Synthesis and relative stability of a series of compounds of type [Fe(II)(bztpen)X]+, where bztpen = pentadentate ligand, N 5, and X- = monodentate anion

Article abstract of DOI:10.1021/ic0620743

The structural and solution characterization of novel Fe(II) compounds of the general formula [Fe(bztpen)X]PF₆ and [Fe(bztpen)CH ₃CN](PF₆)₂ is presented, where bztpen is the pentadentate ligand N-benzyl-N,N′,N′-tris(2-methylpyridyl) ethylenediamine and X^- is a monodentate ligand. All complexes were characterized in solution and in the solid state, employing the usual techniques and single-crystal X-ray diffraction. The results obtained are discussed in terms of the existing information for some previously reported analogous compounds to arrive at a rationalization regarding the influence of a variation in the coordination environment of all compounds and to evaluate their relative stability. The observed magnetic response in the solid state is paramagnetic in the entire temperature range for the Cl^-, Br^-, l ^-, OCN^-, and SCN^- derivatives, while the N(CN)₂^-, CH₃CN, and CN^- derivatives are diamagnetic. The diamagnetic character of these last two compounds is confirmed in acetonitrile solution, while a spin transition step is observed for the N(CN)₂^- derivative. Diffraction data for all compounds as hexafluorophosphates shows that the I^-, Br^-, and OCN^- derivatives crystallize in the orthorhombic space group Pbca, while the CN^-, SCN^-, and CH₃CN compounds crystallize in the triclinic space group P1. Average bond lengths and the trigonal distortion parameter can be correlated to the observed magnetic susceptibility depending on the coordinated monodentate ligand. Solution measurements of electronic properties for the compounds follow the trend established by the spectrochemical series. The relative stability of the Fe(II) complexes can be established in terms of the percentage of dissociation from the voltammetry and conductivity results, which are consistent with those obtained spectrophotometrically, mainly, the larger stability for the CN^- derivative and the lower for the I^- derivative. The redox potential and percentage of dissociation values allow for the estimation of the relative stability constants for the Fe(II) and Fe(III) complexes.

Full text of DOI:10.1021/ic0620743

Inorg. Chem. 2007, 46, 7285−7293
Synthesis and Relative Stability of a Series of Compounds of Type
+
-
[Fe(II)(bztpen)X] , Where bztpen
) Pentadentate Ligand, N5, and X )
Monodentate Anion
Norma Ortega-Villar,‡ V´ıctor M. Ugalde-Sald´ıvar,‡ M. Carmen Mun˜oz,§ Luis A. Ortiz-Frade,‡
Jose´ G. Alvarado-Rodr´ıguez,| Jose´ A. Real,† and Rafael Moreno-Esparza*,‡
UniVersitat de Vale`ncia, Instituto de Ciencia Molecular, Edificio de Institutos de Paterna,
Apartado de correos 22085, 46071 Vale`ncia, Spain, Facultad de Qu´ımica (UNAM), Edificio B.
AV. UniVersidad 3000, Coyoaca´n, Me´xico D. F. 04510, Me´xico, Departament de F´ısica Aplicada.
UniVersitat Polite´cnica de Vale`ncia, Camino de Vera s/n, E-46022 Vale`ncia, Spain, and Centro de
InVestigaciones Qu´ımicas, UniVersidad Auto´noma del Estado de Hidalgo, Ciudad UniVersitaria
km. 4.5, Carretera Pachuca-Tulancingo 42074, Pachuca, Hidalgo, Me´xico
Received October 30, 2006
The structural and solution characterization of novel Fe(II) compounds of the general formula [Fe(bztpen)X]PF6
and [Fe(bztpen)CH3CN](PF6)2 is presented, where bztpen is the pentadentate ligand N-benzyl-N,N ,N -tris(2-
′
′
methylpyridyl)ethylenediamine and X- is a monodentate ligand. All complexes were characterized in solution and
in the solid state, employing the usual techniques and single-crystal X-ray diffraction. The results obtained are
discussed in terms of the existing information for some previously reported analogous compounds to arrive at a
rationalization regarding the influence of a variation in the coordination environment of all compounds and to evaluate
their relative stability. The observed magnetic response in the solid state is paramagnetic in the entire temperature
range for the Cl-, Br-, I-, OCN-, and SCN- derivatives, while the N(CN)2-, CH3CN, and CN- derivatives are
diamagnetic. The diamagnetic character of these last two compounds is confirmed in acetonitrile solution, while a
spin transition step is observed for the N(CN)2- derivative. Diffraction data for all compounds as hexafluorophosphates
shows that the I-, Br-, and OCN- derivatives crystallize in the orthorhombic space group Pbca, while the CN-,
SCN-, and CH3CN compounds crystallize in the triclinic space group P1
h. Average bond lengths and the trigonal
distortion parameter can be correlated to the observed magnetic susceptibility depending on the coordinated
monodentate ligand. Solution measurements of electronic properties for the compounds follow the trend established
by the spectrochemical series. The relative stability of the Fe(II) complexes can be established in terms of the
percentage of dissociation from the voltammetry and conductivity results, which are consistent with those obtained
spectrophotometrically, mainly, the larger stability for the CN- derivative and the lower for the I- derivative. The
redox potential and percentage of dissociation values allow for the estimation of the relative stability constants for
the Fe(II) and Fe(III) complexes.
Introduction
area. To this day, there is a significant database of experi-
mental and theoretical results, continuously mentioned and
frequently employed, which has proven to be extremely
useful to understand and correlate the magnetic behavior of
these complexes with their structural parameters. Although
several reviews on this subject have been published lately,
the ongoing study of these systems does not fail to provide
novel and interesting information.1,2 In addition, solution
studies of the magnetic behavior for these compounds are
extremely scarce.
The study of mononuclear complexes of Fe(II) coordinated
to polydentate ligands through nitrogen atoms (N6 or N5X)
has, during the last fifteen years, yielded a fruitful research
field with excellent results, especially in the magnetochemical
* To whom correspondence should be addressed. E-mail: moresp@
servidor.unam.mx.
† Universitat de Vale`ncia.
‡ Facultad de Qu´ımica (UNAM).
§ Universitat Polite´cnica de Vale`ncia.
| Universidad Auto´noma del Estado de Hidalgo.
10.1021/ic0620743 CCC: $37.00
Published on Web 08/04/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 18, 2007 7285

Ortega-Villar et al.
