Pentadienyl complex

The acid cleavage of l-hydroxy-2,4-pentadiene iron tricarbonyl gives the pentadienyl cation, 4.27 [264]. As shown, the same complex may be made 

The ease of the removal of the hydroxyl group may be compared with the ether cleavage reaction discussed on p. 210. 

Some interesting hydrolysis and reduction reactions of a related 1-hydroxy-2,complex, 4.29, have been described [265], 

It is interesting that some metal complexes of the pentalene system have been prepared [266]. 

The organic chemistry of n-cydopentadienyl transitfaMi metal fmmpiPTO 

The transition metals form many open (9.97a) and closed (9.97b) dienyl complexes. Open complexes comprise those containing the pentadienyl C5H7 ligand and its derivatives CsHy. R, while closed compounds possess cyclohexadienyl, cycloheptadienyl, cyclooctatrienyl, etc. Pentadienyl and its derivatives C5H7 j,R as Se ligands are usually coordinated in the U form (9.97a),although complexes are 

4-C5H5Me2)2(PEt3) (M = Zr, Nb) have synperiplanar conformation, as do titanium and vanadium compounds M(2, 4-C5H5Me2)2L (L = CO, Pen- 

The reaction of 2,4-C5H4Me with C0CI2 leads to the dimer (9.99). Many 

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Lothar Stahl, Pentadienyl Complexes of the Group 4 Transition Metals. 

A scandium complex, Cp ScH, also polymerizes ethylene, but does not polymerize propylene and isobutene [125]. On the other hand, a linked amidocyclo-pentadienyl complex [ Me2Si( / 5-C5 Me4)( /1 -NCMe3) Sc(H)(PMe3)] 2 slowly polymerizes propylene, 1-butene, and 1-pentene to yield atactic polymers with low molecular weight (Mn = 3000-7000) [126, 115]. A chiral, C2-symmetric ansa-metallocene complex of yttrium, [rac-Me2Si(C5H2SiMe3-2-Buf-4)2YH]2, polymerizes propylene, 1-butene, 1-pentene, and 1-hexene slowly over a period of several days at 25°C to afford isotactic polymers with modest molecular weight [114]. 

In 1992, Green reported the unexpected synthesis of an 774-vinylke-tenemolybdenum (II) complex while investigating the reactivity of r 4-buta-dienylmolybdenum (II) species.77 He had previously found that upon deprotonation of the cationic 774-butadienemolybdenum(II) complex 56, the neutral 73-pentadienyl complex 57 was formed.78 He also established that this was in fact a fully reversible process, and that upon treatment with... 

Pentadentate 2,2 6, 2"-terpyridine, 30 107 Pentadiene complexes with iron, 12 255-257 with rhodium, 12 295 with silver, 12 340 Pentadienyl complexes with group VIB metals, 12 224, 225 with group VIIB metals, 12 240 JV,Af,/V,lV",N"-Pentaethyldiethylenetriamine, 42 76... 

In subsequent investigation by the authors (1) other high-activity monocyclo-pentadienyl complexes were prepared, (I), and used in the preparation of polyethylene and poly(ethylene-l-hexene). 

Bis(oxazolinyl)phenylrhodium complex, in styrene asymmetric hydrosilylation, 10, 821 Bis(pentadienyl) complexes preparation and properties, 5, 336 with titanium(II), 4, 265... 

Considering the organometallic side of Scheme III (right), usually all the compounds located on this side are able to produce the neighboring systems on the left by Broensted acid-base-type reactions, e.g., alkyls might readily react with amines, cyclopentadienes and alcohols to yield amide, alkoxide and cyclo-pentadienyl complexes, respectively. The silylamide route has attracted enormous attention for the synthesis of pure alkoxides (A in Scheme III, Eq. 12)... 

Ernst has recently reviewed structural and spectroscopic features of pentadienyl compounds. He discusses experimental and theoretical studies of pentadienyl anions, radicals, and cations and also the structures of transition metal pentadienyl complexes (21). He has also written an account of his own work in this field (22). 

