Magnus Schau-Magnussen

Modifying the properties of 4f single-ion magnets by peripheral ligand functionalisation

We study the ligand-field splittings and magnetic properties of three ErIII single-ion magnets which differ in the peripheral ligand sphere but exhibit similar first coordination spheres by inelastic neutron scattering (INS) and SQUID magnetometry. The INS spectra of the three compounds are profoundly different pointing at a strong response of the magnetic behavior to minor structural changes, as they are e.g. encountered when depositing molecules on surfaces. The observation of several magnetic excitations within the J = 15/2 ground multiplet together with single-crystal magnetic measurements allows for the extraction of the sign and magnitude of all symmetry-allowed Stevens parameters. The parameter values and the energy spectrum derived from INS are compared to the results of state-of-the-art ab initio CASSCF calculations. Temperature-dependent alternating current (ac) susceptibility measurements suggest that the magnetisation relaxation in the investigated temperature range of 1.9 K < T < 5 K is dominated by quantum tunnelling of magnetisation and two-phonon Raman processes. The possibility of observing electron paramagnetic resonance transitions between the ground-state doublet states, which can be suppressed in perfectly axial single-ion magnets, renders the studied systems interesting as representations of quantum bits.

Direct observation of a ferri-to-ferromagnetic transition in a fluoride-bridged 3d–4f molecular cluster

We report on the synthesis, crystal structure and magnetic characterisation of the trinuclear, fluoride-bridged, molecular nanomagnet [Dy(hfac)3(H2O)–CrF2(py)4–Dy(hfac)3(NO3)] (1) (hfacH = 1,1,1,5,5,5-hexafluoroacetylacetone, py = pyridine) and a closely related dinuclear species [Dy(hfac)4–CrF2(py)4]·½CHCl3 (2). Element-specific magnetisation curves obtained on 1 by X-ray magnetic circular dichroism (XMCD) allow us to directly observe the field-induced transition from a ferrimagnetic to a ferromagnetic arrangement of the Dy and Cr magnetic moments. By fitting a spin-Hamiltonian model to the XMCD data we extract a weak antiferromagnetic exchange coupling of j = −0.18 cm−1 between the DyIII and CrIII ions. The value found from XMCD is consistent with SQUID magnetometry and inelastic neutron scattering measurements. Furthermore, alternating current susceptibility and muon-spin relaxation measurements reveal that 1 shows thermally activated relaxation of magnetisation with a small effective barrier for magnetisation reversal of Δeff = 3 cm−1. Density-functional theory calculations show that the Dy–Cr couplings originate from superexchange via the fluoride bridges.

Fluoride-Bridged {GdIII3MIII2} (M=Cr, Fe, Ga) Molecular Magnetic Refrigerants†

The reaction of fac-[M(III)F3(Me3tacn)]⋅x H2O with Gd(NO3)3⋅5H2O affords a series of fluoride-bridged, trigonal bipyramidal {Gd(III)3M(III)2} (M = Cr (1), Fe (2), Ga (3)) complexes without signs of concomitant GdF3 formation, thereby demonstrating the applicability even of labile fluoride-complexes as precursors for 3d-4f systems. Molecular geometry enforces weak exchange interactions, which is rationalized computationally. This, in conjunction with a lightweight ligand sphere, gives rise to large magnetic entropy changes of 38.3 J kg(-1)  K(-1) (1) and 33.1 J kg(-1)  K(-1) (2) for the field change 7 T→0 T. Interestingly, the entropy change, and the magnetocaloric effect, are smaller in 2 than in 1 despite the larger spin ground state of the former secured by intramolecular Fe-Gd ferromagnetic interactions. This observation underlines the necessity of controlling not only the ground state but also close-lying excited states for successful design of molecular refrigerants.

Azatrioxa[8]circulenes: Planar Anti-Aromatic Cyclooctatetraenes

We describe herein the first synthesis of a new class of anti-aromatic planar cyclooctatetraenes: the azatrioxa[8]circulenes. This was achieved by treating a suitably functionalised 3,6-dihydroxycarbazole with 1,4-benzoquinones or a 1,4-naphthoquinone. We fully characterised the azatrioxa[8]circulenes by using optical, electrochemical and computational techniques as well as by single-crystal X-ray crystallography. The results of a computational study (NICS) suggest that the central planar cyclooctatetraene is anti-aromatic when the molecules are in neutral or oxidised states (2+), and that the corresponding dianions are aromatic. We discuss the aromatic/anti-aromatic nature of the planar cyclooctatetraenes and compare them with the isoelectronic tetraoxa[8]circulenes.

