Univalent transition metal complexes of arenes stabilized by a bulky terphenyl ligand: Differences in the stability of Cr(I), Mn(I) or Fe(I) complexes

Article abstract of DOI:10.1039/b715027j

Two univalent transition metal complexes, (μ-η⁶: η⁶-C₇H₈){MnAr*-3,5-Pr ⁱ₂}₂ (1) and (η⁶-C ₆H₆)FeAr*-3,5-Prⁱ₂ (2) (Ar*-3,5-Prⁱ₂ = C₆H-2,6-(C ₆H₂-2,4,6-Prⁱ₃)₂-3,5- Prⁱ₂), that have η⁶ arene coordination were synthesized by reduction of the corresponding metal halides. The complexes are thermally stable in contrast to the corresponding Cr(i) complexes of benzene or toluene which decompose at room temperature. The Royal Society of Chemistry.

Full text of DOI:10.1039/b715027j

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Univalent transition metal complexes of arenes stabilized by a bulky
terphenyl ligand: differences in the stability of Cr(I), Mn(I) or Fe(I)
complexesw
Chengbao Ni,a Bobby D. Ellis,a James C. Fettinger,a Gary J. Longb and
Philip P. Power*a
Received (in Berkeley, CA, USA) 1st October 2007, Accepted 28th November 2007
First published as an Advance Article on the web 20th December 2007
DOI: 10.1039/b715027j
Two univalent transition metal complexes, (l-g6:g6-
C7H8){MnAr*-3,5-Pri2}2 (1) and (g6-C6H6)FeAr*-3,5-Pri2 (2)
of weak secondary Cr(I)–arene interactions in the structure in
the related quintuple bonded Ar0CrCrAr0 (Ar0 = C6H3-2,6-
(C6H3-2,6-Pri2)2).12 Currently, it is not known if the instability
of the interaction of a one-coordinate Cr(I) moiety with an
arene13 is peculiar to chromium (cf. the stability of the Cr(I)
complex (m-Z6:Z6-C7H8){Cr[(C6H3-2,6-Pri2)NCMe]2CH}2)3 or
if it is a more general property of one coordinate transition
metal(I) moieties. We hypothesized that the attempted synth-
esis of the Cr(I) arene complex led in the first instance to the
unstable toluene complex 3,5-Pri2-Ar*Cr(Z6-C7H8), which
decomposed to unidentified products partly as a result of the
presence of only 13 valence electrons. This led to the proposal
that an increase in the number of valence electrons might lead
to analogous complexes of the more electron rich later metals
which would have sufficient stability for isolation. We now
report that this approach has led to the synthesis and char-
acterization of the stable inverted sandwich complex (m-Z6:Z6-
C7H8){MnAr*-3,5-Pri2}2 (1) and the related iron(I) half sand-
wich complex (Z6-C6H6)FeAr*-3,5-Pri2 (2), which are the first
open shell one coordinate metal moieties to be stabilized by
coordination to a neutral arene.
(Ar*-3,5-Pri2
=
C6H-2,6-(C6H2-2,4,6-Pri3)2-3,5-Pri2), that
have g6 arene coordination were synthesized by reduction of
the corresponding metal halides. The complexes are thermally
stable in contrast to the corresponding Cr(I) complexes of
benzene or toluene which decompose at room temperature.
Transition metal complexes in which a six p-electron aromatic
ring, for example benzene or toluene, bridges two metal
centers in an Z6 fashion are rare.1 The first such compound,
(m-Z6-C6H6){V(Z5-C5H5)}2,2 was prepared in 1983 by reaction
of (Z5-C5H5)V(C3H5)2 with an excess of 1,3-cyclohexadiene in
refluxing n-heptane. More recent examples include the
chromium(I) species (m-Z6:Z6-C7H8){Cr[(C6H3-2,6-Pri2)NCMe]2-
3
CH}2
and the
p
and
f
block metal derivatives
4
(m-Z6:Z6-C7H8){Bi[N(CH2C6H2-1-O-But2)3]}2 and (m-Z6:Z6-
C7H8){U(N[R]Ar)2}2 (R = C(CH3)3, Ar = C6H3-3,5-Me2).
