C-H activation of benzene by platinum(II) complexes with cyclometalated phosphine ligands

The bulky phosphine ligands di-tert-butyl(1-naphthyl)phosphine (1) or di-tert-butyl(N-indolyl)phosphine (2) react at room temperature with [(μ-SMe₂)PtMe₂]₂. Coordination of the phosphine and C-H bond activation at an sp² carbon of the ligand with the release of methane takes place to form the PC cyclometalated products [(PC)PtMe(SMe₂)] (3 or 4, respectively). The cyclometalated complexes 3 and 4 have both been characterized by X-ray crystallography. Complexes 3 and 4 were each observed to undergo intermolecular activation of arene C-H bonds. Upon thermolysis in benzene, complexes 3 and 4 react to eliminate methane and yield isolable platinum(II)-phenyl complexes.     

(Pyrazolylmethyl)pyridine platinum(II) and gold(III) complexes: Synthesis, structures and evaluation as anticancer agents

Reactions of 2-(3,5-dimethylpyrazol-1-ylmethyl)pyridine (L1), 2-(3,5-diphenylpyrazol-1-ylmethyl)pyridine (L2), 2-(3,5-di-tert-butylpyrazol-1-ylmethyl)pyridine (L3) and 2-(3-p-tolylpyrazol-1-ylmethyl)pyridine (L4) with K₂[PtCl₄] in a mixture of ethanol and water formed the dichloro platinum complexes [PtCl₂(L1)] (1), [PtCl₂(L2)] (2), [PtCl₂(L3)] (3) and [PtCl₂(L4)] (4). Complex 1, [PtCl₂(L1)], could also be prepared in a mixture of acetone and water. Performing the reactions of L2 and L3 in a mixture of acetone and water, however, led to C-H activation of acetone under mild conditions to form the neutral acetonyl complexes [Pt(CH₂COCH₃)Cl(L2)] (2a) and [Pt(CH₂COCH₃)Cl(L3)] (3a). The same ligands reacted with HAuCl₄ · 4H₂O in a mixture of ethanol and water to form the gold salts [AuCl₂(L1)][AuCl₄] (5) [AuCl₂(L2)][Cl] (6) [AuCl₂(L3)][Cl] (7) and [AuCl₂(L4)][AuCl₄] (8); however, with the pyrazolyl unit in the para position of the pyridinyl ring in 4-(3,5-dimethylpyrazol-1-ylmethyl)pyridine (L5), 4-(3,5-diphenylpyrazol-1-ylmethyl)pyridine (L6) neutral gold complexes [AuCl₃(L5)] (9) and [AuCl₂(L6)] (10) were formed; signifying the role the position of the pyrazolyl group plays in product formation in the gold reactions. X-ray crystallographic structural determination of L6, 2, 33a, 8 and 10 were very important in confirming the structures of these compounds; particularly for 3a and 8 where the presence of the acetonyl group confirmed C-H activation and for 8 where the counter ion is . Cytotoxicity studies of L2, L4 and complexes 1-10 against HeLa cells showed the Au complexes were much less active than the Pt complexes.     

Activation and deactivation of a carbene containing non-classical ruthenium hydride complex in catalytic processes involving C-H bond cleavage

The carbene-containing non-classical ruthenium hydride complex [(PCy₃)Ru(H)₂(H₂)₂(IMes)] 1 (IMes=1,3-Bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene) is an active catalyst for H/D exchange in aromatic ketones. It is also capable of combining sp² C-H bond activation with C-C bond formation. Comparing the chemo- and regio-selectivities of the H/D exchange process and the C-C bond formation clearly indicates that different intermediates are involved in the two processes. High pressure NMR studies provide strong evidence that the key intermediate for the C-C coupling reaction is analogous to that for other ruthenium catalysts reported previously. Catalytic turnover is limited by the instability of this intermediate in the presence of the olefinic coupling partner.     

