Interconversion of Molybdenum or Tungsten d2 Styrene Complexes with d0 1-Phenethylidene Analogues.

Upon addition of 5-15% PhNMe2H+X- (X = B(3,5-(CF3)2C6H3)4 or B(C6F5)4) to Mo(NAr)(styrene)(OSiPh3)2 (Ar = N-2,6-i-Pr2C6H3) in C6D6 an equilibrium mixture of Mo(NAr)(styrene)(OSiPh3)2 and Mo(NAr)(CMePh)(OSiPh3)2 is formed over 36 h at 45 °C (Keq = 0.36). A plausible intermediate in the interconversion of the styrene and 1-phenethylidene complexes is the 1-phenethyl cation, [Mo(NAr)(CHMePh)(OSiPh3)2]+, which can be generated using [(Et2O)2H][B(C6F5)4] as the acid. The interconversion can be modeled as two equilibria involving protonation of Mo(NAr)(styrene)(OSiPh3)2 or Mo(NAr)(CMePh)(OSiPh3)2 and deprotonation of the α or ß phenethyl carbon atom in [Mo(NAr)(CHMePh)(OSiPh3)2]+. The ratio of the rate of deprotonation of [Mo(NAr)(CHMePh)(OSiPh3)2]+ by PhNMe2 in the α position versus the ß position is ∼10, or ∼30 per Hß. The slow step is protonation of Mo(NAr)(styrene)(OSiPh3)2 (k1 = 0.158(4) L/(mol·min)). Proton sources such as (CF3)3COH or Ph3SiOH do not catalyze the interconversion of Mo(NAr)(styrene)(OSiPh3)2 and Mo(NAr)(CMePh)(OSiPh3)2, while the reaction of Mo(NAr)(styrene)(OSiPh3)2 with pyridinium salts generates only a trace (∼2%) of Mo(NAr)(CMePh)(OSiPh3)2 and forms a monopyridine adduct, [Mo(NAr)(CHMePh)(OSiPh3)2(py)]+ (two diastereomers). The structure of [Mo(NAr)(CHMePh)(OSiPh3)2]+ has been confirmed in an X-ray study; there is no structural indication that a ß proton is activated through a CHß interaction with the metal. W(NAr)(CMePh)(OSiPh3)2 is also converted into a mixture of W(NAr)(CMePh)(OSiPh3)2 and W(NAr)(styrene)(OSiPh3)2 (Keq = 0.47 at 45 °C in favor of the styrene complex) with 10% [PhNMe2H][B(C6F5)4] as the catalyst; the time required to reach equilibrium is approximately the same as in the Mo system.

Synthesis and Electrochemical Properties of Aluminum Hexafluorophosphate.

We report the first synthesis of aluminum hexafluorophosphate (Al(PF6)3) and its electrochemical properties in dimethyl sulfoxide (DMSO). The single crystal structure of the synthesized Al(PF6)3 is revealed as [Al(DMSO)6](PF6)3, and 0.25 M Al(PF6)3 in DMSO with high ionic conductivity is obtained. The purity of this electrolyte was further confirmed with nuclear magnetic resonance spectroscopy and electrospray ionization mass spectrometry. We then demonstrated the reversibility of Al deposition-stripping in this electrolyte using scanning electron microscopy and an X-ray photoelectron spectroscopy depth profiling study. The parasitic reaction involving DMSO decomposition during Al deposition is also identified via gas chromatography/electron ionization mass spectrometry.

Ring-Closing Olefin Metathesis Catalyzed by Well-Defined Vanadium Alkylidene Complexes.

Vanadium-based catalysts have shown activity and selectivity in ring-opening metathesis polymerization of strained cyclic olefins comparable to those of Ru, Mo, and W catalysts. However, the application of V alkylidenes in routine organic synthesis is limited. Here, we present the first example of ring-closing olefin metathesis catalyzed by well-defined V chloride alkylidene phosphine complexes. The developed catalysts exhibit tolerance to various functional groups, such as an ether, an ester, a tertiary amide, a tertiary amine, and a sulfonamide. The size and electron-donating properties of the imido group and the phosphine play a crucial role in the stability of active intermediates. Reactions with ethylene and olefins suggest that both ß-hydride elimination of the metallacyclobutene and bimolecular decomposition are responsible for catalyst degradation.

