DFT Study of Molecular Structure, Electronic and Vibrational Spectra of Tetrapyrazinoporphyrazine, Its Perchlorinated Derivative and Their Al, Ga and In Complexes

Abstract

Electronic and geometric structures of metal-free, Al, Ga and In complexes with tetrapyrazinoporphyrazine (TPyzPA) and octachlorotetrapyrazinoporphyrazine (TPyzPACl₈) were investigated by density functional theory (DFT) calculations and compared in order to study the effect of chlorination on the structure and properties of these macrocycles. The nature of the bonds between metal atoms and nitrogen atoms was described using the NBO-analysis. Simulation and interpretation of electronic spectra were performed with the use of time-dependent density functional theory (TDDFT). A description of calculated IR spectra was carried out based on the analysis of the distribution of the potential energy of normal vibrational coordinates.

1. Introduction

Tetrapyrrole macroheterocycles are essential for a wide range of technologies, including optoelectronics and information storage devices [1,2,3,4,5,6,7]. The complexes of tetrapyrazinoporphyrazine (H₂TPyzPA) with metals possessing π-deficient heterocyclic fragments are promising compounds for organic electronics [8,9,10,11]. The results of the recent electrochemical study [8] of the perchlorinated complexes M(Cl)TPyzPACl₈ (M = Al, Ga, In) allow them to be considered as potential acceptor materials. Recently this was also demonstrated for perchlorinated tripyrazinosubporphyrazines [12].

Fine tuning of the physico-chemical properties of the metal complexes with macroheterocyclic ligands requires a deep knowledge of their molecular and electronic structures. However, only a little attention has, of yet, been paid to structural investigations of the metal complexes of TPyzPA [13]. Solid-state X-ray studies of Fe(II) [14] and Sn(IV) complexes [15] revealed that the TPyzPACl₈ macrocycle is nearly planar. The possible planarity of the structures of MTPyzPACl₈ complexes gives rise to the extended π-electron conjugation, which in turn results in an increased packing density.

The present contribution is devoted to the combined computational investigation of the geometry structures, features of the metal–ligand bonding and spectral properties of M(Cl)TPyzPA (M = Al, Ga, In) complexes and their perchlorinated analogues. It also extends our recent study [16] of the tetrabenzoporphyrin (TBP) complexes with the same set of metals. Density functional theory (DFT) [17,18] was used as a theoretical framework since it was earlier established to provide a good description of the features of the geometry and electronic structures of the analogous macroheterocycles and their metal complexes [16,19,20,21,22,23,24,25].

2. Results

2.1. Molecular Structures

According to the performed quantum chemical calculations, the equilibrium structures of the Al(III), Ga(III) and In(III) complexes of tetrapyrazinoporphyrazine (M(Cl)TPyzPA) and their perchlorinated analogues are doming-distorted and possess C_4v symmetry (Figure 1). The distortion is caused by a metal atom as the metal-free H₂TPyzPA and H₂TPyzPACl₈ molecules are planar (D_2h symmetry). The H₂TPyzPA internuclear distances obtained by the authors of [26] are systematically higher compared to our data. This discrepancy can be explained by the use of corrections for the dispersion interaction in this paper and pseudopotentials in [26]. The H₂TPyzPA bond lengths calculated in [27] are also systematically higher than those obtained in this work. The main geometric parameters of compounds under consideration are given in

Table 1. Internuclear distances (r_e, in Å) and valence angles (∠, in deg.) of the equilibrium structures by PBE0-D3/def2-TZVP calculations.

