J. Am. Chem. SOC.1991, 113, 501-512
501
Structures and Photoactivation of the Charge-Transfer Complexes of Bis( arene)iron(11) Dications with Ferrocene and Arene Donors R. E. Lehmann and J. K. Kochi* Contribution from the Department of Chemistry, University of Houston, University Park, Houston, Texas 77204-5641. Received May 31, 1990. Revised Manuscript Received August 24, I990
Abstract: Ferrocene forms a series of unusual charge-transfer crystals with the isoelectronic bis(arene)iron( 11) dication, in which X-ray crystallography establishes the alternate (heterosoric) stacking of sandwich structures comprising the donor-acceptor 630 nm. pairs. In acetonitrile, the 1:l complex [Cp2Fe, ArzFe2+] shows a broad absorption band centered at A,, Bis(arene)iron(11) acceptors also form highly colored crystals with various arene donors, in which the charge-transfer absorption (hiCT)derives from the same heterosoric stacking of donor-acceptor pairs-despite the structurally divergent nature of the arene (planar) and ferrocene (sandwich) donors. These absorptions undergo a predictable red-shift with increasing acceptor strength in the order (BZ)2FeZ+> (MES)2Fez+> (DUR)zFe2+> (HMB)zFe2+,as judged by the reduction potentials Eoled of the benzene, mesitylene, durene, and hexamethylbenzene derivatives, respectively. Common to both classes of 1: 1 bis(arene)iron(II) complexes [Ar2Fe2+,D], where D is either a ferrocene or arene donor, is the efficient photoinduced substitution of the arene ligands that occurs i n solution when the charge-transfer bands are selectively irradiated with monochromatic light. Time-resolved picosecond spectroscopy identifies such a charge-transfer deligation to occur via the direct photoexcitation of the complex to the ion radical pair, Le., [Ar2Fe2+,D] b [ArzFe+,D'+], followed by the spontaneous loss of arene ligands by the acceptor moiety. Indeed, the substitution lability of the 19-electron intermediateAr2Fe+can be independently demonstrated by the application of transient electrochemical methods to the reduction of various bis(arene)iron(II) dications. N
Introduction Ferrocene is a viable electron donor by virtue of its vertical ionization potential of only IP = 6.858 eV in the gas phase and its oxidation potential of E O , , = 0.41 V vs SCE in solution.lq2 Furthermore, the minimal structural change resulting from the oxidative conversion of ferrocene to ferrocenium ion3 is symptomatic of the minor reorganization energy requisite for facile electron transfera4s5 The electron-rich character of ferrocene is chemically manifested in various reactions. Thus, the rapid substitution of the ferrocene ring by electrophiles is akin to electrophilic aromatic substitution of the most electron-rich arenes, and it typically includes alkylation, acylation, metalation, and s ~ l f o n a t i o n . ~However, ~~ various attempts to effect the corresponding halogenation and nitration of ferrocene with the usual electrophilic reagents such as bromine, nitric acid, etc. merely result in its oxidation to ferrocenium ion.*v9 Similar to other electron-rich donors, ferrocene also forms neutral charge-transfer (CT) complexes with electron-poor acceptors such as tetracyanoethylene: quinones,I0 polynitroarenes,' I and various alkyl halide^,'^-'^ including carbon tetrachloride.ls Indeed we believe ( I ) Rabalais, J. W.; Werme, L. 0.;Bergmark, T.; Karlsson, L.; Hussain, M.; Siegbahn, K. J. J . Chem. Phys. 1972, 57, 1185. (2) In acetonitrile; see: Stolzenberg, A. M.; Stershic, M. T. J . Am. Chem. Soc. 1988, 110, 6391.
