Axial Mn-CCN Bonds of Cyano Manganese(II) Porphyrin Complexes: Flexible and Weak?

Article abstract of DOI:10.1021/acs.inorgchem.6b00173

Three five-coordinate high-spin (cyano)manganese(II) complexes, utilized tetraphenylporphyrin (TPP), tetratolylporphyrin (TTP), and tetramesitylporphyrin (TMP) as ligands, are prepared and studied by single-crystal X-ray, FT-IR, UV-vis, and EPR spectrosco

Full text of DOI:10.1021/acs.inorgchem.6b00173

Article
pubs.acs.org/IC
Axial Mn−CCN Bonds of Cyano Manganese(II) Porphyrin Complexes:
Flexible and Weak?
Mingrui He,† Xiangjun Li,† Yanhong Liu,‡ and Jianfeng Li*,†
†College of Materials Science and Optoelectronic Technology, University of Chinese Academy of Sciences, Yanqi Lake, Huairou
District, Beijing 101408, China
‡Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China
S
* Supporting Information
ABSTRACT: Three five-coordinate high-spin (cyano)-
manganese(II) complexes, utilized tetraphenylporphyrin
(TPP), tetratolylporphyrin (TTP), and tetramesitylporphyrin
(TMP) as ligands, are prepared and studied by single-crystal X-
ray, FT-IR, UV−vis, and EPR spectroscopies. The crystal
structure studies revealed noteworthy structural features
including unexpectedly wide tilting angles of the axial Mn−
CCN bonds, which is contrasted to the isoelectronic Fe(III)−
CCN bonds. Solid-state EPR measurements (90 K) and simulations are applied to obtain the ZFS parameters (D, E, and E/D
(λ)), which are compared to Mn(II) porphyrin analogues of hemes to understand the ligand field of the cyanide. The solution
EPR studies gave new insights into the chemical equilibrium of four- and five-coordinate species, which has been monitored by
UV−vis spectroscopy.
In this Article, we report the synthesis and characterization of
the first examples of five-coordinate (cyano)manganese(II)
porphyrin derivatives: [K(222)][Mn(TPP)(CN)] (Cc and
P21/n), [K(222)][Mn(TTP)(CN)], and [K(222)][Mn(TMP)-
(CN)]. The single-crystal structures, UV−vis, FT-IR, and
multitemperature EPR spectra in frozen solution and solid
states are investigated.
INTRODUCTION
■
Although both iron1 and manganese2 porphyrin complexes
have long been used as the models of cytochrome P-450 and
hemoglobins, the reports on manganese(II) porphyrinates are
much less as compared to the extensively studied iron(II,III)
analogues. The first manganese(II) porphyrin structure [Mn-
(TPP)(1-MeIm)]3 was reported by Scheidt and co-workers in
1977.4 The authors claimed that manganese(II) derivatives
have a distinct preference for five coordination irrespective of
EXPERIMENTAL SECTION
■
2
2
General Information. All experimental operations with Mn(II)
complexes, including the reduction of Mn(III) complexes, were carried
out using standard Schlenk ware and cannula techniques under an
atmosphere of argon unless otherwise noted. Benzene (Sinopharm
Chemical Reagent) was distilled over sodium/benzophenone, hexanes
(Beijing Chemical Works) over potassium−sodium alloy, and
chlorobenzene (Sinopharm Chemical Reagent) over P2O5 under
nitrogen. 2,6-Dimethylpyridine and ethanethiol were purified by
distilling before use. MnCl2, dichloromethane, hydrochloric acid,
acetone, methanol, DMF, and propionic acid were used as received.
KCN was recrystallized and purified by a literature procedure.13
Krypotofix 222 (ACROS) was purified by vacuum sublimation.
[K(222)(CN)] was prepared by the reaction of Krypotofix 222 with
KCN (1:1) in THF, which was stirred overnight and evaporated to
dryness. The porphyrin ligands H2TPP and H2TTP were prepared
according to Adler et al.,14 and H2TMP was made with a modified
procedure published by Lindsey et al.15 UV−vis spectra were recorded
on a PerkinElmer Lambda 25 UV−vis spectrometer. FT-IR spectra
were recorded on a Nicolet 6700 FT-IR spectrometer as Nujol mulls.
