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6Porphyrazines ; , Л Paper
Порфиразины http://mhc-isuct.ru Статья
DOI: 10.6060/mhc224831s
Electrochemical Study of the Lanthanide Complexes of Tetra(1r2r5-thiadiazolo)poiphyrazmer [TSDPzLn(acac)] (Ln = SmIII[ EuIIIr DyIIIr Erm, LuIII)
Mahmoud Hamdoush, Sergey Sigunov, Ivan A. Skvortsov, Ekaterina N. Tarakanova, and Pavel A. Stuzhin@
Research Institute of Macroheterocycles, Ivanovo State University of Chemistry and Technology, 153000 Ivanovo, Russia @Corresponding author E-mail: [email protected]
Complexes of tetra(1,2,5-thiadiazolo)porphyrazine with lanthanides (Ln = Sm111, Euln, Dy111, Er111, LuHI) were prepared by complexation of the metal free macrocycle [TSDPzH2] with the corresponding trisacetylacetonates Ln(acac)s in o-dichlorobenzene in the presence of DBU. The analytical and spectral data evidence that complexes were isolated as dihydrates [TSDPzLn(acac)]2H2O. In their IR spectra coupled C^O and C^C stretching vibration bands of acetylacetonate at 1515-1530 and 1580—1590 cm'1, as well as deformation vibrations of coordinated water molecule at 1620—1650 cm'1 are observed. Electronic absorption spectra contain an intense and narrow Q band at ca. 650 nm due to almost pure 3a2^1e* transition and two bands in the UV region having maxima at 292—296 and 364—366 nm which are much broader due to complex configuration interaction with predominant contribution of 5ai^1e* and 4ai^1e* transitions. Cyclovoltammetric study in DMFsolution reveals two reversible reductions at -0.63±0.03 and —1.18+0.03 V, and non-reversible oxidation at +1.22+0.06 V(vs. Ag/AgCl). Negative shift of the reduction potentials in comparison with complexes with p- and d-metals is indicative of stronger ionic character of the coordination bonds between central metal andpyrrolic nitrogen atoms in the lanthanide complexes.
Keywords: Porphyrazines, 1,2,5-thiadiazole, lanthanides, cyclovoltammetry, electronic absorption spectra.
Электрохимическое исследование комплексов лантанидов c тетра(1г2,5-тиадиазоло)порфиразином, [TSDPzLn(acac)] ^п = Smшr Euшr Dyшr Erшr LuIII)
М. Хамдуш, С. Сигунов, И. А. Скворцов, Е. Н. Тараканова, П. А. Стужин@
НИИ Макрогетероциклических соединений, Ивановский государственный химико-технологический университет, 153000Иваново, Россия @Е-таН: [email protected]
Комплексы тетра(1,2,5-тиадиазоло)порфиразина с лантанидами (Ьп = $ти, Ей111-, Еу111, Ег111, Ьи111) получены комплексообразованием безметального макроцикла [TSDPzH2] с соответствующими трис-ацетилацето-натами Ьп(асас)з в о-дихлорбензоле в присутствии DBU. Согласно аналитическим и спектральным данным комплексы выделены в виде дигидратов [TSDPzЬn(acac)] 2Н2О. В ихИК спектрах валентные колебания С^О и С^С связей ацетилацетоната наблюдаются при 1515—1530 и 1580—1590 см'1, а деформационные колебания координированной воды — при 1620—1650 см'1. Электронные спектры поглощения содержат интенсивную узкую Q полосу при ~650 нм (3 а2^1е* переход) и две более широкие полосы в УФ области с максимумами при 292—296и 364—366нм (конфигурационное взаимодействие 5а1^1е*и 4а1^1е*переходов). Цикловольтамп е-рометрические исследования в ДМФА показали наличие двух обратимых процессов восстановления макроцикла при -0.63+0.03 и -1.18+0.03 В и необратимое окисление при +1.22+0.06 В (отн. Ag/AgCl). Катодный сдвиг потенциалов восстановления по сравнению с комплексами р- и d-металлов указывает на более ионный характер связи металла с пиррольными атомами азота в комплексах лантанидов.
