Photochemically initiated oxidative carbon-carbon bond-cleavage reactivity in chlorodiketonate NiII complexes

Article abstract of DOI:10.1002/chem.201101962

Three mononuclear Ni^II complexes containing a 2-chloro-1,3-diketonate ligand and supported by the 6-Ph₂TPA chelate, as well as analogues that lack the 2-chloro substituent on the β-diketonate ligand, have been prepared and characterized. Upon irradiation at 350 nm under aerobic conditions, complexes containing the 2-chloro-substituted ligands undergo reactions to generate products resulting from oxidative cleavage, α-cleavage, and radical-derived reactions involving the 2-chloro-1,3-diketonate ligand. Mechanistic studies suggest that the oxidative cleavage reactivity, which leads to the production of carboxylic acids, is a result of the formation of superoxide, which occurs through reaction of reduced nickel complexes with O₂. The presence of the 2-chloro substituent was found to be a prerequisite for oxidative carbon-carbon bond-cleavage reactivity, as complexes lacking this functional group did not undergo these reactions following prolonged irradiation. The approach toward investigating the oxidative reactivity of metal β-diketonate species outlined herein has yielded results of relevance to the proposed mechanistic pathways of metalloenzyme-catalyzed β-diketonate oxidative cleavage reactions.

Full text of DOI:10.1002/chem.201101962

DOI: 10.1002/chem.201101962
Photochemically Initiated Oxidative Carbon–Carbon Bond-Cleavage
Reactivity in Chlorodiketonate NiII Complexes
Caleb J. Allpress,[a] Atta M. Arif,[b] Dylan T. Houghton,[c] and Lisa M. Berreau*[a]
Abstract: Three mononuclear NiII com-
plexes containing a 2-chloro-1,3-diketo-
nate ligand and supported by the 6-
Ph2TPA chelate, as well as analogues
that lack the 2-chloro substituent on
the b-diketonate ligand, have been pre-
pared and characterized. Upon irradia-
tion at 350 nm under aerobic condi-
tions, complexes containing the 2-
chloro-substituted ligands undergo re-
actions to generate products resulting
from oxidative cleavage, a-cleavage,
and radical-derived reactions involving
the 2-chloro-1,3-diketonate ligand.
Mechanistic studies suggest that the ox-
idative cleavage reactivity, which leads
to the production of carboxylic acids, is
a result of the formation of superoxide,
which occurs through reaction of re-
duced nickel complexes with O2. The
presence of the 2-chloro substituent
was found to be a prerequisite for oxi-
dative carbon–carbon bond-cleavage
reactivity, as complexes lacking this
functional group did not undergo these
reactions following prolonged irradia-
tion. The approach toward investigat-
ing the oxidative reactivity of metal b-
diketonate species outlined herein has
yielded results of relevance to the pro-
posed mechanistic pathways of metal-
loenzyme-catalyzed b-diketonate oxi-
dative cleavage reactions.
Keywords: diketones · dioxygenase ·
iron
· nickel · oxygen · redox
chemistry · superoxides
Introduction
superoxide at the terminal carbon to yield an organoperoxo
species. Attack of the terminal oxygen of the peroxo moiety
at the other carbonyl produces a five-membered dioxetane
ring from which aliphatic oxidative carbon–carbon bond
cleavage and CO release is proposed to occur.
Reaction pathways leading to the oxidative cleavage of the
aliphatic carbon–carbon bonds of b-diketonate-type mole-
cules are of current interest due to the identification of en-
zymes that catalyze the cleavage of these strong bonds.[1] Of
primary interest toward better understanding the chemistry
of such systems is the elucidation of how changes in the
nature of the metal center, or the structural and/or electron-
ic features of the b-diketonate, influence the reaction mech-
anism. An example of a metalloenzyme of this class is acire-
ductone dioxygenase (Ni–ARD), which uses a mononuclear
NiII center as a Lewis acid to stabilize a dianionic form of
the acireductone substrate in a b-diketonate-type coordina-
tion motif (Scheme 1 (top)).[2] The acireductone dianion acts
as a reductant toward O2. Transfer of two electrons from the
enediolate to O2 is proposed to occur through an initial
single-electron transfer to form superoxide and an organic
radical, followed by collapse of the radical pair by attack of
A different reaction pathway is proposed for the FeII-con-
taining acetylacetone dioxygenase Dke1, which catalyzes the
oxidative decomposition of acetylacetone to acetate and
methyl glyoxal.[3] Kinetic studies suggest that the role of the
FeII center is to help overcome the spin forbidden nature of
the reaction between triplet dioxygen and the singlet sub-
strate by delocalization of electron density through orbital
mixing of the HOMOs of the b-diketonate and FeII center
(Scheme 1 (bottom)). Electron transfer from the coordinat-
ed b-diketonate is suggested to occur in a concerted fashion
ꢀ
with C O bond formation at C(2) to give a peroxide species
from which carbon–carbon bond cleavage occurs. It should
be noted that replacing FeII in Dke1 with other divalent
metal ions, including NiII, results in a loss of catalytic activi-
ty.[4] In model studies of relevance to Dke1, Limberg et al.
have shown that although [Tp*Fe
AHCTUNGERTGtNNUN ris(3,5-dimethylpyrazol-1-yl)borato, acac=acetylacetonate)
does not undergo reaction with O2 to give products of acety-
lacetonate oxidative cleavage, a structurally similar complex
of the more electron-rich diethylphenylmalonate b-diketo-
ACHTUNGERTN(NUNG acac)] (Tp*=hydrido-
[a] C. J. Allpress, Dr. L. M. Berreau
Department of Chemistry & Biochemistry, Utah State University
Logan, UT 84322-0300 (USA)
Fax : (+1)435-797-3390
E-mail: lisa.berreau@usu.edu
[b] Dr. A. M. Arif
nate, [Tp*FeACTHNUGTRNEUNG(Phmal)] (Phmal=diethyl phenylmalonate), un-
Department of Chemistry, University of Utah
Salt Lake City, UT 84112 (USA)
dergoes O2-dependent carbon–carbon bond-cleavage to give
Dke1-type products (ethyl benzoylformate, ethoxide, and
[c] D. T. Houghton
ꢀ
CO2 through decomposition of EtOCO2 ).[5] Control studies
Department of Chemistry, University of Wyoming
Laramie, WY 82071 (USA)
indicate that the formation of an FeIII superoxide species is
necessary in this model system for the observed reactivity.
In a very recent study of relevance to Dke1, Nunes et al.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201101962.
14962
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Chem. Eur. J. 2011, 17, 14962 – 14973

FULL PAPER
Complexes that are structur-
ally similar to 1 but instead
have an unsubstituted b-diketo-
nate ligand, such as [(6-
Ph2TPA)Ni
Ph)][ClO4] (9), are stable with
respect to O2 under ambient
conditions.[7]
As described
herein, NiII complexes having a
mildly electron-withdrawing
ACHTUNGTREN(NGNU PhC(O)CHC(O)-
R
ACHTUNGTRENNUNG
chloride at the C(2) position,
for example, [(6-Ph2TPA)Ni-
ACHUTNGRENNU(G PhC(O)C(Cl)C(O)Ph)]ACHTUNGTRENNUNG[ClO4]
(6), are also stable with respect
to O2 under ambient conditions.
