Synthesis and electrochemical studies of organometallic cobalt(III) complexes with substituted benzonitrile chromophores: NMR spectroscopic data as a probe on the second-order non-linear optical properties

Article abstract of DOI:10.1016/j.jorganchem.2005.06.004

The family of organometallic Co(III) benzonitrile derivatives of general formula [CoCp(dppe)(p-NCR)][PF₆]₂ (R = C₆H ₄NMe₂, C₆H₄NH₂, C ₆H₄OMe, C₆H₄C₆H ₅, C₆H₅, C₆H₄C ₆H₄NO₂, and C₆H₄NO ₂) have been synthesized. Spectroscopic and electrochemical data were analyzed in order to evaluate the extent of electronic coupling between the organometallic fragment and the nitrile ligands. An attempt of correlation between NMR spectroscopic data and the second-order non-linear optical properties is presented, based on this work and available published data for related η⁵-monocyclopentadienyliron, ruthenium and nickel complexes.

Full text of DOI:10.1016/j.jorganchem.2005.06.004

Journal of Organometallic Chemistry 690 (2005) 4063–4071
www.elsevier.com/locate/jorganchem
Synthesis and electrochemical studies of organometallic
cobalt(III) complexes with substituted benzonitrile
chromophores: NMR spectroscopic data as a probe
on the second-order non-linear optical properties
a,b,
b
M. Helena Garcia
*, Paulo J. Mendes b,c, A. Roma˜o Dias
a
ˆ
Faculdade de Ciencias da Universidade de Lisboa, Ed. C8, Campo Grande, 1749-016 Lisboa, Portugal
´ ´
Centro de Quımica Estrutural, Instituto Superior Tecnico, Av. Rovisco Pais, 1049-001 Lisboa, Portugal
b
c
´
´
´
Centro de Quı´mica de Evora, Universidade de Evora, Rua Roma˜o Ramalho 59, 7002-554 Evora, Portugal
Received 25 February 2005; received in revised form 3 June 2005; accepted 3 June 2005
Available online 11 July 2005
Abstract
The family of organometallic Co(III) benzonitrile derivatives of general formula [CoCp(dppe)(p-NCR)][PF6]2 (R = C6H4NMe2,
C6H4NH2, C6H4OMe, C6H4C6H5, C6H5, C6H4C6H4NO2, and C6H4NO2) have been synthesized. Spectroscopic and electrochemical
data were analyzed in order to evaluate the extent of electronic coupling between the organometallic fragment and the nitrile ligands.
An attempt of correlation between NMR spectroscopic data and the second-order non-linear optical properties is presented, based
on this work and available published data for related g5-monocyclopentadienyliron, ruthenium and nickel complexes.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Cobalt complexes; Benzonitrile derivatives; Cyclic voltammetry; Electronic coupling; Non-linear optics
1. Introduction
The main studies have been made in push-pull systems
in which the metal center, bonded to a polarizable or-
ganic conjugated backbone (chromophore), acts as an
electron releasing or withdrawing group.
The search for new organometallic materials with
non-linear optical (NLO) properties is currently the sub-
ject of considerable interest due to their potential tech-
nological applications in the area of integrated optics
[1–6]. It is well known that organometallic complexes
can possess low energy, sometimes intense, electronic
metal-to-ligand or ligand-to-metal charge transfer exci-
tations which can be responsible for high values of
molecular first hyperpolarizability b. The value of these
systems is that the energy of the charge transfer excita-
tion can be tuned by variation of the metal itself and
its oxidation state, ligand environment and coordination
geometries to optimize the second-order NLO response.
Among the organometallic compounds presenting
this donor–p-system–acceptor interaction, the family
of g5-monocyclopentadienylmetal derivatives revealed
significant second-order optical non-linearities, espe-
cially those of iron and ruthenium complexes coordi-
nated to p-nitrobenzonitriles, p-nitrobenzoacetylides
and p-nitrothienylnitriles [7–15]. Compared with the
metallocene systems, which were the first organometallic
compounds to be studied for their NLO properties, the
main structural feature of this family of compounds is
the existence of the metal center in the same plane of
the chromophore, thus allowing a better coupling be-
tween the organometallic fragment and the conjugated
chromophores. In fact, the existence of metal–ligand
*
Corresponding author. Tel.: +351 217500972; fax: +351 217500088.
E-mail address: lena.garcia@fc.ul.pt (M.H. Garcia).
0022-328X/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.06.004

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M.H. Garcia et al. / Journal of Organometallic Chemistry 690 (2005) 4063–4071
p-backdonation through dmetal–p*ligand interaction was
found to play an important role on the second-order
NLO response of this family of compounds, where the
driving force is the presence of the strong acceptor
NO2 group on the end of the p-system of the chromo-
phore [16]. In our earlier studies on systematic variation
of the metal ion in g5-monocyclopentadienylmetal com-
plexes with p-substituted benzonitrile chromophores,
the first hyperpolarizability was found to increase along
the sequence Co, Ni, Ru, Fe, where the high values
found for Ru and Fe complexes were attributed to the
occurrence of p-backdonation, clearly supported by IR
and NMR spectroscopic data [16]. Therefore, spectro-
scopic IR and NMR data, besides the employment of
the UV–Vis spectra to probe electronic metal-to-ligand
or ligand-to-metal charge transfer excitations, can be
used to give an insight on the molecular electronic
factors that may be responsible for the second-order
non-linear optical responses.