	Al(Cl)TPyzPA	Ga(Cl)TPyzPA	In(Cl)TPyzPA	H2TPyzPA
Symmetry	C4v	C4v	C4v	D2h
Distances
M-Np/M-Np′	1.981	2.022	2.161	-
M-Clax	2.145	2.187	2.336	-
Np-Cα/Np′-Cα′	1.371	1.367	1.364	1.368/1.355
Cα-Nm/Cα′-Nm	1.308	1.310	1.316	1.304/1.321
Cα-Cβ/Cα′-Cβ′	1.450	1.453	1.458	1.450/1.464
Cβ-Cβ/Cβ′-Cβ′	1.391	1.393	1.399	1.401/1.393
Cβ-Nd/Cβ′-Nd′	1.328	1.328	1.326	1.329/1.324
Nd-Cγ/Nd′-Cγ′	1.322	1.322	1.323	1.320/1.327
Cγ-Cγ/Cγ′-Cγ′	1.408	1.407	1.405	1.408/1.401
Cγ-H/Cγ′-H’	1.086	1.086	1.086	1.086/1.086
(Np…Np)opp	3.859	3.918	4.037	4.054/3.899
(Np…Np)adj	2.728	2.770	2.854	2.812
P 1	21.432	21.416	21.440	21.376
Bond angles
$\angle$(ClaxMNp)	103.1	104.3	110.9	-
$\angle$(MNpCα)	125.2	124.5	123.6	123.2
$\angle$(NpCαNm)/(Np′Cα′Nm)	127.9	128.0	128.0	128.5/128.1
$\angle$(CαNmCα′)	122.5	123.4	125.5	124.2
$\angle$(CαCβNd)/(Cα′Cβ′Nd′)	129.8	129.8	129.8	129.0/130.6
$\angle$(CβNdCγ)/(Cβ′Nd′Cγ′)	113.2	113.3	113.6	113.6/113.3
M-X 2	0.448	0.499	0.772	-
α 3	175.5	174.3	164.7	180
	Al(Cl)TPyzPACl8	Ga(Cl)TPyzPACl8	In(Cl)TPyzPACl8	H2TPyzPACl8
Symmetry	C4v	C4v	C4v	D2h
Distances
M-Np/M-Np′	1.980	2.021	2.162	-
M-Clax	2.142	2.183	2.332	-
Np-Cα/Np′-Cα′	1.372	1.368	1.364	1.368/1.356
Cα-Nm/Cα′-Nm	1.308	1.310	1.316	1.304/1.321
Cα-Cβ/Cα′-Cβ′	1.447	1.450	1.455	1.448/1.461
Cβ-Cβ/Cβ′-Cβ′	1.384	1.387	1.392	1.394/1.386
Cβ-Nd/Cβ′-Nd′	1.330	1.329	1.328	1.330/1.325
Nd-Cγ/Nd′-Cγ′	1.305	1.306	1.307	1.304/1.310
Cγ-Cγ/Cγ′-Cγ′	1.433	1.432	1.430	1.433/1.424
Cγ-Cl/Cγ′-Cl′	1.709	1.709	1.709	1.709/1.712
(Np…Np)opp	3.856	3.915	4.031	4.051/3.892
(Np…Np)adj	2.726	2.768	2.850	2.809
P 1	21.440	21.424	21.440	21.376
Bond angles
$\angle$(ClaxMNp)	103.2	104.5	111.2	-
$\angle$(MNpCα)	125.2	124.5	123.5	123.3
$\angle$(NpCαNm)/(Np′Cα′Nm)	128.0	128.1	128.1	128.5/128.2
$\angle$(CαNmCα′)	122.3	123.3	125.3	124.0
$\angle$(CαCβNd)/(Cα′Cβ′Nd′)	130.3	130.3	130.3	129.5/131.1
$\angle$(CβNdCγ)/(Cβ′Nd′Cγ′)	114.9	115.0	115.2	115.3/114.9
M-X 2	0.451	0.504	0.782	-
A 3	175.3	174.0	164.6	180

¹ P is the coordination cavity perimeter (in Å). ² X is dummy atom located in center between N_p atoms. ³ α is the dihedral angle between planes of opposite pyrrole rings.

.

The degree of the doming-distortion can be described by the distance M-X between the metal atom and the center of the plane formed by four nitrogen atoms N_p and the dihedral angle between the planes of opposite pyrrole rings α. The distance M-X increases in line with the size of a metal atom. Furthermore, the dihedral angle between the opposite pyrrole fragments α decreases by 10 degrees from Ga to In, while a slight change in this parameter is noted from Al to Ga (r_ionic(Al) = 0.39, r_ionic(Ga) = 0.47, r_ionic(In) = 0.62) [28]. Structural parameters of the TPyzPA macrocyclic ligand are practically independent of the nature of a metal atom and peripheral –Cl substitution. The only noticeable change occurs for the peripheral C_γ-C_γ bond that is 0.025 Å longer in the TPyzPACl₈ metal complexes due to the electron-withdrawing effect of the –Cl substituents. A similar picture was previously observed for the complexes of tetrabenzoporphyrin with Al(Cl), Ga(Cl), In(Cl) [10] and porphyrazine, and tetrakis(1,2,5-thiadiazole)porphyrazine with Y(Cl), La(Cl) and Lu(Cl) [29]. The perimeter of the coordination cavity is not affected by the nature of a metal atom and increases by ca. 0.05 Å in the metal complexes as compared to the metal-free H₂TPyzPA and H₂TPyzPACl₈; nevertheless, the distances between the nitrogen atoms of pyrrole fragments (N_p…N_p)_opp and (N_p…N_p)_adj are noticeably different (up to 0.2 Å). This can be explained by the fact that in the Al-Ga-In series, there is a consistent decrease in distances N_p-C_α along with valence angles $\angle$(MN_pC_α) and an increase in C_α-N_m distances and $\angle$(C_αN_mC_α), which leads to the possibility of a significant difference in distances (N_p…N_p)_opp and (N_p…N_p)_adj without noticeable perimeter change.

