Coordination-induced spin crossover (CISCO) through axial bonding of substituted pyridines to nickel-porphyrins: σ-Donor versus π-acceptor effects

Article abstract of DOI:10.1002/chem.201000603

Nickel-porphyrins, with their rigid quadratic planar coordination framework, provide an excellent model to study the coordination-induced spin crossover (CISCO) effect because bonding of one or two axial ligands to the metal center leads to a spin transition from S=0 to S=1. Herein, both equilibrium constants K_1S and K₂, and for the first time also the corresponding thermodynamic parameters ΔH_1S, ΔH₂, ΔS_1S, and ΔS₂, are determined for the reaction of a nickel-porphyrin (Nitetrakis(pentafluorophenyl) porphyrin) with different 4-substituted pyridines by temperature-dependent NMR spectroscopy. The association constants K_1S and K₂ are correlated with the basicity of the 4-substituted pyridines (R: OMe>H>CO₂Et-NO₂) whereas the ΔH_1S values exhibit a completely different order (OMeCO₂Et> NO₂). 4-Nitropyridine exhibits the largest binding enthalpy, which, however, is overcompensated by a large negative binding entropy. We attribute the large association enthalpy of nitropyridine with porphyrin to the back donation of electrons from the Ni d_xz and d_yz orbitals into the π orbitals of pyridine, and the negative association entropy to a decrease in vibrational and internal rotation entropy of the more rigid porphyrin-pyridine complex. Back donation for the nitro-and cyanopyridine complexes is also confirmed by IR spectroscopy, and shows a shift of the N-O and C-N vibrations, respectively, to lower wave numbers. X-ray structures of 2:1 complexes with nitro-, cyano-, and dimethylaminopyridine provide further indication of a back donation. A further trend has been observed: the more basic the pyridine the larger is K_1S relative to K_2. For nitropyridine K₂ is 17 times larger than K_1S and in the case of methoxypyridine K₂ and K_1S are almost equal.

Full text of DOI:10.1002/chem.201000603

DOI: 10.1002/chem.201000603
Coordination-Induced Spin Crossover (CISCO) through Axial Bonding of
Substituted Pyridines to Nickel–Porphyrins: s-Donor versus p-Acceptor
Effects
Steffen Thies,[a] Claudia Bornholdt,[a] Felix Kçhler,[a] Frank D. Sçnnichsen,[a]
Christian Nꢀther,[b] Felix Tuczek,*[b] and Rainer Herges*[a]
Abstract: Nickel-porphyrins, with their
rigid quadratic planar coordination
framework, provide an excellent model
to study the coordination-induced spin
crossover (CISCO) effect because
bonding of one or two axial ligands to
the metal center leads to a spin transi-
tion from S=0 to S=1. Herein, both
equilibrium constants K1S and K2, and
for the first time also the correspond-
ing thermodynamic parameters DH1S,
DH2, DS1S, and DS2, are determined for
the reaction of a nickel-porphyrin (Ni-
tetrakis(pentafluorophenyl)porphyrin)
with different 4-substituted pyridines
by temperature-dependent NMR spec-
troscopy. The association constants K1S
and K2 are correlated with the basicity
of the 4-substituted pyridines (R:
OMe>H>CO2Et>NO2) whereas the
DH1S values exhibit a completely differ-
ent order (OMe<H>CO2Et>NO2).
4-Nitropyridine exhibits the largest
binding enthalpy, which, however, is
overcompensated by a large negative
binding entropy. We attribute the large
association enthalpy of nitropyridine
with porphyrin to the back donation of
electrons from the Ni dxz and dyz orbi-
tals into the p orbitals of pyridine, and
the negative association entropy to a
decrease in vibrational and internal ro-
tation entropy of the more rigid por-
phyrin–pyridine complex. Back dona-
tion for the nitro- and cyanopyridine
complexes is also confirmed by IR
spectroscopy, and shows a shift of the
N–O and C–N vibrations, respectively,
to lower wave numbers. X-ray struc-
tures of 2:1 complexes with nitro-,
cyano-, and dimethylaminopyridine
provide further indication of a back
donation. A further trend has been ob-
served: the more basic the pyridine the
larger is K1S relative to K2. For nitro-
pyridine K2 is 17 times larger than K1S
and in the case of methoxypyridine K2
and K1S are almost equal.
Keywords:
donor–acceptor
systems · nickel · porphyrinoids ·
pyridine · spin crossover
Introduction
bonding of O2 to the heme group in hemoglobin and myo-
globin, which proceeds under conversion of high-spin FeII
(S=2) in the deoxy form to low-spin FeIII (S=1/2) in the
oxy form of these metalloproteins. In the nickel(II)-porphyr-
ins, on the other hand, the metal center is diamagnetic (S=
0) in the axially uncoordinated complexes and paramagnetic
(S=1) in the five- and six-coordinated state. The spin
change provides an opportunity to use these systems as mo-
lecular sensors, through monitoring the bonding of axial li-
gands by electro-optical methods. Alternatively, if the pyri-
dine ligand is suitably tethered to the porphyrin backbone,
the coordination and decoordination of this moiety induced
by an external stimulus can be exploited for the fabrication
of a molecular switch.[2]
The coordination of axial ligands to porphyrins is of funda-
mental importance in many areas of chemistry, biology, and
materials science.[1] The most prominent example is the
[a] S. Thies, Dr. C. Bornholdt, Dr. F. Kçhler, Prof. Dr. F. D. Sçnnichsen,
Prof. Dr. R. Herges
Institut fꢀr Organische Chemie
Universitꢁt Kiel
Otto-Hahn-Platz 4, 24118 Kiel (Germany)
Fax : (+49)431-880-1558
E-mail: rherges@oc.uni-kiel.de
[b] Dr. C. Nꢁther, Prof. Dr. F. Tuczek
Institut fꢀr Anorganische Chemie
Universitꢁt Kiel
The association constants of Ni-porphyrins to a number of
nitrogen donor ligands have been measured.[3] The binding
strength increases with the electron deficiency of the por-
phyrin and the basicity of the nitrogen donor. Electron-with-
Otto-Hahn-Platz 6/7, 24118 Kiel (Germany)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201000603.