In this work, spectroscopic data were used on the
family of compounds [CoCp(dppe)(p-NCR)][PF6]2
(R = C6H4NMe2, C6H4NH2, C6H4OMe, C6H4C6H5,
C6H5, C6H4C6H4NO2, and C6H4NO2) in order to eval-
uate the role played by the organometallic fragment on
their second-order NLO response. Published data for re-
lated iron, ruthenium and nickel complexes were also in-
cluded in an attempt to correlate NMR spectroscopic
data with the corresponding second-order non-linear
optical properties. Electrochemical studies by cyclic
voltammetry were performed in all the complexes to
complement these studies.
towards oxidation in air and to moisture both in the
solid state and in solution. Formulation of the new
compounds was supported by analytical data, IR and
1H, 13C and 31P NMR spectra. The molar conductivities
of ca. 10ꢀ3 M solutions of the complexes in nitrometh-
ane, in the range 165–204 Xꢀ1 cm2 molꢀ1, are consistent
with values reported for 2:1 type electrolytes [17].
Typical IR bands confirm the presence of the cyclo-
pentadienyl ligand (ca. 3050–3080 cmꢀ1), the PF6ꢀ anion
(840 and 550 cmꢀ1) and the coordinated nitrile (2235–
2275 cmꢀ1) in all complexes. A positive shift of (CN)
upon coordination was observed in the range 15–55
cmꢀ1. This means a r-coordinated nitrile in which the
overall N„C bond is strengthened upon coordination.
The same behavior was found previously on [NiCp-
PPh3(p-NCR)[PF6] derivatives [18] and seems to indi-
cate the absence of p backdonation in these cobalt
complexes, in opposite of that was found for the previ-
ously reported Fe(II) and Ru(II) analogs [7–10].
Accordingly, recent density functional theory (DFT)
calculations on some of these cobalt complexes confirm
that the ligand–metal interaction are p mainly and have
no evidence of p back donation contribution [19].
1H NMR chemical shifts for the cyclopentadienyl
ring are in the range usually observed for dicationic co-
balt(III) complexes and seems to be only slightly affected
by the nature of the coordinated nitrile. In fact, as the
electron donor character of the p-substituent in benzo-
nitrile ligands increases, the Cp ring resonance shifts up-
field, consistent with an increased electronic density on
the Cp ring due to the increased electron density at the
metal center. This effect was observed in similar nickel
[18], iron [7,8] and ruthenium [9,20] complexes. In con-
trast to the effect observed for Cp ring, the phosphine
signals are relatively insensitive to the nature of the aro-
matic nitrile. For benzene ring protons of the nitrile li-
2. Results and discussion
The complexes were prepared by the reaction of
[CoCpI2CO] with a mixture of DPPE, TlPF6 and excess
of the appropriate p-benzonitrile derivative in dichloro-
methane at room temperature (Scheme 1). After work-
up, orange-red crystals of [CoCp(dppe)(p-NCR)][PF6]2
(R = C6H4NMe2 (1) [16], C6H4NH2 (2), C6H4OMe (3),
C6H4C6H5 (4) [16], C6H5 (5), C6H4C6H4NO2 (6), and
C6H4NO2 (7)) were obtained with yield in the range of
62–87%. The synthesis of the complexes involves the for-
mation of [CoCp(dppe)I]+ in situ, followed by iodide
abstraction by TlPF6 and coordination of the different
p-benzonitrile ligands. The compounds are quite stable
gands,
a shielding effect upon coordination was
observed. This shielding was found to be in the range
of 0.73–0.87 ppm for ortho (relative to the NC group)
protons and 0.08–0.31 ppm for meta protons. A shield-
ing effect on these protons upon coordination was al-
ready found in similar Ru(II) and Fe(II) complexes
and was explained by the occurrence of p-backdonation
from the organometallic fragment to the coordinated ni-
trile ligand [7–10]. In the present Co(III) complexes no
evidence of p-backdonation was found, as mentioned
above. Thus, this shielding effect should be due to other
factors. The origin of the observed upfield shifts could
be an effect of the change of the N„C anisotropy upon
coordination. Moreover, an additional explanation can
be found by assuming an electronic interaction of the
protons with the counter ion, as observed before on
Ru(II) [9] and Ni(II) [18] compounds.
DPPE, p-NCR, TlPF6
Co
CO
R= C6H4NMe2 (
I
Co NCR PF6
P
2
CH2Cl2, r.t.
I
P
13C NMR data of this family of compounds confirm
the evidence found for proton spectra. The Cp ring
chemical shifts are in the range usually observed for
1
), C6H4NH2 (
2
), C6H4OMe (
3
)
), C6H4C6H5 (
4
),
C6H5 (
5
), C6H4C6H4NO2 (
6
), C6H4NO2 (
7
Scheme 1. Reaction scheme for the synthesis of cobalt complexes.

