Monoligated Complex Research Articles

Isolable dicarbon stabilized by a single phosphine ligand.

In contrast to naturally occurring F2, O2 and N2, diatomic C2 is an intriguing species that has only been observed indirectly in the gas phase, and because of its high reactivity has eluded isolation in the condensed phase. It has previously been stabilized in L→C2←L compounds but the bonding situation of the central C2 in this motif differs remarkably from that of free C2. Here we have prepared and structurally characterized diatomic C2 as a monoligated complex L→C2 using a bulky phosphine ligand bearing two imidazolidin-2-iminato groups (L is (NHCR=N)2(CH3)P, where NHCR is an N-heterocyclic carbene). The compound is stable in solution at ambient temperature and has also been isolated in the solid state. Reactivity studies, in combination with quantum chemical analysis, suggest that the two carbon atoms of the L→C2 complex both have carbene character. The complex underwent intermolecular C-H bond activation upon thermolysis and exhibited hydroalkoxylation-like reactivity with methanol.

Electrostatic, sequential bond energies and structures of Li+·(N2)n complexes: computational study

The MP2 and CCSD calculations of the geometries and binding energies of the Li+·(N2)n (n = 1–4) complexes are obtained. The potential energy surface showed that these complexes exhibit one minimum state and one transition state. The mono- and di-ligated complexes exhibit linear configurations with a binding energy of 11.1 and 21.2 kcal mol−1, respectively. Trigonal planar and tetrahedral configurations are obtained for tri- and tetra-ligated complexes, respectively. The computed sequential bond dissociation energies (BDEs) of Li+·(N2)n (n = 1–4) complexes are also calculated in which the mono-ligated complex has the largest BDE value. The obtained trend is mainly dependent on the variation in the ion-quadrupole interaction of these ion complexes. These calculations predict that these complexes are of purely electrostatic nature.

Structure and potential energy surface of Na+/0·(O2)n (n = 1–3) complexes

The bonding and structures of sodium oxides complexes, Na+/0·(O2)n, were studied at various levels of density functional theory (DFT). The neutral sodium complexes have stronger binding energies than those of cationic sodium complexes. The mono-ligated complex exhibited the shortest Na–O distance and the strongest bond dissociation energy in these two sets of complexes. In addition, the neutral and cationic sodium complexes have identical sequential bond dissociation energy trends that are highly dependent on the strength of the electrostatic interaction within the complex.

Synthesis, structure, and catalytic activity of titanium complexes with chiral biaryl Schiff-base ligands

A series of chiral organo-titanium complexes have been prepared from the reaction between Ti(OiPr)4 and chiral biaryl Schiff-base ligands 1H2–12H. The steric demand of the ligand plays an important role in the formation of the titanium complexes. For example, treatment of ligand 1H2 with 1 equiv of Ti(OiPr)4 in toluene at room temperature gives, after recrystallization from a toluene solution, the chiral bis-ligated titanium complex (L1)2Ti (14). While under similar reaction conditions, the more bulky ligands 2H2, 4H2, and 6H2 form the mono-ligated titanium complexes (L2)Ti(OiPr)2 (15), (L4)Ti(OiPr)2 (19), and (L6)Ti(OiPr)2 (22), respectively, in good yields. The mono-ligated titanium alkoxides can be converted to bis-ligated complex via ligand redistribution reaction. For one instance, treatment of mono-ligated complex (L2)Ti(OiPr)2 (15) in benzene at 60°C results in the isolation of the bis-ligated complex (L2)2Ti (16) in 92% yield. All titanium complexes have been characterized by various spectroscopic techniques and elemental analyses. The solid-state structures of complexes 14–21, 23, 24 and 29 have further been confirmed by X-ray diffraction analyses. The titanium complexes are active catalysts for the asymmetric hydrophosphonylation of aromatic aldehydes with moderate enantioselectivities.

