N3-Ligated nickel(ii) diketonate complexes: synthesis, characterization and evaluation of O2reactivity

Article abstract of DOI:10.1039/d0dt01338b

Interest in O₂-dependent aliphatic carbon-carbon (C-C) bond cleavage reactions of first row divalent metal diketonate complexes stems from the desire to further understand the reaction pathways of enzymes such as DKE1 and to extract information to develop applications in organic synthesis. A recent report of O₂-dependent aliphatic C-C bond cleavage at ambient temperature in Ni(ii) diketonate complexes supported by a tridentate nitrogen donor ligand [(MBBP)Ni(PhC(O)CHC(O)Ph)]Cl (7-Cl; MBBP = 2,6-bis(1-methylbenzimidazol-2-yl)pyridine) in the presence of NEt₃spurred our interest in further examining the chemistry of such complexes. A series of new TERPY-ligated Ni(ii) diketonate complexes of the general formula [(TERPY)Ni(R₂-1,3-diketonate)]ClO₄(1: R = CH₃;2: R = C(CH₃)₃;3: R = Ph) was preparedunder airand characterized using single crystal X-ray crystallography, elemental analysis,¹H NMR, ESI-MS, FTIR, and UV-vis. Analysis of the reaction mixtures in which these complexes were generated using¹H NMR and ESI-MS revealed the presence of both the desired diketonate complex and the bis-TERPY derivative [(TERPY)₂Ni](ClO₄)₂(4). Through selective crystallization1-3were isolated in analytically pure form. Analysis of reaction mixtures leading to the formation of the MBBP analogs [(MBBP)Ni(R₂-1,3-diketonate)]X (X = ClO₄:5: R = CH₃;6: R = C(CH₃)₃;7-ClO₄: R = Ph; X = Cl:7-Cl: R = Ph) using¹H NMR and ESI-MS revealed the presence of [(MBBP)₂Ni](ClO₄)₂(8). Analysis of aerobic acetonitrile solutions of analytically pure1-3,5and6containing NEt₃and in some cases H₂O using¹H NMR and UV-vis revealed evidence for the formation of additional bis-ligand complexes (4and8) but suggested no oxidative diketonate cleavage reactivity. Analysis of the organic products generated from3,7-ClO₄and7-Clrevealed unaltered dibenzoylmethane. Our results therefore indicate that N₃-ligated Ni(ii) complexes of unsubstituted diketonate ligands do not exhibit O₂-dependent aliphatic C-C bond clevage at room temperature, including in the presence of NEt₃and/or H₂O.

Full text of DOI:10.1039/d0dt01338b

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DOI: 10.1039/D0DT01338B
ARTICLE
N3-ligated Nickel(II) Diketonate Complexes: Synthesis,
Characterization and Evaluation of O2 Reactivity
Josiah G.D. Elsberga, Austin Petersona, Amy L. Fullerb and Lisa M. Berreaua*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Interest in O2-dependent aliphatic carbon-carbon (C-C) bond cleavage reactions of first row divalent metal diketonate
complexes stems from the desire to further understand the reaction pathways of enzymes such as DKE1 and to extract
information to develop applications in organic synthesis. A recent report of O2-dependent aliphatic C-C bond cleavage at
ambient temperature in Ni(II) diketonate complexes supported by
a tridentate nitrogen donor ligand
[(MBBP)Ni(PhC(O)CHC(O)Ph)]Cl (7-Cl; MBBP = 2,6-bis(1-methylbenzoimidazol-2-yl)pyridine) in the presence of NEt3
spurred our interest in further examining the chemistry of such complexes. A series of new TERPY-ligated Ni(II) diketonate
complexes of the general formula [(TERPY)Ni(R2-1,3-diketonate)]ClO4 (1: R = CH3; 2: R = C(CH3)3; 3: R = Ph) was prepared
1
under air and characterized using single crystal X-ray crystallography, elemental analysis, H NMR, ESI-MS, FTIR, and UV-
vis. Analysis of the reaction mixtures in which these complexes were generated using 1H NMR and ESI-MS revealed the
presence of both the desired diketonate complex and the bis-TERPY derivative [(TERPY)2Ni](ClO4)2 (4). Through selective
crystallization 1-3 were isolated in analytically pure form. Analysis of reaction mixtures leading to the formation of the
MBBP analogs [(MBBP)Ni(R2-diketonate)]X (X = ClO4: 5: R = -CH3; 6: R = - C(CH3)3; 7-ClO4: R = Ph; X = Cl: 7-Cl: R = Ph) using
1H NMR and ESI-MS revealed the presence of [(MBBP)2Ni](ClO4)2 (8). Analysis of aerobic acetonitrile solutions of
1
analytically pure 1-3, 5 and 6 containing NEt3 and in some cases H2O using H NMR and UV-vis revealed evidence for the
formation of additional bis-ligand complexes (4 and 8) but suggested no oxidative diketonate cleavage reactivity. Analysis
of the organic products generated from 3, 7-ClO4 and 7-Cl revealed unaltered dibenzoylmethane. Our results therefore
indicate that N3-ligated Ni(II) complexes of unsubstituted diketonate ligands do not exhibit O2-dependent aliphatic C-C
bond clevage at room temperature, including in the presence of NEt3 and/or H2O.
et al. used a trispyrazolylborato (Tp) ligand to mimic the 3His
facial triad in a Fe(II) diethylphenylmalonate complex (Scheme
1(b)).12 This complex undergoes O2-dependent aliphatic C-C
cleavage within the diketonate unit to give ethyl carbonate
(which decomposes to give CO2 and ethoxide anion) and ethyl
benzoylformate as products of a dioxygenase-type reaction.
Park et al. reported a similar set of complexes using two
different R2Tp ligands (R = -Me and -Ph; Scheme 1(c)) and a
variety of diketonate ligands.13 Their studies showed that O2-
dependent cleavage only occurred for Ph2Tp complexes
Introduction
Oxidative aliphatic carbon-carbon (C-C) bond cleavage
reactions mediated by first row metals using O2 as the terminal
oxidant are of significant current interest, particularly for
applications in organic synthesis.1-7 To date, examination of
the mechanistic pathways of these reactions has received
minimal attention. This is due in part to the limited number of
systems wherein a well-characterized metal/substrate type
structure can be evaluated in terms of its O2 reactivity. A few
laboratories have developed model systems for enzymes that
mediate O2-dependent aliphatic C-C bond cleavage. DKE1 is an
enzyme found in Acinetobacter johnsonii which catalyzes the
oxidative cleavage of diketonate substrates, including
acetylacetone, using O2 as the terminal oxidant.8 The active
site of DKE1 contains a non-heme Fe(II) center ligated in a
facial array by three histidine donors (Scheme 1(a)).9-11 Siewart
containing
a diethyl malonate or diethylphenylmalonate
diketonate ligand. Complexes that didn’t undergo oxidative
diketonate cleavage exhibited either no reaction or formed a
green peroxo-bridged intermediate that decayed without any
oxidative C-C bond cleavage. Recently, Banerjee and Paine
reported that an Fe(II) dibenzoylmethane complex supported
by the monoanionic tris(2-pyrdiylthio)methanido ligand
undergoes reaction with O2 at ambient temperature to give a
small amount of benzoic acid (11-13%).14 The major product of
this reaction is the Fe(III) diketonate complex.
