Observation of a Chloride-Bridged P–P Bond in the Phosphorus Cation [L(Cl)P(μ-Cl)P(Cl)L]⁺ (L = NHC)

Abstract

A bridging chloride anion between two electrophilic phosphorus centers was observed for the first time in the molecular structure of the novel P−P bonded cation [L Dipp 2 P 2 Cl 3 ] + (2 + ; L Dipp = 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene). Cation 2 + is prepared by the reduction of the monophosphorus species [L Dipp PCl 2 ] + (1 + ) with sodium. DFT calculations, AIM and mechanochemical (compliance constants) analyses are used to examine the bonding situation of this unusual species. The monochloro-substituted cation [L Dipp 2 P 2 Cl] + (3 + ) was likewise observed as a reduction product of 1 + . ■ INTRODUCTIONPhosphorus has the highest homoatomic single bond energy (205 kJ/mol) of any group 15 element and therefore shows a high propensity to form P−P-bonded frameworks. 1 The chemistry of neutral and anionic polyphosphorus compounds has therefore been flourishing for decades. 2 Recently, this area has received fresh impetus from the use of stable N-heterocyclic carbenes (NHCs) for the preparation of P−P-bonded species that display P 2 , P 3 , P 4 , and P 12 frameworks. 3 Several novel types of P−P-bonded phosphorus cations have emerged in recent years. 4 One interesting challenge in this area is the synthesis of cationic polyphosphorus compounds that carry halide substituents. 5 Mono-, di-, and trichlorophosphanylphosphonium ions have been recently described. 6 Chloride substituents have also been incorporated into a few oligophosphonium cations through ion transfer. 7 We recently reported a convenient protocol for the synthesis of cations [L iPr PCl 2 ] + and [L iPr 2 PCl] 2+ (L iPr = 1,3-diisopropylimidazol-2-ylidene), using an imidazolium-transfer reagent [L iPr SiMe 3 ]- [OTf]. 8 The chlorine atoms in these reactive cations can be substituted by cyano and azido groups.Aiming at the synthesis of new polyphosphorus species, we initiated a study of the reaction behavior of our [LPCl 2 ] + cations toward low-valent transition metalates and strong reducing agents. Here, we describe the synthesis of the novel cationic diphosphane derivative [L Dipp 2 P 2 Cl 3 ] + (2 + ; L Dipp = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) and the related cation [L Dipp 2 P 2 Cl] + (3 + ), which are obtained via the reduction of the novel dichloro-substituted cation [L Dipp PCl 2 ] + (1 + ). The molecular structure of cation 2 + shows an unprecedented arrangement where a chloride anion symmetrically bridges two phosphorus centers. The bonding situation in this unusual compound is analyzed by DFT calculations and compliance constant analysis. 9 ■ RESULTS AND DISCUSSION [L Dipp 2 P 2 Cl 3 ][OTf] (2[OTf]) and [L Dipp 2 P 2 Cl][OTf] (3[OTf]) were initially obtained as a product mixture by reacting the triflate salt [L Dipp PCl 2 ][OTf] (1[OTf]) with the recently reported low-valent iron(0) naphthalene complex [K(18-crown-6){Cp*Fe(η 4 -naphthalene)}] (K[Fe]) (Scheme 1). 10 K[Fe] acts as a two-electron reducing agent in this reaction. The cationic complex [Cp*Fe(η 6 -naphthalene)]Cl ([Fe]Cl) was identified as the oxida...

N‐Heterocyclic Carbene Stabilized Boryl Radicals

“…[27,28] In contrast, an almost perpendicular orientation with atwist angle of 85.68 8 between the carbene and boryl moieties is found for 7,a ffording minor localization of spin density on the boron atom. [41,42] NMR spectra of 10 a-c were recorded in CD 2 Cl 2 solution; the 1 HNMR spectra exhibit singlets at ca. [29] In this contribution, we wish to report the preparation of boryl radicals such as 11 c that contain the sterically demanding and strongly nucleophilic carbene 1,3-di-tert-butyl-4,5dimethylimidazolin-2-ylidene (8), [30] which should also lead to perpendicularly oriented, but inversely polarized NHC and boryl fragments,a sd escribed by the canonical form B (Figure 1).…”

NHC‐Mediated Synthesis of an Asymmetric, Cationic Phosphoranide, a Phosphanide, and Coinage‐Metal Phosphanido Complexes

“…[3] Additionally, the intriguing reactivity of certain phosphanyl-substituted imidazolium ions has been discussed. [5] The above-mentioned examples are either described as donor-acceptor complexes of NHC ligands and the respective P-centered moiety or as imidazoliumyl-substituted P compounds that bear a positive charge on the imidazoliumyl substituent. [4] Furthermore, we recently reported on the reduction of cation 7 + to compound 8 + in which a dicationic diphosphane acts as a chelating Lewis acid toward a chloride anion (Scheme 1).…”

Recent Advances in Imidazoliumyl‐Substituted Phosphorus Compounds

“…When 96 c [OTf] is reduced with two equivalents of sodium at 60 °C, the formation of the unique chloride‐bridged P 2 compound 101 [OTf] is observed (Scheme ) . The 31 P NMR spectrum shows a singlet resonance at δ (P)=−34.4 ppm, confirming the identical chemical environment of the two P atoms in solution.…”

“…Our group developed a selective and versatile reaction protocol for the synthesis of mono‐, di‐, and tricationic imidazoliumyl‐substituted P III ‐centered compounds by the reaction of PCl 3 with imidazoliumyl transfer reagents 95 a – d [OTf] . The main advantage of this protocol is to prevent the reductive formation of P I species and anion‐induced side reactions as the Cl − anions are released from the reaction by the formation of Me 3 SiCl.…”