Microstructure characterization and mechanistic insight into polyether polyols and their associated polyurethanes.

This article reviews the analytical tool chest used for characterizing alkoxylates and their associated copolymer mixtures. Specific emphasis will be placed upon the use of mass spectrometry-based techniques as rapid characterization tools for optimizing reaction processes in an industrial R&D setting. An initial tutorial will cover the use of matrix-assisted laser desorption/ionization-mass spectrometry and tandem mass spectrometry fragmentation for detailed component analysis (e.g., polyol and isocyanate) of a model polyurethane-based foam. Next, this critical feedback information will be used with the guidance of mass spectrometry to initiate the development of a new, more efficient, tris(pentafluorophenyl)borane (FAB) catalyst-based alkoxylation process for generating the next generation of glycerin-initiated poly(propylene oxide)-co-poly(ethylene oxide) copolymers. Examples will be provided for each step in the FAB-based optimization process that were required to generate the final product. Following this example, two-dimensional liquid chromatography, supercritical fluid chromatography, and ion mobility separations, along with their coupling to mass spectrometry, will be reviewed for their efficiency in characterizing and quantitating the components within these complex polyether polyol mixtures.

Distinct modes of Si-H binding to Rh in complexes of a phosphine-diarylamido-silane (SiNP) pincer ligand.

Syntheses of Rh complexes of the phosphine-amido-silane SiNP ligand are reported. The reaction of the parent (SiNP)H ligand (4) with 0.5 equiv. [(COE)RhCl]2 (COE = cis-cyclooctene) in the presence of NaN(SiME3)2 resulted in the formation of (SiNP)Rh(COE) (5). Compound 5 was converted to a series of (SiNP)Rh(P(OR)3) complexes 6-10 (R = Ph, iPr, nBu, Et, or Me) by treatment with the corresponding phosphite. NMR and XRD structural data, as well as the DFT computational analysis indicate that compounds 5-10 are divided into two structural Types (A and B), differing in the nature of the interaction of the Si-H bond of the SiNP ligand with Rh.

Synthesis and characterization of rhodium, iridium, and palladium complexes of a diarylamido-based PNSb pincer ligand.

A new diarylmido-based pincer proto ligand (iPrPNHSbPh) with one -PPri2 and one -SbPh2 side donor has been synthesized. Three complexes of its amido form were prepared using standard metalation techniques: (iPrPNSbPh)PdCl, (iPrPNSbPh)RhCO, and (iPrPNSbPh)Ir(COE), where COE = cis-cyclooctene. These complexes were compared with their previously reported analogs incorporating a -PPh2 side donor in place of -SbPh2. The -SbPh2 donor arm is less donating towards the metal and is less strongly trans-influencing, based on the structural and IR spectroscopic analysis of the Rh complexes. The redox potential of the Pd complexes is only marginally affected by the change from -PPh2 to -SbPh2. (iPrPNSbPh)Ir(COE) proved to be a slower and less selective catalyst in the dehydrogenative borylation of terminal alkynes (DHBTA) than its -PPh2 analog.

Dearomatization of the PCP Pincer Ligand in a ReV Oxo Complex.

A high-valent, rhenium(V) oxo complex (PCP)ReOCl2 (1; PCP=bis(2,6-di-tert-butylphosphinomethyl)phenyl) undergoes a deprotonation and "dearomatization" upon treatment with LiN(SiMe3 )2 to give (P*CP)ReOCl (2 a), in which Re is bound to a new dianionic P*CP ligand. Compound 2 a was studied spectroscopically, structurally, and computationally and was determined to have non-negligible Re=C multiple bond character, leading to its formulation as a new pseudo-carbenoid species. Reaction of 2 a or its iodo analogue (P*CP)ReOI (2 b) with CO2 provided access to (PCP)ReOX(CO2 ) (X=Cl or I, 3 a/b), the product of 1,3-cycloaddition and C-C bond formation.

Irreversible Hydrolysis of PCP-Supported Rhenium(V) Acetates.

Complexes (PCPR)Re(O)(OAc)2 [R = iPr (4a) and tBu (4b); PCP = κ3-P,C,P-2,6-(R2PCH2)2C6H3] undergo unexpected irreversible hydrolysis to yield (PCPR)Re(O)(OAc)(OH) (3a/3b) and free AcOH. 3a and 3b are highly fluxional in solution, possibly via AcOH loss and the intermediacy of (PCPR)Re(O)2, which was isolated for R = tBu (5b).

Structure, Electronics and Reactivity of Ce(PNP) Complexes.

