First-Row Transition Metal and Lithium Pyridine-ene-amide Complexes Exhibiting N- and C-Isomers and Ligand-Based Activation of Benzylic C-H Bonds

Article abstract of DOI:10.1021/acs.organomet.5b00385

Ene-amines Z-3-(2-pyridyl)-1-aza(2,6-ⁱPr₂-Ph)propene, (pynac)H, and 2-(2-pyridyl)-1-aza(2,6-R,R′-Ph)propene, (pyEA-ArRR′)H, were synthesized by condensation procedures; corresponding lithium or potassium ene-amides were prepared via standard deprotonation protocols. Addition of 2 equiv of (pynac)H to {(Me₃Si)₂N}₂Fe(THF) or 2 Li(pynac) to FeBr₂(THF)₂ afforded (pynac)₂Fe (1), while treatment of CrCl₂(THF)₂, MnCl₂, FeBr₂(THF)₂, and CoCl₂py₄ with 2 equiv of (pyEA-ArⁱPr₂)K afforded pseudotetrahedral (pyEA-ArⁱPr₂)₂M (2-M, M = Cr, Mn, Fe) and (pyEA-ArⁱPr₂)₂Co-py (2-Co-py). Diamagnetic (κ-C,N-pyEA-ArⁱPr₂)₃Co (3) was prepared in low yield (～7%) from CoCl₂, and its Co-C(sp³) linkages are unusually low in field strength. Reactivity studies yielded little clean reactivity, but thermolysis of 2-Co-py afforded the bis-indolamide derivative {κ-N,N-N(C₆H₃(2-ⁱPr)CMe₂C(Me)(2-py)}₂Co (5-Co), and related thermolyses of 2-M (M = Cr, Mn, Fe), conducted on NMR tube scales, generated related 5-M (M = Cr, Mn, Fe) at roughly the same rates. This observation prompted thermolyses of (pyEA-ArRR′)Li, which rearrange to their corresponding indolamides in >90% yields. Rate studies, accompanied by KIE and EIE observations, revealed that an initial hydrogen transfer is reversible and is likely to correspond to an anionic rearrangement, whereas C-C bond formation is rate-determining, as suggested by accompanying calculations. X-ray structure determinations of 1, 2-Fe, 2-Co-py, 3, and 5-Co were conducted.

Full text of DOI:10.1021/acs.organomet.5b00385

Article
pubs.acs.org/Organometallics
First-Row Transition Metal and Lithium Pyridine-ene-amide
Complexes Exhibiting N- and C‑Isomers and Ligand-Based Activation
of Benzylic C−H Bonds
Brian M. Lindley,† Peter T. Wolczanski,*,† Thomas R. Cundari,‡ and Emil B. Lobkovsky†
†Department of Chemistry & Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853, United States
‡Department of Chemistry, Center for Advanced Scientific Computing and Modeling (CASCaM), University of North Texas,
Box 305070, Denton, Texas 76203-5070, United States
S
* Supporting Information
ABSTRACT: Ene-amines Z-3-(2-pyridyl)-1-aza(2,6-iPr2-Ph)propene, (pynac)H, and 2-(2-pyridyl)-1-aza(2,6-R,R′-Ph)propene,
(pyEA-ArRR′)H, were synthesized by condensation procedures; corresponding lithium or potassium ene-amides were pre-
pared via standard deprotonation protocols. Addition of 2 equiv of (pynac)H to {(Me3Si)2N}2Fe(THF) or 2 Li(pynac) to
FeBr2(THF)2 afforded (pynac)2Fe (1), while treatment of CrCl2(THF)2, MnCl2, FeBr2(THF)2, and CoCl2py4 with 2 equiv of (pyEA-
AriPr2)K afforded pseudotetrahedral (pyEA-AriPr2)2M (2-M, M = Cr, Mn, Fe) and (pyEA-AriPr2)2Co-py (2-Co-py). Diamagnetic
(κ-C,N-pyEA-AriPr2)3Co (3) was prepared in low yield (∼7%) from CoCl2, and its Co−C(sp3) linkages are unusually low in field
strength. Reactivity studies yielded little clean reactivity, but thermolysis of 2-Co-py afforded the bis-indolamide derivative {κ-N,N-
N(C6H3(2-iPr)CMe2C(Me)(2-py)}2Co (5-Co), and related thermolyses of 2-M (M = Cr, Mn, Fe), conducted on NMR tube scales,
generated related 5-M (M = Cr, Mn, Fe) at roughly the same rates. This observation prompted thermolyses of (pyEA-ArRR′)Li, which
rearrange to their corresponding indolamides in >90% yields. Rate studies, accompanied by KIE and EIE observations, revealed that an
initial hydrogen transfer is reversible and is likely to correspond to an anionic rearrangement, whereas C−C bond formation is rate-
determining, as suggested by accompanying calculations. X-ray structure determinations of 1, 2-Fe, 2-Co-py, 3, and 5-Co were conducted.
character, leading to C−C bond formation (C),40 but in related
INTRODUCTION
■
tetradentate ligands, electrostatic stabilization of a 14e− π-system
Transition metal compounds containing pyridine-imine (PI)1−6
afforded very stable Fe(II) complexes (D).41,42 Finally, a
and pyridine-diimine (PDI)7−19 moieties often exhibit redox
tetradentate di-PI ligand revealed five redox states (E) that were
noninnocent (RNI)20−29 behavior due to multiple accessible
quite stable, while the metal formally remained Ni(II).6
charged states of the ligands. This capacity is most evident in
first-row transition metals, where the ionic character of the
metal−ligand bond limits charge distribution via covalency, and
The successful implementation of 2-azaallyls34,35,40,43 in bond-
forming processes44−47 prompted an investigation into correspond-
ing 1-azaallyls,15,46−49 recognizing that such entities are ene-amides
in early transition metals29−32 that have limited redox capability.
that can be readily prepared via condensation routes. Herein is
Ligands designed as PI variants consisting of 2-azaallyls, their
described an initial study of 1-azaallyl ligands, with pyridine utilized
in its usual supporting role in bidentate chelation.
precursors, or related chelates have led to intriguing carbon−
carbon and C−X bond-forming reactions and afford examples of
RNI, as illustrated in Figure 1.
