Structural and Electronic Noninnocence of α-Diimine Ligands on Niobium for Reductive C-Cl Bond Activation and Catalytic Radical Addition Reactions

Article abstract of DOI:10.1021/jacs.7b02710

A d⁰ niobium(V) complex, NbCl₃(α-diimine) (1a), supported by a dianionic redox-active N,N′-bis(2,6-diisopropylphenyl)-1,4-diaza-2,3-dimethyl-1,3-butadiene (α-diimine) ligand (ene-diamido ligand) served as a catalyst for radical addition reactions of CCl₄ to α-olefins and cyclic alkenes, selectively affording 1:1 radical addition products in a regioselective manner. During the catalytic reaction, the α-diimine ligand smoothly released and stored an electron to control the oxidation state of the niobium center by changing between an η⁴-(σ²,π) coordination mode with a folded MN₂C₂ metallacycle and a κ²-(N,N′) coordination mode with a planar MN₂C₂ metallacycle. Kinetic studies of the catalytic reaction elucidated the reaction order in the catalytic cycle: the radical addition reaction rate obeyed first-order kinetics that were dependent on the concentrations of the catalyst, styrene, and CCl₄, while a saturation effect was observed at a high CCl₄ concentration. In the presence of excess amounts of styrene, styrene coordinated in an η²-olefinic manner to the niobium center to decrease the reaction rate. No observation of oligomers or polymers of styrene and high stereoselectivity for the radical addition reaction of CCl₄ to cyclopentene suggested that the C-C bond formation proceeded inside the coordination sphere of niobium, which was in good accordance with the negative entropy value of the radical addition reaction. Furthermore, reaction of 1a with (bromomethyl)cyclopropane confirmed that both the C-Br bond activation and formation proceeded on the α-diimine-coordinated niobium center during transformation of the cyclopropylmethyl radical to a homoallyl radical. With regard to the reaction mechanism, we detected and isolated NbCl₄(α-diimine) (6a) as a transient one-electron oxidized species of 1a during reductive cleavage of the C-X bonds; in addition, the monoanionic α-diimine ligand of 6a adopted a monoanionic canonical form with selective one-electron oxidation of the dianionic ene-diamido form of the ligand in 1a.

Full text of DOI:10.1021/jacs.7b02710

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Article
Structural and Electronic Non-innocence of #-
Diimine Ligands on Niobium for Reductive C-Cl Bond
Activation and Catalytic Radical Addition Reactions
Haruka Nishiyama, Hideaki Ikeda, Teruhiko Saito, Benjamin M.
Kriegel, Hayato Tsurugi, John Arnold, and Kazushi Mashima
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b02710 • Publication Date (Web): 14 Apr 2017
Downloaded from http://pubs.acs.org on April 14, 2017
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Structural and Electronic Non-innocence of -Diimine Ligands on Niobium for
Reductive C-Cl Bond Activation and Catalytic Radical Addition Reactions
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Haruka Nishiyama,a Hideaki Ikeda,a Teruhiko Saito,a Benjamin Kriegel,b Hayato
Tsurugi,*a John Arnold,*b and Kazushi Mashima*a
a Department of Chemistry, Graduate School of Engineering Science, Osaka University,
Toyonaka, Osaka, 5608531, Japan
b Department of Chemistry, University of California, Berkeley, California 94720-1460
E-mail: mashima@chem.es.osaka-u.ac.jp
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Abstract
A d0 niobium(V) complex, NbCl3(-diimine) (1a), supported by a dianionic
redox-active N,N’-bis(2,6-diisopropylphenyl)-1,4-diaza-2,3-dimethyl-1,3-butadiene
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(-diimine) ligand (ene-diamido ligand) served as a catalyst for radical addition
reactions of CCl4 to -olefins and cyclic alkenes, selectively affording 1:1 radical
addition products in a regioselective manner. During the catalytic reaction, the
-diimine ligand smoothly released and stored an electron to control the oxidation state
of the niobium center by changing between an 4-(2,) coordination mode with a
folded MN2C2 metallacycle and a 2-(N,N’) coordination mode with a planar MN2C2
metallacycle. Kinetic studies of the catalytic reaction elucidated the reaction order in
the catalytic cycle: the radical addition reaction rate obeyed first-order kinetics that were
dependent on the concentrations of the catalyst, styrene, and CCl4, while a saturation
effect was observed at a high CCl4 concentration. In the presence of excess amounts
of styrene, styrene coordinated in an 2-olefinic manner to the niobium center to
decrease the reaction rate. No observation of oligomers or polymers of styrene and
high stereoselectivity for the radical addition reaction of CCl4 to cyclopentene suggested
that the C—C bond formation proceeded inside the coordination sphere of niobium,
which was in good accordance with the negative entropy value of the radical addition
reaction. Furthermore, reaction of 1a with (bromomethyl)cyclopropane confirmed that
both the C—Br bond activation and formation proceeded on the -diimine-coordinated
niobium center during transformation of the cyclopropylmethyl radical to a homoallyl
radical.
With regard to the reaction mechanism, we detected and isolated
NbCl4(-diimine) (6a) as a transient one-electron oxidized species of 1a during
reductive cleavage of the C—X bonds; in addition, the monoanionic-diimine ligand of
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6a adopted a monoanionic canonical form with selective one-electron oxidation of the
dianionic ene-diamido form of the ligand in 1a.
