Octahedral gyroscope-like molecules with M(CO)3(X) rotators encased in three-spoked diphosphine stators: Syntheses by alkene metathesis/hydrogenation sequences, structures, dynamic properties, and reactivities

Article abstract of DOI:10.1021/om700625u

Threefold ring-closing metatheses of mer,trans-Re(CO)₃(X) (P((CH₂)₆CH=CH₂)₃)₂ (X = Cl, Br), followed by hydrogenations, yield the gyroscope-like species Re(CO)₃(X)-(P((CH₂

Full text of DOI:10.1021/om700625u

Organometallics 2007, 26, 5129-5131
5129
Octahedral Gyroscope-Like Molecules with M(CO)3(X) Rotators
Encased in Three-Spoked Diphosphine Stators: Syntheses by
Alkene Metathesis/Hydrogenation Sequences, Structures, Dynamic
Properties, and Reactivities
Gisela D. Hess,† Frank Hampel,† and John A. Gladysz*,†,‡
Institut fu¨r Organische Chemie and Interdisciplinary Center for Molecular Materials,
Friedrich-Alexander-UniVersita¨t Erlangen-Nu¨rnberg, Henkestrasse 42, 91054 Erlangen, Germany, and
Department of Chemistry, Texas A&M UniVersity, P.O. Box 30012, College Station, Texas 77842-3012
ReceiVed June 22, 2007
Scheme 1. Previous Syntheses of Gyroscope-Like
Summary: Threefold ring-closing metatheses of mer,trans-Re-
Complexes in Which trans-Phosphorus Atoms Are Linked by
(CO)3(X)(P((CH2)6CHdCH2)3)2 (X ) Cl, Br), followed by
Three Aliphatic Bridges
hydrogenations, yield the gyroscope-like species Re(CO)3(X)-
(P((CH2)14)3P), which react with NaI or Ph2Zn to giVe substitu-
tion products; crystal structures haVe been determined, and low-
temperature NMR data indicate rapid rotation of the Re(CO)3(X)
moieties.
There is rapidly increasing interest in syntheses and applica-
tions of molecular devices that incorporate rotating components.1-3
Such “rotors” can in turn be dissected into “rotators” and
“stators”, the latter being assigned to the moiety with the greater
moment of inertia.1 We have had an ongoing interest in metal-
based rotors that approximate the connectivities and symmetries
of toy gyroscopes.4-8 As sketched in Scheme 1, these have been
accessed from precursors with trans-phosphine ligands (I) via
alkene metathesis/hydrogenation sequences. The overall yields
of the cagelike diphosphine adducts II can be remarkably high,
reflecting the extraordinary effectiveness of modern alkene
metathesis catalysts. Conceptually related nonmetallic systems
have been studied in detail by Garcia-Garibay.2
interligand metathesis. We then found that analogous sequences
could be carried out with square-planar species, provided that
the non-phosphine ligands were small (Cl, CO).5,7 However,
consistent with the “mismatched” metal fragment and phosphine
symmetries (C2 vs C3 axes), the products IV were obtained in
lower yields, or were not detected at all for certain ring sizes.
The question remained as to whether this methodology might
be extended to other coordination geometries with mismatched
symmetries. In this communication, we report the surprisingly
successful application of such sequences to octahedral systems
that feature four non-phosphine ligands, as well as key structural,
dynamic, and chemical properties of the resulting complexes
V.
In standard procedures that have been applied to related
complexes,9 the rhenium pentacarbonyl halides (CO)5Re(X) (X
) Cl, Br) and 2.0-2.1 equiv of the alkene-containing phosphine
P((CH2)6CHdCH2)3 were reacted in chlorobenzene at 140 °C.
Workups gave the substitution products mer,trans-Re(CO)3(X)-
(P((CH2)6CHdCH2)3)2 (X ) Cl (1), Br (2)), shown in Scheme
2, in 70-73% yields. A reaction of 1 and NaI (THF/acetone,
reflux) afforded the corresponding iodide complex 3. These and
all other new complexes below were characterized by mi-
croanalysis and by IR and NMR (1H, 13C, 31P) spectroscopy,
as summarized in the Supporting Information. The mer relation-
Our first successes involved trigonal-bipyramidal iron tricar-
bonyl species (III; Scheme 1).4 Since both the Fe(CO)3 fragment
and phosphine ligands in precursor I possess local C3 axes, the
energetically favorable doubly staggered conformation I′ is
possible. This preorganizes the vinyl groups for intramolecular,
* To whom correspondence should be addressed at Texas A&M
University. E-mail: gladysz@mail.chem.tamu.edu.