Pressure-Thresholded Response in Cylindrically Shocked Cyclotrimethylene Trinitramine (RDX).

We demonstrate a strongly thresholded response in cyclotrimethylene trinitramine (RDX) when it is cylindrically shocked using a novel waveguide geometry. Using ultrafast single-shot multi-frame imaging, we demonstrate that <100 µm diameter single crystals of RDX embedded in a polymer host deform along preferential planes for >100 ns after the shock first arrives in the crystal. We use in situ imaging and time-resolved photoemission to demonstrate that short-lived chemistry occurs with complex deformation pathways. Using scanning electron microscopy and ultra-small-angle X-ray scattering, we demonstrate that the shock-induced dynamics leave behind porous crystals, with pore shapes and sizes that change significantly with shock pressure. A threshold pressure of ∼12 GPa at the center of convergence separated the single-mode planar crystal deformations from the chemistry-coupled multi-plane dynamics at higher pressures. Our observations indicate preferential directions for deformation in our cylindrically shocked system, despite the applied stress along many different crystallographic planes.

Strongly Coordinating Ligands To Form Weakly Coordinating Yet Functional Organometallic Anions.

Weakly coordinating anions (WCAs) are generally tailored to act as spectators with little or no function. Here we describe the implementation of strongly coordinating dianionic carboranyl N-heterocyclic carbenes (NHCs) to create organometallic -ate complexes of Au(I) that serve both as WCAs and functional catalysts. These organometallic WCAs can be utilized to form both heterobimetallic (Au(I)-/Ag(I)+; Au(I)-/Ir(I)+) and organometallic/main group ion pairs (Au(I)-/(CPh3+ or SiEt3+). Because parent unfunctionalized dianionic carboranyl NHC complex 3 is unstable in most solvents when paired with CPh3+, novel synthesis methodology was devised to create polyhalogenated carboranyl NHCs, which show superior stability toward electrophilic substitution and cyclometalation chemistry. Additionally, the WCAs containing polyhalogenated carboranyl NHCs are among the most active catalysts reported for the hydroamination of alkynes. This investigation has also produced the first examples of a low-coordinate Au(III) center with two cis accessible coordination sites and the first true dianionic carbene. These studies pave the way for the design of functional ion pairs that have the potential to participate in tandem or cooperative small-molecule activation and catalysis.

Protonation Studies of Molybdenum(VI) Nitride Complexes That Contain the [2,6-(ArNCH2)2NC5H3]2- Ligand (Ar = 2,6-Diisopropylphenyl).

[Ar2N3]Mo(N)(O- t-Bu) (1), which contains the conformationally rigid pyridine-based diamido ligand [2,6-(ArNCH2)2NC5H3]2- (Ar = 2,6-diisopropylphenyl), is a catalyst for the reduction of dinitrogen with protons and electrons. Various acids have been added in order to explore where and how the first proton adds to the complex. The addition of adamantol to 1 produces a five-coordinate bis(adamantoxide), [HAr2N3]Mo(N)(OAd)2 (2a), in which one of the amido nitrogens in the ligand has been protonated and the resulting aniline nitrogen in the [HAr2N3]- ligand is not bound to the metal. The addition of [Ph2NH2][OTf] to 1 produces {[HAr2N3]Mo(N)(O- t-Bu)}(OTf) (3), in which an amido nitrogen has been protonated, but the aniline in the [HAr2N3]- ligand remains bound to the metal. Last, the addition of (2,6-lutidinium)BArF4 (BArF4 = {B(3,5-(CF3)2C6H3)4}-) to 1 yields {[Ar2N3]Mo(N)(LutH)(O- t-Bu)}BArF4, in which LutH+ is hydrogen-bonded to the nitride in the solid state and in dichloromethane with Keq = 412 ± 94 and Δ G = -3.6 ± 0.8 kcal at 22 °C. A similar hydrogen-bonded adduct was formed through the addition of (2-methylpyridinium)BArF4 to 1, but the addition of (pyridinium)BArF4 to 1 leads to the formation of (inter alia) {[HAr2N3]Mo(N)(O- t-Bu)}(BArF4), in which the amide nitrogen has been protonated. The addition of cobaltocene to 3 or {[Ar2N3]Mo(N)(LutH)(O- t-Bu)}(BArF4) leads only to the re-formation of 1. X-ray structural studies were carried out on 2a, 3, and {[Ar2N3]Mo(N)(LutH)(O- t-Bu)}(BArF4).

