Linkage Isomers of 4-Methylimidazolate Mn(II) Porphyrinates: Hindered or Unhindered?

Article abstract of DOI:10.1021/acs.inorgchem.1c00755

Three different manganese(II) porphyrins have been exploited to react with 4-methylimidazolate (4-MeIm-), and the five-coordinate products are characterized by ultraviolet-visible, single-crystal X-ray, and electronic paramagnetic resonance spectroscopies. Interestingly, 4-MeIm- is found to bond to the metal center through either of the two N atoms (N1 or N3), which yielded two linkage isomers with either an unhindered or a hindered ligand conformation, respectively. Investigations revealed it is the large metal out-of-plane displacements (Δ24 and Δ4 ≥ 0.59 ?) that have rendered the equivalence of two isomers with a small energy difference (5.2-8.3 kJ/mol). The nonbonded intra- and intermolecular interactions thus become crucial factors in the balance of linkage isomerization. All of the products in both solution and solid states show the same characteristic resonances of high-spin Mn(II) (S = 5/2) with g⊥ ≈ 5.9 and g⊥ ≈ 2.0 at 4 K, consistent with the weak effects of the axial ligand on core conformation and metal electronic configurations. Zero-field splitting parameters obtained through simulations are also reported.

Full text of DOI:10.1021/acs.inorgchem.1c00755

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Article
Linkage Isomers of 4‑Methylimidazolate Mn(II) Porphyrinates:
Hindered or Unhindered?
Jianping Zhao, Fei Qian, Wenping Guo, Jianfeng Li,* and Zeyuan Lin
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ABSTRACT: Three different manganese(II) porphyrins have been exploited to react with 4-methylimidazolate (4-MeIm−), and the
five-coordinate products are characterized by ultraviolet−visible, single-crystal X-ray, and electronic paramagnetic resonance
spectroscopies. Interestingly, 4-MeIm− is found to bond to the metal center through either of the two N atoms (N1 or N3), which
yielded two linkage isomers with either an unhindered or a hindered ligand conformation, respectively. Investigations revealed it is
the large metal out-of-plane displacements (Δ24 and Δ4 ≥ 0.59 Å) that have rendered the equivalence of two isomers with a small
energy difference (5.2−8.3 kJ/mol). The nonbonded intra- and intermolecular interactions thus become crucial factors in the
balance of linkage isomerization. All of the products in both solution and solid states show the same characteristic resonances of
5
high-spin Mn(II) (S = /2) with g⊥ ≈ 5.9 and g∥ ≈ 2.0 at 4 K, consistent with the weak effects of the axial ligand on core
conformation and metal electronic configurations. Zero-field splitting parameters obtained through simulations are also reported.
Scheme 1. Comparison of 2-MeIm− and 4-MeIm− (or 5-
INTRODUCTION
■
MeIm−)
Model compound studies of heme iron are useful for
understanding the properties of hemoproteins.1 Five-coordi-
nate heme iron is involved in a large variety of biological
processes, such as catalytic (e.g., cytochrome P450),2 gas
transport (e.g., globins),3 or signal function (e.g., heme-
dependent sensory kinases),4 allowing interaction of heme
with a substrate molecule.5 The functions of these complexes
are modulated by many factors that include the nature and
geometry of the axial ligands.6 Histidyl imidazole that widely
exists in five-coordinate heme complexes has been recognized
as having a more negative charge than a truly neutral imidazole
because of the hydrogen bonding under protein environ-
ments.7 Imidazolate, which is produced by removing the
hydrogen of imidazole to give stronger σ and π donation, is the
extreme case of strong hydrogen bonding. Thus, imidazolate
can be expected to be a stronger field ligand than neutral or H-
bonded imidazole. By using 2-methylimidazolate (2-MeIm−)
and its precursor 2-methylimidazole (2-MeHIm), both of
which have only one hindered ligand conformation through
the N2 atom (Scheme 1), several couples of Fe(II) and Co(II)
porphyrin adduct products have been synthesized and isolated,
respectively.8,9 Interestingly, as opposed to the case of iron(II)
analogues,8 a complete spin state transition of high-spin (S =
3/2) [Co(Porph)(2-MeIm−)]− and low-spin (S = 1/2)
[Co(Porph)(2-MeHIm)] was found for Co(II) species that
2
2
is attributed to the eg(dπ) → b1g(dx −y ) promotion for the
stronger bonding and hindrance of 2-MeIm−.9,10
Manganese(II) (d5) is isoelectronic with iron(III) and, thus,
can be employed as a substitute for iron in studying iron-
Received: March 11, 2021
Published: May 5, 2021
https://doi.org/10.1021/acs.inorgchem.1c00755
© 2021 American Chemical Society
Inorg. Chem. 2021, 60, 7465−7474
