Structural And Spectral Investigation Of Platinum Group Metal Complexes Containing Weakly Bonded Ligands

Author : Charlene Tsay
Publisher :
ISBN 13 :
Total Pages : 266 pages
Book Rating : 4.:/5 (846 download)

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Download or read book Small Molecule Binding to Electrophilic Trigonal Pyramidal Platinum, Palladium, and Nickel written by Charlene Tsay and published by . This book was released on 2013 with total page 266 pages. Available in PDF, EPUB and Kindle. Book excerpt: Chapter 1 A general introduction to the concepts and background of several types of transition metal complexes that motivate and inform the research described herein. These include a-complexes and molecular adducts of dinitrogen, dihydrogen, and carbon dioxide. Chapter 2 Trigonal bipyramidal platinum(II) complexes of the monoanionic, tetradentate, triphosphine [SiPR3 ([SiP3R]- = [(2-R2PC6H4)3Si]-; R = Ph, iPr) ligand are prepared and shown to provide access to cationic species with divergent behavior. The less electron-rich phenyl-substituted ligand renders the platinum center extremely electrophilic, leading to structurally characterized examples of weakly-donating ligands bound in the fifth, apical coordination site. Of particular interest is the structure of the toluene adduct, which suggests a possible interaction between the platinum center and an aryl C-H bond. When the ligand phosphines are instead substituted by the more electron-rich isopropyl groups, the electrophilicity of the cationic platinum is shown to be mitigated, allowing access to a four-coordinate, trigonal pyramidal platinum center. The crystallographically characterized geometry for this divalent platinum is in contrast to the canonical square planar configuration for d8, 16-electron transition metal complexes. The palladium analogue is also synthesized and shown to possess the same coordination. Chapter 3 Cationic nickel complexes of the [SiPR3] ligand are synthesized and, in contrast to their platinum and palladium congeners, facilitate the surprising binding of molecular dinitrogen to electrophilic nickel(II) centers. The extremely high stretching frequencies of these bound N2 moieties attest to their minimal activation, and the stability of these complexes is shown to arise from increased adonation from the N2 to the cationic nickel center, which compensates for the relative lack of it back-bonding that stabilizes N2 adducts in less electrophilic systems. These cationic nickel species are additionally shown to form thermally stable adducts of molecular dihydrogen. The relative binding strengths of N2 and H2 to these nickel centers are explored and shown to be modulated by the ligand phosphine substituents. Furthermore, evidence of linear binding of carbon dioxide is presented, representing an electrophilic approach to carbon dioxide activation that is in contrast to the low-valent, nucleophilic metal paradigm. Chapter 4 The four-coordinate neutral nickel boratrane (TPiPrB = (2-iPr2PC6H4)3B) reported in the literature represents an isostructural counterpart to the cationic {[SiiPr3]Ni}+ species presented in Chapter 3. Though these two compounds are formally separated by two oxidation states of nickel, the Lewis-acidic nature of the Z-type borane ligand in (TP'PrB)Ni renders it valence-isoelectronic with {[SiiPr3]Ni}+. The reactivity toward N2 and H2 of (TPiPr'B)Ni, as well as that of the new compound (TPPhB)Ni, is explored and discussed in context of what is observed for the {[SiPR3]Ni}+ system. The neutral (TPiPr'B)Ni, while presumably a better [pi] back-bonder than cationic {I[SiPip' 3]Ni}T, is demonstrated not to bind N2, though a very weak, fluxional interaction with H2 at low temperature is hypothesized. The more electrophilic (TP PhB)Ni exhibits room temperature interactions with both N2 and H2, though the nature of these interactions has yet to be confirmed. These results thus underline the importance of [sigma]-donation in stabilizing N2 and H2 adducts of poorly 7r back-bonding metal centers. Chapter 5 Cobalt(I) complexes of [SiPR3] provide an additional isostructural, isoelectronic point of comparison to the cationic nickel species presented in Chapter 3. The dinitrogen adducts [SiP'i' 3]Co(N2) and [SiPPh3]Co(N2), previously reported from our laboratory, feature strongly bound N2 ligands that are not labile to vacuum. The corresponding dihydrogen adducts are generated slowly under an H2 atmosphere. The intact nature of both dihydrogen ligands, which also are not labile to vacuum, is reflected in their NMR spectroscopic parameters. The thermal stability of these compounds enabled crystallization of [SiPi'' 3]Co(H2) which, along with the related (TP'i'B)Co(H2) complex also developed in our laboratory, represent the first structurally characterized dihydrogen adducts of cobalt. Additional comparisons are made between the relative N2 and H2 binding strengths of this system and those of the structurally and electronically related family of [SiPR3] and (TpRB) metal complexes. Appendix A The asymmetric dinucleating ligand [NOPPh], designed to contain both a hard, N-donor binding site and a soft-P-donor binding site, is synthesized and shown to form a diiron complex that features asymmetric bonding to the bridging acetates. The corresponding symmetric, allphosphine dinucleating ligand [POPPh], proves to be more conducive to further study, and provides access to the symmetric diiron, di-([mu]-bromide) starting material {[POPPh ]Fe 2Br2} {BArF4 }. Addition of hydrazine generates the asymmetric, unbridged N2H4 adduct, which features localized diamagnetic and paramagnetic iron centers. The conformation of this species additionally demonstrates the flexibility of this ligand framework. Reduction of the diiron(II) starting material in the presence of PMe3 results in formation of a putative asymmetric iron(O)/iron(I) dimetallic complex, in which an N2 molecule is bound to the diamagnetic iron center, while the PMe3 is ligated to the high-spin iron center and rendered NMR silent. The N2 ligand is shown to be reversibly displaced by H2 , suggesting the formation of a dihydrogen adduct, as well as by CO2, which is postulated to bind as a bent, [eta]2(C,O) ligand.