Ключевые слова: Порфиразины, 1,2,5-тиадиазол, лантаниды, цикловольтамперометрия, электронные спектры поглощения.
Introduction
Phthalocyanine analogues bearing four annulated electron deficient 1,2,5-thiadiazole rings instead of benzene fragments, [TSDPzM], attract much attention[1] due to their remarkably enhanced electron affinity as compared to common phthalocyanines [PcM] (Chart 1). Strong electron affinity of the macrocycle combined with their ability to form two-dimensional layered structures in the solid state[2] allow to consider TSDPz complexes as perspective «-type conducting functional materials for organic electronic applications, e.g. as «-type organic semiconductors in prototypes ofthe field-effect transistors and photovoltaic cells.[3-7] Preparation and physicochemical characterization of the metal free macrocycle, [TSDPzH2], and its complexes with Mgn,[8] L?,[9] trivalent ^-metals [TSDPzM(X)] (M = Alm, Gain, iniii)[1o] and first row transition metals [TSDPzM] (M = Mn11, Fe11, Co11, Ni11, Cu11, Zn11, V[V=O)[11-13] have been reported.
Phthalocyanines
_N N .
ft~ N / N M .n
(or^o)
[PcM ]
Tetrapyrazinoporphyrazines
Tetra(1,2,5-thiadiazolo)porphyrazines
_N N -
\ / ~ N M N
/ \ ' = N N -
N — S
OS
_N N
\ / N M
/ S -N N
VÎN N^^ S-N N —S
N
I
N — S
They are convenient precursors for the synthesis of hetero-leptic sandwich-type species with different types of macrocycles (not limited to tetrapyrrolic).[15,31]
S
_N
N — S
OS
n^r \r>N
V_N H N __/
N ' ' N Ln(acac)3
' !/ DBU, o-DCB
= M HN^ reflux, 3h
iOr n^O
S-N N — S
[T S D P z H 2]
_N |i "ff N — S
O >
V_ N ^ \ /_, N _/
N / \ ,n
N N
iO^'^YO
N i
N — S
[TSDPzLn(acac)]
Ln = Sm111, Eu111, Dy111, Er111, Lu111
[TPyzPzM ]
[TSDPzM ]
Scheme 1. Synthesis of lanthanide complexes [TSDPzLn(acac)].
The ability of lanthanides to form neutral or charged homo- and heteroleptic sandwich complexes with tetrapyrrolic macrocycles and their stability is determined by coordination and redox-properties of the macrocycle. Annulation of electron-deficient heterocycles instead of benzene rings in phthalocyanines can strongly influence the coordination ability of porphyrazine macrocycle and its electron affinity. Complexes of lanthanides have so far only been reported for heterocyclic phthalocyanine analogues containing fused pyrazine[32-35] or 2,3-thiophene frag-ments.[36] Recently we have obtained complexes with Y111 and Lu111 as first representatives of the rare earth porphyrazines with annulated 1,2,5-thiadiazole rings, [TSDPzY(acac)] and [TSDPzLu(acac)],[37] and conducted DFT study of their molecular and electronic structure.[38] In this work we present synthesis, spectral and electrochemical study of new complexes from the lanthanide series - tetra(1,2,5-thiadiazolo)porphyrazines containing samarium(III), euro-pium(iii), dysprosium(iii), erbium(iii) and lutetium(iii) as central metal and coordinated acetylacetonate as extralig-and, [TSDPzLn(acac)], Ln = Sm111, Eu111, Dy111, Er111, Lu111 (Scheme 1).
M e
M e
N
N
Chart 1. Structures of phthalocyanine and its heterocyclic analogues.
