You could plant them now, but if you do, you will have to make sure they are well watered for the rest of the growing season (baby them during the summer and early fall). Also, you will probably want to give them a little shade and mulch well to keep the roots cooler.

It is my understanding that clematis can take up to 2 years to germinate. I winter sowed some in December, but do not have any sprouts. I have been luck enough to have about 6 volunteers come up in various garden beds the past couple of years. The seeds that you plant will very likely not be true to the parent plant. It still should be fun seeing the variations you get. Good luck!