Synthesis, structural studies, kinetic stability, and oxidation catalysis of the late first row transition metal complexes of 4,10-dimethyl-1,4,7,10-tetraazabicyclo[6.5.2]pentadecane

Article abstract of DOI:10.1039/c5dt00742a

Synthetic details for 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.5.2]pentadecane, the dimethyl ethylene cross-bridged homocyclen ligand are presented for the first time. Its novel Mn²⁺, Fe²⁺, Mn³⁺, and Fe³⁺ complexes have been synthesized and characterized. X-ray crystal structures were obtained for both manganese complexes, along with five additional Co³⁺, Cu²⁺, and Zn²⁺ structures, the first structural characterization of complexes of this ligand. Each complex has the cis-V configuration of the cross-bridged macrocycle ring, leaving cis labile binding sites for interaction of the complex with oxidants and/or substrates. The copper(II) complex kinetic stability in 5 M HCl and at elevated temperatures was determined and compared to related complexes in the literature. The electronic properties of the manganese and iron complexes were evaluated using solid state magnetic moment determination and acetonitrile solution electronic spectroscopy, revealing high spin metal complexes in all cases. Cyclic voltammetry in acetonitrile of the divalent iron and manganese complexes revealed reversible redox processes, suggesting catalytic reactivity involving electron transfer processes are possible for both complexes. Screening of the Mn²⁺ and Fe²⁺ complexes for oxidation catalysis using hydrogen peroxide as the terminal oxidant showed both complexes are worthy of continued development.

Full text of DOI:10.1039/c5dt00742a

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Synthesis, structural studies, kinetic stability, and
oxidation catalysis of the late first row transition
metal complexes of 4,10-dimethyl-1,4,7,10-
tetraazabicyclo[6.5.2]pentadecane†
Cite this: DOI: 10.1039/c5dt00742a
Dallas L. Matz,a Donald G. Jones,a Kimberly D. Roewe,a Michael-Joseph Gorbet,a
Zhan Zhang,b Zhuqi Chen,b Timothy J. Prior,c Stephen J. Archibald,c
Guochuan Yin*b and Timothy J. Hubin*a
Synthetic details for 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.5.2]pentadecane, the dimethyl ethylene
cross-bridged homocyclen ligand are presented for the first time. Its novel Mn2+, Fe2+, Mn3+, and Fe3+
complexes have been synthesized and characterized. X-ray crystal structures were obtained for both
manganese complexes, along with five additional Co3+, Cu2+, and Zn2+ structures, the first structural
characterization of complexes of this ligand. Each complex has the cis-V configuration of the cross-
bridged macrocycle ring, leaving cis labile binding sites for interaction of the complex with oxidants and/
or substrates. The copper(II) complex kinetic stability in 5 M HCl and at elevated temperatures was deter-
mined and compared to related complexes in the literature. The electronic properties of the manganese
and iron complexes were evaluated using solid state magnetic moment determination and acetonitrile
solution electronic spectroscopy, revealing high spin metal complexes in all cases. Cyclic voltammetry in
acetonitrile of the divalent iron and manganese complexes revealed reversible redox processes,
suggesting catalytic reactivity involving electron transfer processes are possible for both complexes.
Screening of the Mn2+ and Fe2+ complexes for oxidation catalysis using hydrogen peroxide as the terminal
oxidant showed both complexes are worthy of continued development.
Received 19th February 2015,
Accepted 1st April 2015
DOI: 10.1039/c5dt00742a
www.rsc.org/dalton
useful 64Cu PET imaging agents.4–14 Only recently has the
imaging community incorporated the 13-membered macro-
Introduction
Dimethyl ethylene cross-bridged tetraazamacrocycles intro- cycle “homocyclen” with the production of CB-TR2A15 and
duced this important class of topologically constrained ligand another homocyclen-based cross-bridged ligand (L1, Fig. 1)
multiple decades ago with cyclam-based Me2EBC1 and cyclen- produced by another group16 with bioconjugation for targeted
based Me2Bcyclen2 the primordial examples (Fig. 1). As the imaging applications as a stated goal.
field’s founder, Weisman quickly succeeded the dimethyl ver-
Concurrent with the imaging applications of pendant-
sions and established a flexible debenzylation route3 to the armed cross-bridged tetraazamacrocycles, Busch, with some of
“parent” dihydro versions that could be substituted with a us, embraced the original dimethyl ligands Me2EBC and Me2B-
myriad of pendant arms and produced a number of highly cyclen as ideal chelates for manganese and iron for the
purpose of oxidation catalysis in harsh aqueous media.17–30 As
rigidly bridged macrocycles, these small cryptands would
provide complementary binding sites for these redox active
metal ions, yet resist stepwise protic removal and deactivation
of the potential catalysts as their oxide forms. The methyl ver-
aDepartment of Chemistry and Physics, Southwestern Oklahoma State University,
Weatherford, OK 73096, USA. E-mail: tim.hubin@swosu.edu
bKey Laboratory for Large-Format Battery Materials and Systems, Ministry of
Education, School of Chemistry and Chemical Engineering, Hubei Key Laboratory of
sions of the ligands were actually preferred as they made all of
Material Chemistry and Service Failure, Huazhong University of Science and
the nitrogen atoms tertiary, and thus more resistant to ligand
oxidation; did not chelate the metal ion, allowing two cis open
sites on the metal ion for oxidant and substrate binding and
activation; and even provided just enough steric bulk to
prevent dimerization and inactivation of the catalysts as the
oxo-bridged forms.31
Technology, Wuhan 430074, P. R. China
cDepartment of Chemistry, The University of Hull, Cottingham Road, Hull HU6 7RX,
UK
†Electronic supplementary information (ESI) available: Discussion of the hydro-
gen bonding in the crystal structures of this work. CCDC 1049977–1049983. For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c5dt00742a
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Fig. 2 Synthetic scheme for the synthesis of Me2B13N4 and
Me2Bcyclen.
and cyclam is achieved using benzyl bromide, which is fol-
lowed by ring opening and debenzylation to give two second-
ary amines.3 However, this ref. 3 in fact does not describe
Me2Bcyclen or Me2B13N4 as part of the general scheme or at
all. A later publication4 contains synthetic details on Me2EBC
Fig. 1 Ethylene cross-bridged tetraazamacrocycles discussed in this only, but again not on Me2Bcyclen or Me2B13N4. Although we
have been making references in our own publications35–37 to
work.
these papers1,3,4 for nearly two decades for the syntheses of
Me2Bcyclen and Me2B13N4, only during the preparation of
this manuscript did we discover that synthetic details specific
to neither of these two ligands actually appear anywhere in the
With growing understanding of the lead catalyst from this
series, Mn(Me2EBC)Cl2,32,33 we have recently begun to explore
literature, other than Bencini’s original synthesis of Me2Bcy-
the oxidation chemistry of closely related Mn(Me2Bcyclen)-
clen by another route.2 We therefore present those synthetic
Cl2 and found interesting effects of Ca2+ and Cl− on its oxi-
34
details in the Experimental section. Fig. 2 shows the synthetic
dation catalysis. Having published cobalt,35 nickel,36 copper,37
scheme for Me2B13N4 and Me2Bcyclen.
and zinc37 complexes of Me2B13N4 (Fig. 1), we set out to syn-
thesize the manganese and iron analogues and screen them
for catalytic oxidation chemistry for comparison to the cyclam
and cyclen analogues. An additional goal was to obtain X-ray
crystal structures of any transition metal complexes with
Me2B13N4 that we could, as previous attempts in our hands at
structural characterization of complexes of this ligand had
been unsuccessful. Below, we present synthetic details, seven
The chemistry is entirely analogous to the synthesis of
Me2EBC,4 with regiospecific dimethylation in high yields in
both cases the key synthetic step. The dimethyl cyclen glyoxal
has been prepared under different conditions by Handel39 and
Lukes,40 but is included for completeness. Sodium boro-
hydride ring opening reduction, again under conditions
suggested for Me2EBC,4 gives the smaller cross-bridged
ligands in similar yields as colorless oils after vacuum distilla-
tion from KOH.
X-ray crystal structures of Me2B13N4 complexes, and chemical
characterization and oxidation screening of the biomimetically
Complexation of Me2B13N4 with manganese and iron were
relevant Fe(Me2B13N4)Cl2 and Mn(Me2B13N4)Cl2 complexes.
carried out to produce novel complexes that may have interest-
ing catalytic oxidation behavior, as do their Me2EBC17–29 and
Me2Bcyclen34 analogues. Reaction of the ligand in acetonitrile
with anhydrous MnCl2 and FeCl2 in an inert atmosphere glove-
box gave good yields of the M(Me2B13N4)Cl2 complexes. Oxi-
dation of these divalent complexes by molecular bromine in
methanol in the presence of NH4PF6 resulted in the immediate
Results and discussion
Synthesis
Dimethyl cross-bridged cyclam, Me2EBC1 was the first ethylene
precipitation of [M(Me2B13N4)Cl2]PF6 trivalent complexes.
