Synthesis, solid state molecular structures, and cyclic voltammetry of five-coordinate perchlorate manganese porphyrins and a benzamide derivative

Abstract

We report the synthesis and characterization of two perchlorate manganese porphyrins obtained from the reactions of tetra-phenyl based porphyrin manganese chloride precursors, (T(p-Cl)PP)MnCl (T(p-Cl)PP = 5,10,15,20-tetra-p-chlorophenylporphyrinato dianion) and (TPP)MnCl (TPP = 5,10,15,20-tetraphenylporphyrinato dianion), with excess silver perchlorate. A further reaction between (TPP)Mn(OClO₃) and benzamide was performed, resulting in the formation of a six-coordinate benzamide derivative, (TPP)Mn(O=C(NH₂)Ph)(OClO₃). The complexes were characterized by UV-Vis and IR spectroscopy, as well as single-crystal X-ray crystallography. The molecular structures of (T(p-Cl)PP)Mn(OClO₃) and (TPP)Mn(O=C(NH₂)Ph)(OClO₃) reveal monodentate O-binding of the perchlorate and benzamide to the manganese center. The intermolecular interactions within the crystalline packing of both complexes were investigated using Hirshfeld surface analysis. In addition, the redox behavior of (T(p-Cl)PP)Mn(OClO₃) was studied through cyclic voltammetry in THF.

Graphical Abstract

1. Introduction

Synthetic manganese porphyrin complexes have been studied for a range of applications, including their use as catalysts for the degradation of pesticides and pharmaceutical compounds [1,2], as well as for cycloaddition and copolymerization of CO₂ with epoxides [3-5]. In addition to their catalytic roles, certain manganese porphyrin complexes have shown potential as antioxidants, serving as scavengers of reactive oxygen species in treatments of cancers and dry eye disease [6,7]. Moreover, manganese porphyrins have been synthesized as models to study the structures and redox behaviors of heme-containing proteins, providing valuable information of these biological systems [8-12]. Notably, five-coordinate manganese perchlorate porphyrins have been used as key starting compounds for the preparation of both five- and six-coordinate heme model complexes [12,13]. Furthermore, the electrochemistry of many synthetic manganese porphyrin complexes has been studied, revealing the diverse redox behaviors relevant to their catalytic and biomedical applications [10,12,14,15].

Benzamide belongs to the large amide family containing the ─C(=O)N─ group. Benzamide derivatives are used as antipsychotic, prokinetic and anti-inflammatory medications. Their potential applications in fungicide development [16] and treatments for viral [17], parasitic infections [18], as well as cancer [19], have been reported. The coordination chemistry of amides is diverse as demonstrated by the different binding modes reported. Amides are capable of binding to metal centers through the oxygen atom [12], nitrogen atom [20] or both O and N atoms in bimetallic complexes [21]. For instance, benzamide based ligands coordinate to two rhodium ions via both their N and O atoms in a series of dirhodium complexes [21]. Amides are known to bind to both heme models and heme proteins. Previously, we reported the synthesis and structural characterization of acetamide and acrylamide manganese porphyrin complexes [12]. Recently, an X-ray crystal structure of a heme protein with a bound acetamide ligand was reported [22]. To our knowledge, no benzamide heme model complex has been reported. Thus, we are interested in studying the interactions of manganese perchlorate porphyrins with benzamide and exploring the coordination chemistry of benzamide with metalloporphyrins.

In this work, we prepared and characterized two five-coordinate manganese perchlorate porphyrins, tetra(p-chlorophenyl)porphyrin manganese perchlorate (T(p-Cl)PP)Mn(OClO₃) and tetraphenyl porphyrin manganese perchlorate (TPP)Mn(OClO₃). The X-ray crystal structure of (T(p-Cl)PP)Mn(OClO₃) has been obtained and the inter-molecular interactions of the molecules in crystalline have been explored. In addition, the redox behavior of (T(p-Cl)PP)Mn(OClO₃) has been studied using cyclic voltammetry. Reaction of (TPP)Mn(OClO₃) with benzamide was performed and a six-coordinate neutral benzamide derivative, (TPP)Mn(O=C(NH₂)Ph)(OClO₃), was obtained and characterized. The molecular structure of (TPP)Mn(O=C(NH₂)Ph)(OClO₃) was determined using X-ray crystallography. To our knowledge, this is the first report of molecular structures for both (T(p-Cl)PP)Mn(OClO₃) and (TPP)Mn(O=C(NH₂)Ph)(OClO₃), with the latter representing only the second example of a neutral six-coordinate manganese perchlorate porphyrin complex with any ligand.

