  We have successfully synthesized single crystals, solved the crystal
structure, and studied the magnetic properties of a new family of copper
halides (C$_4$H$_{14}$N$_2$)Cu$_2X_6$ ($X$= Cl, Br). These compounds
crystallize in an orthorhombic crystal structure with space group $Pnma$. The
crystal structure features Cu$^{2+}$ dimers arranged parallel to each other
that makes a zig-zag two-leg ladder-like structure. Further, there exists a
diagonal interaction between two adjacent dimers which generates inter-dimer
frustration. Both the compounds manifest a singlet ground state with a large
gap in the excitation spectrum. Magnetic susceptibility is analyzed in terms of
both interacting spin-$1/2$ dimer and two-leg ladder models followed by exact
diagonalization calculations. Our theoretical calculations in conjunction with
the experimental magnetic susceptibility establish that the spin-lattice can be
described well by a frustrated two-leg ladder model with strong rung coupling
($J_0/k_{\rm B} \simeq 116$ K and 300 K), weak leg coupling
($J^{\prime\prime}/k_{\rm B} \simeq 18.6$ K and 105 K), and equally weak
diagonal coupling ($J^{\prime }/k_{\rm B} \simeq 23.2$ K and 90 K) for Cl and
Br compounds, respectively. These exchange couplings set the critical fields
very high, making them experimentally inaccessible. The correlation function
decays exponentially as expected for a gapped spin system. The structural
aspects of both the compounds are correlated with their magnetic properties.
The calculation of entanglement witness divulges strong entanglement in both
the compounds which persists upto high temperatures, even beyond 370~K for the
Br compound.
