The recently reported series of divalent lanthanide complex salts, namely [K(2.2.2-cryptand)][Cp'3Ln] (Ln = Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm; Cp' = C5H4SiMe3) and the analogous trivalent complexes, Cp'3Ln, have been characterized via dc and ac magnetic susceptibility measurements. The salts of the complexes [Cp'3Dy](-) and [Cp'3Ho](-) exhibit magnetic moments of 11.3 and 11.4 μB, respectively, which are the highest moments reported to date for any monometallic molecular species. The magnetic moments measured at room temperature support the assignments of a 4f(n+1) configuration for Ln = Sm, Eu, Tm and a 4f(n)5d(1) configuration for Ln = Y, La, Gd, Tb, Dy, Ho, Er. In the cases of Ln = Ce, Pr, Nd, simple models do not accurately predict the experimental room temperature magnetic moments. Although an LS coupling scheme is a useful starting point, it is not sufficient to describe the complex magnetic behavior and electronic structure of these intriguing molecules. While no slow magnetic relaxation was observed for any member of the series under zero applied dc field, the large moments accessible with such mixed configurations present important case studies in the pursuit of magnetic materials with inherently larger magnetic moments. This is essential for the design of new bulk magnetic materials and for diminishing processes such as quantum tunneling of the magnetization in single-molecule magnets.

The tris(cyclopentadienyl) yttrium complexes Cp 3 Y-(THF), Cp Me 3 Y(THF), Cp″ 3 Y, Cp″ 2 YCp, and Cp″ 2 YCp Me [Cp = C 5 H 5 , Cp Me = C 5 H 4 Me, Cp″ = C 5 H 3 (SiMe 3 ) 2 ] have been treated with potassium graphite in the presence of 2.2.2-cryptand to search for more stable examples of complexes featuring the recently discovered Y 2+ ion first isolated in [K(18-crown-6)][Cp′ 3 Y] and [K(2.2.2-cryptand)][Cp′ 3 Y], 1-Y (Cp′ = C 5 H 4 SiMe 3 ). Reduction of the tris(cyclopentadienyl) complexes generates dark solutions like that of 1-Y, and the EPR spectra contain doublets with g values between 1.990 and 1.991 and hyperfine coupling constants of 34−47 gauss that are consistent with the presence of Y 2+ . [K(2.2.2-cryptand)][Cp″ 2 YCp], 2-Y, was characterizable by X-ray crystallography. Reduction of the Cp″ 3 Gd, Cp″ 2 GdCp, and Cp″ 2 GdCp Me complexes containing the larger metal gadolinium were also examined. In each case, dark solutions and EPR spectra like that of [K(2.2.2-cryptand)][Cp′ 3 Gd], 1-Gd, were obtained, and [K(2.2.2-cryptand)][Cp″ 2GdCp], 2-Gd, was crystallographically characterizable. None of the new yttrium and gadolinium complexes displayed greater stability than 1-Y and 1-Gd. Exploration of this reduction chemistry with indenyl ligands did not give evidence for +2 complexes. The only definitive information obtained from reductions of the Cp In 3 Ln (Cp In = C 9 H 7 , Ln = Y, Ho, Dy) complexes was the X-ray crystal structure of {K(2.2.2-cryptand)} 2 {[(C 9 H 7 ) 2 Dy(μ−η 5 :η 1 -C 9 H 6 )] 2 }, a complex containing the first example of the indenyl dianion, (C 9 H 6 ) 2− , derived from C−H bond activation of the (C 9 H 7 ) 1− monoanion. Density functional theory analysis of these results provides an explanation for the observed hyperfine coupling constants in the yttrium complexes and for the C−H bond activation observed for the indenyl complex. ■ INTRODUCTIONRecent studies of the reduction chemistry of yttrium and the f elements have shown that the +2 ions are available for yttrium, 1 all the lanthanides 2−4 (except promethium, which was not studied due to its radioactivity), uranium, 5 and thorium. 6 These new oxidation states have been obtained by reduction of the tris(cyclopentadienyl) complexes, Cp′ 3 M and Cp″ 3 M [Cp′ = C 5 H 4 SiMe 3 , M = Y, lanthanide, U; Cp″ = C 5 H 3 (SiMe 3 ) 2 , M = La, Ce, Th] to form (Cp′ 3 M) 1− and (Cp″ 3 M) 1− complexes, Schemes 1 and 2.Structural, spectroscopic, and density functional theory analyses suggest that these new ions could be accessed for the first time because the (Cp′ 3 ) 3− and (Cp″ 3 ) 3− ligand sets allow the d z 2 orbital to be populated such that the new ions have 4f n 5d 1 electron configurations for the lanthanides, 5f 3 6d 1 for uranium, 6d 2 for thorium, and 4d 1 for yttrium. This is consistent with numerous theoretical analyses of the f elements in trigonal tris(cyclopentadienyl) coordination environments. 8−13 Whereas reduction of a 4f n Ln 3+ ion to a 4f n+1 Ln 2+ product would be difficult due to the highly negative calculated generic r...

