Evidence for a 1.2-shift mechanism of the 1-5-η-cycloheptatrienyl moiety with respect to the central iron atom of the title compound is presented together with absolute assignments of the ^(13)C n.m.r. chemical shifts of the C_(7)H_(7) ring. A low-temperature rocking motion of both rings can be frozen out at -70 °C

Blackborow, J. Richard

Grubbs, Robert H.

Hildenbrand, Knut

Miyashita, Akira

Scrivanti, Alberto

von Gustorf, E. A. Koerner

English

J. Richard Blackborow

Robert H. Grubbs

Knut Hildenbrand

E. A. Koerner von Gustorf

Akira Miyashita

Alberto Scrivanti

Crossref

Mechanism of the fluxional behaviour in (1–5-η-cycloheptadienyl)-(1–5-η-cycloheptatrienyl)iron

Caltech Authors - Main

1977 2206 
Mechanism of the Fluxional Behaviour in (1 -5-q-Cycloheptadieny1)- 
(I -5-q-cycloheptatrienyl) iron 
By J. Richard Blackborow, Robert H. Grubbs, Knut Hildenbrand," the late E. A. Koerner von Gustorf, 
Akira Miyashita, and Alberto Scrivanti, lnstitut fur Strahlenchemie im Max-Planck-lnstitut fur Kohlen- 
forschung, D-4330 Mulheim-Ruhr, Stiftstrasse 34-36, West Germany 
Evidence for a 1.2-shift mechanism of the 1 -5-q-cycloheptatrienyl moiety with respect to the central iron atom of 
the title compound is presented together with absolute assignments of the 13C n.m.r. chemical shifts of the C,H7 
ring. A low-temperature rocking motion of both rings can be frozen out a t  -70 "C. 
THE previously reported 1 compound [Fe(y5-C7H,)!y5- assigned by comparison with the lH spectrum of the 
C7H,)] (1) exhibits two distinct types of fluxional motion isoelectronic Ru compound and by proton-proton 
over two discrete temperature ranges. It is the purpose double-resonance experiments (Table 2). Increasing the 
of this paper to  describe these processes, principally temperature from -70 to -10 "C caused 12 of the 14 
from evidence obtained from variable-temperature 13C carbon-13 signals to coalesce pairwise whilst two remain 
n.m.r. studies but also with reference to the X-ray unaltered (Figure 2). The explanation of this behaviour 
structure of the compound and its l H  n.m.r. spectra. might be analogous to the description of similar effects 
Fe 
v 
FIGURE 1 Side view of X-ray structure of molecule 1 
RESULTS AND DISCUSSION 
The solid-state structure of (1) is illustrated in Figure 1. 
It contains 14 carbon and 16 hydrogen atoms which are 
all spatially inequivalent. It is particularly interesting 
that the 'open '  non-bonded faces of both rings are 
skewed at  ca. 50" to each other. In  agreement with this 
the n.m.r. spectra of (1) in solution (CDCl, or C6D,CD3) 
showed 14 inequivalent carbon-13 signals and 16 partly 
overlapping inequivalent proton signals a t  low temper- 
atures (Tables 1 and 2). Proton resonances were 
J.  R. Blackborow, K. Hildenbrand, E. Koerner von Gustorf, 
C. Kruger, A. Scrivanti, C. R. Eady, and D. Entholt, J.C.S. Chem. 
Comm., 1976, 16. 
J. Muller, C. G. Kreiter, B. Mertschenck, and S. Schmidt, 
Chem. Ber., 1975, 108, 273. 
in the lH n.m.r. spectra of bis(cyclohexadieny1)iron 
corn pound^:^^^ we can assume the coexistence of two 
enantiomeric forms of (1) in solution, one of which is 
described by the solid-state structure and schematically 
depicted in Figure 3. I t  is evident that by an internal 
rotation of the two rings relative to each other by an 
angle of ca. 2 x 50" = 100" these structures are 
interconverted (Figure 3). Warming increases the rate 
of this rocking motion and apparent planes of symmetry 
are induced in both rings on the n.m.r. time scale. The 
two unchanged 13C signals can then be assigned to C6 and 
