Gerard Parkin

Agostic interactions in transition metal compounds

The impact of agostic interactions (i.e., 3-center–2-electron MHC bonds) on the structures and reactivity of organotransition metal compounds is reviewed.

Zinc Catalysts for On-Demand Hydrogen Generation and Carbon Dioxide Functionalization

[Tris(2-pyridylthio)methyl]zinc hydride, [κ(3)-Tptm]ZnH, is a multifunctional catalyst that is capable of achieving (i) rapid release of hydrogen by protolytic cleavage of silanes with either water or methanol and (ii) hydrosilylation of aldehydes, ketones, and carbon dioxide. For example, [κ(3)-Tptm]ZnH catalyzes the release of 3 equivalents of H(2) by methanolysis of phenylsilane, with a turnover number of 10(5) and a turnover frequency surpassing 10(6) h(-1) for the first 2 equivalents. Furthermore, [κ(3)-Tptm]ZnH also catalyzes the formation of triethoxysilyl formate by hydrosilylation of carbon dioxide with triethoxysilane. Triethoxysilyl formate may be converted into ethyl formate and N,N-dimethylformamide, thereby providing a means for utilizing carbon dioxide as a C(1) feedstock for the synthesis of useful chemicals.

Cleaving carbon-carbon bonds by inserting tungsten into unstrained aromatic rings.

The cleavage of C–H and C–C bonds by transition metal centres is of fundamental interest and plays an important role in the synthesis of complex organic molecules from petroleum feedstocks. But while there are many examples for the oxidative addition of C–H bonds to a metal centre, transformations that feature oxidative addition of C–C bonds are rare. The paucity of transformations that involve the cleavage of C–C rather than C–H bonds is usually attributed to kinetic factors arising from the greater steric hindrance and the directional nature of the spn hybrids that form the C–C bond, and to thermodynamic factors arising from the fact that M–C bonds are weaker than M–H bonds. Not surprisingly, therefore, most examples of C–C bond cleavage either avoid the kinetic limitations by using metal compounds in which the C–C bond is held in close proximity to the metal centre, or avoid the thermodynamic limitations by using organic substrates in which the cleavage is accompanied by either a relief of strain energy or the formation of an aromatic system. Here, we show that a tungsten centre can be used to cleave a strong C–C bond that is a component of an unstrained 6-membered aromatic ring. The cleavage is enabled by the formation of an unusual chelating di(isocyanide) ligand, which suggests that other metal centres with suitable ancillary ligands could also accomplish the cleavage of strong C–C bonds of aromatic substrates and thereby provide new ways of functionalizing such molecules.

Palladium complexes with Pd→B dative bonds: Analysis of the bonding in the palladaboratrane compound [κ4-B(mimBut)3]Pd(PMe3)

The dinuclear complex {[mu-kappa(1),kappa(3)-B(mim(Bu(t)))(3)]Pd}(2), which features a Pd-->B dative bond, may be obtained by the reaction of [Tm(Bu(t))]K with Pd(OAc)(2); treatment of {[mu-kappa(1),kappa(3)-B(mim(Bu(t)))(3)]Pd}(2) with PMe(3) affords the mononuclear boratrane derivative [kappa(4)-B(mim(Bu(t)))(3)]Pd(PMe(3)), for which a molecular orbital analysis indicates that the palladium center possesses a d(8) configuration.

Synthesis, Structure, and Reactivity of a Mononuclear Organozinc Hydride Complex: Facile Insertion of CO2 into a Zn–H Bond and CO2-Promoted Displacement of Siloxide Ligands

Tris(2-pyridylthio)methane, [Tptm]H, has been employed to synthesize the mononuclear alkyl zinc hydride complex, [κ(3)-Tptm]ZnH, which has been structurally characterized by X-ray diffraction. [κ(3)-Tptm]ZnH provides access to a variety of other [Tptm]ZnX derivatives. For example, [κ(3)-Tptm]ZnH reacts with (i) R(3)SiOH (R = Me, Ph) to give [κ(4)-Tptm]ZnOSiR(3), (ii) Me(3)SiX (X = Cl, Br, I) to give [κ(4)-Tptm]ZnX, and (iii) CO(2) to give the formate complex, [κ(4)-Tptm]ZnO(2)CH. The bis(trimethylsilyl)amide complex [κ(3)-Tptm]ZnN(SiMe(3))(2) also reacts with CO(2), but the product obtained is the isocyanate complex, [κ(4)-Tptm]ZnNCO. The formation of [κ(4)-Tptm]ZnNCO is proposed to involve initial insertion of CO(2) into the Zn-N(SiMe(3))(2) bond, followed by migration of a trimethylsilyl group from nitrogen to oxygen to generate [κ(4)-Tptm]ZnOSiMe(3) and Me(3)SiNCO, which subsequently undergo CO(2)-promoted metathesis to give [κ(4)-Tptm]ZnNCO and (Me(3)SiO)(2)CO.

On the Chalcogenophilicity of Mercury: Evidence for a Strong Hg-Se Bond in [TmBut]HgSePh and Its Relevance to the Toxicity of Mercury

One of the reasons for the toxic effects of mercury has been attributed to its influence on the biochemical roles of selenium. For this reason, it is important to understand details pertaining to the nature of Hg-Se interactions and this has been achieved by comparison of a series of mercury chalcogenolate complexes that are supported by tris(2-mercapto-1-t-butyl-imidazolyl)hydroborato ligation, namely [Tm(Bu(t))]HgEPh (E = S, Se, Te). In particular, X-ray diffraction studies on [Tm(Bu(t))]HgEPh demonstrate that although the Hg-S bonds involving the [Tm(Bu(t))] ligand are longer than the corresponding Cd-S bonds of [Tm(Bu(t))]CdEPh, the Hg-EPh bonds are actually shorter than the corresponding Cd-EPh bonds, an observation which indicates that the apparent covalent radii of the metals in these compounds are dependent on the nature of the bonds. Furthermore, the difference in Hg-EPh and Cd-EPh bond lengths is a function of the chalcogen and increases in the sequence S (0.010 A) < Se (0.035 A) < Te (0.057 A). This trend indicates that the chalcogenophilicity of mercury increases in the sequence S < Se < Te. Thus, while mercury is often described as being thiophilic, it is evident that it actually has a greater selenophilicity, a notion that is supported by the observation of facile selenolate transfer from zinc to mercury upon treatment of [Tm(Bu(t))]HgSCH(2)C(O)N(H)Ph with [Tm(Bu(t))]ZnSePh. The significant selenophilicity of mercury is in accord with the aforementioned proposal that one reason for the toxicity of mercury is associated with it reducing the bioavailability of selenium.

Structural and spectroscopic studies on four-, five-, and six-coordinate complexes of zinc, copper, nickel, and cobalt: Structural models for the bicarbonate intermediate of the carbonic anhydrase catalytic cycle

The molecular structures and electronic spectra of a series of four-, five-, and six-coordinate complexes of cobalt, nickel, copper, and zinc, stabilized by tris(pyrazolyl)hydroborato ligands, have been determined. The structural variations that are observed for the series of nitrate and carbonate complexes reveal that the preference for bidentate coordination of these ligands increases across the series Zn < Co ⪡ Cu and Ni.

New Modes for Coordination of Aromatic Heterocyclic Nitrogen Compounds to Molybdenum: Catalytic Hydrogenation of Quinoline, Isoquinoline, and Quinoxaline by Mo(PMe3)4H4

The heterocyclic nitrogen compounds isoquinoline (iQH), quinoxaline (QoxH) and quinazoline (QazH), abbreviated generally as NHetH, react with Mo(PMe3)6 to give (η2-NHet)Mo(PMe3)4H as a result of cleavage of the C−H bond adjacent to the nitrogen atom. The C−H bond cleavage is reversible. For example, in the case of isoquinoline and quinoxaline, treatment of (η2-NHet)Mo(PMe3)4H with PMe3 regenerates Mo(PMe3)6 and NHetH. Furthermore, at elevated temperatures (η2-NHet)Mo(PMe3)4H converts sequentially to isomers of (η6-NHetH)Mo(PMe3)3 in which the N-heteroaromatic ligand coordinates via either the heterocyclic or carbocyclic rings. Isomers of (η6-NHetH)Mo(PMe3)3 in which the heterocyclic ring coordinated to molybdenum may be hydrogenated. Thus, (η6-C5N-iQH)Mo(PMe3)3 and (η6-C4N2-QoxH)Mo(PMe3)3 react with H2 at 90 °C to give Mo(PMe3)4H4 and release 1,2,3,4-tetrahydroisoquinoline and 1,2,3,4-tetrahydroquinoxaline, respectively. Furthermore, Mo(PMe3)4H4 serves as a catalyst precursor for the hydrogenation of quin...

Multiple bonding between germanium and the chalcogens: the syntheses and structures of the terminal chalogenido complexes (η4-Me8taa)GeE (E = S, Se, Te)

A series of terminal chalcogenido complexes of germanium supported by ligation of the macrocyclic octamethyldibenzotetraaza[14]annulene dianion (η4-Me8taa)GeE (E = S, Se, Te) has been prepared by addition of the chalcogen to (η4-Me8taa)Ge; structural studies on these complexes indicate that the bonding of the Ge≈E moiety is best described as intermediate between the Ge–Ē and GeE resonance structures.

Synthesis and characterization of (η5-C5Me5)2TaCl(THF), a useful synthetic precursor for the preparation of oxo, imido and methylidene derivatives of permethyltantalocene

Abstract The synthesis and characterization of Cp*2Ta(O)Cl (Cp*  (η5-C5Me5)), Cp*2Ta(NPh)Cl, Cp*2Ta (O)H, Cp*2Ta(NR)H (R  Ph, CMe3), Cp*2Ta(CH2)H, Cp*2Ta(CH2)Cl, Cp*2Ta(H2 CCH2)H, and the unusual cyclometallated product Cp*(η6-C5Me4CH2)TaH2 from Cp*2TaCI(THF) (THF  tetrahydrofuran) is described. Cp*2TaCl(THF) is prepared by the Na / Hg reduction of Cp*2TaCl2 in THF and used in situ. These synthetic routes are more convenient than those previously described and in most cases give much higher yields and purer products. All attempts to isolate Cp*2TaCl(THF) as a pure crystalline solid have led instead to less reactive [Cp*2TaCl]n, whose structure is uncertain. Although Cp*2TaCl(THF) is only moderately stable in THF at room temperature, it has been characterized in solution by 1H NMR spectroscopy. Both [Cp*2TaCl]n and Cp*2TaCl(THF) react with CO to afford Cp*2TaCl(CO). An X-ray crystal structure determination for Cp*2Ta (NPh)Cl (triclinic space group P 1 (number 2) with Z = 2; a = 8.627(2); b = 9.538(5), c = 16.890(5) A, α = 74.81(3)°, β = 87.12(2)°, γ = 63.79(3)°, with V = 1200.0(8) A3) reveals a linear TaNC group, as had been found previously for the closely related complex Cp*2Ta(NPh)H.