Synthesis, structure, and theoretical studies of a calcium complex of a unique dianion derived from 1-methylpyrrolidin-2-one

The synthesis, structure, and theoretical studies of a calcium complex of a unique dianion derived from N-methyl-2-pyrrolidine are explored.


Chemical context
There has been recent interest in ternary sulfides as two-color IR optical window materials (Jarý et al., 2015) as well as other uses, such as phosphor materials (Sun et al., 1994). In the synthesis of alkaline earth ternary sulfides, reactions using metal thiolates and H 2 S are an obvious avenue of study. An obstacle to such work is the lack of soluble alkaline earth thiolates. As part of a program for the investigation of precursors for the synthesis of a wide variety of metal sulfide materials, the reactions between alkaline earth amides and thiolate ligands were explored (Purdy et al., 1997). When the barium complex, Ba(SCMe 3 ) 2 , was crystallized from a mixed NMP solution over a period of years, an unusual barium sulfur cluster was obtained, [Ba 6 (C 4 H 9 S) 10 S(C 5 H 9 NO) 6 ], containing a central 6 -sulfido atom surrounded by six Ba atoms and NMP ligands (Butcher & Purdy, 2006). On the other hand, when solutions of the analogous calcium complex are substi- ISSN 2056-9890 tuted, these solutions turn blue over time (or more quickly when heated).
The solvent NMP, along with the presence of calcium ions, appears to play a crucial role in this reactivity. Solutions of calcium ions in N-methyl-2-pyrrolidine have shown unusual reactivity in many areas, including the synthesis of thermally stable polyamides (Mallakpour & Kolahdoozan, 2008;Faghihi, 2009;Faghihi et al., 2010;Dewilde et al., 2016), the synthesis and structural studies of functional coordination polymers from calcium carboxylates based on cluster-and rod-like building blocks (Kang et al., 2014), dental applications using calcium hydroxide paste along with NMP (Lim et al., 2017;Kim et al., 2020), the formulation of solid self-nanoemulsifying drug-delivery systems (Agrawal et al., 2015), and in lyotropic liquid crystalline behavior of poly(2-cyano-p-phenylene terephthalamide) in N-methyl-2-pyrrolidone/calcium chloride solutions (Jung et al., 2016).
The results of this unusual reactivity are explored in this paper.

Structural commentary
The title compound, C 30 H 44 CaN 6 O 6 S 2 , 1, crystallizes with the triclinic space group, P1. The Ca atoms are located on centers of inversion. Each Ca atom is surrounded by four NMP ligands and coordinated through one of the two O atoms to two DMTBT dianions. This dianion thus results in the formation of a 1-D polymer, which extends in the [011] direction. Each Ca atom is in a CaO 6 six-coordinate envir-onment with Ca-O bond lengths ranging from 2.308 (6) to 2.341 (6) Å , cis bond angles ranging from 88.2 (2) to 91.8 (2) and the trans angles all 180 due to the Ca atoms being located on centers of inversion. Thus each Ca atom has close to ideal octahedral geometry.
In view of the interest in combinations of NMP with Ca ions as a reaction medium, it is surprising to note that in the literature (Kang et al., 2014;Qinghua, 2018) there are only three instances of structures containing Ca coordinated to NMP. In these structures, the Ca-O bond length varies from 2.244 (4) to 2.305 (3) Å , which match the values in 1. However, there are no previous structures containing the dianion or any related species. This dianion has resulted from the condensation of two molecules of NMP along with the incorporation of two sulfur atoms in the form of C-S À bonds ( Fig. 1). In view of the reactivity of Ca in NMP solutions as mentioned above, it appears that the calcium associated with NMP templates this reaction.
The two five-membered rings of the dianion (Fig. 2) are planar (r.m.s. deviations for C11 to N3 and C16 to N4 of 0.005 and 0.009 Å , respectively) and the two rings are almost coplanar [dihedral angle between rings of only 1.0 (5) ]. The two nitrogen atoms in the ring are almost trigonal [sum of angles about N3 and N4 of 359.5 (7) and 359.8 (7) , respectively] with their attached methyl groups being only 0.157 (15) and 0.051 (15) Å out of the plane of their respective rings. Thus there must be considerable aromatic character in the linked five-membered rings of the dianion. The bond order of both the C-O and C-S moieties in both rings appear to be close to double bond in character with distances of 1.242 (9) and 1.256 (10) Å for C-O and 1.696 (9) and 1.713 (9) Å for C-S (Trinajstić, 1968).

DFT calculations
The calculations for the DMTBT dianions were treated with density functional theory (DFT) within the Gaussian 09 suite (Frisch et al., 2016;Hohenberg & Kohn, 1964  Diagram showing the dianion linking the Ca centers and showing atom labeling for the asymmetric unit. Atomic displacement parameters are at the 30% probability level.