2.2. NBO-Analysis

To gain a deeper insight into the electronic structures of M(Cl)TPyzPA and M(Cl)TPyzPACl₈ complexes and features of the metal–ligand chemical bonding, we performed the NBO-analysis of the electron density distribution. The obtained results suggest that the chemical bonding between a metal atom and TPyzPACl₈ macrocyclic ligand can be described in terms of the donor–acceptor interactions of the types: LP(N) → ns(M) and LP(N) → np(M), where n is the principal quantum number for the valence shell of the metal atom (n = 3 for Al, n = 4 for Ga, n = 5 for In). It should be noted that in the case of Al complexes, the interactions occur between LP(N) and two 3p(Al) orbitals, while for the complexes of Ga and In, all three valence p-orbitals provide a favorable overlap (Figure 2).

Comparing periphery-substituted TPyzPACl₈ complexes with their non-substituted analogues (

Table 2. Selected parameters of M(Cl)PyzPA complexes from NBO calculations.

	Al(Cl) TPyzPA	Al(Cl) TPyzPACl8	Ga(Cl) TPyzPA	Ga(Cl) TPyzPACl8	In(Cl) TPyzPA	In(Cl) TPyzPACl8
E(HOMO),eV	−6.329	−6.803	−6.362	−6.836	−6.411	−6.882
E(LUMO),eV	−3.815	−4.294	−3.829	−4.308	−3.864	−4.343
∆E, eV	2.514	2.509	2.533	2.528	2.547	2.539
q(M) NPA, e	1.718	1.716	1.637	1.635	1.694	1.692
q(N) NPA, e	−0.651	−0.651	−0.631	−0.631	−0.623	−0.623
q(Cl) NPA, e	−0.550	−0.545	−0.523	−0.518	−0.528	−0.521
configuration	3s0.423p0.83	3s0.423p0.83	4s0.534p0.81	4s0.534p0.81	5s0.545p0.76	5s0.545p0.76
∑ E(d-a), kcal mol−1	526.0	526.5	539.6	539.0	510.7	507.8
Q(M-N), e	0.335	0.330	0.343	0.342	0.327	0.325
r(M-N), Å	1.981	1.980	2.022	2.021	2.161	2.162

), we found that peripheral –Cl substituents do not affect the characteristics of the metal–ligand bonding. The enhanced covalent contribution into the Ga–N bond is in line with the trend of the electronegativities of the metal atoms: Al (χ = 1.47), Ga (χ = 1.82) and In (χ = 1.49) [30,31]. According to Table 2, the NPA charge of Al is the most positive, in contrast to the NPA charge of Ga. More than that, an analogical situation is observed for the negativity of NPA charges of chlorine atoms. Therefore, these charges also correlate with the electronegativities of the metals.

2.3. Electronic Absorption Spectra

Analyzing the absorption spectra (Figure 3), we can note that three intense absorption bands are observed in the visible light and near UV in the case of M(Cl)TPyzPACl₈, in contrast to M(Cl)TPyzPA, of which spectra contain only two bands in the visible light and near UV range. The B₁-band in the latter has low oscillator strength, so it is practically imperceptible in the absorption spectrum, while this peak is clearly observed in the perchlorinated complexes. This can be related to the fact that the peripheral Cl atoms contribute significantly to 6a₁ molecular orbital (MO) in M(Cl)TPyzPACl₈ in contrast to the hydrogen atoms in M(Cl)TPyzPA to 3a₁ MO (Figure S1). The B₁-band is predominantly formed by the electron transitions from these orbitals to doubly-degenerated LUMOs (

Table 3. Calculated composition of the lowest excited states and corresponding oscillator strengths for TPyzPA and TPyzPzCl₈ complexes.

