Synthesis and characterization of the metal(I) dimers [Ar′MMAr′ ]: Comparisons with quintuple-bonded [Ar′CrCrAr′]

Article abstract of DOI:10.1002/anie.200802657

Cobalt together: The iron and cobalt analogues of the quintuple-bonded chromium species [Ar′CrCrAr′] (Ar′ = C₆H ₃-2,6-(2,6-iPr₂C₆H₃)₂) are described and their structures investigated by cry

Full text of DOI:10.1002/anie.200802657

Angewandte
Chemie
DOI: 10.1002/anie.200802657
Metal–Metal Interactions
Synthesis and Characterization of the Metal(I) Dimers [Ar’MMAr’]:
Comparisons with Quintuple-Bonded [Ar’CrCrAr’]**
Tailuan Nguyen, W. Alexander Merrill, Chengbao Ni, Hao Lei, James C. Fettinger,
Bobby D. Ellis, Gary J. Long, Marcin Brynda, and Philip P. Power*
Metal–metal bonding in the dimeric chromium complex
[Ar’CrCrAr’] (Ar’ = C6H3-2,6(2,6-iPr2C6H3)2 is derived from
the interaction of two d5 {CrAr’} fragments whose valence
electrons are paired to form a quintuple bond.[1–5] The ready
synthesis[1] of this complex by the simple reduction of an aryl
metal halide precursor[6] suggests the possible formation of
similar complexes by other transition metals. Such complexes
might exhibit unprecedented types of interactions between
metals of various dn configurations, and their isolation and
characterization would permit informative comparisons with
the chromium species. Herein, we report the synthesis,
structural and spectroscopic characterization of the iron and
cobalt derivatives [Ar’MMAr’] (M = Fe (1)or Co ( 2)) and
clarify that, although they bear a structural resemblance to
[Ar’CrCrAr’], their metal–metal bonding has little in
Figure 1. Molecular structure of 1 with thermal ellipsoids presented at
a 30% probability level. Hydrogen atoms are omitted for clarity.
common with that of the chromium analogue.
Selected distances [Š] and angles [8]: Fe1–Fe2 2.5151(9), Fe1–
C1 2.028(4), Fe1–C(flanking ring) 2.166(4), 2.192(4), 2.256(4),
2.286(5), 2.309(4), 2.341(5), Fe1–centroid (CNT2) 1.7625(19), Fe2–
C31 2.048(4), Fe2–C(flanking ring) 2.173(4), 2.232(4), 2.278(4),
2.276(5), 2.232(5), 2.221(4), Fe2–centroid (CNT1) 1.7333(18), C1-Fe1-
Fe2 100.41(13), C1-Fe1-CNT2 139.1(2), Fe1-Fe2-CNT1 119.1(3), C31-
Fe2-Fe1 100.39(13), C31-Fe2-CNT1 137.9(2), Fe2-Fe1-CNT2 119.2(2),
C1-C2-C7 118.4(4), C31-C32-C37 118.3(4).
Complexes 1 and 2 were obtained in low yields by the
reduction of [{Li(OEt2)2Ar’FeI2}2][6] (or [{Ar’Fe(m-Br)}2])[7a]
and [{Li(OEt2)2Ar’CoI2}2][6] (or [{Ar’Co(m-Cl)}2])[7b] with the
potassium–graphite intercalate KC8 in tetrahydrofuran. The
metal dimers crystallized as very dark red (1)or very dark
green (2)solids. They were initially characterized by X-ray
crystallography,[8] which revealed that they had a dimeric
structure (Figure 1 and Figure 2, respectively; selected struc-
tural data are provided in the Figure legends).
The overall trans-bent CipsoMMCipso core configuration is
similar to that in [Ar’CrCrAr’].[1] This resemblance is
misleading, however, and masks profound differences both
in the metal–metal and the metal–ligand interactions. In a
manner which appears to be similar to that in [Ar’CrCrAr’],
the iron and cobalt atoms are h1 bonded to an ipso-carbon
center from the central aryl ring of the terphenyl ligand and
are also h6 bonded to a flanking ring of a terphenyl ligand
[*] T. Nguyen, W. A. Merrill, C. Ni, H. Lei, Dr. J. C. Fettinger,
Dr. B. D. Ellis, Dr. M. Brynda, Prof. P. P. Power
Department of Chemistry
University of California, Davis
One Shields Avenue, CA, 95616 (USA)
Fax: (+1)530-752-8995
E-mail: pppower@ucdavis.edu
Figure 2. Molecular structure of 1 with thermal ellipsoids presented at
a 30% probability level. Hydrogen atoms are omitted for clarity.
