Bis(N-heterocyclic olefin) Derivative: An Efficient Precursor for

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Bis(N-heterocyclic olefin) Derivative: An Efficient Precursor for Isophosphindolylium Species Che Chang Chong,†,‡ Bin Rao,† Rakesh Ganguly,§ Yongxin Li,§ and Rei Kinjo*,† †

Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, and §NTU-CBC Crystallography Facility, Nanyang Technological University (NTU), Singapore 637371, Singapore S Supporting Information *

ABSTRACT: We have developed bis(N-heterocyclic olefin) derivatives 2 and demonstrated that 2 can be utilized as precursors for the synthesis of isophosphindolylium species 3. X-ray diffraction and density functional theory studies indicate the aromatic property of the PC4 five-membered ring in 3. Despite its cationic nature, the P center in 3b exhibits nucleophilic character and thus readily forms a bond with CuCl to afford a copper phosphenium complex 4, demonstrating the potential utility of 3 as a σ-donor ligand.

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INTRODUCTION N-Heterocyclic olefins (NHOs) I have received a surge in interest in recent years because the nucleophilic character induced by the polar exocyclic CC bond (Figure 1a) has been widely utilized in various applications ranging from the stabilization of reactive main-group species to organocatalysis.1−19 It is salient to mention that, by replacing the H atom at the terminal C atom in NHO with a functional group, the electronic property and thus the electron-donating nature of NHOs are finely tunable.20,21 Beller and co-workers reported Pd-catalyzed hydroxylation and amination reactions of aryl halides, in which 2-phosphanylmethylimidazolium salts II are employed as the ligand precursors (Figure 1b).22,23 Very recently, Rivard and co-workers have developed efficient synthesis of NHO-appended phosphines III and amines IV and revealed their unique coordination behavior with Lewis acids.24,25 The deoxy Breslow intermediates V, key species in a myriad of N-heterocyclic carbene-catalyzed conjugated addition reactions,26−36 which can also be classified as fuctionalized NHOs, were independently synthesized by von Wangelin, Mayr, and co-workers.37−39 Although a couple of metal complexes supported by multidentate ligands involving a single NHO moiety such as VI−VIII have also been reported (Figure 1c),40−42 isolable compounds featuring multi-NHO units remain relatively uncommon. Recently, Rivard and co-workers have synthesized a divinylgermylene, IX, featuring two NHO units.43,44 We envisaged that the incorporation of two NHO units at the ortho position of a phenyl ring would lead to a bis(NHO) derivative, Xa, which could exhibit a bidentate character, Xb (Figure 1e). The chelating property of X would be effective for the installation and stabilization of main-group or transition-metal elements (Figure 1d, XI). It is salient to mention that the bis(phosphino ylide), an analogue of bis(NHO) derivatives X, has been shown to stabilize various main-group species (Figure 1e). As a representative compound among these main-group derivatives, the cationic bis(phosphoniophospholide) derivative © 2017 American Chemical Society

XII was elegantly developed by Schmidpeter and co-workers (Figure 1e).45−49 Significantly, it has been demonstrated that the resonance structure for the phosphoniophospholide XII can best be described as shown in XIIb, which features a 6πaromatic phospholide moiety. 50 The same group also synthesized the stannaindene XIII,51 and later its silicon analogue XIV was reported by Driess et al.52 These pioneering studies prompted us to develop a bis(NHO) derivative, IX, and investigate its coordination chemistry with main-group elements. Herein we report the synthesis of bis(NHO) derivatives and its utility as a precursor to develop isophosphindolylium and the reactivity of the latter as a two-electron donor ligand.

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EXPERIMENTAL SECTION

Materials and Methods. All reactions were performed under an atmosphere of argon by using standard Schlenk or drybox techniques; solvents were dried over sodium metal, potassium metal, or CaH2. Reagents were of analytical grade, were obtained from commercial suppliers, and were used without further purification. 1H, 13C, and 31P NMR spectra were obtained with a Bruker AV 300, AV 400, AV 500, or AVIII 400 MHz BBFO1 spectrometer at 298 K unless otherwise stated. The solvent residual of C6D6 or CDCl3 was used as a reference for both 1H and 13C NMR spectra. The 31P NMR spectra were calibrated using 85% aqueous H3PO4 as the reference. NMR multiplicities are abbreviated as follows: s = singlet, d = doublet, t = triplet, sept = septet, m = multiplet, br = broad signal. Coupling constants J are given in hertz. Electrospray ionization mass spectra were obtained at the Mass Spectrometry Laboratory at the Division of Chemistry and Biological Chemistry, NTU. Melting points were measured with OptiMelt (Stanford Research Systems). La and Lb were prepared according to literature procedures.53,54 Elemental analyses were carried out at NTU and National University of Singapore. Synthesis of 2. La or Lb (10 mmol) and α,α′-dibromo-o-xylene (1; 0.65 g, 2.5 mmol) were dissolved in tetrahydrofuran (THF; 50 mL) Special Issue: Advances in Main-Group Inorganic Chemistry Received: December 8, 2016 Published: January 30, 2017 8608

