Metal-phosphido and -phosphinidene complexes in P-E bond-forming reactions

Panoply of P: An Array of Rhenium-Phosphorus Complexes Generated from a Transition Metal Anion

We expand upon the synthetic utility of anionic rhenium complex Na[(BDI)ReCp] (1, BDI = N,N'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate) to generate several rhenium-phosphorus complexes. Complex 1 reacts in a metathetical manner with chlorophosphines Ph2PCl, MeNHP-Cl, and OHP-Cl to generate XL-type phosphido complexes 2, 3, and 4, respectively (MeNHP-Cl = 2-chloro-1,3-dimethyl-1,3,2-diazaphospholidine; OHP-Cl = 2-chloro-1,3,2-dioxaphospholane). Crystallographic and computational investigations of phosphido triad 2, 3, and 4 reveal that increasing the electronegativity of the phosphorus substituent (C < N < O) results in a shortening and strengthening of the rhenium-phosphorus bond. Complex 1 reacts with iminophosphane Mes*NPCl (Mes* = 2,4,6-tritert-butylphenyl) to generate linear iminophosphanyl complex 5. In the presence of a suitable halide abstraction reagent, 1 reacts with the dichlorophosphine iPr2NPCl2 to afford cationic phosphinidene complex 6+. Complex 6+ may be reduced by one electron to form 6•, a rare example of a stable, paramagnetic phosphinidene complex. Spectroscopic and structural investigations, as well as computational analyses, are employed to elucidate the influence of the phosphorus substituent on the nature of the rhenium-phosphorus bond in 2 through 6. Furthermore, we examine several common analogies employed to understand metal phosphido, phosphinidene, and iminophosphanyl complexes.

Rare-Earth Metal Phosphinidene Complexes: A Trip from Bridging One to Terminal One

ConspectusAs phosphorus analogues of alkylidene (or carbene) and imido (or nitrene) complexes, phosphinidene complexes have received great attention not only for their fundamental scientific merits but also for their ability to build new phosphorus-containing molecules. A large number of phosphinidene complexes in bridging, mononuclear, or terminal coordination modes have been synthesized, and their reactivity has been extensively explored. However, the synthesis of rare-earth metal (scandium, yttrium, and lanthanide metal) phosphinidene complexes lagged behind the transition metal and actinide congeners for decades. Rare-earth metal ions are among the hardest Lewis acids, whereas phosphinidene ligands are soft Lewis bases; rare-earth metal-phosphinidene coordination is thus mismatched based on the Pearson's HSAB principle. The bridging rare-earth metal phosphinidene complexes were not reported until 2008, and the synthesis of the mononuclear and terminal species is even more challenging, which has only recently been achieved.Our group reported a bis(μ2-phosphinidene)dineodymium complex in 2008. In the following >10 years, we have been pursuing the terminal rare-earth metal phosphinidene complexes. Due to the high instability of rare-earth metal-phosphorus multiple bonds, the synthesis and stabilization of these complexes are extremely difficult. Finally, by using suitable phosphinidene ligands and supporting ligands, we obtained the first mononuclear rare-earth metal phosphinidene complex in 2018 and the first terminal rare-earth metal phosphinidene complex in 2020. In these more than ten years of research, we have also found some interesting reactivity of the rare-earth metal phosphinidene complexes. The rare-earth metal bridging phosphinidene complexes can act as two-electron reductants based on the oxidative coupling of two phosphinidene ligands into a diphosphene ligand. The mononuclear rare-earth metal phosphinidene complexes catalyze the hydrogenation of terminal alkenes under mild conditions, and the joint experimental/DFT studies indicate that the hydrogenation reaction proceeds in a 1,2-addition/elimination mechanism rather than the common σ-bond metathesis mechanism. These reactivities are new and important for the rare-earth metal complexes. In addition, the ligand design in our study may contribute to the synthesis of rare-earth metal-arsenic multiple bonding complexes and alkaline-earth metal-phosphorus multiple bonding complexes, which have not yet been realized. Herein, we present an account of our investigations into rare-earth metal phosphinidene complexes, a trip from bridging one to terminal one. To give the readers an overall image of the development of the rare-earth metal phosphinidene complexes, some findings from other researchers are also included.

