Base Interactions in 2-(2

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

pubs.acs.org/IC

Exploration of Hypervalent Lewis Acid/Base Interactions in 2‑(2′Thiazolyl)-3-thienylphosphanes Nicole Grenon and Thomas Baumgartner*,† Department of Chemistry and Centre for Advanced Solar Materials, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada S Supporting Information *

ABSTRACT: The synthesis of a series of conjugated organophosphorus materials with intramolecular Lewis acid/base interactions and the exploration of the electronic nature of the bonding around the resulting hypervalent phosphorus centers are reported. To further establish the influence of increasing the size of the π-conjugated backbone, two scaffolds, thiazolyl-thiophene and benzothiazolylthiophene, were included in this study. Single-crystal X-ray crystallography of several of the compounds supports the hypervalent nature of the phosphorus center in the new species. Surprisingly, altering the Lewis acidity of the phosphorus center via oxidation or methylation impacts the coordinating mode of the thiazolyl substituent, which also has considerable impact on the photophysical and electrochemical properties of the πconjugated molecular scaffolds. Through theoretical calculations involving natural bond orbital (NBO) analysis and atom-inmolecules (AIM) correlation, the existence and electronic nature of weak hypervalent bonding interactions around the phosphorus center was solidified as weak 3c−4e and/or σ-hole bonds, depending on the coordination mode of the peripheral thiazolyl substituent as well as the Lewis acidity of the phosphorus center.

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INTRODUCTION Incorporation of phosphorus centers into π-conjugated molecules can provide interesting and versatile functional materials through the modification of a trivalent phosphorus atom.1 This can be achieved, for example, via oxidation, resulting in a tetracoordinate phosphorus environment with negative hyperconjugation that involves back-donation from the filled p-orbital of the substituent (e.g., the lone pair on oxygen) into a σ*-orbital on the phosphorus.2 Phosphorus functionalization thus provides an easily accessible and flexible way to alter the coordination number, geometry, and charge, which in turn can tune the optoelectronic properties of the material, and its electron acceptor character in particular.1,2 We have extensively showcased this for the dithienophosphole and other related scaffolds over the past decade or so.3,4 As part of our ongoing interest in developing new phosphorus-containing conjugated building blocks, our attention was recently drawn to species that exhibit an intramolecular Lewis acid/base (LA/LB) interaction. Yamaguchi and coworkers first successfully introduced this concept in 2006 using thiazolyl-stabilized conjugated thienylboranes (I, Figure 1),5 whose functional properties (i.e., luminescence, electronaccepting ability) were improved through the LA/LB interaction. Along similar lines, Kornev et al. reported a series of intramolecularly pyrazolyl-stabilized phosphorus(III) species (II, Figure 1), showcasing the phosphorus’ ability to act as a Lewis acid in such an environment, however, without commenting on any functional properties.6 In the presence of electron-withdrawing P-substituents, such as halides (X), the resulting species exhibit a pentacoordinate, trigonal bipyramidal © XXXX American Chemical Society

Figure 1. Conjugated main group species with intramolecular LA/LB interaction.

P atom (according to the VSEPR model) with an axial threecenter four-electron (3c−4e) PX2 bond. This type of bond is commonly invoked for pentacoordinate or hexacoordinate molecules with an expanded octet, also referred to as hypervalent.7,8 It should be noted that many such hypervalent main group species have been reported, and similar bonding has been widely accepted to apply to all of them. This scenario complements the more abundant and inherently obvious Lewis acid behavior of phosphonium and phosphenium cations.9 In recent years, the “new” concept of the σ-hole bond evolved in the literature in an attempt to describe certain nonclassical intermolecular interactions, such as halogen bonding.10,11 This interaction is usually defined as a noncovalent interaction occurring between a covalently bonded atom from groups 14−16 and a negative site, for example, a lone pair of a Lewis base. It is based on electrostatic attractions between the σ-holes (regions of positive electrostatic potential) Received: November 27, 2017

A

DOI: 10.1021/acs.inorgchem.7b03008 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Synthesis of Thiazolyl- (a) and Benzothiazolyl-3-thienylphosphoranes (b) and Their P-Functionalization‡

‡

Reagents and conditions: [a] Pd2dba3, P(2-furyl)3, 120 °C, 72 h, toluene;5 [b] n-BuLi, Ph2PCl, −78 °C to RT, 20 h, Et2O; [c] H2O2, RT, CH2Cl2; [d] MeOTf, 0 °C to RT, 20 h, CH2Cl2; [e] Au[tht]Cl, RT, 2 h, CH2Cl2.

and negative sites (e.g., lone pairs).11 By having more electronwithdrawing substituents on the Lewis acid center, the σ-hole becomes more positive due to the formation of polarized covalent bonds that pull electron density away from the LA center.12 Notably, the σ-hole commonly lies along the covalent bond to a central atom,11 and the σ-hole interaction is essentially colinear (at least 175°) to the covalent bond holding the σ-hole.7,13 However, explaining σ-hole-bonding strictly through electrostatic and polarization interpretations tends to be oversimplified. These concepts do not explain some of the most notable features in this noncovalent interaction. As the negative site is attracted to the σ-hole, charge transfer can occur from the negative site to the antibonding (σ*) molecular orbital of the central atom with the positive σ-hole, in essence creating another polar covalent bond between the two.11 Moreover, the σ-hole bond requires linearity of the substitutes involved. By comparing the molecular orbitals of the 3c−4e bond to the molecular orbitals of an interaction between the negative site and a σ*-orbital, we surmise that they are, in fact, quite similar, if not identical in nature, but perhaps at different polarity ends of the same spectrum. In this context, it is well-established that the nature of LA/LB interactions depends on many factors, but foremost the Lewis acidity of the acceptor atom.14 Based on the properties of the Lewis acid and Lewis base involved, these interactions can either be more covalent or electrostatic, which further strengthens the link between 3c−4e and σ-hole bonds. Since phosphorus has been seen to form hypervalent compounds through one or more 3c−4e bonds, we anticipated to be able to exploit these interactions in the construction of new conjugated functional materials, in which phosphorus acts as the Lewis acid. To also establish a stronger correlation between hypervalent 3c−4e and σ-hole bonds, we intended to explore potential N → P intramolecular interactions and investigated the corresponding photophysical and electrochemical properties of the species with pentacoordinate hypervalent phosphorus centers. Two series of compounds, with thiazolylthiophene and benzothiazolyl-thiophene back-

bones, respectively, were synthesized, and the phosphorus center was functionalized systematically to provide further insight into the character of the Lewis acidic phosphorus center and the character of the bonding in these LA/LB interactions.

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RESULTS AND DISCUSSION Synthesis. The precursor synthesis, following a literature procedure reported by Yamaguchi et al. can be seen in Scheme 1.5 It should be noted that we modified the procedure by employing trimethylstannyl functional groups instead of the tributylstannyl analogue used in the literature, as this ultimately resulted in cleaner reaction conditions. Thiazolyl-based 2a was dissolved in toluene and refluxed for 3 days in the presence of 1, Pd2dba3, and P(2-furyl)3. Column chromatography provided pure 3a in a moderate yield (40%). The same synthesis was applied to 2b to produce the benzothiazolylthiophene derivative 3b, again in moderate yield (48%). Both 4a and 4b were synthesized following a similar procedure, where 3a and 3b, respectively, were dissolved in Et2O and lithiated at −78 °C using n-BuLi and left to react at this temperature for 1 h. Chloro(diphenyl)phosphane was added dropwise at −78 °C and reacted overnight, warming to room temperature. This afforded compounds 4a and 4b with yields of 54 and 84%, respectively. Single crystals for 4a and 4b were grown through slow evaporation of the solvent in a mixture of dichloromethane and cyclohexane; their solid-state structures are discussed below. The 31P NMR shifts for both 4a and 4b are −24.7 ppm, in the expected, albeit high-field, range for a trivalent phosphorus center with related substitution pattern.15 To broaden the scope toward a better understanding of the anticipated intramolecular N → P interactions, the compounds were further functionalized through oxidation, methylation, and complexation of the phosphorus center with gold(I) chloride, respectively (Scheme 1, bottom). The trivalent phosphorus species 4a and 4b were oxidized with hydrogen peroxide in chloroform and reacted for 24 h. After purification via B


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Inorganic Chemistry

Figure 2. (a) Molecular structure; 50% probability level, (b) space-fill representation, (c) intermolecular packing of 4a in the solid state. Hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]: P(1)−C(5), 1.8286(15); P(1)−C(14), 1.8371(15); P(1)−C(8), 1.8380(14); C(5)−P(1)−C(8), 102.28(7); C(5)−P(1)−C(14), 101.28(7); C(14)−P(1)−C(8), 101.36(7).

