Structural Chemistry of Titanium (IV) Oxo Clusters, Part 2: Clusters without Carboxylate or Phosphonate Ligands.

Homometallic titanium oxo clusters (TOC) are one of the most important groups of metal oxo clusters. In a previous article, TOC structures with carboxylato and phosphonato ligands were reviewed and categorized. This work is now extended to clusters with other ligands. Comparison of the different cluster types shows how the interplay between condensation of the titanium polyhedra by means of bridging oxygen atoms and the coordination characteristics of the ligands influences the cluster structures and allows working out basic construction principles of the cluster core.

Synthesis and characterization of pyrrole-based group 4 PNP pincer complexes.

The synthesis, characterization, and reactivity of several group 4 metal complexes featuring a central anionic pyrrole moiety connected via CH2 linkers to two phosphine donors is described. Treatment of [P(NH)P-iPr] with [MCl4(THF)2] (M = Zr, Hf) in the presence of base yields the dimeric complexes [M(PNPiPr)(µ-Cl)(Cl)2]2 featuring two bridging chloride ligands. These complexes react with sodium cyclopentadienyl and SiMe3I to give the mononuclear complexes [M(PNPiPr)(η5-Cp)(Cl)2] and [M(PNPiPr)(I)3], respectively. The latter react with MeMgBr to form the trialkyl complexes [M(PNPiPr)(Me)3]. Upon treatment of [Ti(NMe2)4] with [P(NH)P-iPr] a complex with the general formula [Ti(PNPiPr)(NMe2)3] is obtained. DFT calculations revealed that the most stable species is [Ti(κ1N- PNPiPr)(NMe2)3] featuring a κ1N-bound PNP ligand. When [P(NH)P-iPr] is reacted with [Ti(NMe2)4] in CH2Cl2 complex [Ti(PNPiPr)(Cl)2(NMe2)] is formed. Treatment of a solution of [P(NH)P-iPr] and [Zr(NMe2)4] with SiMe3Br affords the anionic seven-coordinate tetrabromo complex [Zr(PNPiPr)(Br)4][H2NMe2]. The corresponding hafnium complex [Hf(PNPiPr)(Br)4][H2NEt2] is obtained in similar fashion by utilizing [Hf(NEt2)4] as metal precursor. All complexes are characterized by means of NMR spectroscopy. Representative complexes were also characterized by X-ray crystallography. Supplementary Information: The online version contains supplementary material available at 10.1007/s00706-024-03171-x.

Hydrogenation of Terminal Alkenes Catalyzed by Air-Stable Mn(I) Complexes Bearing an N-Heterocyclic Carbene-Based PCP Pincer Ligand.

Efficient hydrogenations of terminal alkenes with molecular hydrogen catalyzed by well-defined bench stable Mn(I) complexes containing an N-heterocyclic carbene-based PCP pincer ligand are described. These reactions are environmentally benign and atom economic, implementing an inexpensive, earth abundant non-precious metal catalyst. A range of aromatic and aliphatic alkenes were efficiently converted into alkanes in good to excellent yields. The hydrogenation proceeds at 100 °C with catalyst loadings of 0.25-0.5 mol %, 2.5-5 mol % base (KOt Bu) and a hydrogen pressure of 20 bar. Mechanistic insight into the catalytic reaction is provided by means of DFT calculations.

Cage compound Sc5Pt24B12: a Pt-stuffed variant of filled skutterudite structure. Electronic and structural properties.

The title compound was obtained from elements via arc melting and its crystal structure was determined from single-crystal X-ray diffraction data (space group Im3̄, a = 10.2042(6) Å). The refinement indicated the occupancy of icosahedral 2a and cubooctahedral 8c sites solely by Sc atoms which leads to the composition Sc5Pt24B12 in contrast to the previously reported ternary stannides of Gd3Ni8Sn16 type (RE5-xM12Sn24(+x) compounds). The compound is the first representative of borides crystallizing with a site exchange variant of this stannide structure type. The structural relationships of the boride structure and filled skutterudite LaFe4P12vs. the Remeika phase of Yb3Rh4Sn13-type are discussed. Analysis of chemical bonding classifies Sc5Pt24B12 as a cage compound exhibiting the ionic interaction of cationic scandium species in the cages of anionic framework, formed by covalently bonded B and Pt atoms. Electronic structure calculations show that the electronic states of atoms centered around the cubooctahedral 8c site, i.e. Sc2 3d-, Pt2 5d- and B 2p-states dominate the density of states (DOS) at the Fermi level EF. Strong effect of spin-orbit coupling on the band structure at the gamma point has been found from density functional theory calculations. Sc5Pt24B12 exhibits superconductivity with a transition temperature of TC = 2.45 K.

