Strength of the Zn-N coordination bond in zinc porphyrins on the basis of experimental thermochemistry

Article abstract of DOI:10.1021/ic0702522

The compound, 5,10,5,20-tetrakis(4-methoxyphenyl)porphine zinc(II) (ZnTMPP), was prepared, and its thermochemical properties were experimentally established. The standard molar energy of combustion (Δ_cU° _m) was determined from oxygen rotating-bomb combustion calorimetry experiments. The standard molar enthalpies of combustion (Δ _cH°_m) and formation (Δ_fH° _m) were derived. The enthalpy of sublimation (Δ _cr^gH°_m) was determined by Knudsen effusion at high temperatures. With these results, the standard molar enthalpies of formation and atomization (Δ_atH°_m) in the gas state were calculated. A summary of the results at T = 298.15 K (p° = 0.1 MPa) is shown in Table 1. Using these results and those previously obtained for the free ligand, 5,10,15,20-tetrakis(4-methoxyphenyl)porphine, the mean dissociation enthalpy for the Zn-N coordination bond is obtained as D(Zn-N) = (160 ± 9) kJ-mol^-1. This value is consistent with the results obtained using the same experimental approach in a similar system (5,10,15,20-tetraphenylporphine, TPP/ZnTPP) reported elsewhere. A discussion of the strength for the Zn-N coordination bond is made in terms of the structural and electronic features of the molecules involved.

Full text of DOI:10.1021/ic0702522

Inorg. Chem. 2007, 46, 9332−9336
Strength of the Zn−N Coordination Bond in Zinc Porphyrins on the
Basis of Experimental Thermochemistry
Rodrigo Patin˜o,*,† Myriam Campos,‡ and Luis Alfonso Torres‡
Departamento de F´ısica Aplicada, CinVestaV-Me´rida, Km. 6 carretera antigua a Progreso,
Apartado Postal 73, 97310 Cordemex, Me´rida, Mexico, and Departamento de Qu´ımica,
CinVestaV, Apartado Postal 14-740, 07000 Me´xico, D.F., Mexico
Received February 8, 2007
The compound, 5,10,15,20-tetrakis(4-methoxyphenyl)porphine zinc(II) (ZnTMPP), was prepared, and its thermo-
chemical properties were experimentally established. The standard molar energy of combustion ( cU m) was
determined from oxygen rotating-bomb combustion calorimetry experiments. The standard molar enthalpies of
combustion ( cH m) and formation ( fH m) were derived. The enthalpy of sublimation ( m) was determined
gH
by Knudsen effusion at high temperatures. With these results, the standard molar enthalpies of formation and
atomization ( atH m) in the gas state were calculated. A summary of the results at T 298.15 K (p° ) 0.1 MPa)
is shown in Table 1. Using these results and those previously obtained for the free ligand, 5,10,15,20-tetrakis(4-
methoxyphenyl)porphine, the mean dissociation enthalpy for the Zn N coordination bond is obtained as D(Zn N)
(160 9) kJ
mol-1. This value is consistent with the results obtained using the same experimental approach
in a similar system (5,10,15,20-tetraphenylporphine, TPP/ZnTPP) reported elsewhere. A discussion of the strength
∆
°
∆
°
∆
°
∆
°
cr
∆
°
)
−
−
)
±
‚
for the Zn
−N coordination bond is made in terms of the structural and electronic features of the molecules involved.
Table 1
Introduction
ZnTMPP
(kJ‚mol-1
)
From an energetic point of view, chemical bonding has
been approached using experimental (thermochemical) and
theoretical methods.At present, knowledge of the energy for
typical bonds involved in organic molecules is known, or
estimations with a high reliability are possible. In contrast,
not much research in molecular thermochemistry of coor-
dination or organometallic compounds has been performed
because of the very specialized techniques and procedures
required.1
∆cUom (cr)
∆cHom (cr)
∆fHom (cr)
-24235 ( 14
-24249 ( 14
-135 ( 16
238 ( 4
103 ( 16
45163 ( 27
∆
gHo
m
cr
∆fHom (g)
∆atHom (g)
density functional theory study of Co-C bond dissociation
energy in cobalamins5 has produced correlations between
optimized bond lengths and bond energies calculated from
total energies of gaseous and isolated molecules. However,
assessment of these results is not possible because the
corresponding experimental data are available only in solu-
tion. An ab initio study6 on free porphine and first row
transition-metal porphyrins leading to an indication of the
metal-nitrogen strength has been reported.
