Acknowledgments
N- Based Rare Earth Complexes
4.3 Rare Earth Complexes with N-Heterocyclic Type Ligands
4.3.3 Rare Earth Complexes with Porphyrin Type Ligands
N
NH N
HN
2 8
10
12 13 15 17 18 20
Figure 4.22 The structure of porphyrin.
N
NH N
HN
X X
X X
R
R
R R
N NH
N HN
20 X = C, R = H, H2TPP 21 X = C, R = CH3, H2TTP 22 X = N, TPyP
23 octaethylporphyrin, OEP
Figure 4.23 Four representative porphyrin type ligands.
Ce(acac)3 + 2 H2TTP Ce(TTP)2 + 1_H2O 2 Pr(acac)3 + 2 H2TTP PrH(TTP)2 + 3 Hacac
Figure 4.24 Synthesis of homoleptic sandwich-type double-decker porphyrinato rare earth com- plexes Ce(TTP)2and PrH(TTP)2.
Ln(acac)3
Ln2(OEP)3
Ln(OEP)2 LnH(OEP)2
Ln(OEP)acac H2OEP, 220°°C, 1 h
20 h –Hacac H2OEP 220°°C
LnH(OEP)2 –H+, – e–
oxidant
Figure 4.25 Schematic representation of the formation of double-decker and triple-decker porphyrinato rare earth complexes from monoporphyrins.
of mankind and animals, directly participates in life-maintaining processes by transporting O2
[39]. Nowadays, as a result of the discovery of the large number of applications of the rare earth metals and the conjugated structure of the porphyrin ring, novel porphyrin rare earth complexes display characteristic features that cannot be found in their non-coordinated coun- terparts, enabling them to be used in different areas, for example, as artificial receptors for molecular recognition, as fluorescent probes for the exploration mechanisms of biologically important reactions, as shift reagents in the research of nuclear magnetic resonance, and as catalysts in organic chemistry. As the rare earth metals tend to coordinate to porphyrin with higher coordinate numbers, common sandwich type double- or triple-decker porphyrinato have been synthesized.
Owing to the lack of an effective preparation method, the development of porphyrinato rare earth complexes were relatively restrained in comparison with their phthalocyaninato rare earth analogs. The first sandwich type bis(porphyrinato) rare earth complex was reported in 1983 [40]. By prolonging the reaction time and replacing20, H2TPP with21, H2TTP, Buchleret al.
accidentally obtained double-decker Ce(TTP)2 and HPr(TTP)2when they repeated Wong’s procedure for preparing monomeric porphyrinato rare earth compounds, Figure 4.24 [41]. By adopting the same procedure with 1,2,4-trichlorobenzene (TCB) as the solvent, a series of porphyrinato rare earth complexes were also prepared by the reaction between H2TPP and RE(acac)3·nH2O (acac=acetylacetone in a ratio of 1 : 3 for RE=La–Gd, except for Pm.
The reaction procedure was also suitable for other porphyrin species such as 23, H2OEP.
In 1986, Buchler obtained a substantial amount of the triple-decker RE2(OEP)3in addition to double-decker RE(OEP)2using the same reaction procedure [42]. One step-by-step mechanism has been proposed for the formation of triple-decker RE2(OEP)3, Figure 4.25. Accordingly, the bis(porphyrinato) rare earth complexes were first produced by reaction of H2OEP and
–Liacac Li2Por 220°°C
Ce(Por)2 LiCe(Por)2
–LiOH, – e O2, H2O Por = TPyP, TMAP, TMeCPP
Figure 4.26 Schematic representation of the synthesis of the double-decker bis(porphyrinato) cerium complexes from Li2Por and Ce(acac)3.
