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Simple saturated hydrocarbons, alkanes, are extremely poor ligands and complexes containing alkane molecules acting as discrete ligands are typically very short lived, with lifetimes less than ~100 ms at room temperature. We can observe alkane complexes and other reactive species using NMR spectroscopy by using a combination of photochemistry to generate the reactive alkane complex and low temperatures to stabilise it for sufficient time to allow characterization.

In this case, absorption of a UV photon results in the loss of a carbonyl ligand from CpRe(CO)₃. Cyclopentane replaces the CO as a ligand, forming an σ-alkane complex. These molecules are of interest both from the standpoint of basic coordination chemistry (an agostic interaction) and because alkane complexes are known intermediates in the C-H activation process, a potentially useful route to functionalising these relatively unreactive hydrocarbons found in petroleum and an intense area of research around the world.

We have characterised several types of alkane complexes including the rhenium and tungsten pentane complexes, CpRe(CO)₂(±è±ð²Ô³Ù²¹²Ô±ð-η²-C²,H²) and (η⁶-hexaethylbenzene)W(CO)₂(±è±ð²Ô³Ù²¹²Ô±ð-η²-C¹,H¹) shown below. We have extended our work on alkanes to include binding of xenon in complexes of the [CpRe(CO)(PF₃)(Xe)] type. The structures shown below are all calculated using density functional theory (DFT) methods. We are increasingly employing DFT and ab initio quantum chemical methods in this project to aid the elucidation of the stutructure and reactivity of these fascinating compounds. Frequently, this work is done in collaboration with groups from around Australia and overseas (see references below).

Current research is aimed at answering questions such as:

• How does the alkane bind to the metal centre?
• Can we make more stable alkane complexes?
• Can we do useful chemistry with alkane complexes?
• Can we observe more complexes with ligands that bind even more weakly than alkanes using NMR?

We are constantly seeking to expand the applicability of the in situ photolysis with NMR detection technique to new areas in inorganic and organic chemistry and welcome the opportunity to forge new collaborations in the area.

Key references:

Young, R.D.; Lawes, D.J.; Hill, A.F.; Ball, G.E. "Observation of a tungsten alkane σ-complex showing selective binding of methyl groups using FTIR and NMR spectroscopies" J. Am. Chem. Soc., 2012, 134, Article ASAP DOI: 10.1021/ja300281s.

Ball, G.E.; Darwish, T.A; Geftakis, S.; George, M.W.; Lawes, D.J.; Portius, P.; Rourke, J.P. "Characterization of an Organometallic Xenon Complex using NMR and IR Spectroscopy." Proc. Natl. Acad. Sci. USA., 2005, 102, 1853.

Lawes, D.J.; Geftakis, S.; Ball, G.E. "Insight into binding of alkanes to transition metals from NMR spectroscopy of isomeric pentane and isotopically labelled alkane complexes." J. Am. Chem. Soc., 2005, 127, 4134.

Geftakis, S.; Ball, G.E. "Direct Observation of a Transition Metal Alkane Complex, CpRe(CO)2(cyclopentane), Using NMR Spectroscopy", J. Am. Chem. Soc., 1998, 120, 9953.

Simple saturated hydrocarbons, alkanes, are extremely poor ligands and complexes containing alkane molecules acting as discrete ligands are typically very short lived, with lifetimes less than ~100 ms at room temperature. We can observe alkane complexes and other reactive species using NMR spectroscopy by using a combination of photochemistry to generate the reactive alkane complex and low temperatures to stabilise it for sufficient time to allow characterization.

In this case, absorption of a UV photon results in the loss of a carbonyl ligand from CpRe(CO)₃. Cyclopentane replaces the CO as a ligand, forming an σ-alkane complex. These molecules are of interest both from the standpoint of basic coordination chemistry (an agostic interaction) and because alkane complexes are known intermediates in the C-H activation process, a potentially useful route to functionalising these relatively unreactive hydrocarbons found in petroleum and an intense area of research around the world.

We have characterised several types of alkane complexes including the rhenium and tungsten pentane complexes, CpRe(CO)₂(±è±ð²Ô³Ù²¹²Ô±ð-η²-C²,H²) and (η⁶-hexaethylbenzene)W(CO)₂(±è±ð²Ô³Ù²¹²Ô±ð-η²-C¹,H¹) shown below. We have extended our work on alkanes to include binding of xenon in complexes of the [CpRe(CO)(PF₃)(Xe)] type. The structures shown below are all calculated using density functional theory (DFT) methods. We are increasingly employing DFT and ab initio quantum chemical methods in this project to aid the elucidation of the structure and reactivity of these fascinating compounds. Frequently, this work is done in collaboration with groups from around Australia and overseas (see references below).

Using NMR spectroscopy as our primary tool, backed up wuth computational methods, structures of compounds and the mechanism of fluxional processes is explored. A couple of examples one each from inorganic and organic chemistry are shown below.

Hydrides and dihydrogen complexes

Metal hydride complexes are well known to be excellent catalysts for hydrogenation reactions and have the potential to act as hydrogen storage materials. Dihydrogen complexes contain a hydrogen molecule that is essentially intact but acting as a ligand. We have an ongoing interest in these classes of compounds. We are interested in answering questions such as can we accurately determine the location of the hydrogen atoms in such molecules and interatomic distances? This can be difficult using crystallographic methods.

Hydride and dihydrogen complexes also often show interesting mechanisms of fluxionality, i.e., the hydrogen atoms interchange their positions within the complexes. NMR is ideally suited to revealing how these molecular reorientations take place.

Photochromic organic compounds

Building on some high profile work led by the CSIRO, we have studied certain merocyanine molecules, similar to the type found in shade-changing photochromic lenses of spectacles. We have shown that a relatively rapid isomerization of different isomers can occur and that this process is catalyzed by acid.

We are currently investigating several other photochemically generated organic reactive intermediates with NMR spectroscopy at low temperatures.

References:

Yee, L. H.; Hanley, T.; Evans, R. A.; Davis, T. P.; Ball, G. E. "Photochromic Spirooxazines Functionalized with Oligomers: Investigation of Core-Oligomer Interactions and Photomerocyanine Isomer Interconversion Using NMR Spectroscopy and DFT" J. Org. Chem. 2010, 75, 2851-2860.

Evans, R.A; Hanley, T.L.; Skidmore, M.A.; Davis, T.P.; Such, G.K.; Yee, L.H; Ball, G.E.; Lewis, D.A. "Lubrication and Control of Nano-Mechanical Processes in Polymers." Nature Materials, 2005, 4, 193.

- with A/Prof. Larry Wakelin and Dr Don Thomas

This project is using NMR techniques and molecular modelling (primarily molecular mechanics/dynamics with AMBER) to the investigate the binding of potential anti cancer therapeutics. Precise details of how the drugs bind to the DNA will aid the design of next generation drugs that interact with DNA.