Phillip E. Fanwick

Functionalization of Carbon Dioxide and Carbon Disulfide Using a Stable Uranium(III) Alkyl Complex

A rare uranium(III) alkyl complex, Tp*(2)U(CH(2)Ph) (2) (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate), was synthesized by salt metathesis from Tp*(2)UI (1) and KCH(2)Ph and fully characterized using (1)H NMR, infrared, and electronic absorption spectroscopies as well as X-ray crystallography. This complex has a uranium-carbon distance of 2.57(2) Å, which is comparable to other uranium alkyls reported. Treating this compound with either carbon dioxide or carbon disulfide results in insertion into the uranium-carbon bond to generate Tp*(2)U(κ(2)-O(2)CCH(2)Ph) (3) and Tp*(2)U(SC(S)CH(2)Ph) (4), respectively. These species, characterized spectroscopically and by X-ray crystallography, feature new carboxylate and dithiocarboxylate ligands. Analysis by electronic absorption spectroscopy supports the trivalent oxidation state of the uranium center in both of these derivatives. Addition of trimethylsilylhalides (Me(3)SiX; X = Cl, I) to 3 results in the release of the free silyl ester, Me(3)SiOC(O)CH(2)Ph, forming the initial uranium monohalide species, Tp*(2)UX, which can then be used over multiple cycles for the functionalization of carbon dioxide.

Synthesis, characterization, and multielectron reduction chemistry of uranium supported by redox-active α-diimine ligands.

Uranium compounds supported by redox-active α-diimine ligands, which have methyl groups on the ligand backbone and bulky mesityl substituents on the nitrogen atoms {(Mes)DAB(Me) = [ArN═C(Me)C(Me)═NAr], where Ar = 2,4,6-trimethylphenyl (Mes)}, are reported. The addition of 2 equiv of (Mes)DAB(Me), 3 equiv of KC(8), and 1 equiv of UI(3)(THF)(4) produced the bis(ligand) species ((Mes)DAB(Me))(2)U(THF) (1). The metallocene derivative, Cp(2)U((Mes)DAB(Me)) (2), was generated by the addition of an equimolar ratio of (Mes)DAB(Me) and KC(8) to Cp(3)U. The bond lengths in the molecular structure of both species confirm that the α-diimine ligands have been doubly reduced to form ene-diamide ligands. Characterization by electronic absorption spectroscopy shows weak, sharp transitions in the near-IR region of the spectrum and, in combination with the crystallographic data, is consistent with the formulation that tetravalent uranium ions are present and supported by ene-diamide ligands. This interpretation was verified by U L(III)-edge X-ray absorption near-edge structure (XANES) spectroscopy and by variable-temperature magnetic measurements. The magnetic data are consistent with singlet ground states at low temperature and variable-temperature dependencies that would be expected for uranium(IV) species. However, both complexes exhibit low magnetic moments at room temperature, with values of 1.91 and 1.79 μ(B) for 1 and 2, respectively. Iodomethane was used to test the reactivity of 1 and 2 for multielectron transfer. While 2 showed no reactivity with CH(3)I, the addition of 2 equiv of iodomethane to 1 resulted in the formation of a uranium(IV) monoiodide species, ((Mes)DAB(Me))((Mes)DAB(Me2))UI {3; (Mes)DAB(Me2) = [ArN═C(Me)C(Me(2))NAr]}, which was characterized by single-crystal X-ray diffraction and U M(4)- and M(5)-edge XANES. Confirmation of the structure was also attained by deuterium labeling studies, which showed that a methyl group was added to the ene-diamide ligand carbon backbone.

Harnessing redox activity for the formation of uranium tris(imido) compounds

Classically, late transition-metal organometallic compounds promote multielectron processes solely through the change in oxidation state of the metal centre. In contrast, uranium typically undergoes single-electron chemistry. However, using redox-active ligands can engage multielectron reactivity at this metal in analogy to transition metals. Here we show that a redox-flexible pyridine(diimine) ligand can stabilize a series of highly reduced uranium coordination complexes by storing one, two or three electrons in the ligand. These species reduce organoazides easily to form uranium-nitrogen multiple bonds with the release of dinitrogen. The extent of ligand reduction dictates the formation of uranium mono-, bis- and tris(imido) products. Spectroscopic and structural characterization of these compounds supports the idea that electrons are stored in the ligand framework and used in subsequent reactivity. Computational analyses of the uranium imido products probed their molecular and electronic structures, which facilitated a comparison between the bonding in the tris(imido) structure and its tris(oxo) analogue.

