There were a number of challenges. The interests at that time didn't align well enough for the conclusion of a negotiation. We never launched full-fledged negotiations with Mercosur. We were in discussions with them as part of the negotiations for the Free Trade Area of the Americas. When those negotiations stalled, we tried to proceed with some bilateral discussions with Mercosur. When the Free Trade Area of the Americas negotiations were put on hold more formally, then the negotiations with Mercosur ceased as well.
On October 25th, 2011. See this statement in context.