Indian Journal of Chemistry Vol. 42A, September 2003, pp. 2191-2197

Electronic interactions in meso ferrocenyl porphyrin and its metal derivatives

Sundararaman Venkatraman' , Viswanathan Prabhurajaa, Rajneesh Mishra', Rajeev Kumar", Tavarekere K Chandrashekar*a, Weijie Tengb & Karin Ruhlandt Sengeb

"Department of Chemistry, Indian Institute of Technology, Kanpur 208 016, India Email: tkc@iitk .ac.in, Phone: +91-512-2597282/2597426, Fax:+91-512-2597282/2597436

bDepartment of Chemistry, Syracuse University, Syracus:.:, New York 13244, USA

Received 29 January 2003

Synthesis, characterization, spectroscopic and electrochemical studies on meso tetraferrocenyl porphyrin are reported. Substitution of ferrocenyl groups on the meso carbons affect the electronic structure of the porphyryn as well as the ferrocenyl moiety. This is reflected in the shift of electronic absorption band and redox potentials in the ferrocenyl containing porphyrin relative to the porphyrins without ferrocenyl group. Single crystal X-ray structure of freebase reveals an Ct.,~,Ct.,~ orientation of the ferrocenyl groups relative to the mean plane of the porphyrin ring. Introduction of metal into the porphyrin ring does not alter the electronic structure of the metal as revealed by EPR spectrum for Cu2

+ derivative.

The covalent linkage of a pendant redox and photoactive ligand to a conjugated luminescent system will generate new macromolecules. The photochemical and electrochemical properties of the molecular components of the macromolecules can be modulated via changes in the oxidation state of the redox active group and photochemical activity of the luminescent group'. Direct linkage of ferrocene to the meso carbon bridges of the porphyrin skeleton induces strong electronic coupling between the two systems, which is essential for the application in molecular devices2

. The interest in such molecules are two fold: (i) They can act as molecular or chemical sensors; sensing system usually comprises of a signaling unit linked to a receptor so that the ion binding is read out by a measurable physical change3

.

Thus, linking a porphyrin moiety to ferrocenyl molecule would lead to a sensor, the response of which would be detectable by optical, fluorescence and electrochemical methods. (ii) Inducing electronic coupling between the redox active group of the porphyrin system would lead to changes in the conjugation pathway of the cyclic porphyrin system and such an interaction is essential for the application of these materials in the molecular electronic devices4

,5. In this paper we report on the synthesis and characterization of meso tetraferrocenyl porphyrin and its metal derivatives (Cu" and Zn"). Single crystal X­ray structure of meso tetraferrocenyl porphyrin suggests a,~,a,~ disposition of ferrocenyl groups with respect to porphyrin plane. The electronic

absorption spectral studies suggest strong electronic coupling between porphyrin n-system and the ferrocenyl moiety. The electrochemical data suggests a slight inequivalence of ferrocenes attached to porphyrin molecule and in all the cases harder oxidation of ferrocenyl groups relative to free ferrocene is observed.

Materials and Methods Synthesis of H2TFcP, 2

Meso ferrocenyl dipyrromethane 1 (500 mg, 1.5 mmol) dissolved in dry dichloromethane (150 mL) was stirred under nitrogen atmosphere for 5 min in dark. Trifluoroacetic acid(TFA) (0.06 mL, 0.8 mmol) was added and stirring was continued for a further 90 min. Chloranil (372 mg, 1.5 mmol) was added and refluxed for 90 min. The solvent was evaporated in vacuo. The residue so obtained was chromatographed on a basic alumina (grade III). The first green band eluted with dichloromethane and ethyl acetate (99: 1) gave a dark green crystalline solid on evaporation of solvent, identified as 2. FAB Ms: mlz: 1047 (100%) [M+1t. 'H NMR (400 MHz, CDCh, ppm) 8 = 9.61(s, 8H), 5.34 (s, 4H), 4.77 (s, 8H), 3.98 (s, 20H), -0.49 (s, 2H).

