Three-Fold-Symmetric Selenium-Donor Metallaboratranes of Cobalt

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Cite This: Inorg. Chem. 2017, 56, 12670-12673

Three-Fold-Symmetric Selenium-Donor Metallaboratranes of Cobalt and Nickel Stefan Holler,† Michael Tüchler,† Michaela C. Roschger,† Ferdinand Belaj,† Luis F. Veiros,‡ Karl Kirchner,§ and Nadia C. Mösch-Zanetti*,† †

Institute of Chemistry, University of Graz, Schubertstrasse 1, 8010 Graz, Austria Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais No. 1, 1049-001 Lisboa, Portugal § Institute of Applied Synthetic Chemistry, Vienna University of Technology, Getreidemarkt 9, 1060 Vienna, Austria ‡

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the literature despite their expected diverse electronic features compared to the sulfur derivatives. We have previously found that thiopyridazine-based scorpionate ligands exhibit a significantly higher tendency to form boratrane complexes versus [TmR].24−27 For this reason, we envisioned that a selenopyridazine might enable the formation of elusive boratrane compounds. Therefore, 6-tert-butyl-4-methylpyridazin-3(2H)-one was reacted with a Woollins reagent (WR), which led to the desired HPnse in 52% yield after sublimation under an inert atmosphere (Scheme 1). From the sublimation residue, by washing with

ABSTRACT: A novel selenium-containing pyridazinylbased soft scorpionate ligand (KTnse) was synthesized. It reacts with CoCl2 and NiCl2, yielding the first metallaboratrane complexes with selenium in their donor positions. Further substitution with Ag(OTf) or NaN3 allows isolation of the respective triflate or azide complexes. Reaction with Ag(OTf) leads in the case of nickel to a dinuclear, dicationic complex with a short Ni− Ni distance, while cobalt gave a mononuclear cationic species. Substitution of the chloride by azide yields with both metals the respective azide complexes. All compounds were characterized via single-crystal X-ray diffraction analysis. Density functional theory calculations on the chloride species point to oxidized cobalt(III) and nickel(III) centers.

Scheme 1. Preparation of KTnse [WR = (PhPSe2)2]

omplexes of the Z-type, which are defined containing σacceptor ligands, have gained considerable attention in modern coordination chemistry.1−5 Among them, the class of metallaboratranes exhibiting borane ligands with three donor arms, forming cagelike tricyclic structures with a metal and a boron atom in the bridging head positions, have moved from mere curiosity to a frequently used motif. Corresponding complexes found widespread applications including reduction of dinitrogen6,7 and reversible hydride migration between metal and boron.8−11 Although variation of the donor atoms seems to be the best way of influencing the electronic properties, only a few different metallaboratrane systems are known, including [BN3], [BP3], and [BS3] arrangements.5,9 In this paper, we introduce a new class of boratrane complexes that feature the as-yet-elusive [BSe3] array. In general, tripodal selenium ligands are rare.12,13 While the tris(2-mercapto-1-Rimidazolyl)hydroborato ligand [TmR], introduced by Spicer and Reglinski,14 is commonly used, the analogous selenium version has only been coordinated to few metals, as described by Parkin and co-workers, where they found it to be a better donor than its sulfur counterpart.15−18 Besides these anionic ligands, some examples of metal complexes containing the tripodal seleno ether ligand MeC(CH2SeMe)3 were reported.19−23 However, selenium-containing boratrane compounds are consistently absent in

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toluene, the zwitterionic side product HPnPSe2Ph could be obtained in 18% yield. Experimental data and molecular structures, as determined by single-crystal X-ray diffraction analyses, can be found in the Figures S1 and S2. The potassium salt of the scorpionate compound tris(4methyl-6-tert-butyl-3-selenopyrazinyl)borate (KTnse) was obtained in 72% yield by the reaction of 2.5 equiv of HPnse with 1 equiv of KBH4 in refluxing mesitylene for 24 h under exclusion of light. To ensure complete conversion of HPnse, which is difficult to remove, excess borohydride was used and removed by recrystallization. While the product is stable in the solid state, it decomposes in solution upon exposure to daylight, a behavior that is consistent with its sulfur analogue KTn.28 The 1H NMR spectrum is similar to that of the sulfur analogue KTn, albeit downfield shifted, implying lower electron density of the heterocycles in the selenium ligand (Table S1).24−26 This is in contrast to previous observations where selenium ligands were found to be the better donors.15−18 Received: July 10, 2017 Published: October 16, 2017 12670

DOI: 10.1021/acs.inorgchem.7b01730 Inorg. Chem. 2017, 56, 12670−12673

Communication

Inorganic Chemistry The molecular structure of KTnse, as determined by singlecrystal X-ray diffraction analysis, reveals a dimeric nature with one molecule of tetrahydrofuran (THF; crystallization solvent) coordinated to the potassium atom (Figure 1). This is similar to the dimeric and polymeric structures of sulfur-based pyridazine scorpionate salts.24,25

Figure 2. Molecular views of 1 (left) and 2 (right). Hydrogen atoms and solvent molecules were omitted for clarity.