M.H. Garcia et al. / Journal of Organometallic Chemistry 690 (2005) 4063–4071
4065
Table 1
Optical spectral data for complexes [CoCp(dppe)(p-NCR)][PF6]2 in
dichloromethane solution (ca. 5.0 · 10ꢀ5 M)
dicationic Co(III) complexes and is slightly affected by
the nature of substituent at the end of the coordinated ni-
trile. In spite of the fact that the Cp ring displays signals
in a narrow range, there is a linear correlation between
DCp (DCp = dCp(p-substituted benzonitrile) ꢀ dCp(ben-
zonitrile)) and rp Hammett parameters [21] of the sub-
stituents (Fig. 1). This linear realtionship was already
observed in the analogous iron(II) systems [7]. These
results indicate that as the donating ability of the substit-
uents increases, the metal center transmits electron den-
sity to the Cp ring, moving their resonance chemical
shifts upfield. The slope of this linear correlation could
give an insight of the electronic communication between
the nitrile ligands and the organometallic cobalt frag-
ment. An interesting relationship between Cp-shifts
and strength of donor–acceptor interaction on sesqui-
fulvalene complexes was also reported recently by other
authors [22]. The slope of the straight line (0.476) is lower
than the observed for Fe(II) compounds [7] and seems to
indicate a poorer communicability between the nitrile li-
gands and the organometallic cobalt fragment. For the
nitrile ligand carbons, an expected deshielding on the
carbon of the N„C functional group upon coordination
was observed, and for the aromatic carbons an also ex-
pected more significant changes was observed for the car-
bon C1, next to the coordination place.
Compound [CoCp(dppe)(p-NCR)][PF6]2
k
(nm)
e · 104
(Mꢀ1cmꢀ1
)
R = C6H4NMe2 (1)
233
336
413
495
2.19
4.50
0.20
0.15
R = C6H4NH2 (2)
R = C6H4OCH3 (3)
233
308
1.30
3.30
420 (sh)
495
0.10
234
287
1.54
3.80
325 (sh)
420
495
0.03
0.07
R = C6H4C6H5 (4)
234
306
415
495
1.88
3.60
0.04
0.08
R = C6H5 (5)
234
1.66
245 (sh)
275 (sh)
316
1.11
0.06
0.07
415
495
31P NMR data of the complexes showed an expected
deshielding upon coordination of the phosphine, in
accordance with its donor character, and no significant
changes on phosphorous electron density were observed
by the presence of different coordinated nitriles.
The optical absorption spectra of all complexes were
recorded as 5.0 · 10ꢀ5 M solutions in dichloromethane
(Table 1). The spectra for the compound [CoCp(DP-
PE)(p-NCC6H4NMe2)][PF6]2 (1) typify the electronic
spectra of this family of compounds, obtained in dichlo-
romethane solution (Fig. 2). The spectra are character-
ized by the presence of two weak bands in the visible
region, where d-d transitions are expected to occur,
R = C6H4C6H4NO2 (6)
233
307
415
495
0.97
2.19
0.02
0.05
R = C6H4NO2 (7)
233
2.30
270 (sh)
307
415
1.98
0.02
0.07
495
and another two intense absorption bands in the UV re-
gion, attributed to electronic transitions occurring both
in the organometallic fragment {[CoCp(DPPE)]2+]}
(k ꢁ 235 nm) and coordinated nitrile chromophores
(k ꢁ 280ꢀ340 nm). No bands attributed to any nitrile-
metal charge transfer were clearly identified, despite
0.5
6
7
5
50000
0
-0.5
-1
2500
3
4
2000
1500
1000
500
0
40000
30000
20000
10000
1
2
370 480 590 700
-1
-0.5
0
0.5
1
0
σ
p
200
300
400
500
600
Fig. 1. Plot of DCp vs. rp for complexes [CoCp(dppe)(p-NCR)]-
[PF6]2. (DCp = dCp(Y ¼ H) ꢀ Cp(Y = H); rp: Hammett parameters; R =
C6H4NMe2 (1), C6H4NH2 (2), C6H4OMe (3), C6H4C6H5 (4), C6H5 (5),
C6H4C6H4NO2 (6), and C6H4NO2 (7)).
λ (nm)
Fig. 2. UV–Vis absorption spectra of [CoCp(DPPE)(p-NCC6H4-
NMe2)][PF6]2 recorded in CH2Cl2 (5.0 · 10ꢀ5 M).

4066
M.H. Garcia et al. / Journal of Organometallic Chemistry 690 (2005) 4063–4071
Table 3
Electrochemical data for [CoCp(dppe)(p-NCR)][PF6]2 in acetonitrilea,b
the fact that the presence of small shoulders in the UV
region for all the complexes does not exclude this possi-
bility. Solvatochromic studies were performed to verify
this hypothesis but the results reveal that the position
of all the absorption bands remain almost unchanged.
The solvents used (chloroform, mixtures of dichloro-
methane/n-hexane and methanol) were chosen consider-
ing the stability of the compounds. DFT calculations
performed in some of these complexes [19] showed that
this possible electronic transition will be a ligand to me-
tal charge transfer (LMCT) instead of MLCT found in
related iron and ruthenium complexes [8,10].
Compound Ec
[CoCp(dppe)(p-NCR)][PF6]2 (V)
Ea
(V)
Ep/2
(V)
DEc
(mV)
ia/ic
R = C6H4NMe2 (1)
R = C6H4NH2 (2)
ꢀ0.16 ꢀ0.07 ꢀ0.12 90
ꢀ0.55 ꢀ0.49 ꢀ0.52 60
0.8
0.9
0.2
1.12
1.20
1.16 80
ꢀ0.15 ꢀ0.07 ꢀ0.11 80
ꢀ0.56 ꢀ0.49 ꢀ0.52 70
0.7
0.9
–
–
1.38
–
–
R = C6H4OCH3 (3)
R = C6H4C6H5 (4)
R = C6H5 (5)
ꢀ0.15 ꢀ0.07 ꢀ0.11 80
ꢀ0.56 ꢀ0.49 ꢀ0.52 70
0.8
1.0
ꢀ0.16 ꢀ0.07 ꢀ0.12 90
1.0
1.0
The electrochemistry of this family of compounds
was studied by cyclic voltammetry both in dichloro-
methane and acetonitrile between the limits imposed
by the solvents (Tables 2 and 3).
ꢀ0.56 ꢀ0.50 ꢀ0.53 60
ꢀ0.16 ꢀ0.08 ꢀ0.12 80
ꢀ0.56 ꢀ0.47 ꢀ0.52 90
0.8
0.9
In dichloromethane, the electrochemical behavior of
the compounds is characterized by the presence of two
redox processes, exemplified in Fig. 3. The first (A) is
an irreversible process, presenting one single cathodic
wave in the range from ꢀ0.01 to 0.09 V without the ano-
dic counterpart, and the second (B) is a quasi-reversible
process with Ep/2 = 0.55 V, independent of the coordi-
nated nitrile. For complexes 1, 2, 6 and 7 additional redox
waves were observed and attributed to redox processes
occurring on the coordinated nitrile ligands. The changes
R = C6H4C6H4NO2 (6)
ꢀ0.14 ꢀ0.08 ꢀ0.11 60
ꢀ0.56 ꢀ0.50 ꢀ0.53 60
ꢀ1.04 ꢀ0.98 ꢀ1.01 60
0.8
0.9
0.7
–
ꢀ1.59
–
–
–
R = C6H4NO2 (7)
ꢀ0.14 ꢀ0.08 ꢀ0.11 60
ꢀ0.56 ꢀ0.49 ꢀ0.52 70
ꢀ0.87 ꢀ0.80 ꢀ0.84 70
0.7
0.8
0.8
–
ꢀ1.70
–
–
–
R = CH3 (8)
ꢀ0.15 ꢀ0.08 ꢀ0.12 70
0.9
0.9
ꢀ0.55 ꢀ0.49 ꢀ0.52 60
a
Supporting electrolyte: n-Bu4NPF60.1 M; working electrode: Pt;
reference electrode: calomelane; sweep rate: 200 mV/s; temperature: 20
ꢁC; solute concentration: ca. 1 mM.
Table 2
Electrochemical data for [CoCp(dppe)(p-NCR)][PF6]2 in dichloro-
methanea,b
b
In the same conditions. Ep=2ðFeCp2=FeCpþ2 Þ ¼ 0.41 V.
c
DE = Ea ꢀ Ec.
Compound Ec
[CoCp(dppe)(p-NCR)][PF6]2 (V)
Ea
Ep/2
DEc
(mV)
ia/ic
(V)
(V)
B
R = C6H4NMe2 (1)
R = C6H4NH2 (2)
ꢀ0.01
–
–
–
–
ꢀ0.58 ꢀ0.52 ꢀ0.55
60
1.39 100
0.8
0.8
A
1.34
1.44
0.00
–
–
–
–
ꢀ0.58 ꢀ0.52 ꢀ0.55
60
0.8
–
–
1.42
–
–
R = C6H4OCH3 (3)
R = C6H4C6H5 (4)
R = C6H5 (5)
0.02
–
–
–
–
0.8
ꢀ0.58 ꢀ0.52 ꢀ0.55
0.04
ꢀ0.58 ꢀ0.52 ꢀ0.55
0.05
60
60
60
60
1.2
0.8
0.4
0
-0.4
-0.8
-1.2
-1.6
-2
–
–
–
–
–
–
0.9
E (V)
Fig. 3. Cyclic voltammogram of [CoCp(dppe)(p-NCC6H4C6H5)][PF6]2
in dichloromethane containing 0.1 M n-Bu4NPF6 (T = 20 ꢁC; sweep
rate = 200 mV/s).
–
–
–
0.8
ꢀ0.58 ꢀ0.52 ꢀ0.55
0.05
R = C6H4C6H4NO2 (6)
–
–
–
ꢀ0.58 ꢀ0.52 ꢀ0.55
0.8
0.7
–
ꢀ1.14 ꢀ1.02 ꢀ1.08 120
on corresponding potentials upon coordination are not
significant (10–30 mV) and agree with the expected
behavior resulting from the released electron density.
The cathodic wave A, which potential depends on the
ꢀ1.59
0.09
–
–
–
R = C6H4NO2 (7)
–
–
–
–
ꢀ0.58 ꢀ0.52 ꢀ0.55
ꢀ0.92 ꢀ0.84 ꢀ0.88
60
80
0.8
0.7
–
coordinated nitrile, was attributed to formal CoIII
!
ꢀ1.54
–
–
–
CoII reduction of [CoCp(dppe)(p-NCR)]2+ species. This
cathodic wave has no anodic counterpart even at ꢀ20
ꢁC and the sweep direction was inverted immediately
after the reduction potential. This behavior indicates
that the 19-electron species [CoCp(dppe)(p-NCR)]+,
a
Supporting electrolyte: n-Bu4NPF60.1 M; working electrode: Pt;
reference electrode: calomelane; sweep rate: 200 mV/s; temperature: 20
ꢁC; solute concentration: ca. 1 mM.
b
In the same conditions. Ep=2ðFeCp2=FeCpþ2 Þ ¼ 0.41 V.
c
DE = Ea ꢀ Ec.

M.H. Garcia et al. / Journal of Organometallic Chemistry 690 (2005) 4063–4071
4067
formed on the electrode surface at reduction potential,
B
undergo fast decomposition (in an ECC process) and de-
pleted the diffusion layer of the [CoCp(dppe)(p-NCR)]+
species. The fact that the redox process B has an Ep/2
independent of the coordinated nitrile suggests that it
is originated by decomposition of 19-electron species
[CoCp(dppe)(p-NCR)]+ formed in the process (A) due
to the decoordination of nitrile ligand, leading to the
formation of 17-electron species [CoCp(dppe)]+. These
species are sufficient stable to undergo a second reduc-
tion process (B) leading to a 18-electron CoI complex,
[CoCp(dppe)]. Stable 17-electron monocyclopenta-
dienylcobalt species with large and/or p-acid ligands
were already mentioned in the literature [23–25]. This
hypothesis is corroborated by the Ep/2 value attributed
to [CoCp(dppe)]+/[CoCp(dppe)] redox couple, found in
the electrochemical studies of [CoCp(dppe)] complex
[26]. According to these results, the electrochemical
behavior of the studied complexes follows the suggested
reactions depicted in Scheme 2.
A
0.4
0.2
0
-0.2
-0.4
E (V
Fig. 4. Cyclic voltammogram of [CoCp(dppe)(p-NCC6H4C6H5)][PF6]2
-0.6
-0.8
-1
-1.2
)
in acetonitrile containing 0.1
rate = 200 mV/s).
M n-Bu4NPF6(T = 20 ꢁC; sweep
results confirm the suggested substitution of the nitrile
ligands by the acetonitrile solvent before the electrochem-
ical reductive processes on 1–7 complexes. As in dichloro-
methane, the redox waves B in acetonitrile were
attributed to the [CoCp(dppe)]+/[CoCp(dppe)] redox
process.
In acetonitrile, the anodic counterpart of the redox
process A arises (Fig. 4) and the corresponding Ep/2 is
independent of the coordinated nitrile. These results sug-
gest a substitution of the nitrile ligands by the acetonitrile
solvent, leading to [CoCp(dppe)(NCCH3)]2+ species, be-
fore the reductive processes. In order to confirm this
hypothesis, the complex [CoCp(dppe)(NCCH3)][PF6]2
(8) was synthesized and its electrochemistry studied in
acetonitrile. The Ep/2 values observed for the two reduc-
tive processes are similar of those found for the complexes
1–7 (see Table 3). The lability of nitrile ligands was also
confirmed chemically. The complexes were dissolved in
acetonitrile and the solutions stirred during 5 min. After
As mentioned above, the cathodic potential for the
CoIII ! CoII reduction process in dichloromethane de-
pends on the coordinated benzonitrile derivatives and,
in spite of the narrow range observed, this potential
agrees with the relative donor/acceptor character of
their substituents. As the donor character of the substit-
uents increases, the reduction potential becomes less po-
sitive, indicating an increased richness of the metal
center (Fig. 5). These results are consistent with NMR
spectroscopic data above, where a more shielded Cp ring
results from an enhanced electron richness of the metal
center, and agrees with DFT calculations performed in
some of these cobalt complexes where the energy of
the LUMO, mainly located on the metal fragment, fol-
lows the expected trend with the relative donor/acceptor
1
work-up, the samples were analyzed by H NMR spec-
troscopy and all spectra showed the presence of reso-
nances attributed to both the [CoCp(dppe)(NCCH3)]2+
complex and the corresponding free nitrile ligands. These
2+
+
+ e
III
II
Co NCR
Co NCR
A
P
P
P
P
0.1
7
19 e
18 e
5
6
4
2
- NCR
3
0
1
+
- e
-0.1
I
II
Co
Co
-1
-0.5
0
σ
0.5
1
P
P
P
P
+ e
p
B
Fig. 5. Plot of cathodic potential (Ec) vs. Hammett parameters (rp) for
the complexes [CoCp(dppe)(p-NCR)][PF6]2. (R = C6H4NMe2 (1),
C6H4NH2 (2), C6H4OMe (3), C6H4C6H5 (4), C6H5 (5), C6H4C6H4NO2
(6), and C6H4NO2 (7)).
