Tri(pyridylmethyl)phosphine: The Elusive Congener of TPA Shows Surprisingly Different Coordination Behavior

Tri­(pyridylmethyl)­phosphine
(TPPh), the remarkably elusive congener
of tri­(pyridylmethyl)­amine (TPA), has been prepared, as well as the
relative tri­(N-methyl-pyridylamino)­phosphine (TPAMP).
The coordination properties of these new ligands have been evaluated
for chromium­(III), iron­(II), and ruthenium­(II) complexes and compared
with the related TPA complexes. In all cases, a different coordination
behavior has been observed whereby TPPh and TPAMP always act as tridentate
ligands. A chromium­(III) complex [Cr­(TPPh)­Cl3] has been
prepared, which has shown low ethylene oligomerization activity. Octahedral
low spin iron­(II) complexes [Fe­(TPPh)2]2+ and
[Fe­(TPAMP)2]2+ were obtained with two ligands
bound to the metal center. Ruthenium­(II) chloro complexes of TPA and
TPPh undergo ligand exchange reactions in acetonitrile, and the ruthenium­(II)
complex [Ru­(MeCN)2(TPA)]2+ can be oxidized by m-CPBA in acetonitrile to give a transient ruthenium­(IV)
oxo complex [Ru­(O)­(MeCN)­(TPA)]2+. Attempts to generate
high valent ruthenium­(IV) oxo TPPh or TPAMP complexes could not be
achieved, probably due to insufficient stabilization by these strong
field ligands.