Arene coordination in bis(imino)pyridine iron complexes: Identification of catalyst deactivation pathways in iron-catalyzed hydrogenation and hydrosilation

Article abstract of DOI:10.1021/om060441c

The phenyl-substituted bis(imino)pyridine iron bis(dinitrogen) complex (ⁱPrPhPDI)Fe(N₂)₂ (ⁱPrPhPDI = 2,6-(2,6-ⁱPr₂-C₆H₃N=CPh) ₂C₅H₃N) was prepared by sodium amalgam reduction of the corresponding ferrous dichloride precursor under 4 atm of dinitrogen. Comparison of the infrared stretching frequencies of the bis(dinitrogen), mono(dinitrogen), and related dicarbonyl derivatives to those of the corresponding bis(imino)pyridine iron compounds bearing the methyl-substituted ligand, (ⁱPrPDI)Fe(L)_n ( ⁱPrPDI = 2,6-(2,6-ⁱPr₂-C₆H ₃N=CMe)₂C₅H₃N; L = CO, n = 2; L = N₂, n = 1, 2), established a more electrophilic iron center for the phenyl-substituted cases. Comparing the productivity of (ⁱPrPhPDI) Fe(N₂)₂ to (ⁱPrPDI)Fe(N₂) ₂ in the catalytic hydrogenation and hydrosilation of 1-hexene demonstrated higher turnover frequencies for (ⁱPrPhPDI)Fe(N ₂)₂. For more hindered substrates such as cyclohexene and (+)-(R)-limonene, the opposite trend was observed, where the methyl-substituted precursor, (ⁱPrPDI)Fe(N₂)₂, produced more rapid conversion. The difference in catalytic performance resulted from competitive, irreversible formation of η⁶-aryl and -phenyl compounds with the phenyl-substituted complex. Addition of coordinating solvents such as cyclohexene or THF resulted in exclusive formation of the η⁶- phenyl derivative. When alkoxide substituents are introduced in the bis(imino)pyridine ligand backbone, the formation of η⁶-aryl compounds was exclusive, as alkali metal reduction of (ⁱPrROPDI) FeBr₂ (ⁱPrROPDI = 2,6-(2,6-ⁱPr ₂-C₆H₃N=C(OR))₂C₅H ₃N, R = Me, Et) yielded only the catalytically inactive η⁶-aryl species.

Full text of DOI:10.1021/om060441c

Organometallics 2006, 25, 4269-4278
4269
Arene Coordination in Bis(imino)pyridine Iron Complexes:
Identification of Catalyst Deactivation Pathways in Iron-Catalyzed
Hydrogenation and Hydrosilation
Andrew M. Archer, Marco W. Bouwkamp, Maria-Patricia Cortez, Emil Lobkovsky, and
Paul J. Chirik*
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell UniVersity,
Ithaca, New York 14853
ReceiVed May 19, 2006
The phenyl-substituted bis(imino)pyridine iron bis(dinitrogen) complex (iPrPhPDI)Fe(N2)2 (iPrPhPDI
) 2,6-(2,6-iPr2-C6H3NdCPh)2C5H3N) was prepared by sodium amalgam reduction of the corresponding
ferrous dichloride precursor under 4 atm of dinitrogen. Comparison of the infrared stretching frequencies
of the bis(dinitrogen), mono(dinitrogen), and related dicarbonyl derivatives to those of the corresponding
bis(imino)pyridine iron compounds bearing the methyl-substituted ligand, (iPrPDI)Fe(L)n (iPrPDI
) 2,6-(2,6-iPr2-C6H3NdCMe)2C5H3N; L ) CO, n ) 2; L ) N2, n ) 1, 2), established a more electrophilic
iron center for the phenyl-substituted cases. Comparing the productivity of (iPrPhPDI)Fe(N2)2 to
(iPrPDI)Fe(N2)2 in the catalytic hydrogenation and hydrosilation of 1-hexene demonstrated higher turnover
frequencies for (iPrPhPDI)Fe(N2)2. For more hindered substrates such as cyclohexene and (+)-(R)-
limonene, the opposite trend was observed, where the methyl-substituted precursor, (iPrPDI)Fe(N2)2,
produced more rapid conversion. The difference in catalytic performance resulted from competitive,
irreversible formation of η6-aryl and -phenyl compounds with the phenyl-substituted complex. Addition
of coordinating solvents such as cyclohexene or THF resulted in exclusive formation of the η6-phenyl
derivative. When alkoxide substituents are introduced in the bis(imino)pyridine ligand backbone, the
formation of η6-aryl compounds was exclusive, as alkali metal reduction of (iPrROPDI)FeBr2 (iPrROPDI
) 2,6-(2,6-iPr2-C6H3NdC(OR))2C5H3N, R ) Me, Et) yielded only the catalytically inactive η6-aryl species.
to three electrons from the metal center6sis well-established
for this ligand class.7 Our laboratory has recently described the
synthesis and structural characterization of a bis(imino)pyridine
iron bis(dinitrogen) complex, (iPrPDI)Fe(N2)2 (iPrPDI ) 2,6-
(2,6-iPr2-C6H3NdCMe)2C5H3N), 1-(N2)2).8 Notably, 1-(N2)2
serves as an efficient precatalyst for both the hydrogenation and
hydrosilation of olefins and alkynes (eq 1).8 1-(N2)2 has also
been found to catalyze the hydrogenation of aryl azides to the
corresponding anilines with isolable, crystallographically char-
Introduction
Catalytic bond-forming reactions using well-defined coordi-
nation and organometallic compounds of iron are receiving
increased attention given the relatively low cost and reduced
environmental impact of the first-row metals compared to more
precious second- and third-row counterparts.1 Inspired by the
reports of photocatalytic hydrogenation, hydrosilation, and
isomerization of olefins with Fe(CO)5 by Wrighton and co-
workers,2 our laboratory has been exploring modular, trivial to
prepare, terdentate ligand scaffolds designed to mimic the
proposed [Fe(CO)3] active species.3 Aryl-substituted bis(imino)-
pyridine iron compounds are an attractive starting point given
the large number of variants available by straightforward
synthesis and precedent for catalytic C-C bond formation in
olefin polymerization reactions.4
(4) (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. (g) Bouwkamp, M. W.; Lobkovsky, E.; Chirik, P. J. J. Am.
Chem. Soc. 2005, 127, 9660.
One significant challenge in developing iron catalysts is
controlling unwanted one-electron redox changes, a problem
not typically encountered with the second- and third-row
congeners.5 Aryl-substituted bis(imino)pyridines are also useful
in this regard, as the “redox activity”sthe ability to accept up
(5) Ingleson, M. J.; Pink, M.; Caulton, K. G. J. Am. Chem. Soc. 2006,
128, 4248.
(6) Enright, D.; Gambarotta, S.; Yap, G. P. A.; Budzelaar, P. H. M.
Angew. Chem., Int. Ed. 2002, 41, 3873.
* To whom correspondence should be addressed. E-mail:
pc92@cornell.edu.
(7) (a) de Bruin, B.; Bill, E.; Bothe, E.; Weyhermu¨ller, T.; Wieghardt,
K. Inorg. Chem. 2000, 39, 2936. (b) Budzelaar, P. H. M.; de Bruin, B.;
Gal, A. W.; Wieghardt, K.; van Lenthe, J. H. Inorg. Chem. 2001, 40, 4649.
(c) Sugiyama, I.; Korobkov, I.; Gambarotta, S.; Mueller, A.; Budzelaar, P.
H. M. Inorg. Chem. 2004, 43, 5771. (d) Scott, J.; Gambarotta, S.; Korobkov,
I.; Knijnenburg, Q.; de Bruin, B.; Budzelaar, P. H. M. J. Am. Chem. Soc.
2005, 127, 17204.
(1) Bolm, C.; Legros, J.; Le Paih. J.; Zani, L. Chem. ReV. 2004, 104,
6217.
(2) (a) Schroeder, M. A.; Wrighton, M. S. J. Am. Chem. Soc. 1976, 98,
551. (b) Schroeder, M. A.; Wrighton, M. S. J. Organomet. Chem. 1977,
128, 345.
(3) Kismartoni, L. C.; Weitz, E.; Cedeno, D. L. Organometallics 2005,
24, 4714.
(8) Bart, S. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2004,
126, 13794.
10.1021/om060441c CCC: $33.50 © 2006 American Chemical Society
Publication on Web 08/05/2006

4270 Organometallics, Vol. 25, No. 18, 2006
Archer et al.
Figure 1. Redox-active bis(imino)pyridine ligands in iron chemistry.
