Synthesis, characterisation and X-ray structure of a novel porphyrin array employing Zn-O and O-H...O bonding motifs

Article abstract of DOI:10.1016/j.poly.2006.06.002

The preparation and characterisation of the free-base and zinc metallated derivatives of 5,10,15,20-tetrakis(4-(2-(2-hydroxyethoxy)ethoxy)phenyl)porphyrin 1 is described. The X-ray crystal structure of the Zn(II) adduct 2 dimerises in the solid state via an intermolecular polyether oxygen-Zn(II) interaction (O...Zn = 2.124(4) ?). The porphyrin dimers form discrete layers defined by a distance of 5.10 ? between the porphyrin planes in adjacent layers. A bilayer sheeting arrangement of the porphyrin macrocyclic units is achieved through cooperative hydrogen bonding of the ethoxyethanol arms to form 11-membered macrocycles containing four hydrogen bonds.

Full text of DOI:10.1016/j.poly.2006.06.002

Polyhedron 26 (2007) 338–343
www.elsevier.com/locate/poly
Synthesis, characterisation and X-ray structure of a novel
porphyrin array employing Zn–O and O–H. . .O bonding motifs
*
Steven J. Langford , Clint P. Woodward
School of Chemistry, Monash University, Wellington Road, Clayton, Melbourne, Vic. 3800, Australia
Received 4 April 2006; accepted 2 June 2006
Available online 10 June 2006
Abstract
The preparation and characterisation of the free-base and zinc metallated derivatives of 5,10,15,20-tetrakis(4-(2-(2-hydroxyeth-
oxy)ethoxy)phenyl)porphyrin 1 is described. The X-ray crystal structure of the Zn(II) adduct 2 dimerises in the solid state via an inter-
˚
molecular polyether oxygen–Zn(II) interaction (O. . .Zn = 2.124(4) A). The porphyrin dimers form discrete layers defined by a distance of
˚
5.10 A between the porphyrin planes in adjacent layers. A bilayer sheeting arrangement of the porphyrin macrocyclic units is achieved
through cooperative hydrogen bonding of the ethoxyethanol arms to form 11-membered macrocycles containing four hydrogen bonds.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Keywords: Metalloporphyrins; Zn(II) complex; Hydrogen bonding; Coordination network; X-ray structure
1. Introduction
anol set leading to discrete assemblies or 2-D supramolec-
ular arrays.
Controlling the arrangement of porphyrin macrocycles
into well-defined solid-state arrays through the processes
of self-assembly and self-organisation is necessary to
advance the development of surface bound porphyrin
devices with applications in surface sensing, size exclusion,
light harvesting and heterogeneous porphyrin catalysis [1–
6]. Desired control may be executed by utilising ‘‘intelli-
gent’’ porphyrins in which non-covalent interactions are
pre-programmed into the porphyrin structure to aid molec-
ular alignment in either a pseudo 1-D or 2-D and 3-D
arrays [7–10]. In particular, new porphyrin systems capable
of forming either hydrogen bonded or coordinative 2D
sheet networks from a single molecular entity are desirable.
In this paper we report on preliminary studies using eth-
oxyethanol side chains for this goal (Fig. 1). Such function-
ality could potentially self-assemble at the porphyrin
periphery in a non-covalent way cooperatively, forming
two distinct intermolecular hydrogen bonds per ethoxyeth-
2. Results and discussion
5,10,15,20-Tetrakis(4-(2-(2-hydroxyethoxy)ethoxy)phe-
nyl)porphyrin (1, H2TEEP) was prepared in 83% yield by
the alkylation of 5,10,15,20-tetrakis(4-hydroxyphenyl)por-
phyrin with 2-(2-chloroethoxy)ethanol [11]. Compound 1
is surprisingly insoluble in the usual aprotic solvents used
for dissolving porphyrins (CHCl3, CH2Cl2, toluene, etc).
This insolubility can be attributed to extensive intermolec-
ular hydrogen bonding interactions, which are desirable
but uncontrolled to this point. The problem is alleviated
by the addition of a small amount of a solvent capable of
interfering with hydrogen bonding (e.g., methanol or pyri-
dine)1. Zinc metallation of 1 was achieved using the acetate
method [12] producing 5,10,15,20-tetrakis(4-(2-(2-hydroxy-
ethoxy)ethoxy)phenyl)porphinato zinc(II) (2, ZnTEEP) in
89% yield as a light purple solid. Solubility of the porphy-
1
*
Despite this fix, crystals of 1 suitable for X-ray crystallography have
Corresponding author. Tel.: +61 3 9905 4569; fax: +61 3 9905 4597.
E-mail address: Steven.Langford@sci.monash.edu.au (S.J. Langford).
not been forthcoming.
0277-5387/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2006.06.002

S.J. Langford, C.P. Woodward / Polyhedron 26 (2007) 338–343
339
Fig. 1. Utilising the potential of ethoxyethanol groups on the porphyrin periphery could lead to a range of supramolecular constructs through
concentration and surface dependency.
Scheme 1.
Fig. 2. ORTEP diagram of the molecule displaying the atom labelling scheme used with in paper. Ellipsoids are drawn at 50% probability level.
Coordinating alcohol removed for clarity.

