A five-coordinate cobalt bis(dithiolene)–phosphine complex [Co(pdt)2(PTA)] (pdt = phenyldithiolene; PTA = 1,3,5-triaza-7-phosphaadamantane)

The synthesis and crystal structure are reported for a five-coordinate cobalt dithiolene-phosphine complex [Co(pdt)2(PTA)] (pdt = phenyldithiolene; S2C2Ph2), produced by PTA ligand-induced cleavage of the cobalt bis(dithiolene) dimer [Co2(pdt)4].


Chemical context
Transition-metal complexes of 1,3,5-triaza-7-phosphaadamantane (PTA) and related ligands have attracted much attention because of their potential as water-soluble catalysts, materials, and therapeutic agents (Guerriero et al., 2018). The small cone angle (103 ) of the PTA ligand combined with the high thermal and chemical stability, and high hydrophilicity makes it unique among phosphine ligands (Phillips et al., 2004). Electronically, the PTA ligand is much less electron donating than PMe 3 , while a slightly better electron donor than PPh 3 (Darensbourg et al., 1999). However, the formation of heteroleptic dithiolene-phosphine complexes from the corresponding homoleptic metal-dithiolene has not been fully explored (Natarajan et al., 2017). Reactions of homoleptic metal-dithiolenes with phosphines to produce heteroleptic complexes have exhibited interesting metal-ligand redox interplay as a result of the redox-active or non-innocent nature of dithiolene ligands (Chandrasekaran et al., 2014). In this context, phosphine-induced cleavage of the iron and cobalt bis(dithiolene) dimer to yield five-coordinate bis(dithiolene)phosphine has been explored in depth with PPh 3 and PMe 3 ligands (Selby-Karney et al., 2017;Yu et al., 2007). These complexes were all synthesized from the corresponding bis-(dithiolene) metal dimer complexes followed by addition of an excess of phosphine ligand to form bis(dithiolene) metal complexes bound to an additional phosphine ligand. The resulting [M(adt) 2 (PR 3 )] (M = Co, Fe; adt = para-anisylphenyldithiolene; PR 3 = PMe 3 or PPh 3 ) complexes have ISSN 2056-9890 approximately square-pyramidal geometries at the metal center.

Figure 4
View of two molecules of [Co(C 14 H 10 S 2 ) 2 (C 6 H 12 N 3 P)]Á0.5C 2 H 4 Cl 2 within the unit cell and one additional translation of the unit cell along b, at 50% probability ellipsoids. Planes defined by aryl rings containing C15-C20 along with the corresponding centroids are depicted to highlight parallel displaced -stacking of aryl rings.  Table 1 for symmetry operators). When the unit cell is grown along the b axis, parallel displaced -stacking of the aryl rings is revealed (Fig. 4). Planes defined by atoms C15-C20 (Fig. 4, blue) and C15 v -C20 v [ Fig. 4, purple; symmetry code: (v) 1 À x, Ày, 1 À z] within the unit cell are parallel, with a distance of 2.928 Å between planes and a distance of 4.961 Å between the respective centroids defined by the same atoms. The shortest atomic distance is between the carbon atoms of the aryl rings between unit cells with C17Á Á ÁC18 v being 3.343 (3) Å apart (Figs. 3, #6; Fig. 4).

Database survey
A survey of the Cambridge Structural Database (Web accessed March 26, 2020; Groom et al., 2016) and SciFinder (SciFinder, 2020) yielded no exact matches for reported structures of this complex. Structures with two dithiolene ligands with p-anisyl substitution bound to Co and an additional coordinated phosphine ligand were reported with PMe 3 coordination (Selby-Karney et al., 2017), and PPh 3 coordination (Yu et al., 2007). Both reported complexes also have approximately square pyramidal geometry at the cobalt center with slight deviations. The PPh 3 complex exhibits the largest distortion from planarity with a sum of angles around cobalt of 353.89 (6) , while the sum of the angles is 356.97 (6) for the PMe 3 complex. Similarly, the phosphine in PPh 3 is axially distorted because of the steric bulk of the phenyl groups, resulting in two more obtuse bond angles for S2-Co1-P1 and S3-Co1-P1 of 101.31 (3) and 106.6 (3) , respectively. The other bond angles of 92.81 (3) for S1-Co1-P1 and 97.13 (3) for S4-Co1-P1 are within the range of S-Co1-P1 angles of 91.19 (3) to 99.65 (3) observed for the PMe 3 complex.

