[Fe2{SeC6H2-2,4,6-Ph3}2{N(SiMe3)2}2] and [Fe2{SeC6H2,4,6-Ph3}4], the first three-coordinate selenolatoiron complexes

Article abstract of DOI:10.1002/(SICI)1521-3773(19990201)38:3<377::AID-ANIE377>3.0.CO;2-5

In a controlled manner, half or all the bis(trimethylsilyl)amido ligands of 1 can be replaced by selenolato ligands, resulting in the first selenolatoiron complexes 2 and 3 with three-coordinate iron centers. They are stabilized by secondary metal-framework bonds (here metal-carbon) and could, since they are coordinatively unsaturated, possibly serve as model compounds of the FeMo cofactor of nitrogenases.

Full text of DOI:10.1002/(SICI)1521-3773(19990201)38:3<377::AID-ANIE377>3.0.CO;2-5

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we isolated the novel molecular complexes 1 and 2 in
crystalline form (Scheme 1). Whereas 1 was obtained from
the reaction of iron(ii) bis(trimethylsilyl)amide with 2,4,6-
triphenylselenophenol in the molar ratio of 1:1 in a mixture of
toluene and hexane, the reaction in the 1:2 molar ratio led to
the formation of 2. Complex 1 can also be converted to 2 by
treatment with two further equivalents of 2,4,6-triphenylsele-
nophenol. These results demonstrate that the partial (forma-
tion of 1) or complete (formation of 2) replacement of amide
ligands in the complex iron(ii) bis(trimethylsilyl)amide can be
controlled by varying the reaction conditions.
[Fe2{SeC6H2-2,4,6-Ph3}2{N(SiMe3)2}2] and
[Fe2{SeC6H2-2,4,6-Ph3}4], the First
Three-Coordinate Selenolatoiron Complexes
Ralf Hauptmann, Rainer Kliû, and Gerald Henkel*
Dedicated to Professor Bernt Krebs
on the occasion of his 60th birthday
Iron complexes with sterically nondemanding chalcogen
ligands are characterized by a coordination number of 4 if the
ligand donor functions are negatively charg-
ed. They comprise not only the mixed
sulfidothiolato complexes of composition
/3
[FenSn(SR)4]2
but also the homoleptic
thiolato complexes of general formula [Fe(S-
/2
R)4] , which have attracted considerable
interest as models for the active sites of 2Fe ±
2S (n ˆ 2) and 4Fe ± 4S ferredoxins (n ˆ 4)
and of rubredoxins.[1, 2] Within the ferredox-
Scheme 1. Synthesis of 1 and 2 from [Fe2{N(SiMe3)2}4] and 2,4,6-Ph3C6H2SeH.
in-like complexes, FeS4 tetrahedra are con-
nected through common S ´´´ S edges. Similar
Complex 1 with a mixed selenolatoamido ligand sphere is
the first member of a new class of complexes. Closely related
to 1 is the complex 3, in which iron is replaced by manganese
and which contains additional thf ligands.[11] Interestingly,
complexes with mixed thiolatoamido ligand spheres are also
rare. The few examples known so far include the trinuclear
complexes 4 (M ˆ Zn[12] and Fe[8c]), the dinuclear complex
5,[13] and the mononuclear complex 6.[7] To date, complexes
with mixed tellurolatoamido ligand spheres are completely
unknown.
connectivity patterns also occur in the dinuclear homoleptic
alkane thiolato complexes of general formula [Fe2(SR)6]2 , if
the sulfur atoms are bound to primary or secondary C atoms.
This pattern may change, however, if tert-butane thiolato
ligands are used instead, as illustrated by the unexpected
formation of the dinuclear iron complex [Fe2(SC4H9)5] , in
which the FeS4 tetrahedra share a common face.[3] In the
meantime, related cobalt and nickel complexes are also
known.[4±6] In search of further parameters which may
influence the coordination number and/or the degree of
condensation within thiolatoiron complexes, sterically de-
manding thiolato ligands with bulky substituents have at-
tracted increasing interest in the last few years. Following this
concept, the first complexes of coordination number 2[7] and
3[8] were synthesized and structurally characterized.
