Pentacoordinate Nickel Complexes Research Articles

Structure and spectroscopic properties of nickel benzazolate complexes with hydrotris(pyrazolyl)borate ligand.

The reaction of benzazole ligands 2-(2'-hydroxylphenyl)benzimidazole (Hpbm), 2-(2'-hydroxylphenyl)benzoxazole (Hpbx), and 2-(2'-hydroxylphenyl)benzothiazole (Hpbt), with [Ni(Tp*)(μ-OH)]2 (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate), leads to pentacoordinate nickel complexes [Ni(Tp*)(pbz)] (pbz = pbm (1), pbx (2), pbt (3)). The structures of 1, 2, and 3 were determined by X-ray crystallography. The pentacoordinate nickel complexes have distorted trigonal bipyramidal geometries with Addison's τ parameter values of 0.63, 0.73, and 0.61 for 1, 2 and 3, respectively. The benzazolates are bonded in an η(2)(N,O) fashion to the nickel atoms. DFT calculations are carried out to optimize the structures of the three complexes giving a good agreement with the X-ray structures. The (1)H NMR spectra of complexes 1-3 exhibit sharp isotropically shifted signals. The complete assignment of these signals required an application of two-dimensional {(1)H-(1)H}-COSY techniques. The experimental absorption spectra of the three complexes in chloroform solution each show an intense absorption band in the ultraviolet region ca. 240 nm, followed by three less intense bands, the first two at ∼295 and ∼340 nm, and the last more disperse one, at wavelengths between 360 and 410 nm. The absorption spectra are simulated by TD-DFT and reproduce the main features of the experimental spectra well. The analysis of the electronic transitions by inspection of the frontier molecular orbitals and also the natural transition orbitals allowed us to characterize and assign the observed bands properly. The three complexes are moderately blue luminescent at room temperature, both in the solid state and in solution. Emission spectra at room temperature display broad structureless bands in chloroform solution at 460, 482, and 512 nm for complexes 1, 2 and 3, respectively, and structured emission in solid state with λmax values of 473, 486, and 516 nm. Complexes containing different donor atoms in the benzazole ligand are furthermore observed to give different luminescence responses in the presence of Zn(II), Cd(II), Hg(II), and Cu(II).

Modularly Designed Transition Metal PNP and PCP Pincer Complexes based on Aminophosphines: Synthesis and Catalytic Applications

Transition metal complexes are indispensable tools for any synthetic chemist. Ideally, any metal-mediated process should be fast, clean, efficient, and selective and take place in a catalytic manner. These criteria are especially important considering that many of the transition metals employed in catalysis are rare and expensive. One of the ways of modifying and controlling the properties of transition metal complexes is the use of appropriate ligand systems, such as pincer ligands. Usually consisting of a central aromatic backbone tethered to two two-electron donor groups by different spacers, this class of tridentate ligands have found numerous applications in various areas of chemistry, including catalysis, due to their combination of stability, activity, and variability. As we focused on pincer ligands featuring phosphines as donor groups, the lack of a general method for the preparation of both neutral (PNP) and anionic (PCP) pincer ligands using similar precursor compounds as well as the difficulty of introducing chirality into the structure of pincer ligands prompted us to investigate the use of amines as spacers between the aromatic ring and the phosphines. By introduction of aminophosphine and phosphoramidite moieties into their structure, the synthesis of both PNP and PCP ligands can be achieved via condensation reactions between aromatic diamines and electrophilic chlorophosphines (or chlorophosphites). Moreover, chiral pincer complexes can be easily obtained by using building blocks obtained from the chiral pool. Thus, we have developed a modular synthetic strategy with which the steric, electronic, and stereochemical properties of the ligands can be varied systematically. With the ligands in hand, we studied their reactivity towards different transition metal precursors, such as molybdenum, ruthenium, iron, nickel, palladium, and platinum. This has resulted in the preparation of a range of new pincer complexes, including various iron complexes, as well as the first heptacoordinated molybdenum pincer complexes and several pentacoordinated nickel complexes by using a controlled ligand decomposition pathway. In addition, we have investigated the use of some of the complexes as catalysts in different C-C coupling reactions: for example, the palladium PNP and PCP pincer complexes can be employed as catalysts in the well known Suzuki-Miyaura coupling, while the iron PNP complexes catalyze the coupling of aromatic aldehydes with ethyl diazoacetate under very mild reaction conditions to give selectively 3-hydroxyacrylates, which are otherwise difficult to prepare. While this Account presents an overview of current research on the chemistry of P-N bond containing pincer ligands and complexes, we believe that further investigations will give deeper insights into the reactivity and applicability of aminophosphine-based pincer complexes.

Nickel Complexes Based on Tridentate Pyrazolyl Ligands for Highly Efficient Dimerization of Ethylene to 1-Butene

A series of new pentacoordinated nickel complexes NiCl2(NZN) based on nitrogen-, oxygen-, or sulfur-bridged bis(pyrazolyl) ligands (NZN) were synthesized and characterized by elemental analysis and an X-ray diffraction study for NiCl2{bis[2-(3,5-dimethylpyrazolyl)ethyl)]amine} (1). Upon activation with methylaluminoxane (MAO), these complexes show high activity in ethylene oligomerization (TOF = 7−80 × 103 h-1), which varies according to the ligand environment. Up to 92% selectivity for 1-butene, combined with a TOF of 60 × 103 h-1, has been obtained under moderate conditions (30 °C, 40 bar, MAO-to-Ni = 250) using NiCl2{bis[2-(5-phenylpyrazolyl)ethyl)]ether} (3) as the catalyst precursor.

Syntheses and nickel coordination properties of cyclen substituted with mixed stilbene/poly(ethylene glycol) dendritic arms

AbstractWe synthesized a series of cyclens substituted with mixed stilbene and poly(ethylene glycol) dendritic arms. All dendrimers terminated with different peripheral groups had good solubility in common organic solvents, and dendrimers terminated with CO2H groups (CO2H‐dendrimers) were also soluble in alkaline solutions. The nickel coordination properties of these dendrimers were investigated in organic solvents. Dendrimers terminated with CN groups (CN‐dendrimers) and the second‐generation CO2H‐dendrimer [(CO2H)8L2] could produce pentacoordinated nickel complexes; the third‐generation CO2H‐dendrimer [(CO2H)16L3] could form tetra‐ and pentacoordinated nickel complexes, and the nickel complex of the fourth‐generation CO2H‐dendrimer [(CO2H)32L4] could not be obtained. This result was due to the fact that the globular surface of (CO2H)16L3 formed a hydrogen‐bond network that selectively penetrated cations and inhibited the access of anions to the core. The formation of the hydrogen‐bond network was confirmed by Fourier transform infrared, 1H NMR, and fluorescence data. The CN‐dendrimers could not form hydrogen bonds on the surface, and the first‐ and second‐generation CO2H‐dendrimers could not form intramolecular hydrogen‐bond networks. Therefore, they had no selectivity for positive nickel ions and negative chloride ions. (CO2H)32L4 could not produce a nickel complex because it had a crammed backbone structure that could not penetrate nickel and chloride ions. Therefore, it was possible to control the ion access of cations and anions with the hydrogen‐bond network of (CO2H)16L3. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5414–5428, 2005

Pentacoordinated nickel complexes as intermediates in carbonylation reactions of allyl compounds

Preparation and properties of a new class of halo(carbonyl)(phosphine)(π-allyl)nickel complexes are discussed. All the possible equilibria appear to exist in solution. Evidence is given that pentacoordinated intermediates are involved in carbonylation reactions