Binding Modes and Reactivity of Pyrido[2,1-a]isoindole as a Neutral

Oct 12, 2017 - (9a) The observation of two different methyl peaks for the SMe2 ligand is in agreement with the asymmetric iczH carbon atom after it is...
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Binding Modes and Reactivity of Pyrido[2,1‑a]isoindole as a Neutral Carbon Donor with Main-Group and Transition-Metal Elements Sean M. McDonald, Soren K. Mellerup, Christopher Barran, Xiang Wang, and Suning Wang* Department of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada S Supporting Information *

ABSTRACT: Various binding modes of pyrido[2,1-a]isoindole with main-group and transition-metal elements have been established. The carbon atom at position 6 of pyrido[2,1-a]isoindole is highly nucleophilic, forming a σ complex with Pt(II) ion. The benzene ring of pyrido[2,1a]isoindole forms an η6-π complex with Cr(0). The reaction of pyrido[2,1-a]isoindole with PPh2Cl in the presence of Proton Sponge leads to a PPh2-functionalized product, which can further react with a BH3 molecule, forming a P→B adduct. Pyrido[2,1-a]isoindole was also found to undergo a dehydrogenative C−C coupling reaction in the presence of Cu(I) ions, forming a dimer. These interesting reactivities and binding modes demonstrate the rich chemistry of pyrido[2,1-a]isoindole, as well as its potential application in main-group and transition-metal chemistry.

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INTRODUCTION The investigation of novel ligand types often leads to new and exciting chemistry.1a−e Despite being known since 1966,2a there exist only a handful of studies on pyrido[2,1-a]isoindole (or isocarbazole, iczH).2b,3−5 This is quite surprising, considering the unique electronic structure of iczH (Figure 1). Due to its

Scheme 1. Known Reactivity of iczH with Alkynes and Boranes

structurally related to some of the adducts discussed in this work.3a,4 The unusual reactivity and NHC-like behavior of iczH revealed by these earlier results inspired us to further examine its ability to react with other main-group elements such as phosphorus or transition-metal-based systems. In this work we report the reactivity of iczH with a variety of new substrates, paving the way for future exploration of chemistry based on iczH.

Figure 1. Pyrido[2,1-a]isoindole in two resonance forms and its HOMO orbital diagram.7

additional zwitterionic resonance form, the carbon atom at position 6 of iczH possesses significant electron density and is therefore highly nucleophilic, with reactivities resembling those of N-heterocyclic carbenes. Earlier investigations established that iczH can readily form adducts with boranes (HBR2), which can undergo further 1,1-hydroboration reactions to form BNheterocycles, as depicted in Scheme 1.5b When HB(C6F5)3 is used as the borane source, the adduct can be observed spectroscopically. This reactivity of iczH bears similarity to the ring expansion reactions of NHC−borane adducts reported by several teams.6 In addition, iczH is known to undergo 1,3dipolar cycloaddition with a variety of alkenes and alkynes.3,5a In certain instances, the cycloaddition products can be converted to their respective aromatic heterocycles, while many others (with electron-deficient olefins) could be transformed into their Michael adduct derivatives, which are © XXXX American Chemical Society

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RESULTS AND DISCUSSION Reactivity of Pyrido[2,1-a]isoindole with Transition Metals. The reactivity of iczH with a variety of transition-metal complexes was examined, including Pt(II), Cr(0), Rh(I), Cu(I), and Zn(II). Although NMR data indicated that ZnCl2 does form an adduct with iczH, we failed to isolate the pure compound. Attempts to prepare the adduct Rh(CO)2(iczH)Cl via the reaction of iczH with [Rh(CO)2Cl]2 led to the Received: September 12, 2017

A

DOI: 10.1021/acs.organomet.7b00696 Organometallics XXXX, XXX, XXX−XXX

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Organometallics formation of unidentifiable products, none of which could be assigned as the simple adduct. The reactions of iczH with Pt(II) and Cr(0) led to the isolation of distinct complexes, while the reaction with Cu(I) led to the isolation of the C−Ccoupled dimer (icz)2 (3), as shown in Scheme 2. Scheme 2. Reactivity of iczH with Pt(II), Cu(I), and Cr(0)

Figure 2. (top) Structure of 1 (Pt1 conformer) with 35% thermal ellipsoids and labels for key atoms. Important bond lengths (Å) and angles (deg): Pt(1)−Cl(1) 2.288(2), Pt(1)−Cl(2) 2.310(2), Pt(1)− S(1) 2.326(3), Pt(1)−C(1) 2.077(8), C(1)−C(2) 1.470(12), C(1)− N(1) 1.472(10); Cl(1)−Pt(1)−Cl(2) 175.92(9), C(1)−Pt(1)−S(1) 179.8(3). (bottom) The four conformer structures of 1 viewed down the C−Pt−S axis.

