A Highly-Reduced Cobalt Terminal Carbyne: Divergent Metal- and α

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A Highly-Reduced Cobalt Terminal Carbyne: Divergent Metal- and α‑Carbon-Centered Reactivity Charles C. Mokhtarzadeh, Curtis E. Moore, Arnold L. Rheingold, and Joshua S. Figueroa* Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, Mail Code 0358, La Jolla, California 92093-0358, United States

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has been employed by Peters and co-workers for the generation of terminal carbynes of iron.1,5 However, to prevent cluster formation by carbyne-coupling and related processes,28−30 we focused on the reduction of an encumbered organoisocyanide ligand (isolobal to CO) in order to sterically shield a [Cp*Co≡C] unit from intermolecular aggregation. Accordingly, we report here the synthesis, characterization and reactivity of a dianionic cobalt terminal carbyne derived from the m-terphenyl isocyanide CNArTripp2 (ArTripp2 = 2,6-(2,4,6-(iPr)3C6H2)2(C6H3).31 The reduced nature of this terminal carbyne complex creates an electronic environment where Cocentered reactivity dominates its interactions with electrophilic substrates. However, upon reaction with internal alkynes, [2+2] cycloaddition processes are observed that result in the formation of new C−C bonds. Dissolution of the diiodide complex, Cp*CoI2(CNArTripp2) (1), in benzene followed by passage through a short column of KC8 (10 equiv) resulted in the clean formation of a new diamagnetic species as assayed by 1H NMR spectroscopy (Figure 1). Crystallographic analysis of crystals grown from an n-pentane/Et2O mixture revealed the dipotassium salt, [K2(Et2O)2][Cp*Co≡CNArTripp2] ([K2(Et2O)2][2]), which adopts a one-legged piano stool structural arrangement (Figure 1). Notably, complexes featuring a one-legged piano stool motif, are quite rare in the context of [CpRCoLn] coordination chemistry.32 For [K2(Et2O)2][2], this arrangement is accompanied by contact ion pairing between the potassium cations and the flanking triisopropylphenyl rings of the ArTripp2 unit. In addition, [K2(Et2O)2][2] features long interactions between the K+ ions and the carbon and nitrogen atoms of the isocyano unit. However, FTIR analysis in C6D6 solution reflects that the isocyano unit of the CNArTripp2 ligand has undergone a substantial degree of C−N bond reduction. This is indicated by the presence of an exceedingly low-energy νCN band at 1509 cm−1, which is shifted to 1490 cm−1 when employing selectively labeled 13CNArTripp2. Importantly, the νCN in [K2(Et2O)2][2] band is significantly red-shifted from that of free CNArTripp2 (νCN = 2122 cm−1 (C6D6)),31 and is also far lower in energy than the νCN stretches typically found for imines and metal-bound iminoacyls (νC=N ≈ 1850−1700 cm−1).33 The solid-state structure of [K2(Et2O)2][2] possesses several features consistent with a cobalt terminal carbyne formulation. Most notable is a short Co−C1 bond distance of 1.670(3) Å,

ABSTRACT: Reported here is the isolation of a dianionic cobalt terminal carbyne derived from chemical reduction of an encumbering isocyanide ligand. Crystallographic, spectroscopic and computational data reveal that this carbyne possesses a low-valent cobalt center with an extensively filled d-orbital manifold. This electronic character renders the cobalt center the primary site of nucleophilicity upon reaction with protic substrates and silyl electrophiles. However, reactions with internal alkynes result in [2+2] cycloaddition with the carbyne carbon to form a new C−C bond.

