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.     

X-ray absorption spectroscopic investigation of the spin-transition character in a series of single-site perturbed iron(II) complexes

Select ferrous spin-transition complexes with the pentadentate ligand 2,6-bis(bis(2-pyridyl)methoxymethane)pyridine (PY5) were examined using variable-temperature solution solid-state magnetic susceptibility, crystallography, X-ray absorption spectroscopy (XAS), and UV/vis absorption spectroscopy. Altering the single exogeneous ligand, X, of [Fe(PY5)(X)]n)+ is sufficient to change the spin-state of the complexes. When X is the weak-field ligand Cl-, the resultant Fe complex is high-spin from 4 to 300 K, whereas the stronger-field ligand MeCN generates a low-spin complex over this temperature range. With intermediate-strength exogenous ligands (X = N3-, MeOH), the complexes undergo a spin-transition. [Fe(PY5)(N3)]+, as a crystalline solid, transitions gradually from a high-spin to a low-spin complex as the temperature is decreased, as evidenced by X-ray crystallography and solid-state magnetic susceptibility measurements. The spin-transition is also evident from changes in the pre-edge and EXAFS regions of the XAS Fe K-edge spectra on a ground crystalline sample. The spin-transition observed with [Fe(PY5)(MeOH)]2+ appears abrupt by solid-state magnetic susceptibility measurements, but gradual by XAS analysis, differences attributed to sample preparation. This research highlights the strengths of XAS in determining the electronic and geometric structure of such spin-transition complexes and underscores the importance of identical sample preparation in the investigation of these physical properties.     

Tris(5-methylpyrazolyl)methane: synthesis and properties of its iron(II) complex

The new ligand, tris(5-methylpyrazolyl)methane (1), has been prepared by the reaction of n-butyl lithium with tris(pyrazolyl)methane followed by trimethylation of the tetralithiated species with methyl iodide. The BF(4)(-), ClO(4)(-), and BPh(3)CN(-) salts of the Fe(II) complex of this ligand were also synthesized. The X-ray crystal structure of the BF(4)(-) complex (2) at 100 K had Fe-N bond lengths of 1.976 ?, indicative of a low spin Fe(II) complex, while at room temperature, the structure of this complex had a Fe-N bond distance close to 2.07 ?, indicative of an admixture of approximately 50% low-spin and 50% high-spin. The solid-state structure of the complex with a ClO(4)(-) counterion was determined at 5 different temperatures between 173 and 293 K, which allowed the thermodynamic parameters for the spin-crossover to be estimated. M?ssbauer spectra of the BF(4)(-) complex further support spin-state crossover in the solid state with a transition temperature near 300 K. UV-visible spectroscopy and (1)H NMR studies of 2 show that the transition temperature in solution is closer to 400 K. No spin-crossover was observed for [Fe(1)(2)](2+)·2BPh(3)CN(-). The results allow the separation of effects of groups in the 3-position from those in the 5-position on tpm ligands, and also point toward a small cooperative effect in the spin-crossover for the Fe(II) complex.     

Spin-crossover in an iron(III)-bispidine-alkylperoxide system

The iron(II) complex of a tetradentate bispidine ligand with two tertiary amines and two pyridine groups (L = dimethyl [3,7-dimethyl-9,9'-dihydroxy-2,4-di-(2-pyridyl)-3,7-diazabicyclo nonan-1,5-dicaboxylate]) is oxidized with tert-butyl hydroperoxide to the corresponding end-on tert-butylperoxo complex [Fe(III)(L)(OOtBu)(X)]n+ (X = solvent, anion). UV-vis, resonance Raman, and EPR spectroscopy, as a function of the solvent, show that this is a spin-crossover compound. The experimentally observed Raman vibrations for both low-spin and high-spin isomers are in good agreement with those computed by DFT.     

