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Mit der Entdeckung der resonanzstabilisierten Phosphamethincyanine durch Dimroth und Hoffmann /1/ und Phosohabenzole durch Märkl /2/ Mitte der sechziger Jahre hat eine stürmische Entwicklung auf dem Gebiet der Phosphorverbindungen mit Phosphor-Element-(p-p)-π-Bindungen eingesetzt. Im Verlauf dieser Aktivitäten gelang 1973 in unserem Arbeitskreis /3/ und 1976 Becker /4/ mit der Synthese der ersten Imino- und Methylenphosphane der Zugang zu isolierbaren Systemen mit Phosphor-Stickstoff- und Phosphor-Kohlenstoff-Doppelbindungen. Einen weiteren Höhepunkt dieser Forschungen stellt die Entdeckung des Diphosphensystems durch Yoshifuji und Mltarbeiter 1981 dar /5/, mit der erstmals die Verifizierung einer (3p-3p)-π-Bindung beim Phosphor gelang. Der folgende Bericht gibt eine Zusammenfassung von eigenen Arbeiten über die drei letztgenannten Verbindungssysteme, wobei der Vergleich der Chemie dieser Verbindungen mit bekannten Systemen der organischen Chemie eine zentrale Rolle spielt.

Die UV- und die He-I-Photoelektronenspektren der Methylenphosphane R-P=C(SiMe3)2 3 (R = Cl, F, OBut, NHBut, SBut, NHSiMe3, NEt2, NPri2, 2,2,6,6-Me4C5H6N, N(But)SiMe3, N(SiMe3)2, Me, But) werden diskutiert. Die Zuordnung der Ionisationspotentiale zu π(P=C)- bzw. n(P)-Orbitalen ist konsistent mit den UV-Daten sowie semieempirischen MNDO-Rechnungen. Die π-Akzeptorwirkung der Silylsubstituenten bewirkt bei den alkylsubstituierten Verbindungen 31,m eine Umkehr der Abfolge der π(P=C)- und n(P)–I.P.'s im Vergleich zu bisher untersuchten Phosphaalkenen sowie theoretischen Vorhersagen. Einführung von RO- bzw. RS-Substituenten in 3c,d führt zur Ausbildung eines mesomeriestabilisierten 3-Zentren-4-Elektronen-π-Bindungssystems. Bei den Aminomethylenphosphanen 3e-k können in Abhängigkeit von der Raumerfüllung der Substituenten zwei unterschiedliche Konformere unterschieden werden, die sich durch die Stellung des Aminrestes bezüglich der Molekülsymmetrieebene unterscheiden. Die Koexistenz beider Konformere kann am Beispiel von 3h durch temperaturabhängige UV-Untersuchungen belegt werden. Die bemerkenswerten Unterschiede der 31P- und 13C-NMR-Daten von 3e-k können im Zusammenhang mit der beobachteten Konformationsisomerie erklärt werden., UV and He-I-p.e. spectra of phosphaalkenes R-P=C(SiMe3)2 3 (R = Cl, F, OBut, SBut, NHBut, NHSiMe3, NEt2, NPri2, 2,2,6,6-Me4C5H6N, N(But)SiMe3, N(SiMe3)2, Me, But) are discussed. Assignment of π(P=C)- and n(P) ionization potentials agrees with UV data as well as semiempirical MNDO calculations. The alkyl-substituted species 31,m exhibit an inverse sequence of π(P=C) and n(P)-i.p.'s compared to theoretical predictions and recent spectroscopic studies, due to the π-electron accepting effect of the silyl substituents. Introduction of RO and RS substituents results in formation of a mesomeric 3-center-4-electron-π-system. Two different conformers of amino-phosphaalkenes can be distinguished which depend on the steric demand of the substituent, and differ in the orientation of the amine ligand in relation to the plane of molecular symmetry. Coexistence of both isomers is proved in case of 3h by temperature dependant UV studies. The existence of conformational isomerism provides an explanation for the remarkable differences in n.m.r. data of 3e-k.

Phospha-alkenes X-P=CTMS2(X = F, Cl, I; TMS = SiMe3) display different reaction behaviour towards metal complexes (Bu3P)2Ni(cod)(cod = cyclo-octa-1,5-diene) or (Ph3P)2Pt(C2H4), respectively, undergoing either co-ordination to give η2-phospha-alkene complexes or alternatively oxidative addition of the P-X bond, forming phospha-alkenyl-metal(II) complexes, [(R3P)2M(X)(σ-P=CTMS2)].

Contrary to the [2 + 2] cycloaddition reaction of aminoiminophosphanes with iminoboranes, the title compounds react to yield the heterocyclic products 7 and 9, the structures of which were determined by means of spectroscopic methods and, in the case of 9, additionally by X-ray structural analysis. Further investigations on the reaction mechanism suggest that the first step of the reaction sequence is an oxidative [2 + 1] cycloaddition of the iminophosphane 3 to 2.