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FULL PAPER
drawing substituents at the meso position of the porphyrin
(substituents R in Scheme 1) favor the association of axial li-
gands.[4] Walker et al. observed a Hammett relationship of b
the coordination of two ligands (b). Ni-TPP binds two piper-
idine molecules with an enthalpy DHb =ꢀ5.6 kcalmolꢀ1 and
an entropy DSb =ꢀ21 calmolꢀ1 Kꢀ1.[5] The corresponding
thermodynamic parameters for the system Ni-TPP/pyridine
have also been determined.[10]
For the design of a spin switch based on the coordination-
induced spin crossover (CISCO) effect, that is, by controlled
coordination/decoordination of an axial ligand, we deter-
mined the thermodynamic parameters of both coordination
stages by measuring the temperature dependence of K1S and
K2. In an ideal system K1S should be considerably larger
than K2 to provide the possibility to switch the spin state by
adding/removing a single ligand. To this end we developed a
simple and reliable methodology to experimentally deter-
mine both association constants K1S and K2 (Scheme 1.) The
coordination of 4-substituted pyridines (R=NO2, CO2Et, H,
OMe) to Ni-tetrakis(pentafluorophenyl)porphyrin was in-
vestigated (Scheme 2).
Scheme 1. Definition of the association constants of porphyrins with axial
ligands (L).
(K1S K2) of piperidine with various porphyrins as a function
of the meso substituent (R=p-C6H4R’).[5] A closer look re-
veals that the increase of the association constant induced
by the electron deficiency of the porphyrin and the donor
strength of the axial ligand is more pronounced for K1S than
for K2. In most combinations of porphyrins and nitrogen
donor ligands K2 is considerably larger than K1S. Even with
an electron-deficient porphyrin such as tetra(4-cyanophe-
nyl)porphyrin (R=C4H5CN) and the strong donor piperi-
dine, the first equilibrium constant (K1S =0.4mꢀ1) is much
smaller than the second (K2 =2.5mꢀ1).[6] Hence, the concen-
tration of the five-coordinate 1:1 complex in the equilibrium
does not exceed 15% during titration of the porphyrin with
piperidine.[7] It needs the extremely electron-deficient por-
phyrin nickel tetrakis(2,3,5,6-tetrafluoro-N,N,N-trimethyl-4-
Scheme 2. NiII-tetrakis(pentafluorophenyl)porphyrin 1 and 4-substituted
pyridine derivatives 2a–f used as axial ligands.
In a somewhat simplified fashion, the bonding of the first
ligand can be associated with a bonding constant K1S and
that of the second ligand with a bonding constant K2 (see
below). Herein, K1S and K2 are determined by means of
NMR spectroscopy, by monitoring the paramagnetic shift of
the pyrrole protons as a function of the concentration of
added ligand. Through the temperature dependence of K1S
and K2, bonding enthalpies and entropies are evaluated,
which provides the first experimental determination of these
parameters for both individual stages of axial ligand coordi-
nation to a Ni-porphyrin. The influence of s versus p effects
on the bonding of the substituted pyridine ligands to Ni-por-
phyrins is discussed.
aniliniumyl)porphyrin ([NiACTHUNRGTENNUG ACHUTNGTREN(NUGN Me3))
(TF4TMAP)]4+; R=C6F4N
and the very strong donor ligand methyl imidazole to re-
verse the order of association constants (K1S =550, K2 =
110mꢀ1) and to make the five-coordinate species the pre-
dominant complex in solution.[8]
Another factor affecting the axial coordination is the
degree of nonplanarity of the Ni-porphyrin in its four-coor-
dinate S=0 low-spin state. Upon coordination to one or two
axial ligands, the spin state changes to high spin (S=1) and
induces a flattening of the porphyrin core. Large substitu-
ents in the meso position hinder this decrease in ruffling,
and with it axial coordination.[5] Steric hindrance at the
donor ligand is also a very sensitive parameter. Pyrrolidine,
for example, binds almost ten times more strongly to Ni-
TPP (TPP: tetraphenyl porphyrin; b=23) than 2-methylpyr-
rolidine (b=0.27).[9] Thermodynamic parameters of axial co-
ordination have been determined by measuring association
constants as a function of temperature, however, only for
Results and Discussion
Determination of the association constants K1S and K2: The
most frequently used methods for the determination of asso-
ciation constants of porphyrins with axial ligands are UV
and NMR titration. In preliminary experiments we recorded
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10075

R. Herges, F. Tuczek et al.
the UV spectra of porphyrin 1 as a function of the concen-
tration of added pyridine. The Soret band of porphyrin 1 at
406 nm decreases and a new band at 429 nm appears. How-
ever, there is no clear isosbestic point, which indicates that
there are more than two species involved in the equilibrium
(Figure 1). This is attributed to the formation of a five- and
six-coordinated complex.
porphyrin. In a fast equilibrium of complexation and decom-
plexation, the ratio of diamagnetic and paramagnetic species
can be determined from the average chemical shift of the
pyrrole protons. In free porphyrin 1 the pyrrole protons res-
onate at 8.79 ppm and upon addition of pyridine the chemi-
cal shift attains saturation at 57 ppm (Figure 2).
Figure 2. Plot of the chemical shift of the pyrrole protons of porphyrin 1
as a function of added pyridine in [D8]toluene at 298.03 K.
Figure 1. UV/Vis spectra of 1 as a function of added pyridine 2d. The
Soret band of pure 1 (406 nm) decreases and a new band arises at
429 nm upon addition of pyridine.
An Evans measurement of 1 in pure pyridine yielded a
molar susceptibility of the six-coordinated paramagnetic
Unfortunately, the deviation from an isosbestic plot is not
sufficient for an accurate and separate determination of
both association constants (K1S and K2). However, assuming
complex of cm =3.66ꢃ10ꢀ3 cm3 molꢀ1 and
moment of m=2.9 B.M. (Bohr magneton). This fits very well
to an S=1 state with two unpaired electrons (3.5<m<
2.8 B.M.).[12–14]
a
magnetic
that the paramagnetic complexes 1·2d and 1·ACTHNUGTRENUNG(2d)2 have sim-
ilar absorption maxima (lmax) and extinction coefficients (e),
K1S and K2 can be estimated from the decrease of the Soret
band of the uncoordinated porphyrin at 406 nm.[11] By using
this approximation the association constants of 2d with 1
were determined at 305 K in toluene as: K1S =(5.9ꢁ
0.3) Lmolꢀ1, K2 =(13.7ꢁ1.2) Lmolꢀ1, and b=(81.1ꢁ
2.1) L2 molꢀ2 (for details see the Supporting Information).
