Ring Expansions of Nonpolar Polyphosphorus Rings

Functionalization of Tetraphosphido Ligands by Heterocumulenes

Although numerous polyphosphido complexes have been accessed through the transition-metal-mediated activation and functionalization of white phosphorus (P4), the selective functionalization of the resulting polyphosphorus ligands in these compounds remains underdeveloped. In this study, we explore the reactions between cyclotetraphosphido cobalt complexes and heterocumulenes, leading to functionalized P4 ligands. Specifically, the reaction of carbon disulfide (CS2) with [K(18c-6)][(Ar*BIAN)Co(η4-P4)] ([K(18c-6)]1, 18c-6 = [18]crown-6) affords the adduct [K(18c-6)][(Ar*BIAN)Co(η3:η1-P4CS2)] ([K(18c-6)]3), in which CS2 is attached to a single phosphorus atom (Ar* = 2,6-dibenzhydryl-4-isopropylphenyl, BIAN = 1,2-bis(arylimino)acenaphthene diimine). In contrast, the insertion of bis(trimethylsilyl)sulfur diimide S(NSiMe3)2 into a P-P bond of [K(18c-6)]1 yields [K(18c-6)][(Ar*BIAN)Co(η3:η1-P4SN2(SiMe3)2)] (K(18c-6)]4). This salt further reacts with Me3SiCl to form [(Ar*BIAN)Co(η3:η1-P4SN2(SiMe3)3] (5), featuring a rare azatetraphosphole ligand. Moreover, treatment of the previously reported complex [(Ar*BIAN)Co(η3:η1-P4C(O)tBu)] (2) with isothiocyanates results in P-C bond insertion, yielding [(Ar*BIAN)Co(η3:η1-P4C(S)N(R)C(O)tBu)] (6a,b; R = Cy, Ph).

Cobalt-Mediated [3+1] Fragmentation of White Phosphorus: Access to Acylcyanophosphanides

Despite the accessibility of numerous transition metal polyphosphido complexes through transition-metal-mediated activation of white phosphorus, the targeted functionalization of Pn ligands to obtain functional monophosphorus species remains challenging. In this study, we introduce a new [3+1] fragmentation procedure for cyclo-P4 ligands, leading to the discovery of acylcyanophosphanides and -phosphines. Treatment of the complex [K(18c-6)][(Ar*BIAN)Co(η4 -P4 )] ([K(18c-6)]3, 18c-6=[18]crown-6, Ar*=2,6-dibenzhydryl-4-isopropylphenyl, BIAN=1,2-bis(arylimino)acenaphthene diimine) with acyl chlorides results in the formation of acylated tetraphosphido complexes [(Ar*BIAN)Co(η4 -P4 C(O)R)] (R=tBu, Cy, 1-Ad, Ph; 4 a-d). Subsequent reactions of 4 a-d with cyanide salts yield acylated cyanophosphanides [RC(O)PCN]- (9 a-d- ) and the cyclo-P3 cobaltate anion [(Ar*BIAN)Co(η3 -P3 )(CN)]- (8- ). Further reactions of 4 a-d with trimethylsilyl cyanide (Me3 SiCN) and isocyanides provide insight into a plausible mechanism of this [3+1] fragmentation reaction, as these reagents partially displace the P4 C(O)R ligand from the cobalt center. Several potential intermediates of the [3+1] fragmentation were characterized. Additionally, the introduction of a second acyl substituent was achieved by treating [K(18c-6)]9b with CyC(O)Cl, resulting in the first bis(acyl)monocyanophosphine (CyC(O))2 PCN (10).

