4736
Organometallics 1995, 14, 4736-4741
1,12=Dimethyl[ l.l]ferrocenophane, an Organometallic Cyclohexane Analogue with Extraordinary Flexibility. Molecular Structure of a Third Isomer, exo,endo,syn-1,12=Dimethyl[ 1.11ferrocenophane Jan-Martin Lowendahl*l* Department of Organic Chemistry, Giiteborg University, S-412 96 Goteborg, Sweden
Mikael HBkansson*>$ Department of Inorganic Chemistry, Chalmers University of Technology, S-412 96 Giiteborg, Sweden Received January 23, 1995@
A third isomer of 1,12-dimethyl[l.l]ferrocenophane (DMFCP),i.e. exo,endo,syn-1,12dimethyl[l. llferrocenophane (IC),has been isolated and structurally characterized by X-ray diffraction. The conformational properties of IC and other [ l .llferrocenophanes are discussed and compared with those of cyclohexanes. Introduction
The conformational properties of the [l.llferrocenophanes have been discussed ever since 1,12dimethyl[ 1.llferrocenophane (DMFCP) (Figure 1)was synthesized in 1966.l One of the original issues was if DMFCP preferred the “flexible” syn conformation (e.g. la, Figure 2) or the “rigid” anti conformation (e.g. lb, Figure 1. 1,12-Dimethyl[1.llferrocenophane (DMFCP) Figure 2), and for a long time the anti conformation was consists of two ferrocene units connected with two carbon ruled out due to the alleged inability to relieve internal bridges, i.e. C1 and C12. The ring protons next to the steric strain.2-6 This led Mueller-Westerhoff and cocarbon bridges are named a-protons, and the other ring workers to assign an exo,endo,syn-DMFCP structure to protons are named P-protons. the second isomer to be isolated from the synthesis of DMFCP.7 The set of NMR signals shown by this la-c and the parent compound [l.llferrocenophane (2) orange-red powder in a CDC13 solution is compliant with see Figure 2. a syn-syn interconverting exo,endo,syn-DMFCP as well The parent compound [l.llferrocenophane (2) shows as with a C2h type exo,exo,anti-DMFCP.8 Recently we unusual flexibility, and a complex degenerate interconwere able to crystallize and structurally characterize an version was proposed to explain the simple NMR anti conformer of DMFCP, from a hexane solution of spectrum (only three signals) of 2.6 We have proven this orange-red powder. This structure demonstrated 2 indeed goes through a degenerate internal that that there are ways of relieving steric strain in anti rearrangement and determined the free energy of conformers as well.9 We now report the X-ray structure activation to be 28 f 4 k J mol-l.lo The most likely of crystals obtained from a THF solution of the orangemechanism for this interconversion is a pseudorotation red powder which shows that these crystals consists of of the same type that can be seen in the cyclohexane a third isomer of DMFCP, i.e. exo,endo,syn-1,12-dimethylimethylsystem. In order to clarify some of the conformational [l.llferrocenophane (IC). At present it is not clear properties of the [l.llferrocenophane molecules, we now which conformational isomer is the dominant species introduce a comparison between the cyclohexane and in solution. For a diagrammatical representation of [1.1lferrocenophane systems. E-mail address:
[email protected]. E-mail address:
[email protected]. Abstract published in Advance ACS Abstracts, September 1,1995. (1)Watts, W. E. J.A m . Chem. SOC. 1968,88, 855-856. (2)Mueller-Westerhoff, U. T.; Nazzal, A.; Proessdorf, W. J. Am.
+
Experimental Section
@
Chem. Soc. 1981,103,7678-7681. (3)Kansal, V. K.;Watts, W. E.; Mueller-Westerhoff, U. T.; Nazzal, A. J. Organomet. Chem. 1983,243,443-449. (4)Barr, T. H.; Lentzner, H. L.; Watts, W. E. Tetrahedron 1969, 25,6001-6013. (5) McKechnie, J. S.; Berstedt, B.; Paul, I. C.; Watts, W. E. J. Organomet. Chem. l967,8,29-31. (6)Watts, W. E. J. Organomet. Chem. 1967,10,191-192. (7) Cassens, A.; Eilbracht, P.; Nazzal, A.; Proessdorf, W.; MuellerWesterhoff, U. T. J. Am. Chem. SOC.1981,103,6367-6372. (8) Lowendahl, M. Thesis, Gateborg University, 1995. (9) Lowendahl,
M.; Davidsson, 0.; Ahlberg, P.; HHkansson, M.
