Stereochemically Nonrigid Six-Coordinate Molecules.' III. The Temperature-Dependent 'H and 31P Nuclear Magnetic Resonance Spectra of Some Iron and Ruthenium Dihydrides

A combined experimental and computational study to decipher complexity in the asymmetric hydrogenation of imines with Ru catalysts bearing atropisomerizable ligands

RuCl₂(P–OP)(N–N) complexes containing an atropisomerizable phosphine–phosphite and a chiral diamine are effective catalyst precursors for the asymmetric hydrogenation of N-aryl imines following an outer-sphere mechanism.

Isomerization of the hydride complexes [HFe2(SR)2(PR3)(x)(CO)(6-x)]+ (x = 2, 3, 4) relevant to the active site models for the [FeFe]-hydrogenases

The stepwise formation of bridging (mu-) hydrides of diiron dithiolates is discussed with attention on the pathway for protonation and subsequent isomerizations. Our evidence is consistent with protonations occurring at a single Fe center, followed by isomerization to a series of mu-hydrides. Protonation of Fe(2)(edt)(CO)(4)(dppv) (1) gave a single mu-hydride with dppv spanning apical and basal sites, which isomerized at higher temperatures to place the dppv into a dibasal position. Protonation of Fe(2)(pdt)(CO)(4)(dppv) (2) followed an isomerization pathway similar to that for [1H](+), except that a pair of isomeric terminal hydrides were observed initially, resulting from protonation at the Fe(CO)(3) or Fe(CO)(dppv) site. The first observable product from low temperature protonation of the tris-phosphine Fe(2)(edt)(CO)(3)(PMe(3))(dppv) (3) was a single mu-hydride wherein PMe(3) is apical and the dppv ligand spans apical and basal sites. Upon warming, this isomer converted fully but in a stepwise manner to a mixture of three other isomeric hydrides. Protonation of Fe(2)(pdt)(CO)(3)(PMe(3))(dppv) (4) proceeded similarly to the edt analogue 3, however a terminal hydride was observed, albeit only briefly and at very low temperatures (-90 degrees C). Low-temperature protonation of the bis-chelates Fe(2)(xdt)(CO)(2)(dppv)(2) produced exclusively the terminal hydrides [HFe(2)(xdt)(mu-CO)(CO)(dppv)(2)](+) (xdt = edt and pdt), which subsequently isomerized to a pair of mu-hydrides. At room temperature these (dppv)(2) derivatives convert to an equilibrium of two isomers, one C(2)-symmetric and the other C(s)-symmetric. The stability of the terminal hydrides correlates with the (C(2)-isomer)/(C(s)-isomer) equilibrium ratio, which reflects the size of the dithiolate. The isomerization was found to be unaffected by the presence of excess acid, by solvent polarity, and the presence of D(2)O. This isomerization mechanism is proposed to be intramolecular, involving a 120 degrees rotation of the HFeL(3) subunit to an unobserved terminal basal hydride as the rate-determining step. The observed stability of the hydrides was supported by DFT calculations, which also highlight the instability of the basal terminal hydrides. Isomerization of the mu-hydride isomers occurs on alternating FeL(3) via 120 degree rotations without generating D(2)O-exchangeable intermediates.

Solution NMR Studies of Iron(II) Spin-Crossover Complexes

The 1H NMR spectra of a series of mono- and dinuclear pyridine complexes [FeL1(R1/R2)(py)2] and [Fe2L2(R1/R2)(py)4] have been investigated in a mixed toluene-d8/pyridine-d5 solution. The equatorial tetradentade Schiff base like ligands L1(R1/R2) and L2(R1/R2) with a N2O22- coordination sphere for each metal center have been obtained by condensation of a substituted malonodialdehyde (R1/R2 are Me/COOEt, Me/COMe, or OEt/COOEt) with o-phenylenediamine (L1(R1/R2)) or 1,2,4,5-tetraaminobenzene (L2(R1/R2)). The 1H NMR resonances were assigned by comparison of differently substituted complexes in combination with a line-width comparison. The 1H NMR shifts from 188 to 358 K show a strong influence of the spin state of the iron center. The behavior of the pure high-spin iron(II) complexes is close to ideal Curie behavior. Analysis of the resonance shifts of the spin-transition complexes can be used for determining the high-spin mole fraction of the complex in solution at different temperatures. Magnetic susceptibility measurements in solution using the Evans method were made for all six complexes. Significant differences between the spin-transition behavior of the complexes in solution of those in the solid state were found. However, the plots of microeff as a function of temperature obtained using the Evans method and those obtained by interpretation of the NMR shifts were virtually identical. The isotropic shifts of protons in the complexes proved to be suitable tools for following a spin transition in solution. Comparison of the microeff plots of the mono- and dinuclear complexes in solution reveals slight differences between the steepness of the curves that may be attributable to cooperative interactions between the metal centers in the case of the dinuclear complexes.

