Synthesis and crystal structure of 1,1′-bis{[4-(pyridin-2-yl)-1,2,3-triazol-1-yl]methyl}ferrocene, and its complexation with CuI

The ferrocene-bridged compound 1,1′-bis(pyridyltriazoylmethyl)ferrocene acquires an anti conformation in its solid state but forms a discrete tetranuclear [2 + 2] complex with [Cu(CH3CN)4](PF6) in solution.


Chemical context
Metal-organic supramolecular chemistry is an emerging area in inorganic chemistry: the structurally challenging functional supramolecules can be constructed from the self-assembly of multidentate organic ligands and transition-metal ions in relatively few synthetic steps (Cook & Stang, 2015). Such supramolecules are designed by careful selection of the conformational flexibility of the linker groups in multidentate ligands, and the coordination preference of transition-metal ions. We have recently studied the self-assemblies of m-xylylene-or 2,7-naphthalenebis(methylene)-bridged tetradentate bis(pyridyltriazole) ligands with Cu II ions to give discrete [2 + 2] metallocycles (Pokharel et al., 2013(Pokharel et al., , 2014. In a continuation of our work, we became interested in the design of metalloligands, i.e., metal-containing organic linkers, to produce mixed-metal complexes with different topologies. Ferrocene, a well-known metallocene, exhibits high thermal stability, reversible electrochemistry, and conformational flexibility, making it an ideal precursor for the development of polymetallic metallosupramolecular complexes (Astruc, 2017). Introduction of the iron(II) center as the structural ISSN 2056-9890 component of the ligand allows the study of electronic coupling between metal centers in heterometallic metallosupramolecular assemblies. Although 1,1 0 -disubstituted ferrocenes featuring the pyridyl moiety as a donor group have been exploited in metallosupramolecular assemblies (Quinodoz et al., 2004;Buda et al., 1998;Ion et al., 2002;Lindner et al., 2003;Sachsinger & Hall, 1997), the ferrocenebridged bis(pyridyltriazole)-based tetradentate ligands are relatively new in coordination chemistry (Findlay et al., 2018;Manck et al., 2017;Romero et al., 2011). Herein, we report the synthesis of the 1,1 0 -bis(methylenepyridyltriazole) ferrocene ligand starting from 1,1 0 -ferrocenedicarboxylic acid in a threestep sequence and its complexation with Cu I ions ( Fig. 1).

Structural commentary
The asymmetric unit of the title compound contains one half of the molecule since the Fe II center is on an inversion center, as shown in Fig. 2. The symmetry in the molecule was also apparent in the NMR data where only one set of signals was found for the protons and carbons of the cyclopentadienyl (Cp) rings, methylene groups, and the pyridyltriazole units. The Fe-C(Cp) bond lengths are in the range 2.0349 (12)-2.0471 (13) Å [average 2.0498 (13) Å ] with the FeÁ Á ÁCpcentroid distance being 1.6550 (6) Å . The Fe-C bond to the substituted carbon [Fe-C1 2.0349 (12)] is shorter than the remaining Fe-C bond lengths, as is seen in similar 1,1 0disubstituted ferrocene derivatives (Glidewell et al., 1994). The conformation of the ferrocenyl unit is exactly staggered by inversion symmetry, and the centrosymmetry also makes the Cp-Fe-Cp linkage linear and the Cp rings parallel. The Csp 3 atom, C6, is displaced towards the Fe II center by 0.044 (3) Å from the least-squares plane of the Cp ring. The C Cp -Csp 3 and C Cp -N bond lengths involving C6 are 1.4910 (19) and 1.4700 (18) Å , respectively. The pyridyltriazole moiety is oriented exo from the Fe II center, with the least-squares planes of the Cp and triazole rings forming a dihedral angle of 65.68 (5) . The nitrogen donor atoms of the pyridyltriazole units adopt an anti conformation, as is often observed in this type of chelating ligand . The pyridyl and triazole units deviate slightly from coplanarity, with the N3-C7-C9-N4 torsion angle being 167.64 (13) .

Supramolecular features
The crystal structure of the title compound is consolidated by intermolecular C-HÁ Á ÁN (Table 1) Molecular structure of the title compound showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level. Unlabeled atoms are generated by the symmetry operation 1 À x, 1 À y, 1 À z.

Figure 1
The synthetic scheme showing the formation of the title compound and its complexation with Cu I . triazole N3 (at x À 1, y, z) and the Cp carbon atom C5 forms a C-HÁ Á ÁN interaction with a CÁ Á ÁN distance of 3.4240 (19) Å to triazole N2 (at x À 1, y, z). These two contacts form a ring with graph-set motif R 2 2 (9) (Etter et al., 1990). In addition, the triazole carbon C3 forms a C-HÁ Á ÁN interaction with a CÁ Á ÁN distance of 3.4625 (19) Å to pyridyl nitrogen N4 (at x, y + 1, z). Thus, the C-HÁ Á ÁN contacts form a two-dimensional network normal to [001]. The pyridyltriazole moieties stack in an antiparallel fashion about inversion centers. The pyridyl moiety of one molecule has ainteraction with the triazole moiety of another molecule with a dihedral angle of 11.27 (10) and centroid-centroid distance of 3.790 Å (symmetry operation 2 À x, Ày, Àz). In addition, there are also C-HÁ Á Á interactions between the hydrogen atom of the pyridyl moiety with the cyclopentadienyl ring [H12Á Á ÁCp(centroid) = 2.692 Å ; symmetry operation 2 À x, Ày, Àz;]. These two interactions thus form centrosymmetric dimers, illustrated in Fig. 4.

