New York--Former U.S. Treasury Secretary Robert Rubin has taken a senior executive position with Citigroup. Mr. Rubin, whose title and exact responsibilities at the financial services juggernaut have not been released, will serve as a senior executive in the office of Sanford Weill and John Reed, Citigroup's co-chairmen and CEOs. Mr. Rubin left his spot at the helm of the Treasury earlier this summer. His profile among Wall Street insiders and marketers was generally viewed second only in importance to U.S. Federal Reserve Chairman Alan Greenspan. Prior to joining the Treasury at the beginning of President Clinton's first term, Mr. Rubin had spent 28 years at Goldman, Sachs & Co., New York, as a bond trader.