Synthesis ofZnTFcP, 3 The free base 2 (50 mg, 0 .047 mmol) and zinc

acetate (420 mg, 1.91 mmol) were dissolved in 20 mL of freshly distill~d dimethylformamide containing sodium acetate (4: 1), refluxed under nitrogen and

2192 INDIAN J CHEM, SEC A, SEPTEMBER 2003

monitored spectrophotometrically until there was no residual Soret band from the starting material. Dichloromethane (20 mL) was added when the reaction vessel reached room temperature. The mixture was then washed with brine (3x50 mL) and the metallated porphyrin was extracted with dichloromethane. The organic solution was then dried over sodium sulphate and solvent was evaporated under reduced pressure. Crystallization with dichloromethane and methanol afforded 3. FAB Ms: m1z: 1109 [M+]. 'H NMR (400 MHz, CDCl3, ppm) 8 = 9.85 (s, 8H), 5.38 (s , 8H), 4.77(s, 8H), 4.01 (s, 20H).

Synthesis o/CuTFcP, 4 The above procedure was followed using copper

acetate (100 mg, 0.5 mmol)to get copper derivative 4. FAB Ms: m/z: 1109 [M+lt.

PhysicaL measurements Electronic spectra were recorded on a Perkin

Elmer-Lambda 20 UV-vis spectrophotometer. The data analyses were done using the UV-winlab software package. 'H NMR spectra were obtained by 400 MHz lEOL spectrometer in CDCI3. Chemical shifts were expressed in parts per million with reference to TMS . FAB mass spectra were obtained on a lEOL SX-120/DA 6000 spectrometer using argon (6KV, 10 rnA) as the FAB gas. Cyclic voltammetric studies were done on a EG/G PAR model 273A polarographic analysis interfaced to computer. A three-electrode system consisting of platinum working electrode, a platinum mesh counter electrode and a commercially available saturated calomel electrode (SCE) as the reference electrode were used. This reference electrode was separated from the bulk of solution by a fritted glass bridge filled with the solvent-supporting electrolyte mixture. Half wave potentials were measured as the average of anodic and cathodic peak potentials. EPR spectra were recorded on a Varian E-I09 X-band spectrometer at liquid nitrogen temperature.

Data collection/or X-ray crystallography X-ray quality crystals for 2 were grown from

diffusi ng methanol into dichloromethane containing 2. The crystals were removed from the tube and covered with a layer of viscous hydrocarbon oil (Paratone N, Exxon) . A suitable crystal was selected with the aid of a microscope, attached to a glass fiber,

and immediately placed in the low temperature nitrogen stream of the diffractometer. The intensity data sets for all compounds were collected using a Siemens SMART system, complete with 3-circle goniometer and CCD detector operating at -54°C. The data sets for 2 were collected at 98K, using a custom build low temperature device from Prof. H Hope (UC Davis). In all the cases graphite monochromated MoKa radiation (A = 0.71073 A) was employed. The data collections nominally covered a hemisphere of reciprocal space utilizing a combination of three sets of exposures, each with a different ~ angle and each exposure covering 0.3° in w. Crystal decay was monitored by repeating the initial frames at the end of the data collection and analyzing the duplicate reflections. In all the cases, no decay was observed. An absorption correction was applied for all compounds utilizing the program SADABS. The crystal structures of all compounds were solved by direct methods as included in the SHELXTL-Plus program package. Missing atoms were located in subsequent difference Fourier maps and included in the refinement. The structures of all compounds were refined by full-matrix least-squares refinement on F2 (SHELX-93). Hydrogen atoms were placed geometrically and refined using a riding model. The crystallographic programs used for structure refinement and solution was installed on a Silicon Graphics Indig02 R 10000 Solid Impact or a PC clone. Scattering factors were those provided with the SHELX program system. All non-hydrogen atoms were refined anisotropically.

