Ligand exchange and spin state equilibria of Fe(II)(N4Py) and related complexes in aqueous media

We report the characterization and solution chemistry of a series of Fe(II) complexes based on the pentadentate ligands N4Py (1,1-di(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine), MeN4Py (1,1-di(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)ethanamine), and the tetradentate ligand Bn-N3Py (N-benzyl-1,1-di(pyridin-2-yl)-N-(pyridin-2-ylmethyl)methanamine) ligands, i.e., [Fe(N4Py)(CH(3)CN)](ClO(4))(2) (1), [Fe(MeN4Py)(CH(3)CN)](ClO(4))(2) (2), and [Fe(Bn-N3Py)(CH(3)CN)(2)](ClO(4))(2) (3), respectively. Complexes 2 and 3 are characterized by X-ray crystallography, which indicates that they are low-spin Fe(II) complexes in the solid state. The solution properties of 1-3 are investigated using (1)H NMR, UV/vis absorption, and resonance Raman spectroscopies, cyclic voltammetry, and ESI-MS. These data confirm that in acetonitrile the complexes retain their solid-state structure, but in water immediate ligand exchange of the CH(3)CN ligand(s) for hydroxide or aqua ligands occurs with full dissociation of the polypyridyl ligand at low (<3) and high (>9) pH. pH jumping experiments confirm that over at least several minutes the ligand dissociation observed is fully reversible for complexes 1 and 2. In the pH range between 5 and 8, complexes 1 and 2 show an equilibrium between two different species. Furthermore, the aquated complexes show a spin equilibrium between low- and high-spin states with the equilibrium favoring the high-spin state for 1 but favoring the low-spin state for 2. Complex 3 forms only one species over the pH range 4-8, outside of which ligand dissociation occurs. The speciation analysis and the observation of an equilibrium between spin states in aqueous solution is proposed to be the origin of the effectiveness of complex 1 in cleaving DNA in water with (3)O(2) as terminal oxidant.     

Syntheses, structures, and reactivity of low spin iron(III) complexes containing a single carboxamido nitrogen in a [FeN5L] chromophore

A new pentacoordinate ligand based on TPA (tris-(2-pyridylmethyl)amine), namely, N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-carboxamide (PaPy(3)H), has been synthesized. The iron(III) complexes of this ligand, namely, [Fe(PaPy(3))(CH(3)CN)](ClO(4))(2) (1), [Fe(PaPy(3))(Cl)]ClO(4) (2), [Fe(PaPy(3))(CN)]ClO(4) (3), and [Fe(PaPy(3))(N(3))]ClO(4) (4), have been isolated and complexes 1-3 have been structurally characterized. These complexes are the first examples of monomeric iron(III) complexes with one carboxamido nitrogen in the first coordination sphere. All four complexes are low spin and exhibit rhombic EPR signals around g = 2. The solvent bound species [Fe(PaPy(3))(CH(3)CN)](ClO(4))(2) reacts with H(2)O(2) in acetonitrile at low temperature to afford [Fe(PaPy(3))(OOH)](+) (g = 2.24, 2.14, 1.96). When cyclohexene is allowed to react with 1/H(2)O(2) at room temperature, a significant amount of cyclohexene oxide is produced along with the allylic oxidation products. Analysis of the oxidation products indicates that the allylic oxidation products arise from a radical-driven autoxidation process while the epoxidation is carried out by a distinctly different oxidant. No epoxidation of cyclohexene is observed with 1/TBHP.     

Olefin cis-dihydroxylation versus epoxidation by non-heme iron catalysts: two faces of an Fe(III)-OOH coin

The oxygenation of carbon-carbon double bonds by iron enzymes generally results in the formation of epoxides, except in the case of the Rieske dioxygenases, where cis-diols are produced. Herein we report a systematic study of olefin oxidations with H(2)O(2) catalyzed by a group of non-heme iron complexes, i.e., [Fe(II)(BPMEN)(CH(3)CN)(2)](2+) (1, BPMEN = N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)-1,2-diaminoethane) and [Fe(II)(TPA)(CH(3)CN)(2)](2+) (4, TPA = tris(2-pyridylmethyl)amine) and their 6- and 5-methyl-substituted derivatives. We demonstrate that olefin epoxidation and cis-dihydroxylation are different facets of the reactivity of a common Fe(III)-OOH intermediate, whose spin state can be modulated by the electronic and steric properties of the ligand environment. Highly stereoselective epoxidation is favored by catalysts with no more than one 6-methyl substituent, which give rise to low-spin Fe(III)-OOH species (category A). On the other hand, cis-dihydroxylation is favored by catalysts with more than one 6-methyl substituent, which afford high-spin Fe(III)-OOH species (category B). For catalysts in category A, both the epoxide and the cis-diol product incorporate (18)O from H(2)(18)O, results that implicate a cis-H(18)O-Fe(V)=O species derived from O-O bond heterolysis of a cis-H(2)(18)O-Fe(III)-OOH intermediate. In contrast, catalysts in category B incorporate both oxygen atoms from H(2)(18)O(2) into the dominant cis-diol product, via a putative Fe(III)-eta(2)-OOH species. Thus, a key feature of the catalysts in this family is the availability of two cis labile sites, required for peroxide activation. The olefin epoxidation and cis-dihydroxylation studies described here not only corroborate the mechanistic scheme derived from our earlier studies on alkane hydroxylation by this same family of catalysts (Chen, K.; Que, L, Jr. J. Am. Chem. Soc. 2001, 123, 6327) but also further enhance its credibility. Taken together, these reactions demonstrate the catalytic versatility of these complexes and provide a rationale for Nature's choice of ligand environments in biocatalysts that carry out olefin oxidations.     

