Available online at www.sciencedirect.com JOURNAL OF
Inorganic Biochemistry Journal of Inorganic Biochemistry 102 (2008) 472–479 www.elsevier.com/locate/jinorgbio
Magnetic circular dichroism spectroscopy of cobalt tetraphenyltetraacenaphthoporphyrin John Mack a,b, Yoshiaki Asano b, Nagao Kobayashi b, Martin J. Stillman a,* b
a Department of Chemistry, University of Western Ontario, London, Ontario, Canada N6A 5B7 Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
Received 8 July 2007; received in revised form 16 October 2007; accepted 26 October 2007 Available online 28 November 2007
Abstract The first MCD spectral data for an open shell first row transition metal complex of tetraphenyltetraacenaphthoporphyrin (TPTANP) are reported. The B (or Soret) band of cobalt tetraphenyltetraacenaphthoporphyrin (CoIITPTANP(2)) exhibits an anomalous negative Faraday A1 term as was reported previously in the case of ZnTPTANP, while a positive A1 term is observed for the Q band. INDO/1 geometry optimizations predict that the TPTANP ligand is saddled due to steric hindrance at the ligand periphery to a slightly lesser extent than is the case with ZnTPTANP. The Q and B bands of CoTPTANP arising from the p-system are blue shifted relative to those of ZnTPTANP, based on the ‘‘hypso” effect reported previously for planar porphyrin complexes of d6–9 transition metals. Ó 2007 Elsevier Inc. All rights reserved. Keywords: Magnetic circular dichroism spectroscopy; DFT calculations; Non-planar porphyrin; Red-shifted Q band; Co(II)
1. Introduction We recently reported a study [1] of the magnetic circular dichroism (MCD) spectroscopy of free base tetraphenyltetraacenaphthoporphyrin (H2TPTANP), ZnTPTANP and [H4TPTANP]2+. MIITPTANP complexes were reported by Lash [2–5] to have the reddest shifted B (or Soret) bands ever observed for a porphyrinoid. The quest for red-shifted Q bands coupled with intense absorbances has been triggered by both industrial uses and for photodynamic therapy. The spectroscopic study of the TPTANP complexes revealed further unusual properties. A set of anomalous negative Faraday A1 terms was observed within the MCD spectrum [1], due to a reversal in the alignment of the magnetic moments of the optically accessible pp* excited states due to the saddling of the ligand, which is caused by the steric hindrance between the phenyl sub-
*
Corresponding author. E-mail address:
[email protected] (M.J. Stillman).
0162-0134/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.jinorgbio.2007.10.031
stituents and the fused peripheral acenaphthalene rings. There has been considerable recent controversy over the origin of significant red shifts observed in the major p ? p* bands of peripherally crowded dodecasubstituted porphyrins, which have been used to model the effects of ligand folding on the optical spectroscopy and electronic properties of metal porphyrinoids in proteins [6–21]. Ligand non-planarity is one of the key factors in determining the redox properties, the basicity of the inner nitrogen atoms, and the axial ligand binding affinity of metal porphyrinoids and is, therefore, believed to play a role in controlling the function of heme proteins, and, in particular, methyl-coenzyme M reductase [6,7]. The steric interactions between peripherally-fused acenaphthalene rings and the phenyl substituents in MIITPTANP complexes are directly analogous to those between the substituents of sterically hindered dodecasubstituted porphyrins. In this paper we report the MCD spectroscopy of CoTPTANP, Fig. 1, and explore the effect of inserting an open shell first row transition metal on the optical properties and electronic structure of the highly saddled TPTANP ligand.
J. Mack et al. / Journal of Inorganic Biochemistry 102 (2008) 472–479
Fig. 1. The INDO/1 structure of CoIITPTANP.
