Thomas Wiegand

Reactions of phosphorus/boron frustrated Lewis pairs with SO2

The frustrated Lewis pair tBu3P/B(C6F5)3 (1) readily adds SO2 to yield the zwitterionic adduct tBu3P+–S(O)–OB−(C6F5)3 (3). A series of intramolecular vicinal P/B FLPs Mes2P–(X)–B(C6F5)2 [X = –CH2–CH2– (2a), –CHMe–CH2– (2b), cyclo-C6H10 (5)] add SO2 at −78 °C to yield the corresponding six-membered addition products 4a, 4b, 6. The adducts contain a chiral sulfur center. The [B]–O–(O)S–[P] addition products 3, 4b and 6 were characterized by X-ray diffraction.

Remarkable coordination behavior of alkyl isocyanides toward unsaturated vicinal frustrated P/B Lewis pairs

The conjugated frustrated phosphane/borane Lewis pairs formed by 1,1-carboboration of a substituted diphenylphosphino acetylene, undergo a synergistic 1,1-addition reaction to n-butyl isocyanide with formation of new B–C and P–C bonds to the former isonitrile carbon atom. Using tert-butyl isocyanide dynamic behaviour between the isocyanide–[B] adduct and the 1,1-addition product formation was observed in solution. The different modes of isocyanide binding to the FLPs in the solid state were characterized using X-ray crystal structure analyses and comprehensive 11B and 31P solid-state magic-angle-spinning (MAS-) NMR experiments. The free FLP, the Lewis adduct at the borane group, and the cyclic product resulting from isocyanide addition to both reaction centers, can be differentiated via11B and 31P isotropic chemical shifts, 11B nuclear electric quadrupole coupling constants, isotropic indirect 11B–31P spin–spin coupling constants, and 11B⋯31P internuclear distances measured by rotational echo double resonance.

Binding of Molecular Magnesium Hydrides to a Zirconocene–Enyne Template†

The ternary silicide La2Li2Si3 was synthesized from the elements in a sealed niobium tube. La2Li2Si3 was characterized by powder and single crystal X-ray diffraction: Ce2Li2Ge3 type, Cmcm, a 1⁄4 450.03(8), b 1⁄4 1880.3(4), c 1⁄4 689.6(1) pm, wR2 1⁄4 0.0178, 597 F2 values, and 26 parameters. The La2Li2Si3 structure contains two crystallographically independent silicon sites, both in slightly distorted trigonal prismatic coordination. The Si1 atoms are located in condensed La6 prisms and form cisetrans chains (two-bonded silicon) with Si1eSi1 distances at 238 and 239 pm, indicating single bond character. The Si2 atoms are isolated within La2Li4 prisms. La2Li2Si3 might be formally considered as an electron precise Zintl phase with an electron partition (2La3þ)(2Liþ)(2Si12e)(Si24e). Electronic structure calculations show a trend in this direction based on a charge density analysis with large electron localization around the Si1eSi1 chains. The compound is found weakly metallic with chemical bonding reminiscent of LaSi and additional features brought in by Li and Si2. High resolution solid state 7Li and 29Si MAS-NMR spectra are in agreement with the crystal structural information, however, the 29Si resonance shifts observed suggest strong Knight shift contributions, at variance with the Zintl concept. Variable temperature solid state 7Li spectra indicate the absence of motional narrowing on the kHz timescale within the temperature range 300K < T < 400 K. 2012 Elsevier Masson SAS. All rights reserved.