J. F. Helling and D. M. Braitsch, J .  Amer. Chem. Soc., 1970, 
92, 7209. 
L. M. Jackman and F. A. Cotton, ' Dynamic Nuclear Mag- 
netic Resonance Spectroscopy,' Academic Press, New York, 1975. 
2206 J.C.S. Dalton 
C8 atoms through which the apparent symmetry planes 
must pass. A similar coalescence process was observed 
in the proton spectrum (Table 2). The difference in the 
overall position of the coalesced proton signals with 
respect to their components is attributed to the different 
magnetic shielding experienced by the protons due to a 
small temperature-dependent amplitude change of the 
arc described by the free double bond as the rings rock 
with respect to each other. 
Further increase in temperature from -10 to +70 "C 
caused the partial collapse of four of the eight 13C n.m.r. 
signals (Figure 4) in a way which can be best assigned to 
- 5 5 O C  
-65'C 
-65OC 
I I I I 1 
130 100 70 40 10 
8 l p . p .  m. 
FIGURE 2 (a) - 65 "C; l H  off-resonance decoupling; thirty peaks 
(twelve doublets, two triplets); (b )  -65 "C; 1H noise-decoupling; fourteen peaks; (c) -55 "C and ( d )  -10 "C; lH noise-decoup- 
ling ; coalescence from fourteen signals to eight (twelve pairwise coalescences, two remaining unaltered). Spectral assignment and 
chemical shifts can be obtained from Table 1. 
Low-temperature 13C spectra of [Fe(qs-C7H,) (q5-C7H,)] in CDCl,; 
Pulse sequence 0.6 s, pulse angle 30", 5 000 scans 
1977 2207 
TABLE 1 
Carbon- 13 n.1n.r. spectra of [Fe(q5-C7H7) ($-C,H,)] in 
[2H,]toluene with 1H noise decoupling. Chemical 
shifts are with respect t o  SiMe, (6 = 0) 
-65 "C -10 "C 
7-
Number Number 
6 carbons ment 6 carbons 
of Assign- of 
80 "C 
c-h-7 
Number 
of 
6 carbons 
34.3 
72.9 
x9.0 
91.0 
133.2 h 2 
a Horizontal lines indicate observed coalescence processes. 
Q Broadened by exchange above - 10 *C and unobservable a t  
80 "C. 
a fluxional revolution of the q5-C7H7 ring with respect to 
the iron atom. The collapse of the four 13C signals was 
motion in four different ways: as a 1,Z;  1,3; 1,4; or 
random shift of the uncomplexed double bond with 
8 z 50° 
1 J I G u m  3 Scheme of one of the two interconverting fornis of (1) 
Note that the planes do not imply symmetry of in solution. 
the electronic environments of the carbon nuclei 
respect to some particular direction about which the 
cycloheptadienyliron moiety is We first estab- 
lished that during this motion both rings remained q5- 
bonded. This can be seen by inspection of the 13C n.m.r. 
signals of the l-5-q-cycloheptadienyl residue which 
remain unchanged throughout. We then simulated the 
four distinct mechanisms together with a fifth (an equal 
mixture of the first four) on a computer, using the formal- 
ism developed by K ~ b o ~ ~  and by Sack7b for %-site 
exchange. The lineshapes of the l3C resonances of the 
(1-5-7&H7) moiety can be described by a 4 x 4 
matrixs in the temperature region where the shift 
mechanism is observed. 
Because of intensity problems the temperature- 
dependent spectra were obtained from proton-noise- 
decoupled 13C n.m.r. spectra. In general this is bad 
practice, since the Overhauser effects must be assumed to 
be of the same magnitude for each signal. However, 
the distinction between the proposed mechanism and the 
TABLE 2
Hydrogen-1 n.m.r. (T) of (1) in [2H,]toluene 
-65 "C -10 "C 80 "C 
bosition Inte- Position Integral [Assignment] Position Integral [Assignment] 
A A r \ r 7 
10.0 l [Hll (H12)] 
8.8 1 [H12 (Hll)] 
8.4 2 [Hll* 12)] 8.6 2 [HIIS 1 2 1  b 8.6 2 [Hlh 121  Q 
7.1 1 [H9 (H14)] 7.5 2 [HI1? 12] 7.6 2 [Hlh 121 G 
6.4 1 [H14 (H9)] 6.8 2 [H1, H4] 
