Chelating or bridging? Halide-controlled binding mode of diamido donor ligands in iron(III) complexes

Article abstract of DOI:10.1021/ic061832m

In a series of iron(III) halide complexes of the form {FeX[MesN-(SiMe ₂)]₂O}₂ (Mes = mesityl; X = Cl, Br, I), the ancillary diamidosilylether ligand can either chelate to one metal or instead bridge two metal centers, as a function of the halide coligand. The complexes are prepared from diamidosilyletheriron(II) precursors, which are oxidized with iodine, benzyl bromide, or PhICl₂ to yield the appropriate iron(III) halide. The bromide analogue can also be synthesized by reacting the iron(II) precursor with a bromonium transfer agent (stabilized by adamantylideneadamantane). The latter reaction may proceed via an iron(IV) intermediate, which can oxidize the normally noncoordinating, inert [B(Ar _F)₄]^- counteranion [Ar_F = 3,5-(CF₃)₂Ph].

Full text of DOI:10.1021/ic061832m

Inorg. Chem. 2007, 46, 366−368
Chelating or Bridging? Halide-Controlled Binding Mode of Diamido
Donor Ligands in Iron(III) Complexes
Anjan Kumar Das, Zohreh Moatazedi, Garry Mund, Andrew J. Bennet, Raymond J. Batchelor, and
Daniel B. Leznoff*
Department of Chemistry, Simon Fraser UniVersity, 8888 UniVersity DriVe, Burnaby,
British Columbia V5A 1S6, Canada
Received September 26, 2006
In a series of iron(III) halide complexes of the form
{
FeX[MesN-
(IV) and titanium(IV) centers, they display high activities
toward olefin polymerization,4 while dinitrogen activation
chemistry occurs with chelating diamidophosphine-based
metal complexes.5 We herein describe a series of iron(III)
halide complexes in which the ancillary diamidosilylether
ligand, as a function of the halide coligand, can either chelate
to one metal or instead bridge two metal centers.
The addition of the diamidosilylether ligand {Li2[MesN-
(SiMe2)]2O}8,9 (Mes ) mesityl) to FeCl2 generates amido-
bridged {Fe[MesN(SiMe2)]2O}2 (1), along with LiCl as a
biproduct.10 Each iron(II) center exhibits a distorted trigonal
geometry (Figure 1) in which each Fe atom is bound by one
(SiMe2)]2O (Mes mesityl; X Cl, Br, I), the ancillary
}
)
)
2
diamidosilylether ligand can either chelate to one metal or instead
bridge two metal centers, as a function of the halide coligand.
The complexes are prepared from diamidosilyletheriron(II) precur-
sors, which are oxidized with iodine, benzyl bromide, or PhICl2 to
yield the appropriate iron(III) halide. The bromide analogue can
also be synthesized by reacting the iron(II) precursor with a
bromonium transfer agent (stabilized by adamantylideneadaman-
tane). The latter reaction may proceed via an iron(IV) intermediate,
which can oxidize the normally noncoordinating, inert [B(ArF)4]-
counteranion [ArF
) 3,5-(CF3)2Ph].
(6) Elias, A. J.; Roesky, H. W.; Robinson, W. T.; Sheldrick, G. M. J.
Chem. Soc., Dalton Trans. 1993, 495. Male, N. A. H.; Thornton-Pett,
M.; Bochmann, M. J. Chem. Soc., Dalton Trans. 1997, 2487.
(7) Mund, G.; Batchelor, R. J.; Sharma, R. D.; Jones, C. H. W.; Leznoff,
D. B. J. Chem. Soc., Dalton Trans. 2002, 136.
(8) Mund, G.; Vidovic, D.; Batchelor, R. J.; Britten, J. F.; Sharma, R. D.;
Jones, C. H. W.; Leznoff, D. B. Chem. Eur. J. 2003, 9, 4757.
(9) Mund, G.; Gabert, A. J.; Batchelor, R. J.; Britten, J. F.; Leznoff, D.
B. Chem. Commun. 2002, 2990. Leznoff, D. B.; Mund, G.; Jantunen,
K. C.; Bhatia, P. H.; Gabert, A. J.; Batchelor, R. J. J. Nucl. Sci.
Technol. 2002, Nov (Supplement 3), 406.
