Phenolate complexes of iron(II) in different spin states

Article abstract of DOI:10.1002/anie.200462153

(Chemical Equation Presented) Spin doctored: In reactions with phenolate anions, the (tetraisopropylcyclopentadienyl)iron fragment forms low-spin or high-spin iron (II) complexes with different coordination geometries. The lithium complex 1 allows a direct comparison of low-spin and high-spin cyclopentadienyl-iron fragments within one molecule.

Full text of DOI:10.1002/anie.200462153

Angewandte
Chemie
Organometallic Chemistry
Phenolate Complexes of Iron(ii) in Different Spin
States**
Mark Wallasch, Gotthelf Wolmershꢀuser, and
Helmut Sitzmann*
4
In 1996, [{4CpFe(m-Br)}2] (1a, Cp = h5-C5H(CHMe2)4) was
reported as the first high-spin cyclopentadienyliron complex,
but could not be prepared as single crystals.[1] With the tri(tert-
butyl)cyclopentadienyl derivative [Cp’’’Fe(m-Br)]2 (1b, Cp’’’ =
h5-1,3,4-tBu3C5H2) structural information can now be
obtained.[2] Because the alkylcyclopentadienyl ligands
employed are bulky enough to hinder unwanted ligand
redistribution reactions that result in ferrocene formation,[3]
1a[1] and 1b offer interesting synthetic possibilities. Reactions
of complexes 1 with substituted phenolates reveal a delicate
balance between different coordination modes (Scheme 1).
Figure 1. Crystal structure of the dimer 1b. Selected interatomic dis-
tances [ꢀ] and angles [8]: Fe-C 2.228(6)–2.302(6) ꢀ, Fe1-Br1 2.5110(13),
Fe1-Br2 2.4746(12), Fe2-Br1 2.4810(12), Fe2-Br2 2.5013, Fe-C5 plane
(4Cp) 1.92, Fe···Fe 3.399(2), Br···Br 3.587(1); Br1-Fe1-Br2 92.01(4),
Br1-Fe2-Br2 92.09(4), Fe1-Br1-Fe2 85.81(4), Fe1-Br2-Fe2 86.16(4).
and the ring planes, which is much longer than the value of
1.72 ꢀ found in octaisopropylferrocene,[4] although octaiso-
propylferrocene is more sterically crowded.
Complexes 1 undergo smooth reactions with phenolates at
ambient temperature (Scheme 1). Depending on the sub-
stituent pattern of the phenolate, the products are different in
color, spectroscopic properties, and structure. With potassium
2,4,6-tri(tert-butyl)phenolate, 1a yields red crystals, which are
easily soluble in pentane and other aprotic solvents, extremely
air-sensitive, and have a magnetic moment of 5.15 mB in
solution corresponding to the presence of four unpaired
electrons. An attempted crystal-structure analysis provided
evidence for a structure change. Unfortunately, the structure
could not be resolved well but it shows the monomeric
complex [(h5-C5HiPr4)Fe(OC6H2tBu3-2,4,6)] (2a) as drawn in
Scheme 1. Since the Cp’’’ derivative of 2a, compound 2b, was
very similar and also did not give suitable crystals for X-ray
diffraction, an experiment with 2,6-di(tert-butyl)phenolate
was carried out in an attempt to obtain better crystals of the
same type of complex.
From the reaction of 1a with potassium 2,6-di(tert-
butyl)phenolate, the diamagnetic compound 3a was obtained
in high yield as red-purple crystals, which are moderately air-
stable. The NMR spectroscopic signals of 3a (see Exper-
imental Section) correspond to a sandwich complex with one
tetraisopropylcyclopentadienyl and one oxocyclohexadienyl
ligand. The crystal structure of 3a[5] is shown in Figure 2. The
distances between the iron atom and the two h5-C5 least-
squares planes are 1.69 ꢀ for the cyclopentadienyl and 1.64 ꢀ
for the oxocyclohexadienyl ligands. The angle between the
least-squares planes is 6.68. The carbon atoms of the six-
membered ring show five bond lengths to the iron center
Scheme 1. Synthesis of complexes 1–4; M=Li or Na.
