Crystal structures of two novel iron isocyanides from the reaction of 2,6-dimethylphenyl isocyanide, CNXyl, with bis(anthracene)ferrate(−1)

Three crystal structures from the reaction of 2,6-dimethylphenyl isocyanide with bis(anthracene)ferrate(−1) are presented.


Chemical context
The low-valent bis(anthracene)cobaltate(À1) has been shown to be an excellent source of spin-paired atomic Co(À1) anions in substitution reactions in which both anthracene (C 14 H 10 ) ligands are readily displaced by a wide variety of acceptor ligands (Brennessel et al., 2002;Brennessel & Ellis, 2012). The reaction with four equivalents of CNXyl, Xyl is 2,6-dimethylphenyl, resulted in an excellent yield of the homoleptic isocyanidecobaltate(À1), [Co(CNXyl) 4 ] 1À , first obtained by an alternate synthesis (Warnock & Cooper, 1989). Attempts to prepare the analogous 18-electron iron complex, bis(anthracene)ferrate(À2), afforded only the related 17-electron, paramagnetic bis(anthracene)ferrate(À1) (Brennessel et al., 2007). The latter species was shown to react with carbon monoxide to afford excellent yields of the Fe(À1) complex, [Fe 2 (CO) 8 ] 2À . On this basis, the corresponding reaction with CNXyl in tetrahydrofuran, thf, was examined to determine whether the unknown [Fe 2 (CNXyl) 8 ] 2À could be accessed. Bis(anthracene)ferrate(À1) was also reacted with excess CNXyl in the presence of one equivalent of a reducing agent to see whether the previously reported monometallic [Fe(CNXyl) 4 ] 2À (Brennessel & Ellis, 2007) could be prepared by this facile route. However, in both cases, infrared (IR) spectroscopy indicated predominant formation of the long-known, but only recently structurally authenticated Fe 0 complex, [Fe(CNXyl) 5 ] (Bassett et al., 1979, Brennessel et al., 2019. Because a complex containing formally Fe -I resulted in an oxidation to Fe 0 , it was of interest to determine what other species were produced by the reaction of bis(anthracene)ferrate(À1) with excess CNXyl in THF. First an aliquot was taken from the reaction mixture early on and placed in a 243 K freezer until orange blocks were observed. A single crystal X-ray diffraction experiment revealed these to be [Fe(C 14 H 10 )(CNXyl) 3 ] 1 (Fig. 1). It is thought that this complex is likely a crystallization-trapped intermediate, since [Fe(CNXyl) 5 ] is ultimately produced. Compound 1 is of interest as the first mixed anthracene-isocyanide derivative of the unknown bis(anthracene)iron(0). However, the related carbonyl, [Fe(C 14 H 10 )(CO) 3 ], has been known for more than 50 years (Manuel, 1964).
After the reaction mixture had warmed to room temperature and stirred for a few hours, the solvent was removed and n-heptane was added. The mixture was then filtered and a new species crystallized in the filtrate. A crystal structure revealed the material to be a thf disolvate of [Fe(C 54 H 56 N 9 )(CNXyl) 3 ] 2 (Fig. 2). In this case, six isocyanides had reductively 'coupled' to form a previously unknown tridentate ligand that had been protonated twice at two of the nitrogen atoms (Fig. 3). An IR spectrum obtained from the few crystals that could be harvested showed CN stretches of 2110w and 2055vs cm À1 , consistent with an Fe +2 oxidation state, which would make the ligand formally dianionic. The source of the protons in aprotic media was still a mystery at this point.