Gold(I) and Silver(I) Complexes of Diphosphacyclobutadiene Cobaltate Sandwich Anions

The synthesis and structural characterization of the first coordination compounds of bis(diphosphacyclobutadiene) cobaltate anions [M(P(2)(2)R(2))(2)](-) is described. Reactions of the new potassium salts [K(thf)(3){Co(η(4)-P(2)C(2)tPent(2))(2)}] (1) and [K(thf)(4){Co(η(4)-P(2)C(2)Ad(2))(2)}] (2) with [AuCl(tht)] (tht = tetrahydrothiophene), [AuCl(PPh(3))] and Ag[SbF(6)] afforded the complexes [Au{Co(P(2)C(2)tPent(2))(2)}(PMe(3))(2)] (3), [Au{Co(P(2)C(2)Ad(2))(2)}](x) (4), [Ag{Co(P(2)C(2)Ad(2))(2)}](x) (5), [Au(PMe(3))(4)][Au{Co(P(2)C(2)Ad(2))(2)}(2)] (6), [K([18]crown-6)(thf)(2)][Au{Co(P(2)C(2)Ad(2))(2)}(2)] (7), and [K([18]crown-6)(thf)(2)][M{Co(P(2)C(2)Ad(2))(2)}(2)] (8: M = Au 9: M = Ag) in moderate yields. The molecular structures of 2 and 3, and 6-9 were elucidated by X-ray crystallography. Complexes 4-9 were thoroughly characterized by (31)P and (13)C solid state NMR spectroscopy. The complexes [Au{Co(P(2)C(2)Ad(2))(2)}](x) (4) and [Ag{Co(P(2)C(2)Ad(2))(2)}](x) (5) exist as coordination polymers in the solid state. The linking mode between the monomeric units in the polymers is deduced. The soluble complexes 1-3, 6, and 7 were studied by multinuclear (1)H-, (31)P{(1)H}-, and (13)C{(1)H} NMR spectroscopy in solution. Variable temperature NMR measurements of 3 and 6 in deuterated THF reveal the formation of equilibria between the ionic species [Au(PMe(3))(4)](+), [Au(PMe(3))(2)](+), [Co(P(2)C(2)R(2))(2)](-), and [Au{Co(P(2)C(2)R(2))(2)}(2)](-) (R = tPent and Ad).

Indirect “No-Bond” 31P···31P Spin–Spin Couplings in P,P-[3]Ferrocenophanes: Insights from Solid-State NMR Spectroscopy and DFT Calculations

No-bond (31)P-(31)P indirect dipolar couplings, which arise from the transmission of nuclear spin polarization through interaction of proximal nonbonded electron pairs have been investigated in the solid state for a series of closely related substituted P,P-[3]ferrocenophanes and model systems. Through variation and combination of ligands (phenyl, cyclohexyl, isopropyl) at the two phosphorus sites, the P···P distances in these compounds can be varied from 3.49 to 4.06 Å. Thus, the distance dependence of the indirect no-bond coupling constant J(nb) can be studied in a series of closely related compounds. One- and two-dimensional solid-state NMR experiments serve to establish the character of these couplings and to measure the isotropic coupling constants J(iso), which were found to range between 12 and 250 Hz. To develop an understanding of the magnitude of J(nb) in terms of molecular structure, their dependences on intramolecular internuclear distances and relative orbital orientations is discussed by DFT-calculations on suitable models. In agreement with the literature the dependence of J(nb) on the P···P distance is found to be exponential; however, the steepness of this curve is highly dependent on the internuclear equilibrium distance. For a quantitative description, the off-diagonal elements of the expectation value of the Kohn-Sham-Fock operator in the LMO basis for the LMOs of the two phosphorus lone-pairs is proposed. This parameter correlates linearly with the calculated J(nb) values and possesses the same distance-dependence. In addition, the simulations indicate a distinct dependence of J(nb) on the dihedral angle defined by the two C-P bonds providing ligation to the molecular backbone. For disordered materials or those featuring multiple sites, conformers, and/or polymorphism, a new double-quantum NMR method termed DQ-DRENAR can be used to conveniently measure internuclear (31)P-(31)P distances. If conducted on compounds with known P···P distances, such measurements can also serve to estimate the magnitude of the anisotropy ΔJ of these no-bond indirect spin-spin couplings. The DFT results suggest that in the present series of compounds the magnitude of ΔJ is strongly correlated with that of the isotropic component, as both parameters have analogous distance dependences. While our studies indicate a sizable J-anisotropy for the model compound 1,8-bis(diphenylphosphino)napthalene (ΔJ ~ -70 Hz), the corresponding values for the P,P-[3]ferrocenophanes are significantly smaller, affecting their DQ-DRENAR curves only in a minor way. Based on the above insights, the structural aspects of conformational disorder and polymorphism observed in some of the P,P-[3]ferrocenophanes are discussed.

