Synthesis and characterization of low-coordinate divalent aryl transition-metal halide analogues of grignard reagents: Precursors for reduction to metal-metal-bonded complexes

Article abstract of DOI:10.1021/ic061121o

The synthesis and characterization of the series of divalent first-row aryl transition-metal(II) halide compounds [Cr(μ-Cl)Ar′]₂ (1) and (Li(OEt₂)Ar′Ml₂]₂ (M = Mn (2), Fe (3), and Co (4); Ar′ = C₆H₃-2,6-(C₆H ₃-2,6-ⁱPr₂)₂) are described. 1-4 were prepared by the addition of one equiv of Ar′Li to the respective transition-metal dihalides. They were characterized by UV-vis spectroscopy, magnetic measurements, and by X-ray crystallography. In dimeric 1, each chromium center has quasi-four-coordinate, square-planar geometry, in which the metal is terminally bound to a terphenyl ligand through the ipso carbon of the central ring and to two bridging chloride ligands. There is a further interaction between chromium and an ipso carbon from one of the flanking -C ₆H₃-2,6ⁱPr₂ rings. In contrast, for the iodo derivatives 2-4, Lil is not eliminated upon addition of LiAr′ to Ml₂. Instead, the diethyl ether solvated adducts, [Li(OEt ₂)Ar′MI₂]₂ (M = Mn (2), Fe (3), or Co (4)) were isolated. These possess a distorted cubane Li₂M ₂l₄ core, in which the lithiums are bound to an ether and the transition metals are bound to a terphenyl group. Magnetic measurements between 2 and 300 K reveal the expected weak antiferromagnetic exchange coupling in each of the complexes.

Full text of DOI:10.1021/ic061121o

Inorg. Chem. 2007, 46, 4809−4814
Synthesis and Characterization of Low-Coordinate Divalent Aryl
Transition-Metal Halide Analogues of Grignard Reagents: Precursors
for Reduction to Metal Metal-Bonded Complexes
−
Andrew D. Sutton,† Tailuan Ngyuen,† James C. Fettinger,† Marilyn M. Olmstead,† Gary J. Long,‡ and
Philip P. Power*,†
Department of Chemistry, One Shields AVenue, UniVersity of California, DaVis, California 95616,
and Department of Chemsitry, UniVersity of Missouri-Rolla, Rolla, Missouri 65409
Received June 20, 2006
The synthesis and characterization of the series of divalent first-row aryl transition-metal(II) halide compounds
[Cr( -Cl)Ar (1) and (Li(OEt2)Ar MI2]2 (M
Mn (2), Fe (3), and Co (4); Ar′ ) C6H3-2,6-(C6H3-2,6-iPr2)2) are
described. 1 4 were prepared by the addition of one equiv of Ar Li to the respective transition-metal dihalides.
They were characterized by UV vis spectroscopy, magnetic measurements, and by X-ray crystallography. In dimeric
µ
′
]
′
)
2
−
′
−
1, each chromium center has quasi-four-coordinate, square-planar geometry, in which the metal is terminally bound
to a terphenyl ligand through the ipso carbon of the central ring and to two bridging chloride ligands. There is a
further interaction between chromium and an ipso carbon from one of the flanking
for the iodo derivatives 2 4, LiI is not eliminated upon addition of LiAr to MI2. Instead, the diethyl ether solvated
adducts, [Li(OEt2)Ar MI2]2 (M Mn (2), Fe (3), or Co (4)) were isolated. These possess a distorted cubane Li2M2I4
−
C6H3-2,6iPr2 rings. In contrast,
−
′
′
)
core, in which the lithiums are bound to an ether and the transition metals are bound to a terphenyl group. Magnetic
measurements between 2 and 300 K reveal the expected weak antiferromagnetic exchange coupling in each of
the complexes.
Introduction
major justification for these investigations has been the study
of new metal-metal bonds, the structures and properties of
their terphenyl-halide precursors are also of significant
interest and have been the subject of several investigations.7-9
In contrast, heteroleptic transition-metal halide species of the
formula Ar′MX and Ar′MX2 have received little attention
and only a handful of examples have been well characterized.
These include [Co(µ-Br)Ar#(THF)]2,10 (Ar# ) (-C6H3-2,6-
(C6H3-2,4,6-CH3)2), and the closed shell species (Et2O)Li-
(ICuAr′).11 There are, however, other related heteroleptic
transition-metal halide species that are based on different
sterically large ligands such as the â-diketiminate derivatives
[({(2,6-iPrC6H3)NtBuC}2CH)MCl] (M ) Ni,12 Co12 and Fe13)
Recent work has shown that several main-group terphenyl
derivatives of the type ArMMAr (M ) Al-Tl, Ge-Pb; Ar
) terphenyl) that have formal M-M multiple bonds can be
synthesized and characterized.1-4 Their stability depends on
the use of the large terphenyl ligand Ar′ ) C6H3-2,6(C6H3-
2,6-iPr2)2,5 and their synthesis was usually effected via the
reduction of terphenyl-metal-halide precursors.6 Although the
* To whom correspondence should be addressed. E-mail: pppower@
ucdavis.edu.
