Dietrich Gudat

Hydrolysis of Imidazole-2-ylidenes

The direct reaction of an imidazole-2-ylidene in a predominantly aqueous environment [about 0.1 M solution in a H(2)O (>60%)/THF solvent system] was investigated for the first time. The reaction yielded a stable solution of the corresponding imidazolium-hydroxide of pH 13, which is in agreement with results from an ab initio molecular dynamics simulation. In contrast, hydrolysis of the carbene in a mainly aprotic environment (>80% THF) gives a hydrogen-bridged carbene-water complex which could be detected by NMR and IR spectroscopies for the first time. This complex converts slowly to two isomeric ring opened products and is at higher water concentration in dynamic equilibrium with the imidazolium hydroxide. A computational mechanistic study of the carbene hydrolysis with a gradually increasing number of water molecules revealed that the imidazolium-hydroxide structure can only be optimized with three or more water molecules as reactants, and with the increasing number of water molecules its stability is increasing with respect to the carbene-water complex. In agreement with the experimental results, these findings point out that solvent stabilization and basicity of the hydroxide ion plays a crucial role in the reaction. With increasing number of water molecules the barriers connecting the reaction intermediates are getting smaller, and the ring opened hydrolysis products can be derived from imidazolium-hydroxide type intermediates. Computational studies on the hydrolysis of a nonaromatic imidazolidine-2-ylidene analogue clearly indicated the analogous ring-opened product to be by 10-12 kcal/mol more stable than the appropriate ion pair and the carbene-water complex, in agreement with the known aromatic stabilization of imidazol-2-ylidenes. Accordingly, these molecules hydrolyze with exclusive formation of the ring-opened product.

Stability and Electrophilicity of Phosphorus Analogues of Arduengo Carbenes—An Experimental and Computational Study

A variety of differently substituted 1,3,2-diazaphospholenium salts and P-halogeno-1,3,2-diazaphospholenes (X = F, Cl, Br) were synthesized, and their molecular structures, bonding situation, and Lewis acid properties were characterized by experimental (single-crystal X-ray diffraction, NMR and IR/Raman spectroscopy, MS, conductometry, titrations with Lewis bases) and computational methods. Both experimental and computational investigations confirmed that the structure and bonding in the diazaphospholenium cations of OTf and BF4 salts resembles that of neutral Arduengo carbenes and that the cations should not be described as genuinely aromatic. P-Halogenodiazaphospholenes are, in contrast to earlier assumptions, molecular species with covalent P-X bonds whose bonding situation can be expressed in terms of hyperconjugation between the six pi electrons in the C2N2 unit and the sigma*(P-X) orbital. This interaction induces a weakening of the P-X bonds, whose extent depends subtly on substituent influences and contributes fundamentally to the amazing structural similarity of ionic and covalent diazaphospholene compounds. A further consequence of this effect is the unique polarizability of the P-Cl bonds in P-chlorodiazaphospholenes, which is documented in a considerable spread of P-X distances and bond orders. Measurement of the stability constants for complexes of diazaphospholene compounds with Lewis bases confirmed the lower Lewis acidities and higher stabilities of diazaphospholenium ions as compared with nonconjugated phosphenium ions; this had been inferred from computed energies of isodesmotic halide-transfer reactions, and permitted also to determine equilibrium constants for P-Cl bond dissociation reactions. The results suggest, in accord with conductance measurements, that P-chlorodiazaphospholenes dissociate in solution only to a small extent. On the basis of these findings, the unique solvatochromatic behavior of NMR chemical shifts of these compounds was attributed to solvent-dependent P-Cl bond polarization rather than to shifts in dissociation equilibria.

Diazaphospholenes: N-Heterocyclic Phosphines between Molecules and Lewis Pairs

The interest in geometrically distorted or electronically polarized molecules is often motivated by the realization that unusual structures can engender unprecedented physical or chemical properties. 1,3,2-Diazaphospholenes are N-heterocyclic phosphines (NHPs) that have ring structures similar to N-heterocyclic carbenes (NHCs). Although NHPs were initially mainly of interest as precursors for carbene-analogous phosphenium cations, it was noted that they exhibit quite peculiar structural features and remarkable chemical behavior on their own. In this Account, we discuss both structure and chemistry in connection with the special bonding situation that is characterized by significant n(N)-σ*(P-X) hyperconjugation and induces a strong ionic polarization of the P-X bonds. This induced polarization is surprisingly maintained even when P and X have similar or like electronegativities (for example, X = H, P) and offers the possibility to design compounds with polarized homonuclear bonds. An exemption from the general pattern was only noted for some P-amino-NHPs in which reverse hyperconjugation weakens endocyclic P-N bonds and was predicted to facilitate ring fragmentation and formation of phosphinidenes. An important corollary of the P-X bond polarization in NHPs is a unique bond lengthening, which not only can be fine-tuned by adjusting intramolecular steric and electronic interactions but also responds to intermolecular influences and solvent effects. Insight from crystallographic, spectroscopic, and computational studies allows the development of concepts for controlled manipulation of the bonding, up to a point where P-X bonds are dominated by electrostatic interactions and species behave as Lewis pairs rather than covalent molecules. An appealing aspect lies in the fact that this P-X bond polarization induces reactivities that have hardly any precedence in phosphine chemistry. Examples include (i) reactions of P-hydrogen-substituted NHPs as hydride transfer reagents, (ii) metatheses and additions to multiple bonds (diphosphination) of phosphino-NHPs, which can be used to catalyze P-C cross-coupling reactions and to synthesize 1,2-bisphosphine ligands, (iii) cyclopentadienyl (Cp) transfer reactions of P-Cp-NHPs, and (iv) metal insertion into the P-X bonds of P-halogeno-NHPs. In many aspects, these reactions have potentially useful mechanistic or synthetic implications, and their future exploitation might help to convert NHPs from academically interesting species into useful reagents.

