Bis(imino)pyridine ligand deprotonation promoted by a transient iron amide

Article abstract of DOI:10.1021/ic051839o

Addition of 2 equiv of LiNMe₂ to the bis(imino)pyridine ferrous dichloride, (^iPrPDI)FeCl₂ (^iPrDI = (2,6- ⁱPr₂-C₆H₃N=CMe)₂C ₅H₃N), resulted in deprotonation of the chelate methyl groups, yielding the bis(enamide)pyridine iron dimethylamine adduct, ( ^iPrPDEA)Fe-(NHMe₂) (^iPrPDEA = (2,6- ⁱPr₂-C₆H₃NC=CH₂) ₂C₅H₃N). Performing a similar procedure with KN(SiMe₃)₂ in THF solution afforded the corresponding bis(THF) adduct, (^iPrDEA)Fe(THF)₂. (^iPrPDEA)Fe- (NHMe₂) has also been prepared by addition of the free amine to the iron dialkyl complex, (^iPrPDI)Fe(CH₂SiMe₃) ₂, implicating formation of a transient iron amide that is sufficiently basic to deprotonate the bis(imino)pyridine methyl groups. Deprotonation of the amine ligand in (^iPRPDEA)Fe(NHMe₂) has been accomplished by addition of amide bases to afford the ferrous amide-ate complexes, [(^iPrPDEA)Fe(μ-NMe₂)M] (M = Li, K).

Full text of DOI:10.1021/ic051839o

Inorg. Chem. 2006, 45, 2−4
Bis(imino)pyridine Ligand Deprotonation Promoted by a Transient Iron
Amide
Marco W. Bouwkamp, Emil Lobkovsky, and Paul J. Chirik*
Department of Chemistry and Chemical Biology, Baker Laboratory,
Cornell UniVersity, Ithaca, New York 14853
Received October 24, 2005
Scheme 1
Addition of 2 equiv of LiNMe2 to the bis(imino)pyridine ferrous
dichloride, (iPrPDI)FeCl2 (iPrPDI (2,6-iPr2
C6H3N CMe)2C5H3N),
)
−
d
resulted in deprotonation of the chelate methyl groups, yielding
the bis(enamide)pyridine iron dimethylamine adduct, (iPrPDEA)Fe-
(NHMe2) (iPrPDEA
) −C6H3NCdCH2)2C5H3N). Performing
(2,6-iPr2
a similar procedure with KN(SiMe3)2 in THF solution afforded the
corresponding bis(THF) adduct, (iPrPDEA)Fe(THF)2. (iPrPDEA)Fe-
(NHMe2) has also been prepared by addition of the free amine to
the iron dialkyl complex, (iPrPDI)Fe(CH2SiMe3)2, implicating forma-
tion of a transient iron amide that is sufficiently basic to deprotonate
the bis(imino)pyridine methyl groups. Deprotonation of the amine
ligand in (iPrPDEA)Fe(NHMe2) has been accomplished by addition
of amide bases to afford the ferrous amide-ate complexes,
petency of iron(II).8 As part of our continuing investigations
with iron complexes for catalytic bond-forming reactions,9,10
we are exploring alternative synthetic routes to iron dialkyls
and ultimately the resulting alkyl cations.
[(iPrPDEA)Fe(
µ-NMe2)M] (M ) Li, K).
Bis(imino)pyridine complexes of iron(II) are of interest
given their high productivities for ethylene and R-olefin
polymerization upon activation with methylalumoxane
(MAO).1 While extensively studied,2-4 little is known about
the role of MAO in the activation process, and hence, key
details such as the oxidation state of the propagating iron
species remain speculative. The recent synthesis of bis-
(imino)pyridine iron(II) dialkyl complexes5-7 and the cor-
responding alkyl cations has established the catalytic com-
On the basis of observations from early transition metal
chemistry,11 we targeted the synthesis of iron amide com-
plexes with the goal of treating these compounds with alkyl
aluminums to furnish the desired iron dialkyls. Addition of
2 equiv of LiNMe2 to a diethyl ether slurry of (iPrPDI)FeCl2
(1-Cl2; iPrPDI ) (iPr2-C6H3NdCMe)2C5H3N) followed by
filtration and recrystallization from ether at -35 °C yielded
a red-brown solid identified as (iPrPDEA)Fe(NHMe2)
(2-NHMe2; iPrPDEA ) (2,6-iPr2-C6H3NCdCH2)2C5H3N),
arising from deprotonation of the two bis(imino)pyridine
methyl groups rather than amidation (Scheme 1).
