Methyl-bridged transition metal complexes (M = Cr - Fe) supported by bulky terphenyl ligands

Article abstract of DOI:10.1021/om900724p

The synthesis and characterization of a series of divalent terphenyl transition metal methyl complexes {ArM(μ-Me)}₂ (M = Cr (1), Ar = C₆H-2, 6-(C₆H₂-2, 4, 6-ⁱPr ₃)₂-3,5-ⁱPr₂, abbreviated as 3,5-ⁱPr₂-Ar*; M = Mn (2), Fe (3), Ar = C ₆H₃-2, 6-(C₆H₂-2, 4, 6- ⁱ:Pr₃)₂, abbreviated as Ar*) as well as a phenyl-bridged iron complex {Ar*Fe(μ-Ph)}₂ (4) are described. Complexes 1-4 were prepared by reaction of the corresponding terphenyl metal halide with LiMe or LiPh and have dimeric structures, in which threecoordinate metals are bridged by methyl or phenyl groups. Each chromium atom in 1 features a further interaction with the ipso - carbon of the flanking aryl ring; while the manganese and iron atoms in 2 and 3 have distorted trigonal-planar geometry. Complexes 1-4 are rare examples of low-coordinate transition metal complexes of methyl or phenyl groups, and they were characterized by X-ray crystallography, NMR, and UVna - vis spectroscopy, elemental analysis, and magnetic measurements.

Full text of DOI:10.1021/om900724p

Organometallics 2009, 28, 6541–6545 6541
DOI: 10.1021/om900724p
Methyl-Bridged Transition Metal Complexes (M = Cr-Fe) Supported
by Bulky Terphenyl Ligands
Chengbao Ni and Philip P. Power*
Department of Chemistry, University of California, Davis, California 95616
Received August 18, 2009
The synthesisand characterizationofaseriesof divalent terphenyltransitionmetal methylcomplexes
{ArM(μ-Me)}2 (M = Cr (1), Ar = C6H-2,6-(C6H2-2,4,6-iPr3)2-3,5-iPr2, abbreviated as 3,5-iPr2-Ar*;
M = Mn(2), Fe(3), Ar= C6H3-2,6-(C6H2-2,4,6-iPr3)2, abbreviatedasAr*) aswellasaphenyl-bridged
iron complex {Ar*Fe(μ-Ph)}2 (4) are described. Complexes 1-4 were prepared by reaction of the
corresponding terphenyl metal halide with LiMe or LiPh and have dimeric structures, in which three-
coordinate metals are bridged by methyl or phenyl groups. Each chromium atom in 1 features a further
interaction withthe ipso-carbon of the flanking arylring; whilethe manganese and ironatoms in2 and 3
have distorted trigonal-planar geometry. Complexes 1-4 are rare examples of low-coordinate
transition metal complexes of methyl or phenyl groups, and they were characterized by X-ray
crystallography, NMR, and UV-vis spectroscopy, elemental analysis, and magnetic measurements.
Introduction
[Li(THF)4][Mn2Ph6],13 [{(Et2O)Li}4][FePh4],14 and [{(Et2-
O)2Li}{Li(1,4-dioxane)}{FePh4}].15 A characteristic feature
of all the above complexes is that the metals have a coordina-
tion number of at least 4. Low-coordinate (coordination
number e3) methyl or phenyl complexes are quite scarce.
For Cr-Fe, structurally characterized methyl complexes
are limited to the β-diketiminato M(II) derivatives
HC{C(R)NAr}2M(Me) (Ar = C6H3-2,6-iPr2; R = Me or
tBu; M=Cr or Fe).16-19 The related phenyl species are the
β-diketiminato complexes [HC{C(Me)NAr}2]MPh (M =
Mn20 or Fe21) and the heterometallic derivative {(tmeda)-
Na(μ-tmp)(μ-Ph)Mn(tmp)} (tmp = 2,2,6,6-tetramethylpi-
peridide).22 Herein, we report the preparation and
molecular structures of the dimeric complexes {ArM(μ-
Me)}2 (M = Cr (1), Ar = 3,5-iPr2-Ar*; M = Mn (2), Fe
(3), Ar = Ar*) and {Ar*Fe(μ-Ph)}2 (4) supported by bulky
terphenyl ligands.
