Electrochemical and spectroscopic behavior of dicobalt cofacial diporphyrins. The redox sites revisited

ELSEVIER

lnorganica Chimica Acta 248 (1996) 181-191

Electrochemical and spectroscopic behavior of dicobalt cofacial diporphyrins. The redox sites revisited Y. Le Mest

a,*,

M. L'Her a, j._y. Saillard b

Laboratoire de Chimie, Electrochimie et Chimie Analytiques, Unit~ de Recherche Associie au CNRS 322, Universiti de Bretagne Occidentale, B.P. 809, 29285 Brest Cedex, France b Laboratoire de Chimie du Solide et Inorganique Mol~culaire, Uniti de Recherche Associie au CNRS 1495, Universit~ de Rennes 1, 35042 Rennes Cedex, France

Received 29 July 1995; revised 17 November 1995

Abstract The electrochemical behavior of dicobalt cofacial porphyrins is reported and the EPR and UV-Vis spectra of their oxidized forms are described. The results are discussed in terms of a comparison with the properties of the same complexes bearing non-electroactive metals, which have been described recently. On the basis of UV-Vis spectrophotometry, EPR and electrochemical criteria, an original formal writing is offered for the oxidation pathway of the dicobalt porphyrins in PhCN. It is proposed that the first oxidation process concerns the 7r-rings, the second is the oxidation of the eobalt(II) into cobalt(HI) and a third one the second oxidation of the It-rings. Based on the X-ray structure of the C02FTF4 diporphyrin, extended H~ckel MO calculations can provide a theoretical basis for the electrochemical and spectroscopic behavior. This arises from crossings of the 7r-ring ligand and (cobalt) metallic levels due to a combination of 7r-lr and d-d electronic interactions between the two moieties of the dimers. A steric effect is also emphasized: the size of the interporphyrin cavity exerts a selectivity as regards internal ligation. The present findings afford a completion of the so-called 'cofacial effect' of intramolecular interactions in dimers proposed in a previous paper. Keywords: Electrochemistry; Molecular orbital calculations; Porphyrin complexes; Cobalt complexes; 7r-Cationradical

1. Introduction The wide interest devoted to the cobalt derivatives of dimeric cofacial porphyrins (diporphyrins) and related systems originates from the discovery by the Collman-Anson groups and later by the Chang-Weaver groups that some of these compounds are able to promote the electrocatalytical reduction of dioxygen by a direct four-electron mechanism in an aqueous acidic medium when adsorbed on a graphite electrode [ 1-7]. Interestingly, it appeared that very small modifications in the structures of these dimeric porphyrin derivatives could lead to a partial or total loss of their efficiency in regard to the four-electron reduction process of dioxygen [ 1-7]. Whence it was concluded that the study of these families of compounds could provide evidence of some of the intrinsic properties which generate their efficacy. We have undertaken a study of the electrochemical and spectroscopic properties of these compounds in aprotic solvents [ 8-10]. The first goal was to emphasize any difference

in their behavior in comparison with those of the monomeric cobalt porphyrins, as well as differences between the dimers in the series. Our initial studies concerned the electrochemical behavior of the first discovered active diporphyrins Co2FTF4, and was later extended to two series of dicobalt diporphyrins FTF and DP, shown in Fig. 1 [8]. As postulated earlier [ 1 ], the solution experiments emphasized a mixed valence behavior for the oxidation of the dimers in a very tight configuration (FTF4, FTF5 3-1, FTF5 2-2, DPB) called group 2 compounds: --e

--e

[Co(II)P Co(II)P] ~ [Co(III)P Co(II)P] ÷ [ C o ( I I I ) P C o ( I I I ) P ] z+

For the loosest dimers (FTF6, DPA,) called group 1 compounds, the two components of the molecule are oxidized simultaneously and seem to behave as two independent monomers: -

2e

[ C o ( I I ) P C o ( I I ) P ] ~ [ C o ( I I I ) P C o ( I I I ) P ] 2+ * Corresponding author. 0020-1693/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSDI0020-1693 (95)05004-9

(1)

(2)

182

Y.Le Mest et al. / lnorganica Chimica Acta 248 (1996) 181-191 the tightest dimers, the first oxidation process is split into two redox steps i: --C

--e

[MP MP] ~ [MP.MP] ÷ ~ [MP- - -MP] 2+

CoC2diE } N -n + m+2 2 2 2 I ~ , 1 1 2 Fn~2,2

i

! FI'F4

Fig. 1. Porphyrinsinvestigatedin the presentstudy: FTFnseries and 'Pacman' DP series. On the basis of the extensively described oxidation pathway of cobalt monoporphyrins [ 11-18]: -c