Since the 1990s, hydroxyperoxidase-like behavior for the
[Fe(II)(R-tpen)Cl]+-type compounds has been reported.3 The
proposed mechanism for this behavior includes substitution
of the chloride ligand by the hydroxyperoxo radical (‚OOH),
with the consequent oxidation of Fe(II) to Fe(III).3-8 These
compounds also show superoxydismutase (SOD) activity.9
Experimental Section
Synthesis. All manipulations were performed under an argon
atmosphere, using standard Schlenk techniques. Commercially
available reagents and solvents (analytical grade) were used without
prior purification. Acetonitrile and acetone solvents were HPLC
grade. The ligand bztpen was prepared according to the literature
-
procedure.8 Synthesis of the compound with N(CN)2 (6) was
From the proposed mechanism, it seems reasonable to
assume that a variation of the monodentate ligand should
modulate both stability and redox properties in these
compounds. Because only the chloride and bromide (1 and
2) derivatives have been studied previously,6 the investigation
of the aforementioned properties when different monodentate
substituents X- are employed is an interesting field that may
extend the existing available information.
reported previously.10 The syntheses of [Fe(bztpen)X]PF6 com-
pounds where X- ) Br-, I-, SCN-, OCN-, and CH3CN was carried
out according to the following reaction scheme
[Fe(H2O)6](BF4)2 + bztpen ) [Fe(bztpen)H2O](BF4)2‚nH2O +
X-(excess) + NH4PF6(excess) ) [Fe(bztpen)X]PF6 + 2NH4BF4
For the CH3CN compound, liquid acetonitrile was added instead
of X-, and a slightly different procedure was employed, compared
to that outlined below.
With that perspective in mind, we carried out the synthesis
and characterization in the solid state and in solution of a
new series of [Fe(II)(bztpen)X]n+ compounds, where X )
I- (3), OCN- (4), SCN- (5), CH3CN (7), and CN- (8). The
main aim is to evaluate the variation in properties such as
magnetic behavior both in solid state and solution, Fe-N5
and Fe-L6 average bond distances, electronic absorption
spectra, and redox potentials. The analysis of such parameters
should render more arguments to establish a relationship
between the studied properties. Consequently, these results
may in future be employed in the enzymatic catalysis area
of study.
Preparation of [Fe(bztpen)X]PF6 Compounds. The cationic
complex [Fe(bztpen)CH3OH]2+ was prepared by the dropwise
addition of a solution of Fe(BF4)2‚6H2O (0.08 g, 0.24 mmol in 5
mL methanol) to a solution of bztpen (0.10 g, 0.24 mmol in 10
mL methanol). The coordinated solvent molecule of the precursor
[Fe(bztpen)CH3OH]2+ was then replaced with X- by the in situ
addition of a freshly prepared methanolic solution containing an
excess of the corresponding potassium salt (∼0.1 M, 10-20 mL).
For the CH3CN derivative, 15-20 mL of spectroscopy grade
acetonitrile was added. The resulting yellow (Cl-, Br-, I-, OCN-,
-
and SCN-) or brown (N(CN)2 and CH3CN) mixture was stirred
for 20 min, after which a solution of NH4PF6 (0.12 g, 0.71 mmol)
in methanol was added very slowly (20 mL). In some cases, the
precipitate formed was removed by filtration. The resulting solution
was slowly evaporated under argon during ∼36 h, in which time-
lapse monocrystalline samples of each compound were obtained.
[Fe(bztpen)Cl]PF6 (1). Yield: 0.087 g, 56%. Yellow crystals.
The results obtained in this work will be compared to those
previously reported for the compounds with X- ) Cl-, Br-,6
and N(CN)2- (dicyanamide) (6)10 to analyze the coordination
environment of each compound and to estimate its field
strength. The analysis of the redox potential values should
yield information indicative of the relative stability between
the Fe(II) and Fe(III) complexes. Finally, the percentage of
dissociation data for the monodentate ligand X- exchanging
with the solvent molecule for these complexes will be
presented.
-
FAB MS: m/z 498 (M - PF6 - Cl- + H2O + e-)+, 514 (M -
PF6-)+. Anal. Calcd for C27H29N5ClF6PFe: C, 49.13; N, 10.56; H,
4.40. Found: C, 49.60; N, 10.61; H, 4.46%. This compound has
already been reported.6
[Fe(bztpen)Br]PF6 (2). Yield: 0.063 g, 38%. Yellow crystals.
FAB MS: m/z 498 (M - PF6-Br- + H2O + e-)+, 558 (M -
PF6-)+. Anal. Calcd for C27H29N5BrF6PFe: C, 46.02; N, 9.43; H,
4.12. Found: C, 46.52; N, 9.95; H, 4.07%.
[Fe(bztpen)I]PF6 (3). Yield: 0.084 g, 47%. Yellow crystals.
(1) Batten, S. R.; Bjernemose, J.; Jensen, P.; Leita, B. A.; Murray, K. S.;
Mourabaki, B.; Smith, J. P.; Toftlund, H. Dalton Trans. 2004, 20,
3370-3375.
-
FAB MS: m/z 498 (M - PF6 - I- + H2O + e-)+, 606 (M -
PF6-)+. Anal. Calcd for C27H29N5IF6PFe: C, 43.14; N, 9.32; H,
3.86. Found: C, 43.12; N, 9.17; H, 3.65%.
(2) Toftlund, H.; McGarvey, J. J. Top. Curr. Chem. 2004, 233, 151-
166.
[Fe(bztpen)OCN]PF6, (4). Yield: 0.074 g, 47%. Yellow
crystals. FAB MS: m/z 521 (M - PF6-)+. Anal. Calcd for
C28H29N6OF6PFe: C, 50.45; N, 12.61; H, 4.35. Found: C, 49.62;
(3) Bernal, I.; Jensen, I. M.; Jensen, K. B.; McKenzie, C. J.; Toftlund,
H.; Tuchagues, J. P. Dalton Trans. 1995, 3669-3675.
(4) Roelfes, G.; Vrajmasu, V.; Chen, K.; Ho, R. Y. N.; Rohde, J. U.;
Zondervan, Chapter; la Crois, R. M.; Schudde, E. P.; Lutz, M.; Spek,
A. L.; Hage, R.; Feringa, B. L.; Mu¨nck, E.; Que, L., Jr. Inorg. Chem.
2003, 42, 2639-2653.
(5) Costas, M.; Mehn, M. P.; Jensen, M. P.; Que, L., Jr. Chem. ReV. 2004,
104, 939-986.
(6) Hazell, A.; McKenzie, C. J.; Nielsen, L. P.; Schindler, S.; Weitzer,
M. J. Chem. Soc., Dalton Trans. 2002, 310-317.
N, 12.62; H, 3.87%. IR (KBr): ν(ÃCΝ) 2212 cm-1
.