Large molecules of biological interest like cobalamins have
been studied by kinetics of thermolysis2,3 or iodination.4 Both
of these kinds of studies led to the indirect determination of
mean bond dissociation enthalpies of Co-C in solution. A
* To whom correspondence should be addressed. E-mail: rtarkus@
mda.cinvestav.mx.
† Cinvestav-Me´rida.
The importance of free and metallic synthesized porphyrins
has been described already,7 and the synthesis of new zinc
‡ Cinvestav.
(1) Ribeiro da Silva, M. A. V. New J. Chem. 1997, 21, 671-680.
(2) Ng, F. T. T.; Rempel, G. L.; Halpern, J. Organometallics 1990, 9,
2762-2772.
(3) Martin, B. M.; Finke, J. J. Am Chem. Soc. 1992, 114, 585-592.
(4) Toscano, P. J.; Seligson, A. L.; Curran, M. T.; Skrobutt, A. T.;
Sonnenberger, D. C. Inorg. Chem. 1989, 28, 166-168.
(5) Andruniow, T.; Zgierski, M. Z.; Kozlowski, P. M. J. Am. Chem. Soc.
2001, 123, 2679-2680.
(6) Zwaans, R.; van Lenthe, J. H.; den Boer, D. H. W. J. Mol. Struct.
(THEOCHEM) 1995, 339, 153-160.
9332 Inorganic Chemistry, Vol. 46, No. 22, 2007
10.1021/ic0702522 CCC: $37.00
© 2007 American Chemical Society
Published on Web 10/02/2007

Zn-N Coordination Bond in Zinc Porphyrins
similarities between both ZnTPP and ZnTMPP have been
found in spectroscopic,17,18 electrochemical,18,19 and theoreti-
cal20 studies.
Experimental Section
Preparation and Purification of the Sample. ZnTMPP is not
commercially available and was prepared from 5,10,15,20-tetrakis-
(4-methoxypenyl)-21H, 23H-porphine (TMPP, Aldrich Chemical
Co.). As provided, the sample of TMPP was contaminated with
the corresponding chlorin (a 7,8-dihydroporphyrin). Sample puri-
fication was achieved, and proved by 1H NMR, as previously
described.15,21,22 The chlorin-free TMPP was dissolved in boiling
chloroform and refluxed with a saturated solution of zinc acetate
in methanol, as described by Abraham et al.23 After evaporation of
the solvent to almost dryness, cold methanol was added. Filtration
afforded the pure ZnTMPP as a purple powder, that was dried
overnight at T ) 360 K in a vacuum oven.
There is a characteristic band for free TMPP (λ ) 650 nm) but
not for ZnTMPP when the visible-absorption spectrophotometric
analysis of the sample is performed. It permits the detection of
small amounts of free TMPP in the sample, and the coordination
reaction must be repeated with zinc acetate when necessary.
Figure 1. The general structure of the ZnTMPP (R ) -OCH3, M ) Zn)
reported here and those porphyrins cited previously: ZnTPP7 (R ) -H, M
) Zn), TMPP15,16 (R ) -OCH3, M ) 2H), and TPP15,16 (R ) -H, X )
2H).
porphyrins is today a challenge as a consequence of their
enormous applications.8 Despite its importance in chemical
and biological processes,9 the nature of the Zn-N interaction
is poorly understood. Some efforts have been made from
spectroscopic10 and analytical11 points of view. The reports
from Chagas and Airoldi12-14 have provided approximate
values for the mean dissociation enthalpy of the Zn-N bond
in some coordination compounds. These values were derived
from experimental enthalpies of formation in solution and
estimation of the corresponding enthalpies of sublimation.
