Synthesis and structural characterization of porphyrinic enediynes: Geometric and electronic effects on thermal and photochemical reactivity

Inorg. Chem. 2003, 42, 5158−5172

Synthesis and Structural Characterization of Porphyrinic Enediynes:

Geometric and Electronic Effects on Thermal and Photochemical

Reactivity

Tilak Chandra, Brian J. Kraft, John C. Huffman, and Jeffrey M. Zaleski*

Department of Chemistry and Molecular Structure Center, Indiana UniVersity,

Bloomington, Indiana 47405-7102

Received January 28, 2003

We report the preparation of [5,10,15,20-tetraphenyl-2,3,7,8,12,13,17,18-octakis(phenylethynyl)porphinato] complexes

of Ni(II), H , Zn(II), Mg(II), and Cu(II), as well as select trimethylsilanylethynyl derivatives. The X-ray structures of

2

the octakis(phenylethynyl) compounds show systematic deviations from planarity (Ni(II), 0.2851 Å; Zn(II), 0.0304

Å) as a function of the central cation. These geometric distortions are reflected in bathochromic shifts of the Soret

and Q bands (Ni(II), 497, 604, and 650 nm; Mg(II), 515, 595, 642, and 705 nm) which loosely correlate with

increasing planarity of the structure. Similarly, vibrational modes involving the octasubstituted porphyrin core exhibit

shifts to lower frequency as a function of increasing planarity in the solution-state resonance Raman spectra (λ_exc

) 501.7 nm) of these compounds. Analogous trends are also observed in their solid-state electronic and resonance

Raman spectra, indicating that the structural distortions within the octakis(phenylethynyl) series are preserved in

solution. Comparison of the saddle distortion of the octasubstituted Ni(II) compound with the ruffle/saddle distortions

of the pentakis and hexakis Ni(II) derivatives reveals some influence of asymmetric peripheryl substitution on geometric

structure. These Ni(II) derivatives also exhibit systematic red shifts in their electronic spectra as a function of the

number of conjugated alkyne units (∼13 nm/alkyne), revealing participation of the enediyne units in the electronic

ground and excited states. The solid-state Bergman cyclization temperatures of the phenylethynyl compounds vary

markedly as a function of planarity, and correlate loosely with alkyne termini separation (Ni(PA)₈, 4.00 Å, 281 °C;

MgP(PA)₈, 3.77 Å, 244 °C). In solution, both thermal and photochemical activation of the free-base octakis-

(phenylethynyl) compound lead to formal reduction of the porphyrin backbone via H-atom addition at opposing

meso-positions. Generation of a common product suggests that both thermal and photochemical pathways to

Bergman cyclization in solution contain significant activation barriers to formation of the 1,4-phenyl diradical

intermediate, and under these solution conditions, alternate reaction channels are more thermodynamically favorable.

Introduction

tion, ultimately leading to H-atom abstraction from DNA

and cell death.²³The geometric factors, i.e., distance between

the alkyne termini,^7,10,24and strain in the ground and

Enediynes are intriguing chemical structures that can

undergo rearrangement to form a 1,4-phenyl diradical

intermediate upon thermal^1-12or photochemical^13-22activa-

(4) Golik, J.; Clardy, J.; Dubay, G.; Groenewold, G.; Kawaguchi, H.;

Konishi, M.; Krishnan, B.; Ohkuma, H.; Saitoh, K.-i.; Doyle, T. W.

J. Am. Chem. Soc. 1987, 109, 3461-3462.

* Author to whom correspondence should be addressed. E-mail:

zaleski@indiana.edu.

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5158 Inorganic Chemistry, Vol. 42, No. 17, 2003

Published on Web 07/24/2003

Synthesis and Structure of Porphyrinic Enediynes

transition states,^25-28as well as electronic contribution from

electron-donating or -withdrawing substitution at the ene^29-31

or yne³²positions of the 1,5-diyn-3-ene unit, can lead to

dramatic influences on thermal cyclization temperatures.

More recently, metal ions have also begun to play a key

role in influencing the geometric contributions to enediyne

cyclization by affecting the alkyne termini separation through

chelation^33-44or π-complexation.^45,46

Although a considerable amount is known about the

geometric and electronic factors that drive Bergman cycliza-

tion thermally, by comparison little is known about the origin

of photo-Bergman cyclization,⁴⁷and published examples are

generally restricted to high-energy UV excitation.^15-22This

is due to the ability of high-energy π-π* transition of the

enediyne unit to create a transient electronic structure that

produces a subtle geometric distortion in the excited state,

leading to the formation of the 1,4-phenyl diradical inter-

mediate for some structures. In this sense, the photocycliza-

tion mechanism can be considered as an excited-state thermal

reaction with a specific activation barrier to the formation

of chemical intermediates or products.⁴⁷

In light of the tissue transparency advantages of red and

near-infrared wavelengths, strongly absorbing chromophores

at low energies are highly desirable for in vivo biological

applications such as photodynamic therapy (PDT).^48-51Due

to this requirement, the use of simple enediyne structures in

these applications has not been widespread because of the

contradictory relationship between practical depth penetration

requirements and the need for UV excitation to initiate

Bergman cyclization.

The widespread use of porphyrins in PDT applications,

coupled with our interest in designing long-wavelength-

absorbing enediyne motifs, and the recent enediyne-generated

picenoporphyrin by Smith et al.,⁵²naturally led us to consider

the porphyrin skeleton as a chromophore. Recent interest in

alkynylated porphyrins for biological and materials applica-

tions has made incorporation of alkynes into such macro-

cycles synthetically accessible.^52-57Porphyrins have a rich

electronic structure typically exhibiting strong electronic

transitions throughout the visible spectral region (400-700

nm) with significant absorptivities (ꢀ > 10000 M^-1cm^-1).⁵⁸

In addition, these compounds also adopt distinctive saddled,

ruffled, waved, or domed geometric conformations as a

function of the central cation.^59-61The differing geometric

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Inorganic Chemistry, Vol. 42, No. 17, 2003 5159

Chandra et al.

conformations strongly influence the disposition of substit-

uents at the porphyrin periphery and, in turn, can be affected

by the steric bulk of the R groups.^62-64Remarkably, even

with the extensive study of porphyrin structures, controversy

still exists over how the electronic structure of the porphyrin

skeleton is influenced by these geometric distortions,^65-68

and whether the R groups play an active or passive role in

governing geometric and electronic structure.^65,69

trends that reflect the degree of planarity of the macrocycle.

Finally, solution thermal and photochemical reactivity of the

octasubstituted free-base derivative lead to the same 5,15-

dihydroporphyrin product, indicating that the thermal and

photochemical reaction pathways are connected to a common

product. These results reveal key criteria required for the

design of second-generation porphyrinic enediyne structures

with specific reactivities.

The intriguing geometric and electronic characteristics of

porphyrin macrocycles, and the potential for using these

structural properties to influence the reactivities of enediynes,

have led to several fundamental questions regarding the

development and potential application of porphyrinic ene-

diyne structures. Can more than one enediyne unit be

incorporated into the porphyrin backbone? What are the

geometric structures of porphyrinic enediyne compounds, and

how do the number and nature of the substituents affect the

structures? Conversely, what is the influence of the porphyrin

structure on the thermal reactivities of the enediyne unit?

Additionally, how are the electronic structures of these

compounds affected by conjugated peripheryl substitution?

Are these geometric and electronic characteristics maintained

in solution? Finally, does conjugation of the enediyne unit

into the large chromophore permit enediyne photocyclizaton,

or does it lead to delocalization of the reactive excited state

and alternate reactivity?

Experimental Section

Materials. All chemicals and solvents used were of the highest

purity available from Aldrich and Fluka. Reactions were carried

out under nitrogen using Schlenk techniques, and all air-sensitive

solids were handled in an inert-atmosphere drybox. Benzene,

methylene chloride, and tetrahydrofuran were dried and degassed

according to literature methods.⁷⁰The free-base and metalated

porphyrinic enediynes were purified by flash chromatography using

silica gel (200-440 mesh) or neutral alumina.

Physical Measurements. ¹H NMR and ¹³C NMR were recorded

on a VXR 400 NMR spectrometer using the residual proton

resonance of the solvent as an internal reference. The multiplicities

of the ¹³C NMR signals were determined by the DEPT technique.

Electronic absorption spectra were collected on a Perkin-Elmer

Lambda 19 UV/vis/near-IR spectrometer at ambient temperature.

Infrared spectra (KBr) were recorded on a Nicolet 510P FT IR

spectrometer. Elemental analyses on all samples were obtained from

Robertson Microlit Laboratories, Inc. Mass data (FD, MALDI, and

FAB) were obtained at the University of Illinois using a Micromass

Quattro-I mass spectrometer. Cyclic voltammetry measurements

were made using a BAS E2 Epsilon potentiostat/galvanostat running

at a scan rate of 200 mV/s. The electrochemical cell consisted of

a 0.5 M TBA(PF₆)/THF solution containing a Pt working electrode

and a Ag/AgCl reference, with ferrocene (Fc) added as an internal

standard. Oxidation potentials are irreversible and thus are reported

as peak potentials. Reduction potentials derive from reversible

waves at scan rates of 200 mV/s. Differential scanning calorimetry

(DSC) traces were recorded on a V4.1 Dupont 910 differential

scanning calorimeter coupled to a DuPont thermal analyst 2100 at

a heating rate of 10 °C min^-1. Photolyses were performed using a

1000 W XeHg lamp (Oriel no. 66021) at λ g 420 nm using a series

of long-pass cutoff filters (λ g 295, 345, 395, and 420 nm).

Resonance Raman spectra were collected using an Ar⁺ion laser

(Coherent model I-70) operating at 457.9, 488.0, or 501.7 nm (50

mW at the sample). The backscattered light was collected with a

Nikkor 85 mm 1:1.4 lens focused through a depolarizer (CVI) onto

the entrance slit of an f/4 subtractive double monochromator

equipped with 600 grooves/mm gratings (500 nm blaze) and a 6

mm band-pass slit (7 nm/mm). The output from the double

monochromator was focused onto the entrance slit of an Acton

Spectrapro 500i spectrograph (f/6.5) operating with a 1800 grooves/

mm (500 nm blaze) grating. The disperse scattering was then

recorded with a back-illuminated liquid N₂cooled CCD (Princeton

Instruments) with a 30 mm × 14.4 mm active area (2500 × 600

pixel array).