[Fe(bztpen)SCN]PF6 (5). Yield: 0.104 g, 65%. Orange crystals.
FAB MS: m/z 537 (M - PF6-)+. Anal. Calcd for C28H29N6SF6-
PFe: C, 49.27; N, 12.32; H, 4.25. Found: C, 49.57; N, 12.06; H,
4.06%. IR (KBr): ν(SCN) 2036 cm-1
.
[Fe(bztpen)CH3CN](PF6)2 (7). Yield: 0.047 g, 25%. Brown
(7) (a) Horner, O.; Jeandey, C.; Oddou, J.; Bonville, P.; McKenzie, C. J.;
Lotour, J. M. Eur. J. Inorg. Chem. 2002, 12, 3278-3283. (b) Nielsen,
A.; Larsen, F. B.; Bond, A. D.; McKenzie, C. J. Angew. Chem., Int.
Ed. 2006, 45, 1602-1606.
-
crystals. FAB MS: m/z 497 (M - 2PF6 - CH3CN + H2O +
-
e-)+, 650 (M - PF6 - CH3). Anal. Calcd for C29H32N6F12P2Fe:
C, 42.98; N, 10.37; H, 3.98. Found: C, 42.94; N, 10.47; H, 4.31%.
(8) Duelund, L.; Hazell, R.; McKenzie, C. J.; Nielsen, L. P.; Toftlund,
H. J. Chem. Soc., Dalton Trans. 2001, 152-156.
IR (KBr): ν(CN) 2268 cm-1
.
(9) Tamura, M.; Urano, Y.; Kikuchi, K.; Higuchi, T.; Hirobe, M.; Nagano,
T. J. Organomet. Chem. 2000, 611, 586-592.
A slightly different technique was employed to obtain the cyanide
(X- ) CN-) compound, where a methanol-acetone solution of the
[Fe(bztpen)Cl]PF6 compound was used, with the following reaction
scheme
(10) Ortega-Villar, N.; Thompson, A. L.; Mun˜oz, M. C.; Ugalde-Sald´ıvar,
V. M.; Goeta, A. E.; Moreno-Esparza, R.; Real, J. A. Chem.sEur. J.
2005, 11, 5721-5734.
7286 Inorganic Chemistry, Vol. 46, No. 18, 2007

Series of [Fe(II)(bztpen)X]+-Type Compounds
dispersion corrections were obtained from the International Tables
for Crystallography, Vol. A.14 Unit cell parameters, along with data
collection and refinement details, for all the reported complexes
are listed in Table 1. Selected bond lengths of all the compounds
are listed in Table 2. All molecular structure drawings were
generated using the WINGX suite of crystallographic programs for
Windows.15 The labels of the coordinated atoms of bztpen used in
all the studied compounds are shown in Scheme 1.
[Fe(bztpen)Cl]PF6 + KCN(excess) + NH4PF6(excess)
)
[Fe(bztpen)CN]PF6 + KCl
[Fe(bztpen)CN]PF6‚H2O‚MeOH (8). Yield: 0.090 g, 54%. Red
crystals. FAB MS: m/z 505 (M - PF6- - H2O - CH3OH)+. Anal.
Calcd for C29H35N6O2F6PFe: C, 51.43; N, 12.00; H, 4.57. Found:
C, 50.03; N, 12.06; H, 5.02%. IR (KBr): ν(CN) 2063 cm-1
.
Physical Measurements. All complexes were characterized
using IR (Perkin-Elmer FT-IR spectrophotometer, 1605), FAB+
(JEOL JMS-SX spectrometer, 102 A), elemental analysis (Fissons
EA 1105 CHNS-O microanalyzer), UV-vis (Diode array HP
8493A spectrophotometer), solid-state magnetic susceptibility
(SQUID Quantum Design MPMS2 susceptometer, 5.5 T, 1 T; in
the temperature range of 1.8-300 K), solution magnetic suscep-
tibility (Evans method11,12 in a 400 MHz Varian Unity Inova NMR
spectrometer), where both the internal reference and the sample
(10-2 M) contained TMS dissolved in acetone-d6 in the temperature
range from 190 to 310 K, molar conductivity (YSI 3100 conductiv-
ity instrument with a 3253 cell Pt/Pt, K ) 1.0 cm-1) using
acetonitrile as solvent at room temperature, and cyclic voltammetry
(EG & G PAR potentiostat-galvanostat model 263A). All electro-
chemical measurements were performed in acetonitrile solution
containing 0.1 M N-tetrabutylammonium tetrafluoroborate, (C4H9)4-
NBF4, as the supporting electrolyte. A typical three-electrode array
was employed for all electrochemical measurements: platinum
microdisk (Pt, 2.01 mm2) as working electrode, Pt wire as
counterelectrode, and a pseudoreference electrode of AgCl(s)-Ag-
(wire) immersed in an acetonitrile 0.1 M (C4H9)4NCl solution. All
solutions were deoxygenated with N2 before each measurement.
All voltammograms were started from the current null potential
(Ei)0) and were scanned in both directions, positive and negative.
In agreement with IUPAC convention, the voltammogram of the
ferrocene/ferricinium (Fc/Fc+) system was obtained to establish the
Results and Discussion
All synthetic procedures yielded compounds whose spec-
troscopic characterization, X-ray diffraction, and elemental
analysis is indicative of complexes of Fe(II) with the general
formula [Fe(bztpen)X]n+. Because of the difficulty in pre-
cipitating the compounds with the BF4- counterion, exchange
-
with PF6 was chosen to crystallize all compounds.
The IR spectra of all compounds confirm the coordination
of the pentadentate ligand (bztpen) to the metal; in fact, all
relevant ligand bands show shifts of ∼10-20 cm-1 in the
1600-1300 cm-1 range, when compared to those of the free
ligand.8 Additionally, all spectra clearly display the bands
at 840 and 560 cm-1, corresponding to the vibration modes
for the octahedral hexafluorophosphate unit, PF6-, confirming
its presence as counteranion of the cationic fragment. All
compounds containing the nitrile group display the ν(CN)
absorption in the 2200-2000 cm-1 range.16 For the SCN-
(5) compound, a band is observed consisting of the overlap-
ping of two absorptions occurring at 2052 and 2036 cm-1
and with almost identical intensity patterns. This is consistent
with the existence of two cations in the unit cell with slightly
different values for the Fe-NCS distances and angles. The
OCN- derivative (4) displays the largest ν(OCN) frequency
value at 2212 cm-1. When compared to the values observed
for the SCN- (5) compound, the larger value obtained for
the OCN- (4) compound must take into account the
difference in mass of the oxygen atom relative to the sulfur
atom because the ν(CN) vibration requires displacement of
the CO and CS fragments in the triatomic ligand.
values of half wave potentials (E1/2) from the expression E1/2
)
(Eap + Ecp)/2. To obtain the normalized current for each complex,
the measured current was divided by the exact molar concentration
of the electroactive species.