RE(acac)3·nH2O. Along with prolonging the reaction time, metal free H2OEP will further react with the double-deckers RE(OEP)2to form the triple-decker RE2(OEP)3. However, it must be pointed out that attempts to synthesize double-decker or triple-decker porphyrinato rare earth complexes with smaller ionic radius for RE=Dy–Lu by the same procedure failed. Systematic investigation over a series of sandwich rare earth complexes with porphyrins H2OEP and H2TPP revealed that the rare earth ionic size is a critical factor in determining the ease, species, and stability of sandwich type complexes, double-decker or triple-decker. With the decrease in the rare earth ion radius, the repulsion between the two facing porphyrin rings becomes more enhanced, resulting in an increasing difficulty in inserting the rare earth ion into the center of bis(porphyrinato) compounds.
Encouraged by the synthesis of bis(phthalocyaninato) rare earth double- or triple-decker compounds, a more active porphyrin intermediate, Li2(TPP), generated from metal free H2TPP and butyllithium in TCB under an inert gas such as Ar or N2, was used to prepare sand- wich porphyrinato rare earth complexes [43]. The yield was found to decrease along with the increase in the rare earth ionic radius, from 76% for Eu to only 4% for Lu. After the syn- thesis of bis(porphyrinato) complexes with rare earth metals other than cerium, some neutral and water-soluble bis(porphyrinato) cerium complexes, namely Ce(IV)(Por)2 (Por=TpyP, TMAP, TMeCPP), [Ce(III)(Por)2]7+ (Por=TM4PyP, TE4PyP), and [Ce(IV)(TTM4AP)2]8+ were also synthesized by Jiang and coworkers [44] and Buchler and coworkers [45] by employ- ing the reaction between Li2(Por) (Por=TpyP, TMAP, TMeCPP) and cerium acetylacetonate Ce(acac)3·nH2O in refluxing TCB for a prolonged reaction time, Figure 4.26. Recently, Aida reported the synthesis and optical resolution of someD2-chiral bis(porphyrinato) complexes of cerium and zirconium [46]. These optically active bis(porphyrinato) metal complexes can be used as probes to investigate the rotation dynamics of the porphyrin ligands around the metal center as the ligand rotation corresponds to the racemization. Investigation revealed that rotation of porphyrin ligands in the cerium double-decker complex is relatively simple, result- ing in the easy racemization of the corresponding optically active bis(porphyrinato) cerium complexes. In contrast, the thermally-induced porphyrin ligand rotation hardly occurs in the zirconium analog.
(a) N1
N41 N51 N57 N63 Ce N11 N23
N17
(b)
Figure 4.27 (a) Ortep plot of one molecule and (b) stick bond model projection of Ce(OEP)2[42].
(Reprinted with permission from J.W. Buchler,et al., “Metal complexes with tetrapyrrole ligands. 40.
Cerium(IV) bis(octaethylporphyrinate) and dicerium(III) tris(octaethylporphyrinate): parents of a new family of lanthanoid double-decker and triple-decker molecules,’’Journal of the American Chemical Society,108, no. 13, 3652–3659, 1986. © 1986 American Chemical Society.)
4.3.3.1 Ce(OEP)2
Ce(OEP)2 was the first homoleptic porphyrinato metal double-decker complex to be struc- turally characterized by X-ray crystallography [42]. In the crystalline state, the central cerium ion is eight-coordinate with eight nitrogen atoms from two porphyrin rings. The coordination geometry can be described as a square antiprism. The eight Ce–N bond lengths do not signifi- cantly differ from each other, with an average of about 0.2475(1) nm. The skew angle between two porphyrin ring is 41.8◦, very similar to that for its bis(phthalocyaninato) analog. The four pyrrole nitrogen atoms of each macrocycle are coplanar and the mean separation of these two parallel planes formed by N1–N11–N17–N23 and N41–N51–N57–N63 is 0.2752 nm. The π–πinteraction distance between the two average planes of the 24 atom (C20N4) framework of the OEP rings amounts to 0.3464 nm. As shown in Figure 4.27a, both the macrocycles are severely distorted from planarity with a mean dihedral angleδof 15.5◦.