Synthesis of [K(222)][MnII(Porph)(CN)]. Free-base porphyrin (1
g) and MnCl2 (2 g, 14.7 mmol) were heated under reflux in DMF
the choice of axial ligand and when the dx −y orbital is
populated, as the metal atom is too far out of plane to permit
effective interaction with a sixth ligand. Only five-coordinate
[Mn(II)(Porph)(L)] are isolated despite attempts to force six-
coordination with large excesses of a variety of ligands.5
Cyanide ion (CN−) has been widely investigated both as a
classic inhibitor and as a ligand for exploring the properties of
hemes and hemoproteins.6 The strong binding of cyanide to
ferric hemoproteins has been long recognized, and cyanide is
deeply entrenched as a strong-field ligand.7 All of the known
cyano iron(III) porphyrinates, either bis(cyano) [FeIII(Porph)-
(CN)2] or mixed-ligand [FeIII(Porph)(CN)(L)], are six-
coordinated and low-spin.8 Recently, the first (cyano)iron(II)
porphyrin complex, five-coordinate [K(222)][FeII(TPP)-
(CN)], was isolated, which exhibited unusual temperature-
dependent S = 0 ⇌ S = 2 spin crossover phenomena.9
[K(222)][Fe(TPP)(CN)] represents a case in which π back-
bonding should be maximized and the CN− ligand should
unequivocally lead to a LS species.10 The spin crossover
behavior has been investigated using electronic structure
methods.11 The first cyano manganese(III) porphyrin complex
[MnIII(TPP)(CN)] was reported to be a high-spin derivative.12
Received: January 23, 2016
© XXXX American Chemical Society
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(500 mL) until the TLC showed the absence of free porphyrin red
fluorescence.16 The cooled reaction mixture was poured into a flask
containing 500 mL of water. The resulting green precipitate was
collected by filtration and washed with 500 mL of H2O. The solid was
dissolved in dichloromethane, then vigorously shaken in 3 M solution
of aqueous KOH. The dichloromethane layer was separated from the
water layer, and the solvent was evaporated under vacuum. The
residue was purified by column chromatography of silica gel with
acetone, then eluted with methanol. Rotary evaporation of the solvent
yielded a dark green solid, 0.73 g (73%) of [Mn(TPP)(OH)] (UV−vis
(CH2Cl2): 374.5, 421.5, 472, 532, 578.5, and 616.5 nm), 0.68 g (68%)
of [Mn(TTP)(OH)] (UV−vis (CH2Cl2): 378.5, 420, 470.5, 534, 582,
and 622.5 nm), and 0.6 g (60%) of [Mn(TMP)(OH)] (UV−vis
(CH2Cl2): 372, 397.5, 479, 534, 589, and 625 nm).
one potassium cation coordinated with Kryptofix 222, one ordered
chlorobenzene molecule, and one disordered chlorobenzene molecule.
The disordered chlorobenzene was constrained by means of “DFIX”
command, and the occupancy was found to be 0.63.
EPR. X-band continuous wave (cw) EPR measurements were
carried out on a Bruker E500 EPR spectrometer at a microwave
frequency of 9.45 GHz using a liquid nitrogen cooling system. The
EPR spectra were measured at different temperatures between 4 and
100 K with a modulation amplitude 0.3 mT, a modulation frequency
of 100 kHz, and a microwave power of 10.02 mW. EPR spectra of
[K(222)][Mn(TPP)(CN)] were recorded on a powder sample, which
was obtained by grinding a sufficient quantity of crystals, while
[K(222)[Mn(TMP)(CN)] was recorded on both solid and solution
states and the solution sample was prepared by mixing [MnII(TMP)]
with different equivalents of [K(222)(CN)] in chlorobenzene. The
EPR spectra were simulated with the EasySpin package.20
UV−Vis Titration. Aliquots of the benzene solution of
[K(222)(CN)] (5.9 × 10−3 M) were added to a benzene solution
of [MnII(TPP)] (5.0 × 10−5 M), and the mixture was subjected to
UV−vis spectroscopy at 25 °C. The association constant Ka for the
complex was derived by the nonlinear curve fitting based on the
equations in the Supporting Information.21
[MnII(Porph)] was prepared by reduction of [Mn(Porph)(OH)]
(0.03 mmol) with ethanethiol (1 mL) in benzene (10 mL) for 48 h.
Vacuum evaporation of the solvent yielded a dark purple solid to
which [K(222)(CN)] (12.6 mg, 0.03 mmol) in PhCl was added by
cannula. The mixture was stirred overnight. X-ray quality crystals were
obtained in 8 mm × 500 mm sealed glass tubes by liquid diffusion
using hexanes as nonsolvent.
[K(222)][Mn(TPP)(CN)] (Cc and P21/n). Two polymorphic
crystals have been obtained in the preparation of [K(222)][Mn-
(TPP)(CN)], although the reactions have been conducted in the same
manner. IR ν(C−N): 2198 cm−1 (very weak) for [K(222)][Mn-
(TPP)(CN)] (Cc) and 2198 cm−1 (very weak) for [K(222)][Mn-
(TPP)(CN)] (P21/n). UV−vis (PhCl) λmax: 405, 420, 451, 516, 548,
583, and 626 nm.
Hirshfeld Surface Calculations. Hirshfeld surfaces, fingerprint
plots, and void volumes of [K(222)][Mn(TPP)(CN)] (Cc and P21/n)
were calculated using CrystalExplorer (version 3.1) software from the
crystal structure coordinates supplied as CIF files.22 The Hirshfeld
surface analysis is illustrated in the Supporting Information.23
Hirshfeld surface was calculated at an isovalue of 0.5 e au−3. Void
volumes were calculated using an isovalue of 0.002 e au−3.24
[K(222)][Mn(TTP)(CN)]. IR ν(C−N): 2195 cm−1 (very weak).
UV−vis (PhCl) λmax: 407, 420, 453, 521, 544, 585, and 627 nm.
[K(222)][Mn(TMP)(CN)]. IR ν(C−N): 2173 cm−1 (very weak).
UV−vis (PhCl) λmax: 400, 416, 453, 513, 544, 588, and 628 nm.