Organometallics 1993,12,2417-2419.
General Data. All operations were carried out under nitrogen. Tetrahydrofuran (THF)and hexane were distilled from sodiumhenzophenone shortly prior to use. Preparation of eso,endo,syn-l,la-Dimethyl[1.llferrocenophane (IC). A mixture of the 1,12-dimethyl[l.l]ferrocenophane isomers was prepared according to the literature method.’ This isomer mixture (9 g) was dissolved in 800 mL of hexane by gentle warming, whereafter the temperature was lowered to approximately 4 “C, which resulted in the crystal(10)Uwendahl, M.; Davidsson, 0.;Ahlberg, P. J. Chem. Res., Symp. 1993,1,40-41.
0276-733319512314-4736$09.00/00 1995 American Chemical Society
lJ2-DimethylLl. l]ferrocenophane
Organometallics, Vol. 14, No. 10, 1995 4737 Table 1. Crystallographic Data for exo,endo,syn-l,la-D)imethyl[l.llferrocenophane (IC) formula fw color cryst syst space group a, A
b, A
la
c, A a, deg
C24H24Fez
P! deg
424.15
79 deg
red orthorhombic Pbca (No. 61)
v, A3
14.949(8) 18.745(5) 12.819(5)
2 dcalc, g/cm3 y, cm-1 T,K R
90
RW
90 90
3592(4) 8 1.569 16.19
200 0.035 0.045
2:l. Data were measured for 5 < 20 < 50", using a n o scan rate of 8.0"Imin and a scan width of (1.26 0.30tan 0)'. Weak reflections (Z 1O.OutZ))were rescanned up to three times and counts accumulated t o improve counting statistics. The intensities of three reflections monitored regularly after measurement of 150 reflections confirmed the crystal stability during data collection. Correction was made for Lorentz and polarization effects. No correction was made for the effects of absorption, owing to failure to obtain a more satisfactory structural model from empirically corrected data (by means of azimuthal scans, yielding a transmission factor range of 0.76-1.00with a n average value of 0.94).Cell constants were obtained by least-squares refinement from the setting angles of 20 reflections in the range 15 < 20 < 30". The structure was solved by direct methods (MITHRIL),ll and the hydrogens were located from difference maps. Fullmatrix least-squares refinement, including anisotropic thermal parameters for the iron and carbon atoms, with positional parameters (the isotropic thermal parameters were fixed) for the hydrogen atoms, gave a final R = 0.035 (R,= 0.045)for 307 parameters and 2238 observed reflections. The maximum and minimum values in the final difference map were 0.61 and -0.56 elA3, respectively. Fractional coordinates and thermal parameters are given in Table 2, and selected intramolecular distances and angles are listed in Table 3. The crystallographic numbering is shown in Figure 3. All calculations were carried out with the TEXSAN12 program package. Atomic scattering factors and anomalous dispersion correction factors were taken from ref 13. Figure 3 has been drawn with ORTEP.14
+
lb
IC
a'
P' P"
P"
He,,
a"
a' a"
P' P"
a" Hex0
a"
