Synthesis, structural studies, and oxidation catalysis of the manganese(II), iron(II), and copper(II) complexes of a 2-pyridylmethyl pendant armed side-bridged cyclam

Article abstract of DOI:10.1016/j.inoche.2015.07.002

Abstract The first 2-pyridylmethyl pendant armed structurally reinforced cyclam ligand has been synthesized and successfully complexed to Mn^{2 +}, Fe^{2 +}, and Cu^{2 +} cations. X-ray crystal structures were obtained for the diprotonated ligand and its Cu^{2 +} complex demonstrating pentadentate binding of the ligand with trans-II configuration of the side-bridged cyclam ring, 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 these complexes were determined by means of solid state magnetic moment, with a low value of μ = 3.10 μ_B for the Fe^{2 +} complex suggesting that it has a trigonal bipyramidal coordination geometry, matching the crystal structure of the Cu^{2 +} complex, while the μ = 5.52 μ_B value for the Mn^{2 +} complex suggests that it is high spin octahedral. Cyclic voltammetry in acetonitrile revealed reversible redox processes in all three complexes, suggesting that catalytic reactivity involving electron transfer processes are possible for these complexes. Screening for oxidation catalysis using hydrogen peroxide as the terminal oxidant identified the Fe^{2 +} complex as the oxidation catalysts most worthy of continued development.

Full text of DOI:10.1016/j.inoche.2015.07.002

Inorganic Chemistry Communications 59 (2015) 71–75
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis, structural studies, and oxidation catalysis of the
manganese(II), iron(II), and copper(II) complexes of a 2-pyridylmethyl
pendant armed side-bridged cyclam
Anthony D. Shircliff a, Kevin R. Wilson a, Desiray J. Cannon-Smith a, Donald G. Jones a, Zhan Zhang b,
c,
a,
Zhuqi Chen b, Guochuan Yin b, , Timothy J. Prior , Timothy J. Hubin
⁎
⁎
⁎
a
Department of Chemistry and Physics, Southwestern Oklahoma State University, 100 Campus Drive, Weatherford, OK 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
b
Failure, Huazhong University of Science and Technology, Wuhan 430074, PR China
c
Department of Chemistry, The University of Hull, Cottingham Road, Hull HU6 7RX, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
The first 2-pyridylmethyl pendant armed structurally reinforced cyclam ligand has been synthesized and suc-
cessfully complexed to Mn2+, Fe2+, and Cu2+ cations. X-ray crystal structures were obtained for the
diprotonated ligand and its Cu2+ complex demonstrating pentadentate binding of the ligand with trans-II config-
uration of the side-bridged cyclam ring, leaving a potential labile binding site cis to the pyridine donor for inter-
action of the complex with oxidants and/or substrates. The electronic properties of these complexes were
determined by means of solid state magnetic moment, with a low value of μ = 3.10 μB for the Fe2+ complex sug-
gesting that it has a trigonal bipyramidal coordination geometry, matching the crystal structure of the Cu2+ com-
plex, while the μ = 5.52 μB value for the Mn2+ complex suggests that it is high spin octahedral. Cyclic
voltammetry in acetonitrile revealed reversible redox processes in all three complexes, suggesting that catalytic
reactivity involving electron transfer processes are possible for these complexes. Screening for oxidation catalysis
using hydrogen peroxide as the terminal oxidant identified the Fe2+ complex as the oxidation catalysts most
worthy of continued development.
Received 7 November 2014
Received in revised form 28 March 2015
Accepted 5 July 2015
Available online 11 July 2015
Keywords:
Structurally reinforced cyclam
Side-bridged cyclam
Pendant-arm macrocycle
Oxidation catalysis
2-Pyridylmethyl
© 2015 Elsevier B.V. All rights reserved.