To this end, we have prepared a series of porphyrinic

enediyne structures with five, six, and eight alkyne substit-

uents, and several have been crystallographically character-

ized. The X-ray structures indicate that the identity of the

central cation plays a strong role in determining the degree

of ground-state geometric distortion in the resulting com-

pounds. However, more subtly, the specific nature of the

deviations from planarity is governed by the number of

substituents at the porphyrin periphery. These structural

differences influence the observed Bergman cyclization

temperatures of the compounds, which are shown to loosely

correlate with the degree of planarity of the macrocycle. In

addition, electronic and resonance Raman spectra show

systematic conjugation of the alkyne units into the porphyrin

π-π* transitions, indicating participation by the enediyne

unit in the overall electronic structure. Careful examination

of spectral shifts as a function of structure reveal systematic

(61) Shelnutt, J. A. In The Porphyrin Handbook; Smith, K. M., Guilard,

R., Eds.; Academic: New York, 2000; Vol. 7, pp 167-223.

(62) Alden, R. G.; Crawford, B. A.; Doolen, R.; Ondrais, M. R.; Shelnutt,

J. A. J. Am. Chem. Soc. 1989, 111, 2070-2072.

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J. A. J. Am. Chem. Soc. 1992, 114, 9859-9869.

(64) Jentzen, W.; Simpson, M. C.; Hobbs, J. D.; Song, X.; Ema, T.; Nelson,

N. Y.; Medforth, C. J.; Smith, K. M.; Veyrat, M.; Mazzanti, M.;

Ramasseul, R.; Marchon, J.-C.; Takeuchi, T.; Goddard, W. A., III;

Shelnutt, J. A. J. Am. Chem. Soc. 1995, 117, 11085-11097.

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Soc. 1995, 117, 8279-8280.

(2,3,7,8,12,13,17,18-Octabromo-5,10,15,20-tetraphenylporphi-

nato)nickel(II) (1Ni).⁷¹To a solution of 2,3,7,8,12,13,17,18-

octabromo-5,10,15,20-tetraphenylporphyrin⁷¹(0.420 g, 0.33 mmol)

in chloroform was added Ni(CH₃COO)₂‚4H₂O (0.084 g, 0.33 mmol)

(66) Parusel, A. B. J.; Wondimagegn, T.; Ghosh, A. J. Am. Chem. Soc.

2000, 122, 6371-6374.

(67) Wertsching, A. K.; Koch, A. S.; DiMagno, S. G. J. Am. Chem. Soc.

2001, 123, 3932-3939.

(68) Ryeng, H.; Ghosh, A. J. Am. Chem. Soc. 2002, 124, 8099-8103.

(69) Wasbotten, I. H.; Wondimagegn, T.; Ghosh, A. J. Am. Chem. Soc.

2002, 124, 8104-8116.

(70) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;

Timmers, F. J. Organometallics 1996, 15, 1518-1520.

(71) Bhyrappa, P.; Krishnan, V. Inorg. Chem. 1991, 30, 239-245.

5160 Inorganic Chemistry, Vol. 42, No. 17, 2003

Synthesis and Structure of Porphyrinic Enediynes

in methanol, and the mixture was stirred for 24 h. After complete

metalation, the solvent was removed under reduced pressure. The

crude solid was suspended in methanol (50 mL) and allowed to

stir for 1 h. The remaining solid was filtered, washed with methanol

and subsequently with a mixture of methylene chloride/methanol

(1:1), and dried under vacuum. Yield: 85%. Dark blue powder.

MS (FD): m/z 1302 (M⁺+ 1), 1224, 1145, 1065, 986, 908. Anal.

Calcd for C₄₄H₂₀Br₈N₄Ni‚3H₂O: C, 39.19; H, 1.94; N, 4.15.

Found: C, 38.79; H, 1.76; N, 3.77. IR (KBr, cm^-1): 2967, 1598,

1574, 1443, 1323, 1128, 1069, 1042, 1025, 927, 752, 736, 615.

[5,10,15,20-Tetraphenyl-2,3,8,17,18-pentakis(phenylethynyl)-

porphinato]nickel(II) (NiP(PA)₅, 2Ni). To a crude (multiply

brominated species) mixture of 1Ni (0.5 g, 0.384 mmol) in dry,

degassed THF (20 mL) was added [(C₆H₅)₃P]₄Pd (0.2 g, 0.173

mmol) under nitrogen, and the mixture was stirred for 15 min at

30 °C. A solution of trimethyl(phenylethynyl)tin (0.92 g, 3.4 mmol)

was prepared in degassed THF, and slowly added to the above

mixture at 50 °C. The resulting solution was then heated at 75 °C

for 6 h. Complete conversion of the starting material was confirmed

through a silica gel column and eluted with benzene/hexane (1:1)

to give the desired product in 65% yield. The crude solid was

recrystallized in chloroform/benzene (1:1) to give needle-shaped

green crystals. X-ray-quality crystals were grown from chloroform/

methanol (1:1). R_f0.5. ¹H NMR (CDCl₃): δ 7.23 (br s, 40H, Ar),

7.58 (t, J ) 7.2 Hz, 4H, meso-Ar), 7.69 (t, J ) 7.6 Hz, 8H, meso-

Ar), 8.19 (d, J ) 7.2 Hz, 8H, meso-Ar). ¹³C NMR (CDCl₃): δ

84.19 (Cquat), 105.43 (Cquat), 119.70 (Cquat), 123.53 (Cquat),

127.70 (CH), 127.84 (CH), 128.11 (CH), 129.92 (CH), 131.92 (CH),

132.07 (Cquat), 135.35 (CH), 137.80 (Cquat), 144.34 (Cquat). MS

(FD): m/z 1471 (M⁺+ 1), 1370, 1269, 1094, 925. Anal. Calcd for

C

₁₀₈H₆₀N₄Ni‚H₂O: C, 87.09; H, 4.16; N, 3.76. Found: C, 87.01,

H; 4.09, N; 3.74. IR (KBr, cm^-1): 3051, 2192, 1652, 1596, 1530,

1495, 1441, 1259, 1105, 1003, 752, 688, 667.

[5,10,15,20-Tetraphenyl-2,3,7,8,12,13,17,18-octakis(phenyl-

ethynyl)porphyrin] (H₂P(PA)₈, 4H₂). To a suspension of pure

2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetraphenylporphyrin⁷¹

(1H₂) (1.0 g, 0.80 mmol) and [(C₆H₅)₃P]₄Pd (0.2 g, 0.17 mmol) in

THF (10 mL) was added trimethyl(phenylethynyl)tin (2.0 g, 7.5

mmol), and the reaction mixture was refluxed for 5 h. The reaction

mixture was cooled to room temperature, and the solvent was then

removed under reduced pressure. The residue was passed through

a silica gel column and eluted with methylene chloride/hexane (1:

1) to give the desired product as a dark brown solid. Yield: 70%.

R_f0.3. ¹H NMR (CDCl₃): δ 7.21-7.30 (m, 40H, Ar), 7.65 (t, J )

7.6 Hz, 4H, meso-Ar), 7.77 (t, J ) 7.6 Hz, 8H, meso-Ar), 8.42 (d,

J ) 6.8 Hz, 8H, meso-Ar), 9.62 (br s, 2H, 2NH). ¹³C NMR

(CDCl₃): δ 84.70 (Cquat), 104.09 (Cquat), 119.99 (Cquat), 123.52

(Cquat), 127.85 (CH), 128.17 (CH), 130.12 (CH), 131.26 (CH),

132.06 (CH), 135.01 (Cquat), 136.52 (CH), 139.14 (Cquat), 154.00

(Cquat). MS (FAB): m/z 1416 (M⁺+ 1). Anal. Calcd for

1

by TLC (benzene/hexane, 1:1) and H NMR. After completion,

the reaction mixture was cooled and the THF was removed under

reduced pressure. The crude mixture was dissolved in methylene

chloride (50 mL), and silica gel (20 g) was added to the flask.

Following removal of the solvent under reduced pressure, the

resulting powder was loaded onto a silica gel column. The column

was eluted with a 5-30% benzene/hexane concentration gradient,

which yielded a mixture of penta- and hexasubstituted porphyrins

and pure octaalkynylporphyrin. The penta- and hexaalkynylpor-

phyrins were then separated and purified via preparative thin-layer

chromatography using a benzene/hexane (1:1) cosolvent mobile

phase. Yield: 5% (pentaalkynylporphyrin, 2Ni), 7% (hexaalky-

nylporphyrin, 3Ni), and 40% (octaalkynylporphyrin, 4Ni). Suitable

crystals of 2Ni-4Ni for X-ray diffraction were grown from a

mixture of methanol and chloroform. The following are data for

2Ni: R_f0.7. Green solid. ¹H NMR (CDCl₃): δ 7.27-7.31 (m, 25H,

Ar), 7.59-7.67 (m, 12H, meso-Ar), 7.96-8.02 (m, 8H, meso-Ar),

8.59 (s, 2H, pyrrole), 8.87 (s, 1H, pyrrole). MS (FD): m/z 1170

(M⁺), 1070, 970. Anal. Calcd for C₈₄H₄₈N₄Ni‚H₂O: C, 84.82; H,

4.24; N, 4.71. Found: C, 84.56, H, 4.52, N, 4.16. IR (KBr, cm^-1):

2922, 2187, 1596, 1558, 1539, 1506, 1489, 1456, 1341, 1260, 1101,

1013, 753, 688.

C₁₀₈H₆₂N₄‚H₂O: C, 90.47; H, 4.50; N, 3.90. Found: C, 90.18, H,

4.22, N, 3.46. IR (KBr, cm^-1): 3320, 3051, 2188, 1652, 1596, 1504,

1478, 1441, 1404, 1343, 1275, 1089, 1069, 1000, 753, 688, 678.

[5,10,15,20-Tetraphenyl-2,3,7,8,12,13,17,18-octakis(phenyl-

ethynyl)porphinato]zinc(II) (ZnP(PA)₈, 4Zn). To a solution of

4H₂(0.260 g, 0.183 mmol) in chloroform (20 mL) was added a

solution of Zn(CH₃COO)₂‚2H₂O (0.040 g, 0.182 mmol) in methanol

(5 mL), and the resulting mixture was stirred for 4 h at room

temperature. After the completion of the reaction, the solvent was

removed under reduced pressure and the crude residue was

suspended in methanol/chloroform (3:1) at 5 °C for 2 h. The solid

was filtered, washed with hexane (2 × 20 mL), and then passed

through a plug column using methylene chloride as the eluent.

Suitable crystals for X-ray diffraction were grown from slow

evaporation of chloroform in methanol. Yield: 92%. Metallic green

[5,10,15,20-Tetraphenyl-2,3,7,8,12,13-hexakis(phenylethynyl)-

porphinato]nickel(II) (NiP(PA)₆, 3Ni). R_f0.6. Dark green solid.