Figure 2
Plots of 2 with C-H hydrogen atoms and solvent molecules omitted and with only the major components of disorder shown. Top: anisotropic displacement ellipsoid plot drawn at the 50% probability level. Bottom: ball-and-stick plot in the same orientation featuring the numbering scheme. Warner & Lippard, 1986), although this exact 'coupling' of six isocyanide ligands appears to be new. The protonation of nitrogen atoms has also been observed in such circumstances. For instance, reduction of [Mo(CNR) 6 X] + (many variations on R and X) by Zn in the presence of water generated a bis-(alkylamino)acetylene ligand with protonated nitrogen atoms (Lam et al., 1977;Giandomenico et al., 1982;Warner & Lippard, 1986). The source of protons in the production of 2, however, was not discovered until single crystals grown from the heptane-insoluble component were evaluated. The structure was formulated by X-ray diffraction as [K(18-crown-6)(C 9 H 8 N)] 3 (Fig. 4), a cyclized, reduced form of CNXyl, from which one hydrogen atom was lost. It must be emphasized that examples of trimerization (Yamamoto et al., 1982;Blake et al., 1997;Bashall et al., 2000;Chen et al., 2019), tetramerization (Shen et al., 2014;Altenburger et al., 2016;Kucera et al., 2019), pentamerization (Tanase et al., 1992(Tanase et al., , 1996, hexamerization (Shen et al., 2014), and polymerization (Yamamoto & Yama-zaki, 1972) of isocyanides are well-precedented, but 2 appears to be only the second example in which hexamerization of an organic isocyanide has been established.
Given the speciation observed by the crystal structures and IR spectroscopy, a balanced equation can be written [Equation (1)]. The hydrogen atom lost during the reduction and cyclization that forms 3 is now found in the two protonations in the one-half equivalent of 2.
Interestingly, in support of this equation, when less than eight equivalents of CNXyl were employed (e.g., four), intractable tars resulted. It should be noted, however, that this equation is only speculative and requires further investigation for confirmation.

Structural commentary
The geometry at the formally zerovalent iron center of 1 is nearly identical to those of related molecules with one 1,2,3,4--naphthalene o-anthracene ligand and three excellent acceptor ligands in a tripodal arrangement. The average of the three (XylN)C-Fe-C(NXyl) angles of 1, 95.3 , matches well with that of the average C-Fe-C angle from three carbonyl ligands of the [Fe(1,2,3,4--naphthalene)(CO) 3 ] portion of a trinuclear molecule, 97.5 (Imhof, 1999), and those of the average P-Fe-P angles from [Fe(1,2,3,4--naphthalene)-(P(OMe) 3 ) 3 ], 97.7 (Schä ufele et al., 1989), and [Fe(1,2,3,4-anthracene)(P(OMe) 3 ) 3 ], 97.9 (Brennessel et al., 2007). The 'fold angle' between the iron-coordinating 4 -diene unit and the exo-benzene or -naphthalene portions are 30.7, 30.2, 40.6, and 40.8 , respectively, for the same four structures. The latter two angles are significantly larger than those in molecules containing three CNXyl or CO ligands, and since the Fe-C( 4 -diene) bond lengths (Table 1) in all four structures are comparable, it would be interesting to know if this is an electronic effect due to the different nature of CO/CNXyl versus phosphite and/or due to the bulk of the trimethylphosphite ligands.
The ligand set of 2 is built from nine CNXyl ligands, of which six, with the addition of two protonations at nitrogen atoms, have joined together into one tridentate dianionic ligand. Because this ligand is essentially planar at the core of two fused metallacyclopentanes (Fig. 2), it binds the iron center meridionally. The three remaining CNXyl ligands are also meridional, resulting in a distorted octahedral geometry. The bond lengths in the fused ring core (Table 2) suggest resonance stabilization (Fig. 3). To our knowledge, only one other 'coupling' of six isocyanide ligands has been structurally verified. In this case, six cyclohexyl isocyanide ligands have 'coupled' into a dianionic ligand (without any protonations) that bridges two chromium centers (Shen et al., 2014).

Figure 4
Anisotropic displacement ellipsoid plot of 3 drawn at the 50% probability level with H atoms and the minor component of disorder omitted.
interacting normally with an 18-crown-6 macrocycle, and additionally with the nitrogen atom of the anion (Table 3).