The first mean bond dissociation enthalpy D(Zn-N), deter-
mined from the experimental enthalpy of formation in the
gas phase for such a large and complex molecule like the
zinc(II) 5,10,15,20-tetraphenylporphine (ZnTPP) has recently
been reported.7
1
Purity of the final sample of ZnTMPP was confirmed by H
NMR visible-absorption spectrophotometry and differential scan-
ning calorimetry (DSC). DSC experiments under a nitrogen
atmosphere revealed an endothermic process at temperatures around
T ) 393 K when ZnTMPP is aged some days in a vial at normal
atmosphere conditions or several hours when the sample is
introduced in a vessel with the same conditions of oxygen and
humidity as in the bomb combustions. When the sample is reheated,
the endothermic process does not occur. This could be interpreted
as an axial coordination of a molecule of water to Zn(II) in
ZnTMPP, since there are numerous examples of axial coordinated
ligands to the planar square structure of metal porphyrins; neverthe-
less, further work should be done to confirm the coordination
proposed here. Anyway, the sample was stored in a vacuum oven
at T ) 423 K before every combustion experiment. Moreover, a
reversible process was found by DSC for the purified sample at
temperatures around T ) 320 K, involving a very small energy
change. Finally, decomposition of the ZnTMPP occurs during
melting at T ) 770 K, and DSC was not suitable as a technique
for purity determination of the sample by the melting method.
The simplest synthesized porphyrins, like those studied
in the present and previous work,7,15,16 constitute excellent
models to obtain mean dissociation enthalpies when the
porphyrins are coordinated to the Zn(II) ion. The aim of this
work is to determine the mean bond dissociation enthalpy
D(Zn-N) involved in 5,10,15,20-tetrakis(4-methoxyphenyl)-
porphine (ZnTMPP, see Figure 1) from combustion and
sublimation experiments of this compound. This will allow
us to assess the accuracy and sensitivity of the experimental
approach to observe the effect of four methoxy substituent
groups on D(Zn-N), in relation to the compound studied
previously.7 The results could be used as reference for
theoretical approaches of Zn-N bonding. Differences and
Combustion Calorimetry Experiments. The combustion ex-
periments of a zinc-porphyrin have been detailed elsewhere.7
Benzoic acid (SRM 39j) was used as a combustion calorimetric
standard, and auxiliary material was supplied by NIST. Oxygen (x
) 0.9999) was obtained from Alphagaz (U.S.A.). A rotating-bomb
calorimeter (Argonne National Laboratory design) was used with
the combustions bomb (Parr Instrument Co., 1004C). The experi-
ments were performed in the presence of oxygen at p ) 3.0 MPa
and of a volume of 10 cm3 of a HNO3 (aq) dissolution (c ) 2.0
(7) Patin˜o, R.; Campos, M.; Torres, L. A. J. Chem. Thermodyn. 2002,
34, 193-204.
(8) Khan, M. M.; Ali, H.; van Lier, J. E. Tetrahedron Lett. 2001, 42,
1615-1617.
(9) Sponer, J.; Sabat, M.; Gorb, L.; Leszczynski, J.; Lippert, B.; Hobza,
P. J. Phys. Chem. B 2000, 104, 7535-7544.
(10) Rees, W. S.; Just, O.; Schumann, H.; Weimann, R. Polyhedron 1998,
17, 1001-1004.
(17) Buchler, J. W.; De Cian, A.; Fischer, J.; Kruppa, S. B.; Weiss, R.
Chem. Ber. 1990, 123, 2247-2253.
(18) Ichimori, K.; Ohya-Nishiguchi, H.; Hirota, N. Bull. Chem. Soc. Jpn.
1988, 61, 2753-2762.
(11) Song, B.; Reuber, J.; Ochs, C.; Hahn, F. E.; Lu¨gger, T.; Orvig, C.
Inorg. Chem. 2001, 40, 1527-1535.
(12) Lot, E. F.; Airoldi, C.; Chagas, A. P. Polyhedron 1994, 13, 27-37.
(13) Chagas, A. P.; Airoldi, C. Polyhedron 1989, 8, 1093-1097.
(14) Airoldi, C.; Silva, M. L. C. P.; Chagas, A. P. J. Chem. Soc., Dalton
Trans. 1986, 1913-1916.
(19) Wolberg, A. Isr. J. Chem. 1974, 12, 1031-1035.
(20) Zhang, Y.; You, X. Z. J. Chem. Res., Synop. 1999, 156-157.
(21) Barnett, G. H.; Hudson, M. F.; Smith, K. M. J. Chem. Soc., Perkin
Trans. I 1975, 1401-1403.
(15) Patin˜o, R.; Torres, L. A.; Campos, M. J. Chem. Thermodyn. 1999,
31, 627-634.