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Introduction
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Organic radicals are among the most useful reactive species for carbon-carbon
bond formation in synthetic organic chemistry.1 Over the last several decades, organic
radicals have been accessed by treating alkyl halides with a wide variety of radical
initiators such as AIBN, BEt3, and peroxides:2 Kharasch et al. first discovered that
organic radicals generated in situ from homolytic cleavage of the carbon-halogen bonds
of alkyl halides by these typical radical initiators added across double bonds of olefins.3
Later, one-electron redox processes on metal centers were determined to be an effective
method for generating radical species from alkyl halides smoothly and controllably,
while also suppressing side reactions such as oligomerization and telomerization of
olefins.4-12 Remarkable research efforts have been devoted to atom transfer radical
addition (ATRA) and polymerization (ATRP), in which a radical generation step can be
finely controlled through modulation of the supporting ligand framework in transition
metal complexes to tune redox processes at the metal center.6c-k,8c,9f,h,10,13 A recent
trend is directed toward the introduction of redox-active ligands around transition
metals to avoid any change in the oxidation state at the metal center.14-17 In a
representative example reported by Chirik and coworkers, [2+2] cycloaddition reactions
were catalyzed by {bis(imino)pyridine}iron(II) complexes,14b,15a-d where the ligand
reversibly changed from dianionic to neutral during the reductive cyclization process.
Another typical example was reported by Heyduk and coworkers for the catalytic
application of d0 bis(ortho-aminophenolate)zirconium complexes to the
disproportionation of diphenylhydrazine to a mixture of aniline and azobenzene.17b
The tetranionic bis(ortho-aminophenolate) ligand provided the two electrons needed to
cleave the N—N bond, while allowing the zirconium center to remain in the d0
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oxidation state. Based on these results, we anticipated that systems incorporating
redox-active ligands coordinated to early transition metal d0 centers could efficiently
support catalytic radical addition involving reversible one-electron redox processes.
As exemplified in these two seminal examples, the redox-active ligands often
contain planar -conjugated frameworks that effectively overlap with a d-orbital of the
central metal, allowing for reduction or oxidation of substrates occurring at the metal
center to be mediated by the redox-active ligand without formal oxidation state change
at the metal center. We have investigated the structurally and electronically flexible
coordination behavior of the redox-active -diimine ligand to early transition metals, in
which the -diimine ligand coordinates to the metal in either folded 4-(2,)
coordination or planar 2-(N,N’) coordination modes.18 Herein, we report the
application of group 5 metal d0 complexes MCl3(-diimine) [M = Nb and Ta; -diimine
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=
N,N’-bis(2,6-diisopropylphenyl)-1,4-diaza-2,3-dimethyl-1,3-butadiene (L1) and
N,N’-bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene (L2)] as catalysts for radical
addition reaction of alkyl halides to alkenes. The major role of the -diimine ligand
was to release and store one-electron during the catalytic reaction by reversibly
switching the folding 4-(2,) coordination and planar 2-(N,N’) coordination to the
metal center, in which the planar 2-(N,N’) coordination mode smoothly transferred
one-electron from the ligand to the niobium center to generate Nb(IV) for accelerating
the reductive cleavage of the C—Cl bond. Structural change induced electron transfer
between the metal center and the redox-active ligand is a new strategy for organizing
various catalytic reactions by metal complexes with redox-active ligands. Kinetic
studies for the radical addition reaction as well as control experiments were performed
to elucidate the reaction mechanism.
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Results and Discussion
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Trichloroniobium complexes with ene-diamido ligands
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Niobium and tantalum complexes MCl3(-diimine) (1a: M = Nb, -diimine =
N,N’-bis(2,6-diisopropylphenyl)-1,4-diaza-2,3-dimethyl-1,3-butadiene (L1); 1b: M =
Nb, -diimine = N,N’-bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene (L2); 2a: M =
Ta, -diimine = L1; 2b: M = Ta, -diimine = L2) were prepared by reducing MCl5 (M
= Nb and Ta) with 1.0 equiv. of 1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene
(Si-Me-CHD) in the presence of the corresponding -diimine (L1 and L2) in toluene
(eq 1).18i The -diimine ligands in 1a and 1b coordinated to the niobium atom in a
1
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dianionic ene-diamide manner, as confirmed by the H and C{1H} NMR spectra. In
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the C{1H} NMR spectra of 1a and 1b, a signal assignable to the olefinic carbons of
1
the ene-diamido ligands was observed at δC 110.8 for 1a and 103.8 for 1b, and the H
NMR spectrum of 1b displayed one singlet signal for the imine protons of L2, which
was shifted to a higher field (δH 5.76). Figure 1 shows an ORTEP drawing of complex
1a, and the bond distances and angles of 1a are summarized in Table 1. The lengths of
C1—N1 [1.389(2) Å] and C2—N2 [1.410(3) Å] are longer than those of typical C=N
double bonds and closer to those of the typical C—N single bonds, while the length of
C1—C2 [1.383(2) Å] is between that of C=C double and C—C single bonds. The
5-membered chelate ring is folded with dihedral angles between the N1—C1—C2—N2
and N1—Nb—N2 planes of 126.17(8)º for 1a and 121.8(3)º for 1b, suggesting that the
C1—C2 double bond coordinates through -donation to the niobium center in a
4-(2,) coordination mode, which were typical for dianionic ligation of the
ene-diamido ligands to group 5 metal centers.18b,e,i,19 In sharp contrast to the folded
structure observed for 1a and 1b, the fold angle of the MN2C2 ring for monoanionic
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-diimine complexes (including 6a, vide infra) is typically in the range of
164—179°.18b,e,f,h,i,20 Although we considered that 1a was a Nb(V) species bearing a
dianionic ene-diamido ligand, we could not completely exclude the contribution of a
Nb(IV) species having a monoanionic -diimine ligand in 1a. The 1H NMR spectrum
of 1a in toluene-d8 at room temperature showed a singlet peak at 1.85 ppm for
MeC=CMe, while only the peak shifted to 1.53 ppm at -80 ºC (Figure S3), suggesting
that there was a small contribution of the singlet/triplet niobium complexes, Nb(IV)
species having -diimine monoanionic ligands.21 Niobium complexes 1a and 1b were
close structural analogs to the previously reported tantalum complexes 2a and 2b.18i
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SiMe3
Me3Si
Si-Me-CHD
MCl5
+
Ar
N
Ar N
R
(1.0 equiv.)
