Metathesis reactivity of bis(phosphinite) pincer ligated nickel chloride, isothiocyanate and azide complexes

•A dozen new POCOP pincer nickel isothiocyanate and azide complexes were synthesized.•Metathesis reactivity and ligand exchange reactions of these complexes were studied.•The metathesis reaction is faster with a less electron rich nickel center.•In competing with NCS−, N3−N3− prefers a less electron rich nickel center.•The experimental results were supported by DFT calculations and the HSAB theory.

A series of nickel pincer complexes of the type [4-Z-2,6-(R2PO)2C6H2]NiX (R = tBu, iPr, Ph; Z = H, CO2Me; X = NCS, N3) have been synthesized from the reactions of the corresponding nickel chloride complexes [4-Z-2,6-(R2PO)2C6H2]NiCl and potassium thiocyanate or sodium azide. X-ray structure determinations of these complexes have shown that the thiocyanate ion binds to the nickel center through the nitrogen. A comparable Ni–N bond length (approx. 1.87 Å for the isothiocyanate complexes and 1.91 Å for the azide complexes) and an almost identical Ni–Cipso bond length (approx. 1.89 Å) have been observed for these complexes. Metathesis reactivity of [4-Z-2,6-(R2PO)2C6H2]NiCl and ligand exchange reactions between the nickel isothiocyanate and nickel azide complexes have been investigated. The metathesis reactions with thiocyanate/azide complexes are faster with a less electron rich and more sterically accessible nickel center. The thermodynamic stability of these nickel complexes has been rationalized using hard-soft acid-base theory (HSAB theory); a harder ligand prefers a less electron rich nickel center. These experimental results have been supported by quantum chemical analysis of the coordinating nitrogen atoms in SCN– and N3−N3−.

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