Allenylidene and higher cumulenylidene complexes [M]=C(=C)n=CR1R2 (n = 1, 2, 3) have continuously gained significance in the context of transition metal carbene chemistry. Important developments which have been disclosed during the last two years are reviewed. These include a variety of stoichiometric and catalytic reactions of allenylidene complexes and their utility in organic synthesis. The related chemistry of butatrienylidene (n = 2) and pentatetraenylidene (n = 3) complexes as well as theoretical studies are also reviewed.

The allenylidene complexes [M(CCCR2)(η5-C9H7)L2][PF6] (M = Ru, L = PPh3, L2 = 1,2-bis(diphenylphosphino)ethane (dppe), bis(diphenylphosphino)methane (dppm), R2 = 2 Ph (1a−c), C12H8 (2,2‘-biphenyldiyl) (2a−c); M = Os, L = PPh3, R2 = 2Ph (3), C12H8 (4)) have been prepared by reaction of the complexes [MCl(η5-C9H7)L2] with HC⋮CC(OH)R2 and NaPF6 in refluxing methanol. The crystal structures of [M(CCCPh2)(η5-C9H7)(PPh3)2][PF6]·CH2Cl2 (M = Ru (1a), Os (3)) were determined by X-ray diffraction methods. In the structures the MCCC chains are nearly linear (M−C(1)−C(2) = 168.5(5)° (1a) and 169.3(4)° (3); C(1)−C(2)−C(3) = 168.2(7)° (1a) and 168.0(5)° (3)) with MC(1) distances of 1.878(5) Å (1a) and 1.895(4) Å (3). The indenyl ligand is η5-bonded to the metal with the benzo ring orientated “cis” with respect to the allenylidene group. Extended Hückel molecular orbital calculations have been used to rationalize the preferred “cis” orientation. The reaction of [RuCl(η5-C9H7)L2] (L = PPh3, L2 = dppe, dppm) with HC⋮CCMe(OH)Ph and NaPF6 in refluxing methanol leads to the formation of the allenylidene complexes [Ru{CCC(Me)Ph}(η5-C9H7)L2][PF6] (6a−c) along with the vinylvinylidene isomers [Ru{CC(H)C(Ph)CH2}(η5-C9H7)L2][PF6] (L = PPh3 (5a), L2 = dppe (5b), dppm (5c)). Only complex 6a could be isolated by chromatography (SiO2) from these mixtures along with complex 7a obtained from the deprotonation of the vinylvinylidene complex 5a. The treatment of these reaction mixtures with potassium carbonate yields the neutral σ-enynyl derivatives [Ru{C⋮CC(Ph)CH2}(η5-C9H7)L2] (7a−c). The monosubstituted allenylidene complex [Ru{CCC(H)Ph}(η5-C9H7)(PPh3)2][PF6] (9) has been prepared by the reaction of [RuCl(η5-C9H7)(PPh3)2] with HC⋮CCH(OH)Ph and NaPF6 in methanol. Under similar reaction conditions [RuCl(η5-C9H7)L2] reacts with HC⋮CCH(OH)R and NaPF6 to afford the alkenylmethoxycarbene derivatives [Ru{C(OMe)C(H)CH(R)}(η5-C9H7)L2][PF6] (L2 = dppe, R = Ph (11b); L2 = dppm, R = Ph (11c), H (13)). [RuCl(η5-C9H7)(PPh3)2] also reacts with HC⋮CC(OH)H2 to give the hydroxyvinylidene complex [Ru{CCH(CH2OH)}(η5-C9H7)(PPh3)2][PF6] (12), which is stable toward the dehydration process.

