Influence of water on the formation of O2-reactive divalent metal enolate complexes of relevance to acireductone dioxygenases

Article abstract of DOI:10.1039/c1dt10587f

Reaction conditions were evaluated for the preparation of [(6-PhTPA)Ni(PhC(O)C(OH)C(O)Ph)]ClO₄ (3) and [(6-Ph ₂TPA)Co(PhC(O)C(OH)C(O)Ph)]ClO₄ (7), two complexes of structural relevance to the enzyme/substrate (ES) adduct in Ni(ii)- and Co(ii)-containing forms of acireductone dioxygenase. The presence of water in reactions directed at the preparation of 3 and 7 was found to result in isomerization of the enolate precursor 2-hydroxy-1,3-diphenylpropane-1,3-dione to give the ester 2-oxo-2-phenylethylbenzoate. Performing synthetic procedures under dryer conditions reduced the amount of ester production and enabled the isolation of 3 in high yield. This complex was comprehensively characterized, including by X-ray crystallography. Using similar conditions for the 6-Ph ₂TPACo-containing system, the amount of ester generated was only modestly affected, but the formation of a benzoate complex ([(6-Ph ₂TPA)Co(O₂CPh)]ClO₄, 10) resulting from ester hydrolysis was prevented. The best preparation of 7 was found to involve dry conditions and short reaction times. The approach outlined herein toward determining appropriate reaction conditions for the preparation of 3 and 7 involved the preparation and characterization of several air-stable (6-PhTPA)Ni- and (6-Ph₂TPA)Co-containing analog complexes having enolate, solvent, and benzoate ligands. These complexes were used as paramagnetic ¹H NMR standards for evaluation of reaction mixtures containing 3 and 7.

Full text of DOI:10.1039/c1dt10587f

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PAPER
Influence of water on the formation of O2-reactive divalent metal enolate
complexes of relevance to acireductone dioxygenases†
Katarzyna Grubel,a Gajendrasingh K. Ingle,a Amy L. Fuller,a Atta M. Arifb and Lisa M. Berreau*a
Received 6th April 2011, Accepted 14th June 2011
DOI: 10.1039/c1dt10587f
Reaction conditions were evaluated for the preparation of [(6-PhTPA)Ni(PhC(O)C(OH)C(O)Ph)]ClO4
(3) and [(6-Ph2TPA)Co(PhC(O)C(OH)C(O)Ph)]ClO4 (7), two complexes of structural relevance to the
enzyme/substrate (ES) adduct in Ni(II)- and Co(II)-containing forms of acireductone dioxygenase. The
presence of water in reactions directed at the preparation of 3 and 7 was found to result in isomerization
of the enolate precursor 2-hydroxy-1,3-diphenylpropane-1,3-dione to give the ester 2-oxo-2-
phenylethylbenzoate. Performing synthetic procedures under dryer conditions reduced the amount of
ester production and enabled the isolation of 3 in high yield. This complex was comprehensively
characterized, including by X-ray crystallography. Using similar conditions for the
6-Ph2TPACo-containing system, the amount of ester generated was only modestly affected, but the
formation of a benzoate complex ([(6-Ph2TPA)Co(O2CPh)]ClO4, 10) resulting from ester hydrolysis was
prevented. The best preparation of 7 was found to involve dry conditions and short reaction times. The
approach outlined herein toward determining appropriate reaction conditions for the preparation of 3
and 7 involved the preparation and characterization of several air-stable (6-PhTPA)Ni- and
(6-Ph2TPA)Co-containing analog complexes having enolate, solvent, and benzoate ligands. These
complexes were used as paramagnetic 1H NMR standards for evaluation of reaction mixtures
containing 3 and 7.
ARD enzymatic reaction.2 However, the formation of a small
amount of another organic product, benzil (PhC(O)C(O)Ph),
Introduction
Acireductone dioxygenases (ARDs) catalyze dioxygenase-type
in the reactions involving 1 and 2, suggested the possibility of
oxidative reactions involving the cleavage of one or more
a “hydroperoxide” pathway for oxidative carbon–carbon bond
aliphatic carbon–carbon bonds in an intermediate generated in
cleavage reactivity involving triketone and OOH- intermediates.
the methionine salvage pathway.1 With Ni(II) or Co(II) as the
This proposed pathway is supported by recent computational
active site metal ion (Ni(II)-ARD or Co(II)-ARD), the cleavage
and mechanistic studies for the oxidative carbon–carbon bond
reaction results in the formation of carboxylic acid products
and CO (Scheme 1, top).1 This reaction is proposed to proceed
cleavage reaction involving 1.3 This mechanism differs from a
radical pathway4 available for carbon–carbon bond cleavage in
from an enzyme/substrate (ES) adduct having a coordinated
the native substrate for the enzyme and is a consequence of the
enediolate moiety. To date, our laboratory has produced the
phenyl substituent at the C(1)-carbon. Thus, while our previous
only synthetic complexes of structural relevance to the ES
studies show that the C(1)-phenyl-containing acireductone analog
adduct in Ni(II)-ARD using a C(1)–Ph containing analog of
introduces mechanistic differences in terms of its oxidative cleav-
the native acireductone substrate (Scheme 1, middle).2 These
complexes, [(6-Ph2TPA)Ni(PhC(O)C(OH)C(O)Ph)]ClO4 (1) and
[(bnpapa)Ni(PhC(O)C(OH)C(O)Ph]ClO4 (2) (Scheme 1, bot-
age reactivity, it is a convenient species for exploring bioinspired
coordination chemistry of an acireductone-type molecule. We
note that prior to our preparation and characterization of 1,
tom), undergo reaction with O2 to produce CO and carboxy-
only a single complex having an acireductone type ligand had
late/carboxylic acid products in a reaction similar to the Ni(II)-
been characterized by X-ray crystallography.5 This complex,
[Ru(bipy)2(m-C4H4O3)Ru(bipy)2](PF6)2, was generated as a by-
aDepartment of Chemistry & Biochemistry, Utah State University, Logan,
product in an ethylene glycol-containing reaction mixture and no
UT, USA. E-mail: lisa.berreau@usu.edu; Fax: 435-797-3390; Tel: 435-797-
further chemistry of the complex was reported. The [C4H4O3]2-
3509
bDepartment of Chemistry, University of Utah, Salt Lake City, UT, USA.
E-mail: arif@chem.utah.edu; Fax: 801-581-8433; Tel: 801-581-5320
acireductone-type ligand bridges the two ruthenium centers via
two five-membered chelate rings. We have reported a similar
coordination motif for the dianionic form of the C(1)-phenyl-
containing acireductone analog in Ni(II) trinuclear and cluster
complexes.6,7
† Electronic supplementary information (ESI) available: Table of assign-
ments of 1H NMR resonances of 3–6. CCDC reference numbers 821105–
821109. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c1dt10587f
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 10609–10620 | 10609
©

of [H2O] ~3000 ppm. For the preparation of 3, these condi-
tions proved problematic, as a mixture of Ni(II) complexes was
generated, as determined by 1H NMR (Fig. 1(a)), along with
a significant amount of an isomerization product, the ester 2-
oxo-2-phenylethylbenzoate9 (~46% isolated yield based on dione;
Scheme 2(b)). This ester has been previously reported to form
upon treatment of 2-hydroxy-1,3-diphenylpropane-1,3-dione with
sodium bicarbonate in H2O/methanol.10 In this prior report, a
reaction pathway was proposed wherein the deprotonated keto
form undergoes intramolecular attack of the C(2)–O¢ moiety on
an adjacent carbonyl (Scheme 3). This results in the formation of
a new C–O bond and the cleavage of a C–C bond involving the
C(2) center. We had not previously identified this rearrangement
chemistry in preparing 1. However, we have now reexamined the
organic products remaining following the isolation of 1 and have
found that some ester is generated in this reaction, but it is only
approximately half of what is found in the 6-PhTPA system. The
difference in ester formation for the 6-Ph2TPA and 6-PhTPA
systems might result from differences in interactions of the Ni(II)
center in each complex with the reactants due to modulation of the
hydrophobic microenvironment by the differing chelate ligands.
Specifically, factors such as a greater possible propensity of the
6-PhTPA complex to form dimeric structures, as well as electronic
effects at the metal, and differing anion association constants,
may be responsible for the observed differences in reactivity.
Notably, we discovered that reducing the amount of water in the
reaction mixture involving the 6-PhTPA-supported Ni(II) complex
via predrying of the base and use of extra dry CH3CN (<10 ppm
H2O) enables the isolation of 2 in high yield (94%, Scheme 2(b)).
Additionally, only a trace amount of ester was detected in this
Scheme 1 Top: Reaction catalyzed by Ni(II)-ARD. Middle: Native
and alternative substrates for Ni(II)-ARD; C(1)-phenyl-containing model
substrate. Bottom: Structural drawings of 1 and 2.
In further exploration of the coordination chemistry of
the [PhC(O)C(OH)C(O)Ph]- anion, we describe herein our ef-
forts to prepare analogs of 1 containing a sterically less-
demanding supporting chelating ligand, 6-PhTPA, or a different
metal ion, Co(II). Notably, we have found that the prepara-
tion of [(6-PhTPA)Ni(PhC(O)C(OH)C(O)Ph)]ClO4 (3) and [(6-
Ph2TPA)Co(PhC(O)C(OH)C(O)Ph)]ClO4 (7) is complicated by
water-dependent isomerization chemistry involving the C(1)-
phenyl-containing acireductone analog which results in the for-
mation of an ester. While we were able to modify the reaction
conditions to successfully isolate and characterize 3, the Co(II)
chemistry is more complicated and includes ester hydrolysis reac-
tivity. Our approach toward identifying the best conditions under
which to generate 7 involved the preparation and characterization
of three new 6-Ph2TPA-supported Co(II) complexes that were
1
used as paramagnetic H NMR standards. Overall, these studies
demonstrate that the conditions required for the preparation
of metal enolate complexes of relevance to the ES adduct in
acireductone dioxygenases depend on several factors, including
the presence of water in the reaction mixture.
Results and Discussion
Isomerization reactivity of a C(1)-phenyl containing acireductone
analog and the preparation of [(6-PhTPA)Ni(PhC(O)C(OH)C-
(O)Ph)]ClO4 (3)
Fig.
1
A
region of the 1H NMR spectra for (a) the reaction
mixture produced upon treatment of 6-PhTPA with an equimolar
amount of Ni(II)(ClO4)2·6H2O and slight stoichiometric excesses of
Me4NOH·5H2O and 2-hydroxy-1,3-diphenylpropan-1,3-dione in CH3CN
(~3000 ppm H2O); (b) [(6-PhTPA)Ni(PhC(O)C(OH)C(O)Ph)]ClO4 (3);
(c) [(6-PhTPA)Ni(PhC(O)CHC(O)Ph)]ClO4 (4); (d) [(6-PhTPA)Ni-
(CH3CN)(CH3OH)](ClO4)2 (5); (e) [(6-PhTPA)Ni(O2CPh)]ClO4 (6). Spec-
tra (b-e) are for analytically pure compounds. The 1H NMR spectra were
collected in CD3CN at ambient temperature and were referenced to the 1H
NMR signal of CD2HCN (1.94 ppm).
In our initial attempts to prepare the 6-PhTPA-supported
Ni(II) complex 3,8 we used reaction conditions identical
to those which had enabled us to isolate the O2-reactive
[(6-Ph2TPA)Ni(PhC(O)C(OH)C(O)Ph)]ClO4 (1) in high yield
(>90%).2a These conditions are referred to herein as the “wet”
conditions. Karl Fischer titration of the CaH2-dried acetonitrile
typically used in this type of reaction indicated a concentration
10610 | Dalton Trans., 2011, 40, 10609–10620
This journal is
The Royal Society of Chemistry 2011
©

2a
˚
Ni(1)–N(4) 2.323(2) A), presumably due to reduced steric
hindrance as a second phenyl-appended pyridyl donor is not
present in 3. The remaining bond distances/angles in this
complex are similar to those found in 1. The spectroscopic
properties of 3 are also generally similar to those found for
1. For example, both complexes form orange-brown solutions
with lmax at 393 (6800 M-1cm-1) and 399 nm (10300 M-1cm-1),
respectively.2b The 1H NMR features of 3 in the region of
1
70–10 ppm are shown in Fig. 1(b). Prior H NMR studies of
6-Ph2TPA-ligated Ni(II) complexes have identified this region as
containing pyridyl ring proton resonances that are sensitive to
changes in ligation at the Ni(II) center.11,12 In order to perform
spectral comparisons for 6-PhTPA-supported Ni(II) complexes,
the air stable dibenzoylmethane-derived enolate complex [(6-
PhTPA)Ni(PhC(O)CHC(O)Ph)]ClO4 (4, Fig. 3 (top)), the solvent-
bound
complex
[(6-PhTPA)Ni(CH3CN)(CH3OH)](ClO4)2
(5, Fig. 3(bottom)), and the benzoate complex [(6-
PhTPA)Ni(O2CPh)]ClO4 (6) were prepared and characterized.
Complexes 4 and 5 were investigated using single crystal X-ray
crystallography. Details of the X-ray data collection are given in
Table 1. Selected bond distances and angles for the complexes
are shown in Table 2. Both 4 and 5 exhibit a psuedooctahedral
geometry with the longest Ni–N bond distance being for the
phenyl-appended pyridyl donor.
Scheme 2 (a) Previously reported synthetic route for the preparation of
1. (b) Synthetic routes for the preparation of 3.
1
As is evident by comparison of the H NMR spectra shown
in Fig. 1(b)–(e), complexes having a six-membered ring enolate
ligand (Fig. 1(b) and(c); 3 and 4) exhibit generallysimilar 1H NMR
features that are clearly distinct from those exhibited by solvent-
(Fig. 1(d), 5) or benzoate-coordinated (Fig. 1(e), 6) compounds.13
That being said, we note that all of the complexes exhibit a pattern
of isotropically shifted resonances for the pyridyl ring protons with
chemical shifts in the order a-H > b-H > g-H.11
Scheme
3
Proposed pathway of isomerization of 1,3-diphenyl-
propane-1,3-dione to generate 2-oxo-2-phenylethylbenzoate.10
reaction mixture. These combined results indicate that water plays
a key role in this system in promoting the isomerization reactivity
that results in ester formation, and that a lower amount of ester
formation correlates with a higher yield of the desired enolate
complex 3.
Having the family of complexes 3–6 fully characterized has
enabled us to use 1H NMR to examine the mixture of Ni(II)
complexes in the “wet” reaction conditions wherein a significant
amount of ester is generated. As shown in Fig. 1(a) it is
evident that the desired Ni(II) enolate complex 2 is present, as
resonances at 44.5, 41.6, 34.8, and 13.3 ppm are present. It is
also clear that a Ni(II) solvent-coordinated complex, such as [(6-
PhTPA)Ni(CH3CN)(CH3OH)](ClO4)2 (5), if present, is a very
minor species. This suggests that all of the 6-PhTPA-ligated Ni(II)
species in solution have one or more coordinated non-solvent
ligands. A small amount of the Ni(II)-benzoate complex 6 may
be present in the reaction mixture, as evidenced by signals at
the 49.3 and 36.2 ppm. We note that preliminary O2 reactivity
studies indicate that complex 3 undergoes oxidative carbon-
carbon bond cleavage upon exposure to oxygen to give products
similar to those of [(6-Ph2TPA)Ni(PhC(O)C(OH)C(O)Ph)]ClO4
(1),2b specifically, the Ni(II) monobenzoate complex 6, along with
benzoic acid and benzil. Thus, the Ni(II)-benzoate complex in
the reaction mixture for the preparation of 3 could be due
to oxidative decomposition of the sample. Alternatively, the 6-
PhTPA-supported Ni(II) benzoate complex 6 might be formed via
hydrolysis of the 2-oxo-2-phenylethylbenzoate ester generated in
the reaction mixture. This hypothesis was evaluated via treatment
of the solvent complex 5 with 2-oxo-2-pheylethylbenzoate in the
presence of one equivalent of Me4NOH·5H2O in CH3CN. The
outcome of this reaction indicated the formation of new Ni(II)
species (as determined by 1H NMR), but no ester hydrolysis
Complex 3 (Fig. 2) has a six-coordinate Ni(II) center with the
longest Ni–N interaction for the phenyl-substituted pyridyl donor
˚
(Ni(1)–N(4), 2.202(4) A). This distance is slightly shorter than
˚
that found for the Ni–NPhPy bonds in 1 (Ni(1)–N(3) 2.271(2) A,
Fig. 2 Thermal ellipsoid drawing of the cationic portion of 3. Ellipsoids
are drawn at the 50% probability level. Hydrogen atoms except the hydroxyl
proton of the enolate ligand are not shown for clarity.
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 10609–10620 | 10611
©