2 Figure 2. Diagrammatic representation of compounds la-c, a n d 2. lization of mainly l a (3g) after a few days. The solution was decanted and evaporated to dryness, yielding a n orange-red powder (6 g). A portion of this orange-red powder (100mg) was then dissolved in 5.0 mL of THF a t ambient temperature. The crystallization of large red needles (64 mg) of IC was achieved by cooling the solution to 4 "C for a few days. NTVIR Spectroscopy. All NMR spectra were recorded using a Varian Unity 500 spectrometer. The measuring frequency was 500 MHz THF-ds was used as a solvent. Typically 5 mg of the compound was dissolved in 1 mL of the solvent. Proton NMR: one doublet at 1.23 ( J = 7.1Hz) ppm for the methyl groups; one quartet at 3.60ppm ( J = 7.1 Hz) for the methine protons; four multiplets for the ring protons, with /3-protons at 4.24and 4.27ppm and a-protons at 4.12and 4.16 ppm ( J 1.5 Hz). The THF signal at 3.58 ppm was taken as the reference signal. X-ray Crystallography. Crystal and experimental data for [C24H24Fe~](IC) are summarized in Table 1. Data were collected a t 200 K with a Rigaku AFCGR difiactometer, using a red irregular-shaped crystal with the dimensions 0.30 x 0.30 x 0.30 mm, which was mounted in a Lindemann capillary. Diffracted intensities were measured using graphite-monochromated Mo K a (1= 0.710 73 A) radiation from a n RU200 The rotating anode source operated at 9 kW (50 kV; 180 d). 0/20 scan mode was employed, and stationary background counts were recorded on each side of the reflection, the ratio of peak counting time and background counting time being
Results and Discussion
When considering the conformational space of the [ l . l l f e r r o c e n o p h a n e s y s t e m , it is rewarding to use an analogy with the well-investigated cyclohexane system.15-21 First, w h e n one looks at the boat conformer of cyclohexane and the syn conformer of [ l . l l f e r r o cenophane, it becomes a p p a r e n t that the bow and stern carbon atoms in cyclohexane ( a t o m s 1 and 4 in Figure (11)Gilmore, C. J. J . Appl. Crystallogr. 1984,17, 42. (12)TEXSAN-TEXRAY Structure Analysis Package, Molecular Structure Corp., The Woodlands, TX, 1989. (13)International Tables for X-ray Crystallography; Kynoch Press: Birmingham, England, 1974;Vol. IV. (14)Johnson, C. K.ORTEP. Report ORNL-3794; Oak Ridge National Laboratory, Oak Ridge, TN, 1965. (15)Anet, F.A.L.; Bourn, A. J. R. J . Am. Chem. SOC.1967,89,760768. (16)Burkert, U.;Allinger, N. L. Molecular Mechanics; American Chemical Society: Washington, DC, 1982. (17)Cremer, D.;Szabo, K. J. In Conformational Behauior of SixMembered Rings: Analysis, Dynamics and Stereoelectronic Effects;
Juaristi, E., Ed.; VCH: Weinheim, Germany, in press. We thank Prof. D. Cremer and Ph. D. K. Szabo for making- their manuscript available to us prior to publication. (18)Eliel, E. L.;Wilen, S. H.; Mander, L. N. Stereochemistry of Organic Compounds;Wiley: New York, 1993;pp 13-14,49-51,686-
726. (19)March, J. Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 4th ed.; Wiley-Interscience: New York, 1992; pp 143-146. (20)Sachse, H. Ber. Bunsen-Ges. Phys. Chem. 1890,23,1363. (21)Sachse, H.2.Phys. Chem. 1892,IO, 203.