The use of chelating diamido ligands has recently played
an increasingly important role in transition-metal catalyst
design,1 spurred on by the desire to find alternative ancillary
ligands to the ubiquitous cyclopentadienyl-based systems.2
Diamido and diamidodonor ligands, which usually act as
chelates, are particularly attractive ligands for high-valent
metal centers because of their strong π-donating ability and
their versatility toward steric and electronic modification (via
the nitrogen substituent).3-9 In combination with zirconium-
(10) See the Supporting Information for synthetic procedures and charac-
terization details. Crystal data for compound 1: C44H68Fe2N4O2Si4,
M ) 909.08, triclinic, space group P1h, a ) 11.468(3) Å, b ) 11.575-
(4) Å, c ) 19.840(4) Å, R ) 74.48(3)°, â ) 77.39(2)°, γ ) 74.71-
(3)°, V ) 2416.6 Å3, Z ) 2, µ(Mo KR) ) 0.734 mm-1, T ) 293 K,
6362 unique reflections, 3207 observed [I0 > 2.5σ(I0)]. The final RF
) 0.0390 and RWF ) 0.0354 (obsd data). Crystal data for compound
2: C22H34FeIN2OSi2, M ) 581.44, orthorhombic, space group Pbca,
a ) 16.098(2) Å, b ) 15.199(2) Å, c ) 21.481(2) Å, V ) 5255.8 Å3,
Z ) 8, µ(Mo KR) ) 1.840 mm-1, T ) 293 K, 4400 unique reflections,
* To whom correspondence should be addressed. E-mail: dleznoff@sfu.ca.
Tel.: 1-604-291-4887. Fax: 1-604-291-3765.
(1) Gade, L. H. Chem. Commun. 2000, 173.
(2) Gibson, V. C.; Spitzmesser, S. K. Chem. ReV. 2003, 103, 283.
(3) Bradley, D. C.; Chisholm, M. H. Acc. Chem. Res. 1976, 9, 273. Love,
J. B.; Clark, H. C.; Cloke, F. G. N.; Green, J. C.; Hitchcock, P. B. J.
Am. Chem. Soc. 1999, 121, 6843. Athimoolam, A.; Gambarotta, S.;
Korobkov, I. Organometallics 2005, 24, 1996. Scollard, J. D.;
McConville, D. H.; Vittal, J. J. Organometallics 1997, 16, 4415.
(4) Aizenberg, M.; Turculet, L.; Davis, W. M.; Schattenmann, F.; Schrock,
R. R. Organometallics 1998, 17, 4795. Cloke, F. G. N.; Hitchcock,
P. B.; Love, J. B. J. Chem. Soc., Dalton Trans. 1995, 25. Gue´rin, F.;
McConville, D. H.; Payne, N. C. Organometallics 1996, 15, 5085.
Liang, L.-C.; Schrock, R. R.; Davis, W. M.; McConville, D. H. J.
Am. Chem. Soc. 1999, 121, 5797. Graf, D. D.; Schrock, R. R.; Davis,
W. M.; Stumpf, R. Organometallics 1999, 18, 843. Baumann, R.;
Davis, W. M.; Schrock, R. R. J. Am. Chem. Soc. 1997, 119, 3830.
Liang, L.-C.; Schrock, R. R.; Davis, W. M. Organometallics 2000,
19, 2526.
1729 observed [I0 > 2.5σ(I0)]. The final RF ) 0.0423 and RWF
)
0.0476 (obsd data). Crystal data for compounds 3 and 3a. Prepared
using the bromonium reagent (3a): C22H34FeBr0.63Cl0.37N2OSi2, M
) 518.43, monoclinic, space group P21/c, a ) 15.9792(13) Å, b )
12.0218(11) Å, c ) 15.0858(13) Å, â ) 116.730(2)°, V ) 2588.3
Å3, Z ) 4, µ(Cu KR) ) 7.169 mm-1, T ) 293 K, 8010 unique
reflections, 2028 observed [I0 > 2σ(I0)]. The final RF ) 0.0541 and
RWF ) 0.0648 (obsd data). Prepared using benzyl bromide (3): C22H34-
FeBrN2OSi2, M ) 533.98, monoclinic, space group P21/c, a ) 15.990-
(7) Å, b ) 12.009(2) Å, c ) 15.105(5) Å, â ) 117.06(3)°, V ) 2583.0
Å3, Z ) 4, µ(Mo KR) ) 2.239 mm-1, T ) 293 K, 3614 unique
reflections, 1453 observed [I0 > 2.5σ(I0)]. The final RF ) 0.0482 and
RWF ) 0.0431 (obsd data). Unit cell data for compound 4: primitive
monoclinic, a ) 15.932(9) Å, b ) 11.970(5) Å, c ) 14.973(8) Å, â
) 116.31(2)°, V ) 2559.6 Å3. For detailed X-ray crystallographic
experimental information, see the deposited composite CIF files for
1-3 and 3a.
