Open Coordination Site Research Articles

New Oxorhenium(V) Compound for Catalyzed Oxygen Atom Transfer from Picoline N-Oxide to Triarylphosphines

The synthesis and characterization of a new oxorhenium(V) compound is reported; it is [MeReO(edt)(bpym)], 8, where edt = 1,2-ethanedithiolate and bpym = 2,2'-bipyrimidine. Compound 8 was characterized by NMR spectroscopy and single-crystal X-ray analysis. It exists as a six-coordinate Re(V) compound comparable to the previously known [MeReO(edt)(bpy)] and [MeReO(mtp)(bpy)]. Compound 8 catalyzes the oxygen-atom-transfer reaction PicO + PZ3 --> Pic + Z3PO, whereas the other two do not. The kinetics of this reaction with catalyst 8 follows the rate law -d[PicO]/dt = k[8][PicO]/(1 + c[PZ3]). With different phosphines, the rate law has the same k value, 4.17 L mol(-1) s(-1), but different c values. For tritolylphosphine, c = 67.5 L mol(-1) in benzene at 25 degrees C. A mechanism has been proposed to account for these findings. The data establish that an open coordination site on rhenium is necessary for oxygen-atom-transfer reactions.

New Tetranuclear Ruthenium(I) Carbonyl Trifluoroacetate Complex: Gas-Phase Transformations and Coordination Reactions

A new mixed ligand ruthenium(I) complex of the composition [Ru2(O2CCF3)2(CO)5] (1) has been prepared by refluxing Ru3(CO)12 with trifluoroacetic acid in a dichloromethane/benzene mixture. Crystals of 1 were obtained by gas phase sublimation of the crude product at 110°C. The X-ray diffraction study has revealed a tetranuclear `8dimer of dimers' 9 structure [Ru2(O2CCF3)2(CO)5]2 in 1. Complex 1 can be re-sublimed without decomposition at temperatures up to 130°C, while at higher temperatures fragmentation accompanied by a ligand re-distribution reaction has been observed. As a result, two new ruthenium(I) complexes have been isolated from the gas phase transformation of 1 at 160°C: [Ru2(O2CCF3)2(CO)6] (2) and [Ru2(O2CCF3)2(CO)4] (3). Complex 2 has a dinuclear cis-trifluoroacetato-bridged core, while 3 exhibits a polymeric structure built on axial Ru···O interactions of the dimetal units, [Ru2(O2CCF3)2(CO)4]∞. The above reaction suggests the possible gas phase dissociation of the tetranuclear molecule 1 to the dimetal fragments, [Ru2(O2CCF3)2(CO)5] that have one open axial coordination site. The latter was proved by codeposition of 1 with an aromatic hydrocarbon, [2. 2]paracyclophane, that afforded a new sandwich compound, [Ru2(O2CCF3)2(CO)5·(μ2-C16H16)·Ru2(O2CCF3)2(CO)5] (4), in which the ligand is entrapped between two dimetal complexes. In contrast, when 3 is codeposited with [2. 2]paracyclophane, a new 1D polymeric product, [Ru2(O2CCF3)2(CO)4·(μ2-C16H16)]∞ (5) has been isolated. Complexes 1–5 have been fully characterized by IR and NMR spectroscopy, as well as by X-ray diffraction.

Synthesis, Structure, Theoretical Studies, and Ligand Exchange Reactions of Monomeric, T-Shaped Arylpalladium(II) Halide Complexes with an Additional, Weak Agostic Interaction