Compounds containing 1,3-di-2-pyridyl-2-azaallyl (smif)33
tridentate ligands exhibit reversible and irreversible C−C bond-
forming reactions depending on steric factors (A),34 while the
generation of transient azaallyls within a tetradentate chelate have
afforded [{Me2C(CHNCH-2-py)2}M]2 (M = Cr, Co, Ni) dimers
wherein three new carbon−carbon bonds have been formed around
unique metal−metal bonds (B).35 Incorporating PI precursors into
a nacnac framework36−39 permitted the isolation of carbon radical
RESULTS
■
Ligand Syntheses. The 1-azaallyl ligands used in this study
were prepared as illustrated in Scheme 1. Treatment of 2,6-diisopropyl
Special Issue: Gregory Hillhouse Issue
Received: May 6, 2015
© XXXX American Chemical Society
A
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Figure 1. Pyridine-imine (PI) redox states (red box) and corresponding azaallyl ligand variations: (A) 1,3-di-2-pyridyl-2-azaallyl (smif) compounds
exhibiting C−C bond couplings that can be construed as diradical couplings; (B) tetradentate precursors to 2-azaallyls are activated by metal diamides to
afford three new C−C bonds and six stereocenters in the generation of metal−metal-bonded complexes; (C) incorporating oxidatively destabilized
pyridine-imines within a nacnac group affords C−C coupling; (D) PIs within nacnacs are electrostatically stabilized by Fe(II) and through (4n+2)e−
π-systems; and (E) electrostatically stabilizing pyridine-imines with Ni(II) inhibits reactivity.
Scheme 1
aniline with triethyl orthoformate afforded, upon thermolysis,
the 2,6-diisopropyl-ethoxyimine as a colorless oil in 78% yield.50
2-Lithiomethylpyridine was generated via deprotonation of
2-picoline with LDA and treated with the 2,6-diisopropyl-
ethoxyimine51 to prepare Z-3-(2-pyridyl)-1-aza(2,6-iPr2-Ph)-
propene, (pynac)H, as a light brown solid in 98% yield.
B
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Scheme 2
Standard condensation procedures utilizing 2-acetylpyridine
and 2,6-R,R′-anilines provided the pyridine-methylimines in
good to very good yields, and subsequent deprotonation of a
methylimine proton with LiHMDS or KH led to the lithium and
potassium ene-amides, (pyEA-ArRR′)Li (R, R′ = Me, Et, iPr; R =
Me (CD3), R′ = H) and (pyEA-AriPr2)K, respectively. Somewhat
surprisingly, the ene-amides possessed reproducible, vibrant
colors depending on substituent and main group metal. While
the spectral properties of the species were not studied, it is
worthwhile noting that related di-2-pyridyl-2-azaallyl main group
species are an intense maroon color whose larger crystals are
bronze/gold.33,34 The 1-azaallyl species in Scheme 1 were not as
intense, and their solid colors matched those in solution.
Synthesis of (pynac)2Fe (1). The initial ligand entry into
1-aza-allyl complexes was the pyridyl-based nacnac analogue
pynac. A ferrous derivative containing two pynac ligands was
prepared via two metathesis routes: (1) protonation of
{(Me3Si)2N}2Fe(THF)52 with the free ligand,33−35,40−42 which
afforded (pynac)2Fe (1) in 85% yield, and (2) salt metathesis of
FeBr2(THF)2 with Li(pynac), prepared in situ from (pynac)H
and LDA, which provided 1 in 77% yield. The red complex was
high spin (μeff (Evans’s)53 = 5.2(1) μB) and relatively devoid of
substantive, clean reactivity, despite being exposed to numerous
reagents. Incorporation of the azaallyl into a nacnac analogue was
deemed counterproductive from a reactivity standpoint; hence a
change to ene-amide coordination was warranted.
Structure of (pynac)2Fe (1). A molecular view of (pynac)2Fe
(1) is given in Figure 2, along with pertinent interatomic metric
parameters, and Table 1 lists selected data acquisition and refine-
ment information. The complex is a highly distorted tetrahedron40
caused by pynac bite angles (93.5(7)° av) that are substantially less
than 109.5°. The bulky 2,6-diisopropylphenyl substituents cause
the nitrogens of the pynac imines to splay apart at 128.63(6)°,
whereas the nitrogens of the smaller py components are separated
by only 92.67(6)°. The remaining Nim−Fe-Npy angles average
122.4(47)°; hence the structure is roughly C2-symmetric. The
Fe−N distances corresponding to the pyridines (2.069(2) Å av)
and imines (1.976(2) Å av) are normal in comparison to related
ligands and are consistent with the high-spin ferrous environment.
Syntheses of Ene-amides (pyEA-AriPr2)2M (2-M, M = Cr,
Mn, Fe, Co-py). The shift to ene-amide-pyridine derivatives
was accomplished by salt metathesis, and Scheme 3 summarizes
the results. CrCl2(THF)2, MnCl2, and FeBr2(THF)2 were treated
with 2 equiv of the bulky potassium diisopropyl-eneamide, (pyEA-
AriPr2)K, in THF, and pseudotetrahedral (pyEA-AriPr2)2M (2-M,
M = Cr, Mn, Fe) were prepared in 61−68% yields. All derivatives
were high spin according to magnetic measurements using Evans’s
method,53 with near spin-only μeff values of 4.7(1), 5.5(1), and
5.2(1) μB for 2-Cr, 2-Mn, and 2-Fe, respectively. Interestingly,
treatment of CoCl2 with potassium or lithium ene-amides failed to
cleanly provide the bis-chelate derivatives obtained for the other
Figure 2. Molecular view of (pynac)2Fe (1) and selected interatomic
distances (Å) and angles (deg): Fe1−N1, 2.0678(15); Fe1−N2,
1.9777(15); Fe1−N3, 2.0707(15); Fe1−N4, 1.9743(14); N1−C5,
1.358(2); C5−C6, 1.414(3); C6−C7, 1.370(3); N2−C7, 1.348(2);
N2−C8, 1.434(2); N3−C24, 1.354(2); C24−C25, 1.420(3); C25−
C26, 1.367(3); N4−C26, 1.335(2); N4−C27, 1.441(2); N1−Fe1−N2,
94.00(6); N1−Fe1−N3, 92.67(6); N1−Fe1−N4, 119.06(6); N2−
Fe1−N3, 125.68(6); N2−Fe1−N4, 128.63(6); N3−Fe1−N4, 93.01(6).
metals, but a red-brown pyridine adduct was generated in 57%
yield when CoCl2py4 was utilized as the starting material. Its μeff
was 3.9(1) μB, consistent with an S = 3/2 ground state, and it is
depicted in Scheme 3 as a trigonal bipyramid, as its structural
study indicates.