4738 Organometallics, Vol. 14,No. 10,1995
Lowendahl and Hhkansson
Table 2. Positional Parameters and B(eq) or B Values (A2)for exo,endo,syn-1,12-Dimethyl[l.l]ferroceno-phane (IC) 0.40120(4) 0.15840(4) 0.3715(3) 0.4232(3) 0.4844(3) 0.5341(3) 0.5027(3) 0.4346(3) 0.2702(3) 0.3304(3) 0.3945(3) 0.3740(3) 0.2981(3) 0.2550(3) 0.1805(3) 0.1679(3) 0.0831(3) 0.04 19(3) 0.1025(3) 0.2272(3) 0.1389(3) 0.1303(3) 0.2135(3) 0.2738(3) 0.3855(4) 0.2184(4) 0.396(3) 0.485(3) 0.573(3) 0.524(3) 0.397(3) 0.218(3) 0.326(3) 0.444(3) 0.402(3) 0.300(2) 0.208(3) 0.057(3) -0.004( 3) 0.092(3) 0.252(3) 0.088(3) 0.077(3) 0.229(3) 0.364(3) 0.456(3) 0.358(3) 0.176(3) 0.270(3) 0.202(3)
0.15095(3) 0.09322(3) 0.0473(2) 0.0785(2) 0.1359(3) 0.1384(3) 0.0820(2) 0.0449(3) 0.1697(2) 0.1717(3) 0.2262(3) 0.2566(3) 0.2226(2) 0.2468(2) 0.2011(2) 0.1859(3) 0.1547(3) 0.1498(3) 0.1785(2) 0.0225(2) -0.0029(2) -0.0098( 3) 0.0102(3) 0.0310(2) 0.0916(3) 0.3229(3) O.OOl(2) 0.176(2) 0.180(2) 0.067(2) 0.006(2) 0.141(2) 0.142(2) 0.240(2) 0.293(2) 0.253(2) 0.194(2) 0.138(2) 0.136(3) 0.179(2) 0.031(2) -0.014(2) -0.028(2) 0.0 14(2) 0.139(3) 0.094(3) 0.075(3) 0.319(3) 0.357(3) 0.345(3)
0.67568(5) 0.48136(5) 0.4625(3) 0.554l(3) 0.5499(4) 0.6450(4) 0.7089(4) 0.6532(4) 0.7231(3) 0.8092(3) 0.7906(4) 0.6927(4) 0.6493(3) 0.5487(3) 0.5054(3) 0.3977(4) 0.3842(4) 0.4831(4) 0.5575(4) 0.5756(3) 0.5548(4) 0.4453(4) 0.3990(4) 0.4800(3) 0.3652(4) 0.5646(4) 0.448(3) 0.499(4) 0.662(3) 0.785(4) 0.676(3) 0.723(3) 0.871(3) 0.833(3) 0.667(3) 0.496(3) 0.347(3) 0.321(4) 0.496(4) 0.621(4) 0.641(3) 0.606(4) 0.408(4) 0.325(4) 0.376(4) 0.343(4) 0.303(4) 0.616(4) 0.589(3) 0.510(4)
1.51(3) 1.62(3) 1.8(2) 1.7(2) 2.0(2) 2.2(2) 2.0(2) 1.8(2) 1.7(2) 1.9(2) 2.4(2) 2.0(2) 1.5(2) 1.6(2) 1.6(2) 2.0(2) 2.4(2) 2.1(2) 1.8(2) 1.6(2) 2.1(2) 2.5(2) 2.3(2) 1.9(2) 2.6(2) 2.4(2) 2.2 2.4 2.6 2.4 2.2 2.0 2.3 2.9 2.4 1.9 2.4 2.9 2.5 2.2 1.9 2.5 3.0 2.8 3.1 3.1 3.1 2.9 2.9 2.9
4a) are analogous to the bridge methylene carbon atoms in [l.l]ferrocenophane (atoms 1 and 4 in Figure 4b). Second, the other carbon atoms in the cyclohexane ring (atoms 2, 3, 5, and 6 in Figure 4a) are then analogous to the dummy atoms which are placed at the center of gravity of the cyclopentadienyl moieties (atoms 2, 3, 5, and 6 in Figure 4b). The extent of this analogy is especially clear from Figure 5, where a comparison is made between 1,12dimethyl[ 1.llferrocenophane (DMFCP) and 1,4-dimethylcyclohexane. Consider, for example, exo,exo,synDMFCP (syn = boat), which is analogous to boatl(e),4(e)-dimethylcyclohexane (e = equatorial). It is well-known that boat-l(e),4(e)-dimethylcyclohexane can interconvert into boat-l(a),4(a)-dimethylcyclohexane (a = axial) v i a pseudorotation (or ring inversion), and the same mechanistic pathways are also open to exo,exo,syn-DMFCP, whereby it becomes endo,endo,syn-DMFCP. Consequently, there are eight structures of