Complexation with other metal ions were typically repeated
from previous work that has already been published.35–37 The
goal of re-making cobalt, copper, and zinc complexes was to
try again to obtain X-ray crystal structures to compare with the
ones obtained from the manganese complexes and with
the Me2EBC and Me2Bcyclen complexes from earlier work.
Fortunately, we were successful in a number of cases, where we
cross-bridged tetraazamacrocycle and was first synthesized by
Weisman’s route utilizing the regiospecific bis-methylation of
the cyclam-glyoxal condensate bisaminal, followed by ring-
opening reduction by sodium borohydride.1 Me2Bcyclen was
initially synthesized by another route,2 but was stated38 to have
been described as part of the general synthetic procedure for
producing the “parent” cross-bridged ligands. In this route,
alkylation of the glyoxal condensates of cyclen, homocyclen,
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had been unsuccessful in our earlier works, and we now
present a total of seven X-ray crystal structures of Me2B13N4
below. One new zinc(II) complex was synthesized from zinc
acetate in acetonitrile, followed by anion metathesis to give the
[Zn(Me2B13N4)(OAc)(H2O)]PF6 complex, which is also structu-
rally characterized below.
Single crystal X-ray crystallographic studies
As far as we are aware, the only published crystal structure of
an ethylene cross-bridged homocyclen is the copper complex
of CB-TR2A. With its two acetate pendant arms, this hexa-
dentate ligand coordinatively saturates the copper(II) ion.15 We
report seven new structures of Me2B13N4 complexes here, a
tetradentate ligand allowing for greater variability in coordi-
nation of the two open coordination sites and more similar to
the large number of Me2Bcyclen and Me2EBC complexes pre-
viously published and to which we can turn to for comparison.
Table 1 lists crystal data for these seven new crystal structures.
Table 2 lists selected bond lengths and angles. Fig. 3–11
contain views of the X-ray crystal structures and will be referred
to in the discussion that follows.
General observations about the series of Me2B13N structures
Several general observations can be made about this collection
of crystal structures, before descriptions of the individual
structures are made. First, the cis-V conformation expected to
be dictated by the ligand cross-bridge is apparent for all of the
complexes structurally characterized here. This result is one of
the reasons to use ethylene cross-bridged tetraazamacrocycles,
as the bridge forces the cis-V conformation, allowing two cis
labile cites for binding oxidant and substrate next to each
other, in the case of prospective oxidation catalysts.31
Second, the size of the metal ion influences how much dis-
tortion away from the preferred octahedral coordination geo-
metry is required due to the short two-carbon cross-bridge,
which does not allow the ligand to fully embrace larger
cations. Fig. 3 illustrates the easing of this distortion, most
apparent in the axial Nax–M–Nax bond angle that becomes
more linear, for example from Fig. 3a–c. As the metal ionic
radius decreases from 97 pm for 6-coordinate Mn2+ to 79 pm
for 6-coordinate Mn3+ to 76 pm for 6 coordinate Co3+, the
corresponding Nax–M–Nax bond angles increase from
150.29(10)° to 158.3(4)° to ∼171°, respectively (two examples:
170.59(10)° and 171.2(2)°).
Finally, in comparison to the many published examples of
the closely related Me2EBC and Me2Bcyclen transition metal
complex structures, it is apparent that the ring size of the
Me2B13N4 macrocycle makes a clear difference in how fully
the metal ion is engulfed by the bridged macrocycle. Table 3
illustrates this trend, as well as giving further data illustrating
the influence of the effect of the metal ionic radius within a
given ligand. As expected, but not illustrated by structural data
until now, the Me2B13N4 ligand generally produces Nax–M–Nax
and Neq–M–Neq bond angles intermediate to those of the
larger Me2EBC and smaller Me2Bcyclen ligands, given the
same metal ion and coordination environment.
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Table 2 Selected bond lengths [Å] and angles [°]
Mn(Me2B13N4)Cl2
Table 2 (Contd.)
[Co(Me2B13N4)Cl2]PF6
Co(1)–Cl(2)
Co(1)–Cl(1)
2.2509(8)
2.2785(8)
88.10(10)
83.13(11)
86.42(11)
87.46(11)
92.91(11)
170.59(10)
179.11(8)
91.78(7)
97.74(8)
91.66(8)
92.41(8)
173.42(8)
87.13(8)
93.67(8)
87.81(3)
Mn(1)–N(4)
Mn(1)–N(1)
Mn(1)–N(3)
Mn(1)–N(2)
Mn(1)–Cl(2)
Mn(1)–Cl(1)
N(4)–Mn(1)–N(1)
N(4)–Mn(1)–N(3)
N(1)–Mn(1)–N(3)
N(4)–Mn(1)–N(2)
N(1)–Mn(1)–N(2)
N(3)–Mn(1)–N(2)
N(4)–Mn(1)–Cl(2)
N(1)–Mn(1)–Cl(2)
N(3)–Mn(1)–Cl(2)
N(2)–Mn(1)–Cl(2)
N(4)–Mn(1)–Cl(1)
N(1)–Mn(1)–Cl(1)
N(3)–Mn(1)–Cl(1)
N(2)–Mn(1)–Cl(1)
Cl(2)–Mn(1)–Cl(1)
2.317(3)
2.320(3)
2.324(3)
2.331(3)
2.4396(9)
2.4559(9)
81.97(9)
78.47(9)
74.92(9)
150.29(10)
78.98(10)
74.70(10)
99.12(7)
92.48(7)
167.36(7)
104.25(8)
99.79(7)
168.38(7)
94.10(7)
94.65(8)
98.54(4)
N(1)–Co(1)–N(3)
N(1)–Co(1)–N(2)
N(3)–Co(1)–N(2)
N(1)–Co(1)–N(4)
N(3)–Co(1)–N(4)
N(2)–Co(1)–N(4)
N(1)–Co(1)–Cl(2)
N(3)–Co(1)–Cl(2)
N(2)–Co(1)–Cl(2)
N(4)–Co(1)–Cl(2)
N(1)–Co(1)–Cl(1)
N(3)–Co(1)–Cl(1)
N(2)–Co(1)–Cl(1)
N(4)–Co(1)–Cl(1)
Cl(2)–Co(1)–Cl(1)
[Cu(Me2B13N4)Cl]Cl·2H2O
Cu(1)–N(1)
Cu(1)–N(2)
Cu(1)–N(4)
Cu(1)–N(3B)
Cu(1)–N(3A)
Cu(1)–Cl(1)
N(1)–Cu(1)–N(2)
N(1)–Cu(1)–N(4)
N(2)–Cu(1)–N(4)
N(1)–Cu(1)–N(3B)
N(2)–Cu(1)–N(3B)
N(4)–Cu(1)–N(3B)
N(1)–Cu(1)–N(3A)
N(2)–Cu(1)–N(3A)
N(4)–Cu(1)–N(3A)
N(1)–Cu(1)–Cl(1)
N(2)–Cu(1)–Cl(1)
N(4)–Cu(1)–Cl(1)
N(3B)–Cu(1)–Cl(1)
N(3A)–Cu(1)–Cl(1)
2.046(4)
2.048(3)
2.073(4)
2.118(10)
2.238(10)
2.2645(12)
85.60(14)
84.72(16)
169.88(15)
