Facile and highly efficient light-induced PR3/CO ligand exchange: A novel approach to the synthesis of [(μ-SCH2NnPrCH2S)Fe2(CO)4(PR3)2]

Article abstract of DOI:10.1016/j.jorganchem.2006.12.009

A straightforward and efficient transformation of the Fe-S complex [(μ-SCH₂NⁿPrCH₂S)Fe₂(CO)₆] to its double phosphine coordinated analogues [(μ-SCH₂NⁿPrCH₂S)Fe₂(CO)₄(PR₃)₂] (R = Ph, Me) is described. The single crystal structure of the PPh₃-disubstituted complex [(μ-SCH₂NⁿPrCH₂S)Fe₂(CO)₄(Ph₃P)₂] (3) showed that both of the phosphine ligands take an apical/apical instead of a basal/basal or an apical/basal configuration.

Full text of DOI:10.1016/j.jorganchem.2006.12.009

Journal of Organometallic Chemistry 692 (2007) 1579–1583
www.elsevier.com/locate/jorganchem
Facile and highly efficient light-induced PR3/CO ligand exchange:
A novel approach to the synthesis of
[(l-SCH2NnPrCH2S)Fe2(CO)4(PR3)2]
a
a,
b
a,c,
*
˚
Weiming Gao , Jianhui Liu *, Bjorn Akermark , Licheng Sun
¨
a
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Zhongshan Road 158-40, 116012 Dalian, PR China
b
Department of Organic Chemistry, Arrhenius Laboratory, Stockholm University, 10691 Stockholm, Sweden
c
KTH Chemistry, Organic Chemistry, Royal Institute of Technology, 10044 Stockholm, Sweden
Received 24 October 2006; received in revised form 30 November 2006; accepted 12 December 2006
Available online 20 December 2006
Abstract
A straightforward and efficient transformation of the Fe–S complex [(l-SCH2NnPrCH2S)Fe2(CO)6] to its double phosphine coordi-
nated analogues [(l-SCH2NnPrCH2S)Fe2(CO)4(PR3)2] (R = Ph, Me) is described. The single crystal structure of the PPh3-disubstituted
complex [(l-SCH2NnPrCH2S)Fe2(CO)4(Ph3P)2] (3) showed that both of the phosphine ligands take an apical/apical instead of a basal/
basal or an apical/basal configuration.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Fe-hydrogenases; Phosphine ligands; Bioinorganic chemistry; Hydrogen production
1. Introduction
the case of [(l-SCH2NR0CH2S)Fe2(CO)4L2] (L = PMe3,
R0 = benzyl) complex [2], the prior preparation of [(l-
SCH2NR0CH2S)Fe2(CO)4L2] (L = PMe3, R0 = p-nitro-
phenyl) complex [3] involved in the reaction of the Fe2S2
hexacarbonyl model complex with a large excess of phos-
phine in heated toluene. The propanedithiolato-bridged
dinuclear analogue [(l-PDT)Fe2(CO)4L2] (L = PR3)
(PDT = propanedithiolate, –SCH2CH2CH2S–) was also
prepared in similar manner [4–7]. So far, no reactions have
been reported by using sunlight to induce direct CO-substi-
tution by phosphine. This paper describes a novel and con-
venient protocol, employing light-induced ligand exchange
for the synthesis of the PR3-disubstituted model complexes
[(l-SCH2NnPrCH2S)Fe2(CO)4(L)2] (L = PMe3, PPh3).
Dinuclear butterfly Fe–S complexes [Fe2(l-S-R-S)-
(CO)6] (R = –(CH2)3–, –CH2NR0CH2–) have been exten-
sively studied as models for the active site of Fe-only hydrog-
enases [1]. One significant type of model complex is the bis-
phosphine coordinated derivative. The phosphine ligands
have been found to increase protonation capability at the
Fe–Fe moiety and therefore favour molecular hydrogen for-
mation. Although the carbonyl displacement reactions
between [Fe2(l-S-R-S)(CO)6] and tertiary phosphines
(PMe3, PPhMe2, P(OMe)3, PPh3) have been extensively
studied since the 1970s, the preparation of the
bis-phosphine coordinated product, particularly the bis-
triphenylphosphine, remains difficult. While the preparation
of bis-phosphine complexes of PMe3 proceeds smoothly in
2. Results and discussion
*
2.1. Synthesis
Corresponding authors. Address: State Key Laboratory of Fine
Chemicals, Dalian University of Technology, Zhongshan Road 158-40,
116012 Dalian, PR China. Tel.: +86 411 88993886; fax: +86 411 83702185
(J. Liu).
