Ground- and Excited-State Properties of Zn(II) Tetrakis(4

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Ground- and Excited-State Properties of Zn(II) Tetrakis(4-tetramethylpyridyl) Pophyrin Specifically Encapsulated within a Zn(II) HKUST Metal Organic Framework Randy W. Larsen,*,† Jaroslava Miksovska,‡ Ronald L. Musselman,† and Lukasz Wojtas† † ‡

Department of Chemistry, University of South Florida, 4202 East Fowler Avenue, Tampa, Florida 33620, United States Department of Chemistry and Biochemistry, Florida International University, 11200 South West Eighth Street, Miami, Florida 33199, United States

bS Supporting Information ABSTRACT: We have examined the photophysical properties of Zn(II) tetramethylpyridyl porphyrin (ZnT4MPyP) specifically encapsulated within the cubioctahedral cavities of a ZnHKUST metal organic framework. The encapsulated ZnT4MPyP exhibits a Soret maxima at ∼458 nm that is bathochromically shifted relative to ZnT4PyP in ethanol solution (Soret maxima centered at 440 nm). The corresponding emission spectra of the encapsulated porphyrin exhibit resolvable bands centered at 636 and 677 nm relative to a single broad emission band of the ZnT4MPyP in ethanol solution centered at 636 nm with a shoulder situated near ∼660 nm. The fluorescence lifetime of the encapsulated porphyrin is also perturbed relative to that of the free porphyrin in solution (1.88 ns for the encapsulated porphyrin relative to 1.2 ns in solution). These results are consistent with the ZnT4MPyP being in a more constrained environment in which the peripheral pyridyl groups have restricted rotational motion. The ZnT4MPyP triplet lifetime is also affected by encapsulation, giving rise to a longer lifetime (τ ≈ 3.3 ms) relative to that for the free porphyrin in solution (τ ≈ 1 ms). The triplet-state results indicate that nonplanar vibrational modes of the porphyrin leading to intersystem crossing are retained by encapsulation of the porphyrin but that either the density of vibrational states or the specific nonplanar modes coupling the singlet and triplet states may be perturbed, resulting in the longer observed lifetime.

’ INTRODUCTION Heme proteins represent one of the most diverse class of biomolecules in nature performing a wide array of reactions including electron transfer (cytochromes), mono-oxygenation, peroxide degradation (peroxidases, catalases), small-molecule sensing (FixL, PAS domain sensors, and HemAT sensors), transcription regulation (CooA type proteins), energy transduction (heme/copper oxidases, cytochrome bc1, etc.), oxygen transport and storage (hemoglobins and myoglobins) and polymer synthesis/degradation (lignan peroxidase etc.).1 4 Heme proteins contain an iron protoporphyrin IX active site (or a derivative of this macrocycle) that is coordinated to the protein through histidine, methionine, tyrosine, or cysteine axial ligands. The catalytic diversity of heme proteins is an ongoing target for biomimetic chemistry, and a wide array of systems have been developed to replicate the key catalytic aspects of heme proteins. Early efforts focused on modified metalloporphyrins containing “picket fence” peripheral groups designed to mimic the distal amino acid environment of the protein5 10 as well as “basket handle” porphyrins containing covalently attached bases that would coordinate to the proximal side of the central metal.11 13 Of specific interest has been the ability of the picket fence porphyrins to perform asymmetric epoxidations of alkenes through the addition of chiral groups on the r 2011 American Chemical Society

periphery of the porphyrin ring.14,15 The most widely utilized porphyrins are the iron and manganese derivatives using either hydrogen or alkyl peroxides as the oxidants. In addition, catalytic porphyrin systems have been developed containing functional groups on either one side or both sides of the porphyrin macrocycle. An alternative approach is to encapsulate metalloporphyrins in porous solid-state matrixes, forming heterogenious catalytic systems. Metalloporphyrins have been encapsulated in sol gels,16,17 clay-like layered materials,18,19 synthetic zeolites,20,21 detergent micelles,22 and polymer films,23,24 to name only a few. Many of these materials exhibit significant catalytic activity with a wide variety of substrates, similar to heme proteins. However, they lack important structural features of the protein matrix, including tunable distal and proximal protein pockets and channels that lead from the bulk solvent to the heme active site that not only provide substrate access to the active site but can also regulate the rate of substrate delivery and/or selectivity. An attractive target for heme biomimetic systems are the metal organic framework (MOF) materials that contain polyhedral Received: July 7, 2011 Revised: August 28, 2011 Published: September 06, 2011 11519

