Synthesis, characterization, and catalytic activity of iron(II) and nickel(II) complexes containing the rigid pentadentate ligand PY 5Me2

Article abstract of DOI:10.1002/zaac.201300463

The reactions of PY₅Me₂ with hydrated metal perchlorate salts [M^II(ClO₄)₂] in CH ₃CN afforded two isostructural compounds [M^II(PY ₅Me₂)(CH₃CN)](ClO₄)₂ in high yields [M = Fe (1), Ni (2)]. The crystal structure of 2 was determined by X-ray crystallography. The catalytic activities of 1 and 2 towards the alcohol oxidation and C-H bond activation of organic substrates were studied. Initial experiments revealed that 1 was an active catalyst for the oxidative dehydrogenation and C-H bond activation of various substrates, whereas 2 was not. A Fe^III species [Fe^III(PY₅Me ₂)(OH)]²⁺ was proposed as the active species for the catalytic reactions. Copyright

Full text of DOI:10.1002/zaac.201300463

ARTICLE
DOI: 10.1002/zaac.201300463
Synthesis, Characterization, and Catalytic Activity of Iron(II) and Nickel(II)
Complexes Containing the Rigid Pentadentate Ligand PY5Me2
Jing Xiang,*[a] Hao Li,[a] and Jia-Shou Wu*[b]
Keywords: PY5Me2; Iron; Catalytic activity; C–H bond activation
Abstract. The reactions of PY5Me2 with hydrated metal perchlorate C–H bond activation of organic substrates were studied. Initial experi-
salts [MII(ClO4)2] in CH3CN afforded two isostructural compounds ments revealed that 1 was an active catalyst for the oxidative dehydro-
[MII(PY5Me2)(CH3CN)](ClO4)2 in high yields [M = Fe (1), Ni (2)]. genation and C–H bond activation of various substrates, whereas 2
The crystal structure of 2 was determined by X-ray crystallography. was not. A FeIII species [FeIII(PY5Me2)(OH)]2+ was proposed as the
The catalytic activities of 1 and 2 towards the alcohol oxidation and
active species for the catalytic reactions.
complexes of polypodal ligands have been used as oxidation
catalysts.[16]
Introduction
The coordination chemistry of the semirigid pentadentate
polypyridyl ligands PY5R2, (where R = H, Me, OH, Me) (Fig-
ure 1) has attracted wide attention.[1] These ligands typically
coordinate central metal atoms by one axial and four equatorial
pyridine, imposing a square-pyramidal coordination arrange-
ment, although other coordination modes have also been re-
ported.[2,3] Moreover, they are able to reinforce an octahedral
environment and restrict ligand substitution to a single coordi-
nation site.[4] These properties are particularly important in
controlling the reactivity of kinetically labile first row metals.
A number of transition metal complexes (Mn, Fe, Co, Ni, Cu,
and Zn) bearing these ligands have been reported and applied
in a wide range of areas including single-molecule magnets,[5–9]
and electrocatalytic water reduction.[10,11] The pyridine sub-
units of PY5R2 provide a neutral five-coordinate metal binding
cavity. Its complexation with divalent metal results usually in
a stronger Lewis acidity on the central metal atoms. Several
iron PY5R2 complexes have been reported to mimic the iron
active site in lipoxygenases (Los).[12,13] These iron complexes
were found to activate 1,4-pentadiene subunit-containing fatty
acids and also related model substrates into alkyl perox-
ides.[14,15] However, the catalytic activity of metal complexes
bearing PY5R2 for functional transformations of organic sub-
strates has been seldom studied, although many similar iron
Figure 1. Structure of PY5R2.
Herein, we report the synthesis of nickel and iron complexes
bearing the ligand 2,6-bis[1,1-bis(2-pyridyl)ethyl]pyridine
(PY5Me2) and their catalytic oxidative dehydrogenation of
various alcohols, phenol, hydroquinone, as well as the C–H
bond activation of styrene and cyclohexane using H2O2 as the
terminal oxidant.
