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Edgar Niecke

Publications and source records attributed to Edgar Niecke.

7 recordsLinked to original sources
PubMed2005

Synthesis and structure of K+ [iPrN=C=P]-, a 1-aza-3lambda3-phospha-3-allenide.

2-Isopropyl(trimethylsilyl)amino-1lambda3-phosphaalkyne 1 reacts with potassium tert-butoxide to form potassium 1-isopropyl-1-aza-3lambda3-phospha-3-allenide (2). This compound was structurally characterized as the corresponding 18-crown-6 ether complex 3. The molecular structure of 1 was also determined in order to compare the bonding situation in the anion and the neutral lambda3-phosphaalkyne. Compound 3 contains a nitrogen-carbon-phosphorus group for which the parameters were shown by X-ray structural analysis and quantum chemical calculations to lie between the extrema N-C[triple bond]P and N=C=P, suggesting reactivity typical of an ambident anion. This is indeed the case, as subsequent reaction of 2 with chlorotrimethylsilane at nitrogen regenerates the lambda3-phosphaalkyne 1; with chlorotriphenylsilane the new derivative 4 is formed. In contrast, chlorotrimethylstannane reacts at phosphorus, giving the 1-aza-3lambda3-phosphaallene isopropyliminomethylidene(trimethylstannyl)phosphane 5.

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PubMed2002

1,3-Diphospholene-4-ylidene chromium (tungsten) pentacarbonyl complexes formed by CO insertion into the ring of a 1,3-diphosphacyclobutane-2,4-diyl-2-ide-complexes of a phosphanyl carbene or a phosphonium ylide?

Reaction of the 1,3-diphosphacyclobutane-2,4-diyl-2-ide 1 with chromium or tungsten hexacarbonyl afforded the anionic complexes [cyclo-[P(Mes*)-C(SiMe(3))-P(Mes*)-C(O)-C[M(CO)(5)]]](-) (3 a,b: M=Cr, W) by the formal insertion of CO into the four membered ring. Computational analysis suggests that this reaction proceeds via two intermediates that can be formulated as a cyclic metal acyl and an acyclic ketenyl complex. The anionic complexes 3 a,b further reacted with electrophiles to afford the neutral complexes [cyclo-(P(Mes*)-C(SiMe(3))-P(Mes*)-C(OR)-C[M(CO)(5)])] (4 a,b: M=Cr, W, R=Me; 5, 6: M=Cr, R=SiMe(3), H). All products were characterized by standard spectroscopic (NMR and MS) techniques, and 4 a,b further by extensive one- and two-dimensional multinuclear ((1)H, (13)C, (31)P, (183)W) NMR studies. From these investigations, an unequivocal assignment of chemical shifts and coupling constants was derived, confirming unusually large shielding for the formal carbenic carbon atoms which exceed even those in complexes of imidazoyl carbenes. Single-crystal X-ray diffraction analyses of 3 a, 4 a,b, and 5 revealed that all of these compounds contain planar P(2)C(3) rings. The phosphorus atoms are slightly pyramidal, and the carbon-metal distances (C-Cr 218 pm, C-W 230 pm) suggest low bond orders. Comparison of the structural parameters of 3 a with those of the O-substitution products 4 a, 5 revealed substantial changes in endocyclic P-C bond lengths and the degree of pyramidal character of bonding at the phosphorus atoms. In line with the spectroscopic and computational results, these effects were interpreted in terms of a considerable reorganization of pi electrons in the ring, which induces a substantial degree of aromatic character in the neutral complexes 4-6.

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