Ligand dependence of binding to three-coordinate Fe(II) complexes.

A series of three- and four-coordinate iron(II) complexes with nitrogen, chlorine, oxygen, and sulfur ligands is presented. The electronic variation is explored by measuring the association constant of the neutral ligands and the reduction potential of the iron(II) complexes. Varying the neutral ligand gives large changes in K(eq), which decrease in the order CN(t)Bu > pyridine >2-picoline > DMF > MeCN > THF > PPh(3). These differences can be attributed to a mixture of steric effects and electronic effects (both sigma-donation and pi-backbonding). The binding constants and the reduction potentials are surprisingly insensitive to changes in an anionic spectator ligand. This suggests that three-coordinate iron(II) complexes may have similar binding trends as proposed three-coordinate iron(II) intermediates in the FeMoco of nitrogenase, even though the anionic spectator ligands in the synthetic complexes differ from the sulfides in the FeMoco.

Quantitative geometric descriptions of the belt iron atoms of the iron-molybdenum cofactor of nitrogenase and synthetic iron(II) model complexes.

Six of the seven iron atoms in the iron-molybdenum cofactor of nitrogenase display an unusual geometry, which is distorted from the tetrahedral geometry that is most common in iron-sulfur clusters. This distortion pulls the iron along one C3 axis of the tetrahedron toward a trigonal pyramid. The trigonal pyramidal coordination geometry is rare in four-coordinate transition metal complexes. In order to document this geometry in a systematic fashion in iron(II) chemistry, we have synthesized a range of four-coordinate iron(II) complexes that vary from tetrahedral to trigonal pyramidal. Continuous shape measures are used for a quantitative comparison of the stereochemistry of the Fe atoms in the iron-molybdenum cofactor with those of the presently and previously reported model complexes, as well as with those in polynuclear iron-sulfur compounds. This understanding of the iron coordination geometry is expected to assist in the design of synthetic analogues for intermediates in the nitrogenase catalytic cycle.

Macrocyclic Binucleating ß-Diketiminate Ligands and their Lithium, Aluminum, and Zinc Complexes.

The incorporation of rigid aromatic linkers into ß-diketiminate ligands creates a binucleating scaffold that holds two metals near each other. This paper discloses the synthesis, characterization, and reactivity of mBin(2-), which has a meta-substituted xylylene spacer, and pBin(2-), which has a para-substituted xylylene spacer. Lithium, aluminum, and zinc complexes of each ligand are isolated, and in some cases are characterized by X-ray crystallography. The lithium complexes are coordinated to solvent-derived THF ligands, while the zinc and aluminum complexes have alkyl ligands. Complexes of the mBin(2-) ligand have an anti conformation in which the metals are on opposite sides of the macrocycle, while pBin(2-) complexes prefer a syn conformation. The (1)H NMR spectra of the complexes demonstrate that the conformations rapidly interconvert in the lithium complexes, and less rapidly in the zinc and aluminum complexes.

Studies of low-coordinate iron dinitrogen complexes.

Understanding the interaction of N2 with iron is relevant to the iron catalyst used in the Haber process and to possible roles of the FeMoco active site of nitrogenase. The work reported here uses synthetic compounds to evaluate the extent of NN weakening in low-coordinate iron complexes with an FeNNFe core. The steric effects, oxidation level, presence of alkali metals, and coordination number of the iron atoms are varied, to gain insight into the factors that weaken the NN bond. Diiron complexes with a bridging N2 ligand, L(R)FeNNFeL(R) (L(R) = beta-diketiminate; R = Me, tBu), result from reduction of [L(R)FeCl]n under a dinitrogen atmosphere, and an iron(I) precursor of an N2 complex can be observed. X-ray crystallographic and resonance Raman data for L(R)FeNNFeL(R) show a reduction in the N-N bond order, and calculations (density functional and multireference) indicate that the bond weakening arises from cooperative back-bonding into the N2 pi orbitals. Increasing the coordination number of iron from three to four through binding of pyridines gives compounds with comparable N-N weakening, and both are substantially weakened relative to five-coordinate iron-N2 complexes, even those with a lower oxidation state. Treatment of L(R)FeNNFeL(R) with KC8 gives K2L(R)FeNNFeL(R), and calculations indicate that reduction of the iron and alkali metal coordination cooperatively weaken the N-N bond. The complexes L(R)FeNNFeL(R) react as iron(I) fragments, losing N2 to yield iron(I) phosphine, CO, and benzene complexes. They also reduce ketones and aldehydes to give the products of pinacol coupling. The K2L(R)FeNNFeL(R) compounds can be alkylated at iron, with loss of N2.