Pt(II) Complex 1. Upon the addition of iczH to a solution of 1 equiv of PtCl2(SMe2)28 in CH2Cl2, a slow color change from yellow to orange took place. The speed of this reaction was greatly increased by heating to 60 °C (the reaction time decreased from 4 to 1 h). Crystals were easily grown by layering hexane on top of CH2Cl2 solutions and allowing slow diffusion to occur. The resulting product was found to be an adduct (1) of iczH with the platinum precursor, displacing one dimethyl sulfide unit. The 1H NMR spectrum of 1 bears resemblance to that of the protonated salt [iczH2][Br] (see the Supporting Information), with a low-field aryl doublet similar to that of pyridine. This is a consistent trait for adducts of iczH with a Lewis acceptor, as the disrupted conjugation causes more pyridine-like character in the heterocyclic ring of iczH. In the 1 H NMR spectrum of 1, the SMe2 ligand displays two distinct peaks (2.03 and 2.17 ppm) with satellites due to coupling with the 195Pt nucleus (3JPt−H = 56 and 54 Hz, respectively), similar to the case for related Pt(II) SMe2 complexes reported previously.9a The observation of two different methyl peaks for the SMe2 ligand is in agreement with the asymmetric iczH carbon atom after it is coordinated to the Pt(II) center. Peculiarly, the signal for the proton bound to the C(1) atom (7.86 ppm; 400 MHz) bears platinum satellites peaks with 2 JPt−H ≈ 180 Hz. While the satellite peak at 8.09 ppm is visible, confirmation of the second satellite peak was inferred through integration, due to overlap with other signals. Due to the limited solubility of 1, more detailed NMR studies could not be explored to further elucidate this coupling, which is considerably higher than the typical range9b for 2JPt−H (60− 80 Hz),9c,d and the cause is not understood. The structure of compound 1 was established unambiguously by single-crystal X-ray diffraction analysis and is shown in Figure 2 (full details are available in the Supporting Information). An unusual feature observed in the crystal data of 1 is that there are four independent molecules of 1 in the asymmetric unit of the crystal lattice. The key difference among the four independent molecules is the relative orientation of the SMe2 group versus that of the iczH group, as shown by their projections down the C−Pt−S axis (Figure 2), which leads to the four distinct conformers. In one of the conformers, Pt4, the S atom of the SMe2 group is disordered. There is uncertainty in the assignment of C(2) and N(1) atoms in Pt1 and the related pairs in other conformers. The final assignment of C and N atoms is based on the best refinement results. Structural details and selected bond lengths and angles for the Pt1 conformer are

shown in Figure 2. The crystal data show that the Pt(II) complex 1 has a trans geometry. The Pt−Cl and Pt−S bond lengths are typical and similar to those reported for related compounds in the literature.9a,d The Pt−C bond length is similar to that of Pt−CH3 bonds but slightly longer than the typical Pt−NHC or Pt−Ph bond lengths reported in the literature,9a perhaps due to the lack of π back-donation to the iczH ligand. Nonetheless, compound 1 is stable at ambient temperatures and for several days in air, which supports the idea that the iczH molecule is capable of acting as a neutral σ donor, forming stable Pt(II) complexes. Despite this seemingly strong interaction, attempts to prepare Pt(iczH)2Cl2 by adding 1 equiv of iczH to complex 1 failed and no reaction was observed, possibly due to a strong σ-donating effect. Cr(0) Complex 2. In order to examine if iczH can function as a π donor and whether a preferential π-binding site for transition-metal atoms exists, a piano-stool complex, (CH3CN)3Cr(CO)3, that contains labile acetonitrile ligands was selected for possible π complexation with iczH. Mixing iczH with (CH3CN)3Cr(CO)3 resulted in a deep red solution, yielding the anticipated π complex (η6-iczH)Cr(CO)3 (2) nearly quantitatively. Compound 2 has a poor stability in coordinating solvents such as acetonitrile and THF, which can displace the coordinated iczH ligand, causing decomposition. In the solid state, compound 2 is fairly stable, allowing manipulation in air for hours. 1H NMR data indicate that the benzene ring of iczH is bound to the Cr(0) atom, as all proton peaks on the benzene ring experience a large upfield shift relative to free iczH (see the Supporting Information). No dynamic migration of the Cr(CO)3 between the pyridine and benzene rings was observed at room temperature. This is likely due to the strong π-donating nature of the phenyl ring vs the pyridyl ring (see DFT data in the Supporting Information). Single crystals suitable for X-ray diffraction analysis were obtained from a concentrated solution of dichloromethane. The crystal structure of 2 shown in Figure 3 confirmed the π-bound structure of iczH to the Cr(0) center (see the Supporting Information for full details). B

DOI: 10.1021/acs.organomet.7b00696 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Figure 3. Crystal structure of compound 2 with 35% thermal ellipsoids and labeling schemes. Bond lengths (Å): Cr(1)−C(1) 1.829(3), Cr(1)−C(2) 1.820(3), Cr(1)−C(3) 1.837(3), Cr(1)−C(4) 2.308(2), Cr(1)−C(5) 2.219(3), Cr(1)−C(6) 2.208(3), Cr(1)−C(7) 2.204(3), Cr(1)−C(8) 2.250(3), Cr(1)−C(9) 2.386(3), C(1)−O(1) 1.168(3), C(2)−O(2) 1.162(3), C(3)−O(3) 1.158(3), C(4)−C(5) 1.438(4), C(5)−C(6) 1.397(4), C(6)−C(7) 1.419(4), C(7)−C(8) 1.378(4), C(8)−C(9) 1.418(4), C(4)−C(9) 1.430(4), C(9)−C(10) 1.395(4), C(4)−C(15) 1.429(4), N(1)−C(10) 1.368(3), N(1)−C(11) 1.389(3), C(11)−C(12) 1.350(4), C(12)−C(13) 1.406(4), C(13)− C(14) 1.369(4), C(14)−C(15) 1.375(4).

