An Unusal Case of Facile Non‐Degenerate PC Bond Making and Breaking

On metal coordination of neutral open-shell P-ligands focusing on phosphanoxyls, their electron residence and reactivity

This feature article highlights the discovery and development of phosphanoxyl complex chemistry starting from (neutral) low-coordinate phosphorus radicals and the quest of metal ligation effects. We describe synthesis and reactions of precursors, namely 2,2,6,6-tetramethylpiperidinoxyl (TEMPO) substituted phosphane tungsten(0) complexes. Trapping reactions of transient phosphanoxyl complexes, formed via thermal homolytic N-O bond cleavage, as well as their use in radical polymerisations are illustrated, thus revealing an interesting reactivity dichotomy. DFT calculations provide insight into thermal stabilities of precursors and the resulting spin density distributions (SDDs) in these reactive intermediates. Systematic studies on the dependance of the electron delocalisation in phosphanoxyl complexes have been performed examining different substitution pattern at phosphorus and different co-ligand combinations at the tungsten(0) center. Preliminary results on Mn and Fe complexes are reported.

Three-Coordinate Pd(0) with Rare-Earth Metalloligands: Synergetic CO Activation and Double P-C Bond Cleavage-Formation Reactions

Metalation of β-diketiminato rare-earth metal complexes L^nacnacLn(PhNCH₂PPh₂)₂ (Ln = Y, Yb, Lu) with (COD)Pd(CH₂SiMe₃)₂ afforded three-coordinate Pd(0) complexes supported by two sterically less bulky phosphines and a Pd → Ln dative interaction. The Pd(0) center is prone to ligation with isonitrile and CO; in the latter case, the insertion of a second CO with the Y-N bond was assisted via a precoordination of CO on the Pd(0) center, which led to the formation of an anionic Pd(0) carbamoyl. The reaction of the Pd-Y complex with iodobenzene showed a remarkable double P-C bond cleavage-formation pathway within the heterobimetallic Pd-Y core to afford (Ph₃P)₂PdI(Ph), imine PhNCH₂, and a β-diketiminato yttrium diiodide. In the related reaction of L^nacnacY(PhNCH₂PPh₂)₂ with (Ph₃P)₂PdI(Ph), the P-C bond cleavage following with a N-C bond formation was observed. Computational studies revealed a synergetic bimetallic mechanism for these reactions.

Formation and properties of phosphaquinomethane tungsten(0) complexes - isolation and conversion of primary radical coupling products

A rational approach to phosphaquinomethane metal(0) complexes, based on dearomatization of the phenylene unit in [W(CO)₅](R)P(Cl)-C₆H₅-CPh₂, is described, including theoretical studies on mechanisms and structures. Furthermore, the first phosphaquinone tungsten complex with reversible redox properties is reported thus illustrating the beneficial stabilization of ligation.

Organoiron- and Fluoride-Catalyzed Phosphinidene Transfer to Styrenic Olefins in a Stereoselective Synthesis of Unprotected Phosphiranes

Catalytic phosphiranation has been achieved, allowing preparation of trans-1-R-2-phenylphosphiranes (R = t-Bu: 1-t-Bu; i-Pr: 1-i-Pr) from the corresponding dibenzo-7-(R)-7-phospha-norbornadiene (RPA, A = C₁₄H₁₀, anthracene) and styrene in 73% and 57% isolated yields, respectively. The cocatalyst system requires tetramethylammonium fluoride (TMAF) and [Fp(THF)][BF₄] (Fp = Fe(η⁵-C₅H₅)(CO)₂). In the case of the t-Bu derivative, the reaction mechanism was probed using stoichiometric reaction studies, a Hammett analysis, and a deuterium labeling experiment. Together, these suggest the intermediacy of iron-phosphido FpP(F)(t-Bu) (2), generated independently from the stoichiometric reaction of [Fp(t-BuPA)][BF₄] with TMAF. Two other plausible reaction intermediates, [Fp(t-BuPA)][BF₄] and [Fp(1-t-Bu)][BF₄], were prepared independently and structurally characterized.

Access and unprecedented reaction pathways of Li/Cl phosphinidenoid iron(0) complexes

Complexes [Fe(CO)₄(RPCl₂)] (2) (a: R = CPh₃, b: R = ^tBu) were used to generate the first examples of phosphinidenoid iron(0) complexes [Li(12-crown-4)(solv)_n][Fe(CO)₄(RPCl] (3a,b), characterized by NMR spectroscopy.

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.

Strong Evidence of a Phosphanoxyl Complex: Formation, Bonding, and Reactivity of Ligated Phosphorus Analogues of Nitroxides

Facile access to [W(CO)₅ (Ph₂ P-OTEMP)] is used to initiate a study on the generation, properties, and reactions of transient phosphanoxyl complexes [ML_n (R₂ PO)], the first example of which could be trapped via heterocoupling with the trityl radical. It is also demonstrated that the phosphorus nitroxyl complex acts as radical initiator in the polymerization of styrene. The quest for P-O versus O-N bond homolysis, as well as the initial steps of the polymerization were studied by DFT methods.

Selective phosphanyl complex trapping using TEMPO. Synthesis and reactivity of P-functional P-nitroxyl phosphane complexes

Open-shell phosphanyl complexes (OC)5W{(Me3Si)2HCP(X)} (X = F, H), obtained by one-electron oxidation of lithium phosphanido complexes, were trapped by TEMPO to yield novel thermally labile P-nitroxyl phosphane complexes. First experimental evidence for an open-shell P-F phosphanoxyl complex (OC)5W{(Me3Si)2HCP(O)F}, formed upon thermolysis of corresponding P-nitroxyl derivative, is reported; reaction pathways are also theoretically investigated.

Li/X phosphinidenoid pentacarbonylmetal complexes: a combined experimental and theoretical study on structures and spectroscopic properties

The synthesis of P-F phosphane metal complexes [(CO)5M{RP(H)F}] 2a-c (R = CH(SiMe3)2; a: M = W; b: M = Mo; c: M = Cr) is described using AgBF4 for a Cl/F exchange in P-Cl precursor complexes [(CO)5M{RP(H)Cl}] 3a-c; thermal reaction of 2H-azaphosphirene metal complexes [(CO)5M{RP(C(Ph)═N}] 1a-c with [Et3NH]X led to complexes 3a-c, 4, and 5 (M = W; a-c: X = Cl; 4: X = Br; 5: X = I). Complexes 2a-c, 3a-c, 4, and 5 were deprotonated using lithium diisopropylamide in the presence of 12-crown-4 thus yielding Li/X phosphinidenoid metal complexes [Li(12-crown-4)(Et2O)n][(CO)5M(RPX)] 6a-c, 7a-c, 8, and 9 (6a-c: M = W, Mo, Cr; X = F; 7a-c: M = W, Mo, Cr; X = Cl; 8: M = W; X = Br; 9: M = W; X = I). This first comprehensive study on the synthesis of the title compounds reveals metal and halogen dependencies of NMR parameters as well as thermal stabilities of 6a, 7a, 8, and 9 in solution (F > Cl > Br > I). DOSY NMR experiments on the Li/F phosphinidenoid metal complexes (6a-c; M = W, Mo, Cr) rule out that the cation and anion fragments are part of a persistent molecular complex or tight ion pair (in solution). The X-ray structure of 6a reveals a salt-like structure of [Li(12-crown-4)Et2O][(CO)5W{P(CH(SiMe3)2)F}] with long P-F and P-W bond distances compared to 2a. Density functional theory (DFT) calculations provide additional insight into structures and energetics of cation-free halophosphanido chromium and tungsten complexes and four contact ion pairs of Li/X phosphinidenoid model complexes [Li(12-crown-4)][(CO)5M{P(R)X}] (A-D) that represent principal coordination modes. The significant increase of the compliance constant of the P-F bond in the anionic complex [(CO)5W{P(Me)F}] (10a) revealed that a formal lone pair at phosphorus weakens the P-F bond. This effect is further enhanced by coordination of lithium and/or the Li(12-crown-4) countercation (to 10a) as in type A-D complexes. DFT calculated phosphorus NMR chemical shifts allow for a consistent interpretation of NMR properties and provide a preliminary explanation for the "abnormal" NMR shift of P-Cl derivatives 7a-c. Furthermore, calculated compliance constants reveal the degree of P-F bond weakening in Li/F phosphinidenoid complexes, and it was found that a more negative phosphorus-fluorine coupling constant is associated with a larger relaxed force constant.

First synthesis of chloroformylphosphane complexes

An approach to novel P-functional chloroformylphosphane complexes is described using the synthetic potential of lithium/halogeno phosphinidenoid tungsten(0) complexes. Similarly to transition-metal-free chloroformylphosphanes, the obtained complexes tend to eliminate CO via P–C bond cleavage to give the corresponding P-chlorophosphane complex derivatives. Nevertheless, this method allowed the isolation of the first complex derivatives, which will open new synthetic perspectives in organophosphorus chemistry.