Solvent effects on hydride transfer from Cp?(P-P)FeH to BNA+ cation

Article abstract of DOI:10.1021/acs.organomet.6b00907

Examining of the hydride transfer reaction between Cp^?(Ph₂PN^tBuPPh₂)-FeH (Ph₂PN^tBuPPh₂ = N,N-bis(diphenylphosphanyl)tert-butylamine, 1-H) and 1-benzyl-3-carbamoylpyridinium cation (BNA⁺) in different solvents, we found that the solvents exert considerable influence on the hydride transfer processes. A coordinating solvent molecule such as MeCN is not only a ligand which stabilizes the organo-iron fragment producing [C^p?(Ph₂PN^tBuPPh₂)Fe(NCMe)]⁺ ([1(NCMe)]⁺), but also assists the hydride transfer. In THF, reaction of 1-H with BNA⁺ under high pressure of nitrogen (60 psi) giving the iron(II)-nitrogen complex [Cp^?(Ph₂PN^tBuPPh₂)Fe(N₂)]⁺ ([1-N₂]⁺) and BNAH. In CH₂Cl₂, [1-N₂]⁺ catalyzes the conversion of 1-H to Cp^?(Ph₂PN^tBuPPh₂)- FeCl (1-Cl), which hampers the expected hydride transfer reaction. In the presence of MeCN, the hydride transfer process in THF, CH₂Cl₂, or benzene was achieved affording the reduced BNAH and [1(NCMe)]⁺. New iron complexes in the [Cp^?(Ph₂PN^tBuPPh₂)-FeX]ⁿ⁺ series (where n = 0, X = H or Cl; n = 1, X = MeCN, N₂, or Cl^-) were obtained and well characterized.

Full text of DOI:10.1021/acs.organomet.6b00907

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pubs.acs.org/Organometallics
Solvent Effects on Hydride Transfer from Cp*(P-P)FeH to BNA+
Cation
Fanjun Zhang, Xin Xu, Yingjie Zhao, Jiong Jia, Chen-Ho Tung, and Wenguang Wang*
School of Chemistry and Chemical Engineering, Shandong University, No. 27 South Shanda Road, Jinan 250100, China
S
* Supporting Information
ABSTRACT: Examining of the hydride transfer reaction between Cp*(Ph2PNtBuPPh2)-
FeH (Ph2PNtBuPPh2 = N,N-bis(diphenylphosphanyl)tert-butylamine, 1-H) and 1-benzyl-
3-carbamoylpyridinium cation (BNA+) in different solvents, we found that the solvents
exert considerable influence on the hydride transfer processes. A coordinating solvent
molecule such as MeCN is not only a ligand which stabilizes the organo-iron fragment
producing [Cp*(Ph2PNtBuPPh2)Fe(NCMe)]+ ([1(NCMe)]+), but also assists the
hydride transfer. In THF, reaction of 1-H with BNA+ under high pressure of nitrogen
(60 psi) giving the iron(II)−nitrogen complex [Cp*(Ph2PNtBuPPh2)Fe(N2)]+ ([1-N2]+)
and BNAH. In CH2Cl2, [1-N2]+ catalyzes the conversion of 1-H to Cp*(Ph2PNtBuPPh2)-
FeCl (1-Cl), which hampers the expected hydride transfer reaction. In the presence of
MeCN, the hydride transfer process in THF, CH2Cl2, or benzene was achieved affording
the reduced BNAH and [1(NCMe)]+. New iron complexes in the [Cp*(Ph2PNtBuPPh2)-
FeX]n+ series (where n = 0, X = H or Cl; n = 1, X = MeCN, N2, or Cl−) were obtained
and well characterized.
evidenced by spectroscopic studies.7d For pyridinium cations
INTRODUCTION
■
with a noncoordinating group such as −CF3 or H at the C3
The coenzyme nicotinamide adenine dinucleotide (phos-
phate) (NAD(P)H) provides electrons or energy for
position, the corresponding 1,4-dihydropyridine products were
proposed to coordinate with the Ru(II) center in a η2 mode
carbohydrate synthesis in the dark reactions of photosyn-
through a CC bond at the initial formation stage.7b On the
thesis,1 and in the preparation of nitrogen-containing
basis of the investigation of reducing iminium8 and N-
heterocycles.2 Reduction of pyridinium cations to the
acylpyridinium cations by Cp(P-P)RuH hydrides, in which P-
corresponding dihydropyridine is interesting and important,3
P = chelating diphosphine,5c,9 Norton et al. suggested “a
not only because of its relevance to energy storage and release
in the biological system but also because dihydropyridine
single-step H− transfer” mechanism to form the 1,2-product,
and “a two-step e−/H• process” for only the 1,4-product.
