Hydride Transfer from Iron(II) Hydride Compounds to NAD(P)+ Analogues

Article abstract of DOI:10.1021/acs.organomet.6b00179

Iron(II) hydride complexes of the "piano-stool" type, Cp?(P-P)FeH (P-P = dppe (1H), dppbz (2H), dppm (3H), dcpe (4H)) were examined as hydride donors in the reduction of N-benzylpyridinium and acridinium salts. Two pathways of hydride transfer, "single-step H^-" transfer to pyridinium and a "two-step (e^-/H^?)" transfer for acridinium reduction, were observed. When 1-benzylnicotinamide cation (BNA⁺) was used as an H^- acceptor, kinetic studies suggested that BNA⁺ was reduced at the C6 position, affording 1,6-BNAH, which can be converted to the more thermally stable 1,4-product. The rate constant k of H^- transfer was very sensitive to the bite angle of P-Fe-P in Cp?(P-P)FeH and ranged from 3.23 × 10^-3 M^-1 s^-1 for dppe to 1.74 × 10^-1 M^-1 s^-1 for dppm. The results obtained from reduction of a range of N-benzylpyridinium derivatives suggest that H^- transfer is more likely to be charge controlled. In the reduction of 10-methylacridinium ion (Acr⁺), the reaction was initiated by an e^- transfer (ET) process and then followed by rapid disproportionation reactions to produce Acr₂ dimer and release of H₂. To achieve H^? transfer after ET from [Cp?(P-P)FeH]⁺ to acridine radicals, the bulkier acridinium salt 9-phenyl-10-methylacridinium (PhAcr⁺) was selected as an acceptor. More evidence for this "two-step (e^-/H^?)" process was derived from the characterization of PhAcr^? and [4H]⁺ radicals by EPR spectra and by the crystallographic structure confirmation of [4H]⁺. Our mechanistic understanding of fundamental H^- transfer from iron(II) hydrides to NAD⁺ analogues provides insight into establishing attractive bio-organometallic transformation cycles driven by iron catalysis.

Full text of DOI:10.1021/acs.organomet.6b00179

Article
pubs.acs.org/Organometallics
Hydride Transfer from Iron(II) Hydride Compounds to
+
NAD(P) Analogues
Fanjun Zhang, Jiong Jia, Shuli Dong, Wenguang Wang,* and Chen-Ho Tung
Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering,
Shandong University, No. 27 South Shanda Road, Jinan 250100, People’s Republic of China
*
S Supporting Information
ABSTRACT: Iron(II) hydride complexes of the “piano-stool” type,
Cp*(P-P)FeH (P-P = dppe (1H), dppbz (2H), dppm (3H), dcpe (4H))
were examined as hydride donors in the reduction of N-benzylpyridinium and
−
acridinium salts. Two pathways of hydride transfer, “single-step H ” transfer
−
•
to pyridinium and a “two-step (e /H )” transfer for acridinium reduction,
+
were observed. When 1-benzylnicotinamide cation (BNA ) was used as an
−
+
H acceptor, kinetic studies suggested that BNA was reduced at the C6
position, affording 1,6-BNAH, which can be converted to the more thermally
−
stable 1,4-product. The rate constant k of H transfer was very sensitive to the
−3
−1 −1
bite angle of P−Fe−P in Cp*(P-P)FeH and ranged from 3.23 × 10
M
s
−1
−1 −1
for dppe to 1.74 × 10
M
s
for dppm. The results obtained from−
reduction of a range of N-benzylpyridinium derivatives suggest that H
transfer is more likely to be charge controlled. In the reduction of
+
−
1
0-methylacridinium ion (Acr ), the reaction was initiated by an e transfer
•
(
ET) process and then followed by rapid disproportionation reactions to produce Acr dimer and release of H . To achieve H
2
2
+
+
transfer after ET from [Cp*(P-P)FeH] to acridine radicals, the bulkier acridinium salt 9-phenyl-10-methylacridinium (PhAcr )
−
•
•
was selected as an acceptor. More evidence for this “two-step (e /H )” process was derived from the characterization of PhAcr
+
+
and [4H] radicals by EPR spectra and by the crystallographic structure confirmation of [4H] . Our mechanistic understanding
of fundamental H transfer from iron(II) hydrides to NAD analogues provides insight into establishing attractive bio-
organometallic transformation cycles driven by iron catalysis.
−
+
−
INTRODUCTION
Scheme 1. Two Pathways of H Transfer from M−H to
■
+
a
1
Organic Cation Acceptors (A )
So-called “organo-hydrides”, or nicotinamide cofactor
NAD(P)H analogues such as dihydronicotinamides and
2
,3
1
,4-dihydropyridines, are widely used as reducing agents in
4
−6
hydrogenation and transfer hydrogenation catalysis.
Regen-
+
eration of NAD /NADH model compounds by metal catalysis
7
−10
is significant in the establishment of transformation cycles.
−
In this context, hydride (H ) transfer from metal hydrides
M−H to pyridinium, iminium, acridinium, and other organic
cation salts is a ubiquitous elemental step and a knowledge of
a
Abbreviations: SET, single-electron transfer; HAT, hydrogen atom
transfer; solv, coordinating solvent molecules such as MeCN and H O.
2
1
1−13
−
M−H cleavage
during H transfer is essential, because it
Cp′(P-P)RuH hydrides (Cp′ = C H or C Me5− anion, P-P =
−
provides a qualitative strategy for catalyst design.
5
5
5
−
16
−
In principle, there are two pathways for H transfer in the
chelating diphosphine) and reported that the H transfer
reaction is significantly affected by the electronic and steric
properties of the chelating diphosphine ligand on the Ru(II)
+
reported reactions of M−H and organo-cations A , as shown in
−
Scheme 1. These are the “single-step H ” transfer path (I) and
−
•
14
−
+
the “two-step (e /H )” path (II). Transfer of a hydride (H )
center. When 1-benzylnicotinamide cation (BNA ), resembling
oxidized NAD cofactor, is used as an H acceptor, it gives
mainly the reduction product, 1-benzyldihydronicotinamide
BNAH), as the two isomers 1,6- and 1,4-BNAH. Fish et al.
reported H transfer from [Cp*(bpy)RhH] (bpy =
+
−
is usually achieved via nucleophilic attack on organo-cations.