Within pyridazine boratrane complexes, the bond distances do not vary significantly with changes in the donor atom (Table 1). Table 1. Selected Bond Lengths (Å) and Angles (deg) of 1, 2, and Their Sulfur Analogues [M{B(Pn)3}Cl] [Se3] donor set

Figure 1. Molecular view of KTnse. The methyl and tert-butyl groups as well as hydrogen atoms (except BH) were omitted for clarity. M−B M−Cl B−M−Cl

Treatment of the divalent metal halides CoCl2 and NiCl2 with KTnse under exclusion of light yields [M{B(Pnse)3}Cl] [M = Co (1), Ni (2)] as the first boratrane complexes featuring a Ztype ligand with donating selenium atoms (Scheme 2). Gas

[S3] donor set26

Co1

Ni2

Co

Ni

2.078(6) 2.3043(16) 176.6(2)

2.023(4) 2.3168(10) 169.66(11)

2.064(2) 2.2878(6) 176.56(8)

2.027(3) 2.3193(7) 177.08(9)

While in the cobalt complexes, the [Se3] donor set leads to slightly longer M−B and M−Cl bond lengths compared to the respective [S3], the differences in the nickel complexes are below 3σ. These found small differences prevent a meaningful interpretation of the influence of the change from sulfur to selenium based on the structural data. For a better understanding of their reactivity and stability, to complexes 1 and 2 were added Ag(OTf) and NaN3, respectively (Scheme 3). The triflate complexes [Co{B(Pnse)3}(NCMe)]-

Scheme 2. Preparation of Selenium-Containing Boratrane Complexes 1 and 2

Scheme 3. Substitution Reactions of Complexes 1 and 2

evolution is apparent, consistent with dihydrogen formation delivering the electron for the formal reduction of the metal.27 In contrast to KTnse, both complexes are stable at ambient atmosphere and daylight. As expected for paramagnetic species, compounds 1 and 2 exhibit broad resonances in their 1H NMR spectra and are shifted to higher fields compared to the analogous [S3] boratrane complexes.26 This might indicate better donor properties of the [S3] motif compared to [Se3], consistent with the data found for the potassium salts. The successful formation of 1 and 2 was also confirmed by single-crystal X-ray diffraction analysis. Both complexes reveal five-coordinate metal atoms in a slightly distorted trigonalbipyramidal environment (Figure 2. Metal boron bond lengths are 2.078(6) Å in 1 and 2.023(4) Å in 2 and thus significantly shorter compared to other boratrane systems. While nickel boratranes with methimazolyl-based systems reveal Ni−B bond lengths of 2.08−2.11 Å29,30 and a triphosphineborane-based system 2.168 Å,31 cobalt boratrane complexes are extremely rare. There are six reported examples with a triphosphineborane ligand, revealing Co−B bond lengths between 2.256 and 2.463 Å32,33 and one tris(methimazolyl)borane-based complex with a Co−B bond length of 2.131 Å.34

(OTf) (3a) and [Ni{B(Pnse)3}]2(OTf)2 (4a) were obtained by the reaction of the respective chloride precursor with Ag(OTf) in acetonitrile in moderate yields as brown solids (Scheme 3). In the monomeric cationic complex 3a, as determined by X-ray diffraction analysis (Figure 3), the cobalt center is coordinated in a distorted trigonal-bipyramidal fashion by the ligand and an additional acetonitrile molecule in its axial position [Co1−N1 2.047(2) Å]. Its Co1−B1 distance is, with 2.059(3) Å, significantly shorter compared to that of 1 [2.078(6) Å], indicating a stronger bonding interaction. The structure is 12671

