Cyclopentadienone triisocyanide iron complexes: general synthesis and crystal structures of tris(2,6-dimethylphenyl isocyanide)(η4-tetraphenylcyclopentadienone)iron and tris(naphthalen-2-yl isocyanide)(η4-tetraphenylcyclopentadienone)iron acetone hemisolvate

Cyclopentadienone triisocyanide iron complexes were isolated and fully characterized for the first time. Two of the twelve isolated complexes could be crystallographically characterized.

The isolated complexes were inactive in hydrogenation and transfer hydrogenation reactions of acetophenone in i PrOH (1 mol% catalyst loading, 363 K, 10 bar H 2 ). Addition of Me 3 NO, as is routinely done for activating the corresponding tricarbonyl complexes, did not lead to turnover either. It is assumed that neither Me 3 NO nor elevated temperatures are able to cleave one of the Fe-CNR bonds to free up a coordination site needed for catalysis. While potential applications of these complexes in catalysis were unsuccessful, our studies nevertheless prompted us to seek systematic relationships between the ligand properties and either the structural or the functional properties of the complexes.

Figure 1
Synthetic route to access cyclopentadienone triisocyanide complexes starting from the corresponding tricarbonyl complexes by irradiation with blue LEDs.

Figure 4
The two independent Fe(CN-2-Naphth) 3 -TPCPD molecules in the asymmetric unit viewed along the Fe-Fe axis. In the crystal, the molecules appear in pairs that are rotated by 180 with respect to each other and show an interlocked arrangement of the naphthyl groups.
ligand are shifted ca 5 ppm downfield for C4 and C6/C7 and ca 10 ppm upfield for C5/C8 and follow the same overall trend as the complexes with TPCPD. It can be observed that the signals are all shifted upfield, i.e. to lower chemical shifts, compared to the parent tricarbonyl complex. This observation can be explained by considering that isocyanides are weakeracceptors and stronger -donors compared to CO. They thus render the iron center more electron rich and therefore lead to more electron density and thus shielding in the cyclopentadienone ligand. Complexes with isocyanide ligands bearing electron-withdrawing or aromatic substituents (CH 2 Ts, 2-Naphth, 2,6-DMP) show more deshielded signals compared to isocyanide ligands with electron-donating substituents (CH 2 Ph, Bu, t Bu). Furthermore, 13 C NMR analysis showed that the CNR signals are generally more shielded by 2-5 ppm for complexes bearing the TPCPD ligand compared to the BTTHI ligand, indicative of stronger d Fe to * CN back-bonding with BTTHI, since more back-donation generally leads to higher chemical shifts (Pruchnik & Duraj, 1990). TPCPD can thus be said to be a stronger acceptor than BTTHI, rendering the Fe center less electron rich.

Supramolecular features
In the crystal of Fe(CN-2-Naphth) 3 -TPCPD, the complexes form pairs with the Fe(CNR) 3 fragments facing each other. The complexes are rotated by approximately 180 relative to each other. The naphthyl groups form an interlocked structure. No obvious intermolecular interactions are observed in Fe(CN-2,6-DMP) 3 -TPCPD.

Synthesis and crystallization
The general procedure for the synthesis of the triisocyanide complexes is as follows: Under an atmosphere of N 2 , the iron tricarbonyl complex (1 equiv.) and the isocyanide (4 equiv.) were dissolved in toluene (ca 0.1 M total concentration). Drying or degassing of the solvent was not found to be necessary. The solution was irradiated with blue LEDs 4.8 W,470 nm) at room temperature overnight. The next day, the solution was directly loaded onto a silica packed column and purified by column chromatography using the appropriate eluent as indicated below. The relevant, yellow-colored fractions were combined and concentrated under reduced pressure. For complexes bearing the TPCPD ligands with electron-rich isocyanides (CNCH 2 Ph, CN t Bu, CNBu), it was necessary to perform rotary evaporation at 298 K instead of 313 K because of the thermal instability of these compounds, as evidenced by the observation of the dark-purple color of the TPCPD ligand during thin layer chromatography (TLC) analysis. The complexes were isolated as yellow to orange solids and were characterized by 1 H NMR, 13 C NMR, elemental analysis and HRMS. Single crystals of the compounds Fe(CN-2,6-DMP) 3 -TPCPD and Fe(CN-2-Naphth) 3 -TPCPD were obtained by suspending the solids in acetone to obtain a saturated solution, filtering off the solids and storing the saturated solution at 253 K in a freezer.
The decomposition upon heating, presumably due to the loss of the cyclopentadienone ligand, appears to depend on how electron rich the complex is as a whole, as indicated by 13 C NMR, with more electron density on the cyclopentadienone ligand leading to thermal instability. Combining the observations made above, it can be said that more thermally stable complexes can be expected by combining stronglyaccepting isocyanide ligands with weakly electron-accepting cyclopentadienone ligands, reminiscent of a push-pull interaction between the cyclopentadienone ligand and the isocyanide ligands mitigated by the iron center.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms were positioned geometrically (C-H = 0.95-0.98 Å ) and refined as riding with U iso (H) = 1.2-1.5U eq (H). The crystal for Fe(CN-2-Naphth) 3 -TPCPD was twinned. Two domains, with approximate refined mass fractions of 3:1 and rotated by approximately 179 , were found and integrated simultaneously. The best model in terms of residual densities and their location, R values and weighting scheme was obtained using de-twinned HKLF4 data. program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: Olex2 1.5 (Dolomanov et al., 2009); software used to prepare material for publication: Mercury (Macrae et al., 2020).

Crystal data
[Fe (C 9

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.