Al(Cl)TPyzPA					Al(Cl)TPyzPACl8
State	Composition (%)	λ, nm	f	exp. λ, nm	State	Composition (%)	λ, nm	f	exp. λ, nm
11E	$4a_1\to 1e^{*}$(8) $3a_2\to 1e^{*}$(90)	570	0.31		11E	$5a_1\to 1e^{*}$(5) $3a_2\to 1e^{*}$(90)	576	0.37	643
71E	$3a_1\to 1e^{*}$(54) $2b_2\to 1e^{*}$(17) $4a_1\to 1e^{*}$(22)	327	0.07		41E	$2b_1\to 1e^{*}$(5) $6a_1\to 1e^{*}$(77) $3b_1\to 1e^{*}$(7) $3a_2\to 2e^{*}$(5)	377	0.37
91E	$3a_1\to 1e^{*}$(27) $4a_1\to 1e^{*}$(58) $3a_2\to 1e^{*}$(6) $3a_2\to 2e^{*}$(6)	318	1.00		81E	$5a_1\to 1e^{*}$(70) $6a_1\to 1e^{*}$(7) $3a_2\to 1e^{*}$(7) $3a_2\to 2e^{*}$(9)	331	0.95	363
Ga(Cl)TPyzPA					Ga(Cl)TPyzPACl8
State	Composition (%)	λ, nm	f	exp. λ, nm	State	Composition (%)	λ, nm	f	exp. λ, nm
11E	$4a_1\to 1e^{*}$(8) $3a_2\to 1e^{*}$(90)	565	0.32		11E	$5a_1\to 1e^{*}$(6) $3a_2\to 1e^{*}$(90)	571	0.38	638
71E	$3a_1\to 1e^{*}$(26) $2b_2\to 1e^{*}$(34) $4a_1\to 1e^{*}$(32)	333	0.05		41E	$5a_1\to 1e^{*}$(6) $6a_1\to 1e^{*}$(75) $4b_2\to 1e^{*}$(13)	381	0.31
91E	$3a_1\to 1e^{*}$(47) $4a_1\to 1e^{*}$(38) $3a_2\to 1e^{*}$(6) $3a_2\to 2e^{*}$(5)	319	0.95		81E	$5a_1\to 1e^{*}$(63) $6a_1\to 1e^{*}$(8) $3a_2\to 1e^{*}$(8) $3a_2\to 2e^{*}$(16)	337	1.03	360
In(Cl)TPyzPA					In(Cl)TPyzPACl8
State	Composition (%)	λ, nm	f	exp. λ, nm	State	Composition (%)	λ, nm	f	exp. λ, nm
11E	$4a_1\to 1e^{*}$(8) $3a_2\to 1e^{*}$(89)	562	0.32	589	11E	$3a_2\to 1e^{*}$(90) $5a_1\to 1e^{*}$(6)	569	0.38	649
71E	$3a_1\to 1e^{*}$(36) $2b_2\to 1e^{*}$(26) $4a_1\to 1e^{*}$(28) $3a_2\to 2e^{*}$(7)	336	0.04	335	41E	$6a_1\to 1e^{*}$(76) $5a_1\to 1e^{*}$(6) $3b_2\to 1e^{*}$(14)	386	0.26
91E	$3a_1\to 1e^{*}$(43) $4a_1\to 1e^{*}$(39) $3a_2\to 1e^{*}$(7) $3a_2\to 2e^{*}$(7)	320	0.98		81E	$5a_1\to 1e^{*}$(55) $6a_1\to 1e^{*}$(6) $3a_2\to 1e^{*}$(8) $3a_2\to 2e^{*}$(22)	336	1.05	339
H2TPyzPA					H2 TPyzPACl8
State	Composition (%)	λ, nm	f	exp. λ, nm	State	Composition (%)	λ, nm	f	exp. λ, nm
11B1u	$3a_u\to 1b_{1g}^{*}$(90)	555	0.35	645	11B1u	$3a_u\to 1b_{1g}^{*}$(90)	560	0.43	656
11B3u	$3a_u\to 1b_{3g}^{*}$(86) $2b_{2u}\to 1b_{1g}^{*}$(5) $3b_{2u}\to 1b_{1g}^{*}$(8)	552	0.33	611	11B3u	$1b_{2u}\to 1b_{1g}^{*}$(5) $2b_{2u}\to 1b_{1g}^{*}$(7) $3a_u\to 1b_{3g}^{*}$(86)	560	0.39	626
21B3u	$2b_{2u}\to 1b_{1g}^{*}$(12) $3b_{2u}\to 1b_{1g}^{*}$(80)	382	0.12		31B3u	$1b_{2u}\to 1b_{1g}^{*}$(5) $2b_{2u}\to 1b_{1g}^{*}$(85) $3a_u\to 2b_{3g}^{*}$(5)	364	0.78	358
41B3u	$1b_{2u}\to 1b_{1g}^{*}$(7) $2b_{2u}\to 1b_{1g}^{*}$(66) $3b_{2u}\to 1b_{1g}^{*}$(9) $3a_u\to 1b_{3g}^{*}$(9) $3a_u\to 2b_{3g}^{*}$(6)	315	1.32	330	41B1u	$1b_{2u}\to 1b_{3g}^{*}$(18) $2b_{2u}\to 1b_{3g}^{*}$(78)	362	0.26	358
41B1u	$1b_{2u}\to 1b_{3g}^{*}$(62) $2b_{2u}\to 1b_{3g}^{*}$(33)	311	0.29		41B3u	$1b_{2u}\to 1b_{1g}^{*}$(6) $3a_u\to 2b_{3g}^{*}$(91)	345	0.13
51B1u	$1b_{2u}\to 1b_{3g}^{*}$(33) $2b_{2u}\to 1b_{3g}^{*}$(42) $3b_{2u}\to 1b_{3g}^{*}$(10) $3a_u\to 1b_{1g}^{*}$(6) $3a_u\to 2b_{1g}^{*}$(5)	306	0.94		51B3u	$1b_{2u}\to 1b_{1g}^{*}$(79) $3a_u\to 1b_{3g}^{*}$(8)	319	0.88
					51B1u	$1b_{2u}\to 1b_{3g}^{*}$(68) $2b_{2u}\to 1b_{3g}^{*}$(12)	316	1.34

