"To make all laws which shall be necessary and proper for carrying into execution the foregoing powers, and all other powers vested by this Constitution in the government of the United States, or in any department or officer thereof."1. The authority to create money is limited specifically to the Congress, not an extra-governmental institution created by the Congress.
In order to execute the power of coining money, Congress passed the Federal Reserve Act.
Link?2. The main tool of exercising monetary policy (open market operations) for the Fed is unconstitutional because currency creation is limited specifically to coining money.
General Welfare clause (this really applies to all above points as well, but hey).3. Since no such authority is enumerated, the government does not retain the authority to dictate interest rates.