Synthesis and reactivity of low-coordinate iron(II) fluoride complexes and their use in the catalytic hydrodefluorination of fluorocarbons.

Transition metal fluoride complexes are of interest because they are potentially useful in a multitude of catalytic applications, including C-F bond activation and fluorocarbon functionalization. We report the first crystallographically characterized examples of molecular iron(II) fluorides: [L(Me)Fe(mu-F)]2 (1(2)) and L(tBu)FeF (2) (L = bulky beta-diketiminate). These complexes react with donor molecules (L'), yielding trigonal-pyramidal complexes L(R)FeF(L'). The fluoride ligand is activated by the Lewis acid Et2O.BF3, forming L(tBu)Fe(OEt2)(eta1-BF4) (3), and is also silaphilic, reacting with silyl compounds such as Me3SiSSiMe3, Me3SiCCSiMe3, and Et3SiH to give new thiolate L(tBu)FeSSiMe3 (4), acetylide L(tBu)FeCCSiMe3 (5), and hydride [L(Me)Fe(mu-H)]2 (6(2)) complexes. The hydrodefluorination (HDF) of perfluorinated aromatic compounds (hexafluorobenzene, pentafluoropyridine, and octafluorotoluene) with a silane R3SiH (R3 = (EtO)3, Et3, Ph3, (3,5-(CF3)2C6H3)Me2) is catalyzed by addition of an iron(II) fluoride complex, giving mainly the singly hydrodefluorinated products (pentafluorobenzene, 2,3,5,6-tetrafluoropyridine, and alpha,alpha,alpha,2,3,5,6-heptafluorotoluene, respectively) in up to five turnovers. These catalytic perfluoroarene HDF reactions proceed with activation of the C-F bond para to the most electron-withdrawing group and are dependent on the degree of fluorination and solvent polarity. Kinetic studies suggest that hydride generation is the rate-limiting step in the HDF of octafluorotoluene, but the active intermediate is unknown. Mechanistic considerations argue against oxidative addition and outer-sphere electron transfer pathways for perfluoroarene HDF. Fluorinated olefins are also hydrodefluorinated (up to 10 turnovers for hexafluoropropene), most likely through a hydride insertion/beta-fluoride elimination mechanism. Complexes 1(2) and 2 thus provide a rare example of a homogeneous system that activates C-F bonds without competitive C-H activation and use an inexpensive 3d transition metal.

Facile synthesis, structure, and luminescence properties of Pt(diimine)bis(arylacetylide) chromophore-donor dyads.

The luminescent complex Pt(dpphen)bis(arylacetylide) complex (1) (dpphen = 4,7-diphenylphenanthroline and arylacetylide = 4-ethynylbenzaldehyde) has been synthesized and characterized structurally and spectroscopically. Complex 1 has been employed in the synthesis of donor-chromophore (D-C) dyads through Schiff base condensations of different anilines to give imine-linked dyads 2-4 and through imine reduction with borohydride, to give the corresponding amine-linked dyads 2a-4a. Crystal structure determinations of 1-4 and 4a establish a distorted square-planar geometry around the Pt(II) ion in each system with cis arylacetylide ligands and a diimine-constrained N-Pt-N bond angle of ca. 79.5 degrees. Complex 1 is strongly emissive having a relative quantum yield (phi) of 36% and an excited-state lifetime of 3.1 micros. In accord with the notion of photoinduced electron transfer from the aniline-based donor to the photoexcited chromophore, the emission of dyads 2-4 and 2a-4a is effectively quenched in all solvents tested. The intense absorption at 400 nm (30000-70000 L/mol.cm) for 2 and 2a has been assigned as an intraligand pi-pi* transition, whereas the lowest-energy transitions for all other dyads correspond to Pt-to-pi(diimine) MLCT transitions. Although the dyads can be synthesized in a facile manner, photolysis experiments reveal that both the imine and amine linkages are photochemically unstable, resulting in hydrolysis and regeneration of the aldehyde-containing chromophore 1.