Table 1 Summary of X-ray data collection and refinementa
3·CH2Cl2
4·Et2O
5
8·0.5CH3CN
10
Empirical formula
C39H33ClN4NiO7·CH2Cl2 C39H33ClN4NiO6·C4H10O C27H29Cl2N5NiO9
C45H37ClCoN4O4·0.5 C37H31ClCoN4O6
CH3CN
Formula weight
Crystal system
Space group
848.78
Monoclinic
Cc
23.2096(5)
15.9822(4)
11.9687(2)
90
117.2760(12)
90
3946.02(15)
4
1.429
150(1)
Orange
Prism
821.97
Orthorhombic
Pcab
16.0204(3)
19.2924(6)
25.0918(7)
90
90
90
7755.2(4)
8
1.408
150(1)
Orange
Plate
0.38 ¥ 0.38 ¥ 0.10
Nonius KappaCCD
0.627
697.16
844.69
Tetragonal
I-4
34.4730(5)
34.4730(5)
13.5221(3)
90
722.04
Monoclinic
P21/c
11.0736(2)
24.5657(5)
12.18640(10)
90
100.8097(10)
90
3256.25(9)
4
1.473
150(1)
Green
Orthorhombic
P212121
9.8884(2)
10.8891(2)
28.1201(4)
90
˚
a/A
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
90
90
90
90
3
˚
V/A
3027.85(9)
4
1.529
150(1)
Purple
16069.5(5)
16
1.397
150(1)
Red-Brown
Plate
Z
Dc/Mg m-3
T/K
Color
Crystal habit
Crystal size (mm-1)
Diffractometer
m (mm-1)
Prism
0.33 ¥ 0.25 ¥ 0.20
Nonius KappaCCD Nonius KappaCCD Nonius KappaCCD
0.878
54.96
6591
Plate
0.35 ¥ 0.23 ¥ 0.10
0.25 ¥ 0.23 ¥ 0.20
Nonius KappaCCD
0.749
54.94
8111
0.35 ¥ 0.35 ¥ 0.13
0.550
54.96
15365
0.664
54.96
14646
2qmax (◦)
54.98
16226
Reflections collected
Independent reflections 8104
8868
0.0767
677
0.0552/0.1015
1.015
0.452/-0.465
6591
0.0000
400
0.0654/0.1377
1.040
0.699/-0.976
-0.01(3)
15360
0.0395
1060
0.0638/0.1236
1.072
0.475/-0.586
0.371(16)
7451
0.0374
566
0.0385/0.0811
1.023
0.337/-0.512
Rint
0.0445
498
0.0539/0.1320
1.066
Variable parameters
R1, wR2 (I > 2s(I))b
Goodness-of-fit (F2)
-3
˚
Drmax/min/e A
0.972/-0.684
0.510(5)
Flack parameter
ꢀ
ꢀ
ꢀ
ꢀ
a
2
Radiation used: Mo-Ka (l = 0.71073 A). b R1 = | |Fo| – |Fc| |/ |Fo|; wR2 = [ [w(Fo2–Fc2)2]/[ (Fo2)2]]1/2 where w = 1/[s (Fo2) + (aP)2 + bP].
˚
◦
a
˚
Table 2 Selected bond lengths (A) and angles ( )
3·CH2Cl2
4·Et2O
5
8·0.5CH3CN
10
M–N(1)
M–N(2)
M–N(3)
M–N(4)
M–N(5)
M–O(1)
M–O(2)
2.048(4)
2.084(3)
2.122(4)
2.202(4)
2.077(3)
2.101(3)
2.110(3)
2.149(2)
2.089(5)
2.097(5)
2.090(5)
2.170(5)
2.055(5)
2.077(5)
2.134(5)
2.135(5)
2.291(5)
2.285(5)
2.1117(17)
2.1773(17)
2.1192(18)
2.1437(17)
1.986(3)
1.998(3)
92.31(13)
169.33(14)
92.78(13)
108.01(13)
1.997(2)
1.995(2)
172.53(10)
90.38(9)
91.33(9)
85.18(9)
2.029(4)
1.989(4)
175.44(15)
95.84(16)
82.06(16)
96.88(16)
1.9695(14)
O(1)–M–N(1)
O(1)–M–N(2)
O(1)–M–N(3)
O(1)–M–N(4)
O(1)–M–N(5)
O(1)–M–O(2)
O(2)–M–N(1)
O(2)–M–N(2)
O(2)–M–N(3)
O(2)–M–N(4)
N(1)–M–N(2)
N(1)–M–N(3)
N(1)–M–N(4)
N(1)–M–N(5)
N(2)–M–N(3)
N(2)–M–N(4)
N(2)–M–N(5)
N(3)–M–N(4)
N(3)–M–N(5)
N(4)–M–N(5)
171.61(18)
91.86(18)
88.37(19)
83.81(18)
90.4(2)
100.38(6)
173.83(7)
108.75(6)
102.48(6)
90.84(12)
173.30(14)
94.72(14)
89.15(13)
89.34(14)
83.13(15)
96.59(15)
84.06(15)
91.77(9)
94.96(9)
172.17(9)
91.69(9)
106.72(9)
82.53(10)
85.24(10)
95.97(10)
90.30(15)
93.99(17)
171.34(17)
99.88(17)
105.54(16)
80.06(18)
98.68(18)
80.50(17)
82.77(18)
84.5(2)
101.40(18)
94.8(2)
81.92(19)
79.13(19)
177.2(2)
159.22(19)
96.43(19)
102.82(19)
77.04(7)
108.75(6)
127.56(7)
78.23(14)
81.20(15)
80.73(10)
80.97(10)
75.03(18)
77.45(7)
75.25(6)
159.18(14)
161.33(10)
107.42(6)
a Estimated standard deviations in the last significant figure are given in parentheses.
10612 | Dalton Trans., 2011, 40, 10609–10620
This journal is
The Royal Society of Chemistry 2011
©

Ph2TPA)Co(PhC(O)C(OH)C(O)Ph)] ClO4 (7) involved following
a procedure identical to that employed for the preparation of
[(6-Ph2TPA)Ni(PhC(O)C(OH)C(O)Ph)]ClO4 (1) using the “wet”
conditions. Specifically, equimolar amounts of 6-Ph2TPA and
Co(ClO4)2·6H2O were combined with a slight stoichiometric
excess of Me4NOH·5H2O and 2-hydroxy-1,3-diphenylpropane-
1,3-dione in CH3CN (~3000 ppm H2O; Karl Fischer titration) and
the reaction mixture was stirred under N2 for 16.5 h. We noted
the rapid formation of a green color in this reaction mixture,
which is significantly different from the orange-brown color
that is found for [(6-Ph2TPA)Ni(PhC(O)C(OH)C(O)Ph)]ClO4
1
(1) and [(6-PhTPA)Ni(PhC(O)C(OH)C(O)Ph)]ClO4 (3). The H
NMR spectral features of the green mixture are shown in Fig.
4(a). To interpret this 1H NMR data, we prepared and fully
characterized (including assignment of 1H NMR resonances)
two new Co(II) complexes, the dibenzoylmethane derivative [(6-
Ph2TPA)Co(PhC(O)CHC(O)Ph)]ClO4 (8) and the benzoate com-
plex [(6-Ph2TPA)Co(O2CPh)]ClO4 (10). Additionally, we inves-
tigated the 1H NMR features of the previously reported [(6-
Ph2TPA)Co(CH3CN)](ClO4)2·CH3CN (9).16
Fig. 3 Thermal ellipsoid drawings of the cationic portions of 4 (top) and
5 (bottom). Ellipsoids are plotted at the 50% probability level. Hydrogen
atoms, except the methanol proton of the coordinated methanol ligand in
5, are not shown for clarity.
Fig. 4 Regions of the 1H NMR spectra of (a) the reaction mixture
produced upon treatment of 6-Ph2TPA with an equimolar amount of
Co(II)(ClO4)2·6H2O and slight stoichiometric excesses of Me4NOH·5H2O
and 2-hydroxy-1,3-diphenylpropan-1,3-dione in CH3CN (~3000 ppm
H2O); (b) the reaction mixture generated using conditions identical to
those employed for the preparation of 3 (predrying of base and solvent);
(c) the reaction mixture produced upon predrying of base and solvent and
shorter reaction time; (d) [(6-Ph2TPA)Co(PhC(O)CHC(O)Ph)]ClO4
(8); (e) [(6-Ph2TPA)Co(CH3CN)](ClO4)2·ClO4·CH3CN (9); (f)
[(6-Ph2TPA)Co(O2CPh)]ClO4 (10). Spectra d-f are for analytically
pure compounds. The 1H NMR spectra were collected in CD3CN at
ambient temperature and were referenced to the 1H NMR signal of
CD2HCN (1.94 ppm). The diamagnetic region of each spectrum is
omitted to improve the clarity of the spectral comparison.
(95% recovery of unaltered ester). Interestingly, performing the
same reaction in the absence of the Ni(II) complex results in
ester hydrolysis to give Me4NO2CPh and PhC(O)CH2OH. Thus,
in the Ni(II)-containing reaction, hydroxide anion is unavailable
for ester hydrolysis, presumably due to the formation of Ni-OH
species, which apparently do not promote ester hydrolysis. Overall,
this means that any Ni(II) benzoate complex 6 present in the
reaction mixture shown in Fig. 1(a) would be due to oxidative
decomposition. Finally, we note that at this point we cannot
identify species associated with resonances at 32.4 and 33.7 ppm
in the “wet” reaction mixture (Fig. 1(a)).
The cationic portions of 8 and 10 are shown in Fig. 5. Details
of the X-ray data collection are given in Table 1. Selected
bond distances and angles for the complexes are shown in
Table 2. The Co(II) center in 8 exhibits a distorted octahedral
geometry. The bond distances involving the phenyl-appended
Co(II) Chemistry: The challenges in preparing
[(6-Ph2TPA)Co(PhC(O)C(OH)C(O)Ph)]ClO4 (7)
We were interested in preparing the Co(II) analog of 1 as
it has been reported that reconstitution of apo-ARD with
Co(II) produces enzyme having >90% Ni(II)-ARD-type reac-
tivity (cleavage to give CO and carboxylate products, Scheme
1(top)).14 Additionally, ARD enzyme isolated from K. pneu-
moniae contains ~20% Co(II) (with Ni(II) being present in
~70% and Fe(II) ~10%).15 Our initial attempt to prepare [(6-
˚
pyridyl donors (N(3) and N(4)) are ~0.16 A longer than the other
Co–N distances. The Co(II)–O distances involving the enolate
˚
ligand differ by about ~0.03 A, with the Co–O bond trans to
the unsubstituted pyridyl donor being slightly longer. Similar
to [(6-Ph2TPA)Co(ONHC(O)CH3)]ClO4,16 the bidentate anion
is positioned in a hydrophobic sandwich between the phenyl
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©

