Did you ever think of amending the clay with organic matter. The trouble with planting anything in solid clay is that it is poor draining when it is wet, so plants get waterlogged and in the winter it freezes in a solid lump. When it is dry it bakes into solid clumps that a plant has a hard time spreading it's roots in.

Dig it up when it is moist and add manure or compost and a couple of bags of the cheap non clumping clay kitty litter. It is baked clay in granular form and acts better than gravel or sand to separate the particles of clay.

When you have the soil amended try dogwood. It has white flowers in spring and if you get the variagated one it has white leaves with green edges. Actually there are several varieties of dogwood so look at them all and pick the one you like. They do very well in the shadiest corner. You might want to put Golden creeping jenny in front of the bush. This is a ground cover who's leaves are a bright golden yellow and it has small yellow flowers almost all summer. It really pops in the shade.

HOSTA

Westy

HOSTA

Westy

I'm having good luck with Corsican mint (Mentha requienii) in my own shady clay area. I can't tell what climate zone you're in, however.

If you are not careful amending clay soils, you could end up with concrete. See link for plants that appreciate clay soil.

Here is a link that might be useful: Plants that do well in clay soil