Single-molecule magnets are compounds that exhibit magnetic bistability caused by an energy barrier for the reversal of magnetization (relaxation). Lanthanide compounds are proving promising as single-molecule magnets: recent studies show that terbium phthalocyanine complexes possess large energy barriers, and dysprosium and terbium complexes bridged by an N2(3-) radical ligand exhibit magnetic hysteresis up to 13 K. Magnetic relaxation is typically controlled by single-ion factors rather than magnetic exchange (whether one or more 4f ions are present) and proceeds through thermal relaxation of the lowest excited states. Here we report polylanthanide alkoxide cage complexes, and their doped diamagnetic yttrium analogues, in which competing relaxation pathways are observed and relaxation through the first excited state can be quenched. This leads to energy barriers for relaxation of magnetization that exceed 800 K. We investigated the factors at the lanthanide sites that govern this behaviour.

There has recently been a huge renaissance in the study of the magnetism of 4f-coordination complexes.[1] There have been remarkable results, such as slow relaxation of magnetization in the "single-ion magnets" (Bu 4 N)[Tb(Pc) 2 ] (H 2 Pc = phthalocyanine), for which the thermal energy barrier for relaxation is 330 K.[2] Equally remarkable has been the slow relaxation brought about by the toroidal arrangement of local magnetization vectors in a {Dy 3 } triangle ("spin chirality"). [3] In parallel, studies of polymetallic dysprosium cages have shown slow relaxation in a variety of cages with energy barriers as high as 200 K, [4a] and showing magnetic hysteresis to 8 K.[4b]Much of the fascinating physics of (Bu 4 N)[Tb(Pc) 2 ][2] and other single-ion magnets, such as Na 9 [Er(W 5 O 18 ) 2 ], [5] is associated with their fourfold symmetry. Equally, the toroidal magnetism of the {Dy 3 } cage is associated with the triangular array of 4f-ions.[3] Therefore, we targeted a molecule that had fourfold symmetry and metal triangles. The obvious polyhedron is a square-based pyramid. Oxo-centered {Ln 5 } pyramids of general formula [Ln 5 (m 5 -O)(m 3 -OR) 4 (m 2 -OR) 4 (OR) 5 ] are known for around half of the lanthanoids (R = iPr, Ln = Nd, [6,7] Eu, [8] Gd, [6] Er, [6,9] Yb;[10] R = tBu, Ln = La, [11] Nd [11,12] ) but not with dysprosium. The iso-propoxide-bridged dysprosium square-based pyramid [Dy 5 O(OiPr) 13 ] (1) is made by the reaction of freshly generated KOiPr with DyCl 3 in iPrOH/toluene with a stoichiometric amount of H 2 O (see the Experimental Section). Crystals of 1 form in two different crystal systems: one is isostructural with the previously reported [9] {Er 5 } cage whereas the second has a new unit cell.[13] Both polymorphs have essentially identical magnetic behavior. There is no evidence, either visual or by X-ray diffraction, [14] that samples ever contain a mixture of polymorphs, that is, we have studied pure samples of each. Polymorphs have been previously reported for lanthanide alkoxides. [6,9] In the new structure the square-based pyramid is disordered, with four of the Dy sites common to both disorder forms. The final Dy site is 50:50 disordered over two positions, however it is clear that the square-based pyramid is slightly elongated (Figure 1), with the average distance between the apical dysprosium and the basal dysprosia 3.43 , while the average distance between the adjacent dysprosia within the basal plane is 3.37 in one of the two models and 3.40 in the second. The structure has no crystallographic symmetry.All Dy sites are six-coordinate. The geometry at each site is based on octahedral, but with the Dy shifted towards the terminal alkoxide and away from the central m 5 -oxide-thus each Dy site has local, but non-crystallographic, C 4v symmetry. The distances of the central oxide to the Dy sites fall in the range 2.25-2.60 . The thirteen alkoxides fall into three groups: there is a terminal alkoxide on each metal site; a second group of four alkoxides bridges on each of the four triangul...

A series of homo- and hetero-tri(aryl)boranes incorporating pentafluorophenyl, 3,5-bis(trifluoromethyl)phenyl, and pentachlorophenyl groups, four of which are novel species, have been studied as the acidic component of frustrated Lewis pairs for the heterolytic cleavage of H2. Under mild conditions eight of these will cleave H2; the rate of cleavage depending on both the electrophilicity of the borane and the steric bulk around the boron atom. Electrochemical studies allow comparisons of the electrophilicity with spectroscopic measurements of Lewis acidity for different series of boranes. Discrepancies in the correlation between these two types of measurements, combined with structural characterisation of each borane, reveal that the twist of the aryl rings with respect to the boron-centred trigonal plane is significant from both a steric and electronic perspective, and is an important consideration in the design of tri(aryl)boranes as Lewis acids.

Pentametallic Ln complexes of formula [Ln(5)O(O(i)Pr)(13)] have been made, where Ln(III) = Sm, Gd, Tb, Ho and Er; slow magnetisation relaxation to 33 K is observed for the Ho complex with an energy barrier of ca. 400 K.

Oxidative activation of a B-H bond of a coordinated scorpionate ligand provides an unprecedented route to rhodaboratranes.