(5) Fryzuk, M. D.; MacKay, B. A.; Patrick, B. O. J. Am. Chem. Soc.
2003, 125, 3234. MacKay, B. A.; Patrick, B. O.; Fryzuk, M. D.
Organometallics 2005, 24, 3836.
366 Inorganic Chemistry, Vol. 46, No. 2, 2007
10.1021/ic061832m CCC: $37.00
© 2007 American Chemical Society
Published on Web 12/15/2006

COMMUNICATION
Figure 3. Molecular structure of 3 (ORTEP view, 50% probability
ellipsoids are shown). Selected bond lengths (Å) and angles (deg): Fe1-
Fe1* 3.232(3), Fe1-Br1 2.471(2), Fe1-Br1* 2.503(2), Fe1-N1 1.864(8),
Fe1-N2 1.880(7), Si10-O1 1.651(9), Si11-O1 1.600(10), Si2*-O1 1.626-
(6), Si10-N1 1.761(10), Si11-N1 1.755(11), Si2*-N2* 1.740(8); Br1-
Fe1-Br1* 98.96(7), Br1-Fe1-N1 105.5(2), Br1*-Fe1-N1 109.5(2),
Si10-O1-Si2* 154.0(5), Si11-O1-Si2* 148.3(5). * ) -x + 1, 1 - y -
1, -z + 1.
Figure 1. Molecular structure of 1 (ORTEP, 33% ellipsoids; methyl groups
on aryl rings excluded for clarity). Selected bond lengths (Å) and angles
(deg): Fe1-Fe2 2.5479(13), Fe1-N1 2.128(4), Fe1-N2 2.055(4), Fe2-
N1 2.049(5), Fe2-N2 2.111(4), Fe1-N3 1.912(4), Fe2-N4 1.902(5), Si1-
N3 1.694(5), Si2-N1 1.747(5), Si3-N2 1.751(5), Si4-N4 1.702(5), Si1-
O1 1.634(4), Si2-O1 1.616(4), Si3-O2 1.594(4), Si4-O2 1.625(4); N1-
Fe1-N2 93.09(16), N1-Fe1-N3 120.34(18), N2-Fe1-N3 143.56(18),
Si1-O1-Si2 149.5(3), Si3-O1-Si4 154.8(3).
2.6622(17) and 2.7657(17) Å, respectively. These distances
are longer than those in the few reported iodoiron(III)
complexes that contain terminal Fe-I bond lengths.11,12
The analogous dark-red iron(III) bromide complex
{FeBr1-x(Cl)x[MesN(SiMe2)]2O}2 (3a) can be prepared10 by
the reaction of 1 with the bromonium (Br+) transfer reagent
[AdAdBr][B(ArF)4] [AdAd ) adamantylideneadamantane;
ArF ) 3,5-(CF3)2Ph].13 However, this product also contained
a chloride impurity (x < 0.37), even in the single crystals.
In this case, incorporation of chloride was avoided by
reacting 1 with an excess of benzyl bromide,14 which yielded
pure {FeBr[MesN(SiMe2)]2O}2 (3)10 (the reaction of FeBr2
with {Li2[MesN(SiMe2)]2O} did not generate 1 cleanly). The
structure of 3 (Figure 3) shows a bromide-bridged dimer with
a symmetric Fe2(µ-Br)2 core (Fe-Br ) 2.471(2) and 2.503-
(2) Å) and a shorter Fe-Fe distance of 3.232(3) Å. However,
the most unusual feature is that each diamidosilylether ligand
does not chelate each iron center as in iodo complex 2 but
instead bridges to both Fe atoms and the silylether donors
remain unbound and remote from the metal centers. This
binding mode for diamidodonor-type ligands is extremely
rare;15 related ligands such as para-N,N′-disilyated bis-
(amido)benzene do bridge metal centers but, because of the
para arrangement of the amido donors, cannot act as
chelates.16 An intramolecular π-π-stacked amidoaryl array
(centroid-centroid ) 3.770 Å) is also present in 3 but is
absent in sterically strained 2.