a. Department of Chemistry & Biochemistry, Utah State University, 0300 Old Main
Hill, Logan, UT 84322-0300 USA
b. Department of Chemistry, University of Hawaii Manoa, 2545 McCarthy Mall,
Honolulu, HI 96822-2275 USA
†Electronic Supplementary Information (ESI) available: Paramagnetic 1H NMR, ESI-
MS, and FT-IR spectra; crystallographic data; organic recovery data. CCDC 1191784
1991786 1991787 1992287. For ESI, see DOI: 10.1039/x0xx00000x
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To examine how a change in the metal center affects O2
Journal Name
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DOI: 10.1039/D0DT01338B
activation, Hoof et al. studied the O2 reactivity of a Me2Tp-
ligated Ni(II) diethylphenylmalonate complex.15 Their studies
revealed O2-dependent reactivity leading to hydroxylation and
aliphatic C-C bond cleavage products (Scheme 2(a)). This
reaction required an open coordination position at the Ni(II)
center as using the more sterically hindered Ph2Tp supporting
ligand system did not produce the same type of reactivity. A
hydroxylated diphenyl diketonate ligand in a model system for
Ni(II)-containing acireductone dioxygenase (Scheme 2(b))
undergoes two-electron oxidation upon reaction with O2 to
give a triketone and hydroperoxide anion as intermediates.16,17
Subsequent reaction between these intermediates in the
presence of the remaining Ni(II) complex gives
benzoate/benzoic acid and CO as products of the aliphatic C-C
bond cleavage reaction. An Fe(II) analog produces similar
products as well as an a-keto acid product, which results from
iron-promoted hydration of the triketone intermediate
followed by C-C cleavage.18 Recently, Raje, et al. reported
similar results using
a Ni(II) complex supported by a
tetradentate pyridinophane (L4) ligand.19 This complex was
Scheme 2 a) Reaction of a Me2TpNi(II) diethylphenyl-
malonate complex with O2. b) Reactivity of Ni(II)
hydroxydiketonate complex that mimics the reactivity of Ni(II)-
containing acireductone dioxygenase (ARD). c) UV-light
induced aliphatic C-C bond cleavage of methoxy-substituted
chlorodiketonate complex in the presence of O2.
treated
with
the
hydroxylated
dibenzoylmethane
(PhC(O)C(OH)C(O)Ph) in the presence of base in CH3CN.
Exposure of this solution to O2 followed by ESI-MS analysis
revealed the presence of the [L4Ni(OC(O)Ph)]+ cation. Organic
recovery experiments also revealed the formation of benzoic
acid. These results mirror those previously reported by Szajna
et al.16
Notably, a structurally similar Ni(II) dibenzoylmethane
complex that does not contain the hydroxyl substituent on the
central carbon of the diketonate ligand is air stable.20
However, a Ni(II) chlorodiketonate complex having methoxy-
substituted phenyl appendages (Scheme 2(c)) undergoes O2-
dependent aliphatic C-C bond cleavage reactivity within the
diketonate moiety to give
a carboxylate product upon
illumination with UV light.21 This reactivity is similar to thermal
oxidative C-C bond cleavage identified for structurally-similar
copper chlorodiketonate complexes.22,23
Based on the literature precedent described above, which
shows that only electron rich diketonates, or those containing
a central carbon substituent that can undergo reaction or act
as a leaving group, exhibit significant O2 reactivity, a recent
publication drew our attention.24 Ramasubramanian, et al.
reported that Ni(II) diketonate complexes (Scheme 3(a))
supported by the tridentate 2,6-bis(1-methylbenzoimidazol-2-
yl)pyridine (MBBP) ligand undergo O2-dependent oxidative
Scheme 1 a) Reaction catalyzed and active site iron center
of DKE1. b) Diethylphenylmalonate-containing model complex
for the active site of DKE1. c) Complexes with diketonates e-f
undergo oxidative C-C bond cleavage whereas those with
diketonates a-d do not exhibit this reactivity.
2 | J. Name., 2012, 00, 1-3
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cleavage in the presence of one equivalent of triethylamine to date in synthetic systems. Based on these results we
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(NEt3) and in the absence or presence of water (10% in CH3CN) became interested in further examiningDsOuIc:h10s.1y0s3t9e/mD0sD.TH0e13r3e8inB
(Scheme 3(b)) to give carboxylic acid products (52%, R = Ph) we report synthetic, characterization and reactivity studies of a
along with CO2. These reactions are reported to occur under series of new TERPY-ligated Ni(II) diketonate complexes,
mild conditions, as a 6.00x10-4 M solution of each complex [(TERPY)Ni(diketonate)]ClO4 (1-3). While such complexes could
undergoes complete reaction in <1 h at 25 oC as determined by be isolated by selective crystallization, the reaction mixtures
UV-vis. 18O2 studies showed 43% isotope incorporation in the leading to their formation always also contained
benzoic acid product. A reaction pathway was proposed for [(TERPY)2Ni](ClO4)2 (4). Unlike the MBBP Ni(II) diketonate
the R = Ph derivative (Scheme 3(b)) wherein the diketonate complexes reported by Ramasubramamanin, et al.24, 1-3 are
acts as a two-electron reductant to O2, leading to the stable with respect to O2 over 24 h, showing no aliphatic C-C
formation of an organoperoxo species that is stabilized via bond cleavage reactivity upon exposure to air, even in the
interaction with Ni(II). Intramolecular attack of the terminal presence of base and water. Based on these results, we re-
peroxo oxygen at a diketonate carbonyl moiety followed by examined the chemistry of the MBBP-ligated diketonate
1
aliphatic C-C bond cleavage gives the benzoic acid product and complexes. Using H NMR under paramagnetic conditions we
phenylgloyoxal. Another report in the literature suggested that found that the formation bis-MBBP Ni(II) derivatives is
O2-dependent oxidative cleavage of an acetylacetonate ligand prevalent
under
the
reaction
conditions.24,26
can occur in Ni(acac)2(H2O)2 the presence of base Ramasubramanian et al. briefly mention these complexes, but
(ethylenediamine) (Scheme 3(c)).25 However, this reaction was did not report the chemistry associated with such species.
reported to proceed very slowly at room temperature (18 d Using an organic recovery experiment, we found no evidence
under air).
for oxidative aliphatic C-C bond cleavage upon exposure of
The reactions outlined in Scheme 3 are interesting in that [(MBBP)Ni(PhC(O)CHC(O)Ph)]X (7-X; X = Cl or ClO4) to O2 in the
aliphatic C-C cleavage is reported to occur within an presence of NEt3 or NEt3/H2O after 24 h.
unsubstituted diketonate under mild conditions, which is rare
Experimental
General Methods
The following chemicals were purchased and used as received:
1,3-diphenylpropane-1,3-dione (98%, Sigma-Aldrich), 2,4-
pentanedione (99%, Acros Organics), 2,2,6,6-tetramethyl-3,5-
heptanedione (97%, Combi-Blocks) and 2,2’;6’,2’’-terpyridine
(98%, Sigma-Aldrich). Solvents used in air were used without
further purification unless otherwise stated. Solvents used in
the glovebox were dried following a previously published
procedure prior to use.27 The CD3CN used in 1H NMR
experiments was dried using molecular sieves. 2,6-bis(2’-
benzimidazolyl)pyridine (BBP) was prepared following the
literature procedure.28 Manipulations performed under an
inert N2 atmosphere were conducted in a MBraun Unilab
glovebox.
