Organometallics 2009, 28, 6213–6217 DOI: 10.1021/om900745t
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Meso-Substituent Effects on Redox Properties of the 5,10-PorphodimetheneType P,S,N2-Hybrid Calixphyrins and Their Metal Complexes Yoshihiro Matano,*,† Masato Fujita,† Tooru Miyajima,† and Hiroshi Imahori†,‡,§ †
Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan, ‡Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan, and §Fukui Institute for Fundamental Chemistry, Kyoto University, Sakyo-ku, Kyoto 606-8103, Japan Received August 26, 2009
The 5,10-porphodimethene-type P,S,N2-hybrid calixphyrins bearing p-methoxyphenyl or p-(trifluoromethyl)phenyl groups at the sp2-hybridized meso carbons and their palladium(II) and rhodium(I) complexes were prepared and characterized. The electronic effects of the meso-aryl substituents on the redox properties of the π-conjugated subunits were evaluated for both the free bases and the palladium complexes, and the hemilabile nature of the P,S,N2-calixphyrin platforms in the rhodium complexes was revealed. Calixphyrins1 are a class of macrocyclic ligands that contain multiple pyrrole rings bridged by sp2- and sp3-hybridized meso carbons. It is well-known that the structural, redox, and coordinating properties of calixphyrins vary widely, depending on the number of sp3-hybridized meso carbons as well as the character of peripheral substituents.2 Core modification is also a *To whom correspondence should be addressed. E-mail: matano@ scl.kyoto-u.ac.jp. (1) (a) Kr al, V.; Sessler, J. L.; Zimmerman, R. S.; Seidel, D.; Lynch, V.; Andrioletti, B. Angew. Chem., Int. Ed. 2000, 39, 1055–1058. (b) Sessler, J. L. J. Porphyrins Phthalocyanines 2000, 4, 331–336. (c) Sessler, J. L.; Zimmerman, R. S.; Bucher, C.; Kral, V.; Andrioletti, B. Pure Appl. Chem. 2001, 73, 1041–1057. (2) For example, see: (a) Krattinger, B.; Callot, H. J. Tetrahedron Lett. 1998, 39, 1165–1168. (b) Benech, J.-M.; Bonomo, L.; Solari, E.; Scopelliti, R.; Floriani, C. Angew. Chem., Int. Ed. 1999, 38, 1957–1959. (c) Bonomo, L.; Toraman, G.; Solari, E.; Scopelliti, R.; Floriani, C. Organometallics 1999, 18, 5198–5200. (d) Bucher, C.; Seidel, D.; Lynch, V.; Kral, V.; Sessler, J. L. Org. Lett. 2000, 2, 3103–3106. (e) Senge, M. O.; Kalisch, W. W.; Bischoff, I. Chem. Eur. J. 2000, 6, 2721–2738. (f) Harmjanz, M.; Gill, H. S.; Scott, M. J. J. Org. Chem. 2001, 66, 5374–5383. (g) Furuta, H.; Ishizuka, T.; Osuka, A.; Uwatoko, Y.; Ishikawa, Y. Angew. Chem., Int. Ed. 2001, 40, 2323–2325. (h) Dolensky, B.; Kroulík, J.; Kral, V.; Sessler, J. L.; Dvorakova, H.; Bour, P.; Bernatkova, M.; Bucher, C.; Lynch, V. J. Am. Chem. Soc. 2004, 126, 13714–13722. (i) Senge, M. O. Acc. Chem. Res. 2005, 38, 733–743. (j) Bernatkova, M.; Dvorakova, H.; Andrioletti, B.; Kral, V.; Bour, P. J. Phys. Chem. A 2005, 109, 5518–5526. (k) Bucher, C.; Devillers, C. H.; Moutet, J.-C.; Pecaut, J.; Royal, G.; Saint-Aman, E.; Thomas, F. Dalton Trans. 2005, 3620–3631. (l) O'Brien, A. Y.; McGann, J. P.; Geier, G. R., III J. Org. Chem. 2007, 72, 4084–4092. (3) (a) Stepie n, M.; Latos-Gra_zy nski, L.; Szterenberg, L.; Panek, J.; Latajka, Z. J. Am. Chem. Soc. 2004, 126, 4566–4580. (b) Gupta, I.; Fr€ ohlich, R.; Ravikanth, M. Chem. Commun. 2006, 3726–3728. (c) Hung, C.-H.; Chang, G.-F.; Kumar, A.; Lin, G.-F.; Luo, L.-Y.; Ching, W.-M.; Diau, E. W.-G. Chem. Commun. 2008, 978–980. (d) Chang, G.-F.; Kumar, A.; Ching, W.-M.; Chu, H.-W.; Hung, C.-H. Chem. Asian J. 2009, 4, 164–173. (e) Skonieczny, J.; Latos-Gra_zynski, L.; Szterenberg, L. Chem. Eur. J. 2008, 14, 4861–4874. (4) (a) Matano, Y.; Miyajima, T.; Nakabuchi, T.; Imahori, H.; Ochi, N.; Sakaki, S. J. Am. Chem. Soc. 2006, 128, 11760–11761. (b) Matano, Y.; Miyajima, T.; Ochi, N.; Nakabuchi, T.; Shiro, M.; Nakao, Y.; Sakaki, S.; Imahori, H. J. Am. Chem. Soc. 2008, 130, 990-1002; 2009, 131, 14123 (Additions and Corrections) . (c) Matano, Y.; Miyajima, T.; Ochi, N.; Nakao, Y.; Sakaki, S.; Imahori, H. J. Org. Chem. 2008, 73, 5139–5142. (d) Matano, Y.; Fujita, M.; Miyajima, T.; Imahori, H. Phosphorus, Sulfur Silicon Relat. Elem., in press. (e) Ochi, N.; Nakao, Y.; Sato, H.; Matano, Y.; Imahori, H.; Sakaki, S. J. Am. Chem. Soc. 2009, 131, 10955–10963. (f) Matano, Y.; Imahori, H. Acc. Chem. Res. 2009, 42, 1193–1204. r 2009 American Chemical Society
versatile methodology to drastically change the fundamental properties of the calixphyrin platform.3,4 Recently, we reported the first examples of phosphole-containing P,S,N2-hybrid calixphyrins 1 and 2, in which the phosphorus atom incorporated in the rim plays a crucial role in tightly binding various late transition metals.4a,b Notably, 1 and 2 are interconvertible by redox reactions, and 1 behaves as a redox-active mixed-donor (hybrid) macrocyclic ligand toward palladium.5 However, for a fine tuning of the electronic structure of the P,S,N2-hybrid calixphyrin platform, it is necessary to evaluate the electronic effects of peripheral substituents on the redox properties of the π-conjugated subunits. In this context, we prepared new P,S,N2-hybrid calixphyrins bearing para-substituted phenyl groups on the sp2-hybridized meso carbons. Herein, we report electronic effects of the meso substituents on redox properties of the 5,10-porphodimethene-type P,S,N2-hybrid calixphyrins and their metal complexes. In addition, the unique coordination structures of the Rh(I)-P,S,N2 complexes are described in detail.
