Rainer Streubel

Chemistry of λ3-2H-azaphosphirene metal complexes

Abstract In this review, important aspects of λ 3 -2 H -azaphosphirene metal complexes are discussed in relation to synthesis, physical properties and synthetic applications; ab-initio calculations on relative energies of CH 2 NP isomers and of λ 3 -2 H -azaphosphirene metal complexes (Cr, Mo, W) are also presented. Currently, there are three routes to this unsaturated three-membered ring system, which are discussed in detail: two of them use a rearrangement of metal carbene complexes, whereas the other relies on [2+1] cycloaddition reactions of electrophilic terminal phosphanediyl complexes and carbonitriles. The structural data show characteristics of a very strained heterocyclic ring system, which partially helps to understand the reactivity of this heterocycle complex in solution. The synthetic potential of λ 3 -2 H -azaphosphirene metal complexes is illustrated by selected examples, which demonstrate their ability to serve, under very mild conditions as precursor for various new building blocks, i.e. nitrilium phosphanylide complexes, electrophilic terminal phosphanediyl complexes and phosphavinyl–nitrene complexes.

The strongest bond in the universe? Accurate calculation of compliance matrices for the ions N2H+, HCO+, and HOC+

Compliance matrices of protonated CO and N2 are calculated using coupled cluster methods and basis sets of quadruple zeta quality. Diagonal elements of the compliance matrices are used as unique bond strength descriptors. Going from CO (0.052 A/mdyn) to CO–H+ the C–O bond is weakened (0.062 A/mdyn), while the C–O bond in H–CO+ is getting stronger (0.045 A/mdyn). After protonation, the N–N bond strength is getting stronger (from 0.043 to 0.042 A/mdyn), too. The invariance of compliance matrix elements Cij under completion of (xi,xj) to a complete set (…,xi,…,xj,…) of internal coordinates is demonstrated.

Synthesis and Reactions of the First Room Temperature Stable Li/Cl Phosphinidenoid Complex

P-Trityl substituted Li/Cl phosphinidenoid tungsten(0) complex (OC)5W{Ph3CP(Li/12-crown-4)Cl} (3) was prepared via chlorine/lithium exchange in complex (OC)5W{Ph3CPCl2} (2) using (t)BuLi in the presence of 12-crown-4 in tetrahydrofuran (THF) at low temperature; complex 3 possesses significantly increased thermal stability in contrast to previously reported analogue derivatives. Terminal phosphinidene-like reactivity of 3 was used in reactions with benzaldehyde and isopropyl alcohol as oxaphosphirane complex (OC)5W{Ph3CPC(Ph)O} (5) and phosphinite complex (OC)5W{Ph3CP(H)O(i)Pr} (6) were obtained selectively. Reaction of 3 with phosgene allowed to obtain the first kinetically stabilized chloroformylphosphane complex (OC)5W{Ph3CP(Cl)C(O)Cl} (4). Density functional theory (DFT) calculations revealed remarkable differences in the degree of P-Li bond dissociation 3a-d: using a continuum model 3 displays a covalent character of P-Li bond (COSMO (THF)) (a), which becomes elongated if 12-crown-4 is coordinated to lithium (b) and is cleaved if a dimethylether unit is additionally coordinated to lithium (c). A similar result was obtained for the case of 3(thf)4 in which also a solvent-separated ion pair structure is present (d). All products were unambiguously characterized by various spectroscopic means and, in the case of 2 and 4-6, by single-crystal X-ray diffraction analysis. In all structures very long P-C bonds were determined being in the range from 1.896 to 1.955 Å.

Novel access to azaphosphiridine complexes and first applications using Brønsted acid-induced ring expansion reactions

Synthesis of azaphosphiridine complexes 3a-e was achieved via thermal group transfer reaction using 2H-azaphosphirene complex 1 and N-methyl C-aryl imines 2a-e (i) or via reaction of transient Li/Cl phosphinidenoid complex 5 (prepared from dichloro(organo)phosphane complex 4) using 2a-c (ii), respectively. Reaction of complexes 3a,d and trifluoromethane sulfonic acid in the presence of dimethyl cyanamide led to a highly bond- and regioselective ring expansion yielding 1,3,4sigma3lambda3-diazaphosphol-2-ene complexes 8a,d after deprotonation with NEt3. 31P NMR reaction monitoring revealed that protonation of complex 3a yields the azaphosphiridinium complex 6a, unambiguously identified by NMR spectroscopy at low temperature. All isolated products were characterized by multinuclear NMR spectroscopy, IR and UV/Vis (for 3a,d, 6a, 8a,d), MS and single-crystal X-ray crystallography in the cases of complexes 3b-d, 8a and 8d. DFT studies on the reaction mechanism and compliance constants of the model complex of 6a are presented.

Transient Nitrilium Phosphanylid Complexes — New Versatile Building Blocks in Phosphorus Chemistry

This review shows preparative and theoretical aspects of the formation of transient nitrilium phosphanylid complexes (NPCs) and highlights their synthetic potential in organophosphorus chemistry with special emphasis on unsaturated five-membered N,P-heterocycles. According to mechanistic investigations NPCs are, most often, formed via terminal [1,1]-addition of electrophilic terminal phosphanediyl complexes with carbonitriles. Although theoretical investigations on NPCs show that the HOMO and LUMO are mainly concentrated at the metal center, they react like common organic 1,3-dipole systems. Experimental investigations on the regioselectivity of NPCs towards terminal alkynes, nitriles and phosphaalkynes (a≡b) show a strong preference for the formation of the sterically least hindered products. Furthermore, the five-membered ring formation proceeds with retention of configuration at the carbon centers in the case of stereochemically defined alkenes (a=b); nevertheless, some products undergo subsequent epimerization reactions.