X-ray Structure Determinations. Single-crystal experiments for
[K(222)][Mn(TPP)(CN)] (Cc, P21/n) and [K(222)][Mn(TTP)-
(CN)] were carried out on a Xcalibur, Eos, Gemini system and
[K(222)][Mn(TMP)(CN)] on a Bruker ApexII system, with graphite-
monochromated Mo Kα radiation (λ = 0.71073 Å). The crystalline
samples were placed in inert oil, mounted on a glass pin, and
transferred to the cold gas stream of the diffractometer. Crystal data
were collected at 150 or 100 K. The program SADABS17 was used to
apply an absorption correction. The structures were solved by direct
methods (SHELXS-97)18 and refined against F2 using SHELXL-97.19
Subsequent difference Fourier syntheses led to the location of all of
the remaining non-hydrogen atoms. For the structure refinement, all
data were used including negative intensities. All non-hydrogen atoms
were refined anisotropically if not remarked otherwise below.
Hydrogen atoms were added with the standard SHELXL-97 idealized
methods.
RESULTS
■
The synthesis, UV−vis, IR, molecular structures, and EPR of
several five-coordinate (cyano)manganese(II) porphyrinates
are reported. The UV−vis spectrum of [Mn(TPP)(CN)] in
chlorobenzene solution exhibits Soret bands at 405, 420, 451
nm and Q bands at 516, 548, 583, 626 nm (Figure S1).
[Mn(TTP)(CN)] and [Mn(TMP)(CN)] show similar spectra
with Soret bands at 407, 420, 453 nm and 401, 416, 453 nm
(Figures S2 and S3). FT-IR spectra of these cyano complexes
have been measured, and the spectra are found in Figure S4.
The very weak C−N stretching bands (νCN) were observed at
2198, 2195, and 2173 cm−1 for [(K222)][Mn(TPP)(CN)],
[K(222)][Mn(TTP)(CN)], and [K(222)][Mn(TMP)(CN)],
respectively.
Spectrophotometric titrations of the reaction of [Mn(TPP)]
with [K(222)(CN)] were performed in benzene at room
temperature, and the spectroscopic results are shown in Figure
1. On the basis of the change values of absorbance, the Ka of
[K(222)][MnII(TPP)(CN)] was estimated to be 3.72( 0.2) ×
104 M−1, which is comparable to 1.9( 0.3) × 104 M−1 of the
cyano ferriprotoporphyrin IX.25
The molecular structures of [K(222)][Mn(TPP)(CN)] (Cc
and P21/n), [K(222)][Mn(TTP)(CN)]·PhCl, and [K(222)]-
[Mn(TMP)(CN)]·1.63PhCl have been determined. A brief
summary of the crystallographic data is given in Table 1. Two
polymorphic forms of [K(222)][Mn(TPP)(CN)] in space
groups of Cc and P21/n have been characterized. Thermal
ellipsoid plots of [Mn(TPP)(CN)]− (Cc and P21/n), [Mn-
(TTP)(CN)]−, and [Mn(TMP)(CN)]− are given in Figure 2.
Formal diagrams showing the displacements of atoms (in units
of 0.01 Å) from the 24-atom mean plane of the four complexes
are given in Figure S5. The diagrams also display the average
values for the bond distances and angles. Selected bond
distances and angles of five-coordinated (cyano)manganese(II)
porphyrinates and related complexes are given in Tables 2 and
3.
[K(222)][Mn(TPP)(CN)] (Cc). A dark green crystal with dimensions
of 0.86 × 0.46 × 0.25 mm3 was used for the structure determination.
The asymmetric unit contains one porphyrin complex and one
potassium cation coordinated with Kryptofix 222.
[K(222)][Mn(TPP)(CN)] (P21/n). A dark green crystal with
dimensions of 0.25 × 0.20 × 0.06 mm3 was used for the structure
determination. The asymmetric unit contains one porphyrin complex
and one potassium cation coordinated with Kryptofix 222. The C17
atom of the porphyrin phenyl group exhibited unusual thermal
motions; thus the atom was restrained by the “ISOR” command.
[K(222)][Mn(TTP)(CN)]·PhCl. A dark green crystal with dimen-
sions of 0.50 × 0.39 × 0.30 mm3 was used for the structure
determination. The asymmetric unit contains one porphyrin complex,
one potassium cation coordinated with Kryptofix 222, and one
chlorobenzene molecule. Three carbon atoms (C35, C41, C47) of
Kryptofix 222 exhibited unusual thermal motions; thus the atoms were
restrained by the “ISOR” command. Three components of 222 (O3
and C37, O4 and C39, N7 and C41) were restrained by “DELU” to
constrain the anisotropic displacement parameters (ADPs).
[K(222)][Mn(TMP)(CN)]·1.63PhCl. A dark green crystal with
dimensions of 0.34 × 0.27 × 0.16 mm3 was used for the structure
determination. The asymmetric unit contains one porphyrin complex,
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reactions of 1 equiv of [MnII(Porph)] and [K(222)(CN)] yield
the moderately air-stable products. Thermal ellipsoid diagrams
of the four [Mn(Porph)(CN)]− structures are displayed in
Figure 2, which shows the views parallel to the porphyrin
planes. The top-down diagrams of these complexes will be
found in Figures S6−S9. As can be seen, the new structures
share some common features, which include near planar
porphyrin macrocycles, more or less tilted Mn−CCN axial
bonds, and significantly displaced high-spin Mn(II) atoms that
induced noticeable porphyrin core doming and stretched Mn−
Np bonds.
