Binuclear iron(II) complex from a linked-bis(amidinate) ligand: Synthesis and its reaction with carbon monoxide

Article abstract of DOI:10.1039/b111651g

Binuclear iron(II) complexes supported by a cyclohexane-linked bis(amidinate) ligand have been isolated and structurally characterized.

Full text of DOI:10.1039/b111651g

Binuclear iron(II) complex from a linked-bis(amidinate) ligand: synthesis
and its reaction with carbon monoxide
Hiroyuki Kawaguchi* and Tsukasa Matsuo
Coordination Chemistry Laboratories, Institute for Molecular Science and CREST, Japan Science and
Technology Corporation (JST), Myodaiji, Okazaki 444-8585, Japan. E-mail: hkawa@ims.ac.jp
Received (in Cambridge, UK) 21st December 2001, Accepted 18th March 2002
First published as an Advance Article on the web 3rd April 2002
Binuclear iron(II) complexes supported by a cyclohexane-
linked bis(amidinate) ligand have been isolated and struc-
turally characterized.
yellow crystals in 47% yield.† Complex 2 is air- and moisture-
sensitive, is very soluble in common organic solvents, and is
crystallized from Et2O. Since the racemic L22 ligand is used,
the product is expected to consist of a heterochiral binuclear
complex [Fe2(LRR)(LSS)] and a racemic mixture of homochiral
Bimetallic complexes offer the potential to promote chemical
transformations that could not be achieved in mononuclear
systems.1 On the other hand, many examples of bimetallic
complexes are not able to show high reactivity, due to undesired
bridging interactions between metals or due to the lack of an
available coordination site. Thus, ligands should be designed to
fix two metal centers having sites for binding and activating
substrates. In this context, we chose to employ the linked
bis(amidinate) as a binucleating ligand, because amidinate
complexes display a wide variety of reactivity.2 However,
complexes with the linked bis(amidinate) ligand have yet to be
explored.3–5 Herein we describe binuclear iron(II) complexes
supported by a cyclohexane-linked bis(amidinate) ligand.
The linked benzamidinate was synthesized in three steps that
are entirely analogous to those employed for the preparation of
functionalized benzamidinates Li[Me3NSiC(Ph)NR] (Scheme
1).5,6 The silylamine trans-(Me3SiNH)2C6H10 1 was prepared
by the reaction of racemic trans-1,2-diaminocyclohexane with
ClSiMe3 and NEt3 in Et2O. Treatment of 1 with 2 equiv. of
BunLi, followed by addition of 2 equiv. of benzonitrile in THF,
yielded the 1,2-cyclohexanediyl-linked bis(benzamidinate) salt
Li2[{Me3SiNC(Ph)N}2C6H10] [Li2(L)]. The THF solution of
Li2(L) was used in the synthesis of amidinate complexes,
although we found that the dilithium salt Li2(L) could be
isolated as a THF adduct.
binuclear complexes [L,L-Fe2(LRR
)
and D,D-Fe2(LSS)2].
2
1
However, the H NMR spectrum indicates the formation of a
single species in solution. X-Ray analysis of several crystals
eventually revealed that the product is a racemic mixture of
homochiral binuclear species in the unit cell.‡ An intriguing
aspect of this reaction is ligand self-recognition, resulting in the
formation of two stereospecific complexes, L,L-Fe2(LRR)2 and
D,D-Fe2(LSS)2.
The linked amidinate ligand acts as a binucleating ligand
(Fig. 1). The molecule possesses a crystallographically imposed
2-fold axis passing through two iron atoms. The cyclohexyl
spacer adopts a chair conformation, and the amidinate groups
are in equatorial positions. Each iron atom assumes a distorted
tetrahedral geometry defined by four N atoms of the amidinate
units, which is similar to those of the monomeric bis(amidinate)
iron(II) complexes Fe[FcC(NCy)2]2 (Fc = ferrocenyl)7 and
Fe[ButC(NCy)2]2.8 The benzamidinate groups form four-
membered rings almost coplanar with the iron atoms [torsion
angles Fe–N–C–N of the FeN2C rings = 5.1(2)° for Fe(1) and
2.4(2)° for Fe(2)]. The average Fe–N(1, 4) distances attached to
the SiMe3 group (2.066 Å) are slightly longer than those
attached to the cyclohexyl bridging unit [Fe–N(2, 3), 2.046 Å].
