Chemistry of the late first-row transition metals supported by a silicon-based pincer ligand

Abstract

A number of modern chemical industries utilize precious metals to catalyze chemical transformations. However, the earth-abundant, first-row transition metals have received great interest in recent decades to replace costly precious metal catalysts. In particular, pincer ligands and their metal complexes have been extensively investigated and their chemical reactivity explored, with the aim of producing various candidates as robust metal catalysts. The properties of the pincer metal complexes can be drastically changed by substituting donor atoms onto the ligand. A silicon atom, which has a strong electron-donor ability and great trans influence, has been recently introduced to the pincer ligand scaffolds. This dissertation deals with the coordination chemistry of the first-row late transition metals supported by a silicon-based pincer type ligand, $RSiP_2$ ($[RSiP_2]^$- = [$RSi(2-P^iPr_2-C_6H_4)_2$]-

R = Me or Ph). In Chapter 2 and 3, the preparation and characterization of tridentate silyl pincer complexes of nickel will be described. A synthetically reliable nickel(II) amido complex ($MeSiP_2$)Ni(NHMes) was prepared while an analogous species ($MeSiP_2$)Ni(NHTrip) reveals a chemical interconversion to a metallacyclic compound ($\kappa^2-^iPr_2PC_6H_4SiMe(NHTrip)$)Ni($\kappa^2-^iPr_2PC_6H_4$). The reactivity of ($MeSiP_2$)Ni(NHMes) toward $CO_2$ was further investigated. From an instantaneous reaction with $CO_2$, a new four-coordinate nickel(II) carbamato species ($MeSiP_2$)Ni(OC(O)NHMes) was successfully generated and fully characterized by various spectroscopic techniques and X-ray crystallography. Under aerobic conditions, ($MeSiP_2$)Ni(NHMes) undergoes a $CO_2$ reaction to give a nickel(II) carbonato species ${(MeSiOPP^O)Ni}_2-\mu-CO_3-\kappa^2O,O$. Meanwhile, by changing the ligand to $PhSiP_2$, nickel(II) amido complexes ($PhSiP_2$)Ni(NHMes) and ($PhSiP_2$)Ni(NHTrip) were generated from the reaction of nickel(II) halido complex with the lithiated arylamines. These species could be readily converted to nickel(II) phenyl species ($(MesNH)SiP_2$)Ni(Ph) and ($(TripNH)SiP_2)Ni(Ph)$, possibly via the reductive elimination-oxidative addition pathway. Addition of $CO_2$ to both the nickel(II) amido and the nickel(II) phenyl species resulted in the formation of the nickel(II) carbamato complex ($PhSiP_2$)Ni(OC(O)NHMes), revealing an unusual amido group transfer at the silicon center. In Chapter 4, a series of cobalt complexes supported by a silyl pincer ligand will be discussed. During the metalation, low-temperature stable paramagnetic intermediates were observed. Various spectroscopic techniques including X-ray crystallography and ENDOR spectroscopy unveiled the $\sigma-SiH$ interaction with the cobalt center in the intermediate species, $(MeSi^HP_2)CoBr_2$, which possesses an unusual $Co-(\eta^1-HSi)$ binding mode. Frozen solution ENDOR experiments show a Co-H distance of $~1.53\angstrom$ and Si-Co-H angle of $~15\circ$. Its stepwise deprotonation with base followed by dehalogenation was experimentally and theoretically identified. The restricted coordination geometry of the cobalt center might affect to the acidity of the silane moiety. In Chapter 5, a series of rare ($\eta^2-SiH$) copper complexes were synthesized and structurally characterized. $MeSi^HP_2$ copper carbazolido complex and its derivatives, $(MeSi^HP_2)Cu(Cbz^R)$ emit bright green fluorescence at ~500 nm. This could be an evidence of a small geometry relaxation of the copper complexes due to the reduction of Jahn-Teller distortion in the excited state. In comparison, analogous $PP^{Me}P$ copper complexes $(PP^{Me}P)Cu(Cbz^R)$, which have no sigma interaction to the metal, reveal no visible fluorescence. The $\sigma-SiH$ moiety in the copper carbazolido species might inhibit the geometrical relaxation of the copper center, resulting in enhanced emission.