¹H NMR (CDCl₃): δ 7.26 (br s, 30H, Ar), 7.55-7.72 (m, 12H,

meso-Ar), 8.05 (d, J ) 7.2 Hz, 4H, meso-Ar), 8.16 (d, J ) 7.2 Hz,

4H, meso-Ar), 8.45 (s, 2H, pyrrole). MS (FD): m/z 1271 (M⁺

+

1

1), 970. Anal. Calcd for C₉₂H₅₂N₄Ni‚H₂O: C, 85.69; H, 4.22; N,

4.34. Found: C, 85.97, H, 4.52, N, 4.16. IR (KBr, cm^-1): 3051,

2920, 2850, 2189, 1595, 1518, 1440, 1397, 1338, 1255, 1222, 1103,

1003, 751, 687.

crystalline solid. H NMR (CDCl₃): δ 7.19-7.26 (m, 25H, Ar),

7.32-7.34 (m, 15H, Ar), 7.60 (t, J ) 8 Hz, 4H, meso-Ar), 7.72 (t,

J ) 8 Hz, 8H, meso-Ar), 8.33 (d, J ) 7.2 Hz, 8H, meso-Ar). ¹³C

NMR (CDCl₃): δ 85.50 (Cquat), 104.59 (Cquat), 120.70 (Cquat),

123.91 (Cquat), 127.47 (CH), 127.77 (CH), 127.93 (CH), 130.18

(CH), 132.10 (CH), 132.77 (Cquat), 135.86 (CH), 140.27 (Cquat),

148.00 (Cquat). MS (FAB): m/z 1478 (M⁺+ 1), 1347, 1290, 1240,

1175. Anal. Calcd for C₁₀₈H₆₀N₄Zn‚2H₂O: C, 85.57; H, 4.25; N,

3.69. Found: C, 85.82; H, 4.50; N, 3.80. IR (KBr, cm^-1): 3051,

2189, 1594, 1569, 1511, 1492, 1402, 1384, 1323, 1258, 1174, 1104,

1068, 1001, 828, 752, 686.

[5,10,15,20-Tetraphenyl-2,3,7,8,12,13,17,18-octakis(phenyl-

ethynyl)porphinato]nickel(II) (NiP(PA)₈, 4Ni). This compound

was also prepared from pure 1Ni as follows: To a suspension of

1Ni (0.40 g, 0.30 mmol) in THF (20 mL) was added [(C₆H₅)₃P]₄-

Pd (0.20 g, 0.17 mmol) under nitrogen, and the mixture was stirred

for 15 min. A solution of trimethyl(phenylethynyl)tin (0.97 g, 3.6

mmol) in THF (30 mL) was added dropwise over 30 min at 50 °C.

The temperature was raised to 70 °C and the solution refluxed for

[5,10,15,20-Tetraphenyl-2,3,7,8,12,13,17,18-octakis(phenyl-

ethynyl)porphinato]magnesium(II) (MgP(PA)₈, 4Mg). A solution

of 4H₂(100 mg, 0.070 mmol) in methylene chloride (15 mL) was

charged with MgI₂(140 mg, 0.50 mmol) and diisopropylethylamine

(0.20 g, 1.5 mmol), and the mixture was stirred at 40 °C for 4 h.

The reaction was monitored by TLC. After completion, ethanol

1

8 h. Completion of the reaction was determined by TLC and H

NMR. The solvent was then removed under reduced pressure, and

the residue was extracted with methylene chloride (200 mL). The

organic layer was dried over anhydrous sodium sulfate and

evaporated under reduced pressure. The residue was then passed

Inorganic Chemistry, Vol. 42, No. 17, 2003 5161

Chandra et al.

(1.0 mL) was added to the deeply colored mixture. The solution

was then diluted with methylene chloride (50 mL) and washed with

saturated sodium bicarbonate solution (2 × 50 mL). The organic

layer was washed with water (50 mL), dried over sodium sulfate,

and concentrated under reduced pressure. The residue was purified

on silica gel with methylene chloride/hexane (1:1) to give 4Mg in

80% yield as a metallic green crystalline solid. X-ray-quality crystals

were obtained by slow evaporation of a 1:1 chloroform/methanol

Ar), 7.73 (t, J ) 7.2 Hz, 4H, meso-Ar), 7.95 (d, J ) 7.2 Hz, 8H,

meso-Ar). ¹³C NMR (CDCl₃): δ 0.38 (CH₃), 96.95 (Cquat), 110.71

(Cquat), 120.07 (Cquat), 127.51 (CH), 129.52 (CH), 132.26 (Cquat),

135.54 (CH), 138.33 (Cquat), 145.46 (Cquat). MS (FD): m/z 1440

(M⁺+ 1). Anal. Calcd for C₈₄H₉₂N₄Si₈Ni‚2H₂O: C, 68.36; H, 6.56;

N, 3.79. Found: C, 68.4; H, 6.17; N, 3.86. IR (KBr, cm^-1): 2956,

2897, 2139, 1506, 1414, 1362, 1330, 1246, 1165, 1140, 1007, 927,

865, 845, 757, 716.

1

solution. H NMR (CDCl₃): δ 7.20-7.26 (m, 25H, Ar), 7.32-

[5,10,15,20-Tetraphenyl-2,3,7,8,12,13,17,18-octakis(trimeth-

ylsilanylethynyl)porphinato]zinc(II) (ZnP(TMSA)₈, 5Zn). The

preparation of 5Zn is analogous to that of 4Zn. Yield: 89%. Dark

7.35 (m, 15H, Ar), 7.16 (t, J ) 7.6 Hz, 4H, meso-Ar), 7.73 (t, J )

7.6 Hz, 8H, meso-Ar), 8.36 (d, J ) 7.2 Hz, 8H, meso-Ar). ¹³C

NMR (CDCl₃): δ 85.76 (Cquat), 103.70 (Cquat), 121.75 (Cquat),

123.98 (Cquat), 127.41 (CH), 127.76 (CH), 127.81 (CH), 129.93

(CH), 132.06 (CH), 132.32 (Cquat), 136.31 (CH), 140.86 (Cquat),

148.44 (Cquat). MS (MALDI-TOF): m/z 1438 (M⁺+ 1). Anal.

Calcd for C₁₀₈H₆₀N₄Mg‚4H₂O: C, 85.91; H, 4.54; N, 3.71. Found:

C, 85.54; H, 4.72; N, 3.71. IR (KBr, cm^-1): 3051, 2963, 2186,

1635, 1594, 1491, 1440, 1384, 1319, 1256, 1175, 1102, 1086, 1000,

975, 898, 825, 752, 688.

1

green solid. H NMR (CDCl₃): δ 0.12 (s, 72H, SiCH₃), 7.65 (t, J

) 7.6 Hz, 8H, meso-Ar), 7.79 (t, J ) 7.6 Hz, 4H, meso-Ar), 8.13

(d, J ) 6.8 Hz, 8H, meso-Ar). ¹³C NMR (CDCl₃): δ 0.66 (CH₃),

97.97 (Cquat), 109.24 (Cquat), 121.36 (Cquat), 127.54 (CH), 129.54

(CH), 132.14 (Cquat), 136.66 (CH), 140.50 (Cquat), 148.64 (Cquat).

MS (FD): m/z 1447 (M⁺+ 1). Anal. Calcd for C₈₄H₉₂N₄Si₈Zn‚

2H₂O: C, 68.08; H, 6.53; N, 3.78. Found: C, 68.35; H, 6.62; N,

3.59. IR (KBr, cm^-1): 3056, 2957, 2898, 2136, 1601, 1489, 144,

1322, 1246, 1336, 1002, 901, 864, 756, 692.

[5,10,15,20-Tetraphenyl-2,3,7,8,12,13,17,18-octakis(phenyl-

ethynyl)porphinato]copper(II) (Cu(PA)₈, 4Cu). To a solution of

4H₂(0.060 g, 0.042 mmol) in chloroform was added Cu(CH₃-

COO)₂‚H₂O (0.010 g, 0.050 mmol) in methanol (5 mL), and the

mixture was stirred until the starting material was completely

converted to metalated product (3 h). The solvent was then removed

under vacuum, and the residue was dissolved in methylene chloride

and washed with water. The organic layer was dried over anhydrous

sodium sulfate and concentrated in vacuo. The residue was then

stirred in methanol at room temperature for 2 h. The solid was

filtered, washed with methanol and hexane (2 × 20 mL), and

then passed through a plug column using methanol/methylene

chloride (1:9) as the eluent. Suitable crystals for X-ray diffraction

were obtained by slow evaporation of chloroform in methanol.

Black crystalline solid. R_f0.4. MS (FD): m/z 1476 (M⁺+ 1),

1378, 1277, 1179. Anal. Calcd for C₁₀₈H₆₀N₄Cu‚H₂O: C, 86.78;

H, 4.18; N, 3.75. Found: C, 86.92; H, 4.22; N, 3.85. IR (KBr,

cm^-1): 2963, 2187, 1652, 1594, 1440, 1260, 1102, 1021, 1001,

800, 749, 686.

[5,10,15,20-Tetraphenyl-2,3,7,8,12,13,17,18-octakis(trimeth-

ylsilanylethynyl)porphinato]copper(II) (CuP(TMSA)₈, 5Cu). To

a solution of 5H₂(0.060 g, 0.043 mmol) in chloroform (20 mL)

was added Cu(CH₃COO)₂‚H₂O (0.010 g, 0.050 mmol) in methanol

(5 mL), and the reaction mixture was stirred for 2 h at room

temperature. After the evaporation of the solvent, the crude product

was washed several times with methanol (5 × 20 mL) and then

dried under reduced pressure to afford 5Cu. Yield: 85%. Dark

green solid. MS (FD): m/z 1445 (M⁺+ 1), 1351. Anal. Calcd for

C₈₄H₉₂N₄Si₈Cu‚H₂O: C, 68.97; H, 6.48; N, 3.83. Found: C, 69.29;

H, 6.58; N, 3.65. IR (KBr, cm^-1): 2956, 2897, 2139, 1615, 1500,

1496, 1444, 1354, 1325, 1246, 1168, 1138, 1003, 864, 845, 757,

738, 716, 694.