X-ray crystal structure determination 0/ H2TFcP, 2 Crystal structure formula C60 iL8 Fe4 N4, M =

1048.42, Crystal system Tetragonal, Space group P-42,c (no . 114), a = 12.2479(4) A, b= 12.2479(4) A, c = 14.8572(8) A, ex = ~ = y = 90°, V = 2228.74(16) A3, T = 98(2) K, Z = 2, ).l = 1.324 mm-' . 2289 independent reflections [R(int) = 0.0635] out of 19822 reflections collected with 2.15 < 8 < 26.41 and completeness to 8 = 26.41 ° is 99.9%. Final R indices [1>2sigma(I)] RI = 0.0406, wR2 = 0.1037, R indices(all data) R 1 = 0.0442, wR2 = 0.1058.

Results and Discussion Synthesis of porphyrin containing four ferrocenyl

groups on the meso carbons was reported in 1977 by reaction of ferrocene carboxaJdehye with pyrrole under reflux in propionic acid6

. However, the

VENKATRAMAN et at.: ELECTRONIC INTERACTIONS IN MESO FERROCENYL PORPHYRIN 2193

formation of inseparable atropisomers resulted in the poor resolution in UV-vis, 'H NMR and I3C spectra, which hindered the progress in chemistry of such molecules. Only recently a few reports have appeared on the synthesis of porphyrin systems bearing one or two ferrocenyl groups and the X-ray structure of porphyrin containing two ferrocenyl groups in a,a orientation is reported7.8. Very recently a communication9 from our laboratory described the one pot synthesis of tetraferrocenyl porphyrin using a synthon contaInIng ferrocene group in dipyrromethane 1. Specifically reaction of 1 in the presence of 0.5 equivalents of TFA in dichloromethane followed by oxidation with chloranil under reflux temperature gave crude 5,10,15 ,20-tetraferrocenyl porphyrin (H2TFcP) 2 (Scheme 1). Purification on basic alumina using dichloromethane containing 1 % ethyl acetate gave a green solution, which on solvent evaporation and recrystallization afforded 2 in 30% yield. The formation of 2 as the sole product in this reaction can be accounted in terms of acidolysis and recombination of the fragmented products of 1 under the reaction conditions. The FAB mass spectrum and 'H NMR data confirms the proposed structure. A comparison of 1 H NMR spectrum of 2 along with its zinc complex 3 is made in Fig. 1. The presence of sharp singlet at 9.16 ppm assigned to ~-CH protons reveals the equivalence of all the four pyrrole rings in solution. The inner NH protons appear in the shielded region (-0.49 ppm) suggesting the retainment of aromatic character of 2 upon ferrocene substitution . The ferrocenyl groups resonate in the region 3.8 - 4.5 ppm. The ferrocenyl ring protons (c & d) which are directly, linked to porphyrin moiety are inequivalent among themselves and exhibit resonances at 5.34 ppm and 4.77 ppm corresponding to eight protons each. Cyc1opentadienyl(Cp) ring which is · not linked directly to the porphyrin moiety shows equivalence of all protons and resonate as sharp singlet at 3.98 ppm.

{

--=NH

Fe

f NH -"

TFAlCH2CI2 Fe Chloranil •

Fe

Fe

2

Scheme 1

Upon metallation, in zinc complex 3, the inner NH protons disappear as expected and the ~-CH protons show small downfield shift expected for porphyrin ring upon metallation 10. The observation of sharp singlet for ~-CH protons in 3 also suggests the equivalence of all the pyrrole rings upon metallation.