The synthesis and full magnetic characterization of a new series of N2(3-) radical-bridged lanthanide complexes [{(R2N)2(THF)Ln}2(μ3-η(2):η(2):η(2)-N2)K] [1-Ln; Ln = Gd, Tb, Dy; NR2 = N(SiMe3)2] are described for comprehensive comparison with the previously reported series [K(18-crown-6)(THF)2]{[(R2N)2(THF)Ln]2(μ-η(2):η(2)-N2)} (2-Ln; Ln = Gd, Tb, Dy). Structural characterization of 1-Ln crystals grown with the aid of a Nd2Fe13B magnet reveals inner-sphere coordination of the K(+) counterion within 2.9 Å of the N2(3-) bridge, leading to bending of the planar Ln-(N2(3-))-Ln unit present in 2-Ln. Direct current (dc) magnetic susceptibility measurements performed on 1-Gd reveal antiferromagnetic coupling between the Gd(III) centers and the N2(3-) radical bridge, with a strength matching that obtained previously for 2-Gd at J ∼ -27 cm(-1). Unexpectedly, however, a competing antiferromagnetic Gd(III)-Gd(III) exchange interaction with J ∼ -2 cm(-1) also becomes prominent, dramatically changing the magnetic behavior at low temperatures. Alternating current (ac) magnetic susceptibility characterization of 1-Tb and 1-Dy demonstrates these complexes to be single-molecule magnets under zero applied dc field, albeit with relaxation barriers (Ueff = 41.13(4) and 14.95(8) cm(-1), respectively) and blocking temperatures significantly reduced compared to 2-Tb and 2-Dy. These differences are also likely to be a result of the competing antiferromagnetic Ln(III)-Ln(III) exchange interactions of the type quantified in 1-Gd.

Gaining control of the building blocks of magnetic materials and thereby achieving particular characteristics will make possible the design and growth of bespoke magnetic devices. While progress in the synthesis of molecular materials, and especially coordination polymers, represents a significant step towards this goal, the ability to tune the magnetic interactions within a particular framework remains in its infancy. Here we demonstrate a chemical method which achieves dimensionality selection via preferential inhibition of the magnetic exchange in an S = 1/2 antiferromagnet along one crystal direction, switching the system from being quasi-two-to quasi-one-dimensional while effectively maintaining the nearest-neighbour coupling strength.Coordination polymers are self-organising materials consisting of arrays of metal ions linked via molecular ligands, with non-coordinated counterions supplying charge neutrality. The choice of initial components permits a high level of control over the final product, enabling many different polymeric architectures to be obtained [1]. These materials provide a route to successful crystal engineering, and a number of functionalities are being actively studied, including gas storage [2-4], optoelectronic [5,6], ferroelectric [7,8] and magnetic properties [9-14].Although it is now possible to generate an assortment of disparate magnetic lattices using this method [15,16], true control of magnetic exchange interactions implies an ability to adjust selected parameters while keeping others constant. To this end, a series of coordination polymers based on Cu(II) ions bridged by pyrazine (C 4 H 4 N 2 ) molecules have proven to be highly versatile. In these systems it has been shown that it is possible to alter significantly the primary exchange energies via adjustment of the ligands [17] and the counterions [18, S9], or fine-tune the exchange by a few percent via isotopic substitution [20], all the while maintaining the same basic metal-pyrazine network. In this paper we demonstrate the power of this strategy by chemically engineering a reduction in the dimensionality of a magnetic system. After first designing a material based on coordinated planes of Cu(II), we adapt the recipe such that the ligand bridges are broken along a specific crystal direction, resulting in a chain-like compound. Because the ligand mediating the magnetic interactions in both cases is unchanged, the nearest-neighbour exchange energies of the two materials are found to be equal to each other to within 5%. The difference in numbers of nearest-neighbours, however, means that the strength of the combined exchange interactions acting on each magnetic ion in the quasitwo-dimensional material is twice that of its quasi-onedimensional cousin.Figs. 1(a) and (b) show the crystal structure of orthorhombic [Cu(pyz) 2 (pyO) 2 ](PF 6 ) 2 (where pyz = pyrazine and pyO = pyridine-N -oxide, C 5 H 5 NO) determined using single-crystal x-ray diffraction [21]. S = 1/2 Cu ions are linked by pyz molecules into nearly square planar ...

Using a mixed-ligand synthetic scheme, we create a family of quasi-two-dimensional antiferromagnets, namely, [Cu(HF2)(pyz)2]ClO4 [pyz = pyrazine], [CuL2(pyz)2](ClO4)2 [L = pyO = pyridine-N-oxide and 4-phpyO = 4-phenylpyridine-N-oxide. These materials are shown to possess equivalent two-dimensional [Cu(pyz)2] 2+ nearly square layers, but exhibit interlayer spacings that vary from 6.5713Å to 16.777Å, as dictated by the axial ligands. We present the structural and magnetic properties of this family as determined via x-ray diffraction, electron-spin resonance, pulsed-and quasistatic-field magnetometry and muon-spin rotation, and compare them to those of the prototypical two-dimensional magnetic polymer Cu(pyz)2(ClO4)2. We find that, within the limits of the experimental error, the two-dimensional, intralayer exchange coupling in our family of materials remains largely unaffected by the axial ligand substitution, while the observed magnetic ordering temperature (1.91 K for the material with the HF2 axial ligand, 1.70 K for the pyO and 1.63 K for the 4-phpyO) decreases slowly with increasing layer separation. Despite the structural motifs common to this family and Cu(pyz)2(ClO4)2, the latter has significantly stronger two-dimensional exchange interactions and hence a higher ordering temperature. We discuss these results, as well as the mechanisms that might drive the long-range order in these materials, in terms of departures from the ideal S = 1/2 two-dimensional square-lattice Heisenberg antiferromagnet. In particular, we find that both spin exchange anisotropy in the intralayer interaction and interlayer couplings (exchange, dipolar, or both) are needed to account for the observed ordering temperatures, with the intralayer anisotropy becoming more important as the layers are pulled further apart.