Stereospecific alkane hydroxylation by non-heme iron catalysts: mechanistic evidence for an Fe(V)=O active species.

High-valent iron-oxo species have frequently been invoked in the oxidation of hydrocarbons by both heme and non-heme enzymes. Although a formally Fe(V)=O species, that is, [(Por(*))Fe(IV)=O](+), has been widely accepted as the key oxidant in stereospecific alkane hydroxylation by heme systems, it is not established that such a high-valent state can be accessed by a non-heme ligand environment. Herein we report a systematic study on alkane oxidations with H(2)O(2) catalyzed by a group of non-heme iron complexes, that is, [Fe(II)(TPA)(CH(3)CN)(2)](2+) (1, TPA = tris(2-pyridylmethyl)amine) and its alpha- and beta-substituted analogues. The reactivity patterns of this family of Fe(II)(TPA) catalysts can be modulated by the electronic and steric properties of the ligand environment, which affects the spin states of a common Fe(III)-OOH intermediate. Such an Fe(III)-peroxo species is high-spin when the TPA ligand has two or three alpha-substituents and is proposed to be directly responsible for the selective C-H bond cleavage of the alkane substrate. The thus-generated alkyl radicals, however, have relatively long lifetimes and are susceptible to radical epimerization and trapping by O(2). On the other hand, 1 and the beta-substituted Fe(II)(TPA) complexes catalyze stereospecific alkane hydroxylation by a mechanism involving both a low-spin Fe(III)-OOH intermediate and an Fe(V)=O species derived from O-O bond heterolysis. We propose that the heterolysis pathway is promoted by two factors: (a) the low-spin iron(III) center which weakens the O-O bond and (b) the binding of an adjacent water ligand that can hydrogen bond to the terminal oxygen of the hydroperoxo group and facilitate the departure of the hydroxide. Evidence for the Fe(V)=O species comes from isotope-labeling studies showing incorporation of (18)O from H(2)(18)O into the alcohol products. (18)O-incorporation occurs by H(2)(18)O binding to the low-spin Fe(III)-OOH intermediate, its conversion to a cis-H(18)O-Fe(V)=O species, and then oxo-hydroxo tautomerization. The relative contributions of the two pathways of this dual-oxidant mechanism are affected by both the electron donating ability of the TPA ligand and the strength of the C-H bond to be broken. These studies thus serve as a synthetic precedent for an Fe(V)=O species in the oxygen activation mechanisms postulated for non-heme iron enzymes such as methane monooxygenase and Rieske dioxygenases.     

NMR Investigation of beta-Substituted High-Spin and Low-Spin Iron(III) Tetraphenylporphyrins

The NMR spectra of a series of beta-substituted iron(III) tetraphenylporphyrin (2-X-TPP) complexes have been studied to elucidate the relationship between the electron donating/withdrawing properties of the 2-substituent and the (1)H NMR spectral pattern. The electronic nature of the substituent has been significantly varied and covered the -0.6 to 0.8 Hammett constant range. Both high-spin and low-spin complexes of the general formula (2-X-TPP)Fe(III)Cl and [(2-X-TPP)Fe(III)(CN)(2)](-) have been investigated. The (1)H NMR data for the following substituents (X) have been reported: py(+), NO(2), CN, CH(3), BzO (C(6)H(5)COO), H, D, Br, Cl, CH(3), NH(2), NH(3)(+), NHCH(3), OH, and O(-). The (1)H NMR resonances for low-spin dicyano complexes have been completely assigned by a combination of two-dimensional COSY and NOESY experiments. In the case of selected high-spin complexes, the 3-H resonance has been identified by the selective deuteration of all but the 3-H position. The pattern of unambiguously assigned seven pyrrole resonances reflects the asymmetry imposed by 2-substitution and has been used as an unique (1)H NMR spectroscopic probe to map the spin density distribution. The pyrrole isotropic shifts of [(2-X-TPP)Fe(III)(CN)(2)](-) are dominated by the contact term. In order to quantify the substituent effect, the dependence of isotropic shift of all low-spin pyrrole resonances and 3-H high-spin pyrrole resonance versus Hammett constants has been studied. The electronic effect is strongly localized at the beta-substituted pyrrole. The major change of the isotropic shift has also been noted for only one of two adjacent pyrrole rings, i.e., at 7-H and 8-H positions. These neighboring protons, located on a single pyrrole ring, experienced opposite shift changes when electron withdrawing/donating properties were modified. Two other pyrrole rings for all investigated derivatives revealed considerably smaller, substituent related, isotropic shift changes. A long-range secondary isotopic shift has been observed for [(2-D-TPP)Fe(III)(CN)(2)](-). The effect is consistent with a general spin density distribution mechanism due to beta-substitution. A fairly good correlation between the 3-H isotropic shift of (2-X-TPP)Fe(III)Cl and the Hammett constant has been found as well. The observed contact shift pattern of [(2-X-TPP)Fe(III)(CN)(2)](-) reflects spin pi delocalization into the highest filled MO equivalent to the unsubstituted porphyrin 3e(pi) orbital. To account for the substituent contribution, the semiquantitative Fenske-Hall LCAO method has been used to determine the molecular orbitals involved in the spin density delocalization. For low-spin complexes, (13)C pyrrole resonances of carbons bearing a proton have been identified by means of a (1)H-(13)C HMQC experiment. The reversed order of (13)C resonance patterns as compared to their (1)H NMR counterparts has been determined, e.g., the largest isotropic shift of 3-H has been accompanied by the smallest measured (13)C isotropic shift. Analysis of the isotropic shifts in (2-X-TPP)Fe(III)Cl and [(2-X-TPP)Fe(III)(CN)(2)](-) suggests that the observed regularities of the electronic structure modification due to the beta-substitution should apply to iron(III) natural porphyrin or geoporphyrin complexes.     