The lack of dioxygen reactivity
for 6 and 9 is not unexpected
because the reduced electron
density within the p system of
the b-diketonate moiety makes
these anions poorer reducing
agents than the a-hydroxy-con-
taining analogue.
Previous studies[9] of the pho-
tochemistry of NiII b-diketonate
species led us to consider how
this approach might be used to
induce redox activity at the
Scheme 1. Reactions catalyzed by Ni–ARD and Dke1.
have reported the formation of [Ni(en)
N
ACHTUNGRTEN[NUNG PF6]
(en=ethylenediamine) through treatment of [NiAHCTUNGTRENNUNG
(acac)2-
ACHTUNGTRENNUNG(H2O)2] with ethylenediamine in aerobic, aqueous solution.
The oxidative cleavage of acacꢀ to give acetate is suggested
to involve superoxide generated in the reaction mixture,
however no further details were provided.[6]
As can be seen in the chemistry of Ni–ARD, Dke1, and
related model systems, specific combinations of b-diketonate
ligand and metal ion impart oxidative carbon–carbon bond-
cleavage reactivity. In Ni–ARD, a typically redox-inactive
metal center stabilizes the dianionic form of a b-diketonate-
type substrate having inherent reductive reactivity toward
O2. In Dke1, a potentially redox-active FeII center is part-
nered with the O2-stable acetylacetone substrate. In this
latter case, the metal center is important for redox reactivity
with O2 to initiate the oxidative cleavage process.
Figure 1. Synthetic complex studied as a model system for NiII-contaning
acireductone dioxygenase 1 and analogues 6 and 9 that are O2-stable
under ambient conditions.
We have previously shown that the b-diketonate complex
nickel center of 6 and 9 as a means toward promoting oxida-
[(6-Ph2TPA)Ni
G
G
tive carbon–carbon bond-cleavage reactivity. It is known, for
reacts with O2 to give products (carboxylates and CO) akin
to those generated in the Ni–ARD-catalyzed reaction.[7] The
mechanism by which this occurs involves two-electron oxi-
dation of the enediolate moiety to give an intermediate tri-
ketone species that subsequently undergoes reaction with
HOOꢀ, generated in the reaction mixture, to give aliphatic
carbon–carbon bond-cleavage products.[8] Although this
mechanism differs from that proposed for the enzyme[1], this
system does mimic Ni–ARD in terms of incorporating the
combination of an inherently O2-reactive b-diketonate
ligand with a redox-inactive NiII center.
example, that [NiACTHNGUTERNNU(G acac)2] undergoes photoreduction to pro-
duce transient NiI complexes in the presence of stabilizing
ligands.[9] Based on this literature precedent, if NiI b-diketo-
nate species can be photochemically generated, starting
from complexes, such as 6 and 9, this offers the possibility of
producing a combination of a NiI center and a b-diketonate
radical. The NiI could then activate O2 to generate NiII and
ꢀ
O2 , from which subsequent oxidative chemistry may
occur.[10] This net transfer of one electron from the b-diketo-
nate to O2 would be similar to the mechanism proposed for
the Dke1 enzyme, in which the ligand is also oxidized by
Chem. Eur. J. 2011, 17, 14962 – 14973
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www.chemeurj.org
14963

L. M. Berreau et al.
one electron to form superoxide, albeit in the nickel case
this would involve direct electron transfer rather than orbi-
tal mixing.
Herein, we outline the results of studies into the photo-
chemical reactivity of 6 and 9 and para-substituted ana-
logues. The results show that oxidative cleavage products
are generated for systems containing a 2-chloro substituent
on the coordinated enolate.
and UV/Vis spectroscopy, and in some cases, mass spectrom-
etry and/or X-ray crystallography.
Single-crystal X-ray structures were obtained for 5, 6, and
8. Details of the data collection and refinement are given in
Table 1. Selected bond lengths and angles are given in
Table 2. These complexes all contain a pseudo-octahedral
NiII cation with bidentate coordination of the corresponding
b-diketonate (Figure 2 and Figure S1 in the Supporting In-
formation). Bond lengths within the six-membered chelate
ring in each complex are typical for nickel b-diketonates.[7]
ꢀ
Results and Discussion
The C Cl bond lengths in 5 and 6 are within the range pre-
viously reported for metal-bound 2-chloro-1,3-diketonates
(1.68–1.81 ꢁ), being particularly close to that reported for
Preparation of 2-chloro-1,3-diones: b-diketones can be
chlorinated at the a position by using various reagents.[11]
Recent studies in this area have focused on the use of hyper-
valent iodine reagents[12] and reactions performed by using
N-chlorosuccinimide in an ionic liquid[13] or CCl4.[14] In a
new synthetic route (Scheme 2 (top)), we have found that
treatment of the appropriate b-diketone (2a–2c) with
RuCl3/Oxone/H4NCl in CH3CN/H2O/EtOAc gives the 2-
chloro compounds (3a–3c) as crystalline solids in high
purity, following recrystallization(s).
Complex synthesis and characterization: A series of mono-
nuclear NiII 2-chloro-1,3-diketonate complexes (4–6) sup-
ported by the 6-Ph2TPA ligand were prepared as outlined in
the bottom equation in Scheme 2. Crystalline solids were
isolated in yields greater than 70%. For reactivity compari-
son, structurally similar b-diketonate complexes lacking the
2-chloro substitutent (7–9) were generated. We note that
complex 9 has been reported previously.[7] Each new com-
Figure 2. Thermal ellipsoid representation of the cationic portion of 5. El-
lipsoids are drawn at the 50% probability level. Hydrogen atoms have
been omitted for clarity.
1
plex was characterized by elemental analysis, IR, H NMR,
CuII complexes (1.739(3) and
1.755(3) ꢁ).[15] As can be seen
in Table 2, there are only subtle
ꢀ
changes in the Ni N/O bond
lengths as the nature of the sub-
stituents on the b-diketonate is
changed. Comparison of the
structural features of 6 to those
of 1 (Figure 1) revealed that the
ꢀ
Ni NPhPy bonds are slightly
elongated in the 2-hydroxy-1,3-
diketonate complex relative to
those found in 2-chloro ana-
ꢀ
logue 6 (Ni NPhPy, 1: av. 2.30 ꢁ;
6: av. 2.25 ꢁ).
Selected spectroscopic data
for 4–9 is given in Table 3. Each
b-diketonate complex has an
absorption feature at ꢁ370 nm
that may be assigned as a p!
p* transition involving the b-di-
Scheme 2. Synthesis of 2-chloro-1,3-diones (top) and NiII 2-chloro-1,3-diketonate complexes supported by the
6-Ph2TPA ligand (bottom).
ketonate ligand. Comparison of
the
2-chloro-1,3-diketonate
14964
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 14962 – 14973

Chlorodiketonate NiII Complexes
FULL PAPER
Table 1. Crystallographic data for 5, 6, and 8.
Reactivity studies: It was found that the yellow
color of solutions of the 2-chloro-1,3-diketonate
complexes 4–6 in acetonitrile faded (lirr =350 nm)
over the course of several hours under aerobic con-
ditions upon exposure to UV light. As shown in
Figure 4 for methoxy-substituted compound 4, this
corresponds to loss of the p!p* absorption band
of the b-diketonate at 378 nm. An isosbestic point
for the reaction was identified at 269 nm.