Figure 2. Molecular structure of 2 (ORTEP, 50% ellipsoids). Selected
bond lengths (Å) and angles (deg): Fe1-Fe1* 3.749(3), Fe1-I1 2.6622-
(17), Fe1-I1* 2.7657(17), Fe1-N1 1.887(8), Fe1-N2 1.898(8), Si1-O1
1.630(9), Si1-N1 1.737(8), Si2-O1 1.582(8), Si2-N2 1.719(8); Fe1-
I1-Fe1* 87.36(5), I1-Fe1-I1* 92.64(5), N1-Fe1-N2 110.6(4), I1-Fe1-
N1 120.1(3), I1-Fe1-N2 112.5(3), I1*-Fe1-N1 112.4(3), I1*-Fe1-
N2 106.5(3), Si1-O1-Si2 144.4(5). * ) -x, -y + 1, -z.
terminal and two bridging amido groups; the silylether donor
is not bound. The Fe-Fe distance is 2.5479(13) Å. The
structure of 1 and its metrical parameters are comparable to
those of the related iron(II) dimer {Fe[tBuN(SiMe2)]2O}2,
with the exception that the silylether donor is bound to the
t
iron center in the Bu analogue.7
The reaction of 1 with iodine resulted in an immediate
color change from pale yellow to dark purple, and large
diamond-shaped crystals of {FeI[MesN(SiMe2)]2O}2 (2) were
isolated.10 To eliminate the presence of a chloride impurity
in 2 (from trace unseparated LiCl in the synthesis of 1), the
precursor 1 was prepared from FeI2 (thereby generating only
a LiI biproduct). The structure of 2 (Figure 2) clearly reveals
an iron(III) iodide-bridged dimer in the solid state, with each
four-coordinate, roughly tetrahedral iron center coordinated
to two terminal amido donors and two bridging iodides; the
Fe-Fe distance is 3.749(3) Å, precluding any direct metal-
metal interaction. The diamidosilylether ligands are chelating
the iron centers in 2, although the central silylether donor
remains unbound; a six-membered chelate ring is thereby
formed. The Fe2(µ-I)2 core is slightly asymmetric, as
indicated by the Fe1-I1 and Fe1-I1* bond lengths of
Indeed, the smaller size of bromide vs iodide (van der
Waals radii for Br/I are 1.85/1.96 Å)17 appears to be a key
(11) Stokes, S. L.; Davis, W. M.; Odom, A. L.; Cummins, C. C.
Organometallics 1996, 15, 4521.
(12) Pohl, S.; Bierbach, U.; Saak, W. Angew. Chem., Int. Ed. Engl. 1989,
26, 776.
(13) Nagra, H. K.; Batchelor, R. J.; Bennet, A. J.; Einstein, F. W. B.;
Lathioor, E. C.; Pomeroy, R. K.; Wang, W. J. Am. Chem. Soc. 1996,
118, 1207 and references cited therein.
(14) Fryzuk, M. D.; Leznoff, D. B.; Ma, E. S. F.; Rettig, S. J.; Young, V.
G., Jr. Organometallics 1998, 17, 2313.
(15) Jime´nez, M. V.; Sola, E.; Lahoz, F. J.; Oro, L. A. Organometallics
2005, 24, 2722. Hitchcock, P. B.; Huang, Q.; Lappert, M. F.; Wei,
X.-H.; Zhou, M. Dalton Trans. 2006, 2991.
(16) Zhao, B.; Li, H.; Shen, Q.; Zhang, Y.; Yao, Y.; Lu, C. Organometallics
2006, 25, 1824. Daniele, S.; Hitchcock, P. B.; Lappert, M. F. Chem.
Commun. 1999, 1909.
(17) Bondi, A. J. Phys. Chem. 1964, 68, 441.
