Synthesis, characterization, and reactivity of Fe complexes containing cyclic diazadiphosphine ligands: The role of the pendant base in heterolytic cleavage of H2

Article abstract of DOI:10.1021/ja211193j

The iron complexes CpFe(P^Ph₂N^Bn ₂)Cl (1-Cl), CpFe(P^Ph₂N^Ph ₂)Cl (2-Cl), and CpFe(P^Ph₂C₅)Cl (3-Cl)(where P^Ph₂N^Bn₂ is 1,5-dibenzyl-1,5-diaza-3,7-diphenyl-3,7-diphosphacyclooctane, P ^Ph₂N^Ph₂ is 1,3,5,7-tetraphenyl-1,5- diaza-3,7-diphosphacyclooctane, and P^Ph₂C₅ is 1,4-diphenyl-1,4-diphosphacycloheptane) have been synthesized and characterized by NMR spectroscopy, electrochemical studies, and X-ray diffraction. These chloride derivatives are readily converted to the corresponding hydride complexes [CpFe(P^Ph₂N^Bn₂)H (1-H), CpFe(P^Ph₂N^Ph₂)H (2-H), CpFe(P ^Ph₂C₅)H (3-H)] and H₂ complexes [CpFe(P^Ph₂N^Bn₂)(H ₂)]BAr^F₄, [1-H₂]BAr^F ₄, (where BAr^F₄ is B[(3,5-(CF₃) ₂C₆H₃)₄]^-), [CpFe(P ^Ph₂N^Ph₂)(H₂)]BAr ^F₄, [2-H₂]BAr^F₄, and [CpFe(P^Ph₂C₅)(H₂)]BAr ^F₄, [3-H₂]BAr^F₄, as well as [CpFe(P^Ph₂N^Bn₂)(CO)]BAr ^F₄, [1-CO]Cl. Structural studies are reported for [1-H₂]BAr^F₄, 1-H, 2-H, and [1-CO]Cl. The conformations adopted by the chelate rings of the P^Ph ₂N^Bn₂ ligand in the different complexes are determined by attractive or repulsive interactions between the sixth ligand of these pseudo-octahedral complexes and the pendant N atom of the ring adjacent to the sixth ligand. An example of an attractive interaction is the observation that the distance between the N atom of the pendant amine and the C atom of the coordinated CO ligand for [1-CO]BAr^F₄ is 2.848 A, considerably shorter than the sum of the van der Waals radii of N and C atoms. Studies of H/D exchange by the complexes [1-H₂]⁺, [2-H₂]⁺, and [3-H₂]⁺ carried out using H₂ and D₂ indicate that the relatively rapid H/D exchange observed for [1-H₂]⁺ and [2-H₂] ⁺ compared to [3-H₂]⁺ is consistent with intramolecular heterolytic cleavage of H₂ mediated by the pendant amine. Computational studies indicate a low barrier for heterolytic cleavage of H₂. These mononuclear Fe^II dihydrogen complexes containing pendant amines in the ligands mimic crucial features of the distal Fe site of the active site of the [FeFe]-hydrogenase required for H-H bond formation and cleavage.

Full text of DOI:10.1021/ja211193j

Article
pubs.acs.org/JACS
Synthesis, Characterization, and Reactivity of Fe Complexes
Containing Cyclic Diazadiphosphine Ligands: The Role of the
Pendant Base in Heterolytic Cleavage of H2
Tianbiao Liu, Shentan Chen, Molly J. O’Hagan, Mary Rakowski DuBois, R. Morris Bullock,*
and Daniel L. DuBois*
Chemical and Materials Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, K2-57, Richland, Washington
99352, United States
S
* Supporting Information
ABSTRACT: The iron complexes CpFe(PPh2NBn2)Cl (1-Cl),
CpFe(PPh2NPh2)Cl (2-Cl), and CpFe(PPh2C5)Cl (3-Cl)
(where PPh2NBn is 1,5-dibenzyl-1,5-diaza-3,7-diphenyl-3,7-
2
diphosphacyclooctane, PPh2NPh is 1,3,5,7-tetraphenyl-1,5-
2
diaza-3,7-diphosphacyclooctane, and PPh2C5 is 1,4-diphenyl-
1,4-diphosphacycloheptane) have been synthesized and
characterized by NMR spectroscopy, electrochemical studies,
and X-ray diffraction. These chloride derivatives are readily
converted to the corresponding hydride complexes [CpFe-
(PPh2NBn2)H (1-H), CpFe(PPh2NPh2)H (2-H), CpFe(PPh2C5)H (3-H)] and H2 complexes [CpFe(PPh2NBn2)(H2)]BArF , [1-
4
H2]BArF , (where BArF is B[(3,5-(CF3)2C6H3)4]−), [CpFe(PPh2NPh2)(H2)]BArF , [2-H2]BArF , and [CpFe(PPh2C5)(H2)]-
4
4
4
4
BArF , [3-H2]BArF , as well as [CpFe(PPh2NBn2)(CO)]BArF , [1-CO]Cl. Structural studies are reported for [1-H2]BArF , 1-H,
4
4
4
4
2-H, and [1-CO]Cl. The conformations adopted by the chelate rings of the PPh2NBn ligand in the different complexes are
2
determined by attractive or repulsive interactions between the sixth ligand of these pseudo-octahedral complexes and the pendant N atom
of the ring adjacent to the sixth ligand. An example of an attractive interaction is the observation that the distance between the N atom of
the pendant amine and the C atom of the coordinated CO ligand for [1-CO]BArF4 is 2.848 Å, considerably shorter than the sum of the
van der Waals radii of N and C atoms. Studies of H/D exchange by the complexes [1-H2]+, [2-H2]+, and [3-H2]+ carried out using H2
and D2 indicate that the relatively rapid H/D exchange observed for [1-H2]+ and [2-H2]+ compared to [3-H2]+ is consistent with
intramolecular heterolytic cleavage of H2 mediated by the pendant amine. Computational studies indicate a low barrier for heterolytic
cleavage of H2. These mononuclear FeII dihydrogen complexes containing pendant amines in the ligands mimic crucial features of the
distal Fe site of the active site of the [FeFe]-hydrogenase required for H−H bond formation and cleavage.
Because of its high abundance8 and low toxicity, iron is a particularly
INTRODUCTION
■
attractive metal for incorporation into synthetic catalysts for the
production and oxidation of H2. Recently studied iron complexes
containing an amine base in the second coordination sphere have
included simple mononuclear iron complexes, such as 5 and 6,9,10 as
well as diiron complexes that model the hydrogenase active site, such
as 711 and related derivatives.12 Studies with several of these
compounds have established that the pendant amines can play an
important role in relaying protons between metal and solution, in
coupling proton and electron transfers and in activating dihydrogen.
Nevertheless, the electrocatalytic activity for H2 formation demon-
strated for iron derivatives with pendant amines is often limited by low
rates, low stability, and/or high overpotentials. Furthermore, very few
examples of Fe complexes are known for H2 activation, and iron-based
molecular electrocatalysts for H2 oxidation have not been developed.
In this work we describe the syntheses and characterizations of a
series of new cyclopentadienyl iron derivatives containing a cyclic
Hydrogenase enzymes are fast and energy-efficient molecular catalysts
for hydrogen oxidation and production, and their active sites contain
the inexpensive and abundant metals iron and/or nickel.1,2 These
features have inspired numerous groups to study simpler synthetic
molecular electrocatalysts that mimic the structure and/or function of
these enzymes that are found in nature.3 Research in our laboratories
has focused on transition-metal complexes containing pendant amines
incorporated into diphosphine ligands, such as those shown
in structures 1−3.4−6 These nickel and cobalt complexes are
electrocatalysts for either the oxidation or production of
hydrogen, and the pendant amines have been shown to play
important roles in the catalytic reactions of these complexes by
facilitating: (1) heterolytic H−H bond cleavage/formation; (2)
the coupling of proton and electron transfer reactions; and (3)
proton transfer between acids and bases in solution and the
redox active metal center. It is likely that these same functions
are also facilitated by the pendant amine of the azadithiolate
ligand proposed to be present in the active site of the [FeFe]-
hydrogenase enzyme, structure 4.2,7
Received: December 8, 2011
Published: March 6, 2012
© 2012 American Chemical Society
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octahedral complexes. We also observed that intramolecular
heterolytic cleavage of H2 (as manifested by H/D exchange) is
enhanced by a pendant amine. Complete H2 heterolytic cleavage
can be achieved using an external base as proton acceptor. These
results indicate a ferrous (FeII) center and pendant amines in Fe
complexes are crucial features for H2 bonding and cleavage, as
proposed for the active site of the [FeFe]-hydrogenase.7 Although
electrocatalytic oxidation of H2 is not observed for CpFe-
(PPh2NBn2)H, electrochemical studies indicate that this complex
is capable of performing all of the individual steps required for a
catalytic process. Thus this work provides a useful basis for catalyst
development in this series of CpFe derivatives.
RESULTS
■
Synthesis and Characterization of CpFe(PPh2NR )Cl. A
2
1:1 mixture of CpFe(CO)2Cl and PPh2NBn was dissolved in
2
toluene and irradiated with a UV mercury lamp to give
CpFe(PPh2NBn2)Cl (1-Cl), which was isolated in 81% yield
(Scheme 1) as black crystals. During the early stages of the
photolysis, a yellow solid precipitated that was identified as
[CpFe(PPh2NBn2)(CO)]Cl ([1-CO]Cl). Under prolonged
photolysis, this product loses CO to give 1-Cl. Both 1-Cl
1
and [1-CO]Cl were fully characterized by H and 31P{1H}
NMR spectroscopy, elemental analyses, and single crystal X-ray
diffraction studies. In the 31P{1H} NMR spectrum, CpFe-
1
(PPh2NBn2)Cl displays a singlet at 57.2 ppm. The H NMR
spectrum is also consistent with the formulation of this
complex, with resonances assignable to the Cp and PPh2NBn
2
ligands (see Experimental Section and Figure S1, Supporting
Information). Similar 1H and 31P{1H} NMR spectra were
observed for [1-CO]Cl. Infrared spectroscopy of [1-CO]Cl in
CH2Cl2 shows a single carbonyl absorption at 1964 cm−1,
consistent with the expected monocarbonyl complex (Figure S2,
Supporting Information). A similar photolysis reaction was
PPh2NR2 ligand (1,5-dibenzyl-1,5-diaza-3,7-diphenyl-3,7-diphospha-
cyclooctane, where R = Bn (benzyl), or 1,3,5,7-tetraphenyl-1,5-
diaza-3,7-diphosphacyclooctane, where R = Ph) with the aim of
determining how the complexes might be modified and developed
to function as electrocatalysts for oxidation or formation of H2.
These complexes have a structural motif that should permit facile
tuning of the electronic and steric properties by varying
substituents on both the Cp and diphosphine ligand, while
incorporating positioned pendant bases to promote facile
heterolytic cleavage or formation of the H−H bond. To assess
the importance of the amine bases in the second coordination
sphere, we have also prepared related CpFe derivatives containing
a cyclic diphosphine ligand with no bases in the ligand backbone.
The work presented here shows that the potential of the FeIII/II
couple and ease of H2 addition make these complexes interesting
potential electrocatalysts for H2 oxidation. In addition, our studies
carried out with a mixture of CpFe(CO)2Cl and PPh2NPh to
2
form CpFe(PPh2NPh2)Cl (2-Cl), and spectroscopic data for this
product are given in the Experimental Section.