Carbon Dioxide Hydrogenation Catalyzed by a Ruthenium Dihydride: A DFT and High-Pressure Spectroscopic Investigation

Reaction pathways during CO(2) hydrogenation catalyzed by the Ru dihydride complex [Ru(dmpe)(2)H(2)] (dmpe=Me(2)PCH(2)CH(2)PMe(2)) have been studied by DFT calculations and by IR and NMR spectroscopy up to 120 bar in toluene at 300 K. CO(2) and formic acid readily inserted into or reacted with the complex to form formates. Two formate complexes, cis-[Ru(dmpe)(2)(OCHO)(2)] and trans-[Ru(dmpe)(2)H(OCHO)], were formed at low CO(2) pressure (<5 bar). The latter occurred exclusively when formic acid reacted with the complex. A RuHHOCHO dihydrogen-bonded complex of the trans form was identified at H(2) partial pressure higher than about 50 bar. The trans form of the complex is suggested to play a pivotal role in the reaction pathway. Potential-energy profiles along possible reaction paths have been investigated by static DFT calculations, and lower activation-energy profiles via the trans route were confirmed. The H(2) insertion has been identified as the rate-limiting step of the overall reaction. The high energy of the transition state for H(2) insertion is attributed to the elongated Ru--O bond. The H(2) insertion and the subsequent formation of formic acid proceed via Ru(eta(2)-H(2))-like complexes, in which apparently formate ion and Ru(+) or Ru(eta(2)-H(2))(+) interact. The bond properties of involved Ru complexes were characterized by natural bond orbital analysis, and the highly ionic characters of various complexes and transition states are shown. The stability of the formate ion near the Ru center likely plays a decisive role for catalytic activity. Removal of formic acid from the dihydrogen-bonded complex (RuHHOCHO) seems to be crucial for catalytic efficiency, since formic acid can easily react with the complex to regenerate the original formate complex. Important aspects for the design of highly active catalytic systems are discussed.

Hydrodefluorination of pentafluoropyridine at rhodium using dihydrogen: detection of unusual rhodium hydrido complexes

The pentafluoropyridyl complex [Rh(4-C5NF4)(PEt3)3] (3) reacts with H2 to give initially the dihydrido complex cis-mer-[Rh(H)2(4-C5NF4)(PEt3)3] (6). Within a few hours 2,3,5,6-tetrafluoropyridine as well as two rhodium(III) complexes mer-[Rh(H)3(PEt3)3] (mer-) and fac-[Rh(H)3(PEt3)3] (fac-) are formed. A catalytic C-F activation process for the formation of 2,3,5,6-tetrafluoropyridine starting from pentafluoropyridine and dihydrogen using 3 as a catalyst has been developed. Reaction of [RhH(PEt3)3] (1) with hydrogen affords fac-[Rh(H)3(PEt3)3] (fac-7) and mer-[Rh(H)3(PEt3)3] (mer-7) in a ratio of 1 : 7.25 at 193 K. The latter complex represents the first mononuclear rhodium compound bearing trans-hydrides.

Trigonal-Bipyramidal Geometry Induced by an External Water Ligand in a Sterically Hindered Iron Salen Complex, Related to the Active Site of Protocatechuate 3,4-Dioxygenase

A unique distorted trigonal-bipyramidal geometry observed for the non-heme iron center in protocatechuate 3,4-dioxygenase (3,4-PCD) was carefully examined utilizing a sterically hindered iron salen complex, which well reproduces the endogenous His2Tyr2 donor set with water as an external ligand. X-ray crystal structures of a series of iron model complexes containing bis(3,5-dimesitylsalicylidene)-1,2-dimesitylethylenediamine indicate that a distorted trigonal-bipyramidal geometry is achieved upon binding of water as an external ligand. The extent of a structural change of the iron center from a preferred square-pyramidal to a distorted trigonal-bipyramidal geometry varies with the external ligand that is bound in the order Cl << EtO < H2O, which is consistent with the spectrochemical series. The distortion in the model system is not due to steric repulsions but electronic interactions between the external ligand and the iron center, as evidenced from the X-ray crystal structures of another series of iron model complexes with a less-hindered bis(3-xylylsalicylidene)-1,2-dimesitylethylenediamine ligand, as well as by density functional theory calculations. Further spectroscopic investigations indicate that a unique distorted trigonal-bipyramidal geometry is indeed maintained even in solution. The present model study provides a new viewpoint that a unique distorted trigonal-bipyramidal iron site might not be preorganized by a 3,4-PCD protein but could be electronically induced upon the binding of an external hydroxide ligand to the iron(III) center. The structural change induced by the external water ligand is also discussed in relation to the reaction mechanism of 3,4-PCD.

