1920
Organometallics 1995, 14, 1920-1927
Metal Centers Connected by Conjugated Bridges. A Theoretical Evaluation of Delocalization Effects Michael B. Sponsler Department of Chemistry and W. M. Keck Center for Molecular Electronics, Syracuse University, Syracuse, New York 13244-4100 Received August 30, 1994@
A series of diiron complexes, in which the Fe atoms are linked by a conjugated bridge, have been investigated through Fenske-Hall molecular orbital calculations. The complexes studied were [CpFeL212b-(X),In+, where Cp = q5-C5H5,L2 = (CO):! or q2-H2PCH2PHz,X = -CHCHCHCH-, -CCCC-, P - C ~ H or ~ , -NCHCHN-, m = 1 or 2, and n = 0, 1, or 2 (in selected combinations). For L = CO, the neutral complexes exhibit only weak delocalization, but the radical cationic and dicationic species show relatively strong n delocalization, as measured by population analysis and effective coupling parameter (Vab) for the mixed-valence cations. All bridges studied participate in n delocalization, with the polyenediyl bridges promoting delocalization most effectively. The property of the bridges that most strongly affects the conjugating ability in the cationic and dicationic complexes is the energy of its highest n orbital relative to the metal n d orbital, while the property that most strongly effects the conjugating ability in the neutrals is the energy of the lowest n" orbital. For LO= H2PCH2PH2, delocalization in the neutrals is increased, but the extent of additional delocalization upon oxidation is decreased. In the mononuclear complexes [CpFe(C0)2(CHCH),Hl"+, where m = 1 or 2 and n = 0 or 1, n delocalization is less extensive than in the dinuclear complexes, but not dramatically so. Conjugated organometallic complexes have captured the interest of many research groups in the last several years,l driven in part by potential applications as materials for nonlinear optics, electrooptics, and molecular electronics. A large number of dinuclear complexes with conjugated bridges have been reported, and the extent of delocalization between metal centers has been debated on the basis of experimental and theoretical evidence. In this contribution, Fenske-Hall molecular orbital calculations2 on bridged diiron complexes are used to address several issues pertaining to delocalization, such as the importance of JC vs u interactions and the effects of structural modifications, including the identity of the bridge, the orientation of the bridge, the nature of the other ligands, and the number of metal centers. Some general conclusions that extend beyond the CpFe (Cp = v5-C5H5) systems studied have been made. Recent reports from my laboratories have described a series of butadienediyl-bridged diiron complexes [CpFe(L)(L')12(11-CH=CHCH=CH) (1-4; Scheme 1) that have been chemically and electrochemically oxidized t o produce the corresponding radical cation and dication specie^.^ Cyclic voltammetry showed two reversible oxidation waves for 2-4, suggesting that the monocations are del~calized.~ Spectroscopic data supporting Abstract published in Advance ACS Abstracts, March 1, 1995. (1)(a) Chisholm, M. H.Angew. Chem., Int. Ed. Engl. 1991,30,673674.(b) Biswas, M.; Mukherjee, A. Adu. Polym. Sci. 1994,115,89123. (2) (a) Hall, M. B.; Fenske, R. F. Inorg. Chem. 1972,11, 768-775. (b) Bursten, B. E. Pure Appl. Chem. 1991,63,839-844. (3)(a) Etzenhouser, B. A,; Cavanaugh, M. D.; Spurgeon, H. N.; Sponsler, M. B. J . Am. Chem. SOC. 1994, 116, 2221-2222. (b) Etzenhouser, B. A,; Chen, Q.; Sponsler. M. B. Organometallics 1994, 13, 4176-4178.( c )Compound 1 was previously known: Sanders, A,; Giering, W. P. J . Organomet. Chem. 1976,104, 67-78.
0276-7333/95/2314-1920$09.00/0
Scheme 1
l
L-L'-co
-
1
-e-
-
2
L PMe3, L' CO
3
L PPh3, L' CO
4
L,L'-dppm
I
L"I
L
ground-state delocalization of these mixed-valence radical cations were presented for 2+ (EPR), 3+ (EPR, IR), and 4+ (near-IR absorption). A n X-ray crystal structure of 42+showed bond length alternation in the bridge that was consistent with the bis(carbene1 formulation in Scheme 1. Fenske-Hall calculations were used t o help interpret some of the experimental observations, such as the EPR spectrum of 4+ and the bond distances of 42+.3b The mixed-valence radical cations 1+-4+ may be classified as ( N 1)npolymethines with N = 6,5 and complexes 1-4 are the first organometallic examples of the multistage redox systems first described by Deuchert and Hunig,6 which have the general formula XCH=CHCH=CHX. However, a large body of experimental and theoretical work has been published con-
+
(4)Compound 1 showed a n irreversible oxidation wave at 0 "C, suggesting that 1+ undergoes rapid decomposition at this temperature. At -78 "C, this wave becomes quasi-reversible; a n EPR spectrum for l + has also been obtained at this t e m p e r a t ~ r e . ~ ~ ( 5 )Fabian, J.; Hartmann, H. J . Mol. Struct. 1975,27,67-78. (6)Deuchert, K.;Hiinig, S. Angew. Chem., Int. Ed. Engl. 1978,17, 875-886.
0 1995 American Chemical Society
Metal Centers Connected by Conjugated Bridges
cerning dinuclear complexes with other conjugating bridges, with special attention devoted to experimental and theoretical evaluation of the extent of delocalization in the mixed-valence specie^.^ The most common bridges are those bound through nitrogen, including pyrazine, as found in the Creutz-Taube ion,8 4,4'-bi~yridine,~ bipyridines incorporating additional conjugated linkers,1° and related heteroaromatic groups.ll Conjugated hydrocarbon bridges in di- and polynuclear complexes include (CH), analogues with odd or even n,12 C, and (polyynediyl or cumulenyl),13 ~yclopentadienyl,~~ fulvalene and other Cp-X-Cp species.15 Other bridge types include bis(diketonato),16 linked porphyrin^,'^ linked cyclams,18 and dinitrogen.lg This list is not intended t o be exhaustive. A complex closely related to 1 that has been studied experimentally and theoretically with respect to delo( 7 ) ( a ) Robin, M. B.; Day, P. Adu. Inorg. Chem. Radiochem. 1967, 10,247-422. (b) Hush, N. S. Prog. Inorg. Chem. 1967,8,391-444. (c) Creutz, C. Prog. Inorg. Chem. 1983, 30, 1-73. (d) Mixed Valency Systems: Applications in Chemistry, Physics, and Biology; Prassides, K.. Ed.: Kluwer: Dordrecht. The Netherlands., 1991. '(8)Experimental: (a) Creutz, C.; Taube, H. J. Am. Chem. SOC. 1973, 95, 1086-1094. (b) Fiirholz, U.;Biirgi, H.-B.; Wagner, F. E.; Stebler, A.; Ammeter, J. H.; Krausz, E.; Clark, R. J. H.; Stead, M. J.; Ludi, A. J. Am. Chem. SOC.1984, 106, 121-123. Theoretical: (c) Zhang, L.-T.; KO,J.; Ondrechen, M. J . J . A m . Chem. SOC.1987,109,1666-1671. (d) Piepho, S. B. J. Am. Chem. SOC.1990, 112, 4197-4206. (9)Woitellier, S.; Launay, J. P.; Joachim, C. Chem. Phys. 1989,131, 481-488. (10) (a) Woitellier, S.; Launay, J. P.; Spangler, C. W. Inorg. Chem. 1989,28,758-762. (b) Joachim, C.; Launay, J. P.; Woitellier, S. Chem. Phys. 1990,147,131-141. (c) Reimers, J . R.; Hush, N. S. Inorg. Chem. 1990,29, 4510-4513. (d) Thomas, J . A.; Jones, C. J.; McCleverty, J . A.; Collison, D.; Mabbs, F. E.; Harding, C. J.;Hutchings, M. G. J. Chem. SOC.,Chem. Commun. 1992, 1796-1798. (11) (a) Kaim, W.; Olbrich-Deussner, B. In Organometallic Radical Processes; Trogler, W. C., Ed.; J . Organomet. Chem. Libr. 22; Elsevier: New York, 1990; pp 173-200. (b) Boyde, S.; Strouse, G. F.; Jones, W. E., Jr.; Meyer, T. J . J . Am. Chem. SOC.1990, 112, 7395-7396. (12) ( a ) Davison, A.; Solar, J. P. J . Organomet. Chem. 1978, 155, C8-Cl2. (b) Kolobova, N. Y.; Skripkin, V. V.; Alexandrov, G. G.; Struchkov, Y. T. J . Organomet. Chem. 1979, 169, 293-300. (c) Schaefer, W. P.; Spotts, J . M.; Marder, S. R. Acta Crystallogr. 1992, C48, 811-814. (d) Lemke, F. R.; Szalda, D. J.; Bullock, R. M. J . Am. Chem. SOC.1991, 113, 8466-8477. (e) O'Connor, J. M.; Uhrhammer, R.; Rheingold, A. L.; Roddick, D. M. J . Am. Chem. SOC.1991, 113, 4530-4544. (13) ( a ) Le Narvor, N.; Lapinte, C. J. Chem. Soc., Chem. Commun. 1993, 357-359. (b) Seyler, J. W.; Weng, W.; Zhou, Y.; Gladysz, J . A. Organometallics 1993, 12, 3802-3804. (c) Zhou, Y.; Seyler, J. W.; Weng, W.; Arif, A. M.; Gladysz, J. A. J . A m . Chem. SOC. 1993, 115, 8509-8510. (d) Bruce, M. I.; Hinterding, P.; Tiekink, E. R. T.; Skelton, B. W.; White, A. H. J . Organomet. Chem. 1993, 450, 209-218. (e) Rappert, T.; Niirnberg, 0.; Werner, H. Organometallics 1993, 12, 1359-1364. (0 Lang, H. Angew. Chem., Int. Ed. Engl. 1994,33,547550. (g) Weng, W.; Bartik, T.; Gladysz, J. A. Angew. Chem., Int. Ed. Engl. 1994,33,2199-2202. (h) Brady, M.; Weng, W.; Gladysz, J. A. J . Chem. SOC.. Chem. Commun. 1994. 2655-2656. (14) Schneider, J. J.; Goddard, 8.;Werner, S.; Kriiger, C. Angew. Chem., Int. Ed. Engl. 1991,30, 1124-1126. (15)(a)Sterzo, C. L. Organometallics 1990, 9, 3185-3188. (b) Stephan, M.; Davis, J. H., Jr.; Meng, X.; Chase, K. J.; Hauss, J . ; Zenneck, U.; Pritzkow, H.; Siebert, W.; Grimes, R. N. J . Am. Chem. SOC.1992, 114, 5214-5221. (c) Webb, R. J.; Hagen, P. M.; Wittebort, R. J.; Sorai, M.; Hendrickson, D. N. Inorg. Chem. 1992, 31, 17911801. (d) Delville, M.-H.; Rittinger, S.;Astruc, D. J . Chem. SOC.,Chem. Commun. 1992, 519-520. (e) Hudeczek, P.; Kohler, F. H. Organometallics 1992,11, 1773-1775. (0 Dong, T.-Y.; Huang, C.-H.; Chang, C.K.; Wen, Y.-S.; Lee, S.-L.; Chen, J.-A.; Yeh, W.-A.;Yeh, A. J . A m . Chem. SOC.1993,115,6357-6368. (9)Atwood, C. G.; Geiger, W. E.; Rheingold, A. L. J . Am. Chem. SOC.1993,115,5310-5311. (hj Gilbert, A. M.; Katz, T. J.; Geiger, W. E.; Robben, M. P.; Rheingold, A. L. J. Am. Chem. SOC.1993,115, 3199-3211. (16) Kasahara, Y.; Hoshino, Y.; Kajitani, M.; Shimizu, K.; Sat& G. P. Organometallics 1992, 11, 1968-1971. (17)Arnold, D. P.; Heath, G. A. J . Am. Chem. SOC. 1993, 115, 12197- 12198, and references cited therein. (18)( a ) McAuley, A.; Xu, C. Inorg. Chem. 1992,31, 5549-5554. (b) Mountford, H. S.; Spreer, L. 0.;Otvos, J. W.; Calvin, M.; Brewer, K. J.; Richter, M.; Scott, B. Inorg. Chem. 1992, 31, 717-718. (c) Spreer, L. 0.;Allan, C. B.; MacQueen, D. B.; Otvos, J. W.; Calvin, M. J . Am. Chem. SOC.1994, 116, 2187-2188. (19) Powell, C. B.; Hall, M. B. Inorg. Chem. 1984,23, 4619-4627. ~
