John L. Hubbard

Synthesis and structural investigation of two potential boronate affinity chromatography ligands catechol [2-(diisopropylamino)carbonyl]phenylboronate and catechol [2-(diethylamino)carbonyl, 4-methyl]phenylboronate

Two potential boronate affinity chromatography ligands, catechol [2-(diisopropylamino)carbonyl]phenylboronate (I) and catechol [2-(diethylamino)carbonyl,4-methyl]phenylboronate (II) were synthesized by directed ortholithiation followed by boronation. Single crystal X-ray analyses of compounds I and II demonstrated an internal coordination bond between the boron atom and the carbonyl oxygen atom, rendering the boron atom environment to be tetrahedral. In addition, 1BMR data also indicated that the boron environment is tetrahedral. The coordinated carbonyl oxygen-B bond length is 1.556(9) A compared to an average BO bond length of 1.47 A to the catechol ligand. They are ideal models of a new type of ligands to study boronate affinity chromatography because they may esterify with catechols at neutral pH conditions.

The Jahn–Teller effect in the photoelectron spectrum of iron pentacarbonyl

High quality photoelectron spectra of gaseous Fe(CO)5 excited by HeI, HeII, and ArI photons have been obtained. Major attention is focused on the primarily metal 3d ionizations, which occur in the binding energy region from 7 to 11 eV. Ionization to the 2E′ positive ion state (centered at 8.6 eV) clearly shows the effects of Jahn–Teller distortions in the positive ion. This ionization results in two ionization bands of approximately equal intensity and shape separated by 0.38 eV at room temperature. These bands broaden and the splitting increases to 0.47 eV at 473 °K. Ionization to the 2E″ positive ion state, centered at 9.9 eV, is much less influenced by the Jahn–Tellar effect. There is no discernable splitting of this band at room temperature. These observations are discussed in terms of the electronic structure and bonding of Fe(CO)5. Simple model calculations of the energies of the doubly degenerate electronic states in relation to the appropriate doubly degenerate normal vibrational modes are used to...

Formation and reactivity of the chloromethyl π-allyl complex (η5-C5Me5)Ru(η3-C3H5)(CH2Cl)Cl

Treatment of (η5-C5Me5)Ru(η3-C3H5)Cl2 in CH2Cl2 with ethereal diazomethane in the presence of copper powder produces (η5-C5Me5)Ru(η3-C3H5)(CH2Cl)Cl in moderate yield with no detectable formation of the bis(chloromethyl) derivative. Deuterium labeling with CD2N2 shows no methylene scrambling into the allyl ligand. Evidence from NMR spectroscopy supports an endo allyl conformation in the chloromethyl complex. Mass spectroscopy experiments indicate that the d4 (RuIV) metal center is relatively ineffective in stabilizing the (η5-C5Me5)Ru(η3-C3H5) ( = CH2)Cl]+ fragment. The new chloromethyl complex reacts under photochemical conditions to give polymethylene with the regeneration of the starting dichloride. No intramolecular transfer of methylene to the allyl or (η5-C Me5) ligands is observed.

Facile conversion of (η5-C5R5) M(CO)2-halide complexes to halomethyl, alkoxymethyl, and cyanomethyl derivatives (R H, CH3; M Fe, Ru; halide Cl, Br, I)

The CpM(CO)2-X complexes (M  Fe, Ru; X  Cl, Br; Cp  η5-C5H5, Cp★  η5-C5Me5) are completely converted to the corresponding halomethyl derivatives over a 20–30 min period when ethereal diazomethane is added dropwise in the presence of Cu powder. Product work-up involves only simple extraction of the crude product with hexane followed by recrystallization at −40°C. Formation of iodomethyl derivatives from iodide precursors requires considerably longer CH2N2 addition times and cannot be completely freed of the starting iodide complexes. Fortunately, iodomethyl complexes can be prepared in 80–95% isolated yield by treating the chloromethyl or bromomethyl derivatives with NaI in acetone/Et2O. The CpFe(CO)2CH2I and Cp★Fe(CO)2CH2I complexes are the most sensitive, decomposing rapidly to polymethylene and the parent iodide complexes upon standing at room temperature. Metathetical reactions of the halomethyl complexes to give alkoxymethyl and cyanomethyl derivatives are described.

The electronic structure of nitrosyl and carbonyl supported metal-metal interactions. The photoelectron spectra of [η5-(C5H5)Fe(μ-NO)]2, [η5-(C5(CH3)5)Fe(μ-NO)]2, [η5-(C5H5)Ru(μ-NO)]2 and [η5-(C5(CH3)5)Co(μ-CO)]2

Abstract The valence photoelectron ionizations of[CpFe(μ-NO)] 2 , [Cp*Fe(μ-NO)] 2 , [CpRu(μ-NO)] 2 and [Cp*Co(μ-CO)] 2 (Cp = η 5 -C 5 H 5 and Cp* = η 5 -C 5 (CH 3 ) 5 ) are examined in comparison to several theoretical calculations of the electronic structure and bonding in these complexes. The photoelectron spectra of this group of complexes, when collected together, allow identification of the eight valence metal-based ionizations. There is considerable disparity among the various calculations on the predicted order of these ionizations. The combination of Fenske—Hall calculations with experimental observations of ionization trends between isoelectronic first row complexes [Fe(NO) vs Co(CO)], between first and second row complexes (Fe vs Ru), and between complexes with ring methylation (Cp vs Cp*) allows a consistent assignment of the valence ionizations. The interplay of theory and experiment gives unique insight into the nature of bridging ligand coordination and clarifies the relative strengths of metal—ligand and metal—metal interactions, in each complex. It is found that as many metal—metal antibonding orbitals are occupied as metal—metal bonding orbitals, so that the formal metal—metal bond order is zero. The interactions are important in defining the bonding and stability of the complexes. These interactions lead to large ligand character in two of the valence ionizations.

The structure of crystalline trans-dichlorobis(triphenylphosphine)rhodium(II), a square planar rhodium(II) monomer: isolation of the proposed paramagnetic impurity in Wilkinson's catalyst

trans-Dichlorobis(triphenylphosphine)rhodium(II), a square planar rhodium(II) monomer, has been isolated and characterized spectroscopically and crystallographically.