Dr Chris Adams
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Dr. Chris Adams' Research
The redox chemistry of coordination and organometallic compounds
Molybdenum - alkyne compoundsWorking in collaboration with Professor Connelly and his group, I've been looking at the ESR spectra of some 19- and 17- electron alkyne compounds of (mainly) molybdenum. Ions such as 1 (right) have a relativey low-lying LUMO, and can be chemically reduced by decamethylcobaltocene. The resulting radicals are not stable enough to be isolated, but are readily detected by ESR, and show some interesting properties. At high temperatures the alkyne moves back and forth in a windscreen-wiper fashion, meaning that the two phosphorus atoms are in the same environment, and the resulting ESR pattern is a triplet. As the temperature is lowered though, the motion of the alkyne is slowed until it becomes essentially stationary. At this point it lines up nearer to one phosphorus atom than the other, rendering them inequivalent, and the pattern changes to a doublet. The resonance of an electron is so fast (GHz, compared to MHz for protons in NMR) that for a compound to be fluxional on the ESR timescale is unusual; for this to be temperature dependant is exceedingly rare. See J. Chem. Soc., Chem. Commun., 2001, 2458, for further information. |
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We've also done some experiments with 13C labelled alkynes, in order to better see coupling to the carbon atoms of the alkyne; thus, the bottom spectrum opposite is that of [Cr(C6Me6)(CO)2(Ph-CC-Ph)]+, where the two central carbon atoms of the alkyne are labelled. The alkyne rotates, making these two atoms equivalent, and so a central triplet is seen due to coupling to the two spin 1/2 nuclei, with little chromium (I = 3/2, 9.5 % abundant) satellites peeking out on the sides. |
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Some of the other molybdenum-alkyne chemistry stuff we've done can be seen in Organometallics, 2002, 21, 3454 and J. Chem. Soc., Dalton Trans., 2001, 1284. |
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Ruthenium alkynyl chemistry
There have been a lot of papers in the past three years concerning the chemistry of platinum alkynyl compounds such as 2 (below), which have long lived excited states and luminesce nicely - see J. Chem. Soc., Dalton Trans., 2000, 63 for the start of these. One problem with these compounds is that they're fairly insoluble, and one way to overcome this would be to occupy the axial positions on the metal with solubilising ligands. You can't do this if the metal is platinum, which likes to be square planar, but if you change the metal to ruthenium it should be possible. Thus, I've recently investigated compounds like 3 and 4, which turn out to have interesting properties in their own right. So, for example, the complexes are redox active (unlike the platinum analogues), and the way the electrons move around within the molecule depends upon the oxidation state, thus affecting their physical properties.
We've also tried influencing the way that these molecules pack together in the solid state ("crystal engineering") by appropriate functionalisation of the end groups. 4 was designed with this purpose in mind, the idea being that the nitro and iodo groups will attract each other in the solid state. The result is below, the red and blue bits being the nitro groups, and the purple the iodine atoms; as you can see, we ended up forming chain of molecules that run through the crystal.
My work on this has recently been published in Inorganic Chemistry, 2004, 43, 3492, and Dalton Trans., 2004, 4130.
Last updated 7/12/04