Square planar bis(imino)pyridine iron halide and alkyl complexes

Article abstract of DOI:10.1039/b504063a

Square planar iron methyl complexes containing bis(imino)pyridine (PDI) ligands have been prepared by reductive alkylation of the corresponding ferrous dichloride; dialkylation is observed upon treatment with a larger alkyl lithium. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b504063a

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Square planar bis(imino)pyridine iron halide and alkyl complexes{{
Marco W. Bouwkamp, Suzanne C. Bart, Eric J. Hawrelak,§ Ryan J. Trovitch, Emil Lobkovsky and
Paul J. Chirik*
Received (in Berkeley, CA, USA) 20th March 2005, Accepted 28th April 2005
First published as an Advance Article on the web 27th May 2005
DOI: 10.1039/b504063a
Square planar iron methyl complexes containing bis(imino)-
pyridine (PDI) ligands have been prepared by reductive
alkylation of the corresponding ferrous dichloride;
dialkylation is observed upon treatment with a larger alkyl
lithium.
ð1Þ
Interest in well-defined iron complexes as precious metal
surrogates in catalytic bond-forming reactions relevant to organic
synthesis continues to be an area of ongoing development.1
Solution magnetometry (benzene-d6, 23 uC) on each of the dark
green complexes produced magnetic moments consistent with the
spin-only value for three unpaired electrons and is consistent with
one electron reduction of the ferrous dihalide precursor (eqn. 1).
Reports of catalytic olefin hydrogenation, hydrosilation and
isomerization by a transient [Fe(CO)3] fragment, generated either
by high temperature thermolysis (. 100 uC)2 or continued
photolysis3 of Fe(CO)5, inspired the exploration of related [L3Fe]
species that could be accessed under mild conditions. The
bis(dinitrogen) complex, (iPrPDI)Fe(N2)2 (1-(N2)2), prepared by
reduction of the corresponding ferrous dichloride or dibromide,4
readily dissociates one equivalent of dinitrogen in solution and
serves as an active catalyst for the aforementioned transforma-
tions.5 Significantly, these reactions operate efficiently at part per
million levels of iron without the need for organic solvent. Inspired
by the reports of olefin polymerization with (PDI)FeX2 (X 5 Cl,
Br) when activated by methylalumoxane (MAO),4 we
targeted the preparation of well-defined iron alkyl complexes
for catalytic bond-forming reactions involving small molecule
organic substrates. Here we describe our initial synthetic
efforts toward this goal with the preparation and characteriza-
tion of four- and five-coordinate iron alkyl and dialkyl
derivatives.
1
Paramagnetically broadened and isotropically shifted H NMR
resonances are observed over a 600 ppm chemical shift range.
Hydrogens in the plane of the three nitrogens and hence in
conjugation with the iron center6 exhibit large displacements from
their diamagnetic reference values while those on the orthogonal
aryl groups have smaller isotropic shifts.
Cooling a concentrated diethyl ether solution of 2-Cl to 235 uC
afforded green crystals suitable for X-ray diffraction. The solid
state structure (Fig. 1) reveals coordination of a diethyl ether
ligand in the apical position of the molecule. Dissolving the crystals
in hydrocarbons such as pentane and toluene induced rapid
dissociation of Et2O and allowed isolation of the square planar
complex upon trituration. Spectroscopic characterization of single
crystals of 1-Cl isolated from Et2O provided no evidence for ether
coordination, highlighting the greater steric protection imparted by
the larger 2,6-diisopropyl aryl substituents. The metrical para-
meters (Table 1) for 2-Cl?Et2O reveal slight perturbations (0.05–
During the synthesis of 1-(N2)2 prepared by Na(Hg) reduction
of 1-Cl2, an intermediate C2v symmetric product was identified.
Subsequent experiments revealed that this species could be cleanly
isolated by treatment of 1-Cl2 with one equivalent of NaBEt3H or
by stirring the alkali metal reduction for shorter reaction times. A
˚
0.1 A) from the corresponding five-coordinate ferrous dichloride
complexes.4,7 For the monochloride, the C–N bond lengths are
slightly elongated and the iron–nitrogen bonds are contracted,
consistent with greater reduction of the bis(imino)pyridine ligand
by the low-valent iron center.
