THE PHOTOPHYSICAL CHARACTERIZATION OF N-CONFUSED
TETRAPHENYLPORPHYRIN AND THE CHARACTERIZATION OF ZINC N-
CONFUSED TETRAPHENYLPORPHYRIN
A Thesis
Presented To
The Graduate Faculty of the University of Akron
In Partial Fulfillment
of the Requirements for the Degree
Master of Science
Jeffery Paul Belair
December 2005 THE PHOTOPHYSICAL CHARACTERIZATION OF N-CONFUSED
TETRAPHENYLPORPHYRIN AND THE CHARACTERIZATION OF ZINC N-
CONFUSED TETRAPHENYLPORPHYRIN
Jeffery Paul Belair
Thesis
Approved: Accepted:
______Advisor Dean of the College Dr. Christopher J. Ziegler Dr. Ronald F. Levant
______Faculty Reader Dean of the Graduate School Dr. David A. Modarelli Dr. George R. Newkome
______Interim Chair Date Dr. Michael J. Taschner
ii ACKNOWLEDGEMENTS
Gratitude is extended to the faculty and staff of the University of Akron who have made my academic career exciting, memorable, and rewarding. The friendship, expertise and insight of laboratory coworkers was invaluable. I wish to acknowledge Dr. Ziegler,
Dr. Modarelli, Dr. Rajesh, Bart, Janet, John, Kathy, and Tang who offered their guidance, knowledge, experience, and friendship during the course of this project. I wish to thank
Professor Edward Lim for the use of his picosecond laser for this project. I also wish to thank Dr. Chrys Wesdemiotis and Kathleen Wollyung for use of the group mass spectrometer.
I wish to acknowledge Pfizer Global Research and Development in Ann Arbor,
Michigan for use of its digital library and electronic resources to access journal articles relevant to this research. I wish to thank Kathleen, Heather, Karen, Dennis, Dana, David, and Venkateraman for assisting me in locating rare journal articles, guidance and direction for some analyses, and for the use of their personal reference books.
iii TABLE OF CONTENTS Page
LIST OF TABLES………………………………………………………………………...v
LIST OF FIGURES……………………………………………………………………....vi
CHAPTER
I. A REVIEW OF N-CONFUSED TETRAPHENYLPORPHYRIN AND METALLATED N-CONFUSED TETRAPHENYLPORPHYRINS………………..1
II. THE PHOTOPHYSICAL CHARACTERIZATION OF N-CONFUSED TETRAPHENYLPORPHYRIN……………………………………………………..19
III. THE SYNTHESIS AND CHARACTERIZATION OF ZINC N-CONFUSED TETRAPHENYLPORPHYRIN……………………………………………………..34
REFERENCES…………………………………………………………………………..77
APPENDIX………………………………………………………………………………83
iv LIST OF TABLES
Table Page
1 Summary of absorption data for N-Confused porphyrin tautomers 1e, 1i, porphyrin H2TPP, and chlorin H2TPChl……………..…….….27
2 Summary of fluorescence data for N-Confused porphyrin tautomers 1e, 1i, porphyrin, and chlorin H2TPChl………………………..……..31
3 Peaks and shoulders for the lowest and highest concentrations of Zn(NCTPP)…………………………………………………...41
4 X-Ray Crystal Data for Zn4(NCTPP)2(O2CCH3)3(OH), Zn(NCTPP)(DMSO), and Zn(NCTPP)(Pyridine)……………………………….58
5 Zinc-ligand distances for Zn(NCTPP) and the dizinc core………………………65
6 Zn1(NCTPP) and Zn2(NCTPP) bond angles……………………………....……66
7 Comparison of the Zinc-ligand distances in Zn(NCTPP) dimer model compound and the active site of Aeromonas proteolytica aminopeptidase………………………………………...69
8 Zinc-ligand distances and ligand-zinc-ligand bond angles in Zn(NCTPP)(DMSO)…………………………………………………………..71
9 Zinc-ligand distances and ligand-zinc-ligand bond angles in Zn(NCTPP)(Pyridine)…………………………………………………………74
1 A1 H NMR Proton assignment for Zn(NCTPP) in CDCl3…………………………84
1 A2 H NMR Proton assignment for Zn(NCTPP) in pyridine-d5…………………….85
v LIST OF FIGURES
Figure Page
1 N-Confused tetraphenylporphyrin………………..……..…...……………………2
2 Crystal structure of N-Confused tetraphenylporphyrin……………………..…….3
3 Latos-Grazinski proposed mechanism of electrophilic attack for the formation of NCTPP………………………..………………………3
4 Tautomers of N-Confused tetraphenylporphyrin……………………...……..……5
5 Absorption spectra of H2NCTPP in (a) CH2Cl2 and (b) DMF……………..……...6
6 sp2-hybridized geometry of an interior pyrrolic carbon in a metallated N-Confused porphyrin……………………..……….…...... ………9
7 Crystal structure of Fe(II)(NCTPP)Br……………………………..……………...9
8 Crystal structure of Fe(II)(NCTPP)(S-C7H7)……………………..……………...10
9 The geometry of the agostic interaction………………..………………………...11
10 A suggested method for which NCTPP could stabilize silver(III)…………..…..12
11 Crystal structure of silver(II)NCTPP……………………..………………….….13
12 Cu-assisted oxygenolysis of H2NCTPP…………..……………………………...15
13 Reaction conditions for making the tripyrrolic, tripyrrinone derivative and Cu(II)NCTPP………………………..……………...16
14 Crystal structure of the 14-benzoyl-5,10-diphenyl-1-oxo-tripyrrin Cu(II)…..….17
15 Degradation of Cu(II)NCTPP by molecular oxygen………………………..…...17
16 Structures of externally protonated (1e) and internally protonated (1i) tautomers of N-Confused tetraphenylporphyrin, and tetraphenylporphyrin……………………………..………………………….23 vi 17 Emission spectra of 1i (chloroform) and 1e (dimethylacteamide)……..………...26
18 Emission spectra of 1i (chloroform) and 1e (dimethylacteamide)……..………...30
19 Synthesis of Zinc N-Confused tetraphenylporphyrin……………………...…….35
20 UV-visible absorption spectrum for Zn(NCTPP) in dichloromethane…………..38
21 UV-visible absorption spectrum for Zn(NCTPP) in N,N-dimethylacetamide…..38
22 Normalized UV-visible absorption spectra for Zn(NCTPP) from low concentration to high concentration…………………………………...40
23 Normalized UV-visible absorption spectra of the Q-band region for Zn(NCTPP)…………………………………………………………...41
1 24 H NMR spectrum of Zn(NCTPP) in CDCl3…………………………………….45
1 25 H NMR spectrum of Zn(NCTPP) in pyridine-d5………………………………..46
26 Mass spectrum of purified Zn(NCTPP)………………………………………….48
27 Expanded region of the mass spectrum of purified Zn(NCTPP) for the 677.5 m/z peak showing the naturally occurring zinc, 13C, and 15N isotope patterns……………………………………………………..48
28 UV-visible spectrum of photooxidized Zn(NCTPP) in dichloromethane….……49
29 Mass spectrum of the photooxidation of Zn(NCTPP) from 100 to 1500 m/z…………………………………………………………….52
30 Mass spectrum of the photooxidation of Zn(NCTPP) from 480 to 650 m/z……………………………………………………………...53
31 Mass spectrum of the photooxidation of Zn(NCTPP) from 1020 to 1350 m/z…………………………………………………………...54
32 Candidate chemical structures corresponding to the five main peaks of photooxidized Zn(NCTPP) fragments…………..………………..55
33 Candidate chemical structures corresponding to the four minor peaks of photooxidized Zn(NCTPP) fragments……….………………….56
34 Coordination of the ligands with the zinc metal centers…………………………60
35 Molecular structure of Zn4(NCTPP)2(O2CCH3)3(OH)…………………………..61 36 Molecular structure of Zn4(NCTPP)2(O2CCH3)3(OH)…………………………..62
37 Molecular structure of the Zn4(NCTPP)2(O2CCH3)3(OH) dizinc core with coordinating ligands and inter-atomic distances………………………63
38 Comparison of the dizinc core of the Zn(NCTPP) model compound with the active site of Aeromonas proteolytica aminopeptidase………………………………………...69
39 Molecular structure of zinc N-Confused tetraphenylporphyrin with axially coordinated dimethylsulfoxide……………………………………...71
40 Molecular structure of zinc N-Confused tetraphenylporphyrin with axially coordinated pyridine………………………………………………..73
viii CHAPTER I
A REVIEW OF N-CONFUSED TETRAPHENYLPORPHYRIN AND METALLATED
N-CONFUSED TETRAPHENYLPORPHYRINS
N-Confused tetraphenylporphyrin (H2NCTPP) is an isomer of tetraphenylporphyin (H2TPP) discovered simultaneously by Latos-Grazynski and
Furuta.1,2,3 N-Confused tetraphenylporphyrin contains a modified core consisting of three inner pyrrolic nitrogens and one inner pyrrolic carbon in contrast to four inner pyrrolic nitrogens commonly found in normal tetraphenylporphyin. In addition to being a dianionic macrocyclic ligand for divalent metals, N-Confused tetraphenylporphyrin has the ability to form metal-carbon bonds with the inner pyrrolic carbon.1 N-Confused tetraphenylporphyrin has been metallated with copper, iron, nickel, silver, and zinc as well as other metals.1,4-6 Normal tetraphenylporphyrin has been investigated as a model for coordination centers of heme-type proteins and in metalloporphyrin catalyzed reactions.1 Thus N-Confused tetraphenylporphyrin provides an opportunity to investigate the reason why porphyrin is the preferred macrocycle in biological systems. It also can be investigated for applications such as a photosensitizer in photodynamic therapy.1
While Latos-Grazynski and Furuta discovered N-Confused porphyrin indenpendly, their experiments and observations were not in complete agreement with
1,2 one another. Latos-Grazynski synthesized the tolyl variant H2NCTTP, by reacting p-
1 tolylaldehyde (40 mmol) with excess pyrrole (70 mmol) in dichloromethane using
1 BF3•Et2O (8 mmol) as the catalyst. He reported H2TTP and H2NCTTP as the products in
1 a 4:1 ratio with H2NCTTP having a yield of approximately 4%. Furuta reported the synthesis of H2NCTPP (Figures 1 and 2) by reacting dilute equimolar concentrations of
2 benzaldehyde and pyrrole in t-BuOH/CH2Cl2 (1:1) and concentrated HBr (1 equivalent).
After column chromatography and recrystallization, Furuta reported the products of the reaction as H2TPP and H2NCTPP in approximate yields of 20% and 5-7%, respectively.
Latos-Grazynski proposed a mechanism for H2NCTTP synthesis that involves the formation of two helical conformations of tetrapyrromethane that differ by the orientation of the pyrrole ring around the α-meso carbon.1 The terminal pyrrole ring orients with the nitrogen toward the center of the helical structure and then attacks electrophilicly resulting in the formation of tetraphenylporphyrin.1 When the terminal pyrrole ring orients with the nitrogen on the exterior of the helical structure followed by electrophilic attack, this results in the formation of H2NCTTP (Figure 3).
N
C H NH HN
N
Figure 1. N-Confused tetraphenylporphyrin.2
2
Figure 2. Crystal structure of N-Confused tetraphenylporphyrin.2
H H H H Ar N Ar Ar N Ar H H NH NH HN NH H H H H N N Ar OH Ar OH H Ar H Ar
Ar = p-Tol OXIDATION OXIDATION
TTPH2 CTTPH2
Figure 3. Latos-Grazinski’s proposed mechanism of electrophilic attack for the formation
of NCTPP.1
Furuta proposed a different reaction mechanism for the formation of H2NCTPP that involves the linear tetrapyrrole intermediate wrapping around Br- and Cl-, but not F-,
- - - 2 - - TFA , NO3 , and/or H2PO4 . He postulated that Br and Cl destabilize the transistion
2 state geometry required for the formation of H2TPP by this wrapping.
3 Latos-Grazynski and Furuta suggested similar results with regard to protonation studies. Latos-Grazynski suggested that the inner pyrrolic nitrogens are protonated first,
1 1 as was verified by H NMR using D2O. Further heating of H2NCTTP with
CDCl3/CH3COOD (95:5 volume ratio) resulted in pronotation of the exterior pyrrolic
1 nitrogen. Furuta observed similar protonation when TFA was added to H2NCTPP in
2 CDCl3. Furuta and coworkers reported that the H2NCTPP Soret and Q-bands in the UV- visible spectrum in CH2Cl2 were red-shifted compared to H2TPP, and were also present when protonated with TFA.2 The 20-30 nm red-shifted spectra associated with protonation of TPP correspond to the spectral changes in NCTPP and are thought to be an indication of severe distortion from planarity.1 Furuta concluded that the flexible
2 structure and basicity of H2NCTPP may useful in binding anions.
G. Richard Geier et al. reported a method for increasing the yield of acid-
7 catalyzed H2NCTPP. Lindsey and coworkers experimented with a range of condensation conditions for pyrrole and benzaldehyde using BF3-etherate and TFA, and found that H2NCTPP was an ever-present byproduct under most reaction conditions, composing at least 10% yield of porphyrinoid products. In exploring various acid catalysts, Lindsey and coworkers discovered that methansulfonic acid (MSA) efficiently produces H2NCTPP. N-Confused tetraphenylporphyrin was optimally produced by using methansulfonic acid in the 5-15 mM concentration range. Lindsey and coworkers found that the yields of H2NCTPP and H2TPP inverted by changing the concentration of MSA.