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Table 1. Complete Crystallographic Details for [K(222)][Mn(TPP)(4-MeIm−)]·PhCl, 2{[K(222)][Mn(TTP)(4-MeIm−)]}·
3PhCl, [K(222)][Mn(TTP)(4-MeIm−)]·PhCl·H2O, and [K(222)][Mn(TMP)(4-MeIm−)]·PhCl
[K(222)][Mn(TPP)(4-
2{[K(222)][Mn(TTP)(4-
[K(222)][Mn(TTP)(4-
[K(222)][Mn(TMP)(4-
MeIm−)]·PhCl
MeIm−)]}·3PhCl
MeIm−)]·PhCl·H2O
MeIm−)]·PhCl
chemical formula
FW
C69H71.5Cl0.5KMnN8O6
1220.60
C158H169Cl3K2Mn2N16O12
2778.51
C76H83ClKMnN8O7
1349.99
C162.67H191.82ClK2Mn2N16O12
2786.70
a (Å)
b (Å)
c (Å)
α (deg)
12.3193(8)
21.6450(12)
23.4929(15)
90
28.831(11)
16.886(7)
32.208(12)
90
12.9163(5)
21.9010(8)
24.8329(9)
90
12.7623(5)
24.3339(11)
24.9279(11)
90
β (deg)
γ (deg)
V (Å3)
space group
Z
96.599(2)
90
6222.9(7)
P21/n
96.410(14)
90
15583(11)
C1c1
100.749(2)
90
6901.5(4)
P21/c
90
90
7741.5(6)
P212121
2
2
4
4
crystal color
crystal dimensions (mm)
temp (K)
dark green
0.97 × 0.21 × 0.16
100(2)
dark green
0.59 × 0.26 × 0.12
100(2)
dark green
0.49 × 0.16 × 0.07
100(2)
dark green
0.74 × 0.70 × 0.45
100(2)
total no. of data collected
no. unique data
120489
13253 (Rint = 0.1056)
11130
96267
31066 (Rint = 0.0736)
27309
67069
14127 (Rint = 0.0853)
8878
168308
16494 (Rint = 0.0482)
14562
no. of unique observed data
[I > 2σ(I)]
goodness of fit (based on F2) 1.069
1.028
1.184
0.329
0.995
1.300
0.351
1.599
1.195
0.297
Dcalcd (g cm−3
μ (mm−1
)
1.303
0.360
)
final R indices [I > 2σ(I)]
final R indices (all data)
R1 = 0.0627, wR2 = 0.1593
R1 = 0.0757, wR2 = 0.1708
R1 = 0.0709, wR2 = 0.1844
R1 = 0.0802, wR2 = 0.1904
R1 = 0.0812, wR2 = 0.2015
R1 = 0.1381, wR2 = 0.2421
R1 = 0.0637, wR2 = 0.1870
R1 = 0.0767, wR2 = 0.1998
the method of Adler et al.,17 and H2TMP was made with a modified
procedure published by Lindsey et al.18 Ultraviolet−visible (UV−vis)
spectra were recorded on a PerkinElmer Lambda 25 UV−vis
spectrometer.
Synthesis of [Mn(Porph)Cl]. H2TPP (2 g, 3.26 mmol), 2,6-lutidine
(1 mL), and anhydrous MnCl2 (4.07 g, 32.60 mmol) in THF (200
mL) were heated to reflux under argon. The reaction was completed
in 6 h. The mixture was dried on a rotary evaporator and extracted
with CH2Cl2. The filtrate was treated with a diluted HCl solution and
then washed with distilled water three times, dried over anhydrous
magnesium sulfate, filtered, and evaporated to dryness. The resulting
solid was chromatographed on a column of silica gel (2.5:1 benzene/
diethyl ether). The first fraction was collected to give 1.4 g of
[Mn(TPP)Cl] (61%). Reaction procedures similar to the synthesis of
[Mn(TTP)Cl] and [Mn(TMP)Cl] were performed.
Synthesis of [Mn(Porph)(OH)]. A CH2Cl2 (100 mL) solution of
[Mn(Porph)Cl] (0.5 g, ∼0.7 mmol) was shaken vigorously with a 4
M KOH solution (200 mL) three times and dried over anhydrous
magnesium sulfate. After filtration, the filtrate was dried on a rotary
evaporator. The resulting solid was eluted on a column of silica gel
with benzene and diethyl ether (2.5:1) and then methanol. The last
fraction was collected to give 0.3 g of product (62%).
containing heme proteins.11 Scheidt and co-workers reported
the first Mn(II) porphyrin structure in 1977.12 The high-spin
(S = 5/2) [Mn(TPP)(1-MeIm)] has a large metal out-of-plane
distance (Δ24 and Δ4 ≥ 0.51), and the authors suggested that
2
2
when the dx −y orbital is populated, the metal atom is too far
out of the porphyrin plane to permit effective interaction with
a sixth ligand.12 Thus, Mn(II) porphyrins have a distinct
preference to form five-coordinate but not six-coordinate
complexes, in contrast to its near neighbors Fe(II) and Fe(III)
where a sterically hindered ligand, e.g., 2-methylimidazole, has
to be used to prevent the strong tendency to form six-
coordinate products.8,13 In this paper, we report the isolation
and characterization of the first examples of imidazolate-ligated
Mn(II) porphyrinates [K(222)][Mn(Porph)(4-MeIm−)]
(Porph = TPP, TTP, or TMP). Interestingly, linkage isomers
with either hindered or unhindered ligand conformations are
found for the same porphyrin complex. All of the products are
determined to be in the high-spin state, which is consistent
with cyanide14- and imidazole15-ligated Mn(II) analogues.
Synthesis of [K(222)][Mn(Porph)(4-MeIm−)]. [Mn(Porph)(OH)]
(10 mg, 0.015 mmol) was dried under vacuum for 30 min and
dissolved in 5 mL of benzene. After addition of 1 mL of ethanethiol,
the solution was stirred for 2 days and evacuated under vacuum to
give a purple powder. The purple solid of [Mn(Porph)] (10 mg,
0.015 mmol) was dried for 30 min, and excess [K(4-MeIm−)] (6 mg,
0.050 mmol) and Kryptofix 222 (17 mg, 0.045 mmol) in PhCl (5
mL) were added by cannula. The mixture was stirred for 1 h and
transferred into glass tubes (8 mm × 500 mm) that were layered with
hexanes as the nonsolvent. Several weeks later, X-ray quality crystals
were collected.