These values are in good agreement with the binding con-
stants evaluated by NMR spectroscopy (see Table 1.)
NMR titration turned out to provide a very accurate
method for the evaluation of K1S and K2. Upon addition of
axial ligands the diamagnetic porphyrin forms the paramag-
netic five- and six-coordinated axial complexes. The para-
magnetic NiII induces a strong paramagnetic shift and a
broadening of the NMR signal of the pyrrole protons in the
The titration experiments (analogous to Figure 2) were
performed with the 4-substituted pyridines 2a and 2c–e at
four different temperatures (298, 308, 318, 328 K). Because
of solubility problems, 2b and 2 f could not be used in this
study. K1S as well as K2 were calculated from the corre-
sponding titration curves by linear fitting of Equation (7)
(see below and Table 1).
As expected, the equilibrium constants decrease with in-
creasing temperature (see below). Moreover, both log K1S
and log K2 (and therefore also log b) exhibit linear relation-
ships[5] with respect to the pKa values of the pyridines (2a
1.5, 2c 3.5, 2d 5.2, 2e 6.6; see Figure 3, Figure 4, and
Figure 5).[15]
Electron-withdrawing substituents give rise to smaller as-
sociation constants whereas electron-donating groups lead
to a stronger binding to the porphyrin. With 4-nitropyridine
2a as the axial ligand, K1S and K2 are significantly smaller (2
and 50%, respectively) relative to the most basic ligand
used in our study, 4-methoxypyridine 2e. However, the in-
fluence of the basicity of the pyridines on the association
constant is more pronounced for K1S than for K2: with
ligand 2a K1S is considerably smaller than K2 (K1S/K2 =0.06
at 298 K) whereas for ligand 2e, K1S is larger than K2 (K1S/
K2 =1.34 at 298 K).
Table 1. Association constants [Lmolꢀ1] of porphyrin 1 with pyridine de-
rivatives 2a and 2c–e at different temperatures in [D8]toluene.
T [K]
Association constant
2a
2c
2d
2e
298
K1S
K2
K1S
K2
K1S
K2
K1S
K2
0.68
2.90
19.80
2.16
13.90
1.61
10.40
1.26
8.20
22.40
6.00
15.80
4.50
11.60
3.40
30.40
22.70
21.50
20.20
15.80
14.00
11.60
10.80
11.60
0.48
9.40
0.34
6.60
0.25
4.80
308
318
328
Determination of thermodynamic parameters: From the
temperature dependence of the association constants, the
7.65
8.60
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Coordination-Induced Spin Crossover
FULL PAPER
Table 2. Association enthalpies [kcalmolꢀ1] and entropies [calmolꢀ1 Kꢀ1
]
calculated from the temperature dependence of the association constants
K1S, K2, and b (see Table 1).
2a
2c
2d
2e
DH1S
DS1S
DH2
DS2
DHb
DSb
ꢀ6.85ꢁ0.06
ꢀ23.7ꢁ0.2
ꢀ6.2ꢁ0.2
ꢀ5.45ꢁ0.07
ꢀ16.2ꢁ0.2
ꢀ6.1ꢁ0.1
ꢀ5.3ꢁ0.1
ꢀ13.6ꢁ0.3
ꢀ5.97ꢁ0.04
ꢀ13.9ꢁ0.1
ꢀ11.2ꢁ0.5
ꢀ27.5ꢁ0.4
ꢀ6.19ꢁ0.06
ꢀ14.0ꢁ0.06
ꢀ6.1ꢁ0.2
ꢀ15.7ꢁ0.9
ꢀ13.1ꢁ0.3
ꢀ39.4ꢁ0.8
ꢀ14.5ꢁ0.4
ꢀ11.5ꢁ0.1
ꢀ30.7ꢁ0.3
ꢀ13.8ꢁ0.6
ꢀ12.2ꢁ0.08
ꢀ27.7ꢁ0.3
rate values for the enthalpies and entropies. For 2c–e, the
entropies DS1S and DS2 range from ꢀ13.6 to ꢀ16.2 calm-
olꢀ1 Kꢀ1, whereas the formation of the five-coordinated com-
plex with 4-nitropyridine (2a) is associated with an excep-
tionally large negative entropy DS1S of ꢀ23.67 calmolꢀ1 Kꢀ1
(Table 2). The binding enthalpies, in contrast, exhibit a
much smaller variation; that is, the DH1S values range from
ꢀ5.29 (2d) to ꢀ6.85 kcalmolꢀ1 (2a) and the DH2 values
from ꢀ5.97 (2d) to ꢀ6.18 kcalmolꢀ1 (2a). In spite of the
large differences observed for K1S and K2 (see above), the
associated binding enthalpies are thus quite similar.
Figure 3. Association of 1 with 2a and 2c–e to form high-spin five-coordi-
nated complexes (log K1S) as a function of the basicity of the pyridines 2
(pKa) in [D8]toluene at 298 K.
Also surprising is the relationship of DH1S and DH2 with
respect to the basicity of the para-substituted pyridines. For
pure s binding of the pyridine ligand to the nickel ion, a
linear correlation of the basicity (pKa) of the pyridines with
the enthalpies (DH1S and DH2) would be anticipated. How-
ever, we observe a V- or U-shaped relationship. As expect-
ed, the more basic 4-methoxypyridine (2e, R=OMe) binds
more strongly than the parent pyridine (2d). However, the
binding enthalpy increases again for electron-deficient 2c
(R=CO2Et) and 2a (R=NO2). The least basic pyridine
(2a, R=NO2) binds more strongly (DH1S =ꢀ6.85 kcalmolꢀ1)
than the most basic pyridine (2e, R=OMe; DH1S =
ꢀ6.19 kcalmolꢀ1) (see Figure 6). A similar trend was ob-
served for DH2 (and, as a consequence, for DHb) as a func-
tion of the pKa of the pyridine ligands (Figure 7 and
Figure 8). Importantly, the influence of the binding entropy
converts the V- or U-shaped characteristics of DH versus
Figure 4. Formation of high-spin six-coordinated complexes from the
five-coordinated complex of 1 with 2a and 2c–e (log K2) as a function of
the basicity of the pyridines 2 (pKa) in [D8]toluene at 298 K.