DOI: 10.1021/acs.inorgchem.7b01730 Inorg. Chem. 2017, 56, 12670−12673

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Inorganic Chemistry

are, with 0.83 for cobalt and 0.84 for nickel, quite high, indicating high oxidation states and well established B−M bonds, corresponding to a strong coordination of the boron atom. Wiberg indices for the M−B bonds are 0.40 (cobalt) and 0.44 (nickel). The calculation of different spin states, exemplified by complex 1, revealed the triplet to be the most stable configuration in accordance with the experimental data, while the singlet state is 29.1 kcal/mol higher in energy and leads to a tetrahedral geometry without Co−B interaction. Also, the quintet state is 16.5 kcal/mol less stable. In conclusion, we present a straightforward synthetic route for a selenium-containing soft scorpionate ligand, leading to the first metallaboratrane complexes with the [BSe3] coordinating moiety. The axial position trans to boron of these trigonal bipyramidal complexes is found to be labile towards substitution allowing formation of chloride, azide and triflate compounds. A comparison to the related sulfur ligand is challenging because the differences of the data are relatively small so that the final conclusions must be met with caution. Nevertheless, the data suggest a somewhat weaker donating ligand compared to sulfur. The electronic properties of the cobalt and nickel chloride complexes indicate high oxidized Co(III) and Ni(III) centers with strong B−M bonding interactions. The soft nature of the selenium-donor atom is expected to stabilize metal centers in lower oxidation states and will therefore enable reactions that are less likely with nitrogen-, sulfur-, and phosphorus-based systems.

Figure 3. Molecular structures of 3a (left) and 4a (right). Ligand backbones and triflate anions were omitted for clarity.

isotypic to the previously reported sulfur-coordinated copper boratrane compound [Cu{B(Pn)3}(NCMe)](OTf).25 In 4a, the two distorted trigonal-bipyramidal nickel centers are coordinated by the central boron atom and four selenium-donor atoms, resulting in a short Ni−Ni contact of 2.6669(11) Å (Figure 3). The Ni1−B1 bond length is, with 2.055(6) Å, slightly elongated compared to that of 2 [2.023(4) Å], which might be attributed to the higher coordination number in 4a. As expected, the bridging axial Ni−Se bonds are longer compared to the bridging equatorial ones [Ni1−Se4 2.4753(10) Å vs Ni1−Se1 2.3962(11) Å], and the nonbridging Ni−Se bonds are, with 2.3329(10)−2.3472(9) Å, even shorter. A similar coordination motif, albeit with a tris(thiophenol)phosphane [PS3] donor set, has previously reported a NiIII−NiIII distance of 2.603 Å35 and NiII−NiIII distances of 2.50036 and 2.609 Å.35 The reaction of complexes 1 and 2 with NaN3 in refluxing acetone leads to isolation of the azide complexes 3b and 4b in good-to-moderate yields (Scheme 3). Their formation is clearly evidenced by strong absorptions in the IR spectra at 2035 cm−1 (3b) and 2031 cm−1 (4b), which is typical for the asymmetric stretching frequency of metal azides.37 While 1H NMR spectroscopy revealed full conversion of 1 after 4 h, the respective nickel complex revealed only 66% conversion. They can be recrystallized from a methylene chloride/heptane solution to obtain 4b in 58% yield as brown-orange needles. The X-ray structure of 4b is consistent with the NMR data, revealing the distorted trigonal-bipyramidal nickel complex as a cocrystal of 4b and 2 in the ratio of 2:1 (Figure S8). Structurally characterized azide metallaboratrane complexes are extremely rare because only one nickel29 and two copper25 complexes were previously described. The relatively short M−B bonding distance of 4b [2.029(5) Å] indicates a stronger metal−boron interaction than those of the literature known compounds [2.079(13) Å for nickel complexes and 2.059(4) and 2.068(4) Å for copper complexes].25,29 In complexes with Z-type ligands, electron counting rules are somewhat more challenging because two electrons may be considered to remain at the metal [here M(I)] or to donate to the ligand with a formal increase of the oxidation state [here M(III)].38,39,4 For this reason, the electronic nature of complexes 1 and 2 was investigated by means of DFT/PBE0 calculations. The optimized structures (Figure S9) are in good agreement with the experimentally obtained data, and the frontier orbitals are provided in the Figure S10. The d splittings in both cases show one empty d orbital (z2 if the B−M−Cl direction is the z axis) and two half-filled d orbitals in the case of cobalt and one-half-filled d orbital for nickel that correspond to CoIII d6 (with S = 1) and NiIII d7 (with S = 1/2), respectively. The atomic charges (natural population analysis)

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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.inorgchem.7b01730. Synthesis, experimental details, X-ray structure determination, and DFT calculations (PDF) Accession Codes

CCDC 1554738−1554742 and 1574305−1574307 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Luis F. Veiros: 0000-0001-5841-3519 Nadia C. Mösch-Zanetti: 0000-0002-1349-6725 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge support from NAWI Graz. REFERENCES

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