Reactivity of [NiR(R′)L2] compounds and the crystal structure of [Ni(C2Cl3)(C6H2Me3-2,4,6)(PMe2Ph)2]

Abstract

A series of compounds of the type trans-[NiR(R′)L₂](L = PMe₂Ph, R = C₂Cl₃, R′= Ph, C₆H₄OMe-4, C₆H₄Me-2, or -4, C₆H₂Me₃-2,4,6; R = C₆H₄Cl-2, R′= C₆H₄Me-2 or C₆H₂Me₃-2,4,6; R = C₆H₂Me₃-2,4,6, R′= C₆H₄Me-2 or Ph; L = PEt₃, R = C₂Cl₃, R′= C₆H₄Me-2, Ph, C₆H₄OMe-4, C₆H₄Cl-4, or C₆H₄Me-4; R = C₆H₄Cl-2, R′= Ph, C₆H₄Me-2 or -4; R = C₆H₄Me-2, R′= Ph, or C₆H₄Me-4; R = C₆H₂Me₃-2,4,6, R′= C₆H₄Me-2) and cis-[Ni(C₆H₂Me₃-2,4,6)(C₆H₄Me-2)(bipy)](bipy = 2,2′-bipyridine) have been prepared. The proton n.m.r. signals of the ortho methyls of the mesityl ligand in the most hindered compound of the series [Ni(C₂Cl₃)(C₆H₂Me₃-2,4,6)(PMe₂Ph)₂] appear overlapped despite the asymmetry of C₂Cl₃. The crystal structure of this compound [triclinic, space group P, a=12.570(3), b=12.703(3), c= 9.352(2)Å, α= 91.70(2), β= 90.02(2), γ= 102.65(2)°, Z= 2] indicates that this may be due to the phosphine aromatic rings. Ligandexchange reactions of PMe₂Ph by PEt₃ in benzene, [NiR(R′)(PMe₂Ph)₂]⇌[NiR(R′)(PMe₂Ph)(PEt₃)]⇌[NiR(R′)(PEt₃)₂], are complete in 0.5 h at room temperature, no intermediates being observed. The reverse process requires refiux for 10 h and in this case the intermediates [NiR(R′)(PMe₂Ph)(PEt₃)] can be obtained. If the groups R and R′ contain ortho substituents the reaction does not take place. In refluxing benzene under nitrogen the stability of the organometallics containing PMe₂Ph is greater than those with PEt₃. Decomposition takes place via reductive elimination to give R–R′ or via homolytic cleavage to give R–H and R′–H, the most favoured pathway depending on the size and electronegativity of the groups R and R′. The reductive-elimination process involves a previous step in which a phosphine group is lost. The addition of CO to solutions of [NiR(R′)L₂](when R and R′ possess ortho substituents) results in decomposition of the organometallics giving RCOR′. The compound [Ni(C₆H₂Me₃-2,4,6)(C₆H₄Me-2)(bipy)] is also decomposed but gives R-R′. When one of the ligands (R) contains an ortho substituent, the corresponding two isomers (syn and anti) of the acyl derivative [NiR(COR′)L₂] can be detected. The compounds cis-[NiR(R′)(bipy)] show a favourable associative decomposition pathway when R and R′ are strong donor ligands.