(3) Haaland, A. Ace. Chem. Res. 1979, 12, 415. (4) Yang, E. S.;Chan, M. S.;Wahl, A. C. J . Phys. Chem. 1980,84,3094. ( 5 ) Sharp, M.; Petersson, M.; Edstrom, K. J . Elecrroanal. Chem. 1980, 109, 27 I . (6) (a) Rosenblum, M. E. Chemistry of the Iron Group Metallocenes; Wiley: New York, 1965; Part I. (b) Nesmeyanov, A. N.; Perelova, E. G. Ann. N.Y. Acad. Sci. 1965, 125, 67. (7) Watts, W. E. In Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A,, Abel, E. W., Eds.; Pergamon: New York, 1982; Vol. 8, pp 1013 ff. (8) For other examples of electron transfer, see: Watts, W. E. in ref 7. (9) (a) Rosenblum, M.; Fish, R. W.; Bennett, C. J . Am. Chem. SOC.1964, 86, 5166. (b) Collins, R. L.; Pettit, R. J . Inorg. Nucl. Chem. 1967, 29, 503. (IO) Brandon, R. L.; Osiecki, J. H.; Ottenberg, A. J . Org. Chem. 1966, 3 / , 1214. ( I I ) (a) Hetnarski, B. Bull. Acud. Pol. Sci., Ser. Sci., Chim. 1965, 13, 515, 523; Chem. Abstr. 1966.64, 9093. (b) Hetnarski, B. Ya. Dokl. Akad. Nauk. SSSR 1964, 156, 604. (12) Velichko, F. K.; Vasil'eva, T. T.; Kochetkova, V. A,; Vinogradova, L. V.; Balabanova, L. V.; Kartseva, 0. S.;Shvekhgeimer, G. A. Izv. Akad. Nauk SSR, Ser. Khim. 1967, 659.
0002-7863/91/1513-501%02.50/0
that such weakly bound donor-acceptor pairs are the common intermediates to both substitution and electron transfer of ferrocene-with the electrophile (oxidant) acting as the electron acceptor (A) in the labile precursor complex, i.e.I6 Cp2Fe + A
* [Cp2Fe, A], etc.
Particularly diagnostic of charge-transfer complexes is the immediate appearance of new spectral bands (hiCT)when electron donors are mixed with electron accept~rs.l'*~~ Time-resolved spectroscopic studies have established the charge-transfer bands to derive from the vertical excitation of the donor-acceptor complex to the ion-radical pair, e.g.I9 [Cp2Fe, A]
hvcr
[Cp2Fe+,A'-]
The mechanistic delineation of such ion-radical pairs is especially relevant to the photoactivation of ferrocenez0 and to the design of organometallic solid-state devices.21 Thus, the efficient photochemical conversion of ferrocene to ferrocenium chloride in carbon tetrachloride with a quantum yield of near unity is an example22-zsin which the spontaneous fragmentation within the (13) Zver'kov, V. A.; Petrova, A. A.; Vannikov, A. V. Izu. Akad. Nauk
SSR,Ser. Khim. 1988, 1399.
(14) Velichko, F. K.; Balabanova, L.V.; Vasil'eva, T. T.; Bondarenko, 0. P.; Shvekhgeimer, G. A. Izv. Akad. Nauk SSR,Ser. Khim. 1988, 71 I . (15) Brand, J. C. D.; Snedden, W. Trans. Faraday Soc. 1957, 53, 894. ( I 6) Kochi, J. K. Organometallic Mechanisms and Catalysis; Academic: New York, 1978; Part 3. (1 7) See: Foster, R. Organic Charge-Transfer Complexes; Academic: New York, 1969. ( I 8) Andrews, L. J.; Keefer, R. M. Molecular Complexes in Organic Chemistry; Holden-Day: San Franciso, CA, 1964. (19) (a) Hilinski, E. F.; Masnovi, J. M.; Kochi, J. K.; Rentzepis, P. M.J . Am. Chem. Soc. 1984, 106, 8071. (b) Masnovi, J. M.; Hilinski, E. F.; Rentzepis, P. M.; Kochi, J. K. J . Am. Chem. Soc. 1986, 108, 1126. (20) See: (a) Balzani, V.; Carassiti, V. Photochemistry of Coordination Compounds; Academic: New York, 1970. (b) Geoffroy, G. L.; Wrighton, M. S. Organometallic Photochemistry; Academic: New York, 1979. (21) Ando, Y.; Nishihara, H.; Aramaki, K. J . Chem. Soc., Chem. Commun. 1989, 1430. (22) (a) Koerner von Gustorf, E. A.; Koller, H.; Jun, M. J.; Schenck, G. 0.Chem. Eng. Technol. 1963, 35, 591. (b) Koerner von Gustorf, E. A,; Grevels, F. W. Fortschr. Chem. Forsch. 1969, 13, 366.