Selected distances [Š] and angles [d8]: Co1–Co1A 2.8033(5), Co1–
C1A 2.0058(16), Co1–C(flanking ring) 2.1111(16), 2.1482(16),
2.2041(17), 2.2686(16), 2.2246(16), 2.1624(16), Co1-CNT 1.7638(16),
C1-Co1-Co1A 94.26(5), C1A-Co1-CNT1 143.7(3), Co1A-Co1-CNT1
122.0(3), C1-C2-C7 120.15(14).
Prof. G. J. Long
Department of Chemistry
Missouri University of Science and Technology (USA)
[**] We are grateful to the National Science Foundation (CHE-0641020)
for financial support. B.D.E. thanks the NSERC of Canada for a
postdoctoral fellowship. Ar’=C6H3-2,6-(2,6-iPr2C6H3)2; M=Fe or
Co.
attached to the other metal center within the dimer. However,
in complexes 1 and 2, these flanking-ring interactions are
much stronger than those in [Ar’CrCrAr’], as indicated by the
Supporting Information for this article is available on the WWW
under http://angewandte.org or from the author.
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metal–centroid distances, which are almost 0.5 Š shorter.
(2.8033(5)Š) does not support the existence of a double
bond. A second crystal structure of 2 (crystals obtained from
hexane)afforded a noncentrosymmetric structure which,
although it featured an essentially unchanged Co–Co distance
of 2.8010(6)Š and Co-Co-C ipso angles of 94.1(1)and 94.6(1) 8,
Thus, in 1 the iron–centroid distances are 1.733(2)and
1.763(2)Š and in 2 the cobalt–centroid distance is similar at
1.764(2)Š. These lengths, which may be compared to
2.203(6)Š in [Ar ’CrCrAr’], are similar to the iron– and
À
À
cobalt–centroid distances in the complexes [HC{C(Me)N(2,6-
had a torsion angle of 17.28 between the two Cipso Co
centroid planes. Furthermore, the two cobalt–centroid dis-
tances, 1.672(1)and 1.666(1)Š, were approximately 0.1 Š
shorter than that of the structure in Figure 2. The independ-
ence of the Co–Co distances in the two structures on the
torsion angle supports the view that there is no cobalt–cobalt
bond. The low-spin configuration of the cobalt centers in 2, in
which the valence d electrons are paired, is not in agreement
[9]
iPr2C6H3)}2Fe(h6-C6H6)],
[3,5-iPr2Ar*Fe(h6-C6H6)] (3,5-
iPr2Ar* = 2,6(2,4,6-iPr3C6H2)2-3,5-iPr2C6H),[10]
and
[11]
[HC{C(Me)N(2,6-Me2C6H3)}2Co(h6-C7H8)]. The variation
from the mean metal–ring-carbon distance in 1 and 2 is less
than 0.07 Š, whereas, in [Ar’CrCrAr’], the corresponding
interactions involve only the ipso and ortho carbon atoms of
the flanking rings. A more detailed examination of the
À
structural data for 1 and 2 reveals that the average C C bond
with the high spin (S = 1)configuration found in the mono-
[7b]
length within the metal-coordinated flanking arene rings is
approximately 0.02 Š longer than that in the uncomplexed
ring, thus suggesting significant interactions with the metal
center.
nuclear arene complex [3,5-iPr2Ar*Co(h6-C7H8)],
which
has a Cipso-Co-centroid angle (167.608)that is close to
linearity. The magnetic moment in this mononuclear com-
pound, meff = 3.37mB, is consistent with the presence of two
1
The most prominent structural parameters in 1 and 2 are
the Fe–Fe and Co–Co distances, which present an even
greater contrast to the chromium structure. The Fe–Fe
(2.515(9)Š) and Co–Co (2.8033(5)Š) separations greatly
exceed the Cr–Cr bond length in [Ar’CrCrAr’] (1.8351(4)Š).