Figure 3. (a) Molecular structure; 50% probability level, (b) space-fill representation, (c) intermolecular packing of 4b in the solid state. Hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]: P(1)−C(9), 1.8313(16); P(1)−C(18), 1.8347(15); P(1)−C(12), 1.8361(15); C(9)−P(1)−C(12), 102.78(7); C(9)−P(1)−C(12), 100.59(7); C(18)−P(1)−C(12), 101.62(7).

sonication of the compound in hexanes for 10 min and filtering the solid, 4a-O and 4b-O were isolated cleanly in yields of 67 and 73%, respectively. Methylation of the phosphorus center was achieved by adding methyl trifluoromethanesulfonate

(MeOTf) to the trivalent species in dichloromethane at 0 °C to afford 4a-Me and 4b-Me in near quantitative yields (94, 98%) after reacting for 2 h. The functionalization of the trivalent phosphorus compounds to coordinate AuCl onto the C


Article

Inorganic Chemistry

Figure 4. (a) Molecular structure; 50% probability level, (b) space-fill representation, (c) intermolecular packing of 4a-Me in the solid state. Hydrogen atoms are omitted for clarity; CH3 and triflate anion are omitted for clarity in the space-fill representation. Selected bond lengths [Å] and angles [°]: P(1)−C(5), 1.784(2); P(1)−C(20), 1.786(2); P(1)−C(14), 1.7921(19); P(1)−C(8), 1.802(2); C(5)−P(1)−C(20), 110.38(10); C(5)−P(1)−C(14), 112.72(10); C(20)−P(1)−C(14), 109.28(10); C(5)−P(1)−C(8), 106.91(9); C(20)−P(1)−C(8), 109.44(10); C(14)−P(1)− C(8), 108.02(9).

phosphorus lone pair being the fifth substituent (according to the VSEPR model) as is commonly observed for hypervalent phosphorus compounds.7 The P−S distance is 3.20 Å in 4a and 3.22 Å in 4b, less than the sum of the van der Waals radii of the two elements (3.60 Å) in both cases.18 The space-fill visualizations of the molecular structures clearly show the overlap of the van der Waals radii of the two elements, further suggesting an interaction between P and S. The packing of 4a and 4b is shown in Figure 2c and Figure 3c, respectively. Both structures exhibit π-stacking between the planar conjugated scaffolds with common distances of 3.5 and 3.6 Å for compounds 4a and 4b. The torsion angles between the nitrogen (N1) of the thiazolyl or benzothiazolyl substituent and the sulfur (S2) in the thiophene ring are 9.0(2) and 7.19(19)°, respectively. From these torsion angles, it can be seen that the fused benzene ring on the thiazole adds rigidity and planarity to the backbone, which in turn suggests better πconjugation, providing some insight into the experimentally found photophysical properties of the compounds and the density functional theory (DFT) calculations (vide infra). Trivalent phosphorus species, particularly those without electron-withdrawing substituents, are not commonly known to act as Lewis acids due to the presence of the lone pair. From the X-ray crystallographic data of 4a and 4b, the lone pair can be extrapolated to reside in the equatorial position of the distorted trigonal bipyramid. Notably, the C−P−S angles between the phenyl substituent trans to the S → P interaction and the sulfur atom amount to 174° in 4a and 176° in 4b. While not completely linear, likely due to the repulsion of the perpendicular lone pair with the phosphorus substituents, this interaction supports the notion that there is indeed a Lewis acid/base (LA/LB) interaction, leading to a hypervalent 3c−4e or σ-hole bond. However, the bond lengths of the phosphorus

phosphorus center involved a reaction with chloro(tetrahydrothiophene)gold(I) (Au[tht]Cl). Au[tht]Cl was added to a solution of the trivalent species in dichloromethane and reacted overnight providing 4a-Au in 45% yield (estimated by NMR)16 and 4b-Au in a near quantitative 97% yield, respectively. Single crystals of 4a-Me, 4b-Me, and 4b-Au, suitable for X-ray crystallography, were grown through slow evaporation of the solvent in dichloromethane and cyclohexane; their structural data are discussed below. 31 P NMR spectroscopy of the functionalized compounds revealed downfield-shifted phosphorus peaks (from their trivalent parents) confirming the successful functionalization of the phosphorus centers. The oxide species (4a-O, 4b-O) showed 31P resonances at 22.5 and 22.7 ppm, the methylated species (4a-Me, 4b-Me) showed 31P resonances at 16.1 and 16.5 ppm, and the gold complexes (4a-Au, 4b-Au) showed 31P resonances at 20.1 and 23.2 ppm, respectively. These shifts are in line with related functionalized thienylphosphanes, but again at the high-field edge.15 Molecular Structures in the Solid State. As mentioned above, single crystals of several of the compounds were obtained, allowing for X-ray crystallographic characterization of the species. The molecular structures of 4a and 4b are shown in Figures 2 and 3, respectively. As mentioned above, the expected outcome of this project was to obtain an intramolecular N → P interaction. However, the structures of the trivalent compounds in the solid state reveal intramolecular S → P interactions instead. The sum of the angles around the phosphorus center of both compounds is 305°, which is smaller than the expected angle of 310° for a trigonal pyramidal (or distorted tetrahedral) triarylphosphane,17 suggesting the presence of an additional interaction (S → P) resulting in an overall distorted trigonal bipyramidal geometry about the phosphorus atom, with the D


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Inorganic Chemistry

Figure 5. (a) Molecular structure; 50% probability level, (b) space-fill representation, (c) intermolecular packing of 4b-Me in the solid state. Hydrogen atoms are omitted for clarity; CH3 and triflate anion are omitted for clarity in the space-fill representation. Selected bond lengths [Å] and angles [°]: P(1)−C(1), 1.787(3); P(1)−C(8), 1.793(3); P(1)−C(16), 1.800(3); P(1)−C(2), 1.806(3); C(1)−P(1)−C(8), 110.49(13); C(1)− P(1)−C(16), 111.75(13); C(8)−P(1)−C(16), 112.19(12); C(1)−P(1)−C(2), 107.42(13); C(8)−P(1)−C(2), 108.30(12); C(16)−P(1)−C(2), 106.44(12).

center to the axial phenyl ring are 1.8371(15) Å (4a) and 1.8361(15) Å (4b), very similar to the P−C lengths of the equatorial phenyl ring (4a: P(1)−C(8) = 1.8380(14) Å; 4b: P(1)−C(18) = 1.8347(15) Å), suggesting that the S → P interaction is fairly weak (vide infra). Notably, upon P-methylation, the intramolecular LA/LB interaction switches to involve phosphorus and nitrogen, as originally anticipated for all of the new compounds. The P−N distances of 3.17 Å for 4a-Me and 3.03 Å for 4b-Me are again both shorter than the sum of the van der Waals radii of the elements (3.35 Å).18 The space-fill representations in Figure 4b and Figure 5b visually show the overlap between the phosphorus and nitrogen atoms, with the methyl substituents removed for clarity. The sum of the angles around the phosphorus center involving the equatorial substituents increases with the addition of the methyl substituent (4a-Me: 328°; 4b-Me: 327°). The N → P interaction leads to a close to linear C−P−N triad with angles of 175 and 174° for 4a-Me and 4b-Me, respectively. This again supports the notion that there is a hypervalent 3c−4e bond (or σ-hole bond, vide infra). In this case, the axial P−C bonds (4a-Me: P(1)−C(8) = 1.802(2) Å; 4b-Me: P(1)−C(2) = 1.806(3) Å) involved in the hypervalent bond are noticeably longer than the bonds between the phosphorus and the equatorial phenyl substituent (4a-Me: P(1)−C(14) = 1.7921(19) Å; 4b-Me: P(1)−C(8) = 1.793(3) Å), suggesting a distinct N → P interaction. The N → P interaction also causes a shortening in the bond lengths of the

