J. CHEM. SOC.FARADAY TRANS.,1994, 90(18), 2775-2781 2775 EXAFS Studies of Molecular Geometries of Some Coil and Col" Porphyrins Monica Endregard" and David G. Nicholson Department of Chemistry, University of Trondheim,A VH,N-7055 Trondheim, Norway Raymond J. Abraham and Ian Marsden Robert Robinson Laboratories, University of Liverpool, P.O. Box 147, Liverpool, UK L69 3BX Brian Beagley Department of Chemistry, UMIST, P.O. Box 88,Manchester, UK M60 IQD ~~ Co K-edge EXAFS data of some (porphinato)cobalt(tii) complexes with amine ligands in chloroform solution are reported. Data for the following compounds were collected : chloro(tetraphenylporphinato)cobalt(iit) (TPPCoCI) complexed with pyridine (A), isoquinoline (B) and cyclohexylamine (C) and 4-methylpiperidine complexed with the sterically hindered chloro(tetra-o-dichlorophenylporphinato)cobalt(iii) (TCIPCoCI) (D) and chloro(tetra-o- difluorophenylporphinato)cobalt(iit) (TFPCoCI) (E).In addition to the four N atoms from the macrocyclic porphyrin ligand, the cobalt atom is coordinated to either one amine and a chlorine atom (A and B) or two axial amine ligands with the chloride as a counterion (C-E). An interesting feature is the longer Co-CI distances in A and B relative to those in theosolid state. The Co-N distances are represented by a single composite distance ranging from 1.91(1) to 1.95(1) A. The Co-N bond lengths for the ortho-substituted porphyrins were found to be similar to those in the less hindered TPP complexes. The EXAFS for the cobalt(ii) compounds TClPCo (F) and TFPCo (G) gave distances similar to those of the crystal structure of TPPCo.The equatorial Co-N distances depend more on the conformation of the porphyrin core than on the cobalt oxidation state, as demonstrated by the similarity between the Co"l-N and Co"-N distances. Metalloporphyrins are currently being studied intensively in and used to obtain geometric information on porphyrin-the search for molecular metals and selective oxidative cata- ligand'.' '*' and porphyrin-porphyrin complexes.7~9~'0~12 lysts. Their high chemical and thermal stability and their We have previously shown1' that the cobalt-nitrogen ability to form complexes with many different metals are distances in chloro(pyridine)(phthalocyaninato)cobalt(rn) unique assets.Also their propensity to form stacked com- obtained by EXAFS supported previous assumptions made plexes on the one hand, and the relative ease of synthesising in obtaining a ring-current model for the phthalocyanine ring sterically protected metalloporphyrins on the other, makes and lead to an improved model. Here we investigate the them of considerable potential in these important areas of steric effects of o-phenyl substituents on the porphyrin ring. A research.' The use of porphyrins as molecular metals depends detailed NMR and molecular mechanics investigation of critically on the geometry and interactions between the these compounds has been given," and we will show that the macrocycles;2 similarly their use as oxidative catalysts is EXAFS results complement the NMR studies.largely determined by the steric constraints and geometry of Steric hindrance appears to have a considerable impact on the axial ligand.3 complex formation between porphyrins and amines, as We report here an extended X-ray absorption fine struc- demonstrated by the fact that the sterically hindered N-ture (EXAFS) spectroscopic study of some methylpiperidine ligand quantitatively reduces TPPCo"'C1 (porphinato)cobalt(mI) complexes with amine ligands in chlo- to the Co" species.16 2-Methylpiperidine is also sterically hin- roform solution and two solid (porphinato)cobalt(rr) com- dered (albeit to a lesser extent) and reduction to the Co" pounds. EXAFS was chosen because of its unique ability to complex is only partial. Similar steric effects are absent in the provide precise structural information about a given metal case of 3-methyl- and 4-methyl-piperidine, and reduction does atom in solution