Search

Large separation of magnetic levels and slow relaxation in metal complexes are desirable properties of single-molecule magnets (SMMs). Spin-phonon coupling (interactions of magnetic levels with phonons) is ubiquitous, leading to magnetic relaxation and loss of memory in SMMs and quantum coherence in qubits. Direct observation of magnetic transitions and spin-phonon coupling in molecules is challenging. We have found that far-IR magnetic spectra (FIRMS) of Co(PPh 3 ) 2 X 2 (Co-X; X=Cl, Br, I) reveal rarely observed spin-phonon coupling as avoided crossings between magnetic and u-symmetry phonon transitions. Inelastic neutron scattering (INS) gives phonon spectra. Calculations using VASP and phonopy programs gave phonon symmetries and movies. Magnetic transitions among zero-field split (ZFS) levels of the S=3/2 electronic ground state were probed by INS, high-frequency and -field EPR (HFEPR), FIRMS, and frequency-domain FT terahertz EPR (FD-FT THz-EPR), giving magnetic excitation spectra and determining ZFS parameters (D, E) and g values. Ligand-field theory (LFT) was used to analyze earlier electronic absorption spectra and give calculated ZFS parameters matching those from the experiments. DFT calculations also gave spin densities in Co-X, showing that the larger Co(II) spin density in a molecule, the larger its ZFS magnitude. The current work reveals dynamics of magnetic and phonon excitations in SMMs. Studies of such couplings in the future would help to understand how spin-phonon coupling may lead to magnetic relaxation and develop guidance to control such coupling., (© 2021 Wiley-VCH GmbH.)