E-H Bond Cleavage Processes in Reactions of Heterometallic Phosphinidene-Bridged MoRe and MoMn Complexes with Hydrogen and p-Block Element Hydrides

Reactions of complexes [MoMCp(μ-PMes*)(CO)6] with H2 and several p-block element (E) hydrides mostly resulted in the cleavage of E-H bonds under mild conditions [M = Re (1a) and Mn (1b); Mes* = 2,4,6-C6H2tBu3]. The reaction with H2 (ca. 4 atm) proceeded even at 295 K to give the hydrides [MoMCp(μ-H)(μ-PHMes*)(CO)6]. The same result was obtained in the reactions with H3SiPh and, for 1a, upon reduction with Na(Hg) followed by protonation of the resulting anion [MoReCp(μ-PHMes*)(CO)6]-. The latter reacted with [AuCl{P(p-tol)3}] to yield the related heterotrimetallic cluster [MoReAuCp(μ-PHMes*)(CO)6{P(p-tol)3}]. The reaction of 1a with thiophenol gave the thiolate-bridged complex [MoReCp(μ-PHMes*)(μ-SPh)(CO)6], which evolved readily to the pentacarbonyl derivative [MoReCp(μ-PHMes*)(μ-SPh)(CO)5]. In contrast, no P-H bond cleavage was observed in reactions of complexes 1a,b with PHCy2, which just yielded the substituted derivatives [MoMCp(μ-PMes*)(CO)5(PHCy2)]. Reactions with HSnPh3 again resulted in E-H bond cleavage, but now with the stannyl group terminally bound to M, while 1a reacted with BH3·PPh3 to give the hydride-bridged derivatives [MoReCp(μ-H)(μ-PHMes*)(CO)5(PPh3)] and [MoReCp(μ-H){μ-P(CH2CMe2)C6H2tBu2}(CO)5(PPh3)], which follow from hydrogenation, C-H cleavage, and CO/PPh3 substitution steps. Density functional theory calculations on the PPh-bridged analogue of 1a revealed that hydrogenation likely proceeds through the addition of H2 to the Mo=P double bond of the complex, followed by rearrangement of the Mo fragment to drive the resulting terminal hydride into a bridging position.

A Terminal Yttrium Phosphinidene

Terminal, nondirectional ionic "multiple" bond interactions between group 15 elements and rare-earth metals (Ln) have remained a challenging target until present. Although reports on terminal imide species have accumulated in the meantime, examples of terminal congeners with the higher homologue phosphorus are yet elusive. Herein, we present the synthesis of the first terminal yttrium organophosphinidene complex, TptBu,MeY(═PC6H3iPr2-2,6)(DMAP)2, according to a double-deprotonation sequence previously established for organoimides of the smaller rare-earth metals. Subsequent deprotonation of the primary phosphane H2PC6H3iPr2-2,6 (H2PAriPr) with discrete dimethyl compound TptBu,MeYMe2 in the presence of DMAP under simultaneous methane elimination generated a terminal multiply bonded phosphorus. The primary phosphide intermediates TptBu,MeYMe(HPAriPr) and TptBu,MeYMe(HNPAriPr)(DMAP) are isolable species and were also obtained and fully characterized for holmium and dysprosium. The Lewis acid-stabilized yttrium phosphinidene TptBu,MeY[(μ2-PAriPr)(μ2-Me)AlMe2] was obtained by treatment of H2PAriPr with TptBu,MeYMe(AlMe4) but could not be converted into a terminal phosphinidene via cleavage of trimethylaluminum. The corresponding reaction of H2PAriPr with TptBu,MeYMe(GaMe4) led to adduct [GaMe3(PH2AriPr)] rather than to the formation of a yttrium phosphinidene. The yttrium-phosphorus interaction in the obtained organophosphide and phosphinidene complexes was scrutinized by 31P/89Y NMR spectroscopy and DFT calculations, unambiguously supporting the existence of multiple bonding.

Cycloaddition and C-S Bond Cleavage Processes in Reactions of Heterometallic Phosphinidene-Bridged MoRe and MoMn Complexes with Alkynes and Phenyl Isothiocyanate

Reactions of [MoReCp(μ-PMes*)(CO)6] with internal alkynes RC≡CR yielded the phosphapropenylidene-bridged complexes [MoReCp(μ-κ2P,C:η3-PMes*CRCR)(CO)5] (Mes* = 2,4,6-C6H2tBu3; R = CO2Me, Ph). Terminal alkynes HC≡CR1 gave mixtures of isomers [MoReCp(μ-κ2P,C:η3-PMes*CHCR1)(CO)5] and [MoReCp(μ-κ2P,C:η3-PMes*CR1CH)(CO)5], with the first isomer being major (R1 = CO2Me) or unique (R1 = tBu), indicating the relevance of steric repulsions during the [2 + 2] cycloaddition step between Mo=P and C≡C bonds in these reactions. Similar reactions were observed for [MoMnCp(μ-PMes*)(CO)6]. Addition of ligands to these complexes promoted rearrangement of the phosphapropenylidene ligand into the allyl-like μ-η3:κ1C mode, as shown by the reaction of [MoReCp(μ-κ2P,C:η3-PMes*CHC(CO2Me)}(CO)5] with CN(p-C6H4OMe) to give [MoReCp{μ-η3:κ1C-PMes*CHC(CO2Me)}(CO)5{CN(p-CH4OMe)}2]. The MoRe phosphinidene complex reacted with S=C=NPh to give as major products the phosphametallacyclic complex [MoReCp{μ-κ2P,S:κ2P,S-PMes*C(NPh)S}(CO)5] and its thiophosphinidene-bridged isomer [MoReCp(μ-η2:κ1S-SPMes*)(CO)5(CNPh)]. The first product follows from a [2 + 2] cycloaddition between Mo=P and C=S bonds, with specific formation of P-C bonds, whereas the second one would arise from the alternative cycloaddition involving the formation of P-S bonds, more favored on steric grounds. The prevalence of the μ-η2:κ1S coordination mode of the SPMes* ligand over the μ-η2:κ1p mode was investigated theoretically to conclude that steric congestion favors the first mode, while the kinetic barrier for interconversion between isomers is low in any case.