The optimal conditions for H2NCTPP provide approximately 40% H2NCTPP and less than 10% of H2TPP, and the optical conditions for H2TPP provided approximately 40%
H2TPP and less than 10% H2NCTPP. Lindsey noted that the results of using MSA 4 catalysis contrast with the similar yields of TPP and NCTPP employing the BF3-etherate and TFA catalysis discovered by Latos-Grazynski.
Lindsey and coworkers examined whether exposing pyrrole to MSA prior to
7 benzaldehyde addition affected the yield of H2NCTPP. They found that the H2NCTPP yield decreased whereas the H2TPP yield was unaffected, indicating that some interaction between the pyrrole and the MSA selectively affected the pathway leading to the formation of H2NCTPP. Lindsey and coworkers further investigated previous work by
Latos-Grazynski by studying how excess pyrrole affects H2NCTPP yield. They found the maximum yield of H2NCTPP when equimolar concentrations of benzaldehyde and pyrrole were used. Lindsey and coworkers also found that H2NCTPP yields were maximized under dilute reaction conditions.
In a recent paper by Furuta, et al. three different tautomeric forms of H2NCTPP were investigated.8 Furuta and Latos-Grazynski previously established the existence of the tautomer with the exterior pyrrolic nitrogen deprotonated and the two opposite interior pyrrolic nitrogens protonated (Figure 4, Type A).
N NH N
C C C H H H H NH HN N N N N H H N N N
A B C
Figure 4. Tautomers of N-Confused tetraphenylporphyrin.8
5 The type A tautomer was determined by DFT calculations to be the most stable form of
1,8,19 H2NCTPP. The type B tautomer has the exterior pyrrolic nitrogen and the interior pyrrolic nitrogen protonated and is found in divalent metal complexes such as Ni(II)- and
9 Pd(II)-NCTPP. Furuta noted that H2NCTPP was colored red in dichloromethane and green in dimethylformamide (Figure 5). It was also noted was that some of the Q-bands of the absorbance spectra were red-shifted in DMF compared to dichloromethane.
8 Figure 5. Absorption spectra of H2NCTPP in (a) CH2Cl2 and (b) DMF.
1 Furuta and coworkers investigated the H2NCTPP species in DMF using H NMR.
The type A tautomer found in CDCl3 showed two peaks at -4.99 and -2.41 ppm corresponding to the inner CH and two NH protons, respectively. The type B tautomer in
DMF showed singlets at 0.76, 2.27, and 13.54 ppm corresponding to the inner CH and the inner and outer NH protons, respectively. Upon the addition of D2O, the two peaks at
2.27, and 13.54 ppm disappeared and the 15N signal changed to a doublet when 15N- labeled H2NCTPP was used. These data indicate that the type B tautomer should be
6 stabilized by hydrogen-bonding through the exterior pyrrolic NH. Furuta suggested that the reduced diamagnetic ring current is due to the incomplete π-conjugated system of the type B form tautomer, and results in the upfield shifts of the internal CH and NH signals.
Furuta and coworkers obtained more information about the environment of the nitrogens in the type B tautomer by using 15N NMR. Four 15N signals were observed at 135.64,
137.16, 238.09, and 303.58 ppm using CDCl3 at ambient temperature. The first two sharp peaks at 135.64 and 137.16 ppm corresponded to the inner NH's while the third sharp peak at 238.09 ppm corresponded to the unprotonated inner pyrrolic nitrogen. The fourth broad peak at 303.58 ppm corresponded to the exterior pyrrolic nitrogen that is magnetically coupled to the adjacent α-CH. Four 15N signals at 129.82, 175.72, 267.54, and 271.38 ppm were observed in DMF-d7. The first signal at 129.82 ppm corresponded to the inner NH while the second broader peak at 175.72 ppm corresponded to the outer
NH. The third and fourth signals at 267.54 and 271.38 ppm corresponded to interior unprotonated nitrogens. The weaker aromaticity of the type B tautomer was supported by the smaller chemical shift difference between the interior and exterior nitrogens. Furuta noted that while the type C tautomer was calculated to have similar stability to the type B tautomer, it has not yet been reported in the literature and is currently under study.
Latos-Grazynski et al. reported a brief characterization of Ni(II)NCTTP
1 simultaneously with the discovery of H2NCTTP. Ni(II)NCTTP was made by refluxing a
0.12 mM solution of H2NCTTP in CHCl3 with a 1.2 mM ethanolic solution of NiCl2 •
6H2O for thirty minutes. The solvent was evaporated and the residue purified by column chromatography using silica and dichloromethane. The first green band was collected and the solvent evaporated. Ni(II)NCTTP was recrystalized from pyridine and 7 determined to have a yield of 40%. From the x-ray crystallographic analysis, the structure of Ni(II)NCTTP was determined to be similar to Ni(II)OEP. The Ni-N and Ni-
C average bond distances of 1.955(3) and 1.963(3)Å were comparable to to the Ni-N distances of 1.958(2) in Ni(II)OEP. Latos-Grazynski reports that the 1H NMR spectrum of Ni(II)NCTTP resembles that of NCTTP2+. He also writes that the proton located on the exterior pyrrolic nitrogen is spin-spin-coupled to the proton on the adjacent exterior pyrrolic carbon and is exchangeable for deuterium with D2O. Latos-Grazynski notes the lability of the interior C-H bond with the disappearance of the 21-H resonance in the 1H
NMR spectrum and the coordination of Ni(II) to the unprotonated β–carbon.
In a recent paper, Chen and Hung report the synthesis and x-ray single-crystal analysis of iron N-Confused porphyrins.5 Chen and Hung synthesized Fe(II)(NCTPP)Br by reacting H2NCTPP with 5 equivalents of FeBr2 in CH3CN/THF with a few drops of lutidine under anaerobic conditions at 65°C for 2 h. After concentration and filtration, green Fe(II)(NCTPP)Br was obtained with a yield of 85%. The UV-visible spectrum in dichloromethane of this species revealed a Soret band at 461 nm and a Q-band at 744 nm.
This spectrum was similar to one obtained by Latos-Grazynski for (2-NH-
21CH3CTPP)Ni(II)Cl, however it was dissimilar to the spectra of planar Ag(III)NCTPP reported by Furuta.6 The room-temperature effective magnetic moment obtained by
Evan's method in CDCl3 was measured and found to be µeff = 4.85µB. Chen and Hung report that this value is close to the spin-only value for a high spin d6 Fe(II) center with four unpaired electrons. These results indicate that Fe(II)(NCTPP)Br can be oxidized to
Fe(III)(NCTPP)Br under aerobic conditions. Fe(II)(NCTPP)(S-C7H7) was made by reacting Fe(II)(NCTPP)Br with sodium 4-methylbenzenethiolate under anaerobic 8 conditions. Chen and Hung noted from the x-ray single-crystal analysis of
Fe(II)(NCTPP)Br that C(1) very likely has an sp2-hybridized geometry (Figures 6 and 7).
Ph Ph N X NH N M C N H Ph Ph
3: M = Fe, X = Br 4: M = Fe, X = SC7H7
Figure 6. sp2-hybridized geometry of an interior pyrrolic carbon in a metallated N-
Confused porphyrin.5
Figure 7. Crystal structure of Fe(II)(NCTPP)Br.5
9 When comparing the iron coordination spheres of Fe(II)(NCTPP)Br and
Fe(II)(NCTPP)(S-C7H7), the authors noted that the Fe(1)-H(1) bond distance in
Fe(II)(NCTPP)Br was much shorter than the Fe(1)-H(1) bond in Fe(II)(NCTPP)(S-C7H7)
(Figure 8). Chen and Hung attributed this to the flexibility of the inverted pyrrole ring.
Chen and Hung suggest that an agostic interaction exists between the iron and the pyrrolic C(1)-H(1) bond due to the geometry of Fe(II)(NCTPP)Br. They also postulated that the strength of the agostic interaction is attenuated by the axial ligand. Cheng and
Hung also note that a stronger two-electron C(1)-H(1)-Fe interaction is created by the reduced basicity of bromide in the presence of an electron-deficient iron center in
Fe(II)(NCTPP)Br. Chen and Hung noted that in the Fe(II)(NCTPP)(S-C7H7), the S(1)-
Fe(1)-N(3) bond angle is larger than most other five-coordinated iron porphyrin complexes (Figure 9).
5 Figure 8. Crystal structure of Fe(II)(NCTPP)(S-C7H7).
10 Br S
107.0 118.6 H 126.2 H 116.1 N Fe 2.361 N Fe 2.398 N C1 N C1 1.971 2.334 H1 H1
Figure 9. The geometry of the agostic interaction.5
The Fe-N bonds trans to the inverted pyrrole rings in Fe(II)(NCTPP)Br and
Fe(II)(NCTPP)(S-C7H7) are shorter and suggest greater electron-donation into the iron metal center and supports the agostic interaction between the iron and the C-H bond.
Furuta, et al. have reported the synthesis and characterization of Ag(III)NCTPP using UV-visible spectroscopy, x-ray crystallography, 1H NMR, and cyclic
10 voltammetry. One of the motivating factors for complexing silver with H2NCTPP is that silver can exhibit oxidation states of +1, +2, and +3. H2NCTPP is known to behave as a dianionic ligand, however, Furuta and coworkers suggest that H2NCTPP can act as a trianionic ligand in stabilizing metal oxidation states of +3 by removing all three labile inner ring protons (Figures 10 and 11).
11 H N N
N C N C
N N N N
P2- P3-
Ph Ph H N N
N C N C 2+ 3+ Ph Ag Ph Ph Ag Ph N N N N
Ph Ph
Figure 10. A suggested method for which NCTPP could stabilize Ag(II) or Ag(III).10
Ag(III)NCTPP was made with a 73% yield by reacting silver(I) trifluoroacetate and
H2NCTPP in dichloromethane/methanol in darkness for two hours at ambient temperature. The silver N-Confused porphyrin complex was confirmed by mass spectroscopy with a peak at 719.1 m/z corresponding to the species 107Ag(III)NCTPP -
+ 2H . Furuta and coworkers investigated the possibility of H2NCTPP stabilizing Ag(III) as a trianionic ligand and found supporting evidence in the x-ray crystallographic structure analysis (Figure 11).
12
Figure 11. Crystal structure of Ag(II)NCTPP.10
Furuta found the silver N-Confused porphyrin complex to be nearly planar and the metal-
N bonds to be much shorter than in the Ag(II)TPP complex. The absence of a counteranion in the crystal structure and subsequent experiments with 1H NMR and magnetic susceptibility suggest that NCTPP is trianionic. With regard to the 1H NMR, a sharp singlet at 9.46 ppm assigned to the outer alpha-carbon of the inverted pyrrole ring indicates that exterior pyrrolic nitrogen is not protonated. Furuta noted that if NCTPP is dianionic, it would stabilize silver(II) and should be paramagnetic (d9 configuration). If
H2NCTPP was indeed trianionic, it would stabilize silver(III) and should be diamagnetic
(d8 configuration). The magnetic susceptibility experiment results showed that the silver
N-Confused porphyrin complex was weakly paramagnetic at temperatures less than 80K and primarily diamagnetic in the solid state. The 1H NMR also confirmed that the silver
13 N-Confused porphyrin complex was diamagnetic since the peaks were not broadened as expected for a paramagnetic Ag(II) ion. Furuta postulates that substituting a carbon for a nitrogen in the porphyrin macrocycle makes the ligand more basic while since the carbon would be a better σ-donor than nitrogen and thereby stabilize metals of higher oxidation states. Furuta suggests that the P3- ligand is stabilized by the acidity of the coordinated metal.
Furuta further proposes that dissociation of the exterior pyrrolic NH is due to the covalent bonding interaction between Ag(II) and NCTPP, which is faciliated by the in- plane and out-of-plane π-bonding between the metal d-orbital and the ligand orbitals.
Cyclic voltammograms of H2NCTPP and Ag(III)NCTPP were obtained in solutions of
CH2Cl2 and 0.1M TBAP at ambient temperature. Free-base NCTPP was scanned from
0.1V to positive potentials, where an irreversible anodic peak was observed at Epa = 0.87
V. Three reversible waves were observed at +1.25, +1.47, and +1.85 V vs. SHE for the oxidative process. An irreversible cathodic wave was observed at -0.75V. From the intensity ratios of the CV, H2NCTPP seems to undergo monoelectronic redox processes in contrast to the two oxidation and reduction processes of normal porphyrins. Furuta observed three redox processes in the CVs of the silver complex and noted that the first and second oxidation processes were shifted more than 250 mV to positive potentials relative to free-base H2NCTPP, which agrees with the coordination of silver(III).
Furuta, et al. have also reported on the Cu(II)-assisted oxygenolysis of H2NCTPP to form the product 14-benzoyl-5,10-diphenyl-1-oxo-tripyrrin Cu(II) complex (Figure
12).11
14 Ph Ph Ph Ph Ph Ph N N N Cu(OAc)2, O2 1) H+ NH HN N Cu N N M N toluene 2) "M" O O O O Ph Ph Ph Ph N
NCTPP 3-Cu 3-M
M = H2, Zn(II), Ni(II), Pd(II), Pt(II)
11 Figure 12. Proposed mechanism of Cu-assisted oxygenolysis of H2NCTPP.
Degradation or ring opening of porphyrins by photo or metal oxidation typically results in linear tetrapyrrolic compounds. However, Furuta and coworkers found that when
H2NCTPP is metallated with copper under aerobic conditions, the expected linear tetrapyrrole compound is not formed but rather a tripyrrolic, tripyrrinone derivative results instead. It is of interest to note that one of the terminal pyrroles was benzoylated and both the inverted pyrrole and a meso-phenyl group were removed in the reaction.
