Reaction of aryl diazonium salts and diiron(I) dithiolato carbonyls: Evidence for radical intermediates

Article abstract of DOI:10.1021/om300107s

Treatment of Fe₂(pdt)(CO)₄(dppv) (1) with aryldiazonium salts affords the 34 e^- adducts [Fe₂(pdt) (ν-N₂Ar)(CO)₄(dppv)]⁺ (pdt^2- = 1,3-propanedithiolate, dppv = cis-C₂H₂(PPh ₂)₂). Under some conditions, the same reaction gave substantial amounts of [1]⁺, the product of electron transfer. Consistent with the influence of electron transfer in the reactions of some electrophiles with Fe(I)Fe(I) dithiolates, the reaction of [Me₃S ₂]⁺ and Fe₂(pdt)(CO)₄(dppbz) was found to give [Fe₂(pdt)(CO)₄(dppbz)]⁺ as well as Me₂S and Me₂S₂ (dppbz = 1,2- bis(diphenylphosphino)benzene).

Full text of DOI:10.1021/om300107s

Note
pubs.acs.org/Organometallics
Reaction of Aryl Diazonium Salts and Diiron(I) Dithiolato Carbonyls:
Evidence for Radical Intermediates
Matthew T. Olsen, Thomas B. Rauchfuss,* and Riccardo Zaffaroni
School of Chemical Sciences, University of Illinois, Urbana, Illinois 61801, United States
S
* Supporting Information
ABSTRACT: Treatment of Fe2(pdt)(CO)4(dppv) (1) with
aryldiazonium salts affords the 34 e− adducts [Fe2(pdt)(μ-
N2Ar)(CO)4(dppv)]+ (pdt2− = 1,3-propanedithiolate, dppv =
cis-C2H2(PPh2)2). Under some conditions, the same reaction
gave substantial amounts of [1]+, the product of electron
transfer. Consistent with the influence of electron transfer in
the reactions of some electrophiles with Fe(I)Fe(I) dithiolates, the reaction of [Me3S2]+ and Fe2(pdt)(CO)4(dppbz) was found
to give [Fe2(pdt)(CO)4(dppbz)]+ as well as Me2S and Me2S2 (dppbz = 1,2-bis(diphenylphosphino)benzene).
Scheme 1. Three Pathways Established for Addition of
Electrophiles (E+) to Diiron(I) Dithiolates
INTRODUCTION
■
The reactivity of diiron dithiolato carbonyls has come under
intense scrutiny with the discovery that these diiron
compounds are structurally related to the active site of the
[FeFe]-hydrogenases.1 Many efforts are underway to prepare
low molecular weight analogues of these biocatalysts.2 Since the
diiron dithiolato center in the [FeFe]-hydrogenases features
strong donor ligands in addition to CO, biomimetic modeling
generally focuses on the substituted derivatives of the diiron
dithiolates, especially diphosphine complexes such as Fe2(pdt)-
(CO)4(dppv) (1) and Fe2(pdt)(CO)4(PMe3)2 (2).2
Electrophiles (E+) attack substituted diiron(I) dithiolato
complexes in one of three ways. Most commonly, electrophiles
give adducts of the type [Fe2(μ-E)(SR)2L6]+.3 Many examples
exist; cases include E+ = Cl+,4 SMe+,5−7 and H+.8 The addition
of NO+ to diiron(I) dithiolates results in substitution, although
36 e− adducts are implicated as intermediates.9 Protonation of
diiron dithiolates containing chelating diphosphine ligands
generally gives terminal hydride complexes, which subsequently
isomerize by migration of the hydride to a bridging position.10
Intermediate terminal hydrides are not observed, however, for
the protonation of the symmetrical complex 2.11 It is therefore
unclear if terminal hydrides form upon protonation of 2 and
isomerize very readily or if the protonation occurs at the Fe−Fe
bond. Some electrophiles (O-atom transfer agents,12 alkylating
agents13) attack not at the metal but rather at sulfur (Scheme
1).
more generally probe the reactivity of the diiron complexes, we
extended the range of electrophiles to include diazonium
cations. Some diazonium salts are highly soluble in cold organic
solvents, which makes them amenable to in situ NMR analysis.