Molybdenum Complexes that Contain a Calix[6]azacryptand Ligand as Catalysts for Reduction of N2 to Ammonia.

[CAC(OMe)6]Mo(N) (3, where [CAC]3- is a calix[6]azacryptand ligand derived from a [6]calixarene) has been prepared in a reaction between Li3[CAC(OMe)6] and ( t-BuO)3Mo(N). An X-ray structural study showed 3 to have a structure similar to that of [HIPTN3N]Mo(N) (where [HIPTN3N]3- is [(3,5-(2,4,6-triisopropylphenyl)2C6H3NCH2CH2)3N]3-). The relatively rigid [CAC(OMe)6]3- ligand in 3 forms a bowl-shaped cavity defined by a 24-atom macrocyclic ring. The Mo-Namido-Cipso angles are ∼8° smaller in 3 than they are in [HIPTN3N]Mo(N). Methoxides on the three linking units point into the cavity above the nitride in 3, whereas the three methoxides on phenyl rings attached to the amido nitrogen atoms point away from the cavity. An analogous [CAC(OMe)3(H)3]Mo(N) complex (9) was prepared in which the three methoxides pointing into the cavity in 3 have been replaced by protons. Its structure differs little from that of 3. The nitride could be protonated in 3 to give {[CAC(OMe)6]Mo(NH)}+, which could be reduced (reversibly) to [CAC(OMe)6]Mo(NH). Catalytic reduction of molecular nitrogen under a variety of conditions with either Ph2NH2OTf or HBArf (BArf- = {B[3,5(CF3)2C5H3]4}-) as the acid and a Co metallocene or KC8 as the reducing agent between -78 and 22 °C in diethyl ether shows that 1.20-1.34 equivalents of ammonia are formed starting with either [CAC(OMe)6]Mo(N) (50% 15N) or [CAC(OMe)3(H)3]Mo(N) (50% 15N).

Syntheses of Molybdenum Oxo Benzylidene Complexes.

The reaction between Mo(O)(CHAro)(ORF6)2(PMe3) (Aro = ortho-methoxyphenyl, ORF6 = OCMe(CF3)2) and 2 equiv of LiOHMT (OHMT = O-2,6-(2,4,6-Me3C6H2)2C6H3) leads to Mo(O)(CHAro)(OHMT)2, an X-ray structure of which shows it to be a trigonal bipyramidal anti benzylidene complex in which the o-methoxy oxygen is coordinated to the metal trans to the apical oxo ligand. Addition of 1 equiv of water (in THF) to the benzylidyne complex, Mo(CArp)(OR)3(THF)2 (Arp = para-methoxyphenyl, OR = ORF6 or OC(CF3)3 (ORF9)) leads to formation of {Mo(CArp)(OR)2(µ-OH)(THF)}2(µ-THF) complexes. Addition of 1 equiv of a phosphine (L) to Mo(CArp)(ORF9)3(THF)2 in THF, followed by addition of 1 equiv of water, all at room temperature, yields Mo(O)(CHArp)(ORF9)2(L) complexes in good yields for several phosphines (e.g., PMe2Ph (69% by NMR), PMePh2 (59%), PEt3 (69%), or P( i-Pr)3 (65%)). The reaction between Mo(O)(CHArp)(ORF9)2(PEt3) and 2 equiv of LiOHMT proceeds smoothly at 90 °C in toluene to give Mo(O)(CHArp)(OHMT)2, a four-coordinate syn alkylidene complex. Mo(O)(CHArp)(OHMT)2 reacts with ethylene (1 atm in C6D6) to give (in solution) a mixture of Mo(O)(CHArp)(OHMT)2, Mo(O)(CH2)(OHMT)2, and an unsubstituted square pyramidal metallacyclobutane complex, Mo(O)(CH2CH2CH2)(OHMT)2, along with ethylene and ArpCH═CH2. Mo(O)(CHArp)(OHMT)2 also reacts with 2,3-dicarbomethoxynorbornadiene to yield syn and anti isomers of the "first-insertion" products that contain a cis C═C bond.