(22) Smith, K. M.; Goff, D. A.; Abraham, R. J.; Plant, J. E. J. Org. Magn.
Reson. 1983, 21, 505 - 511.
(16) Torres, L. A.; Campos, M.; Enriquez, E.; Patin˜o, R. J. Chem.
Thermodyn. 2002, 34, 293-302.
(23) Abraham, R. J.; Bedford, G. R.; McNeille, D., Wright, B. Org. Magn.
Reson. 1980, 14, 418 - 425.
Inorganic Chemistry, Vol. 46, No. 22, 2007 9333

Patin˜o et al.
Table 2. Physical Properties at T ) 298.15 Kc
Table 3. Summary of Ideal Reactions Occurring during Combustion
Experimentsa,b
(δu/δp)T
(J‚g-1
MPa-1
cp
Ma
F
‚
(J‚K-1
‚
combustion experiments
substance
ZnTMPP
benzoic acid C7H6O2
cotton CH1.774O0.887
composition (g‚mol-1
)
(g‚cm-1
)
)
g-1
)
1. 10.9 · (C7H6O2 + 7.5O2 ) 7CO2 + 3H2O)
C44H28N4O4Zn 798.2
(1.275) (-0.1)
1.32027 -0.11527 1.2127
-0.28927 1.727
1.353b
2. C48H36N4O4Zn + 55.5O2 ) 48CO2 + 18H2O + ZnO + 2N2
3. ZnO + 45.2HNO3 • 1243.2H2O ) Zn(NO3)2 • 43.2HNO3 • 1244.2H2O
4. C48H36N4O4Zn + 10.9C7H6O2 + 137.3O2 + 45.2HNO3 • 1243.2H2O )
124.3CO2 + Zn(NO3)2 • 43.2HNO3 • 1286.1H2O + 2N2
122.121
27.9903 1.527
a Molar masses are based on the 2005 IUPAC recommendation.26 b Heat
capacity determined by DSC in this work. c M is the molar mass of the
sample; F is the density; (δu/δp)T is the isothermal variation of the massic
energy with pressure; and cp is the massic heat capacity (estimated values
are in parentheses).
a Reaction 4 is the addition of reactions 1, 2, and 3. b Symbol ‚ refers to
a multiplication; symbol • refers to a chemical association.
considered proportional to the corresponding vapor pressure as
presumed by the Knudsen equation. Then, it is possible to obtain
an integrated Clausius-Clapeyron equation
mol‚dm-3). Two hundred seconds after ignition, the combustion
bomb was rotated until the end of each experiment.
During experiments, the temperature of the calorimetric vessel
(with 2708 g of distilled water) was measured every 10 s with a
linearized thermistor connected to a multimeter (Keithley, 2001).
The energy equivalent of the calorimeter has been previously
determined as ꢀ(calor) ) (14039.1 ( 0.4) J‚K-1; the quoted
uncertainty represents the standard deviation of the mean. From
12 comparison experiments combusting benzoic acid in the presence
of ZnO, a different value for the energy equivalent was obtained:
ꢀcomp(calor) ) (14044.9 ( 3.2) J‚K-1. This value includes the
energetic processes after the combustion reaction to obtain, as
closely as possible, the same final state as in the porphyrin
combustion experiments.
The energy of combustion of purified ZnTMPP was determined
from ten independent experiments. To ensure complete combustion,
a pellet of benzoic acid was required as an auxiliary material.
Corrections to standard states for the experimental reaction were
computed according to the usual procedures.24,25 The physical
properties summarized in Table 2 were used to transform apparent
mass to mass and for the calculations of the standard massic energy
of combustion. The energy of combustion of the ZnTMPP was
determined for aged samples, showing differences with fresh
samples with values as low as ∆cu° ) -29415 J‚g-1 when
compared to the mean value ∆cu° ) (-30361.8 ( 18.0) J‚g-1
(Table 5). To diminish the possible effects of aging, all the
experiments were carried out with ignition 1 h after oxygen addition.
Nevertheless, the significant deviation of the experimental values
of energy of combustion could not be diminished.
∆
subH
dL
dt
ln
T
1/2 ) lnB -
RT
where B is a constant value. Plotting ln[(dL/dt)‚T1/2] versus T -1, a
least-squares fitting led to the enthalpy of sublimation.