Ruthenium mediated C-H activation of benzaldehyde thiosemicarbazones: Synthesis, structure and, spectral and electrochemical properties of the resulting complexes

Reaction of five 4R-benzaldehyde thiosemicarbazones (R = OCH₃, CH₃, H, Cl and NO₂) with [Ru(PPh₃)₃(CO)(H)Cl] in refluxing methanol in the presence of a base (NEt₃) affords complexes of two different types, viz. 1-R and 2-R. In the 1-R complexes the thiosemicarbazone is coordinated to ruthenium as a dianionic tridentate C,N,S-donor via C-H bond activation. Two triphenylphosphines and a carbonyl are also coordinated to ruthenium. The tricoordinated thiosemicarbazone ligand is sharing the same equatorial plane with ruthenium and the carbonyl, and the PPh₃ ligands are mutually trans. In the 2-R complexes the thiosemicarbazone ligand is coordinated to ruthenium as a monoanionic bidentate N,S-donor forming a four-membered chelate ring with a bite angle of 63.91(11)°. Two triphenylphosphines, a carbonyl and a hydride are also coordinated to ruthenium. The coordinated thiosemicarbazone ligand, carbonyl and hydride constitute one equatorial plane with the metal at the center, where the carbonyl is trans to the coordinated nitrogen of the thiosemicarbazone and the hydride is trans to the sulfur. The two triphenylphosphines are trans. Structures of the 1-CH₃ and 2-CH₃ complexes have been determined by X-ray crystallography. All the complexes show intense transitions in the visible region, which are assigned, based on DFT calculations, to transitions within orbitals of the thiosemicarbazone ligand. Cyclic voltammetry on the complexes shows two oxidations of the coordinated thiosemicarbazone on the positive side of SCE and a reduction of the same ligand on the negative side.     

Intramolecular and intermolecular C-H activation at a cationic Pt center

Syntheses and C-H bond activation reactions of the novel electrophilic Pt^II complexes [(tmeda)Pt(CH₃)(OEt₂)][BAr₁], [(tmeda)Pt(CH₃)(THF)][BAr_f], and [(tmeda)Pt(CH₃)(NC₅F₅)][BAr_f] are described {[BAr_f]⁻ = [(3,5-C₆H₃(CF₃)₂)₄B]⁻} (tmeda is N,N,N′,N′-tetramethylethylenediamine), [(tmeda)Pt(CH₃)(OEt₂)][BAr_f] and [(tmeda)Pt(CH₃)(THF)][BAr_f] are unstable at room temperature, yielding methane and the Fischer carbene Pt^II hydrides, [(tmeda)Pt(=C(CH₃)(OCH₂CH₃))(H)][BAr_f] and . The methane liberated from [(tmeda)Pt(CH₃)(OEt₂-d₁₀)][BAr_f] consists of an isotopomeric mixture, (CH₄, CH₃D, CH₂D₂ and CHD₃), indicating a multiple H/D exchange reaction following the C-D activation and prior to methane loss. [(tmeda)Pt(CH₃)(THF-d₈)][BAr] liberates CH₄ and CH₃D. Methane-¹³C, cyclohexane, toluene, and benzene react with [(tmeda)Pt(CH₃)(NC₅F₅)][BAr_f] to yield methane and new organoplatinum complexes. Deuterated alkanes and arenes react with [(tmeda)Pt(CH₃)(NC₅F₅] [BAr_f] to give a mixture of methane isotopomers. The relevance of these results to the oxidation of alkanes by aqueous platinum complexes is discussed.     