-c

[Co(II)P] ~ [Co(III)P] ÷ --C

[Co(III)P'] 2+ ~ [Co(III)P] 3+

(3)

those studies of the dimers were devoted exclusively to the first oxidation process (Eqs. (1) and (2)). As ascribed to the oxidation of Co(II) into Co(III), it was assumed to be the one involved in oxygen activation. However, from the electrochemical and spectroscopic (UV-Vis, EPR) properties of these dimers, as compared to those of the cobalt(III) monomers, there was some suspicion about the assignment of the redox site: either an unusual tetra- or pentacoordinate cobalt(III) derivative or a 7r-ring-oxidized form, [Co(II)P'] ÷ [19-24], neither formulation being clearly established at that time [ 8]. Moreover the remarkable affinity of the one-electron oxidized species [Co(II)P Co(III)P] ÷ [ 10] and even more the unexpected reactivity of the twoelectron oxidized derivative [Co(III)P Co(III)P] 2+ [25] towards dioxygen remained especially puzzling. This lead us to the conclusion that the electrochemistry and reactivity of the dicobalt diporphyrins are deeply influenced by their cofacial structure. A subsequent examination of the properties of derivatives with non-electroactive metals in place of cobalt, such as Zn, Cu or the free base derivatives (H2) emphasized that the cofacial configuration induces very strong perturbations in the electronic properties of the porphyrin ~r-rings themselves, which we referred to as a so-called 'cofacial effect' [9]. As in the case of cobalt dimers (Eq. (1) ) it was found that for

(4)

It was shown that (Eq. (4)) these dimers must be considered as a new entity from an electronic standpoint, completely different from a simple juxtaposition of two independent monomers. A total delocalization of the unpaired electron density in the two oxidized (one-electron and two-electron) forms of the dimers induces a bonding interaction between the two rings in the dimer. A qualitative MO diagram consistent with the observed properties was proposed [ 9]. The purpose of the present work is to propose a proper assignation of the redox site in the case of the dicobalt derivatives by scrutinization of their three oxidation processes and comparison with the non-active metal derivatives [9]. MO calculations based on the X-ray structure of one of these compounds (Co2FTF4) account for the original oxidative pathway observed through a combination of the electronic d--d (metal-metal) and ~-rr (ring-ring) interactions. In addition to these electronic components of the so-called 'cofacial effect', it is also shown that a steric component of selectivity in the inter-ring cavity towards the size of the axial ligand must be taken into account when the dimer is complexed by a 'coordinating' metal such as cobalt. The rationalization of the redox pathway for the oxidation of the dicobalt diporphyrins is a prerequisite to a possible interpretation of their unprecedented and unclassical reactivity towards Oz in organic media [10,25]. Concerning the catalytic cycle for 02 reduction, the link from the behavior of the diporphyrins in organic solvents to their properties in aqueous acid may appear uneasy to establish [ 1-7]. However, for the elucidation of this catalysis, the actual electronic properties of the dicobalt dimers, which are likely to be retained in whichever medium, could constitute a better basis than those of monomers. Another attractive area in which the electronic properties of these dimers is involved is the field of molecular conduction, especially in the case of cobalt porphyrin conductors [26,27 ].

2. E x p e r i m e n t a l

Since the compounds used in the present work are obtained at very low yields after long synthetic routes, the available quantities for electrochemistry were very low (a few mg for each). This means that each experiment had to be performed on a very small fraction (0.2--0.3 mg) dissolved in minute volumes ( ~ 300 bd) of solvent. Furthermore, the solubility of these compounds is low ( < ~ 7 × 10 -4 M). For these i Throughout the presentpaper,P designatesthe m-rings of the porphyrin, and M a metalliccenter,non-electroactivesuchas Zn, Cu, ... or alsothe two protons (H2) of the freebase,orthe cobalt.Thedot - designatesan electron delocalizedover the two 7r-rings, and the dashes ( - - - ) the equivalent of a

weak ~r-~" bond obtained by pairing of two single electrons (see Ref. [9] ).

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Y. Le Mest et al. /lnorganica Chimica Acta 248 (1996) 181-191

reasons, some of the quantitative measurements were obtained with a limited precision. Owing to the extreme sensitivity of these dimers to traces of oxygen and water, all experiments were carried out in a dry nitrogen atmosphere box with carefully deoxygenated solvents and chemicals. 2. I. Chemicals

The synthesis, purification and characterization of the diporphyrins of the FTFn series [ 1,3] and the DPX (X = A or B) 'Pacman' series [28] have been described in detail elsewhere. All were synthesized at Stanford. The solvent (mainly benzonitrile=PhCN) and the supporting electrolyte, tetrabutylammonium hexafluorophosphate (Bu4NPF6), were purified as previously described [ 8 ]. Solutions of the supporting electrolyte were prepared in the dry box, stored on molecular sieves (Linde 4/~) and twice percolated through an activated (400 °C under vacuum for 48 h) neutral alumina (Merck) column. The oxidized forms of the derivatives were electrochemically generated; thus all the spectra were recorded in the presence of the supporting electrolyte. To record the spectra, solutions were transferred from the electrochemical cell to EPR tubes or to UV-Vis cuvettes and sealed prior to removal from the dry box. 2.2. Apparatus