We have studied cross-bridged cyclam complexes of manganese
and iron for nearly a decade and a half as oxidation catalysts
[1–14]. The manganese complex of 4,11-dimethyl-1,4,8,11-
tetraazabicyclo[6.6.2]hexadecane [15] (Me2EBC, Scheme 1) in par-
ticular, has a rich oxidation chemistry [5–14]. This compound,
which we propose to call “the Busch catalyst”, was initially synthe-
sized as an oxidation catalyst because the cross-bridged ligand
could rigidly bind the oxygen-reactive manganese metal and stop it
from being lost in the form of MnO2 [1–4]. Que has determined
that the iron complex of Me2EBC is an efficient olefin epoxidation
catalyst with H2O2 oxidant under appropriate conditions as well
[16]. (See Scheme 2.)
Cyclam ligands structurally reinforced with an ethylene bridge be-
tween nitrogens 1 and 4 (i.e. “side-bridged”) have been known since
1980 [17–26]. The additional bridge provides rigidity, configurational
selectivity, and kinetic stability and has been synthetically put in place
by several different methods [17–21]. Like other cyclam ligands, addi-
tional utility can be added by addition of pendant arms, which have in-
cluded in Scheme 1, structure 1: R and/or R′ = H [17,21], Me [19], Et
[20], Bz [19,23,22], Bz-p-NO2 [22,25], Bz-p-NH2 [22], CH2CO2H [24,26],
CH2PO3H [25,26] and others.
An interesting 2-pyridylmethyltrimethylcyclam ligand (Scheme 1,
structure 2) was reported by Que and coworkers [27] and stimulated
our present work. Its iron complex activated dioxogen and formed an
oxoiron(IV) intermediate that was crystallographically characterized,
but has not been pursued further as a catalyst. [27] Although 2-
pyridylmethyl pendant armed unbridged cyclams are ubiquitous
[28–38], no 2-pyridylmethyl pendant armed side-bridged or cross-
bridged cyclams had been reported prior to our work. Due to our expe-
rience with bridged cyclam oxidation catalysts, we set out to synthesize
and characterize side- and cross-bridged cyclam ligands containing a 2-
pyridylmethyl pendant arm, along with their most biologically relevant
oxidation active metal ion complexes: manganese, iron, and copper. Our
cross-bridged work has been reported elsewhere [39]. Here, we report
on the results of our efforts with the side-bridged cyclam ligand,
(Scheme 1, structure 3) and the screening of its complexes as potential
oxidation catalysts.
Alkylation of the cyclam-glyoxal bisaminal 4 in typical SN2 solvents
like acetonitrile was unsuccessful due to self-reaction of the picolyl
chloride. So, we explored the chlorinated solvents [40] to stabilize this
⁎
Corresponding authors.
http://dx.doi.org/10.1016/j.inoche.2015.07.002
1387-7003/© 2015 Elsevier B.V. All rights reserved.

72
A.D. Shircliff et al. / Inorganic Chemistry Communications 59 (2015) 71–75
Scheme 1. Ligands discussed in this paper.
reagent in the presence of the bisaminal and found successful, although
low-yielding, monoalykylation with picolyl chloride in chloroform at
room temperature [41]. We raised the yields of the monoalkyl salt 5 to
66% by addition of KI and increasing the temperature to reflux over
6 days. Typical reduction conditions [23] gave 3 in good yield. Metal
complexation with anhydrous metal chlorides in dry solvents under ni-
trogen gave adequate yields of the desired complexes [41].
complex led us to conclude that the most likely structure is a 6-
coordinate pseudo-octahedral one. If the chloride is not coordinated
and the complex is only 5-coordinate, precipitation by PF−6 would
have led to the di-hexafluorophosphate salt.
