You do not need to worry about planting something in front of it to keep it's roots cool. This is a persistent myth.

They do not like root competition at all. What they need is moist soil and a big well amended hole without competition.

Thanks for your help

I would not plant any perennial closer than a foot and a half of your clematis, however a shallow rooted perennial planted fairly close can really compliment the clematis. For early clematis you could try "Globe Master" allium or dianthus. I saw a beautiful combo of perennial lavender poppies in front of "John Warren" clematis. For late flowering clematis try purple coneflowers or gay feather (liatris).