Results and Discussion
The reactions of equimolar amounts of ligand PY5Me2 and
hydrated FeII and NiII perchlorate salts in CH3CN afforded two
isostructural compounds [MII(PY5Me2)(CH3CN)](ClO4)2 [M =
Fe (1); Ni (2)].[8] Both complexes are stable to O2 and moist-
ure and are isolated as microcrystalline solids at high yields.
IR spectra of 1 and 2 show weak stretching bands at 2007
and 2018 cm–1, respectively, which are assigned to ν(CϵN)
stretches of the coordinated CH3CN. The strong stretching
bands at around 1110 cm–1 are assigned to ν(Cl–O) stretch of
* Dr. J. Xiang
Fax: +86-716-8060650
E-Mail: xiangjing35991@sohu.com
* Dr. J.-S. Wu
E-Mail: jsw79@tzc.edu.cn
[a] College of Chemistry and Environmental Engineering
Yangtze University
Jingzhou 434020, HuBei, P. R. China
[b] School of Pharmaceutical and Chemical Engineering
Taizhou University
–
ClO4 . The cyclic voltammogram (CV) (Figure 2) of 1 in
CH3CN shows a quasi-reversible couple at E1/2 = 0.75 V (ΔEp
Taizhou 317000, P. R. China
= 71 mV) and an irreversible couple at Epa = –2.09 V vs.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201300463 or from the au-
thor.
+/0
FeCp2
in the range of –2.5 to 1.7 V at a scan rate of
100 mV·s–1. These couples are assigned to FeIII/II and FeII/I
1670
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2014, 640, (8-9), 1670–1674

Iron(II) and Nickel(II) Complexes with the Ligand PY5Me2
couples respectively, as their potentials are comparable with
those in other FeII compounds bearing these ligands.[2] Ligand
centered oxidation and reduction occurs at potentials of Ͼ
+1.0 V and Ͻ –2.3 V vs. FeCp2+/0, respectively, as reported in
a previous study.[2] Under similar conditions, CV of 2 shows
a quasi-reversible wave at E1/2 = 1.36 V (ΔEp = 80 mV) and a
reversible wave at E1/2 = –1.57 V (ΔEp = 60 mV), which are
tentatively assigned to NiIII/II and NiII/I couples, respectively
(Figure 2). ESI/MS of 1 in CH3CN (+ve mode) shows two
predominant peaks at m/z 249.6 and 598.2, which are assigned
to the parent cation [M]2+ and [M + ClO4]+ respectively (Fig-
ure S1, Supporting Information). ESI/MS of 2 in CH3CN (+ve
mode) shows the peaks of the corresponding cations at m/z
250.8 and 600.4, respectively (Figure S2).
Figure 3. The ORTEP drawing of cationic structure in 2 (thermal ellip-
soids are drawn at 30% probability and hydrogen atoms are omitted
for clarity).
solvents using H2O2 as the oxidant at room temperature. In
protic solvents, such as tert-butanol, water and trifluoroacetic
acid, only small amount of ketone product were obtained (GC-
FID). A better result was obtained in acetonitrile, where cyclo-
hexanone was formed in 14% yield after 20 h. Acetone was
proved to be the solvent of choice. With 1 mol equiv. H2O2,
cyclohexanone was obtained in 20% yield after 20 h at 25 °C
in an argon atmosphere in acetone solution containing
0.003 mmol 1 at a substrate/catalyst (S/C) ratio of 100 (Table
S2, entry 5). With 5 mol equiv. of H2O2, the yield of cyclohex-
anone was further improved (42%) (Table S2, entry 6). The
investigation was also extended to other alcohols (Table 1).
Figure 2. CVs. of 1 and 2 in 0.1 m [NnBu4]PF6 in CH3CN. Scan rate
= 100 mV s–1.