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The indenyl complex [RuCl(η 5 -C 9 H 7 )(PPh 3 ) 2 ] (1) reacts with monodentate (L: PMePh 2 , PMe 2 Ph, PMe 3 ) or bidentate [L-L: Ph 2 PCH 2 PPh 2 (dppm), Ph 2 P(CH 2 ) 2 PPh 2 (dppe)] phosphines to give monosubstituted [RuCl(η 5 -C 9 H 7 )(PPh 3 )(L)], bisubstituted [RuCl(η 5 -C 9 H 7 )(L) 2 ], or chelated complexes [RuCl(η 5 -C 9 H 7 )(L-L)] in toluene or tetrahydrofuran. The corresponding cyclopentadienyl complex [RuCl(η 5 -C 5 H 5 )(PPh 3 ) 2 ] (2) reacts similarly, at higher temperatures or longer reaction times. In refluxing toluene, PMe 3 and dppm give ionic products [Ru(η 5 -C 9 H 7 )(L) 3 ]Cl. The kinetics of PPh 3 substitution by PMePh 2 and PMe 2 Ph in tetrahydrofuran yield first-order rate constants that are independent of the concentration or the nature of phosphine. Rate decrease in the presence of added PPh 3 or saturation behavior at high [PPh 3 ] indicates that the reaction proceeds by a dissociative mechanism, in which extrusion of PPh 3 is rate determining. Kinetics for the reaction with PMePh 2 in the temperature range 12-40 °C for the indenyl and 20-50 °C for the cyclopentadienyl complex give the following activation parameters: ∆H q) 26 ( 1 kcal mol -1 and ∆S q ) 11 ( 2 cal mol -1 K -1 for 1 and ∆H q) 29 ( 1 kcal mol -1 and ∆S q ) 17 ( 2 cal mol -1 K -1 for 2. Complex 1 is 1 order of magnitude more reactive than 2, indicating more efficient stabilization of 16-electron intermediates RuCl(η 5 -ligand)(PPh 3 ) by the indenyl group. Cyclic voltammetry measurements for [RuCl(η 5 -ligand)(L) 2 ] in dichloromethane indicate that indenyl or pentamethylcyclopentadienyl complexes are oxidized at lower potentials than cyclopentadienyl complexes. Kinetics and electrochemistry suggest that indenyl is electron donating toward the metal fragment, with respect to cyclopentadienyl.

Treatment of complex trans-[RuCl(2)(eta(2)-C(2)H(4))[kappa(3)-N,N,N-(R,R)-Ph-pybox]] [(R,R)-Ph-pybox = 2,6-bis[4'-(R)-phenyloxazolin-2'-yl]pyridine] with phosphines or phosphites in dichloromethane at 50 degrees C leads to the formation of novel ruthenium(II)-pybox complexes trans-[RuCl(2)(L)[kappa(3)-N,N,N-(R,R)-Ph-pybox]] [L = PPh(3) (1 a), PPh(2)Me (2 a), PPh(2)(C(3)H(5)) (3 a), PPh(2)(C(4)H(7)) (4 a), PMe(3) (5 a), PiPr(3) (6 a), P(OMe)(3) (7 a) and P(OPh)(3) (8 a)]. Likewise, reaction of trans-[RuCl(2)(eta(2)-C(2)H(4))[kappa(3)-N,N,N-(R,R)-Ph-pybox]] with PPh(3) or PiPr(3) in refluxing methanol leads to the complexes cis-[RuCl(2)(L)(kappa(3)-N,N,N-(R,R)-Ph-pybox] [L = PPh(3) (1 b), PiPr(3) (6 b)]. No trans-cis isomerisation of complexes 1 a-8 a has been observed. Complexes 1 a-8 a, 1 b, 6 b together with the analogous trans-[RuCl(2)[P(OMe)(3)][kappa(3)-N,N,N-(S,S)-iPr-pybox]] (10 a) and the previously reported trans- and cis-[RuCl(2)(PPh(3))[kappa(3)-N,N,N-(S,S)-iPr-pybox]] (9 a and 9 b, respectively) are active catalysts for the transfer hydrogenation of acetophenone in 2-propanol in the presence of NaOH (ketone/cat/NaOH 500:1:6). cis-Ph-pybox derivatives are the most active catalysts. In particular, cis complexes 1 b and 6 b led to almost quantitative conversions in less than 5 min with a high enantioselectivity (up to 95 %). A variety of aromatic ketones have also been reduced to the corresponding secondary alcohols with very high TOF and ee up to 94 %. The overall catalytic performance seems to be a subtle combination of the steric and/or electronic properties both the phosphines and the ketones. A high TOF (27 300 h(-1)) and excellent ee (94 %) have been found for the reduction of 3-bromoacetophenone with catalyst 6 b. Reductions of alkyl ketones also proceed with high and rapid conversions but low enantioselectivities are achieved.