† University of California, Davis.
‡ University of Missouri-Rolla.
(1) Stender, M.; Phillips, A. D.; Wright, R. J.; Power, P. P. Angew. Chem.,
Int. Ed. 2002, 41, 1785-1787.
(2) Wright, R. J.; Phillips, A. D.; Hino, S.; Power, P. P. J. Am. Chem.
Soc. 2005, 127, 4794-4799.
(7) Simons, R. S.; Pu, L.; Olmstead, M. M.; Power, P. P. Organometallics
1997, 16, 1920.
(8) Pu, L.; Twamley, B.; Power, P. P. Organometallics 2000, 19, 2874.
(9) Eichler, B. E.; Pu, L.; Stender, M.; Power, P. P. Polyhedron 2001,
20, 551.
(3) Pu, L.; Twamley, B.; Power, P. P. J. Am. Chem. Soc. 2000, 122, 3524-
3525.
(4) Phillips, A. D.; Wright, R. J.; Olmstead, M. M.; Power, P. P. J. Am.
Chem. Soc. 2002, 124, 5930-5931.
(10) Ellison, J. J.; Power, P. P. J. Organomet. Chem. 1996, 526, 263-
(5) Schiemenz, B.; Power, P. P. Angew. Chem., Int. Ed. 1996, 35, 2150-
267.
2152.
(11) Hwang, C. S.; Power, P. P. J. Organomet. Chem. 1999, 589, 234-
238.
(12) Holland, P. L.; Cundari, T. R.; Perez, L. L.; Eckert, N. A.; Lachicotte,
R. J. J. Am. Chem. Soc. 2002, 124, 14416-14424.
(6) Pu, L.; Phillips, A. D.; Richards, A. F.; Stender, M.; Simons, R. S.;
Olmstead, M. M.; Power, P. P. J. Am. Chem. Soc. 2003, 125, 11626-
11636.
10.1021/ic061121o CCC: $37.00
Published on Web 05/10/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 12, 2007 4809

Sutton et al.
and [({(2,6-iPrC6H3)NMeC}2CH)Mn(I)(THF)].14 In addition,
there are the recently described heteroleptic thiolates of the
type [tBu3SiSMX]n (M ) Fe, Co, X ) Cl, n ) 12; M ) Fe,
Ni, X ) Br, n ) 12; M ) Fe, X ) I, n ) 14)15,16 We showed
recently that the reduction of the heteroleptic chromium(II)
species, Ar′CrCl with KC8 gave the Cr(I)-Cr(I) dimer
Ar′CrCrAr′, in which the chromium atoms were linked
through a 5-fold bonding interaction.17 We were therefore
anxious to characterize the Ar′CrCl precursor and to isolate
and further characterize examples of heteroleptic Ar′MX (M
) first-row transition metal, X ) halide) Grignard-like
species as potential candidates for reduction to give metal-
metal-bonded species. We now report the synthesis of the
series of metal(II) aryl halides [Cr(µ-Cl)Ar′]2 (1) and [Li-
(OEt2)Ar′MI2]2 (M ) Mn (2), Fe (3), or Co (4)). In addition,
we describe their characterization by X-ray crystallography,
allowed to warm to room temperature and was stirred overnight.
The solvent was removed under reduced pressure, and the resulting
light-brown solid was extracted with toluene (30 mL) and filtered.
The volume was reduced to ca. 5 mL, which afforded pale-pink
X-ray-quality crystals of 2‚n-hexane after 1 day at -20 °C. Yield
1
2.84 g (56%), mp ) 270 °C (dec). H NMR (300 MHz, C7D8, 25
°C, δ): 7.20 (br, s), 2.9 (br, s), 1.2 (br, s). UV (toluene) λmax, nm
(ꢀ, mol-1 L cm-1) 296 (3100), 334 (2100).
[Li(OEt2)Ar′FeI2]2‚n-hexane (3‚n-hexane). (LiAr′)2 (1.62 g, 2.0
mmol) in Et2O (25 mL) was added dropwise to a stirred suspension
of FeI2 (1.24 g, 4.0 mmol) in Et2O (25 mL) with cooling in an ice
bath. The resulting dark-red solution was allowed to warm to room
temperature and stirred was overnight. The solvent was removed
under reduced pressure, and the resultant dark-red solid was
extracted with hexanes (30 mL) and filtered. The volume was
reduced to ca. 5 mL, and storage at ca. -20 °C for 3 days afforded
deep-red X-ray-quality crystals of 3. Yield 2.02 g (61%), mp )
1
1
265 °C (dec). H NMR (300 MHz, C6D6, 25 °C, δ): 43 (br, s),
electronic and H NMR spectroscopy, and magnetic mea-
29.3 (br, s), 12.9 (br, s), 9.6 (br. s), 7.4 (br, s), 3.6 (br, s), -6.2 (br,
s), -14.0 (br, s), -19.4 (br, s), -25.4 (br, s), -35.8 (br, s), -38.1
(br, s), -41.3 (br, s), -52.5 (br, s). UV (hexanes) λmax, nm (ꢀ,
mol-1 L cm-1) 394 (7800).
surements.