† Universita¨t Erlangen-Nu¨rnberg.
‡ Texas A&M University.
(1) Kottas, G. S.; Clarke, L. I.; Horinek, D.; Michl, J. Chem. ReV. 2005,
105, 1281.
(2) (a) Khuong, T.-A. V.; Nun˜ez, J. E.; Godinez, C. E.; Garcia-Garibay,
M. A. Acc. Chem. Res. 2006, 39, 413 and earlier work from this group
cited therein. (b) Khuong, T.-A. V.; Dang, H.; Jarowski, P. D.; Maverick,
E. F.; Garcia-Garibay, M. A. J. Am. Chem. Soc. 2007, 129, 839. (c) Jarowski,
P. D.; Houk, K. N.; Garcia-Garibay, M. A. J. Am. Chem. Soc. 2007, 129,
3110.
(3) For additional leading references to an extensive literature, see: (a)
Kay, E. R.; Leigh, D. A.; Zerbetto, F. Angew. Chem., Int. Ed. 2007, 46,
72; Angew. Chem. 2007, 119, 72. (b) Shirai, Y.; Morin, J.-F.; Sasaki, T.;
Guerrero, J. M.; Tour, J. M. Chem. Soc. ReV. 2006, 35, 1043.
(4) Shima, T.; Hampel, F.; Gladysz, J. A. Angew. Chem., Int. Ed. 2004,
43, 5537; Angew. Chem. 2004, 116, 5653.
(5) Nawara, A. J.; Shima, T.; Hampel, F.; Gladysz, J. A. J. Am. Chem.
Soc. 2006, 128, 4962.
(6) Wang, L.; Hampel, F.; Gladysz, J. A. Angew. Chem., Int. Ed. 2006,
45, 4372; Angew. Chem. 2006, 118, 4479.
(7) Wang, L.; Shima, T.; Hampel, F.; Gladysz, J. A. Chem. Commun.
2006, 4075.
(8) Skopek, K.; Hershberger, M. M.; Gladysz, J. A. Coord. Chem. ReV.
2007, 251, 1723.
(9) Bauer, E. B.; Hampel, F.; Gladysz, J. A. AdV. Synth. Catal. 2004,
346, 812.
10.1021/om700625u CCC: $37.00 © 2007 American Chemical Society
Publication on Web 09/05/2007

5130 Organometallics, Vol. 26, No. 21, 2007
Communications
Scheme 2. Syntheses of Title Complexesa
-75 °C, respectively (toluene-d8). However, no decoalescence
was observed upon further cooling to -100 °C.
We sought to assay for any ligand cage effects on substitution
rates. Thus, in side-by-side experiments, the reactions of the
chloride complexes 1 and 6 with NaI in refluxing acetone/THF
were monitored by NMR. Surprisingly, the acyclic and gyroscope-
like iodide complexes 3 and 8 formed with virtually identical
rates (conversions: 4.25 h, 23-26%; 6.50 h, 32-34%; 11.75
h, 58-59%). Although further study is required, we view this
as inconsistent with a rate-determining step that involves iodide
association. Similar substitutions have been effected with other
nucleophiles (e.g., thiolates), and these will be detailed in our
full paper.
Structural data were sought. Crystals of 6, 7, and 9 (or
solvates thereof) could be grown. The X-ray structures were
solved as described in the Supporting Information, and thermal
ellipsoid diagrams are collected in Figure 1. The bromide ligand
in 7 was disordered into the two cis positions (70:15:15), but
only the dominant conformation is depicted and analyzed below.
The unit cell of 9 exhibited four independent molecules with
similar conformations, all of which are included in the average
values given below.