Figure 5. Formation of high-spin six-coordinated complexes from porphy-
rin 1 with two equivalents of 2a and 2c–e (log b) as a function of the ba-
sicity of the pyridines 2 (pKa) in [D8]toluene at 298 K.
corresponding binding enthalpies (DH1S, DH2) and entropies
(DS1S, DS2) were determined (Table 2).
~
&
Figure 6. Enthalpy DH1S ( ) and free enthalpy DG1S
( ) of complex for-
The plots of ln K as a function of Tꢀ1 exhibit excellent
linear relationships (0.998<R<0.9999), thus providing accu-
mation of the para-substituted pyridines (2a, 2c–e) with porphyrin 1 in
[D8]toluene plotted as a function of the pKa of the pyridines.
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R. Herges, F. Tuczek et al.
than FeII ions due to an increased effective nuclear charge,
such effects are clearly still present for Ni-porphyrins.
IR investigations on back donation: If back donation is the
cause of the unexpected trend in binding energies, the
p density of the electron-deficient pyridines should be larger
in the Ni complexes relative to the free ligand. Electron
transfer into p*-type orbitals should lead to a weakening of
bonds, which in turn should give rise to a decrease in force
constants and thus a shift of normal-mode vibrations of suit-
able symmetry to lower wave numbers. Thus, back donation
of electrons from the dxz or dyz orbital of Ni into the LUMO
of 4-nitropyridine (2a) should give rise to lower wave num-
bers of the asymmetric and symmetric N–O stretching fre-
quency of the nitro group (which has a node between N and
O, see Figure 9).
~
&
Figure 7. Enthalpy DH2 ( ) and free enthalpy DG2 ( ) of complex forma-
tion of the 2:1 complex 1·(2)2 from the 1:1 complex 1·2 by coordination
of a second pyridine ligand (2a, 2c–e) in [D8]toluene plotted as a func-
tion of the pKa of the pyridines.
&
Figure 8. Enthalpy DHb (DH1S +DH2; ) and free enthalpy DGb (DG1S
+
~
DG2; ) of complex formation of the para-substituted pyridines (2a, 2c–
e) with porphyrin 1 in [D8]toluene plotted as a function of the pKa of the
pyridines.
Figure 9. Molecular orbital diagram for the back donation of the Ni dxz
orbital into the LUMO of 4-nitropyridine.
pKa to the almost linear characteristics observed for DG or
log K.
Obviously, the basicity of the pyridine nitrogen lone pair
is not the exclusive factor determining the binding energy of
the corresponding pyridine to the nickel ion. There must be
an additional effect operating in the opposite direction. A
straightforward explanation would be back donation of the
dxz and dyz orbitals into the p orbitals of the nitro-, cyano-,
and ester-substituted pyridines. Back donation of NiII to pyr-
idines has been reported previously.[16,17] Cole et al. deter-
mined the binding enthalpies of several pyridines to FeII-
porphyrins by optical spectroscopy.[18] They also observed
that pyridine (R=H) was the weakest axial ligand, and elec-
tron-donating (e.g., R=NH2) as well as electron-deficient
substituents (e.g., R=CN) in the 4-position of the pyridine
ligands gave rise to larger binding enthalpies, which led to a
V shape of the DH versus pKa plot. These rather unusual re-
sults were explained by a back-donation effect. Although p-
acceptor interactions are expected to be weaker for NiII
To obtain an idea about the magnitude of these s-donor
and p-acceptor effects, we compiled data from model com-
pounds. A suitable model for back donation is the mesomer-
ic (M+) effect of p-donor substituents in the para position
to the electron-withdrawing nitro and cyano groups. Thus, p-
methoxynitrobenzene and p-methoxycyanobenzene should
serve as model structures. Similar to the Ni dxz orbital, the
methoxy group donates electrons into the LUMO of the ar-
omatic system including the nitro or cyano group (Figure 9).
As expected the symmetric (ns) and asymmetric (na) stretch-
ing frequencies of the nitro group as well as the CN stretch-
ing of the cyano group are shifted to lower wave numbers
(Table 3).
The strongest effect is observed for the asymmetric
stretching frequency of the nitro group (Dn=ꢀ27 cmꢀ1), fol-
lowed by the symmetric nitro N–O vibration (Dn=ꢀ8 cmꢀ1)
ꢀ1
ꢂ
and the C N stretch (Dn=ꢀ5 cm ). Electron-withdrawing
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Coordination-Induced Spin Crossover
FULL PAPER
ꢀ
ꢂ
Table 3. Shift of the N O and C N stretching frequencies in nitro and
cyano aromatics upon introduction of a p donor (methoxy) and s accep-
tor (protonation of pyridine) in the para position. The methoxy group
serves as a model for back donation and the protonated pyridine nitrogen
atom as the s-acceptor interaction of a coordinated metal center. Wave
numbers are given in cmꢀ1
.
R
D
D
[a]
na
1526[19]
1349[19]
2220[21]
1499[19]
1341[19]
2215[21]
ꢀ27
ꢀ8
1539[20]
1355[20]
2245[20]
1548[d]
1359[d]
2280[d]
+9
+4
+35
NO2
ns[b]
n[c]
ꢂ
C N
ꢀ5
[a] Asymmetric stretching vibration of the nitro group. [b] Symmetric
stretching vibration of the nitro group. [c] Stretching vibration of the
cyano group. [d] Experimental data from hydrochlorides.
groups in the 4-position (e.g., CN instead of OMe) exhibit a
reverse effect. The s-donor effect from the nitrogen lone
pair of pyridine to the NiII ion can be modeled by protona-
tion of the pyridine. For symmetry reasons there is no direct
effect on the p system; however, by second-order perturba-
tion the electron density of the p system is also decreased.