Crystals of 1-Cl and 2-Cl suitable for X-ray diffraction
analysis were grown from a CH2Cl2 solution layered with
hexane. The structures of the two products are very similar, and
only data for 1-Cl are discussed here. The structure is depicted
in Figure 1a, and selected bond distances and angles are given
in Table 1. Figure 1 and Table 1 also provide information about
other structures to be presented and discussed later in this
paper. The complex 1-Cl adopts a typical three-legged piano-
stool geometry. The Fe−Cl and Fe−P bond lengths are similar
to those reported for Cp*Fe(dppe)Cl (dppe =1,2-bis-
(diphenylphosphino)ethane).13 The bite angle of the PPh2NBn
2
ligand is 81.37(2)°, comparable to the values for this ligand in
show that the conformations of the chelate rings of the PPh2NR
other Fe,10 Ni5, or Co6 complexes. The six-membered ring of
2
the PPh2NBn ligand adjacent to the Cp ligand adopts a boat
ligands are determined by attractive or repulsive interactions
between the N atom and the sixth ligand of these pseudo-
2
conformation with the lone pair of the N atom directed toward
Scheme 1
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and bond distances and angles are listed in Table 1. The overall
structure is similar to 1-Cl, with slightly longer Fe−P bond
distances. A particularly interesting feature of this complex is
that in the PPh2NBn2 ligand, the chelate ring proximal to the Cp
ligand has a chair conformation, while a boat conformation is
observed for the ring adjacent to the CO ligand. This is the
only complex of this class that we have studied that has this
stereochemistry in the solid state. A clue to the origin of the
boat conformation is found in the N···CO distance of 2.848 Å.
This distance is shorter than the sum of van der Waals radii of N
and C atoms (3.25 Å)14 by ca. 0.4 Å, indicating there is a
significant bonding interaction between the N atom of the
P
Ph2NBn ligand and the C atom of the carbonyl ligand. As a
2
result, the Fe−C−O angle, 170.97(17)°, is not linear.
Synthesis and Characterization of [CpFe(PPh2NR )(H2)]-
2
BArF and [CpFe(PPh2NR )(HD)]BArF . The dihydrogen com-
4
2
4
plex, [CpFe(PPh2NBn2)(H2)]BArF , [1-H2]BArF , (ArF = 3,5-
4
4
bis(trifluoromethyl)phenyl) was synthesized from 1-Cl by
abstracting chloride using NaBArF in fluorobenzene (PhF)
4
under an atmosphere of H2 (1.0 atm). This synthesis can also
be carried out in a two-step process by abstracting chloride first
to form a brown solution of a cationic complex
[CpFe(PPh2NBn2)]BArF designated as [1]+, then adding H2
4
gas to form a yellow solution of [1-H2]BArF , as shown in
4
Scheme 2. When CH3CN was used as solvent for the above
Scheme 2
Figure 1. Molecular structures of complexes 1-Cl (a), [1-CO]+ (b),
[1-H2]+ (c), 1-H (d), and 3-Cl-a (e) represented as thermal ellipsoid
drawings at 50% ellipsoid possibility. The atomic positions of the H2
ligand of [1-H2]+ were not refined but were modeled using
appropriate constraints and restraints (see text and Experimental
Section for details). Hydrogen atoms are not shown except for the H2
ligand of [1-H2]+ and the Fe-H in 1-H.
the Cp ring. The second six-membered ring adopts a chair
conformation. This orientation presumably avoids repulsion
between the lone pair of nitrogen and the chloride ligand.
Crystals of [1-CO]Cl were grown from a CH2Cl2 solution
layered with hexane, and an X-ray diffraction analysis was
undertaken. The structure consists of the [CpFe(PPh2NBn2)(CO)]+
cation, chloride anion, and one equivalent of diethyl ether. A
drawing of [CpFe(PPh2NBn2)(CO)]+ is shown in Figure 1b,
Table 1. Selected Bond Distances (Å) and Angles (°) for Complexes 1-Cl, [1-CO]+, [1-H2]+, 1-H, and 3-Cl-a
1-Cl
[1-CO]+
[1-H2]+
1-H
3-Cl-a
Fe-Cp (centroid)
1.7047(9)
2.3180(7)
2.1523(7)
2.1568(7)
125.81
1.7234(10)
1.759(2)
2.1989(6)
2.1772(6)
123.99
1.6986(21)
1.60(2)
1.6886(8)
1.47(2)
1.6930(6)
2.3164(3)
2.1810(3)
2.1672(3)
129.09
a
Fe−X
Fe−P1
Fe−P2
Cp−Fe−P1
Cp−Fe−P2
Cp−Fe−X
P1−Fe−P2
P1−Fe−X
P2−Fe−X
2.1830(14)
2.1799(15)
124.98
2.1008(5)
2.1014(5)
128.37
127.10
126.13
128.43
130.77
128.00
123.03
123.08
121.28
125.73
124.23
81.37(2)
94.74(2)
93.41(2)
81.66(2)
97.94(7)
93.46(6)
81.04(5)
84.0(8)
82.069(19)
87.5(8)
78.426(12)
91.542(13)
91.942(13)
83.8(8)
87.5(8)
a
X represents the atom of the monodentate ligand coordinated to Fe. For [1-H2]+, the distance of Fe−(H2) is 1.535 Å (center of the H−H bond)
and the angle of Cp−Fe−(H2) is 121.82°.
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reactions, [1-H2]BArF4 was not formed because of coordination
of CH3CN to the iron. Complex [1-H2]+ shows a singlet in the
was observed in the dihydrogen resonance at −12.64 ppm.
These results suggest that the pKa of [1-H2]+ can be bracketed
between 20 and 24. Although these reactions were carried out
in fluorobenzene, the pKa values cited are for acetonitrile as the
solvent. 1-H can also be prepared from 1-Cl by reaction with
LiAlH4 in THF (61% isolated yield). Other metal hydrides,
including NaBH4, NaH, PPNBH4, and LiBEt3H, were examined
for the same reaction but gave lower yields. 1-H shows a singlet
at 71.8 ppm in the 31P NMR spectrum and a triplet at −16.0
1
31P NMR spectrum at 58.4 ppm, and the H NMR spectrum
exhibits a broad singlet at −12.64 ppm assigned to the H2
ligand (see Figure 2a). Further support for the assignment of
1
ppm (JPH = 60 Hz, see Figure 2c) for the hydride in the H
NMR spectrum. Crystals of 1-H were grown from an ether
solution cooled to −35 °C. The crystal structure of 1-H (Figure
1d) is very similar to that of 1-Cl, with Fe−P distances and a
P1−Fe−P2 angle comparable to those of the other derivatives
with the PPh2NBn ligand. The Fe−H distance is 1.47(2) Å, a
2
value comparable to that of [CpFe(PPh3)2H] (1.49 Å) and
shorter than [CpFe(dppm)H] (1.59 Å).21 It is noted that the
P−Fe−X angle (87.5°, X = H for 1-H) is smaller than the
values observed for 1-Cl and [1-CO]+, by as much as 6°,
indicating a reduced repulsion between the PPh2NBn2 ligand and
the smaller hydride ligand, though all of these comparisons of
Fe−H distances are subject to uncertainty of determination of
H locations by X-ray crystallography.22
Figure 2. (a) 1H NMR spectrum (PhCl-d5) of [CpFe(PPh2NBn2)-
1
(H2)]BArF , [1-H2]BArF , showing the H2 resonance, (b) H NMR
4
4
spectrum (PhCl-d5) of [CpFe(PPh2NBn2)(HD)]BArF , [1-H2]BArF ,
Iron derivatives containing the PPh2NPh2 ligand ([2-H2]+, [2-
HD]+, and 2-H) were prepared from 2-Cl following procedures
4
4
1
showing the HD resonance, and (c) H NMR spectrum (THF-d8) of
[CpFe(PPh2NBn2)(H)], 1-H, showing the hydride resonance. All
spectra were recorded at 22 °C.
similar to those described above. [CpFe(PPh2NPh2)(H2)]BArF
4
([2-H2]BArF ) shows a singlet at 50.6 ppm in the 31P NMR
4
spectrum and a broad singlet at −12.68 ppm for the dihydrogen
ligand in the 1H NMR spectrum (Figure S6a, Supporting
Information). The corresponding [Fe-HD] species, [CpFe-
[1-H2]+ as a metal dihydrogen complex was obtained by
preparation of the corresponding [Fe(HD)]+ complex, [1-
HD]+, which exhibits a 1:1:1 triplet at −12.71 ppm (JHD = 30
Hz, see Figure 2b) for the HD ligand. This HD resonance
shows an isotopic shift (−0.07 ppm) compared to the H2 ligand
of [1-H2]+.15 The HD coupling constant is comparable to those
of [CpFe(dppp)(HD)]BF4 (30.7 Hz) and [CpFe(dppe)(HD)]-
BF4 (29.0 Hz).16 Based on the JHD, the H−H distance of the
dihydrogen ligand of [1-H2]+ is calculated to be 0.94 Å using
the equation of Heinekey17 or 0.92 Å using the equation of
(PPh2NPh2)(HD)]BArF4 ([2-HD]BArF ), exhibits a 1:1:1 triplet
4
at −12.75 ppm (JHD = 27.5 Hz, see Figure S6b, Supporting
Information), indicating a H−H distance of 0.98 Å17 (or 0.96
Å)18,19 for the coordinated H2 ligand, ca. 0.042 Å longer than
the H2 ligand of [1-H2]+. [CpFe(PPh2NPh2)(H)] (2-H) displays
a singlet at 73.3 ppm in the 31P NMR spectrum and a triplet
hydride signal at −15.74 ppm (JPH = 50 Hz, see Figure S6c,
Supporting Information) in the 1H NMR spectrum. X-ray
diffraction studies of 2-Cl and 2-H indicate that the structural
parameters of these complexes are similar to those of the
Morris.18,19 Due to the low solubility of [1-H2]BArF below
4
−20 °C, the T1 of the dihydrogen ligand was measured at 22 °C
and was found to be 14 ms, consistent with a dihydrogen
complex.
corresponding PPh2NBn derivative (Figure S29, Supporting
2
Information).
Synthesis and Characterization of CpFe(PPh2C5)Cl (3-
Cl), [CpFe(PPh2C5)(H2)]BArF ([3-H2]BArF ), and CpFe-
Crystals of [1-H2]BArF were grown from a fluorobenzene
4
4
4
solution layered with hexane under H2 (1.0 atm). The structure
of the cation [1-H2]+, as determined by an X-ray diffraction
study, is shown in Figure 1c. The overall structure, the Fe−P
bond distances, and the P1−Fe−P2 angle of the cation are
quite similar to its precursor (1-Cl). Since the H2 ligand cannot
be accurately located using X-ray diffraction data, the single
residue ca. 1.60 Å from the Fe atom was modeled and refined
as an H2 ligand. The H−H bond distance was constrained using
the distance (0.94 Å) derived from the NMR study of [1-HD]+.
