Joseph N. Coalter

Coordinated carbenes from electron-rich olefins on RuHCl(PPr3i)2

Dehydrohalogenation of RuH2Cl2L2 (L=PPr3i) gives (RuHClL2)2, shown to be a halide-bridged dimer by X-ray crystallography; the fluoride analog is also a dimer. (RuHClL2)2 reacts with N2, pyridine and C2H4 (L′) to give RuHClL′L2, but with vinyl ether and vinyl amides, H2CCH(E) [E=OR, NRC(O)R′] such olefin binding is followed by isomerization to the heteroatom-substituted carbene complex L2HClRuC(CH3)(E). The reaction mechanism for such rearrangement is established by DFT(B3PW91) computations, for C2H4 as olefin (where it is found to be endothermic), and the structures of intermediates are calculated for H2CC(H)(OCH3) and for cyclic and acyclic amide-substituted olefins. It is found, both experimentally and computationally, that the amide oxygen is bonded to Ru, with a calculated bond energy of approximately 9 kcal mol−1 for an acyclic model. Less electron-rich vinyl amides or amines form η2-olefin complexes, but do not isomerize to carbene complexes. Calculated ΔE values for selected ‘‘ competition’’ reactions reveal that donation by both Ru and the heteroatom-substituted X are necessary to make the carbene complex L2HClRuC(X)(CH3) more stable than the olefin complex L2HClRu(η2-H2CCHX). This originates in part from a diminished endothermicity of the olefin→carbene transformation when the sp2 carbon bears a π-donor substituent. The importance of a hydride on Ru in furnishing a mechanism for this isomerization is discussed. The compositional characteristics of Schrock and Fischer carbenes are detailed, it is suggested that reactivity will not be uniquely determined by these characteristics, and these new carbenes RuHCl[C(X)CH3]L2 are contrasted to Schrock and Fischer carbenes.

R-Group reversal of isomer stability for RuH(X)L2(CCHR) s. Ru(X)L2(CCH2R): access to four-coordinate ruthenium carbenes and carbynes

NaOPh converts equimolar RuHClL2(CCHR) (L = PPr3i and PCy3) first to RuH(OPh)L2(CCHR), but then, only for R = H, these isomerize to the more stable carbynes Ru(OPh)L2(C–CH3); the rate of isomerization is slowed considerably by THF. RuH(OPh)L2(CCHR) can also be synthesized by reaction of RuCl2L2[CH(CH2R)] with 2 NaOPh; again, only when R = H does the hydrido vinylidene isomerize to the carbyne. While phenoxide converts RuCl2L2(CHPh) to Ru(OPh)L2(CPh), ia the observable intermediates RuCl2−n(OPh)nL2(CHPh), alkoxides OBut and OAdamantyl cause phosphine displacement to give the four-coordinate carbenes Ru(OR)2L(CHPh). DFT (B3PW91) calculations show these d6 species have a traditional cis-divacant octahedral structure with trans OR groups.

Geminal dehydrogenation of ether and amine C(sp3)H2 groups by electron-rich Ru(II) and Os

Reaction of [RuHCl(PiPr3)2]2 with THF or dioxolane doubly dehydrogenates the carbon α to the oxygen to yield RuHCl(H2)(PiPr3)2, together with the coordinated cyclic carbene, RuHCl[CO(CH2)2E](PiPr3)2, where E=CH2 or O. In the presence of CH2CHtBu as hydrogen acceptor, all Ru is converted to its carbene complex. The cyclic amines RN(CH2)4 (R=H, Me) react analogously, to produce N-substituted carbene complexes by geminal dehydrogenation; a crystal structure is presented for the carbene complex with R=H, which reveals intermolecular NH⋯Cl hydrogen bonding. Similar chemistry is established for Os(H)3Cl(PiPr3)2, at 25°C, in the presence of H2CCHtBu, to give OsHCl[CO(CH2)3](PiPr3)2. Pyrrolidine reacts rapidly at 25°C to give first a 1∶1 amine adduct, then slowly the trihydride carbene Os(H)3Cl[C(NH)(CH2)3](PiPr3)2, together with H2. Os(H)2Cl2(PiPr3)2 is first dehydrochlorinated by one mole of pyrrolidine, then a second mole of pyrrolidine is geminally dehydrogenated to form Os(H)3Cl[C(NH)(CH2)3](PiPr3)2. The five-coordinated carbene OsHCl[CO(CH2)3](PiPr3)2 will add H2, and the resulting product exists as two isomers, a trihydride with Cl cis to the carbene and a species with Cl trans to carbene and H and “2H” mutually trans. Isomer preferences, strength of H2 binding, and carbene plane orientation are discussed, together with DFT calculations on the thermodynamics of ether dehydrogenation by ruthenium. The latter reveal that coordination of removed H2 to Ru is essential to achieving favorable thermodynamics.

Facile C(sp2)/O2CR bond cleavage by Ru or Os

[RuHClL2]2, L = PiPr3, reacts with H2CCH(O2CR) (R = CH3, CF3, C6H5) during mixing at 20 °C, via two observable intermediates, to give RuCl(O2CR)(CHMe)L2; this carbene complex then redistributes the Cl and O2CR groups. Vinyl tosylate gives RuCl(OTs)(CHMe)L2 already at −60 °C. Vinyl chloroformate, H2CCH(O2CCl) reacts rapidly with [RuHClL2]2 to give the olefin metathesis catalyst RuCl2(CHMe)L2 and CO2. Os(H)3ClL2 (L = PiPr3 or PtBu2Me) reacts with vinyl esters H2CCHE (E = O2CR) to form first an η2-olefin adduct. This is followed by C/O bond cleavage, giving the carbyne OsHCl(O2CCF3)(CMe)L2. Vinyl chloroformate and Os(H)3ClL2 gives OsHCl2(CMe)L2 and CO2. RuHCl(PPh3)3 reacts with vinyl chloroformate, via RuCl(O2CCl)(CHMe)(PPh3)2, to give RuCl2(CHMe)(PPh3)2 while OsHCl(PPh3)3 reacts analogously, through observable OsCl2(CHMe)(PPh3)2, to form OsHCl2(CMe)(PPh3)2. Vinyl trifluoroacetate converts OsHCl(PPh3)3, to OsHCl(O2CCF3)(CMe)(PPh3)2. The less π-basic metal in OsH(CO)(PtBu2Me)2+ reacts with vinyl esters to give only an olefin adduct; detectable binding of the ester oxygen to Os in this adduct suggests a mechanism for carboxylate migration from carbene carbon to metal. The mechanisms of these reactions are explored, and the thermodynamic disparity between Ru and Os is discussed. DFT (B3PW91) calculations have been carried out to establish the energy pattern of possible products. The thermodynamic preference for cleaving the C–O2CR bond is shown to have a thermodynamic origin associated with the energy of the formed Ru–O2CR bond. The calculations also indicate the very large thermodynamic driving force for loss of CO2 in the case of H2CCH(O2CCl). The corresponding loss of CO2 is shown to be thermodynamically unfavorable in the case of H2CCH(O2CR). The energy of the Ru-R bond is a key factor.

Amido/phosphine pincer hydrides of ruthenium

The chemistry of the ligand (R2PCH2SiMe2)2N− (R = cyclohexyl and tBu), “PNP-R”, on ruthenium is developed, including RuH(PNP-Cy)(PPh3) and (HPNP-R)RuH3Cl. The latter contains a protonated nitrogen (i.e., amine as a donor to Ru) and one H2 ligand (X-ray structure for R = tBu). This compound can be dehydrohalogenated to give (PNP-Cy)RuH3, which undergoes H/D exchange of D2 into its cyclohexyl rings, and is itself dehydrogenated by excess H2CCHR to give [Cy2PCH2SiMe2NSiMe2CH2PCy(C6H8)] Ru, which contains a triply dehydrogenated cyclohexyl ring π-allyl bonded to Ru. (PNP-Cy)RuH3 reacts with dihydrofurans to give the heteroatom-stabilized carbene complex (PNP-Cy)RuH[CO(CH2)3].

Carbene transposition involving double dehydrogenation of an sp3 carbon

Two benzylic hydrogens of 2,6-Me2C6H3O− coordinated to RuCl(PCy3)2(CHPh)+ are transferred to the benzylidene ligand, liberating toluene to form a new carbene which is covalently linked to the aryloxide ligand.