Twenty years ago, Hembre et al. reported H transfer from
Cp*(dppm)RuH (Cp* = C Me , dppm = Ph PCH PPh ) to
N-methylquinolinium salts. The presence of an electron-
transfer inhibitor, [MV] (MV = methyl viologen), had no−
effect on methylquinolinium reduction, thus excluding an H
transfer process initiated by an ET step. Norton et al.
investigated the reduction of iminium cations to imines by
−
(
5
5
2
2
2
−
+
1
0
+
2
+
2,2′-bipyridine) to BNA , producing 1,4-BNAH as the kinetic
Received: March 2, 2016
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.6b00179
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Organometallics
Article
−
Scheme 2. H Transfer from Cp*(P-P)FeH to N-Benzylpyridinium and Acridinium Salts
1
7
product. Regioselective production of 1,4-BNAH was
thought to be through a transition state in which the oxygen
and Cp*(dcpe)FeH (dcpe = 1,2-C H (PCy ) ; 4H) were
synthesized in good yields using modified published methods.
2
4
2 2
+
atom of the carbamoyl group in BNA was coordinated to
the metal center and the Cp* ring changed its coordination
mode from η to η . This transition state could cause the C4
position to be more electrophilic toward attack. Ishitani et al.
reported the interactions between [(tpy)(bpy)RuH] (tpy =
FeCl was added to 1 equiv of Cp*Li in THF, and the mixture
2
was stirred at room temperature for 1 h. Two equiv amount of
5
3
KPF in MeCN was added to give [Cp*Fe(NCMe) ]PF , which
6
3
6
was precipitated by diethyl ether as a deep purple solid.
+
This precursor was used to prepare various [Cp*(P-P)Fe-
+
+
2
,2′:6′2″-terpyridine) and BNA salts, forming a [Ru-BNAH]
NCMe)]+ salts by adding the corresponding diphosphine.
The MeCN ligand in [Cp*(P-P)Fe(NCMe)] was displaced by
(
18
adduct. However, such interactions have also been observed in
+
19
NMR studies, even in pyridinium salts with a noncoordinating
a hydride from Bu NBH to give Cp*(P-P)FeH compounds.
−
•
4
4
group at the C3 position. A two-step (e /H ) pathway is more
likely to operate in cases in which the M−H and organo-cations
have close redox potentials. Norton et al. reported that
reduction of acylpyridinium salts by Cp*Ru(dppf)H (dppf =
1
The H NMR spectra of 1H and 2H exhibit hydride resonances
at around δ −16 (t, J
31
= 69 Hz), and the P NMR spectrs
P−H
display a singlet at δ 107.8 for 1H and at δ 102.6 for 2H.
In comparison with 1H and 2H, the hydride signal shifts to
δ −11.9 in 3H and δ −17.4 in 4H.
1
,1′-bis(diphenylphosphino)ferrocene) is initiated by a “single-
electron-transfer” (SET) step, followed by a hydrogen atom
+
14
All of the synthesized complexes of [Cp*(P-P)Fe(NCMe)]
transfer (HAT) step.
Although hydrides (M−H) based on noble metals
M = Ru,
in this field, studies based on abundant metal hydrides are
sparse. In recent years, remarkable progress has been made in
hydrogenation and transfer hydrogenation catalysis by first- and
and Cp*(P-P)FeH were structurally characterized by crystallo-
graphic analysis. The structures of 2H−4H are depicted in
Figure 1, and other structures are shown in Figures S1−S5 in
the Supporting Information. A comparison of P−Fe−P bite
angles among these iron(II) complexes is provided in Table 1.
The bite angle in Cp*(P-P)FeH is larger than those in
the corresponding parent [Cp*(P-P)Fe(NCMe)]+ cations.
The bite angles (∠P−Fe−P) in 1H and 2H differ by only ca.
0.2°. Such minor differences of binding effects between dppe
and dppbz have also been observed in Cp*(P-P)RuH
1
4−16,18−20
21
17,22
(
Ir, Rh
) have been well investigated
23,24
second-row metals.
We have been interested in the
+
reduction of NAD cation models by using hydrido iron(II)
complexes, and previous work with Rauchfuss et al. has
demonstrated the hydridic character of a diiron dihydride
25
compound toward benzimidazolium salts.
16
In this paper, we examine the role of the well-known
iron(II) hydride complexes of the “piano-stool” type,
Cp*(P-P)FeH (P-P = chelating diphosphine), in the reduction
of N-benzylpyridinium and acridinium salts. Significantly, two
different mechanisms were found in the process of H transfer
from Cp*(P-P)FeH to these organo-cations (Scheme 2): a
compounds. In comparison to dppbz, the more bulky dcpe
group increases the bite angle in 4H by about 1.4°, while the
dppm ligand experiences steric constriction at the Fe center in
3H with its bite angle of 75.33(4)°.
−
Single-Step H− Transfer to BNA . Stoichiometric
reactions between Cp*(P-P)FeH (1H or 2H) and BNA+
were conducted in a J. Young tube in CD Cl /CD CN
+
−
−
•
single-step H transfer to pyridinium and a two-step (e /H )
transfer for acridinium reduction.
2
2
3
1
(
v/v 2/1) at room temperature. Analysis of the H NMR
+
spectra suggests that BNA was reduced quantitatively to give
,4-BNAH (yield >90%) and 1,6-BNAH (yield <10%)
RESULTS AND DISCUSSION
1
■
3
1
Synthesis and Characterization. The compound
overnight (eq 1). P NMR spectroscopic studies confirm
2
6
Cp*(dppe)FeH (dppe = 1,2-C H (PPh ) ; 1H) and its
that the organoiron product was exclusively composed of
2
4
2 2
+
new analogues Cp*(dppbz)FeH (dppbz = 1,2-C H (PPh ) ;
[Cp*(P-P)Fe(NCMe)] cations. Interestingly, there no
6
4
2 2
2
H), Cp*(dppm)FeH (dppm = Ph PCH PPh ; 3H),
reaction was observed in THF.