as well as in the solid state.Strong EXAFS not take place. This illustrates that the environment around signals are observed from metalloporphyrins because of the the metal is crucial to the chemistry. local symmetry provided by the macrocyclic ring system. In addition to first-shell nitrogen atoms, distant carbon atoms in the ring also contribute to the EXAFS.4-6 The present investigation includes chloroform solutions of TPPCoCl complexed with pyridine (A), isoquinoline (B) and cyclohexylamine (C). The effect of ortho substituents on the phenyl rings was examined in chloroform solution by analys- ing the 4-methylpiperidine complexes of TClPCoCl and X = H : TPPCoCl TFPCoCl, D and E, respectively.Fig. 1 shows the porphyrins X = F : TFPCOCI referred to in this study. The results for the cobalt(I1) com- X = CI : TClPCoCl pounds TClPCo" (F) and TFPCO" (G) are included for pur- poses of comparison. These experiments are closely connected to NMR measure-ments on Co"' porphyrins and phthalocyanines complexed with axial amine ligand~.~-'~ The NMR spectra are domi- Fig.1 Structures of chloro(tetraphenylporphinato)cobalt(III)nated by the large ring-current shifts produced by the circu- (TPPCoCl), chloro(tetra-o-difluorophenylporphinato)cobalt(m)lating .n electrons of the porphyrin macrocycle.A refined (TFPCoCl)and chloro(tetra-o-dichlorophenylporphinato)cobalt(I~~) the chlorine atom is not shownmodel of the porphyrin ring current has been presented14 (TClPCoCl); The steric effect of o-chloro substituents has recently been revealed by NMR and X-ray crystallography of [(l-methyl- imidazole),TClPCo] +BF4-.' The chloro substituents cause large changes in the 59C0 chemical shifts and linewidths and increased ruming of the porphyrin core with respect to the unhindered analogue [(imidazole),TPPCo] +OAc-.' The chloro groups effectively block the space above and below the porphyrin plane which leads to a fixed ligand orientation. The geometries of hindered and unhindered cobalt(rrr) porphyrins complexed with pyridine, 1-methylimidazole, 4- methylpiperidine and isoquinoline have recently been investi- gated by NMR and molecular mechanics calculations.'3 The preferred orientation of the axial amine ligands with respect to the porphyrin ring was deduced.For the sterically hin- dered porphyrins TClPCoCl and chloro(tetramesity1-porphinato)cobalt(m) (TMPCoCl) complexed with the planar amine ligands, the eclipsed position (ligand over N) is pre- ferred as shown by, for example, the above crystal structure of [( 1-methylimidaz~le)~TClPCo]-tBF, -.l7 The staggered orientation is preferred in the case of the less hindered porphyrins TFPCoCl and TPPCoCl. The non-planar 4-methylpiperidine complexes are more similar, involving relax- ation of the Co-N bond from the vertical.It was suggested that this bond was lengthened so as to accommodate the increasing steric strain. Experimental The preparations of the Cot" porphyrin compounds have been described in more detail el~ewhere.'~*'~ TPP and the ortho-substituted TXP (X = Cl, F) were prepared by the methods of Alder et a1." and Lindsey and Wagner," respec-tively. Cobalt was incorporated into the porphyrins asco",21.22 and the Co"' complexes were prepared from the corresponding Co" complexes by warming the latter with HCl dissolved in a solution of chloroform and methanol fol- lowed by evaporation. Conversions from free porphyrin to the Co" complex and from Co" to Co"' porphyrin were mon- itored by UV-VIS spectroscopy. The Co"' solutions were prepared immediately prior to data collection by dissolving appropriate amounts of the Co"' porphyrin in chloroform (typically 40 mg ml-') and adding the amine.Two types of complexes are formed depending on the molar ratio between porphyrin and amine. A molar ratio of 1 : 1 gives an octahedral complex with the sixth coordi- nation site occupied by a chloride counter ion acting as a ligand (A and B). When the ratio is increased to greater than 1 :2, the chloride is