The crystals of 2 suitable for X-ray crystallography were Cyclopentanol and cyclohexanol were oxidized to correspond-
obtained by slow diffusion of diethyl ether into a CH3CN solu- ing ketone (43% yield) at room temperature (Table 1, entries
tion of 2. The structure of the cation is shown in Figure 3. 2 1 and 2). Benzyl alcohols could also be oxidized. When 1-
¯
crystallizes in the triclinic P1 space group. In each asymmetric phenylethanol was used, acetophenone was formed in a moder-
–
unit, there is one cation and two ClO4 . The divalent nickel ate yield of 49% (Table 1, entry 3). With more reactive and
atom is ligated in a six-coordinate octahedral arrangement by less steric benzyl alcohol, benzaldehyde was obtained selec-
the five nitrogen donors of PY5Me2 and an exogenous CH3CN. tively in 52% yield within 3 h (Table 1, entry 4). For the oxi-
The nitrogen atoms on the four terminal pyridines (N2–5) de- dation of aliphatic acyclic alcohol, poor yield was obtained for
fines the equatorial plane, and the axial positions are occupied n-heptanol (14%), while a relatively better result (33%) was
by the remaining pyridine nitrogen (N6) and an exogenous achieved with the more sterically hindered 2-heptanol (Table 1,
CH3CN (N1). The Ni–N(pyridine) bond lengths are in the nar- entries 6 and 7). Surprisingly, n-butanol was not oxidized
row range of 2.047(2)–2.114(2) Å. The cis N(pyridine)–Ni– (Table 1, entry 5). Hydroquinone was also oxidized to benzo-
N(pyridine) bond angles are in range of 81.87(9)–99.42(8)° quinone (71%) with 1 as catalyst after 4 h, whereas phenol
and the trans N(pyridine)–Ni–N(CH3CN) bond angles are in exhibited no reaction under similar conditions (Table 2, entries
the range of 175.08(8)–177.27(8)°. These bond parameters are 1 and 2).
comparable with those in related nickel compounds[17] (Table
The oxidation of cyclohexane via C–H bond activation with
S1, Supporting Information). The metal ion is slightly (ca. 1 was also investigated and the two products, cyclohexanol
0.07 Å) displaced from the equatorial plane toward the CH3CN and cyclohexanone, were detected at 14% and 11% yield,
unit.
respectively (Table 2, entry 3), whereas the oxidation of styr-
The catalytic activities of 1 and 2 towards the alcohol oxi- ene in acetone and acetonitrile gave benzaldehyde in 18% and
dation and C–H bond activation of various substrates were 9% yield (Table 2, entries 4 and 5).
studied. Initial experiments revealed that complex 1 catalyzes
Stacket et al. has investigated the reaction of [Fe(PY5)-
the oxidative dehydrogenation of cyclohexanol, while complex (MeCN)](OTf)2 [PY = 2,6-bis(bis(2-pyridyl)methoxymethane)
2 is not an active catalyst. Table S2 (Supporting Information) pyridine] with excessive H2O2 in methanol at 298 K, where a
shows the results of cyclohexanol oxidation on 1 in different thermally unstable blue species that was proposed to be a low
Z. Anorg. Allg. Chem. 2014, 1670–1674
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J. Xiang, H. Li, J.-S. Wu
ARTICLE
Table 1. Oxidation of various alcohols by H2O2 catalyzed by m/z 249.6 nearly disappeared, while a predominant peak at m/z
a)
258.2 tentatively assigned to the species [FeIII(PY5Me2)-
