Is it permissible for an employee who was dismissed and harmed, after the company and his colleagues benefited from his experience and effort in bringing in new clients, to demand financial compensation for that?

The lease contract is not perpetual, and either party may terminate it upon its expiration. You are not permitted to take anything from the company's properties, because you rendered a service for a wage, and you are entitled only to what was stipulated in the contract. The company has the right to terminate your service and not renew your contract.