Based on these considerations, we extended our approach
to control the degree of condensation and/or the coordination
number in transition metal chalcogenolate complexes by
using selenolato ligands which combine high nucleophilicities
with large spatial demands. Although we have obtained the
first dinuclear cobalt complexes with face-sharing MSe4
tetrahedra by using this concept,[9] comparable results for
iron complexes have so far remained elusive. More successful,
however, were our efforts to stabilize unusually low coordi-
nation numbers. In this paper we report on the first
selenolatoiron complexes of coordination number 3.
[Mn2(SeC6H2-2,4,6-iPr3)2{N(SiMe3)2}2(thf)2]
[M3(SC6H2-2,4,6-iPr3)4{N(SiMe3)2}4]
[Mo2(StBu)2(NMe2)4]
[Fe{SC6H3-2,6-(C6H2-2,4,6-Me3)2}{N(SiMe3)2}]
3
4
5
6
During the formation of 1, the selenolato ligand evidently
prefers the bridging position probably because it is a softer
base than the leaving amide group. From a chemical point of
view, the mixed selenolatoamido complex is of special interest
because it should be possible to introduce ligands other than
those described here by a controlled replacement of the
terminally bonded bis(trimethylsilyl)amide groups by suitable
acid ± base reactions. Therefore, 1 might serve as a starting
material for the synthesis of complexes with mixed chalcogen
ligand spheres (e.g. sulfur/selenium) or complexes of higher
nuclearities.
During our investigations of the reactivity of 2,4,6-triphe-
nylselenophenol towards iron(ii) bis(trimethylsilyl)amide,[10]
Crystals of 1 (Figure 1) and 2 (Figure 2) consist of isolated
electroneutral [Fe2{SeC6H2-2,4,6-Ph3}2{N(SiMe3)2}2] and [Fe2-
{SeC6H2-2,4,6-Ph3}4] molecules, respectively.[14] In both cases,
the iron atoms possess trigonal coordination geometries with
pyramidally distorted FeSe2N (1) and FeSe3 (2) coordination
sites. The mean bond angles of the iron atoms are 115.88 in 1
and 116.38 in 2, respectively. The iron atoms are brigded by
two selenolato ligands. In 2, each of the iron atoms binds a
further selenolato ligand, whereas in the case of 1, the
[*] Prof. Dr. G. Henkel, Dr. R. Hauptmann, Dipl.-Chem. R. Kliû
Institut für Synthesechemie der Universität
Lotharstrasse 1, D-47048 Duisburg (Germany)
Fax : (‡ 49)203-3792110
E-mail: biohenkel@uni-duisburg.de
[**] Financial support by the Deutsche Forschungsgemeinschaft (DFG),
the Bundesminister für Bildung, Wissenschaft, Forschung und Tech-
nologie (BMBF) and the Fonds der Chemischen Industrie is gratefully
acknowledged.
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tion has been discussed in the complex [Fe(SC6H3-2,6-Mes2)2]
with formally two-coordinate iron centers (Fe ´´´ C 2.470 Š).[7]
In all these cases, however, the additional interactions have to
be considered rather weak because Fe ± C distances observed
in other iron complexes containing h2-h6-coordinated carbon
ligands range from 1.90 to 2.25 Š.[15]
The similar geometries of the central Fe2Se2 units are
reflected in the comparable Fe ´´´ Fe distances in
1
(3.170(1) Š) and 2 (3.207(1) Š). The corresponding (m2-Se)-
Fe-(m2-Se) angles are as small as 101.0(1) and 99.0(1)8, and the
Fe-(m2-Se)-Fe angles are 79.0(1) and 81.0(1)8, respectively.