The UV -vis spectra of ferrocenyl porphyrins exhibit the expected electronic interaction between the porphyrin system and ferrocenyl moiety. A comparison of absorption spectra of 2 with meso tetraphenyl porphyrin (H2TPP) is made in Fig. 2 and the absorption data are given in Table 1. A comparison of absorption spectra reveals the following: (i) Strong red shift of both Soret and Q­bands in 2 relative to H2TPP; (ii) merging of Qx(l,O), Qx(O,O) and Qy(l,O) Qy(O,O) bands in 2, while in H2TPP all the four bands are well resolved"; and (iii) there is small decrease in the £ values in the Soret and increase in Q-bands relative to H2TPP. Upon metallation both the Soret band and Q-bands shift as expected. The observation of more number of Q­bands upon metallation relative to MTPP suggests lower symmetry than the expected D4h symmetry for metalloporphyrins. The electronic interaction between the porphyrin 1t system and the ferrocenyl moietl is seen in the shift of Soret bands in ferrocenyl porphyrins (Table 1). For example, the Soret band shifts by 770 cm· 1 in 2 relative to H2TPP, while the metalloporphyrins also show a similar magnitude of shift suggesting that the electronic interaction is retained upon metallation. This observation is in contrast to that made by Burrell and coworkers7 on the monoferrocenyl substituted porphyrin. This is attributed to the following facts: (a) there is only one ferrocenyl group connected via the ~ pyrrole carbon while in the present case four ferrocenyl groups are linked through meso carbons; and (b) furthermore, the ferrocenyl groups in Burrell's porphyrins7 are linked through a spacer group while in the present case, the

M2+ Fe --~Fe

Fe

Fe

M

-Q Fe Fe = Fe

6>

2194 lNDIAN J CHEM, SEC A, SEPTEMBER 2003

b

~ed Fe

0 ~ a b~r • a C d a

b b

I! I I ! ! , , ! ,

10.0 7.5 ppm 5.0

b

b~b a

~

Fe

ddJ

d 0

* ~ a C d a a

I bq Fe"B e :

b Gt;- d d

, -~ ,'--, , , , 10 8 ppm 6 4

Fig. I-A comparison of I H NMR spectra of 2 and 3 in CDCI) at 25° C (- 10-4 M concentration was used in each case)

porphyrin is directly linked to one of the Cp ring of ferrocene moiety at the meso carbons.

The redox properties of ferrocenyl porphyrins are probed through cyclic voltammetric and differential pulse voltammetric studies w.ith tetrabutylammonium hexafluorophosphate (0.1 M) as supporting electrolyte in dichloromethane as solvent. In general, porphyrins and metaJloporphyrins exhibit two oxidation waves and two reduction waves corresponding to the formation of mono, dications and mono, dianions of porphyrin ring respectively 12. The absolute El/2 values depend on the nature of porphyrin and nature of the

metal inside the cavity. In the ferrocenyl porphyrin an additional redox couple corresponding to the oxidation of ferrocenyl rings are also expected. A comparison of oxidation waves of 2 at different scan rates and its eu derivative 4 is made in Fig. 3. As expected, the porphyrin ring exhibits two quasi reversible oxidations. The ferrocenyl moiety also shows two 2-electron oxidation waves suggesting inequivalence of ferrocenyl moieties oriented in a.,a. and f3,f3 positions 13

• On scanning the potentials negative side, the two reductions expected for porphyrin ring are seen and Fig. 4 shows the

YENKATRAMAN el oi.: ELECTRONIC INTERACTIONS IN MESO FERROCENYL PORPHYRIN 2195

comparison of differential pulse voltammogram for 2, 3 and 4. The redox potential data given in Table 2 indicate the foll owing: (a) harder oxidation for the ferrocenyl groups attached to porphyrin moiety relative to free ferrocene by about IOO-200mY ; and (b) harder porphyrin ring oxidations and reductions. These observations are in contrast to those observed for mono and diferrocenyl porphyrins.8 The electron rich ~ pyrrole substituents in the mono and bis meso ferrocenyl porphyrins make the porphyrin ring electron releasing and thus oxidation becomes easier. In the present case, the absence of ~-substituents probably makes the porphyrin moiety electron deficient and hence harder oxidations are expected for both ferrocenyl and porphyrin ring as observed . The substitution of metal in porphyrin core has very little effect on the ferrocenyl oxidation potential s suggesting little perturbation in the electronic structure of the porphyrin ring. The harder porphyrin ring reductions observed here are attributed to non­planarity of the porphyrin core as revealed by single crystal X-ray analysis l 2

.