~~~
~~
~~~
~~~~
Organometallics, Vol. 14,No. 4,1995 1921
calization effects is the p-phenylene-bridged complex FpCsH4Fp (5; Fp = CpFe(C0)o). Hunter has reported
5
6
a variety of spectroscopic evidence in support of delocalization occurring through the participation of a quinoidal resonance contributor in the neutral complex.20 This evidence includes spectral data (NMR, IR), electrochemical data, and X-ray structural data. However, the interpretation of these data has been questioned by Richardson and who used Fenske-Hall calculations on 5 and several organic models to support their assertion that JT delocalization effects are unimportant in 5. They further argued that small delocalization effects in this complex, if they exist, are better described as 0 effects. Another related bridge that has received considerable recent attention is the butadiynediyl bridge. Le Narvor and L a ~ i n t ehave ' ~ ~ reported the diiron complex 6, and Gladysz and c o - ~ o r k e r shave ~ ~ ~reported ,~ the dirhenium complex 7,each of which contains the C4 bridge. In both cases, oxidation of the neutral complex provided the radical cation and dication species, which were isolated. Strong evidence was presented in both cases for ground-state delocalization in the radical cations. The neutral complex 8, which is included in the present study, has also been prepared.22
7
,
8
An extensive amount of experimental and theoretical work has been reported on complexes with mixed alkyne/phenylene bridges, such as -C=CCsH&=C-. Takahashi and co-workers first described a series of group 10 polymers that exhibit liquid ~rystallinity,~~ and some of these polymers have been found to display strong nonlinear optical effects.24 A wide variety of mono-, di-, and polynuclear complexes have been prepared by Lewis,25Marder,26and Field27 and their coworkers. Lewis and co-workers have used Fenske-Hall calculations to investigate electronic structure.25cFrapper and Kertesz have reported extended Huckel calculations on a series of related complexes of Fe and Pt with (20) Richter-Addo, G. B.; Hunter, A. D. Inorg. Chem. 1989, 28, 4063-4065. (21)Richardson, N. A,; Hall, M. B. Organometallics 1993,12,13381343. (22)(a) Wong, A.; Kang, P. C. W.; Tagge, C. D.; Leon, D. R. Organometallics 1990, 9, 1992-1994. (b) Crescenzi, R.; Sterzo, C. L. Organometallics 1992, 11, 4301-4305. (23)(aj Takahashi, S.; Morimoto, H.; Murata, E.; Kataoka, S.; Sonogashira, K.; Hagihara, N. J . Polym. Sci., Polym. Chem. Ed. 1982, 20,565-573. (b) Kaharu, T.; Matsubara, H.; Takahashi, S. J . Mater. Chem. 1991, 1 , 145-146. (cj Abe, A,; Kimura, N.; Tabata, S. Macromolecules 1991, 24, 6238-6243. (24)Porter, P. L.; Guha, S.; Kang, K.; Frazier, C. C. Polymer 1991, 32, 1756-1760.
1922 Organometallics, Vol. 14, No. 4, 1995
variations in bridge structure and analysis of band structure in the polymers.28 Conjugated mononuclear complexes have been studied both experimentally and theoretically. Radical cations of the form CpFeLz(C=CR)+have been isolated as PF6- salts and characterized spectroscopically.29 Lichtenberger and c o - w ~ r k e r have s ~ ~ recorded photoelectron spectra of several alkynyl Fp complexes and correlated the results with Fenske-Hall calculations. The vinyl complex C ~ F ~ ( P H ~ ) Z C H = and C H the Z ~ ~carbene complex Fp=CH2+ 32 have also been calculated with the Fenske-Hall method. The results are consistent with relatively strong Fe-C n bonding for the carbene and weak n antibonding interactions for the alkynyl and vinyl complexes. The goals of the present theoretical study included the following: (1) to compare some of the bridges mentioned above, along with longer versions of them, in their ability to promote interaction between metal centers, (2) to investigate the bonding changes induced by oxidation to radical cation and dication complexes, (3) to investigate the effects of ancillary ligands, (4) to compare mono- and dinuclear complexes, in order to evaluate the possibility of synergism in the metalbridge interactions, and ( 5 ) to assign any deloca,lization effects as n or u effects. These goals were addressed by comparing calculations for for several Fp complexes, along with one diphosphine analog.