4.4 2 [H3 (H2), aH8] 5.0 2 [H2, H3] 5.6 7 [HI-'] 
5-5.7 7 [H1, H2 (H3), H4, H5 (H'), H6, 5.5- 7 [H5, H6, H7, H9, Hf4 5.65 5 [H8-l0, HI3, H14] 
H10, HI31 d 6.07 HIO, H131 
3.3 1 [H7 (H5)] 4.2 1 [Hal 
'8 See Figure 1. b endo to  iron. C exo to  iron. d Not unambiguously assigned. 
not complete before the compound decomposed, but was, 
however, reversible up to 90 "C. 
It is possible analytically to describe such a fluxional 
5 F. A .  Cotton and D. L. Hunter, J .  Amer. Chem. Soc., 1976, 
6 F. A. Cotton, D. L. Hunter, and P. Lahuerta, J ,  A m e v .  
other possibilities was clear enough for us to disregard 
slight changes in intensity due to Overhauser effects. 
Figure 5 shows the difference between the five distinct 
(a)  R. Kubo, Nuovo Cimento SuppZ., 1957,6, 1063; (b )  R. A. 
Sack, Mol. Phys., 1958, 1, 163. 
See e.g. G. M. Whitesides and J.  S. Fleming, J .  Amer. Chern. 
SOG., 1967, 89, 2855. 
98, 1413. 
Chem. Soc., 1974, 96, 7926. 
2208 J.C.S. Dalton 
mechanisms when one particular assignment for the 
13C signals of the C7H7 ring was taken. (By inspection 
the C6 and C2,C3 signals can easily be assigned; however, 
C1,C4 and C5,C7 can be interchanged.) The lower part 
of Figure 5 shows the effect of the interchange of C5,C7 
and C1,C4 for the 1,Z-shift mechanism; the two cases are 
quite distinct. We therefore simulated five different 
mechanisms (1,2; 1,3; 1,4; random; and mixed) 
I I I 1 
130 100 70 40 
G/p.p.m. 
FIGURE 4 High-temperature l3C spectra of [Fe(yS-C,H,)- 
(y5-C,H,)] in [2H8]toluene; (a) 23 "C; (b)  60 "C; and (c) 90 "C. 
1H noise-decoupling; fluxional collapse of 13C signals of the 
(+C,H,) -ring (indicated by arrows). Spectral assignment and 
chemical shifts can be obtained from Table 1. Pulse sequence 
0.6 s, pulse angle 30", 6 000 scans; S = solvent 
with the two possible 13C shift assignments. The only 
'~ close fit ' from the 10 possible was obtained with a 
1,2-shift mechanism with the 13C assignments given in 
Table 4. Figure 6 illustrates two representative spectra 
and spectral simulations using this mechanism. The 
clear point of distinction is that the left-hand signal 
broadens more slowly than the right-hand signal, and 
that the middle signals eventually become less broadened 
than either of the others. Using the 1,2-shift mechan- 
ism with the other possible l3C assignment, the middle 
signal becomes eventually less broadened than the right 
but more broadened than the left-hand signal. In all 
the other possible mechanisms the left-hand signal is 
I It I 
133.2 95.1 68.3 
94.2 
6 
A comparison of the simulations of the possible 
mechanisms of exchange. (a) 1,2 shift, (b )  random shift, (c) 
1,4 shift, and ( d )  1,3 shift. ( e )  and (f) represent the 1,2 shift 
mechanism a t  two different exchange rates. The simulation 
using the correct chemical-shift assignment of C4,C1 and C5,C7 
(full line) is compared with the simulation using the false 
assignment (dotted line) ; the dotted-line spectrum has been 
off-set to  the right for purposes of clarity. Note that the 1,2 
shift mechanism is the only one with the low-field peak less 
broadened than the high-field peak 
FIGURE 5 
more broadened than the right-hand signal. Kinetic 
parameters of the two fluxional processes and the assign- 
ments of the 13C resonances of the C,H,-ring are presented 
in Tables 3 and 4. 
k = 50 S" 1 
I 
I I I 42OC 
I I I  I1 I I  
95.1 91.0 72.9 68.3 
94.2 89.0 
133.2 
6 
FIGURE 6 Actual and simulated 13C spectra using the 1,2 mech- 
anism with chemical-shift assignments given in Table 1 ; 
( a )  42 "C and (b)  62 "C; pulse sequence 0.6 s;  pulse angle 30"; 