Iridium assisted S-H and C-H activation of benzaldehyde thiosemicarbazones. Synthesis, structure and electrochemical properties of the resulting complexes

Reaction of five 4-R-benzaldehyde thiosemicarbazones (R = OCH₃, CH₃, H, Cl and NO₂) with [Ir(PPh₃)₃Cl] in refluxing ethanol in the presence of a base (NEt₃) affords complexes of three different types, viz. 1-R, 2-R and 3-R. In the 1-R complexes the thiosemicarbazone is coordinated to iridium as a monoanionic bidentate N,S-donor forming a four-membered chelate ring. Two triphenylphosphines, a hydride and a chloride are also coordinated to the metal center. The 2-R complexes are very similar in composition and stereochemistry to the corresponding 1-R complexes, except that a second hydride is bound to iridium instead of the chloride. In the 3-R complexes, the thiosemicarbazones are coordinated to iridium as dianionic tridentate C,N,S-donors forming two adjacent five-membered chelate rings. Two triphenylphosphines and a hydride are also coordinated to the metal center. Structures of the 1-NO₂, 2-NO₂ and 3-NO₂ complexes have been determined by X-ray crystallography. Reaction of the same 4-R-benzaldehyde thiosemicarbazones with [Ir(PPh₃)₃Cl] in refluxing toluene in the presence of NEt₃ affords complexes of two types, viz. 3-R and 4-R. The 4-R complexes are very similar in composition and stereochemistry to the corresponding 3-R complexes, except that a chloride is bound to iridium instead of the hydride. Structure of the 4-CH₃ complex has been determined by X-ray crystallography. In all the complexes the two PPh₃ ligands are trans. All the complexes show intense MLCT transitions in the visible region. Cyclic voltammetry on the complexes shows an Ir(III)-Ir(IV) oxidation on the positive side of SCE followed by an oxidation of the coordinated thiosemicarbazone. A reduction of the coordinated thiosemicarbazone is also observed on the negative side of SCE.     

Bond cleavage reactions in substituted thiophenes by a rhodium complex

The reactions of (C₅Me₅)Rh(PMe₃)(Ph)H with 2-methoxythiophene, 3-methoxythiophene, 2-cyanothiophene, 3-cyanothiophene, 2-trimethylsilylthiophene, ethylenesulfide, trimethylenesulfide, and several polymethylthiophenes have been investigated. These thiophene derivatives give C-S and in one case C-H insertion products. Ethylene sulfide and trimethylene sulfide undergo ring opening or desulfurization.     

Heterogenized polymetallic catalysts Part II. Oxidation of 3,5-di-t-butylphenol by Cu(II), Fe(III) and Pd(II) complexed to a polyphenylene polymer containing ß-di- and triketone surface ligands; an improved catalyst system

Polyphenylene polymer preparation involves the cyclic trimerization polymerization of acetylated methyl benzoate with diacetyl benzene. Since the methyl benzoate groups do not take part in the polymerization they are present in high concentration. The ß-diketone ligands were placed on the surface by reaction of the methylbenzoate group with base and a methyl ketone and the triketone by reaction with base to give the ß-triketone. The ß-triketones can bind two metal ions in a known geometry that is suitable for bimetallic catalysis of the rapid polyelectron oxidation of catechols. The final catalytic surfaces were prepared by treating the chemically modified polymer with copper(II), iron(II) and palladium(II) acetonitrile complexes with non-coordinating BF₄⁻ as the anion. Since the metal ions contain no strongly coordinating ligand, they are very reactive species. These surfaces catalyzed the rapid air oxidation of 3,5-di-tert-butylcatechol (DTBC). The diketone surfaces gave only 3,5-di-tert-butyl-o-quinone (DTBQ) while the triketone surfaces gave ring-cleaved products, confirming the special catalytic effect of the triketone surface. Also, only the triketone catalysts showed any activity for ring cleavage oxidation of DTBQ. These catalysts were much more reactive than previous ones using the same polyphenylene polymer but without the methyl benzoate groups. With these polymers the di- and triketone groups were placed on the surface by chemical modification of the unpolymerized acetyl groups.     

