Redox-Noninnocent Behavior of Tris(2-pyridylmethyl)amine Bound to a Lewis Acidic Rh(III) Ion Induced by C-H Deprotonation

Article abstract of DOI:10.1021/jacs.5b06237

Rh(III) complexes having tris(2-pyridylmethyl)amine (TPA) and its derivative as tetradentate ligands showed reversible deprotonation at a methylene moiety of the TPA ligands upon addition of a strong base as confirmed by spectroscopic measurements and X-ray crystallography. Deprotonation selectively occurred at the axial methylene moiety rather than equatorial counterparts because of the thermodynamic stability of corresponding deprotonated complexes. One-electron oxidation of the deprotonated Rh(III)-TPA complex afforded a unique TPA radical bound to the Rh(III) center by a ligand-centered oxidation. This is the first example to demonstrate emergence of the redox-noninnocent character of the TPA ligand.

Full text of DOI:10.1021/jacs.5b06237

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Communication
Redox Non-Innocent Behavior of Tris(2-Pyridylmethyl)amine
Bound to a Lewis Acidic Rh(III) Ion Induced by C-H Deprotonation
Hiroaki Kotani, Takumi Sugiyama, Tomoya Ishizuka,
Yoshihito Shiota, Kazunari Yoshizawa, and Takahiko Kojima
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b06237 • Publication Date (Web): 24 Aug 2015
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Redox Non-Innocent Behavior of Tris(2-Pyridylmethyl)amine Bound
to a Lewis Acidic Rh(III) Ion Induced by C-H Deprotonation
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Hiroaki Kotani,† Takumi Sugiyama,† Tomoya Ishizuka,† Yoshihito Shiota,‡ Kazunari Yoshizawa,‡,§
and Takahiko Kojima*,†
†Department of Chemistry, Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennoudai, Tsukuba, Ibaraki 305-8571, Japan
‡Institute for Materials Chemistry and Engineering, Kyushu University, Motooka, Nishi-Ku, Fukuoka 819-0395, Japan
§Elements Strategy Initiative for Catalysts & Batteries, Kyoto University, Nishikyo-ku, Kyoto 615-8520, Japan
Supporting Information Placeholder
lengths in the ligand.3
ABSTRACT: Rh(III) complexes having tris(2-pyridylmethyl)-
amine (TPA) and its derivative as tetradentate ligands showed
reversible deprotonation at a methylene moiety of the TPA ligands
upon addition of a strong base as confirmed by spectroscopic
measurements and X-ray crystallography. The deprotonation selec-
tively occurred at the axial methylene moiety rather than equatorial
counterparts because of the thermodynamic stability of corre-
sponding deprotonated complexes. One-electron oxidation of the
deprotonated Rh(III)-TPA complex afforded a unique TPA radical
bound to the Rh(III) center by a ligand-centered oxidation. This is
the first example to demonstrate emergence of the redox-non-
innocent character of the TPA ligand.