Low-coordinate iron(II) amido complexes of beta-diketiminates: synthesis, structure, and reactivity.

The synthesis, structure, and reactivity of a series of low-coordinate Fe(II) diketiminate amido complexes are presented. Complexes L(R)FeNHAr (R = methyl, tert-butyl; Ar = para-tolyl, 2,6-xylyl, and 2,6-diisopropylphenyl) bind Lewis bases to give trigonal pyramidal and trigonal bipyramidal adducts. In the adducts, crystallographic and (1)H NMR evidence supports the existence of agostic interactions in solid and solution states. Complexes L(R)FeNHAr may be oxidized using AgOTf, and the products L(R)Fe(NHAr)(OTf) are characterized with (19)F NMR spectroscopy, UV/vis spectrophotometry, solution magnetic measurements, elemental analysis, and, in one case, X-ray crystallography. In the structures of the iron(III) complexes L(R)Fe(NHAr)(OTf) and L(R)Fe(OtBu)(OTf), the angles at nitrogen and oxygen result from steric effects and not pi-bonding. The reactions of the amido group of L(R)FeNHAr with weak acids (HCCPh and HOtBu) are consistent with a basic nitrogen atom, because the amido group is protonated by terminal alkynes and alcohols to give free H(2)NAr and three-coordinate acetylide and alkoxide complexes. The trends in complex stability give insight into the relative strength of bonds from three-coordinate iron to anionic C-, N-, and O-donor ligands.

N=N bond cleavage by a low-coordinate ironII hydride complex.

Reaction of the three-coordinate complex LFeCl (L = bulky beta-diketiminate) with KBEt3H gives a dark red iron(II) hydride complex. The complex is a dimer in the solid state, but spectroscopy and kinetics suggest that an orange three-coordinate monomer is in equilibrium with the dimer in solution. The double bond of azobenzene is completely cleaved by heating with the hydride complex, and a hydrazido intermediate can be isolated.

Synthesis and characterization of platinum diimine bis(acetylide) complexes containing easily derivatizable aryl acetylide ligands.

New Pt(II) diimine bis(acetylide) complexes where the diimine is a substituted bipyridine or phenanthroline and the arylacetylide is 4-ethynylbenzaldehyde have been prepared in good to excellent yields. Spectroscopic characterization supports a square planar coordination geometry with cis-alkynyl ligands, and the crystal structure of one of the complexes, Pt(phen)(Ctbd1;CC(6)H(4)CHO)(2) (1), confirms the assignment. The new diimine bis(acetylide) complexes exhibit an absorption band ca. 400 nm that corresponds to a Pt(d) --> pi diimine charge transfer transition and are brightly emissive in fluid solution, with excited state lifetimes in the range 100-800 ns. Correlation of diimine substituent with lambda(max) for the 400 nm absorption band gives strong support to the MLCT assignment. Complex 1 undergoes electron transfer quenching, showing good Stern-Volmer behavior with a variety of oxidative and reductive quenchers. Quenching studies conducted with DNA nucleosides (A, T, C, G) were also investigated. Silyl-protected adenosine and guanosine were found to quench the luminescence of 1 better than similarly protected cytidine or thymidine. Since the former are the more easily oxidized bases, the results suggest that the Pt(II) diimine bis(acetylide) complexes are more powerful photooxidants than photoreductants with regard to electron transfer to DNA bases.

Nickel complexes of a bulky beta-diketiminate ligand.