Photolysis of H₂P(PA)₈. Free-base 4H₂(5 mg, 0.0035 mmol)

was placed in a Schlenk flask and dried under vacuum. Degassed

benzene-d₆(0.6 mL) and excess (>100-fold) 1,4-cyclohexadiene

(CHD) were then added, and the solution was transferred to a

J-Young NMR tube. The sealed tube was placed in a temperature-

controlled (13 °C) bath and photolyzed with λ g 420 nm light for

6 h. Upon photolysis, the solution turned from green-brown to dark

red. After completion of the reaction, the solvent was removed and

the product was purified via preparative TLC using methylene

chloride/hexane (1:1). The reaction was repeated on a ∼50 mg scale

in a Schlenk flask (18 h) using the same ratio of solvent to CHD

and 4H₂to CHD under parallel conditions. X-ray-quality crystals

of H₂RP(PA)₈6H₂were obtained by slow evaporation of a 1:1

chlorform/methanol solution. Yield of octasubstituted 5,15-dihy-

droporphyrin product 6H₂: 70%. R_f0.5. ¹H NMR (CDCl₃): δ 6.45

(s, 2H, meso-CHR), 7.13-7.17 (m, 18H, Ar), 7.19-7.23 (m, 14H,

Ar), 7.24-7.25 (m, 3H, Ar), 7.35-7.37 (m, 5H, Ar), 7.43-7.45

(m, 8H, meso-Ar), 7.48-7.53 (m, 4H, meso-Ar), 7.58-7.62 (m,

4H, meso-Ar), 7.65-7.67 (m, 4H, meso-Ar). ¹³C NMR (CDCl₃):

δ 82.23 (Cquat), 83.69 (Cquat), 98.38 (Cquat), 102.53 (Cquat),

119.89 (Cquat), 123.37 (Cquat), 126.33 (Cquat), 127.30 (Cquat),

127.82 (CH), 128.07 (Cquat), 128.17 (CH), 128.29 (CH), 129.19

(CH), 130.15 (CH), 130.76 (CH), 130.96 (CH), 131.52 (CH), 131.99

(CH), 134.59 (Cquat), 139.37 (Cquat), 139.57 (Cquat), 140.31

(Cquat), 157.16 (Cquat). MS (MALDI-TOF): m/z 1417 (M⁺+ 1)

1308, 825, 809, 545, 523. Anal. Calcd for C₁₀₈H₆₄N₄‚2H₂O: C,

89.22; H, 4.71; N, 3.85. Found: C, 89.27; H, 4.23; N, 3.77. IR

(KBr, cm^-1): 2960, 2187, 1733, 1700, 1652, 1596, 1521, 1464,

1384, 1250, 1208, 1139, 1068, 998, 952, 752, 687.

[5,10,15,20-Tetraphenyl-2,3,7,8,12,13,17,18-octakis(trimeth-

ylsilanylethynyl)porphyrin] (H₂P(TMSA)₈, 5H₂). Compound 5H₂

was prepared in a manner similar to that of 4H₂. Freshly prepared

trimethyl(trimethylsilylethynyl)tin was used in the [(C₆H₅)₃P)]₄Pd-

catalyzed cross-coupling reaction. After workup, the crude product

was purified on silica gel using a 1:1 mixture of hexane/methylene

chloride as the eluent. Yield: 60%. Green crystalline solid. ¹H NMR

(CDCl₃): δ 0.12 (s, 72H, 8SiCH₃), 7.68 (t, J ) 7.6 Hz, 8H, meso-

Ar), 7.80 (t, J ) 7.6 Hz, 4H, meso-Ar), 8.18 (d, J ) 6.8 Hz, 8H,

meso-Ar). ¹³C NMR (CDCl₃): δ 0.47 (CH₃), 97.46 (Cquat), 108.77

(Cquat), 120.40 (Cquat), 127.60 (CH), 129.62 (CH), 132.83 (Cquat),

137.03 (CH), 139.57 (Cquat), 153.89 (Cquat). MS (FD): m/z 1383

(M⁺+ 1). Anal. Calcd for C₈₄H₉₄N₄Si₈‚H₂O: C, 71.97; H, 6.90;

N, 3.75. Found: C, 71.78; H, 6.91; N, 3.75. IR (KBr, cm^-1): 3319,

2956, 2897, 2137, 1527, 1476, 1445, 1389, 1246, 1116, 1030, 1001,

926, 861, 842, 757, 732, 715.

[5,10,15,20-Tetraphenyl-2,3,7,8,12,13,17,18-octakis(trimeth-

ylsilanylethynyl)porphinato]nickel(II) (NiP(TMSA)₈, 5Ni). To

a solution of 5H₂(0.230 g, 0.166 mmol) in chloroform (20 mL)

was added a solution of Ni(CH₃COO)₂‚4H₂O (0.041 g, 0.164 mmol)

in methanol (5 mL), and the mixture was stirred at room temperature

for 4 h. The solvent was removed under reduced pressure, and the

crude product was purified on a silica gel column using methylene

1

chloride/hexane (1:1). R_f0.8. Yield: 83%. Green solid. H NMR

(CDCl₃): δ 0.089 (s, 72H, SiCH₃), 7.60 (t, J ) 7.6 Hz, 8H, meso-

5162 Inorganic Chemistry, Vol. 42, No. 17, 2003

Synthesis and Structure of Porphyrinic Enediynes

Scheme 1. Syntheses of NiP(PA)₅, NiP(PA)₆, and NiP(PA)₈(2Ni-

4Ni)^a

Thermolysis of H₂P(PA)₈. Under anaerobic conditions, a

Schlenk flask was charged with 4H₂(30 mg, 0.021 mmol) and 1,4-

cyclohexadiene (1.2 g, 15 mmol) in 1,2,4-trichlorobenzene (20 mL),

and the mixture degassed by three freeze-pump-thaw cycles. The

mixture was transferred into a pressure vessel under nitrogen and

heated at 250 °C in sand for 48 h. The reaction vessel was cooled

and the reaction mixture passed through a silica gel column using

hexane as the eluent for 1,2,4-trichlorobenzene. The product was

then removed from the column with methylene chloride and

repurified on silica gel using methylene chloride/hexane (1:1) as

the eluent. Yield of octasubstituted 5,15-dihydroporphyrin product

6H₂: 67%. Using 2-propanol as a hydrogen donor gives a 73%

yield of 6H₂. The spectroscopic data were identical to those of the

photochemically prepared product 6H₂.

Photolysis and Thermolysis of 5,10,15,20-Tetraphenylpor-

phyrin (H₂TPP). H₂TPP was photolyzed and thermolyzed (96 h)

in the same manner as free-base 4H₂. Standard NMR and

chromatographic analyses of the solutions revealed only the

presence of unreacted H₂TPP starting material with no meso-reduced

product formation.

X-ray Structure Determinations. X-ray-quality crystals of

2Ni-4Ni, 4H₂, 4Zn, 4Cu, and 4Mg were grown by slow evapora-

tion from either a methanol/dichloromethane or methanol/chloro-

form mixture (1:1). Crystals were mounted onto glass fiber

mounting pins using a matrix of high-vacuum silicone grease. In

cases where solvent loss was severe, the crystals were placed in a

0.05 mm monofilament loop with a solvent matrix. The crystals

were immediately placed on a Bruker SMART6000 diffractometer

and cooled using a locally designed nitrogen flow cooling system.

The cooling system utilizes house nitrogen and a recooling Dewar

with a silvered glass delivery tube. The sample temperature can be

maintained from 110 K to room temperature as desired. The data

were collected using a sealed, graphite-monochromatized Mo X-ray

source. Data were collected using a combination of ω and φ scans,

with frame widths of 0.30° and frame times selected on the basis

of the scattering of the individual crystal. Data collection and initial

indexing and cell refinement were handled using SMART soft-

ware.⁷²Frame integration and final cell parameter calculation were

carried out using SAINT software.⁷³Where necessary, the data were

corrected for absorption using the SADABS program.⁷⁴Decay of

reflection intensity was not observed. Structure solution and

refinement, the production of graphics, and the preparation of

publication materials were performed using SHELXTL and various

local programs.⁷⁵

^aReaction conditions: (i) trimethyl(phenylethynyl)tin, Pd(0), THF, reflux.

reactions were prepared from lithium salts of the correspond-

ing alkynes at temperatures ranging from -78 to +20 °C.

All metal-mediated coupling reactions^52-54,79,80were per-

formed in refluxing THF under nitrogen.

Reaction of crude 1Ni with trimethyl(phenylethynyl)tin

over a Pd(0) catalyst in THF leads to the generation of a

mixture of penta-, hexa-, and octaalkynylporphyrinic ene-

diynes 2Ni-4Ni (Scheme 1). The products of the crude

reaction were purified on silica gel using a 1:1 mixture of

benzene/hexane as the eluent. Compound 4Ni was isolated

and recrystallized from a methanol/chloroform mixture (1:

1). The structure was confirmed by ¹H NMR, ¹³C NMR, and

mass spectrometry, as well as X-ray crystallography (vide

Results and Discussion

Syntheses. The starting materials 1Ni and 1H₂were

prepared in good yields by standard literature procedures.⁷¹

Crude 1Ni contains a mixture of penta-, hexa-, and octabro-

minated TPP due to incomplete bromination. Pure 1Ni is

obtained by extracting the hexa- and pentabromo derivatives

by stirring the mixture in CH₂Cl₂for 3 h. The octabromopor-

phyrin product, which is insoluble in CH₂Cl₂, is then isolated

by filtration. Tin alkynes⁷⁶for subsequent Stille coupling^77,78

1

infra). In the H NMR spectrum of 4Ni, a broad singlet

appears at δ 7.23 for the eight phenyl rings of the phenyl-

acetylene units. Two triplets and one doublet are observed

at δ 7.58 and 7.69 and δ 8.19, respectively, for the protons

of the meso-phenyl groups. The presence of the alkyne

carbons is verified by resonances at δ 84.19 and 105.43 in

the ¹³C NMR. Compound 4Ni was also prepared indepen-

(72) SMART, Version 4.210, Bruker Analytical X-ray Systems, 6300

Enterprise Lane, Madison, WI 53719, 1996.

(73) SAINT, Version 4.05, Bruker Analytical X-ray Systems, 6300

Enterprise Lane, Madison, WI 53719, 1996.

(74) Sheldrick, G. SADABS; University of Go¨ttingen: Go¨ttingen, Germany,

1996.

(75) SHELXTL, Version 5.1, Bruker Analytical X-ray Systems, 6300

Enterprise Lane, Madison, WI 53719, 1997.

(77) Stille, J. K. Angew. Chem. 1986, 98, 504-519.

(78) Stille, J. K. Angew. Chem., Int. Ed. Engl. 1986, 25, 508-524.

(79) Heck, R. F. Acc. Chem. Res. 1979, 12, 146-151.

(80) Lin, V. S.-Y.; DiMagno, S. G.; Therien, M. J. Science 1994, 264,

1105-1111.

(76) Menz, G.; Wrackmeyer, B. Z. Naturforsch., B 1977, 32B, 1400-1407.

Inorganic Chemistry, Vol. 42, No. 17, 2003 5163

Chandra et al.