Inorganic Chemistry, Vol. 46, No. 2, 2007 367

COMMUNICATION
Scheme 1. Proposed Route for the Bromonium Reaction with Iron(II)
Diamidoether Complex 1, via an Iron(IV) Intermediate ([NON] )
[MesN(SiMe2)]2O)
phosphinomethanes, which are usually designed to bridge
metal centers, can be induced to chelate a single metal by
altering the alkyl/aryl group steric properties; however, a
single such ligand has not been observed to be capable of
supporting both binding modes as a function of the coli-
gand.19 On the other hand, ansa-metallocenes can act either
as chelates or bridging “fly-over” ligands depending on the
coligands.20
factor that influences the binding mode: the larger steric
strain of adjacent amidoaryl groups in the chelating form
(as exemplified by the long Fe-Fe bonds in 2) can be
relieved by a switch to the bridging mode, as is observed in
3. Consistent with this concept, the same bridging mode is
also observed for the chloride analogue {FeCl[MesN-
(SiMe2)]2O}2 (4), which can be prepared by reacting 1 with
PhICl2.10 The 1H NMR spectra of these iron(III) complexes
have very broad, featureless peaks consistent with their
paramagnetism, and their intense colors are attributable to a
halide-to-metal charge transfer that shifts to lower energy
from Cl-Br-I (424 to 485 to 670 nm, respectively).
The large size and polarizability of iodides are known to
uniquely impact the catalyst reactivity but not necessarily
the catalyst structure.18 In the {FeX[MesN(SiMe2)]2O}2
system, it is the smaller halides (X) that significantly perturb
the structure from the expected arrangement. Unfortunately,
the absence of characteristic NMR or other spectroscopic
data in solution renders an examination of the actual solution
structures difficult, but the fact that both chelating and
bridging binding modes are observed for {FeX[MesN-
(SiMe2)]2O}2 as a function of halide in the solid state
indicates that there is only a small energy difference between
the two forms. As a comparison, ligands such as dialkyl-
The formation of 3 via the bromonium reagent, a formally
two-electron oxidant, may proceed via an iron(IV) intermedi-
ate of the form {FeBr[MesN(SiMe2)]2O}+[B(ArF)4]- (or its
dimer), which then undergoes intramolecular electron transfer
to form the final iron(III) product (Scheme 1). In agreement
with this proposal are the following observations: (1) the
oxidation product B(ArF)3 was isolated (which emphasizes
the noninnocent nature of the noncoordinating, “inert”
counterion);21 (2) if the bromonium addition reaction is
conducted in tetrahydrofuran (THF), rapid solvent polym-
erization occurs, but if 3 is prepared using benzyl bromide,
no polymerization occurs; (3) complexes 1-4 are stable in
THF once formed. Together, these points suggest the
presence of a highly reactive intermediate.
The bridging binding motif for diamidodonor ligands
reported here may have implications in the design of future
diamido-supported and other chelating ligand-supported
metal catalysts because it should not be assumed that a
“chelating ligand” will always chelate.
Acknowledgment. We are grateful to NSERC of Canada
(A.J.B. and D.B.L.) and Simon Fraser University (SFU) for
financial support and to Michael J. Katz (SFU) for assistance
with X-ray crystallography.
(18) Maitlis, P. M.; Haynes, A.; James, B. R.; Catellani, M.; Chiusoli, G.
P. Dalton Trans. 2004, 3409.
(19) Pamplin, C. B.; Rettig, S. J.; Patrick, B. O.; James, B. R. Inorg. Chem.
2003, 42, 4117. Puddephatt, R. J. Chem. Soc. ReV. 1983, 12, 99.
(20) Desurmont, G.; Li, Y.; Yasuda, H.; Maruo, T.; Kanehisa, N.; Kai, Y.
Organometallics 2000, 19, 1811. Stern, D.; Sabat, M.; Marks, T. J. J.
Am. Chem. Soc. 1990, 112, 9558. Qiao, K.; Fischer, R. D.; Paolucci,
G. J. Organomet. Chem. 1993, 456, 185.
Supporting Information Available: Synthetic procedures and
characterization data for 1-4 and crystallographic data for 1-3 in
CIF format. This material is available free of charge via the Internet
at http://pubs.jacs.org.
(21) Konze, W. V.; Scott, B. L.; Kubas, G. J. Chem. Commun. 1999, 1807.
IC061832M
368 Inorganic Chemistry, Vol. 46, No. 2, 2007