(PPh2C5)H (3-H). Attempts were made to prepare CpFe-
(PPh2C6)Cl, where PPh2C6 (1,5-diphenyl-1,5-diphosphacy-
23
clooctane) is an eight-membered cyclic diphosphine ligand
with methylene groups substituted for the amine bases of
PPh2NR ligands. These complexes would provide structurally
2
analogous control complexes for evaluating the effects of the
pendant amines of [1-H2]BArF , 1-H, [2-H2]BArF , and 2-H
4
4
on their structures and reactivities. Unfortunately, the reaction
23
of [CpFe(CO)2Cl] with PPh2C6 did not form the desired
Synthesis and Characterization of CpFe(PPh2NR )(H).
2
mononuclear product CpFe(PPh2C6)Cl. However, reactions
with the closely related seven-membered cyclic diphosphine,
[1-H2]+ can be readily deprotonated by DBU (1,8-diaza-
bicyclo[5.4.0]undec-7-ene, pKa = 24 for H-DBU+ in CH3CN)20
in PhF solvent to generate the orange hydride complex,
[CpFe(PPh2NBn2)(H)] (1-H), which was isolated in 20% yield.
Reversible protonation of 1-H with 1 equiv of HBF4·Et2O leads
to the clean formation of the [Fe−H2]+ complex [1-H2]+, as
indicated by NMR spectroscopy. Attempts to deprotonate
[1-H2]+ with excess Et3N (20 equiv; pKa = 18.8 in CH3CN)20
did not result in the formation of the hydride, 1-H; no change
24
PPh2C5 (1,4-diphenyl-1,4-diphosphacycloheptane), were suc-
cessful. The complexes CpFe(PPh2C5)Cl (3-Cl), [CpFe-
(PPh2C5)(H2)]BArF ([3-H2]BArF ), and CpFe(PPh2C5)H (3-H)
4
4
were prepared using synthetic routes analogous to those used
to prepare 1-Cl, [1-H2]BArF , and 1-H. From the reaction of
4
[CpFe(CO)2Cl] with PPh2C5, two isomeric products, 3-Cl-a
and 3-Cl-b, were observed by 31P{1H} NMR spectroscopy,
with resonances at 80.1 and 85.5 ppm in a ratio of ca. 1.00:0.15.
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smaller than those of 1-Cl and 2-Cl. This is a consequence
of replacing one six-membered chelate ring with a smaller
five-membered ring. The change in ring size also reduces
the steric interactions between the diphosphine ligand
and the chloride and Cp ligands; as a result the P−Fe−Cl
and Cp−Fe−P angles (measured to the centroid of the Cp
ring) are larger for 3-Cl-a than those of 1-Cl and 2-Cl.
When the pure isomer 3-Cl-a was used as the starting
Scheme 3
material in the reaction with NaBArF and H2, two isomers of
4
[CpFe(PPh2C5)(H2)]+, [3-H2-a]+ and [3-H2-b]+ (ca. 1.00: 0.55,
see Scheme 3), were observed by 1H NMR and 31P{1H} NMR
spectroscopy. The major ([3-H2-a]+) and minor ([3-H2-b]+)
isomers exhibit dihydrogen resonances at −13.79 and −13.04
1
ppm, respectively, in the H NMR spectrum (see Figure S7a,
Supporting Information). T1 values measured at 22 °C for the
dihydrogen ligand are 22.9 and 19.1 ms for [3-H2-a]+ and [3-
H2-b]+, respectively, and the two isomers of the HD complex
[CpFe(PPh2C5)(HD)]+ exhibit two triplets of triplets at −13.89
The major isomer (3-Cl-a, Scheme 3) was isolated as a crystalline
material. The H NMR spectrum of 3-Cl-a (see Figure S4,
1
ppm (JHD = 25, JPH = 8.3 Hz) and −13.13 ppm (JHD = 25, JPH
=
4.5 Hz) in the 1H NMR spectrum (see Figure S7b, Supporting
Information), confirming the presence of an H2 ligand. The JHD
value can be used to calculate a H−H distance of 1.02 Å (or
1.00 Å) for both isomers.17−19 The smaller coupling constant is
assigned to the coupling of the proton of H−D to the two
Supporting Information) displays a singlet at 4.08 ppm for the
Cp ligand and multiplets from 2.66 to 1.80 ppm for the
methylene groups of the PPh2C5 ligand (see Experimental
Section). To provide further information on the structure of
this isomer, a single crystal X-ray diffraction study was
undertaken (Figure 1e, Table 1). This isomer has a six-
membered ring in the chair conformation adjacent to the Cp
ring, and the five-membered ring is adjacent to the chloride
ligand. The minor isomer, 3-Cl-b, is therefore assigned a
structure in which the five-membered chelate ring of the
diphosphine is adjacent to the Cp ligand and the six-
membered ring is adjacent to the chloride ligand. From
Table 1 it can be seen that the Fe−Cl and Fe−P bond
distances of 3-Cl-a are comparable to those of 1-Cl and 2-Cl,
but the P−Fe−P bite angle is 78.426(12)°, about 3−4°
1
phosphorus atoms, and was confirmed by a H{31P} NMR
spectrum (see Figure S7c, Supporting Information). Although
P−H coupling in dihydrogen complexes is generally not
observed, it has been found in some cases. Similar P−H
coupling constants have been reported previously for W(HD)-
(CO)3(P(i-Pr)3)2 (2.7 Hz)25 and [CpRu(P2)(HD)]BF4 (3.5
Hz for P2 = 1,2-bis(dimethylphosphino)ethane; 2.3 Hz for P2 =
1,l-dimethyl-2,2-diphenylphosphino)ethane).15 Resonances
corresponding to these two dihydrogen species were observed
in the 31P{1H} NMR spectrum at 83.70 and 85.47 ppm.
Scheme 4
Scheme 5
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1
Figure 3. H NMR spectra at 22 °C of [1-H2]+ in chlorobenzene-d5 under D2 and H2 as a function of time.
Scheme 6
Deprotonation of a mixture of [3-H2-a]+ and [3-H2-b]+ with
DBU resulted in two [Fe−H] isomers, 3-H-a and 3-H-b (see
Scheme 3). These isomers exhibit singlets at 96.1 and 100.0
ppm in a ratio of 1.00:0.22 in the 31P{1H} NMR spectrum.
Corresponding hydride resonances are observed as triplets at
−17.30 (JPH = 65 Hz) and −15.80 ppm (JPH = 65 Hz) in the
1H NMR spectrum (see Figure S8a, Supporting Information).
The isomer with a phosphorus resonance at 96.1 ppm can be
selectively prepared from 3-Cl-a (see Figure S8b, Supporting
Information) by reaction with LiAlH4. As a result, this isomer is
tentatively assigned structure 3-H-a, and the minor isomer is
assigned structure 3-H-b (Scheme 4). In order to assign the
structures of the two isomers of [3-H2]+, protonation of
[CpFe(PPh2C5)H] (3-H-a) with HBF4·Et2O was examined by
1H NMR spectroscopy at −35 °C. At this temperature only, the
major isomer at −13.79 ppm was observed (see Figure S9,
Supporting Information), which suggests structure [3-H2-a]+
(see Scheme 4). Upon warming to −15 °C, the resonance for
the minor isomer assigned to [3-H2-b]+ appeared at −13.04
ppm. It is proposed that the conversion of [3-H2-a]+ to [3-H2-b]+
proceeds through dissociation of H2 from [3-H2-a]+ to form
the coordinately unsaturated [CpFe(PPh2C5)]+ species and
reassociation of H2 to form [3-H2-b]+. The lability of the H2
ligand is indicated by the H2/D2 ligand exchange observed for
[3-H2]+ (vide infra and see Figures S13 and S14, Supporting
Information).
scrambling experiments were carried out with [1-H2]+. In these
experiments, a solution was prepared in situ under 1.0 atm of
H2 in an NMR tube, and D2 (3.0 mL at 1.0 atm) was injected
1
into the NMR tube. Scrambling of H/D was monitored by H
NMR spectroscopy (see Figure 3). The formation of [1-HD]+
was observed after 15 min as a small shoulder at −12.71 ppm
on the H2 resonance of [1-H2]+. After 6 h, [1-HD]+ was the
dominant species, as indicated by its distinctive triplet in the 1H
NMR spectrum. During the course of H/D scrambling, the
total amount of [1-H2]+ and [1-HD]+ decreased due to
2
formation of [1-D2]+, which was confirmed by H NMR (see
Figure S10, Supporting Information). After 12 h, the ratio of
[1-H2]+, [1-HD]+, and [1-D2]+ was 38:51:11. HD gas in
solution was also detected 1H NMR spectroscopy as a triplet at
4.22 ppm (JHD = 45 Hz) (see Figure 3). Based on the
integration of the Cp and HD resonances, the concentration of
HD gas in solution was determined to be 2.1 mM after 12 h.
[2-H2]+ was also examined for H/D exchange under the same
conditions. The formation of [2-HD]+ and HD was established
1
2
by H and H NMR spectroscopy (see Figures S11 and S12,
Supporting Information), and after 12 h the ratio of [2-H2]+,
[2-HD]+, and [2-D2]+ was 28: 51: 20. The concentration of
1
HD gas in the reaction solution was determined by H NMR
spectroscopy to be 2.7 mM after 12 h, which suggests [2-H2]+
undergoes H/D exchange at a rate comparable to that of
[1-H2]+.
Complex [3-H2]+ was also studied for H2/D2 and H/D
exchange under identical conditions to those used for [1-H2]+
and [2-H2]+. Integration of the H2 resonances for both isomers
of [3-H2]+ relative to the Cp resonance indicated a decrease in
H2/D2 and H/D Exchange for [1-H2]BArF , [2-H2]BArF ,
4
4
and [3-H2]BArF . To assess the existence of an equilibrium
4
between [1-H2]+ and its tautomer, [1-FeH-NH]+, in which
heterolytic cleavage of H2 has occurred (Scheme 5), H2/D2
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Scheme 7. DFT Studies of Intramolecular Heterolytic H2 Cleavage of [1-H2]+: Energy Comparison of Isomers of [1-H2]+ and
a
the Product of Heterolytic Cleavage, [1-FeH-NH]+, and Selected Distances
a
For clarity, phenyl substituents of all structures are simplified by only showing ipso carbons.
2
the intensity of the H2 resonance with time, and a H NMR
spectrum recorded after 4 h confirmed the presence of two
[Fe(D2)]+ isomers (see Scheme 6 and Figures S13 and S14,
Supporting Information). The formation of [3-D2]+ via H2/D2
exchange was also observed by 31P{1H} NMR spectroscopy.
However, after 19 h there was no detectable formation of an
[Fe(HD)]+ species or HD gas that would indicate cleavage of
the H−H or D−D bonds or H/D exchange. Addition of 2.0
equiv of either aniline or benzylamine resulted in the slow
formation of [3-HD]+ and HD gas over 12 h.
comparison of the selected distances and angles) with the axis
of the H−H bond parallel to the P−P vector. Structure A is
related to structure C by conformational changes of the two six-
membered rings. The calculated structure for the chair−boat
conformer A also shows the bond of the H2 ligand is parallel to
the P−P vector, and it is found to be 1.1 kcal/mol more stable
than C.34 The small energy difference due to ligand
conformational changes is consistent with the DFT calculations
on related Ni systems and also indicates that both conformers
should be present in solution.