Figure 2. Bis(imino)pyridine iron dihalide precursors and the labeling scheme used in this study.
acterized bis(imino)pyridine iron imides serving as catalytically
competent intermediates.9
may protect the complex from unwanted deprotonation reac-
tions.13,14 Each of the bis(imino)pyridine iron dihalide precursors
used in this study are presented in Figure 2. The phenyl-
substituted ferrous dihalides (iPrPhPDI)FeX2 (iPrPhPDI ) 2,6-
(2,6-iPr2-C6H3NdCPh)2C5H3N; X ) Cl, 2-Cl2; X ) Br, 2-Br2)
were prepared using a modification of a procedure originally
reported by Esteruelas and co-workers15 involving a Schiff-base
condensation of 2,6-diisopropylaniline with 2,5-dibenzoylpyri-
dine in the presence of FeX2 (eq 2). Both 2-Cl2 and 2-Br2 have
also been synthesized by addition of the preformed bis(imino)-
pyridine with the appropriate iron(II) precursor.16
The electronic structures of 1-(N2)2 and related 1-(DMAP)
(DMAP ) N,N-diemthylaminopyridine) have been more thor-
oughly investigated with Mo¨ssbauer spectroscopy and DFT
calculations.10 These data, in combination with high-quality
X-ray structures, establish intermediate spin ferrous ions (SFe
) 1) complexed by a bis(imino)pyridine dianion, [iPrPDI]2-
,
where the singlet (SL ) 0) or triplet (SL ) 1) diradical are nearly
isoenergetic (Figure 1). A doubly reduced bis(imino)pyridine
chelate has also been observed by Gambarotta and co-workers
with the synthesis and characterization of the square planar,
paramagnetic iron methyl anion [(iPrPDI)FeMe]-. While the
magnetic data are not straightforward to interpret (µeff ) 6.45
µB, 23 °C), both the metrical parameters and computational
results support an intermediate spin Fe(II) center complexed to
a two-electron-reduced bis(imino)pyridine dianion (Figure 1).11
Having established the redox activity of the bis(imino)pyridine
ligand in 1-(N2)2, we have become interested in exploring the
role of ligand-centered radicals in catalysis. To address this issue
and to further investigate the mechanism and scope of iron-
catalyzed olefin hydrogenation and hydrosilation reactions, we
have initiated a study whereby the electronic properties of the
bis(imino)pyridine are systematically varied. By favoring (or
disfavoring) the metal-ligand electron-transfer events, the
catalytic activity may also be altered. In this contribution, we
describe the introduction of phenyl and alkoxide substituents
into the bis(imino)pyridine backbone and evaluation of the
changes in structure, electronic properties, and catalyst perfor-
mance. During the course of these investigations, an important
catalyst deactivation pathway arising from intramolecular arene
coordination has been identified.
The alkoxide-substituted compounds (iPrROPDI)FeBr2
(iPrROPDI ) 2,6-(2,6-iPr2-C6H3NdC(OR))2C5H3N, R ) Me
(3-Br2), Et (4-Br2)) were prepared as described previously.16
The 2,6-diisopropylaniline versions of the bis(imino)pyridines
were used throughout this study for direct comparison to 1-(N2)2.
Reduction of either 2-Cl2 or 2-Br2 with an excess of 0.5%
sodium amalgam under 4 atm of N2 furnished the desired iron
bis(dinitrogen) complex (iPrPhPDI)Fe(N2)2 (2-(N2)2 (eq 3)). Four
atmospheres of dinitrogen were necessary, as lower pressures
led to unwanted side products arising from competitive arene
coordination (vide infra).
(9) Bart, S. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2006,
128, 5302.
(10) Bart, S. C.; Chlopek, K.; Bill, E.; Bouwkamp, M. W.; Lobkovsky,
E.; Neese, F.; Weighardt, K.; Chirik, P. J. Submitted.
(11) Scott, J.; Gambarotta, S.; Korobkov, I.; Budzelaar, P. H. M.
Organometallics 2005, 24, 6298.
(12) Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991, 91, 165.
(13) Bouwkamp, M.; Lobkovsky, E.; Chirik, P. J. Inorg. Chem. 2006,
45, 2.
Results and Discussion
(14) Reardon, D.; Aharonian, G.; Gambarotta, S.; Yap, G. P. A.
Organometallics 2002, 21, 786. (b) Sugiyama, H.; Aharonian, G.; Gam-
barotta, S.; Yap, G. P. A.; Budzelaar, P. H. M. J. Am. Chem. Soc. 2002,
124, 12268.
(15) Esteruelas, M. A.; Lo´pez, A. M.; Me´ndez, L.; Oliva´n, M.; On˜ate,
E. Organometallics 2003, 22, 395.
(16) Kleigrewe, N.; Steffen, W.; Blo¨mker, T.; Kehr, G.; Fro¨hlich, R.;
Wibbeling, B.; Erker, G.; Wasilke, J.-C.; Wu, G.; Bazan, G. J. Am. Chem.
Soc. 2005, 127, 13955.
Reduction of Phenyl-Substituted Bis(imino)pyridine Iron
Dihalides. Phenyl- and alkoxide-substituted bis(imino)pyridines
were chosen for this study, as straightforward synthetic routes
to the desired compounds have been developed and the
electronic properties are expected to differ from the original
methyl-substituted compound.12 In addition, replacement of the
backbone methyl group with phenyl or alkoxide substituents

Arene Coordination in Bis(imino)pyridine Iron Complexes
Organometallics, Vol. 25, No. 18, 2006 4271
Table 1. Comparison of Selected Bond Distances (Å) and
Angles (deg) for 1-(N2)2 and 2-(N2)2
1-(N2)2
2-(N2)2
Fe(1)-N(1)
Fe(1)-N(3)
Fe(1)-N(5)
Fe(1)-N(6)
Fe(1)-N(7)
N(1)-N(2)
N(3)-N(4)
C(2)-N(5)
N(6)-C(3)
N(6)-C(7)
C(8)-N(7)
C(2)-C(3)
C(7)-C(8)
1.8341(16)
1.8800(19)
1.9452(16)
1.8362(14)
1.9473(16)
1.090(2)
1.104(3)
1.332(2)
1.376(2)
1.367(2)
1.841(5)
1.865(5)
1.935(5)
1.842(4)
1.927(4)
1.106(6)
1.107(5)
1.355(7)
1.379(6)
1.376(7)
1.344(7)
1.430(7)
1.429(8)
In analogy with 1-(N2)2, dinitrogen coordination in 2-(N2)2
is reversible. Stirring a pentane solution of 2-(N2)2 for 20 min
at 23 °C and collecting the noncombustible liberated gas with
a Toepler pump produced a quantitative amount of dinitrogen
(1 equiv) (eq 4). A second equivalent of N2 gas was collected
when the solution was stirred for an additional 48 h, arising
from formation of a new bis(imino)pyridine iron product (vide
infra). The interconversion of 2-(N2)2 and 2-N2 was also
monitored by in situ infrared spectroscopy. In pentane solution
at -78 °C, 2-(N2)2 exhibited two intense NtN bands centered
at 2086 and 2138 cm-1. Upon warming, these bands gradually
disappeared with concomitant growth of a new, single peak
centered at 2061 cm-1 attributed to 2-N2. This process was
reversible, as recooling the solution regenerated the bands for
2-(N2)2.
1.333(2)
1.428(3)
1.427(2)
Fe(1)-N(1)-N(2)
Fe(1)-N(3)-N(4)
N(1)-Fe(1)-N(3)
N(1)-Fe(1)-N(5)
N(1)-Fe(1)-N(7)
N(5)-Fe(1)-N(6)
N(6)-Fe(1)-N(7)
178.40(19)
171.81(17)
98.02(8)
96.65(7)
97.41(7)
74.49(6)
79.90(6)
178.5(5)
170.4(5)
99.0(2)
97.2(2)
96.6(2)
80.9(2)
79.3(2)
contracted to 1.430(7) and 1.429(8) Å as compared to the values
in the corresponding iron dichloride compound where the ligand
is in its neutral form.16 These values are consistent with
population of molecular orbitals that are antibonding with respect
to the imine but bonding with respect to the C-C backbone.10
Comparing these values to those for 1-(N2)2 demonstrates similar
distortions. Thus the structural data, coupled with the observed
temperature independent paramagnetism (vide infra), suggest
an intermediate spin ferrous ion with a bis(imino)pyridine
dianion.
A relatively sharp and readily assignable 1H NMR spectrum
was observed over a 12 ppm chemical shift range for the
equilibrium mixture of 2-(N2)2 and 2-N2. As with 1-N2,8 the
resonances for the substituents orthogonal to the iron chelate
plane are close to their diamagnetic reference values, while the
hydrogens in the plane and hence in conjugation with the metal
are more dramatically shifted. For example, the orthogonal,
diastereotopic isopropyl methyl groups were observed at -0.71
and 1.16 ppm (1.04 and 1.27 in the free ligand), while the
p-pyridine hydrogen appeared at -0.80 ppm (8.01 ppm in the
free ligand).
The five-coordinate bis(imino)pyridine iron bis(dinitrogen)
complex was also characterized by X-ray diffraction, combustion
analysis, and NMR spectroscopy. A representation of the solid-
state structure is presented in Figure 3 and establishes a pseudo-
square pyramidal geometry with apical and basal N2 ligands.
Both dinitrogen ligands are weakly activated, with NtN bond
lengths of 1.106(6) and 1.107(5) Å (Table 1), consistent with
the solution IR stretching frequencies. As with 1-(N2)2, the apical
dinitrogen ligand is slightly bent, with an Fe(1)-N(3)-N(4)
bond angle of 170.4(5)°. A comparison of the solid-state
structures of 1-(N2)2 and 2-(N2)2 (Figure 3) reveals little
difference.
Comparison of the Electronic Properties of Methyl- versus
Phenyl-Substituted Bis(imino)Pyridine Iron Complexes. To
compare the relative electronic environments imparted by the
methyl- versus phenyl-substituted bis(imino)pyridine iron frag-
ments, the synthesis of (iPrPhPDI)Fe(CO)2 (2-(CO)2) was
The bond distances in the bis(imino)pyridine ligand are
consistent with two-electron reduction of the chelate.10 For
example, the Cimine-Nimine bond lengths are elongated to 1.355-
(7) and 1.344(7) Å while the Cimine-Cipso distances are
Figure 3. Molecular structure of 2-(N2)2 with 30% probability ellipsoids (left). Overlay of the solid-state structures of 1-(N2)2 (dashed) and
2-(N2)2 (solid, right). Hydrogen atoms are omitted for clarity.

