Borepin Rings as âSigma-Freeâ Reporters of Aromaticity within

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Borepin Rings as “Sigma-Free” Reporters of Aromaticity within Polycyclic Aromatic Scaffolds Reid E. Messersmith, and John D. Tovar J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b00125 • Publication Date (Web): 08 Jan 2019 Downloaded from http://pubs.acs.org on January 12, 2019

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The Journal of Physical Chemistry

Borepin rings as “sigma-free” reporters of aromaticity within polycyclic aromatic scaffolds

Reid E. Messersmith[1] and John D. Tovar[1,2]*

[1] Department of Chemistry, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, United States of America [2] Department of Materials Science and Engineering, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, United States of America * corresponding author email: [email protected]

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Abstract

The definition and measurement of local and global aromaticity in fused ring polycyclic aromatic compounds is a complex issue. Historically, these types of molecules have been explored in this capacity by way of experimental (NMR, thermochemistry) and computational (NICS, HOMA) analyses. We previously showed how borepin rings with [b, f] arene fusions can be used as experimental magnetic aromaticity reporters via the remaining protons attached to the borepin rings. In this report, we describe a joint experimental and computational analysis of several borepin-containing polycyclic aromatic molecules in order to draw conclusions about the influence of ring fusion on aromaticity. We find that the borepin ring within these extended structures is a unique motif with limited σ-contribution to aromaticity while still displaying a wide range of structural and magnetic aromatic character.


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Introduction

Aromaticity is a multi-faceted concept1,2 commonly described through by energetic,3,4 structrual,5 and magnetic6 criteria. Fused-ring polycyclic aromatics can maintain multiple ring currents with unequal distributions of local aromatic character within different portions of the molecule.7,8 The evaluation of the local aromaticity of individual rings and the global aromaticity of the whole molecule requires multiple metrics. The harmonic oscillator model of aromaticity (HOMA)5,9 index is one method for quantifying structural aromaticity that evaluates bond length deviation on a normalized scale. Nucleus-independent chemical shifts (NICS) utilize a ghost atom (bq) as a local computational probe for the magnetic aspect of aromaticity,10,11 while plots of the current density (CD) can visualize electron delocalization.12,13 The combined use of HOMA, NICS, and CD aromaticity measures can help to better elucidate the nature of the aromaticity within complex fused-ring systems and is generally recommended for a more complete picture of a particular system versus considering one unique calculation or observable.11,14 The cyclic delocalization of π electrons is traditionally associated with assessments of aromaticity. However, the influence of σ electrons can be detrimental for HOMA calculations on pyrene15 and for NICS calculations on a variety of substrates16 when attempting to quantify aromaticity. The desire to remove σ influences from aromaticity calculations led to the development of the Sigma-Only Model17 and dissected NICS techniques.18,19 These calculations seek to quantify aromaticity by removing the complication of σ electron influences. There are significant contributions from σ effects at ring centers identified using the Sigma-Only Model for benzene,20 pentalenes,21 s-indacene,22 as well as several nitrogen-containing polycyclic aromatics.23 3 ACS Paragon Plus Environment


We show here how borepin rings embedded within polycyclic aromatic cores can inform on local aromaticity with limited inherent impact from σ electron aromaticity. A borepin ring is a seven membered, 6 π-electron system that can be considered as a Hückel aromatic motif similar to the tropylium ion.24 Borepins are chemically sensitive motifs25 that typically require trimethylphenyl (Mes)26 or tri-tert-butylphenyl (Mes*)27 capping groups on the boron atom to maintain ambient stability. Borepin-containing polycyclic aromatics can be further functionalized to tune their properties.28,29 The borepin motif has appeared in radialene-like,30 [b,d,f]-fused systems,31 [bc,ef]-fused systems,32 and has been involved in fascinating skeletal rearrangements.33 Previous computational work indicated that borepin aromaticity could be influenced by a variety of ring fusion orientations,34 and our group has since validated these predictions via the preparation of several mesityl-capped, [b,f]-fused borepins that have specific aromaticity handles. Here, we subjected a selected set of 14 borepin containing molecules synthesized over the past decade to thorough computational analysis to explicate the general abilities of the borepin ring to act as a reporter on the aromaticity within fused polycyclic compounds. We find that the borepin centers are remarkably devoid of σ-electron aromaticity influence and that their relative aromaticites within polycyclic structures spans a large swath of aromaticity-space.