Nickel(II) chloride forms a complex with tetrahydrofuran, NiCl(2)(THF)(1.5), that can be used to prepare nickel chloride complexes of a bulky beta-diketiminate ligand L(Me). [L(Me)NiCl](2) and L(Me)NiCl(2)LiTHF(2), which have tetrahedral geometries in the solid state, are in equilibrium with three-coordinate L(Me)NiCl. Thermodynamic parameters for the equilibrium between [L(Me)NiCl](2) and L(Me)NiCl are DeltaH = 51(5) kJ/mol and DeltaS = 116(11) J/(mol.K). L(Me)NiCl forms a tetrahydrofuran complex with a binding constant of 1.2(2) M(-)(1) at 21 degrees C. The chloride complexes were used to generate a three-coordinate nickel(II)-amido complex. This amido complex, L(Me)NiN(SiMe(3))(2), is compared with L(Me)MN(SiMe(3))(2) (M = Mn, Fe, Co) (Panda, A.; Stender, M.; Wright, R. J.; Olmstead, M. M.; Klavins, P.; Power, P. P. Inorg. Chem. 2002, 41, 3909-3916). Trends in the metrical parameters of the three-coordinate L(Me)M(II) amido compounds are similar to the trends in three-coordinate L(tBu)M(II) chloride compounds (Holland, P. L.; Cundari, T. R.; Perez, L. L.; Eckert, N. A.; Lachicotte, R. J. J. Am. Chem. Soc. 2002, 124, 14416-14424).

Alkyl isomerisation in three-coordinate iron(II) complexes.

The tertiary to iso-butyl isomerisation of three-coordinate iron(II) diketiminate complexes is reported and a hydride intermediate is proposed on the basis of exchange experiments.

Electronically unsaturated three-coordinate chloride and methyl complexes of iron, cobalt, and nickel.

Three-coordinate organometallic complexes are rare, especially with the prototypical methyl ligand. Using a hindered, rigid bidentate ligand (L), it is possible to create 12-electron methyliron(II) and 13-electron methylcobalt(II) complexes. These complexes are thermally stable, and (1)H NMR spectra suggest that the low coordination number is maintained in solution. Attempts to create the 14-electron LNiCH(3) led instead to the three-coordinate nickel(I) complex LNi(THF). Single crystals of LMCH(3) are isomorphous with the new three-coordinate chloride complexes LNiCl and LCoCl. Along with the recently reported LFeCl (Smith, J. M.; Lachicotte, R. J.; Holland, P. L. Chem. Commun. 2001, 1542), these are the only examples of three-coordinate iron(II), cobalt(II), and nickel(II) complexes with terminal chloride ligands, enabling the systematic evaluation of the effect of coordination number and metal identity on M-Cl bond lengths. Electronic structure calculations predict the ground states of the trigonal complexes.

Multiple emissions and brilliant white luminescence from gold(I) O,O'-di(alkyl)dithiophosphate dimers.

The Au(I) dimers Au(2)[S(2)P(OR)(2)](2) for R = Me, Et are found to exhibit a structure in which aurophilic interactions yield one-dimensional Au...Au chains with intermolecular contacts (3.09-3.16 A) similar to the Au...Au distances within the dimers (3.10-3.18 A). The dimers are luminescent in the solid state and become brilliantly emissive at low temperatures. At 77 K, Au(2)[S(2)P(OMe)(2)](2) shows multiple emission bands. The two higher energy bands at 415 and 456 nm are assigned to (1)MC and (3)MC on the basis of lifetime measurements (20 ns and 2.16 micros, respectively) and concentration-related effects, while the lower energy band at 560 nm is attributed to a LMCT excited state. In frozen glasses of different solvents, Au(2)[S(2)P(OMe)(2)](2) as well as the Et and n-Pr derivatives exhibit a bright luminescence of different colors and striking thermochromism of the emission.

A new tantalum dinitrogen complex and a parahydrogen-induced polarization study of its reaction with hydrogen.

Reduction of Cp*(2)TaCl(2) with sodium amalgam in THF under a nitrogen atmosphere results in the formation of the novel complex (Cp*(2)TaCl)(2)(micro-N(2)). This dinuclear complex containing a micro-eta(1):eta(1) dinitrogen bridge has been characterized by NMR and X-ray crystallography. The complex possesses a C(2)-symmetric structure with each Ta bound to diastereotopic Cp* rings and chloride in addition to the micro-N(2) bridge. The Ta-N and N-N distances of 1.885(10) and 1.23(1) A, respectively, suggest modest reduction of the dinitrogen moiety. The two Cp* resonances on each Ta center remain inequivalent in solution, even up to 80 degrees C. Addition of hydrogen results in the formation of two isomers of the dihydride complex Cp*(2)TaH(2)Cl. Under parahydrogen, polarized resonances are observed for the unsymmetrical isomer with adjacent hydrides as the product of H(2) oxidative addition. The symmetric isomer of Cp*(2)TaH(2)Cl also forms, most likely by isomerization of the unsymmetrical kinetic isomer. The reactivity of (Cp*(2)TaCl)(2)(micro-N(2)) was compared to that of the related monomer, Cp*(2)TaCl(THF). The THF adduct yields the same hydrogen addition products, but the reaction is much more facile than for the nitrogen dimer, indicative of the structural integrity of the micro-N(2) complex.

