Molecular gyroscopes: {Fe(CO)3} and {Fe(CO)2(NO)} + rotators encased in three-spoke stators; facile assembly by alkene metatheses

Article abstract of DOI:10.1002/anie.200460534

Complexes to toy with: Ring-closing metathesis of the bis(phosphane) complexes 1a-c (n = 4-6) followed by hydrogenation gives the "molecular gyroscopes" 2a-c. The crystal structure of 2c and the NMR data for the analogous {Fe(CO)₂(NO)}⁺

Full text of DOI:10.1002/anie.200460534

Angewandte
Chemie
Molecular Gyroscopes
Scheme 1, analogous reactions can be conducted with com-
plexes in which each phosphane contains two such substitu-
ents (A), giving doubly bridged assemblies (B).[9c] However,
the yields are lower. Unfortunately, extensions to complexes
bearing phosphanes with three such substituents,
Molecular Gyroscopes: {Fe(CO)3} and
{Fe(CO)2(NO)}+ Rotators Encased in
Three-Spoke Stators; Facile Assembly by
Alkene Metatheses**
=
P((CH2)nCH CH2)3 1, n = 4–6, and triply bridged assemblies
(D) were unsuccessful.[10]
Takanori Shima, Frank Hampel, and J. A. Gladysz*
Molecules that can perform a rotor function are experiencing
increasing attention, particularly in the context of nanoscale
devices.[1–6] These efforts have evolved far beyond the initial
concept stage[7] to materials that are being exploited in
nanofluidics and photonics. All rotors consist of a rotator and
a stator. Importantly, rotators with electric dipole moments
can, in appropriately oriented alternating electric fields, be
rendered unidirectional.[1b] Other types of external stimuli can
also effect unidirectional rotation,[5b,6] which is a necessary
feature of molecular motors.[6]
One subset of molecular rotors has been termed molec-
ular gyroscopes.[3,4] Such compounds feature an axis, an
interior domain of which can rotate independently of the
termini. However, to our knowledge there are no examples in
which this interior rotator is encased by a stator with multiple
“spokes”, such that the connectivity and symmetry (Dnh) of a
toy gyroscope is rigorously duplicated.[8] In this communica-
tion, we report a novel, convenient, and general route to a
family of such compounds that utilizes ring-closing alkene
metathesis. The lengths of the spokes are easily varied, and a
dipole moment can be introduced on the rotator without
affecting its steric properties.
Scheme 1. Elaboration of alkene-containing trans-bis(phosphane) com-
plexes to trans-spanning diphosphane complexes. a) Alkene metathe-
sis, b) hydrogenation. MLn =Cl-Pt-C6F5.
In these efforts, the local symmetry of the metal fragment
in C did not match that of the phosphane 1. Hence, we set out
to prepare trigonal-bipyramidal complexes consisting of an
equatorial ML3 array and two such axial phosphanes. Since
the substituents along the P-M-P axis should prefer staggered
conformations as in E (Scheme 2), such systems are better
preorganized with respect to the target molecules D. Thus,
(benzylideneacetone)iron tricarbonyl and 1a–c[11] were
In previous studies, we established that alkene metathesis
can be used to connect phosphane ligands that contain a
=
(CH2)nCH CH2 substituent (n > 4) and occupy trans posi-
tions in square-planar metal complexes.[9] As shown in
Scheme 2. Syntheses of gyroscope-like molecules.
reacted. Workup gave, in accord with precedent for other
phosphanes,[12]
diaxially
substituted
trans-[Fe(CO)3-
[*] Dr. T. Shima, Dr. F. Hampel, Prof. Dr. J. A. Gladysz
Institut für Organische Chemie
=
{P((CH2)nCH CH2)3}2] (2a–c) as yellow oils in 75–67%
yield.[13] These exhibited a single intense IR n˜CO band
(1860–1856 cmꢀ1), as did all other {Fe(CO)3} species below.
The metathesis of 2a was investigated first. As shown in
Scheme 2, a 0.0075m solution in CH2Cl2 was refluxed with
Friedrich-Alexander-Universität Erlangen-Nürnberg
Henkestrasse 42, 91054 Erlangen (Germany)
Fax: (+49)9131-85-26865
E-mail: gladysz@organik.uni-erlangen.de
[**] We thank the Deutsche Forschungsgemeinschaft (DFG, GL 300/1-
3) and the Humboldt Foundation (fellowship to T.S.) for support.
=
Grubbsꢀ catalyst (13 mol% or 4% per new C C linkage).