(OH)]2+ was observed. However, under the same conditions,
the ESI/MS of 2 remained unchanged after addition of H2O2,
suggesting that the oxidation of [NiII(PY5Me2)]2+ into the cor-
responding [NiIII(PY5Me2)(OH)]2+ species did not occur. This
suggests 2 is not an active catalyst when H2O2 is used as the
oxidant. The oxidation of 1 to [FeIII(PY5Me2)(OH)]2+ with
H2O2 is further supported by the more negative FeIII/II couple
(E1/2 = 0.76 V), whereas the NiIII/II couple in 2 occurs at much
more positive potential (E1/2 = 1.36 V). Attempts to isolate
the [FeIII(PY5Me2)(OH)]2+ species were unsuccessful. On
basis of the above results, [FeIII(PY5Me2)(OH)]2+ was pro-
posed to be the active catalyst formed from the oxidation of
[FeII(PY5Me2)]2+ by H2O2 (Scheme 1). As formation of
[FeIII(PY5Me2)(OH)]2+ in the reaction of [FeII(PY5Me2)]2+ and
hydrogen peroxide would also imply the generation of hy-
droxyl radicals, which in turn may also act as the oxidant. In
order to exclude this possibility, the radical inhibitors such as
CCl3Br or coumarin was added in the reaction system; how-
ever, these reagents do not exhibit significant influence on the
catalytic effect. Thus, we proposed that [FeIII(PY5Me2)(OH)]2+,
herein, functions as the only catalyst for the present alcohol
oxidation. In fact, the structurally related methoxy species
[FeIII(PY5Me2)(OMe)]2+ was also found to activate C–H
bonds of certain organic substrates.[14,15] It is believed that
[FeIII(PY5Me2)(OH)]2+, rather than [FeIII(PY5Me2)(OOH)]2+,
may be responsible for the oxidative dehydrogenation and C–
H bond activation in this study.
[Fe(py5Me2)(H2O)](ClO4)2 in acetone
.
Entry Substrate Time /h
Product
Yield /%b)
a) Reaction conditions: [substrate] = 3 mmol; [H2O2] = 0.3 mmol;
[Fe(py5Me2)(CH3CN)](ClO4)2] = 0.003 mmol; total solution, 3 mL.
The mixture was stirred at room temperature. b) Yield of product is
based on oxidant.
Table 2. Oxidation of phenol, hydroquinone, styrene, and cyclohexane
by H2O2 catalyzed by [Fe(py5Me2)(H2O)](ClO4)2 in acetone
a)
.
Entry Substrate Time /h Product
Yield /%b)
a) Reaction conditions: [substrate] = 3 mmol; [H2O2] = 0.3 mmol;
[Fe(py5Me2)(CH3CN)](ClO4)2] = 0.003 mmol; total solution, 3 mL.
The mixture was stirred at room temperature. b) Yield of product is
based on oxidant. c) Reaction conditions: [substrate] = 0.3 mmol;
[H2O2] = 0.3 mmol; [Fe(py5Me2)(CH3CN)](ClO4)2] = 0.003 mmol; in
3 mL acetone. d) Reaction conditions: [substrate] = 3 mmol; [H2O2]
= 0.3 mmol; [Fe(py5Me2)(CH3CN)](ClO4)2] = 0.003 mmol; in 3 mL
acetonitrile.
Figure 4. (a) and (b): ESI-MS of 1 in acetone (+ ve mode) before and
after addition of H2O2; (c) and (d): ESI-MS of 2 in acetone (+ ve
mode) before and after addition of H2O2.
spin hydroperoxide-bound ferric species, [FeIII(PY5)(OOH)]-
(OTf)2, was generated.[15] To identify the active species in our
system, the ESI/MS of 1 and 2 in the presence of excess H2O2
was carried out in acetone. As shown in Figure 4, a strong
peak at m/z 249.6 assigned to the [Fe(PY5Me2)]2+ cation was
initially observed. After the addition of 100 equiv. H2O2, the
Scheme 1. The proposed active species from the reaction of 1 with
light yellow solution turn green immediately and the peak at H2O2.