Key structural parameters of the four [Mn(Porph)(CN)]−
complexes are given in Table 2. Also given are the structural
parameters of all known cyano Mn(III) and iron(II,III)
porphyrin complexes. The Mn(II) atoms show significantly
large out-of-plane displacements (Δ24 and Δ4), which are
consistent with the high-spin states and long Mn−Np bonds.
Only one cyano Mn(III) porphyrinate [Mn(TPP)(CN)] is
high spin.12 All of the cyano iron(III) porphyrinates, either
bis(cyano) [Fe(Porph)(CN)2] or mixed-ligand [Fe(Porph)-
(CN)(L)], are six-coordinate and low spin. In these complexes,
the iron(III) atoms rest in the porphyrin planes showing tiny
out-of-plane displacements. The two five-coordinate cyano
iron(II) complexes exhibit unusual temperature-dependent S =
0 ⇌ S = 2 spin-crossover, which possess low-spin state in the
temperature range of 2−100 K.9 Therefore, the metal out-of-
plane displacements exhibit the sequence: HS Mn(II) > HS
Mn(III) > LS Fe(II) > LS Fe(III), which follows the same
sequence of the metal ion radius.28
Figure 1. UV−vis spectral change (in benzene at 295 K) of 5 × 10−5
M solution of [MnII(TPP)] upon gradual addition of 0.1−5 equiv of
[K(222)(CN)]. The enlarged spectra from 500 to 700 nm are
measured in the 10 mm UV cell.
EPR spectra of [K(222)][Mn(TPP)(CN)] and [K(222)]-
[Mn(TMP)(CN)] have been measured in solid and solution
states at temperatures between 4 and 100 K. Computer
simulation have been performed to assist in the analysis of the
spectra. Both experimental and simulated spectra are given in
Figures 4, 5, S10, and S11. The EPR parameters are
summarized in Table 4.
DISCUSSION
■
Crystal Structures and IR Spectra. Until recently, only
two manganese(II) porphyrin structures have been charac-
terized, the low-spin nitrosyl [Mn(TTP)(NO)]26 and the high-
spin [Mn(TPP)(1-MeIm)].4 Lately, we reported three five-
coordinate “picket fence” porphyrin complexes [Mn(TpivPP)-
(R-Im)] (R-Im = 1-MeIm, 1-EtIm, and 2-MeHIm) and studied
the EPR properties in both solution and solid states.27 Here,
three cyano Mn(II) porphyrin complexes are reported. The
The CN− ligand bonds to heme complexes in a linear
fashion, just as that of CO. The bending and tilting are key
structural features of a (cylindrical) axial ligand; the strong
bending sometimes, but not always, accompanies the strong
tilting. The tilting angles (τ) of the four new structures are in an
unexpectedly wide range of 2.6−11.2° (Table 2). Usually the σ-
Table 1. Complete Crystallographic Details for [K(222)][Mn(Porph)(CN)]
[K(222)][Mn(TPP)(CN)]
[K(222)][Mn(TPP)(CN)]
[K(222)][Mn(TTP)(CN)]·
[K(222)][Mn(TMP)(CN)]·
(Cc)
(P21/n)
PhCl
1.63PhCl
chemical formula
fw (amu)
a (Å)
C63H64MnKN7O6
1109.25
C63H64MnKN7O6
1109.25
C73H77ClMnKN7O6
1127.91
C84.77H96.15MnKN7O6Cl1.63
1461.05
14.1096(6)
20.8624(8)
20.7212(8)
90
13.2466(8)
25.3523(14)
17.9030(10)
90
11.7796(3)
14.8337(4)
20.0496(6)
81.690(2)
13.2231(6)
b (Å)
24.3062(12)
c (Å)
24.7602(13)
α (deg)
90
β (deg)
103.628(4)
90
109.549(6)
90
74.862(2)
90
γ (deg)
V (Å3)
86.134(2)
90
5930.6(4)
Cc
5665.8(6)
P21/n
3344.68(16)
7958.0(7)
space group
Z
P1
̅
2
P212121
4
4
4
cryst color
cryst dimensions (mm)
temp (K)
total data collected
unique data
dark green
0.86 × 0.46 × 0.25
150
dark green
0.25 × 0.20 × 0.06
150
dark green
0.50 × 0.39 × 0.30
150
dark green
0.34 × 0.27 × 0.16
100
11 644
23 619
40 103
139 376
7710 (Rint = 0.0324)
6732
11 091 (Rint = 0.0847)
5455
13 139 (Rint = 0.0276)
10 371
17 528 (Rint = 0.0362)
16 196
unique obsd data
[I > 2σ(I)]
GOF (based on F2)
1.050
0.894
1.039
1.109
D
calcd, g cm−3
1.242
1.300
1.269
1.219
μ, mm−1
0.348
0.365
0.357
0.329
final R indices [I > 2σ(I)]
final R indices (all data)
R1 = 0.0402, wR2 = 0.0768
R1 = 0.0501, wR2 = 0.0811
R1 = 0.0607, wR2 = 0.0781
R1 = 0.1407, wR2 = 0.1017
R1 = 0.0592, wR2 = 0.1535
R1 = 0.0767, wR2 = 0.1667
R1 = 0.0494, wR2 = 0.1449
R1 = 0.0544, wR2 = 0.1493
C
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and the contacts are illustrated at the bottom panels of Figure 3.