2. Experimental

The synthesis was performed under an atmosphere of ultra-high pure nitrogen using standard Schlenk glassware and/or in a Mbraun Unilab Glovebox. Solvents were distilled from appropriate drying agents under nitrogen just prior to use: CH₂Cl₂ (CaH₂), hexanes (CaH₂), cyclohexane (CaH₂), THF (Na).

2.1. Chemicals and instrumentation

Benzamide (PhCO(NH₂), 98%) and anhydrous silver perchlorate (AgClO₄, 99.5%) were purchased from Alfa Aesar Company and used as received. The manganese chloride porphyrin precursors, (T(p-Cl)PP)MnCl and (TPP)MnCl, were synthesized by published procedures [23]. Infrared spectra were recorded on a Nicolet iS10 FTIR spectrometer from 4000 to 400 cm⁻¹. IR spectra of the three complexes are provided in Figure S1. Spectrophotometric measurements in solution were performed using an Agilent Cary 300 UV-Vis spectrophotometer. Cyclic voltammetry was conducted using a Pine Wave now potentiostat voltammetric analyzer. For cyclic voltammetry, a three-electrode cell with a 3 mm diameter Pt disk working electrode, a Pt wire counter electrode, and a Ag/AgCl reference electrode were utilized. The solutions were deaerated by purging with ultra-high pure nitrogen for 10 min before each set of measurements, and then the nitrogen atmosphere was maintained during the measurements.

2.2. Preparation of complexes

The five-coordinate manganese perchlorate porphyrin complexes, (T(p-Cl)PP)Mn(OClO₃) and (TPP)Mn(OClO₃), were prepared similar to the procedure reported [24] with a minor modification as described below. The six-coordinate complex, (TPP) Mn(O=C(NH₂)Ph)(OClO₃), was prepared by the reaction of excess benzamide with (TPP)Mn(OClO₃).

2.2.1. (T(p-Cl)PP)Mn(OClO₃)

To a THF solution (20 mL) of T(p-Cl)PPMnCl (0.050 g, 0.060 mmol) was added anhydrous silver perchlorate (0.018g, 0.086mmol). The mixture was heated to reflux and stirred for 30 min, during which time the color of the solution changed from brownish purple to reddish purple. The solvent was removed under vacuum. The resulting solid was redissolved in 20 mL CH₂Cl₂ and filtered. The filtrate was reduced to approximately 5 mL and hexane (30 mL) was added. The violet crystalline solid that formed was collected by filtration, washed with hexane (2 × 15 mL), and dried in vacuo to give (T(p-Cl)PP)Mn(OClO₃) (0.027g, 0.030 mmol, 50% isolated yield). Slow evaporation of a CH₂Cl₂/cyclohexane (4:1 ratio; 5 mL) solution of the product under N₂ at room temperature gave suitable crystals for X-ray diffraction studies. IR (KBr, cm⁻¹): 1485s, 1396m, 1340m, 1205m, 1121s, 1090s, 1010s, 884m, 852m, 805s, 719m, 623m, 502m. UV-Vis (CH₂Cl₂, nm): 381(33), 478(52), 584(10), 621(10).

2.2.2. (TPP)Mn(OClO₃)

Isolated yield: 67%. IR (KBr, cm⁻¹): 1487m, 1441m, 1341m, 1203m, 1179w, 1122s, 1074s, 1012s, 914w, 804m, 754m, 703s, 663w, 622m, 456w. UV-Vis (CH₂Cl₂, nm): 392(49), 488(30), 573(11), 610(10).

2.2.3. (TPP)Mn(O=C(NH₂)Ph)(OClO₃)

To a CH₂Cl₂ solution (20 mL) of (TPP)Mn(OClO₃) (0.050g, 0.061 mmol) was added excess benzamide (0.070g, 0.58 mmol). The mixture was heated to reflux and stirred for 3h. The solvent was reduced to approximately 2 mL under vacuum, and hexane (15 mL) was added. The black crystalline solid that formed was collected by filtration, washed with hexane (2 × 15 mL), and dried in vacuo to give (TPP)Mn(O=C(NH₂)Ph)(OClO₃) (0.035 g, 0.040 mmol, 65% isolated yield). Slow evaporation of a CH₂Cl₂/cyclohexane (2:1 ratio; 6 mL) solution of the product under N₂ at room temperature gave suitable crystals for X-ray diffraction studies. IR (KBr, cm⁻¹): 1654s, 1624m, 1577m, 1483m, 1448w, 1396m, 1340w, 1121m, 1091s, 1010s, 803m, 688s, 668w, 623m, 501w.

