In a large bowl stir the water, 1 tablespoon salt and 1 tablespoon syrup until dissolved. Stir in the thyme. Smash 3 cloves of garlic and add to the brine with the chicken. Cover and refrigerate, 2 to 3 hours.
Mince the remaining clove of garlic and add to a medium pot with the tomato sauce, broth, vinegar, remaining 2 tablespoons syrup, chipotles, chili powder and salt, to taste. Bring to a simmer over medium heat. Reduce heat to medium-low and gently simmer until thick, stirring occasionally, 25 to 30 minutes. Reserve 1/2 cup of the sauce for serving.
Prepare the grill or grill pan for direct/high heat. Oil the grill grates. Drain the chicken and very pat dry. Grill, turning, until no longer pink near the bone, 15 to 20 minutes (depending on the size of the chicken pieces). Brush with sauce generously as it cooks during the second half of the cooking time. Serve the chicken with the extra sauce on the side.
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