Supramolecular features
In addition to several intermolcular edge-to-face (C-HÁ Á Á) interactions, pairs of molecules in 1 are linked by offset parallel (slippage, 0.85 Å )interactions (Fig. 5), whose centroid-centroid distances are 3.588 (2) Å . In 2 there is one instance of an intramolecular offset parallel (slippage, 1.24 Å ) interaction between phenyl rings C56-C61 and C65-C70 [ Fig. 2, centroid-centroid distance, 3.614 (9) Å ]. The acceptor for the N7-H7 donor is the -system of phenyl ring C47-C52 and that for the N8-H8 donor is intramolecular acceptor N9 (Table 4). No obvious intermolecular interactions are observed in 2, which may also explain the reason for the significant disorder in the thf molecules (i.e., there are no C-HÁ Á ÁO interactions from the iron complex to anchor them). The intermolecular interactions in 3 are limited to C-HÁ Á Á interactions between methylene hydrogen atoms and the indenyl -system.
To a deep-orange solution of [K(18-crown-6)(thf) 2 ]-[Fe(C 14 H 10 ) 2 ] (1.000 g, 1.163 mmol) in thf (100 mL, 195 K) was added CNXyl (1.373 g, 10.47 mmol) in thf (40 mL, 195 K). The reaction mixture was warmed slowly to room temperature. A solution IR spectrum showed no anionic species, but a broad peak with shoulders that matched the well-known [Fe(CNXyl) 5 ] (Bassett et al., 1979), as well as a sharp peak for free CNXyl. An aliquot taken early in the reaction was placed in a freezer (243 K), from which orange crystals of 1 were structurally determined. The solvent was removed from the main reaction mixture and heptane was added with vigorous stirring. Crystals grown from the filtrate (i.e., heptane-soluble component) were identified as 2 by X-ray diffraction. IR spectroscopy on the crystals (Nujol mull) gave CN = 2110w and 2055vs cm À1 . The filter cake (i.e., heptane-insoluble component) was redissolved in THF and layered with pentane, which resulted in crystals of 3 as determined by a single crystal X-ray experiment. No characterization beyond what is presented above was performed. Table 1 Selected geometric parameters (Å , ) for 1.

Figure 5
Depiction of the offset parallelinteractions between two molecules of 1 whose centroid-centroid (dashed lines) distances are 3.59 Å . The second molecule is generated by inversion operator 1 À x, 1 À y, Àz.
In 3, the anion is modeled as disordered with a planar flip of itself [0.905 (3):0.095 (3)]. The 18-crown-6 macrocycle is also disordered in a similarly lopsided component ratio; the eight largest residual peaks are the two peaks near the K atom and those for six O atoms of the minor component of disorder. However, the data-to-parameter ratio drops below eight if this disorder is modeled. Thus only the anion disorder was modeled.
To model the various disordered species, analogous bond lengths and angles were restrained to be similar and anisotropic displacement parameters for proximal atoms were restrained to be similar. For the THF solvent molecules in 2, bond lengths were restrained toward ideal values and anisotropic displacement parameters were additionally restrained toward the expected motion relative to bond direction.
For 1 the maximum residual peak of 0.36 e À Å À3 and the deepest hole of À0.35 e À Å À3 are found 0.97 and 0.53 Å from atoms C2 and Fe1, respectively.
For 2 the maximum residual peak of 0.38 e À Å À3 and the deepest hole of À0.18 e À Å À3 are found 0.81 and 0.39 Å from atoms H15 and C14, respectively.
For 3 the maximum residual peak of 0.58 e À Å À3 and the deepest hole of À0.23 e À Å À3 are found 1.15 and 1.25 Å from atoms C15 and K1, respectively. The peak is part of the minor component of disorder of the 18-crown-6 ring, which was not modeled (see above).  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.

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. Refinement. Two CNXyl groups were modeled as disordered over two positions each: N1/C2-C9, 0.52 (2) For the various pairs of components of disorder, analogous bond lengths and angles were restrained to be similar and anisotropic displacement parameters for proximal atoms were restrained to be similar. Bond lengths for the thf solvent molecules were restrained toward ideal values. Anisotropic displacement parameters for the thf solvent molecules were also restrained toward the expected motion relative to bond direction.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (