The word agostic was coined about 30 years or so ago to describe how certain two-point bonding occurs. The Greek word agostos (ἀγοστός) means the "flat of the hand" and is apparently very rare and only occurs once in Homer's Iliad (it took me some doing to find that Greek link). The metaphor is that the bonding resembles a man's bent arm with the flat of one hand on a hip such that there are two points of arm attachment to the body: a strong one at the shoulder and a weaker one between the hand and the hip.
Recently, an old friend of mine co-discovered and reported on a remarkable new compound which crystallized in two subtly different ways, one analogous to having the palm on the hip and the other with the back of the hand on the hip. The two isomers are termed "agostomers" and also show the interesting property that one agostomer crystal is orange and the other one is blue. Here's a depiction:
This is really interesting. How could the same compound crystalize differently and more to the point, WHY would it? Does it have to do with temperature?
ReplyDeleteThe two agostomers are structurally different -- ever so slightly -- and thus there really are two different compounds in solution. It takes little energy to interconvert them. As you cool the solution the interconversion freezes out and the two different agostomers find one another and begin building crystals of like-kind.
DeleteThe pharmaceutical industry has long recognized things called crystalmorphs which are the very same drug which can crystalize in different configurations. There are tricks to getting this to happen -- using different solvents for example. You can imagine the economic driving force to do so -- getting a composition of matter patent on esentially the same drug more than once. But there is practical utility as well to justify. Different crystalmorphs may dissolve faster in a solid dosage form and lead to faster uptake.
What's fascinating and new here is the different colors of the two agostomers. The color is due to the molybdenum center which is obviously sensitive to "forehand" or "backhand" contact.
Another possible use could be devising a new method to detect rates of interconversion between such agostomers. I mentioned that in solution you see just a purple color -- but that may just be a visual average or in other words a superposition of two colors orange and blue. A spectrometer able to resolve and detect the two colors might in fact "see" the two different agostomers in solution. In this way, their interconversion in solution could be measured and, as a function of temperature, could yield the energy required to do so.
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