A series of monomeric arylpalladium(II) complexes LPd(Ph)X (L = 1-AdPtBu2, PtBu3, or Ph5FcPtBu2 (Q-phos); X = Br, I, OTf) containing a single phosphine ligand have been prepared. Oxidative addition of aryl bromide or aryl iodide to bis-ligated palladium(0) complexes of bulky, trialkylphosphines or to Pd(dba)2 (dba = dibenzylidene acetone) in the presence of 1 equiv of phosphine produced the corresponding arylpalladium(II) complexes in good yields. In contrast, oxidative addition of phenyl chloride to the bis-ligated palladium(0) complexes did not produce arylpalladium(II) complexes. The oxidative addition of phenyl triflate to PdL2 (L = 1-AdPtBu2, PtBu3, or Q-phos) also did not form arylpalladium(II) complexes. The reaction of silver triflate with (1-AdPtBu2)Pd(Ph)Br furnished the corresponding arylpalladium(II) triflate in good yield. The oxidative addition of phenyl bromide and iodide to Pd(Q-phos)2 was faster than oxidative addition to Pd(1-AdPtBu2)2 or Pd(PtBu3)2. Several of the arylpalladium complexes were characterized by X-ray diffraction. All of the arylpalladium(II) complexes are T-shaped monomers. The phenyl ligand, which has the largest trans influence, is located trans to the open coordination site. The complexes appear to be stabilized by a weak agostic interaction of the metal with a ligand C-H bond positioned at the fourth-coordination site of the palladium center. The strength of the Pd.H bond, as assessed by tools of density functional theory, depended upon the donating properties of the ancillary ligands on palladium.

Crystal Structures and Intercalation Reactions of Three‐Dimensional Coordination Polymers [M(H2O)2]2 [Mo(CN)8]×4H2O (M: Co, Mn).

AbstractFor Abstract see ChemInform Abstract in Full Text.

Crystal Structures and Intercalation Reactions of Three‐Dimensional Coordination Polymers [M(H2O)2]2[Mo(CN)8]·4H2O (M = Co, Mn)

AbstractTwo coordination polymers [MII(H2O)2]2[MoIV(CN)8]·4H2O [MII = Co (1), Mn (2)] were prepared as monocrystals and structurally characterised. These compounds crystallise in the tetragonal systems I4/m (1) and I4/mcm (2) and form three‐dimensional networks containing channels occupied by two types of water molecules, i.e. coordinated to MII and crystallised ones. The dehydration of these materials occurs topochemically to give [MII(H2O)]2[MoIV(CN)8] compounds having an open metal coordination site on the MII ions that may be accessible to coordination by other species and serve as a reaction site. The intercalation of water, ammonia and amines has been studied. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Gas-phase synthesis of charged copper and silver Fischer carbenes from diazomalonates: mechanistic and conformational considerations in metal-mediated wolff rearrangements.

Copper(I) and silver(I) Fischer carbenes are synthesized in the gas phase. Various diazomalonate-based compounds with an attached metal ion are introduced into the gas phase by electrospray ionization and subjected to collisional activation. Loss of N(2) generates a metastable Fischer carbene, which subsequently undergoes Wolff rearrangement and loss of CO. Further excitation leads to the loss of another CO molecule and the generation of a stable Fischer carbene. Isotopically labeled compounds are utilized to confirm the assignment of the products resulting from this process. DFT calculations are used to evaluate various mechanistic possibilities and to quantitatively assess the energetics of reactants and products. Silver(I) is shown to be more effective in facilitating Wolff rearrangement than copper(I), although both are more effective when compared to spectator charges such as sodium or a fixed quaternary nitrogen. Carbenes are not produced when copper(II), nickel(II), or a proton is used to form a quasi-molecular ion from the diazomalonate carbene precursor. Finally, trapping of the Fischer carbene by various functional groups attached through the open coordination site of the metal is investigated.

Effect of [PMDETA]/[Cu(I)] Ratio, Monomer, Solvent, Counterion, Ligand, and Alkyl Bromide on the Activation Rate Constants in Atom Transfer Radical Polymerization

A detailed study on the effect of [PMDETA]/[Cu(I)] ratio, monomer, solvent polarity, counterion, ligand, and alkyl bromide on the activation rate constant (kact) in ATRP was carried out. The highest values of kact for Cu(I)Br were obtained at [PMDETA]/[Cu(I)Br] ∼ 1/1 in more polar solvents and ascribed to a neutral [Cu(PMDETA)Br] structure of the complex. However, in less polar solvent mixtures a relatively fast reaction was observed already at the 0.5/1 ratio with the smaller rate increase up to [PMDETA]/[Cu(I)Br] ∼ 1/1. The highest values of kact for Cu(I)PF6 were observed at [PMDETA]/[Cu(I)Br] ∼ 1/1 in more polar, less polar solvent mixtures and monomer, which was explained by the formation of an ionic [Cu(PMDETA)]+PF6- complex. In both more polar and less polar media, the values of kact were slightely larger for PF6- than Br- counterion. However, in methyl acrylate, kact was 1.9 times larger for Br- than PF6- counterion. This was attributed to monomer coordination through the open coordination site of...

