Article on the subject: Technically it could be considered obstruction if one could legally prove the action was taken with corrupt intent - but that's the hard part itself.

It is not enough to show that a defendant knew the act would have a side consequence of impeding an investigation; achieving that obstruction has to have been the specific intention.
“To prove that he did it not because Comey was grandstanding or showboating or all the other excuses he has given, but because he wanted to impede the investigation, that would be awfully hard to prove
As for impeachment, there are any number of things to construe as impeachable under Trump, but

the Constitution’s standards for impeachment and removal of a president — if he has committed “treason, bribery, or other high crimes and misdemeanors” — are met by anything that a majority of the House and two-thirds of the Senate are willing to vote for. That makes prognostication an exercise in vote counting, not legal analysis. Because Mr. Trump’s fellow Republicans control both chambers of Congress, as things stand, he is exceedingly unlikely to be impeached for firing Mr. Comey.
while the threshold may have been met for Democrats, it has clearly not been met for Republicans.

Possibly if Trump were to do something to personally dishonor or threaten influential Republican Congresspersons - on top of the putative venality.