displaced by an amine and is present as a counter ion (C-E). The solid porphyrin samples were crystallised by evapo- rating a chloroform solution of 4-hydroxypiperidine and TXPCo"'C1 (X = Cl, F). Compounds F and G were formed by the subsequent reduction of Co"' to Con. The reference compound tris(ethylenediamine)cobalt(rIr)chloride hydrate (Co(en),Cl, 3H20) was obtained from Aldrich Chemical Company.X-Ray absorption data were collected at the Daresbury Synchrotron Radiation Source, station 7.1, operating at an energy of 2.0 GeV and an average current of 200 mA. This station is equipped with an order-sorting Si( 1 11) mono- chromator that was offset to 50% of the rocking curve for harmonic rejection. Focussing optics were not used. Room-temperature cobalt K-edge data (E = 7710 eV, A = 1.60811 A) were registered over the energy range 7422- 8307 eV in the fluorescence mode because the samples were dilute in cobalt. An NaI scintillation counter was used. Two scans were collected for each sample. A spectrum of the refer- ence compound was also collected in the transmission mode, J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 the sample being diluted with boron nitride in order to attain log(Io/I) in the range 1-2 just above the absorption edge. The fluorescence sample was also diluted with boron nitride so as to give a suitable fluoresence spectrum. The solid samples were finely ground and held between strips of sellotape. The porphyrin solutions were prepared immediately prior to data collection and injected into a specially designed sample holder with Kapton windows. Data were corrected for dark currents, converted to k-space, summed and background-subtracted using EXCALIB and EXBACK23 to yield the EXAFS function ZbS(k).Model fitting was carried out with EXCURV90,23 using curved- wave theory and ab initio phase shifts. The central atom (Co) and backscattering atom (C and N) phase shifts were calcu- lated within EXCURV90 using a 2 + 1 1s core hole correc- tion; they were not iterated in the least-squares fitting process.Fourier transforms were corrected for the phase shifts of the relevant atoms. The low-energy cut-off fsr all samples was 30 eV. The full energy range was applied for solution B and the solid samples (F and G), but the range for the remaining solution data was reduced to 9.4 (E) and 10.2 A-' (A, C and D) because of lower signal to noise at higher k. The data were k3 weighted in order to compensate for the diminishing amplitude at high k owing to the decay of the photoelectron wave.The solution data (A-E) were Fourier filtered using a Gaussian window function to include only the two, three or four first shells using a convenient cut-off which was not complicated by overlapping Fourier-transform shells. This has been shown not to introduce truncation errors.24 Data for the solid samples (F and G) were Fourier filtered in the range 1.0-25.0 A. This filter removes only the low- frequency contributions below 1 A, but does not remove the high-frequency oscillations, i.e. no noise removal. Only those shells significant at the 99% level2' were included in the final models. Details of the final models are listed in Tables 1 and 2 which give interatomic distances (rJ, Debye-Waller-like factors (20~) and the multiplicities (N),i.e.the number of atoms in a given shell n. The fits are shown in Fig. 2-4. The reference compound, tris(ethylenediamine)cobalt(rII) ~hloride,~~,~~was used to check the validity of the ab initio phase shifts and to establish the general parameters, AFAC (proportion of absorption causing EXAFS) and VPI (accounts for inelastic scattering of the photoelectron).28 These parameters were then transferred into the analyses for the compounds being investigated, thereby reducing any residual systematic error in the multiplicities. The estimated standard deviations (esd) for distances derived by EXAFS are 0.01 A at small r and 0.10 A at large r (ca. 4 A), with +20% accuracy for 2a2. Results and Discussion