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Статья Paper
Early Lanthanides (Porphyrinato)(Crownphthalocyaninates): Efficient Synthesis and NIR Absorption Characteristics
Kirill P. Birin,@ Yulia G. Gorbunova, and Asian Yu. Tsivadze
A.N. Frumkin Institute of Physical Chemistry and Electrochemistry RAS, 119991 Moscow, Russia N.S. Kurnakov Institute of General and Inorganic Chemistry RAS, 119991 Moscow, Russia @1Corresponding author E-mail: [email protected]
Series of early lanthanides heteroleptic complexes of double- and triple-decker sandwich-type structure with tetra(15-crown-5)phthalocyanine [(15C5) 4PcH2] and tetrakis-meso-(4-methoxyphenyl)porphyrin [An^HJ ([An4P] Ln[(15C5)4Pc] and [Anp]Ln[(15C5)4Pc]Ln[Anp]) are synthesized. It is found that the whole series of La-Eu acetylacetonates can be applied for a one-step formation of heteroleptic sandwich-type complexes. Triple-decker heteroleptic complexes [An4P]Ln[(15C5)4Pc]Ln[An4P] are formed regioselectively as a single isomer with internal position of crownphthalocyanine deck. Heteroleptic double- and triple-decker complexes are found to be the only products of the reaction. Ligand scrambling is not observed and no homoleptic complexes are detected as side products. All the synthesized complexes are characterized with a set of physical-chemical methods. It was shown that the lanthanide-sensitive bands are present in NIR region of the spectra of all synthesized complexes. The position of NIR absorption is found to be linearly dependent on lanthanide ionic radius. Analysis of electronic absorption spectra of all synthesized complexes allowed to determine the influence of ligand environment on the oxidation state of cerium metal center. The comparison of the mentioned linear correlation and spectral data for cerium complexes allowed determination of the oxidation state in each particular case. It is found that cerium atom may utilize both +3 and +4 oxidation states in double-decker complex [Anp]Ce[(15C5) 4Pc]. In contrast, coordination environment of tripledecker complex [An p]Ce[(15C5) pc]Ce[An p] effectively stabilizes Ce111 state and prevents its oxidation to CeIV. The comparison of behaviour of Eu and Ce double- and triple-decker complexes upon chemical oxidation is performed, that allowed to determine a set of redox forms and their stability.
Keywords: Porphyrins, phthalocyanines, crown ether, lanthanide, heteroleptic complexes, near-IR absorption.
Introduction
Non-linear optics, multibit information storage and molecular recognition are foreground trends of current molecular design. These topics of science require highly stable molecules of variable and tunable structure, allowing to manipulate their physico-chemical properties. Metal complexes with porphyrins and phthalocyanines attract interest as promising starting compounds for development of materials possessing unique properties. Depending on the coordination features of metal center, diverse architectures can be synthesized. Coordination features of lanthanide ions allow formation of sandwich-type homo- and heteroleptic complexes with porphyrins and phthalocyanines. Peripheral part of the macrocyclic ligands can be easily modified before or after formation of complex. Variation of ionic radius of metal centers in sandwich-type complexes allows to tune the interligand distance and thus the efficiency of n-n-interaction between macrocycles. In turn, optical properties and redox potentials of the molecules are dependent on n-n-interaction between ligands.
All mentioned features of lanthanide heteroleptic complexes with cyclic tetrapyrrolic ligands resulted in a series of investigations of these types of molecules in recent years.[1-6] Selective receptors for ions and molecules were constructed on the basis of sandwich-type complexes.[4,5,7-10]
In recent work[11] selective binding at sandwich-type receptor site was utilized as driving force for contraction of "molecular spring". Series of papers of Lindsey's group were devoted to selective synthesis of heteroleptic complexes containing porphyrin and phthalocyanine ligands and their application for development of molecular storage devices.[3,12-16]
The typical synthetic route to preparation of lanthanide (porphyrinato)(phthalocyaninates) consists in stepwise formation of heteroleptic complexes with a target structure and is usually called "raise-by-one-storey" technique. This method is applicable mostly for the synthesis of complexes with late lanthanides. The main problem of this approach is preparation of lanthanide monoporphyrinates and monophthalocyaninates as intermediates, which are unstable in the case of early lanthanides.