4272 Organometallics, Vol. 25, No. 18, 2006
Archer et al.
Table 2. Comparison of IR Spectroscopic Data for 1-(L)n
and 2-(L)n Compounds
(+)-(R)-limonene. For (+)-(R)-limonene, only the geminal olefin
was reduced. As is typical for transition metal-catalyzed
hydrosilation reactions,17 anti-Markovnikov addition of the
silane to the olefin predominates. For 1-hexene, a 2:1 ratio of
phenylsilane to olefin was required to obtain the monosilylated
product. If lower concentrations of silane were employed,
detectable quantities of the bis(alkylated) product, dihexyl-
(phenyl)silane, were observed.
1-(L)n (cm-1
)
2-(L)n (cm-1
)
ν(N2) (pentane)
ν(N2) (KBr)
2073, 2132
2053, 2124
2046
2086, 2138
2074, 2130
2061
ν(N2) (pentane)a
ν(CO) (pentane)
1914, 1974
1921, 1979
a Value for the four-coordinate bis(imino)pyridine iron mono(dinitrogen)
complex.
Comparing the relative productivities of 1-(N2)2 and 2-(N2)2
demonstrated that the phenyl-substituted compound was more
productive for catalytic 1-hexene hydrogenation and hydrosi-
lation but was inferior for the traditionally more difficult sub-
strates such as cyclohexene and (+)-(R)-limonene. During the
course of these investigations, it was discovered that the produc-
tivity of 1-(N2)2 was higher when pentane rather than toluene
was used as the reaction medium. All of the turnover frequencies
reported in Table 3 were determined in pentane solution.
Identification of Catalyst Deactivation Pathways: Char-
acterization of η6-Aryl and η6-Phenyl Bis(imino)pyridine
Iron Complexes. The difference in the relative hydrogenation
and hydrosilation productivities of 1-(N2)2 and 2-(N2)2 suggested
a competitive catalyst deactivation pathway for the phenyl-
substituted complex. To investigate this possibility, the stability
of 2-(N2)2 in benzene-d6 was monitored as a function of time
Table 3. Performance of 2-(N2)2 in Catalytic Olefin
Hydrogenation and Hydrosilation and Comparison to 1-(N2)2
hydrogenation TOF
hydrosilation TOF
(h-1
a
(h-1
)
)
compound
1-hexene
cyclohexene
(R)-(+)-limonene
2-(N2)2
1-(N2)2
2-(N2)2
1-(N2)2
5300
60
275
3300
1075
1085
930
16
37
330
20
166
a Conditions: 0.3 mol % Fe, 1.25 M substrate in pentane solution at 22
°C. Hydrogenation reactions were conducted at 4 atm H2, hydrosilations
with 2 equiv of PhSiH3.
undertaken. Exposure of 2-(N2)2 to 4 atm of carbon monoxide
followed by solvent removal furnished a brown solid identified
as 2-(CO)2 on the basis of NMR and IR spectroscopies and
combustion analysis. This compound was more routinely
prepared by sodium amalgam reduction of 2-Cl2 in the presence
of carbon monoxide (eq 5).
1
by H NMR spectroscopy. Over the course of hours at 23 °C,
clean and quantitative conversion to two new diamagnetic
products in an 85:15 ratio was observed. A combination of
multinuclear NMR spectroscopy, combustion analysis, and
X-ray diffraction identified the new products as the bis(imino)-
pyridine iron η6-phenyl compound, 2-Phenyl (major, 85%), and
the corresponding η6-2,6-diisopropylphenyl complex, 2-Aryl
(minor, 15%, eq 6). Notably, both iron arene complexes were
unreactive toward dihydrogen, silanes, and olefins. This behavior
most likely accounts for the diminished catalytic productivity
of 2-(N2)2 with more hindered substrates where longer times
are required for complete conversion. The robust nature of the
arene complexes was further demonstrated by the lack of a
reaction with 4 atm of carbon monoxide upon heating to 80 °C
over 24 h.
Compiled in Table 2 are the dinitrogen and carbonyl infrared
stretching frequencies for 1-(L)n and 2-(L)n. Comparing the
pentane solution carbonyl bands reveals a 5 and 7 cm-1 shift
to higher frequencies upon substitution of a phenyl group for
the methyl substituent. A similar trend is observed with the N2
stretches, as 1-(N2)2 has lower frequency bands centered at 2073
and 2132 cm-1 compared to the values of 2086 and 2138 cm-1
for 2-(N2)2. In general, the bands for complexes with the phenyl-
substituted backbone appear at higher frequency than those with
the methyl backbone, suggesting more electrophilic iron centers
in compounds bearing the [iPrPhPDI] ligand.
Catalytic Activity of 2-(N2) in Olefin Hydrogenation and
Hydrosilation. The catalytic activity of the more electrophilic,
phenyl-substituted complex, 2-(N2)2, was evaluated in a series
of olefin hydrogenation and hydrosilation reactions. The hydro-
genation reactions were conducted under standard conditions8
with 0.3 mol % iron at 22 °C in a 1.25 M pentane solution of
the desired substrate. Typically 4 atm of H2 were used; lower
pressures were effective but not used for comparative purposes
to avoid complications from mass-transfer effects. The catalytic
hydrosilation reactions were also conducted with 0.3 mol % of
2-(N2)2 with a 1.25 M solution of substrate and 2 equiv of
PhSiH3.
As we have reported previously with R-diimine iron com-
pounds,18 η6-coordination of the arene ligand was readily
identified by 1H spectroscopy, as the peaks for the coordinated
1
ring shift upfield relative to free arene. In benzene-d6, the H
NMR spectrum of 2-Phenyl exhibited the number of peaks
expected for a molecule with idealized Cs symmetry. Three
upfield shifted phenyl resonances were identified at 3.85 (para),
5.32 (meta) and 5.66 (ortho) ppm. Similar spectroscopic features
were observed for 2-Aryl, with two upfield shifted arene peaks
centered at 4.36 ppm (para) and 5.63 ppm (meta). The 1H and
13C NMR spectra for both compounds were completely assigned
with the aid of two-dimensional NMR experiments and the peak
listings are reported in the Experimental Section.
As an initial catalytic assay, the hydrogenation and hydrosi-
lation of three representative substrates, 1-hexene, cyclohexene,
and (+)-(R)-limonene, were examined (Table 3). The progress
of each catalytic reaction was monitored by gas chromatography,
and the reported turnover frequencies are defined at >98%
conversion (1-hexene) and after 60 min for cyclohexene and
Both η6-aryl complexes were characterized by X-ray diffrac-
tion. The solid-state structures for 2-Phenyl and 2-Aryl are
(17) Jardine, F. H. Prog. Inorg. Chem. 1981, 28, 63.
(18) Bart, S. C.; Hawrelak, E. J.; Lobkovsky, E.; Chirik, P. J. Organo-
metallics 2005, 24, 5518.

Arene Coordination in Bis(imino)pyridine Iron Complexes
Organometallics, Vol. 25, No. 18, 2006 4273
Figure 4. Molecular structure of 2-Phenyl (left) and 2-Aryl (right) with 30% probability ellipsoids. Hydrogen atoms are omitted for
clarity.
Figure 5. (a) Reduction of the bis(imino)pyridine ligands in 2-Aryl and 2-Phenyl and (b) DFT-computed HOMO (left) and LUMO (right)
of 2-Aryl.
presented in Figure 4. Selected metrical parameters are reported
in Table 4. In the structure of 2-Phenyl, one isopropyl group
was positionally disordered and was successfully modeled.
Significant localization of the pyridine ring and contracted
C(2)-C(3) bond distances are observed in both compounds.
These ligand distortions coupled with the observation of
elongated N(1)-C(2) distances are indicative of two-electron
reduction of the bis(imino)pyridine ligand.7 Because one arm
of the chelate is dissociated in both compounds, the bond lengths
associated with this portion of the ligand provide a useful
internal reference for “unperturbed” imine bonds. Consistent
with reduction, the N(1)-C(2) bond length in 2-Aryl elongates
to 1.351(3) Å, compared to 1.283(3) Å in the free imine and
1.297(4) Å in (iPrPDI)FeCl2 (1-Cl2).16 Similar features are
observed in 2-Phenyl with N(1)-C(2) bond elongation to 1.369-
(3) Å compared to 1.267(3) Å for the free imine. The C(2)-
C(3) contractions of 1.419(3) and 1.414(3) Å for 2-Aryl and
2-Phenyl are similar to the distortions observed in the solid-
state structures of R-diimine iron arene complexes.18 Thus, the
metrical parameters in conjunction with the observed diamag-
netism establish the presence of a low-spin ferrous ion com-
plexed by a two-electron-reduced bis(imino)pyridine ligand
(Figure 5).
To confirm the formulation of the ligand as a bis(imino)-
pyridine dianion, the electronic structure of 2-Aryl was further
investigated with full molecule DFT calculations (ADF2003.01,
TZ2P, ZORA). The optimized geometry is in excellent agree-
ment with the experimentally determined solid-state structure.