Anisotropic magnetic exchange is of great value for the design of high performance molecular nanomagnets. In the present work, enhanced single-chain magnet (SCM) behavior is observed for a Mo III -Mn II chain that exhibits anisotropic magnetic exchange. Self-assembly of the pentagonal bipyramidal [Mo(CN) 7 ] 4- anion and the Mn II unit with a tridentate ligand results in a neutral double zigzag 2,4-ribbon structure which exhibits SCM behavior with a high relaxation barrier of 178(4) K. Open magnetic hysteresis loops are observed below 5.2 K, with a coercive field of 1.5 T at 2 K. Interestingly, this SCM can be considered to be a result of a step-wise process based on our previously reported Mn 2 Mo single-molecule magnets (SMMs)., (© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Bifunctional electrically conducting single-molecule magnets are highly promising platforms for non-volatile memory devices and quantum computing applications. The development of these molecular materials, however, has largely been hindered by the lack of straightforward synthetic methods. Herein a facile and modular approach is demonstrated for the realization of bifunctional materials that does not require electrochemical or chemical oxidation to obtain partially charged organic radicals. Magnetic and electrical conductivity studies reveal that the Dy III compound exhibits slow relaxation of the magnetization between 5.0-8.0 K and semiconducting behavior over the range 180-350 K. DC magnetic fields have been found to suppress the quantum tunneling of the magnetization and affect the spin-canted antiferromagnetic interactions., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A family of distorted trigonal pyramidal transition-metal complexes [M II (N 3 N)Li(THF)] (M=Mn, Fe, Co, Ni) have been studied as candidates for mononuclear single-molecule magnets. Magnetic anisotropy of the family depends on the electronic configuration of the central ion, with the Co analogue exhibiting pronounced SMM behavior., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The heptacyanotungstate(IV) anion was isolated in the salt (n-Bu4 N)3 [W(IV) (CN)7 ] (1) from the reduction of (n-Bu4 N)3 [W(V) (CN)8 ] with concomitant loss of one cyanide ligand. The anion in 1 adopts a slightly distorted pentagonal bipyramidal geometry with an average W-C distance of 2.169 Å. The spectroscopic and magnetic properties are described., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The new complex [Ru(pydppn)(biq)(py)](2+) (1) undergoes both py photodissociation in CH3CN with Φ500 =0.0070(4) and (1)O2 production with ΦΔ =0.75(7) in CH3 OH from a long-lived (3) ππ* state centered on the pydppn ligand (pydppn=3-(pyrid-2-yl)benzo[i]dipyrido[3,2-a:2',3'-c]phenazine; biq = 2,2'-biquinoline; py=pyridine). This represents an order of magnitude decrease in the Φ500 compared to the previously reported model compound [Ru(tpy)(biq)(py)](2+) (3) (tpy=2,2':6',2''-terpyridine) that undergoes only ligand exchange. The effect on the quantum yields by the addition of a second deactivation pathway through the low-lying (3) ππ* state necessary for dual reactivity was investigated using ultrafast and nanosecond transient absorption spectroscopy, revealing a significantly shorter (3) MLCT lifetime in 1 relative to that of the model complex 3. Due to the structural similarities between the two compounds, the lower values of Φ500 and ΦΔ compared to that of [Ru(pydppn)(bpy)(py)](2+) (2) (bpy=2,2'-bipyridine) are attributed to a competitive excited state population between the (3) LF states involved in ligand dissociation and the long-lived (3) ππ* state in 1. Complex 1 represents a model compound for dual activity that may be applied to photochemotherapy., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The ligand-centered radical complex [(CoTPMA)2 -μ-bmtz(.-) ](O3 SCF3 )3 ⋅CH3 CN (bmtz=3,6-bis(2'-pyrimidyl)-1,2,4,5-tetrazine, TPMA=tris-(2-pyridylmethyl)amine) has been synthesized from the neutral bmtz precursor. Single-crystal X-ray diffraction studies have confirmed the presence of the ligand-centered radical. The Co(II) complex exhibits slow paramagnetic relaxation in an applied DC field with a barrier to spin reversal of 39 K. This behavior is a result of strong antiferromagnetic metal-radical coupling combined with positive axial and strong rhombic anisotropic contributions from the Co(II) ions., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Given the recent advent of mononuclear single-molecule magnets (SMMs), a rational approach based on lanthanides with axially elongated f-electron charge cloud (prolate) has only recently received attention. We report herein a new SMM, [Li(THF)4[Er{N(SiMe3)2}3Cl]⋅2 THF, which exhibits slow relaxation of the magnetization under zero dc field with an effective barrier to the reversal of magnetization (ΔEeff/kB =63.3 K) and magnetic hysteresis up to 3 K at a magnetic field sweep rate of 34.6 Oe s(-1). This work questions the theory that oblate or prolate lanthanides must be stabilized with the appropriate ligand framework in order for SMM behavior to be favored., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The tape-like chain {[(tptz)Mn(II) (H2 O)Mn(III) (CN)6 ]2 Mn(II) (H2 O)2 }n ⋅4n MeOH⋅2n H2 O based on the anisotropic building block hexacyanomanganate(III) exhibits long-range magnetic ordering below 5.1 K as well as single-chain magnetic behavior at lower temperatures with an effective energy barrier of 40.5(7) K., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The self-assembly of organic TCNQF˙⁻ radicals (2-fluoro-7,7,8,8-tetracyano-p-quinodimethane) and the anisotropic [Tb(valpn)Cu](3+) dinuclear cations produced a single-chain magnet (SCM) involving stacking interactions of TCNQF˙⁻ radicals (H2valpn is the Schiff base from the condensation of o-vanillin with 1,3-diaminopropane). Static and dynamic magnetic characterizations reveal that the effective energy barrier for the reversal of the magnetization in this hetero-tri-spin SCM is significantly larger than the barrier of the isolated single-molecule magnet based on the {TbCu} dinuclear core., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A family of magnetic metal-organic frameworks, (Ph3PMe)2[M2(TCNQ)3] {M=Fe(2+), Co(2+), Ni(2+) and Zn(2+)} have been prepared and structurally characterized. The honeycomb-like "layers" consist of M(II) ions doubly bridged with dinitrilomethane moieties of two 7,7,8,8-tetracyanoquinodimethane (TCNQ) dianions which are further connected through phenyl rings to form a 3D dianionic framework [M2TCNQ3](2-) with Ph3PMe(+) cations filling cavities that run along the c axis. Studies of the magnetic coupling through the TCNQ dianion in these structures revealed that it can promote long-range magnetic ordering despite the long coupling pathway., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Reactions of the main-group cation Tl(I) with anions of 2,5-derivatives of TCNQ (TCNQ = 7,7,8,8-tetracyanoquinodimethane) have led to the isolation of a family of unprecedented semiconducting main-group-metal-organic frameworks, namely, [Tl(TCNQX(2))], (X = H, Cl, Br, I). A comparison of single-crystal and powder X-ray diffraction data revealed the existence of a third polymorph of the previously reported material Tl(TCNQ)] and two distinct polymorphs of [Tl(TCNQCl(2))], whereas only one phase was identified for [Tl(TCNQBr(2))] and [Tl(TCNQI(2))]. These new results are described in the context of the structures of other known binary metal-TCNQ frameworks that display a variety of coordination environments for the central cation, namely, four-, six-, and eight-coordinate, and different arrangements of the adjacent TCNQ radicals-parallel versus perpendicular-in the stacked columns. The halogen substituents affect the structures and the properties of these compounds, owing to both steric and electronic effects as evidenced by the semiconducting properties of crystals of [Tl(TCNQCl(2))] phase I, [Tl(TCNQBr(2))], and [Tl(TCNQI(2))], which correlate well with the distances of adjacent TCNQ radicals in the columns. 1D infinite Hückel model simulations of the band structures of [Tl(TCNQCl(2))] phase I, [Tl(TCNQBr(2))], and [Tl(TCNQI(2))] were conducted with and without consideration of the Tl(I) cations, the results of which indicate that the charge mobility does not strictly occur in one dimension. The modulations of the band structures with various assumptions of the energy difference (Δ) between the Tl(I) 6s orbital and the TCNQ LUMO orbital were calculated and are discussed in light of the observed properties., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Two families of cationic cyanide-bridged complexes, namely, {[Co(triphos)(CN)(2)](2)[M(MeOH)(4)]}(ClO(4))(2) ([Co(2)M] M=Mn, Fe, Co, and Ni; triphos=1,1,1-tris((diphenylphosphino)methyl)ethane) and {[Co(triphos)(CN)(2)](2)[M(MeOH)(4)](2)}(ClO(4))(4) ([Co(2)M(2)] M=Mn and Ni) have been prepared from reactions of [Co(II)(triphos)(CN)(2)] and M(ClO(4))(2).6H(2)O (M=Mn, Fe, Co, Ni) in methanol. The trinuclear complexes [Co(2)Mn], [Co(2)Fe], and [Co(2)Co], as well as both new tetranuclear complexes [Co(2)Mn(2)] and [Co(2)Ni(2)], exhibit antiferromagnetic coupling between metal centers. In contrast, the [Co(2)Ni] is characterized by ferromagnetic interactions between the Co(II) and Ni(II) centers. The magnetic behavior for these complexes was investigated by DFT calculations and was found to derive from overlap patterns of the different magnetic orbitals as influenced by the angles of the cyanide bridges.