We chose azadithiolate containing Fe2S2 system
[{(l-SCH2N(CH2CH2CH3)CH2S)}Fe2(CO)6] (1) as the
E-mail addresses: liujh@dlut.edu.cn (J. Liu), lichengs@kth.se (L. Sun).
0022-328X/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.12.009

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W. Gao et al. / Journal of Organometallic Chemistry 692 (2007) 1579–1583
Table 1
substrate for our studies. The propyl group was used as the
N-substituent because of its simplicity and electron-releas-
ing ability [8]. As expected, the PPh3-monosubstituted
complex 2 could be obtained in good yield by the reaction
of 1 with 1 equiv. of PPh3 and 1 equiv. of Me3NO. As an
oxidative decarbonylation reagent in the CO/phosphine
substitution [9], Me3NO can transfer an oxygen to car-
bonyl and accelerates the substitution reaction for the type
of [(l-SCH2NRCH2S)Fe2(CO)5L] (L = phosphine ligand).
However, further conversion to the bis-PPh3-substituted
complex 3 was difficult even if excessive PPh3 and Me3NO
were involved, perhaps because of the weak oxidation abil-
ity of the latter compared to [FeCp2]PF6 in MeNO2 [10,11].
When the reaction of 1 with only 2 equiv. of PPh3 was car-
ried out under sunlight in the presence of Me3NO, the
PPh3/CO exchange proceeded quite smoothly and complex
1 was transformed into complex 3 in excellent yield
(Scheme 1). It turned out both sunlight and Me3NO were
essential for this reaction. In the absence of sunlight, it
was rather difficult to obtain the disubstituted complex 3
even at high temperature with an excess of PPh3. In con-
trast, in the absence of Me3NO but under sunlight, the
substitution still occurred affording a mixture of the mono-
and disubstituted PPh3 complexes, but the yield was rela-
tively low, ca. 5% mono- and 10% disubstituted product,
respectively. It was also evident that complex 2 was the
intermediate in the formation of a bis-PPh3-substituted
complex 3. Similar results were obtained with PMe3 to give
The control experiments for the synthesis of PR3-disubstituted Fe2S2
complexesa
Complexes
Me3NO onlyb (%)
Light only (%)
Me3NO/lightb (%)
3
5
7
8
a
<1
30
<1
11
10
21
<1
9
95
90
92
87
The mole ratio of PR3/Fe2S2 is 2:1.
The mole ratio of Me3NO/Fe2S2 is 1:1.
b
(5), in yields of 95% and 90%, respectively. Unfortunately,
we did not obtain complexes 4 and 5 in crystalline form
because of their oily state. These results could be further
extended to PDT analogues. As expected, the conversions
from the reported complex 6 to the known complexes 7
and 8 were easily realized in yields of 92% and 87%, respec-
tively; especially for complex 7, much higher than the
reported yield of 20% [7]. Control experiments were also
performed to show the efficiency of combined using of
Me3NO and light (Table 1). Under the action of Me3NO
(Me3NO/Fe2S2 = 1:1), the bis-PPh3 substituted complexes
can barely be detected. When 10-folds of Me3NO was used,
the bis-PPh3 substituted complexes 3 and 7 could be
obtained but still in a very low yield (ca. 10%). In the case
of bis-PMe3 coordinated complexes 5 and 8, the relatively
improved yields of 30% and 11% are probably due to the
more strong electron-donating ability of PMe3 compared
with PPh3. When such reaction was performed by irradia-
tion, it did not show an increasing trend for yields. How-
[{(l-SCH2N(CH2CH2CH3)CH2S)}Fe2(CO)5(PMe3)]
(4)
and [{(l-SCH2N(CH2CH2CH3)CH2S)}Fe2(CO)4(PMe3)2]
N
N
1 equiv. of Me NO
3
S
S
CO
+
2PPh3
S
OC
S
PPh3
CO
Ph3P
OC
h v
-2CO
Fe
Fe
Fe
Fe
CO
( 95%)
OC
OC
CO
OC
CO
1
3
h v
h v
PPh -CO,
PPh -CO,
3,
3,
slow
fast
N
S
S
PPh3
OC
OC
Fe
Fe
CO
OC
CO
2
1 equiv. of Me NO
3
S
S
CO
+
2PR3
S
OC
S
PR3
R3P
h v -2CO
Fe
Fe
Fe
Fe
CO
OC
CO
OC
OC
CO
OC
CO
R = Ph 92%
R = Me 87%
6
7
8
Scheme 1. Synthetic route for the PR3-disubstituted Fe–S complexes.