).

It can be seen that the position of the Q-band maxima in the electronic absorption spectra shifted bathochromically in the series Ga < Al < In. The changes in the theoretical and experimental spectra are similar to each other and therefore allow the nature of the results obtained to be explained. This indicates that the HOMO–LUMO gap is influenced not only by the degree of doming-distortion of the macrocycle, but also by electronegativity of the central metal, which is the highest for Ga. In addition, a minor bathochromic shift of the Q-band occurs for perchlorinated complexes M(Cl)TPyzPACl₈ as compared to M(Cl)TPyzPA. Noteworthy is the single absorption maximum (B-band) in the near UV Soret region (300–420 nm) that appears for M(Cl)TPyzPA, while B₁- and B₂-bands are present for M(Cl)TPyzPACl₈. Experimental spectra of Ga(OH)TPyzPACl₈ and In(OH)TPyzPACl₈ can be found in ESI (Figures S2 and S3).

In addition, attention should be paid to the pronounced hyperchromic effect in metal-free complexes caused by the superposition of several absorption bands with close wavelengths. The composition of the excited states forming the absorption bands is given in Table 3.

It can be seen that the excited states of the compounds under consideration are formed as a result of similar sets of electron transitions. The main composition of the excited states responsible for the Q-band consists of transitions between the frontier π-orbitals. Hence Q-band wavelengths correlate with the HOMO–LUMO gaps. The molecular orbitals level diagram is shown in Figure 4. Stabilization of LUMO upon peripheral chlorination of the TPyzPA macrocycle correlates with the electrochemical data [8], indicating that the reduction potentials for Cl₈TPyzPA complexes are shifted by 0.4–0.5 V in a less negative region as compared to the values typical for unsubstituted TPyzPA complexes [10]. It is also worth noting that the 1¹E states are formed mainly by Gouterman-type transitions a₂ → e* for metal complexes and transitions of the a_u→b_1g and a_u→b_3g types for metal-free ones due to the lower symmetry of the latter [32,33,34]. The situation is typical for a number of macrocycles of similar structure [35,36,37]. Despite the fact that the higher energy states are formed by almost the same electronic transitions, they have a different composition, and therefore energy.

Considering the shapes of the molecular orbitals (MOs) involved in the most probable electronic transitions, we can conclude that the metal nature should not affect the Q-band position since the boundary MOs are the linear combination of atomic orbitals (LCAO) of the macrocyclic ligand. Nevertheless, the size of the coordination cavity depends on the metal nature: changing the geometric structure provides an indirect effect on the position of the Q-band. Moreover, the shape of the LUMOs depends on the metal nature (Figure S1). In the case of Al, it consists of the bonding π-orbitals lying along the C_α-C_β bond and antibonding π-orbitals located around the C_α-N_m and C_α-N_p bonds, while for Ga and In, the C_α-N_p bonds have a significant contribution to the bonding π-orbital, probably due to the influence of the d-sublevel of the metal. The HOMO consists of the AOs localized on carbon atoms of pyrrolic fragments, which is a typical picture for porphyrazines [10,19,25,38]. Note that peripheral Cl atoms contribute to the formation of the MOs involved in the electronic transitions corresponding to the B₁-band in the absorption spectrum. For the rest, the MO’s form is similar for all compounds; therefore, it can be concluded that the ligand rather than metal has a decisive influence on the absorption spectrum.

2.4. IR Spectra

The IR spectra were simulated on the basis of the normal mode frequencies and band intensities, which have been calculated by the DFT (PBE0/def2-TZVP) method in a harmonic approximation.

