Reactivity of (Pyridine-Diimine)Fe Alkyl Complexes with Carbon Dioxide

Article abstract of DOI:10.1021/acs.organomet.6b00470

The reaction of CO₂ with (PDI)FeMe (1), (PDI)Fe(Me)PMe₃ (1-PMe₃) and [(PDI)FeMe][BPh₄] (2, PDI = 2,6-(2,6-ⁱPr₂-C₆H₃-N=CMe)₂-C₅H₃N) gen

Full text of DOI:10.1021/acs.organomet.6b00470

Article
pubs.acs.org/Organometallics
Reactivity of (Pyridine-Diimine)Fe Alkyl Complexes with Carbon
Dioxide
Ka-Cheong Lau and Richard F. Jordan*
Department of Chemistry, The University of Chicago, 5735 South Ellis Avenue, Chicago, Illinois 60637, United States
S
* Supporting Information
ABSTRACT: The reaction of CO2 with (PDI)FeMe (1), (PDI)Fe(Me)-
PMe3 (1-PMe3) and [(PDI)FeMe][BPh4] (2, PDI = 2,6-(2,6-iPr2−C6H3−
NCMe)2−C5H3N) generates (PDI)FeOAc (3), (PDI)Fe(OAc)PMe3
(3-PMe3), and [(PDI)FeOAc][BPh4] (4), respectively. Kinetic data and
solvent effects provide evidence that these reactions occur by
precoordination of CO2 to the Fe center regardless of the charge state
and thus favor an insertion mechanism for carboxylation. Carboxylation of
1-PMe3 requires initial dissociation of PMe3 to generate 1, which reacts
with CO2; 1-PMe3 itself does not react directly with CO2. CO2 reacts 5
times faster with neutral 1 than with cationic 2 (at 0 °C), which is ascribed to the higher nucleophilicity of the Fe−Me group in
1.
The cationic complexes exhibit much higher reactivity due to
the presence of the labile MeB(C6F5)3 or C6D5Cl ligands,
INTRODUCTION
■
−
The reaction of metal alkyls with CO2 to afford metal
carboxylate products plays a key role in strategies to convert
CO2 to value-added chemicals.1−3 Mechanistic studies of these
reactions may provide insights that are useful for the
development of new CO2 conversion processes. Experimental
and computational studies show that the carboxylation of metal
alkyls can occur by two general mechanisms (Scheme 1): (i)
which facilitates CO2 binding, and the cationic charge, which
engenders stronger Zr···OCO interactions compared to neutral
Cp2ZrMe2. Carboxylation reactions of other d0 metal alkyls
species likely proceed by similar mechanisms.5 Kinetic studies
show that dissociation of PMe3 is required for carboxylation of
the oxa-ruthenacycle Ru(PMe3)4(κ2-O,C-OC6H3Me), consis-
tent with a migratory insertion pathway.6 In contrast, reactivity
trends and computational studies show that carboxylation of
group 10 metal (tBuPCP)MMe pincer complexes ([tBuPCP]− =
[2,6-(tBu2PCH2)2−C6H3]−; M = Pd, Ni) occurs by direct SE2
backside attack of CO2 on the M−Me group to generate a M+
−O2CMe ion pair that collapses to the product. In these cases,
the nucleophilicity of the M−Me group controls the reactivity.7
Similarly, CO2 reacts with (PO-iPr)PdMe2− (PO-iPr− = 2-PiPr2-
Scheme 1
−
4-Me−C6H3SO3 ) to yield (PO-iPr)PdMe(OAc)− by direct
SE2 attack of CO2 at the Pd−Me group that is trans to the
phosphine, which is more electron-rich than the methyl group
that is cis to the phosphine.4b
The most thoroughly studied carboxylation reactions are
those of group 6 cis-M(CO)4(L)R− complexes (M = Cr, W; L
= CO, P(OMe)3, PMe3; R = Me, Et, Ph), which yield cis-
M(CO)4L(O2CR)− products.8 Darensbourg and co-workers
showed that these reactions are accelerated by electron-
donating ligands (L = PMe3 > P(OMe)3 > CO), proceed
with retention of configuration at the alkyl carbon, and are not
retarded by excess CO and thus do not proceed via a
coordinatively unsaturated intermediate. On the basis of these
and other observations, Darensbourg proposed a concerted Ia
(associative interchange) mechanism, which may also be
migratory insertion, involving initial CO2 coordination to the
metal center, followed by migration of the alkyl group via a
four-center transition state, and (ii) SE2 or SEi electrophilic
substitution processes in which CO2 directly attacks the metal
alkyl carbon without prior coordination.
Reactivity and computational studies show that cationic
Cp2ZrMe{MeB(C6F5)3} and [Cp2ZrMe(ClC6D5)][B(C6F5)4],
and neutral Cp2ZrMe2, react with CO2 by insertion
mechanisms to afford the corresponding acetate products.4
Received: June 10, 2016
© XXXX American Chemical Society
A
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categorized as an SEi (internal electrophilic substitution)
mechanism, for these reactions. In this mechanism, the CO2
attacks the metal alkyl carbon with the involvement of a M···
OCO interaction, but a M(OCO) adduct is not formed prior to
the transition state (Scheme 1). Similar mechanisms have been
implicated for Ru,9 Rh,10 and Cu11 alkyls.
