin-
phosphonic acids tethered to SiO2 dielectric layers on Si
platforms. Collectively, these studies provide the basis
for the rational synthesis of a wide variety of porphyrins,
including porphyrin building blocks and multiporphyrin
arrays, bearing phenylphosphonic acid tethers and dem-
onstrate the robust electrical characteristics of the mol-
ecule-oxide-semiconductor architectures.
Resu lts a n d Discu ssion
1. Ap p r oa ch . A variety of protecting groups have been
used for phosphonic acids, including methyl, ethyl, allyl,
and tert-butyl groups.2-14,17-23 For our application, a key
issue concerns the stability of the metalloporphyrin
toward conditions employed for protecting group removal,
as inadvertent demetalation of the porphyrin would
complicate the synthesis of mixed-metal multiporphyrin
arrays. Accordingly, the ideal masking agent should meet
the following requirements: (1) compatibility with por-
phyrin forming conditions, including acid catalysis and
DDQ oxidation conditions, (2) stability toward a variety
of metalation conditions, (3) compatibility with Pd-
mediated coupling reactions, and (4) ability to undergo
cleavage without demetalation of the metalloporphyrins.
Mild conditions for the cleavage of a dialkyl phospho-
nate to give the phosphonic acid originate with Rabino-
witz, who first used trimethylsilyl chloride (TMS-Cl)
followed by hydrolysis of the resulting bis(trimethylsilyl)
phosphonate.24 Modifications of this approach have led
(2) Ungashe, S. B.; Wilson, W. L.; Katz, H. E.; Scheller, G. R.;
Putvinski, T. M. J . Am. Chem. Soc. 1992, 114, 8717-8719.
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(4) Deniaud, D.; Scho¨llorn, B.; Mansuy, D.; Rouxel, J .; Battioni, P.;
Bujoli, B. Chem. Mater. 1995, 7, 995-1000.
(5) Deniaud, D.; Spyroulias, G. A.; Bartoli, J .-F.; Battioni, P.;
Mansuy, D.; Pinel, C.; Odobel, F.; Bujoli, B. New J . Chem. 1998, 901-
905.
(15) Buchler, J . W. In Porphyrins and Metalloporphyrins; Smith,
K. M., Ed.; Elsevier Scientific Publishing Co.: Amsterdam, 1975; pp
157-231.
(6) Nixon, C. M.; Le Claire, K.; Odobel, F.; Bujoli, B.; Talham, D. R.
Chem. Mater. 1999, 11, 965-976.
(16) Balakumar, A.; Lysenko, A. B.; Carcel, C.; Malinovskii, V. L.;
Gryko, D. T.; Schweikart, K.-H.; Loewe, R. S.; Yasseri, A. A.; Liu, Z.;
Bocian, D. F.; Lindsey, J . S. J . Org. Chem. 2004, 69, 1435-1443.
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Tetrahedron Lett. 1977, 155-158.
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2621.
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H. K.; Nguyen, S. T.; Hupp, J . T. Polyhedron 2003, 22, 3065-3072.
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Talham, D. R. J . Am. Chem. Soc. 2002, 124, 4363-4370.
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G.; King, B. F.; Burnstock, G.; J acobson, K. A. J . Med. Chem. 2001,
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(24) Rabinowitz, R. J . Org. Chem. 1963, 28, 2975-2978.
J . Org. Chem, Vol. 69, No. 5, 2004 1445

Muthukumaran et al.
SCHEME 1
SCHEME 2
SCHEME 3
to the following conditions: (1) TMS-Br (neat);17 (2) TMS-
Br/CH3CN22 or CH2Cl2;20 (3) TMS-Cl/TEA;25 and (4) TMS-
Br/TEA/CH2Cl2.21 Some of these approaches have been
applied to the cleavage of a diethyl porphyrin-phospho-
nate: (1) TMS-Br/CH2Cl2 (free base porphyrins);6,7,12 (2)
TMS-Br/TEA/DMF (Mn-porphyrin);13 and (3) NaBr/TMS-
Cl/TEA/DMF (Mn-porphyrin).4
In this and the accompanying paper,26 we have exam-
ined methods for the introduction and cleavage of various
phosphonic acid protecting groups that are compatible
with the preparation of diverse porphyrinic compounds.