A series of isostructural compounds with formula [M(TCNQF(4))(2)(H(2)O)(6)]TCNQF(4)3 H(2)O (M=Tb (1), Y (2), Y:Tb (74:26) (3), and Y:Tb (97:3) (4); TCNQF(4)= tetrafluorotetracyanoquinodimethane) were prepared and their magnetic properties investigated. Compounds 1, 3, and 4 show the beginning of a frequency-dependent out-of-phase ac signal, and decreasing intensity of the signal with decreased concentration of Tb(III) ions in the diluted samples is observed. No out-of-phase signal was observed for 2, an indication that the behavior of 1, 3, and 4 is indicative of slow paramagnetic relaxation of Tb(III) ions in the samples. A more detailed micro-SQUID study at low temperature revealed an interplay between single-molecule magnetic (SMM) behavior and a phonon bottleneck (PB) effect, and that these properties depend on the concentration of diamagnetic yttrium ions. A combination of SMM and PB phenomena was found for 1, whereby the PB effect increases with increasing dilution until eventually a pure PB effect is observed for 2. The PB behavior is interpreted as being due to the presence of a "sea of organic S=1/2 radicals" from the TCNQF(4) radicals in these compounds. The present data underscore the fact that the presence of an out-of-phase ac signal may not, in fact, be caused by SMM behavior, particularly when magnetic metal ions are combined with organic radical ligands such as those found in the organocyanide family.

Fixing a hole: Hydrothermal chemistry has been exploited in the preparation of a 3D framework material exhibiting 48% accessible void volume and 1.5% hydrogen uptake by weight at 120 kPa (see picture). The title compound also exhibits single-chain magnetic behavior and reversible changes in magnetic properties upon solvation and desolvation.