exocyclic substituents likely due to the nature of this interaction being more electrostatic (vide infra). It is also noteworthy that the methyl group is essentially oriented perpendicular to the πconjugated scaffold, potentially allowing for σ*−π* hyperconjugation to occur, which would lower the LUMO and enhance the acceptor character of this species (vide infra). Moreover, the methylation of the phosphorus center has an impact on the packing of the molecule in the solid state. While there is intermolecular π-stacking in 4a-Me (distance: 3.8 Å), no such interactions are observed in the more extended benzothiazolyl derivative 4b-Me. This can be attributed to the significant twist found in the backbone of 4a-Me and reflected in the torsion angles of the methylated species (4a-Me: 30.0(2)°; 4b-Me, 3.5(3)°), allowing for the thiophene ring of one molecule to stack with the thiazole ring of its neighbor. The π-stacking is likely also enabled by the position of the triflate anion in the solid state. While the triflate anion is located quite far away from the molecule in 4a-Me, it is in close proximity to the cation in 4b-Me (Figures 4c and 5c). The structure of 4b-Au in the solid state is shown in Figure 6. The addition of AuCl again suggests a molecular structure with a S → P interaction. The P−S distance is 3.44 Å, which again falls within the sum of the van der Waals radii (vdW) of the two elements (3.60 Å),18 albeit with only a very small contraction, suggesting an overall very weak interaction. In fact, an even shorter distance is found between the gold atom and the thiophene S atom (3.31 Å; cf. vdW radii: 3.46 Å),18 E


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Inorganic Chemistry

Figure 6. (a) Molecular structure; 50% probability level, (b) space-fill representation, (c) intermolecular packing of 4b-Au in the solid state. Hydrogen atoms are omitted for clarity; AuCl is omitted for clarity in the space-fill representation. Selected bond lengths [Å] and angles [°]: P(1)− C(9), 1.817(3); P(1)−C(12), 1.819(3); P(1)−C(18), 1.819(3); Au(1)−P(1), 2.2362(8); Au(1)−Cl(1), 2.2903(8); C(9)−P(1)−C(12), 105.20(14); C(9)−P(1)−C(18), 103.57(14); C(12)−P(1)−C(18), 108.66(15); C(9)−P(1)−Au(1), 112.78(10); C(12)−P(1)−Au(1), 109.08(11); C(18)−P(1)−Au(1), 116.83(11); P(1)−Au(1)−Cl(1), 175.23(3).

Table 1. Photophysical Data of the Functionalized Thiazolyl and Benzothiazolyl Series compound

λmax [nm]a (ε [M−1cm−1])b

λem [nm]c

ϕPL [%]d

Stokes shift [cm−1]

λem [nm]e

ϕPL [%]f

3a 3b 4a 4b 4a-O 4b-O 4a-Me 4b-Me 4b-Au

313 (12 500) 332 (26 900) 328 (15 100) 335 (23 000) 325 (12 500) 335 (18 400) 331 (6100) 341 (17 500) 345 (15 000)

397 390 409 393 399 403 404 -

1 4 2 88 20 70 3 -

4930 4850 5400 5320 4790 5400 4570 -

467 503 438 483 -

3.0 2.2 2.2 1.9 -

UV−vis maxima absorption in chloroform. bMolar extinction coefficient. cEmission wavelength measured for a solution in chloroform (c ≈ 10−5 M). dPhotoluminescemce quantum yields in chloroform, measured against a quinine sulfate standard adjusted to the same absorbance, λex = 310 nm. e Emission wavelength measured in the solid state. fAbsolute photoluminescence efficiency, determined by the use of an integrating sphere, λex = 365 nm. a

suggesting the presence of an (additional) intramolecular S → Au interaction. The gold(I) chloride unit shows a relatively linear coordination to the phosphorus center (175.23(3)°); the bond lengths around gold (Au(1)−P(1): 2.362(8) Å; Au(1)− Cl(1): 2.2903(8) Å) are similar to those typically found in other (phosphanyl)gold(I) chloride compounds, but no intermolecular gold−gold interactions are observed.19 The only intermolecular short contact that can be deduced from the X−ray data is another Au−S interaction at 3.45 Å with the thienyl S atom of a neighboring molecule (Figure 6c). The sum of the angles around the phosphorus center is 317°, which could correlate to a distorted tetrahedral geometry, but it also justifies a distorted trigonal bipyramidal geometry. Additionally,

there is also some weak intermolecular π-stacking at a distance of 3.8 Å. Figure 6c also shows the significant twist seen in the backbone of 4b-Au with a torsion angle of 27.2(4)°, representing the largest twist in the backbone for the benzothiazolyl series, significantly larger than the methylated species 4b-Me (3.5(3)°). This is observation can likely be attributed to the intramolecular Au−S interaction. The P(1)− C(12) and P(1)−C(18) bonds of the two phenyl substituents are the same length of 1.819(3) Å, further supporting an only very weak intramolecular S → P interaction at best for this compound. Photophysical Properties. In order to study the effects of the modification of the phosphorus center as well as the two different conjugated backbones of the compounds, the F


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Inorganic Chemistry

Figure 7. Absorption (top) and normalized emission (bottom) spectra of the thiazolylthiophene (a,c) and benzothiazolylthiophene (b,d) series in chloroform solutions.

fluorescence in solution exhibit typical Stokes shifts for organic conjugated fluorophores (3b: 4930 cm−1; 4a: 4850 cm−1; 4b: 5400 cm−1; 4a-O: 5320 cm−1; 4b-O: 4790 cm−1; 4a-Me: 5400 cm−1; 4b-Me: 4570 cm−1).20 As already observed for the absorption features of the compounds, the emission wavelengths do not show a large dependence on the functionalization of the phosphorus centers. All emissive compounds emit over a fairly narrow range between λem = 390−410 nm (Figure 7c,d), and the species with solid-state emission showed a red shift from their emission features in solution (3b: Δλem = 70 nm; 4b: Δλem = 94 nm; 4a-Me: Δλem = 35 nm; 4b-Me: Δλem = 79 nm), due to their intermolecular interactions in the solid state. The quantum yields of the new species were determined in both solution (relative, vs quinine sulfate) and the solid state (absolute, via integrating sphere). The solid-state quantum yields were found to be quite low, with compounds 3b, 4b, 4aMe, and 4b-Me being only slightly emissive. In solution, compounds 3b, 4a, 4b, and 4b-Me were found to be weak emitters (ϕPL = 1.4, 4.2, 1.8, and 3.2%, respectively); 4b-O shows moderate emissive features (ϕPL = 20%), whereas compounds 4a-O and 4a-Me were found to be highly emissive (ϕPL = 88 and 71%, respectively), suggesting that the N → P interaction in 4a-Me that very likely also applies to 4a-O rigidifies the molecular scaffold toward overall beneficial emissive properties. DFT Calculations. We carried out theoretical calculations to gain further details on the role of the LA/LB interactions and

absorption and fluorescence features of the compounds were investigated; the data is summarized in Table 1. The largely featureless UV−vis absorption spectra show absorption maxima in the UV region of the optical spectrum, with the benzothiazolyl series exhibiting a red shift in the λmax of approximately 10 nm in comparison to the thiazolyl series that is concomitant with a significantly higher absorbance compared to the thiazolyl series, as would be expected for larger π-systems (Figure 7a,b). Surprisingly, the incorporation of phosphorus into the πconjugated backbones results only in a very small change in the λmax (4a: λmax = 328 nm; 4b: λmax = 335 nm) when compared to 3a and 3b as a reference (λmax = 313 and 332 nm, respectively). In both series, the oxidation of the phosphorus center has also no effect on the absorption spectra (4a-O: λmax = 325 nm; 4bO: λmax = 334 nm), in contrast to related systems reported by us earlier.3,4,19 On the other hand, the addition of the methyl substituent induces a slight red shift for both species (4a-Me: λmax = 331 nm; 4b-Me: λmax = 341 nm), likely due to an appropriate orientation of the P−CH3 σ*-orbital, which allows for negative hyperconjugation with the π*-system of the backbone.2 Substitution of the P-center with AuCl results in the strongest red shift in this series for 4b-Au (λmax = 345 nm). The fluorescence of the new species was investigated in both solution (chloroform) and the solid state. Compounds 3a and 4b-Au were nonfluorescent both in solution and the solid state, whereas compounds 4a, 4a-O, and 4b-O were only nonfluorescent in the solid state. The compounds that showed G


