Rosa M. Ceder

Coordination features of bis(N-heterocyclic carbenes) and bis(oxazolines) with 1,3-alkylidene-2,4,6-trimethylbenzene spacers. Synthesis of the ligands and silver and palladium complexes

The synthesis of simple imidazolium-based ligand precursors containing a 1,3-alkylidene-2,4,6-trimethylbenzene spacer was examined and different synthetic protocols were applied depending on the nature of the alkylidene arm. For a methylene arm, simple dications 5a,b.2CI were obtained directly. The higher homologue counterparts were conveniently prepared by general multistep routes following a five-step sequence for ethylene dications 6a,b.2Br or a six-step sequence for propylene dications 7a,b.2Br in > or = 52% overall yield. Imidazolium salts based on the shorter methylene spacer were used to prepare palladium complexes (17-20) with N-heterocyclic carbenes via transmetallation from well-defined silver compounds or directly in basic conditions. In order to facilitate spectroscopic characterisation of the palladium species two [Pd(allyl)(bis-oxazoline)]+ (25-26) complexes with the same ligand bridge were synthesized. [PdX2bisL] complexes appeared in solution as mixtures of species, mononuclear with cis- or trans-geometry or oligomeric compounds. The reaction of [PdCl(allyl)]2 and micro-bis(carbene)(AgX)2 complexes in 1 : 1 or in 0.5 : 1 ratio leads to binuclear compounds [Pd2Cl2(allyl)2(micro-bis-carbene)] (19a,19b) and to very labile monomeric [Pd(allyl)(bis-carbene)]+ (20a,20b) compounds, respectively. The preparation of analogous [Pd(allyl)(bis-oxazoline)]+ complexes showed the formation of one of the four possible isomers. [Pd(allyl)(bis-oxazoline)]PF6 complexes were inactive as catalytic precursors in the allylic substitution reaction.

Mono- and di-cycloplatinated derivatives of benzalazines

Abstract The reaction of (p-RC6H4CHN)2 (R = NO2, Cl, H, NMe2) with PtCl42− has been studied. Cyclometallation only occurs when R = Cl, giving mixtures of the mono- and di-metallated compounds. [ PtCl( p -ClC 6 H 3 CHN NCH(p-ClC6H4))]2 and [ PtCl( p -ClC 6 H 3 CHN )]n, that react with phosphines, PR′3 = PPh3, PEt3, 1 2 dppe, to yield [PtCl(p-ClC6H3CHNNCH(p-ClC6H4))(PR′3)2] and [PtCl(p-ClC6H3CHN)(PR′3)2]2, respectively. (For the structures of these products see the Scheme.) The ionic compound {[Pt(p-ClC6H3CHN)(dppe)]2}(ClO4)2 is formed from [ PtCl( p -ClC 6 H 3 CHN )]n and dppe in the presence of NaClO4. The results obtained can be explained in terms of electrophilic attack of the metal on the aromatic rings. Structures for the new compounds are assigned on the basis of IR and NMR (1H, 13C, 31P) spectra.

Binuclear mixed metal cyclometallated complexes

The preparation of the first mixed metal cyclometallated compounds [ClPd(p-RC6H3CHNNCH(p-RC6H3))PtCl]n (R = H, Cl) are reported; they were made from monocyclopalladated [(AcO)Pd(p-RC6H3CHNNCH(p-RC6H4)]2 and PtCl42−.

Nickelacycles with anionic C–N–N′ terdentate α-diimine based ligands. Reaction with ethylene

A series of chloro-substituted 1,4-diaza-1,3-butadiene (a–e) and pyridine-imine (f, g) ligands have been prepared. The oxidative addition of five of the bidentate ligands to [Ni(cod)2] gave a group of neutral complexes containing an anionic terdentate ligand [NiCl(CNN′)] with five-and six-membered nickelacycles (1a, 1b, 1e, 1f, 1g). Two isomers were obtained with the 1,4-diaza-1,3-butadiene ligands a and b due to a 1,3-prototropic rearrangement. The reaction of the neutral complexes (1) with TlBF4 in the presence of 2,4,6-trimethylpyridine yields well-defined cationic complexes [NiL(CNN′)]BF4 (2). The low solubility of the neutral compounds (1) complicates their characterisation which could be completed only after preparation of the ionic complexes (2). The molecular structure of compound 2a [NiL[(2-Ph)CH2–NC(CH3)C(CH3)N–CH2–(2-ClC6H4)]]BF4 has been determined by X-ray diffraction methods. Four ionic complexes [Ni(NCCH3)(CNN′)]BF4 (3) have been obtained and tested as precursors of catalytic species in the reaction of ethylene oligomerisation. The catalytic results are limited, as expected, due to the structural type of the nitrogenated ligands. However it is possible to use these relatively stable nickelacycles as one-component precursors for the oligomerisation reaction. Similar palladacycles are not suitable for this purpose.

Nickelacycles with anionic C–N–N′ terdentate ligands containing (5,5), (5,6), (5,7) and (6,5) membered fused rings. Reaction with ethylene

A series of nickelacycle complexes with di-Schiff bases acting as terdentate CNN− anionic ligands has been prepared. Oxidative addition of one C–X bond in mono- or di-ortho-halogenoaryl substituted ligands, N,N′-dibenzylidenealkane-1,2-diamines (ethane, propane and butanediamines) and N,N′-bis(1,2-diphenylethylidene)ethane-1,2-diamine, to [Ni(cod)2] gives neutral complexes [NiX(CNN)] containing (5,5), (5,6), (5,7) and (6,5)-membered fused chelate rings (NiCN and NiNN respectively). Complexes from monohalogenoaryl substituted ligands are obtained as mixtures of two non-interchanging configurational isomers (E or Z stereochemistry about the non-metallated imine group). Cationic complexes [NiL(CNN)]BF4 are generated by halide abstraction from these neutral complexes by using TlBF4 and 2,4,6-Me3C5H2N or CH3CN as stabilizing ligand. In ionic compounds from monohalogenoaryl substituted ligands the formation of isomers by restricted rotation around the Cphenyl–Cmethinic bond is also observed. The 1H NMR spectra of complexes with (5,5), (5,6), (6,5) fused rings show fluxional behavior, the data being consistent with interconversion of two chelate ring conformations that are associated with the relative position of the non-metallated methinic moiety in the lower or the upper side of the coordination plane. Molecular structures of three ionic complexes [Ni(2,4,6-Me3C5H2N)(CNN)]BF4, were determined by X-ray single-crystal diffraction, revealing the effects of chelate sizes on the distortion of the coordination sphere around nickel. Ionic complexes [Ni(CH3CN)(CNN)]BF4 were tested as precursors of active species in the reaction with ethylene under pressure. The influence of the nickelacycle ring size on the insertion reaction is discussed, with moderate activities being observed only when the Ni–C bond is involved in a six-membered ring.