Results
In Table 3, the total idealized chemical process occurring
during the combustion experiment is displayed. Combustion
reactions of benzoic acid as an auxiliary material and of
ZnTMPP are represented by eqs 1 and 2, respectively. The
energy associated with the combustion of benzoic acid is
precisely known. Equation 3 represents the dissolution of
the zinc oxide formed during combustion in the nitric acid
solution. The small energy involved by dissolution is
considered in ꢀcomp(calor), in addition to other lateral physical
processes such as heating, cooling, mixing, and gas dissolu-
tion. Every effort was made to obtain very similar final states
in both comparison and combustion experiments. The total
idealized chemical process is summarized in eq 4. The
amounts of carbon dioxide, water, and zinc nitrate produced
are closely the same as in comparison experiments. The mean
value7 of ꢀcomp(calor) ) (14044.9 ( 3.2) J‚K-1 was used to
calculate the standard massic energy of combustion of
ZnTMPP.
To follow the scheme of calculation, the typical masses
and energies involved during combustion experiments are
presented in Table 4. The value for entry (16) is just (15)
divided by (1), the value for (15) is obtained by summing
entries (9)+(10)+(11)+(12)+(13)+(14), and the value for
entry (9) is just the experimental ∆T (8) multiplied by the
calorimeter constant, ꢀ(calor). From ten combustion experi-
ments summarized in Table 5, the value ∆cu°(ZnTMPP, cr)
) -(30361.8 ( 18.0) J‚g-1 at T ) 298.15 K was obtained.
The uncertainty represents the standard deviation of the
mean.
Sublimation Experiments. The enthalpy of sublimation was
determined by the Knudsen effusion method. The experimental
procedure and the data analysis have been detailed elsewhere.16
A
special system with metal-glass joints was constructed to work at
temperatures up to T ) 593 K. The effusion orifice diameter was
2.3 mm. Samples of around 20 mg were placed into the effusion
cell and heated near 600 K with an electric resistance. The system
was under vacuum with a turbo molecular pump. The amount of
sublimed ZnTMPP was detected with a piezoelectric quartz crystal
under a cold finger with liquid nitrogen. The thickness increment
of the crystal was directly measured every 120 s. Four series of
experiments were made between T ) 583 K and T ) 593 K.
Experiments at lower temperatures were not sensitive enough. The
rate of sample condensing (dL/dt) was determined at different
temperatures using a least-squares fitting, and the values were
Four series of effusion experiments are summarized in
Table 6. The variation of the thickness of the sublimed
sample was detected by the piezoelectric crystal with time
and computed at different temperatures. Variation of these
values with temperature allows the calculation of the enthalpy
of sublimation for each series. A mean value was obtained
(24) Hubbard, W. N.; Scott, D. W.; Waddington, G. Experimental Ther-
mochemistry; Rossini, F. D., Ed.; Wiley-Interscience: New York,
1956; Chapter 5.
(25) Experimental Thermochemistry; Skinner, H. A., Ed.; Wiley-Inter-
science: New York, 1962; Vol. 2.
(26) Wieser, M. E. Pure Appl. Chem. 2006, 78, 2051-2066.
(27) Good, W. D.; Smith, N. K. J. Chem. Eng. Data 1969, 14, 102-106.