Single-Crystal X-ray Diffraction. Diffraction data of prismatic
crystals for the OCN- (4) complex were collected at 298 K with
an Enraf-Nonius CAD4 diffractometer using graphite-monochro-
mated Mo KR radiation (λ ) 0.71073 Å), while for the I- (3) and
SCN- (5) complexes, the data were collected at 298 K with a Bruker
6000 CCD area detector diffractometer and analyzed using mono-
chromated Mo KR radiation (λ ) 0.71073 Å). Diffraction data for
the Br- (2), CH3CN (7), and CN- (8) complexes were collected at
298 K on a Siemens P4 diffractometer, using the standard procedure
with monochromated Mo KR radiation (λ ) 0.71073 Å). A
semiempirical ψ-scan absorption correction was applied to all the
data. The structures were solved and refined without constraints or
restraints. A minor disorder was observed in the hexafluorophos-
phate ions for all the reported compounds. All structures were solved
by direct methods using SHELXS-97-2. Least-squares refinement
based on F2 was carried out by the full-matrix method with
SHELXL-97-2.13 All non-hydrogen atoms were refined with
anisotropic thermal parameters. The hydrogen atoms for all the
reported structures were located in the difference map and included
in the refinement with an isotropic fixed thermal parameter using
a “riding” model. Neutral atom scattering factors and anomalous
X-ray Structures of the Br-(2), I-(3), OCN-,(4),
SCN-(5), CH3CN (7), and CN-(8) Hexafluorophosphate
Derivatives. Structures were solved partly to study the effect
on the coordination environment of the metal produced by
each monodentate ligand. The magnetic, spectroscopic, and
electrochemical properties can be greatly modified by these
effects. The structures of the complexes with X- ) Cl-, Br-
-
(as a perchlorate), and N(CN)2 were reported in previous
works.6,10
All complexes were solved at 298 K. Complexes 2, 3, and
4 are yellow and crystallize in the orthorhombic space group
Pbca. Orange 5, red brown 7, and dark red 8 crystallize in
the P1h space group. All compounds have hexafluorophos-
phate counteranions which neutralize the charge of the
complex cation. The compound with cyanide has water and
(14) Hahn, T., Ed. International Tables for Crystallography; Kluwer
Academic Publishers: Dordrecht, The Netherlands, 1995; Vol. A.
(15) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837-838.
(16) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coor-
dination Compounds, 5th ed.; John Wiley & Sons: New York, 1997;
Part B.
(11) Evans, D. F. J. Chem. Soc. 1959, 2003-2005.
(12) Shubert, E. M. J. Chem. Educ. 1992, 69, 62-69.
(13) Sheldrick, G. M. SHELX97, Programs for Crystal Structure Analysis,
release 97-2; Institu¨t fu¨r Anorganische Chemie der Universita¨t:
Go¨ttingen, Germany, 1998.
Inorganic Chemistry, Vol. 46, No. 18, 2007 7287

Ortega-Villar et al.
Table 1. Crystal and Structure Refinement Data for [Fe(bztpen)X]+/2+ Cations
parameters
Br- (2) I- (3) OCN- (4)
space group Pbca Pbca
SCN- (5)
CH3CN- (7)
CN- (8)
Pbca
P1h
Pbca
P1h
a (Å)
b (Å)
c (Å)
18.452
16.9375
18.471
90
90
90
17.445
18.465
18.465
90
90
90
18.227
17.703
18.250
90
90
90
8.814
17.921
18.913
19.744
90
90
90
6692.0
8
1.609
9.485
18.842
19.003
91.12
95.70
92.74
3135.78
4
1.446
10.877
16.570
73.26
81.56
76.04
1610.43
2
1.469
R (deg)
â (deg)
γ (deg)
vol (cm3)
Z
5772.7
8
5947.98
8
5888.77
8
D
calcd (Mg m-3
)
1.621
1.678
1.503
T (K)
298(2)
2.028
0.5 × 0.5 × 0.5
1.97-25.99
6712
298(2)
1.663
0.4 × 0.2 × 0.15
1.95-24.00
24 393
4618
298(2)
0.638
0.4 × 0.2 × 0.2
1.95-26.69
40 513
6205
298(2)
0.663
0.1 × 0.1 × 0.1
2.15-24.07
16 912
9709
298(2)
0.645
0.4 × 0.2 × 0.2
2.15-27.48
14 246
7603
298(2)
0.600
0.6 × 0.4 × 0.1
1.29-29.00
9312
µ(Mo KR) (mm-1
cryst size (mm)
θ range (deg)
total reflns
)
independent reflns
5562
7861
data/constraints/params
R1
5562/0/370
0.0733
0.1306
0.655
4618/0/370
0.0473
0.1275
1.195
6205/0/385
0.0716
0.2194
1.023
9709/0/775
0.0545
0.0807
0.595
7603/0/452
0.0782
0.2134
1.159
7861/3/414
0.0582
0.1465
0.610
wR2
∆Fmax (e Å-3
)
)
∆Fmin (e Å-3
-0.471
-0.498
-0.449
-0.267
-0.953
-0.483
Table 2. Selected Fe-L Bond Lengths (Å) and Distortion Parameter (Φ/deg) for [Fe(bztpen)X]+/2+ Cations
Cl- (1)
Br- (2)
I- (3)
OCN- (4)
SCN- (5)
N(CN)2- (6)
CH3CN (7)
CN- (8)
ref
6
this work
2.232(5)
2.244(5)
2.177(5)
2.292(5)
2.278(5)
2.498(1)
2.245
this work
2.209(5)
2.226(4)
2.172(4)
2.283(4)
2.269(5)
2.720(1)
2.232
this work
2.162(5)
2.244(4)
2.233(4)
2.272(4)
2.274(4)
2.007(5)
2.237
this work
2.187(5)
2.252(4)
2.174(5)
2.245(4)
2.241(4)
2.026(5)
2.220
10
this work
1.975(4)
2.000(4)
1.971(4)
2.077(4)
1.994(4)
1.925(4)
2.003
this work
1.972(3)
2.024(2)
1.967(3)
2.079(3)
1.988(3)
1.914(3)
2.006
Fe1-N1
Fe1-N2
Fe1-N3
Fe1-N4
Fe1-N5
[Fe-X]
[Fe-N5]average
[Fe-L6]average
Φ
2.228(3)
2.254(3)
2.175(3)
2.288(3)
2.279(3)
2.329(1)
2.245
1.976(4)
2.002(3)
1.965(3)
2.089(3)
1.995(3)
1.956(4)
2.005
2.259
7.84
2.287
8.29
2.313
8.70
2.199
7.21
2.188
8.51
1.997
3.04
1.990
2.66
1.991
2.75
Scheme 1. General Structure for bztpen Ligand and Numbering
Scheme of the Coordinated Nitrogen Atoms
is occupied by iodide and bromide, for the monatomic anions,
and the nitrogen atom for the OCN- and SCN- anions and
the CH3CN molecule. Coordination to the metal center occurs
via the carbon atom for the CN- anion. For all compounds
obtained, monodentate coordination is observed. The total
average bond distances in the octahedron, [Fe-L6]average, are
shown in Table 2. In all complexes, the bztpen ligand
envelops the iron atom defining a distorted square pyramid
with the nitrogen atom N(2) lying on the axial apex. The
N(2) atom is in the center of a tripod with a distance to the
iron atom shown in Table 2; the arms of the tripod are
defined by two picolylamine moieties [N(2)-C(6)-C(5)-
N(1)] and [N(2)-C(7)-C(8)-N(3)] and one ethylenedi-
amine moiety [N(2)-C(13)-C(14)-N(4)]. These arms are
anchored to the iron atom by the N(1), N(3), and N(4) atoms.