C5H3N}FeCl(OEt2).15,18 The solid state structure of 1-PMe3
was characterized by X-ray diffraction, although the precision of
the analysis is limited by a whole body disorder (see the
Experimental Section and Supporting Information). In the solid
state, 1-PMe3 adopts a square-pyramidal structure, with the
PMe3 ligand bound in the axial position (Figure 1). This
Here, we describe the reactions of the (pyridine-diimine)Fe
alkyl complexes (PDI)FeMe (1), (PDI)Fe(Me)PMe3 (1-
PMe3), and [(PDI)FeMe][BPh4] (2, PDI = 2,6-(2,6-iPr2−
C6H3−NCMe)2−C5H3N) with CO2. (Pyridine-diimine)Fe
complexes have been used in olefin polymerization,12 and many
other catalytic reactions.13 (PDI)FeMe and (PDI)FeMe+ are
unusual examples of metal alkyl species that have the same
composition and nearly the same structure but differ in overall
charge due to the presence of the redox-active PDI ligand.
Thus, this system provides an interesting opportunity to probe
how charge influences the carboxylation process. The reactions
of several other Fe alkyl complexes with CO2 have been
reported.14
RESULTS AND DISCUSSION
■
Figure 1. Molecular structure of (PDI)Fe(Me)PMe3 (1-PMe3).
Selected bond lengths (Å) and angles (deg): Fe1−C1 2.00(4),
Fe1−P1 2.359(8), Fe1−N1 1.962(9), Fe1−N2 1.874(10), Fe1−N3
1.955(10), N1−C18 1.356(11), C18−C19 1.414(11), N3−C24
1.354(13), C24−C23 1.423(11), C1−Fe1−P1 101.1(14), N1−Fe1−
C1 96.8(13), N2−Fe1−C1 174.2(15), N3−Fe1−C1 101.7(12), N1−
Fe1−P1 97.8(4), N2−Fe1−P1 84.3(4), N3−Fe1−P1 98.6(4), N2−
Fe1−N1 80.1(4), N2−Fe1−N3 79.5(5).
Synthesis of (PDI)Fe Methyl Complexes. Complexes 1
and 2 were synthesized by the procedures reported by Chirik
(Scheme 2).15,16 Mossbauer spectroscopy, X-ray diffraction,
̈
Scheme 2
structure is analogous to those of (PDI)FeBr(THF) and {2,6-
(2,6-Et2−C6H3−NCMe)2−C5H3N}FeCl(OEt2), which also
adopt square-pyramidal geometries, with the THF and Et2O
ligands in axial positions. The Fe atom of 1-PMe3 is displaced
by 0.05(1) Å out of the plane formed by the PDI ligand and the
Fe−Me group toward the PMe3 ligand. The Nimine−Cimine
(1.36(1) Å) and Cimine−Cipso (1.42(1) Å) distances in 1-
PMe3 are similar to those in 1 (Nimine−Cimine = 1.335(7) Å,
Cimine−Cipso = 1.435(8) Å)15 and are consistent with the
presence of a reduced PDI ligand, although the low precision of
the data precludes definitive assignment of the PDI oxidation
state. The Fe−Me distance in 1-PMe3 (2.00(4) Å) is also
similar to that in 1 (2.001(6) Å). SQUID measurements
establish that 1-PMe3 has an effective magnetic moment of μeff
= 1.8 μB, indicative of one unpaired electron. The X-band EPR
spectrum of 1-PMe3 in a toluene glass at 15 K gives a rhombic
signal, with gmin = 2.050, gmid = 2.075, gmax = 2.224, showing that
there is significant spin density at the Fe center. On the basis of
these results, we assign 1-PMe3 an electronic configuration
and magnetic measurements show that 1 and 2 contain high-
spin Fe2+ centers (SFe = 2), with a monoreduced PDI− radical
1
anion in 1 (SPDI = /2) and a neutral PDI ligand in 2 (SPDI
=
0).17
Synthesis of (PDI)Fe(Me)PMe3. The reaction of 1 with
PMe3 yields the PMe3 adduct 1-PMe3 (Scheme 3). 1-PMe3 is
an unusual example of a (PDI)FeXL species with anionic X
(Me) and neutral L (PMe3) ligands, two other examples being
(PDI)FeBr(THF) and {2,6-(2,6-Et2−C6H3−NCMe)2−
comprising a PDI− radical anion (SPDI
=
1/2) that is
antiferromagnetically coupled with an intermediate spin state
Fe2+ center (SFe = 1).
Scheme 3
The 1H NMR spectrum of 1-PMe3 in C6D6 exhibits
concentration-dependent behavior. Upon dilution of the
solution, the 1H resonances for 1-PMe3 shift toward the
chemical shifts of base-free 1 and free PMe3 (Figure 2).