2. Magnetic circular dichroism spectroscopy MCD spectroscopy provides a probe for obtaining critical ground and excited state degeneracy information, which is required to fully understand the electronic structure of high symmetry complexes. The additional information provided by the MCD technique is derived from three highly characteristic spectral features, the Faraday A1 , B0 and C0 terms [22]. In the case of metal porphyrinoid complexes with D4h symmetry or lower, these terms arise from temperature-independent transitions between nondegenerate ground states and orbitally degenerate excited states ðA1 Þ or orbitally-nondegenerate excited state ðB0 Þ, and a temperature-dependent transitions between orbitally degenerate ground states and orbitally nondegenerate excited states ðC0 Þ. A1 terms can be readily identified based on their characteristic first derivative-shape, since B0 terms are Gaussian-shaped. The C0 term is observed as a Gaussian-like envelope that is temperature-dependent (the magnitude increasing with decreasing temperature) since it arises from electronic transitions from an orbitally degenerate ground state, with an electronic occupation dependent on the Boltzmann distribution when the degeneracy is removed by application of an applied, external magnetic field. The band envelopes can be of either sign. Gouterman’s 4-orbital [23] and Michl’s perimeter models [24,25] are usually used to describe the optical spectroscopy of porphyrinoids, based on the magnetic quantum number as applied to molecules, ML, which is derived from the orbital angular momentum of each individual molecular orbital, L. The value of L for the D16h symmetry C16 H2 16 parent perimeter of the 16 atom 18 p-electron system on the inner ligand perimeter of metal porphyrinoid complexes ranges from 0 to 8, so ML = 0, ±1, ±2, ±3, ±4, ±5, ±6, ±7, 8, where the sequence increases in energy. Within Gouterman’s 4-orbital model [23], the optical spectroscopy of the tetrapyrrole porphyrinoids is based on an allowed transition to the B state (‘‘DML = ±1”) arising 5 from the W5 4 and W4 excited state configurations and a forbidden transition to the Q state (‘‘DML = ±9”) arising 5 from the W5 4 and W4 excited state configurations, which arise from the four spin allowed transitions linking the HOMOs (ML = ±4) and the LUMOs (ML = ±5). Within
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Michl’s perimeter model [24,25] the induced excited state magnetic moments of these ideal Q and B states are referred to as l+ and l, respectively. Values of 6.24 and 0.01 b, respectively, were calculated based on INDO calculations. If the HOMOs and LUMOs are degenerate (DHOMO, DLUMO = 0), as is the case with the C16 H2 16 parent perimeter, the B and Q bands retain their fully allowed and forbidden character, respectively. The l moment determines the sign sequence observed for the Faraday A1 term associated with the B band. In the case of planar complexes it tends to be largely unaffected by changes to the structure but the moment is small enough that perturbations to the structure can potentially reverse the alignment relative to the applied field. Under the D4h symmetry of most metal porphyrinoids the degeneracy of the HOMO is broken to varying extents, but this is not the case usually for the LUMO. This results in a mixing of the allowed and forbidden character of the Q and B transitions. The degree of mixing, a, and the magnitude of the induced magnetic dipole moments of the Q and B excited states (l(Q), l(B)) can be estimated using Eqs. (1) and (2), within Michl’s perimeter model [24,25], based on the observed dipole strengths, D0 , for the Q and B bands. D0 ðQÞ=D0 ðBÞ ¼ tan2 a; 2
ð1Þ 2
lðBÞ ¼ ðcos aÞl ðsin aÞl; lðQÞ ¼ ðsin2 aÞl ðcos2 aÞl:
ð2Þ
Michl has demonstrated that, due to the contribution of the l+ magnetic moment, the sign sequence in ascending energy for the four Faraday B0 or two Faraday A1 terms associated with Q and B bands of reduced symmetry cyclic polyenes with DHOMO > DLUMO is usually ve, +ve, ve, +ve in terms of the observed DeM intensity values, while the sequence is usually +ve, ve, +ve, ve when DLUMO > DHOMO. The ve, +ve sequence of positive Faraday A1 terms (or pseudo A1 terms) is usually the dominant feature in the spectra of planar porphyrinoids. 3. Results As has been reported previously [1], in contrast to the spectra of other planar, radially symmetric, main group and closed shell d10 metal porphyrinoids complexes, the MCD spectrum of ZnTPTANP exhibits negative Faraday A1 terms at 717 and 552 nm, which can be assigned as the Q00 and B bands, respectively, Fig. 2. The positive A1 term at 666 nm was assigned as a vibrational Q01 band. In this present study, we report the first open shell MCD data for this class of molecule. The MCD spectrum of CoTPTANP exhibits a positive Faraday A1 term centered at 676 nm, 41 nm to the blue of the Q00 band of ZnTPTANP, a broad positive B0 term at 630 nm and a negative A1 term centered at 532 nm, 20 nm to the blue of the B band of ZnTPTANP, Fig. 2 and Table 1. The positive A1 term at 676 nm and the negative A1 term at 532 nm can be assigned to the Q00 and B bands, respectively based
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Fig. 2. Absorption and MCD spectra of ZnTPTANP (LEFT) and CoIITPTANP (CENTER) recorded at 298 K in CH2Cl2. The INDO/s absorption spectrum is plotted against the right hand axis. Spectral deconvolution analysis of the absorption and MCD spectra of CoIITPTANP (RIGHT) recorded in CH2Cl2. The Q00 and B00 bands are highlighted with a broader linewidth. The residual difference between the experimental (black) and calculated spectra (gray) is shown beneath each spectrum. The band parameters are tabulated in Table 1.