Furuta and coworkers formed the 14-benzoyl-5,10-diphenyl-1-oxo-tripyrrin Cu(II) complex by refluxing Cu(OAc)2 and H2NCTPP in toluene under aerobic conditions for
24 hours (Figure 13). The same reaction performed under anaerobic conditions resulted in the expected Cu(II)H2NCTPP (Figure 13).
15 Ph Ph N Cu(OAc)2, O2 N Cu N 34% O O Ph
3-Cu
Ph Ph N
NH HN
Ph Ph N
NCTPP Ph Ph N Cu(OAc)2, -O2 N Cu N ~100% Ph Ph NH
Figure 13. Reaction conditions for making the tripyrrolic,
tripyrrinone derivative and Cu(II)NCTPP.11
From the x-ray crystal structure analysis, the 14-benzoyl-5,10-diphenyl-1-oxo-tripyrrin
Cu(II) complex was found to be planar and the Cu(II) ion was coordinated to three nitrogens and one oxygen. One solvent molecule, a CHCl3, was hydrogen-bonded to the
O1 of one of the pyrrole rings (Figure 14). Furuta used 5,10,15-triphenyl-20-(2'-pyridyl)-
2-aza-21-carbaporphyrin to determine the cleavage site of the fragmentation reaction of
1 H2NCTPP. The pyrridine unit was found in both H NMR and FABMS from the tripyrrinone derivatives with no other tripyrrinone derivatives detected in the reaction.
Therefore, the meso-phenyl moiety next to the confused pyrrole ring was removed by regioselective Cu(II) ion mediated oxygenolysis of NCTPP. Furuta and coworkers suggest a mechanism by which molecular oxygen degrades Cu(II)NCTPP (Figure 15). 16
Figure 14. Crystal structure of the 14-benzoyl-5,10-diphenyl-1-oxo-tripyrrin Cu(II)
complex.11
Ph Ph Ph Ph N N O2 N Cu N N Cu N
O O Ph Ph Ph NH NCTPP-Cu(II) 3-Cu
O2
Ph Ph Ph Ph N N O2 N Cu N N Cu N O O O Ph Ph Ph Ph NH O N O O
Figure 15. Degradation of Cu(II)NCTPP by molecular oxygen.11
17 In the mechanism, Furuta suggests that the Cu(II)NCTPP (3N + C) coordination changes into a Cu(II) trypyrrinone complex with (3N + O) coordination. The activating mechanism for the reaction is electron transfer from Cu(II) to molecular oxygen in the absence of light. Furuta and coworkers metallated 14-benzoyl-5,10-diphenyl-1-oxo- tripyrrin with zinc, nickel, palladium, platinum, and cobalt and obtained UV-visible spectra for each metallated complex. They found that all of the Q-bands of the metallated complexes to be red-shifted relative to the Q-bands of 14-benzoyl-5,10-diphenyl-1-oxo- tripyrrin.
N-Confused tetraphenylporphyrin is an interesting ligand for metal complexation since it differs from tetraphenylphorphyrin by having three interior pyrrolic nitrogens and one interior pyrrolic CH. The interior pyrrolic CH has shown interesting chemistry as the hydrogen has exhibited varying reactivity depending on the metal used. The β-carbon in the macrocycle core has shown to be a good σ-donor for some transition metals.8 Furuta and coworkers have shown with UV-vis spectra and NMR studies that H2NCTPP is less
8 aromatic than H2TPP. Metallated NCTPP may also serve as an effective photosensitizer as has previously studied metallated TPP complexes.2 N-Confused tetraphenylporphyrin can be used to further elucidate why porphyrin is the preferred macrocycle in biological systems through the construction of model compounds. Much work continues in the area of this newly discovered tetrapyrrolic ligand.
18 CHAPTER II
THE PHOTOPHYSICAL CHARACTERIZATION OF N-CONFUSED
TETRAPHENYLPORPHYRIN
Introduction
Photochemistry involves the study of molecules which have electrons that are promoted from the ground state to an excited state by the absorption of light, and the physical and chemical pathways available to the excited molecules.12 These electronic transitions are associated more so with the nature of the two orbitals rather than the molecule itself. The electronic transitions in a molecule correspond to the absorption of light in the visible, UV, and far UV regions of the spectrum. Since the energy levels of a molecule are quantized, only light with a frequency associated with the energy of a transition between the ground state orbital and higher unoccupied orbital can be absorbed by the electron. Molecules not only have quantized electronic states, but also vibrationally and rotationally quantized states. In a population of molecules, the differences in the rotational, vibrational, and electronic states account for the broad rather than the expected sharp peaks in absorption spectra. In non-photochemical reactions, most organic molecules are in the ground state and have all the electrons paired in bonds with opposite spin. However, in photochemical reactions, one electron remains in a ground state orbital while the other electron is promoted to an orbital of higher energy.
19 Electronic promotions, typically π → π* or n → π*, result in the excited electron retaining its spin from the ground state, a S0 → S1 transition which designates the molecule a singlet. Promoted electrons can change spin to that of the formerly paired electron in the ground state through a process known as intersystem crossing, S0 → T1, a transition which designates the molecule as a triplet. Triplet excited states in molecules are usually lower in energy than the corresponding singlet states due to the conservation of angular momentum.
Excited state molecules have a variety of physical and chemical pathways available to them depending on the nature of the molecule and the local environment.12
Most photochemical promotions are from the S0 to the S1 state, however, the S0 to triplet states occur less frequently. While promotions to the S2 and higher singlet states occur, they rapidly decay to the S1 state. The energy of higher singlet states is released to the environment by collisions with neighboring molecules in an energy cascade from the highest through lowest vibrational levels of the S1 state. Therefore, the lowest vibrational level of the S1 state is often of most interest. Internal conversion is a slow process whereby a molecule cascades down through the vibrational levels of the S0 state to the ground state. Fluorescence is a relatively slow process whereby a molecule in the S1 state can drop to a low vibrational level of the S0 state by releasing energy in the form of light.
Fluorescence is usually very weak or undetectable, and when measurable the absorption spectra often are mirror images of the excitation spectra since excitation from the S0 ground state to the various S1 vibrational levels coincides with the drop from the S1 state to the various vibrational levels of the S0 state. Most molecules in the S1 state can enter the T1 state through an intersystem crossing without loss of energy, however, the lowest 20 vibrational level of the T1 state is usually reached through an energy cascade. Molecules in the T1 state may return to the S0 state by releasing energy as heat through an intersystem crossing, or light, known as phosphorescence. In the same molecule, phosphorescence is associated with lower frequencies than fluorescence because of the difference in energy between T1 → S0 and S1 → S0 transitions and the longer life time of the T1 state. Photosensitization is another physical process whereby the excited state of a one molecule is transferred to another. Chemical processes of the excited state molecules include simple cleavage into radicals, decomposition into molecules, intramolecular rearrangement, photoisomerization, and hydrogen atom extraction.
The optical and photophysical properties of porphyrins continue to be an active area of research. Porphyrin macrocycles are found in biological systems, function as photosynthetic reaction centers in purple bacteria and green plants, and play roles in the heme-based metalloproteins hemoglobin and myoglobin.13-27 By studying the photochemistry of N-Confused tetraphenylporphyrin, one may gain insight into why porphyrin is the preferred macrocycle among biological systems. Studying the photodynamics of NCTPP can provide information about the photodynamics of naturally occuring porphyrin macrocycles. Free-base and metalloporphyrins have been investigated for light harvesting purposes,28-37 mimics of photosynthetic reaction centers,38-44 and as electronic devices.45-49 Although the motivation for researching the photochemistry of N-Confused porphyrins is the creation of model compounds for studying the processes coupled with artificial photosynthesis, these candidates further develop knowledge of fundamental photochemical processes such as electronic and vibrational relaxation,50-52 energy transfer,53-56 and solvent dynamics.57-59 21 The photophysics of free-base and metalloporphyrins has been extensively investigated,60 however, these experiments have not been performed on porphyrin analogues and isomers. These isomers and analogues of porphyrins can provide much data on the photophysics of natural porphyrin-related chromophores. N-Confused porphyrin is an isomer of porphyrin which arises from the inversion of a β-carbon and nitrogen in one of the four tetrapyrroles, and accordingly has dissimilar physical and chemical properties. Concerning electronic structure, N-Confused porphyrins have reduced aromaticity and decreased symmetry relative to porphyrin.61-64 Due to these properties, N-Confused porphyrins are intrinsically similar to both chlorophylls and porphyrins, and may potentially be precursors to models for natural light-harvesting systems. In addition to N-Confused porphyrins having a significantly altered electronic structure, the interally protonated tautomer is slightly more sterically demanding than normal porphyrins. Some of this work was carried out to determine the potential of using
N-Confused porphyins as the basis for artificial light-harvesting systems and dendrimers.65,66
This work is the first photophysical study of the solvent-dependent, externally and internally protonated tautomers of 5,10,15,20-tetraphenyl N-Confused porphyrin free- base (Figure 16). The externally protonated tautomer is prefered in highly polar solvents such as DMF while the internally protonated tautomer occurs in aromatic and halogenated solvents such as dichloromethane and chloroform. This work reports the florescence spectra, florescence quantum yields, and florescence lifetimes of both of the exterior and interior protonated tautomers in solution.
22
Figure 16. Structures of externally protonated (1e) and internally protonated (1i)
tautomers of N-Confused tetraphenylporphyrin, and tetraphenylporphyrin.3,8
Experimental Section
General Methods. All solvents used for spectroscopic measurements were of spectrophotometric grade or HPLC grade and were used as received with the exception of chloroform, which was purified with silica gel to remove trace HCl. Absorption experiments were recorded using a Hitachi 3100 single monochromator UV-vis spectrophotometer. N-Confused porphyrin Q-band absorption assignments were correlated with normal porphyrin Q-band absorptions. Steady-state fluorescence
23 measurements were obtained using a ISA Jobin Yvon-SPEX Fluorolog 3-22 fluorometer equipped with dual input and output monochromators. Using argon-saturated solutions, fluorescence spectra were obtained by exciting at the Soret maxima in S/R mode to correct for changes in lamp output intensity. Quantum yield measurements were made
67 relative to H2TPP.
Synthesis. 2-Aza-21-carba-5,10,15,20-tetraphenyl porphyrin (H2NCTPP, 1) was synthesized according to the method developed by Geier, et al.7 Purification of
H2NCTPP was achieved by passage through a column of basic alumina activity 3 using the eluents dichloromethane and hexanes in a 1:1 ratio.
Time-Resolved Fluorescence Measurements. Spectroscopic measurements utilized either HPLC-grade or spectrophotometric grade solvents. All solutions were saturated with argon before experimentation. The time-correlated single-photon counting
(TCSPC) technique was used for time-resolved experiments. The equipment used for these experiments was previously described by Rajesh et al.,65,66 and employed the pulses from a Coherent dye laser pumped by a 532 nm output of a Coherent Antares 76-s CW mode-locked Nd:YAG laser. Fluorescence signals were detected at 55° with an emission polarizer and depolarizer, using a Hamamatsu R3809U-51 red-sensitive microchannel plate detector. An Edinburgh Instruments data collection system was used for data collection and analysis. The time-resolution of this system is estimated at ~7-9 ps after reconvolution. H2TPP was excited with 590 nm light with the decay profile monitored at
655 nm. The exterior and interior protonated tautomers of N-Confused porphyrin were excited with 590 nm light and decay profiles monitored at the emission maxima. Time- correlated single-photon counting (TCSPC) measurements of N-Confused porphyrin did 24 not show a clean single-component decay and were subsequently best fit to a multiexponential decay using the Marquardt algorithm.68 All measurements were fit with values of χ2 < 1.20. Error limits in these measurements are commonly ±10%.
Results and Discussion
Steady State Absorption. The absorption spectra acquired for N-Confused porphyrin are different relative to H2TPP and are very solvent dependent (Figure 17 and
Table 1). The absorption spectra obtained in these experiments were in good agreement with the spectra reported by Furuta et al., who posited that the different spectra occur from the two tautomeric forms of N-Confused porphyrin.72 The exterior protonated tautomer is more stable in polar solvents such as dimethylacetamide (DMAc) due to either hydrogen-bonding or dipole-dipole interactions of the exocyclic N-H bond with the solvent. The interiorly protonated tautomer is more stable in nonpolar solvents such as chloroform. In DMAc, the Soret band for the exterior protonated tautomer is significantly red-shifted (441 nm) compared to the Soret band of H2TPP (419 nm). N-
Confused tetraphenylporphyrin has three Q-band absorptions of increasing intensity that are found at 695, 642, 592, and a small shoulder at 550 nm that are analogous to the
Qx(0,0), Qx(1,0), Qy(0,0), and Qy(1,0) transitions, respectively, in H2TPP. These Q-bands are similar to the red-shifted Soret band in that they are ~37-50 nm lower in energy than the corresponding absorptions in H2TPP. The intensities of these bands increase slightly with decreasing energy, with the lowest-energy absorption being most intense. A high energy N-band absorption is observed at 381 nm.73
25
Figure 17. Absorption spectra of the interiorly protonated tautomer (chloroform, top) and
exterior protonated tautomer (dimethylacetamide, bottom).