Scheme 2. Syntheses of Octaalkynylporphyrinic Enediyne

Derivatives 4 and 5^a

Table 1. Crystallographic Data for 2Ni-4Ni

2Ni

3Ni

4Ni

empirical formula

fw

cryst color

cryst syst

space group

a, Å

b, Å

c, Å

R, deg

â, deg

C₈₁H₅₁Cl₃N₄Ni C₉₆H₆₀Cl₈N₄Ni C₁₁₄H₆₆Cl₁₈N₄Ni

1245.37

metallic black

monoclinic

P2₁/c

1611.87

black

triclinic

P1h

2188.52

black

monoclinic

C2/c

14.5157(7)

20.9515(11)

19.6961(10)

90

108.609(2)

90

13.8989(17)

15.5193(21)

19.0221(24)

102.745(4)

96.614(4)

104.401(3)

3812.9(14)

2

27.913(3)

18.4655(16)

17.9207(14)

90

91.617(2)

90

γ, deg

V, Å³

5683.06

4

9233.2(14)

4

Z

F

_calcd, g/cm³

1.456

1.404

1.574

T, K

115

108

λ, Å

0.71073

F

1.498

R ) 0.073

R_w) 0.086

0.071073

F

1.864

R ) 0.065

R_w) 0.057

0.71073

refined on the basis of

F²

GOF on F²

0.905

final R indices^a

[I > 2σ(I)]

R1 ) 0.076

wR2 ) 0.194

R1 ) 0.162

wR2 ) 0.230

0.973 and -0.547

R indices^a(all data)

largest diff peak and hole 1.064 and -0.83

^aR1 ) ∑(|F_o| - |F_c|)/∑|F_o|. wR2 ) [∑[w(F_o- F_c)²]/∑[w(F_o)²]]^1/2

.

2

R ) ∑|F_o| - |F_c|/∑|F_o|. R_w) [∑w(|F_o| - |F_c|)²/∑w|F_o|²]^1/2. w ) 1/σ²(|F_o|).

prepared by Lindsey’s method.⁸²Briefly, free-base 4H₂was

treated with MgI₂in the presence of excess diisopropylethy-

lamine at 40 °C, producing MgP(PA)₈in 80% yield.

Crystallization of 4Mg from methanol/chloroform (1:1) leads

to axial coordination of a solvent molecule (MeOH) in the

1

X-ray structure (vide infra). In the H NMR, the aromatic

resonances of the phenylacetylene fragment appear at δ

7.20-7.26 as a multiplet for 25 protons. A second multiplet

is also observed at δ 7.32-7.35 corresponding to 15 protons

of the phenylacetylene unit. The meso-phenyl protons are

observed at δ 7.16 and 7.73 as a triplet and δ 8.36 as a

doublet. The identities of 4Zn, 4Mg, 5Ni, and 5Zn were

confirmed by NMR, mass spectrometry, and elemental

analysis, while the structures of 4Zn, 4Mg, and 4Cu were

characterized by X-ray diffraction. The paramagnetic com-

pound 5Cu was characterized by mass and elemental

analysis.

X-ray Crystal Structures of Porphyrinic Enediynes. A

summary of crystallographic data for the Ni-substituted

porphyrinic enediyne structures 2Ni-4Ni is given in Table

1, with the accompanying structures shown in Figure 1.

Compounds 2Ni-4Ni crystallize as black plates in mono-

clinic or triclinic space groups by slow evaporation from

either 1:1 methanol/chloroform (for 2Ni and 4Ni) or methanol/

methylene chloride (for 3Ni). Crystals of the corresponding

free-base, zinc(II), magnesium(II), and copper(II) octaalky-

nylporphyrin derivatives 4H₂, 4Zn, 4Mg, and 4Cu were

grown by slow evaporation from chloroform (4H₂) or 1:1

chloroform/methanol mixtures (4Zn and 4Mg). Crystals of

4Cu were obtained by slow evaporation of a 1:1 chlorform/

methanol cosolvent system. The resulting structures and

crystallographic parameters are given in Figure 2 and Table

2, respectively. Like their Ni(II) counterparts, these com-

^aReaction conditions: (i) trimethyl(phenylethynyl)tin/trimethyl(trimeth-

ylsilylethynyl)tin, Pd(0), THF, reflux; (ii) M(OAc)₂‚H₂O, MeOH, CHCl₃,

rt/MgI₂, diisopropylethylamine, 40 °C, 4 h.

dently in 65% yield by reacting pure 1 with the same tin

reagent under identical coupling conditions (Scheme 2).

The free-base porphyrinic enediynes 4H₂and 5H₂were

similarly prepared in good yields (4H₂, 70%; 5H₂, 60%) from

2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetraphenylporphy-

rin⁷¹as described for 1Ni, without using the metalloporphyrin

template for the coupling reaction.⁸¹Compound 5H₂(H₂P-

(TMSA)₈) was synthesized by reacting freshly prepared

trimethyl(trimethylsilylethynyl)tin⁷⁶with 1H₂under reflux

in THF (Scheme 2). The structures of porphyrinic enediynes

4H₂and 5H₂were confirmed by ¹H NMR, ¹³C NMR, mass

spectrometry, and X-ray crystallography (for 4H₂). In the

¹H NMR of 5H₂, a sharp singlet appears at δ 0.12 for the

72 protons of the trimethylsilyl groups. Two triplets and one

doublet are also observed at δ 7.68 and 7.80 (triplet) and δ

8.18 (doublet) corresponding to resonances of the meso-

phenyl rings. In the ¹³C NMR, the alkyne carbons are

detected at δ 97.46 and 108.77.

The metalated octaalkynylporphyrinic enediynes 4Zn,

4Mg, 4Cu, 5Zn, 5Cu, and 5Ni were readily prepared from

4H₂or 5H₂using the corresponding metal acetate at room

temperature (Scheme 2). The Mg(II) derivative 4Mg was

(81) Shea, K. M.; Jaquinod, L.; Smith, K. M. J. Org. Chem. 1998, 63,

7013-7021.

(82) O’Shea, D. F.; Miller, M. A.; Matsueda, H.; Lindsey, J. S. Inorg. Chem.

1996, 35, 7325-7338.

5164 Inorganic Chemistry, Vol. 42, No. 17, 2003

Synthesis and Structure of Porphyrinic Enediynes

Figure 1. ORTEP representations of the X-ray crystal structures of 2Ni-

4Ni. Thermal ellipsoids are illustrated at 50% probability.

Figure 2. ORTEP representations of the X-ray crystal structures of 4H₂,

4Zn, 4Mg, and 4Cu. Thermal ellipsoids are illustrated at 50% probability.

pounds crystallize as dark brown, black, or green materials

in low-symmetry monoclinic or triclinic space groups (Table

2).

Each porphyrin possesses differing degrees of distortion

depending upon the identity of the central cation and the

number of phenylacetylene substituents at the â-pyrrole

positions (Table 3). For the Ni(II) series 2Ni-4Ni, the

structures are all nonplanar, which is common for substituted

nickel porphyrin derivatives.^63,83-88The pentaalkynyl deriva-

tive 2Ni exhibits the classic ruffled conformation, while the

hexasubstituted porphyrin 3Ni and octaalkynyl compound

Inorganic Chemistry, Vol. 42, No. 17, 2003 5165

Chandra et al.

Table 2. Crystallographic Data for 4H₂, 4Zn, 4Mg, and 4Cu

4H₂

4Zn

4Mg

4Cu

empirical formula

fw

cryst color

cryst syst

space group

a, Å

C

₁₀₈H₆₂N₄

C

₁₁₀H₆₂Cl₆N₄Zn

C₁₁₁H₆₅N₄Cl₆₀Mg

1707.78

metallic green

monoclinic

P2₁/c

15.3782(4)

20.1494(5)

13.8125(4)

90.00(0)

98.713(1)

90

C₁₀₈H₆₀CuN₄

1477.24

black

triclinic

P1h

11.8599(11)

16.5714(15)

20.8422(19)

72.6033(23)

76.0427(23)

84.0341(26)

3791.21

2

1415.71

dark brown

triclinic

1717.84

metallic green

monoclinic

P2₁/c

15.2911(4)

20.1741(5)

13.5494(4)

90.00(0)

99.2212(6)

90

P1h

12.1770(12)

12.3303(13)

14.3679(14)

68.6847(27)

70.9162(25)

72.0008(28)

1854.43

1

b, Å

c, Å

R, deg

â, deg

γ, deg

V, Å³

4125.8(3)

2

1.383

4230.57

2

1.341

Z

F

T, K

_calcd, g/cm³

1.268

122

1.294

121

118

116

λ, Å

0.71073

F

1.355

R ) 0.076

R_w) 0.059

0.55 and -0.59

0.71073

F

1.231

R ) 0.046

R_w) 0.045

0.53 and -0.44

0.71073

F

1.759

R ) 0.065

R_w) 0.056

0.66 and -0.68

0.71073

F

0.680

R ) 0.040

R_w) 0.043

0.58 and -0.68

refined on the basis of

GOF

final R indices^a[I > 2σ(I)]

largest diff peak and hole

2

^aR ) ∑(|F_o| - |F_c|)/∑|F_o|. R_w) [∑w(|F_o| - |F_c|)²]/∑w|F_o| ]^1/2. w ) 1/σ²(|F_o|).

Table 3. Structural Parameters and Average Deviations from Planarity

for 2-4

This trend is also observed in 3Ni (C4-C5 ) 1.362 Å vs

C15-C16 ) 1.385 Å) and 4Ni, where the C_â-C_âdistance

is further increased to 1.388 Å. Lengthening of the C_â-C_â

bond increases the ideal radial size of the porphyrin core,^64,89

which is consistent with the increasing degree of distortion

observed upon additional substitution. Within this Ni(II)

series 2Ni-4Ni, it is apparent that increasing peripheral

substitution is responsible for the enhancement of the

porphyrin ring distortion.

Unlike the highly distorted, partially ruffled/saddled

structures of 2Ni-4Ni, the crystal structure of 4H₂shows a

wave conformation (Figure 2, Table 3).⁸⁴The structure is

only slightly distorted with a mean deviation from planarity

of 0.068 Å (Table 2), revealing that the distortions observed

in 2Ni-4Ni do not solely derive from steric strain between

the peripheral groups. Interestingly, the C_â-C_âbond lengths

of the four pyrrole units are almost identical (C3-C4 )

1.383(7) Å and C8-C9 ) 1.381(7) Å), but the bond angles

C2-N1-C5 (112.3(5)°) and C7-N11-C10 (105.0(4)°)

show a large asymmetry.

The structure of the Zn(II) derivative 4Zn is nearly planar,

exhibiting only a very modest wave conformation with the

trans-phenylacetylenes projecting in opposite directions from

the mean plane of the porphyrin core (Figure 2, Table 3).