NMR and EPR studies of chloroiron(III) tetraphenyl-chlorin and its complexes with imidazoles and pyridines of widely differing basicities

The NMR and EPR spectra of two bisimidazole and three bispyridine complexes of tetraphenylchlorinatoiron(III), [(TPC)Fe(L)2]+ (L = Im-d4, 2-MeHIm, 4-Me2NPy, Py, and 4-CNPy), have been investigated. The full resonance assignments of the [(TPC)Fe(L)2]+ complexes of this study have been made from correlation spectroscopy (COSY) and nuclear Overhauser enhancement spectroscopy (NOESY) experiments and Amsterdam density functional (ADF) calculations. Unlike the [(OEC)Fe(L)2]+ complexes reported previously (Cai, S.; Lichtenberger, D. L.; Walker, F. A. Inorg. Chem. 2005, 44, 1890-1903), the NMR data for the [(TPC)Fe(L)2]+ complexes of this study indicate that the ground state is S = 1/2 for each bisligand complex, whereas a higher spin state was present at NMR temperatures for the Py and 4-CNPy complexes of (OEC)Fe(III). The pyrrole-8,17 and pyrroline-H of all [TPCFe(L)2]+ show large magnitude chemical shifts (hence indicating large spin density on the adjacent carbons that are part of the pi system), while pyrrole-12,13-CH2 and -7,18-CH2 protons show much smaller chemical shifts, as predicted by the spin densities obtained from ADF calculations. The magnitude of the chemical shifts decreases with decreasing donor ability of the substituted pyridine ligands, with the nonhindered imidazole ligand having slightly larger magnitude chemical shifts than the most basic pyridine, even though its basicity is significantly lower (4-Me2NPyH+ pKa = 9.7, H2Im+ pKa = 6.65 (adjusted for the statistical factor of 2 protons)). The temperature dependence of the chemical shifts of all but the 4-Me2NPy bisligand complexes studied over the temperature range of the NMR investigations shows that they have mixed (dxy)2(dxz,dyz)3/(dxzdyz)4(dxy)1 electron configurations that cannot be resolved by temperature-dependent fitting of the proton chemical shifts, with an S = 3/2 excited state in each case that in most cases lies at more than kT at room temperature above the ground state. The observed pattern of chemical shifts of the 4-CNPy complex and analysis of the temperature dependence indicate that it has a pure (dxzdyz)4(dxy)1 ground state and that it is ruffled, because ruffling mixes the a(2u)(pi)-like orbital of the chlorin into the singly occupied molecular orbital (SOMO). This mixing accounts for the negative chemical shift of the pyrroline-H (-6.5 ppm at -40 degrees C) and thus the negative spin density at the pyrroline-alpha-carbons, but the mixing is not to the same extent as observed for [(TPC)Fe(t-BuNC)2]+, whose pyrroline-H chemical shift is -36 ppm at 25 degrees C (Simonneaux, G.; Kobeissi, M. J. Chem. Soc., Dalton Trans. 2001, 1587-1592). Peak assignments for high-spin (TPC)FeCl have been made by saturation transfer techniques that depend on chemical exchange between this complex and its bis-4-Me2NPy adduct.

Contrasting photochemical and thermal reactivity of Ru(CO)2(PPh3)(dppe) towards hydrogen rationalised by parahydrogen NMR and DFT studies

The synthesis, characterisation and thermal and photochemical reactivity of Ru(CO)2(PPh3)(dppe) 1 towards hydrogen are described. Compound proved to exist in both fac (major) and mer forms in solution. Under thermal conditions, PPh3 is lost from 1 in the major reaction pathway and the known complex Ru(CO)2(dppe)(H)2 2 is formed. Photochemically, CO loss is the dominant process, leading to the alternative dihydride Ru(CO)(PPh3)(dppe)(H)2 3. The major isomer of 3, viz. 3a, contains hydride ligands that are trans to CO and trans to one of the phosphorus atoms of the dppe ligand but a second isomer, 3b, where both hydride ligands are trans to distinct phosphines, is also formed. On the NMR timescale, no interconversion of 3a and 3b was observed, although hydride site interchange is evident with activation parameters of DeltaH(double dagger) = 95 +/- 6 kJ mol(-1) and DeltaS(double dagger) = 26 +/- 17 J K(-1) mol(-1). Density functional theory confirms that the observed species are the most stable isomeric forms, and suggests that hydride exchange occurs via a transition state featuring an eta2-coordinated H2 unit.

Parahydrogen studies of H2addition to Ir(i) complexes containing chiral phosphine–thioether ligands: implications for catalysis

Ir(CO)[CpFe{eta5-C5H3(PPh2)CH2SR}]Cl [R = Ph and (t)Bu], containing a kappa2:P,S ligand, undergoes H2 addition across the S-Ir-CO axis under kinetic control to form two distinct diastereoisomeric products, which then rearrange via S dissociation in a process that can be hijacked for useful catalysis, but ultimately form a single diastereoisomer of the thermodynamic product where the hydride ligands are trans to chloride and phosphine.

