Electronic Lock in Chromiumtricarbonyl Complex of Cycloheptatrienyltin

Article abstract of DOI:10.1021/ja0379621

The fast sigmatropic [1,5]-Sn migration in cycloheptatrienyl(triphenyl)tin is effectively locked in the chromiumtricarbonyl complex. A much slower [1,3]-Sn + [1,2]-Cr diatropic rearrangement was detected by spin saturation transfer NMR. Copyright

Full text of DOI:10.1021/ja0379621

Published on Web 11/08/2003
“Electronic Lock” in Chromiumtricarbonyl Complex of Cycloheptatrienyltin
Ilya D. Gridnev*,† and Oleg L. Tok‡
COE Laboratory, Department of Chemistry, Graduate School of Science, Tohoku UniVersity,
Sendai 980-8578, Japan and Laboratory of Inorganic Chemistry, UniVersity of Bayreuth,
D-95440 Bayreuth, Germany
Received August 15, 2003; E-mail: igridnev@mail.cc.tohoku.ac.jp
Metallotropic rearrangements are useful for probing our under-
standing of reactivity and selectivity in chemistry. The minimized
importance of the solvent effects and any other intermolecular
interactions simplifies the overall experimental outcome, making
possible more clear-cut conclusions on the reaction mechanism
compared to the solvent-assisted multicomponent transformations.
We report here a new phenomenon of locking the fast [1,5]-Sn
migrations in cycloheptatrienyl(triphenyl)tin (1) by involving the
π-electrons of cycloheptatrienyl ring into the coordination with the
chromiumtricarbonyl group (in complex 2).
Organometallic derivatives of cycloheptatriene exhibit various
types of intramolecular dynamics. A η1-bonded organometallic
substituent can migrate around the seven-membered ring by the
mechanism of a [1,j] sigmatropic rearrangement.1 On the other hand,
Figure 1. Molecular structure (30% thermal ellipsoids) of complex 2.
ηx (x > 1)-bonded organometallic groups often undergo haptotropic
shifts around the π-system of cycloheptatriene.2 The most interesting
situation emerges when a cycloheptatriene moiety bears several
organometallic groups, since such molecules provoke an interesting
problem of simultaneous intramolecular shift of both substituents
(diatropic migrations), which has been scarcely studied thus far.
In their pioneering work,3 Cotton and Reich analyzed in detail
the temperature-dependent 1H NMR spectra of the bimetallic µ-η3:
η4-Cp(CO)2Mo(C7H7)Fe(CO)3 complex and concluded that the most
probable mechanism of the observed fluxionality is the simultaneous
[1,2] shift of both metals with the probable admixture of the [1,3]
shift mechanism.3 Later, a series of cycloheptatrienyl-bridged homo-
and heterobimetallic complexes with two ηx (x > 1)-bonded
organometallic groups was studied.2a,4-7 All syn-bridged complexes
exhibit ring whizzing of the cycloheptatrienyl moiety,4-7 which in
most cases is too rapid to be frozen in the NMR experiments.4 Some
of these compounds were reported to give low-temperature-limiting
1H and 13C spectra at 190-200 K;5-7 however, no mechanistic
studies have been made thus far.
Selected bond lengths: C(1)-C(2) ) 1.494(4), C(1)-C(7) ) 1.496(5),
C(2)-C(3) ) 1.382(4), C(3)-C(4) ) 1.416(6), C(4)-C(5) ) 1.393(5),
C(5)-C(6) )1.416(4), and C(6)-C(7) ) 1.375(4) Å. A slight bond
alternation is observed in the cycloheptatriene ring. The atoms C(2)-C(3)-
C(4)-C(5)-C(6)-C(7) are almost planar (within 0.0453 Å), and the C(1)-
C(2)-C(7) plane is inclined by 45.63° from the ring.
Ph3Sn(C7H7)Cr(CO)3 2, and the mechanism of diatropic rearrange-
ment in this compound.
Synthesis and Stereochemistry of Complex 2. Reaction of
cycloheptatrienyl(triphenyl)tin (1) with (CH3CN)3Cr(CO)3 in boiling
benzene occurred rapidly and according to NMR control resulted
in complete transformation of 1 to 2 in 20 min (Scheme 1).
Scheme 1
In (η1:η4) complexes Me3XC7H7Fe(CO)3 (X ) Si, Ge), relatively
facile [1,3]-Fe haptotropic shifts have been characterized, whereas
sigmatropic migrations of Me3X groups were not detected.8 In
