Synthesis and crystal structures of bis[1-oxopyridin-2-olato(1−)]bis(pentafluorophenyl)silicon(IV)–tetrahydrofuran–pentane (2/1/1), bis[1-oxopyridin-2-olato(1−)]bis(p-tolyl)silicon(IV), and dimesitylbis[1-oxopyridin-2-olato(1−)]silicon(IV)

Three hexacoordinated bis(aryl)silicon(IV) complexes of 1-oxopyridin-2-one (OPO) are reported, each of which exhibit C/N site disorder in their pyridine rings. In (C6F5)2Si(OPO)2, the equal disorder ratios and solution NMR characterization together indicate the presence of a single totally asymmetric ON-trans-OC isomer. Unequal disorder ratios in p-tolyl2Si(OPO)2 and in mesityl2Si(OPO)2 indicate the presence of up to three isomers.


Chemical context
The intriguing capacity of 1-hydroxypyridin-2-one (HOPO) to dissolve silica to form [Si(OPO) 3 ] + in aqueous solution was reported by Weiss & Harvey in 1964.More recently, related ligand derivatives have been utilized as sequestering agents of lead and rare-earth metals, among others (Lewis & Cohen, 2004;Szigethy & Raymond, 2011;Wang, et al., 2019).In order to further study the powerful chelate effect of the OPO ligand, we have examined the solution-and solid-state structures of silicon complexes with varying organo ancillary ligands.
Previously reported hexacoordinate neutral dialkylsilicon 1-oxopyridin-2-one (OPO) complexes, R 2 Si(OPO) 2 [R = Me, Et, iPr; R 2 = (CH 2 ) 3 ], and one diaryl complex, Ph 2 Si(OPO) 2 , each exhibit co-crystallization of up to three possible isomers due, in part, to the isosteric character of the OPO ligand with the coplanar flip of itself (Kraft & Brennessel, 2014).In solution at room temperature, the dialkyl complexes exhibit only five OPO ligand resonances by NMR spectroscopy, indicating rapid interconversion of isomers that occurs with concomitant Si OC bond dissociation.For Me 2 Si(OPO) 2 , three isomers were observed at 193 K by 1 H NMR spectroscopy.In Ph 2 Si(OPO) 2 , the more electron-withdrawing phenyl groups strengthened the OPO ligand chelate interaction as given by generally shorter Si-O distances, and this resulted also in a slower interconversion between isomers relative to the alkyl derivatives (Kraft & Brennessel, 2014).
In all known R 2 Si(OPO) 2 complexes, the pair of Si-O bond distances trans to alkyl or aryl groups are longer than those cis.This characteristic, together with the observed C/N site disorder, highlights the underlying ambidentate character of the OPO ligand with interchangeability of canonical structures having either 2-pyridinone or N-oxide electronic forms.In contrast with the four known alkyl R 2 Si(OPO) 2 complexes in the crystalline state which favored primarily the ON-trans-ON isomer, the aryl derivative, Ph 2 Si(OPO) 2 , favored primarily the OC-trans-OC isomer and suggested that electron-withdrawing ancillary ligands might favor structures with primarily N-oxide forms.We report here the crystal structures and solution characterization of three additional aryl-substituted R 2 Si(OPO) 2 [R = C 6 F 5 (1), p-tolyl (2), mesityl (3)] complexes.

