Question

It was tough but intellectually satisfying. (Rewrite as complex sentence)

Answer 1

it was tough, even if it was satisfying.
hope it helps you :)

Answer 2

Although it was tough, it was intellectually satisfying'.

The sentence in the question is a compound sentence as it is written with a coordinate conjunction, 'but'.

A complex sentence, on the other hand, is constructed with the help of a subordinate conjunction or a relative pronoun, in this case, 'although'.