5 000 scans; S = solvent ([2H8]toluene); 0 = impurities 
1977 2209 
So far only three other q5-cycloheptatrienylmetal 
complexes have been described, namely [Ru( 1-5-q- 
C7H7)(1-5-q-C7H9)],2 [Fe( 1-5-q-C7H7)(CO),]+,g~10 and 
[Mn(l-5-q-C7H7) (CO),].lo The n.m.r. spectra of these 
species are strongly temperature dependent but no 
TABLE 3 
Kinetic parameters of the exchange processes of 
[Fe(q6-C7H,) (16-C,H9)1 
' Rocking ' motion t 
' 1,2-ShiftJ motion 
Et = 40 kJ mol-l a 
log A = 12.5 
Et = 71 kJ mol-l 
logA = 13.1 
t This is a simple two-site exchange problem. 
Error f 6 k J mol-l. 8 Error & 10 k J mol-l. 
TABLE 4 
Assignment of the l3C resonances of the C7H7 ring 
8lp.p.m. Correct Incorrect 
133.2 c3, c2 c3, ca 
68.3 c4, c1 c5, c7 
94.2 c5, c7 c4, c1 
95.1 C6 C6 
detailed investigations of the mechanisms of rearrange- 
ments have been reported.* 
1,2-Shift mechanisms have been well established how- 
ever in the q4- and -@-metal complexes [M(1--4-q- 
C,H,)(CO),] (M = Fe or Ru) (2) and [M(1-6-q-tetra- 
methyl-C,HJ(CO),] (M = Cr, Mo, or W) (3), with 
activation energies of 34 k J mol-l in [(2) ; M = Fe5] and 
66 kJ mol-l in [(3); M = Cr].4 
The latter value is of the same order of magnitude as 
the value we determined for the activation energy of the 
1,2-shift in (1). 
* Note added in $Yoof: while this paper was in preparation, a 
1,2-shift mechanism has been established for the rearrangement 
of [Mn( 1-5-q-C,H,) (CO),] with an activation energy of about 
63 kJ mole1 (T. H. Whitesides and R. A. Budnik, Inorg. Chew., 
1976, 15, 874). 
As the electronic structures of (1) and (2) or (3) are 
quite different and the structures of the transition states 
of this type of rearrangements are unknown, any explan- 
ation of the behaviour of the kinetic parameters is 
bound to be very speculative. So for further discussions 
we should like to refer t o  lit. cit. (33) inref 5. 
EXPERIMENTAL 
All the work-up procedures were carried out under argon 
(99.997%). Iron (2 g) was co-condensed with cyclohepta- 
triene (40 cm3) on a hexane matrix a t  - 196 "C over a period 
of 1 h in an apparatus described previously. The reaction 
mixture was allowed to warm to room temperature and 
hexane and cycloheptatriene were removed from the filtered 
solution in vacw a t  0 "C. The resulting red solid was 
recrystallised from pentane a t  -80 "C (Found: C, 70.0; 
H, 6.65; Fe, 23.3. Calc. for Cl,H1,Fe: C, 70.0; H, 6.70; 
Fe, 23.3%). 
N.m.r. samples were run in [2H,]toluene in sealed evacu- 
ated 5- or 10-mm tubes on a Bruker W.H. 90 spectrometer. 
The temperature controller was calibrated with ethanol or 
ethylene glycol in the usual way. Operations were per- 
formed in the Fourier-transform mode a t  90 MHz for 
hydrogen and 22.3 MHz for carbon, using solvent-deuterium 
locking. Samples of differing concentration (0.14, 0.2, and 
0.3 mol dmW3) exhibited identical temperature dependences 
within experimental error. Errors in Table 3 for E were 
estimated graphically from a considered temperature error 
of f 2  "C and a rate error of &6%. 
We thank Drs. Mark, Bausch, and Grevels a t  the Institut 
fur Strahlenchemie for helpful discussions. One of us 
(R. H. G.) thanks the Alexander von Humboldt Stiftung for 
a research fellowship. 
[6/1522 Received, 4th August, 19761 
R. Pettit, J .  Amer. Chem. Soc., 1964, 86, 2589. 
lo T. H. Whitesides and K. A. Budnik, Chem. Comm., 1971, 
1514. 


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Mechanism of the fluxional behaviour in (1–5-η-cycloheptadienyl)-(1–5-η-cycloheptatrienyl)iron

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