Structural commentary
There is one silicon complex in a general position per asymmetric unit for all three structures.In 1, there are also solvents of crystallization (see Refinement).Each of the three complexes is hexacoordinate in a distorted octahedral geometry with cis-aryl groups and two chelating OPO ligands (Figs.1-3).Selected bond lengths and angles are summarized in Tables 1, 2 and 3.In all three complexes, the oxygen-bonded C and N atoms of each pyridine ring are modeled as disordered (see Refinement), which indicates the presence of up to three possible diastereomers in each.In 1, the C1/N1 and C6/ N2 disorder ratios indicate approximately equal C/N atom occupancy in both OPO ligand sites.In 2, to an uncertain degree, a larger proportion of the ON-trans-ON arrangement is indicated from the disorder ratios, and in 3, a larger proportion of the OC-trans-OC arrangement is indicated.In our previous work (Kraft & Brennessel, 2014), similarly disordered dialkyl R 2 Si(OPO) 2 [R = Me, Et, iPr; R 2 = (CH 2 ) 3 ] complexes were found to favor a larger proportion of the ONtrans-ON arrangement, whereas the more electron-withdrawing Ph 2 Si(OPO) 2 favored a larger proportion of the OCtrans-OC arrangement.The structures of 1, 2, and 3 indicate no trend in major isomer preference with aryl/electron withdrawing ancillary ligands.As in all other R 2 Si(OPO) 2 complexes, the Si-O bonds trans to alkyl or aryl groups in 1-3 are consistently longer than those cis.

Figure 1
Anisotropic displacement ellipsoid plot of 1 drawn at the 50% probability level with H atoms and solvent omitted.Only the major components of disorder are shown.

Figure 2
Anisotropic displacement ellipsoid plot of 2 drawn at the 50% probability level with H atoms omitted.Only the major components of disorder are shown.
The 29 Si NMR spectrum of 1 in DMSO-d 6 displays a single broadened resonance at À 152.5 ppm, consistent with hexacoordinated silicon.Two sets of sharp OPO ligand resonances in 1:1 ratio are observed in the 13 C NMR spectrum, and two sets of C 6 F 5 ligand resonances in 1:1 ratio are observed in the 19 F NMR spectrum, pointing to magnetic inequivalence of all four ligands.At 298 K, the ortho and meta 19 F NMR resonances are significantly broadened, and each of the ten sharp OPO ligand 13 C NMR resonances appears as a pair of closelyspaced peaks (a total of 20 peaks) separated by � 0.2 ppm.Variable temperature NMR studies at 353 K show coalesced and sharpened meta 19 F resonances, broadened ortho 19 F resonances that approach coalescence, and 1 H and 13 C resonances of the OPO ligands that remain sharp.These observations are consistent with the absence of evidence of interconversion between diastereomers and the presence of two rotamers in 1:1 ratio of the totally asymmetric ON-trans-OC isomer with hindered rotation about the Si-C 6 F 5 bonds.The absence of dynamic stereoisomerism at the observed temperatures is striking in light of that observed with all other known R 2 Si(OPO) 2 complexes.This may be explained by the markedly stronger chelate interaction in 1, manifested by its shorter average Si-O bond lengths (Table 1) and larger O-Si-O 'bite' angles [84.60 (4) and 85.17 (4) � ], which are �1-3 � larger than those of all known R 2 Si(OPO) 2 complexes (Kraft & Brennessel, 2014).As a result, Si OC bond dissociation would be expected to be inhibited as observed, which has been shown as part of the mechanism of isomerization of R 2 Si(OPO) 2 complexes.Similarly, interconversion of fac and mer isomers in the even more strongly chelated [Si(OPO) 3 ] + cation is not observed for likely the same reason (Kraft et al., 2015).Bite angles in homoleptic [Si(OPO) 3 ] + silyl cations range from 87. 0-87.4 � in [Si(OPO)  CN, and [Si(OPO) Structural Database (CSD;Groom et al., 2016), version 5.45, update Nov. 2023; refcodes RUTQUU, RUTRAB (Kraft et al., 2015) and QOXSIF (Tacke, Willeke, & Penka, 2001)], respectively, indicating even stronger chelate interactions in comparison with 1.The presence of only one isomer of 1 in solution is consistent with the crystallographic data having a common disorder ratio of 0.52 (2):0.48(2) for both C1/N1 and C6/N2.The ON-trans-OC isomer and molecular superimposition of the flip of itself (i.e., a C 2 rotation about the axis bisecting the C-Si-C angle) uniquely reverses the positions of C and N atoms in all four oxygen-bonded sites, necessarily resulting in an equal disorder ratio.
The strength of the chelate interaction increases in the complexes in the order 3!2!1 as given by decreasing average Si-O bond distances and increasing O 2 Si bite angles (Tables 1, 2 and 3).This can be explained by the electronwithdrawing effect of the fluoroaryl groups which strengthens the interaction in 1 and the increase in steric hindrance from ortho-methyl substitution, which weakens the interaction in 3.