Physical Methods
1H NMR spectra were collected on a Brüker Advance III HD
Ascend-500 spectrometer. Chemical shifts (ppm) are reported
relative to the residual solvent peak in CD2HCN (1.94 ppm,
quintet) or CHCl3 (7.26 ppm, singlet). 1H NMR spectra for high-
spin paramagnetic Ni(II) complexes were performed using
previously published parameters: 32k data points, 300 scans, a
90° pulse (9.75 µ
s) at 298 K, and a 250 ms relaxation delay.26
During processing, an exponential weighting function (lb = 7
Hz) and a manual baseline correction were performed using
MNova (Version 12.0.4). FTIR spectra were collected as KBr
pellets using a Shimadzu FTIR-8400 spectrometer. UV-vis data
was collected using a Hewlett-Packard 8453A diode array
spectrometer at ambient temperature. ESI mass spectral data
Scheme
3 a) MBBP-ligated Ni(II) diketonate complexes
reported by Ramasubramamanin, et al.24 b) Proposed
pathway of O2 reaction of MBBP-ligated Ni(II) phenyl
was collected using
analyses were performed by Robertson Microlit Laboratories
(Ledgewood, NJ) or Atlantic Microlab (Norcross, GA).
a Shimadzu LCMS-2020. Elemental
diketonate complex.
c) Oxidative cleavage of an
aceytylacetonate ligand.25
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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Journal Name
[(TERPY)Ni(Ph(O)CHC(O)Ph)(H2O)]ClO4 (3). NVi(ieCwlOAr4ti)c2le·6OHnl2inOe
DOI: 10.1039/D0DT01338B
Caution! Perchlorate compounds containing organic ligands (49.4 mg 0.135 mmol) was dissolved in CH3CN (2 mL) under air.
are potentially explosive. These materials should be handled Dibenzoylmethane (30.1 mg, 0.135 mmol) was added to the
with care and in small quantities.29
solution along with triethylamine (19
mixture was stirred for 30 minutes and then added to a vial
Synthesis and Characterization. Unless stated otherwise all containing TERPY (31.6, 0.135 mmol) dissolved in CH3CN (2
μL, 0.135 mmol). This
synthetic reactions were performed in air.
mL). The resulting solution was stirred for 2 h during which
time it became green. Excess Et2O (8 mL) was added and the
(1). solution was placed in the freezer (-15 oC) overnight. The
[(TERPY)Ni(CH3C(O)CHC(O)CH3)(CH3CN)]ClO4
Ni(ClO4)2·6H2O (49.4 mg 0.135 mmol) was dissolved in CH3CN yellow-green solution was decanted away from a tan solid (4
(2 mL) under air. An excess of 2,4-pentanedione (~1 mL) and by 1H NMR; vida infra) and was brought to dryness under
one equivalent of triethylamine (19
μL, 0.135 mmol) were reduced pressure. The resulting thick green oil was dissolved in
subsequently added. The mixture was then stirred for CH2Cl2 (4 mL) and the solution was passed through a celite
approximately 30 min during which time it became blue. This plug. Slow diffusion of Et2O into the CH2Cl2 filtrate produced X-
solution was added to a vial containing TERPY (31.6 mg, 0.135 ray diffraction quality crystals (35 mg, 42% yield). ESI-MS: m/z
mmol) dissolved in CH3CN (2 mL). The resulting solution calc. for C30H22N3NiO2·CH3CN, 555.1 [M-ClO4]+; found 555.2;
became faint yellow. After stirring at ambient temperature for calc. 514.1 [M-CH3CN-ClO4]+; found 514.1. Anal. Calc. for
2 h, Et2O (8 mL) was added and the solution was placed in the C30H22ClN3NiO6·H2O: C, 56.95; H, 3.82; N, 6.64. Found: C, 57.08;
freezer (-15 oC) for 12 h. This produced grayish-green X-ray H, 3.75; N, 6.70. UV-Vis λmax, nm (ε, M−1 cm−1): 236 (20752),
quality crystals. We note that the ratio of Et2O to CH3CN 253 (17386), 270 (17553), 278 (18609), 317 (12050), 329
(~1.4:1) is important to selectively crystallize 1. The solution (13187), 364 (10790) (d-d transitions). FT-IR (KBr, cm-1): 3440
was decanted from the crystals, which were then washed with (νO-H), 3160 (νC-H), 2010 (νC-H), 1600 (νC=C), 1510 (νC=N), 1450
Et2O and dried under vacuum (49 mg, 74% yield). ESI-MS: m/z (νC=O), 1410 (νC=O), 1090 (νClO4), 780(νC-H), 630(νClO4).
calc. for C20H18N3NiO2·CH3CN, 431.1 [M-ClO4]+; found 431.0;
calc. 390.1 [M-CH3CN-ClO4]+; found 390.0. Anal. Calc. for C20- [(TERPY)2Ni](ClO4)2 (4). This compound has been previously
H18ClN3NiO6·CH3CN: C, 49.71; H, 3.98; N, 10.54. Found: C, reported but a new synthesis is shown here.30,31 A mixture of
49.34; H, 3.92; N, 10.35. UV-Vis λmax, nm (ε, M−1 cm−1): 233 Ni(ClO4)2·6H2O (24.7 mg 0.0675 mmol) and TERPY (31.5 mg,
(23532), 272 (17307), 279 (19884), 305 (18457), 319 (17046), 0.135 mmol) in CH3CN (5 mL) was stirred for 12 h under air at
329 (13074). FT-IR (KBr, cm-1): 3420 (νO-H), 3100 (νC-H), 2320 ambient temperature. Slow diffusion of Et2O into the CH3CN
(νC≡N), 2020 (νC-H), 1600 (νC=C), 1520 (νC=N), 1450 (νC=O), 1400 solution produced orange-brown plates. (31 mg, 63% yield)
(νC=O), 1090 (νClO4), 780(νC-H), 620(νClO4).
ESI-MS: m/z calc. for [C30H22N6Ni·ClO4]+, 623.1; found, 623.1;
calc. 262.1 [M-(2·ClO4)]2+; found 262.1. Anal. Calc. for
(2). C30H22Cl2N6NiO8·H2O: C, 48.55; H, 3.26; N, 11.32. Found: C,
[(TERPY)Ni((CH3)3CC(O)CHC(O)C(CH3)3)(H2O)]ClO4
Ni(ClO4)2·6H2O (49.4 mg 0.135 mmol) was dissolved in CH3CN 48.38; H, 2.95; N, 11.30. UV-Vis λmax, nm (ε, M−1 cm−1): 235
(2 mL) under air. An excess of 2,2,6,6-tetramethyl-3,5- (31872), 269 (29970), 310 (18235), 320 (27374), 334 (25642).
heptanedione (141
triethylamine (19
μ
L, 0.675 mmol) was added, along with FT-IR (KBr, cm-1): 3610 (νO-H), 3410 (νO-H), 3120 (νC-H), 2360 (νC-
L, 0.135 mmol). The resulting solution was N), 2010 (νC-H), 1610 (νC=C), 1450 (νC=O), 1090 (νClO4), 770(νC-H),
μ
stirred for 30 min, during which time it became blue. This 620(νClO4).
solution was then added to a vial containing TERPY (31.6 mg,
1
0.135 mmol) dissolved in CH3CN (2 mL). The resulting yellow- Analysis of crystallization filtrates by H NMR. Crystallization
green mixture was stirred for 2 h. Et2O (8 mL) was added and of 1-3 produced materials with purity as determined by
o
1
the solution was placed in the freezer (-15 C) overnight which elemental analysis. Evaluation of the remaining filtrates by H
resulted in the deposition of a fine tan precipitate (4 by 1H NMR revealed the presence of 4 in each reaction mixture.
NMR; vida infra). The yellow-green solution was brought to
dryness under reduced pressure. Crystalline material was Addition of TERPY (1 eq) to 1 in CD3CN. Complex 1 (4.30x10-3
obtained by dissolving the precipitate in CH2Cl2, adding a few mmol) was dissolved in CD3CN (600 µL), and a 1H NMR
drops of CH3CN, followed by slow diffusion of Et2O (41 mg, spectrum was collected under paramagnetic conditions. TERPY
53% yield). ESI-MS: m/z calc for C26H30N3NiO2·CH3CN, 515.2 (4.30x10-3 mmol) was added, the solution was briefly mixed
[M-ClO4]+; found 515.2; calc. 474.2 [M-CH3CN-ClO4]+; found and another 1H NMR spectrum was collected. Additional
474.1. Anal. Calc. for C26H30ClN3NiO6·H2O·0.9 CH2Cl2: C, 48.28; spectra were collected after 3 h and 24 h. The NMR spectra
H, 5.09; N, 6.28. Found: C, 48.48; H, 4.89; N, 6.54. UV-Vis λmax
,
indicated significant formation of 4.
nm (ε, M−1 cm−1): 233 (27182), 270 (18653), 278 (22176), 306
(21052), 316 (23235), 328 (14948). FT-IR (KBr, cm-1): 3530 (νO- Probing for aliphatic C-C bond cleavage reactivity involving 3.