Bands of weak intensity appear in the region up to 600 cm⁻¹, as well as the shift of bands in the range from 600 to 1000 cm⁻¹ occurs with the substitution of hydrogen atoms by the metal. It should be noted that the most intensive vibrational transitions of the metal complexes are degenerate, while the splitting of the bands in the case of the metal-free H₂TPyzPA and H₂TPyzPACl₈ is observed due to the lower symmetry. It results in two peaks with close frequencies formed by the normal vibrations of the pyrrole and pyrrolenine fragments. A medium peak of the N_p-H stretching at 3552 cm⁻¹ and 3554 cm⁻¹ (H₂TPyzPA and H₂TPyzPACl₈, respectively) is typical for macrocycles such as porphyrazines [25]. According to the experimental data [8,26], these frequencies are 3286 and 3287 cm⁻¹, respectively.

In-plane vibrations contribute to the medium band at 1400 cm⁻¹ in the spectra of M(Cl)TPyzPA. Noteworthy are the main differences in the M(Cl)TPyzPA IR spectra observed in the range of 300–600 cm⁻¹ related to the weak bands with a strong contribution of the M-Cl stretching.

It is worth noting that a low-frequency band shift occurs in both M(Cl)TPyzPA and M(Cl)TPyzPACl₈ in the Al→Ga→In series. Among the most intense peaks, the only exception is the ω₉₂-ω₉₃ band at the ~1140 cm⁻¹ for MTPyzPA, which corresponds to in-plane vibrations of the macrocyclic core. Moreover, a significant decrease in these bands’ intensity occurs with an increase in the ionic radius of a metal [28].

The IR spectra of M(Cl)TPyzPA are similar in the range of 600–3500 cm⁻¹ where the influence of the metal is almost absent. The replacement of H by Cl leads to the vanishing of the bands at 3190 cm⁻¹, corresponding to the C-H stretching vibrations, and an increase in the relative intensity of the peaks in the 800–1300 cm⁻¹ region. Simulated spectra are shown in Figure 5. The most intense bands at ~1250 cm⁻¹ of all investigated compounds are predominantly stretching vibrations of the bonds of pyrazine rings. Unlike M(Cl)TPyzPA spectra with one peak with a relative intensity >50%, the M(Cl)TPyzPACl₈ spectra also contain the strong bands ω₁₁₇-ω₁₁₈, composed by N_d-C_γ and C_β-N_d stretching vibrations. Moreover, M(Cl)TPyzPACl₈ spectra include more relatively intense peaks compared to MTPyzPA. A description of the main vibrations is listed in

Table 4. Assignment of the IR vibrations of the M(Cl)TPyzPA and M(Cl)TPyzPACl₈ complexes.