The mean Fe ± (m2-Se) bond lengths in 1 (2.491 Š) and 2
(2.470 Š) differ only slightly. As expected, the terminal Fe ±
Se bonds in 2 are significantly shorter (2.383(1) Š). The Fe ± N
distance in 1 (1.923(2) Š) is in the same range as in the iron(ii)
bis(trimethylsilyl)amide used as starting material.[16]
The results presented here clearly demonstrate that syn-
thetically versatile and sterically demanding selenolato
ligands can be used to prepare novel coordinatively unsatu-
rated complexes that are stabilized by a weak binding
interaction between the metal center and a carbon atom of
a neighboring ligand. This type of interaction could be of
interest to explain the stabilization and activation of the FeMo
cofactor of nitrogenases (binding of small exogenic molecules
such as H2O or N2), which contains coordinatively unsatu-
rated trigonal FeE3 units (E ˆS).[17] Investigations of the
reactivity of related sulfur complexes have already shown that
small neutral molecules with suitable donor functions such as
acetonitrile can be added very easily under expansion of the
coordination sphere of the iron atom.[18]
Figure 1. Structure of
1 in the crystal (methyl groups of the silyl
substituents are omitted for clarity). Selected distances [Š] and angles [8]:
Fe ´´´ Fe 3.170(1), Fe ± Se 2.476(1) and 2.506(1), Fe ± N 1.923(2), Fe ´´´ C
2.602(2); Se-Fe-N 116.6(1), Se-Fe-Se' 101.0(1), N-Fe-Se' 138.7(1), Se-Fe ´´´
C 88.3(1), Se'-Fe ´´´ C 95.6(1), N-Fe ´´´ C 101.7(1).
Experimental Section
All operations were performed in glove boxes under a pure dinitrogen
atmosphere.
1: A solution of 2,4,6-triphenylselenophenol (1.93 g, 5 mmol) in toluene
(20 mL) was added dropwise to a solution of iron(ii) bis(trimethylsilyl)-
amide (1.88 g, 5 mmol) in toluene/hexane (1:1; 20 mL). The red-brown
reaction mixture was stirred for 24 h at ambient temperature and then
filtered. Within a period of three days at 28C, pale brown crystals were
obtained. Yield: 0.63 g (21%).
Figure 2. Structure of 2 in the crystal. Selected distances [Š] and angles [8]:
Fe ´´´ Fe 3.207(1), Fe ± Se(1) 2.491(1), Fe ± Se(2) 2.383(1), Fe ± Se(1)'
2.449(1), Fe ´´´ C 2.530(2); Se(1)-Fe-Se(2) 129.2(1), Se(1)-Fe-Se(1')
99.0(1), Se(2)-Fe-Se(1') 120.7(1), Se(1)-Fe ´´´ C 87.2(3), Se(2)-Fe ´´´ C
119.9(3), Se(1)'-Fe ´´´ C 90.5(3).
2: A solution of 2,4,6-triphenylselenophenol (1.31 g, 3.4 mmol) in toluene
(20 mL) was added slowly to a solution of iron(ii) bis(trimethylsilyl)amide
(0.64 g, 1.7 mmol) in toluene (10 mL). After stirring for 48 h at ambient
temperature, the red-brown reaction mixture was evaporated to dryness.
The residue was recrystallized from hexane/toluene (1:2; 15 mL). Within
two days at ambient temperature, red crystals were obtained. Yield: 0.84 g
(30%).
coordination is completed by a terminal bis(trimethylsilyl)-
amide group.
In both complexes, the iron atoms deviate from the plane
defined by the coordinating atoms by distances of 0.24 Š (1)
and 0.46 Š (2), respectively. This is probably due to a specific
orientation of the ligands which allows for a weak Fe ´´´ C
interaction. One of the phenyl rings of each bridging
selenolato ligand is oriented approximately parallel to the
central Fe2Se2 unit, thus supporting this iron ± carbon contact,
which is roughly normal to the plane defined by the Se2N or
Se3 donor set, respectively. As a consequence, the Fe atom is
shifted from this plane in the direction of the phenyl ring. The
corresponding Fe ´´´ C distances are 2.602(2) (1) and
2.530(3) Š (2), respectively. A similar weak Fe ´´´ C interac-
Received: May 4, 1998
Revised version: September 30, 1998 [Z11809IE]
German version: Angew. Chem. 1999, 111, 389 ± 391
Keywords: iron ´ nitrogen ´ nitrogenase ´ selenium
[1] S. J. Lippard, J. M. Berg, Bioanorganische Chemie, Spektrum, Heidel-
berg, 1995.
[2] a) G. Henkel, B. Krebs, Angew. Chem. 1991, 103, 785; Angew. Chem.
Int. Ed. Engl. 1991, 30, 769; b) R. H. Holm, K. S. Hagen, Inorg. Chem.
1984, 23, 418.
[3] C. Chen, G. Henkel, Inorg. Chem. 1993, 32, 1064.
