Syntheses and characterizations of α-iminopyridine compounds (alkylNCHpy)2Fe(L/Xn), and an assessment of redox non-innocence

Article abstract of DOI:10.1016/j.poly.2012.08.060

Treatment of cis-(Me₃P)₄FeMe₂ with two equiv ^neoPeNCH(2-py) afforded {κ-N,N′- ^neoPeNCH(2-pyridyl)}₂FeMe₂ (2-P). Reductions of FeCl₂ with Na/Hg in the presence of ^neoPeNCH(2-pyridyl) or ^neoHexNCH(2-pyridyl) and L (H₂CCHCO₂Me, PhCCPh, PMe₃) gave tentative evidence of {κ-N,N′- ^neoPeNCH(2-pyridyl)}₂Fe(H₂CCHCO₂Me) (2-P), but {κ-N,N′-^neoPeNCH(2-pyridyl)} ₂Fe(PhCCPh) (5-P), {κ-N,N′-^neoHexNCH(2- pyridyl)}₂Fe(PhCCPh) (5-H), and {κ-N,N′- ^neoHexNCH(2-pyridyl)}₂Fe(PMe₃) (6-H) were all isolated in good yield. Phosphine complex 6-H could be alkylated with CH ₃I or CH₃OTf to afford [{κ-N,N′- ^neoHexNCH(2-pyridyl)}₂Fe(PMe₃)Me] ⁺I^- (7-H-I) or [{κ-N,N′- ^neoHexNCH(2-pyridyl)}₂Fe(PMe₃)Me]⁺ OTf^- (7-H-OTf). [{κ-N,N′-RNCH(2-pyridyl)} ₃Fe][OTf]₂ (R = ^neoPe, 8-P-OTf; ^neoHex, 8-H-OTf) complexes were also prepared from Fe(OTf) ₂ in order to identify potential byproducts in certain reactions. Compounds 2-P and 7-H-OTf were tested for ethylene oligomerization activity, to no avail. Complexes 2-P, 5-H, 6-H, and 7-H-OTf were structurally characterized and evaluated by Mo?ssbauer spectroscopy, and 5-H was shown to possess a redox non-innocent alkyne ligand, while the α-alkyliminopyridine ligands of 6-H were classified as radical anions, rendering all compounds describable as Fe(II).

Full text of DOI:10.1016/j.poly.2012.08.060

Polyhedron 52 (2013) 406–415
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Syntheses and characterizations of
a-iminopyridine compounds (alkylNCHpy)2
Fe(L/Xn), and an assessment of redox non-innocence
⇑
Emily C. Volpe, Peter T. Wolczanski , Jonathan M. Darmon, Emil B. Lobkovsky
Department of Chemistry & Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853, USA
a r t i c l e i n f o
a b s t r a c t
Treatment of cis-(Me3P)4FeMe2 with two equiv neoPeN@CH(2-py) afforded { -N,N0-neoPeN@CH(2-pyri-
j
Article history:
Available online 16 September 2012
dyl)}2FeMe2 (2-P). Reductions of FeCl2 with Na/Hg in the presence of neoPeN@CH(2-pyridyl) or
neoHexN@CH(2-pyridyl) and
-N,N0-neoPeN@CH(2-pyridyl)}2Fe(H2C@CHCO2Me) (2-P), but {
(5-P),
-N,N0-neoHexN@CH(2-pyridyl)}2Fe(PhCCPh) (5-H), and
L
(H2C@CHCO2Me, PhCCPh, PMe3) gave tentative evidence of
-N,N0-neoPeN@CH(2-pyridyl)}2Fe(PhCCPh)
-N,N0-neoHexN@CH(2-pyridyl)}2-
Dedicated to the inspirational father of
coordination chemistry, Alfred Werner, on
the 100th Anniversary of his 1913 Nobel
Prize in Chemistry.
{j
j
{j
{j
Fe(PMe3) (6-H) were all isolated in good yield. Phosphine complex 6-H could be alkylated with CH3I or
CH3OTf to afford [{
j
-N,N0-neoHexN@CH(2-pyridyl)}2Fe(PMe3)Me]+Iꢀ (7-H-I) or [{
j
-N,N0-neoHexN@CH(2-
neo-
pyridyl)}2Fe(PMe3)Me]+ OTfꢀ (7-H-OTf). [{
j
-N,N0-RN@CH(2-pyridyl)}3Fe][OTf]2 (R = neoPe, 8-P-OTf;
Keywords:
Iron
Hex, 8-H-OTf) complexes were also prepared from Fe(OTf)2 in order to identify potential byproducts in
certain reactions. Compounds 2-P and 7-H-OTf were tested for ethylene oligomerization activity, to no
avail. Complexes 2-P, 5-H, 6-H, and 7-H-OTf were structurally characterized and evaluated by Mössbauer
spectroscopy, and 5-H was shown to possess a redox non-innocent alkyne ligand, while the a-alkylimi-
nopyridine ligands of 6-H were classified as radical anions, rendering all compounds describable as Fe(II).
Redox non-innocence
Alkyne complex
Phosphine complex
Alkyl cation
Alkyl
Ó 2012 Elsevier Ltd. All rights reserved.
Iminopyridine
t
1. Introduction
exposed to BuCH2N@CH(2-py), but no evidence of aliphatic CH-
bond activation was obtained. Instead, the generation of bis-a-imi-
The discovery of a new class of Werner complexes, [(smif)2M]n
(n = 0; M = V, Cr, Mn, Fe, Co, Ni, Ru; n = 1; Cr, Co, Rh, Ir) [1], based
on the 1,3-(2-pyridyl)-2-azaallyl anion (i.e., smif) prompted an
investigation into variants where the pyridine components were
replaced with aryls. One approach involved the preparation of
imine complexes that could be deprotonated to afford azaallyl
fragments. As Scheme 1 illustrates, treatment of Karsch’s cis-(Me3-
P)4FeMe2 [2] with various imines resulted in aryl-iminopyridine
and diaryl-imine complexes derived from CH bond activation at
the ortho-position of one or more aryl groups [3–10].
nopyridine iron complexes was noted [19,24], and, given the
importance of imine-chelates in the field of olefin oligomerization
[25–34], a synthetic and structural study of low spin compounds of
the type (RN = CHpy)2Fe(L/Xn) was instigated [36–42]. This investi-
gation is reported herein, including an assessment of the redox
non-innocence of the
a-iminopyridine ligands.
2. Results and discussion
2.1. Preparation of {
j
-N,N0-neoPeN@CH(2-pyridyl)}2FeMe2 (2-P)
The generation of chelates containing the imino-pyridine group
can potentially lead to systems that exhibit redox non-innocence,
i.e., complexes in which the ligands can be redox active. Wieghardt
and co-workers [11–24] have studied complexes of the type Ar–
N@CHpy [19–24], and have noted characteristic changes in the li-
gand bond lengths that clearly indicate their redox character, as
Fig. 1 illustrates. While the studies have been definitive for arylimi-
no species, i.e., ligands where R1 = aryl in Fig. 1, there are fewer
examples of iminopyridine ligands derived from alkylamines (R1 -
t
In an NMR tube, cis-(Me3P)4FeMe2 [2] was exposed to BuCH2-
N@CH(2-py), but no release of methane was noted, hence no CH-
bond activation occurred. According to 1H NMR analysis, the major
product contained a plane of symmetry, consistent with the forma-
tion of
a simple adduct, cis-(Me)2,trans-(PMe3)2Fe{j
-N,N0-neo-
PeN@CH(2-pyridyl)} (1-P, Eq. (1)) [3,35]. However, the amount of
PMe3 released exceeded the expected two equiv, and there was a
considerable amount of a minor product. As a consequence, cis-
(Me3P)4FeMe2 was treated with 2 equiv of tBuCH2N@CH(2-py),
and 4 equiv of PMe3 were released, leading to the bis-iminopyri-
= alkyl) [22,23]. In the context of examining
a-iminopyridine li-
gands for sp3–CH bond activations [3–10], cis-(Me3P)4FeMe2 was
dine complex, {j
-N,N0-neoPeN@CH(2-pyridyl)}2FeMe2 (2-P) [36],
⇑
which was isolated as a purple crystalline solid in 80% yield
(Eq. (2)). One set of diastereotopic methylene protons at d 3.20
Corresponding author.