for the complex (vida infra) suggest that the solution form of
the complex has a six-coordinate Co(II) center. For the green
benzoate complex 10 a ligand field transition is present at 590 nm
(49 M-1cm-1). This suggests an overall coordination number of six
for the benzoate complex in CH3CN.
Assignments for the 1H NMR resonances of 8–10 are given in
Table 3. Initial assignments were made on the basis of integrated
intensity and T1 values. Signals associated with the aryl and
methylene hydrogens of the 6-Ph2TPA ligand were confirmed using
analog complexes prepared with the deuterated ligand analogs 6-
2
(d5-Ph)2TPA and (6-Ph)2-d6-TPA and complementary H NMR
spectroscopy.11 For each complex the overall number of signals is
consistent with an effective plane of symmetry in the cation to give
equivalent phenyl-appended pyridyl donors. This is also consistent
with the identification of three methylene proton resonances.
Selected regions of the spectra of 8–10 are shown in Fig. 4(d–
f). Similar to the 1H NMR features of 6-Ph2TPA-supported Ni(II)
complexes,11 each Co(II) complex exhibits a pattern of isotropically
shifted resonances for the pyridyl ring protons with chemical shifts
in the order a-H > b-H > g-H. Resonances in the region of ~80–
20 ppm clearly distinguish the complexes. Within this region are
signals associated with the b and b¢ hydrogens of the pyridyl rings
(Fig. 6). A characteristic feature of the 1H NMR spectrum of [(6-
Ph2TPA)Co(PhC(O)CHC(O)Ph)]ClO4 (8) is a signal at -49 ppm,
which has been assigned as a phenyl ring proton resonance of
the 6-Ph2TPA ligand. An additional distinct resonance for this
compound is found at -10 ppm and is for the g¢ hydrogen. The
spectrum of the benzoate complex is unique in a clustering of three
of the four b/b¢ resonances in chemical shift range of 50–56 ppm.
Fig. 5 Thermal ellipsoid drawings of the cationic portions of 8 and 10.
Ellipsoids are drawn at the 50% probability level. Hydrogen atoms are not
shown for clarity. Only one of two cations present in the asymmetric unit
of 8 are shown.
appendages of the 6-Ph2TPA ligand. The benzoate complex 10
˚
exhibits monodentate coordination of the carboxylate (Dd = 0.8 A;
Dq = 36.6◦).17 The overall geometry of the Co(II) center is distorted
trigonal bipyramidal (t = 0.77; for a perfect trigonal bipyramid
t = 1),18 with the carboxylate ligand in an axial position. A
similar geometry was found for [(6-Ph2TPA)Co(CH3CN)](ClO4)2·
CH3CN (9) (t = 0.89).16
Fig. 6 Labeling scheme for the 6-Ph2TPA ligand.
The solution magnetic moments for 8 and 10 were measured
in CH3CN at 298 K using the Evans method.19 The meff values
obtained for these complexes, 4.72 and 4.32 mB, respectively,
are within the expected experimental range for high-spin Co(II)
complexes.20
1
Having fully characterized 8–10 and assigned the H NMR
signals of these complexes, we were positioned to evaluate the
“wet” reaction mixture (Fig. 4(a)) generated upon treatment
of 6-Ph2TPA and Co(ClO4)2·6H2O with a slight stoichiometric
excess of Me4NOH·5H2O and 2-hydroxy-1,3-diphenylpropane-
1,3-dione in “wet” acetonitrile under the conditions used to isolate
[(6-Ph2TPA)Ni(PhC(O)C(OH)C(O)Ph)]ClO4 (1).2a It is evident
High-spin Co(II) complexes generally exhibit a ligand field
transition in the range of 500–600 nm. The intensity of this
transition varies with the coordination number of the Co(II)
center, with six-coordinate Co(II) complexes generally having
e < 50 M-1cm-1.21 The absorption spectrum of the red-orange
dibenzoylmethane complex 8 contains a band at 564 nm (e =
77 M-1cm-1). Though the molar absorptivity value for the 564 nm
1
from the H NMR features shown in Fig. 4(a) that the Co(II)
benzoate complex 10 is present with b/b¢-H signals at ~55.6, 53.0,
50.6, and 35.5 ppm. Additionally, it is noteworthy that several
signals are present upfield of 0 ppm. This suggests that multiple
enolate-type complexes are also present (via comparison to the
1
band exceeds the threshold of e < 50 M-1cm-1, H NMR data
10614 | Dalton Trans., 2011, 40, 10609–10620
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©

Table 3 1H NMR chemical shifts for 8–10 in CD3CN at 298 K
Assignment
Chemical shift (ppm)a
Dn1/2(Hz)b
T
1exp(ms)c
Rel area
[(6-Ph2TPA)Co(PhC(O)CHC(O)Ph)]ClO4 (8)
a
151.21
247
2
1
b
77.42, 52.47
27.12, 22.59
18.42
104, 49
71, 62
d
10, 33
20, 52
d
74
2, 2, 10
85, 41, 5
d, d
2, 2
d
b¢
g
g¢
CH2
Ph
-10.06
69
2
193.14, 125.99, 75.82
632, 242, 104
2, 2, d
d, d, 4
8.04, 3.87, -48.69
38, 42, 184
[(6-Ph2TPA)Co(CH3CN)](ClO4)2 (9)
a
153.68
963
141, 141
141, 141
65
d
1
b
69.32, 48.34
73.05, 40.07
-4.28
d, 35
48, 180
150
1, 1
2, 2
2
b¢
g
g¢
-0.91
72
480
d
d, 160, 230
1
CH2
Ph
119.1, 87.60, 53.1
25.68, 5.46, 2.96
d, 2190, d
751, 69, d
2, 2, d
4, 2, d
[(6-Ph2TPA)Co(O2CPh)]ClO4 (10)
a
142.22
867
< 1
11, 5
8, 4
9
11
< 1
14, 7, d
2, 23, 47
1
b
52.99, 50.61
55.62, 35.43
0.64
103, 153
153, 217
72
1, 1
2, 2
1
b¢
g
g¢
-0.36
72
2
CH2
Ph
117.2, 76.35, d
8.59, 4.19, d
27.45, 15.73, 12.92
d, 2190, d
65, 88, d
286, 103, 84
d, d, d
2, 4, d
2, 2, 1
Benzoate
a Chemical shifts in ppm relative to the residual solvent peak of CHD2CN (1H, 1.94 (quintet) ppm). b Line widths are full width at a half-maximum. c T1
values were obtained on 300 MHz.
spectral features with those of the dibenzoylmethane complex 8
(Fig. 4(d)). Work-up of the “wet” reaction mixture resulted in the
isolation of the ester 2-oxo-2-phenylethylbenzoate in ~50% yield
based on the initial amount of 2-hydroxy-1,3-diphenylpropane-
1,3-dione employed. Thus, similar to the chemistry of 6-PhTPA-
supported Ni(II), ester formation occurs in the Co(II) system under
the “wet” reaction conditions. Additionally, we isolated ~21 mg
(58% recovery) of unaltered 6-Ph2TPA from the reaction mixture.
Rationalizing that removal of water might enable
the preparation of the desired enolate complex [(6-
Ph2TPA)Co(PhC(O)C(OH)C(O)Ph)]ClO4 (7), we next attempted
a synthesis using the dry conditions involving predried Me4NOH
and CH3CN that enabled the isolation of [(6-PhTPA)Ni-
(PhC(O)C(OH)C(O)Ph)]ClO4 (3). Admixture of the reagents
followed by stirring for 3 h under N2 and workup resulted in the
isolation of an orange solid with features in the 1H NMR spectrum
as shown in Fig. 4(b). Notably, this spectrum does not indicate
the presence of Co(II) benzoate complex (8), and suggests the for-
mation of a single Co(II) enolate complex as evidenced by signals
at ~-10 and -49 ppm. Based on the 1H NMR features of 8, these
could be signals for the g¢ and phenyl ring protons of the desired
complex 7. Unfortunately, other signals are present indicating
that the isolated product is not pure. Evaluation of the organic
products from the reaction once again revealed the presence of a
mixture of the ester 2-oxo-2-phenylethylbenzoate and 6-Ph2TPA
in a ~1 : 1 ratio. Thus, unlike the (6-PhTPA)Ni(II) system, reducing
the amount of water in the reaction mixture did not significantly
reduce the amount of ester generated. However, removal of water
does prevent formation of the benzoate complex 10.
conditions akin to those described herein for the preparation of
[(6-PhTPA)Ni(PhC(O)C(OH)C(O)Ph)]ClO4 (3) but with a shorter
reaction time. Specifically, admixture of equimolar amounts
of [(6-Ph2TPA)Co(CH3CN)](ClO4)2·CH3CN (9), 2-hydroxy-1,3-
diphenylpropane-1,3-dione, and predried Me4NOH in CH3CN
(<10 ppm H2O) and stirring for 15 min results in the formation
of a deep orange solution with lmax = 397 nm. We note that this is
the same wavelength that was identified for the p→p* transition of
the enolate ligand in [(6-Ph2TPA)Ni(PhC(O)C(OH)C(O)Ph)]ClO4
(1)2b and [(6-PhTPA)Ni(PhC(O)C(OH)C(O)Ph)]ClO4 (3). Imme-
diate removal of the solvent under vacuum, followed by workup,
yielded a dark orange solid with the 1H NMR properties as shown
in Fig. 4(c). The complex generated has many similar features
to the dibenzoylmethane complex 8 suggesting that both likely
contain a six-membered ring enolate ligand. These 1H NMR
similarities, combined with the absorption maximum at 397 nm,
suggest that [(6-Ph2TPA)Co(PhC(O)C(OH)C(O)Ph)]ClO4 (7), has
been formed. However, like all of the reaction mixtures targeting
this complex, the ester 2-oxo-2-phenylethylbenzoate and free 6-
Ph2TPA are also generated. The presence of these byproducts has
prevented our obtaining elemental analysis data for 7. That being
said, evaluation of the O2 reactivity properties of this complex
are underway. Preliminary studies indicate that exposure of a
CH3CN solution of 7 to air results in a rapid color change from
1
orange brown to green. The H NMR features of the reaction
mixture are consistent with formation of the benzoate complex [(6-
Ph2TPA)Co(O2CPh)]ClO4 (10). This reaction is akin to that pre-
viously reported for [(6-Ph2TPA)Ni(PhC(O)CH(OH)CPh)]ClO4
(1).2b
We next explored a variety of reaction conditions in attempts
to prepare pure [(6-Ph2TPA)Co(PhC(O)C(OH)C(O)Ph)]ClO4 (7)
and have found that the best results are obtained using dry
The generation of 2-oxo-2-phenylethylbenzoate in the initial
“wet” reaction mixture directed at the preparation of the Co(II)
enolate complex 7 (Fig. 4(a)), and the disappearance of the
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©