Table 1 Band fitting parameters obtained from the SIMPFIT program [37–39] for CoIITPTANP based on the modified Faraday A1 and B0 term and dipole strength ðD0 Þ definitions recommended by Stephens, Piepho and Schatz [32,33] m/cm1
C/cm1
T
hDeMin
A1 , B 0
13646 14101 14461 14787 14787 15217 15779 16396 16917 17499 18404 18944 18944 19618 20133
629 480 465 527 527 590 778 779 576 932 1008 765 765 814 560
B B B A B B B B B B B A B B B
0.011 0.391 0.089 668.3 0.919 1.212 0.257 1.513 0.068 0.312 0.681 820.4 1.609 0.283 0.102
7.48E05 2.56E03 5.84E04 4.38 6.03E03 7.95E03 1.68E03 9.92E03 4.44E04 2.05E03 4.47E03 5.38 1.06E02 1.86E03 6.66E04
hei0
D0
98.5 184.2 372.3 966.8
0.30 0.56 1.14 2.96
630.3 1374.0 806.5 278.5 1346.3 3958.3 6428.5
1.93 4.21 2.47 0.85 4.12 12.12 19.68
3994.8 1178.2
12.23 3.61
A1 =D0
103ðB0 =D0 Þ
lz
0.25 4.54 0.51 1.48
1.48 4.12 0.40 4.02 0.52 0.50 0.37 0.27
0.27 0.15 0.18
The Q00 and B00 bands are shown in bold. a – Calculated energy of the band center in wavenumbers. b – Band width at half height in wavenumbers. c – Faraday term for MCD band type. d, e – D0 ¼ hei0 =326:6, where the units of D0 (dipole strength) are D2 (Debye units). f, g – A1 , B0 ¼ hDein =152:5, where hDeMi1 and hDeMi0 are the first and zeroth moment of the MCD intensity and are calculated by SIMPFIT for the A1 and B0 terms, respectively. The units of A1 are ⁄ D2 and the units of B0 are ⁄ D2 cm. h lz ¼ A1 =D0 , where lz is the z-component of magnetic moment in Bohr magnetons.
on the fact that A1 terms would be anticipated for these bands based on the anticipated saddled D2d symmetry, since there is an orbitally nondegenerate 2B ground state and orbitally degenerate 2E excited states. The A1 =D0 ratio calculated for the B band of 0.3 ⁄ is similar in magnitude to that reported for ZnTPTANP, while the value of 1.5 ⁄ calculated for the Q band is similar to that observed for the Q band of [H4TPTANP]2, Fig. 2. The energy sepa-
ration between the bands of 4200 cm1 is typical of that expected for the Q and B bands of metal porphyrinoids [1]. There is a broad envelope of weaker overlapping bands in the UV region associated with the higher energy pp* states as is the case with ZnTPTANP. INDO/1 calculations predict that (H2O)CoIITPTANP(2) has a saddled S4 structure similar to that of ZnTPTANP, Fig. 3. The angles formed by the central
J. Mack et al. / Journal of Inorganic Biochemistry 102 (2008) 472–479
metal and the carbons on the periphery of the acenaphthalenes along the x- and y-axes of ZnP(2) are 16.3° and 22.4°, respectively. The INDO/1 structure of ZnTPTANP differs only slightly from those derived from B3LYP geometry optimizations [1]. The INDO/s calculations for ZnP, (H2O)CoIIP(2), ZnTPTANP and (H2O)CoIITPTANP(2) demonstrate clearly that the presence of a d7 central metal has only a minor effect on the electronic absorption spectra of planar porphyrinoids, Fig. 3 and Table 2. In the case of (H2O)CoIITPTANP(2), although the saddling of the ligand is predicted to markedly affect the energies of the d orbitals of the central metal due to the removal of the square planar coordination by the four pyrrole nitrogens, the energies of the four frontier p-MOs are largely unaffected, Fig. 4.