In experiments using aniline as an external base (Figures S15
and S16, Supporting Information), the concentration of HD
gas in the reaction solution after 12 h was determined to be
0.96 mM, less than half of that observed for [1-H2]+ and
2-H2]+, indicating that the pendant bases in the latter com-
plexes are important for catalyzing H/D exchange more
efficiently than [3-H2]+ in the presence of an external base
and much more efficiently than [3-H2]+ alone.
Attempts to orient the H2 ligand so that the H−H bond was
perpendicular to the P−P vector, i.e., one H atom pointing
toward the pendant N base, resulted in the calculations
converging to isomer B, the H2 heterolytic cleavage product,
which lies 4.72 kcal/mol above A. The energy difference
suggests the intramolecular H2 heterolytic cleavage of 1-H2
from isomer A to B is a slightly uphill reaction but
thermodynamically accessible at room temperature, as indicated
by the experimentally observed H−H and D−D cleavage. The
calculated ΔG° (4.72 kcal/mol) reflects an equilibrium
constant of 0.003 between A and B at 22 °C, which explains
why [1-FeH-NH]+ was not observed by NMR spectroscopy.
The transition state for the reaction from A to B was also
optimized. The energy of the transition state is 6.12 kcal/mol
above A, a relatively small reaction barrier, implying a rapid
Deuterium Incorporation from D2O into [CpFe-
(PPh2NR )(H)] (R = Ph, Bn) and [CpFe(PPh2C5)(H)]. In the
2
presence of excess D2O (56 equiv, 0.93 M) in THF, all three
hydrides (0.017 M, 1-H, 2-H, and 3-H-a) undergo exchange of
the hydride ligand for deuterium to form the corresponding
[Fe-D] species within 4 min. The half-life for the reaction can
be estimated as 2 min, but differences in the rates of exchange
are not distinguishable for the three hydride complexes using
this approach.
proton exchange between the N of PPh2NBn and the hydride
2
ligand. The N−H distance of 1.367 Å in the optimized
structure of the transition state suggests a hydrogen bond
between the N and the H2. The H−H bond distance is about
0.89 Å for both A and C, slightly shorter than the value (0.92
Å) derived from our NMR measurements. In addition, the H−
H bond is elongated to 0.99 Å in the transition-state structure
compared to the values of structures A and C and lengthens
further to 1.359 Å in isomer B. In isomer B the N−H distance
is 1.09 Å, and the Fe−H distance is 1.538 Å, suggesting the
formation of a N−H bond and an Fe−H bond as a result of the
heterolytic H−H cleavage. The structure of the transition state
Computational Studies of the Heterolytic Cleavage of
Dihydrogen by [CpFe(PPh2NBn2)(H2)]BArF , [1-H2]BArF .
4
4
Density functional (DFT) calculations28−33 were carried out on
the boat−chair conformer (A) and the chair−boat conformer
(C) of [1-H2]+ as well as the corresponding isomers (B, D, and
E) of the product of heterolytic cleavage, [1-FeH-NH]+ (see
Scheme 7). Optimization of the boat−chair conformer (C) of
[1-H2]+ started from the crystal structure of [1-H2]+. The
optimized structure of conformer C is in good agreement
with the X-ray structure (see Supporting Information for a
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illustrates the role that a pendant base can play in decreasing
the reaction barrier to H−H bond cleavage/formation. It is also
of interest that isomers D and E are 24.71 and 17.12 kcal/mol,
respectively, higher in energy than isomer B. The energy
difference between D and E arises from the position of the Bn
group at the axial vs equatorial position of the six-membered
ring adjacent to the H2. The shorter H−H and longer N−H
distance suggests that the intramolecular H−H interaction
considerably stabilizes isomer B relative to D and E. These
protic−hydridic interactions (i.e., Fe−H···H−N) have been
referred to as “dihydrogen bonding”.35
For the sake of completeness, we also calculated the
possibility of intermolecular H2 heterolytic cleavage between
two molecules of isomer A (see Supporting Information, Figure
S30). It was found that the intermolecular reaction is uphill by
48.00 kcal/mol, ca. 11 times the barrier for the intramolecular
pathway. Efforts to find a transition state for the intermolecular
reaction failed; two molecules of isomer C cannot easily
approach each other, which further suggests a low probability
for an intermolecular pathway.
oxidation process is under diffusion control.36 This wave is
assigned to the FeIII/II couple of 1-Cl. Similar reversible one-
electron oxidation waves are also observed for 2-Cl (−0.51 V,
see Figure S19, Supporting Information) and 3-Cl (−0.68 V,
see Figure S20, Supporting Information).
Figure 4b shows a cyclic voltammogram recorded on a
solution prepared by treating 1-Cl in fluorobenzene with
NaBArF under an argon atmosphere. A reversible oxidation
4
wave is observed at 0.07 V with a peak-to-peak separation
(ΔEp) of 81 mV. Under these conditions, a ΔEp of 76 mV was
observed for the Cp2Fe+/Cp2Fe couple. The scan rate
dependence of the peak current indicates that this oxidation
is diffusion controlled.36 This redox process is assigned to the
oxidation/reduction of the coordinatively unsaturated 16-
electron species [CpFe(PPh2NBn2)]+, [1]+. The analogous
[Cp*Fe(dppe)]+ complex has been reported to have a
reversible oxidation wave at −0.29 V vs the Cp2Fe+/Cp2Fe
couple in tetrahydrofuran.37 A smaller wave is also observed at
−0.55 V that is currently unassigned. There are also two
irreversible reduction waves for [1]+ with peaks at −1.54 and
−1.80 V at 100 mV/s scan rate (see Figure S24, Supporting
Information).
Electrochemical Studies. A cyclic voltammogram of 1-Cl
in fluorobenzene is shown in Figure 4a. It consists of a single
reversible one-electron oxidation wave at −0.61 V vs the
Cp2Fe+/0 couple. A plot of the peak current of the anodic wave vs
the square root of the scan rate is linear, indicating that this
Figure 5 shows cyclic voltammograms of complex [1-H2]+
recorded in fluorobenzene under H2 (1.0 atm). The black trace
Figure 5. Cyclic voltammograms of [1-H2]+ in fluorobenzene. For the
black trace (solid line), the initial scan direction was positive. For the
red trace (dotted line), the initial scan direction was negative.
Conditions: 1.0 mM [1-H2]+; scan rate, 100 mV/s; electrolyte, 0.2 M
n-Bu4NBArF ; 1.0 atm H2.
4
shows a cyclic voltammogram in which the initial scan direction
is positive. An irreversible oxidation wave is observed with an
anodic peak potential of 0.43 V. This wave becomes partially
reversible at higher scan rates (Figure S21, Supporting
Information); it is assigned to the oxidation of [1-H2]+(FeII)
to [1-H2]2+ (FeIII) followed by loss of H2. In support of this
interpretation, a wave is observed at 0.07 V (average of
oxidation and reduction peaks, see Figure S23, Supporting
Information) on the return scan, identical to the potential
observed for the coordinatively unsaturated 16-electron species
[CpFe(PPh2NBn2)]+, [1]+, discussed above. The small waves at
−0.50 and −1.13 V, which are related to the oxidation at 0.43 V
and the reduction at −1.83 (see below), are unassigned. When
Figure 4. (a) Cyclic voltammogram of 1-Cl (1 mM) and (b) [1]+
(1 mM). Conditions: scan rate, 100 mV/s; electrolyte, 0.2 M
n-Bu4NB(C6F5)4 in fluorobenzene.
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initial scan direction is negative, the waves at −0.50 and −1.13 V
are not observed. The dotted red trace in Figure 5 is a cyclic
voltammogram for which the initial scan direction is negative.
In this case, an irreversible reduction wave is observed with a
peak potential of −1.83 V that is assigned to the FeII/I couple
corresponding to the reduction of [1-H2]+ to [1-H2], followed
by hydrogen loss. Again on the return scan, the oxidation
wave at 0.07 V of [1]+ is associated with the reduction wave at
−1.83 V, consistent with H2 loss following reduction.
−0.78 V, and at a scan rate of 25 V/s, the ratio of the cathodic
current to the anodic current, ic/ia, is 0.98. In addition to the
irreversible oxidation wave at −0.72 V, an oxidation wave is
observed at 0.11 V with an associated reduction at 0.04 V that is
assigned to the formation of [1]+ as discussed above. A second
irreversible oxidation wave is observed at +0.43 V that is assigned
to [1-H2]+. These two new oxidation waves are half the height of
the oxidation wave. These data are consistent with the reactions
shown in Scheme 8. The oxidation of 1-H forms [1-H]+, which
reacts with a second molecule of [1-H]+ to form [1]+ and
[1-H2]+. This scheme illustrates the overall stoichiometry of the
reactions but is not meant to imply a mechanism.
The cyclic voltammogram of 1-H shows an irreversible
oxidation at −0.72 V at a scan rate of 0.1 V/s (see Figure 6a) as
Complex 2-H shows similar electrochemical behavior to 1-H,
as shown in Figure S27, Supporting Information, but the
oxidation of 2-H to 2-H+ is somewhat more reversible than for
1-H, as indicated by some reversibility of the 2-H+/2-H couple
(E1/2 = −0.65 V) even at 100 mV/s. Complex 3-H-a exhibits an
irreversible oxidation at −0.82 V. This is a somewhat
unexpected observation, because Cp*Fe(dppe)H is reported
to have a reversible oxidation at −0.75 V at a scan rate of 1.0 V/s.37
At scan rates above 9 V/s, the oxidation of 3-H-a is quasireversible
as a catholic wave can be clearly observed (see Figure S28,
Supporting Information). At 25 V/s, the ic/ia is ca. 0.3. These
electrochemical data suggest 3-H-a undergoes a faster chemical
process following the electron transfer step than either 1-H or 2-H.
The irreversibility may be due to an increased ring strain for 3-H
compared to 1-H and 2-H, as suggested by smaller P−Fe−P
angles for 3-Cl compared to 1-Cl and 2-Cl.
Cyclic voltammograms of 1-H in fluorobenzene solutions
titrated with increasing amounts of DBU are shown in Figure 7.
The addition of 5 equiv of base results in an increase in the
current observed for oxidation wave at −0.72 V by a factor of
2.1−2.4. The increase in current observed for the oxidation
wave at −0.72 V implies that a second electron transfer event
becomes possible in the presence of base, likely as a result of
fast deprotonation of [CpFeIII(PPh2NBn2)(H)]+, [1-H]+, to form
[CpFeI(PPh2NBn2)]. This FeI species should be readily oxidized
at −0.72 V resulting in an ECE reaction as shown in the first
three steps of Scheme 9.
Figure 6. (a) Cyclic voltammogram of 1-H (1.0 mM solution).
Conditions: scan rate, 100 mV/s; electrolyte, 0.2 M n-Bu4NBArF in
4
PhF; under Ar (1.0 atm). (b) The oxidation wave at −0.72 V recorded
at scan rates from 0.1−25 V/s.