Therefore, protonation (as well as s donation) should lead
to a shift of the nitro and cyano stretching frequencies to
higher wave numbers, which is in fact observed (Dna, Dns,
Dn=+9, +4, +35 cmꢀ1, respectively; Table 3). For the Ni-
porphyrin–pyridine complexes, s donation as well as p back
donation are expected. In more basic pyridines such as 2e
(R=OMe), the s-donor effect is dominant and at the lower
end of the pKa scale (e.g., 2a, R=NO2) back donation
should prevail. Unfortunately, both effects give rise to oppo-
site shifts of the nitro and CN stretching modes. Because
s donation is operative in all complexes, a shift to lower
wave numbers should be observed only if the influence of
back donation on the stretching frequencies is stronger than
the s-donation effect.
Figure 10. Experimental (solid state) IR spectrum of the 2:1 complex of
4-nitropyridine (2a; top left) and 4-cyanopyridine (2b; top right) with
porphyrin 1. The corresponding calculated spectra (PBE/DZP) are given
below.
To study the back-donation effect we measured the
stretching frequencies of the nitro group in free 4-nitropyri-
dine (2a) and 4-cyanopyridine (2b) and compared these
data with those for the corresponding 2:1 complexes with
porphyrin 1 in the solid state (Figure 10). Although the in-
terpretation of the normal modes in the free pyridines is
straightforward, the assignment of the stretching frequencies
in the complexes is difficult because of interfering bands
from the porphyrin 1. We therefore calculated the vibration-
al spectra (optimization including harmonic frequency calcu-
lations) at the PBE/DZP level of density functional theory.
According to our calculations the asymmetric as well as the
symmetric stretching frequencies of the nitro group split
into two lines (Dnꢃ1 cmꢀ1) by the coupling of these modes
between the two nitropyridine ligands at the two axial posi-
tions. The frequencies of the calculated and the experimen-
tal spectra agree well (<10 cmꢀ1; for spectra and assign-
ments see the Supporting Information). However, the mea-
sured splittings of the asymmetric (Dna) and symmetric
(Dns) vibrational modes of the nitro group are larger (13
and 9 cmꢀ1, respectively), probably because of packing ef-
fects in the asymmetric unit cell (see crystal structure in the
Supporting Information). We therefore use average values
for ns and na (Table 4).
By comparing the stretching frequencies of the free pyri-
dines and the corresponding vibrations in the 2:1 complex
Table 4. Experimentally determined stretching frequencies of the nitro
and cyano groups in the corresponding 4-substituted pyridines and in
their 2:1 complexes with porphyrin 1. Wave numbers are given in cmꢀ1
.
R
Pyridine 2a
Complex 1·
G
D
[a]
na
1532
1351
2242
1525[d]
1346[d]
2239
ꢀ7
ꢀ5
ꢀ3
NO2
ns[b]
n[c]
ꢂ
C N
[a] Asymmetric stretching mode of the nitro group. [b] Symmetric
stretching mode of the nitro group. [c] Stretching mode of the cyano
group. [d] Bands are split into two in the crystal due to the asymmetric
environment of the nitro groups in the unit cell, therefore the average
value is given.
Chem. Eur. J. 2010, 16, 10074 – 10083
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10079

R. Herges, F. Tuczek et al.
with Ni-porphyrin, we observe a shift to lower wave num-
bers of ꢀ7 and ꢀ5 cmꢀ1 for Dna, and Dns of the nitro group,
respectively, and a shift of ꢀ3 cmꢀ1 for the C N stretching
ꢂ
mode (a shift of ꢀ4 cmꢀ1 was found in the Raman spectra of
the free and complexed ligand). Thus, the magnitude of the
shift is about one third of the effect of the methoxy group in
our model system (Table 3) for all three vibrational modes.
If no back donation was involved in the complexes with the
4-nitro (2a) and 4-cyanopyridine (2b) the reverse shift to
higher wave numbers would be expected, because s dona-
tion is operative in all pyridine complexes (see above).
Hence the effect of back donation on the vibrational modes
investigated for 2a and 2b must exceed that of s donation.
X-ray structures of Ni-tetrakis(pentafluorophenyl)porphyrin
complexes with 4-nitro-, 4-cyano-, and 4-dimethylaminopyri-
dine: Single crystals suitable for X-ray diffraction were ob-
tained for the 2:1 complexes of 4-nitro- (2a), 4-cyano- (2b),
and 4-dimethylaminopyridine (2 f) with porphyrin 1. A se-
lection of geometry parameters is given in Table 5 and these
parameters are defined in Figure 11. Figure 12 shows the
crystal structures of the complexes.
Figure 11. Definition of the geometry parameters listed in Table 5.
with pure s coordination free rotation would be expected.
Back donation, however, with the pyridine ligand breaks the
degeneracy of the dxz and dyz orbitals (see Figure 13c and d)
and should thus induce a rotational barrier. According to
PBE/DZP calculations including dispersion corrections, the
ꢀ
activation barrier to rotation around the pyridine-N Ni
Table 5. Selection of important bond lengths [ꢄ], angles [8], and dihedral
angles [8] of 2a, 2b, and 2 f determined by X-ray structure analysis. For
definitions of the geometry parameters, see Figure 11.
bond of 4-nitropyridine is predicted to be 1.76 kcalmolꢀ1.
As expected, the barrier decreases with decreasing electron
deficiency of the pyridine. The parent pyridine rotates with
R
NO2
CN
NMe2
a
barrier of 1.59 kcalmolꢀ1 and 4-aminopyridine with
a
2.230(2)
2.046(2)
2.050(2)
40.2(2)
86.8(1)
2.220(2)
2.046(2)
2.052(2)
44.1(2)
82.8(1)
2.192(2)
2.060(2)
2.054(2)
21.1(2)
76.8(1)
1.41 kcalmolꢀ1.
b1
b2
V
F
Electron-rich dimethylaminopyridine (which is not ex-
pected to be susceptible to back donation) forms an angle of
21.18. Obviously, in the latter case the orientation of the pyr-
idine rings is influenced by packing effects, whereas in the
electron-deficient pyridines back donation determines the
The bond lengths (a) of the
two axial ligands to the NiII ion
slightly decrease with increasing
basicity of the pyridine ligands.