E-mail address: ptw2@cornell.edu (P.T. Wolczanski).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.08.060

E.C. Volpe et al. / Polyhedron 52 (2013) 406–415
407
H
H
H
PMe3
H
N
H
n
PMe3
Me
N
benzyl-
Fe
Me3P
Me
iminopyridine
N
N
X
Fe
N
Me3P
PMe3
- CH4
PMe3
M
Me
N
N
2-pyridylmethylene-
iminoarene
benzyl-
-2 CH4
X = H, p-OMe, o-Cl
N
iminoarene
H
H
H
H
- CH4
H
H
H
H
(smif)2M
N
N
N
n = 0; M = V, Cr, Mn, Fe, Co, Ni, Ru
n = 1; M = Cr, Co, Rh, Ir
X
Fe
Fe
Me3P
PMe3
Me3P
PMe3
Me
PMe3
X = H, p-CF3, o-OMe
Scheme 1. Illustration of new Werner complexes, (smif)2Mn, and aryl- and diaryl-imine complexes derived from CH bond activation by cis-(Me3P)4FeMe2.2ꢀ10
.
R1
R1
R1
R2
R2
R2
N
N
1.46
N
1.34
+ e-
- e-
+ e-
- e-
1.28
1.47
1.41
1.35
N
N
N
1.40
1.35
1.39
Fig. 1. Redox states of the a-iminopyridine ligand, with characteristic bond lengths
(Å).
(d, J = 11 Hz) and d 3.75 (d, J = 11 Hz) revealed the C2-symmetry of
the complex, but an X-ray crystal structure analysis was need to
determine the relative disposition of the imines versus the pyri-
dines, and the latter were determined to be trans about the Fe
center.
1-P
PMe3
PMe3
Me
N
C6D6
N
ð1Þ
Me3P
Me
Fe
Me3P
Me
N
N
-2 PMe3
Fe
PMe3
PMe3
+
...
Me
Fig. 2. Molecular view of {j
-N,N0-neoPeN@CH(2-pyridyl)}2FeMe2 (2-P).
N
PMe3
PMe3
Fe Me
N
N
C6H6
Me
Me
N
Me3P
Me
Fe
ð2Þ
2
N
-4 PMe3
remaining core angles of the pseudo-octahedron are acute, with /
C–Fe–C = 82.25(7)°, and /Nim–Fe–C = 84.5(4)° (ave).
PMe3
N
2-P
The Npy–Fe–C angles average 90.7(7)°, but the C–Fe–Nim angles
average 164.7(4)° due to the consequence of the chelate bite an-
gles, in addition to the trans-influence of the Me groups. Imine dis-
2.2. Structure of {j
-N,N0-neoPeN = CH(2-pyridyl)}2FeMe2 (2-P)
tances of 1.303(2) and 1.306(2) Å, and
Cim–Cpy distances of
1.424(2) and 1.417(2) Å, leave little doubt that the iminopyridine
ligands are neutral, and the complex can be considered Fe(II).
Fig. 2 provides a molecular view of pseudo-octahedral, C2
j
-N,N0-neoPeN@CH(2-pyridyl)}2FeMe2 (2-P), showing the trans-py
{
groups and the cis-arrangement of the methyls and imines. Table 1
lists pertinent refinement and data collection information, while
selected bond distances and angles are given in Table 2. The aver-
age Fe–Nim distance of 1.980(6) Å is considerably longer than the
average Fe–Npy distance (1.931(2) Å), perhaps an indication of
the strong trans-influence of the opposing methyl groups, whose
average d(Fe–C) of 2.042(5) Å is normal for low spin Fe(II). The
2.3. Reductive approaches to {
j
-N,N0-RN@CH(2-pyridyl)}2FeL
j
-N,N0-RN@CH(2-pyridyl)}2Fe(PhCCPh) (R = neoPe,
2.3.1. Synthesis of {
5-P; neoHex, 5-H)
With
a more general approach to the preparation of {
{j
-N,N0-neoPeN@CH(2-pyridyl)}2FeMe2 (2-P) in hand,
j
-N,N0-RN@CH(2-
N
py–Fe–Npy angle is 178.08(5)°, and the iminopyridine chelate an-
pyridyl)}2FeL/Xn species was sought. Unfortunately, treatment of
a variety of FeX2 with neoPeN@CH(2-pyridyl) or neoHexN@CH
(2-pyridyl), a slightly less congested alternative, failed due to dis-
proportionation, and the ready formation of [{RN@CH(2-pyridyl)}3-
Fe]2+ (vide infra).
gles average 80.99(16)°. A combination of steric repulsion of the
neopentyl groups, and the acute chelate bite angles, cause the
N
im–Fe–Nim angle and Npy–Fe–N0im angles to open to 109.56(5)°
and 97.90(6)° (ave), respectively. As a corollary, some of the

408
E.C. Volpe et al. / Polyhedron 52 (2013) 406–415
Table 1
Selected crystallographic and refinement data for
{j
-N,N0-neoPeN@CH(2-pyridyl)}2FeMe2 (2-P),
{
j
-N,N0-neoHexN@CH(2-pyridyl)}2Fe(PhCCPh) (5-H),
{j
-N,N0-neoHexN@CH
(2-pyridyl)}2Fe(PMe3) (6-H), and [{j
-N,N0-neoHexN@CH(2-pyridyl)}2Fe(PMe3)Me]+OTfꢀ (7-H-OTf).
2-P
5-Ha
6-Ha
7-H-OTf
Formula
Formula weight
Space group
Z
C
24H38N4Fe
C38H46N4Fe
614.64
Ia3d
C27H45N4PFe
512.49
C2/c
C29H48N4O3F3SPFe
676.59
P1
438.43
P21/n
4
ꢀ
48
4
2
a (Å)
b (Å)
c (Å)
a (°)
11.5730(6)
15.2735(10)
13.3733(8)
90
92.646(3)
90
2361.3(2)
1.233
0.655
35.4108(9)
35.4108(9)
35.4108(9)
90
90
90
44402(2)
1.103
0.436
18.3431(8)
7.3184(4)
21.4583(10)
90
102.803(2)
90
2809.0(2)
1.212
0.615
10.0631(6)
11.1129(7)
16.4463(10)
81.295(2)
72.480(2)
73.226(2)
1675.06(18)
1.341
b (°)
g (°)
V (Å3)
qcalc (g cmꢀ3
)
l
(mmꢀ1
)
0.612
T (K)
173(2)
173(2)
173(2)
173(2)
k (Å)
0.71073
R1 = 0.0304
wR2 = 0.0774
R1 = 0.0424
wR2 = 0.0849
1.004
0.71073
R1 = 0.0403
wR2 = 0.0996
R1 = 0.0663
wR2 = 0.1092
1.002
0.71073
R1 = 0.0312
wR2 = 0.0801
R1 = 0.0385
wR2 = 0.0848
1.060
0.71073
R indices
R1 = 0.0399
wR2 = 0.0992
R1 = 0.0546
wR2 = 0.1070
1.063
[I > 2s(I)]b,c
R indices (all data)b,c
Goodness-of-fit (GOF)d
n, number of independent reflections, p, number of parameters.
a
One-half of the molecule is the asymmetric unit.
b
R1
wR2 = [
GOF (all data) = [
=
R
||Fo| ꢀ |Fc||/
R|Fo|.
c
2 1/2
R
w(|Fo| ꢀ |Fc|)2/
R
wFo
]
.
d
R
w(|Fo| ꢀ |Fc|)2/(n ꢀ p)]1/2
.