The dry box was manufactured by Jaram; the nitrogen flow was continuously purified by passage through molecular sieves at ambient temperature and divided copper BTS catalyst (BASF) at 100 °C. The electrochemical cell was specifically designed to fit the rotating disk electrode (EDI Tacussel) for a minimum volume of solution in the main compartment. The auxiliary and reference (ferrocenium/ferrocene = Fc +/Fc) electrodes were in separate compartments connected to the main one through ground joints terminated by frits (Vycor tips from PAR). For voltammetric measurements, a platinum disk (diameter = 2 mm) was employed, and the electrolyses were performed with the same electrode, but rotated, equipped with a 4 mm diameter disk. For the purpose of comparison, the formal potential of Fc ÷/Fc versus SCE is 0.43 V measured ~n the same medium (PhCN, 0.2 M Bu4NPF6). A model PAR 173 potentiostat equipped with a PAR 179 digital coulometric unit was monitored by a PAR 175 programmer; the chart recorder was a T-2Y SEFRAM ENERTEC. UV-Vis spectra were recorded on a CARY 219 spectrophotometer from VARIAN. A JEOL FE3X apparatus was used for the EPR spectroscopy. Spectra were recorded from solutions (V= 40/zl) at a concentration close to 5 × 10 -4 M in quartz tubes with a power of 1 mW and frequency close to 9.2 GHz.

3. R e s u l t s a n d d i s c u s s i o n

Fig. 2 shows a typical voltammogram for one of the dicobalt dimers, Co2FTF4, extended to the entire electrochemical range of the solvent. Apart from the assignment of the redox site to metal versus 7r-ring, a difficulty arises with these types of dimer for a coherent designation of the redox reactions. As in the previous article [9], we designate: (i) by process the global abstraction of two electrons from the dimer, corresponding to one electron for each monomeric unit constituting the dimer as each step in Eq. (3), (ii) by step, each of the individual electrochemical reactions. Each process can thus take place either in two one-electron steps or in one twoelectron step, not excluding that they may be intermingled. All the dimers display one reduction process, as shown in Fig. 2 for the case of Co2FTF4. This has been previously ascribed to the reduction of the cobalt(H) centers into cobalt(I) in two successive one-electron steps [ 8,11-18 ]. As the so-called cofacial effect does not affect the site of this redox reaction, which normally is not involved in 02 reactivity, the reduction of the dimers is not reconsidered in the present paper. As may be postulated from the behavior of the cobalt porphyrin monomers (Eq. (3)), three oxidation processes are observed. Only the first one was examined in our previous reports on the basis of its attribution to the oxidation of the two cobalt centers ((Eqs. (1) and (2)) [8]. The site of oxidation (cobalt versus ring) has been shown to be quite questionable; therefore the three oxidation processes were scrutinized by electrochemical and spectroscopic means.

I 2pA 1=/

-2 /

- ~

o

/

~ E(V)

Fig. 2. Cyclic voltammogram of the Co2FTF4 diporphyrin, ~ 5 X 10-4 M, on the entire electrochemical range of PhCN+Bu4NPFr, displaying the different redox processes observed. Platinum electrode; scan rate O.1 V s - t vs. Fc ÷/Fc system.

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3.1. Spectroscopic criteria

3.1.2. EPR

In the case of EPR, a comparison is more difficult to make, as some derivatives have an unpaired electron only on the zrring system (zinc) and others (cobalt(II) and copper(II)) also have unpaired electrons located on the metals. In the latter case, possible couplings between electrons on the metal and on the 7r-ring systems render the interpretation very difficult. This is most likely the reason why the results obtained in the case of the cobalt derivatives had so far remained unclear [ 8 ]. The neutral derivatives of the cobalt(II) dimers are EPR silent in non-coordinating media, with this apparent diamagnetism of the crude complex likely being ascribable to weak spin interaction [ 8 ] (vide infra). In the presence of a strong axial donor ligand (N-Melm), typical spectra of exchangecoupled systems are observed, due to the interaction of unpaired spin densities in the two d~2 cobalt orbitals activated by the axial ligand [ 1 ]. The one-electron oxidized form of the dimers in pure solvent (PhCN) was shown to display a broad ill-defined spectrum with a central line, g = 2.05 [8], typical neither of cobalt(II) ( g = 2 . 3 ) [30] nor of a ~r-cation (g = 2.0025). It is somewhat comparable to the one described for the 7r-cation radical of cobalt(III) [31 ], although the g value (2.005-2.003) in that case is lower than in the present case. This would indicate a higher metallic versus radical contribution in the present case. Actually, on the basis of our previous account [9], it appears that this spectrum compares well with that which we obtained in the case of the dicopper derivative in the same redox form (g -- 2.05). A comparison of the spectra is shown in Fig. 4. The latter spectrum has been