Structure of (pyEA-AriPr2)2Fe (2-Fe). Data acquisition and
refinement parameters for (pyEA-AriPr2)2Fe (2-Fe) are listed in
Table 1, and its molecular view in Figure 3, with accompanying
metric values, shows a distorted tetrahedral40 arrangement
similar to (pynac)2Fe (1). Bite angles of the pyridine ene-amide
average 79.5(2)°, which allows the nitrogens of the bulky 2,6-
diisopropylphenylamide components to be 136.70(7)° apart,
while those of the pyridines are 100.94(7)°. The remaining
Npy−Fe−Nam angles average 130.6(35)°, as the small bite angles
permit a greater value than in 1, and all core angles are in accord
with a C2-structure. Iron−nitrogen distances corresponding to
the pyridines and amides average 2.114(12) and 1.948(7) Å,
respectively, and are typical for a high-spin environment.
There is little evidence of imine character to the ene-amides, as
d(C6−C7) and d(C24−C25) are 1.340(3) and 1.345(3) Å,
respectively, consistent with standard carbon−carbon double
bonds adjacent to nitrogen.54 Likewise, the NamC distances of
1.374(3) Å are in line with N(sp2)−C(sp2) bond lengths,54 as are
the Nam−Car distances of 1.429(2) and 1.425(2) Å.
Structure of (pyEA-AriPr2)2Co-py (2-Co-py). Data acqui-
sition and refinement information for (pyEA-AriPr2)2Co-py
(2-Co-py) are provided in Table 1, while structural metrics are
given in the caption to Figure 4, which contains a molecular view
C
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Table 1. Select Crystallographic and Refinement Data for (pynac)2Fe (1), (pyEA-AriPr2)2Fe (2-Fe), (pyEA-AriPr2)2Co-py (2-Co-py),
(κ-C,N-pyEA-AriPr2)3Co (3), and {κ-N,N-N(C6H3(2-iPr)CMe2C(Me)(2-py)}2Co (5-Co)
a
1
2-Fe
2-Co-py
3
5-Co
formula
fw
C38H46N4Fe
614.64
C38H46N4Fe
614.64
C47H61N5OCo
770.94
C57H69N6Co·(C6H6)0.5
936.17
C38H46N4Co
617.72
space group
Z
P21/n
P21/n
P21/n
R3
P1
̅
̅
4
4
4
6
4
a, Å
10.6647(10)
14.9567(14)
21.981(2)
90
12.0967(8)
14.8011(11)
20.1037(14)
90
11.3721(5)
23.8329(11)
16.3006(7)
90
19.691(2)
19.691(2)
24.631(3)
90
9.7996(7)
9.8387(7)
38.267(3)
87.205(4)
83.381(4)
89.069(4)
3660.3(5)
1.121
b, Å
c, Å
α, deg
β, deg
101.760(4)
90
107.236(3)
90
101.587(2)
90
90
γ, deg
V, Å3
ρcalc, g cm−3
μ, mm−1
temp, K
λ (Å)
120
3432.6(6)
1.189
3437.8(4)
1.188
4327.9(3)
1.183
8271.2(16)
1.128
0.470
0.469
0.436
0.353
0.498
173(2)
213(2)
203(2)
203(2)
173(2)
0.710 73
R1 = 0.0359
wR2 = 0.0873
R1 = 0.0517
wR2 = 0.0971
1.021
0.710 73
R1 = 0.0463
wR2 = 0.1109
R1 = 0.0786
wR2 = 0.1267
1.022
0.710 73
R1 = 0.0370
wR2 = 0.0923
R1 = 0.0539
wR2 = 0.1015
0.710 73
R1 = 0.0508
wR2 = 0.1193
R1 = 0.1254
wR2 = 0.1710
1.067
0.710 73
R1 = 0.0598
wR2 = 0.1251
R1 = 0.0702
wR2 = 0.1287
1.166
bc
,
R indices [I > 2σ(I)]
bc
,
R indices (all data)
d
GOF
1.012
a
b
c
d
2
Contains Et2O of solvation. R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = [∑w(|Fo| − |Fc|)2/∑wFo ]1/2
. ,
GOF(all data) = [∑w(|Fo| − |Fc|)2/(n − p)]1/2
n = number of independent reflections, p = number of parameters.
Scheme 3
of the C2-symmetric compound. The N1−Co−N3 angle of
172.64(4)° and corresponding N2−Co−N4 angle of 135.83(4)°
lead to an Addison parameter of τ = 0.61,55 a value consistent with
a distorted trigonal bipyramidal geometry where the pyridine and
two amides lie in the equatorial plane and the two chelate-
pyridines are axial. Amide Co−N distances of 1.968(4) Å (av),
chelate-pyridine Co−N bond lengths of 2.183(4) Å (av), and the
pyridine d(Co−N) of 2.1148(11) Å are normal for Co(II) in a
high-spin environment. There is a slight twist to the axial axis
due to 77.91(11)° (av) chelate bite angles and correspond-
ing N2−Co−N3 and N1−Co−N4 angles of 99.05(4)° and
99.55(4)°, respectively. The nitrogen of the unique equatorial
pyridine is 93.7(3)° (av) and 112.1(22)° (av) from the nitrogens
of the axial chelate-pyridines and equatorial amides, respectively.
D
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(3), is an uncommon species in coordination chemistry, as
the vast majority of organometallic Co(III) alkyls are supported
by additional strong-field carbon-based ligands such as cyclo-
pentadienyls.