DOI: 10.1021/acs.inorgchem.6b02991 Inorg. Chem. 2017, 56, 8608−8614

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Inorganic Chemistry

Figure 1. (a) Canonical forms of NHO I, (b) functionalized NHO derivatives II−V, (c) multidentate ligands involving a NHO VI−VIII, and a germylene supported by two NHO IX, (d) canonical forms of bis(NHO) X and its element adduct XI, and (e) main-group species supported by bis(phosphino ylide) XII−XIV. was then stirred at 40 °C for 36 h. Toluene was then removed in vacuo, and the solid residue was dissolved in dichloromethane (DCM). The DCM solution was then washed twice with water (2 × 20 mL). The organic extract was then dried using MgSO4 and removed under vacuum to yield the corresponding 3. Crystals of compound 3a were obtained by the slow evaporation of a saturated ethanol (EtOH) solution of 3a. 3a (3.44 g, 91%): yellow solid. Mp: 288 °C. 1H NMR (500 MHz, CDCl3): δ 7.54 (s, 4H, CH), 7.28 (t, 3J = 8.0 Hz, 4H, ArH), 6.99 (d, 3 J = 8.0 Hz, 8H, ArH), 6.73 (m, 2H, ArH), 6.58 (m, 2H, ArH), 2.59 (sept, 3J = 6.5 Hz, 8H, CHMe2), 1.10 (d, 3J = 6.5 Hz, 24H, CH(CH3)2), 0.64 (d, 3J = 6.5 Hz, 24H, CH(CH3)2). 13C NMR (125 MHz, CDCl3): δ 152.4 (d, 2JP−C = 25.0 Hz, NCN), 145.2 (ArCN), 137.9 (ArCC), 131.6 (ArCC), 130.9 (ArCH), 124.7 (ArCH), 124.4 (ArCH), 119.3 (ArCH), 117.1 (CH), 115.3 (d, 1JP−C = 38.0 Hz, ylide CP), 29.2 (CH), 26.0 (CH), 22.5 (CH3). 31P NMR (200 MHz, CDCl3): δ 184.2. HRMS. Calcd for C62H76N4P: m/z 907.5808 ([M]+). Found: m/z 907.5789. Elem anal. Calcd for C62H76N4PCl: C, 78.91; H, 8.12; N, 5.94. Found: C, 78.95; H, 7.95; N, 5.98. 3b (1.03 g, 45%): pale-orange solid. Mp: 252 °C. 1H NMR (400 MHz, CDCl3): δ 8.00 (dd, 3J = 6.3 Hz, J = 3.2 Hz, 4H, ArH), 7.66 (dd, 3 J = 6.3 Hz, J = 3.2 Hz, 4H, ArH), 7.20−7.13 (m, 4H, ArH), 4.95 (sept, 3J = 7.0 Hz, 8H, CHMe2), 1.79 (d, 3J = 7.0 Hz, 12H, CH(CH3)2), 1.68 (d, 3J = 7.0 Hz, 12H, CH(CH3)2). 13C NMR (100 MHz, CDCl3): δ 152.8 (d, 2JP−C = 24.1 Hz, NCN), 139.0 (ArCN), 130.56 (ArCC), 126.0 (ArCH), 121.0 (ArCH), 117.6 (ArCH), 116.5(d, 1JP−C = 37.2 Hz, ylideC), 114.8 (ArCH), 52.2 (CH), 21.3 (CH3). 31P NMR (160 MHz, CDCl3): δ 170.1. HRMS. Calcd for C34H40N4P: m/z 535.2991 ([M]+). Found: m/z 535.3015. Satisfactory data of elemental analysis could not be obtained despite several attempts, presumably because of rapid decomposition. Synthesis of 4. CuCl (35 mg, 0.35 mmol) and 3b (0.200 g, 0.35 mmol) were dissolved in DCM (20 mL), and the reaction was stirred overnight. The solvent was then removed to yield compound 4 as a yellow solid (0.19 g, 81%). Crystals of compound 4 were obtained by the slow evaporation of a saturated acetonitrile solution of 4. Mp: 255 °C (dec). 1H NMR (400 MHz, CDCl3): δ 7.92 (br, 4H, ArH), 7.62