Photoexcitation and relaxation dynamics of catecholato-iron(III) spin-crossover complexes.

The photophysical properties of the ferric catecholate spin-crossover compounds [(TPA)Fe(R-Cat)]X (TPA=tris(2-pyridylmethyl)amine; X=PF(6) (-), BPh(4) (-); R-Cat=catecholate dianion substituted by R=NO(2), Cl, or H) are investigated in the solid state. The catecholate-to-iron(III) charge-transfer bands are sensitive both to the spin state of the metal ion and the charge-transfer interactions associated with the different catecholate substituents. Vibronic progressions are identified in the near-infrared (NIR) absorption of the low-spin species. Evidence for a low-temperature photoexcitation process is provided. The relaxation dynamics between 10 and 100 K indicate a pure tunneling process below approximately 40 K, and a thermally activated region at higher temperatures. The relaxation rate constants in the tunneling regime at low temperature, k(HL)(T-->0), vary in the range from 0.58 to 8.84 s(-1). These values are in qualitative agreement with the inverse energy-gap law and with structural parameters. A comparison with ferrous spin-crossover complexes shows that the high-spin to low-spin relaxation is generally faster for ferric complexes, owing to the smaller bond length changes for the latter. However, in the present case the corresponding rate constants are smaller than expected based on the single configurational coordinate model. This is attributed to the combined influence of the electronic configuration and the molecular geometry.     

Synthesis, crystal structures, and magnetic properties of a new family of heterometallic cyanide-bridged Fe(III)2M(II)2 (M=Mn, Ni, and Co) square complexes

New heterobimetallic tetranuclear complexes of formula [Fe(III){B(pz)(4)}(CN)(2)(μ-CN)Mn(II)(bpy)(2)](2)(ClO(4))(2)·CH(3)CN (1), [Fe(III){HB(pz)(3)}(CN)(2)(μ-CN)Ni(II)(dmphen)(2)](2)(ClO(4))(2)·2CH(3)OH (2a), [Fe(III){B(pz)(4)}(CN)(2)(μ-CN)Ni(II)(dmphen)(2)](2)(ClO(4))(2)·2CH(3)OH (2b), [Fe(III){HB(pz)(3)}(CN)(2)(μ-CN)Co(II)(dmphen)(2)](2)(ClO(4))(2)·2CH(3)OH (3a), and [Fe(III){B(pz)(4)}(CN)(2)(μ-CN)Co(II)(dmphen)(2)](2)(ClO(4))(2)·2CH(3)OH (3b), [HB(pz)(3)(-) = hydrotris(1-pyrazolyl)borate, B(Pz)(4)(-) = tetrakis(1-pyrazolyl)borate, dmphen = 2,9-dimethyl-1,10-phenanthroline, bpy = 2,2'-bipyridine] have been synthesized and structurally and magnetically characterized. Complexes 1-3b have been prepared by following a rational route based on the self-assembly of the tricyanometalate precursor fac-[Fe(III)(L)(CN)(3)](-) (L = tridentate anionic ligand) and cationic preformed complexes [M(II)(L')(2)(H(2)O)(2)](2+) (L' = bidentate α-diimine type ligand), this last species having four blocked coordination sites and two labile ones located in cis positions. The structures of 1-3b consist of cationic tetranuclear Fe(III)(2)M(II)(2) square complexes [M = Mn (1), Ni (2a and 2b), Co (3a and 3b)] where corners are defined by the metal ions and the edges by the Fe-CN-M units. The charge is balanced by free perchlorate anions. The [Fe(L)(CN)(3)](-) complex in 1-3b acts as a ligand through two cyanide groups toward two divalent metal complexes. The magnetic properties of 1-3b have been investigated in the temperature range 2-300 K. A moderately strong antiferromagnetic interaction between the low-spin Fe(III) (S = 1/2) and high-spin Mn(II) (S = 5/2) ions has been found for 1 leading to an S = 4 ground state (J(1) = -6.2 and J(2) = -2.7 cm(-1)), whereas a moderately strong ferromagnetic interaction between the low-spin Fe(III) (S = 1/2) and high-spin Ni(II) (S = 1) and Co(II) (S = 3/2) ions has been found for complexes 2a-3b with S = 3 (2a and 2b) and S = 4 (3a and 3b) ground spin states [J(1) = +21.4 cm(-1) and J(2) = +19.4 cm(-1) (2a); J(1) = +17.0 cm(-1) and J(2) = +12.5 cm(-1) (2b); J(1) = +5.4 cm(-1) and J(2) = +11.1 cm(-1) (3a); J(1) = +8.1 cm(-1) and J(2) = +11.0 cm(-1) (3b)] [the exchange Hamiltonian being of the type H? = -J(S?(i)·S?(j))]. Density functional theory (DFT) calculations have been used to substantiate the nature and magnitude of the exchange magnetic coupling observed in 1-3b and also to analyze the dependence of the exchange magnetic coupling on the structural parameters of the Fe-C-N-M skeleton.     

