Structure-function correlations in Iron(II) tris(pyrazolyl)borate spin-state crossover complexes

Iron(II) poly(pyrazolyl)borate complexes have been investigated to determine the impact of substituent effects, intramolecular ligand distortions, and intermolecular supramolecular structures on the spin-state crossover (SCO) behavior. The molecular structure of Fe[HB(3,4,5-Me3pz)3]2 (pz = pyrazolyl ring), a complex known to remain high spin when the temperature is lowered, reveals that this complex has an intramolecular ring-twist distortion that is not observed in analogous complexes that do exhibit a SCO at low temperatures, thus indicating that this distortion greatly influences the properties of these complexes. The structure of Fe[B(3-(cy)Prpz)4]2.(CH3OH) ((cy)Pr = cyclopropyl ring) at 294 K has two independent molecules in the unit cell, both of which are high spin; only one of these high-spin iron(II) sites, the site with the lesser ring-twist distortion, is observed to be low-spin iron(II) in the 90 K structure. A careful evaluation of the supramolecular structures of these complexes and several similar complexes reported previously revealed no strong correlation between the supramolecular packing forces and their SCO behavior. Magnetic and M?ssbauer spectral measurements on Fe[B(3-(cy)Prpz)4]2 and Fe[HB(3-(cy)Prpz)3]2 indicate that both complexes exhibit a partial SCO from fully high-spin iron(II) at higher temperatures, respectively, to a 50:50 high-spin/low-spin mixture of iron(II) below 100 K. These results may be understood, in the former case, by the differences in ring-twisting and, in the latter case, by a phase transition; in all complexes in which a phase transition is observed, this change dominates the SCO behavior. A comparison of the M?ssbauer spectral properties of these two complexes and of Fe[HB(3-Mepz)3]2 with that of other complexes reveals correlations between the M?ssbauer-effect isomer shift and the average Fe-N bond distance and between the quadrupole splitting and the average FeN-NB intraligand dihedral torsion angles and the distortion of the average N-Fe-N intraligand bond angles.     

High-spin and low-spin iron(II) complexes with facially-coordinated borohydride ligands

Rare examples of monometallic high-spin and low-spin L3Fe(H3BH) complexes have been characterized, where the two L3 ligands are [TpPh2] and [PhBP3] ([TpPh2] = [HB(3,5-Ph2pz)3]- and [PhBP3] = [PhB(CH2PPh2)3]-). The structures are reported wherein the borohydride ligand is facially coordinated to the iron center in each complex. Density functional methods have been employed to explain the bonding in these unusual iron(II) centers. Despite the differences in spin states, short Fe-B distances are observed in both complexes and there is significant theoretical evidence to support a substantial bonding interaction between the iron and boron nuclei. In light of this interaction, we suggest that these complexes can be described as (L3)Fe(eta4-H3BH) complexes.     

Solid-state structural and magnetic investigations of [M[HC(3,5-Me(2)pz)(3)](2)](BF(4))(2) (M = Fe,Co, Ni,Cu): observation of a thermally induced solid-state phase change controlling an iron(II) spin-state crossover