The frontier molecular orbitals are presented in Figure 5 and
establish the involvement of the ligand. As has been noted pre-
viously,7,10 the LUMO of free aryl-substituted bis(imino)pyridine
ligands are π-antibonding with respect to the Cimine-Nimine bond
but π-bonding with respect to Cipso-Cimine. Examination of the
DFT-computed HOMO of 2-Aryl demonstrates this feature and
clearly establishes the doubly reduced, dianionic form of the
chelate. Notably, the LUMO, which lies 29.8 kcal/mol above
the HOMO, is a similar linear combination of ligand orbitals.
Reduction of Alkoxide-Substituted Bis(imino)pyridine
Iron Dihalides. Attempts to prepare iron dinitrogen complexes
bearing alkoxide-substituted bis(imino)pyridine ligands were

4274 Organometallics, Vol. 25, No. 18, 2006
Archer et al.
Table 4. Selected Metrical Parameters for 2-Aryl and
2-Phenyl
Because the metrical parameters and the DFT calculations on
the η6-aryl compounds established little reduction and hence
π-back-bonding to the arene, the iron-ring interaction is
principally a σ-donation. As a result, the iron-arene bond is
expected to be stronger in the more electron-deficient phenyl-
substituted compound, 2-Aryl, and hence would be less likely
displaced by carbon monoxide.
2-Aryl
2-Phenyl
Fe(1)-N(1)
Fe(1)-N(2)
N(1)-C(2)
N(2)-C(3)
N(2)-C(7)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(5)-C(6)
C(6)-C(7)
C(7)-C(8)
N(3)-C(8)
1.9033(16)
1.8798(16)
1.351(3)
1.405(3)
1.391(3)
1.419(3)
1.401(3)
1.362(3)
1.405(3)
1.378(3)
1.483(3)
1.283(3)
1.8956(18)
1.8478(18)
1.369(3)
1.369(3)
1.380(3)
1.414(3)
1.414(3)
1.364(4)
1.405(3)
1.361(3)
1.485(3)
1.267(3)
Characterization of the bis(imino)pyridine iron dicarbonyl
1
compounds by H and 13C NMR spectroscopy established C2V
symmetric molecules, demonstrating that the carbonyl ligands
are rapidly interchanging basal and apical positions by rocking
through the iron chelate plane. Similar dynamic behavior was
observed with related five-coordinate bis(imino)pyridine iron
compounds.8 The pentane solution infrared spectrum of the
ethoxy-substituted compound 4-(CO)2 was straightforward to
interpret, exhibiting two intense bands centered at 1972 and 1910
cm-1. Comparing these values to those for 1-(CO)2 and 2-(CO)2
(Table 2) demonstrated that introduction of ethoxide substituents
produced the most reducing bis(imino)pyridine iron complex
to date.
N(1)-Fe(1)-N(2)
C(2)-N(1)-Fe(1)
C(3)-C(2)-N(1)
C(7)-C(8)-N(3)
C(7)-C(8)-C(14)
83.02(7)
81.36(8)
116.15(13)
112.62(17)
129.72(18)
112.95(17)
116.29(15)
112.08(19)
120.1(2)
112.14(19)
unsuccessful. Reduction of either 3-Br2 or 4-Br2 with an excess
of 0.5% sodium amalgam afforded red, diamagnetic products
identified as the η6-arene compounds 3-Aryl and 4-Aryl,
respectively, arising from displacement of the imine nitrogen
by the N-aryl substituent (eq 7).
The infrared spectrum of the methoxy-substituted complex
exhibited fiVe carbonyl bands, in contrast to the single C2V
symmetric product suggested by NMR spectroscopy. Two
intense bands, centered at 1974 and 1912 cm-1, are in agreement
with those observed for 4-(CO)2. However, observation of
another strong band at 2013 cm-1 and two weaker bands at
1957 and 1933 cm-1 suggest the presence of other isomers of
the bis(imino)pyridine iron dicarbonyl compound. These ad-
ditional peaks were observed in the same ratio from indepen-
dently prepared samples that were >95% pure as judged by 1H
NMR spectroscopy.
While the experimental data are limited, we tentatively
propose that the other bands arise from isomers where the
oxygen atoms are coordinated to the iron rather than the imines.
Thus, if both “κ3-O,N,O” and “κ3-O,N,N” isomers were also
present, a total of six CO bands would be expected. Isomers
involving a κ2-bis(imino)pyridine ligand also cannot be ex-
cluded. The observation of three additional bands suggests that
two of the peaks overlap. The discrepancy between the NMR
structure, which supports “κ3-N,N,N” coordination, and the IR
data, consistent with additional isomers in solution, may arise
from the difference in time scales of the two experiments. As
stated previously, 3-(CO)2 is dynamic on the NMR time scale
and may undergo dissociation of the imines in solution, resulting
in observation of a time-average C2V symmetric molecule. The
IR time scale, being much shorter, may allow observation of
the discrete components of the dynamic mixture. It should also
be noted that oxygen coordination has been observed in the
ferrous dihalide complex 3-Cl2.16 Observation of only two bands
for 4-(CO)2 suggests that introduction of a slightly larger ethyl
group results in a more crowded oxygen atom that does not
compete with the imine for the coordination sphere of the iron.
As with 2-Phenyl and 2-Aryl, both arene compounds were
readily identified by multinuclear NMR spectroscopy. In
benzene-d6, the number of 1H and 13C resonances expected for
Cs symmetric molecules were observed where the mirror plane
contains the iron center and results in distinct resonances for
the inequivalent alkoxide, pyridine, and aryl environments. For
3-Aryl, diagnostic upfield shifted aryl hydrogens were observed
at 4.11 (para) and 5.53 (meta) ppm, while similar peaks, cen-
tered at 4.19 (para) and 5.57 (meta) ppm, were assigned for
4-Aryl. The coordinatively saturated, diamagnetic arene com-
pounds are robust molecules, failing to convert to the corre-
sponding iron bis(dinitrogen) complexes upon exposure to N2.
To evaluate the electronic environment imparted by the
alkoxy-substituted bis(imino)pyridine ligands, the synthesis of
the corresponding dicarbonyl compounds was targeted. Exposure
of the iron aryl complexes 3-Aryl and 4-Aryl to 4 atm of carbon
monoxide at ambient temperature resulted in slow displacement
of the arene ligand to furnish the desired dicarbonyl derivatives
3-(CO)2 and 4-(CO)2 as green solids, respectively (eq 8).
Insights into the Origin of η6-Arene Coordination. Because
conversion of 2-(N2)2 to 2-Phenyl and 2-Aryl was identified
as a major catalyst deactivation pathway, attempts were made
to understand the origin of this deleterious reaction. Stirring a
benzene or toluene solution of 2-(N2)2 under vacuum furnished
predominantly 2-Phenyl (85-90%) with small, yet detectable
amounts (10-15%) of 2-Aryl. Repeating the experiment in the
presence of dinitrogen inhibited but did not prevent arene
coordination. The half-life for the conversion of 2-(N2)2 into
2-Aryl and 2-Phenyl in benzene-d6 under vacuum was ap-
proximately 7.5 h at 23 °C. Identical experiments performed
Substitution of the arene in the alkoxy-substituted compounds
3-Aryl and 4-Aryl, but not in the phenyl-substituted compounds
2-Aryl may be a result of the relative electronic density at the
iron imparted by the different bis(imino)pyridine ligands.

Arene Coordination in Bis(imino)pyridine Iron Complexes
Organometallics, Vol. 25, No. 18, 2006 4275
Table 5. Solvent Dependence for the Formation of 2-Aryl
Table 6. Relative Rates of Conversion of 2-(THF)2 to 2-
Phenyl in Benzene-d6 at 23 °C as a Function of Added THF
and 2-Phenyl at 23 °C
equiv of added THF
time to completion (h)a
0
2
53
69
6
25
neat
108
193
36
1
a Defined as >98% as judged by H NMR spectroscopy at 23 °C.
solventa
pentane
% 2-Aryl
% 2-Phenyl
Table 7. Formation of 2-Aryl versus 2-Phenyl as a Function
of Added Cyclohexene in a 6.1 mM solution of 2-(N2)2 in
Pentane after Stirring 48 h under Vacuum
100
100
100
100
15
10
0
0
0
0
0
hexane
diethyl ether
mesitylene
benzene
toluene
THF
[cyclohexene] (mM)
% 2-Aryl
% 2-Phenyl
0
0
6.1
12
25
100
75
63
0
25
37
53
85
90
100
100
47
cyclohexene
a All experiments stirred under vacuum for 48 h.
diamagnetic reference values. A complete assignment of the
1H and 13C NMR spectra is reported in the Experimental Section.
