J . Am. Chem. Soc. 1986, 108, 325-327
325
First Realization of Threefold Fluxionality in Polycyclic Conjugated Hydrocarbon-Metal Complexes: Synthesis and Dynamic NMR Study of [Pd(s3-phenalenyl)(tmeda)]+PF6- and Its Methyl Derivative Kazuhiro Nakasuji,* Masakazu Yamaguchi, and Ichiro Murata* Department of Chemistry, Faculty of Science Osaka University, Toyonaka, Osaka 560, Japan Hiroshi Nakanishi B llH1
a 14Nl t
25 Gab33
-
National Chemical Laboratory for Industry Tsukuba, Yatabe, Ibaraki 30.5, Japan
6 70G
= 1
OOG
-
- l n e ~ > a t h i ~ i ! OG
Figure 2. (Top) Second-derivative ESR spectrum of BPheo a* in CH2C1, a t 25 "C obtained by computer averaging.l3 (Bottom) Simulation that demonstrates that two methyl groups, four large protons, and four nitrogens determine the spectral pattern observed.
of BPheo a+ agree within 2% with those observed in the alcoholic solvent, indicating that hydrogen bonding has little effect on the spin distributions of the radicals (Table I). (Comparison of BChl a+ in CH2CI2and C H 3 0 H is complicated by the formation of aggregates in CH2Cl2). The combination of (1) and (2) suggests therefore that oxygen ligation of the Mg in BChls and/or hydrogen bonding of the oxygen peripheral groups do not significantly alter the spin profiles of BChl a+ in vitro and, by extrapolation, the profiles of the primary donors P870' and P960' in vivo. (3) Computer signal averaging yields3 a partially resolved solution ESR spectrum of BPheo a+ with 40 or more lines (Figure 2). This spectrum provides a sensitive test for the assignments of the ENDOR coupling constants. A satisfactory facsimile of the experimental BPheo a+ spectrum is obtained with a computer simulation (Figure 2) that assumes two methyl groups with aH = 1.74 (I-CH3) and 3.10 G (5-CH3), four protons with aH = 4.6, 5.1, 5.7, and 6.7 G ( p protons on rings I1 and IV), and four nitrogens with aN = 1.0 G (approximated from the sole N splitting that is resolved).I3 These results thus support the ENDOR assignments and the similar spin profiles predicted by M O calculations for the BPheo and BChl c a t i ~ n s . ~ ~ ~ J ~ J ~ (4) Changes in the coupling constants of BPheo a+ can be induced near the "freezing" point of CH30H/CH2C12mixtures (Figure 1, T = 153-163 K)). However, only the /3 protons of rings I1 and IV are significantly affected. The effect observed is attributable to conformational changes4-" (twisting) of the flexible saturated ringsI7 induced by the glassy matrix. Analogous variations in rings IV or I1 have been observed in X-ray structures of single crystals of methyl bacteriopheophorbides a and d.17 Similar conformational effects could be induced by packing and protein interactions in vivo and may explain some of the ESR and ENDOR variations observed in different photosynthetic bacteria.'-5 It is intriguing to speculate further that some conformational rearrangements accompany electron transferla and alter both the distances and the relative orientations of BChl donors and acceptors in vivo and thereby help to regulate forward and back electron flow.Ig Acknowledgment. This work was supported by the Division of Chemical Sciences, US Department of Energy, Washington, DC, under Contract DE-AC02-76CH00016. We thank Arthur Forman for the simulation in Figure 2. Registry No. BPheoa+., 60686-68-2. (17) Smith, K. M.; Goff, D. A.; Fajer, J.; Barkigia, K. M. J . Am. Chem. SOC.1983, 105, 1674-1676. Barkigia, K. M.; Fajer, J.; Smith, K. M.; Wil-
liams, G. J. B. Ibid. 1981, 103, 5890-5893. (18) Barkigia, K. M.; Spaulding, L. D.; Fajer, J. Inorg. Chem. 1983, 22, 349-351. (19) Marcus, R. A.; Sutin, N. Biochim. Biophys. Acta 1985,811,265-322. Siders, P.; Cave, R.J.; Marcus, R.A. J . Chem. Phys. 1984,81, 5613-5624.