Organometallic open-shell derivatives of the simplest
alkyl and aryl ligands, methyl and phenyl, are usually
stabilized by a variety of coligands, such as carbonyl, cyclo-
pentadienyl, or phosphine.1 Binary (or homoleptic) transi-
tion metal methyl or phenyl compounds are generally stable
only if the coordination sphere is saturated.2,3 The coordi-
native saturation blocks decomposition pathways that re-
quire open sites at the metal, as exemplified by the isolation
of WMe6.4 The use of Coulombic repulsion, combined with
ionic interactions, has allowed the stabilization of numerous
stable homoleptic transition metal methyl and phenyl
anionic species as contact ion pairs with alkali metal complexes.
Examples include [{Li(THF)}4{Cr2Me8}],5 [{Li(1,4-dioxane)}3-
{CrMe6}],6 [{(tmeda)Li}2{CrMe4}] (tmeda = N,N,N0,N0-tetra-
methylethylene-1,2-diamine),7 [{(tmeda)Li}2{MnMen}] (n =
4-6),8,9 [(tmeda)2Li][MnMe4],10 [{(Et2O)Li)}2{LiMe}-
{FeMe4}],11 [{(THF)Li}2VPh6],12 [{(THF)Li}3CrPh6],12
(14) Bazhenova, T. A.; Lobkovskaya, R. M.; Shibaeva, R. P.; Shilov,
A. E.; Shilova, A. K.; Gruselle, M.; Leny, G.; Tchoubar, B. J. Org-
anomet. Chem. 1983, 244, 265.
(15) Furstner, A.; Martin, R.; Krause, H.; Seidel, G.; Goddard, R.;
Lehmann, C. W. J. Am. Chem. Soc. 2008, 130, 8773.
(16) Fan, H. J.; Adhikari, D.; Saleh, A. A.; Clark, R. L.; Zuno-Cruz,
F. J.; Cabrera, G. S.; Huffman, J. C.; Pink, M.; Mindiola, D. J.; Baik, M.
H. J. Am. Chem. Soc. 2008, 130, 17351.
*Corresponding author. E-mail: pppower@ucdavis.edu.
(1) Elschenbroich, C.; Oliveira, J. Organometallics, 3rd ed.; Wiley-VCH:
Weinheim, 2006.
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Lachicotte, R. J. J. Am. Chem. Soc. 2002, 124, 14416.
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A.; Lachicotte, R. J.; Flaschenriem, C. J.; Holland, P. L. Organometal-
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Organometallics 2004, 23, 1177.
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r
2009 American Chemical Society
Published on Web 10/29/2009
pubs.acs.org/Organometallics

6542 Organometallics, Vol. 28, No. 22, 2009
Ni and Power
Experimental Section
Table 1. Selected Crystallographic Data and Collection Para-
meters for 1-4
General Procedures. All manipulations were carried out using
modified Schlenk techniques under an argon atmosphere or in a
vacuum atmospheres HE-43 drybox. All solvents were dried by
1 2 n-hexane
2a
3 2.5C6H6
4
3
3
formula
fw
color
C98H156Cr2
1438.23
blue
C74H104Mn2 C89H118Fe2
C84H108Fe2
1229.40
orange
the method of Grubbs,23 followed by storage over 3 A molecular
˚
1103.45
colorless
block
1300.54
orange
plate
sieves overnight, and degassed three times (freeze-thaw) prior
to use. The metal halide precursors were prepared according to a
literature procedure.24 LiPh was prepared by treating PhI with
nBuLi in hexanes and isolated as a white powder. LiMe (1.6 M in
Et2O) was purchased from Acros Organics and used as received.
Melting points were recorded in glass capillaries sealed under N2
and are uncorrected. UV-vis data were recorded on a Hitachi-
1200 spectrometer. Magnetic susceptibility measurements were
conducted by Evans’ method25,26 on a Bruker 300 MHz NMR
spectrometer at 23 °C. The 1H NMR spectra were recorded on a
Mercury 300 MHz NMR spectrometer at 20 °C.