85.6(3)
98.0(3)
84.2(3)
82.1(3)
87.1(2)
[Mn(Me2B13N4)Cl2]PF6
Mn(1)–N(3)
Mn(1)–N(1)
Mn(1)–Cl(2)
Mn(1)–Cl(1)
Mn(1)–N(4)
Mn(1)–N(2)
N(3)–Mn(1)–N(1)
N(3)–Mn(1)–Cl(2)
N(1)–Mn(1)–Cl(2)
N(3)–Mn(1)–Cl(1)
N(1)–Mn(1)–Cl(1)
Cl(2)–Mn(1)–Cl(1)
N(3)–Mn(1)–N(4)
N(1)–Mn(1)–N(4)
Cl(2)–Mn(1)–N(4)
Cl(1)–Mn(1)–N(4)
N(3)–Mn(1)–N(2)
N(1)–Mn(1)–N(2)
Cl(2)–Mn(1)–N(2)
Cl(1)–Mn(1)–N(2)
N(4)–Mn(1)–N(2)
2.093(12)
2.100(12)
2.250(4)
2.266(5)
2.275(12)
2.296(10)
83.3(4)
93.2(3)
176.5(4)
173.8(3)
93.1(3)
90.43(16)
90.8(5)
81.8(4)
98.1(4)
93.6(4)
81.7(4)
77.2(4)
102.6(3)
92.6(3)
94.4(2)
168.40(12)
94.01(11)
94.87(12)
105.9(2)
109.5(2)
[Zn(Me2B13N4)Cl(H2O)]2[ZnCl4]·CH3CN
Zn(1)–N(3)
Zn(1)–N(1)
Zn(1)–N(4)
2.15(3)
2.15(3)
2.16(3)
158.3(4)
Zn(1)–O(1)
Zn(1)–N(2)
2.20(2)
2.26(3)
[Co(Me2B13N4)Cl2]PF6·CH3CN
Zn(1)–Cl(1)
2.362(10)
82.1(11)
93.0(11)
84.9(11)
171.1(11)
94.7(10)
95.1(11)
81.4(10)
78.7(10)
163.2(10)
89.8(10)
97.5(7)
178.3(8)
96.8(8)
85.5(7)
99.6(7)
2.12(3)
2.13(3)
2.16(2)
2.19(3)
2.24(3)
2.387(8)
169.6(10)
94.0(11)
95.8(10)
82.5(11)
95.0(10)
84.3(10)
Cl(1)–Co(1)
Cl(2)–Co(1)
Co(1)–N(1)
Co(1)–N(3)
Co(1)–N(2)
Co(1)–N(4)
N(1)–Co(1)–N(3)
N(1)–Co(1)–N(2)
N(3)–Co(1)–N(2)
N(1)–Co(1)–N(4)
N(3)–Co(1)–N(4)
N(2)–Co(1)–N(4)
N(1)–Co(1)–Cl(2)
N(3)–Co(1)–Cl(2)
N(2)–Co(1)–Cl(2)
N(4)–Co(1)–Cl(2)
N(1)–Co(1)–Cl(1)
N(3)–Co(1)–Cl(1)
N(2)–Co(1)–Cl(1)
N(4)–Co(1)–Cl(1)
Cl(2)–Co(1)–Cl(1)
2.2859(17)
2.2541(19)
1.956(5)
1.988(5)
2.029(5)
2.047(4)
88.1(2)
83.6(2)
85.9(2)
87.7(2)
92.4(2)
171.2(2)
179.14(15)
92.43(16)
97.09(15)
91.64(15)
91.04(15)
173.01(15)
87.07(15)
94.54(14)
88.46(6)
N(3)–Zn(1)–N(1)
N(3)–Zn(1)–N(4)
N(1)–Zn(1)–N(4)
N(3)–Zn(1)–O(1)
N(1)–Zn(1)–O(1)
N(4)–Zn(1)–O(1)
N(3)–Zn(1)–N(2)
N(1)–Zn(1)–N(2)
N(4)–Zn(1)–N(2)
O(1)–Zn(1)–N(2)
N(3)–Zn(1)–Cl(1)
N(1)–Zn(1)–Cl(1)
N(4)–Zn(1)–Cl(1)
O(1)–Zn(1)–Cl(1)
N(2)–Zn(1)–Cl(1)
Zn(2)–N(7)
Zn(2)–O(2)
Zn(2)–N(8)
Zn(2)–N(5)
Zn(2)–N(6)
Zn(2)–Cl(2)
N(7)–Zn(2)–O(2)
N(7)–Zn(2)–N(8)
O(2)–Zn(2)–N(8)
N(7)–Zn(2)–N(5)
O(2)–Zn(2)–N(5)
N(8)–Zn(2)–N(5)
[Co(Me2B13N4)Cl2]PF6
Co(1)–N(1)
Co(1)–N(3)
Co(1)–N(2)
Co(1)–N(4)
1.954(2)
1.998(2)
2.035(3)
2.055(3)
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obtained as green hexagonal plates, is isostructural with the
manganese compound, but there are small differences in the
coordination geometry about the metal ion, which are
discussed later.
Table 2 (Contd.)
[Zn(Me2B13N4)Cl(H2O)]2[ZnCl4]·CH3CN
N(7)–Zn(2)–N(6)
O(2)–Zn(2)–N(6)
N(8)–Zn(2)–N(6)
N(5)–Zn(2)–N(6)
N(7)–Zn(2)–Cl(2)
O(2)–Zn(2)–Cl(2)
N(8)–Zn(2)–Cl(2)
N(5)–Zn(2)–Cl(2)
N(6)–Zn(2)–Cl(2)
82.1(10)
87.5(9)
162.1(9)
77.8(9)
95.6(8)
86.5(6)
98.0(7)
177.1(8)
99.8(7)
A second form of the Co3+ complex was obtained as green
needles. This was shown by X-ray diffraction to be the solvate
[Co(Me2B13N4)Cl2]PF6·MeCN (Fig. 6). The asymmetric unit
−
contains a single [Co(Me2B13N4)Cl2]+ ion, two half PF6
anions and one well-determined molecule of acetonitrile. The
coordination about the metal is not significantly altered by
this but the packing is different.
[Zn(Me2B13N4)(OAc)(H2O)]PF6
[Cu(Me2B13N4)Cl]Cl·2H2O is the only structure here to
feature unbound chloride (Fig. 7). This a reflection of the
greater reluctance of Cu2+ to adopt (undistorted) octahedral
geometry compared with the other ions. The copper ion is five
coordinate in an arrangement similar to the other complexes
here, but with apparently one coordinating chloride removed.
The result is a slightly distorted square pyramidal coordination
about Cu2+ (4 × N, 1 × Cl).
Crystallographically, this structure is intriguing because of
the disorder within the Me2B13N4 macrocycle which adopts
two different orientations depending on the location of the
propylene bridge. The crystal is not “bad” – the disorder is a
reflection that the energies for packing each of the two
different forms in the solid are very similar. Fig. 8 below shows
the complete model, with each of the two components high-
lighted in red. Note the difference in the carbon atom
positions.
N(1)–Zn(1)
N(2)–Zn(1)
N(3)–Zn(1)
N(4)–Zn(1)
O(2)–Zn(1)
Zn(1)–O(1)
O(2)–Zn(1)–N(4)
O(2)–Zn(1)–N(2)
N(4)–Zn(1)–N(2)
O(2)–Zn(1)–N(1)
N(4)–Zn(1)–N(1)
N(2)–Zn(1)–N(1)
O(2)–Zn(1)–O(1)
N(4)–Zn(1)–O(1)
N(2)–Zn(1)–O(1)
N(1)–Zn(1)–O(1)
O(2)–Zn(1)–N(3)
N(4)–Zn(1)–N(3)
N(2)–Zn(1)–N(3)
N(1)–Zn(1)–N(3)
O(1)–Zn(1)–N(3)
2.151(5)
2.143(4)
2.222(5)
2.138(4)
2.034(4)
2.152(4)
90.98(16)
174.50(17)
83.52(18)
96.83(18)
91.7(2)
83.3(2)
89.52(15)
169.13(19)
95.90(17)
99.01(19)
99.07(18)
83.3(2)
80.46(19)
163.41(19)
85.86(18)
The structure of [Zn(Me2B13N4)Cl(OH2)]2[ZnCl4]·CH3CN is
non-centrosymmetric and contains four symmetry unique
components: two independent [Zn(Me2B13N4)Cl(OH2)]+ ions,
Descriptions of specific structures
[Mn(Me2B13N4)Cl2] occurs as discrete complexes in the solid
state featuring a distorted octahedral coordination about the a tetrahedral [ZnCl4]2− ion, and one molecule of acetonitrile
Mn2+ ion (Fig. 4). As is the case for all of the 6-coordinate (Fig. 9). The two [Zn(Me2B13N4)Cl(OH2)]+ ions each adopt a
examples described here, the complex adopts the cis-V con-
similar coordination geometry to the other octahedral ions
figuration with metal residing in the pocket of the ligand and described here, with the macrocycle tetradentate to Zn2+ and
the two remaining coordination site cis to each other.
water and chloride completing the coordination in a cis
arrangement. These two symmetry-unique [Zn(Me2B13N4)-
[Mn(Me2B13N4)Cl2]PF6 (Fig. 5) is similar to the analogous
Mn2+ complex in many ways, but with contracted Mn–N bond Cl(OH2)]+ ions form
a hydrogen-bonded dimer through
lengths and altered bond angles appropriate to the smaller O–H⋯Cl interactions that is approximately centrosymmetric.