Like 1a, compound 1b is a high-spin iron(ii) species with
four unpaired electrons and has a magnetic moment in
solution of 5.09 mB (1a: 5.15 mB[1]) The crystal structure of 1b
(Figure 1) shows a distance of 1.92 ꢀ between the Fe centers
[*] Dr. M. Wallasch, Dr. G. Wolmershꢁuser, Prof. Dr. H. Sitzmann
FB Chemie der Universitꢁt Kaiserslautern
Erwin-Schrꢂdinger-Strasse
À
À
ranging from 2.056(2) ꢀ (Fe C54) to 2.252(3) ꢀ (Fe C52)
while the carbonyl carbon atom is found at a nonbonding
distance of 2.414(3) ꢀ from the iron center. The fold angle of
the six-membered ring along the line C52···C56 is 9.58,
whereas for related complexes, fold angles of [Cp*Fe(h5-
C6H5O)] 1.68,[6] (Cp* = h5-C5Me5) for [Cp*Ir(h5-C6H5O)]+
218,[7] for [Cp*Rh(h5-C6H5O)]+ 148,[8] for [Cp*Ru(h5-
C6H5O)] 48,[9] for [Cp*Ru(h5-2,6-tBu2OC6H3)] 19.18[10] and
for [(OC)3Mn(h5-C6H5O)] 21.38[11] have been determined.
67663 Kaiserslautern (Germany)
Fax: (+49)631-205-2187
E-mail: sitzmann@chemie.uni-kl.de
[**] High-spin cyclopentadienyl complexes, part 5. This work was
funded by Deutsche Forschungsgemeinschaft (DFG grant Si366/9-
1 and 9-2). We are grateful to Prof. Dr. O. J. Scherer for his generous
support, and to Dr. G. Hornung for mass spectra. Part 4: H.
Sitzmann, Coord. Chem. Rev. 2001, 214, 287–327.
Angew. Chem. Int. Ed. 2005, 44, 2597 –2599
DOI: 10.1002/anie.200462153
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2597

Communications
of cyclopentadienyl ligand and exhibit different spin states.
4
À
The Fe Cp (ring center) separation for the sandwich part is
1.68 ꢀ, whereas the corresponding value for the other iron–
ring-center separation is 1.97 ꢀ, which is an increase of more
than 17%. This result is comparable to the situation found in
the low-spin and high-spin 1,1’-dimethylmanganocene
(1.73 ꢀ vs. 2.04 ꢀ) by electron diffraction in the gas phase.[15]
Apart from the introduction of other nucleophiles, such as
hydrocarbyl anions, it should be possible to oxidize the readily
available complexes 1 and 2, with their high-spin d6 electron
configuration, to FeIII half-sandwich complexes of the as-yet
unknown type [CpFeX2] with a d5 electron configuration and
without additional donor ligands.
Figure 2. Crystal structure of 3a. Selected interatomic distances [ꢀ]
and angles [8]: Fe-C (C1–C5) 2.074(2)–2.112(2), Fe-C (C52–C56)
2.056(2)–2.252(3), C51-O 1.246(3), Fe···C51 2.414(3); interplanar
angle 6.68.
Experimental Section
The extremely air-sensitive complexes were prepared and handled in
a
glove box (MBraun company, Garching) filled with argon.
The structure of 3a suggests that a third alkyl substituent
in the para position of the phenolate is necessary to stabilize
the s coordination of the phenolate found in 2a. Among the
oxocyclohexadienyl ligands mentioned above, the Ir[7] Rh,[8]
and Ru[9,10] derivatives were formed in direct reactions from
phenolate anions. For cyclopentadienyliron complexes with
oxocyclohexadienyl ligands, indirect synthetic pathways had
to be developed.[6,12]
Several reactions of complexes 1 with 2,6-dimethylphe-
nolate gave oily product mixtures that could not be charac-
terized. Only when the starting compound 1a was prepared
from lithium tetraisopropylcyclopentadienide (and contained
a small amount of lithium bromide thereafter) and the
reaction mixture with 2,6-dimethylphenolate was worked up
after only 10 min of stirring at room temperature, could the
paramagnetic complex 4a be obtained in low yield by
crystallization (Scheme 1).
Measurement of magnetic properties in solution followed a mod-
ification[16] of Evansꢁ method.[17] Complexes 1 were prepared as
described for 1a in ref. [1], obtained in 70–80% yield by evaporation
of the filtered pentane extracts, and were used without further
purification, see ref. [14]. Single crystals of 1b were obtained from
concentrated pentane solution at room temperature.
2a: A solution of K(OC6H2tBu3-2,4,6) (246 mg, 0.82 mmol) in
THF (5 mL) was added to a solution of 1a (300 mg, 0.41 mmol
(dimer)) in THF (5 mL). The solution was stirred for 12 h, then
evaporated to dryness. The solid residue was extracted with pentane
(10 mL) and the filtered extract evaporated to a volume of about
2 mL. At ambient temperature 2a was obtained as red crystals in
252 mg (0.46 mmol, 56%) yield. Elemental analysis (%) calcd for
C35H58FeO (550.70): C 76.34, H 10.62; found C 75.43, H 10.60; EI-MS
(70 eV): m/z (%): 550.2 (6) [M+], 493.2 (5) [M+ÀC4H9], 289.1 (1)
[Fe{C5H(CHMe2)4}+], 262.2 (17) [HOC6H2tBu3+], 247.2 (100)
[HOC6H2tBu3 ÀCH3], 57.1 (88) [C4H9+]; 1H NMR (400 MHz,
+
298 K, C6D6): d = 119.2 (Dn1/2 395 Hz, 12H, CH(CH3)2), 72.9 (Dn1/2
935 Hz, 12H, CH(CH3)2), 28.9 (Dn1/2 404 Hz, 18H, o-C(CH3)3), À10.9
(9H, p-C(CH3)3), À106.0 ppm (Dn1/2 817 Hz, 12H, CH(CH3)2).Signals
for the ring protons of both ligands could not be detected.
The crystal structure of 4a (Figure 3)[13] reveals a lithium
cation[14] coordinated to the oxygen donor sites of one bis(m,k-
O-2,6-dimethylphenolato)(tetraisopropylcyclopentadienyl)-
ferrate(ii) anion and the oxocyclohexadienyl part of another
iron sandwich. Compound 4a is the first example of a
molecule in which two atoms of the same metal in the same
oxidation state within the same molecule carry the same type
3a: A solution of K(OC6H3tBu2-2,6) (200 mg, 0.82 mmol) in THF
(5 mL) was added to a solution of 1a (300 mg, 0.41 mmol) in THF
(5 mL). The solution was stirred for 12 h, then evaporated to dryness.
The solid residue was extracted with toluene (10 mL) and the filtered
extract evaporated to a volume of about 2 mL. At ambient temper-
ature 3a was obtained as violet crystals in 257 mg (0.52 mmol, 64%)
yield. Elemental analysis (%) calcd for C31H50FeO (494.59): C 75.28,
H 10.19; found C 74.20, H 10.19; EI-MS (70 eV): m/z (%): 522.3 (100)
[Fe{C5H(CHMe2)4}2+], 494.2 (3) [M+], 438.2 (7) [M+ÀC4H9], 289.1 (1)
[Fe{C5H(CHMe2)4}+], 181.1 (3) [OC6H3(CH3)2+], 57.1 (24) [C4H9+],
43.0 (25) [C3H7]. 1H NMR (400 MHz, 298 K, C6D6): d = 5.31 (d,
3J(HH) = 6.0 Hz, 2H, meta), 4.90 (t, 3J(HH) = 6.0 Hz, 1H, para), 4.49
(s, 1H, C5H(CHMe2)4), 2.81 (m, 2H, CHMe2), 2.70 (m, 2H, CHMe2),
3
1.71 (s, 18H, C(CH3)3), 1.45 (br, 6H, CH(CH3)2), 1.17 (d, J(HH) =
7.5 Hz, 6H, CH(CH3)2), 1.15 (d, 3J(HH) = 7.5 Hz, 6H, CH(CH3)2),
0.95 ppm (d, 3J(HH) = 6.6 Hz, 6H, CH(CH3)2). 13C{1H}-NMR
(100 MHz, C6D6): d = 152.5 (CO), 105.6, 102.1, 91.4, 83.6, 67.8, 65.8,
36.2 (CMe3), 30.4 (C(CH3)3), 27.2 (CH(CH3)2), 26.9 (CHMe2), 26.7
(CH(CH3)2), 24.2 (CHMe2), 23.6 (CH(CH3)2), 23.2 ppm (CH(CH3)2).
