Mononuclear lanthanide-based single-ion magnets (SIMs) are known since 2003 with the discovery of SIM properties in a bis-(phthalocyaninato)lanthanide complex. A recent report on [Dy(Cp ttt ) 2 ][BC 6 F 5 ] indicating that it exhibits the highest known blocking temperature (60 K) has spurred fresh interest in this area. In this article, we discuss about the various requirements of lanthanide-based SIMs along with representative examples. Specifically, we describe the complexes whose coordination numbers vary from 2 to 8. We also discuss the representative examples of organometallic lanthanide complexes that can function as molecular magnets.

Molecular transition-metal phosphonates are of relatively recent origin and can be assembled by several synthetic strategies. The nuclearity and the structure of the metal aggregates can be modulated by several factors including the stoichiometry of the reactants, nature of the metal precursor and the type of phosphonic acid used. This perspective summarizes some of the recent work carried out on copper(II)-, zinc(II)- and cadmium(II) phosphonates with particular emphasis on their synthesis and structure.

The reaction of the multisite coordination ligand (LH4) with CoX2·nH2O in the presence of tetrabutylammonium hydroxide affords a series of homometallic dinuclear mixed-valence complexes, [Co(III)Co(II)(LH2)2(X)(H2O)](H2O)m (1, X = Cl and m = 4; 2, X = Br and m = 4; 3, X = NO3 and m = 3). All of the complexes have been structurally characterized by X-ray crystallography. Both cobalt ions in these dinuclear complexes are present in a distorted-octahedral geometry. Detailed magnetic studies on 1-3 have been carried out. M vs H data at different temperatures can be fitted with S = 3/2, the best fit leading to D(3/2) = -7.4 cm(-1), |E/D| < 1 × 10(-3), and g = 2.32 for 1 and D(3/2) = -9.7 cm(-1), |E/D| <1 × 10(-4), and g = 2.52 for 2. In contrast to 1 and 2, M vs H data at different temperatures suggest that compound 3 has comparatively little magnetic anisotropy. In accordance with the large negative D values observed for compounds 1 and 2, they are single-molecule magnets (SMMs) and exhibit slow relaxation of magnetization at low temperatures under an applied magnetic field of 1000 Oe with the following energy barriers: 7.9 cm(-1) (τo = 6.1 × 10(-6) s) for 1 and 14.5 cm(-1) (τo = 1.0 × 10(-6) s) for 2. Complex 3 does not show any SMM behavior, as expected from its small magnetic anisotropy. The τo values observed for 1 and 2 are much larger than expected for a SMM, strongly suggesting that the quantum pathway of relaxation at very low temperatures is not fully suppressed by the effects of the applied field.

The reaction of the lanthanide(III) salts [Dy(III), Tb(III), and Gd (III)] with a hetero donor chelating ligand N'-(2-hydroxy-3-methoxybenzylidene)-6-(hydroxymethyl) picolinohydrazide (LH3) and pivalic acid (PivH) in the presence of tetra-n-butylammonium hydroxide (TBAH) afforded the tetranuclear Ln(III) coordination compounds, [Ln4(LH)2(LH2)2(μ2-η(1)η(1)Piv)2(η(1)Piv)4]·2CHCl3 [Ln = Dy(1), Tb(2), and Gd(3)]. The molecular structure of these complexes reveals that the tetranuclear derivatives are composed of two dinuclear subunits which are interconnected through the coordination action of the picolinoyl hydrazine ligand. Within each subunit two different types of Ln(III) ions are present. One of these is eight-coordinate in a distorted triangular dodecahedral geometry while the other is nine-coordinate in a distorted spherical capped square antiprism geometry. Alternating current (ac) susceptibility measurements of complex 1 reveal a frequency- and temperature-dependent two step out-of-phase signals under 1kOe DC field which is characteristic of a single-molecule magnet (SMM) behavior. Analysis of the magnetic data afforded the anisotropic barriers and relaxation times: Δ/kB = 62.6 K, τ0 = 8.7 × 10(-7) s; Δ/kB = 26.3 K, τ0 = 1.26 × 10(-6) s for the slow and fast relaxations respectively.

Heterometallic 3d-4fc omplexes are being investigated, for some time, as being useful in molecular magnetism, particularly as single-molecule magnets (SMMs). This interest is primarily because of the possibility of an increased ligand-mediated super-exchange phenomenon between the 3d and 4f metal ions. Such an interaction, apart from bestowing af avorable ground-state spin to the complex, also assists in reducing quantum tunneling of magnetization that is widely prevalenti nS MMs making them to lose magnetization. However,a ssembling both 3d as well as 4f ions using same ligand system is challenging and involves the design of multi-site coordination ligandsw ith specific coordination compartments for the 3d and the 4f metal ions while at the same time allowing these disparate metal ions to be linked to each other through ab ridging ligatinga tom. This review presents as ummary of the 3d-4f complexes primarily derived from the author's work while alluding to importante xamplesf rom the literature. We also provide an outlookf or the future design of such complexes.[a] Dr.A .Dey, Prof. V. Chandrasekhar 4434Minireview dro-imidazol-2-ylidenea nd DmpN 3 = 2,6-dimesitylphenyl azide] which exhibit slow magnetic relaxation with U eff values of 450 and 413 cm À1 respectively. [13][14][15] Similarly,t here have been reports on mononuclear lanthanide complexes [12a, 16] ever since the seminalp aper on the first lanthanide based SMM in 2003, (Bu 4 N)[Ln III (Pc) 2 ] [17] [Ln III = Tb III (7)a nd Dy III (8); Pc = phthalocyaninato].R ecently,t he complexes [(Cp ttt ) 2 Dy III ][B(C 6 F 5 ) 4 ] [18] (9) (Cp ttt [19] (10)h ave been reported with blockingt emperatures of 60 and 80 Kr espectively.C omplex 10 holds the record for the highest reported energy barrier U eff = 1541 cm À1 for the reversal of magnetization.One of the problemsa ssociated with SMMs is that the magnetization can be lost by av ariety of ways including quantum tunneling mechanism (QTM) which shortcuts the energy barriers of relaxation. [6b] This is an important impediment and progress in this field is dependent on minimizing or eliminating QTM. There are many strategies to do this including assembling mononuclear complexes possessing point-group symmetries such as C 1v , D 1h , S 8 (I 4 ), D 4d , D 5d and D 5h[12a] or by increasing the ferromagnetic coupling between the adjacent lanthanide centers,a ne xample of this being the dinuclearc omplex [K(18crown-6)]{[((Me 3 Si) 2 n) 2 (THF)Dy III ] 2 (m-h 2 :h 2 -N 2 )} (11)w here the radicall igand, N 2 3À , [20] facilitates af erromagnetic exchange between the two lanthanide centers. [21] Another strategy is to involve heterometallic transition metal/lanthanide complexes where as uper-exchange ferromagnetic interaction between the lanthanide and transition metal ion can mitigate QTM. [22] Accordingly,t he first 3d-4fb ased SMM, a[ Cu II 2 Tb III 2 ]( 12) complex, was reported by Osa and co-workers in 2004. [23] Murray and co-workers have successfully utilized this strategy for av ariety of complexes with {Cr II...