Figure 4. Diagram showing the π-stacked dimer of 2 in the crystal lattice and the shortest atomic separation distances.

The C−C bond lengths in the benzene ring bound to the Cr atom are on average longer than those in the pyridyl ring due to the π-electron donation to the Cr(0) center. The Cr−C (C5, C6, C7, and C8 of benzene) bond lengths (2.204(3)−2.250(3) Å) are similar to those reported10 for (η6-C6H6)Cr(CO)3, while Cr−C(4) and Cr−C(9) bonds are significantly longer (2.308(2) and 2.386(3) Å), likely a consequence of the diene-like resonance structure shown in Figure 1 and the lack of π-orbital contribution/low electron density of the C(9) atom in the HOMO of iczH (Figure 1). The Cr−CO bond lengths are typical and similar to those of the related piano stool Cr(0) benzene complex.10 The C−O bond lengths of the carbonyl ligands are normal. The IR values for the carbonyl stretches were found at 1815 and 1925 cm−1, which are lower than those found for (η6-C6H6)Cr(CO)3, which has CO stretches at 1890 and 1970 cm−1.11 This appears to support that iczH is a stronger π donor than benzene. In the crystal lattice, compound 2 forms π-stacked dimers with the shortest atomic separation distances being 3.250(3) and 3.383(3) Å, respectively, as shown in Figure 4. Formation of icz Dimer 3. [Cu(CH3CN)4][PF6] was used in our attempts to prepare a Cu(I) adduct with iczH. Mixing of the two reactants in a THF solution resulted in a swift color change to dark green with the appearance of black precipitates and appearance of copper metal on the walls of the vial. After filtration of the solution from the precipitates (copper), removal of volatiles, and extraction with toluene, a yellow solution with yellow luminescence was obtained. The 1H NMR spectrum showed a set of proton signals with a pattern similar to that of iczH, but lacking the singlet for the proton at position 6 (see the Supporting Information). This product readily crystallizes, forming bright yellow crystals. Single-crystal X-ray diffraction analysis established that it is a dehydrogenatively C− C coupled dimer (3) of iczH, as shown by the crystal structure in Figure 5. In the dimer structure of 3, the C(1)−C(1A) bond length is 1.441(5) Å, somewhat shorter than a typical C−C bond between two aryl rings, perhaps due to some π delocalization

Figure 5. Crystal structure of compound 3 and a packing diagram showing the π-stacking interaction of 3. Important bond lengths (Å): C(1)−C(1A) 1.441(5), C(1)−C(2) 1.389(3), N(1)−C(1) 1.391(3) Å.

between the two icz rings, despite the fairly large dihedral angle between the two icz rings of 55.9(1)°. The presence of extended π delocalization in the dimer 3 is in fact supported by the significant bathochromic shift of both UV−vis and fluorescence spectra of 3, in comparison to that of iczH (see Photophysical Properties). Other bond lengths and angles within the icz ring in 3 are similar to those of iczH.6 In the crystal lattice, there are intermolecular π-stacking interactions between the benzene and py rings of the icz, with the shortest atomic separation distance being 3.56 Å. In addition to [Cu(CH3CN)4][PF6], we also examined the reaction of iczH with [Cu(CH3CN)2(PPh3)2][BF4] in THF, which also produced the same dimer product, 3. When the reaction of iczH with [Cu(CH3CN)4][PF6] was performed in acetonitrile, heating to 75 °C was necessary for the reaction to take place as monitored by 1H NMR in CD3CN, while no reaction was observed with [Cu(CH3CN)2(PPh3)2][BF4] in acetonitrile even with heating. The stabilization effect imposed by the acetonitrile solvent to the Cu(I) reactant is clearly responsible for the low reactivity in this solvent. On the basis of this observation, we suggest that the first step in the reaction pathway toward compound 3 involves binding of iczH to the Cu(I) center, a process that is impeded by excess coordinating acetonitrile. Tracking this reaction in CD3CN further revealed C

DOI: 10.1021/acs.organomet.7b00696 Organometallics XXXX, XXX, XXX−XXX

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unit due to coupling to the 31P nucleus was observed in both 1 H and 13C NMR spectra (see the Supporting Information). The 31P NMR spectrum of 4 has a signal at −37.6 ppm, considerably more upfield than that of triphenylphosphine (∼−6.0 ppm)13 but close to that of tri-o-methoxyphenylphosphine at around −38.5 ppm, indicating a fair amount of π donation in addition to the σ donation from the icz ligand to the phosphorus center. The formation of 4 has some similarity with the formation of 3, in which one iczH molecule acts as a proton acceptor. To improve the yield of 4 and avoid wasting iczH as a sacrificial proton acceptor, we added Proton Sponge, 1,8-bis(dimethylamino)naphthalene, to the reaction of iczH with PPh2Cl, which led to the isolation of compound 4 in about 90% yield (Scheme 4). The crystal structure of 4 was determined by single-crystal Xray diffraction analysis. There are two independent molecules of 4 in the asymmetric unit with similar bond lengths and angles. Only the structure of P1 is shown in Figure 6. The phosphorus