340
S.J. Langford, C.P. Woodward / Polyhedron 26 (2007) 338–343
ꢀ
Table 1
ZnTEEP (2) crystallises in the triclinic space group P1.
The molecular structure of 2 is shown in Fig. 2 and selected
bond distances and angles for 2 are listed in Table 1. The
square pyramidal Zn(II) centre is somewhat distorted with
the four inner peripheral nitrogens of the porphyrin defin-
ing the pyramid base and the apical position occupied by a
coordinated (intermolecularly) alcohol side chain (see
Fig. 3). The metal to nitrogen bond distances vary from
˚
Selected bond lengths (A) and bond angles (ꢁ) for 2
Bond distances
M–N(1)
M–N(2)
M–N(3)
2.056(4)
2.059(4)
2.050(4)
M–N(4)
M–O(6)
2.071(4)
2.124(4)
Bond angles
N1–Zn1–N2
N1–Zn1–N3
N1–Zn1–N4
N2–Zn1–N3
N2–Zn1–N4
N3–Zn1–N4
88.76(14)
163.25(15)
88.86(14)
89.23(14)
163.80(15)
88.45(14)
N3–Zn1–O6
N1–Zn1–O6
N2–Zn1–O6
N4–Zn1–O6
99.34(14)
97.41(14)
97.84(14)
98.35(14)
˚
˚
2.050(4) to 2.071(4) A (average value 2.059 A), with the
axially coordinated Zn–O bond displaying a longer dis-
tance of 2.124(4) A. These values fall into the ranges
observed in other Zn(II) porphyrin systems [13]. The Zn(II)
metal ion resides 0.29 A above the porphyrin least squares
˚
˚
rin 2 was also only possible in a mixed solvent system in
which a solvent capable of competing with ethoxyethanol
hydrogen bonding was employed. Using this technique,
crystals suitable for X-ray crystallography were grown by
the slow diffusion of diethyl ether into dichloromethane/
methanol (see Scheme 1).
plane and toward the coordinating alcohol. The large dihe-
dral angles observed between the meso phenyl rings and the
mean porphyrin plane (69.9–88.3ꢁ) are also typical, the dif-
ference from orthogonal attributed to crystal packing
forces [14]. Despite their inherent flexibility, the ethoxyeth-
anol chains are extended and relatively well ordered. Each
Fig. 3. Representation of the dimer formed in the crystal structure of 2 (H-atoms and CH2Cl2 solvent molecule have been omitted for clarity).
˚
Fig. 4. Bilayer sheeting arrangement observed in the crystal structure of 2. The shaded area highlights the organisation and intra-layer thickness (3.45 A),
˚
with inter-layer separation of 5.10 A (H-atoms have been omitted for clarity).