Stereochemical Influence of the Ligand on the Structure of Manganese Complexes: Implications for Catalytic Epoxidations

Manganese complexes of the ligand HphoxCOOR (R=H or Me) have been synthesized and characterized by X-ray analysis, ESI-MS, ligand-field spectroscopy, electrochemistry, and paramagnetic 1H NMR. The ligands, chirally pure or racemic, influence the structures of the complexes formed. Manganese(III) complexes of the ligand HphoxCOOMe are square-pyramidal or octahedral with two ligands bound in a trans fashion in the solid state. The racemic ligand (RS-HphoxCOOMe) as well as the enantiopure ligand (R-HphoxCOOMe) forms manganese complexes with similar solid-state structures. Ligand-exchange reactions occur in solution giving rise to meso complexes as confirmed by ESI-MS and deuteration studies. The manganese(III) complex of R-HphoxCOOH is octahedral, with two dianionic ligands bound in a fac-cct fashion in a tridentate manner. The manganese(III) complex of RS-HphoxCOOH is also octahedral with two dianionic ligands now bound in a trans fashion in a didentate manner and with two water molecules occupying axial sites. The paramagnetic 1H NMR spectra of the complexes have been interpreted on the basis of the relaxation times with the help of the inversion-recovery pulse technique. The binding of imidazole with the metal center depends on the chirality of the ligands in the metal complexes of HphoxCOOMe. Imidazole coordination was found to occur with the metal complex that contains two ligands with the same chirality (R and R) (R-1), while no imidazole coordination was found upon reaction with the metal complex that contains two ligands with opposite chirality (R and S) (RS-1). Epoxidation reactions of various alkenes with H2O2 as the oxidant reveal that the complexes give turnover numbers in the range of 10-35, the epoxide being the major product. The catalytic activity depends on the additives used, and a clear base effect is observed. The turnover numbers have been found to be higher in the complexes where no binding of N-Meim is observed. The latter fact unambiguously shows that imidazole binding is not a prerequisite for higher turnover numbers, in contrast to the Mn-Schiff base catalysts.

New Class of Verdoheme Analogues with Weakly Coordinating Anions: The Structure of (μ-Oxo)bis[(octaethyloxoporphinato)iron(III)] Hexafluorophosphate

Three new verdoheme analogues with weakly coordinating anions, [OEOPFe(II)X], where OEOP is the monoanion of octaethyloxoporphyrin and X = PF(6), ClO(4), and BF(4), have been synthesized and characterized by spectroscopic methods. (1)H NMR spectroscopy reveals that the [OEOPFe(II)X] species are paramagnetic, and the iron is five-coordinate (S = 2). The oxidation of [OEOPFe(II)PF(6)] with dioxygen yields [(OEOPFe)(2)O](PF(6))(2). The structure of (mu-oxo)bis[(octaethyloxoporphinato)iron(III)] has been determined by X-ray diffraction analysis. The eight Fe-N bond distances have an average value of 2.077(3) Angstroms. The oxygen atom sits on the inversion center, and the average axial Fe-O bond length is 1.756(3) Angstroms. The average displacement of the iron(III) atom from the mean porphinato core is 0.60 Angstroms. Crystal data: crystal system, monoclinic; a = 8.7114(10) Angstroms; b = 26.102(4) Angstroms; c = 15.8323(14) Angstroms; beta = 104.134(6) degrees ; space group P2(1)/c; V = 3491.1(7) Angstroms (3); Z = 2; R1 = 0.0546, wR2 =0.1145 for data with I > 2sigma(I).

NMR and EPR Studies of the Bis(pyridine) and Bis(tert-butyl isocyanide) Complexes of Iron(III) Octaethylchlorin