habit
cryst syst
space group
plate
monoclinic
P21/c
plate
tetragonal
I41/acd
triclinic
P1
triclinic
P1
˚
a, A
16.2132(5)
15.4750(5)
36.8813(11)
90
13.242(2)
13.805(2)
18.525(3)
87.230(2)
85.699(2)
82.118(2)
3342.5(9)
2
13.4414(7)
13.5328(6)
22.4499(12)
17.598(8)
17.598(8)
46.42(2)
˚
b, A
˚
c, A
R, deg
β, deg
γ, deg
103.4480(10) 90
98.112(1)
90
93.703(2)
99.797(2)
3890.6(3)
2
90
90
3
˚
V, A
Z
9160.9(5)
4
1.043
14377(11)
8
{3,5-iPr2-Ar*Cr(μ-Me)}2 (1). A diethyl ether solution of LiMe
(1.6 M, 0.39 mL, 0.62 mmol) was added slowly via a syringe to
{3,5-iPr2-Ar*Cr(μ-Cl)}2 (0.392 g, 0.30 mmol) in Et2O (30 mL)
with cooling to ca. -78 °C. The mixture was warmed slowly to
room temperature and stirred for one day. All volatile materials
were removed, and the blue solid was extracted with hexanes (40
mL). The green-blue filtrate was concentrated to ca. 10 mL,
which afforded X-ray quality blue crystals of 1 after storage for
several days at 7 °C. Yield: 0.243 g (63.9%). Mp: 134-136 °C
d
calcd, Mg/m3
1.096
1.110
1.136
2.62-27.50
0.446
2890
θ range, deg
μ, mm-1
2.59-27.49
0.279
14 328
2.52-25.25
0.416
7420
1.55-27.50
0.415
15 854
obsd data,
I > 2σ(I)
R1 (obsd data) 0.0554
0.0603
0.1740
0.0379
0.1019
0.0341
0.0960
wR2 (all data) 0.1866
a Collected at 190 K.
(dec). 1H NMR (C6D6, 300 MHz, 20 °C): δ 1.14 (s, vbr, ν1/2
=
ca. 120 Hz), 2.95 (s, br). UV-vis (hexane, nm [ε, cm-1 M-1]):
673 (300). Anal. Calcd for C86H128Cr2: C, 81.59; H, 10.19.
Found: C, 81.84; H, 10.35. μeff = 3.71 μB/dimer at 23 °C.
{Ar*Mn(μ-Me)}2 (2). A diethyl ether solution of LiMe (1.6 M,
0.39 mL, 0.62 mmol) was added slowly via a syringe to
{Ar*MnI2Li(THF)}2 (0.522 g, 0.30 mmol) in Et2O (30 mL) with
cooling to ca. -78 °C. The mixture was warmed slowly to room
temperature and stirred for one day. All volatile materials were
removed, and the yellow residue was extracted with hexanes
(60 mL). The pale yellow filtrate was concentrated to ca. 40 mL,
which afforded X-ray quality colorless crystals of 2 after storage
for several weeks at ca. 7 °C. Yield: 0.246 g (74.3%). Mp: 175-
177 °C (dec). 1H NMR (C6D6, 300 MHz, 20 °C): δ 1.22 (s, vbr, ca.
80 Hz), 2.96 (s, br). These two signals integrate to 6:1, corre-
sponding to the isopropyl groups. UV-vis (hexane, nm
[ε, cm-1 M-1]): 387 (200). Anal. Calcd for C74H104Mn2: C, 80.54;
H, 9.50. Found: C, 80.83; H, 9.31. μeff = 5.48 μB/dimer at 23 °C.
