  A molecular Mott insulator $\kappa$-(ET)$_2$B(CN)$_4$ [ET =
bis(ethylenedithio)tetrathiafulvalene] with a distorted triangular lattice
exhibits a quantum disordered state with gapped spin excitation in the ground
state. $^{13}$C nuclear magnetic resonance, magnetization, and magnetic torque
measurements reveal that magnetic field suppresses valence bond order and
induces long-range magnetic order above a critical field $\sim 8$ T. The
nuclear spin-lattice relaxation rate $1/T_1$ shows persistent evolution of
antiferromagnetic correlation above the transition temperature, highlighting a
quantum spin liquid state with fractional excitations. The field-induced
transition as observed in the spin-Peierls phase suggests that the valence bond
order transition is driven through renormalized one-dimensionality and
spin-lattice coupling.