and stirred for 36 h at room temperature. The THF solvent was removed in vacuo, and toluene (80 mL) was added to the resulting residue. After the corresponding imidazolium salt was filtered away, the filtrate was then removed in vacuo to afford nearly pure compound 2. Further purification for 2 can be carried out by washing the residue with a minimum amount of hexane (20 mL). 2a (1.76 g, 80%): yellow solid. Mp: 250 °C (dec). 1H NMR (300 MHz, C6D6): δ 7.32 (t, 3J = 8.0 Hz, 2H, ArH), 7.19 (d, 3J = 8.0 Hz, 4H, ArH), 7.00 (t, 3J = 8.0 Hz, 2H, ArH), 6.85 (d, 3J = 8.0 Hz, 4H, ArH), 6.35 (dd, 3J = 8.0 Hz, J = 3.6 Hz, 2H, ArH), 5.95 (d, 3J = 4.0 Hz, 2H, CH), 5.90 (dd, 3J = 8.0 Hz, J = 3.6 Hz, 2H, ArH), 5.82 (d, 3J = 4.0 Hz, 2H, CH), 3.97 (s, 2H, ylideCH), 3.39 (sept, 3J = 8.0 Hz, 4H, CHMe2), 3.29 (sept, 3J = 8.0 Hz, 4H, CHMe2), 1.24 (d, 3J = 8.0 Hz, 12H, CH(CH3)2), 1.12−1.07 (m, 36H, CH(CH3)2). 13C NMR (75 MHz, C6D6): δ 148.8 (ArCN), 145.9 (NCN), 145.6 (ArCN), 137.3 (ArCC), 135.6 (ArCC), 131.4 (ArCC), 129.3 (ArCH), 128.4 (ArCH), 125.9 (ArCH), 124.9 (ArCH), 123.9 (ArCH), 118.4 (ArCH), 117.7 (CH), 115.6 (CH), 69.2 (ylideCH), 28.8 (CH), 28.3 (CH), 25.7 (CH3), 24.4 (CH3), 23.8 (CH3), 22.4 (CH3). HRMS. Calcd for C62H79N4: m/ z 879.6305 ([M + H]+). Found: m/z 879.6376. Elem anal. Calcd for C62H78N4: C, 84.96; H, 8.94; N, 6.37. Found: C, 84.55; H, 8.91; N, 6.13. 2b (0.56 g, 44%): pale-orange solid. Mp: 147 °C. 1H NMR (400 MHz, C6D6): δ 7.64 (dd, 3J = 5.7 Hz, J = 3.5 Hz, 2H, ArH), 7.09 (dd, 3 J = 5.7 Hz, J = 3.5 Hz, 2H, ArH), 6.91−6.79 (m, 8H, ArH), 4.91 (s, 2H, ylideCH), 4.54 (sept, 3J = 8.0 Hz, 2H, CHMe2), 4.25 (sept, 3J = 8.0 Hz, 2H, CHMe2), 1.23 (d, 3J = 8.0 Hz, 12H, CH(CH3)2), 1.16 (d, 3J = 8.0 Hz, 12H, CH(CH3)2). 13C NMR (100 MHz, C6D6): δ 148.4 (NCN), 138.4 (ArCC), 135.9 (ArCC), 135.5 (ArCC), 126.8 (ArCH), 122.4 (ArCH), 120.4 (ArCH), 119.1 (ArCH), 110.1 (ArCH), 107.4 (ArCH), 71.5 (ylideCH), 50.4 (CH), 47.1 (CH), 19.6 (CH3), 19.2 (CH3). HRMS. Calcd for C34H43N4: m/z 507.3488 ([M + H]+). Found: m/z 507.3504. Elem anal. Calcd for C34H42N4: C, 80.59, H, 8.35, N, 10.98. Found: C, 80.33; H, 8.69; N, 11.05. Synthesis of 3. NEt3 (3.3 mL, 24 mmol) was added to a solution of 2 (4 mmol) in 30 mL of toluene. After which, PCl3 (0.4 mL, 4.6 mmol) was added to the mixture at room temperature. The reaction 8609

