Tetrakis-1,2,4,5-(bis(3,5-dimethylphenyl)phosphino)benzene (Me16tpbz): A soluble and spectroscopically simple variant of the 1,2,4,5-tetraphosphinobenzene ligand platform

Abstract

A preparation of 1,2,4,5-tetrakis(bis(3,5-dimethylphenyl) phosphino)benzene (Me16tpbz) has been devised as a two-step synthesis beginning with bis(3,5- dimethylphenyl) chlorophosphine and 1,2,4,5-tetrafluorobenzene, albeit in low overall yield of 10%. The crystal structure of Me16tpbz reveals slight pushing of the phosphorus atoms above and below the central arene ring, suggesting the effect of slight steric pressure, as compared to the fully planar C6P4 core found in 1,2,4,5-tetrakis (diphenylphosphino) benzene (tpbz). Upon exposure to air, solution samples of Me16tpbz degrade to (3,5-Me2C6H3)2PP(O)(C6H3-3,5-Me2)2, identified by X-ray crystallography, which suggests P–C bond scission is a decomposition pathway. The dinickel compound [(mnt)Ni(Me16tpbz)Ni(mnt)] (mnt(2–) = [(NC)2C2S2]2–) has been structurally characterized by X-ray crystallography and reveals a chair-like conformation defined by a 32.3° angle between the NiP2 planes with respect to the central C6P4 mean plane. Compared to the analogous [(mnt)Ni(tpbz)Ni(mnt)], slightly weaker binding of Me16tpbz to Ni is implied by its modestly longer Ni–P bond lengths. The cyclic voltammogram of [(mnt)Ni(Me16tpbz)Ni(mnt)] in CH2Cl2 shows reversible reductions at ~ −1.01 V and −1.21 V, corresponding to sequential Ni2+ + e– → Ni1+ processes, while in DMF, under conditions where [(mnt)Ni(tpbz)Ni(mnt)] shows multiple, reversible reduction processes, its behavior is irreversible and more complex.