Manab Sharma

Theoretical Investigation into the Mechanism of Reductive Elimination from Bimetallic Palladium Complexes

Reductive elimination of C-Cl and C-C bonds from binuclear organopalladium complexes containing Pd-Pd bonds with overall formal oxidation state +III are explored by density functional theory for dichloromethane and acetonitrile solvent environments. An X-ray crystallographically authenticated neutral complex, [(L-C,N)ClPd(μ-O(2)CMe)](2) (L = benzo[h]quinolinyl) (I), is examined for C-Cl coupling, and the proposed cation, [(L-C,N)PhPd(1)(μ-O(2)CMe)(2)Pd(2)(L-C,N)](+) (II), examined for C-C coupling together with (L-C,N)PhPd(1)(μ-O(2)CMe)(2)Pd(2)Cl(L-C,N) (III) as a neutral analogue of II. In both polar and nonpolar solvents, reaction from III via chloride dissociation from Pd(2) to form II is predicted to be favored. Cation II undergoes Ph-C coupling at Pd(1) with concomitant Pd(1)-Pd(2) lengthening and shortening of the Pd(1)-O bond trans to the carbon atom of L; natural bond orbital analysis indicates that reductive coupling from II involves depopulation of the d(x(2)-y(2)) orbital of Pd(1) and population of the d(z(2)) orbitals of Pd(1) and Pd(2) as the Pd-Pd bond lengthens. Calculations for the symmetrical dichloro complex I indicate that a similar dissociative pathway for C-Cl coupling is competitive with a direct (nondissociative) pathway in acetonitrile, but the direct pathway is favored in dichloromethane. In contrast to the dissociative mechanism, direct coupling for I involves population of the d(x(2)-y(2)) orbital of Pd(1) with Pd(1)-O(1) lengthening, significantly less population occurs for the d(z(2)) orbital of Pd(1) than for the dissociative pathway, and d(z(2)) at Pd(2) is only marginally populated resulting in an intermediate that is formally a Pd(1)(I)-Pd(2)(III) species, (L-Cl-N,Cl)Pd(1)(μ-O(2)CMe)Pd(2)Cl(O(2)CMe)(L-C,N) that releases chloride from Pd(2) with loss of Pd(I)-Pd(III) bonding to form a Pd(II) species. A similar process is formulated for the less competitive direct pathway for C-C coupling from III, in this case involving decreased population of the d(z(2)) orbital of Pd(2) and strengthening of the Pd(I)-Pd(III) interaction in the analogous intermediate with η(2)-coordination at Pd(1) by L-Ph-N, C(1)-C(2).

An alternative strategy to an electron rich phosphine based carbonylation catalystThis paper is dedicated to the memory of Prof. Noel McAuliffe in recognition of his contribution to phosphine chemistry and friendship.

The complexes [Rh(CO)Cl(2-Ph2PC6H4COOMe)], 1, and trans-[Rh(CO)Cl(2-Ph2PC6H4COOMe)2], 2, have been synthesized by the reaction of the dimer [Rh(CO)2Cl]2 with 2 and 4 molar equivalents of 2-(diphenylphosphino)methyl benzoate. The complexes 1 and 2 show terminal ν(CO) bands at 1979 and 1949 cm−1 respectively indicating high electron density at the metal centre. The molecular structure of the complex 2 has been determined by single crystal X-ray diffraction. The rhodium atom is in a square planar coordination environment with the two phosphorus atoms trans to each other; the ester carbonyl oxygen atom of the two phosphine ligands points towards the rhodium centre above and below the vacant axial sites of the planar complex. The rhodium–oxygen distances (Rh⋯O(49) 3.18 A; Rh⋯O(19) 3.08 A) and the angle O(19)⋯Rh⋯O(49) 179° indicate long range intramolecular secondary Rh⋯O interactions leading to a pseudo-hexacoordinated complex. The complexes 1 and 2 undergo oxidative addition (OA) reactions with CH3I to produce acyl complexes [Rh(COCH3)ClI(2-Ph2PC6H4COOMe)], 4, and trans-[Rh(COCH3)ClI(2-Ph2PC6H4COO-Me)(2-Ph2PC6H4COOMe)], 5, and the kinetics of the reactions reveal that the complex 1 undergoes faster OA reaction than that of the complex 2. The catalytic activity of the complexes 1 and 2 in the carbonylation of methanol were higher than that of the well known species [Rh(CO)2I2]− and the complex 1 shows higher activity than 2.

Mono(imidazolin-2-iminato) Titanium Complexes for Ethylene Polymerization at Low Amounts of Methylaluminoxane

The polymerization of ethylene with titanium complexes bearing one bulky imidazolin-2-iminato ligand (L) in the presence of MAO and/or TTPB as cocatalysts have been explored. The complex LTiCl3 and its methylated forms were prepared to shed light on the nature of the active polymerization species. With some of these complexes, the best catalytic activity was obtained at an Al:Ti ratio of 8.

