Chemistry and catalysis advances in organometallic chemistry and catalysis
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24.4 COMPUTATIONAL DETAILS The mechanism of the reduction of sulfoxides promoted by MoO 2 Cl
was calculated using the Gaussian 03 software package [23] , and the PBE0 functional, without symmetry constraints. That functional uses a hybrid, generalized gradient approximation (GGA), including 25% mixture of Hartree–Fock [35] exchange with DFT [22] exchange-correlation, given by the Perdew, Burke, and Ernzerhof functional (PBE) [36]. The optimized geometries were obtained with the LanL2DZ basis 312 SULFOXIDE REDOX CHEMISTRY WITH MOLYBDENUM CATALYSTS Figure 24.11 The catalytic cycle for the reduction of Me 2 SO catalyzed by MoO 2 Cl 2 (H 2 O) 2 (L) in the presence of HBcat ( G with solvent effects given by the PCM model, kcal/mol). set [37] augmented with an f-polarization function [38] for Mo, and a standard 6-31G(d,p) [39] for the remaining elements (basis b1). Transition state optimizations were performed with the synchronous transit-guided quasi-Newton method (STQN) developed by Schlegel et al. [40], following extensive searches of the potential energy surface. Frequency calculations were performed to confirm the nature of the stationary points, yielding one imaginary frequency for the transition states and none for the minima. Each transition state was further confirmed by following its vibrational mode downhill on both sides and obtaining the minima presented on the energy profile. The electronic energies ( E b1 ) obtained at the PBE0/b1 level of theory were converted to free energy at 298.15 K and 1 atm ( G b1
based on structural and vibration frequency data calculated at the same level. Natural population analysis (NPA) [41] and the resulting Wiberg indices [34] were used to study the electronic structure and bonding of the optimized species. Single-point energy calculations were performed using an improved basis set (basis b2) and the geometries optimized at the PBE0/b1 level. Basis b2 consisted of the 3-21G set [42] with an extra f-polarization function [38] for Mo, and a standard 6-311 ++G(d,p) [43] for the remaining elements. Solvent effects (THF) were considered in the PBE0/b2//PBE0/b1 energy calculations using the polarizable continuum model (PCM) initially devised by Tomasi and coworkers [44] as implemented on the Gaussian 03 [45]. The molecular cavity was based on the united atom topological model applied on UAHF radii, optimized for the HF/6-31G(d) level. The free-energy values presented in the text ( G soln
b2 ) were derived from the electronic energy values obtained at the PBE0/b2//PBE0/b1 level, including solvent effects ( E soln b2 ), according to the following expression: G soln
b2 = E
soln b2 + G b1 – E b1 Three-dimensional representations were obtained with Chemcraft [46]. REFERENCES 313 ACKNOWLEDGMENT We thank Fundac¸˜ao para a Ciˆencia e Tecnologia, Portugal, for financial support (Projects PEst-OE/QUI/UI0612/2011-2012, PEst-OE/QUI/UI0100/2011-2012 and PTDC/QUI-QUI/099389/2008).
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45. (a) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005, 105 , 2999; (b) Cossi, M.; Scalmani, G.; Rega, N.; Barone, V. J. Chem. Phys. 2002, 117 , 43. 46. http://www.chemcraftprog.com/index.html (accessed May 2013). 25 A NEW FAMILY OF ZIRCONIUM COMPLEXES ANCHORED BY DIANIONIC CYCLAM-BASED LIGANDS: SYNTHESES, STRUCTURES, AND CATALYTIC APPLICATIONS Ana M. Martins ∗ , Rui F. Munh ´a, Luis G. Alves, and Shanmuga Bharathi Centro de Qu´ımica Estrutural, Instituto Superior T´ecnico, Universidade de Lisboa, Lisboa, Portugal 25.1 INTRODUCTION The field of organometallic chemistry was dominated by cyclopentadienyl-based systems until the 1990s. Since then, a growing interest in new ancillary ligand frameworks suitable to disclose new reactivity patterns became an active research topic. Amido ligands were primarily responsible for “the beginning of a postmetallocene era” [1], and have proved successful in taking the chemistry promoted by metallocene systems a step further [1, 2]. The concept of combining different donor atoms in a chelating array has exponentially increased the diversity of metal complexes available, and the resulting new coordination environments has given way to the discovery of well-defined platforms for the study of elementary organometallic reactions and new metal-mediated chemical transformations [3]. Albeit ligand design has reached a high level of sophistication, the chemistry promoted by the corresponding metal complexes remains in many cases incipient, or poorly understood, when compared to their metallocene counterparts. In the late 1980s, it was anticipated that early transition metal complexes supported by anionic macrocyclic ligands “could establish a chemistry comparable with that of the corresponding bis(cyclopentadienyl) derivatives” [4]. However, it was only recently that macrocyclic ligands have received considerable attention in the context of early transition metal chemistry [4, 5]. The cavity size of the macrocycle can be such that larger metal ions are forced to sit above the plane of the donors, resulting in less sterically hindered reactive sites at the metal center [6]. The out-of-plane coordination mode renders the remaining coordination positions adjacent, which is a prerequisite for many established reaction pathways and catalytic processes. In the case of tetradentate dianionic ancillary ligands, the “(L 2 X 2 )M” fragment can be considered isoelectronic to the “Cp 2 M” scaffold. However, anionic macrocyclic ligands comprise, in most cases, hard ligands such as amido, alkoxido, or amines, which make the metal–ligand bonds very polar and make these systems more electron releasing than the combination of two cycplopentadienyl units. Consequently, the reactivity of the former complexes is affected by the decreased electrophilicity at the metal center [7]. The work presented here describes the chemistry of Zr(IV) complexes anchored by dissymmetric, disubstituted, dianionic cyclam ligands. In comparison with zirconium complexes supported by unsaturated dianionic tetraazamacrocycles [4,7a,8], the compounds derived from dianionic, trans-disubstituted cyclam ancillary ligands have a more robust skeleton, which is not susceptible to nucleophilic attack and rearrangement, and higher flexibility, suitable to fit to the electronic and steric metal requirements. Part of this work has already been published and the reader will be directed to the original publications for the sake of clarity.
