CoIII(cyclam) Oligoynyls: Monomeric ... - ACS Publications

Apr 29, 2016 - counterion were omitted for clarity. Figure 2. ORTEP plot of [1b]+ at the 30% probability level. Hydrogen atoms, solvent molecules, and...
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CoIII(cyclam) Oligoynyls: Monomeric Oligoynyl Complexes and Dimeric Complexes with an Oligoyn-diyl Bridge Timothy D. Cook, Sean N. Natoli, Phillip E. Fanwick, and Tong Ren* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States S Supporting Information *

ABSTRACT: Described in this work are the preparation and characterization of an extensive family of CoIII(cyclam)-oligoynyl compounds (cyclam = 1,4,8,11-tetraazacyclotetradecane) and elucidation of their electronic structures through DFT calculations. Monomeric Co compounds bearing one oligoynyl, namely, [Co(cyclam)(C2nR)Cl]+ with n = 1−3, and two butadiynyls [Co(cyclam)(C4H)2]+ were prepared from the reactions between [Co(cyclam)Cl2]Cl, Et3N or Et2NH, and the corresponding alkynes. The oligoyndiyl-bridged (μ-C2m) dimers of CoIII(cyclam)Cl with m = 2 and 3 were prepared via the same process by varying alkyne stoichiometry, while those with m = 4 and 6 were prepared using the Glaser coupling reaction. The complexes were prepared in moderate yield (with the exception of m = 6) under mild conditions without requiring an anaerobic or anhydrous environment and are generally stable toward ambient atmosphere. Voltammetric analysis of the dimeric complexes revealed a weak Co−Co interaction through the bridge, which is attenuated by the length of the oligoyne. The orbital origin of the Co−Co interaction is rationalized through DFT analysis.

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ince the early works by Nast1 and Hagihara2 nearly four decades ago, there has been extensive exploration of metal alkynyl compounds and their potential use as molecular charge transfer “wires”.3,4 The potential applications of metal alkynyl compounds have since diversified from linear wire-like charge delivery to other electronic domains such as memory devices,5 photovoltaics, and optical power limiting materials.6 While oligomeric metalla-oligoynes remain the true targets for such applications, discrete synthetic surrogates [M]−C2n−[M] allow for more convenient investigation of the desired properties and obviate the complex construction of oligomeric assemblies. As such, a myriad of [M]−C2n−[M] species have been produced with the intent to study charge transfer between the metal end groups, mediated through the oligoyndiyl bridge. Significant through-bridge communication has been demonstrated for [M] = Fe,7 Mn,8 Ru,9 Ru2,10 W,11 and Re.12 While there are abundant examples of [M]−C2n−[M] systems based on 4d or 5d metals, systems based on redoxactive 3d metals are dominated by Fe capping groups,13 with examples of other 3d metal end groups remaining sparse. Our interest has recently shifted from metal alkynyl chemistry based on diruthenium14 to 3d metal complexes based on the M(cyclam) unit (cyclam = 1,4,8,11-tetraazacyclotetradecane)15 with the successful synthesis of new mononuclear Cr(III),16 Fe(III),17 Ni(II),18 and Co(III)19,20 species, including complexes of scarcely studied cross-conjugated alkynes.21 These contributions, along with works from the laboratories of Wagenknecht,22,23 Nishijo,24 Shores,25 and Berben/Long,26 have significantly expanded this previously underexplored © XXXX American Chemical Society

frontier of 3d metal alkynyl chemistry. Besides our contribution, [M]−C2n−[M] complexes containing 3d metal end groups have been constructed only for M = Fe,7,27 Mn,8 and Cr,28 and each of these species requires anaerobic synethetic procedures. In this contribution, the versatility of aerobic Co(cyclam) alkynylation is demonstrated, achieving monomeric mono- and bis-acetylides as well as dimeric monoacetylides under ambient laboratory conditions.

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RESULTS AND DISCUSSION As shown in Scheme 1, [Co(cyclam)Cl2]Cl allows facile access to a variety of alkynyl complexes through use of mild reagents. Previously, the only known oligoyne-bridged dimer of Co(cyclam) was prepared in a multistep reaction using nBuLi and Me3SiC4SiMe3 to prepare the highly sensitive LiC4Li in situ. The approach outlined in Scheme 1 necessitates only mild alkylamine bases, and in some cases CuCl, to achieve a diverse array of transformations under ambient laboratory conditions. Mononuclear and dinuclear Co(cyclam) complexes can be constructed with this approach by careful consideration of the reaction stoichiometry, as was elegantly demonstrated by Shores and co-workers in the construction of both monoand dinuclear complexes of CoIII(cyclam) with para-diethynylbenzene.25 Complexes [1c]Cl and [1d]Cl were synthesized under essentially the same conditions as their dimeric analogues Received: March 18, 2016

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DOI: 10.1021/acs.organomet.6b00219 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 1. Synthesis of [1a−d]Cl and [2a−d]Cl2a

a Conditions: (i) 0.5 equiv of Me3SiC2mSiMe3, Et3N, MeOH/THF, reflux 24 h; (ii) n = 1: R = Si(iPr)3, 1.1 equiv of HC2Si(iPr)3, Et3N, MeOH, reflux 24 h; n = 2: R = H, 1.9 equiv of Me3SiC4SiMe3, Et3N, MeOH/THF, reflux 24 h; n = 3: R = H, 1.3 equiv of Me3SiC6SiMe3, Et3N, MeOH/THF, reflux 0.5 h; (iii) CuCl/TMEDA, O2, MeOH/acetone, 4 h; (iv) 4.7 equiv of Me3SiC4SiMe3, Et2NH, MeOH/THF, reflux 24 h.