* Corresponding author. E-mail: pc92@cornell.edu.
(1) (a) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. ReV. 2000, 100,
1169. (b) Small, B. L.; Brookhart, M.; Bennett, A. M. A. J. Am. Chem.
Soc. 1998, 120, 4049. (c) Britovsek, G. J. P.; Gibson, V. C.; Kimberley,
B. S.; Maddox, P. J.; McTavish, S. J.; Solan, G. A.; White, A. J. P.;
Williams, D. J. Chem. Commun. 1998, 849. (d) Britovsek, G. J. P.;
Bruce, M.; Gibson, V. C.; Kimberley, B. S.; Maddox, P. J.;
Mastroianni, S.; McTavish, S. J.; Redshaw, C.; Solan, G. A.;
Stro¨mberg, S.; White, A. J. P.; Williams, D. J. J. Am. Chem. Soc.
1999, 121, 8728. (e) Gibson, V. C.; Spitzmesser, S. K. Chem. ReV.
2003, 103, 283. (f) Bianchini, C.; Mantovani, G.; Meli, A.; Migliacci,
D.; Zanobini, F.; Laschi, F.; Sommazzi, A. Eur. J. Inorg. Chem. 2003,
1620.
The benzene-d6 1H NMR spectrum of 2-NHMe2 exhibits
nine paramagnetically broadened and shifted resonances over
(5) Bouwkamp, M. W.; Bart, S. C.; Hawrelak, E. J.; Trovitch, R. J.;
Lobkovsky, E.; Chirik, P. J. Chem. Commun. 2005, 3406.
(6) Scott, J.; Gambarotta, S.; Korobkov, I.; Budzelaar, P. H. M. J. Am.
Chem. Soc. 2005, 127, 13019.
(2) (a) Babik, S. T.; Fink, G. J. Mol. Catal. A 2002, 188, 245. (b)
Britovsek, G. J. P.; Gibson, V. C.; Spitzmesser, S. K.; Tellman, K.
P.; White, A. J. P.; Williams, D. J. J. Chem. Soc., Dalton Trans. 2002,
1159. (c) Bryliakov, K. P.; Semikolenova, N. V.; Zakharov, V. A.;
Talsi, E. P. Organometallics 2004, 23, 5375.
(3) Deng, L.; Margl, P.; Ziegler, T. J. Am. Chem. Soc. 1999, 121, 6479.
(4) Britovsek, G. J. P.; Clentsmith, G. K. B.; Gibson, V. C.; Goodgame,
D. M. L.; McTavish, S. J.; Pankhurst, Q. A. Catal. Comm. 2002, 3,
207.
(7) Ca´mpora, J.; Naz, A. M.; Palma, P.; AÄ lvarez, E.; Reyes, M. L.
Organometallics 2005, 24, 4878.
(8) Bouwkamp, M. W.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc.
2005, 127, 9660.
(9) Bart, S. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2004,
126, 13794.
(10) Bart, S. C.; Hawrelak, E. J.; Schmisseur, A. K.; Lobkovsky, E.; Chirik,
P. J. Organometallics 2003, 23, 437.
(11) Kim, I.; Jordan, R. F. Macromolecules 1996, 29, 489.
2
Inorganic Chemistry, Vol. 45, No. 1, 2006
10.1021/ic051839o CCC: $33.50
© 2006 American Chemical Society
Published on Web 11/30/2005

COMMUNICATION
Figure 1. ORTEP represenations of 2-NHMe2, 2-NMe2K, and 2-OHK. Hydrogen atoms except for those attached to the methylene backbone carbons, the
dimethylamine ligand in 2-NHMe2, and the hydroxy ligand in 2-OHK are omitted for clarity. Included is a side-view of 2-OHK (bottom right) revealing
the geometry of the [Fe(OH)]2[µ-K]2 framework.