5·C4H10O
6
8·2CH2Cl2
formula
Mr
T [K]
C47H40Cl2N4NiO6·C4H10O C45H36Cl2N4NiO6 C49H41ClN4NiO6·2CH2Cl2
960.56
858.39
150(1)
1021.85
150(1)
150(1)
crystal size
[mm]
0.30ꢃ0.28ꢃ0.20
0.38ꢃ0.15ꢃ0.13 0.30ꢃ0.22ꢃ0.12
crystal
system
space group P1
triclinic
triclinic triclinic
¯
¯
¯
P1
P1
a [ꢁ]
b [ꢁ]
c [ꢁ]
a [8]
b [8]
g [8]
V [ꢁ3]
Z
11.96810(10)
9.7136(1)
13.2959(2)
16.7754(3)
77.5113(7)
76.7675(10)
68.7334(9)
1944.11(5)
2
13.5383(2)
17.0153(2)
23.0279(3)
97.2779(7)
92.9850(7)
112.8856(7)
4817.82(11)
4
The products of each aerobic photochemical re-
action were determined after irradiating a 0.002m
solution of each complex (4–6) in acetonitrile at
350 nm for 20 h, under an aerobic atmosphere. The
NiII-containing products were identified by using
1H NMR spectroscopy (Figures 5–7) and ESI-MS.
The most reactive compound was found to be me-
thoxy derivative 4 (Figure 5). A major NiII product
of this reaction is the chloride complex [(6-
13.6006(2)
16.1278(3)
108.5037(6)
104.6479(9)
102.9892(9)
2271.55(6)
2
1calcd
1.404
1.466
1.409
ACHTUNGTRENNUNG ]
[gcmꢀ3
m [mmꢀ1
(000)
]
0.603
1004
0.693
888
0.733
2112
[ClO4] (10)[17], as shown
Ph2TPA)Ni(Cl)ACTHNUGTRENN(GU CH3CN)]ACHUTNGTRENNUGN
by the presence of resonances at d=54 and 37 ppm.
Also present are resonances associated with a mon-
F
A
V range [8]
index
ranges
1.91–27.48
ꢀ14ꢂhꢂ15
ꢀ17ꢂkꢂ17
ꢀ20ꢂlꢂ20
19512
2.34–27.49
ꢀ12ꢂhꢂ12
ꢀ17ꢂkꢂ17
ꢀ21ꢂlꢂ21
16726
1.71–27.44
ꢀ16ꢂhꢂ17
ꢀ22ꢂkꢂ21
ꢀ29ꢂlꢂ29
40512
oanisate
OCH3C6H4)}]
[(6-Ph2TPA)NiAHCTUNGTRENNUNG
complex
[ClO4] (11) and the dianisate complex
{O2C(p-OCH3C6H4)}2] (12). Com-
[(6-Ph2TPA)NiACHTUNGTRENNUNG{O2C(p-
reflns
AHCTUNGTRENNUNG
collected
unique
10319
8901
21813
plexes 11 and 12 were independently synthesized
and characterized to confirm the assignment. The
formation of 10–12 in the photochemical reaction
of 4 is also supported by ESI-MS investigations.
reflns
Rint
data/restr./
param.
(Rint=0.0235)
0.0235
10319/24/632
(Rint=0.0237)
0.0237
8901/0/533
(Rint=0.0420)
0.0420
21813/12/1184
GoF (F2)
R1, wR2
[I>2s(I)]
R1, wR2 (all
data)
1.024
0.0427, 0.1055
1.027
0.0364, 0.0816
1.026
0.0497, 0.1106
1
From comparison of H NMR spectra, it is evident
that the aerobic photochemical reaction of 4 goes
to completion over the 20 h irradiation period, as
no signals for the starting material could be identi-
fied. A summary of the reaction of methoxy-substi-
tuted 4 is presented in Scheme 3.
0.0617, 0.1163
0.8889/0.8398
0.441/ꢀ0.664
0.0553, 0.0889
0.9153/0.7786
0.333/ꢀ0.510
0.0944, 0.1293
0.9172/0.8101
0.0820/ꢀ0.862
max/min
transmission
D1(max/min)
[eꢁꢀ3
ACHTUNGTRENNUNG ]
The reaction of methyl-substituted 5 under aero-
complexes (4–6) with their unsubstituted analogues (7–9) re-
vealed a redshift of 4 nm for the former.
1
The H NMR features of 1 and 9 have been reported pre-
1
viously.[7,16] Resonances in selected regions of the H NMR
spectra of 4–6, 7 and 8 can be assigned on the basis of chem-
ical shift, integrated intensity, and comparison to previously
studied complexes.[7,16,17] All of the complexes (4–9) exhibit
Cs symmetry in CD3CN on the NMR time scale, as shown
by the presence of only two resonances for the b’ pyridyl
protons (Figure 3), indicating that the phenyl-appended pyr-
idyl rings are equivalent. Diagnostic resonances for the pyr-
idyl-ring b-Hꢂs and phenyl-appended pyridyl-ring b’-Hꢂs
(Figure 3) are found in the region of d=44–48 ppm
(Table 3). The effect of changing the para-substituent on the
aryl groups of the b-diketonate ligand can be seen in the
shift of the pyridyl g-Hꢂs in the two series 4–6 and 7–9, and
in a shift of the b-diketonate methyne proton in 7–9. In the
latter set of complexes, the pyridyl b, b’, and g proton reso-
nances are shifted upfield by d=1–2 ppm relative to the 2-
chloro analogues.
Figure 3. Labeling scheme for the 6-Ph2TPA ligand.
bic conditions also leads primarily to the formation of
chloro complex 10 (Figure 6).[17] The reaction of 6 (Figure 7)
differs from that of 4 and 5 in that 1H NMR resonances
from the starting material are clearly evident in the
1H NMR and ESI-MS spectra after 20 h of irradiation. Note
that as a control, 0.002m solutions of 4–6 in CD3CN were
stored in the dark under ambient aerobic conditions for sev-
Chem. Eur. J. 2011, 17, 14962 – 14973
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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14965

L. M. Berreau et al.
Table 2. Selected bond lengths [ꢁ] and angles [8].