0 1991 American Chemical Societv
Lehmann and Kochi
502 J . Am. Chem. SOC.,Vol. 113, No. 2, 1991
\
Wavelength,
Figure 2. Molecular diagram of the donor-acceptor pair obtained as the 1:1 charge-transfer complex of ferrocene and (DUR)2Fe2+(PFc)2.
nm
Figure 1. Charge-transfer absorption band (---) obtained as a difference spectrum from the visible absorption (-) attendant upon the mixing of 0.09 M ferrocene and 0.03 M (HMB)2Fe2+(PFc)2(---) in acetonitrile. (-a)
-
first-formed ion pair, Le., [Cp2Fe'+, CCI{-] [Cp2FeC1,CCI3'1, etc., effectively competes with its deactivation by back electron transfer.26 The ion-radical pair pertinent to the configuration mixing of the charge-transfer excited state with the ground state of ferrocene-TCNE complexes has led to bulk ferromagnetic coupling in molecular solids.27 It is in both of the latter contexts that the preliminary report2*of the charge-transfer complexes of ferrocene with the isoelectronic bis(arene)iron(II) dication elicits our further interest-particularly arising from the similarity of the (sandwich) structures of these acceptors to that of the ferrocene donor. Accordingly, in this study, we focus first on the isolation and structural characterization of the bis(arene)iron-ferrocene complex by X-ray crystallography and then examine the photochemistry attendant upon the actinic excitation of the chargetransfer band in solution. The CT behavior of ferrocene in comparison with that of the prototypical aromatic donors29+30 is also an important facet of this study.
Results 1. Charge-Transfer Complexes of Bis(arene)iron(11) and Ferrocene. Spectral Characterization and X-ray Crystallography. When dilute solutions of ferrocene and bis(hexamethy1benzene)iron( 11) hexafluorophosphate (HMB)2Fe2+(PF6-)2in acetonitrile were mixed, the pale yellow-orange mixture instantly took on a dark brown coloration. The spectral change accompanying this dramatic visual transformation is shown in Figure 1 by the obscuration of the visible absorption spectra of ferrocene (dotted curve) and (HMB)2Fe2+(dashed curve) by an enveloping new absorption at lower energy. Spectra (digital) subtraction revealed the latter as a very broad absorption band with A, = (23) (a) Traverso, 0.; Scandola, F. Inorg. Chim. Acta 1970, 4,493. (b) Traverso, 0.; Scandola, F.; Carassiti, V. Inorg. Chim. Acra 1972, 6, 471. (24) (a) Akiyama, T.; Hoshi, Y.;Goto, S.;Sugimori, A. Bull. Chem. SOC. Jpn. 1973, 46, 1851. (b) Akiyama, T.; Sugimori, A,; Hermann, H. Bull.
Chem. SOC.Jpn. 1973, 46, 1855. (25) Compare also: (a) Bock, C. R.; Wrighton, M. S.Inorg. Chem. 1977, 16, 1309. (b) Fields, R.; Godwin, G. L.; Haszeldine, R. N. J . Chem. Soc., Dalton Trans. 1975, 1867. (26) Fox, M. A. Ado. Photochem. 1986, 13, 237. (27) See,e.g.: Miller, J. S.; Eptein, A. J.; Reiff, W. M. Science 1988, 240, 40. Miller, J. S.;OHare, D. M.; Chakraborty, A,; Epstein, A. J. J . Am. Chem. SOC.1989, I II , 7853. (28) Braitsch, D. M. J . Chem. Soc., Chem. Commun. 1974, 460. (29) Howell, J. 0.; Goncalves, J. M.;Amatore, C.; Klasinc, L.; Wightman, R. M.; Kochi, J. K. J . Am. Chem. SOC.1984, 106, 3968. (30) See, for example: (a) Wallis, J. M.; Kochi, J. K. J . Am. Chem. SOC. 1988, 110, 8207. (b) Takahashi, Y.; Sankararaman, S.; Kochi, J. K. J. Am. Chem. SOC.1989, III,2954.
Figure 3. Unit cell of [Cp2Fe, (DUR),Fe2+(PF,-),] showing the vertical heterosoric stacks of the ferrocene donor and bis(arene)iron(II) acceptor.
626 nm. A similar treatment of ferrocene with the analogous durene dication (DUR)2Fe2+yielded a corresponding difference spectrum with A, = 647 The bathochromic shift of 510 cm-' paralleled the relative ease of reduction of the bis(arene)iron(1I) dications as given by their reversible redox potentials (vide infra). The spectral assignment of these broad new absorption bands to intermolecular charge-transfer transitions from the ferrocene donor to the bis(arene)iron(II) acceptors was further confirmed with a variety of aromatic donors, as described in the following section. The charge-transfer complex spontaneously separated from acetonitrile solution as dark brown, almost black crystals when ferrocene and (DUR)2Fe2+were employed at higher (0.1 M) concentrations, and single crystals suitable for X-ray crystallography were obtained by the slow diffusive mixing of these solutions, (31) The instability of bis(benzene)iron(ll) in acetonitrile ( 7 , < 5 h) necessitated the use of the persistent dications (DUR)*FeZ+and (HhB)zFezt for most of our studies, with an occasional use of (MES)zFezt ( T , 21 h).)* (32) Compare: Abdul-Rahman, S.; Houlton, A,; Roberts, M.;Silver, J. J. Organomet. Chem. 1989, 359, 331.
d.
-
Charge- Transfer Complexes of Bis(arene)iron(II)Dications
J. Am. Chem. SOC.,Vol. 113, No. 2, 1991 503
B
A
The structure of the charge-transfer complex was solved in the tetragonal s ace group P4*/nmc with a = 11.200 ( 3 ) A and c = 13.283 ( 5 ) (see full details in the Experimental Section), as a discrete 1 :1 mixture of individual ferrocene and bis(durene)iron(I1) hexafluorophosphate units, i.e.
w
* &
(PF,),
+
Fe
e
e
[ea-* Fe
3
Fa
](PFiI2
(3)
The ferrocene component maintained its structural integrity within the crystalline complex, and it was observed with an unexceptional disorder of the Cp rings about the rotational axis.33 By way of contrast, in the (DUR)2Fe2+acceptor, the coplanar durene rings were rigidly locked in the mutually orthogonal orientation shown in Figure 2. Otherwise the iron-ring (centroid) distance of 1.58 A in the dication was only slightly less than that (1.66 A) extant in ferrocene.34 Importantly, the diagram of the unit cell in Figure 3 shows the stacked alignment of alternating donor-acceptor interactions of Cp2Feand (DUR)2Fe2+units that are separated by the interannular Cp-DUR distance of 3.43 A. The overall Fe-Fe separation of 6.64 A within the stack compares with the closest nonbonded Fe-Fe distance of 7.92 A in the horizontal direction between stacks. 11. Charge-TransferComplexes of Bis(arene)iron(II)Acceptors and Aromatic Donors. In order to generalize the spectral complexation of the bis(arene)iron(II) acceptor in Figure 1, it was exposed to various arenes whose donor characteristics in charge-transfer complexes were already e ~ t a b l i s h e d .As ~ ~a test M solution of procedure, aliquots of a standard 2 X (DUR),Fe2+(PF
e
t
a l
t
GB e 3
e
I-
zv
t
40
I . P. ,
6
eV
Figure 10. Mulliken correlation of the charge-transfer transition energy (hvCT) with the ionization potential (IP) of the ferrocene and arene donors in (DUR)2Fe2tcomplexes. The line is arbitrarily drawn with a slope of unity.