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Inorganic Chemistry

To gain deeper insight into the experimentally determined photophysics, the optical transitions were calculated through time-dependent density functional theory (TD-DFT) calculations at the B3LYP/6-31G(d) level of theory21 and are summarized in Table 2. It should be noted that TD-DFT

how the functionalization of the phosphorus center affects the energy levels of the frontier orbitals. Calculations were performed using B3LYP/6-31G(d) (3a, 3b, 4a, 4b, 4a-O, 4bO, 4a-Me, 4b-Me) and B3LYP/LanL2DZ (4b-Au) levels of theory and carried out through WestGrid using Gaussian.21 The highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) and their corresponding energies are shown in the Supporting Information. The calculations reveal that all of the compounds show a coplanar or slightly twisted backbone, in line with the X-ray crystallographic data (vide supra). In the thiazolylthiophene series, the LUMOs are π*-orbitals that show delocalization over the conjugated backbone. The HOMOs represent π-orbitals and also show delocalization across the backbone with the exception of 4a, which shows the delocalization across the backbone as well as the exocyclic phenyl substituents. The (benzothiazolyl)thiophene series shows a similar trend, representing π and π*-orbitals for the HOMO and LUMO, respectively, showing delocalization across the conjugated backbone with the exception of 4b-O, whose HOMO is a nonbonding orbital. When comparing the frontier orbital levels within the thiazolylthiophene and the benzothiazolylthiophene series, the same trend is observed. The trivalent species (4a, 4b) both show lowered HOMO−LUMO gaps (4a: Eg = 4.00 eV; 4b: Eg = 3.89 eV) when compared to their precursors (3a: Eg = 4.29 eV; 3b: Eg = 4.14 eV). The LUMO levels of 4a and 4b are lowered by 0.10 and 0.11 eV, respectively. The HOMO levels of 4a and 4b are significantly destabilized (0.40 and 0.35 eV, respectively), which results in an overall decrease in the HOMO−LUMO gap of the compounds (ΔEg = 0.29 eV (4a) and 0.25 eV (4b)). In our previously reported related dithienophospholes, the phosphorus center is known to contribute to the LUMO via σ*−π* hyperconjugation, which allows for tuning of the LUMO energy level through functionalization of the lone pair.3 In the present case, however, the HOMO and LUMO energy levels for 4a-O and 4b-O are both destabilized when the phosphorus center is oxidized, causing an overall increase in the HOMO−LUMO gap (4a-O: Eg = 4.31 eV; 4b-O: Eg = 4.12 eV), compared to 4a (Eg = 4.00 eV) and 4b (Eg = 3.89 eV). Upon methylation of the P-center (4a-Me, 4b-Me), both the HOMO and LUMO levels are significantly stabilized resulting in lowered HOMO−LUMO gaps (4a-Me: Eg = 4.08 eV; 4bMe: Eg = 3.86 eV); however, the LUMOs experience a more pronounced drop down to −2.43 and −2.60 eV, respectively. This shows that via methylation of the phosphorus center, the electron-accepting ability of the system increases considerably and correlates well with the optical and electrochemical studies (vide infra). Upon complexation with AuCl, the LUMO energy level of 4b-Au is stabilized, but the HOMO energy level is destabilized, which results in an overall small decrease in the HOMO−LUMO gap of the compound (Eg = 3.79 eV). It should be noted in this context, that in contrast to the majority of the other species reported herein, the HOMO largely corresponds to a lone pair of the Cl atom with some contribution of a gold d-orbital (Supporting Information). And finally, comparing the thiazolyl series to the benzothiazolyl series shows that the HOMO−LUMO gaps for the latter compounds are smaller than those of the former series. This can be attributed to the overall extended benzothiazolylthiophene backbone and correlates well with the observed optical spectroscopy data.

Table 2. Calculated Transitions for the Thiazolyl and Benzothiazolyl Series compound

λmax [nm]a

4a-O 4b-O 4a-Me 4b-Me 4b-Aue

325 334 331 341 346

λμax [nm]b ( f)c 311.9 299.6 332.7 361.8 345.8

(0.316) (0.182) (0.297) (0.394) (0.482)

transitiond HOMO → LUMO HOMO−3 → LUMO HOMO → LUMO HOMO → LUMO HOMO−1 → LUMO

a

UV−vis absorption in chloroform. bAbsorption maxima found through TD-DFT calculation level: B3LYP/6-31G(d). cOscillator strength. dTransitions found through TD-DFT calculation level: B3LYP/6-31G(d). eTransitions found through TD-DFT calculation level: B3LYP/LanL2DZ.

calculations could ultimately not be accomplished for 4a and 4b, as the calculations, while attempted multiple times, continually failed for unknown reasons. Most of the transitions found through the successful calculations are between the HOMO and LUMO energy levels, which correspond to π → π* transitions. By contrast, the TD-DFT calculations for 4b-O revealed that there are several major transitions that occur between HOMO−3/HOMO−1 and the LUMO. Figure 8 shows that the HOMO orbital does not involve the π-conjugated scaffold. In fact, it is governed by the S, N, and O lone pairs, next to some contribution of the π-system on an exocyclic phenyl ring. In this context, the orbital would classify as nonbonding orbital and the corresponding n → π* transition is typically forbidden. The allowed π → π* transitions that are in close proximity energetically (294 and 300 nm) ultimately involve two sets of degenerate orbitals at the HOMO−1 and HOMO−3 levels, which provides a potential explanation as to why only a moderate quantum yield is observed for the fluorescence of 4bO. Electrochemistry. To evaluate the redox properties of the new series of hypervalent compounds, the thiazolyl and benzothiazolyl series were investigated by cyclic voltammetry (CV), and the data are summarized in Table 3. The spectra were recorded in either a dichloromethane (0.1 M) or DMF (0.05 M) solution with tetrabutylammonium hexafluorophosphate ([NBu4][PF6]) as the supporting electrolyte. The parent compounds 3a and 3b, used as a reference again, as well as 4a, 4b, and 4b-Au, largely show a series featureless irreversible reductions between −1.0 and −2.1 V, as well as some irreversible oxidation features between +1.0 and +2.2 V (see Supporting Information). By contrast, converting the phosphorus center into a phosphoryl group leads to a quasi-reversible reduction behavior (4a-O: Ered = −2.46 V; 4b-O: Ered = −2.24 V) overall, suggesting enhanced rigidity and stability of the scaffold, as already suggested by the other characterization methods (Figure 9). It should be noted that the cyclic voltammetry of 4a-O and 4b-O was performed in dichloromethane; however, 4a-O deposited on the working electrode upon reduction resulting in irreversible reduction features (Supporting Information). To circumvent deposition of the species on the H


Article

Inorganic Chemistry

Figure 8. TD-DFT calculation data for 4a-O (left) and 4b-O (right) showing the notable transitions and their intensities ( f = oscillator strength) and the percent composition of each transition as calculated by the basis set B3LYP/6-31G(d).