Experimental Section
General Procedures. All of the manipulations were carried out
using modified Schlenk techniques under an argon atmosphere or
in a Vacuum Atmospheres HE-43 drybox. All of the solvents were
dried over an aluminum column and degassed three times (freeze-
thaw) prior to use. (LiAr′)2 was prepared according to a literature
procedure.5 CrCl2(THF)2 was prepared by literature methods,18 and
CoI2 (Cerac) was used as received. The iodides MnI2 and FeI2
(Aldrich) were further purified before use by washing the crude
[Li(OEt2)Ar′CoI2]2‚n-hexane (4‚n-hexane) (LiAr′)2 (1.62 g, 2.0
mmol) in Et2O (25 mL) was added dropwise to a stirred suspension
of CoI2 (1.25 g, 4.0 mmol) in Et2O (25 mL). The resulting green
solution was allowed to warm to room temperature and stirred
overnight. The solvent was removed under reduced pressure, and
the resultant green solid was extracted with hexanes (30 mL) and
filtered. The volume was reduced to ca. 5 mL, which afforded
emerald-green X-ray-quality crystals upon overnight storage at -20
°C. Yield 1.93 g (58%), mp ) 160-163 °C. 1H NMR (300 MHz,
C6D6, 25 °C, δ): 14.1 (br, s), 9.3 (br, s), 3.1 (br, s). UV (toluene)
1
salts with benzene to remove any excess iodine. H NMR spectra
were recorded on a Varian 300 MHz instrument and referenced to
the residual protio benzene in the C6D6 solvent. Melting points were
recorded in glass capillaries sealed under N2 or Ar and are
uncorrected. UV-vis data were recorded on a Hitachi-1200
spectrometer. Attempts to obtain C, H analytical data gave
inconsistent results probably because of partial desolvation.
[Cr(µ-Cl)Ar′]2‚n-hexane (1‚n-hexane). (LiAr′)2 (1.62 g, 2.0
mmol) in Et2O (25 mL) was added dropwise to a stirred solution
of CrCl2(THF)2 (1.07 g, 4.0 mmol) in Et2O (25 mL) with cooling
to ca. 0 °C. The resulting blue solution was allowed to warm to
room temperature and was stirred overnight. The solvent was
removed under reduced pressure, and the resulting blue solid was
extracted with hexanes (30 mL). The solution was filtered and
concentrated to ca. 5 mL under reduced pressure, which afforded
royal blue X-ray-quality crystals of 1‚n-hexane after storage for
λ
max, nm (ꢀ, mol-1 L cm-1) 294 (6100), 334 (5100), 790 (850).
X-ray Crystallographic Studies. Suitable crystals of the hexane
solvates 1-4 were selected and covered with a layer of hydrocarbon
oil under a rapid flow of argon. They were mounted on a glass
fiber attached to a copper pin and placed in the cold N2 stream on
the diffractometer. X-ray data were collected on a Bruker SMART
1000 diffractometer at 90(2) K using Mo KR radiation (λ ) 0.71073
Å) or on a Bruker SMART Apex II diffractometer at 90(2) K with
Mo KR radiation (λ ) 0.71073 Å). Absorption corrections were
applied using SADABS.19 The structures were solved using direct
methods and refined by the full-matrix least-squares procedure in
SHELX.20 All of the non-hydrogen atoms were refined anisotro-
pically, whereas hydrogens were placed at calculated positions and
included in the refinement using a riding model.
1
24 h at ca. -20 °C. Yield 1.36 g (70%), mp ) 115-119 °C. H
NMR (300 MHz, C6D6, 25 °C, δ): 7.2 (s, br), 2.7 (s, br), 1.0 (s,
Magnetic Studies. The samples for magnetization measurements
wre sealed under N2 in 3 or 4 mm quartz tubing. The sample
magnetization was measured using a Quantum Design MPMSXL7
superconducting quantum interference device (SQUID) magnetom-
eter. For each measurement, the sample was zero-field cooled to 5
K, and the magnetization was measured as a function of field to 2
T. The field was then reduced to 1 T, and the magnetization of the
sample was measured in 0.5 K increments to 4 K, 1 K increments
to 10 K, 2.5 K increments to 60 K, and 10 K increments to 320 K.
br). UV (hexanes) λmax, nm (ꢀ, mol-1 L cm-1) 677 (200).