1
Addition of excess PMe3 to 1-PMe3 shifts the H resonances
for 1-PMe3 away from the chemical shifts of 1 but also
1
broadens them significantly. In addition, the H NMR spectra
of mixtures of 1 and 1-PMe3 contain a single set of resonances
at the mole-fraction-weighted average of the chemical shifts of 1
and 1-PMe3 (Figure 3). These results show that 1-PMe3
undergoes partial dissociation of PMe3 to form 1, and that 1
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1
Figure 2. H NMR spectra of (a) 1 (0.029 M) in toluene-d8, (b) 1-PMe3 (0.010 M) in C6D6, (c) 1-PMe3 (0.012 M) in C6D6, and (d) 1-PMe3
(0.026 M) in C6D6, at 23 °C. Three key regions of the spectra are shown: δ 21−12, δ 5 to −9, and δ −20 to −210; note that the horizontal
expansion of each region is different. The variation of chemical shifts of the resonances of 1-PMe3 with concentration and the correlations of these
resonances with those for 1 are indicated by dashed lines. Other resonances: δ 3.3 (Et2O), 1.3 (pentane), 1.2 (Et2O), 0.9 (pentane). The chemical
shift of free PMe3 (which is not present) is δ 0.8.
Figure 3. 1H NMR spectra of (a) 1 (0.029 M) in toluene-d8, (b) a 1/3 mixture of 1-PMe3 and 1 (0.016 M total) in toluene-d8, (c) a 1/1 mixture of
1-PMe3 and 1 (0.040 M total) in C6D6, and (d) 1-PMe3 (0.026 M) in C6D6, at 23 °C. Two key regions of the spectra are shown: δ 5 to −22 and δ
−10 to −220; note that the horizontal expansion of each region is different. The correlations of the resonances of 1-PMe3 and 1 are indicated by
dashed lines. Other resonances: δ 3.3 (Et2O), 2.1 (toluene-d7), 1.3 (pentane), 1.2 (Et2O), 0.9 (pentane), −198.4 (Fe−Me of 1-PMe3).
and 1-PMe3 undergo rapid exchange on the NMR time scale
(Scheme 4). The equilibrium constant for PMe3 dissociation of
1-PMe3 was determined to be Keq = 1.8(9) × 10−3 M at 23 °C
using the method of Rose and Drago.19
resonance for 1 and moves toward the chemical shift of free
PMe3 (δ 0.8) as the solution of 1-PMe3 is diluted. The
resonance at δ −198.4 for 1-PMe3 was assigned to the Fe−Me
group based on the observations that this resonance disappears
upon mixing 1-PMe3 with 1 and the Fe−Me resonance of 1 is
not observable.
Scheme 4
Carboxylation of 1. Complex 1 reacts with CO2 in C6D6 at
room temperature to afford the known acetate complex
(PDI)FeOAc (3, Scheme 5), which was previously charac-
terized by Chirik and co-workers.20 No intermediates in this
reaction were detected by NMR. The kinetics of the reaction of
1 with CO2 in toluene-d8 were measured by 1H NMR
spectroscopy by monitoring the disappearance of the CHMe2
resonance (δ −11) of 1 (Figure 4). The reaction is first-order in
1 and CO2 (Figure 5)
The 1H NMR spectrum of 1 has been assigned by Chirik and
Scheme 5
1
2
co-workers by analysis of peak intensities and the H and H
15,20
2
1
NMR spectra of H-labeled samples.
The H resonances
for 1-PMe3 were assigned by correlating the resonances of 1, 1-
PMe3, and rapidly exchanging mixtures of these species as
shown in Figure 3. For example, the two CHMe2 resonances of
1 at δ −21 and −11 (Figure 3a) move steadily to δ −5 and −3
as the 1-PMe3/1 ratio is increased, implying that the δ −5 and
−3 resonances of 1-PMe3 are due to the CHMe2 groups. The
resonance at δ 17 for 1-PMe3 (Figure 2) was assigned to the
PMe3 ligand because this resonance is not correlated with any
C
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Figure 4. 1H NMR monitoring of the reaction of 1 with CO2 (1 atm)
in toluene-d8 at 0 °C. Selected spectra are shown. These spectra show
the disappearance of 1 and concomitant formation of 3.
rate = k1′[1] = k1[1]PCO
2
where k1′ is the observed first-order rate constant, k1 is the
second-order rate constant, and PCO is the pressure of CO2.
2
The observed first-order rate constant at 0 °C and PCO = 1
2
atm was determined to be k1′ = 3.63(9) × 10−3 s−1. k1 values
for the conversion of 1 to 3 (PCO = 1 atm) were determined
2
over the temperature range of 0 to −35 °C. An Eyring plot is
shown in Figure 6, and activation parameters are listed in Table
1. Extrapolation from the Eyring plot gave k1 (23 °C) = 2(4) ×
10−2 atm−1 s−1.
Figure 5. Representative kinetic plots for the reaction of 1 with CO2 in
toluene-d8. (a) Plot of ln(I1) versus time (0 °C, PCO = 1 atm); I1 =
2
Carboxylation of 1-PMe3. As noted earlier, carboxylation
reactions of metal alkyl complexes that proceed by insertion
mechanisms require a vacant site for CO2 binding and,
therefore, are expected to be inhibited or quenched by addition
of competing ligands. In contrast, carboxylation reactions that
proceed by Ia or SE2 mechanisms do not require a vacant site
but do require a nucleophilic alkyl ligand, and these reactions
are expected to be unaffected or possibly accelerated by
electron-donating ligands. Accordingly, comparison of the
carboxylation reactions of 1 and 1-PMe3 may provide insight
into the mechanism of carboxylation of the former complex.
