Crystalline Coordination Networks of Zero-Valent Metal Centers

A permanently porous, three-dimensional metal–organic material formed from zero-valent metal nodes is presented. Combination of ditopic m-terphenyl ...
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Crystalline Coordination Networks of Zero-Valent Metal Centers: Formation of a 3-Dimensional Ni(0) Framework with m-Terphenyl Diisocyanides Douglas W. Agnew, Ida M. DiMucci, Alejandra Arroyave, Milan Gembicky, Curtis E. Moore, Samantha N. MacMillan, Arnold L. Rheingold, Kyle M. Lancaster, and Joshua S. Figueroa J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b09569 • Publication Date (Web): 13 Nov 2017 Downloaded from http://pubs.acs.org on November 13, 2017

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Crystalline Coordination Networks of Zero-Valent Metal Centers: Formation of a 3-Dimensional Ni(0) Framework with m-Terphenyl Diisocyanides Douglas W. Agnew,1 Ida M. DiMucci,2 Alejandra Arroyave,1 Milan Gembicky,1 Curtis E. Moore,1 Samantha N. MacMillan,2 Arnold L. Rheingold,1 Kyle M. Lancaster,2 and Joshua S. Figueroa1* 1

Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive MC 0358, La Jolla, CA 92193. 2

Department of Chemistry and Chemical Biology, Cornell University, Ithaca, NY 14853.

Supporting Information Placeholder ABSTRACT: A permanently porous, three-dimensional metal-organic material formed from zero-valent metal nodes is presented. Combination of ditopic m-terphenyl diisocyanide, [CNArMes2]2, and the d10 Ni(0) precursor Ni(COD)2, produces a porous metal-organic material featuring tetrahedral [Ni(CNArMes2)4]n structural sites. X-ray absorption spectroscopy provides firm evidence for the presence of Ni(0) centers, while gas-sorption and differentialscanning calorimetry analysis reveal the characteristics of a robust network with a micro-domain N2-adsorption profile. Permanently porous metal-organic materials have become prominent candidates for a number of targeted applications as a result of their high degree of synthetic flexibilty.1,2 In particular, the ever-growing library of ligands and nodal geometries afforded by single-metal and higher nuclearity nodes presents a seemingly endless array of tunable multi-dimensional structures.3-6 With regards to nodal types, molecular inorganic complexes have provided much of the inspiration for the preparation of numerous coordination polymers.7 However, despite the vast number of low-valent transition metal complexes that have been reported in the literature there are, to our knowledge, no such constructs that have been organized into a well-defined supramolecular network.8-10 This has prevented an analysis of the properties that low-valent species, especially those in zero or negative formal oxidation states, may endow when used as the junction of framework materials. Accordingly, in an extension of our work on coordination frameworks derived from m-terphenyl diisocyanide ligands and four-coordinate Cu(I) nodes,11 we now report the formation and physical properties of a valence isoelectronic network material incorporating four-coordinate Ni(0) centers as structural sites. We have previously shown that addition of four equivalents of the monoisocyanide CNArMes2 to Ni(COD)2 (ArMes2 = 2,6-(2,4,6-Me3C6H2)2C6H3; COD = 1,5-

Scheme 1. Synthesis of Ni-ISOCN-2 from [CNArMes2]2 and Ni(COD)2.

cyclooctadiene) leads to the exclusive formation of the molecular species Ni(CNArMes2)4, which adopts a tetrahedral geometry about Ni(0).12 Importantly, the use of π-acidic isocyanides enabled stabilization13-16 of the electron-rich Ni(0) metal center, and provided a structural, electronic and functional mimic to the highly-reactive tetracarbonyl complex Ni(CO)4.17-19 Based on the unique electronic and chemical properties displayed by both Ni(CO)4 and Ni(CNArMes2)4, we sought to construct a three-dimensional framework in which the nodes are repeating units of a tetra-coordinated Ni(0) construct. To achieve this, we utilized the recently-reported diisocyanide [CNArMes2]2,11 which is a ditopic dimer of CNArMes2 and was shown to be effective in the production of three- and four-coordinate Cu(I) network materials.12,20-21 In a manner similar to the preparation of Ni(CNArMes2)4, 12,22 we found that the slow addition of 3.0 equiv. of [CNArMes2]2 to a stirring THF solution of Ni(COD)2 leads to the precipitation of an amorphous, maroon solid. Subsequent heating of this solid as a THF suspension at 100 ˚C inside a thick-walled glass pressure tube for 2 days generates Ni-ISOCN-2 (Scheme 1; ISOCN = isocyanide coordination network) as a free-flowing solid, with a strong peak at 2q = 7.25˚ in its powder Xray diffraction (PXRD) pattern. ATR-IR analysis of this material reveals a single broad, n(CN) absorbance band centered at 1950 cm-1 (Figure 1, inset), which is slightly redshifted from that of four-coordinate Ni(CNArMes2)4 but