The NMR and EPR spectra of a series of pyridine complexes [(OEC)Fe(L)2]+ (L = 4-Me2NPy, Py, and 4-CNPy) have been investigated. The EPR spectra at 4.2 K suggest that, with a decrease of the donor strength of the axial ligands, the complexes change their ground state from (d(xy))2 (d(xz)d(yz))3 to (d(xz)d(yz))4 (d(xy))1. The NMR data from 303 to 183 K show that at any temperature within this range the chemical shifts of pyrrole-8,17-CH2 protons increase with a decrease in the donor strength of the axial ligands. The full peak assignments of the [(OEC)Fe(L)2]+ complexes of this study have been made from COSY and NOE difference experiments. The pyrrole-8,17-CH2 and pyrroline protons show large chemical shifts (hence indicating large pi spin density on the adjacent carbons which are part of the pi system), while pyrrole-12,13-CH2 and -7,18-CH2 protons show much smaller chemical shifts, as predicted by the spin densities obtained from molecular orbital calculations, both Hückel and DFT; the DFT calculations additionally show close energy spacing of the highest five filled orbitals (of the Fe(II) complex) and strong mixing of metal and chlorin character in these orbitals that is sensitive to the donor strength of the axial substituents. The pattern of chemical shifts of the pyrrole-CH2 protons of [(OEC)Fe(t-BuNC)2]+ looks somewhat like that of [(OEC)Fe(4-Me2NPy)2]+, while the chemical shifts of the meso-protons are qualitatively similar to those of [(OEP)Fe(t-BuNC)2]+. The temperature dependence of the chemical shifts of [(OEC)Fe(t-BuNC)2]+ shows that it has a mixed (d(xz)d(yz))4 (d(xy))1 and (d(xy))2 (d(xz),d(yz))3 electron configuration that cannot be resolved by temperature-dependent fitting of the proton chemical shifts, with a S = 5/2 excited state that lies somewhat more than 2kT at room temperature above the ground state; the observed pattern of chemical shifts is the approximate average of those expected for the two S = 1/2 electronic configurations, which involve the a-symmetry SOMO of a planar chlorin ring with the unpaired electron predominantly in the d(yz) orbital and the b-symmetry SOMO of a ruffled chlorin ring with the unpaired electron predominantly in the d(xy) orbital. A rapid interconversion between the two, with calculated vibrational frequency of 22 cm(-1), explains the observed pattern of chemical shifts, while a favoring of the ruffled conformation explains the negative chemical shift (and thus the negative spin density at the alpha-pyrroline ring carbons), of the pyrroline-H of [TPCFe(t-BuNC)2]CF3SO3 (Simonneaux, G.; Kobeissi, M. J. Chem. Soc., Dalton Trans. 2001, 1587-1592). Peak assignments for high-spin (OEC)FeCl have been made by saturation transfer techniques that depend on chemical exchange between this complex and its bis-4-Me2NPy adduct. The contact shifts of the pyrrole-CH2 and meso protons of the high-spin complex depend on both sigma and pi spin delocalization due to contributions from three of the occupied frontier orbitals of the chlorin ring.

The reaction of M(CO)3(Ph2PCH2CH2PPh2) (M = Fe, Ru) with parahydrogen: probing the electronic structure of reaction intermediates and the internal rearrangement mechanism for the dihydride products

The photochemical reaction of Ru(CO)(3)(dppe) and Fe(CO)(3)(dppe)(dppe = Ph(2)PCH(2)CH(2)PPh(2)) with parahydrogen has been studied by in situ-photochemistry resulting in NMR spectra of Ru(CO)(2)(dppe)(H)(2) that show significant enhancement of the hydride resonances while normal signals are seen in Fe(CO)(2)(dppe)(H)(2). This effect is associated with a singlet electronic state for the key intermediate Ru(CO)(2)(dppe) while Fe(CO)(2)(dppe) is a triplet. DFT calculations reveal electronic ground states consistent with this picture. The fluxionality of Ru(CO)(2)(dppe)(H)(2) and Fe(CO)(2)(dppe)(H)(2) has been examined by NMR spectroscopy and rationalised by theoretical methods which show that two pathways for ligand exchange exist. In the first, the phosphorus and carbonyl centres interchange positions while the two hydride ligands are unaffected. A second pathway involving interchange of all three ligand sets was found at slightly higher energy. The H-H distances in the transition states are consistent with metal-bonded dihydrogen ligands. However, no local minimum (intermediate) was found along the rearrangement pathways.

Symmetry and bonding in metalloporphyrins. A modern implementation for the bonding analyses of five- and six-coordinate high-spin iron(III)-porphyrin complexes through density functional calculation and NMR spectroscopy

Bonding interactions between the iron and the porphyrin macrocycle of five- and six-coordinate high-spin iron(III)-porphyrin complexes are analyzed within the framework of approximate density functional theory with the use of the quantitative energy decomposition scheme in combination with removal of the vacant pi orbitals of the porphyrin from the valence space. Although the relative extent of the iron-porphyrin interactions can be evaluated qualitatively through the spin population and orbital contribution analyses, the bond strengths corresponding to different symmetry representations can be only approximated quantitatively by the orbital interaction energies. In contrast to previous suggestions, there are only limited Fe --> P pi back-bonding interactions in high-spin iron(III)-porphyrin complexes. It is the symmetry-allowed bonding interaction between d(z)2 and a(2u) orbitals that is responsible for the positive pi spin densities at the meso-carbons of five-coordinate iron(III)-porphyrin complexes. Both five- and six-coordinate complexes show significant P --> Fe pi donation, which is further enhanced by the movement of the metal toward the in-plane position for six-coordinate complexes. These bonding characteristics correlate very well with the NMR data reported experimentally. The extraordinary bonding interaction between d(z)2 and a(2u) orbitals in five-coordinate iron(III)-porphyrin complexes offers a novel symmetry-controlled mechanism for spin transfer between the axial ligand sigma system and the porphyrin pi system and may be critical to the electron transfer pathways mediated by hemoproteins.