The reaction of 1-PMe3 with CO2 in C6D6 proceeds at room
temperature to form the acetate complex (PDI)Fe(OAc)PMe3
(3-PMe3), which exists in equilibrium with its PMe3-dissociated
form 3 and PMe3 (Scheme 6). No intermediates in this reaction
were detected by NMR. 3-PMe3 was independently generated
by the reaction of 3 with PMe3.
integral value for the CHMe2 resonance of 1 at δ −11 relative to the
integral value of the Et2O resonance (δ 1.2, internal standard). (b)
Plot of k1′ vs PCO (−25 °C).
2
Vacuum transfer of the volatiles from a solution of 3-PMe3 in
C6D6 afforded 0.9 equiv of PMe3 in the volatile fraction and
essentially pure 3 in the nonvolatile fraction, as determined by
1H and 31P NMR, indicating that the PMe3 of 3-PMe3 is quite
Figure 6. Eyring plot for the reaction of 1 with CO2 (1 atm) in
toluene-d8.
1
labile. The H NMR spectrum of 3-PMe3 in C6D6 at room
Table 1. Activation Parameters for the Reaction of CO2 (1
atm) with 1 and 2
temperature is only slightly shifted from that of base-free 3 (Δδ
ca. 1−2 ppm), and no PMe3 resonance is observed for 3-PMe3
by 1H or 31P NMR. These observations indicate that the PMe3
ligand of 3-PMe3 undergoes extensive dissociation and fast
ΔH⧧ (kcal/mol)
ΔS⧧ (e.u.)
ΔG⧧ at 0 °C (kcal/mol)
1
1
2
10.9(6)
14.9(9)
−29(2)
−17(3)
19(1)
20(2)
exchange. The differences in the observed H NMR chemical
shifts of 3 and 3-PMe3 are too small to allow accurate
D
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Scheme 6
determination of Keq. Addition of excess PMe3 to shift the
1
equilibrium to 3-PMe3 shifts the H NMR resonances of 3-
PMe3 away from those of 3 but also broadens them
significantly. 3-PMe3 could not be isolated.
Kinetics of Carboxylation of 1-PMe3. The observation
that 1-PMe3 undergoes partial dissociation of PMe3 in solution
raises the question of whether the observed reaction of 1-PMe3
and CO2 to form 3-PMe3 proceeds by direct carboxylation of 1-
PMe3 or requires prior PMe3 dissociation. The kinetics of the
reaction of 1-PMe3 with CO2 (1 atm) in the presence of excess
PMe3 were studied to address this issue. Due to the broadening
of the resonances of 1-PMe3 in the presence of PMe3 noted
above, the kinetics were measured by analyzing the appearance
of 3-PMe3, monitoring the CHMe2 resonance at δ −18 (Figure
7).
Figure 8. Representative first-order kinetic plot for the formation of 3-
PMe3 from the reaction of 1-PMe3 with CO2 (1 atm) in C6D6 at 23
°C in the presence of 0.04 M free PMe3. kobs = 6.9(3) × 10−4 s−1.
Table 2. Observed First-Order Rate Constants (kobs) for the
Reaction of 1-PMe3 with CO2 (1 atm) and Various [PMe3]
Values at 23 °C
[PMe3] (M)
kobs (10−4 s−1
)
0.04
0.05
0.09
0.11
0.13
0.17
6.9(3)
5.7(2)
3.1(1)
2.6(2)
2.35(6)
1.69(8)
shown in Scheme 7. In Scheme 7, reversible dissociation of
PMe3 from 1-PMe3 generates an equilibrium mixture of 1, 1-
Scheme 7
Figure 7. 1H NMR monitoring of the reaction of 1-PMe3 with CO2 (1
atm) in the presence of excess PMe3 (0.04 M) in C6D6 at 23 °C. kobs
(23 °C) = 6.9(3) × 10−4 s−1. One resonance for 1-PMe3 is visible at δ
0, and the others are broadened into the baseline due to the presence
of excess PMe3. Other resonance: δ 1.1 (t, Et2O).
The carboxylation of 1-PMe3 is first-order in the total
concentration of Fe−Me species, i.e.
Rate = kobs[Fe−Me] = kobs([1‐PMe3] + [1])
PMe3, and PMe3. 1 reacts with CO2 to form 3, which, in the
presence of PMe3, gives 3-PMe3. 1-PMe3 does not react
directly with CO2.
where kobs = observed first-order rate constant and [Fe−Me] =
[1-PMe3] + [1].
The first-order rate equation for the appearance of 3-PMe3
(exponential form) is given by
The rate equation for this mechanism is
rate = kobs[Fe−Me]
where
IFe = IFe,∞[1 − exp(−kobst)]
where IFe and IFe,∞ are the integral values of the δ −18
resonance of 3-PMe3 (relative to that of an internal standard)
at time = t and the end of the reaction, respectively. A
representative kinetic plot is shown in Figure 8. The conversion
of 1-PMe3 to 3-PMe3 is strongly inhibited by PMe3 (Table 2).