Cl
Cl
(1)
R
R
M
toluene, rt, 16 h
- 2 Me3SiCl
R
Ar N
Ar = 2,6-iPr2C6H3
N Ar
Cl
- C6H5CH3
1a: M = Nb, R = Me (65% yield)
1b: M = Nb, R = H (91% yield)
2a: M = Ta, R = Me (92% yield)
2b: M = Ta, R = H (86% yield)
L1: R = Me
L2: R = H
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Figure 1. Molecular structure of 1a with 50% thermal
ellipsoids. All hydrogen atoms are omitted for clarity.
Table 1. Selected bond distances (Å) and angles (deg.) of 1a.
Nb—N1
Nb—N2
Nb—Cl1
Nb—Cl2
Nb—Cl3
Nb—C1
1.9692(16)
1.9901(16)
2.3873(7)
2.3050(6)
2.3892(8)
2.4747(17)
Nb—C2
2.4810(17)
1.389(2)
1.383(2)
1.410(3)
86.66(6)
126.17(8)
N1—C1
N2—C2
C1—C2
N1—Nb—N2
ene-diamido fold anglea
aene-diamido fold angle: dihedral angle between N1—C1—C2—N2 and N1—Nb—N2
planes
Catalytic radical addition reactions
We found that 1a exhibited catalytic activity for radical addition reactions of CCl4
to various alkenes. A reaction of CCl4 and styrene (3a) was chosen as a model radical
addition reaction to evaluate the optimum catalytic conditions. First, niobium and
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tantalum complexes 1a,1b, and 2a,2b were screened as catalysts (3 mol%) for radical
addition of CCl4 (1.50 mmol, 5.0 equiv.) to 3a (0.300 mmol, 1.0 equiv.) in C6D6 (0.30
mL) at 100 °C for 3 h in J-Young NMR tubes; the results are shown in Table 2.
Niobium complex 1a exhibited excellent catalytic activity for the radical addition
reaction, selectively affording a 1:1 coupling product, 1,3,3,3-tetrachloropropylbenzene
(4a), in quantitative yield without any byproducts, such as styrene oligomers,
polystyrene, or other regioisomers of the radical addition product (entry 1). Niobium
complex 1b showed less activity, giving 4a in 60% yield (entry 2), whereas the tantalum
complexes 2a and 2b produced polystyrene without forming any 4a (entries 3 and 4).
We further examined some low-valent niobium complexes, such as NbCl3(DME) and
NbCl3(TMEDA), as catalysts. Although the coordination of 1,2-dimethoxyethane
(DME) and N,N,N’,N’-tetramethylethylenediamine (TMEDA) to Nb(III) suppressed the
radical polymerization of styrene, NbCl3(DME) had lower catalytic activity for 4a
(55%) than 1a, and NbCl3(TMEDA) exhibited no catalytic activity (entries 5 and 6).
Accordingly, coordination of a dianionic ene-diamido ligand to the Nb(V) moiety
played an essential role in the high catalytic activity without any radical polymerization,
indicating that the catalytic cycle using 1a did not involve free organic radicals. In
contrast to radical addition catalysts based on late transition metal complexes, such as
Ru(III) and Cu(II), the high oxidation state Nb(V) catalyst system did not require any
reductant such as AIBN, although the niobium system exhibited lower catalytic
activity.9,10
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We next screened solvents such as toluene, acetonitrile, and THF for this radical
addition reaction catalyzed by 1a (entries 7—9). The coupling product 4a was
quantitatively obtained in toluene-d8 (entry 7), as in C6D6 (entry 1), whereas the reaction
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in CD3CN afforded 4a in lower yield (19%) (entry 8) and no reaction was observed in
THF-d8 (entry 9). Decreasing the amount of CCl4 from 5.0 equiv. to 3.0 equiv. or 1.0
equiv. lowered the yield of 4a (entries 10 and 11). Therefore, we employed 5.0 equiv.
of CCl4 for further studies.
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Table 2. Catalytic radical addition reaction of CCl4 to
styrene.a
Cl
(3 mol%)
cat.