Figure 3. Molecular view of (pyEA-AriPr2)2Fe (2-Fe) and selected
interatomic distances (Å) and angles (deg): Fe1−N1, 2.1050(17); Fe1−
N2, 1.9532(16); Fe1−N3, 2.1221(17); Fe1−N4, 1.9427(15); N1−C5,
1.342(3); C5−C6, 1.489(3); C6−C7, 1.340(3); N2−C6, 1.374(3);
N2−C8, 1.429(2); N3−C24, 1.348(3); C24−C25, 1.482(3); C25−
C26, 1.345(3); N4−C25, 1.374(3); N4−C27, 1.425(2); N1−Fe1−N2,
79.66(7); N1−Fe1−N3, 100.94(7); N1−Fe1−N4, 128.05(7); N2−
Fe1−N3, 133.07(7); N2−Fe1−N4, 136.70(7); N3−Fe1−N4,
79.37(6).
Figure 5. UV−vis spectrum of (κ-C,N-pyEA-AriPr2)3Co (3) taken in
THF. Concentrations are 10 μM (blue), 100 μM (red), and 200 μM
(green).
Shown in Figure 5, the UV−vis spectrum of (κ-C,N-pyEA-
AriPr2)3Co (3) exhibits two bands at 370 and 493 nm that have
relatively strong intensities of 5600 and 3400 M−1 cm−1,
respectively. Normally such intensities would be attributable to
MLCT or LMCT transitions, but a TDDFT calculation on a
truncated model of 3 (AriPr2 replaced by Me) supports their
1
1
Figure 4. Molecular view of (pyEA-AriPr2)2Co-py (2-Co-py) and
selected interatomic distances (Å) and angles (deg): Co−N1,
2.1864(10); Co−N2, 1.9648(10); Co−N3, 2.1802(10); Co−N4,
1.9709(10); Co−N5, 2.1148(11); N2−C6, 1.3718(16); C6−C7,
1.356(2); C5−C6, 1.4881(18); N1−C5, 1.3481(16); N4−C25,
1.3750(15); C25−C26, 1.3566(18); C24−C25, 1.4878(18); N3−
C24, 1.3467(16); N1−Co−N2, 77.95(4); N1−Co−N3, 172.64(4);
N1−Co−N4, 99.55(4); N1−Co−N5, 93.48(4); N2−Co−N3,
99.05(4); N2−Co−N4, 135.83(4); N2−Co−N5, 110.55(4); N3−
Co−N4, 77.83(4); N3−Co−N5, 93.87(4); N4−Co−N5, 113.61(4);
N2−C6−C7, 126.65(13); N2−C6−C5, 113.28(11); C5−C6−C7,
120.06(13); N4−C25−C26, 126.60(12); N4−C25−C24, 113.17(10);
C24−C25−C26, 120.23(12).
tentative assignments as the A → (A,E) absorptions that
1
correlate to the A1g → 1T1g and 1A1g → 1T2g d−d bands in an
octahedral complex. All CT absorptions are calculated to be at
much higher energy (>35 000 cm−1). The lower symmetry,
which renders both transitions electric dipole-allowed, and
significant intensity stealing from the tail of an MLCT band in the
ultraviolet region are responsible for their rather high extinction
coefficients. Since each axis in the molecule is identical, Δoct
and B values for 3 were estimated from interpolation of a d6
Tanabe−Sugano diagram.56 From a Δ/B ratio of ∼45, Δoct is
∼22 000 cm−1 and B is ∼483 cm−1, which is 44% of the Co3+ free
ion value of 1100 cm−1 and fully consistent with the high
covalency expected for a complex containing three Co−C bonds.
The Δoct for a hypothetical Co(py)63+ complex can be estimated
as ∼24 000 cm−1;56 hence application of the rule of average
environment would suggest that the field strength of the sp3-
alkyls in 3 would amount to Δoct ≈ 20 000 cm−1 for “CoR63−”.
Structure of (κ-C,N-pyEA-AriPr2)3Co (3). A molecular view
of (κ-C,N-pyEA-AriPr2)3Co (3) and its accompanying metric
parameters are given in Figure 6, and selected information
regarding data acquisition and refinement is available in Table 1.
The compound crystallizes in the hexagonal system and has the
As in 2-Fe, the chelate bond distances are descriptive of 1-aza-allyls
with clear CC bonds.54
Synthesis of (κ-C,N-pyEA-AriPr2)3Co (3). When CoCl2 was
used as the starting material, a persistent diamagnetic product
was isolated as a red-purple precipitatedespite its low yield
1
(∼7%)due to its lesser solubility in hydrocarbons. H NMR
spectroscopic studies revealed a single type of ene-amide ligand
with diastereotopic methylene hydrogens and four distinct
isopropyl methyl groups. With consideration of its solubility, the
NMR spectrum hinted at a highly symmetric aggregate structure
or a rigid, mononuclear Co(I) or Co(III) species. An X-ray
diffraction structural examination of the complex showed it to be
a C3-symmetric monomer containing three Co−C bonds, which
are a consequence of the linkage isomerization available to the ene-
amide ligand. Formally a Co(III) tri-sp3-alkyl, (κ-C,N-pyEA-AriPr2)3Co
space group R3, which translates into having one unique κ-C,N-
̅
pyEA-AriPr2 ligand and C3 local symmetry. The cobalt−carbon
distance is 1.963(4) Å, and the d(Co−Npy) is 2.001(3) Å, values
that are consistent with the diamagnetic ground state accorded
Co(III). The bite angle for κ-C,N-pyEA-AriPr2 is 89.39(16)°,
the N−Co−N angle is 95.80(11)°, and the C−Co−C angle is
E
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addition of dihydrogen to the methylene functionality of 2′-Fe
showed each step to be slightly favorable, with the first
ΔG = −7.2 kcal/mol and the second equal to −2.4 kcal/mol.
The enthalpy changes (BDE(H2) = 104 kcal/mol)57 suggest that
the first C−H bond formed has a bond dissociation energy
(BDE) of ∼63 kcal/mol, and the second is ∼59 kcal/mol,
modest values that reflect disruption of the pyridine-imine RNI
character upon HAT. For comparison, note that the BDE of
H−CH2CO(R)57 and, presumably, a corresponding imine is
∼94 kcal/mol. How much of the roughly 30 kcal/mol decline in
BDE is due to RNI vs the effects of coordination is unknown,
but it is significantly attenuated. Numbers for the analogous
Co compound, 2′-Co, are similar, although the second HAT is
endoergic.