EPR, 1H and 2H NMR, and reactivity studies of the iron-oxygen intermediates in bioinspired catalyst systems

Complexes [(BPMEN)Fe(II)(CH(3)CN)(2)](ClO(4))(2) (1, BPMEN = N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)-1,2-diaminoethane) and [(TPA)Fe(II)(CH(3)CN)(2)](ClO(4))(2) (2, TPA = tris(2-pyridylmethyl)amine) are among the best nonheme iron-based catalysts for bioinspired oxidation of hydrocarbons. Using EPR and (1)H and (2)H NMR spectroscopy, the iron-oxygen intermediates formed in the catalyst systems 1,2/H(2)O(2); 1,2/H(2)O(2)/CH(3)COOH; 1,2/CH(3)CO(3)H; 1,2/m-CPBA; 1,2/PhIO; 1,2/(t)BuOOH; and 1,2/(t)BuOOH/CH(3)COOH have been studied (m-CPBA is m-chloroperbenzoic acid). The following intermediates have been observed: [(L)Fe(III)(OOR)(S)](2+), [(L)Fe(IV)═O(S)](2+) (L = BPMEN or TPA, R = H or (t)Bu, S = CH(3)CN or H(2)O), and the iron-oxygen species 1c (L = BPMEN) and 2c (L = TPA). It has been shown that 1c and 2c directly react with cyclohexene to yield cyclohexene oxide, whereas [(L)Fe(IV)═O(S)](2+) react with cyclohexene to yield mainly products of allylic oxidation. [(L)Fe(III)(OOR)(S)](2+) are inert in this reaction. The analysis of EPR and reactivity data shows that only those catalyst systems which display EPR spectra of 1c and 2c are able to selectively epoxidize cyclohexene, thus bearing strong evidence in favor of the key role of 1c and 2c in selective epoxidation. 1c and 2c were tentatively assigned to the oxoiron(V) intermediates.     

Oxidation state dependence of the geometry, electronic structure, and magnetic coupling in mixed oxo- and carboxylato-bridged manganese dimers

Approximate density functional theory has been used to investigate changes in the geometry and electronic structure of the mixed oxo- and carboxylato-bridged dimers [Mn(2)(mu-O)(2)(O(2)CH)(NH(3))(6)](n+)and [Mn(2)(mu-O)(O(2)CH)(2)(NH(3))(6)](n+)in the Mn(IV)Mn(IV), Mn(III)Mn(IV), and Mn(III)Mn(III) oxidation states. The magnetic coupling in the dimer is profoundly affected by changes in both the bridging ligands and Mn oxidation state. In particular, change in the bridging structure has a dramatic effect on the nature of the Jahn-Teller distortion observed for the Mn(III) centers in the III/III and III/IV dimers. The principal magnetic interactions in [Mn(2)(mu-O)(2)(O(2)CH)(NH(3))(6)](n+)() involve the J(xz/xz)and J(yz/yz) pathways but due to the tilt of the Mn(2)O(2) core, they are less efficient than in the planar di-mu-oxo structure and, consequently, the calculated exchange coupling constants are generally smaller. In both the III/III and III/IV dimers, the Mn(III) centers are high-spin, and the Jahn-Teller effect gives rise to axially elongated Mn(III) geometries with the distortion axis along the Mn-O(c) bonds. In the III/IV dimer, the tilt of the Mn(2)O(2) core enhances the crossed exchange J(x)()()2(-)(y)()()2(/)(z)()()2 pathway relative to the planar di-mu-oxo counterpart, leading to significant delocalization of the odd electron. Since this delocalization pathway partially converts the Mn(IV) ion into low-spin Mn(III), the magnetic exchange in the ground state can be considered to arise from two interacting spin ladders, one is the result of coupling between Mn(IV) (S = 3/2) and high-spin Mn(III) (S = 2), the other is the result of coupling between Mn(IV) (S = 3/2) and low-spin Mn(III) (S = 1). In [Mn(2)(mu-O)(O(2)CH)(2)(NH(3))(6)](n+)(), both the III/III dimer and the lowest energy structure for the III/IV dimer involve high-spin Mn(III), but the Jahn-Teller axis is now orientated along the Mn-oxo bond, giving rise to axially compressed Mn(III) geometries with long Mn-O(c) equatorial bonds. In the IV/IV dimer, the ferromagnetic crossed exchange J(yz)()(/)(z)()()2 pathway partially cancels J(yz/yz) and, as a consequence, the antiferromagnetic J(xz/xz) pathway dominates the magnetic coupling. In the III/III dimer, the J(yz/yz) pathway is minimized due to the smaller Mn-O-Mn angle, and since the ferromagnetic J(yz)()(/)(z)()()2 pathway largely negates J(xz/xz), relatively weak overall antiferromagnetic coupling results. In the III/IV dimer, the structures involving high-spin and low-spin Mn(III) are almost degenerate. In the high-spin case, the odd electron is localized on the Mn(III) center, and the resulting antiferromagnetic coupling is similar to that found for the IV/IV dimer. In the alternative low-spin structure, the odd electron is significantly delocalized due to the crossed J(yz)()(/)(z)()()2 pathway, and cancellation between ferromagnetic and antiferromagnetic pathways leads to overall weak magnetic coupling. The delocalization partially converts the Mn(IV) ion into high-spin Mn(III), and consequently, the spin ladders arising from coupling of Mn(IV) (S = 3/2) with high-spin (S = 2) and low-spin (S = 1) Mn(III) are configurationally mixed. Thus, in principle, the ground-state magnetic coupling in the mixed-valence dimer will involve contributions from three spin-ladders, two associated with the delocalized low-spin structure and the third arising from the localized high-spin structure.     

