Synthesis, structural studies, and oxidation catalysis of the late-first-row-transition-metal complexes of a 2-pyridylmethyl pendant-armed ethylene cross-bridged cyclam

Article abstract of DOI:10.1021/ic502699m

The first 2-pyridylmethyl pendant-armed ethylene cross-bridged cyclam ligand has been synthesized and successfully complexed to Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺ cations. X-ray crystal structures were obtained for all six complexes and demonstrate pentadentate binding of the ligand with the requisite cis-V configuration of the cross-bridged cyclam ring in all cases, leaving a potential labile binding site cis to the pyridine donor for interaction of the complex with oxidants and/or substrates. The electronic properties of the complexes were evaluated using solid-state magnetic moment determination and acetonitrile solution electronic spectroscopy, which both agree with the crystal structure determination of high-spin divalent metal complexes in all cases. Cyclic voltammetry in acetonitrile revealed reversible redox processes in all but the Ni²⁺ complex, suggesting that catalytic reactivity involving electron-transfer processes is possible for complexes of this ligand. Kinetic studies of the dissociation of the ligand from the copper(II) complex under strongly acidic conditions and elevated temperatures revealed that the pyridine pendant arm actually destabilizes the complex compared to the parent cross-bridged cyclam complex. Screening for oxidation catalysis using hydrogen peroxide as the terminal oxidant for the most biologically relevant Mn²⁺, Fe²⁺, and Cu²⁺ complexes identified the Mn²⁺ complex as a potential mild oxidation catalyst worthy of continued development.

Full text of DOI:10.1021/ic502699m

Article
pubs.acs.org/IC
Synthesis, Structural Studies, and Oxidation Catalysis of the Late-
First-Row-Transition-Metal Complexes of a 2‑Pyridylmethyl Pendant-
Armed Ethylene Cross-Bridged Cyclam
Donald G. Jones,† Kevin R. Wilson,† Desiray J. Cannon-Smith,† Anthony D. Shircliff,† Zhan Zhang,‡
Zhuqi Chen,‡ Timothy J. Prior,*,§ Guochuan Yin,*,‡ and Timothy J. Hubin*,†
†Department of Chemistry and Physics, Southwestern Oklahoma State University, 100 Campus Drive, Weatherford, Oklahoma
73096, United States
‡Key Laboratory for Large-Format Battery Materials and System, Ministry of Education, School of Chemistry and Chemical
Engineering, Hubei Key Laboratory of Material Chemistry and Service Failure, Huazhong University of Science and Technology,
Wuhan 430074, P. R. China
§Department of Chemistry, The University of Hull, Cottingham Road, Hull HU6 7RX, U.K.
S
* Supporting Information
ABSTRACT: The first 2-pyridylmethyl pendant-armed ethylene cross-
bridged cyclam ligand has been synthesized and successfully complexed to
Mn2+, Fe2+, Co2+, Ni2+, Cu2+, and Zn2+ cations. X-ray crystal structures
were obtained for all six complexes and demonstrate pentadentate binding
of the ligand with the requisite cis-V configuration of the cross-bridged
cyclam ring in all cases, leaving a potential labile binding site cis to the
pyridine donor for interaction of the complex with oxidants and/or
substrates. The electronic properties of the complexes were evaluated using
solid-state magnetic moment determination and acetonitrile solution
electronic spectroscopy, which both agree with the crystal structure
determination of high-spin divalent metal complexes in all cases. Cyclic
voltammetry in acetonitrile revealed reversible redox processes in all but
the Ni2+ complex, suggesting that catalytic reactivity involving electron-
transfer processes is possible for complexes of this ligand. Kinetic studies of
the dissociation of the ligand from the copper(II) complex under strongly acidic conditions and elevated temperatures revealed
that the pyridine pendant arm actually destabilizes the complex compared to the parent cross-bridged cyclam complex. Screening
for oxidation catalysis using hydrogen peroxide as the terminal oxidant for the most biologically relevant Mn2+, Fe2+, and Cu2+
complexes identified the Mn2+ complex as a potential mild oxidation catalyst worthy of continued development.
destruction.1−4 Mn(Me2EBC)Cl2 is a patented bleach catalyst
heavily invested in by the laundry detergent industry because of
INTRODUCTION
■
Oxidation catalysis by cross-bridged cyclam complexes of
its ability to activate O2/H2O2 in water and remove stain
manganese and iron has been studied for nearly a decade and a
molecules from cloth.16−20 However, it has not been fully
half.1−14 The manganese complex of 4,11-dimethyl-1,4,8,11-
tetraazabicyclo[6.6.2]hexadecane15 (Me2EBC, Scheme 1) in
implemented in consumer products.
Recently, Que and co-workers have revisited the iron
complex of Me2EBC and observed efficient olefin epoxidation
particular has a diverse and rich oxidation chemistry utilizing
oxidation mechanisms ranging from hydrogen-atom abstraction
catalysis with hydrogen peroxide (H2O2) oxidant under
(HAT), electron transfer, concerted oxygen-atom transfer
appropriate conditions.21 In this study, it was shown that
(OAT), to the oxygen-rebound mechanism.5−14 This com-
pound, which we propose to call “the Busch catalyst”, was
initially targeted as a potential oxidation catalyst because the
rigid cross-bridged ligand could strongly bind the oxygen-
added acetic acid increases the yield of the epoxide product,
and it was postulated that acetic acid binds to the iron center
and facilitates O−O bond cleavage, which is dependent on the
cis orientation of the two labile sites. Only one ligand-modified
reactive manganese ion and prevent it from being deactivated in
the form of MnO2.1−4 Additional critical ligand properties are
analogue of Me2EBC, the diethyl analogue Et2EBC (Scheme
1), has had its manganese complex oxidation catalysis
thought to be the two available cis labile coordination sites for
explored.22,23 Interestingly, the seemingly simple exchange of
oxidant and substrate interaction, the methyl groups sterically
preventing dimerization, which might deactivate the catalyst,
and the saturated and all-tertiary nitrogen nature of the ligand,
which minimizes the possibility of ligand oxidation and catalyst
Received: November 13, 2014
© XXXX American Chemical Society
A
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Article
in the same ligand, giving an opportunity to examine
coordination preference, and geometric and parametric changes
as the size and electronic properties change throughout the
series; (3) the effect of an added pyridine donor on the
electronic properties of the resulting complexes, most
dramatically seen in the increased range of oxidation states
available and increased reversibility observed in the cyclic
voltammetry; (4) oxidation screening through HAT and OAT
reactions that identify the Mn(PyMeEBC)Cl+ complex as an
active oxidation catalyst that may have significant advantages
over Mn(Me2EBC)Cl2 as a laundry bleach catalyst due to
enhanced oxidation selectivity; (5) kinetic decomplexation
studies of the Cu(PyMeEBC)2+ complex that in comparison
with literature data indicate that the pyridine pendant arm
actually destabilizes the complex with respect to the parent
Me2EBC ligand, even though an additional chelate ring is
added.
Scheme 1. Ligands Discussed in This Paper
methyl for ethyl groups results in a large change in the catalyst
oxidation potential and the surprising oxidation of the two ethyl
groups into chelated methylene carboxylato and ethoxo groups
(Scheme 1), respectively. While demonstrating the large
possible effect of modifying the Me2EBC ligand structure on
the oxidation potential and thus the resulting catalytic behavior,
the hexadentate-modified ligand product reduces the utility of
the ultimate product in this case by coordinatively saturating it.
Additional motivation derived from (trimethylpyridyl)cyclam
(TMC-py, Scheme 1), having all but the desired ethylene cross-
bridge of our preferred ligand characteristics (vide supra). Its
iron complex activated dioxygen and formed an oxoiron(IV)
intermediate, which was crystallographically characterized but
has not been pursued further as a catalyst.24 Although other
pyridyl pendant-armed unbridged cyclams are ubiquitous25−35
and a dipyridyl pendant-armed cross-bridged cyclen has been
published,36 none of these ligands provide all of the desirable
features for an oxidation catalyst (vide supra).
In this report, we offer our initial contribution to this field
with the synthesis and characterization of a 2-pyridylmethyl N-
pendant arm ethylene cross-bridged cyclam (PyMeEBC,
Scheme 1) and its late-first-row-transition-metal (Mn2+, Fe2+,
Co2+, Ni2+, Cu2+, and Zn2+) complexes. We chose to modify
Me2EBC by replacing only one methyl with a single 2-
pyridylmethyl pendant arm, in order to maintain a labile
nonchelated coordination site for interaction with oxidant/
substrate. The additional aromatic nitrogen donor was expected
to modify the steric and electronic properties of the metal ion,
potentially as profoundly as the ethyl groups in Et2EBC,22,23
resulting in modified oxidation chemistry. The pyridine
pendant would also maintain ligand neutrality and steric
prevention of dimerization. It was, furthermore, expected that
the quaternary, aromatic β-carbon of the pyridyl pendant arm
would limit the reactivity of the ligand toward the oxidative
modification found in Et2EBC. Here, we disclose its synthesis,
complexation to a range of late transition metals, the X-ray
crystal structures of all six complexes, their electronic structure
as revealed by magnetic moment, UV−vis spectroscopy, and
cyclic voltammetry, and the initial screening of the most
biomimetically relevant complexes (manganese, iron, and
copper) for the ability to catalyze oxidation reactions.
