Michael Findlater

Proton-catalyzed Hydrogenation of a d 8 Ir(I) Complex Yields a trans Ir(III) Dihydride

Hydrogenation of the (PONOP)Ir(I)CH(3) complex [PONOP = 2,6-bis(di-tert-butylphosphinito)pyridine] yields the unexpected trans-dihydride species (PONOP)IrCH(3)(H)(2). Mechanistic investigations have revealed that this reaction proceeds via proton-catalyzed H(2) cleavage, a pathway that circumvents the intermediacy of the typically invoked cis-dihydride isomer. Protonation yields the cationic (PONOP)Ir(CH(3))(H)(+) complex, which is then trapped by H(2) to yield an eta(2)-H(2) complex. Deprotonation of this species yields the trans-dihydride. Intermediates in the proposed pathway have been confirmed by independent low-temperature syntheses and spectroscopic observations.

1D molecular ladder of the ionic complex of terbium-4-sebacoylbis(1-phenyl- 3-methyl-5-pyrazolonate) and sodium dibenzo-18-crown-6: Synthesis, crystal structure, and photophysical properties

On the basis of the novel heterocyclic beta-diketone, 4-sebacoylbis(1-phenyl-3-methyl-5-pyrazolone (H 2SbBP), three new lanthanide complexes Tb 2(SbBP) 3(H 2O) 2 ( 1), Gd 2(SbBP) 3(H 2O) 2 ( 2), and [Tb(SbBP) 2] [Na(DB18C6)H 2O] ( 3) have been synthesized and characterized by various spectroscopic techniques. The single-crystal X-ray diffraction analysis of 3 reveals that the complex crystallizes in the monoclinic space group C2/ c with a = 25.300(6) A, b = 19.204(7) A, c = 15.391(3) A, beta = 93.17(3) degrees , and V = 7466(4) A (3). The crystal structure of 3 is heterodinuclear and features a Tb (3+) center surrounded by two tetradentate bispyrazolone ligands in a somewhat distorted square-antiprismatic geometry. The Na (+) coordination environment is distorted hexagonal pyramidal and involves six oxygen atoms furnished by DB18C6 and one oxygen atom from a water molecule. The X-ray diffraction study of 3 also revealed an interesting 1D molecular ladder structure based on C-H/pi, intra- and intermolecular hydrogen-bonding interactions. The photophysical properties of 1 and 3 in solid state have been investigated, and the quantum yields and (5)D 4 lifetimes were found to be 4.82 +/- 0.01% and 18.13 +/- 0.82% and 1.11 +/- 0.01 and 2.82 +/- 0.02 ms, respectively.

Dual emission from stoichiometrically mixed lanthanide complexes of 3-phenyl-4-benzoyl-5-isoxazolonate and 2,2′-bipyridine

The luminescence properties of the three new stoichiometrically mixed lanthanide complexes, Sm1/2Eu1/2(PBI)3·bpy·H2O (1), Sm1/2Tb1/2(PBI)3·bpy·H2O (2) and Eu1/2Tb1/2(PBI)3·bpy·H2O (3) [HPBI = 3-phenyl-4-benzoyl-5-isoxazolone; bpy = 2,2′-bipyridine] have been compared with those of the analogous single lanthanide ion systems, Sm(PBI)3·bpy·H2O (4), Eu(PBI)3·bpy·H2O (5) and Tb(PBI)3·bpy·H2O (6). Compound 5 was structurally characterized by single-crystal X-ray diffraction, and crystallizes in the triclinic space groupP with a = 12.839(3) A, b = 13.863(3) A, c = 16.379(3) A, α = 81.66(3)°, β = 73.32(3)°, γ = 89.26(3)° and V = 2762.0(10) A3. The crystal structure of 5 comprises an assembly of mononuclear species, each of which features a central Eu3+ cation coordinated to three bidentate PBI ligands, a bidentate bipy ligand, and a water molecule. The overall geometry of the nonacoordinate array is that of a distorted monocapped trigonal prism. The X-ray diffraction study of 5 also revealed many interesting π–π, interplanar and intermolecular hydrogen-bonding interactions. The mixed lanthanide complexes 1–3 exhibit interesting dual emissions in the visible region. The quantum yields and lifetime measurements for 1–3 support the premise that Ln → Ln energy transfer occurs in these mixed lanthanide systems, along with the usual ligand-to-metal triplet energy pathways.