X-ray Structure Determination. The single-crystal experiment
was carried out on a BRUKER D8 QUEST system with graphite-
monochromated Mo Kα radiation (λ = 0.71073 Å). The crystal
samples were placed in inert oil, mounted on a glass fiber attached to
a brass mounting pin, and transferred to the cold N2 gas steam of the
diffractometer. Crystal data were collected at 100 K and integrated
EXPERIMENTAL SECTION
■
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 and
tetrahydrofuran (Sinopharm Chemical Reagent) were distilled over
sodium/benzophenone. Chlorobenzene (Sinopharm Chemical Re-
agent) was distilled over P2O5 under nitrogen. Hexanes (Beijing
Chemical Works) were distilled over a potassium−sodium alloy. 2,6-
Dimethylpyridine was purified by being distilled before use. MnCl2,
dichloromethane, hydrochloric acid, DMF, and propionic acid were
used as received. KH (Aladdin chemicals) was stored in the drybox
and washed with hexanes before use. Kryptofix 222 (ACROS) was
purified by vacuum sublimation. Potassium 4-methylimidazolate
{[K(4-MeIm−)]} was prepared according to the method of Hu et
al.16 The free base H2TPP and H2TTP were prepared according to
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using a Bruker Apex II system. The structure was determined by a
direct method (SHELXS-2014) and refined against F2 using
SHELXL-2014.19 Subsequent difference Fourier syntheses led to the
location of all remaining non-hydrogen atoms. All non-hydrogen
atoms were refined anisotropically if not remarked upon otherwise
below. For the structure refinement, all data were used, including
negative intensities. All hydrogen atoms were idealized with the
standard SHELX idealization methods useless otherwise noted.
SADABS20 was applied for the absorption correction. Complete
crystallographic details, atomic coordinates, anisotropic thermal
parameters, and fixed hydrogen atom coordinates are given; a brief
summary of crystallographic details is given in Table 1.
into a [MnII(TPP)] solution, and the UV−vis spectra were measured.
Similar procedures were performed for the titrations of [MnII(TMP)]
with [K(4-MeIm−)]. Association constants (K) were derived by the
nonlinear curve fitting based on the equations in the Supporting
Information.22
RESULTS
■
UV−vis titrations of the 4-MeIm− ligand to [MnII(TPP)] were
performed, and the spectra are given in Figure 1. As one can
[K(222)][Mn(TPP)(4-MeIm−)]·PhCl. A dark green crystal with
dimensions of 0.97 mm × 0.21 mm × 0.16 mm was used for
structure determination. The asymmetric unit contains one porphyrin
complex, one potassium cation coordinated with Kryptofix 222, and
one PhCl solvent molecule. The PhCl was found to be disordered
over two positions, and the final SOFs was refined to be 0.50 and
0.50. Hydrogen atoms H1, H4A, H4B, and H4C were found in
Fourier maps, and all coordinates and isotropic temperature factors
were refined. Two outliers were omitted in the last cycles of
refinement.
2{[K(222)][Mn(TTP)(4-MeIm−)]}·3PhCl. A dark green crystal with
dimensions of 0.59 mm × 0.26 mm × 0.12 mm was used for structure
determination. The asymmetric unit contains two porphyrin
complexes with 4-methylimidazolate ligands, two potassium cations
coordinated with Kryptofix 222, and three PhCl solvent molecules.
One Kryptofix 222 molecule and one PhCl molecule are disordered.
These disordered molecules are restrained by “DFIX”, “RIGU”, and
“ISOR” commands. One region was removed with the SQUEEZE
procedure of PLATON after unsuccessful attempts to model it as
plausible solvent molecules.
Figure 1. UV−vis spectral change (in THF at 295 K) of a 5 × 10−5
M
solution of [MnII(TPP)] upon gradual addition of [K(4-MeIm−)-
(222)]. The enlarged spectra from 550 to 650 nm are measured in the
10 mm UV cell.
[K(222)][Mn(TTP)(4-MeIm−)]·PhCl·H2O. A dark green crystal with
dimensions of 0.49 mm × 0.16 mm × 0.07 mm was used for structure
determination. The asymmetric unit contains one porphyrin complex,
one potassium cation coordinated with Kryptofix 222, one PhCl, and
one H2O molecule. PhCl was restrained by “similar Uij” (SIMU) to
constrain the anisotropic displacement parameters (ADPs). The H2O
is fully occupied, and two hydrogen atoms are located from the
difference Fourier map.
see, gradual addition of imidazolate induced Soret (433 nm)
and Q bands (568 and 605 nm) to decrease for four-
coordinate species, and new bands increase (410, 452, 584, and
627 nm) for the five-coordinate product. Similar results are
observed for the titration of [MnII(TMP)] (Figure 2), which
shows new bands at 410, 454, 588, and 630 nm. On the basis
of changes in the spectrum, the K values of [K(222)][Mn-
(TPP)(4-MeIm−)] and [K(222)][Mn(TMP)(4-MeIm−)]
[K(222)][Mn(TMP)(4-MeIm−)]·PhCl. A dark green crystal with
dimensions of 0.74 mm × 0.70 mm × 0.45 mm was used for
structure determination. The asymmetric unit contains one porphyrin
complex, one potassium cation coordinated with Kryptofix 222, and
one PhCl solvent molecule. Four carbon atoms (C3, C4, C1A, and
C4A) of 4-methylimidazolate and two carbon atoms (C27 and C27A)
of the porphyrin phenyl group exhibited unusual thermal motions
and, thus, were restrained by the “ISOR” command. PhCl was
restrained by “similar Uij” (SIMU) and “DELU” to constrain the
anisotropic displacement parameters (ADPs). The disordered 4-
methylimidazolate was constrained by means of the “DFIX”
command. Twenty-four outliers were omitted in the last cycles of
refinement.
were estimated to be (1.12
0.2) × 107 and (3.76
0.2)
EPR Measurements and Simulations. X-Band continuous wave
(cw) EPR measurements were carried out on a Bruker EMXPlus/10−
12 EPR spectrometer at a microwave frequency of 9.45 GHz using a
liquid helium cooling system. The EPR spectra were measured at 4 K
with a modulation amplitude of 0.3 mT, a modulation frequency of
100 kHz, and a microwave power of 10.02 mW. [K(222)][Mn-
(TPP)(4-MeIm−)], [K(222)][Mn(TTP)(4-MeIm−)], and [K(222)]-
[Mn(TMP)(4-MeIm−)] were recorded on both crystalline and
solution states, and the solution samples were prepared by mixing
[MnII(Porph)] with 3 equiv of [K(4-MeIm−)] in chlorobenzene. The
EPR spectra were simulated to the experimental data using
EasySpin,21 which is operated in MATLAB.
UV−Vis Titration. UV−vis spectra were recorded in a specially
designed combined 1 and 10 mm inert atmosphere cell; 1.0 mg (1.5 ×
10−3 mmol) of [MnII(TPP)] was dissolved in 30 mL of THF. The
ligand solution (1.5 × 10−2 M) that was prepared by dissolving an
equivalent amount of [K(4-MeIm−)] and 222 in THF was titrated
Figure 2. UV−vis spectral change (in THF at 295 K) of a 5 × 10−5
M
solution of [MnII(TMP)] upon gradual addition of [K(4-MeIm−)-
(222)]. The enlarged spectra from 550 to 650 nm are measured in the
10 mm UV cell.
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× 106 M−1, respectively.22 It is noteworthy that all of the
imidazolate products exhibit two characteristic peaks at ∼410
and 452 nm, in contrast to the imidazole analogues that usually
show one Soret band {e.g., 442 nm of [Mn(TpivPP)(2-
MeHIm)]}.15 Because Soret and Q bands are attributed to
a1u(π) → eg*(π) and a2u(π) → eg*(π) transitions,
respectively,23 the distinct spectra suggest different ligand-to-
metal and porphyrin-to-metal charge transfer of the two
species.24
Three different Mn(II) porphyrins (TPP, TTP, and TMP)
have been utilized in the reactions with 4-MeIm−, and the
products were isolated as single crystals that were determined
by 100 K X-ray diffraction. Two crystal forms of TTP
derivatives are characterized. In the crystal structure of
2{[K(222)][Mn(TTP)(4-MeIm−)]}·3PhCl, two porphyrin
molecules are found in the asymmetric unit and are named
[Mn(TTP)(4-MeIm−)]− (A or B). Thermal ellipsoid plots of
these complexes are given in Figure 3. The top-down and edge-
on diagrams can be found in Figure S1. Formal diagrams
showing the displacements of atoms (in units of 0.01 Å) from
the 24-atom mean plane are given in Figure S2. The absolute
ligand orientation, given by the dihedral angle between the
axial ligand and the closest Nax−M−Np plane and conven-
tionally denoted by φ, is also shown.
DISCUSSION
■
Crystal Structures. As one can see in the ORTEP
diagrams (Figure 3 and Figure S1), all of the crystal structures
are five-coordinate and show apparent metal out-of-plane
displacements that indicate the high-spin state of Mn(II).
Interestingly, hindered and unhindered linkage isomers are
found, which are consistent with the two different bonding
sites of 4-MeIm− (N1 or N3). In crystals of [K(222)][Mn-
(TPP)(4-MeIm−)]·PhCl and 2{[K(222)][Mn(TTP)(4-
MeIm−)]}·3PhCl, 4-MeIm− bonds to porphyrins in an
unexpected hindered conformation with the methyl group
directed to the porphyrin planes, similar to a 2-
methylimdiaozle(ate) ligand, e.g., [FeII(TPP)(2-MeHIm)]
and [K(222)][FeII(TPP)(2-MeIm−)].8 In contrast, crystals of
[K(222)][Mn(TMP)(4-MeIm−)]·PhCl and [K(222)][Mn-
(TTP)(4-MeIm−)]·PhCl·H2O show the usual unhindered
ligand conformation, the same as those of six-coordinate
[K(222)][FeIII(TPP)(4-MeIm−)2].27 The reaction and linkage
isomerization are illustrated in Scheme 2.
Figure 3. Thermal ellipsoid plots of [Mn(TPP)(4-MeIm−)]−,
[Mn(TTP)(4-MeIm−)]− (A), [Mn(TTP)(4-MeIm−)]− (B), and
[Mn(TMP)(4-MeIm−)]−.
Table 2 gives key crystal structural parameters of all known
imidazole(ate) Mn(II) porphyrinates. Parameters of related
Fe(III) analogues are also given for comparisons. The five new
structures show consonant structural parameters in narrow
ranges, e.g., Δ24 and Δ4 (0.64−0.72 and 0.59−0.64,
respectively) and (Mn−Np)av distances [2.148(15)−2.155(4)
Å]. Regardless of the hindered or unhindered conformation, all
of the imidazolate Mn(II) derivatives always show larger metal
out-of-plane displacements (Δ24 and Δ4 ≥ 0.59) and equatorial
(Mn−Np)av distances [≥2.148(15) Å] longer than those of
Np)av distances has been widely accepted and, in some cases,
applied in the synthesis to obtain five-coordinate high-spin
species, e.g., the high-spin iron(II) [Fe(Porph)(2-MeHIm)]
complexes.16 Thus, in Table 2, [Mn(TpivPP)(2-MeHIm)]
shows larger Δ24 and Δ4 (0.60 and 0.54, respectively) and
longer (Mn−Np)av distances [2.129(3) Å] than [Mn(TPP)(1-
MeIm)], [Mn(TpivPP)(1-MeIm)], and [Mn(TpivPP)(1-
EtIm)] that are ligated with unhindered imidazoles. However,
this effect is not observed (or tiny, if any) for current
imidazolate Mn(II) complexes that already possess large
enough metal out-of-plane displacements. [Mn(TMP)(4-
MeIm−)]− with an unhindered ligand, however, shows the
largest Δ4 and (Mn−Np)av distances, while the expected large
parameter values that should have been induced by the
hindered 4-MeIm− of [Mn(TPP)(4-MeIm−)]− and [Mn-
imidazole analogues [Δ24 and Δ4 ≤ 0.60; (Mn−Np)av
≤
2.129(3) Å]. The imidazolate derivatives also show axial Mn−
NIm bond distances [≤2.139(5) Å] shorter than those of
imidazole analogues [≥2.168(5) Å]. These structural features
are quite consistent with the stronger ligand nature of
imidazolate than imidazole, which has drawn the metal more
out of the porphyrin plane by its stronger bonding.
The steric effect of a hindered axial ligand that would induce
larger metal out-of-plane displacements as well as longer (M−
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Scheme 2. Reactions of 4-MeIm− with Mn(II) Porphyrins
Table 2. Selected Structural Parameters for Manganese(II)/Iron(III) Porphyrin Species
a,
b
a
,
b
a,
c
a
,
d
e
,
f
e,
g
e
,
h
e,i
complex
Δ24
Δ4
(M−Np)av
M−NIm
M−NIm−CIm(2)
M−NIm−CIm(4)
φ
τ
ref
Mn(II) with Hindered Imidazole(ate)
[Mn(TPP)(4-MeIm−)]−
0.71
0.72
0.71
0.60
0.62
0.63
0.61
0.54
2.153(8)
2.152(7)
2.148(15)
2.129(3)
2.118(2)
2.126(6)
2.139(5)
2.177(9)
133.7(2)
132.5(5)
134.7(5)
126.4(7)
122.44(19)
124.6(5)
122.0(5)
128.1(9)
20.9
21.7
5.46
4.3
6.1
6.1
9.8
this work
this work
this work
15
[Mn(TTP)(4-MeIm−)]− (A)
[Mn(TTP)(4-MeIm−)]− (B)
[Mn(TpivPP)(2-MeHIm)]
28.1
Mn(II) with Unhindered Imidazole(ate)
[Mn(TTP)(4-MeIm−)]−
0.64
0.68
0.56
0.56
0.54
0.59
0.64
0.51
0.50
0.49
2.150(4)
2.155(4)
2.128(7)
2.1285(6)
2.122(3)
2.129(4)
2.13(21)
2.192(2)
2.168(5)
2.170(7)
125.3(3)
122.0(4)
130.4(2)
128.8(9)
130.5(13)
131.5(3)
130.0(3)
125.2(2)
125.5(8)
123.0(4)
24.75
15.3
15.4
24.0
23.5
12.3
3.4
10.9
5.2
this work
this work
12
15
15
j
[Mn(TMP)(4-MeIm−)]−
[Mn(TPP)(1-MeIm)]
[Mn(TpivPP)(1-MeIm)]
j
[Mn(TpivPP)(1-EtIm)]
5.5
Fe(III)
[Fe(OEP)(2-MeHIm)]ClO4
[Fe(OEP)(2-MeHIm)]ClO4(A)
[Fe(OEP)(2-MeHIm)]ClO4 (B)
[Fe(oetpp)(2-MeHIm)]ClO4
[Fe(TPP)(4-MeIm−)2]−
0.36
0.34
0.33
0.26
0.00
0.35
0.34
0.31
0.26
0.00
2.038(6)
2.039(28)
2.039(28)
1.966(2)
1.998(25)
2.068(4)
2.09
2.10
2.079(2)
1.928(12)
1.958(12)
131.7(3)
134.6
138.2
132.5(19)
130.9(11)
125.2(11)
122.1(3)
120.3
117.0
120.5(18)
127.9(11)
129.7(11)
4.0
7.3
3.0
3.9
16.7
1.4
5.0
0
2.5
2.5
0
25
25
25
26
27
27
0
a
b
Values in angstroms. Displacement of the metal atom from the 24-atom (Δ24) or the four-pyrrole nitrogen atom (Δ4) mean plane. The positive
c
d
numbers indicate a displacement toward the axial ligand. Average distance between the metal and porphyrin nitrogen atoms. Distance between
e
f
g
the metal and the axial nitrogen atom. Angles in degrees. M−NIm−CIm angle with C2 being the methyl-substituted direction. M−NIm−CIm angle
h
with C4 being the methyl-substituted opposite direction. Dihedral angle between the ligand plane and the plane of the closest Np−M−NIm angle.
i
j
Tilt of the M−NIm vector off the normal to the 24-atom mean plane. Average values between two disordered ligand parts.
(TTP)(4-MeIm−)]− (A and B) are not clear. It is thus
deduced that when the metal out-of-plane displacements are
large enough (e.g., Δ24 and Δ4 ≥ 0.59), the steric effect of
hindered 4-MeIm− is dramatically eliminated, leaving porphyr-
in cores unaffected. One can also see in Table 2 that the axial
distances of picket fence porphyrinates are considerably
shorter than those of simple porphyrins [2.168(5)−2.177(9)
Å vs 2.192(2) Å]. Similar phenomena have been identified by
us for Co(II) and Fe(II) analogues, and we believe it is a
specific effect of picket fence porphyrins.28
Although both are high-spin states, all of the Mn(II)
complexes show metal out-of-plane distances (Δ24 and Δ4 ≥
0.49) that are much larger than those of Fe(III) analogues
(0.26−0.36 Å). This is consistent with the ionic radius of
Mn(II) being larger than that of Fe(III) (0.66 Å vs 0.49 Å),
which does not allow it to fit into the central hole of the
porphyrin ring.29 The smaller metal displacements of iron(III),
however, have facilitated tenser axial bonding and presented
considerably shorter axial distances [1.928(12)−2.10 Å]
compared to those of all of the Mn(II) analogues
[2.118(2)−2.192(2) Å].