oligoyne length (e.g., Me3SiC8SiMe3). Monomeric complexes [1c]Cl and [1d]Cl were employed as starting materials in a Glaser coupling strategy,29 circumventing the issue of handling longer alkyne precursors directly. Glaser coupling of [1c]Cl and [1d]Cl under the Hay conditions indeed led to the formation of the dimeric μ-C8 and μ-C12 complexes (2c[Cl2] and 2d[Cl2]), respectively. While 2c[Cl2] was isolated in reasonable cumulative yield (40% based on [Co(cyclam)Cl2]Cl), the yield of 2d[Cl2] was precipitously lower (11%). This is attributed to the reactivity and instability of the precursor [1d]Cl. As a solid, [1d]Cl degrades from a light orange powder to a sticky brown solid overnight. Compound [1d]Cl also degrades in concentrated solutions, such as when solvent is almost completely removed under reduced pressure. As such, rather than isolating [1d]Cl for the Glaser coupling to yield 2d[Cl2], the filtrate from silica purification is used directly to prevent degradation of the [1d]Cl precursor. Similar instability has been reported for Pd-octatetraynyl monomeric monoalkynyls, with rapid degradation occurring in solution to give an intractable, tarry black material.30 Absorption spectra of all of the complexes contain weak d−d bands between 400 and 500 nm, as well as strong LMCT bands in the UV region, as expected (see Supporting Information). No photoluminescence behavior was observed for any of these complexes. X-ray quality crystals were readily obtained for all complexes through slow diffusion of diethyl ether ([1a−d]Cl, [2a]Cl2, [2b]Cl2), ethyl acetate ([2c]Cl2), or a 1:1 mixture of the two ([2d]Cl2) into a concentrated methanolic solution of the corresponding complex. The structures of the monomeric Co(cyclam) alkynyls [1a−d]+ are shown in Figures 1−4, and the selected geometric parameters are collected in Table 1. Monoalkynyls [1a]+, [1c]+, and [1d]+ have nearly linear Cl− Co−C bond angles, which bisect the equatorial plane constituted by the coordinating amine groups of the cyclam ligand, confirming the expected octahedral coordination geometry. Complexes [1a]+, [1c]+, and [1d]+ have relatively short Co−C bond lengths (1.871(2), 1.898(2), and 1.886(2) Å, respectively), and their unit cells consist of one crystallographically independent molecule. On the other hand, there are two independent molecules in the unit cell of [1b]+, with each possessing an inversion center bisecting the Co(cyclam) unit.

([2a]Cl2 and [2b]Cl2), with the caveat that a slight excess of alkyne is used in preparation of the monomers. The monoC2SiiPr3 derivative [1a]Cl can also be prepared through this route, avoiding the mixture of terminally protected and deprotected products yielded by reaction with HC2SiMe3. When a large excess of Me3SiC4SiMe3 (5 equiv) was utilized in a preliminary attempt to produce [1c]Cl, it was found that while [1c]Cl and a slight amount of [2a]Cl2 were present in solution, the product distribution appeared to be dominated by the monomeric bis-C4H derivative [1b]Cl. The yield of [1b]Cl is increased when Et2NH is used rather than Et3N, suggesting higher reactivity of Et2NH toward alkynylation of the Co(cyclam) center. This is a synthetically unique result, as all previously known bis-alkynyls of Co(cyclam) required the use of lithio-alkynyls to introduce a second alkynyl ligand to the Co center. Oddly, reaction with a large excess of HC2SiiPr3 did not yield the corresponding bis-C2SiiPr3 product, nor did reaction with excess Me3SiC6SiMe3 or HC2Ph produce the corresponding bis-alkynyls. While [1c]Cl and [1d]Cl are interesting for their potential use in constructing longer μ-C2n dimeric complexes (namely, n = 4, 6), [1a]Cl and [1b]Cl do not have this utility but are useful nonetheless as points of comparison to the other monomeric complexes, and [1b]Cl is an interesting candidate as a precursor to forming a trimeric complex. The shorter dimers, namely, the μ-C4 and μ-C6 complexes ([2a]Cl2 and [2b]Cl2), were prepared directly from [Co(cyclam)Cl2]Cl and Me3SiC2nSiMe3 in a one-pot deprotection/ metalation reaction in reasonable yield (57% and 34%, respectively), and high purity was achieved after silica gel purification and recrystallization. Unfortunately, the analogous μ-C2 complex could not be prepared in the same manner. Reactions between [Co(cyclam)Cl2]Cl and Me3SiC2SiMe3 or HC2SiMe3 led to a mixture of [Co(cyclam)(C2SiMe3)Cl]Cl, [Co(cyclam)(C2H)Cl]Cl, and unreacted [Co(cyclam)Cl2]Cl. The absence of a μ-C2 product is likely attributed to the steric repulsion that such a short bridging diynyl would impose upon the Co(cyclam) units. The synthesis of dimeric complexes with longer bridging oligoyn-diyl requires a slightly different strategy, as Me3SiC2nSiMe3 precursors become less stable with increasing B

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oligoynyl complexes [1b−d]+ displays the same bond length alternation phenomena as their dimeric analogues, with the C− C and CC bond lengths falling in the expected range for single and triple bonds, respectively. There is no indication of cumulenic character in the oligoyne moieties for any of the monomeric or dimeric complexes. The molecular structure of [2a]Cl2 (Figure 5) verifies the expected connectivity, wherein the butadiyndiyl moiety bridges the two Co centers in trans position to the Cl ligands. The Co centers clearly exhibit pseudo-octahedral coordination geometry, with four nitrogens of cyclam constituting the equatorial plane. The carbon−carbon distances in the butadiyndiyl moiety are consistent with those previously observed for [{Co(cyclam)(C2Ph)}2(μ-C4)]Cl2,19 with C−C and CC bond lengths falling in the expected range for single and triple bonds, respectively. This suggests that the shift in donation by changing axial ligands from −C2Ph to −Cl does not promote an increased contribution of the cumulenic or carbynic resonance forms of the bridging alkynyl.3 Structural features similar to those of [2a]2+ are observed for [2b−d]2+ (Figures 6−8, Table 2) with the oligoyndiyl moiety exhibiting bond distances characteristic of the canonical acetylenic resonance form. While the two Co centers in [2a]2+ and [2c]2+ are crystallographically independent, [2b]2+ and [2d]2+ possess an inversion center bisecting the oligoyne bridge, and there is only one independent Co. Notably, each of the complexes [2a−d]2+ contains shorter Co−C bonds (1.884(3), 1.873(3), 1.886(9), and 1.866(6) Å, respectively) than were observed for [{Co(cyclam)(C2Ph)}2(μ-C4)]Cl2 (1.95(7) Å). The shortening of Co−C bonds is the result of trans-influence: Cl− is a significantly weaker π donor than phenylacetylide, which enables a much stronger trans-Co−C bond. All of the structures exhibit sigmoidal curvature along the oligoyne bridge rather than concave, which was also observed for [{Co(cyclam)(C2Ph)}2(μ-C4)]Cl2. The cyclic voltammograms of [1a−c]Cl, shown in Figure 9, display basic trends common among Co(cyclam) mono- and bis-alkynyls that have been studied thus far. Complex [1a]Cl has an irreversible first reduction (A, see Scheme 2) and an essentially irreversible second reduction (B), although B displays slight current on the return sweep. These two peaks represent the reduction of CoIII to CoII, followed by further reduction of CoII to CoI. Complex [1c]Cl also displays an irreversible first reduction and a completely reversible second reduction. Both reductions are shifted anodically with respect to [1a]Cl, accentuating the electronic influence of an additional CC unit in the alkynyl ligand of [1c]Cl. While irreversible A waves for [1a]Cl and [1c]Cl can be explained as the dissociation of the weakly coordinating axial chloride ligand, the difference in reversibility for B is somewhat unexpected. It is possible that the C2SiiPr3 ligand dissociates upon reduction to CoI in [1a]Cl, while the more electron deficient butadiynyl in [1c]Cl remains bound to the Co center. Another possibility is cleavage of the −SiiPr3 unit upon reduction to CoI. Complex [1b]Cl highlights the difference in the electrochemical profile of Co(cyclam) bis-alkynyls relative to mono-alkynyls. In comparison with its mono-alkynyl counterpart [1c]Cl, the voltammogram of [1b]Cl displays only a single, reversible reduction. Unlike the monoalkynyls, wave A for complex [1b]Cl has a current magnitude consistent with a 2 e− reduction from CoIII directly to CoI. Wave A is also reversible, compared to the completely irreversible A wave of [1a]Cl and [1c]Cl, further supporting that cleavage of the Co−Cl bond is

Figure 1. ORTEP plot of [1a]+ at the 30% probability level. Hydrogen atoms, solvent molecules, and Cl− counterion were omitted for clarity.

