Developer Larry Silverstein yesterday looked to build momentum for his bid to redevelop ground zero. At a joint press conference with Governor Pataki, Mr. Silverstein named a star British architect, Lord Norman Foster, to design Tower 2, a 65-story office building to stand alongside the Freedom Tower. It is expected to be completed in 2011.

Mr. Silverstein also announced the first lease signing at 7 World Trade Center, the commercial office building that is set to open this spring. The New York Academy of Sciences agreed to a 15-year lease for 40,000 square feet – the entire 40th floor – where it will house its headquarters and a conference center.

Advocates of increased mixed-use development at ground zero, which would include housing, have pointed to a tepid response by potential commercial tenants to the $700 million 7 World Trade Center. Mr. Silverstein has yet to sign any major tenants, although several companies are said to have expressed interest, including China’s Vantone Real Estate Company and Ameriprise Financial.

Tower 2, located at 200 Greenwich St., will contain 2.4 million square feet of office space, just 200,000 square feet less than the Freedom Tower. It will also include about 130,000 square feet of retail space at ground level and in an underground concourse that will link to the PATH train transportation hub next door.

The building will fill an entire block bound by Church Street to the east, Vesey Street to the north, the reconstituted Fulton Street to the south, and Greenwich Street to the west.

Mr. Foster’s recent projects include the Hearst Corporation’s headquarters in Midtown, the Beijing Airport, the German Parliament at the Reichstag in Berlin, the courtyard of the British Museum in London, and the Boston Museum of Fine Arts. He received the prestigious Pritzker Architecture Prize in 1999.

Mr. Foster will join a celebrated list of architects who have signed on to design some part of Daniel Libeskind’s master site plan, including Frank Gehry, David Childs, and Santiago Calatrava. In the original contest to determine the designer of the ground zero centerpiece, Mr. Foster’s touching twin towers lost out to Mr. Child’s design for the Freedom Tower, which is scheduled to begin construction this spring.

Yesterday’s announcements closely followed Mr. Pataki’s decision on Wednesday to break with the city and give the state’s half of the remaining tax-exempt Liberty Bonds to Mr. Silverstein for the construction of the Freedom Tower and Tower 2. The city and state had been working together in their negotiations with Mr. Silverstein over the remaining $3.35 billion.

Mr. Silverstein remains in negotiations with the city over its outstanding allotment of $1.67 billion in Liberty Bonds, with which he plans to finance two more commercial towers at ground zero. But the city has been arguing with the developer over several issues, including a construction timetable and the amount of the developer’s fee.

State officials have indicated that the Port Authority has engaged in preliminary discussions with Silverstein Properties over taking back some portions of ground zero, including other building sites. The Port Authority owns the site and leased it to Silverstein Properties for 99 years in July 2001. Mr. Pataki said Wednesday that the Port Authority and Mr. Silverstein have 90 days to conclude their negotiations.

Yesterday, Mr. Pataki said in a statement, “The selection of Lord Norman Foster to design Tower Two is just the latest signal that the rebuilding of the World Trade Center site is moving forward in achieving its destiny as a world-class central business district.”