Backbone-functionalised ruthenium diphosphine complexes for catalytic upgrading of ethanol and methanol to iso-butanol.

Efficient catalysts for Guerbet-type ethanol/methanol upgrading to iso-butanol have been developed via Michael addition of a variety of amines to ruthenium-coordinated dppen (1,1-bis(diphenylphosphino)ethylene). All catalysts produce over 50% iso-butanol yield with >90% selectivity in 2 h with catalyst 1 showing the best activity (74% yield after this time). The selectivity and turnover number approach 100% and 1000 respectively using catalyst 6. The presence of uncoordinated functionalised donor groups in these complexes results in a more stable catalyst compared to unfunctionalised analogues.

Heterometathesis of diphosphanes (R2P-PR2) with dichalcogenides (R'E-ER', E = O, S, Se, Te).

The reactions of R2P-PR2 with R'E-ER', (where E = Se, S, O, Te) to give R2P-ER' have been explored experimentally and computationally. The reaction of Ph2P-PPh2 with PhSe-SePh gives Ph2P-SePh (1) rapidly and quantitatively. The P-P/Se-Se reaction is inhibited by the addition of the radical scavenger TEMPO which is consistent with a radical mechanism for the heterometathesis reaction. Compound 1 has been fully characterised, including by X-ray crystallography. A range of other Ar2P-SeR (R = Ph, nBu or CH2CH2CO2H) have also been prepared and characterised. The reaction of 1 with [Mo(CO)4(nbd)] (nbd = norbornadiene) gives two products which, from their characteristic 31P NMR data, have been identified as cis-[Mo(CO)4(Ph2PSePh-P)2] (8) and the mixed-donor complex cis-[Mo(CO)4(Ph2P-SePh-P)(Ph2P-SePh-Se)] (9). It is deduced that the P and Se atoms in ligand 1 have comparable capacity to coordinate to Mo(0). The reaction of Ph2P-PPh2 with PhS-SPh gives Ph2P-SPh (2) quantitatively but no reaction was observed between Ph2P-PPh2 and PhTe-TePh. Heterometathesis between Ph2P-PPh2 and tBuO-OtBu does not occur thermally but has been observed under UV irradiation to give Ph2P-OtBu along with P(V) oxidation by-products. DFT calculations have been carried out to illuminate why heterometatheses with dichalcogenides R'E-ER' occur readily when E = S and Se but not when E = O and Te. The calculations show that heterometathesis is predicted to be thermodynamically favourable for E = O, S and Se and unfavourable for E = Te. The fact that a metathesis reaction between Ph2P-PPh2 with tBuO-OtBu is not observed in the absence of UV radiation, is therefore due to kinetics.

Rhenium Complexes Bearing Tridentate and Bidentate Phosphinoamine Ligands in the Production of Biofuel Alcohols via the Guerbet Reaction.

We report a variety of rhenium complexes supported by bidentate and tridentate phosphinoamine ligands and their use in the formation of the advanced biofuel isobutanol from methanol and ethanol. Rhenium pincer complexes 1-3 are effective catalysts for this process, with 2 giving isobutanol in 35% yields, with 97% selectivity in the liquid fraction, over 16 h with catalyst loadings as low as 0.07 mol %. However, these catalysts show poorer overall selectivity, with the formation of a significant amount of carboxylate salt solid byproduct also being observed. Production of the active catalyst 1d has been followed by 31P NMR spectroscopy, and the importance of the presence of base and elevated temperatures to catalyst activation has been established. Complexes supported by diphosphine ligands are inactive for Guerbet chemistry; however, complexes supported by bidentate phosphinoamine ligands show greater selectivity for isobutanol formation over carboxylate salts. The novel complex 7 was able to produce isobutanol in 28% yield over 17 h. The importance of the N-H moiety to the catalytic performance has also been established, giving further weight to the hypothesis that these catalysts operate via a cooperative mechanism.

Radical-initiated P,P-metathesis reactions of diphosphanes: evidence from experimental and computational studies.

