Unsymmetrical Chelation of N‑Thioether-Functionalized Bis(diphenylphosphino)amine-Type Ligands and Substituent Effects on the Nuclearity of Iron(II) Complexes: Structures, Magnetism, and Bonding

Starting
from the short-bite ligands
N-thioether-functionalized bis­(diphenylphosphino)­amine-type
(Ph2P)2N­(CH2)3SMe
(1) and (Ph2P)2N­(p-C6H4)­SMe (2), the Fe­(II)
complexes [FeCl2­(1)]n (3), [FeCl2(2)]2 (4), [Fe­(OAc)­(1)2]­PF6 (5), and [Fe­(OAc)­(2)2]­PF6 (6) were synthesized and characterized by Fourier
transform IR, mass spectrometry, elemental analysis, and also by X-ray
diffraction for 3, 4, and 6. Complex 3 is a coordination polymer in which 1 acts as a P,P-pseudochelate
and a (P,P),S-bridge,
whereas 4 has a chlorido-bridged dinuclear structure
in which 2 acts only as a P,P-pseudochelate. Since these complexes were obtained under
strictly similar synthetic and crystallization conditions, these unexpected
differences were ascribed to the different spacer between the nitrogen
atom and the −SMe group. In both compounds, one Fe–P
bond was found to be unusually long, and a theoretical analysis was
performed to unravel the electronic or steric reasons for this difference.
Density functional theory calculations were performed for a set of
complexes of general formula [FeCl2(SR2)­{R21PN­(R2)­P′R23}] (R = H, Me; R1, R2, and R3 =
H, Me, Ph), to understand the reasons for the significant deviation
of the iron coordination sphere away from tetrahedral as well as from
trigonal bipyramidal and the varying degree of unsymmetry of the two
Fe–P bonds involving pseudochelating PN­(R)P ligands. Electronic
factors nicely explain the observed structures, and steric reasons
were further ruled out by the structural analysis in the solid-state
of the bis-chelated complex 6, which displays usual and
equivalent Fe–P bond lengths. Magnetic susceptibility studies
were performed to examine how the structural differences between 3 and 4 would affect the interactions between
the iron centers, and it was concluded that 3 behaves
as an isolated high-spin Fe­(II) mononuclear complex, while significant
intra- and intermolecular ferromagnetic interactions were evidenced
for 4 at low temperatures. Complexes 3 and 4 were also tested in catalytic ethylene oligomerization but
did not exhibit any significant activity under the studied conditions.