Fig. 3. The INDO/s spectra of ZnP and (H2O)CoIIP (A), of the planar and saddled structures of (H2O)CoII (B), and ZnTPTANP and (H2O)CoIITPTANP (C), and saddled and planar structures of ZnTPTANP (D). The energies of the experimentally observed Q and B bands in CH2Cl2 are shown as vertical lines. Octaethylporphyrin data are used for the P complexes [1,36].
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4. Discussion The negative A1 term observed for the B band in the MCD spectrum of CoIITPTANP(2) is consistent with that observed previously in the case of ZnTPTANP despite the presence of the paramagnetic metal. Our earlier analysis of a reversal in the alignment of the l moment due to the differing effects of ligand folding on the orbital angular momentum (OAM) properties of the HOMOs and the LUMO can be applied here as well. As has been observed previously by Djerassi and coworkers [26–28], structural perturbations, which result in a significant DHOMO value, tend to initially reverse only the sign of the A1 term associated with the Q band. When jDHOMO DLUMOj is small, the introduction of the larger l+ moment into Eqs. (1) and (2) initially has a far greater effect on the Q band. Normally in the case of planar MP(2) complexes where DHOMO ’ 0, the Q band gains intensity from the B band via a vibrational borrowing intensity mechanism [29] in which the signs of Faraday A1 terms associated with the Q00, Q01 and Q02 bands alternate between positive and negative. In the spectrum of CoTPTANP there is instead an intense Faraday B0 term. A possible explanation is that the insertion of the central Co(II) introduces a perturbation to the structure, which results in a significant DHOMO value, due to the fact that the 1a2u orbital from Gouterman’s 4 orbital model has major nodes on the pyrrole nitrogens, while the 1a1u MO has antinodes. If the Q00 band gains significant intensity the vibrational envelope associated with the transition can be expected to be populated by Frank Condon factors as opposed to vibrational borrowing. The presence of a second electronic transition involving the central metal may also be a significant factor, however. Several excited states are predicted with energies reasonably similar to those resulting from the Q and B transitions, Fig. 3. It should be noted that Antipas and Gouterman proposed that the quenching of the emission intensity of Co(II) and Co(III) porphyrins is due to coupling between a low lying CT or 3(d–d) excited state and the ground state [30]. Shelnutt and coworkers have proposed, based on molecular mechanics and INDO/s calculations, that there is a direct correlation between the red shifts observed for the Q and B bands of sterically hindered porphyrins and the degree of ligand non-planarity [6–21]. On the other hand, Di Magno and coworkers [19–21] have pointed to the fact that saddled free base and cobalt tetrakis(perfluoroalkyl)porphyrins do not show a red shift relative to the planar tetraphenylporphyrins and have postulated that red shifts are due to the impact of electron withdrawing or donating effects and in-plane nuclear reorganization induced by bulky substituents on the HOMO–LUMO band gap and the energies of the frontier p-MOs. DFT calculations were reported [19,31], which predicted that ligand deformation via ruffling and saddling need not necessarily result in a significant red shift. Shelnutt and coworkers [7] subsequently pointed out that these calcula-
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Table 2 Calculated electronic excitation spectra of ZnP, (H2O)CoIIP, ZnTPTANP, (H2O)CoIITPTANP, a saddled model structure of (H2O)CoIIP based on the INDO/1 structure of (H2O)CoIITPTANP, Fig. 1, and a planar model structure for ZnTPTANP #a
Sym.b
Calc.c
Wave functiond
Bande
14.9 (0.07) 27.5 (2.38)
65% 1a1u ! 1eg 32% 1a2u ! 1eg 64% 1a2u ! 1eg 31% 1a1u ! 1eg þ . . .