The reaction of M(BF(4))(2).xH(2)O (M = Co, Ni, and Cu) and HC(3,5-Me(2)pz)(3) in a 1:2 ratio yields [Co[HC(3,5-Me(2)pz)(3)](2)](BF(4))(2) (2), [Ni[HC(3,5-Me(2)pz)(3)](2)](BF(4))(2) (3), and [Cu[HC(3,5-Me(2)pz)(3)](2)](BF(4))(2) (4). Over the temperature range from 5 to 350, 345, or 320 K, Curie law behavior is observed for microcrystalline samples of all three compounds showing them to have three, two, and one unpaired electrons, respectively, with no spin-crossover observed for 2. Crystalline samples of these compounds torque in the applied magnetic field the first time the sample is cooled to 5 K. The solid-state structures of all three are isomorphous at 220 K, monoclinic in the space group C2/c. The metal is located on a unique crystallographic site and has a trigonally distorted octahedral structure, with 4 showing the expected Jahn-Teller distortions. Cooling crystals of all three to low temperatures leads to the observation of the same phase change to triclinic in the new space group P(-)1 with nonmerohedral twinning. This change is reversible and yields two crystallographically unique metal sites at low temperature. The bond angles and distances for the two different metal sites for each compound in the low temperature structures are very similar to each other and to those in the 220 K structures. The same phase change, monoclinic to triclinic, has been observed previously for [Fe[HC(3,5-Me(2)pz)(3)](2)](BF(4))(2) (1), except in this case, the phase change results in half of the cations changing over from the high-spin state to the low-spin state while the other half of the cations remain high-spin, with the low-spin form decreasing its Fe-N bond distances by 0.19 A. The new results with 2-4 show that it is the phase transition, which occurs in complexes of the type [M[HC(3,5-Me(2)pz)(3)](2)](BF(4))(2) with first row transition metals, that is driving the unusual spin-crossover behavior of [Fe[HC(3,5-Me(2)pz)(3)](2)](BF(4))(2).     

A synthetic, structural, magnetic, and spectral study of several [Fe[tris(pyrazolyl)methane]2](BF4)2 complexes: observation of an unusual spin-state crossover

The complexes [Fe[HC(3,5-Me2pz)3]2](BF4)2 (1), [Fe[HC(pz)3]2](BF4)2 (2), and [Fe[PhC(pz)2(py)]2](BF4)2 (3) (pz = 1-pyrazolyl ring, py = pyridyl ring) have been synthesized by the reaction of the appropriate ligand with Fe(BF4)2.6H2O. Complex 1 is high-spin in the solid state and in solution at 298 K. In the solid phase, it undergoes a decrease in magnetic moment at lower temperatures, changing at ca. 206 K to a mixture of high-spin and low-spin forms, a spin-state mixture that does not change upon subsequent cooling to 5 K. Crystallographically, there is only one iron(II) site in the ambient-temperature solid-state structure, a structure that clearly shows the complex is high-spin. M?ssbauer spectral studies show conclusively that the magnetic moment change observed at lower temperatures arises from the complex changing from a high-spin state at higher temperatures to a 50:50 mixture of high-spin and low-spin states at lower temperatures. Complexes 2 and 3 are low-spin in the solid phase at room temperature. Complex 2 in the solid phase gradually changes over to the high-spin state upon heating above 295 K and is completely high-spin at ca. 470 K. In solution, variable-temperature 1H NMR spectra of 2 show both high-spin and low-spin forms are present, with the percentage of the paramagnetic form increasing as the temperature increases. Complex 3 is low-spin at all temperatures studied in both the solid phase and solution. An X-ray absorption spectral study has been undertaken to investigate the electronic spin states of [Fe[HC(3,5-Me2pz)3]2](BF4)2 and [Fe[HC(pz)3]2](BF4)2. Crystallographic information: 2 is monoclinic, P2(1)/n, a = 10.1891(2) A, b = 7.6223(2) A, c = 17.2411(4) A, beta = 100.7733(12) degrees, Z = 2; 3 is triclinic, P1, a = 12.4769(2) A, b = 12.7449(2) A, c = 13.0215(2) A, alpha = 83.0105(8) degrees, beta = 84.5554(7) degrees, gamma = 62.5797(2) degrees, Z = 2.     

A second-generation janus scorpionate ligand: controlling coordination modes in iron(II) complexes by steric modulation