Allowing THF-d8 solutions of 2-(THF)2 to stand at 23 °C
resulted in quantitative conversion to 2-Phenyl after 36 h. To
determine the influence of added THF on the rate of 2-Phenyl
formation, the conversion of 2-(THF)2 to 2-Phenyl was
monitored by 1H NMR spectroscopy in benzene-d6 as a function
of added THF. The relative rates were quantified by measuring
the time required to reach >98% conversion because the kinetics
are not straightforward (vide infra). As reported in Table 6,
adding 2-25 equiv of excess THF inhibits formation of
2-Phenyl, similar to the behavior observed with dinitrogen for
the conversion of 2-(N2)2 to 2-Aryl. However, these data do
not fit a simple inverse first-order dependence, and the observa-
tion of a faster rate of conversion in neat THF-d8 implies a
more complex kinetic process perhaps due to competing
solvation effects.19
The observation of exclusive formation of 2-Phenyl in
coordinating solvents prompted investigation of the influence
of potential Lewis bases on the selectivity of the arene
coordination. As presented in Table 7, increasing the concentra-
tion of cyclohexene in a 6.1 mM solution of 2-(N2)2 in pentane
increased the amount of 2-Phenyl observed. Because the two
iron aryl compounds do not interconvert, the conversion is under
kinetic rather than thermodynamic control. Notably, dissolving
2-(N2)2 in neat cyclohexene for 5 min, removing the volatiles,
and dissolving the residue in benzene-d6 resulted in 88%
conversion to 2-Phenyl. The remaining 12% of the material
was identified as residual 2-(N2)2. This observation accounts
for the slow rates of hydrogenation to cyclohexane when 2-(N2)2
was employed as the catalyst precursor.
under 1 and 4 atm of dinitrogen produced half-lives of 19 and
125 h, respectively.
Changing the solvent medium to noncoordinating solvents
such as pentane, hexane, diethyl ether, or mesitylene produced
exclusively 2-Aryl after 48 h at 23 °C (Table 5). In more
coordinating solvents such as THF or cyclohexene, 2-Phenyl
was observed exclusively. Significantly, isolated and purified
samples of 2-Phenyl and 2-Aryl did not interconvert in benzene-
d6 solution, even upon heating to 80 °C, demonstrating that arene
coordination is irreversible. Similar observations were made in
R-diimine iron arene chemistry where the perprotio R-diimine
iron η6-benzene compound does not exchange with benzene-d6
solvent.18
The exclusive formation of 2-Phenyl in THF was studied in
more detail. Dissolution of brown 2-(N2)2 in THF resulted in
immediate formation of a purple solution. Collecting the
noncombustible gas formed from the dissolution process with
a Toepler pump produced 93% of the amount expected for
liberation of 2 equiv of dinitrogen. Removal of the solvent
yielded a purple solid that was identified on the basis of
combustion analysis and NMR spectroscopy as the bis(imino)-
pyridine iron bis(THF) complex 2-(THF)2. Dissolving the
isolated purple solid in benzene-d6 under an atmosphere of
dinitrogen immediately regenerated 2-(N2)2 concomitant with
liberation of 2 equiv of THF (eq 9).
In the absence of a detailed study of the electronic structure
of 2-(N2)2 and observable intermediates, drawing firm mecha-
nistic conclusions from the above data is tenuous. Despite these
limitations, it is clear that at least two competitive pathways
are operative in the formation of 2-Aryl and 2-Phenyl. The
data definitively establish that coordinating solvents (THF,
cyclohexene) lead exclusively to 2-Phenyl, suggesting that under
these conditions, imine dissociation is operative, allowing
competitive Cipso-Cimine bond rotation and phenyl coordination.
The putative κ2-bis(imino)pyridine iron intermediate is most
likely stabilized by solvent coordination. Displacement of the
solvent molecules from the coordination sphere of the iron could
The 1H NMR spectrum of 2-(THF)2 in THF-d8 or benzene-
d6 produced the number of resonances expected for a molecule
with idealized C2V symmetry, arising from rapid rocking of the
THF ligands through the iron-chelate plane on the time scale
of the experiment. Samples prepared in benzene-d6 must be
thoroughly degassed of dinitrogen to avoid formation of 2-(N2)2.
All of the peaks were assigned on the basis of integration and
two-dimensional experiments. As with 2-(N2)2, the protons that
lie in the plane of the iron experience the largest shifts. For
example, the meta pyridine protons appear at 23.83 ppm in THF-
d8, and the para protons were located at 10.61 ppm. The hydro-
gens orthogonal to the metal chelate plane appear close to their
(19) Attempts were made to monitor the conversion of 2-(N2)2 to η6-
arene compounds in the presence of 1,2-dimethyltetrahydrofuran. However,
addition of this solvent produced unidentified iron products.

4276 Organometallics, Vol. 25, No. 18, 2006
Archer et al.
the goniometer head of a Bruker X8 APEX2 system equipped with
a molybdenum X-ray tube (λ ) 0.71073 Å). Preliminary data
revealed the crystal system. A hemisphere routine was used for
data collection and determination of lattice constants. The space
group was identified and the data were processed using the Bruker
SAINT program and corrected for absorption using SADABS. The
structures were solved using direct methods (SHELXS), completed
by subsequent Fourier synthesis, and refined by full-matrix least-
squares procedures. Elemental analyses were performed at Rob-
ertson Microlit Laboratories, Inc., in Madison, NJ. The general
procedures for catalytic olefin hydrogenation and hydrosilation were
carried out as described previously in pentane rather than toluene
solution.8
All calculations were performed with the Amsterdam Density
Functional Theory (ADF2003.01) suite of programs.23-25 Relativ-
istic effects were included using the zero-order regular approxima-
tion. The Vosko, Wilk, and Nusair (VWM) local density approxi-
mation,26 Becke’s exchange,27 and Perdew’s correlation28 (BP86)
were used. The cores of the atoms were frozen up to 1s for C and
N and 2p for Fe. Uncontracted Slater-type orbitals (STOs) of triple-ú
quality with two polarizations were employed. This basis set is
denoted TZ2P in the ADF program. Each geometry optimization
was carried out without symmetry constraints. Orbital representa-
tions were generated using ADF view.
Preparation of (iPrPhPDI)FeCl2 (2-Cl2). A 500 mL round-
bottomed flask was charged with 3.560 g (11.67 mmol) of 2,6-
dibenzoylpyridine. A solution of 4.149 g (23.40 mmol) of 2,6-
diisopropylaniline dissolved in 150 mL of acetic acid and the
resulting yellow solution was heated to reflux. Under a stream of
argon gas, 1.483 g (11.70 mmol) of FeCl2 was added to the stirring
mixture, forming a blue slurry. Following the addition, the reflux
was resumed for 4 h, after which time the mixture was cooled to
room temperature. Acetic acid from the blue mixture was removed
in vacuo on a Schlenk line. Residual solvent was removed under
high vacuum. In the dry box, the resulting blue solid was dissolved
in a minimal amount of anhydrous dichloromethane, precipitated
by layering with pentane, and recovered on a filter. The filter cake
was washed with three 20 mL portions of diethyl ether and three
20 mL portions of pentane and then dried under vacuum, yielding
5.959 g (70%) of a dark blue powder identified as 2-Cl2. Anal.
Calcd for C43H47N3FeCl2: C, 70.50; H, 6.47; N, 5.74. Found: C,
70.39; H, 6.75; N, 5.75. Magnetic susceptibility: µeff ) 5.9 µB
(solid-state balance). 1H NMR (dichloromethane-d2, 22 °C): δ
-18.63 (519, 4H, CHMe2), -9.22 (24, 2H, p-Ar), -6.12 (40, 12H,
CHMe2), 1.41 (d, 88 Hz, 12H, CHMe2), 7.07 (190, 2H, p-Ph), 7.40
(36, 4H, 36, m/o-Ph), 8.08 (21, 4H, m/o-Ph), 14.42 (46, 4H, m-Ar),
77.97 (60, 2H, m-Pyr), 82.36 (40, 1H, p-Pyr).
be rate limiting (or pre-rate limiting) and would account for
the observed inhibition by added THF in benzene. However,
the observed rate acceleration in neat THF suggests a more
complex process. Notably, 2-Phenyl is the major product in
benzene-d6 and toluene, suggesting the intermediacy of η2-arene
compounds.20 If formed, these intermediates must be displaced
by the incoming phenyl substituent of the bis(imino)pyridine
ligand, as η6-benzene or toluene compounds are not observed.
Recall if these complexes were formed, the irreversibility of
arene coordination would allow observation and isolation.
Observation of 2-Aryl and 2-Phenyl from 2-(N2)2 prompted
investigation of the stability of the methyl-substituted complex
1-(N2)2. Benzene-d6 and pentane solutions of 1-(N2)2 (in
equilibrium with 1-N2) are stable indefinitely at ambient
temperature under 1 atm of dinitrogen. Continued cycles of
solvent removal and degassing of a benzene-d6 solution of 1-N2
resulted in gradual formation of a paramagnetic, red solid. While
the paramagnetism of the compound definitively excludes
formation of an η6-arene complex, the identity of this NMR-
silent material remains unknown. Notably, addition of 1 atm of
dinitrogen regenerates 1-(N2)2, suggesting that the NMR-silent
compound is the bis(imino)pyridine iron cyclometalated hydride
implicated in deuterium-labeling studies.8 Experiments are
ongoing to definitively establish the identity of this compound.
In summary, the data reported in this study demonstrate how
subtle manipulation of the backbone architecture of bis(imino)-
pyridine ligands induces arene coordination to form catalytically
inactive η6-arene complexes. For the phenyl-substituted dini-
trogen complex 2-(N2)2, coordination of the imine aryl and
backbone phenyl is solvent dependent. The latter was observed
in the presence of coordinating solvents such as THF, suggesting
stabilization of a κ2-bis(imino)pyridine intermediate allowing
Cipso-Cimine bond rotation.