DOI: 10.1021/acs.inorgchem.6b02991 Inorg. Chem. 2017, 56, 8608−8614

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Inorganic Chemistry (br, 4H, ArH), 7.07 (m, 4H, ArH), 4.86 (sept, 3J = 7.0 Hz, 8H, CHMe2), 1.69 (m, 3J = 7.0 Hz, 12H, CH(CH3)2). 13C NMR (100 MHz, CDCl3): δ 151.7 (d, 2JP−C = 22.0 Hz, NCN), 138.2 (ArCN), 130.7 (ArCC), 125.8 (ArCH), 121.0 (ArCH), 117.8 (ArCH), 115.0 (ArCH), 110.5 (br, ylideCP), 52.4 (CH), 21.6 (CH3). 31P NMR (160 MHz, CDCl3): δ 144.2. HRMS. Calcd for C34H40N4P: m/z 535.2991 ([M]+). Found: m/z 535.2991. Elem anal. Calcd for C34H40N4PCuCl2: C, 60.94; H, 6.02; N, 8.36. Found: C, 60.46; H, 5.94; N, 8.13. X-ray Crystallography, X-ray Data Collection, and Structural Refinement. Intensity data for compounds 3a and 4 were collected using a Bruker APEX II diffractometer. The crystal of 3a was measured at 103(2) K, while that of 4 was measured at 153(2) K. These structures were solved by direct-phase determination (SHELXS-2014) and refined for all data by full-matrix least-squares methods on F2.55,56 A summary of the data collection and structural refinement for all of the crystals is found in the Supporting Information (Table S1). All non-H atoms were subjected to anisotropic refinement. H atoms were generated geometrically and allowed to ride on their respective parent atoms; they were assigned appropriate isotropic thermal parameters and included in the structure-factor calculations. CCDC 1481006− 1481007 contain the supplementary crystallographic data for this paper. The data can be obtained free of charge from the Cambridge Crystallography Data Centre via www.ccdc.cam.ac.uk/data_request/cif

represented by several resonance forms including a phosphenium cation 3i, phosphaalkenes 3ii and 3iii, the phospholide anion 3iv, a bis(carbone) ligand,58−60 and phosphorus(I+) cation adducts 3v and 3vi (Scheme 2). Scheme 2. Selected Resonance Forms of the Cationic Fragment in 3i−3vi

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RESULTS AND DISCUSSION At ambient temperature, the treatment of 1 and 4 equiv of Nheterocyclic carbenes L (La or Lb) in THF afforded a yellow solution. After workup, the bis(NHO) derivatives 2a and 2b were obtained as yellow solids in 80% and 44% yield, respectively (Scheme 1). In the 1H NMR spectra, characteristic

The solid-state molecular structure of 3a was unanimously determined by single-crystal X-ray diffraction analysis, which confirms no interaction between the cationic fragment and chloride (Figure 2). The P atom is two-coordinate, and the PC4

Scheme 1. Syntheses of 2 and 3 (Ar = 2,6Diisopropylphenyl)

Figure 2. Crystal structure of 3a with thermal ellipsoids set at the 30% probability level. Selected bond lengths (Å) and angles (deg): C1−C2 1.443(3), C2−C3 1.427(3), C3−C4 1.429(3), C4−C5 1.428(3), C5− C6 1.449(3), C2−P1 1.739(2), C5−P1 1.737(2); N1−C1−N2 105.7(2), C3−C2−P1 112.41(17), C2−P1−C5 91.51(11), P1−C5− C4 112.40(17), N3−C6−N4 106.0(2). H atoms and solvent (EtOH) molecules are omitted for clarity.

signals were observed at 3.97 ppm (2a) and 4.91 ppm (2b) corresponding to the respective ylideCH proton. However, attempts to obtain single crystals for 2 from various solvents were unsuccessful, which impeded structural characterization. Therefore, to test the coordination property of 2, the installation of a P atom was examined by the reaction of 2 and a stoichiometric amount of phosphorus trichloride in toluene in the presence of excess triethylamine (Scheme 1). The reaction mixture was stirred for 36 h at 40 °C, and after purification, the isophosphindolylium chloride derivatives 3 were obtained in 91% (3a) and 45% (3b) yield.57 The 31P NMR spectra of 3 display singlets at 184.2 ppm for 3a and 170.1 ppm for 3b, which are shifted upfield in comparison to that (242 ppm) of bis(phosphoniophospholide) XII (X = Br), as reported by Schmidpeter and Thiele.49 Compound 3 can be