The synthesis and characterization of a cationic technetium(V) phenylimido complex. The X-ray crystal structure of [TcCl2(NPh)(PMe2Ph)3](BPh4)

The reaction of the neutral Tc(V) phenylimido complex [TcCl₃(NPh)(PPh₃)₂] with excess PMe₂Ph in refluxing MeOH gives the cationic, tris-dimethylphenylphosphine complex [TcCl₂(NPh)(PMe₂Ph)₃]⁺, which is isolated as the tetraphenylborate salt. The IR spectrum of the crystalline product shows a medium intensity band at 1102 cm⁻¹ which is assigned to ν(TcN) from the phenylimido core. The ¹H NMR spectrum of the diamagnetic complex shows a series of multiplets in the aryl region and three distinct signals near 2 ppm from the phosphine methyl groups. The X-ray crystal structure, which is the first for a cationic technetium organoimido complex, shows a meridional arrangement of phosphine ligands with a chloride ligand coordinated trans to the phenylimido unit. The TcN bond length of 1.711(2) Å is consistent with the dianionic nature of the organonitrogen core. The Tc---N---C bond angle of 178.8(2)° reflects the sp hybridization of the phenylimido nitrogen atom. The coordination geometry is best described as a distorted octahedron. Crystal data for C₅₄H₅₈BCl₂NP₃Tc: triclinic space group

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. Structure solution based on 9986 observed reflections converged at R = 3.65%, R_w = 5.43%, GOF = 1.82.     

An unsaturated half-sandwich ruthenium(II) complex containing a dithioimidodiphosphinate ligand

Treatment of [Cp*RuCl₂]_x (Cp* = η⁵-C₅Me₅) with K[N(Ph₂PS)₂] afforded [Cp*Ru{N(Ph₂PS)₂}Cl] (1). Reduction of 1 with Li[BEt₃H] gave the 16-electron half-sandwich Ru(II) complex [Cp*Ru{N(Ph₂PS)₂}] (2). Complexes 1 and 2 have been characterized by X-ray crystallography. The Ru-Cp*(centroid) and average Ru-S distances in 1 are 1.827 and 2.3833(5) Å, respectively. The corresponding bond distances in 2 are 1.739 and 2.379(1) Å. Treatment of 2 with 2-electron ligands L afforded the adducts [Cp*Ru{N(Ph₂PS)₂}L] (L = CO (3), 2,6-Me₂C₆H₄NC (4), MeCO₂CCCO₂Me (5)). Oxidation of 2 with tetramethylthiuram disulfide gave the Ru(IV) complex [Cp*Ru{S₂CNMe₂}₂][N(Ph₂PS)₂] (6). The Ru-Cp*(centroid) and average Ru-S distances in 6 are 1.897 and 2.387(1) Å, respectively.     

Electron transfer. Part 165: Oxidations of Ti(II)(aq) with ligated iron(III) and ruthenium(III)

Titanium(II) solutions, prepared by dissolving titanium wire in triflic acid + HF, contain equimolar quantities of Ti(IV). Treatment of such solutions with excess Fe(III) or Ru(III) complexes yield Ti(IV), but reactions with Ti(II) in excess give Ti(III). Oxidations by (NH₃)₅Ru(III) complexes, but not by Fe(III) species, are catalyzed by titanium(IV) and by fluoride. Stoichiometry is unchanged. The observed rate law for the Ru(III)-Ti(II)-Ti(IV) reactions in fluoride media points to competing reaction paths differing by a single F⁻, with both routes involving a Ti(II)-Ti(IV) complex which is activated by deprotonation. It is suggested that coordination of Ti(IV) to Ti^II(aq) minimizes the mismatch of Jahn-Teller distortions which would be expected to lower the Ti(II,III) self-exchange rate.     

Half-sandwich Mo(III) complexes with asymmetric diazadiene ligands

The asymmetric 1,4-diazadiene ligands R^∗NCHCHNR^∗ [R^∗ = (S)-CH(CH₃)Ph], , and 2,2′-bis(4-ethyloxazoline), as-ox, have been used to generate half-sandwich Mo^III derivatives by addition to Cp₂Mo₂Cl₄. Ligand affords a mononuclear, paramagnetic 17-electron product, , whereas as-ox leads to the isolation of a dinuclear compound where only one molecule of ligand has been added per two Mo atoms, Cp₂Mo₂Cl₄(as-ox). In the presence of free as-ox, this compound coexists with the paramagnetic mononuclear complex in solution. Both products are capable of controlling the radical polymerization of styrene under typical atom transfer radical polymerization (ATRP) conditions. However, the tacticity of the resulting polystyrene does not differ from that given by conventional free radical polymerization.     