EXPERIMENTAL SECTION
■
General Procedures. N,N′-Bis(aminopropyl)ethylenediamine
(98%) and 2-picolyl chloride hydrochloride (98%) were purchased
from Acros Organics. Glyoxal (40 wt % in water), methyl iodide
(99%), and sodium borohydride (NaBH4; 98%), all anhydrous
divalent transition-metal chloride salts, and all anhydrous solvents
used in the glovebox were purchased from Aldrich Chemical Co. All
other solvents were of reagent grade and were used without
modification. Cyclam was prepared according to a modified literature
method from N,N′-bis(aminopropyl)ethylenediamine.37 cis-
3a,5a,8a,10a-Tetraazaperhydropyrene (1, cyclam glyoxal) was prepared
according to a literature method.38 Elemental analyses were performed
by Quantitative Technologies Inc. Electrospray mass spectra were
collected on a Shimadzu LCMS-2020 instrument. All samples were
dissolved in 50% water (H2O)/50% methanol (MeOH). NMR spectra
were obtained on a Varian Bruker AVANCE II 300 MHz NMR
spectrometer. Electronic spectra were recorded using a Shimadzu UV-
3600 UV−vis−near-IR spectrophotometer. Magnetic moments were
obtained on finely ground solid samples at ambient temperatures using
a Johnson Matthey MSB Auto magnetic susceptibility balance.
Electrochemistry. Electrochemical experiments were performed
on a BAS Epsilon EC-USB electrochemical analyzer. A button
platinum electrode was used as the working electrode with a platinum
wire counter electrode and a silver wire pseudoreference electrode.
Scans were taken at 200 mV/s. Acetonitrile solutions of the complexes
(1 mM) with tetrabutylammonium hexafluorophosphate (0.1 M) as
the supporting electrolyte were used. The measured potentials were
referenced to SHE using ferrocene (+0.400 V versus SHE) as an
internal standard. All electrochemical measurements were carried out
under nitrogen.
Acid Decomplexation Studies. [Cu(Me2EBC)Cl]PF6 synthe-
sized according to the literature39 and [Cu(PyMeEBC)][PF6]2
synthesized as below were used at 1 mM. The complex’s lone d−d
absorption was recorded on a Shimadzu UV-3600 UV−vis−near-IR
spectrophotometer in 5 M HCl at both 90 and 50 °C over time.
Typically, isosbestic spectra indicated only one decomposition product
as the absorbance at λmax decreased over time. Pseudo-first-order
conditions allowed the calculation of half-lives from the slopes of the
linear ln(absorbance) versus time plots.
Synthesis. 3a-(Pyridin-2-ylmethyl)decahydro-5a,8a,10a-triaaza-
3a-azoniapyrenium Iodide (2). A total of 13.28 g (0.08096 mol, 2
equiv) of picolyl chloride hydrochloride and 13.60 g (0.1619 mol, 4
equiv) of anhydrous NaHCO3 were stirred in 700 mL of chloroform
for 1 h. Solids were removed by filtration, and the filtrate was added to
9.00 g (0.04048 mol, 1 equiv) of 1 and 13.44 g (0.08096 mol, 2 equiv)
of KI. The reaction was stirred and heated to reflux for 6 days under
nitrogen, during which it became an orange color. After cooling,
minimal solids were removed by filtration and discarded. The filtrate
was evaporated to 100 mL volume, and excess diethyl ether was added
to precipitate the yellow solid product, which was filtered on a glass
Important results and discussion include the following: (1)
synthesis of the first cross-bridged cyclam to include a pendant
arm 2-pyridylmethyl group, which will no doubt be exploited by
many other coordination chemists for a variety of purposes; (2)
X-ray crystal structure characterization of a full series of first-
row transition-metal complexes from manganese through zinc
B
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frit, washed with diethyl ether, and dried under vacuum. Yield: 11.841
g (66%). Electrospray mass spectrometry gave a single peak at m/z
314 corresponding to (M − I)+. Anal. Calcd for C18H28N5I·H2O: C,
47.06; H, 6.58; N, 15.25. Found: C, 46.87; H, 6.54; N, 14.88. 1H NMR
(300 MHz, CDCl3): δ 1.36 (d, 1H), 1.84 (d, 1H), 2.25 (m, 2H), 2.41
(d, 1H), 2.56 (d, 1H), 2.66 (m, 2H), 3.03 (m, 6H), 3.23 (t, 1H), 3.65
(m, 2H), 3.89 (d, 1H), 4.21 (m, 2H), 4.40 (td, 1H), 4.58 (s, 1H), 5.45
(m, 2H), 7.41 (m, 1H), 7.84 (m, 1H), 8.31 (d, 1H), 8.66 (d, 1H).
13C{1H} NMR (75.6 MHz, D2O): δ 18.0, 18.4, 41.9, 46.6, 49.4, 51.3,
dissolved in a minimum of MeOH, and precipitated with NH4PF6 as
for the other metal ions). The filtrates were then evaporated under
vacuum to give crude [M(PyMeEBC)Cl]Cl solid products that were
dissolved in a minimum of MeOH in the glovebox. To each was added
0.815 g (0.005 mol, 5 equiv) of NH4PF6, likewise dissolved in a
minimum of MeOH. Precipitation of the [M(PyMeEBC)Cl]PF6
products was immediate, but the suspensions were allowed to stir
approximately 1 h to complete precipitation. The solid [M-
(PyMeEBC)Cl]PF6 products were filtered off, washed with diethyl
ether, and allowed to dry overnight open to the glovebox atmosphere.
[Mn(PyMeEBC)Cl]PF6. Yield: 0.302 g (53%) of white powder.
Electrospray mass spectrometry gave peaks at m/z 421 corresponding
to (Mn(PyMeEBC)Cl)+ and m/z 193 corresponding to (Mn-
(PyMeEBC))2+. Anal. Calcd for [Mn(C19H33N5)Cl]PF6·0.5H2O: C,
39.63; H, 5.95; N, 12.16. Found: C, 39.46; H, 6.08; N, 12.16. X-ray-
quality crystals were obtained from the slow diffusion of diethyl ether
into an acetonitrile solution in the glovebox.
[Fe(PyMeEBC)Cl]PF6. Yield: 0.357 g (58%) of yellow powder.
Electrospray mass spectrometry gave peaks at m/z 422 corresponding
to (Fe(PyMeEBC)Cl)+ and m/z 194 corresponding to (Fe-
(PyMeEBC))2+. Anal. Calcd for [Fe(C19H33N5)Cl]PF6·0.3NH4PF6:
C, 37.01; H, 5.59; N, 12.04. Found: C, 37.10; H, 5.27; N, 12.09. X-ray-
quality crystals were obtained from the slow evaporation of a MeOH
solution in the glovebox.
[Co(PyMeEBC)Cl]PF6. Yield: 0.401 g (70%) of pink powder.
Electrospray mass spectrometry gave peaks at m/z 425 corresponding
to (CoLCl)+ and m/z 195 corresponding to (CoL)2+. Anal. Calcd for
[Co(C19H33N5)Cl]PF6: C, 39.98; H, 5.83; N, 12.27. Found: C, 39.89;
H, 5.54; N, 12.18. X-ray-quality crystals were obtained from the slow
diffusion of diethyl ether into an acetonitrile solution in the glovebox.
[Ni(PyMeEBC)Cl]PF6. Yield: 0.345 g (60%) of a brown powder.
Electrospray mass spectrometry gave peaks at m/z 424 corresponding
to (Ni(PyMeEBC)Cl)+, m/z 408 corresponding to (NiL(H2O)2)+,
and m/z 195 corresponding to (Ni(PyMeEBC))2+. Anal. Calcd for
[Ni(C19H33N5)Cl]PF6·0.2NH4PF6·1.5H2O: C, 36.21; H, 5.89; N,
11.56. Found: C, 36.36; H, 5.61; N, 11.26. X-ray-quality crystals were
obtained from the slow diffusion of diethyl ether into a dimethyl
sulfoxide solution outside of the glovebox.
[Zn(PyMeEBC)Cl]PF6. Yield: 0.577 g (73%) of a white powder.
Electrospray mass spectrometry gave peaks at m/z 430 corresponding
to (Zn(PyMeEBC)Cl)+ and m/z 198 corresponding to (Zn-
(PyMeEBC))2+. Anal. Calcd for [Zn(C19H33N5)Cl]PF6: C, 39.53;
H, 5.76; N, 12.13. Found: C, 39.22; H, 5.49; N, 11.99. X-ray-quality
crystals were obtained from the slow evaporation of a nitromethane
solution (form 2) and from the slow diffusion of diethyl ether into an
acetonitrile solution (form 1), both outside of the glovebox.
51.9, 53.2, 54.0, 60.5, 62.9, 69.5, 82.2, 125.8, 129.0, 138.6, 146.6, 150.3.