Table 3. Selected Intramolecular Distances (A) and Angles (deg) for [C24H24Fe21 (IC) Fe(l)-C(2) Fe( 1)-C(4) Fe( 1)-C(6) Fe(l)-C(8) Fe(l)-C(lO) Fe(2)-C(13) Fe(2)-C(15) Fe(2)-C(17) Fe(2)-C(19) Fe(2)-C(21) C(l)-C(2) C(ll)-C(l2) C(l)-C(23) C(2)-C(3) C(4)-C(5) C(6)-C(2) C(8)-C(9) C(lO)-C(11) C(2)-C( 1)-C(22) C(2)-C(l)-C(23) C(ll)-C(l2)-H(l2) C(I)-C(2)-C(3) C(2)-C(3)-C(4) C(12)-C( 13)-C( 17)
C15
2.093(4) 2.039(5) 2.070(5) 2.050(4) 2.033(5) 2.073(5) 2.036(5) 2.051(5) 2.054(5) 2.053(5) 1.523(6) 1.511(6) 1.514(7) 1.413(6) 1.418(7) 1.428(6) 1.421(7) 1.416(6) 117.0(4) 110.7(4) llO(2) 126.3(4) 109.3(4) 127.3(4)
Fe(1)-C(3) Fe(l)-C(5) Fe(l)-C(7) Fe(l)-C(9) Fe(l)-C(ll) Fe(2)-C( 14) Fe(2)-C(16) Fe(2)-C(18) Fe(2)-C(20) Fe(2)-C(22 C(l)-C(22) C(12)-C( 13) C(12)-C(24) C(3)-C(4) C(5)-C(6) C(7)-C(8) C(9)-C(lO) C(7)-C( 11) C(2)-C( 1)-H( 1) C(ll)-C(l2)-C(13) C(ll)-C(12)-C(24) C(l)-C(2)-C(6) C(12)-C( 13)-C( 14) C(13)-C( 14)-C( 15)
2.056(4) 2.039(5) 2.081(5) 2.042(5) 2.072(4) 2.047(5) 2.040(5) 2.068(5) 2.029(5) 2.082(4) 1.500(5) 1.500(5) 1.500(5) 1.427(6) 1.425(6) 1.424(6) 1.412(7) 1.432(6) 108(3) 117.3(4) 108.4(4) 125.2(4) 124.6(4) 108.8(4)
c1
Figure 3. ORTEP drawing showing the crystallographic numbering in IC.
1
b Figure 4. Structure elements that are common to both cyclohexane and [1.l]ferrocenophane. DMFCP, with respect to the positions of the bridges (syn or anti) and to the positions of the methyl groups (ex0 or endo). However, of these eight structures, there are two pairs with identical structures, two exo,endo,antiDMFCP isomers (Figure 6A), and two exo,endo,syn-
Organometallics, Vol. 14, No. 10, 1995 4739
1,12)-Dimethyl[l.llferrocenophane
Twist angle
Tilt angle
boat- l(a), 4(a)dimethyl-c yclohexane
boat- l(e), 4(e)dimethyl-c yclohexane
Betweenplanes 1 and 2 Betweenplanes 3 and 4 .
Pseudorotation
Y
Fe
I
Between planei.,l +d 4 Between planes %.and 3
endo,endo,syn1,12-dimethyl[l.l]ferrocenophane
exo,exo,syn-
1,12-dimethyl[l.l]ferrocenophane
Figure 5. Example of the common conformational properties of cyclohexane and [1.llferrocenophane.
I Fa
-
Pseudorotation I
II
Fa
Rotation angle
Between planes 1 and 2 Between planes 3 and 4
Bridge angle
Around carbon C1 Around carbon C 12
Figure 7. Description of the four essential angles in a L1.11ferrocenophane molecule.
-+’
Pseudorotation ca I
Fa
I
endo-endo-anti
Figure 6. The six diastereoisomers of 1,12-dimethyll.l.11ferrocenophane and their interconversion pathways. Flipu and FlipL means a flip of the upper and lower bridges, respectively.
DMFCP isomers (IC,Figure 6B), which are connected via ring inversion and pseudorotation, respectively. This leaves six possible diastereoisomersls of DMFCP. Figure 6 depicts these diastereoisomers as well as their interconversion pathways. For simplicity the comparison is always made between boat and syn structures and chair and anti structures and not, in some cases, the more energetically stable twist forms (see Table 4). Furthermore, if a twist is introduced in the diastereoisomers the complexity increases since there will be enantiomeric pairs of each structure. The optical activity depends on whether the twist is clockwise or anticlockwise relative to the eclipsed starting point.