5
These so-called inverted sandwich complexes are closely re-
lated to half-sandwich compounds that incorporate a neutral
Z6-arene and a s-bonded ligand. Although the latter are
relatively common for organometallic complexes which con-
form to the 18-electron rule, they are relatively scarce for
open-shell species with valence electron counts lower than 18.
These electronically unsaturated complexes have attracted
considerable interest because of their high reactivity and
potential catalytic applications in small molecule activa-
tion.3,5–10 We have shown recently that the very large terphe-
nyl ligand C6H-2,6-(C6H2-2,4,6-Pri3)2-3,5-Pri2 (abbreviated
Ar*-3,5-Pri2) could stabilize the unusual d5, Cr(I), two-coordi-
nate complexes [3,5-Pri2-Ar*Cr(L)] (L = THF or PMe3).11
However, attempts to prepare 3,5-Pri2-Ar*Cr(Z6-arene) com-
plexes were unsuccessful. Reduction of 3,5-Pri2-Ar*CrCl in
toluene with KC8 or the reaction of 3,5-Pri2-Ar*Cr(thf) with
benzene or toluene initially afforded a red color, but decom-
position occurs over several hours to afford intractable brown
mixtures. Understanding the apparent lack of stability of this
Cr(I) arene complex is of importance because of the presence
Compounds 1 and 2 were prepared (Scheme 1) by reduction
of in situ generated 3,5-Pri2-Ar*MnI or 3,5-Pri2-Ar*FeCl with
KC8 in THF and were isolated in modest yields as air-sensitive
brown (1) or orange (2) crystals after extraction with toluene
and benzene, respectively.z
The structures of 1 and 2 were determined by X-ray crystal-
lography.z Complex 1, which crystallizes in a Pna21 space
group, has two molecules with very similar structures in the
asymmetric unit. One of these is illustrated in Fig. 1. The two
manganese atoms are separated by a bridging toluene mole-
cule, which is disordered axially over six positions (only one is
shown in Fig. 1). The C–C bond lengths within the bridging
toluene were constrained to be 1.3900 A, which is essentially
the same as it is in the uncomplexed molecule.14 The
Mn(1)–centroid and Mn(2)–centroid distances, 1.7815(12) A
and 1.7798(12) A, differ only slightly and the Mn–C(toluene)
distances are in the range 2.237(3) A to 2.279(3) A. The
Mn–centroid distances are longer than the usual range found
a Department of Chemistry, University of California, Davis, California
95616, USA. E-mail: power@chem.ucdavis.edu
b Department of Chemistry, University of Missouri-Rolla, Rolla,
Missouri 65409, USA. E-mail: glong@umr.edu
w Electronic supplementary information (ESI) available: Crystallo-
graphic data and magnetic susceptibilities for 1 and 2. See DOI:
10.1039/b715027j
Scheme 1 Reduction of the ligand halide precursors. 1 M = Mn,
R = Me, X = I, n = 2; 2 M = Fe, R = H, X = Cl, n = 1.
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Fig. 1 Molecular structure of 1 with thermal ellipsoids presented at a
30% probability level. All hydrogen atoms, the disordered isopropyl
groups and toluene methyl groups are not shown. Selected bond
lengths (A): Mn(1)–Mn(2) 3.5608(8), Mn(1)–C(1) 2.088(3), Mn(2)–
C(43) 2.089(3), Mn(1)–C(toluene) 2.277(3), 2.275(3), 2.258(3), 2.245(3),
2.243(3), 2.262(3), Mn(2)–C(toluene) 2.279(3), 2.270(3), 2.249(3),
2.237(3), 2.246(3), 2.267(3), Mn(1)–centroid 1.7815(12), Mn(2)–
centroid 1.7798(12).