derivatives are widely used as reductants in organocatalysis.4
Recently, we reported reduction of a range of N-
Intensive studies have focused on noble-metal hydrides such
benzylpyridinium cation derivatives by Cp*(P-P)FeH (P-P =
as Rh(III),5 Ir(III),4c,5c,6 Ru(II),7 and Re(I),7d employed as
bis(diphenylphosphino)methane (dppm), bis(diphenylphos-
hydride donors to reduce pyridinium cations. Mechanistic
phino)ethane (dppe), bis(diphenylphosphino)benzene
insights into factors that determine the regioselectivity of
(dppbz), or 1,2-bis(dicyclohexylphosphino)ethane (dcpe)).
dihydropyridine products (1,4-isomer vs 1,6-isomer) have
Our studies indicated that the N-benzylpyridinium cation is
attracted particular attention (Scheme 1).
favorably reduced at the C6 position, affording the 1,6-isomer
For example, the regioselective reduction of 1-benzyl-3-
as the kinetically controlled product. The stability of the 1,6-
carbamoylpyridinium cation (BNA+) to the 1,4-BNAH
products is very dependent on the substituent (R2) at the C3
product was thought to proceed through a coordinative
position. The results obtained suggested that H− transfer is
intermediate, in which the carbamoyl group in BNA+ interacts
more likely to be charge controlled, which is consistent with
with the metal center.5a,b Ishitani et al. reported such
the “a single-step H− transfer” mechanism.10 Unlike the
interactions in the case of BNA+ and [(tpy)(bpy)RuH]+
systems based on the noble metal hydrides,5c,9 the hydride
transfer reaction between Cp*(P-P)FeH and pyridinium
cations requires the coordinating solvent MeCN. The
Scheme 1. Reduction of Pyridinium Cations to 1,6- and
1,4-Dihydropyridines by Metal Hydrides (M−H)
presence of MeCN seems not only to stabilize the organoiron
products in the [Cp*(P-P)Fe(NCMe)]+ form, but also affect
significantly the hydride transfer process.
Received: December 7, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.6b00907
Organometallics XXXX, XXX, XXX−XXX

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In this paper, we examine the reactions between BNA+ and
1-H in various solvents such as CH2Cl2, THF, and benzene in
the presence or absence of MeCN (Scheme 2). We found the
Scheme 2. Reactions of Cp*(Ph2PNtBuPPh2)FeH with
BNA+ in Different Solvents
Figure 1. Structure (50% probability thermal ellipsoids) of 1-H. For
clarity, the Cp* ring and phenyl groups are drawn as lines. Hydrogen
atoms with the exception of the hydride ligand are omitted. Selected
distances (Å) and angles (deg): Fe−P(1) 2.1169(5), Fe−P(2)
2.1320(5), and P(1)−Fe−P(2) 72.72(2).
critical limit of 1.0 eV.11 On the basis of the reduction
potential of BNA+ (−1.5 V vs Fc+/0), ΔGET for the electron
transfer process for 1-H was calculated to be 1.21 eV and
accordingly, the initial ET from 1-H to BNA+ would appear
to be thermodynamically unfavorable.
solvents exert an influence on such hydride transfer processes,
beyond the rate of the hydride transfer and some side
reactions that hamper the expected reduction of the BNA+
cation. Besides 1-H and [1(NCMe)]+, several new iron
complexes [1-N2]+, 1-Cl, and [1-Cl]+ were isolated and
characterized from these reactions.
Hydride Transfer in THF. The reaction of 1-H with
BNA+ was examined in THF, which was found to be weakly
coordinating solvent for [Cp*(P-P)Fe]+ species.12 To assist
BNA+ dissolving well in THF, we exchanged the counterion,
−
−
replacing PF6 with BPh4 (tetraphenylborate). The stoichio-
metric reaction of 1-H and [BNA]BPh4 (0.04 M) in THF-d8
was conducted in a J. Young tube at room temperature. After
24 h, the reaction mixture was analyzed by NMR at room
temperature. The peaks were broadened in the 1H NMR
spectrum, and the hydride signal for 1-H at −13.5 ppm was
absent. The 31P NMR spectrum exhibited a new signal at
114.9 ppm, and the phosphorus signals for 1-H were not
observed. However, upon cooling down the reaction solution
to −20 °C, both the hydride and phosphorus signals for 1-H
RESULTS AND DISCUSSION
■
Synthesis and Characterization. Compound 1-H was
synthesized according to the reported procedure for Cp*(P-
P)FeH,10 using the acetonitrile complex [1(NCMe)]+ as the
precursor. Treatment of [1(NCMe)]+ with NaBH4 in THF
leads to gradual color changes from dark red to orange. The
product was recrystallized from pentane, and isolated by
1
filtration in a yield of 78%. The H NMR spectrum of 1-H
displays its hydride resonance at −13.5 ppm (JP−H = 63 Hz)
as a triplet.