Perhaps the most interesting contribution to structural information
came from the magnetic moment of [Fe(3)]Cl2. μ = 3.10 μB, which is
most consistent with n = 2 for a low spin 5-coordinate d6 Fe2+ metal
ion [44]. In our previous experience with Mn2+ and Fe2+ complexes
of bridged tetraazamacrocycles [1,3,45], both ions agree in that they
were high spin and 6-coordinate. However, in this case, Mn(3)Cl+ ap-
pears 6-coordinate and high spin, while Fe(3)2+ appears 5-coordinate
and low spin. Two supporting pieces of evidence for this perhaps unex-
pected geometry for the Fe2+ complex are (1) the 5-coordinate
Cu(3)2+ crystal structure presented above for this ligand system; and
(2) an unpublished crystal structure in which Fe2+ is coordinated to
the four nitrogens of a similar ethylene side-bridged ligand having a
methyl group in place of the pyridylmethyl group of 3, and a chloro li-
gand [39]. Interestingly, a six-coordinate 1,8-bispyridylmethylcyclam
Fe2+ complex with spin-crossover properties and a transition tempera-
ture to low spin of 150 K is known [46]. Of course, low spin d6 Fe2+ com-
plexes are ubiquitous in organometallic chemistry. A number of 5-
coordinate low-spin Fe2+ examples have been published [47–50].
In anticipation of carrying out oxidation studies, we obtained cyclic
voltammograms (Fig. 2) on these complexes in acetonitrile looking for
multiple stabilized oxidations states if catalytic processes were to be
likely. Unfortunately, only one reversible redox wave was observed for
each complex. Relative to SHE, Cu(3)2+ gave a reversible Cu1+/2+
redox wave at E1/2 = −0.586 V (ΔE = 77 mV). No oxidation to Cu3+
was observed, which is perhaps not surprising, as there are no negative-
ly charged ligands to help stabilize the Cu3+ cation. Cu(Me2EBC)Cl+ has
an irreversible Cu2+ to Cu+ reduction wave at Ered = −0.544 V and an
irreversible Cu2+ oxidation to Cu3+ at Eox = +1.530 V [51]. In compar-
ison, Cu(3)2+ is more difficult to oxidize (due to lack of negatively
charged ligands) and more reversibly reduced.
The X-ray crystal structures of the H23Cl2 •2 H2O and
[Cu(3)][PF6]2 •C3H6O were obtained and are depicted in Fig. 1 [42].
The ligand is protonated at one of the piperazine tertiary nitrogens,
and at the secondary nitrogen of the cyclam ring. There is an exten-
sive hydrogen bonding network between the chloride anions, the
two waters of crystallization, the cyclam nitrogens, and their pro-
tons. The pyridine nitrogen is oriented away from the center of the
cyclam ring and is not involved in the hydrogen bonding network.
Upon coordination of Cu2+, the pyridine nitrogen is oriented into
the center of the cyclam ring and coordinates the Cu2+ ion located
there. This compound contains Cu2+ in two different coordination ge-
ometries. The ligand coordinates through each of the nitrogen atoms
but in a slightly different orientation. The two copper complexes are ap-
proximately enantiomers, as shown in Fig. 1(c). The coordination geom-
etry around the copper(II) ion is somewhat between trigonal
bipyramidal (as shown in Fig. 1(b)) and square pyramidal (as shown
in Fig. 1(c)). To illustrate the slight differences between the two differ-
ent metal ion coordination geometries of this structure, square pyrami-
dal type bond angles for the two different copper(II) ions are given:
Cu(1) N–Cu–N angles/°: (in plane) 100.9(2), 74.0(2), 94.75(19),
87.79(19); (to N5) 82.91(17), 118.02(18), 101.94(17), 97.03(18).
Cu(2) N–Cu–N angles/°: (in plane) 102.10(19), 73.4(2), 91.7(2),
86.2(2); (to N25) 80.5(2), 107.85(18), 107.37(19), 120.5(2). Complete
metrical parameters for both structures are given in the Supplementary
Information.