{Ar*Fe(μ-Me)}2 (3). A diethyl ether solution of LiMe (1.6 M,
0.39 mL, 0.62 mmol) was added slowly via a syringe to
{Ar*Fe(μ-Br)}2 (0.371 g, 0.30 mmol) in Et2O (30 mL) with
cooling to ca. -78 °C. The amber solution became reddish-
brown immediately. The mixture was warmed slowly to room
temperature and stirred for one day. All volatile materials were
removed, and the brown solid was extracted with benzene (20
mL). The dark orange filtrate was concentrated to ca. 5 mL,
which afforded X-ray quality orange crystals of 3 after storage
for several days at 7 °C. Yield: 0.141 g (42.4%). Mp: 168-170
°C. 1H NMR (C6D6, 300 MHz, 20 °C): δ -10.29 (s, 12H,
{Ar*Fe(μ-Ph)}2 (4). About 30 mL of Et2O was added to a mix-
ture of {Ar*Fe(μ-Br)}2 (0.371 g, 0.30 mmol) and LiPh (0.052 g,
0.62 mmol) at room temperature. The amber solution became
red immediately. The mixture was stirred for one day. All
volatile materials were removed, and the brown residue was
extracted with benzene (20 mL). The dark red filtrate was
concentrated to ca. 10 mL, which afforded X-ray quality orange
crystals of 4 after storage for several days at 7 °C. Yield: 0.235 g
(63.7%). Mp: 187-189 °C. 1H NMR (C6D6, 300 MHz, 20 °C): δ
-5.24 (s, br, 12H, CaH(CH3)2, ν1/2 = ca. 15 Hz), -4.78 (s, br,
2H, CaH(CH3)2), -1.03 (d, 3JH-H = 7.2 Hz, 12H, CbH(CH3)2),
-0.06 (sept, 3JH-H = 7.2 Hz, 2H, CbH(CH3)2), 1.79 (s, br, 2H,
CcH(CH3)2), 2.61 (s, br, 12H, CcH(CH3)2), 2.74 (s, 8H, m-
C6H2), 4.96 (s, br, 4H, C6H5), 7.45 (m, 2H, p-C6H5), 9.19 (t,
3JH-H = 7.2 Hz, 2H, p-C6H3), 14.27 (s, br 4H, C6H5), 20.14 (d,
3JH-H = 7.2 Hz, 4H, m-C6H3). UV-vis (hexane, nm [ε, cm-1
M-1]): 362 (4700), 500 (1300). Anal. Calcd for C84H108Fe2: C,
82.06; H, 8.85. Found: C, 82.37; H, 9.01. μeff = 5.32 μB/dimer
at 23 °C.
X-ray Crystallographic Studies. Suitable crystals of 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 or on a Bruker SMART Apex II
˚
diffractometer with Mo KR radiation (λ = 0.71073 A) at 90(2)
K for 1, 3, and 4 and 190(2) K for 2. Absorption corrections were
applied using SADABS27 for 1, 3, and 4 and TWINABS28 for 2.
The structures were solved using direct methods and refined by
the full-matrix least-squares procedure in SHELXL.29 All non-
hydrogen atoms were refined anisotropically. Hydrogen atoms
for the bridging methyl groups were located by a Fourier
difference map. All other hydrogen atoms in all structures were
placed at calculated positions and included in the refinement
using a riding model.
3
CH(CH3)2), -0.63 (d, JH-H = 7.2 Hz, 12H, CH(CH3)2),
3
-0.66 (sept, JH-H = 7.2 Hz, 2H, CH(CH3)2), 1.20 (m, 2H,
CH(CH3)2), 1.28 (m, 2H, CH(CH3)2), 1.63 (s, 12H, CH(CH3)2),
4.04 (s, 6H, bridging-CH3), 6.01 (s, 8H, m-C6H2), 7.96 (t, 3JH-H
= 7.2 Hz, 2H, p-C6H3), 20.33 (d, 3JH-H = 7.2 Hz, 4H, m-C6H3).
UV-vis (hexane, nm [ε, cm-1 M-1]): 352 (4500), 436 (600).
Anal. Calcd for C74H104Fe2: C, 80.41; H, 9.48. Found: C, 80.77;
H, 9.60. μeff = 3.30 μB/dimer at 23 °C.
(23) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518.
(24) Wolf, R.; Ni, C.; Nguyen, T.; Brynda, M.; Long, G. J.; Sutton,
A. D.; Fischer, R. C.; Fettinger, J. C.; Hellman, M.; Pu, L. H.; Power,
P. P. Inorg. Chem. 2007, 46, 11277.
(27) SADABS, Version 5.0 package; an empirical absorption correc-
tion program from the SAINTPlus NT; Bruker AXS: Madison, WI, 1998.
(28) Sheldrick, G. M. An Empirical Correction for Absorption
€
€
€
Anisotropy Applied to Twinned Crystals; Universitat Gottingen: Gottingen,
Germany, 2003.
(25) Evans, D. F. J. Chem. Soc. 1959, 2003.
(26) Schubert, E. M. J. Chem. Educ. 1992, 69, 62.
(29) SHELXL, Version 5.1; Bruker AXS: Madison, WI, 1998.

