Joseph H. Reibenspies

Carbon Monoxide and Cyanide Ligands in a Classical Organometallic Complex Model for Fe‐Only Hydrogenase

The Fe(I) organometallic complex [(µ-SCH(2)CH(2)CH(2)S)Fe(2)(CO)(6)] provides a structural model for the cyano-carbonyl diiron site of Fe-only hydrogenase as characterized by X-ray crystallography (the picture shows the structure (black) of the model overlaid with that of the Fe-Fe dimetallic site in the hydrogenase isolated from Desulfovibrio desulfuricans). Cyanide substitution of CO occurs readily and provides spectroscopic references for the active site.

Possible Steric Control of the Relative Strength of Chelation Enhanced Fluorescence for Zinc(II) Compared to Cadmium(II): Metal Ion Complexing Properties of Tris(2-quinolylmethyl)amine, a Crystallographic, UV−Visible, and Fluorometric Study

The idea is examined that steric crowding in ligands can lead to diminution of the chelation enhanced fluorescence (CHEF) effect in complexes of the small Zn(II) ion as compared to the larger Cd(II) ion. Steric crowding is less severe for the larger ion and for the smaller Zn(II) ion leads to Zn-N bond length distortion, which allows some quenching of fluorescence by the photoinduced electron transfer (PET) mechanism. Some metal ion complexing properties of the ligand tris(2-quinolylmethyl)amine (TQA) are presented in support of the idea that more sterically efficient ligands, which lead to less M-N bond length distortion with the small Zn(II) ion, will lead to a greater CHEF effect with Zn(II) than Cd(II). The structures of [Zn(TQA)H(2)O](ClO(4))(2).1.5 H(2)O (1), ([Pb(TQA)(NO(3))(2)].C(2)H(5)OH) (2), ([Ag(TQA)(ClO(4))]) (3), and (TQA).C(2)H(5)OH (4) are reported. In 1, the Zn(II) is 5-coordinate, with four N-donors from the ligand and a water molecule making up the coordination sphere. The Zn-N bonds are all of normal length, showing that the level of steric crowding in 1 is not sufficient to cause significant Zn-N bond length distortion. This leads to the observation that, as expected, the CHEF effect in the Zn(II)/TQA complex is much stronger than that in the Cd(II)/TQA complex, in contrast to similar but more sterically crowded ligands, where the CHEF effect is stronger in the Cd(II) complex. The CHEF effect for TQA with the metal ions examined varies as Zn(II) >> Cd(II) >> Ni(II) > Pb(II) > Hg(II) > Cu(II). The structure of 2 shows an 8-coordinate Pb(II), with evidence of a stereochemically active lone pair, and normal Pb-N bond lengths. In 3, the Ag(I) is 5-coordinate, with four N-donors from the TQA and an oxygen from the perchlorate. The Ag(I) shows no distortion toward linear 2-coordinate geometry, and the Ag-N bonds fall slightly into the upper range for Ag-N bonds in 5-coordinate complexes. The structure of 4 shows the TQA ligand to be involved in pi-stacking between quinolyl groups from adjacent TQA molecules. Formation constants determined by UV-visible spectroscopy are reported in 0.1 M NaClO(4) at 25 degrees C for TQA with Zn(II), Cd(II), and Pb(II). When compared with other similar ligands, one sees that, as the level of steric crowding increases, the stability decreases most with the small Zn(II) ion and least with the large Pb(II) ion. This is in accordance with the idea that TQA has a moderate level of steric crowding and that steric crowding increases for TQA analogs tris(2-pyridylmethyl)amine (TPyA) < TQA < tris(6-methyl-2-pyridyl)amine (TMPyA).

Synthetic Support of De Novo Design: Sterically Bulky [FeFe]‐Hydrogenase Models

A twisted mimic: Upon oxidation of [(μ-SCH2C(CH3)2CH2S-){FeI(CO)2PMe3}2], rearrangement yields the mixed-valent FeIFeII cation in a square-pyramid/inverted square-pyramid geometry with a semibridging CO ligand, closely mimicking the [FeFe] hydrogenase enzyme active site. According to de novo design principles, the steric effect of bridgehead bulk in the S–S bridging ligand stabilizes this structure in the absence of the protein matrix.