Experimental
General
Rare earth elements (REE) can form with tetrapyrrolic macrocycles (porphyrins, porphyrazines and phthalocyanines) monomelic (1:1) and sandwich-type (1:2, 2:3) complexes.!14,1^ Due to their magnetic properties and rich redox chemistry sandwich-type lanthanide complexes with phthalocyanines are very interesting for the development of functional magnetic, electronic and optical materials,[16-19! e.g. single molecule magnets (SMMs),[20! molecular switches,^ sensors,[22! organic field effect transistors,!16-23! and nonlinear optical materials.!24! Compared to the sandwichtype 1:2 complexes lanthanide monoporphyrazinates and monophthalocyaninates (1:1 complexes) remain much less studied, although their potential application as semiconduc-tors,[25,26] photo- or electroluminescent materials,!27,28! and SMMs[29! was demonstrated. Lanthanide monoporphyrazi-nates/monophthalocyaninates with unsaturated coordination sphere (n= 5 or 6) are often unstable but can be stabilized by its saturation due to binding of additional cis-ligands.[30!
Electronic absorption spectra were recorded using a Cary 60 spectrophotometer. The IR spectra were obtained on a Cary 630 FT-IR spectrometer. Elemental analyses were performed on a Flash EA 1112 CHN analyzer and mass-spectrometric measurements were carried out on a MALDI TOF Shimadzu Biotech Ax-ima Confidence mass-spectrometer at the Center of the Collective Usage at Ivanovo State University of Chemistry and Technology. Commercially available solvents were dried and distilled prior to use. Tetra(1,2,5-thiadiazolo)porphyrazine [TSDPzH2![8,9! and its LuIn complex [TSDPzLu(acac)![! were prepared as described earlier. Lanthanide acetylacetonates Ln(acac)3-2H2O were commercial products or prepared from the corresponding acetates and acetylacetone.
Synthesis of [TSDPzLn(acac)] (Ln = Sm, Eu, Dy, Er)
In a common procedure the mixture of [TSDPzH2! (0.10.2 mmol) and lanthanide acety lacetonate dihydrate, Ln(acac)3 2H2O, taken in 1:2 or 1:3 molar ratio was refluxed in o-dichlorobenzene (5 mL) in the presence of DBU (0.1-0.15 mL) with intense stir-
ring under Ar for 2h. After cooling the reaction mixture was poured into hexane, the precipitate was filtered and washed with 2 % water solution of acetic acid, then with hot aqueous 75 % methanol and dried overnight at 50 oC.
[TSDPzSm(acac)]. From 0.11 mmol [TSDPZH2] and 0.23 mmol Sm(acac)3. Yield: 72 %. Anal. Found C, 30.23; H, 1.28; N, 27.16; S, 15.20%. Calcd. for C2iH7Ni6O2S4Sm-2H2Ü: C, 30.39; H, 1.34; N, 27.00; S, 15.45%. UV-Vis (DMF) W nm (lge): 365 (4.58), 593 (4.10), 623 (4.32), 649 (4.93). FTIR (KBr) v cm-1: 510m, 675s, 760w, 1094s, 1256vs, 1392w, 1516m, 1580m, 1647m, 2853w, 2927m.
[TSDPzEu(acac)]. From 0.22 mmol [TSDPZH2] and 0.66 mmol Eu(acac)3. Yield: 70 %. Anal. Found C, 30.51; H, 1.22; N, 26.91; S, 15.24%. Calcd. for C21H7N16O2&Eu-2H2O: C, 30.33; H, 1.33; N, 26.95; S, 15.42%. UV-Vis (DMF) W nm (lge): 366 (4.53), 592 (4.09), 623 (4.31), 655 (4.99). FTIR (KBr) v cm-1: 408w, 510m, 675s, 734w, 760w, 818w, 1094s, 1254vs, 1397w, 1526m, 1586m, 1644m, 2857w, 2928m.