The average Zn-N distance is 2.057 Å, consistent with the

nearly planar nature of the 24-atom core. The structure of

MgP(PA)₈, 4Mg, is unique in that the Mg²⁺ion is 5-coor-

dinate with an axially bound methanol which is not detected

in solution by NMR. The structure of 4Mg, however, is very

similar to that of 4Zn in that the 24-atom core is nearly

planar, and exhibits only a slight wave deformation, leading

to only a small deviation from planarity (0.039 Å). For CuP-

(PA)₈, 4Cu, a ruffle/saddle conformation is observed that is

very similar to that of Cu(OETPP) reported by Sparks.⁵⁹The

distortion leads to an overall mean deviation from the plane

of 0.275 Å, which is similar to that observed for 3Ni and

dev from C_meso

/

M-N /

Å

compd plane^a/Å

Å

2Ni

3Ni

4Ni

4Cu

4H₂

4Zn

4Mg

0.285

0.392

0.451

0.275

0.068

0.030

0.039

0.595

0.585

0.617

0.025

0.091

0.034

0.042

1.932

1.917

1.918

2.003

106.13

106.36

106.60

106.65

109.16

107.50

107.64

121.60

120.94

120.67

123.99

126.05

125.69

126.48

2.057

2.072

^aThe deviation from the plane is defined as the mean deviation of the

24 core atoms of the macrocycle from the idealized planar structure.

4Ni exhibit complex distortions containing degrees of both

ruffling and saddling.^60,61Features such as the average Ni-N

bond length, the C_R-C_meso-C_Rand C_R-N-C_Rangles, and

the observed mean deviation of the 24 core atoms of the

macrocycle from planarity are diagnostic of these structural

distortions (Table 3).

Within this series, the pentaalkynyl nickel porphyrin 2Ni

is the least distorted, having a mean deviation from the plane

of 0.285 Å. The value of this parameter increases to 0.451

Å for the octaalkynyl derivative 4Ni, revealing a systematic

increase in distortion as a function of peripheral substitution.

In these structures, the â-carbon of each pyrrole is tilted

above and below the plane, which causes the phenylacetylene

substituents to bend in opposite directions to their nearest

neighbors. Additionally, the unsubstituted C_â-C_â(C4-C5)

bond in 2Ni is shorter (1.329(5) Å) than that of the

substituted â-pyrrolic carbons (C21-C22 ) 1.374(5) Å).

(83) Senge, M. O.; Renner, M. W.; Kalisch, W. W.; Fajer, J. J. Chem.

Soc., Dalton Trans. 2000, 381-386.

(84) Nurco, D. J.; Medforth, C. J.; Forsyth, T. P.; Olmstead, M. M.; Smith,

K. M. J. Am. Chem. Soc. 1996, 118, 10918-10919.

(85) Senge, M. O.; Bischoff, I. Eur. J. Org. Chem. 2001, 1735-1751.

(86) Barkigia, K. M.; Renner, M. W.; Furenlid, L. R.; Medforth, C. J.;

Smith, K. M.; Fajer, J. J. Am. Chem. Soc. 1993, 115, 3627-3635.

(87) Senge, M. O.; Kalisch, W. W. Inorg. Chem. 1997, 36, 6103-6116.

(88) Kalisch, W. W.; Senge, M. O. Angew. Chem., Int. Ed. 1998, 37, 1107-

1109.

(89) Hoard, J. L. Ann. N. Y. Acad. Sci. 1973, 206, 18-31.

5166 Inorganic Chemistry, Vol. 42, No. 17, 2003

Synthesis and Structure of Porphyrinic Enediynes

Table 4. Electronic Absorption Maxima (λ/nm (ꢀ × 10^-4/M^-1cm^-))

for Porphyrinic Enediynes Collected at 25 °C in CH₂Cl₂

compd

B band

Q bands

2Ni

3Ni

4Ni

4Cu

4H₂

4Zn

4Mg

4Ni^a

4Cu^a

456 (29.34)

484 (30.59)

497 (32.80)

500 (25.25)

506 (30.36)

503 (34.02)

515 (42.58)

499

505

508

507

510

487 (31.55)

501 (23.66)

502 (29.42)

505 (25.68)

520 (1.36)

568 (3.14), 611 (2.08)

592 (3.44), 639 (3.01)

604 (3.84), 650 (2.85)

616 (3.86), 655 (1.42)

595 (3.70), 669 (1.10), 761 (0.32)

620 (3.63)

595 (1.82), 642 (3.66), 705 (0.09)

608, 654

615, 653

582, 656, 750

618

585, 635

597 (2.95), 639 (1.35)

618 (3.21), 667 (0.50)

596 (1.96), 651 (1.28), 764 (0.41)

632 (1.61), 699 (0.63)

600 (0.23)

a

4H₂

4Zn^a

4Mg^a

5Ni

5Cu

5H₂

5Zn

6H₂

^aSolid-state spectra collected in KBr at 25 °C.

4Ni, but with an overall distortion that is more comparable

to that of 2Ni in magnitude.

Figure 3. Electronic absorption spectra of porphyrinic enediynes, collected

in CH₂Cl₂at 25 °C: (a) 2Ni (s), 3Ni (‚‚‚), 4Ni (---); (b) 4Zn (---), 4Mg

(s), 4Cu (‚‚‚).

Comparison of the octaalkynyl analogues with various

central cations (4Ni, 4H2, 4Zn, 4Cu, and 4Mg) leads to an

interesting trend in the degree of distortion of the compounds.

Due to the static substitution pattern across the series, the

effect must derive exclusively from the identity of the central

metal ion. Using the mean deviation of the 24 core atoms of

the porphyrin ring as the measure of distortion, the deviation

increases in the order Zn < Mg < H₂< Cu < Ni (Table 3).

Although this order is in slight contrast to that obtained by

other common measures of distortion, including the M-N

bond length, the C_R-C_meso-C_Rangle, and the C_R-N-C_R

angle, it is clear from the magnitude of the mean deviation

that 4Mg and 4Zn are very similar (0.039 and 0.030 Å,

respectively), and as a result, the degree of nonplanarity is

best represented as Mg ≈ Zn < H₂< Cu < Ni.

Comparison of the electronic spectra of the series of

octaalkynylporphyrin derivatives with differing central cat-

ions reveals a sequential red shift of the Soret absorption

band energy in the order Mg < H₂< Zn < Cu < Ni in both

the solid-state (Table 4) and solution (Table 4, Figure 3b)

spectra. This effect clearly does not derive from metal-

porphyrin π-bonding interactions, as both Ni²⁺and Cu²⁺have

very similar back-bonding characteristics, while the π-orbitals

of both Mg²⁺and Zn²⁺are both energetically isolated from

the porphyrin π-system. The modest electronic influence of

the central metal ion on the absorption band energies is

further supported by the negligible differences observed in

the absorption spectra of the M²⁺TPP and M²⁺OEP deriva-

tives, which show only minor shifts of the Soret band upon

substitution of the central metal cation.⁵⁸From Table 3, the

magnitude of the red shifts in the electronic spectra correlates

well with the degree of planarity determined from the

crystallographically characterized structures. The most planar

compounds 4Mg and 4Zn exhibit the most red-shifted Soret

maxima (λ_max) 510 nm), while that of 4Ni is the most blue-

shifted (λ_max) 499 nm). These trends are also retained in

solution (4Ni, λ_max) 497 nm; 4Mg, λ_max) 515 nm),

suggesting that the structural distortion is not crystal packing

in origin. Thus, the dependence of the optical spectra of the

octaalkynylporphyrin derivatives on central macrocycle

substitution appears to be a direct consequence of the out-

of-plane geometric distortions of the porphyrin skeleton.

These observations are contrary to the commonly held,^66,91

and significantly debated,^65-68view that distortions of

porphyrins by definition cause red shifts in the electronic

spectra. Concurrent with this work, Shelnutt et al.⁹²quan-

Electronic Spectroscopy. The electronic absorption spec-

tra of the octaalkynylporphyrins show dramatic red shifts in

both the Soret and Q bands (Table 4). Generally, these

derivatives have electronic spectra that are shifted by ∼100

nm relative to their unsubstituted counterparts.⁹⁰Interestingly,

the â-phenylacetylene substitution has a larger effect on the

optical spectra than benzannulation of the pyrroles, as the

octaalkynyl derivatives have electronic spectra that are red-

shifted by ∼30 nm relative to that of tetraphenyltetraben-

zoporphyrin.⁹⁰The effect of the phenylacetylene substitution

is most clearly demonstrated through comparison of the

electronic spectra of the Ni(II) derivatives 2Ni-4Ni, which

have systematically varying numbers of substituents (Figure

3a). Within this series, the Soret and Q bands shift ∼40 nm

on going from 2Ni to 4Ni, suggesting to a first approximation

that π-delocalization associated with the extent of alkynyl

substitution is worth ∼13 nm/alkyne. However, there is an

accompanying increase in the degree of nonplanar distortion

observed in the crystal structures of 2Ni-4Ni with increasing

alkynyl substitution (Table 3), which is clearly convolved

in the observed trends in the electronic spectra (vide infra).

(91) Miura, M.; Majumder, S. A.; Hobbs, J. D.; Renner, M. W.; Furenlid,

L. R.; Shelnutt, J. A. Inorg. Chem. 1994, 33, 6078-6085.

(92) Haddad, R. E.; Gazeau, S.; Pecaut, J.; Marchon, J.-C.; Medforth, C.

J.; Shelnutt, J. A. J. Am. Chem. Soc. 2003, 125, 1253-1268.

(90) Lash, T. D. J. Porphyrins Phthalocyanines 2001, 5, 267-288.

Inorganic Chemistry, Vol. 42, No. 17, 2003 5167

Chandra et al.

tions from planarity observed in the solid state are not always

retained in solution, the trends observed across the solid-

state Raman spectra are conserved in spectra collected in

CH₂Cl₂solution at room temperature.

Significant resonance enhancement is also observed for

the Raman vibrations at ∼1234 and ∼1095 cm^-1. Neither

of these features show shifts that correlate with distortion

of the porphyrin backbone. In accordance with this observa-

tion, and on the basis of their frequencies, they can be

coarsely assigned as in-plane skeletal modes of the macro-

cycle.⁹⁴The bands at ∼920, ∼1000, and 1600 cm^-1are

unaffected by distortion of the porphyrin ring and demon-

strate only modest resonance enhancement. The absence of

these modes in the spectrum of NiOEP^93,98and their shift

upon perdeuteration in the spectrum of NiTPP⁹⁹both suggest

that these modes are associated with the phenyl rings at the

meso-positions.^94,99In the porphyrinic enediynes, these

vibrational features may also have contributions from the

â-phenylacetylene units as their vibrational frequencies

would be nearly degenerate.

Electronic delocalization of the porphyrin π-π* excited

state through the alkyne units is clearly observed via

enhancement of the alkyne stretch observed in the resonance

Raman spectra obtained with λ_exc) 501.7 nm. Minor shifts

are also observed in the alkyne stretching frequencies as a

function of central cation substitution. 4Ni has the highest

energy alkyne stretch at 2192 cm^-1, while the other deriva-

tives are grouped tightly in the range of 2189-2186 cm^-1

with the order 4Cu ≈ 4H₂≈ 4Zn > 4Mg. These vibrational

frequencies are markedly lower than those observed for

Figure 4. Solid-state resonance Raman (λ ) 501.7 nm) spectra of the

octaalkynylporphyrinic enediynes at 200 K (KBr).

titatively showed that out-of-plane distortions along the 2B_1u

and 3B_1unormal modes can lead to red shifts in the electronic

spectra of substituted porphyrins, and that seemingly small

distortions along specific normal modes can contribute

dramatically to electronic spectral shifts. Within the empirical

correlation of the maximum red shift with planarity, the

mixed ruffle/saddle character of the compounds in Table 3

leads to the possibility that small distortions along key normal

modes may contribute to the observed spectral red shifts.

