Is prayer valid in a hall rented for parties, knowing that there is an intention to build a special prayer room for the Muslim community in the complex?

Your prayer is valid in the hall you mentioned. The principle is that prayer is valid in any place whose impurity is not known, due to the generality of the Prophet's (peace be upon him) saying: "The earth has been made for me a place of prayer and a means of purification. So, any man from my Ummah whom the time of prayer reaches, should pray." Your effort in dedicating a place for prayer is a righteous deed for which you will be rewarded.