Article
Organometallics, Vol. 28, No. 22, 2009 6543
Table 2. Selected Bond Lengths (A) and Angles (deg) for 1-4
˚
1
2
3
4
Cr1 Cr2
3 3 3
2.6941(5)
2.115(2)
2.190(2)
2.614(3)
103.33(9)
102.85(9)
75.72(8)
75.79(8)
M
M
2.7337(7)
2.104(3)
2.214(4)
173.49(9)
175.65(9)
104.38(14)
76.10(13)
2.5832(3)
2.032(2)
2.116(2)
174.27(3)
176.08(4)
103.99(6)
75.30(5)
Fe1 Fe1A
3 3 3
2.7207(14)
2.069(2)
2.1200(19)
129.92(4)
100.17(9)
79.83(9)
3 3 3
Cr1-C(ipso)
M1-C(aryl)
M1-C(Me)
C1-M1-M2
M1-M2-C37
C73-M1-C74
M1-C73-M2
Fe1-C1
Cr1-C(Me)
Fe1-C20
Cr1-C(flanking)
C85-Cr1-C86
C85-Cr2-C86
Cr1-C85-Cr2
Cr1-C86-Cr2
C1-Fe1-C20
C20-Fe1-C20A
Fe1-C20-Fe1
Scheme 1. Preparations of 1-4
360 nm and moderate absorptions (436 nm for 3 and 500 nm
for 4) in the visible region.
Complexes 1-4 are paramagnetic. The 1H NMR spectra
of 1 and 2 show very broad signals and could not be readily
assigned. In contrast, the 1H NMR spectra of 3 and 4 show
rather sharp signals in the range -11 to þ22 ppm. Since the
structures of 3 and 4 in the solid state differ only in the
bridging Me and Ph groups, the 1H NMR spectra have
similarities, and assignment of some signals is possible.
Due to the hindered rotation of the flanking C6H2-
2,4,6-iPr3 rings, the isopropyl groups in both structures
display three sets of signals. However, the meta-hydrogens
of the C6H2-2,4,6-iPr3 rings in both structures display a
relatively broad singlet at 6.01 ppm (in 3) and 2.74 ppm (in
4). In addition, the meta-hydrogens of the central aryl rings
in both compounds are shifted downfield to 20.33 ppm (in 3)
and 20.14 ppm (in 4) as a doublet, and the corres-
ponding para-hydrogen shows a triplet at 7.96 ppm (in 3)
and 9.19 ppm (in 4), respectively. The bridging methyl
hydrogens in 3 have a chemical shift of 4.04 ppm, while the
bridging phenyl ligands in 4 show three sets of signals (4.96,
7.45, and 14.27 ppm), which are consistent with the sym-
metric structure in the solid state.
Figure 1. Thermal ellipsoid (30%) plot of 1 (at 90(2) K). Iso-
propyl groups and H atoms are not shown.
Results and Discussion
Synthesis. The reactions of LiMe with the monoaryl metal
halide precursors {ArM(μ-X)}2 (Ar = 3,5-iPr2-Ar*, M =
Cr; Ar
=
Ar*,
M
=
Fe;
X
=
halide) or
30
{Ar*MnI2Li(THF)}2 in Et2O at ca. -78 °C proceeded
smoothly with elimination of LiX. Blue crystals of 1 and
colorless crystals of 2 were obtained from hexanes in mod-
erate yield. The iron complex 3 has poor solubility in
hexanes, and orange crystals were obtained from benzene.
The monoaryl metal halide species were similarly treated
with LiPh to prepare the heteroleptic diaryl complexes. In the
case of chromium and manganese, no crystalline products
could be isolated. For iron, orange-red crystals of 4 were
obtained from benzene in high yield. Attempts were also
made to prepare the cobalt analogues with bridging methyl
or phenyl groups; however, the products are apparently
unstable at room temperature and no crystalline products
were isolated.
Structures. The structures of 1-4 were determined by
X-ray crystallography. Important data collection and refine-
ment parameters are provided in Table 1. Selected bond
lengths and angles are summarized in Table 2.