The tilting of the axial Mn−NIm bond is a partial result of
minimizing interactions between imidazole(ate) and the
porphyrin plane. Unequal Mn−NIm−CIm angles of imidazole-
(ate) also serve to minimize the steric interactions. As one can
see in Table 2, the hindered Mn(II) complexes have an average
value of 131.8° on the methyl side [M−NIm−CIm(2)] and
124.3° on the other [M−NIm−CIm(4)], and the absolute
difference between the two angles ranges from 1.7 to 12.7°, in
contrast to the range of 3.0−21.2° for Fe(III) analogues. The
much larger angle difference of Fe(III) complexes indicates
more steric hindrance that the axial imidazole has suffered,
which is thus consistent with its smaller metal displacements as
well as tenser and shorter axial distances.
As has been noted, both hindered and unhindered linkage
isomers have been isolated from the same reaction of
[Mn(TTP)] and 4-MeIm−, the phenomenon of which suggests
the two linkage isomers might be very close in energy. Single-
point energy calculations on two isomers of each porphyrin
were thus conducted, and the results of relative energy at
298.15 K are listed in Table S2. It is seen that although for all
three porphyrins, the hindered isomers are predicted to be
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more stable than the unhindered; the energy differences
between them are as small as 5.2−8.3 kJ/mol, which speak to a
nearly energetic equivalence of linkage isomers. Previously, two
crystal forms of ferric bis(imidazole) [Fe(TMP)(5-MeHIm)2]-
ClO4 that have different relative ligand orientations (θ) have
been isolated and investigated.30 The paral-[Fe(TMP)-(5-
MeHIm)2]ClO4 with a θ value of 26° or 30° showed rhombic
EPR g tensors (g1 = 2.69, g2 = 2.34−2.43, and g3 = 1.75) and a
large quadrupole splitting [ΔEQ = 2.557(3) mm/s, 120 K],
while perp-[Fe(TMP)-(5-MeHIm)2]ClO4 with a θ value of 76°
showed a single observable gmax value (3.43 at 4.2 K) and a
small quadrupole splitting [1.78(1) mm/s, 120 K]. Although
molecular mechanics calculations indicated nonbonded inter-
actions between the axial ligands and meso-mesityl groups
destabilize the relative parallel orientation, “the solid-state
structure of paral-[Fe(TMP)(5-MeHIm)2]ClO4 is observed to
be in a hydrogen-bonding network and this feature might be
considered responsible for the parallel orientation of the axial
ligands”.30 The authors concluded the energy balance between
the two isomers is a result of crystal field stabilization effects
and steric strain effects, and the two opposing energetic effects
are estimated to be both <3.0 kcal/mol (12.55 kJ/mol), which
is similar to the range of 5.2−8.3 kJ/mol of current isomers.
Therefore, given the same high-spin state (vide infra) and the
negatively charged 4-MeIm− anion ligand that is very likely to
interact with its surroundings, various nonbonded forces that
play important roles in the balance of linkage isomers are
expected for the case presented here.
Figure 5. Diagram of [K(222)][Mn(TTP)(4-MeIm−)]·PhCl·H2O
showing interactions between 4-MeIm− and the water molecule
bonded to the [K(222)]+ cation.
molecule bonded to the [K(222)]+ cation. As one can see, the
[K(222)]+ cation and the [Mn(TTP)(4-MeIm−)]− anion are
quite close to each other, which accounts for the unusually
large tilt angle of axial 4-MeIm− [12.3° (Table 2)] that is
induced by the strong steric press of [K(222)]+. The distance
between water O1S and N6 of 4-MeIm− and the
corresponding O1S−H1S−N6 angle are 2.963 Å and
150.20°, respectively, in agreement with the (O−)H···N
hydrogen bond (e.g., 2.941 Å and 153.0°, respectively).32
A
We first consider the complexes with unusual hindered
ligand conformations. Figure 4 illustrates the interactions
number of similar intra- and/or intermolecular nonbonded
interactions are found for the remaining crystal structures that
are illustrated and given in the Supporting Information. As one
can see in Figure S5, which illustrates the interactions between
the PhCl solvent and 4-MeIm− ligand of [Mn(TPP)(4-
MeIm−)]−, the C2S−N6 distance and C2S−H2S−N6 angle
are 3.307 Å and 164.69°, respectively, consistent with a C−H···
N interaction.33 Several C−H···π interactions between PhCl
solvents and axial 4-MeIm− are found for [Mn(TTP)(4-
MeIm−)]− (A and B) and illustrated in Figures S6 and S7.
It is interesting to note the previously reported, six-
coordinate bis(4-MeIm−) iron(III) porphyrinate [K(222)]-
[FeIII(TPP)(4-MeIm−)2];27 its synthesis and current Mn(II)
products are illustrated in Scheme 3 for comparison. As one
can see, 4-MeIm− could bond to porphyrin metal centers
through either N1 or N3, which would yield linkage isomers
with an unhindered or a hindered conformation, respectively.