On the other hand, the ligand deprotonation has been known to
induce redox-non-innocent character in an originally redox-
innocent ligand.4 For instance, Grützmacher and co-workers have
reported successful isolation of a Rh(I) complex with an aminyl
radical by one-electron oxidation of a deprotonated Rh(I)-amido
complex.5 Also, de Bruin and co-workers have reported ligand oxi-
dation of a deprotonated Ir(I)-BPA complex (BPA = bis(2-
picolyl)amine) to form a radical ligand.6 However, these sequential
deprotonation and oxidation processes of deprotonated ligands
have been limited to the case of metal complexes having amino
(NH) groups in the ligand as deprotonation sites. In order to ex-
pand the scope of applicable ligands without NH groups as the
deprotonation site, a combination of metal-ligand cooperation and
subsequent ligand oxidation is required because a variety of radical
intermediate-bound metal complexes ([Mn+(L•)]n+) have been
investigated as, for example, models of galactose oxidase to realize
the importance of the radical character of the ligand.7
Metal-ligand cooperation has been recognized as one of new
methods to develop novel and striking functionality of metal com-
plexes, which often derives from pyridine-based ligands.1 Milstein
and co-workers have reported that a novel Ru catalyst having a
pyridine-based PNN pincer ligand is capable of proceeding the C–
H bond activation of alcohols and amines, triggered by deprotona-
tion and synergetic dearomatization processes of the ligand by the
metal-ligand cooperation.2 The deprotonated complex is produced
as a key intermediate and stabilized by the contribution of a
dearomatized resonance structure among plausible resonance
structures as shown in Scheme 1,3 although the free pyridylmethyl
group is not acidic without metal coordination. In fact, the contri-
bution of dearomatized resonance structures was suggested by
crystal structures of deprotonated complexes in the light of bond
The concept of metal-ligand cooperation is expected to be ex-
panded to metal complexes having versatile and well-known chelat-
ing ligands without NH groups. In contrast to the aforementioned
BPA ligand, tripodal pyridylamine ligands such as TPA (= tris(2-
pyridylmethyl)amine) have no easily removable protons and only
methylene protons can be deprotonated.8,9 So far, a large number of
metal-TPA complexes have been reported to investigate their reac-
tivity in oxidation reactions and in O2 activation chemistry.10 How-
ever, there is no report on the observation of a metal complex hav-
ing deprotonated TPA as a ligand and redox non-innocent behavior
Scheme 1
H
H
•
H
H
H
H
–H+
+H+
–e–
X
X
X
X
N
N
N
N
Mn+
Mn+
Mn+
Mn+
A
B
Redox Non-Innocent
Character
H
X = N or P
X
X
N
N
Mn+
Mn+
Figure 1. (a, b) Chemical structures of Rh(III) complexes used in
this study. (c) An ORTEP drawing of [RhIII(Cl)2(L2-H6)]+.
C
D
Metal-Ligand Coorperation
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(a)
pyridine protons
methylene protons
(a)
1.5
1.0
0.5
0
1
2
3
4
293 nm
1.5
1.0
0.5
0
5
0
1
2
(b)
10
6
equiv of KOtBu
7
8
9
300
400
500
600
Wavelength, nm
1.5
1.0
0.5
0
(b)
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0.45
δ, ppm
0.30
0.15
1
Figure 2. H NMR spectra of (a) [RhIII(Cl)2(L1-H6)]+ (2.1 mM)
and (b) [RhIII(L1-H5)(Cl)2] obtained upon addition of KOH (5.0
mM) in DMSO-d6 at 298 K under Ar. Dotted lines denote the chemi-
cal shift of the axial pyridine moiety before and after deprotonation
process.
293 nm
0
5
10
[DBU]0, mM
300
400
500
600
Wavelength, nm
of any metal-bound TPA derivatives.
We report herein unique deprotonation from the TPA ligand
bound to a Lewis-acidic Rh(III) center via metal-ligand coopera-
tion as shown in Scheme 1 and subsequent one-electron oxidation
of a deprotonated complex allows us to investigate redox-non-
innocent behavior of the TPA ligand. The Rh(III) ion was selected
to stabilize the deprotonated species11 toward further chemical
manipulation based on the Lewis acidity and to perform oxidation
of the deprotonated ligand based on the redox-inactive character,
which fixes the oxidation state to be the Rh(III) state due to the
instability of the Rh(II) and Rh(IV) states in the TPA environ-
ment.10c
Figure 4. UV-vis spectra observed in titration of [RhIII(Cl)2(L1-H6)]+
(0.10 mM) with (a) KOtBu or (b) DBU in CH3CN at 294 K under Ar.
Inset: Plots of Abs at 293 nm vs initial concentration of bases. A dotted
line denotes UV-vis spectrum of DBU (15 mM) in CH3CN.
1
nated complexes were assigned by H-1H COSY spectroscopy
1
(Figure S1 in SI). The H NMR signals due to the axial pyridine
moiety of [RhIII(L1-H5)(Cl)2] (L1-H5: deprotonated TPA) were
revealed to show larger change of chemical shifts in comparison
with those assigned to the equatorial pyridine moieties by the
deprotonation. In addition, peak integration of the singlet signal at
4.15 ppm due to the methylene moiety of the axial pyridylmethyl
arm in [RhIII(L1-H5)(Cl)2] decreased to about 50% of the original
value for the singlet peak at 5.33 ppm observed for [Rh(Cl)2(L1-
H6)]+ (Figure 2). These results indicate that the deprotonation of
[RhIII(Cl)2(L1-H6)]+ occurs at the axial pyridylmethyl group rather
than at the equatorial pyridylmethyl groups.