Figure 2. ORTEP plot of [1b]+ at the 30% probability level. Hydrogen atoms, solvent molecules, and Cl− counterion were omitted for clarity.

Figure 3. ORTEP plot of [1c]+ at the 30% probability level. Hydrogen atoms, solvent molecules, and Cl− counterion were omitted for clarity.

Figure 4. ORTEP plot of [1d]+ at the 30% probability level. Hydrogen atoms, solvent molecules, and Cl− counterion were omitted for clarity.

Consequently, the C−Co−C′ bond angle for [1b]+ is exactly 180.0°. Complex [1b]+ also demonstrates the trans-influence that has been established for Co(cyclam) alkynyls (vide inf ra), with a significantly longer Co−C bond (1.926(3) Å) than that of the monobutadiynyl species 1c. Each of the monomeric C

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Organometallics Table 1. Selected Bond Lengths (Å) and Bond Angles (deg) for 1a+−1d+ [1a]+

[1b]+

Co1−N1 Co1−N2 Co1−N3 Co1−N4 Co1−Cl1 Co1−C1 C1−C2 C2−Si1

1.9741(18) 1.9816(18) 1.9715(18) 1.9727(18) 2.3426(6) 1.871(2) 1.207(3) 1.835(2)

C11−Co1−C1

179.63(7)

[1c]+

Co1−N11 Co1−N12

1.983(2) 1.981(2)

Co1−C11 C11−C12 C12−C13 C13−C14

C11−Co1−C11′

[1d]+

1.926(3) 1.222(4) 1.372(4) 1.195(4)

Co1−N1 Co1−N2 Co1−N3 Co1−N4 Co1−Cl1 Co1−C1 C1−C2 C2−C3 C3−C4

1.9726(17) 1.9772(19) 1.9924(18) 1.9710(18) 2.2868(6) 1.898(2) 1.201(3) 1.385(3) 1.190(3)

180.00(10)

C11−Co1−C1

177.71(6)

Co1−N1 Co1−N2 Co1−N3 Co1−N4 Co1−Cl1 Co1−C1 C1−C2 C2−C3 C3−C4 C4−C5 C5−C6 Cl1−Co1−C1

1.9722(18) 1.9793(18) 1.9687(17) 1.9770(17) 2.3106(6) 1.886(2) 1.201(3) 1.374(3) 1.205(3) 1.380(3) 1.199(4) 179.55(7)

of Co−Co interaction, which was traced to the localization of frontier orbitals (namely, HOMO and HOMO−1) on the peripheral phenylacetylene moieties instead of the oligoyndiyl bridge.19 It was hypothesized that replacing the two axial phenylethynyl ligands with a weak π donor such as chloride would reduce electron localization to the terminal axial ligands and enhance the contribution of the bridging oligoyndiyl in the frontier orbitals, and hence strengthen Co−Co interaction. Given the insoluble nature of [2b-d]Cl2 in aprotic solvents and the narrow electrochemical potential window of methanol, counteranion metathesis was performed with an excess of the appropriate Na salt in MeOH to precipitate [2b](BPh4)2, [2b](PF6)2, [2c](PF6)2, and [2d](PF6)2, which are sufficiently soluble in acetonitrile for voltammetric analysis. [2d](PF6)2 is markedly less soluble than its shorter analogues, following the trend of decreasing solubility with increasing bridge length. Though [2a]Cl2 is soluble in acetonitrile, [2a](PF6)2 was also prepared for the sake of consistency in differential pulse voltammetry analysis. As shown in Figure 10, the cyclic voltammogram of [2a]Cl2 displays two irreversible reductive peaks in the potential window allowed by acetonitrile. The wave at Epc = −1.55 V (A) is assigned to the reduction of both Co3+ centers to Co2+, and the wave at Epc = −1.92 V (B) to the further reduction of both Co2+ to Co+. The Epc(A) for [2a]Cl2 (−1.55 V) is significantly more anodic than that of the bis-alkynyl [{Co(cyclam)(C2Ph)}2(μ-C4)]Cl2 (−1.98 V), reflecting the increased electron deficiency of [2a]Cl2. This relative electron deficiency is likely responsible for the appearance of the Co(+2/+1) couple in the acetonitrile potential window, which was not observed for [Co(cyclam)(C2R)2]+-type compounds.23 The electron deficiency of the CC is also reflected in the cyclic voltammograms, with the reduction potentials for A and B shifting anodically as the length of the bridging oligoyndiyl increases. In the voltammograms of the dimeric complexes, wave A appears broader than what is expected for a simple simultaneous two-electron reduction. Instead, A is likely the result of two closely spaced one-electron reductions (A1 and A2) as depicted in Scheme 3, which is indicative of some degree of Co−Co coupling mediated by the oligoyndiyl bridge. Further qualitative assessment of such coupling in compounds [2a−d]2+ was attempted based on the method of Taube and Richardson.31 Under the conditions prescribed by Taube and Richardson, the width at half-height of the first reductive peak for [2a](PF6)2 was measured as 120 mV, corresponding to a ΔEp value of 57 mV. Similar analysis of the differential pulse

Figure 5. ORTEP plot of [2a]2+ at the 30% probability level. Hydrogen atoms, solvent molecules, and Cl− counterions were omitted for clarity. Co−Co distance: 7.53 Å.

Figure 6. ORTEP plot of [2b]2+ at the 30% probability level. Hydrogen atoms, solvent molecules, and Cl− counterions were omitted for clarity. Co−Co distance: 10.1 Å.

Figure 7. ORTEP plot of [2c]2+ at the 15% probability level. Hydrogen atoms, solvent molecules, and Cl− counterions were omitted for clarity. Co−Co distance: 12.6 Å.

responsible for the irreversible CoIII/II couple in monoalkynyl complexes of Co(cyclam). The electrochemical behaviors of [2a−d]2+ were examined to gain insight into their electronic structure and establish the extent of Co−Co interaction through the oligoyndiyl bridge. Previous analysis of voltammetric data of the Co(+3/+2) couple in [{Co(cyclam)(C2Ph)}2(μ-C4)]Cl2 indicated the lack D

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Figure 8. ORTEP plot of [2d]2+ at the 30% probability level. Hydrogen atoms, solvent molecules, and Cl− counterions were omitted for clarity. Co− Co distance: 17.6 Å.

