Turkey has a government which is led by Muslim moderates who seek to integrate their religion with the model of a democratic society. Turkey does not always follow the precepts of democracy and there are numerous violations of human rights. However, Turkey seeks to work for peace in the region and has supported the rights of Syrians in rebellion against the tyranny of President Bashar al-Assad.
Turkish Prime Minister Recep Erdogan is in Iran urging its government to cooperate with western nations such as the US, Rusisa, UK, France, Germany and China in an effor to resolve the nuclear issue of atomic weapons. It would make sense to hold the scheduled meeting in Istanbul since Iranians have a solid relationship with Turkey.
This is one time the US should take a back seat and allow Turkey to lead.