Diazadioxa(8)circulenes: Planar Antiaromatic Cyclooctatetraenes

In this paper we describe a new class of antiaromatic planar cyclooctatetraenes: the diazadioxa[8]circulenes. The synthesis was achieved by means of a new acid-mediated oxidative dimerization of 3,6-dihydroxycarbazoles to yield the diazadioxa[8]circulenes in high yields. The synthetic protocol appears to be general, and is a one-pot transformation in which two C-C bonds and two C-O bonds are formed with the loss of two molecules of water. We also present a detailed characterization of the optical and electrochemical properties of this new class of stable planar cyclooctatetraenes. The properties of the diazadioxa[8]circulenes are compared with the properties of isoelectronic tetraoxa[8]circulenes and azatrioxa[8]circulenes. We discuss the antiaromatic nature of the planar central cyclooctatetraene moiety. The antiaromatic nature of the planar cyclooctatetraenes was studied by using computational methods (NICS calculations), and these calculations reveal that the central eight-membered ring has antiaromatic character.

Heterobimetallic Nitride Complexes from Terminal Chromium(V) Nitride Complexes: Hyperfine Coupling Increases with Distance†

Terminal nitride complexes of rhenium, osmium and molybdenum can form complexes with either alkylating agents, Lewis acidic metal halides, or low-valent, coordinatively unsaturated metal complexes. The few reactions of this type with a first-row transition-metal complex are limited to vanadium. Recently, the nitride chemistry of the chromium(V) cation has been significantly expanded by introduction of a preparative route which is based on nitrogen transfer from [Mn(N)(salen)] (salen=N,N’-bis(salicylidene)ethylenediamine) to the chromium(V) cation. With a range of chromium nitride complexes at hand we have investigated their reactivity and found that nucleophilicity is a general property which can be observed during formation of imide complexes with, for example, the trityl cation, tris(pentafluorophenyl)boron, and methyl triflate. In addition we report that terminal chromium(V) nitride complexes coordinate through the nitride ligand to low-valent complexes of the platinum metals. These compounds are possible precursors to bimetallic nitride phases which are gaining in importance as heterogeneous catalysts in, for example, the Haber–Bosch process. Solutions of terminal chromium nitride complexes in noncoordinating solvents treated with electrophiles such as B(C6F5)3 or C(C6H5)3 + quickly yield intensely colored orangered or green solutions. The reactions proceed cleanly as shown by EPR spectra which display a signal from a single S= =2 spin species. Similar reactivity was observed in reactions with either [Rh(cod)Cl]2 or cis-[PtCl2(dmso)2] (cod= 1,5cyclooctadiene, dmso= dimethyl sulfoxide). Structures of some of these systems, characterized by single-crystal X-ray diffraction, are shown in Scheme 1. Experimental and crystallographic details such as ORTEP drawings andmetric parameters of complexes 1–5 (Scheme 1) are available in the Supporting Information (Tables S1 and S1a). Inspection of the structures reveals a number of general aspects: there is a strong propensity for the chromium center to increase its coordination number from five to six upon coordination of the nitride ligand. This propensity is expected and a consequence of the trans influence of either an imide or a bridging nitride ligand which is significantly lower than that of a terminal nitride ligand. Accompanying this, the displacement of Cr out of the plane spanned by the equatorial ligators is diminished from about 0.5 to about 0.2 . The Cr N bond length is elongated from 1.55 in the terminal nitride complexes to approximately 1.60–1.62 in the functionalized systems. Comparison of structure 1 with that of [Cr(N)(salen)] reveals that the metal–salen ligand bonds are significantly shorter when the nitride ligand is functionalized, as expected when two ligands compete for electron donation. However, for the systems derived from [Cr(N)(dbm)2] the situation is less clear (dbm= dibenzoylmethanolate). In complex 2 all the Cr–dbm bonds are longer than in the parent terminal nitride complex, while they are shorter or similar within the limits of uncertainty in complex 5. The B N and C N bonds in 1, 4, and 5 are unexceptional but the N Rh and N Pt bond lengths in 2 and 3 are at about 1.970 and 1.906 , respectively, and very short; the first value belongs to the top 5% of the shortest Rh N bonds and the second belongs to the top 1% of the shortest Pt N bonds. Table 1 compares the Pt N bond of 3 with Pt N bonds of other cis[PtCl2(dmso)L] structures. Scheme 1. Schematic representation of the chromium(V) imide and chromium(V) bridging-nitride complexes.

X-ray Magnetic Circular Dichroism (XMCD) Study of a Methoxide-Bridged Dy^III-Cr^III Cluster Obtained by Fluoride Abstraction from cis-[Cr^III F_2 (phen)_2]^+

An isostructural series of dinuclear chromium(III)-lanthanide(III) clusters is formed by fluoride abstraction of cis-[CrF2(phen)2](+) by Ln(3+) resulting in LnF3 and methoxide-bridged Cr-Ln clusters (Ln = Nd (1), Tb (2), Dy (3)) of formula [Cr(III)(phen)2(μ-MeO)2Ln(NO3)4]·xMeOH (x = 2-2.73). In contrast to fluoride, methoxide bridges in a nonlinear fashion, which facilitates chelation. For 3, X-ray magnetic circular dichroism (XMCD) provides element-specific magnetization curves that are compared to cluster magnetization and susceptibility data acquired by SQUID magnetometry. The combination of XMCD and SQUID is able to resolve very small magnetic coupling values and reveals a weak Cr(III)-Dy(III) coupling of j = -0.04(3) cm(-1). The Dy(III) ion has a ground-state Kramers doublet of mJ = ±13/2, and the first excited doublet is found to be mJ = ±11/2 at an energy of δ = 57(21) cm(-1). The Cr(III) ion exhibits a uniaxial anisotropy of DCr = -1.7(1.0) cm(-1). Further, we observe that a weak anisotropic coupling of dipolar origin is sufficient to model the data, suggesting that methoxide bridges do not play a significant role in the magnetic coupling for the present systems.

