Two Mn-70 torus-like molecules have been obtained from the alcoholysis in EtOH and 2-ClC2H4OH of [Mn12O12(O2CMe)(16)(H2O)(4)]center dot 4H(2)O center dot 2MeCO(2)H (1) in the presence of (NBu4MnO4)-Mn-n and an excess of MeCO2H. The reaction in EtOH afforded [Mn70O60(O2CMe)(70)(OEt)(20)-(EtOH)(16)(H2O)(22)] (2), whereas the reaction in ClC2H4OH gave [Mn70O60(O2CMe)(70)(OC2H4Cl)(20)(ClC2H4OH)(18)-(H2O)(22)] (3). The complexes are nearly isostructural, each possessing a Mn-70 torus structure consisting of alternating near-linear [Mn-3(mu(3)-O)(4)] and cubic [Mn-4(mu(3)-O)(2)(mu(3)-OR)(2)] (R = OEt, 2; R = OC2H4Cl, 3) subunits, linked together via syn,syn-mu-bridging MeCO2- and mu(3)-bridging O2- groups. 2 and 3 have an overall diameter of similar to 4 nm and crystallize as highly ordered supramolecular nanotubes. Alternating current (ac) magnetic susceptibility measurements, performed on microcrystalline samples in the 1.8-10 K range and a 3.5 G ac field with oscillation frequencies in the 5-1500 Hz range, revealed frequency-dependent out-of-phase signals below similar to 2.4 K for both molecules indicative of the slow magnetization relaxation of single-molecule magnets (SMMs). Single-crystal, magnetization vs field studies on both complexes revealed hysteresis loops below 1.5 K, thus confirming 2 and 3 to be new SMMs. The hysteresis loops do not show the steps that are characteristic of quantum tunneling of magnetization (QTM). However, low-temperature studies revealed temperature-independent relaxation rates below similar to 0.2 K for both compounds, the signature of ground state QTM. Fitting of relaxation data to the Arrhenius equation gave effective barriers for magnetization reversal (U-eff) of 23 and 18 K for 2 and 3, respectively. Because the Mn-70 molecule is close to the classical limit, it was also studied using a method based on the Neel-Brown model of thermally activated magnetization reversal in a classical single-domain magnetic nanoparticle. The field and sweep-rate dependence of the coercive field was investigated and yielded the energy barrier, the spin, the Arrhenius pre-exponential, and the cross-over temperature from the classical to the quantum regime. The validity of this approach emphasizes that large SMMs can be considered as being at or near the quantum-classical nanoparticle interface.