Several attempts were performed in order to simplify the procedure and to decrease the number of experimental steps for the preparation of heteroleptic lanthanide complexes. A pseudo-one-step procedure was developed for selective synthesis of double-decker europium (porphyrinato)-(phthalocyaninates).[17] This method consisted in generation of europium monoporphyrinate [Por]Eu(acac) semiproduct in refluxed trichlorobenzene, which was evaporated and the residual monoporphyrinate was further treated with phthalonitrile and DBU in amyl alcohol. This procedure leads to heteroleptic double-decker compound with moderate
yields. Interaction of octa-p-ethylporphyrin (OEPH2) and naphthalonitrile with lanthanide acetylacetonate (Ln(acac)3) in high-boiling alcohol in the presence of strong base leads to the formation of double-decker complexes [OEP]Ln[Nc] for the whole La-Lu series.[18-20] It was shown[18] that in the case of Nd and Eu additional treatment of [OEP]Ln[Nc] complex with Ln(acac)3 and OEPH2 allows to synthesize triple-decker heteroleptic complexes of [OEP]Ln[Nc] Ln[OEP] structure. Mixtures of heteroleptic triple-decker lanthanide (porphyrinato)(phthalocyaninates) containing different number of porphyrin decks can be prepared through interaction of lanthanide monoporphyrinates and Li2Pc.[21,22]
A stepwise procedure was also applied by group of J. Jiang for synthesis of heteroleptic triple-decker europium complexes [TPP]Eu[(15C5)4Pc]Eu[TPP] and [TPP]Eu[(15C5)4Pc]Eu[(15C5)4Pc].[23] The first step was the synthesis of homo- and heteroleptic double-decker complexes. Heteroleptic double-decker compound was synthesized via tetramerization of corresponding phthalonitrile at monoporphyrinate as template. The double-deckers were further additionally treated with europium monoporphyrinate that resulted in formation of target molecules. Authors specially mentioned that they performed attempts to synthesize heteroleptic triple-decker complex of symmetrical type [TPP]Eu[(15C5)4Pc]Eu[TPP] in 1-octanol at 200oC through interaction of phthalonitrile and europium monoporphyrinate. This attempt was not successful and resulted in formation of desired triple-decker with low yield and its separation from other sandwich-type side products was complicated.
In[313] authors investigated in details and applied the rational routes for preparation of heteroleptic homo- and heterometallic triple-decker (porphyrinato) (phthalocyaninates) with different position of decks. Europium and cerium monoporphyrinates were used as key intermediates for the synthesis. In the case of cerium application of sterically hindered extra-ligands required for stabilization of monoporphyrinic species. The tripledecker complexes were obtained with moderate yields. Different types of heteroleptic triple-decker complexes were synthesized as a statistical mixture resulting in low yields of each particular compound.[1214,15]
Although one-step and pseudo-one-step routs were found for preparation of heteroleptic double-decker lanthanide (porphyrinato)(phthalocyaninates) the selective procedures for preparation of triple-decker complexes are still limited. The addition of the third deck requires a special synthetic step which is performed at high temperature. In turn it results in formation of complicated mixture of complexes owing to ligand scrambling processes. On the other hand triple-decker complexes of early lanthanides cannot be synthesized in terms of this protocol owing to low stability of corresponding monoporphyrinates. These problems forced us to develop effective one-step procedure for regioselective preparation of triple-decker heteroleptic (porphyrinato)-(phthalocyaninates) of early lanthanides.
Recently we have reported on a procedure allowing to obtain early lanthanides heteroleptic triple-decker complexes of symmetrical [Por]Ln[Pc]Ln[Por] structure in one-step process for La, Ce and Pr.[24] The triple-decker compounds were obtained regioselectively as a single isomer and the
only side products were the corresponding double-decker complexes. Herein we report on applicability and synthetic details of the found synthetic route for regioselective preparation of heteroleptic triple-decker (porphyrinato) (phthalocyaninates) of general structure [An4P]Ln[(15C5)4Pc] Ln[An4P] for the whole early lanthanides series (La-Eu).
As it was shown earlier,[25] the valent state of cerium atom in sandwich-type complexes with tetrapyrrolic ligands is very sensitive to coordination environment. This particular feature allows precise tuning of its oxidation state, resulting in modification of electronic properties, redoxpotentials, stability and absorption spectra of the complexes. A comprehensive research to determine the influence of the structure of tetrapyrrolic ligands on the cerium oxidation state was undertaken by the group of J. Jiang for the series of double-decker homo- and heteroleptic complexes.[25] It was shown that electron-donating properties of the ligands stabilize the CeIn state of the metal center. Thus, in the case of cerium bis-phthalocyaninates the CeIV state was determined. Expansion of n-system of the ligand increases the electron-donating properties of the ligand. As a result, in the case of cerium bis-naphthalocyaninate the intermediate valent state between CeIII and CeIV was determined. Recently we have investigated the homoleptic double-decker cerium bis[tetra(15-crown-5)phthalocyaninate] Ce[(15C5)4Pc]2 in order to determine the valent state of metal center.[26] We have shown that the presence of eight alkoxy-substituents in the phthalocyanine macrocycle results in partial electron back-donation to metal center, resulting in the intermediate cerium valent state.