Dihydrogen complexes of iridium and rhodium.

A series of iridium and rhodium pincer complexes have been synthesized and characterized: [(POCOP)Ir(H)(H(2))] [BAr(f)(4)] (1-H(3)), (POCOP)Rh(H(2)) (2-H(2)), [(PONOP)Ir(C(2)H(4))] [BAr(f)(4)] (3-C(2)H(4)), [(PONOP)Ir(H)(2))] [BAr(f)(4)] (3-H(2)), [(PONOP)Rh(C(2)H(4))] [BAr(f)(4)] (4-C(2)H(4)) and [(PONOP)Rh(H(2))] [BAr(f)(4)] (4-H(2)) (POCOP = κ(3)-C(6)H(3)-2,6-[OP(tBu)(2)](2); PONOP = 2,6-(tBu(2)PO)(2)C(5)H(3)N; BAr(f)(4) = tetrakis(3,5-trifluoromethylphenyl)borate). The nature of the dihydrogen-metal interaction was probed using NMR spectroscopic studies. Complexes 1-H(3), 2-H(2), and 4-H(2) retain the H-H bond and are classified as η(2)-dihydrogen adducts. In contrast, complex 3-H(2) is best described as a classical dihydride system. The presence of bound dihydrogen was determined using both T(1) and (1)J(HD) coupling values: T(1) = 14 ms, (1)J(HD) = 33 Hz for the dihydrogen ligand in 1-H(3), T(1)(min) = 23 ms, (1)J(HD) = 32 Hz for 2-H(2), T(1)(min) = 873 ms for 3-H(2), T(1)(min) = 33 ms, (1)J(HD) = 30.1 Hz for 4-H(2).

Boron di- and tri-cations

Previous work on boron di- and tri-cations is reviewed. The structural chemistry of representative examples of these classes of compound has been probed by determination of the single-crystal X-ray structures of [(4-Mepy)4B]Br3 and [py3BH]Br2. The electronic structures of the polycations [(py)3BH]2+, [(py)3BBr]2+, [(4-Mepy)3BH]2+, [(4-Mepy)4B]3+, [(Me3P)3BH]2+ and [(Me3P)4B]3+ have been examined by DFT methods. The atomic charges on these cations were evaluated by Mulliken, natural population analysis (NPA), Hirschfeld and Voronoi deformation density (VDD) methods.

Role of coordination geometry in dictating the barrier to hydride migration in d6 square-pyramidal iridium and rhodium pincer complexes