Addition of H2 to this solution did not result in any further
increase in current. These results indicate that catalytic oxi-
dation of H2 is not occurring. Consistent with this inter-
pretation, chemical oxidation of 1-H using Cp2Fe+ in the pre-
sence of DBU and D2 does not result in the formation of
sufficient DBU-D+ to indicate a catalytic reaction. In addition,
well as waves at 0.11 and 0.43 V that correspond to the
oxidation of [1]+ and [1-H2]+, respectively. The FeIII/II redox
couples for the series 1-H (−0.72 V), 1-Cl (−0.61 V) and [1-
H2]+ (0.43 V) follow the order of Lever’s38 electrochemical
parameters for the ligands H− (−0.4 V), Cl− (−0.24 V), and H2
(0.8 V).39 In addition, the peak current of the irreversible wave
at −0.72 V is almost the same as that observed for the oxidation
wave of 1-Cl under identical conditions and at the same
concentration. This indicates that the oxidation of 1-H at −0.72
V is an one-electron process. The oxidation of 1-H becomes
more reversible at higher scan rates (see Figure 6b), as
indicated by the associated return reduction wave with a peak at
when [1]+ is generated by reaction of 1-Cl with NaBArF and
4
treated with DBU, a single new resonance is observed at 46.4
ppm in the 31P{1H} NMR spectrum. This reaction is
accompanied by a color change from dark brown to pink.
Upon addition of DBU, the oxidation wave at 0.07 V observed
for [1]+ shifts to −0.05 V, and the reduction waves at 1.54 and
1.80 V disappear, consistent with the formation of a new
species (see Figure S25, Supporting Information). Based on
Scheme 8
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bases in facilitating proton transfer between the metal center and
the solution.
The structures of all of the [CpFe(P2N2)L]n+ complexes
reported in this study have diphosphine ligands with one six-
membered chelate ring in a boat conformation and the other in
a chair conformation. This is the most common conformational
arrangement observed for other complexes with this class of
ligand as well. If the ligand L bears a formal negative charge,
such as chloride or hydride, then the interaction between the N
atom of the diphosphine ligand and this ligand is repulsive. This
results in a chair conformation for this chelate ring to minimize
the interaction and a boat conformation for the ring adjacent to
the Cp ligand. However, for the complex with a CO ligand, [1-
CO]+, there is an attractive interaction between the nitrogen
lone pair and the partial positive charge on the carbonyl carbon,
as indicated by the short N···C distance (2.85 Å) that is
significantly less than the sum of the van der Waals radii.
Similar, but much weaker, N···CO interactions were reported
previously for Mn(PPh2NBn2)(dppm)(CO)(H) (where dppm is
1,2-bis(diphenylphosphino)methane, N···C = 3.17 Å)41 and
[Ni(PPh2NBn2)2(CO)](BF4)2 (N−C distances of 3.30 and 3.38 Å
Figure 7. Cyclic voltammogram of 1-H (0.5 mM) in the absence of
DBU (red trace), in the presence of DBU (5 equiv, blue trace), and in
the presence of DBU (5 equiv) and H2 (1.0 atm, green trace).
Conditions: scan rate, 100 mV/s; electrolyte, 0.2 M n-Bu4NBArF in
4
PhF.
these data, the new species is tentatively identified as [1-(DBU)]+,
in which DBU has coordinated to iron to form an 18-electron
complex (reaction C2 of Scheme 9). There are a few examples of
metal complexes that have DBU as a ligand.40 Upon treatment of
[1-(DBU)]+ with H2, no [1-H2]+ or 1-H was formed. However,
addition H2 to [1]+ followed by DBU led to the immediate
formation of 1-H. It can be deduced that formation of
[1-(DBU)]+ upon oxidation of 1-H under H2 in the presence
of DBU would effectively limit catalysis, as observed. Similar
results were obtained for 2-H and 3-H.
for both PPh2NBn ligands).42 The much shorter N···C distance
2
observed for [1-CO]+ implies a stronger interaction. Complex
[1-CO]+ is also of interest because it suggests an arrested
nucleophilic attack on the coordinated CO ligand, related to
previously reported nucleophilic attacks of primary amines on
coordinated CO.43
Morris18,19 and Heinekey17 have shown that the value of JHD
is correlated with the H−D distance of the bound HD ligand,
with smaller JHD values indicating longer H−D distances. The
JHD value of 25 Hz that we found in [CpFe(PPh2C5)(HD)]+ is
smaller than those reported for other Fe(H2)+ complexes.
Morris and co-workers found JHD = 32 Hz for [Fe(HD)(H)-
(dppe)2]+.44 A slightly smaller JHD of 29.8 Hz was found in the
related water-soluble iron complex [Fe(HD)(H)-
(DMeOPrPE)2]+ (DMeOPrPE = 1,2-bis(bis(methoxypropyl)-
phosphino)ethane) studied by Tyler and co-workers.45 A small
JHD value of 27 Hz was reported for [Cp*Fe(dppe)(HD)]+.46
For the dihydrogen complex [1-H2]+, the energies of the
chair/boat conformations of the chelate rings appear to be
nearly balanced. A crystal structure determination indicates that
the six-membered ring adjacent to the H2 ligand adopts a chair
conformation, while DFT calculations suggest that in solution
the boat conformation is more stable. For the latter
DISCUSSION
■
A series of new CpFe derivatives containing the PPh2NR2 ligands
(R = Ph, Bn) and the base-free PPh2C5 ligand have been
synthesized and characterized by X-ray diffraction, electrochemical,
and spectroscopic techniques. The features of primary interest in
this study are characteristics of the complexes that are relevant to
the development of electrocatalytic activity for H2 oxidation or
formation. These include the conformations of the ligand chelate
rings and the positioning of the amine bases relative to the active
coordination site at iron, the ability of the complexes to coordinate
and effect heterolytic cleavage of H2, and the role of the pendant
Scheme 9
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conformation, the pendant base is positioned to participate in
H2 cleavage with a low activation energy.
All three of the dihydrogen complexes can be deprotonated
by the base DBU (pKa = 24 for H-DBU+ in CH3CN)20 to form
the corresponding hydride derivatives 1-H, 2-H, and 3-H. Depro-
tonation with excess triethylamine (pKa = 18.8 for H-NEt3+ in
CH3CN)20 is not observed, and this brackets the effective pKa of
the coordinated H2 ligand in this series in the range of 20−24 in
acetonitrile. As noted before, our experiments were conducted in
fluorobenzene since the complexes react with CH3CN, but the
comparison of pKa values cited are for CH3CN solvent. These
complexes are less acidic than previously reported cationic iron
dihydrogen complexes reported by Morris and co-workers, such as
trans-[Fe(H2)H(dppe)2]BPh4 (pKa in CH3CN estimated as 18.39
based on measurements in THF).49
Each of the complexes 1-Cl, 2-Cl, and 3-Cl undergoes
chloride abstraction under 1.0 atm of H2 in fluorobenzene to
rapidly form the corresponding dihydrogen complex. Further
studies with a mixture of H2 and D2 provide a method to evaluate
activity for heterolytic cleavage of H2. The hydrogenase enzymes47
have been shown to undergo exchange reactions to produce HD
under an H2/D2 atmosphere. Diiron model compounds reported
by Darensbourg and co-workers48 show H2/D2 scrambling but
involves photolytically induced CO loss, so it is significantly
different than the reactivity of the enzyme and the Fe complexes
reported here. Exposure of the dihydrogen complexes prepared in
this study to mixtures of D2 and H2 gas results in the exchange of
the H2 ligand with D2 for all three complexes. However, only
complexes [1-H2]+ and [2-H2]+ with the P2N2 ligands exhibited
H/D exchange to form HD gas and coordinated HD, detected
over a period of minutes. The relatively rapid H/D exchange
observed for [1-H2]+ and [2-H2]+ is consistent with intra-
molecular heterolytic cleavage of H2 to form [1-FeH-NH]+ and
[2-FeH-NH]+ as intermediates. This rapid heterolytic cleavage,
followed by proton transfer between species in solution, leads
to the observed H/D exchange.
Our experimental results are supported by DFT calculations
that suggest the barrier (ΔG‡) and the free energy (ΔG°) of
heterolytic H−H cleavage are small (ΔG‡ = 6.12 and ΔG°=
4.72 kcal/mol). DFT calculations on the diiron (FeII) active site
model reported by Hall et al. indicate that the bridging
di(thiomethyl)amine ligand can facilitate heterolytic H−H
bond cleavage with comparable free energies, ΔG‡ = 4.9 and
ΔG° = 3.5 kcal/mol.7 Both calculations also reveal almost
identical reaction coordinates for H−H cleavage. As sketched in
Scheme 7 (and Figure 1 in ref 7), the H−H bond length
gradually increases from 0.89 Å for the dihydrogen complex to
0.99 Å for the transition state and finally to 1.36 Å for the H2
heterolytic cleavage product. This increase in the H−H distance
is accompanied by the disappearance of Fe−(H2) σ bond and
the formation of Fe−H and N−H bonds as well as an Fe−
H···H−N “dihydrogen bond” in the transition-state structure
and the heterolytic cleavage product. As displayed in Schemes 5
and 7 and the calculations from Hall,7 the fundamental
requirements for heterolytic H2 cleavage at the active site of
[FeFe]-hydrogenases and synthetic catalysts based on Fe are:
(1) a ferrous (FeII) center for H2 bonding and activation; (2) a
pendant amine base for H−H bond cleavage; and (3) energy
matching of the hydride acceptor abilities of the Fe and the
proton acceptor abilities of the pendant amine. Therefore,
complexes [1-H2]+ and [2-H2]+ with positioned pendant
amines serve as models of the distal Fe site coordinated with
the H2 ligand of the enzymatic active site, suggesting mono-
nuclear Fe complexes can be competent models of the active
site of [FeFe]-hydrogenase enzymes.
Electrochemical oxidation of 1-H produces [1-H2]+ and [1]+,
as shown in Scheme 8. Paramagnetic complexes similar to [1]+,
[Cp*Fe(dppe)]+26 and [Cp*Fe(dippe)]+27 (dippe =1,2-bis-
(diisopropylphosphino)ethane), have been crystallographically
characterized. However, in the presence of a base, such as DBU,
[1-H]+ is deprotonated (step C1 of Scheme 9), followed by a
second electron transfer reaction (step E2). Coordination of H2
would lead to regeneration of [1-H2]+ and completion of a
catalytic cycle. However, H2 binding to [1]+ is not competitive
with the coordination of DBU, which is present in solution in a
relatively high concentration. The preferential coordination of
base rather than H2 prevents the completion of the desired
cycle, and catalysis is not observed for 1-H. Similar inhibition
by base is also observed for 2-H and 3-H. Although we have
not yet found conditions for which 1-H is an electrocatalyst for
oxidation of H2, this complex does meet important require-
ments for catalytic activity, including rapid binding and
heterolytic activation of H2, followed by proton transfer to
solution. The results presented here suggest a rationale for
designing related iron derivatives with bulky ligands and
appropriate pKa values to favor coordination and heterolytic
cleavage of H2 in the presence of a less coordinating base. Our
studies of such ligand modifications and their impact on
catalytic activity for H2 oxidation will be reported.
SUMMARY AND CONCLUSIONS
■
A series of new CpFe(diphosphine)H, [CpFe(diphosphine)-
(H2)]BArF , and related complexes have been prepared
4
containing PPh2NBn2 and PPh2NPh2 ligands with pendant amines
to explore the utility of this structural motif for developing
electrocatalysts for H2 oxidation and production. Based on the
structural studies of several of these complexes and DFT cal-
culations, it is concluded that attractive or repulsive interactions
between the sixth ligand and the pendant amine of the adjacent
six-membered ring of the cyclic diphosphine determine the
conformations adopted by these ligand chelate rings. An
attractive interaction between the H2 ligand in [1-H2]BArF
4
and [2-H2]BArF and the pendant base of the diphosphine
4
ligand facilitates the intramolecular heterolytic cleavage of H2.