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n recent years, there has been increasing attention on the synthesis and properties of metal−carbon multiple bonds of the late 3d transition metals.1−15 This interest has originated from several distinct applications in reactivity, which include olefin cyclopropanation reactions,14,16−18 C−H activation and functionalization processes13 and the development of homogeneous Fischer−Tropsch catalysts.5,19 For the molecular Fischer−Tropsch systems in particular, there has been an emphasis on investigations of metal−carbon multiple bonds of iron and cobalt due to the fact that these metals are the primary components of the most active heterogeneous catalysts.20,21 Whereas mononuclear iron terminal carbenes have been long known,22−24 more recently a number of unique cobalt terminal carbenes have been isolated in a range of formal oxidation states.2−4,9,13 Likewise, new examples of iron terminal carbynes have been reported recently,1,5,11,12 some of which have direct relevance to carbon monoxide (CO) reduction and homologation.5 In this context, however, it is notable that well-defined mononuclear terminal carbynes of cobalt have remained elusive toward isolation, despite the wellestablished structural and reaction chemistry of μ3-carbyne cobalt clusters as Fischer−Tropsch surface models.25,26 As a strategy for accessing a cobalt terminal carbyne, it was reasoned that an axially symmetric electronic environment could provide a straightforward means of supporting a 3-fold Co−C bonding interaction. In analogy to Bergman’s seminal studies on the terminal iridum-imido complex Cp*Ir(NtBu),27 which by orbital symmetry possesses an Ir−N triple bond, we targeted a simple [Cp*Co] fragment for installation of a carbyne unit due to the accessibility of a- and e-symmetry orbitals. In addition, we deemed that reduction of a carbondonor ligand, such as CO, may provide a successful synthetic route to a heteroatom-containing cobalt carbyne. This strategy © XXXX American Chemical Society

Received: May 13, 2018 Published: June 15, 2018 A

DOI: 10.1021/jacs.8b05019 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society

Scheme 1. Reactivity Profile of Carbyne [K2(Et2O)2][2]

Figure 1. Synthesis and molecular structure of carbyne [K2(Et2O)2][2].

carbene (d(Co=C)= 1.740(1) Å).2 In addition, the Co−C1 bond distance in [K2(Et2O)2][2] also compares well with structurally characterized iron terminal carbynes, which have Fe≡C bond distances ranging between 1.639 and 1.734 Å.1,5,11,12,34 Importantly, [K2(Et2O)2][2] possesses a nearly linear Cp(centroid)−Co−C1 angle (175.0(5)°). This feature is similar to the Cp(centroid)−Ir−N angle found in Bergman-type iridium-imido complexes (i.e., Cp*Ir(NR)),27,35 thereby strongly indicating that an appropriate orbital symmetry environment for a σ+2π Co≡C bonding interaction is present. In fact, it is important to note that [K2(Et2O)2][2] gives rise to a broad singlet at δ = 181.0 ppm in its 13C{1H} NMR spectrum, which is assigned to the Ccarbyne atom based on 13C labeling. This Ccarbyne chemical shift is upfield of those reported for Fe terminal carbyne complexes (i.e., δ(Ccarbyne) = 225−280 ppm),1,5,11,12,34 which we tentatively attribute to a shielding effect induced by the axially symmetric 3-fold bonding environment within [K2(Et2O)2][2]. While a Co≡C bonding interaction can imply a depletion of electron density around the Co center, there are additional structural characteristics of [K2(Et2O)2][2] that suggest it is best formulated as a reduced, low-valent carbyne complex. Most instructive is the comparison between [K2(Et2O)2][2] and Fischer’s original iron terminal carbyne cation, [Fe(≡CN(i-Pr)2)(CO)3(PPh3)]+,34 which likewise possesses a strongly π-donating nitrogen-based substituent on the carbyne carbon. It has been well established that N-donor substituents of aminocarbynes (i.e., M≡C−NR2) can elongate M≡C distances through N → Ccarbyne π-donor interactions.12,34 For [Fe(≡CN(i-Pr)2)(CO)3(PPh3)]+, the relatively long Fe≡C (1.734(6) Å) and short Ccarbyne−N (1.266(7) Å) bond distances have been rationalized as a result of this effect. However, for [K2(Et2O)2][2], the Ccarbyne−N bond distance (d(N1−C1) = 1.307(3) Å; Figure 1) is longer than that found in [Fe(≡CN(i-Pr)2)(CO)3(PPh3)]+, despite the fact that the [NArTripp2] group can be viewed as an anionic amido-type substituent (i.e., [M≡C− NR]−)). Furthermore, the bond distance between the nitrogen

Figure 2. Resonance forms of dianion [2]2− (top) and DFTcalculated molecular orbitals for the model [2m]2− (bottom). Note that the lower Co character in HOMO-2 relative to HOMO-3 is accompanied by delocalization of electron density over the aryl ring of the carbyne unit. This feature is further indicative of the contribution from the azabenzallyl resonance form A.

