Juan Olguín

Reversible Switching of a Cobalt Complex by Thermal, Pressure, and Electrochemical Stimuli: Abrupt, Complete, Hysteretic Spin Crossover

Triply switchable [Co(II)(dpzca)(2)] shows an abrupt, reversible, and hysteretic spin crossover (T(1/2)↓ = 168 K, T(1/2)↑ = 179 K, and ΔT(1/2) = 11 K) between the high-spin (HS) and low-spin (LS) states of cobalt(II), both of which have been structurally characterized. The spin transition is also reversibly triggered by pressure changes. Moreover, in a third reversible switching mechanism for this complex, the magnetic properties can be switched between HS cobalt(II) and LS cobalt(III) by redox.

Effect of N4-Substituent Choice on Spin Crossover in Dinuclear Iron(II) Complexes of Bis-Terdentate 1,2,4-Triazole-Based Ligands

Seven new dinuclear iron(II) complexes of the general formula [Fe(II)2(PMRT)2](BF4)4·solvent, where PMRT is a 4-substituted-3,5-bis{[(2-pyridylmethyl)-amino]methyl}-4H-1,2,4-triazole, have been prepared in order to investigate the substituent effect on the spin crossover event. Variable temperature magnetic susceptibility and (57)Fe Mössbauer spectroscopy studies show that two of the complexes, [Fe(II)2(PMPT)2](BF4)4·H2O (N(4) substituent is pyrrolyl) and [Fe(II)2(PM(Ph)AT)2](BF4)4 (N(4) is N,N-diphenylamine), are stabilized in the [HS-HS] state between 300 and 2 K with weak antiferromagnetic interactions between the iron(II) centers. Five of the complexes showed gradual half spin crossover, from [HS-HS] to [HS-LS], with the following T(1/2) (K) values: 234 for [Fe(II)2(PMibT)2](BF4)4·3H2O (N(4) is isobutyl), 147 for [Fe(II)2(PMBzT)2](BF4)4 (N(4) is benzyl), 133 for [Fe(II)2(PM(CF3)PhT)2](BF4)4·DMF·H2O (N(4) is 3,5-bis(trifluoromethyl)phenyl), 187 for [Fe(II)2(PMPhT)2](BF4)4 (N(4) is phenyl), and 224 for [Fe(II)2(PMC16T)2](BF4)4 (N(4) is hexadecyl). Structure determinations carried out for three complexes, [Fe(II)2(PMPT)2](BF4)4·4DMF, [Fe(II)2(PMBzT)2](BF4)4·CH3CN, and [Fe(II)2(PM(Ph)AT)2](BF4)4·solvent, revealed that in all three complexes both iron(II) centers are stabilized in the high spin state at 90 K. A general and reliable 4-step route to PMRT ligands is also detailed.

Doubly Pyrazolate-Bridged Dinuclear Complexes of a Highly Constrained Bis-terdentate Ligand: Observation of a [High Spin-Low Spin] State for [FeII2(PMAP)2][SbF6]2·2.25(C3H8O) (PMAP = 3,5-bis{[N-(2-pyridylmethyl)amino]-methyl}-1H-pyrazolate)

The bis-terdentate pyrazole-based ligand 3,5-bis{[N-(2-pyridylmethyl)amino]methyl}-1H-pyrazole (PMAPH) was synthesized from 3,5-(1H)-pyrazoledicarbaldehyde and 2 equiv of 2-(aminomethyl)pyridine, using sodium borohydride to reduce the imine intermediate. A family of dinuclear complexes [M(II/III)(2)(PMAP)(2)](X)(2/4) was prepared by 2:2:2 reactions of MX(2)/PMAPH/base, where M = Zn(II) and X = BF(4)(-); M = Cu(II) and X = ClO(4)(-), BF(4)(-), OAc(-), NO(3)(-); M = Ni(II), Fe(III) and X = ClO(4)(-), BF(4)(-); M = Fe(II) and X = SbF(6)(-). Single crystal X-ray structure determinations on four complexes: [Fe(III)(2)(PMAP)(2)](BF(4))(4).2MeCN, [Ni(II)(2)(PMAP)(2)](ClO(4))(2).2MeCN, [Cu(II)(2)(PMAP)(2)](BF(4))(2).2MeCN, and [Zn(II)(2)(PMAP)(2)](BF(4))(2).2MeCN confirmed a dinuclear doubly pyrazolate-bridged structure for each. The two metal centers in these complexes have similar N(6) distorted octahedral coordination spheres, with all donors provided by the two deprotonated PMAP(-) ligands. Magnetic measurements reveal intra-dinuclear antiferromagnetic interactions for both the M = Cu(II) and Ni(II) [M(2)(PMAP)(2)](BF(4))(4) complexes, with J/k(B) = -252(2) K and J/k(B) = -24.7(2) K (H = -2JS(M)S(M)), respectively. Interestingly magnetic measurements show that the complex [Fe(2)(II)(PMAP)(2)](SbF(6))(2).2.25(C(3)H(8)O) is in a mixed high spin (HS)-low spin (LS) spin state, [HS-LS], from 300 to 1.8 K, with no sign of spin crossover to a fully low spin form [LS-LS] even at 1.8 K.