First Edition. Edited by Armando J. L. Pombeiro. © 2014 John Wiley & Sons, Inc. Published 2014 by John Wiley & Sons, Inc.
316 A NEW FAMILY OF ZIRCONIUM COMPLEXES ANCHORED BY DIANIONIC CYCLAM-BASED LIGANDS 25.2 SYNTHESES AND MOLECULAR STRUCTURES Trans-disubstituted cyclam-based ligand precursors of the general formulas H 2 (Bn
2 Cyclam) and H 2 (
Bn 2 Cyclam) (Bn = C 6 H 5 CH 2 , 1a, 3,5-Me Bn
2 C 6 H 3 CH 2 , 1b) were obtained in high yields in a multistep synthetic procedure starting from cyclam. Reactions of ZrCl 2 (CH 2 SiMe
3 ) 2 (Et 2 O) 2 with either 1a or 1b resulted in the elimination of tetramethylsilane and formation of (Bn 2 Cyclam)ZrCl 2 (2a) and ( 3,5-Me Bn
Cyclam)ZrCl 2 (2b) in high yields [9]. The latter were used as precursors in the syntheses of alkoxido, thioalkoxido, amido, and alkyl derivatives via chloride metatheses using the appropriate lithium or Grignard reagents (Scheme 25.1) [10]. The zirconium complexes shown in Scheme 25.1 fall into three distinct coordination geometries: (i) trigonal prismatic (P) where the four nitrogen donors of the macrocyclic ligand are ligated to zirconium and define one rectangular face of the prism, (ii) tetrahedral capped (T), for which the distances between zirconium and the two cyclam amine functions fall out of the bonding range observed in zirconium complexes, and (iii) distorted octahedral (O), characterized by a conformational twist of the macrocycle such that it is not possible to define an average plane containing the four nitrogen donors of the cyclam ligand. The molecular structures of all alkoxido and thioalkoxido complexes, as well as those of mono-amido compounds show trigonal prismatic coordination environment around the zirconium. The Zr–N Cyclam bond lengths are consistent with values reported for Zr–N amine
and Zr–N amido
in other Zr(IV) complexes and do not vary significantly with the type of the coligands. Figures 25.1 and 25.2 show ORTEP drawings of the molecular structures of (Bn 2 Cyclam)Zr(OPh) 2 (4) and (Bn 2
t Bu) (9), as examples of what has been described above. The figure captions include selected bond lengths and angles [10a,c]. The molecular structures of complexes with bulky diamido ligands, such as (Bn 2 Cyclam)Zr(NH t Bu)
2 (7) and (Bn 2
2,6-Me Ph)
2 (8), display two elongated Zr–N amine distances triggered by steric constraints. The solid-state structure of 7 and relevant bond distances and angles are depicted in Fig. 25.3. The bonding of two bulky amido ligands leads to capped tetrahedral zirconium complexes. Taking into account the distances between zirconium and the two cyclam amine units, these complexes may be described as masked tetraamido zirconium derivatives. Density functional theroy (DFT) calculations have shown that the change from trigonal prismatic (P) to capped tetrahedral (T) environments in complexes (Bn 2 Cyclam)ZrXY is associated to the angle between the X–Zr–Y plane and the N amido
–Zr–N amido
plane ( α, Fig. 25.4). On going from P to T, the two N amine atoms of the macrocyclic ligand are pushed away from the metal (from 2.51 ˚ A to 2.81 ˚ A) and an increase of the angle α from 55 ◦ to 81
◦ is observed, approaching the value of a perfect tetrahedron (90 ◦ ) [10a]. The molecular structures of (Bn 2 Cyclam)Zr(CH 2 Ph)
2 (12a) and ( 3,5-Me Bn
Cyclam)Zr(CH 2 Ph) 2 (12b) disclose the κ 4
N 2 N’ 2 coordination of the macrocycle to the zirconium, complemented by the bonding of two benzyl ligands. The methyl substituents of the pendant benzyl groups in 12b have no effect on the structural parameters and the molecular structures of both compounds are identical. However, these compounds do differ from all the previously describe zirconium complexes, as the geometry around the metal is best described as distorted octahedral. The molecular structure of 12a reveals that the macrocycle underwent a conformational twist that places one of the cyclam amido nitrogens and one of the benzyl ligands in the octahedron axial positions, defining an angle of 151.29(6) ◦ . The remaining cyclam nitrogens and benzyl ligand occupy a slightly twisted square plane displaying a combined equatorial angle of 364.1 ◦ [10b]. Download 11.05 Mb. Do'stlaringiz bilan baham: 
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