Mn3+ cation. The cobalt analogue, [CoMe2B13N4Cl2]PF6
There are no intramolecular O–H⋯Cl hydrogen bonds but
Fig. 3 X-ray crystal structures of (a) Mn(Me2B13N4)Cl2 (b) Mn(Me2B13N4)Cl2+ and (c) Co(Me2B13N4)Cl2+ (green block, without CH3CN).
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Fig. 4 Crystal structure of [Mn(Me2B13N4)Cl2].
Fig. 6 Representation of the molecular units within [Co(Me2B13N4)Cl2]-
PF6·MeCN. Symmetry equivalent atoms are generated by the operators:
i = 1 − x, −y, 1 − z; ii = 2 − x, −y, −z.
Fig. 7 Complete asymmetric unit for [Cu(Me2B13N4)Cl]Cl·2H2O.
Atoms are drawn as 50% probability ellipsoids. Hydrogen atoms have
been omitted for clarity.
Fig. 5 Crystal structure of [Mn(Me2B13N4)Cl2]PF6.
each bound water forms a second interaction: O1–H1C forms a
hydrogen bond to N100 (acetonitrile) and O2–H2D forms a
hydrogen bond to the [ZnCL4]2− ion.
Kinetic stability of [Cu(Me2B13N4)Cl]Cl
Cross-bridged tetraazamacrocycle complexes typically gain
stability compared to unbridged analogues under harsh con-
For [Zn(Me2B13N4)OAc(OH2)]PF6, the use of zinc acetate
rather than zinc chloride leads to a different ligand set about ditions due to their topological complexity and the rigidity
caused by their small size.18–20,31 Yet, the proton-sponge basi-
the zinc ion (Fig. 10). The macrocycle is tetradentate to octa-
hedral Zn2+ and the coordination is completed by cis water
city of many of these ligands precludes aqueous titration in
and acetate. Charge balancing is completed by the PF6− anion. the presence of metal ions to produce formation constants. As
The structure contains intramolecular and intermolecular
hydrogen bonds involving bound water and acetate. There is
an alternative, kinetic decomplexation studies using high acid
concentration under pseudo first order conditions are used to
an intramolecular hydrogen bond between O3 and H1C give some quantification of complex stability. Typically, only
(O3⋯O1 distance is 2.669(6)Å). Hydrogen-bonded trimers of
copper(II) complexes are subjected to these studies and have
yielded published data for comparison. They are often the first
[Zn(Me2B13N4)OAc(OH2)]+ are formed from three symmetry-
equivalent complexes held together by O–H⋯O interactions (or only) transition metal complex synthesized due to the inter-
est in PET imaging applications of 64Cu. Weisman has standar-
(Fig. 11).
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Fig. 10 Crystal structure of [Zn(Me2B13N4)OAc(OH2)]PF6.
Fig. 8 (a) Component 1 of the macrocycle shown in red (b) component
2 of the macrocycle shown in red.
Fig. 11 Intramolecular and intermolecular hydrogen bonding in [Zn-
(Me2B13N4)OAc(OH2)]PF6. Dashed lines show hydrogen bonds.
Results of the decomplexation studies of [Cu(Me2B13N4)Cl]-
Cl and related complexes are presented in Table 4.
One major trend easily visible in the kinetic stability data is
the increased kinetic stability towards strong acid decomplexa-
tion with increased macrocycle ring size. The cyclam based
complexes are stable in 5 M HCl at 90 °C with measurable
half-lives. The Me2B13N4 ligand which is the subject of this
work gave a half-life under these conditions of approximately
2 minutes, which was just at the edge of what was measurable
with our instrumentation. We did not attempt this tempera-
ture with the cyclen based complexes at this temperature since
they gave half-lives that were not measurable at the lower temp-
erature of 50 °C with 5 M HCl. Clearly the smallest cyclen
cross-bridged tetraazamacrocycles do not provide a comp-
lementary size/geometry match for Cu2+ to withstand these
Fig. 9 Crystal structure of [Zn(Me2B13N4)Cl(OH2)]2[ZnCl4]·CH3CN.
Dashed lines are hydrogen bonds.
dized9,11,13,15 probing the stability of new cross-bridged
ligands using the copper(II) complex. Although established by
Busch17,18 using different conditions, Weisman has chosen
5 M HCl and various temperature points, such as 30 °C, 50 °C,
and 90 °C as benchmarks for kinetic stability of new Cu cross-
bridged complexes.
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Table 3 Comparison of Nax–M–Nax and Neq–M–Neq bond angles across metal ions of differing ionic radius and ethylene cross-bridged tetraaza-
macrocycles of differing sizea
Me2EBC
ax–M–Nax
Me2B13N4 (this work)
ax–M–Nax eq–M–Neq
Me2Bcyclen
ax–M–Nax
H.S. 6-coord.
ionic radius
Ref.
Ref.
Metal ion
N
Neq–M–Neq
N
N
N
Neq–M–Neq
Mn2+
Fe2+
Co2+
Zn2+
Ni2+
Cu2+
Mn3+
Fe3+
Cr3+
Cr3+
Co3+
97
92
89
88
83
79d
79
79
76
76
75
158.0(2)
161.88(5)
172.4(2)
—
75.6(2)
78.36(5)
81.11(13)
—
18
18
35
150.29(10)
—
74.92(9)
—
144.0(2)
74.1(2)
18
18
35
37
36
37
42
43
44
45
35
145.78(7)
149.81(9)
150.75(7)
161.58(13)c
164.85(13)e
155.01(11)
153.20(9)
160.83(19)
161.62(11)
168.8(4)
77.31(7)
80.86(8)
78.32(7)
85.46(13)c
83.92(14)e
81.29(11)
77.81
83.50(18)
83.23(10)
87.2(4)
—
—
163.41(19)b
—
83.52(18)b
—
174.56(10)c
175.16(13)d
170.1(2)
—
85.07(9)c
85.30(12)d
81.7(2)
—
36
41
42
169.88(15)d
158.3(4)
—
82.1(3)d
83.3(4)
—
172.46(11)
171.44(14)
—
84.63(11)
84.18(13)
—
44
45
—
—
—
—
170.59(10)
171.2(2)
88.10(10)
88.1(2)
a Complex is MLCl2 unless otherwise designated. b Zn(L)(OAc)(OH2)+. c Ni(L)(acac)+. d Ionic radius and bond angles are for 5-coord complex
Cu(L)Cl+. e Cu(L)(NCCH3)2.
n+
36 minute vs. <1 minute half-life for Cu-Me2Bcyclen and Cu-
H2Bcyclen, respectively. Making all nitrogen donors tertiary
increases the rigidity of the ligand and provides some steric
protection of the nitrogen atom from protons in solution,
slowing down protonation, and thus decomplexation. The
dihydro ligands have secondary nitrogen atoms with ionizable
hydrogen atoms that easily exchange with protons in solution,
providing for much faster interaction with the highly acidic
solution and as demonstrated, faster decomplexation under
strong acid conditions.
Table 4 Half-lives of selected copper(II) complexes in HCl
30 °C 1 M 40 °C 1 M 50 °C 5 M 90 °C 5 M
Ligand
HCl
HClO4
HCl
HCl
Ref.
a
H2Bcyclen
Me2Bcyclen 36 min
H2B13N4
Me2B13N4
H2EBC
<1 min
—
30 h
—
—
—
<1 min
<1 min
—
30.1 min
—
—
—
—
37a
4.8 hour
7.7 day
—
15,46
a
<2 min
11.8 min
79 min
9
Me2EBC
—
>6 years
7.3 day
37,47
a This work.
Finally, there appears to be some role for the chloride of
HCl in the decomplexation of the copper complexes as well.
The original acid decomplexation of cross-bridged tetraaza-
macrocycle copper complexes37 used 1 M HClO4 at 40 °C to
determine how stable these complexes were. Cu-Me2EBC gave
a half-life estimated at >6 years and Cu-Me2Bcyclen gave a half-
life of 30 h under these conditions. Due to the extreme stabi-
lity of Cu-Me2EBC, we did not attempt to repeat this experi-
ment at 30 °C in 1 M HCl, but we did carry out this experiment
for Cu-Me2Bcyclen. At the same H+ concentration and 10 °C
lower temperature, the half-life in HCl was found to be only
36 minutes, much shorter than the 30 hours in 1 HClO4 at
40 °C. Since Cl− is a coordinating anion and is found bound to
the metal ions in many of the crystal structures in Table 3, its
involvement in the decomplexation process should perhaps be
harsh conditions, which can also be seen in the X-ray crystal
structural data given in highly distorted bond angles of
Table 3. There is an impressive amount of stability gained by
adding just one (–CH2–) to the size of the parent macrocycle
for the dimethyl complexes: 7.7 days vs. 36 minutes for Cu-
Me2B13N4 vs. Cu-Me2Bcyclen at 30 °C in 1 M HCl, suggesting
modest gains in bond angles (see Table 3) can yield important
amounts
of
complex
stability
due
to
increased
complementarity.
