Hans Toftlund

Systematic study of spin crossover and structure in [Co(terpyRX)2](Y)2 systems (terpyRX = 4'-alkoxy-2,2':6',2''-terpyridine, X = 4, 8, 12, Y = BF4(-), ClO4(-), PF6(-), BPh4(-)).

A family of spin crossover cobalt(II) complexes of the type [Co(terpyRX)(2)](Y)(2) x nH(2)O (X = 4, 8, 12 and Y = BF(4)(-), ClO(4)(-), PF(6)(-), BPh(4)(-)) has been synthesized, whereby the alkyl chain length, RX, and counteranion, Y, have been systematically varied. The structural (single crystal X-ray diffraction) and electronic (magnetic susceptibility, electron paramagnetic resonance (EPR)) properties have been investigated within this family of compounds. Single crystal X-ray diffraction analysis of [Co(terpyR8)(2)](ClO(4))(2), [Co(terpyR8)(2)](BF(4))(2) x H(2)O, and [Co(terpyR4)(2)](PF(6))(2) x 3 H(2)O, at 123 K, revealed compressed octahedral low spin Co(II) environments and showed varying extents of disorder in the alkyl tail portions of the terpyRX ligands. The magnetic and EPR studies were focused on the BF(4)(-) family and, for polycrystalline solid samples, revealed that the spin transition onset temperature (from low to high spin) decreased as the alkyl chain lengthened. EPR studies of polycrystalline powder samples confirmed these results, showing signals only due to the low spin state at the temperatures seen in magnetic measurements. Further to this, simultaneous simulation of the EPR spectra of frozen solutions of [Co(terpyR8)(2)](BF(4))(2) x H(2)O, recorded at S-, X-, and Q-band frequencies, allowed accurate determination of the g and A values of the low spin ground state. The temperature dependence of the polycrystalline powder EPR spectra of this and the R4 and R12 complexes is explained in terms of Jahn-Teller effects using the warped Mexican hat potential energy surface model perturbed by the low symmetry of the ligands. While well recognized in Cu(II) systems, this is one of the few times this approach has been used for Co(II).

Synthesis and characterization of ruthenium(II) complexes with polypicolylamine ligands

Abstract A series of ruthenium(II) complexes of polypicolylamine ligands have been prepared. The reaction of Ru(PhCN)4Cl2 with the tridentate ligand N,N-bis(2-pyridylmethyl)aniline (phdpa) followed by precipitation with PF6xa0− salts affords the complex, [Ru(phdpa)2](PF6)2. The crystal structure of [Ru(phdpa)2](PF6)2 shows that the ligand coordinates in a facial mode, with sp3-nitrogens located cis to each other. The reaction of Ru(dmso)4Cl2 with the tetradentate ligand tris(2-pyridylmethyl)amine (tpa) in ethanol yielded the (Cl, Namine)-trans [Ru(tpa)(dmso)Cl](PF6) complex which was characterized by X-ray crystallography. [Ru(tpa)(dmso)Cl](PF6) reacts with bipyridine, tpa and tricyanomethane anion (tcm) affording the [Ru(tpa)(bipy)](PF6)2, [Ru(tpa)2](PF6)2 and Ru(tpa)(tcm)2 complexes, respectively. The structures of [Ru(tpa)(bipy)](PF6)2 and Ru(tpa)(tcm)2 show that tpa acts as a tetradentate ligand, while in [Ru(tpa)2](PF6)2 it is tridentate, facially coordinated, with one non-coordinated pyridine.

