Complex sentence: He subsequently served as British Ambassador to Russia where he took part in negotiations leading to the 1825 Treaty of Saint Petersburg and as British Ambassador to the Netherlands where he was involved in negotiations leading to the establishment of Belgium in 1831.
Split into simpler sentences: He subsequently served as British Ambassador to Russia, where he took part in negotiations leading to the 1825 Treaty of Saint Petersburg. Then, he served as British Ambassador to the Netherlands, where he was involved in negotiations leading to the establishment of Belgium in 1831.