g
as ∆cr H°m (ZnTMPP) ) (223.7 ( 4.4) kJ‚mol-1 at the mean
temperature T ) 588 K. Correction of this value to T )
9334 Inorganic Chemistry, Vol. 46, No. 22, 2007

Zn-N Coordination Bond in Zinc Porphyrins
Table 4. Typical Masses and Energy Quantities for the Combustion
Experiments at T ) 298.15 Ka
Table 6. Experimental Values for Determination of the Molar Enthalpy
of the Sublimation of ZnTMPP Using the Quartz-Crystal Microbalancec
ZnTMPP
T/K 103‚(dL/dt)/nm‚s-1 102‚∆ T/K 103‚(dL/dt)/nm‚s-1 102‚∆
Series 1: ∆crgH°m) (217.7 ( 4.1) kJ‚mol-1 at <T> ) 588.2 Ka
1) m′(ZnTPP)/g
2) m′′(benzoic acid)/g
3) m′′′(cotton)/g
4) m(Pt)/g
5) nl(Zn)/mol
0.303516
0.499727
0.002316
12.804822
0.00038
0.52
583.2
585.2
587.2
2.6163
2.9815
3.5659
0.33 589.2
-1.77 591.2
1.45 593.2
4.1337
4.7164
5.4990
0.90
-0.76
-0.16
Series 2: ∆crgH°m) (215.6 ( 10.8) kJ‚mol-1 at <T> ) 588.2 K
6) nl(H20)/mol
7) nl(HN03)/mol
584.2
586.2
588.2
2.5255
3.0696
3.4019
-1.65 590.2
2.89 592.2
-1.71
4.0696
4.6018
1.44
0.02
-0.94
8) ∆T/K ) (tl - tf + ∆Tcorr)/K
9) ꢀ(calor) (∆T)/kJ
10) ꢀ(cont)(∆T)/kJ
11) ∆udec(HNO3)/KJ
12) ∆U(corr. to std. states)/kJ
13) -m′′′‚∆cu°(cotton)/kJ
14) -m′′‚∆cu°(benzoic acid)/kJ
15) m′‚∆cu°(ZnTPP)/kJ
-1.59613
-22.4175
-0.0812
0.0196
Series 3: ∆crgH°m) (226.5 ( 2.8) kJ‚mol-1 at <T> ) 588.2 K
583.2
585.2
587.2
2.0296
2.4267
2.8068
-0.91 589.2
1.16 591.2
0.02 593.2
3.2695
3.8448
4.4459
-0.30
0.43
0.0335
0.0392
-0.42
13.1996
-9.2068
-30.3338
Series 4: ∆crgH°m) (230.2 ( 9.6) kJ‚mol-1 at <T> ) 588.2 K
584.2
586.2
588.2
2.1094
2.4177
2.9425
0.72 590.2
-1.63 592.2
2.13
3.3014
3.9835
-2.13
16) ∆cu°(ZnTPP)/(kJ‚g-1
)
0.98
a p° ) 0.1 MPa. The quantities are reported according to the thermo-
chemical protocol; m represents the mass of various samples, and nl is used
for the initial molar quantities of substances (before combustion), ∆T is
the corrected temperature rise during the combustion process, ꢀ(calor) is
the heat capacity of the combustion bomb (also known as the calorimeter
constant), ꢀ(cont) is the heat capacity of the different substances inside the
combustion bomb, ∆U(corr. to std. states) is the contribution to the internal
energy change of the process when corrections to the standard states are
applied, and ∆cu° is the standard energy of combustion for every substance.
Mean Value: ∆crgH°m) (223.7 ( 4.4) kJ‚mol-1 at <T> ) 588.2 Kb
a Uncertainty σi of individual values represents the standard deviation
of the linear fitting. b The weighted average µ and the corresponding
uncertainty σs were calculated from µ ) Σ(xl/σl2)/Σ(σl-2) and σs
)
[Σ(σl-2)]-1/2‚N1/2 with N being the number of series. c L is the thickness of
the sample deposited on the piezoelectric crystal, and ∆ is the uncertainty
from a least-squares fitting. Effusion-orifice diameter: 2.3 mm.
A reversible transformation in the solid phase was found
for ZnTMPP by DSC experiments. When samples were
heated, an endothermic process began at temperatures over
T ) 313 K with ∆H ) (2.36 ( 0.07) kJ‚mol-1. When
cooling, an exothermic process began at temperatures below
T ) 328 K with ∆H ) -(2.5 ( 0.3) kJ‚mol-1. In all five
heating/cooling experiments, the mass of the samples was
conserved. The low energy involved in the process and the
reversibility suggest a structural rearrangement.
