Low-Coordinate Cobalt(I) Complexes Stabilized by an Extremely

Communication pubs.acs.org/Organometallics

Low-Coordinate Cobalt(I) Complexes Stabilized by an Extremely Bulky Amide Ligand Jamie Hicks and Cameron Jones* School of Chemistry, Monash University, P.O. Box 23, Melbourne, Victoria 3800, Australia S Supporting Information *

ABSTRACT: A series of low-coordinate, high-spin, mono- and dinuclear cobalt(I) complexes bearing an extremely bulky amide (L″ = N(Ar*)(SiPh3); Ar* = C6H2{C(H)Ph2}2Me-2,6,4) ligand have been synthesized and characterized. These include the first example of a neutral two-coordinate cobalt(I) complex, [L″Co(IPriMe)] (IPriMe = :C{N(Pri)C(Me)}2), which has a near-linear cobalt coordination geometry.

he chemistry of low-coordinate, carbonyl-free first-row transition-metal(I) complexes has rapidly emerged over the past decade, and these highly reactive systems have found a variety of applications in, for example, synthesis, catalysis, and small-molecule activations.1 Low-coordinate cobalt(I) compounds have proved especially useful in these areas, and as a result, they are more prevalent than the majority of their first row d-block counterparts. While multidentate anionic ligands are effective at stabilizing cobalt(I) complexes, more coordinatively unsaturated complexes incorporating bulky lower-dentate ligands have proved more reactive in their further chemistry.2 Important examples here include the threecoordinate guanidinate-bridged compound 13 and arenecapped β-diketiminato cobalt(I) complexes, e.g. the “masked” two-coordinate system 2 (Figure 1).4−6 The low metal

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coordination numbers in these species have led to their use as effective reagents in inorganic synthesis, N2 activation, C−F bond activation, and so forth. Of course, related complexes bearing monodentate ligands should be more coordinatively unsaturated and, therefore, more reactive. However, as far as we are aware, the only anionic monodentate ligands to be applied to the stabilization of neutral, low-coordinate cobalt(I) complexes have been bulky terphenyls. These have been utilized by Power and co-workers in the synthesis of the arenecapped species 37 and the weakly cobalt−cobalt bonded dimer 4.8,9 We have developed an extremely bulky class of amide ligands (e.g., −N(Ar*)(SiR3); Ar* = C6H2{C(H)Ph2}2Me-2,6,4; R = Me (L′), Ph (L″)),10 which we have shown to possess stabilizing properties similar to those of Power’s terphenyls in studies of low-oxidation-state p-block compounds.11 Recently, we have extended this work to the d block, with the stabilization of a range of low-oxidation-state manganese complexes, including unprecedented examples of two-coordinate MnI dimers, e.g. [L′MnMnL′].12 It seemed reasonable that these bulky amides could be similarly applied to boosting the sparse ranks of very low-coordinate cobalt(I) compounds bearing monodentate ligands. Here, we report on several such compounds. Reductions of the cobalt(II) chloride precursor complex [{L″Co(THF)(μ-Cl)}2]13 with either KC8 or the magnesium(I) dimer [{(MesNacnac)Mg}2] (MesNacnac = [(MesNCMe)2CH]−, Mes = mesityl)14 under an atmosphere of dinitrogen afforded good yields of the green benzene-capped cobalt(I) complex 5 when the former reduction was carried out Special Issue: Mike Lappert Memorial Issue

Figure 1. Examples of low-coordinate cobalt(I) complexes (Dip = C6H3Pri2-2,6; Tip = C6H2Pri3-2,4,6; Cy = cyclohexyl).3−9 © XXXX American Chemical Society

Received: December 3, 2014

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DOI: 10.1021/om501233f Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics

center of which can coordinate THF and will activate the C−F bond of fluorobenzene under mild conditions.5,6 So as to access an adduct of the L″Co fragment, a benzene solution of compound 5 was treated with the strongly nucleophilic N-heterocyclic carbene (NHC) IPriMe (:C{N(Pri)C(Me)}2), which afforded a low isolated yield of the yellow two-coordinate cobalt(I) complex 7 after recrystallization of the crude product from diethyl ether (Scheme 1).15 Interestingly, when this compound was recrystallized from pentane, it was isolated as a red crystalline solid in a different, intramolecularly arene coordinated, isomeric form. Little useful information could be gained from the 1H NMR spectra of 5−7, all of which exhibit broad signals over wide chemical shift ranges due to the paramagnetic nature of the compounds. It is of note that both isomeric forms of 7, on dissolution in d6-benzene, yielded yellow solutions that exhibited identical 1H NMR spectra, with 10 observable resonances. This implies that the compound exists in solution, most likely as the more symmetrical two-coordinate form. The room-temperature solution-state magnetic moments (Evans method) of the compounds were determined to be 2.8 ± 0.1 μB for 5 (d6-benzene solution), 2.8 ± 0.1 μB per Co center for 6 (d8-THF solution), and 2.6 ± 0.1 μB for 7 (d6benzene solution). All of these values are consistent with the compounds possessing high-spin (S = 1, μso = 2.83 μB) electronic configurations for their d 8 cobalt(I) atoms. Furthermore, the measured magnetic moment for 6 remains essentially constant over the temperature range −65 to +55 °C, suggesting that there is minimal magnetic communication between the two Co centers. The magnetic moment for 5 is comparable to those for 1 (2.7 μB),1 2 (3.07 μB),5 and 3 (R = H, 2.35 μB),7 all of which were proposed to contain high-spin (S = 1) cobalt(I) centers. In contrast, the dimeric compound 4, which is most closely related to 6, was found to have a very low magnetic moment, due either to strong antiferromagnetic coupling between its cobalt centers or to those centers having isolated low-spin d8 electronic configurations. Although the Co···Co distance in 4 is quite short (2.8033(5) Å), it is well outside the sum of two covalent radii for two Co atoms (viz. 2.46 Å16). As a result, the latter explanation was seen as most likely. In fact, prior calculations on [MeCo(η6-C6H6)] showed that this compound is high-spin when linear, but when the C− Co-centroid angle reaches 135° it becomes low-spin.17

in benzene or when the product from the latter reduction was recrystallized from that solvent (Scheme 1). In contrast, when Scheme 1. Synthesis of 5−7a

a

Ar* = C6H2{C(H)Ph2}2Me-2,6,4.

the reduction of [{L″Co(THF)(μ-Cl)}2] with KC8 was carried out in THF, the pale green-brown dimeric complex 6 was formed in good yield. This indicates that intermolecular coordination of the cobalt center of the L″Co fragment by one phenyl group from the amide ligand is preferential to coordination by THF. Similarly, dissolution of 5 in THF did not lead to displacement of the coordinated benzene molecule by the ether but instead, upon heating to 60 °C, benzene loss occurred, and compound 6 was generated. Dissolution of 6 in benzene, under a dinitrogen atmosphere, led to the rapid regeneration of 5. The lack of reactivity of 5 and 6 toward THF is comparable to the stability of solutions of the closely related compounds 3 and 4 in that solvent.7,8 It is also worthy of mention that treating solutions of either 5 or 6 with toluene, fluorobenzene, 1,2-difluorobenzene, or hexafluorobenzene led to no reaction (other than the slow conversion of 5 to 6). In this respect, 5 and 6 are apparently broadly less reactive than Holland’s “masked” two-coordinate compound 2, the cobalt(I)

Figure 2. Thermal ellipsoid plots (20% probability surface) of (a) 5 and (b) 6. B

DOI: 10.1021/om501233f Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics

Figure 3. Thermal ellipsoid plot (20% probability surface) of the (a) two-coordinate and (b) higher coordinate isomeric forms of 7.

circumvented in the near-linear form of 7, as the compound packs in the crystal lattice with relatively close intermolecular contacts between the p-methyl group of one molecule and the Co center of another (closest methyl proton···Co distance: 3.27 Å). It is worthy of mention that the near-linear form of 7 includes a diethyl ether of solvation in its crystal lattice, while the crystal lattice of the other form incorporates a pentane of solvation. This difference could also affect the crystal packing of the respective isomeric forms of the compound. In summary, a series of low-coordinate mono- and dinuclear cobalt(I) complexes bearing an extremely bulky amide ligand have been synthesized. These encompass the benzene-capped system [L″Co(η6-C6H6)] (5), which readily loses its benzene ligand in solution to give the dimer [{L″Co}2] (6). The benzene ligand of 5 can also be displaced by the NHC IPriMe to give the first example of a neutral two-coordinate cobalt(I) complex, [L″Co(IPriMe)] (7), which has a near-linear coordination geometry. A higher coordinate, nonlinear isomeric form of this compound has also been characterized. All prepared complexes possess high-spin (S = 1) cobalt centers in solution, and there appears to be little magnetic communication between both of those centers in the dimer [{L″Co}2]. We are currently exploring the solid-state magnetic properties and further reactivity of the low-coordinate cobalt(I) complexes described here. Our efforts in this direction will be reported in due course.

