Phosphinidenoid Complex Research Articles

Organophosphorus Synthesis beyond P-Cl Bond: The Development of Shelf-stable Reagents for [RP] Transfer

: The direct chlorine-free incorporation of P1 units into organic molecules has very important synthetical value owing to environmental considerations and the prospect of accessing unique compounds with fascinating structures and useful properties. This selective survey presents a panorama of phosphorus species that are synthetic equivalents of free singlet phosphinidenes [R-P] and highlights the state-of-art of the [RP]-transfer reactions with emphasis on the synthesis of molecular architectures difficult to reach using traditional methods. Among stabilized phosphinidene precursors capable of RP-transfer are terminal transition-metal phosphinidene and phosphinidenoid complexes, dibenzo-7λ3- phosphinobornadienes, phosphinidene-phosphoranes, inversely polarized phosphaalkenes, phosphaketenes, intramolecularly base-stabilized phosphinidenes, (cyclo)polyphosphines and diphosphenes.

Regioselective N- versus P-Deprotonation of Aminophosphane Tungsten(0) Complexes

1,2-Bifunctional ligands are rare, in general, which holds especially for those with a P-N linkage. Herein, we report on the synthesis of P-tert-butyl substituted aminophosphane W(CO)5 complexes 3a-f (a: R = R’ = H; b: R = H, R = Me; c: R = H, R’ = ally; d: R = H, R’ = i-Pr; e: R = H, R’ = t-Bu; f: R = R’ = Me) obtained via formal N-H insertion reactions of Li/Cl phosphinidenoid complex 2 into NH bonds of ammonia and different amines. The 1,2-bifunctionality of 3b was addressed in targeted regioselective deprotonation reactions leading to amidophosphane complexes M-4b or M/N(H)Me phosphinidenoid complexes M-5b, respectively (M = Li, K). Remarkable was the observation that reactions of M-4b and M-5b with MeI as the electrophile resulted in the formation of the same product 7b. The constitution of all the compounds has been established by means of NMR and IR spectroscopy and mass spectrometry. Two possible reaction pathways were studied in detail using high-level DFT calculations.

M/X Phosphinidenoid Metal Complex Chemistry.

ConspectusLike singlet carbenes and silylenes, transient electrophilic terminal phosphinidene complexes enabled highly selective synthetic transformations, but the required multistep synthetic protocols precluded widespread use of these P1 building blocks. By contrast, nucleophilic M/Cl phosphinidenoid complexes can be easily accessed in one step from [M(CO)n(RPCl2)] complexes. This advantage and the mild reaction conditions opened broad synthetic applicability that enabled access to a variety of novel compounds. The chemistry will be described in this Account, including bonding and mechanistic considerations derived from high-level density functional theory calculations.In 2007, we gained the first strong evidence for the formation of these thermally labile complexes using two different synthetic approaches: P-H deprotonation and Cl/Li exchange; the latter has become the preferred method. Intense studies revealed that steric demand of the P substituents in combination with metal complexation, a donor solvent, and/or the presence of a crown ether are necessary prerequisites for the formation and especially the usability of these intermediates as novel P1 building blocks. Solution-phase NMR spectroscopy and solid-state X-ray diffraction studies revealed the bonding situation, i.e., a solvent-separated ion pair structure, and typical 31P NMR signatures of the anions. To date, we have established the following reactivity patterns for Li/Cl phosphinidenoid complexes: self-condensations (I), electrophilic and nucleophilic reactions (II), 1,1-additions (III), [2 + 1] cycloadditions (IV), ring expansions (V), and redox reactions (VI). For example, self-condensations can yield dinuclear acyclic or polycyclic diphosphane or diphosphene complexes. Their use as nucleophiles and electrophiles can be employed to access functional phosphane ligands with mixed substitution patterns. 1,1-Addition reactions were a puzzling discovery because the resulting products resembled classical P-C π-bond structures but the bonding was more of a donor-to-phosphorus adduct with significant differences in bonding parameters. Into the same category and also surprising fall formal E-H insertion reactions leading to 1,1'-bifunctional phosphane complexes. To date, the most important synthetic impact was achieved in the chemistry of strained P-heterocyclic ligands such as oxaphosphiranes and azaphosphiridines, obtained via [2 + 1] cycloadditions of the title compounds with carbonyls and imines, respectively. Ring expansions have been shown to yield 1,2-oxaphosphetanes and 1,2-thiaphosphetanes, and because of the pool of industrially important epoxides, this provides straightforward and affordable access to these novel P-heterocyclic ligands, which also promise to be of interest in catalytic applications. Recent developments describe redox transformations of Li/Cl phosphinidenoid complexes into new reactive intermediates such as complexes with open-shell P-functional phosphanyl ligands via oxidative single electron transfer reactions or into terminal electrophilic phosphinidene complexes via chloride elimination. The latter is clearly restricted to P-amino derivatives because of their enhanced π-donation capability, as evidenced in a recent study on umpolung of these reactive intermediates. While our efforts to expand M/X phosphinidenoid complex chemistry are ongoing, we want to emphasize that the development of new reactive intermediates not only improves our understanding of bonding and reactivity but also opens new perspectives in organoelement chemistry.

