Reactions of diaminophosphenium and 1,3,2‐diazaphospholenium triflates with silver salts

Abstract

Reaction of bis(diisopropy1amino)phosphenium triflate (l[CF3S03]) with silver salts AgY [Y-= CF3C0, (Za), CF:{-SO, (2b), CH3C0y (2c)l affords neutral complexes [[(iPr,N),(Y)P) Ag(OS02CF3)] (3a-c). While 3a, c are stable up to ambient temperature, 3b is only stable at low temperature and in equilibrium with the cation 1. The constitution of 3a-c was derived from multinuclear (IH, I3C, "P, ""Ag) NMR spectra. Compound 3a was further studied by X-ray single-crystal diffractommetry which showed the presence of dimeric units with p2-bridging triflate ligands and three-coordinate silver atoms with a planar T-shaped coordination geome try. Reaction of 4 -chloro-l ,3-bis (2,6-dime thylphenyl) -1,3,2-diazaphospholenium triflate (4[CF3S03]) with 2a, c affords equilibrium mixtures whose 31P-and IogAg-NMR spectra suggested the presence of silver complexes 5a, c with constitutions similar to those of 3a, c besides unreacted cation 4. No reaction was observed between 4[CF,SO,] and 2b. A common mechanism accounting for the different outcome of the individual reactions will be discussed.The ability of phosphenium ions RZP+ to form stable cationic transition-metal complexes of type I has long been recognised[ll. Considering that the free cations are isoelectronic and isolobal with neutral carbenes, the complexes I share the same relation with the carbene complexes I1 which are of considerable importance in organic and organometallic chemistry[2].Scheme 1 However, whereas carbene complexes are known for most d-block transition metals, examples of phosphenium complexes have hitherto been restricted mainly to group 6 (Mo, w) and group 8 (Fe) metals; lately, a single nickel phosphenium complex was reported [?]. The recently renewed interest in carbene complexes of the coinage m e t a l~[~~~] stimulated us to investigate if similar phosphenium complexes of these metals can be obtained. Knowing that many phosphorus ligands form stable coordination compounds with Ag+, we studied the reactivity of stable diaminophosphenium and 1,3,2-diazaphospholenium cationsL61 (which can be regarded as the isolobal and isoelectronic counterparts to the stable imidazoyl carbenesf71) towards silver salts AgY (Y-= CF3SOT. CF,CO,, CH3CO;). ReactionsReaction of bis(diisopropy1amino)phosphenium triflate (1[CF3S03]) [s] with one equivalent of CF3C02Ag (2a) in CH2C12 afforded according to an "P-NMR spectroscopic assay a single product which was isolated in moderate yield as colourless, air-and moisture-sensitive crystals. The product was identified by NMR spectroscopy and by X-ray crystal structure analysis as the phosphane complex 3a featuring a four-coordinate phosphorus centre, rather than an anticipated three-coordinate phosphenium complex. In contrast to the known behaviour of phosphanes and p h o s p h i t e~ [~~~~] , no evidence for the formation of complexes of the type [Ag(PR3),]+ (pz > 1) was obtained when 3a was treated with an excess of phosphenium salt. Similar reactions as with CF3C02Ag were observed with 1[CF3S03] and C F 3 S 0 3 h g (2b) or CH...

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

“…Syntheses: The main synthetic strategies [20] to l3-1,3,2-diazaphospholene rings include base-induced addition of phosphorus trichloride to 1,4-diazadienes to give 2,4-dichloro-1,3-dialkyl(diaryl)-substituted derivatives (Scheme 1, route a), [12,21] Abstract in German: Eine Reihe unterschiedlich substituierter 1,3,2-Diazaphospholeniumsalze und P-Halogeno-1,3,2-diazaphospholene (X F, Cl, Br) wurde synthetisiert und hinsichtlich ihrer Molekülstrukturen, Bindungssituation und LewisSäure-Eigenschaften durch experimentelle (Einkristallröntgen-strukturanalyse, NMR, IR/Raman, MS, Konduktometrie, Titrationen mit Lewis-Basen) und theoretische Methoden charakterisiert. Sowohl experimentelle und theoretische Befunde legen nahe, daû die Diazaphospholeniumkationen in OTf-und BF 4 -Salzen hinsichtlich Struktur und Bindungssituation enge Analogien zu neutralen Arduengo-Carbenen zeigen und nicht als genuine aromatische Systeme beschrieben werden sollten.…”

“…or metathesis of PCl 3 with 2,2-dichloro-1,2-dihydro-1,3,2-diazasiloles [13, 16a] or diazadiene dianions [16b] to give 2-chloro-1,3-dialkyl-substituted compounds (Scheme 1, route b). For the present study, we used both routes to obtain in addition to the known compounds 1 a [13, 16a] and 3 a [21] the N-mesityl derivatives 1 c and 3 c (Scheme 1). The required sila-heterocycles 4 a and 4 c were prepared as published by Karsch.…”

Recent Developments in the Chemistry of N-Heterocyclic Phosphines

“…Neutral species do not seem to have been the target of systematic investigations, apart from a study describing the spectroscopic detection of highly unstable and rather poorly characterized NHP-silver complexes [78] , and the reaction of [W(bipy)(CO) 3 (MeCN)] with a P-chloro-NHP to give a likewise unstable yet spectroscopically characterized complex 24 (Scheme 15 ) which decarbonylates at room temperature to produce the isolable neutral phosphenium complex 25 [79] . This reaction was considered remarkable as the occurrence of spontaneous P-Cl heterolysis without assistance of a Lewis acid contradicts a common reaction pattern of chlorophosphines which are normally reluctant to undergo chloride abstraction reactions when coordinated to a metal atom; the observed formation of 24 owes presumably to a pronounced p -basicity of the metal fragment.…”

Synthesis of β‐Lapachone, a Potential Anticancer Agent from the Lapacho Tree

“…Interestingly, Ag I coordination to phosphenium, the heavier analogue of nitrenium, has not been observed with either monodentate10 or pincer‐type11 ligands. It appears that the interaction with silver ion renders phosphenium highly Lewis acidic, so that the vacant p ‐orbital of its central P atom is attacked even by weakly nucleophilic counterions (acetate, trifluoroacetate), resulting in the formation of a trivalent phosphorus species.…”

“…The substantial obstacle is that the same low‐energy empty p orbital, which makes these cationic ligands good π‐acceptors, also renders them highly Lewis acidic, both in the free ligand form or when coordinated to metals. Thus, cationic NHC analogues are susceptible to nucleophilic attack (on the central atom) even by weak nucleophiles, which leads to changes in electronic properties of the ligand and results in a loss of the desired π‐back‐bonding10 (Figure 3 a). Such an attack can also occur in an intramolecular fashion via a halide ion migration from the metal’s coordination sphere to the free ligand p ‐orbital11 (Figure 3 b).…”