While N(1) is trans to the N(4) atom of ethylenediamine,
the N(3) atom from the other picolylamine fragment is trans
to the fifth nitrogen atom of the square pyramid, N(5). The
X- ligand is in a trans configuration to N(2). Fe-X bond
distances are shown in Table 2. Bond angles around the iron
atom show distortion from the expected 90 and 180° for a
perfect octahedron. This distortion is imposed by the
geometrical constraints of the pentadentate ligand, as well
as by the characteristics of each X- ligand.
methanol crystallization molecules, and in this case, hydrogen
bonds between water and methanol molecules were observed.
In addition, the water molecules form four-membered rings
with the nitrogen from the cyanide. The rest of the complexes
show no solvent crystallization molecules.
Because there are no relevant intermolecular interactions,
hydrogen bonding (except for 8) or π stacking, in the cell,
cohesion is attributed essentially to the electrostatic interac-
tion of the [Fe(bztpen)X]+ cations and the slightly disordered
-
PF6 anions for the rest of the complexes.
Complexes 2, 3, 4, 7, and 8 consist of one [Fe(II)-
(bztpen)X]n+ cation per asymmetric unit, while complex 5
consists of two [Fe(bztpen)SCN]+ equivalent cations in the
asymmetric unit.
In all the complexes, the iron atom is in a slightly distorted
octahedral [FeN5X] environment, where five of the nitrogen
atoms belong to the pentadentate bztpen ligand, with an
average Fe-N bond distance, [FeN5]average, shown in Table
2. The remaining coordination site in the [FeN5X] octahedron
The molecular structures of the CN- (8) and the OCN-
(4) complexes are displayed in Figures 1 and 2, respectively,
together with the atom numbering scheme for the octahedral
site employed for all the described compounds. Relevant
7288 Inorganic Chemistry, Vol. 46, No. 18, 2007

Series of [Fe(II)(bztpen)X]+-Type Compounds
whole temperature range, with an approximate magnetic
moment value of 5.0 µB, corresponding to the high-spin (HS)
configuration for Fe(II). The magnetic study of the latter
complex from 300 to 1.8 K has been included in the
Supporting Information. These compounds have a charac-
teristic yellow color, except for the SCN- which is orange.
It must be mentioned, however, that the Br- (2) complex
with the perchlorate anion has been previously obtained6 as
paramagnetic brown crystals. The complexes with CN-, CH3-
CN, and N(CN)2- are diamagnetic with a magnetic moment
value of ∼0 µB, corresponding to the low-spin (LS) Fe(II)
configuration. These crystals have a characteristic and very
intense red-brown color. None of the compounds studied is
a spin-crossover system.
As can be seen in Table 2, the [Fe-N5]average distances
for the paramagnetic complexes 1, 2, 3, 4, and 5 are in the
range of 2.22-2.25 Å, while the diamagnetic complexes of
6, 7, and 8 are in the range of 2.00-2.01 Å. This trend is
also observed for the [Fe-L6]average distances, which are in
the 2.19-2.31 Å range for the paramagnetic complexes and
1.99-2.01 Å for the diamagnetic complexes.
Figure 1. ORTEP diagram of the [Fe(bztpen)CN]PF6 compound. The
thermal ellipsoids are shown at 50% probability.
The analysis of bond angles around the Fe(II) ion shows
a great deviation from the expected regular octahedron. The
fact that the iron atoms in each complex are in a distorted
octahedral environment, is imposed by two factors. First, the
geometrical constraints imposed by the bztpen ligand and,
second, the basicity of the monodentate anion. The trigonal
distortion parameter Φ as defined by Purcell19 and used by
McCusker20 has been calculated to arrive at a quantitative
measurement regarding the extent of distortion for each
compound. [The average trigonal distortion parameter can
24
be defined as Φ ) θ)1(|60 - θ|)/24, where θ represents
∑
the trigonal complementary angles of all triangles defined
by eight faces of the octahedron, yielding a total of 24
trigonal angles.] Calculated values for this parameter are
shown in Table 2.
Figure 2. ORTEP diagram of the [Fe(bztpen)OCN]PF6 compound. The
thermal ellipsoids are shown at 50% probability.
Once again, two distinct groups appear: the first in the
7.21-8.70° range and the second in the 2.66-3.04° range.
The former group corresponds to HS Fe(II) complexes, while
the latter group corresponds to the LS Fe(II) complexes.18,20,21
The average bond distances, [Fe-N5]average and [Fe-
L6]average, and the trigonal distortion parameter Φ, thus appear
to be directly related to the magnetic properties of the
complexes.17,18,21
From the above discussion, it seems clear that structural
parameters are useful in pointing out the existence of two
groups of complexes. To determine if there is any modulation
of the electronic properties caused by the monodentate
ligands, both the spectroscopic study and the electrochemical
behavior of all complexes is required.
crystal data for these complexes are shown in Table 1. CIF
files of the six new complexes and ORTEP diagrams with
the molecular structure for the rest of the reported compounds
are included as Supporting Information.