The separation of the iron atoms is 3.516(1) Å, indicative of no
metal–metal interaction. The closely related, linked bis(amidi-
nate) ligand was reported to coordinate solely to one titanium
metal center.3
Reaction of FeCl2 with 1 equiv. of Li2(L) in THF afforded a
red, homogeneous solution from which a paramagnetic binu-
clear iron(II) amidinate complex Fe2(L)2 2 was obtained as
To test the robustness of the binuclear structure of 2, we
carried out the reaction of 2 with carbon monoxide. Complex 2
was found to readily react with 1 atm of CO even in the solid
Fig. 1 Structure of D,D-Fe2(LSS
) 2. Phenyl groups (except for the ipso
2
carbon) and methyl groups on Si are omitted for clarity. Selected
interatomic distances (Å) and angles (°): Fe(1)–Fe(2) 3.516(1), Fe(1)–N(1)
2.069(2), Fe(1)–N(2) 2.043(2), Fe(2)–N(3) 2.049(2), Fe(2)–N(4) 2.062(2),
N(1)–C(1) 1.346(3), N(2)–C(1) 1.320(3), N(3)–C(2) 1.321(3), N(4)–C(2)
1.339(3); N(1)–Fe(1)–N(2) 66.03(7), N(3)–Fe(2)–N(4) 66.19(7), N(1)–
C(1)–N(2) 114.4(2), N(3)–C(2)–N(4) 115.2(2).
Scheme 1
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CHEM. COMMUN., 2002, 958–959
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state as well as in Et2O to afford a paramagnetic orange
compound Fe2(L)2(CO)2 3 quantitatively. Irradiation of 3
resulted in regeneration of 2 concomitant with the loss of CO.
According to X-ray analysis (Fig. 2),‡ the homochiral binuclear
frame was retained during the reaction. A crystallographic C2
axis again runs though the two iron atoms. Complex 3 contains
the two different iron atoms: one iron atom [Fe(1)] is placed in
the center of a disordered octahedron defined by four nitrogen
atoms of two benzamidinate groups and two carbonyl ligands;
the other iron atom [Fe(2)] adopts a distorted tetrahedral
geometry with two amidinate groups. The solid structure of 3 is
consistent with its 1H NMR spectrum in solution, which shows
two SiMe3 protons as well as 16 resonances attributable to
phenyl and cyclohexane groups. In the IR spectrum, the n(CO)
bands of 3 are observed at 2018 and 1944 cm21 and are shifted
to higher frequencies than those of Fe[ButC(NCy)2]2(CO)2
(1999, 1929 cm21).8
The dihedral angles between the two Fe–amidinates of 85.4
and 84.8° for Fe(1) and Fe(2) in 3 are lager than those of 2 (59.9
and 68.1°). The geometrical parameters within the FeN2C rings
in 3 are comparable to those found in 2. However, the tethering
of the amidinate groups in 3 restricts the N(4)–Fe(2)–N(4A)
angle at the tetrahedral iron site, which is 128.9(2)° and acute
compared with the corresponding angles at 3 [154.2(1) and
140.2(1)°]. The Fe–Fe separation of 3 was elongated by 0.94 Å
compared with that of the parent complex 2, indicating that the
size of the cage created by the L22 ligands is flexible. The
stability of 3 deserves comment. Although 3 contains a
14-electron iron site, it is found to be stable in the solid state for
a few minutes under air. In contrast, 2 immediately decomposes
in air. This is ascribed to the rigid conformation, which is locked
by the CO ligands and prevents 3 from undergoing further
reaction. This reason is related to the formation of 3, and
coordination of additional CO is hampered in the remaining
tetrahedral iron site. Studies of the reactivity of binuclear iron
complexes 2 and 3 are in progress.