2.3. Solid-state structural determinations

Single-crystal X-ray diffraction data were collected for (T(p-Cl)PP)Mn(OClO₃)·CH₂Cl₂ and (TPP)Mn(O=C(NH₂)Ph)(OClO₃)·CH₂Cl₂ on two different Bruker D8 X-ray diffractometers equipped with either a CCD area detector [25,26] or a PHOTON detector and a sealedtube Mo Ka source (λ = 0.71073 Å) at 100 K. The structures were solved by direct methods using the SHELXTL system and refined by full-matrix least-squares methods on F² [27,28]. Details of the crystal data and refinement for both structures are given in Table 1.

Table 1.

Crystal data and structure refinement.

Empirical formula	(C44H24Cl5MnN4O4)·(CH2Cl2)	(C51H35ClMnN5O5)·(CH2Cl2)
Formula weight	989.79	971.15
Crystal system	Triclinic	Triclinic
Space group	$P\overline{1}$	P1
Unit cell dimensions
a (Å)	10.468(2)	10.4722(12)
b (Å)	13.892(3)	10.6282(11)
c (Å)	14.912(3)	11.5239(13)
α (°)	73.004(3)	105.878(4)
β (°)	75.577(3)	105.097(4)
γ (°)	81.581(3)	108.232(4)
Volume (Å3), Z	2002.1(7), 2	1084.4(2), 1
Density (calculated)	1.642 g/cm3	1.490 g/cm3
Temperature (K)	100(2)	100(2)
F(000)	556	500
Crystal size (mm)	0.170 × 0.330 × 0.370	0.210 × 0.230 × 0.440
Theta range for data collection (°)	1.463 to 27.539	1.99 to 28.70
Reflections collected	42207	55038
Independent reflections	9200 [R(int) = 0.0238]	11165 [R(int) = 0.0516]
Goodness-of-fit on F2	1.006	1.048
R(F observed data)	R1 = 0.0402	R1 = 0.0487
wR(F2 all data)	wR2 = 0.1160	wR2 = 0.1243
Largest diff. peak and hole (e/Å3)	1.134 and −0.657	1.184 and −0.450

2.3.1. (T(p-Cl)PP)Mn(OClO₃)·CH₂Cl₂

Cell parameters were determined from a least-squares fit of 9769 peaks in the range 2.22 < θ < 27.47°. A total of 42207 data were measured in the range 1.463 < θ < 27.539° using phi and omega oscillation frames. The data were corrected for absorption by the empirical method [29] giving minimum and maximum transmission factors of 0.625 and 0.746. The data were merged to form a set of 9200 independent data with R(int) = 0.0238 and a coverage of 100.0%.

The triclinic space group $P\overline{1}$ was determined by systematic absences and statistical tests and verified by subsequent refinement. The position of hydrogen bonded to carbons were initially determined by geometry and were refined using a riding model. Non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atom displacement parameters were set to 1.2 times the isotropic equivalent displacement parameters of the bonded atoms. A total of 550 parameters were refined against 9200 data to give wR(F²) = 0.1160 and S = 1.006 for weights of w = 1/[σ² (F²) + (0.0670 P)²], where P = [F_o² + 2F_c²]/3. The final R(F) was 0.0402 for the 6927 observed, [F > 4σ(F)], data. The largest shift/s.u. was 0.001 in the final refinement cycle. The final difference map had maxima and minima of 1.134 and −0.657 e/ų, respectively.

2.3.2. (TPP)Mn(O=C(NH₂)Ph)(OClO₃)·CH₂Cl₂

A total of 2059 frames were collected. The total exposure time was 2.06 h. The frames were integrated with the Bruker SAINT software package using a narrow-frame algorithm. The structure was solved and refined using the Bruker SHELXTL Software Package [28,30]. The integration of the data using a triclinic unit cell yielded a total of 55038 reflections to a maximum θ angle of 28.70° (0.74 Å resolution), of which 11165 were independent (average redundancy 4.930, completeness = 100.0%, R_int = 5.16%, R_sig = 4.04%) and 10418 (93.31%) were greater than 2σ(F²). Data were corrected for absorption effects using the Multi-Scan method (SADABS). The ratio of minimum to maximum apparent transmission was 0.851. The calculated minimum and maximum transmission coefficients (based on crystal size) are 0.7950 and 0.8940.