Experimental Section
General Considerations. All air- and moisture-sensitive ma-
nipulations were carried out using standard vacuum line, Schlenk,
and cannula techniques or in an MBraun inert atmosphere drybox
containing an atmosphere of purified nitrogen. The MBraun drybox
was equipped with a cold well designed for freezing samples in
liquid nitrogen. Solvents for air- and moisture-sensitive manipula-
tions were initially dried and deoxygenated using literature proce-
dures.21 Argon and hydrogen gas were purchased from Airgas
Incorporated and passed through a column containing manganese
oxide supported on vermiculite and 4 Å molecular sieves before
admission to the high-vacuum line. Benzene-d6 was purchased from
Cambridge Isotope Laboratories and distilled from sodium metal
under an atmosphere of argon and stored over 4 Å molecular sieves
or sodium metal.
Preparation of (iPrPhPDI)FeBr2 (2-Br2). This compound was
prepared using the method described for 2-Cl2 with 1.270 g (4.49
mmol) of 2,6-dibenzoylpyridine, 1.770 g (9.97 mmol) of 2,6-
1H NMR spectra were recorded on Varian Mercury 300 and
Inova 400 and 500 spectrometers operating at 299.763, 399.780,
and 500.62 MHz, respectively. All chemical shifts are reported
relative to SiMe4 using 1H (residual) chemical shifts of the solvent
as a secondary standard. For paramagnetic molecules, the 1H NMR
data are reported with the chemical shift followed by the peak width
at half-height in hertz or multiplicity, followed by integration value
and, where possible, peak assignment. Magnetic moments as
determined by the Evans method22 using a ferrocene standard and
are the mean value of at least two independent measurements, unless
stated otherwise.
(22) Sur, S. K. J. Magn. Reson. 1989, 82, 169.
(23) Velde, G. T.; Bickelhaupt, F. M.; Baerends, E. J.; Guerra, C. F.;
van Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. J. Comput. Chem. 2001,
22, 931.
(24) Fonseca Guerra, C.; Snijders, J. G.; te Velde, G.; Baerend E. J.
Theor. Chem. Acc. 1998, 99, 391.
(25) Baerends, E. J.; Autschbach, J. A.; Berces, A.; Bo, C.; Boerrigter,
P. M.; Cavallo, L.; Chong, D. P.; Deng, L.; Dickson, R. M.; Ellis, D. E.;
Fan, L.; Fischer, T. H.; Guerra Fonseca, C.; van Gisbergen, S. J. A.;
Groeneveld, J. A.; Gritsenko, O. V.; Gru¨ning, M.; Harris, F. E.; van den
Hoek, P.; Jacobsen, H.; van Kessel, G.; Koostra, F.; van Lenthe, E.; Osinga,
V. P.; Patchkovshii, S.; Philipsen, P. H. T.; Post, D.; Pye, C. C.; Ravenek,
W.; Ros, P.; Schipper, P. R. T.; Schreckenbach, G.; Snijders, J. G.; Sola,
M.; Swart, M.; Swerhone, D.; te Velde, G.; Vernooijs, P.; Versluis, L.;
Visser, O.; van Wezenbeek, E.; Wiesenekker, G.; Wolff, S. K.; Woo, T.
K.; Ziegler, T. ADF 2002.03; SCM Theoretical Chemistry, Vrije Univer-
siteit: Amsterdam, The Netherlands, http://www.scm.com/.
(26) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1990, 58, 1200.
(27) Becke, A. D. Phys. ReV. 1988, A38, 2398.
Single crystals suitable for X-ray diffraction were coated with
polyisobutylene oil in a drybox and were quickly transferred to
(20) For an example of η2-coordination in iron chemistry see: Sciarone,
T. J. J.; Meetsma, A.; Hessen, B.; Teuben, J. H. Chem. Commun. 2002,
1580.
(21) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518.
(28) Perdew, J. P. Phys. ReV. 1986, B33, 8822.

Arene Coordination in Bis(imino)pyridine Iron Complexes
Organometallics, Vol. 25, No. 18, 2006 4277
diisopropylaniline, and 0.970 g (4.49 mmol) of FeBr2, yielding
2.093 g (57%) of a dark blue powder identified as 2-Br2. Anal.
Calcd for C43H47N3FeBr2: C, 62.87; H, 5.77; N, 5.12. Found: C,
62.69; H, 5.54; N, 5.00. Magnetic susceptibility: µeff ) 5.4 µB
(solid-state balance). 1H NMR (benzene-d6, 22 °C): δ -16.38 (445,
4H, CHMe2), -9.86 (26, 2H, p-Ar), -5.86 (55, 12H, CHMe2),
-5.44 (303, 12H, CHMe2), 6.87 (36, 4H, m/o-Ph), 8.89 (22, 2H,
p-Ph), 9.20 (237, 4H, m/o-Ph), 14.35 (40, 4H, m-Ar), 74.64 (74,
2H, m-Pyr), 101.56 (60, 1H, p-Pyr).
remaining product was extracted from the amalgam with pentane,
and the extracts were combined and filtered through Celite to
remove NaCl. The filtrate was collected and the solvent removed
in vacuo to yield 0.130 g (72%) of a red solid identified as 2-Aryl.
Anal. Calcd for C43H47N3Fe: C, 78.05; H, 7.16; N, 6.35. Found:
1
C, 77.95; H, 7.43; N, 6.59. H NMR (benzene-d6): δ 0.71 (d, 7
Hz, 6H, CHMe2*), 1.17 (d, 7 Hz, 6H, CHMe2), 1.27 (d, 7 Hz, 6H,
CHMe2), 1.49 (d, 7 Hz, 6H, CHMe2*), 3.45 (spt, 7 Hz, 2H, CHMe2),
3.65 (spt, 7 Hz, 2H, CHMe2*), 4.36 (t, 6 Hz, 1H, p-Ar*), 5.63 (d,
6 Hz, 2H, m-Ar*), 6.32 (dd, 8 and 7 Hz, 1H, p-Pyr), 6.74-6.83
(m, 5H, Ph*), 6.92 (dd, 8 Hz & 1 Hz, 1H, m-Pyr*), 7.03 (d, 7 Hz,
2H, o-Ph), 7.25 (t, 7 Hz, 2H, m-Ph), 7.32 (dd, 7 and 1 Hz, 1H,
m-Pyr), 7.35 (m, 2H, p-Ar and p-Ph), 7.94 (d, 8 Hz, 2H, m-Ar).
13C NMR (benzene-d6, 22 °C): δ 22.39 (CHMe2), 23.15 (CHMe2),
24.10 (CHMe2*), 25.84 (CHMe2*), 27.90 (CHMe2*), 28.30 (CHMe2),
79.49 (p-Ar*), 80.34 (m-Ar*), 83.48 (o-Ar*), 109.38 (ipso-Ar*),
118.59 (p-Pyr), 123.52 (Ph), 126.02 (Ph), 126.11 (Ph), 127.49 (Ph),
128.97 (m-Pyr), 129.46 (m-Ar), 130.42 (Ph), 132.42 (Ph), 135.81,
141.43, 143.88, 144.09, 148.19, 149.37, 153.16, 175.31. * denotes
resonances on the portion of the molecule where the arene is
coordinated to iron.
Preparation of (iPrPhPDI)Fe(N2)2 (2-(N2)2). A thick walled glass
vessel was charged with 44.62 g of mercury, approximately 100
mL of pentane, and a magnetic stir bar. Sodium metal (0.224 g,
9.76 mmol, 0.5 wt %) was added to the vessel in small (∼20 mg)
portions. The resulting slurry was stirred for 15 min to completely
dissolve the metal. After this time, a pentane slurry containing 1.429
g (1.024 mmol) of 2-Cl2 was added to the vessel. The vessel was
sealed, cooled to liquid nitrogen temperature, and evacuated on a
Schlenk line. At -196 °C, 1 atm of N2 was added. The vessel was
resealed and warmed to ambient temperature. The resulting reaction
mixture was stirred for 24 h, after which time a brown solution
was decanted from the amalgam. The remaining product was
extracted from the amalgam into pentane and filtered through Celite,
and the solvent from the combined organic layers was removed in
vacuo. The resulting brown solid was recrystallized from pentane
at -35 °C, yielding 0.558 g (40%) of 2-(N2)2. Anal. Calcd for
C43H47N7Fe: C, 71.96; H, 6.60; N, 13.66. Found: C, 71.98; H,
6.64; N, 13.36. IR (KBr) νN2: 2074, 2130 cm-1. IR (pentane, -78
°C) νN2: 2086, 2138 cm-1. Characterization of 2-N2: IR (pentane,
23 °C): νN2 ) 2061 cm-1. 1H NMR (benzene-d6, 23 °C): δ -0.80
(t, 8 Hz, 1H, p-Pyr), -0.71 (d, 6 Hz, 12H, CHMe2), 0.05 (sept, 6
Hz, 4H, CHMe2), 1.16 (d, 6 Hz, 12H, CHMe2), 4.61 (t, 8 Hz, 2H,
p-Ph), 5.82 (d, 8 Hz, 4H, o-Ph), 7.80 (t, 8 Hz, 2H, p-Ar), 8.05 (d,
8 Hz, 4H, m-Ar), 8.13 (t, 8 Hz, 4H, m-Ph), 11.80 (d, 8 Hz, 2H,
m-Pyr). 13C NMR (benzene-d6, 22 °C): δ 24.79 (CHMe2), 40.27
(CHMe2), 52.18 (CHMe2), 122.85, 124.80, 125.25, 128.06, 130.51,
144.15, 173.57.
