Conformationally Constrained N‑Heterocyclic Phosphine–Diimine with Dual Functionality

Condensation of octahydro-2,2′-bipyrimidine with P­(NMe₂)₃ gave a 1,3,2-diazaphospholidine–4,5-diimine <b>4a</b> in which the “open” (exo/exo) conformation of the diimine unit was enforced by incorporation into a tricyclic molecular backbone. The coordination behavior of this potentially ambident ligand was sampled in reactions with ([(nbd)­W­(CO)₄] and [CpCo­(CO)₂]) and pnictogen halides ECl₃ (E = P, As, Sb). While PCl₃ reacted under ring metathesis, all other reactions gave isolable complexes of composition (<b>4a</b>)­ML_<i>n</i> (ML_<i>n</i> = W­(CO)₅, CpCo­(CO), AsCl₃, SbCl₃); attempted recrystallization of the As-adduct yielded a complex (<b>4a</b>)­(AsCl₃)₂ which was also accessible from reaction of <b>4a</b> with 2 equiv of AsCl₃. Single-crystal X-ray diffraction studies revealed that the ligand in [(<b>4a</b>)­W­(CO)₅] and [(<b>4a</b>)­CpCo­(CO)] binds through its phosphorus lone-pair; [(<b>4a</b>)­SbCl₃] and [(<b>4a</b>)­(AsCl₃)₂] contain a T-shaped ECl₃ unit which binds to the chelating diimine moiety, and associate further via chloride bridges to give centrosymmetric dimers. Reactions of <b>4a</b> with excess metal substrates gave no evidence that formation of bimetallic complexes with μ-bridging 1κ²(N,N′)-2κP-coordination is feasible; the extra AsCl₃ moiety in [(<b>4a</b>)­(AsCl₃)₂] avoids this coordination mode by interacting with the peripheral chlorides of the central core. The observed selectivity suggests that ligand <b>4a</b> specifically addresses transition metal centers with low positive charge and some back-bonding capacity through the phosphorus lone-pair, and electrophiles that behave essentially as “pure” Lewis acids through the diimine unit. This assumption was confirmed by DFT studies which disclosed further that binding of the first metal center deactivates the opposite binding site and thus strongly inhibits the formation of dinuclear complexes

Donor-Free Phosphenium–Metal(0)–Halides with Unsymmetrically Bridging Phosphenium Ligands

Reactions of (cod)­MCl₂ (cod = 1,5 cyclooctadiene, M = Pd, Pt) with <i>N</i>-heterocyclic secondary phosphines or diphosphines produced complexes [(NHP)­MCl]₂ (NHP = <i>N</i>-heterocyclic phosphenium). The Pd complex was also accessible from a chlorophosphine precursor and Pd₂(dba)₃. Single-crystal X-ray diffraction studies established the presence of dinuclear complexes that contain μ-bridging NHP ligands in an unsymmetrical binding mode and display a surprising change in metal coordination geometry from distorted trigonal (M = Pd) to T-shaped (M = Pt). DFT calculations on model compounds reproduced these structural features for the Pt complex but predicted an unusual <i>C</i>_2<i>v</i>-symmetric molecular structure with two different metal coordination environments for the Pd species. The deviation between this structure and the actual centrosymmetric geometry is accounted for by the prediction of a flat energy hypersurface, which permits large distortions in the orientation of the NHP ligands at very low energetic cost. The DFT results and spectroscopic studies suggest that the title compounds should be described as phosphenium–metal(0)–halides rather than conventional phosphido complexes of divalent metal cations and indicate that the NHP ligands receive net charge donation from the metals but retain a distinct cationic character. The unsymmetric NHP binding mode is associated with an unequal distribution of σ-donor/π-acceptor contributions in the two M–P bonds. Preliminary studies indicate that reactions of the Pd complex with phosphine donors provide a viable source of ligand-stabilized, zerovalent metal atoms and metal(0)–halide fragments

Reactivity of Phosphanylphosphinidene Complex of Tungsten(VI) toward Phosphines: A New Method of Synthesis of <i>catena</i>-Polyphosphorus Ligands

The reactivity of an anionic phosphanylphosphinidene complex of tungsten­(VI), [(2,6-<i>i-</i>Pr₂C₆H₃N)₂(Cl)­W­(η²-<i>t</i>-Bu₂PP)]­Li·3DME toward PMe₃, halogenophosphines, and iodine was investigated. Reaction of the starting complex with Me₃P led to formation of a new neutral phosphanylphosphinidene complex, [(2,6-<i>i-</i>Pr₂C₆H₃N)₂(Me₃P)­W­(η²-<i>t-</i>Bu₂PP)]. Reactions with halogenophosphines yielded new <i>catena</i>-phosphorus complexes. From reaction with Ph₂PCl and Ph₂PBr, a complex with an anionic triphosphorus ligand <i>t-</i>Bu₂P–P⁽⁻⁾–PPh₂ was isolated. The main product of reaction with PhPCl₂ was a tungsten­(VI) complex with a pentaphosphorus ligand, <i>t</i>-Bu₂P–P⁽⁻⁾–P­(Ph)–P⁽⁻⁾–P-<i>t</i>-Bu₂. Iodine reacted with the starting complex as an electrophile under splitting of the P–P bond in the <i>t-</i>Bu₂PP unit to yield [(1,2-η-<i>t-</i>Bu₂P–P–P-<i>t-</i>Bu₂)­W­(2,6-<i>i-</i>Pr₂C₆H₃N)₂Cl], <i>t</i>-Bu₂PI, and phosphorus polymers. The molecular structures of the isolated products in the solid state and in solution were established by single crystal X-ray diffraction and NMR spectroscopy