Solution Data The results are listed in Table 1, and the fits are shown in Fig.2 and 3. All coordinated nitrogen atoms are averaged to a single weighted distance, which varies from 1.91 (1) to 1.95 (1) A. Thus, the Co-N bonds to porphyrin and axial amine ligand could not be resolved by EXAFS. Samples A and B have one axial amine ligand with the sixth octahedral posi- tion being occupied by a chlorine atom. This corresponds to the formulae (pyridine)TPPCoCl and (isoquino1ine)-TPPCoCl. The other solutions (C-E) gave distances and coordination numbers consistent with complex salts with for- mulae [(cyc1ohexy1amine),TPPCo] 'C1 -(C) and C(4-methyl- piperidine),TXPCo]+Cl-, where X = C1 and F for D and E, respec ti vel y. As a result of the relatively rigid macrocyclic system, con- tributions from more distant atoms in the porphyrin ring are J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Results of EXAFS curve fitting for A-E, all samples in chloroform solution (ca.40 g 1-') multiplicity r/A 2a'lA' E,/eV R(%) A Co-N 5 1.911 (2) 0.0072 (2) 19.1 (4) 13 co-Cl 1 2.569 (3) 0.005 (1) co. . -c 10 2.880 (7) 0.035 (2) B Co-N 5 1.913 (1) 0.0046 (1) 19.5 (3) 11 co-Cl 1 2.552 (5) 0.013 (1) co. . .c 8 2.948 (2) 0.0088 (5) co. . -c 6 3.266 (3) 0.007 (1) C Co-N 6 1.924 (2) 0.0071 (3) 17.7 (3) 14 CO. . -c 8 2.981 (4) 0.012 (1) D Co-N 6 1.930 (2) 0.0088 (2) 17.7 (3) 13 co. * *c 12 2.971 (3) 0.018 (1) co. ..c 4 3.323 (8) 0.012 (2) E Co-N 6 1.947 (2) 0.0099 (3) 16.0 (4) 16 CO. .*c 12 3.015 (4) 0.016 (1) CO...c 4 3.357 (7) 0.004 (2) Each distance (r, Co-X for bonding and Co...X for non-bonding) is associated with a multiplicity (or coordination number) and thermal vibration (Debye-Waller-like factor, 20'). E, is the refined correction to the threshold energy of the absorption edge. The multi- plicities are assigned on structural chemical grounds and the spectra Fourier filtered using a Gaussian window function and a convenient cut-off which includes two, three or four shells. The standard devi- ation in the least significant digit as calculated by EXCURV90 is given in parentheses. This is a measure of the precision (as opposed to accuracy) to which the parameters are determined within the con- straints of the model.Systematic errors will degrade the accuracy, with major contributions stemming from inexact treatment in the theory. Thus, the estimates of precision overestimate the accuracy, particularly in cases of high correlation between parameters. The estimated standard deviations for distances are 0.01 A at small r and 0.04 A at larger (ca. 3 A) r, with +20% accuracy for 20'. The residual index, R, was calculated as R= seen in the Fourier transforms. The multiplicities and approximate distances for these atoms were deduced from reported crystal structures. Table 3 compares the EXAFS and crystallographic distances. Distances extracted from EXAFS are often shorter than those obtained from crystallography. Table 2 Results of EXAFS curve fitting for the solid Co" com-pounds F and G; selected distances out to 4.5 A deduced from the crystal structure of TPPCo" are included for comparison with the EXAFS results multiplicity r/A 2a2/A2 E,/eV R(%) EXAFS" F Co-N 4 1.945 (2) 0.0060 (3) 17.1 (5) 22 co..*c 8 2.980 (4) 0.010 (1) co.. -c 4 3.323 (10) 0.011 (3) co-* .c 4 3.747 (15) 0.007 (3) co-. .c 8 3.946 (14) 0.014 (3) G Co-N 4 1.920 (3) 0.0071 (4) 17.7 (5) 26 co. . .c 8 2.966 (5) 0.011 (1) CO. * *c 4 3.295 (9) 0.010 (2) co*. -c 4 3.741 (15) 0.011 (3) co. . .c 8 4.363 (12) 0.012 (3) X-ray TPPCo" Co-N 1.949 (3) CO. * *c 2.998 (5) co-..c 3.403 (5) CO. .