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and Zn(II) tetrakis(4-methylpyridyl)porphyrin (ZnT4MPyP), within the octahedral cage, while the remaining cavities allow small molecules to reach the active site for catalysis, much like channels in heme proteins. More importantly, the porphyrin encapsulated HKUST-1 materials demonstrate crystallographically resolved porphyrin macrocycles in cavities reminiscent of heme pockets. In the current study, the photophysical properties of ZnT4MPyP encapsulated in a ZnHKUST-1 framework are examined in order to probe the porphyrin environment within the octahedral cage. These studies provide a basic photophysical understanding of the effects of specific porphyrin encapsulation within a MOF that can be utilized in the future development of photocatalytic porphyrin-MOF based materials.

’ MATERIALS AND METHODS Figure 1. Diagram of the two MOF heme biomimetic systems. (Left) Metallo-T4MPyP (red) encapsulated in the large cavities of rhoZMOF. (Right) Structural diagram of Cu HKUST-1, illustrating the different types of polyhedral cavities, including the octahedral cavity encapsulating the metal porphyrin.

cages.25 29 Unlike mesoporous materials previously utilized for porphyrin encapsulation, polyhedral MOFs share common structural features with heme proteins, including large pockets that can accommodate the catalytic metalloporphyrin as well as channels that connect the bulk solvent to the various interior cages within the MOF. In addition, both the topology and the structural versatility of the MOF can be tuned through application of the molecular building blocks (MBB) approach in which metal ligand clusters form building units through which a wide array of three-dimensional topologies can be prepared. Coupling the three-dimensional structure with the ability to functionalize the organic ligand component of the MBB affords enormous flexibility in catalytic tuneability. Currently, two systems have been reported in which a metalloporphyrin has been encapsulated within the cages of a MOF (Figure 1). The first involved the encapsulation of free base tetrakis(N-methyl pyridyl)porphyrin (T4MPyP) into the large cages of rhoZMOF using a “ship-in-a-bottle” approach (i.e., the porphyrin is encapsulated during the synthesis of the MOF).30 The T4MPyP could be readily metalated, giving rise to a plethora of catalytic materials. As an example of the catalytic capability of the T4MPyP rhoZMOF materials, the Mn(III)T4MPyP-rhoZMOF derivative exhibited limited biomimetic activity toward the monooxygenation of organic molecules using organic peroxide as a substrate. However, the porphyrin was not crystallographically resolvable, indicating conformational disorder and the absence of well-defined proximal or distal heme pockets, a critical feature of heme proteins. The second system is based on a prototypal MOF, HKUST-1, into which catalytically active metalloporphyrins have been selectively encapsulated in the ship-in-abottle fashion within one of the three polyhedral cages that exist in HKUST-1.31 HKUST-1, formed through the assembly of benzene-1,3,5-tricarboxylate anions and either copper(II)32 or zinc(II)33 cations, contains several features that are attractive toward heme biomimetic chemistry. Specifically, the HKUST-1 structure contains three structurally distinct polyhedral cages capable of entrapping guest molecules. The HKUST-1 framework has now been shown to selectively encapsulate metalloporphyrins, including Fe(III)tetrakis(4-sulphonatophenyl)porphyrin (Fe4SP), Mn(III)tetrakis(4-sulphonatophenyl)porphyrin (Mn4SP),