The Fe–Fe distance slightly exceeds the sum of the single-
bond covalent radii for iron (2.48 Š),[12] whereas the Co–Co
distance is more than 0.3 Š longer than that predicted for a
cobalt–cobalt single bond (2.46 Š).[12] On the basis of these
distances an iron–iron single bond could be expected for 1,
whereas, in 2, the cobalt centers are either weakly bonded or
nonbonded. However, the metal–metal distance, by itself,
provides only limited information on possible bonding
interactions. We used magnetic and computational data for
further insights into the nature of bonding in 1 and 2.
Magnetic measurements for 1 were impeded by the
persistent presence of low levels of impurities with high
magnetic moments, in spite of numerous attempts to elimi-
nate these by recrystallization. Magnetic measurements on
several samples, obtained by separate syntheses, reveal that
they essentially exhibit Curie behavior. However, the prox-
imity of two FeI centers strongly suggests an exchange
interaction between them. As a result we came to the
conclusion that 1 is essentially diamagnetic and that the
magnetic component which furnishes the Curie behavior may
be a superparamagnetic iron oxide or carbide impurity.
Diamagnetism is consistent with an Fe–Fe single bond with
the two remaining electrons at each iron center becoming
paired. Alternatively, the iron centers may be linked by a
triple bond, formed by pairing of the three d electrons from
each iron center. However, the Fe–Fe distance is more
consistent with a single bond. The pairing of the nonbonding
electrons at the iron centers may be a consequence of the bent
nature of the geometry at iron, in contrast to the linear
geometry of the analogous, more crowded, monomer [3,5-
unpaired electrons. H NMR spectroscopic analysis of 2 in
[D8]THF or C6D6 displays signals in the region 0–10 ppm.
The low-spin configuration of the cobalt centers in 2
differs from experimental findings for the mononuclear
cobalt(I)-arene complex [3,5-iPr2Ar*Co(h6-C7H8)][7] and the
theoretical data for [Co(h6-C6H6)] half-sandwich complexes,
which indicate that they are high spin.[13,14] Recent calcula-
tions[5] for the model species [MeCo(h6-C6H6)] also predict a
high spin configuration, that is, S = 1 for a linear Me-Co-
centroid geometry. However, when the Me-Co-centroid angle
is 1358 (a bending of 458 from linearity), electron pairing is
induced and the configuration becomes low spin. In 2, the
C
ipso-Co-centroid angle is 143.78 (a bending of 36.38 from
linearity), which may be sufficient to induce the low-spin
configuration. The presence of paired electrons at each cobalt
center might also account for the increased metal–metal
separation of 2, in comparison to that in 1, as a result of
increased interelectronic repulsion.
Density functional (DFT)calculations, optimized at the
B3LYP/DVZp level on the model species [{2,6-(2,6-
Me2C6H3)2-C6H3}CoCo{2,6-(2,6-Me2C6H3)2-C6H3}], indicated
that the high-spin state is most stable for the molecule (see the
Supporting Information), contrary to experimental data. Such
a discrepancy may be connected with the strongly multi-
configurational character of the arene–metal–aryl moieties,
which is not accounted for in the DFT approach. Further-
more, DFT methods are known to often favor states with
higher multiplicities.[17] Similar calculation on the analogous
model diiron species have not afforded definitive results to
date. The 3B state was found to be lowest in energy but the 5A
state was found to only be approximately 3 kcalmolÀ1 less
stable. Efforts to synthesize a series of diiron and dicobalt
species related to 1 and 2, by using a variety of terphenyl
ligands to fully explain their unusual bonding and electronic
properties, are currently underway.
[10]
iPr2Ar*Fe(h6-C6H6)], which has three unpaired electrons.
The magnetic data for 2 suggest that the two CoI, d8 centers
are either very strongly antiferromagnetically coupled (essen-
tially double-bonded through pairing of two electrons from
Experimental Section
All manipulations were carried out under strictly anhydrous and
anaerobic conditions.
8
each cobalt center)or each have a diamagnetic, low-spin d
configuration. However, the longer Co–Co separation
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Angew. Chem. Int. Ed. 2008, 47, 9115 –9117

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[Ar’FeFeAr’] (1): A light tan solution of [{Ar’FeBr}2], (0.906 g,
1.7 mmol, synthesized from LiAr’ and FeBr2)in THF (30 mL)was
added dropwise over 30 min to a freshly prepared suspension of KC8
(K 0.067 g, 1.7 mmol; C 0.182 g, 15.2 mmol)in THF (25 mL)at 0 8C.