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© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2014, 1670–1674

Iron(II) and Nickel(II) Complexes with the Ligand PY5Me2
basis of riding mode with thermal parameters equal to 1.2 times that
of the associated carbon atoms, and participated in the calculation of
final R indices. All calculations were performed using the teXsan crys-
tallographic software.[23]
Conclusions
We found that the FeII complex bearing the semi-rigid ligand
PY5Me2 catalyzes the oxidation of various alcohols and even
alkanes by H2O2 under mild conditions. The in situ generated
[FeIII(PY5Me2)(OH)]+ was proposed to be the active species
involved. No encouraging results could be obtained with the
analogous NiII complex, possibly as a result of the more posi-
tive NiIII/II couple, which discourages the formation of the cor-
responding NiIII–OH species. To our best knowledge, this is
the first study, which demonstrates the catalytic activity of iron
complexes bearing PY5Me2 ligand towards the functional
transformations of organic substrates. It is well known that
high-valent Ru complexes are usually more stable than their
Fe analogues and Ru-oxo species oxidizes alcohols rapidly via
a two-electron hydride abstraction process.[18,19] Thus, we will
try to synthesize the ruthenium compound bearing PY5Me2 li-
gand and investigate their catalytic activity towards C–H bond
activation of various substrates and develop more functional
transformations in organic synthesis.
General Procedure for Catalytic Study: In a typical reaction, the
catalyst (0.003 mmol) in solvent (acetone, 3 mL) was placed in a
25 mL Schlenk tube and stirred for 10 min at 298 K in an argon atmo-
sphere. The respective substrates (3 mmol) and chlorobenzene (GC in-
ternal standard) were added into the catalyst solution whilst stirring
conditions. The oxidant (0.3 mmol H2O2) was added over a period of
20 h through a syringe pump.
[Fe(PY5Me2)(CH3CN)](ClO4)2 (1): Equimolar amounts of PY5Me2
(88.6 mg, 0.2 mmol) and Fe(ClO4)2·6H2O (72.6 mg, 0.2 mmol) were
dissolved in CH3CN (15 mL) and the solution was stirred at room
temperature for 2 h in a nitrogen atmosphere. Addition of diethyl ether
into the solution resulted in the precipitation of a yellow solid, which
was collected by filtration. The compound was further purified by slow
diffusion of diethyl ether into a CH3CN solution of 1. Yield: 90%
(133.1 mg). ESI/MS (+ve mode): m/z 249.6 [M – CH3CN]2+; 598.2
[M + ClO4 – CH3CN]+. IR (selected bands, KBr): ν˜ = 2007 w, 1597
m, 1467 m, 1440 m, 1413 w, 1392 w, 1090 s, 864 w, 763 m, 623 m
cm–1. C31H28Cl2FeN6O8: calcd. C 50.36; H 3.82; N 11.37%; found:
C, 50.40; H, 3.90; N, 11.31%. UV/Vis (CH3CN): λmax [nm]
(ε[mol–1dm3cm–1]) 251 (21260), 353 (6703), 421 (7620).
Experimental Section
The ligand 2,6-bis(1,1-bis(2-pyridyl)ethyl)pyridine (PY5Me2) was syn-
thesized by a reported procedure.[20] [nBu4N]PF6 (Aldrich) used for
cyclic voltammetry was recrystallized three times from boiling ethanol
and dried in a vacuum at 120 °C for 24 h. Acetonitrile (Aldrich) was
distilled over calcium hydride. All other chemicals were of reagent
grade and used without further purification. All manipulations were
performed with no precaution to exclude air or moisture unless other-
wise stated.
[Ni(PY5Me2)(CH3CN)](ClO4)2 (2): Equimolar amounts of PY5Me2
(88.6 mg, 0.2 mmol) and Ni(ClO4)2·6H2O (73.1 mg, 0.2 mmol) were
dissolved in CH3CN (15 mL) and the solution was stirred at room
temperature for 2 h. Addition of diethyl ether resulted in the precipi-
tation of a green solid. The compound was further purified by slow
diffusion of diethyl ether into a CH3CN solution of 2. Yield: 78%
(115.8 mg). ESI/MS (+ mode): m/z 250.8 [M – CH3CN]2+; 600.4 [M
+ ClO4–CH3CN]+. IR (selected bands, KBr): ν˜ = 2018 w, 1596 m,
1465 m, 1454 m, 1440 m, 1389 w, 1299 w, 1108 s, 865 w, 764 m,
628 s cm–1. C31H28Cl2N6NiO8: calcd. C 50.17; H 3.80; N 11.32%;
found: C 50.20; H 3.78; N 11.20%. UV/Vis (CH3CN): λmax [nm]
(ε[mol–1dm3cm–1]) 261 (17000), 309 (940).