Chelating Effect as a Driving Force for the Selective Formation of Heteroligated Pt(II) Complexes with Bidentate Phosphino-Chalcoether Ligands

The halide-induced ligand rearrangement reaction (HILR) has been employed to provide selective and exclusive in situ formation of heteroligated Rh(I), Pd(II), and Pt(II) complexes with bidentate phosphino-chalcoether ligands. To gain insights on the nature of this unique reaction, we explored this process via the stepwise addition of bidentate phosphino-chalcoether (P, X; X = S or Se) and relevant monodentate phosphine ligands with a Pt(II) metal precursor. The corresponding monoligated complexes were obtained in quantitative yields by reacting 1 equiv of a P, X bidentate ligand with Pt(II) and were fully characterized via single crystal X-ray diffraction studies and heteronuclear ((31)P, (77)Se, and (195)Pt) NMR spectroscopy in solution. These species were further reacted with a second equivalent of either a bidentate ligand or the monodentate ethyl diphenylphosphine ligand, resulting in the clean formation of the heteroligated species or, in the case of the monodentate ligand with an electron-withdrawing bidentate ligand, a mixture of products. On the basis of competitive exchange reactions between these heteroligated, homoligated, and monoligated complexes, we conclude that ligand chelation plays a crucial role in the Pt(II) HILR. The in situ preferable formation of the stable monoligated complex allows for ligand sorting to occur in these systems. In all cases where the heteroligated product results, the driving force to these species is ligand chelation.

Surveying Sterically Demanding N-Heterocyclic Carbene Ligands with Restricted Flexibility for Palladium-catalyzed Cross-Coupling Reactions

Heterocyclic carbenes (NHCs), especially monodentate ones, have become the ligand of choice for many transition-metal-catalyzed transformations. They generally form highly stable complexes, have strong sigma-donor character, and have a unique shape that can be used to generate sterically demanding ligands.In this Account, we survey recent developments in the design and synthesis of some sterically demanding NHCs with a particularly strong influence on the metal's coordination sphere. We show the successful and insightful application of these ligands in transition-metal catalysis. First, we discuss methods for determining and classifying the electronic and steric properties of NHCs. In addition, we present data on the most important NHC ligands.The selective variation of either electronic or steric parameters of NHCs, and therefore of the catalyst, allows for the optimization of the reaction. Thus, we prepared several series of differentially substituted NHC derivatives. However, because the substituents varied were not directly connected to the carbene carbon, it was difficult to induce a large electronic variation. In contrast, an independent variation of the ligands' steric properties was more straightforward. We highlight three different classes of very sterically demanding NHCs that allow this kind of a steric variation: imidazo[1,5-a]pyridine-3-ylidenes, bioxazoline-derived carbenes (IBiox), and cyclic (alkyl)(amino)carbenes (CAAC).These latter NHC ligands can facilitate a number of challenging cross-coupling reactions. Successful transformations often require a monoligated palladium complex as the catalytically active species, and the sterically demanding NHC ligand favors this monoligated complex. In addition, the electron-rich NHC facilitates difficult oxidative addition steps. Moreover, the conformational flexibility of the ligands can facilitate the formation of catalytically active species and hemilabile interactions, such as agostic or anagostic bonds, as well as stabilize coordinatively unsaturated catalyst species. The increasing level of understanding of the role of NHC ligands in transition-metal catalysis will soon allow the design of even more sophisticated ligand systems.

In-situ gas-phase luminescence and time-of-flight mass spectroscopic detection of photofragments during photochemical synthesis of copper particles from bis(tert-butylacetoacetato)copper.

During the 308 nm laser-driven photochemical synthesis of Cu particles from bis(tert-butylacetoacetato)copper, gas-phase photogenerated intermediates are identified by luminescence and time-of-flight mass spectroscopies. Pure Cu deposits are obtained as homogeneous, granular 200 nm particles. In the gas phase, luminescent photoproducts are observed and atomic Cu, Cu2, and dissociated ligand are identified spectroscopically. In addition, mass spectroscopy identifies Cu atoms, the dissociated ligand, a monoligated complex, and fragments of the ligands. The implications of the photofragmentation that produces copper atoms and dimers for the laser-assisted production of the Cu deposits are discussed.

REACTIVITY OF p-DIKESTONES STRUCTURE AND STABILITY CONSTANT OF COMPLEXES OF DIMEDONE -2- PHENYLAZO WITH Cu2+, Ni2+, Co2+, Zn2+, AND Mn2+.

The formation of chelate compounds between dimedone-2-phenylazo and some tran- sition metal ions including Cu, Ni, Co, Zn and Mn was investigated spectrophotometrically, conductometrically and potentiometrically. Solid complexes were prepared, investigated by ir and analyscd for their elements. The results claimed to formation of a monoligated complex for the studied ions except in case of Co where a bilagated complex predominates. The stability constant were calculated and found to be in the order: Cu > Ni > Zn > Co > Mn