The larger void volume in [K(222)][Mn(TPP)(CN)] (Cc)
accompanies the shorter contacts, and the close proximity
restricts further tilting of the CN ligand. In contrast,
[K(222)][Mn(TPP)(CN)] (P21/n), with its larger room for
the axial ligand, induces a larger tilting angle. These analyses are
consistent with the void-volume percentage calculations. The
smaller void-volume percentage of [K(222)][Mn(TPP)(CN)]
(P21/n), 15.25% versus 19.20%, is consistent with its more
compact crystal lattice. Furthermore, a considerable number of
points beyond 2.8 Å in the 2-D fingerprint plots of
[K(222)][Mn(TPP)(CN)] (Cc) (left middle panel of Figure
3) is further evidence of “cavities” in the structure. Thus, the
forces such as nonbonded interactions and/or crystal packing
play important roles in the geometry of the Mn−C−N
moieties.
As given in Table 2, a total of 17 cyano iron(III) porphyrin
structures have been reported. For such a large number of
structures, the τ angles of the cyanide are in a small range of
1.6−8.6°. For example, the basket-handle porphyrin complex
[K(222)][Fe(BH(Bipy)2P)(CN)2] has two kinked cyanide
ligands bonding to the iron(III) center.33 The extremely strong
hindrance to the axial ligands induces a very large bending Fe−
C−N angle of 170.8(5)° (average value of two cyanide ligands
as reported), similar to 171.3(4)° in [Mn(TPP)(CN)]− (P21/
n). However, the strong bending does not lead to the strong
tilting, as [Fe(BH(Bipy)2P)(CN)2]− shows much less tilting
angle than that of [Mn(TPP)(CN)]− (P21/n) (5.2° vs 11.2°).
This difference led us to inspect the isoelectronic FeIII−CCN
and MnII−CCN bonds: Is the MnII−CCN bond more flexible to
tilt?
Figure 2. Thermal ellipsoid plots of [Mn(Porph)(CN)]− with partial
atom labeling scheme. The off-axis tilt of the axial ligand and the Mn−
C−N angles are shown. The dashed line is the position of the normal
to the porphyrin plane. Thermal ellipsoids are contoured at the 40%
probability level. Hydrogen atoms have been omitted for clarity. (a)
[Mn(TPP)(CN)]− (Cc). (b) [Mn(TPP)(CN)]− (P21/n). (c) [Mn-
(TTP)(CN)]−. (d) [Mn(TMP)(CN)]−.
Apparently, the significantly out-of-plane high-spin Mn(II)
atom is more able to cause a tilted axial bond than the low-spin
iron(III) atom, which deeply rests in the porphyrin plane.
However, more insights are provided by the vibrational
properties of the cyanide ligand, which are available in Table
2. Cyano Mn(II) complexes shows 40−90 cm−1 higher C−N
stretches than the Fe(III) analogues. It has been agreed that the
σ-donation has the effect of increasing C−N stretching
frequency, while the π-back-bonding tends to decrease the
interaction, which requires donations of lone-pair electrons
8b,39
2
value of νCN
.
Thus, the higher νCN of the Mn(II)−CCN
from a ligand to the dz orbital of the metal center, is favored by
“linear” coordination of the ligands. CN− is a better σ-donor
but a poorer π-acceptor; thus deviations from the linear
conformation are more tolerated for the CN− ligand. As a
result, the bonding geometry of the cyanide is governed to a
great extent by the environment of the heme center and varies
considerably.36 Here, we used the recently developed program
of CrystalExplorer to do the Hirshfeld surface analysis,22,23,37
which provides the structural comparison by examining
interactions of an individual molecule with its nearest neighbor
molecules.
bonds suggests they are more dominated by σ-bonding than the
Fe(III)−CCN bonds. This suggestion is in agreement with their
wider range of tilting angles (and flexibility). Among the four
new complexes, a decreased νCN sequence is seen with the
increase of electron-donating substituents at the meso-phenyl
groups of the porphyrins. The increase of electron-donation of
porphyrin substituents will in turn increase the π donation of
the Mn(II) atom to the cyanide ligands, which has the effect of
decreasing the νCN. As a consequence, a decreasing νCN was
observed from [Mn(TPP)(CN)]− (2198 cm−1) to [Mn(TTP)-
(CN)]− (2195 cm−1) and [Mn(TMP)(CN)]− (2173 cm−1).