Unfortunately, crystal structures of the Mn2+ and Fe2+ complexes
were not obtained. We sought other hints at their structures and turned
to solid state magnetic moments to help determine them. First, the
magnetic moment of [Cu(3)][PF6]2 was determined to be 2.32 μB,
which is consistent with n = 1 for a d9 Cu2+ ion [43]. [Mn(3)Cl]PF6
was determined to have μ = 5.52 μB, which is consistent with n = 5
for a high spin octahedral d5 Mn2+ metal ion [43]. High spin n = 5 is
not diagnostic of a particular coordination geometry for Mn2+, but the
consistent presence of one chloride in the elemental analysis of this
Mn(3)Cl+ gave a reversible Mn2+/3+ redox wave at E1/2 = +0.856 V
(ΔE = 93 mV) and an irreversible reduction at Ered = −0.685 V. This can
be compared to the well-known Mn(Me2EBC)Cl2 catalyst [1] which has
reversible Mn2+/3+ and Mn3+/4+ waves at E1/2 = +0.585 V (ΔE =
61 mV) and E1/2 = +1.343 (ΔE = 65 mV), respectively. The single chloro
ligand allows only Mn3+ to be accessed for Mn(3)Cl+, and at a higher po-
tential, since only one negatively charged chloro ligand is present to
Scheme 2. Synthesis of 3. a) i. CHCl3, 2 eq. picolyl chloride hydrochloride, 4 eq. NaHCO3, 1 h, filter to remove solids; ii. 1 eq. of (1), 2 eq. KI, reflux 6 days; b) i. 95% EtOH, 5 eq. NaBH4, N2,
reflux, 1.5 h; ii. 12 M HCl(aq), 30% KOH(aq), benzene extraction.

A.D. Shircliff et al. / Inorganic Chemistry Communications 59 (2015) 71–75
73
Fig. 1. (a) Structure of H23Cl2 •2 H2O, (b) structure of Cu(3)2+ depicted as an approximate trigonal bipyramidal geometry and (c) structure of Cu(3)2+ depicted as an approximate square-
based pyramidal geometry and showing that the two slightly different ligand orientations are approximately enantiomers.
stabilize the growing positive charge. Reduction is possible here, when
not observed for Mn(Me2EBC)Cl2, but not reversible, likely due to the
loss of the chloro ligand upon reduction to Mn+.
5-Coordinate Fe(3)2+ exhibits a complex cyclic voltammogram with
a reversible Fe2+/3+ wave at E1/2 = +0.456 V (ΔE = 78 mV), two irre-
versible reductions at Ered = −0.802 V and −1.671 V, respectively, and
a large return oxidation at Eox = −0.252 V. A much simpler behavior is
observed for Fe(Me2EBC)Cl2, with only E1/2 = 0.110 (ΔE = 63 mV)
observed for Fe2+/3+ [1]. Oxidation is obviously much easier for the
latter complex, where two negatively charged chloro ligands stabilize
the positive charge. Reduction is observed only for Fe(3)2+, where no
negatively charged ligands inhibit it. The complex behavior of Fe(3)2+
after the initial reduction is the subject of ongoing study.
Finally, we present initial oxidation screening data on these
complexes. As shown in Table 1, compared with its analog,
Mn(Me2EBC)Cl2 complex, the redox activities of Cu(3)2+, and
Mn(3)Cl+ are quite poor. In sulfide oxidation using H2O2 as oxidant,
while the Mn(Me2EBC)Cl2 catalyst provided nearly complete
conversion of thioanisole (99.8%) with 44.3% yield of sulfoxide and
46.5% of sulfone, the Cu(3)2+ and Mn(3)Cl+ complexes are almost
inactive for sulfide oxidation. However, Fe(3)2+ demonstrated
some activity, providing 9.7% yield of sulfoxide, and 2.9% of sulfone
with 15.6% conversion. Similarly, in hydrogen abstraction from 1,4-
cyclohexadiene, Mn(Me2EBC)Cl2 gave 71.4% yield of benzene with
86.2% conversion, and Cu(3)2+ and Mn(3)Cl+ complexes were still
Fig. 2. Cyclic voltammograms in acetonitrile vs. SHE for (a) Cu(3)2+, (b) Fe(3)2+, and
(c) Mn(3)Cl+
.

74
A.D. Shircliff et al. / Inorganic Chemistry Communications 59 (2015) 71–75
Table 1
Oxidation catalysis screening results for manganese(II), iron(II), and copper(II) complexes of 3, in comparison to Mn(Me2EBC)Cl2.