The diphenylallenylidene complexes [Ru(CCCPh2)(η5-C9H7)L2][PF6] (L = PPh3; L2 = 1,2-bis(diphenylphosphino)ethane (dppe), bis(diphenylphosphino)methane (dppm)) (1 a−c) react with NaOMe to yield the methoxyalkynyl derivatives [Ru{C⋮CC(OMe)Ph2}(η5-C9H7)L2] (3 a−c). Protonation of these species gives back the starting allenylidene derivatives. Regioselective additions on the Cγ are also observed when 1 a,b are treated with LiR (R = Me, nBu), giving the alkynyl complexes [Ru{C⋮CC(R)Ph2}(η5-C9H7)L2] (4a,b, 5 a,b). Vinylidene derivatives [Ru{CC(H)C(R)Ph2}(η5-C9H7)(PPh3)2][BF4] (6a, 7a) can be prepared by protonation of complexes 4a and 5a with HBF4. The diphenylallenylidene compound 1c reacts with LitBu to yield the metallacycle (8c). The alkynyl complexes [Ru{C⋮CC(C⋮CR)Ph2}(η5-C9H7)(PPh3)2] (R = Ph, nPr, H) (9a−11a) have been obtained by reaction of 1a with lithium or sodium acetylides. Protonation of these derivatives yields the vinylidene complexes [Ru{CC(H)C(C⋮CR)Ph2}(η5-C9H7)(PPh3)2][BF4] (12a−14a). The crystal structure of [Ru{C⋮CC(C⋮CH)Ph2}(η5-C9H7)(PPh3)2] (11a) was determined by X-ray diffraction methods. In the structure the alkynyl chain is nearly linear (Ru−C(1)−C(2) = 175.0(2)°) with Ru−C(1) and C(1)−C(2) distances of 1.993(2) and 1.209(3) Å, respectively. The monosubstituted allenylidene complex [Ru{CCC(H)Ph}(η5-C9H7)(PPh3)2][PF6] (2a) reacts with PMe3, PMe2Ph, PMePh2, and PPh3 to yield the cationic alkynyl−phosphonio derivatives [Ru{C⋮CC(PR3)(H)Ph}(η5-C9H7)(PPh3)2][PF6] (17a−20a) in a regioselective way. Similarly, allenylidene complexes 1 a−c add PMe3 to give the corresponding alkynyl−phosphonio derivatives 15 a−c. [Ru{C⋮CC(PMe3)Ph2}(η5-C9H7)(dppm)][PF6] (15c) undergoes an isomerization process to yield the thermodynamically more stable allenyl−phosphonio complex [Ru{C(PMe3)CCPh2}(η5-C9H7)(dppm)][PF6] (21c). [Ru{C(PMe2Ph)CCPh2}(η5-C9H7)(dppm)][PF6] (22c) can be obtained directly by addition of PMe2Ph to the Cα atom of 1c. The behavior of the diphenylallenylidene complexes 1 a−c toward sodium 2-methylthiophenolate is also discussed.

Heating under reflux solutions of the monosubstituted vinylidene complex [Ru{CC(H)Ph}(η5-C9H7)(PPh3)2][PF6] (1) in nitriles yields the complexes [Ru(N⋮CR)(η5-C9H7)(PPh3)2][PF6] (R = Me (2a), Et (2b), Ph (2c)) and phenylacetylene. The process proceeds via an initial η1-vinylidene-η2-alkyne tautomerization followed by the displacement of the coordinated π-alkyne by the solvent. Vinylidene complexes [Ru{CC(H)R}(η5-C9H7)(PPh3)2][PF6] (R = (η5-C5H4)Fe(η5-C5H5) (3), 4-NO2-C6H4 (4)) also react with acetonitrile to yield the nitrile derivative 2a and the corresponding terminal alkynes HC⋮CR. Cationic alkenyl−vinylidene derivatives [Ru{CC(H)CHCR1R2}(η5-C9H7)(PPh3)2][BF4] (R1 = R2 = Ph (7a), R1 = H; R2 = 4-OMe-C6H4 [(Z)-7b], 4-NO2-C6H4 [(E,Z)-7c], (η5-C5H4)Fe(η5-C5H5) [(E)-7d]) behave similarly. Thus, the treatment of 7a−d with acetonitrile at reflux results in the formation of complex 2a and the liberation of the corresponding terminal 1,3-enyne HC⋮CCHCR1R2 (8a−d). The formation of the enynes 8b−d is stereoselective, giving rise to the E stereoisomer. The allenylidene complex [Ru{CCC(C13H20)}(η5-C9H7)(PPh3)2][PF6] (9), containing the bicyclic [3.3.1]non-2-en-9-ylidene moiety C13H20, reacts with NaC⋮CH in THF at −20 °C to yield the neutral σ-alkynyl derivative [Ru{C⋮CC(C⋮CH)C13H20}(η5-C9H7)(PPh3)2] (10) in a regioselective manner. Protonation of 10 with HBF4·Et2O, in diethyl ether at −20 °C, affords the vinylidene complex [Ru{CC(H)C(C⋮CH)C13H20}(η5-C9H7)(PPh3)2][BF4] (11), which can be easily demetalated by heating in refluxing acetonitrile to give 2a and the unprecedented diyne (HC⋮C)2CC13H20 (12). These demetalation processes allow the quantitative recovery of the metal auxiliary as the labile complex 2a, which can be used as starting material for further reactions. Ab initio molecular orbital calculations on the η1-vinylidene to η2-alkyne tautomerization have been performed. It is shown that the process proceeds through a 1,2-[H] shift mechanism showing that the conversion requires an energy barrier of 29.9 kcal/mol. This is a value low enough to be overcome under the experimental reaction conditions allowing the formation of the labile η2-alkyne complex and the subsequent exchange of the coordinated alkyne by acetonitrile.