H), 3430 (νO-H), 2960 (νC-H), 2020 (νC-H), 1590 (νC=C), 1410 (νC=O), Complex 3 (10 mg, 0.02 mmol) was dissolved in CH3CN (2.7 ml)
1110 (νClO4), 780(νC-H), 630(νClO4).
in a glass scintillation vial with a stir bar. Following the addition
of H2O (300 µL) and NEt3 (279 µL, 2.00 mmol) and purging with
O2 with for 1 min, the mixture was stirred for 24 h. The solvent
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was then removed under reduced pressure. CH2Cl2 (8 ml) and 1400 (νC=O), 1110 (νClO4), 750(νC-H), 620(νClO4). 6:VEieSwIA-MrticSle:Omnli/nez
+
DOI: 10.1039/D0DT01338B
HCl (8 ml, 1 M) were added, followed by stirring for 1 h and calc. for C32H36N5NiO2·CH3CN, 621.3 [M-ClO4] ; found 621.2;
subsequently the organic layer was separated. The organic calc. 580.2 [M-CH3CN-ClO4]+; found 580.2. Anal. Calc. for
layer was dried over Na2SO4 and brought to dryness under C32H36ClN5NiO6·0.4 H2O: C, 55.61; H, 5.34; N, 10.12: Found: C,
vacuum to give a white solid (3.2 mg, 71% based on 55.86; H, 5.39; N, 10.18. UV-Vis λmax, nm (ε, M−1 cm−1): 249
1
dibenzoylmethane). The H NMR (CDCl3) spectrum of the solid (11864), 305 (29644), 348 (17675), 366 (10896). FT-IR (KBr, cm-
showed only dibenzoylmethane and no aliphatic C-C bond 1): 3410 (νO-H), 3150 (νC-H), 2980 (νC-H), 2020 (νC-H), 1590(νC=C),
cleavage products.
1410 (νC=O), 1120 (νClO4), 750(νC-H), 630(νClO4).
In a separate experiment, a similar reaction was performed Analysis of filtrates. Analysis of the filtrates remaining
1
with the only difference being 3 (10 mg, 0.02 mmol) was following removal of the green solids of 5 and 6 using H NMR
dissolved in dry CH3CN (3 mL) and one equivalent of NEt3 (~3 and ESI-MS revealed the presence of the diketonate complexes
µL, 0.02 mmol) was added. An aliquot was removed before O2 (5 and 6) as well as [(MBBP)2Ni](ClO4)2 (8), which was
purge and after stirring for an additional 24 h. ESI-MS of both independently synthesized and characterized (vida infra).
aliquots only contained patterns for 3 and 4. After workup, a
1
white solid was collected (4.1 mg, 91%). The H NMR of this Synthesis of 6 following previously published procedures.
solid indicated recovering of dibenzoylmethane.
Under N2, Ni(ClO4)2·6H2O (24.7 mg, 0.0675 mmol) was
dissolved in degassed CH3CN (2 mL). Degassed 2,2,6,6-
2,6-bis(1-methylbenzoimidazol-2-yl)pyridine (MBBP). This tetramethyl-3,5-heptanedione (13.8
μL, 0.0675 mmol) and
procedure includes slight modifications from the previously degassed triethylamine (9.4 L, 0.0675 mmol) was then added
μ
published synthesis.28 2,6-bis(2’-benzimidazolyl)pyridine (1.00 to the vial and stirred for 30 minutes. MBBP (22.9 mg, 0.0675
g, 3.21 mmol) was added to a round bottom flask containing mmol) dissolved in degassed CH3CN (2 mL) was added, which
acetone (30 mL) and powdered potassium hydroxide (0.90 g, resulted in the precipitation of a green solid. This solution was
16 mmol). This mixture was stirred at room temperature for 30 stirred for 2 h, then degassed Et2O (8 mL) was added to the
min during which time it became brown-red. Methyl iodide mixture. This mixture was then placed in the freezer overnight
(0.60 ml, 9.6 mmol) was added and the reaction stirred (-12 °C). The green precipitate collected on the bottom, so the
overnight. Following the addition of water, a light tan solution was decanted off and the precipitate washed with
precipitate was produced. This solid was removed via more Et2O and dried. The 1H NMR under paramagnetic
filtration, dissolved in dichloromethane, and the solution was conditions and the ESI-MS of the solid indicated that 6 is
dried over sodium sulfate. Following filtration to remove the present along with 8.
drying agent, the filtrate was collected and brought to dryness
under reduced pressure to give a tan solid (0.50 g, 46% yield) Attempted Synthesis of [MBBP)Ni(PhC(O)CHC(O)Ph)]ClO4 (7-
1H NMR (CDCl3) δ (ppm): 8.40 (d, 2H, 8.5 Hz), 8.04 (t, 1H, 7.9 ClO4). The previously reported synthesis for this complex was
Hz), 7.87 (d, 2H, 7.9), 7.46 (d, 2H 7.37 Hz), 7.36 (m, 4H), 4.24 (s, used in an attempt to make this complex.24 Ni(ClO4)2·6H2O
6H). These 1H NMR features match those previously (24.7 mg, 0.0675 mmol) was dissolved in CH3CN (~2 mL). This
reported.28
solution was combined with dibenzoylmethane (15.2 mg,
0.0675 mmol) and triethylamine (9.4 L, 0.0675 mmol). The
Synthesis of [(MBBP)Ni(CH3C(O)CHC(O)CH3)]ClO4 (5) and resulting mixture was stirred for 30 min at which point it was
[(MBBP)Ni((CH3)3CC(O)CHC(O)C(CH3)3)]ClO4 (6). These transferred to vial containing MBBP (22.9 mg, 0.0675 mmol).
μ
complexes were prepared under air. Ni(ClO4)2·6H2O (24.7 mg, This resulted in the formation of a clear green solution. After
0.0675 mmol) was dissolved in CH3CN (2 mL). The diketone, stirring for 2 h, Et2O (8 mL) was added, which resulted in the
2,4 pentanedione (7.0
tetramethyl-3,5-heptanedione (13.8
added along with triethylamine (9.4
μL, 0.0675 mmol) or 2,2,6,6- deposition of a green-yellow precipitate. The solid was
μ
μ
L, 0.0675 mmol), was collected and dried under vacuum. ESI-MS (CH3CN) and 1H
L, 0.0675 mmol). The NMR (CD3CN) under paramagnetic conditions showed that
resulting blue solution was stirred for 30 min at ambient both [MBBP)Ni(PhC(O)CHC(O)Ph)]ClO4 (7-ClO4) and 8 were
temperature. MBBP (22.9 mg, 0.0675 mmol) dissolved in present. We were unable to selectively precipitate 7-ClO4 as
CH3CN (2 mL) was added, which resulted in the precipitation of had been possible with 5 and 6.
