1. Mössbauer studies of the interaction of oxygen with solid β-Fe II phthalocyanine Ernő Kuzmann 1,2, Zoltán Homonnay 2, Attila Vértes 1,2, Shuxi Li 3, Houping Yin 3, Yen Wei 3, Amar Nath 3, Xuean Chen 4, and Jing Li 4 1 Laboratory of Nuclear Chemistry, Chemical Research Center, Hungarian Academy of Sciences, Budapest, Hungary 2 Chemical Institute, Eötvös Loránd University, Budapest, Hungary 3 Department of Chemistry, Drexel University, Philadelphia, PA 19104, USA 4 Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey,, NJ 08854, USA E-mail: kuzmann@ludens.elte.hu

2. Encapsulated iron carbide nanoparticles (carbon nanotubes) with a uniform 2.7 nm wide carbon coating were prepared with controlled pyrolysis of flat layered phthalocyanines. - Kopelev, N., Chechersky, V., Nath, A., Wang, Z. L., Kuzmann, E., Zhang, B. and Via, G. H., Chemistry of Materials C 95 (1995), 19. A key step of the synthesis is the oxygenation of the iron phthalocyanine complex (FePc). 57 Fe Mössbauer spectroscopy and X-ray diffractometry were used to investigate the nature of the oxygen bonding after the oxygenation of the α and β polymorphs of FePc. - E. Kuzmann, A. Nath, V. Chechersky, S. Li, Y. Wei, X. Chen, J. Li, Z. Homonnay, M. Gál, V.K. Garg, Z. Klencsár, A. Vértes: Hyperfine Interactions, 139/140, 631-639 (2002) - E. Kuzmann, Z. Homonnay, A. Vértes, S. Li, H. Yin, Y. Wei, A. Nath, X. Chen, J. Li: J. Solid State Chem., 170, 118-123 (2003)

3. Phthalocyanines can acquire several polymorphic forms of which  and β are fairly well characterized. The difference in the  - and β-polymorph lies essentially in the orientation of the flat molecules with respect to the crystallographic axes, where the perpendicular distance between the stacks of molecules remains approximately the same, viz., 3.38 Å: The inclination of the molecular stacks in the a-form is much less than in the b-form. Therefore, the distance between the central metal atoms in the adjoining stacks is about 3.8 Å in the  - form and about 4.8 Å in the β-form. In the β polymorph, the nitrogen atoms of the neighboring molecules are present axially above and below the central metal atom at a distance of about 3.38 Å : In the  -form, the nitrogens of the nearest molecules are not in axial positions. Structure of α- and β- polymorphs of iron phthalocyanines.

4. Mössbauer spectra, recorded at 295 K, of pristine, P, and oxygenated phthalocyanine samples A: O 2 /H 2 O at RT 14 days B: O 2 /H 2 O at 50 o C 14 days D: O 2 /dry at 50 o C 92 days The Mössbauer spectra for samples oxygenated at room temperature (A) can be fit with three different species including the parent β-Fe II Pc: Only 24 % of the compound is oxygenated. However, if the oxygenation is carried out at 50 o C in aqueous suspension, or in the dry state, for prolonged periods, then two additional oxygen adducts are formed and a larger fraction of the compound is oxygenated.

5. Conditions of oxygenation Species ISpecies IISpecies IIISpecies IVSpecies VWidth Temperature:RT80 KRT80 KRT80 KRT80 KRT80 KRT80 K O 2 /H 2 O, at RT 14 days  (mm/s) 0.470.540.170.280.3730.4720.330.29  (mm/s) 0.800.680.720.582.5442.625 Area fraction (%) 12.514.511.313.676.271.9 O 2 /H 2 O, at 50 o C 14 days  (mm/s) 0.390.550.130.300.120.190.410.620.3790.4760.310.28  (mm/s) 0.590.670.560.550.791.061.001.042.5732.638 Area fraction (%) 24.118.512.116.22.98.87.46.153.550.4 O 2 /H 2 O, at 50 o C 44 days,  (mm/s) 0.390.530.130.300.120.190.410.480.3880.4790.320.29  (mm/s) 0.590.610.46 0.841.090.961.052.6342.655 Area fraction (%) 31.020.14.09.910.314.515.610.339.045.2 O 2 /dry 50 oC 92 days  (mm/s) 0.490.600.160.300.140.240.460.570.3900.4700.410.40  (mm/s) 0.740.850.64 1.131.211.251.422.5562.631 Area fraction (%) 20.221.818.720.010.414.414.112.136.631.7 Mössbauer parameters for the different oxygen adducts formed by oxygenation of β-Fe II Pc