18 e
17 e
Scheme 2. Electrochemical behavior of the complexes [CoCp(dppe)(p-
NCR)][PF6]2 in dichloromethane.

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M.H. Garcia et al. / Journal of Organometallic Chemistry 690 (2005) 4063–4071
6.5
6
character of the benzonitrile substituents [19]. The same
DFT calculations showed that the LUMOs are not sig-
nificantly affected by the substituents of the nitrile li-
gands, which explains the narrow range of cathodic
potentials observed in these cobalt complexes and the
low slope (0.064) of the linear relationship showed in
Fig. 5. As was already recognized above by NMR spec-
troscopic data, this means that the electronic p-coupling
between the cyclopentadienyl metal fragment and the p-
system of the benzonitrile ligands is relatively weak.
The poor electronic communication between the
organometallic fragment and the nitrile ligands, deduced
from both spectroscopic and electrochemical data, lead
to a slight effect of the conjugation length upon the first
hyperpolarizability of these complexes. Moreover, the
absence of low energy charge transfer bands between
the organometallic fragment and the nitrile ligands also
limits the magnitude of their second-order NLO
response. These facts could explain the previously
reported low first hyperpolarizabilities for two of these
cobalt complexes [16]. As discussed above, the slope of
the linear correlation between DCp (DCp = dCp(p-substi-
tuted benzonitrile) ꢀ dCp(benzonitrile)) and rp Hammett
parameters of the nitrile substituents could give an in-
sight of the electronic communicability between the ni-
trile ligands and the organometallic cobalt fragment.
Comparison of the slope magnitude obtained from
available spectroscopic data of related g5-monocyclo-
pentadienyliron, ruthenium and nickel derivatives
[7,8,16,18,20], show that the electronic communicability
between the nitrile ligands and the organometallic frag-
ments increases along the sequence Codppe < NiPPh3 <
Rudppe < Fediop < Fedppe. A similar trend was found
for the first hyperpolarizabilities of complexes contain-
ing these organometallic fragments and p-NCC6H4NO2
and p-NC(C6H4)2NO2 ligands [8,16]. An attempt to cor-
relate the second-order NLO response of these com-
plexes and the degree of electronic communicability
between the organometallic fragments and the nitrile
chromophores gave the linear relationships showed in
Fig. 6. These results seem to indicate that as the elec-
tronic communication between the nitrile ligands and
the organometallic fragments increases, the magnitude
of first hyperpolarizability increases exponentially.
These results seem to indicate that NMR spectroscopic
data might be used to give an insight of the role played
by the organometallic fragment on the first hyperpolar-
izability response of complexes containing the same
chromophore. Within the same scope to relate NMR
spectroscopic parameters with first hyperpolarizabilities,
other authors reported a correlation between 1H–1H
coupling constants and the first hyperpolarizability of
organometallic merocyanines [27]. The linear trend
found in this work can be useful to predict first hyperpo-
larizabilities of other monocyclopentadienylmetal com-
plexes containing p-NCC6H4NO2 or p-NC(C6H4)2NO2
Fediop Fedppe
5.5
5
Rudppe
NiPPh3
4.5
4
Codppe
3.5
3
0.4
0.6
0.8
slope of ∆Cp vs σ
1
1.2
p
Fig. 6. Plot of lnb vs. 13C NMR data of Cp ring in the complexes
[NiCp(PPh3)L][PF6], [CoCp(dppe)L][PF6]2, [RuCp(dppe)L][PF6] and
[FeCp(P-P)L][PF6]. (L = p-NCC6H4NO2 (r) and p-NC(C6H4)2NO2
(h); b: first hyperpolarizability; DCp = dCp(Y ¼ H) ꢀ Cp(Y = H)
; rp:
Hammett parameters).
chromophores if the degree of electronic communication
between the corresponding organometallic fragment and
nitrile derivatives is known. Keeping this in mind, the
synthesis of other monocyclopentadienylmetal com-
plexes by varying the organometallic fragment (i.e., me-
tal, their formal oxidation state and geometry) is
currently in progress in order to confirm the quantitative
relationship found in this work. Furthermore, different
chromophores and metal centers are subject of similar
studies in order to ensure the practicability of using
NMR spectroscopic data as a probe on the second-order
non-linear optical properties of different families of
complexes.
3. Conclusion
The family of [CoCp(dppe)(p-NCR)][PF6]2 com-
plexes with different benzonitrile derivatives has been
synthesized. IR spectroscopic data for these complexes
shows that the coordination of the nitrile ligands to
the metal centre has mainly a r-type character and has
no evidence of metal–ligand p-backdonation contribu-
tion. NMR and cyclic voltammetry data suggest a poor
electronic p-coupling between the g5-cyclopentadienyl-
metal fragment and the p-system of the nitrile ligands.