Resonance Raman Spectroscopy. Resonance Raman

spectra (λ_exc) 501.7 nm) of the octaalkynyl series of

compounds 4Ni, 4H₂, 4Zn, 4Cu, and 4Mg are shown in

Figure 4. Systematic shifts to lower energy of the two

dominant resonance Raman vibrations at ∼1365 and ∼1530

cm^-1are observed as a function of central macrocycle

substitution. Frequency shifts of the structure-sensitive

Raman bands (ν₂, ν₃, ν₄, ν₁₀, and ν₁₉as defined by the

notation of Kyogoku and co-workers⁹³) of the core macro-

cycle are known to occur upon distortion from planarity.^91,94-96

In the octaalkynyl derivatives, structural distortion derives

from a decrease in the atomic core size proceeding along

the series Ni²⁺to Mg²⁺. This effect is reflected in the X-ray

structures as an increase in the average M-N bond lengths

from 1.918 Å for Ni²⁺to 2.072 Å for Mg²⁺, as well as a

decrease in the C_R-N-C_Rangle from 107.6° in 4Mg to

106.6° in 4Ni (Table 3). Shifts to lower energy of the ∼1365

cm^-1vibration (ν₄, pyrrole breathing mode) and the ∼1530

cm^-1band (ν₃, in-plane C_meso-C_R-C_â)^93-95,97across the

octaalkynyl series display a negative linear relationship (R²

) 0.96 and 0.98, respectively) when plotted relative to the

C_R-N-C_Rangle, which correlates loosely with the overall

degree of deviation from the mean plane.^64,89While devia-

simple nonconjugated, nonbenzannulated enediynes (ν_CC

≈

2230 cm^-1), as well as their benzannulated counterparts with

conjugated quinoline functionalities at the alkyne termini (ν_CC

≈ 2209 cm^-1).^38,39,41,100Thus, the shift of ν_CCto lower

frequency in the porphyrinic enediynes derives from elec-

tronic delocalization of the porphyrin macrocycle through

the alkynes, which leads to a decrease in the -CtC-

bonding character and hence a decrease in the alkyne

stretching frequency. The degree of delocalization and

corresponding shift in the alkyne stretching frequency can

be modestly altered by changing the central porphyrin cation,

which produces a systematic distortion in the macrocycle

structure (Table 3). Therefore, the frequency of ν_CC(4Ni >

4Cu ≈ 4H2 ≈ 4Zn > 4Mg) is weakly related to the degree

of distortion of the macrocycle in the order 4Ni > 4Cu >

4H2 > 4Zn ≈ 4Mg (Table 3).

Changes in the resonance Raman spectrum of 2Ni-4Ni

are also observed upon increasing the number of phenly-

acetylene substituents (Figure 5). A sequential shift of the

dominant features at ∼1370 and ∼1530 cm^-1to lower energy

is observed upon increasing the number of alkynyl substit-

uents from five to eight. These are the same modes that shift

to higher energy with increasing macrocycle distortion

(93) Abe, M.; Kitagawa, T.; Kyogoku, Y. J. Chem. Phys. 1978, 69, 4526-

4534.

(94) Spiro, T. G. In Physical Bioinorgic Chemistry Series: Iron Porphyrins,

Part 2; Lever, A. B. P., Gray, H. B., Eds.; Addison-Wesley: London,

1983; pp 89-159.

(95) Shelnutt, J. A.; Medforth, C. J.; Berber, M. D.; Barkigia, K. M.; Smith,

K. M. J. Am. Chem. Soc. 1991, 113, 4077-4087.

(96) Piffat, C.; Melamed, D.; Spiro, T. G. J. Phys. Chem. 1993, 97, 7441-

7450.

(97) Sparks, L. D.; Anderson, K. K.; Medforth, C. J.; Smith, K. M.;

Shelnutt, J. A. Inorg. Chem. 1994, 33, 2297-2302.

(98) Li, X. Y.; Czernuszewicz, R. S.; Kincaid, J. R.; Su, Y. O.; Spiro, T.

G. J. Phys. Chem. 1990, 94, 31-47.

(99) Li, X. Y.; Czernuszewicz, R. S.; Kincaid, J. R.; Stein, P.; Spiro, T. G.

J. Phys. Chem. 1990, 94, 47-61.

(100) Kraft, B. J.; Zaleski, J. M. Unpublished results.

5168 Inorganic Chemistry, Vol. 42, No. 17, 2003

Synthesis and Structure of Porphyrinic Enediynes

Table 5. Average Alkyne Termini Separation and Bergman Cyclization

Temperatures for 2-14 Determined by DSC

av alkyne termini temp/

compd

separation/Å

°C

compd

separation/Å

°C

2Ni

3Ni

4Ni

4Cu

4H₂

4Zn

4.09

3.95

4.00

3.79

3.82

3.79

275

261

281

247

248

249

4Mg

5Ni

5H₂

5Zn

5Cu

6H₂

3.77

244

361

351

347

345

302

4.23

distortion. Conversely, increased substitution from 2Ni to

4Ni (Figures 3a and 5) leads to a pronounced red shift in

the electronic spectra, and a corresponding shift in the Raman

marker bands to lower frequency. Hence, π-delocalization

throughout the porphyrin framework causes a shift of the

marker bands to lower frequency, despite the competing

effect of the geometric distortion, which in the octaalkynyl

series leads to an increase in these vibrational frequencies.

Solid-State Thermal Reactivity. Differential scanning

calorimetry has been shown to be an effective tool for

measuring Bergman cyclization temperatures of metalloene-

diyne structures in the solid state (absence of a H donor),

and effectively correlating the temperature with the distance

between the alkyne termini.^51-62Within this theme, the

thermal Bergman cyclization temperatures for 2Ni-4Ni, 4H₂,

4Cu, 4Zn, 4Mg, 5Ni, 5Zn, 5Cu, and 6H₂determined in the

solid state by DSC are given in Table 5. Where applicable,

the cyclization temperatures are compared to crystallographi-

cally measured alkyne termini separation distances in an

effort to evaluate the dependence of reactivity upon structural

differences at the porphyrin periphery. Table 3 reveals a

systematic distortion of the macrocycle as a function of the

numbers of alkyne substituents, and the nature of the central

macrocycle cation. For 2Ni-4Ni, the X-ray structures show

only limited differences in the average alkyne termini

separation (2Ni, 4.09 Å; 3Ni, 3.95 Å; 4Ni, 4.00 Å).

Interestingly, the thermal Bergman cyclization temperatures

(2Ni, 275 °C; 3Ni, 261 °C; 4Ni, 281 °C) loosely parallel

these average distances. Since termini separation is an

averaged quantity over several independent units, the fact

that a measurable, albeit weak, correlation exists is signifi-

cant, and therefore reflects the importance of the distance

dependence to the Bergman cyclization reaction within this

class of structures.

The pronounced differences in the degrees of distortion

along the octaalkynyl series 4Ni, 4H₂, 4Cu, 4Zn, and 4Mg

(Table 3) generates increased variability in the average alkyne

termini separation (4Ni, 4.00 Å; 4H₂, 3.82 Å; 4Zn, 3.79 Å;

4Mg, 3.77 Å; 4Cu, 3.79 Å; Table 5). In light of the structure/

reactivity relationship in 2Ni-4Ni, these differences would

be expected to lead to enhanced variation of the Bergman

cyclization temperature across the series. Indeed this is the

case as the average alkyne termini separation in 4H₂(3.82

Å) is considerably less than in 2Ni-4Ni despite the asym-

metry in the independent units (3.77 and 3.87 Å), and

consequently, 4H₂possesses a cyclization temperature that

is markedly lower than those of 2Ni-4Ni (248 °C, Figure

6). This general trend continues across 4Cu, 4Zn, and 4Mg

as the Bergman cyclization temperatures are all grouped very

Figure 5. Resonance Raman spectra of the nickel penta-, hexa-, and

octaalkynylporphyrinic enediynes collected with 457.9, 488.0, and 501.7

nm excitation, respectively. All spectra were recorded at 200 K in KBr.

Figure 6. DSC traces for the Bergman cyclization of 4Ni (281 °C) and

4H₂(248 °C) within the solid state.

(Figure 5). From the crystallographic data for 2Ni-4Ni,

the degree of distortion increases with increasing substitu-

tion number. Thus, one would expect, as is well pre-

cendented,^91,94-96that these structure-sensitive Raman bands

would shift to higher energy with increasing substitution.

Interestingly, the opposite trend is observed in the Raman

spectra of 2Ni-4Ni (Figure 6). From the dramatic red shifts

in the optical spectra of these compounds, it is clear that

there are electronic consequences to increasing the number

of peripheryl substitutents. Thus, one could conclude that

both of these parameters will influence the Raman spectrum.

The observed shifts in the resonance Raman spectra of

the porphyrinic enediynes would suggest there are two

parameters that affect the observed frequencies of the

structure-sensitive Raman bands. The first is a well-prece-

dented geometric effect, which causes a shift of these bands

to higher energy with increasing macrocycle distortion. The

second is an electronic effect that results in a shift of these

features to lower energy with increasing delocalization.

Across the octaalkynyl series (Figures 3b and 4), there is a

relatively modest blue shift in the electronic spectra, and the

Raman marker bands move to higher energy with increasing

macrocycle distortion. For this group, shifts in the Raman

spectra appear to be dominated by the degree of geometric

Inorganic Chemistry, Vol. 42, No. 17, 2003 5169

Chandra et al.

Scheme 3. Comparison of the Photolysis and Thermolysis of

close to that of 4H₂(4Zn, 249 °C; 4Mg, 244 °C; 4Cu, 247

°C) but are thermally well-separated from that of the highly

distorted 4Ni. Finally, the cyclization temperatures for the

(TMS)₈analogues 5H₂, 5Ni, 5Zn, and 5Cu are considerably

higher than those of their corresponding phenylacetylene

counterparts (5H₂, 351 °C; 5Ni, 361 °C; 5Zn, 347 °C; 5Cu,

345 °C). The (TMS)₈derivatives would be expected to create

significant steric hindrance and possibly electronic contribu-

tions to the activation barrier to Bergman cyclization. Since

the crystal structures for these compounds were not obtained,

drawing structure/reactivity relationships within this series

is not appropriate. Although these correlations are not without

minor inconsistencies, the overarching trend in the stability

of the enediyne unit with increasing termini separation

appears to hold. Perhaps more importantly, deviations in

macrocyle structure and planarity can induce marked per-

turbations in the observed cyclization temperatures.

