prov:value
| - The compound of claim 3 wherein R is an aromatic group which is unsubstituted or inertly substituted; and X is independently an oxygen atom, a sulfur atom, a carboxylic or thiocarboxylic ester group, a sulfone, a sulfoxide, perfluoroalkylene, perfluoroalkylene ether, alkylene, acetylene, a phosphine, a carbonyl or thio carbonyl group; seleno; telluro; nitrido; a boranediyl or methylboranediyl grou
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