A pre-equilibrium reaction scheme consistent with these
results and the known dissociation of PMe3 from 1-PMe3 is
k P Keq
1 CO2
kobs
=
Keq + [PMe3]
and
[Fe−Me] = [1‐PMe3] + [1]
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Rearranging, and at PCO = 1 atm
2
1
kobs
1
1
k1
=
[PMe3] +
k1Keq
(1)
A plot of 1/kobs versus [PMe3] over the range of [PMe3] =
0.04−0.17 M is shown in Figure 9. Consistent with Scheme 7
Figure 10. Two views of the molecular structure of [(PDI)FeOAc]-
[BPh4] (4). The BPh4− anion and hydrogen atoms are omitted in both
views and the 2,6-iPr2−C6H3 groups are omitted in the side view.
Selected bond lengths (Å) and angles (deg): Fe1−O1 2.0869(19),
Fe1−O2 2.0462(19), Fe1−N1 2.153(2), Fe1−N2 2.082(2), Fe1−N3
2.169(2), N1−C2 1.282(3), C2−C3 1.492(4), N3−C8 1.276(3), C8−
C7 1.493(4), O2−Fe1−O1 63.94(9), N1−Fe1−O1 104.35(8), N1−
Fe1−O2 109.93(8), N2−Fe1−O1 143.44(9), N2−Fe1−O2
151.91(8), N3−Fe1−O1 98.36(8), N3−Fe1−O2 99.47(8), N2−
Fe1−N1 74.36(8), N2−Fe1−N3 74.41(8), N3−Fe1−N1 148.53(8).
Figure 9. Plot of 1/kobs versus [PMe3] for the reaction of 1-PMe3 with
CO2 (1 atm) to produce 3-PMe3 in C6D6 at 23 °C.
and eq 1, the plot is linear and gives k1 (23 °C) = 2(3) × 10−2
atm−1 s−1 and Keq (23 °C) = 2(3) × 10−3 M. Importantly, these
results are in excellent agreement with the values determined
bond lengths within the PDI ligand of 4 are consistent with a
neutral (nonreduced) PDI ligand. A SQUID analysis gave μeff
=
1
independently by the kinetic studies of 1 and the H NMR
5.3 μB, indicative of 4 unpaired electrons in 4. These results are
consistent with the presence of a high-spin Fe2+ (SFe = 2) with
neutral PDI ligand (SPDI = 0) in 4. The IR νCO values for 4
analysis of 1-PMe3 (k1 = 2(4) × 10−2 atm−1 s−1 and Keq
=
1.8(9) × 10−3 M), which confirms the mechanism in Scheme 7,
and, in particular, that 1-PMe3 does not react directly with CO2
under these conditions.
(νasym = 1548 cm−1, νsym = 1472 cm−1, ΔνCO = νasym − νsym
=
76 cm−1) are also consistent with a κ2 binding mode for the
acetate ligand.21
Carboxylation of 2 in a Noncoordinating Solvent. The
reaction of cationic complex 2 with CO2 in C6D5F at room
temperature affords [(PDI)FeOAc][BPh4] (4, Scheme 8). No
The kinetics of the reaction of 2 with CO2 were measured by
1H NMR spectroscopy, monitoring the disappearance of the δ
−25 resonance of 2 (Figure 11). The reaction is first-order in 2
and CO2 (see Figures S-7 and S-8 in the Supporting
Information)
Scheme 8
intermediates in this reaction were detected by NMR. Complex
4 was independently synthesized by the oxidation of 3 with
[Cp2Fe][BPh4] and characterized by X-ray diffraction (Figure
10). The (PDI)Fe(OAc)+ cation of 4 is structurally similar to
the neutral analogue 3.20 The coordination geometry at the Fe
center of 4 is trigonal-bipyramidal, as observed for 3. The
acetate unit is bound in a symmetrical κ2 fashion, and the plane
of the acetate unit is orthogonal to the (PDI)Fe chelate plane.
The Fe−O bond distances in 4 are ca. 0.04 Å shorter than
those in 3 (2.1271(14), 2.0837(16) Å), which is expected for
stronger Fe−O bonding in cationic 4 versus neutral 3. The
Figure 11. 1H NMR monitoring of the reaction of 2 with CO2 (1 atm)
in C6D5F at 0 °C to produce 4. Other resonances: δ 1.2 (pentane), 0.9
(pentane).
F
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rate = k2′[2] = k2[2]PCO
EXPERIMENTAL SECTION
2
■
General Procedures. All experiments were performed using
drybox or Schlenk techniques under a nitrogen atmosphere. Nitrogen
was purified by passage through activated molecular sieves and Q-5
oxygen scavenger. C6D5F and C6H5F were distilled from and stored
over P2O5. C6D6, toluene-d8, and THF-d8 were distilled from Na/
benzophenone. Pentane, hexanes, and toluene were purified by
passage through activated alumina and BASF R3-11 oxygen scavenger.