Li/X Phosphinidenoid Pentacarbonylmetal Complexes: A Combined Experimental and Theoretical Study on Structures and Spectroscopic Properties

The synthesis of <i>P</i>-F phosphane metal complexes [(CO)₅M­{RP­(H)­F}] <b>2a</b>–<b>c</b> (R = CH­(SiMe₃)₂; <b>a</b>: M = W; <b>b</b>: M = Mo; <b>c</b>: M = Cr) is described using AgBF₄ for a Cl/F exchange in <i>P</i>-Cl precursor complexes [(CO)₅M­{RP­(H)­Cl}] <b>3a</b>–<b>c</b>; thermal reaction of 2<i>H</i>-azaphosphirene metal complexes [(CO)₅M­{RP­(C­(Ph)N}] <b>1a</b>–<b>c</b> with [Et₃NH]­X led to complexes <b>3a</b>–<b>c</b>, <b>4</b>, and <b>5</b> (M = W; <b>a</b>–<b>c</b>: X = Cl; <b>4</b>: X = Br; <b>5</b>: X = I). Complexes <b>2a</b>–<b>c</b>, <b>3a</b>–<b>c</b>, <b>4</b>, and <b>5</b> were deprotonated using lithium diisopropylamide in the presence of 12-crown-4 thus yielding Li/X phosphinidenoid metal complexes [Li­(12-crown-4)­(Et₂O)_<i>n</i>]­[(CO)₅M­(RPX)] <b>6a</b>–<b>c</b>, <b>7a</b>–<b>c</b>, <b>8</b>, and <b>9</b> (<b>6a</b>–<b>c</b>: M = W, Mo, Cr; X = F; <b>7a</b>–<b>c</b>: M = W, Mo, Cr; X = Cl; <b>8</b>: M = W; X = Br; <b>9</b>: 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 <b>6a</b>, <b>7a</b>, <b>8</b>, and <b>9</b> in solution (F > Cl > Br > I). DOSY NMR experiments on the Li/F phosphinidenoid metal complexes (<b>6a</b>–<b>c</b>; 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 <b>6a</b> 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 <b>2a</b>. 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}] (<b>A-D</b>) 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}] (<b>10a</b>) 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 <b>10a</b>) as in type <b>A-D</b> 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 <i>P</i>-Cl derivatives <b>7a</b>–<b>c</b>. 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

Reactivity of Phosphanylphosphinidene Complex of Tungsten(VI) toward Phosphines: A New Method of Synthesis of <i>catena</i>-Polyphosphorus Ligands

The reactivity of an anionic phosphanylphosphinidene complex of tungsten­(VI), [(2,6-<i>i-</i>Pr₂C₆H₃N)₂(Cl)­W­(η²-<i>t</i>-Bu₂PP)]­Li·3DME toward PMe₃, halogenophosphines, and iodine was investigated. Reaction of the starting complex with Me₃P led to formation of a new neutral phosphanylphosphinidene complex, [(2,6-<i>i-</i>Pr₂C₆H₃N)₂(Me₃P)­W­(η²-<i>t-</i>Bu₂PP)]. Reactions with halogenophosphines yielded new <i>catena</i>-phosphorus complexes. From reaction with Ph₂PCl and Ph₂PBr, a complex with an anionic triphosphorus ligand <i>t-</i>Bu₂P–P⁽⁻⁾–PPh₂ was isolated. The main product of reaction with PhPCl₂ was a tungsten­(VI) complex with a pentaphosphorus ligand, <i>t</i>-Bu₂P–P⁽⁻⁾–P­(Ph)–P⁽⁻⁾–P-<i>t</i>-Bu₂. Iodine reacted with the starting complex as an electrophile under splitting of the P–P bond in the <i>t-</i>Bu₂PP unit to yield [(1,2-η-<i>t-</i>Bu₂P–P–P-<i>t-</i>Bu₂)­W­(2,6-<i>i-</i>Pr₂C₆H₃N)₂Cl], <i>t</i>-Bu₂PI, and phosphorus polymers. The molecular structures of the isolated products in the solid state and in solution were established by single crystal X-ray diffraction and NMR spectroscopy