Table 1. Selected Bond Distances (Å) and Angles (deg) for 2-NHMe2,
2-NMe2K, and 2-OHK
value typically encountered in bis(imino)pyridine ligands
(∼1.30 Å).1,5
2-NHMe2
2-NMe2K
2-OHK
Performing the deprotonation of 1-Cl2 in THF with larger
amide bases such as KN(SiMe3)2 or LiNiPr2 furnished the
bis-THF compound, (iPrPDEA)Fe(THF)2 (2-(THF)2) as a
green-brown solid (Scheme 1). Exposure of 2-(THF)2 to
vacuum, either in solution or the solid state, yielded 2-(THF)
arising from THF loss. Substitution of the amine or THF
ligands in 2-NHMe2 and 2-(THF)2 has been accomplished
by addition of more potent ligands. Treatment of either
compound with pyridine resulted in isolation of a brown solid
Fe(1)-Xa
2.040(3)
1.931(3)
1.906(2)
1.944(3)
1.9773(16)
2.0746(15)
2.1113(14)
2.0780(15)
1.9347(17)
2.0035(19)
2.0900(18)
2.0162(17)
1.9333(18)
2.0252(18)
2.0683(19)
2.0366(18)
1.371(3)
1.360(3)
1.375(3)
1.379(3)
1.335(4)
Fe(1)-N(1)
Fe(1)-N(2)
Fe(1)-N(3)
Fe(1)-O(5)
Fe(1)-N(4)
Fe(1)-N(5)
Fe(1)-N(6)
N(1)-C(2)
N(3)-C(8)
N(4)-C(35)
N(6)-C(41)
C(1)-C(2)
C(8)-C(9)
C(34)-C(35)
C(41)-C(42)
1.386(4)
1.378(4)
1.370(2)
1.380(2)
t
identified as 2-(py) while addition of BuNC formed
1.350(5)
1.345(5)
1.355(3)
1.346(3)
2-(CNtBu) (Scheme 1). All of these compounds have similar
spectroscopic features to 2-NHMe2, although 2-(CNtBu) has
only been observed in solution as formation of an unidenti-
fied brown precipitate occurred over time.
1.340(4)
1.333(4)
1.336(4)
X-Fe(1)-N(2)a
N(1)-Fe(1)-N(3)
O(5)-Fe(1)-N(5)
N(4)-Fe(1)-N(6)
164.32(14)
161.88(11)
165.05(6)
148.18(6)
175.56(8)
Each adduct prepared in this study is paramagnetic. For
the complex containing the strongest-field ligand, 2-(CNtBu),
a magnetic moment (Evans method, C6D6, 23 °C) of 2.9 µB
was measured, consistent with the spin-only value for two
unpaired electrons and an intermediate-spin iron(II) center.
Weaker-field ligands such as THF yielded a high-spin
complex with a µeff of 4.7 µB. A solution and solid-state
magnetic moment of 3.6 µB was repeatedly (nine indepen-
dent trials) measured for 2-NHMe2, suggesting an admixture
of high- and intermediate-spin complexes at 23 °C.
It is now well-documented that bis(imino)pyridine chelates
are “non-innocent” ligands, participating in redox processes
with the metal9,12 or reaction chemistry including alkylation
at the pyridine6,13 or imine positions14 and C-C dimeriza-
150.77(7)
172.40(8)
151.22(8)
a For 2-NHMe2 and 2-NMe2K: X ) N(4); for 2-OHK: X ) O(1).
a 170 ppm chemical shift range, consistent with a
C2V-symmetric ligand environment. Notably, two singlets are
observed at -58.9 and -39.4 ppm for the methylene protons
on the tridentate ligand backbone, indicating their coplanarity
1
with the chelate. Although not located in the H NMR
spectrum, an N-H stretch was observed at 3258 cm-1 in
the solid-state (KBr) infrared spectrum, confirming a neutral
NHMe2 ligand.