3a-(Pyridin-2-ylmethyl)-8a-methyldecahydro-5a,10a-diaaza-
3a,8a-diazoniapyrenium Diiodide (3). A total of 11.841 g (0.02683
mol, 1 equiv) of 2 was suspended in 690 mL of dry acetonitrile in a 1 L
round-bottomed flask. A total of 15 equiv (0.4025 mol, 57.13 g, 25.17
mL) of iodomethane was added, the flask was stoppered, and the
reaction was stirred at room temperature for 7 days. The light-brown
solid product was obtained by filtration on a glass frit, washing with
diethyl ether, and drying under vacuum. A second crop was obtained
by the addition of excess ether to the filtrate and added to the first
crop. Yield: 12.921 g (83%). Electrospray mass spectrometry gave
peaks at m/z 456 corresponding to (M − I)+, m/z 328 corresponding
to (M − 2I)+, and m/z 165 corresponding to (M − 2I)2+. Anal. Calcd
for C19H31N5I2·0.5CH3CN: C, 39.78; H, 5.43; N, 12.76. Found: C,
1
39.51; H, 5.63; N, 13.09. H NMR (300 MHz, CDCl3): δ 1.80 (m,
2H), 1.97 (s, 3H), 2.32 (m, 3H), 2.79 (td, 1H), 3.10 (m, 5H), 3.38 (m,
5H), 3.55 (d, 2H), 3.72 (d, 2H), 4.51 (m, 2H), 5.18 (d, 2H), 7.52 (m,
1H), 7.65 (d, 1H), 7.93 (m, 1H), 8.65 (d, 1H). 13C{1H} NMR (75.6
MHz, D2O): δ 17.1, 17.5, 45.3, 45.6, 47.4, 48.4, 48.7, 49.8, 50.2, 59.6,
62.0, 64.1, 73.6, 75.7, 125.0, 128.2, 137.7, 145.3, 149.5.
4-Methyl-11-(pyridin-2-ylmethyl)-1,4,8,11-tetraazabicyclo[6.6.2]-
hexadecane (PyMeEBC). A total of 9.010 g (0.0154 mol, 1 equiv) of 3
was dissolved in 550 mL of 95% ethanol (EtOH). A total of 8.739 g
(0.2310 mol, 15 equiv) of NaBH4 was added over 5 min. The reaction
was then stirred under nitrogen for 5 days; copious white precipitate
formed during the course of the reaction. Excess NaBH4 was
decomposed by the slow addition of concentrated HCl. EtOH was
removed under vacuum, after which 500 mL of 30% aqueous KOH
was added. This basic solution was extracted with five 200 mL portions
of benzene. The combined benzene layers were dried over sodium
sulfate, filtered, and evaporated to a pale-yellow oil, which was not
purified further. Yield: 4.550 g (89%). Electrospray mass spectrometry
gave a single peak at m/z 332 corresponding to (MH)+. Anal. Calcd
for C19H33N5·0.3H2O·0.1C6H6: C, 68.29; H, 10.00; N, 20.32. Found:
C, 68.40; H, 10.52; N, 20.33. 1H NMR (300 MHz, CDCl3): δ 1.68 (m,
4H), 2.30 (s, 3H), 2.85 (m, 18H), 3.58 (m, 4H), 7.16 (m, 2H), 7.61
(m, 1H), 8.49 (m, 1H). 13C{1H} NMR (75.6 MHz, CDCl3): δ 22.9,
23.6, 41.5, 50.3, 50.6, 51.3, 51.8, 52.2, 52.8, 53.3, 55.7, 56.8, 57.6, 58.6,
122.0, 123.1, 135.6, 148.4, 156.1.
[Mn(PyMeEBC)Cl]Cl. A total of 1.000 g (0.003017 mol) of
PyMeEBC and 0.380 g (0.003016 mol) anhydrous MnCl2 were added
to 20 mL of anhydrous acetonitrile in an inert-atmosphere glovebox.
The reaction was stirred at room temperature for 2 days, during which
the pink MnCl2 beads dissolved and the white precipitate powder
product precipitated. This product was obtained by filtration on a glass
frit, washed with ether, and allowed to dry open to the atmosphere of
the glovebox for 4 days. Yield: 0.930 g (67%). Electrospray mass
spectrometry gave peaks at m/z 421 corresponding to (Mn-
(PyMeEBC)Cl)+ and m/z 193 corresponding to (Mn(PyMeEBC))2+.
Anal. Calcd for [Mn(C19H33N5)Cl]Cl·0.1H2O: C, 49.70; H, 7.29; N,
15.25. Found: C, 49.35; H, 7.21; N, 15.09.
[Cu(PyMeEBC)][PF6]2. This complex was synthesized following
the general synthesis of [M(PyMeEBC)Cl]PF6. However, no chloride
is bound to the metal ion and two hexafluorophosphate anions are
required. Yield: 0.302 g (44%) of a bright blue powder. Electrospray
mass spectrometry gave a peak at m/z 197 corresponding to
(Cu(PyMeEBC))2+. Anal. Calcd for [Cu(C19H33N5)][PF6]2: C,
33.32; H, 4.86; N, 10.22. Found C, 32.96; H, 4.65; N, 10.11. X-ray-
quality crystals were obtained from the slow diffusion of diethyl ether
into an acetonitrile solution outside of the glovebox.
Crystal Structure Analysis. Single-crystal X-ray diffraction data
were collected in a series of ω scans using a Stoe IPSD2 image-plate
diffractometer utilizing monochromated Mo radiation (λ = 0.71073
Å). Standard procedures were employed for integration and processing
of the data using X-RED.40 Samples were coated in a thin film of
perfluoropolyether oil and mounted at the tip of a glass fiber located
on a goniometer. Data were collected from crystals held at 150 K in an
Oxford Instruments nitrogen gas cryostream.
[M(PyMeEBC)Cl]PF6 where M = Mn2+, Fe2+, Co2+, Ni2+, and
Zn2+. The general procedure for [Mn(PyMeEBC)Cl]Cl was followed
using 0.001 mol (0.332 g) of PyMeEBC and 0.001 mol of the
respective anhydrous divalent metal chloride salt. [Because NiCl2 has
little solubility in acetonitrile, N,N-dimethylformamide (DMF) was
used as the solvent for this reaction only. The reaction was removed
from the glovebox and refluxed overnight before workup.] These
reactions, other than manganese, gave little or no precipitation. All
were filtered to remove trace solids, which were discarded (other than
manganese, which gave a copious white solid, which was filtered,
Crystal structures were solved using routine automatic direct
methods implemented within SHELXS-97.41 Completion of structures
was achieved by performing least-squares refinement against all unique
F2 values using SHELXL-97.41 All non-hydrogen atoms were refined
with anisotropic displacement parameters. Hydrogen atoms were
placed using a riding model. Where the location of hydrogen atoms
C
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a
Scheme 2. Synthesis of PyMeEBC
a
(a) (i) CHCl3, 2 equiv of picolyl chloride hydrochloride, 4 equiv of NaHCO3, 1 h, filter to remove solids; (ii) 1 equiv of 1, 2 equiv of KI, reflux 6
days; 66% yield. (b) CH3CN, 15 equiv of CH3I, 7 days, rt; 83% yield. (c) (i) 95% EtOH, 15 equiv of NaBH4, N2, 5 days, rt; (ii) 12 M HCl(aq), 30%
KOH(aq), benzene extraction; 89% yield.
was obvious from difference Fourier maps, C−H bond lengths were
refined subject to chemically sensible restraints.
divalent metal salts in an inert-atmosphere glovebox. The white
powder manganese complex [Mn(PyMeEBC)Cl]Cl precipi-
tated from the reaction and was obtained in pure form by
filtration. The other complexes were more soluble, so only trace
solids were filtered off, and the solvent was evaporated. The
crude chloride salts were not pure, so they were redissolved in
MeOH, and the chloride anion was replaced by the addition of
NH4PF6 to precipitate the pure [M(PyMeEBC)Cl]PF6 powder
products. An exception to this formulation was the copper(II)
complex, which remained 5-coordinate with no chloro ligand
and precipitated as [Cu(PyMeEBC)][PF6]2. To aid in
crystallization, anion metathesis was similarly achieved to
produce [Mn(PyMeEBC)Cl]PF6.
Complexation by cross-bridged tetraazamacrocycles is some-
times difficult, requiring heat or long reaction times.4
Comparatively, complexation with PyMeEBC appeared qual-
itatively to be faster than usual in most cases. For example, a
crude [Mn(PyMeEBC)Cl]Cl white powder began to precip-
itate from the room temperature acetonitrile complexation
solution within minutes. Similarly, the dramatic color changes
indicating Cu2+ (dark blue-green) and Co2+ (dark purple)
coordination with similar ligands were observed immediately
upon the addition of metal chloride salts to the room
temperature acetonitrile ligand solutions. Therefore, no heat
was added to any of the complexation reaction except for NiCl2
complexation, which was done in DMF to aid the solubility of
NiCl2, which required heat to fully dissolve this sparingly
soluble salt. Certain pendant arms have been noted to aid the
speed of complexation of tetraazamacocycles,47−50 and it is
possible that the pyridine pendant arm plays such a role here.
X-ray Crystal Structures. [M(PyMeEBC)Cl]PF6 (M = Mn,
Fe, or Co). The metal(II) complexes [M(PyMeEBC)Cl]PF6
(M = Mn, Fe, or Co) are isostructural and crystallize in the
noncentrosymmetric space group Pca21. The unit cell volume
decreases across this series from manganese to cobalt, in line
with reduction in the metal ion radius. The manganese
structure is representative of the others, and that one will be
discussed in detail to represent the set.
Catalytic Sulfide Oxidation by Complexes. In 5 mL of dry
acetonitrile containing 0.1 M thioanisole and 0.33 mM complex, 0.2
mL of 30% H2O2 was added to initialize the reaction. The reaction
mixture was stirred in a water bath at 303 K for 6 h, and product
analysis was performed by gas chromatography (GC) using the
internal standard method. A parallel experiment without catalyst was
conducted as a control.