Methine (CH) transfer via a chlorine atom abstraction/benzene-elimination strategy: molybdenum methylidyne synthesis and elaboration to a phosphaisocyanide complex.

Methine (CH) transfer to an open coordination site was achieved in one pot by titanium(III) abstraction of Cl from 7-chloronorbornadiene, radical capture by Mo, and benzene extrusion. This efficient Mo methylidyne synthesis permitted elaboration to an anionic phosphaisocyanide derivative upon deprotonation, functionalization with dichlorophenylphosphine, and ultimate reduction.

Nitrosocyanin, a red cupredoxin-like protein from Nitrosomonas europaea.

Nitrosocyanin (NC), a soluble, red Cu protein isolated from the ammonia-oxidizing autotrophic bacterium Nitrosomonas europaea, is shown to be a homo-oligomer of 12 kDa Cu-containing monomers. Oligonucleotides based on the amino acid sequence of the N-terminus and of the C-terminal tryptic peptide were used to sequence the gene by PCR. The translated protein sequence was significantly homologous with the mononuclear cupredoxins such as plastocyanin, azurin, or rusticyanin, the type 1 copper-binding region of nitrite reductase, and the binuclear CuA binding region of N(2)O reductase or cytochrome oxidase. The gene for NC contains a leader sequence indicating a periplasmic location. Optical bands for the red Cu center at 280, 390, 500, and 720 nm have extinction coefficients of 13.9, 7.0, 2.2, and 0.9 mM(-1), respectively. The reduction potential of NC (85 mV vs SHE) is much lower than those for known cupredoxins. Sequence alignments with homologous blue copper proteins suggested copper ligation by Cys95, His98, His103, and Glu60. Ligation by these residues (and a water), a trimeric protein structure, and a cupredoxin beta-barrel fold have been established by X-ray crystallography of the protein [Lieberman, R. L., Arciero, D. M., Hooper, A. B., and Rosenzweig, A. C. (2001) Biochemistry 40, 5674-5681]. EPR spectra of the red copper center indicated a Cu(II) species with a g(parallel) of 2.25 and an A(parallel) of 13.8 mT (144 x 10(-4) cm(-1)), typical of Cu in a type 2 copper environment. NC is the first example of a type 2 copper center in a cupredoxin fold. The open coordination site and type 2 copper suggest a possible catalytic rather than electron transfer function.

Copper complexes of novel superbasic peralkylguanidine derivatives of tris(2-aminoethyl)amine as constraint geometry ligands.

The coordination chemistry of copper(I) and copper(II) ions with novel tripodal peralkylguanidine derivatives of the tris(2-aminoethyl)amine (tren) backbone TMG(3)tren (tetramethylguanidino-tren) N[CH(2)CH(2)N=C(NMe(2))(2)](3) (1) and cyclic DMPG(3)tren (dimethylpropyleneguanidino-tren) N[CH(2)CH(2)N=C[NMe(CH(2))(3)NMe]](3) (2) is reported. These sterically demanding ligands form complexes of constraint trigonal geometry. Their superbasic character with estimated pK(BH)+ values 6 orders of magnitude higher than that of the known Me(6)tren and their softer N-donor character compared to tert-amine ligands stabilize cationic mononuclear Cu(I) and Cu(II) ions by delocalization of charge into the guanidine functionalities. The crystal structures and spectroscopic features of two cationic copper(I) complexes with an uncommon trigonal-pyramidal [N(4)Cu](+) coordination sphere and a sterically protected open coordination site and of two cationic copper(II) complexes with the characteristic trigonal-bipyramidal coordination geometry [N(4)CuCl](+) and [N(5)Cu](2+) are reported.

How does heme axial ligand deletion affect the structure and the function of cytochrome b562?

