Magnetization Slow Dynamics in Ferrocenium Complexes.

The single-molecule magnet (SMM) properties of a series of ferrocenium complexes, [Fe(η5 -C5 R5 )2 ]+ (R=Me, Bn), are reported. In the presence of an applied dc field, the slow dynamics of the magnetization in [Fe(η5 -C5 Me5 )2 ]BArF are revealed. Multireference quantum mechanical calculations show a large energy difference between the ground and first excited states, excluding the commonly invoked, thermally activated (Orbach-like) mechanism of relaxation. In contrast, a detailed analysis of the relaxation time highlights that both direct and Raman processes are responsible for the SMM properties. Similarly, the bulky ferrocenium complexes, [Fe(η5 -C5 Bn5 )2 ]BF4 and [Fe(η5 -C5 Bn5 )2 ]PF6 , also exhibit magnetization slow dynamics, however an additional relaxation process is clearly detected for these analogous systems.

Crystal structures of two cross-bridged chromium(III) tetra-aza-macrocycles.

The crystal structure of di-chlorido-(4,10-dimethyl-1,4,7,10-tetra-aza-bicyclo-[5.5.2]tetra-deca-ne)chromium(III) hexa-fluorido-phosphate, [CrCl2(C12H26N4)]PF6, (I), has monoclinic symmetry (space group P21/n) at 150 K. The structure of the related di-chlorido-(4,11-dimethyl-1,4,8,11-tetra-aza-bicyclo-[6.6.2]hexa-deca-ne)chromium(III) hexa-fluorido-phosphate, [CrCl2(C14H30N4)]PF6, (II), also displays monoclinic symmetry (space group P21/c) at 150 K. In each case, the Cr(III) ion is hexa-coordinate with two cis chloride ions and two non-adjacent N atoms bound cis equatorially and the other two non-adjacent N atoms bound trans axially in a cis-V conformation of the macrocycle. The extent of the distortion from the preferred octa-hedral coordination geometry of the Cr(III) ion is determined by the parent macrocycle ring size, with the larger cross-bridged cyclam ring in (II) better able to accommodate this preference and the smaller cross-bridged cyclen ring in (I) requiring more distortion away from octa-hedral geometry.

Production of H2 at fast rates using a nickel electrocatalyst in water-acetonitrile solutions.

We report a synthetic nickel complex containing proton relays, [Ni(P(Ph)2N(C6H4OH)2)2](BF4)2 (P(Ph)2N(C6H4OH)2 = 1,5-bis(p-hydroxyphenyl)-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclo-octane), that catalyzes the production of H2 in aqueous acetonitrile with turnover frequencies of 750-170,000 s(-1) at experimentally determined overpotentials of 310-470 mV.

Two pathways for electrocatalytic oxidation of hydrogen by a nickel bis(diphosphine) complex with pendant amines in the second coordination sphere.

A nickel bis(diphosphine) complex containing pendant amines in the second coordination sphere, [Ni(P(Cy)2N(t-Bu)2)2](BF4)2 (P(Cy)2N(t-Bu)2 = 1,5-di(tert-butyl)-3,7-dicyclohexyl-1,5-diaza-3,7-diphosphacyclooctane), is an electrocatalyst for hydrogen oxidation. The addition of hydrogen to the Ni(II) complex gives three isomers of the doubly protonated Ni(0) complex [Ni(P(Cy)2N(t-Bu)2H)2](BF4)2. Using the pKa values and Ni(II/I) and Ni(I/0) redox potentials in a thermochemical cycle, the free energy of hydrogen addition to [Ni(P(Cy)2N(t-Bu)2)2](2+) was determined to be -7.9 kcal mol(-1). The catalytic rate observed in dry acetonitrile for the oxidation of H2 depends on base size, with larger bases (NEt3, t-BuNH2) resulting in much slower catalysis than n-BuNH2. The addition of water accelerates the rate of catalysis by facilitating deprotonation of the hydrogen addition product before oxidation, especially for the larger bases NEt3 and t-BuNH2. This catalytic pathway, where deprotonation occurs prior to oxidation, leads to an overpotential that is 0.38 V lower compared to the pathway where oxidation precedes proton movement. Under the optimal conditions of 1.0 atm H2 using n-BuNH2 as a base and with added water, a turnover frequency of 58 s(-1) is observed at 23 °C.