Cl3C
+
CCl4
solvent, 100 ºC, 3 h
3a (0.300 mmol) (X equiv.)
4a
entry
1
cat.
solvent
C6D6
CCl4 (equiv.)
yieldb (%)
1a
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
3.0
1.0
>99 (97)
2
1b
C6D6
60
0
3
2a
C6D6
4
2b
C6D6
0
5
NbCl3(DME)
C6D6
55
0
6
NbCl3(TMEDA)
C6D6
7
1a
1a
1a
1a
1a
toluene-d8
CD3CN
THF-d8
C6D6
>99
19
0
8
9
10
62
11
C6D6
33
aAll reactions were carried out in J-Young NMR tubes. bDetermined by 1H NMR analysis
using 1,4-dioxane as an internal standard. cIsolated yield was given in parentheses.
Complex 1a (3 mol%) catalyzed the reactions of various alkene substrates 3b—j
(0.300 mmol) with CCl4 (5.0 equiv.) in C6D6 at 100 ºC, although a longer reaction time
was required than for styrene (Table 3). The 1:1 coupling product 4b was obtained in
93% yield in the reaction of para-methylstyrene 3b, whereas incorporation of an
electron-withdrawing group at the para-position of the styrene decreased the yield of
the product due to the contamination of oligomers as byproducts (entries 1—3).
Reaction with meta-methylstyrene 3e proceeded in 81% yield; but no product was
obtained for ortho-methylstyrene 3f, likely due to steric congestion around the alkene
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moiety (entries 4 and 5). -Olefins with alkyl substituents were also amenable to the
radical addition reactions: alkyl olefins such as 1-octene (3g), trimethyl(vinyl)silane
(3h), vinylcyclohexane (3i), and allylbenzene (3j) produced the corresponding 1:1
coupling compounds 4g—j in moderate to high yields (entries 6—9).
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Table 3. Radical addition reaction of CCl4 to alkenes.a
Cl
(3 mol%)
1a
Cl3C
R1
R1
+
CCl4
C6D6, 100 ºC, 14 h
3 (0.300 mmol) (5.0 equiv.)
4
entry
R1
conv. (%)b
yield (%)b
1
2
3
4
5
6
7
8
9
p-MeC6H4
p-ClC6H4
3b
3c
94
100
76
93 (88)
82d (78)
40d (38)
81 (81)
0e
p-CF3C6H4 3d
m-MeC6H4 3e
86
o-MeC6H4
nHex
3f
95
3g
3h
3i
100
80
>99 (98)
77 (70)
58 (58)
48 (40)
Me3Si
Cy
58
PhCH2
3j
56
aAll reactions were carried out in J-Young NMR tubes. bDetermined by 1H NMR
analysis using 1,4-dioxane as an internal standard. cIsolated yield was given in
parentheses. dOligomer was observed as byproduct. e1-methyl-2-(3,3,3-trichloroprop-
1-en-1-yl)benzene was observed as byproduct.
As shown in Scheme 1, a reaction of cyclopentene with CCl4 was also catalyzed
using either 1a (3 mol%) or NbCl3(DME) (3 mol%) in C6D6 at 100 ºC for 14 h. The
reaction using 1a afforded only trans-1-chloro-2-(trichloromethyl)cyclopentane in 62%
yield without contamination by other stereoisomers (Scheme 1a). In contrast, the
reaction using NbCl3(DME) resulted in
a
5:1 mixture of trans- and
cis-1-chloro-2-(trichloromethyl)cyclopentane (Scheme 1b), indicating that free radical
addition was partially involved in the reaction using NbCl3(DME) (vide infra).22
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Scheme 1.
Radical addition reaction of CCl4 to
cyclopentene catalyzed by (a) 1a and (b) NbCl3(DME).
9
(a)
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1a (3 mol%)
C6D6, 100 ºC, 14 h
Cl
CCl3
(62% yield)
+
CCl4
(5.0 equiv.)
(b)
Cl
CCl3
(46% yield)
+
NbCl3(DME) (3 mol%)
C6D6, 100 ºC, 14 h
Cl
CCl3
(9% yield)
Kinetics of radical addition reactions
To gain further insight into the reaction mechanism, we carried out kinetic
experiments for the radical addition reaction of CCl4 to styrene catalyzed by 1a at 100
ºC in C6D6 with varying concentrations of 1a, styrene, and CCl4. The reaction rate
constants kobs were determined by monitoring the concentrations of the coupling product
1
4a by H NMR spectroscopy in C6D6 containing hexamethylbenzene as an internal
standard. A plot of kobs obtained from six different catalyst concentrations under
conditions of [styrene]0 = 0.60 M and [CCl4]0 = 3.0 M showed a linear (first-order)
dependence of the kobs on the concentration of 1a in a catalyst concentration range of
18—72 mM (Figure 2).
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Figure 2. Dependence of kobs on the concentration of 1a
as the catalyst for the radical addition reaction of CCl4 and
styrene. Reaction conditions: [styrene]0 = 0.60 M, [CCl4]0
= 3.0 M, temperature 100 ºC.