Preparation of (iPrPhPDI)Fe(CO)2 (2-(CO)2). A thick walled
glass vessel was charged with 6.245 g of mercury, approximately
10 mL of pentane, and a stir bar. Sodium metal (0.031 g, 1.4 mmol)
was added to the vessel in small (∼15 mg) portions. The resulting
slurry was stirred for 10 min to ensure complete dissolution of the
metal. A pentane slurry containing 0.200 g (0.273 mmol) of 2-Cl2
was added to the flask containing the amalgam. The vessel was
sealed, cooled to liquid nitrogen temperature, and evacuated. At
this temperature 1 atm of CO was added. The reaction mixture
was stirred vigorously for 24 h. The yellowish-brown solution in
the resulting mixture was decanted away from the amalgam. Pentane
was used to extract the remaining product from the amalgam, and
both portions were combined and filtered through Celite to remove
NaCl. The filtrate was collected and the solvent was removed in
vacuo, yielding 0.121 g (62%) a yellowish-brown solid identified
as 2-(CO)2. Anal. Calcd for C45H47N3O2Fe: C, 75.31; H, 6.60; N,
Preparation of 2-Phenyl. A J. Young NMR tube was charged
with 0.017 g (0.024 mmol) of 2-(N2)2 and approximately 0.5 mL
of dry tetrahydrofuran. The tube was immersed in a liquid nitrogen
bath and evacuated on the high-vacuum line. The contents were
then warmed to ambient temperature and shaken for 48 h. Solvent
was removed in vacuo, yielding a red oil. A minimal quantity of
pentane was used to extract the oil from the tube and was placed
in a 20 mL scintillation vial. The pentane was removed in vacuo,
affording 0.015 g (94%) of an orange oil identified as 2-Phenyl.
Crystals suitable for X-ray analysis were grown from a pentane
solution at -35 °C. Anal. Calcd for C43H47N3Fe: C, 78.05; H, 7.16;
N, 6.35. Found: C, 77.72; H, 6.85; N, 5.95. 1H NMR (benzene-d6,
22 °C): δ 0.80 (d, 7 Hz, 6H, CHMe2), 1.27 (d, 6H, 7 Hz, CHMe2),
1.37 (d, 12H, 7 Hz, CHMe2), 3.26 (spt, 2H, 7 Hz, CHMe2), 3.55
(spt, 7 Hz, 2H, CHMe2), 3.85 (t, 6 Hz, 1H, p-Ph*), 5.32 (t, 6 Hz,
2H, m-Ph*), 5.66 (d, 6 Hz, 2H, o-Ph*), 6.47 (dd, 8 and 7 Hz, 1H,
p-Pyr), 6.83 (d, 7 Hz, 2H, o-Ph), 6.87 (t, 7 Hz, 1H, p-Ph), 6.93 (t,
7 Hz, 2H, m-Ph), 7.05 (t, 7 Hz, 1H, p-Ar), 7.11 (d, 7 Hz, 2H, m-Ar),
7.14 (d, 9 Hz, 2H, m-Ar), 7.16-7.25 (m, 2H, p-Ar and m-Pyr),
7.58 (dd, 7 and 1 Hz, 1H, m-Pyr). 13C NMR (benzene-d6, 22 °C):
δ 22.90 (CHMe2), 24.61 (CHMe2), 24.88 (CHMe2), 26.59 (CHMe2),
28.52 (CHMe2), 29.66 (CHMe2), 80.27 (p-Ph*), 83.96 (o-Ph*),
85.91 (m-Ph*), 89.71 (ipso-Ph*), 109.81, 122.89, 123.96, 124.07,
125.45, 126.41, 126.47, 129.03, 129.51, 135.87, 136.50, 142.33,
144.54, 147.47, 149.60, 153.35, 166.72, 167.95. * denotes reso-
nances on the portion of the molecule where the phenyl group is
coordinated to iron.
Preparation of (iPrPhPDI)Fe(THF)2 (2-(THF)2). A 20 mL
scintillation vial was charged with 0.017 g (0.024 mmol) of 2-(N2)2
and approximately 2 mL of THF. The resulting purple solution was
stirred for 5 min. The volatiles were removed in vacuo, yielding
0.019 g (97%) of purple solid identified as 2-(THF)2. Anal. Calcd
for C51H61N3O2Fe: C, 76.20; H, 7.65; N, 5.23. Found: C, 75.87;
5.85. Found: C, 74.97; H, 7.00; N, 5.98. IR (KBr, 23 °C) νCO
:
1979, 1921 cm-1. IR (pentane, 23 °C) νCO: 1979, 1921 cm-1. H
NMR (benzene-d6, 22 °C): δ 0.92 (d, 7 Hz, 12H, CHMe2), 1.25
(t, 8 Hz, 1H, p-Pyr), 1.55 (d, 7 Hz, 12H, CHMe2), 3.18 (spt, 7 Hz,
4H, CHMe2), 7.02-7.27 (16H, Ph and Ar), 7.62 (d, 8 Hz, 2H,
m-Pyr). 13C NMR (benzene-d6, 22 °C): δ 24.45 (CHMe2), 26.84
(CHMe2), 28.93 (CHMe2), 121.49, 124.39, 125.69, 127.50, 128.45,
128.90, 131.78, 135.24, 141.11, 148.10, 151.05, 158.68.
1
1
H, 7.75; N, 4.83. H NMR (THF-d8, 22 °C): δ -2.42 (30, 4H,
CHMe2), -0.99 (13, 12H, CHMe2), 0.39 (20, 12H, CHMe2), 6.67
(26, 4H, m-Ph), 6.84 (26, 2H, p-Ph), 7.68 (19, 4H, o-Ph), 9.51 (25,
2H, p-Ar), 10.29 (23, 4H, m-Ar), 10.61 (25, 1H, p-Pyr), 22.83 (24,
2H, m-Pyr). 2H NMR (THF-d8, 22 °C): δ 1.14 (66, 4D, 2,3-THF),
3.00 (5, 4D, 1,4-THF). 13C NMR (THF-d8, 22 °C): δ 14.45
(CHMe2), 23.28 (CHMe2), 24.06 (broad, 13 Hz, 2,3-THF), 30.00
(broad, 19 Hz, 1,4-THF), 35.13 (CHMe2), 93.98, 116.17, 119.89,
Preparation of 2-Aryl. A 50 mL round-bottomed flask was
charged with 6.245 g of Hg and approximately 500 mL of pentane.
Sodium metal (0.031 g, 1.37 mmol) was added to the flask. The
resulting amalgam was stirred for 10 min. After this time, a pentane
slurry containing 0.200 g (0.273 mmol) of 2-Cl2 was added to the
amalgam. The resulting blue reaction mixture was stirred vigorously
for 72 h and the solution decanted away from the amalgam. The
1
132.83, 134.51, 140.97. H NMR (benzene-d6, 23 °C): δ -2.26
(25, 4H, CHMe2), -0.84 (11, 12H, CHMe2), 0.51 (10, 12H,
CHMe2), 1.44 (69, 8H, 2,3-THF), 3.59 (63, 8H, 1,4-THF), 6.67 (t,
7.7 Hz, 4H, m-Ph), 6.96 (t, 7.7 Hz, 2H, p-Ph), 7.63 (d, 7.6 Hz, 4H,

4278 Organometallics, Vol. 25, No. 18, 2006
Archer et al.
o-Ph), 9.13 (t, 7.6 Hz, 2H, p-Ar), 10.12 (d, 7.6 Hz, 4H, m-Ar),
10.44 (t, 7.6 Hz, 1H, p-Pyr), 21.35 (17, 2H, m-Pyr).
CHMe2), 1.14 (d, 7.1 Hz, 12H, CHMe2), 1.34 (br, 4H, OCH2CH3),
2.90 (spt, 7.1 Hz, 4H, CHMe2), 4.32 (br, 4H, OCH2CH3), 7.0-7.1
(m-Ar, p-Ar), 7.2-7.6 (br, m-Pyr, p-Pyr).13C NMR (chloroform-d,
22 °C): δ 14.5 (OCH2CH3), 22.4 (CHMe2), 23.4 (CHMe2), 28.1
(CHMe2), 62.7 (OCH2CH3), 122.6 (m-aryl), 124,4 (p-aryl), 136.2
(p-Pyr), 137.0 (m-Pyr), 142.7 (o-Ar), 149.8 (ipso-Ar), 153.8 (o-
Pyr).
Preparation of (iPrEtOPDI)FeBr2 (4-Br2). This compound was
prepared in a manner identical to 3-Br2 with 1.10 g (5.10 mmol)
of FeBr2 and 2.79 g (5.13 mmol) of iPrEtOPDI and yielded 3.70 g
(95%) of a blue solid identified as 4-Br2. 1H NMR (dichlo-
romethane-d2): δ -32.8 (309.1, 4H), -16.4 (832.2, 4H), -10.7
(349.7, 12H, CHMe2), -8.6 to -5.0 (2 broad overlapping, 14H),
-3.5 (489.8, 6H, OCH2CH3), 13.3 (300.9, 4H), 31.2 (198.7, 1H,
p-Pyr), 83.7 (253.6, 2H). Magnetic susceptibility (solid-state
balance, 23 °C): µeff ) 5.2 µB. MS (m/z) calcd ) 755.13847, found
) 755.13811.