upon the dication reduction. The rate of deligation was related to the lifetime of the m o n ~ c a t i o n as , ~ qualitatively ~ evaluated in Table 1V by the minimum scan rate (umin) sufficient to achieve electrochemical reversibility of the first cyclic voltammetric wave PI. Discussion Bis(arene)iron(lI) dications (Ar),FeZ+ can effectively serve as electron acceptors in the facile formation of various types of weak charge-transfer complexes, as spectrally detected in solution by the spontaneous appearance of new visible absorption bands. The linear correlation shown in Figure 10, arbitrarily drawn with a slope of unity, confirms the charge-transfer absorption (huCT)to apply equally to such structurally diverse donors as ferrocene and the various arenes (Table I)” according to the Mulliken formulation,” i.e. hVCT
= IP - EA - w
(9)
when the electron affinity (EA) of the bis(durene)iron(lI) acceptor and the work term ( w ) are constant.58 Most importantly, the successful isolation of single crystals of the 1:1 molecular complexes with ferrocene as well as electron-rich arenes (see the Experimental Section) allows X-ray crystallography to identify the relevant charge-transfer interactions of the bis(arene)iron(II) acceptor with
-
t
(56)The increasing lifetimes of the 19-electron radical ion bis(arene)iron(1) in the order (MES)2Fe+ (DUR)2Fe+ 3o(I) 501, total variables 58; R = x:IFol - IF,II/CIFol 0.073; R, = [Cw(lFol - IFc1)2/xwlFo12]i/20.064; weights w = U(F)-~.The Laue symmetry was determined to be 4/mmm, and (from the systematic absences noted) the space group was shown unambiguously to be P42/nmc. Intensities were measured by the B scan technique, with the scan rate depending on the count obtained in rapid prescans of each reflection. Two standard reflections were monitored after every 2 h or every 100 data collected, and these showed no significant variation. During data reduction, Lorentz and polarization corrections were applied; however, no correction for absorption was made due to the small absorption coefficient. Since there were only two formula units in the unit cell, the structure was solved by placing the Fe atoms on 4m2 sites 2a and 2b. This site symmetry presupposed massive disorder of the ferrocene molecule, and the Cp ring was treated as an ideal rigid body, with each carbon and hydrogen atom having 25% occupancy. The remaining non-hydrogen atoms were located in subsequent difference Fourier syntheses. The methyl groups were treated as ideal rigid bodies and allowed to rotate freely. The anion was also found to be massively
-
-
Charge- Transfer Complexes of Bis(areneJiron(1I)Dications Table VI. Atomic Coordinates ( X IO4) and Equivalent Isotropic Disulacement Parameters (A2 X IO3) of ICu,Fe. DUR,Fe2+(PFr-),l X Y z U(eq)" Fe(l) 7500 2500 2500 7500 2500 7500 2554 (43) 2526 (43) 1109 (27) I637 3221 446 3472 1771 1832 2713 1741 2049 2340 3368 168 3720 3216 1347 1333 1894 870 2442 (36) 1217 (21) 2506 (39) 1654 2918 2122 3231 2095 313 1474 I570 919 3412 3443 1515 3452 I826 529 1905 1561 3059 2492 (33) 2504 (55) 1109 (24) 3395 3476 749 1589 1533 1468 3046 2550 21 86 31 I939 2459 808 3397 I507 1409 1588 3502 1300 (6) 6853 (7) 3646 (7) 2500 6239 (12) 1320 (9) 4757 (7) 1294 (6) 6158 (7) 3695 (24) 6327 (7) 7193 (16) 2730 6245 6377 1654 6202 7047 1955 6257 8277 C(8 j 3217 6334 8367 a Equivalent isotropic CJ defined as one-third of the trace of the orthogonalized U,,tensor. disordered about a 2-mm site. Eventually, by use of ideal rigid body models, three different orientations of PF6- were identified (before site symmetry considerations). Each of these was refined independently with occupancy factors of 8.33% ( I /l2). Fixed isotropic temperature factors were used for the fluorine atoms. The centers of these anion models were not constrained and were refined to three slightly different locations. After all shift/esd ratios were less than 0.5, convergence was reached at the agreement factors listed above. No unusually high correlations were noted between any of the variables in the last