A Large [Mn10Na]4 Loop of Four Linked Mn10 Loops

Smart Ligands for Efficient 3d-, 4d- and 5d-Metal Single-Molecule Magnets and Single-Ion Magnets

There has been a renaissance in the interdisciplinary field of Molecular Magnetism since ~2000, due to the discovery of the impressive properties and potential applications of d- and f-metal Single-Molecule Magnets (SMMs) and Single-Ion Magnets (SIMs) or Monometallic Single-Molecule Magnets. One of the consequences of this discovery has been an explosive growth in synthetic molecular inorganic and organometallic chemistry. In SMM and SIM chemistry, inorganic and organic ligands play a decisive role, sometimes equally important to that of the magnetic metal ion(s). In SMM chemistry, bridging ligands that propagate strong ferromagnetic exchange interactions between the metal ions resulting in large spin ground states, well isolated from excited states, are preferable; however, antiferromagnetic coupling can also lead to SMM behavior. In SIM chemistry, ligands that create a strong axial crystal field are highly desirable for metal ions with oblate electron density, e.g., TbIII and DyIII, whereas equatorial crystal fields lead to SMM behavior in complexes based on metal ions with prolate electron density, e.g., ErIII. In this review, we have attempted to highlight the use of few, efficient ligands in the chemistry of transition-metal SMMs and SIMs, through selected examples. The content of the review is purely chemical and it is assumed that the reader has a good knowledge of synthetic, structural and physical inorganic chemistry, as well as of the properties of SIMs and SMMs and the techniques of their study. The ligands that will be discussed are the azide ion, the cyanido group, the tris(trimethylsilyl)methanide, the cyclopentanienido group, soft (based on the Hard-Soft Acid-Base model) ligands, metallacrowns combined with click chemistry, deprotonated aliphatic diols, and the family of 2-pyridyl ketoximes, including some of its elaborate derivatives. The rationale behind the selection of the ligands will be emphasized.

Giant Heterometallic [Mn36Ni4]0/2- and [Mn32Co8] "Loops-of-Loops-and-Supertetrahedra" Molecular Aggregates

We report the synthesis, crystal structures and magnetic properties of the giant heterometallic [Mn₃₆Ni₄]^2−/0 (compounds 1, 2)/[Mn₃₂Co₈] (compound 3) “loops-of-loops-and-supertetrahedra” molecular aggregates and of a [Mn₂Ni₆]²⁺ compound (cation of 4) that is structurally related with the cation co-crystallizing with the anion of 1. In particular, after the initial preparation and characterization of compound [Mn₂Ni₆(μ₄-O)₂(μ₃-OH)₃(μ₃-Cl)₃(O₂CCH₃)₆(py)₈]²⁺[Mn₃₆Ni₄(μ₄-O)₈(μ₃-O)₄(μ₃-Cl)₈Cl₄(O₂CCH₃)₂₆(pd)₂₄(py)₄]²⁻ (1) we targeted the isolation of (i) both the cationic and the anionic aggregates of 1 in a discrete form and (ii) the Mn/Co analog of [Mn₃₆Ni₄]²⁻ aggregate. Our synthetic efforts toward these directions afforded the discrete [Mn₃₆Ni₄] “loops-of-loops-and-supertetrahedra” aggregate [Mn₃₆Ni₄(μ₄-O)₈(μ₃-O)₄(μ₃-Cl)₈Cl₂(O₂CCH₃)₂₆(pd)₂₄(py)₄(H₂O)₂] (2), the heterometallic Mn/Co analog [Mn₃₂Co₈(μ₄-O)₈(μ₃-O)₄(μ₃-Cl)₈Cl₂(μ₂-OCH₂CH₃)₂(O₂CCH₃)₂₈(pd)₂₂(py)₆] (3) and the discrete [Mn₂Ni₆]²⁺ cation [Mn₂Ni₆(μ₄-O)₂(μ₃-OH)₄(μ₃-Cl)₂(O₂CCH₃)₆(py)₈](ClO₄)(OH) (4). The structure of 1 consists of a mixed valence [Mn28IIIMn8IINi4II]²⁻ molecular aggregate that contains two Mn8IIINi2II loops separated by two Mn6IIIMn4II supertetrahedral units and a [Mn2IIINi6II]²⁺ cation based on two [Mn^IIINi3II(μ₄-O)(μ₃-OH)_1.5(μ₃-Cl)_1.5]⁴⁺ cubane sub-units connected through both mono- and tri-atomic bridges provided by the μ₄-O²⁻ and carboxylate anions. The structures of 2–4 are related to those of the compounds co-crystallized in 1 exhibiting however some differences that shall be discussed in detail in the manuscript. Magnetism studies revealed the presence of dominant ferromagnetic interactions in 1–3 that lead to large ground state spin (S_T) values for the “loops-of-loops-and-supertetrahedra” aggregates and antiferromagnetic exchange interactions in 4 that lead to a low (and possibly zero) S_T value. In particular, dc and ac magnetic susceptibility studies revealed that the discrete [Mn₃₆Ni₄] aggregate exhibits a large S_T value ~ 26 but is not a new SMM. The ac magnetic susceptibility studies of the [Mn₃₂Co₈] analog revealed an extremely weak beginning of an out-of-phase tail indicating the presence of a very small relaxation barrier assignable to the anisotropic Co²⁺ions and a resulting out-of-phase ac signal whose peak is at very low T.