Workup afforded a homogeneous white solid (3a) in 60%
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
yield. Reaction aliquots were assayed by 31P NMR spectros-
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DOI: 10.1002/anie.200460534
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5537

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copy. The spectra showed two intermediates that gave a
iron are unexceptional. However, the space-filling represen-
tations suggest there is enough room for the {Fe(CO)3} moiety
to rotate within the three P(CH2)14P spokes. In this regard, it
is useful to compare the iron–oxygen distances (2.93 Š) with
those from the iron to the distal carbon atoms of the
macrocycles (7.86–6.64 Š). When the van der Waals radius
of oxygen is added to the former, and that of carbon is
subtracted from the latter, considerable “clearance” remains
(4.45 Š vs. 6.10–4.94 Š). The molecules pack in layers with
the vertically aligned P-Fe-P axes. Those in one layer are
offset in both in-plane dimensions from those in adjacent
layers.
monotonic chemical-shift sequence from 2a to 3a (d = 64.2,
67.9, 75.0, 84.7 ppm, C6D6), and only minor amounts of by-
products. The mass spectrum of 3a exhibited a molecular ion
signal appropriate for the product of a three-fold intra-
molecular metathesis and no peaks of higher mass.[13] The
13C NMR spectrum (C6D6) indicated a highly symmetric
species, with a single FeCO signal coupled to both phosphorus
=
atoms (213.9 ppm, t, JCP = 28 Hz), a single C C signal
(132.0 ppm), and four CH2 signals. Hence, 3a was assigned
the gyroscope-like structure depicted in Scheme 2, with three
[14]
=
13-membered rings containing E C C linkages.
Next, a solution of 3a and Wilkinsonꢀs catalyst (8 mol%)
in toluene was treated with H2 (5 atm) at 808C . Workup gave
the hydrogenation product 4a as a white solid in 84% yield.
In accord with the high idealized molecular symmetry (D3h),
the 13C NMR spectrum exhibited only five CH2 signals.[13]
When reaction aliquots were assayed by 31P NMR spectros-
copy, two intermediates could again be detected, with
chemical shifts changing monotonically from 3a to 4a (d =
84.7, 81.1, 78.1, 75.5 ppm, C6D6). Complexes 3a and 4a were
moderately air sensitive, but showed no mass loss at temper-
atures less than 3008C (TGA). The latter melted at 165–
1708C (capillary; 1748C, DSC).
The generality of the preceding sequence was tested with
2b and 2c. Metatheses under similar conditions gave 3b and
3c, with three 15- and 17-membered rings, in yields of 73%
and 81%, respectively. The mass spectra showed molecular
ions and no peaks of higher mass. However, mixtures of E and
The symmetries of 3a and 4a–c are too high to probe the
rate of {Fe(CO)3} rotation by conventional solution-phase
NMR techniques. Thus, derivatives of lower symmetry were
sought. As shown in Scheme 3, 3a and 4a were combined with
CF3SO3H in CDCl3 in an NMR tube. In accord with precedent
for closely related complexes,[12] the cationic iron hydrides
5a+·CF3SO3ꢀ and 6a+·CF3SO3ꢀ formed in quantitative yields,
1
as evidenced by diagnostic H NMR signals (d = ꢀ9.41 and
ꢀ9.42, t, JHP = 21.4 and 21.8 Hz). The 13C NMR spectra
recorded at ambient temperature showed two sets of
P(CH2)4C signals (2:1), indicating a slow-exchange limit
with respect to any process (e.g., {HFe(CO)3} rotation) that
can render the bridges equivalent.
We next sought to replace one carbonyl ligand of 4a–c by
a similarly sized ligand. As shown in Scheme 3, reactions with
NO+·BF4 gave the isoelectronic and isosteric dicarbonyl
ꢀ
nitrosyl cations 7a–c+·BF4 in 98–81% yield.[13] Although
ꢀ
Z C C bonds were present, as evidenced by multiple
many {Fe(CO)3} species behave similarly,[16] it is of mecha-
nistic note that substitution is not inhibited by the three
=
31P NMR signals. When the second-generation Grubbsꢀ
catalyst was used,[15] the yield of 3c was unaffected, but that
of 3a increased to 76%.
methylene bridges. The IR spectra of 7a–c+·BF4 exhibited
ꢀ
Analogous hydrogena-
tions of 3b and 3c gave
4b and 4c in yields of
86% and 79%, respec-
tively. Interestingly, the
IR n˜CO values of 4a–c
exhibited a monotonic
trend
(1841,
1853,
1861 cmꢀ1).
Prisms of 4c were
grown, and the crystal
structure was solved as
described in the Sup-
porting
Information.