The calculations suggested that dehydrogenation of 9,10-
dihydroanthracene by (pyEA-AriPr2)2Fe (2-Fe) is slightly
downhill, although the first HAT is likely to be ∼15 kcal/mol
unfavorable; yet no observed transfer of H2 was noted upon
extended thermolysis. The same rough thermodynamics apply to
HAT from HSnnBu3, and this reagent was unreactive as well.
While it is possible that the degradation noted upon thermolysis,
later recognized as a rearrangement, was promoted by HAT, no
byproducts were observed that support this contention, and
stable bis-pyridine-imine Fe(II) complexes have been prepared.1
One reaction that appeared promising was the thermolysis of
(pyEA-AriPr2)2Fe (2-Fe) in the presence of tBuI, which afforded
a mixture of paramagnetic material (4) and isobutylene, con-
sistent with the elimination of HI. It is conceivable that the
products resulted from HAT,18,19,58 yet thermolyses of 2-Fe and
tBuX (X = Cl, Br) failed to elicit related products, prompting a
study of the thermal stabilities of 2-M.
Figure 6. Molecular view of (κ-C,N-pyEA-AriPr2)3Co (3), whose
isopropyl methyl groups have been removed for clarity, and selected
interatomic distances (Å) and angles (deg): Co1−N1, 2.001(3); Co1−
C6, 1.963(4); N1−C5, 1.342(5); N1−C1, 1.365(4); C1−C7, 1.476(5);
C6−C7, 1.502(5); N2−C7, 1.282(4); N1−Co1−N1, 95.80(11); C6−
Co1−C6, 89.30(18); N1−Co1−C6 (bite angle), 89.39(16); N1−Co1−
C6, 85.37(14); N1−Co1−C6, 174.53(17); Co1−N1−C1, 114.6(2);
N1−C1−C7, 114.3(3); C1−C7−N2, 117.0(3); C1−C7−C6, 115.5(3);
C6−C7−N2, 127.5(3); Co1−C6−C7, 109.8(3).
89.30(18)°, rendering the molecule nearly octahedral. The
Co−C−C angle of 109.8(3)°, the C6−C7 distance of 1.502(5) Å,
and the 360° sum of the angles about C7 are consistent with
a C(sp2)−C(sp3)−Co linkage in the chelate.54 The C7−N2
distance of 1.282(4) Å is short and ascribed to an imine, and
d(C1−C7) = 1.476(5) Å, in line with an sp2−sp2 connection.54
In summary, the cobalt−carbon interaction appears to be purely
sigma and corresponds to an α-imino-carbyl.
Thermolyses of Ene-amides (pyEA-AriPr2)2M (2-M, M =
Cr, Mn, Fe, Co-py). In testing the thermal stability of ene-
amides (pyEA-AriPr2)2M (2-M, M = Cr, Mn, Fe, Co-py), a
surprising rearrangement occurred as illustrated in eqs 1 and 2.
High-temperature extended thermolysis of 2-Co-py affected the
addition of two methine C−H bonds across the carbon−carbon
double bonds of the ene-amides, affording the bis-indolamide
{κ-N,N-NC6H3(6-iPr)CMe2C(Me)(2-py)}2Co (5-Co, eq 1), in
Reactivity of Ene-amides (pyEA-AriPr2)2M (2-M, M = Cr,
Mn, Fe, Co-py). The transfer of a hydrogen atom (HAT) to the
ene-amide methylene group would generate a pyridine-imine
that is likely to be a radical anion, as this ligand is known for its
redox noninnocent behavior in stabilizing pseudotetrahedral
M(II) complexes.1 The feasibility of HAT to (pyEA-AriPr2)2Fe
(2-Fe) was assessed by calculation (primes) and tested by
experiment, as illustrated in Scheme 4. A hypothetical, stepwise
Scheme 4
F
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Article
distances and angles. The distorted tetrahedral C2 core of 5 is
related to those of (pynac)2Fe (1) and (pyEA-AriPr2)2Fe (2-Fe),
again reflecting the bite angle of the pyridine-amide chelate
(82.21(11)°, 82.44(11)°). Nitrogens of the pyridine ligands are
closer to each other (N1−Co1−N3 = 111.37(12)°) than those of
the amides (N2−Co1−N4 = 148.41(12)°), presumably due to the
same steric factors that affect the similarly distorted structures of 1
and 2-Fe. The disparity is somewhat greater, probably because of
the tertiary carbon center resulting from C−H bond activation by
the ene-amide. All of the carbon−carbon and carbon−nitrogen
bonds within the chelate are normal, as are the d(Npy−Co) and
d(Nam−Co) of 2.051(3) and 1.908(3) Å, respectively.
Table 2. Kinetics of Li(pyEA-ArRR′) (ArRR′ = 2,6-R2-C6H3
(R = iPr, Et, Me), 2-Me-C6H4) and (pyEA-AriPr2)2M (2-M, M =
Cr, Mn, Fe, Co-py) Rearrangement to Corresponding
Indolamides Li{κ-N,N-NC6H3(6-R)CR′R″C(Me)(2-py)) (R =
iPr, R′ = R″ = Me; R = Et, R′ = H, R″ = Me; R = Me, R′ = R″ = H;
R = R′ = R″ = H) and {κ-N,N-NC6H3(6-iPr)CMe2C(Me)(2-
py)}2M (5-M, M = Cr, Mn, Fe, Co)
an essentially clean (>95%) conversion. Related thermolyses
of 2-M (M = Cr, Mn, Fe), performed on NMR tube scales in
C6D6 or THF-d8, gave evidence of similar transformations to
{κ-N,N-NC6H3(6-iPr)CMe2C(Me)(2-py)}2M (5-M; M = Cr,
Mn, Fe, eq 2), according to NMR spectroscopy and aqueous
quenching studies. After 36 h at 140 °C in benzene, the Cr, Mn,
and Fe complex rearrangements were estimated to have occurred
to 83%, 80%, and 44% conversion, according to aqueous
quenching studies.