Six-coordinate and five-coordinate Fe(II)(CN)(2)(CO)(x) thiolate complexes (x = 1, 2): synthetic advances for iron sites of [NiFe] hydrogenases

The dicyanodicarbonyliron(II) thiolate complexes trans,cis-[(CN)(2)(CO)(2)Fe(S,S-C-R)](-) (R = OEt (2), N(Et)(2) (3)) were prepared by the reaction of [Na][S-C(S)-R] and [Fe(CN)(2)(CO)(3)(Br)](-) (1). Complex 1 was obtained from oxidative addition of cyanogen bromide to [Fe(CN)(CO)(4)](-). In a similar fashion, reaction of complex 1 with [Na][S,O-C(5)H(4)N], and [Na][S,N-C(5)H(4)] produced the six-coordinate trans,cis-[(CN)(2)(CO)(2)Fe(S,O-C(5)H(4)N)](-) (6) and trans,cis-[(CN)(2)(CO)(2)Fe(S,N-C(5)H(4))](-) (7) individually. Photolysis of tetrahydrofuran (THF) solution of complexes 2, 3, and 7 under CO led to formation of the coordinatively unsaturated iron(II) dicyanocarbonyl thiolate compounds [(CN)(2)(CO)Fe(S,S-C-R)](-) (R = OEt (4), N(Et)(2) (5)) and [(CN)(2)(CO)Fe(S,N-C(5)H(4))](-) (8), respectively. The IR v(CN) stretching frequencies and patterns of complexes 4, 5, and 8 have unambiguously identified two CN(-) ligands occupying cis positions. In addition, density functional theory calculations suggest that the architecture of five-coordinate complexes 4, 5, and 8 with a vacant site trans to the CO ligand and two CN(-) ligands occupying cis positions serves as a conformational preference. Complexes 2, 3, and 7 were reobtained when the THF solution of complexes 4, 5, and 8 were exposed to CO atmosphere at 25 degrees C individually. Obviously, CO ligand can be reversibly bound to the Fe(II) site in these model compounds. Isotopic shift experiments demonstrated the lability of carbonyl ligands of complexes 2, 3, 4, 5, 7, and 8. Complexes [(CN)(2)(CO)Fe(S,S-C-R)](-) and NiA/NiC states [NiFe] hydrogenases from D. gigas exhibit a similar one-band pattern in the v(CO) region and two-band pattern in the v(CN) region individually, but in different positions, which may be accounted for by the distinct electronic effects between [S,S-C-R](-) and cysteine ligands. Also, the facile formations of five-coordinate complexes 4, 5, and 8 imply that the strong sigma-donor, weak pi-acceptor CN(-) ligands play a key role in creating/stabilizing five-coordinate iron(II) [(CN)(2)(CO)Fe(S,S-C-R)](-) complexes with a vacant coordination site trans to the CO ligand.     

Supramolecular spin-crossover iron complexes based on imidazole-imidazolate hydrogen bonds

The [Fe(II)(H(3)L)](BF(4))(2).3H(2)O (1) complex was synthesized, where H(3)L (tris[[2-[(imidazole-4-yl)methylidene]amino]ethyl]amine) is a tripodal ligand obtained by condensation of tris(2-aminoethyl)amine and 4-formylimidazole (fim) in a 1:3 molar ratio. Starting from 1, a series of complexes, [Fe(II)(H(1.5)L)](BF(4))(0.5) (2) (=[Fe(II)(H(3)L)][Fe(II)(L)]BF(4)), [Fe(H(1.5)L)]BF(4) (3) (=[Fe(II)(H(3)L)][Fe(III)(L)](BF(4))(2)), [Fe(III)(H(3)L)](BF(4))(3).fim.H(2)O (4), and [Fe(III)(L)].2.5H(2)O (5), has been synthesized and characterized. The single-crystal X-ray structure of each complex has been determined. The Fe(II) compound, 2, and a mixed valence Fe(II)-Fe(III) compound, 3, involve formally hemi-deprotonated ligands, H(1.5)L. The structure of 3 consists of a homochiral two-dimensional assembled sheet, arising from the intermolecular hydrogen bonds between [Fe(II)(H(3)L)](2+) and [Fe(III)(L)](0) (3). All but 5 exhibit spin crossover between low-spin (LS) and high-spin (HS) states. This is a rare case where both Fe(II) and Fe(III) complexes containing the same ligand exhibit spin-crossover behavior. Magnetic susceptibility and M?ssbauer studies showed that 3 has three accessible electronic states: LS Fe(II)-LS Fe(III), HS Fe(II)-LS Fe(III), and HS Fe(II)-HS Fe(III). Compounds 1-3 show the light-induced excited spin-state trapping effect at the Fe(II) sites upon irradiation with green light. The solution magnetic properties, electronic spectra, and electrochemical properties of 1, 4, and 5 were also studied.     