Complex
Conversion %
Yield % sulfoxide
Yield % sulfone
Mn(Me2EBC)Cl2
[Mn(3)Cl]+
[Fe(3)]2+
99.8
8.3
15.6
6.1
44.3
0.2
9.7
46.5
0.02
2.9
[Cu(3)]2+
3.4
1.9
Complex
Conversion %
Yield % benzene
Mn(Me2EBC)Cl2
[Mn(3)Cl]+
[Fe(3)]2+
86.2
23.1
44.5
29.6
71.4
3.5
23.6
4
[Cu(3)]2+
inactive for benzene formation (~3% yields represent natural ben-
zene content in commercial 1,4-cyclohexadiene). Again, Fe(3)2+
provides more encouraging results, generating 23.6% yield of
benzene with 44.5% conversion.
Acknowledgments
TJH acknowledges the Donors of the American Chemical Society Pe-
troleum 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 In-
stitutes of Health (NIH) for partial support of this research. TJH also ac-
knowledges the Henry Dreyfus Teacher-Scholar Awards Program for
support of this work.
These results are perhaps not surprising, when reconciled with the
electrochemical studies (Fig. 2). Mn(3)Cl+ has only a redox couple of
Mn2+/3+ at E1/2 = +0.856 V, which is much higher than that of the
corresponding Mn2+/3+ couple of Mn(Me2EBC)Cl2 (E1/2
=
+0.585 V), and no access to Mn(IV). Therefore, the catalytic cycle
of the Mn(3)Cl+ complex is very sluggish in oxidations. Similar
sluggish redox behavior is observed in Cu(3)2+ since its oxidation
to Cu(3)3+ is not observed. For the Fe(3)2+ complex, although its
catalytic activity in sulfide oxidation, an oxygen transfer process, is
poor, it still demonstrates a relatively good activity in hydrogen
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.inoche.2015.07.002.
abstraction because its redox couple for Fe2+/3+ is modest (E1/2
=
+0.456 V) and even lower than that of Mn(Me2EBC)Cl2. Consistent
with the hydrogen abstraction activity of the Fe(3)2+ complex,
Stack has reported [55] that Fe(PY5)(OH)3+, which has a potential
of Epc = +0.555 V, is capable of stoichiometric hydrogen abstrac-
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cyclam ligand, 3, has been synthesized, and its diprotonated salt struc-
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Roisnel, Eur. J. Inorg. Chem. (2006) 3472–3481.
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Patinec, M. Le Baccon, H. Handel, T. Roisnel, New J. Chem. 30 (2006) 392–398.
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Barbier, M. Aplincourt, New J. Chem. 25 (2001) 1168–1174.
[43] X-ray crystallographic details: Single crystal X-ray diffraction data were collected in
series of ω-scans using
a Stoe IPSD2 image plate diffractometer utilising
[38] J.-F. Morfin, R. Tripier, M. Le Baccon, H. Handel, Polyhedron 28 (2009) 3691–3698.
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monochromated Mo radiation (λ = 0.71073 Å). Standard procedures 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 K in an
Oxford Instruments nitrogen gas cryostream.Crystal structures were solved using
routine automatic direct methods implemented within SHELXS-97 [54]. Completion
of structures was achieved by performing least squares refinement against all
unique F2 values using SHELXL-97 [54]. All non-H atoms were refined with aniso-
tropic displacement parameters. Hydrogen atoms were placed using a riding
model. Where the location of hydrogen atoms was obvious from difference Fourier
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287–296.
[41] Synthetic details3a-(pyridin-2-ylmethyl)-decahydro-5a,8a,10a-triaaza-3a-azoniapyre-
nium iodide (5). 13. g (0.08096 mol, 2 eq.) of picolyl chloride hydrochloride and
13.60 g (0.1619 mol, 4 eq.) 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 eq.) of (4) [52] and 13.44 g (0.08096 mol, 2 eq.) of KI. The reaction
was stirred and heated to reflux for 6 d under nitrogen, during which it became an or-
ange 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 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.
