Preparation, facile deprotonation, and rapid H/D exchange of the μ-hydride diiron model complexes of the [FeFe]-hydrogenase containing a pendant amine in a chelating diphosphine ligand

Article abstract of DOI:10.1021/ic901154m

The CO-displacement of [(μ-pdt)Fe₂(CO)₆] with (Ph₂PCH₂)₂N(n-Pr) in refluxing toluene gave an unsymmetrical chelating complex [(7μ-pdt){Fe(CO)₃}{Fe(CO) (κ²-Ph₂PCH₂

Full text of DOI:10.1021/ic901154m

Inorg. Chem. 2009, 48, 11551–11558 11551
DOI: 10.1021/ic901154m
Preparation, Facile Deprotonation, and Rapid H/D Exchange of the μ-Hydride Diiron
Model Complexes of the [FeFe]-Hydrogenase Containing a Pendant Amine in a
Chelating Diphosphine Ligand
Ning Wang,† Mei Wang,*,† Jihong Liu,† Kun Jin,† Lin Chen,† and Licheng Sun*,†,‡
†State Key Laboratory of Fine Chemicals, DUT-KTH Joint Education and Research Center on Molecular
‡
Devices, Dalian University of Technology (DUT), Dalian 116012, China, and Department of Chemistry, Royal
Institute of Technology (KTH), 10044, Stockholm, Sweden
Received June 16, 2009
The CO-displacement of [(μ-pdt)Fe2(CO)6] with (Ph2PCH2)2N(n-Pr) in refluxing toluene gave an unsymmetrical
chelating complex [(μ-pdt){Fe(CO)3}{Fe(CO)(κ2-Ph2PCH2N(n-Pr)CH2PPh2}] (1) as a major product, together with
a small amount of the symmetrical intramolecular bridging complex [(μ-pdt){μ-Ph2PCH2N(n-Pr)CH2PPh2}{Fe-
(CO)2}2] (2) and the intermolecular bridging complex [{μ,κ1,κ1-Ph2PCH2N(n-Pr)CH2PPh2}{(μ-pdt)Fe2(CO)5}2]
(3). In contrast, the reaction of [(μ-pdt)Fe2(CO)6] with (Ph2PCH2)2NR (R=n-Pr, Ph) afforded the intermolecular
bridging isomers 3 and 4 in the presence of a CO-removing reagent Me3NO 2H2O in CH3CN at room temperature.
The molecular structures of 1, 3, and 4, as well as the doubly protonated complex [1(HNHμ)](OTf)2] were determined
3
by X-ray analyses. The protonation processes of 1 with HBF4 Et2O and HOTf were studied in different solvents. The
3
presence of the Hμ HN interaction in [1(HNHμ)]2þ was studied by relaxation time T1 and spin saturation transfer
3 3 3
measurements. The μ-hydride of [1(Hμ)]þ and [1(HNHμ)]2þ undergo facile deprotonation with aniline and rapid H/D
exchange with deuterons in solution. In contrast, neither deprotonation nor H/D exchange was detected for [(μ-H)-
(μ-pdt){Fe(CO)3}{Fe(CO)(κ2-dppp)}]þ ([5(Hμ)]þ, dppp=Ph2PCH2CH2CH2PPh2) without internal base.
Introduction
ing a pendant basic site,5-9 it is proposed that the internal
base near the iron centers could act as a proton transfer relay
to provide a low energy pathway for H-H bond heterolytic
cleavage and formation at the [FeFe]-H2ase active site.
Therefore, the preparation and the related chemistry of
diiron dithiolate complexes containing internal bases have
drawn intense attention for better understanding the
mechanism of enzymatic hydrogen evolution. In recent years,
protophilicity of the bridging-N atom of diiron azadithio-
late complexes has been well-studied.10-13 Very recently,
Rauchfuss and co-workers reported the great influence of
the bridging X (X=NH, O, CH2) group of the dithiolate
bridge on the protonation and deprotonation of the diiron
model complexes.14 Lubitz and co-workers have reported
some strong experimental evidence from advanced electron
paramagnetic resonance (EPR) studies, showing that the
X-ray crystal structures show that the active site of [FeFe]-
hydrogenases ([FeFe]-H2ases) is composed of a [4Fe4S]
cluster linked through a cysteine residue to the [2Fe2S]
subcluster, in which the two iron atoms are bridged by a
dithiolate cofactor.1,2 Some spectroscopic and theoretical
studies suggest that the dithiolate bridge is an azadithiolate
(adt, (SCH2)2NH) or an oxadithiolate (odt, (SCH2)2O).3,4
One interesting issue is the function of this internal base,
either the bridging-N or -O atom, in the [FeFe]-H2ase active
site. On the basis of density functional theory (DFT) calcula-
tions and in the light of numerous examples for heterolytic H2
activation mediated by transition metal complexes contain-
*To whom correspondence should be addressed. E-mail: symbueno@
dlut.edu.cn (M.W.); lichengs@kth.se (L.S.).
(1) Peters, J. W.; Lanzilotta, W. N.; Lemon, B. J.; Seefeldt, L. C. Science
1998, 282, 1853–1858.
(2) Nicolet, Y.; Piras, C.; Legrand, P.; Hatchikian, C. E.; Fontecilla-
Camps, J. C. Structure 1999, 7, 13–23.
(9) Yang, J. Y.; Morris Bullock, R.; Shaw, W. J.; Twamley, B.; Rakowski
DuBois, M.; DuBois, D. L. J. Am. Chem. Soc. 2009, 131, 5935–5945.
ꢀ
(10) Lawrence, J. D.; Li, H.; Rauchfuss, T. B.; Benard, M.; Rohmer, M.-
(3) Nicolet, Y.; Lacey, A. L.; Vernede, X.; Fernandez, V. M.; Hatchikian,
E. C.; Fontecilla-Camps, J. C. J. Am. Chem. Soc. 2001, 123, 1596–1602.
(4) Pandey, A. S.; Harris, T. V.; Giles, L. J.; Peters, J. W.; Szilagyi, R. K.
J. Am. Chem. Soc. 2008, 130, 4533–4540.
(5) Fan, H.-J.; Hall, M. B. J. Am. Chem. Soc. 2001, 123, 3828–3829.
(6) Custelcean, R.; Jackson, J. E. Chem. Rev. 2001, 101, 1963–1980.
(7) Rakowski DuBois, M.; DuBois, D. L. Chem. Soc. Rev. 2009, 38, 62–72.
(8) Ishiwata, K.; Kuwata, S.; Ikariya, T. J. Am. Chem. Soc. 2009, 131,
5001–5009.