W. Gao et al. / Journal of Organometallic Chemistry 692 (2007) 1579–1583
1581
ever, all target bisphosphine products could be obtained in
good yields with the combined use of Me3NO and light.
has been characterized by single crystal X-ray analysis. The
1H NMR signals for most CH3CH2CH2 and P(CH3)3
groups are reported as singlets usually attribute to the
intrinsic property of the iron-containing compounds.
Some control experiments at an early stage clarified the
formation of phosphine sulfide in the reaction of complexes
1 or 6 with phosphine ligand (Scheme 2). Two requisite
conditions were the presence of the bridge nitrogen and
Me3NO, since the reaction of 6 failed to give phosphine
sulfide. The mechanism might possibly involve the nucleo-
philic attack by a bridging sulfur atom or the direct oxida-
tion of iron (I) by Me3NO, followed by reaction of Ph3P
with any sulfur formed.
The coordination configuration of diphosphine complex
had been structurally characterized (Fig. 2), and compared
to the structures of its analogues [Fe2(l-S-R-S)(CO)4(L)2]
(L = PMe3, or PMe2Ph) [7–12]. The single crystal X-ray
analysis showed that the PPh3-disubstituted complex 3 pos-
sesses an ap/ap (apical/apical, trans position relative to the
2.2. Characterization
All of the new complexes had been characterized by their
1H NMR and mass spectra. The most intense peaks were
observed for 2 and 3 in the mass spectra at m/z 664 and
898 respectively, corresponding to [M+H]+ (positive
mode). In order to have a better understanding of the reac-
tion process, the 31P NMR spectroscopic changes were
recorded (Fig. 1). The 31P NMR signals observed were first
ꢀ5.5 ppm (PPh3), then 65.6 ppm (PPh3-monosubstituted
complex 2) and finally 62.7 ppm (PPh3-disubstituted com-
plex 3). After 10 min, the PPh3-monosubstituted complex
2 was observed with some PPh3 left. When the reaction mix-
ture was kept in the dark, the 31P NMR spectrum remained
unchanged for a further 20 min or even longer. However, if
the reaction mixture was then irradiated with visible light,
most of the PPh3 had coordinated to iron within 30 min.
The weak signal at 43.1 ppm was due to the S = PPh3 which
10min
30min
1 h
100
80
60
40
20
0
-20
Fig. 2. ORTEP view of [{(l-SCH2NnPrCH2S)}Fe2(CO)4(PPh3)2] (3) with
30% ellipsoids. Selected bond distances (A) and angles (deg): Fe(1)–Fe(2)
δ (ppm)
˚
Fig. 1. Time dependence of 31P NMR spectra of 1+PPh3 (molar ratio:
1:2): (a) SPPh3 (43.1 ppm) and PPh3 (before reaction), (b) at room
temperature without light and (c) under sunlight irradiation.
2.5181(12), Fe(1)–S(1) 2.269(2), Fe(1)–S(2) 2.2662(17), Fe(1)–P(2)
2.2459(17), Fe(1)–S(1)–Fe(2) 67.36(6), Fe(1)–S(2)–Fe(2) 67.27(5), P(2)–
Fe(1)–Fe(2) 156.92(6).
N
N
+
+ Me3NO
PPh3
S
S
CO
S
S
CO
OC
OC
PPh3
+
Fe
Fe
Fe
Fe
Fe
Fe
CO
CO
OC
OC
OC
OC
CO
CO
1
1
S=PPh3 as by product
S
S
CO
CO
S
S
CO
CO
OC
OC
Fe
Fe
+
+ Me3NO
PPh3
+
PPh3
OC
OC
OC
OC
CO
CO
6
6
Scheme 2. Different substrates and reaction conditions relating to the formation of Ph3PS.

1582
W. Gao et al. / Journal of Organometallic Chemistry 692 (2007) 1579–1583
Fe–Fe bond) coordination configuration, which was identi-
cal to that of the complexes containing the large isocyanide
ligand tBuNC [13] and PMe2Ph-disubstituted complex [7],
but in contrast to the transoid ba/ba (basal/basal, cis to the
Fe–Fe bond) configuration of [(l-PDT)Fe2(CO)4(PMe3)2]
[12] and [(l-PDT)Fe2(CO)4(PTA)2] [14] (PTA = 1,3,5-tri-
aza-7-phosphaadamantane). This feature showed that for
larger tertiary phosphine ligands, a ba/ba or an ap/ba con-
figuration should be more sterically crowded than an ap/ap
configuration. The newly reported PMe3-disubstituted
Me3NO Æ 2H2O (11 mg, 0.1 mmol) at room temperature.