Synthesis of [RhIII(Cl)2(L1-H6)](Cl)12a and [RhIII(Cl)2(L2-
H6)](PF6) (L2-H6: TPACOOMe = tris(5-methoxycarbonyl-2-
pyridylmethyl)amine12b) was accomplished by synthetic routes
described in Supporting Information (SI). The methoxycarbonyl
(COOMe) groups were introduced to the 5-positions of all pyri-
dine rings to stabilize the deprotonated complex (vide infra). The
crystal structure of [RhIII(Cl)2(L2-H6)](PF6) was determined by X-
ray crystallography as shown in Figure 1c. It should be noted that
the impact of introduction of the COOMe groups on the structural
parameters of [RhIII(Cl)2(L2-H6)]+ is negligible; there is no differ-
ence (< 0.01 Å) of bond lengths of Rh–Nx (x = 1-4) between
[RhIII(Cl)2(L1-H6)]+ and [RhIII(Cl)2(L2-H6)]+ (see Table S2 in SI).
Upon addition of a strong base such as potassium tert-butoxide
In order to clarify the reason for the selectivity of the deprotona-
tion at the axial methylene moiety, we evaluated the thermodynam-
ic stability of [RhIII(L1-H5)(Cl)2] (deprotonation from the meth-
ylene moiety of the axial pyridylmethyl arm) and [RhIII(L1-
H5eq)(Cl)2] (deprotonation from the methylene moiety of one of
the equatorial pyridylmethyl arms), using density functional theory
(DFT) calculations. As a result, [RhIII(L1-H5)(Cl)2] was indicated
to be more stable by 9.8 kcal/mol than [RhIII(L1-H5eq)(Cl)2]. This
difference should be derived from the conformational distortion of
[RhIII(L1-H5eq)(Cl)2] as depicted in Figure S3 in SI: The distor-
tion of [RhIII(L1-H5eq)(Cl)2] would be derived from the rotation
around an N-Rh axis due to the sp2 character of the deprotonated
equatorial methylene carbon.
The structure of [RhIII(L1-H5)(Cl)2] was further confirmed by
X-ray crystallography as shown in Figure 3.14 The bond length of
C1–C2 in [RhIII(L1-H5)(Cl)2] (1.364(8) Å) is much shorter than
that of [RhIII(Cl)2(L1-H6)]+ (1.480(9) Å12a), as described in Table
S3 and Figure S4 in the SI. Judging from the structural change,
difference of bond lengths was mainly observed on the axial pyri-
dine moiety in comparison with those of equatorial pyridine moie-
ties even in the crystal. This result is consistent with those obtained
from the spectroscopic measurements. The structure of [RhIII(L1-
H5)(Cl)2] also indicate that the dearomatized resonance structures
of B, C, and D (Scheme 1) of the axial pyridylmethyl moiety
(KOtBu) or KOH into
a DMSO-d6 solution containing
[RhIII(Cl)2(L1-H6)]+, drastic change of the H NMR spectrum was
observed as shown in Figure 2.13 The 1H NMR signals of deproto-
1
Figure 3. A schematic description of [RhIII(L1-H5)(Cl)2] and an
ORTEP drawing of the crystal structure.
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should be dominant in the actual structure of [RhIII(L1-H5)(Cl)2]
compared to the aromatic structure of A as shown in Scheme 1. In
addition, the coordination of the pyridine N atom to a highly Lew-
is-acidic Rh(III) center should stabilize the dearomatized resonan-
ce structure D (Scheme 1) of [RhIII(L1-H5)(Cl)2] by trapping the
negative charge on the N atom.