that about 50% of iczH became the protonated salt [iczH2]+, presumably with the PF6− counterion. Once optimized, yields of 3 improved to 72% (based on the assumption that two iczH molecules produce one molecule of 3) though the reaction is wasteful in both ligand and copper, and we opted for a iczH to copper ratio of 1.7:1. Investigating the reactivity with other group 11 metals yielded similar results. For example, the reaction of AgSO3CF3 or Au(THT)Cl (THT = tetrahydrothiophene) with iczH yielded 3 and [iczH2]+ as the products, accompanied by the deposition of silver metal or gold metal. This led us to suggest that the group 11 M(I) ions act as both an anchor for proton transfer from a bound iczH to an unbound iczH and an oxidant for C−C coupling of icz anions to form the dimer. The proposed mechanism is shown in Scheme 3. Oxidative C−C coupling reactions of arenes Scheme 3. Proposed Mechanism for the Formation of Compound 3 in the Presence of Group 11 Metal(I) Ions

facilitated by metals are well-known in the literature but in general require powerful oxidants or harsh reaction conditions.12 The facile formation of 3 using weak oxidants such as Cu(I) can be attributed to the highly nucleophilic/electron donating nature of iczH, which enables it to function not only as a reductant but also as a proton acceptor in the C−C coupling process. Reactivity of Pyrido[2,1-a]isoindole with PPh2Cl. We made various attempts to attach iczH to a main-group element such as Si(IV) and P(III) by reacting iczH with Ph2SiCl2, Me3SiCl, or PPh2Cl. We only succeeded in isolating and identifying the product from the reaction with PPh2Cl, which is described below. When iczH was mixed with 1 equiv of PPh2Cl in diethyl ether, a pale solid precipitate immediately formed along with the formation of a yellow solution. The precipitate was found to be [iczH2][Cl], and interestingly the product in solution was identified to be the adduct of the deprotonated iczH with PPh2+ (4; Scheme 4). Significant splitting of the icz

Figure 6. Crystal structure of compound 4 with 35% thermal ellipsoids and labeling schemes. Important bond lengths (Å): P(1)−C(1) 1.790(2), P(1)−C(13) 1.838(2), P(1)−C(19) 1.833(2), N(1)−C(1) 1.405(2), C(1)−C(2) 1.397(2) Å.

atom has a typical pyramidal geometry with a P(1)−C(1) bond length of 1.790(2) Å between the carbon atom of the icz moiety and the phosphorus center, which is significantly shorter than those of P(1)−C(13) and P(1)−C(19) (1.838(2) and 1.833(2) Å) and that of PPh3 (1.854 Å).14 Natural bond orbital (NBO) analysis indicates that the C1 atom has more s character than expected, supporting the shorter distance between P(1) and C(1) atoms. This is also in agreement with the significantly upfield shifted 31P NMR chemical shift of 4. Much like the case for iczH, compound 4 degrades slowly in the solid state and quickly in solution on exposure to oxygen. Adduct of BH3 with Compound 4. DFT calculation data indicate that the HOMO of 4 is mostly localized on the nucleophilic C(1) atom, whereas HOMO-1 localizes mostly on the phosphorus atom (see the Supporting Information), indicating that both sites are still nucleophilic and perhaps the carbon would react first with Lewis acids. To test the reactivity of 4 with Lewis acids, a simple reaction of 4 with BH3· SMe2 was performed. The resulting mixture has the same color as that of 4 but displays bright blue fluorescence. The isolated product 5 has a 11B NMR signal at −33.6 ppm, which is comparable to that (−37.8 ppm) of BH3·PPh3 (see the Supporting Information).15 The 31P NMR signal (5.8 ppm) of

Scheme 4. Syntheses of 4 and 5

D

DOI: 10.1021/acs.organomet.7b00696 Organometallics XXXX, XXX, XXX−XXX

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Organometallics 5 is shifted more than 40 ppm downfield from that of 4 and close to that of BH3·PPh3 (21.1 ppm).15 These data are consistent with the coordination of the BH3 group to the P center in 5, which was confirmed by the crystal structure of 5 shown in Figure 7. The P(1)−B(1) bond length in 5 is

Figure 8. Absorption (solid line) and emission (dashed line) spectra of iczH and 1−5. The absorption spectra of 1 and 2 were recorded in CH2Cl2 at 0.1 mM, while the absorption and emission spectra of other compounds were recorded in THF at 0.1 mM for iczH, 0.015 mM for compounds 3 and 4, and 0.025 mM for 5.

