Cyclopentadiene complexes with cobalt

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. 

Sneddon et at. (106) described the reaction of cyclopentadiene, pen-taborane(9), and 2-butyne with cobalt atoms, which yielded complex 76 (R = R = CH3, 1.3%), as well as two other metallocarbaboranes. 

Cobaltacyclobutene 39 reacts with alkynes in a net insertion/reductive elimination process to yield stereoisomeric cobalt-77-cyclopentadiene complexes 55 (Equation 17) <1995JA8029>. 

The chiral iron(III) Lewis acid 3, derived from an oxazoline ligand, catalyzes Diels-Alder reactions of A -acryloyl-l,3-oxazolidinone (1) and cyclopentadiene (2) with good enantiomeric excess30. Nickel complexes of chiral phosphanes also catalyze Diels-Alder reactions albeit with low enantiomeric excess, not exceeding 15% cc31. Much better results are achieved for cobalt complexes with chiral phosphanes in the presence of a Lewis acid31,32. 

The cationic aqua complexes prepared from traws-chelating tridentate ligand, R,R-DBFOX/Ph, and various transition metal(II) perchlorates induce absolute enantio-selectivity in the Diels-Alder reactions of cyclopentadiene with 3-alkenoyl-2-oxazoli-dinone dienophiles. Unlike other bisoxazoline type complex catalysts [38, 43-54], the J ,J -DBFOX/Ph complex of Ni(C104)2-6H20, which has an octahedral structure with three aqua ligands, is isolable and can be stored in air for months without loss of catalytic activity. Iron(II), cobalt(II), copper(II), and zinc(II) complexes are similarly active. 

The cationic aqua complexes of the C2-symmetric trans-chelating tridentate ligand 447 proved also highly effective chiral catalysts. The complexes involving the metal(II) perchlorates of iron, cobalt, nickel, copper and zinc produced the main endo adduct of cyclopentadiene and N - aery loy 1-1,3 -oxazo I i din -2 -one with very high ee values281. 

Of the cyclic olefins, norbornadiene replaces two CO groups from one Co to yield a labile complex 159, 160, 235), cyclooctatetraene replaces the axial CO ligands from all three cobalt atoms 53) and is itself replaced by other Lewis bases 330), and cyclopentadiene forms the unusual complex [95] with Co3(CO)gCMe 159,160). A few catalytic reactions were observed with methinyltricobalt enneacarbonyls including the dimerization of norbornadiene 160, 235) and the polymerization of functional olefins 312) with different Co3(CO)9CX. 

A complex of Fe2+ with cyclopentadiene and the B9C2Hii2 ion (a) and the cobalt complex, [Co(B9C2H11)2] (b). There is one hydrogen atom attached to each boron atom in these structures. The hydrogen atoms have been omitted to simplify the drawing. 

They noted that in dimethoxyethane or in iso-octane (path a), the major product was dicarbonylcyclopentadienylcobalt (2) which must arise as a result of a retro Diels-Alder reaction of the norbornadiene (which would lead to the formation of acetylene and cyclopentadiene). When the solvent was changed to an aromatic hydrocarbon such as benzene or toluene (path b), the major cobalt-containing product was shown to be a complex derived from Co4(CO)i2, with three CO ligands on an apical cobalt being replaced by a molecule of the aromatic solvent (3). The group noted that they were also obtaining hydrocarbon and ketonic products derived from norbornadiene, acetylene and carbon monoxide .1,2... 

Borole (IV in Figure 14) has been used to synthesize a plethora of sandwich and half-sandwich stmctures it is the bora analog of cyclopentadiene. Borole is an antiaromatic system see Antiaromatic Compound), and only the blue pentaphenyl derivative has been prepared. The general route to borole complexes consists of complexation under dehydrogenation of 2- or 3-borolenes, which are prepared from [Mg(butadiene)]x and RBX2. Thus, the heating of either isomer of borolene with Co2(CO)g produces [(borole)Co(CO)2]2 (47) (equation 60) from which further cobalt borole complexes are prepared (equation 61). CpCo(borole) (48) can be made from (47) by Cp transfer from the labile 20-electron nickelocene (equation 61). Pyrolysis of (48) at 160-180 °C gives a triple-decker complex (49) in nearly quantitative yield, which in turn can be cleaved by Cp into (48) and the bis(borole)cobaltate anion (50 ) (Scheme 34). 

Another cobalt complex having only hydrocarbon ligands, (1,3-butadiene)cyclopentadienylcobalt, has been obtained as a volatile red solid melting at 103°-105°C by reaction of the diene with dicyclopenta-dienylcobalt or (1-benzoyl-1,3-cyclopentadiene)cyclopentadienylcobalt (489). The compound decomposes slowly in air. 

An increasing volume in the literature is devoted to pseudo-cyclopentadienes and pseudo--arenes. A general paper describes complexation of the cyclo-Ps anion. The synthesis of the first diazacobaltocene has been described. Tert-butyl-substituted diazaferrocenes have been prepared from substituted pyrroles. Bis-(borole)nickel complexes have been prepared from dilithium txnelediides. Carborane stabilised iron indole and iron thiophene complexes have been made. The electrophilic stacking of borataferrocene and bis(borole)cobaltate ions with -Cr(CO)3 and Mn(00)3 has been described. The synthesis and crystal structure of... 

A second one-step synthesis of ferrocene is achieved by the direct action of metal halides on cyclopentadiene in the presence of a base such as di- or tri-ethylamine. This method also can be applied to the preparation of cobalt, nickel, copper, and palladium cyclopentadienyl 7r-complexes, although the yields with these metals are not as high as with iron. 

The cobalt complex 117 reacted with but-2-yne in the presence of HBF, to afford the cycloheptadienyl complex 118 (Scheme 7.45) [63]. The alkyne moiety is inserted into the C-C bond of the cyclopentadiene ring. 

Schemes 58-62. A new non-rigid phosphine ligand was synthesized and reacted with Fe(CO)5 to form the mononuclear iron complex (Equation (81)). Phosphino-oxazoline ligands were used as assembling ligands for hetero-metallic complexes, where the phosphorus atom binds to iron and the nitrogen atoms act as donor atoms to copper, cobalt, or palladium (Scheme 58). The copper complex catalyzes cyclopropanation and Diels-Alder reactions. When 2-(A -diphenylphosphinomethyl-A -cyclohexyl)aminopyridine (NNP) reacts with Fe(CO)5 in ethanol, /ra .r-(OC)3Fe(NNP)2 is formed (Scheme 59). This monometallic complex can then be reacted with a copper salt in CH2GI2 to form a complex having an Fe-Cu dative bond. The complex was demonstrated to be an efficient catalyst for the cyclopropanation of styrene by ethyl diazoacetate and for the Diels-Alder reaction of cyclopentadiene and methacrolein. No other heterometallic complexes have been shown to have such reactivity. Previously known...

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