5·C4H10O
6
8·2CH2Cl2
ꢀ
Ni(1) N(1)
2.0569(18)
2.0635(17)
2.3041(17)
2.2261(18)
1.9709(14)
1.9968(14)
1.418(3)
1.411(3)
1.760(2)
88.78(6)
97.65(6)
172.50(6)
178.06(6)
89.74(6)
83.91(7)
100.33(6)
85.78(6)
96.80(7)
78.31(7)
101.07(6)
94.50(6)
80.52(7)
80.30(7)
158.60(7)
88.78(6)
97.65(6)
172.50(6)
178.06(6)
89.74(6)
83.91(7)
100.33(6)
85.78(6)
96.80(7)
2.0404(15)
2.0882(15)
2.3174(15)
2.1890(15)
1.9748(12)
1.9883(13)
1.422(3)
1.397(3)
1.7571(18)
89.13(5)
2.068(2)
2.085(2)
2.283(2)
2.238(2)
1.9796(16)
1.9885(17)
1.401(3)
1.405(3)
–
ꢀ
Ni(1) N(2)
ꢀ
Ni(1) N(3)
ꢀ
Ni(1) N(4)
ꢀ
Ni(1) O(1)
ꢀ
Ni(1) O(2)
ꢀ
C(37) C(38)
ꢀ
C(38) C(39)
ꢀ
C(38) Cl(1)
O(1)-Ni(1)-O(2)
O(1)-Ni(1)-N(1)
O(2)-Ni(1)-N(1)
O(1)-Ni(1)-N(2)
O(2)-Ni(1)-N(2)
N(1)-Ni(1)-N(2)
O(1)-Ni(1)-N(4)
O(2)-Ni(1)-N(4)
N(1)-Ni(1)-N(4)
N(2)-Ni(1)-N(4)
O(1)-Ni(1)-N(3)
O(2)-Ni(1)-N(3)
N(1)-Ni(1)-N(3)
N(2)-Ni(1)-N(3)
N(4)-Ni(1)-N(3)
O(1)-Ni(1)-O(2)
O(1)-Ni(1)-N(1)
O(2)-Ni(1)-N(1)
O(1)-Ni(1)-N(2)
O(2)-Ni(1)-N(2)
N(1)-Ni(1)-N(2)
O(1)-Ni(1)-N(4)
O(2)-Ni(1)-N(4)
N(1)-Ni(1)-N(4)
91.57(7)
90.92(8)
175.39(8)
171.45(8)
95.57(7)
82.29(8)
103.07(8)
93.47(7)
82.18(8)
81.23(8)
100.47(7)
82.25(7)
101.11(8)
75.92(8)
156.18(7)
91.57(7)
90.92(8)
175.39(8)
171.45(8)
95.57(7)
82.29(8)
103.07(8)
93.47(7)
82.18(8)
94.17(6)
176.66(6)
174.91(5)
93.15(6)
83.51(6)
102.77(5)
92.08(5)
87.69(6)
81.71(6)
98.63(5)
84.69(5)
94.29(6)
77.08(6)
158.31(6)
89.13(5)
94.17(6)
176.66(6)
174.91(5)
93.15(6)
83.51(6)
102.77(5)
92.08(5)
87.69(6)
Figure 4. Absorption changes over time observed for the aerobic photo-
chemical reaction of 4.
4–6, and the anisate ligands in 11 and 12, are released upon
passage of the complexes through the silica plug. Thus, the
presence of 2-chloro-1,3-dione in the organic products is fur-
ther evidence for incomplete reactions. The inorganic resi-
due on the silica is eluted by washing with acetonitrile, fol-
1
lowed by methanol. H NMR analysis of the inorganic frac-
tions showed the presence of [(6-Ph2TPA)Ni
[ClO4]2 (13, Figure 5), which was not present in the reaction
mixtures prior to work-up.
ACHTUGNERTN(NUNG CH3CN)2]-
AHCTUNGTRENNUNG
Upon irradiation of 4 for 20 h under aerobic con-
ditions, the primary organic product generated is
para-anisic acid (I, Figure 8), along with lesser
amounts of para-anisil methyl ketone (II), anisalde-
hyde, deoxyanisoin (III), and halogenated species
(for example, 2-chloro-1-(4-methoxyphenyl)propan-
1-one; Figure 8 (top)). Notably, performing the
same reaction with 4 under N2 results in the forma-
tion of only a-cleavage products (for example II
and III, Figure 8 (bottom)). Thus, oxygen is re-
quired for the generation of the carboxylic acid
product.
Table 3. Selected spectroscopic features for 4–9.
4
5
6
7
8
9
lmax [nm][a]
378
1.03
374
1.16
372
0.80
374
2.38
370
2.16
368
1.30
e [104 mꢀ1 cmꢀ1
]
b-py[b]
b’-py
48.4
42.8
46.0
34.8
15.4
–
48.1
43.5
46.0
34.6
15.5
–
48.4
43.5
46.4
34.7
15.5
–
46.0
44.1
44.1
33.8
14.9
ꢀ14.1
47.0
44.4
44.1
33.9
15.0
ꢀ13.9
46.6
44.1
44.9
34.3
15.1
ꢀ13.6
g-py
a-CH
[a] UV/Vis spectra obtained in CH3CN. [b] 1H NMR spectra obtained in CD3CN at
ambient temperature at 400 MHz. Chemical shifts are reported in ppm.
The photoreactivity of the methoxy-substituted 2-
chloro-1,3-dione 3a has previously been evalutated
eral days and produced no decomposition products, as deter-
mined by H NMR spectroscopy.
by Kosmrlj et al.[18] After irradiation of a 0.002m solution of
the diketone at 350 nm for 2 h under aerobic conditions,
they noted the formation of only a-cleavage products
(Scheme 4). Thus, the presence of nickel is required for oxi-
dative cleavage reactivity involving the 2-chloro-1,3-dione.
The aerobic photochemical reaction involving the methyl-
containing NiII 2-chloro-1,3-diketonate complex 5 results in
the formation of multiple products (Figure 8) including fla-
vone and deoxytoluoin, with lesser amounts of para-toluic
acid, para-tolualdehyde and methyl-para-tolyl ketone.
Under a N2 atmosphere, the reaction involving 5 reached
only approximately 80% completion. The organic products
1
For each reaction described above, the organic products
could not be analyzed in the presence of the NiII com-
plex(es). Therefore, these products were separated by filtra-
tion of each reaction mixture through a short silica plug
using ethyl acetate as the eluent. Each sample was analyzed
1
by H NMR spectroscopy and GC-MS. The amount of or-
ganic material isolated from each reaction mixture corre-
sponds to >80% by mass of that expected from the respec-
tive starting 2-chloro-1,3-diketonate ligand. Control reac-
tions indicate that the coordinated 1,3-diketonate ligand in
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Chlorodiketonate NiII Complexes
FULL PAPER
Figure 5. Selected features of the 1H NMR spectra of analytically pure 4
and 10–13 and the reaction mixture produced upon irradiation of 4 under
aerobic conditions at 350 nm for 20 h in CH3CN. All spectra were ob-
tained at 400 MHz in CD3CN.
Scheme 3. NiII complexes generated upon irradiation of 4 at 350 nm for
20 h in aerobic CH3CN.
Figure 6. Selected features of the 1H NMR spectra of analytically pure 5
and 10 and the reaction mixture produced upon irradiation of 5 under
aerobic conditions at 350 nm for 20 h in CH3CN. All spectra were ob-
tained at 400 MHz in CD3CN.
were a similar mixture to that found under aerobic condi-
tions, except para-toluic acid is not generated. As the photo-
reactivity of the methyl-substituted 3b had not previously
been reported, we performed this reaction under the condi-
tions previously employed for 3a and 3c (0.002m in CH3CN
under aerobic conditions with irradiation at 350 nm for
2 h).[18] The primary reaction products generated were the
a-cleavage products methyl-para-tolyl ketone and para-tol-
ualdehyde, with no para-toluic acid.
Figure 7. Selected features of the 1H NMR spectra of analytically pure 6
and 10 and the reaction mixture produced upon irradiation of 6 under
aerobic conditions at 350 nm for 20 h in CH3CN. All spectra were ob-
tained at 400 MHz in CD3CN.