Several possible masking agents [2-trimethylsilylethyl,
(25) Sekine, M.; Iimura, S.; Nakanishi, T. Tetrahedron Lett. 1991,
32, 395-398.
(26) Loewe, R. S.; Ambroise, A.; Muthukumaran, K.; Padmaja, K.;
Lysenko, A. B.; Mathur, G.; Li, Q.; Bocian, D. F.; Misra, V.; Lindsey,
J . S. J . Org. Chem. 2004, 69, 1453-1460.
1446 J . Org. Chem., Vol. 69, No. 5, 2004

Porphyrins Bearing Arylphosphonic Acid Tethers
SCHEME 4
SCHEME 5
ryl)benzaldehyde4 (Scheme 1). Acetals can be used di-
rectly (like aldehydes) in the synthesis of porphyrins28
and dipyrromethanes.29 The mixed-aldehyde condensa-
tion30 of acetal 2, mesitaldehyde, and pyrrole with BF3‚
O(Et)2/ethanol cocatalysis31 afforded porphyrin 3a in 6.8%
yield after chromatography. Metalation of 3a using Zn-
(OAc)2‚2H2O in CHCl3/methanol gave zinc porphyrin
Zn 3a in 73% yield (Scheme 1). Cleavage of the tert-butyl
groups was achieved by following the known procedure25
with TMS-Cl/TEA in refluxing CHCl3 (stabilized with
amylenes). In this manner, the zinc porphyrin-phospho-
nic acid Zn 4 was obtained without demetalation in 89%
yield. Note that CHCl3 stabilized with amylenes rather
than ethanol was used to avoid the possible reaction of
ethanol with TMS-Cl.
The synthesis of magnesium porphyrin Mg3a was first
tried using the heterogeneous magnesium insertion
procedure.32 Porphyrin 3a was treated with MgI2 and
DIEA in CH2Cl2 at room temperature, but these condi-
tions resulted in cleavage of the tert-butyl groups.
However, magnesium insertion using the homogeneous
procedure33 (ethereal MgI2-DIEA reagent in CH2Cl2) gave
2-cyanoethyl, 2-chloroethyl, methyl] were examined but
found inapplicable for the preparation of porphyrin-
phosphonic acids (see Supporting Information). The tert-
butyl group has been used for the protection of phos-
phates in nucleotide syntheses with facile removal under
mild nonacidic conditions (TMS-Cl/TEA).25 We therefore
employed the tert-butyl group as masking agent in the
work described herein.
2. Syn th esis. The synthesis of porphyrins bearing a
phosphonate group was achieved following three different
strategies: (1) a mixed-aldehyde condensation of a phos-
phonate-derivatized benzaldehyde and an aldehyde of
choice (mesitaldehyde) with pyrrole, (2) condensation of
a phosphonate-derivatized dipyrromethane with a dipyr-
romethane-dicarbinol, and (3) derivatization of a bromo-
or iodo-substituted porphyrin with a phosphonate re-
agent.
A. F u n ction a liza tion of P r ecu r sor s of th e P or -
p h yr in . For porphyrin formation using a mixed-aldehyde
condensation, we required a phosphonate-derivatized
benzaldehyde. Thus, the Pd-mediated coupling of 4-bro-
mobenzaldehyde dimethylacetal (1)27 with di-tert-butyl
phosphite afforded 4-(di-tert-butyloxyphosphoryl)benzal-
dehyde dimethylacetal (2) in 53% yield following a
procedure used for the synthesis of 4-(diethoxyphospho-
(28) (a) Zhang, H.-Y.; Blasko´, A.; Yu, J .-Q.; Bruice, T. C. J . Am.
Chem. Soc. 1992, 114, 6621-6630. (b) Zhang, H.-Y.; Bruice, T. C. Inorg.
Chim. Acta 1996, 247, 195-202.
(29) (a) Paolesse, R.; J aquinod, L.; Senge, M. O.; Smith, K. M. J .