Sample Preparation and Loading. All solvents as well as 1,3,5-benzene tricarboxylic acid (BTCA), aniline, and Zn(II)(NO3)2 3 6H2O were obtained from Sigma-Aldrich and used without further purification. ZnT4MPyP was obtained from Frontier Scientific (Logan, Utah) and used without further purification. The ZnT4MPyP encapsulated ZnHKUST-1 was prepared by carefully layering 10 mL of a solution containing 0.220 g of BTCA, 0.220 g of Zn(II)(NO3)2 3 6H2O, and 30 mg of the porphyrin of interest over 10 mL of nitrobenzene containing 230 μL of pyridine in a 25 mL scintillation vial. After ∼4 days, dark crystals appeared on the side of the vial. The solution was then decanted, and the crystals were collected by centrifugation. The crystals were washed extensively with a 1:1 (V:V) solution of methanol/nitrobenzene. Porphyrin loading into the cavities of the ZnHKUST-1 was determined using X-ray diffraction data (site occupancy refinement of the metal atom) and spectroscopically. The spectroscopic method involves dissolving a known weight of ZnT4MPyP ZnHKUST-1 in water containing 250 mM imidazole. Under these conditions, the material is completely solubilized, and the resulting solution concentration of ZnT4MPyP is determined spectrophotometrically. The number of ZnT4MPyP molecules per cavity can then be determined. Single-Crystal Specular Reflectance and Diffuse Reflectance Spectroscopy. Single-crystal UV/vis spectra were obtained using a polarized specular reflectance spectrophotometer.34 This is a single-beam, wide-range, fast acquisition spectrophotometer. The optics retain focus over a wide range of wavelengths (mid-IR to far-UV) through the use of reflecting optics in all instances except the polarizer. Light sources are a xenon arc lamp and a tungsten halogen lamp, the polarizer is a MgF2 Rochon prism, and optics are spherical and planar reflectors with an Ealing Optics reflecting objective. The image beam size is 30 μm (0.030 mm), sample, reference mirror, and beam-directing mirror motions are normally computer-controlled but are temporarily manually controlled, UV and visible dispersion is through an Acton Research SpectraPro 275 spectrograph, and detection is with a Princeton Instruments 1152  296 EEV (English Electric Valve) CCD (charge-coupled device), maintained at 110 K. All instrument control and data collection are through a Macintosh computer. Spectra are recorded from selected highly reflective natural faces of crystals. The average of 50 spectra is reported in each case; the exposure time for each ranges from 0.01 to 20 s, depending on the spectral region. The data are corrected for percent reflectivity relative to a NIST standard mirror. Diffuse reflectance spectra were obtained from a home-built spectrometer in which the light from an Oriel 150 W Xe arc lamp is 11520

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The Journal of Physical Chemistry A fed through a 1/4 m fiber optic cable mounted ∼2 cm from a glass coverslip containing the solid material. The slide was mounted on a solid-state cell holder provided by ISS Inc. The reflected light was collected at 45 from the excitation source and fed into another fiber optic cable leading to an Ocean Optics UV2400 CCD camera. Spectra were collected using Ocean Optics software. Transient Absorption and Fluorescence. Transient absorption (TA) and fluorescence data were obtained using particle suspensions in ethanol. For TA, samples were subjected to 532 nm excitation from a frequency-doubled Nd:YAG laser (Continuum MiniLite I, 7 ns fwhm, ∼1 mJ/pulse) while being probed with white light from a Xe arc lamp (Oriel) filtered through a 1/4 m single monochromator (Yvon-Jobin) and detected with a photomultiplier tube (R928, Hamamatsu). The signal was amplified using a Mels Griot 3AMP005 preamplifier followed by a Stanford Instruments SR445A 350 post amplifier and digitized using a Tektronix TDS7404 4 GHz digitizer. Spectra were the average of 25 pulses. Samples were deaerated using an Ar purge. Curve fitting was accomplished through Origin. The quality of the fit was judged using χ2 analysis, residuals, and autocorrelation. Steady-state emission and polarization data were obtained using an ISS PC1 spectrofluorometer (Champaign, IL). Phase and modulation have been described, in detail, in the review provided in ref 35. Measurements were performed on a ChronosFD fluorometer (ISS, Champaign, IL) using a 370 nm laser diode as an excitation source, and the emitted light was collected through a 400 nm long path filter (Andover Corp.). 1,4-Bis(5-phenyloxazole-2-yl)benzene (POPOP) solubilized in ethanol was used as a reference compound (τ = 1.35 ns). The data were analyzed using a single exponential decay model (Vinci software, IIS, Champaign, IL). X-ray Diffraction. The crystal structure of ZnT4MPyP ZnHKUST-1 was determined using single-crystal X-ray diffraction. The X-ray diffraction data were collected using a BrukerAXS SMART-APEXII CCD diffractometer (CuKα, λ = 1.54178 Å). Indexing was performed using APEX2.36 Data integration and reduction was performed using SaintPlus 6.01.37 Absorption correction was performed by a multiscan method implemented in SADABS.38 The space group was determined using XPREP implemented in APEX2.36 The structure was solved using SHELXS-97, expanded using Fourier methods and refined on F2 using nonlinear least-squares techniques with SHELXL-97 contained in APEX236 and WinGX v1.70.01 program packages.39 42 The Fourier map was calculated using the Fourier Map routine in Wingx40 and plotted using MCE, version 2005 2.2.0.43 All nonhydrogen framework atoms were refined anisotropically. The metal atom in the core of the porphyrin was found on a Fourier difference map and refined anisotropically. The remaining nonhydrogen atoms of porphyrin were found from a difference Fourier map and refined isotropically using geometry restraints. Highly disordered solvent/anion moieties were modeled as oxygen atoms. Although the porphyrin is disordered over three positions, the porphyrin planes can be clearly seen due to the fact that the D4h symmetry of the porphyrin’s core is a subgroup of symmetry of the cage (Oh). The presence of porphyrin is unambiguous due to the fact that it is actually locked in the octahemioctahedral cages with 4-N-methylpyridyl groups oriented through the square windows of these cages.