The second-generation Janus scorpionate ligand [HB(mtda (Me)) 3] (-) (mtda (Me) = 2-mercapto-5-methyl-1,3,4-thiadiazolyl) with conjoined ( N, N, N-) and ( S, S, S-) donor faces has been prepared. This second-generation Janus scorpionate ligand [HB(mtda (Me)) 3] (-) differs from the first-generation [HB(mtda) 3] (-) ligand by the replacement of hydrogens on the heterocyclic rings proximal to the nitrogenous face with methyl groups. This study probed whether steric interactions introduced by such methyl group substitution could modulate the reactivity and coordination preferences of these ambidentate ligands. The crystal structures of a sodium complex Na[HB(mtda (Me)) 3].3(MeOH), the potassium complexes K[HB(mtda) 3].MeOH, and K 2[HB(mtda (Me)) 3] 2.3MeOH, and several iron complexes were obtained. The difference between first- and second-generation Janus scorpionate ligands is most obvious from the discrepancy between the properties and structures of the two iron(II) compounds with the formula Fe[HB(mtda (R)) 3] 2.4DMF (R = H or Me). The complex with the first-generation ligand (R = H) is pink and diamagnetic. An X-ray structural study revealed two facially coordinated kappa (3)N-scorpionates with no bound solvent molecules. The average Fe-N bond distance of 1.97 A is indicative of the low-spin t 2g (6)e g* (0) electron configuration. In contrast, the iron(II) complex of the second-generation ligand (R = Me) is yellow and paramagnetic. This structure shows two trans-kappa (1)S-scorpionates and four equatorial-bound DMF where the average Fe-O and Fe-S distances of 2.12 and 2.51 A, respectively, are indicative of the high-spin t 2g (4)e g* (2) electron configuration. The discrepancy in binding modes and spin-states of iron(II) is carried over to the solvent-free Fe[HB(mtda (R)) 3] 2 (R = H, Me) complexes, as determined from Mossbauer spectral studies. The Mossbauer spectral parameters for Fe[HB(mtda) 3] 2 are fully consistent with low-spin iron(II) in a FeN 6 environment, whereas those for Fe[HB(mtda (Me)) 3] 2 are most consistent with high-spin iron(II) in a FeS 6 environment. Interestingly, when either complex is dissolved in highly polar solvents (DMF, DMSO, or H 2O), the ligand completely dissociates forming [Fe(solvent) 6][HB(mtda (R)) 3] 2 (R = H, Me).     

Ground-state singlet L3Fe-(mu-N)-FeL3 and L3Fe(NR) complexes featuring pseudotetrahedral Fe(II) centers

Pseudotetrahedral iron(II) coordination complexes that contain bridged nitride and terminal imide linkages, and exhibit singlet ground-state electronic configurations, are described. Sodium amalgam reduction of the ferromagnetically coupled dimer, {[PhBP(3)]Fe(mu-1,3-N(3))}(2) (2) ([PhBP(3)] = [PhB(CH(2)PPh(2))(3)](-)), yields the diamagnetic bridging nitride species [{[PhBP(3)]Fe}(2)(mu-N)][Na(THF)(5)] (3). The Fe-N-Fe linkage featured in the anion of 3 exhibits an unusually bent angle of approximately 135 degrees , and the short Fe-N bond distances (Fe-N(av) approximately equal to 1.70 A) suggest substantial Fe-N multiple bond character. The diamagnetic imide complex {[PhBP(3)]Fe(II)(triple bond)N(1-Ad)}{(n)()Bu(4)N} (4) has been prepared by sodium amalgam reduction of its low-spin iron(III) precursor, [PhBP(3)]Fe(III)(triple bond)N(1-Ad) (5). Complexes 4 and 5 have been structurally characterized, and their respective electronic structures are discussed in the context of a supporting DFT calculation. Diamagnetic 4 provides a bona fide example of a pseudotetrahedral iron(II) center in a low-spin ground-state configuration. Comparative optical data strongly suggest that dinuclear 3 is best described as containing two high-spin iron(II) centers that are strongly antiferromagnetically coupled to give rise to a singlet ground-state at room temperature.     