Reactions of Heterometallic Phosphinidene-Bridged MoMn and MoRe Complexes with Sulfur and Selenium: From Chalcogenophosphinidene- to Trithiophosphonate-Bridged Derivatives

Reactions of [MoReCp(μ-PR*)(CO)₆] with S₈ were strongly dependent on experimental conditions (R* = 2,4,6-C₆H₂^tBu₃). When using 1 equiv of sulfur, complex [MoReCp(μ-η²:κ¹_S-SPR*)(CO)₆] was slowly formed at 313 K, with a thiophosphinidene ligand unexpectedly bridging the dimetal center in the novel μ-κ¹_S:η² coordination mode, as opposed to the μ-κ¹_P:η² mode usually found in related complexes. The latter underwent fast decarbonylation at 363 K to give [MoReCp(μ-η²:η²-SPR*)(CO)₅], with a six-electron donor thiophosphinidene ligand rearranged into the rare μ-η²:η² coordination mode. Depending on reaction conditions, reactions with excess sulfur involved the addition of two or three S atoms to the phosphinidene ligand to give new complexes identified as the dithiophosphinidene-bridged complex [MoReCp(μ-η²:κ²_S,S'-S₂PR*)(CO)₅], its dithiophosphonite-bridged isomer [MoReCp(μ-κ²_S,S':κ²_S,S'-S₂PR*)(CO)₅], or the trithiophosphonate-bridged derivative [MoReCp(μ-κ²_S,S':κ²_S,S'-S₃PR*)(CO)₅], all of them displaying novel coordination modes of their PRS₂ and PRS₃ ligands, as determined by X-ray diffraction studies. In contrast, the related MoMn complex yielded [MoMnCp(μ-η²:η²-SPR*)(CO)₅] under most conditions. A similar output was obtained in reactions with gray selenium for either MoRe or MoMn phosphinidene complexes, which under different conditions only gave the pentacarbonyl complexes [MoMCp(μ-η²:η²-SePR*)(CO)₅] (M = Re, Mn), these providing a new coordination mode for selenophosphinidene ligands.

P–C, P–N, and M–N Bond Formation Processes in Reactions of Heterometallic Phosphinidene-Bridged MoMn and MoRe Complexes with Diazoalkanes and Organic Azides to Build Three- to Five-Membered Phosphametallacycles

Reactions of the heterometallic MoRe complex [MoReCp(μ-PR*)(CO)₆] and its MoMn analogue with some small molecules having N–N multiple bonds, such as diazoalkanes and organic azides, were investigated (R* = 2,4,6-C₆H₂^tBu₃). Reactions with excess ethyl diazoacetate proceeded slowly and with concomitant denitrogenation to give complexes [MoMCp(μ-η²_P,C:κ²_P,O-PR*CHCO₂Et)(CO)₅], which display a bridging phosphaalkene ligand in a novel μ-η²:κ² coordination mode, while reactions with other diazoalkanes resulted only in the decomposition of the organic reagent. The MoRe complex reacted with benzyl- or p-tolyl azide at room temperature to give the green complexes [MoReCp(μ-η²_P,N:κ_P,N′²-PR*N₃R)(CO)₆] [R = Bn, p-tol], which display bridging phosphatriazadiene ligands in a novel 6-electron donor coordination mode as a result of a formal [2 + 1] cycloaddition of the terminal N atom of the azide to the Mo–P double bond of the parent complex, followed by coordination of the distal NR nitrogen to the rhenium center. Denitrogenation was only observed for the p-tolyl azide derivative, which upon heating at 333 K yielded [MoReCp{μ-κ_P:κ_N-PR*N(p-tol)}(CO)₆], a molecule displaying a bridging phosphaimine ligand in a rare κ_P:κ_N coordination mode. Analogous reactions of the MoMn phosphinidene complex proceeded similarly at 273 K to give the phosphatriazadiene-bridged derivatives [MoMnCp(μ-η²_P,N:κ²_P,N′-PR*N₃R)(CO)₆], but these were thermally unstable and degraded at room temperature to give the mononuclear triazenylphosphanyl complexes [Mn²(κ_P,N-PR*NHNNR)(CO)₃] as major products, along with small amounts of the phosphaimine-bridged complex [MoMnCp{μ-κ_P:κ_N-PR*N(p-tol)}(CO)₆] in the case of the p-tolyl azide derivative. The structure of the new complexes was analyzed in light of spectroscopic data and single-crystal diffraction studies on selected examples of each type of complex.