H₂P(PA)₈and H₂TPP^a

Thermal and Photochemical Solution Reactivity. Con-

jugation of the enediyne unit into the porphyrin π-π*

electronic transitions leads to fundamental questions regard-

ing the viability for photochemical Bergman cyclization upon

macrocycle-centered π-π* excitation. Due to lifetime

quenching by open d shell metalated compounds, the free-

base derivative 4H₂was photolyzed with λ g 420 nm light

in benzene-d₆using (>100-fold) degassed 1,4-cyclohexadi-

ene at 13 °C. Upon photolysis, the solution turns a deep red

color, and a sharp singlet appears at δ 6.44 in the ¹H NMR

spectrum, indicating reduction in macrocycle aromaticity.

Commensurate with this new proton signal, the splitting

^aReaction conditions: (i) benzene, CHD, 420 nm/1,2,4-trichlorobenzene,

CHD/IPA, 250 °C; (ii) benzene, CHD, 420 nm/1,2,4-trichlorobenzene, CHD/

IPA, 250 °C.

1

pattern of the phenyl protons in the photoproduct H NMR

spectrum is dramatically altered relative to that of 4H₂. In

the ¹³C NMR spectrum, four distinct alkyne carbons are

observed at δ 82.23, 83.63, 98.38, and 102.53, consistent

with the decrease in symmetry of the macrocycle caused by

reduction of two of the four meso-positions (Scheme 3).

Reduction of the macrocycle was confirmed by X-ray

crystallography (Figure 7, Table 6) and was also detected

by measurable shifts in the electronic spectrum, as well as a

significant decrease in the extinction coefficient of the final

photoproduct 6H₂(Figure 8).

The conformation of 6H₂can be described as a rooflike

structure in which the core skeleton of 6H₂is folded along

the line joining the two reduced meso-carbon atoms, with

the two meso-phenyl antennae in the syn-conformation.

Figure 7. ORTEP of the X-ray crystal structure of 6H₂. Thermal ellipsoids

are illustrated at 50% probability.

Compound 6H₂also has the lowest C_R-C_meso-C_Rangles

(110.13° and 111.81° for the reduced centers), which are

significantly smaller than those observed for all of the

phenylacetylene-subsitituted derivatives. This decreased angle

is reflected in the N‚‚‚N distance of neighboring pyrroles

(N1‚‚‚N11 and N11‚‚‚N23) which have nearest-neighbor

contacts of 3.040 and 2.729 Å, respectively. These are

significantly different from those observed for 4H₂(2.967

and 2.934 Å), further illustrating the rooflike structure of

6H₂. The reduced meso-carbon to meso-phenyl distances are

1.516 Å (C5-C41) and 1.527 Å (C18-C85), which are

slightly longer than those observed in 4H₂(1.490 and 1.512

Å), reflecting a decreased bonding interaction between these

two centers. The lack of photoreactivity of 4H₂in the absence

of an external H-atom donor reveals that reduction at the

meso-positions must involve an external H-atom donor.

Finally, the average alkyne termini separation is dramatically

increased to 4.23 Å relative to those of the other porphyrinic

enediynes (∼3.8 Å, Table 5). This is reflected in a significant

increase in the thermal Bergman cyclization temperature

measured by DSC (302 °C, Table 5), which further supports

the alkyne termini separation/cyclization temperature cor-

relation for the octalkynyl derivatives (Table 5).

As a control, 4H₂was thermolyzed (250 °C) anaerobically

in 1,2,4-trichlorobenzene using CHD as well as 2-propanol

as H-atom donors. The thermolysis reaction leads to the

5170 Inorganic Chemistry, Vol. 42, No. 17, 2003

Synthesis and Structure of Porphyrinic Enediynes

Table 6. Crystallographic Data for 6H₂

empirical formula C₁₀₈H₆₂N₄‚CHCl₃V, Å³

state population onto the enediyne linkage via the out-of-

plane π-orbitals. However, unlike simple enediynes without

an adjacent π-system, the highly conjugated chromophore

leads to (i) rapid internal conversion to lower-lying chro-

mophore-centered excited states due to the overall increase

in the density of states, which (ii) decreases the population

of a reactive configuration that yields Bergman-cyclized

product, and favors other photophysical (fluorescence,

phosphorescence) or photochemical side processes. The result

of these is (iii) a decrease in the lifetime of a photoinduced

geometric distortion required to generate Bergman product

upon electronic excitation. Thus, one approach to driving

photochemical and thermal Bergman product formation in

solution for a system such as 4H₂may be reduction of the

activation barrier to cyclization by a decrease in the alkyne

termini separation via organic¹⁰or inorganic^{33,37-39,41,42}ring-

closing strategies, thereby channeling energy into the specific

Bergman cyclization reaction coordinate. Clearly, more

experimental and theoretical work is needed to properly

evaluate this model; however, on the basis of the established

relationship between the alkyne termini separation and

Bergman cyclization thermodynamics, this concept is plau-

sible.

3926.4(13)

2

1.531

82

fw

1206.38

black

monoclinic

P2₁

14.962(3)

16.981(3)

15.520(3)

Z

cryst color

cryst syst

space group

a, Å

b, Å

c, Å

F

T, K

λ, Å

_calcd, g/cm³

0.71073

refined on the basis of F²

GOF

1.048

R1 ) 0.115,

wR2 ) 0.294

R1 ) 0.166,

final R indices^a

[I > 2σ(I)]

R, deg

â, deg

γ, deg

90

R indices^a(all data)

wR2 ) 0.326

95.289(6)

90

largest diff peak

and hole

0.973 and

-0.568

^aR1 ) ∑(|F_o| - |F_c|)/∑|F_o|. wR2 ) [∑[w(F_o- F_c)²]/∑[w(F_o)²]^1/2

.

2

Conclusion

Figure 8. Electronic absorption spectra of 4H₂(s), and the meso-reduced

photoproduct 6H₂(---) collected in CH₂Cl₂at 25 °C.

Reaction of the appropriate 2,3,7,8,12,13,17,18-octabromo-

5,10,15,20-tetraphenylporphyrin with trimethyl(phenylethy-

nyl)tin leads to a series of substituted porphyrinic enediyne

structures with varying numbers (five to eight) of alkynyl

substitutents. The crystal structures of several of these

compounds reveal systematic ruffle, saddle, or wave distor-

tions of the porphyrin macrocycle based on the number of

alkyne substituents and the central cation. These distortions

are also observed in the solid-state and solution optical

spectra as a systematic red shift in the electronic transitions

as a function of increasing π-delocalization and decreasing

macrocycle distortion. The trend is also coarsely detected

in the Raman spectra of these compounds as traditional

Raman core modes, as well as the alkyne stretch, shift to

lower frequency with decreasing distortion. The compounds

represent a unique structural series from which trends in

electronic properties of substituted porphyrins can be evalu-

ated and compared to those previously proposed in the

literature.

In addition to the influence of structure on the electronic

properties of these compounds, decreasing distortion also

leads to a corresponding decrease in the alkyne termini

separation at the porphyrin periphery, which is reflected in

a commensurate decrease in the solid-state Bergman cy-

clization temperatures. This trend further emphasizes the

correlation between alkyne termini separation and Bergman

cyclization reactivity. In solution, both π-π* photoexcitation

and thermolysis of the free-base porphyrinic enediyne in the

presence of a H-atom donor lead to meso-carbon reduction

of the macrocycle, whereas the corresponding H₂TPP

compound is unreactive under identical conditions. The

disparity indicates that alkyne substitution in 4H₂activates

alternate photochemical and thermal reaction pathways to

identical octasubstituted 5,15-dihydroporphyrin product 6H₂

in 67-73% yield depending upon the H-atom donor. Thus,

both the photochemical and thermal reactions ultimately

converge to a common chemical product, despite the vast

differences in these two reaction conditions. Interestingly,

neither thermolysis nor photolysis of H₂TPP (7H₂) under

identical conditions leads to meso-carbon reduction (Scheme

3). This suggests that the extensive peripheral alkyne

substitution may increase the electrophilicity of the meso-

position in 4H₂via a decrease in the redox potential (4H₂,

E^+/0) 0.56 V, E^0/-) -1.11 V, E^-/2-) -1.27 V; 7H₂,

E^+/0) 0.71 V, E^0/-) -1.58 V, E^-/2-) -1.98 V vs Fc⁺/

Fc), as well as enhance the ability of the macrocycle to

stabilize a potential transient porphyrin radical upon initial

H-atom addition to the double bond.

The propensity for both photochemical and thermal

reduction of 4H₂at the meso-carbon, and the inability to

form Bergman product upon visible region chromophore

excitation, generates questions regarding the fundamental

pathways by which structures such as 4H₂react. To date,

spectroscopic or theoretical models describing the excited-

state dynamics upon enediyne excitation are scarce,⁴⁷and

do not yet provide sufficient predictive insight into the design

and corresponding reactivity of enediynes incorporated into

extended chromophores. Within this context, the photo-

chemical and thermal solution reactivity results described

herein can be explained by a coarse experimental model that

lends insight into the future design of chromophore-

enediyne constructs for photo-Bergman cyclization. In refer-

ence to structures such as 4H₂, excitation of a predominantly

chromophore-centered π-π* state delocalizes the excited-

Inorganic Chemistry, Vol. 42, No. 17, 2003 5171

Chandra et al.

photo-Bergman cyclization. The results show that photocy-

clization in such extended π-structures can be challenging,

and suggest that reduction in the activation barrier to

Bergman cyclization by reducing the alkyne termini separa-

tion may be required to force enediyne-centered reactivity

to become the dominant reaction coordinate.

fully acknowledged. We also thank Dr. Maren Pink for

assistance with the structural analysis and Professor Dan

Mindiola for use of electrochemical equipment.

Supporting Information Available: Crystallographic data

including complete tables of bond distances and angles, final

fractional coordinates, and thermal parameters and ORTEP drawings

of the X-ray crystal structures. This information is available free

of charge via the Internet at http://pubs.acs.org.

Acknowledgment. The generous support of the National

Institutes of Health (Grant R01 GM62541-01A1) and

National Science Foundation (Grant CHE-0077942) is grate-

IC030035A

5172 Inorganic Chemistry, Vol. 42, No. 17, 2003

Article Doi

Synthesis and structural characterization of porphyrinic enediynes: Geometric and electronic effects on thermal and photochemical reactivity

DOI: 10.1021/ic030035a

Source and publish data:

Authors:

Article abstract of DOI:10.1021/ic030035a

Full text of DOI:10.1021/ic030035a

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