Results and Discussion

Structural Criteria of Aromaticity. Over the past decade, several borepin-containing compounds with varying ring fusions (shown in Figure 1) were synthesized by our group and by Piers’ group.26,29,35-38 This collection spans a wide range of aromatic character in the borepin ring 4 ACS Paragon Plus Environment

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while maintaining several key features such as a B-Mes capping group, arene fusions to the b and f faces of the borepin core, and hydrogens on the base of the borepin ring. The continuity between these structures allows for comparisons of aromaticity in specific rings of these polycyclic aromatic compounds versus other formal inclusions of borepins within other more complex scaffolds such as those reported by Yamaguchi and by Wagner.30,32

Figure 1. Borepin containing polyclic aromatic compounds with explicit Hγ shown in 1.

The HOMA index ranges from ca. 0 for non-aromatic compounds to ca. 1 for aromatic compounds, while formally antiaromatic compounds have negative HOMA values. We recently used the HOMA index to explain the difference in aromatic character between 12 and 13,37 and here, we apply it more generally to 1-14 as a method to compare the experimental single crystal X-ray crystallographic data with calculated geometry optimized structures (B3LYP/6311+G(d,p)) of the borepin ring (Figure S1). Starting with the lowest HOMA value, compound 3 5 ACS Paragon Plus Environment


has a bend in the crystalline state (11° between the plane of the borepin ring and the naphthalene ring) with a HOMA value of -0.15. This molecule is predicted to be planar in gas-phase geometry optimized structures with a HOMA value of -0.18, indicating an absence of structural aromaticity in the borepin ring. This surprising result displays the lack of influence of molecular packing parameters imposed in the crystalline solid state on local HOMA values compared to gas-phase geometry optimized structures. Differentially-fused borepins with thiophene and carbocyclic arene fusions (4-7) display more structurally aromatic borepin rings with HOMA values from the geometry optimized structures of 0.25 ±0.03 compared with their all-carbocyclic congeners (e.g. 1 – 3, 0.04 to -0.18). This can be rationalized by the energetic preference for the benzo fusions to localize aromaticity therefore driving circulating π-electron density away from the borepin core. Compound 11 is the only molecule whose borepin subunit is not locally aromatic in the major resonance contributor, and it has crystalline (0.16) and geometry optimized (0.19) HOMA values that reflect the lessened structural aromaticity within the borepin ring. The other borepin structures that are flanked by two thiophene rings (8-10, 12-14) have borepin ring HOMA values of 0.5 ±0.1, which are significantly more structurally aromatic than 11, further affirming the responsiveness of the borepin ring as a local reporter of structural aromaticity. At the same time, all 7 dithieno flanked compounds (8-14) have similar HOMA values when calculated at the periphery of the polycyclic structures (ca. 0.5 ±0.1), which indicates that HOMA can also act as a global reporter of structural aromaticity. Compound 14 had slightly different HOMA values in the borepin ring when recrystallized from chloroform/methanol (0.61), or ortho-dichlorobenzene (0.64), and each value is plotted in Figure 2 against the geometry optimized structure (0.61). This is the most structurally aromatic borepin to date that fits within the parameters of this study. 6 ACS Paragon Plus Environment

