Cyclopentadiene complexes with metals

The cyclopentadienide ion, C H , is a common organic anion that forms very stable complexes with metal cations. The anion is derived by removing a proton from cyclopentadiene, QH, with strong base. The molecule has a five-memhered ring of carbon atoms, with four carbon atoms attached to only one proton and one carbon atom bonded to two. Draw the Lewis... 

Cyclopentadiene reacts with metal atoms in a combination of oxidative addition and simple orbital mixing processes. The very reactive M-H species form tetrahapto and trihapto complexes under loss of hydrogen. Important representative complexes are listed in Table 9. 

Some heterocyclic cyclopentadiene derivatives form n complexes with metal carbonyls (47, 55, 373, 384) ... 

Cyclopentadiene is known to be a relatively strong Bronsted acid (piC = 16 ), and as such it is able to form salts. It also forms covalent compounds with most nonmetal elements as well as complexes with metals. 

As mentioned earlier, ruthenium cymene, rhodium and iridium pentamethyl-cyclopentadiene are good metal complexes, to start with, in a screen. It is diffi-... 

Standard organolithium reagents such as butyllithium, ec-butyllithium or tert-butyllithium deprotonate rapidly, if not instantaneously, the relatively acidic hydrocarbons of the 1,4-diene, diaryhnethane, triarylmethane, fluorene, indene and cyclopentadiene families and all terminal acetylenes (1-alkynes) as well. Butyllithium alone is ineffective toward toluene but its coordination complex with A/ ,A/ ,iV, iV-tetramethylethylenediamine does produce benzyllithium in high yield when heated to 80 To introduce metal into less reactive hydrocarbons one has either to rely on neighboring group-assistance or to employ so-called superbases. 

The condensation of aldehyde 2 with cyclopentadiene yields the fulvene 20 which can be transformed to the respective cyclopentadienyl anion 21 by addition of a hydride anion (Scheme 1.5.8). The anion 21 opens up the possibility of the formation of sandwich and half-sandwich type of complexes with suitable metal fragments. 

This review deals with metal-hydrocarbon complexes under the following headings (1) the nature of the metal-olefin and -acetylene bond (2) olefin complexes (3) acetylene complexes (4) rr-allylic complexes and (5) complexes in which the ligand is not the original olefin or acetylene, but a molecule produced from it during complex formation. ir-Cyclopentadienyl complexes, formed by reaction of cyclopentadiene or its derivatives with metal salts or carbonyls (78, 217), are not discussed in this review, neither are complexes derived from aromatic systems, e.g., benzene, the cyclo-pentadienyl anion, and the cycloheptatrienyl cation (74, 78, 217), and from acetylides (169, 170), which have been reviewed elsewhere. 

Several preparations of silylated cyclopentadienylmetal complexes involve the formation of a triorganosilylcyclopentadienyl anion (by treatment of a silylated cyclopentadiene with an alkali metal in tetrahy-drofuran or by metalation with n-butyllithium), followed by reaction with metal chlorides. This type of reaction has been used for the synthesis of silylated ferrocenes (41, 43, 58, 83, 84, 103, 107, 116, 135, 142, 171, 172), cobaltocenes (135), nickelocene (135), titanium cyclopen-tadienyls (46, 145), and cyclopentadienylmanganese tricarbonyl (30) [Eqs. (19) and (20)]. It is remarkable that Si—C5H6 bonds are not... 

The strategies used to synthesize metal complexes with the cyclopentadienyl ligands portrayed in Scheme 13 are briefly described below. The most widely used procedure begins with the corresponding cyclopentadiene derivative, which can be transformed into a cyclopentadienyl metal complex by a variety of metallation methods as described in... 

There is a certain analogy between the aromatic anions of cyclopentadienide (C5H5 ) and boratabenzene (C5H6B ). l-Methylbora-2,5-cyclohexadienehas a more acidic proton connected to the, sp3-hybridized ring carbon atom than cyclopentadi-ene, due to the same tendency of aromatic anion formation [252, 253]. The related 1-phenyl-1,4-dihydroborabenzene affords the lithium salt of 1-phenylborataben-zene on treatment with tert-butyllithium. Like metallic complexes such as ferrocene formed by cyclopentadiene, boratabenzene also forms such sandwich -complexes with iron and cobalt. The iron complex can be acetylated under Friedel-Crafts conditions. 

Treatment of hydrogermanium cyclopentadiene transition metal complexes with LDA can lead initially to a competition between the deprotonation of the hydrogen linked to germanium or to the cyclopentadienyl ring, but a migration of the germyl group to cyclopentadiene was actually observed (equation 192)9. 

Apart from providing direct spectroscopic evidence for the various catalytic steps, experiments with 6.15 and other lanthanide cyclopentadiene complexes also established that paramagnetic metal ions do not have any special effect on the oligomerization reaction. The mechanism proposed on the basis of these studies is as shown in Fig. 6.5. 

The 1,4-diphospha-l,3-butadienes are suitable as ligands for cr-coordi-nated complexes with transition metals. Attempts to carry out pericy-clic reactions with maleinic anhydride, acetylene dicarboxylate esters, dimethylbutadiene, or cyclopentadiene failed, but Diels-Alder reactions with norbornadienes were successful (94). Earlier attempts to synthesize 1,4-diphosphabuta-l,3-dienes with oxalychloride and phenyl-bis(silyl)phosphane proceeded via ring closure [Eq. (42)] (89), where R = Ph (a) or [Pg.285]

The metallole ring is shown theoretically to be an analogue of cyclopentadiene and can form both rf - and 7 -complexes with transition metals. Thus, siloles and germoles react with Fe(CO)s or Fe2(CO)9 to give... 

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