The reaction of the functionalized P/C double bond systems X-P=C(R)SiMe3 [R = SiMe3 and X = Cl (1a), Cp* (5a) or R = Ph and X = Br (1b), Cp* (5b)] with alkali carbonylmetallates or carbonyl complexes of group 6, 8, and 10 metals leads to the formation of the transition metal-substituted phosphaalkenes [{M}-P=C(R)SiMe3 3a-j and 4a-i], in which the P-C fragment acts as a one-electron donor. Depending on the nucleophilicity of the transition metal anion used, but also on the methylene carbon substituents [1a: (SiMe3)2; 1b: (Ph)-SiMe3], a different stability of the (methylenephosphanyl)1-metal complexes is observed, which results in a broad spectrum of reactivities. As demonstrated by the reactions of the (methylenephosphanyl)metal complexes with sulfonic acid derivatives (10a-c), diverse transition metal complexes (11a-c), as well as chalcogenes (14a-c, 15), a change in the reactivity is observed, indicating a transition from an olefinic to a carbenic character of the methylenephosphane. In comparison to the phenyl-substituted derivatives 4a-i the bis(trimethyl)1-substituted species 3a-j exhibit an increased reactivity. This is reflected in the easy decarbonylation of 3a-d to the phosphavinylidene complexes 12a-d.

Reaction of (Me5C5)P=NBut with [(R3P)2Ni(cod)](cod = cyclo-octa-1,5-diene)(R = Et, Bu, or Ph) produces complexes [(R3P)2Ni(C5Me5)P=NBut], which rearrange to yield the novel metal-substituted iminophosphanes, [(R3P)(η5-C5Me5)Ni–P=NBut] and free phosphane R3P, the new complexes being identified by means of n.m.r. spectroscopy; similarly, the metallophospha-alkenes, [(R3P)(η5-C5Me5)Ni–P=C(SiMe3)2] are formed via reaction of (Me5C5)P=C(SiMe3)2 and [(R3P)Ni(alkene)2].

Interessante Abfangprodukte mit Phosphanen und Chlortrimethylsilan bildet das neuartige intermediär gebildete Phosphoranylidencarbenoid 1: Mit PPh3 entsteht das Phosphoniomethanid 2, mit PMe3 das Dihydrodiphosphet 3 und mit ClSiMe3 bildet sich das Imino(methylen)phosphoran 4. R = 2,4,6-tBu3C6H2, R′ = Ph, Me.

In this chapter, we attempt to give a systematic analysis of the influence on the 31P chemical shift and coupling 1JPN in terms of substituent effects, double-bond stereochemistry, and molecular dynamics for iminaphosphines and related compounds containing partial phosphorus-nitrogen triple-bond character. Although the main focus is on properties in the isotropic phase, reflecting that the majority of measurements were made in solution, the tensorial properties of the chemical shift as obtained from solid-state NMR measurements are also mentioned.

Novel iminophosphanes of the type (Me5C5)P=NR [R = C6H2tBu3 (5a), Si-i-Pr3 (5b), SiMe3 (5c)] are prepared via either thermolytic or base-promoted elimination reactions and characterized by analytical and spectroscopic methods. The x-ray crystal structures of 5a,b establish the presence of coordination isomerism of the cyclopentadienyl ring, which at the same time strongly alters the characteristics of the P-N multiple bond. For 5a, η1-attachment of the Me5C5 moiety to the phosphorus and a localized phosphorus-carbon σ-bond are found, and the P-N double bond compares to that of alkylated iminophosphanes (rPN = 155.1 (8) pm). 5b exhibits η2-coordination of the cyclopentadienyl ring leading to increased P-N triple-bond character, as evidenced by shortening of the P-N distance (153.3 (3) pm) and the remarkable opening of the nitrogen valence angle (153.3 (2)°). The structure is discussed as an intramolecular π-complex between a formal cyclopentadienyl anion and an iminophosphenium cation. The 1H and 13C NMR spectra show that rapid elementotropic rearrangements around the five-membered ring take place both in solution and in the solid state. The unique temperature dependence of δ31P in solution together with the considerable differences in δ 31P between the solution and solid states are interpreted in a model assuming a "haptotropic" mechanism for the fluxionality involving dynamic η1/η2-coordination isomerizations in solution. As expected, nucleophilic displacement of the Me5C5 moiety takes place in the reaction of 5b,c with LiC6H2tBu3, yielding the iminophosphanes tBu3H2C6P=NR (R = Si-i-Pr3 (8b), SiMe3 (c)) as products.

A number of aminotetraphosphetanes could be obtained by reductive dehalogenation of dichlorophosphines with magnesium as well as by elimination of chlorotrimethylsilane from vicinal chlorosily-functionalized biphosphines and from [2+2]-cyclodimerization of aminodiphosphenes, respectively. The molecular structures of several tetraphosphetanes are reported and discussed. The torsion-angle of the four-membered rings varies from 22,5 to 45,7 depending on the size und electronegativity of the exocyclic ligands.