Chemistry of oxaphosphirane complexes

Abstract In this review, the chemistry of σ3λ3-oxaphosphirane metal complexes is presented which was developed over the last 30 years. Important aspects like the improvements in the synthetic accessibility, starting from epoxidation of phosphaalkene complexes in the early 90s to the usage of Li/Cl phosphinidenoid complexes in formal [2+1] cycloaddition reactions with aldehydes and ketones are discussed. Furthermore, physical properties, depending on the P-substituent and metal fragment, as well as the results of quantum chemical calculations are presented, with a special focus on the ring strain energy. The existence of three different polar bonds in the oxaphosphirane ligand open a great variety of synthetic opportunities such as the use of these metal complexes in ring rearrangements, ring expansions catalyzed by strong Bronsted acids, ring opening reactions with protic substrates giving access to 1,2-difunctional phosphane complexes or “ring deconstruction” with different nucleophiles.

Synthesis of the First Oxaphosphirane Iron Complexes

AbstractSynthesis of the first P‐CPh3 substituted oxaphosphirane iron(0) complexes was achieved, representing a more cost‐efficient access to these versatile ligands. While reactions with very electron‐deficient aldehydes such as pentafluoro‐ and 3,5‐bis(trifluoromethyl)‐benzaldehyde were selective to give oxaphosphirane complexes 3 a,b, those with less electronically activated benzaldehydes were very sensitive towards traces of water, and oxaphosphirane complexes 6 a–d were obtained as mixtures with the hydroxyphosphane iron complex 5 in competing reactions. As a case in point, complex 3 a was converted into 1,3,4‐oxazaphospholane complex 6 using an acid‐induced ring‐expansion protocol and acetonitrile. The formation of oxaphosphirane complexes from Li/Cl phosphinidenoid complexes and carbonyl units was studied for the first time through theoretical calculations using appropriate model species.

A synthetic equivalent for unknown 1,3-zwitterions? - A K/OR phosphinidenoid complex with an additional Si-Cl function.

Although the chemistry of frustrated Lewis pairs (FLPs) has seen tremendous developments, investigations on anionic, mono-molecular FLPs are still scarce and 1,3-zwitterions are unknown. Herein, synthesis of a K/OR phosphinidenoid complex bearing a Si-Cl function is reported and, hence, can serve as a synthetic equivalent for a 1,3-zwitterion to allow access to rare and novel heterocyclic P-ligands.