(TPP)Mn(O=C(NH₂)Ph)(OClO₃)·CH₂Cl₂ crystallizes in the triclinic space group P1. The asymmetric unit consists of a (TPP)Mn(O=C(NH₂)Ph)(OClO₃) molecule and a dichloromethane solvent molecule. Both molecules are well-ordered and well-separated from each other. All non-hydrogen atoms were located in successive Fourier maps and refined anisotropically. Most of the H atoms were also located in the Fourier maps but were refined isotropically. Two of the benzamide H atoms were placed in calculated positions and refined isotropically adapting a riding model with fixed thermal parameters.

The final anisotropic full-matrix least-squares refinement on F² with 731 variables converged at R₁ = 4.87% for the observed data and wR₂ = 12.43% for all data. The goodness-of-fit was 1.048. The largest peak in the final difference electron density synthesis was 1.184 e⁻/ų and the largest hole was −0.450 e⁻/ų with an RMS deviation of 0.115 e⁻/ų. On the basis of the final model, the calculated density was 1.490 g/cm³ and F(000), 500 e⁻.

3. Results and discussion

3.1. Synthesis and characterization

Bis-THF manganese perchlorate porphyrin complexes have previously been prepared by reacting the respective $(\text{por})\mathsf{MnCI}$ precursors with anhydrous ${\mathsf{AgCIO}}_4$ in THF [12,24]. The five-coordinate $(\text{por})\mathsf{Mn}({\mathsf{OCIO}}_3)$ (por = TPP and T(p-Cl)PP) complexes were obtained by dissociation of the weakly bonded THF ligand in a diluted CH₂Cl₂ solution (Equation 1).

$$\begin{gathered} (\text{por})\mathsf{MnCI}+{\mathsf{AgCIO}}_4+\mathsf{excess}\mathsf{THF}\to [(\text{por})\mathsf{Mn}{(\mathsf{THF})}_2]({\mathsf{CIO}}_4) \\ \to (\text{por})\mathsf{Mn}({\mathsf{OCIO}}_3)+2\mathsf{THF} \end{gathered}$$ (1)

It is known that the coordination of perchlorate to metals results in the splitting of the 1100cm⁻¹ perchlorate band in the IR spectrum due to a geometry change from T_d to C_3v [31]. Hydrogen bonding involving perchlorate may also cause distortion and further complicate the splitting. IR spectra of (T(p-Cl)PP)Mn(OClO₃) and (TPP)Mn(OClO₃) show multiple bands near 1100 cm⁻¹ and 627 cm⁻¹, indicating the presence of the perchlorate ligand [32]. The UV–Vis spectra of (T(p-Cl)PP)Mn(OClO₃) and (TPP)Mn(OClO₃), obtained in CH₂Cl₂, closely resemble those of the tetraphenyl based manganese perchlorate porphyrin complexes reported [33].

Further reaction of (TPP)Mn(OClO₃) with excess benzamide in CH₂Cl₂ resulted in the formation of a six-coordinate neutral complex, (TPP)Mn(O=C(NH₂)Ph)(OClO₃), as shown in Equation (2).

$$(\mathsf{TPP})\mathsf{Mn}({\mathsf{OCIO}}_3)+\mathsf{O}=\mathsf{C}({\mathsf{NH}}_2)\mathsf{Ph}\to (\mathsf{TPP})\mathsf{Mn}(\mathsf{O}=\mathsf{C}({\mathsf{NH}}_2)\mathsf{Ph})({\mathsf{OCIO}}_3)$$ (2)

The IR spectrum of $(\mathsf{TPP})\mathsf{Mn}(\mathsf{O}=\mathsf{C}({\mathsf{NH}}_2)\mathsf{Ph})({\mathsf{OCIO}}_3)$ shows characteristic bands at 1100 cm⁻¹ and 627 cm⁻¹, attributed to the perchlorate ligand. Additionally, bands at 1654 cm⁻¹ and 1396 cm⁻¹ are assigned to the C=O and =C-N stretches from the bound benzamide ligand, respectively. We note that the infrared spectrum of free benzamide in Nujol mull shows υ_CO at 1658 cm⁻¹ and υ_(=C-N) at 1401cm⁻¹, correspondingly [34].