1
appeared as broad peaks in the H and 31P NMR spectra. At
1
−60 °C, the H NMR spectrum features a triplet signal at
Crystals of 1-H suitable for X-ray diffraction were obtained
by cooling the saturated pentane solution to −30 °C. The
structure of 1-H was confirmed crystallographically, and the
hydride ligand was located (Figure 1). The framework Cp*(P-
P)FeX is similar to the Cp*(P-P)FeH series described
previously.10 The bite angle ∠P−Fe−P in 1-H is 72.72(2)°,
2.6° smaller than that in Cp*(dppm)FeH (75.33°). It has
been suggested that the bite angle in Cp*(P-P)MX
compounds reflects the steric hindrance around the metal
center.8b Compared with the Cp*(P-P)FeH (P-P = dppm,
dppe, dppbz, or dcpe),10 1-H exhibits the smallest bite angle.
The cyclic voltammogram of 1-H exhibits a one-electron
reversible oxidation process at −0.29 V vs Fc+/0 for the [1-
H]+/0 couple. Compared to Cp*(dppm)FeH, the oxidation
potential of 1-H is 60 mV less negative, which indicates that
dppm has better electron-donating properties than the
Ph2PNtBuPPh2 ligand for the Cp*FeH motif. For the
reduction of BNA+ or relative organocations, Cheng et al.
suggested that a multistep mechanism (e−/H• or e−/H+/e−)
would be followed if the energy gap (ΔGET) of the initial
electron transfer process (ET) between the cations and the
reducing substrate is considerably smaller than the empirical
−13.5 ppm (JP−H = 63 Hz) corresponding to the 31P NMR
signal at δ 128.4 for 1-H. As shown in Figure 2, we recorded
the 31P NMR spectra at various temperatures from 20 °C to
−60 °C. Besides the 31P signal for 1-H, a new signal at 114.9
ppm was also observed, indicating the production of a new
organo-iron species.
+
[FeII−H]0 + BNA + N2 → [FeII−N2]+ + BNAH
(1)
This new species was thought to be [1-N2]+. To prove this
hypothesis (eq 1), we conducted the reaction of 1-H and
BNA+ in THF under higher pressure (60 psi) of nitrogen at
room temperature. After 48 h, 1-H was almost completely
converted to [1-N2]+, which was isolated and further
characterized (eq 1). In the IR spectrum, [1-N2]+ exhibits a
strong νNN band at 2132 cm−1, which is close to the N−N
stretching frequency of 2130 cm−1 for [HFe(dppe)2(N2)]-
BPh4,13 but much higher than that for [{CpFe-
(dppe)}2(N2)]2+ (2040 cm−1),14 in which N2 is as a bridging
ligand coordinated to the two metals.
The structure of [1-N2]BPh4 was confirmed by X-ray
crystallography (Figure 3). In the [1-N2]+ cation, N2 is
B
DOI: 10.1021/acs.organomet.6b00907
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1
Figure 2. 31P NMR (left) and H NMR (right, hydride region) spectra recorded for the reaction of 1-H and BNA+ in THF-d8 at various
temperatures from 293 to 213 K.
interaction between the unreacted 1-H and the organo-iron
product [1-N2]+ (eq 2). Such interaction were also observed
for the THF solutions of 1-H and [1-N2]+ (20 mol %) in the
independent NMR experiments (Figure S12). It should be
noticed that the NMR spectra of 1-H recorded in THF
features the distinguishable signals for the hydride and the
diphosphines ligands.