M. Angew. Chem., Int. Ed. 2001, 40, 1768–1771.
(11) Barton, B. E.; Rauchfuss, T. B. Inorg. Chem. 2008, 47, 2261–2263.
(12) Schwartz, L.; Eilers, G.; Eriksson, L.; Gogoll, A.; Lomoth, R.; Ott, S.
Chem. Commun. 2006, 520–522.
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r
2009 American Chemical Society
Published on Web 11/24/2009
pubs.acs.org/IC

11552 Inorganic Chemistry, Vol. 48, No. 24, 2009
Wang et al.
central atom of the bridging ligand in the active site of FeFe-
hydrogenases is indeed a nitrogen atom.15
Scheme 1
In addition to the [FeFe] complexes with bridging aza-
dithiolate ligands, DuBois and co-workers has demonstrated
that the tertiary amine in the chelating diphosphine ligand
(R2PCH2)2NR0 (PNP) of mononuclear iron and nickel com-
plexes significantly accelerates both H2 uptake and
production.16-18 Similarly, a recent report by Talarmin and
co-workers describes the protonation processes of [(μ-pdt)-
{Fe(CO)3}{Fe(CO)(κ2-Ph2PCH2N(Me)CH2PPh2)}] (pdt =
propane-1,3-dithiolate) with HBF4 Et2O in acetone and
3
CH2Cl2, respectively, indicating that the great effect of sol-
vents can even switch the N-protonated product to the μ-
hydride. These reports show that the pendant amine of such a
chelating diphosphine ligand can play a similar role as the N
atom in the active site of the [FeFe] hydrogenase enzyme.19
Ourstudiesdescribedin this paper provide moreimportant
evidence for the function of the pendant amine in the dipho-
sphine ligand of the diiron complex as relays for proton
transfer to and from the iron centers. Here we report the CO-
displacement of [(μ-pdt)Fe2(CO)6] with (Ph2PCH2)2N(R)
(PNP, R = n-Pr, Ph) under different conditions for selec-
tive preparation of the unsymmetrical chelating complex [(μ-
pdt){Fe(CO)3}{Fe(CO)(κ2-Ph2PCH2N(n-Pr)CH2PPh2)}] (1)
and the intermolecular bridging complex [(μ,κ1,κ1-
Ph2PCH2N(R)CH2PPh2){(μ-pdt)Fe2(CO)5}2] (3, R = n-Pr;
(1) was obtained as a major product, together with ca. 3%
of the symmetrical intramolecular bridging complex [(μ-
pdt)(μ-Ph2PCH2N(n-Pr)CH2PPh2){Fe(CO)2}2] (2) and
15% of the intermolecular bridging complex [{(μ-
pdt)Fe2(CO)5}2(μ,κ1,κ1-Ph2PCH2N(n-Pr)CH2PPh2)] (3).
In the presence of a CO-removing reagent Me3-
NO 2H2O, the reaction of [(μ-pdt)Fe2(CO)6] and
3
(Ph2PCH2)2N(n-Pr) in CH3CN at room temperature for
3 h gave complex 3 in ca. 65% yield.22 Therefore, the
unsymmetrical chelating complex 1 and the intermole-
cular bridging complex 3 can be selectively prepared from
the CO-displacement of [(μ-pdt)Fe2(CO)6] by controlling
the reaction condition. When ligand (Ph2PCH2)2N(Ph)
was used for the CO-displacement of [(μ-pdt)Fe2(CO)6],
the intermolecular bridging complex [{(μ-pdt)Fe2-
(CO)5}2(μ,κ1,κ1-Ph2PCH2N(Ph)CH2PPh2)] (4) was the
sole product either in refluxing toluene or in the presence
4, Ph). The protonation processes of 1 with HBF4 Et2O and
3
HOTf as the proton source, the facile deprotonation of the
μ-hydride of [1(Hμ)]þ and [1(HNHμ)]2þ in the presence of
weak bases, the rapid H/D exchange of the μ-hydride of
[1(Hμ)]þ and [1(HNHμ)]2þ with deuterons in solution, and the
crystal structures of 1, [1(HNHμ)](OTf)2,20 3, and 4 are
described in this paper. These systems are compared to an
analogue system without internal base. The activity of the
μ-hydride of [1(Hμ)]þ and [1(HNHμ)]2þ containing a pendant
amine in the chelating diphosphine ligand contrasts with the
relative inertness of the μ-hydride in the analogous complex
[(μ-H)(μ-pdt){Fe(CO)3}{Fe(CO)(κ2-dppp)}] [5(Hμ)]þ and
other previously reported diiron dithiolate complexes toward
organic bases.13,14,21,22
of Me3NO 2H2O in CH3CN at room temperature
3
(Scheme 2), indicating that the group on the N atom
has a great influence on the coordination style of the PNP
ligand. Selective preparation of these coordination iso-
mers and the chemistry of unsymmetric diphosphine
diiron complexes are of particular interest as the unsym-
metric [2Fe2S] complexes are attractive functional mod-
els of the [FeFe]-hydrogenase active site.
Complex 1 was doubly protonated in diethyl ether
upon addition of 3 equiv of HOTf. The proton-hydride
diiron complex [1(HNHμ)](OTf)2 was obtained as a pur-
ple crystalline solid in good yield after washing with
diethyl ether and recrystallized in hexane/CH2Cl2. Com-
plex 1 is stable in the solid state and in solution under an
N2 atmosphere, while its doubly protonated species
[1(HNHμ)](OTf)2 is very sensitive to moisture.
Complexes 1-4 and [1(HNHμ)](OTf)2 were character-
ized by MS, IR, 1H and 31P NMR spectroscopy, and 1, 3,
4, and [1(HNHμ)](OTf)2 were also identified by elemental
analysis. The IR data of the ν(CO) bands of 1-4 are listed
in Table 1 together with the ν(CO) data of the reference
complex [(μ-pdt){Fe(CO)3}{Fe(CO)(κ2-dppp)}] (5; dppp=
Ph2PCH2CH2CH2PPh2).22 The band at 1894 cm-1 for
1 is attributed to the vibration of the CO ligand in the
Fe(CO)(κ2-Ph2PCH2N(n-Pr)CH2PPh2) unit of 1, while the
bands at 1948 and 2020 cm-1 are assigned to the CO
vibrations of the Fe(CO)3 unit.23 A comparison of the
Results and Discussion
CO-Displacement of [(μ-pdt)Fe2(CO)6] by PNP Li-
gands and Spectroscopic Characterization of Complexes
1-4. The products of three coordination modes were
afforded by the CO-displacement of [(μ-pdt)Fe2(CO)6]
with (Ph2PCH2)2N(n-Pr) in refluxing toluene for 3 h
(Scheme 1). The unsymmetrical chelating complex [(μ-
pdt){Fe(CO)3}{Fe(CO)(κ2-Ph2PCH2N(n-Pr)CH2PPh2)}]
(15) Silakov, A.; Wenk, B.; Reijerse, E.; Lubitz, W. Phys. Chem. Chem.