The results serve as a reminder that some electrophiles function
as electron-transfer agents, and the stereochemistry of the
adduct reflects the stereochemistry of the open-shell diiron
intermediate.
A new perspective on the stereochemistry of electrophilic
attack at diiron(I) dithiolates came from our recent study of the
reaction of 2 with [S2Me3]+, a source of the electrophile SMe+.7
The initially observed product, [Fe2(pdt)(SMe)-
(CO)4(PMe3)2]+, features a terminal MeS ligand. This complex
isomerizes by a first-order pathway to the corresponding μ-
thiolato isomer (eq 1).
RESULTS AND DISCUSSION
■
Treatment of 1 with [PhN2]BF4 at 0 °C resulted in good yields
of the adduct [Fe2(pdt)(μ-N2Ph)(CO)4(dppv)]BF4 ([1(μ-
1
N2Ph)]BF4). H and 31P{1H} NMR spectra of this salt are
simple, indicating a symmetrical product. The formula of the
cation was confirmed by ESI-mass spectrometry. Binding of the
This result suggests that protonation of 2 may occur also at a
single Fe center followed by rapid rearrangement to the
observed μ-hydride complex. To address this question and to
Received: February 9, 2012
Published: March 29, 2012
© 2012 American Chemical Society
3447
dx.doi.org/10.1021/om300107s | Organometallics 2012, 31, 3447−3450

Organometallics
Note
solubility of the typical diazonium salts (and, to a lesser extent,
the low solubility of 1) in cold dichloromethane. This problem
was addressed by the use of the organoborate salt [PhN2]BArF
4
(ArF = C6H3-3,5-(CF3)2). The off-white solid [PhN2]BArF is
4
highly soluble in CH2Cl2 at low temperatures. The reaction of
[PhN2]BArF and 1 at −90 °C was monitoried by 31P NMR
4
spectroscopy. The initial spectrum reveals the immediate
formation of a single new, unsymmetrical intermediate.
Under our reaction conditions, ca. 50% of 1 remains
undissolved. Upon warming the sample, signals for this
intermediate disappear and only weak broad signals are
observed. Above −30 °C, signals for [1(μ-N2Ph)]+ appear,
and at room temperature [1(μ-N2Ph)]+ is the exclusive
product. The observations suggest that (i) an unsymmetrical
diamagnetic adduct forms at low temperature that is not [1(μ-
N2Ph)]+, and (ii) upon warming, this intermediate rearranges
concomitant with the formation of additional species that
convert to [1(μ-N2Ph)]+. We propose the sequence shown in
eq 2.
diazonium cation to the diiron center causes a shift in νCO(av) of
∼72 cm−1 to 2030 cm−1. For comparison, protonation of 1
shifts νCO(av) by 60 cm−1.14 The reaction of 1 and [4-
ClC6H4N2]PF6 gave the corresponding chlorophenyldiazonium
+
complex. The spectroscopic data on the 4-ClC6H4N2 and
+
PhN2 derivatives are similar. We found that [PhN2]BF4 also
reacts rapidly with 2, but we were unable to purify or identify
products.
The solid-state structure of [Fe2(pdt)(μ-N2C6H4-4-Cl)-
(CO)4(dppv)]PF6 was confirmed crystallographically (Figure
1). The complex has idealized Cs symmetry, and the diazonium
Significantly, when the reaction of 1 and [PhN2]BArF was
4
monitored by IR spectroscopy at room temperature, we
observed a ∼1:1 ratio of [1(μ-N2Ph)]+ and the previously
characterized17 S = 1/2 species [1]+. It therefore is likely that
the broadened NMR spectra arise from the presence of [1]+.
Such mixed-valence cations are known to adopt the structures
that feature a vacant apical site on one Fe center.17,18
Figure 1. Structure of the cation in [1(μ-N2C6H4Cl)]PF6 with thermal
ellipsoids set at 35%. Hydrogen atoms, phenyl carbon thermal
ellipsoids, and the counteranion were omitted for clarity. Selected
bond distances (Å): Fe1−Fe2, 2.9653(6); Fe1−N1, 1.941(2); Fe2−
N1, 1.973(3); Fe1−P1, 2.2256(9); Fe1−P2, 2.2271(9); Fe1−C39,
1.789(3); Fe1−S1, 2.3280(8); Fe1−S2, 2.3244(9); Fe2−S2,
2.3321(8); Fe2−S1, 2.3176(9); Fe2−C1, 1.827(4); Fe2−C3,
1.824(3); Fe2−C2, 1.823(4); N1−N2, 1.235(3); C35−Cl42,
1.744(3).