Ruthenium dihydride complexes: NMR studies of intramolecular isomerization and fluxionality including the detection of minor isomers by parahydrogen-induced polarization

NMR studies reveal that complexes Ru(CO)(2)(H)(2)L(2) (L = PMe(3), PMe(2)Ph, and AsMe(2)Ph) can have three geometries, ccc, cct-L, and cct-CO, with equilibrium ratios that are highly dependent on the electronic properties of L; the cct-L form is favored, because the sigma-only hydride donor is located trans to CO rather than L. When L = PMe(3), the ccc form is only visible when p-H(2) is used to amplify its spectral features. In contrast, when L = AsMe(2)Ph, the ccc and cct-L forms are present in similar quantities and, hence, must have similar free energies; for this complex, however, the cct-CO isomer is also detectable. These complexes undergo a number of dynamic processes. For L(2) = dppe, an interchange of the hydride positions within the ccc form is shown to be accompanied by synchronized CO exchange and interchange of the two phosphorus atoms. This process is believed to involve the formation of a trigonal bipyramidal transition state containing an eta(2)-H(2) ligand; in view of the fact that k(HH)/k(DD) is 1.04 and the synchronized rotation when L(2) = dppe, this transition state must contain little H-H bonding character. Pathways leading to isomer interconversion are suggested to involve related structures containing eta(2)-H(2) ligands. The inverse kinetic isotope effect, k(HH)/k(DD) = 0.5, observed for the reductive elimination of dihydrogen from Ru(CO)(2)(H)(2)dppe suggests that substantial H-H bond formation occurs before the H(2) is actually released from the complex. Evidence for a substantial steric influence on the entropy of activation explains why Ru(CO)(2)(H)(2)dppe undergoes the most rapid hydride exchange. Our studies also indicate that the species [Ru(CO)(2)L(2)], involved in the addition of H(2) to form Ru(CO)(2)(H)(2)L(2), must have singlet electron configurations.

Molecular structures and magnetic resonance spectroscopic investigations of highly distorted six-coordinate low-spin iron(III) porphyrinate complexes

Three bis-axially ligated complexes of iron(III) octaethyltetraphenylporphyrin, (OETPP)Fe(III), have been prepared, which are low-spin complexes, each with two axial nitrogen-donor ligands (N-methylimidazole (N-MeIm), 4-(dimethylamino)pyridine (4-NMe(2)Py), and 2-methylimidazole (2-MeImH)). The crystal and molecular structure of the bis-(2-MeImH) complex shows the macrocycle to be in a saddled conformation, with the ligands in perpendicular planes aligned at 14 degrees to the porphyrin nitrogens so as to relieve the steric interaction between the 2-methyl groups and the porphyrin. The Fe-N(por) bond lengths are typical of nonplanar six-coordinate low-spin Fe(III) complexes, while the axial Fe-N(ax) bond lengths are substantially longer than those of [(TPP)Fe(2-MeImH)(2)](+) (2.09(2) A as compared to 2.015(4) and 2.010(4) A). The crystal and molecular structure of the bis-(4-NMe(2)Py) complex also shows the macrocycle to be in a mainly saddled conformation, but with a significant ruffled component. As a result, the average Fe-N(por) bonds are significantly shorter (1.951 A as compared to 1.974 A) than those of the bis-(2-MeImH) complex. One ligand is aligned at 9 degrees to two trans porphyrin nitrogens, while the other is at 79 degrees to the same porphyrin nitrogens, producing a dihedral angle of 70 degrees between the ligand planes. The EPR spectrum of this complex, like that of the bis-(2-MeImH) complex, is of the "large g(max)" type, with g(max) = 3.29 and 3.26, respectively. However, in frozen CD(2)Cl(2), [(OETPP)Fe(N-MeIm)(2)](+) exhibits both "large g(max)" and normal rhombic signals, suggesting the presence of both "perpendicular" and "parallel" ligand orientations. The 1- and 2D (1)H NMR spectra of each of these complexes, as well as the chloroiron(III) starting material, were investigated as a function of temperature. The COSY and NOESY/EXSY spectra of the chloride complex are consistent with the expected J-coupling and saddle inversion dynamics, respectively. Complete spectral assignments for the bis-(N-MeIm) and -(4-NMe(2)Py) complexes have been made using 2D (1)H NMR techniques. In each case, the number of resonances due to methylene (two) and phenyl protons (one each) is consistent with D(2)(d)() symmetry, and therefore an effective perpendicular orientation of the axial ligands on the time scale of the NMR experiments. The temperature dependences of the (1)H resonances of these complexes show significant deviations from Curie behavior, and also evidence of extensive ligand exchange and rotation. Spectral assignment of the eight methylene resonances of the bis-(2-MeImH) complex to the four ethyl groups was possible through the use of 2D (1)H NMR techniques. The complex is fluxional, even at -90 degrees C, and ROESY data suggest that the predominant process is saddle inversion accompanied by simultaneous rotation of the axial ligands. Saddle inversion becomes slow on the 2D NMR time scale as the temperature is lowered in the ligand order of N-MeIm > 4-NMe(2)Py > 2-MeImH, probably due mainly to progressive destabilization of the ground state rather than progressive stabilization of the transition state of the increasingly "hindered" bis-ligand complexes.