Phys. 2009, 6592–6599.
(16) Curtis, C. J.; Miedaner, A.; Ciancanelli, R. F.; Ellis, W. W.; Noll, B.
C.; Rakowski DuBois, M.; DuBois, D. L. Inorg. Chem. 2003, 42, 216–227.
(17) Henry, R. M.; Shoemaker, R. K.; Newell, R. H.; Jacobsen, G. M.;
DuBois, D. L.; Rakowski DuBois, M. Organometallics 2005, 24, 2481–2491.
(18) Henry, R. M.; Shoemaker, R. K.; DuBois, D. L.; DuBois, M. R. J.
Am. Chem. Soc. 2006, 128, 3002–3010.
ꢀ
(19) Ezzaher, S.; Capon, J.-F.; Gloaguen, F.; Petillon, F.; Schollhammer,
P.; Talarmin, J. Inorg. Chem. 2009, 48, 2–4.
(20) Wang, N.; Wang, M.; Zhang, T.; Li, P.; Liu, J.; Sun, L. Chem.
Commun. 2008, 5800–5802.
(21) Gloaguen, F.; Lawrence, J. D.; Rauchfuss, T. B. J. Am. Chem. Soc.
2001, 123, 9476–9477.
(22) Adam, F. I.; Hogarth, G.; Kabir, S. E.; Richards, I. C. R. Chim. 2008,
11, 890–905.
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Article
Inorganic Chemistry, Vol. 48, No. 24, 2009 11553
Scheme 2
Table 1. Comparison of ν(CO) Bands of 1-5
complex
ν(CO) (cm-1) in CH2Cl2
1
2
3
4
5
2020, 1948, 1894
1981, 1943, 1909
2040, 1973, 1924
2041, 1975, 1924
2019, 1946, 1890
ν(CO) data of 1 and 2 shows that the frequency of the first
ν(CO) band of 1 is apparently higher than that of the
corresponding band displayed by the symmetrically disub-
stituted diiron complex 2, whereas the frequency of the last
ν(CO) band of 1 is lower than the corresponding ν(CO)
frequency of 2. Complexes 3 and 4 display similar ν(CO)
data in the range of 1924-2041 cm-1. A comparison of the
ν(CO) absorptions of 1 and 5 shows that the electron
density of their iron centers is similar.
Molecular Structures of 1, 3, 4, and [1(HNHμ)](OTf)2.
The molecular structures of 1, 3, 4, and [1(HNHμ)](OTf)2
are determined by X-ray diffraction studies of single
crystals, which are shown in Figure 1 as ball and stick
drawings. Selected bond lengths and angles for 1, 3, 4, and
[1(HNHμ)](OTf)2 are listed in Table 2.
The central [2Fe2S] structure of complex 1 has a
butterfly framework, and each iron atom is coordinated
with a pseudosquare-pyramidal geometry. The iron
atoms of complex [1(HNHμ)](OTf)2 are coordinated with
a distorted octahedral geometry as previously reported
μ-hydride diiron dithiolate models.12,13 In the solid state,
the (Ph2PCH2)2N(n-Pr) ligand lies in the apical/basal
configuration at the Fe(2) atom of 1, while the orientation
of the two phosphorus atoms in the crystal of
[1(HNHμ)](OTf)2 changes to a basal/basal position. The
different coordination modes lead to the single bond
rotation of the Fe(2)PCNCP cyclohexane ring. The Fe(2)
atom points down and the N atom is up in the chair
conformation of 1, while a rotated chair conformation is
observed for [1(HNHμ)]2þ with the hydrogen attached to
the N atom on the axial bond. The Fe(2)-C(4) bonds of
Figure 1. Molecular structures of 1, 3, 4, and [1(HNHμ)](OTf)2 as ball
and stick drawings.
phosphorus atom to the Fe center in a basal position.24
˚
The Fe-Fe distances are 2.5202(11) and 2.5129(7) A in 3
and 4, respectively.
Protonation and Deprotonation Processes of 1 and 5. To
explore the influence of solvents and acids on the forma-
tion of [1(Hμ)]þ and [1(HN)]þ, we first studied the pro-
tonation of 1 with HOTf in different solvents (d6-acetone
and CD2Cl2). As 1 equiv of HOTf was dropped into the
d6-acetone solution of 1, three new signals at δ 37.3,
57.7, and 58.4 ppm appeared in the 31P NMR spectrum
(Figure 2b) in addition to the 31P NMR signal of 1 at δ
52.5 ppm (Figure 2a).20 The 31P NMR signals at δ 57.7
and 58.4 ppm are attributed to the HN-endo/exo isomers
of [1(HN)]þ, and the 31P NMR signal at δ 37.3 ppm is
assigned to the phosphorus atoms of the μ-hydride com-
plex [1(Hμ)]þ. Further addition of HOTf up to 3 equiv
2þ
˚
˚
1.740(4) A for 1 and 1.737(4) A for [1(HNHμ)] are
notably shorter than the three Fe(1)-C bonds
˚
1.781-1.810 A. Accordingly, the C(4)-O(4) distance is
longer than the C-O distances in the Fe(CO)3 unit of 1
and [1(HNHμ)]2þ. A reasonable explanation is that the
higher electron density of the Fe(2) atom leads to the
stronger electron back-donation to the C(4)O(4) as com-
pared to the back-donation from the Fe(1) center to the
CO ligands. After protonation at the iron atoms, the
˚
Fe-Fe distance is increased by 0.056 A and the HN Hμ
distance is 3.934 A in the molecule of the doubly proto-
nated species [1(HNHμ)](OTf)2.
3 3 3
˚
The main framework of complexes 3 and 4 are very
similar. Each end of the zigzag P-C-N-C-P bridge is
linked with a [2Fe2S] butterfly unit by coordination of the
(24) Gao, W.; Ekstrom, J.; Liu, J.; Chen, C.; Eriksson, L.; Weng, L.;
Akermark, B.; Sun, L. Inorg. Chem. 2007, 46, 1981–1991.
˚

11554 Inorganic Chemistry, Vol. 48, No. 24, 2009
Wang et al.
Figure 2. Selected regions of the 31P{1H} (left) and 1H (right) NMR
spectra of 1 in d6-acetone with (a) þ0 equiv, (b) þ1 equiv HOTf, and
(c) þ2 equiv HOTf to that in part b at 298 K.