Solvothermal synthesis of cobalt PCP pincer complexes from [Co2(CO)8].

Treatment of [Co2(CO)8] with the ipso-substituted P(C-X)PY ligands (X = Br, Cl; R = iPr, tBu) bearing Y = NH and CH2 linkers under solvothermal conditions affords the five-coordinate Co(I) and Co(III) complexes [CoI(PCPY-R)(CO)2] and [CoIII(PCPY-R)X2]. The later are paramagnetic exhibiting a solution magnetic moment in the range of 3.0-3.3 µB which is consistent with a d6 intermediate spin system corresponding to two unpaired electrons. In the case of P(C-X)PY ligands (X = Br, Cl; R = tBu; Y = NH) the formation of the square planar Co(II) complex [Co(PCPNH-tBu)X] was favored. This complex gives rise to a magnetic moment of 1.8 µB being consistent with a d7 low spin system corresponding to one unpaired electron. All complexes are characterized by means of spectroscopic techniques (NMR, IR), HR-MS. Representative complexes were also characterized by X-ray crystallography. Supplementary Information: The online version contains supplementary material available at 10.1007/s00706-023-03123-x.

Cr(II) and Cr(III) NCN pincer complexes: synthesis, structure, and catalytic reactivity.

The synthesis, characterization, and reactivity of several new Cr(II) and Cr(III) complexes featuring an NCN pincer ligand with an arene backbone connected to amine donors NEt2 and NiPr2 via CH2-linkers is described. Reacting the in situ lithiated ligand precursor N(C-Br)NCH2-Et with [CrCl3(THF)3] resulted in the formation of the Cr(III) complex trans-[Cr(κ3NCN-NCNCH2-Et)(Cl)2(THF)]. Upon reaction of lithiated N(C-Br)NCH2-iPr with a suspension of anhydrous CrCl2, the Cr(II) complex [Cr(κ2NC-NCNCH2-iPr)2] is formed featuring two NCN ligands bound in κ2NC-fashion. In contrast, when lithiated N(C-Br)NCH2-iPr is reacted with a homogeneous solution of anhydrous CrX2 (X = Cl, Br), complexes [Cr(κ3NCN-NCNCH2-iPr)X] are obtained. Treatment of [Cr(κ3NCN-NCNCH2-iPr)Cl] with 1 equiv of PhCH2MgCl and LiCH2SiMe3 afforded the alkyl complexes [Cr(κ3NCN-NCNCH2-iPr)(CH2Ph)] and [Cr(κ3NCN-NCNCH2-iPr)(CH2SiMe3)]. All Cr(II) complexes exhibit effective magnetic moments in the range of 4.7-4.9 µB which is indicative for d4 high spin systems. If a solution of lithiated N(C-Br)NCH2-iPr is treated with CrCl2, followed by addition of an excess of Na[HB(Et)3], the dimeric complex [Cr(κ2NC-NCNCH2-iPr)(µ2-H)]2 is obtained bearing two bridging hydride ligands. [Cr(κ3NCN-NCNCH2-iPr)(CH2SiMe3)] turned out to be catalytically active for the hydrosilylation of ketones at room temperature with a catalyst loading of 1 mol%. X-ray structures of all complexes are presented. Supplementary Information: The online version contains supplementary material available at 10.1007/s00706-023-03128-6.

Selective Transfer Semihydrogenation of Alkynes Catalyzed by an Iron PCP Pincer Alkyl Complex.

