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Electron Donor-Acceptor Compounds. Synthesis and Structure of 5-(l,4-Benzoquinone-2-yl)-10,15,20-trialkylporphyrinsSteffen Runge, M athias O. Senge*Institut für Organische Chemie (WE02), Fachbereich Chemie, Freie Universität Berlin, Takustraße 3, D-14195 Berlin, GermanyZ. Naturforsch. 53b, 1021-1030 (1998); received June 15, 1998Porphyrins, Quinones, Electron Transfer Compounds, Crystal Structure, Conformation

A series of 5-(benzoquinone)-10,15,20-trialkylporphyrins was synthesized via cross conden­sation of the respective aldehydes, 2,5-dimethoxybenzaldehyde and pyrrole followed by de- methylation with BBr3 and oxidation with P b02. This method worked reasonably well for compounds bearing the benzoquinone substituent and butyl, isopropyl, 1 -methylpropyl and2-ethylpropyl residues (2a-d). The free base porphyrin quinones were converted into the zinc(II) complexes (3a-d) all of which showed remarkable stability for porphyrin quinones. The zinc(II) complex 3c bearing isopropyl residues was investigated by X-ray crystallography and showed a supramolecular structure consisting of polymeric chains facilitated by coordina­tion of a benzoquinone oxygen to a neighboring zinc(II) center. Attempts to synthesize a 5- (benzoquinone)-10,15,20-tris(terr-butylporphyrin) resulted in the formation of a yellow por- phomethene (4), which could not be oxidized further. A crystal structure analysis of 4, the first for a free base porphomethene, shows an extremely twisted conformation with syn- orientation of the three rm-butyl groups. The results indicate that new methods will have to be developed for the synthesis of nonplanar porphyrin quinones.

Introduction

E lectron transfer involving porphyrin-type do­nors and quinone acceptors presents one of the underlying principles of photosynthesis. The eluci­dation of the first crystal structure of a bacterial photosynthetic reaction center has provided a de­tailed picture of the geometrical arrangem ent of the cofactors involved [1], Despite enorm ous ef­forts aim ed at the synthesis and characterization of suitable porphyrin-quinone model com pounds m any questions regarding the photoinduced elec­tron transfer rem ain unanswered or are discussed controversially [2-4]. O pen problem s involve the sequence, activation, efficiency, and speed of the electron transfer in m ulticom ponent biomimetic systems. O ne question, which has been com pletely overlooked so far, involves the influence of the tetrapyrro le macrocycle conform ation on electron transfer processes. During the last years convinc­ing evidence was accumulated that the conform a­tional flexibility of porphyrins [5] and nonplanar macrocycles plays an im portant role in fine-tuning

* Reprint requests to Priv.-Doz. Dr. M. O. Senge. E-mail: mosenge@chemie.fu-berlin.de

1a R = CH2CH2CH2CH3 1b R = CH2CH2(CH3)21c R = CH(CH3)21d R = CH(CH2CH3)2

3a R = CH2CH2CH2CH33b R = CH2CH2(CH3)23c R = CH(CH3)23d R = CH(CH2CH3)2

2a R = CH2CH2CH2CH32b R = CH2CH2(CH3)22c R = CH(CH3)22d R = CH(CH2CH3)2

4 R = C(CH3)3

0932-0776/98/0900-1021 $06.00 © 1998 Verlag der Zeitschrift für Naturforschung, Tübingen • www.znaturforsch.com

1022 St. R unge-M . O. Senge • Electron Donor-Acceptor Compounds

the physicochemical properties of tetrapyrrole- based cofactors in vivo [6 ]. Since conform ational distortion of porphyrins results in significant a lte r­ations in their redox chemistry, photophysics and photochemistry, it follows that conform ational ef­fects should exert considerable influence on pho­toinduced electron transfer reactions. As early as 1988 Jortner and coworkers invoked conform a­tional effects in the special pair as one possible explanation for the unidirectionality of the elec­tron transfer along the L-branch of the photosyn­thetic reaction center [7], N evertheless, m ost of the m odel com pounds prepared to date are based on simple, planar octa-/3-alkylporphyrins or tetra- meso-arylporphyrins. M ore e laborate systems have been based on phytochlorin derivatives [8 ].

We have now em barked on a program aim ed at the synthesis of covalently linked porphyrin qui- nones with tailor-m ade distortion m odes suitable for investigation of the influence of the macrocycle conform ation on the electron transfer. The two m ost common perturbation modes for porphyrins are saddle distortion and ruffling of the macrocy­cle [9,10]. The latter involves significant out-of- plane distortion of the raeso-carbons and in-plane ro tation of the pyrrole rings. Such porphyrins are easily accessible via the synthesis of 5,10,15,20- tetraalkylporphyrins with bulky alkyl residues[11]. W hile num erous porphyrin-quinones with 5- quinone-10,15,20-triflry/porphyrin fram ew ork have been synthesized [3], we chose 5-(benzoqui- none-2-yl)-10,15,20-trialkylporphyrins as initial targets to investigate if published synthetic m eth ­ods can easily be applied for the synthesis of non- planar-porphyrin quinones or if new synthetic stra­tegies have to be developed by us.

Results and Discussion

A lthough crude and requiring laborious chro­m atographic work-up, the standard m ethod for the synthesis of covalently linked (protected) triarylp- orphyrin quinones is a mixed condensation of the required aldehydes and pyrrole. This approach fol­lows the procedures developed by Adler-Longo[12] and Lindsey [13] for symmetric porphyrins. The choice of m ethod depends on the reactivity of the required aldehydes and on the stability of the porphyrin quinone precursor form ed. This ap ­proach has been used to synthesize sterically un­

hindered 5-triptycenequinone-10,15,20-tripentyl- porphyrins [14]. For sterically demanding tetraalkylporphyrins a modification of the Lindsey conditions proved necessary [1 1 a].