X-ray absorption spectroscopic studies of high-spin nonheme (alkylperoxo)iron(III) intermediates

The reactions of iron(II) complexes [Fe(T(pt-Bu,i-Pr))(OH)] (1a, Tp(t-Bu,i-Pr) = hydrotris(3-tert-butyl-5-isopropyl-1-pyrazolyl)borate), [Fe(6-Me2BPMCN)(OTf)2] (1b, 6-Me2BPMCN = N,N'-bis((2-methylpyridin-6-yl)methyl)-N,N'-dimethyl-trans-1,2-diaminocyclohexane), and [Fe(L8Py2)(OTf)](OTf) (1c, L8Py2 = 1,5-bis(pyridin-2-ylmethyl)-1,5-diazacyclooctane) with tert-BuOOH give rise to high-spin FeIII-OOR complexes. X-ray absorption spectra (XAS) of these high-spin species show characteristic features, distinct from those of low-spin Fe-OOR complexes (Rohde, J.-U.; et al. J. Am. Chem. Soc. 2004, 126, 16750-16761). These include (1) an intense 1s --> 3d preedge feature, with an area around 20 units, (2) an edge energy, ranging from 7122 to 7126 eV, that is affected by the coordination environment, and (3) a 1.86-1.96 A Fe-OOR bond, compared to the 1.78 A Fe-OOR bond in low-spin complexes. These unique features likely arise from a flexible first coordination sphere in those complexes. The difference in Fe-OOR bond length may rationalize differences in reactivity between low-spin and high-spin FeIII-OOR species.     

A high-pressure iron K-edge x-ray absorption spectral study of the spin-state crossover in (Fe[HC(3,5-(CH(3))(2)pz)(3)](2))I(2) and (Fe[HC(3,5-(CH(3))(2)pz)(3)](2))(BF(4))(2)

The room temperature iron K-edge X-ray absorption near edge structure spectra of (Fe[HC(3,5-(CH(3))(2)pz)(3)](2))I(2) and (Fe[HC(3,5-(CH(3))(2)pz)(3)](2))(BF(4))(2) have been measured between ambient and 88 and 94 kbar, respectively, in an opposed diamond anvil cell. The iron(II) in (Fe[HC(3,5-(CH(3))(2)pz)(3)](2))I(2)undergoes the expected gradual spin-state crossover from the high-spin state to the low-spin state with increasing pressure. In contrast, the iron(II) in (Fe[HC(3,5-(CH(3))(2)pz)(3)](2))(BF(4))(2) remains high-spin between ambient and 78 kbar and is only transformed to the low-spin state at an applied pressure of between 78 and 94 kbar. No visible change is observed in the preedge peak in the spectra of (Fe[HC(3,5-(CH(3))(2)pz)(3)](2))I(2) with increasing pressure, whereas the preedge peak in the spectra of ((e[HC(3,5-(CH(3))(2)pz)(3)](2))(BF(4))(2) changes as expected for a high-spin to low-spin crossover with increasing pressure. The difference in the spin-state crossover behavior of these two complexes is likely related to the unusual behavior of (Fe[HC(3,5-(CH(3))(2)pz)(3)](2))(BF(4))(2) upon cooling.     

Electronic configuration of five-coordinate high-spin pyrazole-ligated iron(II) porphyrinates

Pyrazole, a neutral nitrogen ligand and an isomer of imidazole, has been used as a fifth ligand to prepare two new species, [Fe(TPP)(Hdmpz)] and [Fe(Tp-OCH(3)PP)(Hdmpz)] (Hdmpz = 3,5-dimethylpyrazole), the first structurally characterized examples of five-coordinate iron(II) porphyrinates with a nonimidazole neutral ligand. Both complexes are characterized by X-ray crystallography, and structures show common features for five-coordinate iron(II) species, such as an expanded porphyrinato core, large equatorial Fe-N(p) bond distances, and a significant out-of-plane displacement of the iron(II) atom. The Fe-N(pyrazole) and Fe-N(p) bond distances are similar to those in imidazole-ligated species. These suggest that the coordination abilities to iron(II) for imidazole and pyrazole are very similar even though pyrazole is less basic than imidazole. Mo?ssbauer studies reveal that [Fe(TPP)(Hdmpz)] has the same behavior as those of imidazole-ligated species, such as negative quadrupole splitting values and relative large asymmetry parameters. Both the structures and the Mo?ssbauer spectra suggest pyrazole-ligated five-coordinate iron(II) porphyrinates have the same electronic configuration as imidazole-ligated species.     