Theoretical Methods The Fenske-Hall program, version 5.1, was used for all calculations. The standard basis functions included with the program were used. The exponent was 2.0 for the 4s and 4p orbitals of Fe. The geometry of the Fp fragment was taken from the idealized to C, symmetry. The structure of 1,4-FpC~F4Fp,~~ geometry of Fp’ (CpFe(r2-H2PCH2PH~)) was also idealized to C, symmetry on the basis of the structure of [CpFe(dppm)lz(p-CHCHCHCH)2t(PFs-)2.3b Bridge geometries were idealized from X-ray crystal structures of appropriate neutral models: FpCH=CHCH=CHFp3* for butadienediyl and diazabutadienediyl, and 713c(for C-C bond lengths: 1.20, 1.39 A) and F p c ~ c P h(for ~ ~the Fe-C bond length: 1.92 A) for butadiynediyl. For phenylene bridges, C-C bond lengths were 1.39 A, the Fe-C bond length was 1.987 (the same as for butadienediyl), and bond angles were 120”. All C-H bond lengths were 1.08 A. The geometry used for cation and
A
(25) (a)Khan, M. S.; Davies, S. J.; Kakkar, A. K.; Schwartz, D.; Lin, B.; Johnson, B. F. G.; Lewis, J. J . Organomet. Chem. 1992,424, 8797. (b) Atherton, Z.; Faulkner, C. W.; Ingham, S. L.; Kakkar, A. K.; Khan, M. S.; Lewis, J.; Long, N. J.;Raithby, P. R. J. Organomet. Chem. 1993, 462, 265-270. (c) Khan, M. S.; Kakkar, A. K.; Ingham, S. L.; Raithby, P. R.; Lewis, J.; Spencer, B.; Wittmann, F.; Friend, R. H. J . Organomet. Chem. 1994, 472,247-255. (26) Fyfe, H. B.; Mlekuz, M.; Zargarian, D.; Taylor, N. J.; Marder, T. B. J . Chem. Soc., Chem. Commun. 1991, 188-190. (27) (a) Field, L. D.; George, A. V.; Laschi, F.; Malouf, E. Y.; Zanello, P. J . Organomet. Chem. 1992,435, 347-356. (b) Field, L. D.; George, A. V.; Hambly, T. W.; Malouf, E. Y.; Young, D. J. J . Chem. SOC.,Chem. Commun. 1990, 931-933. (28) Frapper, G.; Kertesz, M. Inorg. Chem. 1993,32, 732-740. (29) Connelly, N. G.; Gamasa, M. P.; Gimeno, J.; Lapinte, C.; Lastra, E.; Maher, J . P.; Le Narvor, N.; Rieger, A. L.; Rieger, P. H. J. Chem. Soc., Dalton Trans. 1993, 2575-2578. (30) Lichtenberger, D. L.; Renshaw, S. K.; Bullock, R. M. J. Am. Chem. Soc. 1993, 115, 3276-3285. (31) Kostic, N. M.; Fenske, R. F. Organometallics 1982,1,974-982. (32) Schilling, B. E. R.; Hoffmann, R.; Lichtenberger, D. L. J . A m . Chem. Soc. 1979, 101, 585-591. (33)Chukwu, R.; Hunter, A. D.; Santarsiero, B. D.; Bott, S. G.; Atwood, J. L.; Chassaignac, J . Organometallics 1992, 1 I , 589-597. (34) C=C = 1.34 A,C-C = 1.45 A, Fe-C = 1.987 A, Fe-C-C = 131.8”: Churchill, M. R.; Wormald, J . Inorg. Chem. 1989, 8, 19361941.
Sponsler
Table 1. Fragment morbital Energy Gaps and Populations: Variation of Ligand, Charge, and Geometry energy gap, eV
Fp#Fp
charge
n-zd
n*-nd
x
n*
0
2.1 2.2 2.2
9.0 9.0 9.0
2.00 1.73 1.48
0.13 0.13 0.14
5.1 4.3 3.8 3.9 3.3 3.1‘
5.8 6.7 7.3 5.7 5.1 5.0
2.00 1.90 1.73 1.84 1.52
a
C?h.neutral geometry
1
+
2+ F
P
Mulliken population
’
M FP’
C?/,,neutral geometry
4h
CZ,, cation geometry
4hl
+ 2+ +
Cze, dication geometry C?,X-ray geometry
4h2 4hx
2+ 2+
0
0.20 0.17 0.17 0.26 0.36 1.50‘ 0.38
‘I Fp = CpFe(C0)z. Fp’ = CpFe(r2-H2PCH2PH2). ‘ Subject to a small uncertainty due to u-n mixing.
dication complexes was generally the same as that of the neutral. The different geometries of 4h have different compound labels (4h, 4h1, 4h2,4hx). The results in Table 4 (effective coupling parameter and HOMO-LUMO gaps) were obtained from calculations with output in the atomic basis, as were overlap populations given in text. Transformed-basis calculations were performed in order to obtain the results in Tables 1and 2 (fragment orbital energy gaps and populations) and 3 (organic fragment n orbital energy gaps and overlaps with n d ) . In the transformed-basis calculations, Mulliken population analysis36 was done with respect to the orbitals of two fragments: (1)the bridge and (2)the Fe centers with their other ligands. Fragment orbitals were obtained from calculations of the charged fragments [Fp1z2+or [Fp’1zzt and [bridge12-. Energies of the fragment orbitals were obtained from the corresponding diagonal element of the self-consistent field matrix from a transformedbasis calculation of the entire molecule and, therefore, may differ somewhat from the energies of the same fragment orbitals in a different molecule. In other words, the fragment orbitals are “prepared”for bonding with the other fragments, and the energies reflect the bonding environment of the complete mo1ecule.2a,21
Results and Discussion
FpCHCHCHCHFp (1). The results from this study are summarized in Tables 1-4 regarding the different bridges and oxidation levels. Before these results are diagram for FpCHanalyzed, the molecular orbital (MO) CHCHCHFp (1) will be presented as a basis for the comparisons that follow. The Fe-C bonding in 1 was analyzed in terms of fragment orbitals, the most important of which are shown in Figure 1. The bridge orbitals that interact significantly with the Fp2 orbitals are the nonbonding symmetric (n (ag)),and antisymmetric (n (bJ), orbitals, which consist primarily of lone pairs on C1 and Cq, and the frontier n orbitals, n (b,) and the n* (ad. (Symmetry labels are from the C 2 h point group.) The Fp2 orbitals32 come in degenerate pairs, since the Fp groups are too far apart to interact directly. Four of the frontier orbital pairs are composed mainly of d orbital combinations: dz2, which has o symmetry with respect to the bridge, d,, and d,,, which have n symmetry, and dx2-y2,which has 6 symmetry. The fifth d orbital pair, dxy(6 symmetry), lies at much higher energy due to u donation from the carbonyl ligands. (35) Goddard, R.; Howard, J.; Woodward, P. J. Chem. Soc., Dalton Trans. 1074, 2025-2027. (36) Mulliken, R. S. J. Chem. Phys. 1955,23,1833-1840. Mulliken, R. S. J. Chem. Phys. 1955,23, 1841-1846.