On the bromination of the dihydroazulene/vinylheptafulvene photo-/thermoswitch.

Summary Background: The dihydroazulene (DHA)/vinylheptafulvene (VHF) system (with two cyano groups at C1) functions as a photo-/thermoswitch. Direct ionic bromination of DHA has previously furnished a regioselective route to a 7,8-dibromide, which by elimination was converted to a 7-bromo-substituted DHA. This compound has served as a central building block for functionalization of the DHA by palladium-catalyzed cross-coupling reactions. The current work explores another bromination protocol for achieving the isomeric 3-bromo-DHA and also explores the outcome of additional bromination of this compound as well as of the known 7-bromo-DHA. Results: Radical bromination on two different VHFs by using N-bromosuccinimide/benzoyl peroxide and light, followed by a ring-closure reaction generated the corresponding 3-bromo-DHAs, as confirmed in one case by X-ray crystallography. According to a 1H NMR spectroscopic study, the ring closure of the brominated VHF seemed to occur readily under the reaction conditions. A subsequent bromination–elimination protocol provided a 3,7-dibromo-DHA. In contrast, treating the known 7-bromo-DHA with bromine generated a very labile species that was converted to a new 3,7-dibromoazulene, i.e., the fully unsaturated species. Azulenes were also found to form from brominated compounds when left standing for a long time in the solid state. Kinetics measurements reveal that the 3-bromo substituent enhances the rate of the thermal conversion of the VHF to DHA, which is opposite to the effect exerted by a bromo substituent in the seven-membered ring. Conclusion: Two general procedures for functionalizing the DHA core with a bromo substituent (at positions 3 and 7, respectively) are now available with the DHA as starting material.

cyclo-Tetra-μ-fluorido-1:2κF;2:3κF;3:4κF;1:4κF-octa-nitrato-1κO,O';3κO,O'-tetra-kis-(1,10-phenanthroline)-2κN,N';4κN,N'-2,4-dichromium(III)-1,3-dineodymium(III) methanol tetra-solvate monohydrate.

In the title compound, [Cr2Nd2F4(NO2)8(C12H8N2)4]·4CH3OH·H2O, two cis-difluoridobis(1,10-phenanthroline)chromium(III) fragments containing octahedrally coordinated chromium(III) bridge via fluoride ions to two tetranitratoneodymate(III) fragments, forming an uncharged tetranuclear square-like core. The fluoride bridges are fairly linear, with Cr—F—Nd angles of 168.74 (8)°. Cr—F bond lengths are 1.8815 (15) Å, slightly elongated compared to those of the parent chromium(III) complex, which has bond lengths ranging from 1.8444 (10) to 1.8621 (10) Å. The tetranuclear complex is centered at a fourfold rotoinversion axis, with the Cr and Nd atoms situated on two perpendicular twofold rotation axes. The uncoordinated water molecule resides on a fourfold rotation axis. The four methanol solvent molecules are located around this axis, forming a cyclic hydrogen-bonded arrangement. The title compound is the first structurally characterized example of unsupported fluoride bridges between lanthanide and transition metal ions.

Salicylaldoxime-Supported Nona- and Tetrametallic Fe III Cages

The syntheses, structures and magnetic properties of seven new iron complexes, (Fe4O2(sao)4(tacn)2)·2MeOH·H2O (1·2MeOH·H2O), (Fe4O2(Me-sao)4(tacn)2)·2MeCN (2·2MeCN), (Fe4O2(Et- sao)4(tacn)2)·MeOH (3·MeOH), (Fe9NaO4(Et-sao)6(hmp)8)·3MeCN·Et2O (4·3MeCN·Et2O), (Fe4(Et-sao)4(hmp)4)·Et-saoH2 (5·Et-saoH2), (Fe4(Ph-sao)4(hmp)4)·2MeCN (6·2MeCN) (Fe9O3(sao)(pdm)6(N3)7(H2O)) (7), stabilised with salicylal- doxime (saoH2) or derivatised salicylaldoxime (R-saoH2) ligands in conjunction with either 1,4,7-triazocyclononane (tacn), 2-(hydroxymethyl)pyridine (hmpH) or 2,6-pyridinedimethanol (pdmH2) are discussed.