Evaluating the identity and diiron core transformations of a (μ-oxo)diiron(III) complex supported by electron-rich tris(pyridyl-2-methyl)amine ligands

The composition of a (μ-oxo)diiron(III) complex coordinated by tris[(3,5-dimethyl-4-methoxy)pyridyl-2-methyl]amine (R(3)TPA) ligands was investigated. Characterization using a variety of spectroscopic methods and X-ray crystallography indicated that the reaction of iron(III) perchlorate, sodium hydroxide, and R(3)TPA affords [Fe(2)(μ-O)(μ-OH)(R(3)TPA)(2)](ClO(4))(3) (2) rather than the previously reported species [Fe(2)(μ-O)(OH)(H(2)O)(R(3)TPA)(2)](ClO(4))(3) (1). Facile conversion of the (μ-oxo)(μ-hydroxo)diiron(III) core of 2 to the (μ-oxo)(hydroxo)(aqua)diiron(III) core of 1 occurs in the presence of water and at low temperature. When 2 is exposed to wet acetonitrile at room temperature, the CH(3)CN adduct is hydrolyzed to CH(3)COO(-), which forms the compound [Fe(2)(μ-O)(μ-CH(3)COO)(R(3)TPA)(2)](ClO(4))(3) (10). The identity of 10 was confirmed by comparison of its spectroscopic properties with those of an independently prepared sample. To evaluate whether or not 1 and 2 are capable of generating the diiron(IV) species [Fe(2)(μ-O)(OH)(O)(R(3)TPA)(2)](3+) (4), which has previously been generated as a synthetic model for high-valent diiron protein oxygenated intermediates, studies were performed to investigate their reactivity with hydrogen peroxide. Because 2 reacts rapidly with hydrogen peroxide in CH(3)CN but not in CH(3)CN/H(2)O, conditions that favor conversion to 1, complex 1 is not a likely precursor to 4. Compound 4 also forms in the reaction of 2 with H(2)O(2) in solvents lacking a nitrile, suggesting that hydrolysis of CH(3)CN is not involved in the H(2)O(2) activation reaction. These findings shed light on the formation of several diiron complexes of electron-rich R(3)TPA ligands and elaborate on conditions required to generate synthetic models of diiron(IV) protein intermediates with this ligand framework.     

Syntheses, structures, spectroscopy, and chromotropism of new complexes arising from the reaction of nickel(II) nitrate with diphenyl(dipyrazolyl)methane

The complexes [(dpdpm)Ni(2-NO3)2] (1), [(dpdpm)Ni(2-NO3)(1-NO3)(CH3CN)] (2), [(dpdpm)2Ni(1-NO3)(H2O)]NO3 (3), and [(dpdpm)2Ni(H2O)2][NO3]2 (4) (dpdpm = diphenyl(dipyrazolyl)methane, Ph2C(C3N2H3)2), have been prepared and characterized by IR and UV-vis-NIR spectroscopy and X-ray diffraction studies. X-ray studies have confirmed that complexes 1-4 all adopt variously distorted octahedral structures in the solid state, the largest distortions arising from the small bite-angle of the bidentate nitrate ligand in 1 and 2. Magnetic moment measurements indicate that these solids are paramagnetic with two unpaired electrons. The solution 1H NMR data show that the paramagnetism is maintained in solution. Absorption spectra of 1-4 show three main bands in the region of 350-1000 nm representing spin allowed (d-d) transitions from the ground state 3A2g to the excited states 3T2g, 3T1g(3F), and 3T1g(3P). A weak shoulder was also detected at about 700-800 nm in most spectra, representing spin-forbidden transitions 3A2g 1Eg. A comparison of the crystal field parameters 10Dq and B for 1-4 to the corresponding values for related complexes indicated that these parameters are fairly insensitive to structural variations within this family of complexes. The 10Dq/B ratios show greater variations, but no clear correlations are apparent between 10Dq/B and such structural features as the nature of ligator atoms (N:O ratio), the bonding mode of the nitrate ligand, or the overall charge. Complexes 1 (green) and 2 (blue) interconvert as a function of temperature (solutions and solid samples), concentration of CH3CN (solutions), or CH3CN vapor pressure (solid samples).     

Ferromagnetic coupling between low- and high-spin iron(III) ions in the tetranuclear complex fac-[[FeIII[HB(pz)3](CN)2(mu-CN)]3FeIII(H2O)3]* 6H2O ([HB(pz)3]- = hydrotris(1-pyrazolyl)borate)

The novel mononuclear PPh4-fac-[FeIII[HB(pz)3](CN)3]*H2O (1) [PPh4+= tetraphenylphosphonium cation; (HB(pz)3)- = hydrotris(1-pyrazolyl)borate] and tetranuclear fac-[[FeIII[HB(pz)3](CN)2(mu-CN)]3FeIII(H2O)3]*6H2O (2) have been prepared and characterized by X-ray diffraction analysis. Crystal data for compound 1: monoclinic, space group P21/c, a = 9.575(3) A, b = 21.984(4) A, c = 16.863(3) A, beta = 100.34(2) degrees, U = 3486(1) A3, Z = 4. Crystal data for compound 2: orthorhombic, space group Pnam, a = 14.084(3) A, b = 14.799(4) A, c = 25.725(5) A, U = 5362(2) A3, Z = 4. Compound 1 is a low-spin iron(III) compound with three cyanide ligands in fac arrangement and a tridentate pyrazolylborate ligand building a distorted octahedral environment around the iron atom. Compound 2 is the first example of a molecular species containing three peripheral low-spin iron(III) ions linked to a central high-spin iron(III) cation by single cyanide bridges, the anion of 1 acting as a monodentate ligand in 2. Variable-temperature magnetic susceptibility measurements of 2 reveal the occurrence of a significant ferromagnetic coupling between the three peripheral low-spin iron(III) centers and the central high-spin iron(III) ion cations leading to a low-lying nonet spin state.     