Initial attem pts to utilize the Lindsey conditions for the synthesis of our target compounds did not m eet success. We finally adapted the conditions used by Em a et al. for the synthesis of sterically dem anding 5,10,15,20-tetraalkylporphyrins [11a] and were able to prepare the series of 5-(2,5-di- methoxyphenyl)-10,15,20-trialkylporphyrins (la ­id) using valeraldehyde, isovaleraldehyde, isobu- tyraldehyde, and 2 -ethyl-butyraldehyde as source for the m eso -alkyl groups. The choice of alkyl al­dehydes for la -ld was governed by our earlier work on symmetric tetraalkylporphyrins [1 1 c]. Using a variety of alkyl groups we have shown that while tetraalkylporphyrins with residues such as butyl, isopropyl, 2 -methylpropyl or 1 -ethylpropyl are planar or m oderately distorted, tert-butyl groups induce significant ruffling in the porphyrin. The respective m etal complexes showed a higher degree of conform ation flexibility and by using the substituents shown we hoped to gain access to a series of porphyrin quinones with graded degree of conform ational distortion. For the synthesis of com pound lb we also applied the Adler-Longo m ethod. This resulted in an almost doubled yield, albeit offset by a m ore time consuming work-up. A pplication of this m ethod to the synthesis of the m ore sterically hindered porphyrins with isopropyl (lc) or 1 -ethyl-propyl groups (Id) failed.

The next step towards the synthesis of the de­sired porphyrin quinones involved removal of the m ethyl protection group with boron tribrom ide to yield the respective hydroquinones [15] which were oxidized in situ with lead(IV ) oxide [16]. Using this m ethod, the free base porphyrin qui­nones 2a-2d were obtained in 60-65% yield. Fi­nally, the free base porphyrins were converted to the respective zinc(II) complexes (3a-3d) with zin- c(II) oxide using the m ethod developed by Dieks et al. [17]. B oth the free bases and zinc(II) com ­plexes are relative stable com pared to o ther por­phyrin quinones.

D espite num erous attem pts we were able to grow crystals suitable for single-crystal X-ray crys­tallography only for com pound 3c. A view of the m olecular structure is given in Fig. 1. A lthough of low resolution, the structure illustrates several im-

5 t . R un ge-M . O. Senge • Electron Donor-Acceptor Compounds 1023

Fig. 1. Computer generated plot of the monomeric unit in the crystal structure of 3c. Hydrogen atoms have been omitted for clarity. Selected bond lengths (A) and angles (°): Zn-N21 2.074(13), Zn-N22 2.026(15), Zn-N23 2.053(12), Zn-N24 2.024, Zn-02(ax) 2.265(13), C52-01 1.26(2), C55-02 1.25(2), N24-Zn-02 98.1(5), N22-Zn-02 92.1(5), N23 Zn-02 103.8(5), N21-Zn-02 85.6.

portan t points. The distance between the donor (defined as the center of the four pyrrole nitrogen atom s) and the quinone (defined as the geom etric center of the six ring carbon atom s) is 6.36 A, while the benzoquinone ring forms an angle of 76.5° with the 4N-plane of the tetrapyrrole. The overall conform ation of 3c is characterized by a m oderate degree of ruffling (see Fig. 2). The average deviation of the 24 macrocycle atom s (z!24) from their least-squares plane is 0.124 A, while the largest out-of-plane displacem ents are observed for the Cm-atoms (C5 = 0.33 A, CIO =

Fig. 2. View of the deviations of macrocycle atoms from the 4N-plane in 3c [Ä x 102].

0.26 A, C15 = 0.22 A , C20 = 0.16 Ä). As expected, the sterically less dem anding benzoquinone sub­stituent leads to the smallest Cm-displacements at C20. The perturbation of the macrocycle is also indicated by the pyrrole tilt angles against the 4N- plane which are 9.0°, 8.4°, 8.2°, and 6 .6 ° for the pyrroles with N21, N22, N23, and N24, respec­tively. For com parison, (pyridine)(5,10,15,20-tetra- (isopropyl)porphyrinato)zinc(II) is slightly more ruffled (ZI24 = 0.19 Ä ) with Cm-displacem ents of 0.34 A [11c]. In solution all com pounds exhibit very similar absorption spectra within each series (1, 2 or 3) with a slight bathochrom ic shift for com ­pounds c and d. Since the degree of distortion cor­relates with absorption maxim a [6 ] the com pounds will have very similar conform ations ranging from planar to m oderately ruffled.

The crystal structure of 3c is characterized by the form ation of a supram olecular system con­sisting of polym eric chains of molecules. The chains are form ed via coordination of one benzo­quinone oxygen atom (0 2 ) to the zinc center of a neighboring molecule [Z n -0 2 = 2.265(13) Ä]. Thus, the zinc(II) centers are pentacoordinated and displaced from the 4N-plane by 0.17 A. This arrangem ent leads to chains consisting of porphy­rin units in a zig-zag array (Fig. 3). Polymeric chains involving peripheral heteroatom s and por­phyrin m etal centers have been described before[18] but the present exam ple shows how easily such effects can take place in porphyrin quinones with all the associated im plications on photophysi­cal studies of such com pounds. In this context it should be m entioned that isolated quinones have been shown to coordinate to isolated porphyrins forming highly o rdered 3d-systems [19].

Fig. 3. Partial view of the polymeric chains formed in the crystal structure of 3c.

1024 St. R unge-M . O. Senge • Electron Donor-Acceptor C om pounds

The success in the synthesis of 2a-2d and 3a-3d, involving porphyrins with no or only a m oderate degree of conform ational distortion, prom pted us to attem pt the synthesis of a severely ruffled po r­phyrin quinone, namely 5-(l,4-benzoquinone-2- yl)-10,15,20-tris(terr-butyl)porphyrin. The synthe­sis of the symmetric 5,10,15,20-tetrakis(terf-bu- tyl)porphyrin bearing four m eso tert-butyl groups is easily achieved [1 1 ] and thus, we expected no problem s with the synthesis of a porphyrin bearing three m eso rm -butyl groups and one sterically less dem anding aryl group. Surprisingly, all attem pts to obtain a 5-(2,5-dimethoxyphenyl)-10,15,20-tris- (rert-butyl)porphyrin from the reaction of pivalal- dehyde, 2,5-dim ethoxybenzaldehyde and pyrrole failed. In all cases, a small am ount of a yellow com pound was the sole soluble product that could be separated from the reaction mixture.

On the basis of spectroscopic data and a crystal structure determ ination (Fig. 4) the yellow com ­pound was identified as the porphom ethene (5,10,15,22-tetrahydroporphyrin) 4. D espite addi­tion of D D Q during the reaction, only one oxida­tion step had occurred involving the sterically least h indered 2 0 -position of the porphyrinogen (the prim ary product of the aldehyde and pyrrole con­densation reaction). Subsequent attem pts to oxi­dize 4 to the porphyrin using p-chloranil, brom ine, lead(II) oxide or D D Q while heating under reflux, e ither gave no change or resulted in com plete d e­com position of the product. Similarly, attem pts to p repare 5-phenyl-10,15,20-tris(terr-butyl)porphy- rin utilizing analogous reactions did not give the desired result.

