Kinetics study of the substitution reaction of fac-[FeII(CN) 2(CO)3I]- with PPh3

Article abstract of DOI:10.1016/j.ica.2011.01.069

Substitution reaction of fac-[Fe^II(CN)₂(CO) ₃I]^- with triphenylphosphine (PPh₃) produced mono phosphine substituted complex cis-cis-[Fe^II(CN) ₂(CO)₂(PPh₃)I]^-. Crystal structure of the product showed that carbonyl positioned trans- to iodide was replaced by PPh₃. The substitution reaction was monitored by quantitative infrared spectroscopic method, and the rate law for the substitution reaction was determined to be rate = k[[Fe^II(CN)₂(CO) ₂(PPh₃)I]^-][PPh₃]. Transition state enthalpy and entropy changes were obtained from Eyring equation k = (k _BT/h)exp(-ΔH^≠/RT + ΔS^≠/R) with ΔH^≠ = 119(4) kJ mol^-1 and ΔS ^≠ = 102(10) J mol^-1 K^-1. Positive transition state entropy change suggests that the substitution reaction went through a dissociative pathway.

Full text of DOI:10.1016/j.ica.2011.01.069

Inorganica Chimica Acta 370 (2011) 243–246
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Inorganica Chimica Acta
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Kinetics study of the substitution reaction of fac-[FeII(CN)2(CO)3I]ꢀ with PPh3
Benjamin Schwartz a, Ofir Azran a, Jonathan Solaimanzadeh a, Joshua Fluss a, Wenfeng Lo b, Jianfeng Jiang a,
⇑
a Department of Chemistry, Yeshiva University, New York, NY 10033, USA
b Department of Chemistry of Chemical Biology, Harvard University, Cambridge, MA 02138, USA
a r t i c l e i n f o
a b s t r a c t
Substitution reaction of fac-[FeII(CN)2(CO)3I]ꢀ with triphenylphosphine (PPh3) produced mono phosphine
substituted complex cis-cis-[FeII(CN)2(CO)2(PPh3)I]ꢀ. Crystal structure of the product showed that
carbonyl positioned trans- to iodide was replaced by PPh3. The substitution reaction was monitored by
quantitative infrared spectroscopic method, and the rate law for the substitution reaction was
determined to be rate = k[[FeII(CN)2(CO)2(PPh3)I]ꢀ][PPh3]. Transition state enthalpy and entropy changes
Article history:
Received 22 September 2010
Received in revised form 19 January 2011
Accepted 20 January 2011
Available online 4 February 2011
were obtained from Eyring equation k = (kBT/h)exp(ꢀ
D D D
H–/RT + S–/R) with H– = 119(4) kJ molꢀ1 and
Keywords:
Cyano
Substitution
Kinetics
D
S– = 102(10) J molꢀ1 Kꢀ1
.
Positive transition state entropy change suggests that the substitution
reaction went through a dissociative pathway.
Ó 2011 Elsevier B.V. All rights reserved.
Crystal structure
1. Introduction
2. Experimental
Mixed iron cyano carbonyl complexes with exclusive replaceable li-