Two bench-stable Fe(II) alkyl complexes [Fe(κ3PCP-PCP-iPr)(CO)2(R)] (R = CH2CH2CH3, CH3) were obtained by the treatment of [Fe(κ3PCP-PCP-iPr)(CO)2(H)] with NaNH2 and subsequent addition of CH3CH2CH2Br and CH3I, respectively. The reaction proceeds via the anionic Fe(0) intermediate Na[Fe(κ3PCP-PCP-iPr)(CO)2]. The catalytic performance of both alkyl complexes was investigated for the transfer hydrogenation of terminal and internal alkynes utilizing PhSiH3 and iPrOH as a hydrogen source. Precatalyst activation is initiated by migration of the alkyl ligand to the carbonyl C atom of an adjacent CO ligand. In agreement with previous findings, the rate of alkyl migration follows the order nPr > Me. Accordingly, [Fe(κ3PCP-PCP-iPr)(CO)2(CH2CH2CH3)] is the more active catalyst. The reaction takes place at 25 °C with a catalyst loading of 0.5 mol%. There was no overhydrogenation, and in the case of internal alkynes, exclusively, Z-alkenes are formed. The implemented protocol tolerates a variety of electron-donating and electron-withdrawing functional groups including halides, nitriles, unprotected amines, and heterocycles. Mechanistic investigations including deuterium labeling studies and DFT calculations were undertaken to provide a reasonable reaction mechanism.

Electronic and Structural Properties of MPtxB6-2x (M = Y, Yb): Structural Disorder in an Octahedral Boron Framework.

Two new ternary platinum borides, YPtxB6-2x and YbPtxB6-2x, were obtained by argon-arc melting of the elements followed by annealing at 780 °C (750 °C). The structures of these compounds combine the fragments of CaB6- and AuCu3-type structures [space group Pm3̅m; x = 1.15, a = 4.0550(4) Å and x = 1.34, a = 4.0449(2) Å for YPtxB6-2x and YbPtxB6-2x, respectively; single-crystal X-ray diffraction]. Two possible variants of B/Pt ordering (space group P4/mmm) were created via a group-subgroup approach targeting the derived stoichiometry. The architecture of the type-I YPtxB6-2x structure model (a' = a, b' = b, c' = c) combines the 4.82 boron nets alternating with the layers of Y and Pt; the type-II YPtxB6-2x structure model (a' = 2a, b' = 2b, c' = c) exhibits columns of linked [B24] truncated cubes filled with Y running along the c axis. The striking features of both structural models are [B4Pt2] octahedra. The structural similarities with hitherto reported structures (YB2C2, M2Ni21B20, MNi21B20, and ErNiB4) were drawn supporting the verity of these models. A chemical bonding analysis for type-I and type-II YPtxB6-2x based on electron localization function distribution revealed a two-center interaction forming the 4.82 boron nets for type-I YPtxB6-2x and a covalent bonding within [B4Pt2] octahedra as well as a two-center interaction for B-B intraoctahedral bonds for type-II YPtxB6-2x. Analysis of Bader charges revealed the cationic character of the yttrium atoms. The interactions for nondistorted areas of the structures agree well with the bonding picture calculated for constituent building structures, YB6 and YPt3. Electronic structure calculations predict YPtxB6-2x to be a metal with the density of states of around N(EF) = 1 states eV-1 f.u.-1. The exploration of the Y-Pt-B system in the relevant concentration range elucidated the homogeneity field of YPtxB6-2x (0.90 ≤ x ≤ 1.40) and revealed the existence of three more ternary phases at 780 °C: YPt2B (space group P6222), YPt3B (space group P4mm), and YPt5B2 (space group C2/m).

Order-disorder (OD) polytypism of K3FeTe2O8(OH)2(H2O)1+x.

K3FeTe2O8(OH)2(H2O)2 was synthesized under hydrothermal conditions from Te(OH)6, FeSO4·7H2O and 85 wt% KOH in a 1:2:6 molar ratio. The crystal structure is built of a triperiodic network. One disordered water molecule per formula unit is located in a channel and can be partially removed by heating. Systematic one-dimensional diffuse scattering indicates a polytypic character, which is best described by application of the order-disorder theory. The major polytype is monoclinic with pseudo-orthorhombic metrics. It is interrupted by fragments of an orthorhombic polytype. The diffraction intensities are analyzed using structure factor calculations.