The m olecular structure of 4 exhibits a very nonplanar macrocycle conform ation (Fig. 4). The average deviation of the 24 macrocycle atom s from their least-squares plane is 0.442 A. Indivi­dual pyrrole tilt angles with the 4N-plane are quite different; the angles are 23.4°, 47.3°, 48.7°, and 97.8° for the pyrroles with N21, N22, N23, and N24, respectively. The porphom ethene character of the com pound is clearly evidenced by the dipyr- rom ethene structure of the N21,N24-dipyrrin unit. The eleven atom s comprising this d ipyrrom ethene are planar with an average deviation from their least-squares plane of 0.026 A. O ne notable fea­ture of this structure is sy/i-orientation of the three rm -butyl groups involving the three sp3 -hybrid- ized m eso positions. To our knowledge, this struc-

Fig. 4. Upper panel: Top view of the molecular structure of 4 (50% occupancy ellipsoids). All hydrogen atoms have been omitted for clarity. Lower panel: Side view of 4 with the Cm- and nitrogen hydrogen atoms.

ture presents the first example of a crystal struc­ture of a free base porphom ethene. Only two examples of m etalloporphom ethenes obtained via oxidation of 5,5,10,15,15,20,20-octaethylporphyri- nogen have been described in the literature [2 0 ]. Note, that porphom ethenes are putative in term e­diates of the biosynthetic pathway leading to p o r­phyrins and this structure presents the first view of such a com pound.

One possible explanation for the vexing result that a seemingly sterically less hindered com pound [the putative 5-(2,5-dimethoxyphenyl)-10,15,20- tris(terf-butyl)porphyrinogen] can not be oxidized to the porphyrin while the m ore hindered [5,10,15,20-tetrakis(ferr-butyl)porphyrinogen] can, might be found in the specific configuration of the interm ediates. Possibly, the first oxidation step

S t. R u nge-M . O. Senge • Electron Donor-Acceptor Compounds 1025

le ad in g to 4 and involving the dimethoxyphenyl- ■quadrant results in the form ation of a porpho- zmethene with a configuration m ore difficult to oxi- udize than those formed during the synthesis of :5,10,15,20-tetrakis(terf-butyl)porphyrin. Recently, ■we had similar experiences during the synthesis of «extremely ruffled dodecaalkylporphyrins where "we found indications that the oxidizability of por- ^phodim ethenes strongly depends on their configu­ra tio n [2 1 ].

O ur presen t results indicate that it is possible to synthesize m eso-substituted porphyrin quinones bearing sterically undem anding alkyl residues via m odification of existing methodologies. N everthe­less, cross condensation m ethods fail for the p rep­aration of highly ruffled porphyrin quinones. Such com pounds will have to be prepared via either bi- ladiene-fl,c cyclization reactions, M acDonald con­densations or preferably, via modification of p o r­phyrins [21]. Such approaches are currently tested in our laboratory.

ExperimentalG eneral

All chem icals used were of analytical grade and were purchased from Aldrich Co. unless stated otherwise. - M elting points were m easured on a Biichi m elting point apparatus and are uncor­rected. - Silica gel 60 (M erck) was used for col­um n chrom atography. Analytical thin-layer chro­m atography (TLC ) was carried out using M erck silica gel 60 plates (precoated sheets, 0 . 2 mm thick, fluorescence indicator F254). - 1 H-NM R spectra w ere recorded at frequencies of 250 MHz (AC 250) or 500 M H z (Bruker, A M X 500). All chem i­cal shifts are given in ppm, referenced on the d scale dow nfield from the TMS signal used as in­ternal. standard. - Electronic absorption spectra w ere recorded with a Specord S10 (Carl Zeiss) spectropho tom eter using dichlorom ethane as solvent. - M ass spectra were recorded with a Var- ian M AT 711 mass spectrom eter using E l tech­nique with a d irect insertion probe and an excita­tion energy of 80 eV. - E lem ental Analyses were perform ed with a Perkin-E lm er 240 Analyzer.

Synthesis o f 5 ,10,15-Trialky 1-20- (2 ,5-dim ethoxyphenyl)porphyrins

M ethod A: Pyrrole (0.04 m ol), aldehyde (0.03 m ol), and 2,5-dim ethoxybenzaldehyde (0 .0 1 mol) were dissolved under argon in 2 0 0 ml degassed

and dried m ethylene chloride. B oron trifluoride d iethyletherate (0.5 ml) was added dropwise and the m ixture stirred for 12 h. For oxidation 0.16 mol D D Q was added followed by stirring for 1 h. The reaction m ixture was filtered through alumina (neutral, B rockm ann grade III) and chrom ato­graphed on silica gel, eluting with C H 2 Cl2/n-hex- ane (1:2, v/v). In o rder of elution the following four fractions were obtained: 5,10,15,20-tetraal- kylporphyrin, 5-(2,5-dim ethoxyphenyl)-10,15,20- trialkylporphyrin, a m ixture of 5,10- and 5,15- bis(2,5-dim ethoxyphenyl)-dialkylporphyrin, and5.10.15-tris(2,5-dimethoxyphenyl)-20-alkylpor- phyrin. The second fraction, containing the desired product, was further purified by recrystallization.

M ethod B: Pyrrole (0.2 mol), aldehyde (0.15 mol), and 2,5-dim ethoxybenzaldehyde (0.05 mol) were dissolved in 750 ml refluxing propionic acid and heated for 45 min under reflux while bubbling air through the solution. The reaction m ixture was concentrated and the residue dissolved in m ethy­lene chloride. A fter neutralization with saturated sodium carbonate solution, washing with water, and drying over sodium sulfate the product was purified as described for m ethod A.

