Fabrizio Zanobini

Synthesis and Characterisation of tetrahedro-Tetraphosphorus Complexes of Rhenium – Evidence for the First Bridging Complex of White Phosphorus

Reaction of white phosphorus with [(triphos)Re(CO)2(OTf)] (1) in dichloromethane affords the new tetraphosphorus complex [(triphos)Re(CO)2(η1-P4)](OTf) (2) [triphos = MeC(CH2PPh2)3; OTf = OSO2CF3]. Compound 2 reacts with a second equivalent of 1 to give the binuclear complex [{(triphos)Re(CO)2}2(μ,η1,η1-P4)](OTf)2 (3) in which a tetrahedro-P4 ligand behaves as tethering unit between two [(triphos)Re(CO)2]+ moieties. Complexes 2 and 3 represent the first soluble metal complexes of the tetraphosphorus molecule where the P4 ligand has not undergone any major modification.

Regio- and stereoselective dimerization of 1-alkynes catalyzed by an Os(II) complex

Abstract The complex [(PP 3 )OsH(N 2 )]BPh 4 is a catalyst precursor for the regio- and stereoselective dimerization of HCCR (RPh, SiMe 3 ) to ( Z )-1,4-disubstituted-but-3-en-l-ynes (PP 3 P(CH 2 CH 2 PPh 2 ) 3 ). In the presence of H 2 O or C 2 H 5 OH, the catalytic reaction with HCCSiMe 3 selectively gives but-3-en-l-ynyl-trimethyisilane. A detailed study under different experimental conditions, the detection of some intermediates, and the use of isolated complexes in independent reactions, taken altogether, permit mechanistic conclusions which account for the observed products. A key-role is played by (vinylidene)σ-alkynyl complexes which transform into η 3 -butenynyl derivatives via intramolecular CC bond formation. The Os(II) η 3 -butenynyl complexes are likely reagents in the rate determining step of the catalytic cycle, and produce free ( Z )-1,4-disubstituted-but-3-en-l-ynes upon σ-bond metathesis reaction with HCCR. The 16-electron fragments [(PP 3 )OsX] + (X  H, Cl, CCR) are capable of promoting the 1-alkyne to vinylidene tautomerism. In particular, the (vinylidene)hydride [(PP 3 )OsH{CC(H)-SiMe 3 }]BPh 4 has been isolated and properly characterized. Since the stoichiometric reaction of the latter compound with HCCSiMe 3 gives vinyltrimethylsilane, the formation of (vinylidene)hydride species is suggested to be an effective step, alternative to 1-alkyne insertion, in the reduction of 1-alkynes to alkenes assisted by hydrido metal complexes.

An exceptionally stable cis-(hydride)(η2-dihydrogen) complex of iron

Abstract The cis -(H)(H 2 complex [(PP 3 )Fe(H)(H 2 )]BPh 4 (PP 3  P(CH 2 CH 2 PPh 2 ) 3 ) has been made by reaction of the chloride [(PP 3 )FECl]BPh 4 in THF with NaBH 4 under 1 atm of H 2 . In the solid state and in solution at low temperature the complex is octahedral, and the hydride and dihydrogen ligands occupy mutually cis positions. At ambient temperature in solution the complex is trigonal-bipyramidal, and an “H 3 ” unit occupies an axial position trans to the bridgehead phosphorus atom of PP 3 ; this results in exceptional thermal and chemical stability of the complex.

Catalytic amine-borane dehydrogenation by a PCP-pincer palladium complex: a combined experimental and DFT analysis of the reaction mechanism

Catalytic dehydrogenation of ammonia-borane (NH(3)·BH(3), AB) and dimethylamine borane (NHMe(2)·BH(3), DMAB) by the Pd(II) complex [((tBu)PCP)Pd(H(2)O)]PF(6) [(tBu)PCP = 2,6-C(6)H(3)(CH(2)P(t)Bu(2))(2)] leads to oligomerization and formation of spent fuels of general formula cyclo-[BH(2)-NR(2)](n) (n = 2,3; R = H, Me) as reaction byproducts, while one equivalent of H(2) is released per amine-borane equivalent. The processes were followed through multinuclear ((31)P, (1)H, (11)B) variable temperature NMR spectroscopy; kinetic measurements on the hydrogen production rate and the relative rate constants were also carried out. One non-hydridic intermediate could be detected at low temperature, whose chemical nature was explored through a DFT modeling of the reaction mechanism, at the M06//6-31+G(d,p) computational level. The computational output was of help to propose a reliable mechanistic picture of the process.