The derived enthalpies of formation and atomization in
gas phase of ZnTMPP at T ) 298.15 K were calculated from
the combustion and sublimation experimental results and
using the following values:29 ∆fHm°[CO2(g)] ) -(393.51
( 0.13) kJ‚mol-1, ∆fHm°[H2O(l)] ) -(285.83 ( 0.04)
kJ‚mol-1, ∆fHm°[ZnO(cr)] ) -(350.46 ( 0.27) kJ‚mol-1,
∆fHm°[C(g)] ) (716.68 ( 0.45) kJ‚mol-1, ∆fHm°[H(g)] )
(217.998 ( 0.006) kJ‚mol-1, ∆fHm°[N(g)] ) (472.68 ( 0.40)
kJ‚mol-1, ∆fHm°[O(g)] ) (249.18 ( 0.10) kJ‚mol-1, and
∆fHm°[Zn(g)] ) (130.40 ( 0.40) kJ‚mol-1. Results are given
in Table 7. Columns in the same table contain the corre-
sponding quantities for the free ligands TMPP and TPP as
well as for the ZnTPP, previously reported.7,15,16
Table 5. Summary of the Standard Massic Energies of Combustion of
ZnTMPP at T ) 298.15 Ka
combustion experiments
∆cu°(ZnTMPP)/(J‚g-1
)
-30307.8
-30331.2
-30292.2
-30419.4
-30333.8
-30400.9
-30433.5
-30432.1
-30372.2
-30294.4
-30361.8
18.0
mean value
standard deviation of the mean
a p° ) 0.1 MPa.
298.15 K requires knowledge of the heat capacity of the
gaseous compound.
Experimental values of this heat capacity are not available,
and estimation from group contributions is not possible with
the available data. For this reason, the classical value
proposed by Burkinshaw and Mortimer28 ∆cp ) cp(g) -
cp(cr) ) -50 J‚mol-1‚K-1 for a number of different
molecular structures, as suggested by Pedley in a personal
communication, was applied. A correction of 14.5 kJ‚mol-1
for the molar enthalpy of sublimation of ZnTMPP leads to
Discussion
Previous reports for combustion and sublimation experi-
ments of ZnTMPP are not known. Not even the elucidation
of either the TMPP or ZnTMPP structures by X-ray
diffraction analysis have been found in the literature.
Comparison of their enthalpies of formation shows a
stabilization of the zinc porphyrin in relation to the free
TMPP, in the solid phase (204 kJ‚mol-1) as in the gas phase
(192 kJ‚mol-1). However, this stabilization is around 100
kJ‚mol-1 lower than in the pair TPP/ZnTPP.7
g
a value of ∆cr H°m(298.15 K) ) (238.2 ( 4.4) kJ‚mol-1. It
is important to keep in mind that this is an approximate but
necessary correction. However, since this same correction
was made with the other three porphyrins in previous
reports,7,15,16 it is possible to compare without higher errors
the corresponding values for the enthalpies of sublimation,
of formation in gas phase, and of atomization.
(28) Burkinswhaw, P. M.; Mortimer, C. T. J. Chem. Soc., Dalton Trans.
1984, 75-77.
(29) CODATA Key Values for Thermodynamics; Cox, J. D., Wagman, D.
D., Medvedev, V. A., Eds.; Hemisphere: New York, 1989.
Inorganic Chemistry, Vol. 46, No. 22, 2007 9335

Patin˜o et al.
Table 7. Standard Molar Thermochemical Functions for TMPP and
ZnTMPP at T ) 298.15 Ka
not coplanar to the porphine ring, as observed from the X-ray
structures for TPP31 and ZnTPP.32 Instead, an inductive effect
of the methoxy groups in ZnTMPP could be the reason for
the weakness of the Zn-N bond, wholly assessed as (25 (
11) kJ‚mol-1 in relation to ZnTPP. The diminishing of Zn-N
bond energy in ZnTMPP could favor the axial coordination
of water molecules to ZnTMPP at room temperature, as
discussed from the observations by DSC.
ZnTMPP
TMPP
ZnTPP7
TPP
∆cU°m(cr)/ -24235 ( 14 -24375 ( 715 -22007 ( 7 -22244 ( 715
kJ‚mol-1
∆cH°m(cr)/ -24249 ( 14 -24388 ( 715 -22021 ( 7 -22257 ( 715
kJ‚mol-1
∆fH°m(cr)/
kJ‚mol-1
-135 ( 16
238 ( 4
69 ( 915
226 ( 1016
295 ( 1416
45277 ( 2516
354 ( 9
196 ( 3
655 ( 915
184 ( 216
∆
gH°m/
crkJ‚mol-1
839 ( 916
The large effects that meso substituents have on the
π-system of zinc porphyrins have been observed previously
by EPR studies.18 Particularly, differences in half-wave
potentials of first oxidation of TPP and ZnTPP are larger
than the corresponding to TMPP and ZnTMPP.19 This has
been confirmed when both, ZnTPP and TMPP, are used as
catalysts.33,34 In addition, when they are axially coordinated
with 1-methylimidazole, the reaction with ZnTMPP is more
exothermic than the reaction with ZnTPP.35
The ab initio study of Zwaans et al.6 suggest that the Zn-N
bonds are weaker than the N-H bonds in the porphine ring,
as experimentally deduced in the porphyrins studied. The
bond energy Zn-N of 55.4 kJ‚mol-1 obtained in the same
study is also consistent with our results because phenyl or
methoxyphenyl groups stabilize the porphine ring, leading
to stronger Zn-N bonding, when coordinated.