4.3.3.2 Ce2(OEP)3
Ce2(OEP)3is the only homoleptic porphyrinato triple-decker whose molecular structure has been determined by the X-ray crystallographic method [42]. The crystal structure of Ce2(OEP)3
and comparison of the crystallographic data between Ce(OEP)2and Ce2(OEP)3are shown in Figure 4.28. As can be seen, for each Ce2(OEP)3 molecule, there are three octaethylpor- phyrinate (OEP) dianions, which are surrounded by two Ce(III) ions, resulting in a neutral
(a) N4′
N5′ N6′ N5
N3 N2 N1 N4
N3
N1′
Ce′
Ce
(b)
Figure 4.28 (a) Ortep plot of one molecule and (b) stick bond model projection of Ce2(OEP)3[42].
(Reprinted with permission from J.W. Buchler,et al., “Metal complexes with tetrapyrrole ligands. 40.
Cerium(IV) bis(octaethylporphyrinate) and dicerium(III) tris(octaethylporphyrinate): parents of a new family of lanthanoid double-decker and triple-decker molecules,’’Journal of the American Chemical Society,108, no. 13, 3652–3659, 1986. © 1986 American Chemical Society.)
triple-decker molecule, Figure 4.28. Compared with the double-decker counterpart Ce(OEP)2, the triple-decker Ce2(OEP)3is more distorted from the ideal square antiprism. The two exter- nal OEP rings have the same orientation with respect to the planar internal macrocycle. The distance between the central cerium and the mean 4Np plane of the external and internal OEP ring is 0.1394 and 0.1876 nm, respectively. Clearly, the Ce(III) ions are not equidistant from their neighboring macrocycle rings as the internal OEP ring is shared by two metal ions, which cannot coordinate with the metal ion as effectively as the external OEP. Similarly, the Ce–N bond lengths are divided into two classes. The average Ce–N distance, connected to the pyr- role nitrogen atoms of the internal ring, is 0.2758 nm, slightly longer in comparison with those associated with the pyrrole atoms of the external rings, 0.2501 nm. The mean Ce–N bond length is 0.263 nm, slightly longer than that found in double-decker Ce(OEP)2because of the stronger coordination bonding interaction of pyrrole nitrogen with Ce(IV) than with Ce(III).
The mean planes of the 4Np atoms and the independent 12 core atoms of the internal porphyrin are quasicoincident coplanar. The separation between the mean planes of the 24 core atoms of the external ring and the 12 core atoms of the internal ring is approximately 0.354 nm.
Compared with that in the double-decker analog Ce(OEP)2, the distance indicates a weaker π–πinteraction between neighboring OEP rings in the triple-decker compound.
1.2 E
0.8
0.4
0.0
300 400
387
467
530 573
639 571
= Ce2(OEP)3
= Ce(OEP)2 378
500 600 nm
Figure 4.29 Electronic absorption spectra of Ce(OEP)2(solid line) and Ce2(OEP)3(dashed line) [42].
(Reprinted with permission from J.W. Buchler,et al., “Metal complexes with tetrapyrrole ligands. 40.
Cerium(IV) bis(octaethylporphyrinate) and dicerium(III) tris(octaethylporphyrinate): parents of a new family of lanthanoid double-decker and triple-decker molecules,’’Journal of the American Chemical Society,108, no. 13, 3652–3659, 1986. © 1986 American Chemical Society.)