Syntheses of Molybdenum Oxo Alkylidene Complexes through Addition of Water to an Alkylidyne Complex.

Addition of one equiv of water to Mo(CAr)[OCMe(CF3)2]3(1,2-dimethoxyethane) (2, Ar = o-(OMe)C6H4) in the presence of PPhMe2 leads to formation of Mo(O)(CHAr)[OCMe(CF3)2]2(PPhMe2) (3(PPhMe2)) in 34% yield. Addition of one equiv of water alone to 2 produces the dimeric alkylidyne hydroxide complex, {Mo(CAr)[OCMe(CF3)2]2(µ-OH)}2(dme) (4(dme)) in which each bridging hydroxide proton points toward an oxygen atom in an arylmethoxy group. Addition of PMe3 to 4(dme) gives the alkylidene oxo complex, (3(PMe3)), an analogue of 3(PPhMe2) (95% conversion, 66% isolated). Treatment of 3(PMe3) with two equiv of HCl gave Mo(O)(CHAr)Cl2(PMe3) (5), which upon addition of LiO-2,6-(2,4,6-i-Pr3C6H2)2C6H3 (LiOHIPT) gave Mo(O)(CHAr)(OHIPT)Cl(PMe3) (6). Compound 6 in the presence of B(C6F5)3 will initiate the ring-opening metathesis polymerization of cyclooctene, 5,6-dicarbomethoxynorbornadiene (DCMNBD), and rac-5,6-dicarbomethoxynorbornene (DCMNBE), and the homocoupling of 1-decene to 9-octadecene. The poly(DCMNBD) has a cis,syndiotactic structure, whereas poly(DCMNBE) has a cis,syndiotactic,alt structure. X-ray structures were obtained for 3(PPhMe2), 4(dme), and 6.

Synthesis of High-Oxidation-State Mo═CHX Complexes, Where X = Cl, CF3, Phosphonium, CN.

Reactions between Z-XCH=CHX where X = Cl, CF3, or CN and Mo(N-t-Bu)(CH-t-Bu)(OHIPT)Cl(PPh2Me) (OHIPT = O-2,6-(2,4,6-i-Pr3C6H2)2C6H3) produce Mo(N-t-Bu)(CHX)(OHIPT)Cl(PPh2Me) complexes. Addition of 2,2'-bipyridyl (Bipy) yields Mo(N-t-Bu)(CHX)(OHIPT)Cl(Bipy) complexes, which could be isolated and structurally characterized. The reaction between Mo(N-t-Bu)(CH-t-Bu)(OHMT)Cl(PPh2Me) (OHMT = O-2,6-(2,4,6-Me3C6H2)2C6H3) and Z-ClCH=CHCl in the presence of Bipy produces a mixture that contains both Mo(N-t-Bu)(CHCl)(OHMT)Cl(PPh2Me) and Mo(N-t-Bu)(CHCl)(OHMT)Cl(Bipy), but the relatively insoluble product that crystallizes from toluene-d 8 is the phosphoniomethylidene complex, [Mo(N-t-Bu)(CHPPh2Me)(OHMT)(Cl)(Bipy)]Cl. The Mo(N-t-Bu)(CHX)(OHIPT)Cl(PPh2Me) complexes (X = Cl or CF3) were confirmed to initiate the stereoselective cross-metathesis between Z-5-decene and Z-XCH=CHX.

CO2 reduction or HCO2- oxidation? Solvent-dependent thermochemistry of a nickel hydride complex.