Figure 4. Selected region of the 31P{1H} NMR of 1 in CD2Cl2 with
(a) þ0 equiv, (b) þ2 equiv HBF4 Et2O, (c) þ5 equiv HBF4 Et2O to that
3
3
in part b, (d) þ5 equiv aniline to that in part c, and (e) þ2 equiv aniline to
that in part d at 298 K.
report by Talarmin and co-workers.19 The results indicate
that, in addition to the solvent, the strength of the acid
used also has a certain influence on the formation of the
singly protonated species of 1. Further addition of
Figure 3. Selected regions of the 31P{1H} (left) and 1H (right) NMR
spectra of 1 in CD2Cl2 with (a) þ0 equiv, (b) þ1 equiv HOTf, and (c) þ2
equiv HOTf to that in part b at 298 K.
HBF4 Et2O up to 7 equiv results in immediate disap-
˚
Table 2. Selected Bond Lengths (A) and Angles (deg) for 1, 3, 4, and
[1(HNHμ)](OTf)2
3
pearance of the 31P NMR signal for [1(Hμ)]þ and simul-
taneous appearance of a broad 31P NMR signal at
δ 45.5 ppm (Figure 4c), which is ascribed to the doubly
protonated complex [1(HNHμ)]2þ (Scheme 3).
1
[1(HNHμ)]2þ
3
4
Fe(1)-Fe(2)
Fe(1)-S(1)
Fe(1)-S(2)
Fe(2)-S(1)
Fe(2)-S(2)
Fe-P(1)
2.5564(7)
2.6121(16)
2.5202(11) 2.5129(7)
2.2518(17) 2.2603(10)
2.2472(16) 2.2554(11)
2.2647(18) 2.2685(10)
2.2547(16) 2.2628(10)
2.2395(16) 2.2324(9)
2.2244(9)
It is noticeable a broad 31P NMR signal for the two
2.2594(10) 2.2497(16)
2.2643(11) 2.2527(16)
2.2525(10) 2.2595(16)
2.2592(10) 2.2500(16)
2.2132(11) 2.2182(17)
2.1981(10) 2.2313(15)
isomers appears in Figure 4c when HBF4 Et2O is used at
3
room temperature in CD2Cl2. In contrast, when HOTf is
used as a proton source under the same condition, two
sharp 31P NMR signals for the endo and exo isomers of
[1(HNHμ)]2þ are observed (Figure 3c), indicating that the
interconversion of the endo and exo isomers is slow. The
Fe-P(2)
Fe(1)-Cav
Fe(2)-Cav
Fe-H(100)av
N(1)-H(101)
Fe-S-Fe
1.785
1.740(4)
1.806
1.737(4)
1.659
0.893(18)
70.87
84.11
95.82(6)
113.85(4)
113.01(5)
1.791
1.757
1.788
1.771
-
result implies that the BF4 anion could assist inter-
molecular proton transfer.18 The interconversion of the
endo and exo isomers can be speeded up as temperature is
increased. The two 31P NMR signals gradually merge into
a relatively broad signal (see Supporting Information
Figure S1) with increase of the temperature from 25 to
70 °C in CD3CN, which can only occur by intermolecular
proton exchange.
68.92
84.62
67.96
84.56
67.45
84.71
S-Fe-S
P-Fe(2)-P
Fe-Fe-P(1)
Fe-Fe-P(2)
92.51(4)
109.54(3)
154.00(4)
153.02(6)
155.69
results in complete disappearance of the 31P NMR signals
at δ 37.3, 52.5, 57.7, and 58.4 ppm for [1(Hμ)]þ, 1, and the
endo/exo isomers of [1(HN)]þ, respectively, accompanied
with the appearance of two new signals at δ 45.1 and
46.2 ppm ascribed to the doubly protonated species
[1(HNHμ)]2þ (Figure 2c).
To find the influence of the internal amine of the PNP
ligand on the protonation of the diiron complexes, a
comparative study of the reference complex [(μ-pdt){Fe-
(CO)3}{Fe(CO)(κ2-dppp)}] (5) without internal base in
the chelating diphosphine ligand was made in parallel
For the protonation of 1 in the presence of 1 equiv of
HOTf in CD2Cl2, the [1(Hμ)]þ was formed as a dominant
with complex 1. With addition of 7 equiv of HBF4 Et2O
to the CD2Cl2 solution of 5, the signals for the μ-hydride
3
product contaminated with [1(HN)]þ as assayed by H
1
and 31P NMR spectroscopy (Figure 3b). Upon addition
of HOTf up to 3 equiv, the HN-endo/exo isomers of the
doubly protonated species were formed (Figure 3c). The
results show that the formation of [1(HN)](OTf) is pre-
ferred in acetone over methylene chloride.
1
species of 5 are not observed in the H and 31P NMR
spectra. Complex 5 can be protonated by 1.5 equiv
of HOTf or by a large excess of HBF4 Et2O to form
3
the μ-hydride diiron complex [5(Hμ)]þ (structure C of
Figure 5).22
Then we studied the protonation of 1 with HBF4 Et2O
in CD2Cl2. When 2 equiv of HBF4 Et2O was added to the
Formation of the μ-hydride on the Fe-Fe center makes
the first and the last ν(CO) absorptions of 1 shift by 77
and 66 cm-1 to higher energy (Table 3), respectively. For
the further protonation at the bridged N atom of the PNP
ligand, the shift of 8 cm-1 for the first ν(CO) band of
[1(Hμ)]þ is identical to the shift reported for the proton-
ation at the N atom of [(μ-pdt){Fe(CO)3}{Fe(CO)-
(κ2-Ph2PCH2N(Me)CH2PPh2)}] (6).19 As the similarity
3
3
CD2Cl2 solution of 1, the only new signal at δ 38.0 ppm in
the 31P NMR spectrum (Figure 4b) and the signal at
1
δ -13.0 ppm in the H NMR spectrum show that the
μ-hydride diiron complex [1(Hμ)]þ is formed instantly
and quantitatively,11-14 and the N-protonated species
[1(HN)]þ is not detected. This is in agreement with the

Article
Inorganic Chemistry, Vol. 48, No. 24, 2009 11555
Scheme 3
Table 3. Comparison of ν(CO) Bands of [1(Hμ)]BF4, [1(HNHμ)](OTf)2, and
Analogous Protonated Diiron Complexes
ν(CO) in CD2Cl2
(cm-1
Δν(CO)first
(cm-1 a
Δν(CO)last
(cm-1 b
complex
)
)
)
[1(Hμ)]BF4 2097, 2037, 2003, 1960
[1(HNHμ)](OTf)2 2105, 2057, 2043, 1990
77
85
77
79
66
96
66
68
[5(Hμ)]OTf
2095, 2040, 2003, 1956
2099, 2052, 2036, 1961
c
Figure 5. Structures of the proposed [2Fe2S] subcluster with a t-hydride
(A, t = terminal), the μ-hydride diiron mimic [1(HNHμ)]2þ (B), and the
μ-hydride complex [5(Hμ)]þ (C).