[Li(OEt2)Ar′MnI2]2‚n-hexane (2‚n-hexane). (LiAr′)2 (1.62 g,
2.0 mmol) in Et2O (25 mL) was added dropwise to a stirred
suspension of MnI2 (1.23 g, 4.0 mmol) in Et2O (25 mL) with
cooling in an ice bath. The resulting pale-yellow solution was
(13) Smith, J. M.; Lachicotte, R. J.; Holland, P. L. Chem. Commun. 2001,
1542-1543.
(14) Chai, J.; Zhu, H.; Most, K.; Roesky, H. W.; Vidovic, D.; Schmidt,
H.-G.; Noltemeyer, M. Eur. J. Inorg. Chem. 2003, 4332-4337.
(15) Sydora, O. L.; Wolczanski, P. T.; Lobkovsky, E. B. Angew. Chem.,
Int. Ed. 2003, 42, 2685-2687.
(16) Sydora, O. L.; Henry, T. P.; Wolczanski, P. T.; Lobkovsky, E. B.;
Rumberger, E.; Hendrickson, D. N. Inorg. Chem. 2006, 45, 609-
626.
(17) Nguyen, T.; Sutton, A. D.; Brynda, M.; Fettinger, J. C.; Long, G. J.;
Power, P. P. Science 2005, 310, 844-847.
(18) Job, R.; Earl, R. Inorg. Nucl. Chem. 1979, 15, 81-83.
(19) SADABS, version 5.0 package; an empirical absorption correction
program from the SAINTPlus NT; Bruker AXS: Madison, WI 1998.
(20) SHELXL, version 5.1; Bruker AXS: Madison WI 1998.
(21) Chai, J.; Zhu, H.; Stueckl, A. C.; Roesky, H. W.; Magull, J.; Bencini,
A.; Caneschi, A.; Gatteschi, D. J. Am. Chem. Soc. 2005, 127, 9201-
9206.
(22) Borras, J. J.; Clemente, J. M. J. Comput. Chem. 2001, 22, 985-991.
4810 Inorganic Chemistry, Vol. 46, No. 12, 2007

Precursors for Reduction to Metal-Metal-Bonded Complexes
Results and Discussion
Synthesis and Spectroscopy. Recent work has shown that
low-valent transition-metal halide derivatives related to the
target compounds, Ar′MX, could be synthesized by using
sterically hindering bidentate â-diketiminate ligands to afford
complexes such as [({2,6-iPr2C6H3)N(tBu)C}2CH)MCl] (M
) Ni,12 Co12, and Fe13) and [({2,6-iPr2C6H3)N(Me)C}2CH)-
Mn(I)]2.14 The latter species can be reduced with potassium
metal to afford the Mn-Mn singly bonded complex (N-
N)MnMn(N-N) (N-N ) {(2,6-iPr2C6H3)N(Me)C}2CH),21
in which the manganese atoms are coordinated by two
nitrogens from the â-diketiminate ligands. However, in order
to prepare species in which the multiple metal-metal
bonding is maximized, the coordination number must be
minimized in order to maintain the largest-possible number
of metal d orbitals available for metal-metal bonding. This
requirement can be met by using bulky monodentate ligands,
such as Ar′, which can both maintain a low-coordination
number at the metal center and stabilize the requisite metal-
halide precursors. 1-4 were synthesized in good yield (ca.
60-70%) by simple addition of one equiv of Ar′Li to the
respective metal dihalide at low temperature to afford a series
of air- and moisture-sensitive complexes. The preparation
of [Cr(µ-Cl)Ar′]2 (1) proceeded smoothly with complete
elimination of LiCl to yield blue crystals of the product in
70% yield (eq 1). Extension of this synthetic protocol to
include late transition-metal analogues has not yet led to
products with sufficient crystal quality for X-ray crystal-
lography. In contrast, the use of M(II) iodides led to the
formation of the cubanes 2-4, which are formed by the
inclusion of LiI in the product (eq 2) as shown by X-ray
crystallography. To date, none of the complexes have
displayed a tendency to undergo equilibria involving dia-
rylmetal species. Our investigations have shown that both
types of complexes are suitable precursors for new metal-
metal-bonded species exemplified by the synthesis and
characterization of a Ar′CrCrAr′ complex with an unprec-
edented Cr-Cr bond formed by a quintuple orbital interac-
tion.17
Figure 1. Temperature dependence of the molar magnetic susceptibility
of [Li(OEt2)Ar′MnI2]2, 2. The black line corresponds to the total fit
comprising the antiferromagnetically coupled dimer, green line, and a trace
of paramagnetic impurity, red line.
any, of the magnetic exchange within the dimeric complexes.
