Article Cite This: Organometallics XXXX, XXX, XXX−XXX
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Square-Planar Iron(II) Silyl Complexes: Synthesis, Characterization, and Insertion Reactivity C. Vance Thompson, Hadi D. Arman, and Zachary J. Tonzetich* Department of Chemistry, University of Texas at San Antonio (UTSA), San Antonio, Texas 78249, United States
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S Supporting Information *
ABSTRACT: Treatment of [Fe(BH4)(CyPNP)] (CyPNP = bis(dicyclohexylphosphinomethyl)pyrrole) with 2 or 3 equiv of phosphine afforded the octahedral low-spin iron hydride compounds [FeH(PMe2Ph)(N2)(CyPNP)] and [FeH(PMe3)2(CyPNP)], respectively. These new hydrides, as well as the previously reported [FeMe(CyPNP)], react with silanes to form a new class of squareplanar, intermediate-spin (S = 1) iron silyl complexes, [Fe(SiRPh2)(CyPNP)] (R = H, Me, F). The more sterically encumbered iron hydride [FeH(tBuPNP)] (tBuPNP = bis(di-tertbutylphosphinomethyl)pyrrole) was also employed to isolate [Fe(SiH2Ph)(tBuPNP)] via treatment with PhSiH3. [Fe(SiRPh2)(CyPNP)] exists in an equilibrium with low-spin [Fe(N2)(SiRPh2)(CyPNP)] under an atmosphere of molecular nitrogen, whereas [Fe(SiH2Ph)(tBuPNP)] does not. Introducing an atmosphere of CO to [Fe(SiHPh2)(CyPNP)] yielded the octahedral transdicarbonyl species [Fe(CO)2(SiHPh2)(CyPNP)], but removal of that atmosphere resulted in loss of one CO to afford the square-pyramidal complex [Fe(CO)(SiHPh2)(CyPNP)]. The insertion chemistry of the silyl complexes with unsaturated molecules was examined to shed light on possible reaction pathways relevant to iron-catalyzed hydrofunctionalization protocols involving silanes. The reaction of [Fe(SiHPh2)(CyPNP)] with phenylacetylene was found to generate the square-planar, intermediate-spin vinylsilane complex [Fe(C(SiHPh2)C(H)Ph)(CyPNP)], corresponding to a net 1,1-insertion of the phenylvinylidene tautomer, whereas reaction with 2-butyne afforded the expected 1,2-insertion product [Fe(C(Me)C(SiHPh2)Me)(CyPNP)]. Under similar conditions, [Fe(SiHPh2)(CyPNP)] reacted with p-trifluoromethylbenzaldehyde to generate a new intermediate-spin complex assigned as [Fe(CH(OSiHPh2)Ar)(CyPNP)] (Ar = 4-CF3C6H4), corresponding to a 2,1-insertion of the aldehyde. Finally, the reaction of CO2 with [Fe(SiHPh2)(CyPNP)] resulted in reduction of CO2 to CO and formation of [Fe(CO)(SiHPh2)(CyPNP)]. The stoichiometric reactivity of this new class of iron silyl complexes indicates diverse insertion behavior applicable to iron-mediated hydrosilylation catalysis.
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INTRODUCTION
Other mechanisms have been put forward for transitionmetal-mediated hydrosilylation that feature iron silyl complexes as the active catalyst.20 The first example of an iron silyl compound was synthesized by Wilkinson in 1956, employing the CpFe(CO)2 fragment.21 Unfortunately this compound and many other reported examples of iron silyl species to date feature coordinatively and electronically saturated metal centers, mitigating their usefulness in subsequent chemistry.22−28 Silyls have also been employed in the construction of supporting ligands,29−33 although in this capacity they are not primed for reactivity with incoming substrates. Consequently, there are relatively few examples of the synthesis and application of iron silyl compounds as hydrosilylation catalysts.19,34,35 Further convoluting the role of silyl species in catalysis is the fact that they are often generated by oxidative addition of a Si−H bond to a reduced metal center. In this process a metal hydride is also created, providing for a
