Preheat oven to 350°.
Combine first 7 ingredients in a large bowl; stir well. Add chicken, stirring to coat with yogurt mixture. Cover and marinate in refrigerator 3 hours, stirring occasionally.
Place chicken and marinade in a 13 x 9-inch baking dish coated with cooking spray. Sprinkle sumac over chicken mixture. Bake at 350° for 35 minutes. Turn chicken over, and bake an additional 35 minutes or until done. Serve over couscous.