The temperature dependence of the molar magnetic suscep-
tibility of the manganese(II) dimer, 2, is shown in Figure 1;
the corresponding inverse molar magnetic susceptibility is
linear above ca. 150 K and yields a Weiss temperature of
-90 K; below 50 K, the inverse molar susceptibility
increases dramatically. Both the maximum in the susceptibil-
ity observed at ca. 45 K, (Figure 1), and the negative Weiss
temperature are consistent with the expected presence of
weak antiferromagnetic Mn(II)-Mn(II) exchange in 2. Thus,
the magnetic susceptibilities have been fit with the standard-
exchange vector-coupled model22-25 (see Supporting Infor-
mation for details of the Hamiltonian used) with g ) 2.00
as would be expected for this high-spin 3d5 manganese(II)
ion. Initial fits indicated that the molar susceptibility at the
lowest temperatures was somewhat higher than predicted by
the exchange model presumably because of traces of a
paramagnetic Mn(II) or Mn(III) monomer. Thus, a para-
magnetic contribution to the molar magnetic susceptibility
with J ) 0 cm-1 was included in the fits. A least-squares fit
to the observed molar magnetic susceptibility of 2 leads to
a virtually perfect fit, the black line in Figure 1, with an
exchange coupling constant of J ) -6.57 ( 0.02 cm-1, the
green line, and a 1.3% contribution, the red line, from the
paramagnetic impurity. The rather-small negative J value is
reasonable for 2, in which the superexchange through iodides
I1 and I2 involves two 87.04(5)° Mn-I-Mn bond angles;
little or no exchange would be expected for a 90° bond angle.
Et2O, 0 °C
[Cr(µ-Cl)Ar′] + LiCl
2CrCl2(THF)2 + 2Ar′Li
8
2
1
(1)
2Ml2 + 2Ar′Li Et O, 0 °C8 [Li(OEt2)Ar′Ml2]2
M ) Mn (2), Fe (3), Co (4)
2
(2)
The UV-visible spectra of 1-4 are characterized by
intense absorptions below 250 nm corresponding to π-π*
transitions within the terphenyl ligand. In addition, 1 and 3
exhibit low-intensity absorptions centered at 677 nm (ꢀ )
200 mol-1 L cm-1) and 790 nm (ꢀ ) 850 mol-1 L cm-1),
respectively, corresponding to Laporte forbidden d-d transi-
tions. Also, 3 and 4 have bands located at 394 (ꢀ ) 7800
mol-1 L cm-1) and 334 nm (ꢀ ) 5100 mol-1 L cm-1)
respectively. 2 has a featureless spectrum in the visible
region, which is expected for a high-spin d5 system.
(23) Zeigler, H. J., Pratt, G. W. Magnetic Interaction In Solids; Clarendon
Press: Oxford, 1973.
(24) Martin, R. L. In New Pathways In Inorganic Chemistry; Ebsworth,
E. A. V., Maddock, A. G., Sharpe, A. G., Eds.; Cambridge University
Press: Cambridge, 1968, p 175.
(25) Kahn, O. Molecular Magnetism; VCH Publishers: New York, 1993,
pp 38-43.
Magnetism. Magnetic susceptibility studies were carried
out on crystalline samples of 1-4 to evaluate the extent, if
Inorganic Chemistry, Vol. 46, No. 12, 2007 4811

Sutton et al.
Table 1. Selected Crystallographic Data and Collection Parameters for 1-4
1‚n-hexane
2a
3‚n-hexane
4‚n-hexane
formula
fw
color, habit
cryst syst
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
C66H88Cr2Cl2
1056.26
blue, prism
monoclinic
P21/n
11.919(1)
20.682(2)
12.489(1)
90
98.55(1)
90
3044.6(5)
2
C68H94Mn2I4Li2O2
1574.79
pale yellow, block
monoclinic
P21/c
14.487(5)
22.018(6)
25.736(8)
90
104.23(3)
90
7957(4)
4
C74H108Fe2I4Li2O2
1662.78
dark red, plate
monoclinic
P21/c
14.438(3)
22.045(4)
26.090(5)
90
105.86(7)
90
7988(2)
4
C74H108Co2I4Li2O2
1668.94
green, plate
monoclinic
P21/c
14.426(2)
21.985(3)
25.938(4)
90
105.77(8)
90
7916.7(19)
4
Z
d
calcd, Mg/m3
1.152
1.314
1.383
1.400
θ range, deg
1.92-31.5
1.23-27.48
1.23-27.48
1.23-27.48
µ, mm-1
0.482
1.904
1.947
2.017
obs data, I > 2 σ(I)
R1 (obs data)
wR2 (all data)
9572
0.0848
0.2566
18 238
0.0967
0.2570
18 290
0.0367
0.1001
18 139
0.0380
0.1124
a n-hexane is not included in the formula because of extensive desolvation.