Q B
15.1 (0.06) 26.7 (2.17)
62% 1a1u ! 1eg 35% 1a2u ! 1eg 58% 1a2u ! 1eg 33% 1a1u ! 1eg þ . . .
Q B
(H2O)CoIIP (D2d model structure) 2 1 A1 2 11, [12] Ex 2 Ex 27, [28]
14.8 (0.06) 26.3 (1.81)
64% 1b1 ? 1e* 32% 1b2 ? 1e* 58% 1b2 ? 1e* 30% 1b1 ? 1e* + . . .
Q B
ZnTPTANP (S4) 1 2, [3] 4, [5]
12.7 (0.03) 20.6 (2.00)
55% 1b ? 1e* 37% 2b ? 1e* 52% 2b ? 1e* 36% 1b ? 1e* + . . .
Q B
ZnTPTANP (D4h model structure) 1 1 A1g 1 2, [3] Eux 1 4, [5] Eux
13.2 (0.03) 21.1 (2.65)
55% 1a1u ! 1eg 38% 1a2u ! 1eg 49% 1a2u ! 1eg 35%1a1u ! 1eg þ . . .
Q B
(H2O)CoIITPTANP (S4) 1 17, [18] 33, [34]
12.9 (0.06) 20.3 (2.36)
59% 1b ? 1e* 32% 2b ? 1e* 50% 2b ! 1eg 35% 1b ! 1eg þ . . .
Q B
ZnP (D4h) 1 2, [3] 4, [5] (H2O)CoIIP (D4h) 1 12, [13] 27, [28]
a b c d e
1
A1g Eux 1 Eux 1
2
A1g Eux 2 Eux 2
1
A Ex 1 Ex 1
2
B Ex 1 Ex 1
The number of the state assigned in terms of ascending energy within the INDO/s calculation. Symmetry of the excited state. A correlation table enables comparison of MOs of different symmetry, Table 4. Calculated band energies (103 cm1) and oscillator strength. The wave functions based on the eigenvectors predicted by INDO/s. Only eignenvectors greater than 5% are included. Band assignment described in the text.
tions involved artificially constraining the molecular geometry, and reported a series of INDO/s calculations for ruffled structures of tetraalkylporphyrins that clearly showed a direct correlation between red shifts of the Q and B bands and the degree of ligand folding. We recently demonstrated during a study of a wide range of planar and saddled free base core modified tetrabenzopenzoporphyrins that the wavelengths of the Q and B bands can be correlated with the average HOMO–LUMO band gap for all four frontier p-MOs within Gouterman’s 4-orbital model based on a wide range of structural perturbations such as phenylation of the meso-carbons, partial peripheral substitution with fused benzenes and substitution of protonated pyrrole nitrogens with sulfur, oxygen and carbon atoms. One-electron transitions involving all four frontier p-MOs contribute to the Qx, Qy, Bx and By transitions, Table 2, since the OAM properties of the cyclic polyene perimeter are based on complex rather than real MOs. Structural perturbations, such as ligand saddling, that systematically alter the energy of one or more of the four frontier p-MOs can, therefore, be expected to show the trends reported by Shelnutt and coworkers [7]. We recently reported that ligand saddling has this effect in the case of ZnTPTANP and a series of similar peripherally crowded porphyrinoids based on a comparison of the TD-DFT and INDO/s spec-
tra of saddled optimized structures with planar model structures, Fig. 3 [1]. Similar trends in band energies with HOMO–LUMO band gap are not observed when the INDO/s spectra of (H2O)CoIIP(2) and (H2O)CoIITPTANP(2) are compared to the spectra of ZnP and ZnTPTANP and the experimentally observed blue shift of both the Q and B bands of the Co(II) complexes is not replicated, Fig. 3 and Tables 2 and 3. The fact that the INDO/1 structure of (H2O)CoII TPTANP(2) is predicted to be slightly less saddled than that of ZnTPTANP could be used to rationalize the marked blue shift of the Q and B bands. The magnitude of the blue shifts makes this explanation unlikely, however. Planar metal porphyrin complexes of the d6–9 metals including CoIITPP(2) are known to exhibit the ‘‘hypso” spectral pattern (Gouterman’s terminology [23] for a blue shift in porphyrin spectra) relative to Zn(II) porphyrin complexes and those of other closed shell central metals. The ‘‘hypso” blue shift pattern is usually observed for metals with a filled t2g shell and primarily involves a shift to higher energy of the major spectral bands rather than the introduction of major new charge transfer bands in the visible region that are typically observed with the ‘‘hyper” spectral pattern associated with d1–6 metals with a partially filled the t2g shell [23]. Lash has reported significant p orbital related ‘‘hyper” red shift
J. Mack et al. / Journal of Inorganic Biochemistry 102 (2008) 472–479
477
Fig. 4. The MO energies of ZnP and planar and saddled structures of (H2O)CoIIP(2) (TOP) and ZnTPTANP and (H2O)CoIITPTANP (BOTTOM). The d orbitals associated with Co(II) are offset slightly to the right. The orbitals associated with Gouterman’s 4-orbital [23] and Michl’s perimeter [24,25] models are highlighted with diamonds. These MOs are well separated from other p-MOs even in the case of the radially fused ring expanded TPTANP complexes. The DHOMO values associated with the energy separation of the 1a1u and 1a2u or 1b and 2b MOs, which help to determine the optical spectroscopy are tabulated in Table 2.
