5,10,15,20-Tetrakis(diphenylmethyl)porphyrin - A Nonplanar Porphyrin with Intermediate Degree of Ruffling Steffen Runge3, Mathias O. Senge 3 *, Karin Ruhlandt-Sengeb a Institut für Organische Chemie, Freie Universität Berlin, Takustraße 3, D-14195 Berlin, Germany b Department of Chemistry, Syracuse University, Syracuse, NY 13244, U SA Z. Naturforsch. 54b, 6 6 2 -6 6 6 (1999); received February 17, 1999 Porphyrins, Steric Strain, Cobalt Porphyrins, Crystal Structure, Conformation The title compound was prepared by condensation of pyrrole and diphenylacetaldehyde in 16% yield. Based on spectroscopic data, the free base porphyrin exhibits a nonplanar macrocycle in solution with a degree of distortion between that of 5,10,15,20-tetracyclohexylporphyrin and 5,10,15,20-tetra(r-butyl)porphyrin. A crystal structure determination of the cobalt(II) complex reveals a ruffled macrocycle conformation with an average out-of-plane displacement of the m eso carbon atoms by 0.55 A.
Much attention has focused in recent years on porphyrins bearing sterically demanding substitu ents with nonplanar conformations. It has been established that different macrocycle conform a tions result in altered physicochemical properties of the porphyrin cofactors in vivo and technical applications of this concept have been described as well [1-4]. Of special interest are 5,10,15,20tetraalkylporphyrins with a ruffled conformation or 2,3,7,8,12,13,17,18-octaalkyl/aryl-5,10,15,20tetraarylporphyrins with a saddle conformation [5-8]. In order to elucidate the mutual interrelation ship between macrocycle conform ation and prop erties it is necessary to prepare series of porphy rins with gradually altered distortion modes for detailed physicochemical studies [9]. While nu merous 5,10,15,20-tetraalkylporphyrins have been prepared these are either only m oderately dis torted (e.g. with isopropyl) or highly nonplanar (with /-butyl residues) and prone to side reactions [5-7,10,11]. The only alkylporphyrins with an in term ediate degree of ruffling were 5,10,15,20tetracyclohexylporphyrin [1 1 , 1 2 ] and the so-called chiroporphyrins, all based on residues with sp 2 hy bridized ipso carbon atoms [13-15]. In order to obtain a tetraalkylporphyrin with a degree of con
* Reprint requests to Priv.-Doz. Dr. M. O. Senge. Fax: 0 3 0 -8 3 8 -4 2 4 8 E-mail:
[email protected] 0932-0776/99/0500-0662 $ 06.00
formational distortion between these porphyrins and tetra(r-butyl)porphyrin we targeted 5,10,15,20tetrakis(diphenylmethyl)porphyrin ( 1 ) as one such compound. Reaction of pyrrole with diphenylacetaldehyde and TFA under Lindsey conditions [16] followed by oxidation with DDQ yielded the desired free base 1 in acceptable yield. After a first crude puri fication several different atropisomers were found, a situation similar to that described for tetracyclohexylporphyrin [12]. After the second purification step, using neutral alumina, only one product was observed, presumably due to interconversion of the different atropisomers into the most stable form. Utilizing standard metallation techniques various metal complexes of the free base porphy rin are accessible and the complexes 2 - 4 were prepared in the hope to obtain crystals suitable for X-ray crystallography. Unfortunately, only the Co(II) complex 4 gave crystals of marginal size suitable for analysis (see below). Despite various syntheses of the Ni(II) complex 3 we were unable to obtain a meaningful 'H NMR spectrum; always formation of a mixture of atropisomers was ob served. In no case we observed side reactions re sulting in disruption of the aromatic system as have been described for 5,10,15,20-tetra(r-butyl)porphyrin [6,10]. With access to the title com pound, a relatively ruffled tetraalkylporphyrin is now in hand that is amenable for further synthetic modifications without the problems associated with tetra(f-butyl)porphyrin.
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St. Runge et al. ■5,10,15,20-Tetrakis(diphenylmethylporphyrin
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Zn(ll) Ni(ll) Co(ll)
On the basis of the spectroscopic data in methy lene chloride the free base porphyrin 1 (ASoret = 433 nm) takes an interm ediate position between tetracyclohexylporphyrin (ASoret = 422 nm) and tetra(?-butyl)porphyrin (ASoret = 446 nm) with re gard to its conformational distortion. For compari son, the planar 5,10,15,20-tetra(«-butyl)porphyrin exhibits a ASoret ° f 416 nm [10]. The bathochromic shift of the absorption maxima is known to di rectly correlate with the degree of conformational distortion [8 a]. The situation is not as clear for the related metal complexes. Both the Ni(II) and Co(II) complexes of 1 show a small hypsochromic shift compared to the respective 5,10,15,20-tetracyclohexylporphyrin metal complexes. Presuma bly, the metal complexes have a large degree of conformational flexibility. A similar observation can be made on the basis of the crystal structure of 4. The cobalt(II) com plex crystallized as the pentacoordinated complex with a m ethanol serving as axial ligand to the co balt center (Fig. 1). Selected structural param eters are compiled in Table I. The macrocycle confor mation is characterized by a typical ruffled distor tion mode with significant out-of-plane displace ments of the meso carbon positions {A Cm = 0.55 A) and twisting of the pyrrole rings about the CbCb axis. The degree of nonplanarity is similar to that found in {5,10,15,20-tetraphenylporphyrinato)cobalt(II) [17a]. Thus, contrary to the situa tion found for the free base, the solid state struc ture of 4 gives no evidence for significant steric strain in this compound. In the absence of any re lated cobalt(II) tetraalkylporphyrin structures for comparison we can only surmise that metal com-
Fig. 1. Computer generated top and side views of 4 in the crystal. Hydrogen atoms have been omitted for clarity.