Reactions of [MoMCp(μ-PR)(CO)₆] (M = Mn, Re) with N₂CHCO₂Et involved denitrogenation and decarbonylation to give derivatives with a phosphaalkene ligand in a novel μ-κ²:η² coordination mode. However, reactions with organic azides involved [2 + 1] cycloaddition of the terminal nitrogen to the Mo−P double bond of the parent complex and coordination of the distal NR nitrogen without decarbonylation to give phosphatriazadiene derivatives, which evolved differently depending on R and M, to give either phosphaimine or triazenylphosphanyl derivatives.

2,6-Bis[bis(1,1-dimethylethyl)phosphinito-κP]phenyl-κC]-trans-chlorohydro(phenylphosphine)iridium(III)

The molecular structure of an iridium complex featuring a phenylphosphine ligand is described. The reaction of (POCOP)IrHCl (1, POCOP = 2,6-(tBu2PO)2C6H3–) with phenylphosphine gives (POCOP)IrHCl(PH2Ph) (2) under mild conditions. The structural features are consistent with a classic pseudo-octahedral iridium compound with three neutral phosphine donors. Compound 1 is unreactive at elevated temperatures and is unreactive toward excess phenylphosphine under the sampled conditions.

Cyclo-Tetrakis(μ-diphenylphosphido)-1,5-bis(tri-tert-butylphosphine)-Tetracopper

Copper phosphido compound Cu4(μ-PPh2)4(PtBu3)2 was synthesized by three synthetic methods and structurally characterized by X-ray diffraction and 1H, 31P, 13C and 31P HMBC NMR spectroscopy. Cu4(μ-PPh2)4(PtBu3)2 was also demonstrated to be a hydrophosphination pre-catalyst.

Metathesis of P=C Bonds Catalyzed by N-Heterocyclic Carbenes

The catalytic metathesis of C=C bonds is a textbook reaction that has no parallel in the widely studied area of multiple bonds involving heavier p-block elements. A high-yielding P=C bond metathesis of phosphaalkenes (ArP=CPh₂ , Ar=Mes, o-Tol, Ph) has been discovered that is catalyzed by N-heterocyclic carbenes (NHC=Me₂ IMe, Me₂ Iⁱ Pr). The products are cyclic oligomers formally derived from ArP=PAr [i. e. cyclo-(ArP)_n ; n=3, 4, 5, 6] and Ph₂ C=CPh₂ . Preliminary mechanistic studies of this remarkable transformation have established NHC=PAr (Ar=Mes, o-Tol, Ph) as key phosphinidene transfer agents. In addition, novel cyclic intermediates, such as, cyclo-(ArP)₂ CPh₂ and cyclo-(ArP)₄ CPh₂ have also been observed. This work represents a rare application of non-metal-based catalysts for transformations involving main-group elements.

Synthesis of a carborane-substituted bis(phosphanido) cobaltate(i), ligand substitution, and unusual P4 fragmentation

Oxidative addition of the P-P single bond of an ortho-carborane-derived 1,2-diphosphetane (1,2-C2(PMes)2B10H10) (Mes = 2,4,6-Me3C6H2) to cobalt(-i) and nickel(0) sources affords the first heteroleptic complexes of a carborane-bridged bis(phosphanido) ligand. The complexes also incorporate labile ligands suitable for further functionalisation. Thus, the cobalt(i) complex [K([18]crown-6)][Co{1,2-(PMes)2C2B10H10}(cod)] (cod = 1,5-cyclooctadiene) bearing a labile cyclooctadiene ligand undergoes facile ligand exchange reactions with isonitriles and tert-butyl phosphaalkyne with retention of the bis(phosphanido) ligand. However, in the reaction with one equivalent of P4, the electron-rich bis(phosphanido) moiety abstracts a single phosphorus atom with formation of a new P3 chain, while the remaining three P atoms derived from P4 form an η3-coordinating cyclo-P3 ligand. In contrast, when the same reaction is performed with two equivalents of the cobalt(i) complex, a dinuclear product is formed which features an unusual P4 chain in its molecular structure.