Heteronuclear cobalt(iii)/sodium complexes with salen type compartmental Schiff base ligands: methylene spacer regulated variation in nuclearity

Three unique heteronuclear cobalt(iii)/sodium Schiff base complexes have been synthesized and characterized. Nuclearity of these complexes changes as a result of alteration of the steric hindrance in the ligand moiety.

Filling the gap between the quantum and classical worlds of nanoscale magnetism: giant molecular aggregates based on paramagnetic 3d metal ions

In this review, aspects of the syntheses, structures and magnetic properties of giant 3d and 3d/4f paramagnetic metal clusters in moderate oxidation states are discussed. The term "giant clusters" is used herein to denote metal clusters with nuclearity of 30 or greater. Many synthetic strategies towards such species have been developed and are discussed in this paper. Attempts are made to categorize some of the most successful methods to giant clusters, but it will be pointed out that the characteristics of the crystal structures of such compounds including nuclearity, shape, architecture, etc. are unpredictable depending on the specific structural features of the included organic ligands, reaction conditions and other factors. The majority of the described compounds in this review are of special interest not only for their fascinating nanosized structures but also because they sometimes display interesting magnetic phenomena, such as ferromagnetic exchange interactions, large ground state spin values, single-molecule magnetism behaviour or impressively large magnetocaloric effects. In addition, they often possess the properties of both the quantum and the classical world, and thus their systematic study offers the potential for the discovery of new physical phenomena, as well as a better understanding of the existing ones. The research field of giant clusters is under continuous evolution and their intriguing structural characteristics and magnetism properties that attract the interest of synthetic Inorganic Chemists promise a brilliant future for this class of compounds.

Self-assembly of linear [Mn(II)2Mn(III)] units with end-on azido bridges: the construction of a ferromagnetic chain using ST = 7 high-spin trimers

The controlled organization of high-spin complexes into 1D coordination polymers is a challenge in molecular magnetism. In this work, we report a ferromagnetic Mn trimer Mn3(HL)2(CH3OH)6(Br)4·Br·(CH3OH)2 1 (H2L = 2-[(9H-fluoren-9-yl)amino]propane-1,3-diol) with the ground spin state of ST = 7 that can be assembled into a one-dimensional coordination chain [Mn3(HL)2(CH3OH)2(Br)4(N3)(H2O)·CH3OH]∞ 2 using azido bridging ligands. Interestingly, the ferromagnetic nature of 1 is well retained in 2. However, due to the negligible magnetic anisotropy in 1, both 1 and 2 do not show slow-relaxation of magnetization, which indicates that during the process of molecular assembly not only the intratrimer magnetic interaction but also the magnetic anisotropy of the trimer can be reserved.