(5) Metal complexes coordinated by phosphole-containing macrocyclic ligands have been reported by Mathey and co-workers: (a) Mathey, F.; Mercier, F.; Nief, F.; Fischer, J.; Mitschler, A. J. Am. Chem. Soc. 1982, 104, 2077–2079. (b) Bevierre, M.-O.; Mercier, F.; Ricard, L.; Mathey, F. Angew. Chem., Int. Ed. Engl. 1990, 29, 655–657. (c) Laporte, F.; Mercier, F.; Ricard, L.; Mathey, F. J. Am. Chem. Soc. 1994, 116, 3306– 3311. (d) Deschamps, E.; Ricard, L.; Mathey, F. J. Chem. Soc., Chem. Commun. 1995, 1561. (e) Mercier, F.; Laporte, F.; Ricard, L.; Mathey, F.; Schr€oder, M.; Regitz, M. Angew. Chem., Int. Ed. Engl. 1997, 36, 2364– 2366. Published on Web 10/14/2009
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Scheme 1. Synthesis of σ3-P,S,N2-Hybrid Calixphyrins 9 and 10
Results and Discussion The new σ3-P,S,N2-calixphyrins 9 and 10 were prepared from σ4-phosphatripyrrane 36 and the corresponding 2,5-bis[hydroxy(aryl)methyl]thiophenes 4b,c7 (Scheme 1) according to the reported procedure for the synthesis of 1 and 2.4a,b Addition of BF3 3 OEt2 to a CH2Cl2 solution containing 3 and 4b, followed by treatment with 2.2 equiv of 2,3dichloro-5,6-dicyanobenzoquinone (DDQ), afforded the 4e-oxidized σ4-P,S,N2-calixphyrin 5 as a major product. When 4c was used in place of 4b, the 2e-oxidized σ4-P,S,N2-calixphyrin 6 was obtained as the sole isolable product in low yield. Compound 6 was produced in better yield via BF3-promoted condensation of phosphole 78 with thiatripyrrane 8 (eq 1).
Treatment of 5 and 6 with excess P(NMe2)3 in refluxing toluene yielded the 4e-oxidized σ3-P,S,N2-calixphyrin 9 and the 2e-oxidized σ3-P,S,N2-calixphyrin 10, respectively. Probably due to the electronic effects of the para substituents (vide infra), 2e-oxidized counterparts of 5 and 9 and 4e-oxidized counter(6) (a) Matano, Y.; Nakabuchi, T.; Miyajima, T.; Imahori, H. Organometallics 2006, 25, 3105–3107. (b) Nakabuchi, T.; Matano, Y.; Imahori, H. Organometallics 2008, 27, 3142–3152. (7) Hilmey, D. G.; Abe, M.; Nelen, M. I.; Stilts, C. E.; Baker, G. A.; Baker, S. N.; Bright, F. V.; Davies, S. R.; Gollnick, S. O.; Oseroff, A. R.; Gibson, S. L.; Hilf, R.; Detty, M. R. J. Med. Chem. 2002, 45, 449–461. (8) Matano, Y.; Miyajima, T.; Nakabuchi, T.; Matsutani, Y.; Imahori, H. J. Org. Chem. 2006, 71, 5792–5795.
Matano et al. Scheme 2. Synthesis of Hybrid Calixphyrin-Palladium Complexes
parts of 6 and 10 were not obtained at all. Apparently, the p-methoxyphenyl groups stabilize the 4e-oxidized form of the pyrrole-thiophene-pyrrole (N-S-N) subunit, whereas the p-(trifluoromethyl)phenyl groups stabilize the 2e-oxidized form. Newly prepared calixphyrins were characterized by 1H and 31P NMR spectroscopy and high-resolution mass (HRMS) spectrometry. The 31P chemical shifts of 9 (δ 26.7) and 10 (δ 31.3) are almost identical with those of 1 (δ 26.7) and 2 (δ 31.0), respectively, indicating that the mesoaryl groups at the N-S-N subunits do not affect the electronic character of the phosphorus nucleus. In the 1H NMR spectra, 9 shows the pyrrole-β and thiophene-β protons at δ 6.60-6.68 and 6.67 ppm (in CDCl3), respectively, whereas 10 displays the corresponding protons at δ 5.74-5.99 and 6.39 ppm. The difference in chemical shifts of the heterole-β protons between 9 and 10 clearly supports the different oxidation states of their N-S-N subunits. The 2e-oxidized P,S,N2-calixphyrin 10 shows reversible two-step oxidation processes at Eox,1=þ0.46 V and Eox,2=þ0.64 V (vs FeCp*2/FeCp*2þ; Cp* = C5Me5). These values are more positive than the corresponding values of 2 (Eox,1 = þ0.34 V and Eox,2 =þ0.54 V), indicating that the introduction