The structure of 1 (Figure 1) is dimeric, and the metal
atoms are linked by almost symmetrically bridging methyl
groups. The Cr2(μ-C)2 unit is not planar but possesses a fold
angle of ca. 165.6° along the Cr Cr axis. The bridging
3 3 3
˚
Spectroscopy. The UV-visible spectra of 1-4 are char-
acterized by intense absorptions below 250 nm, which corre-
spond to the π-π* transitions within the aryl ligands. The
blue complex 1 exhibits a weak absorption centered at 673
nm, corresponding to the transitions of the 5D ground state.
Complex 2 displays a featureless spectrum in the visible
region, which is expected for the high-spin 3d5 system (6S
ground state). Complexes 3 (5D ground state) and 4
(5D ground state) both show very intense absorptions near
Cr-C distances are 2.160(2) and 2.229(2) A, which are on
˚
average ca. 0.08 A longer than the mean Cr-C(ipso) bond
distance to the 3,5-iPr2-Ar* ligand. These distances are
significantly longer than those in {Ar0Cr(μ-Cl)}2 (Cr-C(1)
30
= 2.041(3) A) and Ar0Cr(μ-O)2Cr(O)Ar0 (Cr-C(1) =
˚
1.999(4) A),31 which can be attributed in part to the greater
˚
size of the 3,5-iPr2-Ar* ligand in 1 and the higher oxidation
state of chromium in Ar0Cr(μ-O)2Cr(O)Ar0. The Cr Cr
3 3 3
˚
separation in 1 is 2.6941(5) A, and the structure differs
(30) Sutton, A. D.; Ngyuen, T.; Fettinger, J. C.; Olmstead, M. M.;
Long, G. J.; Power, P. P. Inorg. Chem. 2007, 46, 4809.
(31) Ni, C. B.; Ellis, B. D.; Long, G. J.; Power, P. P. Chem. Commun.
2009, 2332.

6544 Organometallics, Vol. 28, No. 22, 2009
Ni and Power
Figure 2. Thermal ellipsoid (30%) plots of 2 (left, at 190(2) K) and 3 (right, at 90(2) K). Isopropyl groups and H atoms are not shown.
dramatically from that of {(η5-C5Me5)Cr(μ-Me)}2,32 which
Mn3(Mes)6 (Mes = C6H2-2,4,6-Me3; Mn-C(terminal) =
34
˚
is highly folded (fold angle = 119.5°) along the Me Me
2.119(2) A). The Mn-C(methyl) distances (2.202(4) and
2.226(4) A) are almost equal and somewhat shorter than that
3 3 3
˚
axis rather than the Cr-Cr vector and possesses a short
Cr Cr distance of 2.263(3) A. The chromium in 1 is
in {[HC(CMeNAr)2]Mn(μ-Me)}2 (2.306(2) A).20 This may be
˚
˚
3 3 3
terminally bound to the ipso-carbon of the 3,5-iPr2-Ar*
due to the higher coordination number in the latter species.
˚
˚
ligand (Cr(1)-C(1) = 2.115(2) A) and has a further inter-
The Mn Mn separation (2.7337(7) A) is similar to that in
3 3 3
action with the flanking C6H2-2,4,6-iPr3 ring (Cr(1) C(13)
33
{Mn(μ-Me)[N(SiMe3)2AlMe3]}2 (2.714(2) A), but slightly
˚
3 3 3
˚
= 2.614(3) A). Thus, the coordination geometry of the metal
˚
longer than that of 2.809(1) A in {HC(CMeNAr)2Mn(μ-
center can be viewed as quasi-square planar. Similar me-
tal-ligand secondary interactions are also observed in the
Me)}2,20 and may indicate some interactions between the
two metal atoms. Magnetic measurements showed that 2
has an effective magnetic moment of 5.48 μB/dimer at 23 °C,
due to the antiferromagnetic interactions between the two
manganese ions. Similar antiferromagnetic interactions were
also observed in the dimethylamido-bridged complex
{Ar0Mn(μ-NMe2)}2.35
analogous chloride species {Ar0Cr(μ-Cl)}2 (Ar0 = C6H3-
30
2,6-(C6H3-2,6-iPr2)2; Cr C = 2.435(3) A) and the oxo
˚
3 3 3
31
complex Ar0Cr(μ-O)2Cr(O)Ar0 (Cr C = 2.358(3) A).
