J . Am. Chem. Soc. 1985, 107, 510-512
510
by Caz+,and Sakurai and Kawamura studied the growth mechanism of myelin figures. These results are in sharp contrast with ours in that helices are formed only in the chain-melting phase (above TJ and do not exhibit enantiomorphism. Recently. however, Weis et a1.I0 reported twedimensional chiral crystals of phospholipid at the air/water interface. I n the case of non-bilayer-forming amphiphiles. helix formation has already been reported from 12-hydroxystearic acid, both in organic gel and from a collapsed m ~ n o l a y e r . ’ ~ - ’ ~ In conclusion, helical superstructures are formed from synthetic. chiral bilayer membranes. The physical state and the chemical structure are crucial for generation and transformation of these helices. It is now possible to prepare helical superstructures from designed component amphiphiles. Electron microscopic studies of these and related systems are published elsewhere.s,16 Acknowledgment. We are grateful to Dr. Hirokazu Hotani of Kyoto University for his helpful discussion. The present study was supported in part by Special Coordination Funds for Promoting Science and Technology from Science and Technology Agency of Japan. Registry No. I. 94061-40-2; 2,93965-91-4; 3. 93965-92-5; 4, 9396593-6; 5, 93965-94-7; 6, 93965-95-8.
Figure 3. Shape changes of hclices into vcsiclcs via tubular s1ruc1urcs. 2C,+Glu-C,,N+. I X IO-’ M. Micrographs (a) and (b) areobtained within an interval of I s during slow heating of the solution beyond T,. Noticeable shape changes are denoted by arrows. Magnification 1300 times (scale bars, 10 ,”. Enantiomeric ~C,,-D-GIU-C, , N t produces similar helical structures. The sense of the helix of the L amphiphile is always right handed, and that of the D amphiphile is left handed. They do not change, even when the helix is bent. Helices are not found for aqueous dispersions of racemic 2CI,-o~-Glu-C,,N+;instead, elastic fibers are present. The helical superstructures are stable only a t temperatures below the phase transition temperature (T,)of the respective The helices bilayers (T, = 34 ‘C for helical 2ClZ-~-GIu-C,,NC).S of Figure 1 change instantaneously to spherical vesicles and flexible filaments, when the solution is warmed to 40-50 OC. The conversion processes can be seen more clearly by slower heating. At temperatures near T,, the pitch of a long helix of 2CI2-~-GluC,,N+ becomes shorter (from 3.2 to 2.8 fim) from one end (Figure 3a, I). The helices are then transformed into tubes (Figure 3a, II), which are immediately constricted and separated into vesicles. Figure 3, parts a and b a r e photographed within an interval of I s, and the rapid constriction process is apparent from comparisons of II with II’, 111 with Ill’, and IV with IV’. Some flexible filaments remain intact. All helices are eventually transformed into vesicles or flexible filaments. The chemical structure of component amphiphiles in addition to their chirality plays crucial roles in morphology. 2C,,-rGlu-C, IN+ gives helical aggregates, whereas 2C,,-r-Glu-C2N+ and 2C,,-r-Glu-C2N+ form spherical vesicles (diameter, several micrometers) at all temperatures. Long spacer methylenes (C,) are required for helix formation. It is known that biological lipids (lecithins) form helical myelin figures?+ Lin et al. reported the induction of helical liposomes Another possible mechanism far this transformation is the double-helix formation. ( 7 ) Virchow. R. Virchows Arch Porhol. Anal. Physiol. 1854.6.571. Cited (6)
in Kclkecr, H. Mol. Crysf. Liq. Cry$t. 1973, 21. 1-48.