5.10.15- Tributyl-20- (2,5- d im eth oxyphenyl)porphyrin ( la )

Yield: m ethod A: 0.26 g (0.43 mm ol, 3% ); pu r­ple crystals from C H 2C12/az-hexane; m. p. 165-170 C °C. UV/vis (C H 2C12): 2max (lg e) = 230 nm (3.85), 303 (3.95), 354 (4.11), 418 (5.47), 517 (4.02), 552 (3.67), 597 (3.15), 654 (3.61). ]H NM R (250 MHz, CDC13, TMS): ö = -2.61 (s, 2H, N H ), 1 .10-1.19 (q, J = 7.4 Hz, 9H, C H 2 C H 2 C H 2C //3), 1 ,74-1.92 (hept, J = 7.2 Hz, 6 H, C H 2 C H 2C // 2C H 3),2.44-2 .60 (hept, J = 7.9 Hz, 6 H, C H 2C // 2 C H 2 C H 3), 3.50 (s, 3H, O C //3), 3.91 (s, 3H, O C H 3), 4 .90-5 .02 (quin, J = I A Hz, 6 H, C // 2C H 2 C H 2 C H 3), 7.26 (s, 1H, H „henvl), 7.28-7.29 (d, J = 3.0 Hz, 1H, H phenyl), 7 .58-7 .59 (d, J = 2.2 Hz, 1H, Hphenyi), 8 .79-8.81 (d, J = 4.4 Hz, 2H, ß- H), 9 ,35-9,37 (d, J = 5.1 Hz, 2H, ß-H ), 9.47-9.53 (q, J = 4.9 Hz, 4H, ß-H ). MS (40 eV ), m /z (% ): 614 (100) [M+], 571 (6 8 ) [M+ - C H 3/CO], 528 (7) [M+ - C H 3/CO], 499 (5) [M+ - C 2 H 5],

C4oH46N 40 2Calcd 614.3710 Found 614.3635 (H RM S)

C4oH46N 40 2 (614.8)Calcd C 78.14 H 7.54 N 9 . l l 0 5.20%, Found C 78.04 H 7.42 N 8.98 0 5.57%.

1026 St. R unge-M . O. Senge • Electron Donor-Acceptor C om pounde

5-(2,5-D im ethoxyphenyl)-10,15,20-tris(2-m ethyl- propyl)porph yrin ( lb )

Yield: m ethod A: 0.26 g (0.429 mmol, 3% ); m ethod B: 1.43 g (2.3 mmol, 5% ); purple crystals from C H 2Cl2/n-hexane; m. p. 185-189 °C. - UV/ vis (C H 2 C12): Amax (lg e) = 230 nm (4.33), 301 (4.18), 367(4.26), 418 (5.36), 516 (3.97), 549 (3.47), 593 (3.36), 650 (3.49). - !H N M R (250 MHz, CDC13, TMS): (5 = -2.62 (s, 2H, N H ), 1 .17-1.19 (d, J = 5.2 Hz, 12H, C H 2 C H (C //3)2), 1.20-1.22 (d, J =6.0 Hz, 6 H, C H (C //3)2), 2 .69-2 .84 (sept, J = 6.0 Hz, 3H, C H 2C //(C H 3)2), 3.49 (s, 3H, O C //3), 3.91 (s, 3H, O C //3), 4 .81-4 .84 (d, J = 7.0 Hz, 4H, C H 2C H (C H 3)2), 4 .88-4.91 (d, J = 7.0 Hz, 2H, C / /2 C H (C H 3)2), 7.26 (s, 1H, H phenyl), 7.28-7.29 (d, J = 2.3 Hz, 1H, H phenyl), 7 .58-7 .59 (d, J = 2.3 Hz, 1H, H phenyl), 8 .79-8.81 (d, J = 4.6 Hz, 2H, H^. pyrrole), 9.35-9.37 (d, J = 4.2 Hz, 2H, Hß.pyrroje),9 .47-9.53 (q, J = 5.1 Hz, 4H, H ^ y ^ e ) . - MS (40 eV), m /z (% ): 614 (16) [M+], 571 (23) [M+-c2H3o].C4oH46N 40 2

Calcd 614.3710 Found 614.3636 (H RM S)

C40H 46N 4 O 2 (614.8)Calcd C 78.14 H 7.54 N 9 . l l 0 5.20%, Found C 78.50 H 7.52 N 9.16 0 4.82%.

5-(2,5-D im ethoxypheny l)-10,15,20- tri(isopropyl)porph yrin ( lc )

Yield: m ethod A: 0.164 g (0.29 mmol, 2% ); p u r­ple crystals from C H 2Cl2/«-hexane; m. p. 280 °C. - UV/vis (C H 2C12): Amax (lg e) = 230 nm (3.96), 302 (3.99), 354 (4.11), 419 (5.38), 519 (3.84), 556 (2.87), 598 (2.70), 654 (2.86). - !H N M R (250 MHz, CDC13, TMS): <3 = -2.19 (s, 2H, N H ), 2 .34-2.38 (d, J = 7.0 Hz, 6 H, C H (C //3)2), 2 .35-2 .38 (d, J = 7.0 Hz, 12H, C H (C //3)2), 3.47 (s, 3H, O C //3), 3.89 (s, 3H, O C H 3), 5 .43-5.62 (m, 3H, C H (C H 3)2), 7.23 (s, 1H, H phenyl), 7 .26-7 .27 ( d , / = 2.5 Hz, 1H, H phe. nyl), 7 .54-7.56 (d, J = 2.5 Hz, 1H, H phenyl), 8 .71- 8.73 (d, J = 5.1 Hz, 2H, H ^ pym>le), 9 .41-9.43 (d, J = 5.01 Hz, 2H, H^.pyrro,e), 9 .54-9 .56 (d, J = 5.01 Hz, 2H, Hß_pyrrole), 9.59-9.61 (d, J = 5.01 Hz, 2H, H ^ pyrrole). - MS (40 eV ), m /z (% ): 572 (100) [M+], 557 (73) [M+ - C H 3], 499 (5) [M+ - C3H 80 ] .

c 3 7h 4 0n 4o 2

Calcd 572.3241 Found 572.3157 (H RM S)

C3 7H 4 0N 4O 2 (572.7)Calcd C 77.59 H 7.03 N 9.78 0 5.58%, Found C 77.21 H 7. N 9.61 0 6.18%.