a green solid. This solid was carefully collected and dried by
gravity filtration using a Buchner funnel and filter paper (5: 19 Attempted Synthesis of [(MBBP)Ni(PhC(O)CHC(O)Ph)]Cl (7-
mg, 46%; 6: 18 mg, 40%). 1H NMR analysis of each solid Cl). To a solution of NiCl2 (16.0 mg, 0.0675 mmol) in CH3CN (2
indicated the presence of a single compound. 5: ESI-MS: m/z mL) was added dibenzoylmethane (15.2 mg, 0.0675 mmol) and
calc. for C26H24N5NiO2·CH3CN, 537.2 [M-ClO4]+; found 537.1; triethylamine (9.4
μL, 0.0675 mmol). The mixture was stirred
496.1 [M-CH3CN-ClO4]+; found 496.3. Anal. Calc. for for about 30 min at which point it was added to a vial
C26H24ClN5NiO6: C, 52.34; H, 4.05; N, 11.74. Found: C, 52.28; H, containing MBBP (22.9 mg, 0.0675 mmol). A green precipitate
4.11; N, 11.66. UV-Vis λmax, nm (ε, M−1 cm−1): 249 (13720), 304 formed which was separated by filtration (11.3 mg). Analysis of
(40254), 350 (22999), 366 (16041). FT-IR (KBr, cm-1): 3440 (νO- this solid by 1H NMR (d6-DMSO) showed the presence of a
1
H), 3120 (νC-H), 2360 (νC-N), 2030 (νC-H), 1930 (νC-H), 1600 (νC=C), mixture of 7-Cl and 8. Analysis of the filtrate by H NMR (d6-
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DMSO) also showed the presence of both complexes. We were isolated (4.0 mg, 88%). The 1H NMR of this solid indicated only
View Article Online
DOI: 10.1039/D0DT01338B
unable to selectively crystallize 7-Cl as was possible with 5 and dibenzoylmethane was isolated.
6. We note that while 7-Cl is insoluble in CD3CN, complex 8 is
soluble.
Evaluation of the reactivity of 1-7 in the presence of D2O and
1
NEt3 using H NMR. Each isolated complex (4.09x10-3 mmol)
1
Synthesis of [(MBBP)2Ni](ClO4)2 (8). Ni(ClO4)2·6H2O (12.4 mg, was evaluated by H NMR over 24 h under aerobic conditions
0.0338 mmol) was combined with MBBP (22.9, 0.0675) in in dry CD3CN only (600 μL), dry CD3CN (600 μL) containing NEt3
CH3CN (3 mL). This solution was stirred for 12 h at room (5 eq), dry CD3CN (540 μL) containing 10% added D2O (60 μL) ,
temperature. Slow diffusion of Et2O into the CH3CN solution or dry CD3CN (540 μL) containing 10% added D2O (60 μL) and
produced small brown crystals suitable for single crystal X-ray NEt3 (5 eq). Evaluations for complexes 5 and 6 were also done
crystallographic characterization. (26 mg, 83% yield). ESI-MS: under previously published conditions (i.e. dried solvent, 1 eq
m/z calc. for C42H34N10Ni·ClO4, 835.1 [M-ClO4]+, found 835.2, NEt3).24 Each isolated complex was evaluated by UV-vis over 24
calc. 368.1 [M-(2ClO4)]2+; found 368.5 [M-(2ClO4)]2+. Anal. Calc. h in CH3CN:H2O (9:1) in the presence of 5 equivalents of NEt3.
for C42H34Cl2N10NiO8·0.4 H2O: C, 53.46; H, 3.71; N, 14.84.
Found: C, 53.08; H, 3.77; N, 14.66. UV-Vis λmax, nm (ε, M−1 Addition of MBBP (1 eq) to 5 in CD3CN. Complex 5 (2.0 mg,
cm−1): 249 (12825), 304 (33046), 357 (26261), 366 (20572). FT- 3.40x10-3 mmol) was dissolved in CD3CN (600 μL) and a 1H
IR (KBr, cm-1): 3420 (νO-H), 3130 (νC-H), 2020 (νC-H), 1600 (νC=C), NMR spectrum was collected under paramagnetic conditions.
1480 (νC=O), 1060 (νClO4), 750 (νC-H), 620 (νClO4).
MBBP (1.1 mg, 3.40x10-3 mmol) was added, the solution was
briefly mixed and another 1H NMR spectrum was collected.
Probing for aliphatic C-C bond cleavage reactivity involving 7- Additional spectra were collected after 3 h and 24 h. The NMR
ClO4. Ni(ClO4)2·6H2O (7.3 mg, 0.02 mmol) was dissolved in spectra indicated significant formation of 8.
CH3CN (2.7 ml). This solution was added to a mixture of
X-Ray Crystallography. Single crystals of 1-3 and 8 were used for X-
dibenzoylmethane (4.5 mg, 0.02 mmol) and NEt3 (2.8 µL, 0.02
mmol). The resulting mixtures was stirred for 30 min at which
point it was added to MBBP (6.8 mg, 0.02 mmol). This clear
green solution was stirred for 30 min. To this solution was
added H2O (300 µL) and NEt3 (279 µL, 2.00 mmol) followed by
purging with O2 and stirring for 24 h at ambient temperature.
CH2Cl2 (8 ml) and HCl (8 ml, 1 M) were then added, followed by
stirring for 1 h and subsequently the organic layer was
separated. The organic layer was dried over Na2SO4 and
brought to dryness under vacuum to give a white solid (3.3 mg,
73% based on dibenzoylmethane). The 1H NMR (CDCl3)
spectrum of the solid showed only dibenzoylmethane and no
aliphatic C-C bond cleavage products.
ray crystallography studies. Structures of 1, and were
3
8
determined at the University of Montana (UM) where a crystal of
each was mounted on a glass fiber using viscous oil and transferred
to a Bruker D8 Venture using Mo Kα (λ = 0.71073 Å) radiation for
data collection at 100 K. The structure of 2 was determined at Utah
State University. Single crystals of 2 were mounted on a glass fiber
using viscous oil and transferred to a Rigaku XtaLAB Mini II
Diffractometer using Mo Kα (λ = 0.71073 Å) radiation for data
collection at 100 K.
1, 3 and 8. Collected data were corrected for absorption using
the SADABS.32 Using Olex233, the structure of each was solved
with the SHELXT34 structure solution program using direct
In a separate experiment, a similar reaction was performed
with the only difference being after stirring the mixture of 7-
ClO4 for 30 minutes in dry CH3CN, one equivalent of NEt3 (~3
µL, 0.02 mmol) was added. An aliquot was taken before O2
purging and then again after stirring for 24 h at room
temperature. ESI-MS of both aliquots only contained patterns
of 7 and 8. After workup using HCl and CH2Cl2 extraction as
described above, a white solid was isolated (4.1 mg, 91%). The
1H NMR of this solid indicated only dibenzoylmethane was
isolated.
Probing for aliphatic C-C bond cleavage reactivity involving 7-
Cl. The same conditions as 7-ClO4 were used, except
NiCl2·6H2O (4.8 mg, 0.02 mmol) was used instead of
Ni(ClO4)2·6H2O in dry CH3CN. After adding one equivalent of
NEt3 (~3 µL, 0.02 mmol), the reaction was stirred for 10 min.
An aliquot was taken before O2 purging and then again after
stirring for 24 h at room temperature. ESI-MS of both aliquots
only contained patterns of 7 and 8. After workup using HCl and
CH2Cl2 extraction as described above, a white solid was
Scheme 4 The synthetic pathway for the preparation of 1-3.
Compound 4 is a contaminating species in each reaction mixture.