of two p-(trifluoromethyl)phenyl groups makes the π-conjugated N-S-N subunit less oxidizable by ca. 0.1 V.9 With the new P,S,N2-hybrid ligands 9 and 10 in hand, we investigated their coordination behavior toward palladium and rhodium. As previously reported, the hybrid ligands 1 and 2 reacted with Pd(dba)2 (dba=dibenzylideneacetone) and Pd(OAc)2, respectively, yielding the same palladium complex 11a (Scheme 2).4a,b Both the experimental and theoretical results revealed that the formal oxidation state of the palladium center in 11a is þ2. Thus, the redoxcoupled complexation occurs between 1 and Pd(dba)2, whereas the metathesis takes place between 2 and Pd(OAc)2. Similar reactivities were observed for 9 and 10. The p-methoxyphenyl derivative 9 underwent redoxcoupled complexation with Pd(dba)2 to give palladium(II) complex 11b, whereas the p-(trifluoromethyl)phenyl derivative 10 underwent metathesis with Pd(OAc)2 to afford (9) Provided that the electronic effect is quantitatively evaluated on the basis of ΔEox value per one meso-aryl substituent, the electronic effect observed for 10 vs 2 (ca. 0.05-0.06 V) is comparable to that reported for TPP derivatives (TPP=5,10,15,20-tetraphenylporphyrin) bearing the same para substituents (ca. 0.04 V). For Eox values of the TPP derivatives under the same CV measurement conditions, see: (a) Wolberg, A. Isr. J. Chem. 1974, 12, 1031–1035. (b) Hariprasad, G.; Dahal, S.; Maiya, B. G. J. Chem. Soc., Dalton Trans. 1996, 3429–3436. (c) Richter-Addo, G. B.; Hodge, S. J.; Yi, G.-B.; Khan, M. A.; Ma, T.; Van Caemelbecke, E.; Guo, N.; Kadish, K. M. Inorg. Chem. 1996, 35, 6530–6538.
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11c (Scheme 2). The spectral features of 11b,c are essentially the same as those of 11a, suggesting that the palladium center in 11b,c takes a þ2 oxidation state. The UV-vis absorption spectra of 11b,c (Figure S2 in the Supporting Information) also show the similarity of their electronic structures. Evidently, the hybrid calixphyrin platforms in 11b,c behave as dianionic P,S,N2-tetradentate ligands. The 31P chemical shifts of 11b (δ 45.9) and 11c (δ 47.3) are close to the 31P chemical shift of 11a (δ 47.0), indicating that the para substituents have little impact on the electronic character of the phosphorus nucleus, even in the palladium(II) complexes. To examine the electronic effects of meso-aryl substituents on the N-S-N π-conjugated subunits of the palladium complexes, the first and second oxidation potentials (Eox,1 and Eox,2) of 11b,c were compared with those of 11a. Both Eox,1 and Eox,2 of 11b (þ0.17 and þ0.28 V vs FeCp*2/ FeCp*2þ) are shifted to the negative side, while Eox,1 and Eox,2 of 11c (þ0.32 and þ0.42 V) are shifted to the positive side, as compared to the corresponding potentials of the phenyl derivative 11a (þ0.23 and þ0.34 V). These data simply reflect the electron-donating (for 11b) and electronwithdrawing (for 11c) abilities of the para substituents on the meso phenyl groups.9 The differences in the oxidation potentials between 11c and 11a (þ0.08 to þ0.09 V) are comparable to those observed for the free bases 2 and 10 (þ0.10 to 0.12 V). It seems likely that the electronic effects of the para substituents on the N-S-N subunit are little affected by the complexation with palladium. Additionally, it is evident that the palladation at the core raises the HOMO level of the P,S,N2-hybrid calixphyrin platform by ca. 0.1 V. Similar to 1, free base 9 reacted with [RhCl(CO)2]2 in CH2Cl2 to give rhodium complex 12b in high yield (Scheme 3).10 As described below, 12b was found to possess the neutral 