3.1.1. UV-Vis spectrophotometry

Fig. 3 displays the comparison between the previously described spectra of the different redox forms (neutral, oneelectron and two-electron oxidized) of a cobalt [8] and a zinc [11] dimer derivative (FTF4) for the first oxidation process. While both compounds have a normal spectrum in the neutral form, the oxidized forms of the cobalt derivatives displayed spectra radically different from the standard spectrum of a Co(III) porphyrin derivative, shown in the inset of Fig. 3, for the CoC2diE monomer in the same medium. Conversely, it is clear that the spectra of the oxidized forms of the dicobalt derivatives are very close to those obtained for the zinc dimer, and both are perfectly characteristic of 1rcation radicals [29]. The same is also true for the whole series of dimers [8,9]. Noticeably, the spectra of the oneand two-electron oxidized forms of the dicobalt derivatives display a Soret band located at ,-" 350-370 nm with a strongly decreased (at least 50%) molar absorption coefficient as compared to the parent neutral Co(II) compounds. This observation is identical with that reported for the formation of Co(H) ~-cation radical derivatives [Co(II)P'] ÷ [19,24b,c]. These spectra contrast with those reported for [PCo(III)L] ÷ ( L = H 2 0 , MeOH, CO, CH3CN ) or [PCo(III)L2] ÷ (L = solvent, nitrogenous axial base .... ), which have spectra similar to the neutral Co(II) forms with a red-shifted Soret band ( ~ 4 3 0 nm) and a only slightly decreased intensity [ 11-18,24].

/

i.

o~

II ii

xl,, !l]!

® ,,nan

400 .tO

:

"°. A I

400

'

I

I

500

600

I

nm

~00

/

500

I

600

nm

Fig. 3. Comparisonof the UV-Visspectraof the Zn2FTF4(set of curves I) and of the Co2FTF4(set 2) diporphyrins, ~ 5 × 10-+ M, beforeand afteroxidation: (a) neutral form; (b) one-electronoxidizedform; (c) two-electronoxidized; solventPhCN, 0.2 M Bu+NPF~.Inset: (a) Co(II)C2diEand (b) Co(III)C2diE monoporphyrinin PhCN, 0.2 M Bu+NPF6.

Y. Le Mest et al. / Inorganica Chimica Acta 248 (1996) 181-191

185

electroactive metal derivatives of the diporphyrins is in total agreement with an assignment of the redox site to the 7r-ring orbitals for the first oxidation process of the dicobalt derivatives.

g= 2.05

(Cu(|)2FTF4°)+~

3.2. Electrochemistry

(b)

(Co2FTF4)+

~G

From our previous accounts on the cobalt and non-electroactive metal dimer derivatives [ 8,9], electrochemical indications could also be gained in support of the 7r-ring versus cobalt redox site attribution. (i) The redox systems corresponding to the first oxidation process are always electrochemically reversible. This runs counter to the widely described EC behavior of the Co (III) / Co (II) process, which results from associated ligand exchange reactions due to a change in coordination when Co(II) and Co(III) electrochemically interconvert (Fig. 5) [ 11-18,24]. (ii) When the first oxidation process is split into two one-electron steps (group 2 compounds, Eq. (1) ), comparison of the results for the cobalt [ 8] and the non-electroactive metals derivatives [9] shows that the redox splitting A Eox differences between the potentials of the two one-electron steps, follows exactly the same order no matter what the metal is:

,~OG

FTF5 3-1 > FTF4 > FIT5 2-2 > FTF6

g =2.05 f

(c)

gj. = 2.31

(~F-rF4)+ .in PhCN + N-Ivlelm

9#=2.02

_J

Fig. 4. EPR spectra of the one-electron oxidized form of the F r F 4 diporphyrins, ~ 5 × 10 - a M in PhCN 0.2 M Bu4NPFr, frozen at 130 K: (a) copper derivative; (b) cobalt derivative in the pure solvent; (c) in the presence of an excess of N-Melm.

clearly demonstrated to be associated with an average coupling situation of a species in which formally three unpaired electrons are present, one totally delocalized on the ~r-ring orbitals and two on the metals such as in: [M(II)P. PM(II) ] + T

~

M=Cu(II)

(5)

T

corresponding to an average S = 1/2 configuration. After the addition of a strong donor ligand (N-Melm) to a solution of the one-electron oxidized species a spectrum typical of a pentacoordinated cobalt(II) (Fig. 4) was observed, indicating unambiguously that the presence of the nitrogenous base generates a [Co(II)P PCo(III) ] + configuration with no spin density on the w-ring orbitals. The spin pairing of the two single electrons in the two-electron oxidized form demonstrated in the case of Zn and Cu derivatives also accounts for the EPR inactivity of [Co(II) P PCo (II) ] z ÷ with the two electrons removed from the it-ring orbitals, as it has been observed above that the two Co (II) centers remain EPR silent most likely due to weak spin interactions. It thus appears that a comparison of the spectroscopic properties of the cobalt derivatives to those of the non-

(6)