Bis(1,3-di-tert-butylimidazolin-2-iminato) Titanium Complexes as Effective Catalysts for the Monodisperse Polymerization of Propylene

The use of bis(1,3-di-tert-butylimidazolin-2-iminato) titanium dichloride (1) and dimethyl (2) complexes in the polymerization of propylene is presented. The complexes were activated using different amounts of methylalumoxane (MAO), giving in each case a very active catalytic mixture and producing polymers with a narrow molecular weight distribution (polydispersity = 1.10). The use of the cocatalyst triphenylcarbenium (trityl) tetra(pentafluorophenyl)borate totally inhibits the reaction, producing the corresponding bis(1,3-di-tert-butylimidazolin-2-iminato) titanium(III) methyl complex, the trityl radical ((•)CPh(3)), the anionic MeB(C(6)F(5))(4)(-), B(C(6)F(5))(3), and the bis(1,3-di-tert-butylimidazolin-2-iminato) titanium(IV) dimethyl·B(C(6)F(5))(3) complex. The use of a combination of physical methods such as NMR, ESR-C(60), and MALDI-TOF analyses enabled us to propose a plausible mechanism for the polymerization of propylene, presenting that the polymerization is mainly carried out in a living fashion. In addition, we present a slow equilibrium toward a small amount of a dormant species responsible for 2,1-misinsertions and chain transfer processes.

Ligand effects in bimetallic high oxidation state palladium systems

Ligand effects in bimetallic high oxidation state systems containing a X-Pd-Pd-Y framework have been explored with density functional theory (DFT). The ligand X has a strong effect on the dissociation reaction of Y to form [X-Pd-Pd](+) + Y(-). In the model system examined where Y is a weak σ-donor ligand and a good leaving group, we find that dissociation of Y is facilitated by greater σ-donor character of X relative to Y. We find that there is a linear correlation of the Pd-Y and Pd-Pd bond lengths with Pd-Y bond dissociation energy, and with the σ-donating ability of X. These results can be explained by the observation that the Pd d(z(2)) population in the PdY fragment increases as the donor ability of X increases. In these systems, the Pd(III)-Pd(III) arrangement is favored when X is a weak σ-donor ligand, while the Pd(IV)-Pd(II) arrangement is favored when X is a strong σ-donor ligand. Finally, we demonstrate that ligand exchange to form a bimetallic cationic species in which each Pd is six-coordinate should be feasible in a high polarity solvent.

A Golden Ring: Molecular Gold Carbido Complexes

Stannylcarbynes [M(≡CSnMe3)(CO)2(Tp*)] [M = Mo, W; Tp* = hydrotris(dimethylpyrazol-1-yl)borate], which are readily obtained via the successive treatment of [M(≡CBr)(CO)2(Tp*)] with (n)BuLi and ClSnMe3, serve as effective carbyne transmetalation agents for the preparation of heteronuclear molecular gold carbido complexes such as [M(≡CAuPPh3)(CO)2(Tp*)] and the tetrameric golden ring complex [W(≡CAu)(CO)2(Tp*)]4, which are in turn able to transfer the carbido unit to palladium.

Structural chemistry of [MX 2 (bipy)] (M = Pd, Pt; X = Cl, Br, I): the yellow polymorph of dichlorido(2,2'-bipyridine) platinum(II) and diiodido(2,2'-bipyridine)palladium(II), and overview of this system

Synchrotron data have been used to determine the structures of the yellow polymorph of dichlorido(2,2′-bipyridine)platinum(ii) (1), and diiodido(2,2′-bipyridine)palladium(ii) (2), allowing a detailed comparison of the crystal chemistry of [MX2(bipy)] (M = Pd, Pt; X = Cl, Br, I), which exhibit polymorphism involving nine structures distributed over five space groups. Complex 1 crystallizes in space group Pbca (a = 18.2540(7), b = 15.6970(7), c = 7.3560(6) A), and 2 in space group C2/c (a = 17.149(1), b = 9.8050(8), c = 7.548(1) A, β = 110.72(1) °).

η1-Alkynyl Chemistry for the Higher Oxidation States of Palladium and Platinum

This chapter reviews the organometallic chemistry of palladium and platinum, in which the metal atom is bonded to alkynyl ligands and the formal oxidation state of the metal atom is greater than two. Several synthetic methods have been reported, where the most generally applicable involve oxidation of alkynylmetal(II) complexes and reactions of organometal(II) complexes with alkynyl(aryl)iodine(III) reagents. Metal(IV) complexes obtained have octahedral geometry and some have been shown to decompose via reductive elimination processes to generate carbon–carbon bonds. Unsymmetrical metal–metal bonded species formally represented as PtIII–PtIII ↔ PtIV–PtII have been characterised as intermediates in oxidation of PtII to PtIV. Potential implications for mechanisms of organic reactions mediated by higher oxidation state metal centres are discussed.