Phosphenium Hydride Reduction of [(cod)MX₂] (M = Pd, Pt; X = Cl, Br): Snapshots on the Way to Phosphenium Metal(0) Halides and Synthesis of Metal Nanoparticles

The outcome of the reduction of [(cod)­PtX₂] (X = Cl, Br; cod = 1,5-cyclooctadiene) with N-heterocyclic phosphenium hydrides ^RNHP–H depends strongly on the steric demand of the <i>N</i>-aryl group R and the nature of X. Reaction of [(cod)­PtCl₂] with ^DippNHP–H featuring bulky N-Dipp groups produced an unprecedented monomeric phosphenium metal(0) halide [(^DippNHP)­(^DippNHP–H)­PtCl] stabilized by a single phosphine ligand. The phosphenium unit exhibits a pyramidal coordination geometry at the phosphorus atom and may according to DFT calculations be classified as a Z-type ligand. In contrast, reaction of [(cod)­PtBr₂] with the sterically less protected ^MesNHP–H afforded a mixture of donor-ligand free oligonuclear complexes [{(^MesNHP)­PtBr}_<i>n</i>] (<i>n</i> = 2, 3), which are structural analogues of known palladium complexes with μ₂-bridging phosphenium units. All reductions studied proceed via spectroscopically detectable intermediates, several of which could be unambiguously identified by means of multinuclear (¹H, ³¹P, ¹⁹⁵Pt) NMR spectroscopy and computational studies. The experimental findings reveal that the phosphenium hydrides in these multistep processes adopt a dual function as ligands and hydride transfer reagents. The preference for the observed intricate pathways over seemingly simpler ligand exchange processes is presumably due to kinetic reasons. The attempt to exchange the bulky phosphine ligand in [(^DippNHP)­(^DippNHP–H)­PtCl] by Me₃P resulted in an unexpected isomerization to a platinum(0) chlorophosphine complex via a formal chloride migration from platinum to phosphorus, which accentuates the electrophilic nature of the phosphenium ligand. Phosphenium metal(0) halides of platinum further show a surprising thermal stability, whereas the palladium complexes easily disintegrate upon gentle heating in dimethyl sulfoxide to yield metal nanoparticles, which were characterized by TEM and XRD studies

Multinuclear Solid-State NMR and DFT Studies on Phosphanido-Bridged Diplatinum Complexes

Multinuclear (³¹P, ¹⁹⁵Pt, ¹⁹F) solid-state NMR experiments on (<i>n</i>Bu₄N)₂[(C₆F₅)₂Pt­(μ-PPh₂)₂Pt­(C₆F₅)₂] (<b>1</b>), [(C₆F₅)₂Pt­(μ-PPh₂)₂Pt­(C₆F₅)₂]­(<i>Pt–Pt</i>) (<b>2</b>), and <i>cis</i>-Pt­(C₆F₅)₂(PHPh₂)₂ (<b>3</b>) were carried out under cross-polarization/magic-angle-spinning conditions or with the cross-polarization/Carr–Purcell Meiboom–Gill pulse sequence. Analysis of the principal components of the ³¹P and ¹⁹⁵Pt chemical shift (CS) tensors of <b>1</b> and <b>2</b> reveals that the variations observed comparing the isotropic chemical shifts of <b>1</b> and <b>2</b>, commonly referred to as “ring effect”, are mainly due to changes in the principal components oriented along the direction perpendicular to the Pt₂P₂ plane. DFT calculations of ³¹P and ¹⁹⁵Pt CS tensors confirmed the tensor orientation proposed from experimental data and symmetry arguments and revealed that the different values of the isotropic shieldings stem from differences in the paramagnetic and spin–orbit contributions

New Selective Synthesis of Dithiaboroles as a Viable Pathway to Functionalized Benzenedithiolenes and Their Complexes

A synthetic protocol to synthesize 2-bromobenzo-1,3,2-dithiaboroles in one step from easily accessible benzene bis­(isopropyl thioether)­s has been developed. The reaction is remarkably specific in converting substrates with two adjacent ^<i>i</i>PrS moieties while leaving isolated thioether functions and other functional groups intact. On the basis of the spectroscopic detection or isolation of reaction intermediates, a mechanistic explanation involving a neighbor-group-assisted dealkylation as a key step is proposed. The resulting products featuring one or two dithiaborole units were isolated in good yields and fully characterized. Subsequent methanolysis, which was carried out either as a separate reaction step or in the manner of a one-pot reaction, gave rise to functionally substituted benzenedithiols. The feasibility of a methylphosphoryl-substituted benzenedithiol to act as a dianionic S,S-chelating ligand was demonstrated with the formation of paramagnetic Ni­(III) and Co­(III) complexes. Selective reduction of the phosphoryl group afforded a rare example of a phosphino dithiol which was shown to act as a monoanionic P,S-bidentate ligand toward Pd­(II). All complexes were characterized by spectral data and X-ray diffraction studies, and the paramagnetic ones also by superconducting quantum interference device magnetometry