2
2
2
B
DOI: 10.1021/acs.organomet.6b00179
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Organometallics
Article
Figure 1. Structures (50% probability thermal ellipsoids) of 2H (left), 3H (center), and 4H (right). For clarity, the four phenyl groups bound to
phosphorus are drawn as lines and hydrogen atoms are omitted. Selected distances (Å) and angles (deg): 2H, Fe−P(1) 2.1368(8), Fe−P(2)
2
.1289(8), P(1)−Fe−P(2) 85.99(3); 3H, Fe−P(1) 2.1479(10), Fe−P(2) 2.1531(10), P(1)−Fe−P(2) 75.33(4); 4H, Fe−P(1) 2.1712(7),
Fe−P(2) 2.1855(7), P(1)−Fe−P(2) 87.44(3).
Table 1. NMR Spectroscopic Properties and Bite Angles of P−Fe−P in Iron Complexes
1
31
+
a
complex
ligand
δ( H, Fe−H) (JP−H, Hz)
δ( P)
∠P−Fe−P (deg) (vs [Cp*(P-P)Fe(NCMe)] )
1
2
3
4
H
H
H
H
dppe
dppbz
dppm
dcpe
−16.7 (t, 69)
−16.1 (t, 69)
−11.9 (td, 63)
−17.4 (t, 72)
107.8
102.6
45.9
86.22(7) (85.48(7))
85.99(3) (83.90(4))
75.33(4) (74.47(4))
113.7
87.44(3) (85.36(4))
a
+
Comparison of the bite angle of P−Fe−P with that of the corresponding [Cp*(P-P)Fe(NCMe)] cation.
relationship between ln{1 + [3H] /[3H] )} and t, and
0
t
the y intercept (0.746) is consistent with ln 2. The rate constant
−
+
k for H transfer from 3H to BNA derived from the slope is
.74 × 10−
1
M
−1 −1
s , which is nearly 40 times that for 2H
1
3
−1 −1
(
k = 4.45 × 10−
M
s ). This is mainly due to a 10.7° smaller
bite angle (∠P−Fe−P) in 3H than in 2H. The significance of
−
the bite angle effect on H transfer rates was also found in the
16a
case of CpRu(dppm)H and an iminium cation.
To explore whether substituents (R) at the C3 position affect
H− transfer, a broad class of pyridinium cations were
investigated as hydride acceptors of 2H. Table 2 shows the
results for the reactions between 2H and various BNA cation
analogues under the same conditions. N-Benzylpyridium salts
In order to gain insight into the possible mechanistic steps
of 1,4-BNAH (major) and 1,6-BNAH (minor) formation,
+
we followed the kinetics for reactions of Cp*(P-P)FeH with
0 equiv of BNA+ at various concentrations by H NMR
1
with a −CONEt substituent at the C3 position decreased the
1
2
hydride transfer rate considerably. Only 65% conversion was
spectroscopy. For 2H, the concentrations [1,4-BNAH],
1,6-BNAH], and [BNAH] observed over time are shown in
−
achieved in 7 days, which suggests that the H transfer reaction
[
−
is affected by steric factors in the substrates. An electronic effect
also plays an important role in this reaction. Generally,
Figure 2. During the H transfer reaction, both [1,4-BNAH] and
1,6-BNAH] increased slowly and, after the amount of reduced
[
electron-withdrawing groups such as −COCH , −COOCH ,
BNAH reached a plateau, the isomerization process of 1,6-BNAH
to 1,4-BNAH continued. The results suggest that the 1,4-product
3
3
and −CF accelerate H− transfer, while electron-donating
3
2
a,27
groups (e.g. −CH ) have the opposite effect.
is more thermally stable
,6-BNAH was clearly demonstrated for 2H. For the production
of BNAH, a pseudo-first-order rate constant (k ) can be linearly
and the kinetic production of
3
−
In the most of these H transfer reactions, N-benzyl-1,4-
1
dihydropyridine derivatives were observed as the major
product. However, the thermodynamic stability of the
obs
+
−
correlated with [BNA ]. The rate constant of H transfer of
−3
−1 −1
1
,6-product seems very dependent on an electron-withdrawing
group at C3. For −COOCH and −CF groups (Table 2,
entries d and e), 1,6-products remain dominant after 10 h of
reaction. A slow conversion of 1,6-BzPyH-CF to 1,4-BzPyH-CF
was observed, as shown in Figure 4, from the kinetics of the
BNAH reduction for 2H is 4.45 × 10
M
s , which is
−3
−1 −1
comparable to 3.23 × 10
M
s
for 1H (Figure S25 in the
3
3
Supporting Information) at 298 K.
+
Surprisingly, the reaction of 3H and BNA was complete
in 10 min at room temperature, and the yields of 1,6−-
and 1,4-BNAH were 36% and 64%, respectively. The H
transfer process was too fast to allow measurement of kobs
under pseudo-first-order conditions. The rate constant was
measured using a 3H/BNA+ ratio of 1/2. Under these
conditions, a second-order reaction would follow the equation
3
3
+
3
reaction between 2H and BzPy-CF .
With the exception of the N-benzyl-3-CN-pyridinum salt
+
(BzPy-CN ), containing a −CN group at C3, there was no
interaction observed between the pyridinium salts and
+
Cp*(P-P)FeH. For the reaction of BzPy-CN with 2H, the
16a,22b
31
shown in Figure 3.
The plot of the result confirms a linear
P NMR spectrum displays not only a singlet at δ 96.2 for
C
DOI: 10.1021/acs.organomet.6b00179
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Article
+
Figure 2. (left) Concentrations of BNAH, 1,4-BNAH, and 1,6-BNAH over time detected in the reaction of 2H and BNA in CD Cl /CD CN (2/1)
2
2
3
+
−3
−2
+
at 298 K. The initial [2H] and [BNA ] are 6.09 × 10 M and 6.09 × 10 M, respectively. (right) Plot of kobs vs [BNA ] for the reaction of 2H and
+
−3
−1 −1
2
BNA . Results: k = 4.45 × 10
M
s , R = 0.9847.