Metal Centers Connected by Conjugated Bridges
Organometallics, Vol. 14, No. 4, 1995 1923
x (a,)
--+ Figure 1. Partial molecular orbital mixing diagram for 1, with fragment orbitals of (CH)d2- on the left and (Fp')z on the right. A solid line indicates that the fragment orbital contributes at least 50% to the resulting MO, and a dashed line indicates a contribution of less than 50%. Energy spacings shown are proportional to the relative orbital energies from the calculation. Table 3. Organic Fragment n-Orbital Energy Gaps and Overlaps with Jth
Table 2. Fragment n-Orbital Energy Gaps and Populations: Bridge Variation energy gap3 x-xd x*-xd F Fp Fp
p
m
F
p
==
Fp
Fp
Mulliken population
x*
1
2.1
9.0
0.13
lr
2.0
9. I
0.10
8
2.4
12.4
0.1 1
Fp Fp
F
5
3.5
11.3
Fp
== P
O
F N,
0.15
0. I4
14.8
0. I5
0.15
5
14.8
0.15
0. I3
9
8.6
0.13
0.1 1
Fp
P
0. 10 FPxN//\//
11.0
1
FP
9
1.8
6.9
0.13
F P d
10
15.7
0.12
0.12
F P d
10
3.8
11.9
0.07
Fp&
11
11.5
0.1 1
0.10
Fp &
11
2.5
9.0
0.09
10.9
0.13
0.12
F P x N / w N,
FP
In Figure 1, the MO energy levels of 1 are shown in the center, the fragment orbital levels for the butadienediyl bridge are on the left, and the fragment orbital levels for Fp are on the right. The bridge orbitals participate in two CJ and two n interactions with the Fp orbitals. The nonbonding (lone pair) orbitals each interact with a Fp2 CJ orbital, which is principally a dz2 combination with the appropriate symmetry, to form CJ bonding and antibonding orbitals. (The a, bonding combination also has a contribution from a Fp2 orbital that is principally a Cp-Fe bonding orbital. The CJ antibonding orbitals do not appear in the figure due to their high energy.) Two n interactions are also present: a filled-filled interaction between the n (b,) bridge orbital and a Fp2 n d orbital, consisting primarily of the b, combination of d,, orbitals, and an emptyfilled interaction between the n* (a,) bridge orbital
(I
Fp' = CpFe($-H?PCH?PH?).
and the nd (a,) orbital. The filled-filled interaction is stronger, because the n-nd energy gap, 2.1 ev, is much smaller than the n*-nd gap, 9.0 eV. (One should be cautious about drawing conclusions from the magnitudes of these numbers or any others given by the calculations, but trends in the values are more meaningful.) The overlaps between these pairs of orbitals are similar (0.15 and 0.14, respectively; see Table 3). The MOs that result from these interactions (middle of Figure 1) follow the expected pattern for a d6, pseudooctahedral complex. The nonbonding d block contains six MOs that are all filled (Le., the tz,-like set), with the somewhat antibonding n-nd orbital appearing higher than the rest as the highest occupied MO (HOMO). The lowest unoccupied MO (LUMO) is composed primarily of the bridge n* orbital, mixed with
1924 Organometallics, Vol. 14,No.4, 1995
Sponsler
Table 4. EffectiveCoupling Parameters (Vab) and HOMO-LUMO Energy Gaps"
vah,e~ /
Fp
M
/ F
P
FP
Fp
F
P
~
F
P
HOMO-LUMO gap, eV neutral cation dication
n
label
cation
1 2
1
0.74 0.60
8.4 6.3
8.5 6.2
1.2
la
0.68 0.44
8.9 8.7
9.4 8.8
0.6
0.60 0.45
8.9 8.2
9.3 8.4
0.7 0.9
1
8
2
Sa
I 2
Sa
5
FP&
10
9.7
10.4
Fp
11
8.7
9.4
6.3
7.0
4h
FP -'
0.48
1.2 0.5
0.7
FP'
'>
The geometries were identical for each oxidation level. Fp' = CpFe(q2-H2PCH?PH2). ('
small amounts of the n d (a,) and a few high-lying Fp orbitals (not shown). The primary features of this diagram are common t o all of the complexes studied, with differences occurring in the relative energies of the orbitals. Mulliken's population analysis is valuable in determining the importance of these interactions (Table 1). In 1, the bridge n* orbital is populated by 0.13 electron, indicating that its interaction with the n d (a,) orbital is relatively weak. Since the n orbital interacts with another filled orbital, its population remains 2.00 electrons, even though the interaction is stronger. The nonbonding bridge orbitals, which form the C-Fe u bonds, are depopulated to the level of 1.23 and 1.09 electrons, reflecting the formation of C-Fe bonds which are largely covalent but are polarized toward C. Overlap populations between appropriate atomic orbitals are also informative as a measure of bond orders.36 The overlap population between the C1 pn (i.e., py) orbital and the adjacent Fe d, orbital3' is -0.01, indicating that the interaction is slightly antibonding. Thus, the filled-filled interaction dominates the filledempty interaction. The pz-p, overlap populations for c1-C~and cZ-c3 are 0.49 and 0.11, respectively, which are virtually identical with the corresponding values for 1 , 3 - b ~ t a d i e n eof~0.49 ~ and 0.10. The u overlap populations (obtained by summing all s, px, and pz contributions) are 0.87 and 0.79 for ( 3 1 4 2 and cZ-c3, while the corresponding values for 1,3butadiene are 0.82 and 0.80. The c 1 - C ~ difference reflects, in part, changes in electron density at C1. This effect is very localized and does not indicate that u delocalization between Fe centers exists. This is more apparent from calculations on related complexes with longer bridges, Fp2k-(CH),), n = 8 or 12. In these cases, an identical effect is observed for c1-C~(and Cn-l-Cn), while no differences in u overlap population are observed for any other C-C bonds with respect to the corresponding polyene. These results are in line with Richardson and Hall's calculations for 5.21 In both 1 and 5, n interactions between Fe and C have little effect on the total energy of the complexes. This is not true, however, for oxidized forms of these complexes (see below). (37) The p,-d, overlap population was taken a s the sum of the contributions from C1 p,-Fe dy2and C1 p,-Fe p,. In other words, the d, orbital is actually a d-p mixture (mostly d). (38) Calculated with the same bond distances used for 1.
Fp'CHCHCHCHFp' (4h). Because phosphine derivatives of l have been studied experimentally, calculations were also done for Fp'CHCHCHCHFp' (4h;Fp' = CpFe(r2-H2PCH2PH2)). The diphosphine is a more effective donor than the carbonyls, and the energies of the Fp'z orbitals are therefore higher relative t o the bridge orbitals. The n-nd energy gap is thus increased to 5.1 eV in 4h,while the n*-nd gap is reduced to 5.8 eV (see Table 1). This leads to a weakening of the filled-filled interaction and a strengthening of the filled-empty interaction, and the p,-d, overlap population is now 0.0, corresponding to a nonbonding interacexpected, the n* bridge orbital becomes t i ~ n .As~ ~ populated to a larger extent, 0.20 electron, and the pnp, overlap populations for c1-C~and cZ-c3, 0.47 and 0.13, show a slightly increased delocalization of n bonding. The u overlap populations for c1-C~and CZC3 are 0.86 and 0.80.