Electronic and resonance Raman spectra of mixed-valence, linear-chain complexes of platinum and palladium with 1,2-diaminocycloalkanes (N–N), [ MII(N–N)2][ MIV(N–N)2X2]X4(X = halogen)

The electronic and resonance Raman spectra of a series of halogen-bridged, linear-chain, mixed-valence complexes, [PtII(N–N)2][PtIV(N–N)2X2]X4[N–N = 1,2-diaminocyclohexane (dach); X = Cl, Br, or I], as well as of the complexes [PdII(dach)2][PdIV(dach)2Cl2]Cl4 and [PtII(N–N)2][PtIV(N–N)2Br2]Br4[N–N = 1,2-diaminocyclopentane (dacp)], have been recorded at ca. 295, 80, and 10 K. Excitation within the contours of the axially polarized MIVâ†� MII intervalence band of each complex leads to the appearance of long overtone progressions, v1ν1, in the resonance Raman spectrum, where ν1 is the totally symmetric axial metal–halogen stretching mode. The excitation profile of the ν1 band maximizes in each case on the low-energy side of the intervalence band maximum. The wavenumbers of the ν1 band, intervalence band maximum, and excitation profile maximum of the complexes decrease in the order Cl > Br > I, Pt > Pd, and dach > dacp. Although the mixed-valence complexes are chemically pure, they form as mixtures of conformational isomers unless the resolved ligand is used in their preparations. Such conformers have different intervalence band maxima and different ν1 values and, in consequence, as the exciting-line wavenumber (0) is changed, different conformers have their ν1 bands resonance-enhanced, and the apparent value of ν1 and its overtones change. These observations are discussed with reference to the steric hindrance between the cycloalkane rings in mixed-valence linear-chain complexes.

Designing dinuclear iron(II) spin crossover complexes. Structure and magnetism of dinitrile-, dicyanamido-, tricyanomethanide-, bipyrimidine- and tetrazine-bridged compounds

In order to expand the few known examples of dinuclear iron(II) compounds displaying (weak) intradinuclear exchange coupling and spin-crossover on one or both of the iron(II) centres, various dinuclear compounds have been synthesised and assessed for their spin-crossover and exchange coupling behaviour. The key aim of the work was to prepare and structurally characterise weakly linked and covalently bridged systems incorporating bridging ligands such as alkyldinitriles (e.g.NC(CH(2))(4)CN), bipyrimidine (bpym), dicyanamide (dca(-)), tricyanomethanide (tcm(-)), 3,6-bis(2-pyridyl)tetrazine (bptz) and 3,6-bis(2-pyridyl)2,5-dihydrotetrazine (H(2)bptz). The end groups, which complete the Fe(ii)N(6) chromophores, include tris(2-pyridylmethyl)amine (tpa), di(2-pyridylethyl)(2-pyridylmethyl)amine (tpa), 3-(2-pyridyl)pyrazole (pypzH), 1,10-phenanthroline (1,10-phen), tris(pyrazolyl)methane (tpm) and NCX(-)(X = S, Se). It was quite difficult to achieve the spin-crossover condition, many ligand combinations yielding high-spin/high-spin (HS-HS) Fe(II)Fe(II) spin states at all temperatures (300-2 K) with very weak antiferromagnetic coupling (J < -1 cm(-1)), two such being the crystallographically characterised [(dca)(tpm)Fe(mu(1,5)-dca)(2)Fe(tpm)(dca)], 5, and [(tpa)Fe(mu(1,5)-tcm)(2)Fe(tpa)](tcm)(2)(H(2)O)(2), 6. In contrast, a strong field was created around the Fe(II) centres in [(tpa)Fe(mu-(NC(CH(2))(4)CN))(2)Fe(tpa)](ClO(4))(4).NC(CH(2))(4)CN, 1, and the Fe-N bond distances, at 173 K, reflected this. This weakly-linked dinitrile example showed a spin-crossover beginning above 300 K. Half crossover examples, yielding HS-LS states below the spin transition, similar to those noted by Real and coworkers in some mu-bpym systems, were noted for [(1,10-phen)(NCS)(2)Fe(mu-bpym)Fe(NCS)(2)(1,10-phen)], 2, [(pypzH)(NCSe)(2)Fe(mu-bpym)Fe(NCSe)(2)(pypzH)], 4, and [(tpa)Fe(mu-H(2)bptz)Fe(tpa)](ClO(4))(4), 8. Interestingly, the mu-bptz analogue, 7, remained LS-LS at all temperatures with the start of a broad spin crossover evident above 300 K. No thermal hysteresis was evident in the spin transitions of these new dinuclear crossover species indicating a lack of intra- or interdinuclear cooperativity.