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14967

L. M. Berreau et al.
As noted above, the photochemical reaction of 6 under
aerobic conditions does not go to completion after 20 h of
irradiation at 350 nm. Hence, the major organic product
isolated from the reaction mixture is the unreacted 2-
chloro-1,3-dione 3c. However, of the remaining organic
compounds generated, flavone is the primary product
(Figure 8), along with a small amount of benzoic acid. Per-
forming the irradiation of 6 under a N2 atmosphere result-
ed in less than 10% of the starting material undergoing
reaction, with only trace amounts of flavone generated.
Kosmrlj et al. previously reported that irradiation of un-
substituted 2-chloro-1,3-dione 3c yielded the photocycliza-
tion product IV as the sole product in approximately 50%
yield, regardless of conditions (air, argon or oxygen at-
mosphere).[18]
Complexes 7–9 were tested for photoreactivity under
both aerobic and anaerobic conditions identical to those
used for the 2-chloro-1,3-diketonate complexes 4–6. Anal-
ysis of the product mixtures by 1H NMR spectroscopy
using paramagnetic parameters showed no change from
the starting material.
Mechanistic experiments: The differing product distribu-
tions found for the reactions of 4–6 and the overall lack of
reactivity found for 7–9 was evaluated through consider-
ation of literature precedent and the results of additional
mechanistic experiments. In pioneering work by Lintvedt
et al., it was found that when [NiACHTUNRGTNEUNG(acac)2] is irradiated at
252 nm in ethanol in the absence of O2, free Hacac is pro-
duced along with a colloidal suspension of Ni0 or a Ni0
film (Scheme 5).[19,20] The mechanism for reduction of the
NiII center in [Ni
N
transition to a vibrationally excited p* state from which
the reduction can take place. However, the involvement
of ligand-to-metal charge transfer has also been proposed.
In either scenario, an intial single-electron reduction of
NiII to NiI can occur (Scheme 5).[9] The NiI formed could
then react with a second NiI species in solution to dispro-
portionate and form NiII and Ni0. Interestingly, photo-
Figure 8. Top) Organic products generated in photochemical reactions of
4–6 under aerobic conditions with irradiation at 350 nm for 20 h in
CH3CN. Bottom) Products generated in the same reactions performed
under N2. In each graph the column labeled completion corresponds to
the relative percentage of reaction completion observed after 20 h. The
relative percentages of I–IV refer to the amount of product generated as
a percentage of the overall amount of 2-chloro-1,3-diketonate that under-
went reaction under the prescribed conditions. Other refers to species
such as para-R-arylaldehyde, chloro-containing compounds, and unidenti-
fied compounds.
chemical reduction of [NiACTHNUTRGNE(UNG acac)2] is not observed to occur
in the presence of O2.[19] However, this observed overall
lack of reactivity may represent the sum of a photochemi-
cal reduction followed by rapid reoxidation of the reduced
nickel and complexation to give [Ni
This is consistent with the fact that the anaerobically gen-
erated photoreduction products of [Ni(acac)2] readily oxi-
dize in O2 to regenerate the starting material.[20]
Given the literature prece-
ACHTUNGTERN(NNUG acac)2] (Scheme 5).
AHCTUNGTRENNUNG
dent described above, we hy-
pothesize that low-valent nickel
complexes, likely NiI, are gener-
ated upon photoirradiation of
the 6-Ph2TPA-ligated NiII b-di-
ketonate complexes. Irradiation
on the high-energy shoulder of
the p!p* transition that is
present at 368–380 nm could
Scheme 4. a-Cleavage products formed from the photoirradiation of 3a at 352 nm in aerobic CH3CN for
2 h.[18]
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Chlorodiketonate NiII Complexes
FULL PAPER
dicates the formation of a possibly electrode-deposited
nickel species. Complex 7 exhibits a quasireversible cathodic
wave at approximately ꢀ1.93 V (Figure S2 (bottom) in the
Supporting Information), indicating that the stability of the
NiI b-diketonate anion species is influenced by the nature of
the enolate ligand. In the presence of O2, solutions of 4 and
7 exhibit only a quasireversible cathodic wave consistent
ꢀ [21]
with the reduction of O2 to O2 . The reversibility of this
feature improves in the absence of the complex, indicating
that the electrochemically generated superoxide is reacting
with the complex (either 4 or 7). Notably, we have found
that the NiII b-diketonate complexes 4 and 7 are reactive
with potassium superoxide (solubilized by 18-crown-6) in
acetonitrile. In the reaction of 4, the carboxylic acid product
(I) was detected as the major b-diketonate-derived product.
Literature precedent, as well as experimental evidence,
suggests that the oxidative reactivity leading to carboxylic
acid formation in the photoreactions of 4–6 likely involves
the formation of a trione intermediate. Tada et al. have pro-
posed that in situ generated a-I-b-diketones undergo reac-
tion with O2 upon irradiation with fluorescent light to pro-
duce 1,3-diphenylpropanetrione, PhC(O)C(O)C(O)Ph.[22]
We have shown that the same trione is generated as a reac-
tive intermediate upon reaction of 1 with O2.[8] The trione is
a transient intermediate that may then react with in situ
generated peroxide to form the observed carboxylate cleav-
age products. Alternatively, triones with aryl groups in the
C(1) and C(3) positions may undergo a Lewis acid promot-
ed benzoyl migration and subsequent decarbonylation to
form benzil (PhC(O)C(O)Ph), making the detection of
benzil a potential probe for the detection of triketone inter-
mediates. Careful examination
Scheme 5. Proposed reaction sequence for photoreduction of NiACTHNUTRGNEUNG(acac)2.
produce a high-energy electronic state on the b-diketonate
with reducing character, leading to one-electron reduction
of NiII to NiI and the formation of a b-di
ACTHNUTRGENNkGU eACHTUGNTRENtNUGN onate radical
(Scheme 6). Although we cannot directly probe the NiI/b-di-
ketonate radical species, cyclic voltammetry studies were
performed to provide insight into the redox properties of
the NiII 2-chloro-1,3-diketonate complex 4, as well as an un-
substituted analogue 7. Complex 4 exhibits two poorly re-
versible cathodic features at approximately ꢀ1.82 and
ꢀ1.91 V versus Fc/Fc+ (Figure S2 (top) in the Supporting
Information), suggesting that an initially formed NiI b-di
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
or a chemical reactivity that limits reversiblity. A new oxida-
tive wave at ꢀ0.44 mV, generated after the cathodic scan, in-
of the products generated in
the photoreaction of 6 (labeled
as “other” in Figure 8) shows
the presence of a small amount
of benzil, strongly suggesting
the formation of the triketone
intermediate.[23]
To probe the formation of a
trione intermediate, 18O-label-
ing studies were undertaken
and the results obtained for the
aerobic photoreaction of 6 are
consistent with previous re-
ports. Specifically, the amount
of benzoic acid containing one
18O label (36%) is generally
similar to that found for the re-
action of 1 (ꢁ50%).[8] Labeling
studies were also undertaken to
further explore the photoin-
duced oxidative cleavage reac-
tivity of 4 under aerobic condi-
tions. Irradiation of 4 in the
presence of 18O2 (99%) resulted
Scheme 6. Proposed photochemical reaction pathways for 4–6 leading to the formation of I–IV (diket=diketo-
nate).