cycle of full-matrix least-squares refinement; and the final difference density map showed a maximum peak of about 0.75 e A-3, located quite close to PI. All c a h l a t i o n s were made using Nicolet's SHELXTL PLUS (1987) series of crystallographic programs to yield the final atomic coordinates listed in Table VI. A reddish-orange prismatic block of the (HMB)2FeZt complex with durene having approximate dimensions 0.50 X 0.45 X 0.35 mm was mounted in a random orientation on a Nicolet R3m/V automatic diffractometer. The sample was immediately coated with a thin layer of epoxy in order to retard the loss of solvent since the crystals were known to decompose within a few minutes after removal from the mother liquor. The crystallographic data were as follows: space group PT (triclinic); cell constants a = 9.270 (4) A, b = 11.040 (S), c = 12.130 (S), a = 71.72 (3)", /3 = 67.30 (3), y = 77.41 (3)", V = 1081 A3;molecular formula (C24H36Fe2') (2PF6-)(c,,H,4)(2C,H,o); formula weight 920.8 1; formula units per cell Z = 1; density p = 1.41 g ~ m - absorption ~; coefficient p = 5.03 cm-'; radiation (Mo Ka) X = 0.71073 A; collection range 4O I 20 I 50"; scan width PO = 1.20 t (Ka2- Ka,)O; scan speed range 2.0-1 5.0' min-'; total data collected 3804; independent data, I > 3a(I) 3241; total variables 313; R = xlFoI - ~ F o ~ ~0.066; / ~ ~R,F=o[xw(lFoI ~ - IFc1)2/CwIFo12]1/2 0.058; weights w = U ( F ) - ~The . Laue symmetry was determined to be 1, and the space group was shown to be either PI or Pi. Intensities were measured by the 0 scan technique, with the scan rate depending on the count obtained in rapid prescans of each reflection. Two standard reflections were monitored after every 2 h or every 100 data collected, and these showed a 15% decay over the course of the experiment. A normalization factor ns a function of X-ray exposure time was applied to account for this changc. Since each of the nonsolvent molecules in the compound was capable of inversion symmetry, space group PT was assumed from the outset. The structure was solved by placing the Fc atom on an inversion center. Remaining non-hydrogen atoms werc locatcd in subsequent difference Fourier syntheses. The asymmetric unit was found to consist of half dication and half durene
J . Am. Chem. SOC.,Vol. 113, No. 2, 1991 5 1 1 Table VII. Atomic Coordinates (X104) and Equivalent Isotropic Displacement Parameters (AZ X 10') of [Durene, (HMB)2FeZ+(PF6-)2]
Y 5000 2199 (6) 3579 819 2639 I759 2579 1820 2139 (7) 3452 827 1786 2493 2768
z CJ(eqY 5000 26 (1) 1044 (5) 55 (1) 342 120 (12) I746 122 (12) 2263 114 (9) -177 180 (18) 730 183 (15) 1357 141 (11) 1017 (6) 55 (1) 627 115 (11 144 (14 1406 2253 292 (18 172 (14 -219 1649 161 (17 1511 168 (19 385 2060 (IO) 1014 (12) 55 (1) 957 130 (6) 1949 3164 81 130 (6) 1 I92 1116 130 (6) 2930 914 130 (6) 2580 2109 130 (6) 1542 -7 9 130 (6) 3401 (5) 7423 (4) 69 (2) 3072 (5) 5712 (5) 35 (2) 3019 (5) 5715 (5) 39 (2) 4611 (5) 3599 (5) 36 (2) 4189 (5) 3523 (5) 36 (2) 4222 (5) 3528 (5) 38 (2) 3671 (5) 4619 (5) 35 (2) 2441 (6) 6853 (5) 56 (3) 2309 (6) 6855 (5) 58 (3) 3524 (6) 4584 (6) 56 (3) 4737 (6) 2345 (5) 56 (3) 2328 (5) 4807 (6) 60 (3) 3691 (6) 4595 (6) 53 (3) 5933 (5) -565 (5) 51 (3) -578 (5) 6181 (5) 50 (3) 5213 (6) -2 (5) 52 (3) -1207 (7) 7478 (6) 80 (4) 5423 (7) 55 (7) 75 (4) 2859 (6) 8516 (6) 54 (3) 1460 (7) 110 (6) 8953 (7) 9473 (6) 88 (4) "Equivalent isotropic U defined as one-third of the trace of the orthogonalized Ut, tensor. X
Fe
5000 2429 (7) 1942 2919 1834 3026 4096 764 2351 (9) 1400 3300 952 3748 3064 1636 2592 (12) 1597 3585 4130 1053 2469 2714 9146 (5) 4313 (6) 5859 (6) 7058 (6) 6700 (6) 5145 (6) 3950 (6) 3062 (7) 6274 (8) 8701 (6) 7999 (7) 4780 (8) 2342 (6) 5581 (8) 4015 (7) 3386 (7) 2950 (9) 1693 (7) 8702 (7) 8840 (11)
