Coordination isomerism in N-heterocyclic phosphenium thiocyanates

Two N-heterocyclic phosphines with exocyclic SCN substituents were synthesised via metathesis of chlorophosphine precursors with KSCN and fully characterised. The crystallographic studies reveal that the products exhibit pronounced structural differences. The thiocyanato unit binds in one case via the nitrogen atom to yield a molecular structure with a slightly elongated P-N single bond and, in the other case, via the sulfur atom to form a structure that is best described as an ion pair and forms a one-dimensional coordination polymer in the crystal. DFT calculations suggest that the P-N and P center dot center dot center dot S interactions can be described as covalent and dative bonds, respectively, and that the structural differences correlate with the different cation stabilities of the individual phosphenium cation fragments.Peer reviewe

A Ditopic Phosphane-decorated Benzenedithiol as Scaffold for Di- and Trinuclear Complexes of Group-10 Metals and Gold

The ability of 3-(diphenylphosphinomethyl)-benzene-1,2-dithiol (pbdtH(2)) to act as ditopic ligand was probed in reactions with selected group-10-metal complexes. Reactions with [(cod)PdCl2] afforded a mixture of products identified as [Pd(pbdtH)(2)], [Pd-2(mu(2)-pbdt)(2)] and [Pd-3(mu(2)-pbdt)(2)Cl-2]. The polynuclear complexes could be isolated after suitably adjusting the reaction conditions, and heating of a mixture in a microwave reactor effected partial conversion into a further complex [Pd-3(mu(2)-pbdt)(3)]. Reaction of pbdtH(2) with [Ni(H2O)(6)Cl-2] gave rise to a complex [Ni-2(mu(2)-pbdt)(2)], which was shown to undergo two reversible 1e(-)-reduction steps. Reaction of [Pd(pbdtH)(2)] with [Au(PPh3)Cl] afforded heterotrinuclear [PdAu2(mu(2)-pbdt)(2)(PPh3)]. All complexes were characterized by analytical, spectroscopic and single-crystal X-ray diffraction studies. Their molecular structures confirm the ability of the pbdt(2-) unit to support simultaneous P,S- and S,S-chelating coordination to two metal centers.Peer reviewe

Comparing the Ligand Behavior of N-Heterocyclic Phosphenium and Nitrosyl Units in Iron and Chromium Complexes

N-Heterocyclic phosphenium (NHP) and nitro-sonium (NO+) ligands are often viewed as isolobal analogues that share the capability to switch between different charge states and thus display redox "noninnocent" behavior. We report here on mixed complexes [(NHP)M(CO)(n)(NO)] (M = Fe, Cr; n = 2, 3), which permit evaluating the donor/acceptor properties of both types of ligands and their interplay in a single complex. The crystalline target compounds were obtained from reactions of N-heterocyclic phosphenium triflates with PPN[Fe(CO)(3)(NO)] or PPN[Cr(CO)(4)-(NO)], respectively, and fully characterized (PPN = nitride-bistriphenylphosphonium cation). The structural and spectroscopic (IR, UV-vis) data support the presence of carbene-analogue NHP ligands with an overall positive charge state and pi-acceptor character. Even if the structural features of the M-NO unit were in all but one product blurred by crystallographic CO/NO disorder, spectroscopic studies and the structural data of the remaining compound suggest that the NO units exhibit nitroxide (NO-) character. This assignment was validated by computational studies, which reveal also that the electronic structure of iron NHP/ NO complexes is closely akin to that of the Hieber anion, [Fe(CO)(3)(NO)](-). The electrophilic character of the NHP units is further reflected in the chemical behavior of the mixed complexes. Cyclic voltammetry and IR-SEC studies revealed that complex [(NHP)Fe(CO)(2) (NO)] (4) undergoes chemically reversible one-electron reduction. Computational studies indicate that the NHP unit in the resulting product carries significant radical character, and the reduction may thus be classified as predominantly ligand-centered. Reaction of 4 with sodium azide proceeded likewise under nucleophilic attack at phosphorus and decomplexation, while super hydride and methyl lithium reacted with all chromium and iron complexes via transfer of a hydride or methyl anion to the NHP unit to afford anionic phosphine complexes. Some of these species were isolated after cation exchange or trapped with electrophiles (H+, SnPh3(+)) to afford neutral complexes representing the products of a formal hydrogenation or hydrostannylation of the original M=P double bond.Peer reviewe