We have recently generated a new mutant of cytochrome b(562) (cytb(562)) in which Met7, one of the axial heme ligands, is replaced by Ala (M7A cytb(562)). The M7A cytb(562) can bind heme and the UV-visible absorption spectrum is of a typical high-spin ferric heme. To investigate the effect of the lack of Met7 ligation on the structural integrity of cytb(562), thermal transition analyses of M7A cytb(562) were conducted. From the thermodynamic parameters obtained, it is concluded that the folding of M7A cytb(562) is comparable to the apoprotein despite the presence of heme. On the other hand, exogenous ligands such as cyanide and azide ions are readily bound to the heme iron, indicating that the axial coordination site is available for substrate binding. The peroxidase activity of this mutant is thus examined to evaluate new enzymatic function at this site and M7A cytb(562) was found to catalyze an oxidation reaction of aromatic substrates with hydrogen peroxide. These observations demonstrate that the Met7/His102 bis-ligation to the heme iron is crucial for the stable folding of cytb(562), whereas the functional conversion of cytb(562) is successfully achieved by the loose folding together with the open coordination site.

Modeling the active sites in metalloenzymes. 3. Density functional calculations on models for [Fe]-hydrogenase: structures and vibrational frequencies of the observed redox forms and the reaction mechanism at the Diiron Active Center.

Optimized structures for the redox species of the diiron active site in [Fe]-hydrogenase as observed by FTIR and for species in the catalytic cycle for the reversible H(2) oxidation have been determined by density-functional calculations on the active site model, [(L)(CO)(CN)Fe(mu-PDT)(mu-CO)Fe(CO)(CN)(L')](q)(L = H(2)O, CO, H(2), H(-); PDT = SCH(2)CH(2)CH(2)S, L' = CH(3)S(-), CH(3)SH; q = 0, 1-, 2-, 3-). Analytical DFT frequencies on model complexes (mu-PDT)Fe(2)(CO)(6) and [(mu-PDT)Fe(2)(CO)(4)(CN)(2)](2)(-) are used to calibrate the calculated CN(-) and CO frequencies against the measured FTIR bands in these model compounds. By comparing the predicted CN(-) and CO frequencies from DFT frequency calculations on the active site model with the observed bands of D. vulgaris [Fe]-hydrogenase under various conditions, the oxidation states and structures for the diiron active site are proposed. The fully oxidized, EPR-silent form is an Fe(II)-Fe(II) species. Coordination of H(2)O to the empty site in the enzyme's diiron active center results in an oxidized inactive form (H(2)O)Fe(II)-Fe(II). The calculations show that reduction of this inactive form releases the H(2)O to provide an open coordination site for H(2). The partially oxidized active state, which has an S = (1)/(2) EPR signal, is an Fe(I)-Fe(II) species. Fe(I)-Fe(I) species with and without bridging CO account for the fully reduced, EPR-silent state. For this fully reduced state, the species without the bridging CO is slightly more stable than the structure with the bridging CO. The correlation coefficient between the predicted CN(-) and CO frequencies for the proposed model species and the measured CN(-) and CO frequencies in the enzyme is 0.964. The proposed species are also consistent with the EPR, ENDOR, and Mössbauer spectroscopies for the enzyme states. Our results preclude the presence of Fe(III)-Fe(II) or Fe(III)-Fe(III) states among those observed by FTIR. A proposed reaction mechanism (catalytic cycle) based on the DFT calculations shows that heterolytic cleavage of H(2) can occur from (eta(2)-H(2))Fe(II)-Fe(II) via a proton transfer to "spectator" ligands. Proton transfer to a CN(-) ligand is thermodynamically favored but kinetically unfavorable over proton transfer to the bridging S of the PDT. Proton migration from a metal hydride to a base (S, CN, or basic protein site) results in a two-electron reduction at the metals and explains in part the active site's dimetal requirement and ligand framework which supports low-oxidation-state metals. The calculations also suggest that species with a protonated Fe-Fe bond could be involved if the protein could accommodate such species.

