Influence of η1-Acetylide Ligands on the Spin State and Catalytic Activity of Iron(II) PNP Pincer Complexes

Here, we report the preparation, structures and reactivity of iron(II) PNP pincer complexes bearing one, two or three η¹-acetylide ligands bound to the metal center. The described complexes are accessible through the reaction of either one, two or ≥3 equivalent of LiCCR (R=SiMe₃, Ph) with the dibromide complex [Fe(PNP)Br₂] (1). Through increasing the number of η¹-acetylides the overall ligand field strength can be modulated resulting in S=2, S=1 and S=0 spin states for mono-, bis- and tris-acetylated complexes, respectively. These findings are supported by DFT calculations on the spin state spitting in these complexes and appear to affect their binding affinity towards additional co-ligands. The Fe(II) mono-, bis- and tris-acetylides were tested as catalyst in the catalytic dimerization of terminal alkynes. These test reactions revealed that only the bis-acetylated complexes are catalytically competent while mono-acetylated species are inactive. The tris-acetylated complexes were found to be off-cycle intermediates in the presence of lithium acetylides.