Boston-based private equity firm Rockpoint Group has an agreement to buy 75-101 Federal St., an 800,000-square-foot office complex in the Financial District, from Chicago-based Pearlmark Real Estate Partners.

The price was in the $320 million range, according to a source. Pearlmark acquired the property for $260 million in 2007. It recently hired Cushman & Wakefield to market the property a year after exploring a sale but receiving a disappointing level of interest, according to a real estate source.

The acquisition will be on behalf of Rockpoint’s $950-million Core Plus Fund, which closed to investors in September and pursues a low-risk strategy.

The occupancy rate is 84 percent, with major tenants including law firm Sherin and Lodgen, Xerox Corp. and Electronic Data Systems, according to research by Avison Young.

Built in 1929, the original tower at 75 Federal St. is an example of high-rise setback architecture, in which New York City zoning was changed to minimize shadow impacts from skyscrapers. Similar building designs were exported to Boston and other cities, said Brendan Carroll, vice president of research for Avison Young. Other local examples include 50 Post Office Square and the Landmark building at 160 Federal St.

The L-shaped office tower overlooks the city-owned Winthrop Square garage, which could be redeveloped into a massive mixed-use project.

Developer Steve Belkin has floated plans for a 740-foot tower on the garage site at 115 Federal St. The project would cost approximately $900 million and contain offices, a 300-room hotel, retail space and possibly condos on the upper floors.

The city recently issued a request for interest from developers for the property, which are due April 13. The garage was closed due to safety concerns in May 2013.