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When we titrated [RhIII(Cl)2(L1-H6)]+ with KOtBu by using
UV-vis spectroscopy in CH3CN at 294 K, one equivalent of KOtBu
was consumed to complete the spectral change, which should be
derived from the deprotonation of [RhIII(Cl)2(L1-H6)]+, as shown
in Figure 4a. This result indicates that the further deprotonation of
[RhIII(L1-H5)(Cl)2] is negligible. Upon addition of 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU) as a weaker base (pKa =
24.3 in CH3CN)15 than KOtBu, saturation behavior was observed
in a titration plot as shown in Figure 4b. The pKa value of the axial
methylene proton of [RhIII(Cl)2(L1-H6)]+ was thus determined to
be 27.3 in CH3CN at 294 K, according to the equation of the acid-
base equilibrium (Experimental Section in SI). Similarly, the pKa
value of [RhIII(Cl)2(L1-H6)]+ in DMSO was also determined to be
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Figure 5. Methylation of [RhIII(L1-H5)(Cl)2] and an ORTEP
drawing of [RhIII(Cl)2(L1-H5Me)]+.
doublets due to the methyl group and the methyne proton of the
reacted axial pyridylmethyl arm with the same coupling constant
(6.8 Hz), together with two AX doublets (15.3 and 16.5 Hz) as-
signed to the equatorial methylene protons (Figure S8 in SI). The-
se results indicate that the methylation of [RhIII(L1-H5)(Cl)2] only
occurs at the axial methylene position, although the negative charge
should delocalize on the pyridine moiety as well.17 The site selectiv-
ity of the methylation reaction mainly depends on the stability of
the methylated product rather than the population of the negative
charge, because the methylation at the 3 or 5-position of the axial
pyridine ring produces an unstable dearomatized product. The
selective methylation of the axial methylene moiety is expected to
embark a new method to provide a chiral TPA derivative by using a
chiral reagent toward the development of TPA-based chiral cata-
lysts for asymmetric transformation.
1
16.1 by H NMR titration with potassium acetylacetonate (pKa =
15.1 in DMSO)16 (Figure S5 in SI). This pKa value in DMSO was
significantly smaller than that of 2-benzylpyridine as a reference
compound without a Rh(III) ion (pKa = 28.8 in DMSO).16 The
large decrease of the pKa value is derived from the stabilization of
[RhIII(L1-H5)(Cl)2] by coordination of deprotonated axial pyridine
to a highly Lewis-acidic Rh(III) center as evidenced by the analysis
on the crystal structure shown in Figure 3.
If the localized negative charge of the dearomatized resonance
structure B or C depicted in Scheme 1 would be further stabilized
by electron-withdrawing substituents introduced to the corre-
sponding positions, the deprotonated complex should be more
stabilized. According to this hypothesis, COOMe groups as elec-
tron-withdrawing substituents were introduced to the 5-positions
of all pyridyl groups of TPA as described in Figure 1b. Similar spec-
tral change was detected for the 1H NMR spectrum of [Rh(Cl)2(L2-
H6)]+ as observed for [Rh(Cl)2(L1-H6)]+, as depicted in Figure S6
(SI). This result indicates that one methylene proton of the axial
pyridylmethyl arm in [Rh(Cl)2(L2-H6)]+ is removed to afford
[Rh(Cl)2(L2-H5)]+. The pKa value of [RhIII(Cl)2(L2-H6)]+ was
determined to be 20.9 in CH3CN at 296 K by UV-vis titration with
triethyl amine (pKa = 18.8 in CH3CN),15 as shown in Figure S7 in
SI. The lower pKa value indicates that stabilization of the deproto-
nated complex is made through delocalization of the negative
charge at the 5-position (dearomatized structure B in Scheme 1)
into the methoxycarbonyl group. On the basis of the pKa values of
[RhIII(Cl)2(L1-H6)]+ (27.3) and [RhIII(Cl)2(L2-H6)]+ (20.9) in
CH3CN, the deprotonated species derived from the latter is more
stable by 8.7 kcal mol–1 (ΔΔG°) than that derived from the former.