The preparation of pentadienylsodium was being studied at about the same time as ferrocene was discovered. It was not until 1968, however, that the first binary pentadienyl complex of a transition element, bis(pentadienyl)-chromium, was obtained from PINa and CrCl2 (121). This compound forms green, air-sensitive crystals, and like chromocene it has two unpaired electrons (Ht = 2.74 BM). This discovery was shortly followed by that of the curious complex, bis(pentadienyl)dinickel (12) which was prepared from NiCl2 and triethylaluminium in 1,4-pentadiene (122). The pentadienyl ligands in 12... 

In principle pentadienyls can bond to transition elements in at least three basic ways, tj3, and tjs (Fig. 1). These can be further subdivided when geometrical factors are considered. If r 5 coordination could be converted to rj3 orr/1, one or two coordination sites could become available at the metal center, and perhaps coordinate substrate molecules in catalytic processes. Little is known about the ability of pentadienyl complexes to act as catalysts. Bis(pentadienyl)iron derivatives apparently show naked iron activity in the oligomerization of olefins (144), resembling that exhibited by naked nickel (13). The pentadienyl groups are displaced from acyclic ferrocenes by PF3 to give Fe(PF3)5 in a way reminiscent of the formation of Ni(PF3)4 from bis(allyl)nickel (144). 

The molecular structures of three >/3-pentadienyl complexes having W, S, and U configurations, respectively, are shown in Fig. 5. Apart from showing the short uncoordinated C=C bond of the vinyl system a common feature is the unsymmetrical bonding of the fj3-allyl group with the longest M—C bond to the atom which bears the vinyl substituent (149). 

Some f/3-pentadienyl complexes of nickel, [CpNi(>j3-H2CCMeCHCMe= CH2)] (syn and anti isomers) (18 and 19) (150) have arisen in the course of work on the insertion of alkenes into metal-alkyl bonds, relevant to Ziegler... 

There has been one report of an rj1 -pentadienyl complex of a transition element, Cp2Zr(2-MeC5H6)2, prepared from Cp2ZrCl2 and 2-methyl-pentadienylpotassium (155). Cp2ZrHCl, however, give -complexes of 1,3-pentadiene and its higher homologs, when treated with pentadienyl-potassiums. The reactions of these products with alkenes, alkynes, dienes, and carbonyl compounds are synthetically important (156). 

The first acyclic pentadienyl complexes to be prepared were pentadienyl-tricarbonyliron salts. They were discovered by Mahler and Pettit (158), who... 

Some nucleophiles (OMe, H ) add to the central carbon atom of the dienyl unit in 55, yielding 1,4-pentadienyl complexes (207). Halide ions, however, attack the metal center to form -pentadienyl complexes. A singlecrystal X-ray diffraction study of RhCp(>/5-l-PhC5H6)+PF6 reveals an essentially planar pentadienyl ligand with the phenyl group twisted by 26° out of the plane (213). 

A versatile method for the preparation of allyl complexes of transition elements involves addition of allyl halides or acetates to low oxidation state coordinatively unsaturated, or potentially unsaturated, complexes (214). A few attempts have been made to extend the method to halogenopentadienes. l-Chloro-5-phenyl-2,4-pentadiene adds to Pd2(dba)3 (dba = dibenzoylac-etone) to give the >/3-pentadienyl complex 61 [Eq. (30)] (215). A similar palladium complex has also been obtained from PdCl2, and 2,5-dimethyl-2,4-hexadiene in isopropanol (216). 

Mo(CH3CN)3(CO)3 reacts with allyl chloride to form MoCl( /3-C3H5)-(CH3CN)2(CO)2 (214). 1-Bromo- and l-chloro-2,4-hexadienes give analogous f 3-pentadienyl complexes (219). 

Although >/5-pentadienyl complexes have been known for a long time, the first examples being [Fe(C0)3(f/5-hexadienyl)][C104] 183) or [Cr( js-pentadienyl)2] and [Cr(//5-pentadienyl)2(CO)] (184), increasing interest in these ligands has arisen in the last few years. Homoleptic and mixed f/5-dienyl complexes of Ti (185), V, Cr (186), Mo (187), Mn (188-190), and Fe (191) have been prepared by metathetical reactions. Photochemistry provides an easy access to Mn derivatives. Quite recent is the development of syntheses for >/3-dienyl complexes. Examples are known for Mn (192,193), Fe (795), Co (194), and Ni (195,196). 

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