(located on separate inversion centers), with one anion and one molecule of acetone solvent (located in general positions). After the usual sequence of isotropic and anisotropic refinement, all hydrogens were entered into ideal calculated positions and constrained to riding motion, with a single variable isotropic temperature factor for all of them. The methyl groups were treated as ideal rigid bodies and allowed to rotate freely. The anion was found to be massively disordered, and by employing ideal rigid body models, three different orientations of PFC were identified. On the basis of analysis of the isotropic thermal motion, the separate models were refined independently with occupancy factors of 42%, 35%, and 23% for the P, P', and P" models, respectively, Eventually the fluorine atoms in the more populous P and P' models were converted to anisotropic thermal motion, but fixed isotropic temperature factors were used for the fluorine atoms in P". The centers of these anion models were not constrained, and refined to three slightly different locations. After all shift/esd ratios were less than 0.3 (except for those involving the rigid-body hydrogens), convergence was reached at the agreement factors listed above. The final difference density map showed a maximum peak of about 0.6 e A-3, Calculations were made with the aid of the Nicolet programs above, and the final atomic coordinates are listed in Table VII. Time-Resolved Spectroscopy of Bis(arene)iron(II) Complexes with Ferrocene and Arene Donors. Solutions of the charge-transfer complexes were made up in 4 mL of acetonitrile to yield an optimum absorbance at 532 nm of unity as follows: (HMB)2Fe2+(PF6-)2(9.0 mg) and 9methylanthracene (1 57 mg), (DUR)2Fe2t (8.0 mg) and 9-methylanthracene ( 1 5 1 mg), and 56 mg Cp2Fe (56 mg) and (HMB)2Fe2t( P F 0 2 (60 mg). The time-resolved picosecond spectrum were obtained by flash photolysis using the frequency-doubled (KDP crystal) output at 532 nm of a Nd?YAG Q-switched laser (Quantel 501C-IO) with 18-ps
512
J . Am. Chem. SOC.1991, 113, 512-517 Acknowledgment. We thank T. M . Bockman for providing critical ideas and suggestions, J. D. Korp for crystallographic assistance, Z. Karpinski for the electrochemical measurements, S. Sankararaman for the time-resolved spectra, and the National Science Foundation, Robert A. Welch Foundation, and the Texas Advanced Research Program for financial support.
(fwhm) pulses. The white light (1:l DzO/H20)probe beam was detected by a dual diode array (Princeton Instruments DDA512) consisting of 512 elements as described elsewhere.s1 The transient spectra in Figures 7 and 8 were obtained by averaging 300-500 shots, and the system was
calibrated relative to the transient spectrum of the anthracene cation radical.82
Supplementary Material Available: Tables of the observed and calculated structure factors for [Cp2Fe, (DUR)2Fe2+(,PF6-)2]and [durene, (HMB)2Fe2'(PF6-)2] (1 7 pages). Ordering information is given on any current masthead page.
(81) Sankararaman, S.; Kochi, J. K. J . Chem. SOC.,Perkin Trans. 2, in
press.
(82) Compare: Mataga, N.; Shioyama, H.; Kanda, Y . J . Phys. Chem. 1987, 91, 314. See also: References 19 and 30.
Factors Determining the Site of Electroreduction in Nickel Metalloporphyrins. Spectral Characterization of Ni( I) Porphyrins, Ni( 11) Porphyrin T-Anion Radicals, and Ni( 11) Porphyrin T-Anion Radicals with Some Ni( I) Character K. M. Kadish,* M. M. Franzen, B. C. Han, C. Araullo-McAdams, and D. Sazou Contribution from the Department of Chemistry, University of Houson, Houston, Texas 77204-5641. Received July 2, 1990
Abstract: The effect of temperature, porphyrin macrocycle, and solvent on the electroreduction of (P)Ni" where P is the dianion of meso-tetrakisb-(diethy1amino)phenyl)porphyrin (T(p-Et,N)PP) or meso-tetrakis(o,o,m,m-tetrafluoro-p(dimethy1amin0)phenyl)porphyrin (T(p-Me2N)F4PP)is reported. Both compounds were reduced by controlled potential electrolysis and characterized by ESR and UV-visible spectroscopy. The site of electron transfer varied as a function of experimental conditions and the stable electrogenerated products were assigned as Ni(1) porphyrins, Ni(1I) porphyrin n-anion radicals, or Ni(l1) porphyrin n-anion radicals with some Ni(1) character. Ni(1) ESR spectra were obtained for [(T(p-Et2N)PP)Ni]and [(T(p-MezN)F4PP)Ni]-at low temperature in THF under a CO atmosphere as well as in DMF or pyridine under N,. In contrast, ESR spectra of Ni(I1) porphyrin r-anion radicals or Ni(I1) porphyrin ?r-anion radicals with some Ni(1) character were obtained in THF under N, at room and low temperature, respectively. [ (Tb-Et,N)PP)Ni]- catalytically reduces CH31 at room temperature, consistent with the presence of some Ni(1) character in the reactive species. [(T@-Me,N)F,PP)Ni]also reacts with CH31, but this reaction is not catalytic at room temperature and one or more p pyrrole methylated nickel chlorins are formed as stable products. These species were characterized by UV-visible spectroscopy, 'H NMR spectroscopy, and mass spectrometry and give data which suggests a high electron density on the porphyrin macrocycle of [(T@-Me2N)F4PP)N]-. Introduction The electroreduction of synthetic nickel(I1) ~ h l o r i n sand ~ ~isobacteriochlorins6-8 ~~~ proceeds via one or two one-electron transfer reactions in nonaqueous media. The product of the first reduction may be assigned as a Ni(I1) porphyrin *-anion radical, a Ni(1) porphyrin anion, or a mixture of both as shown in Scheme I where L is the dianion of a given porphyrin, chlorin, or isobacteriochlorin ring. The assignment of Ni(1) in reduced nickel F430I0 and isobacteriochlorins6-8 is unambiguous, but similar definitive conclusions have not been reached for reduced nickel porphyrins, chlorins, or hydroporphyrins. Initial assignments of reduced nickel porphyrins and chlorins as wanion radicals were made primarily on the basis of room temperature ESR and UV-visible spe~tra,"~,~ both of which seemed to be definitive. However, more recent results for different Ni(1I) porphyrins under different solution ( I ) Kadish, K. M. Prog. Inorg. Chem. 1987, 34, 435-605. (2) Kadish, K. M.; Morrison, M. M. Inorg. Chem. 1976, 15, 980. (3) Chang. D.; Malinski, T.; Ulman, A.; Kadish, K. M. Inorg. Chem. 1984, 23, 8 17. (4) Kadish, K. M.; Sazou, D.; Liu, Y.M.; Saoiabi, A.; Ferhat, M.; Guilard, R. Inorg Chem. 1988, 27, 1198. ( 5 ) Kadish, K. M.; Sazou, D.; Maiya, G. B.; Han, 8. C.; Liu, Y . M.; Saorabi, A.; Ferhat, M.; Guilard, R. Inorg. Chem. 1989, 28, 2542. (6) Stolzenberg, A. M.; Stershic, M. T. J . Am. Chem. SOC.1988, 110, 6391,
(7) Stolzenberg, A. M.; Stershic, M. T. Inorg. Chem. 1987, 26, 3082. (8) Renner, M. W.; Forman, A.; Fajer, J.; Simpson, D.; Smith, K. M.; Barkigia, K. M. Biophys. J . 1988, 53, 277a. (9) Lexa, D.; Momenteau, M.; Mispelter, J.; Sav€ant, J.-M.; Inorg. Chem. 1989, 28, 30. (IO) Jaun, 8.; Pfaltz, A. J . Chem. SOC.,Chem. Commun. 1986, 1327.
0002-7863/91/1513-512%02.50/0
Scheme I
..
A
[(L)Ni']-
conditions indicate that the site of the electroreduction may be all9 or partially5**at the metal center. A review of the overall data in the literature seems to suggest that variations in experimental conditions or porphyrin macrocycle can influence the site of electron transfer and/or the electron density on a given singly reduced nickel porphyrin, but little attention has been given to the effect of solvent and the degree of axial ligation. This is discussed in the present paper for two Ni(I1) porphyrins which are quite similar in structure, yet have very different half-wave potentials for formation of the singly reduced complex. The structures of these compounds are schematically shown in Figure 1 where (T@-Et,N)PP) and ( T b Me2N)F4PP) are the dianions of meso-tetrakisb-(diethylamin0)phenyl)porphyrin and meso-tetrakis(o,o,m,m-tetrafluorop-(dimethylamino)phenyl)porphyrin, respectively. The two complexes were reduced by controlled potential electrolysis and the resulting products characterized by ESR and thin-layer UV-visible spectroscopy under different solution conditions. Reactions between singly reduced [(P)Ni]- and CHJI were also investigated in order to better understand relationships that might exist between thermodynamic potentials, the site of electroreduction, and catalytic reactivity of the reduced complex with alkyl halides.
0 1991 American Chemical Society