Org. Chem. 1997, 62, 6193-6198. (b) Collman, J . P.; Chong, A. O.;
J ameson, G. B.; Oakley, R. T.; Rose, E.; Schmittou, E. R.; Ibers, J . A.
J . Am. Chem. Soc. 1981, 103, 516-533.
(30) Lindsey, J . S.; Prathapan, S.; J ohnson, T. E.; Wagner, R. W.
Tetrahedron 1994, 50, 8941-8968.
(31) Lindsey, J . S.; Wagner, R. W. J . Org. Chem. 1989, 54, 828-
836.
(27) Clerici, A.; Pastori, N.; Porta, O. Tetrahedron 1998, 54, 15679-
15690.
(32) Lindsey, J . S.; Woodford, J . N. Inorg. Chem. 1995, 34, 1063-
1069.
J . Org. Chem, Vol. 69, No. 5, 2004 1447

Muthukumaran et al.
SCHEME 6
SCHEME 7
the ethyl groups, affording the free base porphyrin-
phosphonic acid 4 in 79% yield (Scheme 3). Porphyrin 4
was metalated with Zn(OAc)2‚2H2O under standard
conditions, giving Zn 4 in 77% yield. The successful
metalation of a porphyrin bearing a free phosphonic acid
was surprising and may stem in part from the suppres-
sion of aggregation afforded by the three mesityl groups.
The success of the above studies prompted us to employ
the di-tert-butyl phosphonate in a variety of porphyrin
precursors. Accordingly, reaction of acetal 2 with excess
pyrrole following a standard procedure34 afforded dipyr-
romethane 6 in 31% yield (Scheme 4). Alternatively, Pd-
mediated coupling of 5-(4-bromophenyl)dipyrromethane
(5)34 with di-tert-butyl phosphite under conditions similar
to those for the synthesis of 2 gave 6. Dipyrromethane 6
is a valuable synthon for use in porphyrin chemistry.
Mg3a in 71% yield. Treatment of Mg3a with TMS-Cl/
TEA in refluxing CHCl3 or THF did not cause cleavage
of the tert-butyl groups. However, use of TMS-Br/TEA
in refluxing CHCl3 gave porphyrin-phosphonic acid Mg4
in 77% yield (Scheme 2).
Dipyrromethane 6 was treated with various dipyr-
romethane-dicarbinols (7-d iol, 8-d iol, and 9-d iol)35 in a
rational route35 to afford the corresponding porphyrins
10, 11, and 12 in 4.6%, 7.8%, and 25% yields, respectively
(Scheme 5). The synthesis of 10 was achieved in CH2Cl2
containing Yb(OTf)3 for acid catalysis,36 and that of 11
and 12 was carried out in CH3CN containing TFA.
We also examined the diethyl porphyrin-phosphonate
Zn 3b, which was prepared from 4-(diethoxyphosphoryl)-
benzaldehyde4 in the same manner as for Zn 3a . We
found that Zn 3b could be deprotected without affecting
the metalation state by using the TMS-Br/TEA reagent
in refluxing CHCl3. On the other hand, treatment of
Zn 3b with TMS-Br in the absence of TEA (employed for
the deprotection of diethyl phosphonates)6 at room tem-
perature caused demetalation in addition to cleavage of
(34) Littler, B. J .; Miller, M. A.; Hung, C.-H.; Wagner, R. W.; O’Shea,
D. F.; Boyle, P. D.; Lindsey, J . S. J . Org. Chem. 1999, 64, 1391-1396.
(35) Rao, P. D.; Dhanalekshmi, S.; Littler, B. J .; Lindsey, J . S. J .
Org. Chem. 2000, 65, 7323-7344.
(36) Geier, G. R., III; Callinan, J . B.; Rao, P. D.; Lindsey, J . S. J .
Porphyrins Phthalocyanines 2001, 5, 810-823.
(33) O’Shea, D. F.; Miller, M. A.; Matsueda, H.; Lindsey, J . S. Inorg.
Chem. 1996, 35, 7325-7338.