The a, ~,a,~ orientation of the ferrocenyl moiety in

., () c

"' .0 (; U) , .0 ..::

Sorel

400

a-bands :.',

:' ..... " ........... -, ......... .

600

Wavelength (nm)

--2 ----- · H TPP ,

X5

800 2,0

4

25mVs"

100 mVs'

1.5 1.0 0,5

E vs SeE (V) 0.0

Fig. 2-Electron ic spectra of 2(-) and H2 TPP( ----)in dichloromethane in Soret and Q-band region.

Fig. 3--Comparison of cyclic voltammogram of 2 and 4 on scanning to positive potentials

Table I-UY-visible absorption data for 2,3 and 4 in Soret and Q-bands region in dichloromethane

Compound

2

3

4

Sorel Amax (nm)

(E x 10'5) M' cm,1

433 (1.46).4 19'

432 ( Ll 3), 422'

430 ( 1.43). 4 17 '

Shi ft re lative to H2TPP (cm")

770

505

725

*These numbers correspond to Soret maxi ma of H2TPPfMTPP

Q-Bands Amax (nm)

(E x 10'5) M' cm,1

495 (0.26), 664«0.15), 728 (0. 13)

492 (0,26), 607 (0,06), 676 (0. 17)

493 (0.36), 669 (0.23), 6 11 (0,08)

2196 INDIAN J CHEM. SEC A, SEPTEMBER 2003

Table 2-Redox potentia ls for 2.3 and 4 with tetrabutylaJ1JJ11oniuJ11 hexafIuorophosphate (0. 1M) as supporti ng electro lyte in dichloromethane as so lvent

Porphyrin ring Porphyrin ring FelTocenyl ring Compound Ellz""1 ( I)V E Il2 rcd (2) V E II20

X (I)V E 1I2o, (2)V E llzOX ( I*)V E ll20

X ( I '*)V

HzTPP - 1.23 -1.55 1.03 2 - 1.35 - 1.60 1.23 1.43 0.45 0.63

ZnTPP - 1.39 -1.84 0.78 1.11

3 - 1.39 -1.70 1.3 1 0.47 0.63 C uTPP - 1.33 - 1.80 0.98 1.2 1

4 - 1.46 1.25 1.42 0.44 0.58

* I & l ' corrcspond to the ferrocenyl groups in cx,cx and p,p orientations respecti vely.

1600 -- 2 -- 3

1400 - 4

f,'. 1200

1000

« 000 "-

600

400 , .'-

200

-1.8 -1.6 ·1 .4 ·1.2 · 1.0

E vs SCE(V)

Fig. 4----CoJ11parison of differenti al pul se voltam mogram of 2(-), 3( ... ) and 4(---) on scanning to negative potential s

2 is further confirmed by single crystal X-ray structure as shown in Fig. 5. Substitution of the ferrocenyl moiety leads to puckering of the porphyrin ring (side view). The observation that the CJ3-Cp distance (l.369A) is shorter than the Ca-Ca di stance (1.443A) suggests that the aromatic nature is retained upon ferrocene substitution l4

. The carbon-carbon distance linking meso carbon of the porphyrin to the ferrocenyl ring (in present case C5-C6) is 1.480A which is significantly shorter than that observed for monoferrocenyl linked porphyrin (1.505 A).8 The Cp ring of ferrocene and porphyrin plane are twisted, the dihedral angle is - 35°. The Fe-carbon distance in 2 is not altered much upon anchoring to porphyrin ring relative to free ferrocene. Some selected bond lengths and bond angles are listed in Table 3.