[TSDPzDy(acac)]. From 0.18 mmol [TSDPZH2] and 0.55 mmol Dy(acac)3. Yield: 75 %. Anal. Found C, 29.83; H, 1.40; N, 26.47; S, 14.97%. Calcd. for C21H7N16O2&Dy 2H2O: C, 29.95; H, 1.32; N, 26.61; S, 15.23%. UV-Vis (DMF) W nm (lge): 364 (4.50), 592 (4.02), 623 (4.22), 651 (4.88). FTIR (KBr) v cm-1: 409w, 511m, 676s, 734w, 761w, 1098s, 1256vs, 1323, 1397w, 1528m, 1588m, 1610w, 1643m, 2853w, 2928m.
[TSDPzEr(acac)]. From 0.11 mmol [TSDPZH2] and 0.33 mmol Er(acac)3. Yield: 73 %. Anal. Found C, 29.74; H, 1.39; N, 26.32; S, 14.88%. Calcd. for C21H7N16O2&Er-2H2O: C, 29.78; H, 1.31; N, 26.46; S, 15.14%. UV-Vis (DMF) W nm (lge): 364 (4.53), 592 (4.01), 624 (4.22), 651 (4.86). FTIR (KBr) v cm-1: 411w, 510m, 676s, 760w, 1088s, 1260vs, 1373w, 1532m, 1550m, 1620m, 2853w, 2924m.
Cyclovoltammetric Measurements
Cyclovoltammetric measurements were performed in dime-thylformamide (DMF) using a three-electrode electrochemical cell with a glassy carbon working electrode, a Pt wire counter electrode and an Ag/AgCl reference electrode on a potentiostat Elins P-4. The solutions containing 0.001 M [TSDPzLn(acac)] and 0.1 M tetrabutylammonium perchlorate as supporting electrolyte were deoxygenated with argon. The reference Fc/Fc+ couple in DMF
Table 1. Assignment of the IR vibrations (in cm-1) of the TSDPz macrocycle.
was observed at +0.567 V vs. Ag/AgCl. The obtained potential values are shifted versus Standard Calomel Electrode (SCE) by -0.045 V.[39]
Results and Discussion
Synthesis and Spectral Characterization
Complexes of TSDPz macrocycle with Smm, Eu111, Dy111 and Er111 were prepared similarly to the previously reported complexes with Y111 and LuIII[37] by complexation of the metal free macrocycle [TSDPZH2] with corresponding lanthanide acetylacetonate (1:2 - 1:3 molar ratio) in o-dichlorobenzene in the presence of DBU under reflux The target complexes were isolated from the reaction mixture by precipitation into hexane and excess of unreacted acetylacetonate and by-products was removed by washing with slightly acidified water and aqueous methanol. According to analysis results the products were obtained as hydrated materials [TSDPzLn(acac)]-2H2Û. Two water molecules complete the coordination sphere of the lanthanide ions which are able to bind up to eight donor atoms as was shown by X-ray study for the Lu111 phthalocyanine with bidentate coordinated acetate [PcLu(OAc)(H2o)2].[40] In the following discussion water will be omitted in the abbreviations .
IR Spectra
IR spectra ofthe lanthanide complexes [TSDPzLn(acac)] contain intense bands at 1260, 1090-1100, 675 and 510 cm-1 (Figure 1). These bands are also typical for the spectra of TSDPz complexss with p- and d-metals[8,10,11] and were assigned to skeletal vibrations 1,2,5-thiadiazolo-pyrrole fragments constituting the macrocycle.[38,41-43] It should be noted that above 1400 cm-1 for tetracoordinated complexes with transition metals [TSDPzM] (M = Zn, Cu, Ni, Fe, Mn) only one band of medium intensity is observed at 15351555 cm-1[11] (Table 1).