THF, CH2Cl2, and Et2O were purified by passage through activated
alumina. CO2 (99.999%) was purchased from Airgas and used as
received. [Cp2Fe][BPh4],23 (PDI)FeCl, (PDI)FeMe (1),15 and
[(PDI)FeMe][BPh4] (2)16 were synthesized by literature procedures.
NMR spectra were recorded on a Bruker DMX-500 spectrometer in
Teflon-valved J. Young tubes at ambient temperature unless otherwise
where k2′ is the observed first-order rate constant and k2 is the
second-order rate constant.
The value for k2′ at 0 °C and PCO = 1 atm is k2′ = 7.10(9) ×
2
10−4 s−1. Therefore, under these conditions, the carboxylation
of 2 is 5-fold slower than that of 1 (k1′ (0 °C) = 3.63(9) × 10−3
s−1).
Values for k2 for the conversion of 2 to 4 were determined
over the temperature range of 23 to −10 °C, and activation
parameters determined from an Eyring plot are given in Table 1
(see Figure S-9 in the Supporting Information).
Carboxylation of 2 in a Coordinating Solvent. To
probe whether CO2 precoordination to the Fe center is
important for the carboxylation of 2, the reaction of 2 with CO2
(1 atm) was carried out in the coordinating solvent THF-d8 at
room temperature. Several observations show that 2 binds THF
to form, presumably, 2-THF. First, the color of a solution of 2
in C6D5F changes from red to blue upon addition of THF,
consistent with the color change associated with the conversion
of the analogous alkyl complexes [(PDI)FeR][BPh4] to
[(PDI)FeR(THF)][BPh4] (R = CH2CMe3, CH2SiMe3).16,22
1
indicated. H NMR chemical shifts are reported relative to SiMe4 and
1
were determined by reference to the residual H solvent resonances.
Mass spectrometry experiments were performed on Agilent 6224
TOF-MS (high resolution) or Agilent 6130 LCMS (low resolution)
instruments. Magnetization data were recorded with a SQUID
magnetometer (Quantum Design). Magnetic susceptibility values
were corrected for the underlying diamagnetic increment by using
tabulated Pascal constants and the effect of the blank sample holders.
EPR spectra were collected on a Bruker Elexsys-II E500 spectrometer
and simulated using Easyspin software.24
X-ray Crystallography. Crystallographic data are summarized in
Table S-1 in the Supporting Information and the accompanying CIF
files. Data were collected on a Bruker D8 Venture PHOTON 100
CMOS Diffractometer using Mo Kα radiation (0.71073 Å). Direct
methods were used to locate many atoms from the E-map. Repeated
difference Fourier maps allowed location of all expected non-hydrogen
atoms. Following anisotropic refinement of all non-H atoms, ideal H
atom positions were calculated. Final refinement was anisotropic for all
non-H atoms, and isotropic-riding for H atoms. ORTEP diagrams are
drawn with 40% probability ellipsoids. Specific comments for each
structure are as follows. 1-PMe3: Crystals of 1-PMe3 were obtained by
crystallization from a saturated solution of 1-PMe3 in pentane at −35
°C. Significant elongation of most of the thermal ellipsoids was
observed and was modeled as a whole body disorder of two equivalent
complexes sitting at almost the same location (ratio: 60/40). All atoms
were refined with anisotropic thermal parameters utilizing constraints
on thermal parameters (RIGU, SIMU, EADP). While geometric
restraints were initially used to support the refinement, geometric
parameters were not restrained during the final refinement cycles
1
Second, the H NMR spectrum of 2 in THF-d8 is significantly
shifted from that of 2 in C6D5F (see Figure S-2 in the
1
Supporting Information). H NMR monitoring experiments
show that 2 does not react with CO2 to produce 4 in THF-d8 at
room temperature but rather partially decomposes over time.
The rate of decomposition is approximately the same as that in
the absence of CO2. These results show that the coordination
of THF to 2 blocks the carboxylation reaction.
Comparative Reactivity of 1, 2, and Other Metal
Alkyls with CO2. The observation that PMe3 and THF block
the carboxylation of 1 and 2, respectively, implies that these
reactions proceed by initial coordination of CO2 to the Fe
center and thus favors an insertion mechanism in both cases.
The similarity in the rates, activation parameters, and effects of
added ligands of these reactions reflects the structural and
electronic commonalities of 1 and 2, both of which are square-
planar, 4-coordinate, high-spin FeII species. The modest (5-
fold) rate enhancement observed for 1 versus 2 can be ascribed
to the presence of the reduced PDI ligand in the former species,
which renders the Fe−Me group more nucleophilic than that in
2.
i
except for soft SADI restraints that were imposed on the Pr groups.
After modeling the disorder, the data/parameter ratio was about 9. 4:
Crystals of 4 were obtained by layering hexanes onto a solution of 4 in
C6H5F at room temperature. No anomalous bond lengths or thermal
parameters were noted.