378
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[4] S. Weiûgräber, G. Henkel, Angew. Chem. 1992, 104, 1382; Angew.
Chem. Int. Ed. Engl. 1992, 31, 1368.
[5] K. Ruhlandt-Senge, P. P. Power, J. Chem. Soc. Dalton Trans. 1993, 649.
[6] A. Silver, M. Millar, J. Chem. Soc. Chem. Commun. 1992, 948.
[7] J. J. Ellison, K. Ruhlandt-Senge, P. P. Power, Angew. Chem. 1994, 106,
1248; Angew. Chem. Int. Ed. Engl. 1994, 33, 1178.
Copper(i)-Catalyzed Enantioselective
Substitution of Allyl Chlorides with
Diorganozinc Compounds**
Frank Dübner and Paul Knochel*
[8] a) P. P. Power, S. C. Shoner, Angew. Chem. 1991, 103, 308; Angew.
Chem. Int. Ed. Engl. 1991, 30, 330; b) K. Ruhlandt-Senge, P. P. Power,
Bull. Soc. Chim. Fr. 1992, 129, 594; c) F. M. MacDonnell, K. Ruhlandt-
Senge, J. J. Ellison, R. H. Holm, P. P. Power, Inorg. Chem. 1995, 34,
1815.
[9] G. Henkel, S. Weiûgräber, unpublished results.
[10] R. A. Andersen, K. Faegri, J. C. Green, A. Haaland, W.-P. Leung, K.
Rypdal, Inorg. Chem. 1988, 27, 1782.
[11] N. Bochmann, A. K. Powell, X. Song, Inorg. Chem. 1994, 33, 400.
[12] H. Grützmacher, K. Steiner, H. Pritzkow, L. Zsolnai, G. Huttner, A.
Sebald, Chem. Ber. 1992, 125, 2199.
[13] M. H. Chisholm, J. F. Corning, J. C. Huffman, Inorg. Chem. 1983, 22,
38.
Catalytic asymmetric allylic substitutions are potentially
useful methods for the preparation of a wide range of chiral
molecules. Several highly enantioselective palladium(0)-cata-
lyzed allylic substitutions have been described in recent
years;[1] however, these reactions are generally limited to
symmetrical substrates. Only recently, several regioselective
palladium(0)-catalyzed asymmetric allylation reactions have
been reported.[2] Alternatively, copper(i)-catalyzed asymmet-
ric allylations can be performed. These reactions proceed with
high SN2' regioselectivity and therefore accommodate unsym-
metrical allylic substrates as starting materials. A further
advantage is that a broad range of organometallic compounds
(organolithium, Grignard, and organozinc reagents) can be
used in these allylic substitutions.[3] Unfortunately, only
moderate enantioselectivities are usually obtained in these
copper(i)-catalyzed allylations.[4] To develop such an asym-
metric reaction, we have screened a broad range of ligands
(amines, diamines, phosphanes, phosphites, sulfur derivatives)
and have observed that primary amines show the highest
catalytic activity. Further fine-tuning of chiral primary amines
indicated that chiral ferrocenyl amines of type 1 are highly
effective ligands. Thus, the substitution of various unsym-
metrical allyl chlorides of type 2 with diorganozinc reagents 3
in the presence of CuBr´ Me2S (1 mol%) and 1 (10 mol%) as
catalyst leads to the desired allylated products 4. The reaction
is highly regioselective (ratio 4:5 > 90:10) and affords prod-
ucts 4 with up to 87% ee (Scheme 1 and Table 1).
[14] Crystal data: Siemens P4RA four circle diffractometer, MoKa
radiation (l ˆ 0.71073 Š), graphite monochromator, rotating-anode
generator, scintillation counter, 150 K, empirical absorption correc-
tions, SHELXTL-PLUS programs, direct methods, least-squares
refinements, one scaling factor, one isotropic extinction parameter.
