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Patrick L Holland

Publications and source records attributed to Patrick L Holland.

At least 19 recordsLinked to original sources
PubMed2007

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.

Iron↗
PubMed2006

Mössbauer and computational study of an N2-bridged diiron diketiminate complex: parallel alignment of the iron spins by direct antiferromagnetic exchange with activated dinitrogen.

This work reports Mössbauer and DFT studies of the diiron-N2 complex LMeFeNNFeLMe (L = beta-diketiminate), 1a. Complex 1a, formally diiron(I), has a system spin S = 3 with an isolated MS = +/-3 quasi-doublet as a ground state; the MS = +/-2 doublet is >100 cm-1 higher in energy. Complex 1a exhibits at 4.2 K a large, positive magnetic hyperfine field, Bint = +68.1 T, and an effective g value of 16 +/- 2 along the easy magnetization axis of the ground doublet; this value is significantly larger than the spin-only value (g = 12). These results have been rationalized by DFT calculations, which show that each Fe site donates significant electron density into the pi* orbitals of dinitrogen, resulting in a configuration best described as two high-spin FeII (Sa = Sb = 2) bridged by triplet N22- (Sc = 1). In this description the minority spin electron of each iron is accommodated by two nonbonding, closely spaced 3d orbitals, z2 and yz (z is perpendicular to the diketiminate planes, x is along the Fe...Fe vector). Spin-orbit coupling between these orbital states generates a large unquenched orbital momentum along the iron-iron vector. The S = 3 ground state of 1a results from strong antiferromagnetic direct exchange couplings of the Fe spins (Sa = Sb = 2) to the N22- spin (Sc = 1) and can be formulated as ((Sa,Sb)Sab = 4, Sc = 1), S = 3>; H = J(Sa + Sb).Sc with J approximately 3500 cm-1.

Iron↗
PubMed2006

Binding affinity of alkynes and alkenes to low-coordinate iron.

We report the synthesis, spectroscopy, and structural characterization of iron-alkyne and -alkene complexes of the type L(Me)Fe(ligand) [L(Me) = bulky beta-diketiminate, ligand = HCCPh, EtCCEt, CH2CHPh, EtCHCHEt, HCC(p-C6H4OCH3), HCC(p-C6H4CF3)]. The neutral ligand exchanges rapidly at room temperature, and the equilibrium constants have been measured or estimated. The binding affinity toward the low-coordinate Fe follows the trend HCCPh > EtCCEt > CH2CHPh > EtCHCHEt approximately PPh3 > benzene >> N2. This trend is consistent with a model in which pi back-bonding from the formally Fe(I) center is the dominant interaction in determining the relative binding affinities. In nitrogenase, alkynes are reduced while alkenes are unreactive, and this work suggests that the different binding affinities to low-coordinate Fe might explain the differential activity of the enzyme toward these two substrates.

Alkenes↗
PubMed2006

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.

Crystallography, X-Ray↗
PubMed2005

A T-shaped three-coordinate nickel(I) carbonyl complex and the geometric preferences of three-coordinate d9 complexes.

A three-coordinate diketiminate-nickel(I) complex with a carbonyl ligand has been characterized using EPR and IR spectroscopies and X-ray crystallography. The T geometry (bending from the sterically favored C(2)(v)() structure) contrasts with that of isosteric d(9) copper(II) complexes. DFT calculations on a truncated model reproduce experimental geometries, implying that the geometric differences are electronic in nature. Analysis of the charge distribution in the complexes shows that the geometry of the three-coordinate d(9) complexes is affected by differential charge donation of the ligands to the metal center.

Journal Article↗
PubMed2005

Mössbauer, electron paramagnetic resonance, and crystallographic characterization of a high-spin Fe(I) diketiminate complex with orbital degeneracy.