Table 2. Selected Bond Lengths (angstroms) and Angles (deg) for 1
The temperature dependence of the molar magnetic
susceptibility of 1, see Figure S1 in the Supporting Informa-
tion, is rather similar to that shown in Figure 1; the
corresponding inverse molar magnetic susceptibility is linear
above ca. 100 K and yields a Weiss temperature of -100
K, and below ca. 60 K the inverse molar susceptibility
increases dramatically. Again, both the maximum in the
susceptibility observed at ca. 40 K, see Figure S1, and the
negative Weiss temperature are consistent with weak anti-
ferromagnetic Cr(II)-Cr(II) exchange, and the magnetic
susceptibility has been fit as discussed above for 2. An
acceptable least-squares fit leads to an exchange coupling
constant of J ) -7.0 ( 0.1 cm-1 and ca. 2% of a
paramagnetic impurity. In this case, the fit at the lowest
temperatures is not perfect, presumably because some portion
of the impurity may be a Cr(III)-Cr(III) exchange-coupled
dimer. However, again the rather-small negative J value is
reasonable for 1, in which the superexchange through Cl
involves two 88.61(4)° Cr-Cl-Cr bond angles.
The temperature dependence of the molar magnetic
susceptibility of 4, see Figure S2, is somewhat similar to
that shown in Figure 1 except that the peak corresponding
to the Ne´el point is narrower and shifted down to 10 K; the
corresponding inverse molar magnetic susceptibility is linear
above ca. 150 K and yields a Weiss temperature of -342
K; below 150 K the inverse molar susceptibility first
decreases and then increases below 10 K (see the inset to
Figure S2). As is often the case for polynuclear cobalt(II)
complexes, a determination of the magnetic exchange present
in 4 required the inclusion of a large pseudo-zero-field
splitting for the nominal 4A2g ground state of cobalt(II) in a
low-symmetry environment. The best fit obtained for the
molar magnetic susceptibility of 4 observed between 2 and
60 K (see Figure S3) yields a g of 1.90, a zero-field splitting,
D, of ca. -100 cm-1, and an exchange coupling constant, J,
of ca. -1 cm-1; values that are typical of cobalt(II) in the
low-symmetry environment found in 4. Rather surprisingly,
Cr(1)-Cl(1)-Cr(1A)
Cl(1)-Cr(1)-Cl(1A)
C(6)-C(1)-Cr(1)
C(2)-C(1)-Cr(1)
C(1)-Cr(1)-Cl(1)
C(1)-Cr(1)-Cl(1A)
C(1)-Cr(1)-C(7)
88.61(4)
91.39(4)
142.0(3)
99.9(2)
100.20(9)
167.9(1)
65.0(1)
Cr(1)-Cl(1)
Cr(1)-Cl(1A)
C(1)-Cr(1)
C(7)-Cr(1)
Cr(1)-Cr(1A)
2.375(2)
2.417(3)
2.041(3)
2.435(3)
3.301(2)
in 4, which has a Co-I-Co bond angle of 85.58(2)°, the
antiferromagnetic exchange coupling is weaker than in 2,
which has a Mn-I-Mn bond angle of 87.04(5)°, a value
that is closer to 90°.
The magnetic properties of 3 resemble those of a para-
magnetic iron cluster, see Figure S4, and are similar to those
observed for [Fe(µ-Br)(µ-SSitBu3)]12.15,16 The inverse molar
magnetic susceptibility of 3 is linear above 70 K and yields
a Weiss temperature of -36 K, a Curie constant of 4.38 mol
Fe/(emu K), and a corresponding µeff per iron of 5.92µB. The
latter value seems to indicate that in spite of careful handling
of the complex the iron(II) expected in 3 has been oxidized
to iron(III). However, it should be noted that the molar
magnetic susceptibility of 3 is also rather consistent12,13 with
the presence of high-spin iron(II) with a small J value of ca.
-0.05 cm-1, a g of ca. 2, and a large iron(II) zero-field
splitting, D, of ca. -20 to -40 cm-1. Although the fit is far
from perfect and poorer than that shown in Figure S3, it is
possible that 3 has not been or has only been partially
oxidized to iron(III).
Structures. Important data collection and refinement
parameters for 1-4 are provided in Table 1, and selected
structural data are given in Tables 2 and 3. The structures
of 1-4 are illustrated by those shown in Figures 2 and 3,
respectively.
The structure of 1 is dimerized through bridging of the
chromium atoms by chlorides and is characterized by an
inversion center midway between the metals. The central Cr2-
Cl2 unit is planar with Cr-Cl-Cr and Cl-Cr-Cl angles of
88.61(4) and 91.39(4)°, respectively. The chromium atom
is bound terminally to a C(1) (or C(1A)) ipso carbon of the
central aryl ring of the terphenyl ligand (Cr(1)-C(1) )
(26) Edema, J. J. H.; Gambarotta, S.; Van Bolhuis, F.; Smeets, W. J. J.;
Spek, A. L.; Chiang, M. Y. J. Organomet. Chem. 1990, 389, 47-59.
