Coordination complexes of chromium(0) with a series of 1,3-diphenyl-6-arylfulvenes

The synthesis and structural properties of a series of chromium tricarbonyl ‘piano-stool’ complexes bearing substituted pentafulvene ligands were studied. Significant deviation of the exocyclic fulvene double bond from the cyclopentadiene plane accompanies coordination. Evidence of non-covalent π-π interactions was observed.


Chemical context
Pentafulvenes have been investigated because of their unique cross-conjugated electronic system. Despite the ability to tune the fulvene's steric and electronic properties through substitution, their coordination chemistry remains relatively unexplored. As a result of their electronic structure, fulvenes display a variety of coordination behaviors with metals, ranging from -2 , typically with late transition metals (O'Conner et al., 1997), to -5 :-1 ,which is more common with early transition metals (Ebert et al., 2014). Metal-fulvene complexes have been probed for hydroamination catalysis (Janssen et al., 2010), olefin metathesis (Erker et al., 1991), and cytotoxicity (Deally et al., 2011). Reduction to yield a cyclopentadiene ligand (Gó mez-Ruiz et al., 2005) or reductive coupling to form ansa bis-cyclopentadiene ligands (Adas & Balaich, 2018) are the most common examples of fulvene reaction chemistry. Herein, we report on the synthesis and structural properties of a series of chromium(0) complexes formed from 1,3-diphenyl-6-aryl fulvenes. ISSN 2056-9890

Structural commentary
Complex I crystallizes in the monoclinic space group P2 1 /n, (Fig. 1), complex II in the monoclinic space group P2 1 /c (Fig. 2), and complex III in the triclinic space group P1 (Fig. 3), each with one molecule per asymmetric unit. A benzene molecule was found co-crystallized and located on an inversion center in the structure of I. In each complex, the coordination geometry around the chromium(0) atom is distorted octahedral, with the midpoints of the three formal fulvene double bonds and the three carbonyl carbons describing the six verticies of the octahedra. Analysis of the fulvene bond lengths when compared to the previously reported uncomplexed fulvenes reveals nearly unchanged C-C single bonds (C1-C5, C4-C5, and C2-C3) with slight elongation of the C C double bonds (C1 C2, C3 C4, and C5 C6) ( Table 1). This double-bond elongation is typical uponcoordination to a metal atom. Based upon the alternating short and long bond distances, the coordination mode of the fulvene to the chromium atom is best described as -2 :-2: -2 in nature. Additionally, the coordination of the fulvene exocyclic double bond (C5 C6) results in the bending of this bond from the cyclopentadiene plane by 33.22 (18) to 34.2 (3) . This is in agreement with a previously reported chromium complex with 6,6-dimethylfulvene (Konietzny et al., 2010). The molecular structure of II. Displacement ellipsoids are shown at the 50% probability level.

Figure 1
The molecular structure of I. Displacement ellipsoids are shown at the 50% probability level.

Supramolecular features
Evidence forinteractions in the solid state is observed in I and III. In both I and III, the molecules are arranged in layers in which the system composed of the cyclopentadiene core (head) of the fulvene and the 3-phenyl substituent (tail) adopt a head-to-tail (Peterson et al., 1999)stacked arrangement. The interplanar contact distance is 3.420 (17) Å in I (Fig. 4) and 3.330 (8) Å in and III (Fig. 5), both well within the distance expected for a non-covalentinteration (Gruber et al., 2008). In I, the centroid of each cyclopentadiene ring is slipped by 0.470 (17) Å end-to-end and 1.505 (17) Å edge-toedge with respect to the opposing 3-phenyl substituent centroid. This results in a near perfect alignment of the fulvene C2 atom over the centroid of the opposing phenyl ring, with angles betwen the cyclopentadiene-phenyl ring normal and the C2 to phenyl ring centroid vector of only 2.05 (2) end-to-end and 5.85 (3) edge-to-edge. In complex III, the centroid of each cyclopentadiene ring is by slipped 0.286 (8) Å end-to-end and 0.761 (7) Å edge-to-edge with respect to the opposing 3-phenyl substituent centroid. Again, the C2 fulvene atom is brought into near perfect alignment over the centroid of the opposing phenyl ring, with angles betwen the cyclopentadiene-phenyl ring normal and the C2 to phenyl ring centroid vector of 7.67 (9) end-to-end and 6.16 (9) edge-to-edge. Further non-covalentinteractions are observed in III between the pyrene units. The interplanar contact distance is 3.494 (8) Å (Fig. 6)  Thestacking arrangement of I, viewed in the plane (left) and normal to the place (right) of the cyclopentadiene-phenyl rings. Displacement ellipsoids are shown at the 50% probability level. Hydrogen atoms have been omitted for clarity.

Figure 6
Thestacking arrangement of III, viewed in the plane (left) and normal to the place (right) of the pyrene rings. Displacement ellipsoids are shown at the 50% probability level. Hydrogen atoms have been omitted for clarity.

Figure 5
Thestacking arrangement of III, viewed in the plane (left) and normal to the place (right) of the cyclopentadiene-phenyl rings. Displacement ellipsoids are shown at the 50% probability level. Hydrogen atoms have been omitted for clarity.
-stacked dimers slipped by 2.352 (7) Å in the end-to-end direction when viewed down the normals of the pyrene rings (Fig. 6). The ring centroids remain aligned in the edge-to-edge direction. The carbon atoms of opposing pyrene rings are brought close to perfect alignment with carbon atoms in the opposing ring system, slipped by one half a ring width. This is in contrast to the stacking arrangement observed in the uncomplexed fulvene, where the overlap is intermediate between full carbon-to-carbon alignment and carbon-to-ringcentroid alignment (Peloquin et al. 2012).
(1,3,6-Triphenylfulvene)tricarbonylchromium(0) (I). A solution of 1,3,6-triphenylfulvene (0.518 g, 1.69 mmol) in THF (10 mL) was added to a stirred suspension of Cr(CO) 3 (-MeCN) 3 (0.499 g, 1.93 mmol) in THF (15 mL) under N 2 . The solution quickly turned from pale yellow to dark red. The reaction mixture was allowed to stir at room temperature for 24 h before removal of the solvent in vacuo. The residue was dissolved in diethyl ether (100 mL), filtered under ambient conditions, and the solvent removed in vacuo. Crystals suitable for single-crystal X-ray diffraction were obtained by dissolving the crude product in benzene and layering with pentane.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were positioned with idealized geometry and refined using a riding model, with C-H = 0.95 Å , and with U iso (H) = 1.2 U eq (C). In I, an outlier (101) was omitted in the last cycles of refinement.   For all structures, data collection: APEX3 (Bruker, 2017); cell refinement: SAINT (Bruker, 2017); data reduction: SAINT (Bruker, 2017); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL (Sheldrick, 2015b); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and Mercury (Macrae et al., 2008); software used to prepare material for publication: publCIF (Westrip, 2010).  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.

Tricarbonyl(1,3,6-triphenylfulvene)chromium(0) benzene hemisolvate (I)
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 ) x y z U iso */U eq Cr1 0.55485 (4) 0.83445 (9) 0.67800 (3) 0.0316 (2) (2) 0.0358 (10) Atomic displacement parameters (Å 2 ) (18)   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.