Pn Ring Size Dependence on NHC-Induced Ring Contraction Reactions of [CoCp‴(η4-P4)], [Ta(CO)2Cp″(η4-P4)], and [FeCp*(η5-P5)]: A DFT Study

The facile ring contraction of [CoCp‴(η⁴-P₄)] and [Ta(CO)₂Cp″(η⁴-P₄)] to [CoCp‴(η³-P₃)][(^MeNHC)₂P] and [Ta(CO)₂Cp″(η³-P₃)] [(^MeNHC)₂P] induced by ^MeNHC and the absence of the ring contraction of [FeCp*(η⁵-P₅)] under the same conditions are studied by density functional theory (DFT) computations. The latter is estimated to be thermodynamically the least favorable reaction and also has a very high energy barrier. The similar strain energies of P₃ and P₄ rings and the lower strain energy of the P₅ ring play a decisive role in the ring contraction capability of these [TM-cyclo-P_n] complexes. Theoretical approaches involving NBO and IBO analysis have been employed to provide a qualitative picture of the overall reactions. The role of substituents and the nature of transition metals in determining the energetics of these reactions has also been studied and an isolobal perspective on these systems affords a simplified picture.

Synthesis and Multiple Subsequent Reactivity of Anionic cyclo-E3 Ligand Complexes (E=P, As)

A synthetic pathway for the synthesis of novel anionic sandwich complexes with a cyclo-E3 (E=P, As) ligand as an end deck was developed giving [Cp'''Co(η3 -E3 )]- (Cp'''=1,2,4-tri-tert-butylcyclopentadienyl, E=P ([5]), As ([6])) in good yields suitable for further reactivity studies. In the reaction with the chlorophosphanes R2 PCl (R=Ph, Cy, t Bu), neutral complexes with a disubstituted cyclo-E3 P (E=P, As) ligand in [Cp'''Co(η3 -E3 PR2 )] (E=P (7 a-c), As (9 a-c)) were obtained. These compounds can be partially or completely converted into complexes with a cyclo-E3 (E=P, As) ligand with an exocyclic {PR2 } unit in [Cp'''Co(η2 :η1 -E3 PR2 )] (E=P (8 a-c), As (10 a-c)). Additionally, the insertion of the chlorosilylene [LSiCl] (L=(t BuN)2 CPh) into the cyclo-E3 ligand of [5] and [6] was achieved and the novel heteroatomic complexes [Cp'''Co(η3 -E3 SiL)] (E=P (11), As (12)) could be isolated. The reaction pathway was elucidated by DFT calculations.

Transition-Metal-Mediated Functionalization of White Phosphorus

Recently there has been great interest in the reactivity of transition-metal (TM) centers towards white phosphorus (P4 ). This has ultimately been motivated by a desire to find TM-mediated alternatives to the current industrial routes used to transform P4 into myriad useful P-containing products, which are typically indirect, wasteful, and highly hazardous. Such a TM-mediated process can be divided into two steps: activation of P4 to generate a polyphosphorus complex TM-Pn , and subsequent functionalization of this complex to release the desired phosphorus-containing product. The former step has by now become well established, allowing the isolation of many different TM-Pn products. In contrast, productive functionalization of these complexes has proven extremely challenging and has been achieved only in a relative handful of cases. In this review we provide a comprehensive summary of successful TM-Pn functionalization reactions, where TM-Pn must be accessible by reaction of a TM precursor with P4 . We hope that this will provide a useful resource for continuing efforts that are working towards this highly challenging goal of modern synthetic chemistry.

Stabilization of Pentaphospholes as η5 -Coordinating Ligands

Electrophilic functionalisation of [Cp*Fe(η5 -P5 )] (1) yields the first transition-metal complexes of pentaphospholes (cyclo-P5 R). Silylation of 1 with [(Et3 Si)2 (μ-H)][B(C6 F5 )4 ] leads to the ionic species [Cp*Fe(η5 -P5 SiEt3 )][B(C6 F5 )4 ] (2), whose subsequent reaction with H2 O yields the parent compound [Cp*Fe(η5 -P5 H)][B(C6 F5 )4 ] (3). The synthesis of a carbon-substituted derivative [Cp*Fe(η5 -P5 Me)][X] ([X]- =[FB(C6 F5 )3 ]- (4 a), [B(C6 F5 )4 ]- (4 b)) is achieved by methylation of 1 employing [Me3 O][BF4 ] and B(C6 F5 )3 or a combination of MeOTf and [Li(OEt2 )2 ][B(C6 F5 )4 ]. The structural characterisation of these compounds reveals a slight envelope structure for the cyclo-P5 R ligand. Detailed NMR-spectroscopic studies suggest a highly dynamic behaviour and thus a distinct lability for 2 and 3 in solution. DFT calculations shed light on the electronic structure and bonding situation of this unprecedented class of compounds.

