Relaxation rates for 2-Dy and 3-Dy do not plateau and instead reach rates slower than the SIP below 6 K. Complex 2-Dy cannot be fit with a QTM term (τQTM > 10 4 s; Figure ) and τQTM of 10 3.6(1) s for 3-Dy (Figure ) is unfeasibly slow compared to the SIP. Both 2-Dy and 3-Dy have increasing rate distributions at low temperature and non-mono-exponential decay at 2 K (β = 0.48; Figure ) which is one of the hallmarks of a phonon-bottleneck effect. The log-log relaxation profile of 2-Dy (Figure ) has two linear regions that cross around 6 K, with a steeper slope at lower temperatures; fitting these rates to Equation S9