Table 2. Selected Bond Lengths (Å) and Bond Angles (deg) for 2a2+−2d2+ [2a]2+

[2b]2+

[2c]2+

Co1−N1 Co1−N2 Co1−N3 Co1−N4 Co1−Cl1 Co1−C1 C1−C2 C2−C2′

1.982(2) 1.975(2) 1.966(2) 1.979(2) 2.3122(7) 1.884(3) 1.205(4) 1.387(5)

Co11−N11 Co11−N12 Co11−N13 Co11−N14 Co11−Cl11 Co11−C11 C11−C12 C12−C13 C13−C13′

1.974(2) 1.981(2) 1.984(2) 1.984(2) 2.3041(8) 1.873(3) 1.223(4) 1.376(4) 1.206(6)

C11−Co1−C1

177.42(8)

C11−Co1−Cl11

177.51(9)

Co1−N11 Co1−N12 Co1−N13 Co1−N14 Co1−Cl1 Co1−C1 C1−C2 C2−C3 C3−C4 C4−C5 C5−C6 C6−C7 C7−C8 C11−Co1−C1

[2d]2+ 1.967(7) 1.9730(7) 1.978(8) 1.977(7) 2.324(2) 1.886(9) 1.191(12) 1.356(11) 1.209(12) 1.358(13) 1.209(13) 1.374(12) 1.196(11) 178.0(3)

Co11−N11 Co11−N12 Co11−N13 Co11−N14 Co11−Cl11 Co11−C11 C11−C12 C12−C13 C13−C14 C14−C15 C15−C16 C16−C16′

1.976(4) 1.987(4) 1.985(4) 1.982(4) 2.2858(14) 1.866(6) 1.219(7) 1.359(8) 1.203(8) 1.353(8) 1.211(8) 1.352(12)

Cl1−Co1−C1

177.17(15)

Scheme 2. General Reduction Pathways for Mono (top) and Bis (bottom) Alkynyl Complexes of CoIII(cyclam)

as the oligoyndiyl bridge elongates, based on this rough comparison. To rationalize the variation in the Co−Co interaction among dimeric compounds [2a]2+, [2b]2+, [2c]2+, and [2d]2+ and the previously studied [{Co(cyclam)(C2Ph)}2(μ-C4)]Cl2, density functional theory calculations were performed on their geometry-optimized structures at the B3LYP/LANL2DZ level using the Gaussian03 suite.32 Bond lengths and angles obtained from the optimization are in good agreement with the crystallographically determined parameters (Supporting Information). The computed contour plots and energy levels for the frontier molecular orbitals are shown in Figure 11. For [2a]2+, [2b]2+, [2c]2+, and [2d]2+, the LUMO and LUMO+1 are primarily based on the dz2 orbitals, part of the unoccupied eg set for a strong field d6 ion. These unoccupied orbitals also exhibit some σ-conjugation onto the oligoyne bridge, which decreases with extension of the bridge. The HOMO and HOMO−1 contain the Co dyz and dxz orbitals, as part of the filled t2g set. Both HOMO and HOMO−1 feature substantial contribution from the antibonding combination of π(CC) orbitals of the oligoyndiyl bridge, a characteristic that is essential for through-bridge Co−Co interaction.33 In keeping

Figure 9. Cyclic voltammograms of [1a]Cl (top), [1c]Cl (middle, ΔEp(B) = 91 mV), and [1b]Cl (bottom, ΔEp(A) = 68 mV) at 0.1 V/s in a 0.1 M MeCN solution of Bu4NPF6.

voltammograms under identical conditions revealed ΔEp values of 51 mV for [2b](PF6)2, 49 mV for [2c](PF6)2, and 32 mV for [2d](PF6)2. The weak Co−Co coupling gradually decreases E

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The contrast in the composition of frontier orbitals in [2a]2+ compared to [{Co(cyclam)(C2Ph)}2(μ-C4)]2+ helps to explain the contrast in ΔEp,c observed upon the first reduction of the two dimers. Complex [2a]2+ displays substantial π−dπ interactions between the Co centers and the C4 bridge in both the HOMO and HOMO−1, which is expected to facilitate electron delocalization across the bridge. However, this conjugation in occupied orbitals is more suggestive of mixed valency upon oxidation, as reduction would populate the LUMO and not significantly affect the HOMO and HOMO−1. Thus, contribution to delocalization from these orbitals is likely to be minimal. Although the LUMO and LUMO+1 contain some contribution from the bridge, especially in the case of [2a]2+, these orbitals are σ-conjugated rather than π-conjugated, and as such delocalization upon reduction is expected to be minimally supported by these orbitals. In contrast, [{Co(cyclam)(C2Ph)}2(μ-C4)]2+ has π−dπ contributions buried in the HOMO−4 and HOMO−5. At such lower energies, these orbitals are less likely to engender communication that is observable by voltammetric methods. Further, the LUMO and LUMO+1 of [{Co(cyclam)(C2Ph)}2(μ-C4)]2+ contain no contribution from the oligoyne bridge and are completely localized to the Co(cyclam) end groups.

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CONCLUSIONS In summary, a series of oligoyndiyl-bridged dimers with Co end groups (Co−C2n−Co) were synthesized under mild conditions, leaving terminal −Cl groups intact to allow for further functionalization or incorporation into existing frameworks. Complexes [2b]Cl2, [2c]Cl2, and [2d]Cl2 represent the first hexatriyndiyl-, octatetrayndiyl-, and dodecahexayndiyl-bridged dinuclear Co species. Electrochemistry of [2a−d] seems to indicate modest Co−Co interaction in [2a-c]Cl2 and no interaction in [2d]Cl2. This behavior is in contrast to that of [{Co(cyclam)(C2Ph)}2(μ-C4)]Cl2, which lacks any observable Co−Co coupling, highlighting the sensitivity of M−C2n−M systems to the nature of ligands trans to the oligoyndiyl. Additionally, several monomeric Co(cyclam) were prepared, which have synthetic utility in constructing longer oligomeric systems. Most notable was the novel preparation of the bisbutadiynyl complex [1b]Cl, which demonstrates the feasibility of constructing bis-alkynyls of Co(cyclam) using the mild alkylamine base approach rather than lithio-acetylide reagents. Complex [1b]Cl may serve to attenuate conductivity of longer oligomeric assemblies based on other metals, acting as an intervening site to tune the overall conductivity of the system, as its two terminal butadiynyl moieties should allow for facile incorporation into oligoyne-based constructs.