26 Table 1: Summary of absorption data for N-Confused porphyrin tautomers 1e, 1i,
69,70,71 porphyrin H2TPP, and chlorin H2TPChl.
Compound Solvent Soret (nm) Q-bands (nm) (ε x 104 M-1 (ε x 103 M-1 cm-1) cm-1)
1i CHCl3 438 (15.9) 539 (7.8) 580 (10.8) 665 (2.7) 724 (10.4) 1e DMAc 442 (11.9) 550 (2) 595 (6.1) 644 (9.5) 699 (12.4) 69 H2TPP CHCl3 419 (41.1) 515 (17.3) 550 (8.1) 590 (6.4) 645 (6.2) 70 H2TPP DMAc 417 (48.5) 513 (20.8) 548 (9.67) 591 (6.69) 646 (6.05) 71 H2TChl C6H6 419 (1.85) 520 (15) 545 (5.5) 595 (5.5) 650 (42)
The Soret band for the interiorly protonated tautomer in CHCl3 is likewise red- shifted (438 nm). In CHCl3, three Q-band absorptions are recorded at 539, 580, and 734 nm, with two minor absorptions at 504 and 665 nm. The interiorly protonated tautomer exhibits an increase in the intensities of the Q-band absorptions with a decrease in the absorption band energy. The absorption at 724 nm is assigned to the Qx(0,0) band and the small absorption at 665 nm to the Qx(1,0) band. The Qy(1,0) and Qy(0,0) bands are due to the absorptions at 539 and 580 nm, respectively. The N-band corresponds to the weak absorption at 353 nm. The intensity profile of the Q-band region (724 nm) is similar to absorption spectra of reduced porphyrins, such as chlorophylls a and b and chlorins.
The decreased symmetry of the macrocycle caused by the inverted pyrole ring, and the expected variation in electronic structure demands a comparison with reduced porphyrins such as chlorin and chlorophyll b. The four-orbital four-electron model in reduced porphyrins is altered to account for the loss of degeneracy of the occupied a1u and a2u and unoccupied eg orbitals. In benzene, the absorption spectrum of H2TPChl is
27 similar in intensity pattern to the interiorly protonated tautomer of N-Confused porphyrin, where a broad Soret band is found at ~419 nm and an accompanying Q-band region exhibited by the intense Qx(0,0) band at 650 nm, a very weak Qx(1,0) absorption at 595 nm, and intense Qy(0,0) and Qy(1,0) bands at 545 and ~520 nm, respectively (Table
71,74 1). While the bands for H2TPP and H2TPChl are not considerably shifted from one another, their absorption intensities are appreciably dissimilar. Weiss, using the four- electron four-orbital model, attributed the change in the intensity pattern in the Q-band region of H2TPChl to the low energy Qx(0,0) band becoming weakly allowed due to the
75 decreased symmetry resulting from the loss of degeneracy in the eg orbitals. The relative intensities of the Q-bands in H2TPChl and the internally protonated tautomer of
H2NCTPP establishes the ratio Qx(0,0)/Qx(1,0) to be 7.6 for H2TPChl and 3.9 for the internally protonated tautomer of H2NCTPP, whereas the same ratio for H2TPP is 0.97 in
76 CHCl3. The ratio of Qy(0,0)/Qy(1,0) for H2TPChl and the internally protonated tautomer of H2NCTPP are 0.73 and 0.64, respectively.
The absorption spectrum of the externally protonated tautomer of H2NCTPP has
Soret and Q-band regions that are substantially red-shifted, which compares favorably with H2TPP more so than with H2TPChl. The Q-band oscillator strengths increase with decreasing energy in a linear fashion, with Qy(1,0) being the weakest and Qx(0,0) being the most intense. Integration of the Qx(0,0) bands in the internally protonated tautomer of
H2NCTPP and H2TPP showed a ~53% increase in the ground state ↔ Q-state excited- state transition. Relative to H2TPP, the ground state ↔ Q-state excited-state transition in the externally protonated tautomer of H2NCTPP was increased ~64%.
28 The S0 → S1 forbidden transitions for the externally and internally protonated tauotomers of H2NCTPP that are promoted by the eg orbital degeneracy are supported by these results.
Fluorescence Experiments. For the internally protonated tautomer dissolved in chloroform, steady-state fluorescence measurements show a large Qx(0,0) emission band at 744 nm, and a small low energy Qx(0,1) shoulder at 815 nm (Figure 18 and Table 2).
The externally protonated tautomer dissolved in DMAc exhibited a prevalent Qx(0,0) emission 713 nm, and a Qx(0,1) shoulder 783 nm (Figure 18 and Table 2). Relative to
H2TPP (651 and 715 nm in either solvent) and H2TPChl (660 and 730 nm in benzene),
71,77,78 the fluorescence spectra of H2NCTPP are considerably red-shifted. These shifts are in agreement with the red-shifts in the absorption spectra. The ratio of the Qx(0,0) and
Qx(0,1) emission bands is similar to the Qx(0,0) and Qx(1,0) absorption bands, and is increased with respect to H2TPP. These changes are characteristic of the ground state ↔
Q-state excited state transitions for both the externally and internally protonated tautomer of H2NCTPP. The radiative rate constant is the product of the oscillator strength and the square of the transition energy. The values of the transition energy are substantially
-1 -1 larger for H2TPP (15504 cm ) than either 1i (13777 cm ) or 1e (14318 cm-1), resulting in smaller radiative lifetimes for the N-Confused porphyrins. The natural radiative rates for the externally and internally protonated N-Confused porphyrins is ~(50 ns)-1 and ~(70
-1 ns) , respectively, and are significantly reduced from the natural radiative rate for H2TPP
(~120 ns-1).60
29 The time-correlated single-photon counting (TC-SPC) technique was used to determine the fluorescence lifetimes (Table 2) for both the internally and externally protonated tautomers of N-Confused porphyrin. In CHCl3, the internally protonated tautomer of N-Confused porphyrin has a fluorescence lifetime of 1.60 ns, which is approximately one-sixth that of H2TPP (~9.25 ns) in the same solvent. In DMAc, the fluorescence lifetime of N-Confused porphyrin is 1.98 ns, which is much shorter than that of H2TPP in the same solvent (12.2 ns). The decay trace had two very short-lived observable components in DMAc (680 ps, 9%, and 25 ps, 4%).
Figure 18. Emission spectra of 1i (chloroform) and 1e (dimethylacteamide).
30 Table 2. Summary of fluorescence data for N-Confused porphyrin tautomers 1e, 1i, porphyrin, and chlorin H2TPChl.
b c c c Compound Solvent Fluorescence ΦFl τ1 (ns) τ2 (ns) τ3 (ns) Stokes shift maxima (nm)a (cm-1)
1i CHCl3 744/815 0.00156 1.60 (95%) 371 1e DMAc 713/783 0.03648 1.98 (87%) 0.68 (9%) 0.025 (4%) 281
H2TPP CHCl3 651/715 0.11 9.25 143 H2TPP DMAc 650/715 0.11 12.2 95 H2TPChl C6H6 660/730 233 a Excited at the Soret bands to avoid aggregation due to concentration effects. b Relative to the fluorescence of H 2TPP. Quantum yields were calculated using standard methods. c Fluorescence lifetimes were determined by exciting argon-purged and stirred solutions of the respective porphyrins with the 570 nm output of a Coherent 700 dye laser pumped by the 532 nm output of a Coherent Antares 76-s CW mode- locked Nd:YAG laser. Emission was detected at the appropriate wavelength and measured at 55° with an emission polarizer and depolarizer, using a Hamamatsu R3809U51 multichannel plate detector. Values in parentheses are the relative contribution from each component. d Taken from reference 74.
Fluorescence quantum yields (Table 2, ΦFl) for the externally and internally protonated tautomers of N-Confused porphyrin are considerably less than the ΦFl for
67 H2TPP (ΦFl = 0.11). The reduced rate of fluorescence N-Confused porphyrins is likely the cause for the difference in fluorescence quantum yields. While the fluorescence lifetime values for the externally and internally protonated tautomers were similar, the quantum yield for the externally protonated tautomer (ΦFl = 0.036) was more than twenty times greater than that of the internally protonated tautomer (ΦFl = 0.0016). The different natural radiative rates between the two N-Confused porphyrins can partially be ascribed to the fluorescence quantum yields.67 The differences in the intersystem crossing rates or internal conversion is likely the reason for the changes in the quantum yield. The three hydrogens in the core of the internally protonated tautomer of N-Confused porphyrin promote ring deformation and accommodate a mechanism for efficient intersystem crossing or internal conversion. The dissimilarity between the two non-radiative
31 pathways is likely accountable for the externally protonated tautomer being so much more emissive than the internally protonated tautomer.
Conclusions
The photophysical properties of N-Confused porphyrins were studied for potential usefulness in molecular photonic devices and understanding the biological selection of chlorophylls and chlorins as light harvesting macrocycles in natural photosynthetic systems. While the absorption spectra of N-Confused porphyrins and normal porphyrins are distinctly dissimilar, the absorption characteristics are similar for N-Confused porphyrins and reduced normal porphyrins, which is due to the decreased symmetry of the N-Confused porphyrin ring. Additionally, the fluorescence lifetimes and fluorescence quantum yields are significantly smaller than those found in H2TPP. Relative to chlorophylls and chlorins, N-Confused porphyrins have significantly reduced fluorescent lifetimes and fluorescence quantum yields. From these experiments, the nonplanar structure of the internally protonated tautomer of N-Confused porphyrin likely induces an increase in the rate constants for internal conversion (kIC) and intersystem crossing (kISC).
Additional research into the characteristics that describe the reduced fluorescence lifetimes and fluorescence quantum yields observed in N-Confused porphyrins should afford a better understanding of the efficient light harvesting macrocycles found in nature, and provide knowledge for reproducing this chemistry with model compounds for artificial photosynthesis.
The transitory excited states observed for the externally protonated and internally protonated tautomers of N-Confused porphyrin relative to normal porphyrin free-base do not preclude their possible use as polarity dependent control devices or as model 32 compounds for studying the processes coupled with artificial photosynthesis. N-
Confused porphyrins have exceptional absorption properties in the visible region of the absorption spectrum compared with normal porphyrins and have fluorescence lifetimes similar to the metalloporphyrin, ZnTPP. The differences in electronic structure and planarity of the two tautomers may influence how they participate in energy and electron transfer to acceptor molecules. These processes might be controlled by the solvent dependent protonation in N-Confused porphyrins.
33 CHAPTER III
THE SYNTHESIS AND CHARACTERIZATION OF ZINC N-CONFUSED
TETRAPHENYLPORPHYRIN
Introduction
It has been shown in the literature that N-Confused tetraphenylporphyrin and subsequent metallation reactions are easily prepared in the laboratory. The objectives of this research were to metallate N-Confused tetraphenylporphyrin with biologically relevant transition metals, coordinate derivatives of biological ligands to the metal center, and characterize the compounds using UV-visible absorption spectroscopy, 1H NMR spectroscopy, mass spectroscopy, photooxidation, and x-ray crystallography. It was anticipated that metallating NCTPP with zinc acetate dihydrate would lead to zinc N-
Confused tetraphenylporphyrin (Zn(NCTPP)) with the metal center coordinated to three interior pyrrolic nitrogens and the interior pyrrolic ß-carbon, however, structure elucidation via x-ray crystallography revealed that the zinc metal center has pseudo- tetrahedral geometry, coordinating with the three interior pyrrolic nitrogens and a ligand at the axial location of the porphyrin. The interior pyrrolic ß-carbon remains protonated contributing to the steric hindrance in the interior of the macrocycle that rotates the N-
Confused pyrrole out of the porphyrin plane.
34 It is possible that an agostic interaction exists between the zinc metal center and the C-H bond of the out-of-plane rotated N-Confused pyrrole. By varying the reagents, solvents, and degree of purification, three distinct crystallographic compounds were made by slow solvent diffusion.
Experimental Section
Synthesis. N-Confused tetraphenylporphyrin was synthesized and purified as described by Geier et al., and subsequently metallated using a modified metallation reaction of
Girolami, Rauchfuss, and Angelici.7,79 Zn(NCTPP) was synthesized in a 250 mL round bottom flask by refluxing 0.16 mol of H2NCTPP and 0.8 mol of Zn(O2CCH3)2•2H2O in
N,N-dimethylformamide for 30 min. using a heated magnetic stir plate, a sand trap for even heating, a Teflon-coated magnetic stir bar for mixing, and a water cooled reflux condenser (Figure 19). The reaction in the flask was isolated from the ambient light of the laboratory. The percent yield of the metallation reaction was determined to be 85%.
Experimentation and characterization of Zn(NCTPP) were performed on both purified and unpurified preparations.
Ph H Ph N N L N,N-dimethylformamide N (100mL), Reflux, 30 min. N Ph Ph + Zn(O CCH ) •2H O Ph Zn Ph H 2 3 2 2 N N N N
Ph Ph Zinc acetate N-Confused Tetraphenylporphyrin dihydrate Zinc N-Confused tetraphenylporphyrin 0.16 mol 0.8 mol 0.136 mol (85% yield)
Figure 19. Synthesis of Zinc N-Confused tetraphenylporphyrin.
L corresponds to a ligand at the axial site.