electrode, the solvent was changed to DMF for 4a-O, resulting in quasi-reversible reduction features. To provide a suitable comparator, 4b-O was also measured in DMF and referenced to ferrocene/ferricenium, enabling us to compare 4a-O with the data obtained for the other compounds in dichloromethane. Similarly, methylation of the phosphorus center provides quasireversible reduction features at Ered = −1.96 and −1.81 V, respectively, supporting the further enhanced electron-accepting properties of 4a-Me and 4b-Me, with similar stability of the scaffold to that of the oxide congeners. The lower reduction threshold seen in 4a-Me and 4b-Me in comparison to 4a-O and 4b-O correlates with the theoretically calculated LUMO energy levels from DFT. The methylated species show lowered LUMO energy levels (4a-O: −1.72 eV; 4b-O: −1.86 eV; 4aMe: −2.43 eV; 4b-Me: −2.60 eV) when compared to the oxide species, which can be attributed to the increased σ*−π* hyperconjugation from the exocyclic CH3 substituent (vide supra). The DFT representations of the LUMO energy levels show a contribution from the σ*-orbital of the P−CH3 group, which causes a lowering of the LUMO energy seen in Table 3. This also is confirmed by the experimental UV−vis absorption

Table 3. Electrochemical Data of the Thiazolyl and Benzothiazolyl Series of Compounds compound 3a 3b 4a 4b 4a-Od 4b-O 4a-Me 4b-Me 4b-Aue

Ered [V]a §

§

−1.7 , −2.1 −1.6§ −1.0§, −1.5§ −2.46 −2.24 −1.96 −1.81 −1.8§

LUMO [eV]b

LUMO [eV]c

−2.34 −2.55 −2.84 −2.99 -

−1.72 −1.87 −1.62 −1.76 −1.72 −1.86 −2.43 −2.60 −2.60

a

CV in dichloromethane solution with [NBu4][PF6] (0.1M) as supporting electrolyte, referenced to Fc/Fc+. bLUMO energy levels calculated from the reduction potentials found from CV using the peak potentials. cLUMO energy levels found from DFT calculations (B3LYP/6-31G(d)) dIn DMF solution with [NBu4][PF6] (0.05M) as supporting electrolyte; Ered was reference to Fc/Fc+ and altered based on the difference of 4b-O Ered found in dichloromethane vs DMF. eB3LYP/LanL2DZ. §irreversible.

I


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Inorganic Chemistry

Figure 9. Cyclic voltammograms of compounds 4a-O (A), 4b-O (B), 4a-Me (C), and 4a-Me (D) at different scan rates in dichloromethane solution with [NBu4][PF6] (0.1 M) as the supporting electrolyte. (A) was performed in DMF solution with [NBu4][PF6] (0.05 M) as the supporting electrolyte.

(S: 2.58; P: 2.19).18a Functionalization of the phosphorus center draws electron density from the central atom into the substituents. This makes the phosphorus relatively nonpolarizable, thus changing its character from a soft acid to a hard acid. Hard acids and bases commonly interact through electrostatic means due to the difference in electronegativities of the atoms, allowing for primarily ionic interactions,14a which should be predominant in the N → P coordination mode of the oxide and methyl phosphonium species of this series. To verify these hypotheses, we performed further theoretical studies using natural bond orbital (NBO) analyses22 and the atom-in-molecules (AIM) method23 to establish bond-critical points as well as to determine potential differences in the bonding in addition to obtaining a gauge for the bond strengths in these interactions. To our satisfaction, the calculations provide a good match for the conclusions drawn from the experimental data. In all cases, intramolecular LA/LB interactions as well as bond-critical points are observed, and the observed differences can be justified along the lines of an HSAB approach. For the trivalent species, NBO analysis does show the expected lp(S) → σ*(PC) interaction, albeit with a small stabilization energy of 2.22 kcal/ mol for 4a and only 1.28 kcal/mol for 4b. However, NBO analysis also reveals a series of other (weak) LB/LA interactions that add to the overall stabilization of the observed conformation. Next to an inverse lp(P) → σ*(SC) interaction (4a: 1.65 kcal/mol; 4b: 1.95 kcal/mol) that suggests that the P lone pair does in fact participate in bonding, there are

spectra and is evident in a red shift in the absorbance of 4a-Me and 4b-Me. Electronic Nature of the Intramolecular Dative Bonds. Overall, the relatively stable electrochemical reduction features (as well as their stronger emission properties) of the oxidized and methylated congeners of this new series of compounds (as opposed to the irreversible redox and weakly emissive features of the remaining species) appears to correlate well with their coordination mode featuring an intramolecular N → P interaction, warranting a deeper look into the electronic nature of the intramolecular interactions that lead to the hypervalent phosphorus centers. As a starting point, the nature of the dative bond can be analyzed through discussion of the Lewis acidity of the phosphorus center. Through functionalization, the phosphorus center Lewis acidity can be tuned, and the polarizability of the phosphorus center is also altered. According to the HSAB theory, hard bases tend to be weakly polarizable and have small radii whereas soft bases contain donor atoms that are larger and relatively polarizable.14a The trivalent phosphorus center in 4a and 4b is quite large and fairly polarizable. Due to its positioning in the equatorial plane of the trigonal bipyramidal geometry seen in the solidstate structures, we expected the lone pair not to participate in donation, thus the phosphorus center should act as a soft Lewis acid according to the HSAB theory. As mentioned, soft acids and soft bases are more likely to interact with each other through covalent means due to their similar electronegativities J


Article

Inorganic Chemistry

Figure 10. AIM calculations23b showing bond-critical points for S → P interaction in 4a (A), 4b (B), as well as N → P interaction in 4a-O (C), 4b-O (D; note: a much weaker π →H interaction is also detected); (E) electron density surface of 4b-O with electrostatic potential mapped onto it; (F) electrostatic potential surface of 4b-O.

potentially two more lp(N) → σ*(SC) interactions (E < 1 kcal/mol for both) each between the benzothiazole nitrogen lone pair and two different σ*-bonds of the thiophene ring. Altogether these interactions amount to a stabilization energy of more than 4.5 kcal/mol for the S → P coordination mode. The main interaction between the sulfur and the phosphorus atoms was further ascertained by the AIM method that provided S−P bond-critical points at electron densities of 0.0119 (4a) and 0.0125 (4b) e/bohr3, respectively (Figure 10a,b). In case of the oxide species 4a-O and 4b-O, NBO analysis provided evidence for lp(N) → σ*(PC), but the stabilization energies were surprisingly low (4a-O: 0.75 kcal/mol; 4b-O: 1.01 kcal/mol). Notwithstanding, bond-critical points were found for both species (4a-O: 0.0084 e/bohr3; 4b-O: 0.0088 e/ bohr3; Figure 10c,d). The phosphorus center in these pentavalent species should be considered a hard Lewis acid, and the resulting LA/LB interactions would consequently have a major electrostatic component to them as well, as mentioned above. When looking at the NBO charges of the relevant atoms (N: −0.44 (4a-O), −0.42 (4b-O); P: + 2.01 (4a-O), + 2.04 (4b-O)), the large differences (>2.4) clearly lend themselves to an electrostatic interpretation of the bonding in these species.

In fact, the electrostatic potential map for the two species (Figure 10e,f), suggests strong electrostatic interactions between P and N in this coordination mode, which would correspond more to a σ-hole bond type for the hypervalent P center in this case. Similar to the oxide species, NBO analysis for the methylated congeners 4a-Me and 4b-Me revealed weak lp(N) → σ*(PC) LA/LB interactions with 1.37 and 1.57 kcal/mol, respectively. However, an additional lp(N) → σ*(CH) LA/LB interaction involving the antibonding orbital of the P-Me group with stabilization energies of 2.82 and 2.32 kcal/mol, respectively, is also detected. This interaction can also be correlated with the twisted scaffold observed via X-ray crystallography for 4a-Me, placing the thiazole N atom closer to the P-Me group (vide supra). Concurrently, two bond-critical points were determined for each species via AIM at 0.0093 (N−P, 4a-Me), 0.0100 (N− P, 4b-Me), 0.0140 (N-Me, 4a-Me), and 0.0136 (N-Me, 4bMe) e/bohr3 (Supporting Information). An electrostatic component to the dative bonding should also be considered in this case, since the P-center can again be classified as hard Lewis acid, and the NBO charge difference was found to be 2.2 for 4a-Me and 2.1 for 4b-Me, respectively. This is again K