4a: A solution of K(OC6H3Me2-2,6) (131 mg, 0.82 mmol) in THF
(5 mL) was added to a solution of 1a (300 mg, 0.41 mmol) in THF
(5 mL). The solution was stirred for 10 min, then evaporated to
dryness. The solid residue was extracted with pentane (25 mL), the
extract was filtered and evaporated to dryness. Crystals suitable for
X-ray diffraction were grown by slow evaporation of a saturated
pentane solution at room temperature, yield approximately 5 mg
Figure 3. Crystal structure of 4a. Selected interatomic distances [ꢀ]
and angles [8]: Fe1-C (C1–C5) 2.036(4)–2.093(4), Fe1-C (C15–C19)
2.051(5)–2.171(5), Fe1-dienyl plane (C15–C19) 1.58, Fe2-C (C6–C10)
2.276(4)–2.400(4), Li-O1 1.728(8), Li-O (O2–O3) 1.874(8) and
1.860(8), Fe2-O (O2–O3) 1.934(3) and 1.955(3); O2-Fe2-O3 83.97(11),
Li-O-Fe (O2–O3) 93.3(2) and 93.6(3); sum of angles O-Li-O- 359.2
2598
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Angewandte
Chemie
(0.0053 mmol, 1.3%) of green blocks immersed in an oily product
mixture. (Observations and a comment on the crystallization of
extremely soluble complexes from viscous solutions can be found in
ref. [18]).
[10] S. D. Loren, B. K. Campion, R. H. Heyn, T. Don Tilley, B. E.
Bursten, K. W. Luth, J. Am. Chem. Soc. 1989, 111, 4712.
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Received: September 29, 2004
Revised: December 21, 2004
Published online: March 22, 2005
[13] Crystal structure determination of 4a, Fe2LiC58H85O3, Mr =
948.90, IPDS diffractometer with imaging system (Stoe), MoKa
radiation, l = 71.073 pm, 2Vmax = 51.368, 359 images with 08 ꢀ
f ꢀ 3598 and Df = 1.08, T= 293(2) K, crystal dimensions 0.44 ꢃ
0.44 ꢃ 0.08 mm3, monoclinic, space group P21/n, lattice parame-
ters: a = 17.857(3), b = 17.645(2), c = 18.355(3) ꢀ, b =
111.407(18)8, V= 5384.5(15) ꢀ3, Z = 4, 1calcd = 1.171 gcmÀ3, m-
(MoKa) = 5.79 cmÀ1, numerical absorption correction (ABST/
PLATON 98), transmission factors 0.79820 to 0.95565, structure
solution: direct methods (SIR97), full-matrix least-squares
refinement based on F2o (SHELXL-97), 73047 reflections, 9790
unique reflections, 610 parameters, 3 restraints, R1(Fo>2s(Fo)) =
0.0435, wR2(all) = 0.0884, residual electron density + 0.299 to
À0.194 eꢀÀ3, CH and CH3 groups have been refined as rigid
Keywords: coordination chemistry · cyclopentadienyl
.
complexes · iron · magnetic properties · phenolate
[1] H. Sitzmann, T. Dezember, W. Kaim, F. Baumann, D. Stalke, J.
Kꢂrcher, E. Dormann, H. Winter, C. Wachter, M. Kelemen,
Angew. Chem. 1996, 108, 3013 – 3016; Angew. Chem. Int. Ed.
Engl. 1996, 35, 2872 – 2874.
