July 31 (Reuters) - Puerto Rico will miss a payment on debt due Aug. 1, the governor's chief of staff said on Friday, an event that will be considered a default by investors as the commonwealth lurches towards what could be one of the largest U.S. municipal debt restructurings in history. The island faces a number of debt payments that day but had signaled in recent weeks that it may miss the $58 million payment on Public Finance Corporation (PFC) bonds. "Tomorrow is Aug.