It is well known that interactions of metalloporphyrin perchlorate complexes with various ligands result in the formation of cationic five- or six-coordinate metalloporphyrin moiety with perchlorate as counter anion [12,35,36]. For instance, the reactions of [(por)Mn(THF)₂]ClO₄ with acetamide and acrylamide resulted in the formation of six-coordinate ionic complexes, [(por)Mn(acetamide)₂]ClO₄ and [(por)Mn(acrylamide)₂]ClO₄ [12]. The similar ionic iron porphyrin amide complexes, [(por)Fe(acetamide)₂]ClO₄ and [(por)Fe(acrylamide)₂]ClO₄, were also obtained using a similar synthetic method [36]. Besides the above ionic form of the ligated metalloporphyrin perchlorate complexes, a neutral six-coordinate perchlorate pyridine manganese porphyrin complex, (TPP)Mn(py)(OClO₃), was reported and its molecular structure determined by X-ray crystallography [37].

To unambiguously establish the binding modes of perchlorate and benzamide to the manganese center, as well as to elucidate the overall structures of the two complexes, we sought to determine the X-ray crystal structures of (T(p-Cl)PP)Mn(OClO₃) and (TPP)Mn(O=C(NH₂)Ph)(OClO₃).

3.2. Crystallography

X-ray diffraction quality crystals of (T(p-Cl)PP)Mn(OClO₃) were obtained by slow evaporation from the CH₂Cl₂/cyclohexane solution of the complex. The molecular structure and the porphyrin atom displacements are shown in Figure 1(a,b).

Figure 1.

(a) The structure of the five-coordinate perchlorate complex (T(p-Cl)PP)Mn(OClO₃). Two oxygen atoms of axial perchlorate ligand are involved in H-bonding of a solvent, CH₂Cl₂, molecule. Hydrogen atoms attached to carbon atoms have been omitted for clarity. (b) Perpendicular atom displacements (in units of 0.01 Å) of the porphyrin core from the 24-atom mean porphyrin plane.

As shown in Figure 1(a), the structure of the complex reveals a monodentate O-binding mode of perchlorate ligand to the central Mn ion. The Mn is displaced by 0.12Å out of the 24-atom mean porphyrin plane towards the ligand. The average equatorial Mn–N(por) bond length is 2.005(2) Å, consistent with those observed in five-coordinate high-spin manganese(III) porphyrin complexes [38-41]. It is expected, as the OEP (OEPMnClO₄) and TPP (TPPMnClO₄) derivatives have been reported as having a high-spin ground state [42,43]. The axial Mn-O bond length is 2.142(2) Å and is tilted 4.7° from the normal to the 24-atom porphyrin plane. The mean plane displacement (Figure 1b) exhibits a saddled distortion of the porphyrin moiety. Within the perchlorate ligand, the bond distance between Cl and the donor O atom (Cl5-O1) is 1.493(2) Å, which is significantly longer than those between Cl and non-bonding O atoms, ranging from 1.426(2) Å to 1.430(2) Å. The structure also shows hydrogen bonding interactions of the two perchlorate O atoms with the H-atoms of a nearby solvent CH₂Cl₂ molecule (C2S···O3 = 3.24Å, C2S···O4 = 3.27Å).

The packing of the molecules reveals close interactions between the face-to-face adjacent macrocycles (Figure 2). The mean plane separation (M.P.S.) between the 24-atom porphyrin planes is 3.70 Å, and the Mn-Mn distance is 4.44 Å. The lateral shift representing the horizontal distance between the two Mn centers of the porphyrins is ~2.04 Å. Based on the magnitudes of the MPS and the lateral shift, this compound is categorized in Class S with a strong interaction between the rings [44]. The detailed intermolecular interactions among the neighboring molecules in the crystalline packing of (T(p-Cl)PP)Mn(OClO₃)·CH₂Cl₂ were investigated using Hirshfeld surface analysis.

Figure 2.

The packing diagram of two adjacent molecules in crystals of (T(p-Cl)PP)Mn(OClO₃). Hydrogen atoms attached to carbon atoms have been omitted for clarity.