Next, we evaluated the effects of [styrene]0 under the conditions of [CCl4]0 = 4.5
M and [1a]0 = 18 mM. When [styrene]0 was less than 0.90 M, kobs increased linearly,
suggesting that the reaction rate obeyed first-order kinetics depending on the styrene
concentration, whereas kobs decreased when the amount of [styrene]0 was greater than
0.90 M (Figure 3). The reaction rate was also slower for 1-octene (Figure S10).23 In
addition, a kinetic studies of CCl4 to trimethyl(vinyl)silane showed a deviation from
linearity (Figure S12).23
To investigate these observations at higher olefin
concentrations, we measured the UV-vis spectra of 1a in styrene (neat), 1-octene (neat),
and trimethyl(vinyl)silane (neat) (Figure 4). A UV-vis spectrum of 1a in styrene
displayed a new absorption (max = 748 nm) characteristic of complexes containing the
monoanionic -radical -diimine ligand bound to a metal center,20n,24 as well as a
shoulder around 465 nm, which was attributed to the contribution of the filled
p-orbitals of the nitrogen atoms to a vacant d-orbital of the niobium, typically
observed for (ene-diamido)metal complexes with planar MN2C2 metallacycles.18b
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These UV-vis spectral data suggested that the coordination mode of the -diimine in 1a
switched from dianionic4-(2,) coordination to monoanionic κ2-(N,N’) -ligation upon
interaction with the alkene substrates (eq 2). In addition, the -acidic character of the
coordinating alkenes accelerated the electron transfer from the dianionic ligand to the
metal center to form Nb(IV) species A, which is in contrast to anionic niobium
complexes [nBu4N][NbCl4(-diimine)] (5a:, -diimine = L1; 5b: -diimine = L2),
where -diimine coordinated with a 2-(N,N’)-binding mode to the niobium center and
showed a typical ligand-to-metal charge-transfer (LMCT) band at 468 nm (see
Supporting Information). In the case of 1-octene, an absorption band also appeared at
735 nm, while there was no detectable band in the range of 730—750 nm for the
solution of 1a in trimethyl(vinyl)silane. Furthermore, the different intensities of
absorptions in the range of 730—750 nm for these three alkenes were due to the
strength of the interaction of the niobium atom with alkenes; accordingly, the order of
the observed intensity was 1-octene > styrene > trimethyl(vinyl)silane. This correlated
with the change in kobs observed at a higher concentration of alkenes during the reaction
kinetics, which also decreased kobs at a high concentration in the order 1-octene >
styrene > trimethyl(vinyl)silane. In addition, 1a partially decomposed in the presence
of excess styrene (1.8 M) at 100 °C after 4 h and free ligand L1 was detected.25 We
thus concluded that a decrease in kobs value was observed when the amount of
[styrene] : [CCl4] was greater than [styrene]0 : [CCl4]0 = 1 : 5 due to both interaction of
1a with alkenes and decomposition of 1a.
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Figure 3. Dependence of kobs on the concentration of
styrene for the radical addition reaction of CCl4 and styrene.
Reaction conditions: [1a]0 = 18 mM, [CCl4]0 = 4.5 M,
temperature 100 ºC.
Figure 4. UV-vis spectra of complex 1a in benzene (red
trace), styrene (green trace), 1-octene (blue trace), and
trimethyl(vinyl)silane (orange trace).
R
Ar
Ar
N
N
Ar N
Me
Me
R
Cl
Cl
Cl
Cl
(2)
Nb
Cl
Nb
N
-
Me
R
Me
Ar
Cl
1a
A
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Finally, under fixed concentrations of [1a]0 = 18 mM and [styrene]0 = 0.60 M, the
initial concentration of CCl4 was changed: the values of kobs for product formation
increased linearly up to [CCl4]0 = 4.8 M, whereas the reaction rate was almost constant
when more than 4.8 M of [CCl4]0 was used (Figure 5). This saturation behavior of the
rate constant on CCl4 concentration can be rationalized by assuming an equilibrium
between 1a and any dormant species involving CCl4 (vide infra).8a,e In fact, a solution
of 1a with Ph2CCl2 in benzene showed a shoulder at 475 nm due to a LMCT band as
well as an absorbance at 760 nm assignable to the band due to the planar-ligated
monoanionic -diimine, which suggested the formation of alkylhalide-coordinated
species B, as shown in Scheme 2 (vide infra).
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60
Figure 5. Dependence of kobs on the concentration of CCl4
for the radical addition reaction of CCl4 and styrene.
Reaction conditions: [1a]0 = 18 mM, [styrene]0 = 0.60 M,
temperature 100 ºC.
We thus propose the following rate laws based on our kinetic study. First, we
show the rate law as eq 3 under the conditions of lower concentrations of both [CCl4]
and [styrene] ([CCl4] < 4.8 M and [styrene] < 0.90 M). Under these conditions, the
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reaction rate was first-order for [Nb], [CCl4], and [styrene].
8
9
[
][ ]
][
Rate = 푘 Nb CCl4 styrene
(3)
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At the higher concentrations of [CCl4] ([CCl4] > 4.8 M), the rate law is given as eq 4,
which suggested that the reaction rate was first-order for [Nb] and [styrene], and
zero-order for [CCl4].
[
][ ]
Rate = 푘 Nb styrene
(4)
In the case of a higher concentration of [styrene] ([styrene] > 0.90 M), we cannot clearly
show the reaction rate because of the formation of A as well as the decomposition of 1a.
