  We report the crystal growth and structural and magnetic properties of quasi
two-dimensional $S=1/2$ quantum magnet
Cu[C$_6$H$_2$(COO)$_4$][H$_3$N-(CH$_2$)$_2$-NH$_3$]$\cdot$3H$_2$O. It is found
to crystallize in a monoclinic structure with space group $C2/m$. The CuO$_4$
plaquettes are connected into a two-dimensional framework in the $ab$-plane
through the anions of [C$_6$H$_2$(COO)$_4$]$^{4-}$ (pyromellitic acid). The
[H$_3$N-(CH$_2$)$_2$-NH$_3$]$^{2+}$$\cdot$3H$_2$O groups are located between
the layers and provide a weak interlayer connection via hydrogen (H...O) bonds.
The temperature dependent magnetic susceptibility is well described by $S=1/2$
frustrated square lattice ($J_1-J_2$) model with nearest-neighbor interaction
$J_1/k_{\rm B} \simeq 5.35$ K and next-nearest-neighbor interaction $J_2/k_{\rm
B} \simeq -0.01$ K. Even, our analysis using frustrated rectangular lattice
($J_{1a,b}-J_2$) model confirms almost isotropic nearest-neighbour interactions
($J_{\rm 1a}/k_{\rm B} \simeq 5.31$ K and $J_{\rm 1b}/k_{\rm B} \simeq 5.38$ K)
in the $ab$-plane and $J_2/k_{\rm B}\simeq-0.24$ K. Further, the isothermal
magnetization at $T=1.9$ K is also well described by a non-frustrated square
lattice model with $J_1/k_{\rm B} \simeq 5.2$ K. Based on the $J_2/J_1$ ratio,
the compound can be placed in the N\'{e}el antiferromagnetic state of the $J_1
- J_2$ phase diagram. No signature of magnetic long-range-order was detected
down to 2 K.