Physical Measurements: IR spectra were obtained as KBr discs with
a Nicolet 360 FTIR spectrophotometer. Elemental analysis was per-
formed with an Elementar Vario EL Analyzer. Electrospray ionization
mass spectrometry (ESI-MS) was performed with a PE-SCIEX API
365 triple quadruple mass spectrometer. Cyclic voltammetry (CV) was
performed with a PAR model 273 potentiostat. Cyclic voltammetry
was performed in acetonitrile containing 0.1 m [nBu4N]PF6 as the sup-
porting electrode at room temperature. An Ag/AgNO3 (0.1 m in
CH3CN) electrode was used as reference electrode and the working
Crystallographic data (excluding structure factors) for the structure in
this paper have been deposited with the Cambridge Crystallographic
Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies
of the data can be obtained free of charge on quoting the depository
number CCDC-930612 (2) (Fax: +44-1223-336-033; E-Mail: deposit
@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk).
electrode was a glassy carbon electrode (CH Instruments, Inc.) with a
+/0
platinum-wire auxiliary electrode. Ferrocenium/ferrocene (FeCp2
)
was used as the internal reference. All solutions for electrochemical
studies were degassed with pre-purified argon gas prior to measure-
ments. Gas chromatographic analyses were performed with a HP5890
GC/FID equipped with HP-5MS (30 mϫ0.25 mm i.d.) or a DB-FFAP
(30 mϫ0.25 mm i.d.) column. GC/MS measurements were carried out
on a HP6890 gas chromatograph interfaced to a HP 5795 mass selec-
tive detector.
Supporting Information (see footnote on the first page of this article):
Selected bond parameters /Å,° for compound 2 (Table S1); Oxidation
of cyclohexanol with H2O2 with 1 in various solvent (Table S2); A
summary of crystal data, data collection, and structure refinement for
2 (Table S3). ESI-MS (+ ve mode) of 1 in CH3CN [inset shows the
expanded (top) and calculated (bottom) isotopic pattern of the peak at
m/z at 598.2] (Figure S1). ESI-MS (+ ve mode) of 2 in CH3CN [inset
shows the expanded (top) and calculated (bottom) isotopic pattern of
the peak at m/z at 600.6] (Figure S2).
X-ray Crystallography: Measurement was collected with an Oxford
CCD diffractometer using graphite-monochromated Mo-Kα radiation
(λ = 0.71073 Å) for 1. Details of the intensity data collection and crys-
tal data are given in Table S3 (Supporting Information). Absorption
corrections were done by the multiscan method. The structure was
resolved by the heavy-atom Patterson method or direct methods and
Acknowledgements
refined by full-matrix least-squares using SHELX-97 and expanded The authors gratefully acknowledge the financial support of National
using Fourier techniques.[21,22] All non-hydrogen atoms were refined Natural Science Foundation of China (21201023) and the Scientific
anisotropically. Hydrogen atoms were generated by the program
Research Fund of HuBei Provincial Education Department
SHELXL-97. The positions of hydrogen atoms were calculated on the (D20131202).
Z. Anorg. Allg. Chem. 2014, 1670–1674
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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J. Xiang, H. Li, J.-S. Wu
ARTICLE
[15] C. R. Goldsmith, R. T. Jonas, T. D. P. Stack, J. Am. Chem. Soc.
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Published Online: February 4, 2014
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