Electron paramagnetic resonance spectrum, recorded for ClI derivative 4 (CuTFcP) in dichloromethane at liquid nitrogen temperature, is

Fig . 5--Single crystal X-ray structure of 2 (a) plane view (b) side view

typical of that expected for CUll ion in tetragonal sy mmetry (Fig. 6) 15. Of the four paralJellines, two are very well resolved and in the perpendicular region the

VEN KATRAMAN et 01.: ELECTRON IC INTERACTI ONS IN MESO FERROCEN YL PORPH YRIN 2 197

Tabl e 3--Se lec ted bo nd lengths rAJ and ang les [0) for 2

N( I)-C(4) 1.357(4) C(8)-C(9) 1.429(5) N( I )-C( I) 1.367(5) C(9)-C( 10) 1.4 18(5) Fe( I )-C( 13) 2.025(4) C( II )-C(I 5) 1.40 I (6) Fe( I )-C( 12) 2.030(4) C( 11 )-C(l2) 1.42 i (7) Fe( I )-C(7) 2.034(3) C( 12)-C( 13) 1.434(6) Fe( I )-C(8) 2.034(4) C( 13)-C( 14) 1.425(6) Fe(l)-C(9) 2 .037(4) C( 14)-C( 15) 1.424 (6) Fe( I )-C( I I) 2.040(4) Fe( I )-C( 14) 2.042(4) C(4)-N( I)-C(I) 109 .0(3) Fe( I )-C( I 0) 2.053(4) N( I )-C( I )-C(2) 108.2(3) Fe( I )-C( I 5) 2.05'1(4) C(3)-C(2)-C( I) 107.4(3) Fe( 1)-C(6) 2 .062(3) C(2)-C(3)-C(4) 106.7(3) C( 1)-C(2) 1.443(5) N( I )-C( 4 )-C(5) 126.5(3) C(2)-C(3) 1.369(5) N( I )-C(4)-C(3) 108.5(3) C(3)-C(4) 1.454(5) C(5)-C( 4 )-C(3) 124.9(3) C(4)-C(5) 1.404(5) C( 4 )-C(5)-C(6) 11 6.4(3) C(5)-C(6) 1.48 1(5) C( 10)-C(6)-C(7) 106.7(3) C(6)-C( I 0) 1.4 17(5) C( 10)-C(6)-C(5) 129.1 (3) C(6)-C(7) 1.444(4) C(7)-C(6)-C(5) 124.0(3) C(7)-C(8) 1.420(5)

gil

I

2500

Magnetic Fietd (Gauss)

Fig. 6--EPR spectrum of 4 in d ichloromethane at 140 K

super hyperfine interactions between the CUll ion and coordinated nilrogens are seen. The EPR parameters (g il = 2. 14; g.l = 2.004; A li

cli = 201.3 X 10-4 cm - I ; A.l CU

= 35 X 10-4 cm-I ; A/ = 16.32 X 10-4 em- I ; a?clI = 0.738) are not very much di fferent relati ve to CuTPP suggesting that the substitu tion o f ferrocenyl groups at meso carbon of porphyrin ring has little effect on the electronic structure of the Cu" ion.

In conclusion, we have descri bed a successful sy nthes is of tetrafen'ocenyl containing porphyrins in good yie ld by eas~ and efficient synthetic methodology. Our approach of linking the ferrocenyl group into dipyrromethane unit prevents the fo rmation of atropisomers, which are d ifficult to separate. Formation of single isomer aJlowed us to synthesize metal derivati ve and study the electron ic structure. The UV -vis and redox potent ials data clearly suggest the presence of strong electronic

interactions between porphyrin n-system and the ferrocenyl moiety. Further studi es are in progress to explo it their properties for the application 111

molecular dev ices .

Acknowledgement We thank the Department of Science & Technology

and the CSIR, New Delhi, Government of rndia for the research grants provided . We thank Prof J Subramanian , Department of Chemistry, Pondicherry University for allowing us to record EPR spectrum .

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13 T he fe rrocenyl oxidation in 2 is very sensi tive to the solvent, nature of the supporting e lectrolyte and the can rate. At faster scan rates only one ox idat io n wave is seen for ferroceny l moiety.

14 Anand V G, Pushpan S K, Venkatraman S, Dey A, Chandrashekar T K. Joshi B S, Roy R, Teng W & Senge K R, J Alii chelll Soc, 123 (200 I) 8620.

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