gands are an excellent starting material for the synthesis of structural
mimics for the active sites of Ni–Fe and Fe–Fe hydrogenase enzymes
[1–3]. Last year, we have reported the synthesis of fac-[FeII(CN)2
(CO)3I]ꢀ and its subsequent reactions to make Ni–Fe dimers (dppe)-
Ni(-SR)2Fe(CN)2(CO)2 (dppe = 1,2-bis(diphenylphosphino)ethane and
SR = ethanethiolate or 1,3-propanedithiolate) [3]. However, we have
observed that in (dppe)Ni(-pdt)Fe(CN)2(CO)2 (pdt = 1,3-propanedithi-
olate), cyano ligands have rearranged to a trans-geometry. The unex-
pected rearrangement has prompted us to further study the intrinsic
substitution reactivity of fac-[FeII(CN)2(CO)3I]ꢀ.
2.1. Preparations of compounds
All reactions were performed in air. Anhydrous THF, diethyl
ether and acetonitrile were acquired from Acros. Triphenylphos-
phine was acquired from Strem. All solvents and chemicals were
used as received. (Et4N)fac-[FeII(CN)2(CO)3I] was prepared as de-
scribed [3].
2.2. (Et4N)[Fe(CN)2(CO)2(PPh3)I]
To a solution of 449 mg (1.00 mmol) of (Et4N)fac-[FeII(CN)2
(CO)3I] in 20 mL acetonitrile was added 325 mg (1.24 mmol) of
PPh3. The reaction mixture was kept stirring at 40 °C overnight to
give a brown color solution. The solution was cooled to room
temperature and covered by 60 mL of ditheyl ether. Light brown
color crystalline materials were obtained in one day. The materials
were collected by filtration, washed with 3 ꢁ 10 mL of diethyl
ether and dried under vacuum to give 366 mg (54% yield) of prod-
We have found that fac-[FeII(CN)2(CO)3I]ꢀ can react with a ser-
ies of ligands such as thiolates, cyanide, phosphines and alkoxides.
Specifically, we have chosen to focus our study on the reaction of
fac-[FeII(CN)2(CO)3I]ꢀ with PPh3 in detail to identify the ligand(s)
substituted and the kinetics of the substitution. The PPh3 substitu-
tion reaction is very clean, fairly air-stable and shows little compli-
cation in terms of reaction rate, which allows for it to be
conveniently monitored by infra-red spectroscopic method.
uct. Absorption spectrum (acetonitrile): kmax (eM) 453 nm (660),
337 nm (4700), 274 nm (17 000). 1H NMR (CDCl3, anion): d
7.24 ppm (1H), 7.38 ppm (2H), 7.80 ppm (2H). 31P NMR (CDCl3):
d
70.86 ppm. IR (acetonitrile):
m , mCN
CO 1998, 2039 cmꢀ1
2114 cmꢀ1. IR (THF): CO 2004, 2042 cmꢀ1 CN 2115 cmꢀ1
m
, m .
3. X-ray structure determination
⇑
The product was structurally identified by X-ray crystallogra-
phy. Suitable crystals of (BzPPh3)[FeII(CN)2(CO)2(PPh3)I] {brown
Corresponding author.
E-mail address: jiang@yu.edu (J. Jiang).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.01.069