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As shown in Figure 2, there is a linear relationship (slope=0.95, intercept=0.00, R2=0.96) between the experimental and geometry optimized structures covering a large portion of the HOMA index. This suggests that the borepin ring is a particularly useful motif for differentiating structural aromaticity within a polycyclic aromatic compound. The HOMA values of each unique ring in 1-14, and the perimeter of the polycyclic structure (excluding the mesityl ring) can be found in the supporting information (Figure S1). The HOMA values of the other rings are ca. 0.7 and the average perimeter HOMA values are 0.5 ±0.1 (for the 14 geometry optimized structures). Substituted benzene derivatives display a limited HOMA response,39 which can be interpreted as a shortcoming of the index or a display of strength of the aromaticity of the benzene ring.40 The small deviation in the perimeter values (0.5 ±0.1) within this series (1-14) demonstrates the strength of HOMA to describe similar structural aromatic qualities on a global scale. However, this also reinforces the idea that the borepin ring is a special motif with the ability to entertain a wide range (-0.18 to 0.61) of structural aromaticity and therefore the capability to analytically discriminate among different degrees of local aromaticity.

Figure 2. HOMA values for the borepin ring from geometry optimized structures (calculated) 114 plotted against corresponding rings from crystallographic data (experimental). 7 ACS Paragon Plus Environment


Nuclear Magnetic Resonance. Nuclear magnetic resonance (NMR) spectroscopy can be used to correlate proton chemical shifts with ring aromaticities.41 The deshielding of protons on the exterior of fused polycyclic rings containing 4n+2 π-electrons has been attributed to the magnetic field induced by circulating electrons.42 The extent of deshielding is complicated by atomic electronegativity considerations that also influence local electron density around these peripheral protons. A comparison of experimental work with computational models has yielded many interesting insights into the complex relationship between induced ring currents and aromatic character.43,44 While 1H NMR chemical shifts are not without their limits,45,46 the Hγ at the base of the borepin ring can be added to this ongoing conversation. The 1H NMR spectra for 1-14 generally showed negligible chemical shift variations in different solvents or at different concentrations. NMR calculations on geometry optimized structures (B3LYP/6-311+G(d,p)) yielded shielding tensors for the Hγ at the base of the borepin ring that were corrected with the 1H isotropic value of tetramethylsilane for comparison with observed values. The observed 1Hγ chemical shifts are plotted against calculated 1Hγ signals in Figure 3. For borepins without C2v symmetry (2, 4-7, 10, 11), the two chemically distinct Hγ protons, which were not unambiguously identified, were assigned to calculated NMR shielding tensors based on relative chemical shift. There is a linear correlation (slope=1.05, intercept=0.20, R2=0.98) between observed and calculated 1Hγ chemical shifts indicating that NMR calculations are good representations of observable phenomena for this series.47 Compounds 13 and 14 are clearly separated from the other 12 compounds but remain on the same linear trendline. Compound 14 has the most deshielded 1Hγ NMR signal of any of the borepins studied (8.65 ppm).


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Figure 3. Calculated 1Hγ NMR shielding tensors corrected with tetramethylsilane plotted against corresponding experimental 1Hγ NMR signals in CDCl3 for 1-14.

NICS. Since the original NICS calculations10 (e.g. NICS(0) where the bq ghost atom is in the plane of the ring), a series of modifications were made in order to mitigate the σ influence that perturbs the NICS(0) calculation.6,11,16 For example, moving the bq atom 1 Å above the plane of the ring (NICS(1)) reduces the σ contribution.8 However, this value is still isotropic, and better correlation with other computational indices was found with the zz-tensor of the bq atom 1 Å above the plane of the ring (NICS(1)ZZ) or 1.7 Å above the plane of the ring (NICS(1.7)ZZ).48 A more recent calculation is NICSπZZ, where the  contribution to the zz-tensor as derived from the Sigma-Only Model (obtained from the in silico hydrogenated π-system) is subtracted from the NICSZZ data calculated for the initial π-system17 (the case where bq is held at 1.7 Å is referred to as NICS(1.7)πZZ49). NICS-Z-Scans were calculated from the plane of the ring to 3.9 Å above the center of the borepin ring along with the corresponding Sigma-Only Model (Figure S2). The difference between the out-of-plane components of the native molecule and the in silico hydrogenated system averaged with three times the difference of the in-plane components can 9 ACS Paragon Plus Environment