Spectroscopic analysis
The UV-vis characterization of [Co(pdt) 2 (PTA)] was conducted in dichloromethane ( Fig. 5) and revealed a strong absorption at 877 nm with a molar absorptivitiy of 6428 M À1 cm À1 . In the related p-anisyl-substituted cobalt complex bound to PMe 3 a similar absorption was observed at 905 nm. This is attributed to a ligand-to-ligand charge-transfer (LLCT) transition, based on comparison with the related iron complex with PPh 3 (Yu et al., 2007) and related dithiolene metal complexes (Ray et al., 2005). In the iron PPh 3 complex, the absorption occurred at 720 nm and disappeared upon conversion to the homoleptic iron bis(dithiolene) complex. The IR signal for [Co(pdt) 2 (PTA)] at 1157.61 cm À1 is characteristic of monoanionic dithiolenes with a -radical when coordinated to metals, and is attributed to (C SÁ) (Patra et al., 2006). Combined, the IR and UV-vis characterization are consistent with two monoanionic dithiolene ligands bound to a Co II center.

Electrochemical analysis
The cyclic voltammogram (CV) of [Co(pdt) 2 (PTA)] was collected in a solution of dichoromethane with a platinum working electrode (Fig. 6) and a glassy carbon working electrode (Fig. 7). Both CVs display two reversible waves with the first one at E 1/2 = +0.62 with both electrodes, and a second one at E 1/2 = À0.17 V with the platinum electrode and E 1/2 = À0.16 V with the glassy carbon electrode. The reversible oxidation wave at +0.62 V is attributed to a metal-centered redox event. The second oxidation at À0.17 V is attributed to ligand oxidation, by comparison to other metal dithiolene complexes (Patra et al., 2006).

Synthesis and crystallization
A 50 mL Schlenk flask containing a stir bar was charged with [Co 2 (pdt) 4 ] (0.300 g, 0.275 mmol) and PTA (0.144 g; 0.551 mmol) under an N 2 atmosphere. To this mixture of solids, 20 mL of CH 2 Cl 2 were added and stirred for 4 h at room temperature. The solvent was removed under reduced pressure and the resulting dark-orange solid was washed with 3 Â 5 mL of Et 2 O and dried under vacuum. The product was stable under reduced pressure and at room temperature. Yield: 92% (0.357 g, 0.509 mmol). Crystals suitable for X-ray diffraction were grown by the vapor diffusion method with diffusion of pentane over a 1,2-dichloroethane solution of the compound. UV-Vis spectra were obtained at ambient temperature with a Varian Cary 50 diode array spectrometer, while IR spectra

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Hydrogen atoms were placed in calculated positions with C-H distances of 0.95 and 0.99 Å for CH and CH 2 , respectively, and refined using a riding model with U iso (H) = 1.2 U eq (C) for CH and CH 2 . Cyclic voltammetry of [Co(pdt) 2 (PTA)] in CH 2 Cl 2 recorded using a platinum working electrode and [ n Bu 4 N][PF 6 ] as electrolyte with a scan rate of 100 mV s À1 at 25 C.

Figure 7
Cyclic voltammetry of [Co(pdt) 2 (PTA)] in CH 2 Cl 2 recorded using a glassy carbon working electrode and [ n Bu 4 N][PF 6 ] as electrolyte with a scan rate of 100 mV s À1 at 25 C.  software used to prepare material for publication: publCIF (Westrip, 2010).

Crystal data
[Co(C 14 H 10 S 2 ) 2 (C 6 H 12 N 3 P)·0.5C 2 H 4 Cl 2 M r = 750.24 Triclinic, P1 a = 9.0954 (7)  Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.