35 Purification of Zn(NC-TPP). Initial purification required removal the DMF reaction solvent using rotary evaporation that resulted in a dry black-green solid. The dry solid was solvated with dichloromethane and filtered with a fine frit under vacuum to separate the product from the reactants. Purification of Zn(NCTPP) was accomplished by silica gel column chromatography, 60 Å, 63-200 µm, with dimensions 2 inch diameter by 18 inch length, employing acetone/hexanes (1:1) as the eluent. This procedure was established by selecting the optimal separation of Zn(NCTPP) from impurities through trials utilizing thin layer silica gel plates chromatographed in various ratios of acetone, dichloromethane, ethanol, and hexanes.
Results and Discussion
UV-visible Absorption Spectroscopy Analysis. UV-visible absorption spectra were acquired in the region from 350 to 850 nm in both coordinating and non-coordinating solvents with matched quartz cuvettes of 1 cm path length and a Hitachi U-3310 single monochrometer UV-visible spectrophotometer. All solvents were used as received from the manufacturer. Where applicable, the UV-visible absorption spectra were processed with Savitsky-Golay filtering due to the introduction of analog-to-digital noise in the spectra at low analyte concentrations.80 Peaks and shoulders in the spectra were identified by Savitsky-Golay smoothed 1st and 2nd derivative curves.80
The UV-visible absorption spectrum for Zn(NCTPP) is dependent on both solvent and concentration. In dichloromethane, a non-coordinating solvent, the UV-visible absorption spectrum is characterized by a prominent Soret band at 462 nm and four Q- bands at 540, 629, 720, and 770 nm, ascribed to π → π* transitions. Consistent with some porphyrin absorption spectra, the Soret band and Q-bands for the metallated macrocycle 36 81 are red-shifted relative to the corresponding absorptions for unmetallated H2NCTPP.
These minor perturbations of the electronic spectra from higher energies of the free-base macrocycle are brought about by the effect of the central substituent on the porphyrin ring π electrons.82 Four distinct Q-band absorptions in the visible region of the spectrum for the metallated macrocycle are attributed to the preservation of the D1h symmetry of the parent ligand. Less prominent features of the absorption spectrum include an N-band absorption at 393 nm, two Soret shoulders at 475 and 485 nm, and a Q-band shoulder at
541 nm. The higher energy shoulders at 437 and 415 nm in the region of the Zn(NCTPP)
Soret band correspond to absorptions that are identical to the Soret bands for the interiorly protonated tautomer of H2NCTPP and H2TPP, respectively. Q-band shoulders at 541 nm and 662 nm have identical absorptions to the Qy(1,0) and Qx(1,0) transitions, respectively, of unmetallated H2NCTPP (Figure 20). In N,N-dimethylacetamide, a coordinating solvent, the absorption spectrum is characterized by a marked Soret band at
464 nm, a shoulder and two Q-bands at 582 nm, 632 nm, and 698 nm. The lowest energy
Q-band absorption analogous to the Qx(0,0) transition in H2NCTPP is absent from the absorption spectra in coordinating solvents due to the solvent interacting with the unprotonated N-Confused pyrrole of the metallated macrocycle. It is noted that a low intensity high-energy shoulder on base of the Soret band at 441 nm has an analogous absorption to the exterior protonated tautomer of H2NCTPP. A well-defined N-band is observed at 398 nm that is attributed to a porphyrin π → π* transition (Figure 21).81
37
Figure 20. UV-visible absorption spectrum for Zn(NCTPP) in dichloromethane.
Figure 21. UV-visible absorption spectrum for Zn(NCTPP) in N,N-dimethylacetamide.
38 Coordination Experiment. A coordination experiment was performed to study the
Zn(NCTPP) dimerization by concentration variation through coordination at the zinc metal center with the exterior pyrrolic nitrogen of an adjacent Zn(NCTPP) macrocycle.
The motivation for this experiment resulted from the observation that during the process of column chromatography purification, the Q-band region of the UV-visible absorption spectra for Zn(NCTPP) varied in intensity pattern with concentration. The experiment was carried out using unspecified concentrations of a purified preparation of Zn(NCTPP) in dichloromethane. A concentrated solution of Zn(NCTPP) was added successively via disposable pastuer pipet to a cuvette followed by acquisition of the UV-visible absorption spectrum. The volume of the experimental cuvette was maintained by allowing for the evaporation of the solvent or the addition of solvent. All UV-visible absorption spectra were normalized relative to the Soret band at 462 nm and subsequently processed with
Savitsky-Golay filtering.80
In dichloromethane, the Zn(NCTPP) Q-band absorptions have varying intensities that are dependent on concentration. At low concentrations, the UV-visible absorption spectrum for Zn(NCTPP) strongly resembles the absorption intensity band pattern of the exterior protonated tautomer of NCTPP, with the exception that the absorption bands are red-shifted. Q-band absorptions at the lowest concentration in the experiment were observed at 586, 629, 720, and 774 nm corresponding to π → π* transitions, respectively, with Q-band oscillator strengths increasing with decreasing energy. At the highest concentration in the experiment, the Q-band absorptions were observed at 582, 629, 720, and 766 nm, respectively, with the absorption band intensity pattern of 629 > 720 > 766 >
582. The trends observed for the normalized Q-band absorption intensity patterns in the 39 transition from low to high concentration were that the highest and lowest energy Q-band absorptions are blue-shifted from 774 to 766 nm and 586 to 582 nm, respectively, the two intermediate Q-band absorptions at 720 and 629 nm, respectively, remain unchanged in wavelength, the lowest energy Q-band absorption and lower energy intermediate Q-band absorption interchange from peak to shoulder, respectively, and the lowest energy Q- band absorption progressively decreases as the other Q-band absorptions increase
(Figures 22 and 23, Table 3). These spectral changes can be interpreted as dimerization of the Zn(NCTPP) macrocycles. The observed decrease in the lowest energy Q-band absorption is due to the coordination of the N-Confused pyrrole of Zn(NCTPP) to an adjacent macrocycle. Pseudo-isosbectic points in the range from 726 to 755 nm are indicative of interconvertible absorbing forms of two species at various concentrations.
Figure 22. Normalized UV-visible absorption spectra for Zn(NCTPP) from low
concentration to high concentration.
40
Figure 23. Normalized UV-visible absorption spectra of the Q-band region for
Zn(NCTPP).
Table 3. Peaks and shoulders for the lowest and highest concentrations of Zn(NCTPP).
N-Band Soret Band Soret Shoulder Soret Shoulder Q-Bands Concentration (nm) (nm) (nm) (nm) (nm) (nm) (nm) (nm) Low 394 462 472 486 586 629 720 774 High 386 462 471 485 582 629 720 766 ∆ Wavelength 8 0 1 1 4 0 0 8 (Low – High)
Examination of the Soret band region of the Zn(NCTPP) absorption spectra revealed underlying bands that are red-shifted relative to the primary Soret absorption.
Application of higher degree polynomial-based 1st and 2nd derivatives to the Soret band region allowed for the elucidation of two Soret shoulders in the region of 471 to 486 nm.
At the lowest concentration in dichloromethane, the first red-shifted low intensity shoulder from the primary Soret band was located at 472 nm while the second lower energy low intensity absorption was located at 486 nm. At the highest concentration in 41 the experiment, the first lower energy shoulder on the main Soret peak was observed at
471 nm while the second shoulder remained at 485 nm. At higher energies, the N-band absorption was blue-shifted from 394 to 386 nm from low to high concentration, respectively. The lower energy shoulders in the region of 471 to 486 nm of the primary
Soret absorption mostly are unaffected by analyte concentration whereas the lowest and highest energy Q-band absorptions are concentration dependent. It is noted that in the transition from low to high concentration that both the N-band absorption and the lowest energy Q-band transition blue-shift by 8 nm. The lower energy shoulders of the Soret absorption seem to be unaffected by dimerization (Figure 22).
Nuclear Magnetic Resonance Spectroscopy. 1D 1H NMR spectroscopy was performed on a column chromatography purified preparation of Zn(NCTPP) using a Varian 300
MHz NMR spectrometer. 1D 1H NMR spectra of zinc N-Confused tetraphenylporphyrin were obtained separately in deuterated chloroform and deuterated pyridine. Numerical values for the chemical shifts of peaks of interest were obtained from digitally scanned images of printed spectra using Plot Digitizer.83 Impurities in the 1H NMR spectra were identified using the published work of Gottlieb, Kotlyar, and Nudelman.84
1 Analysis of the H NMR spectrum of Zn(NCTPP) in CDCl3 suggests the presence of several species in solution despite efforts to purify the product from the reactants using column chromatography and evaporation of the eluents used for chromatographic purification (Figure 24 and Table A1). Peaks relevant to the Zn(NCTPP) macrocycle include the peak at –3.09 ppm corresponding to the interior pyrrolic C-H, the peak at 5.64 ppm corresponding to proton exchange of the N-Confused pyrrolic nitrogen with water, and the aromatic region of the spectrum from 6.55 ppm to 8.68 ppm with the exception of 42 the peak at 7.24 ppm corresponding to chloroform. Evidence of the
Zn4(NCTPP)2(O2CCH3)3(OH) dimer is observed in the spectrum from the peaks at –1.32 ppm and 0.69 ppm in a 2:1 ratio, corresponding to methyl protons of the axially coordinated acetates to zinc 1 and zinc 2 of the porphyrin macrocycles and the bridged acetate of zinc 3 and zinc 4 of the dizinc core, respectively. The methyl protons of the axially coordinated acetate are located at –1.32 ppm due to the anti-stereochemistry of the ligand that directs the methyl protons into the ring current of the porphyrin macrocycle. The peaks at 6.55 ppm and 1.98 ppm suggest the presence of two distinct dimers in solution and correspond to the N-Confused α-pyrrolic C-H in
Zn4(NCTPP)2(O2CCH3)3(OH) and (Zn(NCTPP))2, respectively. The α-pyrrolic C-H of
(Zn(NCTPP))2 dimer is shifted up field into the aliphatic region due to the ring current effects of the adjacent Zn(NCTPP). Peak broadening in the aromatic region of the spectrum further suggests the presence of two types of dimeric and monomeric
Zn(NCTPP) coordinated to polar ligands in solution, however, this effect generally is indicative of fast T2 relaxation which is dependent on choice of solvent and can be due to transient ligand binding. The relatively high concentration of water, a contaminant, is responsible for the large broad peak at 1.63 ppm. The effect of the water concentration in the sample provides an exchangeable proton to interact with the confused pyrrolic nitrogen that is observed at 5.64 ppm. This exchangeable proton interaction alters the resonance of the metallated macrocycle and creates magnetically inequivalent protons, and thereby contributes to peak broadening in the aromatic region of the spectrum. This peak broadening is in contrast to the well-defined peaks in the aliphatic region of the spectrum. One of the reactants for synthesizing Zn(NCTPP), zinc acetate dihydrate, may 43 contribute to the likelihood of an axially coordinated hydroxide corresponding to the peak at –0.87 ppm.
1 Analysis of the H NMR spectrum of Zn(NCTPP) in pyridine-d5 suggests the presence of monomeric Zn(NCTPP) with axially coordinated pyridine (Figure 25 and
Table A2). Peaks relevant to the Zn(NCTPP) macrocycle include –3.11 ppm, 5.71 ppm, and the aromatic region from 7.23 ppm to 9.29 ppm. The peak at 8.38 ppm corresponds to the residual ortho protons on axially coordinated deuterated pyridine, shifted up field due to the relative proximity to the ring current effects of the metallated porphyrin macrocycle. The peaks at 7.21 ppm, 7.58 ppm, and 8.73 ppm correspond to the residual meta, para, and ortho protons of deuterated pyridine, respectively.
44
. 3 l C D C
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um r t c pe s
R M N
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24.
e gur i F
45
. 5 d - ne di i yr p
n i
) P P T C N ( n Z
of
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46 Mass Spectroscopy. Electrospray ionization mass spectroscopy was performed on a purified preparation of Zn(NCTPP) solvated in methanol and analyzed on a Bruker
Esquire LC ion trap mass spectrometer. The acquisition parameters for obtaining the spectra include a scanning range from 100 to 800 m/z, a positive ion polarity, an accumulation time of 1000 µs, and the averaging of 20 spectra. A main peak at 677.5 m/z corresponds to Zn(NCTPP) as a positive molecular ion without an axially coordinated ligand (Figure 26). The group of peaks from 677.5 to 685.3 m/z correspond predominantly to the natural abundance of the various isotopes of zinc coordinated with the macrocycle (Figure 27). The peak at 677.5 m/z corresponds to the average mass for zinc coordinated with NCTPP while the peaks at 678.4, 679.4, 680.3, and 682.2 m/z correspond to 66Zn, 67Zn, 68Zn, 70Zn coordinated with the macrocycle with the relative abundances of 27.9, 4.1, 18.8, and 0.6 percent. The peak at 681.3 m/z corresponds to
68Zn coordinated with NCTPP that have 13C and 15N with natural abundances of 1.10 and
0.37 percent, respectively. Likewise, the low amplitude peaks at 683.2, 684.2, and 685.3 m/z correspond to 70Zn coordinated with the macrocycle that have various combinations of 13C and 15N in low natural abundance.
47
Figure 26. Mass spectrum of purified Zn(NCTPP).
Figure 27. Expanded region of the mass spectrum of purified Zn(NCTPP) for the 677.5
m/z peak showing the naturally occurring zinc, 13C, and 15N isotope patterns.
