Synthesis and crystal structure of an iron triazole complex resulting from the unexpected ligand cleavage of a triazolium carbene precursor.

Typically reactions of N-heterocyclic carbenes with transition metals are straightforward and require a carbene salt, a base strong enough to deprotonate such a salt and a metal. Yet when carbene precursors are in the form of triazolium salts, reaction may not proceed as easily as expected. In our work, we intended to obtain a triazolylidene complex of iron(II) chloride, but due to the presence of small amounts of water in the tetrahydrofuran solvent used, bis(acetonitrile)tetrakis(1-benzyl-1H-1,2,4-triazole-κN4)iron(II) µ-oxido-bis[trichloridoferrate(III)] acetonitrile disolvate, [Fe(C9H9N3)4(CH3CN)2][Fe2Cl6O]·2CH3CN - an interesting anion with a linear geometry of the O atom - was formed instead of the iron carbene complex. Reaction proceeded via cleavage of the alkyl N-substituent of the triazolium salt. The formation of the product was confirmed by X-ray crystallography. The crystal structure and possible reaction pathways are discussed.

Axially chiral racemic half-sandwich nickel(ii) complexes by ring-closing metathesis.

A remarkable nickelacycle has been synthesised via olefin metathesis of the α,ω-diene complex [Ni(η5-C5H4R)(Br)(NHC)] (R = C(CH3)2CH2CH[double bond, length as m-dash]CH2, NHC = 1-(6-hexenyl)-3-(2,4,6-trimethylphenyl)-imidazol-2-ylidene). Single-crystal X-ray analysis reveals a helical shape of the molecule and stretching of some interatomic distances in the Ni(ii) coordination sphere. The Cp-NHC tethered complex shows catalytic activity resembling that of the parent complexes.

Novel imidazolium and imidazolinium salts containing the 9-nickelafluorenyl anion--synthesis, structures and reactivity.

Investigation of the properties of carbene complexes is one of the most important fields of modern coordination chemistry. In this paper, we propose the convenient synthesis of NHC-nickel compounds. The 9-nickelafluorenyllithium complex reacts with imidazolium or imidazolinium salts to afford 9-nickelafluorenyl-NHC salts via ionic metathesis with very good yields (66-92%). These compounds can be isomerised at elevated temperatures to give Ni-NHC complexes with excellent yields (88-91%), probably via nickel mediated hydrogen transfer to the biphenyl moiety. In this reaction, the nickelacyclic ring itself serves as a base in the deprotonation of the carbene precursor. DFT calculations show the thermodynamic instability of the synthesized salts, with Gibbs free energy differences for 1 of -84 kJ mol(-1) at 298 K and -167 kJ mol(-1) at 374 K. The obtained salts and carbene complexes are relatively air and moisture stable in the solid state.

Anion exchange in [Ni(η(5)-C(5)H(4)R)(Cl)(NHC)]. Counterion effect on the structure and catalytic activity.

A series of novel complexes [Ni(η(5)-C5H4R)(L)(NHC)](+)A(-)2a-2j and [Ni(η(5)-C5H5)(A)(NHC)] 3a-3c has been obtained by anion metathesis from the corresponding chlorides 1a-1d, depending on the anion binding properties and reaction conditions. Solid-state structures of two cationic complexes (2c, 2j) and two complexes with a coordinated anion (3a, 3c) have been determined by X-ray diffraction revealing a trigonal planar geometry in all cases. Unexpectedly, 3c displayed unprecedented for this type of compounds temperature-dependent NMR spectra that were interpreted in terms of spin equilibrium. The cationic complexes 2 were less efficient in styrene polymerization than the parent chlorides 1. However, the activity of 2 and 3 in Suzuki cross-coupling did not depend considerably on the counterion.

Nickelacyclic-cobaltocene vs. nickelacyclic-nickelocene. Synthesis, X-ray structures, electron transfer activity, EPR spectroscopy, and theoretical calculations.

Reactions of 9-nickelafluorenyllithium with cobalt and nickel pentamethylcyclopentadienyl-acetylacetonates resulted in the formation of the novel nickelacyclic-cobaltocene 2 and nickelacyclic-nickelocene 3, respectively, in which the central metal atom is bonded to the nickelafluorenyl ring. On the basis of their redox propensity, compounds 2 and 3 were oxidized to the corresponding monocations [2](+) and [3](+). The crystal and molecular structures of both the redox couples were determined by single-crystal X-ray analysis. In spite of their structural similarity, they display a rather different electron transfer ability. To throw light on such an aspect, the pertinent redox couples have been examined by EPR spectroscopy and the nature of the frontier orbitals involved in the redox activity of the neutral precursors has been supported by extended Huckel theoretical calculations.

[(8a,9,9a-eta)-9-(eta5-cyclopentadienyl)-9-nickelafluorenyl](eta5-pentamethylcyclopentadienyl)nickel(II).

The title compound, [Ni(2)(C(5)H(5))(C(10)H(15))(C(12)H(8))] or [Ni(C(10)H(15)){Ni(C(5)H(5))(C(12)H(8))}], is a rare example (and the first obtained from nickelafluorenyllithium) of an analogue of nickelocene in which the central Ni atom is coordinated to one pentamethylcyclopentadienyl ring and one nickelafluorenyl ring. Both rings lie almost parallel to one another: the dihedral angle between the planes which include these rings is 4.4 (1) degrees . Slip parameter analysis indicates that the bonding mode of the central Ni atom to the nickelacyclic ring is between eta(3) and eta(5). Two-dimensional layers of molecules are formed by C-H...pi interactions.

Electron transfer activity of nickelacyclic complex analogues of nickelocene: synthesis of (eta(5)-R-cyclopentadienyl){eta(4)-[1-(eta(5)-R-cyclopentadienyl)]-2,3,4,5-tetraphenyl-1-nickela-2-cyclopentenyl}nickel complexes (R = H, CH(3)) and crystal structures of the redox couples [(eta(5)- methylcyclopentadienyl){eta(4)-[1-(eta(5)-methylcyclopentadienyl)]-2,3,4,5-tetraphenyl-1-nickela-2-cyclopentenyl}nickel]((0/+)) and [(eta(5)-methylcyclopentadienyl){eta(5)-[1-(eta(5)-methylcyclopentadienyl)-1-nickelafluorenyl}nickel]((0/+)).

Following previous reports on the synthesis and structure of different nickelacyclic complexes analogues of nickelocene, we now deal with the new metallacyclic compounds (eta5-R-cyclopentadienyl){eta4-[1-(eta5-R-cyclopentadienyl)]-2,3,4,5-tetraphenyl-1-nickela-2-cyclopentenyl}nickel (R = H, CH3). The redox ability of the whole series of nickelacyclic derivatives has been also investigated by electrochemical and spectroelectrochemical techniques, and the nature of the frontier orbitals responsible for the rich electron transfer activity of this class of compounds has been supported by theoretical considerations. On the basis of the redox properties of a few neutral members of the series, their chemical oxidation afforded the corresponding monocations and the crystal structures of the pertinent redox couples were determined by X-ray single-crystal analysis.