In the reaction of Fe(III) species, when 4-MeIm− bonds
through N3, the metal displacements and porphyrin plane
deformation required by the steric hindrance of the methyl
group make the hindered conformation energetically unfavor-
able. On the contrary, when 4-MeIm− bonds through N1, low-
spin six-coordinate [K(222)][FeIII(TPP)(4-MeIm−)2] could
be formed with unhindered ligands on both porphyrin sides
owing to the small ion radius of Fe(III) that allows it to sit
inside the porphyrin core. In contrast to Fe(III), Mn(II) with a
larger ionic radius always stays outside of the porphyrin core in
a high-spin state. As we suggested, the very large metal
displacement has dramatically eliminated the steric effect of the
methyl group. This feature has allowed 4-MeIm− to be bonded
in either a hindered or an unhindered conformation, resulting
in an energetic equivalence of linkage isomers in solution.
Under this circumstance, kinds of different intra- and
intermolecular nonbonded forces are available to further
Figure 4. Diagram of [Mn(TPP)(4-MeIm−)]− showing the geo-
metrical parameters (θ and d) of C−H···π interactions.
between 4-MeIm− and five-membered atom planes of the
porphyrin core of [Mn(TPP)(4-MeIm−)]−. The distances
between carbon atoms and the centroid of the close five-
membered plane (d) and the C−H···Xcent angles (θ) are also
given. The corresponding structural parameters of [Mn(TPP)-
(4-MeIm−)]− and [Mn(TTP)(4-MeIm−)]− (A and B) are
summarized in Table S1. As one can see, in a hindered
conformation 4-MeIm− afforded a total of three -CH groups
toward the porphyrin core (one from the imidazole hydrogen
and two from methyl hydrogens). These -CH groups have d
distances (3.625−4.156 Å) of <4.3 Å and θ angles (110.94−
140.33°) similar to (or larger than) 120°, in agreement with
the existence of nonbonded (C−)H···π interactions.31
Similarly, nonbonded interactions are also found for the
ligand with the unhindered conformation. In Figure 5, the
ORTEP diagram of [K(222)][Mn(TTP)(4-MeIm−)]·PhCl·
H2O shows the interactions between 4-MeIm− and the water
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Scheme 3. Reactions of 4-MeIm− with [FeIII(Porph)]+ and [MnII(Porph)]
stabilize the products and both isomers can be isolated, e.g.,
the intermolecular (O−)H···N bond of unhindered [Mn-
(TTP)(4-MeIm−)]− (Figure 5). The case of hindered
[Mn(TPP)(4-MeIm−)]− with intramolecular (C−)H···π in-
teractions, however, is very comparable to the known Lennard-
Jones potential model of van der Waals interactions, which is a
function of the distance between the centers of two atoms
(Figure S8).34 As the separation distance decreases below
equilibrium, the potential energy becomes positive, which
indicates a repulsive force caused by the overlapping of atomic
orbitals. This corresponds to the steric effect of the methyl
group in a hindered conformation. However, at long separation
distances, the potential energy is negative and approaches zero
as the separation distance increases to infinity, which indicates
an attractive force. This corresponds to the attractive
(C−)H···π interactions of [Mn(TPP)(4-MeIm−)]− (Figure
4). Finally, as the separation between the two particles reaches
a distance where the potential energy reaches a minimum, the
pair of particles is most stable and will remain in that
orientation until an external force is exerted upon it.35
EPR Spectroscopy. X-Band EPR measurements have been
performed on all three complexes in both solid and solution
states at 4 K, and the spectra are shown in Figure 6. As one can
see, all of the spectra show two characteristic signals with g⊥ ≈
5
5.9 and g∥ ≈ 2.0, which are typical of a high-spin (S = /2) d5
system [e.g., Fe(III)36 and Mn(II)15,37] and suggest a strong
tetragonal field and transitions within the lowest-lying Kramers
(| 1/2⟩) doublet.38 Six lines of hyperfine splittings expected for
a Mn(II) nucleus (I = 5/2) are seen at g⊥ ≈ 5.9 for glassy state
[Mn(TMP)(4-MeIm−)]−, while it is not well resolved for
[Mn(TPP)(4-MeIm−)]−. For all of the solid state samples,
hyperfine splittings are not observed. Computer simulations
have been performed for [Mn(TMP)(4-MeIm−)]− to yield
spin Hamiltonian parameters. D, E, and the E/D ratio (λ) of
related Mn(II) heme complexes are listed in Table 3. As one
can see, the D of [Mn(TMP)(4-MeIm−)]− is slightly smaller
than those of the imidazole derivatives (0.67 cm−1 vs 0.68 and
0.69 cm−1), and both of them have D values larger than those
of the analogues with stronger cyanide and carbene ligands in
both solution and solid states.14,39
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ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00755.
Thermal ellipsoid diagrams of [K(222)][Mn(TPP)(4-
MeIm−)], [K(222)][Mn(TTP)(4-MeIm−)] (A),
[K(222)][Mn(TTP)(4-MeIm−)] (B), [K(222)][Mn-
(TTP)(4-MeIm−)], and [K(222)][Mn(TMP)(4-
MeIm−)] (Figure S1); formal diagram of the porphyrin
core of [K(222)][Mn(TPP)(4-MeIm−)], [K(222)]-
[Mn(TTP)(4-MeIm−)] (A and B), [K(222)][Mn-
(TPP)(4-MeIm−)], and [K(222)][Mn(TMP)(4-
MeIm−)] (Figure S2); space filling diagrams (side-on
view from the porphyrin plane) of [Mn(TPP)(4-
MeIm−)]−, [Mn(TTP)(4-MeIm−)]− (A), and [Mn-
(TMP)(4-MeIm−)]− (Figure S3); X-band EPR spec-
trum of [K(222)][Mn(TPP)(4-MeIm−)] (4 K) (Figure
S4); diagrams of [Mn(TPP)(4-MeIm−)]− and [Mn-
(TTP)(4-MeIm−)]− (A and B) showing the geometrical
parameters (θ and d) of C−H···N or C−H···π
interactions (Figures S5−S7); selected structural and
C−H···π parameters (Table S1); single-point energy
calculations; and a graphical representation of the
Lennard-Jones potential (Figure S8) (PDF)
Accession Codes
CCDC 1964144−1964145, 1994678, and 2069670 contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
Figure 6. Experimental (red and black lines) and simulated (blue
line) X-band EPR spectra of MnII porphyrinates at 4 K.