A second clear trend is the advantage in stability gained by
the dimethyl ligands over the dihydro ligands. For example,
Me2EBC at 90 °C in 5 M HCl gave a half-life for its Cu2+
complex of 79 minutes, while the Cu-H2EBC complex is
reported to have a half-life under the same conditions of just
11.8 minutes.9 The smaller, less stable ligand complexes tested
at 30 °C in 1 M HCl demonstrated a similar trend. Cu-
Me2B13N4 had an impressive half-life under these conditions
of 7.7 days, while Cu-H2B13N4 gave a half-life of only
4.8 hours.15,46 Cu-Me2Bcyclen gave a half-life of 36 minutes
under these conditions, while Cu-H2Bcyclen had a half-life too
short to be measure and thus is listed as less than 1 minute.
Interestingly, 7.7 days is 38.5 times longer than 4.8 hours,
approximately the same fold gain in stability as seen in the
−
expected. Typically non-coordinating ClO4 is less likely to
interact with the metal complex. Clearly, choice of which
strong acid to use in these decomplexation experiments can
make a significant difference in the resulting half-lives.
Electronic characterization of iron and manganese complexes
We have previously published the synthesis and characteriz-
ation of cobalt,35 nickel,36 copper,37 and zinc37 complexes of
Me2B13N4 except for the X-ray crystal structures presented
above. However, the iron and manganese complexes syn-
thesized for this study are novel and are of interest to us as
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potential oxidation catalysts. We have synthesized them
initially in their divalent forms from the corresponding chlor-
ide metal salt, and then oxidized them to the trivalent oxi-
dation state using molecular bromine as the oxidant. Here, we
present characterization of the electronic structure of these
species prior to, and as a way to help understand, the oxi-
dation screening experiments that will conclude this work.
The magnetic moments of the solid state divalent and tri-
valent iron and manganese complexes were determined in
order to probe the high spin or low spin nature of the com-
plexes, and thus indicate the relative ligand field strength. The
divalent species were air sensitive, so they were introduced
into the sample tube in an inert atmosphere glovebox and the
tubes sealed with a septum prior to taking the measurements.
All four complexes Fe(Me2B13N4)Cl2, Mn(Me2B13N4)Cl2,
[Fe(Me2B13N4)Cl2]PF6, and [Mn(Me2B13N4)Cl2]PF6 gave values
(see Experimental section) consistent with high spin behavior,
as was observed for their previously published analogues with
the Me2Bcyclen and Me2EBC ligands.18,42,48 This is consistent
with the relatively weak field folded tetraazamacrocycle ligands
used in these studies.
Table 5 Redox potential verses SHE for MnLCl2 and FeLCl2 complexes
of Me2EBC, Me2Bcyclen, and Me2B13N4
M2+/3+ E1/2 [V]
(ΔE [mV])
M3+/4+ E1/2 [V]
(ΔE [mV])
Complex
18
Mn(Me2Bcyclen)Cl2
Mn(Me2B13N4)Cl2
Mn(Me2EBC)Cl2
+0.466 (70)
+0.526 (92)
+0.585 (61)
+0.036 (64)
+0.076 (114)
+0.110 (63)
+1.232 (102)
+1.296 (95)
+1.343 (65)
—
—
—
18
18
Fe(Me2Bcyclen)Cl2
Fe(Me2B13N4)Cl2
18
Fe(Me2EBC)Cl2
The electronic spectra of all four iron and manganese com-
plexes of Me2B13N4 were obtained in acetonitrile. The Experi-
mental sections contains the absorbances and extinction
coefficients. As expected for high spin d5 and d6 complexes
with ligands of this type,49 neither divalent complex exhibit
intense spectral features, except for tailing absorbances at
highest energy. Fe(Me2B13N4)Cl2 does have one discernable
Fig. 12 Cyclic voltammograms in acetonitrile for (a) Fe(Me2B13N4)Cl2
and (b) Mn(Me2B13N4)Cl2.
peak with λmax = 342 nm (ε = 623 M−1 cm−1) most likely to be mechanisms known. Consistent with the need for stability of
of a low energy charge transfer nature. Similar absorbances the catalyst at multiple oxidation states is the electrochemistry
were observed for Fe(Me2EBC)Cl2 and Fe(Me2Bcyclen)Cl2.18
Me2B13N4 in acetonitrile. The high spin d5 Fe3+ spectrum
of these complexes, shown in Table 5.
We expected that if the cyclic voltammetry of the close ana-
of [Fe(Me2B13N4)Cl2]PF6 and the high spin d4 Mn3+ spectrum logues to these known oxidation catalysts, manganese(II) and
of [Mn(Me2B13N4)Cl2]PF6 in acetonitrile were obtained. Both iron(II) Me2B13N4, gave similar results, that they too might be
metal ions have several charge transfer bands, with the iron(III) active oxidation catalyst. The results of the electrochemical
complex having no other absorptions, which is similar to both study of these complexes are also listed in Table 5 and shown
[Fe(Me2EBC)Cl2]PF6 and [Fe(Me2Bcyclen)Cl2]PF6.42 However, in Fig. 12, and fit exactly in the gaps between the Me2Bcyclen
the manganese(III) complex also has two low energy bands and Me2EBC complexes of a given metal ion. Mn(Me2B13N4)-
typical of this ion.49 These bands are fairly intense for d–d Cl2 has a reversible Mn2+/3+ couple at 0.526 V vs. SHE with ΔE
transitions, are characteristically broad, and may result from = 92 mV and a reversible Mn3+/4+ couple at +1.296 V with ΔE =
electronic transitions involving the tetragonally split B1g(Eg) 95 mV. Fe(Me2B13N4)Cl2 has a quasi-reversible Fe2+/3+ couple
5
5
and B2g(T2g) states.49 Both [Mn(Me2EBC)Cl2]PF6 and [Mn- at 0.076 V vs. SHE with ΔE = 114 mV. Based on the trends
(Me2Bcyclen)Cl2]PF6 gave similar low energy electronic absorp- apparent in comparison with the other cross-bridged ligands,
tions.42 Interestingly, these previously published complexes it appears that the redox potentials are closely tied to the ring
only gave one d–d transition at 534 nm (540 M−1 cm−1) and size of the parent macrocycle. The smaller the ring size, the
530 nm (190 M−1 cm−1), respectively, whereas the present easier to oxidize the metal. As the higher oxidation state metal
Me2B13N4 complex has a similar absorption at 526 nm ion is smaller, it would fit the smaller ligand cavity better. The
(512 M−1 cm−1). But it also shows another shoulder at 400 nm intermediate ring size of Me2B13N4 leads to intermediate
(960 M−1 cm−1) that was not observable for the cyclam and redox potentials for both the manganese and iron complexes.
cyclen derivative. Perhaps the less symmetric Me2B13N4
ligand allows observation of this additional peak.
As observed for the Me2Bcyclen and Me2EBC complexes,
the Me2B13N4 ligand stabilizes manganese in three oxidation
states from Mn2+/Mn3+/Mn4+ with good reversibility. Also
similar to the other cross-bridged ligands, only Fe2+ and Fe3+
Electrochemistry
Mn(Me2EBC)Cl2,17–30,32,33 Fe(Me2EBC)(OTf)2,50 and Me(Me2- are observed in acetonitrile for Fe(Me2B13N4)Cl2. Based on
34
Bcyclen)Cl2 have all been established as oxidation catalysts these experiments, we expected that Mn(Me2B13N4)Cl2 would
with varying degrees of detail about their respective oxidation be the most likely candidate to perform well as an oxidation
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(Me2B13N4)Cl2 has the intermediate redox potential not only
of the Mn3+/4+ redox couple, but also of the Mn2+/3+ couple.
Perhaps a value not too low and not too high, (a “Goldilocks”
zone) is just right for sulfone production. Determination of
these additional factors must await further in-depth mechanis-
tic studies.