[TSDPzLn(acac)]
[TSDPzM]
Vibrational mode Sm Eu Dy Er Lu[38] Li[9] Zn[48] Ni[43]
TSDPz m acrocycle
fold(Nt-Nt) 510m 510m 511m 510m 513m 503s 512m 511m
p(NpCaCp), p(NmCaCp) 675s 675s 676s 676s 678s 684s 684s 689s
r(S-Nt), p(CaNmCa) 736w 734w 734w 738w 727w 730w 741w
r(S-Nt), p(CaNmCa), p(CaCpNt) 762w 760w 761w 760w 764w 760w 766w 763w
In-plane bending (pyrrole+thiaziazole) 821m 820w 820w 820w 822w 818m 825w 827w
Breathing (pyrrole+thiadiazole) 867w 865w 866 866w 868w 870w 872w 895w
r(Np-Ca), p(NpCaCp) r(Ca-Cp), r(Ca-Nm), p(NpCaCp), r(Cp-Nt) 1094s 1094s 1098s 1088s 1090s 1092s 1087s 1108s 1118s
p(NmCaCp), p(CaNmCa) 1256vs 1254vs 1256vs 1260vs 1262vs 1260vs 1269vs 1269vs
r(Ca-Nm), p(NpCaCp); r(Cp-Nt), r(Ca-Cp) * * * * * 1523m 1530m 1552m
Axial ligands
v (C»O) 1516m 1526m 1528m 1532m 1522vs
v (C-C) 1580m 1586m 1588m 1550m
S(H2O) 1647m 1644m 1643m 1620m
v (CH3) 2853w 2927m 2857w 2928m 2853w 2928m 2853w 2924m
aAssignment is based on the theoretical calculation for the Li1 [9] and Lu111 [38] complexes.
* Masked by stronger vibrations of acac ligand.
This band is predominantly contributed by CaNmeso stretching vibrations.[41-43] In the case of the lanthanide complexes [TSDPzLn(acac)] this vibration is masked by three strong-to-medium intensity bands appearing in the 1500-1650 cm-1 region. The bands at 1515-1530 and 1580-1590 cm-1 can be assigned to characteristic coupled C^O and C^C stretching vibrations of the acetylacetonate ligand.[44-45] Similar bands of the coordinated acac ligand appear also in the IR spectra of the corresponding lanthanide complexes of phthalocyanine [PcLn(acac)][46] and meso-tetraphenyl-porphyrin [TPPLn(acac)].[47] Deformation vibrations of coordinated water appear at 1620-1650 cm-1.
Figure 1. IR spectrum of [TSDPzSm(acac)] in KBr.
.1.1. ■ 1 364 1 1 1 Er 65' ■ A. :
651 ■
■ 293 364 Dy A, :
650 ■ A
■ 294 ^ 366 "V Eu A :
649 . A
^ 296 365 Sm i -
1 -1 ^ A. ■
300 400 500 600 700 Wavelength, nm
800
1e*
3a2
4ai
5a1
Figure 2. Electronic absorption spectra of [TSDPzLn(acac)] (Ln = Smm, Eu111, Dym, Er111) in DMF.