ESR and resonance Raman spectroscopic studies of peroxide intermediates derived from iron diazaoxacyclononane

A peroxide-Fe 3+ intermediate generation during the Fenton reaction of iron chelate involving a ligating N, N′-di-2-picolyl-4, 7-diaza-1-oxacyclononane (DPC), H 2O 2/[Fe 2+DPC] 2+, is reported. The identity of this peroxide complex is confirmed by resonance Raman (RR) and electron spin resonance (ESR) spectroscopies. The RR spectrum of [Fe 2+DPC] 2+ treated with H 2O 2 shows a frequency at 854 cm −1 ascribable to ν(OO) vibrational modes of the peroxide-Fe 3+ complex with a side-on geometry. On the other hand, the ESR spectrum of H 2O 2/[Fe 2+DPC] 2+ acquired at 77 K exhibits the resonance transition at g=2.196 and 2.017 due to the peroxide-Fe 3+ complex with an axial symmetry. It has been concluded that the H 2O 2/[Fe 2+DPC] 2+ reaction proceeds by rapid bonding of H 2O 2 to an open coordination site on the central Fe 2+ cation.

Synthesis and characterization of mixed-ligand oxorhenium complexes with the SNN type of ligand. Isolation of a novel ReO[SN][S][S] complex.

A new series of mixed-ligand oxorhenium complexes 4-9, with ligands 1-3 (L1H2) containing the SNN donor set and monodentate thiols as coligands (L2H), is reported. All complexes were synthesized using ReOCl3(PPh3)2 as precursor. They were isolated as crystalline products and characterized by elemental analysis and IR and NMR spectroscopy. The ligands 1 and 2 (general formula RCH2CH2NHCH2CH2SH, where R = N(C2H5)2 in 1 and pyrrolidin-1-yl in 2) act as tridentate SNN chelates to the ReO3+ core, leaving one open coordination site cis to the oxo group. The fourth coordination site is occupied by a monodentate aromatic thiol which acts as a coligand. Thus, three new "3 + 1" [SNN][S] oxorhenium complexes 4-6 (general formula ReO[RCH2CH2NCH2CH2S][SX], where R = N(C2H5)2 and X = phenyl in 4, R = N(C2H5)2 and X = p-methylphenyl in 5, and R = pyrrolidinlyl and X = p-methylphenyl in 6) were prepared in high yield. Complex 4 adopts an almost perfect square pyramidal geometry (tau = 0.07), while 6 forms a distorted square pyramidal geometry (tau = 0.24). In both complexes 4 and 6, the basal plane is formed by the SNN donor set of the tridentate ligand and the S of the monodentate thiol. On the other hand, the ligand 3, [(CH3)2CH]2NCH2CH2NHCH2CH2SH, acts as a bidentate ligand, probably due to steric hindrance, and it coordinates to the ReO3+ core through the SN atoms, leaving two open coordination sites cis to the oxo group. These two vacant positions are occupied by two molecules of the monodentate thiol coligand, producing a novel type of "2 + 1 + 1" [SN][S][S] oxorhenium mixed-ligand complexes 7-9 (general formula ReO[[(CH3)2CH]2NCH2CH2NHCH2CH2S][SX][SX], where X = phenyl in 7, p-methylphenyl in 8, and benzyl in 9). The coordination sphere about rhenium in 7 and 8 consists of the SN donor set of ligand 3, two sulfurs of the two monodentate thiols, and the doubly bonded oxygen atom in a trigonally distorted square pyramidal geometry (tau = 0.44 and 0.45 for 7 and 8, respectively). Detailed NMR assignments were determined for complexes 5 and 8.

Of Ketenes and Carbenes

CHEMISTRY A number of important synthetic reactions take advantage of the formation of a ketene (which contains a C=C=O group) from a carbene (containing M=C) and a carbonyl (M-CO) that are coordinated to the same metal center. Adding a new ligand augments the electronic sphere of the metal and allows the carbene and carbonyl to combine with each other. Grotjahn et al. now have observed this reaction running in reverse in a complex containing a single iridium atom. The key was finding a ligand that would create an open coordination site next to the ketene, in this case, the chelating diphosphine (tert-butyl)2PCH2P(tert-butyl)2. Removal of the chloride ligand through the formation of AgCl yielded the carbene-Ir-carbonyl complex. This reaction could be driven backwards to the ketene by addition of chloride. The synthesis of other analogs of this system should reveal insights into the steric and electronic factors that control this interconversion.— PDS J. Am. Chem. Soc., in press

