J . Am. Chem. Soc. 1994,116, 3657-3658
3657
A Molecular Brake T. Ross Kelly,' Michael C. Bowyer. K. Vijaya Bhaskar, David Bebbington, Albert0 Garcia, Fengrui Lang, Min H. Kim, and Michael P. Jette
a (+60°)
Department of Chemistry. Eugene F. Merkert Chemistry Center, Boston College Chestnut Hill. Massachusetts 02167 Received Nouember 29, 1993 Both the arrest and thecreationofmovement are fundamental aspects of dynamics on macroscopic as well as microscopic levels. Brakes and motors dominate the operation of machines, be they those of daily life, such as vehicles and appliances, or those of living systems. like muscles and Ilagellae. On the molecular level motion is thenorm;spontanwus freerotationaroundsinglebonds is thus the rule, not the exception. In machines of ordinary experience, such as automobiles, the brake is often as important as the accelerator. We now report the first molecular analog: a reversible molecular brake. Figure I presents theconcept inbothgeneral andspecific terms.
8.4
8.2
8.4
I.# 7..
8.2
d
*.e
7.8
7.2
7.8
6.8
p p
8.6
*
7.6
7.4
7.2
7.0
6.8
6.6
6.49-
.
c (-40')
8.e
7.4
7.6
7.4
7.2
7.1
6.8
6.6
~p
(-70')
3 (brako on)
1 (biako ofl)
-
7.6
7.8
b (+30°)
8.4
"W
2 (brake ott)
8.e
4
8.4
8.2
8.I
7.8
7.6
7.4
7.2
7.1
6.8
6.6
8.4
8.E
1..
7..
7.6
7.4
7.L
7.)
6.'
6.6
6.4-
(brakeon)
Figure 1. Conceptualandaetualdepictionoftheoperationofamolecular
brake. With the brakedisengaged. the wheel-(a) representedasa threetoothed gear ( I ) and (b) constructed as a triptycene (2)' -spins rapidly. Engagement ofthe brake (3and4) slowsorstopsrotation. With the actual system, the brake is activated remotely (2 4) by addition of Hg2+ion. In the absence of Hgz+ (or other metal ions), the triptycene wheel spins rapidly a t 30 OC, as evidenced by the simplicity of the 'H NMR spectrum of 2 (Figure 2b). wherein by virtue of C, symmetry arising from relatively rapid rotation, the 12 triptycene aromatic protons give rise to only four sets (asterisked) of resonances. Addition of Hg(OzCCF3)zto 2 results in profound changes in the 30 OC (and other) 'H NMR spectrum (Figure 3a). Most noteworthy are the change in the extraordinarychemicalshiftoftheB-ringmethoxyin2 (6 2.1 to a normal 6 4. I 3 value (not shown) and the obvious broadening of the four resonances attributable to the hydrogens in the three benzo rings of the triptycene. Variable-temperature 'HNMR experiments3 (see Figure 3) document the engagement of the brake. In particular, a t -30 OC (contrast Figure 2),thethreearomaticringsofthetriptyceneare nolongerequivalent becauseofthearrest ofrotation on theNMR . .
-
( I ) For the use of triptycenaas gean see, inter alia (a) Guenzi.A,: Johnson. C. A.:Corzi. F.: Mirlau. K. J. Am. Chem.Soe.1983. l05. 1438. (b) Kawada, Y.:Iwamura.H.J.Am.Ch~m.Soe.1983.l05. 1449.Kawada.Y.:Sakai.H.: Oguri. M.; Kaga. G. Terrohedron Lett. 1994. 35. 135. (c) Nakamura, M.; 0ki.M. Bull. Chem. Soe. Jpn. 1975.18. 2106.0ki. M.:Takuchi.Y.:Toyota, S. Bull. Chem. Soe. Jpn. 1992, 65. 2616. (d) Yamamoto, G.Terrohcdron
--
losn . ..., 46 .-,>16' . ..