6. It is interesting to note that the relative area of β-Fe II Pc is observed to be higher at room temperature (RT) as compared to 80 K for all samples except D (Table). It suggests that Fe II Pc has a higher Lamb-Mössbauer factor as compared to the oxygen adducts, which manifests itself as an ‘‘apparent’’ increase in its relative abundance at RT. However, for sample D which has undergone very prolonged oxygenation, the situation changes presumably due to stronger interaction between the layers. In principle, one could estimate the local Debye temperature of iron located in each of the species by observing the change in their relative areas as a function of temperature; however, that possibility is frustrated by some interesting inter-conversions between the species with temperature.

7. Mössbauer spectra, of oxygenated phthalocyanine sample C, recorded at 78 K (a) and 295 K (b): One can see quite clearly the evidence of interconversion of oxygen adducts (changes in the resonance signals between the two outermost peaks) as a function of temperature.

8. The contraction of the cell volume on oxygenation is clearly evident from XRD measurements Cell volume of pristine (P) and oxygenated (A–C) phthalocyaninesamples

9. In Fe II Pc and for most of its adducts, the electric field gradient (EFG) is generated mostly by the highly covalent in-plane bonding and is positive. The major axis of the EFG lies along z; the molecular axis. There is also considerable π-back donation from d xz,d yz into the macrocycle. The Mössbauer parameters of all the four oxygen adducts (Table) are consistent with Fe III high spin configuration. Species II and III are probably five-coordinate, however species IV and V having considerably smaller isomer shifts associated with not too large D are indicative of sixfold coordination. Six coordinate Fe III high spin compounds for phthalocyanine have not been reported earlier. The solid matrix tends to stabilize species which would ordinarily be regarded as metastable. Five coordinate Fe III in species II and III is likely to be displaced from the plane of the ring. Despite the cell contraction and the out-of-plane displacement of the Fe atom, the Fe–O– Fe oxo-bridge formation would involve some stretching of bonds. However, species similar to III and V have been observed as an intermediate as well as in a peroxo compound. For species IV and V, interaction of O 2 at the sixth coordination site to form superoxo compound is rather weak. At higher temperatures (RT) the diatomic oxygen, either end-on or side-on, is detached from the sixth axial position and becomes free to migrate. On lowering the temperature, any O 2 molecules floating in the vicinity of Fe III of species II and III are captured. Chemical analysis shows that oxygen is present in great abundance indicating the presence of several O 2 molecules unattached to Fe even at 80 K: The detachment of O 2 is reflected in the decrease of abundance of species IV and V and corresponding increase in abundance of species II and III at RT. This interconversion is rather small in the oxygenated compound which has undergone dry thermal treatment, namely D. In principle, longer Fe–O–Fe (or Fe–O–O–Fe) chains are possible with the end members being five-coordinate.

10. Suggested structures of species found in oxygenated β-FeIIPc: In the six-coordinate species (upper Fe), end-on or side-on positions of the dioxygen ligand may be possible for the  - oxo rather the  - peroxo form. The domed structure of the [FeN 4 ] unit may be preserved to some extent after attachment of O 2 (not shown in the figure).

11. The assignment of different species receives support from the IR spectra of the oxygenated compounds. In the figure, by comparison of the two spectra, one can see that noticeable changes occur in the 1600–1740, 1200 and the 890 cm -1 region. The first region can be ascribed to diatomic oxygen physisorbed to the phthalocyanine rings, the second to superoxo attachment in a coordinate position (either end-on or side-on) and the third region is due to Fe–O bridging. Infra-red spectra of pristine b-FeIIPc (top) and oxygenated b-FeIIIPc (bottom, sample C). Novel oxygen adducts were found which are stabilized in the solid matrix of β-Fe II Pc: Some tentative assignments of the species are made on the basis of their Mössbauer parameters.