The synthesis, Characterization, and Reactivity of First-Row Late Transition Metal Complexes Containing Tridentate Pincer-Type N-Heterocyclic Phosphenium Ligands

by Sadie E. Knight

Institution: Brandeis University
Year: 2015
Keywords: ligand; coordination; transition metal; synthesis; NHP; phosphenium
Record ID: 2058033
Full text PDF: http://hdl.handle.net/10192/29092


First-row late transition metal complexes featuring N-heterocyclic phosphine- and phosphenium-containing tridentate pincer ligands have been synthesized and their reactivity has been investigated. The coordination chemistry of a diphosphine pincer ligand incorporating an N-heterocyclic phosphenium cation ([PPP]+) and its halophosphine precursors ([PP(X)P] has been explored using cobalt(II), iron(II) and copper(I) reagents. Treatment of CoCl2 with [PP(Cl)P] led to the neutral coordination complex [PP(Cl)P]CoCl2, which led to the fluoride-abstracted species [PP(F)P]CoCl2 upon exposure to AgPF6 instead of forming a phosphenium-cobalt species. This behavior highlights the electrophilic nature of the [PPP]+ ligand within a [(PPP)CoCl2][PF6] intermediate. When the [PP(X)P] precursors were treated with copper(I) halides, simple [PP(X)P]CuY coordination compounds formed (X and Y = Cl or I). The iodophosphine-copper complex [PP(I)P]CuI was shown to react with THF, resulting in a ring-opening insertion to form the alkoxyphosphine complex [PP(O-CH2CH2CH2CH2-I]CuI, a behavior that was also observed for iron species. Reacting the halide-free phosphenium ligand [PPP][BPh4] with CuCl led to [PP(Ph)P]CuCl, the product of phenyl group abstraction from [BPh4]- by the electrophilic phosphenium-copper intermediate species, [(PPP)CuCl][BPh4]. Arylphosphine products [PP(Mes)P]CuCl and [(PP(Mes)P)-Cu][BArF20] were obtained via transmetallation between starting materials and mesityl migration, respectively. Treatment of CuCl with [PPP][BArF20] led to the dimerization product [(PP(Cl)P-Cu)2][BArF20]. Preliminary density functional theory (DFT) calculations suggest that [(PP(Cl)P-Cu)2][BArF20] resulted from the coupling of cationic chlorophosphine- and phosphenium-copper monomer intermediates. Finally, a new tridentate hemilable mixed-donor ligand containing a central cationic N-heterocyclic phosphenium donor with a rigid phenylene backbone flanked by two ethylenepyridine side arms ([NPN]+) was synthesized. Using DFT methods, it was shown that [NPN]+ was less electrophilic and a weaker π-acceptor than [PPP]+. The coordination chemistry of the phosphenium precursor ([NP(Cl)N]) with a Pd(PPh)3 resulted in a trimetallic NHP-bridged complex [NPN]2Pd3(PPh3)3Cl2, where the pyridyl groups do not take part in any coordination. Structural and theoretical studies suggest an NHP+ phosphenium description for the μ-NHP units, bound to zero valent palladium centers. Formation of diamagnetic species [NP(Cl)N]CuCl occurs upon treatment of CuCl with [NP(Cl)N], while the reaction between [NPN][BArF20] and CuCl yields a paramagnetic product, proposed to be either [(NPN)CuICl][BArF20] or diradical [(NPN)CuIICl][BArF¬20].