The hydricity (ΔGH-) of a newly synthesized nickel hydride was experimentally determined in acetonitrile (50.6 kcal mol-1), dimethyl sulfoxide (47.1 kcal mol-1), and water (22.8 kcal mol-1). The hydricity values indicate hydride transfer from [HNi(TMEPE)2][BF4] (TMEPE = 1,2-bis[di(methoxyethyl)phosphino]ethane) to CO2 is exergonic in water and endergonic in the organic solvents.

Redox Potential and Electronic Structure Effects of Proximal Nonredox Active Cations in Cobalt Schiff Base Complexes.

Redox inactive Lewis acidic cations are thought to facilitate the reactivity of metalloenzymes and their synthetic analogues by tuning the redox potential and electronic structure of the redox active site. To explore and quantify this effect, we report the synthesis and characterization of a series of tetradentate Schiff base ligands appended with a crown-like cavity incorporating a series of alkali and alkaline earth Lewis acidic cations (1M, where M = Na+, K+, Ca2+, Sr2+, and Ba2+) and their corresponding Co(II) complexes (2M). Cyclic voltammetry of the 2M complexes revealed that the Co(II/I) redox potentials are 130 mV more positive for M = Na+ and K+ and 230-270 mV more positive for M = Ca2+, Sr2+, and Ba2+compared to Co(salen-OMe) (salen-OMe = N,N'-bis(3-methoxysalicylidene)-1,2-diaminoethane), which lacks a proximal cation. The Co(II/I) redox potentials for the dicationic compounds also correlate with the ionic size and Lewis acidity of the alkaline metal. Electronic absorption and infrared spectra indicate that the Lewis acid cations have a minor effect on the electronic structure of the Co(II) ion, which suggests the shifts in redox potential are primarily a result of electrostatic effects due to the cationic charge.

Syntheses of Molybdenum Adamantylimido and t-Butylimido Alkylidene Chloride Complexes Using HCI and Diphenylmethylphosphine.

Reactions between Mo(N-t-Bu)2(CH2-t-Bu)2 or Mo(NAdamantyl)2(CH2CMe2Ph)2 and 3 equiv of HCl in the presence of 1 equiv of PPh2Me yield Mo(NR)(CHR')(PPh2Me)Cl2 complexes, from which Mo(NR)(CHR')(PPh2Me)(OAr)Cl complexes (OAr = a 2,6-terphenoxide) can be prepared. The Mo(NR)(CHR')(PPh2Me)(OAr)Cl complexes were evaluated as cross-metathesis catalysts between cyclooctene and Z-1,2-dichloroethylene. The efficiencies of the test reaction for complexes in which OAr = OTPP, OHMT, OHIPT, or OHTBT (where OTPP is 2,3,5,6-tetraphenylphenoxide, OHMT is hexamethylterphenoxide, OHIPT is hexaisopropylterphenoxide, and OHTBT is hexa-t-butylterphenoxide) maximize when OAr is OHMT or OHIPT. Mo(N-t-Bu)(CH-t-Bu)(PPh2Me)Cl2 is essentially inactive for the reaction between cyclooctene and Z-1,2-dichloroethylene. X-ray structural studies were carried out on Mo(NAd)(CHCMe2Ph)(PPh2Me)Cl2, Mo(N-t-Bu)(CH-t-Bu)(PPh2Me)(OHMT)Cl, Mo(NAd)(CHCMe2Ph)(Cl)(OHTBT)(PMe3), and [Mo(NAd)(CHCMe2Ph)(PMe3)(Cl)]2(µ-O), the product of the reaction between Mo(NAd)(CHCMe2Ph)(Cl)(OHTBT)(PMe3) and 0.5 equiv of water.

Spin-state diversity in a series of Co(ii) PNP pincer bromide complexes.