The P-Re-P angles in the three structures range from 169.4
to 174.5° (average 171.3°), indicating slightly more distorted
octahedral coordination geometries as compared to analogues
without trans-spanning diphosphine ligands (177-180°);10
angles between the other trans ligands average 176.4°. The trans
P(CH2)3 moieties are approximately eclipsed, as can be seen in
Figure 1 (upper right) and quantified by the corresponding CH2-
P-P-CH2 torsion angles (15.6-35.3° for the syn segments;
average 24.2°). In each structure, the non-carbonyl ligand
approximately bisects two macrocycles. For the halide com-
plexes 6 and 7, this translates into two X-P-P-CH2 torsion
angles of 48.3-48.9° and two of 70.3-71.5° (sum ca. 120°).
As a result, two of the OC-P-P-CH2 torsion angles for the
carbonyl ligand trans to the halide ligand are close to 0° (11.0-
14.0°).
The halide complexes 6 and 7 crystallize in identical space
groups (P21/n) and with very similar lattice constants (Z ) 4,
V ) 4739.02(19) and 4765.79(15) Å3) and macrocycle confor-
mations (Figure 1, upper and lower left). There are two sets of
molecules with parallel P-Re-P axes, but with a nonparallel
relationship to each other. Depending upon the exact measure
employed, the axes define angles of 31-34°. The phenyl
complex 9 crystallizes in a more complicated lattice, with four
sets of molecules that will be more fully analyzed in our full
paper. Interestingly, the DSC trace of 6 showed a significant
endotherm, not corresponding to melting, at 41 °C.
a Legend: (a) 5 mol % Grubbs’ catalyst, chlorobenzene, N2 stream;
(b) 10 mol % PtO2, 5 bar of H2, THF; (c) NaI, THF, acetone, reflux;
(d) Ph2Zn, THF, reflux.
ship of the carbonyl ligands was reflected by the IR νCO pattern,
and the trans relationship of the phosphine ligands was indicated
by the single 31P NMR signal.
Chlorobenzene solutions of 1 and 2 (0.0011-0.0012 M) were
treated with Grubbs’ catalyst (5-6 mol %) and aspirated with
N2 to aid removal of the byproduct ethene. After 2 days, alumina
filtrations gave the crude metathesis products (4 and 5) in 76-
90% yields. Mass spectra showed the expected parent ions.
Hydrogenations (5 bar, PtO2) afforded the target gyroscope-
like molecules 6 and 7 (Scheme 2) in 81-41% yields (overall:
6, 61%; 7, 37%). The two most intense IR νCO bands were only
slightly shifted versus those of 1 and 2 (0-8 cm-1), indicating
a small ligand cage effect. Ambient-temperature 13C NMR
spectra exhibited (1) two CO signals (2:1 area ratio) that were
phosphorus-coupled and (2) seven CH2 signals, or one type of
P(CH2)14P bridge. If Re(CO)3(X) rotation were slow on the
NMR time scale, two sets of CH2 signals (2:1) should be
observed.
The structural data provide a framework for analyzing the
barriers to Re(CO)3(X) rotation. The rhenium-oxygen (ReCO)
distances in 6 and 7 range from 2.96 to 3.08 Å. When the van
der Waals radius of oxygen is added (1.52 Å),11 a maximum
value of 4.60 Å is obtained. The rhenium-chloride and
-bromide distances are somewhat shorter (2.53 and 2.66 Å),
and when the van der Waals radii of the halides are added (1.75
and 1.85 Å), slightly lower values are obtained (4.28 and 4.51
Å). Hence, the effective radius of the rotator in 6 and 7 can be
taken as 4.60 Å.
Substitution reactions were attempted. As shown in Scheme
2, 6 and 7 were combined with NaI in refluxing acetone. After
1-8 days, workups gave the corresponding iodide complex 8
in 34-75% yields. Similarly, 6 and Ph2Zn were combined in
refluxing THF. After 2 days, workup gave the phenyl complex
9 in 85% yield. The 13C NMR spectrum of 8 also exhibited
seven CH2 signals. However, that of 9 exhibited 12 (theory:
14), indicating that the phenyl ligand brakes the rotation of the
Re(CO)3(X) moiety. Low-temperature 13C NMR spectra of 7
and 8 were recorded. The CH2 signals broadened at -40 and
Distances can also be calculated from rhenium to the remote
carbon atoms of the macrocycle that are closest to the plane of
(10) Structurally characterized complexes of the formula mer,trans-Re-
(CO)3(Cl)(PR3)2: (a) Bucknor, S.; Cotton, F. A.; Falvello, L. R.; Reid, A.