S.J. Langford, C.P. Woodward / Polyhedron 26 (2007) 338–343
341
Fig. 5. Representation of the hydrogen bonding motif observed above the zinc coordination site (hydrogen atoms have been omitted for clarity).
Table 2
Hydrogen bonding parameters of 2
a
˚
˚
˚
O–H
O
O–H (A)
HÁ Á ÁO (A)
OÁ Á ÁO (A)
O–HÁ Á ÁO (ꢁ)
O–H(3)
O–H(6)
O–H(9)
O–H(12)
a
O5
O9
O12
O3
0.84
0.84
0.84
0.84
2.16
1.92
1.89
1.82
2.791(6)
2.626(6)
2.677(6)
2.686(6)
132.2
159.8
155.9
150.2
All non-hydrogen atoms were refined anisotropic, while all hydrogen atoms were fixed in idealised positions and allowed to ride on the atom to which
they are attached.
OCH2CH2OH group forms a staggered gauche arrange-
ment while the internal OCH2CH2O groups are not as well
defined, forming both extended and gauche arrangements.
Interestingly, ZnTEEP forms a C2 symmetric dimer in
which the OH group from the ethoxyethanol side chain
of one porphyrin coordinates to the Zn(II) centre of an
adjacent porphyrin molecule [15]. A representation of this
dimer is shown in Fig. 3. The observed Zn. . .Zn separation
hydrogen bonding motif involving the ethoxyethanol
chains not involved in dimerisation. A representation of
this motif and the extended layer is shown in Fig. 5.
Selected hydrogen bonding data are listed in Table 2.
The bilayer sheeting arrangement of the porphyrin mac-
rocyclic units is achieved through cooperative hydrogen
bonding of the ethoxyethanol arms to form 11-membered
macrocycles containing four hydrogen bonds. Hence, all
ethoxyethanol groups are employed in the formation of
the overall structure. The longest of the hydrogen bonds
O3–H. . .O5 differs from the others (Table 2) in that it is
an ether oxygen rather than hydroxy oxygen atom that par-
takes in the noncovalent bonding.
˚
between the two porphyrins within the dimer is 13.39 A,
with the mean plane of the porphyrins aligned parallel to
˚
one another at a distance of 3.45 A (Fig. 3). The two meso
phenyl groups used to form the 28-membered macrocycle
are parallel to each other but do not provide further Van
der Waals stabilisation through p–p interactions.
The dimeric arrays occur within a plane represented by
the grey regions in Fig. 4 and alternate in an up-and-down
fashion. Each bilayer is separated by an intervening space
of thickness 5.10 A (porphyrin plane to porphyrin plane).
The dimers within one layer are interconnected through a
3. Concluding remarks
A novel supramolecular structure utilising hydrogen
bonding and Zn(II)–N and Zn(II)–O coordination was
determined. Despite the use of hydrogen bonding (avail-
˚