[6(Hμ)]BF4
a Δν(CO)first = ν(CO)(first)protonated - ν(CO)(first)nonprotonated
.
b Δν-
(CO)1ast = ν(CO)(last)protonated - ν(CO)(last)nonprotonated
.
c Reference 19.
[1(HNHμ)]2þ is supported by relaxation time T1. The T1
for the hydride signal of [1(Hμ)]þ at δ -13.0 ppm was
measured in CD2Cl2 at room temperature and was found
to be 1243 ms. In comparison, the T1 values for the two
hydride signals at δ -13.6 and -13.9 ppm for the endo
and exo isomers of [1(HNHμ)]2þ, respectively, were found
to be 724 and 963 ms. The shorter relaxation time T1
(724 ms) for the HN-endo isomer than 1243 and 963 ms
for [1(Hμ)]þ and the HN-exo isomer, respectively, suggests
some degree of hydrogen-bonding interaction between
of the ν(CO) bands for complexes 1 and 5, the CO
vibrations of [1(Hμ)]þ are quite close to those of [5(Hμ)]þ.
As 5 equiv of aniline was added to the CD2Cl2 solution
of [1(HNHμ)]2þ, the proton at the amine N atom was
selectively deprotonated to regenerate [1(Hμ)]þ quantita-
tively (Figure 4d). The μ-hydride [1(Hμ)]þ was further
deprotonated to the parent complex 1 when another 2
equiv of aniline was successively added (Figure 4e,
Scheme 3). It is interesting to note that the μ-hydride of
[1(Hμ)]þ is so active that even a small amount of water
results in the immediate deprotonation to form 1. In a
striking contrast, upon addition of 20 equiv of aniline to
the CD2Cl2 solution of [5(Hμ)]þ, no deprotonated com-
plex was detected during 5 h by the in situ IR and
31P NMR.21 In terms of the close similarity of the IR
spectra in the ν(CO) region for [1(Hμ)]þ and [5(Hμ)]þ
(Table 3), the electron density of their iron centers should
be quite similar. The disparate activity of [1(Hμ)]þ from
[5(Hμ)]þ is undoubtedly caused by the presence of an
internal amine in the diphosphine ligand and more im-
portantly by the short distance between the hydride and
the internal base in [1(Hμ)]þ. It is reported that the
terminal hydride in [Fe2(t-H)(μ-adt)(CO)2(dppv)2]þ
(dppv=cis-1,2-bis(diphenylphosphino)ethylene) was im-
mediately deprotonated with PBu3 even at -90 °C, while
its analogue [Fe2(t-H)(μ-pdt)(CO)2(dppv)2] without in-
ternal base could not be deprotonated.14 If we compare
the structure of [1(Hμ)]þ (structure B of Figure 5) with
that of the terminal hydride in a diiron azadithiolate
complex (structure A of Figure 5), we can find a con-
formation of the (μ-H)FePCN unit in [1(Hμ)]þ similar to
the (t-H)FeSCN unit in the t-hydride diiron azadithiolate
model complexes. By single bond rotation, the μ-hydride
of [1(Hμ)]þ can stand proximal to the amine N atom in the
chelating diphosphine ligand, just like a t-hydride located
near the amine N atom in the azadithiolate bridge. This
could be a rational explanation for the similar deprotona-
tion reactivity of the μ-hydride in [1(Hμ)]þ with that of the
t-hydride in the diiron azadithiolate complexes.
the hydride (Hμ) and the proton (HN) in [1(HNHμ)]2þ 25
.
In addition, it was observed that irradiation of the
hydride signal at δ -13.6 ppm led to an approximately
40% decrease in the intensity of the HN-endo signal at
1
δ 9.48 ppm in H NMR at -70 °C in d6-acetone, while
irradiation of the hydride signal at δ -13.9 ppm under the
same condition could not result in any decrease of the
intensity of the HN-exo signal at δ 10.45 ppm. The spin
saturation transfer experiment gives another evidence for
the Hμ HN interaction in [1(HNHμ)]2þ
.
3 3 3
The catalytic activity for H/D exchange is one of the
important properties of hydrogenases.26,27 The previous
reported H/D exchange reactions via the μ-hydrides
diiron model complexes without pendant bases require
a preformed open site on the Fe center created by photo-
lysis, and the rate of the H/D exchange is quite slow.28-30
The deprotonation activity of the μ-hydride derived from
1 prompted us to study the H/D exchange of [1(Hμ)]þ and
[1(HNHμ)]2þ with deuterons in solution. The protonated
complexes [1(Hμ)](BF4) and [1(HNHμ)](OTf)2 were pre-
pared respectively by the reaction of 1 with HBF4 Et2O
3
or HOTf. Since [1(Hμ)]þ and [1(HNHμ)]2þ are completely
deprotonated in the presence of water, we chose a weak
(25) Chu, H. S.; Lau, C. P.; Wong, K. Y. Organometallics 1998, 17, 2768–
2777.
(26) Albracht, S. P. J. Biochim. Biophys. Acta 1994, 1188, 167–204.
(27) Frey, M. ChemBioChem 2002, 3, 153–160.
(28) Zhao, X.; Georgakaki, I. P.; Miller, M. L.; Mejia-Rodriguez, R.;
Chiang, C. Y.; Darensbourg, M. Y. Inorg. Chem. 2002, 41, 3917–3928.
(29) Zhao, X.; Georgakaki, I. P.; Miller, M. L.; Yarbrough, J. C.;
Darensbourg, M. Y. J. Am. Chem. Soc. 2001, 123, 9710–9711.
(30) Georgakaki, I. P.; Miller, M. L.; Darensbourg, M. Y. Inorg. Chem.
2003, 42, 2489–2494.
Studies on the Hμ HN Interaction and the H/D Ex-
change. The presence of an Hμ HN interaction in
3 3 3
3 3 3

11556 Inorganic Chemistry, Vol. 48, No. 24, 2009
Wang et al.
