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Synthesis of Organometallic Compounds

Advanced Inorganic Chemistry 92/2

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Ruthenium Complexes

• Recently, the chemistry of ruthenium complexes has been extensively explored.

• less application in organic synthesis than palladium compounds, probably because their chemistry is more complicated.

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Ruthenium Complexes

• Ruthenium complexes generally have 5- or 6-coordinated geometry and their oxidation state can vary between -2 to 6.

• This complexity, however, leads to many interesting reactions and further developments in this field are expected.

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Ruthenium Complexes

• A wide variety of organoruthenium complexes is known.

• They can be roughly divided into 4 groups according to their supporting ligands.

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1. Ru3(CO)12

1. Carbonyl complexes which are generally derived from Ru3(CO)12.

• Air stable compound, easy to handle

• The precursor of an active catalyst for reduction of nitro groups, C—H bond activation or carbonylation.

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2. Ruthenium complexes with tertiary phosphine ligands

• RuCl2L4, RuHClL4, or RuH2L4

• useful for organic synthesis, catalytic reactions, asymmetric reactions.

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3. Cyclopentadienyl complexes

• Cyclopentadienyl and pentamethylcyclopentadienyl ligands effectively stabilize alkyl-ruthenium bonds, whereas in phosphine complexes the alkyl group tends to undergo b-hydrogen elimination.

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Ruthenium complexes having arenes or dienes

• Low valent ruthenium starting materials via replacement of arene or diene ligands

• Catalysts for olefin dimerization, hydrogenation of arenes, or C—C bond cleavage reaction.

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Preparation of these ruthenium complexes

• RuCl3.3H2O and Ru3(CO)12

• They are relatively inexpensive and stable against oxygen.

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Dichlororuthenium Complexes

• RuCl2(PPh3)3

• Coordinatively unsaturated.

• Agostic C-H bond

• A common Ru precursor

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Dichlororuthenium complexes

• Dichlororuthenium complexes are formed by the reduction of RuCl3.3H2O in the presence of the ligand.

• RuCl2(PPh3)3 is obtained by treatment of RuCl3.3H2O with an excess of PPh3 in methanol as air-stable shiny black crystals.

• Reaction of RuCl3.3H2O with PRR’2 or PR2R’ (R = phenyl, R’ = alkyl) gives cationic dinuclear complexes [Ru2Cl3(PRnR’3-n)6]Cl under similar conditions.

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RuCl2(PPh3)3

• The X-ray crystallography of RuCl2(PPh3)3 showed that it has a distorted octahedral geometry with a vacant site which is occupied by an agostic proton of a phenyl group.

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Reactivities of RuCl2(PPh3)3

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N-Alkylation of Amines by Primary Alcohols

• RuCl2(PPh3)3 or RuCl3.3H2O/P(OBu)3 effectively catalyze the N-alkylation of aromatic amines.

• N-alkylation of aliphatic amines with a primary alcohol is carried out in high yield by the use of RuH2(PPh3)4 as catalyst.

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Preparation of heterocycles

N-alkyl piperidine

pyrrolidine

pyrrole

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Oxidation of Amines, Amides, and Diols

• RuCl2(PPh3)3 is also a catalyst for the oxidation of nitriles, amides and lactams under moderate conditions.

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A coordinatively unsaturated 16e- ruthenium(0) complex

• Reduction of RuCl2(CO)2(PtBu2Me)2 with magnesium affords an isolable 16e ruthenium(0) complex Ru(CO)2(PtBu2Me)2.

• Highly reactive toward hydrogen, acetylenes and phosphines to give coordinatively saturated complexes.

Trans phosphines

Two COs are bent.

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RuHCl(CO)(PPh3)3

• Formed by the reduction of RuCl3.3H2O with alcohol in the presence of tertiary phosphines.

• Similarly prepared as Vaska's complex, IrCl(CO)(PPh3)2

• Where does the CO ligand come from?