Fig. 2 Molecular structure of 2 with thermal ellipsoids presented at a
30% probability level. All hydrogen atoms and disordered isopropyl
groups have been omitted for clarity. Selected bond lengths (A) and
angles (1): Fe1–C1 2.029(4), Fe1–C(benzene) 2.163(3), 2.166(3),
2.163(3), Fe(1)–centroid 1.6427(13), C(23)–C(24)–C(25) 119.9(3),
C(24)–C(23)–C(25A) 120.1(3), C(23A)–C(25)–C(24) 120.0(3).
in complexes having Z-coordination to arene rings,15 due
probably to the bridging nature of the interacting toluene in
1. The Mn–C(ipso) distances to the Z1 terphenyl ligand are
very similar at 2.088(3) A and 2.089(3) A and are close to those
found in Z1-aryl complexes, such as {[(C6H3-2,6-Pri2)-
NCMe]2CH}Mn(C6H5)16 at 2.076(5) A and Mn(C6H2-2,4,6-
But3)217 at 2.108(2) A, but somewhat shorter than the 2.181(4)
The meff of 1 increases virtually linearly from 4.78 to 5.05 mB
0
between 2 and 320 K whilst wM ꢁ T increases almost linearly
from 2.87 to 3.19 emu K molꢂ1 per Mn between 2 and 320 K.
5 2
In 2, Fe(I) has a nominal 3d7 t2 e electronic ground state and
S = 3/2. A plot of the inverse molar magnetic susceptibility
versus temperature is linear in the range of 200–320 K and the
0
corresponding meff is 4.68 mB. However, a plot of wM ꢁ T
0
A
distance in (C6H2-2,4,6-Me3)Mn{MeC(O)CHC(Me)N-
versus temperature indicates that wM ꢁ T increases virtually
CH2}2Li(1,2-dimethoxyethane)18 because of its anionic
charge. The core C(1)–Mn(1)–centroid–Mn(2)–C(43) deviates
slightly from linearity with C(1)–Mn(1)–centroid and
C(43)–Mn(2)–centroid angles of 178.15(10)1 and 176.76(10)1,
respectively.
0
linearly between 20 and 320 K; below 20 K wM ꢁ T decreases
sharply as a result of the extensive zero-field splitting arising
from the very low-symmetry of the iron(I) coordination
environment.
In conclusion, the toluene bridged inverted sandwich Mn(I)
complex (1) and the half sandwich Fe(I) complex (2) have been
The complex 2 crystallizes in the Pbcn space group, with
half of the molecule in the asymmetric unit. The structure of 2
is illustrated in Fig. 2. The iron is coordinated to the ipso
carbon of the terphenyl ligand, with Fe(1)–C(1) distance of
2.029(4) A, and to a benzene in an Z6 fashion, with Fe(1)–cen-
troid distance of 1.6427(13) A, which is in the expected range
of 1.455 A to 1.704 A,19 and which is slightly longer than the
Fe–centroid distance of 1.6245(8) A in the related b-diketimi-
nate Fe(I) complex [{(C6H3-2,6-Pri2)NC(Me)}2CH]Fe-
(Z6-C6H6)9 and considerably longer than that of 1.5445(12)
A in [{RNQC(Me)}2]Fe(Z6-C6H5Me) (R = C6H3-2,6-Pri2),10
which is short owing to the contribution of the ene-diamide
canonical form to these structures. The average C–C bond
length within the benzene ring is 1.408(5) A, which is very
similar to that in [{(C6H3-2,6-Pri2)NC(Me)}2CH]Fe(Z6-C6H6).
Studies of the magnetic properties of 1 and 2 were under-
taken to throw further light on their bonding. In 1, each
manganese atom is present in a highly distorted environment
4 2
as Mn(I) with a nominal 3d6 t2 e electronic ground state, and
S = 2. A plot of the inverse molar magnetic susceptibility
versus temperature (Fig. 3) is linear in the range of 150–320 K
and the corresponding meff is 5.17 mB per manganese(I) ion,
which is in agreement with 5.2 mB expected for two d6 Mn(I)
centers with no Mn–Mn bonding or any magnetic exchange.