4812 Inorganic Chemistry, Vol. 46, No. 12, 2007

Precursors for Reduction to Metal-Metal-Bonded Complexes
Table 3. Selected Bond Lengths (angstroms) and Angles (deg) for 2-4
2
3
4
M(1)-I(1)
2.793(5)
2.847(2)
2.819(2)
2.109(1)
2.847(2)
2.811(4)
2.813(8)
2.091(1)
124.2(3)
128.0(3)
111.5(3)
123.7(3)
126.8(3)
113.5(3)
87.04(5)
86.70(6)
92.77(6)
92.39(6)
2.734(2)
2.776(9)
2.754(1)
2.044(3)
2.772(1)
2.748(9)
2.755(7)
2.038(4)
123.61(9)
128.18(10)
110.26(9)
124.94(10)
124.99(11)
111.04(10)
86.79(3)
86.40(6)
92.59(2)
92.38(2)
2.675(6)
2.714(2)
2.692(1)
1.998(5)
2.709(2)
2.691(1)
2.691(4)
2.004(5)
122.40(1)
126.41(1)
108.87(1)
124.10(1)
122.86(1)
110.01(1)
85.58(2)
85.18(2)
99.14(2)
92.84(2)
M(1)-I(2)
M(1)-I(3)
M(1)-C(1)
M(2)-I(1)
M(2)-I(2)
M(2)-I(4)
M(2)-C(31)
C(1)-M(1)-I(1)
C(1)-M(1)-I(2)
C(1)-M(1)-I(3)
C(31)-M(2)-I(1)
C(31)-M(2)-I(2)
C(31)-M(2)-I(4)
M(1)-I(1)-M(2)
M(1)-I(2)-M(2)
I(1)-M(1)-I(2)
I(1)-M(2)-I(2)
Figure 2. Thermal ellipsoid (30%) plot of [Ar′Cr(µ-Cl)]2‚n-hexane
(1‚n-hexane) (Ar′ ) C6H3-2,6-(C6H3-2,6-iPr2)2)). Hydrogen atoms and
hexane molecules are not shown.
pent-2-ene) (2.3877(9) and 2.3851(9) Å)27 and similar to
those in [{HC(PPh2NSiMe3)2}Cr(µ-Cl)]2 (2.405(2) and
2.437(1) Å).28
2-4 were synthesized by the reaction of the metal diiodide
with one equiv of Ar′Li. The expected byproduct, LiI, does
not separate. Instead, it remains incorporated in the structure
even upon extraction with hexane. 2-4 are isostructural and
isomorphous and adopt a heterocubane structure consisting
of two transition metals, M (M ) Mn (2), Fe (3), or Co
(4)), and two lithium and four iodine atoms. Each transition
metal is four-coordinate and is bound terminally to a carbon
(C(1)) from a terphenyl group and three bridging iodines.
The lithiums are also four-coordinate and are bound to three
bridging iodines and a diethyl ether donor molecule. No
further interactions between the metals and the terphenyls
are apparent. The bond lengths involving the transition-metal
decrease in the sequence Mn > Fe > Co and are in
agreement with the decrease in size of these metals.29
For 2, the Mn-C bond lengths are essentially equal (2.11-
(1) and 2.09(1) Å) and are similar to those in the homoleptic
Mn(II) derivatives [Mn(Mes*)2] (Mes* ) C6H2-2,4,6-tBu3)
(2.108(2) Å)30,31 and [Mn{C(SiMe3)2}2] (2.102(4) Å).32 No
further interactions between the metal center and the flanking
aryl rings of Ar′ are apparent. The Mn-I bonds are in the
narrow range of 2.793(2) to 2.847(2) Å and are slightly
longer than those in the â-diketiminato complex [HC{(Me)C-
(2,6-iPr2H3C6N)}2Mn(µ-I)2Li(OEt2)2]33 (2.7354(9) and 2.7230-
Figure 3. Thermal ellipsoid (50%) plot of [Li(OEt2)Ar′FeI2]2‚hexane (3‚n-
hexane) (Ar′ ) C6H3-2,6-(C6H3-2,6-iPr2)2)). Hydrogen atoms are not shown.
2.041(3) Å) as well as to the two bridging chlorides (Cr-
(µ-Cl) ) 2.375(2) and 2.417(3) Å). The Cr-C bond length
is slightly shorter than those in [Mes2Cr(THF)2].THF26
(2.083(11) Å) and [Mes2Cr(bipy)]26 (2.098(14) Å). However,
there is an additional interaction with an ipso carbon (Cr-
(1)-C(7) ) 2.435(3) Å) of one of the flanking C6H3-2,6-i-
Pr2 rings of the terphenyl group and the metal, which is more
than 0.4 Å longer than that to the ipso carbon. Thus, the
chromium can be considered to have a quasi-four-coordinate,
square-planar environment rather than a three-coordinate
geometry. The interaction of the chromium and the flanking
aryl ring (attached to C(2) of the central ring) is also evident
in the distortion of the bond angles at the ipso carbon. Thus,
the Cr-C(1)-C(2) bond angle (99.9(2)°) is over 40°
narrower than the Cr-C(1)-C(6) angle (142.0(3)°), presum-
ably a result of the Cr(1)-C(7) interaction. The bridging
Cr-Cl bond lengths are almost equal, and they are within
range of the reported Cr-Cl bond lengths for the dichro-
mium(II) complex [(DDP)Cr(µ-Cl)]2 (DDPH ) 2-{(2,6-
diisopropylphenyl)amino}-4-{(2,6-diisopropylphenyl)imino}-
(9) Å) and in [(HC{(Me)C(2,6-iPr2H3C6)2N}Mn(µ-I)]2
14
(2.7688(7) and 2.7484(8) Å). The Li-I bond lengths range
from 2.778(19) to 2.899(19) Å with the longer bonds
corresponding to iodides involved in bridging to the man-
ganese centers. These Li-I bond lengths are similar to those
(27) Gibson, V. C.; Newton, C.; Redshaw, C.; Solan, G. A.; White, A. J.
