Remarkable Transformation of 2,4,6-Tri-tert-butyl-1,3,5-triphosphabenzene into the 2,4,5-Tri-tert-butyl-1,3-diphosphacyclopentadienide Anion

Abstract

In an unprecedented reaction, the reduction of 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene over a potassium mirror in toluene resulted in the abstraction of P 3-and generation of the 2,4,5-tri-tert-butyl-1,3-diphosphacyclopentadienide anion, P 2 C 3 Bu t 3 -(3). The potassium salt of this anion was characterized by an X-ray crystallographic study.There is considerable current interest in the synthesis and ligating properties of compounds containing P-C multiple bonds, 1-4 and very recently 5,6 synthetic routes have been developed for 2,4,6-tri-tert-butyl-1,3,5-triphosphabenzene (1) (Figure 1), whose structure and PE spectrum have also been reported. 7 We recently described the first structurally characterized η 6 -ligated complex of 1 in the triple-decker sandwich compound [Sc 2 (η 5 -P 3 C 2 Bu t 2 ) 2 (η 6 -P 3 C 3 Bu t 3 )] (2), 8 and Binger and co-workers have reported the complexes ML n (η 6 -P 3 C 3 Bu t 3 )] (ML n ) Mo(CO) 3 ,W(CO) 3 , Mn(η 5 -C 5 H 5 ), Ru(COD)), 9 whose structures have been proposed on the basis of NMR spectroscopic studies.In attempting to extend the range of transition-metal complexes of this novel triphosphabenzene ring system via synthesis of the corresponding P 3 C 3 Bu t 3 radical anion, we reacted a THF solution of 1 with a potassium mirror at -78°C. The solution immediately turned green and on warming to room temperature became red.The 31 P{ 1 H} NMR spectrum of the red solution showed that all the triphosphabenzene had been consumed and unexpectedly exhibited the characteristic singlet (δ P 180 ppm) of the known 2,4,5-tri-tert-butyl-1,3-diphosphacyclopentadienide anion, P 2 C 3 Bu t 3 -, 10 indicating the unprecedented reaction shown in Figure 2.Concentration of the reaction mixture, filtration, and addition of hexanes yielded [K(THF)][P 2 C 3 Bu t 3 ] (3) as an off-white powder (40% yield). 21 Evidence for the formation of the phosphide anion P 3-, among the other products, came from observation of the 31 P{ 1 H} resonance of the P(SiMe 3 ) 2 anion (δ P -296 ppm), 11 formed by treatment of the solution mixture with Me 3 SiCl.Confirmation of the formation of 3 came from a singlecrystal X-ray diffraction study of the air-sensitive white crystals, which were grown from a saturated THF solution at 4°C, and the molecular structure is shown in Figure 3. 22 This is the first reported structural characterization of any 1,3-diphosphacyclopentadienide anion, although in unpublished work 12,13 we have determined the structures of both the corresponding lithium and potassium salts of the 1,2,4-triphosphacyclopentadienide anion P 3 C 2 Bu t 2 -. The structure of 3 consists of chains of alternating K(THF) cations and P 2 C 3 Bu t 3 anions, the alkali metal interacting in an η 5 -bonding mode with the rings. Particularly noteworthy are the short P-C bond distances in the ring (average 1.753(5) Å) and the C(2)-C(3) distance (1.416(6) Å), both indicative of significant (1) Dillon, K. B.; Mathey, F.; Nixon, J. F. Lange, H.; Binger, P.; Stannek, J.; Kruger, C.; Bruckmann, J.; Zenneck, U.; K...

Facile preparation of trimethylsilylphosphaalkyne and its conversion to polyphospholide anions

“…As expected, the P-C and C-C bond lengths in (3) are suggestive of aromatic delocalisation and the bond dimensions are very similar to those seen in the relatively rare examples of tert-butyl substituted diphospholide anions [27,29].…”

“…In contrast, reaction of MeLi with 2 in toluene led selectively to the formation of the lithium diphospholide, 3Li (Scheme 3), the reaction being fairly independent of stoichiometry as using anywhere between half to one equivalent of MeLi generates 3Li with only very minor by-products (by 31 P NMR spectroscopy). The lithium phosphide that is presumed to be the by-product of the reaction was not observed experimentally, but we note literature precedent of similar reactions in the elimination of phosphorus species [27][28][29]. Addition of 12-crown-4 and storage of a concentrated THF solution layered with n-hexane gave crystals of 3Li(12-crown-4) 2 that were suitable for an Xray diffraction study.…”

Synthesis and Structure of a 1,3‐Diphosphacyclobutadienediide: An Aniomesolytic Fragmentation of a 1,3‐Diphosphetane‐2,4‐diyl in Solution

“…The characteristic deshielding of the atoms in the central P 2 C 2 unit indicates a PC π‐bonding system ( δ 31 P=200.3, δ 13 C Ring =126.0 ppm (t, 2 J C,P =59.2 Hz)). These resonance signals lie in the range typical for cyclic π‐delocalized phospha‐aromatics, such as P 2 ( t BuC) 3 ⋅[K(thf)] ( δ 31 P=187.6 ppm)13 and( t BuC) 3 P 3 ( δ 31 P=232.6, δ 13 C=218 ppm, 1 J P,C =77 Hz) 14. The shielding of the silicon atoms of the SiMe 3 group ( δ 29 Si=−17.3 ppm (t, 2 J Si,P =18.3 Hz)) supports the interpretation of a “negative hyperconjugation” in accord with the X‐ray analysis.…”

Synthese und Struktur eines 1,3‐Diphosphacyclobutadiendiids – aniomesolytische Fragmentierung eines 1,3‐Diphosphetan‐2,4‐diyls in Lösung

“…Die charakteristische Entschirmung der Atome der zentralen P 2 C 2 ‐Einheit spricht für ein PC‐π‐Bindungssystem ( δ 31 P=200.3, δ 13 C Ring =126.0 ppm (t, 2 J C,P =59.2 Hz)). Die Signallagen sind im erwarteten Bereich für cyclisch π‐delokalisierte Phosphorarene wie P 2 ( t BuC) 3 ⋅[K(thf)] ( δ 31 P=187.6 ppm)13 und ( t BuC) 3 P 3 ( δ 31 P=232.6, δ 13 C=218.0 ppm ( 1 J C,P =77 Hz)) 14. Die Abschirmung der Siliciumatome der SiMe 3 ‐Gruppen ( δ 29 Si=−17.3 ppm (t, 2 J Si,P =18.3 Hz)) stützt die Interpretation als “negative Hyperkonjugation” in Übereinstimmung mit den Ergebnissen der Röntgenstrukturanalyse.…”