The synthesis and X-ray structure of the low-coordinate, high-spin Fe(I) compound LFe(HCCPh) (L = HC(C[tBu]N[2,6-diisopropylphenyl])2]-), 1, are reported. Low-temperature Mössbauer and electron paramagnetic resonance (EPR) spectroscopies reveal that the electronic ground state is a Kramers doublet with uniaxial magnetic properties (effective g values g(x) = 8.9, 0 < g(y), g(z) < 0.3) that is well isolated from the excited states. The observation of a large and positive magnetic hyperfine field, B(int) = +68.8(3) T, demonstrates that the orbital angular moment is essentially unquenched along one spatial direction. Relaxation rates obtained from variable-temperature Mössbauer spectra were fit to an Orbach process, yielding delta = 130-190 cm(-1) for the energy gap ("zero-field splitting") between the two Kramers doublets of the S = 3/2 multiplet. Density functional theory (DFT) and time-dependent DFT calculations give insight into the electronic structures of the ground and excited states. The oxidation state of the iron and the bond order of the phenylacetylene ligand in complex 1 are analyzed using DFT, showing a substantial back-bonding interaction. Spin-orbit coupling acting in the subspace of quasi-degenerate z2 and yz orbitals provides a consistent description of both the zero-field splitting and magnetic hyperfine parameters that fits the EPR and Mössbauer data for 1. Interestingly, the spin-orbit coupling involves the same two orbitals (z2, yz) as in the analogous three-coordinate Fe(II) compounds, because back-bonding significantly lowers the energy of the xy orbital, making it the lowest doubly occupied d orbital. Despite the different oxidation state and different number of atoms in the first coordination sphere, the electronic structure of LFe(I)(HCCPh) can be interpreted similarly to that of three-coordinate Fe(II) diketiminate complexes, but with a substantial effect of back-bonding. To our knowledge, this is the first detailed Mössbauer and EPR study of a structurally characterized high-spin Fe(I) complex.

Journal Article↗
PubMed2005

Synthesis, structure, and spectroscopy of an oxodiiron(II) complex.

Bridging oxo species are important in the synthetic and biological chemistry of iron, and are found with iron oxidation states from +2 to +4. We report the first oxodiiron(II) complex that has been crystallographically characterized. It has been examined by NMR, IR, and Mössbauer spectroscopies as well as density-functional calculations.

Iron Compounds↗
PubMed2005

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.

Journal Article↗
PubMed2004

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.

Journal Article↗
PubMed2004

A sulfido-bridged diiron(II) compound and its reactions with nitrogenase-relevant substrates.

The active site iron-molybdenum cofactor of nitrogenase has sulfide-bridged pairs of redox-active, trigonal pyramidal iron atoms that are postulated to be the site of N2 transformation. A synthetic compound is described in which two three-coordinate iron(II) ions are bridged similarly by sulfide. The compound binds nitrogen donors to become trigonal pyramidal and cleaves the N-N bond of phenylhydrazine with oxidation of iron(II) to iron(III).

Ferrous Compounds↗
PubMed2003

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.

Journal Article↗
PubMed2003

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).

Journal Article↗
PubMed2003

Copper(I)-phenolate complexes as models of the reduced active site of galactose oxidase: synthesis, characterization, and O2 reactivity.

The Cu(I)-phenolate complexes (1)LCu and (2)LCu and the Cu(I)-phenol complex [H(2)LCu(CNC(6)H(3)Me(2))]BArF(4) were prepared and structurally characterized by X-ray crystallography, where (1)L(-) and (2)L(-) are ligands comprised of a 2,4-di- tert-butylphenolate linked to 1-isopropyl-1,5-diazacyclooctane or 1,4-diisopropyl-1,4,7-triazacyclononane, respectively. The reduced galactose oxidase (GAO) structural models (1)LCu and (2)LCu were found to be highly reactive with O(2), and through combined stopped-flow kinetic and EPR, UV-vis, and resonance Raman spectroscopic studies of the oxygenation of (2)LCu at low temperature, new intermediates relevant to those postulated for the active site oxidation step of the GAO catalytic cycle were identified. The oxygenation was shown by kinetics experiments to proceed via initial binding of O(2) to yield a green, unusually thermodynamically stable 1:1 adduct, (2)LCu(O(2)). Symmetric (eta(2)) binding of a superoxo ligand was indicated by oxygen-isotope-sensitive features in resonance Raman spectra obtained in batch experiments; peaks at nu((16)O(2))=1120 cm(-1), nu((18)O(16)O)=1093 cm(-1), and nu((18)O(2))=1058 cm(-1) were assigned as O-O stretching vibrations. These data represent the first experimental evidence for such superoxide coordination in complexes of tetradentate tripodal ligands and provide new precedent for how O(2) may bind at the reduced GAO active site. The 1:1 Cu/O(2) adduct subsequently evolves into a metastable purple species that is only observable under conditions of substoichiometric O(2). The kinetics of formation of this transient species are second order overall (rate= k'(2)[(2)LCu(O(2))][(2)LCu]). It exhibits an absorption band with lambda(max)=565 nm (epsilon=17900 M(-1) cm(-1)) and multiple oxygen-isotope-sensitive nu(Cu-O) and nu(O-O) features in the respective regions 500-550 cm(-1) and 700-850 cm(-1) in Raman spectra, with excitation-wavelength-dependent intensities that correlate with the 565 nm absorption feature. On the basis of the combined data available, the presence of multiple isomeric peroxodicopper species in the transient purple solution is postulated.

Binding Sites↗
PubMed2002

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.

Journal Article↗