Anion-solvent dependence of bistability in a family of meridional N-donor-ligand-containing iron(II) spin crossover complexes

Five mononuclear spin crossover iron(II) bis-meridional ligand complexes of the general formula [Fe(L)(2)](X)(2).solvent, have been synthesized, where X = BF(4)- or ClO(4)-; L = 2-(1-pyridin-2-ylmethyl-1H-pyrazol-3-yl)-pyrazine (picpzpz) or 2-(3-(2-pyridyl)pyrazol-1-ylmethyl)pyridine) (picpypz); solvent = MeOH or EtOH. The magnetic and structural consequences of systematic variation of meridional ligand, solvent, and anion, including a desolvated species, have been investigated. The complex [Fe(picpzpz)(2)](BF(4))(2).MeOH, 1.MeOH, displays several unique properties including a two-step spin transition with a gradual higher-temperature step ((1)T(1/2) = 197 K) and an abrupt low-temperature step with hysteresis ((2)T(1/2) = 91/98 K) and a metastable intermediate spin state below 70 K with quench-cooling. Removal of the solvent methanol results in the loss of the abrupt step and associated hysteresis (T(1/2) = 150 K). The complexes [Fe(picpzpz)(2)](BF(4))(2).EtOH (1.EtOH), [Fe(picpzpz)(2)](ClO(4))(2).MeOH (2.MeOH), [Fe(picpzpz)(2)](ClO(4))(2).EtOH (2.EtOH), and [Fe(picpypz)(2)](BF(4))(2).MeOH (3.MeOH) all show gradual one-step spin transitions with T(1/2) values in the range 210-250 K. Photomagnetic LIESST measurements on 1.MeOH reveal a near-quantitative excitation of high-spin sites and a unique two-step relaxation process related to the two-step thermal spin transition ((1)T(LIESST) = 49 K and (2)T(LIESST) = 70 K). The structural consequences of the unusual spin transition displayed by 1.MeOH have been investigated by single-crystal X-ray diffraction structural analyses between 25 and 293 K. Detailed characterization of the unit cell parameter evolution vs temperature reflects both the gradual high-temperature step and abrupt low-temperature step, including the thermal hysteresis, observed magnetically.     

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.     

Cooperative spin crossover and order-disorder phenomena in a mononuclear compound [Fe(DAPP)(abpt)](ClO(4))(2) [DAPP = [bis(3-aminopropyl)(2-pyridylmethyl)amine], abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole

The synthesis and detailed characterization of the new spin crossover mononuclear complex [Fe(II)(DAPP)(abpt)](ClO(4))(2), where DAPP = [bis(3-aminopropyl)(2-pyridylmethyl)amine] and abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole, are reported. Variable-temperature magnetic susceptibility measurements and M?ssbauer spectroscopy have revealed the occurrence of an abrupt spin transition with a hysteresis loop. The hysteresis width derived from magnetic susceptibility measurements is 10 K, the transition being centered at T(c) downward arrow = 171 K for decreasing and T(c) upward arrow = 181 K for increasing temperatures. The crystal structure was resolved in the high-spin (293 and 183 K) and low-spin (123 K) states. Both spin-state structures belong to the monoclinic space group P2(1)/n (Z = 4). The thermal spin transition is accompanied by the shortening of the mean Fe-N distances by 0.177 A. The two main structural characteristics of [Fe(DAPP)(abpt)](ClO(4))(2) are a branched network of intermolecular links in the crystal lattice and the occurrence of two types of order-disorder transitions (in the DAPP ligand and in the perchlorate anions) accompanying the thermal spin change. These features are discussed relative to the magnetic properties of the complex. The electronic structure calculations show that the structural disorder in the DAPP ligand modulates the energy gap between the HS and LS states. In line with previous studies, the order-disorder phenomena and the spin transition in [Fe(DAPP)(abpt)](ClO(4))(2) are found to be interrelated.     