5-(2 ,5-D im ethoxyphenyl)-l 0,15,20-tris( 1 -ethyl- propyl)porph yrin (Id)

Yield: m ethod A: 0.281 g (0.43 mmol, 3% ); pu r­ple crystals from CH 2Cl2//7-hexane: m. p. 290 °C. - UV/vis (C H 2C12): Amax (lg e) = 230 nm (4.43), 302: (4.31), 366 (4.44), 419 (5.53), 518 (4.15), 552 (3 .85). 597 (3.76), 655 (3.84). - 'H NMR (250 M H z . CDC13, TMS): (3 = -2.43 (s, 2H, N H), 0 .93-0 .98 ( t , J = 7.5 Hz, 12H, C H (C H 2C //3)2), 0 .99-1 .03(t, J =7.5 Hz, 6 H, C H (C H 2C //3)2), 2.67-3.01 (m, 12H,„ C H (C //,C H 3)2), 3.50 (s, 3H, O C //3), 3.90 (s, 3H,„ O C //3), 4.83-5.05 (m, 3H, C //(C H 2C H 3)2), 7.25 (s, 1H, H phenyI), 7.28-7.29 (d, J = 2.5 Hz, 1H, H phe. nyl), 7 .56-7.57 (d, J = 3.4 Hz, 1H, H phenyl), 8 .75 - 8.77 (d, J = 4.2 Hz, 2H, H ^ ^ e ) , 9 .45-9 .47 (d,. J = 4.3 Hz, 2H, % pyrrole), 9.57-9.59 (d, J = 4.3 Hz, 2H, H ^ pyrrole), 9/62-9.64 (d, J = 5.2 Hz, 2H, Hß-pyrroie)- - MS (40 eV), m /z (%): 656 (100) [M+], 627 (97) [M+ - CHO], 585 (14) [M+ - C3H 4 0 ] .

c 43h 52n 4 o 2

Calcd 656.4179Found 656.4092 (H RM S)

C43H 52N 4 0 2 (656.9)Calcd C 78.62 H 7.97 N 8.52 0 4.87%,Found C 78.58 H 8.15 N 8.1 0 5.17%.

Synthesis o f 5 -(l,4 -ben zoqu in on e-2 -y l)-l0,15,20- trialky Iporphyrins

The respective porphyrin (0.4 mmol) was dis­solved in 100 ml m ethylene chloride. The solution was cooled to -50 °C and 2.5 ml BBr3 w ere added dropwise under argon. The green solution was slowly w arm ed to room tem perature and stirred for 10 h. A fter cooling to 0 °C the m ixture was neutralized with saturated aq. solution of sodium hydrogencarbonate. The organic phase was diluted with m ethylene chloride to 250 ml, washed twice with 1 0 0 ml water, and dried over sodium sulfate. The hydroquinol form ed was im m ediately oxi­dized by addition of 1 g P b 0 2 and purified by col­umn chrom atography on silica gel eluting with neat C H 2C12. The main fraction was concentrated and further purified by recrystallization.

5 -( l ,4-B enzoquinone-2-yl)-10,l 5,20- tributylporphyrin (2 a)

Yield: 0.114 g (0. 195 mmol, 60%); purple crys­tals from C H 2Cl2/n-hexane; m. p. > 300 °C. - UV/ vis (C H 2 C12): Amax (lg e) = 248 nm (4.62), 304 (4.33), 368 (4.54), 415 (5.66), 516 (4.32), 547 (3.95), 596 (3.87), 656 (3.81). - 'H NM R (250 MHz, CDC13, TMS): d = -2.63 (s, 2H, N H ), 1 .08-1.17 (q. J = 7.1 Hz, 9H, C H ,C H 2CH 2C //3), 1.71-1.87

St. R unge-M . O. Senge • Electron Donor-Acceptor Compounds 1027

(m, 6 H, C H 2C H 2C //2 CH 3), 2.40-2.57 (m, 6 H, C H 2 C //2 C H 2 C H 3), 4.86-4.95 (m, 6 H, C / /2 C H 2 C H 2 C H 3), 7.26 (s, 1H, H Dhenvl), 7.26-7.27 (d. J = 1.7 Hz, 1H, H phenyl), 7 .53-7 .54 (d, J = 1.9 Hz, 1H, H phenyl), 8.88-8.91 (d, J = 5.0 Hz, 2H, H^. p y rro le ) , 9.41-9.43 (d, J = 5.0 Hz, 2H, H^.pyrroie), 9.45 (s, 2H, Hß_pyrrole), 9.48-9.50 (d, J = 5.0 Hz, 2H, H^.pyrrole). - MS (40 eV), m /z (% ): 585 (100) [M+ + 2H], 543 (21) [(M+ + 2H) - C3H 7], 533 (21) [(M + + 2H ) - Q H /C O ],

C 3 8 H 4 oN 4 0 2

Calcd 586.3337 Found 586.3308 (HRM S)

C 38H 40N 4O 2 (584.8)Calcd C 78.05 H 6.89 N 9.58 0 5.47%, Found C 77.74 H 6.93 N 9.09 0 6.24%.

5-(l,4-B enzoquinone2-yl)-10,15,20-tris(2- m ethylpropyl)porphyrin (2 b)

Yield: 1.105 g (1.89 mmol, 65%); purple crystals from C H 2 Cl2 /«-hexane; m. p. > 300 °C. - UV/vis (C H 2C12): Amax (lg e) = 249 nm (4.64), 305 (4.41),367 (4.60), 399 (5.16), 416 (5.74), 516 (4.28), 547(3.59), 594 (3.44), 655 (3.42). - 'H NM R (250 M Hz, CDC13, TMS): ö = -2.62 (s, 2H, N H ), 1 .15- 1.17 (d, J = 4.3 Hz, 12H, C H 2 C H (C //3)2), 1.18-1.20 (d, J = 4.3 Hz, 6 H, C H 2 C H (C //3)2), 2.64-2.81 (sept., J = 7.0 Hz, 3H, CH 2 C //(C H 3)2), 4.78-4.81 (d, J = 7.0 Hz, 4H, C / /2C H (C H 3)2), 4.85-4.87 (d, / = 7.0 Hz, 2H, C / /2C H (C H 3)2), 7.25 (s, 1H, H phe_ nyl), 7 .26-7 .27 (d, J = 1.8 Hz, 1H, H phenyl), 7 .53 - 7.54 (d, / = 1.8 Hz, 1H, H phenyl), 8.90-8.92 (d, J =5.1 Hz, 2H , H^.pyrro.e), 9.41-9.43 (d, J = 5.3 Hz, 2H, H ^ pyrrole), 9.44-9.46 (d, J = 5.3 Hz, 2H, H/3. p y r r o le ) , 9.49-9.51 (d, J = 5.3 Hz, 2H, H^pyrrote). - MS (40 eV ), m /z (% ): 584 (8 8 ) [M+ + 2H], 543 (100) [(M + + 2H) - C3H 7], 499 (9) [(M+ + 2H) - 2x C3H 7], 457 (26) [(M+ + 2H) - 3x C3H 7],

c38H40N4o2Calcd 586.3307 Found 586.3327 (HRM S)

C38H 40N 4 O 2 (584.8)Calcd C 78.05 H 6.89 N 9.58 0 5.47%, Found C 77.66 H 6.92 N 9.18 0 6.24%.