’ RESULTS AND DISCUSSION Structural Aspects of ZnT4MPyP ZnHKUST-1. The first report of porphyrin encapsulated in HKUST-1 materials

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Figure 2. X-ray crystal structures of the ZnT4MPyP ZnHKUST-1 material (top) and the cavities containing the encapsulated porphyrin (bottom).

demonstrated that anionic and cationic porphyrins could be encapsulated via ship-in-a-bottle methods and that the porphyrin is encapsulated within one of the three polyhedral cavities within the HKUST-1 framework. The ZnT4MPyP ZnHKUST-1 examined here displays structural features identical to those obtained previously for the porphyrin HKUST-1 material (Figure 2). Specifically, the porphyrin is encapsulated in the octahedral cages with 4-N-methylpyridyl groups oriented through the square windows of these cages. The axial sites on both sides of the porphyrin are exposed to the structural channels because they lie directly above and below the other two square windows of the cage. Due to the symmetry of the cage (Oh), the porphyrin core (D4h symmetry) is statistically disordered over three positions throughout the structure. Regardless of disorder, the movement of porphyrin is limited due to the size of square windows surrounding the 4-N-methylpirydyl groups. Steady-State Absorption, Emission, and Emission Lifetime. The steady-state optical absorption spectra for ZnT4MPyP in ethanol solution as well as diffuse reflectance and single-crystal optical absorption data for the ZnT4MPyP ZnHKUST-1 materials in the Soret region are displayed in Figure 3. The Soret maximum of ZnT4MPyP in solution is centered at 440 nm and is bathochromically shifted to 458 nm in both the single-crystal spectra and the diffuse reflectance spectra of ∼5 mg of crystalline material. The bathochromic shift may have several origins. The hydrophobic nature of the octahedral cavity may contribute to the shift as the Soret band of ZnT4MPyP is sensitive to the dielectric constant of the solvent. For example, in water (ε = 78), the Soret band is found at 436 nm and shifts to ∼440 nm in ethanol (ε = 24.5).44 However, this small shift in Soret position with a relatively large change in dielectric constant would suggest 11521

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Figure 3. Normalized steady-state optical spectra of Zn(II)T4MPyP in solution (dashed line) and encapsulated within the octahedral cavities of ZnHKUST-1 (dotted line, from diffuse reflectance data of crystalline materials; solid line, from single-crystal specular reflectance data). Figure 5. Phase (circles) and modulation (squares) of Zn(II)T4MPyP (solid symbols) in ethanol and ZnHKUST-1 (open symbols). Solid lines represent the fit to experimental data. The data were best fit to a single exponential decay model, giving a lifetime of 1.2 ns for Zn(II)T4MPyP in ethanol and 1.8 ns for ZnHKUST-1.

Figure 4. Steady-state emission spectra of Zn(II)T4MPyP in solution (dashed line) and encapsulated within the octahedral cavities of ZnHKUST-1 (solid line).