1448 J . Org. Chem., Vol. 69, No. 5, 2004

Porphyrins Bearing Arylphosphonic Acid Tethers
SCHEME 8
Application of the procedure for monoacylation of a
dipyrromethane37 to 6 by treatment with EtMgBr and
pyridyl thioester 1337 or 1435 gave the corresponding
monoacyl product 15 or 16 in 48% or 69% yield, respec-
tively (eq 1). Reduction of 15 with NaBH4 in THF/
methanol afforded the monocarbinol 15-OH. Self-con-
densation37 of 15-OH on treatment with TFA in CH3CN
followed by oxidation with DDQ afforded the bis-phos-
phonate-porphyrin 17 in 28% yield (eq 2). The self-
in CHCl3 at reflux afforded the zinc chlorin-phosphonic
acid Zn 21 in 88% yield.
B. F u n ction a liza tion of th e P or p h yr in . An alterna-
tive approach for preparing porphyrin-phenylphosphonic
acids entails the Pd-mediated coupling of di-tert-butyl
phosphite with a bromophenyl- or iodophenyl-substituted
porphyrin. Thus, reaction of iodophenyl-porphyrin Zn 2239
and di-tert-butyl phosphite in toluene/TEA (5:1) contain-
ing Pd(PPh3)4 at 80 °C afforded phosphonate-porphyrin
Zn 23 in 46% yield. The TMS-ethyne group in Zn 23 was
cleaved with TBAF in CHCl3/THF, giving ethynyl-por-
phyrin Zn 24 in 84% yield (Scheme 7). Porphyrin Zn 24
condensation of 15-OH using InCl3 as catalyst36 resulted
in a very low yield (3%) of the required porphyrin 17.
A chlorin bearing a phenylphosphonic acid tether also
was prepared. Monoacyl-dipyrromethane 15 was treated
with NBS to afford 18 in 78% yield. Reduction of 18 with
NaBH4 gave the monocarbinol 18-OH (Eastern half),
which on reaction with tetrahydrodipyrrin 1938 (Western
half) under one-flask chlorin-forming conditions38 gave
the phosphonate-substituted zinc chlorin Zn 20 in 17%
yield (Scheme 6). Treatment of Zn 20 with TMS-Cl/TEA
(38) Taniguchi, M.; Ra, D.; Mo, G.; Balasubramanian, T.; Lindsey,
J . S. J . Org. Chem. 2001, 66, 7342-7354.
(39) Yu, L.; Lindsey, J . S. J . Org. Chem. 2001, 66, 7402-7419.
(37) Rao, P. D.; Littler, B. J .; Geier, G. R., III.; Lindsey, J . S. J . Org.
Chem. 2000, 65, 1084-1092.
J . Org. Chem, Vol. 69, No. 5, 2004 1449

Muthukumaran et al.
SCHEME 9
is an important building block as the free ethyne group
can be exploited in the construction of multiporphyrin
arrays.
solution (5 mM each in toluene/TEA) at 35 °C with
catalysis by Pd2(dba)3 and tri-o-tolylphosphine [P(o-tol)3]
without any copper cocatalysts. Thus, the coupling of
Zn 24 with Zn 2842 afforded Dya d -1 in 54% yield upon
chromatographic workup including size exclusion chro-
matography43 (SEC). Treatment of Dya d -1 with TMS-
Cl/TEA in refluxing CHCl3 afforded Dya d -2 bearing a
free phosphonic acid in 82% yield. No demetalation of
the zinc porphyrins was observed.
D. Ben ch m a r k Com p ou n d . For comparison with the
properties of the various porphyrinic phosphonic acids,
we sought an analogous ferrocene-phosphonic acid. Fer-
rocenes bearing alkylphosphonic acid groups have been
prepared previously.20,21 The reaction of 4-iodophenylfer-
rocene44 with bis(trimethylsilyl)phosphite in the presence
of Pd(PPh3)4 followed by treatment with water afforded
4-(ferrocenyl)phenylphosphonic acid (29) in 40% yield (eq
3). Because no further synthetic transformations were
The same derivatization approach was applied to a
magnesium porphyrin. Dipyrromethane-dicarbinol 25-
d iol (derived from diacyldipyrromethane 2535) and dipyr-
romethane 2634 were subjected to the standard sequence
of condensation and oxidation, affording porphyrin 27 in
22% yield. Use of the heterogeneous magnesium insertion
conditions gave Mg27, which was chromatographed on
alumina rather than silica to avoid demetalation.40 The
Pd-mediated coupling of di-tert-butyl phosphite with
Mg27 afforded Mg11 in 63% yield (Scheme 8).