5-(l,4 -B en zoqu in on e-2 -yl)-l 0,15,20- tri(isopropyl)porph yrin (2 c)

Yield: 0.12 g (0. 214 mmol, 61%); purple crystals from C H 2 Cl2/«-hexane; m. p. > 300 °C. - UV/vis (C H 2 C12): 2max (lg e) = 248 nm (4.45), 306 (4.19), 371 (4.36), 400 (4.90), 416 (5.73), 516 (4.14), 547 (3.72), 597 (3.68), 659 (3.54). - lH NM R (250

MHz, CDC13, TMS): ö = -2.19 (s, 2H, N H ), 2 .33- 2.36 (d, J = 7.8 Hz, 12H, C H (C //3)2), 2 .36-2.39 (d, J = 7.8 Hz, 6 H, C H (C //3)2), 5 .39-5.49 (m, 3H, C //(C H 3)2), 7 .22-7 .24 (d, J = 2.0 Hz, 2H, H p h e n y i) ,

7.49-7.50 (d, J = 2.0 Hz, 1 H , H phenyl), 8.81-8.83 (d, J = 4.1 Hz, 2H, H^.pyrrole), 9 .48-9.50 (d, J = 5.1 Hz, 2H, Hy3_pyrrole), 9 .52-9 .54 (d, J = 5.1 Hz, 2H, H^py^oie), 9 .58-9.60 (d, J = 5.1 Hz, 2H, H^.pyr. role). - MS (40 eV), m /z (% ): 544 (100) [M+ + 2H], 529 (40) [(M + + 2H) - C H 3],

C35H 34N 4 0 2

Calcd 544.2878 Found 544.2838 (H RM S)

C35H 34N 40 2o H ?0 (560.7)Calcd C 74.97 H 6 . l l N 9.99 0 5.89%, Found C 74.99 H 6.22 N 9.65 0 5.14%.

5-( 1,4-B en zoqu in on e-2-yl)-10 ,l 5,20-tris (1 -ethyl- p ropyl)porph yrin (2 d)

Yield: 0.18 g (0.287 mol, 65% ); purple crystals from C H 2Cl2 /«-hexane; m. p. > 300 °C. - UV/vis (C H 2 C12): Amax (lg e) = 248 nm (4.40), 306 (4.13),368 (4.32), 417 (5.42), 516 (4.06), 555 (3.61), 594(3.59), 659 (3.46). - 'H N M R (250 MHz, CDC13, TMS): <3 = -2.41 (s, 2H, N //) , 0 .90-0 .97 (t, J = 7.7 Hz, 12H, C H (C H 2C H 3)2), 0 .95-1.01 ( t, / = 7.7 Hz, 6 H, C H (C H 2 C //3)2), 2 .68-2 .93 (m, 12H, C H (C //2 C H 3)2), 4 .84-5.05 (m, 3H, C //(C H 2C H 3)2), 7.23 (s, 1H, H phenv,), 7 .25-7.26 (d, > = 2.5 H z,1H . Hphenyi), 7.50-7.51 (d, J = 2.5 Hz, 1H, H phenyl), 8 .86-8 .89 (d, J = 5.2 Hz, 2H, H^. p y rro le ), 9.51-9.53 (d, J = 5.2 Hz, 2H, Hß_pyrrole),9 .54-9.56 (d, J = 5.2 Hz, 2H, H ^ p y ^ e ) , 9.61-9.63 (d, J = 6.1 Hz, 2H, H^.pyrrole). - MS (40 eV); m /z (% ): 628 (100) [M+ + 2H], 599 (42) [(M+ + 2H) - C2H 5], 558 (22) [(M + + 2H) - C 2H 5 - C3H 5], 529 (11) [(M + + 2H) - C5H 9 0 2],

c 41h 46n 4o 2

Calcd 628.3777 Found 628.3789 (H RM S)

C4 1H 46N 4 0 2 (626.8)Calcd C 78.56 H 7.39 N 8.94 0 5.10%, Found C 78.56 H 7.30 N 8 . 6 8 0 5.46%.

Synthesis o f {5-(l,4 -ben zoqu in on e-2-y l)-10 ,15,20- tria lkylporphyrinato)zinc(II)

A pproxim ately 0.085 mmol of the porphyrin quinone was dissolved in 50 ml m ethylene chlo­ride. Zinc oxide (250 mg) and three drops of triflu- oroacetic acid were added under stirring. A fter 1 0 m inutes the color changed from green to violet

1028 St. R unge-M . O. Senge ■ Electron Donor-Acceptor Compounds

and the reaction m ixture was filtered through sil­ica gel, followed by recrystallization.

{5-(l,4 -B en zoqu inon e-2-yl)-10 ,15,20- tribu tylporph yrinato jzin c(U ) (3a)

Yield: 0.05 g (0. 071 mmol, 85% ); red crystals from C H 2Cl2//?-hexane; m. p. > 300 °C. - UV/vis (C H 2 C12): 2max (lg e ) = 249 nm (4.58), 307 (4.35), 350 (4.22), 399 (4.81), 418 (5.69), 549 (4.23). - !H N M R (250 M Hz, CDC13, TMS): (3 = 1.02-1.08 (t, J = 7.7 Hz, 3H, C H 2C H 2 C H 2 C //3), 1.07-1.13 (t, J = 7.7 Hz, 6 H, C H 2C H 2 C H 2 C //3), 1.62-1.80 (sext., J = 7.5 Hz, 2H, C H 2 C H 2 C //2 C H 3), 1 .64- 1.84 (sext., J = 7.5 Hz, 4H, C H 2 C H 2C // 2C H 3), 2 .14-2.27 (quin., J = 7.5 Hz, 2H, C H 2 C //2 C H 2 C H 3), 2 .29-2.42 (quin., J = 7.5 Hz, 4H, C H 2C //2C H 2C H 3), 4 .26-4 .33 (t, J = 7.5 Hz, 2H, C / / 2C H 2 CH 2 C H 3), 4.48-4.71 8 (m, 4H, C //2 CH 2 C H 2 C H 3), 7.27 (s, 1H, H phenyl), 7.28-7.29 (d, J = 1.7 Hz, 1H, H phenyl), 7 .53-7 .54 (d, J = 1.7 Hz, 1H, H phenyl), 8 .92-8.94 (d, J = 4.3 Hz, 2H, H ^ p y rro le ), 8.93-8.95 (d, J = 4.3 Hz, 2H, H ^ pyrrole), 8 .98-9.00 (d, J = 4.3 Hz, 2H, H ^ pyrrole), 9.37-9.39 (d, J = 5.1 Hz, 2H, H/J.pyrro.e), - MS (40 eV), m /z (% ): 648 (100) [M+], 605 (73) [M+ - C 3H 7], 562(14) [M+ - 2x C3H 7], 519 (13) [M+ - 3x C3H 7],

c 38h 38n 4 o 2

Calcd 648.2443 Found 648.2452 (H RM S)