4e-oxidized N-S-N subunit. To our surprise, 10 also reacted with [RhCl(CO)2]2 to produce a similar type of rhodium complex 12c, albeit in low yield. Presumably, the 2e-oxidized N-S-N subunit was converted to the 4eoxidized form under the reaction conditions employed. The structures of 12a-c were characterized by NMR and IR spectroscopy, MS spectrometry, and X-ray crystallography (for 12a,b). The diagnostic spectral features of 12a-c are as follows. In the 31P NMR spectra, the characteristic doublet peaks are observed at δ 69.6 (for 12a), δ 69.4 (for 12b), and δ 69.6 (for 12c) with 31P-103Rh coupling constants of 171 Hz (for 12a), 177 Hz (for 12b), and 171 Hz (for 12c), indicating that the oxidation state of the rhodium center in 12a-c is þ1.11 The IR spectra display sharp CO stretching bands at ν 1977 cm-1 (for 12a), 1978 cm-1 (for 12b), and 1981 cm-1 (for 12c), which are at the shorter end of the reported values (ca. 1980-2000 cm-1) of typical Rh(I) (10) In our previous paper,4b we assigned 12a (complex 9 in ref 4b ) as a cationic rhodium(I) complex coordinated by the four core elements (P, S, and two N) on the basis of only spectral data (1H NMR, 31P NMR, and MS). However, this assignment has proven to be incorrect. New data (IR and X-ray) unambiguously elucidated that 12a is a neutral, square-planar rhodium(I) complex coordinated by two core elements (P and N), a chlorine atom, and a CO group, as mentioned in the text. Reinvestigation of the reported spectral data also supports this assignment. Accordingly, the spectral features of 12a are described again in the present paper. The correction of the structure of 12a (compound 9 in ref 4b) has been published in J. Am. Chem. Soc. as an Addition and Correction.4b (11) Brown, T. H.; Green, P. J. J. Am. Chem. Soc. 1970, 92, 2359– 2362.
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Scheme 3. Synthesis of Hybrid Calixphyrin-Rhodium(I) Complexes
carbonyl complexes.12 In the mass spectra, intense peaks due to the fragment ions ([M - (CO, Cl)]þ) are detected at m/z 774 (for 12a), 833 (12b), and 909 (12c), together with weak fragment [M - (CO)]þ ion peaks. In the 1H NMR spectra of 12b,c in CDCl3 at room temperature, the pyrrole-β protons are observed as significantly broadened peaks, which suggests the occurrence of dynamic exchange processes between two conformers. This spectroscopic feature will be discussed later. The rhodium(I) complexes 12a,b show irreversible oxidation processes at þ0.56 and þ0.52 V (vs FeCp*2/FeCp*2þ), respectively. The electrochemical oxidation behavior of 12a,b is close to that of 13 and 14, which is reported to involve a Rh(I)to-Rh(II) transition followed by a chemical reaction.13 The difference in the oxidation potentials between 12a and 12b is very small, suggesting that the electronic effect of the mesoaryl substituents on the P,N-chelated rhodium(I) center is small. The structures of 12a,b were unambiguously elucidated by X-ray crystallographic analyses.10,14 As shown in Figure 1 and Figure S3 (Supporting Information), the P, S,N2-hybrid ligands in these complexes provide essentially the same coordination environments. In each complex, the rhodium center adopts a square-planar geometry P ( L-Rh-L0 = ca. 360°) supported by the phosphorus, nitrogen, chlorine, and carbonyl carbon atoms. The Cl and CO ligands occupy positions trans to the P and N(1) atoms, respectively. The CO stretching frequencies of 12a-c (see above) reflect the trans influence of the azafulvene-type N ligand. One of the nitrogen atoms, N(1), coordinates to the rhodium center, while the other, N(2), does not. Accordingly, the N-S-N π-conjugated subunit is highly twisted, and the phosphole ring is almost perpendicular to a mean plane formed by the four meso carbons. All the structural features observed for 12a,b are completely different from those reported for octahedral (12) Conradie, M. M.; Conradie, J. Inorg. Chim. Acta 2008, 361, 2285–2295. (13) Yao, C.