Syntheses of the olefin hydride complexes [(POCOP)M(H)(olefin)][BAr(f)(4)] (6a-M, M = Ir or Rh, olefin = C(2)H(4); 6b-M, M = Ir or Rh, olefin = C(3)H(6); POCOP = 2,6-bis(di-tert-butylphosphinito)benzene; BAr(f) = tetrakis(3,5-trifluoromethylphenyl)borate) are reported. A single-crystal X-ray structure determination of 6b-Ir shows a square-pyramidal coordination geometry for Ir, with the hydride ligand occupying the apical position. Dynamic NMR techniques were used to characterize these complexes. The rates of site exchange between the hydride and the olefinic hydrogens yielded ΔG(++) = 15.6 (6a-Ir), 16.8 (6b-Ir), 12.0 (6a-Rh), and 13.7 (6b-Rh) kcal/mol. The NMR exchange data also established that hydride migration in the propylene complexes yields exclusively the primary alkyl intermediate arising from 1,2-insertion. Unexpectedly, no averaging of the top and bottom faces of the square-pyramidal complexes is observed in the NMR spectra at high temperatures, indicating that the barrier for facial equilibration is >20 kcal/mol for both the Ir and Rh complexes. A DFT computational study was used to characterize the free energy surface for the hydride migration reactions. The classical terminal hydride complexes, [M(POCOP)(olefin)H](+), are calculated to be the global minima for both Rh and Ir, in accord with experimental results. In both the Rh ethylene and propylene complexes, the transition state for hydride migration (TS1) to form the agostic species is higher on the energy surface than the transition state for in-place rotation of the coordinated C-H bond (TS2), while for Ir, TS2 is the high point on the energy surface. Therefore, only for the case of the Rh complexes is the NMR exchange rate a direct measure of the hydride migration barrier. The trends in the experimental barriers as a function of M and olefin are in good agreement with the trends in the calculated exchange barriers. The calculated barriers for the hydride migration reaction in the Rh complexes are ∼2 kcal/mol higher than for the Ir complexes, despite the fact that the energy difference between the olefin hydride ground state and the agostic alkyl structure is ∼4 kcal/mol larger for Ir than for Rh. This feature, together with the high barrier for interchange of the top and bottom faces of the complexes, is proposed to arise from the unique coordination geometry of the agostic complexes and the strong preference for a cis-divacant octahedral geometry in four-coordinate intermediates.

A β-diketiminato hydroxyphosphenium cation: phosphinous acid–secondary phosphine oxidetautomerism revisited

The first example of a β-diketiminato-supported hydroxyphosphenium cation has been prepared, structurally characterized, and modeled by DFT calculations.

The synthesis and characterization of [IMesH]+[(η3-C5H5)V(N)Cl2]−: An anionic vanadium(V) complex with a terminal nitrido ligand

A series of new half-sandwich vanadium complexes have been prepared. The structures of two new anionic vanadium half-sandwich complexes, [CpVCl(3)](-) and [CpV(N)Cl(2)](-) are presented. (15)N isotopic labelling studies have been conducted to unambiguously assign the V[triple bond, length as m-dash]N infra red stretching frequencies of both a neutral and an anionic (cyclopentadienyl)vanadium nitrido complex. The influence of strongly pi-basic coligands on the hapticity of the cyclopentadienyl ligands in half-sandwich complexes of vanadium is discussed.

Structural diversity in schiff base complexes of Ga(III), In(III), Pb(II), and Zn(II): Precursors and model systems for conducting metallopolymers

A variety of Schiff base ligands have been synthesized in an effort to study the coordination chemistry of these ligands when reacted with various metal salts. By varying the structural features of the ligand as well as the synthetic route used to make the corresponding metal complexes, a number of different structural motifs have been observed. Schiff base ligands were chosen due to their ease of synthesis as well as their ability to bind a variety of metal salts. These complexes serve as model systems for and precursors to monomers for the preparation of conducting metallopolymers. Further functionalization of the Schiff base ligand with bithiophene end groups has resulted in electropolymerizable monomers. When polymerized these monomers give conducting metallopolymers which can then be used for a variety of applications. The products were characterized by multinuclear NMR, UV-Vis, and IR spectroscopy, and mass spectrometry. Solid-state structures were determined by single crystal X-ray diffraction studies. Electropolymerization yielded novel conducting polymers with embedded metals.

Iron Catalyzed Hydroboration of Aldehydes and Ketones

We report an operationally convenient room temperature hydroboration of aldehydes and ketones employing Fe(acac)3 as precatalyst. The hydroboration of aldehydes and ketones proceeded efficiently at room temperature to yield, after work up, 1° and 2° alcohols; chemoselective hydroboration of aldehydes over ketones is attained under these conditions. We propose a σ-bond metathesis mechanism in which an Fe-H intermediate is postulated to be a key reactive species.