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Inorganic Chemistry

electrostatic interactions. Consequently, these new compounds show promise as stable emissive electron-acceptors when functionalized appropriately; however, they also open the door for several future directions toward species with stronger intramolecular LA/LB interactions that are currently being explored in our laboratory.

reflected in the electrostatic potential of the electron density (see Supporting Information). Lastly, NBO analysis of the gold complex reveals a dominant S → Au coordination, rather than S → P coordination. And this is in fact in line with the X-ray crystallographic data for 4b-Au. According to the NBO analysis several S−Au interactions lead to a total stabilization energy of 7.8 kcal/mol, while there is only a very weak component involving a lp(S) → σ*(PC) interaction (0.53 kcal/mol); AIM provides a bond-critical point only between S and Au at 0.0106 e/bohr3. Altogether, these theoretical calculations support the notion that the dative bonding in the trivalent species 4a and 4b favors soft LA/LB interactions leading to a more covalent, albeit weak, 3c−4e bond type for the hypervalent phosphorus center. A similar explanation can be applied to the gold complex 4b-Au, for that matter, despite the fact that the Lewis acid is in fact the gold atom, and not phosphorus, with the LA orbitals being gold-based orbitals. In case of the species involving oxidized or methylated P centers, their character is switched to hard Lewis acid, which favors interaction with the hard Lewis base nitrogen involving a considerable amount of electrostatic interaction. This scenario gives rise to a hypervalent P center that participates in a σ-hole type bond that lies more closely at the ionic end of the spectrum for hypervalent bonding.

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EXPERIMENTAL SECTION

General. All reactions were carried out in dry glassware under an inert atmosphere of purified nitrogen by employing standard Schlenk techniques unless otherwise specified. Solvents were dried using an MBraun Solvent Purification System. Compound 1 was synthesized according to a literature procedure. 24 Commercially available chemicals were purchased from Sigma-Aldrich, Oakwood Chemicals, Alfa-Aesar, and Strem and were used as received unless otherwise noted. Column chromatography was carried out by using silica gel (230−400 mesh, 60 Å). NMR solvents were purchased from SigmaAldrich. 31P{1H}, 1H, 19F{1H}, 119Sn{1H}, and 13C{1H} NMR were recorded on Bruker Advance (-II,-III) 400 MHz spectrometers. Chemical shifts were referenced to external 85% H3PO4 (31P), C6F6 (19F), SnMe4 (119Sn), and external SiMe4 (1H, 13C) or residual nondeuterated solvent peaks (1H, 13C). Elemental analyses and mass spectrometry measurements were performed in the Department of Chemistry at the University of Calgary. All photophysical experiments were carried out on a Jasco FP-6600 spectrofluorometer equipped with an ISF-513 integrating sphere and a UV−Vis−NIR Cary 5000 spectrophotometer in CHCl3 solutions. Cyclic voltammetry analyses were performed on an Autolab PGSTAT302 instrument, with a polished glassy carbon electrode as the working electrode, a Pt wire as counter electrode, and an Ag wire as reference electrode, using ferrocene/ferrocenium (Fc/Fc+) as the internal standard. If not otherwise noted, cyclic voltammetry experiments were performed in dichloromethane with [NBu4][PF6] (0.1M) as supporting electrolyte. Theoretical calculations have been carried out at the B3LYP/6-31G(d) level and B3LYP/LanL2DZ level through WestGrid and Gaussian.21 Crystal data and details of the data collection are provided in Supporting Information. Diffraction data for 4a, 4b, 4a-Me, 4b-Me, and 4b-Au were collected on an APEXII CCD diffractometer, using Cu Kα radiation (λ = 1.54178 Å). The structures were solved by direct methods (SHELXS, SHELXL-2014/7) and refined anisotropically on F by full-matrix least-squares techniques. The crystal data (CCDC 1587542−1587546) are available from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Synthesis of 2a. To a solution of 2-bromothiazole (5.61 g, 34.2 mmol) in diethyl ether at −78 °C, n-BuLi (14.4 mL, 35.9 mmol) was added dropwise and reacted for 1 h. Chloro(trimethyl)stannane (7.18 g, 35.9 mmol; caution: toxic!) was added in one batch and reacted for 1 h at −78 °C. The reaction was warmed to room temperature overnight. The solvent was removed via rotovap and dissolved in cyclohexane and filtered using vacuum filtration. The solvent was removed to produce a light brown oil; yield: 6.70 g (79%); 1H NMR (400 MHz, CDCl3, δ): 8.14 (d, 1H, J = 4.0 Hz), 7.53 (d, 1H, J = 3.0 Hz), 0.46 (s, 9H) ppm; 119Sn{1H} NMR (149 MHz, CDCl3, δ): −29.5 ppm. The 1H NMR data was consistent with reported literature, and thus, full characterization was not performed. Synthesis of 2b. To a solution of benzothiazole (1.59 g, 11.8 mmol) in diethyl ether (150 mL) at −78 °C, n-BuLi (4.90 mL, 12.4 mmol) was added dropwise and reacted for 1 h. Chloro(trimethyl)stannane (2.48 g, 12.4 mmol; caution: toxic!) was added in one batch and reacted at −78 °C for 1 h. The reaction was warmed to room temperature overnight. The solvent was removed via rotovap and dissolved in cyclohexane and filtered using vacuum filtration. The solvent was removed to produce a light brown oil; yield: 3.62 g (98%); 1 H NMR (400 MHz, CDCl3, δ): 8.18 (d, 1H, J = 8.0 Hz), 7.96 (d, 1H, J = 8.0 Hz), 7.49−7.44 ppm (m, 1H), 7.39−7.35 ppm (m, 1H), 0.61− 0.47 (s, 9H) ppm; 119Sn{1H} NMR (149 MHz, CDCl3, δ): −27.0 ppm. The 1H NMR data was consistent with reported literature, and thus, full characterization was not performed.

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CONCLUSION In conclusion, two series of hypervalent phosphorus compounds, thiazolyl- and benzothiazolyl-stabilized 3-thienylphosphanes were successfully synthesized. Two distinctly different types of LA/LB interactions were achieved and confirmed via single-crystal X-ray crystallography. S → P interactions were observed for the trivalent phosphorus species, whereas Pfunctionalization with oxygen or a methyl substituent resulted in a N → P interaction. By contrast, in the case to the gold complex, the major intramolecular LA/LB interaction involved a S → Au interaction, which could conceptually be correlated with that found for the trivalent species. The solid-state structures provide unequivocal evidence that the LA/LB interaction is part of the 3c−4e hypervalent bond, increasing confidence that the σ-hole bond of LA/LB interactions reported in the literature are in fact closely related to the well-known hypervalent 3c−4e bonds of main group chemistry. The absorption and emission features of these compounds are red-shifted with increasing conjugation of the main scaffold. However, modification of the phosphorus center was seen to have less of an effect on the absorption maxima. The methylated compounds show a small red shift in both the absorption and emission maxima due to the σ*−π* interaction that is seen in the LUMO through theoretical calculations. The electrochemical studies show similar results where the methylated and oxidized compounds both exhibit quasireversible reductions, with the methylated species showing lower reduction thresholds than the oxides, thus confirming that this functionalization provided the strongest electronaccepting properties in this series. Theoretical calculations allowed for a further understanding of the effect of the Lewis acidity of the phosphorus center on the bonding. It is clear from these calculations that the observed LA/LB interaction is controlled by the character of the phosphorus center. Being able to show either soft or hard Lewis acid character, the phosphorus enables a more covalent type hypervalent bonding between the soft acids and bases, while the bonding in the hard acid/base interactions is dominated by L