In order to develop reactivity of the Rh(III) complex having the
deprotonated TPA ligand, we examined a reaction of the deproto-
nated Rh(III)-TPA complex with methyl iodide (MeI). Upon addi-
tion of MeI into the DMF solution of [RhIII(L1-H5)(Cl)2], methyl-
ation of [RhIII(L1-H5)(Cl)2] occurred at 298 K. The structure of
the isolated methylated product ([RhIII(Cl)(L1-H5Me)]+ (L1-
H5Me = methylated TPA) was determined by X-ray crystallog-
raphy to reveal that the methylation occurred at the deprotonated
methylene moiety as shown in Figure 5. Since the methylation
induces the chirality at the methyne carbon, in the monoclinic crys-
tal, both enantiomers (the R- and S-form) existed as a racemic mix-
ture. The 1H NMR spectrum of [RhIII(Cl)2(L1-H5Me)]+ showed
Redox properties of the deprotonated Rh(III)-TPA complexes
were examined by cyclic voltammogram (CV) measurements in
CH3CN. The CV trace of [RhIII(L1-H5)(Cl)2] exhibited only one
irreversible anodic wave at 0.08 V vs SCE even at 233 K under Ar as
shown in Figure S10 in SI. In sharp contrast, a pseudo-reversible redox
wave was observed for [RhIII(L2-H5)(Cl)2] with a peak separation
(ΔEp = 75 mV) as shown in Figure 6a.18 The one-electron oxidation
potential (E1/2) of [RhIII(L2-H5)(Cl)2] was determined to be 0.40
V vs SCE in CH3CN at 233 K. The reversibility was drastically
improved by the introduction of the electron-withdrawing
COOMe groups to the 5-positions of pyridine rings to lower the
SOMO level, because the SOMO of one-electron oxidized
[RhIII(L2-H5)(Cl)2] ([RhIII(Cl)2(L2-H5•)]+) was indicated to de-
localize over the axial methylene moiety and the 3- and 5-position
of the pyridine ring by DFT calculations as shown in Figure S12 in
SI. This reversibility allows us to elucidate the oxidation reaction as
discussed below. In order to confirm that the oxidation of [RhIII(L2-
H5)(Cl)2] occurs at the metal center or the deprotonated TPA
ligand, ESR measurements were performed on the solution of
[RhIII(L2-H5)(Cl)2] with addition of 1 equiv of one-electron oxi-
dant such as [RuIII(bpy)3]3+ (bpy = 2,2’-bipyridyl) at 100 K. As a
result, a featureless ESR signal was observed at g = 2.0043 as shown
Figure 6. (a) CV of [RhIII(L2-H5)(Cl)2] in CH3CN at 233 K and (b)
ESR spectrum of [RhIII(Cl)2(L2-H5•)]+ measured at 100 K after the
reaction of [RuIII(bpy)3]3+ and [RhIII(L2-H5)(Cl)2].
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in Figure 6b. Judging from the g value, the one-electron oxidation
of [RhIII(L2-H5)(Cl)2] takes place on the ligand to form a Rh(III)
complex with a carbon-centered ligand radical ([RhIII(Cl)2(L2-
H5•)]+) rather than at the Rh(III) center, in the light of the g value
reported for Rh(IV)Cl6 (g = 2.08).19 The carbon-centered radical
was further confirmed by a reaction with a nitroxyl radical (2,2,6,6-
tetramethylpiperidine 1-oxyl = TEMPO) in CH3CN at 233 K; the
spin adduct, [Rh(Cl)2(L2-H5-TEMPO)]+, was characterized by
1
2
3
4
5
6
7
8
1
ESI-MS and H NMR measurements (Figure S13 in the SI).20,21
Thus, this is the first observation of redox non-innocent behavior of
a metal-bound TPA ligand, after deprotonation of a methylene
proton of the ligand.
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In summary, we have successfully demonstrated selective and
reversible deprotonation of the methylene moiety of the axially
coordinated pyridylmethyl arm of TPA ligands and the reactivity of
the deprotonated TPA ligands in the Rh(III) coordination sphere.
The deprotonation is governed by the stabilization of the dearoma-
tized resonance structures (Scheme 1): For this reason, the coordi-
nation of a Lewis-acidic metal ion and the introduction of electron-
withdrawing groups to the position where the negative charge can
be delivered in the resonance structures are effective. The deproto-
nation of the TPA ligand allows us to encounter unprecedented
redox-non-innocent characteristics of the versatile tetradentate
ligands involving a carbon-centered radical of metal-bound TPA.