Preparation of (iPrMeOPDI)FeBr2 (3-Br2). A 100 mL round-
bottomed flask was charged with 0.30 g (1.4 mmol) of FeBr2 and
i
0.72 g (1.4 mmol) of PrMeOPDI and fitted with a 180° needle
valve. On the vacuum line, approximately 50 mL of THF was
condensed onto the solids at -78 °C, and the resulting mixture
was warmed to ambient temperature and stirred for 2 days. During
this time, a pink reaction mixture formed. The solvent was removed
in vacuo and the resulting pink residue was washed with pentane
to afford 0.89 g (88%) of a pink solid identified as 3-Br2. 1H NMR
(dichloromethane-d2): δ -30.9 (1765.0), -18.9 (1451.2), 0 to -15
(at least 3 broad overlapping), 12.5 (927.7), 24.6 (851.0), 29.1
(1005.8), 56.7 (793.8), 82.0-87.5 (3 broad overlapping), 90.1
(932.2). Magnetic susceptibility: µeff ) 5.1 µB (solid-state balance).
MS (m/z) calcd ) 727.10717, found ) 727.10570.
Preparation of 3-Aryl. A 20 mL scintillation vial was charged
with 6.40 g (32.0 mmol) of mercury and approximately 15 mL of
pentane. To this suspension was added 0.036 g (1.57 mmol) of
sodium. The resulting amalgam was stirred for 10 min. A pentane
slurry containing 0.200 g (0.28 mmol) of 3-Br2 was added to the
flask containing the amalgam. The resulting reaction mixture was
stirred vigorously, forming a red-brown solution. After 18 h, the
solution was decanted from the amalgam and the maroon liquid
was filtered through Celite. The solvent was removed in vacuo and
the resulting residue recrystallizated from a pentane/toluene (∼2:
1) mixture, yielding 0.060 g (39%) of a maroon solid identified as
3-Aryl. Anal. Calcd for C33H43N3O2Fe: C, 69.94; H, 7.64; N, 7.42.
Preparation of 4-Aryl. A 100 mL round-bottomed flask was
charged with 14.15 g (70.7 mmol) of Hg and approximately 50
mL of pentane. To the slurry was added 0.075 g (3.3 mmol) of
sodium metal. The resulting amalgam was stirred for 10 min to
completely dissolve the metal. A pentane slurry containing 0.451
g (0.59 mmol) of 4-Br2 was added and the resulting reaction mixture
stirred vigorously, forming a red-brown mixture. After 26 h, the
solution was decanted from the amalgam and filtered through Celite.
The Celite was extracted with pentane and the solvent from the
combined extracts removed to yield a maroon solid. Recrystalli-
zation from a pentane/toluene (2:1) mixture yielded 0.190 g (53%)
of a forest green solid identified as 4-Aryl. Anal. Calcd for
C35H47N3O2Fe: C, 70.04; H, 7.90; N, 7.00. Found: C, 69.83; H,
1
Found: C, 69.17; H, 7.29; N, 7.01. H NMR (benzene-d6, 22 °C)
δ 1.16 (d, 7.2 Hz, 3H, CHMe2), 1.20 (d, 7.2 Hz, 3H, CHMe2), 1.26
(d, 7.2 Hz, 3H, CHMe2), 1.52 (d, 7.2 Hz, 3H, CHMe2), 2.96 (s,
3H, OCH3), 3.31 (spt, 7.2 Hz, 2H, CHMe2), 3.43 (spt, 7.2 Hz, 2H,
CHMe2), 4.02 (s, 3H, OCH3), 4.11 (t, 6.4 Hz, 1H, p-aryl), 5.53 (d,
6.4 Hz, 2H, m-aryl), 6.53 (t, 8.4 Hz, 1H, p-Pyr), 6.66 (d, 8.4 Hz,
2H, m-Pyr), 7.26-7.38 (m-aryl, p-aryl), 7.94 (d, 7.2 Hz, 1H, m-Pyr).
13C NMR (benzene-d, 22 °C): δ 22.32 (CHMe2), 23.50 (CHMe2),
25.17 (CHMe2), 25.64 (CHMe2), 28.00 (CHMe2), 28.18 (CHMe2),
54.31 (OCH3), 59.56 (OCH3), 78.33 (m-Ar), 78.75 (p-Ar), 116.04
(p-Pyr), 122.056 (m-Pyr), 123.36 (m-Ar), 126.05 (p-Ar), 129.74
(m-Pyr).
1
7.81; N, 8.77. H NMR (benzene-d6, 22 °C): δ 0.63 (t, 7.1 Hz,
3H, OCH2CH3), 1.18 (d, 7.1 Hz, 3H, CHMe2), 1.22 (d, 7.1 Hz,
3H, CHMe2), 1.29 (d, 7.1 Hz, 3H, CHMe2), 1.41 (t, 7.1 Hz, 3H,
OCH2CH3), 1.53 (d, 7.1 Hz, 3H, CHMe2), 3.28 (q, 7.1 Hz, 2H,
OCH2CH3), 3.38 (spt, 6.7 Hz, 2H, CHMe2), 3.49 (spt, 6.7 Hz, 2H,
CHMe2), 4.19 (t, 6.7 Hz, 1H, p-Ar), 4.64 (q, 7.1 Hz, 2H, OCH2-
CH3), 5.57 (d, 6.65 Hz, 2H, m-Ar), 6.58 (ps t, 7.2 8.2 Hz, 1H,
p-Pyr), 6.77 (dd, 8.2 1.8 Hz, 1H, m-Pyr), 7.27-7.38 (3H, m-Ar,
p-Ar), 8.02 (dd, 7.2 1.8 Hz, 1H, m-Pyr). 13C NMR (benzene-d6, 22
°C): δ 22.52 (CHMe2), 23.56 (CHMe2), 24.88 (CHMe2), 26.05
(CHMe2), 27.96 (CHMe2), 28.24 (CHMe2), 62.82 (OCH2CH3),
67.59 (OCH2CH3), 78.28 (p-aryl), 78.46 (m-aryl), 116.22 (p-Pyr),
122.26 (m-Pyr), 123.41 (m-Ar), 126.00 (p-Ar), 129.80 (m-Pyr).
Preparation of (iPrEtOPDI)Fe(CO)2 (4-(CO)2). On a high-
vacuum line, 4 atm of CO were added to an NMR tube charged
with a benzene-d6 solution of 4-Aryl. The resulting reaction mixture
was shaken overnight, affording a dark green solution of a
compound identified as 4-(CO)2. 1H NMR (benzene-d6, 22 °C): δ
0.78 (t, 7.1 Hz, 3H, OCH2CH3), 1.16 (d, 7.0 Hz, 12H, CHMe2),
1.47 (d, 7.0 Hz, 12H, CHMe2), 2.91 (sept, 7.0 Hz, 4H, CHMe2),
3.72 (q, 7.1 Hz, 4H, OCH2CH3), 6.94 (t, 7.7 H, 2H, p-Ar), 7.0-
7.2 (m-Ar, p-Pyr), 7.75 (d, 7.8, 2H, m-Pyr). IR (pentane) νCO: 2013
Preparation of (iPrMeOPDI)Fe(CO)2 (3-(CO)2). A J. Young
NMR tube was charged with a solution of 3-Aryl in benzene-d6
and attached to a high-vacuum line. The solution was degassed,
and 4 atm of CO was added. The solution was shaken overnight,
affording a green solution of a compound identified as 3-(CO)2.
1H NMR (benzene-d6): δ 1.18 (d, 7 Hz, 12H, CH2Me), 1.50 (d, 7
Hz, 12H, CH2Me), 2.93 (sept, 7 Hz, 4H, CHMe2), 3.39 (s, 6H,
OMe), 6.89 (t, 8 Hz, p-pyr), 7.0-7.2 (p-aryl and m-aryl), 7.69 (d,
8 Hz, 2H, m-pyr). IR (pentane): νCO ) 2013, 1974, 1957, 1933,
1912 cm-1
.
Preparation of 2,6-(2,6-iPr2C6H3NdC(OEt))2C5H3N. A 250
mL round-bottomed flask was charged with 2.50 g (0.037 mol) of
NaOEt and 3.57 g (6.80 mmol) of N,N′-bis(2,6-diisopropylphenyl)-
pyridine-2,6-carboximidoyl chloride. A reflux condenser and 180°
needle valve were attached. On the Schlenk line, the contents of
the assembly were degassed, and approximately 100 mL of THF
was added to the solids by vacuum transfer. The resulting solution
was heated to reflux temperature for 3 days. After this time, the
volatiles were removed in vacuo and the resulting yellow residue
was dissolved in approximately 50 mL of CH2Cl2 and washed with
approximately 40 mL of 2 M NaOH, 40 mL of H2O, and 40 mL of
brine. The organic layer was dried over Na2SO4 and the solvent
removed by Rotovap, yielding 2.88 g (78%) of a burnt-orange solid
(s), 1974 (s), 1957 (m), 1933 (m), 1912 (s) cm-1
.
Acknowledgment. We thank the David and Lucile Packard
Foundation for financial support. P.J.C. is a Cottrell Scholar of
the Research Corporation and a Camille Dreyfus Teacher-
Scholar. We also thank Mr. Jim McNeil (Ithaca College) for
access to a magnetic susceptibility balance.
Supporting Information Available: Structural data for 2-(N2)2,
2-Aryl, and 2-Phenyl as CIF files. This material is available free
of charge via the Internet at http://pubs.acs.org.
i
identified as PrEtOPDI. MS (m/z) calcd ) 541.3668, found )
541.3670. 1H NMR (chloroform-d, 22 °C): δ 0.94 (d, 7.1 Hz, 12H,
OM060441C