Views of the molecular
structure are collected
in Figure 1. The P-Fe-P
substituents are arrayed
in staggered conforma-
tions as in E, and a
crystallographic C2 axis
passes through the Fe-
C1-O1 linkage, exchang-
ing two of the 17-mem-
bered rings. The bond
Figure 1. ORTEP (top) and space-filling (bottom) representations of the crystal structure of 4c. Selected bond
lengths [Š] and angles [8]: Fe-C1 1.761(3), Fe-C2 1.764(2), Fe-P1 2.2056(5), C1-O1 1.162(3), C2-O2 1.164(3);
Fe-C1-O1 180.000(1), Fe-C2-O2 179.3(2), C1-Fe-C2 118.99(7), C2-Fe-C2a 122.02(14), C1-Fe-P1 90.565(17), C2-Fe-P1
lengths and angles about 90.32(6), P1-Fe-P1a 178.87(3).
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Angewandte
Chemie
future reports, together with addi-
tional properties of the complexes
prepared above. Finally, the design
insights realized in conjunction with
Scheme 1 have advanced the utility of
alkene metatheses in the synthesis of
topologically novel organometallic
and inorganic molecules, a number
of other elegant examples of which
have recently appeared.[20]
Scheme 3. Symmetry reduction of gyroscope-like molecules.
Received: May 1, 2004
Published Online: August 27, 2004
two n˜CO bands (2023–2030 cmꢀ1, m, 1953–1965 cmꢀ1 s) and a
n˜NO band (1752–1764 cmꢀ1, s). The 13C NMR spectra of
7a,b+·BF4 gave two sets of P(CH2)n+1 signals (2:1), while
ꢀ
Keywords: alkene metathesis · iron · N ligands · phosphine
ligands · ring-closing metathesis
.
the spectrum of 7c+·BF4 gave only one. This indicates, in
ꢀ
accord with expectations from the crystal structure of 4c, that
{Fe(CO)2(NO)}+ rotation is rapid on the NMR timescale at
ꢀ
room temperature for 7c+·BF4ꢀ, but slow for 7a,b+·BF4
.
[1] a) J. Vacek, J. Michl, NewJ. Chem. 1997, 21, 1259; b) X. Zheng,
M. E. Mulcahy, D. Horinek, F. Galeotti, T. F. Magnera, J. Michl,
J. Am. Chem. Soc. 2004, 126, 4540, and references therein.
[2] T. C. Bedard, J. S. Moore, J. Am. Chem. Soc. 1995, 117, 10662.
[3] a) Z. Dominguez, H. Dang, J. M. Strouse, M. A. Garcia-Garibay,
J. Am. Chem. Soc. 2002, 124, 7719; b) Z. Dominguez, T.-A.
Khuong, H. Dang, C. N. Sanrame, J. E. Nuæez, M. A. Garcia-
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Zepeda, C. J. Mortko, H. Dang, M. A. Garcia-Garibay, J. Org.
Chem. 2004, 69, 1652.
Importantly, both phosphorus atoms of 7c+·BF4 retain
coupling to the PCH2CH2CH2 and CO signals, excluding the
most probable dissociative mechanisms for rendering the
bridges equivalent.
ꢀ
The dynamic behavior of 7a–c+·BF4 was further probed
ꢀ
by variable-temperature 13C NMR spectroscopy. Asolution
of 7c+·BF4 in CDFCl2 was cooled from ꢀ10 to ꢀ808C. The
ꢀ
chemical shifts of the PCH2CH2CH2 carbons remained
essentially constant, while those of the other methylene
carbons shifted. The PCH2CH2 signal decoalesced in a well-
defined manner, enabling spectra and line-shape simulation.
These data gave DH° and DS° values of 9.5 kcalmolꢀ1 and
ꢀ6.5 eu for {Fe(CO)2(NO)}+ rotation. If CO or NO dissoci-
ation were required to render the bridges equivalent, a large
positive DS° value would be expected. Finally, a solution of
[4] C. A. Schalley, Angew. Chem. 2002, 114, 1417; Angew. Chem. Int.
Ed. 2002, 41, 1513.
[5] For additional representative molecular rotors with covalently
attached axles, see: a) T. R. Kelly, Acc. Chem. Res. 2001, 34, 514;
b) R. A. van Delden, J. H. Hurenkamp, B. L. Feringa, Chem.
Eur. J. 2003, 9, 2845, and references therein; c) H. Jian, J. M.
Tour, J. Org. Chem. 2003, 68, 5091; d) M. F. Hawthorne, J. I.
Zink, J. M. Skelton, M. J. Bayer, C. Liu, E. Livshits, R. Baer, D.
Neuhauser, Science 2004, 303, 1849.
[6] Recent review of molecular motors: C. P. Mandl, B. König,
Angew. Chem. 2004, 116, 1650; Angew. Chem. Int. Ed. 2004, 43,
1622.
7b+·BF4 in C6D5Br was warmed from 258C to 1008C.
ꢀ
Although coalescence was apparent, lines broadened, pre-
sumably due to a small amount of concomitant decomposi-
tion. Quantitative analyses were not possible.
[7] P. Kaszynski, A. C. Friedli, J. Michl, J. Am. Chem. Soc. 1992, 114,
601.
