  Measurements of proton Nuclear Magnetic Resonance (1H NMR) spectra and
relaxation and of Muon Spin Relaxation ($\mu^{+}$SR) have been performed as a
function of temperature and external magnetic field on two isostructural
lanthanide complexes, Er(trensal) and Dy(trensal) featuring
crystallographically imposed trigonal symmetry. Both the nuclear 1/T1 and muon
$\lambda$ longitudinal relaxation rates, LRR, exhibit a peak for temperatures T
lower than 30K, associated to the slowing down of the spin dynamics, and the
width of the NMR absorption spectra starts to increase significantly at T ca.
50K, a temperature sizably higher than the one of the LRR peaks. The LRR peaks
have a field and temperature dependence different from those previously
reported for all Molecular Nanomagnets. They do not follow the
Bloembergen-Purcell-Pound scaling of the amplitude and position in temperature
and field and thus cannot be explained in terms of a single dominating
correlation time $\tau$c determined by the spin slowing down at low
temperature. Further, for T lower than 50K the spectral width does not follow
the temperature behavior of the magnetic susceptibility chi. We suggest, using
simple qualitative considerations, that the observed behavior is due to a
combination of two different relaxation processes characterized by the
correlation times $\tau$LT and $\tau$HT, dominating for T lower than 30K and T
higher than 50K, respectively. Finally, the observed flattening of LRR for T
lower than 5K is suggested to have a quantum origin.