thereby signifying the presence of considerable Co−C multiple bonding. This Co−C distance is significantly contracted relative to the structurally characterized difluorocarbene complex, CpCo(=CF2)(PPh3), which currently possesses the shortest reported Co−C separation for a cobalt terminal B

DOI: 10.1021/jacs.8b05019 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

Journal of the American Chemical Society

Figure 3. Molecular structures of [K(Et2O)][3] (A), [K(Et2O)][4] (B) and the anionic component of [K(Et2O)][5] (C). For [5]−, some isopropyl groups have been omitted for clarity.

addition, HOMO-2 and HOMO-3 possess significant Co atomic d-orbital character (Figure 2). This suggests that a Fischer-type metal−carbon multiple-bonding scenario, with significant π-backdonation from Co to empty C-based atomic p-orbitals, is a reasonable electronic description for this system. Indeed, the calculations on [2m]2− also reveal a low-lying Co− Ccarbyne σ-interaction with predominant carbon character (HOMO-15; Figure 2). The presence of such a molecular orbital is expected for a Fischer-type carbyne, where strong σdonation from an ostensible carbon-based lone pair is an important component to the bonding framework. Notably, a similar bonding description has been proposed by Peters for the iron alkoxycarbyne (SiPi‑Pr3)Fe≡COSiMe3 (SiPi‑Pr3 = [Si(oC6H4P(i-Pr2))3]−, for which Mössbauer and computational data indicated a Fischer-type interaction between a low-valent (d8 or d10) Fe center and a [C−O−SiMe3]+ carbyne fragment.1 Accordingly, we believe that the electronic structure of [K2(Et2O)2][2] finds analogy with this low-valent iron terminal carbyne and likewise possess an extensively filled dorbital manifold. Consistent with its formulation as a low-valent terminal carbyne complex, a survey of the chemistry available to [K2(Et2O)2][2] with terminal alkynes and silyl electrophiles demonstrated that its Co center possesses significant nucleophilicity. For example, treatment of [K2(Et2O)2][2] with 1.05 equiv of trimethylsilylacetylene (HCCSiMe3) selectively generates the terminal monohydride salt [K(Et2O)][Cp*Co(H)(CNArTripp2)]] ([K(Et2O)][3], Scheme 1). Crystallographic characterization of [K(Et2O)][3] revealed a classic two-legged piano stool structural motif (Figure 3), while 1H NMR spectroscopic analysis in C6D6 solution revealed a sharp singlet at −14.65 ppm assignable to a Co− H unit. This result indicates that simple Co-based protonation is the preferential mode of action with this terminal acetylene. In addition, protonation of the Co center in [K2(Et2O)2][2], rather than the carbyne N atom, further reflects that the [C− N−ArTripp2]− unit is stabilized in an azabenzallyl form and is therefore not the primary site of nucleophilicity. More importantly, monohydride [K(Et2O)][3] gives rise to a νCN stretch of 1710 cm−1, which is consistent with an isocyanide,36 rather than a carbyne formulation for the CNArTripp2 group, and reflects that electronic reorganization accompanies formal oxidation of the Co center. Notably, [K(Et2O)][3] can be considered as a hydride (H−) adduct of the formally Co(I), terminal-isocyanide fragment, Cp*Co(CNArTripp2), and as such would be expected to provide significant π-backbonding interaction to a π-acidic ancillary ligand. Indeed, [K(Et2O)][3] gives rise to a downfield

Scheme 2. Proposed Mechanism for the Formation of Vinyliminacyl [K(Et2O)][5]