1: C60H70N2Si4Fe2Se2, formula weight 1201.16, monoclinic, space group
P21/n, a ˆ 10.822(1), b ˆ 17.705(3), c ˆ 15.720(2) Š, b ˆ 106.34(1)8,
3
1
Vˆ 2890.35 Š3, Z ˆ 2, 1calcd ˆ 1.380 gcm
,
m(MoKa) ˆ 1.88 mm
,
transmission range 0.818 ± 0.754, crystal dimensions ca. 0.77 Â 0.32 Â
0.28 mm, w scan, 2qmax ˆ 48o, 4729 unique reflections, R(Rw) ˆ 0.0266
(0.0285) for 3833 observed reflections (I > 2s(I)), 323 variables, non-
hydrogen atoms anisotropic,
H atoms at idealized positions.
2: C96H68Fe2Se4, formula weight 1649.04, monoclinic, space group
P21/n, a ˆ 12.073(4), b ˆ 18.531(7), c ˆ 16.563(6) Š, b ˆ 98.39(3)8, Vˆ
3665.90 Š3, Z ˆ 2, 1calcd ˆ 1.494 gcm 3, m(MoKa) ˆ 2.43 mm 1, trans-
mission range 0.654 ± 0.471, crystal dimensions ca. 0.62 Â 0.25 Â
0.19 mm, w scan, 2qmax ˆ 54o, 7953 unique reflections, R(Rw) ˆ 0.0461
(0.0376) for 4749 observed reflections (I > 2s(I)), 462 variables, non-
hydrogen atoms anisotropic, H atoms at idealized positions. Crystallo-
graphic data (excluding structure factors) for the structures reported
in this paper have been deposited with the Cambridge Crystallo-
graphic Data Center as supplementary publication no. CCDC-101550
(1) and CCDC-101551 (2). Copies of the data can be obtained free of
charge on application to CCDC, 12 Union Road, Cambridge
CB21EZ, UK (fax: (‡44)1223-336-033; e-mail: deposit@ccdc.cam.
ac.uk).
NH2
R
Fe
R2
1 : 10 mol %
+
R22Zn
+
R1
Cl
R1
R2
[15] A. J. Deeming in Comprehensive Organometallic Chemistry (Eds.: G.
Wilkinson, F. G. A. Stone, E. W. Abel), Pergamon, Oxford, 1984.
[16] M. M. Olmstead, P. P. Power, S. C. Shoner, Inorg. Chem. 1991, 30,
2547.
[17] a) J. Kim, D. C. Rees, Nature 1992, 360, 553; b) J. Kim, D. C. Rees,
Science 1992, 257, 1667; c) M. K. Chan, J. Kim, D. C. Rees, Science
1993, 260, 792; d) J. T. Bolin, A. E. Ronco, T. V. Morgan, L. E.
Mortenson, Proc. Natl. Acad. Sci. USA 1993, 90, 1078; e) H.
Schindelin, C. Kisker, J. L. Schlessman, J. B. Howard, D. C. Rees,
Nature 1997, 387, 370.
R1
CuBr Me2S (1 mol %)
THF, -50°C to -90°C, 18h
2
3
4
5
Scheme 1. Copper(i)-catalyzed enantioselective allylation. The groups R1
and R2 are given in Table 1, and the group R is defined in Scheme 2.
The ligands 1 are readily prepared from the ferrocenyl
ketones 6 by Corey ± Bakshi ± Shibata (CBS) reduction, lead-
ing to the ferrocenyl alcohols 8 with high enantioselectivity
(>99% ee).[5] Acylation of alcohols 8a ± d with acetic anhy-
dride furnishes in quantitative yield the intermediate ferro-
cenyl acetates 9a ± d. Treatment with an aqueous NH3
solution in CH3CN provides the desired ferrocenyl amines
1a ± d in 44 ± 66% yield (Scheme 2).
[18] R. Hauptmann, R. Kliû, J. Schneider, G. Henkel, Z. Anorg. Allg.
Chem. 1998, 624, 1927.
[*] Prof. Dr. P. Knochel, Dipl.-Chem. F. Dübner
Fachbereich Chemie der Universität
Hans-Meerwein-Strasse, D-35032 Marburg (Germany)
Fax: (‡49)6421-28-21-89
E-mail: knochel@ps1515.chemie.uni-marburg.de
[**] F.D. thanks Zeneca Ltd. for
a fellowship and the Fonds der
Chemischen Industrie for generous support. We thank the BASF AG,
Degussa AG, Bayer AG, and Chemetall GmbH for generous gift of
chemicals.
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379