The generic nature of the rearrangement and observations of
similar C−H additions across ligands with related character-
istics59−62 prompted an investigation into corresponding lithium
ene-amides, as eq 3 indicates. Ene-amides with 2,6-R,R′-aryl
T
k
ΔG⧧
solv (°C( 1)) (×104 s−1) (kcal/mol)
cmpd
a
Li(pyEA-ArMe2)
THF-d8
THF-d8
THF-d8
THF-d8
THF-d8
THF-d8
THF-d8
THF-d8
THF-d8
THF-d8
THF-d8
THF-d8
THF-d8
THF-d8
100
120
130
140
150
130
130
130
130
130
130
130
130
130
130
130
0.072(1) 30.8(1)
a
a
a
a
Li(pyEA-ArMe2)
Li(pyEA-ArMe2)
Li(pyEA-ArMe2)
Li(pyEA-ArMe2)
Li(pyEA-ArEt2)
Li(pyEA-AriPr2)
0.47(1)
1.2(1)
2.7(1)
5.9(1)
6.3(1)
31.0(1)
31.1(1)
31.2(1)
31.3(1)
29.7(1)
0.019(1) 34.4(1)
b
Li(pyEA-o-tol)
0.29(1)
0.25(1)
0.33(1)
0.27(1)
32.2(1)
32.3(1)
32.1(1)
32.3(1)
b
Li(py(EA-o-tol)-d5)
c
Li(pyEA-o-tol)
c
Li(pyEA-o-tol-d3)
d
(pyEA-AriPr2)2Cr (2-Cr)
0.040(4) 33.8(1)
de
,
(pyEA-AriPr2)2Mn (2-Mn)
d
(pyEA-AriPr2)2Fe (2-Fe)
0.06(2)
0.13(1)
33.5(3)
32.8(1)
df
,
(pyEA-AriPr2)2Co-py (2-Co-py) THF-d8
g
g
THF-d8
0.034(6) 33.9(2)
a
b
d
From an Eyring plot, ΔH⧧ = 26.9(2) kcal/mol, ΔS⧧ = −10.3(5) cal/mol·K.
c
Tandem runs: kH/kD5 = 1.16(9). Tandem runs: kHkD3 = 1.22(9).
Analytical problems associated with NMR spectral integration of
e
paramagnetic substances hampered accuracies. Overlapping, broad
resonances prevented analysis by H NMR integration. For 5-Co, py
f
1
inhibits the rate of rearrangement, indicating that its dissociation is not
g
rate-determining. Conducted with 10 equiv of py present.
(R, R′ = Me, Et, iPr; R = Me (CD3), R′ = H) substitution proved
to rearrange on similar time scales, and each generated an
indolamide with a quaternary center adjacent to the amide
nitrogen, which renders the transformation potentially useful.
Since the product amide can serve as a base to deprotonate an
imine, the rearrangement to the indoline can be catalyzed, and
LiN(SiMe3)2 can be used as indicated in eq 4. Unfortunately, the
reaction is quite sluggish at 140 °C, and since generating pure lithium
ene-amide is inexpensive, the stoichiometric reaction is preferable.
Attempts to expand the scope of the cyclization were made via
replacement of the pyridine with an aryl group. Clean rearrange-
ments were no longer observed, and while withdrawing groups other
than pyridine may be effective, they have not been assessed.
i
Ene-amide (Li(pyEA-ArR2) (R = Me, Et, Pr) and (pyEA-
AriPr2)2M (2-M, M = Cr, Mn, Fe, Co-py)) Rearrangement
i
Kinetics. Kinetics of the Li(pyEA-ArR2) (R = Me, Et, Pr)
rearrangement to the corresponding indolamide eq 3 were
conducted in THF-d8 after efforts in C6D6 revealed biexponential
decays that hinted at potential aggregation issues or interference
from the product. In THF-d8, the kinetics exhibited smooth first-
order decays, and the rate constants are given in Table 2. A study
of the temperature dependence of the rearrangement from 100 to
150 °C afforded activation parameters of ΔH⧧ = 26.9(2) kcal/mol
and ΔS⧧ = −10.3(5) eu, indicative of substantial bond breaking in a
transition state that is moderately ordered.
Calculations (vide infra) prompted an investigation into the
kinetic isotope effect (KIE) for the rearrangement, and the rate of
Li(pyEA-o-tol) was measured relative to (2-py)C(CD2)NLi-
(C6H4-o-CD3) in a tandem kinetics experiment (kH/kD5, eqs 5, 6).
Somewhat surprisingly, the kH/kD5 obtained was 1.16(9), a value
Structure of {κ-N,N-NC6H3(6-iPr)CMe2C(Me)(2-py)}2Co
(5-Co). Information pertaining to data acquisition and refine-
ment for {κ-N,N-NC6H3(6-iPr)CMe2C(Me)(2-py)}2Co (5-Co)
are given in Table 1, while molecular views of both enantiomers of
5 are illustrated in Figure 7, accompanied by selected interatomic
G
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Article
measured relative to the all-protio material. As Scheme 5 reveals,
scrambling was detected between the o-CD3-tolyl and the
methylene of the eneamide, and the overall phenomenological
kH/kD3 was ∼1.22(9).
A rough analysis of the equilibrium isotope effect (EIE)63 was
1
conducted by H (0.89 H in C(H/D)2 vs 1.11 H in o-Me)
and 2D (1.1 D in C(H/D)2 vs 1.9 D in o-Me) NMR spectral
integration of the starting material once equilibrium was
established during the course of rearrangement. The values
deviate from those predicted from a purely statistical distribution
(A:B:C = 1:6:3; 0.8 H in C(H/D)2 vs 1.2 H in o-Me;1.2 D in
C(H/D)2 vs 1.8 D in o-Me) and lead to an EIE (z/z′ in
Scheme 5) of 1.20, which was calculated assuming secondary
isotope effects are negligible and that the methylene positions
were indistinguishable. The data show that transfer of hydrogen
is unlikely to be the rate-determining step. Unfortunately, the
intrinsic KIE associated with the hydrogen transfer cannot be
assessed because intramolecular KIE experiments (i.e., 2,6-
CH3,CD3-C6H3) that would provide the number are upset by the
scrambling process.
commensurate with a secondary KIE.63 A value substantially
larger was expected for a mechanism in which hydrogen atom,
hydride, or proton transfer was rate-determining. In support, the
rearrangement of (2-py)C(CH2)NLi(C6H4-o-CD3) was also
Scheme 5
Figure 7. (a) Enantiomeric {κ-N,N-NC6H3(6-iPr)CMe2C(Me)(2-py)}2Co (5-Co (S,S), upper left, and (R,R), lower right) molecules in the asymmetric
unit. (b) Molecular view of 5-Co (S,S) and selected interatomic distances (Å) and angles (deg): Co1−N1, 2.051(3); Co1−N2, 1.908(3); Co1−N3,
2.050(3); Co1−N4, 1.907(3); N1−C5, 1.339(4); N3−C24, 1.342(4); C5−C6, 1.515(5); C24−C25, 1.524(5); C6−C14, 1.538(4); C25−C33,
1.538(5); N2−C6, 1.492(4); N4−C25, 1.481(4); N2−C13, 1.385(4); N4−C32, 1.385(4); N1−Co1−N2, 82.21(11); N1−Co1−N3, 111.37(12); N1−
Co1−N4, 116.19(11); N2−Co1−N3, 116.07(12); N2−Co1−N4, 148.41(12); N3−Co1−N4, 82.44(11).