Interaction of 1a with alkenes
As shown in Figure 4, a new absorption band for the solution of 1a in styrene
(neat) and 1-octene (neat) was observed around 740 nm, suggesting that alkenes
interacted with the niobium center to induce the planar ligation of L1 to form A having
a monoanionic -diimine.20n,24 When we monitored a mixture of 1a (18 mM) and
1-octene (0.90 M) in benzene (1.0 mL) by UV-vis spectroscopy at room temperature,
the same absorption band (735 nm) was observed. However, the band decreased upon
addition of increasing amounts of CCl4 (Figure 6). Under the condition, no radical
addition product was observed, suggesting that the alkene was replaced to CCl4.
Accordingly, the alkene coordinated species A is likely not involved in the catalytic
cycle.
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Figure 6. UV-vis spectrum of 1a (18 mM) in benzene (a) with
1-octene (0.90 M) (blue line), (b) with 1-octene (0.90 M) and CCl4
(0.90 M) (green line) (c) with 1-octene (0.90 M) and CCl4 (2.7 M)
(orange line), and (d) with 1-octene (0.90 M) and CCl4 (4.5 M) (red
line).
Proposed mechanism of the radical addition reaction
Based on NMR kinetic experiments and UV-vis spectroscopic experiments, we
propose a mechanism of the radical addition reaction for styrene as outlined in Scheme
2. Initially, CCl4 interacts with the niobium center of 1a to form Nb(IV) species B
with a monoanionic -diimine ligand, and then the niobium complex B reversibly
cleaves the C—Cl bond of CCl4 via a one-electron redox process to generate a dormant
species C. In this process, one electron in the ene-diamido ligand in 1a is effectively
transferred to CCl4 through the metal center, resulting in a change in the coordination
mode of a folded ene-diamido ligand to a planar radical monoanionic ligand by the
interaction of CCl4 with the metal center. Styrene reacts with the organic radical
within the coordination sphere of C to generate a new radical (D). Then, the radical
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abstracts a chloride atom bound to the niobium center in D, leading to product 4a with
regeneration of complex 1a. Overall, the redox-active -diimine ligand plays an
important role in the redox events to form B and regenerate 1a by flexibly changing the
folding and planar MN2C2 metallacycle moiety. The large negative activation entropy
value is in good accordance with the mechanism, in which 1a, CCl4, and styrene are
involved in the transition state going from C to 1a. The absence of styrene oligomers
or polymers during the radical addition reaction indicated the formations of C27 and D
rather than free radicals. Scheme 2 also shows that A is not involved in the catalytic
cycle.
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Scheme 2.
Proposed reaction mechanism for radical
addition reactions.
Ph
Cl
Ar
N
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Me
Me
Nb
Cl
N
Cl
Ar
A
-
+
Ph
Ph
Ar
Ph
N
Ar
N
CCl3
Cl
Cl
Cl
4a
Nb
+
Cl CCl3
Me
Me
Cl
-
Cl CCl3
1a
Ph
Cl
CCl3
Cl CCl3
Ar
N
Ar
N
Me
Me
Me
Me
Cl
Cl
Cl
Cl
Nb
Nb
Cl
N
N
Cl
Ar
Ar
D
B
Cl
Ar
N
CCl3
Cl
Me
Me
Nb
Cl
Ph
N
Ar
C
Cl
The stereoselective reaction of CCl4 to cyclopentene provides further evidence
for the inner sphere radical formation (D) because the cis/trans ratio of the products is
dependent on the degree of freedom associated with the intermediate
2-(trichloromethyl)cyclopentyl radical. In the case of radical addition initiated by free
radicals, no stereoselectivity is generally observed.22 On the other hand, addition of
CCl3 and Cl radicals proceeded stereoselectively within the metal coordination sphere
through ordered transition state D. Lower stereoselectivity was observed when
NbCl3(DME) was used as a catalyst during the reaction (Scheme 1b); however, using 1a,
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the redox-active -diimine ligand was able to support the redox events on the d0
8
9
niobium
center
during
the
catalytic
reaction,
thus
giving
only
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trans-1-chloro-2-(trichloromethyl)cyclopentane in a selective manner (Scheme 1a).