The values for the mean dissociation enthalpies D(N-
Zn) of 153, 116, and 116 kJ‚mol-1, reported by Chagas and
Airoldi14 and derived from the coordination compounds of
ZnCl2 with pyridine, 2,2′-bipyridine, and 1,10-fenantroline,
respectively, are weaker than those reported here, because
the two coordinated chlorine ions produce a weakness in the
Zn-N bonds. In addition a macrocyclic effect in ZnTPP and
ZnTMPP is present, increasing the strength of the Zn-N
bond in these porphyrins.
∆fH°m(g)/
kJ‚mol-1
∆atH°m(g)/
kJ‚mol-1
103 ( 16
45163 ( 27
550 ( 9
39109 ( 22
39125 ( 2216
a p° ) 0.1 MPa. ∆cU°m, ∆cH°m, ∆fH°m, ∆crgH°m, and ∆atH°m denote
standard molar energy of combustion, enthalpy of combustion, enthalpy of
formation, enthalpy of sublimation, and enthalpy of atomization, respec-
tively.
When comparing the differences between the experimental
values for the enthalpy of sublimation at T ) 298.15 K for
the ZnTMPP and the ZnTPP, and for the TMPP and TMPP,
a value of 42 kJ‚mol-1 is obtained in both cases. Furthermore,
when comparing the values between ZnTMPP and TMPP
as well as the values between ZnTPP and TPP, a difference
of 12 kJ‚mol-1 is observed. Since the trend for the enthalpies
of sublimation follows ZnTMPP > TMPP > ZnTPP > TPP,
the coordination to Zn(II) ion in both cases increases the
intermolecular interactions correspondingly. In addition, the
enthalpies of sublimation for free and coordinated TMPP
are greater than for free and coordinated TPP. Finally, the
experimental Knudsen method at high temperatures appears
to be reliable and sensitive for observing molecular structure
influences on the values for the enthalpy of sublimation.
When the enthalpies of atomization are compared for the
pair TMPP/ZnTMPP, the differences are attributed princi-
pally to the contribution of four Zn-N coordination bonds
(in ZnTMPP) in relation with two N-H bonds (in TMPP).
Using the recommended value of Laidler’s parameters30 for
the contribution to the enthalpy of atomization by two N-H
bonds, D(N-H) ) 377.4 kJ‚mol-1, the corresponding value
for the Zn-N coordination bond is calculated as D(Zn-N)
) (160 ( 9) kJ‚mol-1 in the ZnTMPP. This value can be
compared to that obtained for ZnTPP, D(Zn-N) ) (185 (
6) kJ‚mol-1. The differences in these results cannot be
attributed to a resonance effect since phenyl substituents are
Acknowledgment. R. Patin˜o thanks Conacyt (Mexico)
for his scholarship. The authors acknowledge the comments
of the reviewers which have improved the original manuscript.
IC0702522
(32) Scheidt, W. R.; Kastner, M. E.; Hatano, K. Inorg. Chem. 1978, 17,
706-710.
(33) Ito, Y.; Tone, M.; Yokoya, H.; Matsuura, T.; Schuster, G. B. J. Org.
Chem. 1986, 51, 2240-2245.
(34) Schmidt, S. P.; Schuster, G. B. J. Am. Chem. Soc. 1980, 102, 7100-
(30) Cox, J. D.; Pilcher, G. Thermochemistry of Organic and Organome-
tallic Compounds; Academic: London, 1970.
(31) Silvers, S.; Tulinski, A. J. Am. Chem. Soc. 1967, 89, 3331-3337.
7103.
(35) Marbury, G.S.; Brewer, C.; Brewer, G. J. Chem. Soc., Dalton Trans.
1991, 1377-1378.
9336 Inorganic Chemistry, Vol. 46, No. 22, 2007