in the isolation of a sample of
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14969

L. M. Berreau et al.
para-anisic acid that contains approximately 31% unlabeled
O, 46% with one 18O, and 23% with two 18O atoms (Table
S1 in the Supporting Information). We are cautious to avoid
overinterpreting this labeling data in the absence of a more
thorough kinetic analysis of the reactions. However, we ten-
tatively propose that the observed formation of double-la-
beled anisic acid in the reaction of 4 could be a result of
previously reported scrambling reactions in the presence of
the electron-donating methoxy substituent.[24]
To gain additional insight into the effect of the para-sub-
stituent on the reactivity of the 2-chloro-1,3-diketonate li-
gands in 4–6, the rates of the aerobic photoreactions were
investigated and compared. Monitoring the change in ab-
sorbance at 378 nm (Figure 4) as a function of time for the
photochemical reaction of 4 under aerobic conditions indi-
cated a pseudo first-order process (Figure S3 in the Support-
ing Information). Similar experiments performed by using 5
and 6 demonstrated that the reaction slows in the order 4>
5>6. This is consistent with quantum yields measured for
the reactions in which 4 (0.00038ꢃ0.00002)>5 (0.00032ꢃ
0.00003)>6 (0.00020ꢃ0.00007). Thus, enhanced electron
density within the 2-chloro-1,3-diketonate ligand increases
the quantum yield and the rate of the reaction. As shown in
Scheme 6, the overall low quantum yield of these reactions
may be attributed to recombination of the NiI species with
the b-diketonate radical.
mixture containing 9,10-dihydroanthracene were determined
and qualitatively support the proposed reaction scheme. The
reactivity trend 4>5>6 described earlier can be explained
either in terms of the increased electron-rich character de-
creasing the magnitude of kꢀ1 or increasing the magnitude
of at least one of k2–k4. In fact, the two effects may be
viewed as synergistic. Specifically, the electron-donating me-
thoxy groups will stabilize the one-electron oxidized form of
the b-diketonate with respect to reduction, decreasing kꢀ1,
and the increased electron density should facilitate oxida-
tion, thereby increasing k2.
The observed lack of reactivity for the unsubstituted NiII
b-diketonate complexes 7–9 was investigated by using a
crossover-type experiment. Specifically, an acetonitrile solu-
tion containing equimolar amounts of methoxy-substituted 7
and the sodium salt of dibenzoylmethane was irradiated for
20 h, after which time the reaction was evaluated by using
1H NMR spectroscopy under paramagnetic conditions. At
this point, a mixture of 7 and 9 was present. In the absence
of irradiation, formation of a similar mixture took >7 days
to form. These combined results indicate that in a similar
manner to 4–6, the unsubstituted complexes 7–9 undergo a
photochemical reaction that labilizes the b-diketonate
ligand. The lack of formation of oxidation, a-cleavage, or
flavone products in these systems appears to be due to the
lack of the reactive 2-chloro moiety in the labilized b-diACHTUNGTRENNUNGke-
The formation of the other organic products (II–IV) in
the photoreactions of 4–6 can be rationalized on the basis of
the chemistry of the b-diketonate radical. The production of
II and IV suggests that free neutral 2-chloro-1,3-dione is
generated in the reaction mixture and subsequently under-
goes the photochemical reactions previously reported by
Kosmrlj et al.[18] The formation of free neutral 3a–3c most
likely occurs through hydrogen-atom abstraction reactivity
involving the b-diketonate radical. Although we have not
definitively identified the H-atom donor in the system, we
have found that an aerobic photochemical experiment in-
volving 4 in the presence of the H-atom donor 9,10-dihy-
droanthracene significantly increases the rate of disappear-
ance of the 2-chloro-1,3-diketonate complex. The formation
of III has not been reported in the photochemical reactions
of 3a and 3c,[18] although it has been detected as a product
in the photochemical reactions of dibenzoyldiazomethane,
suggesting decomposition involving the b-diketonate radi-
cal.[25]
As depicted in Scheme 6, we propose that the observed
rate of reaction for 4–6 will be influenced by the magnitude
of the pseudo-first-order rate constants k2, k3, and k4 as com-
pared to kꢀ1. For example, the rate should increase in the
presence of O2 as the pathway represented by k2 becomes
operative. Addition of an H-atom donor compound, such as
9,10-dihydroanthracene, would also be expected to increase
the rate, as the magnitude for k3 would increase and k2 may
also increase due to the ability of a hydrogen-atom donor to
trap a nascent superoxide radical. The relative rates of reac-
tion for 4 under an aerobic or nitrogen atmosphere (Fig-
ure S4 in the Supporting Information) and in an aerobic
ACHTUNGTRENtNUNG onate radical species. The chloro substituent may be neces-
sary as a leaving group[26] to enable the formation of a vici-
nal triketone, or to enhance the electrophilic character of
the a-carbon center.
Conclusion
Oxidative carbon–carbon bond-cleavage reactions of rele-
vance to biological processes are of current interest, includ-
ing those involving aliphatic carbon–carbon bond cleavage
in b-diketone substrates. Herein, we demonstrate that this
type of oxidative chemistry can be achieved through photo-
chemical reduction of NiII 2-chloro-1,3-diketonate com-
plexes under aerobic conditions. We propose that the re-
duced nickel center generated in these reactions activates
dioxygen to form superoxide and initiate a reaction se-
quence that ultimately results in the formation of carboxylic
acid products. This novel reactivity has relevance to that
proposed for the iron-containing Dke1 enzyme in which the
metal center mediates electron transfer from the b-di
ACHTUNGTRENNUNGke-
AHCTUNGTREGNUNtN onate ligand to O2, leading to the formation of superoxide.
Overall, this work outlines a new approach toward examin-
ing chemistry of relevance to metal-containing dioxygenase
enzymes that cleave a b-diketone ligand.
Experimental Section
General methods: All reagents and solvents were obtained from commer-
cial sources, and were used without further purification unless otherwise
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Chem. Eur. J. 2011, 17, 14962 – 14973

Chlorodiketonate NiII Complexes
FULL PAPER
noted. The 6-Ph2TPA ligand (N,N-bis-[(6-phenyl-2-pyridyl)methyl]-N-[(2-
4H; Ar-H), 6.97 ppm (s, 1H; CH); IR (KBr): n˜ =1699 (C=O), 1680,
1595, 1580 cmꢀ1; GC-MS: m/z (%): 258 (1.5), 260 (0.5).
pyridyl)methyl]amine),
Ph2TPA)Ni{PhC(O)CHC(O)Ph}]
(CH3CN)][ClO4] (10), [(6-Ph2TPA)Ni
sized according to literature procedures.[7,17,21,27] 1,3-Di(4-methoxy-
phenyl)propane-1,3-dione and 1,3-diphenylpropane-1,3-dione were pur-
1,3-diACHNUTGTRENNUNG
A
ACHTUNGTRENNUNG
Caution! Perchlorate salts of metal complexes with organic ligands are
potentially explosive. Only small amounts of material should be pre-
pared, and these should be handled with great care.[31]
A
E
N
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
Preparation
of
[(6-Ph2TPA)Ni
ACHTUGTNREN{NUG (4-OCH3Ph)C(O)C(Cl)C(O)ACHTUNGTRENNUNG(4-
chased from TCI and ACROS, respectively, and were used as received.
Dry acetonitrile was prepared according to a literature procedure[28] and
was used in the metal complex syntheses.