Figure 4. Concentrations of BzPyH-CF , 1,4-BzPyH-CF and
3
3
Figure 3. Plot of ln{1 + [3H] /[3H] )} vs time (t) for hydride transfer
0
t
+
1,6-BzPyH-CF3 over time detected for the reaction of 2H and
from 3H to BNA in CD Cl /CD CN (2/1) at 298 K. Rate equation:
2
2
3
+
+
BzPy-CF3 in CD Cl /CD CN (2:1) at 298 K. The initial [2H] and
2
2
3
ln{1 + [3H] /[3H] )} = ln 2 + k[3H] t, where [BNA ] and [3H] are
the initial concentrations of BNA (1.39 × 10
0
t
0
0
0
+
−3
−2
+
−2
[BzPy-CF ] are 8.81 × 10 and 8.81 × 10 M, respectively.
3
M) and
−3
3
H (6.95 × 10 M), [3H] is the concentration of 3H at time t, and
t
1
−
−1 −1
k is the second-order rate constant. Results: k = 1.74 × 10
M
s ,
1H NMR spectrum of the reaction mixture displayed proton
2
R = 0.9886.
signals not only at the C4 position for [1,4-BzPyH-CN-2]+
(
δ 2.40) but also for free 1,4-BzPyH-CN (δ 3.09) and proton
Table 2. Pseudo-First-Order Rates of Hydride Transfer from
+
signals on the C6 position for [1,6-BzPyH-CN-2] (δ 3.91) and
free 1,6-BzPyH-CN (δ 4.01). These results suggest that
[1,4-BzPyH-CN-2] and [1,6-BzPyH-CN-2] exhibit the same
+
2
H to BNA Cation Analogues at 298 K
b
+
+
conversion
(%)
ratio
(1,6/1,4 isomer)
a
kobs (s−1
)
c
entry
R
phosphorus nuclear magnetic resonance.
a
b
c
d
e
f
−CONH2
--CONEt2
−COCH3
−COOCH3
−CF3
100
24/76
9/53
4.78 × 10−
very slow
2.64 × 10−
3.27 × 10
3.76 × 10−
4
We next examined the use of 1H as an H donor for
−
d
d
+
31
62
BzPy-CN . The P NMR spectrum displays a singlet at
3
3
3
+
100
100
100
20/80
57/43
91/9
δ 90.2 for [1(NCMe)] and also a new singlet at δ 89.7.
−
Fortunately, [1,4-BzPyH-CN-1]+ from the reaction could
be crystallographically characterized (Figure 5). In this
new complex, reduced N-benzyl-1,4-dihydropyridine-3-carbon-
itrile (1,4-BzPyH-CN) was coordinated to the iron center
e
−CH3
−H
NR
g
100
7/93
3.92 × 10−
4
a
b
+
R is the substituent on C3 of N-benzylpyridinium. The conversions
and ratios of products were determined by H NMR analysis after 12 h
of reaction. Initial concentrations of 2H and BNA analogues were the
of 1 .
1
We found that both 1,4-BzPyH-CN and BzPy-CN+
+
+
replaced the MeCN ligand in [Cp*(P-P)Fe(NCMe)] , giving
−3
c
+
same at 8.81 × 10 M. The k values were determined under
obs
[Cp*Fe(P-P)(1,4-BzPyH-CN)] and [Cp*Fe(P-P)
+
pseudo-first-order conditions. Initial concentrations of 2H and BNA
2+
(
BzPy-CN)] complexes with the crystal structures shown in
−3
−2
d
analogues were 8.81 × 10 and 8.81 × 10 M, respectively. 168 h.
+
Figure 5. Furthermore, [Cp*Fe(P-P)(1,4-BzPyH-CN)] re-
leased 1,4-BzPyH-CN in MeCN to afford [Cp*(P-P)Fe-
e
No reaction was observed in 72 h.
+
(
NCMe)] , and finally an reaction equilibrium was observed,
[
2(NCMe)]+ but also a new singlet at δ 92. These new
judging from NMR spectral studies (Figures S44−S47 in the
+
Supporting Information). Though a H− transfer mechanism
products were thought to be [BzPyH-CN-2] according to the
ESI-MS spectral analysis, which showed an ion peak at m/z
based on coordination or interaction between pyridinium salts
+
17−19
833.3255 corresponding to [2 + BzPyH-CN] . However, the
to Ru and Rh hydride complexes was considered,
the
D
DOI: 10.1021/acs.organomet.6b00179
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Article
Figure 5. Structures (50% probability thermal ellipsoids) of [1,4-BzPyH-CN-1]+ (left) and [BzPy-CN-1] (right) cations. For clarity, the
2+
−
PF6 counteranions are not shown, the four phenyl groups bonded to phosphorus are drawn as lines, and hydrogen atoms are omitted. Selected
+
distances (Å) and angles (deg): [1,4-BzPyH-CN-1] , Fe−P(1) 2.2505(19), Fe−P(2) 2.2155(18), Fe−N(1) 1.888(5), N(1)−C(37) 1.154(7),
C(37)−C(38) 1.414(8), C(38)−C(39) 1.513(8), C(38)−C(42) 1.329(8), C(41)−N(2) 1.411(7), N(2)−C(42) 1.363(7), P(1)−Fe−P(2)
2
+
8
5.38(6); [BzPy-CN-1] , Fe−P(1) 2.223(3), Fe−P(2) 2.232(3), Fe−N(1) 1.872(9), N(1)−C(37) 1.133(12), C(37)−C(38) 1.432(15),
C(38)−C(39) 1.440(17), C(38)−C(42) 1.350(15), C(41)−N(2) 1.372(16), N(2)−C(42) 1.351(13), P(1)−Fe−P(2) 85.77(11).
that no 10-methyl-9,10-dihydroacridine was produced. Further-
more, the hydrido iron(II) complexes were found to react readily
+
with Acr with evolution of H . The yield of H , quantified by
2
2
GC analysis, was 43% (three experiments, standard deviation of
3
%) relative to the amounts of Cp*(P-P)FeH (1H−3H).