H
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Scheme 6
The nature of the hydrogen transfer is hidden by its occurrence
within the preequilibrium; hence the varied rates (130 °C)
corresponding to the aryl substituents (e.g., Et (ΔG⧧ = 29.7(1)
such as cyclopentadienyls and/or phosphines, 3 is a rare example
of a cobalt tris-hydrocarbyl species. Berben and Long have pre-
64
3−
pared Co(CCSiMe3)6
and via analysis of its UV−vis spec-
i
kcal/mol) > Me (ΔG⧧ = 31.1(1) kcal/mol) ≫ Pr (ΔG⧧ =
trum concluded that Δoct = 32 500 cm−1 and B ≈ 516 cm−1,
which is 47% of the Co3+ free ion value (1100 cm−1). The
34.4(1) kcal/mol)) cannot be attributed to a clear trend. It is
tempting to conclude that steric factors are important, given the
higher activation energy for the isopropyl, but the observation of
Et > Me may invoke additional factors. In examining HAT vs
H+ vs H− transfer mechanisms, it becomes clear that conjuga-
tion within the system leads to the same intermediates in each
case (vide infra), which likely obscures any particular character
of the transfer. For the reversible transfer of the hydrogen, the
perpendicular orientation of the ArR2 plane relative to the
LiN,Npy plane suggests that the o-Me C−H bond is already
directed toward the methylene unit; hence a large reorganization
energy is not necessary. Since the reversible hydrogen exchange
between CH2 and −CH3 is unlikely to possess a significant
entropy change, the ΔS⧧ of −10.3(5) eu likely reflects the
orientation required for C−C bond formation.
The transition metal rearrangements are slightly more
sluggish than those with lithium, and the ΔG⧧’s are roughly
33.4(5) kcal/mol. The data are fairly crude, as overlapping
resonances in the paramagnetic compounds hampered the
accuracy of NMR spectral integrations, and that of (pyEA-
AriPr2)2Co-py (2-Co) was shown to be inhibited by added
pyridine, consistent with reversible dissociation of py prior to
rearrangement steps. Note that no intermediates were detected
in the transition metal rearrangements, suggesting that the
second ene-amide to indolamide rearrangement occurs faster
than the first. No obvious trend is evident, and it is reasonable to
assume that the transition metal is not particularly relevant to
what is intrinsically the rearrangement of an anion.
3−
hexaacetylide approaches Co(CN)6 in field strength (Δoct
=
34 000 cm−1) and is fairly similar in nepelauxetic character
(B = 430 cm−1).64
Successful application of the Tanabe−Sugano diagram to the
UV−vis features of (κ-C,N-pyEA-AriPr2)3Co (3) in Figure 4
supports the assignment of the 493 and 370 nm bands as d−d
transitions, but the field strength of ∼20 000 cm−1 thus derived is
significantly lower than that of Co(CCSiMe3)63−. An sp3-alkyl
might be expected to impart a stronger field on the basis of a
better energy match with the cobalt 3d-orbitalsassuming all
carbon-based σ-orbitals are lower than those of the cobaltthan
the sp-hybridized alkynyl ligand, but this is not the case. Note
that the d(CoIII−C) for Co(CCSiMe3)6 is 1.908(3) Å, the
3−
d(CoIII−C(sp2) for Co(ppz)3 (ppz = 2-phenylpyrazolato) is
1.921(6) Å,65 and the values of more traditional organometallic
d(CoIII−C(sp2)) average 1.950(15) Å.66 Since field strength
typically correlates with metal−ligand distance, the value of
20 000 cm−1 affiliated with the sp3-alkyl of 3 is in line with its
1.963(4) Å distance.
Ene-amide to Indoline Rearrangement. One reaction of
consequence, and potentially useful in the construction of
indoline fragments containing quaternary centers, was the
rearrangement of the ene-amides to indolamides (eqs 1−6).
Similar types of rearrangements have been observed in systems
that feature PDI and nacnac ligands that are known to support
RNI reactivity. Scheme 6 illustrates a PDI rhodium azide59 that
ultimately generates an indoline ring via a purported isopropyl
radical attack at one imine. In a related case, the 2,6-iPr2-Ar
substituent of an iron nacnac chelate is attacked at an imine
position by a similarly disposed isopropyl radical.60 The RNI
character of PDI and nacnac implicated radical character in these
rearrangements, but there is no direct evidence that these
processes cannot have considerable hydride-transfer character, as
might be expected for anionic ligands.
DISCUSSION
■
A Cobalt(III) Tris-alkyl, (κ-C,N-pyEA-AriPr2)3Co (3). The
synthesis of several ene-amide complexes was readily affected via
metathesis methods, and the most interesting was a low-yield
product, (κ-C,N-pyEA-AriPr2)3Co (3). Diamagnetic 3 is formally
Co(III) by virtue of three bidentate ligands bound as C,N-
isomers that generate the C3-symmetric pseudo-octahedral
product. Aside from organometallic examples involving ligands
Consider the rearrangement of the lithium ene-amides
featured and the initial preequilibrium in which the hydrogen
I
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Article
Scheme 7
Figure 8. Calculations of Li(pyEA-ArMe2) rearrangement conducted at MP2 and M06 levels of theory at 403.15 K. Note that the C−C bond-forming
step is rate-determining because the intermediate azapentadienyl anion is at a high energy.
is reversibly transferred. Scheme 7 shows that the conjugation in
the system does not permit ready differentiation between hydro-
gen atom, proton, or hydride transfer character. Since the transfer
is not rate-determining, variation in the ortho-substituents on the
phenyl ring provides limited information and does not implicate
any of the three limiting processes.
