The exchange of pyridine on Bis(β-alkanedionato)dipyridinenickel(II) complexes. A nitrogen-14 magnetic resonance study

Abstract

A ¹⁴N N.M.R. study is reported of the exchange of pyridine (py) on [Ni(bzac)₂(py)₂] [k(298 K) = (4.6±0.5)x10⁴ s^-1, ΔH^? = 38±3 kJ mol^-1; ΔS^? = -28±11 J K^-1 mol^-1], [Ni(bzbz)₂(py)₂] [k(298 K) = (2.7±0.3)x10⁴ s^-1; ΔH^? = 38±3 kJ mol^-1; ΔS^? = -34±12 J K^-1 mol^-1], [Ni(tfac)₂(py)₂] [k(298 K) = (8.4±0.9)x10³ s^-1]; ΔH? = 32±3 kJ mol^-1; ΔS? = -58±11 J K^-1 mol^-1], and [Ni(hfac)₂(py)2] [k(298 K) < 2x10³ s^-1] in pyridine solution where the rate of pyridine exchange = 2k[nickel(II) complex], bzac = 1- phenylbutane-1,3-dione; bzbz = 1,3-diphenylpropane-1,3-dione; tfac = 1,1,1-trifluoropentane-2,4-dione; and hfac = 1,1,1,5,5,5- hexafluoropentane-2,4-dione. The lability (298 K) of pyridine in the nickel(II) complexes is observed to decrease as the electron withdrawing power, as assessed from appropriate Taft parameters, of the β-alkanedionato ligand increases.