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Figure 1. Ni K-edge XANES spectra of Ni-ISOCN-2, Ni(CNArMes2)4 and[Ni(CNArMes2)4]OTf. (Inset) ATR-IR spectrum of Ni-ISOCN-2 in the n(CN) region.

nonetheless indicative of a highly-reduced Ni center.12,13,20,22 To provide additional rationale for the assignment of a Ni(0) formal oxidation state for the nodes of Ni-ISOCN-2, and to assess the local coordination about the metal center, we performed X-ray absorption spectroscopy (XAS) at the Ni K-edge on the isolated solid. We also obtained XAS data for the molecular standards Ni(CNArMes2)4 and [Ni(CNArMes2)4]OTf (See SI; OTf = [O3SCF3]–), which represent nickel tetraisocyanide centers in formal oxidation states of 0 and +1, respectively, and therefore serve as ideal spectroscopic benchmarks for a similarly-constituted network material. As shown in Figure 1, a small, broadened band is observed in the near-edge portion of the spectrum (XANES) at 8334.4 eV for Ni-ISOCN-2, with an otherwise featureless rising edge. DFT calculations indicate this band arises from a spin-allowed, X-ray promoted Ni 1s ® CNR π* excitation,23-25 rather than a Ni-based 1s ® 3d transition. This is verified by the XANES spectrum of zero-valent Ni(CNArMes2)4, which also displays a single band at 8334.4 eV resulting from a Ni 1s ® CNR π* excitation. By comparison, the near-edge spectrum of the Ni(I) complex, [Ni(CNArMes2)4]OTf, exhibits two distinct bands at 8330.8 eV and 8334.7 eV, which are assigned as Ni 1s ® 3d and Ni 1s ® CNR π* transitions, respectively. Importantly, the absence of a second near-edge band in the spectrum of NiISO CN-2 provides strong evidence that formally Ni(0) centers are present within the material. Information regarding the structural geometry about the Ni(0) centers can also be gained from the rising edge profile of Ni-ISOCN-2. Both [Ni(CNArMes2)4]OTf and Ni(CNArMes2)4 exhibit a sloping edge that terminates at 8446.4 eV and 8446.5 eV, respectively. A similar profile is observed for Ni-ISOCN-2 (Figure 1), indicating the primary coordination spheres of the Ni centers within Ni-ISOCN-2 are similar to these molecular species, both of which adopt neartetrahedral coordination environments. Accordingly, the IR and X-ray spectroscopic properties of Ni-ISOCN-2 indicate that tetrahedral, d10 Ni(0) centers are the main building unit within the material and are akin to the well-known organometallics Ni(CO)4 and Ni(PPh3)4. Attempts to prepare a crystalline material of