The determination of oxidation state of metal center was done through analysis of electron absorption spectra of the series of isostructural lanthanide complexes.[25,26] In the case of all types of compounds lanthanide-sensitive bands were found, which gradually shift with variation of metal center. The determination of the mentioned dependencies is important for development of novel materials. Thus, we have found that in the case of cerium double-decker bis[tetra15-crown-5)phthalocyaninate] the electrochemical switching of cerium valent state is possible in Langmuir-Blodgett films of the compound.[27] The alteration of cerium oxidation state results in contraction and expansion of metal center and the physical dimensions of the molecule itself. The found system can be called the "electronic muscle", which may perform mechanical work under influence of electric field.
Herein we report on the synthesis of series of double- and triple-decker heteroleptic (porphyrinato)-(phthalocyaninato) early lanthanides. The investigation of synthesis of the whole La-Eu subgroup complexes allows to prove the possible application of the developed route. The preparation of a series of the related lanthanide complexes allows to analyze the absorption characteristics of the compounds, particularly the absorptions in NIR region. The analysis of spectral characteristics of the series of complexes allowed determining the influence of ligand environment onto oxidation state of cerium atom.
Experimental
1-Octanol (OctOH, Acros Organics, 98%) was used freshly distilled over Na. Chloroform was dried over CaCL and used
freshly distilled over CaH2. MeOH (99%, Merk), CDCl3 (99.8%, Aldrich) and hexane (reagent grade) were used as obtained without further purification. Acetylacetonates of La-Eu (99%, Aldrich), hydrazine monohydrate (100%, Acros Organics) and propionic acid (Riedel de Haen, 99%) were also used as obtained. Pyrrole (99%, Acros Organics), 4-methoxybenzaldehyde (99%, Aldrich) and 1,8-diazabicyclo[5.4.0]undec-7-en (DBU, Merck, >97%) were used freshly distilled over CaH2. Tetrakis-meso-(4-methoxyphenyl)porphyrin (An4PH2)[28] and 4,5-dicyanobenzo-(15-crown-5) (DCB-15C5) were prepared according to described
procedures.[29]
Chromatographic separation and purification of the complexes were performed at cylindrical glass columns filled with neutral alumina (Merck, 0.063-0.2 mm). UV-vis absorption spectra were recorded in 250-900 nm spectral region with Varian Cary-100 spectrophotometer in 1-10 mm rectangular quartz cells. MALDI-TOF mass-spectra were obtained on Bruker Daltonics Ultraflex mass-spectrometer in positive ion mode with nicotinic acid as a matrix. 1H-NMR spectra were recorded at Bruker Avance-II spectrometer with 300.21 MHz frequency. Samples with concentration ca. 10"5-10"6M were prepared in CDCl3. Chemical shifts were measured at T = 298 K relatively to external standard (tetramethylsilane, 8 = 0.00 ppm). Acquired spectra were processed with line broadening factor of 1 Hz.
General procedure for preparation of heteroleptic complexes [An4P]Ln[(15C5)4Pc]Ln[An4P] was equal to one described earlier.[24] The mixture of Ln(acac)3 (0.1 mmol), 5,10,15,20-tetrakis (4-methoxyphenyl)porphyrin (An4PH2) (37 mg, 0.05 mmol), 4,5-dicyanobenzo-15-crown-5 (127 mg, 0.4 mmol) and DBU (50 |l, 0.33 mmol) was refluxed in 1-octanol (4 ml) for 18 hours under slow stream of dry argon. The reaction mixture was cooled to room temperature and added dropwise into 50 ml of hexane. The dark precipitate was filtered and washed with hexane. Chloroform solution of the residue was applied on chromatographic column filled with neutral alumina. The column was eluted with CHCl3-MeOH mixture (0-1% v/v MeOH). Brown fraction of triple-decker complex Ln2[An4P]2[(15C5)4Pc] was collected at 0.5-1% of MeOH in eluent. Green fraction of double-decker complex Ln[An4P] [(15C5)4Pc] was collected at further elution with 4-5% MeOH in CHCl3. 4
La[An f][(15C5) fc] (DDI). Yield 26%. UV-vis (CHCl3) Xmax nm: 292, 368, 422, 486, 600, 756. 1H NMR (CDCl3) 8 ppm (J,Hz): 8.56 (s, 8H, HJ, 8.13 (s, 8H, HPyrr), 8.03 (s, 4H, HJ, 7.36 (s, 4H, HJ, 6.84 (s, 8h, HJ, 4.86 (s^H, a-Cr), 4.63 (s, 8H, a'-Cr), 4.224 (s, 16H, ß-Cr), 4.00 (s, 32H, y+8-Cr), 4.09 (s, 12H, OMe). MALDI-TOF MS (m/z): 2145.3, calcd. for C112H108N12O24La 2145.28.