The organic product isolated from the reactions was identified as
the Acr dimer by H NMR analysis. The structure of the Acr
1
2
2
dimer was confirmed crystallographically, in which the two
Acr planes enjoy parallel head to head bonding by C9−C9′
(
Figure S52 in the Supporting Information). Apparently, there is
+
an electron transfer from iron(II) hydrides to Acr with
generation of an Acr radical, which produces Acr dimer.
•
2
+
Oxidation of Cp*(P-P)FeH (1H−3H) by Acr salts in
+
principle gives the [Cp*(P-P)FeH] radical. Tilset et al.
reaction path depicted in eq 2 cannot be excluded for
reported that oxidation of group 6 cyclopentadienyl metal
hydrides leads to the corresponding hydride cation radicals,
+
BzPy-CN and Cp*(P-P)FeH.
Our studies indicate that the reactions between Cp*(P-P)FeH
which are more acidic than their neutral parent by a relatively
+
−
and BNA proceed by a single-step H transfer pathway, in which
the hydride ligand as a nucleophile attacks the C6 position in
28
constant 20.6 ± 1.5 pK units. Most of the 17-electron
a
cationic hydride radicals are short-lived and undergo deproto-
+
the BNA cation, affording 1,6-BNAH as a kinetic product. The
29
30,31
nation,
disproportionation, and H2 elimination.
steric factor arising from the diphosphine chelating ligand
+
32
Though [1H] has been isolated at −60 °C in CH Cl , we
found that oxidation of 1H (or 2H, 3H) by [FeCp ]BF in
−
2
2
affects the reaction rates significantly. The transfer of H to
+
14
2
4
BNA analogues is more likely to be charge controlled,
CH Cl /MeCN (v/v 100/1) at room temperature resulted in
2
2
although steric factors also influence the rates. N-Benzylpyr-
idinum cations with electron-withdrawing substituents at C3
favor the positive charge at C6, allowing nucleophilic attack by
+
release of H and formation of [Cp*(P-P)Fe(NCMe)] , as
2
shown in eq 4.
−
H , which is reflected in increased reaction rates.
−
•
Two-Step “e /H ” Transfer to Acridinium Salts. When
acridinium salts were used as the acceptors, the chemistry differs
greatly from that described for N-benzylpyridium salts. Treat-
ment of the Cp*(P-P)FeH (1H−3H) with 10-methylacridinium
+
(
Acr ) salts in CD Cl /MeCN (v/v 100/1) led quantitatively to
+
2
2
To confirm the [Cp*(P-P)FeH] radical postulated as an
+
organoiron products of [Cp*(P-P)Fe(NCMe)] , as indicated by
+
intermediate resulting from ET of iron hydride to Acr , 4H was
3
1
1
P NMR analysis (eq 3). However, H NMR spectra indicated
selected as a hydride donor. The bulkier and more electron rich
dcpe ligand [4H]+ was expected to be stable at room
temperature and to allow further characterization. After the
+
reaction of 4H and Acr , the solvent was removed under
vacuum (eq 5). The Acr was isolated by washing the residue
2
with toluene. The organoiron complex in the residue was
redissolved in CH Cl and crystallized by allowing a hexane
2
2
layer to diffuse into the CH Cl solution.
2
2
Crystallographic analysis reveals an ionic complex of the type
[
Cp*(P-P)FeX]PF (Figure 6). This framework is similar to
6
that of the neutral hydride of 4H. The bite angle of P−Fe−P in
+
4
H is 89.172(19)°, 1.7° larger than that of 4H. The hydride
E
DOI: 10.1021/acs.organomet.6b00179
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
values that contrast with those determined for the 17-electron
+
32,33
1
H (g = 1.9944, g = 2.0430, g = 2.4487)
The initial ET
1
2
3
+
+
•
step from 4H to Acr generates 4H and Acr radical species,
and the latter is highly susceptible to dimerization. While using
+
bulky 10-methyl-9-phenylacridinium (PhAcr ), we expected
that the phenyl substituent at the C9 position would stabilize
•
•
PhAcr in contrast to the Acr radical. Indeed, the X-band EPR
+
spectrum for the reaction mixture of 4H and PhAcr clearly
showed the formation of 4H . The new sharp signal with a
+
g value of 2.0029 is close to the free-electron g value of
•
2
.0023, implying the formation of PhAcr .
+
The reaction mixture of 4H and PhAcr was always NMR
+
silent at room temperature, probably because 4H was too
•
•
stable to donate the H atom to PhAcr after the initial ET
process. To explore the second step of hydrogen atom transfer
(
HAT) from [Cp*(P-P)FeH] to the PhAcr• radical, we
+
+
examined the reaction of PhAcr with 1H−3H. PhAcrH was
produced unambiguously in yields ranging from 75 to 87%, as
shown in Scheme 3. A small amount of H was also released
2
+
Scheme 3. Reactions Observed between PhAcr and Iron
Hydrides
Figure 6. Structure (50% probability thermal ellipsoids) of [4H]PF6
salts. For clarity, four phenyl groups bound to phosphorus are drawn as
lines and hydrogen atoms are omitted. Selected distances (Å) and angles
(
8
deg): Fe−P(1) 2.2481(5), Fe−P(2) 2.2561(5), P(1)−Fe−P(2)
(
yield <10%), as shown by GC analysis, while the conversions
9.172(19).
+
of Cp*(P-P)FeH and PhAcr are nearly quantitative. The
above results indicate that the second step of HAT from
+
•
•
ligand was located in a different map, and its position and
isotropic displacement parameters were refined. Reduction of
[
indicated by H and P NMR analysis. These results prove that
the hydride ligand persists in the chemical redox reaction of
[
Cp*(P-P)FeH] to PhAcr competes with PhAcr dimeriza-
+
tion and intermolecular H evolution from [Cp*(P-P)FeH]
(Scheme 3).