244
B. Schwartz et al. / Inorganica Chimica Acta 370 (2011) 243–246
blocks} were obtained by vapor diffusing diethyl ether into a
dichloromethane solution of (Et4N)[FeII(CN)2(CO)2(PPh3)I] and
(BzPPh3)Cl and allowing the solutions to stand over 2 weeks at
ꢀ20 °C in refrigerator. A piece of crystal was coated in grease
and mounted on a Siemens (Bruker) SMART CCD area detector
5. Kinetics measurements
The kinetics of the substitution reaction was studied in THF
solution by a quantitative infrared spectroscopic method. Concen-
trations of (Et4N)fac-[FeII(CN)2(CO)3I] were kept at 0.010–0.030 M
and concentrations of PPh3 were kept at 0.10–1.00 M. In each mea-
surement, the concentration of PPh3 was at least 10 times more
than the concentration of (Et4N)fac-[FeII(CN)2(CO)3I]. Reactions
were performed in 100 mL round bottom flasks immersed in a
temperature controlled water bath ( 0.1 °C) at five different tem-
peratures: 20.7, 25.0, 30.0, 35.0 and 40.0 °C. At each temperature,
experiments were carried out at least five times with different
PPh3 concentrations. Infrared spectra for each experiment were ta-
ken periodically. The substitution reaction was monitored by the
decreasing of the carbonyl peak (2098 cmꢀ1) over time. Isosbestic
points were observed in each experiment, indicating no significant
accumulation of an intermediate. Plots of Ln(Abst ꢀ Abs1) against
time were linear over 3–5 half-lives, indicating the substitution
reaction was under pseudo-first-order condition. The rate con-
stants kobs were obtained by the slope of these plots. Plots of kobs
against [PPh3] were linear and yielded an overall second-order rate
constant k at each temperature. Activation parameters were deter-
mined from linear plots of the Eyring equation k = (kBT/h)[ex-
instrument with Mo K
a radiation. The data were collected at
173 K with scans of 0.3° per frame, with 10 s per frame such
x
that 1271 frames were collected for a hemisphere of data. The
first 50 frames were recollected at the end of the data collection
to monitor for decay; no significant decay was detected. Data out
to 2h of 56.00° were used. Cell parameters were retrieved using
SMART software and refined using SAINT software on all ob-
served reflections between 2h of 3° and the upper thresholds
[4,5]. Data reduction was performed with the SAINT software,
which corrects for Lorentz polarization and decay. Absorption
corrections were applied using SADABS [6]. The space groups for
all of the compounds were assigned unambiguously by analysis
of symmetry and systematic absences determined by the pro-
gram XPREP [7]. The crystal parameters are listed in Table 1. The
structure was solved by the direct method with SHELXS-97 and
subsequently refined against all data in the 2h ranges by full
matrix least squares on F2 using SHELXL-97 [8]. All non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were
attached at idealized positions on carbon or nitrogen atoms ex-
cept for the disordered dichloromethane molecule and were
checked for missing symmetry by the PLATON program. Disordered
solvent molecules (CH2Cl2 and diethyl ether) were removed using
SQUEEZE [9].
p(ꢀ
D D
H–/RT + S–/R)]. Standard deviations of rate constants were
estimated by using linear least-squares error analysis with uniform
weighting of data points.
6. Results and discussions
4. Other physical measurement
Synthesis of the product (Et4N)[FeII(CN)2(CO)2(PPh3)I] was per-
formed in acetonitrile solution with ca. 25% excess of PPh3 at 40 °C.
Quantitative IR experiment showed that the conversion to the
product is almost 100%; however, the isolated yield was 54%.
[FeII(CN)2(CO)2(PPh3)I]ꢀ did not react further with PPh3, as ob-
served in the kinetics study, 10- to 100-fold PPh3 did not cause fur-
ther reaction. IR spectra of the reaction mixture and the isolated
product were identical, suggesting no other isomers were formed
during substitution.
Infrared spectra were recorded by a Thermo Nicolet 380 spec-
trometer. A NaCl cell with 0.12 mm path length was used for all
the solution IR measurements. Absorption spectra were recorded
by a Varian Cary 50 spectrophotometer at 200–800 nm range.
1H NMR and 31P NMR spectra were recorded by
Mercury400 MHz spectrometer.
a Varian
Crystal structure of [FeII(CN)2(CO)2(PPh3)I]ꢀ was established by
Table 1
its BzPPh3þ salt. (BzPPh3)[FeII(CN)2(CO)2(PPh3)I] was crystallized in
Crystal data and structure refinement for (BzPPh3)[FeII(CN)2(CO)2(PPh3)I].
ꢀ
a triclinic cell P1. The product anion has an octahedral geometry.
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
C47H37FeIN2O2P2
906.48
173(2)
0.71073
triclinic
There are four diatomic ligands in the equatorial plane which are
assigned as two cis-cyanide and two cis-carbonyl ligands. The
distinctive difference between the Fe–CN and Fe–CO bond dis-
tances confirms our assignments [3,10–12]. PPh3 and iodide
ꢀ
P1
12.233(5)
13.154(5)
16.251(7)
75.820(7)
71.492(7)
74.862(7)
2356.0(17)
2
1.278
1.079
916
0.35 ꢁ 0.30 ꢁ 0.20
1.93–28.00
29171
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
DCalc. (mg/m3)
Absorption coefficient (mmꢀ1
F(0 0 0)
)
Crystal size (mm3)
Theta range for data collection (°)
Reflections collected
Independent reflections (Rint
Completeness to theta = 28.33°
Refinement method
)
11 167 (0.0881)
98.0%
Full-matrix least-squares on F2
11 167/16/484
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
0.997
Final R indices [I > 2
R indices (all data)
Largest difference in peak and hole
r
(I)]
R1 = 0.0758, wR2 = 0.2243
R1 = 0.0976, wR2 = 0.2401
3.957 and ꢀ0.993 e Åꢀ3
Fig. 1. ORTEP drawing of [FeII(CN)2(CO)2(PPh3)I]ꢀ at 50% probability level. Hydrogen
atoms are removed for clarity.