Synthetic methods for the coordination of the monoanionic bis[2-(diisopropylphosphino)-4-methylphenyl]amido (PNP) ligand framework to the cerium(III) cation have been developed and applied for the isolation of a series of {(PNP)Ce} and {(PNP)2 Ce} type complexes. The structures of the complexes were studied by X-ray diffraction and multinuclear NMR spectroscopy. We found that the cerium(III) ion can induce the elimination of one of the iPr groups at phosphorus to yield a new dianionic PNP tridentate framework (PNP-iPr ) featuring a phosphido-donor functionality, which is bound to the cerium ion with the shortest known Ce-P bond of 2.7884(14) Šfor molecular compounds. The reaction of the complex [(PNP)Ce{N(SiMe3 )2 }2 ] (1) with Ph2 CO gave the Ce-bound product of C-C coupling, - N(SiMe3 )SiMe2 CH2 -CPh2 O- , through the C-H bond activation of a SiMe3 group. 31 P NMR spectroscopy was used to estimate the presence of a vacant coordination position at the cerium ion in the CeIII -PNP complexes by the examination of the δ(31 P) shift recorded both in non-polar (C6 D6 ) and polar ([D8 ]THF) solvents. Moreover, 31 P NMR spectroscopy was also found to be a useful tool for the estimation of the Ce-P bond distances in {(PNP)CeIII } and {(PNP)2 CeIII } systems. Electrochemical and computational studies for 1 and its lanthanum analogue containing a redox-innocent metal center revealed the stabilization of the CeIII oxidation state by the PNP ligand.

N-H cleavage as a route to new pincer complexes of high-valent rhenium.

A novel PNN-type pincer ligand has been accessed via imine reduction with LiAlH4 to provide the phosphine diamino proligand, PNHNH (1). The ligand, 1, as well as PNHP can be metallated directly, via N-H cleavage, with L2ReOX2(OEt) precursors to access six-coordinate (PNP)ReOCl2 (2) and (PNNH)ReOX2 (3-Cl, X = Cl; 3-Br, X = Br) in good yield. 3-Cl and 3-Br undergo dehydrohalogenation upon treatment with NEt3, furnishing the five-coordinate phosphine/diamido PNN-type compounds (PNN)ReOX (4-Cl, X = Cl; 4-Br, X = Br) in excellent yield, presenting as a mixture of rotameric diasteromers. The reversibility of this deprotonation, and the coordinative unsaturation of 4-Cl is shown in reactions with HCl(aq) and PMe3 providing 3-Cl and 4-PMe3, respectively. Treatment of 4-Cl with AgOAc, AgOTf, or NaHBEt3 lead to formation of (PNN)ReO(OAc) (4-OAc), (PNN)ReO(OTf) (4-OTf), and (PNN)ReO(H) (4-H), all isolated in excellent yields in varying diasteromeric ratios. The nature of the isomerism was analyzed based on solid-state structural studies and solution NMR data.

High-Turnover Aromatic C-H Borylation Catalyzed by POCOP-Type Pincer Complexes of Iridium.

The catalytic C-H borylation of arenes with HBpin (pin = pinacolate) using POCOP-type pincer complexes of Ir has been demonstrated, with turnover numbers exceeding 10 000 in some cases. The selectivity of C-H activation was based on steric preferences and largely mirrored that found in other Ir borylation catalysts. Catalysis in the (POCOP)Ir system depends on the presence of stoichiometric quantities of sacrificial olefin, which is hydrogenated to consume the H2 equivalents generated in the borylation of C-H bonds with HBpin. Smaller olefins such as ethylene or 1-hexene were more advantageous to catalysis than sterically encumbered tert-butylethylene (TBE). Olefin hydroboration is a competing side reaction. The synthesis and isolation of multiple complexes potentially relevant to catalysis permitted examination of several key elementary reactions. These experiments indicate that the C-H activation step in catalysis ostensibly involves oxidative addition of an aromatic C-H bond to the three-coordinate (POCOP)Ir species. The olefin is mechanistically critical to gain access to this 14-electron, monovalent Ir intermediate. C-H activation at Ir(I) here is in contrast to the olefin-free catalysis with state-of-the-art Ir complexes supported by neutral bidentate ligands, where the C-H activating step is understood to involve trivalent Ir-boryl intermediates.

Chalcogen extrusion from heteroallenes and carbon monoxide by a three-coordinate rh(i) disilylamide.

We report the reactions of several heteroallenes (carbon disulfide, carbonyl sulfide, and phenyl isocyanate) and carbon monoxide with a three-coordinate, bis(phosphine)-supported Rh(I) disilylamide (1). Carbon disulfide reacts with 1 to afford a silyltrithiocarbonate complex similar to an intermediate previously invoked in the deoxygenation of CO2 by 1, and prolonged heating affords a structurally unusual µ-κ(2)(S,S'):κ(2)(S,S')-trithiocarbonate dimer. Carbonyl sulfide reacts with 1 to afford a structurally unique Rh(SCNCS) metallacycle derived from two insertions of OCS and N-to-O silyl-group migrations. Phenyl isocyanate reacts with 1 to afford a dimeric bis(phenylcyanamido)-bridged complex resulting from multiple silyl-group migrations and nitrogen-for-oxygen metathesis, akin to reactivity previously observed with carbon dioxide. The ability of 1 to activate carbon-chalcogen multiple bonds via silyl-group migration is further supported by its reactivity with carbon monoxide, where a nitrogen-for-oxygen metathesis is also observed with expulsion of hexamethyldisiloxane. For all reported reactions, intermediates are observable under appropriate conditions, allowing the formulation of mechanisms where insertion of the unsaturated substrate is followed by one or more silyl-group migrations to afford the observed products. This rich variety of reactivity confirms the ability of metal silylamides to activate exceptionally strong carbon-element multiple bonds and suggests that silylamides may be useful intermediates in nitrogen-atom and nitrene-group-transfer schemes.