Catalytic HAT by Complexes. In 3 mL of MeOH/H2O (1;1, v/
v) containing 0.05 M 1,4-cyclohexadiene and 1 mM complex, 0.02 mL
of 30% H2O2 was added to initialize the reaction. The resulting
reaction mixture was stirred in a water bath at 303 K for 4 h, and
product analysis was conducted by GC using the internal standard
method. A parallel experiment without catalyst was conducted as a
control.
RESULTS AND DISCUSSION
■
Synthesis. Attaching 2-pyridylmethyl N-pendant arms to
ethylene cross-bridged tetraazamacrocycles would most effi-
ciently be accomplished by reaction of a halomethylpyridyl
moiety with a cyclam−glyoxal bisaminal following the teachings
of Weisman et al.15,42,43 and Handel et al.38 Initial attempts
with picolyl chloride under the typical acetonitrile conditions
for such reactions resulted in unsatisfactory red products where
the color appeared because of reaction of the picolyl group with
itself rather than bisaminal. Literature investigation of the
reactivity of halomethylpyridines and halomethylpyrazines led
to work where these compounds were stabilized by storage in
nonpolar chlorinated solvents.44 Unsuccessful at alkylation of
the bisaminals in typical SN2 solvents like acetonitrile, we
explored the chlorinated solvents and found successful,
although low-yielding, monoalykylation of the bisaminal 1
with picolyl chloride in chloroform at room temperature. It
should be pointed out that others have alkylated cyclam−
glyoxal condensates successfully in chloroform, as well.45
Although we had apparently succeeded in retarding the self-
reaction of picolyl chloride because little of the characteristic
red color was produced and pure product was obtained, the
yields were disappointing at ∼10%. We were systematically able
to raise the yields through activation of the electrophile by the
addition of KI to the reaction, although stoichiometric amounts
were required. Additional increases in the temperature (to
reflux) and time (to 6 days) eventually led to respectable yields
of the monoalkyl salt 2 (66% based on the bisaminal 1). Again,
The metal ion lies in an approximately octahedral pocket,
coordinated by one chloride anion and five nitrogen atoms
from the ligand. The four nitrogen atoms from the macrocycle
coordinate to the metal in an “all-cis” arrangement with the
pyridyl nitrogen of the pendant arm and the chloride also in a
cis arrangement (see Figure 1). This coordination is akin to the
cis-V coordination displayed by tetraazamacrocycles bound to a
metal bearing two chloride anions.1,51 Although the four
nitrogen atoms of the macrocyle bind to the metal with four-
and five-membered chelate rings and N−Mn−N bite angles
around 80° and 88°, respectively, the four-membered chelate
ring involving the pyridine is noticeably strained, with an N−
Mn−N bite angle of 76.79(9)°.
46
the uses of I− and heat45 to increase the reactivity of alkyl
agents with macrocycle glyoxal condensates are known
strategies. Methylation of the nonadjacent nitrogen to produce
3 and NaBH4 ring-opening reduction to produce PyMeEBC
successfully followed Weisman et al.’s typical procedures.15,42,43
Complexation of PyMeEBC was carried out in acetonitrile
(or DMF for acetonitrile-insoluble NiCl2) with anhydrous
D
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(PyMeEBC)Cl]+. They are each 6-coordinate but differ in
the arrangement of the alkyl backbone of the ligand.
Figure 2 shows the crystallographic asymmetric unit, and the
geometry about each metal ion is further illustrated in Figure 3.
Figure 3. Representation of the coordination about the two symmetry-
unique metal ions in [Ni(PyMeEBC)Cl]PF6.
The coordination about the nickel is completed by one chloride
ion and five nitrogen atoms of the ligand. The two symmetry-
unique metal ions display the same basic coordination, but they
are effectively, although not crystallographically or even
parametrically, mirror images. Essentially, the ligand can bind
in right- or left-handed orientations, and each is separately
present in Ni1 and Ni2. The N−Ni−N bite angles for the
pendant pyridyl arm are 81.15(12)° and 81.38(14)° in the two
complexes, suggesting a better fit of the Ni2+ ion to the
macrocycle than for the larger Mn2+.
Figure 1. Asymmetric unit of [Mn(PyMeEBC)Cl]PF6 with atoms
drawn as 50% probability ellipsoids. Atom colors: Mn, pink; Cl, green;
C, gray; N, blue; F, yellow-green.
The second polymorph of [Zn(PyMeEBC)Cl]PF6 contains a
single, octahedrally coordinated Zn2+ ion in the asymmetric
unit. The zinc ion is bound by five nitrogen atoms of the ligand
and one chloride ion, as shown in Figure 4. The coordination
geometry is very similar to that of Ni1 in [Ni(PyMeEBC)Cl]-
PF6 (and hence Zn1 in Zn-1). The center of inversion within
the structure means that the second enantiomer (akin to Ni2)
is generated by symmetry.
[Cu(PyMeEBC)](PF6)2. The ligand is again found to bind to
the metal through each of the five nitrogen atoms, Cu2+ is
formally 5-coordinate, and no chloride is present in the crystal
structure. The geometry about the metal may be described as a
[M(PyMeEBC)Cl]PF6 (M = Ni or Zn). This macrocycle has
also been shown to be a good ligand to smaller metal ions in
the complex [Ni(PyMeEBC)Cl]PF6 and two different
compounds containing the complex ion [Zn(PyMeEBC)Cl]-
PF6, hereafter labeled Zn-1 (from diffusion of Et2O into a
CH3NO2 solution) and Zn-2 (from evaporation of CH3NO2).
The nickel compound is isostructural with Zn-1. Therefore,
only the nickel compound and not Zn-1 is described in detail,
although the structural discussion applies equally for Zn-1.
Although this is centrosymmetric, the asymmetric unit
features two symmetry-unique metal complexes, [Ni-
Figure 2. ORTEP representation of the asymmetric unit of [NiLCl]PF6 with atoms drawn as 50% probability ellipsoids. Selected atoms are labeled.
Atom colors: Ni, deep green; Cl, pale green; C, gray; N, blue; P, orange; F, yellow-green.
E
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such that F12 is near the vacant coordination site at the copper
ion and the Cu1···F12 distance is 3.104(3) Å. This is rather
longer than the sum of the van der Waals radii53 of these two
ions (2.87 Å), and it is rather longer than the shortest such
interactions present in similar systems reported in the
Cambridge Structural Database.54 For a copper ion, coordi-
nated by four or five nitrogen atoms, Cu···F distances of around
2.7 Å are commonly observed, and some much shorter
examples are known; e.g., refcode MINKAV has a Cu···F
distance of 2.472 Å. It may be the case that this genuinely
represents a weak interaction between the Cu2+ ion and the
fluorine, but the fluorine is not very polarizable, which suggests
otherwise. It seems more likely that this close approach
−
facilitates C−H···F interactions between the ligand and PF6
anion, i.e., between F12 and H6B, H13B, and H15B and other
similar interactions involving the fluorine atoms of this anion.
A final comparison (Table 1) of all of the PyMeEBC
structures is of interest because all six divalent metal ions from
Mn−Zn were coordinated and crystallized in very similar
environments. A similar range of metal ions were crystallized in
the parent ligand Me2EBC,1,55,39,51,56 and a similar table,
although less complete, has appeared previously.56 In this
comparison, the size of the metal ion is correlated to the Nax−
M−Nax and Neq−M−Neq bond angles and generally shows a
steady increase in these bond angles as the metal’s ionic radius
decreases and the bridged macrocycle is better able to engulf
the metal ion with less distortion of the expected bond angles.
Interestingly, for the larger ions Mn2+ and Fe2+, the pyridine
pendant arm of PyMeEBC allows coordination with much less
distortion (6−7° larger Nax−M−Nax bond angles) than in their
complexes with Me2EBC. However, this pendent arm effect is
not present in the smaller metal ions, or is at least more difficult
to discern, because the identities of the nonmacrocyclic ligands
are not as directly comparable in some cases.
Figure 4. ORTEP representation of the asymmetric unit in Zn-2,
[Zn(PyMeEBC)Cl]PF6. Atoms are drawn as 50% probability
ellipsoids. Atom colors: Zn, yellow; Cl, pale green; C, gray; N, blue;
P, orange; F, yellow-green.
distorted trigonal bipyramid, with N2 and N4 occupying the
axial positions and N1, N3, and N5 located in a trigonal plane.
The coordination might also be described as a distorted square-
based pyramid (N3 as the peak), which highlights the vacant
coordination site at the metal (see Figure 5). The Addison
parameter52 (τ) attempts to quantify the degree to which a 5-
coordinate structure favors the ideal trigonal-bipyramidal
geometry (τ = 1) or the ideal square-pyramidal geometry (τ
= 0). For this structure, t = 0.48, which indicates that the
structure is nearly perfectly intermediate between these two
ideals, and thus characterization as either extreme is inadequate.
The charge balancing in this structure is afforded by two
PF6− anions, one of which forms a close approach to the metal,
Figure 5. ORTEP representation of the asymmetric unit in [Cu(PyMeEBC)](PF6)2. Atoms are drawn as 50% probability ellipsoids. Atom colors:
Cu, bronze; C, gray; N, blue; P, orange; F, yellow-green.
