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Creating, Varying, and Growing Single-Site Molecular Contacts

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Science  22 Jul 2005:
Vol. 309, Issue 5734, pp. 588-590
DOI: 10.1126/science.1113667

Abstract

The known range of chemisorption bonds forms the toolbox for the design of electrical contacts in molecular electronics devices. Double-bond contacts to technologically relevant materials would be attractive for a number of reasons. They are truly single-site, bonding to a single surface atom. They obviate the need for a thiol linkage, and they may be amenable to further modification through olefin-metathesis methodologies. We report olefin-metathesis methods for establishing, varying, and growing thermally stable double-bond contacts to molybdenum carbide, a conducting material.

Establishing efficient electrical contact from metallic surfaces to molecules is a key challenge in the development of molecular electronics devices. These contacts should be both robust and well defined. Most experimental work has focused on contacts to metals such as gold, but transition metal carbides are potentially important for the development of nanoelectronic devices. In particular, Zhang et al. reported on the formation of carbide hetero-junctions between carbon nanotubes and transition metals (1). The latter approach was exploited to prepare highly effective titanium carbide and cobalt carbide ohmic contacts in carbon nanotube devices (2, 3).

Well-defined metal-ligand structures are a feature of organometallic complexes and have long served as a guide to understanding chemisorption bonding. However, one entire area of organometallic chemistry, that of metal-ligand multiple bonds, appears to display very little in common with surface science observations; there are few reports (4, 5) of surface alkylidene, M=CR1R2, species where M is a single metal atom. Metal alkylidenes serve as initiating and propagating sites for olefin metathesis. Although olefin metathesis is a powerful synthetic tool in homogeneous chemistry (6), steric hindrance and competing chemisorption of the olefin may inhibit the reaction for alkylidene groups situated directly on an extended planar surface. However, we report the spectroscopic observation of facile cross-metathesis reactions between propene and cyclopentylidene on β-Mo2C leading to the simultaneous isolation of surface methylidene and ethylidene (Scheme 1), the two primary propagator species in the Hérisson-Chauvin mechanism (7) for propene metathesis.

Furthermore, we report the observation of ring-opening metathesis polymerization (ROMP) from a surface. The latter method is particularly important in that it involves both a double-bond contact to the surface and chain growth away from the carbide surface.

Surface cyclopentylidene initiator sites were prepared on a clean β-Mo2C foil under ultrahigh vacuum conditions by the dissociative chemisorption of cyclopentanone in a surface reaction (8), which is directly analogous to the oxidative addition of cyclopentanone to the single-atom organometallic complex WCl2(PMePh2)4 reported by Bryan and Mayer (9).

We used reflection-absorption infrared spectroscopy (RAIRS) to search for alkylidene propagators and ring-opening polymerization products on the surface, and thermal desorption mass spectrometry to search for gas-phase metathesis products. Transalkylidenation between surface cyclopentylidene and simple gas-phase olefins was investigated by annealing the cyclopentylidene functionalized sample from 100 K at a rate of 1 K/s in a background pressure of 2 × 10-8 Torr propene. New surface species formed at 470 K that were identified as methylidene and ethylidene propagators by RAIRS experiments performed using deuterium-labeled propene (Fig. 1). Interaction with CH2CHCH3 (Fig. 1, spectrum C) led to new CH stretching bands at 3075, 2976, 2938, 2922, and 2861 cm-1.

Fig. 1.

RAIRS spectra for the interaction of propene and ethene with cyclopentylidene on β-Mo2C: (C) CH2CHCH3, (D) CD2CHCH3, (E) CD2CDCD3, (F) CH2CHCD3, and (G) C2H4. Reference data for cyclopentylidene and ethylidene on β-Mo2C are included as spectra (A) and (B), respectively. The top panel displays the difference spectrum [(D) minus (E)]. The observed reactivity is summarized in Scheme 1.

The band at ∼3075 cm-1 is absent for experiments performed using CD2CDCD3 (spectrum 1E) and CD2CHCH3 (spectrum 1D) but is present for experiments performed with CH2CHCH3, CH2CHCD3 (spectrum 1F), and C2H4 (spectrum 1G). The latter spectrum shows that annealing the cyclopentylidene functionalized surface to 470 K in 2 × 10-8 Torr ethene yields new bands at 3077 and 2955 cm-1, in addition to the two bands that arise from residual unreacted surface cyclopentylidene. Spectrum 1A is the reference spectrum of surface cyclopentylidene. The bands at ∼3077 and ∼2955 cm-1 were also resolved for experiments performed using CH2CHCD3 (spectrum 1F) and are attributed to the νas(CH2) and νs(CH2) vibrations of surface methylidene. This assignment is confirmed by reference to data for matrix-isolated H2C=Re(O)2OH (10). The latter system displays ν(CH2) bands at 3080 and 2986 cm-1.

