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Organometallic and Inorganic–Organic Polymers
Published in Charles E. Carraher, Carraher's Polymer Chemistry, 2017
Other inorganic and metal-containing polymers have been formed using the addition approach. These include polyphosphazenes, polyphosphonitriles, and poly(sulfur nitride). Phosphonitrilic polymers (11.50) have been known for many years, but since they lacked resistance to water, they were not of interest as commercial polymers. However, when the pendant chlorine groups are replaced by fluorine atoms, amino, alkoxy, or phenoxy groups, these polymers are more resistant to hydrolysis. Allcock and coworkers have pioneered these efforts. Phosphonitrile fluoroelastomers are useful throughout a temperature range of −56°C to 180°C. Phosphazenes are produced by the thermal cleavage of a cyclic trimer obtained from the reaction of phosphorus pentachloride and ammonium chloride.
Phosphazenes
Published in Leslie R. Rudnick, Synthetics, Mineral Oils, and Bio-Based Lubricants, 2020
Robert E. Singler, Frank J. Gomba
Phosphazenes are ring or chain compounds consisting of alternating phosphorus–nitrogen atoms with two substituents attached to phosphorus. Representative structures are shown below. In these structures, R can be a halogen, organo, or organometallic substituent; X is generally a halide or metal halide counterion. The physical properties of phosphazenes vary considerably with molecular weight and choice of substituents (R). Many of the cyclic phosphazenes are either liquids or low-melting crystalline solids. As the molecular weight is increased, either by the size of the substituent or the number of P–N repeat units, one can obtain oils and greases; ultimately, elastomers and thermoplastics are formed.
N − H bond dissociation free energy of a terminal iron phosphinimine
Published in Journal of Coordination Chemistry, 2022
Heui Beom Lee, R. David Britt, Jonathan Rittle
To obtain further insight into the thermodynamics of HAT, the pKa of 3 was determined by spectrophotometric titration of 2 with diverse acids. Treatment of 2 in THF with the conjugate acid of py, DBU, or TBD (pKa = 5.5, 16.9, 21, respectively) [32,33] leads to effective protonation of 2 to 3. The conjugate acid of Verkade’s base (pKa = 26.6) was not effective, constraining the pKa of 3 between 21 and 26.6 [34]. Using the phosphonium ylide precursor (pKa = 22.7) shown in Figure 6, an equilibrium was established between 2 and 3, and a value of pKa = 24.1 was obtained after mass balance considerations (Figure S12). Using the experimentally determined values of Eo and pKa, application of the Bordwell equation leads to an estimated N − H BDFE of 92 ± 2 kcal/mol in THF. The high BDFE obtained here can be attributed to the simultaneously high oxidation potential and basicity of 2. This is an interesting observation since Eo and pKa are often inversely proportional; a high basicity is typically related to a low redox potential and vice versa. Considering the lower pKa = 19.7 of the phosphazene base (Me2N)3PNH in THF, the higher pKa of the PN − FeIII moiety in 2 is noteworthy. A pKa value of 20.65 has been reported for Ph3PNH, albeit in CH3NO2 [35]. Considering the similar dielectric constants of CH3NO2 and CH3CN (35.9 and 37.5, respectively), a solvent for which pKa values have been extensively tabulated, we extrapolate the pKa of Ph3PNH to be in the range of 13 ∼ 15 in THF using an empirical correlation between pKa values in CH3CN and THF. Thus, we hypothesize that the much higher pKa of the PN − FeIII moiety in 2 is intimately connected to its electronic structure (vide infra). While the HOMO of Ph3PNH assumes a weak P − N π-bonding character, the HOMO of 2 assumes an additional Fe − N π-antibonding character, resulting in increased basicity. Further inductive effects with the previously described 1-adamantyl-substituted version of H3L may increase the basicity of the corresponding PN − Fe species, and efforts on this front are ongoing. Taken together, these data suggest that certain metal-PN species can behave as redox-active superbases apt to engage in hydrogen atom abstraction processes [36].