Iron-promoted nucleophilic additions to diimine-type ligands: a synthetic and structural study

We report here three examples of the reactivity of protic nucleophiles with diimine-type ligands in the presence of Fe(II) salts. In the first case, the iron-promoted alcoholysis reaction of one nitrile group of the ligand 2,3-dicyano-5,6-bis(2-pyridyl)-pyrazine (L1) permitted the isolation of an stable E-imido-ester, [Fe(L1')(2)](CF(3)SO(3))(2) (1), which has been characterized by spectroscopic studies (IR, ES-MS, M?ssbauer), elemental analysis, and crystallographically. Compound 1 consists of mononuclear octahedrally coordinated Fe(II) complexes where the Fe(II) ion is in its low-spin state. The iron-mediated nucleophilic attack of water to the asymmetric ligand 2,3-bis(2-pyridyl)pyrido[3,4-b]pyrazine (L2) has also been studied. In this context, the crystal structures of two hydration-oxidation Fe(III) products, [Fe(L2')(2)](ClO(4))(3).3CH(3)CN (2) and trans-[FeL2"Cl(2)] (3), are described. Compounds 2 and 3 are both mononuclear Fe(III) complexes where the metals occupy octahedral positions. In principle, L2 is expected to coordinate to metal ions through its bipyridine-type units to form a five-membered ring; however, this is not the case in compounds 2 and 3. In 2, the ligand coordinates through its pyridines and through the hydroxyl group attached to the pyrazine imino carbon after hydration, that is, in an N,O,N tridentate manner. In compound 3, the ligand has suffered further transformations leading to a very stable diamido complex. In this case, the metal ion achieves its octahedral geometry by means of two pyridines, two amido N atoms, and two axial chlorine atoms. Magnetic susceptibility measurements confirmed the spin state of these two Fe(III) species: compounds 2 and 3 are low-spin and high-spin, respectively.     

Valence Delocalization in Prussian Blue Fe[FeII(CN)6]3 · XD2O,by Polarized Neutron Diffraction

Polarized neutron diffraction has been used to investigate the spin delocalization from the high-spin Fe(III) sites to the low-spin Fe(II) in deuteriated Prussian Blue, Fe₄[Fe(CN)₆]₃ · xD₂O. Measurements of the 111, 200, and 400 reflections were made on a powdered sample at 3 K and 4.8 T using a neutron wavelength of 1.074 Å. The expectation value of S at the Fe(II) site is - 0.008 ± 0.028 corresponding to an upper limit of about 5% of an electron for the spin delocalization.     

Ligand Recruitment and Spin Transitions in the Solid-State Photochemistry of Fe((III))TPPCl

We report evidence for the formation of long-lived photoproducts following excitation of iron(III) tetraphenylporphyrin chloride (Fe((III))TPPCl) in a 1:1 glass of toluene and CH(2)Cl(2) at 77 K. The formation of these photoproducts is dependent on solvent environment and temperature, appearing only in the presence of toluene. No long-lived product is observed in neat CH(2)Cl(2) solvent. A 2-photon absorption model is proposed to account for the power-dependent photoproduct populations. The products are formed in a mixture of spin states of the central iron(III) metal atom. Metastable six-coordinate high-spin and low-spin complexes and a five-coordinate high-spin complex of iron(III) tetraphenylporphyrin are assigned using structure-sensitive vibrations in the resonance Raman spectrum. These species appear in conjunction with resonantly enhanced toluene solvent vibrations, indicating that the Fe((III)) compound formed following photoexcitation recruits a toluene ligand from the surrounding environment. Low-temperature transient absorption (TA) measurements are used to explain the dependence of product formation on excitation frequency in this photochemical model. The six-coordinate photoproduct is initially formed in the high-spin Fe((III)) state, but population relaxes into both high-spin and low-spin state at 77 K. This is the first demonstration of coupling between the optical and magnetic properties of an iron-centered porphyrin molecule.     

NH/pi interaction in a spin-crossover complex [FeII(HLMe)2](BPh4)2.2CH3CN (BPh4-=Tetraphenylborate; HLMe=2-Methylimidazol-4-yl-methylideneamino-2-ethylpyridine)

Three FeII complexes, [Fe(HLR)2](BPh4)2.solvent (R=H, Me, Ph), were synthesized, where BPh4-=tetraphenylborate and HLR=2-substituted-imidazol-4-yl-methylideneamino-2-ethylpyridine. The magnetic susceptibility measurements in 5-300 K revealed that [Fe(HLH)2](BPh4)2.H2O, [Fe(HLMe)2](BPh4)2.2CH3CN, and [Fe(HLPh)2](BPh4)2.CH3CN are low-spin (LS), spin-crossover (SC), and high-spin (HS) FeII complexes, respectively, indicating that the spin state can be effectively tuned by the bulkiness of the substituent. Complex shows a steep SC around 250 K, where it assumes a cyclic structure of {[Fe(HLMe)2]BPh4}2 constructed by four NH/pi bonds between the imidazole group and the phenyl ring of BPh4- in the HS state and a deformed structure with NH/pi bonds and linear CH3CN...HN hydrogen bonds at the terminals in the LS state.     