Synthesis and reactivity of the uranium phosphinidene metallocene [η5-1,3-(Me3Si)2C5H3]2U([double bond, length as m-dash]P-2,4,6-iPr3C6H2)(OPMe3): influence of the coordinated Lewis base

This paper describes the synthesis and reactivity of [η5-1,3-(Me3Si)2C5H3]2U([double bond, length as m-dash]P-2,4,6-iPr3C6H2)(OPMe3) (6) which is accessible from a ligand exchange reaction between [η5-1,3-(Me3Si)2C5H3]2U([double bond, length as m-dash]P-2,4,6-iPr3C6H2)(OPPh3) (2) and Me3PO at ambient temperature. Phosphinidene 6 exhibits no reactivity towards internal alkynes, but readily reacts with various hetero-unsaturated molecules such as isothiocyanates, aldehydes, nitriles, isonitriles, and organic azides, forming uranium sulfido, oxido, imido, and uranaheterocyclic compounds. Nevertheless, with the bidentate ortho-dicyanobenzene o-C6H4(CN)2 the zwitterionic species [η5-1,3-(Me3Si)2C5H3]2U[NHC(N){C6H4CP(2,4,6-iPr3C6H2)CH2PMe2O}] (13) is isolated in good yield. Moreover, 6 converts with Ph2S2 to the uranium(iii) phenylthiolate compound [η5-1,3-(Me3Si)2C5H3]2USPh(OPMe3) (7) in good isolated yield. Furthermore, the influence of the Lewis base on the reactivity of the uranium phosphinidene metallocenes has also been evaluated.

Rhodium Complexes in P-C Bond Formation: Key Role of a Hydrido Ligand

Olefin hydrophosphanation is an attractive route for the atom-economical synthesis of functionalized phosphanes. This reaction involves the formation of P-C and H-C bonds. Thus, complexes that contain both hydrido and phosphanido functionalities are of great interest for the development of effective and fast catalysts. Herein, we showcase the excellent activity of one of them, [Rh(Tp)H(PMe₃)(PPh₂)] (1), in the hydrophosphanation of a wide range of olefins. In addition to the required nucleophilicity of the phosphanido moiety to accomplish the P-C bond formation, the key role of the hydride ligand in 1 has been disclosed by both experimental results and DFT calculations. An additional Rh-H···C stabilization in some intermediates or transition states favors the hydrogen transfer reaction from rhodium to carbon to form the H-C bond. Further support for our proposal arises from the poor activity exhibited by the related chloride complex [Rh(Tp)Cl(PMe₃)(PPh₂)] as well as from stoichiometric and kinetic studies.

Titanocene pnictinidene complexes

The phospha-Wittig reagent MesTerPPMe3 (MesTer = 2,6-{2,4,6-Me3-C6H2}-C6H3) and arsa-Wittig reagent DipTerAsPMe3 (DipTer = 2,6-{2,6-iPr2-C6H3}-C6H3) have been employed to synthesize the titanocene complexes Cp2Ti(PMe3)PnAr (Pn = P, As) with terminal phosphinidene or arsinidene ligands, respectively. Ab initio studies show that the description as singlet biradicaloids in their ground state is warranted.

Reactivity studies involving a Lewis base supported terminal uranium phosphinidene metallocene [η5-1,3-(Me3C)2C5H3]2U(P-2,4,6-iPr3C6H2)(OPMe3)

Small variations in the phosphinidene substituents, but significant change the reactivity of the uranium phosphinidene complexes.

N-Heterocyclic Carbene Analogues of Nucleophilic Phosphinidene Transition Metal Complexes

Chloride abstraction from the complexes [(η6 -p-cymene){(IDipp)P}MCl] (2 a, M=Ru; 2 b, M=Os) and [(η5 -C5 Me5 ){(IDipp)P}IrCl] (3 b, IDipp=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) with sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaBArF ) in the presence of trimethylphosphine (PMe3 ), 1,3,4,5-tetramethylimidazolin-2-ylidene (Me IMe) or carbon monoxide (CO) afforded the complexes [(η6 -p-cymene){(IDipp)P}M(PMe3 )]BArF ] (4 a, M=Ru; 4 b, M=Os), [(η6 -p-cymene){(IDipp)P}Os(Me IMe)]BArF ] (5) and [(η5 -C5 Me5 ){(IDipp)P}IrL][BArF ] (6, L=PMe3 ; 7, L=Me IMe; 8, L=CO). These cationic N-heterocyclic carbene-phosphinidene complexes feature very similar structural and spectroscopic properties as prototypic nucleophilic arylphosphinidene complexes such as low-field 31 P NMR resonances and short metal-phosphorus double bonds. Density functional theory (DFT) calculations reveal that the metal-phosphorus bond can be described in terms of an interaction between a triplet [(IDipp)P]+ cation and a triplet metal complex fragment ligand with highly covalent σ- and π-contributions. Crystals of the C-H activated complex 9 were isolated from solutions containing the PMe3 complex, and its formation can be rationalized by PMe3 dissociation and formation of a putative 16-electron intermediate [(η5 -C5 Me5 )Ir{P(IDipp)}I][BArF ], which undergoes C-H activation at one of the Dipp isopropyl groups and addition along the iridium-phosphorus bond to afford an unusual η3 -benzyl coordination mode.