Calc. 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, 51.9, 53.2, 54.0, 60.5, 62.9, 69.5, 82.2, 125.8, 129.0, 138.6, 146.6, 150.3.5-
maps,
C H bond lengths were refined subject to chemically sensible
restraints.Crystal Data for H23Cl2 •2 H2O: C18H37N5Cl2O2, Mr = 426.42, Z = 4, T =
150(2) K, Monoclinic, P 21/c, a = 17.1551(16) Å, b = 10.2884(11) Å, c =
13.0825(11) Å, α = 90°, β = 100.328(7)°, γ = 90°, V = 2271.6(4) Å3, F(000) =
920, GOF = 0.822. A total of 12083 reflections were collected, of which 4630
were unique ((R(int)) = 0.0828). R1 (I N 2σ(I)) = 0.0421, wR2 = 0.1297. Crystal
Data for [Cu(3)][PF6]2 •C3H6O: Cu2C39H68N10OP4F24, Mr = 1399.99, Z = 4, T =
150(2) K, Orthorhombic, P c a 21, a = 17.6639(8) Å, b = 9.8148(5) Å, c =
31.3088(18) Å, α = 90°, β = 90°, γ = 90°, V = 5427.6(4) Å3, F(000) = 2856,
GOF = 0.798. A total of 33224 reflections were collected, of which 9229 were
unique ((R(int)) = 0.0554). R1 (I N 2σ(I)) = 0.0318, wR2 = 0.0569.
[44] X-AREA v 1.64, STOE & Cie GmbH, Darmstadt, 2012.
(pyridin-2-ylmethyl)-1,5,8,12-tetraazabicyclo[10.2.2]hexadecane (3). 14.138
g
[45] G. Sheldrick, Acta Crystallogr. Sect. A: Found. Crystallogr. 64 (2008) 112–122.
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1973.
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Gomez-Garcia, E. Coronado, Inorg. Chem. 48 (2009) 10416–10423.
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(0.0320 mol, 1 eq.) of 5 was stirred in 1 L of 95% EtOH in a 2 L roundbottom flask.
6.059 g (0.160 mol, 5 eq.) of NaBH4 was added and the reaction was stirred at reflux
for 1.5 h under N2. Upon cooling, 12 M HCl was added to a pH of 2. Solvent was re-
moved under vacuum and the residue was dissolved in 100 ml of water to which
200 ml of 30% KOH was added. The cloudy white suspension was extracted into ben-
zene, dried over Na2SO4, and evaporated to a tan oil. Yield = 7.947 g (78%).
Electrospray mass spectrometry gave a single peak at m/z = 318 corresponding to
(MH)+. Anal. Calc. for C18H31N5 •1.3H2O •0.1C6H6: C 64.07, H 9.89, N 20.08; found: C
63.80, H 10.00, N 19.87. 1H NMR (300 MHz, CDCl3) δ 1.65 (m, 4H), 2.16 (m, 2H),
2.45–2.65 (m, 11H), 2.86 (m, 4H), 3.14 (m, 2H), 3.73 (s, 2H), 3.82 (m, 2H), 7.09 (m,
1H), 7.30 (t, 1H), 7.56 (t, 1H), 8.45 (d, 1H). 13C{1H} NMR (75.6 MHz, D2O) δ 23.5,
26.1, 48.0, 48.1, 50.4, 51.0, 55.0, 55.1, 55.7, 56.9, 58.9, 121.8, 123.6, 135.9, 148.9,
159.1. X-ray quality crystals of the diprotonated salt H232+ were grown from evapo-
ration of the acetonitrile solvent from a complexation reaction of 3 with FeCl2 which
yielded, in addition to the complex product, a small amount of colorless crystals of
H23Cl2 •2 H2O.[Fe(3)]Cl2: 0.317 g (0.001 mol) of (3) and 0.127 g anhydrous FeCl2
were added to 10 ml of anhydrous acetonitrile in an inert atmosphere glovebox. The
Zanello, F. Zanobini, J. Am. Chem. Soc. 115 (1993) 2723–2730.
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33–40.
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