benzoate complex [(6-Ph2TPA)Co(O2CPh)]ClO4 (10) once the
reagents (base and solvent) were predried, led us to hypothesize
that the presence of water promotes ester hydrolysis to provide the
benzoate ligand in 10. In an independent experiment admixture
of the ester with [(6-Ph2TPA)Co(CH3CN)](ClO4)2·CH3CN (9)
and Me4NOH·5H2O resulted in the formation of the Co(II)
benzoate complex [(6-Ph2TPA)Co(O2CPh)]ClO4 (10) thus con-
firming our notion. As noted above, an independent experiment
involving [(6-PhTPA)Ni(CH3CN)(CH3OH)](ClO4)2 (5), 2-oxo-
2-phenylethyl benzoate, and Me4NOH·5H2O showed no ester
hydrolysis reactivating. Thus the two different metal/ligand com-
binations (6-PhTPANi and 6-Ph2TPACo) exhibit different ester
hydrolysis reactivity.
Overall, our results have provided insight into the conditions
required to prepare two new complexes of relevance to the ES
adduct in acireductone dioxygenases. The differing effects of water
on the reactions involving (6-PhTPA)Ni- and (6-Ph2TPA)Co-
supported complexes (isomerization and/or ester hydrolysis) can-
not be fully explained at this point, but may relate to differences in
M-OH/H2O chemistry for the two metal/ligand combinations. As
our group also has a strong interest in metal-promoted hydrolysis
reactions,22 this area is under further investigation.
Experiments
General comments
All reagents and solvents were obtained from commercial sources
and were used as received unless otherwise noted. Solvents were
dried according to published procedures23 and were distilled
under N2 prior to use. Air-sensitive reactions were performed
in a MBraun Unilab or Vacuum Atmospheres glovebox un-
der an N2 atmosphere. The acireductone analog 2-hydroxy-
1,3-diphenylpropan-1,3-dione,2a the Ni(II) enolate complex [(6-
Ph2TPA)Ni(PhC(O)C(OH)C(O)Ph)]ClO4 (1),2a and the Co(II)
complex [(6-Ph2TPA)Co(CH3CN)]ClO4 (9)16 were prepared as
previously described.
Conclusions
The coordination chemistry of acireductone-type ligands reported
to date is quite limited.2,3,5,6 In our laboratory we have pursued
synthetic studies directed at the preparation of metal complexes
of relevance to the ES adduct in Ni(II)-ARD, which catalyzes
a CO-producing reaction. As the Ni(II) center in this enzyme
can be replaced with Co(II) with almost full retention of enzyme
activity,15 we are also interested in examining the chemistry of
Co(II) complexes of acireductone-type ligands.
Physical methods
The research described herein outlines our discovery that the
presence of water influences the preparation of a Ni(II) enolate
complex of an acireductone-type ligand. Reducing the amount of
water present in the reaction mixture via predrying of the base
and solvent significantly reduced the amount of isomerization
of 2-hydroxy-1,3-diphenylpropane-1,3-dione to the ester 2-oxo-
2-phenylethylbenzoate and increases the yield of the desired Ni(II)
enolate complex, [(6-PhTPA)Ni(PhC(O)CH(OH)CPh)]ClO4 (3).
We propose that water is involved in the acid/base chemistry
necessary to promote isomerization of the reaction (Scheme 3).
It is known that isomerization of 2-hydroxy-1,3-diphenylpropane-
1,3-dione to ester will occur in aqueous NaHCO3/methanol
solution.10 In our system, a combination of a Ni(II)–OH moiety
and H2O may be necessary to promote isomerization reactivity in
a general base/general acid type reaction sequence.
UV-vis spectra were recorded on a Hewlett-Packard 8453 diode
array spectrophotometer. IR spectra were recorded on a Shimadzu
FTIR-8400 spectrometer as KBr pellets. 1H NMR spectra of
diamagnetic species were recorded on a Bruker ARX-400 spec-
trometer and the chemical shifts (in ppm) are referenced to the
residual solvent peak(s) in CHD2CN (1H, 1.94 (quintet) ppm).
GC-MS data was obtained using a Shimadzu QP5000 with an
Alltech EC-5 column and ultra high purity helium as the carrier
gas. FAB-MS data was obtained at the Mass Spectrometry Facility,
Department of Chemistry, University of California, Riverside.
Elemental analyses were performed by Atlantic Microlabs Inc.,
Norcross, GA or Canadian Microanalytical, Service, Inc., British
Columbia, Canada.
The 6-Ph2TPA-supported Co(II) system exhibits a more
multifaceted water-dependent chemistry. Like the (6-
PhTPA)Ni-containing system, isomerization of 2-hydroxy-
1,3-diphenylpropane-1,3-dione to 2-oxo-2-phenylethylbenzoate
occurs in reaction mixtures containing water. However, predrying
of the base and solvent in this system does not significantly
reduce the amount of ester generated. Instead, a dryer reaction
environment reduces the amount of a 6-Ph2TPA-supported Co(II)
benzoate complex generated relative to the “wet” reaction mixture.
Independent studies of ester hydrolysis reactivity showed that
the combination of [(6-Ph2TPA)Co(CH3CN)](ClO4)2·CH3CN
(9), Me4NOH·5H2O and 2-oxo-2-phenylethylbenzoate in
CH3CN results in ester hydrolysis and the liberation of
benzoate anion. Notably, a similar type of ester hydrolysis
reactivity does not occur for a reaction mixture containing
[(6-PhTPA)Ni(CH3CN)(CH3OH)](ClO4)2 (5), Me4NOH·5H2O,
and 2-oxo-2-phenylethylbenzoate in CH3CN. This provides a
rationale for why little, if any, Ni(II) benzoate complex is generated
in the “wet” reaction mixture.
NMR methods for paramagnetic Ni(II) and Co(II) complexes
A typical one-dimensional 1H NMR spectrum consisted of 16 K
data points, 300 scans, and a 250 ms relaxation delay. An
exponential weighting function (lb = 30 Hz) was used during
processing. The 90◦ pulse (14.1 ms) was calibrated at 298 K.
Longitudinal relaxation times (T1) were measured using the
inversion-recovery pulse sequence (180◦-t-90◦) method on a JEOL
ECX-300. 2H NMR spectra were obtained at 298 K on a
Bruker ARX-400 operating at 61.43 MHz using an unlocked
system. Overall, methods of data collection and processing for
paramagnetic complexes followed closely from those previously
described.11
6 - N - (6 - phenyl - 2 - pyridyl)methyl) -N,N-((2-pyridyl)methyl)-
amine (6-PhTPA). This ligand has been previously reported,8
however, the preparative method employed herein is an alternative
route. A round bottomed flask was charged with an acetonitrile
slurry of 2-(chloromethyl)-6-phenylpyridine hydrochloride (6.0 g,
10616 | Dalton Trans., 2011, 40, 10609–10620
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©