Pyrrole-Based Ti(III) and Ti(IV) PNP Pincer Complexes: Insertion of Ketones into the Ti(IV)-Phosphorus Bond.

The synthesis, characterization, and reactivity of pyrrole-based Ti(III) and Ti(IV) PNP pincer complexes are described. [P(NH)P-iPr] (1) reacts with [TiCl4(THF)2] at room temperature in the presence of NEt3 to afford the Ti(IV) complex [Ti(PNPiPr)(Cl)3]. This complex reacts with acetone and cyclopentanone to give complexes [Ti(PNOacet-iPr)(Cl)3] and [Ti(PNOcyclo-iPr)(Cl)3], respectively. Insertion of the ketone into the Ti(IV)-P bond took place, forming a new tridendate PNO-ligand. Treatment of [TiCl3(THF)3] with the lithium salt of [P(NH)P-iPr] afforded, upon workup, complex [Ti(PNP-iPr)(Cl)2(THF)], a paramagnetic complex with an µeff value of 1.8(1) µB which corresponds to one unpaired electron and a formal oxidation state of +III. This compound does not react with ketones. A mechanistic proposal based on DFT calculations is presented. Ketone insertion proceeds via an associative reaction initiated by ketone coordination at the metal center, followed by the opening of the five-membered chelate ring, and finally an intramolecular nucleophilic attack of the noncoordinated phosphine arm at the carbonyl atom of the ketone. All complexes were characterized by X-ray crystallography.

Base metal complexes featuring a new pyrazole-derived PCP pincer ligand.

A new pyrazole-derived PCP pincer ligand featuring a 1-methylpyrazole backbone tethered to two di(isopropyl)phosphine moieties via phenylene spacers (P(CH)P-iPr) was prepared. When reacting the ligand with group six carbonyl complexes [M(CO)6] (M = Cr, Mo, W) at 130 °C, complexes of the type [M(κ2PN-PCP-iPr)(CO)4] were obtained featuring a κ2P,N-bound ligand with a pendant phosphine arm. Upon an increase of the reaction temperature to 150 °C, in the case of molybdenum, the formation of the complex [Mo(κ3PCP-PCP-iPr)(CO)3] was observed featuring a weak Mo-C bond. DFT calculations reveal that there is no agostic η2-C-H interaction. Treatment of [Mn2(CO)10], [Fe2(CO)9], [Co2(CO)8] and [Ni(COD)2] afforded complexes [Mn(κ3PCP-PCP-iPr)(CO)3], [Fe(κ3PCP-PCP-iPr)(H)(CO)2], [Co(κ3PCP-PCP-iPr)(CO)2] and [Ni(κ3PCP-PCP-iPr)(H)], respectively, where the PCP ligand is coordinated in the typical meridional κ3-fashion. Postfunctionalization of the anionic PCP pincer ligand was possible via N-methylation of the second nitrogen atom of the pyrazole unit with the oxonium salt [Me3O]BF4. Treatment of [Mn(κ3PCP-PCP-iPr)(CO)3] and [Fe(κ3PCP-PCP-iPr)(H)(CO)2] with [Me3O]BF4 resulted in the formation of the cationic complexes [Mn(κ3PCP-PCPMe-iPr)(CO)3]+ and [Fe(κ3PCP-PCPMe-iPr)(Cl)(CO)2]+. In the case of the latter, the chloride ligand seems to originate from the solvent CH2Cl2 undergoing a hydride chloride exchange. All complexes were characterized by means of 1H, 13C{1H}, and 31P{1H} NMR spectroscopy, IR spectroscopy and HR-MS. In addition, the structures of representative complexes were determined by X-ray crystallography.

A fresh view on the structure and twinning of owyheeite, a rod-polytype and twofold superstructure.

Owyheeite [Cu0.09 (1)Ag2.77 (4)Pb10.23 (4)Sb10.89 (5)S28.00 (5)] crystallizes as a twofold superstructure with P21/n symmetry and pseudo-orthorhombic metrics [a = 8.1882 (3) Å, b = 27.2641 (7) Å, c = 22.8679 (7) Å, ß = 90.293 (3)°, V = 5105.0 (3) Å3, Z = 4]. Owyheeite is systematically twinned by reflection at (021) or equivalently (021). Twinning is explained by describing a simplified Pmcn archetype structure as polytype built of two kinds of rods, which contact via electron-pair micelles. A procedure of generating hypothetical polytypes by tiling space with partially overlapping equivalent regions is described.