Table 6 Oxidation catalysis screening results for manganese(II) and
iron(II) complexes of Me2B13N4 in comparison to Mn(Me2EBC)Cl2 and
Mn(Me2Bcyclen)Cl2
The Fe(Me2B13N4)Cl2 complex is sluggish in sulfide oxy-
genation, generating only 28.8% conversion of thioanisole
with 18.1% yield of sulfoxide and 4.4% of the sulfone product.
Its redox potential for the FeII/FeIII couple is fairly low, +0.076
V vs. SHE, with no detected FeIII/FeIV couple, which is consist-
ent with the demonstrated low catalytic sulfide oxidation.
In hydrogen abstraction from 1,4-cyclohexadiene in metha-
nol–water (1 : 1, v/v), Mn(Me2B13N4)Cl2 demonstrated more
similar reactivity to its analogues, providing 74.1% conversion
of substrate with 63.1% of the benzene product in 4 h at 303 K
when using H2O2 as oxidant. Mn(Me2EBC)Cl2 gave 86.2% con-
version with 71.4% yield, and Mn(Me2Bcyclen)Cl2 generated
51.2% yield of benzene with 88.0% conversion. In this reac-
tion, the intermediate ligand size and redox potential of
Me2B13N4 leads to the intermediate yield of benzene.
Yield %
sulfoxide
Yield %
sulfone
Complex
Conversion %
Mn(Me2Bcyclen)Cl2
Mn(Me2B13N4)Cl2
Mn(Me2EBC)Cl2
65.9
99.8
99.8
28.8
32.6
13.3
44.3
18.1
25.9
74.8
46.5
4.4
Fe(Me2B13N4)Cl2
Yield %
benzene
Complex
Conversion %
Mn(Me2Bcyclen)Cl2
Mn(Me2B13N4)Cl2
Mn(Me2EBC)Cl2
88.0
74.1
86.2
77.2
51.2
63.1
71.4
61.3
Fe(Me2B13N4)Cl2
Although the manganese(IV) complex of the Me2B13N4
ligand has not been isolated yet, its manganese(IV) analogues,
[Mn(Me2EBC)(OH)2](PF6)2 and [Mn(Me2Bcyclen)(OH)2](PF6)2
have been well characterized22,34 Accordingly, having a similar
ligand structure, a similar manganese(IV) species of the
Me2B13N4 complex can be expected to occur under these oxi-
dative conditions like its analogues, and is likely responsible
catalyst, due to the range of oxidations states that can be rever-
sibly reached.
Catalytic oxidation studies
Iron and manganese bearing synthetic ligands generally have for the hydrogen atom abstraction observed here.23,25
rich redox chemistry, and exploring their catalytic activity in Unlike in the sulfide oxidation, the Fe(Me2B13N4)Cl2
oxidation catalysis is often fruitful. Herein, preliminary studies complex demonstrated comparable catalytic activity with its
to investigate the divalent Mn and Fe Me2B13N4 complexes’ manganese(II) analogue, offering 77.2% conversion of 1,4-
activity in catalytic hydrogen abstraction and oxygen transfer cyclohexadiene with 61.3% yield of benzene. The distinct reac-
were conducted using 1,4-cyclohexadiene and thioanisole as tivity of Fe(Me2B13N4)Cl2 complex in sulfide oxygenation and
substrates with H2O2 oxidant, respectively.
hydrogen abstraction has not been clarified yet, possibly
As shown in Table 6, at 303 K, the Mn(Me2B13N4)Cl2 related to its different mechanisms. One potential explanation
complex demonstrates high activity in sulfide oxidation. After is that, under the oxidative conditions with H2O2 oxidant, the
6 hours of reaction in acetonitrile, it provided almost complete iron(IV) species does not occur as suggested in the electro-
conversion of thioanisole with 13.3% yield of sulfoxide and chemical studies (vide supra) because of the poor electron
74.8% of sulfone. Notably, compared with well-investigated donation ability of Me2B13N4 ligand, thus the iron(III) species
Mn(Me2EBC)Cl2,17–30,32,33 and Mn(Me2Bcyclen)Cl2
com- dominates in solution. Such an iron(III) species is very sluggish
34
plexes which provided 99.8% and 65.9% conversion of sulfide for oxygenation, but active for hydrogen abstraction for
with 44.3% and 32.6% of sulfoxide, 46.5% and 25.9% of similar iron(III) species in lipoxygenases and their synthetic
sulfone, respectively, the Mn(Me2B13N4)Cl2 complex can more models.51,52 To our knowledge, the hydrogen atom abstraction
effectively oxidize sulfoxide to sulfone. This reactivity differ- ability of Fe(Me2EBC)Cl2 and Fe(Me2Bcyclen)Cl2 have not been
ence can’t be solely assigned to the redox potential of the published. It would be of interest to determine if these other
MnIII/MnIV couple. As shown in Table 5, the potential of the low redox potential species are active hydrogen atom abstrac-
Mn(Me2B13N4)Cl2 complex (+1.296 V vs. SHE) is higher than tion oxidation catalysts and to determine by what mechanism
that of Mn(Me2Bcyclen)Cl2 (+1.232 V), but lower than the Mn that reaction occurs if so.
(Me2EBC)Cl2 (+1.343 V), which should demonstrate the
highest oxidizing power amongst its analogues.
Perhaps some combination of redox potential along with
the steric and/or other electronic properties provided by the
Conclusions
Me2B13N4 ligand are optimized for the production of sulfone. Divalent and trivalent Mn and Fe complexes of Me2B13N4
Another possibility is the involvement of the Mn2+/3+ couple were synthesized and characterized, both Mn complexes
in the mechanism of sulfoxide to sulfone oxidation. Mn- by X-ray crystallography. Five additional Co, Cu, and Zn
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Me2B13N4 structures were also presented, and in all seven electrolyte were used. The measured potentials were referenced
structures Me2B13N4 binds the metal ion in a cis-V configur- to SHE using ferrocene (+0.400 V versus SHE) as an internal
ation of the homocyclen ring. Kinetic studies focusing on the standard. All electrochemical measurements were carried out
decomposition of copper complexes in strong acid and at elev- under N2.
ated temperatures show that the Me2B13N4 ligand releases
Ligand synthesis
Cu2+ much slower than either the smaller Me2Bcyclen ligand
or the H2B13N4 secondary nitrogen containing ligand. Further Although we have published Me2B13N4 and Me2Bcyclen com-
studies focused on the manganese and iron complexes as they plexes previously,35–37 only general syntheses following that of
were designed as potential oxidation catalysts. Solid state mag- Weisman1,3 has been referred to in those publications. Below,
netic moment and acetonitrile solution electronic spectro- we present for the first time, full details in the syntheses of
scopy experiments revealed high spin, octahedral, metal Me2B13N4 and Me2Bcyclen.
complexes in all cases. Electrochemical studies of the divalent
2a,7a-Dimethyl-cis-decahydro-2a,7a-diaza-4a,9a-diazonia-
species under an inert atmosphere and in acetonitrile revealed cyclopenta[cd]phenalene diiodide (2). To a solution of cis-
reversible access to multiple oxidation states, a prerequisite for decahydro-5H-2a,4a,7a,9a-tetraazacyclopenta[cd]phenalene (1),
successful oxidation catalysis, in both cases. In particular, Mn- prepared according to
a
literature method,15 (1.822 g,
(Me2B13N4)Cl2 was stabilized in Mn2+/Mn3+/Mn4+ oxidation 8.75 mmol) in acetonitrile (50 mL), iodomethane (9.2 mL,
states. Finally, preliminary screens for oxidation catalysis using 148 mmol) was added and the flask was then sealed with a
H2O2 as the oxidant were carried out on the biomimetically septum and the reaction mixture was stirred at room tempera-
important Mn and Fe complexes and showed promising ture for seven days. The course white solid product precipi-
results, particularly for Mn(Me2B13N4)Cl2, in the oxidation of tated and was collected on a glass frit and was washed with
thioanisole and for both Mn(Me2B13N4)Cl2 and Fe(Me2B13N4)- acetonitrile (2 × 10 mL) and diethyl ether (2 × 10 mL) prior to
Cl2 in the hydrogen atom abstraction of 1,4-cyclohexadiene. drying under vacuum overnight. Yield = 3.023 g (70%). Electro-
Future work will include expanding the range of oxidation spray mass spectrometry gave peaks at m/z = 365 corres-
reactions possible with these catalysts and determination of ponding to (M − I)+ and m/z = 119 corresponding to (M − 2I)2+.
their oxidation catalysis mechanisms.