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Ni-ISOCN-2 suitable for single-crystal X-ray diffraction via various crystallization procedures failed to provide crystallites of suitable size. However, it was found that an alternate synthesis, involving the mechanical grinding of Ni(COD)2 and with an excess of [CNArMes2]2, followed by heating at 100 ˚C in the presence of THF, led to the formation of large black/red single crystals which adopt a cube-type habit by visual inspection. Crystallographic analysis of these crystals at 100 K revealed a doubly-interpenetrated diamondoid26,27 framework in the C2/c space group (Figure 2), with the [CNArMes2]2 linkages providing a Ni-Ni separation of 15.539 Å between direct neighbors. As indicated by the spectroscopic data, the framework architecture is derived from tetrakis-isocyanide Ni nodal sites possessing near-ideal tetrahedral geometries (Houser t4 index = 0.98),28 which is directly analogous to the molecular congener, Ni(CNArMes2)4.12 This coordination geometry results in the formation of ca. 4.2 x 3.6 Å and 6.3 x 4.5 Å channels along the 001 axis; the pore size in this case being limited by the m-terphenyl groups of [CNArMes2]2. Notably, this material represents the first example of a structurally characterized metal-organic material derived from zero-valent metal nodes and signifies that the combination of moderate s-donation and good p-acidity of organoisocyanides can be harnessed in reticular synthesis of unique frameworks. Examination of the void space generated by this 3-D architecture indicates partial occupation of the larger pore by free [CNArMes2]2 (SOF = 50%; Figure 2A and 2C). This is confirmed by ATR-IR analysis of these crystals showing two bands in the n(CN) region with peak maxima at 2114 cm-1 and 1960 cm-1, the former of which corresponds to free [CNArMes2]2.11 We contend that the inclusion of free [CNArMes2]2 in this structure potentially mitigates framework collapse, and consequently, enables the formation of large single crystals. However, as a result of this feature the crystal symmetry is lowered from the expected high-symmetry for a 2-fold interpenetrated diamondoid net.29,30 Importantly, comparison of the PXRD pattern of NiISO CN-2 with the predicted pattern derived from the [CNArMes2]2-included single crystals, which we denote as Ni-ISOCN-2·([CNArMes2]2)0.5, suggests that in the absence of free [CNArMes2]2 the bulk material conforms to a higher symmetry lattice. Indeed, only a single, anisotropically broadened peak is observed at 2q = 7.25˚, compared with the three peaks predicted from the monoclinic lattice of Ni-ISOCN2·([CNArMes2]2)0.5 (Figure 3, middle and bottom). Interestingly, this peak position is close to the 220 and 022 reflections of the previously reported valence isoelectronic isocyanide coordination-network Cu-ISOCN-1,11 which overlap to produce a single observed peak at 2q = 7.0˚ in the PXRD pattern (Figure 3, top). Cu-ISOCN-1 adopts a 2-fold interpenetrated diamondoid framework architecture that is identical to Ni-ISOCN-2·([CNArMes2]2)0.5, but the open void spaces in Cu-ISOCN-1 consequently lead to higher crystal symmetry (Fdd2 vs. C2/c). Accordingly, we contend that the characterized structure of Ni-ISOCN-2·([CNArMes2]2)0.5 provides a reasonable model for the framework architecture of NiISO CN-2, despite the fact that in bulk, Ni-ISOCN-2 lacks a high degree of crystallinity.

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Figure 2. Structure of Ni-ISOCN-2·([CNArMes2]2)0.5. (A) Extended 3-D lattice with [CNArMes2]2 omitted for clarity (001 axis). (B) Tetrahedral Ni(0) tetrakis-isocyanide node of Ni-ISOCN-2·([CNArMes2]2)0.5, which is geometrically identical to the molecular analogue Ni(CNArMes2)4. (C) View of [CNArMes2]2 (blue) located at 50% site occupancy inside the larger voids of Ni-ISOCN-2·([CNArMes2]2)0.5

Figure 3. PXRD patterns of (a) Cu-ISOCN-1 and (b) Ni-ISOCN2 collected at 100 K. The predicted pattern derived from singlecrystal X-ray analysis of Ni-ISOCN-2·([CNArMes2]2)0.5, collected at 100 K, is shown in (c).

Consistent with its formulation as a Ni(0)-containing material, solid Ni-ISOCN-2 is unstable under aerobic conditions, slowly liberating free [CNArMes2]2 in either solid state or as an acetonitrile slurry, as determined by ATR-IR and 1H NMR spectroscopy. However, ATR-IR analysis of NiISO CN-2 indicates excellent stability as a solid under a dinitrogen atmosphere over the course of several months. In addition, Ni-ISOCN-2 does not degrade as a suspension in a variety of common organic solvents, including benzene, acetonitrile, and diethyl ether when O2 is excluded. Examination of the thermal stability of solid Ni-ISOCN-2 by thermogravimetric analysis (TGA) under a dinitrogen atmosphere indicates stability to 302 ˚C, thereby revealing it to be a thermally robust material. This behavior contrasts with that of the thermally-sensitive Cu(I) analogue, Cu-ISOCN-1, and highlights the structural integrity that can be gained from strong p-backbonding interactions between an electron-rich metal node and an isocyanide linker. Given our structural analysis of Ni-ISOCN2·([CNArMes2]2)0.5 and Ni-ISOCN-2, we hypothesized that the latter would exhibit permanent porosity as a result of the voids in the diamondoid structure. After activating

Figure 4. N2 adsorption isotherms (77 K) for Ni-ISOCN-2.

Ni-ISOCN-2 at 80 ˚C under vacuum to remove residual MeCN, we performed dinitrogen sorption experiments at 77 K. The results, shown in Figure 4, indicate permanent porosity in Ni-ISOCN-2 by virtue of the steep N2 uptake in the lowpressure region. Based on this data, the surface area of NiISO CN-2 is estimated to be 580 m2/g using the Brunauer-Emmet-Teller (BET) theorem. Notably, this result is consistent with with Düren-type accessible surface area calculations on Ni-ISOCN-2·([CNArMes2]2)0.5 (ASA = 492 m2/g),31 when reflection data for the co-crystallized [CNArMes2]2 ligand is removed from the diamondoid void space.32 We believe this positive correlation provides further evidence for a diamondoid net structural formulation for Ni-ISOCN-2. Most importantly, the non-standard behavior exhibited in the high-pressure region (P/Po = 0.96) of the adsorption isotherm, as well as the stepped desorption isotherm profile, strongly suggest the presence of microporous domains in NiISO CN-2 (Figure 4).33 Additionally, the significant hysteretic loop indicates substantial pore blocking occurs in some of the mesoporous regions of Ni-ISOCN-2, which is similar to the observed effect of encapsulated micropores in zeolitic solids.34 We contend that this observed effect results from the m-terphenyl mesityl substituents along the biphenyl backbone of [CNArMes2]2, which creates multiple micro-domains for N2 entrapment. In addition to the structural data derived from Ni-ISOCN-2·([CNArMes2]2)0.5, we base this

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upon the three unique features observed in the desorption isotherm. In the initial step (I), N2 is quickly desorbed upon moving to lower pressure until P/Po = 0.94, at which point N2 desorption decreases. At P/Po = 0.48 (II), N2 is abruptly desorbed; however, this does not lead to closure of the hysteresis loop, but rather to a steady desorption of N2 (III) until very low partial pressure (P/Po = 0.003). Most surprisingly, the data in Figure 4 indicate that despite full evacuation of the sample chamber, 8.6 mol N2 per mol Ni-ISOCN-2 remains trapped. While unusual for metal-organic systems, this phenomenon has been observed in certain mesoporous materials that display structural flexibility under pressure flux. 35,36 Accordingly, a full investigation of this behavior towards N2 and other guest molecules is ongoing. In conclusion, we report a porous metal-organic material constructed from zero-valent-metal nodes. While direct structural characterization of Ni-ISOCN-2 was not available, we developed an understanding of its framework architecture with the isolation of Ni-ISOCN-2·([CNArMes2]2)0.5, showing the network to likely be of a two-fold interpenetrated, diamondoid type. ATR-IR and XANES data indicate that Ni-ISOCN-2 features a tetrakis-isocyanide Ni(0) node in a tetrahedral geometry, similar to that found for Ni-ISOCN2·([CNArMes2]2)0.5 and the analogous molecular species Ni(CNArMes2)4. Importantly, this work demonstrates the feasibility of constructing metal-organic materials that possesses very low-valent metal nodes through the use of isocyanide linker groups. We believe the inclusion of low-valent metal sites as part of the structural units within coordination frameworks will enable new avenues to customizable materials with emergent properties.

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ASSOCIATED CONTENT Supporting Information. Synthetic procedures, characterization and spectroscopic data (PDF and CIF). This material is available via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

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Corresponding Author (26) (27)

[email protected] Funding Sources No competing financial interests have been declared.

ACKNOWLEDGMENT This work was supported by the U.S. Department of Energy (DE-SC0008058 to J.S.F.) and the Department of Education (via a GAANN Fellowship to D.W.A.). K.M.L acknowledges support from the U.S. National Science Foundation (CHE1454455) and the Alfred P. Sloan Foundation. XAS data were obtained at SSRL, which is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Contract No. DE-AC02-76SF00515. The SSRL Structural Molecular Biology Program is supported by the Department of Energy’s Office of Biological and Environmental Research, and by NIH/HIGMS (including P41GM103393).

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REFERENCES (1)

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