Ce[Anf][(15C5) fc] (DD2). Yield 20%. UV-vis (CHCl3) X nm: CeIV - 293, 368, 405, 621, 824; CeIn - 291, 368, 421, 624,
max
763. 1H NMR (CDCl3) 8 ppm (J, Hz): 8.62 (s, 8H, HP ), 8.37 (s, 8H, HPyrr), 7.53 (s, 4H, HJ, 7.40 (s, 4H, HJ, 6.75(s, 4Hc HJ, 6.39 (s, 4H, HJ, 4.96 (s, 8H, a-Cr), 4.68(s, 8H, a'-Cr), 4.25 (s, 16H, ß-Cr), 4.00 (s, 32H, y+8-Cr), 4.07 (s, 12H, OMe). MALDI-TOF MS (m/z): 2146.3, calcd. for C112H1n8N12O24Ce 2146.24.
112 108 12 24
Pr(Anf)[(15C5) fc] (DD3). Yield 18%. UV-vis (CHCl3) X nm: 292, 368, 421, 485, 599, 777. MALDI-TOF MS (m/z):
max
2147.9, calcd. for C112H108N12O24Pr 2147.03.
112 108 12 24
Nd(An f)[(15C5) fc] (DD4). Yield 15%. UV-vis (CHCl3) X nm: 291, 370, 420, 487, 601, 783. MALDI-TOF MS (m/z):
max
2150.2, calcd. for C]],H].„N],O,/,Nd 2150.36.
112 108 12 24
Sm(An f)[(15C5) fc] (DD5). Yield 16%. UV-vis (CHCl3) Xmax nm: 289, 368, 417, 485, 600, 795. 1H NMR (CDCl3) 8 ppm (J, Hz): 8.16 (s, 8H, HPc), 7.36 (s, 8H, HPyrT), 6.77 (s, 8H, HJ, 6.63 (s, 4H, HJ, 6.18 (s, 4IH, HJ, 4.60 (s, 8H, a-Cr), 4.43 (s, 8H, a'-Cr), 4.09 (s, 16H, ß-Cr), 3.88 (s, 44H, y+8-Cr + OMe). MALDI-TOF MS (m/z): 2156.4, calcd. for C112H108N12O24Sm 2156.68.
Eu(An f)[(15C5) fc] (DD6). Yield 30%. UV-vis (CHCl3) X nm: 291 , 368, 417, 486, 605, 813; 1H NMR (CDCl3) 8 ppm (J,
Hz): 10.93 (s, 4H, HJ, 8.28 (s, 4H, HJ, 6.75 (s, 4H, HJ, 5.98 (s, 4H, HJ, 9.88 (s, 8H, HPc), 7.16 (s, 8H, HPyrr), 5.64 (s, 8H, a-Cr), 5.02 (s, 8H, a'-Cr), 4.61 (s,c8H, P-Cr), 4.45 (s, 8H, P'-Cr), 4.33 (s, 12H, OMe) 4.16 (s, 32H, y+8-Cr); MALDI-TOF MS (m/z): 2158.1, calcd. for C112H108N12O24Eu 2158.09.
La2[Anf]2[(15C5) fc] (TD1). Yield 32%. UV-vis (CHCl3) Xmax nm: 291, 373, 422, 5 56, 608. 1H NMR (CDCl3) 8 ppm (J, Hz): 9m84 (d, 3J=8.7, 8H, HJ, 8.65 (s, 8H, HJ, 7.85 (dd, 3J=8.4, 4J=2.2, 8H, H ), 7.40 (s, 8H, H ), 6.80 (dd, 3J=8.6, 4J=2.0, 8H, H ), 6.75
' mi7' v ' ' ^yrr7' v ' ' ' ' mo7'
(dd, 3J=8.1,4J=1.9, 8H, Hoo), 4.83 (s, 16H, HaCr), 4.38 (s, 16H, Hp J, 4.14 (s, 24H, H ), 4°.°06 (m, 32H, H "). MALDI-TOF MS-
Cr7' v 5 5 OMe7' v 5 5 y+8-Cr7
(m/z): 3016.8, calcd. for C^H^N, O La, 3016.76.
v 7 7 160 144 16 28 2
Ce[Anf][(15C5)fc] (TD2). Yield 20%. UV-vis (CHCl3) Xmax nm: 289, 372, 420, 5 54, 607. 1H NMR (CDCl3) 8 ppm (J, Hz): 10"08 (s, 8H, H ), 6.67 (s, 8H, H ), 3.15 (s, 16H, HP ), 2.77 (s, 24H, H0Me), 2.75 (s, 8H, Hmi), 2.28 (s, 16H, H^), 2.1^:2 (s, 16H, H ), 1.32 (s, 16H, Hp.Cr), -0.35 (s, 16H, Ha.Cr), -2.53 (s, 8H, HJ, -2.65 (s, 8H, H ). MALDI-TOF MS (m/z): 3019.2, calcd. for
^«Hu^OCC 301918
Pr[An f][(15C5)fc] (TD3). Yield 24%. UV-vis (CHCl3) Xmax nm: 287, 372, 420, 5 55, 607. 1H NMR (CDCl3) 8 ppm (J, Hz): 9j04 (d, 8H, 3J=8.2, H ), 6.74 (dd, 3J=8.4, 4J=2.0, 8H, H ), 5.79
oo mo
(s, 16H, H^), 4.33 (d, 3J=8.1, 8H, Hmi), 3.19 (s, 24H, homi), 2.83 (t, 3J=4.7, 16H, H8Cr), 2.73 (t, 3J=4.7, 16H, H ), 2.27 (s, 16H, Hp_), 1.30 (s, 16H, H J, 1.12 (d, 3J=8.2, 8H,YH ), 0.88 (s, 8H,
P-Cr7' v 5 5 a-Cr7' v 5 55 oi7' v 5 5
HJ. MALDI-TOF MS (m/z): 3021.7, calcd. for C160H144N16028Pr2 3020.76.