Figure 3
Anisotropic displacement ellipsoid plot of 3 drawn at the 50% probability level with H atoms omitted.Only the major components of disorder are shown.

Table 1
Selected geometric parameters (A ˚, � ) for 1.For 2 in CDCl 3 solution, a single set of OPO and p-tolyl ligand resonances was observed by 1 H and 13 C NMR spectroscopy with varying extents of broadened OPO ligand and ptolyl peaks that sharpen further at higher temperature.These observations are consistent with stereodynamic isomerization occurring similar to that observed with Ph 2 Si(OPO) 2 (Kraft & Brennessel, 2014).Complex 3 could not be characterized in solution due to its poor solubility.

Supramolecular features
In 1 there is an offset parallel �-� interaction between ring C11-C16 from pairs of inverted molecules (Fig. 4), with a centroid-centroid distance of 3.8613 (8) A ˚and an interplanar distance of 3.7876 (13) A ˚. Further �-� interactions may have been inhibited during crystal growth by the presence of solvent.There are a few short intermolecular C-H� � �F-C(aromatic) contacts, the strongest of which are listed in Table 4.However, it should be noted that only two [C2-H2� � �F1( 3 2 À x, 1 2 + y, 3 2 À z)] and C10-H10� � �F8(À 1 + x, y, z)] have H� � �F distances of significance compared with the sum of the individual van der Waals radii (2.56A ˚; Rowland & Taylor, 1996) and that these attractions tend to be very weakof the order of the energies of van der Waals complexes (Howard et al., 1996).
The packing of 2 features sheets of molecules parallel to the ac plane (Figs. 5 and 6).Inverted pairs of ring N1/C1-C5 Offset parallel �-� interaction between inverted pairs of molecules of 1.The second molecule is generated by the symmetry operation 1 À x, 1 À y, 1 À z.Centroid-centroid distance, 3.86 A ˚.

Figure 5
Packing plot of 2 with H atoms omitted.Rows of interlocking molecules along the [001] direction create two-dimensional sheets.Centroidcentroid distances are 3.76, 4.17, and 5.05 A ˚, for which the smaller two may allow for offset parallel �-� interactions.
Mesityl 2 Si(OPO) 2 (3): To a filtered solution of Me 3 Si(OPO) (0.0904 g, 0.493 mmol) in 4 ml of CH 3 CN was added a filtered solution of mesityl 2 SiCl 2 (0.0832 g, 0.247 mmol) in 4 ml of CH 3 CN.Colorless crystals deposited after one day at room temperature.Decantation and drying under vacuum afforded 0.0633 g (52.8%) of product that was insoluble in hot chloroform and hot acetonitrile.An attempt to dissolve 3 in DMSO-d 6 with heating resulted in dissolution with complete decomposition into unidentified products.NMR analysis of a CDCl 3 solution prior to precipitation showed severely broadened indecipherable peaks.Analysis calculated for C 28 H 30 N 2 O 4 Si: C, 69.11; H, 6.21; N, 5 H 8 F 10 N 2 O 4 Si•0.5C 5 H 12 •0.5C 4 H 8 O, C 24 H 22 N 2 O 4 Si and C 28 H 30 N 2 O 4 Si 323