Electronic and structural properties of Y6Pt13X4, site occupancy variants of the Ba6Na16N subnitride (X = Al, Ga).

The existence of new ternary compounds Y6Pt13X4 (X = Al, Ga) with the site occupancy variant of the subnitride Ba6Na16N (space group Im3̄m, no. 229) has been established for the first time by single crystal and powder X-ray diffraction from alloys annealed at 600 °C. The striking structural units in these compounds are platinum centered [PtY6] octahedra interconnected via Y-Y bonds and embedded in the XPt3 framework. The structural similarities with the Ce3Ni6Si2-type compounds are discussed. Electronic density of states calculated in the framework of DFT claims the compounds to be metals. The electronic band structures of both compounds resemble each other due to intrinsic similarities in the crystal structures. The analysis of bonding within and between structural fragments based on evaluation of electronic structures, Bader charges and ELF distributions in Y6Pt13X4 (X = Al, Ga) suggests the overall picture of these phases as polar intermetallics, containing a mixture of electrostatically driven interactions (such as those between complementary charged yttrium and platinum) and metallic bonding (such as Pt-Al (Ga), Pt-Pt and Y-Al (Ga)).

The crystal structure of the selenide-based synthetic sulfosalt CuPbSb3Se6.

Single crystals of copper lead tri-anti-mony hexa-selenide, CuPbSb3Se6, were obtained as a minor phase during systematic studies of the formation conditions of selenide-based sulfosalts. The crystal structure is an unusual representative of the family of sulfosalts. Instead of the expected galena-like slabs with octa-hedral coordination, it features mono and double-capped trigonal-prismatic (Pb), square-pyramidal (Sb) and trigonal-bipyramidal (Cu) coordination. All metal positions are occupationally and/or positionally disordered.

Oxidative Desymmetrization Enables the Concise Synthesis of a trans-Cyclooctene Linker for Bioorthogonal Bond Cleavage.

Modified trans-cyclooctenes (TCO) are capable of highly efficient molecular manipulations in biological environments, driven by the bioorthogonal reaction with tetrazines (Tz). The development of click-cleavable TCO has fueled the field of in vivo chemistry and enabled the design of therapeutic strategies that have already started to enter the clinic. A key element for most of these approaches is the implementation of a cleavable TCO linker. So far, only one member of this class has been developed, a compound that requires a high synthetic effort, mainly to fulfill the multilayered demands on its chemical structure. To tackle this limitation, we developed a dioxolane-fused cleavable TCO linker (dcTCO) that can be prepared in only five steps by applying an oxidative desymmetrization to achieve diastereoselective introduction of the required functionalities. Based on investigation of the structure, reaction kinetics, stability, and hydrophilicity of dcTCO, we demonstrate its bioorthogonal application in the design of a caged prodrug that can be activated by in-situ Tz-triggered cleavage to achieve a remarkable >1000-fold increase in cytotoxicity.

Late-stage derivatization of a (B,O)2-doped perylene.

Regioselective di- and tetrabrominations of the (B,O)2-perylene 1 afford derivatives 2-4. Despite their poor solubility, 2 and 4 could be used in Stille-type coupling reactions to introduce two CCMe (5) or four CC(p-C6H4tBu) substituents (6), respectively. The alkynylated derivatives show blue-green photoluminescence with appreciable quantum efficiencies.

The channel structure of trithallium penta-anti-m-on-ate(V), Tl3Sb5O14.

Single crystals of Tl3Sb5O14 were grown by solid-state reaction in a corundum crucible under air (1273 K, 12 h). The structure was determined by single-crystal X-ray diffraction. It is isotypic to the K3Sb5O14, Rb3Sb5O14 and Cs3Sb5O14 analogues with ortho-rhom-bic Pbam symmetry and cell parameters a = 24.2899 (9) Å, b = 7.1931 (3) Å, c = 7.4182 (3) Å. The Sb atoms form irregular [SbO6] octa-hedra, which are linked via edges and corners into a triperiodic network. The Tl+ ions are located in distinct channels of the network extending along [010] and [001].