*c 3.779 (5) CO. .c 4.215 (8) " This study; the standard deviation in the least significant digit, as calculated by EXCURV90, is given in parentheses.The data were Fourier filtered over the range 1.0-25.0 A using a Gaussian window function (i.e.no noise removal). See also notes to Table 1. Ref. 36. This is largely due to the inexact treatment in the theory29 coupled with the lack of resolution of similar distances [i.e. the Co-(N), and Co-amine distances]. It is therefore most appropriate when considering the Co-N lengths in these molecules to compare the relative distances as determined by EXAFS rather than the absolute values. The radius of the central porphyrin hole can be altered by puckering or ruffling the rings and ranges between 1.929 and 2.098 (Porphinato)cobalt(rrI) crystal structures have average Co-N(porphyrin) bond lengths that range from 1.954 (4)3' to 1.985 (9) a.32The distances reported in the present study are short, although equal to the smallest porphyrin hole (1.929 A). The sterically hindered (porphinato)cobalt(IIr) complex [( 1-methylimidazole),-TClPCo]+BF,-has a similar axial Co-N distance; 1.942 (6) A.17 The phthalocyanines generally have shorter Co-N (macrocycle) distances than porphyrins, but similar distances to the axial ligand~.~~.,~ Thus, the EXAFS study of chloro(phthalocyaninato)cobalt(rn)(s) and chloro(pyridine)-(phthalocyaninato)cobalt(III)(s)15 shows that the correspond- ing distances are 1.91 (1) and 1.89 (1) A, respectively.The axial Co--N distance for TPPCo"' complexes with amine ligands varies from 1.93 (2)" to 2.060 (3) A.34 Hence, the dis- tances in the present study are equal to the shortest reported value within experimental error.Structures of Chluro(pyridine)(tetruphenylporphinato)cobu~t(ili) (A)and Chloro(isoquinoline)(tetruphenylporphinato)cobalt(IrI) (B) The Co-N(porphyrin) and Co-"(amine) distances were averaged to a single distance at 1.91 (1) I$ in both com-pounds. The chlorine atom is directly bonded to cobalt C2.57 (1) and 2.55 (1) A, for A and B, respectively]. The corre- sponding crystallographic distances for A35 are 1.976 (6) (TPP), 1.978 (8) (pyridine) and 2.251 (3) 8, (CI). Ruffled (porphinato)metal cores generally have shorter Co-N dis-tances than planar ones.36 This expresses the desire of the system to minimise the metal-N distances within the con- straints of the macrocycle.Thus, the distances reported here are consistent with a puckered ring system. An NMR study using the ring-current program DIPCALCi4 on the bispyridine complex of TPPCoCl gave much better agreement for the freely rotating than for a fixed- ligand orientation,' and it was suggested that the ligand has considerable rotational mobility. The even shorter distance obtained by EXAFS for the monopyridine complex C1.91 (1) A] supports this conclusion. The Co-Cl distances are significantly longer in the com- plexes dissolved in chloroform than in the crystal structure of A. In this connection, note that an unusually long Co-Cl distance has been reported for a paramagnetic five-coordinate Co"' compound C2.507 (2) Of particular interest is the fact that the five-coordinate porphyrin TPPCoCl is diamagnetic in the solid state, but changes partly to paramagnetic species in inert solvents such as chloroform when the temperature is raised.38 Certainly, the analogous non-planar OETPPCO"'C~ has been found to be paramagne- tic.Structure of Bis(cyclohexyfamineMtetruphenylporphinuto) cobalt(II1)Chloride (C) The corresponding crystal structure has not been reported. The short Co-N(porphyrin) distance c1.92 (1) 14) indicates a ruffled (N), grouping. The axial Co-N(amine) distance C1.92 (1) A] is also short, but similar within error limits to distances reported for other amine ligands."*" J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 1 1 1 J 4 6 8 10 1 2345 k/A-RIA 1.6I Q,U w.-c 1.2 0,2 0.8 'c C g 0.4 r m.---. . . . . 