6 | J. Name., 2012, 00, 1-3
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methods and refined with the SHELXL35 refinement package complexes were prepared and characterized. Complexes 1-3
View Article Online
DOI: 10.1039/D0DT01338B
using least squares minimization. All non-hydrogen atoms were prepared as outlined in Scheme 4 under air, with the
were refined with anisotropic thermal parameters. For 1 the synthesis of 1 and 2 being performed using excess diketone.
hydrogen atoms were placed in geometrically calculated Isolation of the desired diketonate complex required
positions and refined using a riding model. For 3 the hydrogen crystallization or precipitation of the product to separate it
atoms attached to the heteroatoms were located in the from [(TERPY)2Ni](ClO4)2 (4), which was also formed in the
residual electron density maps, placed, and refined with reaction mixture (vida infra). Complexes 1-4 were
1
isotropic thermal parameters. All other hydrogen atoms were characterized by X-ray crystallography, elemental analysis, H
placed in geometrically calculated positions and refined using a NMR, ESI mass spectrometry, UV-vis, and FTIR (Figures S1-
riding model. For 8 all the hydrogen atoms were placed in S19).
geometrically calculated positions and refined using a riding
model. Isotropic thermal parameters of riding hydrogen atoms X-ray structures of 1-3. Details of the X-ray data collection and
were fixed to 1.2 times the U value of the atoms they are refinement for 1-3 is provided in Table S1. Selected bond
linked to (1.5 times for methyl groups). Calculations and distances and angles are given in Tables S2 and S3.
refinement of structures were carried out using APEX236, Representations of the cationic portions of the X-ray structures
SHELXTL36, and Olex232 software.
of 1-3 are shown in Figure 1. Each cation contains a pseudo-
The structure of 8 was found to contain indistinguishable
solvent molecules within voids in the lattice. Attempts at
modelling this solvent were not able to produce a suitable
model. The SQUEEZE routine within PLATON was utilized to
account for the residual, diffuse electron density and the
model is refined against these data. A total of 27 electrons per
unit cell were corrected.
2: Data were corrected for absorption using Gaussian grid
(numerical integration) and a 0.5 mm 1D horizontal Gaussian beam
correction for the graphite monochromator. Using Olex233, the
structure was solved with the SHELXT34 structure solution program
using intrinsic phasing and refined with the SHELXL35 refinement
package using least squares minimization. All non-hydrogen atoms
were refined with anisotropic thermal parameters. Hydrogen atoms
for 2 attached to heteroatoms were found in the residual electron
density maps, placed, and refined with isotropic thermal
parameters. All other hydrogen atoms for 2 were placed in
geometrically calculated positions and refined using a riding model.
Isotropic thermal parameters of the placed hydrogen atoms were
fixed to 1.2 times the U value of the atoms they are linked to (1.5
for methyl groups). Calculations and refinement of structures were
carried out using CrysAlisPro38, SHELXL35 and Olex233 software.
Figure 1 Cationic portions of a) 1, b) 2 and c) 3. Hydrogen
atoms except for those of coordinated water molecules
have been omitted for clarity. Ellipsoids are plotted at
the 50% probability level.
octahedral Ni(II) center with the meridional coordinated TERPY
ligand and one diketonate oxygen donor occupying
Compounds 1 and 3 crystallize in the monoclinic crystal coordination positions in the equatorial plane. The other
system, while 2 and 8 are triclinic. The space groups for each diketonate oxygen donor occupies an axial position trans to a
complex were the following: 1: 1C2/c1, 2: P-1, 3: P121/c1, 8: P1. coordinated solvent molecule (CH3CN (1) or H2O (2 and 3)).
In 8, three perchlorate oxygen atoms were modelled as The Ni-N bond lengths for the TERPY ligand are in the range of
disordered over two positions (O30/O31 0.42:0.58, O17/O19 2.00-2.14 Å. The Ni-O(diketonate) distances are in the range of
0.67:0.33, O7/O8 0.66:0.34).
1.98-2.05 Å. The Ni-N(CH3CN) distance in 1 (2.09 Å) is similar to
the Ni-O(H2O) distances in 2 and 3 (2.14 and 2.09 Å,
respectively). Comparison of the bond distances to those
Results and Discussion
reported
by
Ramasubramanian
et
al.
(5)
for
and
[(MBBP)Ni((CH3)3CC(O)CHC(O)C(CH3)3)]ClO4
Synthesis and characterization of TERPY-ligated Ni(II)
diketonate complexes. To evaluate the solution and O2
reactivity properties of Ni(II) diketonate complexes supported
by a tridentate chelate ligand, three new TERPY-ligated
[(MBBP)Ni(PhC(O)CHC(O)Ph)]Cl (8) revealed similar Ni-O
distances and Ni-N distances.24
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DOI: 10.1039/D0DT01338B
Figure 2 1H-NMR spectra of TERPY-ligated a) 1, b) 2, c) 3 and d) 4 in CD3CN.
5.3 ppm, with an integration of six hydrogens, which is
Solution characterization of 1-3. Based on prior results from assigned to the methyl hydrogens of the acetoacetonate
our laboratory26, we hypothesized that the pseudo-octahedral diketonate ligand. Complex 2 has a similar singlet with an
TERPY-ligated Ni(II) diketonate complexes would be amenable integration of 18 hydrogens for the trimethylacetyacetonate
to characterization by 1H NMR under paramagnetic conditions.
1
We also hypothesized that H NMR would be a useful way to
monitor the solution reactivity of these diketonate complexes.
We therefore pursued studies of the 1H NMR features of
crystalline samples 1-3 in CD3CN. As shown in Figure 2, each
complex exhibits signals over a range of -15 to +75 ppm. An
additional broad peak is found at 132 ppm (Figures S1-S3).
Assignment of resonances was made on the basis of integrated
intensity and chemical shift considering
a contact shift
model.26 The signal at 132 ppm is tentatively assigned to the
pyridyl C-H atoms adjacent to the nitrogen donors. Due to the
broad nature of this signal, the integrated intensity could not
be determined. The signals b and c for each complex (Figure 2)
integrate to two hydrogen atoms and are assigned to the meta
C-H positions on the terminal pyridyl donors of the TERPY
ligand. A resonance at 38-39 ppm (d), which integrates to one
hydrogen, is assigned to the meta C-H on the central pyridyl
ring. Resonances at 16 (e, 1H) and 12 (f, 2H) ppm, respectively,
are assigned to the para C-H positions of the central and
terminal pyridyl rings. A signal integrating to one hydrogen at -
15 ppm is assigned as the central C-H moiety of each β-
diketonate ligand. The spectrum for 1 also contains a singlet at
Scheme 5. Products identified in the syntheses of 5-7.
Complex 8 was identified in each reaction mixture using 1H
NMR based on an independently generated sample.
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Figure 3 1H NMR spectra of MBBP-ligated a) 5, b) 6 c) 7-ClO4, d) 7-Cl and e) 8 in CD3CN.
[(MBBP)Ni(diketonate)]ClO4 complexes? (b) Can 1H NMR be
used to characterize the solution properties of the MBBP-
ligand. The 1H NMR spectrum of 3 contains three signals in the ligated diketonate complexes? Our interest in the first
range of 7.7–10.7 ppm which integrate to 10 hydrogens for the question stems in part from color descriptions of these
phenyl hydrogens of the diketonate ligand.
previously reported compounds.24 Specifically, the diketonate
derivatives
[(MBBP)Ni(CH3C(O)CHC(O)CH3)]ClO4
(6),
(5),
and
1
Careful analysis of the H NMR features of analytically pure 1 [(MBBP)Ni((CH3)3CC(O)CHC(O)C(CH3)3)]ClO4
(Figure 2(a)) revealed the presence of a minor impurity. The [(MBBP)Ni(PhC(O)CHC(O)Ph)]ClO4 (7-ClO4) were reported as
signals for this impurity match those of the bis-TERPY complex, being orange-brown solids whereas the CIF files for 6 and 7-
[(TERPY)2Ni](ClO4)2 (4, Figure S4)).29,30 Notably, evaluation of ClO4 list green crystals.24 Our experience with the
the reaction mixture filtrates (Figures S5-S7) remaining [(TERPY)Ni(diketonate)]ClO4 complexes suggested that the
1
following isolation of the precipitated/crystalline 1-3 using H desired diketonate complexes should be green whereas the a
NMR and ESI-MS indicated the presence of the diketonate bis-MBBP complex [(MBBP)2Ni](ClO4)2 (8) may be tan to
complexes 1-3 as well as 4. Analysis of crystalline samples of 1- orange-brown. In their paper, Ramasubramanian, et al. did
3 in CH3CN by ESI-MS revealed molecular ions for the briefly mentioned that the [(MBBP)2Ni](ClO4)2 (8) complex is
[(TERPY)Ni(diketonate)]+ and [(TERPY)Ni(diketonate)(CH3CN)]+ detected by ESI-MS following oxygenation of the diketonate
cations (Figure S8-S10) as well as an isotope cluster for the complexes. We hypothesized that it also could be present in
[(TERPY)2Ni(ClO4)]2+ (4, Figure S11) cation. Overall, it is clear the reaction mixtures leading to the isolation of 5-7.
that 4 is formed in the reaction mixtures leading to the
formation of 1-3.