4.3.3.3 Electronic Absorption Spectra
The electronic absorption spectrum of Ce(IV)(OEP)2exhibits similar UV–vis (ultraviolet–
visible) features to that of monomeric metalloporphyrins except for some new optical bands resulting from the strongπ–πinteraction between the por macrocycles, Figure 4.29. There is a strong porphyrin Soret band at 378 nm, blue-shifted as compared with the monoporphyrinato rare earth complexes. Meanwhile, there are two weak porphyrin Q-bands with maximums at 530 and 573 nm, respectively. In comparison with the monoporphyrinato rare earth com- plexes, a new absorption band appears at 467 nm in the electronic absorption spectrum of Ce(IV)(OEP)2, which is attributed to aπ–π transition arising from the molecular orbitals delocalized over both OEP macrocycles. A similar phenomenon was also observed for the triple-deckers RE2(OEP)3(RE=La–Gd). However, the porphyrin Soret band is located at 387 nm, less blue-shifted in comparison with that for the double-decker analog Ce(IV)(OEP)2. Additionally, the band centered at 467 nm in the double-decker disappears in the triple-decker, indicating the relatively weakenedπ–πinteraction in the triple-decker because of the larger OEP–OEP distance. This was also revealed by the X-ray molecular structural analysis result of the double- and triple-deckers as detailed above.
4.3.3.4 Near-IR Spectra
Oxidation of the neutral tetravalent cerium OEP double-decker into its mono-oxidized form results in the appearance of a new absorption peak at 1240 nm in the near-IR
Figure 4.30 Schematic representation of three reversible one-electron processes.
(near-infrared) region. In fact, all the neutral porphyrinato rare earth(III) double-decker com- plexes RE(III)(Por)2exhibit a characteristic IR band at 1 200–1 500 nm. However, this band disappears in the porphyrinato double-decker complexes in which a monoanion radical Por·−
does not exist, such as Ce(IV)(Por)2and LnH(TTP)2. This absorption band can be ascribed to the intramolecular charge transfer between the (OEP)2−donor and the (OEP)−acceptor.
Systematic study over the series of RE(OEP)2(RE=La, Pr–Lu) complexes indicates that a good linear correlation exists between the energy of the near-IR absorption band and the radius of the central trivalent rare earth metal. With the decreasing distance between the porphyrin rings due to the dwindling of rare earth ionic radius, the intramolecular charge transfer energy increases in the same order.
4.3.3.5 Electrochemical Properties
Buchler and coworkers studied the electrochemistry of OEP double-deckers for the series of trivalent rare earth elements (RE=La–Lu) and observed three reversible monoelectron processes, namely one monoelectron reversible reduction and two monoelectron oxidations, Figure 4.30 [43c]. Figure 4.31 displays the change in the oxidation and reduction potentials (E1, E2, and E3) together with the wavenumber of the near-IR absorption maxima of the double-deckers RE(OEP)2as a function of the rare earth ionic radius. As can be seen, along with the increase in the ionic radius, both of the oxidation potentials show a linear increase, but the reduction potential takes a relatively slight change. Comparison reveals that the change in the energy of near-IR absorption as a function of the rare earth ionic size obeys the similar trend as shown by the first oxidation potential, reflecting the correlation between the electrochemical and spectroscopic properties. The linear relationship revealed also indicates the porphyrin ring- centered nature of the oxidation and reduction processes for RE(OEP)2. Alternatively, if these oxidation and reduction processes are metal ion-centered, Lu(OEP)2should display the highest oxidation potential because of the highest electron negativity of the lutetium cation among the whole series of trivalent rare earth ions. As shown in Figure 4.31, the lowest oxidation potential was observed for Lu(OEP)2. It is worth pointing out that the first electrochemical reduction of Ce(IV)(OEP)2 is metal ion-centered rather than porphyirn ring-centered because of the presence of tetravalent cerium in this complex.