X-ray diffraction quality crystals of (TPP)Mn(O=C(NH₂)Ph)(OClO₃) were also obtained by slow evaporation from the CH₂Cl₂/cyclohexane solution of the complex. The molecular structure and the porphyrin atom displacements are shown in Figures 3(a,b). As shown in Figure 3(a), the structure of the complex reveals a six-coordinate neutral complex, with both benzamide and perchlorate binding to the manganese center through their oxygen atoms. The Mn ion is essentially sitting on the 24-atom mean porphyrin plane, displaced by 0.01 Å towards the benzamide ligand. The mean plane displacement (Figure 3b) exhibits a slight wave distortion of the porphyrin moiety. The average equatorial Mn–N(por) bond length is 2.008(3) Å, which is similar to the 2.000 Å found in its pyridine analogue, (TPP)Mn(py)(OClO₃) [37], and falling within the bnod length range of Mn-N(por) for high-spin, six-coordinate Mn(III) porphyrin complexes reported [45-49]. The two axial Mn-O bond lengths are 2.245(3) Å for benzamide and 2.291(3) Å for perchlorate. The shorter equatorial Mn-N(por) bond lengths and longer axial Mn-O bond lengths are characteristic of the tetragonal elongation geometry commonly observed in high-spin, six-coordinate Mn(III) porphyrin complexes [46-48]. The longer Mn-O(perchlorate) bond in (TPP)Mn(O=C(NH₂)Ph)(OClO₃) compared to that in (T(p-Cl)PP)Mn(OClO₃) indicates a weaker coordination of the perchlorate ligand in this six-coordinate complex. The Mn-O (benzamide) bond and Mn-O (perchlorate) are tilted 7.5° and 3.3° from the normal to the 24-atom porphyrin plane. The bond angle between the two Mn-O bonds is 169.6(1)°. The C-O and C-N bond lengths of the bound benzamide are 1.246(5) Å and 1.336(5) Å, respectively. For the bound perchlorate ligand, the bond distance between the chlorine atom and the donor oxygen atom (Cl1-O1) is 1.450(3) Å, which is comparable to one of the other three Cl-O (non-bonding) bond distances at 1.453(3) Å, and only slightly longer than the other two, determined at 1.437(3) Å and 1.428(3) Å. We note that the Cl-O(bonding) bond length of 1.450(3) Å in (TPP)Mn(O=C(NH₂)Ph)(OClO₃) is significantly shorter than 1.493(2) Å found in (T(p-Cl)PP)Mn(OClO₃), but is similar to the 1.443 Å reported for (TPP)Mn(py)(OClO₃) [37]. The structure also shows a hydrogen bonding interaction between a chlorine atom from a solvent (CH₂Cl₂) molecule and a hydrogen atom from the amide group (Cl2···N5 = 3.624 Å). To the best of our knowledge, the X-ray structure of (TPP)Mn(O=C(NH₂)Ph)(OClO₃) represents the second example of a neutral six-coordinate manganese perchlorate porphyrin complex with any ligand.

Figure 3.

(a) The structure of (TPP)Mn(O=C(NH₂)Ph)(OClO₃). One hydrogen atom of the bound benzamide ligand is involved in H-bonding of a solvent, CH₂Cl₂, molecule. Hydrogen atoms attached to carbon atoms have been omitted for clarity. (b) Perpendicular atom displacements (in units of 0.01 Å) of the porphyrin core from the 24-atom mean porphyrin plane.

The packing of the molecules reveals weak interactions between the face-to-face adjacent macrocycles (Figure 4). The mean plane separation (M.P.S.) between the 24-atom porphyrin planes is 4.69 Å, and the Mn-Mn distance is 10.47 Å. The lateral shift is ~9.36 Å which is significantly larger than those observed in (T(p-Cl)PP)Mn(OClO₃) and other porphyrin complexes [44,50] reported. It is probably due to the bound bulky benzamide ligands, which prevent close face-to-face interaction between the nearby porphyrin rings. Based on the magnitudes of the MPS and the lateral shift, this compound is categorized in Class W with a weak interaction between the rings [44].

Figure 4.

The packing diagram of two adjacent molecules in crystals of (TPP)Mn(O=C(NH₂)Ph)(OClO₃). Hydrogen atoms attached to carbon atoms, perchlorate and benzamide ligands are removed for clarity.