Figure 3. n-Molecular orbitals involved in the electronic transitions. Molecular model for [TSDPzLu(Cl)].[38]
Electronic Absorption Spectra
Electronic absorption spectra of all lanthanide complexes [TSDPzM(acac)] (M = SmIn, EuIn, DynI, Er111) are very similar to one another (Figure 2) and to previously reported complexes with Y111, Lum.[37] In the visible region they contain an intense Q band at ~650 nm with vibronic satellites at 624 and 593 nm In the UV-region two broad bands at ~365 and 290-300 nm are observed. According to TD-DFT calculations performed for the rare earth complexes [TSDPzM(Cl)] (M = Y, La, Lu)[38] the Q band in the visible region arise from an almost pure Gouterman type HOMO-LUMO nn*- transition 3a2^ie*, while two intense excited states observed in the UV-region denoted as Bi and B2 are mainly contributed by strong configuration interaction of 5ai^1e* and 4ai^1e* transitions. The coordinating pyrrolic nitrogen atoms have no contribution to the 3a2 orbital, minor to the 5ai orbital and strong to the 4ai orbital (Figure 3). Therefore, the metal center should have practically no influence on the position of the Q band, and its effect on the Bi and B2 bands reflects contribution of the 4ai^ie* transition. Similar situation is expected for TSDPz complexes with alkali earth metals - MgII,[4i] CaII[42] forming substantially ionic M-Npyr bonds. Complexes with d-metals have more covalent M-Npyr bonds due to interaction of macrocyclic n-MOs with d% orbitals and in the case of ZnII[42] and NiII[43] the energy of the Q band depends on the central metal and the configuration composition of transition in the UV-region is dfferent.[8,ii]
In the case of monophthalocyaninates [PcLn(acac)] (Ln = EunI, Gdm, TbIn, DynI, HoIn, ErnI, TmIn, LuIn) the maximum of the Q band is also independent of the lanthanide ion and observed at ~675 nm.[46] The hypsochromic shift by ca. 25 nm observed for [TSDPzLn(acac)] complexes as
compared to the corresponding [PcLn(acac)] complexes is typical for heteroatom substitution of C4H4 fragments in benzene rings of phthalocyanines by NSN moiety in 1,2.5-thiadiazole rings leading to widening of the HOMO-LUMO gap due to stronger stabilization of HOMO.[1] In the case of pyrazine fused analogues [TPyzPzLn(acac)] (Ln = = Er111, Lu111) and their alkyl substituted derivatives
[RsTPyzPzLn(acac)] (Ln = ErIn, Tm[II, YbIn, LuIn; R = Et, or H, tert-Bu) the maximum of the Q band is observed at ca 640 nm[33] indicating larger HOMO-LUMO gap. However, octaphenyl substituted tetrapyrazinopor-phyrazine phsTPyzPzLn(OAc)] (Ln = YIn, EuIII, GdIII, DyIII, ErIII, YbIII, LuIII, TmIII)[33,35,49] has the Q-band maximum at 663-665 nm due to destabilization of HOMO.
Table 2. UV-Vis spectra of [TSDPzM(acac)] (M = Y, Sm, Eu, Dy, Er, Lu) and related porphyrazine complexes.
Absorption maximum, nm
Porphyrazine complex Solvent B2 Soret region B1 Q band Ref.
[TSDPzZn] DMF 320 372,400sh 588 617 644 [11,48]
[TSDPzAl(Cl)] Py 340 364sh, 393sh 590 622 647 [10]
[TSDPzGa(Cl)] Py 337 367sh, 400sh 585 617 645 [10]
[TSDPzIn(OAc)] Py 321 365sh, 390sh 602 658 [10]
[TSDPzCa] Py 647 [42]
[TSDPzLi]- DMF 298 372 585 611 639 [9]
[TSDPzY(acac)] CH2Cl2 360 (4.20) 590 (3.70) 624 (4.00) 651 (4.60) [37]
[T SDPz Sm(acac)] DMF 296 365 (4.58) 593 (4.10) 623 (4.32) 649 (4.93) tw
[TSDPzEu(acac)] DMF 294 366 (4.53) 592 (4.09) 623 (4.31) 650 (4.99) tw
[T SDPzDy (acac)] DMF 293 364 (4.50) 592 (4.02) 623 (4.22) 651 (4.88) tw
[TSDPzEr(acac)] DMF 292 364 (4.53) 592 (4.01) 624 (4.22) 651 (4.86) tw
[TSDPzLu(acac)] CH2Cl2 360 592 623 650 [37]
[PcLu(Cl)] o-DCB 344 606 672 [50]
[TPyzPzLu(acac)] DMF 365 581 638 [33]
[tBu4TPyzPzLu(acac)] DMF 341 584 640 [33]
[Ph8TPyzPzLu(acac)] DMF 367 598 657 [33]
Table 3. Half-wave potentials (vs. Ag/AgCl) of lanthanide complexes of tetra(1,2,5-thiadiazolo)porphyrazine and related complexes of porphyrazines, phthalocy anine and meso-tetrapheny lporphyrin.