Synthesis and Characterization of Cobalt NCN Pincer Complexes.

A series of cobalt complexes, stabilized by a monoanionic tridentate NCN pincer ligand, was synthetized and characterized. Preparation of the paramagnetic 15 VE complex [Co(NCNCH2-Et)Br] (1) was accomplished by transmetalation of Li[2,6-(Et2NCH2)2C6H3] with CoBr2 in THF. Treatment of this air-sensitive compound with NO gas resulted in the formation of the diamagnetic Co(III) species [Co(NCNCH2-Et)(NO)Br] (2) as confirmed by X-ray diffraction. This complex features a strongly bent NO ligand (Co-N-O∠135.0°). The νNO is observed at 1609 cm-1 which is typical for a bent metal-N-O arrangement. Coordinatively unsaturated 1 could further be treated with pyridine, isocyanides, phosphines and CO to form five-coordinate 17 VE complexes. Oxidation of 1 with CuBr2 led to the formation of the Co(III) complex [Co(NCNCH2-Et)Br2]. Treatment of [Co(NCNCH2-Et)Br2] with TlBF4 as halide scavenger in acetonitrile led to the formation of the cationic octahedral complex [Co(NCNCH2-Et)(MeCN)3](BF4)2. A combination of X-ray crystallography, IR-, NMR- and EPR-spectroscopy as well as DFT/CAS-SCF calculations were used to characterize all compounds.

Manganese-Catalyzed Dehydrogenative Silylation of Alkenes Following Two Parallel Inner-Sphere Pathways.

We report on an additive-free Mn(I)-catalyzed dehydrogenative silylation of terminal alkenes. The most active precatalyst is the bench-stable alkyl bisphosphine Mn(I) complex fac-[Mn(dippe)(CO)3(CH2CH2CH3)]. The catalytic process is initiated by migratory insertion of a CO ligand into the Mn-alkyl bond to yield an acyl intermediate which undergoes rapid Si-H bond cleavage of the silane HSiR3 forming the active 16e- Mn(I) silyl catalyst [Mn(dippe)(CO)2(SiR3)] together with liberated butanal. A broad variety of aromatic and aliphatic alkenes was efficiently and selectively converted into E-vinylsilanes and allylsilanes, respectively, at room temperature. Mechanistic insights are provided based on experimental data and DFT calculations revealing that two parallel reaction pathways are operative: an acceptorless reaction pathway involving dihydrogen release and a pathway requiring an alkene as sacrificial hydrogen acceptor.

Manganese and iron PCP pincer complexes - the influence of sterics on structure and reactivity.

The syntheses of various manganese and iron PCP pincer complexes via a solvothermal oxidative addition methodology is described. Upon reacting [Mn2(CO)10] with the ligands (P(C-Br)PCH2-iPr) (1a) and (P(C-Br)PO-iPr) (1b), Mn(I) PCP pincer complexes [Mn(PCPCH2-iPr)(CO)3] (2a) and [Mn(-PCPO-iPr)(CO)3] (2b) were obtained. Protonation of 2a with HBF4·Et2O led to the formation of [Mn(κ3P,CH,P-P(CH)PCH2-iPr)(CO)3]BF4 (3) featuring an η2-Caryl-H agostic bond. The solvothermal reaction of 1a with [Fe2(CO)9] afforded the Fe(II) PCP pincer complex [Fe(PCPCH2-iPr)(CO)2Br] (4). Treatment of 4 with Li[HBEt3] afforded the Fe(I) complex [Fe(PCPCH2-iPr)(CO)2] (5a). When using the sterically more demanding ligands (P(C-Br)PCH2-tBu) (1c) and (P(C-Br)PO-tBu)(1d) striking differences in reactivity were observed. While neither 1c nor 1d showed any reactivity towards [Mn2(CO)10], the reaction with [Fe2(CO)9] and [Fe(CO)5] led to the formation of the Fe(I) complexes [Fe(PCPCH2-tBu)(CO)2] (5b) and [Fe(PCPO-tBu)(CO)2] (5c). X-ray structures of representative complexes are provided.