The results in Table 2 show that, on the basis of the [Fe-
N5]average and [Fe-L6]average bond distances, the compounds
may be classified in two different groups. Although variations
are observed in the magnetic, spectroscopic, and electro-
chemical properties for these two groups of compounds, it
will be attempted to correlate structural parameters with each
property to rationalize the observed effects arising from X-
substitution.
Magnetic Response and Structural Parameters. It is
well-known that there is a clear relationship between the
magnetic response of the octahedral complexes of Fe(II) and
the metal ligand distances.17,18
The magnetic response of solid samples of the derivatives
with Cl-, Br-, I-, OCN-, and SCN- is paramagnetic in the
Solution Studies. All complexes were characterized using
UV-vis spectroscopy and cyclic voltammetry. In addition,
a solution study of the magnetic properties in acetonitrile
(19) Purcell, K. F. Am. Chem. Soc. 1979, 101/18, 5147-5152.
(20) McCusker, J. K.; Rheingold, A. L.; Hendrickson, D. N. Inorg. Chem.
1996, 35, 2100-2112.
(21) Guionneau, P.; Marchivie, M.; Bravic, G.; Le´tard, J.-F.; Chasseau, D.
Top. Curr. Chem. 2004, 234, 97-128.
(17) Gu¨tlich, P.; Hauser, A.; Spiering, H. Angew. Chem., Int. Ed. Engl.
1994, 33, 2024-2054.
(18) Hauser, A. Top. Curr. Chem. 2004, 233, 49-58.
Inorganic Chemistry, Vol. 46, No. 18, 2007 7289

Ortega-Villar et al.
Figure 3. UV-vis spectra of [Fe(bztpen)X]PF6 compounds in CH3CN solutions obtained in the ranges of (a) 190-600 nm and (b) 500-1100 nm.
Table 3. Molar Absorption Coefficients for [Fe(bztpen)X](PF6)n
Table 4. Voltammetric Parameters and Percentage of Dissociation (%
dissoc) Data for ∼10-3 M [Fe(bztpen)X]+ in Acetonitrile Solutionsa
Compounds in CH3CN Solutions at 293 Ka
b
charger tranfer (CT)
d-d
Epa
(V)
Epc
(V)
Ep
(V)
E1/2
(V)
E1/2
(V)
ipc/ipa
λmax
ν
ꢀ
λmax
ν
ꢀ
(nm) (cm-1
)
(L cm-1 mol-1
)
(nm) (cm-1 (L cm-1 mol-1
) )
CH3CN (7)
I- (3)
0.613
0.387
0.297
0.253
0.181
0.267
0.400
0.539
0.313
0.213
0.179
0.109
0.179
0.312
0.90 0.074
0.576 0.000
0.350 0.226
0.255 0.321
0.216 0.360
0.145 0.431
0.223 0.353
0.356 0.220
-
0.074
0.88 0.084
0.75 0.074
0.70 0.072
0.98 0.088
0.93 0.088
CH3CN (7)
I- (3)
252 39 682
390 25 641
535 18 692
380 26 316
542 18 450
392 25 510
400 25 000
404 24 752
394 25 381
531 18 832
15 425
8892
117
7233
77
2626
24 572
2569
2903
53
Br- (2)
Cl- (1)
OCN- (4)
SCN- (5)
N(CN)2- (6)
CN- (8)
959 10 428
5.6
Br- (2)
959 10 428
929 10 764
860 11 628
858 11 655
12.0
14.0
16.0
14.7
Cl-(1)
-0.040 -0.108
0.93 0.070 -0.074 0.650
OCN- (4)
SCN- (5)
a All potential values are in V vs Fc+-Fc. b ∆E1/2 ) E1/2 - E° ,
1/2
whereE1/2 is the half wave potential for X- derivatives and E° is that for
1/2
N(CN)2- (6) 393 25 445
2461
60
12 295
860 11 628
15.6
the CH3CN derivative).
529 18 903
443 22 573
CN- (8)
For all the paramagnetic compounds in solution, the
absorption maxima associated with the d-d transition in
solution was observed at a λ value in the range from 860
nm (11 628 cm-1) to 959 nm (10 428 cm-1), with molar
absorption coefficients values (ꢀ) that fall in the range from
16.0 to 5.6 L cm-1 mol-1. This behavior is typical of high-
spin octahedral Fe(II) compounds. Molar absorption coef-
ficients and their associated d-d frequency values for each
compound are shown in Table 3. The observed shifts to larger
frequency values, corresponding to the d-d absorption in
all the studied complexes, confirm the relative stability order
also observed in the structural analysis and magnetic
behavior.
The spectroscopic UV-vis experiments show that com-
plexes 1-5 present the expected d-d transition for HS Fe-
(II) compounds, whereas complexes 7 and 8 do not exhibit
such a transition. However, complex 6 does exhibit a d-d
transition, suggesting that the spin state of this complex in
solution is altered because of solvent effects.
The NMR Evans method was employed to evaluate the
nature of this change, and the aforementioned transition was
found in the studied temperature range. While the displace-
ment obtained for the TMS signal confirmed the diamagnetic
behavior of complexes 7 and 8, for compound 6, the
magnetic moment value estimated from the observed dis-
placement of the TMS signal is 5.0 µB at room temperature,
corresponding to the high spin (HS) state for Fe(II), and ∼0.6
µB at 190 K.
a The absorptions correspond to charge-transfer band (CT) and d-d
bands.
for the diamagnetic compounds 7, 8, and 6 was carried out
employing the variable-temperature NMR Evans method.
While the diamagnetic character of the first two complexes
was confirmed, the latter compound becomes paramagnetic
in solution but displays a slight, smooth spin transition as
the temperature decreases (310-190 K); this probably caused
by the relaxation of the octahedral environment produced
by the solvation of the complex cation.
Electronic Absorption Spectra. A very important feature
of the Fe(II) complexes is that, because of their electronic
structure, the paramagnetic complexes always present a d-d
band in the visible region, while this band is absent in the
diamagnetic complexes. The spectra obtained in acetonitrile
solutions of each Fe(II) compound are shown in Figure 3a
in the ultraviolet region and 3b in the visible region.