Anal. Calc. for C13H26N4I2: C 31.72, H 5.33, N 11.38; found: C
31.77, H 5.26, N 11.25. 1H NMR (300 MHz, D2O) δ 1.96 (d, 1H),
2.50–2.29 (m, 1H), 2.76 (td, 1H), 3.39–3.06 (m, 6H), 3.40 (s, 3H,
–CH3), 3.47 (s, 3H, –CH3), 3.79–3.49 (m, 4H), 3.95–3.83 (m,
2H), 4.16–3.98 (m, 2H), 4.44 (td, 1H), 4.68 (d, 1H), 4.73 (d, 1H).
13C{1H} NMR (75.6 MHz, D2O) δ 19.0, 42.3, 45.9, 46.2, 46.9,
Experimental
General
Methyl iodide (99%), and sodium borohydride (98%), all anhy- 48.6, 50.5, 51.6, 56.5, 64.2, 65.1, 76.6, 78.2.
drous divalent transition metal salts, and all anhydrous sol-
4,11-Dimethyl-1,4,8,11-tetraazabicyclo[6.5.2]pentadecane
vents used in the glovebox were purchased from Aldrich (Me2B13N4). (3.023 g, 6.14 mmol) of 2 was added to a 500 mL
Chemical Co. All other solvents were of reagent grade and were single neck round bottom flask and suspended in 95%
used without modification. 1,4,7,10-Tetraazacyclotridecane ethanol (260 mL). The flask was purged with nitrogen for
(homocyclen) was purchased from Strem Chemicals. cis-Deca- 15 minutes. While stirring, sodium borohydride (6.97 g,
hydro-5H-2a,4a,7a,9a-tetraazacyclopenta[cd]phenalene
(1) 184 mmol) was added over 5 minutes. The frothy white solu-
(homocyclen glyoxal) was prepared according to a literature tion was allowed to stir under nitrogen for 7 days. The excess
method.15 Elemental analyses were performed by Quantitative borohydride was quenched with hydrochloric acid (6 M,
Technologies Inc. Electrospray Mass spectra were collected on 80 mL) and additional HCl was added until the solution
a Shimadzu LCMS-2020 instrument. All samples were dis- reached a pH of two. The ethanol was then removed from the
solved in 50% H2O/50% MeOH. Electron impact mass spectra clear solution in vacuo. The remaining aqueous solution was
were taken on solid samples using a Shimadzu QP2010S made basic with potassium hydroxide (20 ml, 30% w/w) until
GCMS equipped with a direct insertion probe. NMR spectra it reached pH 14 at which point an additional portion of pot-
were obtained on a Varian Bruker AVANCE II 300 MHz NMR assium hydroxide was added (5 g). This solution was then
Spectrometer. Electronic spectra were recorded using a Shi- extracted with benzene (5 × 150 mL). The benzene extracts
madzu UV-3600 UV-Vis-NIR Spectrophotometer. Magnetic were collected and dried over sodium sulfate. After gravity fil-
moments were obtained on finely ground solid samples at tration, the excess benzene was removed in vacuo. Vacuum dis-
ambient temperatures using a Johnson Matthey MSB Auto tillation from KOH gave a clear oil product. Yield = 1.444 g
magnetic susceptibility balance. Electrochemical experiments (98%). Electrospray mass spectrometry gave a single peak at
1
were performed on a BAS Epsilon EC-USB Electrochemical m/z = 241 corresponding to (MH)+. H NMR (300 MHz, CDCl3)
Analyzer. A button Pt electrode was used as the working elec- δ 1.44 (p, 2H, β-CH2), 2.00 (dt, 1H), 2.13 (s, 3H, –CH3), 2.31 (s,
trode with a Pt-wire counter electrode and a Ag-wire pseudo- 3H, –CH3), 2.36–2.80 (m, 14H), 2.88–2.96 (m, 2H), 3.03–3.21
reference electrode. Scans were taken at 200 mV s−1. Aceto- (m, 2H), 3.22–3.35 (m, 1H). 13C{1H} NMR (75.6 MHz, CDCl3) δ
nitrile solutions of the complexes (1 mM) with tetrabutyl- 25.6, 42.4, 42.9, 51.4, 51.8, 52.9, 53.7, 54.7, 54.8, 55.2, 56.3,
ammonium hexafluorophosphate (0.1 M) as a supporting 58.0, 59.1. As previously published,36 the free ligand can be
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made into the analytically pure hydrochloride salt nitrile in a 4-dram vial. The reaction mixture was stirred at
(Me2B13N4·3HCl·H2O) by reaction with concentrated HCl in room temperature for 4 days, during which the MnCl2 and
ethanol to produce a white solid which can be recrystallized ligand dissolved to give a clear solution. The solution was fil-
from methanol–ether. Anal. Calc. for C13H33N4Cl3O: C 42.45, tered to remove trace solids, which were discarded, and the
H 9.04, N 15.23; found: C 42.39, H 9.19, N 14.88.36
solvent evaporated to give a white solid. Yield = 0.329 g (90%).
3a,8a-Dimethyl-cis-decahydro-5a,10a-diaza-3a,8a-diazonia- Electrospray mass spectrometry gave a large peak at m/z = 330
pyrene diiodide (4). To a solution of cis-decahydro-2a,4a,6a,8a- corresponding to (MnLCl)+. Solid state magnetic moment
tetraazacyclopent[fg]acenaphthylene (3), prepared according to 298 K: μ = 5.95μB. Electronic spectrum, acetonitrile, λmax (nm)
a literature method,39 (9.00 g, 46.0 mmol) in acetonitrile [ε (M−1 cm−1)]: <237 nm (>2000). Anal. Calc. for Mn(C13H26N4)-
(300 mL), iodomethane (43.0 mL, 690 mmol) was added and Cl2: C 42.63, H 7.71, N 15.30; found: C 42.30, H 7.92, N 15.37.
the flask was then sealed with a septum and the reaction X-ray quality crystals were grown from ether diffusion into an
mixture was stirred at room temperature for three days. The acetonitrile solution.
white solid product precipitated and was collected on a glass
Fe(Me2B13N4)Cl2. The same procedure as for Mn-
frit and was washed with acetonitrile (2 × 20 mL) and diethyl (Me2B13N4)Cl2 was followed, except 0.127 g (1.0 mmol) of
ether (2 × 20 mL) prior to drying under vacuum overnight. anhydrous FeCl2 was used. The product was a tan solid. Yield
Yield = 20.76 g (94%). Electrospray mass spectrometry gave = 0.335 g (91%). Electrospray mass spectrometry gave a large
peaks at m/z = 351 corresponding to (M − I)+ and m/z = 112 peak at m/z = 332 corresponding to (FeLCl)+. Solid state mag-
corresponding to (M − 2I)2+. Anal. Calc. for C12H24N4I2: C netic moment 298 K: μ = 5.20μB. Electronic spectrum, aceto-
30.14, H 5.06, N 11.72; found: C 30.50, H 5.00, N 11.86. 1H nitrile, λmax (nm) [ε (M−1 cm−1)]: <237 [>2000]; 342 [623]. Anal.
NMR (300 MHz, D2O) δ 2.98–3.22 (m, 4H), 3.31 (s, 6H, –CH3), Calc. for Fe(C13H26N4)Cl2: C 42.53, H 7.69, N 15.26; found:
3.32–3.33 (m, 2H), 3.51–3.61 (m, 4H), 3.37–3.86 (m, 2H) C 42.42, H 7.85, N 15.07.
3.90–4.02 (m, 4H), 4.42 (s, 2H). 13C{1H} NMR (75.6 MHz, D2O)
δ 43.1, 46.6, 46.9, 59.2, 65.0, 78.0.