This order is also exactly the same as that of the NMR 6NH internal pyrrolic protons [9] for the free base (I-I4) derivatives which is another independent measure of the extent of the interactions between the two 7r-rings. This also strongly indicates that only the It-ring porphyrin orbitals are involved in this oxidation process of the dicobalt derivatives, In addition to these indications based on the previous observations, the second and third oxidation processes for the cobalt derivatives have been scrutinized. Electrochemical data for these two processes are reported in Table 1, together with those corresponding to the first oxidation process. In Fig. 5, the cyclic voltammogram of the dicobalt cofacial dimer Co2FTF4 is compared to that of the corresponding zinc derivative, and for comparison the voltammograms of the zinc and cobalt monomers are also displayed. For every dicobalt dimer, all of the redox systems are quasireversible. The number of electrons exchanged has been obtained by comparison of the voltammetric peak and wave heights with those of the first process. The instability and reactivity of the oxidized forms precluded coulometry and reliable spectroscopic characterization. As was the case for the previously discussed first oxidation process, the second process also splits in some instances into two one-electron steps (see Table 1 ). However, in this case the splitting is less clearly influenced by the geometrical factors than the first process. For example, the Co2FTF4 and Co2DPB dimers have similar inter-ring separations, but for the former the second oxidation process is split while the latter displays a one-step process. In the case of the two Co2FTF5 compounds, the second process is split and not the third; the second oneelectron step of the second process is intermingled with the

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Y. Le Mest et al. / Inorganica Chimica Acta 248 (1996) 181-191

Monoporphyrins

I

2/~A

Zinc,~ -;

L____~

Diporphydns k2 ~ A

^

oo

,

zn

~,,/

,

/,

..j,

J ,

~,v

Fig. 5. Comparative view of the oxidative voltammetry of the cobalt and zinc C2diE monoporphyrins and of the dicobalt and dizinc FTF4 diporphyrins, ~ 5 × 10- 4 M in PhCN + Bu4NPF~.Platinum electrode; scan rate 0.1 V s - J; vs, Fc +/Fc system. Table 1 Electrochemical data for the oxidation of the cobalt diporphyrins Porphyrins

Oxidation E '° (V) • (AEp (mV)) b, ne ~ I st process

Oxidation E '° (V) b (AEp (mV)) c ne a 2nd process

Oxidation E '° (V)b (AF~ (mV)) ~, ned 3rd process

-0.06 (110), le 0.07 (60), 2e -0,03 (60), le; 0.13 (60), le -0.03 (60), le; 0.13 (70), le - 0.03 (60) le; 0.18 (100), le

0.60 (60), le 0.56 (70), 2e 0.56 (100), le 0,56(100), le 0,60 (80), le; 0.77 (100), le

0.92 (90), le 0.88 (140), 2e [ ~-,0.90 (180) a, n.~ 3 , ~ l e [ ~,0.90 (180) a, n = 3,---*] e 0.98 (140) a, 2e

-0.10 (200), 2e 0.05 (70), 2e 0.00 (70), le; 0.165 (70), le

0.66 (In'), 2e 0.59 (70), 2e 0.65 (70), 2e

0.77 (100), 2e 0.88 (90), 2e

FTF series

CoC~diE Co2FTF6 Co2FIT5 2-2 Co2FTF5 3-1 Co2FTF4 Pacman series

Cozl,5DPB Co2DPA Co2DPB

a Obtained from cyclic voltammetry in PhCN + Bu4NPF6;Pt electrode; 100 mV s- '; E'* vs. Fc +/Fc. ¢ ne = number of electrons exchanged, measured by coulometry for the first oxidation processes with an uncertainty less than 5%. d n values for the other systems were derived from a comparison of the diffusion currents (on the wave of the RDE voltammetry) and/or CV peak heights to those of the first process; uncertainty less than 20%. e This n value indicates that the second process is split with the second one-electron oxidation step being at a potential close to the third two-electron process. third process, g i v i n g a one-electron and a three-electron step. The data o f Table 1 indicate that, in any case, the third oxidation process is a two-electron process in a single step. The fact that for these processes the AEp value is close to 60 m V or higher a n d n o t lower indicates that it reflects the oxidation of two n o n - i n t e r a c t i n g centers at the s.ame potential [ 32]. C o n s i d e r i n g the determination o f the site o f these redox processes consistently with the spectroscopic criteria, the v o l t a m r n o g r a m s o f Fig. 5 seem to indicate that the additional oxidation process ( w h e n a non-electroactive metal is replaced by a c o b a l t ( I I ) ) occurs b e t w e e n the two ring oxidation processes. This suggested the utilization o f an electrochemical criterion proposed by W o l b e r g and M a n a s s e n to determine the site o f the redox reaction on metalloporphyrins

[ 17]. These authors have s h o w n that w h e n a metallic center in metalloporphyrins is the site o f the redox reaction, the redox potential varies linearly with the third ionization potential of the metallic cation (3rd I P ) , while the redox potentials o f the x - r i n g systems are not clearly affected by this constant of the metal. This effect is displayed in Fig. 6, curve a, for the case o f different derivatives of a m o n o p o r p h y r i n , the octaethylporphyrin 2. The d i a g r a m clearly shows that the 2The potential values are those reported by Fuhrhop et al. [15], in butyronitrile expressed vs. the ferrocene reference. These values ate very close to those obtained with the derivatives of the C2diE monomer, which is structurally closer to the octaethylporphyrin than to the tetraphenylporphyrin originally used by Wolberg and Manassen to demonstrate the correlation.

Y. Le Mest et al. / Inorganica Chimica Acta 248 (1996) 181-191

187

First oxidation process: a

•

b

[co~Pco~l'] . ' ~

tc,0~. ~ r ~

~

tcor~-PC~
(7)

or.~.