Database survey
A search of the Cambridge Structural Database (Version 5.41, update of March 2020; Groom et al., 2016) for bis(pyridyltriazole) with a ferrocene linker gave no results. However, the structure of ferrocene attached to one methylenepyridyltriazole, BULQIJ  has been reported. The two pyridyltriazole units connected with organic linkers, namely m-xylylene, VAJVIN (Najar et al., 2010), and p-xylylene as chloroform solvate, FUJJOK  have also been reported.

Complexation with Cu I
Complexation of the ligand 4 with Cu I was performed under a nitrogen atmosphere. When [Cu(CH 3 CN) 4 ]PF 6 was added to a suspension of the ligand in DMF in a 1:1 ratio, the mixture was completely soluble, indicating the formation of a complex. To avoid oxidation of the complex, the resultant solution was diffused with diethyl ether vapor under nitrogen for 3 d. Under these conditions, a bright-yellow microcrystalline solid was formed. At room temperature, the 1 H NMR spectrum of the complex showed a simple pattern containing the same set of signals for the ligand, indicative of the presence of one single species in solution. Compared to the spectrum of the free ligand, the proton signals of the complex, especially for Acta Cryst. (2020). E76, 1582-1586 research communications  Table 1 Hydrogen-bond geometry (Å , ). Symmetry codes: (i) x À 1; y; z; (ii) x; y þ 1; z; (iii) Àx þ 2; Ày; Àz.

Figure 4
The C-HÁ Á Á andinteractions, viewed along the a axis. Displacement ellipsoids are shown at the 50% probability level.

Figure 3
The C-HÁ Á ÁN network, with displacement ellipsoids at the 50% probability level.

Figure 6
A portion of the high-resolution ESI-MS spectrum of complex 5 the pyridyltriazole coordination pocket, are shifted downfield (Fig. 5). Similar retaining of the number of signals and the coupling patterns in the 1 H NMR spectrum was observed in xylylene-linked bis(pyridyltriazole) ligands and their Ag I complexes . To further explore the nature of the complex in solution, we examined a MeOH/ DMSO solution of the complex by positive-ion electrospray mass spectrometry (ESMS). The ESMS spectrum of the complex contains a peak at 1273.0915 corresponding to [Cu 2 L 2 ](PF 6 ) + with a similar isotopic pattern as the theoretical simulation (Fig. 6), indicating the formation of the [2 + 2] complex. Disappointingly, despite obtaining crystalline material, our attempts to obtain crystals suitable for single-crystal X-ray analysis failed.
Synthesis of 1,1 0 0 0 -bis(pyridyltriazolylmethyl)ferrocene, 4. To a stirred solution of 1,1 0 -bis(azidomethyl)ferrocene (1.00 g, 3.34 mmol) in a mixture of DMF and water (4:1) (20 mL), Na 2 CO 3 (354 mg, 3.34 mmol), CuSO 4 Á5H 2 O (333 mg, 1.33 mmol), ascorbic acid (468 mg, 2.66 mmol), and 2ethynylpyridine (862 mg, 8.36 mmol) were added in sequence. The reaction mixture was stirred for 20 h at room temperature, and then poured into an NH 3 /EDTA solution (2.00 g of Na 2 H 2 EDTAÁ2H 2 O in 5 mL of 28% aqueous NH 3 , diluted to 100 mL with H 2 O) and the mixture extracted with chloroform (3 x 100 mL). The organic layer was collected, dried over MgSO 4 , and evaporated to dryness. The crude product was purified by trituration with cold diethyl ether to give 4 (1.26 g, 75%) as a light-brown solid. X-ray quality crystals of the compound were obtained by vapor diffusion of diethyl ether into its solution in chloroform, m.p.: decomposes above 463 K. Synthesis of Cu I complex of 1,1 0 0 0 -bis(pyridyltriazolylmethyl)ferrocene, 5. To a nitrogen-purged stirred suspension of 4 (100 mg, 0.20 mmol) in DMF (10 mL), [Cu(CH 3 CN) 4 ](PF 6 ) (77 mg, 0.20 mmol) was added. The reaction produced a clear yellow solution, which was stirred for 2 h at room temperature. The reaction mixture was diffused with nitrogen-purged diethyl ether using a cannula for 3 d. The solution was decanted and the product was washed with diethyl ether and dried under a slow stream of nitrogen to give 5 (142 mg, 100%) as a yellow microcrystalline solid. A 1 H NMR sample was prepared by dissolving the compound in DMSO-d

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were located in difference maps and then treated as riding in geometrically idealized positions with C-H distances of 0.95 Å (0.99 Å for CH 2 ) and with U iso (H) =1.2U eq for the attached C atom. Acta Cryst. (2020). E76, 1582-1586 research communications  program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014/7 (Sheldrick, 2015); molecular graphics: Mercury (Macrae et al., 2020); software used to prepare material for publication: publCIF (Westrip, 2010).