Solid-state EPR strategies for the structural characterization of paramagnetic NO adducts of frustrated Lewis pairs (FLPs)

Anisotropic interactions present in three new nitroxide radicals prepared by N,N addition of NO to various borane-phosphane frustrated Lewis pairs (FLPs) have been characterized by continuous-wave (cw) and pulsed X-band EPR spectroscopies in solid FLP-hydroxylamine matrices at 100 K. Anisotropic g-tensor values and (11)B, (14)N, and (31)P hyperfine coupling tensor components have been extracted from continuous-wave lineshape analyses, electron spin echo envelope modulation (ESEEM), and hyperfine sublevel correlation spectroscopy (HYSCORE) experiments with the help of computer simulation techniques. Suitable fitting constraints are developed on the basis of density functional theory (DFT) calculations. These calculations reveal that different from the situation in standard nitroxide radicals (TEMPO), the g-tensors are non-coincident with any of the nuclear hyperfine interaction tensors. The determination of these interaction parameters turns out to be successful, as the cw- and pulse EPR experiments are highly complementary in informational content. While the continuous-wave lineshape is largely influenced by the anisotropic hyperfine coupling to (14)N and (31)P, the ESEEM and HYSCORE spectra contain important information about the (11)B hyperfine coupling and nuclear electric quadrupolar interaction. The set of cw- and pulsed EPR experiments, with fitting constraints developed by DFT calculations, defines an efficient strategy for the structural analysis of paramagnetic FLP adducts.

Solid-state NMR strategies for the structural characterization of paramagnetic NO adducts of Frustrated Lewis Pairs (FLPs).

By N,N addition of NO to the norbonane annulated borane-phosphane Frustrated Lewis pair (FLP) 1 a five-membered heterocyclic persistent aminoxyl radical 2 and its diamagnetic hydroxylamine reduction product 3 are prepared, and the comprehensive multinuclear solid state NMR characterization ((1)H, (11)B, (19)F, (31)P) of these FLP adducts is reported. Signal quantification experiments using a standard addition method reveal that the (11)B and (31)P NMR signals observed in 2 actually arise from molecular impurities of 3 embedded in the paramagnetic crystal. In contrast analogous quantification experiments reveal that the (1)H and (19)F MAS-NMR spectra originate from spin-carrying molecules. Peak assignments are based on DFT-calculated Mulliken spin densities, which lead to the surprising result that the largest paramagnetic shift affecting a proton NMR resonance in 2 originates from intermolecular interactions. For the (19)F nuclei, experiments and calculations indicate that paramagnetic shift effects are very small. In this case, assignments are based on DFT chemical shift calculations carried out on diamagnetic 3 and (19)F((11)B) Rotational Echo Adiabatic Passage DOuble Resonance (REAPDOR) experiments. The set of experiments described here defines an efficient strategy for the structural analysis of paramagnetic FLP adducts.

Preparation of an Organometallic Molecular Square by Self-Assembly of Phosphorus-Containing Building Blocks†

Molecular squares are among the most common supramolecular architectures, but phospha-organometallic complexes have not been used as building blocks for this type of structure. Herein we describe the formation of the molecular square [Au{Co(P2C2tBu2)2}]4 (1) by the self-assembly of anionic 1,3-diphosphacyclobutadiene cobalt complexes and gold(I) cations. The X-ray crystallographic determination of the molecular structure of 1 is complemented by solid-state (31)P and (13)C NMR investigations. High-level DFT calculations confirm the assignment of the (31)P and (13)C NMR resonances.