C38H 38N 4 0 2Z noH 20 (702.2)Calcd C 65.00 H 5.45 N 7 .0% ,Found C 65.26 H 5.59 N 7.52% .

{5-(l,4-B enzoqu inone-2-yl)-10,15,20-tris(2-m ethyl- p ro p y l)p o rp h yrin a to jz in c(ll) (3b)

Yield: 0.046 g (0.071 mmol, 85% ); red crystals from C H 2Cl2/n-hexane; m. p. > 300 °C. - UV/vis (C H 2C12): ^ max (lg e) = 249 nm (4.58), 306 (4.30), 399 (4.80), 418 (5.74), 549 (4.18). - !H N M R (250 MHz, CDC13, TMS): (3 = 1.09-1.16 (m, 18H, CH 2C H (C //3)2), 2 .59-2.76 (sept., J = 7.0 Hz, 3H, CH 2C //(C H 3)2), 4 .53-4.56 (d, J = 6 . 8 Hz, 2H, C //2C H (C H 3)2), 4 .66-4.68 (d, J = 6 . 8 Hz, 2H, C H 2C H (C H 3)2), 4 .67-4.69 (d, J = 6 . 8 Hz, 2H, C //2C H (C H 3)2), 7.26 (br s, 2H, H phenyl), 7 .53-7.54 (d, J = 1.7 Hz, 1H, H phenyl), 8 .97-8.99 (d, J = 5.2 Hz, 2H, H ^ pyrrole), 9.25 (s, 4H, H ^ p y ,^ ) , 9 .47- 9.49 (d, J = 5.2 Hz, 2H, H ^ pyrrole). - MS (40 eV), m /z (% ): 648 (11) [M+], 605 (24) [M+ - C3H 7], 562 (8 ) [M+ - 2x C3 H 7], 519 (13) [M+ - 3x C 3H 7].

C38H 38N 4 0 2ZnCalcd 648.2443 Found 648.2465 (H RM S)

C 38H 38N 4 0 2Zn (648.1)Calcd C 70.42 H 5.91 N 8.64%,Found C 70.34 H 5.83 N 8.31% .

{5-(l,4-B enzoquinone-2-yl)-10,15,20- tri(isopropyl)porph yrinato}zin c(U ) (3c)

Yield: 0.047 g (0.0738 mmol, 83%); purple crys­tals from C H 2Cl2/«-hexane; m. p. >300 °C. - U V / vis (C H 2 C12): Amax (lg e) - 250 nm (4.44), 308 (4.21), 351 (404), 400 (4.68), 419 (5.77), 550 (4.14). - ]H NM R (250 MHz, CDC13, TMS): 6 = 2 .42-2.45 (d, J = 6.9 Hz, 12H, C H (C //3)2), 2 .46 - 2.48 (d, J = 6.9 Hz, 6 H, C H (C //3)2), 5 .61-5.72 (sept., J = 6.7 Hz, 3H, C //(C H 3)2), 7 .14-7.16 (d, J = 1.7 Hz, 1H, H phenyl), 7 .18-7.20 (d, J = 1.7 Hz, 1H, H phenyl), 7.38-7.39 (d, J = 2.5 Hz, 1H, H phenyl), 8 .94-8.96 (d, J = 5.1 Hz, 2H, H ^ pyrrole), 9 .70-9.72 (d, J = 5.2 Hz, 2H, H^_pyrrole), 9.74-9.76 (d, J = 5.2 Hz, 2H, H ^ p y ^ e ) , 9.77-9.79 (d, J = 5.2 Hz, 2H, FI/3-pyrrole ). - 'MS (40 eV), m /z (% ): 606 (97) [M+], 591 (37) [M+-CH 3],

c 35H 32n 4 o 2

Calcd 606.1959 Found 606.1973 (HRM S)

C35H 32N 4 0 2Zn (606.1)Calcd C 69.36 H 5.32 N 9.24%,Found C 68.39 H 5.46 N 8.91% .

{5-(l,4 -B en zoqu inon e-2-yl)-10,15,20-tris(l-eth yl- propyl)porphyrin ato}zin c(II) (3d)

Yield: 0.046 g (0.0667 mmol, 83%); red crystals from CH 2 Cl2/«-hexane; m. p. > 300 °C. - UV/vis (C H 2C12): Amax (lg e ) = 253 nm (3.94), 311 (3.78), 419 (5.28), 552 (3.82). - lH NM R (250 M Hz, CDC13, TMS): (3 = 0.91-0.97 (t, J = 7.6 Hz, 18H, C H (C H 2 C //3)2), 2.71-3.03 (m, 12H, C H (C //2 C H 3)2), 4.98-5.15 (m, 3H, C //(C H 2 C H 3)2), 7.22 (s, 2H, H phenyl), 7.49 (br s, 1H, H phenyl) 8 .96-8.98 (d, J JT o H z, 2H, pyrrole), 9.74 (br s, 6 H, H^.pyr^e). - MS (40 eV), m /z (% ): 606 (97) [M+], 591 (37) [M+-CH 3],

C41H 44N 40 2ZnCalcd 690.2946 Found 690.2912 (HRM S)

C41H 44N 40 2Zn (690.2)Calcd C 71.35 H 6.42 N 8.12% ,Found C 71.95 H 6.51 N 7.98% .