This poor electronic communication and the absence
of any charge transfer band between the organometallic
fragment and the nitrile ligands, also recognized by their
optical absorption spectra, are consistent with the previ-
ously reported low first hyperpolarizabilities of these
complexes. The degree of electronic coupling between
different g5-cyclopentadienylmetal fragments and the ni-
trile ligands, which were achieved by using NMR spec-
troscopic data, were correlated to the logarithm of the
first hyperpolarizability of the complexes containing p-
NCC6H4NO2 or p-NC(C6H4)2NO2 chromophores. The
linear relationship that was found can be helpful to

M.H. Garcia et al. / Journal of Organometallic Chemistry 690 (2005) 4063–4071
4069
predict first hyperpolarizabilities of related complexes
4.2. Preparation of [CoCp(dppe)(p-NCR)][PF6]2
with different organometallic fragments and seems to
demonstrate that NMR spectroscopic data can be used
as a probe on the second-order non-linear optical re-
sponses of the family of g5-cyclopentadienylmetal com-
plexes with substituted benzonitrile chromophores.
All the complexes were prepared as follows. A solu-
tion of [CoCpI2CO] (0.98 mmol) in dichloromethane
(20 ml) was added to a mixture of TlPF6 (3.00 mmol),
dppe (1.10 mmol) and the appropriate p-benzonitrile
derivative (5.00 mmol) in the same solvent (20 ml). The
resultant suspension was stirred at room temperature
for 72–120 h. A change was observed from dark-violet
to reddish with simultaneous precipitation of TlI. After
filtration, the solvent was evaporated under vacuum
and the solid residue washed several times with diethyl
ether to remove the excess of p-benzonitrile derivative.
The crude solid was recrystallized from acetone/ethyl
ether or dichloromethane/ethyl ether affording the
desired complexes as orange-red microcrystalline
products.
4. Experimental
4.1. General procedures
All preparations and manipulations of the complexes
described in this work were carried out under nitrogen
or argon atmosphere using standard Schlenck tech-
niques. Solvents were purified according to the usual
methods [28].
The starting materials [CoCp(CO)2] and [CoCpI2CO]
were prepared as described in the literature [29,30]. The
complexes [CoCp(dppe)(p-NCC6H4NMe2)][PF6]2 (1)
and [CoCp(dppe)(p-NCC6H4C6H5)][PF6]2 (4) were pre-
viously characterized [16]. The p-substituted benzonitrile
ligands, except p-NC(C6H4)2NO2, were purchased from
Sigma–Aldrich and used as received. The ligand
p-NC(C6H4)2NO2 was prepared by nitration of p-NC-
C6H4C6H5 by standard procedures.
1H and 13C resonances due to the coordinated DPPE
ligand are very similar for all of the complexes and are
found at: 1H NMR (d-acetone) d 3.37–3.62 (m, 4H,
CH2), 7.68–7.84 (m, 16H, C6H5), 8.24–8.36 (m, 4H,
1
C6H5); 13C NMR (d-acetone) d 28.03 (t, CH2, JCP
21.0), 126.86–127.41 (t, Cipso, JCP = 26.0), 131.05 (m,
=
1
C
meta), 132.81 (s, Cpara), 134.06 (m, Cortho). The PF6ꢀ
conterion gives rise to a resonance at d ꢀ142.7 (sept,
JPF = 708.0) in the 31P NMR spectra of all complexes.
Solid-state IR spectra were measured on a Perkin–
Elmer 457 spectrophotometer with KBr pellets; only
4.2.1. [CoCp(dppe)(p-NCC6H4NH2)][PF6]2 (2)
Red crystals; 70% yield, m.p. 183 ꢁC (dec.). Molar
conductivity (Xꢀ1 cm2 molꢀ1): 204.2. IR (KBr, cmꢀ1):
(N„C): 2240. 1H NMR (d-acetone, d): 6.12 (b, 2H,
NH2), 6.30 (s, 5H, g5-C5H5), 6.52 (d, 2H, H3, H5,
1
significant bands are cited in the text. H, 13C and 31P
NMR spectra were recorded on a Varian Unity 300
spectrometer at probe temperature. The 1H and 13C
chemical shifts are reported in parts per million (ppm)
downfield from internal Me4Si and the 31P NMR spec-
tra are reported in ppm downfield from external stan-
dard 85% H3PO4. Coupling constants are reported in
Hz. Spectral assignments follow the numbering scheme
shown in Fig. 7.
Electronic spectra were recorded at room tempera-
ture on a Shimadzu UV-1202 spectrometer in the range
of 200–1100 nm. Melting points were obtained using a
Reichert Thermovar. The molar conductivities of 1
mM solutions of the complexes in nitromethane were re-
corded with a Schott CGB55 Konduktometer at room
temperature. Microanalyses were performed at Instituto
3
3JHH = 8.7), 6.56 (d, 2H, H2, H6, JHH = 9.0). 13C
NMR (d-acetone, d): 93.64 (C1), 94.45 (g5-C5H5),
113.98 (C3, C5), 135.80 (C2, C6), 140.05 (NC), 155.91
(C4). 31P NMR (d-acetone, d): 81.16. Anal. Calc. for
C38H35N2P4F12Co: C, 49.05; H, 3.79; N, 3.01. Found:
C, 48.57; H, 3.86; N, 2.97%.