The hydrosilylation of unsaturated substrates catalyzed by transition metals has become essential to both the production of silicon-containing materials and organic syntheses.1,2 Precious metals have proven to be especially efficacious as catalysts in these processes. However, the toxicity, scarcity, and high cost of precious metals have created an impetus for the examination of earth-abundant transition-metal replacements.3,4 Among the base metals, iron has become a popular target for catalysis, and examples of methodologies featuring iron have proliferated across a range of hydrofunctionalization protocols.5,6 A number of iron-based systems have also proven to be efficient and selective hydrosilylation catalysts for unsaturated substrates.7−15 The most common pathway invoked for these iron-mediated transformations is the canonical Chalk-Harrod mechanism involving insertion of the unsaturated molecules into the Fe−H bond. As such, iron hydrides have become attractive targets for synthesis and mechanistic examination.16−19 © XXXX American Chemical Society
Received: May 20, 2019
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DOI: 10.1021/acs.organomet.9b00335 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics competitive site of reactivity. Fortunately, silanes are also known to undergo σ-bond metathesis with metal complexes such as hydrides and alkyls, affording a means to synthesize such species in the absence of other potentially reactive moieties.27,36 To this end, the stabilization of discrete, well-defined iron hydride and iron silyl complexes within a common ligand framework would provide a valuable means of scrutinizing their individual reactivities in the context of hydrosilylation. Pincer ligands are ideal supporting frameworks in this regard due to their high kinetic and thermodynamic stability, which makes possible the isolation of these otherwise reactive compounds.37 Pincers have also been employed previously to stabilize a number of different iron hydride species, several of which demonstrate excellent activity toward hydrosilylation.38−46 Our laboratory and others have utilized the anionic pyrrole-based pincer ligand RPNP (RPNP = anion of bis(dialkylphosphinomethyl)pyrrole), to isolate a variety of different first-row transition-metal complexes containing potentially reactive ligands such as hydrides and alkyls.47−54 Herein we describe the synthesis, characterization, and insertion behavior of a new class of square-planar, intermediate-spin iron silyl complexes derived from σ-bond metathesis reactions of both iron hydride and alkyl complexes.
Figure 1. Thermal ellipsoid drawing (50%) of 2. Carbon-bound hydrogens are omitted for clarity. Selected distances (Å) and angles (deg): Fe(1)−H = 1.45(3); Fe(1)−N(1) = 1.969(2); Fe(1)−P(1) = 2.2502(6); Fe(1)−P(2) = 2.399(5); Fe(1)−P(3) = 2.2721(5); Fe(1)−N(2) = 1.792(2); N(2)−N(3) = 1.123(3); P(1)−Fe(1)− P(2) = 153.86(2); N(1)−Fe(1)−N(2) = 174.36(7).
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RESULTS AND DISCUSSION As a route to iron hydride complexes of the CyPNP ligand, the previously reported borohydride species [Fe(BH4)(CyPNP)]48 serves as an effective precursor. We have now obtained its crystallographic structure, which displays the expected κ2 binding of the borohydride ligand (see the Supporting Information).55 We had earlier found that reactions of [Fe(BH4)(CyPNP)] with Lewis bases such as CO and bipyridine provided access to low-spin, octahedral hydride complexes of the type [FeH(CO)2(CyPNP)] and [FeH(bipy)(CyPNP)]. However, such species exhibited little to no insertion reactivity at the Fe−H moiety, likely as a consequence of their coordinative saturation coupled with the tight binding of the CO or bipy ligands. In order to engender more ligand lability, we turned our attention to monodentate phosphines. The reaction of 3 equiv of PMe3 with [Fe(BH4)(CyPNP)] produced [FeH(PMe3)2(CyPNP)] (1) and Me3P·BH3 (see the Supporting Information for the solid-state structure of 1). Samples of 1 were observed to contain small quantities of a second iron hydride species that we assign as [FeH(PMe3)(N2)(CyPNP)]. Accordingly, reaction with the slightly bulkier phosphine PMe2Ph produced the analogous monophosphine−hydride species [FeH(PMe2Ph)(N2)(CyPNP)] (2, eq 1) as the sole product.