(8) Lin, K.-C.: Weis, R. M.: McConnell. H. M. Nolure (London) 1982. 296. 164-165. (9) Sakurai. I.: Kawamura, Y.. personal communication. ( I O ) Weis, R. M.; McConncll, H. M. Nolure (London)1984.310.47-50. ( I I ) Tachibana, T.:Kayama, K.: Takcno. H.Bull. Chem. Sm.Jpn. 1969, 42, 3422-3424. (12) UT”, Y . ; Sugiura. T.J. Colloid Inrerfoee Sei. 1975, 51, 346-349. (13) Tachibana, T.;Hori. K. 3. Colloid lnrerfoee Sri. 1977.61.398-400. (14) Tachibana, T.;Yashizumi. T.:Hori. K . Bull. Chew Sor. Jpn. 1979. 52. 34-41. ( I S ) Stcwarl, M. V.; Amctt. E. M. Top. Sfrremhem. 1981. 13. 195-262. (16) Yamada. K.: Ihara, H.: Idc. T.:Fukumoto, T.:Hirayama. C. Chem. Lell. 1984. 1713-1716.
Magnetic and Spectroscopic Properties of the 66* Excited States of MoZCI4(PMe3),and ~-MozC14(MezPCH2CHzPMe2)2: Experimental Determination of the Energy of a Triplet 66* State Michael D. Hopkins,’ Thomas C. Zietlow, Vincent M. Miskowski, and Harry B. Gray‘ Conrriburion No. 7100. Arrhur Amos Noyes Laborarory Calfornia lnsrirure o/ Technology Pasadena, Calijornia 91 125 Receiued Seprember 20. 1984
The lowest lying electronic excited state of complexes containing quadruple metal-metal bonds is a spin triplet of configuration [a*lr466*].Although the energy of this state has yet to be directly determined by magnetic or spectroscopic measurements, simple theoretical considerations of weakly interacting orbitals.’-3 as well as indirect experimental evidence,’;J suggest that it lies much closer to the (6’) ground state than to its singlet (66‘) counterpart. Ab initio generalized valence bond calculations on Re2C12- have borne this out, placing the ’(W) state of this complex 3200 cm-’ above the ground state and I I200 cm-l below ‘(66’)> Over the past several years, a number of d4-d4 dimers have been reported in
(3) Hanren. A. E.; Ballhausen. C. J. Twrons. Farodov Sm. 1965. 61. 631-639. (4) Zietlow. T. C.: Hopkins. M. D.; Gray, H B. J. SoiidSfac Chrm., in
press. (5) Hay.
P. J. J . Am. Chrm. Sor. 1982. 104. 7007-7017.
0002-7863/85/1507-0510~01.50/0 0 1985 American Chemical Society
J . Am. Chem. SOC.,Vol, 107, No. 2, 1985 51 1
Communications to the Editor
3' 9
3
C
3
\
c
c
i o4
c 3 0
rn
c
1
st2
Temceroture [ K )
E Y
Figure 1. Temperature dependence of the corrected magnetic susceptibility (0) and magnetic moment (0)of a powdered sample of pMo,Cl,(dmpe), (SQUIDdetection, 10-kG field). The room temperature magnetic susceptibility of Mo,C14(PMe3)4 was used a s the diamagnetic correction.
F
which the two ML, units that make up these complexes are partially or completely staggered with respect to each other due to bridging-ligand-induced rotation about the metal-metal
X
iovic limit
-
'(86')
-
- 17200-
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
z
W
5200-
3186' -)
0-
6' -
covalent limit
7=
Mo2C141PMe&
P-Mo2Chidmpe)2
X
While the rotationally symmetric [ u2r4]triple-bond core remains intact in these twisted systems, the extent of &interaction between adjacent d, functions is progressively reduced, relative to that found in eclipsed dimers, as the torsional angle increases, with no overlap remaining at 45O.' Herein we report the magnetic and spectroscopic properties of two d4-d4complexes that nearly span the range of &interactions: P - M ~ ~ C l ~ ( d m(dmpe p e ) ~ = 1,2-bis(dimethylphosphino)ethane), with a twist angle of 40°, and Mo2Cl4(PMe,),, its