A mononuclear iron(II) complex is readily formed when combining equimolar amounts of iron(II) tetrafluoroborate hexahydrate, the pyridine-derived triphosphane C5H3N{2-[CMe(CH2PMe2)2]}{6-[CMe2(CH2PMe2)]} (2) and diethylphosphane (Et2PH) in methanol at room temperature. The chelate ligand is in fact pentadentate, as one of the methyl groups of the neopentyl-like sidearm engages in a C–H-bonded contact (agostic interaction) with the metal centre, in addition to the expected NP3 coordination. The remaining site of what is a distorted coordination octahedron is occupied by monodentate Et2PH. The autoclave reaction of this purple complex with CO (10 bar, ethanol solvent, 65 °C) yields a yellow microcrystalline precipitate, whose analysis reveals a mixture of two products, in an approximate ratio of 40:1. One of the products is the cis-dicarbonyl complex[Fe(2)(CO)2](BF4)2 (4), in which the chelate ligand acts as an NP3 donor, and the other is the monocarbonyl complex of the tetrapodal pentadentate NP4 ligand 1. The latter ligand is formed from 2 by incorporation of an additional PMe2 donor, in what, in effect, is a metal-mediated phosphenium group transfer. A mechanism is suggested for this reaction. Other species in the reaction mixture have been identified on the basis of mass spectra, and the full NMR spectroscopic assignment of complex 4 (1H, 31P, 13C) is reported.

A pyridine-derived tetraphosphane ligand (donor set: NP4) has been found to undergo remarkably specific C-P bond cleavage reactions, thereby producing a ligand with an NP3 donor set. The reaction may be reversed under suitable conditions, with regeneration of the original NP4 ligand. In order to investigate the mechanism of this reaction, the NP3 donor ligand C5H3N[CMe(CH2PMe2)2][CMe2(CH2PMe2)] (11) was prepared, and its iron(II) complex 4 generated from Fe(BF4)2 ・6H2O, with methyl diethylphosphinite (7) as an additional monodentate ligand. Ligand 11 has, in addition to the NP3 donor set, one methyl group in close contact with the iron center, reminiscent of an agostic M・ ・ ・H-C interaction. Depending on the stoichiometric amount of iron(II) salt, a side product 15 is formed, which has a diethylphosphane ligand instead of the phosphinite 7 coordinated to iron(II). While attempts to deprotonate the metal-coordinated methyl group in 4 were unsuccessful, the reaction was shown to occur in an alternative complex (18), which is similar to 4 but has a trimethylphosphane ligand instead of the phosphinite 7. The reaction of complex 15 with CO gave two different products, which were both characterized by single-crystal X-ray diffraction. One (19) is the dicarbonyl iron(II) complex of the triphosphane ligand 11, the other (3) is the carbonyl iron(II) complex of the tetraphosphane C5H3N[CMe(CH2PMe2)2]2 (1). This suggests an intermolecular mechanism for the C-P bond formation in question.

Chapter 6 Square-Pyramidal Coordinated Phosphine Iron Fragments: A Tale of the Unexpected Andreas Grohmann, Andreas Grohmann Institut für Chemie, Technische Unversität Berlin, Straße des 17. Juni 135, 10623 Berlin, GermanySearch for more papers by this authorStephan Kohl, Stephan Kohl Institut für Chemie, Technische Unversität Berlin, Straße des 17. Juni 135, 10623 Berlin, GermanySearch for more papers by this author Andreas Grohmann, Andreas Grohmann Institut für Chemie, Technische Unversität Berlin, Straße des 17. Juni 135, 10623 Berlin, GermanySearch for more papers by this authorStephan Kohl, Stephan Kohl Institut für Chemie, Technische Unversität Berlin, Straße des 17. Juni 135, 10623 Berlin, GermanySearch for more papers by this author Book Editor(s):Prof. Dr. Carsten Bolm, Prof. Dr. Carsten Bolm RWTH Aachen, Institut für Organische Chemie, Landoltweg 1, 52056 Aachen, GermanySearch for more papers by this authorProf. Dr. F. Ekkehardt Hahn, Prof. Dr. F. Ekkehardt Hahn Institut für Anorganische und Analytische Chemie, Westfälische Wilhelms-Universität, Münster, Corrensstraße 36, 48149 Münster, GermanySearch for more papers by this author First published: 28 January 2009 https://doi.org/10.1002/9783527625468.ch6Citations: 3 AboutPDFPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShareShare a linkShare onFacebookTwitterLinked InRedditWechat Summary This chapter contains sections titled: Introduction Polyphosphine Ligands with Three and Four Coordinating Arms CP Bond Activation and Agostic Interactions in Iron Complexes of Polypodal Phosphine Ligands Mechanistic Considerations Conclusion References Citing Literature Activating Unreactive Substrates: The Role of Secondary Interactions RelatedInformation

Two improved routes to synthesize 1-benzyl-1,4,7,10-tetraazacyclododecane (6) and 1,4,7,10- tetraazacyclododecane-1-acetic acid ethyl ester (11) are described as well as the synthesis of 1-{2-[4-(maleimido-N-propylacetamidobutyl)amino]-2-oxoethyl}-1,4,7,10-tetraazacyclododecane- 4,7,10-triacetic acid (17) and its Y, Ho, Tm, and Lu complexes. The 1H and 13C NMR spectra of the new compounds as well as the single crystal X-ray structure analyses of the intermediates 4-benzyl-1,7-bis(p-toluenesulfonyl)diethylenetriamine (3) and 1,4,7-tris(p-toluenesulfonyl)diethylenetriamine (7) are reported and discussed. The rare earth complexes of 17 have been characterized by 1H NMR spectroscopy and MALDI-TOF mass spectrometry.