Although the C−Co−centroid angles in 4 are more open (143.7°), they are not greatly so. So as to draw comparisons between the geometries of 5 and 6 and those of 3 and 4, the former were crystallographically characterized. The molecular structure of 5 (Figure 2a) shows it to be a benzene-capped monomer similar to 3. Its N−Co distance (1.9166(19) Å) is close to that in the precursor complex [{L″Co(THF)(μ-Cl)}2] (1.936(2) Å),13 and its N center is planar (∑(angles) = 359.9°). The Co−centroid distance in the compound (1.647(3) Å) is almost equivalent to that in 3 (R = H: 1.634(2) Å), while the N−Co−centroid angle is 166.3°. Compound 6 (Figure 2b) is a dimer with one phenyl group from each monomeric unit coordinated to the Co center of the other in an η6 fashion. The Co−N and Co−centroid distances (1.933 Å mean, and 1.634 Å mean, respectively) in the compound are reminiscent of those in 5, while the N−Co− centroid angles in 7 (159.2° mean) are smaller than those in 5 but significantly more open than the angles in 4.8 This, combined with the much larger Co···Co separation in 6 (3.633 Å), offers an explanation as to why 6 possesses two isolated high-spin cobalt(I) centers, whereas 4 is essentially diamagnetic. The molecular structures of both isomeric forms of 7 are depicted in Figure 3. That for the two-coordinate variant shows it to have a near-linear N−Co−C fragment (175.2(1)°), with an N−Co distance (1.879(2) Å) that is, not surprisingly, slightly shorter than those in the higher coordinate complexes 5, 6, and the arene-coordinated isomeric form of 7 (1.979(3) Å). The Co−C separation in the compound (1.962(3) Å) is similar to those in other IPriMe complexes of cobalt, e.g. 1.962 Å (mean) in [CoPh2(IPriMe)2],18 though shorter than the equivalent bond in arene-coordinated 7 (2.009(4) Å). It is worthy of note that a closely related, near-linear, twocoordinate amido/NHC ligated nickel(I) complex, [Ni{N(SiMe3)(Dip)}(IPr)] (IPr = :C{N(Dip)C(H)}2), has recently been reported by Tilley and co-workers,19 while several twocoordinate homoleptic complexes of cobalt(I) and cobalt(0) have been described.9,20 That said, as far as we are aware, 7 represents the first neutral two-coordinate cobalt(I) complex to be structurally characterized. The other isomeric form of 7 is considerably distorted from linear (N−Co−C: 140.16(13)°), presumably due to a close interaction with the ipso carbon of one phenyl group from the amide ligand (Co(1)−C(22): 2.103(3) Å). It is apparent that such an interaction is

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ASSOCIATED CONTENT

S Supporting Information *

Text, figures, a table, and CIF files giving details of the synthesis and characterization data for all new compounds, full details and references for the crystallographic studies, and crystallographic data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for C.J.: [email protected]. Notes

The authors declare no competing financial interest. C

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successful, its X-ray crystal structure is included in the Supporting Information. (16) Pauling, L. Proc. Natl. Acad. Sci. U.S.A. 1976, 73, 4290. (17) La Macchia, G.; Gagliardi, L.; Power, P. P.; Brynda, M. J. Am. Chem. Soc. 2008, 130, 5104. (18) Mo, Z.; Liu, Y.; Deng, L. Angew. Chem., Int. Ed. 2013, 52, 10845. (19) Lipschutz, M. I.; Tilley, T. D. Organometallics 2014, 33, 5566. (20) (a) Mondal, K. C.; Roy, S.; De, S.; Parameswaran, P.; Dittrich, B.; Ehret, F.; Kaim, W.; Roesky, H. W. Chem. Eur. J. 2014, 20, 11646. (b) Mo, Z.; Chen, D.; Leng, X.; Deng, L. Organometallics 2012, 31, 7040.

ACKNOWLEDGMENTS Financial support from the Australian Research Council and the U.S. Air Force Asian Office of Aerospace Research and Development (FA2386-14-1-4043) is acknowledged. The EPSRC National Mass Spectrometry Facility is also thanked. Part of this research was undertaken on the MX1 beamline at the Australian Synchrotron, Victoria, Australia.

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DEDICATION

Dedicated to the memory of Michael F. Lappert.

REFERENCES

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DOI: 10.1021/om501233f Organometallics XXXX, XXX, XXX−XXX