Research in the EM Clusters Lab

Our research is centred round organometallic cluster chemistry; we are interested in all aspects of organometallic clusters and their applications in diverse fields. Although the delineation is somewhat arbitrary, the following are some of our areas of exploration:

Heteronuclear and Intermetallic Clusters

We are interested in the synthesis and properties of organometallic clusters containing two or more different metals, either all transition elements (heteronuclear) or containing the heavier main group elements (intermetallic). It is our belief that these clusters will exhibit interesting new chemistry compared to their mononuclear or homonuclear analogues.

Examples include the first higher nuclearity osmium-selenium cluster and our recent report on the first examples of osmium and ruthenium clusters containing a m₅-Sb moiety,

ring formation via metal-metal bond cleavage,

and arrested orthometallation.

Our work on heterometallic clusters include the stereoselective binding of alkynes by an iridium-triruthenium cluster,

and the observation of phosphine migration in the cluster RuOs₃(m-H)₂(CO)₁₂(PPh₃).

Nanomaterials and Heterogeneous Catalysis

Our interest in the interface between organometallic clusters and surface and materials science include:

1. The potential of organometallic clusters as precursors for size-controlled deposition of nanoparticles, thin films and metallic phases.
2. The employment of mixed-metal clusters as precursors of metallic nanoparticles with controlled composition for employment as heterogeneous catalysts.
3. Study of the substrate-surface interface via molecular models.

For example, we have recently reported the use of ToF-SIMS to characterize organometallic cluster species anchored onto gold and silver surfaces. The figure below shows a typical ToF-SIMS (positive ion mode) spectrum of Os₃(m-H)(CO)₁₀(m-SCH₂CH₂SH) anchored onto silver foil.

We have also constructed molecular models of this by preparing colloidal silver nanoparticles stabilized with the water soluble organometallic surfactant [Os₃(m-H)(CO)₁₀S(CH₂)₁₀COO]Na. Shown below is a TEM image of one such spherical nanoparticle and typical UV-VIS spectra which show hypsochromic shift in the surface plasmon band from 390 nm to 408 nm which provide clear evidence for the absorption of the organometallic cluster onto the Ag surface.

We have shown that the pyrolysis of organometallic clusters as a method of preparing metallic nanoparticles of osmium and/or ruthenium leads to aggregation to particles in the 1 to 10 nm size range. This appears to be fairly independent of the precursor cluster size, but is correlated to its ligand set. On the other hand, polymeric organometallic compounds such as [Ru(CO)₄]_¥ can be pyrolysed to afford metallic nanofibres. Thus the molecular structure, in particular, the presence of metal-metal bonds may allow for retention of the rod-like arrangement of the metal-atom chain which then direct the aggregation towards the formation of nanofibres rather than spherical or irregularly shaped nanoparticles.

Bioorganometallic Chemistry

We have a rapidly growing interest in the use of organometallic compounds, particularly organometallic clusters, in the biological sciences. We have collaborations with a number of groups around the world, and within Singapore, on the development of organometallic compounds as potential drugs. An example is that with the research group of Professor Jaouen in Paris on the synthesis of cluster derivatives of Tamoxifen:

We are also the first to report the use of organometallic compounds as an infrared tag for cell imaging. For example, a phospholipid-like cluster (top) was used to obtain an image of a cell in the mid-infrared (bottom).

Most surprisingly of all, we have found that some osmium cluster species can exhibit good anticancer properties. In particular, the cluster Os₃(CO)₁₀(NCCH₃)₂ is significantly more cytotoxic towards both ER+ and ER- breast cancer cells than towards normal cells.

Compound	MDA-MB-231	MCF-7	HT29	SW620	Hepg2	MCF-10A
	4.8 ± 0.7	7.2 ± 1.2	31.3 ± 1.6	8.7 ± 1.0	8.8 ± 0.8	18 ± 2
	6.0 ± 0.7	6.8 ± 1.5	15.0 ± 1.8	8.2 ± 1.2	6.6 ± 1.2	8.6 ± 1.5

Bond Activation Chemistry

We have interests in the development, and mechanistic understanding, of new organometallic catalysts for effecting bond activation reactions, including catalytic reactions.

Examples of our work in this area are:

(1) The direct addition of carboxylic acids to terminal alkynes catalysed by [CpRu(CO)₂Cl] (1) or [{CpRu(CO)₂}₂] (2) to afford the anti-Markovnikov adducts in high selectivity.

(2) The reductive homocoupling of 9-bromofluorene in refluxing xylene catalysed by Ru₃(CO)₁₂, with a TON in excess of 3000. A stoichiometric reaction afforded a number of products (shown in figure below), and investigations suggest that 3 also catalysed the homocoupling.

(3) The hydrosilylation of terminal alkynes is catalysed by [Cp*IrCl₂]₂ to afford selectively the b-(Z)-vinylsilanes in high yields. The catalytic cycle has been studied by a combination of experimental and computational techniques, and suggests that a trans silyl migration step is involved.