The President cannot be charged with obstruction of justice under normal criminal law standards, but he can be impeached if Congress determines that what he did fits the broad Constitutional definition of impeachable offenses. One example is Nixon and Watergate. Nixon obstructed justice by refusing to turnover the Watergate tapes but he was not arrested and charged with that offense. He was impeached for it. Impeachment is a constitutional check and balance between the President and Congress. Nixon resigned before being tried by the US Senate where he would have been found guilty. Impeachment and not some Federal statute, is why Trump has put so much explaining into the question of why he fired Comey and whether it had anything to do with the investigation of Trump associates.