Rapid H/D exchange is observed for these complexes under an
H2/D2 atmosphere, whereas under the same conditions HD
In contrast to [1-H2]+ and [2-H2]+, [3-H2]+ with no pendant
amines exhibited no detectable H/D exchange after 19 h,
although slow exchange was observed for this complex in the
presence of two equivalents of an external base, such as aniline
or benzylamine. The heterolytic cleavage of H2 and proton
transfer to solution observed for the complexes with the
pendant amines, [1-H2]+ and [2-H2]+, suggest that complexes
containing positioned pendant amines are likely to promote
higher catalytic activity for H2 oxidation than systems that lack
these proton relays.
formation is not observed for [3-H2]BArF , which does not
4
have a base incorporated in its second coordination sphere.
Together with the DFT calculations, the experimental
observations indicate crucial structural features for the intra-
molecular heterolytic cleavage of H2 are a ferrous (FeII) center,
a pendant amine, and matched hydride and proton acceptor
abilities.
All of the cationic dihydrogen complexes, [1-H2]BArF ,
4
[2-H2]BArF , and [3-H2]BArF , can be deprotonated by DBU
4
4
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Journal of the American Chemical Society
Article
prepared according to published procedures. All other reagents were
used as received.
to form the corresponding neutral hydrides. Electrochemical
studies have shown that oxidation of 1-H in the presence of
DBU as a base results in the electrochemical and chemical steps
required for a catalytic system (see Scheme 9); however,
preferential binding of DBU rather than H2 at the vacant
coordination site of [CpFe(PPh2NBn2)]BArF , 1+-BArF , prevents
the completion of a catalytic cycle. Rational catalyst design will
involve further synthetic modifications to control the steric bulk of
the ligands on the iron complex, the redox potential of the catalyst,
and the acidity of the coordinated H2 molecule in order to achieve
active molecular electrocatalysts for H2 oxidation based on iron.
Computational Details. Molecular structures were optimized
using DFT theory with the hybrid B3P8628 exchange and correlation
functional using the Stuttgart−Dresden relativistic ECP and associated
basis set (SDD) for Fe29 and 6-31G** for all nonmetal atoms.30
Harmonic vibrational frequencies were calculated at the optimized
geometries using the same level of theory to estimate the zero-point
energy (ZPE) and the thermal contributions (298 K and 1.0 atm) to
the gas-phase free energy. Free energies of solvation in toluene were
then computed using the conductor-like polarizable continuum model
(CPCM)31 with Bondi radii.33 All calculations were carried out with
Gaussian 09.32
4
4
CpFe(PPh2NBn2)Cl, (1-Cl). [CpFe(CO)2Cl] (0.440 g, 2.07 mmol)
and PPh2NBn (1.000 g, 2.07 mmol) were dissolved in toluene
EXPERIMENTAL SECTION
2
■
1
2
General Experimental Procedures. H, H, 15N, and 31P NMR
(150 mL) in a 250 mL quartz flask. The solution was photolyzed for
approximately 48 h. The solution gradually changed from red to black,
with concomitant formation of a yellow precipitate, [CpFe(PPh2NBn2)-
(CO)]Cl, [1-CO]Cl. The reaction was complete when the yellow
intermediate, [1-CO]Cl, disappeared, which was further confirmed by
IR and 31P NMR spectroscopy. The resulting black solution was
filtered through Celite to remove insoluble residues, and the filtrate
was evaporated under vacuum to give a black solid. Recrystallization of
the crude product from 20 mL of PhF layered with 200 mL of hexane
yielded 1.07 g (81%) of black crystals suitable for X-ray diffraction.
Anal. calcd. for C35H37N2FeP2Cl: C, 65.79; H, 5.84; N, 4.38. Found:
spectra were recorded on a Varian Inova spectrometer (500 MHz
for H) at 20 °C unless otherwise noted. All H chemical shifts have
1
1
been internally calibrated to the monoprotio impurity of the
2
deuterated solvent. The H NMR spectra were internally calibrated
to the deuterated solvent. The 31P NMR spectra were proton
decoupled and referenced to external phosphoric acid.
Electrochemical experiments were carried out under an atmosphere
of argon, or hydrogen when indicated, in fluorobenzene solutions
containing 0.2 M n-Bu4NB(C6F5)4. Cyclic voltammetry experiments
were carried out with a CH Instruments model 660C potentiostat. The
working electrode (1 mm PEEK-encased glassy carbon, Cypress
Systems EE040) was polished using Al2O3 (BAS CF-1050, dried at
150 °C under vacuum), suspended in acetonitrile, and then rinsed with
neat PhF. A glassy carbon rod (Structure Probe, Inc.) was used as the
counter electrode, and a silver wire suspended in a solution of 0.2 M
n-Bu4NB(C6F5)4 in PhF and separated from the analyte solution by a
Vycor frit was used as the pseudoreference electrode. Cobaltocenium
hexafluorophosphate (Cp2CoPF6) or bis(pentamethylcyclo-
pentadienyl)iron (Cp*2Fe) was used as an internal standard, and all
potentials are referenced to the Cp2Fe+/0 couple at 0 V.
1
C, 65.75; H, 5.77; N, 4.59. H NMR (CD2Cl2): δ 7.95 (m, 4 H,
CH2C6H5), 7.47 (m, 6 H, CH2C6H5), 7.20 (m, 10 H, PC6H5), 3.97
(s, 5 H, C5H5), 3.86 (s, 2 H, NCH2C6H5), 3.50 (s, 2 H, NCH2C6H5),
3.38 (dt, JHH = 12.8 Hz, JPH = 5.6 Hz, 2 H, NCH2P), 3.16 (d, JHH
=
12.8 Hz, 2 H, NCH2P), 3.03 (d, JHH = 11.9 Hz, 2 H, NCH2P), 2.52
(dt, JHH = 11.9 Hz, JPH = 8.3 Hz, 2 H, NCH2P). 31P{1H} NMR
(CD2Cl2): δ 57.2 (s).
[CpFe(PPh2NBn2)(CO)]Cl, [1-CO]Cl. The yellow precipitate that
formed in the photolytic preparation of [CpFe(PPh2NBn2)Cl] described
above was separated by filtration, washed with toluene, and dried
under vacuum to give a yellow powder (0.028 g, 20% based on 0.060 g
[CpFe(CO)2Cl]). Single crystals suitable for X-ray diffraction were
grown from a CH2Cl2 solution layered with hexane. Anal. calcd. for
C35H37N2FeP2Cl: C, 64.83; H, 5.59; N, 4.20. Found: C, 64.15; H,
5.33; N, 4.68. IR (ν (CO), CH2Cl2, cm−1): 1964. 1H NMR (CD2Cl2):
δ 7.50 (m, 10 H, CH2C6H5), 7.35 (m, 8 H, CH2C6H5), 7.25 (m, 2 H,
PC6H5), 4.69 (s, 5 H, C5H5), 4.08 (s, 2 H, NCH2C6H5), 3.90 (d, 2 H,
JHH = 13 Hz, NCH2P), 3.81 (s, 2 H, NCH2C6H5), 3.42 (dt, JPH = 5 Hz,
JHH = 13 Hz, 2 H, NCH2P), 3.05 (dt, JPH = 4 Hz, JHH = 13 Hz 2 H,
NCH2P), 2.86 (d, JHH = 13 Hz, 4.01 H, NCH2P). 31P{1H} NMR
(CD2Cl2): δ 52.7 (s).
X-ray Structures. For all reported structures, a 10× microscope
was used to identify suitable crystals of the same habit. Each crystal
was coated in Paratone, affixed to a nylon loop and placed under
streaming nitrogen (100 K) in a Bruker KAPPA APEX II CCD
diffractometer with 0.71073 Å Mo Kα radiation (see details in cif files
in the Supporting Information). The space groups were determined on
the basis of systematic absences and intensity statistics. The structures
were solved by direct methods and refined by full-matrix least-squares
on F2. Anisotropic displacement parameters were determined for all
nonhydrogen atoms. Hydrogen atoms were placed at idealized
positions and refined with fixed isotropic displacement parameters.
Disorder fragments (i.e., the disordered cocrystallized ether in
structure of [CpFe(PPh2NBn2)(CO)]Cl, [1-CO]Cl, and the disordered
BArF ‑ anion of [CpFe(PPh2NBn2)(H2)]BArF , (1-H2) was modeled
Generation of [CpFe(PPh2NBn2)]BArF (1+-BArF4) and
CpFePPh2NPh2]BArF (2+-BArF ). Under Ar, a 4mixture of NaBArF
4
4
4
(0.018 g, 0.020 mmol) and 1-Cl (0.013 g, 0.020 mmol) (or 2-Cl) was
dissolved in 1 mL of fluorobenzene (PhF) and stirred for 0.5 h to give
a dark-brown solution of 1+-BArF (or 2+-BArF ). This solution
4
4
using appropriate restrains and constraints. The H2 ligand of [1-H2]+
cannot be accurately located using X-ray diffraction data; the single
residue ca. 1.60 Å from the Fe atom was modeled and refined as an H2
ligand. Programs used were: data reduction, SAINTPLUS, version
6.63;50 absorption correction, SADAB;51 structural solution, SHELXS-
97;52 structural refinement, SHELXL-97;53 graphics, Ortep-3(V2) for
Windows.54 Crystallographic information (available as electronic files
cif format) is provided in the Supporting Information.
4
4
prepared in situ was used for reactions or cyclic voltammetry (CV)
studies. ESI-MS (e/z) for 1+: observed at 603.1785, predicted at
603.1782.
[CpFe(PPh2NBn2)(H2)]BArF , [1-H2]+. Under an atmosphere of H2
4
(1.0 atm), a solution of [CpFe(PPh2NBn2)Cl] (0.128 mg, 0.200 mmol)
in 5 mL of PhF was added via cannula to solid NaBArF (sodium
4
tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) (0.180 g, 0.200 mmol).
The mixture was stirred for 15 min, leading to a color change from
black to yellow. The workup was carried out under an argon
atmosphere in a glovebox. After filtration through Celite, 30 mL of
hexane was added to precipitate the product as a yellow solid (0.230 g,
Synthesis and Materials. All reactions and manipulations were
conducted under a N2, Ar, or H2 atmosphere using standard Schlenk
techniques or a glovebox. Solvents were dried using an activated
alumina column. All NMR solvents were purified according to
standard methods. Photolysis reactions were conducted using a water-
jacketed medium pressure mercury lamp (Ace Hanovia 450 W).