By combining the diphosphanes Ar2P-PAr2, where Ar = C6H5, 4-C6H4Me, 4-C6H4OMe, 3,5-C6H3(CF3)2, it has been shown that P,P-metathesis generally occurs rapidly under ambient conditions. DFT calculations have shown that the stability of unsymmetrical diphosphanes Z2P-PZ'2 is a function of the difference between the Z and Z' substituents in terms of size and electronegativity. Of the mechanisms that were calculated for the P,P-metathesis, the most likely was considered to be one involving Ar2P˙ radicals. The observations that photolysis increases the rate of the P,P-metatheses and TEMPO inhibits it, are consistent with a radical chain process. The P,P-metathesis reactions that involve (o-Tol)2P-P(o-Tol)2 are anomalously slow and, in the absence of photolysis, were only observed to take place in CHCl3 and CH2Cl2. The role of the chlorinated solvent is ascribed to the formation of Ar2PCl which catalyses the P,P-metathesis. The slow kinetics observed with (o-Tol)2P-P(o-Tol)2 is tentatively attributed to the o-CH3 groups quenching the (o-Tol)2P˙ radicals or inhibiting the metathesis reaction sterically.

Manganese Diphosphine and Phosphinoamine Complexes Are Effective Catalysts for the Production of Biofuel Alcohols via the Guerbet Reaction.

We report a variety of manganese-based catalysts containing both chelating diphosphine (bis(diphenylphosphino)methane (dppm: 1, 2, and 7) or 1,2-bis(diphenylphosphino)ethane (dppe: 3)), and mixed-donor phosphinoamine (2-(diphenylphosphino)ethylamine (dppea: 4-6)) ligands for the upgrading of ethanol and methanol to the advanced biofuel isobutanol. These catalysts show moderate selectivity up to 74% along with turnover numbers greater than 100 over 90 h, with catalyst 2 supported by dppm demonstrating superior performance. The positive effect of substituting the ligand backbone was also displayed with a catalyst supported by C-phenyl-substituted dppm (8) having markedly improved performance compared to the parent dppm catalysts. Catalysts supported by the phosphinoamine ligand dppea are also active for the upgrading of ethanol to n-butanol. These results show that so-called PNP-pincer ligands are not a prerequisite for the use of manganese catalysts in Guerbet chemistry and that simple chelates can be used effectively.

Zirconium-Nitrogen Intermolecular Frustrated Lewis Pairs.

A series of intermolecular transition metal frustrated Lewis pairs (FLPs) based on zirconocene alkoxide complexes ([Cp2Zr(OMes)]+ 1 or ([Cp*2Zr(OMes)]+ 2) with nitrogen Lewis bases (NEt3, NEtiPr2, pyridine, 2-methylpyridine, 2,6-lutidine) are reported. The interaction between Zr and N depends on the specific derivatives used, in general more sterically encumbered pairs leading to a more frustrated interaction; however, DOSY NMR spectroscopy reveals these interactions to be dynamic in nature. The pairs undergo typical FLP-type reactivity with D2, CO2, THF, and PhCCD. The catalytic dehydrocoupling of Me2NH·BH3 is also reported. Comparisons can be made with previous work employing phosphines as Lewis bases suggesting that hard-hard or hard-soft acid-base considerations are of little importance compared to the more prominent roles of steric bulk and basicity.

Unlocking the catalytic potential of tris(3,4,5-trifluorophenyl)borane with microwave irradiation.

The catalytic activity of tris(3,4,5-trifluorophenyl)borane has been explored in the 1,2-hydroboration reactions of unsaturated substrates. Under conventional conditions, the borane was found to be active only in the hydroboration of aldehyde, ketone and imine substrates, with alkenes and alkynes not being reduced effectively. The use of microwave irradiation on the other hand has permitted alkenes and alkynes to be hydroborated in good yields.

Catalytic conversion of methanol/ethanol to isobutanol--a highly selective route to an advanced biofuel.

Catalysts based on ruthenium diphosphine complexes convert methanol/ethanol mixtures to the advanced biofuel isobutanol, with extremely high selectivity (>99%) at good (>75%) conversion via a Guerbet-type mechanism.

Unexpected Formation of Early Late Heterobimetallic Complexes from Transition Metal Frustrated Lewis Pairs.

Reaction of transition metal "frustrated" Lewis pair compounds of the type [Cp2Zr(Me)(OC(CF3)2CH2P(t)Bu2)] with the low valent platinum species [Pt(norbornene)3] leads to the unexpected formation of a heterobimetallic species [Cp2Zr{ Pt(Me)}(OC(CF3)2CH2 P(t)Bu2)]. Single crystal X-ray analysis reveals an unusual T-shaped geometry at the platinum center, with a relevant C-Pt-P angle of 163.3(3)°. Treatment of this compound with PMe3 yields [Pt(PMe3)4] and regenerates the zirconium precursor. Treatment with [(Et2O)2H][B(C6F5)4] protonates off the methyl ligand to give an ether adduct at platinum. Analogous observations are made with titanium-platinum species. We propose the chemistry is best rationalized as a formal insertion of Pt(0) into a Zr-C or Ti-Cl bond.

