This paper deals with the problem of collision-free formation tracking of second-order Multi-Agent Systems (MAS) under communication constraints. It is assumed that an agent can only transmit its position to its neighbors at asynchronous and aperiodic discrete time instants. Velocity and acceleration of the agents are not available. Moreover, the dynamics of the leader can be controlled through an external input. Using continuous-discrete time observers, an output feedback formation controller is designed to drive the agents to a desired formation shape and to ensure that the formation tracks the leader trajectory under directed communication topology. A potential function based mechanism is incorporated into the proposed formation tracking algorithm to avoid collision between agents. The simulation results have shown the efficacy of the proposed algorithm.