Table 3 gives the structural parameters of all known five-
coordinate Mn(II) porphyrin complexes. All of the complexes
are high spin except [Mn(TTP)(NO)]. The cyanide, imidazole,
and chloride, three types of derivatives, are seen. In each type,
the complexes show consonant structural parameters. Cyano
complexes show the largest metal out-of-plane displacements
and the longest Mn−NP distances, while the chloride
complexes show the longest axial distance of the Mn−Cl
bonds. It is probably surprising to see that the axial Mn−CCN
bond is slightly longer than the Mn−NIm bond, because the
cyanide has been known to be a stronger ligand than the
The two polymorphic structures [Mn(TPP)(CN)]− (Cc and
P21/n) are picked as examples, and the Hirshfeld surfaces and
fingerprint plots are presented in Figure 3. It is seen that the
closest intermolecular contacts around the cyano group in
[K(222)][Mn(TPP)(CN)] (Cc) are three N···H interactions of
2.492, 2.543, and 2.604 Å, from the surrounding porphyrin
phenyl groups with a void volume of 1138.60 Å3. This is
compared to the structure of [K(222)][Mn(TPP)(CN)] (P21/
n), where the two closest N···H contacts, 2.571 and 2.717 Å,
are from the [K(222)]+ and phenyl groups with a void volume
of 863.92 Å3. All of these distances are smaller than the sum of
the van der Waals radii of H (1.20 Å) and N (1.55 Å) atoms,38
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a
Table 3. Selected Structural Parameters for [MnII(Porph)(L)]
b
b
c
d
complex
Δ24
Δ4
(Mn−Np)av
Mn−L
(Mn−L)av
ref
[K(222)][Mn(TPP)(CN)] (Cc)
[K(222)][Mn(TPP)(CN)] (P21/n)
[K(222)][Mn(TTP)(CN)]
[K(222)][Mn(TMP)(CN)]
[Mn(TPP)(1-MeIm)]
0.70
0.81
0.83
0.81
0.56
0.56
0.54
0.60
0.74
0.70
0.40
0.63
0.66
0.70
0.70
0.51
0.50
0.49
0.54
0.64
0.63
2.163(9)
2.176(4)
2.173(6)
2.167(5)
2.128(7)
2.1285(6)
2.122(3)
2.129(3)
2.160(10)
2.152(4)
2.004(4)
2.220(4)
2.193(4)
2.210(3)
2.245(4)
2.192(2)
2.168(5)
2.17(7)
2.217(22)
2.177(11)
2.367(5)
tw
tw
tw
tw
4
[Mn(TpivPP)(1-MeIm)]
[Mn(TpivPP)(1-EtIm)]
27
27
27
40
41
26
[Mn(TpivPP)(2-MeHIm)]
[K(222)][Mn(TPP)Cl]
2.177(9)
2.364(2)
2.3704(6)
1.641(2)
[K(222)][Mn(TpivPP)Cl]
[Mn(TTP)(NO)](LS)
a
b
Values in angstroms. Displacement of manganese atom from the 24-atom mean plane (Δ24) or the mean plane of the four pyrrole nitrogen atoms
c
d
(Δ4). The positive values are toward the axial ligand. Averaged values of Mn−L distances. tw = this work.
Table 4. EPR Parameters of High-Spin Five-Coordinate Manganese(II) Porphyrin Complexes
a
b
b
c
d
complex
g1
g2
g3
g4
g5
T
phase
D
E
λ
ref
e
[K(222)][Mn(TPP)(CN)]
5.95
5.95
5.95
5.95
5.95
5.95
5.95
2.00
2.00
2.00
2.00
2.00
2.00
2.00
2.0
4
powder
powder
powder
powder
powder
solution
solution
powder
powder
powder
solution
solution
solution
solution
solution
solution
solution
solution
solution
solution
solution
solution
solution
0.53
0.53
0.53
0.53
0.53
0.67
0
0.0026
0.0026
0.0026
0.0026
0.0026
5
tw
tw
tw
tw
tw
tw
tw
50
41
41
41
51
52
52
52
52
53
53
53
53
53
53
53
20
50
100
90
90
90
50
90
90
90
77
77
77
77
77
2
5
5
5
5
e
[K(222)][Mn(TMP)(CN)]
[K(222)][Mn(TMP)(CN)]
ef
,
ef
,
[Mn(TMP)]
g
[Mn(TPP)]
0.79
0.68
0.69
0.67
[Mn(TpivPP)(1-MeIm)]
[Mn(TpivPP)(2-MeHIm)]
5.94
5.95
5.97
5.96
5.9
1.98
2.00
1.99
2.0
1.24
1.23
1.24
0.78
0.77
0.77
0.55
0.54
0.55
0.0034
0.0035
0.0034
5
5
5
[Mn(TPP)(Py)]
>0.3
h
Mn-CCP
2.0
1.21
0.77
0.56
0.54
∼20
10
h
Mn-Hb
5.9
2.0
h
Mn-HRP
5.9
2.0
0.5
<2
h
Mn-Mb
5.9
2.0
0.56
<5
Mn(II)·S
Mn(II)·Pf
Mn(II)·PO4
E· Mn(II)
E·Mn(II)·PO4
E·Mn(II)·Pf
2.0
∼0.03
∼0.03
∼0.03
0.105
0.105
0.090
0.235
2.0
2
2.0
2
2.0
2
0.020
0.020
0.003
0.022
2.0
2
2.0
2
E·Mn(II)·S
2.0
2
a
b
c
d
e
f
Values in K. Values in cm−1. Values in ×10−3. tw = this work. D, E were estimated from EasySpin simulation. The computer simulations are
given in Figure S11. ZFS was determined by inelastic neutron scattering (INS). D was estimated from positions of X- and K-band transitions at 77
K; λ was estimated from the number and line shape of hyperfine lines in the X-band transitions at g⊥ = 5.9.
g
h
imidazoles. This has aroused our interest to study the
complexes by means of EPR, which may give new insights
into this issue.