Using the synthetic procedures under air used for the preparation
of 1-3, we attempted the synthesis of 5-7 (Scheme 5).24 We
identified the following differences from the prior report by
Synthesis and solution characterization of MBBP-ligated Ni
diketonate complexes. Based on our experience in preparing Ramasubramanian et al., noting that their reactions were run
and characterizing the [(TERPY)Ni(diketonate)]ClO4 complexes under N2.24 In our hands, in each reaction mixture, a green
(1-3), we examined the previously reported MPPB-ligated precipitate is produced prior to Et2O addition. As shown via
diketonate complexes 5-7 to evaluate: (a) Is [(MBBPNi)2](ClO4)2 synthetic investigations for 6, this occurs whether the reaction
(8) similarly present in the reaction mixtures of the was run under air or N2. Isolation and evaluation of the green
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1
solid as well as the filtrate by H NMR (Figure 3; Figures S20 (R resonance is found at -15 ppm. We note that aVsiseiwgnArmticleenOtnlionef
1
= -CH3; solid) and S21 (R = -CH3; filtrate); Figures S22 (R = - the H NMR spectral features of 6 enablDeOdI:u1s0.t1o03c9o/Dm0pDaT0re13t3h8Be
C(CH3)3; solid) and S23 (R = -C(CH3)3; filtrate)) and ESI-MS results of synthetic reactions performed for this complex
(Figures S24 and S25) indicated that the green solids were the under air and N2. For the latter, as is shown in Figures S31 and
desired [(MBBP)Ni(diketonate)]ClO4 complexes (5 and 6). ESI- S32, the 1H NMR and ESI-MS features of the compound
MS analysis of solutions of 5 and 6 showed the presence of the isolated under N2 indicate that some 8 is present. The
[(MBBP)2Ni]2+ cation (Figure S24 and S25). The presence of this presence of the bis-MBBP complex is due to a different work
ion suggested that a trace amount may be present in the up procedure being employed under N2 that didn’t enable
samples or that ligand exchange was occurring under the physical separation of 6 and 8.
conditions of the mass spectral experiment. To fully
characterize [(MBBP)2Ni](ClO4)2 (8) we independently Our attempts to generate the MBBP-ligated Ni(II)
prepared the complex which was then characterized using X- dibenzoylmethane derivative 7-ClO4 always resulted in the
1
ray crystallography (Figure 4), 1H NMR (Figures S26 (CD3CN) isolation of mixtures of 7-ClO4 with 8. Representative H NMR
and S27 (d6-DMSO)), ESI-MS (Figure S28), UV-vis (Figure S29), spectra of the filtrate showed neither complex (Figure S33),
and FTIR (Figure S30). The structural features of 8 are similar but isolated precipitate (Figure 3(c); Figure S34) suggested the
to those of [(TERPY)2Ni](ClO4)2 with Ni-N distances of 2.03-2.12 presence of both complexes. ESI-MS data shows the filtrate
Å.30
contains trace amounts of 7-ClO4, while the precipitate is
consistent with a mixture of 7-ClO4 and 8 (Figure S35).
Attempted
syntheses
of
the
chloro
derivative
[(MBBP)Ni(PhC(O)CHC(O)Ph)]Cl (7-Cl) produced similar results.
1
By H NMR, the precipitated solid contains a mixture of 7-Cl
and 8 (Figure 3(d); Figure S36). The remaining filtrate also
contains trace amounts of both compounds (Figure S37). ESI-
MS of the isolated solid and filtrate also indicates the presence
of both compounds (Figures S37).
Exposure of complexes to H2O/NEt3 under aerobic conditions.
Ramasubramanian et al. reported that exposure of 5-7 to O2 in
CH3CN in the presence of NEt3 (1 eq) led to oxidative aliphatic
carbon-carbon bond cleavage within the diketonate unit (52%
yield). They additionally reported that the presence of 10% water
produced a slightly lower yield (41%) of the aliphatic C-C bond
cleavage product (benzoic acid). As we prepared and isolated
analytically pure 1-6 under aerobic conditions, it was evident that
Figure 4 Thermal ellipsoid representation of a cationic
portion of the X-ray structure of 8. The complex crystallizes
with two cations per asymmetric unit. Hydrogen atoms
have been omitted for clarity. Ellipsoids are plotted at the
50% probability level.
1
The H NMR features of 5 and 6 (Figure 3a) and b)) are like
those of the TERPY diketonate complexes. Resonances for the
MBBP derivatives can be assigned based on integrated
intensity and consideration of
a primary contact shift
contribution to the chemical shifts.26 A signal at 65 ppm
integrating to two hydrogens is assigned to the aryl C-H closest
to each benzimidazole nitrogen donor. A sharp resonance
integrating to two hydrogens at ~26 ppm is assigned to the
adjacent aryl C-H moiety. Overlapping signals integrating to
three hydrogens at ~15 ppm correspond to the meta- and para
hydrogens of the central pyridyl donor of the MBBP ligand. The
remaining benzimidazole aryl hydrogens as well as the
diketonate substituent hydrogens (except the central C-H) are
found in the region of 0-10 ppm. The diketonate central C-H
1
Figure 5 H-NMR spectra of 1 in dry CD3CN with 10% D2O
(v:v) and excess NEt3 (5 eq) over time, starting with a
preaddition spectrum (NEt3 and D2O not added) (a), a spectrum
after addition of all reagents (b), 3 hours later (c) and 24 hours
later (d). The blue arrow highlights the growth of 4, and the red
arrow represents the disappearance of the diketonate C-H
proton.
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the complexes were not reactive with O2. We therefore next previously described conditions (as was observedVfieowrA4rt;iclFeiOgnulirnee
examined whether the addition of NEt3 and/or water would lead to S62). As was done for 1 and 4, the reactiDviOtyI: 1w0a.1s03a9l/sDo0aDnTa0l1y3s3e8dB
diketonate cleavage reactivity. 1H NMR samples were prepared by UV-visible spectroscopy for 5 and 8 under the same
under the following conditions and were evaluated after being conditions (CD3CN, 10% D2O, NEt3 (5 eq; Figure S64). While 8
stored for 24 h at ambient temperature under air: a) dry CD3CN displayed no change over 24 hours, 5 exhibited a small band
only; b) dry CD3CN + 10% D2O; c) dry CD3CN + 1 eq NEt3 (6 and 7 shift after 30 minutes, with no change being observed
only); d) dry CD3CN + 5 eq NEt3; e) dry CD3CN + 10% D2O + 5 eq afterward (both at 3 hr and 24 hr).
