Electronic and steric Tolman parameters for proazaphosphatranes, the superbase core of the tri(pyridylmethyl)azaphosphatrane (TPAP) ligand

Abstract

The Tolman electronic parameters (TEP) and cone angles were experimentally measured for a series of substituted proazaphosphatrane ligands by synthesizing their respective Ni(L^R)(CO)₃ complexes, where L = P(RNCH₂CH₂)₃N and R = Me, iPr, iBu and Bz. The complexes Ni(L^Me)(CO)₃ (1), Ni(L^iPr)(CO)₃ (2), Ni(L^iBu)(CO)₃ (3) and Ni(L^Bz)(CO)₃ (4) display CO vibrational frequencies (A1 mode) at 2057.0, 2054.6, 2054.9 and 2059.1 cm⁻¹, respectively. The TEPs for the phosphine ligands in 1–3 are among the lowest measured, with values close to P(tBu)₃ the most donating phosphine measured by Tolman. The cone angles of L^R measured in 1–4 are 152, 179, 200 and 207° for R = Me, iPr, iBu and Bz, respectively. The substituted proazaphosphatranes have larger cone angles compared to the analogous trialkyl subsituted monophosphines. Our study demonstrates that while the cone angles have a significant dependence on R, all of the substituted proazaphosphatranes are strong electron donors. Additionally, in order to determine the electronic donor strength of our previously reported multidentate ligand, TPAP, Ni(TPAP)(CO)₂ (5) (TPAP = tris(2-pyridylmethyl)azaphosphatrane) and Ni(L^Me)₂(CO)₂ (6) were also synthesized and evaluated in a similar fashion.