4.3.3.6 Heteroleptic Bis(porphyrinato) Rare Earth Double-Decker Complexes
Except for the homoleptic bi(porphyrinato) and tri(porphyrinato) rare earth complexes, com- plexes with different porphyrin macrocycles were also investigated. To date, some sandwich type heteroleptic bis(porphyrinato) rare earth double-deckers have been synthesized. How- ever, few have been structurally characterized via single crystal X-ray diffraction analysis [47]. Among these, Ce(IV)(OEP)(TPP) is the only structurally characterized neutral heterolep- tic bis(porphyrinato) rare earth double-decker complex [48]. Using an improved short route, Coutsolelos and coworkers prepared and isolated a series of heteroleptic bis(porphyrinato)
E/V
v–/cm–1
–1.5
–1.0
–0.5
0
+0.5
100 Yb
Lu Tm Y Dy Gd Sm Pr
r1/pm
Er Ho Tb Eu Nd La
105 110 115
10 000
| M(OEP)2 |– | M(OEP)2 |2– (3c)
Wave number of NIR Band e–
| M(OEP)2 |+ e– M(OEP)2 (3a) M(OEP)2 e– | M(OEP)2 |– (3b)
5 000
0
Figure 4.31 Redox potentialsEand wavenumberiof the near-IR absorption maxima of the sandwich complexes M(OEP)2as functions of the ionic radiir1of the trivalent central metal M [43c]. (Reprinted with permission from J.W. Buchler, and B. Scharbert, “Metal complexes with tetrapyrrole ligands. 50.
Redox potentials of sandwichlike metal bis(octaethylporphyrinates) and their correlation with ring-ring distances,’’Journal of the American Chemical Society,110, no. 13, 4272–4276, 1988. © 1988 American Chemical Society.)
rare earth double-decker complexes in the reduced form RE(III)H(OEP)(TPP) (RE=Nd–
Lu). In this series, the two compounds for RE=Sm and Gd were structurally characterized.
Figure 4.32 shows the crystal structure of HSm(III)(OEP)(TPP). As can be seen, the coordi- nation polyhedron of the Sm(III) is a square antiprism containing one+3 charged samarium ion and two different porphyrin rings. In comparison with the TPP ligand, the OEP ligand is more deformed. The skew angle between two porphyrin rings is 45.016◦. The central samar- ium ion is eight-coordinate with four nitrogen atoms from TPP pyrroles and four nitrogen atoms from OEP pyrroles. Contrary to the homoleptic analog Ce(OEP)2, the Sm(III) ion in HSm(III)(OEP)(TPP) is not equidistant from its neighboring porphyrin rings. The mean Sm–N(TPP) bond length is 0.2538(4) nm, slightly shorter than that for Sm–N(OEP), 0.2563 nm.
C24 C25
C26
C22 C21
C70 C69 C51
C71
C3 C2
C55 C56
C73
C44 C41
C40 C20
C39
C57 C75 C76 C58
C18
C17 C60
C61 C78
C77 C16
C59 C19 C52 C53
C54
C50 C6 C7
C8
C47
C10 C46 C27 C28 C32
C31
C30 C29
C66 C65
C12 C13 C11
C64 C45
N3 C14
C63 C62
C34 C80 C79
C35
C37 C36 C38 C33 C15 N8
N4 N6
N1 N7 C1 Sm
C48 N2
N5 C68
C67
C49
C9
C5 C4
Figure 4.32 The structure of complex Sm(III)H(OEP)(TPP) [48]. (Reprinted with permission from G.A.
Spyroulias,et al., “Synthesis, characterization, and X-ray study of a heteroleptic samarium(III) porphyrin double decker complex,’’Inorganic Chemistry,34, no. 9, 2476–2479, 1995. © 1995 American Chemical Society.)
This is also true for HGd(OEP)(TPP). However, these results are in good contrast to those found for Ce(IV)(OEP)(TPP). In the neutral heteroleptic bis(porphyrinato) cerium double- decker, the mean Ce–N(TPP) bond length, 0.2480(1) nm, is slightly longer than Ce–N(OEP), 0.2471(1) nm, suggesting that the proton may locate on the OEP ring and the complex may be denoted as Sm(HOEP)(TPP).