2
4H]PF by 1 equiv of Cp Co in THF afforded 4H, as
6
2
1
31
To understand the plausible electron-transfer process, redox
properties were evaluated for these iron(II) hydride com-
pounds and NAD+ cation analogues (Figure 8). Cyclic
voltammograms of 1H−4H in CH Cl exhibit reversible
redox events involving the [Fe H] /[Fe H] couple. The
oxidation potentials of 1H−3H are all approximately −0.36 V,
mainly due to the similar electon-donating abilities of dppe,
dppbz, and dppm. With a more electron rich dcpe ligand,
+
4
H/4H .
+
More evidence of 4H formation was derived from EPR studies
Figure 7). The X-band EPR spectrum of 4H , recorded at 90 K
2
2
+
II
0
III
+
(
4
H was oxidized at the more negative potential of −0.44 V.
The oxidation potentials of Cp(dppe)RuH and Cp*(dppe)-
6
RuH are −0.16 and −0.51 V, suggesting that Cp*(P-P)FeH is
more electron rich than Cp(dppe)RuH but less so than
Cp*(dppe)RuH hydrides. The reduction potentials of NAD+
cation analogues are more negative than the potentials of
Cp*(P-P)FeH]0 couples. BNA+ exhibits an irreversible
/+
[
+
reduction event at −1.50 V in CH Cl solution, while Acr
2
2
and PhAcr+ salts have reduction potentials of −0.89 and
−
0.94 V, respectively.
According to the redox properties, the energy gap (ΔG ) of
ET
Figure 7. X-band EPR spectra of [4H]PF (black dotted line) and
6
directly processing electron transfer from Cp*(P-P)FeH to
acridinium ranges from 0.48 to 0.58 eV, which is considerably
smaller than the empirical critical limit of 1.0 eV for an
endothermic ET process. Such endergonic ET especially
becomes possible when it is followed by rapid disproportion
+
reaction of 4H with PhAcr (blue line) recorded in THF at 90 K.
34
in THF, exhibits three well-separated signals corresponding to the
three g tensor components (g = 1.980, g = 2.036, g = 2.395),
1
2
3
F
DOI: 10.1021/acs.organomet.6b00179
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
spectra were obtained on a PerkinElmer FT-IR Spectrometer
−1
Spectrum Two in the range of 4000−450 cm but are reported for
the ν region only. Cyclic voltammetry was performed under
Fe−H
nitrogen at room temperature using a CHI 760e electrochemical
workstation (Shanghai Chen Hua Instrument Co., Ltd.) with a glassy-
carbon working electrode, Pt-wire counter electrode, and an Ag-wire
pseudoreference electrode.
+
36
BNA salt substrates, 10-methylacridinium iodide, and 9-phenyl-
3
7
1
0-methylacridinium iodide were synthesized according to published
procedures. Cp*(dppe)FeH and [Cp*Fe(NCMe) ]PF
26 38
were
3
6
synthesized in good yields by modified literature methods. Anion
exchange reactions were conducted in water with saturated NH PF
4
6
aqueous solution.
Synthesis of [Cp*Fe(NCMe) ]PF . [Cp*Fe(NCMe) ]PF was
3
6
3
6
3
8
prepared by modification of the reported methods. To a slurry of
50 mg (5.98 mmol) of Cp*Li in 20 mL of THF was added 760 mg
5.98 mmol) of FeCl , and the mixture was stirred for 1 h at room
8
(
2
temperature. The solution gradually turned dark green. Then KPF6
1.10 g, 5.98 mmol) in 10 mL of CH CN was added. The solution
(
3
turned deep purple within 5 min. Then the solvent was removed under
vacuum and the residue was washed with Et O. The residue was
2
+
Figure 8. Redox potentials of Cp*(P-P)FeH and NAD cation
recrystallized from CH CN/Et O, providing 2.19 g (80%) of
3
2
analogues investigated.
1
[
Cp*Fe(NCMe) ]PF6 as purple crystals. H NMR (300 MHz,
3
CD COCD ): δ 2.42 (s, 9H, FeNCCH ), 1.66 (s, 15H, Cp*).
3
3
3
3
5
General Procedure for Preparation of [Cp*(P-P)Fe(NCMe)]PF6.
reactions. In our case, the subsequent highly exergonic
To a purple solution of 230 mg (0.50 mmol) of [Cp*Fe(NCMe) ]PF
3
6
•
processes are H2 evolution, dimerization of Acr radicals,
and HAT. These force the ET reduction of acridinium by
Cp*(P-P)FeH to go to completion.
in 40 mL of CH CN was added 1 equiv of the diphosphine ligand at
3
room temperature. The mixture turned from purple to deep red
immediately. After the solution was stirred for 3 h, the solvent was
removed under vacuum and the residue was extracted with CH Cl .
2
2
CONCLUSIONS
In conclusion, we investigated the reactions between iron(II)
Recrystallization from CH Cl /pentane afforded red microcrystals.
■
2
2
1
[
Cp*(dppm)Fe(NCMe)]PF . Yield: 83%. H NMR (300 MHz,
6
+
CD CN): δ 7.44−7.59 (m, 20H, Ph), 4.58−4.71 (m, 1H, CH ),
3
2
hydride compounds of Cp*(P-P)FeH and NAD analogues.
II
18,19
3.72−3.84 (m, 1H, CH
2
), 1.96 (s, 3H, FeNCCH
3
), 1.50 (s, 15H,
Unlike reported Ru −H cases,
there were no interaction−s
Cp*). 31P NMR (121 MHz, CD CN): δ 34.8. ESI-MS: calcd for
II
+
3
between Fe −H and BNA analogues observed during H
transfer reactions. Kinetics studies suggest that the BNA+
model is reduced at the C6 position, affording 1,6-BNAH,
which is converted to the more thermally stable 1,4-product.
+
[
Cp*Fe(dppm)] , 575.1720; found, 575.1837. Anal. Calcd for
C H F FeNP : C, 58.33; H, 5.30; N, 1.84. Found: C, 57.82;
37
40
6
3
H, 5.90; N, 1.55.