[2] Crystal structure analysis of 1b, Fe2C34H58Br2Fe2, Mr = 738.32:
IPDS diffractometer with imaging system (Stoe), MoKa radia-
tion, l = 71.073 pm, 2Vmax = 51.368, 359 images with 08 ꢀ f ꢀ
3598 and Df = 1.08, T= 293(2) K, crystal dimensions 0.40 ꢃ
0.24 ꢃ 0.12 mm3, monoclinic, space group P21/c, lattice parame-
ters:, a = 13.9684(9), b = 18.6529(16), c = 15.1953(9) ꢀ, b =
À
groups with variable torsion angles (C H 0.96 ꢀ; C-C-H and H-
C-H 109.58).
[14] When complexes 1, isolated by pentane extraction, filtration,
and evaporation, are redissolved in pentane, there is always a
small amount of insoluble material left, which is presumably
alkali halide carried into pentane solution. After two repetitions
of this procedure a visible turbidity can still be observed upon
redissolution. For the reactions described herein the material
from the first pentane extract has been used.
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[17] D. F. Evans, J. Chem. Soc. 1959, 2003.
113.680(7)8, V= 3625.8(4) ꢀ3, Z = 4, 1calcd = 1.353 gcmÀ3
, m-
(MoKa) = 30.24 cmÀ1, numerical absorption correction (ABST/
PLATON 98), transmission factors 0.45615 to 0.70370, structure
solution: direct methods (SIR97), full-matrix least-squares
refinement based on F2o (SHELXL-97), 46839 reflections, 6637
unique reflections, 361 parameters, 0 restraints, R1(Fo>2s(Fo)) =
0.0459, wR2(all) = 0.1093, residual electron density + 0.625 to
À0.335 eꢀÀ3, CH and CH3 groups have been refined as rigid
À
groups (C H 0.96 ꢀ) with variable torsion angles (C-C-H and
[18] F. Baumann, E. Dormann, Y. Ehleiter, W. Kaim, J. Kꢂrcher, M.
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Wolmershꢂuser, H. Sitzmann, J. Organomet. Chem. 1999, 587,
267 – 283.
H-C-H 109.58). Further details on the crystal structure inves-
tigation may be obtained from the Fachinformationszentrum
Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax:
(+ 49)7247-808-666; e-mail: crysdata@fiz-karlsruhe.de), on
quoting the depository numbers CSD-264484 1b, -264486 3a,
and -264485 4a.
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[5] Crystal structure determination of 3a FeC31H50O Mr = 494.59:
IPDS diffractometer with imaging system (Stoe), MoKa radia-
tion, l = 71.073 pm, 2Vmax = 51.368, 359 images with 08 ꢀ f ꢀ
3598 and Df = 1.08, T= 293(2) K, crystal dimensions 0.60 ꢃ
0.48 ꢃ 0.24 mm3, monoclinic, space group P21/n, lattice parame-
ters: a = 9.7545(6), b = 15.7666(8), c = 18.2239(12) ꢀ, b =
91.063(7)8, V= 2802.3(3) ꢀ3, Z = 4, 1calcd = 1.172 gcmÀ3
, m-
(MoKa) = 5.58 cmÀ1, absorption correction with the multi-scan
method (ABST/PLATON 98), transmission factors 0.74642 to
0.89045, structure solution: direct methods (SIR97), full-matrix
least-squares refinement based on F2o (SHELXL-97), 25361
reflections, 5245 unique reflections, 312 parameters, no
restraints, R1(Fo>2s(Fo)) = 0.0365, wR2(all) = 0.0977, residual
electron density + 0.221 to À0.258 eꢀÀ3, CH and CH3 groups
have been refined as rigid groups with variable torsion angles
À
(C H 0.96 ꢀ; C-C-H and H-C-H 109.58).
[6] F. Moulines, L. Djakovitch, M.-H. Delville-Desbois, F. Robert, P.
Gouzerh, D. Astruc, J. Chem. Soc. Chem. Commun. 1995, 463.
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Angew. Chem. Int. Ed. 2005, 44, 2597 –2599
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ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2599