The distance a is 0.04 ꢄ shorter
in dimethylaminopyridine 2 f
than in nitropyridine 2a. The
two axial bond lengths in each
complex are identical. The
bond lengths b1 and b2 between
the NiII ion and the pyrrole ni-
trogen atoms increase with in-
creasing basicity. In all three
complexes both axial pyridine
ligands are coplanar. Interest-
ingly, in the nitro- and cyano-
pyridine complexes the pyridine
molecules are rotated by almost
458 (V) with respect to the pyr-
ꢀ
role-N Ni bond, and provide
optimal overlap and back dona-
tion with the dxz and dyz orbitals
of Ni (Figure 13). For a ligand Figure 12. Crystal structures of complexes of 2a, 2b, and 2 f with porphyrin 1.
10080
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Chem. Eur. J. 2010, 16, 10074 – 10083

Coordination-Induced Spin Crossover
FULL PAPER
ever, this linear dependence is fortuitous. In contrast to
DG1S and DG2, the binding enthalpies (DH1S, DH2) are not a
linear function of the pKa. DH1S and DH2 are lowest in the
pyridine complex (2d, R=H). Electron-donating (R=
OMe) as well as electron-withdrawing substituents (R=
CO2Et, NO2) at the 4-position of the pyridine give rise to
larger binding enthalpies. We explain this unusual behavior
by back donation of electrons from the occupied Ni dxz orbi-
tal into the LUMO of the electron-deficient pyridines (R=
CO2Et, NO2). The gain in binding enthalpy from back dona-
tion overcompensates the loss in s donation (which should
be a linear function of the pKa). Unexpected as well is the
large binding entropy, particularly DS1S, of the nitropyridine
complex (ꢀ23.7 calmolꢀ1 Kꢀ1) relative to that of the elec-
tron-rich pyridine complexes (methoxypyridine: ꢀ14.0 calm-
olꢀ1 Kꢀ1). This large negative binding entropy renders the
coordination of nitropyridine comparatively unfavorable, in
spite of the large binding enthalpy. A more rigid geometry
and hindered rotation of the axial ligands could be an ex-
planation. The X-ray structures of the nitro- (2a), cyano-
(2b), and dimethylaminopyridine (2 f) complexes are in
agreement with this argument. DFT calculations also predict
a larger rotational barrier for the electron-deficient nitropyr-
idine (2a) than for pyridine (2d) and dimethylaminopyri-
dine (2 f). Moreover, 2 f binds with a coordination angle that
is inclined from perpendicular binding (V=76.88). This
hardly affects the s coordination but would be unfavorable
for p back donation. Independent evidence for back dona-
tion comes from IR spectroscopy; that is, upon coordination
to the Ni-porphyrin 1 the stretching frequencies of the nitro
Figure 13. a,b) Molecular orbital plots of the degenerate nickel dxz and
dyz orbitals in the triplet state of parent nickel-porphyrin (PBE/DZP).
c,d) Molecular orbital plots of HOMO-3 and HOMO-4 of the five-coor-
dinate complex of nickel-porphyrin with one axial pyridine ligand in the
triplet state (PBE/DZP). HOMO-4 represents the back-donation effect
ꢀ
(see also Figure 9; note the antibonding character of the N O bonds in
the nitro group).
orientation. Similar effects are observed for the coordina-
tion angle F of the pyridine ligands with respect to the por-
phyrin plane (F is the angle between the pyridine-N–R axis
and porphyrin plane, see Figure 11). In the electron-rich 4-
dimethylaminopyridine 2 f, the deviation from orthogonal
coordination is largest (F=76.88) whereas the smallest devi-
ation from perpendicular binding is observed for the most
electron-deficient ligand nitropyridine (F=86.88). The s do-
nation is less sensitive to a distortion from orthogonal coor-
dination than p interaction (back donation). Therefore the
electron-rich pyridine 2 f is more easily distorted by packing
effects, whereas 2a and 2b are fixed in an upright position
by back donation.
ꢂ
group in nitropyridine and the C N stretching in cyanopyri-
dine shift to lower wave numbers. This is expected for an
electron transfer from occupied Ni orbitals into the LUMO
of the electron-deficient pyridines.
Experimental Section
CCDC-766988 (2a), 766989 (2b), and 766990 (2 f) contain
the supplementary crystallographic data for this paper.
These data can be obtained free of charge from the Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
Spectroscopy: 1H NMR spectra were measured on a 500 MHz FT NMR
spectrometer DRX 500 (Bruker). IR spectra were recorded with a 1600
series FTIR spectrometer from Perkin–Elmer by using a golden-gate-dia-
mond attenuated total reflection (ATR) unit A531-G.
Materials and solvents: [D8]Toluene, [D5]pyridine, 4-methoxypyridine,
and ethyl 4-pyridinecarboxylate were commercially available and used as
received.
Conclusion
Synthesis of starting materials: 4-Nitropyridine was synthesized by oxida-
tion of 4-aminopyridine.[22] 5,10,15,20-Tetrakis(pentafluorophenyl)por-
phyrin was prepared by the method of Iida et al.[23]
We have developed a new method for the determination of
the association constants K1S and K2 in 2:1 Ni-porphyrin
complexes based on paramagnetic NMR shift data. By eval-
uation of more than 400 NMR spectra, binding enthalpies
(DH1S, DH2) and entropies (DS1S, DS2) of both coordination
steps could be determined with high accuracy (DDH1S <
(ꢁ0.1), DDH2 <(ꢁ0.2) kcalmolꢀ1, DDS1S <(ꢁ0.34), DDS2 <
(ꢁ0.9) calmolꢀ1 Kꢀ1). The free binding enthalpies (DG1S and
DG2) exhibit a linear relationship with respect to the pKa
values of the axial para-substituted pyridine ligands. How-
Nickel-5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (NiTPPF20) (1):
Nickel(II) acetylacetonate (7.00 g, 27.2 mmol) was added to a solution of
5,10,15,20-tetrakis(pentafluorophenyl)porphyrin (975 mg, 1.00 mmol) in
toluene (100 mL) and the mixture was heated at reflux for 5 days. After
removal of the solvent in vacuo, the crude solid was purified by column
chromatography (silica gel 0.04–0.063 mesh) with CH2Cl2 as eluent to
afford the product in 90% yield. 1H NMR (500 MHz, [D]CHCl3, 258C,
TMS): d=8.79 ppm (s, 8H; pyrrole-H); 19F NMR (470 MHz, [D8]toluene,
3
3
258C, CFCl3): d=ꢀ137.35 (d, J
G
ACHTUNGTRENNUNG
20 Hz; Fpara), ꢀ161.66 ppm (t, 3J
ACHTUNGTRENNUNG
paramagnetic complex of pyridine 2d with porphyrin
1
ACHTUNGTRENNUNG
Chem. Eur. J. 2010, 16, 10074 – 10083
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10081