The observed maxima with its respective molar absorption
coefficients, ꢀ (L mol-1 cm-1), are shown in Table 3. Each
spectrum displays an absorption maximum at wavelength,
λ, ≈ 390 nm (25 641 cm-1), with a molar absorption
coefficient (ꢀ (L cm-1 mol-1)) of ∼104. These absorptions
are usually associated to MLCT (metal ligand charge
transfer) transitions for Fe-bztpen, t2g-πL*.22
(22) Lever, A. B. P. Inorganic Electronic Spectroscopy, 2nd ed.; Elssev-
ier: New York, 1986; pp 457-470.
7290 Inorganic Chemistry, Vol. 46, No. 18, 2007

Series of [Fe(II)(bztpen)X]+-Type Compounds
Figure 4. Typical cyclic voltammograms obtained for ∼1.0 mM (a) [Fe-
(bztpen)N(CN)2]PF6, (c) [Fe(bztpen)CN]PF6, and (b) [Fe(bztpen)CH3CN]-
(PF6)2 in 0.1 M of Bu4NPF6 acetonitrile solution at 100 mV s-1 on a carbon
glass electrode. In all cases, the potential scan was initiated in the positive
direction from Ei)0
.
Electrochemistry (Cyclic Voltammetry). The analysis
of the complexes with only one typical monoelectronic quasi-
reversible process will be discussed initially, followed by
the analysis of those systems which exhibit additional redox
-
processes. The study of the complex with N(CN)2 (6) is
Figure 5. Typical cyclic voltammograms obtained for ∼1.0 mM of the
(a) Cl-, Br-, OCN-, and SCN- derivatives and the (b) I- and CH3CN
derivatives in 0.1 M Bu4NPF6 acetonitrile solution at 100 mV s-1 on a
carbon glass electrode. In all cases, the potential scan was initiated in the
reported elsewere.10
-
The complexes with N(CN)2 (6), CH3CN (7), and CN-
(8), see Figure 4, exhibit a voltammogram with only one
quasi-reversible process (I), according to the ipc/ipa ratio
shown in Table 4. From the values obtained for difference
between anodic and cathodic peak potentials (∆Ep), it is
possible to suggest that only one electron-transfer processes
take place (∆Ep ) 0.074, 0.088, and 0.070 V respectively,
see also Table 4). Half-wave potential values (E1/2) for these
complexes are also shown in Table 4.
For the CN- (8) complex (Figure 4c), the voltammogram
displays an oxidation signal with an anodic peak potential
of Epa ) -0.040 V. A reduction signal appears at a cathodic
peak potential of Epc ) -0.108 V. The difference between
both values is ∆Ep ) 0.070 V, and the half wave potential
value is E1/2 ) -0.074 V. The ratio of cathodic (ipc) and
anodic (ipa) peak current values is ipc/ipa ) 0.93. Analysis of
the redox process for compound 8 confirms that it is a quasi-
reversible one-electron transfer process. The asterisked (*)
signal that appears at Eap ) 0.253 V, belongs to the Cl-
derivative, employed in the synthesis of 8.
positive direction from Ei)0
.
However, there is another process in E1/2 ) 0.350 V
identified as I. Along with these two processes, there are
also one oxidation and at least three reduction signals. Values
of E1/2, ipc/ipa, and ∆Ep were obtained for each complex and
are shown in Table 4.
Because of the similarity of the E1/2 values for the
processes labeled as II with the process observed for the
acetonitrile complex, it is possible to suggest the presence
of the [Fe(bztpen)CH3CN] complex in the solutions of 1-5.
It follows that this signal is produced as a result of the
dissociation of each complex in the acetonitrile solution,
according to the equation
[FeII(bztpen)X]+ + CH3CN ) [FeII(bztpen)CH3CN]2+ + X-
Consequently, process I can be assigned to the [Fe(III)-
(bztpen)X]2+/[Fe(II)(bztpen)X]+ system in all complexes.
The remaining observed signals for complex 3 can be
attributed to the free iodide anion, which is electroactive in
these conditions. The electrochemical study of a 1 mM KI
solution (not shown) was performed, and those signals
labeled with an asterisk (*) belong to the free iodide (I-)
oxidation and reduction processes (Figure 5b.)
To explain these results quantitatively, the normalized
anodic intensity for both signals (I, iIpa, and II, iIIpa) obtained
for each complex is required. From these data, the percentage
of dissociation for each complex was obtained employing
the equation
The voltammograms of the complexes of Cl- (1), Br- (2),
OCN- (4), and SCN- (5), Figure 5a, and the ∆Ep and ipc/ipa
ratios obtained are consistent with monoelectronic quasi-
reversible processes equivalent to the one previously de-
scribed. The corresponding E1/2 values are shown in Table
4. However, these voltamperograms also show another
process (II) which is, in all cases, less intense. This process
is also quasi-reversible and has E1/2 ) 0.570 V (Figure 5a).
Finally, the voltammogram for the I- (3) complex (Figure
5b) displays three oxidation and four reduction signals. The
main process appears at E1/2 ) 0.570 V and is labeled as II.
Inorganic Chemistry, Vol. 46, No. 18, 2007 7291

Ortega-Villar et al.
Table 5. Values of Dissociation Percent and Estimated Formation
Equilibrium Constants for All Complexes in Acetonitrile Solution at 293
K
iIIpa
% dissoc )
× 100
iIpa + iIIpa
III
FeIILX
FeIIL,X
IILX
FeIIILX
K
FeIIIL,X
% dissoc
K
(KFf e LX)/(KFf e
)
The calculated value for each complex is shown in Table
5. The values obtained fall in the range of 8-12% for the
Cl- (1), Br- (2), OCN- (4), and SCN- (5) compounds.
However, the value for the I- (3) complex is as large as
69%, indicating that [Fe(bztpen)I]+ is highly dissociated and
that roughly 31% of the undissociated species remains in
solution.
CH3CN (7)
I- (3)
68.7
11.1
10.8
7.90
8.60
,8
102.8
104.9
104.9
105.2
105.1
103.9
105.5
106.1
107.4
106.0
103.8
1011.1
101.0
100.6
101.3
102.2
100.9
Br- (2)
Cl- (1)
OCN- (4)
SCN- (5)
N(CN)2- (6)
CN- (8)
.105
.105
.109
,8
.1016
This fact was confirmed by obtaining the molar conductiv-
ity value (Λ) of an approximately 1 mM acetonitrile solution
of the CH3CN (7) and I- (3) complexes, where the value of
264 S mol-1cm2 obtained for compound 7 is consistent with
a 2:1 electrolyte. However, the value of 186 S mol-1cm2
obtained for compound 3 is greater than the expected value
for a 1:1 electrolyte and corresponds to approximately 2/3
of the iodide dissociated from the complex.