Figure 10. Cyclic voltammograms of [2a]Cl2 (top) and [2b](BPh4)2 (top-middle) at 0.1 V/s in a 0.2 M MeCN solution of Bu4NPF6 and of [2c](PF6)2 (bottom-middle) and [2d](PF6)2 at 0.1 V/s in a 0.1 M MeCN solution of Bu4NBF4.

Scheme 3. Possible Pathways for the Reduction Wave A in Compounds [2a−d]2+

with the observation of Lichtenberger for metal-alkynyl complexes, all calculated π-bonding interactions are dominated by mixing of a filled d orbital on Co with filled π(CC) orbitals.33,34 For [2a]2+ and [2b]+, the HOMO−2 and HOMO−3 are predominately based on the Cl lone pair (py) orbitals, with some contribution from the Co dyz orbitals. Complex [2c] also has substantial contribution from the Cl lone pair orbitals, but the oligoyne bridge orbitals are involved as well. For [2d]2+, the oligoyndiyl bridge orbitals begin to dominate the HOMO−2 and HOMO−3, with minimal contribution from the Cl lone pairs. These bridge orbitals are assigned as the bonding combination of the π*(CC) orbitals. In accordance with simple Hückel theory, the number of nodal planes in these orbitals increases with increasing bridge length, raising their energy relative to the Cl lone pair orbitals and resulting in a larger contribution of the bridge in the HOMO−2 and HOMO−3. As the oligoyndiyl bridge lengthens, Eg decreases, which is reflected in the cyclic voltammetry as Ep,c(A) becomes less negative with increasing bridge length.

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EXPERIMENTAL SECTION

General Procedures. Bis(trimethylsilyl)-1,4-butadiyne and HC2Si(iPr)3 were obtained from GFS Chemicals. Bis(trimethylsilyl)1,3,5-hexatriyne was synthesized according to literature procedure.35 [Co(cyclam)Cl2]Cl was prepared according to literature procedure.36 All reagents were used as received. UV−vis spectra were obtained with a Jasco V-670 spectrophotometer. FT-IR spectra were measured on a Jasco FT/IR-6300 as neat samples. 1H NMR spectra were obtained using a Varian Mercury300 NMR, with chemical shifts (δ) referenced to the residual CH3OH signal (CH3 multiplet at 3.30 ppm). Voltammograms were recorded on a CHI620A voltammetric analyzer with a glassy carbon working electrode (diameter = 2 mm), a Pt-wire auxiliary electrode, and a Ag/AgNO3 reference electrode filled with 10 mM AgNO3 and 0.1 M Bu4NPF6 in dry MeCN. The open-circuit potential of all solutions analyzed was −20 mV. The concentration of F

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Organometallics

Figure 11. Molecular orbital diagrams for [2a]2+, [2b]2+, [2c]2+, and [2d]2+ from DFT calculations. The isovalue of the contour plots was set at 0.02. Anal. Found (calcd) for C15H31N4O2CoCl2 (1c·H2O·MeOH): C, 42.85 (41.97); H, 6.95 (7.28); N, 13.36 (13.05). FT-IR (neat, ν (cm−1)): 2155, 2002 (CC stretch). 1H NMR (CD3OD, δ): 5.27 (br s, 2H, NH), 5.15 (br s, 2H, NH), 2.97−2.40 (m, 16H, CH2), 2.10 (s, 1H, CCH), 1.96 (t, J = 0.042, 2H, CH2), 1.65 (q, J = 0.031, 2H, CH2). Absorption spectrum (MeOH): λmax (εmax, L mol−1 cm−1) 482 (84); 379 (sh, 66); 294 (1600). Preparation of [Co(cyclam)(C6H)Cl]Cl (1d). [Co(cyclam)Cl2]Cl (0.120 g, 0.328 mmol) was dissolved in 15 mL of MeOH. Me3SiC6SiMe3 (0.095 g, 0.436 mmol) was dissolved in 1 mL of THF, and the mixture added to the MeOH solution. Et3N (0.9 mL, 6 mmol) was added, and the solution was refluxed for 30 min. Solvent was carefully reduced on the Rotovap, and the residue was transferred to a small silica pad with DCM. The product was eluted with 1:5 MeOH/CH2Cl2 to collect a light orange fraction. Solvent was reduced, and the product crystallized via slow diffusion of Et2O to give orange block crystals. The product decomposes quickly as a solid, changing from light orange to dark brown overnight in a sealed, N2-purged vial, and the crude product decomposes if solvent is removed to dryness. Yield: 0.028 g, 21% (based on Co). ESI-MS (MeOH): 367 − [Co(cyclam)(C6H)Cl]+. 1H NMR (CD3OD, δ): 5.32 (br s, 2H, NH), 5.20 (br s, 2H, NH), 2.95−2.40 (m, 17H, CH2, CCH), 1.94 (t, J = 0.068, 2H, CH2), 1.60 (q, J = 0.042, 2H, CH2). Absorption spectrum (MeOH): λmax (εmax, L mol−1 cm−1) 482 (86); 388 (sh, 92); 311 (700). Preparation of [{Co(cyclam)Cl}2(μ-C4)]Cl2 ([2a]Cl2). [Co(cyclam)Cl2]Cl (0.500 g, 1.37 mmol) was dissolved in 50 mL of MeOH. Me3SiC4SiMe3 (0.133 g, 0.684 mmol) was dissolved in 5 mL of THF and added to the MeOH solution. Et3N (3.5 mL) was added, and the solution was refluxed for 24 h. Solvent was removed, and the residue was transferred to a small silica pad with DCM. After rinsing with 1:5 MeOH/DCM to remove any unreacted starting material, the product was eluted with 1:3 MeOH/DCM to collect a dark red band. Solvent was removed under reduced pressure, and the residue was recrystallized by addition of Et2O to a concentrated solution in MeOH to give dark red-orange needle crystals. Yield: 0.275 g, 56.8% (based on Co). ESI-MS(MeOH): 318 − [{Co(cyclam)Cl}2(μ-C4)]2+, 671 − [{{Co(cyclam)Cl} 2 (μ-C 4 )}Cl] + . Anal. Found (calcd) for C24H56N8O4Co2Cl4 ([2a]Cl2·4H2O): C, 37.37 (36.94); H, 7.05 (7.23); N, 14.00 (14.36). FT-IR (neat, ν (cm−1)): 2007 (CC stretch). 1H NMR (CD3OD, δ): 5.16 (br s, 4H, NH), 5.06 (br s, 4H, NH), 2.96−2.43 (m, 32H, CH2), 1.89 (d, J = 0.049, 4H, CH2), 1.63 (q, J = 0.043, 4H, CH2). Absorption spectrum (MeOH): λmax (εmax, L mol−1 cm−1) 503 (353); 334 (2640). Preparation of [{Co(cyclam)Cl}2(μ-C4)](PF6)2 ([2a](PF6)2). [{Co(cyclam)Cl}2(μ-C4)]Cl2 (0.030 g, 0.042 mmol) was dissolved in 1 mL of MeOH. A concentrated solution of NaPF6 (0.100 g, 6.92 mmol) in