Article

Inorganic Chemistry Synthesis of 3a. 2-(Trimethylstannyl)thiazole (3.64 g, 14.7 mmol), 3-bromo-2-iodothiophene (4.24 g, 14.7 mmol), tri(2-furyl)phosphine (307 mg, 1.32 mmol), and tris(dibenzylidene-acetone)dipalladium(0) (404 mg, 0.44 mmol) were added to toluene and refluxed for 72 h. The solution was filtered, and the solvent was removed. The resulting oil was purified via column chromatography using 1:3 hexanes/ chloroform resulting in a yellow viscous oil; yield: 1.45 g (40%); 1H NMR (400 MHz, CDCl3, δ): 7.89−7.88 ppm (d, 1H, J = 4.0 Hz), 7.42−7.41 ppm (d, 1H, J = 4.0 Hz), 7.37−7.36 ppm (d,1H, J = 4.0 Hz), 7.11−7.10 ppm (d, 1H, J = 4.0 Hz). The procedure was adapted from literature.5 The 1H NMR data was consistent with reported literature, and thus, full characterization was not performed. Synthesis of 3b. 2-(Trimethylstannyl)benzothiazole (3.62 g, 12.1 mmol), 3-bromo-2-iodothiophene (3.85 g, 13.3 mmol), tri(2-furyl)phosphine (0.25 g, 1.09 mmol), and tris(dibenzylideneacetone)dipalladium(0) (0.33 g, 0.36 mmol) were added to toluene and refluxed for 72 h. The solution was filtered, and the solvent was removed. The resulting oil was run through a short silica plug with chloroform to produce a yellow solid; yield: 1.72 g (48%); 1H NMR (400 MHz, CDCl3, δ): 8.08−8.06 (m, 1H), 7.94−7.91 (m, 1H,), 7.53−7.49 (m, 1H), 7.46 (d, 1H, J = 4.0 MHz), 7.43−7.39 (m, 1H), 7.14 (d, 1H, J = 4 MHz); 13C {1H} NMR (101 MHz, CDCl3, δ): 158.97 (s), 152.12 (s), 134.94 (s), 133.55 (s), 132.25 (s), 128.94 (s), 126.52 (s), 125.37 (s), 122.95 (s), 121.43 (s), 111.68 (s) ppm; HRESI-MS: m/z = 294.9132 ([M + H]+, C11H6BrNS2 Calcd 294.9125); elemental analysis calcd (%) for C11H6BrNS2: C, 44.61; H, 2.04; N, 4.73; found: C, 44.87; H, 2.10; N, 4.78. Synthesis of 4a. To a solution of 3a (495 mg, 2.01 mmol) in diethyl ether at −78 °C, n-BuLi (0.840 mL, 2.11 mmol) was added dropwise and reacted for 1 h. Chloro(diphenyl)phosphane (464 mg, 2.11 mmol) was added dropwise at −78 °C and reacted for 1 h. The reaction was warmed to room temperature overnight. The solution was filtered via cannula, and the solvent was removed. The remaining solid was washed with acetonitrile and dried to produce a light brown solid; yield: 0.71 g (54%); 1H NMR (400 MHz, CDCl3, δ): 7.82 (t, 1H, J = 8.0 Hz), 7.38−7.31 (m, 13H), 6.64−6.62 (dd, 1H, J = 8.0 Hz, 2 JHP = 4.0 Hz) ppm; 31P{1H} NMR (162 MHz, CDCl3, δ): −24.7 ppm; 13C{1H} NMR (101 MHz, CDCl3, δ): 160.26 (s), 143.00 (d, JCP = 30.3 Hz), 142.45 (s), 136.33 (d, JCP = 9.09 Hz), 135.00 (d, JCP = 21.2 Hz), 133.48 (s), 133.41 (d, JCP = 20.2 Hz), 128.96 (s), 128.66 (d, JCP =7.07 Hz), 126.63 (d, JCP = 2.02 Hz), 120.34 (d, JCP = 12.1 Hz); HRESI-MS: m/z = 352.0381 ([M + H]+, C19H15NPS2 Calcd 352.0378); elemental analysis calcd (%) for C19H14NPS2: C, 64.94; H, 4.02; N, 3.99; found: C, 64.72; H, 4.14; N, 3.98. Synthesis of 4b. To a solution of 3b (379 mg, 1.29 mmol) in diethyl ether at −78 °C, n-BuLi (0.810 mL, 1.29 mmol) was added dropwise and reacted for 1 h. Chloro(diphenyl)phosphane (284 mg, 1.29 mmol) was added dropwise at −78 °C and reacted for 1 h. The reaction was warmed to room temperature overnight. The solution was filtered via cannula, and the solvent was removed. The remaining solid was washed with pentane and dried to produce a light brown solid; yield: 0.44 g (84%); 1H NMR (400 MHz, CDCl3, δ): 8.04 (d, 1H, J = 8.0 Hz), 7.85 (d, 1H, J = 8.0 Hz), 7.49−7.45 (m, 1H) 7.43 (d, 1H, J = 8.0 Hz), 7.42−7.34 (m, 11H), 6.69−6.67 (dd, 1H, J = 8.0 Hz, 2 JHP = 4.0 Hz); 31P{1H} NMR (162 MHz, CDCl3, δ): −24.7 ppm; 13 C{1H} NMR (101 MHz, CDCl3, δ): 160.16 (s), 152.42 (s), 142.86 (d, JCP = 30.3 Hz), 137.35 (d, JCP = 23.2 Hz), 136.39 (d, JCP = 10.1 Hz), 136.30 (d, JCP = 15.2 Hz), 133.84 (s), 133.40 (d, JCP = 20.2 Hz), 128.98 (s), 128.68 (d, JCP = 7.1 Hz), 128.14 (d, 2.02 Hz), 126.28 (s), 125.08 (s), 122.94 (s), 121.42 (s); HR-ESI-MS: m/z = 402.0529 ([M + H]+, C23H17NPS2 Calcd 402.0535); elemental analysis calcd (%) for C23H16NPS2: C, 68.81; H, 4.02; N, 3.49; found: C, 67.04; H, 4.06; N, 3.47. Synthesis of 4a-O. To a solution of 4a (110 mg, 0.31 mmol) in chloroform, hydrogen peroxide was added (1 mL, 40% (w/w)). The solution was left to react overnight. The organic layer was washed with water, dried with magnesium sulfate, and the solvent was removed. The resulting solid was sonicated with hexanes and filtered, leaving an orange-yellow solid; yield: 76 mg (67%); 1H NMR (400 MHz, CD3OD, δ): 7.75−7.69 (m, 6H), 7.67−7.62 (m, 2H), 7.57−7.53 (m,