Access to 1,1′‐Bifunctional Phosphane Iron(0) Complexes via P–N Bond‐Forming Reactions and Selective P‐Functionalizations

Reaction of Li/Cl phosphinidenoid Fe(CO)4 complex 2 with ammonia and different amines R1,R2NH yielded complexes 3–5 (3: R1 = R2 = H; 4a: R1 = H, R2 = Me; 4b: R1 = H, R2 = iPr; 4c: R1 = H, R2 = tBu; 4d: R1 = H, R2 = allyl; 4e: R1 = H, R2 = Ph; 5: R1 = Me, R2 = Me) having 1,1′‐bifunctional phosphane ligands via formal N–H bond insertion of the phosphorus center. Studies on deprotonation enabled access to the first structurally characterized K/N(H)R phosphinidenoid complex 6. The latter reacted with phosgene to provide selective access to the P‐chloro complex 7. Theoretical calculations provide insight into the mechanism of the formal N–H bond insertion reaction involving a nucleophilic displacement of chloride at phosphorus and a higher molecularity of a stepwise proton transfer mechanism.

Effects of diminished steric protection at phosphorus on stability and reactivity of oxaphosphirane complexes.

Readily available P-tBu substituted Li/Cl phosphinidenoid complexes react with carbonyl compounds to furnish sterically almost unhindered oxaphosphirane complexes that reveal new and surprisingly facile intra- and intermolecular ring expansion reactions. 1,3,2-Dioxaphosphole complex formation is explained by DFT calculations through diastereoselective carbonyl group-induced ring cleavage of an oxaphosphirane intermediate.

1,1′-Bifunctional Aminophosphane Complexes via N–H Bond Insertions of a Li/Cl Phosphinidenoid Complex and First Studies on N/P Mono Functionalizations

Syntheses of 1,1′-bifunctional aminophosphane complexes 3–10 was achieved by reacting Li/Cl phosphinidenoid complex 2 (R = CPh3) with secondary (Me2NH, Et2NH, Bn2NH, pyrrolidine, and piperidine), primary amines (EtNH2 and allyl amine), and ammonia. Furthermore, regioselective N-silylation of 11 (R = Me) was achieved using MeLi and Me3SiCl to give complex 12. In contrast, treatment of complex 6 with KHMDS in the presence of 18-crown-6 yielded K/NMe2 phosphinidenoid complex 13 subsequently reacted with MeI to give P–Me substituted 14. Complex 5 was deprotonated with KHMDS in the presence of 18-crown-6 and reacted with [Ph3C]BF4 to yield complex 17 via single-electron transfer reaction and P,C-heterocoupling. Finally, selective P–N bond cleavage was achieved by treating complex 18 (R = t-Bu) with HCl(g) to afford chlorophosphane complex 19.

Ambiguous reactivity of Li/Cl phosphinidenoid complexes under redox conditions - a novel dichotomy in phosphorus chemistry.

A novel ambiguous reactivity of Li/Cl phosphinidenoid complexes under redox conditions is described. The outcome of the reaction with hexafluoroacetone is highly dependent on the P-substituent as fluoride substitution occurred in the case of R = CPh3 and C5Me5via a radical pathway, whereas for R = CH(SiMe3)2 a complex having a novel 1,2-diol-type P-ligand was obtained via a closed-shell pathway. DFT calculations reveal a new SET pathway starting with a noncovalent π-hole complex between the phosphinidenoid anion and hexafluoroacetone followed by an elimination of LiF. The second, closed-shell reaction course is strongly influenced by a noncovalent OSi interaction established after the initial nucleophilic attack.

Synthesis and Deprotonation of Aminophosphane Complexes: First K/N(H)R Phosphinidenoid Complexes and Access to a Complex with a P2 N-Ring Ligand.

Synthesis of 1,1'-bifunctional aminophosphane complexes 3 a-e was achieved by the reaction of Li/Cl phosphinidenoid complex 2 with various primary amines (R=Me, iPr, tBu, Cy, Ph). Deprotonation of complex 3 a (R=Me) with potassium hexamethyldisilazide yielded a mixture of K/NHMe phosphinidenoid complex 4 a and potassium phosphanylamido complex 4 a'. Treatment of complex 3 c (R=tBu) and e (R=Ph) with KHMDS afforded the first examples of K/NHR phosphinidenoid complexes 4 c and e. The reaction of complex 3 c with 2 molar equivalents of KHMDS followed by PhPCl2 afforded complexes 5 c,c', which possess a P2 N-ring ligand. All complexes were characterized by NMR, IR, MS, and microanalysis, and additionally, complexes 3 b-e and 5 c' were scrutinized by single-crystal X-ray crystallography.