Electronic Factors for Protonation of an Organometallic Molecule. Photoelectron Spectroscopy and Electron Paramagnetic Resonance Study of [(η6-C6H6)Mo(TRIPOD)]0/+

We have previously shown that the arene complex (η6-C6H6)Mo(TRIPOD), where TRIPOD = 1,1,1-tris((diphenylphosphino)methyl)ethane, is protonated by exo addition of H+ to the arene ring to give the transient cyclohexadienyl complex [(η5-C6H7)Mo(TRIPOD)]+, which eventually yields the thermodynamic molybdenum hydride [(η6-C6H6)Mo(TRIPOD)(H)]+. The present study is a combined experimental and theoretical investigation that reveals the fundamental basis for this mechanism. Photoelectron spectroscopy (PES) is used to probe the electronic structure of (η6-C6H6)Mo(TRIPOD) and the production of the [(η6-C6H6)Mo(TRIPOD)]+ cation in the gas phase. The initial ionizations of (η6-C6H6)Mo(TRIPOD) are from energetically closely spaced orbitals of predominantly metal d character (2A1 and 2E cation states using C3v symmetry) that are shifted over 2 eV to lower energy with respect to the comparable ionizations of (η6-C6H6)Mo(CO)3. The oxidized species [(η6-C6H6)Mo(TRIPOD)]+ is also prepared in solution by electrochemical means and through the use of chemical oxidants. The electron paramagnetic resonance (EPR) spectrum of this cation shows arene-proton hyperfine coupling that indicates substantial arene character in the highest occupied orbital. The photoelectron and EPR spectra both provide evidence for Jahn−Teller distortion of the 2E positive ion states. Electronic structure calculations show that this distortion involves opening of one L−Mo−L angle, which effectively creates an open coordination site on the metal for the hydride to occupy in the final thermodynamic product. These experimental and computational results show that, in terms of their energy, the e symmetry and a1 symmetry metal-based orbitals are similarly favored for oxidative protonation directly at the metal. The e symmetry orbital has a portion of its density on the arene ring, making access to this orbital by proton approach to the exo position of the arene ring possible. For (η6-C6H6)Mo(TRIPOD), exo attack at the arene is favored because the TRIPOD ligand shields the e symmetry orbital from direct attack at the metal by the solvated proton. Thus, exo attack is not initiated by proton interaction with an arene-based orbital but is initiated by proton interaction with the arene portion of the same e symmetry orbital that directs attack at the metal. Calculations predict low barriers for both direct attack at the metal and exo attack at the arene, with attack at the arene favored for longer metal−proton distances.

Synthesis and crystal structures of five new rhenium acetamidate complexes

Syntheses are reported of five rhenium(III) cluster complexes with acetamidate ligands: [(C 4H 9) 4N] 2[{Re 2Cl 6(μ-1,2-NHCOCH 3)} 2] ( 1), [(C 2H 5) 4N][Re 3Cl 8(μ-1,2-NHCOCH 3)(μ-2,3-NHCOCH 3)]·2.5(C 4H 10O) ( 2), [(C 4H 9) 4N] 2[Re 3Cl 10(μ-1,2-NHCOCH 3)] ( 3), [(C 4H 9) 4N] 2[{Re 3Cl 8(μ-1,2-NHCOCH 3)(OH)} 2]·1.7(C 4H 10O) ( 4), and (C 4H 9) 4N[Re 3Cl 9(H 2O)(μ-1,2-NHCOCH 3)]·C 3H 6O ( 5). The acetamidate ligands in these complexes arise via reactions of acetonitrile–rhenium halide species (either dirhenium or trirhenium chloride clusters) and water. Coordination activates the nitrile to hydrolysis; proton loss and chelation of the acetamidate ligand with displacement of chloride ion leads to the products. Initial formation of a rhenium(III) acetonitrile complex can be assisted using a halide acceptor, Ag +, to generate an open coordination site on the metal. The reaction with K 4Mo 2Cl 8 and Ag + ion in acetonitrile gives Mo 2Cl 4(NCCH 3) 4 ( 6). The use of MALDI mass spectrometry in support of synthetic efforts and as a characterization tool is described, and the molecular structures of 1– 6 were established using X-ray crystallography.