C. P or p h yr in Dya d s Bea r in g a P h en ylp h osp h on ic
Acid Teth er . The synthesis of a porphyrin dyad bearing
a single phosphonic acid tether is shown in Scheme 9.
The Sonogashira coupling of Zn 24 with Zn 28 was carried
out under the conditions developed for synthesis of
multiporphyrin arrays.41 The conditions employ equimo-
lar amounts of the two porphyrins in relatively dilute
(42) Ravikanth, M.; Strachan, J .-P.; Li, F.; Lindsey, J . S. Tetrahe-
dron 1998, 54, 7721-7734.
(43) Wagner, R. W.; J ohnson, T. E.; Lindsey, J . S. J . Am. Chem.
Soc. 1996, 118, 11166-11180.
(40) Li, F.; Gentemann, S.; Kalsbeck, W. A.; Seth, J .; Lindsey, J . S.;
Holten, D.; Bocian, D. F. J . Mater. Chem. 1997, 7, 1245-1262.
(41) Wagner, R. W.; Ciringh, Y.; Clausen, C.; Lindsey, J . S. Chem.
Mater. 1999, 11, 2974-2983.
(44) Ambroise, A.; Wagner, R. W.; Rao, P. D.; Riggs, J . A.; Hascoat,
P.; Diers, J . R.; Seth, J .; Lammi, R. K.; Bocian, D. F.; Holten, D.;
Lindsey, J . S. Chem. Mater. 2001, 13, 1023-1034.
1450 J . Org. Chem., Vol. 69, No. 5, 2004

Porphyrins Bearing Arylphosphonic Acid Tethers
F IGURE 2. Cyclic voltammetry of Zn 4 on SiO2 (Tox ) 1.3 nm)
with scan rates of 2, 5, 10, and 20 V s-1 (from lowest to highest
amplitude).
required and the ferrocene-phosphonic acid was readily
purified by crystallization, the trimethylsilyl protecting
group could be employed. This situation is in contrast to
that with the porphyrins, where the dialkyl protecting
group for the phenylphosphonic acid facilitates synthetic
transformations and purification yet can be readily
removed when desired.
E. Ch a r a cter iza tion . The porphyrins and chlorins
were characterized by absorption spectroscopy, 1H NMR
spectroscopy, LDMS,45 and FABMS. The phosphonate-
containing compounds were also characterized by 31P
NMR spectroscopy using H3PO4 as an external standard.
The 31P NMR spectrum of each of the phosphonate-
containing compounds yielded a singlet. The 1H NMR and
13C NMR spectra of the molecules bearing phosphonate
groups showed splitting of some signals originating from
atoms in the adjacent phenylene or alkyl phosphonate
1
unit due to coupling with the phosphorus nucleus. H
NMR and 31P NMR spectra for Zn 21 were not obtained
(in CDCl3, THF-d8, CD3OD, or DMSO-d6) due to aggrega-
tion.
3. E lect r och em ica l St u d ies of Mon ola yer s on
SiO2. The electrochemical behavior was investigated for
a variety of porphyrin-phosphonic acid complexes teth-
ered to thin layers of SiO2 grown on (100) p-type Si
substrates (doping density 1 × 1018 cm-3). Representative
fast-scan cyclic voltammograms of monolayers of Zn 4 on
the SiO2 layer (thickness, Tox ) 1.3 nm) are shown in
Figure 2 as a function of scan rate (2-20 V s-1). Two
distinct anodic and cathodic current peaks are observed
at all scan rates, which correspond to the formation/
neutralization of the mono- and dication radicals of the
porphyrin. At a given scan rate, the integrated current
in each of the waves is approximately the same, indicat-
ing that the same amount of charge is being transferred
in and out of the monolayers during both oxidation and
reduction steps. The integrated current corresponds to a
molecular coverage on the SiO2 surface of ∼1.7 × 10-11
mol cm-2. The electrochemical behavior of Mg4 is very
similar to that of Zn 4. These voltammetric characteristics
indicate that the porphyrin-phosphonic acids form robust,
electrically well-behaved monolayers on the SiO2 surface.