Scheme 4
Figure 6. Comparison of the Structures of [(μ-H)Fe(μ-pdt)(CO)-
{Ph2PCH2N(n-Pr)CH2PPh2}]2þ unit (D) in [1(HNHμ)]2þ and the DuBois’
complex trans-[HFe(dmpm)(CO){Et2PCH2N(CH3)CH2PEt2}]2þ (E).
deuterons in solution presumably involves a rapid inter-
molecular H/D exchange between the protonated built-in
ammonium and CH3COOD followed by a fast intra-
molecular proton/deuteride swap. As the N atom of the
Ph2PCH2N(n-Pr)CH2PPh2 ligand cannot be protonated
by acetic acid in CH2Cl2, the observed H/D exchange of
the μ-hydride of [1(Hμ)]þ may involve an electrostatic
interaction of the pendant amine and the deuteron of
CH3COOD in the initial stage. Several examples of
intramolecular proton/hydride swap in pendant amine-
or pyridine-containing complexes have been reported
previously.6,18 A dihydrogen intermediate is proposed
for intramolecular proton/hydride exchange in trans-
[HFe(PNHP)(dmpm)(CO)]2þ (PNP=Et2PCH2N(CH3)-
CH2PEt2, dmpm=Me2PCH2PMe2).18 The intermediate
in our case is still not clear up to now. The dihydrogen
intermediate is preferred in the light of the close similarity
in the coordination structure of the [(μ-H)Fe(μ-pdt)(CO)-
{Ph2PCH2N(n-Pr)CH2PPh2}]2þ unit (structure D of
Figure 6), if the Fe(CO)3 moiety in [1(HNHμ)]2þ is ignored,
with that of the complex trans-[HFe(dmpm)(CO){Et2-
PCH2N(CH3)CH2PEt2}]2þ (structure E of Figure 6).18
acid, CH3COOD, as a deuteron source for H/D exchange
reaction. The H/D exchange reactions of [1(Hμ)](BF4)
and [1(HNHμ)](OTf)2 with CH3COOD in CH2Cl2 were
1
2
monitored by H and H NMR spectroscopy at room
temperature. Upon addition of 10 equiv of CH3CO-
OD to the CH2Cl2 solutions of [1(Hμ)](BF4) and [1(HN-
Hμ)](OTf)2, respectively, the signals at δ -13.0 ppm for
the μ-hydride of [1(Hμ)]þ and δ -13.6 and -13.9 ppm for
[1(HNHμ)]2þ disappeared instantly in the 1H NMR spec-
tra. In the meantime, a signal at high field appeared in
2
each H NMR spectrum, which are attributed to the
μ-deuterides of [1(Dμ)]þ and [1(DNDμ)]2þ. In addition,
the broad signal at δ = 13.0 ppm is ascribed to the
D-enriched CH3COOD, which overlaps the signal of
the deuteron on N atom in [1(DNDμ)]2þ. The spectro-
scopic evidence shows that the quick intermolecular H/D
exchange does occur between the μ-hydride from [1(Hμ)]þ
or [1(HNHμ)]2þ and the deuteron from CH3COOD in
solution.
Conclusions
The unsymmetrical chelating complex 1 and the intermo-
lecular bridging complex 3 can be selectively prepared from
the CO-displacement of [(μ-pdt)Fe2(CO)6] with (Ph2PC-
H2)2N(n-Pr) by controlling the reaction condition. The
doubly protonated diiron complex [1(HNHμ)](OTf)2 was
successfully isolated and crystallographically characterþized,
To probe the role of the pendant amine of the diiron
dithiolate models in the intermolecular H/D exchange,
the control reaction was made with the in situ formed
complex [5(Hμ)](OTf), which was characterized by IR,
1H, and 31P{1H} NMR spectroscopy (see the Experimen-
tal Section).22 After addition of 10 equiv of CH3COOD to
the solution of [5(Hμ)](OTf), the high-field signal at
δ -12.8 ppm for [5(Hμ)]þ did not show observable change
during 5 h of monitoring by 1H NMR, and except for the
broad signal at δ 13.0 ppm for CH3COOD, no other
signal appeared in the 2H NMR spectrum. The inertness
of the μ-hydride in [5(Hμ)]þ toward H/D exchange un-
doubtedly indicates the crucial role of the pendant amine
proximal to the μ-hydride in the rapid intermolecular H/
D exchange of the μ-hydride in [1(Hμ)]þ and [1(HNHμ)]2þ
with deuterons in solution. To the best of our knowledge,
the rapid H/D exchange has not been reported for the
μ-hydride in the diiron dithiolate complexes.
-
which shows the hydride Hμ and the proton HN in
[1(HNHμ)]2þ are close to each other (with a distance of
˚
3.934 A). The relaxation time T1 and the results obtained
from spin saturation transfer measurements support the
presence of an Hμ HN interaction in [1(HNHμ)]2þ. The
3 3 3
facile deprotonation and rapid H/D exchange of the
μ-hydride in the diiron complexes [1(Hμ)]þ and [1(HNHμ)]2þ
with deuterons in solution provide an effective model of the
active μ-hydride for the [FeFe]-H2ase active site. The results
obtained from comparative studies of analogous complexes
[1(Hμ)]þ and [5(Hμ)]þ suggest that the built-in amine base in
the flexible PNP ligand is capable of serving as an efficient
proton transfer relay in the diiron μ-hydride complex. The
built-in amine in the PNP ligand may approach the μ-hydride
by the conformational change and make the μ-hydride in the
diiron complex so active as to be deprotonated by weak bases
and to occur via rapid H/D exchange with deuterons in
solution, while the amine unit in the adt-bridge, being in the
opposite position of the μ-hydride, can hardly influence
the reactivity of the μ-hydride of the diiron dithiolate com-
plex.13,14,21,22 These results provide a new active model for the
Because H/D exchange was not observed for the pro-
tonated reference complex [5(Hμ)]þ in the presence of
CH3COOD, the mechanism by direct exchange of
the μ-hydride with deuterons in solution can be exclu-
ded. A proposed process for H/D exchange between
[1(HNHμ)]2þ and CH3COOD is shown in Scheme 4. The
observed exchange of the μ-hydride of [1(HNHμ)]2þ with

Article
Inorganic Chemistry, Vol. 48, No. 24, 2009 11557
proton transfer either to or from the iron center at the [2Fe2S]
subcluster of the [FeFe]-H2ase active site.