on 2 as a good candidate for subsequent reactivity on the basis of the lability of the N2 ligand. As noted above, iron hydrides have become popular targets for hydrosilylation catalysis. In this regard, we sought to probe the reactivity of 2 with different silanes. Addition of PhSiH3 to 2 was found to yield a new intermediate-spin iron species, as judged by 1H NMR spectroscopy, but attempts to purify this species led to decomposition. Switching to the bulkier silane H2SiPh2 resulted in a similar reaction giving a mixture of 2, molecular hydrogen, and a new intermediate-spin iron species, 3. The 1H NMR features of 3 closely resembled those of other four-coordinate iron(II) complexes of CyPNP reported previously,47 pointing to its identity as the iron(II) silyl complex [Fe(SiHPh2)(CyPNP)]. Addition of 10 equiv of Ph2SiH2 to 2 resulted in nearly complete formation of 3, permitting its isolation. Crystallographic analysis of the material revealed it to be the phosphine adduct of the iron(II) silyl, [Fe(SiHPh2)(PMe2Ph)(CyPNP)] (3-PMe2Ph, see Supporting Information), confirming that the reaction of 2 did yield a new iron silyl species. As transition-metal silyl complexes are known to result from the reaction of silanes with the corresponding alkyl species,57 we next targeted [FeMe(CyPNP)] as a potential precursor to the silyl species 3. The methyl complex offers the added advantage that, unlike 2, it lacks a pendant Lewis base, simplifying isolation of the desired four-coordinate silyl. Gratifyingly, treatment of [FeMe(CyPNP)] with Ph2SiH2 led to clean formation of 3 (eq 2) with concomitant release of CH4. Analogous reactions employing the phenyl and benzyl derivatives also yielded 3 with production of the corresponding hydrocarbon. Complex 3 can be isolated from the reaction in eq 2 as a red microcrystalline solid after recrystallization from heptane. Magnetic susceptibility measurements display an apparent μeff value of 3.3(2) μB at room temperature (vide infra), consistent with an intermediate-spin (S = 1) compound. The solid-state structure of 3 is pictured in Figure 2. The compound features a
The solution IR spectrum of 2 displays a strong peak at 2053 cm−1 corresponding to the νNN mode of the coordinated N2 ligand. This relatively high value for the N−N stretch coupled with the short N−N bond length of 1.12 Å determined crystallographically (see Figure 1) suggests relatively weak back-bonding by the iron(II) fragment.56 We therefore focused B
DOI: 10.1021/acs.organomet.9b00335 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
Information and Figure 3). Somewhat surprisingly, coordination of N2 causes a movement of the silyl ligand to the apical
Figure 3. Thermal ellipsoid (35%) rendering of 4·N2. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Fe(1)−N(1) = 1.899(3); Fe(1)−P(1) = 2.265(1); Fe(1)− P(2) = 2.228(1); Fe(1)−Si(1) = 2.268(2); P(1)−Fe(1)−P(2) = 157.72(5); N(1)−Fe(1)−N(2) = 176.1(2). Figure 2. Thermal ellipsoid drawing (50%) of compound 3. Carbonbound hydrogen atoms and the minor components of two disordered cyclohexyl rings are omitted for clarity. Selected bond distances (Å) and angles (deg): Fe(1)−N(1) = 1.925(3); Fe(1)−P(1) = 2.258(1); Fe(1)−P(2) = 2.253(1); Fe(1)−Si(1) = 2.384(1); P(1)−Fe(1)− P(2) = 162.49(4); N(1)−Fe(1)−Si(1) = 172.13(9).
position of the resulting square pyramid. In addition, the Fe−Si bond distance in 3·N2 contracts from that in 3 to a value of 2.268(1) Å. The structure of 3·N2 is intriguing, as it represents a model for the approach of π-acidic substrates to the iron− silyl species. The N2 adduct is diamagnetic and displays NMR features consistent with a change in symmetry from C2v to Cs, in accord with the observed five-coordinate square-planar geometry found in the solid state. A triplet resonance for the silyl ligand was identified at 37.94 ppm (2JSiP = 40.7 Hz) by 29 Si NMR. Variable-temperature 1H NMR experiments with 3 demonstrated that the compound exists in equilibrium with 3·N2 in solution (eq 3). A van ’t Hoff analysis (Table 1) of the
distorted-square-planar geometry about iron (τ4 = 0.18)58 with an Fe−Si distance of 2.384(1) Å. This distance is similar to those reported previously by Deng for pseudo-square-planar iron complexes bearing chelating carbene−silyls (cf. 2.396 and 2.419 Å).59 The compound also resembles a recently reported PNP-pincer-ligated Co(II) silyl described by Lee, which displays a slightly shorter Co−Si distance of 2.302 Å.60 The synthetic method in eq 2 was next applied to other substituted diphenylsilanes. HSiFPh2 was found to react readily with [FeMe(CyPNP)] at room temperature to afford [Fe(SiFPh2)(CyPNP)] (5), whereas HSiMePh2 required elevated temperatures to generate [Fe(SiMePh2)(CyPNP)] (4). These observations are consistent with the steric requirements of the corresponding silanes. Along the same lines, treatment of 2 or [FeMe(CyPNP)] with Ph3SiH for several hours at elevated temperature failed to generate a species of the type [Fe(SiPh3)(CyPNP)], as judged by 1H NMR spectroscopy. Therefore, silanes of sufficient steric bulk appear unable to react in a metathetical fashion to generate the corresponding silyl complex. Solutions of compound 3 display thermochromism in hydrocarbon solvents, appearing red at higher temperatures (>40 °C) and green at lower temperatures (