Second oxidation process:

tc*0~-- coanal2+ ' .~ /

tc~m~p---

Pcotn)13+

lco(n~--- PcooI~l4+ 0~)

or-2e

o,.

O0 Third process:

tcoomP--cooed* ~

tCo~ pco(m~l ~

(9)

Scheme 1. Proposition for a formal writing for the oxidation of the cobalt cofacial diporphyrins in PhCN2.

-0.5

? Fig. 6. Plots of the redox potemials (vs. Fc +/Fc) for the two, or three (for the cobalt and iron), oxidation processes of different metal complexes of porphyrins and diporphyrins against the third ionization potentials of the metal atoms [19]. (a) C2diE derivatives; (b) FI'F4 derivatives; redox potentials quoted vs. Fc/Fe + . Oxidation ascribed to the metal site: * ; to the first oxidation process on the w-ring: O; to the second oxidation process of the ring: A. Potential values other than those of the cobalt derivatives are extracted from Ref. [ 11 ].

potentials of the M ( I I I ) / M ( I I ) systems, designated by a * in the figure, vary linearly with the 3rd IP; however, the potentials of the first and second oxidation process of the rings, shown respectively as a • and a • on the curves, follow a curved parallel trend not clearly defined with regard to this constant. It appears that the potentials for the oxidation of the metals are completely out of the range observed for the ring oxidations. In curve b is shown the variation of the points obtained in the case of the different derivatives of the M2FTF4 diporphyrins. The dots ( Q ) indicate the potentials of the two oneelectron steps of the first split oxidation process. The triangles ( • ) represent the potentials of the second oxidation process for the non-electroactivemetal derivatives, and the third process in the case of the cobalt derivatives. It is clear from this picture that these two series of points follow a variation very similar to that observed in the case of the two ,r-ring oxidations of the monomers. Importantly, it is seen that the points corresponding to the second oxidation process of the cobalt derivative, shown by the stars ( * ), are totally out of the range of the other series and cannot be associated with them. Just as is the case for monomeric porphyrins, a similar correlation is obtained with the other cofacial compounds.

3.3. Proposition for a formal writing for the oxidation pathway of the dicobah diporphyrins in PhCN The concurrence of this ultimate electrochemical criterion with the other electrochemical and spectroscopic observations leads us to conclude that Scheme 1 may be now offered for the oxidation pathway of the cobalt face-to-face diporphyrins in PhCN. Scheme 1 proposes that the first oxidation

process concerns the orbital 7r-rings. In the case of the group 2 compounds (split process), the first electron abstraction leads to a dicobalt(II) species in which the remaining odd electron is delocalized over the two 7r-ring orbitals, and the second abstraction to a species in which two 7r-ring electrons are paired giving rise to a weak 7r-Tr bond, represented by a dashed line [9]. This delocalization process had been demonstrated with the derivatives bearing non-electroactive centers (zinc and copper), and thus explains the strong similarity of their UV-Vis and EPR spectra with those obtained in the case of the cobalt derivatives. The second process consists of the sequential or simultaneous oxidation of the two cobalt (II) centers into cobalt(III). The second oxidation of the rings, leading to dicationic ring derivatives of cobalt(Ill), is the third and last oxidation process. Direct evidence for the formation of 7r-cationic forms of cobalt(II) [Co(II)F] +, such as the observation of the 7rcation radical marker band [ 20-23 ] reported by Chang and co-workers for cobalt monomers [ 19] has not been obtained so far in the case of these diporphyrins due to the lack of an adapted equipment (e.g. FTIR spectroelectrochemical cell). Until now, this marker band has been clearly described in the case of the monomeric octaethylporphyrins (OEP) and tetraphenylporphyrins (TPP) obtained by chemical oxidation [ 19 ]. In the case of cobalt (II) 7r-cation radicals electrochemically generated in CH2C12, its observation has not been clearly indicated [24b,c]. However, in the case of cobalt monomers, this kind of derivative is only obtained in a very poorly coordinating medium, and even weak donors such as H20 or MeOH (L) induce an intramolecular electron transfer from the Co(II) to the ring: L

[Co(II)P'] + ~ [LCo(III)P] +

(10)

In the case of the diporphyrins dimers, such a localization of the redox site in a coordinating medium like PhCN seems puzzling 3. Nevertheless, the concurrence of the UV-Vis 3 It has been verifiedthat the electrochemical oxidation of CoC2diE in anhydrous CH2CI2 leads to species with a UV-Vis spectrum qui~ close to that described by Chang et al. [ 19] for [Co(II)ff] + on which the addition of a minute quantity of PhCN gives rise to a cobalt(Ill) typical spectrum attesting a reaction such as in Eq. (10).