• Mechansim?

• Stereochemistry: Cl trans to CO

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• Recent developments

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C-H Bond activation

• The generation of coordinatively unsaturated species play an important role.

• These species are usually produced by thermal or photo-mediated reductive elimination of dihydrogen, alkanes, alkenes or arenes.

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• Mechanistic expect

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Dihydridoruthenium Complexes

• Dihydridoruthenum complexes are reported to be catalysts for either the direct or transfer hydrogenation of olefins.

• Ruthenium hydride complexes are also catalysts for organic reactions such as the coupling reaction of alkenes with terminal alkynes, the [2 + 2] cycloaddition of norbornene with alkynes, Tishchenko-type reactions, and the catalytic insertion of olefins into the ortho C—H bond of aromatic ketones.

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Preparation of RuH2(PPh3)4

• RuH2(PPh3)4 is prepared by the reaction of RuCl2(PPh

3)3 with NaBH4 in the presence of PPh3 in refluxing methanol.

• Or by the direct reaction of RuCl3.3H2O with NaBH4 and PPh3 in refluxing ethanol.

• It is formed as an off-yellow powder and should be kept under argon, not nitrogen, because a PPh3 ligand is readily replaced by dinitrogen.

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Reactivities of RuH2(PPh3)4

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Chemoselective aldol reactions

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Coupling reactions of acetylenes with dienes

• The reaction of l-octyne with 1,3-butadiene catalyzed by RuH2(PBu3)4 affords 2- dodecen-5-yne. A similar coupling reaction is also catalyzed by RuCl(C5H5)(C8H12).

Mechanism?

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Tishchenko-type dimerization.

• RuH2(PPh3)4 reacts with aldehydes to give esters via Tishchenko-type dimerization. For example, benzaldehyde is converted to benzyl benzoate by RuH2(PPh3)4. This reaction involves C—H bond activation of the formyl proton followed by formation of a ruthenium acyl alkoxide complex Ru(OCH2Ph)(COPh)(PPh3)4.

Mechanism?

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RuH2(CO)(PPh3)3 catalyze olefin coupling reactions of aromatic ketones via C—H bond activation

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A possible intermediate in theolefin coupling reaction ofaromatic ketone catalyzed byRuH2(CO)(PPh3)3. Other ligandsare omitted.

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Reactivities of RuH2(PPh3)4

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Catalytic reactions

Intermediate:

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Ruthenium Complexes with Chiral Ligand

• the chemistry of ruthenium complexes with the chiral ligands BINAP and PYBOX are described.

Atropisomers of the BINAP Ligand

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Ruthenium Complexes Having Cyclopentadienyl Ligands

• Ruthenocene is relatively un-reactive

• The dinuclear complex [RuCl2(C5Me5)]2 is a versatile reagent.

• prepared by the reaction of RuCl3.3H2O with pentamethylcyclopentadiene in ethanol

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Treatment of Ru2H4(C5Me5)2 with ethylene results in the formation of a divinyl(ethylene)diruthenium complex under ambient conditions. This is an interesting reaction because there are few examples of vinylic C—H bond activation with metal polyhydride complexes.

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A unique reaction probably proceeds via an acetylide-vinylidene intermediate.

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Ruthenium Complexes with Arene/Diene Ligands

• Ru(cod)(cot) is prepared by the reduction of RuCl3.3H2O with zinc powder in the presence of 1,5-cyclooctadiene in methanol [192].

It is used in several catalytic reactions and as a convenient precursor to various zero- or multi-valent ruthenium complexes

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Reactivities of Ru(cod)(cot)

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• Dimerization of NBD

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For example, ruthenium complexes sometimes show ambiphilic reactivity

allyl carbonate

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• Ruthenium-catalyzed allylations are often show quite different reactivities and selectivities from those of palladium-catalyzed allylations.

The detailed mechanism of the regiocontrolling step is still unclear.

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Useful Ru precursors

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