Investigation of anion-dependence in the spin-state switching properties of [(H2bip)2Fe(6-Mebpy)]X2

We report the syntheses, characterisations and magnetic properties of salts of the heteroleptic Fe(II) complex [(H(2)bip)(2)Fe(6-Mebpy)]X(2) (X = Br (1), BPh(4) (2), H(2)bip = 2,2'-bi-1,4,5,6-tetrahydropyrimidine, 6-Mebpy = 6-methyl-2,2'-bipyridine). The ditopic H(2)bip ligand serves as an anion binding group while 6-Mebpy is intended to adjust the complex ligand field. Thermally induced spin-crossover properties are observed in the solid state and solution, and are heavily influenced by the nature of the anion. Anion-triggered spin-state switching is observed for the heteroleptic complex in dichloromethane solution at 193 K, above background processes of ligand dissociation and rearrangement. Although substitution of 6-Mebpy appears to increase the ligand field encountered by the Fe(II) ion, salt 2 responds to bromide at a significantly lower temperature than the parent homoleptic complex salt [Fe(H(2)bip)(3)](BPh(4))(2).     

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.     

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

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

Spin crossover in a tetranuclear Cr(III)-Fe(III)(3) complex

A novel heteronuclear exchange-coupled complex [Cr(III)[(CN)Fe(III)((5)L)](3)(CN)(3)] containing a pentadentate blocking ligand (5)L was synthesized. The X-ray structure shows that a meridional isomer applies with inequivalent Fe(III) centers. The complex exhibits a thermally induced spin crossover along with the exchange coupling. M?ssbauer spectra indicate a spin transition between S = (1)/(2) and S = (5)/(2) states although a considerable amount of Fe(III) centers stays high-spin at T = 6 K. The magnetization, the magnetic susceptibility, and the M?ssbauer data were fitted in one run with a spin crossover model taking into account exchange interactions among all metal centers.     

Mechanism of the photochemical ligand substitution reactions of fac-[Re(bpy)(CO)(3)(PR(3))](+) complexes and the properties of their triplet ligand-field excited states

We report herein the mechanism of the photochemical ligand substitution reactions of a series of fac-[Re(X(2)bpy)(CO)(3)(PR(3))](+) complexes (1) and the properties of their triplet ligand-field ((3)LF) excited states. The reason for the photostability of the rhenium complexes [Re(X(2)bpy)(CO)(3)(py)](+) (3) and [Re(X(2)bpy)(CO)(3)Cl] (4) was also investigated. Irradiation of an acetonitrile solution of 1 selectively gave the biscarbonyl complexes cis,trans-[Re(X(2)bpy)(CO)(2)(PR(3))(CH(3)CN)](+) (2). Isotope experiments clearly showed that the CO ligand trans to the PR(3) ligand was selectively substituted. The photochemical reactions proceeded via a dissociative mechanism from the (3)LF excited state. The thermodynamical data for the (3)LF excited states of complexes 1 and the corrective nonradiative decay rate constants for the triplet metal-to-ligand charge-transfer ((3)MLCT) states were obtained from temperature-dependence data for the emission lifetimes and for the quantum yields of the photochemical reactions and the emission. Comparison of 1 with [Re(X(2)bpy)(CO)(3)(py)](+) (3) and [Re(X(2)bpy)(CO)(3)Cl] (4) indicated that the (3)LF states of some 3- and 4-type complexes are probably accessible from the (3)MLCT state even at ambient temperature, but these complexes were stable to irradiation at 365 nm. The photostability of 3 and 4, in contrast to 1, can be explained by differences in the trans effects of the PR(3), py, and Cl(-) ligands.     