Solvent-Dependent Switch of Ligand Donor Ability and Catalytic Activity of Ruthenium(II) Complexes Containing Pyridinylidene Amide (PYA) N-Heterocyclic Carbene Hybrid Ligands

Chelating ligands incorporating both N-[1-alkylpyridin-4(1H)-ylidene]amide (PYA) and N-heterocyclic carbene (NHC) donor sites were prepared and used for the synthesis of ruthenium(II) complexes. Cyclic voltammetry, NMR, and UV-vis spectroscopy of the complexes indicate a solvent-dependent contribution of the limiting resonance structures associated with the ligand in solution. The neutral pyridylidene imine structure is more pronounced in apolar solvents (CH2Cl2), while the mesoionic pyridinium amide form is predominant in polar solvents (MeOH, DMSO). The distinct electronic properties of these hybrid PYA-NHC ligands in different solvents have a direct influence on the catalytic activity of the ruthenium center, e.g., in the dehydrogenation of benzyl alcohol to benzaldehyde. The activity in different solvents qualitatively correlates with the solvent permittivity.

Synthesis of 3- and 5-formyl-4-phenyl-1H-pyrazoles: promising head units for the generation of asymmetric imine ligands and mixed metal polynuclear complexes

Two synthetic methodologies are reported for the generation of 4-phenyl-1H-pyrazoles substituted at the 3- and/or 5-positions. Functionalisation of the 4-position of dimethyl-4-iodo-1-(tetrahydropyran-2-yl)-3,5-pyrazolecarboxylate (10) to produce dimethyl-4-phenyl-1-(tetrahydropyran-2-yl)-3,5-pyrazolecarboxylate (14) was achieved by a C–C Suzuki–Miyaura cross coupling reaction in water. However, low yields for this reaction led us to develop a second methodology wherein functionalisation of N-(tetrahydropyran-2-yl)-4-phenylpyrazole (18), synthesised from inexpensive phenylacetic acid, with formyl or hydroxymethyl groups was achieved by lithiation methods. The resulting monoaldehydes, 4-phenyl-5-pyrazole carbaldehyde (20) and 5-formyl-3-(2′-tetrahydropyranyloxymethyl)-4-phenyl-1-(tetrahydropyran-2-yl)pyrazole (28), should facilitate access to new, asymmetric, imine ligands based on a 4-phenyl-1H-pyrazole moiety. This was proven by the successful synthesis of the heterometallic tetranuclear complex [FeII(NiIIL2)3](BF4)2·solvents. Likewise, the alcohol isolated en route to 28, N-(tetrahydropyran-2-yl)-5-(hydroxymethyl)-4-phenylpyrazole (24), should facilitate access to new, asymmetric, amine ligands.

Carbohydrate-Functionalized 1,2,3-Triazolylidene Complexes for Application in Base-Free Alcohol and Amine Oxidation

Acetylglucose- and acetylgalactose-functionalized triazolylideneruthenium(II) and -iridium(III) complexes were synthesized and fully characterized. Subsequent carbohydrate deprotection yielded the first examples of glucose- and galactose-functionalized 1,2,3-triazolylideneiridium complexes. Base-free oxidation of alcohols and amines was used to probe the catalytic potential of the metal complexes and the influence of the carbohydrate wingtip group. Generally, the performance of these complexes is higher in amine oxidation than in alcohol oxidation. While the stereochemistry at the carbohydrate C4 position had no marked influence (galactose vs glucose), the ruthenium complexes typically exhibited higher substrate selectivity and product specificity compared to the analogous iridium species. Most noteworthy is the fact that the catalytic performance is significantly enhanced when the carbohydrate functionality is deprotected, suggesting an active role of the carbohydrate substituent in these transformations.

Synthesis and characterisation of two high spin iron(II) complexes of 3,4-diphenyl-5-(2-pyridyl)-1,2,4-triazole

The stepwise synthesis of the new ligand 4,5-diphenyl-3-pyridyl-1,2,4-triazole (phppt) from N-phenyl-pyridine-2-thiocarboxamide and benzohydrazide is reported. Two iron(II) complexes, [Fe(phppt)2(SCN)2]·2MeOH and [Fe(phppt)2(SeCN)2]·Et2O, have been prepared and structurally characterised at 90 K, showing that both complexes are high spin (HS). The Mössbauer spectrum of [Fe(phppt)2(SeCN)2]·1.5MeOH is consistent with the iron(II) ion being HS at 5.2 K.

Two mononuclear iron(II) complexes of 4-phenylpyrazole-5-carbaldehyde derived ligands are stabilised in different spin states

The polydentate acyclic ligand 4-phenylpyrazole-5-carbaldehyde azine, H2L1, has been prepared and characterised. A second 4-phenylpyrazole-5-carbaldehyde derived ligand, H2L2, was prepared in situ by condensation with o-phenylendiamine. Two mononuclear iron(II) complexes, [FeII(HL2)(MeOH)(NCSe)]·H2O and [FeII(H2L1)2](BF4)2·solvents, have been synthesised from 1 : 1 and 2 : 3 M:L reactions, respectively. No dinuclear, helical, species were obtained from 2 : 3 M:L reactions regardless of the solvent employed (MeOH, MeCN or NO2Me). An X-ray structure determination on the latter complex reveals that it is low spin at 90 K. Mossbauer spectra confirm this, and show that the former complex is, in contrast, high spin, even at 4.6 K.