Synthesis o f 5,10,15,22-Tetrahydro-20-(2,5-dim eth- oxyphenyl)-5,10,15-tris(tert-butyl)porphyrin (4)

2-Ethyl-butyraldehyde (0.03 mol), pyrrole and 2,5-dimethoxybenzaldehyde were reacted as de-

St. R unge-M . O. Senge • Electron Donor-Acceptor Compounds 1029

scribed above for m ethod A. A fter filtration of the reaction m ixture through neutral alumina, fol­lowed by column chrom atography on silica gel (C H 2Cl2//i-hexane, 1:2, v/v) a red solution was ob­tained as main fraction. Yield: 0.08 g (0.13 mmol, 1%); yellow crystals from C H 2 Cl2/«-hexane; m. p. 230-235 °C. - UV/vis (C H 2 C12): Amax (lg e) = 232 nm (3.49), 291 (3.10), 454 (3.52). - ’H NM R (250 MHz, CDC13, TMS): Ö = 0 .96-0.97 (d, J = 1.7 Hz, 9H, C (C //3)3), 1.09-1.10 (d, J = 2.5 Hz, 18H, C(C H 3)3), 3.56 (s, 3H, OC//3), 3.67 (s, 1H, C m-H ), 3.69 (s, 1H, C m-H ), 3.77 (s, 1H, C m-H ), 3.88 (s, 3H, OC//3 ), 5.89-5.90 (d, J = 2.6 Hz, 4H, H ^ p y ^ ) ,6.09-6.11 (t, J = 3.2 Hz, 2H, H^.pyrrole), 6 .26-6.28 (d, J = 3.2 Hz, 2H, H /3.pyrrole), 6.85 (s, 1H, H phenyI), 6 .87-6.89 (t, J = 1.7 Hz, 2H , H phenyl), 8.69 (br s, 1H, NH) , 8.75 (br. s, 2H, NH) . - MS (40 eV); m / z (% ): 618 (5) [M+], 561 (74) [M+ - C4H 9],

c 41h 46n 4o 2

Calcd 618.3934Found 618.3946 (H R M S)

C41H 46N 40 2 (626.8)Calcd C 77.63 H 8.14 N 9.05 0 5.17%,Found C 77.13 H 8.41 N 9.01 0 5.45%.

C rystallography

X-ray quality crystals were grown by liquid dif­fusion. The crystals were rem oved from solution and covered with a layer of P araton N®. A suitable crystal was selected, attached to a glass fiber and im m ediately placed into the low -tem perature ni­trogen stream as described by H ope [22]. Cell param eters were determ ined from 2 5 -3 0 au to ­matically centered reflections in the range 2 0 ° < 6< 25°. D uring the data collections two standard reflections were m easured every 198 reflections and showed only statistical variation of the in ten­sities (<1% ). The intensities were corrected for Lorentz and polarization effects. For both struc­tures an absorption correction was applied using the program XABS2 [23], while extinction effects were disregarded. The structure of 3c was solved via a Patterson synthesis followed by structure ex­pansion, while the structure of 4 was solved using D irect M ethods. For both structure solutions the program SHELXS-93 was used [24], Refinem ents were carried out by full-m atrix least-squares on IF2I using the program SHELXL-93 [25a] for 4 and SHELXL-97 [25b] for 3c. H ydrogen atoms were included at calculated o positions using o a riding m odel with C-H = 0.96 A and N -H = 0.9 A. Details for the crystal data, data collection and refinem ent are given in Table I. For the structure of 4 all non-

Table I. Summary of crystal data, data collection and re­finement for the crystal structure determinations.

Compound 3c 4Chemical formula [C35H32N40 2Zn],, C4()H5f)N40 2Mol. wt. 606.02 618.84Color blue pale yellowHabit plate parallelepipedCrystal size (mm) 0.34 x 0.26 x 0.05 0.25 x 0.05 x 0.05Space group P2, P2]/na (A) 11.359(10) 15.623(4)b (A) 10.998(8) 14.838(6)c (A) 12.60(2) 16.283(3)ß(°) 111.82(9) 109.32(2)V (A ) 1462(3) 3562(2)Z 2 4dcaic (Mg m~3) H (mm ')

1.377 1.1541.472 0.553

Tmax, Tmin 0.93, 0.64 0.97, 0.87Radiation CuKa CuKaA (A) 1.54178 1.54178T (K) 130(2) 130(2)Diffractometer Syntex P2[ Syntex P2,nu max 57.12 57.03Octants collected ±h,+k.+l ±h,+k,+lCollec. reflections 2225 5238Indep. reflections 2101 4797Reflections with 1638 2877

F > 4.0 ct(F)flint 0.0880 0.0699

y 0.1310, 6.9324 0.1537, 0.1052No. of parameters 196 417dlomax 0.008 0.001d lQ m a x (£ Ä-3) 1.176 0.306fl 1 [F > 4.0 a(F)] 0.0974 0.0957w R2 [F > 4.0 a(F)] 0.2318 0.2334fl 1 (all data) a 0.1285 0.1628w R2 (all data) 0.2567 0.3081S 1.105 1.056

a f l l = ZIIF0-FCII/2F0I, wR2 = [2 (w (F 02-F c2)2) /2 [ w (F 02) ]1/2;w 1 = c r (F 02) + (xP)~ + yP.; w h ere P -: (F 02 + 2F c2)/3.

hydrogen atoms were refined with anisotropic therm al param eters. The crystals of 3c were of low quality with fairly broad reflections profiles. D es­pite repeated attem pts to grow better crystals and several attem pted data collections the present structure is the result of the best crystals found. Except for the zinc, nitrogen and benzoquinone carbon and oxygen atom s all nonhydrogen atoms were refined with isotropic therm al param eters. The m eso-carbon atom s were refined with a com ­m on isotropic therm al param eter. The residual electron density is located near the zinc center. Com plete details on the crystal structure investi­gations, including atom ic coordinates, therm al param eters and com plete bond lengths and angles have been deposited at the Cam bridge Crystallo­graphic D ata C entre (CCD C, 12 U nion Road, Cambridge, CB2 1EZ, U K). Copies can be ob­tained on request by quoting the publication cita­tion and the deposition num bers CSD-102147 for 3c and CSD-102148 for 4.

1030 St. R unge-M . O. Senge • Electron Donor-Acceptor Compounds

A ckn owledgem ents

This work was generously funded by the D eutsche Forschungsgem einschaft (Se543/2-4 and Heisenberg-Scholarship S e5 4 3 /3 -l) and the

Fonds der Chemischen Industrie and enjoyed the continuing support of Prof. H. Kurreck. We grate­fully acknowledge the cooperation of the UC Davis crystallographic facility (M. M. O lm stead, director).

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