five-membered ring is nearly planar. The P1−C2 [1.739(2) Å] and P1−C5 [1.737(2) Å] distances are shorter than typical P− C single bonds (1.85 Å) but longer compared with PC double bonds (1.61−1.71 Å) in the reported phosphaalkenes.61 Importantly, the endocyclic P−C bonds in 3a are slightly longer than the corresponding P−C bond distances [1.728(4) and 1.729(4) Å] reported for XII (X = Br).46,50 The C2−C3 [1.427(3) Å] and C4−C5 [1.428(3) Å] bond distances are slightly shorter than the average C(sp2)−C(Ar) single bond (1.470 Å).62 Meanwhile, the C1−C2 and C5−C6 bonds [1.443(3) and 1.449(3) Å, respectively] are slightly longer than 8610

DOI: 10.1021/acs.inorgchem.6b02991 Inorg. Chem. 2017, 56, 8608−8614

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Inorganic Chemistry the average C(sp2)−C(sp2) double bond (1.317 Å).62 The PC4 framework in 3a resembles XII (X = Br) in the bonding form, indicating their similar electronic nature.46,50 Thus, these data support the delocalization of π electrons over the PC4 fivemembered ring concomitant with polarization of the exocyclic C1−C2 and C5−C6 bonds, which is in line with the resonance forms 3ii−3iv over 3i. The high weights for the resonance structures 3ii−3iv indicate the low barriers to the rotation of the exocyclic imidazolio substituents, which is in line with the fact that the 1H and 13C NMR spectra for an imidazolio substituent moiety exhibit symmetric signals with respect to the benzo−PC4 plane. To gain insight into the electronic structures of 2 and 3, we carried out computational studies on the model compound 2 and the cationic part of 3. Figure 3a shows the optimized

structures of 2b and 3b calculated at the B3PW/6-31G(d,p) level of theory. There is significant lengthening of the exocyclic C1−C2/C5−C6 bonds as well as C1−N/C6−N bonds and a slight shortening of the C2−C3/C4−C5 bond of 3b, compared to those of 2b. Natural bond orbital analysis on 3b performed at the same level of theory gave Wiberg bond index (WBI) values of 1.23, 1.25, and 1.21 for the P−C2, C2−C3, and C3−C4 bonds, respectively, indicating partial multiple-bond character of those bonds in the PC4 five-membered ring (see the Supporting Information, Table S2). The nucleus-independent chemical-shift (NICS) values for 3b calculated at the B3PW/631G(d,p) level of theory are estimated to be −11.15 [NICS(0)] and −10.65 [NICS(1)], which are comparable to those of the isoindole derivative IsoN+ and the isoindene derivative IsoC+ (Figure 3b).63,64 These data propose the aromatic nature of 3, which is in sharp contrast to the neutral nonaromatic phospholes featuring the pyramidal P center62 but is in line with the aromatic nature of the PC 4 five-membered phospholide ring observed for the bis(phosphoniophosphindolide) derivative XII.50 The highest occupied molecular orbital (HOMO) of 2b is a π system including the exocyclic CC bonds (Figure 4a). Similarly, the HOMO of 3b is a π-type orbital of the cationic benzophosphole moiety with some contribution from the two imidazole groups, while the LUMO+2 consists of the p orbital on the P atom and π systems on the imidazole rings involving the exocyclic C−C π-bonding orbitals (Figure 4c,d). Meanwhile, the HOMO−1 locates mainly on the C−P−C fragment (Figure 4b), implying the nucleophilic nature of the P center. We also performed calculations on compound 2a and cationic 3a (see the Supporting Information, Figures S2 and S4). Both HOMO−1 and HOMO for compound 3a correspond well with those of 3b. Similarly, the LUMO for compound 3a is comparable to the LUMO+2 for compound 3b. Hence, to

Figure 3. (a) Selected bond distances in the optimized structures of compound 2b and the cationic part of 3b. (b) NICS values for 3b, isoindole (IsoN+), and isoindene (IsoC+) derivatives.