342
S.J. Langford, C.P. Woodward / Polyhedron 26 (2007) 338–343
able through the ethoxyethanol side chains) to form a lay-
ered structure, this present result was more complex than
anticipated by design. We are continuing our quest to form
hydrogen bonded arrays based on the ethoxyethanol motif
for surface adhesion, but recognise that the use of free-base
or metalloporphyrins in which axial coordination is not
possible (i.e. using four coordinate metal ions or strong
donor ligands) may be necessary to achieve this goal. These
experiments are underway.
rotary evaporation. Water and THF were added and the
THF was removed by rotary evaporation to precipitate
the porphyrin. The purple suspension was filtered and
washed with water (3 · 5 mL). The sample was dried under
high vacuum pump to remove the residual solvent giving 2
as a purple solid (14.2 mg, 89%). X-ray quality crystals
were grown by the slow diffusion of diethyl ether into a
CH2Cl2/MeOH solvent system (4:1), resulting in elongated
hexagonal red plates. m.p. 223–225 ꢁC. UV–Vis (THF)
kmax (loge): 406 sh (4.63); 426 (6.07); 558 (4.94); 599 nm
(4.60). 1H n.m.r (400 MHz, CDCl3/MeOD 9:1): d 3.53
(m, 16H, OCH2); 3.76 (m, 8H, OCH2); 4.14 (m, 8H,
OCH2); 7.00 (ABq, J 8.6 Hz, 8H, ArH); 7.83 (ABq, J
8.6 Hz, 8H, ArH); 8.56 (s, 8H, b-pyrrolic-H). 13C n.m.r
(75 MHz, CDCl3/MeOD 9:1): d 61.5, 67.8, 69.9, 73.0,
112.6, 120.1, 131.5, 135.6, 136.4, 150.3, 158.2. HR-MS
(ESI, +ve, DCM/MeOH): calculated m/z 1115.3397;
observed m/z 1115.3414 [M+Na]+.
4. Experimental
4.1. General
Synthesis reagents including solvents were used as pur-
chased. Melting points (m.p.) were measured on a Stuart
Scientific SMP 3 melting point apparatus. UV–Vis spectra
were recorded on a Varian model Cary 100 Bio UV–Visible
Spectrophotometer, with the solvent systems used stated in
the synthesis description. 1H and 13C nuclear magnetic res-
onance (n.m.r.) spectra were recorded using a Bruker DPX
4.1.3. X-ray structure determination
4.1.3.1. Data collection and processing. X-ray measure-
ments were made using a Bruker X8 Apex II CCD diffrac-
tometer using monochromated Mo Ka radiation
1
300 MHz spectrometer (300 MHz H, 75 MHz 13C) or a
Bruker DRX 400 MHz spectrometer (400 MHz 1H,
100 MHz 13C), as solutions in the deuterated solvents spec-
ified. Deuterated chloroform (CDCl3) was base washed
with anhydrous Na2CO3 prior to use. High-resolution elec-
trospray mass spectra (HRMS) were recorded on a Bruker
BioApex 47e Fourier transform mass spectrometer.
˚
(0.71073 A). The collection temperature was maintained
at 123 K using an Oxford Cryostream open-flow N2 cryo-
stat. Solution was obtained by direct methods (SHELXS
97)[15] followed by successive Fourier-difference methods,
and refined by full matrix least squares on F 2obs (SHELXL
97) [16]. Hydrogen atom thermal parameters were tied to
those of the atom to which they are attached.
4.1.1. 5,10,15,20-Tetrakis(4-(2-(2-hydroxyethoxy)
ethoxy)phenyl)porphyrin (1)
Crystal data for 2: C61H62Cl2N4O12Zn, M = 1179.42,
ꢀ
To a stirred DMF solution (50 mL) of 5,10,15,20-tetra-
kis(4-hydroxyphenyl)porphyrin (100 mg, 0.15 mmol),
anhydrous K2CO3 (410 mg, 2.96 mmol) and 2-(2-chloro-
ethoxy)ethanol (0.2 mL, 1.89 mmol) were added and the
resulting solution was heated to 80 ꢁC and left to stir for
4 days. After cooling, the porphyrin was precipitated by
the addition of water (50 mL), filtered and washed with
water (3 · 5 mL). Further purification by column chroma-
tography (SiO2, CHCl3/MeOH, 9:1 ratio) eluted 1 as a pur-
ple solid (128.5 mg, 83%). m.p. 261–262 ꢁC, UV–Vis
(CHCl3/MeOH, 9:1) kmax (loge): 403 sh (4.79); 422 (560);
red plate, 0.20 · 0.20 · 0.02 mm, triclinic, space group P1,
˚
a = 11.6110(5), b = 13.3604(5), c = 17.6703(8) A, a =
3
˚
87.943(2), b = 86.938(2), c = 88.317(2)ꢁ, V = 2734.4(2) A ,
Z = 2, Dc = 1.432 g/cm3, F000 = 1232, T = 123(2) K,
2hmax = 55.1ꢁ, 60320 reflections collected, 12539 unique
(Rint = 0.0751). Final Goodness-of-fit = 1.117, R1 =
0.0891, wR2 = 0.1995, R indices based on 9409 reflections
with I > 2r(I) (refinement on F2), 721 parameters. Lp and
absorption corrections applied, l = 0.616 mmÀ1
.
Acknowledgements
1
520 (4.16); 556 (4.09); 594 (3.88); 649 nm (3.90). H n.m.r
(400 MHz, CDCl3/MeOH, 4:1): d 3.66 (m, 8H, OCH2);
3.72 (m, 8H, OCH2); 3.19 (t, J 4.6 Hz, 8H, OCH2); 4.29
(t, J 4.5 Hz, 8H, OCH2); 7.17 (ABq, J 8.2 Hz, 8H, ArH);
7.98 (ABq, J 8.2 Hz, 8H, ArH); 8.69 (s, 8H, b-pyrrolic-
H); inner NH not observed. HR-MS (ESI, +ve, DCM/
MeOH): calculated m/z 1053.4262; observed m/z
1053.4230 [M+Na]+.
We gratefully acknowledge Dr. Craig Forsyth and Dr.
Paul Jensen for their contributions to structure determina-
tion. This research was supported by the Australia Re-
search Council Discovery Grant (DP0556313). C.P.W.
thanks the Monash University Faculty of Science for sup-
port through a Dean’s postgraduate scholarship.
Appendix A. Supplementary material
4.1.2. [5,10,15,20-Tetrakis(4-(2-(2-hydroxyethoxy)
ethoxy)phenyl)porphinato]zinc(II) (2)
To a solution of 1 (15 mg, 0.015 mmol) in CHCl3/
MeOH (3 mL, 4:1 ratio) was added a saturated Zn(OAc)2
in MeOH (5 mL) and stirred at reflux for 3 h in darkness.
Upon completion, the solvent system was removed by
Crystallographic data for the structure analyses have
been deposited with the Cambridge Crystallographic Data
Centre, CCDC (awaiting No.) for structures of 2. Copies of
this information may be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ,

S.J. Langford, C.P. Woodward / Polyhedron 26 (2007) 338–343
343
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1
c.uk or http://www.ccdc.cam.ac.uk). The H n.m.r. spec-
trum of is also presented. Supplementary data
2
associated with this article can be found, in the online ver-
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(2002) 1895.
[10] M.E. Kosal, J.-H. Chou, S.R. Wilson, K.S. Suslick, Nat. Mater. 1
(2002) 118.
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