1
combination of H NMR spectroscopy, combustion analysis and
solution magnetometry identified the green product as the iron
monochloride complex, 1-Cl (eqn. 1). This synthetic procedure has
been extended to include compounds containing the 2,6-diethyl-
and 2,4,6-trimethylphenyl substituted bis(imino)pyridine ligands as
well as the monobromide complex, 1-Br, of the 2,6-diisopropyl
variant.
With well-defined monohalide complexes in hand, salt metathe-
sis reactions with alkyl lithiums were explored. Methylation of each
of the bis(imino)pyridine iron monochloride complexes was
accomplished by addition of one equivalent of MeLi in diethyl
ether (eqn. 2). The monomethyl derivatives, 1-Me, 2-Me and 3-Me,
were isolated as green solids in modest to good yields. Using a
similar procedure, 2-CH2SiMe3 was prepared from 2-Cl. Attempts
to synthesize 1-CH2SiMe3 in this manner produced no reaction,
again demonstrating the impact of the aryl substituents on the
reactivity of the monochloride complexes. Solution magnetometry
(benzene-d6, 23 uC) of each alkyl derivative produced moments
consistent with the spin only value for three unpaired electrons,
similar to the electronic structure of the monochloride compounds.
{ Electronic supplementary information (ESI) available: experimental
details and details of the refinement and crystal structures. See http://
www.rsc.org/suppdata/cc/b5/b504063a/
{ This communication is dedicated to Ian Rothwell.
§ Present address: Department of Chemistry, 237A Hartline Science
Center, Bloomsburg University, Bloomsburg, PA, USA.
*pc92@cornell.edu
3406 | Chem. Commun., 2005, 3406–3408
This journal is ß The Royal Society of Chemistry 2005

View Article Online
Fig. 1 Solid state structures of 2-Cl?Et2O, 1-Me and 1-(CH2SiMe3)2 at 30% probability ellipsoids.
˚
Table 1 Selected bond distances (A) and angles (u)
consequence of replacing an inductively withdrawing chloride
ligand with a strong field alkyl substituent.
2-Cl?Et2O
1-Me
1-(CH2SiMe3)2
Synthesis of a family of four-coordinate bis(imino)pyridine iron
alkyl complexes by straightforward salt metathesis reactions
prompted investigation into the alkylation chemistry of the
corresponding ferrous dichloride complexes. While ferrous mono-
alkyl8 and dialkyl9 complexes are known, those containing
bis(imino)pyridine ligands have remained elusive. Interest in these
molecules stems from their potential to serve as precursors for
single component olefin polymerization catalysts and ability to
provide insight into the nature of the propagating species.7,10 It
should be noted that related monomethyl and dimethyl derivatives
of a bis(imino)carbazolide iron complex have been reported.11
Treatment of either of these alkyls or the halide precursors with a
variety of activators did not induce ethylene polymerization
activity.
Fe(1)–N(1)
Fe(1)–N(2)
Fe(1)–N(3)
Fe(1)–Cl(1)
Fe(1)–O(1)
Fe(1)–C(10)
Fe(1)–C(14)
N(1)–C(2)
N(3)–C(8)
C(2)–C(3)
C(7)–C(8)
N(1)–Fe(1)–N(3)
N(1)–Fe(1)–N(2)
N(2)–Fe(1)–N(3)
N(2)–Fe(1)–Cl(1)
N(2)–Fe(1)–O(1)
N(2)–Fe(1)–C(10)
N(2)–Fe(1)–C(14)
2.181(2)
2.009(2)
2.209(2)
2.2668(8)
2.213(2)
1.968(5)
1.893(4)
1.952(5)
2.2030(19)
2.0133(19)
2.263(2)
2.001(6)
2.062(3)
2.054(3)
1.302(3)
1.301(3)
1.448(3)
1.454(3)
141.07(7)
74.03(7)
72.88(7)
1.313(3)
1.301(3)
1.443(4)
1.453(4)
149.56(8)
75.22(8)
74.57(8)
150.96(6)
108.99(8)
1.337(7)
1.332(7)
1.432(8)
1.442(8)
159.2(2)
79.1(2)
80.1(2)
176.0(2)
107.93(9)
140.02(10)
Addition of two equivalents of MeLi to a THF or diethyl ether
slurry of 1-Cl2 did not furnish the corresponding dimethyl
complex. Rather, 1-Me, the product of reductive alkylation, was
isolated in moderate yield (eqn. 3). Following an identical
procedure with 2-Cl2 and 3-Cl2 afforded 2-Me and 3-Me,
respectively, demonstrating the generality of the transformation.