Formation of third generation poly(pyrazolyl)borate ligands from alkyne coupling reactions of Fe[(p-IC6H4)B(3-Rpz)3]2 (R = H, Me; pz = pyrazolyl): pathways toward controlling an iron(II) electronic spin-state crossover

Sonogashira coupling reactions of terminal alkynes with Fe[(p-IC6H4)B(3-Mepz)3]2 (pz = pyrazolyl ring) yield Fe[(p-PhC2C6H4)B(3-Mepz)3]2 (2), Fe[(p-Me3SiC2C6H4)B(3-Rpz)3]2 (R = H, 3a, R = Me, 3b), and Fe[(p-HC2C6H4)B(3-Mepz)3]2 (R = H, 4a, R = Me, 4b), a series of new complexes containing "third generation" poly(pyrazolyl)borate ligands. Complex 2 undergoes a fairly gradual iron(II) electronic spin-state crossover with a 30 K hysteresis, whereas complex 3b is an unusual example of a complex with equivalent iron(II) sites in the high-spin form that shows an abrupt 50% spin crossover. For complex 4b, 50% of the iron(II) sites undergo a gradual spin-state transition between 185 and 350 K with an activation energy of 1590 +/- 30 cm(-1) and a T(1/2) = 280 K and, for the remaining iron(II) sites, an abrupt cooperative spin-state crossover between 106 and 114 K. The crystal structures of 4b obtained for each of the three distinct electronic spin states reveal two crystallographically different iron(II) sites, and analysis of the molecular/supramolecular structures indicates that the difference in the degree of pyrazolyl ring tilting in the ligands between the two sites, rather than the strength of the intermolecular forces, play a prominent role in determining the temperature of the spin-state crossover.     

Low-spin bis(2-methylimidazole)(octaethylporphyrinato)iron(III) chloride (perp-[Fe(OEP)(2-MeHIm)(2)]Cl): a consequence of hydrogen bonding?

The synthesis and characterization of low-spin bis(2-methylimidazole)(octaethylporphyrinato)iron(III) chloride (perp[Fe(OEP)(2-MeHIm)2]Cl) is reported. The structure shows that the cation is a low-spin species with two imidazole ligands having a relative perpendicular orientation. The porphyrin core is very ruffled, which leads to shortened equatorial bonds of 1.974(4) A and slightly elongated axial Fe-N bond lengths of 2.005(10) A that are about 0.02 A shorter and 0.03 A longer, respectively, in comparison to bis-imidazole ligated iron(III) species with parallel oriented axial ligands. A one-dimensional hydrogen-bond chain is formed between chloride anions and uncoordinated imidazole nitrogen atoms. Compared with paral-[Fe(OEP)(2-MeHIm)2]ClO4, hydrogen bonding may play an important role in the differences in the two structures. M?ssbauer spectra show broadened quadrupole doublets with quadrupole splittings of 1.81 mm/s at RT and 1.94 mm/s at 20 K. The isomer shift ranges from 0.26 to 0.36 mm/s. These confirm that the title complex is a low-spin iron(III) species with the ground state (dxy)2(dxz,dyz)3. Crystal data: monoclinic, space group P2(1)/c, a = 14.066(3) A, b, 20.883(4) A, c = 19.245(4) A, beta = 109.67 degrees , and Z = 4.     

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.     

Polymorphism in Fe[(p-IC6H4)B(3-Mepz)3]2 (pz = pyrazolyl): impact of supramolecular structure on an iron(II) electronic spin-state crossover