Table 3 The energies of the four frontier p-MOs, the DHOMO value and the average HOMO–LUMO band gap (eV) of ZnP, (H2O)CoIIP, ZnTPTANP, (H2O)CoIITPTANP and the saddled and planar model structures for (H2O)CoIIP (a) and ZnTPTANP
ZnP (H2O)CoIIP (a) (H2O)CoIIP (b) (H2O)CoIIP (a) (D2d) (H2O)CoIIP (b) (D2d) ZnTPTANP ZnTPTANP (D4h) (H2O)CoIITPTANP (a) (H2O)CoIITPTANP (b)
1a2u
1a1u
1egx
1egy
DHOMO Band gap
7.05 6.93 6.94 6.92 6.91 6.33 6.35 6.37 6.37
6.41 6.38 6.40 6.41 6.43 6.18 6.23 6.20 6.22
1.53 1.47 1.46 1.53 1.52 1.78 1.77 1.85 1.84
1.53 1.47 1.46 1.46 1.45 1.78 1.77 1.77 1.76
0.64 0.55 0.54 0.51 0.48 0.15 0.12 0.17 0.15
5.20 5.19 5.21 5.17 5.19 4.47 4.52 4.48 4.49
effects for PbIITPTANP [3]. The Q and B bands lie at 853 and 604 nm, respectively. The data for CoIITPTANP, Fig. 2, and those that have been reported previously by Lash [2b] for Ni(II), Cu(II) and Zn(II) TPTANP complexes are consistent
with a ‘‘hypso” effect since the B bands lie at 528, 545 and 555 nm, respectively.In the case of planar porphyrin complexes of d6–9 transition metals the ‘‘hypso” effect has usually been described as being due to a destabilization of the 1eg LUMO based on mixing with the dp eg MO arising from the dxz and dyz orbitals of the central metal resulting in a blue shift of both the Q and B bands due to an increase in the HOMO–LUMO band gap [23]. The effect becomes less significant as the number of d electrons increases, since this stabilizes the dp MO and reduces the interaction with the 1eg LUMO. 5. Conclusions The assignment of the MCD spectrum of CoIITPTANP on the basis of Gouterman’s 4-orbital model is straightforward. The signs of the Faraday A1 terms can be rationalized on the basis of Michl’s perimeter model. The spectra of CoIITPTANP and ZnTPTANP provide direct evidence that substantial ‘‘hypso” blue shifts can be anticipated in
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the spectra of highly saddled porphyrinoids. The MCD spectroscopy of the transition metal complexes of sterically hindered synthetic porphyrinoids clearly merits further investigation, since new insights into the biochemical role of non-planar porphyrinoids can be derived using the model compound approach that has been developed in recent years by Shelnutt and coworkers [6–18].