[Mn(Me2B13N4)Cl2]PF6. In an inert atmosphere glovebox,
the corresponding divalent complex (0.183 g, 0.50 mmol) was
4,10-Dimethyl-1,4,7,10-tetraazabicyclo[5.5.2]tetraadecane dissolved in methanol (10 mL) and ammonium hexafluoro-
(Me2Bcyclen). (10.00 g, 21.0 mmol) of 4 was added to a 2-L phosphate (5 eq., 0.408 g, 2.50 mmol) was added and dis-
single neck round bottom flask and suspended in 95% solved. This slightly tan solution was then removed from the
ethanol (800 mL). The flask was purged with nitrogen for glove box and a flow of nitrogen was immediately placed over
15 minutes. While stirring, sodium borohydride (23.80 g, the surface of the solution. Bromine (2–3 drops) was added
630 mmol) was added over 5 minutes. The frothy white solu- and reaction was allowed to stir. (CAUTION! Bromine is toxic.
tion was allowed to stir under nitrogen for 5 days. The excess Use only in a chemical fume hood with proper ventilation.) An
borohydride was quenched with 6 M hydrochloric acid, then immediate color change from pale tan to blood red occurred
additional HCl was added until the solution reached a pH of and red precipitate product formed. After 15 minutes, nitrogen
two. The ethanol was then removed from the clear solution was bubbled through the deep red/brown solutions to remove
in vacuo. The remaining aqueous solution was made basic excess bromine. After the excess bromine had been removed
with potassium hydroxide (150 ml, 30% w/w) until it reached (∼30 minutes), the solution was placed in a freezer for
pH 14 at which point an additional portion of potassium 30 minutes to finish precipitating the product. The blood red
hydroxide was added (15 g). This solution was then extracted solid product was collected on a glass frit and washed with
with benzene (5 × 150 mL). The benzene extracts were collected cold methanol (10 mL) and diethyl either (2 × 10 mL). Yield =
and dried over sodium sulfate. After gravity filtration, the 0.170 g (67%). Electron impact mass spectrometry gave peaks
excess benzene was removed in vacuo. Vacuum distillation at m/z = 365 corresponding to (MnLCl2)+ and m/z = 330 corres-
from KOH gave a clear oil product. Yield = 4.51 g (95%). Electro- ponding to (MnLCl)+. Solid state magnetic moment 298 K: μ =
spray mass spectrometry gave a single peak at m/z = 227 4.91μB. Electronic spectrum, acetonitrile, λmax (nm) [ε (M−1
corresponding to (MH)+. 1H NMR (300 MHz, C6D6) δ 2.37 (s, cm−1)]: 214 [12 100]; 295 [8600]; 400sh [960]; 526 [512]. Anal.
6H, –CH3), 2.48–2.55 (m, 4H), 2.75–2.94 (m, 12H), 3.10 (s, 4H). Calc. for [Mn(C13H28N4)Cl2]PF6: C 30.54, H 5.52, N 10.96;
13C{1H} NMR (75.6 MHz, C6D6) δ 44.6, 57.1, 57.5, 60.8. As pre- found: C 30.31, H 5.36, N 10.72. X-ray quality crystals were
viously published,36 the free ligand can be made into the ana- grown from acetone diffusion into a water solution.
lytically pure hydrochloride salt (Me2Bcyclen·3HCl·2H2O) by
[Fe(Me2B13N4)Cl2]PF6. The same procedure as for [Mn-
reaction with concentrated HCl in ethanol to produce a white (Me2B13N4)Cl2]PF6 was followed, except that 0.184 g,
solid which can be recrystallized from methanol–ether. Anal. (0.50 mmol) of the divalent iron complex was used. A bright
Calc. for C12H33N4Cl3O2: C 38.77, H 8.60, N 15.07; found: C orange/yellow solid product was obtained. Yield = 0.201 g
39.13, H 8.76, N 15.04.36
(79%). Electron impact mass spectrometry gave peaks at m/z =
366 corresponding to (FeLCl2)+ and m/z = 331 corresponding to
(FeLCl)+. Solid state magnetic moment 298 K: μ = 5.93μB. Elec-
Metal complexation
Mn(Me2B13N4)Cl2. In an inert atmosphere glovebox, tronic spectrum, acetonitrile, λmax (nm) [ε (M−1 cm−1)]: 215
0.240 g (1.0 mmol) of Me2B13N4 and 0.126 g (1.0 mmol) of [7000]; 254 [7400]; 358 [3350]. Anal. Calc. for [Fe(C13H28N4)Cl2]-
anhydrous MnCl2 were added to 10 ml of anhydrous aceto- PF6: C 30.49, H 5.51, N 10.49; found: C 30.73, H 5.73, N 10.59.
Dalton Trans.
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[Zn(Me2B13N4)(OAc)(H2O)]PF6. In an inert atmosphere glo- Where the location of hydrogen atoms was obvious from differ-
vebox, 0.165 g (0.90 mmol) of anhydrous zinc(II) acetate was ence Fourier maps, C–H bond lengths were refined subject to
suspended in 20 ml anhydrous acetonitrile in a 50 ml round chemically sensible restraints.
bottom flask. 0.216 g (0.90 mmol) of Me2B13N4 was separately
Acid decomplexation studies
dissolved in 10 ml of anhydrous acetonitrile and added by
pipet to the stirring metal salt suspension. The reaction
mixture was stirred at room temperature under nitrogen for 1
day. The clear solution was then removed from the glovebox
and evaporated to a colorless oil, which was the crude acetate
complex. The oil was dissolved in 3 ml of methanol. A solution
of 5 equivalents (4.50 mmol, 0.734 g) of NH4PF6 in 5 ml of
methanol was added to the stirring complex, which immedi-
ately precipitated a white solid. After stirring for 5 minutes,
the suspension was stored at −5 °C overnight to complete pre-
cipitation of the product. The product was filtered on a glass
frit, washed with cold methanol followed by diethyl ether, then
dried under vacuum. Yield = 0.211 g (46%). Electrospray mass
spectrometry gave a single peak at m/z = 363 corresponding to
ZnL(OAc)+. Anal. Calc. for [Zn(C13H28N4(C2H3O2)(H2O)]PF6: C
34.13, H 6.30, N 10.62; found: C 34.46, H 6.10, N 10.87. X-ray
quality crystals were grown by slow evaporation of a water
solution.
The designated complexes were used at 1 mM. The complexes’
lone d–d absorption was recorded on a Shimadzu UV-3600
UV-Vis-NIR Spectrophotometer in 1 M or 5 M HCl at 90 °C,
50 °C, or 30 °C over time. Typically, isosbestic spectra indi-
cated 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) vs. time plots.
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 the product analysis was performed by
GC using the internal standard method. A parallel experiment
without catalyst was conducted as control.
[Zn(Me2B13N4)Cl2]. This complex was synthesized as pre-
viously described and analyzed as formulated.37 X-ray quality
crystals were grown from ether diffusion into an acetonitrile
solution and were formulated as [Zn(Me2B13N4)Cl-
(H2O)]2[ZnCl4]·CH3CN, which may reflect either an impurity or
a decomposition product.
[Co(Me2B13N4)Cl2]PF6. This complex was synthesized as
previously described and analyzed as formulated.35 X-ray
quality crystals of two types were grown from ether diffusion
into an acetonitrile solution. Green needles, formulated as
[Co(Me2B13N4)Cl2]PF6·CH3CN, and green plates, formulated
as [Co(Me2B13N4)Cl2]PF6, were isolated and solved from the
same crystallization sample.
Catalytic hydrogen abstraction by complexes
In 3 mL of methanol–water (1 : 1, v/v) containing 0.05 M 1,4-
cyclohexadiene and 1 mM complex, 0.02 mL of 30% H2O2 were
added to initialize the reaction. The resulting reaction mixture
was stirred in a water bath at 303 K for 4 h, and product ana-
lysis was conducted by GC using the internal standard
method. A parallel experiment without catalyst was conducted
as control.
Acknowledgements
TJH acknowledges Southwestern Oklahoma State University
for internal funding through a Proposal Development Award.
TJH acknowledges the Donors of the American Chemical
Society Petroleum Research Fund; Health Research award for
project number HR13-157, from the Oklahoma Center for the
Advancement of Science and Technology; and Grant Number
P20RR016478 from the National Center for Research Resources
(NCRR), a component of the National Institutes of Health
(NIH) for partial support of this research. TJH also acknowl-
edges the Henry Dreyfus Teacher-Scholar Awards Program for
support of this work. GY acknowledges support from the
National Natural Science Foundation of China (NSFC no.
21273086).
[Cu(Me2B13N4)Cl]Cl. This complex was synthesized as pre-
viously described and analyzed as formulated.37 X-ray quality
crystals were grown from ether diffusion in to a nitromethane
solution.
Crystal structure analysis
Single crystal X-ray diffraction data were collected in series of
ω-scans using a Stoe IPSD2 image plate diffractometer utilizing
monochromated Mo radiation (λ = 0.71073 Å). Standard pro-
cedures were employed for the integration and processing of
the data using X-RED.53 Samples were coated in a thin film of
perfluoropolyether oil and mounted at the tip of a glass fibre
located on a goniometer. Data were collected from crystals
held at 150
cryostream.
K in an Oxford Cryosystems nitrogen gas
Notes and references
Crystal structures were solved using routine automatic
direct methods implemented within SHELXS-97.54 Completion
of structures was achieved by performing least squares refine-
ment against all unique F2 values using SHELXL-97.54 All non-
H atoms were refined with anisotropic displacement para-
meters. Hydrogen atoms were placed using a riding model.
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