3.3. Hirshfeld surface analysis

For a more comprehensive understanding of the intermolecular interactions among neighboring molecules in the crystalline packing of (T(p-Cl)PP)Mn(OClO₃)·CH₂Cl₂ and (TPP)Mn(O=C(NH₂)Ph)(OClO₃)·CH₂Cl₂, we conducted Hirshfeld surface analysis of the complexes using CrystalExplorer 17 [51]. The top and bottom views of the molecular surface maps of T(p-Cl)PP)Mn(OClO₃)·CH₂Cl₂ are shown in Figure 5. Red spots on the maps indicate close contacts between atoms in the mapped molecule and surrounding molecules. The intensity of the color suggests the strength of the interactions.

Figure 5.

Top view (a) and bottom view (b) of the d_norm Hirshfeld surface map for (T(p-Cl)PP)Mn(OClO₃)·CH₂Cl₂.

As shown on the top view of the surface map of T(p-Cl)PP)Mn(OClO₃)·CH₂Cl₂ in Figure 5(a), the two dark red spots (C─-H) on the left and right sides represent the close contacts between a pyrrolic carbon (_pyrC) and a phenyl hydrogen (_phH) atom, while the two lighter ones (C─-C) represent the interactions between two _pyrC atoms from face-to-face adjacent porphyrin molecules. The lighter red dots (H─-Cl) on the top and bottom sides suggest intermolecular contacts of pyrrolic and phenyl hydrogen atoms (_pyrH and _phH) with chlorine atoms from a phenyl group and solvent (CH₂Cl₂), respectively. A single red dot (H─-O) on the top indicates an additional interaction between a _phH atom and an oxygen atom from a perchlorate ligand of a neighboring molecule.

Significant intermolecular interactions are also observed on the bottom side of the molecule. As shown in Figure 5(b), two twin red dots (C─-H) to the top represent the interactions between a _phH atom and two _pyrC atoms. Two bright red dots (O─-H) highlight close contacts between a perchlorate O atom and a _phH atom. An intense red spot (O─-H(CH₂Cl₂)) on the right side suggests a substantial interaction between a perchlorate O atom and a hydrogen atom from a solvent (CH₂Cl₂) molecule. Additionally, a red spot on the left side (H─-Cl) indicates a close interaction of a pyrrolic hydrogen with a phenyl chlorine atom.

To assess the percentage contribution of various intermolecular interactions, 2D fingerprint diagrams were generated as shown in Figure 6. The interactions involving Cl─-H and H─-H contribute approximately 26.2% and 21.8% of total interactions, respectively. The C─-H interactions are shown as a pair of side wings and contribute 21.0%. The O─-H interactions contribute 12.6% which are in the region of ~2.35 Å. Interactions such as C─-N and C─-C, which can be viewed as π···π stacking interactions between two adjacent molecules [52], also contribute 2.8% and 2.4%, respectively, to the overall intermolecular interactions.

Figure 6.

Selected 2D fingerprint diagrams of Hirshfeld surface showing the percentage contribution of respective interactions of (T(p-Cl)PP)Mn(OClO₃)·CH₂Cl₂.

The top and bottom views of the molecular surface map of (TPP)Mn(O=C(NH₂)Ph)(OClO₃)·CH₂Cl₂ are shown in Figure 7. In the top view (Figure 7a), three intense red spots (O--H) near the center of the map indicate substantial interactions of two O atoms from a nearby perchlorate ligand with an amide hydrogen, as well as a _phH from the bound benzamide ligand. A lighter red spot (O─-H) on the top right represents intermolecular contacts between a porphyrin _phH and a perchlorate oxygen atom. Other red spots on the lower left side and a faint red spot in the middle right (C─-H) represent interactions between C and H atoms on the phenyl and pyrrole rings from the included porphyrin rings and neighboring molecular rings. These interactions in the top-view have corresponding reciprocal interactions on the bottom side, which are labeled with a subscript “r” in Figure 7(b). In addition to these reciprocal interactions, an interaction between a solvent (CH₂Cl₂) hydrogen and a perchlorate oxygen (O─-H_CH2Cl2) was observed in the bottom view map.

Figure 7.

Top view (a) and bottom view (b) of the d_norm Hirshfeld surface map for (TPP)Mn(O=C(NH₂)Ph)(OClO₃)·CH₂Cl₂.