Topological and electronic influences on magnetic exchange coupling in Fe(III) ethynylbenzene dendritic building blocks.

Significant variance in the magnitude of reported exchange coupling parameters (both experimental and computed) for paramagnetic transition metal-ethynylbenzene complexes suggests that nuances of the magnetostructural relationship in this class of compounds remain to be understood and controlled, toward maximizing the stability of high-spin ground states. We report the preparation, electrochemical behavior, magnetic properties, and results of computational investigations of a series of iron ethynylbenzene complexes with coordination environments suitable for metallodendrimer assembly: [(dmpe)(2)FeCl(C(2)Ph)](OTf) (1), [(dmpe)(4)Fe(2)Cl(2)(µ-p-DEB)](BAr(F)(4))(2) (2), [(dmpe)(6)Fe(3)Cl(3)(TEB)] (3), [(dmpe)(6)Fe(3)Cl(3)(µ(3)-TEB)](OTf)(3) (4), and [(dmpe)(4)Fe(2)Cl(2)(µ-m-DEB)](BAr(F)(4))(2) (5) [dmpe = 1,2-bis(dimethylphosphino)ethane; p-H(2)DEB = 1,4-diethynylbenzene; BAr(F)(4) = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate; H(3)TEB = 1,3,5-triethynylbenzene; m-H(2)DEB = 1,3-diethynylbenzene]. As expected, the ligand topology drives the antiferromagnetic coupling in 2 (J = -134 cm(-1) using the H = -2JS(1)·S(2) convention) and the ferromagnetic coupling in 4 and 5 (J = +37 cm(-1), J' = +5 cm(-1) for 4; J = +11 cm(-1) for 5); the coupling is comparable to but deviates significantly from values reported for related Cp*-containing species (Cp* = η(5)-C(5)Me(5)). The origins of these differences are explored computationally: a density functional theory (DFT) approach for treating the coupling of three spin centers as a linear combination of single-determinantal descriptions is developed and described, and the results of these computations can be generalized to other paramagnetic systems. Unrestricted B3LYP hybrid DFT calculations performed on rotamers of 4 and 5 and related complexes, as well as Cp* analogues, provide J values that correlate with the experimental values. We find that geometric considerations dominate the magnetism of the Cp* complexes, while topology and alkynyl ligand electronics combine more subtly to drive the magnetism of the new complexes reported here. These calculations imply that substantial magnetic exchange parameters, with accompanying well-isolated high-spin ground states, are achievable for ethynylbenzene-bridged paramagnetic metallodendrimers.

Crystallographic coincidence of two bridging species in a dinuclear Co ethynyl-benzene complex.

In the title compound, trans,trans-[µ-(m-phenyl-ene)bis-(ethyne-1,2-di-yl)]bis-[chlorido(1,4,8,11-tetra-aza-cyclo-tetra-deca-ne)cobalt(III)]-trans,trans-[µ-(5-bromo-m-phenyl-ene)bis-(ethyne-1,2-di-yl)]bis-[chlorido(1,4,8,11-tetra-aza-cyclo-tetra-deca-ne)cobalt(III)]-tetra-phenyl-borate-acetone (0.88/0.12/2/4), [Co(2)(C(12)H(4))Cl(2)(C(10)H(24)N(4))(2)](0.88)[Co(2)(C(10)H(3)Br)Cl(2)(C(10)H(24)N(4))(2)](0.12)(C(24)H(20)B)(2)·4C(3)H(6)O, with the exception of the acetyl-ene and bromine groups, all atomic postitions are the same in the two compounds and are modeled at full occupancy. The Co(III) ions are six-coordinate with acetyl-ide and chloride ligands bound to the axial sites and the N atoms from the cyclam rings coordinated at the equatorial positions. N-H⋯O and N-H⋯Cl hydrogen-bonding interactions help to consolidate the crystal packing.

Unusual electronic effects imparted by bridging dinitrogen: an experimental and theoretical investigation.