Calculation of the Li-ene-amide Rearrangement.
Calculations were performed on the rearrangement of
Li(pyEA-ArMe2) at 403.15 K, and the results are depicted
in Figure 8. The experimental ΔG⧧ of 31.1(1) kcal/mol is
bracketed by values of 28.7 and 35.5 kcal/mol, respectively, from
the M06 and MP2 methods utilized. Using either M06 or MP2
techniques,67−70 the highest transition state is predicted to
correspond to the C−C bond-forming step, in concert with
the observed methylene/o-methyl H/D scrambling, and the
low, experimental KIE of 1.16(9). The preequilibrium in which
hydrogen transfer is reversibly affected relates the starting ene-
amide to an azapentadienyl anion (labeled intermediate) that is
quite high in energy; hence its ΔH° is the primary component of
the experimental ΔH⧧ of 26.9(2) kcal/mol measured for the
J
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Article
overall process. As a consequence, while the barrier to C−C bond
formation is modest at 4.3 (MP2) and 12.4 (M06) kcal/mol, its
transition state (TS2) is the highest in the system.
hydrogen transfer prior to a rate-determining C−C bond-
forming reaction. Related transition metal rearrangements in
pyridine-diimine and nacnac systems have invoked radical char-
acter, but anionic, etc., alternative paths may also be plausible.
(κ-C,N-pyEA-AriPr2)3Co (3), a low-yielding diamagnetic by-
product of metathesis, possesses Co−C(sp3) bonds that have an
anomalously low field strength.
There is another calculated intermediate, labeled as rotamer,
in which C−N bond rotation occurs to afford a species with the
newly formed CH2 bound to Li in addition to imine- and
pyridine-N coordination. There is no experimental support for
this complex, which is calculated to be considerably lower in
energy than the first-formed intermediate generated via the
hydrogen transfer (TS1), and there is some question whether it
would form in competition with THF solvation. Attempts to
model the rearrangement with two THF molecules coordinated
(e.g., (THF)2Li(pyEA-ArMe2)) led to similar free energy values
for ene-amide and indolamide ground-state species and nearly
equal free energies for the intermediate and rotamer, but
transition-state geometries could not be located. Calculations
of a single THF-bound lithium ene-amide (e.g., (THF)Li(pyEA-
ArMe2)) afforded related results, including undiscovered
transition-state conformations.
Calculation of the EIEs pertaining to the isotopomers in
Scheme 5 was conducted by using the energies for Li(pyEA-
ArMe2) and ignoring one of the methyl groups, since the
experiments were conducted on Li(pyEA-o-tol). The static
geometry of Li(pyEA-ArMe2) was used, H’s were appropriately
replaced with D’s for the calculations, and the energies of
rotational isomers were averaged. The approximation was
deemed appropriate since the positioning of the deuteria
among the three methyl positions in −CH2D and −CHD2 did
not substantially change the energy of each isotopomer, nor did
changes in the CHD positions. The calculated EIE values
pertaining to A ⇄ B and B ⇄ C of Scheme 5 were 1.08 and 1.09,
respectively, in good agreement with experiment.
Since H/D scrambling between o-Me and methylene groups
was observed, the KIE (d0 vs d5, eqs 5 and 6) for rearrangement
of 1.16(9) must correspond to a preequilibrium between ene-
amide and the intermediate or rotamer, followed by rate-
determining C−C bond formation. In support of this statement,
the calculated KIE for formation of the intermediate (i.e., barrier
TS1) is kH/kD5 ≈ 2.95. The experimental overall KIE is then the
product of the EIE for the preequilibrium along the reaction
coordinate and the KIE for C−C bond formation, which
likely reflects two α-secondary and three β-secondary isotope
effects.63 EIEs calculated for Li(pyEA-ArMe2) to the intermediate
and rotamer are 1.00 and 1.04, respectively, While it is not feasible
to experimentally verify which equilibrium is consequential,
the intermediate is most likely along the reaction coordinate, and
the calculated KIE (using TS2 in Figure 8) is 1.06, consistent with
experiment.
EXPERIMENTAL SECTION
■
General Considerations. Qualitative descriptions of the synthetic
experiments and crystallographic data collection and refinements are
given in the schemes and tables. For the kinetics experiments, 1H NMR
spectroscopy was used to monitor the progress of the reactions. For
details concerning procedures, NMR spectroscopy, kinetics, and
calculations,67−70 consult the Supporting Information.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details regarding syntheses, reactivity studies,
kinetics measurements and KIE derivations, single-crystal X-ray
structure determination, and computations. The Supporting
Information is available free of charge on the ACS Publications
website at DOI: 10.1021/acs.organomet.5b00385.
AUTHOR INFORMATION
Corresponding Author
*E-mail (P. T. Wolczanski): ptw2@cornell.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Support from the National Science Foundation (P.T.W., CHE-
1402149) is gratefully acknowledged, as is the U.S. Department
of Energy, Office of Basic Energy Sciences, for partial support of
this research (T.R.C., DE-FG02-03ER15387).
DEDICATION
■
This paper is dedicated to the memory of Gregory L. Hillhouse,
friend, colleague, collaborator, and an innovative and significant
organometallic chemist.
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CONCLUSIONS
■
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Ene-amide coordination complexes, alternatively described as
1-azaallyls, have been synthesized via metathetical procedures.
While the reactivity of high-spin (pynac)2Fe (1) and (pyEA-
AriPr2)2M (2-M, M = Cr, Mn, Fe, Co-py)) is limited, 2-M (M =
Cr, Mn, Fe, Co-py) exhibited rearrangements to bis-indolamide
complexes {κ-N,N-NC6H3(6-iPr)CMe2C(Me)(2-py)}2M (5-M,
M = Cr, Mn, Fe, Co). The same rearrangement was found for the
corresponding Li species, Li(pyEA-ArR2/R) (ArR2/R = 2,6-R2-
C6H3 (R = iPr, Et, Me), 2-Me-C6H4), which afforded Li{κ-N,N-
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