Reduction
Oxidation
Compound £'1/2 E2m Epc Solvent
[TSDPzZn] -0.44 -0.87 DMF [50]
[TSDPzAl(Cl)] -0.10 -0.54 DMF [10]
[TSDPzLi]- -0.94 -1.24 DMSO [9]
[T SDPz Sm(acac)] -0.59 -1.15 1.16 DMF tw
[TSDPzEu(acac)] -0.59 -1.16 1.18 DMF tw
[T SDPzDy (acac)] -0.65 -1.21 1.26 DMF tw
[TSDPzEr(acac)] -0.61 -1.17 1.28 DMF tw
[TSDPzLu(acac)] -0.62 -1.20 (quasi) 1.26 DMF tw
[Ph8TPyzPzEr(OAc)] -0.74 -0.98 0.86 Py [35]
[tBu4TPyzPzLu(Cl)] -0.96 -1.68 1.02 DCM [32]
[PcEu(acac)] -1.08 -1.44 0.56 DMSO, SCE [46]
[acacDyPc] -1.09 -1.39 0.55 DMSO, SCE [46]
[PcEr(acac)] -1.09 -1.39 0.55 DMSO, SCE [46]
[PcLu(acac)] -1.07 -1.33 0.55 DMSO, SCE [46]
[TPPSm(acac)] -1.51 0.41 DMSO, SCE [47]
[TPPEu(acac)] -1.37 -1.67 0.60 DMSO, SCE [47]
[TPPDy(acac)] -1.33 -1.68 0.63 DMSO, SCE [47]
[TPPEr(acac)] -1.32 -1.69 0.69 DMSO, SCE [47]
[TPPLu(acac)] -1.33 -1.69 0.76 DMSO, SCE [47]
Electrochemical Properties
Cyclic voltammetry measurements were performed for 1 mM solutions ofthe lanthanide complexes [TSDPzLn(acac)] in DMF in the potential range from -1.5 to 1.5 V vs. Ag/AgCl. In the cathodic region for all compounds two reversible reduction processes are observed at Elm= -0.63+0.03 V and E21/2= -1.18+0.03 V, while non-reversible oxidation is observed in the anodic region with Epc = +1.22+0.06 V. The central lanthanide ion has only minor effect on the redox potentials (Figure 4). However, the reduction potentials of the lanthanide complexes are by 0.2-0.3 V more negative than those of the complexes with bivalent p- and d-metals [TSDPzM] (M = Mg11, Zn11, CuII),[51] and by 0.5 V than those of complexes with tervalent p-metals [TSDPzM(Cl)] (M = Alm, GaIII).[10] This can be explained by more ionic character of the coordination bond of the lanthanide ions with TSDPz macrocycle as compared to the M-NPyr bond in the complexes of p- and d-metals. As a result the negative charge of the coordinating dianion TSDPz2 is not effectively compensated by the lanthanide ions and macrocycle in [TSDPzLn(acac)] is more difficultly reduced than in more covalent complexes such as [TSDPzZn] or [TSDPzAl(Cl)] (see Table 3). It is noteworthy, that anionic lithate complex [TSDPzLi]- having full negative charge on the macrocycle is reduced at potential of -0.94 V,[9] i.e. by 0.4 V more difficult than the lanthanide complexes.
