“…Solvents were purified by standard procedures and stored under argon. Literature procedures were employed to prepare NaCp N and triphenylphosphinegold(I) chloride. , The reagents M(CO) 6 (M = Cr, Mo, W), concentrated aqueous HCl, trifluoroacetic acid, phosphoric acid, and glacial acetic acid were used as received (Aldrich). Solution infrared spectra were acquired on a Nicolet Magna 550 FTIR spectrometer with samples sealed in 0.1 mm gastight NaCl cells.…”
Protonation of (2-(dimethylamino)ethyl)cyclopentadienyl (CpN) group VI metal carbonyl
anions with acetic acid proceeds at the amine to provide zwitterionic (2-(dimethylammonium)ethyl)cyclopentadienyltricarbonylmetalates, M(CO)3(η5-CpNH) (M = Cr (1), Mo (2), W (3)).
Few zwitterionic organometalates with anionic metal centers and positive charges that cannot
be delocalized have been reported to date. The impact of the intramolecular charge separation
on the structures and reactivity of zwitterionic organometalates is of current interest. The
M(CO)3 units of structurally characterized 1−3 are significantly perturbed in solution and
the solid state due to ion-pairing between the pendant ammonium cation and anionic metal
center. The concentration and solvent dependence of the 1H NMR ammonium hydrogen
chemical shifts of 1−3 and the intramolecular N···M separations of these zwitterions in the
solid state are consistent with three-center−four-electron N−H···M hydrogen bonding. One
other hydrogen bond with a zerovalent group VI metal acceptor has been characterized to
date. The intramolecular charge separation attenuates the reactivity of the anionic metal
centers of 1−3 relative to that of [M(CO)3(η5-Cp)]-. Structurally characterized [WH(CO)3(η5-CpNH)]Cl (4) was the only isolable salt obtained from protonation of 1−3; the Cr and Mo
analogues are unstable relative to 1 and 2, respectively. Structurally characterized
[M(AuPPh3)(CO)3(η5-CpNH)]Cl (M = Mo (5), W(6)) were synthesized via reactions of 2 and 3,
respectively, with Ph3PAuCl and protonation of M(AuPPh3)(CO)3(η5-CpN) (M = Mo (8), W
(9)) with HCl. Pure samples of the analogous chromium salt were inaccessible from either
1 or structurally characterized Cr(AuPPh3)(CO)3(η5-CpN) (7).
“…Solvents were purified by standard procedures and stored under argon. Literature procedures were employed to prepare NaCp N and triphenylphosphinegold(I) chloride. , The reagents M(CO) 6 (M = Cr, Mo, W), concentrated aqueous HCl, trifluoroacetic acid, phosphoric acid, and glacial acetic acid were used as received (Aldrich). Solution infrared spectra were acquired on a Nicolet Magna 550 FTIR spectrometer with samples sealed in 0.1 mm gastight NaCl cells.…”
Protonation of (2-(dimethylamino)ethyl)cyclopentadienyl (CpN) group VI metal carbonyl
anions with acetic acid proceeds at the amine to provide zwitterionic (2-(dimethylammonium)ethyl)cyclopentadienyltricarbonylmetalates, M(CO)3(η5-CpNH) (M = Cr (1), Mo (2), W (3)).