Table 1. Photophysical Data Figure 7. Crystal structure of compound 5 with 35% thermal ellipsoids and labeling schemes. Important bond lengths (Å) and angles (deg): P(1)−B(1) 1.9309(16), P(1)−C(1) 1.7812(14), P(1)−C(13) 1.8079(14), P(1)−C(19) 1.8145(14), N(1)−C(1) 1.396(2), C(1)− C(2) 1.419(2); C(1)−P(1)−B(1) 116.69(7), C(13)−P(1)−B(1) 110.70(7), C(19)−P(1)−B(1) 111.60(6), C(1)−P(1)−C(13) 104.66(6), C(1)−P(1)−C(19) 106.61(6), C(13)−P(1)−C(19) 105.82(6).

compd

λabs (nm)

ε (103 M−1 cm−1)

λem (nm)

ΦFL (%)

iczH 1 2 3 4 5

368 303 360 382 370 368

31. 9 11.0 8.7 5.8 6.1 3.3

498

14a

544 501 468

8b 9b 16b

a Absolute quantum yield. bDetermined using quinine sulfate as in ref 17.

1.9309(16) Å, in line with the average phosphine−borane adduct length of ∼1.92 Å.16 The P(1)−C(1) (1.781(1) Å), P(1)−C(13), and P(1)−C(19) bonds are shorter than those of 4 due to the reduced electron density on the P atom in 5 that enhances aryl−P bond strengths. The average C−P−B angle in 5 is 113°, while that of the C−P−C angle is 106°, indicating a slightly distorted tetrahedral geometry around the P atom. The fact that the BH3 is bound to the P atom instead of the C atom of icz could be explained by both kinetic and thermodynamic factors. In the kinetic aspect, due to the steric congestion, the P center is likely more accessible than the carbon atom for the boron atom to bind. In the thermodynamic aspect, the P-bound product is likely more stable than the C-bound product due mainly to the strengthened P−C bonds. DFT calculation data in fact support this, as the optimized structure for BH3 adduct formation at the carbon center was found to be ∼15 kcal/mol higher in energy in comparison to that for BH3 adduct formation at the phosphorus atom (5; see the Supporting Information). Photophysical Properties. Compounds 1 and 2 are nonemissive. In comparison to the absorption spectrum of iczH, the low-energy absorption band with well-resolved vibrational features in iczH attributable to the π → π* transition of the fully conjugated iczH molecule has been replaced with a weak and featureless band in the same region (Figure 8 and Table 1), which can be attributed to MLCT transitions of the Pt(II) compound. The disruption of π conjugation in the σ-bound iczH Pt(II) complex 1 is clearly responsible for the distinct absorption spectral change from

iczH to 1. The absorption spectrum of compound 2 has a weak band in the 500−580 nm region, which is likely caused by LCMT transitions involving the iczH ligand and the Cr(0) atom. For the dimer compound 3, both absorption and emission spectra are shifted by about 50 nm toward lower energy, in comparison to that of iczH, which is in agreement with the extended π-conjugated structure of 3. The emission quantum efficiency (ΦFL) of 3 was determined to be 0.08 in THF. The low energy absorption bands of 4 and 5 are considerably weaker than those of iczH, in agreement with the lower oscillator strengths of S0 → Sx transitions based on TDDFT data (see the Supporting Information). Both 4 and 5 are fluorescent with modest Φ FL values (0.09 and 0.16, respectively). As shown in Figure 8, the emission profiles of 4 and 5 are similar to that of iczH, indicating that their origin is likely the same, namely a π → π* transition of the icz unit. The emission energy difference of these three compounds is clearly caused by the different substituent groups on the carbon atom of iczH.

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CONCLUSIONS In summary, the reactivity of pyrido[2,1-a]isoindole has been explored with a variety of different substrates, testing the limits of the type of compounds it can activate. Its use as a neutral ligand for transition metals has been established directly with Pt(II) and Cr(0), forming η1 and η6 complexes, respectively. However, iczH is shown to be redox sensitive and reactions E

DOI: 10.1021/acs.organomet.7b00696 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

