Combined Experimental and Theoretical Study of Bis(diphenylphosphino)(N‑thioether)amine-Type Ligands in Nickel(II) Complexes for Catalytic Ethylene Oligomerization

Starting from the new ligands bis­(diphenylphosphino)­(N-4-(methylthio)­phenyl)­amine (4, N­(PPh2)2(p-C6H4)­SMe) and its monosulfide derivative (Ph2P)­N­{P­(S)­Ph2}­(p-C6H4)­SMe (4·S), we have prepared and characterized, including by X-ray crystallographic studies, their Ni­(II) complexes [NiCl2{(Ph2P)2N­(p-C6H4)­SMe-P,P}] (5) and [NiCl2{(Ph2P)­N­{P­(S)­Ph2}­(p-C6H4)­SMe-P,S}] (6), respectively. The bis-sulfide compound N­{P­(S)­Ph2}2(p-C6H4)­SMe (4·S2) was also prepared and structurally characterized. Computational studies showed that the combined influence of stronger P donors and a four-membered-ring P,P chelate leads to complex 5 being thermodynamically more stable than 6, which contains one weaker PS donor group but a five-membered P,PS chelate ring. For comparison, the bis-chelate complex [Ni­{(Ph2P)­N­{P­(S)­Ph2}­(p-C6H4)­SMe-P,S}2]­(BF4)2 (7), the monochelate complexes [NiBr2{(Ph2P)­N­{P­(S)­Ph2}­(p-C6H4)­SMe-P,S}] (8) and the Pd­(II) analogue of 6, [PdCl2{(Ph2P)­N­{P­(S)­Ph2}­(p-C6H4)­SMe-P,S}] (9), were synthesized and structurally characterized and their solution behavior was investigated. The catalytic activity and selectivity in ethylene oligomerization of the Ni­(II) complexes 5 and 6 and their known N-(methylthio)­propyl analogues [NiCl2{(Ph2P)2N­(CH2)3SMe-P,P}] (2) and [NiCl2{(Ph2P)­N­{P­(S)­Ph2}­(CH2)3SMe-P,S}] (3), which were obtained from the bis­(diphenylphosphino)­(N-(methylthio)­propyl)­amine ligand N­(PPh2)2(CH2)3SMe (1) and its monosulfide derivative (Ph2P)­N­{P­(S)­Ph2}­(CH2)3SMe (1·S), respectively, revealed a significant influence of the nature of the N-substituent (aryl vs alkyl thioether) and of the chelate ring size (P,P vs P,PS). DFT calculations showed that the trend in ΔErel, [NiCl2(P,P)] > [NiCl2(P,PS)] > [NiCl2(PS,PS)], results from the stronger covalent character of the Ni–P vs Ni–S bond. Using AlEtCl2 as cocatalyst, mostly ethylene dimers were produced, with significant amounts of trimers (selectivity in the range 11–36%). Productivities up to 40400 and 48200 g of C2H4/((g of Ni) h), with corresponding TOF values of 84800 and 101100 mol of C2H4/ ((mol of Ni) h), were obtained with precatalysts 2 and 3, respectively.