Synthesis and Structural Characterization of (Carbene)ruthenium Complexes Binding Nucleobases

The new (carbene)ruthenium(II) nucleobase derivatives fac,cis-[(PNP)RuCl{C(NHC4H3N2O2)(CH2Ph)}]Cl (5) and fac,cis-[(PNP)RuCl{C(NHC5H3N4)(CH2Ph)}]Cl (6) [PNP = CH3CH2CH2N(CH2CH2PPh2)2] were synthesised by treating (vinylidene)ruthenium(II) complex fac,cis-[(PNP)RuCl2{C=C(H)Ph}] (1) with 5-aminouracil or adenine, respectively. Both complexes were characterized by spectroscopic techniques (IR and NMR) and elemental analyses, which confirmed the formation of the aminocarbene moieties incorporating 1 equiv. of the nucleobase in the complex framework. Crystal and molecular structure determination by X-ray diffraction analysis of the uracil derivative 5 revealed a very unusual O-coordination of the exocyclic C=O group on the C(4) atom of the uracil ring to the ruthenium atom leading to an unprecedented six-membered aminocarbene metallacycle. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Dihydrogen Bonding in Complex (PP3)RuH(η1-BH4) Featuring Two Proton-Accepting Hydride Sites: Experimental and Theoretical Studies

Combining variable-temperature infrared and NMR spectroscopic studies with quantum-chemical calculations (density functional theory (DFT) and natural bond orbital) allowed us to address the problem of competition between MH (M = transition metal) and BH hydrogens as proton-accepting sites in dihydrogen bond (DHB) and to unravel the mechanism of proton transfer to complex (PP3)RuH(η(1)-BH4) (1, PP3 = κ(4)-P(CH2CH2PPh2)3). Interaction of complex 1 with CH3OH, fluorinated alcohols of variable acid strength [CH2FCH2OH, CF3CH2OH, (CF3)2CHOH (HFIP), (CF3)3COH], and CF3COOH leads to the medium-strength DHB complexes involving BH bonds (3-5 kcal/mol), whereas DHB complexes with RuH were not observed experimentally. The two proton-transfer pathways were considered in DFT/M06 calculations. The first one goes via more favorable bifurcate complexes to BHterm and high activation barriers (38.2 and 28.4 kcal/mol in case of HFIP) and leads directly to the thermodynamic product [(PP3)RuHeq(H2)](+)[OR](-). The second pathway starts from the less-favorable complex with RuH ligand but shows a lower activation barrier (23.5 kcal/mol for HFIP) and eventually leads to the final product via the isomerization of intermediate [(PP3)RuHax(H2)](+)[OR](-). The B-Hbr bond breaking is the common key step of all pathways investigated.

Synthesis, Characterization, and Interconversion of the Rhenium Polyhydrides [ReH3(η4-NP3)] and [ReH4(η4-NP3)]+ {NP3 = tris[2-(diphenylphosphanyl)ethyl]amine}

The rhenium(III) dichloride complex [ReCl2(η4-NP3)]Cl (1) was prepared from [ReCl3(CH3CN)(PPh3)2] by treatment with the tripodal tetradentate ligand N(CH2CH2PPh2)3 (NP3) in ethanol. The reaction of 1 with LiAlH4 in THF gave the rhenium(III) trihydride [ReH3(η4-NP3)] (2), which was converted into the rhenium(V) tetrahydride [ReH4(η4-NP3)]BPh4 (3) by protonation in CH2Cl2 with HBF4·OMe2, followed by a metathetical reaction with NaBPh4. The classical polyhydride nature of 2 and 3, as well as the overall molecular structures in solution, were determined by NMR spectroscopy, 1H NMR relaxation, and IR spectroscopy. The polyhydride complexes 2 and 3 are stereochemically nonrigid in solution, and the thermodynamic parameters associated with the fluxional processes were determined by variable-temperature NMR studies. A single-crystal X-ray analysis of 3 has shown the complex cation [ReH4(η4-NP3)]+ to be eight-coordinated by the four donor atoms of NP3 and by four terminal hydride ligands in a distorted dodecahedral geometry. An in situ IR study in CH2Cl2 has shown that the protonation of 3 occurs regioselectively at the metal center with no formation of a dihydrogen complex. Kinetic hydrogen bond products of the formula [(η4-NP3)H3Re···HOR] (ROH = C2H5OH, CFH2CH2OH, CF3CH2OH) were intercepted by IR spectroscopy at low temperature. The thermodynamic parameters associated with the formation of the hydrogen bond adducts were determined by either IR spectroscopy applying the Iogansen equation or van’t Hoff plots of the formation constant vs. temperature.

Asymmetric hydrogen-transfer reduction of prochiral and α,β-unsaturated ketones by iridium complexes containing optically pure aminodiphosphine ligands

Abstract This work reports the catalytic reduction, via hydrogen-transfer from propan-2-ol, of various α , β -unsaturated ketones or unsymmetrical ketones in the presence of [Ir(COD)(OMe)] 2 and the chiral aminodiphosphine ligands ( R )-PhC * H(Me)N(CH 2 CH 2 PR 2 ) 2 (R=Ph, Cy) and ( R )-(Et)C * H(Me)N(CH 2 CH 2 PPh 2 ) 2 . In general, these catalyst systems show good activity but modest enantioselectivity. Both the chemoselectivity and the enantioselectivity for the reduction of α , β -unsaturated ketones to optically pure allylic alcohols depend on the nature of the substituent(s) at either the carbon stereocentre or the phosphorus substituents.

SYNTHESIS AND CHARACTERIZATION OF RHENIUM POLYHYDRIDES STABILIZED BY THE TRIPODAL LIGAND MEC(CH2PPH2)3

Abstract The reaction of CH 3 C(CH 2 PPh 2 ) 3 (triphos) with [Re(O)Cl 3 (PPh 3 ) 2 ] in THF at reflux temperature yields [(η 2 -triphos) Re(O)Cl 3 ] ( 3 ). Compound 3 is transformed into [(η 2 -triphos)ReH 7 ] ( 4 ) by treatment with LiAlH 4 in refluxing THF. The reaction of [Re(MeCN)Cl 3 (PPh 3 ) 2 ] with triphos in refluxing toluene gives fac -[(η 3 -triphos)ReCl 3 ] ( 5 ) in excellent yield. Complex 5 is converted to the classical pentahydride [(η 3 -triphos)ReH 5 ] ( 6 ) by treatment with NaBH 4 at room temperature. The reductive elimination of H 2 from 6 is promoted by monodentate ligands such as PPh 3 and CO, giving substitution products of the formula [(η 3 -triphos)ReH 3 (L)] (L = PPh 3 ( 7 ) or CO ( 8 )). All the rhenium polyhydrides obtained have been characterized by spectroscopic techniques including IR and multinuclear variable-temperature NMR analysis. A detailed study of the spin-lattice relaxation time ( T 1 ) at variable temperature has shown that T 1 cannot be used unambiguously to discriminate between classical and non-classical structures of rhenium polyhydrides.

Mechanistic Studies on the Interaction of [(κ3-P,P,P-NP3)IrH3] [NP3 = N(CH2CH2PPh2)3] with HBF4 and Fluorinated Alcohols by Combined NMR, IR, and DFT Techniques

The novel iridium(III) hydride [(kappa(3)-P,P,P-NP(3))IrH(3)] [NP(3) = N(CH(2)CH(2)PPh(2))(3)] was synthesized and characterized by spectroscopic methods and X-ray crystallography. Its reactivity with strong (HBF(4)) and medium-strength [the fluorinated alcohols 1,1,1-trifluoroethanol (TFE) and 1,1,1,3,3,3-hexafluoroisopropanol (HFIP)] proton donors was investigated through low-temperature IR and multinuclear NMR spectroscopy. In the case of the weak acid TFE, the only species observed in the 190-298 K temperature range was the dihydrogen-bonded adduct between the hydride and the alcohol, while with the stronger acid HBF(4), the proton transfer was complete, giving rise to a new intermediate [(kappa(3)-P,P,P-NP(3))IrH(4)](+). With a medium-strength acid like HFIP, two different sets of signals for the intermediate species were observed besides dihydrogen bond formation. In all cases, the final reaction product at ambient temperature was found to be the stable dihydride [(kappa(4)-NP(3))IrH(2)](+), after slow molecular dihydrogen release. The nature of the short-living species was investigated with the help of density functional theory calculations at the M05-2X//6-31++G(df,pd) level of theory.