Frequency, cm−1	Irel, %	Symmetry	Assignment 1	Exp, cm−1
H2TPyzPA				KBr [26]
1091 (ω86)	78	B1u	r(Np-Cα) (29), r(Nm-Cα) (18), r(Nd-Cγ) (10), r(Cγ-Cγ) (12)	1040
1157 (ω90)	81	B2u	r(Np-Cα) (23), r(Nm-Cα) (18), r(Cα-Cβ) (6), r(Cβ-Nd) (8), r(Nd-Cγ) (6), φ(Cα-Np-Hc) (8), φ(Cβ-Nd-Cγ) (7)	1121
1242 (ω95)	100	B1u	r(Cα-Cβ) (14), r(Cβ-Cβ) (10), r(Nd-Cγ) (10), φ(Nd-Cγ-Hs) (7)r(Cβ-Nd) (6), r(Cβ-Cβ) (42), r(Cγ-Cl) (26), φ(Nd-Cγ-Cl) (6)	1198
3193 (ω141)	24	B2u	r(Hs-Cγ) (99)	3051
3552 (ω143)	45	B1u	r(Hc-Np) (99)	3286
H2TPyzPACl8				KBr [8]
1242 (ω107)	100	B1u	r(Cβ-Nd) (6), r(Cβ-Cβ) (42), r(Cγ-Cl) (26), φ(Nd-Cγ-Cl) (6)	1192
1255 (ω109)	98	B2u	r(Cβ-Nd) (8), r(Cγ-Cγ) (46), r(Cγ-Cl) (21), φ(Nd-Cγ-Cl) (6)	1235
1281 (ω111)	67	B1u	r(Cα-Cβ) (8), r(Cβ-Cβ) (19), r(Nd-Cγ) (13), r(Cγ-Cγ) (13)
1328 (ω116)	52	B1u	r(Np-Cα) (7), r(Nm-Cα) (11), r(Cβ-Nd) (12), r(Nd-Cγ) (45)
1340 (ω117)	83	B2u	r(Np-Cα) (8), r(Cα-Cβ) (10), r(Cβ-Cβ) (8), r(Cβ-Nd) (19), φ(Cα-Np-Hc) (28)
3554 (ω143)	27	B1u	r(Hc-Np) (99)	3287
Al(Cl)TPyzPA
1140 (ω92-ω93)	53	E	r(Np-Cα) (24), r(Nm-Cα) (14), r(Cα-Cβ) (10), r(Cβ-Nd) (12), r(Nd-Cγ) (9)
1255 (ω98-ω99)	100	E	r(Np-Cα) (5), r(Cα-Cβ) (15), r(Cβ-Cβ) (14), r(Nd-Cγ) (20), φ(Cβ-Nd-Cγ) (9)
1404(ω113-ω114)	36	E	r(Cβ-Cβ) (12), r(Cγ-Cγ) (13), φ(Nd-Cγ-Hs) (36), φ(Cγ-Cγ-Hs) (23)
3189 (ω142-ω143)	18	E	r(Hs-Np) (99)
Al(Cl)TPyzPACl8				KBr [8]
1246 (ω111-ω112)	100	E	r(Cγ-Cγ) (43), r(Cγ-Cl) (25), φ(Nd-Cγ-Cl) (6)	1168
1293 (ω115-ω116)	52	E	r(Np-Cα) (8), r(Cβ-Cβ) (15), r(Nd-Cγ) (25), r(Cγ-Cγ) (13)	1260
1331 (ω117-ω118)	97	E	r(Cα-Cβ) (8), r(Cβ-Nd) (23), r(Nd-Cγ) (49)	1323
Ga(Cl)TPyzPA
1142 (ω92-ω93)	44	E	r(Np-Cα) (49), r(Nm-Cα) (11), r(Cα-Cβ) (9), r(Nd-Cγ) (9)
1254 (ω98-ω99)	100	E	r(Np-Cα) (6), r(Cα-Cβ) (12), r(Cβ-Cβ) (11), r(Cβ-Nd) (9), r(Nd-Cγ) (18)
1401 (ω113-ω114)	35	E	r(Cβ-Cβ) (11), r(Cγ-Cγ) (12), φ(Nd-Cγ-Hs) (36), φ(Cγ-Cγ-Hs) (23)
3194 (ω142-ω143)	17	E	r(Hs-Np) (99)
Ga(Cl)TPyzPACl8				this work
1248 (ω111-ω112)	100	E	r(Cγ-Cγ) (41), r(Cγ-Cl) (24), φ(Nd-Cγ-Cl) (6)	1232
1290 (ω115-ω116)	50	E	r(Np-Cα) (6), r(Cα-Cβ) (7), r(Cβ-Cβ) (17), r(Nd-Cγ) (26), r(Cγ-Cγ) (13)
1328 (ω117-ω118)	94	E	r(Cα-Cβ) (8), r(Cβ-Nd) (22), r(Nd-Cγ) (50)	1349
In(Cl)TPyzPA				KBr [39]
1143 (ω92-ω93)	36	E	r(Np-Cα) (49), r(Nm-Cα) (10), r(Cα-Cβ) (6), r(Nd-Cγ) (10)	1100
1247 (ω98-ω99)	100	E	r(Np-Cα) (5), r(Cα-Cβ) (15), r(Cβ-Cβ) (14), r(Nd-Cγ) (20)	1213
1396 (ω113-ω114)	34	E	r(Cβ-Cβ) (10), r(Cγ-Cγ) (10), φ(Nd-Cγ-Hs) (34), φ(Cγ-Cγ-Hs) (22)	1364
3194 (ω142-ω143)	17	E	r(Hs-Np) (99)	3316
In(Cl)TPyzPACl8				this work
1250 (ω111-ω112)	100	E	r(Cγ-Cγ) (44), r(Cγ-Cl) (26), φ(Nd-Cγ-Cl) (6)	1264
1280 (ω114-ω115)	50	E	r(Np-Cα) (5), r(Cα-Cβ) (7), r(Cβ-Cβ) (19), r(Nd-Cγ) (26)	1323
1325 (ω117-ω118)	83	E	r(Cα-Cβ) (8), r(Cβ-Nd) (23), r(Nd-Cγ) (49)	1364

¹ Coordinates are listed provided that their contributions (shown in parentheses) are greater than ~5%. Assignment of vibrational modes based on potential energy distribution. The following designations of the coordinates are used: r—stretching of the bond; φ—bending, a change in the angle; OPB—out-of-plane bending; θ—a change in the dihedral angle. Experimental frequencies are given for metal complexes with an axial–OH ligand.

(full list of the most active vibrations can be found in Supplementary Materials, Table S1). It is important to mention that the calculated spectra of Ga(Cl)TPyzPACl₈ and In(Cl)TPyzPACl₈ are consistent with the experimental ones with a factor ~0.95. Actually, the main differences may be caused by another axial ligand (OH instead of Cl). Experimental spectra can be found in ESI (Figures S4 and S5).