We describe the preparation, structural and magnetic characterizations, and electronic structure calculations for a redox-related family of dinitrogen-bridged chromium acetylide complexes containing the [RC(2)Cr(µ-N(2))CrC(2)R](n+) (R = Ph-, (i)Pr(3)Si-; n = 0, 1, 2) backbone: [(dmpe)(4)Cr(2)(C(2)Ph)(2)(µ-N(2))] (1), [(dmpe)(4)Cr(2)(C(2)Si(i)Pr(3))(2)(µ-N(2))] (2), [(dmpe)(4)Cr(2)(C(2)Si(i)Pr(3))(2)(µ-N(2))]BAr(F)(4) (3), and [(dmpe)(4)Cr(2)(C(2)Si(i)Pr(3))(2)(µ-N(2))](BAr(F)(4))(2) (4). Compounds 3 and 4 are synthesized via chemical oxidation of 2 with [Cp(2)Co](+) and [Cp*(2)Fe](+), respectively. X-ray structural analyses show that the alteration of the formal Cr oxidation states does not appreciably change the Cr-N-N-Cr skeletal structures. Magnetic data collected for 2 and 4 are consistent with high-spin triplet and quintet ground states, respectively. The mixed-valent complex 3 exhibits temperature dependent magnetic behavior consistent with a quartet ⇌ doublet two-center spin equilibrium. Electronic structure calculations (B3LYP) performed on the full complexes in 2 and 4 suggest that the high-spin states arise from singly occupied orthogonal π* orbitals coupled with a variable occupation of dδ orbitals. Significant N-N and Cr-N π-bonding pins the occupation of the π manifold, leading to variable occupation of the dδ space. In contrast, mixed-valent 3 is not well described by a B3LYP hybrid density functional model. A [9,11] CAS-SORCI study on a simplified model of 3 reproduces the observed Hund's rule violation for the S = 1/2 ground state and places the lowest quartet 1.45 kcal/mol above the doublet ground state.

Intramolecular charge transfer in a trinuclear iron ene-triyne complex.

A trinuclear redox-active complex with an unprecedented ene-triyne backbone has been isolated and structurally characterized. The mono-oxidized mixed-valent species exhibits electronic delocalization necessary for molecular circuitry.

Chelating tris(amidate) ligands: versatile scaffolds for nickel(II).

The synthesis and characterization of nickel complexes supported by a family of open-chain, tetradentate, tris(amidate) ligands, [N(o-PhNC(O)R)(3)](3-) ([L(R)](3-) where R = (i)Pr, (t)Bu, and Ph) is described. The complexes [Ni(L(iPr))](-), [Ni(L(tBu))](-), and [Ni(L(Ph))(CH(3)CN)](-) have been characterized by solution-state spectroscopic methods and single crystal X-ray diffraction. Each ligand gives rise to a different primary coordination sphere about the nickel centre. These studies indicate that the ligands' acyl substituents can be used to regulate the coordination mode of the amidate donors to nickel and the coordination number of the nickel centres. In addition, the ability of these complexes to bind cyanide has been explored. These experiments demonstrate that only one of these complexes, [Ni(L(iPr))](-), is able to irreversibly bind cyanide and can be used to assemble [Et(4)N](3)[Ni(L(iPr))(mu(2)-CN)Co(L(iPr))], a cyanide bridged, heterobimetallic complex. The synthesis and characterization of the cyanide containing complexes, including magnetic susceptibility studies, are described.

Synthesis and Characterization of the Chromium(III) complexes of Ethylene Cross-Bridged Cyclam and Cyclen Ligands.

Dichloro(4,10-dimethyl-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane)chromium(III) chloride, Dichloro(4,10-dibenzyl-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane) chromium(III) chloride, and Dichloro(4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2] hexadecane)chromium)(III) chloride have been prepared by the reaction of anhydrous chromium(III) chloride with the appropriate cross-bridged tetraazamacrocycle. Aquation of these complexes proved difficult, but Chlorohydroxo(4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane)chromium)(III) chloride was synthesized directly from chromium(II) chloride complexation followed by exposure or the reaction to air in the presence of water. The four complexes were characterized by X-ray crystal structure determination. All contain the chromium(III) ion in a distorted octahedral geometry and the macrocycle in the cis-V configuration, as dictated by the ethylene cross-bridge. Further characterization of the hydroxo complex reveals a magnetic moment of mu(eff) = 3.95 B.M. and electronic absorbtions in acetonitrile at lambda(max) = 583nm (epsilon = 65.8 L/cm.mol), 431nm (epsilon = 34.8 L/cm.mol) and 369nm (epsilon = 17 L/cm.mol).