eclipsed "rotamern.8-10 While Mo2C1,(PMeJ4 displays the rigorous diamagnetism (x, = -280 X emu/mol; crystalline sample, Faraday balance, 300 K) typical of eclipsed d4-d4 dimers of this type, P-Mo2C14( d m ~ e is) ~paramagnetic under the same conditions. The temperature dependence of the magnetic susceptibility of the latter complex (Figure 1) is consistent with an antiferromagnetically coupled two-spin system. Although the N k l point is not observed (6) Best, S. A,; Smith, T. J.; Walton, R. A. Inorg. Chem. 1978,17,99-104. Abbott, E. H.; Bose, K. S.; Cotton, F. A,; Hall, W. T.; Sekutowski, J. C. Inorg. Chem. 1978, 1 7 , 3240-3245. Cotton, F. A,; Fanwick, P. E.; Fitch, J. W.; Glicksman, H . D.; Walton, R. A. J . Am. Chem. Sor. 1979, 101, 1752-1757. Schrock, R. R.; Sturgoff, L. G.; Sharp, P. R. Inorg. Chem. 1983, 22, 2801-2806. Agaskar, P. A,; Cotton, F. A,; Fraser, I. F.; Peacock, R. D. J . Am. Chem. SOC.1984, 106, 1851-1853. (7) Cotton, F. A,; Powell, G. L. Inorg. Chem. 1983, 22, 1507-1510. (8) While the small stoichiometric difference between dmpe and 2(PMe3) prevents B-Mo2C14(dmpe)2and Mo2C14(PMe3)4from being considered rotamers in the strictest sense, the electronic similarity of the two phosphines, in addition to the close structural correlation of the ML4 subunits of each complex? suggests that the metals of both species lie in nearly identical ligand fields, with the fundamental difference between the two being the torsion angle. (9) Structural comparison of M O ~ C I , ( P M ~and , ) ~(p-M~,CI~(dmpe)~)': ~~ d(MoMo), 2.13 (2.18) A; d(MoCI), 2.41 (2.40) A; d(MoP), 2.55 (2.53) A; L(MoMoCl), 112 (11 1)'; L(MoMoP), 102 (98)O; L(MoMo, torsion), 0 (40)'. (10) Cotton, F. A.; Extine, M. W.; Felthouse, T.R.; Kolthammar, B. W. S.; Lay, D. G. J . Am. Chem. Sor. 1981, 103, 4040-4045.
(11) Sample decomposition occurs above -375 K, as evidenced by an increase in the signal attributable to the paramagnetic impurity. (12) Earnshaw, A. "Introduction to Magnetcchemistry"; Academic Press: New York, 1968. (13) Miskowski, V. M.; Hopkins, M. D.; Gray, H. B., unpublished results. (14) Hopkins, M. D.; Gray, H. B. J . Am. Chem. SOC.1984, 106, 2468-2469. (15) No other bands were observed in the electronic spectrum of pMo2CI4(dmpe), between 1000 and 1600 nm. (16) Trogler, W. C.; Gray, H . B. Arc. Chem. Res. 1978, 1 1 , 232-239.
J. Am. Chem. SOC.1985, 107, 512-513
512
limit, P-MoZCl4(dmpe),displays a '(6 6*) transition at 12500 sufficient to supply the water molecules for the disproportionation cm-l, only 4700 cm-' red shifted from that of M O ~ C ~ ~ ( P M ~ reaction.) ~)~. Thus, CICr'I'TPP may be used as a trap for O= CrV(C1)TPP. The rapid trapping of the chromium(V) species by The sum of this red shift and the 3(&3*) energy of @-MozC14( d m ~ e ) -5200 ~, cm-I (15 kcal/mol), which approximates the the chromium(II1) species has allowed us to study oxygen transfer position of the 3(6S*) state of Mo2CI4(PMe3),(Figure 3), is an from percarboxylic acids and hydroperoxides to CICr"'TPP (eq experimental measure of the &bond component of the barrier to 2). rotation about a quadruple bond. ROOH + C1Cr"'TPP ROH + O=CrV(C1)TPP (2a) Acknowledgment. This research was supported by National k2 Science Foundation Grant CHE8 1-20419. Magnetic susceptibility CICr'I'TPP f ~ 2 20=CrIVTPP 0 + 2HCl O=CrV(C1)TPP measurements were made at the