Spectrum 1B is a reference spectrum for ethylidene formed from acetaldehyde on β-Mo2C (5). Comparison of spectra 1B and 1C shows that the new bands at ∼2976, 2938, 2922, and 2861 cm-1 may be attributed to surface ethylidene. This attribution is confirmed by the difference spectrum (1D - 1E), shown in the top panel of Fig. 1, for experiments performed using CD2CHCH3 (spectrum 1E) and CD2CDCD3 (spectrum 1D). It is also in agreement with calculated spectra for Mo=CHCH3 species (5). Hence, we may conclude that chemisorbed methylidene and ethylidene propagators, as predicted by the Hérisson-Chauvin mechanism, are formed by the interaction of propene with surface cyclopentylidene. The primary gas-phase products predicted by the Hérisson-Chauvin mechanism are methylidene cyclopentane and ethylidene cyclopentane.

The mass spectrometry data in Fig. 2 show that both products are detected in desorption experiments performed by annealing the cyclopentylidene functionalized carbide at 1 K/s in the presence of 2 × 10-8 Torr propene. The parent peaks for methylidene cyclopentane and ethylidene cyclopentane appear at 430 and 490 K, respectively. The difference of 60 K in reaction temperature may be attributed to the combined effect of steric 1,2 versus 1,3 substitution (11) and adsorption interactions in the Hérisson-Chauvin metallacyclobutane. In particular, the metallacyclobutane leading to methylidene cyclopentane formation will be stabilized by the noncovalent interaction of the methyl group with the surface. Molybdenum carbide is a conducting material, and the noncovalent interaction of the CH3 group with the surface could reduce the activation energy for metathesis by as much as 6 kJ/mol (12).

Fig. 2.

Temperature-programmed reaction mass spectrometry data (m/e = 67) for the interaction of cyclopentylidene with propene (A) and ethene (B) on β-Mo2C. The experiments were performed by heating the alkylidene functionalized carbide at 1 K/s in the presence of 2 × 10-8 Torr olefin. The desorption products are those predicted by the Hérisson-Chauvin mechanism for olefin metathesis (7).

Having established that transalkylidenation (Scheme 1) can be carried out on β-Mo2C, we then investigated ring-opening metathesis at alkylidene sites on the carbide surface. ROMP is a particularly powerful method (13, 14) in materials science, and its application to the growth of polymers directly from surface sites would be a major advance. Growth from the surface is a necessary condition to achieve direct molecular contact between a polymer chain and a conducting material. We report that ring-opening metathesis can be carried out at cyclopentylidene initiator sites on the surface of β-Mo2C. Figure 3 shows data for ring-opening metathesis reactions of cyclopentene and norbornene on the cyclopentylidene functionalized carbide surface. Exposure to gas-phase norbornene at 500 K leads to a gradual replacement of the cyclopentylidene spectrum (spectrum 3A) with bands at 3058, 2953, 2914, and 2877 cm-1 through monomer insertion (Fig. 3). Polynorbornene films grown using a Grubbs catalyst tethered to the native oxide layer on Si display a weak band at ∼3050 cm-1 and three strong bands at 2946, 2910, and 2866 cm-1 (15). The agreement between the vibrational data confirms that the ROMP reaction occurs on β-Mo2C and leads to polynorbornene formation. By comparing spectra for norbornene ring-opening (spectra 3C and 3D) with data for cyclopentene (spectrum 3B), the νs(CH2) vibrational signature of the linear C3 alkane segment formed from cyclopentene ring-opening is resolved at 2830 cm-1.

Fig. 3.

RAIRS spectra for the ring-opening metathesis reactions of norbornene (C and D) and cyclopentene (B) monomers with cyclopentylidene sites on β-Mo2C. The reaction with norbornene was carried out by heating from 100 to 500 K in 7 × 10-8 Torr. One heating cycle yielded spectrum (C), and two cycles yielded spectrum (D). The reaction with cyclopentene was carried out at 470 K in 2 × 10-8 Torr. Spectrum (A) is included as a reference spectrum for surface cyclopentylidene.

Scheme 1.

Observed surface and gas-phase products for cross-metathesis reactions between propene (b) and cyclopentylidene (a) groups on β-Mo2C: (c) ethylidene cyclopentane; (d) methylidene cyclopentane.

Olefin metathesis can be used to manipulate double-bond contacts on the surface of a technologically relevant material. The propensity to form double bonds suggests that the carbide surface behaves as an array of isolated metal atoms leading to the inhibition of bridged chemisorption configurations. The most likely reason alkylidenes are not readily observed on metal surfaces is that adsorbates generally tend to occupy high-coordination surface sites. Thus, for example, although the diradical CH2 would react with an isolated metal (M) atom to form an M=CH2 methylidene species, it would be expected to react with an extended metal surface to form a bridging methylene species (16). The latter adsorption geometry is advantageous in that it permits tetravalent coordination of the carbon atom (17). In contrast, the truly single-site bonding observed for the surface alkylidene systems implies that identical local bonding will occur at all size scales from a single-atom organometallic complex up to a macroscopic surface. This highly selective bonding to the carbide surface suggests applications such as double-bond contacts to carbided STM tips or double-bond connections between carbide sources and drains. Hence, we anticipate that the demonstrated ability to control and monitor olefin metathesis in chemisorbed layers will play a role in the development of new nanoscale devices.

Supporting Online Material

www.sciencemag.org/cgi/content/full/309/5734/588/DC1

Materials and Methods

Figs. S1 and S2

References

References and Notes

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