Reasoning that in situ reductions might prevent the dispropor-
tionation path, FeCl2 and 2 equiv of Na/Hg were combined in THF
at ꢀ78 °C along with 2 equiv of RN@CH(2-pyridyl) (R = tBuCH2,
tBuCH2CH2) and an appropriate Fe(0) [15,19,41–43] trapping
agent. Initially, olefins were utilized, but the use of excess (P20
equiv) ethylene, cyclohexene, norbornene, and styrene led to
intractable brown mixtures. The only stable olefin complex came
Table 2
Selected distances (Å) and angles (°) for {
j
-N,N0-neoPeN@CH(2-pyridyl)}2FeMe2 (2-P),
{
j
-N,N0-neoHexN@CH(2-pyridyl)}2Fe(PhCCPh) (5-H),
{j
-N,N0-neoHexN@CH(2-pyri-
dyl)}2Fe(P-Me3) (6-H), and [{
(7-H-OTf).
j
-N,N0-neoHexN@CH(2-pyridyl)}2Fe(PMe3)Me]+OTfꢀ
2-P
5-Ha
6-Ha
7-H-OTf
from methyl acrylate, but the yield of putative
{j
-N,N0-neo-
Fe–Npy
Fe–N0py
Fe–Nim
Fe–N0im
Fe–C
1.930(2)
1.932(2)
1.984(2)
1.976(2)
2.046(2)
2.038(2)
1.303(2)
1.306(2)
1.424(2)
1.417(2)
1.368(2)
1.370(2)
1.9294(13)
1.949(2)
1.950(2)
1.965(2)
1.949(2)
1.967(2)
2.060(2)
PeN@CH(2-pyridyl)}2Fe(H2C@CHCO2Me) (3-P) was too low to war-
rant pursuit, and its tentative identification came from 1H NMR
analysis and a protolytic quench, as shown in Scheme 2.
1.9376(13)
1.9789(16)
1.310(2)
1.911(2)
Alkynes were next considered as trapping agents, and interest-
ing results were obtained. With a terminal alkyne, HCC(Ph-p-tBu),
no alkyne derivative was isolated from FeCl2, neoHexN@CH(2-pyri-
dyl), and 2 equiv sodium amalgam, but a standard work-up re-
vealed the presence of an organic product resulting from the
reductive coupling of the imine, [(neoHexNH)(2-pyridyl)HC]2 (4-
H). While labeling studies were not conducted, it is plausible that
the slightly acidic proton of the terminal alkyne was the source
of one NH, but since only one equiv HCC(Ph-p-tBu) was used and
Fe–C0
N
im–Cim
N0im–C0im
im–Cpy
C0im–C0py
1.337(2)
1.396(2)
1.384(2)
2.235(2)
1.284(2)
1.291(2)
1.429(3)
1.435(3)
1.367(3)
1.364(3)
2.255(2)
C
1.423(2)
C
py–Npy
1.380(2)
C0py–N0py
Fe–P
C–C0
1.289(3)
177.54(8)
108.09(8)
80.71(5)
N
N
N
py–Fe–N0py
im–Fe–N0im
py–Fe–Nim
178.08(5)
109.56(5)
80.88(5)
81.10(5)
97.85(5)
97.94(5)
82.25(7)
176.11(6)
140.15(6)
81.24(4)
173.92(6)
100.13(6)
80.57(6)
80.16(6)
94.99(7)
96.46(6)
the crude yield was ꢁ80%, the source of the second NH is unknown.
neo-
Nonetheless, an intermediate Fe(0) complex such as
[
N0py–Fe–N0im
HexN@CH(2-pyridyl)]2Fe could reductively couple its imines, and
subsequent protonations would permit the production of 4-H;
insertion paths are also possible.
N
py–Fe–N0im
97.83(5)
38.02(9)
89.50(6)
92.82(6)
144.84(6)
107.02(6)
97.42(4)
N0py–Fe–Nim
C–Fe–C0
C–Fe–P
82.80(6)
90.19(8)
A switch to diphenylacetylene in the reductive trapping reac-
tion afforded adducts with both the neoPe and neoHex versions
N
py–Fe–C
N0py–Fe–C0
py–Fe–C0
N0py–Fe–C
91.45(6)
91.27(6)
90.13(6)
90.04(6)
164.94(6)
164.44(6)
84.21(6)
84.82(6)
of the imine. Dark brown {
Fe(PhCCPh) (5-H) was isolated in 60% yield upon crystallization
from Et2O, while
-N,N0-neoPeN@CH(2-pyridyl)}2Fe(PhCCPh)
(5-P) was obtained in 45%. 1H and 13C{1H} NMR spectra revealed
one set of -iminopyridine resonances for both derivatives, con-
j
-N,N0-neoHexN@CH(2-pyridyl)}2-
N
93.55(8)
84.82(8)
{
j
N
im–Fe–C
N0im–Fe–C0
N0im–Fe–C
172.27(7)
a
N
N
im–Fe–C0
py–Fe–P
sistent with C2 symmetry, and single and double sets of diaste-
reotopic protons for the CH2 and CH2CH2 groups of 5-P and 5-
H were also noted.
91.94(3)
96.64(5)
88.59(5)
167.30(5)
92.49(5)
N0py–Fe–P
Nim–Fe–P
109.93(3)
N’im–Fe–P
Fe–C–Cipso
C0–C–Cipso
139.9(2)
148.3(2)
2.3.2. Structure of {
Diphenylacetylene adduct
dyl)}2Fe(PhCCPh) (5-H) crystallized in a cubic space group (Ia3d)
j
-N,N0-neoHexN@CH(2-pyridyl)}2Fe(PhCCPh) (5-H)
-N
-N,N0-neoHexN@CH(2-pyri-
{
j j
a
a-Iminopyridine ligands are equivalent by symmetry.

E.C. Volpe et al. / Polyhedron 52 (2013) 406–415
409
N
N
2
THF
CO2Me
N
FeCl2
- 2 NaCl
N
Fe
-78 C
N
CO2Me
3-P
2 Na/Hg
(excess)
N
H+ - "FeXn"
THF
-78 C
~10%
2
CO2Me
N
N
N
N
2
NH
HN
pentane work-up
& filter
N
N
4-H
~80%
N
n
N
2
THF
N
FeCl2
- 2 NaCl
Ph
n = 1,2
N
N
n
n
Fe
N
-78 C
2 Na/Hg
Ph
n = 1, 45%
n = 2, 60%
5-P
5-H
-78 ->23 C, 12 h
Scheme 2. Reductive attempts at olefin and alkyne bis-
neoHex, 5-H).
a-iminopyridine complexes; successful preparations of {j
-N,N0-RN@CH(2-pyridyl)}2Fe(PhCCPh) (R = neoPe, 5-P;
with 48 molecules in the unit cell; one half of one molecule consti-
tutes the asymmetric unit. Selected refinement and data collection
details are listed in Table 1, and metric parameters are given in
Table 2. The compound can be considered 5-coordinate, but since
the alkyne carbons lie roughly in the plane containing the imine
nitrogens, the molecule can also be construed as pseudo-octahe-
dral, as Fig. 3 illustrates. The pyridine nitrogens are 177.54(8)°
apart, and the Fe–Npy and Fe–Nim distances of 1.929(2) and
1.938(2) Å are much more similar than in the case of the dimethyl
derivative, 2-P. The alkyne carbons are 1.979(2) Å from the iron,
and the d(CC) of 1.289(3) Å and C0–C–Cipso angles of 148.33(10)°
convey evidence of significant backbonding from the formal Fe(0)
center. Since the alkyne has some ‘‘metalacyclopropene’’ character,
the
a-iminopyridines remain relatively innocent of any redox
changes [19,22,23], with d(Nim–Cim) = 1.310(2) Å, d(Cim
–
C
py) = 1.423(2) Å, and d(Cpy–Npy) = 1.380(2) Å. The Nim–Fe–N0im
and Npy–Fe–N0im angles of 108.09(8)° and 97.83(5)°, respectively,
are again partly a consequence of the chelate bite angle of
80.71(5)°, hence the {j
-N,N0-neoHexN@CH(2-pyridyl)}2Fe part of
the core is remarkably similar to that of 2-P in terms of the bond
angles. The alkyne carbons are 144.84(6)° from the opposing imine
nitrogens, and 107.82(6)° from the adjacent ones, while 89.50(6)°
and 92.52(6)° from the pyridine nitrogens, revealing a subtle C2
twist away from the Nim–Fe–N0im unit.