Reactions of cis-[Rh(2)(DTolF)(2)(NCCH(3))(6)](BF(4))(2) with the dinucleotides d(GpA) and d(ApG) proceed to form [Rh(2)(DTolF)(2){d(GpA)}] and [Rh(2)(DTolF)(2){d(ApG)}], respectively, with bridging purine bases spanning the Rh-Rh unit in the equatorial positions. Both dirhodium adducts exhibit head-to-head (HH) arrangement of the bases, as indicated by the presence of H8/H8 NOE cross-peaks in the 2D ROESY NMR spectra. The guanine bases bind to the dirhodium core at positions N7 and O6, a conclusion that is supported by the absence of N7 protonation at low pH values and the notable increase in the acidity of the guanine N1H sites (pK(a) approximately 7.4 in 4:1 CD(3)CN/D(2)O), inferred from the pH-dependence titrations of the guanine H8 proton resonances. In both dirhodium adducts, the adenine bases coordinate to the metal atoms through N6 and N7, which induces stabilization of the rare imino tautomer of the bases with a concomitant substantial decrease in the basicity of the N1H adenine sites (pK(a) approximately 7.0-7.1 in 4:1 CD(3)CN/D(2)O), as compared to the imino form of free adenosine. The presence of the adenine bases in the rare imino form is further corroborated by the observation of DQF-COSY H2/N1H and ROE N1H/N6H cross-peaks in the 2D NMR spectra of [Rh(2)(DTolF)(2){d(GpA)}] and [Rh(2)(DTolF)(2){d(ApG)}] in CD(3)CN at -38 degrees C. The 2D NMR spectroscopic data and the molecular modeling results suggest the presence of right-handed variants, HH1R, in solution for both adducts (HH1R refers to the relative base canting and the direction of propagation of the phosphodiester backbone with respect to the 5' base). Complete characterization of [Rh(2)(DTolF)(2){d(GpA)}] and [Rh(2)(DTolF)(2){d(ApG)}] by 2D NMR spectroscopy and molecular modeling supports anti-orientation of the sugar residues for both adducts about the glycosyl bonds as well as N- and S-type conformations for the 5'- and 3'-deoxyribose residues, respectively.

Reactions of [W(CN)(8)](3-/4-) anions with complexes of Mn(2+) ion with tridentate organic ligand 2,4,6-tris(2-pyridyl)-1,3,5-triazine (tptz) lead to a series of heterobimetallic complexes. The crystal structures of these compounds are derived from the same basic structural fragment, namely a W(2)Mn(2) square constructed of alternating cyanide-bridged W and Mn ions. In [Mn(II)(tptz)(OAc)(H(2)O)(2)](2){[Mn(II)(tptz)(MeOH)(1.58)(H(2)O)(0.42)](2)[W(V)(CN)(8)](2)}.5 MeOH.9.85 H(2)O (3), isolated molecular squares are co-crystallized with mononuclear cationic Mn(II) complexes. The structure of {[Mn(II)(tptz)(MeOH)](2)[W(IV)(CN)(8)].2 MeOH}(infinity) (4) is based on an infinite chain of vertex-sharing squares, while {[Mn(II) (2)(tptz)(2)(MeOH)(3)(OAc)][W(V)(CN)(8)].3.5 MeOH0.25 H(2)O}(infinity) (5) and {[Mn(II) (2)(tptz)(2)(MeOH)(3)W(V)(CN)(8)][Mn(II)(tptz)(MeOH)W(V)(CN)(8)].2 H(2).OMeOH}(8) (7) are derived from such an infinite chain by removing one of the W-C[triple bond]N-Mn linkages in each of the squares. The decanuclear cluster [Mn(II) (6)(tptz)(6)(MeOH)(4)(DMF)(2)W(V) (4)(CN)(32)].8.2 H(2)O.2.3 MeOH (6) is a truncated version of structure 4 and consists of three vertex-sharing W(2)Mn(2) squares. The structure of [Mn(II)(tptz)(MeOH)(NO(3))](2)[Mn(II)(tptz)(MeOH) (DMF)](2)[W(V)(CN)(8)](2).6 MeOH (8) consists of a hexanuclear cluster, in which the central W(2)Mn(2) square is extended by two Mn side-arms attached via CN(-) ligands to the W corners of the square. The magnetic behavior of these heterobimetallic complexes (except for 4) is dominated by antiferromagnetic coupling between Mn(II) and W(V) ions mediated by cyanide bridges. Compounds 3, 6, and 8 exhibit high spin ground states of S=4, 13, and 9, respectively, while 5 and 7 exhibit behavior typical of a ferrimagnetic chain with alternating spin centers. Complex 4 contains diamagnetic W(IV) centers but holds promise as a potential photomagnetic solid.