Nd[Anf]2[(15C5)fc] (TD4). Yield 20%. UV-vis (CHCl3) Xmax nm: 295, 375, 421, 606. 1H NMR (CDCl3) 8 ppm (J, Hz): 8.16 (cf,a>3J=8.0, 8H, H ), 7.45 (s, 16H, H ), 6.77 (d, 3J=8.2, 8H, H ),
oo Pyrr mo
5.57 (s, 8H, HJ, 4.12 (s, 8H, Hoi), 3.55 (s, 8H, HJ, 3.52 (s, 24H, Hn„ ), 3.25 (m",i 16H, H8_), 3.20oi(m, 16H, H ), 2*.99 (s, 16H, Hp
OMe7' v 5 5 8-Cr7' v 5 5 y-Cr7' v 5 P-
C), 2.57 (s, 16H, H J MALDI-TOF MS (m/z): 3027.6, calcd. for
C160H144N16028Nd2 3027.43.
Sm2[An f]2[(15C5) fc] (TD5). Yield 35%. UV-vis (CHCl3) Xmax nm: 293, 375, 420, 608. 1H NMR (CDCl3) 8 ppm (J, Hz): 8T6 (d, 3J=8.4, 8H, Hoi), 7.32 (s, 8H, HPc), 7.16 (dd, 3J=8.0, 3J=2.3, 8H, H ), 6.94 (dd, 3J=8.2, 3J=1.8,c8H, H ), 6.73 (dd,
' ' oo7' v ' ' ' ' mi7' v '
3J=8.5, 3J=2.5, 8H, H ), 6.60 (s, 8H, H ), 4.22 (s, 16H, H ),
mo Pyrr a-Cr
4.01 (s, 16H, Hp„), 3.94 (s, 24H, H ), 3.85 (s, 32H, H ).
v 5 5 p-Cr7' v 5 5 OMe7' v 5 5 y+8-Cr7
MALDI-TOF MS (m/z): 3039.7, calcd. for C^H^N, A.Sm,
v 7 7 160 144 16 28 2
3039.67.
Eu[Anf]2[(15C5)fc] (TD6). Yield 12%, UV-vis (CHCl3) Xmax nm: 290, 376, 419, 606. 1H NMR (CDCl3) 8 ppm (J, Hz): 131)0 (s, 8H, Hoi), 12.01 (s, 8H, HJ, 9.01 (s, 8H, HJ, 6.57 (d, 3J=7.8, 8H, H ), 6.23 (s, 16H, H „ ),c 5.32 (s, 24H, Hp„"+H ), 4.68
5 5 mo7' v 5 5 a-Cr7' v 5 5 p-Cr oo7'
(s, 16H, hy-ct), 4.61 (s, 16H, H8 J, 4.45 (s, 24H, ho№), 4.30 (s, 16H, HPyrr). MALDI-TOF MS (m/z): 3043.1, calcd. for C^H^N^O^ 3042.87.
It is difficult to obtain satisfactory elemental analysis for the crownphthalocyanine complexes, because the voids of crown ether substituents can contain residual molecules of the solvents,[30] which can distort the results of the analysis.
Results and Discussion
Synthesis
All the peculiarities of the synthesis of heteroleptic (porphyrinato)(phthalocyaninato) lanthanides, mentioned above, forced us to investigate the possible application of the single step regioselective synthetic route. The synthetic procedure applied in current work for preparation of Nd, Sm and Eu complexes was described previously for synthesis of La, Ce and Pr compounds.1241 Variation of lanthanides allows to investigate the possible application of
IXcf3
DCB-15C5
DD1-DD6, Ln=La-Eu 15-30%
An--l>-o
TD1-TD6, Ln=La-Eu 12-35%
Scheme 1. Synthetic pathway for preparation of double- and triple-decker (porphyrinato)(phthalocyaninato) early lanthanides.
the procedure for preparation of double- and triple-decker (porphyrinato)(phthalocyaninato) complexes. Synthetic details and designation of the complexes are summarized at Scheme 1.