The percentage of dissociation (% dissoc) represents a
quantitative measurement of the relative stability of a [Fe-
(bztpen)X]+ complex in solution. From these values, the
corresponding formation equilibrium constant for each
complex can be obtained (see Table 5) according to the
equilibrium
FeIILX
K
> 105 and that ∆E1/2 ) 0.220 V, we can conclude
FeIIL,X
FeIIILX
that the Fe(III) complex has a K
derivative.
> 106 for this
FeIIIIL,X
With the above arguments, the equilibrium constant for
FeIIILX
FeIIIIL,X
the CN- derivative of Fe(III) is the highest (K
>
1016).
If the stability constant of Fe(III) with X- is larger than
that for Fe(II), the E1/2 value will be lower than the E1°/2 value.
Hence, the more stable the generated Fe(III) complex, the
larger the difference will be between E1/2 and E° . The
differences obtained for these compounds follow the trend
described in the literature for similar systems.23
1/2
Conclusions
Fe(II)LS + X- ) Fe(II)LX + S
The synthesis and complete characterization of five new
complexes of Fe(II) with general formula [Fe(bztpen)X]PF6,
where bztpen is the pentadentate ligand N-benzyl-N,N′,N′-
tris (2-methylpyridyl)-ethylenediamine and X- is an anionic
monodentate ligand, Br-, I-, OCN-, SCN-, and CN- (2, 3,
4, 5, and 8, respectively), and [Fe(bztpen)CH3CN](PF6)2
(complex 7) were achieved. All compounds were analyzed,
[Fe(II)LX]+
[Fe(II)LS]2+ [X-]
FeIILX
FeIIL,X
K
)
-
The N(CN)2 (6) and CN- (8) derivatives show no trace
of the signal associated with the [Fe(bztpen)CH3CN]2+
species. In this case, the sensitivity of the voltammetric
experiment does not allow for the estimation of this quantity.
Consequently, it is feasible to assume that the equilibrium
constant for these two compounds is much larger than 105.
Shifts in the E1/2 value for the [Fe(bztpen)X]+ compounds
can be obtained in terms of the Nernst equation, from the
-
including previously reported Cl- (1) and N(CN)2 (6)
derivatives.6,10 Spectroscopic and X-ray structural analyses
show that all compounds are composed of the monomeric
-
[Fe(bztpen)X]n+ unit neutralized with PF6 anions.
From the analysis of the crystallographic parameters, it is
clear that distortion of the octahedral coordination environ-
ment for the metal center is greater for compounds 1-5 than
for compounds 6-8, as shown by the larger values of Φ
and the longer [Fe-L6]average distances obtained for the former
group of compounds than for the latter.
The observed magnetic susceptibility behavior in solid-
state experiments shows that complexes 1-5 are all para-
magnetic, while 6-8 are diamagnetic.
E° value for the [Fe(bztpen)CH3CN]3+/[Fe(bztpen)CH3-
1/2
CN]2+ redox pair, and the stability constants of the [Fe-
(bztpen)X]2+ and [Fe(bztpen)X]+ systems. The resulting
expression is
FeIIILX
K
RT
nF
FeIIIL,X
FeIILX
E1/2 ) E° -
ln
1/2
K
FeIIL,X
The spectroscopic UV-vis experiments show that com-
plexes 1-5 present the expected d-d transition for HS Fe-
(II) compounds, while complexes 7 and 8 do not exhibit such
a transition. However, complex 6 does present a d-d
absorption, suggesting a change in its spin state caused by
the solvent effect. This was confirmed with the NMR Evans
technique in acetone solution of this complex and was also
used to corroborate the diamagnetic behavior of complexes
7 and 8.
From this equation, the ratio of the equilibrium constants
may be obtained by the calculated values of ∆E ) E1/2
E1°/2. The ratio values at 293 K for each derivative are shown
-
1/2
in Table 5. From this ratio and the values obtained for the
II
Fe(II) formation constant KFe LX
of the Fe(III) formation constant K L,X can be determined
FeIIL,X, the corresponding value
FeIIILX
FeIII
(Table 5). For the paramagnetic derivatives, the Fe(II)
equilibrium constant was higher than the corresponding Fe-
-
(III) constant. For the N(CN)2 (6) derivative, the lowest
(23) Howker, P. N.; Twigg, M. V. ComprehensiVe Inorganic Chemistry;
Wilkinson, G., ed.; Pergamon: Oxford, 1987; Vol. IV, pp 1179-
1288.
∆E1/2 value was obtained, indicating that the stability of the
Fe(II) and Fe(III) complexes is similar. Considering that
7292 Inorganic Chemistry, Vol. 46, No. 18, 2007

Series of [Fe(II)(bztpen)X]+-Type Compounds
From these results, it is possible to conclude that the
observed properties are imposed not only by the geometrical
constraints of the pentadentate ligand and the characteristics
of the monodentate ligand X- but also by the solvent effect.24
Values of E1/2 for the the Fe(III)/Fe(II) redox systems
verify that the least-stable Fe(II) compound is 3, while the
most-stable compound is 8.
the most stable Fe(III) complex, as expected for the derivative
with CN- ligand (∆E1/2 ) 0.650 V). The lowest difference
corresponds to the I- derivative, which is the least stable of
all the compounds: a fact that is consistent with the
previously mentioned results. Additionally, the N(CN)2
derivative is the most stable of the Fe(II) compounds in
regard to its stability toward oxidation by O2.
-
From the normalized anodic intensity of the acetonitrile
complex signal observed in the voltammogram for each
complex, it is possible to obtain the percentage of dissociation
for complexes 1-5. From this estimate, the stability constant
for the Fe(II) complexes is consistent with the trend
established by the spectrochemical series. To improve the
discussion concerning the relative stability of these systems,
a comparison of the half-wave potentials differences (∆E1/2)
of all the complexes was performed. The analysis of these
values establishes that the largest difference is obtained for
Acknowledgment. We thank the Universitat de Valencia-
UNAM agreement for a predoctoral fellowship (N.O.V.). We
are grateful to M. en C. Rosa Isela del Villar for NMR spectra
studies, to Q. Marisela Gutierrez Franco for the IR spectra,
and to Dr. Gerardo Medina-Dickinson for his kind revision
of this manuscript.
Supporting Information Available: Figures S1-S5 and vali-
dated CIF files. This material is available free of charge via the
Internet at http://pubs.acs.org.
(24) Murray, K. S.; Kepert, C. J. Top. Curr. Chem. 2004, 233, 195-228.
IC0620743
Inorganic Chemistry, Vol. 46, No. 18, 2007 7293