Similar results have been reported with an analogous cobalt
complex, as treatment of (iPrPDI)CoCl2 resulted in isolation of
(2)
iPrPDI)CoMe.12 For the iron reactions, methane and ethane have
The benzene-d6 1H NMR spectra of these molecules exhibit
the number of peaks anticipated for C2v symmetric compounds,
with resonances ranging between 2170 and 225 ppm. While
this chemical shift dispersion is slightly less than that observed
with the monochloride complexes, the hydrogens attached to
carbons in conjugation with the iron center still display larger
isotropic shifts than those on the orthogonal aryl substituents.
Resonances for the iron–methyl groups or the methylene
hydrogens on the carbon bound to iron in 2-CH2SiMe3 were
not located.
(
1
been identified by H NMR spectroscopy as byproducts of the
reductive alkylation procedure.
ð3Þ
Alkylation of 1-Cl2 with two equivalents of LiCH2SiMe3
produced a different outcome. In this case, a purple solid identified
as the bis(imino)pyridine ferrous dialkyl, (iPrPDI)Fe(CH2SiMe3)2
(1-(CH2SiMe3)2), was isolated in 45% yield (eqn. 3). The benzene-
d6 1H NMR spectrum of the compound exhibited the number of
resonances expected for a C2v symmetric molecule, with iso-
tropically shifted imine methyl groups centered at 2150.8 ppm and
a para pyridine resonance at 281.9 ppm. The SiMe3 groups are
equivalent, appearing as a broad singlet centered at approximately
15 ppm. The methylene protons on the carbons bound to the iron
were not located. Solution magnetometry (benzene-d6, 23 uC)
The solid state structure of 1-Me (Fig. 1) was elucidated by
X-ray diffraction. A square planar molecular geometry is observed
with the bond angles around the iron summing to 360.0(4)u. The
˚
iron methyl group lies 0.112 A below the idealized square plane
flanked by the orthogonal aryl substituents. The metrical
distortions (Table 1) in the bis(imino)pyridine ligand are more
severe than those observed in 2-Cl?Et2O, with more contracted
iron–nitrogen bonds, elongated imine carbon–nitrogen distances
and an expanded N(1)–Fe(1)–N(3) bond angle. These observations
are consistent with increased iron to ligand backdonation, a
This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 3406–3408 | 3407

View Article Online
produced a magnetic moment of 4.8 mB, consistent with the spin
only value for four unpaired electrons.
species, as well as the corresponding ferrous dialkyl, in olefin
polymerization and bond-forming reactions relevant to organic
synthesis will be the subject of future studies in our laboratory."
This work has been supported by Cornell University, the
Research Corporation (Cottrell Scholarship to P. J. C.), the
Packard Foundation, the National Institutes of Health (Training
Grant to S. C. B.) and the Netherlands Organization for Scientific
Research (TALENT Fellowship to M. W. B.).
The solid state structure of 1-(CH2SiMe3)2 was determined by
X-ray diffraction (Fig. 1). Unlike 2-Cl?Et2O and 1-Me, the iron
˚
atom in 1-(CH2SiMe3)2 lies 0.543 A out of the plane of the three
nitrogen atoms of the bis(imino)pyridine ligand. As a result,
relatively long Fe(1)–N(1) and Fe(1)–N(2) bond distances of
˚
2.2030(19) and 2.0133(19) A are observed, indicative of little
backdonation from the metal center. Accordingly, contracted
N(1)–C(2) and N(3)–C(8) distances of 1.302(3) and 1.301(3) are
also noted, in agreement with the metrical parameters reported for
the corresponding dichloride complex.4 The two alkyl substituents
are oriented above and below the plane of the bis(imino)pyridine
ligand, rendering the geometry of the molecule intermediate
between the square planar and trigonal bipyramidal extremes.