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Y./2 Mest et al. / lnorganica Chimica Acta 248 (1996) 181-191

spectrophotometry, EPR and electrochemical observations leads us to conclude that the ~'-ring electrons are those involved during the first oxidation process of these cobalt diporphyrins even in PhCN, and theoretical considerations rationalize this observation. 3.4. M o l e c u l a r orbital considerations

From the combination of these results with extended Htickel MO calculations [33], it is possible to propose a general qualitative MO diagram which can provide a theoretical basis for the electrochemical and spectroscopic behavior of the dicobalt diporphyrin systems. This diagram, shown in Fig. 7, can be generated from the interaction of two cobalt monoporphyrinic units. In such a mononuclear square-planar unit there are four low-lying d-type orbitals; among them the highest is likely to be of d~2character [ 34]. Our calculations on the C o ( N 4 C 2 o H t 2 ) model found that the highest occupied level of porphyrinic character is a ~r-type orbital of a~, symmetry. When two mononuclear units are brought together, each monomer level generates a bonding and an anfibonding combination. At a Co-Co separation of ~ 3.4 .A, and in the staggered conformation experimentally observed for the

CoeFTF4 dicobalt diporphyrin [ 35 ], the interaction is rather weak and only the bonding and antibonding combinations of the metallic dz2 and of the ligand al, orbitals are expected to exhibit some significant splitting. This is supported by our extended Hiickel MO calculations using a standard parametrization [36] on the idealized [Co(N4CeoHI2)] 2 model, which indicates a splitting of 0.60 and 0.34 eV for the d~2and at, combinations, respectively. These approximate calculations do not reproduce properly the correct relative ordering of the ligand and metal levels in this type of transition-metal porphyrin. In particular, the Co 3d levels are found much too deep in energy when the standard Hii [ 36] are used. However from our experimental data and most of the data reported on the electrochemistry of cobalt porphyrins [ 11], in a moderately coordinating medium, the difference in energy between the cobalt and porphyrin highest occupied levels can be reasonably estimated to be ~ 0.3 eV. The highest level is the antibonding dz~ combination, so that it remains unoccupied in the [Co(II)P Co(II)P] species. The fact that these neutral systems are EPR silent agrees with a singlet ground-state secured by a significant splitting between the two dz2 combinations. This apparent diamagnetism was also observed by dz 2 Imdbondlng

/ e"

/

(m-z) nlibomling dz2bomding (x-.) bonl.ng

~

ICd~l

U, roO~

IC0
[C~l~...~lOp +

IC~,,.--~

+ ~

l,C,m B ~

(a) dimers in their different redox stnta

monomer

.° dz2 entibondJng

,'"

/ /s / ......

/ dz2 bonding

CO)

--~f

ICo(ll)P FC,o(II)]

~

W_.,o01)I.,I

L Fig. 7. General qualitative molecular orbital diagram for the cobalt face-to-face diporphyrins resulting from the extended Htickel calculations showing: (a) the effects of the cobalt-cobalt and ring ¢r-~r interactions on the electronic configurations of the dimers in their different redox states; (b) the effect of the presence of strong axial ligand (L) on the electronic configuration described in (a).

Y. Le Mest et al. / lnorganica Chimica Acta 248 (1996) 181-191

Ibers and co-workers in the case of cobalt porphyrins stacked molecular conductors [ 26]. It should be noted that from the present analysis this situation indicates that some Co-Co bonding interaction is present in the neutral species. The first oxidation process should correspond to the depopulation of the ligand antibonding combination, leading to the creation of a 7r-~r bond between the two ligands. The formation of this bond has been fully emphasized in the case of derivatives of non-electroactive metals such as Zn or Cu [9], and the present calculations corroborate the experimental results. In the case of cobalt, at this oxidation state, the bonding interaction between the two monomeric subunits is maximum. The second oxidation process corresponds to the depopulation of the bonding dz2 combination, thus to the loss of the Co-Co bonding interaction. Finally, the third oxidation process is expected to empty the ligand bonding combination, thus cancelling the ~--7r bond. Of course the 'rigid level' diagram of Fig. 7 does not take into account the electronic relaxation which could occur during the oxidation processes. We believe, however, that the electron configurations given in this diagram are correct, even if some crossings between occupied levels could perhaps happen. An uncertainty remains, however, concerning the nature of the lowest metallic level of Fig. 7. It is possible that the bonding dz~ combination is too low in energy, and that the d-type level involved in the second oxidation process is rather of d= or d~ character, therefore essentially non-bonding between the two metals. In such a case, some Co-Co bonding character should remain in the two highest oxidation states of Scheme 1. Therefore it can be concluded that the theoretical aspects developed here are in total agreement and corroborate the original redox pathway proposed in Scheme 1.

3.5. Effect of axial ligands The redox pathway proposed in Scheme 1 allows one to interpret the effect of the presence of axial ligands on the spectra and electrochemistry of the previously described species [8], as being identical to the intramolecular electron transfer from cobalt(II) to the ring described by Chang et al. (Eq. (10)). Although H20 is not a stronger ligand than PhCN, it has been shown that it can interact inside the cavity as it competes with 02 binding [ 10], allowing the cobalt to adopt a hexacoordinated configuration by complexation of H20 inside and, likely, outside the cavity.