4h R = H
The MO diagram of Figure 1 makes clear that the HOMO of 1 (and of 4h) has a significant amount of Fe-C n antibonding character. Removal of electrons from these compounds should therefore provide radical cation and dication species that show significantly enhanced n bonding. Indeed, calculations for 4h+ and 4h2+(at the same geometry as 4h)do show positive pnd, overlap populations (0.03 and 0.07) and changed n overlap populations for c1-C~and c2-c3 (0.45 and 0.15 for 4h+,0.41 and 0.18 for 4h2+). The changes, as expected, are related mainly to the n-nd interaction. Thus, the C-C u overlap populations remain unchanged. The population of the n* orbital drops to 0.17 electron in both species due to increases in the n*-Xd energy gap (6.7 eV in 4h+ and 7.3 eV in 4h2+). The effects of oxidation upon bonding are markedly attenuated from reality if one holds the bond lengths constant, as was done for the calculations above. Since the X-ray crystal structure of 42+is known,3ba better indication of the actual magnitude of these bonding effects was obtained by using the crystal structure geometry for q2+ (designated 4hx2+).The X-ray geometry has only CZ symmetry, but it is not far from C 2 h . The Fe-C-C-C-C-Fe x system is bent somewhat dihedral angle is from planarity: the Fe-C-C-C 161.8'. Most importantly, the C-C bond distances in the bridge show a long-short-long alternation (1.418 A for c1-C~and 1.366 A for c2-c3), which is opposite to the alternation in 1. The Fe-C bond distance is 1.841 A, considerably shorter than in 1 (and 4h: 1.987 A). The results of a calculation for 4hx2+ show the expected changes with respect to 4h2+,given the structural differences noted above. The p3-d3 overlap population is doubled to 0.15 due to the shorter Fe-C distance. The n* orbital is 2.3 eV lower in energy due t o the changes in the C-C bond lengths, and the population of the n* orbital is almost doubled to 0.38 electron. Consistent with the C-C distances, the p.,(39) The same bond lengths were used for 1 and 4h: t t i c w h r c b . t h r population analysis probably underestimates t h r difliwnc-csh I ) ( ~( ~H( I I I the two complexes.
Metal Centers Connected by Conjugated Bridges
pXoverlap populations also show a reversed alternation with respect to 4h: 0.27 for Cl-Cz and 0.31 for C2-C3. The reduced symmetry leads to strong mixing between the bridge n orbital and the b,-like nonbonding orbital, complicating interpretation. In order to provide a cleaner evaluation of the bond distance changes through the bridge, the bond distances were varied while C2h symmetry was kept. 4h22+has the same geometry as 4h, except that the Fe-C1, C1C2, and cZ-c3 bond distances were taken from the X-ray structure of 42+. For 4hl+, the bridge bond distances of 4h and 4h2%+ were averaged. From Table 1, one can see that the differences between 4hx2+and 4h22+are very small, indicating that the effects of the bond distance changes outweigh those of the loss of symmetry in q2+. Also, 4hl+ gives results that are between those for 4h and 4m2+,as expected. This series is almost certainly closer t o reality than the one with a constant geometry.40 These results demonstrate that adjusting the bridge bond lengths is very important if one wishes to correlate properties with experimental data. However, for most of the calculations in this paper, bond lengths have been held constant, because experimental structural models are unavailable for the bridges studied in the different oxidation levels. When the geometry is held constant for all oxidation levels, the bonding changes are attenuated but still display trends that can be used to make comparisons. Conformational Analysis. Several previous studies have dealt with the rotational preference of unsaturated ligands coordinated to Fp.31,32In the case of a-bound vinyl and phenyl groups, the rotational preference is driven by the repulsive n-nd interaction. This results in a slight preference for the conformation in which the plane of the organic group is perpendicular to the Fp symmetry plane, because a slightly weaker n interaction occurs in this c ~ n f o r m a t i o n . For ~ ~ ,carbene ~~ and other ligands in which the n interaction is bonding, the plane of the ligand coincides with the Fp symmetry plane, and the rotation barrier is larger. These preferences are in agreement with several experimental structures, inThis dication has cluding the X-ray structure for 42+.3b a bonding n interaction, as shown by the calculation for 4hx2+,and the experimental structure has near-Czh symmetry, with the bridge plane approximately bisecting the Cp ring. In this study, comparisons between bridges and oxidation levels in this work were made in the C 2 h geometry, similar to that found for 42+. The similarity of the Fp n d (dYAand Z d ( d d orbitals for n bonding suggests that similar results would have been obtained had the bridges all been rotated by 90”. In order to verify this idea, calculations of 1 and its oxidized forms were repeated for the conformation in which the bridge has been rotated 90” into the “horizontal” yz plane (lr; Ci symmetry). As shown in Table 2 , the n energy gaps and n* Mulliken population are very similar in the two conformations. The most significant difference is a 0.03 electron reduction in the n* population. Therefore, the same trends among bridges and oxidation levels would almost certainly be observed for either conformation. (40) This series probably slightly exaggerates the differences between oxidation states, since the bridge geometry for 4h was taken from the X-ray structure for 1.The actual bond lengths in 4h probably show less alternation, due to the delocalizing effect of the phosphines.
Organometallics, Vol. 14,No. 4,1995 1925
@
H,
OC’I Fe
H
co
H ‘a
oc
I,co
H a
Comparisons between Bridges. For purposes of evaluating the ability of different bridges to promote delocalization, calculations were performed on Fp-XFp, with X = -CHCHCHCH- (11, -CCCC- (81, and p-C6H4 (51, along with their oxidized forms. These complexes show some significant differences, and these differences can be primarily attributed to variations in relative orbital energies, since the overlaps are very similar (Table 3). The n orbital energy gaps (Table 2) show that all of the Fp complexes have considerably smaller gaps for n-nd than for n*-Zd. The n-nd gaps are similar for 1 (2.1 eV) and 8 (2.4 eV), but the gap is a bit higher for the phenylene case (3.5 eV). As a result, the complexes show comparable extents of n delocalization upon oxidation, as measured by populations and overlap populations, with a somewhat smaller effect for the phenylene case. Thus, for example, the population of the bridge n orbital drops by 0.52 electron upon oxidation of 1 t o 12+,by 0.47 electron upon oxidation of 8 to g2+,and by 0.39 electron upon oxidation of 5 to 52+. Similarly, the Fe-C n overlap populations increase by 0.10 for 1 to 12+,by 0.10 for 8 to g2+,and by 0.08 for 5 to 52+. The Z*-nd energy gaps also vary, with 1 having a smaller gap (9.0 eV) than 8 (12.4 eV) or 5 (11.3 eV). These differences are manifested in the n*-nd bonding interactions, with n* populations of 0.13 electron for 1, 0.11 electron for 8 , and 0.10 electron for 5. These populations remain nearly constant in all three oxidation levels. The interactions involving the n and n* bridge orbitals are simultaneously described by the quantity v a b , known as the effective coupling parameter for mixedvalence c ~ m p l e x e s .Vab, ~ ~ which can also be thought of as the resonance stabilization energy, represents half of the intervalence transition energy for delocalized mixed-valence complexes.7c It was simply obtained by using the (‘dimer splitting” m e t h ~ d which , ~ ~ ~sets ~ Vab equal to half of the energy difference between the two MOs that are primarily Xd (dyz)in character. Values of Vsb for the various cations, shown in Table 4, are all greater than 0.4 eV, indicating that strong coupling persists even when the bridge length is doubled. The order is the same as that identified above, with the extent of coupling decreasing from 1+to 8+ to 5+. The double-length octatetraenediyl bridge (la+)promotes coupling with equal effectiveness as the single-length phenylene bridge (5+). As noted above for 1, u effects appear to be unimportant for delocalization in any of the bridges. In 1 and 5, the c 1 - C ~ u overlap populations are increased relative to the corresponding organic compound (with H replacing F P ) , while ~ ~ for 8, no change is found with respect to the hydrocarbon. In every case, the u overlap (41)Also denoted in the literature as HAB,Cew,c, etc. See ref 7c for a general discussion. (42) Caution is advised in the direct comparison of these values to others in the literature, due to differences in method (e.g., FenskeHall vs extended Huckel theory) and in geometry conventions. (43) This result contradicts the report of Richardson and Hall,*] possibly due to differences in basis sets.
1926 Organometallics, Vol. 14, No. 4, 1995
populations are unaffected by oxidation. In the case of the butadiynediyl complex (81, the “in-plane” (xz) n overlap populations are also invariant with oxidation; the only changes that occur are in the yz n system. Since many conjugated dinuclear transition-metal complexes are known in which the bridge is attached through nitrogen atoms,8-11 calculation of the diazabutadienediyl complex 9 was also done. This complex
0
oc
I ,co
Fe-N-N29
oc’ I co
9
exhibits lower energy gaps for both n-nd (1.8 eV) and (6.9eV). However, the overlaps for these orbital pairs are also reduced (see Table 31, due to the more contracted py orbital on N. As a result of these competing effects, the n* population is the same as that for 1 at 0.13 electron, but Vab is considerably reduced to 0.55 eV.44A complicating feature of 9 is that the N lone pair orbitals lie at higher energy than the n orbital, so that upon oxidation electrons are removed from the lone pairs instead of the n system.45 This complication does not exist in most of the known complexes with Ncontaining bridges such as pyrazine, since the nitrogen centers have no free lone pairs. Nonetheless, the results show that replacement of CH with N somewhat reduces the bridge’s ability to promote delocalization. By comparing the n-n* energy gaps and the X-Zd and 3z*-nd overlaps of the Fp complexes for the different bridges (Table 31, one may estimate the relative conjugating abilities of the bridges for any choice of metal and ligands. As discussed above, the n and n* orbitals play somewhat different roles in delocalization, at least with respect to oxidation of the Fp complexes, but decreases in the n-n* gap tend to enhance delocalization by increasing the interaction involving one or both of the orbitals with the metal centers. While differences in overlaps among the bridges will depend on the metal/ ligand environment, the trends established in the table are likely to remain, since the differences mostly reflect the coefficients and diffuseness of the py orbital on the atom attached to the metal. The values in Table 3 show that the four conjugated bridges studied show minor to moderate differences in overlap and energy matching as discussed above. The table also shows that phosphine substitution leads to a small decrease in overlap. Examination of the mononuclear vinyl and butadienyl complexes 10 and 11 allows evaluation of the possibility of synergism in the interaction between metal centers. Both complexes have smaller n interactions than 1, as measured by population of n* (Table 2) and depopulation of n in the cations (to 1.83 electrons in 10+ and 1.66 electrons in 11+). For 10,the organic fragment n-n* gap is considerably larger and the overlap is smaller than that for 1. For 11, the n-n* gap is similar to that for 1, but the overlap is lower. The reduction in the number of Fe-C interactions contributes to the decrease in overlap, and to this degree the delocalization in 1 and n*-nd
(44)Computed for 9+ by using the MOs with d,, character, even though the singly occupied orbital had d, character. See text. (45) In gz+,the lone pair and J’Z-Xd antibonding orbitals are nearly degenerate, and the program was unable to converge.
Sponsler
Scheme 2
its oxidized forms can be considered synergistic. Nonetheless, the cationic mononuclear complexes are both significantly delocalized. A different measure of a bridge’s effectiveness at promoting delocalization is the HOMO-LUMO gap of the dinuclear complex. As the molecules are extended to polymers, this becomes the band gap, which relates directly to electrical and electrooptic properties. Analyses of HOMO-LUMO gaps and band gaps have been presented for a number of transition-metal alkynyl polymer^.^^^,^^ HOMO-LUMO gaps for the complexes studied herein are presented in Table 4. The gaps for 1,8,and 5 are within the narrow range 8.4-8.9 eV, as is the gap for the mononuclear 11. The vinyl complex 10 has a larger gap of 9.7 eV, due mainly to a lower energy HOMO. Phosphine coordination in 4h destabilizes both frontier orbitals but primarily the more Febased HOMO, resulting in a markedly lower gap of 6.3 eV. A similar effect was noted for the alkynyl complexes.25c Doubling the length of the bridge has the effect of lowering the HOMO-LUMO gap. Both orbitals are stabilized, but the LUMO is affected more strongly, since it is more localized on the bridge. This effect is much more significant for the octatetraenediyl complex la, with a 25% reduction, than it is for the octatetraynediyl (8a) and biphenylene (5a) complexes, with reductions of 2% and 6%, respectively. Only minor changes in the HOMO-LUMO gaps were found upon removal of one electron to give the radical cations. However, with removal of a second electron, the HOMO becomes the LUMO, and the HOMOLUMO gaps are very small for the dications. This suggests the possibility of open-shell states in the dinuclear cations and very small band gap materials for oxidized polymers. For the butadiynediyl complex (@+), the HOMO and LUMO are very similar antibonding 7C-m orbitals: the former in the xz plane and the latter in the yz plane. While the orbital energies are not accurate enough to make predictions about spin states in particular cases, they do suggest that open-shell ground states might be observed in some of these dications. As a practical matter, one should consider that butadiynediyl has the advantage that its interactions with the metal center are unaffected by rotations of the bridge with respect to the metal fragments. However, with d6 metal fragments such as Fp, the effect of rotation of the other bridges is less important than it might be with fragments that do not have two similar d orbitals with n symmetry. In the absence of steric hindrances, the planar bridges can rotate in order to maximize the n-bonding interactions. Delocalization of Spin. Examination of the singly occupied HOMOS of the radical cations 1+,lo+,and 11+ reveals a trend that is consistent with simple valencebond resonance ideas. Resonance structures for 10+ (Scheme 2) and 11+ are suggestive of higher spin densities on alternant atoms k e . , Fe and p- and 6-C atoms), in analogy to the organic allyl and pentadienyl radicals. The a-and y-C atoms in the radical cations can gain some spin density through resonance structures that have both spin and charge on the ligand (e.g.,
Organometallics, Vol. 14, No. 4, 1995 1927
Metal Centers Connected by Conjugated Bridges
“
W
F
p
-
Scheme 3 +
.
F p \ W F p
-
+ -” \
FP
the last structure in Scheme 2), but these structures should be relatively disfavored. The calculated singly occupied orbitals indeed show an alternation in spin densities for both 10+ (Fe 0.59, a 0.03, 0.15) and 11+ (Fe 0.50, a 0.07,p 0.12, y 0.02, 6 0.13).46 In contrast, the situation is much different for 1+,in which the C atom a to one Fe atom is also 6 to the other. Thus, each C atom is separated from Fe atoms by both an odd and even number of bonds, resulting in efficient delocalization of spin to each C atom (Scheme 3). Due to the 2-fold symmetry, alternation cannot occur, and a more even distribution of spin densities is observed for 1+ (Fe 0.28, C1 0.08, CZ0.06, C3 0.06, C4 0.08, Fe 0.28). Uniform spin densities have been predicted as a general of which 1 is an phenomenon for radical p~lymethines,~ example. In fact, spin densities were found to be relatively evenly distributed for all of the dinuclear cations in this study. An important question concerning the mixed-valence complexes is whether they are spin-delocalized (class 111)or spin-trapped (class II).’ The large V a b values in Table 4 indicate that all of the mixed-valence species examined are potentially delocalized. Experimental and evidence also favors a delocalized structure for 4+,3b the Vab value of 0.48 eV calculated for 4h+is fortuitously identical47with the experimental Vab value obtained for 4+ from its near-IR intervalence transition and is also close to the comproportionation energy of 0.44 eV obtained for eq 1 from cyclic vo1tammet1-y.~~
Conclusions This study allows a number of conclusions to be drawn. (1) All of the conjugated bridges promote significant delocalization in the cations and dications, while delocalization effects are minimized in the neutral complexes. (2) Delocalization in the oxidized species is dominated by n interactions. Changes in (T bonding are small in comparison. (3) Among the hydrocarbon bridges, the polyenediyl bridges most effectively promote delocalization, due mainly to better energy matching of n orbitals. Among the bridges studied, the polyenediyls (46) These spin densities should be viewed as highly approximate, given the problematic nature of open-shell calculations. (47) The value of Vab obtained depends strongly on geometry. For example, a value of 0.75 eV was obtained for 4hl+. (48)The comproportionation energy was taken as the separation between E“‘ values (0.44 V) times the charge of one electron.7c
Fp
etc.
+
/ Fp-
promote by far the strongest reduction in HOMOL U M O gap as the bridge is lengthened. (4) Replacement of the a-CH groups in butadienediyl with N atoms somewhat reduces the ability of the bridge to promote delocalization. ( 5 ) Phosphine substituents have the expected effect of increasing the energy of the 3rd orbitals, which leads to a stronger 9 - m interaction and a weaker JG-J’cd interaction. This translates into stronger delocalization in the neutral complex but a weaker additional effect upon oxidation. This also leads to a reduced effective coupling parameter (Vab). (6) Mononuclear polyenyl complexes also show appreciable delocalization when oxidized, suggesting that synergistic effects between metal centers are relatively minor. (7) The dinuclear dications have very small HOMO-LUMO gaps, suggesting that some of these species might have open-shell ground states. Very low band gap materials might be realized in the polymeric forms of these dications. (8) The “weak l i n k in intermetal communication is the M-C interaction, which suffers from low overlap relative to the C-C interactions. This is probably the reason that Spreer’s diruthenium mixedvalence complek 12,having twice as many M-C inter-
12
actions, shows such strong intermetal communication.l” This complex has the highest reported comproportionation constant for any mixed-valence compound at 3.5 x 1015. Finally, the approach taken in this study of holding bond lengths constant for different oxidation levels has utility in identifying trends among the levels and among different complexes. However, the actual bonding changes that take place upon oxidation are undoubtedly much greater than those represented in most of the tables presented herein, as shown by the calculational comparison of 4h and several alternate geometries and by the experimental (X-ray structure) comparison of 1 and 42+.
Acknowledgment is made to the Camille and Henry Dreyfus Foundation New Faculty Award Program for partial support of this work. OM9406916
