Mechanochemical Synthesis of Chromium(III) Complexes Containing Bidentate PN and Tridentate P-NH-P and P-NH-P' Ligands

Abstract

Chromium(III) complexes bearing bidentate {NH₂(CH₂)₂PPh₂: PN, (S,S)-[NH₂(CHPh)₂PPh₂]: P'N} and tridentate [Ph₂P(CH₂)₂N(H)(CH₂)₂PPh₂: P-NH-P, (S,S)-(iPr)₂PCH₂CH₂N(H)CH(Ph)CH(Ph)PPh₂: P-NH-P'] ligands have been synthesized using a mechanochemical approach. The complexes {cis-[Cr(PN)Cl₂]Cl (1), cis-[Cr(P'N)Cl₂]Cl (2), mer-Cr(P-NH-P)Cl₃ (3), and mer-Cr(P-NH-P')Cl₃ (4)} were obtained in high yield (95-97%) via the grinding of the respective ligands andthe solid Cr(III) ion precursor [CrCl₃(THF)₃] with the aid of a pestle and mortar, followed by recrystallization in acetonitrile. The isolated complexes are high spin. A single-crystal X-ray diffraction study of 2 revealed a cationic chromium complex with two P'N ligands in a cis configuration with P' trans to P' with chloride as the counteranion. The X-ray study of 4 shows a neutral Cr(III) complex with the P-NH-P' ligand in a mer configuration. The difference in molecular structures and bulkiness of the ligands influence the electronic, magnetic, and electrochemical properties of the complexes as exhibited by the bathochromic shifts in the electronic absorption peaks of the complexes and the relative increase in the magnetic moment of 3 (4.19 μ_β) and 4 (4.15 μ_β) above the spin only value (3.88 μ_β) for a d³ electronic configuration. Complexes 1-4 were found to be inactive in the hydrogenation of an aldimine [(E)-1-(4-fluorophenyl)-N-phenylmethanimine] under a variety of activating conditions. The addition of magnesium and trimethylsilyl chloride in THF did cause hydrogenation at room temperature, but this occurred even in the absence of the chromium complex. The hydrogen in the amine product came from the THF solvent in this novel reaction, as determined by deuterium incorporation into the product when deuterated THF was used.