plexes of the title com pound have a high degree of conformational flexibility whose conformation is easily affected by other factors, for example packing forces. The present structure shows some short interm olecular contacts, e.g. between aryl ring hydrogen atoms and cobalt centers (2 .7 -3 A), and the axial m ethanol molecule forms a hydrogen bond to a methanol of solvation (0 1 S -0 2 S = 2.803 A). The overall structural param eters are close to those of other cobalt(II) porphyrins [18]. The Co-N bonds are significantly shorter than in the planar {2,3,7,8,12,13,17,18-octaethylporphyrinato}cobalt(II) [1.971(6) A] [17b] and slightly longer than in {5,10,15,20-tetraphenylporphyrinato}cobalt(II) [1.949(3) Ä] [17a]. Com pared to highly saddle distorted cobalt porphyrins the Co-N bond lengths in 4 are slightly shorter than in {2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetranitro-
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St. Runge et al. • 5,10,15,20-Tetrakis(diphenylmethylporphyrin
Table I. Structural and geometrical parameters for 4.
Bond lengths [A]
Co-Oax Co-N N-Ca Ca-Cb Ca-Cm Cb-Cb
2.230(5) 1.954(5) 1.385(8) 1.439(9) 1.392(9) 1.338(9)
Bond angles [deg]
N-Co-N opp N-Co-N adj Co-N-Ca N-Ca-Cm N-Ca-Cb Ca-N-Ca Ca-Cm-Ca Ca-Ch-Cb C
C C
174.6(2) 89.9(2) 126.9(4) 125.4(6) 109.3(5) 106.1(5) 121.9(6) 107.6(6) 125.1(6)
Structural parameters [A]
A24 a ZlCo b
zIN b ZlCmb zlCa b ^C bb
0.268 0.09 0.03 0.55 0.31 0.20
a Average deviation of the macrocycle atoms from their least squares plane; b average deviation from the 4Nplane.
porphyrinato}cobalt(II) [1.964(5) Ä] [19a] and longer than in {2,3,7,8,12,13,17,18-octaethyl5,10,15,20-tetraphenylporphyrinato}cobalt(II) [1.929(3) A] [19b]. For comparison, the only re lated tetraalkylporphyrin structure, (5,10,15,20tetraheptafluoropropyl)porphyrinato)cobalt(II), albeit with a saddle distorted macrocycle, exhibits an average Co-N bond length of 1.936 A [19c]. Experimental General experim ental and instrum ental tech niques used were as described before [2 0 ]. Synthesis o f 5,10,15,20-tetrakis(diphenylm ethyl) porphyrin ( 1 )
Pyrrole (1.4 ml, 0.02 mol) and 0.02 mol diphenylacetaldehyde (2.5 ml, 3.96 g) were dissolved under an A r atm osphere in 2 1 absolute m ethylene chlo ride. A fter dropwise addition of 0.02 mol TFA (1.5 ml) the reaction mixture was stirred for 12 h at room tem perature. This was followed by addi tion of 0.016 mol D D Q (3.36 g) and stirring for an additional hour. The crude reaction mixture was filtered through neutral alumina (Brockmann grade III) and chrom atographed on alumina
(Brockmann grade III) eluting with C H 2 Cl2 /«-hexane (2:1, v/v). Recrystallization from C H 2 C12/ CH3OH yielded 779 mg (0.81 mmol, 16%) darkpurple crystals; m.p. 210-215 °C. - UV/vis (CH 2 C12): Amax (Is e) = 433 nm (5.26), 530 (4.08), 565 (3.60), 607 (3.55), 665 (3.24). - {H NM R (250 MHz, CDCI3 , TMS): Ö = -1.60 (s, 2H, NH), 7.167.28 (m, 24H, Hpw_phenyl), 7.41-7.44 (d ,7 = 6 . 8 Hz, 16H, H 0 _phenyi), 8.11 (s, 4H, C //(C 6 H 5)2), 8.94 (s, 8 H, H^.pyrrole). - MS (40 eV); m /z (% ): 974 (100) [M+], 808 (77) [M+ - C 1 3 H „ ] 642 (23) [M+ - 2x C 1 3 H „]. C 7 2 H 54 N 4 -3H20 (1029.248) Calcd C 84.02 H 5.29 Found C 84.31 H 5.47 c 72h 54n 4 Calcd Found
N 5.44% , N 5.04% .
974.43485, 974.43409 (HRMS).
Synthesis o f {5,10,15,20-tetrakis(diphenylmethyl)porphyrinato}zinc(II) (2)
The free base 1 (0.1 mmol, 100 mg) was dis solved in 50 ml CH 2 C12 and treated under stirring with 250 mg ZnO and 3 drops of TFA. A fter 1 0 minutes a color changes from green to purple was observed. The crude mixture was filtered through silica gel and recrystallized from C H 2 C12/ CH 3 OH. Yield: 60 mg (0.058 mmol, 57%) red-purple crystals; m.p. 230-232 °C. - UV/vis (C H 2 C12): Amax (lg e) = 434 nm (5.8), 565 (4.66). - ]H NM R (250 MHz, CDCI3 . TMS): 2.0a(I), 761 param eter, A /g max = 0.846 e A - \ R 1 [I > 2s(I)] = 0.1107, R 1 (all data) = 0.215, wR 2 (all data) = 0.2666, S = 1.016. A ckn ow ledgem en ts
This work was generously funded by the Deutsche Forschungsgemeinschaft (Se543/2-4 and Heisenberg-Scholarship S e543/3-l) and the Fonds der Chemischen Industrie and by the Na tional Science Foundation (NSF CHE-9702246, C H E-95-27898), the W. M. Keck Foundation and Syracuse University.
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