NMR (400 MHz, CD2Cl2): δ 8.29 (d, 1H, 3J = 6.0 Hz), 7.98 (d, 1H, 3J = 7.7 Hz), 7.41 (s, 1H), 7.12 (m, 2H), 6.76 (d, 1H, 3J = 6.5 Hz), 6.40 (d, 1H, 3J = 6.8 Hz), 5.69 (t, 1H, 3J = 6.5, 6.0 Hz), 5.39 (t, 1H, 3J = 6.8, 6.0 Hz) ppm. 13C{1H} NMR (100 MHz, CD2Cl2): δ 212.1, 148.8, 140.0, 125.6, 120.6, 118.4, 118.3, 118.0, 101.1, 94.0, 87.6, 87.0, 82.8. Anal. Calcd for C15H9CrNO3: C, 59.41; H, 2.99; N, 4.62. Found: C, 58.61; H, 2.70; N, 4.48. Synthesis of (icz)2 (3). To a THF solution of 307 mg (1.83 mmol) of iczH was added 404 mg (1.08 mmol) of [(CH3CN)4Cu][PF6] at ambient temperature. The solution instantly became dark green. Copper metal began to form a mirror on the glass wall of the vial used for the reaction, and a dark precipitate began to form. The reaction mixture was stirred for 5 h before the solvent was removed in vacuo. The residue was extracted with toluene (15 mL), and the dark solid was removed by filtration. The toluene in the filtrate was removed in vacuo, and the residue was extracted with hexanes (15 mL). After the solvent was removed in vacuo, an orange solid was obtained and found to be pure compound 3 by NMR (116 mg, 72%). Crystals suitable for X-ray crystallographic analysis were obtained by cooling concentrated hexanes solutions to −35 °C. 1H NMR (400 MHz, C6D6): δ 8.18 (d, 1H, 3J = 8.0 Hz), 7.88 (d, 1H, 3J = 8.6 Hz), 7.42 (d, 1H, 3J = 8.3 Hz) 7.30 (dd, 1H, 3J = 8.3, 6.5 Hz), 7.25 (dd, 1H, 3J = 8.0, 6.5 Hz), 7.24 (d, 1H, 3J = 6.8 Hz), 6.57 (dd, 1H, 3J = 8.6, 6.8 Hz), 6.36 (t, 1H, 3J = 6.8 Hz) ppm. 13C{1H} NMR (100 MHz, C6D6): δ 125.5, 124.7, 123.7, 122.2, 121.1, 119.9, 119.6, 118.4, 118.1, 117.9, 115.4, 114.2 ppm. Anal. Calcd for C24H16N2: C, 86.72; H, 4.85; N, 8.43. Found: C, 86.66; H, 4.73; N, 8.42. Synthesis of (icz)PPh2 (4). A 153 mg portion (0.92 mmol) of iczH was loaded into a Schlenk flask along with 176 mg (0.82 mmol) of Proton Sponge (i.e., 1,8-bis(dimethylamino)naphthalene). The mixture was dissolved in 100 mL of diethyl ether. A 0.18 mL portion (0.97 mmol) of PPh2Cl was added dropwise to this solution, and a white precipitate was observed immediately. The reaction mixture was stirred for 3 h at ambient temperature. After filtration, the filtrate was evaporated to dryness, giving 292 mg (91%) of 4 as a yellow solid which was found to be pure by NMR. X-ray-quality crystals were obtained by cooling concentrated hexanes solutions to −35 °C. 1H NMR (400 MHz, C6D6): δ 8.62 (dd, 1H, 3J = 7.1 3JH−P = 3.5 Hz), 8.01 (d, 1H, 3J = 8.1 Hz), 7.80 (d, 1H, 3J = 8.1 Hz), 7.66 (d, 1H, 3J = 8.1), 7.43 (m, 4H), 7.25 (dd, 1H, 3J = 8.1, 7.1 Hz), 7.14 (dd, 1H, 3J = 8.1, 7.1 Hz), 6.98 (m, 6H), 6.49 (dd, 1H, 3J = 8.1, 7.1 Hz), 6.30 (t, 1H, 3J = 7.1 Hz) ppm. 13C{1H} NMR (100 MHz, C6D6): δ 135.4 (d, 2JCP = 5.9 Hz), 132.3 (d, 2JCP = 18.3 Hz), ∼131.2 (via HMBC), 128.7 (d, 3JCP = 6.6 Hz), ∼128.1 (via HSQC), 126.0, 124.3, 119.8, 119.6, 118.9, 118.1, 116.7, 115.1, ∼103.4 (via HMBC) ppm. 31P{1H} NMR (161 MHz, C6D6): δ −37.6 ppm. HRMS (EI): calcd for C24H19NP [M + 1]+ 352.1255, found 352.1259. Synthesis of (icz)PPh2(BH3) (5). A 48 mg portion (0.14 mmol) of 4 was loaded into a Schlenk flask and dissolved in 50 mL of THF. To the solution was added 0.15 mL (0.15 mmol) of a 1.0 M solution of BH3·SMe2 dropwise. The fluorescence of the solution changed from green to blue. The reaction mixture was stirred for 3 h before the volatiles were removed and the crude mixture was brought into a glovebox. The contents were redissolved in THF and transferred to a vial. Volatiles were again removed in vacuo to give 48 mg (95%) of 5 as a yellow solid, which was found to be pure by NMR. X-ray-quality crystals were grown by cooling a concentrated THF solution to −35 °C. 1H NMR (400 MHz, C6D6): δ 9.28 (d, 1H, 3J = 7.1 Hz), 7.90 (d, 1H, 3J = 7.6), 7.68 (m, 4H), 7.54 (d, 1H, 3J = 8.6 Hz), 7.09−6.98 (m, 5H), 6.93 (m, 4H), 6.48 (dd, 1H, 3J = 8.6, 6.8 Hz), 6.31 (dd, 1H, 3J = 7.1, 6.8 Hz), 3.01−2.17 (m, 3H) ppm.13C{1H} NMR (100 MHz, C6D6): δ 135.4, 133.2 (d, 2JCP = 10.2 Hz), 132.1 (d, 1JCP = 2.2 Hz), 131.2 (d, 5JCP = 2.9 Hz), 130.4, 129.8, 129.2 (d, 3JCP = 10.2 Hz), 127.0, 120.1, 120.0, 119.8, 119.8, 119.0, 118.3, 118.0, 116.1 ppm. 31P{1H} NMR (161 MHz, C6D6): δ 5.8 ppm. 11B{1H} NMR (128 MHz, C6D6): δ −34.1 ppm. HRMS (EI): calcd for C24H22BNP [M + 1]+ 366.1583, found 366.1593.

with Cu(I), Ag(I), and Au(I) all led to the same dimer of iczH, 3. A new reaction pathway was discovered during the investigation of the reactivity of ClPPh2, where the loss of “HCl” from the adduct resulted in a new type of fluorescent phosphine which was then used to bind BH3. This new reaction pathway opens the door to a variety of systems and potential ligands that could be used to support active catalysts or rare molecular species.