Then a CH2Cl2 (5 mL) solution of PPh3 (26 mg, 0.1 mmol)
was added to the above solution during 10 min. After
another 30 min, the solvent was removed and the residue
was purified by silica gel chromatography (CH2Cl2/hexane
1:2) to give two components: complex 2 (64 mg, 97%) as a
1
red solid: mp 78–80 ꢁC. H NMR (CD3COCD3): d (ppm)
0.61 (t, J = 7.2 Hz, 3H, CH3), 1.01–1.07 (m, 2H,
CH3CH2CH2), 1.29 (s, b, 2H, CH3CH2CH2N), 2.39 (d,
J = 11.2 Hz, 2H, NCH2S), 2.92 (d, J = 14.4 Hz, 2H,
NCH2S), 7.28–7.32 (m, 3H, Ph), 7.39–7.40 (m, 6H, Ph),
7.54–7.55 (m, 3H, Ph), 7.70–7.74 (m, 3H, Ph). 31P NMR
(CH2Cl2): d (ppm) 65.60 (s). IR (KBr): m(CO) 2041, 1989,
1961, 1930 cmꢀ1. MS (API-ES): m/z 664.0 [M+H]+. Ele-
mental Anal. Calc. for C28H26Fe2NO5PS2: C, 50.70; H,
3.95; N, 2.11. Found: C, 50.92; H, 4.14; N, 2.33%. SPPh3
(1.5 mg, 5%) as a white solid: mp 160–162 ꢁC (lit. [15] mp
162–164 ꢁC). 1H NMR (CDCl3): d (ppm) 7.41–7.45 (m,
6H, Ph), 7.48–7.50 (m, 3H, Ph), 7.69–7.74 (m, 6H, Ph).
31P NMR (CH2Cl2): d (ppm) 43.1 (s).
derivatives
[{(l-SCH2N(R0)CH2S)}Fe2(CO)4(PMe3)2]
(R0 = p-nitrophenyl or benzyl) usually take a ba/ba or an
ap/ba configurations [2,3]. However, the coordination con-
figurations of other PR3-disubstituted complexes [(l-
SCH2N(R0)CH2S)Fe2(CO)4L2] (L = PMe2Ph, PPh3) were
unknown prior to the preparation of the PPh3-disubsti-
tuted derivative reported here.
3. Conclusion
In summary, PR3-disubstituted azadithiolato diiron spe-
cies were prepared in excellent yield via the direct irradia-
tion in the presence of Me3NO, which provided a new
and efficient access for synthesizing the disubstituted phos-
phine Fe-only hydrogenases model complexes. The conver-
sion showed excellent yield with remarkably site-selective
properties, with the PPh3 taking uniquely ap/ap configura-
tion. The results may be biosynthetically relevant, espe-
cially in view of the importance of its site-selectivity and
further protonation.
4.3. [{(l-SCH2N(CH2CH2CH3)CH2S)}Fe2(CO)4-
(PPh3)2] (3)
This complex was prepared by a similar procedure, with
2 equiv. PPh3 except that the reaction was carried out under
sunlight or irradiated with a 500 W Xe lamp. After 30 min
reaction time, the solvent was removed and the residue was
purified by silica gel chromatography with CH2Cl2/hexane
(1:2) to give complex 3 (84 mg, 95%) as a red solid: mp
120 ꢁC (dec.). 1H NMR (CD3COCD3):
d 0.88 (t,
4. Experimental
J = 6.8 Hz, 3H, CH3), 1.07–1.10 (m, 2H, CH3CH2CH2),
1.76 (s, 2H, CH3CH2CH2N), 2.95 (s, 4H, NCH2S), 7.48–
7.49 (m, 18H, Ph), 7.69–7.73 (m, 12H, Ph) ppm. 31P NMR
(CH2Cl2): d 62.67 (s) ppm. IR (KBr): m(CO) 1995, 1952
and 1932 cmꢀ1. MS (API-ES): m/z 898.0 [M+H]+. Elemen-
tal Anal. Calc. for C45H41Fe2NO4P2S2: C, 60.22; H, 4.60; N,
1.56. Found: C, 60.45; H, 4.70; N, 1.78%.
4.1. General considerations
All reactions and operations related to organometallic
complexes were carried out under a dry, oxygen-free nitro-
gen atmosphere using standard Schlenk techniques. All sol-
vents were dried and distilled prior to use according to the
standard methods. Commercially available chemicals,
including n-propylamine, paraformaldehyde, trimethyla-
mine oxide and triphenylphosphine were used without fur-
ther purification. The reagents LiEt3BH, triflic acid and
trimethylphosphine (1 M in THF) were purchased from
Aldrich. Infrared spectra were recorded on a JASCO FT/
IR 430 spectrophotometer. NMR spectra were collected
on a Varian INOVA 400NMR spectrometer at 400 MHz
(1H) and at 161 MHz (31P). Mass spectra were recorded
on an HP1100 MSD instrument. Elemental analyses were
performed on a CARLO ERBA MOD-1106 elemental
analyzer.