The spin-transition properties of Fe(III) complexes with hetarylformazan in an ion-exchange polymer: An EPR study

For the first time, the EPR method was used to reveal and study the special features of spin-transition processes between high-spin and low-spin iron(III) complexes with hetarylformazans immobilized on an ion-exchange polymer. An analysis of completely reversible temperature dependences of EPR line positions, widths, and integral intensities in the spectra of high-spin and low-spin complexes allowed four temperature intervals to be identified. These intervals corresponded to preparative periods of spin-transition processes (450–275 K, intervals I and II), their appearance (275–230 K, interval III), and occurrence (230–100 K, interval IV). Local concentrations and spin exchange frequencies in clusters were estimated. Effects related to high-spin complex EPR signal shifts during temperature changes and to the duration of sample storage were revealed. High-spin complexes were found to be very sensitive to external actions, as distinct from very stable low-spin complexes. Experimental EPR observations obtained for ion-exchange polymer satisfied the concept of the nucleation and growth of domains in the spin-transition process.     

Coordination algorithms control molecular architecture: [CuI4(L2)4]4+ grid complex versus [MII2(L2)2X4]y+ side-by-side complexes (M=Mn,Co, Ni,Zn; X=solvent or anion) and [FeII(L2)3][Cl3FeIIIOFeIIICl3

The synthesis and characterisation of a pyridazine-containing two-armed grid ligand L2 (prepared from one equivalent of 3,6-diformylpyridazine and two equivalents of p-anisidine) and the resulting transition metal (Zn, Cu, Ni, Co, Fe, Mn) complexes (1-9) are reported. Single-crystal X-ray structure determinations revealed that the copper(I) complex had self-assembled as a [2 x 2] grid, [Cu(I) (4)(L2)(4)][PF(6)](4).(CH(3)CN)(H(2)O)(CH(3)CH(2)OCH(2)CH(3))(0.25) (2.(CH(3)CN)(H(2)O)(CH(3)CH(2)OCH(2)CH(3))(0.25)), whereas the [Zn(2)(L2)(2)(CH(3)CN)(2)(H(2)O)(2)][ClO(4)](4).CH(3)CN (1.CH(3)CN), [Ni(II) (2)(L2)(2)(CH(3)CN)(4)][BF(4)](4).(CH(3)CH(2)OCH(2)CH(3))(0.25) (5 a.(CH(3)CH(2)OCH(2)CH(3))(0.25)) and [Co(II) (2)(L2)(2)(H(2)O)(2)(CH(3)CN)(2)][ClO(4)](4).(H(2)O)(CH(3)CN)(0.5) (6 a.(H(2)O)(CH(3)CN)(0.5)) complexes adopt a side-by-side architecture; iron(II) forms a monometallic cation binding three L2 ligands, [Fe(II)(L2)(3)][Fe(III)Cl(3)OCl(3)Fe(III)].CH(3)CN (7.CH(3)CN). A more soluble salt of the cation of 7, the diamagnetic complex [Fe(II)(L2)(3)][BF(4)](2).2 H(2)O (8), was prepared, as well as two derivatives of 2, [Cu(I) (2)(L2)(2)(NCS)(2)].H(2)O (3) and [Cu(I) (2)(L2)(NCS)(2)] (4). The manganese complex, [Mn(II) (2)(L2)(2)Cl(4)].3 H(2)O (9), was not structurally characterised, but is proposed to adopt a side-by-side architecture. Variable temperature magnetic susceptibility studies yielded small negative J values for the side-by-side complexes: J=-21.6 cm(-1) and g=2.17 for S=1 dinickel(II) complex [Ni(II) (2)(L2)(2)(H(2)O)(4)][BF(4)](4) (5 b) (fraction monomer 0.02); J=-7.6 cm(-1) and g=2.44 for S= 3/2 dicobalt(II) complex [Co(II) (2)(L2)(2)(H(2)O)(4)][ClO(4)](4) (6 b) (fraction monomer 0.02); J=-3.2 cm(-1) and g=1.95 for S= 5/2 dimanganese(II) complex 9 (fraction monomer 0.02). The double salt, mixed valent iron complex 7.H(2)O gave J=-75 cm(-1) and g=1.81 for the S= 5/2 diiron(III) anion (fraction monomer=0.025). These parameters are lower than normal for Fe(III)OFe(III) species because of fitting of superimposed monomer and dimer susceptibilities arising from trace impurities. The iron(II) centre in 7.H(2)O is low spin and hence diamagnetic, a fact confirmed by the preparation and characterisation of the simple diamagnetic iron(II) complex 8. M?ssbauer measurements at 77 K confirmed that there are two iron sites in 7.H(2)O, a low-spin iron(II) site and a high-spin diiron(III) site. A full electrochemical investigation was undertaken for complexes 1, 2, 5 b, 6 b and 8 and this showed that multiple redox processes are a feature of all of them.     