[FeII−N2]+ + *[FeII−H]0 → [FeII−H]0 + *[FeII−N2]+
(2)
Norton et al. reported the formation of bimetallic bridging
hydride complex {[Cp*(2-(2-pyridyl)phenyl)Rh]2(μ-H)}+ by
the reaction of Cp*(2-(2-pyridyl)phenyl)RhH and [Cp*(2-(2-
pyridyl)phenyl)Rh(NCMe)]+ in C6H6.17 Dissolving the
bridging hydride complex in MeCN resulted in the recovery
of Cp*(2-(2-pyridyl)phenyl)RhH and [Cp*(2-(2-pyridyl)-
phenyl)Rh(NCMe)]+. The related bimetallic complex, {[Cp*-
(CO)2Ru]2(μ-H)}+ was proposed as an intermediate in the
reduction of [Ph3C]BF4 by Cp*(CO)2RuH by Bullock.18
Owing to the lability of the N2 ligand, the equilibrium
between [1-N2]+ and the 16-electron cation complex
[Cp*(Ph2PNtBuPPh2)Fe]+ and N2 is possible. The hydride
abstraction from 1-H by [Cp*(Ph2PNtBuPPh2)Fe]+ can be
through an intermediate of [FeII−H−FeII]+ (unobserved)
species.
Figure 3. Structure (50% probability thermal ellipsoids) of [1-N2]+.
For clarity, the four phenyl groups bound to phosphorus are drawn
as lines and hydrogen atoms are omitted. Selected distances (Å) and
angles (deg): Fe−N(2) 1.833(9), N(2)−N(3) 1.12(1), Fe−P(1)
2.210(3), Fe−P(2) 2.233(3), P(1)−Fe−P(2) 70.9(1), and
Fe−N(2)−N(3) 173.4(8).
unambiguously coordinated to the iron(II) center in an end-
on binding mode.15 The N−N bond distance 1.12(1) Å is
comparable to 1.13 Å observed for [Cp(dippe)Fe(N2)]+
(dippe =1,2-bis(diisopropylphosphino)ethane).16 Compared
to [Cp(dippe)Fe(N2)]+, the Fe−N distance 1.833(9) Å is
about 0.07 Å longer, while the Fe−N−N angle 173.4(8)° is
almost unchanged. Combined with the N−N stretching
frequency, these crystallographic data for [1-N2]+ indicate
that the N2 molecule was activated minimally at the iron
center.
Hydride Transfer in THF in the Presence of MeCN.
When the reaction of 1-H with BNA+ was performed in THF-
d8 in the presence of 10 equiv of MeCN, stoichiometric
hydride transfer proceeded and produced [1(NCMe)]+ and
BNAH as both 1,6- and 1,4-isomers (eq 3).
The N2 ligand in [1-N2]+ is very labile, and can be replaced
by CO or MeCN affording [Cp*(Ph2PNtBuPPh2)Fe(CO)]+
(Figures S10 and S11 of the Supporting Information) and
[1(NCMe)]+ respectively. Especially, the reaction of [1-N2]+
with Cp*(dcpe)FeH in THF gave 1-H and 16-electron
cationic complex [Cp*(dcpe)Fe]+, which is stable toward N2.
The structure of [Cp*(dcpe)Fe]+ was established by single-
crystal X-ray diffraction (Figure S25).
+
1‐H + BNA + MeCN
→ [1(NCMe)]+ + 1, 6‐BNAH + 1, 4‐BNAH
(3)
The reaction of 1-H with excess BNA+ (10 equiv) in THF-
d8/CD3CN (2/1) was monitored by 1H NMR at 298 K,
giving a pseudo-first-order rate constant kobs. Variation of
[BNA+] from 0.146 to 0.379 M showed that kobs for 1-H was
linearly related to [BNA+], indicating that hydride transfer
from 1-H to BNA+ is a second order reaction,8b and the rate
For the reaction of 1-H and BNA+ in THF, variable-
temperature 31P NMR spectra showing coalescences and
decoalescences probably belong to the intermolecular
C
DOI: 10.1021/acs.organomet.6b00907
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Figure 4. Left, concentrations of BNAH, 1,4-BNAH and 1,6-BNAH over time during the reaction of 1-H with BNA+ in THF-d8/CD3CN (2/1)
at 298 K. The initial [1-H] and [BNA+] are 3.30 × 10−2 M and 3.30 × 10−1 M, respectively. Results: kobs = 6.2(3) × 10−6 s−1, R2 = 0.997. Right,
plot of kobs vs [BNA+] for the reaction of 1-H with BNA+. Results: k = 1.9(1) × 10−5 M−1 s−1, R2 = 0.992.
constant k was determined to be 1.9(1) × 10−5 M−1 s−1
(Figure 4).
this product was over 90%. Additionally, we found the
reaction between 1-H and BNA+ in CH2Cl2 releases H2
identified by GC analysis.