The new ligands Na[(p-IC6H4)B(3-Rpz)3] (R = H, Me) have been prepared by converting I2C6H4 to IC6H4SiMe3 with Li(t)Bu and SiMe3Cl, and then to IC6H4BBr2 with BBr3 and subsequent reaction with 3 equiv of (un)substituted pyrazole and 1 equiv of NaO(t)Bu. These new ligands react with FeBr2 to give either purple, low-spin Fe[(p-IC6H4)B(pz)3]2 or colorless, high-spin Fe[(p-IC6H4)B(3-Mepz)3]2. Depending upon the crystallization conditions, Fe[(p-IC6H4)B(3-Mepz)3]2 can exist both as two polymorphs and as a methylene chloride solvate. An examination of these polymorphs by variable-temperature X-ray crystallography, magnetic susceptibility, and Mossbauer spectroscopy has revealed different electronic spin-state crossover properties for each polymorph and yields insight into the influence of crystal packing, independent of other electronic perturbations, on the spin-state crossover. The first polymorph of Fe[(p-IC6H4)B(3-Mepz)3]2 has a highly organized three-dimensional supramolecular structure and does not undergo a spin-state crossover upon cooling to 4 K. The second polymorph of Fe[(p-IC6H4)B(3-Mepz)3]2 has a stacked two-dimensional supramolecular structure, a structure that is clearly less well organized than that of the first polymorph, and undergoes an abrupt iron(II) spin-state crossover from high spin to low spin upon cooling below ca. 130 K. The crystal structure of the methylene chloride solvate of Fe[(p-IC6H4)B(3-Mepz)3]2 has a similar stacked two-dimensional supramolecular structure, but the crystals readily lose the solvate. The resulting desolvate undergoes a gradual spin-state crossover to the low-spin state upon cooling below ca. 235 K. It is clear from a comparison of the structures that the long-range solid-state organization of the molecules, which is controlled by noncovalent supramolecular interactions, has a strong impact upon the spin-state crossover, with the more highly organized structures having lower spin-crossover temperatures and more abrupt spin-crossover behavior.     

Variable-temperature x-ray structural investigation of [Fe[HC(3,5-Me2pz)3]2](BF4)2 (pz= pyrazolyl ring): observation of a thermally induced spin state change from all high spin to an equal high spin-low spin mixture, concomitant with the onset of nonmerohedral twinning

The complex [Fe[HC(3,5-Me(2)pz)(3)](2)](BF(4))(2) (pz = pyrazolyl ring) undergoes a phase transition that occurs concomitantly with a thermally induced spin conversion between the high-spin (HS, S = 2) and low-spin (LS, S = 0) states. Above 204 K the compound is completely HS with the structure in the C2/c space group with Z = 4. A crystal structure determination of this phase was performed at 220 K yielding the cell constants a = 20.338(2) A, b = 10.332(1) A, c = 19.644(2) A, beta = 111.097(2) degrees, and V = 3851.5(6) A(3). There is one unique iron(II) site at this temperature. Below 206 K the compound converts to a 50:50 mixture of HS and LS. The radical change in the coordination sphere for half of the iron(II) sites, most notably a shortening of the Fe-N bond distances by ca. 0.2 A, that accompanies this magnetic transition causes a phase transition. The crystal system changes from C-centered monoclinic to primitive triclinic with Z = 2 with two half-molecules on independent inversion centers. A crystal structure determination was performed at 173 K in space group P1 with a = 10.287(2) A, b = 11.355(3) A, c = 18.949(4) A, alpha = 90.852(4) degrees, beta = 105.245(4) degrees, gamma = 116.304(4) degrees, and V = 1892.3(8) A(3). All specimens investigated below the phase transition temperature were determined to be nonmerohedral twins. Temperature cycling between these two forms does not appear to degrade crystal quality. Previous magnetic susceptibility measurements indicate a second, irreversible increase in the magnetic moment the first time the crystals are cooled below 85 K. A crystal structure determination at 220 K of a specimen precooled to 78 K was not significantly different from those not cooled below 220 K.     

Synthesis and characterization of two intensely colored tris(benzoylcyanoxime)iron(II) anionic complexes

