Key bindings should be added to the user key binding settings. Adding default settings will also work, but the settings will be overwritten when installing a new version of Sublime Text 2.
Your attempt follows the general Sublime Text binding structure, but "open_recent" is not a valid command. From what I can find in the documentation, I think that the "Open Recent" menu keeps track of different file names and then opens openly depending on which file is selected, instead of calling the open_recent command.
If there is no way to get the name of the most recent file, perhaps you can write an extension that tracked the most recent file and then open it, but that was probably a pretty dumb solution.
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