that the 18 nm bathochromic shift observed between ZnT4MPyP in solution and that encapsulated in ZnHKUST-1 does not arise exclusively from the hydrophobicity of the octahedral pocket. Alternatively, the dihedral angle between the pyridyl rings and the porphyrin plane can have a significant impact on the electronic states of the porphyirn. Rotation of the pyridyl ring by up to 30 relative to the porphyrin plane can bathochromically shift the Soret band by up to 35 nm.44 46 In the case of the ZnT4MPyP ZnHKUST-1 material, the pyridyl ring has a dihedral angle with the porphyrin plane of 45 relative to the 90 energy minima assumed for the porphyrin in solution. A 45 dihedral would be expected to result in a bathochromic shift of ∼20 nm, close to the observed 18 nm.44 46 The corresponding emission spectra for ZnT4MPyP in ethanol solution and encapsulated within the ZnHKUST-1 material are displayed in Figure 4. In ethanol solution, Soret excitation gives rise to an emission maximum centered at 636 nm with a pronounced shoulder centered at 660 nm. Soret excitation of a suspension of ZnT4MPyP ZnHKUST-1 material in ethanol

gives rise to an emission spectra with well-resolved maxima centered at 636 and 677 nm. The excited state of ZnT4MPyP involves the coupling of the porphyrin S1 state with a close-lying charge-transfer (CT) state between the porphyrin ring π system and the π system localized on the pyridinium group. This coupling is facilitated by rotational freedom of the pyridinium group and high-polarity solvents, giving rise to a broad featureless emission band.45,46 In the case of the ZnT4MPyP ZnHKUST-1 material, two emission bands are clearly resolved. Such splitting is also observed upon T4MPyP binding to nucleic acids and is attributed to reduced coupling of the S1 CT states of the bound porphyrin as well as to reduced polarity of the porphyrin environment.45,46 The emission spectra of the encapsulated ZnT4MPyP is thus consistent with porphyrin confinement within a conformationally restricted low-polarity environment. In the case of the ZnHKUST-1 material, porphyrin encapsulation restricts the motion of the peripheral pyridyl groups (they are likely fixed at ∼45 relative to the plane of the porphyrin ring). In addition, the octahedral cage in which the porphyrin is encapsulated is hydrophobic due to the organic ligands making up the HKUST-1 framework. The emission lifetimes of the porphyrin are likewise affected by encapsulation. In ethanol solution, ZnT4MPyP exhibits an emission lifetime of 1.2 ns, while that of the ZnHKUST-1 encapsulated porphyrin exhibits an emission lifetime of 1.8 ns (Figure 5). The longer emission lifetime relative to solution also matches that of ZnT4MPyP bound to polynucleotides and is further indication of S1 CT decoupling.46 48 It is also of note that the emission lifetime of the encapsulated porphyrin could be fit to a single exponential decay, indicating conformational homogeneity within the octahedral cavity of the ZnHKUST-1 framework. Triplet-State Lifetimes. The triplet states of free base and metalloporphyrins are also sensitive to the local environment of the macrocycle in terms of both the T1 Tn absorption spectrum and the T1 lifetime. In the case of ISC, the rate (i.e., singlet- to tripletstate conversion) is dependent on the spin orbit coupling 11522

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The fact that the T1 lifetime of the ZnT4MPyP encapsulated within the ZnHKUST-1 material is longer by a factor of 3 relative to that of the free porphyrin could arise from a change in population of the out-of-plane b1u type vibrations. A change in the population of vibrational states would also be reflected in the Franck Condon factor, F, which could further reduce knr. A reduction in magnitude of ÆψT1(π,π*)|Hso|ψS0(π,π*)æ also reduces the nonradiative decay rate. Thus, an increase in the T1 lifetime could also arise from a complete damping of the b1u like modes while retaining out-of-plane vibrational modes with different symmetry, resulting in smaller coupling integrals. Regardless of the mechanism through which the T1 lifetime is modulated, the key result is that specific encapsulation of the ZnT4MPyP within the ZnHKUST-1 octahedral cavities can modulate its electronic properties. Figure 6. Normalized transient absorption in the Soret region of Zn(II)T4MPyP (λ = 430 nm) in solution (solid line) and encapsulated within the octahedral cages of ZnHKUST-1 (λ = 450 nm).