189

series, its electron donating strength seems to be sufficient to induce the electron transfer from cobalt to the ring with only one ligand coordinated outside the cavity. This may be concluded from the evolution of the UV-Vis spectra of the oneelectron and two-electron oxidized derivatives of the cobalt dimers when N-MeIm is added to a pure solution of these redox derivatives [ 8]. The ligand fixation reactions may be proposed to be written as: L

[Co(II)P-PCo(II) ] + ~ [LPCo(III) Co(II)PL] +

(13)

L

[Co(II)P- - -PCo(II) ] z+ ~ [LPCo(III)Co(III)PL] 2+ (14) with L ---strong donor ligand, e.g. N-Melm. The addition of a fifth ligand to a square planar complex is known to have a strong destabilizing effect on its dz2 level [ 30,34 ]. Consequently, the coordination of two additional ligands on the external axial positions of the diporphyrin complexes is expected to destabilize both dz2 combinations, the bonding one being probably the most destabilized. Then, they pass above the ligand levels, transferring their electrons to the ligand 1r-~ antibonding level, as shown in Fig. 7(b). This proposition is strongly corroborated by the fact that, by addition of N-Melm, the EPR spectrum of the one-electron oxidized species, from a ring-like spectrum of a delocalized coupled systems (vide supra), becomes an absolutely typical axial spectrum of a cobalt (II) porphyrin with a nitrogenous base on the fifth position ( see Fig. 4) [ 30]. This also accounts for the interconversion between the 7r-ring and cobalt redox processes when a base such as N-Melm is added in solution, the Co(III)/Co(II) process becoming the more negative one and electrochemically irreversible with a reduction peak close to that of the C o ( I I ) / C o ( I ) process [ 8]. Contrarily to what is observed in the presence of small ligands like H20 which can enter the cavity, or a strongly donating ligand like N-Melm, the formation of the ~r-cationic form of cobalt(II) in the case of the diporphyrins in PhCN (Eq. (7)) suggests that, while this molecule cannot interact inside the cavity, its coordinating strength is too weak to counterbalance the electronic effect, and to stabilize a cobalt(III) electronic configuration (Eq. (10) ).

4. Concluding comments

H20

[Co(II) P-PCo(II) ] + [ (H20) PCo(III) (HzO),Co(II) P] ÷

( 11 )

H20

[Co(II) P- - -PCo(II) ]2+ [ (H20) PCo(III) (H20)nCo(III) P(H20) ]2+

(12)

n=l or2 Even though the nitrogenous base, N-Melm, is not likely to interact inside the cavity, at least in the case of the FTF

The present findings on the electrochemical behavior of the cobalt face-to-face porphyrins and on the control exerted by the axial ligation on the localization of the redox site therefore implies a completion of the description of the socalled 'cofacial effect' proposed in our previous paper [9]. In that report was evidenced a 7r-Tr electronic interactive component between the two 7r systems of the porphyrin rings. When an electroactive metal is present, another effect that must be taken into account is the metal-metal electronic component due to the interactions between the two metals

190

Y./2 Mest et al. / lnorganica Chimica Acta 248 (1996) 181-191

and especially between the two dz2 orbitals. A steric component must also be taken into account: the protecting role against bulky axial ligands that the size of the interporphyrin cavity exerts. The theoretical analysis developed here affords a rationalization of the so-called 'cofacial' effect of intramolecular interactions inside these dimers. Below a certain distance between the two moieties constituting the dimers, coarsely estimated to = 4-3.5 ~, the combination of 7r-Tr (ring-ring) and d--d (metal-metal) electronic interactions leads to a totally new entity with unique properties as compared to those of the monomers. In the case of the cobalt derivatives this property explains why the 7r-rings become easier to oxidize than the metal itself, even in a moderately coordinating medium like PhCN, with stabilization of the 7r-cation radical form of cobalt(II). Only the presence of small ligands like H20 or 02 which can enter the cavity [ 10a], or bulky but very strongly donating ligands like N-Melm, can counterbalance the electronic effects. As far as the reactivity of these dimers towards dioxygen is concerned, particularly noteworthy is the fact that the present results afford a rationale for the unexpected and remarkable reactivity of 02 for the form [Co(II)P- - -PCo(II)]2+ [25], as it is now clear that this species bears two cobalt(ii) and not cobalt(iiI) as earlier postulated [8-10]. Yet this explanation leads to another fundamental aspect concerning the nature and structure of the 02 complexes formed from the [Co(II)P.PCo(II)] + and [Co(II)P---PCo(II)] 2+ derivatives, which is the subject of a forthcoming paper. More generally in the chemistry of porphyrins this 'cofacial' effect appears therefore as a new one among the various possibilities for the control and modulation of the electronic properties and reactivity of the metalloporphyrins, such as the choice of the metal, of the substituents on the 7r-ring (equatorial effect), of the axial ligands (axial effect), and of the supramolecular or proteinic environment of the active center (microenvironment effect). This effect could be of interest in fields such as multielectron activation and catalysis but also in molecular conduction [26,27].

Acknowledgements This work was supported by the C.N.R.S. (Unit6 Associ6e 322 and Unit6 AssociEe 1495). Professor J.P. Collman of Stanford University was the initiator of this project; we are gratefully indebted to him for the gift of the diporphyrins synthesized in his laboratory, as well as for his encouragements and pertinent comments.

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