VALENCE ISOMERIZATION IN THE SOLID STATE : FROM 1,3-DIPHOSPHACYCLOBUTANE-2,4-DIYL TO 1,2-DIHYDRO-1,2-DIPHOSPHETE

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.

CATION STABILITIES, ELECTROPHILICITIES, AND CARBENE ANALOGUE CHARACTER OF LOW COORDINATE PHOSPHORUS CATIONS

The stabilities of low coordinated phosphorus cations can be expressed in the frame of the HSAB concept by the transferred charge density Δq(N) which a cation receives upon formation of a donor-acceptor adduct with a Lewis base N. This concept allows to differentiate between relative stabilities towards different reaction partners, and to compare the electrophilicities of phosphenium ions to those of isoelectronic carbenes and silylenes. An analysis of substituent influences on Δq(H) in cations [P(R)2]+suggests an increasing stabilizing power of substituents in the series R = Cl < CH3 < OH, SH < NH2. The same ordering was derived from isodesmic hydride transfer reactions. Interpretation of population analyses suggests that the individual substituent contributions to cation stabilities result from a balance between π-donation into the empty p(P) orbital and electrostatic stabilization by polar P–R σ-bonds. A further stabilizing effect, which is of similar magnitude as in isoelectronic carbenes or silylenes, may arise from cyclic π-conjugation between a diaminophosphenium fragment and an adjacent double bond. Substituent effects influence further the nature of the frontier orbitals of phosphenium ions, resulting in orbital sequences which resemble those of carbenes, allyl anions, or phospholides, respectively. The absence of frontier orbital related changes in reactivity patterns suggests that in all reactions, including metal complex formation, phosphenium ions behave as purely electrophilic rather than ambiphilic species.

The Homoleptic Sandwich Anion [Co(P2C2tBu2)2]−: A Versatile Building Block for Phosphaorganometallic Chemistry†

Phosphaalkynes (RC=P) are valuable starting materials for a wide range of low-coordinate phosphorus compounds.[1] Similar to alkynes,[2] these reactive, triply-bonded molecules oligomerize in the coordination sphere of transition metals to give diphosphacyclobutadienes, triphosphabenzenes, or higher oligomers.[1, 3] The reactions of phosphaalkynes with “vaporized” metal atoms are particularly intriguing, as illustrated by the reaction of cobalt atoms with tBuC=P which affords a mixture of three complexes (A–C) that are difficult to prepare by conventional synthetic methods.[4] Although fascinating compounds can be obtained by metal vapor (MV) synthesis,[5, 6] its applicability is limited due to the low product yields and the need for a special experimental setup.

Valenzisomerisierung im Festkörper: vom 1,3-Diphosphacyclobutan-2,4-diyl zum 1,2-Dihydro-1,2-diphosphet

Das butadienanaloge Phosphanylcarben 2 ist Ab-initio-Rechnungen zufolge das Intermediat der Umlagerung von 1 zu 3 im Feststoff [Gl. (a)]. Seine Energieliegt nur 1.3 kJ mol−1 hoher als die von 1, und es sind bei dieser Umlagerung wahrend der gesamten Reaktion lediglich kleine Anderungen bezuglich der endocyclischen Bindungen erforderlich. R2N = 2,2,6,6-Me4C5H6N.

Complexes with phosphorus analogues of imidazoyl carbenes: unprecedented formation of phosphenium complexes by coordination induced PCl bond heterolysis

Abstract Diazaphospholenium complexes are readily accessible from reactions of complexes [M(bipy)(CO) 3 (L)] (L=CO, MeCN, M=Mo, W) with both the 1,3-dimesityl-4-chloro-1,3,2-diazaphospholenium triflate ( 1 ) [OTf] and the corresponding p -chloro-diazaphospholene ( 8 ). The latter reaction proceeds via an unprecedented coordination induced ionisation of PCl bonds, which requires no further assistance by an external electrophile. The complexes were found to be configurationally stable, but may undergo selective substitution of trans -ligands with retention of the phosphenium moiety. All compounds were characterised by analytical and spectroscopic techniques, and two of the complexes were investigated by single-crystal X-ray diffraction. The spectroscopic and structural data provide evidence for considerable MP double bond character which, leads to a marked reduction of π-delocalization in the diazaphospholenium unit. Studies of metal NMR spectra of tungsten complexes revealed further, a linear correlation between δ 183 W and 1 J WP which allows monitoring of trends in metalphosphorus multiple bonding. Surprisingly, spectroscopic and structural data suggest that the cation 1 displays a higher π-acceptor ability than an analogous CC-saturated heterocyclic phosphenium ligand, which contrasts with the lower electrophilicity of 1 with respect to unconjugated diaminophosphenium species.

Template controlled self-assembly of bidentate phosphine complexes with hemilabile coordination behaviour

Template-assisted self-assembly of ditopic catechol phosphines creates complexes containing a chelating diphosphine ligand, which display hemilabile coordination properties with prospects for applications in catalysis.

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.