Synthesis, Characterization, and Reactivity of Palladium(II) Salen and Oxazoline Complexes.

Two methods for the syntheses of palladium(II) salen complexes are described. The first involves template synthesis of the ligand in which bis(salicylaldehydato)palladium(II), 2, is initially synthesized and reacted with the appropriate diamine bridge to yield the desired palladium salen. The template synthesis approach suffers, however, from low overall yields (17-30%). Alternatively, treatment of bis(acetonitrile)palladium(II) chloride with the appropriate salen ligand under inert atmosphere yields the palladium salen complex in high yields (80-85%). [2-(2'-Hydroxyphenyl)-2-oxazolinato]palladium(II), 7, was also synthesized. All the palladium complexes were fully characterized spectroscopically. A single-crystal X-ray structure of 7 has been solved. In contrast to previous reports, complex 2 was found to be stable over weeks and remained suitable for the template syntheses. The catalytic activity of complex 2 in cyclopropanation of alkenes with ethyl diazoacetate (EDA) was investigated. The catalyst is not functional group tolerant and works best for styrene; turnover numbers (TON's) of 45 were achieved. The availability of an open coordination site seems to be a prerequisite for catalytic decomposition of EDA.

Reversible Dioxygen Binding to Hemerythrin. 1. Electronic Structures of Deoxy- and Oxyhemerythrin

Studies on the electronic structure of the two physiologically relevant forms of hemerythrin (Hr), i.e., deoxyHr (possessing a hydroxo-bridged diferrous site) and oxyHr (having an oxo-bridged diferric site with a terminal hydroperoxide), are presented and discussed. SQUID magnetic susceptibility data for deoxyHr confirm that the two ferrous ions are weakly antiferromagnetically coupled, J = −14(2) cm-1 (ℋ = −2JS1·S2), indicating that at 300 K the entire manifold of ground-state spin sublevels is available for the reaction with O2. From density functional calculations on deoxyHr, evaluated on the basis of experimental data, the redox active orbital on the five-coordinate iron (Fe2) is favorably oriented for a π-bonding interaction with O2 approaching along the open coordination site of that center, and reorientation of the redox active orbital on the six-coordinate iron (Fe1) for electron transfer to O2 through superexchange with Fe2 is energetically accessible. Analysis of existing spectroscopic data for oxyHr using Heller's time-dependent theory leads to the proposal that the peculiar behavior of the UV resonance Raman excitation profile for the symmetric Fe−O−Fe stretching mode ν(Fe−O) to peak with fairly minor absorption features arises from interference effects between oxo-to-Fe charge-transfer excited states. From density functional calculations on oxyHr and related structures the low frequency of the Fe−oxo stretch is ascribed to the hydrogen bond between the hydroperoxide and the bridging oxide, whereas the small |J| value appears to be due primarily to the strong hydroperoxide → Fe2 π-donor interaction that reduces Fe1 → Fe2 electron delocalization.

Mechanistic and thermodynamic aspects of methylene transfer from CH2N2 to MHCl(CO)L2 (M=Ru, Os; L=tertiary phosphine): non-least motion behavior and extreme dependence on phosphine identity

Reaction of MHCl(CO)(PBut2Me)2 (M=Ru and Os) with CH2N2 was studied from -78 to 25°C, revealing first the formation of MHCl(CH2)(CO)(PBut2Me)2, where the carbene ligand CH2 occupies what was the open coordination site of MHCl(CO)(PBut2Me)2, which lies trans to the hydride. This intermediate then isomerizes to M(CH3)Cl(CO)(PBut2Me)2, below 25°C for each metal. The analogous reaction of MHCl(CO)(PPri3)2 with CH2N2 does indeed give MHCl(CH2)(CO)(PPri3)2, which then ‘decomposes’ unselectively; when M=Os, C2H4 and OsHCl(CO)(PPri3)2 are among the products. This extreme phosphine dependence is attributed to the H–MCH2 to M(CH3) isomerization requiring phosphine dissociation; the smaller PPri3 fails to dissociate at a rate competitive with alternative decomposition reactions.