Computational studies on azaphosphiridines, or how to effect ring-opening processes through selective bond activation.

The relative energies of azaphosphiridine and its isomers, the ring stability towards valence isomerization, and the ring strain, as well as the kinetics and thermodynamics of possible ring-opening reactions of P(III) derivatives (1-5) and P(V) chalcogenides (6-9; O to Te), were studied at high levels of theory (up to CCSD(T)). The barrier to inversion at the nitrogen atom in the trimethyl-substituted P(III) derivative 5 increases from 12.11 to 15.25 kcal mol(-1) in the P-oxide derivative 6 (P(V)); the relatively high barrier to inversion at the phosphorus in 5 (75.38 kcal mol(-1)) points to a configurationally stable center (MP2/def2-TZVPP//BP86/def2-TZVP). The ring strain for azaphosphiridine 5 (av. 22.6 kcal mol(-1)) was found to increase upon P-oxidation (6) (30.8 kcal mol(-1); same level of theory). Various ring-bond-activation processes were studied: N-protonation of P(III) (5) and P(V) (6, 7) derivatives leads to highly activated species that readily undergo P-N bond cleavage. In contrast, metal chlorides such as LiCl, CuCl, CuCl(2), BeCl(2), BCl(3), AlCl(3), TiCl(3), and TiCl(4) show little P-N bond activation in 5 and 7. Remarkably, TiCl(3) selectively activates the C-N bond, and induces stronger bond activation for P(V) (6, 7) than for P(III) azaphosphiridines (5). The ring-expanding rearrangement of P(V) azaphosphiridines 6-9 to yield P(III) 1,3,2-chalcogena-azaphosphetidines 32 a-d is predicted to be preferred for the heavier chalcogenides 7-9, but not for the P-oxide 6. The first comparative analysis of three bond strength parameters is presented: 1) the electron density at bond critical points, 2) Wibergs bond index, and 3) the relaxed force constant. This reveals the usefulness of these parameters in assessing the degree of ring bond activation.

The Quest for Ring Opening of Oxaphosphirane Complexes : A Coupled-Cluster and Density Functional Study of CH3PO Isomers and Their Cr(CO)5 Complexes

Opening gambit: A high-level theoretical study on the relative stabilities of oxaphosphirane isomers and their Cr(CO)(5) complexes is reported (see picture). Furthermore, thermodynamics and kinetics of possible ring-opening reactions of these complexes in the presence of a {Cp(2)Ti(III)Cl} fragment are theoretically investigated. The C--O bond cleavage is predicted to be the most efficient pathway, thus leading to reactive intermediates that are attractive for synthetic applications.A high-level theoretical study on the relative stabilities of oxaphosphirane isomers and their Cr(CO)(5) complexes is reported. Furthermore, thermodynamics and kinetics of possible ring-opening reactions of these complexes in the presence of a {Cp(2)Ti(III)Cl} fragment are theoretically investigated. The C--O bond cleavage is predicted to be the most efficient pathway thus leading to reactive intermediates that are attractive for synthetic applications. The ring-opening reaction is predicted to not lead to the most favorable product (a coordinated phosphinidene oxide species). Rather, the ring-opening product is separated by a substantial barrier of about 24 kcal mol(-1) from the thermodynamically most favorable species.

Synthesis and Spectroscopic Data ofpara-Substituted (3-Phenyl-2H-azaphosphirene)tungsten Complexes

High-yield synthesis of para-substituted pentacarbonyl(3-phenyl-2H-azaphosphirene)tungsten complexes is reported, using a multi-step rearrangement reaction. Spectroscopic and mass-spectrometric data of these heterocyclic complexes are discussed; the 31P-NMR-chemical shifts clearly reflect the electronic influence of the para-phenyl substituents and the correlation with Hammett σp-constants is almost linear.

Reactivity of terminal phosphinidene versus Li–Cl phosphinidenoid complexes in cycloaddition chemistry

Reaction of the thermally generated transient phosphinidene complex 2 and the Li-Cl phosphinidenoid complex 7 with N-methyl-thiophene-3-carbaldimine (3) was investigated. In the first case a mixture of azaphosphiridine complex 4 and bicyclic azaphospholene complex 5 was obtained; the reaction of complex 7 yielded selectively 5. DFT modelling of the reaction of 7 with 3 supports a nucleophilic formal [4+1] cycloaddition.

Synthesis, Structure, and Reactions of 1-tert-Butyl-2-diphenylphosphino-imidazole

Metalation reactions were studied of a sterically demanding imidazole derivative, namely, 1-tert-butylimidazole (1), with different metalation reagents and subsequent reaction with diphenylchlorophosphane. The reaction product, 1-tert-butyl-2-diphenylphosphino-imidazole (2), was subjected to oxidation and complexation reactions to yield the corresponding products Ph(2)(Imi)P-E (E = O (3), S (4), Se (5), W(CO)(5) (8)) and in the case of borane-THF the N-BH(3) coordination product 10 was obtained. The analytical data of the new compounds are discussed, including X-ray diffraction studies of 3-5.