We describe the structural and electronic impacts of modifying the bridging atom in a family of Co(ii) pincer complexes with the formula Co(t-Bu)2PEPyEP(t-Bu)2Br2 (Py = pyridine, E = CH2, NH, and O for compounds 1-3, respectively). Structural characterization by single crystal X-ray diffraction indicates that compounds 1 and 3 are 5-coordinate complexes with both bromides bound to the Co(ii) ion, while compound 2 is square planar with one bromide in the outer coordination sphere. The reduction potentials of 1-3, characterized by cyclic voltammetry, are consistent with the increasing electron-withdrawing character of the pincer ligand as the linker (E) between the pyridine and phosphine arms becomes more electronegative. Magnetic property studies of compounds 1 and 2 confirm high- and low-spin behavior, respectively, through a broad temperature range. However, complex 3 features an unusual combination of high spin S = 3/2 Co(ii) and temperature dependent spin-crossover between S = 3/2 and S = 1/2 states. The different magnetic behaviors observed among the three CoBr2 pincer complexes reflects the importance of small ligand perturbations on overall coordination geometry and resulting spin state properties.

Electrocatalytic Hydrogen Evolution under Acidic Aqueous Conditions and Mechanistic Studies of a Highly Stable Molecular Catalyst.

Electrocatalytic activity of a water-soluble nickel complex, [Ni(DHMPE)2]2+ (DHMPE = 2-bis(di(hydroxymethyl)phosphino)ethane), for the hydrogen evolution reaction (HER) at pH 1 is reported. The catalyst functions at a rate of ∼103 s-1 (kobs) with high Faradaic efficiency. Quantification of the complex before and after 18+ hours of electrolysis reveals negligible decomposition under catalytic conditions. Although highly acidic conditions are common in electrolytic cells, this is a rare example of a homogeneous catalyst for HER that functions with high stability at low pH. The stability of the compound and proposed catalytic intermediates enabled detailed mechanistic studies. The thermodynamic parameters governing electron and proton transfer were used to determine the appropriate reductants and acids to access the catalytic cycle in a stepwise fashion, permitting direct spectroscopic identification of intermediates. These studies support a mechanism for proton reduction that proceeds through two-electron reduction of the nickel(II) complex, protonation to generate [HNi(DHMPE)2]+, and further protonation to initiate hydrogen bond formation.

Flexibility is Key: Synthesis of a Tripyridylamine (TPA) Congener with a Phosphorus Apical Donor and Coordination to Cobalt(II).

Tripyridylamine (TPA), a tetradentate ligand that forms 5-membered chelate rings upon metal coordination, has demonstrated significant utility in synthetic inorganic chemistry. An analogue with a phosphorus apical donor is a desirable target for tuning electronic structure and enhancing reactivity. However, this congener has been synthetically elusive. Prior attempts have resulted in tridentate coordination to transition metal ions due to a lack of ligand flexibility. Herein, we report the successful synthesis of tris(2-pyridylmethyl)proazaphosphatrane (TPAP), a more accommodating tripyridyl ligand containing an apical phosphorus donor. The TPAP ligand forms 6-membered chelate rings upon coordination and binds in the desired tetradentate fashion to a Co(II) ion. Structural studies elucidate the importance of ligand flexibility in tripodal ligands featuring phosphorus donors. Cyclic voltammetry, UV-vis, and solution magnetic susceptibility experiments of [Co(TPAP)(CH3CN)](2+) are also reported and compared to [Co(TPA)(CH3CN)](2+). Notably, magnetic susceptibility measurements of [Co(TPAP)(CH3CN)](2+) indicate a low spin electronic configuration, in contrast to [Co(TPA)(CH3CN)](2+), which is high spin.

Solvation effects on transition metal hydricity.

The free energy of hydride donation (hydricity) for [HNi(DHMPE)2][BF4] (DHMPE = 1,2-bis(dihydroxymethylphosphino)ethane was experimentally determined versus the heterolytic cleavage energy of hydrogen in acetonitrile, dimethyl sulfoxide, and water to be 57.4, 55.5, and 30.0 kcal/mol, respectively. This work represents the first reported hydricity values for a transition metal hydride donor in three different solvents. A comparison between our values and the hydricity of hydrogen and formate reveals a narrowing in the range of values with increasing solvent polarity. The thermochemical values also reveal solvation effects that impact the overall thermodynamic favorability of hydride generation from hydrogen and transfer to carbon dioxide. The quantitative solvation effects described herein have important consequences to the design and reactivity of catalysts for transformations that have hydride transfer steps throughout synthetic chemistry.