atom and the m-terphenyl-ipso carbon in [K2(Et2O)2][2] (d(N1−C2) = 1.372(3) Å), is shorter than that expected for an N−C single bond, while the central aryl ring of the mterphenyl group possesses metrical parameters indicative of dearomatization (Figure 1). These features reflect that N → C π-donation is significant in the direction opposite of the Co≡Ccarbyne interaction, and strongly suggest the presence of a reduced Co center in [K2(Et2O)2][2] that resists further accumulation of electron density. Indeed, of the resonance forms that may contribute to the electronic structure of dianion [2]2−, we postulate the azabenzallyl form A in Figure 2 is most consistent with the solid-state structure of [K2(Et2O)2][2]. DFT calculations on the model complex [Cp*Co≡CNXyl]2− ([2m]2−; Xyl = 2,6-Me2C6H3) also support the notion of a 3-fold Co≡C bonding interaction in a low-valent environment. As shown in Figure 2, the calculated HOMO and HOMO-1 for [2m]2− are a near-degenerate set of Co-based dx2-y2 and dxy orbitals. The HOMO-2 and HOMO-3 are also nearly degenerate and correspond to a set of Co−C π-bonding molecular orbitals with Co-based dxz and dyz parentage. The presence of these latter two molecular orbitals indicates that a resonance form such as C in Figure 2, which features only one Co−C π-bonding interaction, does not adequately reflect the Co−C multiple-bonding environment in this complex. In C

DOI: 10.1021/jacs.8b05019 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

Journal of the American Chemical Society isocyanide 13C{1H} chemical shift of δ = 207.5 pm, which is consistent with other isocyanide complexes36−39 featuring substantial M → π*(CN) backbonding and further punctuates the upfield Ccarbyne 13C{1H} chemical shift of [K2(Et2O)2][2] when an axially symmetrical bonding environment is present in this system. More generally, Co-centered nucleophilicity and carbyne electronic reorganization also result upon the addition of other electrophilic substrates. Accordingly, treatment of [K2(Et2O)2][2] with trimethylsilyl chloride (Me3SiCl) results in the anionic cobalt-silyl complex, [K(Et2O)][Cp*Co(SiMe3)(CNArTripp2)]] ([K(Et2O)][4]; Figure 3). Similar to monohydride [K(Et2O)][3], [K(Et2O)][4] gives rise to a νCN stretch of 1707 cm−1 and a 13C{1H} chemical shift of 209.3 ppm, which are both indicative significant Co-to-isocyanide πbackbonding from a formally Co(I) center bound by a terminal [SiMe3]− anion. Whereas Co-centered reactivity is predominant with the substrates outlined above, it is more remarkable that internal alkynes engage the carbyne carbon of [K2(Et2O)2][2] to establish a new C−C bond. Treatment of [K2(Et2O)2][2] with diphenylacetylene in benzene solution generates the monopotassium, vinyliminacyl40,41 salt [K(Et2O)][5], as determined by X-ray diffraction (Scheme 1, Figure 3). Complex [K(Et2O)][5] can be viewed as a [2+2] cycloaddition product of diphenylacetylene to the Co≡C unit in [K2(Et2O)2][2], concomitant with α-carbon protonation of the resulting metallacycle. When this reaction is performed with 0.5 equiv of PhCCPh in C6D6 solution, [K2(Et2O)2][2] is completely consumed and the protio isotopomer of [K(Et2O)][5] is produced. This result signifies that a highly Brønsted-basic carbanion intermediate is produced during the transformation, which can deprotonate a sacrificial equivalent of [K2(Et2O)2][2]. Accordingly, we currently hypothesize that a mechanism such as that shown in Scheme 2, where metallacycle formation results in charge localization at the former alkyne carbon atom, may plausibly account for the formation of [K(Et2O)][5]. Most importantly, however, the generation of [K(Et2O)][5] indicates that 2e− [2+2] alkyne-metathesis-type reactivity may be accessible to a low-valent Co-carbyne such as [K2(Et2O)2][2].42 This reactivity pattern may be especially promising if charge localization on metallacycle carbon atoms can be mitigated. We are currently investigating alternative substrates and conditions with the aim of meeting this goal. However, we postulate the results established for [K2(Et2O)2][2] suggest that low-valent late 3d-metal carbynes may enable new bondformation reactions in a manner distinct from the 1e− reaction pathways associated with high-valent 3d metal complexes featuring multiple bonds to carbon.7,9,14,15

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the U.S. National Science Foundation for support of this work (CHE-1464978) and a Graduate Research Fellowship to C.C.M.

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b05019. Synthetic procedures, characterization and spectroscopic data (PDF) Crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*jsfi[email protected] ORCID

Joshua S. Figueroa: 0000-0003-2099-5984 D

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DOI: 10.1021/jacs.8b05019 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX