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Zn···Zn interactions at nickel and palladium centers.

ABSTRACT: The analogy between ZnR fragments and the hydrogen radical represents a fruitful concept in organometallic synthesis. The organozinc(ii) and -zinc(i) sources ZnMe₂ (Me = methyl) and [Zn₂Cp*₂] (Cp* = pentamethylcyclopentadienyl) provide one-electron fragments ·ZnR (R = Me, Cp*), which can be trapped by transition metal complexes [L _a M], yielding [L _b (ZnR) _n ]. The addition of the dizinc compound [Zn₂Cp*₂] to coordinatively unsaturated [L _a M] by the homolytic cleavage of the Zn-Zn bond can be compared to the classic oxidative addition reaction of H₂, forming dihydride complexes [L _a M(H)₂]. It has also been widely shown that dihydrogen coordinates under preservation of the H-H bond in the case of certain electronic properties of the transition metal fragment. The σ-aromatic triangular clusters [Zn₃Cp*₃]⁺ and [Zn₂CuCp*₃] may be regarded as the first indication of this so far unknown, side-on coordination mode of [Zn₂Cp*₂]. With this background in mind the question arises if a series of complexes featuring the Zn₂M structural motif can be prepared exhibiting a (more or less) intact Zn-Zn interaction, i.e. di-zinc complexes which are analogous to non-classical dihydrogen complexes of the Kubas type. In order to probe this idea, a series of interrelated organozinc nickel and palladium complexes and clusters were synthesized and characterized as model compounds: [Ni(ZnCp*)(ZnMe)(PMe₃)₃] (1), [Ni(ZnCp*)₂(ZnMe)₂(PMe₃)₂] (2), [{Ni(CN ^t Bu)₂(μ₂-ZnCp*)(μ₂-ZnMe)}₂] (3), [Pd(ZnCp*)₄(CN ^t Bu)₂] (4) and [Pd₃Zn₆(PCy₃)₂(Cp*)₄] (5). The dependence of Zn···Zn interactions as a function of the ligand environments and the metal centers was studied. Experimental X-ray crystallographic structural data and DFT calculations support the analogy between dihydrogen and dizinc transition metal complexes.

SUBMITTER: Freitag K

PROVIDER: S-EPMC5355958 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:Here we report that capping the molecule TTFtt (TTFtt = tetrathiafulvalene-2,3,6,7-tetrathiolate) with dialkyl tin groups enables the isolation of a stable series of redox congeners and facile transmetalation to Ni and Pd. TTFtt has been proposed as an attractive building block for molecular materials for two decades as it combines the redox chemistry of TTF and dithiolene units. TTFttH4, however, is inherently unstable and the incorporation of TTFtt units into complexes or materials typically proceeds through the in situ generation of the tetraanion TTFtt4-. Capping of TTFtt4- with Bu2Sn2+ units dramatically improves the stability of the TTFtt moiety and furthermore enables the isolation of a redox series where the TTF core carries the formal charges of 0, +1, and +2. All of these redox congeners show efficient and clean transmetalation to Ni and Pd resulting in an analogous series of bimetallic complexes capped by 1,2-bis(diphenylphosphino)ethane (dppe) ligands. Furthermore, by using the same transmetalation method, we synthesized analogous palladium complexes capped by 1,1'-bis(diphenylphosphino)ferrocene (dppf) which had been previously reported. All of these species have been thoroughly characterized through a systematic survey of chemical and electronic properties by techniques including cyclic voltammetry (CV), ultraviolet-visible-near infrared spectroscopy (UV-vis-NIR), electron paramagnetic resonance spectroscopy (EPR), nuclear magnetic resonance spectroscopy (NMR) and X-ray diffraction (XRD). These detailed synthetic and spectroscopic studies highlight important differences between the transmetalation strategy presented here and previously reported synthetic methods for the installation of TTFtt. In addition, the utility of this stabilization strategy can be illustrated by the observation of unusual TTF radical-radical packing in the solid state and dimerization in the solution state. Theoretical calculations based on variational 2-electron reduced density matrix methods have been used to investigate these unusual interactions and illustrate fundamentally different levels of covalency and overlap depending on the orientations of the TTF cores. Taken together, this work demonstrates that tin-capped TTFtt units are ideal reagents for the installation of redox-tunable TTFtt ligands enabling the generation of entirely new geometric and electronic structures.

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Zn···Zn interactions at nickel and palladium centers.

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