contrast, in Pr2B(C7H7)Fe(CO)3 a diatropic rearrangement involving
simultaneous [1,7]-B and [1,2]-Fe migrations is significantly faster
than the [1,3]-Fe haptotropic shift and [1,3]-B sigmatropic migra-
tion.9
Recrystallization of the raw material from ether yielded complex
2 as a purple solid in 65% yield. At ambient temperature, the H
NMR spectrum of 2 is well resolved and shows the presence of a
(η1:η6)-bonded cycloheptatriene unit. The 8.6 Hz coupling between
H4 and H1 attests for the exo configuration of 2.11 This was explicitly
proved by X-ray structure of 2 shown in Figure 1.
1
Dynamic Behavior of Complex 2. A facile [1,5]-Sn sigmatropic
shift occurs in triphenyl(cycloheptatrienyl)tin 1.1a-d Activation
Several examples of (η1:η6) complexes of cycloheptatriene have
been recently described.10 The results of a 13C spin saturation
transfer (SST) experiment for the [Mo(CO)3(µ-η6:σ1-C7H7)Ru(CO)2-
Cp] were interpreted in favor of the 1,2-shift process, whereas the
barrier of this dynamic process (∆Gq300 ) 15.4 ( 0.1 kcal mol-1 1d
)
is low enough to cause an extensive broadening of the resonances
1
in the H NMR spectrum of 1 at 348 K (Figure 2). On the other
hand, the line shape in the 1H NMR spectrum of complex 2 at the
same temperature is only slightly affected by the dynamic effects.
SST experiments on protons suggest that the 1,3-diatropic rear-
rangement occurs (Figure 3).
1
observations of the line shape changes in the H NMR spectra of
other compounds could not be satisfactorily accounted for. Here
we report the synthesis of a new cycloheptatriene (η1:η6) complex,
Thus, the diatropic migrations in (η1:η6) complex 2 proceed much
slower and by different topology compared to the 1,5-Sn shift in
† Tohoku University.
‡ University of Bayreuth.
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J. AM. CHEM. SOC. 2003, 125, 14700-14701
10.1021/ja0379621 CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
of the Ph3Sn group is exactly the same as in the compound 1.
Therefore, the reason for the significantly slower sigmatropic
migrations of tin in 2 must be purely electronic. Any diatropic
rearrangement in 2 requires a reorganization of the π-system of
the cycloheptatriene ring and, therefore, significant perturbations
of the π-donative bonds between the cycloheptatrienyl ring and
the chromium atom. We conclude that this is the reason for the
significantly decreased mobility of the triphenylstannyl group. This
corresponds to the topology of the observed rearrangements, since
the [1,3] migration requires the reorganization of a single double
bond compared to two double bonds in the case of the [1,5]-Sn
migration or three in the case of the [1,7]-Sn shift.
In conclusion, we have shown that the chromiumtricarbonyl
group acts as a selective “electronic lock” upon coordination to
cycloheptatrienyltin, efficiently blocking the [1,5]-Sn migrations
but participating in a slower [1,3]-Sn + [1,2]-Cr rearrangement.
Further work on diatropic rearrangements in cyclic organometallic
systems is in progress.
Figure 2. “Electronic lock”: Comparison of the 1H NMR spectra (300
MHz, benzene-d6, 348 K) of cycloheptatrienyl(triphenyl)tin 1 (below) and
its chromiumtricarbonyl complex 2 (above).
Acknowledgment. This work was financially supported by the
Chemistry COE Program of Tohoku University. We thank Dr.
Chizuko Kabuto (Tohoku University) for the X-ray analysis of 2.
Supporting Information Available: Experimental details (PDF)
and crystallographic analysis of 2 (CIF). This material is available free
of charge via the Internet at http://pubs.acs.org.
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Figure 3. Difference SST spectra of complex 2 (300 MHz, benzene-d6,
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A sigmatropic shift of the Ph3Sn group in 2 would induce only
slight (if any) movement of the Cr(CO)3 moiety, and the motion
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