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.
Photochemical generation of an alkane complex.
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(pentane-η2-C2,H2) and (η6-hexaethylbenzene)W(CO)2(pentane-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 ths 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).
Set up for an in situ photolysis experiment at low temperature using an excimer laser and a 600 MHz NMR spectrometer.
Monitoring the formation and disappearance of an alkane complex at -90 °C.
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?
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.
amcrasto@gmail.comhttp://newdrugapprovals.org/
DRUG APPROVALS BY DR ANTHONY MELVIN CRASTO amcrasto@gmail.comhttp://newdrugapprovals.org/
DRUG APPROVALS BY DR ANTHONY MELVIN CRASTO