Few zwitterionic organometalates with anionic metal centers and positive charges that cannot
be delocalized have been reported to date. The impact of the intramolecular charge separation
on the structures and reactivity of zwitterionic organometalates is of current interest. The
M(CO)3 units of structurally characterized 1−3 are significantly perturbed in solution and
the solid state due to ion-pairing between the pendant ammonium cation and anionic metal
center. The concentration and solvent dependence of the 1H NMR ammonium hydrogen
chemical shifts of 1−3 and the intramolecular N···M separations of these zwitterions in the
solid state are consistent with three-center−four-electron N−H···M hydrogen bonding. One
other hydrogen bond with a zerovalent group VI metal acceptor has been characterized to
date. The intramolecular charge separation attenuates the reactivity of the anionic metal
centers of 1−3 relative to that of [M(CO)3(η5-Cp)]-. Structurally characterized [WH(CO)3(η5-CpNH)]Cl (4) was the only isolable salt obtained from protonation of 1−3; the Cr and Mo
analogues are unstable relative to 1 and 2, respectively. Structurally characterized
[M(AuPPh3)(CO)3(η5-CpNH)]Cl (M = Mo (5), W(6)) were synthesized via reactions of 2 and 3,
respectively, with Ph3PAuCl and protonation of M(AuPPh3)(CO)3(η5-CpN) (M = Mo (8), W
(9)) with HCl. Pure samples of the analogous chromium salt were inaccessible from either
1 or structurally characterized Cr(AuPPh3)(CO)3(η5-CpN) (7).
“…However, by their very nature, Cp 2 TiCl 2 and Cp 2 VCl 2 are flawed as drugs because of hydrolysis problems. , The consequence of this is that the nature of the active species is unknown and the administration of the compound as a drug can be difficult. We have been interested in the synthesis of water-soluble cyclopentadienyl compounds for some time. − The compounds presented here will help to solve some of the problems normally associated with the water insolubility and instability of this type of compound. 1 Synthesis of Compounds 5 and 7…”
A number of new ionic titanocene compounds have been isolated and characterized. These
exhibit significant cytotoxicity against a number of different human tumor cell lines including
a defined cisplatin-resistant cell line. One of the compounds is an order of magnitude more
active than its nonfunctionalized equivalent.
“…Recently, the modifi cation of the Cp ring with pendant functional groups designed to turn the ligand into a hemilabile or constrained geometry bidentate chelate ligand moved into the area of ligand design [ 125 , 126 ]. Among the most important examples are Cp ligands with tethered phosphanes [ 125 , 126 ], amines [ 127 ], thioethers [ 126 , 128 ], ansa -bis -Cp ligands [ 129 -133 ] and of course NHC modifi ed Cp ligands [ 134 -137 ]. A closely related, but neutral, ligand donating six rather than fi ve (Cp) electrons is the tethered benzene ligand family [ 138 ].…”
We have already seen that many functionalised NHC ligands are modelled on previously known functionalised phosphanes. In the case of an oxygen -containing functional group where the oxygen atom can carry a (formal) negative charge, we can expect functionalised NHC ligands that resemble phosphinophenols [ 1 , 2 ], phosphinoalcohols [ 3 -5 ] or phosphino carboxylic acids [ 6 ]. All these functionalised phosphane groups were employed as ligands in catalytic reactions.The rationale behind the concept of NHC ligands carrying anionic oxygen functional groups goes beyond the application in catalysis. NHC ligands do not bond strongly to the early transition metals [ 7 ] or alkaline earth metals [ 8 ]. However, both these groups of metals bind strongly to alkoxides or aryloxides. It is therefore reasonable to design functionalised NHC ligands with a view to an oxygen (nitrogen) based anchor group in addition to the somewhat loosely binding carbene unit. This results in the formation of a hemilabile chelate ligand [ 9 ].Conversely, NHC bind strongly to the late transition metals (group 9 and group 10), metals that have only a low affi nity to oxygen. Here, the oxygen -containing functional group serves as the hemilabile ligand [ 10 ]. We will therefore see a great versatility in the anionic tether and in the range of (transition) metal employed.Thiel and coworkers reacted imidazole with epoxycyclohexane to form the racemic hydroxycyclohexyl imidazole. Attempts to separate the enantiomers by kinetic resolution with lipase B of Candida antarctica and isopropenyl acetate as acylating agent [ 11 , 12 ] failed, but gave the racemic ester in high yields (see Figure 4.1 ). Alkylation was then
Functionalised N-Heterocyclic Carbene ComplexesOlaf Kühl
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