Synthesis of an S(T) = 7 [Mn3] mixed-valence complex based on 1,3-propanediol ligand derivatives and its one-dimensional assemblies

Controlled organization of high-spin complexes and single-molecule magnets is a great challenge in molecular magnetism in order to study the effect of the intercomplex magnetic interactions on the intrinsic properties of a given magnetic object. In this work, a new S(T) = 7 trinuclear mixed-valence Mn complex, [Mn(III)Mn(II)2(LA)2(Br)4(CH3OH)6] ·Br·(CH3OH)(1.5)·(H2O)(0.5) (1), is reported using a pyridinium-functionalized 1,3-propanediol ligand (H2L(A)Br = 1-(3-bromo-2,2-bis(hydroxymethyl)propyl)pyridinium bromide). Using azido anions as bridging ligands and different pyridinium-functionalized 1,3-propanediol ligands (H2L(B)Br = 1-(3-bromo-2,2-bis(hydroxymethyl)propyl)-4-picolinium bromide; H2L(C)Br = 1-(3-bromo-2,2-bis(hydroxymethyl)propyl)-3,5-lutidinium bromide), the linear [Mn(III)Mn(II)2L2X4](+) building block has been assembled into one-dimensional coordination networks: [Mn(III)Mn(II)2(LA)2(Br)4(CH3OH)4(N3)]·((C2H5)2O)1.25 (2∞), [Mn(III)Mn(II)2(L(B))2(Br)4(C2H5OH)(CH3OH)(H2O)2(N3)]·(H2O)0.25 (3∞), and [Mn(III)Mn(II)2(L(C))2(Cl)(3.8)(Br)(0.2)(C2H5OH)3(CH3OH)(N3)] (4∞). The syntheses, characterization, crystal structures, and magnetic properties of these new [Mn3]-based materials are reported.

Syntheses, crystal structures and magnetic properties of a novel family of penta-manganese complexes derived from an assembly system containing polydentate hydroxy-rich Schiff-base ligands

The syntheses, crystal structures, and magnetochemical characterization are reported of a novel family of four pentanuclear manganese complexes, namely, [Mn(III)(5)(mu(3)-O)(2)(L(1))(4)(O(2)CMe)(3)(CH(3)OH)].1.5CH(3)OH.2.5H(2)O (1), [Mn(III)(5)(mu(3)-O)(2)(L(1))(4)(O(2)CPh)(3)(CH(3)OH)].2CH(3)OH.2.25CH(3)CN.1.5H(2)O (), [Mn(II)Mn(III)(4)(HL(2))(2)(L(2))(2)(O(2)C Me)(4))].CH(3)OH.H(2)O (3) and [Mn(II)Mn(III)(4)(HL(2))(2)(L(2))(2)(O(2)CPh)(4))].1.5H(2)O (4), where H(2)L(1) is 3,5-dibromosalicylidene-2-ethanolamine and H(3)L(2) is 3-(2-hydroxy-3,5-dibromobenzylideneamino)propane-1,2-diol. All the complexes can be obtained from a reaction system containing the trinuclear species [Mn(3)O(O(2)CR)(6)(Py)(3)](0/+) (R = Me or Ph) and H(2)L(1) or H(3)L(2) with similar procedures. Both cores in complexes 1 and 2 feature two mu(3)-O(2-) atoms, four L(1)(2-) ligands together with three RCO(2)(-) groups (R = Me or Ph) bridging five Mn(III) atoms to form an incomplete cubane extended at one face by an incomplete adamantane unit, which is an unprecedented structural type in Mn chemistry. Complexes 3 and 4 both have a rare [Mn(II)Mn(III)(4)(mu(2)-O(alkoxide))(6)](8+) core, which can be regarded as two {Mn(II)Mn(III)(2)(mu(2)-O(alkoxide))(3)} scalene triangles, sharing the Mn(II) vertex. The dc magnetic susceptibility studies in the 2-300 K range for complexes 1-4 reveal the presence of overall antiferromagnetic intracluster interactions. A simple 3-J model was found to be adequate to describe the variable-temperature dc susceptibility data of complexes 1-4. The results have allowed us to compare the obtained magnetic exchange with magneto-structural correlations found previously for manganese-oxo clusters.