OCH3Ph)}][ClO4] (4): NiACHTGNUTRENNUNG
N
(ClO4)2·6H2O (29 mg, 79 mmol) was dissolved
in dry acetonitrile (2 mL), and this solution was added to solid 6-Ph2TPA
(35 mg, 79 mmol). The resulting solution was stirred for 15 min, during
which time it became purple. This solution was added to solid 2-chloro-
1,3-di(4-methoxyphenyl)propane-1,3-dione (25 mg, 79 mmol) and the re-
sulting mixture was stirred until everything had dissolved. The pale-
purple solution was then added to solid Me4NOH·5H2O (13 mg, 79
mmol) and the mixture was stirred overnight at ambient temperature,
during which time it became yellow. The solvent was removed from the
reaction mixture under vacuum, the remaining solid was redissolved in
CH2Cl2, and the solution was filtered through a glass wool/celite plug.
Concentration of the filtrate under vacuum, followed by the addition of
an excess of hexanes resulted in the deposition of a yellow solid (50 mg,
70%) that was dried under vacuum. IR (KBr): n˜ =1603, 1346, 1094
(ClO4), 623 cmꢀ1 (ClO4); HRMS (ESI): m/z calcd for C47H40N4ClO4Ni+:
817.209 [MꢀClO4]+; found: 817.209; elemental analysis calcd (%) for
C47H40N4Cl2O8Ni: C 61.45, H 4.39, N 6.10; found: C 61.48, H 4.90,
N 5.99.
1
Physical methods: H NMR spectra of organic compounds were obtained
by using a JEOL ECX-300 or Bruker ARX-400 NMR spectrometer.
Chemical shifts were referenced to the residual solvent peak in CD2HCN
(d=1.94 ppm, quintet). 1H NMR spectra of paramagnetic NiII complexes
were obtained by using a Bruker ARX-400 spectrometer and parameters
as previously described.[17] FT-IR data were collected on a Shimadzu
FTIR-8400 spectrometer as KBr pellets. UV/Vis data was collected on a
HP8453A spectrometer at ambient temperature. Photoreactions were
carried out in
a Srinivasan–Griffin Rayonet photochemical reactor
equipped with 8 RPR-3500 lamps, having lmax =350 nm. GC-MS data was
obtained with a Shimadzu GCMS-QP5000 gas chromatograph/mass spec-
trometer with a GC-17A gas chromatograph, by using an Alltech EC-
5 30 mmꢃ0.25 mmꢃ0.25 mm thin film capillary column and temperature
program: TInitial: 308C (3 min); temperature gradient: 238Cminꢀ1; TFinal
:
2508C (10 min). Quantum yields were determined by ferrioxalate actino-
metry, by using an integrative analysis method.[29] Anaerobic electro-
chemical measurements were carried out in a drybox under a N2 atmos-
Preparation of [(6-Ph2TPA)Ni
ACHTUNGTNERNUG{(4-CH3Ph)C(O)C(Cl)C(O)ACHTUNGTERN(NUGN 4-CHsPh)}]-
AHCTUNGTRENNUNG
phere in CH3CN with [Bu4N]ACTHNUTRGNE[NUG ClO4] (0.1m) as the supporting electrolyte
manner similar to that described for 4 by using the appropriate starting
materials. IR (KBr): n˜ =1346, 1094 (ClO4), 623 cmꢀ1 (ClO4); HRMS
(ESI): m/z calcd for C47H40N4ClO2Ni+: 785.219 [MꢀClO4]+; found:
785.219; elemental analysis calcd (%) for C47H40N4Cl2O6Ni·1.5CH2Cl2:
C 57.46, H 4.27, N 5.53; found: C 57.19, H 4.26, N 5.50.
by using a model ED401 computer-controlled potentiostat (eDAQ). A
three-electrode configuration with a glassy carbon working electrode, a
Ag wire quasi-reference electrode with an Fc/Fc+ internal reference, and
a platinum wire auxiliary electrode was used. Aerobic electrochemical
studies were performed after purging the cell with O2. The potential
values were referenced to an internal ferrocenium/ferrocene couple,
which is reported to be +0.38 V versus a saturated calomel electrode
Preparation of [(6-Ph2TPA)NiACTHUNGRTENN{UG PhC(O)C(Cl)C(O)Ph}]AHCUTGNTREN[NNGU ClO4] (6): Com-
pound 6 (83%) was prepared and crystallized in a manner similar to that
described for 4 by using the appropriate starting materials. IR (KBr): n˜ =
1352, 1096 (ClO4), 623 cmꢀ1 (ClO4); HRMS (ESI): m/z calcd for
C45H36N4ClO2Ni+: 757.188 [MꢀClO4]+; found: 757.189; elemental analy-
sis calcd (%) for C45H36N4Cl2O6Ni: C 62.95, H 4.23, N 6.53; found:
C 63.22, H 4.18, N 6.65.
(SCE) with [NBu4]ACHTUNGTRENNUNG
[ClO4] in CH3CN.[30]
Preparation of 2-chloro-1,3-di(4-methoxyphenyl)propane-1,3-dione (3a):
A solution comprised of acetonitrile (20 mL), aqueous NH4Cl (1.0m,
20 mL), and aqueous RuCl3 (0.10m, 250 mL) was cooled in an ice-bath.
Oxone (4.08 g, 6.64 mmol) was added to this solution, which resulted in
the formation of a yellow suspension. The mixture was then warmed to
room temperature. Dropwise addition of a solution of 1,3-di(4-methoxy-
phenyl)propane-1,3-dione (0.382 g, 1.34 mmol) in ethyl acetate (10 mL)
resulted in darkening to a purplish color. After stirring overnight at am-
bient temperature, the solution was again a yellow color. The suspension
was then diluted with water (100 mL) and the mixture was extracted with
ethyl acetate (3ꢃ100 mL). The combined organic fractions were dried
over anhydrous Na2SO4, filtered, and the filtrate was brought to dryness
under reduced pressure. The pale yellow solid was recrystallized from
hot ethanol to give pale yellow crystals (338 mg, 79%). M.p. 94–968C;
Preparation of [(6-Ph2TPA)Ni
ACHTUNGTRNENUG{(4-OCH3Ph)C(O)CHC(O)ACHTUNGTNER(NUGN 4-OCH3Ph)}]-
AHCTUNGTRENNUNG
manner similar to that described for 4 by using the appropriate starting
materials. IR (KBr): n˜ =1094 (ClO4), 623 cmꢀ1 (ClO4); elemental analysis
calcd (%) for C47H41N4ClO8Ni·0.2(CH2Cl2): C 62.90, H 4.63, N 6.22;
found: C 62.84, H 4.65, N 6.18.
Preparation of [(6-Ph2TPA)Ni
ACHTUNGTNERNUG{(4-CH3Ph)C(O)CHC(O)ACHTUNGTNER(NUGN 4-CH3Ph)}]-
AHCTUNGTRENNUNG
manner similar to that described for 4 by using the appropriate starting
materials. IR (KBr): n˜ =1094 (ClO4), 623 cmꢀ1 (ClO4); elemental analysis
calcd (%) for C47H41N4ClO6Ni: C 66.67, H 5.14, N 6.42; found: C 66.80,
H 5.50, N 6.48.
