Crystal structures of the polymer precursors 3-(2,5-dimethoxy-3,4,6-trimethylphenyl)propyl methacrylate and 3-(2,4,5-trimethyl-3,6-dioxocyclohexa-1,4-dienyl)propyl methacrylate

The molecular and crystal structures of 3-(2,5-dimethoxy-3,4,6-trimethylphenyl)propyl methacrylate and 3-(2,4,5-trimethyl-3,6-dioxocyclohexa-1,4-dienyl)propyl methacrylate, synthesized as precursors to redox-active polymer gel systems, are reported.


Chemical context
The title compounds, (I) and (II), were synthesised as part of our continuing interest in redox polymers and electrochemical actuators (Dana et al., 2007;McAdam et al., 2008;Goswami et al., 2013Goswami et al., , 2015. Redox-active polymers containing 2,2,6,6tetramethylpiperidin-1-oxyl-4-yl (TEMPO) and ferrocene as pendant groups are well documented (Gracia & Mecerreyes, 2013;Tamura et al., 2008;Schattling et al., 2014). In contrast, polymers with pendant quinone units are less well explored (Hodge & Gautrot, 2009;Hä upler et al., 2014). Reasons for this include their free-radical-scavenging properties in free radical polymerization (FRP), and the incompatibility of the quinone carbonyl groups in typical living polymerization such as anionic or cationic polymerization. In previous work (Goswami et al., 2013) we successfully demonstrated that steric hindrance by alkyl groups around a quinone unit prevents radical addition to the ring or the carbonyl oxygen atom, thus enabling FRP synthesis of homo-and co-polymers of quinone-appended methacrylate monomers.

Structural commentary
Compound (I), a tetra-alkylated p-dimethoxybenzene is shown in Fig. 1. The methoxy substituents are in the typical trans conformation (Wickramasinhage et al., 2016;Wiedenfeld et al., 2003;Wieczorek et al., 1975) with a C111-O1Á Á ÁO4-C41 torsion angle of approximately 179.24 . Three methyl groups and a propyl methacrylate occupy the other four sites on the benzene ring. Compound (II), shown in Fig. 2, is the quinone analogue of (I). As expected, the oxidation destroys the aromaticity of the six-carbon ring, reflected in a shortening The molecular structure of compound (I), with displacement ellipsoids drawn at the 50% probability level.

Figure 2
The molecular structure of compound (II), with displacement ellipsoids drawn at the 50% probability level. Table 1 Hydrogen-bond geometry (Å , ) for (I).

Crystal packing for (II)
For (II), an extensive series of C-HÁ Á ÁO hydrogen bonds and a C-HÁ Á Á(ring) contact generate the three-dimensional structure. These contacts include O10 acting as a trifurcated acceptor; C9-H9BÁ Á ÁO10 hydrogen bonds, supported by C31-H31BÁ Á ÁCg contacts (Cg is the centroid of the C1-C6 ring), Table 2, form chains along the a-axis direction, Fig. 6. The other two components of the trifurcate, the inversionrelated C9-H9AÁ Á ÁO10 and C51-H51BÁ Á ÁO10 hydrogen bonds form R 2 2 (10) and R 2 2 (20) rings, respectively. A third inversion dimer results from C12-H12AÁ Á ÁO1 contacts and forms R 2 2 (22) rings. O4 acts as a bifurcated acceptor, forming C21-H21AÁ Á ÁO4 and C31-H31CÁ Á ÁO4 hydrogen bonds that enclose R 1 2 (7) rings, completing an extensive sheet of molecules parallel to (105), Fig. 7. This eclectic array of contacts combine to produce a three-dimensional network with molecules stacked along the a axis, Fig. 8 A double sheet of molecules of (I) in the ac plane.

Figure 3
Chains of molecules of (I) along the a-axis direction.

Figure 5
Overall packing for (I) viewed along the a-axis direction.

Figure 6
Chains of molecules of (II) formed along a.

Figure 7
Sheets of molecules of (II) viewed along a.

Figure 8
Overall packing for (II) viewed along the a-axis direction.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All H atoms were refined using a riding model with d(C-H) = 0.95 Å , U iso = 1.2U eq (C) for vinyl, 0.99 Å , U iso = 1.2U eq (C) for CH 2 H atoms and 0.98 Å , U iso = 1.5U eq (C) for CH 3 H atoms. The hydrogen atoms of the C13 and C51 methyl groups of (I) were equally disordered over two sites. Idealized disorder models were applied using AFIX123 in SHELXL2014/7. For (I), a low-angle reflection with F o << F c , that may have been affected by the beam-stop, was omitted from the final refinement cycles.   For both compounds, data collection: APEX2 (Bruker, 2013); cell refinement: APEX2 and SAINT (Bruker, 2013); data reduction: SAINT (Bruker, 2013); program(s) used to solve structure: SHELXS2013 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014/7 (Sheldrick, 2015) and TITAN (Hunter & Simpson, 1999); molecular graphics: Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXL2014/7 (Sheldrick, 2015), enCIFer (Allen et al., 2004), PLATON (Spek, 2009), publCIF (Westrip 2010). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.34 e Å −3 Δρ min = −0.24 e Å −3 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. One low angle reflection with Fo << Fc, that may have been affected by the beam-stop, was omitted from the final refinement cycles.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. ( where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.34 e Å −3 Δρ min = −0.32 e Å −3 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.