The Aon Center in Chicago, the city’s third-largest skyscraper, has been sold for $712 million to a private real estate investment company. Atlanta-based Piedmont Office Realty Trust said that it has entered into a binding agreement to sell its largest asset, Aon Center, at 200 E. Randolph St., to the 601W Companies, a New York private real-estate investment company. The price equates to $260 per square foot. The sale is expected to close early in the fourth quarter, Piedmont said. Aon Center, built in 1972, is an 83-story, 2.7 million-square-foot building on 3.5 acres in Chicago’s East Loop. It is 86 percent leased, according to Piedmont. Aon, which has had its name on the building since late 1999, remains the building’s largest tenant with about 420,000 square feet. Other tenants include KPMG, Microsoft, United Health Group, Integrys, the Federal Home Loan Bank of Chicago, JLL, AT&T and Edelman, Miller said. Aon Center, is the third-tallest building in Chicago, behind Willis Tower (the tallest) and Trump Tower.