University of Southern California (2b) SQUID Instrumentation Facility (supported by NSF Grant The kinetics of the reactions of percarboxylic acids and hyCHE82-11349). M.D.H. and T.C.Z. acknowledge graduate droperoxides with CICr'I'TPP were carried out in CHzC12(25 OC, fellowships from the Sun Co. Nz atmosphere), under the pseudo-first-order conditions of Registry No. Mo2CI,(PMe3),, 67619-17-4; @-M~,Cl,(dmpe)~, 851 15[ROOH] >> [CICrl"TPP]. The reactions were monitored by 86-2. observing the decrease in C1Cr'"TPP at 446 nm and were found to follow the first-order rate law to 6 to 7 half-lives. Plots of initial [ROOH] vs. the determined pseudo-first-order rate constant (koM) Mechanism of "Oxygen Atom" Transfer to were linear. The slopes of such plots provided the second-order rate constants kRooH of eq 2a. In Figure 1 there is plotted the (Tetrapheny1porphinato)chromic Chloride log kRooH values for the reaction of ROOH species with CILung-Chi Yuan and Thomas C. Bruice* Cr'I'TPP vs. the pK, of ROH. The lack of a break in the plot Department of Chemistry, University of California is in accord with consistency of mechanism for species of great at Santa Barbara, Santa Barbara, California 931 06 monooxygen-donation potential (m-CIC6H4C03H)and little oxygen-donation potential (t-BuOOH). That this mechanism inReceived July 9, 1984 volves RO-OH bond heterolysis follows from the recovery of Herein are described results obtained in a study of the reaction >90% phenylacetic acid (as methyl ester with CHzN,) when of C1Cr"'TPP with a variety of oxygen-donor molecules. This ROOH is phenylperacetic acid. Homolysis would yield phenyinvestigation is an extension of studies of oxygen transfer from lacetoxyl radical which decarboxylates rapidly." aniline N-oxides to (porphinato)Fe"'X and (porphinato)Mn"'X The log of the second-order rate constants (kNuc)for heterosalts'-3 and to cytochrome P-450 enzyme^.^ The objectives of lytic-nucleophilic cleavage of the 0-0 bond of a series of hythe present study are to ascertain the effect of donor structure droperoxides and percarboxylic acids [ ROOH] have previously on the ease and the mechanism of oxygen transfer. been shown to be a linear function (PI, = -0.6) of the pK, of the The reaction of iodosylbenzene with C1Cr"'TPP is not well leaving groups (ROH) when the nucleophilic species are thioxane, In this study, the reaction of C1Cr"'TPP with N,N-dimethylbenzylamine, and I-.1o The reaction of C1Cr"'TPP PhIO was initiated (spectral grade dichloromethane, 25 OC, N2 with ROOH follows the same linear free energy relationship with atmosphere) by mixing various volumes of a freshly prepared PI, = -0.34. If one wishes to ignore the difference in solvent for solution saturated in PhIO with a solution of C1Cr"'TPP (1 X nucleophilic attack by I-, : N f , :S< (t-BuOH) and by C1Cr"'TPP M). As the ratio of PhIO/C1Cr1"TPP, present at the time (CH2C1,), though probably not wise, one would conclude that 0-0 of mixing, was increased from 1 to 20 the Soret band of the bond cleavage has not proceeded as far in the transition state in product was found to shift from 429 (O=Cr'VTPP) to 415 nm the instance where the Cr"' moiety of C1Cr"'TPP is the nucleo(O=CrV(Cl)TPP). Thus, at high concentrations of PhIO relative phile. With the percarboxylic acids the product is a mixture of to ClCr'I'TPP, there is