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The pyridine-derived tetrapodal tetraphosphane C5H3N[CMe(CH2PMe2)2]2 is susceptible to selective protonolysis of a phosphorus-carbon bond in the presence of iron(II) salts. Water produces dimethylphosphinic acid, Me2POH, and protonates the anionic remainder of the tetraphosphane. The resulting iron(II) complexes and (tetrafluoroborate and perchlorate salts, respectively) contain the residual chelate ligand in which a methyl group, derived from the ligand skeleton, is in agostic interaction with the metal centre, and in which Me2POH, unavailable in the free state owing to rapid tautomerisation, is metal-coordinated and thus stabilised. Full NMR details are presented, including 31P simulations. The reactivity towards alcohols is similar (compounds), and has been studied using deuterium labels (NMR). P-C bond cleavage may be suppressed only if all protic agents are rigorously excluded, as in the reaction of with Fe(SO3CF3)2.2CH3CN in acetonitrile solution, which produces the complex [Fe(NCMe)](SO3CF3)2. In it, the ligand acts as an NP4 coordination cap but is severely distorted from square-pyramidal geometry. The reaction of with anhydrous ferrous bromide, FeBr2, in methanol again produces a dimethylphosphinic acid ester ligand, but the complex now contains ferric iron coordinated by a carbanionic residual chelate ligand, implicating H+ as the oxidising agent under these conditions. Full spectroscopic and X-ray structural details are presented for all compounds.

The reaction of the 2,6 -diethylpyridine-derived tetraphosphane ligand C5H3N[CMe(CH2PMe2)(2)](2) (1(Me)) with nickel(II) or cobalt(II) perchlorate or tetrafluorob orate in methanol produces complexes of approximate square-pyramidal geometry (NP4 coordination). The nickel complexes are diamagnetic, with NMR spectral features ((31)p) that reflect a tetrahedral distortion of diametrically opposite phosphorus donors; this is also found in the solid state. In terms of bond lengths and angles, all complexes differ significantly from their analogues containing the phenyl- substituted ligand C5H3N[CMe(CH2PPh2)(2)](2). The ligand 1(Me) is readily and completely oxidised to the corresponding tetrakis(phosphane oxide) upon reaction with NO. Slow oxidation of the cobalt(II) tetrafluoroborate complex of 1(Me) with aerobic oxygen in acetonitrile produces a trinuclear cobalt complex containing two equivalents of partially oxidised ligand (C-2-symmetrical; donor set: [PMe2](2)[P(O)Me-2](2)). Four phosphane oxide oxygen atoms coordinate the central cobalt(II) ion in tetrahedral fashion, whereas the lateral cobalt(II) ions are in a square-pyramidal environment provided by two PMe2 donors and three acetonitrile ligands in each case. The reaction of the cobalt(II) perchlorate complex of 1(Me) with carbon monoxide gives an octahedral, 19-valence-electron dicarbonyl complex in which one of the dimethylphosphanyl groups is uncoordinated. A structural relative of 1(Me)., C5H3N[CMe(CH2PMe2)(2)][CMe2(CH2PMe2)] (2), which contains one fewer dimethylphosphanyl donor, can be prepared in a straightforward manner from 2-ethyl-6-isopropylpyridine in a four-stage process. In a series of mononuclear nickel(II) complexes, 2 employs only its three phosphane donors which, together with a monodentate ligand (acetonitrile, acetamide, or bromide), provide a tetragonal coordination environment for the metal ion which is intermediate between square planar and tetrahedral. The acetamide complex is generated from the acetonitrile complex by slow hydrolysis of the coordinated ligand. Full spectroscopic details for the complexes, as well as X-ray structure analyses, are reported.

Complex formation between FeX(2)(.)6H(2)O (X=BF4 or ClO4) and the pyridine-derived tetrapodal tetraphosphane C5H3N[CMe(CH2PMe2)(2)](2) (1) in methanol proceeds with solvent-induced cleavage of one PMe2 group. Depending on the reaction temperature and the nature of the counterion, iron(II) is coordinated, in distorted square-pyramidal fashion, by the anionic remainder of the chelating ligand, C5H3N[CMe(CH2PMe2)(2)][CMe(CH2PMe2)(CH2-)] (Np3C- donor set: X = BF4, -50 degrees C: 2; X = ClO4, RT: 4) or its protonated form C5H3N[CMe(CH2PMe2)(2)][CMe(CH2PMe2)(CH3)], in which the methyl group is in agostic interaction with the metal centre (X=BF, RT: 3; X=ClO4, +50 degrees C: 5). A monodentate phosphinite ligand Me2POMe, formed from the cleaved PMe2 group and methanol, completes the coordination octahedron in both cases. Working in CD3OD (X=BF4, RT) gives the deuterium-substituted analogue of 3, with ligands L(CH2D) (L=residual chelating ligand) and Me2POCD3. A mechanism for the observed phosphorus-carbon bond cleavage is suggested. Complex 2, when isolated at -50 degrees C, is stable in the solid state even at room temperature. The reaction of 2 in methanol with carbon monoxide (10.5 bar) at elevated temperature forms, in addition to as yet unidentified side products, the carbonyl complex [(1)Fe(CO)](BF4)(2) (7), in which the previous P-C bond cleavage has been reversed, reforming the original tetrapodal pentadentate NP4 ligand 1. All compounds have been fully characterised, including X-ray structure analyses in most cases.