At the same time, as can be seen from Table 3, the redox potentials for [TSDPzLn(acac)] are considerably shifted as compared to the corresponding lanthanide complexes of phthalocyanines [PcLn(acac)] (ERed11/2 ~ -1.10 and EOx11/2 ~ +0.56 V vs. SCEt46]) and porphyrins [TPPLn(acac)] (ERed11/2~ -1.35+0.03 and EOx11/2 ~ +0.68+0.08 V vs. SCEt47]). 1,2,5-Thiadiazole fused porphyrazine macrocycle in the lanthanide complexes can be easier reduced as compared with the corresponding phthalocyanine and porphyrin complexes (by 0.5 and 0.6 V, respectively), while its oxidation is by 0.65 and 0.55 V more difficult. It is especially important and indicates that formation of the neutral sandwich-type lanthanide complexes [(TSDPz)2LnIn], in which one macrocyclic unit is oxidized, should be much less favorable than in the case of sandwich bisphthalocyaninates [(Pc)2LnIn] and bisporphyrinates [(TPP)2LnnI]. Indeed, we have not so far observed the formation of homoleptic bisporphyrazinates [(TSDPz)2LnIn] neither as by-products in the complexation reaction between [TSDPzH2] and excess of Ln(acac)3 nor in the reaction of the monoporphy-razinates [TSDPzLn(acac)] with the lithate complex [TSDPzLi]-. Probably heteroleptic sandwich complexss, such as [(TSDPz)Ln(Pc)] or [TSDPz)Ln(TPP)] combining TSDPz2- deck and oxidized Pc-' or TPP-' deck, can be more easily prepared. Available electrochemical data on lanthanide complexes of pyrazine fused porphyrazines [tBu4TPyzPzLu(Cl)][32] and [PhsTPyzPzEr(OAc)][35] show that TPyzPz macrocycle is by 0.15-0.35 more difficult to reduce and by 0.15-0.40 V more easy to oxidize. And, indeed, sandwich complex [(tBu4TPyzPz)2Lu] was reported.[32]
The difference between first reduction and oxidation potential AERed/Ox is 1.88 V is in a good correspondence with the optical HOMO-LUMO gap AEopt = 1.91 eV calculated from the energy of the long wave nn* -transition (Q band 650 nm, 15385 cm-1 for [TSDPzLu(acac)]. These ex-
perimental values are smaller than the theoretical HOMO-LUMO gap 2.248 eV obtained from DFT calculations.[38] This is not surprising since DFT calculations overestimate the value of the HOMO-LUMO gap and failed to predict the correct level of the frontier orbitals. Thus, the LUMO level can be estimated using its correlation with the ERed11/2
potential:[52]
Elumo(CVA) = 1.19 (E./2(Fc/Fc+) - ERed11/2) - 4.78
The ELUMO value obtained from this correlation for [TSDPzLu(acac)] is -3.37. eV, which is much higher than -4.15 eV obtained by DFT modelling of [TSDPzLu(Cl)].[38]
■1500 -1000 -500 0 500 1000 1500 Potential vs Ag/AgCl, mV
Figure 4. Cyclic voltammograms for the lanthanide complexes [TSDPzLn(acac)] (Ln = Smm, Eura, Dyra, Erm, Lum) in DMF with 0.1 M tbaClO4 as supporting electrolyte. Scan rate - 50 mV/s.
Conclusions
Lanthanide complexes of tetra(1,2,5-thiadiazolo)-porphyrazine [TSDPzLn(acac)]-2№O (Ln = SmF, Eu111, Dyiii, Er111) can be prepared by complexation of the metal-
free macrocycle [TSDPZH2] with corresponding acety-lacetonate Ln(acac)3 in the presence of DBU as a base in dichlorobenzene. An intense and narrow Q band at ca. 650 nm in the electronic absorption spectra is due to almost pure Gouterman type 3a2^1e* transition, while two broader bands in the UV-region with maxima at 292-296 (B2) and 364-366 (Bi) nm have a complex configuration interaction structure with predominant contribution of 5ai^1e* and 4ai^1e* transitions. Cyclovoltammertric study in DMF solution reveals two reversible reductions at -0.63+0.03 and -1.18+0.03 V, and non-reversible oxidation at +1.22+0.06 V. Negative shift of the reduction potentials in comparison to complexes with p- and d-metals is indicative about stronger ionic character of the coordination bonds between central metal and pyrrolic nitrogen atoms in the lanthanide complexes.
Acknowledgements. The reported study was funded by RFBR (project number 19-33-60099).
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Received 30.10.2022 Accepted 01.12.2022