We sought evidence for electron transfer in other reactions of
+
diiron(I) dithiolates. As the electrophile, we selected Me3S2 ,
which has been used by us and others.6,7,19 IR analysis of its
reaction with 1 at low temperatures revealed the clean
formation of [1]+. A similar result was obtained for the
reaction of [Me3S2]BF4 and Fe2(pdt)(CO)4(dppbz) (3), except
that [3]+ is particularly stable (eq 3).
ligand is bridging. Clusters with μ-η1,η1-RN2 ligands are
precedented.15 The diazonium ligand is strongly bent (N1−
N2−C38 = 120.4°) with the ClC6H4 group oriented away from
the bulky Fe(dppv)(CO) center. The dppv ligand is bound at
the two basal sites on one Fe center. The Fe- - -Fe distance of
2.9653(6) Å is assigned as nonbonding. Similar Fe- - -Fe
distances have been observed for other 36 e− diiron dithiolate
complexes with bridging alkylidene ligands.16
+
Fe (pdt)(CO) (dppbz) + [Me S ]
2
4
3 2
3
+
→ [Fe (pdt)(CO) (dppbz)] + Me S + 0.5Me S
2
4
2
2 2
+
[3]
(3)
Using 1H NMR spectroscopy, we also confirmed that
[Me3S2]BF4 reacts with ferrocene to give ferrocenium and a
2:1 mixture of Me2S and Me2S2. Treating 3 with [PhN2]BF4
also afforded [3]+.
The reaction of 1 and [PhN2]+ was examined in situ to gain
insights into the reaction pathway, i.e., the possible formation of
intermediates. Such experiments were hindered by the poor
3448
dx.doi.org/10.1021/om300107s | Organometallics 2012, 31, 3447−3450

Organometallics
Note
CH2CH2CH2), 2.69 (m, CH2CH2CH2). IR (CH2Cl2, cm−1): 2088,
2039, 1972. ESI-MS (m/z): 613.2 ([Fe2(pdt)(CO)2(dppv)(N2Ph)]+),
803.2 ([Fe2(pdt)(CO)3(dppv)(N2Ph)]+), 831.21 ([Fe2(pdt)-
(CO)4 (dppv)(N2 Ph)]+ ). Anal. Calcd (Found) for
C39H33BF4Fe2N2O4P2S2: C, 51.01 (50.12); H, 3.62 (3.86); N, 3.05
(2.92).
[Fe2(pdt)(μ-N2C6H4Cl)(CO)4(dppv)]PF6. As in the preceding
procedure, a CH2Cl2 solution of 0.397 g (0.55 mmol) of 1 was
treated with 0.156 g (0.55 mmol) of [N2C6H4-4-Cl]PF6. Standard
workup afforded the product. Yield: 0.437 g (92.5%). 31P{1H} NMR
(CD2Cl2): δ −145.2 (sept, JP−F = 733), 81.9 (s). 1H NMR (CD2Cl2):
δ 8.5 − 6.9 (m, Ph), 4.47 (bs, C2H2), 3.26 (m, JH−H = 9, SCH2), 2.93
(m, JH−H = 9, SCH2), 2.85 (m, CH2CH2CH2), 2.69 (m,
CH2CH2CH2). IR (CH2Cl2, cm−1): 2090, 2040, 1975. Anal. Calcd
(Found) for C39H32ClF6Fe2N2O4P3S2: C, 46.34 (45.84); H, 3.19
(3.82); N, 2.77 (2.16).
Fe2(pdt)(CO)4(dppbz) (3). This complex was prepared analo-
gously to 1. A solution of 0.432 g (1.12 mmol) of Fe2(pdt)(CO)6 in
50 mL of toluene was treated with a solution of 0.084 g (1.12 mmol)
of Me3NO in 15 mL of MeCN. After stirring for 10 min, the reaction
mixture was treated with a solution of 0.50 g (1.12 mmol) of dppbz in
50 mL of toluene. The solution was strirred at 70 °C for 5 h. The
solvent was removed under vacuum. The residue, a green-brown solid,
was extracted into 10 mL of CH2Cl2, and the product precipitated as a
light-green-brown powder upon the addition of 100 mL of hexanes.
