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Introduction to Organometallics
Published in Samir H. Chikkali, Metal-Catalyzed Polymerization, 2017
Samir H. Chikkali, Sandeep Netalkar
The significance of bite angle was realized much later and the concept of Natural Bite Angle (βn) was introduced to describe the properties of chelating bidentate ligands. The natural bite angle can be simply defined as an angle with which the two donor atoms of a bidentate ligand bite into the metal.9,10 The bite angle is a geometric factor associated with the bi- and higher dentate ligands used to describe the angle of the ring made at the metal center by the chelating ligand (as depicted in Figure 1.11). This angle is a measure of distortion from a perfect geometry due to electronic and steric factors present in the ligand itself or the overall complex including the nature of the metal in the complex. Thus, ligands with denticity of more 1.3.8 than one can potentially coordinate to a metal with certain bite angle. The ligand bite angle can be calculated by molecular modeling of the backbone.
Living Polymerizations of π-Conjugated Semiconductors
Published in John R. Reynolds, Barry C. Thompson, Terje A. Skotheim, Conjugated Polymers, 2019
Jeffrey Buenaflor, Christine Luscombe
McNeil et al. investigated the ligand effects on the Ni catalyst by altering the electronics or sterics of the phosphine groups. They found that the rate-determining step in KCTP can change depending on the catalyst employed in the polymerization. Figure 6.5 displays the Ni catalysts used in the electronic and steric studies.41–44Table 6.1 shows the rate-determining step for KCTP for each of the employed catalysts. Ni(dppp)Cl2 and Ni(dppe)Cl2, which are commonly used initiators for KCTP, influence the mechanism differently. The rate-determining steps with the two catalysts are transmetalation and reductive elimination, respectively. Sterics are attributed to be the major influence these ligands have on the mechanism of KCTP. The main structural differences between the two bidentate ligands are bite angle (dppp = 91°; dppe = 85°) and chelate ring size (six vs. five). Bite angles and cone angles are influenced by the steric bulk of ligands. Different observations seen through the addition of LiCl confirmed the effect of having different rate-determining steps in KCTP. When transmetalation is the rate-determining step, the monomer is directly involved. The addition of LiCl increases the propagation of polymerization through the formation of Grignard ate complexes.45,46 The addition of LiCl to KCTP initiated by Ni(dppe)Cl2 has no effect on the polymerization rate, since the monomer is not involved in reductive elimination. Further studies on catalysts with bis(diakylphosphino) ethane-based ligands [dmpe = 1,2-bis(dimethylphosphino)ethane, depe = 1,2-bis(diethylphosphino)ethane, and dcpe = 1,2-bis(dicyclohexylphosphino)ethane] of varying steric bulk were investigated. Polymerization with Ni(dmpe)Cl2 is ineffective due to catalyst decomposition. Oligomers were primarily formed with the more sterically bulky Ni(dcpe)Cl2. This observation was due to the dissociation of the Ni(0) π-aryl complex, allowing for side reactions to occur. The rate-determining step for Ni(dcpe)Cl2 is transmetalation, which was expected since steric bulk increases the rate of reductive elimination to relieve the strain while slowing transmetalation. Ni(depe)Cl2 synthesized P3HT and poly-(para-phenylene) (PPP) through chain-growth similar to Ni(dppe)Cl2, albeit with different polymerization rates. These two catalysts have similar bite and cone angles, suggesting the influence of ligand-based electronics, since the depe ligand is more electron donating than dppe.
Synthesis, spectroscopic characterization and solid-state electrical behavior of [Ni(L)2] [L = 1,2-di(4-methoxyphenyl)ethene-1,2-dithiolate]: a computational investigation on UV–vis-NIR absorption and electrical transport
Published in Journal of Coordination Chemistry, 2023
Arghya Dutta, Sanjay Mondal, Soumya Biswas, Vinayak B Kamble, Shubhamoy Chowdhury, Rajarshi Ghosh
In 1, the nickel center has square planar geometry coordinated by four sulfur donors of the two dithiolene ligands. The optimized structure of 1 is shown in Figure 1. The calculated bond distances and bond angles for 1 are listed in Tables 1 and 2, respectively. The Ni-S bond lengths in optimized 1 are 2.2099 and 2.2103 Å and are found in the typical range of NiS4 complexes, 2.10 to 2.14 Å [3a, 9, 17]. All distances of the Ni―S bonds are almost equal in 1. The bite angle of the two ligands is 91.500° and bite angles are typically 90° to 92° [1, 3a, 9]. The average bond lengths for S―C and C〓C are 1.767 and 1.399 Å, respectively. The benzyl groups are not in the same plane with the square-planar chelate rings and the dihedral angle C3-C1-C2-C9 is 10.351°. Good agreement between the calculated geometry of the probable structure of 1 and the X-ray crystal structure geometry reported in the literature [3a, 9, 17] is found.