0 4 6 8 10 12 12345 k1A-I RIA 1.6 0.8 0* C2 0.4 0 4 6 0 10 12345 k/A-' RIA Fig. 2 Observed (-) and calculated (---) Fourier-filtered k3-weighted EXAFS and their Fourier transforms for (a) A, (b) B and (c) C; parameters in Table 1 Structuresof Bis(4-methyZpiperidine)(tetra-o-dichZorophenyl-that these sterically hindered complexes (D and E) exist as porphinato)cobaZt(III) Chloride (0)and Bis(4-methyZpiperidine) six-coordinate cations in chloroform solution with Co-N (tetra-o-diJiuorophenyZporphinato)cobuZt(IIr)Chloride (E) distances which are similar to those of the unhindered com- The fits are shown in Fig. 3, and the parameters listed in plexes A, B and C (Table 1).These results agree with the Table 1. Table 3 compares the obtained distances with rele- crystal structures of [( 1-methylimidazole),TClPCo]+BF4-'' vant crystal structures. The multiplicities and distances show and the unhindered analogue [(imidazole),-Table 3 Intramolecular distances (A) (Co-X for bonding and Coo .-X for non-bonding) from the solution study and from the crystal struc- tures of A"' and the cations [(piperidine),TPPCo'"] + ' and [( l-methylimidazole)2TC1PCo111]the multiplicities (N)are included for the + ;d crystal-structure data + +PyrTPPCoClb (piperidine),TPPCo (1-rnethylimidaz0le)~TClPCoA" B" C" D" E" r/A r/A N r/A r/A r/A r/A N rlA N rlA Co-N 1.91 (1) 1.91 (1) 4 1.976 (6) 1.92 (1) 1.93 (1) 1.95 (1) 4 1.978 (5) 4 1.977 (5) Co-N 1.91 (1) 1.91 (1) 1 1.978 (8) 1.92 (1) 1.93 (1) 1.95 (1) 2 2.060 (3) 2 1.942 (6) CO-Cl 2.57 (1) 2.55 (1) 1 2.251 (3) -CO-* .C 2.88 (2) 2.95 (2) 8 3.018' 2.98 (2) 2.97 (2) 3.02 (2) 8 3.028 (14) 8 3.018 (22) CO.*-C2.88 (2) 3.27 (4) 2 2.909 (9) -2.97 (2) 3.02 (2) 4 3.022 (5) 4 2.9468 CO*--C 2.88 (2) 3.27 (4) 4 3.402' -3.32 (4) 3.36 (4) 4 3.433 (6) 4 3.408 (14) ~ This study, with esds in parentheses.The corresponding multiplicities and Debye-Waller-like factors are given in Table 1. Ref. 35. Ref. 34. Ref. 17. 'Average of eight distances within the range 2.994-3.052 A. Average of four distances within the range 3.388-3.420 A. 9 Average of four distances within the range 2.907-2.978 A. J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 L I t c 4 6 a 10 12345 k/A-' RIA Observed (-) and calculated (---) Fourier-filtered k3-weighted EXAFS and their Fourier transforms for (a)D and (b)E; parameters Fig. 3 in Table 1 TPPCo]+OAc-for which the distances are only slightly different. However, the steric requirements greatly affect the deviations of the core atoms from the mean plane of the porphyrin, the largest displacements in the hindered and unhindered complexes being 0.23 and 0.07 A, respectively. The o-chloro groups combined with crystal packing forces constrain the phenyl groups to be nearly perpendicular to the macrocyclic ring and also fix the axial ligand orientation. This effect has also been shown in a recent molecular mechanics and NMR study on various hindered and unhindered Co"' porphyrins complexed with amine ligands.l3 Planar ligands complex edge-on to the porphyrin with a fixed r 1 1 I 6 h s ,"o .y -6 4 6 8 10 12 1 2 3 4 5 k/A-' RIA Fig.4 Observed (-) and calculated (---) Fourier-filtered (1.0-25.0 A) k3-weighted EXAFS and their Fourier transforms for (a)F and (b)G; parameters in Table 2 orientation, staggered (ligand over N atoms) for the less steri- cally hindered complexes (o-fluoro substituents) and eclipsed ligand over rneso-carbon atoms) for the complexes with o-chloro and -methyl substituted porphyrins. Increased steric strain and a possible lengthening in the axial Co-N bond are suggested in the complexes with the non-planar 4-methylpiperidine complexes.No clear evidence of such a lengthening in the axial bond for D and E with respect to the unhindered complexes A-C could be found in the present study. A complicating factor in this context is that the equa- torial and axial Co-N bonds had to be averaged to one distance to avoid severe correlation between EXAFS param- eters. However, it shows that the axial Co-N bonds are shorter than ca. 2.1 A. Solid Samples The results are listed in Table 2 and the fits shown in Fig. 4. The EXAFS data show that there are only four N atoms in the first shell. The absence of axial amine ligands and/or a bonded chloride is consistent with reduction to the cobalt(I1) species TClPCo" (F) and TFPCo" (G).We note that Kastner and Scheidt,' found that (acety1)TPPCo"' and some other derivatives are unstable in chloroform solution, decomposing within a few