Structural and spectral studies of copper(II) and nickel(II) complexes of pyruvaldehyde mixed bis{N(4)-substituted thiosemicarbazones}

Abstract Pyruvaldehyde mixed bis(thiosemicarbazones) have been prepared in which the two thiosemicarbazone moieties have different N(4)-substituents. The mixed bis(thiosemicarbazones) and their copper(II) and nickel(II) complexes have been characterized with IR, electronic, mass, 1H NMR (Ni) and EPR (Cu) spectra. Representative crystal structures have been solved of nickel(II) complexes with either a pyruvaldehyde mixed bis(thiosemicarbazone) or a bis(thiosemicarbazone) with identical N(4)-substituents acting as a tetradentate ligand. [Ni(Pu4M4DE)] has an N(4)-methylthiosemicarbazone substituent on the keto “arm” and N(4)-diethylthiosemicarbazone substituent on the aldehyde arm. [Ni(Pu4M)] contains two N(4)-methylthiosemicarbazone moieties. Both bis(thiosemicarbazones) form square-planar N2S2 complexes with nickel(II) and copper(II).

Diruthenium–Polyyn-diyl–Diruthenium Wires: Electronic Coupling in the Long Distance Regime

Reported herein is a series of Ru2(Xap)4 capped polyyn-diyl compounds, where Xap is either 2-anilinopyridinate (ap) or its aniline substituted derivatives. Symmetric [Ru2(Xap)4](μ-C4k)[Ru2(Xap)4] (compounds 4ka (X = 3-isobutoxy) and 4kc (X = 3,5-dimethoxy) with k = 2, 3, 4, and 5) was obtained from the Glaser coupling reaction of Ru2(Xap)4(C2kH). Unsymmetric [Ru2(Xap)4](μ-C(4k+2))[Ru2(ap)4] (compounds 4k+2b with k = 2, 3, and 4) were prepared from the Glaser coupling reaction between Ru2(Xap)4(C(2k+2)H) and Ru2(ap)4(C2kH). X-ray diffraction study of compound 12c revealed both the sigmoidal topology of the polyyn-diyl bridge and the fine structural detail about the Ru2 cores. Cyclic and differential pulse voltammetric (CV and DPV) measurements and spectroelectrochemical studies revealed that (i) the reduced monoanions [Ru2-C2m-Ru2](-1) (m = 4-8) belong to the Robin-Day class II mixed valent ions and (ii) the electronic coupling between Ru2 termini depends on the length of the polyyn-diyl bridge with an attenuation constant (γ) between 0.12 and 0.15 Å(-1). In addition, spin-unrestricted DFT calculations provide insight about the nature of orbitals that mediate the long distance electronic coupling.

Carbon–Carbon Reductive Elimination from Homoleptic Uranium(IV) Alkyls Induced by Redox-Active Ligands

The synthesis, characterization, and reactivity of the homoleptic uranium(IV) alkyls U(CH(2)C(6)H(5))(4) (1-Ph), U(CH(2)-p-CH(3)C(6)H(4))(4) (1-p-Me), and U(CH(2)-m-(CH(3))(2)C(6)H(3))(4) (1-m-Me(2)) are reported. The addition of 4 equiv of K(CH(2)Ar) (Ar = Ph, p-CH(3)C(6)H(4), m-(CH(3))(2)C(6)H(3)) to UCl(4) at -108 °C produces 1-Ph in good yields and 1-p-Me and 1-m-Me(2) in moderate yields. Further characterization of 1-Ph by X-ray crystallography confirmed η(4)-coordination of each benzyl ligand to the uranium center. Magnetic studies produced an effective magnetic moment of 2.60 μ(B) at 23 °C, which is consistent with a tetravalent uranium 5f(2) electronic configuration. Addition of 1 equiv of the redox-active α-diimine (Mes)DAB(Me) ((Mes)DAB(Me) = [ArN═C(Me)C(Me)═NAr]; Ar = 2,4,6-trimethylphenyl (Mes)) to 1-Ph results in reductive elimination of 1 equiv of bibenzyl (PhCH(2)CH(2)Ph), affording ((Mes)DAB(Me))U(CH(2)C(6)H(5))(2) (2-Ph). Treating an equimolar mixture of 1-Ph and 1-Ph-d(28) with (Mes)DAB(Me) forms the products from monomolecular reductive elimination, 2-Ph, 2-Ph-d(14), bibenzyl, and bibenzyl-d(14). This is confirmed by (1)H NMR spectroscopy and GC/MS analysis of both organometallic and organic products. Addition of 1 equiv of 1,2-bis(dimethylphosphino)ethane (dmpe) to 1-Ph results in formation of the previously synthesized (dmpe)U(CH(2)C(6)H(5))(4) (3-Ph), indicating the redox-innocent chelating phosphine stabilizes the uranium center in 3-Ph and prevents reductive elimination of bibenzyl. Full characterization for 3-Ph, including X-ray crystallography, is reported.

Solid state structure of the tantalum bis-aryl compounds Ta(OAr-2,6R2)3(C6H5)2 (R = CH3, Pri; OAr-2,6R2 = 2,6-dialkylphenoxide): Observation of a lack of correlation of MOAr distances and MOAr angles for aryloxide derivatives of niobium(V) and tantalum(V)

Abstract Treatment of the tantalum dichloride compounds Ta(OAr-2,6Me2)3Cl2 or Ta(OAr-2,6Pr2i)3Cl2 (OAr-2,6Me2 = 2,6-dimethylphenoxide; OAr-2,6Pr2i = 2,6-diisopropylphenoxide) with phenyl lithium (2 equiv) leads to the formation of the bis-phenyl compounds Ta(OAr-2,6Me2)3(C6H5)2 (1) and Ta(OAr-2,6Pr2i)3(C6H5)2 (2), respectively, in moderate yields. Single crystal X-ray diffraction analyses of both 1 and 2 show them to be isostructural. In both compounds a trigonal bipyramidal environment about the metal centre is evident with trans, axial aryloxide oxygen atoms. Short TaO distances in the range of 1.848(5)-1.881(3) A and almost linear TaOAr angles are present. The central coordination environments in 1 and 2 are strikingly similar to those found for the biscyclometallated compound Ta(OC6H3Ph-C6H4)2(OAr-2,6Ph2). Combining the structural data for 1 and 2 with those of other alkyl, aryloxide compounds of niobium(V) and tantalum(V) shows that the MOAr distances span a narrow range of 1.85-1.95 A, while the corresponding MOOAr angles vary from 135 to 180° with no correlation between the two parameters. Crystal data for TaO3C36H37 (1) at 19°C: a = 19.313(3), b = 11.652(1), c = 15.288(1) A, β = 113.13(1)°, Z = 4, dcalc = 1.446 g cm−3 in space C2/c; for TaO3C48H61 (2) at 21°C: a = 11.955(2), b = 19.585(2), c = 19.337(2) A, β = 103.742(9)°, Z = 4, dcalc = 1.309 g cm−3 in space group P21/n.

Cationic Group 4 metal alkyl compounds containing aryloxide ligation: synthesis, structure, reactivity and polymerization studies

A series of bis(alkyl) derivatives of titanium and zirconium [(ArO) 2 MR 2 ] (M=Ti, Zr; R=Me, CH 2 Ph; ArO=various 2,6-di-substituted phenoxides) has been synthesized and their reactivity towards the Lewis acid [B(C 6 F 5 ) 3 ] examined. The benzyl compounds generate stable zwitterionic species such as [M(OC 6 HPh 2 -2,6-R 2 -3,5) 2 (CH 2 Ph)][η 6 -C 6 H 5 CH 2 B(C 6 F 5 ) 3 ] (M=Ti, R=H, 12 ; Me, 13 : M=Zr, R=Me, 15 ). Structural studies of 12 and 15 show the boron anion π-bound to the metal center through the original benzyl phenyl ring. In contrast, treatment of the benzyl compound [Zr(OC 6 H 3 Bu t 2 -2,6) 2 (CH 2 Ph) 2 ] with [B(C 6 F 5 ) 3 ] leads to the cyclometallated compound [Zr(OC 6 H 3 Bu t -CMe 2 CH 2 )(OC 6 H 3 Bu t 2 -2,6)][η 6 -C 6 H 5 CH 2 B(C 6 F 5 ) 3 ] 16 which was structurally characterized. In contrast to this behavior the bis(methyl) species react with [B(C 6 F 5 ) 3 ] to produce unstable methyl cationic intermediates which decompose to a mixture of [Ti(OAr) 2 (CH 3 )(C 6 F 5 )] and [CH 3 B(C 6 F 5 ) 2 ]. The titanium zwitterionic benzyl compounds will react with alkynes and α-olefins to produce mono-insertion products such as [Ti(OC 6 H 3 Ph 2 -2,6) 2 {C(CH 3 )C(Ph)CH 2 (η 6 -C 6 H 5 )}][PhCH 2 B(C 6 F 5 ) 3 ] 24 . In these compounds 1,2-insertion of olefins occurs followed by chelation of the original benzyl group to the metal center. Spectroscopic studies show the boron anion is non-coordinated to the metal center. Despite their thermal instability, the methyl cations can be generated in situ in the presence of olefins to produce polymers (ethene and propene) and oligomers (1-hexene). Studies show that the molecular weight of the polymers or oligomers increases systematically with the bulk of the aryloxide ligand. Spectroscopic studies of the polypropylene indicate 1,2-insertion is occurring with β-hydrogen abstraction to produce vinylidene end groups as the termination step.

New bioactive heptenes from melodorum fruticosum (annonaceae)

Abstract By guiding fractionation with brine shrimp lethality, three novel compounds, with cytotoxic activities against human tumor cell lines, have been isolated from the bark of Melodorum fruticosum Lour.(Annonaccae). These compounds have benzoyl moieties in common with a c7 dienone or lactone terminal which appears to arise from a heptose or the equivalents. They were named melodienone, isomelodienone, and acetylmelodorinol.

Aryl imido complexes of the group 4 metals : structural aspects and mechanistic study of formation

Abstract The addition of aniline (PhNH2 ⩾ 2 equivalents) to the organometallic compounds [Ti(OC6H3Pri2-2,6)2(η2-ButNCCH2Ph)(CH2Ph)], [Ti(OC6H3Ph2-2,6)2(C4Et4)] and [Zr(OC6H3But2-2,6)2(CH3)2] in hydrocarbon solvents leads to the formation of the mononuclear bis(phenylamido) derivatives [M(OAr)2(NHPh)2] [M = Ti, OAr = OC6H3Pri2-2,6 (1); M = Ti, OAr = OC6H3Ph2-2,6 (2); M = Zr, OAr = OC6H3But2-2,6 (3)]. Treatment of [Hf(CH2Ph)4] first with PhNH2 (4 equivalents), followed by HOC6H3But2-2,6 (2 equivalents), leads to the related bis(phenylamido) compound 4 (M = Hf; OAr = OC6H3But2-2,6). The two homoleptic aryl amido compounds [M(NHC6H3Pri2-2,6)4] [M = Zr (5); Hf (6)] have also been obtained by addition of 2,6-diisopropylaniline to the tetra-benzyl compounds [M(CH2Ph)4] (M = Zr, Hf). The addition of 4-pyrrolidinopyridine (py′) to all of the aryl amido compounds except 4 leads to elimination of 1 equivalent of aryl amine and the formation of a series of five-coordinate aryl amido derivatives of the general formula [M(OAr)2(NPh)(py′)2] [M = Ti, OAr = OC6H3Pri2-2,6 (7); M = Ti, OAr = OC6H3Ph2-2,6 (8); M = Zr, OAr = OC6H3But2-2,6 (9); and [M(NHAr)2(NAr)(py′)2]; M = Zr, Ar = C6H3Pri2-2,6 (10); M = Hf, Ar = C6H3Pri2-2,6 (11)]. In the case of the hafnium bis(phenylamido) complex 4, addition of 4-pyrrolidinopyridine resulted in the formation of a simple adduct. [Hf(OC6H3But2-2,6)2(NHPh)2(py′)], [4·py′]. (A similar adduct, [4·py′], was detected in the conversion of 3–9.) No elimination of aniline from [4·py′] and formation of a phenylimido derivative were observed. Both 2,2′-bipyridine and 1,10-phenanthroline were found to eliminate aniline from compounds 1–3 to produce insoluble products. Addition of 3,4,7,8-tetramethyl-1,10-phenanthroline to 2, however, yielded a soluble phenylimido derivative (12). The four-coordinate aryl amido compounds 2, 3 and 5 were found to be pseudo-tetrahedral in the solid state, while the five-coordinate aryl imido compounds 7, 9, 10 and 11 are best described as distorted trigonal-bipyramidal with trans-axial pyridine ligands. In the phenanthroline derivative 12 a distorted trigonal-bipyramidal geometry exists about the titanium atom with an aryloxide oxygen atom trans to a phenanthroline nitrogen. The bonding of the aryl imido, aryl amido and pyridine groups is described. A detailed study of the reaction of a series of bis(aryl amido) complexes, [Zr(OAr)2(NHC6H4-4X)2] (3)X (X = H, F, CH3, OMe, Br), with a variety of pyridine ligands was undertaken. The mono-pyridine adduct [3X·py] is rapidly formed, followed by the slow formation of the corresponding aryl imido complex [Zr(OAr)2(NHC6H4-4X)(py)2] [9X] and an equivalent of substituted aniline. Equilibrium constants for the reaction [3Xpy] + py = [9X] + ArNH2 were measured. Formation of the aryl imido ligand was found to be favoured by a more basic pyridine ligand and by electron-withdrawing substituents on the aryl ring of the initial aryl amido group. The rate of attainment of the equilibrium situation from [3] and py was investigated and various pathways for the reaction are considered. X-ray crystal structures were determined for 2, 3, 5, 7, 9, 10, 11 and 12.