AUTHOR INFORMATION
Corresponding Author
■
CONCLUSIONS
■
Jianfeng Li − College of Materials Science and Optoelectronic
Technology, CAS Center for Excellence in Topological
Quantum Computation, and Center of Materials Science and
Optoelectronics Engineering, University of Chinese Academy
of Sciences, Beijing 101408, China; orcid.org/0000-0002-
4876-8970; Email: jfli@ucas.ac.cn
Several single-crystal structures of the first examples of
imidazolate-ligated manganese(II) porphyrinates are deter-
mined to be five-coordinate and high-spin states. The very
large metal out-of-plane displacements of Mn(II) have
dramatically eliminated the steric, repulsive interaction
between imidazolate and porphyrin cores, which accommodate
the hindered or unhindered ligand conformation in the
coordination sphere revealing a very unique example of linkage
isomerism. The isolation of both crystal forms as well as the
consonant high-spin state of all products in both solid and
solution states as confirmed by EPR is in agreement with the
small energy difference (5.2−8.3 kJ/mol) between the isomers.
Authors
Jianping Zhao − College of Materials Science and
Optoelectronic Technology, CAS Center for Excellence in
Topological Quantum Computation, and Center of Materials
Science and Optoelectronics Engineering, University of
Chinese Academy of Sciences, Beijing 101408, China
Table 3. EPR Parameters of High-Spin Five-Coordinate Manganese(II) Porphyrin Complexes
a
b
,c
b,
c
c
complex
g1
g2
g3
g4
g5
T
phase
D
E
λ
ref
[Mn(TMP)(4-MeIm−)]−
[Mn(TpivPP)(1-MeIm)]
[Mn(TpivPP)(2-MeHIm)]
[Mn(TPP)(Py)]
Mn-CCP
[Mn(TPP)(CN)]−
5.98
5.94
5.95
5.96
5.9
2.00
1.98
2.0
2.0
2.0
4
90
90
77
77
4
solution
powder
powder
solution
solution
powder
powder
0.67
0.68
0.69
0.00134
0.00338
0.00347
0.002
0.005
0.005
this work
15
15
37a
37b
14
1.24
1.23
0.78
0.77
0.55
0.54
>0.3
∼0.02
d
1.21
0.77
0.56
0.53
0.58
5.95
5.95
2.00
1.94
0.0026
0.00696
0.005
0.012
[Mn(TPP)(1,3-Me2Imd)]
90
39a
a
b
c
d
Values in kelvin. Values in inverse centimeters. D, E, and λ were estimated from EasySpin simulations. D was estimated from positions of X-
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.
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horseradish peroxidase; Mb, myoglobin; 1-MeIm, 1-methylimidazole;
1-EtIm, 1-ethylimidazole; 2-MeHIm, 2-methylimidazole; 2-MeIm−, 2-
methylimidazolate; 4-MeIm−, 4-methylimidazolate; 222 or Krypotofix
222, 4,7,13,16,21,24-hexaoxo-1,10-diazabicyclo[8.8.8]hexacosane; Np,
porphyrin nitrogen; NIm, nitrogen of the imidazole/imidazolate ring;
PhCl, chlorobenzene; DMF, N,N-dimethylformamide; EPR, electron
paramagnetic resonance.
(11) Hoffman, B. M.; Gibson, Q. H.; Bull, C.; Crepeau, R. H.;
Edelstein, S. J.; Fisher, R. G.; McDonald, M. J. MANGANESE-
SUBSTITUTED HEMOGLOBIN AND MYOGLOBIN*. Ann. N. Y.
Acad. Sci. 1975, 244, 174−186.
Fei Qian − State Key Laboratory of Coal Conversion, Institute
of Coal Chemistry, Chinese Academy of Sciences, Taiyuan
030001, China
Wenping Guo − National Energy Center for Coal to Liquids,
Synfuels China Company, Ltd., Beijing 101400, China
Zeyuan Lin − The University of Chinese Academy of Sciences,
Beijing 100049, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c00755
(12) Kirner, J. F.; Reed, C. A.; Scheidt, W. R. Stereochemistry of
manganese porphyrins. 3. Molecular stereochemistry of α,β,γ,δ-
tetraphenylporphinato(1-methylimidazole)manganese(II). J. Am.
Chem. Soc. 1977, 99, 2557−2563.
(13) Reed, C. A.; Kouba, J. K.; Grimes, C. J.; Cheung, S. K.
Manganese(II) and chromium(II) porphyrin complexes: synthesis
and characterization. Inorg. Chem. 1978, 17, 2666−2670.
(14) He, M.; Li, X.; Liu, Y.; Li, J. Axial Mn−CCN Bonds of Cyano
Manganese(II) Porphyrin Complexes: Flexible and Weak? Inorg.
Chem. 2016, 55, 5871−5879.
(15) Yu, Q.; Liu, Y.; Liu, D.; Li, J. Geometric and electronic
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the National Natural Science Foundation of
China (Grants 21771176 and 21977093 to J.L.). A portion of
this work was performed on the Steady High Magnetic Field
Facilities, High Magnetic Field Laboratory, CAS. This work is
supported in part by the Strategic Priority Research Program of
Chinese Academy of Sciences (Grant XDB28000000).
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