We have shown that the resulting reaction mixture contains only two compounds in all cases: the heteroleptic double-decker Ln[An4P][(15C5)4Pc] and triple-decker Ln2[An4P]2[(15C5)4Pc] complexes. The yields of double-and triple-decker complexes are similar and overall conversion may exceed 50%. Only traces of porphyrin were recovered from reaction mixtures, that can be explained by thermal decomposition of the compounds during reaction process. The yields and ratio of double- and triple-decker compounds are highly dependent on maintenance of inert atmosphere during the reaction. In our research freshly distilled from sodium 1-octanol was used. The presence of trace amounts of water in the solvent significantly suppresses the formation of triple-decker complex. Application of wet solvent may also result in decrease of yield of double-decker complex. Since the interaction occurs during 18 hours the presence of trace amounts of oxygen leads to decomposition of complexes.
The process is presumed to be stepwise. The first stage is in situ generation of monoporphyrinic species Ln[An4P] (acac), which further acts as template for crown-phthalonitrile tetramerization. This stage gives rise to heteroleptic double-decker compounds [An4P]Ln[(15C5)4Pc]. The formed
double-decker complex may again interact with lanthanide monoporphyrinate which is present in the reaction mixture. This interaction occurs regiospecifically leading to formation of triple-decker compound as a single isomer with internal position of phthalocyanine deck.
The observed selectivity of the process was discussed in details in [24]. The calculations of localization of HOMO in double-decker complex showed dissymmetry of the orbital depending on the nature of substituents in phthalocyanine ligand. In the presence of electron-donating alkoxy-groups the coefficients of HOMO achieve ca. 40% and 60% at porphyrin and phthalocyanine ligands, respectively. Polarization of the molecule results in selective interaction of monoporphyrinate at phthalocyanine site of the double-decker complex.
The products in the reaction mixture are easily separated by column chromatography owing to difference in chromatographic affinities of double- and triple-decker compounds. Neutral alumina was used for purification of the complexes with CHCl3-MeOH eluent. Electroneutral symmetrical triple-decker complex is eluted first with 0.5-1% of MeOH in eluent. Double-decker complex is eluted as anionic form [An4P2-]Ln[(15C5)4Pc2-] at higher concentrations of methanol as a result of polarisation of the unsymmetrical molecule. The yields of the obtained complexes and their MALDI-TOF MS data are summarized in Table 1.
Table 1. Yields and MALDI TOF MS data of synthesized double- and triple-decker complexes.
Yield
MALDI TOF MS
Ln[An4P][(15C5)4Pc]
LnJAn4Pl[(15C5)4Pc]
Ln[An4P][(15C5)4Pc] Ln2[An4Py(15C5)4Pc] Calculated for C 112H108N12O24Ln Found Calculated for C160H144N16O28Ln2 Found
La DDI, 26% TD1, 32% 2145.28 2145.3 3016.76 3016.8
Ce DD2, 20% TD2, 20% 2146.24 2146.3 3019.18 3019.2
Pr DD3, 18% TD3, 24% 2147.03 2147.9 3020.76 3021.7
Nd DD4, 15% TD4, 20% 2150.36 2150.2 3027.43 3027.6
Sm DD5, 16% TD5, 35% 2156.68 2156.4 3039.67 3039.7
Eu DD6, 30% TD6, 12% 2158.09 2158.1 3042.87 3043.1
Spectral Investigation of the Complexes
All the synthesized compounds were characterized by MALDI-TOF MS and UV-vis spectroscopy. Triple-decker complexes were also investigated by NMR spectroscopy. Application of various correlation techniques allowed the unambiguous assignment of 'H-NMR spectra and determination of lanthanide-induced paramagnetic shifts for each type of protons in the molecule. Precise analysis of lanthanide-induced shifts allowed us to develop a general approach for structural characterization of lanthanide tripledecker heteroleptic complexes.131,321
The polarization of the molecule of double-decker complex results in highly effective concentration-dependent aggregation of the compound. Dilution of double-decker complex solution up to 10-6 M level does not lead to complete suppression of aggregation, that is determined by UV-vis spectroscopy. NMR spectra of compounds at this concentration are significantly broadened that testifies the formation of different types of unordered aggregates. We succeeded to register 1H NMR spectra at low concentrations for all synthesized double-decker complexes except Nd and Pr. These two lanthanides demonstrate large upfield shift of signals and effective relaxation enhancement. In the case of partial aggregation of compounds and fast exchange within aggregates the signals of the compounds are extremely broadened.