The MeCN in the hydride transfer from Cp*(P-P)FeH
(FeII−H) to BNA+ could behave as (i) a ligand stabilizing the
organo iron(II) fragment after the reaction, or (ii) a
nucleophilic agent giving impetus to the hydride transfer. In
principle, there are two possible pathways for “a single-step
H− transfer” mechanism. One is FeII−H attacking the C6
position of BNA+ nucleophilically, affording BNAH and the
16 e− unsaturated [Cp*(P-P)Fe]+ species ([FeII]+), which was
trapped by MeCN, producing [Cp*(P-P)Fe(NCMe)]+
([FeII−NCMe]+, eqs 4 and 5)
+
[FeII−H]0 + BNA → [FeII]+ + BNAH
(4)
[FeII]+ + MeCN → [FeII−NCMe]+
(5)
The other possible pathway could involve an intermediate
of [FeII−H---BNA]+ arising from the weak static interaction
between FeIIH and BNA+. Coordinative molecules such as
MeCN or N2 attacks the iron center of [FeII−H---BNA]+
driving the hydride transfer (eq 6).
Figure 5. Structure (50% probability thermal ellipsoids) of 1-Cl. For
clarity, the four phenyl groups bound to phosphorus are drawn as
lines and hydrogen atoms are omitted. Selected distances (Å) and
angles (deg): Fe−P(1) 2.183(2), Fe−P(2) 2.209(2), Fe−Cl
2.318(2), and P(1)−Fe−P(2) 71.75(6).
+
[FeII−H]0 + BNA + MeCN
→ [FeII−NCMe]+ + BNAH
According to these results, we proposed the reaction
pathways shown in eqs 1 and 7−10. The very slow hydride
transfer from 1-H to BNA+ under the N2 atmosphere
generated a small amount of [1-N2]+ and BNAH.
(6)
To understand the behavior of MeCN in the hydride
transfer process, we examined the reaction of 1-H with BNA+
in CD2Cl2 and C6D6 in the presence of MeCN.
+
very slow: [FeII−H]0 + BNA + N2
Hydride Transfer in CH2Cl2. It has been reported that
the hydride transfer from Cp*(CO)2FeH to [Ph3C]BF4 in
CH2Cl2 produced Ph3CH and [Cp*(CO)2Fe−F−BF3].18 For
the reaction of 1-H with BNA+ in CD2Cl2, the color of the
solution turned to dark green after 24 h, and the NMR
spectral analysis suggest the reaction mixture is paramagnetic.
By contrast, 1-H is very stable toward CH2Cl2 in solution
under a N2 atmosphere. At least in 48 h there was no reaction
→ [FeII−N2]+ + BNAH
(1)
Once [1-N2]+ produced, it catalyzes the degeneration of 1-
H to 1-Cl in CH2Cl2. Though 1-H is stable in CH2Cl2, the
cationic complex [1-N2]+ is highly reactive toward CH2Cl2
resulting in formation of ferric complex [1-Cl]+ and C2H4Cl2
identified by GC-MS analysis (eq 7). Alternatively, the
complex [1-Cl]+ was prepared by dissolving [1-N2]+ in
CH2Cl2, and its structure was characterized by X-ray
crystallography (Figure 6).
1
observed, judging from 31P and H NMR studies.
To understand precisely the reactions that occur, we
conducted the reaction in a larger scale in CH2Cl2 using 50
mg 1-H (0.079 mmol) and 1 equiv [BNA]BPh4. After 36 h,
the solvent was removed under vacuum and the residue was
extracted with hexane. The dark green compound dissolves
readily in hexane, and the undissolved white solid was
As determined by cyclic voltammograms, the oxidation
potential for [1-Cl]+/0 couple is −0.04 V vs Fc+/0, which can
oxidize 1-H (E1/2 = −0.29 V) producing 1-Cl and [1-H]+
ox
(eq 8). Additionally, [1-H]+ are unstable, and undergo H2
elimination via a disproportionation pathway (eq 9).3a Direct
evidence is derived from oxidization of 1-H by FcBF4 in THF,
producing H2 and [1-N2]+ under an N2 atmosphere.
1
confirmed by H NMR spectra to be BNA+. The structure of
the new green compound, 1-Cl, was revealed by X-ray
crystallography (Figure 5). It is surprising that the yield of
D
DOI: 10.1021/acs.organomet.6b00907
Organometallics XXXX, XXX, XXX−XXX