Utilization of Phosphinoamide Ligands in Homobimetallic Fe and Mn Complexes: The Effect of Disparate Coordination Environments on Metal–Metal Interactions and Magnetic and Redox Properties

A series of homobimetallic phosphinoamide-bridged diiron
and dimanganese
complexes in which the two metals maintain different coordination
environments have been synthesized. Systematic variation of the steric
and electronic properties of the phosphinoamide phosphorus and nitrogen
substituents leads to structurally different complexes. Reaction of
[iPrNKPPh2] (1) with MCl2 (M = Mn, Fe) affords the phosphinoamide-bridged
bimetallic complexes [Mn­(iPrNPPh2)3Mn­(iPrNPPh2)]
(3) and [Fe­(iPrNPPh2)3Fe­(iPrNPPh2)]
(4). Complexes 3 and 4 are
iso-structural, with one metal center preferentially binding to the
three amide ligands in a trigonal planar arrangement while the second
metal center is ligated by three phosphine donors. A fourth phosphinoamide
ligand caps the tetrahedral coordination sphere of the phosphine-ligated
metal center. Mössbauer spectroscopy of complex 4 suggests that the metals in these complexes are best described as
FeII centers. In
contrast, treatment of MnCl2 or FeI2 with [MesNKPiPr2] (2) leads to
the formation of the halide-bridged species [(THF)­Mn­(μ-Cl)­(MesNPiPr2)2Mn­(MesNPiPr2)] (5) and [(THF)­Fe­(μ-I)­(MesNPiPr2)2FeI (7), respectively. Utilization of FeCl2 in place of FeI2, however, leads exclusively to the C3-symmetric
complex [Fe­(MesNPiPr2)3FeCl] (6), structurally similar to 4 but with a halide bound to the phosphine-ligated Fe center. The
Mössbauer spectrum of 6 is also consistent with
high spin FeII centers. Thus, in the case of the [iPrNPPh2]− and
[MesNPiPr2]− ligands, zwitterionic complexes with the two metals in disparate
coordination environments are preferentially formed. In the case of
the more electron-rich ligand [iPrNPiPr2]−, complexes
with a 2:1 mixed donor ligand arrangement, in which one of the ligand
arms has reversed orientation relative to the previous examples, are
formed exclusively when [iPrNLiPiPr2] (generated in situ) is treated with
MCl2 (M = Mn, Fe): (THF)3LiCl­[Mn­(NiPrPiPr2)2(PiPr2NiPr)­MnCl] (8) and [Fe­(NiPrPiPr2)2(PiPr2NiPr)­FeCl] (9). Bimetallic complexes 3–9 have been structurally characterized using
X-ray crystallography, revealing Fe–Fe interatomic distances
indicative of metal–metal bonding in complexes 6 and 9 (and perhaps 4, to a lesser extent).
All of the complexes appear to adopt high spin electron configurations,
and magnetic measurements indicate significant antiferromagnetic interactions
in Mn2 complexes 5 and 8 and
no discernible magnetic superexchange in Fe2 complex 4. The redox behavior of complexes 3–9 has also been investigated using cyclic voltammetry, and
theoretical investigations (DFT) were performed to gain insight into
the metal–metal interactions in these unique asymmetric complexes.