3. Materials and Methods

3.1. Synthesis

Octachlorotetrapyrazinoporphyrazinatogallium(III) hydroxide, [Ga(OH)TPyzPACl₈]. A mixture of 5,6-dichloropyrazine-2,3-dicarbonitrile (200 mg, 1 mmol) and gallium(III) hydroxydiacetate (50 mg, 0.24 mmol) was melted at 200 °C for 10 min. The obtained solid was powdered, washed with CH₂Cl₂, then dissolved in conc. H₂SO₄, precipitated by pouring into ice-water, centrifuged and washed with MeOH. After drying, the complex was obtained as dark green hydrated material.

Octachlorotetrapyrazinoporphyrazinatoindium(III) hydroxide, [In(OH)TPyzPACl₈]. A mixture of 5,6-dichloropyrazine-2,3-dicarbonitrile (1) (200 mg, 1 mmol) and indium(III) hydroxydiacetate (50 mg, 0.2 mmol) was melted at 200 °C for 10 min. The obtained solid was powdered, washed with CH₂Cl₂, then dissolved in THF and chromatographed on silica with THF as eluent. The eluted product was precipitated by pouring into water, centrifuged and washed with MeOH. After drying, the complex was obtained as dark green hydrated material [8].

The IR spectra were measured on an IR-spectrometer Cary 630 FT-IR using KBr pellets. UV–Vis spectra were recorded using a Cary UV-Vis spectrophotometer in THF solution and can be found in ESI.

3.2. Computational Details

The DFT study of M(Cl)TPyzPA and M(Cl)TPyzPACl₈ included geometry optimization and calculations of the harmonic vibrations, followed by calculation of the electronic absorption spectrum by the TDDFT method. The number of the calculated excited states was 30. The calculations were performed using the PBE0 functional with the density functional dispersion correction D3 provided by Grimme [40] with the def2-TZVP basis set [41] taken from the EMSL BSE library [42,43,44]. Firefly QC package [45], which is partially based on the GAMESS (US) [46] source code, was used to obtain the optimized geometry, electronic absorption spectra and NBO-analysis. The optimized Cartesian coordinates of H₂TPyzPA, H₂TPyzPACl₈ and their metal complexes with Al, Ga, In are available in the Supplementary Materials.

The calculations of IR spectra were carried out with use of Gaussian09 [47] software package due to the fact it applies analytical functions in the second derivatives computing. The molecular models and orbitals demonstrated in the paper were visualized by means of the Chemcraft program [48].

4. Conclusions

The geometry and electronic structure of TPyzPA were investigated using PBE0-D3 functional with basis set def2-TZVP. The distance M-X between the metal atom and the center of the plane formed by four nitrogen atoms N_p increases in line with the size of a metal atom. Structural parameters of the TPyzPA macrocyclic ligand are practically independent of the nature of a metal atom and peripheral −Cl substitution. The fact that (N_p…N_p)_opp and (N_p…N_p)_adj are substantially different for Al, Ga and In complexes, while the perimeter of the coordination cavity almost remains invariant for both M(Cl)TPyzPA and M(Cl)TPyzPACl₈ complexes, indicates that the degree of distortion increases from Al to In. At the same time, the substitution of H-atoms for Cl mainly affects the internuclear distances of pyrazine rings and has a negligible effect on the N_p-C_α and C_α-N_m.

According to the NBO-analysis of electron density distribution, the strong donor–acceptor interactions of the types: LP(N) → ns(M) and LP(N) → np(M), where n is the principal quantum number for the valence shell of the metal atom, stabilize the complexes.

It was found that the HOMO–LUMO gap is slightly affected by the nature of the metal and increases in the Al-Ga-In series. The substitution of H-atoms for Cl leads to a decrease in the energy of the frontier MOs of M(Cl)TPyzPACl₈ as compared to M(Cl)TPyzPA. Furthermore, the LUMO is stabilized much more than the HOMO, resulting in a smaller HOMO–LUMO gap.

Simulated electronic absorption spectra show that the metal nature slightly influences the position of the band’s position. In the series Al→Ga→In→H₂, the Q-band is shifted to a shorter wavelength. In addition, a minor bathochromic shift occurs in M(Cl)TPyzPA compared to M(Cl)TPyzPACl₈. The excited states corresponding to the Q-bands are composed mainly of Gouterman-type transitions. The electronic absorption spectra of TPyzPACl₈ contain three intense maxima, while only two peaks were found for TPyzPA. The additional B₁ band in the M(Cl)TPyzPACl₈ spectra is caused by the transitions from MOs with a contribution of AOs of the peripheral Cl atoms.

The calculated IR spectra of TPyzPACl₈ have a significant difference in comparison with TPyzPA due to heavy Cl atoms being involved in the vibrations that form medium and strong bands in the region of 1000–1500 cm⁻¹.