Notes and references
†
Preparation of 2: all manipulations were carried out under an
atmosphere of argon. A solution of Li2(L) (2.31 mmol) in THF (40 mL) was
added to a slurry of FeCl2 (0.29 g, 2.31 mmol) in THF (20 mL) at 0 °C. The
solution was stirred for 18 h. The solvent was evaporated to dryness, and the
red residue was extracted with Et2O. Concentration and cooling to 230 °C
gave 0.56 g of 2 as yellow crystals in 47% yield. 1H NMR (C6D6, 500 MHz):
d 127.8 (2H), 31.6 (2H), 21.1 (4H, Ph), 13.3 (4H, Ph), 10.6 (2H), 5.9 (2H),
2.5 (18H, SiMe3 + 2H), 2.0 (2H). Anal. calc. for C52H76N8Si4Fe2: C, 60.21;
H, 7.39; N, 10.80. Found: C, 59.67; H, 7.33; N, 10.51%.
Preparation of 3: a solution of 2 (0.14 g, 0.14 mmol) in Et2O (20 mL) was
stirred under 1atm of CO at room temperature for 16 h, after which the
solvent was removed in vacuo. The remaining orange crystalline solid was
rinsed with hexane and dried to give 0.14 g of 3 in 95% yield. IR (Nujol,
KBr)/cm21: 2018 (s), 1944 (s). 1H NMR (C6D6, 500 MHz): d 127.5 (1H),
26.3 (1H), 23.5 (2H, Ph), 22.8 (1H), 19.3 (2H, Ph), 11.0 (1H), 3.6 (9H,
SiMe3), 2.6 (1H), 2.0 (1H), 0.9 (1H), 0.2 (2H, Ph), 0.1 (2H, Ph), 20.1 (1H),
22.0 (9H, SiMe3), 24.4 (1H), 25.6 (1H), 26.8 (1H), 27.6 (1H). Anal.
calc. for C54H76N8O2Si4Fe2: C, 59.33; H, 7.01; N, 10.25. Found: C, 59.02;
H, 6.85; N, 9.89%.
‡
Crystal data: for 2: C52H76N8Si4Fe2, M = 1037.26, monoclinic, space
group C2/c, a = 23.6648(9), b = 18.6585(9), c = 13.5164(5) Å, b =
92.4516(10)°, V = 5962.7(4) Å3, Z = 4, T = 193 K, m(Mo-Ka) = 6.05
cm21, Rigaku Mercury, 28 006 measured reflections (2qmax = 55°), 6822
unique, 337 variables, R1 = 0.038 [I > 2s(I)], wR2 = 0.111 (all data), and
GOF = 1.03.
For 3: C54H76N8O2Si4Fe2, M = 1093.28, monoclinic, space group C2/c,
a = 23.912(4), b = 18.384(2), c = 13.4886(5) Å, b = 92.6946(9)°, V =
5923.0(9) Å3, Z = 4, T = 193 K, m(Mo-Ka) = 6.15 cm21, Rigaku
Mercury, 27 945 measured reflections (2qmax = 55°), 6690 unique, 355
variables, R1 = 0.041 [I > 2s(I)], wR2 = 0.127 (all data), and GOF =
1.00. All structures were solved by direct methods and refined on F2 by full-
matrix, least squares using the CrystalStructure software package.
CCDC reference numbers 176896 and 176897. See http://www.rsc.org/
suppdata/cc/b1/b111651g/ for crystallographic data in CIF or other
electronic format.
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Fig. 2 Structure of D,D-Fe2(LSS)2(CO)2 3. Phenyl groups (except for the
ipso carbon) and methyl groups on Si are omitted for clarity. Selected
interatomic distances (Å) and angles (°): Fe(1)–Fe(2) 4.4550(10), Fe(1)–
N(1) 2.029(3), Fe(1)–N(2) 2.036(3), Fe(2)–N(3) 2.034(3), Fe(2)–N(4)
2.076(3), N(1)–C(1) 1.344(4), N(2)–C(1) 1.313(4), N(3)–C(2) 1.322(4),
N(4)–C(2) 1.340(4), Fe(1)–C(27) 1.764(4), C(27)–O(1) 1.146(4); N(1)–
Fe(1)–N(2) 65.77(11), N(3)–Fe(2)–N(4) 65.80(11), N(1)–C(1)–N(2)
112.3(3), N(3)–C(2)–N(4) 114.0(3), Fe(1)–C(27)–O(1) 176.1(3), C(27)–
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