field splitting (D) being comparable to, or larger than, hν at this
frequency.44 The manganese(II) ion has the electron
configuration of high-spin 3d5; in general, there are six spin
levels (MS = 5/2, 3/2, 1/2) between which a maximum of
five allowed transitions are possible according to the selection
rule ΔMS = 1. When the magnitude of D is comparable to, or
larger than, the energy of the microwave quantum (hν), the
magnetic MS levels are significantly mixed and MS is not a
strictly good quantum number.45 In such instances, besides the
usual Kramers doublet transitions, the interdoublet transitions
also contribute significantly to resonances in the magnetic field
range.44,46 Therefore, the spectra are complicated by
resonances from multiple overlapping transitions and off-
principle-axis signals (looping transition).47 Temperature-
dependent measurements from 4 to 100 K have also been
performed on the powder [K(222)][Mn(TPP)(CN)], and the
EPR Spectroscopy. EPR measurements have been
performed on [K(222)][Mn(TPP)(CN)] (powder) and
[K(222)][Mn(TMP)(CN)] (powder and solution) at multiple
temperatures between 4 and 100 K. The powder spectra of
[K(222)][Mn(TPP)(CN)] and [K(222)][Mn(TMP)(CN)]
and the simulations are given in Figure 4. Two features are
noted on the experimental spectra. The characteristic signals at
g⊥ ≈ 5.9 and g∥ ≈ 2.0 are typical of a high-spin (S = 5/2) d5
system, for example, high-spin Fe(III) porphyrin complexes,42
and suggest the strong tetragonal field and transitions within
the lowest lying Kramers doublet (| 1/2⟩).43 The spectra also
appear highly complicated and spread over the full field range
of the electromagnet (0−1.5 T). This is evidence for the zero-
F
DOI: 10.1021/acs.inorgchem.6b00173
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
resulting D, E, and the ratio of E/D (λ) are given in Table 4.
Also given are the parameters of all known manganese(II)
heme complexes. It is seen the two cyano complexes
([K(222)][Mn(TPP)(CN)] and [K(222)][Mn(TMP)(CN)])
show consonant D and E values (D = 0.53, E = 0.0026 cm−1),
both of which are smaller than those of the imidazole
derivatives [Mn(TpivPP)(1-MeIm)] (D = 0.68, E = 0.0034
cm−1) and [Mn(TpivPP)(2-MeHIm)] (D = 0.69, E = 0.0035
cm−1). It has been shown that D is inversely proportional to the
energy difference between the ground and the excited states.48
For such a case of 5-coordinate square pyramidal, high-spin d5
complex, the smaller D of the cyanide suggests its stronger field
than the imidazole ligand. In an elegant research of high-spin
iron(III) porphyrin halides, the trend of the increased D was
reported from ligand = F, Cl, Br to I.49
Now we consider the EPR spectra in the glassy solution state.
Four-coordinate [Mn(TMP)] and the mixture of [Mn(TMP)]
with equivalents of [K(222)(CN)] in chlorobenzene have been
measured at 90 K, and the spectra are given in Figure 5. The
Figure 3. Hirshfeld surface (top-down view, the top panels) of
[K(222)][Mn(TPP)(CN)] (Cc, left and P21/n, right) showing
normalized close distance of the promolecule, 2D fingerprint plots
(the middle panels), and the distances of the N···H contacts (the
bottom panels) for the two structures. Highlighted regions on the
Hirshfeld surface indicate the intermolecular contacts (N···H) around
the cyan group (yellow for H atoms of the phenyl to NCN and black for
H atom of [K(222)]+ to NCN).
Figure 5. X-band EPR spectra of [MnII(TMP)] with different
equivalents of [K(222)(CN)] in chlorobenzene at 90 K. (a)
[MnII(TMP)] only. (b) 0.5 equiv of [K(222)(CN)]. (c) 1 equiv of
[K(222)(CN)]. (d) 3 equiv of [K(222)(CN)]. (e) [K(222)(CN)]
saturated.
sample of four-coordinate [Mn(TMP)], without any addition
of cyanide ligand, shows a distinct resonance at g = 2.0 (Figure
5a). This is an idealized case of pure axial distortion, D = 0.
Each transition satisfies the selection rule ΔMS = 1, and the
levels are equally separated at all values of H0.54 Upon the
addition of [K(222)(CN)], the resonance at 5.95 increases
gradually, suggesting the increase of five-coordinate species
[Mn(TMP)(CN)]− (Figure 5b−e). Hyperfine splitting from
the Mn nucleus (I = 5/2) also emerges gradually at both
positions of 5.9 and 2.0. The much stronger resonance at 2.0
from the beginning to the last measurement where the solution
was saturated with [K(222)(CN)] (Figure 5e) suggests the
existence of four-coordinate [Mn(TMP)] because of the
chemical equilibrium. This equilibrium has been monitored
by the UV−vis spectra of the [Mn(TPP)] solution, which was
titrated by [K(222)(CN)], and the resulting spectra are
available in Figure 1. Hence, two Mn(II) nuclei, four- and
five-coordinate, with different ZFS parameters (D = 0 and 0.67
Figure 4. X-band EPR spectra of powder [K(222)][Mn(TPP)(CN)]
(a) and [K(222)][Mn(TMP)(CN)] (b) at 90 K (black lines) and
their simulations (red lines). EPR simulation conditions: D = 0.53
cm−1, E = 0.0026 cm−1.
spectra are given in Figure S10. The very similar spectra suggest
a stable molecular structure in the temperature range.