Six new divalent metal selenites have been synthesized by hydro-/solvothermal methods which leads to the incorporation of the organic template as a cation or a ligand. The structure of [H(2)pip][Cu(SeO(3))(2)] (1) (pip=piperazine) features 1D anionic chains of [Cu(SeO(3))(2)](2-) which are cross-linked by the template cations through hydrogen bonds into a 2D layer. In [Cu(C(3)H(4)N(2))(SeO(3))] (2) the organic template is coordinated to the copper(II) ion of the inorganic Cu(SeO(3)) layer. The isostructural compounds [H(2)en][M(HSeO(3))(2)Cl(2)] (en=ethylenediamine; M=Cu (3), Co (4)) contain layers of [MCl(2)(HSeO(3))(2)](2-) units (M=Cu, Co), which are cross-linked by the template cations via hydrogen bonds into a 3D network. The structure of [H(2)en][Cu(2)(SeO(3))(2)(HSeO(3))](2)H(2)O (5), consists of a pillared layered architecture in which the Cu(SeO(3)) layers are further interconnected by bridging hydrogen selenite groups (the pillar). The compound [H(2)pip][Cu(2)(Se(2)O(5))(3)] (6), which crystallizes as a 3D open framework represents the first organically templated metal diselenite. These new compounds are thermally stable up to at least 170 degrees C. All of the compounds exhibit fairly strong antiferromagnetic interactions. More interestingly, compounds 3 and 4 behave as a weak ferromagnets below the critical temperatures of T(c)=12 and 8 K, respectively, and both of them exhibit spin-flop phase transitions around 800+/-100 Oe.

Reactions of the compound cis-[Rh2(DTolF)2(CH3CN)6](BF4)2, a formamidinate derivative of the class of antitumor compounds [Rh2(O2CR)4] (R=Me, Et, Pr), with 9-ethylguanine (9-EtGuaH) or the dinucleotide d(GpG) proceed by substitution of the acetonitrile groups, with the guanine bases spanning the Rh--Rh bond, in a bridging fashion, through sites N7/O6. In the case of 9-EtGuaH, both head-to-head (HH) and head-to-tail (HT) isomers are formed, whereas with the tethered bases in d(GpG), only one right-handed conformer HH1R [Rh2(DTolF)2{d(GpG)}] is present in solution. For both cis-[Rh2(DTolF)2(9-EtGuaH)2](BF4)2 and [Rh2(DTolF)2{d(GpG)}], the absence of N7 protonation at low pH and the substantial decrease of the pKa values for N1-H deprotonation, support N7/O6 binding of the bases to the dirhodium core. The N7/O6 binding of the bases is further corroborated by the downfield shift by Deltadelta approximately 4.0 ppm of the 13C NMR resonances for the C6 nuclei as compared to the corresponding resonances of the free ligands. The HH arrangement of the guanine bases in [Rh2(DTolF)2{d(GpG)}] is indicated by the intense H8/H8 ROE cross-peaks in the 2D ROESY NMR spectrum. Complete characterization of the [Rh2(DTolF)2{d(GpG)}] conformer by 2D NMR spectroscopy supports anti-orientation and N (C3'-endo) conformation for both deoxyribose residues. The N-pucker for the 5'-G base is universal in such cross-links, but it is very unusual for platinum and unprecedented for dirhodium HH cross-linked adducts to have both deoxyribose residues in the N-type conformation. The bulk, the nonlabile character, and the electron-donating ability of the formamidinate bridging groups spanning the dirhodium core affect the nature of the preferred dirhodium DNA adducts. Molecular modeling studies performed on [Rh2(DTolF)2{d(GpG)}] corroborate the structural features obtained by NMR spectroscopy.

A paramagnetic Ni-containing molecular square has been synthesized in high yield from divergent precursors. An X-ray structure (shown in the picture) reveals the presence of an encapsulated tetrafluoroborate anion, which appears to be a requisite condition for the formation of the cyclic oligomer.