Two-dimensional system of hard cyclic heptamers is studied by Monte Carlo simulations. Isotherms of various crystalline structures formed by the heptamers are obtained. Melting as well as structural phase transitions between crystalline structures are localized. Above melting a rotational phase is observed of hexagonal lattice and isotropic distribution of molecular orientations. With increasing density the coupling between the orientational and rotational degrees of freedom leads to appearance of clearly anisotropic patterns of the atomic density distribution around the lattice sites. These patterns exhibit 6-fold symmetry. At high densities the heptamers form crystalline structures of rectangular lattice and without molecular rotation. Elastic constants of the dense crystalline structures are computed by using an algorithm based on the strain-fluctuation method. It is shown that the densest known crystalline structures of the heptamers exhibit negative Poisson's ratios near close packing.