F
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Table 1. Comparison of N−M−N Bond Angles for First-Row Divalent Metal Ions with Me2EBC and PyMeEBC from Crystal
a
Structures
Me2EBC
PyMeEBC
metal ion
high-spin 6-coordinate ionic radius
Nax−M−Nax
Neq−M−Neq
ref
Nax−M−Nax
Neq−M−Neq
Mn2+
Fe2+
Co2+
Zn2+
Ni2+
Ni2+
Cu2+
97
92
89
88
83
83
158.0(2)
161.88(5)
172.4(2)
75.6(2)
1
164.74(9)
168.27(8)
172.34(12)
78.40(10)
80.82(8)
78.36(5)
81.11(13)
1
51
55
56
56
39
82.88(13)
81.84 (avg)
84.85 (avg)
b
b
171.89(12)
83.90(11)
168.66 (avg)
173.56 (avg)
c
c
175.39(5)
86.16(5)
d
d
174.56(10)
85.07(9)
e
e
e
e
e
79
175.16(13)
85.30(12)
175.66(13)
85.24(12)
a
b
Except where noted, all complexes are high-spin 6-coordinate M(Me2EBC)Cl2 and M(PyMeEBC)Cl+, respectively. Zn(Me2EBC)(OAc)(OH2)+.
c
d
e
Ni(Me2EBC)(OH2)2+. Ni(Me2EBC)(acac)+. Ionic radius and bond angles are for 5-coordinate complexes.
Table 2. Crystallographic Data (L = PyMeEBC)
[MnLCl]PF6
[FeLCl]PF6
[CoLCl]PF6
[NiLCl]PF6
[CuL](PF6)2
[ZnLCl]PF6 sja1_14b [ZnLCl]PF6 sja1_14c
sja7_14 DGJ010B sja3_14 DGJ011 sja4_14 DGJ012 sja5_14 DGJ013 sja2_14 DGJ014
DGJ015 form 1
DGJ015 form 2
chemical
formula
[Mn(C19H33 N5)
Cl]PF6
[Fe(C19H33 N5) [Co(C19H33 N5) [Ni(C19H33 N5) [Cu(C19H33 N5)] [Zn(C19H33 N5)Cl]
[Zn(C19H33 N5)Cl]
PF6
Cl]PF6
18.5989(9)
8.6434(6)
14.8745(7)
90
Cl]PF6
18.6711(13)
8.6259(10)
14.8088(10)
90
Cl]PF6
22.758(2)
13.730(2)
15.0185(16)
90
(PF6)2
10.5546(6)
13.2043(11)
18.6149(10)
90
PF6
a (Å)
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Ζ
18.5536(10)
8.6768(4)
14.9938(6)
90
22.794(3)
13.7535(9)
15.1489(15)
90
8.1050(6)
22.7534(14)
12.5612(10)
90
90
90
90
93.047(8)
90
98.149(5)
90
93.512(9)
90
97.594(6)
90
90
90
90
2413.8(2)
4
2391.2(2)
4
2385.0(4)
4
4686.0(10)
8
2568.1(3)
4
4740.3(8)
8
2296.2(3)
4
fw
566.86
Pca21
567.77
Pca21
570.85
Pca21
570.62
684.98
577.29
P21/c
577.29
P21/c
space
group
P21/c
P21/c
T (°C)
λ (Å)
150(2)
0.71073
1.560
150(2)
0.71073
1.577
150(2)
0.71073
1.590
150(2)
0.71073
1.618
150(2)
0.71073
1.772
150(2)
0.71073
1.618
150(2)
0.71073
1.670
Dcalcd
(g cm−3
)
μ (mm−1
)
0.788
0.874
0.963
1.076
1.083
1.282
1.323
a
2
R1(Fo )
0.0337
0.0263
0.0551
0.0329
0.0637
0.0452
0.1062
0.0476
0.1265
0.0504
0.1185
0.0553
0.1451
a
2
wR2 (Fo ) 0.0848
a
2
2
R1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = {∑[w(Fo − Fc2)2]/∑[w(Fo )2]}1/2
.
Crystallographic details for the seven new crystal structures
in this work, along with selected bond lengths and angles, are
presented in Tables 2 and 3.
Electronic Structure. Solid-state magnetic moments were
determined for all five paramagnetic PyMeEBC complexes.
ion-based absorbances of the other complexes. Zn(PyMeEBC)-
Cl+ had two peaks at 317 nm (ε = 220 M−1 cm−1) and 264 nm
(ε = 4120 M−1 cm−1). The high-spin Mn2+ and Fe2+ complexes
showed no d−d bands, which is typical for ligands of this type,
and the same behavior is observed for Mn(Me2EBC)Cl2 and
Fe(Me2EBC)Cl2.1 Fe(Me2EBC)Cl2 has a weak shoulder at 350
nm (ε = 260 M−1 cm−1) that is not present in the manganese
complex. Likewise, there is an additional absorbance for
Fe(PyMeEBC)Cl+ at 403 nm (ε = 225 M−1 cm−1). This
band is likely associated with the chloro ligand because the
presence of a similar band in Fe(Me2EBC)Cl2 is not dependent
on the pyridine.
[Mn(PyMeEBC)Cl]PF6 gave a magnetic moment of μeff
=
5.47, which is slightly lower than the expected value (5.65−
6.10)57 but clearly indicative of a high-spin Mn2+ d5 ion. The
other paramagnetic complexes had magnetic moments within
the expected ranges for high-spin divalent metal ions:
[Fe(PyMeEBC)Cl]PF6, μeff = 5.28; [Co(PyMeEBC)Cl]PF6,
μeff = 4.61; [Ni(PyMeEBC)Cl]PF6, μeff = 2.91; [Cu-
(PyMeEBC)][PF6]2, μeff = 1.79. This high-spin behavior is
similar to that of the divalent first-row transition-metal
complexes of Me2EBC.1,39,51,56 Apparently, the addition of
the pyridine donor, in place of a typically chloro ligand or
oxygen donor in the Me2EBC complexes, does not cause a
change from high spin to low spin, even though pyridine is
typically a stronger-field ligand.
The electronic spectrum of the cobalt complex is similar to
those of other high-spin octahedral cobalt(II) complexes,
having a single major absorption typically between about 500
4
4
and 600 nm. This band is due to the T1g(P) → T1g(D)
transition.58 The Co(PyMeEBC)Cl+ absorbance is at 499 nm
(ε = 34 M−1 cm−1), which is similar to the Co(Me2EBC)Cl2
complex, which absorbs at 540 nm (ε = 24 M−1 cm−1).51
Ni(PyMeEBC)Cl+ exhibits a classic octahedral Ni2+
spectrum with three major absorbances between 300 and
1100 nm in acetonitrile.58 This spectrum allows us to
determine the ligand field strength, which is taken as the
Table 4 contains absorbances and extinction coefficients for
the six complexes of PyMeEBC. The electronic spectra of the
zinc complex, with only ligand-based absorbances likely, was
obtained to help differentiate these absorbances from the metal-
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Table 3. Selected Bond Lengths [Å] and Angles [deg]
[Mn(PyMeEBC)Cl]PF6
Mn1−N1
Mn1−N3
Mn1−N4
Mn1−N2
Mn1−N5
Mn1−Cl1
2.256(2)
2.257(3)
2.305(3)
2.299(3)
2.307(3)
2.4330(9)
N1−Mn1−N3
N1−Mn1−N2
N3−Mn1−N2
N1−Mn1−N4
N3−Mn1−N4
163.43(11)
N2−Mn1−N4
N1−Mn1−N5
N3−Mn1−N5
N2−Mn1−N5
N4−Mn1−N5
164.74(9)
90.39(10)
78.40(10)
78.91(9)
88.01(9)
N1−Mn1−Cl1
N3−Mn1−Cl1
N2−Mn1−Cl1
N4−Mn1−Cl1
N5−Mn1−Cl1
89.44(7)
76.79(9)
88.99(10)
111.45(9)
80.63(10)
99.53(7)
91.53(7)
101.15(7)
170.22(7)
[Fe(PyMeEBC)Cl]PF6
Fe1−N3
Fe1−N1
Fe1−N5
Fe1−N2
Fe1−N4
Fe1−Cl1
2.204(2)
2.215(2)
2.232(2)
2.246(2)
2.285(2)
2.3805(7)
N3−Fe1−N1
N3−Fe1−N5
N1−Fe1−N5
N3−Fe1−N2
N1−Fe1−N2
167.98(9)
N5−Fe1−N2
N3−Fe1−N4