Interestingly, while the carboxylation of 1 and 2 appears to
require initial CO2 coordination, consistent with an insertion
process, the rate trend (1 faster than 2) is opposite to that
observed for zirconocene species, which react with CO2 by an
insertion mechanism, and parallel to that for group 6 and 10
metal alkyls that react with CO2 by Ia/SE2 mechanisms. One
explanation for this observation is that 1 and 2 have similar
CO2 binding properties and, as a result, the nucleophilic
character of the Fe−Me group exerts a controlling influence on
the carboxylation rate. These comparisons showcase the
spectrum of mechanisms and reactivity trends that are possible
for the carboxylation of metal alkyls.
(PDI)Fe(Me)PMe3 (1-PMe3). A solution of PMe3 in Et2O (0.098
M, 1.15 mL, 0.11 mmol) was added to a solution of 1 in Et2O (0.054
M, 2.0 mL, 0.11 mmol), and the mixture was stirred at room
1
temperature for 5 min. H NMR analysis showed that an equilibrium
mixture of 1-PMe3 and 1 had formed. The volatiles were removed
under vacuum to afford a brown solid. Recrystallization of the solid
from a saturated pentane solution at −35 °C afforded dark green
1
crystals that were identified by H NMR and X-ray diffraction as 1-
1
PMe3 (45 mg, 0.072 mmol, 66%). H NMR (C6D6, 0.027 M): δ 16.9
(Fe−PMe3), 2.8 (p-Haryl), 2.2 (m-Haryl), −2.7 (CHMe2), −5.3
(CHMe2), −22.1 (CHMe2), −100.2 (NCMe), −198.4 (Fe−Me).
μeff (SQUID, 27 °C): 1.8 μB. Multiple elemental analyses of crystalline
samples of 1-PMe3 gave irreproducible results for C but accurate
results for H, N and P possibly due to incomplete combustion. Anal.
Calcd for C37H55FeN3P: C, 70.69; H, 8.82; N, 6.68; P, 4.93. Found: C,
69.81; H, 9.21; N, 6.49; P, 4.88.
Reversible Dissociation of PMe3 from 1-PMe3. An NMR tube
was charged with 1-PMe3 (8.0 mg, 0.013 mmol). C6D6 (0.5 mL) was
added by vacuum transfer at −196 °C. The mixture was thawed at
room temperature. This solution was serially diluted by addition of
C6D6, and 1H NMR spectra were recorded at each concentration.
CONCLUSION
■
(PDI)FeMe (1) and [(PDI)FeMe][BPh4] (2) react with CO2
to yield the corresponding acetate products (PDI)FeOAc (3)
and [(PDI)FeOAc][BPh4] (4). Donor ligands (PMe3, THF)
block these reactions, which implies that they proceed by initial
coordination of CO2 to the Fe center.
G
DOI: 10.1021/acs.organomet.6b00470
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Organometallics
Article
Upon dilution, the 1H NMR resonances for 1-PMe3 shifted toward the
chemical shifts of 1 and free PMe3. These results are consistent with
partial dissociation of PMe3 to form 1, and rapid exchange between 1-
PMe3 and 1 on the NMR time scale (Figure 2). The fast exchange
between 1-PMe3 and 1 was confirmed by 1H NMR spectra of mixtures
of 1-PMe3 and 1 (Figure 3 and Figure S-1 in the Supporting
Information). The 1H NMR resonances for mixtures of 1-PMe3 and 1
appear at the mole-fraction-weighted-averaged chemical shifts of the
resonances of 1-PMe3 and 1.
Generation of [(PDI)Fe(OAc)THF][BPh4] (4-THF). THF (0.01
mmol) was added by syringe to a solution of 4 (9.1 mg, 0.0099 mmol)
in C6D5F to generate 4-THF. The formation of 4-THF is supported
1
by the H NMR spectra of mixtures of 4 and THF, which exhibit
resonances at the mole-fraction-weighted average of the chemical shifts
of 4 and 4-THF. These results imply that 4-THF is formed and
exchanges with 4 rapidly on the NMR time scale (see Figure S-3 in the
Supporting Information). 1H NMR (C6D5F): δ 199.2, 112.9, 12.4, 9.3,
1
4.3, 2.9, 2.4, 1.2, −1.3, −11.8, −13.7, −16.0, −78.9. H NMR (THF-
d8): δ 156.8, 78.7, 13.9, 8.7, 7.8, 7.3, −4.5, −11.1, −13.5, −35.1.
Kinetic Analysis of the Reaction of Fe Alkyl Complexes with
CO2. An NMR tube was charged with the Fe alkyl complex, and the
solvent was added by vacuum transfer at −196 °C. The mixture was
warmed to the desired temperature and exposed to CO2 at the desired
pressure. The tube was then rapidly inserted into an NMR probe that
Generation of [(PDI)Fe(Me)THF][BPh4] (2-THF). An NMR tube
was charged with 2 (8.7 mg, 0.010 mmol), and C6D5F (0.5 mL) was
added by vacuum transfer at −196 °C. The mixture was thawed at
room temperature to produce a red solution. THF (0.010 mmol) was
added by syringe to afford a blue solution. The formation of 2-THF is
supported by several observations as discussed in the text (Figure S-2
1
1
had been equilibrated at the desired temperature. H NMR spectra
in the Supporting Information). H NMR (THF-d8): δ 82.37, 78.79,
1
were recorded periodically. Specific details for each complex are
included in the Supporting Information.