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5.In all three structures, both bidentate ligands are disordered with the coplanar flips of themselves.For the rings containing C1/N1 and C6/N2, respectively, the disorder ratios are 0.52 (2):0.48(2) and 0.52 (2):0.48(2), 0.66 (2):0.34(2) and 0.61 (2):0.39(2), and 0.68 (3):0.32(3) and 0.61 (3):0.39(3), for structures 1, 2, and 3, respectively.Due to resolution limitations, the disorder model did not include the entire ring, but was modeled by refining the occupancies of the two atoms types (C and N) at the oxygen-coordinating portions of the rings.The occupancies at each site were constrained to sum to one and additionally to sum to one C and one N atom between the two sites on each ring.The positional and anisotropic displacement parameters, respectively, at each site of disorder were constrained to be equivalent.It is understood that this type of disorder model will likely exhibit a weighted average of Si-O bond lengths, trending with the disorder ratios.
In 1, the solvent volume contains one each of THF and npentane disordered over a crystallographic inversion center (0.50:0.50).Analogous bond lengths and angles in both directions along each solvent molecule were restrained to be similar.Anisotropic displacement parameters for proximal atoms were restrained to be similar.
For 1 the maximum residual peak of 0.61 e À A ˚À 3 and the deepest hole of À 0.58 e À A ˚À 3 are found 0.69 and 0.35 A ˚from atoms C21 and C25, respectively.
For 2 the maximum residual peak of 1.01 e À A ˚À 3 and the deepest hole of À 0.43 e À A ˚À 3 are found 0.92 and 0.61 A ˚from atom Si1.
For 3 the maximum residual peak of 0.27 e À A ˚À 3 and the deepest hole of À 0.25 e À A ˚À 3 are found 0.92 and 0.58 A ˚from atoms C20 and Si1, respectively.

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.Both bidentate ligands are disordered with the coplanar flips of themselves (0.524 (16):0.476(16) and 0.516 (15):0.484( 16) for the rings containing C1/N1 and C6/N2, respectively).Due to resolution limitations, the disorder was modeled by refining the occupancies of the two atoms types (C and N) at the oxygen-coordinating portions of the rings.The occupancies at each site were constrained to sum to one and additionally sum to one C and one N atom between the two sites on each ring.The positional and anisotropic displacement parameters,respectively, at each site of disorder were constrained to be equivalent.The solvent volume contains once each of n-pentane and tetrahydrofuran disordered over a crystallographic inversion center (0.50:0.50).Analogous bond lengths and angles in both directions along each solvent molecule were restrained to be similar.Anisotropic displacement parameters for proximal atoms were restrained to be similar.

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.Both bidentate ligands are disordered with the coplanar flips of themselves (0.658 (19):0.342 (19) and 0.612 (19):0.388 (19) for the rings containing C1/N1 and C6/N2, respectively).Due to resolution limitations, the disorder model did not include the entire ring, but was modeled by refining the occupancies of the two atoms types (C and N) at the oxygen-coordinating portions of the rings.The occupancies at each site were constrained to sum to one and additionally to sum to one C and one N atom between the two sites on each ring.The positional and anisotropic displacement parameters, espectively, at each site of disorder were constrained to be equivalent.It is understood that this type of disorder model will likely exhibit a weighted average of Si-O bond lengths, trending with the disorder ratios.

Table 5
Experimental details.
Both bidentate ligands are disordered with the coplanar flips of themselves (0.68 (3):0.32(3) and 0.61 (3):0.39(3) for the rings containing C1/N1 and C6/N2, respectively).Due to resolution limitations, the disorder was modeled by refining the occupancies of the two atoms types (C and N) at the oxygen-coordinating portions of the rings.The occupancies at each site were constrained to sum to one and additionally sum to one C and one N atom between the two sites on each ring.The positional and anisotropic displacement parameters,respectively, at each site of disorder were constrained to be equivalent.It is understood that this type of disorder model will likely exhibit a weighted average of Si-O bond lengths, trending with the disorder ratios.