Intramolecular rearrangements in six-coordinate ruthenium and iron dihydrides

Molecular orbital calculations at the ab initio level are used to study polytopal rearrangements in H2Ru(PH3)4 and H2Fe(CO)4 as models of 18-electron, octahedral metal dihydrides. It is found that, in both cases, the transition state for these rearrangements is a dihydrogen species. For H2Fe(CO)4, this is a square pyramidal complex where the H2 ligand occupies an apical position and is rotated by 45 degrees from its original orientation. This is precisely analogous to the transition state for Fe-olefin rotation in (olefin)Fe(CO)4 complexes and has a very similar electronic origin. Another transition state very close in energy is found wherein the basic coordination geometry is a trigonal bipyramid and the H2 ligand is coordinated in the axial position. For H2Ru(PH3)4, the former stationary point lies at a much higher energy and the latter clearly serves as the transition state for hydride exchange. The reason for this difference is discussed along with the roles of electron correlation in the two compounds.

Synthesis and characterization of iridium 1,3,5-triaza-7-phosphaadamantane (PTA) complexes. X-ray crystal and molecular structures of [Ir(PTA)4(CO)]Cl and [Ir(PTAH)3(PTAH2)(H)2]Cl6

The first 1,3,5-triaza-7-phosphaadamantane (PTA) ligated iridium compounds have been synthesized. The reaction of PTA with [Ir(COD)Cl]2 (COD = 1,5-cyclooctadiene) under a CO atmosphere produces an inseparable mixture of [Ir(PTA)3(CO)Cl] (1) and the PTA analogue of Vaska's compound, [Ir(PTA)2(CO)Cl] (2). Compound 1 and [Ir(PTA)4(CO)]Cl (3) were prepared via ligand substitution reactions of PTA with Vaska's compound, trans-Ir(PPh3)2(CO)Cl, in absolute and 95% ethanol, respectively. Complex 3 crystallizes in the orthorhombic space group Pbca with a = 20.3619(4) A, b = 14.0345(3) A, c = 24.1575(5) A, and Z = 8. Single-crystal X-ray diffraction studies show that 3 has a trigonal bipyramidal structure in which the CO occupies an axial position. This is the first crystallographically characterized [IrP4(CO)]+ complex in which the CO is axially ligated. Compound 1 was converted into 3 by ligand substitution with 1 equiv of PTA in water. Interestingly, the reaction of 3 with excess NaCl did not result in the production of 1, but instead the formation of the dichloro species, [Ir(PTAH)2(PTA)2Cl2]Cl3 (4) (PTAH = protonated PTA). Dissolution of 1 or 3 in dilute HCl produced 4 and a dihydrido species, [Ir(PTAH)4(H)2]Cl5 (5), which were readily separated by inspection due to their different crystal habits. Compound 5 crystallizes in the triclinic space group P1 with a = 12.4432(9) A, b = 12.5921(9) A, c = 16.3231(12) A, alpha = 76.004(1) degrees, beta = 71.605(1) degrees, gamma = 69.177(1) degrees, and Z = 2. Complex 5 exhibits a distorted octahedral geometry with two hydride ligands in a cis configuration. A rationale consistent with these reactions is presented by consideration of the steric and electronic properties of the PTA ligand.

N-porphyrinylamino and -amido compounds by addition of an amino or amido nitrogen to a porphyrin meso position