-L.; Anderson, J. E.; Kadish, K. M. Inorg. Chem. 1987, 26, 2725–2727. (14) Crystal data for 12a: C46H39ClN2OPRhS, MW=837.18, monoclinic, P21/c, a=13.669(6) A˚, b=14.829(7) A˚, c=18.824(8) A˚, β=91.964(8)°, V=3814(3) A˚3, Z=4, Dc=1.458 g cm-3, 10 161 reflections collected, 8467 independent reflections, 479 variables, Rw = 0.2285, R = 0.1127 (I > 2.00σ(I)), GOF = 1.264. The CIF file of 12a was submitted as an Addition/Correction of ref 4b. Crystal data for 12b: C48H43ClN2O3PRhS 3 0.5CH2Cl2, MW = 939.70, monoclinic, P21/c, a = 16.168(5) A˚, b=14.487(4) A˚, c=18.921(6) A˚, β=100.612(6)°, V=4356(2) A˚3, Z=4, Dc=1.433 g cm-3, 33 314 collected reflections, 9943 independent reflections, 542 variables, Rw=0.1576, R=0.1001 (I > 2.00σ(I)), GOF= 1.233.
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Matano et al. Table 1. Selected Bond Lengths and Distances (A˚) of 12a,b and 11aa
a, a0 b, b0 c, c0 d, d0 e, e0 f, f0 g N3 3 3N P3 3 3S
Figure 1. Top and side views of 12b (30% probability ellipsoids). Hydrogen atoms are omitted for clarity. Selected bond lengths (A˚) and bond angles (deg): Rh-P, 2.2348(16); Rh-N(1), 2.177(5); Rh-Cl, 2.4116(16); Rh-C(46), 1.792(6); C(46)-O(1), 1.152(7); Rh-C(46)-O(1), 176.3(7); P-Rh-N(1), 91.15(13); P-RhC(46), 92.5(2); Cl-Rh-N(1), 89.59(13); Cl-Rh-C(46), 86.4(2).
P,N3-calixphyrin-rhodium(III) complexes,4b in which all of the core elements occupy the basal positions coordinating to the rhodium center. Table 1 summarizes carbon-carbon bond lengths at the N-S-N subunits and selected interatomic distances of 12a,b together with those of the previously reported Pd(II) complex 11a.4b The pattern of carbon-carbon bond length alternation at the N-S-N subunit of 12a,b well explains the 4e-oxidation state depicted as the canonical structure in Scheme 3 and is in sharp contrast to that observed for 11a containing a dianionic N-S-N subunit with the 2e-oxidation state. The difference in coordination structures between 12a and 11a is also reflected in their N 3 3 3 N and P 3 3 3 S distances. The Rh-N (core nitrogen atom) and Rh-C (trans to the nitrogen atom) bond lengths of dinuclear rhodium(I) porphyrins 1315 and 1416 and rhodium(I) N-confused porphyrin 1516c were reported to be 2.069(3)-2.098(3) and 1.837(10)1.860(4) A˚, respectively. The Rh-N bond lengths of the present Rh(I) complexes 12a,b (2.185(6) and 2.177(5) A˚) are considerably longer than those of 13-15, which may reflect differences in chelating structures (P,N versus N,N) and Lewis acidity at the rhodium center. The Rh-N bond lengths of 12a,b are also longer than the Rh-N bond lengths observed for the octahedral P,N3-calixphyrin-Rh(III) complexes (2.062(4)-2.082(4) A˚).4b On the other hand, the Rh-C bond (15) (a) Yoshida, Z.; Ogoshi, H.; Omura, T.; Watanabe, E.; Kurosaki, T. Tetrahedron Lett. 1972, 13, 1077–1080. (b) Takenaka, A.; Sasada, Y.; Omura, T.; Ogoshi, H.; Yoshida, Z. Chem. Commun. 1973, 792–793. (c) Takenaka, A.; Sasada, Y.; Omura, T.; Ogoshi, H.; Yoshida, Z. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1975, B31, 1–6. (16) (a) Fleischer, E. B.; Dixon, F. Bioinorg. Chem. 1977, 7, 129–139. (b) Srinivasan, A.; Furuta, H.; Osuka, A. Chem. Commun. 2001, 1666–1667. (c) Srinivasan, A.; Toganoh, M.; Niino, T.; Osuka, A.; Furuta, H. Inorg. Chem. 2008, 47, 11305–11313.