First row divalent transition metal complexes of aryl-appended tris((pyridyl)methyl)amine ligands: syntheses, structures, electrochemistry, and hydroxamate binding properties

Divalent manganese, cobalt, nickel, and zinc complexes of 6-Ph(2)TPA (N,N-bis((6-phenyl-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine; [(6-Ph(2)TPA)Mn(CH(3)OH)(3)](ClO(4))(2) (1), [(6-Ph(2)TPA)Co(CH(3)CN)](ClO(4))(2) (2), [(6-Ph(2)TPA)Ni(CH(3)CN)(CH(3)OH)](ClO(4))(2) (3), [(6-Ph(2)TPA)Zn(CH(3)CN)](ClO(4))(2) (4)) and 6-(Me(2)Ph)(2)TPA (N,N-bis((6-(3,5-dimethyl)phenyl-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine; [(6-(Me(2)Ph)(2)TPA)Ni(CH(3)CN)(2)](ClO(4))(2) (5) and [(6-(Me(2)Ph)(2)TPA)Zn(CH(3)CN)](ClO(4))(2) (6)) have been prepared and characterized. X-ray crystallographic characterization of 1A.CH(3)()OH and 1B.2CH(3)()OH (differing solvates of 1), 2.2CH(3)()CN, 3.CH(3)()OH, 4.2CH(3)()CN, and 6.2.5CH(3)()CN revealed mononuclear cations with one to three coordinated solvent molecules. In 1A.CH(3)()OH and 1B.2CH(3)()OH, one phenyl-substituted pyridyl arm is not coordinated and forms a secondary hydrogen-bonding interaction with a manganese bound methanol molecule. In 2.2CH(3)()CN, 3.CH(3)()OH, 4.2CH(3)()CN, and 6.2.5CH(3)()CN, all pyridyl donors of the 6-Ph(2)TPA and 6-(Me(2)Ph)(2)TPA ligands are coordinated to the divalent metal center. In the cobalt, nickel, and zinc derivatives, CH/pi interactions are found between a bound acetonitrile molecule and the aryl appendages of the 6-Ph(2)TPA and 6-(Me(2)Ph)(2)TPA ligands. (1)H NMR spectra of 4 and 6 in CD(3)NO(2) solution indicate the presence of CH/pi interactions, as an upfield-shifted methyl resonance for a bound acetonitrile molecule is present. Examination of the cyclic voltammetry of 1-3 and 5 revealed no oxidative (M(II)/M(III)) couples. Admixture of equimolar amounts of 6-Ph(2)TPA, M(ClO(4))(2).6H(2)O, and Me(4)NOH.5H(2)O, followed by the addition of an equimolar amount of acetohydroxamic acid, yielded the acetohydroxamate complexes [((6-Ph(2)TPA)Mn)(2)(micro-ONHC(O)CH(3))(2)](ClO(4))(2) (8), [(6-Ph(2)TPA)Co(ONHC(O)CH(3))](ClO(4))(2) (9), [(6-Ph(2)TPA)Ni(ONHC(O)CH(3))](ClO(4))(2) (10), and [(6-Ph(2)TPA)Zn(ONHC(O)CH(3))](ClO(4))(2) (11), all of which were characterized by X-ray crystallography. The Mn(II) complex 8.0.75CH(3)()CN.0.75Et(2)()O exhibits a dinuclear structure with bridging hydroxamate ligands, whereas the Co(II), Ni(II), and Zn(II) derivatives all exhibit mononuclear six-coordinate structures with a chelating hydroxamate ligand.     

Tuning the electronic structure of octahedral iron complexes [FeL(X)] (L = 1-alkyl-4,7-bis(4-tert-butyl-2-mercaptobenzyl)-1,4,7-triazacyclononane,X = Cl,CH(3)O,CN, NO). The S = 1/2 <==>3/2 Spin equilibrium of [FeL(Pr)(NO)

Two new pentadentate, pendent arm macrocyclic ligands of the type 1-alkyl-4,7-bis(4-tert-butyl-2-mercaptobenzyl)-1,4,7-triazacyclononane where alkyl represents an isopropyl, (L(Pr))(2-), or an ethyl group, (L(Et))(2-), have been synthesized. It is shown that they bind strongly to ferric ions generating six-coordinate species of the type [Fe(L(alk))X]. The ground state of these complexes is governed by the nature of the sixth ligand, X: [Fe(III)(L(Et))Cl] (2) possesses an S = 5/2 ground state as do [Fe(III)(L(Et))(OCH(3))] (3) and [Fe(III)(L(Pr))(OCH(3))] (4). In contrast, the cyano complexes [Fe(III)(L(Et))(CN)] (5) and [Fe(III)(L(Pr))(CN)] (6) are low spin ferric species (S = 1/2). The octahedral [FeNO](7) nitrosyl complex [Fe(L(Pr))(NO)] (7) displays spin equilibrium behavior S = 1/2<==>S = (3)/(2) in the solid state. Complexes [Zn(L(Pr))] (1), 4.CH(3)OH, 5.0.5toluene.CH(2)Cl(2), and 7.2.5CH(2)Cl(2) have been structurally characterized by low-temperature (100 K) X-ray crystallography. All iron complexes have been carefully studied by zero- and applied-field M?ssbauer spectroscopy. In addition, Sellmann's complexes [Fe(pyS(4))(NO)](0/1+) and [Fe(pyS(4))X] (X = PR(3), CO, SR(2)) have been studied by EPR and M?ssbauer spectroscopies and DFT calculations (pyS(4) = 2,6-bis(2-mercaptophenylthiomethyl)pyridine(2-)). It is concluded that the electronic structure of 7 with an S = 1/2 ground state is low spin ferrous (S(Fe) = 0) with a coordinated neutral NO radical (Fe(II)-NO) whereas the S = 3/2 state corresponds to a high spin ferric (S(Fe) = 5/2) antiferromagnetically coupled to an NO(-) anion (S = 1). The S = 1/2<==>S = 3/2 equilibrium is then that of valence tautomers rather than that of a simple high spin<==>low spin crossover.