48 Photooxidation. A photooxidation study was performed on a purified preparation of
Zn(NCTPP) solvated in dichloromethane. The motivation for this experiment resulted from the observation that a solution of Zn(NCTPP) solvated with dichloromethane noticeably changes color from green to dark green with concomitant degradation upon exposure to fluorescent light in the presence of oxygen in approximately twelve hours. A solution of Zn(NCTPP) in dichloromethane was placed in a glass screw-cap vial and exposed to direct sun light for approximately six hours. The color of the solution changed from green to reddish-purple. The UV-visible absorption spectrum was filtered using Savitsky-Golay smoothing due to analog to digital noise introduced into the spectra at low analyte concentrations.80 Peaks and shoulders in the spectra were identified by
Savitsky-Golay smoothed 1st and 2nd derivative curves.80 The UV-visible absorption spectrum of this solution showed a broader Soret band and significantly reduced Q-band structure relative to a purified preparation of Zn(NCTPP) (Figure 28).
Figure 28. UV-visible spectrum of photooxidized Zn(NCTPP) in dichloromethane. 49 The UV-visible absorption spectrum for photooxidized Zn(NCTPP) is markedly different from the absorption spectrum of non-photooxidized Zn(NCTPP). The wavelength of the Soret band and a single well-defined single Q-band at 774 nm resembles the UV-visible absorption spectrum of reduced porphyrins with a characteristically strong far-red absorption due to the reduction of one pyrrole ring to the chlorin level.85 The well-defined band at 419 nm and the shoulder at 439 nm absorb at the same energies as the Soret bands of H2TPP and H2NCTPP, respectively. Likewise, the low intensity Q-bands at 551 nm and 594 nm absorb at the same energies as the
Qy(0,0) and Qx(1,0) transitions of H2TPP, respectively. Low intensity bands at 580 nm and 721 nm absorb at energies identical to the Qy(0,0) and Qx(0,0) transitions of
H2NCTPP, respectively.
The proposed mechanism by which Zn(NCTPP) decomposes in the presence of light and oxygen is given in the following series of reactions.
h" Zn(NCTPP) ## $ S1 S ## $ T 1 1 3 1 T1+ O2 ## $ S0 + O2 1 Zn(NCTPP)+ O2 ## $ Oxidized Products
Zn(NCTPP) l!ike ly absorbs a photon and is excited to the S1 state where it can readily
86 undergo intersystem crossing to the corresponding T1 state. The T1 excited Zn(NCTPP) reacts with ground state triplet molecular oxygen in a photosensitization process whereby the excited Zn(NCTPP) drops to the S0 state and ground state triplet molecular oxygen is promoted to the reactive excited singlet state. Ground state Zn(NCTPP) then likely reacts 50 with singlet state molecular oxygen where it is oxidized at the α- and β- positions of the
N-Confused pyrrole that allow for further oxidation of the remnants of the macrocycle.
Furuta et al. observed NCTPP oxidation, and reported the mechanism and structure of the
Cu(II)-assisted oxygenolysis of H2NCTPP to form the 14-benzoyl-5,10-diphenyl-1-oxo- tripyrrin Cu(II) complex.11 It is possible that Zn(NCTPP) oxidation undergoes a similar
11 reaction to the Cu(II)-assisted oxygenolysis of H2NCTPP.
The products of the photooxidation reaction were analyzed via electrospray ionization mass spectroscopy on a Bruker Esquire LC ion trap mass spectrometer. The acquisition parameters for obtaining the spectra include a scanning range from 50 to 1500 m/z, alternating ion polarity, an accumulation time of 463 µs, and an averaging of 30 spectra. Five peaks of interest that correspond to degraded Zn(NCTPP) molecular fragments and listed in order of decreasing intensity are 644.5, 491.4, 539.3, 1048.5,
1203.4 m/z (Figure 29). The peak at 1203.4 m/z has an isotope pattern consistent with two zinc atoms coordinated with degraded NCTPP molecular fragments while the other peaks at 644.5, 491.4, 539.3, and 1048.5 m/z show evidence of an isotope pattern corresponding to a single zinc atom coordinated with degraded NCTPP derivatives
(Figures 30 and 31). Due to the limitations of this spectroscopic technique and lack of further analyses, only candidate chemical structures resulting from metallated NCTPP photooxidation can be proposed that match the m/z present in the spectra. Candidate chemical structures corresponding to the five main peaks of interest are listed in Figure
32. Four peaks of much lower intensity relative to the five main peaks are 712.2, 986.2,
1135.5, and 1310.2 m/z with candidate chemical structures listed in Figure 33. The group of peaks near 1135.5 m/z suggest multiple species with similar molecular masses. The 51 five peaks, 644.5, 491.4, 539.3, 1048.5, and 1203.4 m/z, likely correspond to intermediates with relative stability to other short-lived species or are stable end products of the photooxidation process.
Figure 29. Mass spectrum of the photooxidation of Zn(NCTPP) from 100 to 1500 m/z.
52
Figure 30. Mass spectrum of the photooxidation of Zn(NCTPP) from 480 to 650 m/z.
53
Figure 31. Mass spectrum of the photooxidation of Zn(NCTPP) from 1020 to 1350 m/z.
54 Ph Ph Ph OH OH O N O N N Ph Zn Ph Ph Zn Ph Zn N N N N N O Ph Ph Ph
C40H26N3O2Zn C29H20N2O2Zn C33H21N3OZn Exact Mass: 644.13 Exact Mass: 492.08 Exact Mass: 539.10 Mol. Wt.: 646.04 Mol. Wt.: 493.87 Mol. Wt.: 540.93 C, 74.36; H, 4.06; C, 70.53; H, 4.08; N, 5.67; O, C, 73.27; H, 3.91; N, 7.77; N, 6.50; O, 4.95; 6.48; Zn, 13.24 O, 2.96; Zn, 12.09 Zn, 10.12
Ph Ph Ph Ph O O OH N N N N N O H H N Ph Zn O O Ph Ph Zn Ph H H N N N N N N HO O Ph
Ph Ph Ph
C66H44N6O4Zn C66H46N6O4Zn Exact Mass: 1048.27 Exact Mass: 1050.29 Mol. Wt.: 1050.48 Mol. Wt.: 1052.50 C, 75.46; H, 4.22; N, 8.00; O, 6.09; Zn, 6.22 C, 75.32; H, 4.41; N, 7.98; O, 6.08; Zn, 6.21
Ph Ph Ph Ph Ph O OH N N N Phe N N O Zn O Ph Zn OH Zn Ph Ph Ph Zn N N O N N N N N O O Ph Ph Ph Ph
C73H49N6O4Zn2 C73H48N6O4Zn2 Exact Mass: 1201.24 Exact Mass: 1200.23 Mol. Wt.: 1204.99 Mol. Wt.: 1203.98 C, 72.76; H, 4.10; N, 6.97; O, 5.31; Zn, 10.85 C, 72.82; H, 4.02; N, 6.98; O, 5.32; Zn, 10.86
Figure 32. Candidate chemical structures corresponding to the five main peaks of
photooxidized Zn(NCTPP) fragments.
55 Ph Ph N HO N Ph Ph OH O O N N HOOH O O Ph Zn Ph Ph Zn Ph N N Ph Zn N N N N N N Zn Ph N N
Ph Ph Ph Ph
C44H30N4O2Zn C68H45N6O4Zn2 Exact Mass: 710.17 Exact Mass: 1137.21 Mol. Wt.: 712.12 Mol. Wt.: 1140.90 C, 74.21; H, 4.25; N, 7.87; O, 4.49; Zn, 9.18 C, 71.59; H, 3.98; N, 7.37; O, 5.61; Zn, 11.46
Ph Ph Ph Ph Ph O OH OH N N O N N H N Ph Zn Zn Ph Ph Zn H N N N O N O OH O Ph
Ph Ph Ph
C58H40N4O4Zn2 C62H43N5O4Zn Exact Mass: 984.16 Exact Mass: 985.26 Mol. Wt.: 987.74 Mol. Wt.: 987.43 C, 70.53; H, 4.08; N, 5.67; O, 6.48; Zn, 13.24 C, 75.41; H, 4.39; N, 7.09; O, 6.48; Zn, 6.62
Ph Ph Ph Ph OH Ph N N N N O O O Zn H N O Ph Zn Ph Ph N N Ph H Zn O O N N N N O N Ph Ph Ph Ph
C73H50N6O4Zn C68H42N6O4Zn2 Exact Mass: 1138.32 Exact Mass: 1134.19 Mol. Wt.: 1140.61 Mol. Wt.: 1137.88 C, 76.87; H, 4.42; N, 7.37; O, 5.61; Zn, 5.73 C, 71.78; H, 3.72; N, 7.39; O, 5.62; Zn, 11.49
Ph Ph O Ph N OH N Zn Ph Ph N N N N HO Ph OH Ph C73H50N6O4Zn Exact Mass: 1138.32 Mol. Wt.: 1140.61 C, 76.87; H, 4.42; N, 7.37; O, 5.61; Zn, 5.73
Figure 33. Candidate chemical structures corresponding to the four minor peaks of
photooxidized Zn(NCTPP) fragments.
56 Crystallography. All crystallography experiments were performed using slow solvent diffusion techniques in capped NMR tubes and darkness. All solvents were used as received from the manufacturer. Data for Zn4(NCTPP)2(O2CCH3)3(OH) and
Zn(NCTPP)(Pyr) were collected at 100K on a (Bruker KRYO-FLEX) Bruker SMART
APEX CCD-based X-ray diffractometer system. Data for Zn(NCTPP)(DMSO) was collected on a P21 Syntex X-ray diffractometer with a point detector. Molecular graphics images were produced using the UCSF Chimera package from the Computer Graphics
Laboratory, University of California, San Francisco.87 Crystallographic data for the crystals of Zn4(NCTPP)2(O2CCH3)3(OH), Zn(NCTPP)(DMSO), and Zn(NCTPP)(Pyr) can be found in Table 4.
57 Table 4. X-Ray Crystal Data for Zn4(NCTPP)2(O2CCH3)3(OH), Zn(NCTPP)(DMSO), and Zn(NCTPP)(Pyridine).
Compound Zn4(NCTPP)2(O2CCH3)3(OH) Zn(NCTPP)(DMSO) Zn(NCTPP)(Pyridine) Crystals grown by Hexanes/CH2Cl2 Methanol/DMSO Hexanes/Pyridine slow diffusion (1.45:1)/(2.3:1) (1:1.14) (2.4:1)/(3.25:1) Crystal habit Black-green platelets Black-green needles Black-green platelets Empirical formula Zn4C94H66N8O7 ZnC46H34N4SO ZnC49H33N5 Formula weight 1681.20 756.28 757.25 Temperature [K] 100(2) 293(2) 373(2) Wavelength [Å] 0.71073 0.71073 0.71073 Crystal system Triclinic Triclinic Triclinic Space group P-1 P-1 P-1 Unit cell dimensions a [Å] = 13.0462(8) a [Å] = 11.387(2) a [Å] = 12.9230(8) b [Å] = 14.5580(9) b [Å] = 14.011(3) b [Å] = 13.1402(8) c [Å] = 22.3987(14) c [Å] = 14.807(3) c [Å] = 14.7780(9) α [°] = 87.2080(10) α [°] = 83.483(5) α [°] = 73.4000(10) β [°] = 84.7900(10) β [°] = 73.165(6) β [°] =70.1310(10) γ [°] = 81.9040(10) γ [°] = 85.752(7) γ [°] = 66.9550(10) Volume [Å3] 4191.5(4) 2244.5(7) 2137.7(2) Z 2 2 2 Density (calculated) 1.332 1.119 1.176 [Mg/m3] Absorption 1.264 0.626 0.899 coefficient [mm-1] F(000) 1326 748 958 Crystal size [mm3] 0.4 x 0.2 x 0.02 0.17 x 0.01 x 0.01 0.50 x 0.10 x 0.10 Theta range for data 0.91 to 23.47 2.33 to 23.23 1.49 to 25.00 collection [°] Index ranges -14 ≤ h ≤ 14 -12 ≤ h ≤ 12 -15 ≤ h ≤ 15 -16 ≤ k ≤ 16 -15 ≤ k ≤ 14 -15 ≤ k ≤ 15 -24 ≤ l ≤ 24 -16 ≤ l ≤ 16 -17 ≤ l ≤ 15 Reflections collected 26891 12042 16108
Independent 12149 12042 7487 Reflections Absorption SADABS SADABS SADABS correction Refinement method Full-matrix least-squares on Full-matrix least- Full-matrix least- F2 squares on F2 squares on F2 Data / restraints / 12149 / 633 / 1007 12042 / 0 / 580 7487 / 18 / 627 parameters Goodness-of-fit on 1.009 0.901 1.130 F2 Final R indices R1 = 0.0890, R1 = 0.1863, R1 = 0.0676, [I>2sigma(I)] wR2 = 0.1723 wR2 = 0.3903 wR2 = 0.1453 R indices (all data) R1 = 0.1705, R1 = 0.3854, R1 = 0.0807, wR2 = 0.2075 wR2 = 0.4420 wR2 = 0.1514 Largest diff peak and 1.346 and -0.765 0.961 and –0.955 0.663 and –0.652 hole [e.Å-3]
58 Zn4(NCTPP)2(O2CCH3)3(OH). A single crystal of dimeric zinc N-Confused tetetraphenylporphyrin from an unpurified metallation reaction of NCTPP and
3 Zn(O2CCH3)2•2H2O, with dimensions 0.4 x 0.2 x 0.02 mm , was used for structure elucidation. Upon completion of the metallation reaction, N,N-dimethylformamide was removed via rotary evaporation resulting in a dark green solid consisting of Zn(NCTPP) and excess Zn(O2CCH3)2•2H2O. The unpurified product, Zn(NCTPP) and excess
Zn(O2CCH3)2•2H2O, was solvated with dichloromethane at high concentration and placed in two NMR tubes with hexanes layered over the Zn(NCTPP)-
Zn(O2CCH3)2•2H2O/CH2Cl2 solution in ratios of 1.45:1 and 2.3:1. Crystals were observed in both NMR tubes after a duration of one month under darkness. Structure elucidation revealed a tetra-nuclear zinc(II) N-Confused tetraphenylporphyrin dimer with three bridging acetates and one µ-hydroxide in accord with C2 symmetry. Each zinc atom in the compound has distorted tetrahedral coordination. The N-Confused pyrrole with the intact interior C-H bond adjacent to the zinc metal center is rotated 40 degrees out of the plane of the porphyrin. A dizinc core with colinear bridging acetate and µ- hydroxide is interjacent the zinc N-Confused tetetraphenylporphyrins. The dizinc core is coordinated by the exterior pyrrolic nitrogen from each zinc N-Confused tetetraphenylporphyrin and one oxygen from each colinear acetate. The second oxygen from each colinear acetate is axially coordinated to the zinc metal center of each
Zn(NCTPP). The colinear bridging acetate and µ-hydroxide of the dizinc core are approximately perpendicular to the colinear and axially coordinated acetates (Figures 34,
35, 36, and 37).