Table 4 Correlation table for MOs with D4h, D2d and S4 symmetry D4h
D2d
S4
a1u a2u b1u B2u eg
b1 b2 a2 a1 e
b b a a e
6. Experimental section 6.1. Cobalt tetraphenyltetraacenaphthoporphyrin A mixture of tetraphenyltetraacenaphthoporphyrin (20 mg, 0.018 mmol) and cobalt dichloride hexahydrate (128 mg, 0.54 mmol) was reacted in DMF (3 ml) at ca. 150 °C for ca. 30 min. After cooling, the solution was poured into water, and the resulting precipitate collected by filtration, washed with water and methanol, and chromatographed using silica-gel and chloroform containing a small amount of methanol. The first eluting purple fraction was collected and passed through a membrane filter to remove contaminated silica-gel. After removal of the solvent, approximately 10 mg of CoIITPTANP(2) was obtained as a purple colored powder. A small portion of this sample was further imposed on a gel-permeation column (Bio-beads SX-1, Bio-rad) using CH2Cl2 as eluent. After evaporation of the solvent, the residue was recrystallized from CH2Cl2-methanol. Anal. found: C, 82.54; H, 4.12; N, 4.59%. Calc. for C84H44N4Co1 3H2O: C, 82.50; H, 4.28; N, 4.70%, m/z = 1167.3 was obtained for the parent peak of CoTPTANP(2) from ESI–MS recorded on a CHCl3 solution. 6.2. Optical spectroscopy Electronic absorption spectra were measured with a Cary 5G spectrophotometer. MCD spectra were recorded using a Jasco J-725 spectrodichrometer and a Jasco electromagnet that produces a magnetic field of up to 1.09 T or using a Jasco J-810 and an Oxford Instruments SM2 cryomagnet with a field strength of 5.0 T. The Faraday terms A1 and B0 and the dipole strength of the corresponding absorption band, D0 , are quantified based on the modified conventions of Stephens, Piepho and Schatz [22,32,33]. 6.3. Theoretical calculations Molecular structures were refined for ZnP, ZnTPTANP, (H2O)CoIIP, (H2O)CoIITPTANP(2), a D2d symmetry saddled model structure for (H2O)CoIIP based on the structure of (H2O)CoIITPTANP(2) and a D4h model structure for ZnTPTANP [1] through the use of the CAChe workstation (Fujitsu America Inc.) [34] software package based on INDO/1 geometry optimizations. INDO/s calculations were carried out based on all the MOs from HOMO 100 to LUMO + 100 and from HOMO 50 to LUMO + 50, in the case of P and TPTANP complexes,
respectively, using the Gaussian software package [35]. A correlation table enables comparison of the MOs associated with the Q and B transitions for complexes of different symmetry, Table 4. In the case of ZnTPTANP the structures derived from geometry optimizations and the spectral trends observed in the calculated spectra based on the more computationally expensive DFT and TD-DFT techniques were broadly similar to those derived from semi-empirical INDO calculations [1]. Attempts to carry out calculations with H2O as an axial ligand for Zn(II) complexes resulted in a lifting of the orbital degeneracy of the LUMO, which is not consistent with the observed spectral data. The degree of ligand saddling predicted by the INDO/1 geometry optimization calculation for (H2O)ZnTPTANP was similar to that observed in the case of ZnTPTANP. 7. Abbreviations DFT density functional theory HOMO highest occupied molecular orbital INDO intermediate neglect differential overlap LUMO lowest unoccupied molecular orbital MCD magnetic circular dichroism OAM orbital angular momentum P porphyrin TD-DFT time dependent density functional theory TPP tetraphenylporphyrin TPTANP tetraphenyltetraacenaphthoporphyrin Acknowledgements We thank NSERC of Canada for Operating and Equipment grants (to MJS) and support from Fujitsu America Inc. in providing the CAChe Workstation software. MJS is a member of the Centre for Chemical Physics at UWO. This research was partially supported by the Ministry of Education, Science, Sports and Culture, with a Grant-inAid for the COE project, Giant Molecules and Complex Systems, 2006 and a Grant-in-Aid for Exploratory Research (No. 19655045) (to NK). References [1] J. Mack, Y. Asano, N. Kobayashi, M.J. Stillman, J. Am. Chem. Soc. 127 (2005) 17697–17711. [2] T.D. Lash, P. Chandrasekar, J. Am. Chem. Soc. 118 (1996) 8767– 8768. [3] J.D. Spence, T.D. Lash, J. Org. Chem. 65 (2000) 1530–1539.
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