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EXPERIMENTAL SECTION

General Comments. All experiments were performed under a nitrogen atmosphere unless otherwise noted, where solvents were purified using appropriate drying agents and freshly distilled prior to use. The 1H, 13C, 11B 2D-COSY, 2D-NOESY, 2D-HSQC, and 2DHMBC NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer. Excitation and emission spectra were recorded using a Photon Technologies International QuantaMaster Model 2 spectrometer, while UV/vis spectra were recorded using a Varian Cary 50 UV/ vis absorbance spectrophotometer. Solution quantum yields for 3−5 were calculated using optically dilute solutions (A ≈ 0.1) relative to quinine sulfate.17 The quantum yield iczH was obtained directly on a Hamamatsu Quantaurus-QY spectrometer. High-resolution mass spectra (HRMS) were obtained using a Micromass GCT TOF-EI mass spectrometer. IR spectra were obtained on a Bruker Alpha Diamond ATR IR spectrometer. Elemental analyses were performed at the elemental analysis laboratory, University of Montreal, Montreal, Quebec, Canada. Pyrido[2,1-a]isoindole,3a PtCl2(SMe2)2,8 and (CH3CN)3Cr(CO)318 were prepared according to literature procedures, while [Cu(CH3CN)4][PF6] was purchased from Sigma-Aldrich. The purity of all new compounds has been established by NMR following recrystallization in a N2-filled glovebox. For compounds 1−3 their purity was also confirmed by elemental analysis. DFT/TD-DFT Computational Work. All calculations were performed using the Gaussian 09 suite of programs19 on the High Performance Computing Virtual Laboratory (HPCVL) at Queen’s University. Initial input coordinates were taken from crystal structure data where applicable, while others were generated using appropriate optimized geometries of related molecules. Ground state geometry optimizations and TD-DFT vertical excitations were computed using the cam-B3LYP20 level of theory and 6-31(d)21 Pople-style basis set for all atoms. X-ray Crystallographic Analysis. The crystal data of compounds 1−5 were collected on a Bruker D8-Venture diffractometer with Mo target at 180 K. Data were processed on a PC with the aid of the Bruker SHELXTL software package and corrected for absorption effects.22 Complete crystal structural data have been deposited at the Cambridge Crystallographic Data Centre (CCDC No. 1574103− 1574107 for compounds 1−5, respectively). These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Synthesis of Pt(η1-iczH)(SMe2)Cl2 (1). A 3 mL portion of a CH2Cl2 solution containing 101 mg (0.60 mmol) of iczH was added to 7 mL of a CH2Cl2 solution containing 211 mg (0.54 mmol) of PtCl2(SMe2)2, and the mixture was stirred for 4 h at room temperature. Hexanes were layered over the solution and allowed to defuse slowly overnight. Dark orange crystals of 1 formed and were collected by filtration (233 g, 78% yield). 1H NMR (400 MHz, CD2Cl2): δ 9.40 (d, 1H, 3J = 6.4 Hz), 8.17 (m, 2H), 8.01 (d, 1H, 3J = 7.5 Hz), 7.94 (d, 1H, 3J = 7.7), 7.86 (s, 1H, 2JH−Pt = 194.5 Hz), 7.69 (t, 1H, 3J = 7.7, 7.2 Hz), 7.61 (t, 1H, 3J = 7.3, 6.4 Hz), 7.51 (t, 1H, 3J = 7.5, 7.2 Hz) 2.17 (s, 3H, 3JH−Pt = 56 Hz), 2.03 (s, 3H, 3JH−Pt = 54 Hz) ppm. Anal. Calcd for C14H15Cl2NPtS: C, 33.95; H, 3.05; N, 2.83; S, 6.47. Found: C, 33.91; H, 2.87; N, 2.71; S, 6.33. Synthesis of (η6-iczH)Cr(CO)3 (2). To a THF solution containing 49 mg (0.29 mmol) of iczH was added 76 mg (0.29 mmol) of (CH3CN)3Cr(CO)3, and the mixture was stirred for 4 h at room temperature. The volatiles were removed, yielding 87 mg (97%) of NMR-pure 2 as a red solid. X-ray-quality crystals were obtained by cooling a concentrated solution of 2 in diethyl ether to −35 °C. 1H F

DOI: 10.1021/acs.organomet.7b00696 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