Aliphatic and aromatic C-H activation of benzo[h]quinolines by Rh(I). Unique precursor dependent formation of mono-, di- and trinuclear complexes

Treatment of 7,8-benzo[h]quinoline (bhq-H, 1) and 10-methyl benzo[h]quinoline (bhq-Me, 3) with [Rh(C₂H₄)₂(THF)₂][BF₄] resulted in double C-H activation of aliphatic and aromatic C-H bonds, yielding the Rh(III) complexes 4 and 5, respectively. The structures of 4 and 5 were revealed by X-ray diffraction. The reaction of 1 with two other slightly different rhodium precursors, [Rh(olefin)_n(THF)₂][BF₄] (COE (n = 2), COD (n = 1)), led to completely different products, a dinuclear complex 7 and a trinuclear complex 6, respectively, which were characterized by X-ray diffraction. Complex 6 exhibits a rare linear Rh-Rh-Rh structure. Utilizing excess of 1 with [Rh(COD)(THF)₂][BF₄] led to the formation of a new product 8 with no C-H bond activation taking place. Additional C-H activation products of 1, cationic and neutral, in the presence of PⁱPr₃ (9a, 9b and 10) are also presented.     

Ligand dynamics controlled reverse spin cross over in bis pyrazolyl pyridine based Fe(II) complex cation with metallodithiolato anions with an example of a ferromagnetic 2:1 cocrystal of mixed Ni(III)/Ni(II) oxidation states

We report here the crystal and molecular structures of three compounds [FeL₂] [Ni(mnt)₂] (1), [FeL₂]₂ [Ni(mnt)₂]₃·2H₂O (2) and [FeL₂] [Cu(mnt)₂]·2CH₃CN (3) where L = 2,6-bis(3,5-dimethylpyrazol-1-ylmethyl)pyridine and mnt = maleonitriledithiolate, and their detailed spectroscopic and magnetic properties using variable temperature Mössbauer, EPR, susceptibility studies, along with room temperature electron spectroscopy for chemical analysis (ESCA) studies. The observed temperature dependant high spin/low spin (HS/LS) ratios of [FeL₂]²⁺ cations in these lattices, exhibiting ‘reverse spin cross-over’ measured unequivocally by Mössbauer, have been interpreted as resulting from differing amount of ‘void space’ in the lattice, a measure of the ease of lattice dynamics originating from ligand L. Differential scanning calorimetric data points this HS/LS transition to order-disorder type of second order phase transitions. While trying to test this lattice dynamics controlled property of [FeL₂]²⁺ cations an unusual behavior of cocrystallization of two planar complex anions of the same type in two different oxidation states, viz. [Ni(mnt)₂]²⁻ and [Ni(mnt)₂]⁻, was observed in [FeL₂]₂ [Ni(mnt₂)]₃, supported by crystallography, ESCA chemical shifts of Ni 2p_3/2 and EPR. The susceptibility data in combination with ESCA chemical shifts of S 2p_3/2 and Ni 2p_3/2 on all the compounds reveal the importance of charge transfer between the two counter ions.     