Platinum(IV) analogues of AMD473 (cis-[PtCl2(NH3)(2-picoline)]): preparative, structural, and electrochemical studies

The preparation and oxidation of the anticancer drug AMD473, cis-[PtCl2(NH3)(2-pic)] (2-pic = 2-methylpyridine), has been investigated. cis-[PtCl2(NH3)(2-pic)] is readily oxidized with peroxide to give the trans-dihydroxoplatinum(IV) complex cis,trans,cis-[PtCl2(OH)2(NH3)(2-pic)]. The crystal structure of this complex reveals that it is highly strained as a result of a steric clash between the methyl group of the 2-picoline ligand and an axial hydroxo ligand, with the Pt-N-C angle adjacent to this clash opened up to an unprecedented 138.6(6) degrees . Attempts at converting the dihydroxoplatinum(IV) complex to dichloro and diacetato analogues were unsuccessful with reaction with HCl leading to loss and protonation of the 2-picoline ligand to form the salt (2-picH)[PtCl5(NH3)] and the platinum(II) complex cis-[PtCl2(NH3)(2-pic)], both confirmed by crystallography. Electrochemical studies revealed that cis,trans,cis-[PtCl2(OH)2(NH3)(2-pic)] is reduced more readily (-714 mV vs Ag/AgCl) than its pyridine analogue cis,trans,cis-[PtCl2(OH)2(NH3)(pyridine)] (-770 mV vs Ag/AgCl) consistent with the steric clash in the former complex destabilizing the platinum(IV) oxidation state.     

Correlation of spin states and spin delocalization with the dioxygen reactivity of catecholatoiron(III) complexes

A series of catecholatoiron(III) complexes, [Fe(III)L(4Cl-cat)]BPh4 (L = (4-MeO)2TPA (1), TPA (2), (4-Cl)2TPA (3), (4-NO2)TPA (4), (4-NO2)2TPA (5); TPA = tris(pyridin-2-ylmethyl)amine; 4Cl-cat = 4-chlorocatecholate), have been characterized by magnetic susceptibility measurements and EPR, 1H NMR, and UV-vis-NIR spectroscopies to clarify the correlation of the spin delocalization on the catecholate ligand with the O2 reactivity as well as the spin-state dependence of the O2 reactivity. EPR spectra in frozen CH3CN at 123 K clearly showed that introduction of electron-withdrawing groups effectively shifts the spin equilibrium from a high-spin to a low-spin state. The effective magnetic moments determined by the Evans method in a CH3CN solution showed that 5 contains 36% of low-spin species at 243 K, while 1-4 are predominantly in a high-spin state. Evaluation of spin delocalization on the 4Cl-cat ligand by paramagnetic 1H NMR shifts revealed that the semiquinonatoiron(II) character is more significant in the low-spin species than in the high-spin species. The logarithm of the reaction rate constant is linearly correlated with the energy gap between the catecholatoiron(III) and semiquinonatoiron(II) states for the high-spin complexes 1-3, although complexes 4 and 5 deviate negatively from linearity. The lower reactivity of the low-spin complex, despite its higher spin density on the catecholate ligand compared with the high-spin analogues, suggests the involvement of the iron(III) center, rather than the catecholate ligand, in the reaction with O2.     

Fe(bipy)(CN)(4)](-) as a versatile building block for the design of heterometallic systems: synthesis, crystal structure, and magnetic properties of PPh(4)[Fe(III)(bipy)(CN)(4)] x H(2)O, [[Fe(III)(bipy)(CN)(4)](2)M(II)(H(2)O)(4)] x 4H(2)O, and [[Fe(III)(bipy)(CN)(4)](2)Zn(II)] x 2H(2)O [bipy = 2,2'-Bipyridine; M = Mn and Zn