Dihydrogen binding to isostructural S = ½ and S = 0 cobalt complexes.

Two isostructural, nonclassical Co(H(2)) complexes are prepared from their Co(N(2)) precursors using tris(phosphino)silyl and tris(phosphino)borane ancillary ligands. Comproportionation of CoBr(2) and Co metal in the presence of TPB (tris-(o-diisopropylphophinophenyl)borane) gives (TPB)CoBr (4). One-electron reduction of 4 triggers N(2) binding to give (TPB)Co(N(2)) (2-N(2)) which is isostructural to previously reported [SiP(3)]Co(N(2)) (1-N(2)) ([SiP(3)] = tris-(o-diisopropylphosphinophenyl)silyl). Both 1-N(2) and 2-N(2) react with 1 atm H(2) to generate thermally stable H(2) complexes 1-H(2) and 2-H(2), respectively. Both complexes are characterized by a suite of spectroscopic techniques in solution and by X-ray crystallography. The H(2) and N(2) ligands in 2-H(2) and 2-N(2) are labile under ambient conditions and the binding equilibria are observable by temperature-dependent UV/vis. A van't Hoff analysis allows for the ligand binding energetics to be determined (H(2): ΔH(o) = -12.5(3) kcal mol(-1) and ΔS(o) = -26(3) cal K(-1) mol(-1); N(2): ΔH(o) = -13.9(7) kcal mol(-1) and ΔS(o) = -32(5) cal K(-1) mol(-1)).

Four-coordinate, trigonal pyramidal Pt(II) and Pd(II) complexes.

We report herein the characterization of electrophilic, trigonal bipyramidal {[SiP(3)(R)]Pt(L)}(+) cations ([SiP(3)(R)] = [(2-R(2)PC(6)H(4))(3)Si]; R = Ph, (i)Pr) that feature weakly coordinated ligands including CH(2)Cl(2), Et(2)O, toluene, and H(2). A cationic toluene adduct that shows a close platinum aryl C-H σ-contact is perhaps most noteworthy in this context. For the isopropyl-substituted ligand, [SiP(3)(iPr)], it has proven possible to exclude the fifth axial donor to afford the rigorously four-coordinate, trigonal pyramidal (TP) complex {[SiP(3)(iPr)]Pt}(+). An isostructural TP palladium complex {[SiP(3)(iPr)]Pd}(+) is also accessible. Prototypical four-coordinate d(8) platinum and palladium complexes are square planar. The TP d(8) cations described herein are hence geometrically distinct.

Expanded sodalite-type metal-organic frameworks: increased stability and H(2) adsorption through ligand-directed catenation.

The torsion between the central benzene ring and the outer aromatic rings in 1,3,5-tri-p-(tetrazol-5-yl)phenylbenzene (H3TPB-3tz) and the absence of such strain in 2,4,6-tri-p-(tetrazol-5-yl)phenyl-s-triazine (H3TPT-3tz) are shown to allow the selective synthesis of noncatenated and catenated versions of expanded sodalite-type metal-organic frameworks. The reaction of H3TPB-3tz with CuCl2.2H2O affords the noncatenated compound Cu3[(Cu4Cl)3(TPB-3tz)8]2.11CuCl2.8H2O.120DMF (2), while the reaction of H3TPT-3tz with MnCl2.4H2O or CuCl2.2H2O generates the catenated compounds Mn3[(Mn4Cl)3(TPT-3tz)8]2.25H2O.15CH3OH.95DMF (3) and Cu3[(Cu4Cl)3(TPT-3tz)8]2.xsolvent (4). Significantly, catenation helps to stabilize the framework toward collapse upon desolvation, leading to an increase in the surface area from 1120 to 1580 m2/g and an increase in the hydrogen storage capacity from 2.8 to 3.7 excess wt % at 77 K for 2 and 3, respectively. The total hydrogen uptake in desolvated 3 reaches 4.5 wt % and 37 g/L at 80 bar and 77 K, demonstrating that control of catenation can be an important factor in the generation of hydrogen storage materials.