H.; Schmulbach, C. D., Jr. Inorg. Chem. 1986, 25, 1021 (R ) Et). (b) Florke,
U.; Haupt, H.-J. Z. Kristallogr. 1993, 204, 316 (R ) Ph).
(11) Bondi, A. J. Phys. Chem. 1964, 68, 441.

Communications
Organometallics, Vol. 26, No. 21, 2007 5131
Figure 1. Molecular structures of 6 (top left and right), 7 (bottom left; dominant Re(CO)3(Br) rotamer), and 9‚4THF (bottom right; one
of four similar independent molecules in the unit cell).
the rotatorsi.e., the two carbon atoms in the middle of the
methylene chain (6.54-7.25 Å). When the van der Waals radius
of sp3 carbon (1.70 Å)11 is subtracted from the shorter distance,
a minimum clearance is obtained (4.84 Å). Thus, there is
sufficient horizontal clearance for rotation in the halide com-
plexes. However, with the phenyl complex 8, analogous
calculations indicate an effective rotator radius of 7.24 Å
(average of four molecules in the unit cell, including the van
der Waals radius of hydrogen). Hence, there is insufficient
clearance for rotation, in accord with the 13C NMR data.12
Other considerations are relevant to the apparently facile Re-
(CO)3(X) rotation in 6 and 7. We showed earlier that the barrier
to Rh(CO)(I) rotation in complexes of the type IV (Scheme 1)
is much lower than that for Rh(CO)2(I) rotation in the trigonal-
bipyramidal analogue.7 Accordingly, the transition state for the
latter requires a threefold eclipsing interaction (VII), but the
former requires only a single eclipsing interaction (VI). Said
differently, there is a larger energy difference between the
staggered ground state and the triply eclipsed transition state
for the latter and a smaller energy difference between ground
and transition states in the former. Both 6 and 7 would also
have singly eclipsed transition states (VIII). However, as
reflected in the torsion angle analyses above, nearly eclipsed
groups cannot be avoided in the ground state, due to the higher
coordination number. These destabilizing interactions should
further diminish the energy barriers.
In summary, gyroscope-like complexes that feature tetrasub-
stituted Re(CO)3(X) rotators encased within three-spoked
P((CH2)14)3P stators are easily prepared in remarkably good
yields by alkene metathesis/hydrogenation sequences. Such
octahedral systems have heretofore been available only by
protonations of the Fe(CO)3 centers in III4 or by oxidative
additions to Rh(CO)(I) centers in IV.7 When X is chloride or
bromide, rotation about the P-Re-P axis is facile, and
substitution reactions can be effected. Future reports will detail
(1) similar syntheses of analogous M(CO)4 species,13 (2)
extensions of Scheme 2 to other types of stators, and (3)
exploitation of the dipole moments of the rotators to effect
directed rotation.1
Acknowledgment. We thank the Deutsche Forschungsge-
meinschaft (DFG, Grant No. GL 300/9-1) for support.
Supporting Information Available: Text giving experimental
procedures and characterization for all complexes, tables giving
crystallographic data, and figures giving crystallographic, IR, and
rate data. This material is available free of charge via the Internet
at http://pubs.acs.org.
(12) A related issue is vertical clearance. The van der Waals diameters
of the chloride and bromide atoms (3.50 and 3.70 Å)11 are less than the
P-Re-P distances (4.84-4.85 Å) and the eclipsed CH2-P-Re-P-CH2
distances (6.19-6.69 Å). However, when twice the van der Waals radius
of carbon (3.40 Å) is subtracted from the last range, the clearance is
insufficient. Presumably, rotation is correlated to conformational changes
in the P(CH2)14P linkages.
OM700625U
(13) Skaper, D., Nun˜ez, J. E. Unpublished results, Universita¨t Erlangen-
Nu¨rnberg.