In light of the observations with MeLi, isolation of
1-(CH2SiMe3)2 by alkylation of 1-Cl2 seemed puzzling and
prompted further experimentation. Addition of one equivalent of
LiCH2SiMe3 to 1-Cl2 induced reduction, resulting in the clean
isolation of 1-Cl in good yield. Because 1-Cl does not undergo
facile alkylation upon addition of LiCH2SiMe3 in either THF or
diethyl ether, the monochloride is not an intermediate on the
pathway to 1-(CH2SiMe3)2. This observation contrasts the
chemistry with MeLi, where the monochloride compounds serve
as precursors for the observed reductive alkylation products.
To further explore the reactivity difference between the two
alkyl lithiums, the alkylation of 2-Cl2 was investigated in more
detail. In this case, the monochloride complex, 2-Cl, undergoes
alkylation with LiCH2SiMe3 and the product of reductive
alkylation, 2-CH2SiMe3, may be accessible. Addition of two
equivalents of LiCH2SiMe3 to a THF solution of 2-Cl2 furnished
both green and purple solids after workup. Attempts to separate
the two products by washing or recrystallization have not been
successful. However, analysis of the mixture by 1H NMR
spectroscopy revealed formation of a 4 : 1 ratio of 2-CH2SiMe3,
the green product of reductive alkylation, and 2-(CH2SiMe3)2, the
purple material arising from dialkylation (eqn. 4).
Marco W. Bouwkamp, Suzanne C. Bart, Eric J. Hawrelak,§
Ryan J. Trovitch, Emil Lobkovsky and Paul J. Chirik*
Department of Chemistry and Chemical Biology, Baker Laboratory,
Cornell University, Ithaca, NY, USA 14853. E-mail: pc92@cornell.edu;
Fax: 607-255-4137; Tel: 607-254-4538
Notes and references
" Crystal data for 2-Cl?Et2O, 1-Me, and 1-(CH2SiMe3)2. C33H45ClFeN3O,
˚
M 5 591.02, monoclinic, a 5 13.1161(5), b 5 19.2048(8), c 5 13.5204(6) A,
3
˚
b 5 115.1460(10)u, U 5 3082.9(2) A , T 5 173(2) K, space group P2(1)/n,
Z 5 4, m(Mo-Ka) 5 0.606 mm21, 25123 reflections measured, 7615 unique
(Rint 5 0.0483) which were used in all calculations. The final R1 was 0.0522
(all data). C34H46FeN3, M 5 552.59, triclinic, a 5 16.0725(12),
˚
b 5 16.2819(11), c 5 17.0074(12) A, a 5 86.074(2), b 5 64.426(2),
3
˚
c 5 62.003(2)u, U 5 3494.2(4) A , T 5 173(2) K, space group P-1, Z 5 4,
m(Mo-Ka) 5 0.454 mm21, 17737 reflections measured, 7309 unique
(Rint 5 0.0689) which were used in all calculations. The final R1 was 0.0674
(all data). C41H65FeN3Si2, M 5 711.99, orthorhombic, a 5 15.8771(7),
3
˚
˚
b 5 20.0007(7), c 5 26.177(1) A, U 5 8312.6(6) A , T 5 173(2) K, space
group Pbca, Z 5 8, m(Mo-Ka) 5 0.450 mm21, 67211 reflections measured,
5980 unique (Rint 5 0.0446) which were used in all calculations. The final
R1 was 0.0340 (all data). CCDC 267323–267325. See http://www.rsc.org/
suppdata/cc/b5/b504063a/ for crystallographic data in CIF or other
electronic format.
