RESUMEN
The electronic nature of 1,3-diphosphacyclobutane-2,4-diyl is explored with wavefunction based and density functional methods. According to MCSCF calculations the singlet state of the title compound is a biradicaloid with closed shell character, the number of unpaired electrons, assigned upon the analysis of the natural orbitals, is close to one. The participation of closed shell contributions in the overall wavefunction arises from a strong mixing of canonical structures, which emphasizes (a) the phosphorane type of bonding as well as (b) π-delocalization within the ring system. The bonding situation changes when σ-attracting substituents, e.g. amino groups, are attached to the phosphorus atoms. They inhibit possible cyclic π-delocalization and enhance the biradical character within the ring system.
RESUMEN
Deprotonation of aminophosphaalkenes (RMe(2)Si)(2)C=PN(H)(R') (R=Me, iPr; R'=tBu, 1-adamantyl (1-Ada), 2,4,6-tBu(3)C(6)H(2) (Mes*)) followed by reactions of the corresponding Li salts Li[(RMe(2)Si)(2)C=P(M)(R')] with one equivalent of the corresponding P-chlorophosphaalkenes (RMe(2)Si)(2)C=PCl provides bisphosphaalkenes (2,4-diphospha-3-azapentadienes) [(RMe(2)Si)(2)C=P](2)NR'. The thermally unstable tert-butyliminobisphosphaalkene [(Me(3)Si)(2)C=P](2)NtBu (4 a) undergoes isomerisation reactions by Me(3)Si-group migration that lead to mixtures of four-membered heterocyles, but in the presence of an excess amount of (Me(3)Si)(2)C=PCl, 4 a furnishes an azatriphosphabicyclohexene C(3)(SiMe(3))(5)P(3)NtBu (5) that gave red single crystals. Compound 5 contains a diphosphirane ring condensed with an azatriphospholene system that exhibits an endocylic P=C double bond and an exocyclic ylidic P((+))-C((-))(SiMe(3))(2) unit. Using the bulkier iPrMe(2)Si substituents at three-coordinated carbon leads to slightly enhanced thermal stability of 2,4-diphospha-3-azapentadienes [(iPrMe(2)Si)(2)C=P](2)NR' (R'=tBu: 4 b; R'=1-Ada: 8). According to a low-temperature crystal-structure determination, 8 adopts a non-planar structure with two distinctly differently oriented P=C sites, but (31)P NMR spectra in solution exhibit singlet signals. (31)P NMR spectra also reveal that bulky Mes* groups (Mes*=2,4,6-tBu(3)C(6)H(2)) at the central imino function lead to mixtures of symmetric and unsymmetric rotamers, thus implying hindered rotation around the P-N bonds in persistent compounds [(RMe(2)Si)(2)C=P](2)NMes* (11 a, 11 b). DFT calculations for the parent molecule [(H(3)Si)(2)C=P](2)NCH(3) suggest that the non-planar distortion of compound 8 will have steric grounds.
RESUMEN
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.
RESUMEN
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.
RESUMEN
The first oligomeric phosphazene in which each phosphorus center features a PH functionality (3) was obtained from the amidophosphane 1 or its zirconium complex 2.
RESUMEN
The sterically demanding groups on the tricoordinate phosphorus atom, the π-electron acceptors substituted on the ring, and the dicoordinate phosphorus atoms within the ring are the most significant factors contributing to the planarity and aromaticity of the 1,2,4-triphosphole ring in 1. The Bird aromaticity index for 1 shows that it has the most pronounced aromatic character of all known phospholes.
RESUMEN
The butadiene-like phosphanylcarbene 2 is, according to ab initio calculations, the intermediate in the conversion of 1 into 3 in the solid state [Eq. (a)]; it is only 1.3 kJ mol-1 higher in energy than 1. For this conversion, only minor changes in the endocyclic bonds are required throughout the entire reaction. R2 N=2,2,6,6-Me4 C5 H6 N.