Experimental and Computational Studies on a Base-Free Terminal Uranium Phosphinidene Metallocene

The first stable base‐free terminal uranium phosphinidene metallocene is presented; and its structure and reactivity have been studied in detail and compared to that of the corresponding thorium derivative. Salt metathesis reaction of the methyl iodide uranium metallocene Cp’’’₂U(I)Me (2, Cp’’’=η⁵‐1,2,4‐(Me₃C)₃C₅H₂) with Mes*PHK (Mes*=2,4,6‐(Me₃C)₃C₆H₂) in THF yields the base‐free terminal uranium phosphinidene metallocene, Cp’’’₂U=PMes* (3). In addition, density functional theory (DFT) studies suggest substantial 5f orbital contributions to the bonding within the uranium phosphinidene [U]=PAr moiety, which results in a more covalent bonding between the [Cp’’’₂U]²⁺ and [Mes*P]²⁻ fragments than that for the related thorium derivative. This difference in bonding besides steric reasons causes different reactivity patterns for both molecules. Therefore, the uranium derivative 3 may act as a Cp’’’₂U(II) synthon releasing the phosphinidene moiety (Mes*P:) when treated with alkynes or a variety of hetero‐unsaturated molecules such as imines, thiazoles, ketazines, bipy, organic azides, diazene derivatives, ketones, and carbodiimides.

Homoleptic mono-, di-, and tetra-iron complexes featuring phosphido ligands: a synthetic, structural, and spectroscopic study

We report the first series of homoleptic phosphido iron complexes synthesized by treating either the β-diketiminato complex [(Dippnacnac)FeCl₂Li(dme)₂] (Dippnacnac = HC[(CMe)N(C₆H₃-2,6-iPr₂)]₂) or [FeBr₂(thf)₂] with an excess of phosphides R₂PLi (R = tBu, tBuPh, Cy, iPr). Reaction outcomes depend strongly on the bulkiness of the phosphido ligands. The use of tBu₂PLi precursor led to an anionic diiron complex 1 encompassing a planar Fe₂P₂ core with two bridging and two terminal phosphido ligands. An analogous reaction employing less sterically demanding phosphides, tBuPhPLi and Cy₂PLi yielded diiron anionic complexes 2 and 3, respectively, featuring a short Fe-Fe interaction supported by three bridging phosphido groups and one additional terminal R₂P^- ligand at each iron center. Further tuning of the P-substrates bulkiness gave a neutral phosphido complex 4 possessing a tetrahedral Fe₄ cluster core held together by six bridging iPr₂P moieties. Moreover, we also describe the first homoleptic phosphanylphosphido iron complex 5, which features an iron center with low coordination provided by three tBu₂P-P(SiMe₃)^- ligands. The structures of compounds 1-5 were determined by single-crystal X-ray diffraction and 1-3 by ¹H NMR spectroscopy. Moreover, the electronic structures of 1-3 were interrogated using zero-field Mössbauer spectroscopy and DFT methods.

Efficient Synthesis and Multisite Reactivity of a Phosphinidene-Bridged Mo-Re Complex. A Platform Combining Nucleophilic and Electrophilic Features

The heterometallic complex [MoReCp(μ-PR*)(CO)₆] (3) was prepared in 60% overall yield from syn-[MoCp(PHR*)(CO)₂] via a three-step procedure involving complexes syn-[MoCp(PClR*)(CO)₂] and [MoReCp(μ-PR*)(CO)₇] as intermediate species (R* = 2,4,6-C₆H₂^tBu₃). The PR* ligand in 3 displays a novel asymmetric interaction with the dimetal center, involving a double bond with one atom (Mo) and a dative single bond with the other one (Re). Compound 3 underwent thermal isomerization involving a C-H bond cleavage to yield the hydride [MoReCp(μ-H){μ-P(CH₂CMe₂)C₆H₂^tBu₂}(CO)₆] and reacted with I₂ to give [MoReCpI₂(μ-PR*)(CO)₆], which displays a symmetrical phosphinidene bridge. Its reaction with methyl propiolate at 293 K proceeded with [2 + 2] cycloaddition of the alkyne and decarbonylation to yield the phosphapropenylidene-bridged complex [MoReCp{μ-κ²_P,C:η³-PR*CHC(CO₂Me)}(CO)₅] as the major product, whereas its reaction with excess CN(4-C₆H₄OMe) at 273 K proceeded with formal [2 + 1] cycloaddition of the isocyanide and further isocyanide addition at the Re site to yield the complex [MoReCp{μ-η²_P,C:κ¹_P-PR*CN(4-C₆H₄OMe)}(CO)₆{CN(4-C₆H₄OMe)}], which displays an azaphosphaallene ligand in a novel bridging coordination mode.

A Terminal Chlorophosphinidene Complex

Terminal, electrophilic phosphinidene complexes (M=PR) are attractive platforms for PR-transfer to organic substrates. In contrast to aryl- or alkylphosphinidene complexes terminal chlorophosphinidenes (M=PCl) have only been proposed as transient intermediates but isolable example remain elusive. Here we present the transfer of PCl from chloro-substituted dibenzo-7λ3-phosphanorbornadiene to a square-planar osmium(II) PNP pincer complex to give the first isolable, terminal chlorophosphinidene complex with remarkable thermal stability. Os=P bonding was examined computationally giving rise to highly covalent {OsII=PICl} double bonding.

Isolation of Elusive Electrophilic Phosphinidene Complexes with π-Donor N-Heterocyclic Vinyl Substituents

Phosphinidene complexes of the general formula RPM(CO)_n (R = an alkyl or aryl group; M = a transition metal) are electrophilic and thermally unstable. Thus, the isolation of these elusive species for structural elucidations remains a challenge. Herein, we report the first terminal phosphinidene complexes [{(NHC)C(Ph)}P]Fe(CO)₄ [NHC = IPr = C{(NDipp)CH}₂ for 3; Me-IPr = C{(NDipp)CMe}₂ for 4; Dipp = 2,6-iPr₂C₆H₃; NHC = N-heterocyclic carbene] as red crystalline solids containing a π-donor N-heterocyclic vinyl (NHV) substituent at the phosphorus atom. Calculations reveal donor-acceptor type bonding between phosphorus and iron atoms in 3 and 4. The P → Fe donation represents ∼70% of the orbital interaction, whereas the Fe → P π-back-donation corresponds to ∼15% of the orbital interaction. The phosphorus atoms in 3 and 4 carry charges of +0.65e and +0.64e, respectively, indicating the electrophilic character of the phosphinidene {(NHC)C(Ph)}P moiety. Accordingly, 3 reacts with an NHC nucleophile (IMe₄) to yield the Lewis adduct [{(NHC)C(Ph)}P(IMe₄)]Fe(CO)₄ (5) [IMe₄ = C(NMeCMe)₂]. The coordination of an electron-rich NHC (IMe₄) to the phosphorus atom in 5 precludes the π-electron density transfer from the NHV to the phosphorus atom. Thus, the C_IPr-C_vinyl and C_vinyl-P bonds of 5 become shorter and longer, respectively, compared to those of 3.

The Reactivity of Phosphanylphosphinidene Complexes of Transition Metals Toward Terminal Dihaloalkanes

The reactivities of phosphanylphosphinidene complexes [(DippN)₂W(Cl)(η²-P–PtBu₂)]⁻ (1), [(pTol₃P)₂Pt(η²-P=PtBu₂)] (2), and [(dppe)Pt(η²-P=PtBu₂)] (3) toward dihaloalkanes and methyl iodide were investigated. The reactions of the anionic tungsten complex (1) with stochiometric Br(CH₂)_nBr (n = 3, 4, 6) led to the formation of neutral complexes with a tBu₂PP(CH₂)₃Br ligand or neutral dinuclear complexes with unusual tetradentate tBu₂PP(CH₂)_nPPtBu₂ ligands (n = 4, 6). The methylation of platinum complexes 2 and 3 with MeI yielded neutral or cationic complexes bearing side-on coordinated tBu₂P–P-Me moieties. The reaction of 2 with I(CH₂)₂I gave a platinum complex with a tBu₂P—P—I ligand. When the same dihaloalkane was reacted with 3, the P—P bond in the phosphanylphosphinidene ligand was cleaved to yield tBu₂PI, phosphorus polymers, [(dppe)PtI₂] and C₂H₄. Furthermore, the reaction of 3 with Br(CH₂)₂Br yielded dinuclear complex bearing a tetraphosphorus tBu₂PPPPtBu₂ ligand in the coordination sphere of the platinum. The molecular structures of the isolated products were established in the solid state and in solution by single-crystal X-ray diffraction and NMR spectroscopy. DFT studies indicated that the polyphosphorus ligands in the obtained complexes possess structures similar to free phosphenium cations tBu₂P⁺=P−R (R = Me, I) or (tBu₂P⁺=P)₂.

The phosphanylphosphinidene complexes of tungsten and platinum as building blocks for syntheses of polydentate phosphorus ligands.

One-step synthesis and P-H bond cleavage reactions of the phosphanyl complex syn-[MoCp{PH(2,4,6-C6H2tBu3)}(CO)2] to give heterometallic phosphinidene-bridged derivatives

Photolysis of [Mo2Cp2(CO)6] and PH2R* (R* = 2,4,6-C6H2tBu3) yielded the title complex, which turned out to be a versatile precursor of novel heterometallic phosphinidene-bridged complexes via three different P-H bond activation processes: photolysis, deprotonation and reduction. In this way the new complexes [MoReCp(μ-PR*)(CO)n] (n = 6,7), [MoFeCp2(μ-PR*)(CO)m], (m = 3, 4) and [MoAuCp(μ-PR*)(CO)2{P(p-tol)3}] were prepared.