12ab
12b
11ac
1.42, 1.45 1.36, 1.34 1.45, 1.46 1.37, 1.36 1.45, 1.47 1.42, 1.38 1.38 4.93 3.97
1.42, 1.44 1.34, 1.34 1.44, 1.45 1.38, 1.36 1.44, 1.45 1.38, 1.39 1.38 4.88 3.98
1.38, 1.39 1.39, 1.39 1.40, 1.39 1.45, 1.46 1.36, 1.36 1.45, 1.45 1.35 4.15 4.45
a The values are rounded off to the second decimal place. b Data from ref 4b (Addition/Correction) . c Data from ref 4b.
lengths of 12a,b (1.783(9) and 1.792(6) A˚) are appreciably shorter than those of 13-15, implying that the back-donation from Rh to CO in 12a,b is more prominent than that in 13-15. The structural features observed for 12a,b represent the characteristic role of the phosphole ligand incorporated in the calixphyrin platform.
To get an insight into the dynamic behavior of the P,S, N2-calixphyrin-Rh(I) complexes, we measured variabletemperature (VT) 1H NMR spectra of 12b in 1,1,2,2-tetrachloroethane-d2 from -40 to 100 °C (Figure S4 in the Supporting Information). As mentioned above, at room temperature, the pyrrole-β protons were observed as broad peaks. At -40 °C, however, the pyrrole-β protons became sharpened and split into four separate peaks (δ 6.50, 6.56, 6.64, and 6.83) with the same intensity (each d, 1H, J = 3.9 Hz) implying that the two pyrrole rings are placed in nonequivalent magnetic environments. With an increase in the temperature, the peak pattern gradually changed, and the pyrrole-β protons coalesced at ca. 50 °C and appeared as two peaks (δ 6.61 and 6.76, each br s, 2H) at 100 °C. It should be noted that the 31P chemical shift is little affected by temperature. These observations can be interpreted by considering an interconversion between two coordination structures, 121 and 122 (eq 2). At low temperatures, the exchange rate between 121 and 122 is slow on the NMR time scale, whereas at high temperatures it is fast. The dynamic behavior observed by
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VT-NMR measurements exemplifies the hemilabile nature of the P,S,N2-calixphyrin platform.
In summary, we have revealed electronic effects of the meso substituent on the redox properties of the 5,10porphodimethene-type P,S,N 2 -hybrid calixphyrins and their metal complexes for the first time. The electrondonating p-methoxyphenyl groups stabilize the 4e-oxidation state of the π-conjugated N-S-N subunit, whereas the electron-withdrawing p-(trifluoromethyl)phenyl groups stabilize the 2e-oxidation state. The meso-substituent
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effects on the electrochemical oxidation processes of the π-conjugated subunits are clearly observed for a series of palladium(II)-P,S,N2 -calixphyrin complexes. In addition, we have unambiguously characterized the structures of the rhodium(I)-P,S,N2 -calixphyrin complexes by X-ray crystallography and disclosed the hemilabile nature of their macrocyclic platforms by NMR spectroscopy.
Acknowledgment. This work was supported by a Grant-in-Aid for Science Research on Priority Areas (No. 20036028, Synergy of Elements) from the Ministry of Education, Culture, Sports, Science and Technology of Japan and the Sumitomo Foundation. Supporting Information Available: Text and figures giving experimental details, characterization data, and 1H NMR charts for new compounds and a CIF file giving crystallographic data for 12b. This material is available free of charge via the Internet at http://pubs.acs.org.
