Chameleon-like Nature of Anagostic Interactions and Its Impact on

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Chameleon-like Nature of Anagostic Interactions and Its Impact on Metalloaromaticity in Square-Planar Nickel Complexes Mariusz P. Mitoraj,*,† Maria G. Babashkina,‡,§ Koen Robeyns,‡ Filip Sagan,† Dariusz W. Szczepanik,† Yulia V. Seredina,§ Yann Garcia,‡ and Damir A. Safin*,‡,§ †

Department of Theoretical Chemistry, Faculty of Chemistry, Jagiellonian University, Gronostajowa 2, 30-387 Cracow, Poland Institute of Condensed Matter and Nanosciences, Université Catholique de Louvain, Place L. Pasteur 1, 1348 Louvain-la-Neuve, Belgium § Institute of Chemistry, University of Tyumen, Perekopskaya Street 15a, 625003 Tyumen, Russian Federation Downloaded via AUBURN UNIV on April 24, 2019 at 14:41:09 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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S Supporting Information *

ABSTRACT: Anagostic C−H···M (M = a metal center) intramolecular interactions, one of the most fundamental and elusive forces in organometallic chemistry, are intuitively considered as repulsive and purely electrostatic in nature because of significant metal−hydrogen distances (∼2.3−3.0 Å). Contrary to the current state of knowledge, it is shown herein by quantum chemical computations based on the case study of new square-planar NiII isomers based on Nthiophosphorylated thiourea that despite significant metal− hydrogen anagostic distances, the covalent-type charge delocalization contribution [Ni(dz2) → σ*(C−H) and σ(C− H) → Ni(dz2)] exists and it covers, together with the London dispersion energy, up to ∼40% of the overall anagostic stabilization. This charge delocalization component is found to amplify the metalloaromaticity phenomenon although a lack of any stabilizing charge transfer is expected at such long metalhydrogen distances (>3 Å). Remarkably, for the relatively short regime ( 3 Å) anagostic distances (Figures 3, S6, S8−S10 in the Supporting Information). Such unfavorable Coulomb forces for d(C−H···Ni) < 3 Å together with the destabilizing self-atomic deformation energies make the overall anagostic binding energies in the optimized monomers 1 and 2 positive (destabilizing), ΔEbind = 2.95 kcal/mol, ΔEbind = 2.53 kcal/mol, respectively (Figures S6, S8−S10 in the Supporting Information). Note that for longer anagostic crystal-based distances, anagostic binding D

DOI: 10.1021/acs.organomet.9b00062 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Figure 5. EDDB contours and populations for the crystal monomers of 1 (top) and 2 (bottom). Additionally, contributions to the metal populations are depicted from σ, π, and anagostic charge delocalizations (cyan numbers refer to the corresponding systems with the isopropyl groups replaced by hydrogen atoms).

corresponding contribution of 0.03 |e| (Figure 5). Similar conclusions are valid for the optimized monomers (Figure S11 in the Supporting Information). It should be noted that no similar Ni(dz2)-based anagostic contributions to the metalloaromaticity are identified in the corresponding systems with the isopropyl groups replaced by hydrogen atoms, giving rise to a dramatic reduction of the global aromaticity for about 7.7% (∼2.6 |e|) as well as the drop of metal contribution to πmetalloaromaticity by about 13% in both complexes (Figure 5, Tables S6 and S7 in the Supporting Information). The calculated negative NICS values as well as the ACID plots further suggest (weakly) the metalloaromatic character of these species (Figure S12 in the Supporting Information). Finally, to provide qualitative and quantitative pictures of noncovalent interactions present in the R22(16) dimer of 2, the ETS-NOCV charge and energy decomposition calculations based on ADF/BLYP-D3/TZP83,84 were performed. It is evident that a very high dimeric stability, as indicated by the overall interaction energy ΔEtotal = −46.29 kcal/mol, originates mostly from the London dispersion forces, which covers 50% of the overall stability (ΔEdisp + ΔEorb + ΔEelstat), followed by quite similarly important charge delocalization (20%) and ionic contributions (30%) (Figure S13 in the Supporting Information). The overall deformation density Δρorb demonstrates that the dimer of 2 is formed due to bifurcated intermolecular hydrogen bonds N−H···O and N−H···S, N− H···π interactions, and additionally supported by charge delocalizations (containing also monomers polarizations)

Figure 4. The intermolecular model of anagostic C−H···Ni interactions in 1 containing methane and the Ni fragment (the iPr units are replaced by the corresponding hydrogen atoms) (top) together with the ETS-NOCV-based results (bottom).

delocalization is much less marked (∼27% in 1 and 2) (Figure 5). Notably, no δ-type bonding topology, characteristic for Möbius aromatics,78−81 is observed in both cases. It is crucial to emphasize that 0.050 |e| in 1 (4.9%) and 0.031 |e| in 2 (3.1%), delocalized through the metallic center, are associated with the presence of anagostic interactions (Figure 5). Clearly, it was identified in the previous paragraphs (Figures 2 and S3 in the Supporting Information) subtle covalent-type charge delocalization channel due to anagostic C−H···Ni contacts leads to accumulation of the additional electron density around the metal center causing amplification of the metalloaromaticity phenomena (Figure 5). Consistently, 1, exhibiting two anagostic contacts, shows a higher contribution of 0.05 |e|, whereas 2 with only one such contact leads to the E

DOI: 10.1021/acs.organomet.9b00062 Organometallics XXXX, XXX, XXX−XXX

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Organometallics within π···π and C−H···O contacts (Figure S13 in the Supporting Information). Finally, we briefly complement that the presence of four bulky iPr units in each monomer not only leads to the formation of intramolecular anagostic contacts but also makes the crystals highly dominated by intermolecular H···H, H···F, H···S, and H···C contacts due dihydrogen C− H···H−C and other noncovalent interactions as demonstrated by our experimental (Figures S14 and S15, and Table S8 in the Supporting Information) and theoretical results (Figures S13 and S16 in the Supporting Information). Here, it is worth summarizing that London dispersion forces primarily glue the obtained crystal structures, in line with the recent discoveries delineating the true nature of steric crowding in bulky systems.38−42

emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. CCDC 1454954 and 1454955 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 e-mailing [email protected] or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.; Fax: +44 1223 336033.

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Corresponding Authors

*E-mail: [email protected] (M.P.M.). *E-mail: damir.a.safi[email protected], d.a.safi[email protected] (D.A.S.).

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CONCLUSIONS In summary, the anagostic contacts C−H···Ni in the newly synthesized square-planar NiII isomers [Ni{trans-L-1,5-S,S′)2] and [Ni{cis-L-1,5-S,S′)2], one of the most elusive and poorly understood interactions in chemistry including catalysis and organometallics, are found to be determined, similar to other noncovalent interactions, by typical constituents: the covalenttype charge delocalizations Ni(dz2) → σ*(C−H) and σ(C−H) → Ni(dz2), and London dispersion and electrostatic forces. The former two jointly cover ∼40% of the overall stabilization. This is a remarkable result in the light of the fact that it occurs at extremely large metal−hydrogen distances (>3 Å) where lack of any covalent-type bonding is intuitively expected. Anagostic interactions have, till now, been intuitively considered as repulsive and purely electrostatic in nature. Remarkably, the latter Coulomb contribution is found to be repulsive or attractive depending on a metal−hydrogen distance, what in turn, together with the steric (Pauli/kinetic) component, dictates the overall chameleon-like nature of anagostic interactions. It was found that for the relatively short regime (