Figure 6A shows exo,exo,syn-DMFCP (la),endo,endo,syn-DMFCP, and exo,endo,anti-DMFCP which all are conformational isomers18 i.e. they can interconvert without breaking any bonds. Figure 6B shows exo,exo,anti-DMFCP (lb),endo,endo,anti-DMFCP, and exo,endo,syn-DMFCP (IC), which also are conformational isomers. The conformers within parts A and B of Figure 6 belong to different configurational isomers;ls i.e., they cannot interconvert unless there is a bond broken (or inversion at a carbon). In Figure 6, the term “flip” means a flip of one of the bridge carbons from one side to the other, which is the mechanistic equivalent of half a ring inversion. Out of the six diastereoisomers, three have been crystallized so far. The first isomer to be structurally characterized by X-ray diffraction was exo,e~o,syn-DMFCP~7~~ (la),and the second isomer was exo,exo,anti-DMFCPg (lb). The crystal structure of the third isomer, exo,endo,syn-DMFCP (IC),is reported in this work. The relatively large variation in the Fe-C bond distances (Table 3) can be rationalized in terms of the twist and tilt experienced by IC. Still, the carbons in each cyclopentadienyl ring are essentially coplanar. The Fel- eFe2 intramolecular distance of 4.533(2) does not suggest any metal-metal interaction. We have previously suggested that the main structural features of a [l.l]ferrocenophane molecule is given by four essential angles: the twist, the rotation, the tilt, and the bridge angle (see Figure 7).9 The twist angle is defined as the dihedral angle between the best planes of the cyclopentadienyl rings of the same organic ligand. The rotation angle is defined by the [CA-(center of gravity of ring (22) McKechnie, J. S.; Maier, C. A.; Bersted, B.; Paul, I. C. J. Chem. SOC.,Perkin Trans. 2 1973,138-143.
Lowendahl and HlEkansson
4740 Organometallics, Vol. 14, No. 10,1995 Table 4. The Four Essential Angles (deg) in Different Isomers of 1,12-Dimethyl[1.1lferrocenophane Together with T w o Different Forms of [l.l]Ferrocenophane twist angle
rotation angle
30.2 31.5 l b ( e x o , e ~ o , a n t i ) ~ 36.1 34.0 IC (exo,endo,syn) 39.4 (this work) 38.2 2a (syn)23 13.8 12.7 2b (synP4 1.6 3.2
21.5 23.9 3.4 53.9= 32.0 31.4 -10 -10 0.5 0.6
isomer la (exo,exo,syn)22
tilt angle 3.1 3.0 4.1 22.7 7.3 6.2 2.4 1.4 3.6 1.0
bridge angle 115.8 117.5 116.9 118.1 117.0 117.3 121.3 121.7 121.8 122.5
a A rotation angle of 53.9” is reasonable only with reference to a [l.l]ferrocenophane molecule, but for a comparison with ferrocene derivatives a value of 18.1” (72-53.9) is the adequate rotation angle. This angle is close to an ideal midpoint (18.0”) between an eclipsed and a staggered conformation.
A)-(center of gravity of ring B)-CBI torsion angle, where rings A and B are the cyclopentadienyl rings of one “ferrocene unit”. The tilt angle can then be defined as the dihedral angle between the two rings (i.e. from different organic ligands) of such a ferrocene unit. Finally, the bridge angle is defined as the bond angle around the bridging carbon with respect to the two quaternary ring carbons in the same organic ligand. In Table 4, these angles are listed for la-c and for the two known phases of the unsubstituted parent L1.11ferrocenophane (2).23,24 From the data in Table 4, some general relationships involving the essential angles can be established (there are a few special exceptions). 1. The bridge angle seems not to be coupled to any other angle. 2. The twist and rotation angles are directly linked as components of a “pure” pseudorotation. 3. The tilt angle automatically induces a change in the twist angle which usually, but not always, leads to a change in the rotation angle. 4. The tilt angle is instrumental in the ring inversion mechanism. Furthermore, the data in Table 4 indicate three different mechanisms to relieve internal steric strain in [ 1.1lferrocenophane molecules, bearing in mind that packing effects-i.e. intermolecular contacts-also will be of significance for the solid-state conformation. There are several intermolecular H* .H distances in ICin the order of 2.3-2.7 A, which can be compared with several intramolecular contacts that lie in the same range. The first mechanism involves an opening of the bridge angle in order to reduce He *H intramolecular repulsive interactions of the a’ protons. This mechanism is utilized by all the [l.llferrocenophanes but is especially visible in 2b, which exhibits a bridge angle of 122”.The second mechanism increases the twist and rotation angles, i.e. starts a movement along a pseudo-rotational path, which initially results in an increase of the distances between the a‘ protons in the same organic ligand. In this mechanism, both the twist angles and (23)Rheingold, A. L.; Mueller-Westerhoff,U. T.; Swiegers, G. F.; Haas, T. J. Organometallics 1992, 11, 3411-3417. (24) Hlkansson, M.; Lowendahl, M.; Davidsson, 0.; Ahlberg, P. organometallics 1993, 12, 2841-2844.