2.3.3. Synthesis of {j
-N,N0-neoHexN@CH(2-pyridyl)}2Fe(PMe3) (6-H)
Standard reductive trapping procedures utilizing neoHexN@
CH(2-pyridyl) in the presence of PMe3 netted the formally Fe(0)
phosphine adduct, {j
-N,N0-neoHexN@CH(2-pyridyl)}2Fe(PMe3) (6-
H) in 45% yield as metallic green crystals from a plum colored ether
solution. Scheme 3 illustrates the synthesis of C2 6-H, which proved
to be a disappointing starting material for oxidative procedures. No
reaction was observed when the complex was treated azides RN3
(R = adamantyl, 2,6-iPr2–C6H3, 2,4,6-Me3–C6H3), N2@CPh2, H2-
C@PPh3, and HCC(phenyl-p-tBu), despite indications that the
Fig. 3. Molecular view of {
j
-N,N0-neoHexN@CH(2-pyridyl)}2Fe(PhCCPh) (5-H).
compound could be construed as a 16 eꢀ species, { -N,N0-neo-
j

410
E.C. Volpe et al. / Polyhedron 52 (2013) 406–415
N
6-H
N
THF
2
FeCl2
N
- 2 NaCl
N
45%
Fe
PMe3
-78 C
1.5 PMe3
-78 ->23 C, 12 h
N
2 Na/Hg
N
CH3I
or
CH3OTf
various
reagents
No
Fe=X bond
formation
THF
X
PMAO
N
N
N
PMe3
CH3
No
7-H-I
76%
polymerization
activity
Fe
variable conditions
7-H-OTf 82%
N
Scheme 3. Reductive route to {
(X = I, 7-H-I; OTf, 7-H-OTf).
j
-N,N0-neoHexN@CH(2-pyridyl)}2FePMe3 (6-H), and subsequent methylations to give [{
j
-N,N0-neoHexN@CH(2-pyridyl)}2Fe(PMe3)CH3]+Xꢀ
HexN@CH(2-pyridyl)1ꢀ}2Fe2+(PMe3) (6-H). In addition, no reac-
tions were observed in the presence of H2 (1 atm) or HSiPh3 (ex-
cess), even when heated.
structure, the axial Fe–Npy distance of 1.949(2) Å is longer than
the d(Fe–Nim) of 1.911(2) Å, in slight contrast the other structures.
The iron–phosphine bond length of 2.235(2) Å is normal, and the
phosphine is 91.94(3)° from the pyridine nitrogens. The bite angle
of the a
-iminopyridine is 81.24(4)°, and the remaining Npy–Fe–N0im
2.3.4. Structure of {
j
-N,N0-neoHexN@CH(2-pyridyl)}2Fe(PMe3) (6-H)
-N,N0-neoHexN@CH(2-pyridyl)}2Fe(PMe3)
angle is 97.42(4)°, in line with the previous examples. The differ-
ence in this structure are the Nim–Cim, Cim–Cpy, and Cpy–Npy dis-
tances of 1.337(2), 1.396(2), and 1.384(2) Å, respectively, which
The structure of
{j
(6-H) is pseudo-tbp, with the pyridines occupying the axial sites
and the imines and phosphine in equatorial positions, as Fig. 4
illustrates. Limited data collection and refinement information
for 6-H is listed in Table 1, while selected metric bond distances
and angles are given in Table 2. The Nim–Fe–N0im angle of
140.15(6)° and the Nim–Fe–P angles of 109.93(3)° reveal a signifi-
cant distortion from trigonal, although they are planar, as the angles
sum to 360.01°. The pyridines exhibit little deviation from linearity
as the Npy–Fe–Npy angle is 176.11(6)°, and as is typical for a tbp
indicate that the
a-iminopyridine units are monoanions [19,22,23].
2.4. Oxidations of formal Fe(0) species
2.4.1. Methylations of {
j
-N,N0-neoHexN@CH(2-pyridyl)}2Fe(PMe3)
(6-H)
As Scheme 3 illustrates, oxidation of {
j
-N,N0-neoHexN@CH
(2-pyridyl)}2Fe(PMe3) (6-H) via the generation of an Fe@X bond was
not accomplished, so methylation was considered as an option,
especially since cations of the type [{j
-N,N0-neoHexN@CH(2-pyri-
dyl)}2Fe(olefin)R]+ were plausible polymerization catalysts [25–
34]. Treatment of 6-H with either CH3I or CH3OTf provided dark
green [{j
-N,N0-neoHexN@CH(2-pyridyl)}2Fe(PMe3)Me]+Xꢀ (X = I,
7-H-I; OTf, 7-H-OTf) in 76% and 82% yields, respectively. Unlike
the previous C2 examples, the dissymmetric 7-H-X species exhib-
ited two distinct
a
-iminopyridines in their 1H and 13C{1H} NMR
spectra. The iodide complex was similarly characterized, but its
limited solubility restricted its spectral analysis to CD2Cl2 rather
than THF-d8.
2.4.2. Polymerization attempts with [{
pyridyl)}2Fe(PMe3)Me]+Xꢀ (X = I, 7-H-I; OTf, 7-H-OTf) and
-N,N0-neoPeN@CH(2-pyridyl)}2FeMe2 (2-P)
Phosphine adducts [{
j
-N,N0-neoHexN@CH(2-
{
j
j
-N,N0-neoHexN@CH(2-pyridyl)}2-
Fe(PMe3)Me]+Xꢀ (X = I, 7-H-I; OTf, 7-H-OTf) were subjected to
modest pressures of ethylene (1–4 atm, 60 °C, added PMAO, typi-
cally 270 equiv) in an effort to effect polymerization via loss of
PMe3, but no polyethylene resulted from the trials despite the
Fig. 4. Molecular view of {j
-N,N0-neoHexN@CH(2-pyridyl)}2FePMe3 (6-H).

E.C. Volpe et al. / Polyhedron 52 (2013) 406–415
411
trans-influence of the two ligands. The Fe–Npy and Fe–Nim
distances average 1.958(10) and 1.958(13) Å, respectively, in
contrast to {j
-N,N0-neoPeN@CH(2-pyridyl)}2FeMe2 (2-P), where
the pyridine nitrogens were ꢁ0.05 Å farther away from the iron.
The d(Fe–C) of 2.060(2) Å is slightly longer than the corresponding
distances in 2-P, and the Nim–Fe–C, Npy–Fe–C, and P–Fe–C angles of
84.82(8)°, 91.3(16)° (ave), and 82.80(6), respectively, reveal the
methyl to be squeezed by its bulky neighbors. The PMe3 group
exhibits a modest steric influence about the core, with P–Fe–Nim
,
P–Fe–Npy,
and P–Fe–N0py, angles of 92.49(5)°, 88.59(5)° and
96.64(5)°, respectively. The opposing C–Fe–N0im and P–Fe–Nim
angles of 172.27(7)° and 167.30(5)° differ due to the steric
repulsion of the phosphine and its adjacent imine.
2.4.4. Synthesis of [{j
-N,N0-RN@CH(2-pyridyl)}3Fe][OTf]2 (R = neoPe,
8-P-OTf; neoHex, 8-H-OTf)
Attempted one-electron oxidations of either {j
-N,N0-neoHexN =
CH(2-pyridyl)}2Fe(PhCCPh) (5-H) or {j
-N,N0-neoHexN@CH(2-pyri-
dyl)}2Fe(PMe3) (6-H) led to intensely purple solutions. 1H NMR
analysis of the crude reactions showed multiple products, but the
major product isolated from either starting material possessed
three distinct
a-iminopyridine environments in a 1:1:1 ratio. The
Fig. 5. Molecular view of the cation pertaining to [{
dyl)}2Fe(PMe3)Me]+OTfꢀ (7-H-OTf).
j
-N,N0-neoHexN@CH(2-pyri-
same spectral signature was observed in the reaction of 5-H with
CH3OTf or when the reaction of 6-H was not performed at
low-temperatures. Disproportionation to the tris-a-iminopyridine
dication was suspected, and confirmation by independent synthe-
sis was sought.