Solid and solution state structures of mono- and di-nuclear iron(III) complexes of related hexadentate and pentadentate aminopyridyl ligands

Two mononuclear iron(III) complexes formed with the related ligands N,N,N′,N′-tetrakis(2-pyridylmethyl)ethylenediamine (tpen) and N-benzyl-N,N′,N′-tris(2-pyridylmethyl)ethylenediamine (bztpen) have been studied. X-Ray crystallography reveals for the complex [Fe(tpen)][ClO4]3 that tpen acts as a hexadentate ligand in the solid state. In methanol or water containing solutions it was shown by EPR and UV-Vis spectroscopy that one pyridyl arm is exchanged by a solvent molecule. In dmf solution only partial exchange of a pyridyl arm was observed. The complex with the related pentadentate ligand bztpen [Fe(bztpen)Cl][ClO4]2 showed exchange of the coordinated chloride with methanol and water, but not with dmf. In water containing solutions both complexes are slowly converted into the dimeric μ-oxo complexes [(tpen)FeIII(Cl)OFeIII(Cl)(tpen)]2+ or [(bztpen)FeIII(Cl)OFeIII(Cl)(bztpen)]2+. This reaction is accelerated by addition of an excess of chloride.

Electron paramagnetic resonance characteristics of some non-heme low-spin iron(III) complexes

We have recorded the powder EPR-spectra of some near octahedral iron(III) complexes with tridentate ligands donors and analysed their spectra with simple ligand field analysis and for some cases with the angular overlap model (AOM). We have determined the electron praramagnetic resonance (EPR) characteristic of bis 1,4,7-triazacyclonane iron(III)chloride at 4 K and found that it was similar to the characteristics of the so-called highly anisotropic low spin complexes. We have recorded the powder spectra of bis (2,6-bis(benzimidazoly-2-yl)pyridine) iron(III) perchlorate and made an AOM-analyses of the structural similar complex bis-(2,6 (N-carbamoyl)-pyridine) iron(III). With a combination of ligand field analyses and AOM, we could determine the pi-donor properties of these ligands. The same approach have been used to determine the pi-donor properties of the hydroperoxo ligand. Finally we have recorded the powder EPR-spectrum of [Fe(CN)6]3- doped in K3[Co(CN)6] and [Co(NH3)6][Co(CN)6] at 4 and 100 K and in water at 4 K. The spectra are interpreted as the effect of a dynamic Jahn-Teller distortion.

Fe(II) complex with the octadentate btpa ligand: a DFT study on a spin-crossover system that reveals two distinct high-spin states