Figure 1
Diagram showing how the dianion has resulted from the condensation of two molecules of NMP along with the incorporation of two sulfur atoms in the form of C-S À bonds. the exchange-correlation functional for this compound we used the Heyd-Scuseria-Ernzerhof (HSE) screened hybrid HSE06 functional within an unrestricted self-consistent field for the singlet dianion ground state (Heyd et al., 2005). The elements composing the compound are expanded in the 6-311+G(d,p) Gaussian basis set, which is included in the geometry optimization with tight convergence criteria and ultrafine integration grid (McLean & Chandler, 1980;Curtiss et al., 1995). The ground-state equilibrium structure for the dianion state is shown in Fig. 3 with bond lengths in Å overlaid. The optimized geometry was used in all subsequent calculations. The charge distribution is shown in Fig. 4 and from this it can be seen that the negative charge is distributed between the S and O atoms, with the O atom having the major part in each ring. To understand aromaticity in this compound, the ring currents were computed starting from the gaugeindependent atomic orbitals (GIAO) method (London, 1937;Cheeseman et al., 1996). The GIAO results were used to generate the signed modulus of the current density and average induced current with gauge-including magnetically induced current code (GIMIC) on a dense grid (Johansson et al., 2005;Taubert et al., 2008;Fliegl et al., 2009Fliegl et al., , 2011Fliegl et al., , 2015Fliegl et al., , 2016. The results are shown in Fig. 5 and show that there likely is some resonance across the central bond between both azapentyl rings, but this is not sufficient to establish a ring current (Peeks et al., 2017). The UV-vis spectrum ( Fig. 6) is computed with time-dependent self-consistent density functional theory (TD-SCF) with 1000 additional states (Casida et al., 1998;Furche & Ahlrichs, 2002). This shows a peak at 625 nm, originating from the HOMO-LUMO transition ( Fig. 6), which accounts for the deep blue-purple color of solutions of the complex. The experimental max of the blue solution is at 671 nm, which may include colored compounds besides the title compound, as what crystallizes is not necessarily representative of the remaining solution. Thus, while we could obtain a spectrum similar to that generated from calculations, we cannot be sure that what is in solution is the Ground state charge density for the DMTBT dianion. The electric potential ranges from À0.2 atomic units (red) to 0.2 atomic units (blue).

Figure 5
Signed modulus of the magnetically induced current density in the DMTBT dianion. Note, the total diatropic current is found to be 9.53 na T À1 , paratropic is À8.44 na T À1 , and total is 1.08 na T À1 .

Figure 6
Calculated UV-vis spectrum of the DMTBT dianion from TD-SCF. The HOMO-LUMO states featuring the dominant transition are shown above the spectrum.

Figure 3
Ground state equilibrium structure for the DMTBTdianion. The bond lengths, in units of Å , are overlaid. same material that is in the crystals. The oxidation of NMP to the title dianion requires removal of ten hydrogen atoms, and this process must involve multiple steps that produce many different intermediates. An attempt to prepare this dianion by oxidation of NMP with S 8 in the presence of CaS under an inert atmosphere produced purple-and blue-colored compounds, which have yet to be identified.

Supramolecular features
The Ca atoms are located on centers of inversion. Each Ca is surrounded by 4 NMP ligands and coordinated through one of the two O atoms to two DMTBT dianions. This dianion thus facilitates the formation of 1-D ribbons, which propagate in the [011] direction. These ribbons are linked by C-HÁ Á ÁS interactions (Table 1)

Synthesis and crystallization
Ca(SCMe 3 ) 2 (Purdy et al., 1997) was dissolved in N-methyl-2pyrrolidone containing about 10% C 6 D 6 and a drop of tetramethylsilane and sealed in an NMR tube. After $6.5 years, a mass of deep-blue crystals was discovered in the NMR tube. One was selected and transferred to the cold stream of the diffractometer at 100 K. While perfectly stable under an inert atmosphere, the color changes in a few minutes after exposure to air. 13 C NMR spectra of the solution showed nothing that can be attributed to the title compound, so it is likely that the concentration is too low to be observed. Symmetry codes: (i) Àx; Ày; Àz þ 1; (ii) Àx þ 1; Ày; Àz þ 1; (iii) x; y þ 1; z; (iv) Àx þ 1; Ày þ 1; Àz.

Figure 7
Diagram showing how the dianion links the Ca centers into ribbons in the [011] direction. All hydrogen atoms omitted except those involved in C-HÁ Á ÁS interactions. Dashed lines indicate the inter-ribbon C-HÁ Á ÁS interactions linking these ribbons.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 2. All hydrogen atoms for the major component were located in difference Fourier maps and included in idealized positions using a riding model with atomic displacement parameters of U iso (H) = 1.2U eq (C, N) [1.5U eq (C) for CH 3 ], with C-H distances ranging from 0.95 to 0.99 Å . The crystal was twinned by non-merohedry via two different twofold operations, about the normals to (001) and (110) Data collection: APEX2 (Bruker, 2005); cell refinement: SAINT (Bruker, 2002); data reduction: SAINT (Bruker, 2002); program(s) used to solve structure: SHELXT (Sheldrick 2015a); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: SHELXTL (Sheldrick 2008); software used to prepare material for publication: SHELXTL (Sheldrick 2008).