Phosphide Oxides RE2AuP2O (RE = La, Ce, Pr, Nd): Synthesis, Structure, Chemical Bonding, Magnetism, and 31P and 139La Solid State NMR

Polycrystalline samples of the phosphide oxides RE(2)AuP(2)O (RE = La, Ce, Pr, Nd) were obtained from mixtures of the rare earth elements, binary rare earth oxides, gold powder, and red phosphorus in sealed silica tubes. Small single crystals were grown in NaCl/KCl fluxes. The samples were studied by powder X-ray diffraction, and the structures were refined from single crystal diffractometer data: La(2)AuP(2)O type, space group C2/m, a = 1515.2(4), b = 424.63(8), c = 999.2(2) pm, β = 130.90(2)°, wR2 = 0.0410, 1050 F(2) values for Ce(2)AuP(2)O, and a = 1503.6(4), b = 422.77(8), c = 993.0(2) pm, β = 130.88(2)°, wR2 = 0.0401, 1037 F(2) values for Pr(2)AuP(2)O, and a = 1501.87(5), b = 420.85(5), c = 990.3(3) pm, β = 131.12(1)°, wR2 = 0.0944, 1143 F(2) values for Nd(2)AuP(2)O with 38 variables per refinement. The structures are composed of [RE(2)O](4+) polycationic chains of cis-edge-sharing ORE(4/2) tetrahedra and polyanionic strands [AuP(2)](4-), which contain gold in almost trigonal-planar phosphorus coordination by P(3-) and P(2)(4-) entities. The isolated phosphorus atoms and the P(2) pairs in La(2)AuP(2)O could clearly be distinguished by (31)P solid state NMR spectroscopy and assigned on the basis of a double quantum NMR technique. Also, the two crystallographically inequivalent La sites could be distinguished by static (139)La NMR in conjunction with theoretical electric field gradient calculations. Temperature-dependent magnetic susceptibility measurements show diamagnetic behavior for La(2)AuP(2)O. Ce(2)AuP(2)O and Pr(2)AuP(2)O are Curie-Weiss paramagnets with experimental magnetic moments of 2.35 and 3.48 μ(B) per rare earth atom, respectively. Their solid state (31)P MAS NMR spectra are strongly influenced by paramagnetic interactions. Ce(2)AuP(2)O orders antiferromagnetically at 13.1(5) K and shows a metamagnetic transition at 11.5 kOe. Pr(2)AuP(2)O orders ferromagnetically at 7.0 K.

Structural Characterization of Phosphorus‐Based Networks and Clusters: 31P MAS NMR Spectroscopy and Magnetic Shielding Calculations on Hittorf’s Phosphorus

The (31)P MAS NMR spectrum of Hittorfs phosphorus has been measured and assigned to the 21 crystallographically distinct phosphorus atoms based on two-dimensional dipolar correlation spectroscopies. Application of such 2D techniques to phosphorus-based networks is particularly challenging owing to the wide chemical shift dispersions, rapid irreversible decay of transverse magnetization, and extremely slow spin-lattice relaxation in these systems. Nevertheless, a complete assignment was possible by using the combination of correlated spectroscopy (COSY) and radiofrequency-driven dipolar recoupling (RFDR). The assignment is supported further by DFT-based ab initio chemical shift calculations using a cluster-model approach, which gives good agreement between experimental and calculated chemical shift values. The (31)P chemical shifts appear to be strongly correlated with the average P-P bond lengths within the P(P(1/3))(3) coordination environments, whereas no clear dependence on average P-P-P bond angles can be detected. Calculations of localized Kohn-Sham orbitals reveal that this bond-length dependence is reflected in energy variations in the corresponding localized p-p-σ orbitals influencing the paramagnetic deshielding contribution in Ramseys equation. In contrast, the contributions of the lone pairs to shielding differences are small and/or do not vary in a systematic manner for the different crystallographically distinct phosphorus sites. The combined spectroscopic and quantum chemical approach applied here and the increased theoretical understanding of (31)P chemical shifts will facilitate the structural elucidation of other phosphorus-based clusters and networks.