C-Trifluoromethyl-Substituted 1,2-Oxaphosphetane Complexes: Synthetic and Structural Study

Recently, the first synthetic route to 1,2-oxaphosphetane complexes was described, but the formation of too many isomers was a clear drawback and hampered further studies. Herein, we present significant advances with this problem using trifluoromethyl epoxide (4), 1,1′-bis(trifluoromethyl) epoxide (5), and differently substituted Li/Cl phosphinidenoid complexes 1–3 (R = CH(SiMe3)2, CPh3, C5Me5), thus giving 1,2-oxaphosphetane complexes 6–10 with high selectivity.

A novel route to C-unsubstituted 1,2-oxaphosphetane and 1,2-oxaphospholane complexes.

The synthesis of 1,2-oxaphosphetane complexes and 1,2-oxaphospholane complex bearing only substituents at phosphorus is reported using the reaction of Li/Cl phosphinidenoid complex with 2-iodoethanol or 3-bromo-propane-1-ol and the subsequent dehydrohalogenation using KHMDS. In contrast, the reaction of complex with (t)BuLi leads selectively to the formation of phosphinito complex .

CPh3 as a functional group in P-heterocyclic chemistry: elimination of HCPh3 in the reaction of P-CPh3 substituted Li/Cl phosphinidenoid complexes with Ph2C=O.

P-CPh3 substituted oxaphosphirane complexes 3 were prepared using Li/Cl phosphinidenoid complexes 2 (M = Cr, Mo, W) and benzaldehyde. Employing 2 and benzophenone resulted in the formation of oxaphospholane complexes 4 and 5, the former bearing a benzo[c]-1,2-oxaphospholane and the latter a novel pentacyclic P-ligand. According to DFT studies the latter P-heterocycle arises from formal dimerization of a transient benzofused 2-phosphafurane complex 8, one of the fragments undergoing water-catalyzed [1,3]H shift (4) and the other (11) formed via elimination of HCPh3.

Reactions of Li/Cl Phosphinidenoid Complexes with 1,3,4,5‐Tetramethylimidazol‐2‐ylidene: A New Route to N‐Heterocyclic Carbene Adducts of Terminal Phosphin­idene Complexes and an Unprecedented Transformation of an Oxaphosphirane Complex

AbstractThe reaction of Li/Cl phosphinidenoid tungsten(0) complex 2 [R = CH(SiMe3)2] with 1,3,4,5‐tetramethylimidazol‐2‐ylidene (3) yielded the zwitterionic phosphaalkene complex 4. A comparative study on chloro‐ (5) or dichloro(organo)phosphane complexes 1 [R = CH(SiMe3)2] and NHC 3 revealed the formation of 4 at ambient temperature. First evidence for the formation of phosphinidenoid complex 7, having a C‐Cl imidazolium cation, and its reaction with benzaldehyde to give oxaphosphirane complex 9 are presented. Furthermore, the first example of a formal exchange at the phosphorus center of an oxaphosphirane complex to give zwitterion 4 is demonstrated.

Unprecedented ring-ring interconversion of N,P,C-cage ligands.

The novel N,P,C-cage complexes 5 a-f and 6 a-f have been obtained by the reaction of the P-pentamethylcyclopentadienylphosphinidene complex 2, generated thermally from 2H-azaphosphirene complex 1, with N-methyl-C-arylcarbaldimines 3 a-f. Li/Cl phosphinidenoid complex 8 reacted with 3 a,b to give N,P,C-cage complexes 6 a,b, whereas with 3 c-f, complexes 6 c-f were obtained in negligible amounts only. Both types of ligand N,P,C-cage structures 5 and 6 were found to be in an unprecedented equilibrium, with 5 a,f as the predominant species. Transient electrophilic terminal phosphinidene complexes 10 a-f serve as intermediates in both ligand interconversions (5 a,f↔6 a,f), as evidenced through trapping reactions with phenylacetylene and N-methyl-C-phenylcarbaldimine, thus leading to the novel N,P,C-cage complexes 13 b and 15. DFT calculations predicted a small difference in the relative energies of the two types of N,P,C-cage ligands, and a remarkable stabilisation of the aminophosphinidene complex 10 as the common precursor, thereby providing an insight into this surprising 5-ring-3-ring interconversion. In depth analysis of intermediate 10 revealed the occurrence of both through-bond (conventional inductive/mesomeric effects) and through-space (non-covalent interactions) mechanisms, which amount to 67.8 and 14.4 kcal mol(-1), respectively, and account for the remarkable stabilisation of this intermediate.