Synthesis of [{Fe2(CO)5(μ-pdt)}2(μ,K1,K1-Ph2PCH2N(Ph)-
CH2PPh2)] (4). CO-removing reagent Me3NO 2H2O (0.29 g,
3
2.6 mmol) was added to the solution of 1 (1.0 g, 2.6 mmol) in
CH3CN (40 mL). The solution was stirred for 5 min, and then
Ph2PCH2N(Ph)CH2PPh2 (0.636 g, 1.3 mmol) was added. After
the mixture was reacted for 3 h at room temperature, a red
powder was deposited from the solvent and washed several
times with acetonitrile and hexane. Yield: 1.57 g (86%). Single
crystals suitable for X-ray analysis were obtained by recrystalli-
Experimental Section
Reagents and Instruments. All reactions and operations re-
lated to organometallic complexes were carried out under dry
and oxygen-free dinitrogen with standard Schlenk techniques.
Solvents were dried and distilled prior to use according to the
standard methods. Commercially available chemicals, 1,3-pro-
panedithiol, Fe(CO)5, PHPh2, n-propylamine, aniline, dppp,
1
zation from hexane/CH2Cl2. H NMR (CDCl3): δ 7.64-7.29
(m, 20H, 4Ph), 6.62-6.34 (m, 5H, Ph), 4.35 (s, 4H, PCH2N),
1.67-1.25 (m, 12H, SCH2CH2CH2S). 31P NMR (CDCl3): δ
60.3. IR (CH2Cl2): νCO=2041, 1975, 1924 cm-1. ESI-MS: m/z
1227.8 (100%) [M þ Na]þ, 1205.8 (25%) [M þ H]þ. Elem. anal.
calcd for C48H41Fe4NO10P2S4: C, 47.83; H, 3.43; N, 1.16.
Found: C, 47.68; H, 3.50; N, 1.21.
and Me3NO 2H2O were reagent grade and used as received.
3
Compounds Ph2PCH2N(n-Pr)CH2PPh2, Ph2PCH2N(Ph)CH2-
PPh2, [(μ-pdt)Fe2(CO)6], and [(μ-pdt){Fe(CO)3}{Fe(CO)(κ2-
dppp)}] (5) were prepared according to literature methods.22,31-33
Infrared spectra were recorded on JASCO FT/IR 430 spec-
trometer. Proton and 31P NMR spectra were collected with a
Varian INOVA 400NMR instrument. Mass spectra were re-
corded on an HP1100 MSD mass spectrometer. Elemental
analyses were performed with a Thermoquest-Flash EA 1112
elemental analyzer.
Synthesis of [(μ-H)(μ-pdt){Fe(CO)3}{Fe(CO)(K2-Ph2PCH2-
N(n-Pr)CH2PPh2)}](BF4) [1(Hμ)](BF4). A 2 equiv portion of
HBF4 Et2O was added to the solution of 1 (0.79 g, 1.0 mmol) in
3
CH2Cl2 (30 mL), and the red solution was stirred for 5 min. The
solvent was removed under reduced pressure. The residue was
washed several times with hexane/CH2Cl2 (10:1, v/v). Yield:
0.60 g (68%). 1H NMR (CD2Cl2): δ 7.57-7.32 (m, 20H, 4Ph),
4.29 (m, 2H, PCH2N), 3.25 (m, 2H, PCH2N), 3.07 (br s, 2H,
NCH2C), 2.39-1.58 (m, 8H, SCH2CH2CH2S, NCCH2), 0.86
(br s, 3H, CCH3), -13.0 (br s, μ-H). 31P NMR (CD2Cl2): δ 37.3.
Synthesis of [(μ-pdt){Fe(CO)3}{Fe(CO)(K2-Ph2PCH2N(n-
Pr)CH2PPh2)}] (1), [(μ-pdt)(μ-Ph2PCH2N(n-Pr)CH2PPh2)Fe2-
(CO)4] (2), and [{Fe2(CO)5(μ-pdt)}2(μ,κ1,κ1- Ph2PCH2N(n-
Pr)CH2PPh2)] (3). Ligand Ph2PCH2N(n-Pr)CH2PPh2 (0.47 g,
1.0 mmol) was added to the toluene solution (20 mL) of [(μ-
pdt)Fe2(CO)6] (0.39 g, 1.0 mmol). The red solution was refluxed
for 3 h, and the color turned dark red. The solvent was removed
under reduced pressure. The solid was extracted several times
with hexane/CH2Cl2 (5:1, v/v). The residue was then purified by
chromatography on a silica gel column with hexane/CH2Cl2
(3:1, v/v) as eluent. Complex 1 was obtained as a brown powder
from the collected yellow band after removal of solvent. Yield:
0.61 g (78%). Single crystals suitable for X-ray analysis were
IR (CH2Cl2): νCO=2097, 2037, 2003, 1960 cm-1
.
Synthesis of [(μ-H)(μ-pdt){Fe(CO)3}{Fe(CO)(K2-Ph2PCH2-
NH(n-Pr)CH2PPh2)}](OTf)2, [1(HNHμ)](OTf)2. A 3 equiv por-
tion of HOTf was added to a solution of 1 (0.79 g, 1.0 mmol) in
diethyl ether (100 mL), and the yellow solution was stirred for
5 min. A purple powder was deposited from the solvent. It was
washed several times with diethyl ether and dried in vacuum
(yield: 0.95 g, 88%). Single crystals suitable for X-ray analysis
was obtained by gentle addition of hexane to the CH2Cl2
solution of the freshly formed [1(HNHμ)](OTf)2 (hexane/
CH2Cl2=5:1, v/v). After the solution stood for one day at room
temperature, some dark purple crystals of [1(HNHμ)](OTf)2
1
obtained by recrystallization from hexane/CH2Cl2. H NMR
(CDCl3): δ 7.71-7.20 (m, 20H, 4Ph), 3.76 (s, 2H, PCH2N), 3.28
(s, 2H, PCH2N), 2.44 (br s, 2H, NCH2C), 1.68 (br s, 2H,
SCCH2CS), 1.45 (br s, 4H, SCH2C), 1.30 (br s, 2H, NCCH2),
0.87 (br s, 3H, CCH3). 31P NMR (CDCl3): δ 52.5. IR (CH2Cl2):
1
CO=2020, 1948, 1894 cm-1. ESI-MS: m/z 786.1 (100%) [M þ
appeared in the Schlenk tube. H NMR (d6-acetone): δ 10.1
ν
H]þ. Elem. anal. calcd for C36H37Fe2NO4P2S2: C, 55.05; H,
4.75; N, 1.78. Found: C, 55.05; H, 4.73; N, 1.85.
(br, 1H, NH), 7.71-7.21 (m, 20H, 4Ph), 5.47-4.95 (m, 4H,
PCH2N), 3.97-3.76 (br s, 2H, NCH2C), 3.28-1.88 (m, 8H,
SCH2CH2CH2S, NCCH2), 0.98 (br s, 3H, CCH3), -13.6 (t, μ-H,
The extracts are combined and the solvent was removed
under reduced pressure. The crude products were further sepa-
rated by rechromatography on a silica gel column with hexane/
CH2Cl2 (5:1, v/v) as eluent. Complexes 2 and 3 were respectively
obtained.