11.52, 8.70, 7.66, −4.17, −9.95, −34.84. H NMR (C6D5F): δ 85.58,
81.26, 9.54, 8.18, 4.40, 2.35, 1.17, −5.67, −10.46, −42.71. 2-THF is
thermally unstable in THF-d8 and C6D5F at room temperature and
decomposed over 1 day to give a black precipitate.
Attempted Reaction of 2-THF-d8 with CO2. An NMR tube was
charged with 2 (4.8 mg, 0.0055 mmol). THF-d8 (0.5 mL) was added
by vacuum transfer at −196 °C. The mixture was thawed at room
temperature, exposed to CO2 (1 atm, 19 equiv), and allowed to stand
(PDI)FeOAc (3). This compound was previously synthesized by the
reaction of (PDI)Fe(N2)2 with EtOAc, or with MeOAc and H2.20 In
the present work, 3 was prepared by carboxylation of 1. A Schlenk
flask was charged with 1 (210 mg, 0.379 mmol). The flask was
evacuated and exposed to CO2 (1 atm). Et2O (10 mL) was added by
syringe, and the mixture was stirred for 15 min at room temperature
under a CO2 atmosphere. The volatiles were removed under vacuum
to afford 3 as a green solid (212 mg, 0.354 mmol, 93%). 1H NMR data
for 3 matched the literature data. IR νCO (KBr): 1515, 1459 cm−1. IR
assignments were made by inspection of the IR spectra of 3, 3-13C1
(synthesized from 1 and 13CO2), and (PDI)FeCl (Figure S-10 in the
Supporting Information).
1
at room temperature for 2 days. H NMR spectra were recorded and
showed that 2-THF-d8 had partially decomposed to the same
unidentified products at approximately the same rate as in the absence
of CO2. There was no evidence for the formation of [(PDI)Fe(OAc)-
(THF-d8)][BPh4] (4-THF-d8).
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00470.
Generation of (PDI)Fe(OAc)PMe3 (3-PMe3). (a) An NMR tube
was charged with 1-PMe3 (5.2 mg, 0.0083 mmol). Toluene-d8 (0.6
mL) was added by vacuum transfer at −196 °C. The mixture was
thawed and exposed to CO2 (1 atm, 12 equiv) at room temperature.
Additional experimental details, NMR and IR spectra, X-
ray diffraction data, SQUID data, EPR spectra, figures,
and tables (PDF)
1
The tube was agitated for 15 min at room temperature. H NMR
analysis showed that an equilibrium mixture of 3-PMe3 and 3 had
Crystallographic data for 1-PMe3 and 4 (CIF)
formed. These species exchange rapidly on the NMR time scale in
1
toluene-d8. The H resonances for 3-PMe3 were assigned using the
approach described in the text for 1-PMe3. 1H NMR (toluene-d8,
0.017 M): δ 184.5 (OC(O)Me), 118.9 (m-py), −2.9 (m-Ar), −16.2 (p-
Ar), −19.3 (CHMe2), −29.7 (CHMe2), −112.2 (CHMe2), −282.4
(NCMe); the Fe−PMe3 resonance was not observed. (b) An NMR
tube was charged with 3 (10.7 mg, 0.0179 mmol), and a solution of
PMe3 in C6D6 (0.036 M, 0.5 mL, 0.018 mmol) was added. The tube
was agitated at room temperature for 5 min. 1H NMR analysis showed
that an equilibrium mixture of 3-PMe3 and 3 had formed.
AUTHOR INFORMATION
Corresponding Author
*E-mail: rfjordan@uchicago.edu.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
[(PDI)FeOAc][BPh4] (4). Compound 3 (61 mg, 0.10 mmol) and
[Cp2Fe][BPh4] (45 mg, 0.089 mmol) were taken up in benzene (5
mL), and the green slurry was stirred at room temperature for 45 min.
Pentane (10 mL) was added, and the mixture was stirred for 15 min to
afford an orange-red slurry. The yellow supernatant was removed by
pipet, and the solid was washed with pentane until the washes were
colorless. The solid was dried under vacuum to afford a pale red
powder (81 mg, 0.088 mmol, 99%). The use of a substoichiometric
amount of [Cp2Fe][BPh4], which is insoluble under reaction
conditions, ensured that it was fully consumed, which facilitated the
isolation of 4. The unreacted 3 was removed by the pentane washes.
1H NMR (C6D5F): δ 273.3, 167.5, 17.7, 16.2, 10.9, 3.8, −18.1, −23.9,
−35.1, −146.3. HR-ESI-MS (C6H5F, positive ion mode) m/z
This work was supported by the National Science Foundation
(CHE-0911180 and CHE-1048528) and the University of
Chicago Women’s Board. We thank Drs. Antoni Jurkiewicz and
C. Jin Qin for assistance with NMR spectroscopy and mass
spectrometry, respectively, Drs. Ian Steele and Alexander
Filatov for assistance with X-ray diffraction, and Prof. John
Anderson and Airi Kawamura for SQUID measurements and
EPR spectroscopy.
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