Slow evaporation of a diethyl ether solution of 2-NHMe2
produced single crystals suitable for X-ray diffraction. A
representation of the molecular structure is presented in
Figure 1, while selected metrical parameters are reported in
Table 1. The data were of sufficient quality such that all of
the hydrogen atoms were located and refined, reaffirming
the presence of a neutral dimethylamine ligand and meth-
ylene carbons at C(1) and C(9). The C(1)-C(2) and C(8)-
C(9) bond distances of 1.350(5) and 1.345(5) Å are in the
range expected for a CdC bond, while the C-N bond
lengths of 1.386(4) and 1.378(4) Å are elongated from the
(12) (a) de Bruin, B.; Bill, E.; Bothe, E.; Weyermu¨ller, T.; Wieghardt, K.
Inorg. Chem. 2000, 39, 2936. (b) Vidaratne, I.; Gambarotta, S.;
Korobkov, I.; Budzelaar, P. H. M. Inorg. Chem. 2005, 44, 1187.
(13) (a) Reardon, D.; Conan, F.; Gambarotta, S.; Yap, G. P. A.; Wang, Q.
J. Am. Chem. Soc. 1999, 121, 9318. (b) Sugiyama, Aharonian, G.;
Gambarotta, S.; Yap, G. P. A.; Budzelaar, P. H. M. J. Am. Chem.
Soc. 2002, 124, 12268. (c) Clentsmith, G. K. B.; Gibson, V. C.;
Hitchcock, P. B.; Kimberely, B. S.; Rees, R. W. Chem. Commun. 2002,
1498. (d) Blackmore, I. J.; Gibson, V. C.; Hitchcock, P. B.; Rees, C.
W.; Williams, D. J.; White, A. J. P. J. Am. Chem. Soc. 2005, 127,
6012.
(14) Bruce, M.; Gibson, V. C.; Redshaw, C.; Solan, G. A.; White, A. J.
P.; Williams, D. J. Chem. Commun. 1998, 2523.
Inorganic Chemistry, Vol. 45, No. 1, 2006
3

COMMUNICATION
tion.15 Deprotonation of the methyl positions in the chelate
backbone has also been previously observed during low-
temperature alkylation of 1-Cl2 with LiCH2SiMe3,6 in related
manganese chemistry,13b and upon treatment of the free
ligand with alkyllithium bases16 and alkali metal reductants.17
To gain insight into the ligand-deprotonation reaction with
amide bases, additional experiments were conducted. Ad-
Scheme 2
5
dition of 1 equiv of NHMe2 to 1-(CH2SiMe3)2 resulted in
clean conversion to 2-NHMe2 along with liberation of SiMe4
(eq 1). Formation of 2-NHMe2 by this procedure is much
In benzene-d6 solution, both 2-NMe2M complexes exhibit
the number of resonances for C2V-symmetric compounds
along with free diethyl ether, suggesting dissociation of the
cation. It should be noted that a symmetric dimeric structure
(vide infra), while unlikely, has not been ruled out. The
solution magnetic moment of 4.3 µB measured for 2-NMe2K
is consistent with high-spin iron(II).
The protonation of the iron amide-ate complexes was also
studied. Addition of an acid with a weakly coordinating
conjugate base, [PhNHMe2][BPh4], to 2-NMe2K cleanly and
quantitatively regenerated 2-NHMe2. However, when a
more-nucleophilic conjugate base is present, displacement
of the amide ligand was observed. Treatment of 2-NMe2K
with water resulted in isolation of the dimeric bis(hydroxy)
iron compound, [(iPrPDEA)FeOH]2(µ-K)2 (2-OHK, Scheme
2). The solid-state structure of 2-OHK (Figure 1) reveals a
dimeric molecule with bridging potassium ions ligated by
the hydroxide oxygen and aryl rings from the iPrPDEA ligand.
The data were of sufficient quality such that the hydrogen
atoms on the hydroxide ligands were located and refined.
