- Synthesis, Structures and Properties of Half-Sandwich Nickel(II) Complexes with Backbone-Modified NHC Ligands
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A series of seven nickel complexes [Ni(η5-C5H5)(X)(NHC)] (X = Cl or Br; NHC = N-heterocyclic carbene) containing backbone-modified NHC ligands was prepared by treatment of nickelocene with the corresponding NHC·HX salts or, in the case of complex 1h [Ni(η5-C5H5)(Cl)(IMesO)] (IMesO = 4-oxo-1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene), by deprotection of the corresponding NHC ligand in complex 1g [Ni(η5-C5H5)(Cl)(IMesOPiv)] [IMesOPiv = 4-(pivaloyloxy)-1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene]. Single-crystal X-ray structures were determined for complexes 1d [Ni(η5-C5H5)(Cl)(Bn2-bimy)] (Bn2-bimy = 1,3-dibenzylbenzimidazolin-2-ylidene), 1g and 1h. The coordination spheres around the nickel atoms resemble those of d8 NiL4 square-planar complexes, with Ni-C(Cp) bond length differentiation and C-C bond alternation in Cp ligands being taken into account. Desymmetrization of the five-membered heterocyclic ring and a tautomeric equilibrium were observed for complex 1h. All new complexes are efficient precatalysts in three C-C bond-forming reactions (polymerization of styrene, polymerization of methyl methacrylate, Suzuki-Miyaura cross-coupling). Complexes [Ni(η5-C5H5)(X)(NHC)] featuring backbone-modified NHC ligands were synthesized. Their solid-state structures, solution dynamics, keto-enol equilibria and catalytic activity in C-C bond-forming reactions (polymerization of styrene, polymerization of methyl methacrylate, Suzuki-Miyaura cross-coupling) were investigated.
- Banach, ?ukasz,Guńka, Piotr A.,G?rska, Dominika,Podlewska, Maja,Zachara, Janusz,Buchowicz, W?odzimierz
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- Facile derivatization of a chemo-active NHC incorporating an enolate backbone and relevant tuning of its electronic properties
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The present report discloses a modular synthetic route to a new type of anionic N-heterocyclic carbene ligand incorporating an enolate group as a reactive backbone component of its heterocyclic framework. The presence of such a reactive unit facilitates further tailoring of the ligand, even after its complexation to transition metals, with concomitant tuning of the electronic donor properties of the carbene center. Simple acylation of the formamidine Ar-NH-CH=NAr (1a,b) (a: Ar = mesityl (Mes); b: Ar = 2,6-diisopropylphenyl (DIPP)) by chloroacetyl chloride gives an acylated formamidine (2a,b), which undergoes a thermally induced cyclization to afford the 4-hydroxyimidazolium salt 3a,b. Single deprotonation by NEt3 gives the zwitterionic imidazolium-4-olate 4, whereas double deprotonation by 2 equiv of LiHMDS gives the imidazolin-2-ylidene-4-olate ligand 5-, which reacts with 0.5 equiv of [RhCl(1,5-COD)]2 to give, after acidification with HCl, the complex [(5H)RhCl(COD)] (7), in which the acidified carbene ligand noted 5H adopts the keto form. By contrast, treatment of 4 with electrophilic reagents E-X (E1 = tBuCO; E2 = Me; E3 = Tf) results in the O-functionalized imidazolium derivatives [4E1-3]+X-, which can be subsequently complexed under the form of the carbene 5E1-3 to give [(5E1-3)RhCl(COD)] (7E1-3). Alternatively, the carbene ligand 5 can be post-functionalized from its complex [(5H)RhCl(COD)] (7) by a simple sequence involving (i) base-induced deprotonation and (ii) addition of E-X, exemplified by the generation of [(5E4-5)RhCl(COD)] (7E4-5) (with E4 = Ph2PO; E5 = tBuSiMe2 (TBDMS)). In parallel, C-functionalization of the metal-bound ligand can be achieved from 7 by deprotonation with LiHMDS, leading to [(5-)RhCl(COD)]Li(thf)n+ followed by a classical aldolization/elimination sequence leading to [(5CH2) RhCl(COD)] (8), in which the C5 carbon of the NHC bears a methylene group. Parallel direct transformations can be achieved after ligand complexation to copper, as exemplified by the generation of [(5H)CuCl] (9) or [(5E5)CuCl] (9E5) (E5 = TBDMS). The donor properties of all the above NHC ligands were evaluated from the standard complexes (NHC)RhCl(CO)2 against the classical IR ν(CO) scale. The resulting order of nucleophilicities for the modulated carbenes is 5 CH2 H ? 5E4 E1 E2 E5 ? 5-. The X-ray structures analyses of 7E2 and 7E4 are reported.
- Benhamou, Laure,Vujkovic, Nadia,Cesar, Vincent,Gornitzka, Heinz,Lugan, Noel,Lavigne, Guy
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experimental part
p. 2616 - 2630
(2010/08/04)
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- Imidazol-2-ylidene-4-olate: An anionic N-heterocyclic carbene pre-programmed for further derivatization
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The 4-hydroxyimidazolium salt, readily prepared in two steps by acylation of a formamidine and quaternization of the second nitrogen, affords, after deprotonation, the anionic imidazol-2-ylidene-4-olate, which can be complexed to a transition metal and st
- Benhamou, Laure,Cesar, Vincent,Gornitzka, Heinz,Lugan, Noel,Lavigne, Guy
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supporting information; experimental part
p. 4720 - 4722
(2010/01/15)
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