Small Molecule Activation by Intermolecular Zr(IV)-Phosphine Frustrated Lewis Pairs.

We report intermolecular transition metal frustrated Lewis pairs (FLPs) based on zirconocene aryloxide and phosphine moieties that exhibit a broad range of small molecule activation chemistry that has previously been the preserve of only intramolecular pairs. Reactions with D2, CO2, THF, and PhCCH are reported. By contrast with previous intramolecular examples, these systems allow facile access to a variety of steric and electronic characteristics at the Lewis acidic and Lewis basic components, with the three-step syntheses of 10 new intermolecular transition metal FLPs being reported. Systematic variation to the phosphine Lewis base is used to unravel steric considerations, with the surprising conclusion that phosphines with relatively small Tolman steric parameters not only give highly reactive FLPs but are often seen to have the highest selectivity for the desired product. DOSY NMR spectroscopic studies on these systems reveal for the first time the nature of the Lewis acid/Lewis base interactions in transition metal FLPs of this type.

Robust synthesis of epoxy resin-filled microcapsules for application to self-healing materials.

Mechanically and thermally robust microcapsules containing diglycidyl ether bisphenol A-based epoxy resin and a high-boiling-point organic solvent were synthesized in high yield using in situ polymerization of urea and formaldehyde in an oil-in-water emulsion. Microcapsules were characterized in terms of their size and size distribution, shell surface morphology and thermal resistance to the curing cycles of commercially used epoxy polymers. The size distribution of the capsules and characteristics such as shell thickness can be controlled by the specific parameters of microencapsulation, including concentrations of reagents, stirrer speed and sonication. Selected microcapsules, and separated core and shell materials, were analysed using thermogravimetric analysis and differential scanning calorimetry. It is demonstrated that capsules lose minimal 2.5 wt% at temperatures no higher than 120°C. These microcapsules can be applied to self-healing carbon fibre composite structural materials, with preliminary results showing promising performance.

Cooperative Lewis pairs based on late transition metals: activation of small molecules by platinum(0) and B(C6 F5 )3.

A Lewis basic platinum(0)-CO complex supported by a diphosphine ligand and B(C6 F5 )3 act cooperatively, in a manner reminiscent of a frustrated Lewis pair, to activate small molecules such as hydrogen, CO2 , and ethene. This cooperative Lewis pair facilitates the coupling of CO and ethene in a new way.

Reversible CO exchange at platinum(0). An example of similar complex properties produced by ligands with very different stereoelectronic characteristics.

The ligands 1,2-C6H4(CH2P(t)Bu2)2 (La) and 1,2-C6H4(P(t)Bu2)(CH2P(t)Bu2) (Lb) displace norbornene (nbe) from [Pt(η(2)-nbe)3] to give [PtL(η(2)-nbe)] where L = La (1a) or Lb (1b); 1a is fluxional on the NMR timescale. Reaction of 1a,b with CO gives the corresponding monocarbonyls [PtL(CO)] where L = La (2a) or Lb (2b) which then react further, and reversibly, to give the dicarbonyls [PtL(CO)2] where L = La (3a) or Lb (3b). The CO interchange between 2a,b and 3a,b is compared with the only other such system (2f and 3f), which are complexes of (C2F5)2PCH2CH2P(C2F5)2 (Lf). Ethene reacts smoothly with 2a to give (4a) and H2 with 2a generates some [PtH2(La)]. Protonation of 2a gives [Pt(La)(H)(CO)][B(C6F5)4] (5a) whose crystal structure has been determined. Similarly protonation of 2b gives [Pt(Lb)(H)(CO)][B(C6F5)4] as a mixture of geometric isomers 5b­6b.

Generation of aminoborane monomers RR'N=BH2 from amine-boronium cations [RR'NH-BH2L](+): metal catalyst-free formation of polyaminoboranes at ambient temperature.

Protonation of MeRNH·BH3 (R = Me or H) with HX (X = B(C6F5)4, OTf, or Cl), followed by immediate, spontaneous H2 elimination, yielded the amine-boronium cation salt [MeRNH·BH2(OEt2)][B(C6F5)4] and related polar covalent analogs, MeRNH·BH2X (X = OTf or Cl). These species can be deprotonated to conveniently generate reactive aminoborane monomers MeRN=BH2 which oligomerize or polymerize; in the case of MeNH2·BH3, the two step process gave poly(N-methylaminoborane), [MeNH-BH2]n.