4.2.2. [CoCp(dppe)(p-NCC6H4OMe)][PF6]2 (3)
Red crystals; 70% yield, m.p. 181 ꢁC (dec.). Melting
point (ꢁC): 181 (dec.). Molar conductivity (Xꢀ1 cm2
molꢀ1) 169.0. IR (KBr, cmꢀ1): (N„C): 2250. 1H
NMR (d-acetone, d): 3.86 (s, 3H, OMe), 6.32 (s, 5H,
3
Superior Tecnico Laboratories using a Fisons Instru-
g5-C5H5), 6.89 (d, 2H, H3, H5, JHH = 9.0), 6.95 (d,
´
3
ments EA1108 system. Data acquisition, integration
and handling were performed using a PC with the soft-
ware package Eager-200 (Carbo Erba Instruments).
2H, H2, H6, JHH = 9.0).13C NMR (d-acetone, d):
56.45 (OMe), 94.72 (g5-C5H5), 100.69 (C1), 115.65
(C3, C5), 136.34 (C2, C6), 137.79 (NC), 166.70 (C4);
31P NMR (d-acetone, d): 81.13. Anal. Calc. for
C39H36NOP4F12Co: C, 49.54; H, 3.84; N, 1.48. Found:
C, 49.48; H, 3.84; N, 1.50%.
2
3
2
3
9
8
NC
X
NC
Y
4
4
1
1
7
10
11
5
5
12
6
6
4.2.3. [CoCp(dppe)(p-NCC6H5)][PF6]2 (5)
Red crystals; 75% yield, m.p. 186 ꢁC (dec.). Molar
Fig. 7. Numbering scheme for NMR spectral assignments of
[CoCp(dppe)(p-NCR)][PF6]2 complexes. (X = NMe2, NH2, OMe, H,
NO2; Y = H, NO2).
conductivity (Xꢀ1 cm2 molꢀ1): 186.5. IR (CH2Cl2,
1
cmꢀ1): (N„C): 2280. H NMR (d-acetone, d): 6.35 (s,

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M.H. Garcia et al. / Journal of Organometallic Chemistry 690 (2005) 4063–4071
5H, g5-C5H5), 6.95 (d, 2H, H2, H6, 3JHH = 8.7), 7.45 (t,
2H, H3, H5, 3JHH = 7.8), 7.72 (d, H, H4). 13C NMR (d-
acetone, d): 94.84 (g5-C5H5), 109.75 (C1), 129.82 (C3,
C5), 134.14 (C2, C6), 136.47 (C4), 140.36 (NC). 31P
NMR (d-acetone, d): 81.04. Anal. Calc. for
C38H34NP4F12Co: C, 49.80; H, 3.74; N, 1.53. Found:
C, 49.70; H, 3.90; N, 1.53%.
in solute and 0.1 M in tetrabutylammonium hexafluoro-
phosphate as supporting electrolyte. The electrochemi-
cal system was checked with a 1 mM solution of
ferrocene in acetonitrile and dichloromethane for which
the ferrocinium/ferrocene electrochemical parameters
(Ep/2 = 0.38 V in acetonitrile and Ep/2 = 0.41 V in dichlo-
romethane; DE = 60ꢀ70 mV; ia/ic = 1) were in good
agreement with the literature [31,32].
4.2.4. [CoCp(dppe)(p-NCC6H4C6H4NO2)][PF6]2 (6)
Red crystals; 81% yield, m.p. 154 ꢁC (dec.). Molar
conductivity (Xꢀ1 cm2 molꢀ1): 177.8. IR (KBr, cmꢀ1):
(N„C): 2265. 1H NMR (d-acetone, d): 6.38 (s, 5H,
The electrolyte was purchased from Sigma–Aldrich,
recrystallized from ethanol, washed with diethyl ether,
and dried under vacuum at 110 ꢁC for 24 h. Reagent
grade acetonitrile and dichloromethane were dried over
P2O5 and CaH2, respectively, and distilled before use
under argon atmosphere. An argon atmosphere was
maintained over the solution during the experiments.
3
g5-C5H5), 7.11 (d, 2H, H2, H6, JHH = 8.7), 7.88 (d,
3
2H, H3, H5, JHH = 9.0), 7.98 (d, 2H, H8, H12,
3JHH = 8.7), 8.35 (d, 2H, H9, H11). 13C NMR (d-ace-
tone, d): 94.95 (g5-C5H5), 109.77 (C1), 124.94 (C9,
C11), 128.66 (C3, C5), 129.44 (C8, C12), 134.83 (C2,
C6), 141.08 (NC), 145.16 (C4), 145.95 (C7), 148.99
(C10). 31P NMR (d-acetone, d): 80.85. Anal. Calc. for
C44H37N2O2P4F12Co: C, 50.93; H, 3.59; N, 2.70. Found:
C, 51.46; H, 3.56; N, 2.82%.
Acknowledgements
ˆ
We thank to Fundac¸a˜o para a Ciencia e Tecnologia
for finantial support (POCTI/QUI/48433/2002).
4.2.5. [CoCp(dppe)(p-NCC6H4NO2)][PF6]2 (7)
Red crystals; 65% yield, m.p. 174 ꢁC (dec.). Molar
conductivity (ꢀ1cm2molꢀ1): 173.2. IR (KBr, cmꢀ1):
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