Synthesis and Characterization of a Linear [Mn(3)(O(2)CMe)(4)(py)(8)] Complex

Two new compounds that consist of the linear trinuclear manganese(II) cation [Mn₃(O₂CMe)₄(py)₈]²⁺ cocrystallizing with different counteranions (I₃⁻, [1]; ClO₄⁻, [2]) are reported. Complex 1 was prepared from the reaction of [Mn(O₂CMe)₂] · 4H₂O with I₂ in MeCO₂H/py, whereas complex 2 was isolated from the reaction of [Mn₃O(O₂CMe)₆(py)₃] · py with [Mn(ClO₄)₂] · 6H₂O in MeCN/py. The crystal structures of both compounds were determined by single crystal X-ray crystallography. Magnetic susceptibility studies that were performed in microcrystalline powder of 1 in the 2–300 K range revealed the presence of antiferromagnetic exchange interactions that resulted in an S = 5/2 ground spin state.

Structural motifs and topological representation of Mn coordination clusters

Polynuclear coordination clusters have become of particular interest in recent times as a result of their relevance to bioinorganic chemistry and to the special area of molecule-based magnetic materials where cluster compounds behave as single-molecule magnets (SMMs). In this review we have focused on describing Mn coordination cluster complexes. Adopting our topological approach for the description of coordination clusters we present a means of classifying the structural motifs found in manganese clusters which range in nuclearity from 5 to 84, as well as some representative heterometallic Mn-M (M = K, Na, Ca, Sr, Ln) cluster complexes that have been reported. This sheds new light on the classification of the types of core structure accessible which, in turn, provides a useful means for developing the so-far missing magneto-structural correlation algorithm for these finite 0-D systems (212 references).

High nuclearity single-molecule magnets: a mixed-valence Mn26 cluster containing the di-2-pyridylketone diolate dianion

The employment of the dianion (dpkd(2-)) of the gem-diol form of di-2-pyridylketone (dpk) as a tetradentate chelate in manganese chemistry is reported, and the synthesis, crystal structure, and magnetochemical characterization of [Mn26O16(OMe)12(dpkd)12(MeOH)6](OH)6 x solv (3 x solv) are described. The reaction of Mn(ClO4)2 x 6 H2O, dpk, NaOMe, and NEt3 (2:1:4:2) in MeCN/MeOH affords complex 3, which possesses a rare metal topology and is mixed-valence (4 Mn(II), 22 Mn(III)). The complicated [Mn26(mu4-O)10(mu3-O)6(mu3-OMe)12(mu-OR)12](18+) core of 3 consists of an internal Mn(III)16 cage of adjacent Mn4 tetrahedra surrounded by an external Mn(II)4Mn(III)6 shell. The latter is held together by the alkoxide arms of twelve eta(1):eta(2):eta(1):eta(1):mu3 dpkd(2-) groups. Variable-temperature, solid-state direct current (dc), and alternating current (ac) magnetization studies were carried out on 3 in the 1.8-300 K range. Complex 3 is predominantly antiferromagnetically coupled with a resulting S = 6 ground state, a conclusion confirmed by the in-phase (chi'(M)) ac susceptibility data. The observation of out-of-phase (chi''(M)) ac susceptibility signals suggested that 3 might be a single-molecule magnet, and this was confirmed by single-crystal magnetization vs dc field sweeps that exhibited hysteresis, the diagnostic property of a magnet. Combined ac chi''(M) and magnetization decay vs time data collected below 1.1 K were used to construct an Arrhenius plot; the fit of the thermally activated region above approximately 0.1 K gave U(eff) = 30 K, where U(eff) is the effective relaxation barrier. At lower temperatures, the complex exhibits temperature-independent relaxation, characteristic of ground-state quantum tunneling of magnetization between the lowest-lying M(s) = +/-6 levels. The combined work demonstrates the ligating flexibility of dipyridyl-diolate chelates and their usefulness in the synthesis of polynuclear Mn(x) clusters with interesting magnetic properties, without requiring the co-presence of carboxylate ligands.