A glimpse into the chemical reactivity of the unsaturated hydride [MoWCp2(H)(μ-PCy2)(CO)2]
Graphical abstract
Synopsis
The heterometallic hydride [MoWCp2(H)(μ-PCy2)(CO)2] (1) displays a rich and varied chemical behavior derived from its unsaturated nature, this allowing it to react with small organic molecules containing C−C, C−O, C−N or N−N multiple bonds, such as alkynes, aldehydes, isonitriles and diazo compounds. In most of these reactions it behaves similarly to its W2 analogue, yet some genuine heterometallic effects have also been observed.
Pictogram for graphical abstract
Introduction
Transition-metal complexes having two different metal atoms (heterometallic complexes) constitute a family of compounds that have attracted substantial interest over the years, due to their potential to promote novel reactions which depart significantly from those of their corresponding homometallic analogues, thanks to the presence of synergistic effects enabled by the unique electronic and coordinative preferences of different metals in close proximity [1]. In fact, nature has been exploiting such effects to accomplish many relevant biological processes, which are then effectively catalyzed by enzymes having heterometallic active centers (ie. Cu−Mo, Cu−Fe, Mo−Fe, Ni−Fe, Fe−Zn, etc.) [2]. As commonly found for homometallic systems, the presence of metal-metal multiple bonds in heterometallic compounds is a natural way to further enhance the chemical reactivity of these complexes [3], [4], [5], [6], [7] and, not surprisingly, this approach has been successfully exploited for the development of new catalytic processes in which the combination of both, electronic unsaturation (multiple bond) and bond polarity (heterometallic bond) are key enabling factors [5,8]. However, the current knowledge of the chemical behavior of these complexes is rather limited (especially for organometallic compounds), mainly due to the synthetic difficulties associated to their preparation or due to the low stability of most of these compounds. In this area of work, our group recently reported the synthesis of the heterometallic anion [MoWCp2(μ-PCy2)(μ-CO)2]−, which reacted rapidly with different electrophiles to give new Mo−W heterometallic complexes having double and triple metal-metal bonds in combination with alkyl, hydride, or carbyne ligands [9]. Amongst all these new compounds, stands out the heterometallic hydride [MoWCp2(H)(μ-PCy2)(CO)2] (1) (Scheme 1), which in solution exists as an equilibrium mixture of two isomers displaying either a bridging (1B) or a terminal (1T) hydride ligand, in a compound also featuring a metal-metal triple bond [10,11]. Although this compound is structurally related to the corresponding homometallic hydrides [Mo2Cp2(μ-H)(μ-PCy2)(CO)2] (bridging isomer only) and [W2Cp2(H)(μ-PCy2)(CO)2] (bridging and terminal isomers), at the time we identified a genuine heterometallic structural effect, as in solution compound 1 displays a prevalence of the terminal isomer when compared to both homometallic analogues. Furthermore, DFT calculations allowed us to estimate the existence of a ca. 20 kJ·mol−1 thermodynamic preference for the terminal coordination of the hydride ligand at the W site (vs. Mo) [9]. The consequences of all these structural features on the chemical reactivity of this compound are yet to be explored even if, as nicely exemplified by all the chemistry developed around its homometallic Mo2 and W2 analogues [[12], [13], [14], [15], [16]], the combined presence of a metal-metal triple bond and a hydride ligand in 1 is expected to confer great synthetic potential to this compound. In addition, the above-mentioned studies on the homometallic complexes revealed a remarkable influence of the metal atom (Mo vs. W) on the chemical behavior of these unsaturated compounds. For instance, while the protonation of the Mo2 hydride led to complex mixtures of uncharacterized products, the protonation of the W2 compound led to the formation of new cationic dihydrides which remained stable in the absence of coordinating anions [13]. A comparable situation was also found in the reactions with diazoalkanes, which only for the W2 system led to new isolable compounds having N-bound diazoalkanes [14], while the Mo2 hydride just catalyzed the decomposition of the organic reagent [15]. Finally, the reactions with some isocyanides and alkynes were also substantially different, as the W2 compound displayed an uncommon preference to incorporate two molecules of the reagent (rather than one for the Mo2 compound), then leading to the formation of products following from C−N or C−C coupling processes [15,16]. It was then clear that compound 1 was an ideal substrate to further explore the chemical effects of the nature of the metals present in the dimetallic center of this family of unsaturated hydrides. Although similar comparative studies between homometallic and heterometallic complexes for related unsaturated hydrides are extremely scarce, we can quote here the comparative study of Suzuki et al. on the reactivity of the 30-electron hydride [Cp*Ru(μ-H)4OsCp*], which in fact undergoes addition reactions much faster than its homometallic (Ru2 or Os2) analogues, then pointing to the synthetic utility of the heterometallic compound [17]. In this paper we report the reactions of the unsaturated hydride 1 toward some small organic molecules containing C−C, C−O, C−N or N−N multiple bonds, such as alkynes, aldehydes, isonitriles and diazo compounds. These reactions provide a glimpse of the wide synthetic potential of this hydride complex for the preparation of new heterometallic compounds having a wide variety of functional groups, while also pinpointing some genuine heterometallic effects even if the overall chemical behavior of this heterometallic complex rather resembles the known chemistry of its W2 analogue.
Section snippets
Reactions of compound 1 with isocyanides
Isocyanides give addition products easily in their reactions with binuclear unsaturated hydrides, although in most cases the initial addition products evolve through insertion processes to finally give complexes with formimidoyl ligands. The latter ligands commonly adopt a 3-electron donor bridging coordination (A in Chart 1), as found in the reactions of the hydrides [Os3(μ-H)2(CO)10] [18], [Re2(μ-H)2(CO)6(μ-L2)] [19], [Mn2(μ-H)2(CO)6(μ-L2)] [20] and [Mo2Cp2(μ-H)(μ-PPh2)(μ-SMe)2] [21], with
Conclusions
The heterometallic hydride [MoWCp2(H)(μ-PCy2)(CO)2] (1) displays a rich and varied chemical behavior derived from its unsaturated nature, as it is the case of their homometallic analogues. In reactions with isocyanides it behaves essentially in a similar way to the W2 analogue, it being able to reproduce uncommon C−N couplings between formimidoyl and isocyanide ligands for the CN(4-C6H4OMe) derivatives. Yet in these reactions we have also identified some genuine heterometallic effects: i) the
Experimental
All manipulations and reactions were carried out under an argon (99.995%) atmosphere using standard Schlenk techniques. Solvents were purified according to literature procedures, and distilled prior to use [43]. Compound 1 was prepared as described previously [9]. Petroleum ether refers to that fraction distilling in the range 338−343 K. Filtrations were carried out through diatomaceous earth unless otherwise stated. Chromatographic separations were carried out using jacketed columns cooled by
Appendix A. Supplementary data
An XYZ file containing the Cartesian coordinates for all computed species. CCDC 2041372 contains the supplementary crystallographic data for compound 10; these data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We thank the MICINN of Spain and FEDER for financial support (Projects CTQ2015-63726-P and PGC2018-097366-B-I00) and a grant (to E. H.), the SCBI of the Universidad de Málaga and the CMC of the Universidad de Oviedo for access to computing facilities, and the X-Ray unit of the Universidad de Oviedo for acquisition of diffraction data.
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