Going for strain: synthesis of the first 3-imino-azaphosphiridine complexes and their conversion into oxaphosphirane complex valence isomers.

Reaction of a Li/Cl phosphinidenoid complex with N,N'-dialkyl carbodiimides yielded the novel 3-imino-azaphosphiridine complexes; reaction with water led selectively to the first stable valence isomer of an oxaphosphirane complex.

Synthesis and reaction of the first 1,2-oxaphosphetane complexes.

While P(V) 1,2-oxaphosphetanes are well known from the Wittig reaction, their P(III) analogues are still unexplored. Herein, the synthesis and reactions of the first 1,2-oxaphosphetane complexes are presented, which were achieved by reaction of the phosphinidenoid complex [Li(12-crown-4)(solv)][(OC)5W{(Me3Si)2HCPCl}] with different epoxides. The title compounds appeared to be stable in toluene up to 100 °C, before unselective decomposition started. Acid-induced ring expansion with benzonitrile resulted in selective formation of the first complex bearing a 1,3,4-oxazaphosphacyclohex-2-ene ligand.

Theoretical Study on Novel Mixed Valence, P‐H Functional P‐Ligands, and Their Tautomerization

ABSTRACTThe synthesis of a novel mixed valence P‐ligand system (II/I) with a PP bond bearing phosphonite and PH phosphanyl subunits was achieved via the reaction of a Li/Cl phosphinidenoid complex with a phosphite‐substituted ketone. Density functional theory (DFT) calculations provide insight into bonding features of the novel R(H)PP(OR′)2 ligand system and its tautomer RP = PH(OR′)2. Wiberg bond index, Mayer bond order, and electron density at bond critical points are reported for both the free and ligated forms, pointing to a high degree of PP double bond character of the free ligand with a significant bond order decrease in end‐on pentacarbonyltungsten complexation.

Synthesis of Li/OR phosphinidenoid complexes: on the evidence for intramolecular O–Li donation and the effect of cation encapsulation

Synthesis of phosphinite complexes 12-14a-c, 15a and 16a was achieved via reaction of transient Li/Cl phosphinidenoid complexes 6-10, prepared from dichloro(organo)phosphane complexes [(OC)5M{RPCl2}] 1-5 (1,6: R = CPh3, 2,7: R = C5Me5, 3-5, 8-10: R = CH(SiMe3)2, 1-3, 6-8: M = W, 9: M = Mo, 10: M = Cr), with different alcohols 11a-c (a: R = allyl, b: R = methyl, c: R = isopropyl). Deprotonation of complexes 12b, 13b with MeLi or (t)BuLi in the presence of two equivalents of 12-crown-4 led to the selective formation of phosphinidenoid complexes [Li(12-crown-4)2] [(OC)5W{RP(OCH3)}] (18a R = CPh3 and 18b R = C5Me5) which were stable in solution at ambient temperature, in contrast to Li/OMe phosphinidenoid complexes without 12-crown-4. To our surprise attempts to crystallise complex 18b yielded complex 21 having a Li-O-P subunit. The reaction of complex 17c with [Ph3C]BF4 yielded the P-C coupling product 26 and, hence, the first evidence for an oxidative SET reaction. All isolated products were characterised by multinuclear NMR spectroscopy, IR, MS and single-crystal X-ray crystallography in the case of complexes 12a,b, 13b, 14a-c, 15a, 16a and 21.