hours.Certainly, our results are compatible with the observation that (porphinato)cobalt(ur) complexes with sterically hindered amines are reduced in chloroform solu- tion.16 On the other hand, freshly made chloroform solutions of the ortho-substituted porphyrins form six-coordinate Co"' cations with 4-methylpiperidine (D and E). On the basis of these observations, the requirement for Co"' complexes to crystallise as such would seem to be that the axial ligand must orientate in a sterically favourable manner with respect to the phenyl rings to minimise steric repulsions. Planar imid- azole ligands crystallise as octahedral complex salts with TClPFe"'C10, and TClPCo"'BF, , respectively.17e41 In both compounds the axial amine ligands have fixed orientations.The fact that piperidine is not planar combined with unfa- vourable crystal packing forces may explain the failure of complexes with ortho-substituted TPPCo"' to crystallise. An important factor in EXAFS is the phenomenon of multiple scattering which enhances contributions from distant atoms due to the electron wave being focussed by the first-shell atoms with a concomitant phase shift and, in turn, a displacement in distance. This leads to apparently shorter or longer distances as the program moves that particular wave in an attempt to correct the phase, and higher multi- plicities and/or low Debye-Waller factors.Multiple scat-tering effects are known to be important for ring ligands (such as the present compounds) and near collinear (150-180") atomic arrangment~.~' The Debye-Waller-like factors and the errors in the distances out from ca. 3.5 8, are to be regarded in this context (the multiplicities being fixed). That multiple scattering pathways do not significantly affect the results for the first three-shells was tested by repeating the calculations on data that were Fourier filtered to include only these shells. This cut-off can be made without causing large truncation effects, and is one that is not complicated by over- lapping Fourier-transform shells. It also eliminates com-plications in data analysis due to multiple scattering effects which could arise in more distant shells.Structure of( Tetra-o-dichZorophenylporphinato)co~aIt(II)(F) and (Tetra-o-dij?uorophenyZporphinato)cobaZt(II)(G) Table 3 shows that the distances and multiplicities obtained for samples F and G are in good agreement with the crystal structure of TPPCO".~~ C0'I-N bond distances are generally longer than those in the corresponding Co'" compounds.42 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 However, the porphyrin core conformation has a greater impact on the equatorial Co-N distances than the oxidation state, as demonstrated by the distance in TPPCo" which is 1.949 (3) and the short distances derived by EXAFS, 1.95 (1) and 1.92 (1) 8, for F and G, respectively.These dis- tances are similar to the corresponding co-N composite distances observed in the solutions. The authors thank the Norwegian Science and Research Council (NAVF) and VISTA for financial support, the Royal Norwegian Council for Industrial and Scientific Research (NTNF) for a stipend to M.E. and the Science and Engineer- ing Research Council for an earmarked studentship to I.M. We especially thank Mr. Jostein Rise for designing and con- structing our EXAFS cells. References 1 B. M. Hoffman and J. A. Ibers, Acc. Chem. Res., 1983,16, 15. 2 J. P. Collman, J. T. McDevitt, C. R. Leidner, G. T.Yee, J. B. Torrance and W. A. Little, J. Am. Chem. SOC., 1987,109,4606. 3 P. Battioni, J-P. Renaud, J. F. Bartoli and D. Mansuy, J.Chem. SOC.,Chem. Commun., 1986,341. 4 S. P. Cramer, in X-Ray Absorption, ed. D. C. Koningsberger and R. Prins, Wiley, 1988, p. 271. 5 R. W. Joyner, J. A. R. van Veen and W. M. H. Sachtler, J. Chem. SOC., Faruday Trans. 1, 1982,78, 1021. 6 B. van Wingerden, J. A. R. van Veen and C. T. J. Mensch, J. Chem. SOC.,Faruday Trans. 1, 1988,84,65. 7 R. J. Abraham, S. C. M. 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