0.025 mol), di-(2-picolyl)-amine (5.0 g, 0.025 mol), sodium
bicarbonate (13.3 g, 0.125 mol), and a catalytic amount of
tetrabutylammonium bromide. The reaction was then purged with
N2 and refluxed for 24 h. After this time, the mixture was cooled
to room temperature and 0.1 M NaOH was added until the pH >
11. The resulting mixture was extracted with methylene chloride
(3 ¥ 200 mL). The organic fractions were combined, dried over
anhydrous Na2SO4, and the solvent was removed under reduced
pressure. The resulting brown oil was purified using column
chromatography using ethyl acetate : methanol (1 : 1) as the eluent.
Yield: 8.56 g (93%). 1H NMR (CDCl3, 400 MHz): d 8.54 (d, J =
4.8 Hz, 2 H), d 8.00 (d, J = 7.4 Hz, 2 H), d 7.72 (t, J = 7. 8 Hz, 1
H), d 7.68–7.65 (m, 3 H), d 7.58 (d, J = 7. 8 Hz, 2H), d 7.50 (d,
J = 7. 8 Hz, 1H), d 7.46 (t, J = 7.7 Hz, 2 H), d 7.39 (dt, J = 7.3
Hz, J = 2.3 Hz, 1 H), d 7.14 (dd, J = 7.8 Hz, J = 4.7 Hz, 2 H), d
3.97 (s, 2 H), d 3.95 (s, 4 H). These 1H NMR features match those
previously reported.8
1,3-diphenylpropane-1,3-dione (20 mg, 8.3 ¥ 10-5 mol) was added
and the mixture was stirred overnight at ambient temperature to
give a deep orange solution. After removal of the solvent under
vacuum, the residue was dissolved in dichloromethane (~5 mL)
and the solution was filtered through a celite/glass wool plug.
The filtrate was collected and the solvent volume was reduced
to ~1 mL under vacuum. Addition of excess hexanes (~20 mL)
resulted in the deposition of 1 as an orange-brown solid which
was dried under vacuum. (48.2 mg, 76% yield). The formulation
1
of the complex was confirmed by H NMR. The solution from
which the precipitate was obtained was then filtered through a
celite/glass wool plug to remove any residual metal complex and
the filtrate was brought to dryness under vacuum yielding a white
solid (11 mg). 1H NMR analysis of this solid indicated the presence
of 2-oxo-2-phenylethylbenzoate and 6-Ph2TPA in a 2 : 1 ratio.
Isolation of analytically pure [(6-PhTPA)Ni(PhC(O)C(OH)C-
(O)Ph]ClO4 (3) using predried base and solvent. In this entire pro-
cedure extra dry CH3CN purchased from ACROS Organics (<10
ppm H2O) was used. Under a N2 atmosphere, Me4NOH·5H2O
(16 mg, 9.0 ¥ 10-5 mol) was dissolved in methanol in a Schlenk
flask and the mixture was dried under vacuum for >24 h. To
the remaining solid was added an acetonitrile solution (~2 mL)
containing 6-PhTPA (30 mg, 8.2 ¥ 10-5 mol) and Ni(ClO4)2·6H2O
(30 mg, 8.2 ¥ 10-5 mol). The resulting mixture was stirred for
~1 min. At this time, a CH3CN solution (~1 mL) of 2-hydroxy-
1,3-diphenylpropane-1,3-dione (22 mg, 9.0 ¥ 10-5 mol) was added
and the reaction mixture was immediately put under vacuum.
Upon stirring for ~3 h at ambient temperature the solution became
deep orange/brown. After removal of the solvent under reduced
pressure, the remaining orange/brown solid was dissolved in
CH2Cl2 (~5 mL) and the solution was filtered through a celite/glass
wool plug. The filtrate was concentrated under reduced pressure
to a volume of ~1 mL which was then added to excess hexanes
(~20 mL). This resulted in the deposition of a orange/brown solid
which was dried for ~ 2 h under vacuum. Yield: 59 mg (94%). X-
ray quality crystals were obtained from CH2Cl2/n-pentane. Anal.
Calcd for: C39H33ClN4NiO7·1/6 CH2Cl2: C, 60.56; H, 4.33; N, 7.22.
Found: C, 60.48; H, 4.02; N, 7.29. The presence of trace CH2Cl2
in the elemental analysis sample was confirmed by 1H NMR.
FTIR: 3439 (nO-H), 1095 (nClO4), 621 (nClO4); UV-vis (CH3CN) nm
(e, M-1cm-1) 393 (10600).
Caution! Perchlorate salts of metal complexes with organic
ligands are potentially explosive. Only small amounts of material
should be prepared, and these should be handled with great care.24
Attempted preparation of [(6-PhTPA)Ni(PhC(O)C(OH)C-
(O)Ph]ClO4 (3) under the conditions used to prepare 1. Isolation of
isomerization product. In a glovebox, equimolar amounts of 6-
PhTPA (30 mg, 8.2 ¥ 10-5 mol) and Ni(ClO4)2·6H2O (30 mg, 8.2 ¥
10-5 mol) were combined in ~3 mL of acetonitrile (dried from
CaH2; ~3000 ppm water determined via Karl Fischer titration)
and stirred until everything had dissolved. This resulted in the
formation of a purple-brown solution. This solution was then
added to solid Me4NOH·5H2O (16 mg, 9.0 ¥ 10-5 mol) and the
mixture was stirred for an additional ~1 min. At this time, 2-
hydroxy-1,3-diphenylpropane-1,3-dione (22 mg, 9.0 ¥ 10-5 mol),
dissolved in ~1 mL of acetonitrile, was added and the deep orange
solution was stirred overnight at ambient temperature. The solvent
was then removed under vacuum and the remaining solid was
dissolved in ~5 mL of dichloromethane. This solution was passed
through a celite/glass wool plug and the filtrate was then reduced
in volume to ~1 mL. Addition of excess hexanes (~20 mL) resulted
in the deposition of an orange powder, which was dried under
1
vacuum. H NMR analysis indicated the presence of the desired
enolate complex ([(6-PhTPA)Ni(PhC(O)C(OH)C(O)Ph)]ClO4 (3)
and other (6-PhTPA)Ni(II) containing products. To identify the
organic products in the reaction mixture, the CH2Cl2/hexanes
solution from which the metal complexes were precipitated was
filtered through a celite/glass wool plug. The filtrate was then
dried under vacuum leaving a white solid (10 mg). 1H NMR and
GC-MS analysis of the white solid confirmed its identity as a
2-oxo-2-phenylethylbenzoate via comparison with independently
synthesized sample. The amount of solid recovered is consistent
with a 46% yield starting from 2-hydroxy-1,3-diphenylpropane-
1,3-dione.
[(6-PhTPA)Ni(PhC(O)CHC(O)Ph)]ClO4 (4). A solution of
Ni(ClO4)2·6H2O (35 mg, 1.0 ¥ 10-4 mol) in acetonitrile (~3 mL)
was added to solid 6-PhTPA (37 mg, 1.0 ¥ 10-4 mol) and the
resulting mixture was stirred until all of the solids had dissolved.
An acetonitrile solution (~2 mL) containing dibenzoylmethane
(21 mg, 1.0 ¥ 10-4 mol) and Me4NOH·5H2O (17 mg, 1.0 ¥ 10-4 mol)
was then added and the mixture was stirred overnight at ambient
temperature. The solvent was removed under reduced pressure
and the remaining solid was dissolved in CH2Cl2. The solution
was filtered through a celite/glass wool plug and the solvent
was subsequently removed from the filtrate under vacuum. The
remaining solid was dissolved in a small amount of acetonitrile
and excess diethyl ether (~20 mL) was added to precipitate the
desired product. Yield: 40 mg (54%). X-ray quality crystals were
obtained by diethyl ether diffusion into an acetonitrile solution
of the complex. Anal. Calcd for: C39H33ClN4NiO6·0.95C4H10O:
C, 62.82; H, 5.23; N, 6.85. Found: C, 62.98; H, 5.27; N, 7.00.
Evaluation of ester formation in the reaction leading to the forma-
tion of [(6-Ph2TPA)]Ni(PhC(O)C(OH)C(O)Ph]ClO4 (1). In the
glovebox, equimolar amounts of 6-Ph2TPA (34 mg, 7.6 ¥ 10-5 mol)
and Ni(ClO4)2·6H2O (28 mg, 7.6 ¥ 10-5 mol) were mixed in ~3 mL
of acetonitrile and stirred until everything had dissolved, which
gave a purple solution. This solution was then combined with solid
Me4NOH·5H2O (15 mg, 8.3 ¥ 10-5 mol) and the mixture was stirred
for ~1 min. At this time, a CH3CN solution (~1 mL) of 2-hydroxy-
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The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 10609–10620 | 10617
©