Electronic absorption spectra are quite informative for macrocyclic tetrapyrrolic compounds and their metal complexes. The origin of electronic transitions for double-and triple-decker (porphyrinato)(phthalocyaninates) are discussed in details in [21>33>34]. UV-vis spectra of the synthesized lanthanum double- and triple-decker complexes are shown at Figure 1 as examples. The positions and relative intensities of absorption bands in the spectra allow to distinguish double- and triple-decker complexes. Despite the fact that the most absorption bands are not lanthanide-sensitive some of them in spectrum may be express and reliable marker of composition of complexes. The sets of bands in the spectrum are generally similar and are determined by electronic transitions of each particular ligand. In contrast, relative intensities of bands are determined by ratio of ligands of different nature. As a result, the comparison of intensities of bands, originating from porphyrin and phthalocyanine electron transitions shows that phthalocyanine absorptions
are diminished in the case of triple-deckers. Similarly, in [34] the spectra of heteroleptic triple-decker (porphyrinato)-(phthalocyaninates) with different number of porphyrin and phthalocyanine ligands were considered as superposition of spectra of the corresponding double-decker complexes.
In the spectra of the both types of complexes lanthanide-sensitive bands are present in the NIR region. In the case of the synthesized double-deckers lanthanide-sensitive band occupies position in 750-850 nm range. In the spectra of triple-deckers this band is red-shifted up to 860-950 nm region. We have found that the positions of lanthanide-sensitive bands in NIR region are linearly dependent on lanthanide ionic radius and thus on interligand distance in the complex (Figures 2, 3).
Figure 2. Plot of X Q absorption maxima positions vs lanthanide
° max r F
ionic radii in heteroleptic double-decker complexes Ln[An4P][(15C5)4Pc]- (DD1-DD6).
Analysis of the spectra of the synthesized herein double-and triple-decker complexes allowed us to determine the oxidation state of cerium in given coordination environment. Figure 3 shows the linear correlation between ionic radii of metal centers of the double-decker complexes and position of NIR absorption band in the UV-vis spectra. Data for ionic radii of octacoordinated lanthanide ions are taken from the literature.[35] The discussed absorption is mentioned as X Q
A max
since it originates from the similar transition as phthalo-
X, nm X, nm
Figure 1. UV-vis spectra of lanthanum double- and triple-decker (porphyrinato)(phthalocyaninates).
Figure 3. Plot of X MR absorption maxima positions vs lanthanide ionic radii in heteroleptic triple-decker complexes Ln2[An4P]2[(15C5)4Pc] (TD1-TD6). The spectra are normalized at 606 nm absorption.
cyanine Q band. The absorption maxima are gradually shifted to the longer wavelength region in La^Eu series. The behavior of cerium complex differs from the other synthesized complexes of trivalent lanthanides. The position of the discussed band in the spectrum of chromatographically pure complex is 824 nm, while for Ce111 complex this band is expected in 756-774 nm range, between positions of absorptions of lanthanum and praseodymium complexes, respectively. Taking into account the general tendency of bathochromic shift of band with decrease of lanthanide ionic radius this position of absorption can be attributed to CeIV valent state. This data point complies with general linear correlation in the case of CeIV ionic radius.
An assumption of the tetravalent state of cerium in heteroleptic double-decker complex is also proved by behavior of NIR absorption band upon treatment of the complex with reducing agents. Titration of cerium complex solution with N2H4 in the presence of DBU results in gradual hypsochromic shift of band up to 763 nm. According to the found correlation this process can be attributed to the reduction of cerium center CeIV^-CeIn.
In the case of triple-decker heteroleptic complexes similar correlation was determined. The characteristic lanthanide-sensitive absorption band (XmaxNIR) is also gradually shifted with decrease of lanthanide(III) ionic radius. Figure 3 shows a series of spectra of the synthesized triple-deckers in NIR region and the obtained linear correlation.
NIR absorption maximum of cerium triple-decker complex satisfy the trend formed by other lanthanides with CeIII ionic radius. The shape of the band is also gradually changed in La^Eu series. In the case of lanthanum complex a slight splitting of this band is observed. With decrease of the lanthanide ion size the band becomes broader and the splitting disappears.
Since the observed range of absorption maxima of NIR bands is ca. 100 nm both in the case of double- and tripledecker complexes this band can be considered as a sensitive marker of metal center in the complexes of discussed types. Synthesized series of complexes allow to cover almost 200 nm range of wavelengths in NIR region that is promising
for development of non-linear optics devices, similarly to [36]. The possibility of cerium to facilitate both +3 and +4 oxidation states in double-decker heteroleptic complex allowed to develop molecular switching devices with optical response on the basis of cerium bis[tetra(15-crown-5) phthalocyaninate].[27]
The behavior of the complexes upon chemical oxidation differs depending on the metal center and structure of the complex. We have compared the oxidation processes for cerium and europium double- and triple-decker complexes as representatives of the series. Chemical oxidation was performed in CHCl3 with N-bromosuccinimide (NBS), containing trace amounts of bromine. First, we have compared the redox behavior of cerium and europium heteroleptic double-decker complexes Ln[An4P][(15C5)4Pc]. Europium double-decker complex is able to form two stable redox forms. The anionic form can be oxidized with loss of one electron to free radical electroneutral form. Upon oxidation with traces of bromine the absorption band at 807 nm disappears and two bands in NIR region appear at 996 and 1336 nm (Figure 4). These bands correspond to the
417 -Eu[An4P][(15C5)4Pcr
j410 ----{Eu[An4P][(15C5)4Pc]}'
. 1336
Í 996
800 900 1000 1100 1200 1300 1400 1500 1600
996 lnm_1336 •"' --- ■<•----
400 600 800 1000 1200 1400 1600 X, nm
Figure 4. Titration of europium double-decker complex Eu3+[An4P2"][(15C5)4Pc2"] with NBS solution in CHCl3.
transitions of one-electron oxidized phthalocyanine ligands. [37] The observation of porphyrin ligand absorption bands without major changes confirms the preferential localization of unpaired electron at phthalocyanine ligand.