Two intensely blue-colored complexes, P(C 6H 5) 4[Fe(BCO) 3] ( 1) and Na[Fe(BCO) 3] ( 2), where BCO (-) is the benzoylcyanoxime anion, have been prepared and characterized in solution and in the solid state. The crystal structure of 1 has been determined at several temperatures (100, 155, 225, and 293 K) and consists of layers of P(C 6H 5) 4 (+) cations and [Fe(BCO) 3] (-) anions. The latter exist as a pair of fac-Delta and Lambda enantiomers in a monoclinic unit cell in the P2(1)/ n space group. Iron(II) has a trigonal-prismatic N 3O 3 coordination environment with average Fe-N and Fe-O bond distances of 1.866 and 1.956 A, respectively, bonds that are unusually short and indicate a (1)A 1g low-spin ground state for iron(II). A sample of 1 prepared with iron-57 has been studied by Mossbauer spectroscopy between 4.2 and 430 K and found to be low-spin iron(II) in studied temperature range. The stepwise formation constants for 1 in aqueous solution at 296 K and pH of 7 are log beta 1 = 0.85 +/- 0.1, log beta 2 = 3.55 +/- 0.15, and log beta 3 = 6.36 +/- 0.15. Both 1 and 2 exhibit irreversible oxidation of iron(II) at approximately 1.0 V, indicating a significant degree of the ligand-to-iron charge transfer. Thus, 1 and 2 are rare examples of highly colored iron(II) anionic complexes that do not contain aromatic heterocyclic amine ligands, such as bipyridine or phenanthroline.     

Solid-state (67)zn NMR spectroscopy in bioinorganic chemistry. Spectra of four- and six-coordinate zinc pyrazolylborate complexes obtained by management of proton relaxation rates with a paramagnetic dopant

Solid-state (67)Zn NMR spectra of model compounds for metalloproteins, such as [H(2)B(3,5-Me(2)pz)(2)](2)Zn (pz denotes pyrazolyl ring), have been obtained using low temperatures (10 K) to enhance the Boltzmann factor in combination with cross polarization (CP) from (1)H to (67)Zn. Attempts to observe spectra of other model compounds, such as [H(2)B(pz)(2)](2)Zn, were hindered by long relaxation times of the protons. To decrease the proton relaxation times, the high-spin six-coordinate complex [HB(3,4,5-Me(3)pz)(3)](2)Fe has been investigated as a dopant. NMR and EPR measurements have shown that this Fe(II) dopant effectively reduces the (1)H spin lattice relaxation time, T(1), of the zinc samples in the temperature range 5-10 K with minimal perturbations of the (1)H spin lattice relaxation time in the rotating frame, T(1)(rho). Using this methodology, we have determined the (67)Zn NMR parameters of four- and six-coordinate zinc(II) poly(pyrazolyl)borate complexes that are useful models for systems of biological importance. The (67)Zn NMR parameters are contrasted to the corresponding changes in the (113)Cd NMR parameters for the analogous compounds. Further, these investigations have demonstrated that a temperature-dependent phase transition occurs in the neighborhood of 185 K for [HB(3,5-Me(2)pz)(3)](2)Zn; the other poly(pyrazolyl)borate complexes we investigated did not show this temperature-dependent behavior. This conclusion is confirmed by a combination of room-temperature high-field (18.8 T) solid-state (67)Zn NMR spectroscopy and low-temperature X-ray methods. The utilization of paramagnetic dopants should enable low-temperature cross polarization experiments to be performed on a wide variety of nuclides that are important in bioinorganic chemistry, for example, (25)Mg, (43)Ca, and (67)Zn.     

Homodinuclear iron thiolate nitrosyl compounds [(ON)Fe(S,S-C6H4)2 Fe(NO)2]- and [(ON)Fe(SO2,S-C6H4)(S,S-C6H4)Fe(NO)2]- with {Fe(NO)}7-{Fe(NO)2}9 electronic coupling: new members of a class of dinitrosyl iron complexes