between the states according to ÆψS(π,π*)|Hso|ψT(π,π)æÆνS|νTæ, where Hso is the spin orbit coupling element, νS is the lowestenergy vibrational mode in the S1 state, and νT is the vibrational mode of the T state that overlaps with the νS mode.49,50 The spin orbit coupling integral will only be nonzero for transitions between states with different configurations. The most comprehensive studies of intersystem crossing have involved free base porphyrins, but the essential features can be applied to metalloporphyrins as well. In the case of free base porphyrins, the lowest-energy S1 state is of B3u symmetry, while the two closest triplet states are of B2u (T1) and B3u (T2) symmetry.51 The fact that the symmetry of the close-lying T2 and the S1 state are equivalent makes the ÆψS1(π,π*)|Hso|ψT2(π,π*)æ = 0. Similarly, the ÆψS1(π,π*)|Hso|ψT1(π,π*)æ integral has also been shown to be very small and cannot account for the high intersystem crossing rates. What facilitates the intersystem crossing is the population of vibronic modes in the S1, which alter the symmetry of this state, giving rise to much larger values of ÆψS1(π,π*)|Hso|ψT1(π,π*)æ and rate constants for interconversion that compete with S1 emission rates. Specifically, the porphyrin Cβ Cβ stretching mode (back pyrole carbons) that stabilizes the S1 nπ* and T1 ππ* facilitates intersystem crossing. In the case of the ZnT4MPyP encapsulated within the ZnHKUST-1 material, the kinetic difference spectrum between the So f Sn and T1 f Tn transitions is significantly bathochromically shifted relative to that of the ZnT4MPyP in ethanol (data not shown), consistent with the corresponding bathochromic shift in the So f Sn Soret transition. The corresponding lifetime of the T1 state is also significantly longer for the ZnT4MPyP encapsulated within the ZnHKUST-1 (τ = 3.4 ms) relative to ZnT4MPyP in solution (τ = 983 μs) (Figure 6). The nonradiative relaxation rate between the T1 and S0 states also involves spin orbit coupling and is related to the magnitude of ÆψT1(π,π*)|Hso|ψS0(π,π*)æ through the energy gap law according to knr = (2π/p)ÆψT1(π,π*)|Hso|ψS0(π,π*)æ2FF, where F is the Franck Condon factor and F is the density of states per unit energy.52,53 For porphyrin systems, out-of-plane vibrational modes of b1u symmetry provide the most effective coupling of the potential energy surfaces of the T1 and S0, giving rise to nonzero values of ÆψT1(π,π*)|Hso|ψS0(π,π*)æ.51

’ SUMMARY In summary, the results presented here demonstrate the effects of encapsulation of ZnT4MPyP within the octahedral cages of ZnHKUST-1 on the ground and excited states of the porphyrin. Encapsulation appears to restrict the conformational flexibility of the porphyrin peripheral pyridyl groups, giving rise to a well-resolved emission spectrum from the S1 state and an increase in the emission lifetime. Encapsulation also appears to alter the population of the out-of-plane vibrational modes of the porphyrin that couple the S1 and T1 as well as the T1 to S0 ππ* transitions, resulting in formation of the T1 but with a longer relaxation time. ’ ASSOCIATED CONTENT

bS

Supporting Information. CIF file. This material is available free of charge via the Internet at http://pubs.acs.org.

’ ACKNOWLEDGMENT This work was supported by the Department of Defense Defense Threat Reduction Agency (DoD-DTRA) through HDTRA1-08-C-0035. ’ ABBREVIATIONS T4MPyP, tetrakis(N-methylpyridyl)porphyrin; MOF, metal organic framework; Fe4SP or Mn4SP, Fe(III) or Mn(III) tetrakis(4-sulphonatophenyl)porphyrin ’ REFERENCES (1) Groves, J. T. Models and Mechanisms of Cytochrome P-450 Action. In Cytochrome P450: Structure, Mechanism and Biochemistry; Ortiz de Montellano, P. R., Ed.; Kluwer Academic/Plenum Publishers: New York, 2005; pp 1 44. (2) Reedy, C. J.; Gibney, D. R. Chem. Rev. 2004, 104, 617. (3) Morgan, B.; Dolphin, D. Struct. Bonding 2004, 64, 115–199. (4) Collman, J. P.; Boulatov, R.; Sunderland, C. J.; Fu, L. Chem. Rev. 2004, 104, 561–588. (5) Collman, J. P.; Brauman, J. I.; Doxsee, K. M. Proc. Natl. Acad. Sic. U.S.A. 1979, 76, 6035–6039. (6) Imai, H.; Kyuno, E. Inorg. Chem. 1990, 29, 2416–2422. (7) Collman, J. P.; Gagne, R. R.; Reed, C.; Halbert, T. R.; Lang, G.; Robinson, W. T. J. Am. Chem. Soc. 1975, 97, 1427–1439. (8) Collman, J. P.; Brauman, J. I.; Doxsee, K. M.; Halbert, T. R.; Hayes, S. E.; Suslick, K. S. J. Am. Chem. Soc. 1978, 100, 2761–2766. 11523

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