analyte is always 1.0 mM in 4 mL of dry MeCN (thoroughly degassed by N2 or Ar purging), with the exception of [2d](PF6)2 due to its low solubility. Preparation of [Co(cyclam)(C2Si(iPr)3)Cl]Cl (1a). [Co(cyclam)Cl2]Cl (0.750 g, 2.05 mmol) and HC2Si(iPr)3 (0.50 mL, 2.2 mmol) were dissolved in 80 mL of MeOH. Et3N (2.0 mL, 14 mmol) was added, and the solution was refluxed for 24 h. Solvent was removed, and the residue was transferred to a small silica pad with dichloromethane (DCM). The product was eluted with 1:5 MeOH/ EtOAc to collect a yellow band. Solvent was removed, and the residue was recrystallized by slow diffusion of Et2O into a concentrated solution in MeOH to give bronze-colored needle crystals. Yield: 0.521 g, 51.0% (based on Co). ESI-MS(MeOH): 475 − [Co(cyclam)(C2Si(iPr)3)Cl]+. Anal. Found (calcd) for C21H47N4OCoCl2 (1a·H2O): C, 47.64 (47.63); H, 8.76 (8.95); N, 10.32 (10.58). FT-IR (neat, ν (cm−1)): 2047 (CC stretch). 1H NMR (CD3OD, δ): 5.00 (br s, 2H, NH), 4.31 (br s, 2H, NH), 2.91−2.45 (m, 16H, CH2), 1.87 (d, J = 0.053, 2H, CH2), 1.59−1.26 (m, 2H, CH2), 1.00 (m, 21H, Si(iPr)3). Absorption spectrum (MeOH): λmax (εmax, L mol−1 cm−1) 479 (77); 372 (sh, 68); 284 (2400). Preparation of [Co(cyclam)(C4H)2]Cl (1b). [Co(cyclam)Cl2]Cl (0.400 g, 1.09 mmol) was dissolved in 50 mL of MeOH. Me3SiC4SiMe3 (1.00 g, 5.14 mmol) was dissolved in 10 mL of THF and added to the MeOH solution. Et2NH (3.5 mL, 34 mmol) was added, and the solution was refluxed for 24 h. Solvent was removed, and the residue was transferred to a small silica pad with DCM. The product was eluted with 1:5 MeOH/EtOAc to collect a yellow band. Solvent was removed, and the residue was recrystallized by slow diffusion of Et2O into a concentrated solution in MeOH to give yellow needle crystals. Yield: 0.220 g, 51.4% (based on Co). ESIMS(MeOH): 357 − [Co(cyclam)(C4H)2]+. Anal. Found (calcd) for C18H26N4OCoCl2 (1b·H2O): C, 52.69 (52.62); H, 7.00 (6.87); N, 13.21 (13.64). FT-IR (neat, ν (cm−1)): 2143, 1991 (CC stretch). 1 H NMR (CD3OD, δ): 4.68 (br s, 4H, NH), 2.71−2.29 (m, 16H, CH2), 1.97 (s, 2H, CCH), 1.75 (d, J = 0.051 2H, CH2), 1.27 (q, J = 0.035, 2H, CH2). Absorption spectrum (MeOH): λmax (εmax, L mol−1 cm−1) 446 (108); 360 (sh, 219); 311 (1160). Preparation of [Co(cyclam)(C4H)Cl]Cl (1c). [Co(cyclam)Cl2]Cl (0.500 g, 1.37 mmol) was dissolved in 50 mL of MeOH. Me3SiC4SiMe3 (0.300 g, 1.54 mmol) was dissolved in 5 mL of THF and added to the MeOH solution. Et3N (2.6 mL, 18 mmol) was added, and the solution was refluxed for 24 h. Solvent was removed, and the residue was transferred to a small silica pad with DCM. The product was eluted with 1:5 MeOH/EtOAc to collect a light orange band. The MeOH was removed under reduced pressure, resulting in the formation of light orange needle crystals. Yield: 0.390 g, 75.3% (based on Co). ESI-MS(MeOH): 343 − [Co(cyclam)(C4H)Cl]+. G

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Preparation of [{Co(cyclam)Cl}2(μ-C8)](PF6)2 ([2c](PF6)2). [{Co(cyclam)Cl}2(μ-C8)]Cl2 (0.030 g, 0.040 mmol) was dissolved in 1 mL of MeOH. A concentrated solution of NaPF6 (0.100 g, 6.92 mmol) in MeOH was added, causing a light orange solid to precipitate. The solid was filtered off, transferred to a small silica pad with DCM, and eluted with 1:2 MeCN/DCM. Solvent was removed, and the residue recrystallized by addition of Et2O to a concentrated solution in MeCN to give orange needle crystals. Yield: 0.035 g, 90% (based on Co). ESIMS(MeCN): 342 − [{Co(cyclam)Cl}2(μ-C8)]2+, 829 − [{{Co(cyclam)Cl}2(μ-C8)}PF6]+. Preparation of [{Co(cyclam)Cl}2(μ-C12)]Cl2 ([2d]Cl2). [Co(cyclam)Cl2]Cl (0.115 g, 0.315 mmol) was dissolved in 30 mL of MeOH. Me3SiC6SiMe3 (0.092 g, 0.42 mmol) was dissolved in 2 mL of THF and added to the MeOH solution. Et3N (1 mL, 7 mmol) was added, and the solution was refluxed for 30 min. Solvent was carefully reduced on the Rotovap, and the residue was transferred to a small silica pad with DCM. The product was eluted with 1:5 MeOH/ CH2Cl2 to collect a light orange fraction. This fraction was carefully reduced to about 5 mL in volume. In a separate vial, a Hay catalyst solution was prepared by suspending CuCl (0.100 g, 1.01 mmol) in 5 mL of acetone and adding 0.5 mL of N,N,N′,N′-tetramethylethylenediamine (3 mmol) to give a dark blue solution. A 1 mL portion of this Hay catalyst solution was centrifuged and added to the reducedvolume filtrate, which was then purged with O2 for 4 h. The flask was then sealed and allowed to stir overnight. Solvent was removed, and the residue was rinsed copiously with CH2Cl2, leaving behind an orange powder. This orange powder was transferred to a small silica pad with CH2Cl2, and some blue-green residual catalyst was eluted with 1:5 MeOH/CH2Cl2 before eluting the product with 1:2 MeOH/ CH2Cl2. The product was then precipitated by addition of Et2O to give an orange powder. Yield: 0.029 g, 11% (based on Co). ESI-MS (MeOH): 366 − [{Co(cyclam)Cl}2(μ-C12)]2+. Anal. Found (calcd) for C36H64N8O6Co2Cl4 ([2d](Cl2)·4H2O·EtOAc): C, 44.46 (44.83); H, 6.42 (6.69); N, 11.44 (11.62). FT-IR (neat, ν (cm−1)): 2152, 2096, 2000 (CC stretch). 1H NMR (CD3OD, δ): 5.41 (br s, 4H, NH), 5.06 (br s, 4H, NH), 2.95−2.43 (m, 32H, CH2), 1.98 (t, J = 0.052, 4H, CH2), 1.61 (q, J = 0.043, 4H, CH2). Absorption spectrum (MeOH): λmax (εmax, L mol−1 cm−1) 489 (626); 385 (sh, 1740). Preparation of [{Co(cyclam)Cl}2(μ-C12)](PF6)2 ([2d](PF6)2). [{Co(cyclam)Cl}2(μ-C12)]Cl2 (0.015 g, 0.019 mmol) was dissolved in 2 mL of MeOH. NaPF6 (0.1 g, 0.6 mmol) was dissolved in 1 mL of MeOH and added, causing a light orange solid to precipitate. The solid was filtered off and recrystallized by slow diffusion of Et2O into MeCN to give light orange needle crystals. Yield: 0.016 g, 82% (based on Co). ESI-MS (MeCN): 366 − [{Co(cyclam)Cl}2(μ-C12)]2+. X-ray Data Collection, Processing, and Structure Analysis and Refinement for Crystals. X-ray diffraction data were collected on either a Rigaku RAPID-II image plate diffractometer using Cu Kα (λ = 1.541 84 Å) radiation ([1a]Cl, [1b]Cl, [1d]Cl, [2c]Cl2, [2d]Cl2) or a Nonius Kappa CCD using Mo Kα (λ = 0.710 73 Å) radiation ([1c]Cl, [2a]Cl2, [2b]Cl2). The structures were solved using the structure solution program DIRDIF200837 and refined using SHELXTLC.38 Computational Details. The geometries of [2a]2+, [2b]2+, and [2d]2+ in the ground state were fully optimized from the crystal structures reported in this work, using the density functional method B3LYP (Beck’s three-parameter hybrid functional using the Lee− Yang−Parr correlation functional) and employing the LanL2DZ basis sets. The geometry of [2c]2+ was optimized by adding a single CC bond to the optimized structure of [2b]2+. The calculation was accomplished by using the Gaussian03 program package.32