Alkene to carbyne: tandem Lewis acid activation and dehydrogenation of a molybdenum ethylene complex.

Carbyne formation: Treatment of a molybdenum ethylene complex with B(C6 F5 )3 induces ditopic activation of an ethylene ligand and acceptor-assisted ethane elimination to generate a novel type of zwitterionic carbyne complex.

Frustrated Lewis pairs beyond the main group: transition metal-containing systems.

The concept of the "frustrated" Lewis pair (FLP) offers a valuable paradigm for transition metal complexes. This chapter describes recent developments in FLP chemistry where one or both of the Lewis acidic or Lewis basic components are based on a transition metal. At the forefront of these developments have been zirconocene-phosphinoaryloxide complexes; the activation of small molecules, including H2, CO2, ethers, olefins and alkyl chlorides or fluorides, with such zirconocene (or group 4 metallocene in general) phosphine pairs is described. Nascent catalytic applications for such complexes in dehydrogenation reactions and future possibilities for catalytic reactivity are also highlighted. A wider discussion of how the FLP concept may rationalise previous examples of cooperative reactivity in transition metal complexes, such as the heterolytic cleavage of hydrogen, suggests a strong and useful analogy.

New Cu(I)-ethylene complexes based on tridentate imine ligands: synthesis and structure.

A new bulky facially coordinating N3-donor tach-based ligand (tach: cis,cis-1,3,5-triaminocyclohexane) [1: cis,cis-1,3,5-tris(2-fluoro-6-(trifluoromethyl)benzylideneamino)cyclohexane] has been obtained from the condensation of tach with 3 equiv of the appropriate benzaldehyde. Reaction of 1 with [Cu(NCMe)4][PF6] gave the complex [(1)Cu(NCMe)][PF6]. Displacement of the acetonitrile ligand is possible with CO and C2H4 (3-5 bar). Cu(I)-ethylene complexes of ligands 1 and 2 [2: cis,cis-1,3,5-(mesitylideneamino)cyclohexane] were prepared successfully by treatment of the ligands with CuBr and AgSbF6 in the presence of ethylene. These complexes display reversible complexation of the ethylene molecule under mild changes to pressure, suggesting possible application in olefin separation and extraction.

A new class of remote N-heterocyclic carbenes with exceptionally strong σ-donor properties: introducing benzo[c]quinolin-6-ylidene.

We make the case for benzo[c]quinolin-6-ylidene (1) as a strongly electron-donating carbene ligand. The facile synthesis of 6-trifluoromethanesulfonylbenzo[c]quinolizinium trifluoromethanesulfonate (2) gives straightforward access to a useful precursor for oxidative addition to low-valent metals, to yield the desired carbene complexes. This concept has been achieved in the case of [Mn(benzo[c]quinolin-6-ylidene)(CO)5](+) (15) and [Pd(benzo[c]quinolin-6-ylidene)(PPh3)2(L)](2+) L = THF (21), OTf (22) or pyridine (23). Attempts to coordinate to nickel result in coupling products from two carbene precursor fragments. The CO IR-stretching-frequency data for the manganese compound suggests benzo[c]quinolin-6-ylidene is at least as strong a donor as any heteroatom-stabilised carbene ligand reported.

Cationic Ti(IV) and neutral Ti(III) titanocene-phosphinoaryloxide frustrated Lewis pairs: hydrogen activation and catalytic amine-borane dehydrogenation.

Titanium-phosphorus frustrated Lewis pairs (FLPs) based on titanocene-phosphinoaryloxide complexes have been synthesised. The cationic titanium(IV) complex [Cp(2)TiOC(6)H(4)P((t)Bu)(2)][B(C(6)F(5))(4)] 2 reacts with hydrogen to yield the reduced titanium(III) complex [Cp(2)TiOC(6)H(4)PH((t)Bu)(2)][B(C(6)F(5))(4)] 5. The titanium(III)-phosphorus FLP [Cp(2)TiOC(6)H(4)P((t)Bu)(2)] 6 has been synthesised either by chemical reduction of [Cp(2)Ti(Cl)OC(6)H(4)P((t)Bu)(2)] 1 with [CoCp*(2)] or by reaction of [Cp(2)Ti{N(SiMe(3))(2)}] with 2-C(6)H(4)(OH){P((t)Bu)(2)}. Both 2 and 6 catalyse the dehydrogenation of Me(2)HN·BH(3).