the rotation angles change in a coupled manner, and it is used by all [l.l]ferrocenophanes with the exception of 2b ( l b comprises a special case, vide infra). In the third mechanism, the tilt angle is altered, which seems to be the mechanism with the steepest energy potential. The effect of tilting is divided into two parts. First, as in the case of l b (exo,exo,anti), where the anti conformation of the carbon bridges makes a pseudorotation impossible, the asymmetric tilt (tilting of only one ferrocene unit) induces a twist. This tilt-induced twist has the same effect as in a syn conformer. Second, a symmetric tilt (tilting both the ferrocene units to the same degree and in the same direction respectively, e.g. outwards) has the effect of making the carbon bridges come apart. This reduces the steric interaction between the endo methine protons in e.g. l a (exo,exo,syn) and is especially important in IC (exo,endo,syn) in order to reduce the steric tension between the endo methyl protons (e.g. H23A) and the endo methine proton (e.g. H12), referring to Figure 3. The phenomena which are hardest to rationalize with a cyclohexane analogy are the conformational properties of the ferrocene units. However, the effects of movements in the “organometallicball bearing“, which is how ferrocene can be considered, may be explained in terms of bond rotation and bond angle bending. First, consider the cyclopentadienyl moieties as two carbon rings floating on the “iron ball”, i.e. Fe2+. When they are parallel t o each other, the Gibbs free energy of activation for their rotation, relative t o each other, is 0.9 kcaVm01,~~ which is much less than 2.9 kcaVmol, the Gibbs free energy of activation for the C-C bond rotation in ethane.lg The movement of cyclopentadienyl moieties toward each other on the surface of the “iron ball” represents a tilt in the ferrocene unit, or a bending over the iron atom. This would mean a bending of, for example, the bond angle defined by atoms 1, 2, and 3 in Figure 4a, which is the prerequisite for a ring inversion mechanism. One difference with regard to the cyclohexane system is that in [l.llferrocenophane the 2-3 distance has t o become shorter with an opening of the 1-2-3 angle (see Figure 4b) and that the 2-3-4 angle has to be equal to the 1-2-3 angle. To somehow put the energetic requirements for bending in ferrocene in perspective, the MM2 type bending force constants of ferrocene in a ferrocenophane and a C-C-C bond angle have been set to 0.500 mdyddeg (73.2 kcaV deg)26,27 and 0.450 mdyddeg (65.9 kcal/deg),16respectively, where the natural bond angles are set to 111.01 and 109.5”, respectively. The N M R spectrum of ICin THF indicates a degenerate interconversion (or accidental chemical shift equiva, ~ only lence), since only six signals can be r e ~ o r d e de.g. one doublet for the methyl protons at 1.23 ppm. However, whether this interconversion occurs via a pseudorotation, or via ring inversion with l b as an intermediate, is not possible to deduce. Computational chemistry may be able to model the [l.l]ferrocenophane system well enough t o give us the answers in the future, (25) Haaland, A. Acc. Chem. Res. 1979, 12, 415-422. (26) The units have been corrected relative to the original paper after personal communication with Dr. Rudzinski. (27) Rudzinski, J. M.; Osawa, E. J.Phys.-Org. Chem. 1992,5,382394.
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1, 12-DimethyUl.llferrocenophane
although a number of problems are currently encountered, such as the lack of well-developed molecularmechanics parametrization, the size (220 electrons), and the notorious difficulties in ab initio calculations on ferrocene itself.28 One might, however, hope that the experimental data gathered so far in the [l.llferrocenophane system and the parallels that can be drawn with the cyclohexane system will be an incentive to advance the computational methods sufficiently in order t o handle this kind of system. (28) Park, C.; Almlbf, J. J. Chen. Phys. 1991,95, 1829-1833.
Acknowledgment. We thank Professor Per Ahlberg for fruitful discussions and the Swedish Natural Science Research Council (NFR) for financial support. Supporting InformationAvailable: Tables of anisotropic thermal parameters for the non-hydrogen atoms, all bond distances and angles, intermolecular contacts, and leastsquares planes for IC (20 pages). Ordering information is given on any current masthead page. A QUICKTIME film illustrating the pseudorotation of [ l .llferrocenophane has been produced. Contact J.-M.L. for information on how to retrieve it. OM9500511