Figure 4. Plots of (a) the HOMO of 2b and the (b) HOMO−1, (c) HOMO, and (d) LUMO+2 of 3b. 8611

DOI: 10.1021/acs.inorgchem.6b02991 Inorg. Chem. 2017, 56, 8608−8614

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Inorganic Chemistry Scheme 3. Synthesis and Crystal Structure of 4 with Thermal Ellipsoids Set at the 30% Probability Levela

a

H atoms and solvent molecules are omitted for clarity.

investigate the chemical behavior of 3, we next examined the reaction between 3 and a metal complex. A stoichiometric amount of copper chloride was added to a dichlormethane solution of 3a. After allowing it to stand overnight, however, no pronounced change was observed in the 31 P NMR spectrum, suggesting no interaction between the cationic fragment of 3a and CuCl, which is presumably due to the steric hindrance around the P center. In contrast, the addition of CuCl (1 equiv) to a DCM solution of 3b led to a yellow solution, which displays a broad singlet at 144.2 ppm in the 31P NMR spectrum. After workup, compound 4 was obtained as a yellow solid in 81% yield (Scheme 3). Single crystals of 4 were obtained by recrystallization from a saturated acetonitrile solution, and the structure of 4 was confirmed by a single-crystal X-ray diffraction study. The P center exhibits a trigonal-planar geometry with the sum of the bond angles of 360.0°, which is characteristic of an sp2 hybridization. In the case of the bis(phosphoniophosphindolide) XII, the reaction with a copper complex resulted in the formation of dinuclear copper complexes, which also feature planar P centers.65−68 Moreover, these dinuclear copper complexes of the phosphoniophosphindolide XII were shown to be able to dissociate partially in solution and are in dynamic equilibrium with a small amount of free ligand XII and mononuclear copper complexes.69 The coordination of the P atom to the Cu center confirms the electron-donating nature of the P atom, which is consistent with the resonance forms 3iv−3vi (Scheme 2). The P1−Cu1 bond distance of 2.1784(13) Å is shorter than that in (Ph3P)3CuCl [mean bond length of 2.351(4) Å for trigonal and 2.326(1) Å for the triclinic crystal of (Ph3P)3CuCl] but comparable to that [2.177(1) Å] in a tris(2,4,6trimethoxyphenyl)phosphinecopper(I) chloride complex.70,71 This P1−Cu1 bond in 4 is slightly shorter than the P−Cu bonds [2.199(2)−2.351(1) Å] observed for dinuclear copper complexes of the bis(phosphoniophosphindolide) XI66 as well as the P−Cu bonds [2.2262(12)−2.4332(12) Å] of the relevant CuI complex supported by two zwitterionic benzo[c]phospholides.72 To ascertain the electronic structure of 4, density functional theory (DFT) calculations were performed for compound 4 at the B3PW/6-31G(d,p) level of theory with the LANL2DZ pseudopotential applied for the Cu atom. The HOMO of 4 mainly lies on the P atom in the plane of the benzophosphole ring and a d orbital on the Cu center (Figure 5a). Wiberg bond order values show partial double-bond character for the PC4 five-membered phosphole moiety (Figure 5b), suggesting that the aromatic property was retained (see the Supporting Information, Table S2). The second-order perturbation analysis confirms that two characteristic donor−acceptor interactions between the lone pairs on the P atom and the d orbitals on the

Figure 5. (a) HOMO of compound 4 and (b) WBI values of the selected bonds in 4.

Cu atom are manifested with stabilization energy values of 107.97 and 93.42 kcal·mol−1, which indicates the potential application of 3 as ligands for transition-metal complexes.

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CONCLUSION In summary, we have developed bis(NHO) ligands 2, from which aromatic isophosphindolylium derivatives 3 have been synthesized. The isophosphindolylium derivative 3a has been successfully characterized by X-ray diffraction analysis, which shows a free phosphenium cation with chloride as the anion. Theoretical studies on the parent compound 3 indicate an aromaticity around the five-membered PC4 phosphole moiety. Despite being of a cationic nature, the P center in 3b can serve as a σ donor, as demonstrated by the reaction with CuCl, which afforded the copper phosphenium complex 4. The installation of other main-group elements into 2 and complexation of 3 with other metals are currently under investigation in our laboratory.

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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.6b02991. X-ray data for compounds 3a and 4, DFT calculation results, optimized structures and frontier molecular orbitals for compounds 2a, 2b, 3a, 3b, and 4, selected bond lengths and WBI values, XYZ coordinates, and NMR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Rei Kinjo: 0000-0002-4425-3937 8612

DOI: 10.1021/acs.inorgchem.6b02991 Inorg. Chem. 2017, 56, 8608−8614

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‡

C.C.C.: Energetics Research Institute, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the NTU and Ministry of Education, Singapore (Grant MOE2015-T1-001-029), for financial support.

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DOI: 10.1021/acs.inorgchem.6b02991 Inorg. Chem. 2017, 56, 8608−8614