The new cyano complexes of formulas PPh(4)[Fe(III)(bipy)(CN)(4)] x H(2)O (1), [[Fe(III)(bipy)(CN)(4)](2)M(II)(H(2)O)(4)] x 4H(2)O with M = Mn (2) and Zn (3), and [[Fe(III)(bipy)(CN)(4)](2)Zn(II)] x 2H(2)O (4) [bipy = 2,2'-bipyridine and PPh(4) = tetraphenylphosphonium cation] have been synthesized and structurally characterized. The structure of complex 1 is made up of mononuclear [Fe(bipy)(CN)(4)](-) anions, tetraphenyphosphonium cations, and water molecules of crystallization. The iron(III) is hexacoordinated with two nitrogen atoms of a chelating bipy and four carbon atoms of four terminal cyanide groups, building a distorted octahedron around the metal atom. The structure of complexes 2 and 3 consists of neutral centrosymmetric [[Fe(III)(bipy)(CN)(4)](2)M(II)(H(2)O)(4)] heterotrinuclear units and crystallization water molecules. The [Fe(bipy)(CN)(4)](-) entity of 1 is present in 2 and 3 acting as a monodentate ligand toward M(H(2)O)(4) units [M = Mn(II) (2) and Zn(II) (3)] through one cyanide group, the other three cyanides remaining terminal. Four water molecules and two cyanide nitrogen atoms from two [Fe(bipy)(CN)(4)](-) units in trans positions build a distorted octahedron surrounding Mn(II) (2) and Zn(II) (3). The structure of the [Fe(phen)(CN)(4)](-) complex ligand in 2 and 3 is close to that of the one in 1. The intramolecular Fe-M distances are 5.126(1) and 5.018(1) A in 2 and 3, respectively. 4 exhibits a neutral one-dimensional polymeric structure containing two types of [Fe(bipy)(CN)(4)](-) units acting as bismonodentate (Fe(1)) and trismonodentate (Fe(2)) ligands versus the divalent zinc cations through two cis-cyanide (Fe(1)) and three fac-cyanide (Fe(2)) groups. The environment of the iron atoms in 4 is distorted octahedral as in 1-3, whereas the zinc atom is pentacoordinated with five cyanide nitrogen atoms, describing a very distorted square pyramid. The iron-zinc separations across the single bridging cyanides are 5.013(1) and 5.142(1) A at Fe(1) and 5.028(1), 5.076(1), and 5.176(1) A at Fe(2). The magnetic properties of 1-3 have been investigated in the temperature range 2.0-300 K. 1 is a low-spin iron(III) complex with an important orbital contribution. The magnetic properties of 3 correspond to the sum of two magnetically isolated spin triplets, the antiferromagnetic coupling between the low-spin iron(III) centers through the -CN-Zn-NC- bridging skeleton (iron-iron separation larger than 10 A) being very weak. More interestingly, 2 exhibits a significant intramolecular antiferromagnetic interaction between the central spin sextet and peripheral spin doublets, leading to a low-lying spin quartet.     

Ligand topology variations and the importance of ligand field strength in non-heme iron catalyzed oxidations of alkanes

A series of iron(II)-bis(triflate) complexes [Fe(L)(OTf)2] containing linear tetradentate bis(quinolyl)-diamine and bis(quinolylmethyl)-diamine ligands with a range of ligand backbones has been prepared. The coordination geometries of these complexes have been investigated in the solid state by X-ray crystallography and in solution by 1H and 19F NMR spectroscopy. Because of the labile nature of high-spin iron(II) complexes in solution, dynamic equilibria of complexes with different coordination geometries (cis-alpha, cis-beta, and trans) are observed with certain ligand systems. In these cases, the geometry observed in the solid-state does not necessarily represent the only or even the major geometry present in solution. The ligand field strength in the various complexes has been investigated by variable-temperature (VT) magnetic moment measurements and by UV-vis spectroscopy. The strongest ligand field is observed with the most rigid ligand that generates [Fe(L)(OTf)2] complexes with a cis-alpha coordination geometry, and the corresponding [Fe(L)(CH3CN)2]2+ complex displays spin crossover behavior. The catalytic properties of the complexes for the oxidation of cyclohexane have been investigated using hydrogen peroxide as the oxidant. An increased flexibility in the ligand results in a weaker ligand field, which increases the lability of the complexes. The activity and selectivity of the catalysts appear to be related to the strength of the ligand field and the stability of the catalyst.     