R. Herges, F. Tuczek et al.
19F NMR (470 MHz, [D8]toluene, 258C, CFCl3): d=ꢀ136.82 (br, Fortho),
ꢀ152.97 (br, Fpara), ꢀ163.21 ppm (br, Fmeta).
sis, and the plot of cm (cm3 molꢀ1) versus Tꢀ1 (Kꢀ1) exhibits a straight line
with a slope of 1.15ꢁ0.03.[13] According to Equation (4):
Titration: The paramagnetism of the Ni2+ in the penta- and hexacoordi-
nated complexes gave rise to a strong downfield shift and a broadening
of the pyrrole proton resonance in porphyrins such as NiTPPF20 (1). Due
to the very rapid coordination/decoordination process, the time-averaged
Nm2B
1
Nm2
3k
1
T
ð4Þ
cm
¼
m2
¼
B g2SðS þ 1Þ
3k eff T
1
in which N is the Avogadro constant (6.022ꢃ1023 molꢀ1), mB the Bohr
magneton, meff the effective magnetic moment, k the Boltzmann constant,
g the Landꢅ factor, S the spin quantum number, and T the temperature
(K), a slope of 1 is expected for two unpaired electrons. The observed
slope of 1.15 corresponds to a magnetic moment of 3.0, in good agree-
ment with the above room-temperature value of cm.
signal of the dia- and paramagnetic species was observed in the H NMR
spectra. The signal of the pyrrole protons was followed as a function of
the axial ligand concentration (25 different ligand concentrations and
four different pyridines: 2a, 2c–e). K1S and K2 were determined from
these data by using the linear analysis outlined below. All titration ex-
periments were performed at four different temperatures (298, 308, 318,
and 328 K) to determine reaction enthalpies and entropies. More than
Single-crystal structure analysis: The investigation was performed with an
imaging plate diffraction system (IPDS-1) with MoKa radiation from
STOE & CIE. The structure solutions were carried out by direct methods
1
400 H NMR spectra were measured with an automatic sampler. A differ-
ent NMR tube was used for each ligand concentration. Each 1H NMR
tube was filled with 200 mL of a porphyrin solution (10.0 mmolLꢀ1) in
2
with SHELXS-97. These refinements were performed against jFj by
[D8]toluene. Then different amounts of
a solution of ligand in
using SHELXL-97.[29] All non-hydrogen atoms except some of the disor-
dered C atoms in compound 2b were refined with anisotropic displace-
ment parameters. All hydrogen atoms were positioned with idealized ge-
ometry and were refined with fixed isotropic displacement parameters
[Ueq(H)=ꢀ1.2ꢃUeq(C)] by using a riding model with dCꢀH =0.93 ꢄ. In
compound 2a one of the two crystallographically independent toluene
molecules is disordered and in compound 2b both molecules are disor-
dered. They were refined by using a split model. Details of the structure
determination are given in the Supporting Information. CCDC 766988
(2a), 766989 (2b) and 766990 (2 f) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
[D8]toluene (10 vol%) were added and the tubes were filled up to
600 mL with [D8]toluene and sealed. NMR titration experiments could
not be performed for the pyridines 2b and 2 f because of solubility prob-
lems.
Susceptibility measurements after Evans: To quantify the paramagnetic
properties of our NiII complexes, we determined their magnetic suscepti-
bility by using the “Evans method”.[11, 24–26] A set of two coaxial NMR
tubes consisting of an ordinary outer NMR tube and an inner tube with a
smaller diameter was used. The inner tube contained the reference sol-
vents [D5]pyridine and 2 vol% tert-butanol. The outer tube was filled
with the same mixture and additionally with paramagnetic complex.
[D5]Pyridine was used as the solvent as well as the axial ligand to make
sure that the hexacoordinated 2:1 complex was formed. A concentration
of m=0.0113 gcmꢀ3 of porphyrin 1 in [D5]pyridine was applied for the
actual Evans experiment. The mass susceptibility cg of the complex was
calculated from the difference of the chemical shifts Df of the tert-butanol
proton signals in the inner and outer tubes. A downfield shift of 60.5 Hz
was measured for the paramagnetic outer solution. The susceptibility was
corrected by the diamagnetic susceptibility of the solvent and the Ni-por-
phyrin [Eq. (1)]:
Derivation of Equation (7): We derived an equation for the determina-
tion of the equilibrium constants K1S and K2 from NMR and UV titration
data. Usually the coordination of one ligand without spin change is de-
scribed by K1, and if there is also a spin crossover during this process the
equilibrium constant is denoted K1S.[10] Equilibrium constant K2 describes
coordination of a second ligand to the five-coordinate, high-spin (HS)
nickel complex. The simultaneous coordination of two ligands to a four-
coordinate Ni center is described by b [Eqs. (a)–(d)]:
3Df
K1
ð1Þ
cg
¼
þ cg;pyr ꢀ cg;Ni
4pfm
Ni þ L
Ni þ L
NiL
ðaÞ
ðbÞ
ðcÞ
ðdÞ
ƒ!
LS
K1S
NiL
ƒ!
HS
in which f is the frequency of the NMR instrument (Hz), Df the frequen-
cy shift of tert-butanol (Hz), m the concentration of porphyrin 1 (gcmꢀ1),
cg,pyr the mass susceptibility of solvent [D5]pyridine (cm3 gꢀ1), and cg,Ni the
K2
NiLHS þ L
NiL
ƒ!