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(7) The mesomeric zwitterionic carbene form of iczH bears some resemblance to isoelectronic pyridyl annulated imidazolium salts: (a) Weiss, R.; Reichel, S. Eur. J. Inorg. Chem. 2000, 2000, 1935. (b) Weiss, R.; Reichel, S.; Handke, M.; Hampel, F. Angew. Chem., Int. Ed. 1998, 37, 344. (c) Nonnenmacher, M.; Kunz, D.; Rominger, F.; Oeser, T. Chem. Commun. 2006, 1378. (d) Gierz, V.; Melomedov, J.; Förster, C.; Deißler, C.; Rominger, F.; Kunz, D.; Heinze, K. Chem. Eur. J. 2012, 18, 10677. (8) Prignano, A. L.; Trogler, W. C. J. Am. Chem. Soc. 1987, 109, 3586. (9) (a) Song, D.; Wang, S. Organometallics 2003, 22, 2187. (b) Priqueler, J. R. L.; Butler, I. S.; Rochon, F. D. Appl. Spectrosc. Rev. 2006, 41, 185. (c) Crosby, S.; Clarkson, G. J.; Rourke, J. P. J. Am. Chem. Soc. 2009, 131, 14142. (d) Al-Najjar, I. M.; Abdulrahman, A. M.; Al-Refai, J. K.; Al-Shabanah, L. A.; Al-Mutabagani, L. A. Transition Met. Chem. 2002, 27, 799. (10) Wang, Y.; Angermund, K.; Goddard, R.; Kruger, C. J. Am. Chem. Soc. 1987, 109, 587. (11) Camire Ohrenberg, N.; Paradee, L. M.; DeWitte, R. J.; Chong, D.; Geiger, W. E. Organometallics 2010, 29, 3179. (12) (a) Li, Z.; Li, C. J. J. Am. Chem. Soc. 2004, 126, 11810. (b) Li, Z.; Li, C. J. J. Am. Chem. Soc. 2005, 127, 3672. (c) Li, Z.; Li, C. J. Eur. J. Org. Chem. 2005, 2005, 3173. (d) Li, Z.; Bohle, D. S.; Li, C. J. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 8928. (13) Grim, S. O.; Yankowsky, A. W. J. Org. Chem. 1977, 42, 1236. (14) Howells, E. R. Acta Crystallogr. 1950, 3, 317. (15) Schirmer, M.-L.; Jopp, S.; Holz, J.; Spannenberg, A.; Werner, T. Adv. Synth. Catal. 2016, 358, 26. (16) (a) Bayer, M. J.; Pritzkow, H.; Siebert, W. Eur. J. Inorg. Chem. 2002, 2002, 2069. (b) Jacobsen, H.; Berke, H.; Döring, S.; Kehr, G.; Erker, G.; Fröhlich, R.; Meyer, O. Organometallics 1999, 18, 1724. (c) Spies, P.; Fröhlich, R.; Kehr, G.; Erker, G.; Grimme, S. Chem. - Eur. J. 2008, 14, 333. (17) Brouwer, A. M. Pure Appl. Chem. 2011, 83, 2213. (18) Merlic, C. A.; Walsh, J. C. J. Org. Chem. 2001, 66, 2265. (19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A. H.; Nakatsuji, M.; Caricato, X.; Li, H. P. F.; Hratchian, A.; Izmaylov, J.; Bloino, G.; Zheng, J. L.; Sonnenberg, M.; Hada, M.; Ehara, K.; Toyota, R.; Fukuda, J.; Hasegawa, M.; Ishida, T.; Nakajima, Y.; Honda, O.; Kitao, H.; Nakai, T.; Vreven, J. A.; Montgomery, J. E., Jr.; Peralta, F.; Ogliaro, M.; Bearpark, J. J.; Heyd, E.; Brothers, K. N.; Kudin, V. N.; Staroverov, T.; Keith, R.; Kobayashi, J.; Normand, K.; Raghavachari, A.; Rendell, J. C.; Burant, S. S.; Iyengar, J.; Tomasi, M.; Cossi, N.; Rega, J. M.; Millam, M.; Klene, J. E.; Knox, J. B.; Cross, V.; Bakken, C.; Adamo, J.; Jaramillo, R.; Gomperts, R. E.; Stratmann, O.; Yazyev, A. J.; Austin, R.; Cammi, C.; Pomelli, J. W.; Ochterski, R. L.; Martin, K.; Morokuma, V. G.; Zakrzewski, G. A.; Voth, P.; Salvador, J. J.; Dannenberg, S.; Dapprich, A. D.; Daniels, O.; Farkas, J. B.; Foresman, J. V.; Ortiz, J.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision C.01; Gaussian Inc., Wallingford, CT, 2010. (20) Yanai, T.; Tew, D.; Handy, N. Chem. Phys. Lett. 2004, 393, 51. (21) (a) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54, 724. (b) McLean, A. D.; Chandler, G. S. J. Chem. Phys. 1980, 72, 5639. (22) SHELXTL-2014/7; Bruker AXS, Madison. WI, 2000−2003.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00696. NMR characterization data, X-ray crystal structures, TDDFT calculations, and additional UV−vis/fluorescence data (PDF) Cartesian coordinates for calculated structures (XYZ) Accession Codes

CCDC 1574103−1574107 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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AUTHOR INFORMATION

Corresponding Author

*E-mail for S.W.: [email protected]. ORCID

Suning Wang: 0000-0003-0889-251X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Natural Science and Engineering Research Council of Canada (RGPIN1193993-2013) for financial support. S.M.M. thanks the NSERC for a PGS-D Scholarship. S.K.M. thanks the Canadian Government for a Vanier CGS Scholarship.

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REFERENCES

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DOI: 10.1021/acs.organomet.7b00696 Organometallics XXXX, XXX, XXX−XXX