Reaction of Fe(CO)2(NO)2 and [(ON)Fe(S,S-C6H3R)2]- (R = H (1), CH3 (1-Me))/[(ON)Fe(SO2,S-C6H4)(S,S-C6H4)]- (4) in THF afforded the diiron thiolate/sulfinate nitrosyl complexes [(ON)Fe(S,S-C6H3R)2 Fe(NO)2]- (R = H (2), CH3 (2-Me)) and [(ON)Fe(S,SO2-C6H4)(S,S-C6H4)Fe(NO)2]- (3), respectively. The average N-O bond lengths ([Fe(NO)2] unit) of 1.167(3) and 1.162(4) A in complexes 2 and 3 are consistent with the average N-O bond length of 1.165 A observed in the other structurally characterized dinitrosyl iron complexes with an {Fe(NO)2}9 core. The lower nu(15NO) value (1682 cm(-1) (KBr)) of the [(15NO)FeS4] fragment of [(15NO)Fe(S,S-C6H3CH3)2 Fe(NO)2]- (2-Me-15N), compared to that of [(15NO)Fe(S,S-C6H3CH3)2]- (1-Me-15N) (1727 cm(-1) (KBr)), implicates the electron transfer from {Fe(NO)2}10 Fe(CO)2(NO)2 to complex 1-Me/1 may occur in the process of formation of complex 2-Me/2. Then, the electronic structures of the [(NO)FeS4] and [S2Fe(NO)2] cores of complexes 2, 2-Me, and 3 were best assigned according to the Feltham-Enemark notation as the {Fe(NO)}7-{Fe(NO)2}9 coupling (antiferromagnetic interaction with a J value of -182 cm(-1) for complex 2) to account for the absence of paramagnetism (SQUID) and the EPR signal. On the basis of Fe-N(O) and N-O bond distances, the dinitrosyliron {L2Fe(NO)2} derivatives having an Fe-N(O) distance of approximately 1.670 A and a N-O distance of approximately 1.165 A are best assigned as {Fe(NO)2}9 electronic structures, whereas the Fe-N(O) distance of approximately 1.650 A and N-O distance of approximately 1.190 A probably imply an {Fe(NO)2}10 electronic structure.     

Electronic, magnetic, and structural characterization of the five-coordinate, high-spin iron(II) nitrato complex [Fe(TpivPP)(NO3)]-

The preparation and characterization of the five-coordinate iron(II) porphyrinate derivative [Fe(TpivPP)(NO3)]- (TpivPP = picket-fence porphyrin) is described. Structural and magnetic susceptibility data support a high-spin state (S = 2) assignment for this species. The anionic axial nitrate ligand is O-bound, through a single O atom, with an Fe-O bond length of 2.069(4) A. The planar nitrate ligand bisects a N(p)-Fe-N(p) angle. The average Fe-N(p) bond length is 2.070(16) A. The Fe atom is located 0.49 A out of the 24-atom mean porphyrin plane toward the nitrate ligand. From solid-state M?ssbauer data, the isomer shift of 0.98 mm/s at 77 K is entirely consistent with high-spin iron(II). However the quadrupole splitting of 3.59 mm/s at 77 K is unusually high for iron(II), S = 2 systems but within the range of other five-coordinate high-spin ferrous complexes with a single anionic axial ligand. Crystal data for [K(222)][Fe(TpivPP)(NO3)] x C6H5Cl: a = 17.888 (5) A, b = 21.500 (10) A, c = 22.514 (11) A, beta = 100.32 (3) degrees, monoclinic, space group P2(1)/n, V = 8519 A3, Z = 4.     

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.     

金属有机框架材料对[Fe（HB（pz）3）2] 自旋转换行为的影响

通过在介孔结构金属有机框架材料MIL^-101（Cr）和MIL^-100（Al）的孔洞中合成自旋交叉化合物[Fe（HB（pz）₃）₂] 的方法,可以得到SCO@MOF复合物。通过红外光谱（FTIR）、粉末X射线衍射（PXRD）、原子吸收光谱（AAS）以及气体吸附-脱附等进行了进一步测试。通过变温磁测量对复合材料的温度诱导自旋转换行为的研究表明,复合材料的自旋转换行为发生改变甚至是消失了。复合材料的这一现象可以解释为[Fe（HB（pz）3）2] 在MOF主体材料的孔洞中形成了一种新的结晶相,且孔壁压力将会阻碍[Fe（HB（pz）₃）₂] 从低自旋态向高自旋态转变。不同SCO@MOF复合物得到了相似的自旋转换行为结果。这确认了当自旋交叉化合物在金属有机框架材料孔洞中形成时,MOFs材料的限制压力或基体效应对其自旋转换行为的影响显然是至关重要的。