[Cp*(dppbz)Fe(NCMe)]PF . Yield: 80%. 1H NMR (400 MHz,
6
−
The reaction rate of this single-step H transfer process was
CD Cl ): δ 7.54−7.39 (m, 20H, Ph), 7.36−7.32 (m, 4H, Ph),
2
2
3
1
highly improved by Cp*(dppm)FeH, with its less bulky
1.37 (s, 3H, FeNCCH
CD Cl
found, 637.1851. Anal. Calcd for C H F FeNP : C, 61.22; H, 5.14;
), 1.31 (s, 15H, Cp*). P NMR (162 MHz,
3
+
diphosphine ligand. The results obtained from reduction of
2
): δ 96.2. ESI-MS: calcd for [Cp*Fe(dppbz)] , 637.1876;
2
+
−
NAD derivatives suggest that H transfer is more likely to be
42 42
6
3
+
N, 1.70. Found: C, 61.63; H, 5.32; N, 1.53.
charge controlled. In the reduction of Acr , the reaction is
initiated by an ET process that is then followed by rapid
disproportionation reactions. To achieve HAT after ET, bulkier
[
Cp*(dcpe)Fe(NCMe)]PF . Yield: 82%. 1H NMR (400 MHz,
6
CD CN): δ 2.27 (t, 2H, CH ), 2.20 (t, 2H, CH ), 1.99 (s, 3H,
3
2
2
FeNCH ), 1.88−1.66 (m, 20H, C H ), 1.59 (s, 15H, Cp*), 1.50−1.30
+
3
6
20
acridinium salts such as PhAcr should be used as acceptors.
31
(
m, 24H, C H ). P NMR (162 MHz, CD CN): δ 83.9. ESI-MS:
calcd for [Cp*Fe(dcpe)] , 613.3754; found, 613.3735. Anal. Calcd for
6 24 3
Thus, our current mechanistic understanding of fundamental
+
−
transfer from iron(II) hydride complexes to NAD+
H
C H F FeNP : C, 57.05; H, 8.32; N, 1.75. Found: C, 57.68; H,
38
66
6
3
analogues provides valuable insight into establishing bio-
organometallic transformation cycles driven by iron catalysis.
8.56; N, 1.61.
General Procedure for Preparation of Cp*(P-P)FeH. To a
THF solution of [Cp*(P-P)Fe(NCMe)]PF at −30 °C was slowly
6
EXPERIMENTAL SECTION
Materials and Methods. All reagents were purchased from
Sigma-Aldrich and used as received. All air-sensitive compounds were
prepared and handled using standard Schlenk techniques or in a
added 1 equiv of solid Bu NBH . The temperature was increased to
4
4
■
room temperature over a period of 3 h, and the solution turned
orange. The solvent was removed under vacuum, leaving a gummy
residue, which was extracted with pentane. The product was
recrystallized from pentane and isolated by filtration.
glovebox under an N atmosphere. THF, pentane, and ether (dried by
2
1
Cp*(dppm)FeH. Yield: 70% of red solid. H NMR (300 MHz,
distillation over sodium) and CH Cl2 and CH CN (dried by
2
3
C D ): δ 7.80−7.87 (m, 4H, Ph), 7.65−7.71 (m, 4H, Ph), 7.18−7.26
distillation over CaH ) for general use were of AR grade and were
6
6
2
(
m, 12H, Ph), 3.90−4.03 (m, 1H, CH ), 3.30−3.40 (m, 1H, CH ),
stored under N . CD Cl , CD CN, C D , and THF-d were dried
2
2
2
2
2
3
6
6
8
using molecular sieves (4 Å) and degassed by three thaw−freeze
cycles. NMR spectra were recorded in J. Young NMR tubes on Bruker
Avance 400 and Avance 300 spectrometers. H NMR chemical shifts
2.04 (s, 15 H, Cp*), −11.91(td, 1H, JH−H = 6 Hz, JP−H = 63.0 Hz,
Fe−H). P NMR (162 MHz, C
31
D ): δ 45.9. Anal. Calcd for
6 6
1
C
H
38FeP
(solid, νFe−H): 1827 cm .
Cp*(dppbz)FeH. Yield: 80% of red solid. H NMR (400 MHz,
C D ): δ 8.05 (t, 4H, Ph), 7.59 (t, 4H, Ph), 7.39−7.43 (m, 2H, Ph),
6 6
: C, 72.89; H, 6.65. Found: C, 72.38; H, 6.35. IR
35
2
−1
are referenced to the residual proton signal of the deuterated solvent.
The 31P NMR spectra were referenced to external H PO . Single-
1
3
4
crystal X-ray diffraction data were collected using a Bruker
SMART APEX II diffractometer with a CCD area detector
graphite-monochromated Mo Kα radiation) at 173 K. Infrared
7.24−7.32 (m, 6H, Ph), 7.11−7.17 (m, 6H, Ph), 1.78 (s, 15H, Cp*),
−16.07 (t, 1H, J
31
(
= 69.0 Hz, Fe−H). P NMR (162 MHz,
P−H
G
DOI: 10.1021/acs.organomet.6b00179
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
C D ): δ 102.6. Anal. Calcd for C H FeP : C, 75.21; H, 6.63. Found:
6
6
40 40
2
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.organomet.6b00179.
−1
■
*
C, 75.62; H, 6.52. IR (solid, νFe−H): 1839, 1831 cm .
S
1
Cp*(dcpe)FeH. Yield: 75% of yellow solid. H NMR (400 MHz,
C D ): δ 2.05−1.89 (m, 16H, Cy), 2.13 (s, 15H, Cp*), 1.85−1.82
6
6
(
t, 4H, CH ), 1.49−1.29 (m, 24H, Cy), −17.4 (t, 1H, J
= 73.1 Hz,
2
P−H
31
Experimental details, X-ray structures, NMR ( H, 31P)
spectra, and kinetics data (PDF)
X-ray crystallographic data (CIF)
1
Fe−H). P NMR (162 MHz, C D ): δ 113.7. Anal. Calcd for
C H FeP : C, 41.82; H, 5.27. Found: C, 41.26; H, 5.54. IR
(
6
6
3
6
64
2
−1
solid, νFe−H): 1894 cm .