Iron-oxidation-state-dependent O-O bond cleavage of meta-chloroperbenzoic acid to form an iron(IV)-oxo complex

The mechanism of formation of [Fe^IV(O)(N4Py)]²⁺ (2, N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) from the reaction of [Fe^II(N4Py)(CH₃CN)]²⁺ (1) with m-chloroperbenzoic acid (mCPBA) in CH₂Cl₂ at −30 °C has been studied on the basis of the visible spectral changes observed and the reaction stoichiometry. It is shown that the conversion of 1 to 2 in 90% yield requires 1.5 equiv. peracid and takes place in two successive one-electron steps via an [Fe^III(N4Py)OH]²⁺(3) intermediate. The first oxidation step uses 0.5 equiv. peracid and produces 0.5 equiv. 3-chlorobenzoic acid, while the second step uses 1 equiv. peracid and affords byproducts derived from chlorophenyl radical. We conclude that the Fe^II(N4Py) center promotes O-O bond heterolysis, while the Fe^III(N4Py) center favors O-O bond homolysis, so the nature of O-O bond cleavage is dependent on the iron oxidation state.     

Synthesis and characterisation of a series of 1,2-phenylenedioxoborylcyclopentadienyl-metal complexes [Ti(IV), Zr(IV), Hf(IV), La(III), Ce(III), Yb(III), Sn(II) and Fe(II)]

The preparation of a series of 1,2-phenylenedioxoborylcyclopentadienyl-metal complexes is described. These are of formula [M{η⁵-C₅H₄(BX)}Cl₃] [M = Ti and X = CAT (2a), CAT^t (2b) or CAT^tt (2c); X = CAT^tt and M = Zr (4a) or Hf (4b)], [M{η⁵-C₅H₄(BX)}₂Cl₂] [M = Zr, X = CAT (3a) or CAT^t (3c); or M = Hf, X = CAT (3b) or CAT^t (3d)], [M{(μ-η⁵-C₅H₃BCAT)₂ SiMe₂}Cl₂] [M = Zr (5a) or Hf (5b)], [M{η⁵-C₅H₃(BCAT)₂}Cl₃] [M = Zr (6a) or Hf (6b)], [M{η⁵-C₅H₄BCAT}₃(THF)] [M = La (7a), Ce (7b) or Yb (7c)], [Sn{η⁵-C₅ H₄(BCAT^t)}Cl](8) and [Fe{η⁵-C₅H₄(BCAT^t)}₂] (9). The abbreviations refer to BO₂C₆H₄-1,2 (BCAT) and the 4-Bu^t (BCAT^t) and the (BCAT^tt) analogues. The compounds 2a-9 have been characterised by microanalysis, multinuclear NMR and mass spectra. The single crystal X-ray structure of the lanthanum compound 7a is presented.     

Synthesis, structure, magnetic and electrochemical properties of an oxydiacetate iron(II) complex

Compound [{Fe(oda)(H₂O)₂} · H₂O]_n (1) [oda=O(CH₂COO)₂ ²⁻] has been obtained by reaction of FeCl₂ · 4H₂O with a 1:1 mixture of O(CH₂COOH)₂ and Na₂CO₃ in water. The structure is polymeric with concatenated {Fe(oda)(H₂O)₂} units extended in one direction. Each iron centre is six-coordinated by the tridentate planar oda ligand, two mutually trans water molecules and one oda oxygen atom from an adjacent {Fe(oda)(H₂O)₂} unit. Complex 1 is isomorphous with cobalt and zinc analogues. Magnetic susceptibility measurements down to 2 K showed high-spin non-correlated Fe(II) ions with a typical Curie-Weiss behaviour. Differential-pulse voltammetry establishes a value of 0.488 V versus NHE for the Fe(III)/Fe(II) redox potential of this iron complex in 0.25 mol dm⁻³ NaNO₃.     