Complex 2. Yield: 0.03 g (3%). 1H NMR (CDCl3): δ
7.75-7.40 (m, 20H, 4Ph), 3.57-3.42 (s, 4H, PCH2N),
2.06-1.19 (m, 10H, SCH2CH2CH2S, NCH2CH2), 0.87 (s, 3H,
CCH3). 31P NMR (CDCl3): δ 61.6. IR (CH2Cl2): νCO=1981,
1943, 1909 cm-1. ESI-MS: m/z 786.1 (100%) [M þ H]þ. Elem.
anal. calcd for C36H37Fe2NO4P2S2: C, 55.05; H, 4.75; N, 1.78.
Found: C, 55.57; H, 4.83; N, 1.73.
J
H-P=19.9 Hz, HN-endo isomer), -13.9 (t, μ-H, JH-P=20.1
Hz, HN-exo isomer). 31P NMR (d6-acetone): δ 46.2, 45.1. IR
(CH2Cl2): νCO=2105, 2057, 2043, 1990 cm-1. Elem. anal. calcd
for C38H41F6Fe2NO11P2S4: C, 41.36; H, 3.74; N, 1.27. Found:
C, 41.30; H, 3.78; N, 1.24.
Synthesis of [(μ-H)(μ-pdt){Fe(CO)3}{Fe(CO)(K2-dppp)}]-
(OTf) (dppp=Ph2PCH2CH2CH2PPh2), [5(Hμ)](OTf). Complex
[5(Hμ)](OTf) was formed in situ by addition of 1.5 equiv of
HOTf to the solution of 5 in CD2Cl2. The color of the solution
1
immediately turned red. H NMR (CD2Cl2): δ 7.75-7.10 (m,
20H, 4Ph), 2.80-1.65 (m, 12H, all CH2), -12.8 (t, μ-H, JH-P
=
=
15.8 Hz). 31P NMR (CD2Cl2): δ 43.5, 38.6. IR (CH2Cl2): νCO
Complex 3. Yield: 0.12 g (15%). Single crystals suitable for
X-ray analysis were obtained by recrystallization from hexane/
CH2Cl2. 1H NMR (CDCl3): δ 7.75-7.40 (m, 20H, 4Ph), 3.91 (s,
4H, PCH2N), 2.19 (s, 2H, NCH2C), 1.72-1.26 (m, 14H,
SCH2CH2CH2S, NCCH2), 0.88 (s, 3H, CCH3). 31P NMR
2095, 2040, 2003, 1956 cm-1
.
Measurements of Relaxation Time T1 and Spin Saturation
Transfer Experiments (400 MHz). Relaxation times (T1) of the
μ-hydride signals, at δ -13.0 for [1(Hμ)]þ, δ -13.6 for the HN-
endo isomer of [1(HNHμ)]2þ, and δ -13.9 for the HN-exo
isomer, were measured in CD2Cl2 solution at room temperature
on a Bruker AVANCE II 400 instrument by the inversion-
recovery method using the standard 180°-τ-90° pulse sequence
and analyzing with the spectrometer T1 routine. Spin saturation
transfer experiments were carried out on the d6-acetone solution
of [1(HNHμ)]2þ. After the sample was cooled to -70 °C, the
δ -13.6 resonance was irradiated for 10 s (>5T1) with irradia-
tion power of 47 dB and the integrated intensity of the δ 9.48
resonance for the HN-endo signal was recorded with the δ 4.95
signal (PCH2N) as a reference. Subsequently, the δ -13.9
(CDCl3): δ 56.3. IR (CH2Cl2): νCO =2040, 1973, 1924 cm-1
.
ESI-MS: m/z 1171.8 (100%) [M þ H]þ, 1193.8 (45%) [M þ
Na]þ. Elem. anal. calcd for C45H43Fe4NO10P2S4: C, 46.14; H,
3.70; N, 1.20. Found: C, 45.97; H, 3.75; N, 1.23.
(31) Wu, W.; Li, C. Chem. Commun. 2003, 1668–1669.
(32) Balch, A. L.; Olmstead, M. M.; Rowley, S. P. Inorg. Chim. Acta 1990,
168, 255–264.
(33) Lyon, E. J.; Georgakaki, I. P.; Reibenspies, J. H.; Darensbourg, M.
Y. Angew. Chem., Int. Ed. 1999, 38, 3178–3180.

11558 Inorganic Chemistry, Vol. 48, No. 24, 2009
Wang et al.
resonance was irradiated under the same experimental condi-
tion and the intensity of the δ 10.45 resonance for the HN-exo
signal was integrated. Diminution of the HN-endo signal at
δ 9.48 ppm was observed with irradiation of δ -13.6 ppm signal,
while the integrated intensity of HN-exo signal at δ 10.45 ppm
did not show any detectable suppression with irradiation of
δ -13.9 ppm signal.
anisotropically. Hydrogen atoms were placed by geometrical
calculation and refined in a riding model. The μ-hydride and the
proton on the N atom of [1(HNHμ)](OTf)2 were located by the
difference Fourier map.
Acknowledgment. This work was supported by the
National Natural Science Foundation of China (Grant no.
20633020), the National Basic Research Program of China
(Grant No. 2009CB220009), the Program for Changjiang
Scholars and Innovative Research Team in Uni-
versity (IRT0711), the Swedish Energy Agency, the Swedish
Research Council, and the K & A Wallenberg Foundation
of Sweden.
Crystal Structure Determination of 1, 3, 4, and [1(HNHμ)]-
(OTf)2. Crystallographic data of 1, 3, 4, and [1(HNHμ)](OTf)2
were collected on a Bruker Smart Apex II CCD diffractometer
˚
with graphite monochromated Mo KR radiation (λ=0.71073 A)
at 298 K using the ω-2θ scan mode. Data processing was
accomplished with the SAINT processing program. Intensity
data were corrected for absorption by the SADABS program.
The structures were solved by direct methods and refined on F2
against full-matrix least-squares methods using the SHELX-97
program package.34 All non-hydrogen atoms were refined
Supporting Information Available: X-ray crystallographic
data of complexes 3 and 4 in CIF format. Selected region of
variable-temperature 31P NMR spectra of the endo and exo
isomers of [1(HNHμ)](OTf)2. This material is available free of
charge via the Internet at http://pubs.acs.org.
€
(34) Sheldrick, G. M. SHELXTL97; University of Gottingen, Germany, 1997.