˚
3 3 3
The two Cr-ligand secondary interactions adopt a trans
geometry with respect to the Cr2(μ-C)2 unit. The interaction
causes a difference of ca. 37.2° between the Cr(1)-C(1)-C
(2) and Cr(1)-C(1)-C(6) angles. Due to the intramolecular
antiferromagnetic exchange interactions between the two
chromium(II) atoms, the effective magnetic moment (μeff)
of 1, ca. 3.71 μB/dimer at 23 °C, is significantly lower than the
spin-only value of 9.80 μB expected for two isolated
chromium(II) ions. The μeff is also somewhat smaller than
that observed in the analogous chloride species {Ar0Cr(μ-
The structure of complex 3 (Figure 2) is very similar to that
of 2. It is dimerized through almost symmetric bridging of
two Ar*Fe units by two methyl groups, and the iron atoms
have a distorted trigonal-planar geometry. Notably, both
the Fe-C(aryl) and Fe-C(methyl) bond lengths in 3 are ca.
˚
0.1 A shorter than the corresponding Mn-C distances in 2,
which is consistent with the smaller radius of the Fe(II) ion
compared to that of Mn(II).36 The Fe2(μ-C)2 four-membered
Cl)}2,30 which is consistent with the shorter Cr Cr separa-
ring is slightly folded along the Fe Fe axis by 1.5°, and the
3 3 3
3 3 3
tion, thus stronger metal-metal interactions in 1.
torsion angles between the Fe2(μ-C)2 plane and the central
aryl rings are 42.6° and 47.3°. Thus, the planes of the central
aryl rings of the terphenyl ligands are almost perpendicular.
The angles at the bridging methyl groups are 75.30(5)° and
75.88(6)°. These acute angles are consistent with the rela-
Crystals of complex 2 were extremely air-sensitive, and they
shattered at 90(2) K. Thus, an X-ray data set was collected at
the higher temperature of 190 (2) K. The structure (Figure 2)
shows that it is dimerized through almost symmetric bridging
of the manganese atoms by two methyl groups. The Ar* group
is terminally bound to yield a distorted trigonal-planar metal
coordination geometry, in contrast to the quasi-square-planar
coordination induced by metal-ligand secondary interactions
in 1. Manganese complexes with two bridging methyl groups
are quite rare, and structurally characterized examples are
limited to {[HC(CMeNAr)2]Mn(μ-Me)}2 (Ar = C6H3-
2,6-iPr2)20 and {Mn(μ-Me)[N(SiMe3)2AlMe3]}2,33 which have
either four-coordinate or quasi-four-coordinate manganese
atoms. The Mn2(μ-C)2 four-membered ring in 2 is folded along
˚
tively short Fe Fe separation (2.5832(3) A), which may
3 3 3
indicate some interactions between the two metal atoms.
Indeed, the effective magnetic moment of 3 is 3.30 μB/dimer
at 23 °C, which is due to the antiferromagnetic coupling
interactions between the two irons. The Fe-C(aryl) dis-
tances (av 2.032 A) are very close to that in FeAr 2 (Ar# =
˚
#
37
˚
C6H3-2,6-(C6H2-2,4,6-Me3)2; 2.040(3) A).
The Fe-C
(methyl) distances are almost identical and are in the narrow
˚
range 2.0932(17) to 2.1187(16) A, which are, as expected,
considerably longer than the terminal Fe-C(methyl) dis-
the Mn Mn axis by ca. 4.2°, which is similar but different
from the planar cores observed in the methyl-bridged
tance in the three-coordinate complex HC(CtBuNAr)2FeMe
3 3 3
17-19
˚
(2.009(1) A).
The average Fe-C(methyl) distance
complexes mentioned above. The Mn-C(ipso) distances (av
2.104 A) are similar to those observed in the three-coordinate
˚
Mn(II) complexes {HC(CMeNAr)2}MnPh (2.077(6) A)20 and
˚
(34) Gambarotta, S.; Floriani, C.; Chiesi-Villa, A.; Guastini, C.
J. Chem. Soc., Chem. Commun. 1983, 1128.
(35) Ni, C. B.; Fettinger, J. C.; Long, G. J.; Grandjean, F.; Power, P.
(32) Heintz, R. A.; Ostrander, R. L.; Rheingold, A. L.; Theopold,
K. H. J. Am. Chem. Soc. 1994, 116, 11387.
(33) Niemeyer, M.; Power, P. P. Chem. Commun. 1996, 1573.