MeOH was added, causing a light orange solid to precipitate. The solid was filtered off, transferred to a small silica pad with DCM, and eluted with 1:2 MeCN/DCM. Solvent was removed, and the residue recrystallized by addition of Et2O to a concentrated solution in MeCN to give red-orange needle crystals. Yield: 0.034 g, 86% (based on Co). ESI-MS(MeCN): 318 − [{Co(cyclam)Cl}2(μ-C4)]2+, 781 − [{{Co(cyclam)Cl}2(μ-C4)}PF6]+. Preparation of [{Co(cyclam)Cl}2(μ-C6)]Cl2 ([2b]Cl2). [Co(cyclam)Cl2]Cl (0.070 g, 0.19 mmol) was dissolved in 15 mL of MeOH. Me3SiC6SiMe3 (0.020 g, 0.092 mmol) was dissolved in 2 mL of THF and added to the MeOH solution. Et3N (0.6 mL) was added, and the solution was refluxed for 24 h. Solvent was removed, and the residue was transferred to a small silica pad with DCM. After rinsing with 1:5 MeOH/DCM to remove any unreacted starting material, the product was eluted with 1:3 MeOH/DCM to collect a dark red band. Solvent was removed, and the residue recrystallized by addition of Et2O to a concentrated solution in MeOH to give dark red-orange needle crystals. Yield: 0.024 g, 34% (based on Co). ESI-MS(MeOH): 330 − [{Co(cyclam)Cl}2(μ-C6)]2+, 695 − [{{Co(cyclam)Cl}2(μC6)}Cl]+. Anal. Found (calcd) for C24H56N8O4Co2Cl4 ([2b]Cl2· 5H2O): C, 38.38 (37.97); H, 6.48 (7.11); N, 12.66 (13.33). FT-IR (neat, ν (cm−1)): 2187, 2123 (CC stretch). 1H NMR (CD3OD, δ): 5.24 (br s, 4H, NH), 5.17 (br s, 4H, NH), 2.95−2.39 (m, 32H, CH2), 1.94 (t, J = 0.054, 4H, CH2), 1.58 (q, J = 0.051, 4H, CH2). Absorption spectrum (MeOH): λmax (εmax, L mol−1 cm−1) 495 (508); 343 (sh, 2,640); 307 (4030). Preparation of [{Co(cyclam)Cl}2(μ-C6)](PF6)2 ([2b](PF6)2). [{Co(cyclam)Cl}2(μ-C6)]Cl2 (0.030 g, 0.041 mmol) was dissolved in 1 mL of MeOH. A concentrated solution of NaPF6 (0.100 g, 6.92 mmol) in MeOH was added, causing a light orange solid to precipitate. The solid was filtered off, transferred to a small silica pad with DCM, and eluted with 1:2 MeCN/DCM. Solvent was removed, and the residue recrystallized by addition of Et2O to a concentrated solution in MeCN to give orange needle crystals. Yield: 0.036 g, 92% (based on Co). ESI-MS(MeCN): 330 − [{Co(cyclam)Cl}2(μ-C6)]2+, 805 − [{{Co(cyclam)Cl}2(μ-C6)}PF6]+. Preparation of [{Co(cyclam)Cl}2(μ-C6)](BPh4)2 ([2b](BPh4)2). [{Co(cyclam)Cl}2(μ-C6)]Cl2 (0.010 mg, 0.0014 mmol) was dissolved in 1 mL of MeOH. A concentrated solution of NaBPh4 (0.050 g, 0.146 mmol) in MeOH was added, causing a light orange solid to precipitate. The solid was filtered off, transferred to a small silica pad with DCM, and eluted with neat DCM. Solvent was removed, and the residue recrystallized by addition of hexanes to a concentrated solution in acetone to give light orange needle crystals. Yield: 0.017 g, 96% (based on Co). ESI-MS(MeCN): 330 − [{Co(cyclam)Cl}2(μ-C6)]2+, 979 − [{{Co(cyclam)Cl}2(μ-C6)}BPh4]+. Preparation of [{Co(cyclam)Cl}2(μ-C8)]Cl2 (2c[Cl2]). To a suspension of CuCl (0.100 g, 1.01 mmol) in 5 mL of acetone was added 0.5 mL of N,N,N′,N′-tetramethyl ethylenediamine (3.34 mmol) to give a dark blue solution. The solution was stirred for 1 min and centrifuged, and the dark blue supernatant was added to an O2saturated solution of [Co(cyclam)(C4H)Cl]Cl (0.085 g, 0.22 mmol) in 10 mL of MeOH. After purging with O2 for 4 h, the solvent was removed under reduced pressure and the dark blue residue was rinsed with DCM to remove most of the Cu catalyst. The remaining solid was transferred to a silica pad with DCM and rinsed with 1:6 MeOH/ DCM to elute any remaining catalyst and unreacted starting material. The product was then eluted with 1:2 MeOH/DCM to collect a dark orange band. Solvent was removed under reduced pressure, and the residue was recrystallized by addition of EtOAc to a concentrated solution in MeOH to give thin red-orange plate crystals. Yield: 0.045 g, 53.2% (based on Co). ESI-MS(MeOH): 342 − [{Co(cyclam)Cl}2(μC8)]2+. Anal. Found (calcd) for C29H56N8O3Co2Cl4 ([2c]Cl2·2H2O· MeOH): C, 43.25 (42.25); H, 6.57 (6.85); N, 13.25 (13.59). FT-IR (neat, ν (cm−1)): 2149, 2137(CC stretch). 1H NMR (CD3OD, δ): 5.32 (br s, 4H, NH), 4.62 (br s, 4H, NH), 2.92−2.40 (m, 32H, CH2), 1.96 (t, J = 0.046, 4H, CH2), 1.57 (q, J = 0.051, 4H, CH2). Absorption spectrum (MeOH): λmax (εmax, L mol−1 cm−1) 491 (616); 356 (sh, 2,400); 330 (3,120).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00219. UV−vis spectra of all complexes presented, differential pulse voltammograms of [2a−d](PF6)2, crystal data H