The results described in this paper will provide a basis toward the
development of new functionality of transition metal-TPA com-
plexes.
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(11) Investigation on deprotonation of other metal-TPA complexes
is currently ongoing in our laboratory.
ASSOCIATED CONTENT
Supporting Information. Figures S1–S13, Tables S1–S3 and
crystallographic data for [RhIII(Cl)2(L2-H6)]+, [RhIII(L1-H5)(Cl)2],
and [RhIII(Cl)2(L1-H5Me)]+ in the CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
(12) (a) Lonnon, D. G.; Craig, D. C.; Colbran, S. B. Dalton Trans.
2006, 3785–3797. (b) RuII complexes of L2-H6 have been re-
ported: Yamaguchi, M.; Kousaka, H.; Izawa, S.; Ichii, Y.; Ku-
mano, T.; Masui, D.; Yamagishi, T.; Inorg. Chem. 2006, 45,
8342–8354.
AUTHOR INFORMATION
Corresponding Author
1
(13) Similar H NMR spectral change due to the deprotonation of
kojima@chem.tsukuba.ac.jp
[RhIII(Cl)2(L1-H6)]+ was also observed in CD3CN, although
the line broadening of the signals assigned to the axial pyridyl-
methyl moiety occurred by fast proton exchange between water
and the highly basic deprotonated complex.
ACKNOWLEDGMENT
This work was partially supported by a Grant-in-Aid (No
24245011) from the Japan Society of Promotion of Science (JSPS,
MEXT) of Japan, and the Cooperative Research Program of "Net-
work Joint Research Center for Materials and Devices". We appre-
ciate Dr. Motoo Shiro (Rigaku Corp., Japan) for his kind help in X-
ray analysis and Prof. S. Yamago (Kyoto University) for his helpful
suggestion on the radical trapping.
(14) One of three independent [RhIII(L1-H5)(Cl)2] structures in the
asymmetric unit was selected. The ORTEP drawing containing
the other [RhIII(L1-H5)(Cl)2] structures also revealed a depro-
tonated form because there is no counter anions in the asym-
metric unit.
(15) Kaljurand, I.; Kütt, A.; Sooväli, L.; Rodima, T.; Mäemets, V.;
Leito, I.; Koppel, I. A. J. Org. Chem. 2005, 70, 1019–1028.
(16) Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456–463.
(17) The HOMO of [RhIII(L1-H5)(Cl)2] was shown to delocalize on
the methylene carbon, the 3- and 5-positions as well as the N-
atom of the axial pyridine ring by DFT calculations (Figure S9
in SI).
REFERENCES
(1) (a) Grützmacher, H. Angew. Chem., Int. Ed. 2008, 47, 1814–
1818. (b) Gunanathan, C.; Milstein, D. Acc. Chem. Res. 2011,
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(2) Gunanathan, C.; Ben-David, Y.; Milstein, D. Science 2007, 317,
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(3) (a) Ben-Ari, E.; Leitus, G.; Shimon, L. J. W.; Milstein, D. J. Am.
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(4) (a) Lu, C. C.; Bill, E.; Weyhermüller, T.; Bothe, E.; Wieghardt,
K. J. Am. Chem. Soc. 2008, 130, 3181–3197. (b) Shaffer, D.
(18) The lifetime of [Rh(Cl)2(L2-H5•)] was estimated to be 8.3 s on
the basis of CV measurements (Figure S12 in the SI).
(19) Ellison, I. J.; Gillard, R. D. Polyhedron 1996, 15, 339–348.
(20) Beckwith, A. L. J.; Bowry, V. W.; Ingold, K. U. J. Am. Chem. Soc.
1992, 114, 4983–4992.
1
(21) [RhCl2(L2-H5-TEMPO)]+: ESI-MS, m/z = 792.00 ; H NMR
(270 MHz, CD3CN), 6.32 ppm (s, CH), 4.98 and 5.72 ppm (2
ABq, 16 Hz, CH2)).
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