Nickel(II) and copper(II) complexes of mixed benzene-triazolehemiporphyrazines

The kinetics of substitution reactions of [η-CpFe(CO)₃]PF₆ with PPh₃ in the presence of R-PyOs have been studied. For all the R-PyOs (R = 4-OMe, 4-Me, 3,4-(CH)₄, 4-Ph, 3-Me, 2,3-(CH)₄, 2,6-Me₂, 2-Me), the reactions yeild the same product [η⁵-CpFe(CO)₂PPh₃]PF₆, according to a second-order rate law that is first order in concentrations of [η⁵-CpFe(CO)₃]PF₆ and of R-PyO but zero order in PPh₃ concentration. These results, along with the dependence of the reaction rate on the nature of R-PyO, are consistent with an associative mechanism. Activation parameters further support the bimmolecular nature of the reactions: ΔH^≠ = 13.4 ± 0.4 kcal mol⁻¹, ΔS^≠ = −19.1 ± 1.3 cal k⁻¹ mol⁻¹ for 4-PhPyO; ΔH^≠ = 12.3 ± 0.3 kcal mol⁻¹, ΔS^≠ = 24.7 ±1.0 cal K⁻¹ mol⁻¹ for 2-MePyO. For the various substituted pyridine N-oxides studied in this paper, the rates of reaction increase with the increasing electron-donating abilities of the substituents on the pyridine ring or N-oxide basicities, but decrease with increasing ¹⁷O chemical shifts of the N-oxides. Electronic and steric factors contributing to the reactivity of pyridine N-oxides have been quantitatively assessed.     

Syntheses and characterization of iron(II) and iron(III) complexes of a tripodal ligand derived from tris(2-aminoethyl)methane

The trihydrochloride salt of tris(2-aminoethyl)methane (tram·3HCl) was deprotonated in methanolic potassium hydroxide and reacted with three molar equivalents of imidazole-2-carboxaldehyde to give a new Schiff base ligand, HC(CH₂CH₂NCH-2ImH)₃. The ligand, H₃(1), was reacted in situ with iron(II)chloride tetrahydrate. Addition of excess sodium perchlorate resulted in the isolation of the dark red [FeH₃(1)](ClO₄)₂·KClO₄. The neutral emerald green iron(III) tripodal complex, Fe(1), was prepared by the aerial oxidation of the iron (II) complex on addition of three equivalents of potassium hydroxide. The complexes are characterized by EA, IR, ESI-MS, Mössbauer, magnetic susceptibility and single crystal XRD. The spectroscopic and structural data support a low spin assignment for both the iron(II) and iron(III) complexes at 295 K. The overall conformation of the tram backbone in these complexes has the apical carbon atom, C_ap, pointed away from the iron atom with an average non-bonded distance of 3.83 Å. However, C_ap is distorted from tetrahedral geometry toward trigonal monopyramidal. This is indicated by a narrowing of the H-C_ap-C angles, an expansion of the C-C_ap-C angles and a compression along the C-H axis so that C_ap approaches the plane defined by its three carbon substituents. Two unusual supramolecular features are exhibited in [FeH₃(1)](ClO₄)₂·KClO₄. These are a polymeric [K(ClO₄)₃²⁻]_n anion and a bidentate hydrogen bonding donor, N_imineCH-C_imidazole-N_imidazoleH, on each arm of the tripodal ligand. Density Functional Theory (DFT) calculations using the B3LYP functional were performed on the low spin and high spin states of both complexes. B3LYP correctly predicts that the low spin state is favored in both systems and closely matches the important metrical parameters that are indicative of spin state. B3LYP shows that the C_ap-out conformation of the tram backbone would be nearly identical in the low and high spin forms.     

Iron and cobalt complexes with the ligand (2-aminoethyl)bis(2-pyridylmethyl)amine (uns-penp) and derivatives

A series of iron(II)/(III) and cobalt(II)/(III) complexes with the tetradendate tripodal ligands (2-aminoethyl)bis(2-pyridylmethyl)amine (uns-penp), its methylated derivatives Me₂-uns-penp and Me₄-uns-penp as well as the amide ligand N-acetyl-N,N-bis[(2-pyridyl)methyl]ethylenediamine (acetyl-uns-penp) were synthesized and structurally characterized. They have been investigated in regard to their reactivity towards dioxygen and/or hydrogen peroxide. Complexes of this type seem to have a high potential to be useful in the activation of dioxygen for selective oxidation reactions of organic substrates.