2+ (OTf-)2
N
n
n = 1, 87%
n = 2, 93%
N
8-P-OTf
n
2
N
N
N
N
N
THF
mer:fac = 2:1
n = 1,2
n
n
Fe
ð3Þ
23 C, 4 h
8-H-OTf
mer only
N
Fe(OTf)2
varied conditions employed. The dimethyl derivative, {
j
-N,N0-neo-
Fe(OTf)2 was added to three equiv of RN@CH(2-pyridyl) (R@neo-
PeN@ CH(2-pyridyl)}2FeMe2 (2-P), was subjected to methide
abstraction agents PMAO (polymethylalumoxane) and [Ph3C][B(C6-
F5)4] under related conditions, but in only 2 of 4 trials was any
polyethylene detected, and it was in trace amounts. No further ole-
fin oligomerizations were attempted.
Pe; neoHex) in THF for 4 h, resulting in a bright purple precipitate
that was identified as the tris-
NMR spectroscopy (Eq. (3)). Interestingly, the neohexyl derivative,
[mer-{
-N,N0-neoHexN@CH(2-pyridyl)}3Fe][OTf]2 (8-H-OTf) was
obtained as only the mer-isomer in 93% yield [44,45], whereas
the neopentyl species [{
-N,N0-neoPeN@CH(2-pyridyl)}3Fe][OTf]2
a
-iminopyridine dication by 1H
j
j
2.4.3. Structure of [{j
-N,N0-neoHexN@CH(2-
(8-P-OTf) was obtained as a 1:2 ratio of fac and mer isomers in
87% total yield.
pyridyl)}2Fe(PMe3)Me]+OTfꢀ (7-H-OTf)
A structural study was conducted on [{j
-N,N0-neoHexN@CH(2-
pyridyl)}2Fe(PMe3)Me]+OTfꢀ (7-H-OTf), and Fig. 5 illustrates the
expected trans-pyridine and cis-imines groups of the pseudo-
octahedral compound. Table 1 lists some data collection and
refinement details, while the metric parameters are found in Table
2. The pyridine nitrogens are 173.92(6)° apart, and the bite angles
2.5. Mössbauer spectroscopy
Zero-field Mössbauer spectra for a select group of a-iminopyri-
dine complexes is given in Fig. 6, and a table is included that gives
the standard parameters for each species, which are listed in order
of decreasing isomer shift, d (mm/s). In general, the isomer shifts
for all species can be considered reasonable for low spin Fe(II)
(typically -0.2 to 0.4) [46,47], although the ordering is somewhat
for the
a-iminopyridines average 80.4(3)°, causing the opposing
Npy–Fe–N0im angles to average 95.7(10)°, which is slightly less than
the previous examples, due to the greater deviation from 180° of
the Npy–Fe–N0py angle. Presumably this is due in part to the sterics
of the 6-coordinate metal center and its bulky PMe3 ligand, which
is bound 2.255(2) Å from the iron. The imine nitrogen opposite the
methyl group is 1.967(2) Å from the iron, only slightly longer than
the N0im opposite the PMe3 (1.949(2) Å), an indication of the similar
peculiar. On the high end of the list is [{
idyl)}3Fe][OTf]2 (8-P-OTf), whose d = 0.37 and
similar to related compounds such as Wieghardt’s [([{
PhN@CMeCMe@NH)3Fe]2+ (d = 0.37,
EQ = 0.54), and tris-pyridine
complexes [(bipy)3Fe][X]n (n = 2, X = ClO4ꢀ, d = 0.37,
EQ = 0.48;
j
-N,N0-neoPeN@CH(2-pyr-
EQ = 0.47 values are
-N,N0-
D
j
D
D

412
E.C. Volpe et al. / Polyhedron 52 (2013) 406–415
Fig. 6. Zero-field Mössbauer spectra for the labeled compounds (Velocity (mm/s) vs. Relative Absorbance (y-axis)). The table lists the complexes in decreasing isomer shift, d
(mm/s); quadrupole splitting (
D
EQ, mm/s) and linewidth (C, mm/s) parameters are also given.
c
R = R' = CH3
R = R' =
k
d
b
Ph
e
j
m
a
f
h
l
N
n
o
j
i
k
k
f
i
g
h
NH
HN
N
R
g
(or L)
Fe
e
h'
R'
a'
b'
g'
N
L = P(CH3)3
l
N
N
4-H
i'
d
a
f'
O
N
n
j'
m
L =
e'
d'
H
H
c
OCH3
b
H
c'
j
k
Fig. 7. Labeling key for NMR data; diastereotopic protons are given subscripts a or b.
n = 1, X = SO42ꢀ, d = 0.41,
D
EQ = 0.37) [48–51]. Since the field
the dipyridine iron axis. In the cation, the contraction of the d-orbitals
renders the electron density at the iron center less susceptible to
axis variation, and in 7-H-OTf, the Nim–Fe–N0im angle is the closest
to octahedral (100.13(6)°) of all those structurally characterized,
hence the asymmetry in this complex is attenuated.
strengths along each axis are basically the same, the quadrupole
splittings are relatively small, and the isomer shifts are essentially
equivalent.
Two formally ‘‘Fe(0)’’ compounds, {j
-N,N0-neoHexN@CH(2-pyri-
dyl)}2Fe(PhCCPh) (5-H) and (6-H), possess isomer shifts of 0.25
and 0.32 mm/s, respectively, consistent with the partial metalacy-
clopropene character of 5-H, and the radical anion character of the
3. Conclusions
a
-iminopyridines in {
j
-N,N0-neoHexN = CH(2-pyridyl)ꢀÅ}2FeII(PMe3)
A series of a-alkyliminopyridine complexes has been synthe-
(6-H). For reference, Fe(0) complexes such as Fe(CO)5 [52] and
Fe(CO)4PPh3 [53] have isomer shifts ꢁꢀ0.10 mm/s. The quadrupole
splitting of 6-H is by far the greatest of the set at 1.63 mm/s, a value
considerably large than that of 5-H, which is 1.10 mm/s, and one
commensurate with its structure, which is a distorted trigonal
bipyramid with an Nim–Fe–Nim angle of 140.15(6)°. Wieghardt has
sized by substitution processes, by reduction of iron(II) salts in
the presence of neoPeN@CH(2-pyridyl) or neoHexN@CH(2-pyridyl)
and L (L = PMe3, PhCCPh), and by methylation of {
HexN@CH(2-pyridyl)}2Fe(PMe3) (6-H). The study was conducted
to explore the potential redox noninnocence of the -alkylimino-
j
-N,N0-neo-
a
pyridine ligand, and to generate species capable of oligomerizing
olefins. While the former question was answered to some extent,
the oligomerization potential of these derivatives has yet to be
realized.
prepared a related ‘‘Fe(0)’’ complex, (j
-N,N0-C6H4-o-NPh,NH)2FePn-
Bu3 [11], whose metrics suggest phenyl-dimine redox non-
innocence and a true Fe(II) oxidation state. Its Mössbauer isomer
shift of d 0.13 mm/s, while closer to Fe(0) (typically ꢀ0.2 to
Structural and Mössbauer spectroscopic studies point toward
ꢀ0.1 mm/s), is best construed as Fe(II), while its
D
EQ value of
the putative ‘‘Fe(0)’’ complex {
Fe(PMe3) (6-H) as containing redox non-innocent
pyridine ligands; it is best formulated as {
-N,N0-neoHexN@CH
(2-pyridyl)ꢀÅ}2FeII(PMe3) (6-H). While the
-alkyliminopyridine
ligands in {
-N,N0-neoHexN@CH(2-pyridyl)}2Fe(PhCCPh) (5-H) are
j
-N,N0-neoHexN@CH(2-pyridyl)}2-
2.96 mm/s surely reflects the asymmetry in this square pyramidal
complex.