FTIR (KBr, cm-1) 1092 (nClO4), 621 (nClO4). UV-vis (CH3CN) nm
(e, M-1cm-1) 543 (26), 793 (22), 907 (30); FAB-MS m/z (relative
intensity) 647 ([M-ClO4]+, 100%).
N2-filled glovebox, 6-Ph2TPA (36 mg, 8.2 ¥ 10-5 mol) and
Co(ClO4)2·6H2O (30 mg, 8.2 ¥ 10-5 mol) were mixed in ~3 mL
of acetonitrile (~3000 ppm H2O via Karl Fischer titration) and
the solution was stirred until everything had dissolved. This
purple solution was then combined with solid Me4NOH·5H2O
(16 mg, 9.0 ¥ 10-5 mol) and the mixture was stirred for ~1 min.
To this solution was added a CH3CN solution (1 mL) of 2-
hydroxy-1,3-diphenylpropane-1,3-dione (22 mg, 9.0 ¥ 10-5 mol).
The deep orange solution was then stirred overnight at ambient
temperature during which time the color changed to green. The
solvent was removed under vacuum and the remaining solid was
dissolved in CH2Cl2 (~5 mL). This solution was passed through
a celite/glass wool plug and the filtrate was reduced to ~1 mL
under vacuum. Addition of excess hexanes (~20 mL) resulted in the
deposition of a brown solid (40 mg). 1H NMR analysis indicates
the presence of [(6-Ph2TPA)Co(O2CPh)]ClO4 (10) and other Co(II)
compounds (Fig. 4(a)). The solution remaining after the isolation
of the metal complexes was filtered through a celite/glass wool
plug and the filtrate was brought to dryness under vacuum
[(6-PhTPA)Ni(CH3CN)(CH3OH)](ClO4)2 (5). A solution of
6-Ph2TPA (48 mg, 1.3 ¥ 10-4 mol) and Ni(ClO4)2·6H2O (45 mg,
1.3 ¥ 10-4 mol) in acetonitrile (~3 mL) was prepared and stirred
until all solids had dissolved. The solution was then brought to
dryness under vacuum and the residue was dissolved in MeOH.
Crystals were grown by Et2O diffusion into the methanol solution.
Yield: 72 mg (84%). Anal. Calcd for: C27H29Cl2N5NiO9: C, 46.52;
H, 4.19; N, 10.05. Found: C, 46.17; H, 4.30; N, 10.00. FTIR (KBr,
cm-1) 1092 (nClO4), 623 (nClO4). UV-vis (CH3CN) nm (e, M-1cm-1)
541 (20), 793 (17), 913 (23); FAB-MS m/z (relative intensity) 424
([M-2(ClO4)-CH3CN-CH3OH-H+]+, 9%)
[(6-PhTPA)Ni(O2CPh)]ClO4 (6). An acetonitrile solution
(~3 mL) containing 6-PhTPA (33 mg, 8.9 ¥ 10-5 mol) and
Ni(ClO4)2·6H2O (32 mg, 8.9 ¥ 10-5 mol) was prepared and stirred
until all solids had dissolved. This solution was then combined
with solid sodium benzoate (13 mg, 8.9 ¥ 10-5 mol) and ~1 mL of
MeOH was added. This mixture was stirred for ~1 h at ambient
temperature and the solvent was then removed under reduced
pressure. The remaining solid was dissolved in CH2Cl2 and the
solution was passed through a celite/glass wool plug. The solvent
was removed from the filtrate under reduced pressure. The desired
complex were obtained by dissolving the solid in minimal CH2Cl2
followed by addition of excess n-pentane. The process was repeated
three times to obtain analytically pure complex Yield: 30 mg
(53% yield). X-ray quality crystals were obtained by diffusion of a
CH2Cl2/i-PrOH/EtOAc solution of the complex into n-pentane.
Anal. Calcd for: C31H27ClN4NiO6·1/3 CH2Cl2: C, 55.85; H, 4.14;
N, 8.32. Found: C, 55.59; H, 4.31; N, 8.31. The presence of CH2Cl2
in the elemental analysis sample was confirmed by 1H NMR.
FTIR (KBr, cm-1) 1092 (nClO4), 623 (nClO4); UV-vis (CH3CN) nm
(e, M-1cm-1) 543 (14), 802 (7), 953 (15); FAB-MS m/z (relative
intensity) 545 ([M-ClO4]+, 75%).
1
leaving a white solid (32 mg). H NMR (in CD3CN) and GC-
MS analysis of this solid confirmed the presence of the ester 2-
oxo-2-phenylethylbenzoate (~11 mg, 50% based on 2-hydroxy-
1,3-diphenylpropane-1,3-dione) and 6-Ph2TPA (~21 mg, ~58%
recovery).
Attempted preparation of [(6-Ph2TPA)Co(PhC(O)C(OH)C-
(O)Ph]ClO4 (7) under conditions used to isolate 3. In this entire
procedure extra dry CH3CN purchased from ACROS Organics
(<10 ppm H2O) was used. Prior to the synthesis, Me4NOH·5H2O
was dissolved in ~2 mL methanol, transferred to a Schlenk
flask, and dried under vacuum for >24 h. In the glovebox, [(6-
Ph2TPA)Co(CH3CN)](ClO4)2 (9) (64 mg, 8.2 ¥ 10-5 mol) was
dissolved in acetonitrile and stirred until everything had dissolved,
yielding a purple solution. Next, 2-hydroxy-1,3-diphenylpropane-
1,3-dione (22 mg, 9.0 ¥ 10-5 mol) dissolved in ~1 mL of acetonitrile
was added to the solid, pre-dried Me4NOH·5H2O (16 mg;
9.0 ¥ 10-5 mol). This mixture was immediately combined with the
solution of [(6-Ph2TPA)Co(CH3CN)](ClO4)2 and the mixture was
stirred for 3 h, under vacuum, yielding a deep orange/red solution.
The solvent was then removed under reduced pressure. The re-
maining orange solid was dissolved in ~5 mL of dichloromethane,
and the solution was filtered through a celite/glass wool plug.
The filtrate was concentrated to ~1 mL under vacuum and the
Co(II)-containing products were precipitated by the addition of
excess hexanes (~20 mL). This resulted in the isolation of 54 mg
of an orange solid. The 1H NMR features of this solid are shown
in Fig. 4(b). The hexane layer was filtered through a celite/glass
wool plug and the filtrate was brought to dryness under vacuum.
Evaluation of 2-oxo-2-phenylethylbenzoate hydrolysis in the
presence of [(6-PhTPA)Ni(CH3CN)(CH3OH)](ClO4)2 (9) and
Me4NOH·5H2O. In
a
glovebox, [(6-PhTPA)Ni(CH3CN)-
(MeOH)](ClO4)2 (5) (23 mg, 3.3
¥
10-5 mol), was
dissolved in acetonitrile and combined with equimolar
amounts of Me4NOH·6H2O (6.0 mg, 3.29 ¥ 10-5 mol) and
PhC(O)OCH2C(O)Ph (7.9 mg, 3.29 ¥ 10-5 mol) and the mixture
was stirred for 16.5 h. The solvent was then removed under
reduced pressure and the remaining solid was dissolved in CH2Cl2
and the solution was filtered through a celite/glass wool plug. The
filtrate was concentrated under vacuum and precipitation of the
metal complex(es) was induced by the addition of excess hexane.
1
A white solid was obtained (22 mg). H NMR analysis of this
1
The H NMR of this solid indicated the presence of multiple
solid showed the presence of 2-oxo-2-phenylethylbenzoate and 6-
Ni(II) complexes, as evidenced by several overlapping signals in
the region of 80-20 pm. The solution from the precipitate was
isolated was filtered through a celite/glass wool plug and the
filtrate was brought to dryness leaving a white solid (7.5 mg).
1H NMR of this solid indicated the presence of unaltered ester
PhC(O)OCH2C(O)Ph in 95% yield. No other organic products
were detected.
Ph2TPA in ~1 : 1 ratio.
Attempted preparation of [(6-Ph2TPA)Co(PhC(O)C(OH)C-
(O)Ph]ClO4 (7) under dry conditions and using a short reaction
time. In this entire procedure extra dry CH3CN purchased from
ACROS Organics (<10 ppm H2O) was used. Me4NOH·5H2O
(5.9 mg, 3.3 ¥ 10-5 mol) was dissolved in ~3 mL acetonitrile.
This solution was brought to dryness and the remaining solid
was dried under vacuum for 2 h. This step was repeated twice.
Next, [(6-Ph2TPA)Co(CH3CN)](ClO4)2 (9) (26 mg, 3.3 ¥ 10-5 mol)
Attempted preparation of [(6-Ph2TPA)Co(PhC(O)C(OH)C-
(O)Ph]ClO4 (7) under the conditions used to prepare 1. In a
10618 | Dalton Trans., 2011, 40, 10609–10620
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©