N1−Fe1−N4
N5−Fe1−N4
N2−Fe1−N4
80.29(8)
80.31(9)
108.49(8)
89.58(8)
168.27(8)
N3−Fe1−Cl1
N1−Fe1−Cl1
N5−Fe1−Cl1
N2−Fe1−Cl1
N4−Fe1−Cl1
98.15(6)
88.72(6)
171.26(6)
91.10(6)
98.83(6)
80.82(8)
90.89(8)
92.17(9)
77.78(8)
[Co(PyMeEBC)Cl]PF6
Co1−N3
Co1−N5
Co1−N1
Co1−N2
Co1−N4
Co1−Cl1
2.157(3)
2.175(3)
2.177(3)
2.206(3)
2.263(3)
2.3989(12)
N3−Co1−N5
N3−Co1−N1
N5−Co1−N1
N3−Co1−N2
N5−Co1−N2
82.88(13)
N1−Co1−N2
N3−Co1−N4
N5−Co1−N4
N1−Co1−N4
N2−Co1−N4
78.78(12)
81.55(13)
91.59(12)
105.68(12)
172.34(12)
N3−Co1−Cl1
N5−Co1−Cl1
N1−Co1−Cl1
N2−Co1−Cl1
N4−Co1−Cl1
95.69(10)
171.36(9)
87.64(9)
89.71(9)
96.63(9)
171.69(14)
92.69(13)
93.59(13)
81.90(12)
[Cu(PyMeEBC)](PF6)2
Cu1−N2
Cu1−N4
Cu1−N1
Cu1−N5
Cu1−N3
2.046(3)
2.079(3)
2.125(3)
2.135(3)
2.136(3)
N2−Cu1−N4
N2−Cu1−N1
N4−Cu1−N1
N2−Cu1−N5
175.66(13)
N4−Cu1−N5
N1−Cu1−N5
N2−Cu1−N3
91.27(13)
146.87(12)
91.64(12)
N4−Cu1−N3
N1−Cu1−N3
N5−Cu1−N3
86.85(12)
123.66(12)
85.24(12)
79.27(12)
104.95(12)
84.54(13)
[Ni(PyMeEBC)Cl]PF6
Ni1−N3
Ni1−N5
Ni1−N1
Ni1−N2
Ni1−N4
Ni1−Cl1
Ni2−N13
Ni2−N15
Ni2−N12
Ni2−N11
Ni2−N14
Ni2−Cl2
2.101(3)
2.127(3)
2.129(3)
2.153(3)
2.200(3)
2.4310(9)
2.095(3)
2.122(3)
2.125(3)
2.135(3)
2.223(3)
2.3923(9)
N3−Ni1−N5
N3−Ni1−N1
N5−Ni1−N1
N3−Ni1−N2
N5−Ni1−N2
N1−Ni1−N2
N3−Ni1−N4
N5−Ni1−N4
N1−Ni1−N4
N2−Ni1−N4
84.75(11)
N3−Ni1−Cl1
N5−Ni1−Cl1
N1−Ni1−Cl1
N2−Ni1−Cl1
N4−Ni1−Cl1
N13−Ni2−N15
N13−Ni2−N12
N15−Ni2−N12
N13−Ni2−N11
N15−Ni2−N11
94.18(8)
174.34(9)
87.00(8)
90.10(9)
95.16(8)
84.95(11)
93.68(14)
84.49(12)
174.96(13)
93.49(11)
N12−Ni2−N11
N13−Ni2−N14
N15−Ni2−N14
N12−Ni2−N14
N11−Ni2−N14
N13−Ni2−Cl2
N15−Ni2−Cl2
N12−Ni2−Cl2
N11−Ni2−Cl2
N14−Ni2−Cl2
81.39(14)
82.95(12)
89.46(11)
173.32(11)
101.85(12)
94.79(8)
172.93(12)
93.39(12)
91.87(12)
84.38(12)
81.15(12)
84.48(12)
90.28(12)
102.37(12)
173.80(11)
176.45(8)
91.99(9)
86.47(8)
94.02(8)
[Zn(PyMeEBC)Cl]PF6 Form 1
Zn1−N3
Zn1−N1
Zn1−N4
Zn1−N5
Zn1−N2
Zn1−Cl1
Zn2−N13
Zn2−N11
Zn2−N12
Zn2−N15
Zn2−N14
Zn2−Cl2
2.157(4)
2.182(4)
2.221(4)
2.222(4)
2.225(4)
2.4123(14)
2.146(4)
2.196(4)
2.210(4)
2.222(3)
2.229(4)
2.3818(12)
N3−Zn1−N1
N3−Zn1−N4
N1−Zn1−N4
N3−Zn1−N5
N1−Zn1−N5
N4−Zn1−N5
N3−Zn1−N2
N1−Zn1−N2
N4−Zn1−N2
N5−Zn1−N2
168.60(15)
83.94(15)
105.60(15)
81.59(14)
92.25(15)
88.59(15)
90.17(15)
79.40(14)
169.23(15)
81.62(15)
N3−Zn1−Cl1
N1−Zn1−Cl1
N4−Zn1−Cl1
N5−Zn1−Cl1
N2−Zn1−Cl1
N13−Zn2−N11
N13−Zn2−N12
N11−Zn2−N12
N13−Zn2−N15
N11−Zn2−N15
96.48(11)
88.61(11)
97.19(12)
173.70(11)
92.42(12)
170.23(15)
92.06(16)
79.30(15)
82.17(14)
91.96(14)
N12−Zn2−N15
N13−Zn2−N14
N11−Zn2−N14
N12−Zn2−N14
N15−Zn2−N14
N13−Zn2−Cl2
N11−Zn2−Cl2
N12−Zn2−Cl2
N15−Zn2−Cl2
N14−Zn2−Cl2
81.55(14)
83.38(14)
104.37(14)
169.59(14)
88.55(13)
96.97(10)
88.12(10)
93.06(11)
174.49(10)
96.77(10)
[Zn(PyMeEBC)Cl]PF6 Form 2
Zn1−N3
Zn1−N4
Zn1−N5
Zn1−N2
Zn1−N1
Zn1−Cl1
2.163(3)
2.256(3)
2.249(3)
2.210(3)
2.183(3)
2.4043(10)
N3−Zn1−N1
N3−Zn1−N2
N1−Zn1−N2
N3−Zn1−N5
N1−Zn1−N5
167.98(12)
89.81(11)
79.94(11)
81.76(11)
90.42(10)
N2−Zn1−N5
N3−Zn1−N4
N1−Zn1−N4
N2−Zn1−N4
N5−Zn1−N4
81.26(11)
83.32(11)
105.49(11)
167.17(11)
87.02(11)
N3−Zn1−Cl1
N1−Zn1−Cl1
N2−Zn1−Cl1
N5−Zn1−Cl1
N4−Zn1−Cl1
97.38(8)
89.60(8)
93.77(8)
174.94(8)
97.84(8)
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Table 4. Electronic Spectra of M(PyMeEBC) Complexes in
Acetonitrile
Table 5. Redox Potentials (vs SHE) with Peak Separations
for PyMeEBC and Me2EBC Complexes
a
ligand-based and charge-transfer d−d bands, nm (ε,
redox
process
peak separation
(mV)
complex
bands, nm (ε, M−1 cm−1
)
M−1 cm−1
)
complex
potential (V)
ref
Zn(PyMeEBC)Cl+
317 (220), 264 (4120)
322 (266), 263 (4460)
Mn(PyMeEBC)
Mn+/Mn2+
E1/2 = −0.526
246
Cl+
Mn(PyMeEBC)
Cl+
Mn2+/
Mn3+
E1/2 = +0.812
E1/2 = +1.744
E1/2 = +0.585
E1/2 = +1.343
74
Fe(PyMeEBC)Cl+
403 (225), 317 (230), 263
(4550)
Mn3+/
286
61
Mn4+
Co(PyMeEBC)Cl+
Ni(PyMeEBC)Cl+
323 (650), 259 (8150)
310 (177), 264 (3730)
499 (34)
Mn(Me2EBC)Cl2
Mn2+/
Mn3+
1
1
891 (13), 551sh
(19), 422sh (42)
Mn3+/
65
Cu(PyMeEBC)2+
286 (5240), 264 (7430)
639 (101), 1042
(42)
Mn4+
Fe(PyMeEBC)Cl+ Fe+/Fe2+
E1/2 = −1.969
E1/2 = +0.563
E1/2 = +0.110
E1/2 = −1.386
Eox = −1.200
126
82
Fe2+/Fe3+
Fe2+/Fe3+
Fe(Me2EBC)Cl2
Co(PyMeEBC)Cl+ Co+/Co2+
Co+
Co2+
Co2+/Co3+
Co2+
Co+
Co2+/Co3+
63
1
energy of the lowest-energy absorption band.59 In this case, the
absorption at 891 nm (ε = 13 M−1 cm−1) converts to Δ0 =
11223 cm−1. In comparison, Ni(Me2EBC)Cl2 gave Δ0 = 10215
cm−1 from a similar calculation. Clearly, the pendant pyridine
donor increases the ligand field strength, as might be expected
for replacement of one chloro ligand by a pyridine. Yet, as
shown by the magnetic moment data above, all of the
complexes remain high spin.
143
→
E1/2 = +0.657
Ered = −2.198
154
103
Co(Me2EBC)Cl2
→
51
51
E1/2 = +0.173
Ered = −1.026
Ni(PyMeEBC)Cl+ Ni2+
Ni+
→
The Cu(PyMeEBC)2+ complex exhibits the expected Cu2+
d9 d−d band at 639 nm (ε = 101 M−1 cm−1). This compares to
the analogous absorption at 671 nm (ε = 100 M−1 cm−1) for
Cu(Me2EBC)Cl+.39 Interestingly, Cu(PyMeEBC)2+ has an
additional broad peak stretching from 830 to 1200 nm, with its
maximum at 1042 nm (ε = 42 M−1 cm−1), which is not
normally observed for this ion. Cu2+ complexes typically have
only a single absorbance in the visible region representing up to
three different unresolved transitions.58 However, trigonal and
tetragonal distortions can lead to one higher-intensity band in
the visible range and one lower-intensity band in the near-IR.58
The exact nature of the distortion is difficult to assign based on
the spectrum alone. Fortunately, the obtained X-ray crystal
structure confirms a tetragonally distorted 5-coordinate
structure for Cu(PyMeEBC)2+ that may give rise to its
electronic spectrum.