a-alkylimino-
j
a
The remaining complexes,
{
j
-N,N0-neoPeN@CH(2-pyridyl)}2-
FeMe2 (2-P) and [{
j
-N,N0-neoHexN@CH(2-pyridyl)}2Fe(PMe3)Me]+-
j
OTfꢀ (7-H-OTf), possess isomer shifts of 0.19 and 0.27 mm/s, but
surprisingly the quadrupolar splitting of the symmetric dimethyl
species (1.24 mm/s) is greater than that of the phosphine methyl
cation (1.03 mm/s). The large value is likely to be a consequence
of the far greater field strength along the N–Fe–C axes relative to
best construed as ‘‘normal’’, Mössbauer spectroscopy indicates that
the iron is best considered Fe(II), rendering the alkyne as redox
non-innocent in its capacity to bind in a metalacyclopropene fash-
ion. With the possible exceptions of [{
Fe][OTf]2 (R = neoPe, 8-P-OTf; neoHex, 8-H-OTf), the remaining
j
-N,N0-RN@CH(2-pyridyl)}3-

E.C. Volpe et al. / Polyhedron 52 (2013) 406–415
413
derivatives are normal iron(II) compounds, although they possess
considerable covalent character and are all low spin: {
-N,N0-
neoPeN@CH(2-pyridyl)}2FeMe2 -N,N0-neoHexN@CH
(2-P), [{
(2-pyridyl)}2Fe(PMe3) (7-H-OTf), [{
Me]+OTfꢀ -N,N0-neo-
HexN@CH(2-pyridyl)}2Fe(PMe3)Me]+Iꢀ (7-H-I).
-N,N0-neoHexN@CH(2-pyridyl)}2-
to stir at 23 °C for 18 h. After filtration, the solvent was removed,
and the solids were triturated with Et2O (3 ꢂ 8 mL). The solids
were dissolved in 5 mL Et2O and concentrated under vacuum at
ꢀ78 °C to afford a dark purple crystalline solid (125 mg, 80%). 1H
NMR (400 MHz, benzene-d6): d 8.28 (d, 3, a), 6.62 (t, 7, b), 6.90 (t,
8, c), 7.03 (d, 8, d), 9.09 (br s, f), 3.75 (d, 11, ga), 3.20 (d, 11, gb),
0.45 (s, j), 0.97 (s, k). 13C{1H} NMR (400 MHz, benzene-d6): d
152.71 (a), 124.83 (b), 126.24 (c), 117.26 (d), 157.83 (e), 158.00
(f), 68.76 (g), 32.47 (i), 28.19 (j), 21.14 (k). Anal. Calc. for C24H38N4-
Fe: C, 65.75; H, 8.74; N, 12.78. Found: C, 65.54; H, 8.60; N, 12.60%.
j
j
j
Attempts
to
utilize
[{j
Fe(PMe3)Me]+OTfꢀ (7-H-OTf) as an olefin oligomerization catalyst
failed, despite a variety of conditions. Since it was conceivable
the phosphine did not dissociate, or sterically hampered oligomer-
ization if an arm of the
a-iminopyridine dissociated, the dimethyl
derivative, {
j
-N,N0-neoPeN@CH(2-pyridyl)}2FeMe2 (2-P), was also
tested. Even under conditions where a methide abstracting reagent
was employed (PMAO, [Ph3C][B(C6F5)4]), no ethylene oligomeriza-
tion was detected. It is not clear what is preventing oligomeriza-
tion, but it is likely that the 18 eꢀ starting materials, or the
putative 16 eꢀ methyl cations, are still not electrophilic enough
to minimize backbonding to olefins and permit oligomerization
[27,28,32]. There is considerable evidence that 14 eꢀ or lower elec-
tron counts of the alkyl cations are necessary. Additionally, the
4.2.2. {j
-N,N0-neoPeN@CH(2-pyridyl)}2Fe(H2C@CHCO2Me) (3-P)
Ten milliliters THF were vacuum transferred to a flask contain-
ing FeCl2 (35 mg, 0.28 mmol) and 2.2 equiv sodium amalgam
(1.77 g, 0.79%) in a ꢀ78 °C bath. Under argon purge, methyl acry-
late (0.50 mL, 5.5 mmol) and a solution of neoPeN@CH(2-pyridyl)
(97 mg, 0.55 mmol) in THF were added via syringe. The mixture
was allowed to warm slowly to 23 °C over 12 h. The volatiles were
removed, and the solids were redissolved in Et2O and filtered
through Celite. Ether was replaced with hexane and concentration
at ꢀ78 °C yielded ꢁ15 mg olive green solid. 1H NMR (400 MHz,
benzene-d6): d 8.52 (br s, a, a0), 6.47 (t, 6, b), 6.41 (t, 6, b0), 6.69–
6.75 (m, c, c0), 6.83 (d, 7, d), 6.78 (d, 7, d0), 8.05 (s, f), 8.02 (s, f0),
4.29–4.18 (m, ga, g0a, l), 3.53 (d, 11, gb), 3.41 (d, 12, g0b), 0.56 (j),
0.54 (j0), 1.88 (d, 7, k), 4.96 (t, 10, m), 2.84 (s, n).
redistribution of
a-iminopyridine ligands to afford [{j
-N,N0-
RN@CH(2-pyridyl)}3Fe]2+ dications (R = neoPe, neoHex) is a potential
serious drawback.
4. Experimental
4.1. General considerations
4.2.3. Generation of [(neoHexNH)(2-pyridyl)HC]2 (4-H)
Ten milliliters THF were vacuum transferred to a flask contain-
ing FeCl2 (48 mg, 0.38 mmol) and 2.2 equiv sodium amalgam
(2.68 g, 0.72%) in a ꢀ78 °C bath. Under argon purge, a solution of
neoHexN@CH(2-pyridyl) (145 mg, 0.762 mmol) and p-(tert-
butyl)phenylacetylene (60 mg, 0.38 mmol) were added via syringe.
The mixture was allowed to warm slowly to 23 °C over 12 h. The
volatiles were removed, and the solids were redissolved in THF
and filtered through Celite. The volatiles were removed and the
solids were triturated with pentane (3 ꢂ 5 mL). The material was
dissolved in hexane and concentrated at ꢀ78 °C to afford a brown
solid (125 mg). 1H NMR (500 MHz, benzene-d6): d 8.41 (d, 4, a),
6.56 (t, 6, b), 7.02 (t, 8, c), 7.17 (d), 4.28 (t, 4, f), 2.54 (m, g), 1.47
(m, h), 0.77 (s, j), 2.88 (d, 6, k). 13C NMR (500 MHz, benzene-d6):
d 149.55 (a), 122.02 (b), 135.69 (c), 123.53 (d), 163.32 (e), 70.64
(f), 45.16 (g), 45.18 (h), 30.23 (i), 30.10 (j).
4.1.1. General handling
All manipulations were performed using either glovebox or
high vacuum line techniques. Hydrocarbon solvents containing
1–2 mL of added tetraglyme, and ethereal solvents were distilled
under nitrogen from purple sodium benzophenone ketyl and vac-
uum transferred from same prior to use. Benzene-d6 and toluene-
d8 were dried over sodium, vacuum transferred and stored under
N2. THF-d8 was dried over sodium benzophenone ketyl. Methylene
chloride-d2 was dried over CaH2, vacuum transferred and stored
over activated 4 Å molecular sieves. Trimethylphosphine was dried
and stored over activated 4 Å molecular sieves. cis-(Me3P)4FeMe2
was prepared according to the literature procedure.2 All other
chemicals were commercially available and used as received. All
glassware was oven dried.
NMR spectra were obtained using Varian XL-400, INOVA 400,
and Unity-500 spectrometers. Chemical shifts are reported relative
to benzene-d6 (1H d, 7.16; 13C{1H} d 128.39) THF-d8 (1H d 3.58;
13C{1H} d 67.57) and CD2Cl2 (1H d 5.32; 13C{1H} d 54.00). Assign-
ments are given according to Fig. 7. Infrared spectra were recorded
on a Nicolet Avatar 370 DTGX spectrophotometer interfaced to an
IBM PC (OMNIC software). UV–Vis spectra were obtained on a
Shimadzu UV-2102 interfaced to an IBM PC (UV Probe software).