Fig. 3 A plot of the molar magnetic susceptibility of 1 versus
temperature.
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isolated and characterized. Unlike the chromium 3,5-Pri2-
Ar*Cr moiety, whose complexes with PhMe or benzene
decompose at room temperature,13 the corresponding manga-
nese(I) and iron(I) moieties readily afford stable arene com-
plexes.20 The reasons for the apparent instability13 of the
putative 3,5-Pri2-Ar*Cr(Z6-C7H8) intermediate species are
not well understood currently, but may be connected with
the greater electron deficiency and the favorable exchange
energy of the d5 electron configuration. Although an extremely
bulky ligand such as 3,5-Pri2-Ar* creates sufficient protection
at the metal centers for stability in 1 and 2, the unsaturated
geometry and low number of valence electrons (for Mn, 14
electrons; Fe, 15 electrons) make the compounds attractive
substrates for small molecule activation. The investigation of
their chemistry is now underway.
After an exhaustive series of attempts, it was determined that no
sensible solution was forthcoming, as a result of which the structure
was refined to convergence with the bridging groups as benzene rings,
the carbon atoms at full occupancy and the peripheral hydrogen atoms
input at 5/6th occupancy since the terminal methyl group’s multiple
locations could not be ascertained reliably. The missing terminal –CH3
group has been included in the overall formula.
2: C48H67Fe, T = 90(2) K with MoKa (l = 0.71073 A), Fw = 699.87,
orthorhombic, space group Pbcn, orange block, a = 22.0866(14), b =
10.9907(7), c = 17.4459(11) A, V = 4234.9(5) A3, Z = 4, m = 0.386
mmꢂ1, 43 524 total reflections, 3842 independent, Rint = 0.1351,
goodness-of-fit on F2 = 0.998, final R indices [I 4 2s(I)] R1 =
0.0428 and wR2 = 0.1227 (all data). CCDC 662781. For crystal-
lographic data in CIF or other electronic format, see DOI: 10.1039/
b715027j
1 V. Beck and D. O’Hare, J. Organomet. Chem., 2004, 689, 3920.
2 W. Duff, K. Jonas, R. Goddard, H.-J. Kraus and C. Kruger, J.
Am. Chem. Soc., 1983, 105, 5479.
We thank the National Science Foundation for financial
support.
3 Y.-C. Tsai, P.-Y. Wang, S.-A. Chen and J.-M. Chen, J. Am. Chem.
Soc., 2007, 129, 8066.
4 L. E. Turner, M. G. Davidson, M. D. Jones, H. Ott, V. S. Schulz
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Notes and references
z All manipulations were carried out under strictly anhydrous and
anaerobic conditions.
Preparation of (m-Z6:Z6-C7H8){MnAr*-3,5-Pri2}2 (1): a pink solution
of (3,5-iPr2-Ar*)MnI (2.24 g, 1.50 mmol), which was prepared in situ
from (3,5-iPr2-Ar*)Li(Et2O) and MnI2 in diethyl ether, in ca. 30 mL of
THF was added dropwise to a freshly prepared suspension of KC8
(0.405 g, 3.00 mmol) in ca. 20 mL THF at 0 1C. The solution turned to
yellowish brown immediately. The mixture was stirred for 2 days. The
solvent was removed under reduced pressure and the resulting dark
solid was extracted with toluene (ca. 40 mL). The solution was filtered
and the reddish brown filtrate was concentrated to ca. 15 mL, which
afforded X-ray quality reddish brown crystals of 1 after storage for 1
day at ꢂ18 1C. Yield 0.510 g (28.5%). The solid turned brown around
200 1C and melted at 216B218 1C. UV-vis (hexane, nm [e, cmꢂ1
8 X. Dai, P. Kapoor and T. H. Warren, J. Am. Chem. Soc., 2004,
126, 4798.