Ni2+
→
Eox = +1.290
Ered = −1.894
Ni3+
Ni(Me2EBC)Cl2
Ni2+
→
56
56
Ni+
Ni2+/Ni3+
Cu+/Cu2+
E1/2 = +0.991
E1/2 = −0.402
Ered = −0.544
154
80
Cu(PyMeEBC)2+
Cu(Me2EBC)Cl+
Cu2+
Cu+
→
60
60
Cu2+
→
Eox= +1.530
Cu3+
a
If no reference is listed, the data are from this work.
Electrochemical Studies. In selecting PyMeEBC as a
target for the development of oxidation catalysts based on
Me2EBC, we expected the addition of the pyridine donor to
lead to changes in the electrochemistry of the resulting
complexes. Table 5 shows the potential and peak separation
for the Mn−Cu complexes of both of these ligands. Figure 6
shows the cyclic voltammograms of all PyMeEBC complexes.
Three main trends can be observed in examining this data. The
two manganese complexes, Mn(PyMeEBC)Cl+ and Mn-
(Me2EBC)Cl2, will be used to illustrate the trends, which
generally hold for all five metals (Mn−Cu) examined.
(1) Oxidation is significantly more positive for the
PyMeEBC complexes. The Mn2+/Mn3+ redox couple is at
+0.812 V for the PyMeEBC complex, while this value is +0.585
V for the Me2EBC complex. Similarly, the Mn3+/Mn4+ couples
are at +1.744 and +1.343 V for PyMeEBC and Me2EBC,
respectively. A simple explanation for this behavior is the
replacement of a negatively charged chloro ligand with a neutral
pyridine donor in PyMeEBC. The two negatively charged
chloro ligands in the Me2EBC complexes clearly favor
oxidation compared to the single chloro and pyridine donor
in PyMeEBC. Access to stable species at higher oxidation
potentials may allow oxidation processes with more difficult-to-
Figure 6. Cyclic voltammograms for Cu(PyMeEBC)2+, Ni-
(PyMeEBC)Cl+, Co(PyMeEBC)Cl+, Fe(PyMeEBC)Cl+, and Mn-
(PyMeEBC)Cl+.
oxidize substrates if this property is present in catalytically
active species of the PyMeEBC complexes.
(2) Reduction is significantly easier for the PyMeEBC
complexes. One less negatively charged chloro ligand allows
reversible reduction to occur in acetonitrile for the manganese
complex of PyMeEBC (at −0.526 V), whereas this reduction is
not observed at all for the manganese complex of Me2EBC.
(3) A general trend toward reversible access to a larger range
of oxidation states is observed in most cases for the PyMeEBC
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Table 6. Oxidation Catalysis Screening Results for MnII, FeII, and CuII Complexes of PyMeEBC in Comparison to
Mn(Me2EBC)Cl2
ligand (see Figure 6). Reversible access to Mn+/Mn2+/Mn3+/
Mn4+, Fe+/Fe2+/Fe3+, Co+/Co2+/Co3+, and Cu+/Cu2+ is
observed for the PyMeEBC complexes, whereas reversible
access for Me2EBC complexes is restricted to Mn2+/Mn3+/
Mn4+, Fe2+/Fe3+, Co2+/Co3+, and only Cu2+. Catalytic processes
would require reversible access to multiple oxidation states,
although the utility for oxidation catalysis of reversibly accessing
the M+ oxidation states in the PyMeEBC complexes is not
obvious.
differences clearly have electronic/electrochemical rather than
structural origins. The higher oxidation potential of the Mn2+/3+
couple compared to the Fe2+/3+ couple and access to Mn4+
while Fe4+ is not available, as discussed in the Electrochemistry
section, likely explains the more efficient HAT.
Compared with the high conversion, the relatively low yield
of benzene with the MnII complex PyMeEBC is possibly
related to its OAT activity, which leads to the formation of
epoxide (vide infra), but the products were not identified in GC
analysis. In comparison, the MnII complex with the Me2EBC
ligand is much more active and gives 71.4% yield of benzene
with 86.2% conversion. A multitude of other synthetic
manganese complexes have shown excellent catalytic HAT
ability, so this is not unique or unexpected. However, it is
reassuring that the addition of a fifth donor atom to the cross-
bridged cyclam chelate has not eliminated this reactivity for
Mn(PyMeEBC), and future studies will aim at elucidating the
mechanism of HAT by Mn(PyMeEBC).
Interestingly, neither oxidation to Ni3+ nor reduction to Ni+
is reversible for Ni(PyMeEBC)Cl+, even though the Ni2+/Ni3+
redox couple is reversible for Ni(Me2EBC)Cl2. Perhaps two
chloro ligands are required to stabilize the Ni3+ ion, while only
one is present in Ni(PyMeEBC)Cl+. Similarly, the lack of even
one chloro ligand in Cu(PyMeEBC)2+ eliminates the oxidation
to Cu3+ altogether, even though a nonreversible oxidation to
Cu3+ is observed (+1.530 V) for Cu(Me2EBC)Cl+. However,
the Cu+/Cu2+ redox process is reversible for Cu(PyMeEBC)2+,
although nonreversible for Cu(Me2EBC)Cl+. In the latter
complex, loss of the chloro ligand upon reduction to Cu+ is
reasonable and is supported by a related 4-coordinate copper(I)
complex crystal structure.61 Loss of the fifth (pyridine) donor is
not likely for Cu(PyMeEBC)2+ because the pyridine donor is
neutral and, probably more important, because it is covalently
bound to the rest of the ligand.
Catalytic Oxidation Screening. Screening for the
oxidation reactivity of the biologically relevant manganese,
iron, and copper complexes in catalytic processes may not only
help to understand the behaviors of related metalloenzymes but
also guide the exploration of these complexes’ potential
applications. In the electrochemical studies above, it has been
observed that PyMeEBC-ligated metal ions demonstrate
reversible redox behaviors, which provides the basis to explore
their catalytic activity in oxidations.
The CuII complex of PyMeEBC is very sluggish as a catalyst
for HAT: the yield of benzene is close to its natural content in
commercial 1,4-cyclohexadiene (∼3%). The lack of reactivity of
the copper complex is most logically explained not by its
structure, which is, in fact, different from the isostructural iron
and manganese complexes yet retains the entire 5-coordinate
PyMeEBC ligand, but rather its electrochemistry, which lacks
any oxidation states above Cu2+ and suggests that this is just
not reactive enough to result in substrate oxidation.
OAT. For OAT screening, thioanisole was selected as the
substrate and acetonitrile was employed as the solvent with
H2O2 oxidant. After reaction at 303 K for 6 h, the MnII complex
PyMeEBC gives 53.9% yield of sulfoxide with 14.1% of sulfone,
while the conversion is 75.3%. The FeII complex gives 30.1% of
sulfoxide and 12.5% of sulfone with 47.2% conversion. Similar
to the HAT experiment, the CuII complex is inactive for sulfide
oxidation, with results comparable to those from the control
experiment without catalyst.
HAT. The catalytic oxidation properties of the Mn2+, Fe2+,
and Cu2+ complexes were first investigated in HAT (Table 6),
which is the most fundamental process in redox chemistry.62
Using 1,4-cyclohexadiene as the substrate with H2O2 oxidant,
after reaction at 303 K for 4 h in acetonitrile, the MnII complex
of PyMeEBC produces 51.2% yield of benzene with 88.0%
conversion, while the corresponding FeII complex yields only
33.3% of benzene with 55.3% conversion. Because these two
complexes are isostructural (vide supra), these functional
Again, Mn(Me2EBC) is more active in sulfide oxidation,
yielding 44.3% of sulfoxide and 46.5% of sulfone with almost
complete conversion of sulfide. In comparison, Mn-
(PyMeEBC) only produces 14.1% of sulfone, with most of
its product (53.9%) the sulfoxide and only 75.3% conversion. It
appears that the addition of the 2-pyridylmethyl pendant arm
lowered the overall reactivity (less conversion and less sulfone)
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Table 7. Half-lives of Selected CuII Complexes in 5 M HCl
but improved the selectivity for the sulfoxide product. As
shown in Table 5, the redox potential of the Mn3+/Mn4+ couple
for Mn(Me2EBC)Cl2 is substantially lower than that for
Mn(PyMeEBC)Cl+ (+1.343 vs +1.744 V). Also, the peak
separation shows that Mn(Me2EBC)Cl2 is electrochemically
reversible, while the reversibility is not so good for Mn-
(PyMeEBC)Cl+. Clearly, oxidizing Mn(PyMeEBC)Cl+ to the
corresponding MnIV complex is not as easy as Mn(Me2EBC)-
Cl2, where the MnIV complex can be obtained in large scale, and
this may explain its relatively poor efficiency in sulfide
oxygenation in which the MnIV species may play a significant
role.12
In terms of utility, this selectivity for sulfoxide production
may be advantageous. Organic sulfoxides are useful syntheti-
cally for the production of pharmaceuticals and other valuable
chemical compounds.63,64 A number of synthetic manganese
catalysts have been developed that can efficiently transform
sulfides into sulfoxides and/or sulfones.64−69 The closest
literature analogues to Mn(Me2EBC) and Mn(PyMeEBC)
are probably the manganese complexes of 1,4,7-trimethyl-1,4,7-
triazacyclononane (TMTACN),67−69 which generally demon-
strate a more efficient sulfide oxygenation activity. In this
catalyst, a manganese(V) oxo intermediate was proposed to
serve as the oxygenation species by a concerted OAT or
electron-transfer mechanism, leading to sulfoxide and then
sulfone products in two oxidation steps. The Mn(TMTACN)
catalysts were developed as bleach catalysts for the laundry
industry and were actually introduced into consumer products.
Yet, their lack of selectivity in oxidation reactivity contributed
to their removal from those consumer products because of their
damage to cloth.69 Although Mn(PyMeEBC) does not appear
to be as highly efficient as some of these other catalysts based
on the above initial screening data, we look forward to future
studies aimed at fully understanding its oxidation mechanisms
and attempting to optimize its sulfoxidation selectivity.
Although these catalytic oxidations are very preliminary, with
further mechanistic elucidation the subject of future work, they
have provided useful information regarding the potential
applications of these metal complexes with the PyMeEBC
ligand. Another potential application with selective mild
oxidation, in addition to the formation of sulfoxides rather
than sulfones, is the laundry industry. Here, a good catalyst
should be efficient enough to oxygenate the soil for its removal
from clothes but must be sluggish enough in HAT to avoid
damage to cotton or other textiles.16−20 The MnII complex with
Me2EBC has demonstrated its ability to serve as a redox
catalyst in detergent but has not yet been embraced by the
industry, perhaps because it is too reactive.16−20 Here, the MnII
complex with PyMeEBC seems to be even more attractive as a
detergent catalyst because it has an oxygenation efficiency
comparable to that of the Mn(Me2EBC)2+ complex, while its
HAT power is desirably more sluggish. More detailed studies
on this issue are still in progress to explore the potentially
useful redox chemistry of PyMeEBC complexes.
Kinetic Stability. A particularly useful property of cross-
bridged tetraazamacroycle complexes, because of their topo-
logical complexity and rigidity caused by their small size, is their
typically extreme stability under harsh conditions.1−4 As
established by Busch et al.1,70 and standardized by Weisman
et al.,71−74 probing the stability of new cross-bridged ligands is
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now routinely done using the CuII complex, which is often the
first (or only) transition-metal complex synthesized because of
the interest in positron emission tomography (PET) imaging
applications of 64Cu. Weisman et al. have chosen 5 M HCl and
various temperature points, such as 30, 50, and 90 °C, as
benchmarks for the kinetic stability of new copper cross-
bridged complexes. Unfortunately, the proton-sponge nature of
these ligands prevents aqueous titration in the presence of a
metal ion to yield formation constants because the ligand never
gives up its last proton(s) and the complex is not formed.
Instead, kinetic decomplexation studies in the presence of high
acid concentration under pseudo-first-order conditions are used
to give some measure of complex stability. Currently, only CuII
complexes have undergone these studies and yielded published
data for comparison.
Figure 7. Cross-bridged cyclen ligands discussed.
M HClO4 with a half-life of 30 h.60 No crystal structure of the
Cu(CRpy2) ligand was obtained, so strain arguments cannot be
used in this case. Why one pyridine pendant arm destabilizes a
given copper complex while two pyridine pendant arms
stabilize another is a compelling problem.
Even though its copper complex is destabilized toward strong
acid decomplexation, the CuII, FeII, and MnII complexes of
PyMeEBC gave no visual signs of decomplexation during the
oxidation catalysis screening reactions discussed above.
Manganese and iron complexes typically give insoluble oxide
precipitates under oxidizing conditions, such as the H2O2
conditions used in the oxidation reactions tested. No
precipitates were observed during these reactions, qualitatively
indicating sufficient stability under the conditions used.
Additional stability experiments for these complexes will
accompany the mechanistic experiments planned to fully
understand their redox catalysis.
Although Cu(Me2EBC)Cl+ was the first cross-bridged
complex to undergo this sort of experiment,50 the 1 M
HClO4 and 40 °C conditions used make comparison to
Weisman’s database of compounds difficult. We have, therefore,
resynthesized this complex and tested it, as well as Cu-
(PyMeEBC)2+, in order to determine the effects of adding the
2-pyridylmethyl pendant arm to the ethylene cross-bridged
ligand. The results of the decomplexation studies are presented
in Table 7, along with relevant literature data for comparison.
According to the teachings of coordination chemistry,75 if we
add additional chelate rings, a polydentate ligand typically
forms more stable transition-metal complexes, as long as the
metal−ligand match in size, geometry, and electronics is not
perturbed. For example, Weisman et al.’s bis(carboxylate)
pendant-armed cross-bridged ligand CB-TE2A (Table 7) gains
up to 4 orders of magnitude in stability versus the H2EBC
complex, as seen in Table 7. In contrast, the stability of the CuII
complex decreases, from a 79 min half-life for tetradentate
Me2EBC at 90 °C in 5 M HCl to less than 2 min for
pentadentate PyMeEBC, even though an additional chelate
ring involving the pyridine pendant arm has been added to the
structure. A similar, and more easily measurable, decrease in the
stability was observed at 50 °C from 7.3 days to 14.7 min.
This appears to be the first example of subtraction of stability
upon the addition of chelating groups to a cross-bridged
tetraazamacrocycle parent structure. Perhaps the strain noted
for the four-membered chelate rings involving the pyridine
donors [N2−Cu1−N1 = 79.27(12)°] in the discussion of the
X-ray crystal structures above reveals that the geometry match
between metal and ligand is disrupted by the pyridine pendant
arm, making dissociation easier. Weisman’s Cu(CB-TE2A) can
actually protonate one of the acetate pendant arms without
dissociating either chelate arm, likely a key to its acid stability.
Perhaps telling is that the chelate bond angle for the O−Cu−N
chelate ring of the acetate pendant arm that does not protonate
is much less strained at 84.03(7)°, according to a crystal
structure of the monoprotonated complex.71 Whatever the
mechanism, the addition of the pyridine pendant arm clearly
destabilizes its CuII complex toward acid decomplexation
compared to other cross-bridged cyclam ligands, and even the
unbridged ligand TETA is more stable toward decomplexation
under the same conditions (Table 7).
CONCLUSIONS
■
A new pyridylmethyl N-pendant-armed cross-bridged cyclam
ligand, PyMeEBC, has been synthesized with a key synthetic
step using nonpolar chloroform as the solvent to reduce the
self-reactivity of picolyl chloride in the presence of the cyclam−
glyoxal nucleophile. Divalent manganese, iron, cobalt, nickel,
copper, and zinc complexes were synthesized and structurally
characterized by X-ray crystallography. PyMeEBC binds each
metal ion in a cis-V configuration of the cyclam ring, with the
chelated pyridine nitrogen bound to each metal ion and a
chloro ligand occupying the sixth coordination site in all but the
5-coordinate Cu2+ complex. Solid-state magnetic moment and
acetonitrile solution electronic spectroscopy experiments
revealed high-spin, octahedral, divalent metal complexes in all
cases, again with the exception of the 5-coordinate Cu2+
complex. Electrochemical studies under an inert atmosphere
and in acetonitrile revealed reversible access to multiple
oxidation states, a prerequisite for successful oxidation catalysis,
in most cases. In particular, Mn(PyMeEBC)Cl+ was stabilized
in Mn+/Mn2+/Mn3+/Mn4+ oxidation states. Finally, preliminary
screens for oxidation catalysis using H2O2 as the oxidant were
carried out on the biomimetically important manganese, iron,
and copper complexes and showed promising results,
particularly for Mn(PyMeEBC)Cl+, in the oxidation of
thianisole and HAT of 1,4-cyclohexadiene. Importantly,
Mn(PyMeEBC)Cl+ demonstrates powerful, yet selective
oxidation reactivity that may lead to improved applications
where the Mn(Me2EBC)Cl2 catalyst may be too reactive, such
as in laundry bleaching. Surprisingly, the CuII complex is
actually destabilized toward acid decomplexation by the
addition of the pyridine pendant arm compared to the parent
Me2EBC complex, which may be a result of a strained
structure. Future work will include expanding the range of
oxidation reactions possible with this catalyst and determi-
nation of its oxidation catalysis mechanisms.
Interestingly, the Cu2+ complex of a bis(2-pyridylmethyl)
pendant-armed cross-bridged cyclen (CRpy2; Figure 7) ligand
is, in fact, stabilized toward acid decomplexation compared to
its dimethyl analogue (Me2Bcyclen).36 In this case, CRpy2 is
lost at 25 °C in 3 M HClO4 with an estimated half-life of 7
days, while the dimethyl ligand is lost from Cu2+ at 25 °C in 1
L
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(17) Busch, D. H.; Collinson, S. R.; Hubin, T. J.; Perkins, C. M.;
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(22) Yin, G.; Danby, A. M.; Day, V.; Roy, S. B.; Carter, J.; Scheper,
W. M.; Busch, D. H. J. Coord. Chem. 2011, 64, 4−17.
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ASSOCIATED CONTENT
■
S
* Supporting Information
X-ray crystallographic data in CIF format and detailed
crystallographic data. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: T.Prior@hull.ac.uk.
*E-mail: gyin@hust.edu.cn.
*E-mail: tim.hubin@swosu.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
T.J.H. acknowledges Southwestern Oklahoma State University
for internal funding through a Proposal Development Award;
the donors of the American Chemical Society Petroleum
Research Fund, Health Research award for Project HR13-157,
from the Oklahoma Center for the Advancement of Science
and Technology, and Grant P20RR016478 from the National
Center for Research Resources, a component of the National
Institutes of Health, for partial support of this research; and also
the Henry Dreyfus Teacher−Scholar Awards Program for
support of this work. G.Y. acknowledges support from the
National Natural Science Foundation of China (Grant
21273086).
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