(NEt(4))(2)[Fe(CN)(2)(CO)('S(3)')]: an iron thiolate complex modeling the [Fe(CN)(2)(CO)(S-Cys)(2)] site of [NiFe] hydrogenase centers

In the search for complexes modeling the [Fe(CN)(2)(CO)(cysteinate)(2)] cores of the active centers of [NiFe] hydrogenases, the complex (NEt(4))(2)[Fe(CN)(2)(CO)('S(3)')] (4) was found ('S(3)'(2-)=bis(2-mercaptophenyl)sulfide(2-)). Starting complex for the synthesis of 4 was [Fe(CO)(2)('S(3)')](2) (1). Complex 1 formed from [Fe(CO)(3)(PhCH=CHCOMe)] and neutral 'S(3)'-H(2). Reactions of 1 with PCy(3) or DPPE (1,2-bis(diphenylphosphino)ethane) yielded diastereoselectively [Fe(CO)(2)(PCy(3))('S(3)')] (2) and [Fe(CO)(dppe)('S(3)')] (3). The diastereoselective formation of 2 and 3 is rationalized by the trans influence of the 'S(3)'(2-) thiolate and thioether S atoms which act as pi donors and pi acceptors, respectively. The trans influence of the 'S(3)'(2-) sulfur donors also rationalizes the diastereoselective formation of the C(1) symmetrical anion of 4, when 1 is treated with four equivalents of NEt(4)CN. The molecular structures of 1, 3 x 0.5 C(7)H(8), and (AsPh(4))(2)[Fe(CN)(2)(CO)('S(3)')] x acetone (4 a x C(3)H(6)O) were determined by X-ray structure analyses. Complex 4 is the first complex that models the unusual 2:1 cyano/carbonyl and dithiolate coordination of the [NiFe] hydrogenase iron site. Complex 4 can be reversibly oxidized electrochemically; chemical oxidation of 4 by [Fe(Cp)(2)PF(6)], however, led to loss of the CO ligand and yielded only products, which could not be characterized. When dissolved in solvents of increasing proton activity (from CH(3)CN to buffered H(2)O), complex 4 exhibits drastic nu(CO) blue shifts of up to 44 cm(-1), and relatively small nu(CN) red shifts of approximately 10 cm(-1). The nu(CO) frequency of 4 in H(2)O (1973 cm(-1)) is higher than that of any hydrogenase state (1952 cm(-1)). In addition, the nu(CO) frequency shift of 4 in various solvents is larger than that of [NiFe] hydrogenase in its most reduced or oxidized state. These results demonstrate that complexes modeling properly the nu(CO) frequencies of [NiFe] hydrogenase probably need a [Ni(thiolate)(2)] unit. The results also demonstrate that the nu(CO) frequency of [Fe(CN)(2)(CO)(thiolate)(2)] complexes is more significantly shifted by changing the solvent than the nu(CO) frequency of [NiFe] hydrogenases by coupled-proton and electron-transfer reactions. The "iron-wheel" complex [Fe(6)[Fe('S(3)')(2)](6)] (6) resulting as a minor by-product from the recrystallization of 2 in boiling toluene could be characterized by X-ray structure analysis.     

Partial spin crossover behaviour in a dinuclear iron(II) triple helicate

Reported herein are the synthesis, structural, magnetic and M?ssbauer spectroscopic characterisation of a dinuclear Fe(II) triple helicate complex [Fe(2)(L)(3)](ClO(4))(4).xH(2)O (x = 1-4), 1(H(2)O), where L is a bis-bidentate imidazolimine ligand. Low temperature structural analysis (150 K) and M?ssbauer spectroscopy (4.5 K) are consistent with one of the Fe(II) centres within the helicate being in the low spin (LS) state with the other being in the high-spin (HS) state resulting in a [LS:HS] species. However, M?ssbauer spectroscopy (295 K) and variable temperature magnetic susceptibility measurements (4.5-300 K) reveal that 1(H(2)O) undergoes a reversible single step spin crossover at one Fe(II) centre at higher temperatures resulting in a [HS:HS] species. Indeed, the T(1/2)(SCO) values at this Fe(II) centre also vary as the degree of hydration, x, within 1(H(2)O) changes from 1 to 4 and are centred between ca. 210 K-265 K, respectively. The dehydration/hydration cycle is reversible and the fully hydrated phase of 1(H(2)O) may be recovered on exposure to water vapour. This magnetic behaviour is in contrast to that observed in the related compound [Fe(2)(L)(3)](ClO(4))(4)·2MeCN, 1(MeCN), whereby fully reversible SCO was observed at each Fe(II) centre to give [LS:LS] species at low temperature and [HS:HS] species at higher temperatures. Reasons for this differing behaviour between 1(H(2)O) and 1(MeCN) are discussed.