F IGUR E 3. Cyclic voltammetry of Dya d -2 on SiO2 (Tox
)
1.3 nm). (a) Voltammetry at a scan rate of 5 V s-1. (b)
Voltammetry with scan rates of 5, 10, 20, 50, and 100 V s-1
(from lowest to highest amplitude). Note that three peaks are
observed at slower scan rates, whereas two peaks are observed
at higher scan rates.
Figure 3 shows the voltammetric characteristics of a
monolayer of Dya d -2 on SiO2. In the monolayer, the two
porphyrins may be electrically inequivalent if there is a
voltage drop across the long axis of the dyad. This
electrical inequivalence could afford the possibility of
doubling the number of accessible redox states. As can
be seen in Figure 3a, three distinct waves are indeed
observed in the voltammogram. However, at higher scan
rates (Figure 3b), the third peak disappears and only two
peaks are observed. A plausible explanation for this
observation is that (1) the porphyrin distal to the surface
undergoes slower electron-transfer kinetics than the
proximal porphyrin, and (2) the higher scan rates over-
(45) (a) Fenyo, D.; Chait, B. T.; J ohnson, T. E.; Lindsey, J . S. J .
Porphyrins Phthalocyanines 1997, 1, 93-99. (b) Srinivasan, N.; Haney,
C. A.; Lindsey, J . S.; Zhang, W.; Chait, B. T. J . Porphyrins Phthalo-
cyanines 1999, 3, 283-291.
J . Org. Chem, Vol. 69, No. 5, 2004 1451

Muthukumaran et al.
take the slower kinetic component but not the faster
component. At present, we are investigating in detail the
electron-transfer and charge-retention characteristics of
the porphyrin-phosphonic acids tethered to SiO2. A key
objective is to understand how these properties are
affected by the SiO2 thickness (i.e., the dielectric barrier)
as well as the tether barrier. The next paper in this series
describes porphyrinic molecules bearing longer tethers,
including benzylphosphonic acid and tripodal benzylphos-
phonic acid tethers.26 Gaining an understanding of how
both the dielectric barrier and the tether barrier affect
the electron-transfer and charge-retention characteristics
is essential for applications in molecular information
storage.
porphyrinic phenylphosphonates: (1) use of phosphonate-
substituted precursors in the rational synthesis of por-
phyrinic compounds and (2) derivatization of an iodo-
phenyl-substituted porphyrin with di-tert-butyl phosphite
in a Pd-mediated coupling process. The voltammetric
characteristics of monolayers of the porphyrin-phosphonic
acids indicate that these molecules are robust, electrically
well-behaved species when tethered to an oxide layer
deposited on a (semi)conducting medium.
Ack n ow led gm en t. This work was supported by the
DARPA Moletronics Program (MDA972-01-C-0072) and
by ZettaCore, Inc. Mass spectra were obtained at the
Mass Spectrometry Laboratory for Biotechnology at
North Carolina State University. Partial funding for the
Facility was obtained from the North Carolina Biotech-
nology Center and the NSF. We thank Prof. J oseph
Hupp for making available a preprint of his work on
porphyrin-phosphonic acid chemistry.
Con clu sion
The di-tert-butyl-masked phenylphosphonic acid group
shows high stability toward the diverse reaction condi-
tions that are employed in the synthesis of porphyrins
and chlorins. The mild nonacidic deprotection conditions
(TMS-Br or TMS-Cl/TEA in refluxing CHCl3) enable
cleavage of the tert-butyl group without affecting the
metalation state of class III or IV metalloporphyrins (zinc
or magnesium chelates) or a zinc chlorin. Two comple-
mentary routes were developed for preparing di-tert-butyl
Su p p or tin g In for m a tion Ava ila ble: Complete experi-
mental procedures, description of screening experiments for
possible protecting groups, and NMR and mass spectra for all
new compounds. This material is available free of charge via
the Internet at http://pubs.acs.org.
J O034945L
1452 J . Org. Chem., Vol. 69, No. 5, 2004