5H), 6.89 (t, 1H, t, J = 8.0 Hz) ppm; 31P{1H} NMR (162 MHz, CDCl3, δ): 22.5 ppm; 13C{1H} NMR (101 MHz, CDCl3, δ): 158.70 (s), 145.87 (d, JCP = 10.1 Hz), 142.55 (s), 133.76 (d, JCP = 16.2 Hz), 133.22 (s), 132.15 (s), 132.02 (d, JCP = 3.0 Hz), 131.79 (d, JCP =10.1 Hz), 131.16 (s), 130.15 (s), 128.51 (d, JCP = 13.1 Hz), 126.70 (d, JCP = 15.2 Hz), 122.23 (s) ppm; HR-ESI-MS: m/z = 368.0334 ([M + H]+, C19H15NOPS2 Calcd 368.0327); elemental analysis calcd (%) for C19H14NOPS2: C, 62.11; H, 3.84; N, 3.81; found: C, 60.55; H, 3.85; N, 3.43. Synthesis of 4b-O. To a solution of 4b (124 mg, 0.31 mmol) in chloroform, hydrogen peroxide (1 mL, 40% (w/w)) was added. The solution was left to react overnight. The organic layer was washed with water, dried with magnesium sulfate, and the solvent was removed. The resultant solid was sonicated with hexanes and filtered, leaving a yellow/brown solid; yield: 94 mg (73%); 1H NMR (400 MHz, CDCl3, δ): 7.92 (d, 1H, J = 8.0 Hz), 7.77−7.70 (m, 5H), 7.49−7.38 (m, 8H), 7.33−7.29 (m, 1H), 6.84 (t, 1H, t, J = 12.0 Hz) ppm; 31P{1H} NMR (162 MHz, CDCl3, δ): 22.7 ppm; 13C{1H} NMR (101 MHz, CDCl3, δ): 158.91 (s), 152.46 (s), 145.30 (d, JCP = 10.1 Hz), 137.10 (s), 134.30 (d, JCP = 16.2 Hz), 132.40 (d, JCP = 18.2 Hz), 132.21 (d, JCP = 11.1 Hz), 131.98 (d, JCP = 3.03 Hz), 131.80 (d, JCP = 10.1 Hz), 128.46 (d, JCP = 12.1 Hz), 127.90 (d, JCP = 16.1 Hz), 126.21 (s), 125.35 (s), 123.14 (s), 121.46 (s); HR-ESI-MS: m/z = 418.0493 ([M + H]+, C23H17NOPS2 Calcd 418.0484); elemental analysis calcd (%) for C23H17NOPS2: C, 66.17; H, 3.86; N, 3.36; found: C, 63.93; H, 3.88; N, 3.38. Synthesis of 4a-Me. To a solution of 4a (166 mg, 0.47 mmol) in dichloromethane (50 mL), methyl trifluoromethanesulfonate (76 mg, 0.46 mmol) was added dropwise at 0 °C. The reaction was left at 0 °C for 2 h and then left to warm to room temperature overnight. The solvent was pumped off, affording light yellow solid flakes; yield: 0.22 g (94%); 1H NMR (400 MHz, CDCl3, δ): 7.72−7.57 (m, 11H), 7.49 (d, 1H, J = 4 Hz), 7.32 (d, 1H, J = 4 Hz), 6.90 (m, 1H, J = 8 Hz), 3.11 (d, 3H, J = 12 Hz); 31P{1H} NMR (162 MHz, CDCl3, δ): 16.1 ppm; 19 1 F{ H} NMR (376 MHz, CDCl3): δ = −78.2 ppm; 13C{1H} NMR (101 MHz, CDCl3, δ): 157.19 (s), 146.70 (d, JCP = 9.09 Hz), 143.76 (s), 134.67 (s), 134.48 (s) 132.30 (d, JCP = 10.1 Hz), 130.15 (d, JCP = 13.1 Hz), 128.73 (d, JCP = 17.7 Hz), 122.16 (s), 121.13 (s), 120.22 (s), 116.25 (s), 115.32 (s), 10.6 (d, JCP = 61.6 Hz) ppm. HR-ESI-MS: m/z = 366.0531 ([M*]+, C20H17NPS2+ Calcd 366.0535); elemental analysis calcd (%) for C21H17F3NO3PS3: C, 48.93; H, 3.32; N, 2.72; found: C, 50.49; H, 3.56; N, 2.74. Synthesis of 4b-Me. To a solution of 4b (125 mg, 0.31 mmol) in dichloromethane (50 mL), methyl trifluoromethanesulfonate (50 mg, 0.305 mmol) was added dropwise at 0 °C. The reaction was left at 0 °C for 2 h and then left to warm to room temperature overnight. The solvent was pumped off, affording light yellow solid flakes; yield: 169 mg (98%); 1H NMR (400 MHz, CDCl3, δ): 7.82−7.73 (m, 6H), 7.66−7.64 (m, 2H), 7.62−7.57 (m, 5H), 7.42−7.39 (m, 2H), 7.00 (m, 1H, J = 8.0 Hz), 3.20 (d, 3H, J = 12.0 Hz) ppm; 31P{1H} NMR (162 MHz, CDCl3, δ): 16.5 ppm; 19F{1H} NMR (376 MHz, CDCl3, δ): −78.2 ppm; 13C{1H} NMR (101 MHz, CDCl3, δ): 156.69 (s), 152.37 (s), 146.43 (d, JCP = 9.09 Hz), 135.21 (s), 135.07 (d, JCP = 3.03 Hz), 134.46 (d, JCP = 3.03 Hz), 132.24 (d, JCP = 10.1 Hz), 130.31 (s), 130.17 (d, JCP = 13.1 Hz), 127.16 (d, JCP = 41.1 Hz), 123.24 (s), 121.76 (s), 121.23 (s), 120.31 (s), 117.89 (s), 116.96 (s), 10.50 (d, JCP = 61.2 Hz); HR-ESI-MS: m/z = 416.0691 ([M*]+, C24H19NPS2+ Calcd 416.0682); elemental analysis calcd (%) for C24H19NPS2+: C, 53.09; H, 3.39; N, 2.48; found: C, 52.53; H, 3.42; N, 2.58. Synthesis for 4a-Au. Chloro(tetrahydrothiophene)gold(I) (87 mg, 0.25 mmol) was added to a solution of 4a (79 mg, 0.25 mmol) in CH2Cl2 (30 mL), and the mixture was stirred for 2 h at room temperature. After evaporation of the solvent, the crude product was obtained as a yellow powder, which was washed with n-pentane and dried under vacuo; yield: 66 mg (45%). Note: the yield is based on NMR spectroscopy, as the compound could ultimately not be purified; 1 H NMR (400 MHz, CDCl3, δ): 7.91 (d, 1H, J = 8.0 Hz), 7.79 (d, 1H, J = 8.0 Hz), 7.71−7.65 (m, 4H), 7.52−7.41 (m, 8H), 7.36−7.32 (m, 1H), 6.54 (t, 1H, J = 12.0 Hz) ppm; 31P{1H} NMR (162 MHz, CDCl3, δ): 20.1 ppm. M


Article

Inorganic Chemistry Synthesis for 4b-Au. Chloro(tetrahydrothiophene)gold(I) (189 mg, 0.59 mmol) was added to a solution of 4b (237 mg, 0.59 mmol) in CH2Cl2 (30 mL), and the mixture was stirred for 2 h at room temperature. After evaporation of the solvent, the crude product was obtained as a yellow powder, which was washed with n-pentane and dried under vacuo; yield: 0.36 g (97%); 1H NMR (400 MHz, CDCl3, δ): 7.91 (d, 1H, J = 8.0 Hz), 7.79 (d, 1H, J = 8.0 Hz), 7.71−7.65 (m, 4H), 7.52−7.41 (m, 8H), 7.36−7.32 (m, 1H), 6.54 (t, 1H, J = 12.0 Hz) ppm; 31P{1H} NMR (162 MHz, CDCl3, δ): 23.2 ppm; 13C{1H} NMR (101 MHz, CDCl3, δ): 157.27 (s), 152.83 (s), 142.60 (d, JCP = 12.1 Hz), 135.21 (s), 133.98 (d, JCP = 15.2 Hz), 131.80 (d, JCP = 3.03 Hz), 130.13 (s), 129.49 (s), 129.12 (d, JCP = 12.1 Hz), 128.45 (s), 127.79 (d, JCP = 13.1 Hz), 126.81 (s), 125.99 (s), 123.67 (s), 121.45 (s) ppm; HR-ESI-MS: m/z = 633.9874 ([M + H]+, C23H16AuClNPS2 Calcd 633.9889); elemental analysis calcd (%) for C23H16AuClNPS2: C, 39.09; H, 2.42; N, 2.40; found: C, 40.46; H, 2.68; N, 1.94.

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Sutherland for helpful discussions and access to his instrumentation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b03008. Frontier orbitals and energies of the new species obtained via DFT calculations; cyclic voltammograms; additional electrostatic potential maps and bond-critical points provided via AIM calculations; crystal data and structure refinement parameters for 4a, 4b, 4a-Me, 4bMe, and 4b-Au; heteronuclear NMR spectra for all new species; Cartesian coordinates for minimized structures obtained via DFT for the new species (PDF) Accession Codes

CCDC 1587542−1587546 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Thomas Baumgartner: 0000-0001-8066-0559 Present Address

† Department of Chemistry, York University, 4700 Keele Street, Toronto, ON, M3J 1P3, Canada

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the Natural Sciences and Engineering Research Council (NSERC) of Canada and the Canada Foundation for Innovation is gratefully acknowledged. The DFT calculations were enabled in part by support provided by WestGrid (www.westgrid.ca) and Compute Canada Calcul Canada (www.computecanada.ca). N.G. thanks the University of Calgary for a Queen Elizabeth II Graduate Scholarship. We thank Dr. W. Bi for the X-ray crystallography and Dr. T. C. N


Article

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Base Interactions in 2-(2

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