9 J. M. Smith, A. R. Sadique, T. R. Cundari, K. R. Rodgers, G.
Lukat-Rodgers, R. J. Lachicotte, C. J. Flaschenriem, J. Vela and
P. L. Holland, J. Am. Chem. Soc., 2006, 128, 756.
10 S. C. Bart, E. J. Hawrelak, E. Lobkovsky and P. J. Chirik,
Organometallics, 2005, 24, 5518.
11 R. Wolf, M. Brynda, C. Ni, G. J. Long and P. P. Power, J. Am.
Chem. Soc., 2007, 129, 6076.
12 T. Nguyen, A. D. Sutton, M. Brynda, J. C. Fettinger, G. J. Long
and P. P. Power, Science, 2005, 310, 844.
M
ꢂ1]): 312 (3050), 408 (shoulder).
Preparation of (Z6-C6H6)FeAr*-3,5-Pri2 (2): a pale yellow solution of
(3,5-iPr2-Ar*)FeCl (0.99 g, 0.75 mmol), which was prepared in situ
from (3,5-iPr2-Ar*)Li(Et2O) and FeCl2 in diethyl ether, in ca. 25 mL
THF was added dropwise to a freshly prepared suspension of
KC8 (0.202 g, 1.5 mmol) in ca. 15 mL THF at 0 1C. The solution
turned orange brown immediately. The mixture was stirred for
ca. 24 h. The solvent was removed and the black solid was extracted
with benzene (ca. 30 mL). The orange filtrate was concentrated to ca.
10 mL, which afforded X-ray quality orange crystals of 2 after cooling
for 1 day at ꢂ18 1C. Yield 0.160 g (15.2%). The compound
decomposes at 162 1C and melts at 208 1C. UV-vis (hexane, nm
13 R. Wolf, C. Ni, T. Nguyen, M. Brynda, G. J. Long, A. D. Sutton,
R. C. Fisher, J. C. Fettinger, M. Hellman, L. Pu and P. P. Power,
Inorg. Chem., 2007, 46, 11277.
14 Landolt-Bornstein: Numerical Data and Functional Relationships in
Science and Technology, ed. O. Madelung, Springer, Berlin, 1987,
vol. 15.
15 The Mn–centroid (benzene or toluene) distances of 79 manganese
complexes in the Cambridge Crystal Structure Database (Version
5.28, August 2006) range from 1.546 A to 1.765 A.
16 J. Chai, H. Zhu, H. Fan, H. W. Roesky and J. Magull, Organo-
metallics, 2004, 23, 1177.
[e, cmꢂ1
M
ꢂ1]): 436 (4800).
17 R. J. Wehmschulte and P. P. Power, Organometallics, 1995, 14,
3264.
18 E. Gallo, E. Solari, C. Floriani, A. Chiesi-Villa and C. Rizzoli,
Organometallics, 1995, 14, 2156.
1ꢃ4toluene: C118H159Mn2, T = 90(2) K with MoKa (l = 0.71073 A),
Fw = 1687.40, orthorhombic, space group Pna21, orange block, a =
23.2481(11), b = 18.7415(9), c = 46.380(2) A, V = 20 208.1(17) A3, Z
= 8, m = 0.297 mmꢂ1, 169 346 total reflections, 44 483 independent,
Rint = 0.0866, goodness-of-fit on F2 = 1.046, final R indices [I 4
2s(I)] R1 = 0.0595 and wR2 = 0.1501 (all data). CCDC 662780. An
extensive series of difference-Fourier maps were now implemented to
locate the 8 toluene solvent molecules and to refine the bridging
toluene molecules that were randomly disordered in both complexes.
19 The Fe–centroid (benzene or toluene) distances of iron complexes
in the Cambridge Crystal Structure Database (Version 5.28,
August 2006) range from 1.455 A to 1.704 A.
20 A cobalt(I)–arene complex, 3,5-Pri2-Ar*Co(Z6-C6H6) can also be
isolated but full structural details cannot be provided owing to
crystallographic difficulties.
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This journal is The Royal Society of Chemistry 2008
1016 | Chem. Commun., 2008, 1014–1016