Cerium double-decker complex was reduced with N2H4/DBU before investigation. In this case the starting redox form of the compound was the anionic one with metal center in +3 oxidation state. Oxidation processes in the case of cerium double-decker complex differ from ones of europium analogue. Two steps of oxidation were detected giving 2 stable oxidized forms. The first oxidation occurs at metal center and results in formation of electroneutral form of CeIV complex. This process is detected by bathochromic shift of NIR absorption band 763^824 nm, corresponding to contraction of lanthanide atom. The next oxidation step affects the n-system of the ligand. The changes in absorption spectrum are similar to ones in the case of oxidized europium complex. The band at 824 nm is shifted to 978 nm and new band at 1165 nm appears, originating from intermolecular electron transition between ligands (Figure 5). Soret bands of porphyrin ligands in the oxidized complexes are hypsochromically shifted at each oxidation step. This can be explained by increasing n-n-interaction between ligands in oxidized forms of the compounds. The interaction is
Figure 5. Titration of cerium double-decker complex Ce[An4P] [(15C5)4Pc] with NBS solution in CHCl3.
additionally enhanced by contraction of cerium metal center upon oxidation. Since the absorption spectrum is determined by electronic states of ligands and their electron transitions the general charge of the molecule doesn't influence the number and origin of the absorption bands. Nevertheless the size of metal center regulates the interligand distance in the molecule and thus efficiency of n-n-interaction and position of bands arising in NIR region.
Surprisingly, the redox behavior of triple-decker cerium and europium complexes were found to be equal. Only one chemical oxidation process is determined for these compounds affecting n-system of ligands in the investigated conditions. This process results in the formation of one electron oxidized species both in the case of europium and cerium compounds. The bands at 891 nm (Ce complex) and 945 nm (Eu complex) disappear upon oxidation (Figure 6). The further treatment with NBS does not reveal any more processes affecting CeIn metal center.
Comparison of oxidation processes of double- and triple-decker heteroleptic complexes allows to determine the peculiarities of ligand environment and their influence on the oxidation state of cerium atom. Oxidation processes of double-decker complex may involve both metal center (in the case of cerium) and n-system of ligands. Oxidation of triple-decker compound may occur only at ligand system of the molecule. This behavior testifies that coordination environment of triple-decker compound effectively stabilizes trivalent state of cerium atom. Oxidation of metal center to CeIV state in the case of triple-decker compound should lead to decomposition of molecule giving electroneutral double-decker complex. Formation of double-decker CeIV complex could be easily detected by electronic absorption spectrum. Nevertheless the decomposition of compound upon oxidation was not observed. The observed set of redox transformations of the complexes and stability of the compounds upon oxidation prove their possible application as starting materials for development of multibit information storage devices, sensing and electrochromic systems.
Conclusions
Summarizing, we have developed a regioselective synthetic approach allowing to prepare triple-decker
Figure 6. Titration of cerium and europium triple-decker complexes Ln2[An4P]2[(15C5)4Pc] with NBS solution in CHCl3,
(porphyrinato)(phthalocyaninates) of early lanthanides in one-step procedure. The triple-decker complexes are formed as a single isomer of symmetrical structure [An4P] Ln[(15C5)4Pc]Ln[An4P]. The double-decker co-product can be easily separated that also simplifies the synthetic procedure.
The NIR absorption of the discussed types of complexes can be tuned finely through variation of metal center. The variation of metal center in the synthesized series of complexes within La-Eu subgroup allows to cover ca. 200 nm range of NIR absorption. Presence of NIR-absorption in the spectra of compounds makes them promising starting materials for nonlinear optics.
A set of macroaromatic ligands in the complexes determines the variety of redox transitions of the compounds. Chemical oxidation of both double- and tripledecker complexes is reversible and oxidized compounds do not show any decomposition. The observed set of redox transformations and stability of compounds upon oxidation demonstrate their possible application for development of multibit information storage devices, sensing and electrochromic systems.
Acknowledgements. We thank Russian Academy of Sciences, Russian Foundation for Basic research (grant .№0803-00835) and foundation of Russian President for support of young scientists (grant MK-212.2010.3) for financial support.
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Received 17.11.2010 Accepted 09.12.2010