P.; Williams, D. J. Eur. J. Inorg. Chem. 2001, 1895-1903.
(28) Wei, P.; Stephan, D. W. Organometallics 2002, 21, 1308-1310.
(29) Emsley, J. The Elements; Oxford University Press: Oxford, 1998.
(30) Wehmschulte, R. J.; Power, P. P. Organometallics 1995, 14, 3264-
3267.
(31) Mueller, H.; Seidel, W.; Goerls, H. Angew. Chem., Int. Ed. 1995, 34,
325-327.
(32) Buttrus, N. H.; Eaborn, C.; Hitchcock, P. B.; Smith, J. D.; Sullivan,
A. C. Chem. Commun. 1985, 1380-1381.
(33) Prust, J.; Most, K.; Muller, I.; Stasch, A.; Roesky, H. W.; Uson, I.
Eur. J. Inorg. Chem. 2001, 1613-1616.
Inorganic Chemistry, Vol. 46, No. 12, 2007 4813

Sutton et al.
in the â-diketiminate complexes14,33 (2.817(8) and 2.920(9)
Å). The internal angles of the heterocubane core range from
79.7(6) (Li(1)-I(3)-Li(2)) to 100.4(6)° (I(4)-Li(1)-I(3)),
illustrating the distortion in the heterocubane structure.
The Fe-C bond lengths in 3 (2.044(3) and 2.038(4) Å)
are similar to the Fe-C bonds in the Fe(II) aryl [Fe(Mes*)2]
(2.058(6) Å).30,31 The Fe-I bonds are in the range of 2.7341-
(7) to 2.7769(7) Å and are similar to the Fe-I bond lengths
in [{Ph3PSFeI(µ-I)}2]34 (2.732 and 2.644 Å). As in 2, the
Li-I bonds are in the range of 2.772(7) to 2.831(7) Å with
the longer bonds involving iodides bridging the two iron
atoms. These Li-I bond lengths are comparable to those in
[{(Et3N)LiI}4]35 (2.822(6) to 2.840(6) Å), which has a central
Li4I4 core. The heterocubane core has internal angles between
78.93(13) (Fe(2)-I(2)-Li(2)) and 101.699(19)° (I(2)-Fe(2)-
I(4)), a similar range for 2.
The structure of 4 closely resembles those of 2 and 3. The
Co-C bond lengths are equivalent (1.998(5) and 2.004(5)
Å) and the Co-I bonds are in the range of 2.6755(7) to
2.7141(8) Å, which is also seen for [{Ph3POCoI(µ-I)}2]36
(2.647 and 2.652 Å). The Li-I bonds between 2.768(9) and
2.853(10) Å resemble those in [{(Et3N)LiI}4]35 (2.822(6) to
2.840(6) Å). As with 2 and 3, the longer Li-I bonds involve
coordination to the bridging iodide. The internal angles of
the heterocubane core lie between 77.73(19) and 104.33-
(2)° and show the largest range of such angles in the series
2-4.
Conclusions
A series of first-row transition-metal aryl halides have been
prepared by the reaction of the respective transition-metal
dihalides with the bulky terphenyl lithium reagent Ar′Li (Ar′
) C6H3-2,6-(C6H3-2,6-iPr2)2). These species represent some
of the first well-characterized divalent organo transition-metal
halide complexes. Their isolation underlines the stabilizing
influence of the large terphenyl ligands
Acknowledgment. We thank the donors of the Petroleum
Research Fund administered by the American Chemical
Society for financial support and Peter Klavins for assistance
in magnetization measurements.
Supporting Information Available: Magnetic susceptibility
plots of 1, 3, and 4. Hamiltonian used for the determination of the
exchange coupling in 2. 1H NMR spectra of 1-4. This material is
available free of charge via the Internet at http://pubs.acs.org.
IC061121O
(34) Saak, W.; Haase, D.; Pohl, S. Z. Naturforsch., B: Chem. Sci. 1988,
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4814 Inorganic Chemistry, Vol. 46, No. 12, 2007