Quartz glassware was used for all photolysis reactions. [CpFe-
(CO)2Cl],55 1,3-diphenyl-5,7-dibenzyl-1,5-diaza-3,7-diphos-
phacyclooctane (PPh2NBn and PPh 15NBn2),56 1,3,5,7-tetraphenyl-1,
80%). [CpFe(PPh2NBn2)(H2)]BArF is stable under H2 at room
4
temperature for several weeks. [1-H2]+ can also be prepared by
addition of H2 to 1+-BArF . Anal. calcd. for C67H51BN2F24FeP2: C,
4
1
54.79; H, 3.50; N, 1.91. Found: C, 54.77; H, 3.46; N, 1.86. H NMR
(PhCl-d5): δ 8.02 (s, 8 H, B(C8F6H3)4), 7.34 (s, 4 H, B(C8F6H3)4),
7.03 (m, 8 H, C6H5), 6.96 (m, 10 H, C6H5), 6.72 (d, 2 H, C6H5), 3.75
(s, 5 H, C5H5), 3.41 (s, 2 H, NCH2C6H5), 3.04 (s, 2 H, NCH2C6H5),
2
2
5-diaza-3,7-diphosphacyclooctane (PPh2NPh2),5 1,5-diphenyl-1,5-diphospho-
ctane (PPh2C6),23 and 1,4-diphenyl-1,4-diphospheptane (PPh2C5)24 were
6268
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Journal of the American Chemical Society
Article
2.85 (dt, JHH = 15 Hz, JPH = 5 Hz, 2 H, NCH2P), 2.75 (d, 2 H, JHH
=
J. Young NMR tube using 0.6 mL of PhCl-d5 solvent according to the
procedure used for [1-HD]+. 1H NMR (PhCl-d5) for [2-HD]+: δ
−12.75 (t, JHD = 27.5 Hz, 1 H, Fe(HD)).
10 Hz NCH2P), 2.39 (dt, JHH = 15 Hz, JPH = 5 Hz, 2 H, NCH2P), 1.74
(d, JHH = 15 Hz, 2 H, NCH2P), −12.64 (s, 2.00 H, FeH2) . 31P{1H}
NMR (PhCl-d5): δ 58.4 (s).
CpFe(PPh2NPh2)(H), (2-H). Complex 2-H was prepared by both
methods described above for 1-H. Method A gave an isolated yield of
75% (0.173 g, based on 2-Cl, 0.243 g, 0.400 mmol). Single crystals
suitable for X-ray diffraction study were grown from an ether solution
stored at −35 °C. Anal. calcd. for C35H38N2FeP2: C, 69.54; H, 6.34; N,
[CpFe(PPh2NBn2)(HD)]BArF , ([1-HD]+). 1-Cl (0.013 g, 0.020
4
mmol) and NaBArF (0.022, 0.024 mmol) were dissolved in 0.6 mL of
4
PhCl-d5 in a J. Young NMR tube under Ar. The solution was frozen using
liquid N2, and the tube was evacuated. The NMR tube was refilled with
HD gas (1.0 atm). After warming to room temperature, the reaction was
1
4.63. Found: C, 69.26; H, 6.28; N, 4.63. H NMR (THF-d8): δ 7.88
1
1
(m, 4 H, C6H5), 7.47 (m, 6 H, C6H5), 7.15 (m, 4 H, C6H5), 6.96 (d,
JHH = 25 Hz, 2 H, C6H5), 6.87 (d, JHH = 25 Hz, 2 H, C6H5), 6.72
(t, JHH = 10 Hz, 1 H, C6H5), 6.61 (t, JHH = 10 Hz, 2 H, C6H5), 4.31 (d,
monitored by H NMR spectroscopy. H NMR (PhCl-d5): δ −12.71
(t, JHD = 30 Hz, 1 H, Fe(HD)).
CpFe(PPh2NBn2)(H), (1-H). Method A. A mixture of [CpFe-
(PPh2NBn2)Cl] (0.255 g, 0.400 mmol) and 3 equiv of LiAlH4 (ca.
0.046 g) in 20 mL of THF was stirred at 22 °C for 1 h, resulting
in a color change from black to orange. THF was removed by
evaporation to give an orange residue. The orange residue was
extracted with 20 mL of toluene. After filtration, water was
added dropwise to the filtrate to quench any remaining LiAlH4
until no further gas evolution was observed. The precipitate was
removed by filtration. The solvent was evaporated, and the
orange powder was dried under vacuum to give analytically
pure product (0.140 g, 61%). Single crystals suitable for an
X-ray diffraction study were grown from an ether solution at
−35 °C. Anal. calcd. for C35H38N2FeP2: C, 69.54; H, 6.34; N,
4.63. Found: C, 69.26; H, 6.28; N, 4.63. 1H NMR (THF-d8): δ
7.58 (br, 4 H, C6H5), 7.44 (d, 2 H, C6H5), 7.30 (m, 10 H,
C6H5), 7.19 (m, 4 H, C6H5), 4.00 (s, 2H, NCH2C6H5), 4.23 (s,
5 H, C5H5), 3.49 (s, 2 H, NCH2C6H5), 3.20 (d, JHH = 15 Hz, 2
H, NCH2P), 2.98 (m, 4 H, NCH2P), 2.13 (d, JHH = 15 Hz, 2 H,
NCH2P), −16.0 (t, JPH = 60 Hz, 1 H, FeH). 31P{1H} NMR
(THF-d8): δ 71.8 (s).
JHH = 20 Hz, 2 H, NCH2P), 3.90 (m, 4 H, NCH2P), 3.82 (d, JHH
=
20 Hz, 2 H, NCH2P), 3.72 (s, 5 H, C5H5), 2.90 (d, JHH = 20 Hz, 2 H,
NCH2P), −15.74 (t, JPH = 50 Hz, 1 H, FeH) . 31P NMR (THF-d8): δ
73.3 (s).
Protonation of 1-H and 2-H. HBF4·Et2O (30 μL of a suspen-
sion prepared by adding 0.1 mL of HBF4·Et2O to PhF, then diluting to
1.00 mL) was added dropwise to an NMR tube containing a solution
of 1-H (or 2-H) (0.06 g, 0.001 mmol, in 0.6 mL of PhCl-d5) under H2
1
(1.0 atm), until the reaction was complete, as indicated by H NMR
spectroscopy. The formation of the [1-H2]+ and [2-H2]+ was
1
confirmed by H and 31P NMR spectroscopy.
CpFe(PPh2C5)Cl, (3-Cl-a). The complex was synthesized according
to the procedure for 1-Cl using [CpFe(CO)2Cl] (0.467 g, 2.20 mmol)
and PPh2C5 (0.8 g, 2.20 mmol). Two isomers were identified by 31P
NMR spectroscopy in a ratio of about 1:0.15. The minor isomer was
removed by washing with a solvent mixture (2:1 THF:hexane, 2 ×
20 mL). The yield for the major isomer is 0.566 g (61%). Single crystals
suitable for X-ray diffraction were grown from a CH2Cl2 solution
layered with hexane. Anal. calcd. for C22H25N2P2FeCl: C, 59.69; H,
1
5.69. Found: C, 59.61; H, 5.46; N, 5.46. For the major isomer: H
Method B. 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU, 74 μL, 0.075 g,
NMR (CD2Cl2): δ 8.1 (m, 4 H, C6H5), 7.54 (m, 6 H, C6H5), 4.07 (s, 5
H, C5H5), 2.66 (m, 2 H, P(CH2)3P), 2.41 (m, 1 H, P(CH2)3P), 2.07
(m, 1 H, P(CH2)3P), 2.01 (m, 2 H, P(CH2)2P), 1.97 (m, 2 H,
P(CH2)2P), 1.80 (m, 2 H, P(CH2)3P). 31P NMR (CD2Cl2): δ 80.11
(s). For the minor isomer: 31P NMR (CD2Cl2): δ 85.50 (s).
0.48 mmol, ca. 1.2 equiv) was added dropwise to a solution of
[CpFe(PPh2NBn2)(H2)]BArF ([1-H2]+) (generated in situ from 1-Cl
4
(0.255 g, 0.400 mmol) in 10 mL of PhF. The reaction was monitored
by 31P NMR spectroscopy to determine when it had reached
completion. After removal of solvent, extraction with 30 mL of
hexane yielded an orange powder (0.048 g, 20%). Its identity was
[CpFe(PPh2C5)(H2)], [3-H2]+. Complex [3-H2]+ was generated from
3-Cl (0.009 g, 0.020 mmol) and H2 (1.0 atm.) using the same
1
confirmed by H NMR and 31P{1H} NMR.
procedure as described for [CpFe(PPh2NBn2)(H2)]BArF (1-H2). Two
Reaction of [CpFe(PPh2NBn2)]BArF (1+-BArF ) with DBU. 1+-
4
4
4
1
isomers ([3-H2-a]+ and [3-H2-b]+) of [3-H2]+ were identified by H
BArF4 was generated in situ from NaBArF4 (0.018 g, 0.020 mmol) and
1-Cl (0.013 g, 0.020 mmol) in 0.5 mL PhCl-d5 in a NMR tube.
Addition of DBU (6 μL, 0.040 mmol, ca. 2 equiv) to the dark-brown
solution led to an immediate color change to pink. The resultant
species showed a resonance at δ 46.4 in the 31P{1H} NMR spectrum,
NMR and 31P NMR spectroscopy in a ratio of about 1:0.5. In the H
1
NMR spectrum, the two isomers are distinguished by the Cp ring
proton resonances and the H2 ligand resonance, while the signals of
the aromatic protons (from δ 6.7 to 8.01) and methylene protons
(from δ 6 to 1.89) appear as overlapping resonances. For the major
isomer ([3-H2-a]+), 1H NMR (PhCl-d5): δ 3.83(s, 5 H, C5H5), −13.79
(s, 2 H, FeH2). 31P{1H} NMR (PhCl-d5): δ 83.70 (s). For the minor
isomer ([3-H2-b]+), 1H NMR (PhCl-d5): δ 3.57 (s, 5 H, C5H5), −13.04
(s, 2 H, FeH2). 31P{1H} NMR (PhCl-d5): δ 85.47 (s).
tentatively assigned as [CpFe(PPh2NBn2)(DBU)]BArF (1-DBU).
4
Subsequent treatment of H2 did not form [1-H2]+ or 1-H.
CpFe(PPh2NPh2)Cl, (2-Cl). The complex was synthesized in a
manner similar to 1-Cl, using [CpFe(CO)2Cl] (0.467 g, 2.20 mmol)
and PPh2NPh (1.000 g, 2.20 mmol). Recrystallization of the crude
2
[CpFe(PPh2C5)(HD)], [3-HD]+. Complex [3-HD]+ was prepared
from 3-Cl (0.009 g, 0.02 mmol) and HD gas (1.0 atm.) in a J. Young
NMR tube using 0.6 mL PhCl-d5 solvent according to the procedure
of [1-HD]+. The reaction was followed by 1H NMR spectroscopy. For
product in 20 mL of PhF layered with 200 mL of hexane yielded
1.070 g (81%) of black crystals suitable for X-ray diffraction. Anal.
calcd. for C33H33N2FeP2Cl: C, 64.88; H, 5.45; N, 4.59. Found: C,
64.67; H, 5.46; N, 4.55. 1H NMR (toluene-d8): δ 8.15 (m, 3 H, C6H5),
7.27 (m, 4 H, C6H5), 7.21 (m, 2 H, C6H5), 7.08 (m, 2 H, C6H5), 6.75
(m, 4 H, C6H5), 6.57 (m, 2 H, C6H5), 4.08 (s, 5 H, C5H5), 4 (tt, 4 H,
JHH = 12 Hz, JPH = 3 Hz, NCH2P), 3.57 (d, JHH = 13 Hz, 2 H,
NCH2P), 3.18 (dt, JHH = 12.5 Hz, JPH = 8 Hz, 2 H, NCH2P). 31P{1H}
NMR (CD2Cl2): δ 61.1 (s).
the major isomer ([3-HD-a]+), 1H NMR (PhCl-d5): δ −13.89 (tt, JHD
=
25 Hz, JPH = 8.3 Hz, 1 H, Fe(HD)). For the minor isomer ([3-HD-b]+),
1H NMR (PhCl-d5): δ −13.13 (tt, JHD = 25 Hz, JPH = 4.5 Hz,
1
1 H, Fe(HD)). A H{31P} NMR spectrum was recorded to confirm the
phosphorus and HD coupling. The triplet of triplets of the HD ligand
resonance for each isomer observed in the 1H NMR spectrum collapsed
to a triplet.