The Iceland Microcontinent and a continental Greenland-Iceland-Faroe Ridge

The breakup of Laurasia to form the Northeast Atlantic Realm was the culmination of a long period of tectonic unrest extending back to the Late Palaeozoic. Breakup was prolonged and complex and disintegrated an inhomogeneous collage of cratons sutured by cross-cutting orogens. Volcanic rifted margins formed, which are blanketed by lavas and underlain variously by magma-inflated, extended continental crust and mafic high-velocity lower crust of ambiguous and probably partly continental provenance. New rifts formed by diachronous propagation along old zones of weakness. North of the Greenland-Iceland-Faroe Ridge the newly forming rift propagated south along the Caledonian suture. South of the Greenland-Iceland-Faroe Ridge it propagated north through the North Atlantic Craton along an axis displaced ~ 150 km to the west of the northern rift. Both propagators stalled where the confluence of the Nagssugtoqidian and Caledonian orogens formed a transverse barrier. Thereafter, the ~ 400-km-wide latitudinal zone between the stalled rift tips extended in a distributed, unstable manner along multiple axes of extension that frequently migrated or jumped laterally with shearing occurring between them in diffuse transfer zones. This style of deformation continues to the present day. It is the surface expression of underlying magma-assisted stretching of ductile mid- and lower continental crust which comprises the Icelandic-type lower crust that underlies the Greenland-Iceland-Faroe Ridge. This, and probably also one or more full-crustal-thickness microcontinents incorporated in the Ridge, are capped by surface lavas. The Greenland-Iceland-Faroe Ridge thus has a similar structure to some zones of seaward-dipping reflectors. The contemporaneous melt layer corresponds to the 3–10 km thick Icelandic-type upper crust plus magma emplaced in the ~ 10–30-km-thick Icelandic-type lower crust. This model can account for seismic and gravity data that are inconsistent with a gabbroic composition for Icelandic-type lower crust, and petrological data that show no reasonable temperature or source composition could generate the full ~ 40-km thickness of Icelandic-type crust observed. Numerical modeling confirms that extension of the continental crust can continue for many tens of Myr by lower-crustal flow from beneath the adjacent continents. Petrological estimates of the maximum potential temperature of the source of Icelandic lavas are up to 1450 °C, no more than ~ 100 °C hotter than MORB source. The geochemistry is compatible with a source comprising hydrous peridotite/pyroxenite with a component of continental mid- and lower crust. The fusible petrology, high source volatile contents, and frequent formation of new rifts can account for the true ~ 15–20 km melt thickness at the moderate temperatures observed. A continuous swathe of magma-inflated continental material beneath the 1200-km-wide Greenland-Iceland-Faroe Ridge implies that full continental breakup has not yet occurred at this latitude. Ongoing tectonic instability on the Ridge is manifest in long-term tectonic disequilibrium on the adjacent rifted margins and on the Reykjanes Ridge, where southerly migrating propagators that initiate at Iceland are associated with diachronous swathes of unusually thick oceanic crust. Magmatic volumes in the NE Atlantic Realm have likely been overestimated and the concept of a monogenetic North Atlantic Igneous Province needs to be reappraised. A model of complex, piecemeal breakup controlled by pre-existing structures that produces anomalous volcanism at barriers to rift propagation and distributes continental material in the growing oceans fits other oceanic regions including the Davis Strait and the South Atlantic and West Indian oceans

Small Variations, Big Impact : Structural Diversity of the Complexes of a Phosphane-Decorated Benzenedithiol with Group-11 Metals