Reactivity of niobium complex 1a toward radical clock substrates
To elucidate the C—X bond activation step, we examined the reaction of 1a with
(bromomethyl)cyclopropane and (bromomethyl)cyclobutane. Treatment of
(bromomethyl)cyclopropane with 1a (10 mol%) in C6D6 at 120 ºC for 14 h resulted in a
mixture of 4-bromobutene (59%) and bromocyclobutane (33%) along with
(bromomethyl)cyclopropane (eq 5), indicating the formation of a cyclopropylmethyl
radical, which is rapidly rearranged to the homoallyl radical in less than 1.1 x 10-8 s
before reformation of a C—Br bond to give 4-bromobutene.28 We carried out the
addition of TEMPO to trap the cyclopropylmethyl radical; however, we observed only
the TEMPO-induced decomposition of 1a. Although the mechanism for the formation
of bromocyclobutane in eq 5 is not clear, we postulate that cyclization to the
four-membered ring proceeded through the generation of a transient homoallyl cation.29
Br
1a (10 mol%)
Br
Br
+
(5)
C6D6, 120 ºC, 14 h
(59% yield)
(33% yield)
In contrast to the results observed for (bromomethyl)cyclopropane, no
rearrangement products were detected in the reaction of (bromomethyl)cyclobutane
with 1a (10 mol%). The cyclobutylmethyl radical is known to rearrange to the
pent-4-enyl radical within 2.0 x 10-4 s (eq 6).28 Thus, the C—Br bond was reversibly
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cleaved and formed by 1a in the range of 5.0 x 103—9.4 x 107 s-1.28
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Br
1a (10 mol%)
(6)
no reaction
C6D6, 120 ºC, 14 h
Oxidation of 1a by alkyl halides
As shown in Scheme 2, the catalytic cycle involves Nb(V) species bearing a
monoanionic L1 after cleavage of the C—X bond. To detect such Nb(V) species, we
monitored the reaction of 1a with excess Ph2CCl2 in benzene at 60 ºC by UV-vis
spectroscopy (Figure 7, eq 7), as the Ph2CCl radical derived from Ph2CCl2 is more
stable than the CCl3 radical. After addition of excess Ph2CCl2 to 1a in benzene, a new
absorption band was observed around 760 nm as well as a shoulder at 475 nm, which
were both assigned to a Ph2CCl2-interacted species involving a planar monoanionic
-diimine ligand similar to the alkene- and CCl4-coordinated species A and B. An
additional evidence for the interaction of halogenated compounds of the halogenation
interaction was the broad absorption at 748 nm for the green solution of 1a in
fluorobenzene in the same manner as B (Figure S16). Further heating at 60 °C resulted
in the formation of a new band at 785 nm assignable to a new niobium complex. This
niobium species was alternatively prepared by reducing NbCl5 with 0.5 equiv. of
Si-Me-CHD in the presence of -diimine ligand L1 at room temperature in toluene for
14 h, producing 6a with a monoanionic L1 in 59% isolated yield (eq 8), whose spectral
data were superimposable with those of the detected niobium species. Accordingly,
we concluded that the reductive cleavage of C—X bonds induced oxidation of L1 in 1a
from its dianionic form to its monoanionic form, which was in sharp contrast to the
metal-centered redox processes observed in previous reports of ATRP by Mo(III)
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complexes bearing-diimine ligands.30
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7a 381 nm
6a 785 nm
Figure 7. Time-dependent UV-vis transition for the
reaction of 1a and Ph2CCl2 in benzene at 60 ºC, giving 6a,
in the range of 0–60 min.
Cl
Ar
Me
N
Cl
Cl
Nb
Cl
Ar
Cl CPh2Cl
Ar
N
N
N
Ph2CCl2
Me
Ar N
Me
Me
Cl
Cl
Cl
Ar
Nb
(7)
Nb
N
Cl
Me
Me
- Ph2CCl2
6a
Cl
Cl
Ar
+
1a
B
Ph2CCl
SiMe3
NbCl5
Cl
Ar
N
Me3Si
R
R
+
Si-Me-CHD (0.50 equiv.)
Cl
Cl
Nb
Cl
(8)
R
R
N
toluene, rt, 14 h
- Me3SiCl
Ar N
N Ar
Ar
Ar = 2,6-iPr2C6H3
- 0.5 C6H5CH3
6a: R = Me (59% yield)
6b: R = H (65% yield)
L1: R = Me
L2: R = H
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The solid-state structure of 6a was determined by X-ray diffraction (Figure 8 and
Table 4). The niobium is surrounded by four chlorides and two nitrogen atoms of the
monoanoinic α-diimine ligand; the C—N and C—C bond lengths of the L1 backbone
are intermediate between those of the neutral31 and dianionic forms of L1:18b,e,i,19 the
elongated C—N [1.309(17) and 1.318(17) Å] and shortened C—C [1.457(14) Å] bonds
compared with the fully oxidized α-diimine form suggests the partial reduction of L1 to
the monoanionic form.18b,e,f,h,i,20 In contrast to 1a, the five-membered metallacycle
containing L1 and the niobium atom is planar (178.1(5)º), in good accordance with a
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radical anionic canonical form.18b,e,f,h,i,20
The Nb—N distances [2.228(9) and
2.235(10) Å] are slightly longer than typical niobium-nitrogen single bonds
(1.935—2.102 Å).18b,e,f,h,i,20 The absorption maximum at 785 nm (ε = 2.5 x 103 M-1
cm-1) is also consistent with a complex containing a planar metallacycle with a
monoanionic -diimine ligand.20n,24
Figure 8. Molecular structure of 6a with 50% thermal
ellipsoids. All hydrogen atoms are omitted for clarity.
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Table 4. Selected bond distances (Å) and angles (deg) of 6a.