2
diamagnetic correction for porphyrin 1 (cm3 gꢀ1).
b
Ni þ 2L NiL
!
The molar susceptibility of pyridine is known (ꢀ49.2ꢃ10ꢀ4 cm3 molꢀ1
)
[27]
2HS
and the diamagnetic correction of porphyrin 1 was calculated from an in-
crement system.[28] The molar susceptibility cg =3.14ꢃ10ꢀ3 cm3 molꢀ1 was
calculated from the mass susceptibility and the molecular weight M of
the complex [Eq. (2)]:
It is generally accepted that the quadratic pyramidal 1:1 complexes of Ni-
porphyrins as well as the quadratic bipyramidal 2:1 complexes are high
spin. Thus, the complexation of the first axial ligand is accompanied by a
spin crossover from low to high spin. The validity of this approach is
shown by the excellent regression coefficient obtained when fitting exper-
imental data to equations based on these assumptions. La Mar and
Walker derived an equation which describes the ratio between paramag-
netic and diamagnetic species [Eq. (5)]:[11]
cm ¼ cgM
ð2Þ
The magnetic moment m in Bohr magnetons (B.M.), with T=tempera-
ture, is defined as follows [Eq. (3)]:
½paraꢄ
½diaꢄ
d ꢀ d0
½NiL2HSꢄ þ ½NiLHSꢄ
pffiffiffiffiffiffiffiffiffi
¼
¼
ð5Þ
ð3Þ
m ¼ 2:828 cmT
dmax ꢀ d
½NiLSꢄ þ ½NiLLSꢄ
The magnetic moment calculated from our data at 300 K of 2.9 B.M. is in
good agreement with the literature values for two unpaired electrons in
an octahedral ligand field. Depending on the contribution of the orbital
moment, values of 2.8 to 3.5 B.M. were found for the magnetic moment
m.
in which d0 is the chemical shift of the pyrrole protons of the porphyrin
in the absence of an axial ligand, d is the chemical shift of the pyrrole
protons at different ligand concentrations, dmax is the chemical shift of the
pyrrole protons of completely coordinated porphyrin, [NiL2HS] is the con-
centration of the hexacoordinate high-spin complex, [NiLHS] is the con-
centration of the pentacoordinate high-spin complex, [NiLS] is the con-
centration of the low-spin porphyrin, [NiLLS] is the concentration of the
pentacoordinate low-spin complex, and [L] is the concentration of the
ligand.
The magnetic moment m was also measured at 12 different temperatures
(from 293 to 283 K in increments of 10 K) to make sure that the number
of unpaired electrons was not temperature dependent. A plot of the ob-
served shift difference Df as a function of T does not exhibit any hystere-
10082
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Chem. Eur. J. 2010, 16, 10074 – 10083

Coordination-Induced Spin Crossover
FULL PAPER
We assume that [NiLS], [NiLLS], [NiLHS], and [NiL2HS] exhibit similar
Pecaut, J.-C. Marchon, C. J. Medforth, J. A. Shelnutt, J. Am. Chem.
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½NiLHS
ꢄ
chemical shifts of the pyrrole protons. By using K1S
¼
½NiLSꢄ½Lꢄ
½NiL2HS
ꢄ
½NiL2HSꢄ
K2
¼
b ¼
¼ K1SK2, we obtain [Eq. (6)]:
2
½NiLHSꢄ½Lꢄ
½NiLSꢄ½Lꢄ
2
½paraꢄ
½diaꢄ
d ꢀ d0
¼
K1S½Lꢄ þ b2½Lꢄ
1 þ K1½Lꢄ
ð6Þ
¼
dmax ꢀ d
With the assumption that K1 [L]=0, we obtain [Eq. (7)]:
[11] S.-L. Jia, W. Jentzen, M. Shang, X.-Z. Song, J.-G. Ma, W. R. Scheidt,
J. A. Shelnutt, Inorg. Chem. 1998, 37, 4402.
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½paraꢄ
d ꢀ d0
ð7Þ
¼
¼ K1S þ K2K1S½Lꢄ
½diaꢄ½Lꢄ ðdmax ꢀ dÞ½Lꢄ
d ꢀ d0
A plot of
versus [L] should give a straight line if the above
ðdmax ꢀ dÞ½Lꢄ
preconditions are met. The intercept corresponds to [K1S] and the slope
gives b=K1S K2. (At large concentrations of the ligand L deviations from
the linear behavior are observed.)
[16] P. T. T. Wong, D. G. Brewer, Can. J. Chem. 1968, 46, 139.
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Computational details: The geometries of the pyridines 2a and 2b (C2v
symmetry) and of the complexes 1·ACHTUNRGTENNUG(2a)2 and 1·AHCTUNTGRENN(GUN 2b)2 (C1 symmetry) were
optimized at the DFT functional PBE[30–33] with a double-zeta Ahlrichs
basis DZP[35] by using the TURBOMOLE program[32] and the multipole
accelerated resolution of identity method.[34] Harmonic frequency analy-
ses were performed at the same level of theory. The stretching frequen-
cies of the nitro and cyano group could be unambiguously assigned in the
free pyridines 2a and 2b. The calculated stretching frequencies of the
complexes 1·ACHTUNGTRENNUNG(2a)2 and 1·ACHTUTGNREN(NUNG 2b)2 were linearly scaled between 1300 and
1600 cmꢀ1 with the parameters determined by a linear fitting of the theo-
retical and experimental IR spectra of 2a and 2b within the same spec-
tral region.
[25] C. Piguet, J. Chem. Educ. 1997, 74, 815.
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[32] TURBOMOLE, version 6.0: R. Ahlrichs et al., Universitꢁt Karls-
ruhe 2009, see http://www.turbomole.com.
Acknowledgements
We gratefully acknowledge support by the Deutsche Forschungsgemein-
schaft via SFB 677 (Function by Switching).
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Received: March 8, 2010
Revised: May 31, 2010
Published online: July 20, 2010
Chem. Eur. J. 2010, 16, 10074 – 10083
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10083