Stoichiometric Hydride Transfer from Cp*(P-P)FeH to NAD+
Analogues. In a typical experiment, Cp*(P-P)FeH and 1 equiv of
AUTHOR INFORMATION
Corresponding Author
*E-mail for W.W.: wwg@sdu.edu.cn.
Notes
the organic cation salts were placed in a vial in an N -filled glovebox.
■
2
A 0.5 mL portion of CD Cl /MeCN (v/v 100/1) was added by
2
2
gastight syringe, and the solution was transferred to a J. Young NMR
tube before analysis by NMR.
[
Cp*Fe(dppe)(1,4-BzPyH-CN)]PF . To a solution of
6
The authors declare no competing financial interest.
Cp*(dppe)FeH (40 mg, 0.066 mmol) in 2 mL of CH Cl2 was
2
added a solution of [BzPy-CN]PF (22.5 mg, 0.066 mmol) in 3 mL of
6
ACKNOWLEDGMENTS
MeCN. After 1 h, the solvent was removed under vacuum. The residue
was redissolved in CH Cl , and crystals were grown by allowing a
■
We gratefully acknowledge the Natural Science Foundation
of China and Shandong Province (21402107, 91427303
ZR2014BM011), fundamental research and subject construc-
tion funds, and the Young Scholars Program of Shandong
University (2015WLJH23). We also thank Prof. Di Sun for
assistance with the X-ray crystallography and Dr. Xuewang Gao
for EPR experiments.
2
2
1
hexane layer to diffuse into the CH Cl solution at −20 °C. H NMR
2
2
(
300 MHz, CD Cl ): δ 7.58−7.35 (m, 23H, Ph), 7.19−7.16 (m, 2H,
2
2
Ph), 5.65−5.62 (m, 1H, C6-H), 5.46 (s, 1H, C2-H), 4.62−4.60
(
2
dt, 1H, C5-H), 4.13 (s, 2H, PhCH ), 2.55−2.54 (d, 2H, C4-H),
2
31
.14−2.08 (m, 4H, CH ), 1.31 (s, 15H, Cp*). P NMR (162 MHz,
2
+
CD Cl ): δ 89.7. ESI-MS: calcd for [Cp*Fe(dppe)(1,4-BzPyH-CN)] ,
2
2
7
85.2877; found, 785.3177.
[
Cp*Fe(dppbz)(1,4-BzPyH-CN)]PF6. 1H NMR (300 MHz,
REFERENCES
■
CD Cl ): δ 7.43−7.00 (m, 29H, Ph), 5.54−5.49 (m, 1H, C6-H),
2
2
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2
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2
5
31
.32−2.31 (d, 2H, C4-H), 1.24 (s, 15H, Cp*). P NMR (162 MHz,
+
CD Cl ): δ 92.3. ESI-MS: calcd for [Cp*Fe(dppbz)(1,4-BzPyH-CN)] ,
2
2
833.2877; found 833.3255.
(
2
[
Cp*(dcpe)FeH]PF . To a solution of Cp*(dcpe)FeH (30.7 mg,
6
0.05 mmol) in 2 mL of THF was added dropwise 3 mL of a THF
solution of AcrPF (17 mg, 0.05 mmol). After 5 min, the solvent was
6
removed under vacuum. The residue was washed by toluene to remove
(
Acr . The organoiron complex in the residue was redissolved in
2
CH Cl and crystallized by allowing a hexane layer to diffuse into the
2
2
−1
CH Cl solution. IR (solid, ν ): 1934 cm .
2
2
Fe−H
NMR Measurements of Rate Constants for Hydride Transfer
+
from Cp*(P-P)FeH to NAD Analogues. In a typical experiment,
1
H (4 mg, 0.0066 mmol) and [BNA]PF (23.7 mg, 0.066 mmol) were
6
placed in a vial under a nitrogen atmosphere in a glovebox. CD Cl2
̈
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(
0.5 mL) and CD CN (0.25 mL) were added by gastight syringe, and
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3
the solution was immediately transferred to a J. Young NMR tube.
1
The first H NMR spectrum was recorded within 5 min, and then
single-pulse spectra were taken every 300 s until the completion of the
reaction. The integral values of the CH2 of the benzyl group
corresponding to 1,4-BNAH (4.21 ppm) and 1,6-BNAH (4.14 ppm)
were compared to that of residual diethyl ether (m, 3.317−3.386 ppm).
EPR Experiments. EPR samples were prepared in a glovebox.
The sample concentration was approximately 2 mM in THF.
EPR spectra were recorded by using a Bruker ESP-300E spectrometer
at 9.8 GHz, X-band, with 100 Hz field modulation.
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Clarkson, G. J.; Deeth, R. J.; Sadler, P. J. Organometallics 2012, 31,
958.
(
5
H2 Quantification by GC. The amount of evolved H2 was
quantified by a Techcomp 7890 II gas chromatograph (GC) equipped
with a 5 Å molecular sieve column using argon as the carrier gas and a
thermal conductivity detector. In a typical experiment, a Pyrex
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Amsterdam, 2001.
+
tube was filled with 2H (4 mg, 0.0061 mmol) and Acr (2.08 mg,
0.0061 mmol) in a glovebox. The tube was closed with a rubber plug
and sealed with wax. Subsequently, 1 mL of CH Cl /CH CN
2
2
3
(
v/v 100/1) was injected into the tube. After the mixture was stirred
for 10 min, CH (200 μL) was injected into the tube as an internal
4
standard. A 200 μL portion of the gas in the headspace was sampled by
a Hamilton (1750 SL) gastight microliter syringe and then analyzed
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2
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H
DOI: 10.1021/acs.organomet.6b00179
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DOI: 10.1021/acs.organomet.6b00179
Organometallics XXXX, XXX, XXX−XXX