59 O O O O O O
Ph Ph Zn Zn N N N N Ph Zn Ph O Ph Zn Ph N N H N N
Ph Ph
Figure 34. Coordination of the ligands with the zinc metal centers.
The nitrogen of the N-Confused pyrrole and the zinc atom with axially coordinated acetate are on the same side of the plane of the porphyrin, resulting in all four zinc atoms of the dimer being located between the planes of the porphyrin macrocycles (Figure 36).
60
Figure 35. Molecular structure of Zn4(NCTPP)2(O2CCH3)3(OH). The dizinc core is in
the center of the image with three coordinated acetates colored black and one µ- hydroxide interjacent the zinc N-Confused tetraphenylporphyrins. Exterior pyrrole rings
coordinated with the dizinc core are colored black.
61
Figure 36. Molecular structure of Zn4(NCTPP)2(O2CCH3)3(OH). Perspective along the
edges of the zinc N-Confused tetraphenylporphyrin rings. The dizinc core is located in the center of the image with bridging acetate (oxygens 5 and 6), µ-hydroxide (oxygen 7),
and exterior pyrrolic nitrogens (nitrogens 4 and 8). Acetates and N-Confused pyrroles
coordinating with the dizinc core are colored black.
62
Figure 37. Molecular structure of the Zn4(NCTPP)2(O2CCH3)3(OH) dizinc core with coordinating ligands and inter-atomic distances. Chemical bonds are represented by dark
dashed lines and inter-atomic distances are represented by dashed lines (Left). Bond lengths and interatomic distances are in angstroms. The dizinc core is in the hydrophobic
environment of the Zn(NCTPP) dimer (Right).
The zinc atoms in Zn4(NCTPP)2(O2CCH3)3(OH) have a flexible coordination geometry, which is due to the unique chemistry of zinc and its ability to accommodate various electronegative atoms in its coordination sphere.88 The zinc atoms of the N-
Confused porphyrins are coordinated by three pyrrolic nitrogens and an axial oxygen of acetate while the zinc atoms of the dizinc core are coordinated by three oxygens of acetates and one exterior pyrrolic nitrogen. The zinc atoms of each N-Confused tetraphenyporphyrin have more distorted tetrahedral geometry than the zinc atoms of the
63 dizinc core. The Zn1-ligand bond lengths for interior pyrrolic N1, N2, and N3 of the macrocycle and O1 of axial acetate are 2.091(7), 2.001(7), 2.098(8), and 2.017(7) Å, respectively (Table 5). The Zn2-ligand bond lengths for interior pyrrolic N5, N6, and N7 of the macrocycle and O3 of axial acetate are 2.069(8), 2.005(8), 2.088(8), and 2.039(6)
Å, respectively (Table 5). In both zinc N-Confused porphyrins, the metal-ligand bond length for the interior pyrrolic nitrogen between the two adjacent pyrrolic nitrogens is approximately 4 percent shorter than the two metal-ligand bonds lengths of the adjacent pyrrolic nitrogens. Further evidence of the distorted tetrahedral geometry about the zinc metal centers in the macrocycles is observed in the bond angles between the interior pyrrolic nitrogens and axially coordinated acetates (Table 6). The bond angles formed between the axially coordinated acetate, zinc metal center, and the three pyrrolic nitrogens range from 99.7(3) to 116.9(3) degrees while the bond angles between the interior pyrrolic nitrogens and the zinc metal center are approximately 90.7(3) degrees and 150.6(3) degrees (Table 6). The bond angle between Zn1, O1, and C89 of the axially coordinated acetate is 132.4(6) degrees, while the corresponding bond angle for Zn2, O3, and C91 of the axially coordinated acetate is 131.9(6) degrees. The zinc atoms of the dizinc core are separated by a distance of approximately 3.228 Å (Figure 37). The Zn- ligand bond lengths for Zn3 and acetate O2, acetate O5, µ-hydroxide O7, and N-
Confused pyrrolic N8 are 1.973(6), 2.005(7), 1.923(7), and 2.019(8) Å, respectively
(Figure 37 and Table 5). The Zn-ligand bond lengths for Zn4 and acetate O4, acetate O6,
µ-hydroxide O7, and N-Confused pyrrolic N4 are 1.952(6), 1.996(8), 1.928(7), and
2.019(8) Å, respectively (Figure 37 and Table 5). The µ-hydroxide O7 has a shorter bond length with each zinc atom than that observed for the acetate ligands due to the 64 sharing of a singlely bonded oxygen between the two metal centers. Both zinc atoms in the core have N-Confused pyrrolic nitrogen bond lengths of 2.019(8) Å and have out-of- plane torsion angles slightly greater than 30 degrees. The bond angles for the zinc metal centers of the dizinc core and the tetrahedrally coordinated ligands range from 97.7(3) to
99.4(3) degrees, 100.4(3) to 109.3(3) degrees, and 119.5(3) to 128.0(3) degrees (Table 6).
Bond angles for the bridging acetate C93-O5-Zn3 and C93-O6-Zn4 are 128.5(7) and
133.2(7) degrees, respectively, and are greater than an 120 degrees due to steric strain resulting from the zinc interatomic distance of the dizinc core.
Table 5. Zinc-ligand distances for Zn(NCTPP) and the dizinc core.
Zn(NCTPP) Zn-Ligand Distances (Å) Zn1 Zn2 Ligand 2.091(7) NCTPP Interior N1 2.001(7) NCTPP Interior N2 2.098(8) NCTPP Interior N3 2.017(7) Acetate O1 2.069(8) NCTPP Interior N5 2.005(8) NCTPP Interior N6 2.088(8) NCTPP Interior N7 2.039(6) Acetate O3
Dizinc core Zn-Ligand Distances (Å) Zn3 Zn4 Ligand 1.973(6) Acetate O2 2.005(7) Acetate O5 1.923(7) µ-hydroxide O7 2.019(8) NCTPP Exterior N8 1.952(6) Acetate O4 1.996(8) Acetate O6 1.928(7) µ-hydroxide O7 2.019(8) NCTPP Exterior N4
65 Table 6. Zn1(NCTPP) and Zn2(NCTPP) bond angles.
Ligand-Zn-Ligand Bond Angles (degrees) Bond Angle Ligand-Zn-Ligand 99.7(3) Acetate O1 - Zn1 - NCTPP Interior N1 116.9(3) Acetate O1 - Zn1 - NCTPP Interior N2 105.7(3) Acetate O1 - Zn1 - NCTPP Interior N3 90.7(3) NCTPP Interior N1 - Zn1 - NCTPP Interior N2 150.6(3) NCTPP Interior N1 - Zn1 - NCTPP Interior N3 90.8(3) NCTPP Interior N2 - Zn1 - NCTPP Interior N3
Ligand-Zn-Ligand Bond Angles (degrees) Bond Angle Ligand-Zn-Ligand 99.8(3) Acetate O3 - Zn2 - NCTPP Interior N5 117.7(3) Acetate O3 - Zn2 - NCTPP Interior N6 103.8(3) Acetate O3 - Zn2 - NCTPP Interior N7 91.2(3) NCTPP Interior N5 - Zn2 - NCTPP Interior N6 152.3(3) NCTPP Interior N5 - Zn2 - NCTPP Interior N7 90.5(3) NCTPP Interior N6 - Zn2 - NCTPP Interior N 7
Zn3 and Zn4 Ligand Bond Angles (degrees) Bond Angle Ligand-Zn-Ligand 97.7(3) Acetate O2 - Zn3 - Acetate O5 109.3(3) Acetate O5 - Zn3 - µ-hydroxide O7 104.5(3) Acetate O5 - Zn3 - NCTPP Exterior N8 119.5(3) Acetate O2 - Zn3 - µ-hydroxide O7 125.1(3) Acetate O2 - Zn3 - NCTPP Exterior N8 99.4(3) µ-hydroxide O7 - Z3 - NCTPP Exterior N8
Bond Angle Ligand-Zn-Ligand 98.5(3) Acetate O4 - Zn4 - Acetate O6 106.6(3) Acetate O6 - Zn4 - µ-hydroxide O7 99.6(3) Acetate O6 - Zn4 - NCTPP Exterior N4 128.0(3) Acetate O4 - Zn4 - µ-hydroxide O7 119.5(3) Acetate O4 - Zn4 - NCTPP Exterior N4 100.4(3) µ-hydroxide O7 - Zn4 - NCTPP Exterior N4
66 Zn4(NCTPP)2(O2CCH3)3(OH) is a model compound for the active site of the zinc- dependent aminopeptidase in Aeromonas proteolytica.89,90 Chevrier et al. reported the
“Crystal structure of Aeromonas proteolytica aminopeptidase: a prototypical member of the co-catalytic zinc enzyme family” at 1.8 Å resolution.90 The dizinc core in the
Zn(NCTPP) dimer is analogous to the active site in the enzyme, with the exception that the carboxylates in the model compound have unidentate coordination to each metal center while the carboxylates of glutamate-152 and aspartate-179 in the enzyme coordinate to each metal in bidentate tetrahedral fashion. The active site of the protein consists of the metal-binding site with the two zinc ions 3.5 Å apart bound in a loop region near the carboxy-terminal edge of the central parallel strands 3 and 5 and well- defined hydrophobic specificity pocket. In the enzyme, the zinc ions are bridged by the carboxylate oxygens of aspartate 117 and by a water molecule. In the model compound,
Zn3 and Zn4 are bridged by the acetate carboxylate O5 and O6 and O7 of µ-hydroxide
(Figure 38 and Table 7). Zn1 in AAP is coordinated to the Oε1 of glutamate and to the
Nε2 of histidine 256, while Zn2 is coordinated to the Oδ1 of aspartate 179 and to the Nε2 of histidine 97 (Figure 38 and Table 7).91 In the Zn(NCTPP) dimer, Zn3 is coordinated to the carboxylate O2 of acetate and the N-Confused pyrrolic N8, whereas Zn4 is coordinated to carboxylate O4 of acetate and N-Confused pyrrolic N4 (Figure 38 and
Table 7). Both zinc ions in the active site have pseudo-tetrahedral coordination with the understanding that the second oxygen of the carboxylate groups of aspartate 179 and glutamate 152 form a weaker bidentate coordination to the metals. Unlike the protein, the second carboxylate oxygens of the collinear acetates axially coordinate to the zinc metal centers of the porphyrin macrocycles instead of the zinc atoms of the dizinc core. 67 The interatomic distances of the second carboxylic oxygens of the acetates are approximately 3.0 Å from the zinc atoms in the dizinc core and are too great to be considered as bidentate coordination. Carrell et al. surveyed the Cambridge Structural
Database, performed a sterochemical analysis of 67 zinc-carboxylate interactions, and concluded that for cation binding 78 percent of the metal-ligand interactions had a preference for the syn-oriented lone electron pair of the carboxylate oxygen, while 22 percent had a preference for the anti-oriented lone electron pair of the carboxylate oxygen.92,93 Consistent with the carboxylate-zinc interactions observed by Carrell et al. and Christianson and Lipscomb, the colinear acetates in the model compound preferentially bind to the metal ions of the dizinc core with syn stereochemistry, however, the same acetates axially coordinate through a second carboxylic oxygen to the zinc metal centers of the macrocycles with anti stereochemistry (Figure 38).89,92,93 Vedani and
Huhta conducted an investigation of the Cambridge Structural Database and discovered that zinc and cobalt interactions of the 2s2p2 lone electron pair of nitrogen heterocycles, such as imidazole and its derivatives, prefer a head-on and in-plane approach to the lone electron pair.94 As with the findings of naturally occurring zinc-dependent enzymes, this zinc-pyrrolic-nitrogen lone electron pair interaction in the model compound is present with the exception that both N-Confused pyrroles have a zinc-pyrrolic-nitrogen bond angle slightly larger than the ±30 degree C-N-Zn bisector as described by Vedani and
Huhta.94
68
Figure 38. Comparison of the dizinc core of the Zn(NCTPP) model compound
with the active site of Aeromonas proteolytica aminopeptidase.89,90
Table 7. Comparison of the Zinc-ligand distances in Zn(NCTPP) dimer model compound and the active site of Aeromonas proteolytica aminopeptidase.89
Model Compound Zinc-ligand distances (Å). Aeromonas proteolytica Zinc-ligand distances (Å).