A Cyclodextrin Host/Guest Approach to a Hydrogenase Active Site Biomimetic Cavity

The hydrophobic cavity of the active site of [FeFe]-Hydrogenase is mimicked by two cyclodextrin molecules which surround a 2Fe2S synthetic analogue of the active site, outfitted with an aryl sulfonate to promote inclusion into beta-cyclodextrin. The X-ray crystal structure of the clathrate shows an increased torsion angle between the apical CO ligands indicating that the supramolecular cage destabilizes the eclipsed geometry typical of diiron model complexes. Inclusion in the model second coordination sphere also has important effects on the electrochemical properties of the model complex including an approximately 80 mV shift of the Fe(I)Fe(I)/Fe(I)Fe(0) reduction and a change in the potential at which electrocatalytic reduction of protons by the diiron complex occurs.

Mechanistic Studies of the Copolymerization Reaction of Oxetane and Carbon Dioxide to Provide Aliphatic Polycarbonates Catalyzed by (Salen)CrX Complexes

Chromium salen derivatives in the presence of anionic initiators have been shown to be very effective catalytic systems for the selective coupling of oxetane and carbon dioxide to provide the corresponding polycarbonate with a minimal amount of ether linkages. Optimization of the chromium(III) system was achieved utilizing a salen ligand with tert-butyl groups in the 3,5-positions of the phenolate rings and a cyclohexylene backbone for the diimine along with an azide ion initiator. The mechanism for the coupling reaction of oxetane and carbon dioxide has been studied. Based on binding studies done by infrared spectroscopy, X-ray crystallography, kinetic data, end group analysis done by (1)H NMR, and infrared spectroscopy, a mechanism of the copolymerization reaction is proposed. The formation of the copolymer is shown to proceed in part by way of the intermediacy of trimethylene carbonate, which was observed as a minor product of the coupling reaction, and by the direct enchainment of oxetane and CO 2. The parity of the determined free energies of activation for these two processes, namely 101.9 kJ x mol (-1) for ring-opening polymerization of trimethylene carbonate and 107.6 kJ x mol (-1) for copolymerization of oxetane and carbon dioxide supports this conclusion.

Enantioselective Hydrogenations of Arylalkenes Mediated by [Ir(cod)(JM‐Phos)]+ Complexes

Phosphine oxazoline ligands la-j were converted to the corresponding [Ir(cod)(phosphine oxazoline)]+ complexes 2a-j. X-ray diffraction analyses of complexes 2b, 2h, 2i, and 2j were performed. The tert-butyl-, 1,1-diphenylethyl-, and phenyl-oxazoline complexes (2b, 2h, and 2i, respectively) had typical square planar metal environments with chair-like metallocyclic rings. However, the 3,5-di-tert-butylphenyl oxazoline complex 2j was distorted toward a tetrahedral metal geometry. This library of complexes was tested in asymmetric hydrogenations of several arylalkenes. High enantioselectivities and conversions were observed for some substrates. A possible special role for the HPh2C-oxazoline substituent in asymmetric hydrogenations was identified and is discussed. In attempts to rationalize why high enantioselectivities were not observed for some alkenes, a series of deuterium labeling experiments were performed to probe for competing reactions that occurred prior to the hydrogenation step. Double bond migrations were inferred for several substrates, and this is a significant complication in asymmetric hydrogenations of arylalkenes that had not been discussed prior to this study. A mechanistic rationale is proposed involving competing double bond migration for some but not all substrates. Appreciation of this complication will be valuable in further studies aimed at optimization of enantioselection in asymmetric hydrogenations of unfunctionalized alkenes.

Water-soluble organometallic compounds. 5. The regio-selective catalytic hydrogenation of unsaturated aldehydes to saturated aldehydes in an aqueous two-phase solvent system using 1,3,5-triaza-7-phosphaadamantane complexes of rhodium☆

The water-soluble phosphine complex of Rh(I), [Rh(PTAH)(PTA)2Cl]Cl(1), has been synthesized from RhCl3 and 1,3,5,-triaza-7-phosphaadamantane (PTA) in 96% ethanol. This complex is an effective catalyst for the regioselective reduction of unsaturated aldehydes to saturated aldehydes. The rate of hydrogenation of trans-cinnamaldehyde with sodium formate as reductant was studied as a function of catalyst, substrate, and sodium formate concentration. The presence of an excess of PTA was found to inhibit hydrogenations completely. This reaction was also found to be partially inhibited by cyclo-octatetraene and Hg(0), leading to the conclusion that both heterogenous and homogenous mechanisms are operating. Recycling experiments show complex 1 to be quite robust, with minimal leaching into the organic phase in a biphasic system. The complex resulting from the addition of an excess of PTA to 1, [Rh(PTAH)3(PTA)Cl]Cl3 (2) has been prepared and structurally characterized by a single crystal X-ray diffraction study. 31P and 1H NMR, IR and UV-VIS spectroscopy and pH titrametric measurements were employed to study the reactivity of the various rhodium PTA complexes in aqueous solutions.

Efficient and controlled polymerization of lactide under mild conditions with a sodium-based catalyst

A common phenolic antioxidant provides the ligand scaffold in the first discrete sodium-based catalyst for the highly active and controlled ring-opening polymerization of lactide.

Structural and Spectroscopic Features of Mixed Valent FeIIFeI Complexes and Factors Related to the Rotated Configuration of Diiron Hydrogenase

The compounds of this study have yielded to complementary structural, spectroscopic (Mössbauer, EPR/ENDOR, IR), and computational probes that illustrate the fine control of electronic and steric features that are involved in the two structural forms of (μ-SRS)[Fe(CO)2PMe3]2(0,+) complexes. The installation of bridgehead bulk in the -SCH2CR2CH2S- dithiolate (R = Me, Et) model complexes produces 6-membered FeS2C3 cyclohexane-type rings that produce substantial distortions in Fe(I)Fe(I) precursors. Both the innocent (Fc(+)) and the noninnocent or incipient (NO(+)/CO exchange) oxidations result in complexes with inequivalent iron centers in contrast to the Fe(I)Fe(I) derivatives. In the Fe(II)Fe(I) complexes of S = 1/2, there is complete inversion of one square pyramid relative to the other with strong super hyperfine coupling to one PMe3 and weak SHFC to the other. Remarkably, diamagnetic complexes deriving from isoelectronic replacement of CO by NO(+), {(μ-SRS)[Fe(CO)2PMe3] [Fe(CO)(NO)PMe3](+)}, are also rotated and exist in only one isomeric form with the -SCH2CR2CH2S- dithiolates, in contrast to R = H ( Olsen , M. T. ; Bruschi , M. ; De Gioia , L. ; Rauchfuss , T. B. ; Wilson , S. R. J. Am. Chem. Soc. 2008 , 130 , 12021 -12030 ). The results and redox levels determined from the extensive spectroscopic analyses have been corroborated by gas-phase DFT calculations, with the primary spin density either localized on the rotated iron in the case of the S = 1/2 compound, or delocalized over the {Fe(NO)} unit in the S = 0 complex. In the latter case, the nitrosyl has effectively shifted electron density from the Fe(I)Fe(I) bond, repositioning it onto the spin coupled Fe-N-O unit such that steric repulsion is sufficient to induce the rotated structure in the Fe(II)-{Fe(I)((•)NO)}(8) derivatives.

A new type of insulated molecular wire: a rotaxane derived from a metal-capped conjugated tetrayne.

The platinum butadiynyl complex trans-(C(6)F(5))(p-tol(3)P)(2)Pt(C≡C)(2)H and a CuI adduct of a 1,10-phenanthroline based 33-membered macrocycle react in the presence of K(2)CO(3) and I(2) or O(2) to give a rotaxane (ca. 9%) in which the macrocycle is threaded by the sp carbon chain of trans,trans-(C(6)F(5))(p-tol(3)P)(2)Pt(C≡C)(4)Pt(Pp-tol(3))(2)(C(6)F(5)). The crystal structure and macrocycle/axle electronic interactions are analyzed in detail.