Crystal structure of tricarbonyl[η4-6-exo-(triphenylphosphino)cyclohepta-2,4-dien-1-one]iron(0) tetrafluoroborate

The crystal structure of tricarbonyl[η4-6-exo-(triphenylphosphino)cyclohepta-2,4-dien-1-one]iron(0) tetrafluoroborate is described. The two independent tricarbonyl[η4-6-exo-(triphenylphosphino)cyclohepta-2,4-dien-1-one] iron(0) cations and their corresponding anions form dimers, which constitute the asymmetric unit of the structure within the (100) plane.


Chemical context
This compound was prepared as part of a Course-based Undergraduate Research Experience (CURE) (Stone et al., 2020;Huang et al., 2019).The foundation of this CURE was to further examine addition reactions to tricarbonyl(tropone)iron(0) (I) and tricarbonyl(� 5 -ketocycloheptadienyl)iron(0) tetrafluoroborate (II) (Fig. 1).The research focus of one author lies in the synthesis of unique and diverse azapolycyclic skeletons from common synthetic building blocks such as compound I due to the biological importance of such scaffolds.Although seven-membered carbocyclic rings are found in a number of biologically active natural products (Shoemaker & Griffith, 2021), their synthesis tends to present a greater challenge compared to similar five-or six-membered rings because of the increased enthalpic and entropic barriers associated with their formation (Phelan et al., 2020;Huang et al., 2018).The addition of a number of different nucleophiles to compound II has previously been reported, including amines (Phelan et al., 2020), azide, and cyanide (Eisenstadt, 1975).This raised the question as to whether or not triphenylphosphine would be sufficiently nucleophilic to react with compound II.Previously, the reaction of several phosphines (PEt 3 , P n Pr 3 , P n Bu 3 or PMe 2 Ph) with tricarbonyl(� 5cycloheptadienyl)iron(II) tetrafluoroborate in methylene chloride resulted in the formation of the corresponding tricarbonyl[�4 -(5-exo-phosphine)cycloheptadiene]iron(0) tetrafluoroborate (Brown et al., 1982).Similar to that system, the reaction of compound II and triphenylphosphine resulted in the formation of tricarbonyl[� 4 -6-exo-(triphenylphosphino) cyclohepta-2,4-dien-1-one]iron(0) tetrafluoroborate (III) (Fig. 1).Ultimately, this and similar phosphonium salts could be a precursor for Wittig olefinations that would provide efficient access to tropone rings with diverse substituents.
The resulting 3D network was also found to contain solventaccessible voids of �101 A ˚3 within which the interstitial CH 2 Cl 2 was located (Fig. 5).
The Z 0 > 1 nature of the structural model for III suggests the presence of structural differences between molecules within the asymmetric unit.Barring differences in the thermal parameters for the various atoms within the independent components, overlaying the tricarbonyl[� 4 -6-exo-(triphenylphosphino)cyclohepta-2,4-dien-1-one]iron(0) cations and

Figure 6
Molecular overlay of between both cations constituting the symmetric unit of III demonstrating that the greatest disparity between them exists within the torsion angles of the the phenyl rings.The first tricarbonyl[� 4 -6-exo-(triphenylphosphino)cyclohepta-2,4-dien-1-one]iron(0) tetrafluorrborate is in black while the second is in yellow.Anisotropic displacement ellipsoids have been set to the 50% probability level.

Database survey
The structure of this report is not found in the Cambridge Structural Database (CSD version 5.43;Groom et al., 2016).To date, the structures of six tricarbonyl(� 4 -tropone derivative)iron(0) compounds have been reported.In addition to the structure of compound I (Dodge, 1964), three of the remaining reports have one additional substituent in the 6-position, H (Sotokawa et al., 1987), t-Bu (Coquerel et al., 2002) and morpholi-4-yl (Huang et al., 2018).From the various reports, comparison of their structural features suggested that the presence of the formally cationic phosphorous had minimal impact on the bond lengths.

Synthesis and crystallization
All chemicals were purchased from commercial vendors and used as is.Compounds I and II were prepared according to literature procedures (Huang et al., 2019).NMR spectra were obtained in d 3 -acetonitrile using a Bruker Avance III HD 400 FT-NMR spectrometer.The synthesis was performed using standard Schlenk conditions as outlined in Fig. 1, but all subsequent manipulations of the product were conducted in air.Compound II (0.0054 g, 0.016 mmol) and triphenylphosphine (0.0043 g, 0.016 mmol) were added to a 50 mL round-bottom flask along with a stir bar.Methylene chloride (12 mL) was added, and the reaction mixture was stirred at room temperature for 30 minutes.A color change from pastel yellow to a darker yellow was observed.The solution was reduced in vacuo to approximately 5 mL and the resulting solution was layered with diethyl ether (7 mL) before being placed in the freezer for 48 h.The sample formed a pastel yellow solid and was filtered via cannula.The solid was dried in vacuo to give the desired product (0.0085 g, 88% yield).Crystals were grown by slow vapor diffusion of diethyl ether at room temperature into a solution of the compound in methylene chloride. 1H NMR (400 MHz, CD 3 CN): � 7.87 (m, 9H, H meta , H para ), 7.75 (m, 6H, H ortho ), 5.80 (t, J = 6.2 Hz, 1H, H4), 5.20 (t, J = 7.0 Hz, 1H, H1), 4.86 (td, J = 12.7, 4.7 Hz, 1H, H7), 3.21 (dd, J = 13.0,7.5 Hz, 1H, H3), 3.16 (d, J = 6.6 Hz, 1H, H2), 2.18 (m, 1H, H6A/B), 1.99 (q, J = 12.2 Hz, 1H, H6A/B); 31 P{ 1 H} NMR ( 162

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4.All non-hydrogen atoms were refined anisotropically.H atoms bound to carbon were positioned geometrically and constrained to ride on their parent atoms.U iso (H) values were set to a multiple of U eq (C) with 1.2 times all CH and CH 2 groups.

dichloromethane hemisolvate
Crystal data

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.

Figure 3
Figure 3 View of the C-H� � �O and C-H� � �F hydrogen bonds from the (101) plane of III.When coupled with the C-H� � �� and Y-X� � �� (Y = B, C; X = F, O) interactions, this repeat unit extends into a three-dimensional network.Anisotropic displacement ellipsoids have been set to the 50% probability level.

Figure 4
Figure 4 Projection of the C-H� � �� and Y-X� � �� (Y = B, C; X = F, O) interactions in the ac plane of III.Their combination with the hydrogen bonds yields a three-dimensional extended network in the solid state.Anisotropic displacement ellipsoids have been set to the 50% probability level (C g = ring centroids).

Figure 5
Figure 5 View into the (120) plane showing the interstitial CH 2 Cl 2 solvent molecules lying within the solvent-accessible voids of III.These voids are generated from the packing supported by the C-H� � �O and C-H� � �F hydrogen bonds and C-H� � �� and Y-X� � �� (Y = B, C; X = F, O) interactions.The anisotropic displacement ellipsoids for CH 2 Cl 2 have been set to the 50% probability level.

Table 4
Experimental details.