quantitatively describe the ring currents (known here as NICS-Z-mean).17 The latest modification is the NICS-XY-Scan20 which allows for a series of bq atoms placed 1.7 Å above the plane of the molecule in order to sample different local chemical environments. This can be done with or without corrections from  contributions. Representative data for 9, 13 and 14 are shown in Figure 4 with the NICS(1.7)ZZ, NICS(1.7)ZZ Sigma-Only Model (the hydrogenated system), and NICS(1.7)πZZ. All NICS(1.7)πZZ scans (red triangles) show negatively-signed local minima at the center of the borepin ring (0 on the x-axis) indicating aromaticity in the borepin ring. Compounds 13 and 14 also show minima at the center of the fused benzene rings, which are more negative than the borepin ring indicating an increase in aromatic character at those ring centers. There are also global maxima (ca. 0 ppm) in the NICS(1.7)ZZ Sigma-Only Model (blue squares) at the center of the borepin rings in 9, 13 and 14 indicating a lack of σ influence. This causes the scans of the NICS(1.7)ZZ (green circles) and the NICS(1.7)πZZ (red triangles) to nearly intersect at the center of the borepin ring. The complicating  influences are apparent in the divergence of the NICS(1.7)ZZ and the NICS(1.7)πZZ scans over the other local aromatic centers of the molecule. Similar behavior was noted for all other borepins in this study (see Supporting Information), with the minor exception of 8 which shows only a 0.89 ppm difference in the NICS(1.7)ZZ and NICS(1.7)πZZ data. Several related but not yet synthetically realized compounds were examined in silico for comparison, and the results are displayed in the Supporting Information. For example, reduced σ contributions were observed in a related carbocyclic tropylium ion within a fused network. However, the synthetic and isolation challenges associated with tropylium-based molecules might hinder the experimental realization of more complex polycyclic aromatic structures based on this unit. This makes the neutral and isolable borepin ring an interesting case study for aromatic character 10 ACS Paragon Plus Environment

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because the σ influence is inherently not a significant part of the borepin ring when embedded into larger polycyclic aromatics.

Figure 4. NICS-XY-Scans of NICS(1.7)ZZ (green circles), NICS(1.7)ZZ Sigma-Only Model (blue squares) and NICS(1.7)πZZ (red triangles) of 9 (a), 13 (b), and 14 (c) with the x-axis set to zero in the center of the borepin ring.

Current Density. The anisotropy of the induced current density50 (AICD) and the magnetically induced current density51,52 (MICD) of 9, 13 and 14 as representative examples are displayed in Figure 5. The AICD findings for these select molecules mirror what we have shown previously for other thiophene-fused borepins.37 The NMR shielding tensors are calculated with the Continuous Set of Gauge Transformations (CSGT) method for AICD and the GaugeIndependent Atomic Orbital (GIAO) method for MICD. The AICD plots are made directly by the AICD program while the MICD streamline representations are made with the tube filter in Paraview. Both representations display a current around the perimeter of the polycyclic aromatic structure. The vectors become very short near the boron atom but the current density vectors increase from 9 to 13 to 14 across the boron atom in both representations. There are significant current density vectors at the base of the borepin ring in all six panels of Figure 5. These plots 11 ACS Paragon Plus Environment


show diatropic (clockwise) current around the perimeter of the ring indicative of polycyclic aromatic compounds.

Figure 5. AICD plots of 9 (a), 13 (b), 14 (c) of π-orbital contribution of the polycyclic backbone with isosurface values of 0.03 and superimposed arrows showing the direction of electron current. MICD streamline representations of 9 (d), 13 (e), 14 (f) with total current 1 bohr above the molecular plane.