This report describes the synthesis and characterization of a series of octaethylporphyrin derivatives in which the porphyrin pi-network is connected to phenyl, 3-fluoranthenyl, or 1-pyrenyl aromatic systems through a meso amino or amido nitrogen. Metal-free bases and zinc(II) and iron(III) complexes have been obtained. These compounds represent the first examples of linkages between porphyrins and extended pi-networks through a nitrogen atom directly attached to a porphyrin meso position. 1H NMR studies of the metal-free bases and zinc complexes showed that in the amido-linked adducts, the plane containing the aryl substituent was oriented perpendicular to the plane of the porphyrin. Linkage through the secondary amino nitrogen, however, allowed the aryl plane to rotate toward coplanarity with the porphyrin plane, resulting in conjugation of the highest occupied aryl and porphyrin molecular orbitals through the nitrogen lone pair. In developing routes to the amino-linked compounds, the facile formation of fused azaaryl chlorins via an oxidative intramolecular cycloaddition was observed. An aryl carbon ortho to the meso linkage attacked the beta-carbon of an adjacent pyrrole ring, accompanied by 1,2-migration of a pyrrole beta-ethyl substituent and a two-electron oxidation of the initially formed macrocycle. The resulting structures are analogous to benzochlorins. The electronic spectra of the metal-free bases are characterized by intense, long-wavelength bands in the visible region. Molecular structures of the chloroferric complexes of the azabenzofluorantheno- and azabenzpyrenoporphyrin macrocycles (derived from fusion of the fluoranthenyl and pyrenyl substituents, respectively) were obtained by X-ray diffraction. The porphyrin moiety in the azabenzofluoranthenoporphyrin adopted a gable structure, with a 22 degrees fold along a diagonal including the pyrrole-ring C4 and C16 alpha-carbons. By contrast, the azabenzpyrenoporphyrin was virtually planar.

Role of the Invariant Peptide Fragment Forming NH.S Hydrogen Bonds in the Active Site of Cytochrome P-450 and Chloroperoxidase: Synthesis and Properties of Cys-Containing Peptide Fe(III) and Ga(III) (Octaethylporphinato) Complexes as Models

The primary sequence of Cys-X-Gly-Y- (X, hydrophobic residue; Y, hydrophilic residue) is highly conserved in cytochrome P-450s. The amide NHs of Leu, Gly, and X are assumed to form NH.S hydrogen bonds which are also found in the active site fragment, Cys-Pro-Ala-Leu, of chloroperoxidase (CPO). [Fe(III)(OEP)(Z-cys-Leu-Gly-Leu-OMe)] (OEP, octaethylporphinato; Z, benzyloxycarbonyl) and [Fe(III)(OEP)(Z-cys-Pro-Ala-Leu-OMe)] were synthesized as P-450 and CPO model complexes containing the invariant amino acid fragment of the active site. The corresponding gallium(III) complexes were also synthesized to investigate the solution structures using two-dimensional (2D) NMR experiments because the Ga(III) ion is similar to the Fe(III) ion in the ionic radii and in the coordination geometry. The solution structures of the peptide part of the gallium complexes indicate that the invariant fragments maintain a beta I-turn-like conformation and then form NH.S hydrogen bonds between S(gamma)Cys and NH of the third and fourth amino acid residues. The hydrogen bonds have also been confirmed by the (2)H NMR spectra of N(2)H-substituted Fe(III) peptide complexes. The Fe(III)/Fe(II) redox potentials of the Fe(III) complexes indicate that the NH.S hydrogen bonds in the fragments causes a slight positive shift of the redox potential. The tri- and tetrapeptide Fe(III) complexes containing the invariant fragments of P-450 are kinetically stable at 30 degrees C in CH(2)Cl(2). In contrast, [Fe(III)(OEP)(Z-cys-Leu-OMe)] decomposed to give [Fe(II)(OEP)] (22%) and the corresponding disulfide immediately in CD(2)Cl(2) at 30 degrees C for 1 h. These results indicate that the invariant fragments involving the hydrogen bonds cause the stabilization of the high-spin Fe(III) resting state rather than the positive shift of Fe(III)/Fe(II) redox potential.

Iron Complexes Containing the Tripodal Tetraphosphine Ligand P(CH(2)CH(2)PMe(2))(3)

The preparation and characterization of iron(II) complexes containing the tripodal tetraphosphine ligand tris[2-(dimethylphosphino)ethyl]phosphine, P(CH(2)CH(2)PMe(2))(3) (PP(3), 1), are reported. The complex FeCl(2)(PP(3)) (2) was formed by the reaction of PP(3) with anhydrous iron(II) chloride. The complexes FeHCl(PP(3)) (3) and FeH(2)(PP(3)) (4) were formed by the reaction of 2 with lithium aluminum hydride. Likewise, the complexes FeMeCl(PP(3)) (5) and FeMe(2)(PP(3)) (6) were formed by the reaction of 2 with methyllithium or dimethylmagnesium. Reaction of 2 with CO afforded a mixture of isomeric carbonyl chloride complexes [Fe(CO)Cl(PP(3))](+) (7 and 8). Reaction of 2 with PPh(3) afforded [Fe(PPh(3))Cl(PP(3))](+) (9). The air-sensitive complexes 2-9 were characterized by multinuclear NMR spectroscopy, and 9 was characterized by X-ray crystallography. Crystals of 9 (BPh(4) salt), C(54)H(65)BClFeP(5), M 971.09, are monoclinic, space group P2(1)/c, a = 13.246(3) Å, b = 30.314(2) Å, c = 14.338(2) Å, beta = 100.92(2) degrees, Z = 4; R = 0.058.