Zn3 Zn4 Ligand Zn1 Zn2 Ligand 2.005(7) Acetate O5 2.01 Asp117 Oδ1 1.996(8) Acetate O6 2.05 Asp117 Oδ2 1.923(7) 1.928(7) µ-hydroxide O7 2.25 2.29 H2O 2.019(8) Confused Pyrrole N8 2.21 His97 Nε2 2.019(8) Confused Pyrrole N4 2.32 His256 Nε2 1.973(6) Acetate O2 2.04 Glu152 Oε1 2.986 Acetate O1 2.38 Glu152 Oε2 1.952(6) Acetate O4 2.05 Asp179 Oδ1 2.960 Acetate O3 2.34 Asp179 Oδ2
69 Zn(NCTPP)(DMSO). A single crystal of monomeric zinc N-Confused tetetraphenylporphyrin with axially coordinated dimethyl sulfoxide (DMSO) was grown from slow solvent diffusion of methanol and DMSO (1:1.14) using a column chromatography purified preparation of Zn(NCTPP), with dimensions 0.17 x 0.01 x 0.01 mm3, was used for structure elucidation. Single crystals were observed after a duration of one month undisturbed in darkness. The zinc atom has distorted tetrahedral coordination with three pyrrolic nitrogens and axial dimethyl sulfoxide oxygen. The N-Confused pyrrole with the intact interior C-H bond adjacent to the zinc metal center is rotated 13.77 degrees out of the plane of the porphyrin. Axially coordinated DMSO is located between the zinc metal center and the N-Confused pyrrole with the methyl groups directed away from the porphyrin plane. Relative to the interior pyrrolic β-carbon of the N-Confused pyrrole, the sulfur-oxygen bond is directed 14.99 degrees toward the exterior pyrrolic α- carbon and is nearly parallel to the plane of the N-Confused pyrrole. Relative to the metal-oxygen bond, the sulfur lone electron pair is directed 28.41 degrees toward the exterior pyrrolic nitrogen (Figure 39). The metal-ligand bond lengths for interior pyrrolic
N1, N2, and N3 of the macrocycle and axial O of dimethyl sulfoxide are 1.998(17),
2.007(17), 2.111(15), and 2.011(13) Å, respectively. Evidence of the distorted tetrahedral geometry about the zinc metal center is observed in the bond angles between the interior pyrrolic nitrogens and axially coordinated DMSO (Table 8). The bond angles formed between the axially coordinated DMSO, zinc metal center, and the three pyrrolic nitrogens range from 101.9(5) to 105.3(6) degrees while the bond angles between the interior pyrrolic nitrogens and the zinc metal center are 92.4(6), 92.6(6), and 152.8(6) degrees, respectively (Table 8). 70
Figure 39. Molecular structure of zinc N-Confused tetraphenylporphyrin with axially
coordinated dimethylsulfoxide.
Table 8. Zinc-ligand distances and ligand-zinc-ligand bond angles in
Zn(NCTPP)(DMSO).
Zn-Ligand Distances (Å) Zn Ligand 1.998(17) NCTPP Interior N1 2.007(17) NCTPP Interior N2 2.111(15) NCTPP Interior N3 2.011(13) Acetate O1
Ligand-Zn-Ligand Bond Angles (degrees) Bond Angle Ligand-Zn-Ligand 92.4(6) N1 - Zn - N2 92.6(6) N2 - Zn - N3 101.9(5) N3 - Zn - O1 152.8(6) N1 - Zn - N3 102.5(6) N1 - Zn - O1 105.3(6) N2 - Zn - O1 71 Zn(NCTPP)(Pyr). A single crystal of monomeric zinc N-Confused tetetraphenylporphyrin with axially coordinated pyridine (Pyr) grown from an unpurified metallation reaction of NCTPP and Zn(O2CCH3)2•2H2O, with dimensions 0.1 x 0.1 x
0.05 mm3, was used for structure elucidation. Upon completion of the metallation reaction, N,N-dimethylformamide was removed via rotary evaporation resulting in a dark green solid consisting of Zn(NCTPP) and excess Zn(O2CCH3)2•2H2O. The unpurified product, Zn(NCTPP) and excess Zn(O2CCH3)2•2H2O, was solvated with pyridine at high concentration and placed in two NMR tubes with hexanes layered over the Zn(NCTPP)-
Zn(O2CCH3)2•2H2O/pyridine solution in ratios of 2.4:1 and 3.25:1. Structure elucidation revealed a distorted tetrahedral coordinated zinc N-Confused tetraphenylporphyrin with pyridine in the axial position. The N-Confused pyrrole with the intact interior C-H bond is rotated 33.2 degrees out of the plane of the porphyrin. Unlike the previous zinc N-
Confused tetraphenylporphyrin compounds, Zn(NCTPP)(Pyr) has a metal center that is coordinated entirely by nitrogen. The metal-ligand bond lengths for interior pyrrolic N1,
N2, and N3 of the macrocycle and axial N of pyridine are 2.082(3), 1.996(3), 2.081(3), and 2.053(4) Å, respectively. As with Zn(NCTPP) dimer, the interior pyrrolic N2 between the adjacent pyrrolic N1 and N3 has a metal ligand bond length that is nearly 4 percent shorter relative to the metal ligand bond lengths of adjacent pyrrolic nitrogens.
The distortion of the tetrahedral coordination sphere is evident in the bond angles of the nitrogens (Table 9). Axially coordinated pyridine has bond angles with the zinc metal center and the interior macrocyclic nitrogens ranging from 95.5(2) degrees to 111.4(2) degrees while the bond angles between the interior pyrrolic nitrogens are approximately
91 degrees and 153 degrees (Figure 40 and Table 9). 72
Figure 40. Molecular structure of zinc N-Confused tetraphenylporphyrin with axially
coordinated pyridine.
73 Table 9. Zinc-ligand distances and ligand-zinc-ligand bond angles in
Zn(NCTPP)(Pyridine).
Zn-Ligand Distances (Å) Zinc Ligand 2.082(3) NCTPP Interior N1 1.996(3) NCTPP Interior N2 2.081(3) NCTPP Interior N3 2.053(4) Pyridine N5
Ligand-Zn-Ligand Bond Angles (degrees) Bond Angle Ligand-Zinc-Ligand 91.42(13) N1 - Zn - N2 91.49(13) N2 - Zn - N3 98.5(2) N3 - Zn - N5 153.44(13) N1 - Zn - N3 104.9(2) N1 - Zn - N5 111.4(2) N2 - Zn - N5
74 Conclusion. The investigations of Zn(NCTPP) proved useful compounds to study bioinorganic chemistry with regard to the metal-ligand interactions and provided insight into the chemistry of N-Confused porphyrin. Zn(NCTPP) was characterized extensively using UV-visible absorption spectroscopy, 1H NMR spectroscopy, mass spectroscopy, photooxidation, and x-ray crystallography. These characterization techniques revealed that Zn(NCTPP) forms not only monomeric compounds with different axially coordinated ligands, but also dimeric structures consisting of a simple homo-dimer in solution and a dimer with bridging ligands and metal centers isolated in single crystals.
Zn4(NCTPP)2(O2CCH3)3(OH), a dimer synthesized from excess starting reagents, is a model compound for the active site of aminopeptidase in Aeromonas proteolytica.
Indeed, the distorted tetrahedral coordination sphere of zinc and the manner in which it binds to the interior pyrrolic nitrogens allows for Zn(NCTPP) to serve as a substructure for constructing model compounds for the active sites of enzymes. The choice of acetate, a bidentate ligand and derivative of acidic side chain amino acids, allowed for bridging between metal centers in the model compound. Consistent with metal-ligand binding in metalloproteins, the bridging acetates in Zn4(NCTPP)2(O2CCH3)3(OH) have both syn and anti orientation to each zinc atom in the compound. Zn(NCTPP)(DMSO) and
Zn(NCTPP)(Pyr), both of which are monomers, have dimethylsulfoxide and pyridine, respectively, coordinated at the axial site. While DMSO is not a ligand of biological interest, the lone electron pair of sulfur seem to have an interaction with the N-Confused pyrrolic nitrogen. Zn(NCTPP)(Pyr) is unique among the three compounds in that the zinc metal center is coordinated entirely by nitrogen whereas the other compounds have both oxygen and nitrogen in the tetrahedral coordination sphere. Zn(NCTPP) is an 75 interesting and novel chromophoric compound that could serve as a foundation, binding ligands with oxygen, nitrogen, and possibly other electronegative elements, for synthesizing compounds with specific chemical functionality.
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82 APPENDIX
The appendix contains tables of the chemical shifts and proton assignments for
Zn(NCTPP) in CDCl3 and pyridine-d5, respectively.
83 1 Table A1. H NMR Proton assignment for Zn(NCTPP) in CDCl3.
Chemical Shift Proton Assignment δ (ppm)
-3.09 Zn4(NCTPP)2(O2CCH3)3(OH) interior pyrrolic proton on β-carbon of N- Confused pyrrole (C-H). -1.32 Zn4(NCTPP)2(O2CCH3)3(OH) axially coordinated acetate (CH3) -0.87 Zn(NCTPP) axially coordinated OH 0.04 Tetramethylsilane (internal standard) 0.69 CH3 from bridged acetate of dizinc core (Zn4(NCTPP)2(O2CCH3)3(OH)). 0.83 Hexanes 1.09 Hexanes 1.23 Hexanes 1.39 Hexanes 1.63 Water 2.15 Acetone 2.85 Confused pyrrole outer α-H ((Zn(NCTPP))2). Residual dimethylformamide from the metallation reaction cannot be ruled out. 2.93 Residual dimethylformamide from the metallation reaction. 5.28 Dichloromethane 5.64 N-Confused pyrrolic N-H exhanging with water. 6.55 Outer alpha proton on confused pyrrole (Zn4(NCTPP)2(O2CCH3)3(OH)). 6.64 Proton of exterior pyrrolic nitrogen (N-H). 7.20 Zn(NCTPP) phenyl ring (p-H). 7.24 Chloroform; trace pyridine (m-H) cannot be ruled out. 7.38 Zn(NCTPP) phenyl ring (m-H). 7.55 Zn(NCTPP) aromatic protons 7.60 Zn(NCTPP) aromatic protons 7.68 Trace pyridine (p-H) cannot be ruled out. 7.78 Zn(NCTPP) phenyl ring (o-H). 7.95 Monomeric Zn(NCTPP) with axially coordinated ligand; outer alpha proton on confused pyrrole. Appears as a doublet due to proton exchange with adjacent exterior pyrrolic nitrogen. 8.00 Monomeric Zn(NCTPP) with axially coordinated ligand; outer alpha proton on N-Confused pyrrole. Appears as a doublet due to proton exchange with adjacent exterior pyrrolic nitrogen. Residual dimethylformamide from the metallation reaction cannot be ruled out. 8.10 Zn(NCTPP) aromatic protons 8.30 Zn(NCTPP) aromatic protons 8.43 Zn(NCTPP) aromatic protons 8.49 Zn(NCTPP) aromatic protons 8.60 Trace pyridine (o-H) cannot be ruled out. 8.68 Interior pyrrolic proton on ß-carbon of confused pyrrole(C-H).
84 1 Table A2. H NMR Proton assignment for Zn(NCTPP) in pyridine-d5.
Chemical Shift Proton Assignment δ (ppm) -3.11 Interior pyrrolic proton on ß-carbon of confused pyrrole (C-H). 0.00 Tetramethylsilane (internal standard) 0.88 Hexanes 1.27 Hexanes 1.34 Hexanes/water 2.30 Acetone 4.98 Dichloromethane 5.71 N-Confused pyrrolic N-H exhanging with water. 7.21 Pyridine (m-H) from deuterated solvent, not the ligand. 7.39 Zn(NCTPP) phenyl ring (m-H) (broad due to ring rotation) 7.43 Zn(NCTPP) phenyl ring (m-H) (broad due to ring rotation) 7.47 Zn(NCTPP) phenyl ring (m-H) (broad due to ring rotation) 7.49 Zn(NCTPP) phenyl ring (p-H) 7.58 Pyridine (p-H) from deuterated solvent, not the ligand. 7.73 Zn(NCTPP) aromatic protons 7.75 Zn(NCTPP) aromatic protons 7.80 Zn(NCTPP) aromatic protons 7.86 Zn(NCTPP) aromatic protons 7.90 Dimethylformamide 8.07 Dimethylformamide 8.32 Zn(NCTPP) aromatic protons 8.38 Zn(NCTPP) aromatic protons/o-protons on axially coordinated pyridine. 8.44 Zn(NCTPP) aromatic protons 8.73 Pyridine (o-H) from deuterated solvent, not the ligand. 8.85 Zn(NCTPP) aromatic protons 8.93 Zn(NCTPP) aromatic protons 8.95 Zn(NCTPP) aromatic protons 8.96 Zn(NCTPP) aromatic protons 9.21 Zn(NCTPP) aromatic protons 9.23 Zn(NCTPP) aromatic protons 9.26 Zn(NCTPP) aromatic protons 9.29 Zn(NCTPP) aromatic protons
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