It's very difficult to claim that Robert Mueller wouldn't be impartial in this investigation when it was G.W. Bush that appointed him to FBI director during his term.  Let's face it, Trump  didn't get what he wanted. After firing Comey he wanted to pick a new FBI director, one that would pledge loyalty to him and drop the Russian and Flynn investigations.  Instead he got Robert Mueller, a special prosecutor that is well respected by both Republicans and Democrats.
http://www.cnn.com/2017/06/14/politics/robert-mueller-donald-trump/index.html