[CpFe(PPh2NPh2)(H2)]BArF , ([2-H2]+). [2-H2]+ was prepared using
4
the same procedure as described for [CpFe(PPh2NBn2)(H2)]BArF4 ([1-
Deprotonation of [CpFe(PPh2C5)(H2)]+, ([3-H2]+). To the
H2]+) (yield, 79% based on 0.200 mmol (0.122 g) [CpFe(PPh2NPh2)-
solution of [3-H2]+ (prepared in situ in a NMR tube) was added
1
Cl], (2-Cl)). H NMR (PhCl-d5): δ 8.02 (s, 8 H, B(C8F6H3)4), 7.33
1
4 μL of DBU (ca. 4 equiv). After 10 min shaking, both H NMR and
(s, 4 H, B(C8F6H3)4), 7.20 (m, 8 H, C6H5), 7.05 (t, 2 H, C6H5), 6.70
(m, 8 H, C6H5), 6.30 (d, 2 H, C6H5), 3.78 (s, 5 H, C5H5), 3.52 (d,
JHH = 15 Hz, 2H, NCH2P), 3.36 (d, JHH = 15, 2 H, Hz NCH2P), 3.08
(d, JHH = 15 Hz, 2 H, NCH2P), 2.59 (d, 2 H, JHH = 15 Hz, NCH2P),
−12.68 (s, 2 H, FeH2) . 31P{1H} NMR (PhCl-d5): δ 56.0 (s).
31P{1H} NMR spectroscopic monitoring indicated two isomers (3-H-a
and 3-H-b) of 3-H formed in a ratio of about 1:0.22. For the major
isomer, [3-H-a]+, 1H NMR (PhCl-d5): δ −17.30 (t, JPH = 65 Hz, 1 H,
Fe(H)). 31P{1H} NMR (PhCl-d5): δ 96.12 (s). For the minor isomer,
[3-H-b]+, 1H NMR (PhCl-d5): δ −13.13 (t, JPH = 65 Hz, 1 H, Fe(H)).
31P{1H} NMR (PhCl-d5): δ 100.95 (s).
[CpFe(PPh2NPh2)(HD)]BArF , ([2-HD]+). Complex [2-HD]+ was
4
prepared from 2-Cl (0.012 g, 0.020 mmol) and HD gas (1.0 atm.) in a
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Journal of the American Chemical Society
Article
CpFe(PPh2C5)(H), (3-H-a). Complex 3-H-a was prepared from 3-
Cl-a (0.133 g, 0.300 mmol) and LiAlH4 (0.035 g, ca. 3 equiv) by
following method A described above for 1-H. There was only a trace
amount of the minor isomer (3-H-b) present, as indicated by 1H
NMR and 31P{1H} NMR spectroscopy. The minor isomer was
removed by washing with Et2O (2 × 5 mL). The major isomer, 3-H-a,
was isolated in a yield of 0.091 g (74%). Anal. calcd. for C22H26P2Fe: C,
64.73; H, 6.42. Found: C, 64.94; H, 6.58. 1H NMR (THF-d8): δ 7.86 (s,
4 H, C6H5), 7.39 (m, 6 H, C6H5), 4.02 (s, 5 H, C5H5), 2.30 (m, 3 H,
P(CH2)3P, P(CH2)2P), 1.96 (m, 5 H, P(CH2)3P, P(CH2)2P), 1.38 (m,
2 H, P(CH2)3P), −17.30 (t, JPH = 28 Hz, 1 H, Fe(H)). 31P{1H} NMR
(CD2Cl2, ppm): δ 96.12 (s).
be about 2 min. The formation of the Fe-D species was confirmed by
2
2
31P NMR and H NMR. H NMR for 1-D, −16.03 (t, JPD = 10 Hz,
1 D, Fe(D)); 2H NMR for 2-D, −15.71 (t, JPD = 10 Hz, 1 D, Fe(D));
2H NMR for 3-D, −17.30 (t, JPD = 10 Hz, 1 D, Fe(D)).
Attempted Electrochemical H2 Oxidation Catalyzed by 1-H.
An 1 mL of PhF solution containing 1.0 mM of 1-H was maintained
under atm argon. The first CV trace was recorded for 1-H at 100 mV/s.
Five equiv of base (DBU) were added, and cyclic voltammograms were
recorded. Then the atmosphere was replaced by 1.0 atm H2, and the third
CV was recorded. No apparent catalytic current was observed under H2
(see Figure 7). Similar results were obtained in the experiment in which
DBU was added in the presence of H2.
Protonation of 3-H-a. A solution of 3-H-a (0.008 g, 0.020 mmol,
in 0.6 mL of PhCl-d5) under H2 (1.0 atm) in an NMR tube was cooled
to −60 °C using a dry ice/CHCl3 slush bath. One equiv HBF4·Et2O
(30 μL of a suspension prepared by adding 0.1 mL of HBF4·Et2O to
PhF, then diluting to 1.00 mL) was added. The NMR tube was placed
in a NMR spectrometer precooled to −40 °C. A series of spectra (1H
NMR and 31P{1H} NMR) were recorded at −35, −25, −15, 0, 15, and
25 °C. Only [3-H2-a]+ was observed at −35 and −25 °C. The second
isomer, [3-H2-b]+ started to appear at −15 °C.
Attempted Chemical Oxidation of D2 Catalyzed by 1-H. 1-H
(0.006 g, 0.010 mmol) and 5 equiv of the oxidant, [Cp2Fe]PF6 (0.016
mg, 0.050 mmol) were loaded into a NMR tube under 1.0 atm D2.
Then 1.0 mL of PhF solution containing DBU (0.050 mmol) was
2
injected into NMR tube. The reaction solution was monitored by H
NMR spectroscopy for several hours but provided no indication of the
formation of [DBU-D+].
ASSOCIATED CONTENT
■
H2/D2 Scrambling of [1-H2]+ and [2-H2]+. A NMR tube was
loaded with 0.029 g (0.020 mmol) of 1-H2 (or 2-H2) under H2 (1.0
S
* Supporting Information
NMR data for 1-Cl, [1-CO]Cl, [1-H2]BArF , [2-H2]BArF , [2-
1
(HD)]BArF , 2-H, 3-Cl, [3-H2]BArF , and [3-H2]BArF4; IR
4
atm) in 0.6 mL of PhCl-d5. After the first H NMR spectrum was
recorded, the NMR tube was injected with D2 (1.0 atm, 3 mL) to fill
the headspace. D2 was mixed with the solution by shaking for 10 min.
4
4
4
spectrum of [1-CO]Cl, ESI-MS data of 1+-BArF ; electro-
4
1
chemical data for 1-Cl, [1-H2]BArF , 1+-BArF , 1-H, 2-Cl, 2-H,
The H2/D2 scrambling was monitored by H NMR over 12 h. The
4
4
formation of [1-HD]+ (or [2-HD]+) and HD were established by
observing a triplet at δ −12.71 (or −12.75 for [2-HD]+) and another
triplet at δ 4.22, respectively. In the case of [1-H2]+, a 2H NMR
3-Cl-a, and 3-H-a; X-ray structures and selected distances and
angles of 2-Cl and 2-H; structural comparison of the X-ray
structure and the DFT calculated structure of [1-H2]+ and 1-H;
DFT calculation on the intermolecular H2 cleavage by [1-H2]+;
crystallographic data and .cif files for 1-Cl, [1-CO]Cl, [1-
2
spectrum was recorded after 2 h. H NMR: δ −12.64 for [1-HD]+,
−12.90 for [1-D2]+.
H2/D2 Exchange of [3-H2]+ without External Base. Complex
[3-H2]+ (0.027 g, 0.020 mmol) under H2 (1.0 atm.) in 0.6 mL of
PhCl-d5 was prepared in an NMR tube. After the first 1H NMR
spectrum was recorded, the NMR tube was injected with D2 (1.0 atm,
3 mL) to fill the headspace. D2 was mixed with the solution by shaking
H2]BArF , 1-H, 2-Cl, 2-H, and 3-Cl-a; and complete ref 32.
4
This material is available free of charge via the Internet at
http://pubs.acs.org.
1
for 10 min. The H NMR and 31P{1H} spectra were monitored by
AUTHOR INFORMATION
Corresponding Author
*daniel.dubois@pnnl.gov; morris.bullock@pnnl.gov
■
NMR spectroscopy over 19 h. No H2/D2 scrambling occurred; [3-
D2]+ instead of [3-HD]+ was identified by 31P{1H} NMR spectros-
2
copy over time and an H NMR spectrum recorded at 15 h. 31P{1H}
NMR (PhCl-d5) for [3-H2]+: δ 85.47 (s) (minor isomer), 83.70 (s) (major
isomer); 31P{1H} NMR (PhCl-d5) for [3-D2]+: δ 85.40 (s) (minor
isomer), 83.58 (s) (major isomer). 2H NMR for [3-D2]+: δ −13.32 (minor
isomer, s, 2 D, Fe-D2), −14.08 (major isomer, s, 2 D, Fe-D2).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
H2/D2 Scrambling of [3-H2]+ in the Presence of an External
Base (Aniline or Benzylamine). Complex [3-H2]+ (0.027 g, 0.020
mmol) under H2 (1.0 atm.) in 0.6 mL of PhCl-d5 was prepared in an
NMR tube. After the first 1H NMR spectrum was recorded, the NMR
tube was injected with D2 (1.0 atm, 3 mL) to fill its headspace, and
subsequently, 2 equiv aniline (0.040 mmol, 40 μL, 1.0 M in PhF) or
benzylamine (0.040 mmol, 44 μL, 0.92 M in PhF) was added. The
mixture was shaken for 10 min, then H2/D2 scrambling was monitored
by 1H NMR spectroscopy over 15 h. Two triplets attributed to the two
isomers of Fe(HD) gradually grew in; HD was characterized as the
triplet at δ 4.33. 31P NMR spectra were recorded before adding D2, at
10 min and at 15 h after adding D2, respectively. In the case of aniline,
a 2H NMR spectrum was recorded after 15 h. 2H NMR for [3-HD]+: δ
−13.00 (minor isomer, s, 1 D, Fe−HD), −13.77 (major isomer, s, 1 D,
We thank the U.S. Department of Energy, Office of Basic
Energy Sciences, Division of Chemical Sciences, Geosciences
and Biosciences, for support of this research. S.C. (DFT
computations) and M.J.O. (NMR experiments) were supported
by the Center for Molecular Electrocatalysis, an Energy
Frontier Research Center funded by the U.S. Department of
Energy, Office of Science, Office of Basic Energy Sciences.
Pacific Northwest National Laboratory is operated by Battelle
for the U.S. Department of Energy.
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