Rapid synthesis of the 7-deoxy zaragozic acid core.

[reaction: see text] We have developed an efficient synthesis of the 7-deoxy zaragozic acid core. The synthesis begins with a Feist-Bénary reaction that assembles all three carbons of the polycarboxylic acid portion of the core. This reaction is followed by highly diastereoselective aldol and dihydroxylation reactions that set the remaining stereocenters of the core. The synthesis finishes with lactol oxidation and lactone alcoholysis/ketal formation reactions to construct the bicyclic ring system of the core.

Syntheses, Molecular Structures, and Spectroscopy of Gold(III) Dithiolate Complexes.

Two Au(III) dithiolate complexes, [Au(dbbpy)(tdt)]PF(6) and Au(eta(2)-C,N-ppy)(tdt), (dbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine; tdt = 3,4-toluenedithiolate; ppy = C-deprotonated 2-phenylpyridine), have been prepared and structurally characterized by X-ray crystallography. The complexes have low-energy absorption bands that exhibit mild solvatochromism (lambda(max) = 444 nm (epsilon, 2310 M(-)(1) cm(-)(1)) in CH(2)Cl(2) and 406 nm (epsilon, 3170 M(-)(1) cm(-)(1)) in DMSO) and are tentatively assigned to a charge-transfer-to-diimine transition. This transition occurs at higher energy than the analogous charge-transfer transition in related Pt(II) complexes (e.g., Pt(dbbpy)(tdt), lambda(max) = 563 nm (epsilon, 7200 M(-)(1) cm(-)(1)). Whereas neither Au(III) complex is emissive, their respective dichloride precursors, [Au(dbbpy)Cl(2)]PF(6) and Au(eta(2)-C,N-ppy)Cl(2), luminesce in low-temperature glass matrixes from an excited state that is tentatively assigned as intraligand pi-pi. The neutral complex, Au(eta(2)-C,N-ppy)(tdt), is more easily oxidized (E(ox) = 0.925 V vs 1.589 V (vs NHE)) and less easily reduced (E(red) = -1.339 V vs -0.255 V (vs NHE)) than the cationic complex, [Au(dbbpy)(tdt)]PF(6). Both dithiolate complexes exhibit approximately square planar coordination. Yellow crystals of [Au(dbbpy)(tdt)]PF(6) (C(25)H(30)AuF(6)N(2)PS(2)) are triclinic, space group P&onemacr; (No. 2), with a = 7.1977(2) Å, b = 11.7292(1) Å, c = 17.5820(5) Å, alpha = 104.537(2) degrees, beta = 96.592(2) degrees, gamma = 102.455(2) degrees, V = 1380.41(6) Å(3), Z = 2, and final R = 0.050 (R(w) = 0.1062) for 3446 unique reflections. Orange crystals of Au(eta(2)-C,N-ppy)(tdt) (C(18)H(14)AuNS(2)) are monoclinic, space group P2(1)/c (No. 14), with a = 8.852(1) Å, b = 11.726(2) Å, c = 15.499(5) Å, beta = 101.34(2) degrees, V = 1577.5(6) Å(3), Z = 4, and final R = 0.0438 (R(w) = 0.0759) for 3685 unique reflections. A structural trans effect in Au(eta(2)-C,N-ppy)(tdt) results in a significantly longer Au-S distance (ca. 0.1 Å) trans to the Au-C bond than that trans to the Au-N bond. In the solid state, the [Au(dbbpy)(tdt)](+) cations are arranged in stacks with alternating intermolecular Au.Au separations of 3.60 and 3.75 Å while the Au(eta(2)-C,N-ppy)(tdt) molecules form stacks with an intermolecular Au.Au separation of 3.81 Å.