B. Schwartz et al. / Inorganica Chimica Acta 370 (2011) 243–246
245
in intensity and peaks at 2042 and 2004 cmꢀ1 began to appear.
Fig. 2 shows the IR spectra over the course of the substitution
reaction.
Table 2
Selected bond lengths (Å) and angles (°) for
(BzPPh3)[FeII(CN)2(CO)2(PPh3)I].
Fe(1)–C(1)
Fe(1)–C(2)
Fe(1)–C(3)
Fe(1)–C(4)
Fe(1)–P(1)
Fe(1)–I(1)
C(2)–Fe(1)–C(1)
C(4)–Fe(1)–C(3)
P(1)–Fe(1)–I(1)
1.778(4)
1.772(5)
1.957(4)
1.954(5)
2.262(2)
2.666(2)
95.5(3)
The consumption of the reactant was monitored by the disap-
pearance of the carbonyl peak at 2098 cmꢀ1 since there was no
product absorption peak near this range. Plots of Ln(Abst ꢀ Abs1
)
against time were linear over 3–5 half-lives, indicating that the
substitution reaction was first order with respect to fac-
[FeII(CN)2(CO)3I]ꢀ. Observed rate constants kobs were given by the
slope of the least-square best-fit linear trend lines and they were
proportional to the concentration of PPh3, indicating the substitu-
tion reaction is also first order with respect to PPh3. Thus the rate
law of the reaction can be expressed as:
86.0(2)
172.52(5)
occupy the remaining axial positions. An
ORTEP drawing of
fac-½FeIIðCNÞ2ðCOÞ3Iꢂꢀ þ PPh3 ! ½FeIIðCNÞ ðCOÞ ðPPh3ÞIꢂꢀ þ CO
[FeII(CN)2(CO)2(PPh3)I]ꢀ anion is shown in Fig. 1. Selected bond dis-
tances and bond angles are listed in Table 2.
2
2
rate ¼ ꢀd½½FeIIðCNÞ ðCOÞ Iꢂꢀꢂ=dt ¼ k½½FeIIðCNÞ ðCOÞ Iꢂꢀꢂ½PPh3ꢂ
2
3
2
3
7. Kinetics studies
Overall rate constant was obtained from the slope of the least
square best fit trend line of the plot of the observed rate constant
against PPh3 concentration. The substitution reactions were studied
at five different temperatures from 20 to 40 °C and the rate constant
at each temperature was obtained. Detailed kinetics data such as
transition state enthalpy and entropy changes can be achieved
using the Eyring equation:
There were two equal intense peaks in the infrared spectrum of
the product at 2004 and 2042 cmꢀ1 in THF solution, which were
assigned to the two cis-CO vibrations. A broad and much smaller
peak was observed at 2115 cmꢀ1, which was assigned to CN vibra-
tions. Since there was not much overlap between the product and
the fac-[FeII(CN)2(CO)3I]ꢀ starting material in their infrared spectra,
the substitution reaction could be conveniently studied by IR. The
substitution reaction was studied in THF solution because PPh3 has
much better solubility in THF than in acetonitrile. The tetraethy-
lammonium salt of fac-[FeII(CN)2(CO)3I]ꢀ and the product could
both dissolve in THF relatively well.
k ¼ ðkBT=hÞ expðꢀ
D
H–=RT þ
D
S–=RÞ or its log form lnðk=TÞ
¼ ꢀ
D
H–=RT þ
D
S–=R þ lnðkB=hÞ
where kB is Boltzmann constant, T is temperature, h is Plank con-
stant, R is gas constant and
D D
H– and S– are transition state enthal-
The reaction was setup as described in Section 2. Concentra-
tions of PPh3 were kept at least 10 times more than the concentra-
py and entropy changes).
Rate constants at five different temperatures are listed in Table
tion of fac-[FeII(CN)2(CO)3I]ꢀ to maintain
a
near constant
3 and the Eyring plot of ln(k/T) against 1/T is shown in Fig. 3.
concentration of PPh3 throughout kinetics measurement. As the
D D
H– = 119(4) kJ molꢀ1 and S– = 102(10) J molꢀ1 Kꢀ1 were
reaction proceeded, peaks at 2098, 2065, 2055(sh) cmꢀ1 decreased
obtained from the plot. Positive transition state entropy change
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
2200
2150
2100
2050
2000
1950
1900
Fig. 2. Infrared spectra of the reaction of 0.020 M fac-[FeII(CN)2(CO)3I]ꢀ with 0.60 M PPh3 in THF at 35.0 °C. Spectra were collected every 2 min except for the last one, which
was collected 2 h after the mixing of reactants.
Table 3
Rate constants at five different temperatures.
Temperature (K)
293.9
298.2
303.2
308.2
313.2
Rate constant (sꢀ1 Mꢀ1
)
7.38 ꢁ 10ꢀ4
1.56 ꢁ 10ꢀ3
3.85 ꢁ 10ꢀ3
7.99 ꢁ 10ꢀ3
1.57 ꢁ 10ꢀ2

246
B. Schwartz et al. / Inorganica Chimica Acta 370 (2011) 243–246
Eyring Plot
0
3.15E-03
3.20E-03
3.25E-03
3.30E-03
3.35E-03
3.40E-03
3.45E-03
-2
-4
-6
-8
-10
-12
-14
1/T (K-1)
Fig. 3. Eyring plot of ln(k/T) against 1/T.
suggests that the substitution reaction went through a dissociative
pathway. In the past, We have reported fac-[FeII(CN)2(CO)3I]ꢀ to
slowly decompose by loss of CO [3]. This study shows that in pres-
ence of PPh3, however, the dissociation of CO is followed by the for-
mation of [FeII(CN)2(CO)2(PPh3)I]ꢀ.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2011.01.069.
References
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Am. Chem. Soc. 124 (2002) 1680.
8. Conclusion
Reaction of fac-[FeII(CN)2(CO)3I]ꢀ with PPh3 leads the formation
of the product [FeII(CN)2(CO)2(PPh3)I]ꢀ. Kinetics of this well-
behaved substitution reaction is studied and transition state
enthalpy and entropy changes are obtained. This substitution
reaction goes through a dissociative reaction pathway.
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This research is supported by Yeshiva University and Petroleum
Research Fund (UNI #49424).