and 2-hydroxy-1,3-diphenylpropane-1,3-dione (7.8 mg, 3.3 ¥
10-5 mol) were independently dissolved in CH3CN (2 mL).
The solution of 2-hydroxy-1,3-diphenylpropane-1,3-dione was
added to the solid Me4NOH and the CH3CN solution of [(6-
Ph2TPA)Co(CH3CN)](ClO4)2 (9) was subsequently added. The
reaction immediately turned dark orange (lmax 397 nm). It was
stirred for 15 min and then the solvent was removed under
reduced pressure. The remaining dark orange solid was dissolved
in dichloromethane (~3 mL) and the solution was filtered through
a celite/glass wool plug. The filtrate was reduced in volume to
~1 mL and the Co(II) product was precipitated via the addition of
prepared to enable the identification of the benzoate phenyl
resonances.
Evaluation of 2-oxo-2-phenylethylbenzoate hydrolysis in the
presence of [(6-Ph2TPA)Co(CH3CN)](ClO4)2·CH3CN (9) and
Me4NOH·5H2O. Under a nitrogen atmosphere, a solution of
[(Ph2TPA)Co(CH3CN)](ClO4)2·CH3CN (9) (26 mg, 3.3 ¥ 10-5 mol)
in acetonitrile (~3 mL) was added to solid Me4NOH·5H2O (6.0 mg,
3.3 ¥ 10-5 mol) and 2-oxo-2-phenylethylbenzoate (7.9 mg, 3.3 ¥
10-5 mol) and the mixture was stirred for 16.5 h at ambient temper-
ature. The color of the solution changed from purple to green. The
solvent was removed under reduced pressure and the remaining
solid was dissolved in CH2Cl2. The solution was filtered through a
glass wool/celite plug. The filtrate was then concentrated to ~1 mL
under reduced pressure and excess hexanes (~20 mL) was added
1
excess of hexanes (~20 mL). The H NMR features of this solid
are shown in Fig. 4(c). UV-vis (CH3CN) 397 nm. The hexane layer
was filtered through a celite/glass wool plug and the filtrate was
brought to dryness under vacuum. A white solid was obtained
(12 mg). 1H NMR analysis of this solid indicated the presence of
the ester 2-oxo-2-phenylethylbenzoate and 6-Ph2TPA in a ratio of
3 : 1.
1
to induce precipitation of a green solid. Yield: 22 mg. H NMR
analysis confirmed the presence of [(Ph2TPA)Co(O2CPh)]ClO4
(10, major species) and [(Ph2TPA)Co(CH3CN)](ClO4)2·CH3CN
(9, minor species). The solution remaining following the isolation
of the solid was passed through a celite/glass wool plug and the
filtrate was brought to dryness under reduced pressure. A white
solid was isolated (5.7 mg). By 1H NMR this solid is a mixture of
unaltered ester (minor) and 6-Ph2TPA (major).
[(6-Ph2TPA)Co(PhC(O)CHC(O)Ph)]ClO4 (8). A mixture of
6-Ph2TPA (18 mg, 4.1 ¥ 10-5 mol) and Co(ClO4)2·6H2O (14 mg,
3.7 ¥ 10-5 mol) in acetonitrile (~3 mL) was prepared and stirred
until everything had dissolved. To this solution was added
dibenzoylmethane (10 mg, 4.6 ¥ 10-5 mol) as a solution in
acetonitrile (~1 mL). The resulting mixture was stirred for 10 min
at which point it was transferred to a glass vial containing solid
Me4NOH·5H2O (7.5 mg, 4.1 ¥ 10-5 mol). This mixture was then
stirred overnight at ambient temperature. After removal of the
solvent under reduced pressure, the remaining dark orange-red
solid was dissolved in CH2Cl2 and filtered through a celite/glass
wool plug. The solvent was removed from the filtrate under
vacuum and the remaining solid was recrystallized by Et2O
diffusion into the acetonitrile solution. Yield: 24 mg (79%). Anal.
Calcd for: C45H37ClCoN4O6: C, 65.60; H, 4.52; N, 6.80. Found: C,
65.46; H, 4.39; N, 7.15. FTIR (KBr, cm-1) 1094 (nClO4), 623 (nClO4).
UV-vis (CH3CN) nm (e, M-1cm-1) 350 (7500), 564 (77); FAB-MS
m/z (relative intensity) 724 ([M-ClO4]+, 45%).
X-ray crystallography. A single crystal of each complex was
mounted on a glass fiber with viscous oil and then transferred to
˚
a Nonius Kappa CCD diffractometer (Mo-Ka, l = 0.71073 A)
for data collection at 150(1) K. For each compound an initial
set of cell constants was obtained from 10 frames of data that
were collected with an oscillation range of 1◦/frame and an
exposure time of 20s/frame. Indexing and unit cell refinement
based on all observed reflections from those ten frames indicated
monoclinic lattices for 3·CH2Cl2 and 10, orthorhombic lattices for
4·Et2O and 5, and a tetragonal lattice for 8·0.5CH3CN. Final cell
constants for each complex were determined from a set of strong
reflections from the actual data collection. For each data set, these
reflections were indexed, integrated, and corrected for Lorentz,
polarization, and absorption effects using DENZO-SMN and
SCALEPAC.25 The structures were solved by a combination of
direct methods and heavy atom using SIR97. All non-hydrogen
atoms were refined with anisotropic displacement coefficients.
Unless otherwise noted, hydrogen atoms were assigned isotropic
displacement coefficients (U(H) = 1.2 U(C) or 1.5U(Cmethyl) and
their coordinates were allowed to ride on their respective carbons
using SHELXL97.26
The Ni(II) enolate complex 3·CH2Cl2 crystallizes in the mono-
clinic crystal system in the space group Cc with one molecule of
methylene chloride per formula unit. The Ni(II) dibenzoylmethane
complex 4·Et2O crystallizes in the space group Pcab with one
molecule of diethyl ether per formula unit. All hydrogen atoms in
this structure were located and refined independently. The Ni(II)
solvent-coordinated complex 5 crystallizes in the space group
P212121. The Co(II) dibenzoylmethane complex 8·0.5CH3CN
crystallizes in the space group I-4. There are two independent
formula units per asymmetric unit, with the second being denoted
by “A”. One molecule of acetonitrile is present per asymmetric
unit. The perchlorate anions exhibits disorder. The Co(II) benzoate
complex 10 crystallizes in the space group P21/c. All hydrogen
atoms in this structure were located and refined independently.
[(6-Ph2TPA)Co(O2CPh)]ClO4 (10). A mixture of 6-Ph2TPA
(36 mg, 8.1 ¥ 10-5 mol) and Co(ClO4)2·6 H2O (27 mg, 7.3 ¥
10-5 mol) in acetonitrile (~3 mL) was prepared and stirred until
everything had dissolved. This solution was then added to solid
sodium benzoate (13 mg, 8.9 ¥ 10-5 mol) and the resulting mixture
was stirred overnight at room temperature. The solvent was then
removed under reduced pressure. The remaining green solid was
dissolved in CH2Cl2 and filtered through a celite/glass wool plug.
The filtrate was brought to dryness under reduced pressure and
the remaining solid was recrystallized by Et2O diffusion into
a acetonitrile solution. Yield: 51 mg (98%). Anal. Calcd for:
C37H31ClCoN4O6: C, 61.55; H, 4.33; N, 7.76. Found: C, 61.39;
H, 4.34; N, 7.86. FTIR (KBr, cm-1) 1097 (nClO4), 621 (nClO4); UV-
vis (CH3CN) nm (e, M-1cm-1) 462 (103), 619 (66); FAB-MS m/z
(relative intensity) 622 ([M-ClO4]+, 14%).
Preparation of deuterated analogs of 8–10 of either the 6-(d5-
Ph)2TPA or (6-Ph)2-d6-TPA ligand. These analogs were prepared
using 6-(d5-Ph)2TPA and (6-Ph)2-d6-TPA11 and following the
2
procedures for their protio analogs. H NMR spectra of these
complexes were obtained as previously described. We note that
a d5-analog of 10, [(6-Ph2TPA)Co(O2C-d5-Ph)]ClO4, was also
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Dalton Trans., 2011, 40, 10609–10620 | 10619
©

10 P. Karrer, J. Kebrle and R. M. Thakkar, Helv. Chim. Acta, 1950, 33,
1711–1724.
Acknowledgements
11 E. Szajna, P. Dobrowolski, A. L. Fuller, A. M. Arif and L. M. Berreau,
Inorg. Chem., 2004, 43, 3988–3997.
The authors thank the National Institutes of Health
(1R15GM072509) and the National Science Foundation (Grant
CHE-0848858) for support of this research.
12 Methylene proton resonances of the chelate ligand can also appear in
this region11.
13 A table of the chemical shifts of selected 1H NMR resonances of 3–6 is
given in the Supporting Information.
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◦
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9 Keto form of the C(1)-phenyl acireductone analog shown in Scheme 1
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10620 | Dalton Trans., 2011, 40, 10609–10620
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The Royal Society of Chemistry 2011
©