4.3.3.7 Monomeric Porphyrinato Rare Earth Complexes
The first monomeric porphyrinato rare earth complex was reported in 1974 [49]. However, only a few reports on these systems have appeared in the literature since then. In 1991, Schaverien and Orpent reported the synthesis of the monomeric porphyrinato lutetium com- plex Lu(OEP)[CH(Si(CH3)2] from the reaction between Lu{CH[Si(CH3)3]2}3 and H2OEP in toluene [50]. Figure 4.33 displays its molecular structure. The complex belongs to the monoclinic system and crystallizes in a space group P21/c with a=1.4879(6) nm, b=2.0644(10) nm, c=1.4161(5) nm, β= 96.38(3)◦, V=4.323(3) nm3, and Z=4. The coordination geometry can be described as square-pyramidal. In the crystal, the porphyrin
Si(2) Si(1)
C(1) H(1)
N(1)
N(4) N(3) Lu N(2)
Figure 4.33 Structure of monomeric complex Lu(OEP)[CH(Si(CH3)2] [50]. (Reprinted with per- mission from C.J. Schaverien, and A.G. Orpen, “Chemistry of (octaethylporphyrinato)lutetium and -yttrium complexes: synthesis and reactivity of (OEP)MX derivatives and the selective activation of O2by (OEP)Y(µ-Me)2AlMe2,’’Inorganic Chemistry,30, no. 26, 4968–4978, 1991. © 1991 American Chemical Society.)
skeleton is highly distorted. The central ion was five-coordinate, surrounded by four nitrogen atoms from porphyrin pyrroles and one carbon atom from the group of [CH(Si(CH3)2]. The Lu–N(OEP) bond length is 0.2236(7), 0.2253(6), 0.2296(7), and 0.2256(6) nm, respectively, with an average of 0.226 nm. The distance between the lutetium atom and the mean N4 plane of porphyrin ligand is 0.0918 nm.
The first cationic monomeric porphyrinato rare earth complex was reported in 1999 by Wonget al. via the protonlysis of a rare earth amide with porphyrin [51]. An excess amount of RE[N(SiMe3)2]3·x[LiCl(THF)3], generatedin situfrom the reaction of anhydrous RECl3with 3 equiv of Li[N(SiMe3)2] in THF, was treated with H2TMPP, leading to the cationic monomeric porphyrinato rare earth complexes [RE(III)(TMPP)(H2O)3]Cl·4THF (RE=Yb, Er, Y). The crystal structure of [Yb(III)(TMPP)(H2O)3]Cl·4THF is shown in Figure 4.34. The crystal is a square-antiprism and crystallizes in the monoclinic space groupCc. The central terbium ion is eight-coordinate with four porphyrin nitrogen atoms and four oxygen atoms from THF. The mean bond length for Tb–N is 0.2301 nm and for Tb–O, 0.2307 nm. Similar to the double- or triple-decker counterparts, in this monnomeric porphyrinato rare earth compound the porphyrin ring also exhibits a distorted saddle. The separation of the terbium ion from the mean N4 plane of porphyrin ligand and four oxygen atoms is 0.1082 and 0.164 nm, respectively. It is worth noting that [Yb(III)(TMPP)(H2O)3]Cl·4THF has been revealed to exhibit a catalytic role in the cyclotrimerization of phenyl isocyanate.
(a) (b) O5
O2
O4
O6
O7 O7
O6
O3 O5
O4O2
O8 Yb1
Cl1
O1O9 O12 N4N2
N3 N1 O11
O10
O1N1 O3 N2 N3
N4 Yb1
Figure 4.34 (a) Perspective view and (b) side view of compound [Yb(III)(TMPP)(H2O)3]Cl [51].
(Reproduced from W. Wonget al., “Synthesis and crystal structures of cationic lanthanide (β) mono- porphyrinate complexes,’’ Journal of the Chemical Society, Dalton Transactions, 615, 1999 (doi:
10.1039/a809696a), by permission of The Royal Society of Chemistry.)