Computer simulations on the experimental spectra have been
performed to give the spin Hamiltonian parameters. The
G
DOI: 10.1021/acs.inorgchem.6b00173
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
cm−1) are combined in the computer simulations, and the
resulting data are given in Table 4 and Figure S11. Such
apparent coexistence of four- and five-coordinate species is
contrasted to the imidazole derivatives where the four-
coordinate species is undetectable by the EPR spectra of
[Mn(TpivPP)(2-MeHIm)] (CH2Cl2 solution).27
(c) Walker, F. A. Chem. Rev. 2004, 104, 589−616. (d) Scheidt, W. R.;
Reed, C. A. Chem. Rev. 1981, 81, 543−555.
(2) (a) Hoffman, B. M.; Gibson, Q. H.; Bull, C.; Crepeau, R. H.;
Edelstein, S. J.; Fisher, R. G.; McDonald, M. J. Ann. N. Y. Acad. Sci.
1975, 244, 174−186. (b) Gibson, Q. H.; Hoffman, B. M.; Crepeau, R.
H.; Edelstein, S. J.; Bull, C. Biochem. Biophys. Res. Commun. 1974, 59,
146−151. (c) La Mar, G. N.; Walker, F. A. J. Am. Chem. Soc. 1975, 97,
5103−5107. (d) Gunter, M. J.; Turner, P. Coord. Chem. Rev. 1991,
108, 115−161.
CONCLUSIONS
■
(3) Abbreviations: Porph, generalized porphyrin dianion; TPP, meso-
tetraphenylporphyrin dianion; TTP, meso-tetratolylporphyrin dianion;
TMP, meso-tetramesitylporphyrin dianion; TpivPP, α,α,α,α-tetrakis(o-
pivalamidophenyl)porphyrin dianion; OEOP, octaethyloxoporphyrin
dianion; BH(Bipy)2P, bis(6,6′-diphenylbipyridine) basket-handle
porphyrin dianion; OEP, 2,3,7,8,12,13,17,18-octaethylporphyrin dia-
nion; 1-MeIm, 1-methylimidazole; 1-EtIm, 1-ethylimidazole; 2-
MeHIm, 2-methylimidazole; Py, pyridine; 18-C-6, 1,4,7,10,13,16-
hexaoxacyclooctadecane; 222 or Krypotofix 222, 4,7,13,16,21,24-
hexaoxo-1,10-diazabicyclo[8.8.8]hexacosane; Np, porphyrinato nitro-
gen; NIm, nitrogen of imidazole ring; CCN, carbon of cyanide; ZFS,
zero field splitting; PhCl, chlorobenzene; DMF, N,N-dimethylforma-
mide; THF, tetrahydrofuran; DCM, dichloromethane; UV−vis,
ultraviolet−visible; TLC, thin layer chromatography; EPR, electron
paramagnetic resonance; CCP, cytochrome c peroxidase; Hb,
hemoglobin; HRP, horseradish peroxidase; Mb, myoglobin; Mn(II)·
X, is frozen aqueous MnCl2 solutions, where X = S (fosfomycin), Pf
(phosphonoformate), PO4 (in phosphate buffer); E·Mn(II)·X, without
indication of the enzymes’ origin, is genomically encoded FosA
(PA1129) from P. aeruginosa, where X = S, Pf, PO4.
Three, five-coordinate cyano manganese(II) porphyrinates have
been investigated by single-crystal X-ray, FT-IR, UV−vis, and
multitemperature EPR spectroscopies, all of which are
consistent with the high-spin state of the metal centers.
Although the axial Mn−CCN bonds appear flexible and weak for
the unexpectedly wide tilting angles and relatively longer bond
distances, the ZFS parameters presented by the solid-state EPR
investigations are consistent with the stronger field of the
cyanide. The strong coexistence of four- and five-coordinate
species even in saturated solutions is revealed by EPR and UV−
vis experiments. The research sheds new light on the nature of
the cyanide ligand.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b00173.
(4) Kirner, J. F.; Reed, C. A.; Scheidt, W. R. J. Am. Chem. Soc. 1977,
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UV−vis and FT-IR spectra (Figures S1−S4), formal
diagrams of the porphyrin cores (Figure S5), figures with
ORTEP diagrams (Figures S6−S9), EPR spectra of
[K(222)][Mn(TPP)(CN)] for crystalline powder at
different temperature (Figure S10), EPR spectra of
[Mn(TMP)] with different equivalents of
[K(222)(CN)] at 90 K and their simulations (Figure
S11), Hirshfeld surface (down-top and edge-on) of
[K(222)][Mn(TPP)(CN)] (Cc and P21/n) (Figure
S12), equation for association constants Ka, and
Hirshfeld surface analysis PDF)
X-ray data for compound [K(222)][Mn(TMP)(CN)]·
1.63PhCl (CIF)
X-ray data for compound [K(222)][Mn(TPP)(CN)]
(Cc) (CIF)
X-ray data for compound [K(222)][Mn(TPP)(CN)]
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(P21/n) (CIF)
X-ray data for compound [K(222)][Mn(TTP)(CN)]·
PhCl (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: jfli@ucas.ac.cn.
■
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the CAS Hundred Talent Program and the National
Natural Science Foundation of China (Grant no. 21371167 to
J.L.).
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DOI: 10.1021/acs.inorgchem.6b00173
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