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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

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posted on 06.08.2012 by Subramaniam Kuppuswamy, Mark W. Bezpalko, Tamara M. Powers, Mark M. Turnbull, Bruce M. Foxman, Christine M. Thomas
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 39 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 39 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.