1H NMR (300 MHz, CD3CN, 258C): d=7.96 (d, 3J
N
3
Ar-H), 7.03 (d, J
G
(s, 6H; CH3); 13C{1H} NMR (100 MHz, CD3CN, 258C): d=189.8 (C=O),
166.0 (CqAr), 132.8 (CHAr), 128.1 (CqAr), 115.7 (CHAr), 62.8 (CHCl),
56.9 ppm (CH3); IR (KBr): n˜ =1687 (C=O), 1659, 1601, 1572 cmꢀ1; GC-
MS: m/z (%): 320 (0.38), 318 (0.18).
Preparation of [(6-Ph2TPA)Ni
ACHTNUGTNER{NUGN O2CACHTUNRTGENNG(UN 4-OCH3Ph)}]CAHTNUGTRENN[UGN ClO4] (11): Solid Ni-
AHCTUNGTRENNUNG
lution was added to 6-Ph2TPA (27 mg, 61 mmol). The resulting mixture
was stirred for one hour to form a homogeneous purple solution. Anisic
acid (9.3 mg, 61 mmol) was dissolved in methanol (1 mL) and this solution
was added to the solution of the NiII complex. The resulting mixture was
stirred for 15 min and then added to solid Me4NOH·5H2O (11 mg,
61 mmol). The mixture was stirred overnight during which time a blue/
green solution formed. The solvent was removed under reduced pressure,
and the residue was dissolved in CH2Cl2. This solution was passed
through a glass wool/celite plug twice and then the filtrate was reduced
in volume under reduced pressure. The final product was precipitated by
the addition of an excess of hexanes and was dried under vacuum
(26 mg, 47%). IR (KBr): n˜ =1607, 1096 (ClO4), 623 cmꢀ1 (ClO4); HRMS
(ESI): m/z calcd for C38H33ClN4NiO7+: 651.1906 [MꢀClO4]+; found:
Preparation of 2-chloro-1,3-bis(4-methylphenyl)propane-1,3-dione (3b):
Compound 3b (32%) was prepared in a similar manner to 3a. M.p. 140–
1
3
1418C; H NMR (300 MHz, CD3CN, 258C): d=7.87 (d, JACHTNUTRGNE(NUG H,H)=8.5 Hz,
4H; Ar-H), 7.35 (d, 3J
ACHTUNGTRENNUNG(H,H)=8.5 Hz, 4H; Ar-H), 6.89 (s, 1H; CH),
2.45 ppm (s, 6H; CH3); 13C{1H} NMR (100 MHz, CD3CN, 258C): d=
191.0 (C=O), 147.3 (CqAr), 132.8 (CqAr), 131.1 (CHAr), 130.4 (CHAr), 62.7
(CHCl), 22.1 ppm (CH3); IR (KBr): n˜ =1695 (C=O), 1674, 1604 cmꢀ1
;
GC-MS: m/z (%): 286 (1.1), 288 (0.4); elemental analysis calcd (%) for
C17H15ClO2·0.2H2O: C 70.31, H 5.35; found: C 70.50, H 5.31.
Preparation of 2-chloro-1,3-bisphenylpropane-1,3-dione (3c): Compound
3c (83%) was prepared in a similar manner to 3a. M.p. 71–728C;
1H NMR (300 MHz, CD3CN, 258C): d=7.98 (d, 3J
N
651.1913; elemental analysis calcd (%) for C38H33ClN4NiO7·0.2C6H14:
C 61.83, H 4.76, N 7.25; found: C 61.73, H 4.62, N 7.66.
Ar-H), 7.69 (t, 3J
G
A
Chem. Eur. J. 2011, 17, 14962 – 14973
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
14971

L. M. Berreau et al.
Preparation
of
[(6-Ph2TPA)Ni
{O2C
E
(12):
Solid
in the structure of 5. In the structure of 6, two oxygen atoms of the per-
chlorate anion are disorded over two positions (0.86:0.14). For 8, there
are two molecules of CH2Cl2 per formula unit. CCDC-827741
(5·C4H10O), CCDC-827742 (6), and CCDC-827743 (8·2CH2Cl2) contain
the supplementary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Ni(ClO4)2·6H2O (30 mg, 81 mmol) dissolved in acetonitrile (1 mL) was
ACHTUNGTRENNUNG
added to 6-Ph2TPA (40 mg, 80 mmol) and the resulting solution was
stirred for one hour during which time it became pale purple. Anisic acid
(25 mg, 162 mmol) was dissolved in acetonitrile (1 mL) and this solution
was added to solid Me4NOH·5H2O (29 mg, 162 mmol). The resulting mix-
ture was stirred for 1 h during which time the solution became yellow.
The NiII-containing solution was combined with the anisic acid/
Me4NOH·5H2O solution and the mixture was stirred overnight, forming
a blue/green homogeneous solution. The solvent was then removed
under reduced pressure and the remaining solid was dissolved in CH2Cl2
and filtered through a glass wool/celite plug. The filtrate was brought to
approximately 1 mL in volume under reduced pressure and the final
Acknowledgements
product was precipitated by the addition of hexanes (41 mg, 63%). IR
The authors thank the National Science Foundation (Grant CHE-
0848858 to LMB) for support of this research. DTH thanks Mark P.
Mehn, his advisor, for support of this project with funding from the
Wyoming NASA Space Grant Consortium through a Faculty Research
Initiation Grant (NASA Grant NNX10AO95H). DTH was also support-
ed by an NSFGK-12 (Grant DGE-0948027) fellowship.
+
(KBr): n˜ =1605 cmꢀ1; HRMS (ESI): m/z calcd for C38H33ClN4NiO7
:
651.1906 [MꢀCH3OC6H4CO2]+; found: 651.1918; elemental analysis
calcd (%) for C46H40N4O6Ni·1.5H2O: C 66.51, H 5.22, N 6.75; found: C
66.72, H 5.18, N 6.81.
Product recovery for photochemical reactions: To identify products of
the photo-reactions, solutions of complexes 4–6 in acetonitrile (5.0 mL,
2.0 mm) were irradiated for 20 h. The solvent was then removed under re-
duced pressure and the crude reaction mixtures were analyzed by
1H NMR spectroscopy by using paramagnetic parameters. To extract the
organic component, each crude reaction mixture was passed through a
short silica column, eluting with ethyl acetate, providing yields greater
than 80% (ꢁ2.5 mg) of the expected mass of the b-diketonate in the
starting compound. The inorganic fraction was also recovered by eluting
with acetonitrile, followed by methanol. The organic fraction was then
analyzed by 1H NMR spectroscopy and GC-MS and the inorganic frac-
tion by 1H NMR spectroscopy by using paramagnetic parameters. As a
control reaction, complex 4 (0.01 mmol) was passed through a short silica
column by using the same solvents, and dissociation of the complex to
form the 2-chloro-1,3-dione (3a, 85% recovery) and [(6-Ph2TPA)Ni-
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to be stable in the absence of light under aerobic and anaerobic condi-
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¯
space group P1. All hydrogen atoms in these complexes were assigned
isotropic displacement coefficients U(H)=1.2U(C) or 1.5U(Cmethyl) and
AHCTUNGTRENNUNG
their coordinates were allowed to ride on their respective carbon by
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14972
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 14962 – 14973

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Received: June 24, 2011
Published online: December 8, 2011
Chem. Eur. J. 2011, 17, 14962 – 14973
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
14973