Recently, metal-ligand cooperation has attracted much attention in multi electron catalytic reac-tions. Especially, non-innocent behaviors of the $\pi$ -conjugated ligands have been researched as a new method-ology to expand the role of transition metal. Most of known systems employ a noninnocent motif such as aryl-substituted bis(imino)pyridine and pyridine/acridine studied by the Chirik group as well as aromatiza-tion/dearomatization to tightly couple with the redox change of metal studied by the Milstein group which is therefore known as metal-ligand cooperation. Although various phosphine ligands have been widely utilized in organometallic chemistry, examples of a noninnocent phosphine behavior are relatively rare. Recently, the Thomas group has reported a central cationic N-heterocyclic phosphenium (NHP) ligand which has a planar three-coordinate phosphorus center.
We have established the interconversion between $(PPP)Ni(II) (PPP^{-} = P[2-P^{i}Pr^{2}-C_6H_4]_2^{-})$ and $(PP^{OR}P)Ni(0)$ via P-O bond formation and cleavage. Dinitrogen species, $(PP^{OMe}P)Ni(N_2)$ and ${(PP^{OMe}P)Ni}_2(\mu -N_2)$ were synthesized from the reaction of (PPP)NiCl with a methoxy anion. Under CO(g) atmosphere, a dinitrogen ligand was substituted by a carbonyl ligand at a nickel(0) center producing a monocarbonyl nickel complex $(PP^{OMe}P)Ni(CO)$. However, by exposing $CO_2(g)$, metal-ligand cooperation occurs to generate (PPP)Ni(OCOOMe) via a P-O bond cleavage followed by a migratory methoxy group transfer.
In this study, the mechanisms of a reversible conversion between phosphide-nickel(II) and phosphinite-nickel(0) via P-O bond formation and cleavage and methylcarbonate generation with two equivalent of iodomethane were explained. Dinitrogen species, $(PP${OMe}P)Ni(N_2)$ and ${(PP%{OMe}P)Ni}_2(\mu -N_2) were produced from the reaction of (PPP)NiCl with a methoxy anion generated from the solution of tetramethylammonium hydroxide in methanol. Under Ar(g) atmosphere, $(PPP)Ni(II)-OR (R = Me and ^{i}Pr)$ species were generated and converted to the corresponding $(PP^{OR}P)Ni(0)-L$ by $N_2$ and CO exposure, respec-tively. To understand the reactivity of phosphide-Ni(II) species, several reactions of $(PPP)Ni(O^{i}Pr)$ with organic azides and isocyanides were conducted.
금속-리간드 협력 반응을 통한 다전자 촉매 반응을 위한 전이금속 착물에 대한 연구가 최근들어 주목 받고 있다. Chirik 교수진에서 보고한 bis(imino)pyridine이나 acridine 기반 리간드의 산화-환원 활성 리간드 시스템과 Milstein 교수진에서 연구되고 있는 aromatiza-tion/dearomatization을 통한 금속-리간드 협력 시스템 등이 잘 알려져 있는 예이다. 또한 N-heterocyclic phosphenium을 이용한 포스핀 기반 리간드의 금속-리간드 협력 시스템이 Thomas 교수진에 의해 발표되었다. 한편, PPP리간드 지지형 저원자가 니켈 착물의P-O 결합 형성과 절단을 통한 (PPP)Nil(II) $(PPP = P[2-P^{i}Pr^{2}-C_6H_4]_2^{-})$와 $(PP^{OR}P)Ni(0)$ 의 가역적 변환을 통한 금속-리간드 협력 반응이 본 연구실에서 보고된 바 있다. $(PP^{OMe}P)NiN_2$ 는 이산화탄소와 반응하여 (PPP)Ni(OCOOMe)와 디메틸카보네이트를 형성한다. 본 연구에서는 이러한 금속-리간드 협력 반응을 통한 포스피나이트-니켈(0)/포스파이드 니켈(II)의 변환 메카니즘을 연구하였다. 알곤 기체 하에서, $(PPP)Ni(II)-OR (R = Me, ^{i}Pr)$ 의 형성되고, 이 포스파이드-니켈(II) 착물은 질소나 일산화탄소와 반응하여 금속-리간드 협력을 통한 알콕시기의 이동으로 각각의 포스피나이트-니켈(0) 착물을 형성하게 된다. 이러한 포스파이드-니켈(II) 착물의 반응성을 이해하기 위한 다양한 azide와 isocyanide와의 반응을 설명 하였다.