4.4. [{(l-SCH2N(CH2CH2CH3)CH2S)}Fe2(CO)5-
(PMe3)] (4)
A solution of 1 (43 mg, 0.1 mmol) in CH3CN (20 mL)
was added dropwise to a CH3CN (3 mL) solution of
Me3NO Æ 2H2O (11 mg, 0.1 mmol) at room temperature.
Then a 1 M THF solution of PMe3 (0.1 mL, 0.1 mmol)
was added to the above solution during 10 min. After
30 min, the solvent was removed and the residue was puri-
fied by silica gel chromatography with CH2Cl2/hexane (1:2)
to give complex 4 (46 mg, 95%) as red oil: 1H NMR
(CDCl3): d (ppm) 0.80 (s, 3H, CH3), 1.30 (s, 2H,
CH3CH2CH2), 1.49 (s, 9H, P(CH3)3), 2.60 (s, 2H,
CH3CH2CH2N), 3.46 (s, 4H, 2 · NCH2S). 31P NMR
(CDCl3): d (ppm) 25.1 (d). IR (CHCl3): m(CO) 2035,
4.2. [{(l-SCH2N(CH2CH2CH3)CH2S)}Fe2(CO)5-
(PPh3)] (2)
1978, 1942, 1908 cmꢀ1
. Elemental Anal. Calc. for
A solution of 1 (43 mg, 0.1 mmol) in CH3CN (20 mL)
was added dropwise to a CH3CN (3 mL) solution of
C13H20Fe2NO5PS2: C, 32.73; H, 4.23; N, 2.94. Found: C,
33.10; H, 4.44; N, 2.86%.

W. Gao et al. / Journal of Organometallic Chemistry 692 (2007) 1579–1583
1583
4.5. [{(l-SCH2N(CH2CH2CH3)CH2S)}Fe2(CO)4-
(PMe3)2] (5)
Talents of Discipline to Universities and the Swedish Re-
search Council. We thank Mr. Hexiang Zhang, University
of Texas at Arlington, for helpful discussion.
This complex was prepared by a similar procedure, with
2 equiv. PMe3 except that the reaction was carried out
under sunlight or irradiated with a 500 W Xe lamp. After
30 min reaction time, the solvent was removed and the res-
idue was purified by silica gel chromatography with
CH2Cl2/hexane/Et3N (100:200:1) to give a red oily com-
Appendix A. Supplementary material
CCDC 299478 and 288105 contain the supplementary
crystallographic data for 3 and SPPh3. These data can be
obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: (+44) 1223-336-033; or e-mail: depos-
it@ccdc.cam.ac.uk. Supplementary data associated with
this article can be found, in the online version, at
doi:10.1016/j.jorganchem.2006.12.009.
1
plex 5 (47 mg, 90%). H NMR (CDCl3): d (ppm) 0.74 (s,
3H, CH3), 1.93 (s, 2H, CH3CH2CH2), 1.49 (s, 18H,
P(CH3)3), 2.36 (s, 2H, CH3CH2CH2N), 3.14 (s, 4H,
2 · NCH2S). 31P NMR (CD3CN): d (ppm) 23.12 (s). IR
(CHCl3): m(CO) 1998, 1979, 1940, 1898 cmꢀ1. Elemental
Anal. Calc. for C15H29Fe2NO4P2S2: C, 34.31; H, 5.57; N,
2.67. Found: C, 34.51; H, 5.25; N, 2.39%.
References
4.6. Crystal data for 3 and SPPh3
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˚
˚
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˚
ꢀ
897.55, space group: P1, a = 10.3380(19) A, b =
˚
˚
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˚
(3)ꢁ, c = 119.182(2)ꢁ, V = 2135.9(7) A , Z = 2, T = 273 K,
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3
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We acknowledge the following foundation for financial
support of this work: the Chinese National Natural Science
Foundation (Grant Nos. 20633020 and 20672017), the
Swedish Energy Agency, the Programme of Introducing