The product was rinsed with 60 mL of hexanes. Yield: 0.69 g (80%).
31P{1H} NMR (CD2Cl2, 25 °C): δ 89.1 (s) (apical−basal 94.5%), δ
81.1 (s) (basal−basal 5.5%). 1H NMR (CD2Cl2): δ 7.7−7.1 (m, C6Hx
24H), 1.99 (m, SCH2 2H), 1.67 (m, SCH2 2H), 0.48 (bs,
CH2CH2CH2 2H). IR (CH2Cl2): νCO 2020 (m), 1950 (m), 1905
(s) cm−1. Anal. Calcd (Found) for C37H30Fe2O4P2S2: C, 57.24
(57.15); H, 3.89 (3.94).
CONCLUSIONS
■
This work describes the first diazonium derivative of a diiron
dithiolate. The adducts show no tendency to decarbonylate to
the 34 e− derivatives, in contrast to the lability of isoelectronic
[Fe2(pdt)(NO)(CO)4(dppv)]+.9 The new reagent [PhN2]-
BArF represents a useful derivative of the time-honored
4
diazonium salts. According to our spectroscopic measurements,
the properties of the PhN2 component of the salt are
unaffected by the change in counterion.
+
The reaction of diazonium salts with 1 afforded apparent
adducts, including one proposed to feature a terminal
diazonium ligand. Also observed are products resulting from
electron-transfer reactions. Connelly and Geiger have pre-
viously indicated that single electron transfer is associated with
the use of diazonium salts, not unlike related reactions
involving NO+.20 It is well known that diazonium salts are
good oxidants (e.g., for [FC6H4N2]+/0 E = −0.07 V for Fc+/0).20
The detection of odd-electron intermediates expands the
range of reactions of diiron dithiolates. It is well known that 1
e− oxidation of Fe2(pdt)(CO)6−xLx gives products wherein one
Fe center adopts a “rotated structure”.17,18 The rotated
structure is geometrically predisposed to bind both Lewis
bases such as CO21 as well as the radicals:
Selected in Situ and IR and NMR Studies. Several experiments
were conducted (solvent: dichloromethane) to probe the role of
electron-transfer reactions.
We propose that such S = 1/2 species are intermediates in
other reactions of diiron(I) dithiolates.
One surprising and puzzling observation in these studies is
the differing behavior of the unsymmetrical versus symmetrical
diphosphine complexes, such as 1 versus 2. Although exhibiting
similar cyclic voltammograms,17,22 [1]+ and [2]+ differ in terms
of their stability, with [2]+ being highly unstable and the dppv
(and dppbz) cation being readily detectable.
(1) A solution of 15 mg (0.019 mmol) of 3 and 18.8 mg (0.019
mmol) of [Me3S2]BArF was prepared at −78 °C. Upon
4
warming to room temperature, the IR spectrum (νCO region)
confirmed clean formation of [3]+. Very similar results were
obtained using [PhN2]BF4 in place of [Me3S2]BArF .
4
(2) Treatment of a solution of 3 with 1 equiv of FcBF4 gave an IR
spectrum (νCO region) that matched that assigned to [3]+ in
experiment 1.
EXPERIMENTAL SECTION
■
Methods have been recently reported.23 The diazonium salts
[N2Ph]BF4 and [4-ClC6H4N2]PF6 were prepared according to
literature procedures.24
(3) Addition of 4.7 mg (0.025 mmol) of ferrocene to a solution of
5.0 mg (0.025 mmol) of [Me3S2]BF4 in 0.8 mL of CD2Cl2
resulted in the slow (5 min) development of a deep blue-green
color. 1H NMR analysis of the mixture confirmed the formation
of a 2:1 mixture of Me2S (δ 2.00) and Me2S2 (δ 2.46).