P., submitted.
(36) Emsley, J. The Elements; Oxford University Press: Oxford, 1998.
(37) Kays, D. L.; Cowley, A. R. Chem. Commun. 2007, 1053.

Article
Organometallics, Vol. 28, No. 22, 2009 6545
central aryl ring of the terphenyl ligand and the bridging
phenyl ligand. The Fe(1)-C(20)-Fe(1A) angle is 79.83(9)°
˚
and the Fe Fe separation is 2.7207(14) A, which is slightly
3 3 3
longer than those in the homoleptic dimers {Fe(Mes)(μ-
39,40
and {Fe(C6H2-2,4,6-iPr3)(μ-C6H2-
˚
Mes)}2 (2.617(1) A)
i
2,4,6- Pr3)}2 (2.666(3) A).40 This is probably due to the steric
˚
effect of the Ar* ligand. The Fe-C(1) σ-bond length
˚
(2.069(2) A) is comparable to those terminal Fe-C(aryl)
39
˚
distances in {Fe(Mes)(μ-Mes)}2 (2.023(5) A)
and
40
{Fe(C6H2-2,4,6-iPr3)(μ-C6H2-2,4,6-iPr3)}2 (2.083(9) A).
˚
˚
The Fe-C(20) bridging σ-bond length (2.120(2) A) is also
very close to the bridging Fe-C distances in the a
bove complexes.39,40 The effective magnetic moment of 4 is
5.32 μB/dimer at 23 °C. This value is consistent with that
39,40
observed in {Fe(Mes)(μ-Mes)}2
and, as expected, con-
siderably larger than that of 3.30 μB/dimer in 3, which
features shorter Fe Fe separation and thus stronger inter-
actions between the Fe(II) atoms.
Figure 3. Thermal ellipsoid (30%) plot of 4 (at 90(2) K).
Isopropyl groups and H atoms are not shown.
3 3 3
Conclusion
˚
(2.106 A) is significantly longer than one Fe-C σ-bond
˚
distance of 2.025 A (av) in [Fe2(μ-Me)(μ-CO)(μ-dppm)(η-
In conclusion, the low-coordinate heteroleptic transition
metal complexes 1-4 were synthesized and characterized by
using bulky terphenyl ligands.41 Complexes 1-3 feature
bridging methyl groups and are rare examples of low-co-
ordinate complexes of a methyl ligand. Currently, the chem-
istry of these complexes is under investigation.
C5H5)2][PF6] (dppm = Ph2PCH2PPh2),38 but similar to the
˚
other Fe-C distance of 2.113(3) A in that complex, where the
Fe atom has further interaction with one hydrogen of the
methyl group (Fe H 1.64(4) A). However, this Fe-η C-H
2
˚
3 3 3
interaction with the bridging methyl group is not observed in
3 on the basis of the positions of the hydrogens, which are
located directly from a Fourier difference map.
The phenyl-bridged complex 4 crystallizes in an I41/acd
space group, and there is a quarter of the molecule in the
asymmetric unit cell. The two Ar*Fe moieties are bridged
symmetrically by two Ph groups, and each iron atom has a
distorted trigonal-planar geometry. The central Fe2(μ-C)2
unit is planar with torsion angles of 57.1° and 83.6° to the
Acknowledgment. We thank the National Science
Foundation for financial support and Dr. James C.
Fettinger for crystallographic assistance.
Supporting Information Available: 1H NMR spectra for 1-4
and the crystallographic information files (CIFs) for 1-4. These
materials are available free of charge via the Internet at http://
pubs.acs.org.
(38) Dawkins, G. M.; Green, M.; Orpen, A. G.; Stone, F. G. A.
J. Chem. Soc., Chem. Commun. 1982, 41.
(39) Muller, H.; Seidel, W.; Gorls, H. J. Organomet. Chem. 1993, 445,
133.
(40) Klose, A.; Solari, E.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C.;
€
€
(41) Similar reactions with different terphenyl ligandswere attempted
to synthesize the cobalt analogues of bridging methyl or phenyl groups;
however, the desired products were unstable and decomposition was
observed upon warming the reaction mixture.
Re, N. J. Am. Chem. Soc. 1994, 116, 9123.