NEt3. Spectra associated with these experiments are provided in the
Supporting Information (1: Figures S39-S42; 2: Figures S43-S46; 3: Notably, addition of TERPY (1 eq) to a CD3CN solution of 1
Figures S47-S50; 5: Figures S51-S55; 6: Figures S56-S60). As shown showed significant conversion to 4 (Figure S65). This indicates
for the TERPY-ligated Ni(II) acetylacetonate complex 1 in Figures that free TERPY appears to be able to displace the diketonate
S39-S42, in the presence of D2O (10%) or NEt3 (5 eq) a small amount ligand. In a similar experiment involving addition of MBBP (1
of the bis-TERPY complex 4 appears to form over 24 h relative to eq) to a CD3CN solution of 5, growth of signals for 8 can also be
that seen in CD3CN alone. The amount of 4 present appears to be observed (Figure S66).
influenced more by the presence of NEt3 than water. Notably, a
solution containing both D2O and NEt3 shows the most significant Evaluation of aliphatic C-C bond cleavage reactivity. To probe
amount of bis-TERPY complex (4) formation after 24 h (Figure 5, for aliphatic C-C bond cleavage, we stirred an CH3CN solution
Figure S42). The diketonate complexes 2 and 3 exhibit similar of 3 containing 10% H2O (v:v) and NEt3 (5 eq) for 24 h at
reactivity albeit resulting in the presence of an overall smaller ambient temperature after purging with O2. Exposure of the
amount of 4 relative to the starting material. By 1H NMR, 4 is stable solution to 1M HCl followed by extraction with CH2Cl2 resulted
with respect to the presence of D2O (10%) and NEt3 (5 eq) over 24 h in the isolation of unaltered dibenzoylmethane (71% recovery,
(Figure S61). Analysis of the reactivity of 1 and 4 in CH3CN in the Figure S67). A similar experiment performed using an in situ-
presence of D2O (10%) and NEt3 (5 eq) at ambient temperature generated sample of [(MBBP)Ni(PhC(O)CHC(O)Ph)]ClO4 (7-
using UV-visible spectroscopy (Figure S63) shows that 4 is stable ClO4) also resulted only in the isolation of unaltered
whereas
1 exhibits minor changes, possibly suggesting the dibenzoylmethane (73% recovery, Figure S68). Notably, an
formation of 4.
experiment performed using 3, 7-ClO4, or 7-Cl in the presence
of 1 eq NEt3 as reported by Ramasubramanian et al.24
produced only unreacted dibenzoylmethane (71%-91%,
Figures S69-S74) after 24 h stirring and acidic workup.
The MBBP-ligated complexes 5 and 6 were exposed to the
same conditions (CD3CN, 10% D2O, NEt3 (5 eq) for 24 h. As was
the case for the TERPY complexes, the acetoacetonate
derivative 5 showed an increase in the amount of 8 present
(Figure 6). The observed reactivity was complete with 3 h but
was monitored over 24 h. The formation of 8 is also observed
in solutions of 6, although the reaction proceeds more slowly
than for 5. By 1H NMR, 8 shows no change under the
Stability of Ni(II) diketonate complexes under CH3CN/
aqueous basic conditions. Both sets of tridentate Ni(II)
diketonate complexes are stable when exposed to O2 both as
solids and in CH3CN solution. This is evident as: 1) these
complexes can be synthesized and isolated in air; and 2) no
oxidative products were identified or isolated from solutions
under aerobic conditions. These results are consistent with
literature precedent, which suggests that only Ni(II) complexes
containing an electron rich diketonate ligand exhibit oxidative
C-C bond cleavage reactivity with O2.15-23 Some evidence was
found for potential ligand exchange reactivity wherein the
tridentate Ni(II) diketonate complexes were converted to bis
MBBP or bis TERPY Ni(II) complexes in solution. The exchange
reactivity is enhanced for diketonates with less steric
hinderance.
Conclusions
Oxidative aliphatic C-C bond cleavage reactions of diketones
mediated by first-row divalent metals and involving O2 are of
current interest.9-23 The highest level of oxidative cleavage
reactivity has been found for electron rich Ni(II) diketonates.
The recent reactivity reported for a MBBP-ligated Ni(II) aryl
diketonate complex drew our interest due to the oxidative
cleavage of a non-substituted diketone upon reaction with
O2.24 Typically Ni(II) complexes of an unsubstituted aryl
diketonate are air stable.20 We have been unable to reproduce
1
Figure 6 H-NMR stacked spectra of 5 in CD3CN with 10% D2O
and excess NEt3 over time, starting with a pre-addition
spectrum (NEt3 and D2O not added) (a), a spectrum after
addition of all reagents (b), 3 hours later (c) and 24 hours later
(d). The blue arrow highlights the growth of 8, and the red
arrow represents the disappearance of the α proton.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 11
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Page 12 of 13
ARTICLE
Journal Name
these results with either MBBP- or TERPY-supported Ni(II) 16 E. Szajna, A. M. Arif, L. M. Berreau, J. Am. Chem. Soc. 2005,
View Article Online
DOI: 10.1039/D0DT01338B
diketonate complexes. Instead, these complexes are stable in
127, 17186-17187.
air, and in aerobic CH3CN/aqueous basic conditions or 17 L. M. Berreau, T. Borowski, K. Grubel, C. J. Allpress, J. P.
CD3CN/NEt3 only exhibit some possible ligand exchange
reactivity.
Wikstrom, M. E. Germain, E. V. Rybak-Akimova, D. L.
Tierney, Inorg. Chem., 2011, 50, 1047-1057.
18 C. J. Allpress, K. Grubel, E. Szajna-Fuller, A.M. Arif, L.M.
Berreau, J. Am. Chem. Soc., 2013, 135, 659-668.
19 S. Raje, K. Mani, P. Kandasamy, R.J. Butcher, R. Angamuthu,
Eur. J. Inorg. Chem., 2019, 2019, 2164-2167.
20 E. Szajna-Fuller, K. Rudzka, A. M. Arif, L. M. Berreau, Inorg.
Chem. 2007, 46, 5499-5507.
Conflict of Interest
The authors declare no conflict of interest.
21 C. Allpress, A. M. Arif, D. T. Houghton, L. M. Berreau Chem.
Eur. J. 2011, 17, 14962-14963.
22 C. Allpress, A. Milaczewska, T. Borowski, J. R. Bennett, D. L.
Tierney, A. M. Arif, L. M. Berreau, J. Am. Chem. Soc. 2014,
136, 7821-7824.
23 S. Saraf, A. Milaczewska, T. Borowski, C. D. James, D. L.
Tierney, M. Popova, A. A. Arif, L. M. Berreau, Inorg. Chem.
2016, 55, 6916-6928.
24 R. Ramasubramanian, K. Anandababu, M. Kumar, R.
Mayilmurugan, Dalton Trans., 2018, 47, 4049-4053.
25 M. da Graca, M. B. Martin, M. Horner, M. Boneberge,
Behm, F. S. Nunes Z. Anorg. Allg. Chem., 2011, 637, 1229-
1233.
Acknowledgements
We thank the National Science Foundation (CHE-1664977 to LMB;
CHE-1429195 for Brüker Avance III HD 500 MHz NMR; CHE-1828764
for Rigaku Benchtop XtaLAB Mini II X-ray diffractometer). X-ray
crystallographic studies of 1, 3 and 8 were performed at the
University of Montana X-ray diffraction core facility, which is
supported by the Center for Biomolecular Structure and Dynamics
CoBRE (National Institutes of Health, CoBRE NIGMS
P20GM103546). Single crystal X-ray diffraction data were
collected using a Bruker D8 Venture (NSF MRI CHE-1337908).
26 E. Szajna, P. Dobrowolski, A. L. Fuller, A. M. Arif, L. M.
Berreau, Inorg. Chem., 2004, 43, 3988-3997.
27 B.G. Williams, M. Lawton, J. Org. Chem. 2010, 75, 8531-
8354.
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DOI: 10.1039/D0DT01338B
TOC Figure:
N3-ligated Ni(II) diketonate complexes are air stable and do not exhibit oxidative cleavage within the diketonate ligand under
aerobic conditions.