(4) Treatment of a CH2Cl2 solution of 13.5 mg (0.028 mmol) of 2
at −78 °C with a solution of 27.5 mg (0.028 mmol) of
[N2Ph]BArF . A mixture of 0.380 g (2.5 mmol) of [N2Ph]BF4 and
4
1.799 g (2.5 mmol) of KBArF was precooled to −30 °C and then
4
treated with 20 mL of CH2Cl2. This mixture was allowed to warm to 0
°C and then vigorously stirred for 60 min. The resulting cloudy yellow
mixture was filtered to remove KBF4, and the supernatant was
concentrated to ∼5 mL. An off-white precipitate formed upon addition
of 30 mL of hexane and was collected by filtration. Yield: 1.94 g (80%
[PhN2]BArF in 5 mL of CH2Cl2 resulted in a rapid color
4
change from red to black. The mixture was allowed to warm to
room temperature, and the 31P NMR spectrum revealed many
signals. ESI-MS analysis showed strong peak envelopes at m/z
= 587 ([2N2Ph]+) and 559 ([2Ph]+).
1
based on KBArF ). H NMR (CD2Cl2): δ 8.31 (t, JH−H = 8, 1H, p-H
from [N2C6H5]+4), 8.35 (d, JH−H = 8, 2H, o-H from [N2C6H5]+), 8.02
(dd, JH−H = 8, 2H, m-H from [N2Ph]+), 7.72 (m, 8H, BArF −), 7.57
4
(bs, 4H, BArF −). Anal. Calcd for C38H17BF24N2 (Found): C, 47.13
4
(47.69); H, 1.77 (1.75); N, 2.89 (2.83). IR (CH2Cl2): νNN = 1567
ASSOCIATED CONTENT
cm−1.
■
[Fe2(pdt)(μ-N2Ph)(CO)4(dppv)]BF4. A mixture of 0.509 g (0.70
mmol) of 125 and 0.150 g (0.78 mmol) of [N2Ph]BF4 was cooled to 0
°C and dissolved in 10 mL of CH2Cl2. The resulting dark red reaction
mixture was stirred until the IR spectrum indicated the complete
consumption of starting materials (∼45 min). The product
precipitated as a deep red powder upon addition of 30 mL of hexane.
An extract of the crude product in CH2Cl2 was filtered through Celite
and diluted with hexane to give the product. Yield: 0.59 g (86%).
S
* Supporting Information
Selected spectroscopic details. Crystallographic analysis. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
1
31P{1H} NMR (CD2Cl2): δ 81.4 (s). H NMR (CD2Cl2): δ 7.3−6.9
Notes
(m, Ph), 4.56 (s, C2H2), 3.28 (m, SCH2), 2.90 (m, SCH2), 2.86 (m,
The authors declare no competing financial interest.
3449
dx.doi.org/10.1021/om300107s | Organometallics 2012, 31, 3447−3450

Organometallics
Note
(25) Justice, A. K.; Zampella, G.; De Gioia, L.; Rauchfuss, T. B.; van
der Vlugt, J. I.; Wilson, S. R. Inorg. Chem. 2007, 46, 1655.
ACKNOWLEDGMENTS
■
This research was sponsored by the National Institutes of
Health through grant GM061153.
REFERENCES
■
(1) Fontecilla-Camps, J. C.; Volbeda, A.; Cavazza, C.; Nicolet, Y.
Chem. Rev. 2007, 107, 4273.
(2) Tard, C.; Pickett, C. J. Chem. Rev. 2009, 109, 2245.
(3) Arabi, M. S.; Mathieu, R.; Poilblanc, R. Inorg. Chim. Acta 1977,
23, L17.
(4) Haines, R. J.; de Beer, J. A.; Greatrex, R. J. Chem. Soc., Dalton
Trans. 1976, 1749.
(5) Treichel, P. M.; Crane, R. A.; Matthews, R.; Bonnin, K. R.;
Powell, D. J. Organomet. Chem. 1991, 402, 233.
(6) Georgakaki, I. P.; Miller, M. L.; Darensbourg, M. Y. Inorg. Chem.
2003, 42, 2489.
(7) Olsen, M. T.; Gray, D. L.; Rauchfuss, T. B.; De Gioia, L.;
Zampella, G. Chem. Commun. 2011, 47, 6554.
(8) Tschierlei, S.; Ott, S.; Lomoth, R. Energy Environ. Sci. 2011, 4,
2340.