Comparison of observed 1Hγ chemical shift and NICS calculations. Proton chemical shifts have been previously compared with calculated NICS values to evaluate the aromaticity of 12 ACS Paragon Plus Environment

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dimethyldihydropyrenes,53 fulvalenes,47 and boroles.54 Because of the many factors that influence experimental and calculated chemical shifts, there are arguments against using 1H NMR data55,56 as well as against NICS indices57,58 to evaluate aromaticity. Aromaticity is generally considered a global (molecular) parameter, so the single-point NICS calculations (which are local probes) may be inherently flawed without considering other variables.57,58 In Figure 6, various NICS indices are plotted against their corresponding 1Hγ NMR signals for compounds with C2v symmetry (1, 3, 8, 9, and 12-14). For compounds without C2V symmetry (2, 4, 6, 10, and 11) the two distinct 1Hγ NMR signals were averaged before plotting against the NICS indices. Compounds 5 and 7 had too much deviation (>15°) from planarity for quantitative NICS-Z-mean17 and were excluded from Figure 6 (all panels). There is a high degree of resemblance between the four panels in Figure 6, which we attribute to a general lack of σ influence in the borepin ring. There appears to be two independent linear relationships: one showing a direct relationship between the calculated NICS parameter and the observed chemical shift for borepins with a carbocyclic ring fusion (1-7), and one showing a constant NICS value (ca. -5 for NICS(1), Figure 6a) over varying chemical shift for borepins bearing two [b]thieno ring fusions (8-10 and 12-14). Thus, there is an apparent lack of responsiveness for the various NICS indices as pertaining to the thieno-fused borepins that present 1Hγ NMR variations of over a chemical shift range of 7.7-8.7 ppm. NICS is a measure of π-electron current, which may be the same for the thiophene-fused borepin units, and other factors such as the presence of the sulfur atoms or nearby diatropic ring currents within the fused arenes (see Supporting Information) may influence the observed 1Hγ proton chemical shifts of these molecules.



Figure 6. Isotropic NICS(1) (a), NICS(1)ZZ (b), NICS-Z-mean (c), and the NICS(1.7)πZZ (d) for 1-4, 6, and 8-14 plotted against corresponding experimental 1Hγ NMR signals in CDCl3. Calculations were obtained at the center of the borepin ring of each molecule.

Conclusions

Borepin rings with various ring fusions display a wide range of structural (HOMA: -0.18 to 0.61) and magnetic (NICS(1): 0.20 to -5.06) aromatic character. The analysis of the 14


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borepin-containing fused polycyclic aromatic compounds provided here adds to the ongoing discussion of aromaticity with attention to reconciling experimental and computational evaluation. Identifying and isolating individual ring impacts on the aromatic character of the systems is challenging as small modifications inherently change the nature of the system. Isomeric modifications and benzannulation are two approaches to systematically vary the aromatic relationships between fused rings. This is a useful tool for establishing local and global impacts of aromaticity of the π-conjugated aromatic molecules, provided that the inherent  influences can be corrected for. The lack of inherent  contribution from the borepin center makes this subunit an interesting aromaticity probe worthy of continued investigation.

Supporting Information Copies of HOMA values, NICS-Scans and DFT calculated Cartesian coordinates.

Acknowledgement The authors thank Professor Henrik Ottosson (Uppsala University) for providing critical feedback on this manuscript and Professor Lan Cheng (Johns Hopkins) for helpful advice with GIMIC. This research was supported by the National Science Foundation (CHE-1464798). R.E.M. acknowledges the support of the William Hooper Grafflin Fellowship.

Experimental Details



Synthesis and NMR spectroscopy. All molecules were published previously, and the proton NMR data presented here has been taken from these prior reports. Computational Considerations. Quantum chemical calculations59 were performed at the B3LYP hybrid density functional theory level60 using the 6-311+G(d,p) split valence triplet-zeta basis set61 using computational resources at the Maryland Advanced Research Computing Center (MARCC). Optimized geometries were assessed through frequency calculations confirming that the stationary points calculated correspond to minima on the potential energy surface. NMR chemical shift calculations were carried out using the Gauge Including Atomic Orbital (GIAO) method. Nucleus Independent Chemical Shift Scans (NICS) were performed using the Aroma program.62 Calculations of the Anisotropy Induced Current Density were performed using the AICD program with the π current on the mesityl ring omitted from the calculation because it is perpendicular to the applied magnetic vector and not under investigation.12 Calculations of the Magnetically Induced Current Density (MICD) were performed using GIMIC.51

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