0002-7863/86/ 1508-0325$01.50/0
Received August 26, 198.5 A major goal of current research in the area of dynamic behaviors in organometallic chemistry is the synthesis of new metal complexes that exhibit migration of an ML, unit from one ring to another in a polycyclic conjugated hydrocarbon A case in point would be the phenalenyl-ML, ~ystem."~If the ML, unit in an q3-phenalenylcomplex 1 migrates around the phenalenyl skeleton through the pathway of 1 .G 1' G l", such a stereo-
PiMLn
m 1
&=&
MLn
MLn
1"
1'
chemical nonrigid behavior provides a novel example of a 3-fold fluxionality in the metal complexes of polycyclic conjugated hyd r o c a r b ~ n . ~ .We ~ now report the first realization of such a
(1) Much experimental information for this type of process is confined to simple cyclic polyene complexes. (a) Cotton, F. A. "Dynamic Nuclear Magnetic Resonance Spectroscopy"; Jackman, L. M., Cotton, F. A,, Eds.; Academic Press: New York, 1975; Chapter 10. (b) Faller, J. W . Adu. Organomet. Chem. 1977, 16, 211. (c) Deganello, G. "Transition Metal Complexes of Cyclic Polyolefins"; Academic Press: New York, 1979. (d) Fedorov, L. A. Russ. Chem. Rev. 1973, 42, 678. (2) For a leading review and theoretical treatment of the interring migration of an ML, unit in polycyclic hydrocarbon-metal complexes, see: Albright, T. A,; Hofmann, P.; Hoffmann, R.; Lillya, C. P.; Dobosh, P. A. J . Am. Chem. SOC.1983,105,3396 and references cited therein. Note Added In Proof. After submittion of this paper Silvesta and Albright ( J . Am. Chem. SOC.1985, 107,6829) have reported some aspects of the haptotropic shift in phenalenium-ML, complex. (3) (a) For the migration of a Cr(CO), group between two nonadjacent six-membered rings, see: Cunningham, S. D.; Ofele, K.; Willeford, 8. R. J . Am. Chem. SOC.1983, 105, 3724. (b) Crabtree, R. H.; Parnell, C. P. Organometallics 1984, 3, 1727. (4) (a) q'-u- and q6-n-:Lin, S.; Boudjouk, P. J . Organomet. Chem. 1980, 187, C11. (b) q 2 - r - : Woell, J. B.; Boudjouk, P. J . Organomet. Chem. 1979, 172, C43. ( 5 ) q3-allyl-: Keasey, A.; Bailey, P. M.; Maitlis, P. M. J . Chem. Soc.. Chem. Commun. 1978, 142. (6) (a) Nakasuji, K.; Yamaguchi, M.; Murata, I.; Tatsumi, K; Nakamura, A. Chem. Lett. 1983, 1489; (b) Organometallics 1984, 3, 1257. (7) For the postulated intermediacy of an (q)-phenalenyl)rhodium complex, see: Paquette, L. A,; Cree, R. J . Organomet. Chem. 1978, 146, 319. For the thermally and photochemically induced sigmatropic shifts of the trimethylsilyl group in 1-(trimethylsilyl)phenalene, see: Butcher, J. A,, Jr.; Pagni, R. M. J . Am. Chem. SOC.1979, 101, 3997. (8) For attempts to detect such a behavior, see ref 5 and 6.
0 1986 American Chemical Society
Communications to the Editor
326 J . Am. Chem. Soc., Vol. 108, No.2, 1986 behavior. We have recently prepared the acetylacetonate (acac) complex Pd($-phenalenyl)(acac) (1, ML, = Pd(acac)) and characterized it by its 'H NMR spectrum? Unfortunately, however, the complex was too labile to detect such fluxionality at elevated temperatures. Accordingly, in order to enhance the thermal stability, we prepared the N,N,N',N'-tetramethylethylenediamine (tmeda) complex [Pd( q3-phenalenyl) (tmeda)]+PF,- (3a), from bis(pch1oro) bis-
Calculated
Observed k I 5-' 87.1
J
r'
120.4
is i
d
8 . 3 2 ~ 1 6 ~ \ ~
i
_ii-
9.52
L
\ f
PF6-