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(19) Cook, T. D.; Fanwick, P. E.; Ren, T. Organometallics 2014, 33, 4621−4624. (20) Cook, T. D.; Natoli, S. N.; Fanwick, P. E.; Ren, T. Organometallics 2015, 34, 686−689. Banziger, S. D.; Cook, T. D.; Natoli, S. N.; Fanwick, P. E.; Ren, T. J. Organomet. Chem. 2015, 799− 800, 1−6. (21) Natoli, S. N.; Cook, T. D.; Abraham, T. R.; Kiernicki, J. J.; Fanwick, P. E.; Ren, T. Organometallics 2015, 34, 5207−5209. (22) Grisenti, D. L.; Thomas, W. W.; Turlington, C. R.; Newsom, M. D.; Priedemann, C. J.; VanDerveer, D. G.; Wagenknecht, P. S. Inorg. Chem. 2008, 47, 11452−11454. Sun, C.; Turlington, C. R.; Thomas, W. W.; Wade, J. H.; Stout, W. M.; Grisenti, D. L.; Forrest, W. P.; VanDerveer, D. G.; Wagenknecht, P. S. Inorg. Chem. 2011, 50, 9354− 9364. Sun, C.; Thakker, P. U.; Khulordava, L.; Tobben, D. J.; Greenstein, S. M.; Grisenti, D. L.; Kantor, A. G.; Wagenknecht, P. S. Inorg. Chem. 2012, 51, 10477−10479. Thakker, P. U.; Sun, C.; Khulordava, L.; McMillen, C. D.; Wagenknecht, P. S. J. Organomet. Chem. 2014, 772, 107−112. (23) Thakker, P. U.; Aru, R. G.; Sun, C.; Pennington, W. T.; Siegfried, A. M.; Marder, E. C.; Wagenknecht, P. S. Inorg. Chim. Acta 2014, 411, 158−164. (24) Nishijo, J.; Judai, K.; Numao, S.; Nishi, N. Inorg. Chem. 2009, 48, 9402−9408. Nishijo, J.; Judai, K.; Nishi, N. Inorg. Chem. 2011, 50, 3464−3470. Nishijo, J.; Enomoto, M. Inorg. Chem. 2013, 52, 13263− 13268. (25) Hoffert, W. A.; Kabir, M. K.; Hill, E. A.; Mueller, S. M.; Shores, M. P. Inorg. Chim. Acta 2012, 380, 174−180. (26) Berben, L. A. Toward acetylide- and N-heterocycle-bridged materials with strong electronic and magnetic coupling. Ph.D. Dissertation, University of California, Berkeley, 2005. Berben, L. A.; Kozimor, S. A. Inorg. Chem. 2008, 47, 4639−4647. (27) Le Narvor, N.; Toupet, L.; Lapinte, C. J. Am. Chem. Soc. 1995, 117, 7129−7138. Lissel, F.; Fox, T.; Blacque, O.; Polit, W.; Winter, R. F.; Venkatesan, K.; Berke, H. J. Am. Chem. Soc. 2013, 135, 4051−4060. (28) Egler-Lucas, C.; Blacque, O.; Venkatesan, K.; Lopez-Hernandez, A.; Berke, H. Eur. J. Inorg. Chem. 2012, 2012, 1536−1545. (29) Siemsen, P.; Livingston, R. C.; Diederich, F. Angew. Chem., Int. Ed. 2000, 39, 2632−2657. (30) Pigulski, B.; Gulia, N.; Szafert, S. Chem. - Eur. J. 2015, 21, 17769−17778. (31) Richardson, D. E.; Taube, H. Inorg. Chem. 1981, 20, 1278− 1285. (32) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision D.02; Gaussian, Inc.: Wallingford, CT, 2003. (33) Lichtenberger, D. L.; Renshaw, S. K.; Bullock, R. M. J. Am. Chem. Soc. 1993, 115, 3276−3285. (34) Lichtenberger, D. L.; Renshaw, S. K.; Wong, A.; Tagge, C. D. Organometallics 1993, 12, 3522−3526. (35) Rubin, Y.; Lin, S. S.; Knobler, C. B.; Anthony, J.; Boldi, A. M.; Diederich, F. J. Am. Chem. Soc. 1991, 113, 6943−6949. (36) Bosnich, B.; Poon, C. K.; Tobe, M. L. Inorg. Chem. 1965, 4, 1102−1108.

tables, computational details and relevant geometric parameters for the optimized structures of [2a−d]2+ (PDF) Cartesian coordinates in a format for convenient visualization (XYZ) CIF files of all complexes presented giving X-ray crystallographic data for their structural determination (CIF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Science Foundation (CHE 1362214).

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REFERENCES

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DOI: 10.1021/acs.organomet.6b00219 Organometallics XXXX, XXX, XXX−XXX

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Organometallics (37) Beurskens, P. T.; Beurskens, G.; deGelder, R.; Garcia-Granda, S.; Gould, R. O.; Smits, J. M. M. The DIRDIF2008 Program System; Crystallography Laboratory, University of Nijmegen: The Netherlands, 2008. (38) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122.

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DOI: 10.1021/acs.organomet.6b00219 Organometallics XXXX, XXX, XXX−XXX