Triamidoamine-Supported Zirconium Compounds in Main Group Bond-Formation Catalysis

The rationale to pursue long-term study of any system must be sound. Quick discoveries and emergent fields are more than temptations. They remind us to ask what are we gaining through continued study of any system. For triamidoamine-supported zirconium, there has been a great deal gained with yet more ahead. Initial study of the system taught much that is applied to catalysis. Cyclometalation of a trimethylsilyl substituent of the ancillary ligand, abbreviated (N₃N) when not metalated for simplicity, via C-H bond activation is facile and highly reversible. It has allowed for the synthesis of a range of Zr-E bonds, which are of fundamental interest. More germane, cyclometalation has emerged as our primary product liberation step in catalysis. Cyclometalation also appears to be a catalyst resting state, despite how cyclometalation is a known deactivation step for many a compound in other circumstances. Catalysis with triamidoamine-supported zirconium has been rich. Rather than summarizing the breadth of reactions, a more detailed report on the dehydrocoupling of phosphines and hydrophosphination is provided. Both reactions demonstrate the outward impact that the study of (N₃N)Zr-based catalysis has afforded. Dehydrocoupling catalysis, or bond formation via loss of hydrogen, is particular to 3p and heavier main group elements. The reaction has been important in the formation of E-E and E-E' bonds in the main group for molecular species and materials. While study of this reaction at (N₃N)Zr compounds provides key insights into mechanism, discoveries in the area of P-P and Si-Si bond formation with (N₃N)Zr derivatives as catalysts have greater reach than merely the synthesis of main group element containing products. For example, that work has informed design principles for the identification of catalysts that transfer low-valent fragments. The successful application of these principles was evident in the discovery of a catalyst that transfers phosphinidene ("PR") to unsaturated substrates. Hydrophosphination exhibits perfect atom economy in the formation of P-C bonds. The reaction can proceed without a catalyst, but the purpose of a catalyst is enhanced reactivity and selectivity. Nevertheless, significant challenges in this reaction remain. In particular, (N₃N)Zr compounds have demonstrated high activity in hydrophosphination and readily utilize unactivated unsaturated organic molecules, challenging substrates for any heterofunctionalization reaction. This activity has led to not only impressive metrics in the catalysis but access to previously untouched substrates and formation of unique products. The particular properties of the (N₃N)Zr system that engage in this reactivity may influence other heterofunctionalization reactions. The recently discovered photocatalytic hydrophosphination with (N₃N)ZrPRR' compounds already appears to be general rather than unique and may drive additional bond formation catalysis among early transition-metal compounds.

Interconversion of Phosphinyl Radical and Phosphinidene Complexes by Proton Coupled Electron Transfer

The isolable complex [Os(PHMes*)H(PNP)] (Mes*=2,4,6-t Bu3 C6 H3 ; PNP=N{CHCHPt Bu2 }2 ) exhibits high phosphinyl radical character. This compound offers access to the phosphinidene complex [Os(PMes*)H(PNP)] by P-H proton coupled electron transfer (PCET). The P-H bond dissociation energy (BDE) was determined by isothermal titration calorimetry and supporting DFT computations. The phosphinidene product exhibits electrophilic reactivity as demonstrated by intramolecular C-H activation.

The Taming of Redox-Labile Phosphidotitanocene Cations

Tame d⁰ phosphidotitanocene cations stabilized with a pendant tertiary phosphane arm are reported. These compounds were obtained by one-electron oxidation of d¹ precursors with [Cp₂ Fe][BPh₄ ]. The electronic structure of these compounds was studied experimentally (EPR, UV/Vis, and NMR spectroscopy, X-ray diffraction analysis) and through DFT calculations. The theoretical analysis of the bonding situation by using the electron localization function (ELF) shows the presence of π-interactions between the phosphido ligand and Ti in the d⁰ complexes, whereas dπ-pπ repulsion prevents such interactions in the d¹ complexes. In addition, CH-π interactions were observed in several complexes, both in solution and in the solid state, between the phosphido ligand and the phosphane arm. The d⁰ complexes were found to be light sensitive, and decompose through Ti-P bond homolysis to give Ti^III species. A naked d⁰ phosphidotitanocene cation has been trapped by reaction with diphenylacetylene, yielding a Ti/P frustrated Lewis pair (FLP), which was found to be less reactive than a previously reported Zr analog.

Experimental and computational studies on a three-membered diphosphido thorium metallaheterocycle [η5-1,3-(Me3C)2C5H3]2Th[η2-P2(2,4,6-iPr3C6H2)2]

A three-membered diphosphido thorium metallaheterocycle complex was prepared and its reactivity was investigated.

Preparation of a potassium chloride bridged thorium phosphinidiide complex and its reactivity towards small organic molecules

The steric and electronic properties of the coordinated ligands modulate the reactivity of thorium phosphinidene complexes.

A base-free terminal thorium phosphinidene metallocene and its reactivity toward selected organic molecules

Small molecule activation mediated by a base-free terminal phosphinidene thorium metallocene is reported.