1 C. Bolm, J. Legros, J. Le Paih and L. Zani, Chem. Rev., 2004, 104, 6217.
2 R. E. Harmon, S. K. Gupta and D. J. Brown, Chem. Rev., 1973, 73, 21.
3 M. A. Schroeder and M. S. Wrighton, J. Am. Chem. Soc., 1976, 98, 551.
4 (a) B. L. Small, M. Brookhart and A. M. A. Bennett, J. Am. Chem.
Soc., 1998, 120, 4049; (b) G. J. P. Britovsek, M. Bruce, V. C. Gibson,
B. S. Kimberely, P. J. Maddox, S. Mastroianni, S. J. McTavish,
C. Redshaw, G. A. Solan, S. Stromberg, A. J. P. White and A. D.
J. Williams, J. Am. Chem. Soc., 1999, 121, 8728.
5 S. C. Bart, E. Lobkovsky and P. J. Chirik, J. Am. Chem. Soc., 2004, 126,
13794.
6 (a) Q. Knijnenburg, D. Hetterscheid, T. M. Kooistra and P. H.
M. Budzelaar, Eur. J. Inorg. Chem., 2004, 1204; (b) D. Enright,
S. Gambarotta, G. P. A. Yap and P. H. M. Budzelaar, Angew. Chem.,
Int. Ed., 2002, 41, 3873; (c) P. H. M. Budzelaar, B. de Bruin, A. W. Gal,
K. Weighardt and J. H. van Lenthe, Inorg. Chem., 2001, 40, 4649; (d)
B. de Bruin, E. Bill, E. Bothe, T. Weyermu¨ller and K. Wieghardt, Inorg.
Chem., 2000, 39, 2936.
7 (a) S. T. Babik and G. Fink, J. Mol. Catal. A, 2002, 188, 245; (b) G. J.
P. Britovsek, V. C. Gibson, S. K. Spitzmesser, K. P. Tellman, A. J.
P. White and D. Williams, J. Chem. Soc., Dalton Trans., 2002, 3, 207.
8 (a) J. L. Kisko, T. Hascall and G. Parkin, J. Am. Chem. Soc., 1998, 120,
10561; (b) E. J. Daida and J. C. Peters, Inorg. Chem., 2004, 43, 7474; (c)
A. Yamamoto, K. Morifuji, S. Ikeda, T. Saito, Y. Uchida and
A. Misono, J. Am. Chem. Soc., 1965, 87, 4652.
9 S. C. Bart, E. J. Hawrelak, A. K. Schmisseur, E. Lobkovsky and
P. J. Chirik, Organometallics, 2004, 23, 247.
10 G. J. P. Britovsek, G. K. B. Clentsmith, V. C. Gibson, D. M.
L. Goodgame, S. J. McTavish and Q. A. Pankhurst, Catal. Commun.,
2002, 3, 207.
11 V. C. Gibson, S. K. Spitzmesser, A. J. P. White and D. J. Williams,
J. Chem. Soc., Dalton Trans., 2003, 13, 2718.
12 V. C. Gibson, M. J. Humphries, K. P. Tellman, D. F. Wass, A. J.
P. White and D. J. Williams, Chem. Commun., 2001, 2252.
ð4Þ
Based on these initial observations, it appears that the reactivity
of the putative (PDI)Fe(R)Cl intermediate dictates the outcome of
the alkylation reaction. Additional alkyl lithium can serve to either
alkylate or reduce this compound. Smaller, more reducing MeLi
affords solely the products of reductive alkylation, whereas the
more sterically hindered LiCH2SiMe3 either reduces or alkylates,
depending on the steric environment of the ligand. In the absence
of either alkyl lithium, (PDI)Fe(R)Cl undergoes ejection of the
alkyl radical to yield the monochloride complex. The origin of
these differences remains an active area of investigation in our
laboratory.
In summary, a family of square planar bis(imino)pyridine iron
monohalide and monomethyl complexes have been prepared and
isolated. The electronic structure and catalytic activity of these
3408 | Chem. Commun., 2005, 3406–3408
This journal is ß The Royal Society of Chemistry 2005