Reaction of a phosphane-decorated benzenedithiol (pbdtH(2)) with coinage metal salts furnished polynuclear complexes [M-2(pbdtH)(2)] (M=Au-I) or [cat][M-5(pbdt)(3)] (cat=unipositive cation, M=Ag-I, Cu-I), which were characterized by analytical and spectroscopic techniques and single-crystal X-ray diffraction studies. Furthermore, a double salt with an anion [Ag-5(pbdt)(3)(PPh3)](-) that proved unstable in solution was characterized crystallographically. The spectroscopic and crystallographic data revealed that the Cu(I) and Ag(I) complexes exhibit, despite their like stoichiometric composition, isomeric molecular structures. The observed disparities were reproduced by DFT studies. The dinuclear Au(I) complex was found to undergo air-oxidation to furnish a mixed-valent complex [(Au-III)(2)(Au-I)(2)(pbdt)(4)]. The copper(I) - but not the isomeric silver(I) complexes - showed luminescence in the solid state.Peer reviewe

Cationic Diiodo-Phosphoranides through Oxidative I-2 Addition to Tricyclic Phosphamethine Cyanines

Reaction of methylene- and ethylene-bridged bis-imidazolium salts with white phosphorus in the presence of KOtBu furnished moderate yields of tricyclic bis-imidazolio-phosphanide halides. Further oxidation of the products with one equivalent of I-2 gave bis-imidazolio-diiodophosphoranide iodides. All newly prepared compounds were characterized by analytical and spectroscopic data and single-crystal XRD studies. DFT calculations provide evidence for substantial stabilization of cyclic bis-imidazolio-phosphanide cations by pi-conjugation effects and suggest describing the bis-imidazolio-diiodophosphoranide cations as charge-transfer complexes of dicationic iodophosphines with iodide.Peer reviewe

Phosphines with N-Heterocyclic Boryl-Substituents : Ligands for Coordination Chemistry and Catalysis

Boryl-substituted phosphines NHB-P(R)Ph (R = H, Ph, NHB = N-heterocyclic boryl substituent) react with Fe-2(CO)(9) to give isolable Fe(CO)(4) complexes, two of which were characterized by single-crystal XRD studies. The electronic and steric properties for a series of the boryl phosphines were further assessed by evaluation of TEPs for in-situ formed complexes [RhCl(NHB-(PRR2)-R-1)(CO)(2)] (R-1, R-2 = H, Ph, Me, NMe2), and calculations of buried volumes for Fe(CO)(4) complexes. The results imply that the NHB-phosphines exhibit due to their conformational flexibility some variability in their steric bulk, and that some specimens may exhibit similar electron releasing power and steric demand as tBu(3)P. Studies of the amination of bromobenzene with 2,6-diisopropylaniline confirmed that these properties can be exploited to promote Pd-catalyzed C-N cross coupling reactions, and that formal replacement of a phenyl by a NHB substituent in the auxiliary phosphine has a beneficial effect on catalyst performance.Peer reviewe

A Ferrocenophane-Based Diaminophosphenium Ion

Reactions of P-chloro-1,3,2-diazaphospha[3]ferrocenophanes with ECl3 (E = Al, Ga) under solvent-free conditions and with Na[Mn(CO)(5)] furnished salts featuring a ferrocenophane-based phosphenium cation and neutral phosphenium complexes, respectively. All products were characterized by spectroscopic studies. Single-crystal X-ray diffraction studies confirmed the ionic nature of the phosphenium tetrachloroaluminate and the structural analogy between the phosphenium complexes and Fischer-type carbene complexes. Distinct deviations in the conformation of the ansa bridge suggest electronic stabilization of the electrophilic phosphorus atom by phosphorus-nitrogen pi interactions in the free cation and by phosphorus-metal pi bonding in the complexes. The observation of short intermolecular contacts in the crystalline phosphenium salt and its chemical behavior toward donor solvents attest to the cation having an unusually high degree of Lewis acidity, which was confirmed by DFT studies and related to the presence of a rather large N-P-N angle. Computational studies indicate further that the free phosphenium cation exhibits a closed-shell electronic structure with a formal Fe(II) oxidation state and is thus a true analogue to ferrocenophane-based diaminotetrylenes.Peer reviewe

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