(2) The 9-ring mcthary group is evidently (see models) in the shielding zone of the lriplycene knzo rings; the B-ring melhory group in 9 is similarly shielded (6 2.W).' (3) See supplementary material for specifis.
p p
Figure2. Aromaticrcgionof the 500-MHrlH N M R s p e c " (acetoned,) of fatvariaustempcratura. Note that at 30 OC. lhcasterisked p k s for the I2 triptycene aromatic protons a p p r as four sets of resonances. indicatingquivalence (due to relatively rapid rotation). A t 4 OC. p k broadening reflects slowed rotation. but even at -70 OC (2 days). the broadencdpksindicatcthat rotation hasnot stoppion theNMRtime IcalC.
timescale. That nonquivalency ismostclearlyseen (Figure3d) in the two asterisked doublets a t 6 7.67 and 7.53 (in a 2 1 ratio) due to the two H. and one H.. protons (see structure inset in Figure 3) of the nonrotating (on the NMR time scale) form; the 2:l ratio of the H. to H.. r e m n a n m in Figure 3d indicates that engagement ofthe brake results in the planeofsymmetry implied in 4. By contrast. in the absence of metal, restricted rotation at low temperature (Figure 2e) renders all I 2 triptycene protons nonequivalent. As the Figure 3d sample is warmed and the brake begins to slip, the H. and H., resonances coalesce (-2 1 "C), but the complex irreversibly decomposes (-70 "C) before the spectrum has sharpened sufficiently to indicate rapid rotation.
Treatment'oftheHg2+complex4withEDTAremovestheHgz+ and releases the brake, restoring 2. Othertemperaturedependent changes in the spectra in Figure 3 mirror those just described; peak assignments are corroborated by COSY3 and various decoupling experiments. Thesynthesis of2, despite the molecule'sapparent complexity, is quite straightforward (Scheme I),' thanks to the enlistment of
0002-7863/94/1516-3657$04.50/0 0 1994 American Chemical Society
3658 J. Am. Chem. SOC.,Vol. 116, No. 8,1994
Communications to the Editor
a (+30°)
models indicated that coordination of the bipyridine unit in 10 by a metal would activate the brake (eq 1). Reaction of 10 with ,cI
b (+21°)
9.8
8.8
8.6
8.4
8.2
8.8
7.8
7.6
7.2
7.4
7.8
10 (brake off)
p p
(brake on) 11, M IZn;12, M = Hg 13, M = Pd; 14, "M' -CH&Hr
-
8.8
8.6
8.4
8.E
7.8
8.0
7.6
7.2
7.4
7.8
p p
/
8.8
8.6
8.4
8.2
8.0
w
7.8
Y
7.6
7.2
7.4
Y W Y
0.86
p p
Y Y Y
8.11 8.12
0.06
7.0
0.868.86
Y
0.12 0.11 0.24 0.0s
"3. Aromaticregionof the 500-MHz1HNMRspectrum(acetoned6) of 4 (M = Hg2+).Contrast the broad peaks at 30 OC with the sharp peaks at 30 O C in Figure 2b and the sharp peaks at -30 OC in this figure (indicating frozen rotation) with the broad peaks at 4 0 OC and -70 OC in Figure 2. Heating of the complex (4, M = Hg2+) led to irreversible decomposition at -70 O C before the spectrum had sharpened to reflect rapid triptycene rotation.
Scheme 13 O M X
1
-
1.MeO;DMSO
Br 5, X=Br 6. XIOMe
-2
Zn(OTf)z, Hg(02CCF3)2, and Pd(02CCF& leads to enormous changes in the lH NMR spectra consistent with the formation of complexes 11-13 (for example, the resonance for H, is shifted 1.7 ppm downfield, consistent with H, being forced into the deshielding region of the triple bond), but no peak broadening (let alone cessation of free rotation) was observed, even at temperatures as low as -120 OC. Replacing the relatively weak nitrogen-metal coordination bonds with covalent bonds (-+14) by reactions of 10 with BrCHZCHzBr also did not detectably hinder rotation of the triptycene. We believe that in 11-14, the intended brake acts more like a playing card fastened to a wheel of a child's bicycle, being continually dislodged by each passing spoke. The conversion of 2 to 4 (M = Hg2+)is exoergic by several kilocalories per mole (K- 2 1.5 X IO5 M-1).3.9 It is noteworthy that unlike the case with 10, it is impossible to construct CPK models of 2. If one regards the bipyridine bonds in 2 and 10 as fulcrums, then the acetylene unit's extension of the lever arm in 10 provides greater leverage for the spinning triptycene to overcome the binding interaction and dislodge the brake in 10 compared to in 2.10 In summary, we report the first example of a molecular brake. The brake operates by coordination of a metal at a remote site, which brings about a conformational change that reversibly halts rotation of a molecular-scale gear.
benzyne
Pd(PPh&
'
7,X=H 8,X-Br
1. Nbromosuccinimide 2. 9-(Me3Sn)anthracene,
WPW4
two palladium-catalyzed biaryl coupling reactions? The two methoxy groups are included largely for synthetic purposes: in the absence of either (or both), the yield of the benzyne reaction is extremely low (
