  We investigate the dynamics of a large anisotropic spin whose easy-axis
component is coupled to a bosonic bath with a spectral function $J(\w)\propto
\omega^s$. Such a spin complex might be realized in a single-molecular magnet.
Using the non-perturbative renormalization group, we calculate the line of
quantum-phase transitions in the sub-ohmic regime ($s<1$). These quantum-phase
transitions only occur for integer spin $J$. For half-integer $J$, the low
temperature fixed-point is identical to the fixed-point of the spin-boson model
without quantum-tunneling between the two levels. Short-time coherent
oscillations in the spin decay prevail even into the localized phase in the
sub-ohmic regime. The influence of the reorganization energy and the recurrence
time on the decoherence in the absence of quantum-tunneling is discussed.