obtained as product O=CrV(C1)TPP. An porphinato Cr" and CrV species, whereas with all the hydroinvestigation of the kinetics of the reaction was not attempted due peroxides the A,, of the Soret peak at t, corresponds to the Cr" to the known polymeric nature of PhIO e t c 9 Nevertheless, it species. This result is as should be expected from the results with would appear that the logical explanation for these experimental iodosylbenzene. As the pK, of ROH increases the ease of oxyresults is found in eq 1 . When the product kl [PhIO] is sufficiently gen-oxygen bond breaking in R M H decreases as does the value ki kRooHIROOH] (eq 2a) and this allows the term k2[0=CrvPhI=O C1Cr"'TPP PhI O=CrV(C1)TPP ( l a ) (Cl)TPP] to compete for the C1Cr"'TPP species to yield O= k2 Cr'VTPP. O=CrV(C1)TPP C1Cr"'TPP 20=CrIVTPP + 2HCl 2-(Phenylsulfonyl)-3-(p-nitrophenyl)oxaziridine (Oxa) has been (1b) employed in the epoxidation of alkenes12 (60 OC, CHCI,) and on this basis would appear as a worthy agent to explore as an oxygen large, all C1Cr"'TPP is converted into O=CrV(C1)TPP. At lower donor to metalloporphyrins. The kinetics of reaction of Oxa (1.9 concentrations of PhIO the oxidant O=CrV(C1)TPP competes X 1 r 2to 1.9 X M) with C1Cr"'TPP (1.3 X M) to provide with PhIO for C1Cr"'TPP and a disproportionation reaction enslies O=CrIVTPP were followed at 429 nm (CH2CI2,20 "C). Reto yield O=CrIVTPP. (Traces of moisture in the solvent were actions were first order to at least 6 half-lives and plots of kobsd vs. [Oxa] were linear with slope 8.25 M-' s-l. The appearance ( 1 ) Shannon, P.; Bruice, T. C. J . Am. Chem. SOC.1981, 103, 4500. (2) Nee, M. W.; Bruice, T. C. J . Am. Chem. SOC.1982, 104, 6123. of O=CrIVTPP rather than O=CrV(C1)TPP as product is ex(3) Powell, M. F.; Pai, E. F.; Bruice, T. C. J . Am. Chem. SOC.1984, 106, pected since the rate constant is but -2 times greater than that 3277 ._ for the reaction of triphenylmethyl hydrogen peroxide with (4) Heimbrook, D. C.; Murray, R . I.; Egeberg, H. D.; Sligar, S. G.; Nee, ClCr'I'TPP (Figure 1). M . W .; Bruice, T. C. J . Am. Chem. SOC.1984, 106, 1514. Though p-cyano-N,N-dimethylaniline N-oxide @-CNDMA( 5 ) Groves, J. T.; Haushalter, R. C. J . Chem. SOC.,Chem. Commun.1981, 1165. Groves, J. T.; Kruper, W. J., Jr. J . Am. Chem. SOC.1979, 101,7613. NO) is comparable to PhIO as an oxygen donor to ClFeII'TPP (6) Budge, J. R.; Gatehouse, B. M. K.; Nesbit, M. C.; West, B. 0. J . in the epoxidation of alkenes,2 it does not react with CICr"'TPP Chem. SOC.,Chem. Commun. 1981, 370. (25 OC, CHzC12solvent, N z atmosphere). With photoexcitation (7) Buchler, J. W.; Lay, K. L.; Castle, L.; Ullrick, V. Inorg. Chem. 1982, 21. 842. of the reaction mixture, p-CNDMANO does transfer an oxygen (8) Groves, J . T.; Kruper, W. J., Jr.; Haushalter, R. C.; Butler, W. M. to yield O=CrTVTPP.The photoreaction is presumably associated -+
-
+
-
+
+
+
3
Inorg. Chem. 1982, 21, 1363. (9) Bell, R.; Morgan, K. J. J . Chem. SOC.1960, 1209. (10) Bruice, T. C. J . Chem. SOC.,Chem. Commun.1983, 14. Bruice, T. C.; Noar, J. B.; Ball, S. S.; Venkataram, U. V. J . A m . Chem. SOC.1983, 105, 2452.
0002-7863/85/1507-0512$01.50/0
(11) Bartlett, P. D.; Ruchardt, C. J . Am. Chem. SOC.1960, 82. 1756. (12) Davis, F. A.; Abdul-Malik, N. F.; Awad, S. B.; Harakal, M . E. Tetrahedron Lett. 1981, 22, 917.
0 1985 American Chemical Society