Density functional theory calculations (DFT) were performed for the spin-crossover system [Fe(btpa)]2+ (btpa = N,N,N,N-tetrakis(2-pyridylmethyl)-6,6-bis(aminomethyl)-2,2-bipyridine), and for the predominantly low-spin [Fe(b(bdpa))]2+ complex (in the solid state) (b(bdpa) = N,N-bis(benzyl)-N,N-bis(2-pyridylmethyl)-6,6-bis(aminomethyl)-2,2-bipyridine). The calculations confirmed that the former complex exhibits two high-spin isomers of the complexes, i.e. with C1 quasi hepta-coordinated (long-lived isomer) and C2 hexa-coordinated (short-lived isomer) structures that have been suggested previously based on time-resolved Raman and flash photolysis experiments. Application of B3LYP and B3LYP* functionals together with the CEP-31G basis yielded reasonable estimates of electronic energies (E(el) = E(el)(HS) - E(el)(LS)) for both isomers (calculated E(el) of ca. 24 and 31 kJ mol(-1) for long- and short-lived HS isomers, respectively, vs. the experimentally determined value of 27.5 kJ mol(-1)). Further calculations yielded the electronic structure of the low-spin isomer together with lowest lying singlet and triplet excited states of the [Fe(btpa)]2+ as well as the energy profile of the C2 <--> C1 isomerisation pathway for the high-spin [Fe(btpa)]2+ within the framework of the QST (quadratic synchronous transit) approach. The data obtained are discussed in relation to the observed ultrafast intersystem crossing in Fe(II) polypyridine complexes. The importance of ligand strain in relation to the destabilisation of the low-spin isomers is also discussed. In that context, calculations for a further 15 Fe(II) spin-crossover complexes of hexa-coordinating nitrogen-donor ligands have shown that the LS-HS conversion is associated with a release of ligand stress of 95 +/-16 kJ mol(-1), on average. On the basis of the calculations presented in this paper we propose that octahedral high-spin d-6 isomers are far more elastic regarding the angular distortions (equatorial and meridional strain, i.e. the declination of cis- and trans-L-M-L angle from the regular values of 90 and 180 degrees, respectively) than their low-spin counterparts.

Unusual metal coordination chemistry from an amino-amide derivative of 4-nitrophenol, a surprising ligand

The simple ligand N-(2-aminoethyl)-2-hydroxy-5-nitrobenzamide () exhibits several coordination modes depending on the reaction conditions, acting as a zwitterion on its own or being ionic in the presence of acid and depending on the concentration of metal present in a reaction, it can coordinate to the metal in either a 1:1 or a 1:2 metal:ligand mode. Furthermore, the role of solvent plays an important role in these complexation reactions with both four and six coordinate copper complexes being obtained using water as solvent but only six coordinate copper complexes obtained using acetonitrile as solvent.

(2,2′-Bi­pyridine)­di­chloro­gold(III) nitrate

The title compound, [AuCl2(C10H8N2)]NO3, is layered parallel to (overline 101) by π–π stacking. The individual {overline 101} layers are held together by extensive C—H⋯O and C—H⋯Cl hydrogen bonding.

Strain-induced substitutional lability in a Ru(II) complex of a hypodentate polypyridine ligand.

The ruthenium(II) complex of heptadentate N,N,N,N-tetrakis(2-pyridylmethyl)-2,6-bis(aminomethyl)pyridine (tpap) was isolated as the hexafluorophosphate complex Ru(tpap)(PF6)2. The crystal structure has been determined for the triflate salt Ru(tpap)(CF3SO3)2.2H2O, which crystallizes in the monoclinic space group P2(1)/n. The structure was refined to a final R value of 0.0549 for 5894 observed reflections. The heptadentate ligand coordinates with six nitrogens, i.e. with two tertiary nitrogens and four pyridine nitrogens, one of the pyridines remaining un-coordinated. The resulting structure is significantly distorted from octahedral geometry with an equatorial Nsp3-Ru-Npyridine angle of 120 degrees. The consequence of the above steric strain is a labilization of the system and fluxional behaviour involving exchange between equatorially coordinated and non-coordinated pyridines has been observed by 1H NMR for Ru(tpap)(PF6)2 in d6-acetone solution. The activation parameters of DeltaG(not equal to 298) = 53 kJ mol(-1), DeltaH(not equal) = 56 +/- 1 kJ mol(-1) and DeltaS(not equal) = -10 +/- 3 J mol(-1) K(-1) were determined on the basis of NMR experiments. In addition electronic structure calculations applying density functional theory (DFT) have been performed in order to identify a transition state and to estimate the activation barrier. On the basis of NMR and DFT results the mechanism of isoexchange involving a hepta-coordinated intermediate has been proposed.