Compared to the molecular surface map of (T(p-Cl)PP)Mn(OClO₃)·CH₂Cl₂, no notable C─-C interaction is observed on the top or bottom surfaces of (TPP)Mn(O=C(NH₂)Ph)(OClO₃)·CH₂Cl₂, suggesting an absence of significant π···π stacking interactions between any two adjacent molecules. This observation is further supported by the 2D fingerprint diagrams of the complex, which show that interactions involving C─-C contribute approximately 0.2% of total interactions and no contribution from C─-N interaction is observed (Figure S2).

3.4. Electrochemistry

The reduction behavior of (T(p-Cl)PP)Mn(OClO₃) was examined in THF at room temperature using NBu₄ClO₄ as the supporting electrolyte. The resulting cyclic voltammogram is shown in Figure 8. The complex exhibits a well-defined reversible reduction couple in THF. The first reduction potential is at −0.63 V versus Fc/Fc⁺ in THF. The separations in peak potentials, ΔE_p = ∣ E_pa – E_pc ∣, is 216 mV for the reduction at scan rate of 200 mV/s. The DE_p value is similar to 206 mV determined for the ferrocene–ferrocenium couple under identical conditions. We thus ascribed the reductions to single-electron processes. In addition, the reduction is a chemically reversible process, with anodic to cathodic peak current ratio (i_pa/i_pc) of ~1.0. The cyclic voltammetry of a TPP (tetraphenyl porphyrin) analogue, (TPP)Mn(OClO₃), was reported previously [14]. In the study, the first reduction potential of the complex is at −0.12 V (THF) versus SCE, equivalent to −0.59 V when using Fc/Fc⁺ as the reference (E^o′_Fc/Fc+ = +0.47V versus SCE [53]). Comparing the reduction potentials of the two complexes, we observed that (TPP)Mn(OClO₃) has a slightly less negative reduction potential than (T(p-Cl)PP)Mn(OClO₃), despite the more electron-rich porphyrin ligand in (TPP)Mn(OClO₃). To confirm this difference, cyclic voltammetry of (T(p-Cl)PP)Mn(OClO₃) was conducted in CH₂Cl₂, revealing a first reduction potential of −0.65 V versus Fc/Fc⁺ (Figure S3). Compared to the reported −0.16 V versus SCE (−0.63V versus Fc/Fc⁺) for TPP derivatives in CH₂Cl₂ [14], a similar difference was observed, possibly due to variations in experimental conditions during the electrochemical studies. Nevertheless, the comparable first reduction potential values of (T(p-Cl)PP)Mn(OClO₃) and (TPP)Mn(OClO₃) suggest that reduction does not occur at the porphyrin ligand, and that the electron-donating ability of the porphyrin ligands has minimal impact on the reduction potentials of the complexes. Thus, the first reduction of (T(p-Cl)PP)Mn(OClO₃) occurs on the manganese center, reducing Mn(III) to Mn(II), which is consistent with the study of other five-coordinate manganese porphyrin complexes [14,54,55].

Figure 8.

Cyclic voltammograms showing the first reductions of (T(p-Cl)PP)Mn(OClO₃) in THF containing 0.1 M NBu₄ClO₄ at a scan rate of 200 mV/s.

4. Conclusion

We have synthesized and characterized two five-coordinate perchlorate manganese porphyrin complexes, (T(p-Cl)PP)Mn(OClO₃) and (TPP)Mn(OClO₃), and a neutral six-coordinate benzamide perchlorate manganese porphyrin complex, (TPP)Mn (O=C(NH₂)Ph)(OClO₃). The molecular structures of (T(p-Cl)PP)Mn(OClO₃) and (TPP)Mn(O=C(NH₂)-Ph)(OClO₃) have been determined by single-crystal X-ray crystallography, revealing monodentate O-binding of the perchlorate anion to the manganese center in each of the complexes. The benzamide ligand binds to the manganese center through its O atom of the amide group in the structure of (TPP)Mn (O=C(NH₂)Ph)(OClO₃). To the best of our knowledge, the X-ray structure of (TPP)Mn(O=C(NH₂)Ph)(OClO₃) represents only the second example of a neutral six-coordinate manganese perchlorate porphyrin complex with any ligand. In addition, the intermolecular interactions of (T(p-Cl)PP)Mn(OClO₃) and (TPP)Mn(O=C(NH₂)Ph)(OClO₃) in crystalline packing were investigated using Hirshfeld surface analysis. The redox behaviors of (T(p-Cl)PP)Mn(OClO₃) were studied through cyclic voltammetry in THF, showing a well-defined reversible reduction couple.