A pyridine-derived tetraphosphane ligand (donor set: NP 4 ) has been found to undergo remarkably specific C-P bond cleavage reactions, thereby producing a ligand with an NP 3 donor set. The reaction may be reversed under suitable conditions, with regeneration of the original NP 4 ligand. In order to investigate the mechanism of this reaction, the NP 3 donor ligand C 5 H 3 N[CMe(CH 2 PMe 2 ) 2 ][CMe 2 (CH 2 PMe 2 )] (11) was prepared, and its iron(II) complex 4 generated from Fe(BF 4 ) 2 · 6 H 2 O, with methyl diethylphosphinite (7) as an additional monodentate ligand. Ligand 11 has, in addition to the NP 3 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 C 5 H 3 N[CMe(CH 2 PMe 2 ) 2 ] 2 (1). This suggests an intermolecular mechanism for the C-P bond formation in question.
Keywords:Bond activation / N,P ligands / Iron / NMR spectroscopy / Phosphane ligands A mononuclear iron(II) complex is readily formed when combining equimolar amounts of iron(II) tetrafluoroborate hexahydrate, the pyridine-derived triphosphane C 5 H 3 N{2-[CMe(CH 2 PMe 2 ) 2 ]}{6-[CMe 2 (CH 2 PMe 2 )]} (2) and diethylphosphane (Et 2 PH) 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-Hbonded contact (agostic interaction) with the metal centre, in addition to the expected NP 3 coordination. The remaining site of what is a distorted coordination octahedron is occupied by monodentate Et 2 PH. The autoclave reaction of this purple complex with CO (10 bar, ethanol solvent, 65°C)
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