In summary, we have described a facile synthesis of a
family of molecules that duplicate the symmetry, connectivity,
and rotator function of standard toy gyroscopes. The stator
consists of three spokes that span the termini of the gyroscope
axis. Their lengths can be adjusted to modulate the rotational
barrier of the interior rotator. The rotators consist of either an
{Fe(CO)3} moiety, or an isoelectronic and isosteric
{Fe(CO)2(NO)}+ moiety. The latter introduces a dipole
moment, which represents a possible means of driving
unidirectional rotation. However, the idealized limit of a
frictionless gyroscope is not closely modeled by the present
systems, and much different architectures will be required to
approach this objective.
ꢁ
[8] Mooreꢀs “molecular turnstiles”, which consist of a C C-C6H2R2-
[2]
ꢁ
C C axis and a stator with two spokes, nearly fulfill these
criteria. The addition or removal of two tert-butyl groups from
the stator would, coupled with a C6H4 rotator, afford the proper
D2h symmetry.
[9] a) J. Ruwwe, J. M. Martín-Alvarez, C. R. Horn, E. B. Bauer, S.
Szafert, T. Lis, F. Hampel, P. C. Cagle, J. A. Gladysz, Chem. Eur.
J. 2001, 7, 3931; b) E. B. Bauer, F. Hampel, J. A. Gladysz,
Organometallics 2003, 22, 5567; c) T. Shima, E. B. Bauer, F.
Hampel, J. A. Gladysz, Dalton Trans. 2004, 1012.
[10] E. B. Bauer, T. Shima, unpublished results, Universität Erlan-
gen-Nürnberg.
[11] Phosphanes 1a–c were prepared in yields of 87–56% by
=
reactions of the Grignard reagents BrMg(CH2)nCH CH2 (gen-
The title molecules can also be regarded as a special class
of cryptates.[17] If the {Fe(CO)3} moieties were to be excised
from 4a–c, a reaction type with good precedent, the resulting
bicyclic bridgehead diphosphanes would be in,in isomers.
Such species are unusual and normally accessible only by
lengthy syntheses or in low yields.[18] Importantly, similar
methodology can be applied to generate gyroscope-like
molecules with two spokes.[10,19] These will be described in
erated from the readily available alkyl bromides) and PCl3.
Spectroscopic properties were similar to those of the homo-
[9c]
=
logues PhP((CH2)nCH CH2)2 reported previously.
[12] J. R. Sowa Jr., V. Zanotti, G. Facchin, R. J. Angelici, J. Am.
Chem. Soc. 1991, 113, 9185.
[13] NMR (1H, 13C, 31P), IR, MS, and microanalytical data are
summarized in the Supporting Information. CCDC 237814
contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge via www.ccdc.
Angew. Chem. Int. Ed. 2004, 43, 5537 –5540
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ac.uk/conts/retrieving.html (or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+ 44) 1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).
=
[14] The E C C stereochemistry is assumed by analogy to that
established for ring-closing metatheses of other complexes with
[9b,c]
=
trans P(CH2)4CH CH2 moieties (e.g., A!B).
=
[15] [Ru( CHPh)(H2IMes)(PCy3)(Cl)2]; H2IMes = 1,3-dimesityl-4,5-
dihydroimidazol-2-ylidene.
[16] B. F. G. Johnson, J. A. Segal, J. Chem. Soc. Dalton Trans. 1972,
1268.
[17] For some conceptionally related systems, see a) V. J. Catalano,
B. L. Bennett, R. L. Yson, B. C. Noll, J. Am. Chem. Soc. 2000,
122, 10056; b) V. J. Catalano, B. L. Bennett, M. A. Malwitz,
R. L. Yson, H. M. Kar, S. Muratidis, S. J. Horner, Comments
Inorg. Chem. 2003, 24, 39.
[18] a) I. Bauer, O. Rademacher, M. Gruner, W. D. Habicher, Chem.
Eur. J. 2000, 6, 3043; b) R. W. Alder, D. Read, Angew. Chem.
2000, 112, 3001; Angew. Chem. Int. Ed. 2000, 39, 2879.
[19] For an interesting molecule that can be regarded as a progenitor
of a family of such gyroscopes, and was prepared by alkene
metathesis of a trans-bis(pyridine)palladium dichloride complex,
see P. L. Ng, J. N. Lambert, Synlett 1999, 1749. The ring size in
this compound precludes rotation of the PdCl2 moiety.
[20] a) Review: E. B. Bauer, J. A. Gladysz in Handbook of Meta-
thesis, Vol. 2 (Ed.: R. H. Grubbs), Wiley-VCH, New York, 2003,
p. 403; b) V. Martinez, J.-C. Blais, D. Astruc, Angew. Chem. 2003,
115, 4502; Angew. Chem. Int. Ed. 2003, 42, 4366.
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