(9) Olsen, M. T.; Bruschi, M.; De Gioia, L.; Rauchfuss, T. B.; Wilson,
S. R. J. Am. Chem. Soc. 2008, 130, 12021. Olsen, M. T.; Justice, A. K.;
Gloaguen, F.; Rauchfuss, T. B.; Wilson, S. R. Inorg. Chem. 2008, 47,
11816.
(10) Ezzaher, S.; Capon, J.-F.; Gloaguen, F.; Kervarec, N.; Pet
Y.; Pichon, R.; Schollhammer, P.; Talarmin, J. C. R. Chim. 2008, 11,
906. Ezzaher, S.; Capon, J.-F.; Gloaguen, F.; Petillon, F. Y.;
́
illon, F.
́
Schollhammer, P.; Talarmin, J.; Pichon, R.; Kervarec, N. Inorg.
Chem. 2007, 46, 3426. Barton, B. E.; Rauchfuss, T. B. Inorg. Chem.
2008, 47, 2261. Adam, F. I.; Hogarth, G.; Kabir, S. E.; Richards, I. C. R.
Chim. 2008, 11, 890.
(11) Fauvel, K.; Mathieu, R.; Poilblanc, R. Inorg. Chem. 1976, 15,
976. Jablonskyte, A.; Wright, J. A.; Pickett, C. J. Dalton Trans. 2010,
̇
39, 3026.
(12) Kramer, A.; Lingnau, R.; Lorenz, I. P.; Mayer, H. A. Chem. Ber.
1990, 123, 1821. Darensbourg, M. Y.; Weigand, W. Eur. J. Inorg. Chem.
2010, 994. Liu, T.; Li, B.; Singleton, M. L.; Hall, M. B.; Darensbourg,
M. Y. J. Am. Chem. Soc. 2009, 131, 8296.
(13) Zhao, X.; Chiang, C.-Y.; Miller, M. L.; Rampersad, M. V.;
Darensbourg, M. Y. J. Am. Chem. Soc. 2003, 125, 518.
(14) Barton, B. E.; Zampella, G.; Justice, A. K.; De Gioia, L.;
Rauchfuss, T. B.; Wilson, S. R. Dalton Trans. 2010, 39, 3011.
(15) Samkoff, D. E.; Shapley, J. R.; Churchill, M. R.; Wasserman, H.
J. Inorg. Chem. 1984, 23, 397. Yan, X.; Batchelor, R. J.; Einstein, F. W.
B.; Sutton, D. Inorg. Chem. 1996, 35, 7818. Sutton, D. Chem. Rev.
1993, 93, 905.
(16) Bonnet, J. J.; Mathieu, R.; Poilblanc, R.; Ibers, J. A. J. Am. Chem.
Soc. 1979, 101, 7487.
(17) Justice, A. K.; De Gioia, L.; Nilges, M. J.; Rauchfuss, T. B.;
Wilson, S. R.; Zampella, G. Inorg. Chem. 2008, 47, 7405.
(18) Liu, T.; Darensbourg, M. Y. J. Am. Chem. Soc. 2007, 129, 7008.
Justice, A. K.; Rauchfuss, T. B.; Wilson, S. R. Angew. Chem., Int. Ed.
2007, 46, 6152.
(19) Treichel, P. M.; Rublein, E. K. J. Organomet. Chem. 1996, 512,
157.
(20) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877.
(21) Justice, A. K.; Nilges, M.; Rauchfuss, T. B.; Wilson, S. R.; De
Gioia, L.; Zampella, G. J. Am. Chem. Soc. 2008, 130, 5293. Thomas, C.
M.; Liu, T.; Hall, M. B.; Darensbourg, M. Y. Inorg. Chem. 2008, 47,
7009.
(22) Singleton, M. L.; Jenkins, R. M.; Klemashevich, C. L.;
Darensbourg, M. Y. C. R. Chim. 2008, 11, 861.
(23) Olsen, M. T.; Rauchfuss, T. B.; Wilson, S. R. J. Am. Chem. Soc.
2010, 132, 17733.
(24) Flood, D. T. Organic Syntheses; Wiley: New York, 1943; Collect.
Vol. 2, p 295. Rutherford, K. G.; Redmond, W.; Rigamonti, J. J. Org.
Chem. 1961, 26, 5149.
3450
dx.doi.org/10.1021/om300107s | Organometallics 2012, 31, 3447−3450