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Nb—N1
Nb—N2
Nb—Cl1
Nb—Cl2
Nb—Cl3
Nb—Cl4
2.228(9)
2.235(10)
2.326(3)
2.386(3)
2.354(3)
2.378(3)
N1—C1
1.309(17)
1.318(17)
1.457(14)
71.1(3)
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N2—C2
C1—C2
N1—Nb—N2
N1—Nb1—Cl1
N1—Nb1—Cl2
88.6(3)
95.4(2)
α-diimine-fold anglea 178.1(5)
aα-diimine-fold angle: dihedral angle between N1—C1—C2—N2 and N1—Nb—N2
planes
In addition to complex 6a, we prepared NbCl4(L2) (6b) in a similar manner (eq 8),
and measured the ESR spectra of 6a and 6b. The ESR spectrum of 6a displayed a
93
ten-line splitting signal due to the 9/2 nuclear spin of Nb (g = 1.918, Aiso = 105 G)
(Figure 9a). The isotropic hyperfine coupling constant was lower than that of the
purely localized niobium-centered radical complex, NbCl4(PEt3)2 (Aiso = 141.5 G),32
probably due to the coordination of the electron-accepting -diimine ligand to the metal
center. Although this implied that the ligand supported some of the radical character,
any hyperfine coupling to the ligand only served to broaden the peaks. In contrast, the
ESR spectrum for 6b showed a nine-line with a much weaker hyperfine coupling
constant consistent with an organic radical (g = 2.003, Aiso = 6.56 G). This was
simulated by taking into account hyperfine coupling with two virtually identical
nitrogen atoms (aN = 6.80 G) and four equivalents hydrogen atoms of N=CH as well as
the para-hydrogen atom of the N-aryl group (aH = 6.31 G) (Figure 9b, c and S18).18i
Accordingly, 6b possessed a purely -diimine ligand-localized radical. Different
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electron-accepting character of -diimine ligands L1 and L2 was similarly observed for
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the related tantalum and tungsten complexes systems.18i,k
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(a)
(b)
(c)
Figure 9. ESR spectra in toluene at room temperature for (a) 6a
in the range of 2800–4200 G (red line), (b) 6b in the range of
3000–4000 G (blue line), and (c) 6b in the range of 3440 – 3560 G
(blue line).
Deactivation process to give 7a
Upon prolonged reaction of 1a with Ph2CCl2 in benzene, UV-vis measurement
revealed a decrease in the intensity of the absorption band at 785 nm corresponding to
6a with an increase in the absorption at 381 nm assignable to niobium species 7a
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(Figure 10, eq 9), the product of hydrogen atom abstraction from the ligand of 6a by a
Ph2CCl radical. Alternatively, treatment of 1a with excess Ph3CCl at 60 ºC in benzene
for 20 h led to the formation of 7a in 82% yield (eq 10). In the 1H NMR spectrum of
7a, two broad doublet resonances were observed at H4.44 and 4.08 corresponding to
the vinylidene moiety of the ligand backbone. We previously reported a similar
reaction product from the reaction of the tantalum complex 2a with CCl4.18i The
13C{1H} NMR spectrum of 7a displayed a signal due to an imine carbon at C 179.7 as
well as signals due to the vinylidene carbons at C 156.1 and 112.7, suggesting the
formation of an enamido-imine ligand framework. In contrast, the reaction of 1a with
Ph2CCl2 in the presence of styrene afforded the corresponding radical addition product
without any formation of 7a, suggesting that the deactivation process for producing 7a
did not occur in the presence of alkene substrates.33
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7a 381 nm
6a 785 nm
Figure 10. Time-dependent UV-vis transition for the
reaction of 1a and Ph2CCl2 in benzene at 60 ºC in the range
of 60–180 min, corresponding to the conversion of 6a to 7a.
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Cl
Cl
Ar
N
Ar
N
Me
Me
Ph2CCl
Cl
Cl
Cl
Cl
Nb
Cl
Nb
Cl
(9)
H
H
N
N
- Ph2C(H)Cl
H
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H
Ar
H
Ar
7a
6a
Ar
Cl
Ar
N
Ar
N
Me
Ph3CCl (3.0 equiv.)
N
Cl
Cl
Cl
Cl
Nb
Cl
(10)
Nb
H
N
benzene, 60 ºC, 20 h
Me
Me
Cl
H
Ar
- Ph3CH
1a
7a (82% yield)
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Conclusions
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We investigated a radical addition of CCl4 to various alkenes catalyzed by d0
niobium complex 1a with a redox-active -diimine ligand. In this catalytic
transformation, the -diimine ligand played an important role in releasing and storing
one electron during the C—X bond activation and re-forming steps without changing
the oxidation state of the niobium center of 1a. A key event in the electron transfer
was the flexible coordination of the ene-diamido ligand: the 4-(2,) coordination of
the ene-diamido ligand stored an electron in the ligand framework while the planar
2-(N,N’) coordination of the ene-diamido ligand released one-electron to the niobium
center. Effective overlap of the -electrons of the dianionic ene-diamido ligand and
the d-orbital of the niobium center in the planar coordination mode accelerated the
one-electron transfer from the ligand to the niobium center to afford a Nb(IV) species
that cleaved a C—Cl bond of CCl4. Kinetic studies of the radical addition revealed
that the reaction rate obeyed first-order kinetics with respect to the concentrations of 1a,
styrene, and CCl4 under low concentrations of styrene and CCl4. Organic radicals
always stayed in the inner sphere of the niobium center during the catalytic reactions,
which was in good agreement with the negative activation entropy and high
stereoselectivity in the radical addition of CCl4 to cyclopentene. Thus, we anticipate
that the combination of a d0 metal center with a redox-active ligand is a good candidate
for controlling one-electron redox processes to mediate various catalytic radical
reactions. Further studies of the catalytic application of d0 metal complexes with
redox-active ligands are ongoing in our laboratory.
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