Elemental analyses were performed by Robertson Microlit Labora-
tories, Madison, New Jersey.
4.2.4. {j
-N,N0-RN = CH(2-pyridyl)}2Fe(PhCCPh) (R = neoPe, 5-P; neoHex,
5-H)
A 50 mL round bottom flask was charged with 75 mg FeCl2
(0.592 mmol), diphenylacetylene (116 mg, 0.65 mmol), and 2.2
equiv sodium amalgam (3.79 g, 0.79%). Twenty milliliters THF
were vacuum distilled at - 78 °C, and
a solution of imine
(1.18 mmol) in 5 mL THF was added via syringe. The reaction mix-
ture was stirred and warmed to 23 °C over 12 h. The volatiles were
removed, and the solids were redissolved in Et2O and filtered
through Celite. The volatiles were removed and the solids were
triturated with Et2O (3 ꢂ 5 mL), and dissolved in 5 mL Et2O. The
solution was cooled to ꢀ78 °C and concentrated to afford a dark
brown crystalline solid. (a) 5-H: 220 mg, 60%. 1H NMR (500 MHz,
benzene-d6): d 8.67 (d, 5, a), 6.27 (t, 6, b), 6.63 (t, 7, c), 6.83 (d, 8,
d), 8.27 (s, f), 4.25 (td, 12, 4, ga), 4.09 (td, 11, 4, gb), 1.20 (td, 12,
5, ha), 0.56 (td, 13, 3, hb), 0.64 (s, j), 7.61 (d, 8, m), 7.22 (t, 7, n),
7.02 (t, 7, o). 13C NMR (500 MHz, benzene-d6): d 150.88 (a),
122.78 (b), 125.24 (c), 117.83 (d), 155.12 (e), 155.88 (f), 58.79 (g),
46.80 (h), 29.73 (i, j), 138.66 (k), 120.86 (l), 128.73 (m), 129.79
(n), 125.41 (o). Anal. Calc. for C40H46N4Fe: C, 74.26; H, 7.54; N,
9.12. Found: C, 72.03; H, 7.41; N, 8.43. (b) 5-P: 165 mg, 45%. 1H
NMR (400 MHz, benzene-d6): d 8.63 (a), 6.25 (b), 6.62 (c), 6.81
(d), 8.16 (f), 4.45 (ga), 3.72 (gb), 0.53 (j), 7.49 (m), 7.18 (n), 7.04
4.1.2. General procedure for imine synthesis
To a suspension of MgSO4 (5–8 equiv.) in CH2Cl2 were added
1.5 mmol of 2-pyridinecarboxaldehyde and 1.5 mmol of amine.
After stirring for 2 h, the mixture was filtered and concentrated
to yield a pale yellow oil in >98% purity (by 1H NMR). A spectrum
and synthesis for neoPeN@CH(2-pyridyl) has been reported [54].
4.2. Syntheses of
a-iminopyridine complexes
4.2.1. {
j
-N,N0-neoPeN = CH(2-pyridyl)}2FeMe2 (2-P)
To a solution of (PMe3)4FeMe2 (140 mg, 0.359 mmol) in ben-
zene was added a solution of neoPeN@CH(2-pyridyl) (127 mg,
0.718 mmol) in benzene. The bright purple solution was allowed

414
E.C. Volpe et al. / Polyhedron 52 (2013) 406–415
(o). Anal. Calc. for C38H42N4Fe: C, 73.71; H, 7.22; N, 9.55. Found: C,
73.48; H, 7.50; N, 9.27%.
13, h6), 0.83 (s, j1), 0.61 (s, j2, j3). 13C NMR (400 MHz, CD3CN): d
156.89 (a1), 156.42 (a2), 155.88 (a3), 130.13 (b1), 129.93 (b2),
129.42 (b3), 139.65 (c1, c2), 139.54 (c3), 128.93 (d1), 128.86 (d2),
128.73 (d3), 160.18 (e1), 159.49 (e2), 159.06 (e3), 174.09 (f1),
173.80 (f2), 173.49 (f3), 57.38 (g1), 57.07 (g2), 56.80 (g3), 45.24
(h1), 44.82 (h2, h3), 30.77 (i1), 30.30 (i2, i3), 29.34 (j1), 29.05 (j2, j3).
(b) 8-P-OTf: 218 mg bright purple solid (87%). The product con-
tained a mixture of fac and mer isomers in a 1:2 ratio. 1H NMR
(400 MHz, CD3CN): d (fac) 8.36 (d, 8, a), 7.54 (t, 6, b), 8.15 (t, 8,
c), 7.04 (d, 6, d), 9.12 (s, f), 3.85 (d, 15, g), 3.25 (s, 15, g0), 0.77 (s,
j); (mer) 8.87 (br s, a1, a2), 8.76 (d, 8, a3), 7.96 (t, 6, b1), 7.77 (t, 7,
b2), 7.85–7.88 (m, b3, c1), 8.01–8.10 (m, c2, c3, d1), 8.53 (d, 8, d2),
8.50 (d, 8, d3), 10.30 (br s, f1, f2), 10.15 (s, f3), 4.73 (d, 14, g1), 4.39
(d, 16, g2), 4.29 (d, 14, g3), 3.69 (d, 14, g4), 3.10 (d, 16, g5), 2.94
(d, 16, g6), 0.88 (s, j1), 0.82 (s, j2), 0.69 (s, j3). Anal. Calc. for
4.2.5. {j
-N,N0-neoHexN@CH(2-pyridyl)}2Fe(PMe3) (6-H)
A round bottom flask was charged with FeCl2 (100–166 mg) and
sodium amalgam (2.2 equiv). THF (15 mL) and 1.5 equiv PMe3 were
vacuum distilled at ꢀ78 °C, and a solution of imine (2 equiv) in
10 mL THF were added via syringe. The reaction mixture was al-
lowed to stir and slowly warm to 23 °C over 12 h. The volatiles
were removed, 15 mL Et2O was added and the solution was filtered
through Celite. The volatiles were removed and the purple solid
was triturated with Et2O (3 ꢂ 15 mL) and dissolved in 15 mL
Et2O. Green microcrystals of 6-H were grown from concentrated
Et2O and isolated by filtration (300 mg, 45%). 1H NMR (300 MHz,
benzene-d6): d 8.40 (a), 6.95 (b), 6.95 (c), 7.25 (d), 8.90 (f), 4.25
(ga), 3.95 (gb), 1.66 (ha), 1.25 (hb), 0.89 (j), 0.62 (d, 7, l). 13C NMR
(500 MHz, benzene-d6): d 146.24 (a), 123.21 (b), 120.62 (c),
111.44 (d), 153.76 (e), 157.40 (f), 58.15 (g), 48.56 (h), 30.62 (i),
30.15 (j), 14.45 (l). 31P NMR (40 0 MHz, benzene-d6): d 3.65 (s).
Anal. Calc. for C27H45N4PFe: C, 63.28; H, 8.85; N, 10.93. Found: C,
61.31; H, 7.80; N, 10.94%.
C35H48N6F6 O6S2Fe: C, 47.62; H, 5.48; N, 9.52. Found: C, 47.51; H,
5.54; N, 9.48%.
4.3. Polymerization screening procedure
In a typical screening, a reaction vessel was charged with olefin
and a suspension of the iron complex in 25 mL toluene. A solution
of PMAO (270 equiv) in toluene was added via syringe. After the
appropriate reaction time, the reactor was vented, and the mixture
quenched with methanol/HCl to precipitate any polymer present. If
no precipitation occurred, no further characterization was at-
tempted. When the borate additive was employed, a reaction ves-
sel was charged with olefin and a suspension of [Ph3C][B(C6F5)4] in
toluene. A suspension of the iron complex in toluene was added via
syringe. Workup was as described above.