The mixture was stirred for 4 days at 258C, after which the solvent was
removed under reduced pressure. The black residue was extracted
with hexanes (60 mL)and filtered through a cannula. The black
filtrate was concentrated to approximately 15 mL and stored for
4 days at À138C, affording 1 as small red/black blocks (0.030 g, 3.8%
based on Fe). Alternatively, the residue can be extracted with
benzene, concentrated, and stored at 78C to yield 1 as red/black
crystals in 3.3% yield. M.p. 155–1578C, sweats at 1258C. UV/Vis
(hexanes), featureless with increasing absorption at shorter wave-
lengths.
[5] G. La Macchia, L. Gagliardi, P. P. Power, M. Brynda, J. Am.
Chem. Soc. 2008, 130, 5104.
[6] A. D. Sutton, T. Nguyen, J. C. Fettinger, M. M. Olmstead, G. J.
Long, P. P. Power, Inorg. Chem. 2007, 46, 4809.
[7] a)A sample preparation of [{Ar ’Fe(m-Br)}2] is given in the
Supporting Information. Full details and characterization will be
published in a full account of this work; b)H. Lei, C. Ni, J. C.
Fettinger, P. P. Power, unpublished results.
[8] Crystallographic data for 1 and 2 at 90 K with MoKa (l =
0.71073 Š)radiation:
1: a = 11.942(1), b = 18.586(2), c =
21.926(2)Š, V= 4866.5(7)Š 3, Mr = 906.89, orthorhombic,
space group P212121, Z = 4, R1 = 0.0539 (I > 2s(I)), wR2 =
0.1267 for all data. 2·2C7H8: a = 9.7688(4), b = 10.9496(4), c =
14.5087(6)Š, a = 88.104(1)8, b = 84.035(1), d = 76.608(1)8, V=
¯
[Ar’CoCoAr’] (2): A royal blue solution of [{Ar’CoCl}2] (1.48 g,
1.50 mmol)in THF (30 mL)was added dropwise to a freshly prepared
suspension of KC8 (4.404 g, 3.0 mmol)in THF (20 mL)at 0 8C. The
solution turned green immediately. The mixture was stirred for
approximately 24 h. The solvent was removed under reduced pressure
and the black solid residue was extracted with toluene (40 mL). The
dark green organic phase was concentrated to approximately 15 mL,
which, after standing for 3 days at À188C, afforded 2 as green crystals
1501.5(1)Š, Mr = 1097.32, triclinic, space group P1, Z = 1, R1 =
0.0342 (I > 2s(I)), wR2 = 0.0848 all data. CCDC 689809, 689810,
689811, 689812 contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif.
[9] J. M. Smith, A. R. Sadique, T. R. Cundari, K. R. Rodgers, G.
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Holland, J. Am. Chem. Soc. 2006, 128, 756.
1
(0.095 g, 7.0% based on Co). H NMR (300 MHz, [D8]THF, 328C):
d = 1.18 (brs, 12H CH(CH3)2), 1.38 (brs, 12H CH(CH3)2), 2.20 (brs,
12H CH(CH3)2), 2.35 (brs, 12H CH(CH3)2), 2.66 (brs, 4H CH-
(CH3)2), 3.35 (brs, 4H CH(CH3)2), 6.11 (brs, 4H ArHm, (2,6-
iPr2C6H3)), 6.88 (brs, 2H, ArHp, (2,6-iPr2C6H3)), 7.06 (brs, 4H,
ArHm, (C6H3)), 8.10 (brs, 2H ArHp, (C6H3)), 8.85 (brs, 4H, ArHm,
(2,6-iPr2C6H3)), 10.45 ppm (brs, 2H, ArHp, (2,6-iPr2C6H3)); UV/Vis
(hexanes) lmax = 446 nm (e 430 cmÀ1mÀ1).
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Received: June 5, 2008
Revised: July 31, 2008
Published online: October 16, 2008
Keywords: cobalt · density functional calculations · iron · metal–
.
metal interactions · structure elucidation
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