Debt-plagued Puerto Rico defaulted on a bond payment for the first time Monday, triggering what is likely to be a long battle with creditors as it seeks to restructure more than $72 billion in loans.The U.S. territory, whose governor has declared its debts “unpayable” and is seeking the largest restructuring ever in the country’s municipal bond market, paid just $628,000 of a $58 million payment owed by its Public Finance Corp.