The metrical parameters (Table 1) of the iPrPDEA ligand are
comparable to those found in the solid-state structures of
2-NHMe2 and 2-NMe2K.
In summary, a series of ferrous complexes bearing a bis-
(enamide)pyridine ligand has been prepared via double
deprotonation of the corresponding bis(imino)pyridine iron
dichloride complex by reaction with amide bases. Similar
products are obtained by addition of free amine to the related
ferrous dialkyl, implicating a transient iron-amide as the base
for chelate deprotonation. Direct attack by an external amide
base in the salt metathesis procedures is also plausible. Using
this ligand motif, iron-amide- and hydroxy-“ate” complexes
have also been isolated and crystallographically character-
ized, demonstrating the rich coordination chemistry associ-
ated with this class of molecules.
slower than the corresponding salt metathesis reaction. The
analogous experiment with NDMe2 again formed 2-NHMe2
along with deuterated SiMe4. These observations suggest that
the base in these reactions is a transient ferrous amide
complex, either (iPrPDI)Fe(NMe2)(CH2SiMe3) or (iPrPDI)Fe-
(NMe2)2, that promotes the ligand deprotonation, either
intramolecularly by imine dissociation and bond rotation or
through a bimolecular process. Precedent for such chemistry
has been provided by Group 8 amide complexes, such as
(dmpe)2M(NH2)H (M ) Fe, Ru; dmpe ) 1,2-bis(dimethyl-
phosphino)ethane), which are known to deprotonate weak
carbon acids such as fluorene.18
Treatment of 2-NHMe2 with additional KN(SiMe3)2 or
LiNMe2 resulted in formation of the ferrous amide-ate
complexes, [(iPrPDEA)Fe(µ-NMe2)M] (2-NMe2K; M ) K;
2-NMe2Li; M ) Li) (Scheme 2). Interestingly, 2-NMe2Li
was also prepared from treatment of 1-(CH2SiMe3)2 with 1
equiv of LiNMe2. The potassium derivative, 2-NMe2K, has
been characterized by X-ray diffraction (Figure 1) and reveals
an essentially square planar iron center with a dimethylamide
ligand occupying one of the coordination sites. Both the
olefinic and C-N bond distances (Table 1) are comparable
to those found in 2-NHMe2, confirming a doubly deproto-
nated tridentate ligand. The Fe(1)-N(4) bond distance of
1.9773(16) Å is contracted from the value of 2.040(3) Å
found in the neutral precursor. The potassium cation is ligated
by two molecules of diethyl ether and is also coordinated to
the amide nitrogen and to one of the aryl groups of the
tridentate chelate.
Acknowledgment. M.W.B. thanks The Netherlands
Organization for Scientific Research for a TALENT post-
doctoral fellowship. P.J.C. is a David and Lucile Packard
Foundation Fellow in Science and Engineering and a Cottrell
Scholar of the Research Corporation. Both organizations are
acknowledged for generous financial support.
(15) (a) Scott, J.; Gambarotta, S.; Korobkov, I. Can. J. Chem. 2005, 83,
279. (b) Sugiyama, H.; Korobkov, I.; Gambarotta, S.; Moeller, A.;
Budzelaar, P. H. M. Inorg. Chem. 2004, 43, 5771.
(16) Khorobkov, I.; Gambarotta, S.; Yap, G. P. A. Organometallics 2002,
21, 3088.
(17) Enright, D.; Gambarotta, S.; Yap, G. P. A.; Budzelaar, P. H. M. Angew.
Chem., Int. Ed. 2002, 41, 3873.
(18) (a) Fulton, J. R.; Sklenak, S.; Bouwkamp, M. W.; Bergman, R. G. J.
Am. Chem. Soc. 2002, 124, 4722. (b) Fox, D. J.; Bergman, R. G.
Organometallics 2004, 23, 1656.
Supporting Information Available: Complete experimental
detail and crystallographic data for 2-NHMe2, 2-NMe2K, and
2-OHK. This material is available free of charge via the Internet
at http://pubs.acs.org.
IC051839O
4
Inorganic Chemistry, Vol. 45, No. 1, 2006