Cationic Functionalisation by Phosphenium Ion Insertion

The reaction of [Cp'''Ni(η3 -P3 )] (1) with in situ generated phosphenium ions [RR'P]+ yields the unprecedented polyphosphorus cations of the type [Cp'''Ni(η3 -P4 R2 )][X] (R=Ph (2 a), Mes (2 b), Cy (2 c), 2,2'-biphen (2 d), Me (2 e); [X]- =[OTf]- , [SbF6 ]- , [GaCl4 ]- , [BArF ]- , [TEF]- ) and [Cp'''Ni(η3 -P4 RCl)][TEF] (R=Ph (2 f), tBu (2 g)). In the reaction of 1 with [Br2 P]+ , an analogous compound is observed only as an intermediate and the final product is an unexpected dinuclear complex [{Cp'''Ni}2 (μ,η3 :η1 :η1 -P4 Br3 )][TEF] (3 a). A similar product [{Cp'''Ni}2 (μ,η3 :η1 :η1 -P4 (2,2'-biphen)Cl)][GaCl4 ] (3 b) is obtained, when 2 d[GaCl4 ] is kept in solution for prolonged times. Although the central structural motif of 2 a-g consists of a "butterfly-like" folded P4 ring attached to a {Cp'''Ni} fragment, the structures of 3 a and 3 b exhibit a unique asymmetrically substituted and distorted P4 chain stabilised by two {Cp'''Ni} fragments. Additional DFT calculations shed light on the reaction pathway for the formation of 2 a-2 g and the bonding situation in 3 a.

Transformations of the cyclo-P4 ligand in [Cp'''Co(η4-P4)]

The reactivity of the cyclo-P₄ ligand complex [Cp′′′Co(η⁴-P₄)] (1) (Cp′′′ = 1,2,4-tri-tert-butyl-cyclopentadienyl) towards reduction and main group nucleophiles was investigated. By using K[CpFe(CO)₂], a selective reduction to the dianionic complex [(Cp′′′Co)₂(μ,η³:η³-P₈)]²⁻ (2) was achieved. The reaction of 1 with ^tBuLi and LiCH₂SiMe₃ as carbon-based nucleophiles yielded [Cp′′′Co(η³-P₄R)]⁻ (R = ^tBu (4), CH₂SiMe₃ (7)), which, depending on the reaction conditions, undergo subsequent reactions with another equivalent of 1 to form [(Cp′′′Co)₂(μ,η³:η³-P₈R)]⁻ (R = ^tBu (5), CH₂SiMe₃ (8)). In the case of 4, a different pathway was observed, namely a dimerisation followed by a fragmentation into [Cp′′′Co(η³-P₅^tBu₂)]⁻ (6) and [Cp′′′Co(η³-P₃)]⁻ (3). With OH⁻ as an oxygen-based nucleophile, the synthesis of [Cp′′′Co(η³-P₄(O)H)]⁻ (9) was achieved. All compounds were characterized by X-ray crystal structure analysis, NMR spectroscopy and mass spectrometry. Their electronic structures and reaction behavior were elucidated by DFT calculations.

The reactivity of the cyclo-P₄ ligand complex [Cp′′′Co(η⁴-P₄)] (1) (Cp′′′ = 1,2,4-tri-tert-butyl-cyclopentadienyl) towards reduction and main group nucleophiles was investigated.