4.2.6. [{
To a cold solution of 6-H (70 mg, 0.136 mmol) in 10 mL THF un-
der an argon blanket was syringed CH3OTf (15 L, 0.136 mmol).
j
-N,N0-neoHexN@CH(2-pyridyl)}2Fe(PMe3)Me]+OTfꢀ (7-H-OTf)
l
The mixture was allowed to stir at ꢀ78 °C for 6 h, then slowly
warmed to 23 °C over 12 h. The dark green solution was triturated
with THF (3 ꢂ 10 mL), and the volatiles were removed to yield
75 mg dark green solid (82%). Green-black crystals were grown
from slow diffusion of Et2O into
a
THF solution. 1H NMR
(400 MHz, THF-d8): d 8.85 (d, 4, a), 7.45 (t, 6, b), 7.81 (t, 7, c),
8.07 (d, 8, d), 9.54 (s, f), 3.89 (m, ga), 3.38 (m, gb), 1.19 (m, ha),
0.41 (m, hb), 0.52 (s, j), 0.14 (d, 10, k), 0.83 (d, 9, l), 8.90 (d, 5, a0),
7.38 (t, 6, b0), 7.68 (t, 7, c0), 7.88 (d, 8, d0), 8.68 (br s, f0), 4.33 (m,
g0a), 3.70 (m, g0 ), 0.99 (m, ha0 ), 0.85 (m, h0 ), 0.45 (s, j0). 13C NMR
(500 MHz, THF-d8): d 154.47 (a), 125.03 (b), 127.76 (c), 134.27
(d), 161.27 (e), 167.26 (f), 57.14 (g), 46.01 (h), 30.21 (i), 29.49 (j),
6.88 (d, 30, k), 12.17 (d, 23, l), 161.52 (a0), 124.04 (b0), 127.59 (c0),
132.35 (d0), 159.98 (e0), 157.76 (f0), 59.25 (g0), 46.83 (h0), 30.37 (i0),
29.59 (j0). 31P NMR (400 MHz, THF-d8): d 13.36 (s). Anal. Calc. for
C29H48N4F3O3PSFe: C, 51.48; H, 7.15; N, 8.28. Found: C, 50.05; H,
6.18; N, 8.22%.
4.4. Mössbauer spectroscopy
57Fe Mössbauer spectra were recorded on a SEE Co. Mössbauer
spectrometer (MS4) at 80 K in constant–acceleration mode. 57Co/
Rh was used as the radiation source. WMOSS software was used
for the quantitative evaluation of the spectral parameters (least-
squares fitting to Lorentzian peaks). The temperature of the
samples was controlled by a Janis Research Co. CCS-850 He/N2
cryostat within an accuracy of 1 K. Isomer shifts were determined
b
b
relative to a-Fe at 298 K.
4.5. Single crystal X-ray diffraction studies
4.2.7. [{j
-N,N0-neoHexN@CH(2-pyridyl)}2Fe(PMe3)Me]+Iꢀ (7-H-I)
Procedure 4.2.6. was followed, except that CH3I was used (76%).
1H NMR (400 MHz, CD2Cl2): d 8.63 (d, 6, a), 7.42 (t, 7, b), 7.79 (m, c),
7.92 (d, 8, d), 9.35 (s, f), 3.79 (t, 11, ga), 3.27 (t, 11, gb), 1.12 (td, 11,5,
ha), 0.41 (td, 13,4, hb), 0.54 (s, j), 0.09 (d, 9, k), 0.85 (d, 8, l), 8.68
(d, 5, a0), 7.28 (t, 7, b0), 7.68 (t, 8, c0), 7.79 (m, d0), 8.58 (d, 6, f0),
4.21 (t, 11, g0a), 3.60 (t, 11, gb0 ), 1.01 (td, 13,6, h0a), 0.36 (td, 14,5,
h0b), 0.51 (s, j0). 31P NMR (400 MHz, CD2Cl2): d 13.10 (s).
Upon isolation, the crystals were covered in polyisobutenes and
placed under a 173 K N2 stream on the goniometer head of a Sie-
mens P4 SMART CCD area detector (graphite-monochromated
MoKa radiation, k = 0.71073 Å). The structures were solved by di-
rect methods (SHELXS). All non-hydrogen atoms were refined aniso-
tropically unless stated, and hydrogen atoms were treated as
idealized contributions (Riding model).
4.2.8. [{j
-N,N0-RN@CH(2-pyridyl)}3Fe][OTf]2 (R = neoPe, 8-P-OTf;
4.5.1. {j
-N,N0-neoPeN@CH(2-pyridyl)}2FeMe2 (2-P)
neoHex, 8-H-OTf)
A black block (0.40 ꢂ 0.25 ꢂ 0.15 mm) was obtained from Et2O.
A total of 17,741 reflections were collected with 5842 determined
to be symmetry independent (Rint = 0.0311), and 4779 were greater
than 2s(I). A semi-empirical absorption correction from equiva-
lents was applied, and the refinement utilized wꢀ1 = s2(Fo2) +
(0.0418p)2 + 0.7577p, where p = ((Fo2 + 2Fc2)/3).
A solution of imine (0.26–0.85 mmol) and Fe(OTf)2 (31–100 mg,
0.088–0.28 mmol) in 15 mL THF was stirred for 4 h, resulting in a
purple precipitate. The purple material was filtered and washed
with CH2Cl2. The CH2Cl2 was removed and the solids were tritu-
rated with Et2O. The solids were slurried in Et2O and filtered to
yield a purple solid. (a) 8-H-OTf: 75 mg pink-purple solid (93%).
The product contained only the mer isomer. 1H NMR (400 MHz,
CD3CN): d 8.27 (d, 7, a1), 8.17–8.19 (m, a2, a3), 7.58 (t, 6, b1), 7.52
(t, 6, b2), 7.44 (t, 6, b3), 8.03–8.12 (m, c1, c2, c3, d1, d2), 7.21 (d, 6,
d3), 9.23 (s, f1), 9.19 (s, f2), 9.04 (s, f3), 3.85 (td, 5, 13, g1), 3.48 (td,
5, 13, g2, g3), 3.27 (td, 4, 13, g4), 3.08 (td, 4, 13, g5, g6), 1.27–1.43
(m, h1, h2, h3), 1.15 (td, 5, 13, h4), 1.04 (td, 4, 13, h5), 0.66 (td, 5,
4.5.2. {j
-N,N0- neoHex N@CH(2-pyridyl)}2Fe(PhCCPh) (5-H)
A red block (0.40 ꢂ 0.30 ꢂ 0.25 mm) was obtained from Et2O/
THF. A total of 29,730 reflections were collected with 3799 deter-
mined to be symmetry independent (Rint = 0.0661), and 2599 were
greater than 2s(I). A semi-empirical absorption correction from

E.C. Volpe et al. / Polyhedron 52 (2013) 406–415
415
equivalents was applied, and the refinement utilized wꢀ1 = s2(Fo2) +
(0.0621p)2 + 0.0000p, where p = ((Fo2 + 2Fc2)/3).
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4.5.3. {j
-N,N0-neoHexN@ CH(2-pyridyl)}2Fe(PMe3) (6-H)
A green block (0.40 ꢂ 0.20 ꢂ 0.10 mm) was obtained from Et2O.
A total of 12,529 reflections were collected with 3761 determined
to be symmetry independent (Rint = 0.0293), and 3272 were greater
than 2s(I). A semi-empirical absorption correction from equiva-
lents was applied, and the refinement utilized
w
ꢀ1 = s2
(Fo2) + (0.0398p)2 + 1.8440p, where p = ((Fo2 + 2Fc2)/3).
4.5.4. [{j
-N,N0-neoHexN@CH(2-pyridyl)}2Fe(PMe3)Me]+OTfꢀ (7-H-OTf)
A red block (0.30 ꢂ 0.20 ꢂ 0.15 mm) was obtained from THF/
hexanes. A total of 25,651 reflections were collected with 8102
determined to be symmetry independent (Rint = 0.0315), and
6436 were greater than 2s(I). A semi-empirical absorption correc-
tion from equivalents was applied, and the refinement utilized
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Acknowledgements
We would like to acknowledge funding from the National Sci-
ence Foundation (CHE-0718030, CHE-1055505) and Cornell Uni-
versity, and thank Prof. Paul J. Chirik for use of the Mössbauer
spectrometer.
Appendix A. Supplementary material
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