Coordination polymers of Ag(I) based on iminocarbene ligands involving metal-carbon and metal-heteroatom interactions

Journal of Molecular Structure 1108 (2016) 458e466

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Coordination polymers of Ag(I) based on iminocarbene ligands involving metal-carbon and metal-heteroatom interactions Sandeep P. Netalkar, Priya P. Netalkar, Vidyanand K. Revankar* Department of Chemistry, Karnatak University, Pavate Nagar, Dharwad, 580 003, Karnataka, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 November 2015 Received in revised form 3 December 2015 Accepted 4 December 2015 Available online 17 December 2015

The reaction of Ag2O with three novel imino-NHC ligands derived from 2-chloroacetophenone with pendant N-donor functional group incorporated by reaction with methoxyamine and 1-methyl/ethyl/nbutyl-substituted imidazoles afforded one-dimensional coordination polymers with [(-NHCarbene) Ag(NHCarbene-)PF6]n formulation involving both carbon-metal and heteroatom-metal interactions, the carbon and heteroatom involved in coordination to silver being from different molecule of the ligand. The complexes as well as the ligands were characterized by spectroscopic methods as well as the solid state structures determined in case of 2a, 3a and complex 5. The iminocarbene ligands serve as nonchelating building block for supramolecular silver assemblies. © 2015 Elsevier B.V. All rights reserved.

Keywords: N-Heterocyclic carbene ligands Coordination polymers X-ray crystallography Iminocarbene Ag(I) complexes Ag(I) supramolecular architectures

1. Introduction Since the reported synthesis and isolation of stable N-heterocyclic carbenes of the imidazolin-2-ylidene type, in the early 1990's [1,2] and NHCeAg(I) complex in 1993 [3], by Arduengo et al., the NHC carbene chemistry has covered a long distance in relatively short span of time, establishing N-Heterocyclic carbenes (NHCs) derived from imidazolium salts as an extraordinary and flexible source of ligands in the preparation of carbene-metal complexes due to their unique properties such as strong s donor ability with little or no p back bonding properties, stability towards air and moisture, attractive structural diversity and also their coordination affinity towards a wide range of transition metals, find many application from biomedical application [4] to several homogeneous catalytic processes, such as olefin metathesis [5] and CeC cross-coupling reactions [6]. Despite being relatively new, the area has gained so much ground that we have access to large library of these NHC derived metal complexes with different conformations and topologies. These distinctive coordinating abilities also put these NHC ligands at the forefront as an appealing organic building block for the integration into supramolecular-metal architectures [7,8], but astoundingly the NHCs have not received the kind of

* Corresponding author. E-mail address: [email protected] (V.K. Revankar). http://dx.doi.org/10.1016/j.molstruc.2015.12.012 0022-2860/© 2015 Elsevier B.V. All rights reserved.

attention they deserve owing to their special bonding properties in this area and only quite a few reports are available: rigid benzimidazole based biseNHCebased main chain organometallic polymers have been illustrated by Bielawski [9] and a few examples of coordination polymers of Silver(I) [10]. These supramolecular architecture exclusively involve either heteroatomemetal or carbonemetal interactions, but the incorporation of both these interaction in single molecule has been less studied [10c,11]. In this present contribution we report novel ditopic N-heterocyclic carbene ligands derived from 2-chloroacetophenone with pendant N-donor functional group incorporated by reaction with methoxyamine that forms coordination polymers with Ag(I) ions. Since Ag(I) are known to form linear structures the designed ditopic NHC ligands cannot function as a bidentate ligand for Ag(I) ions and hence should be ideal candidate as building block for bigger welldefined supramolecular architectures with suitable metal ions like Ag(I) ions. 2. Experimental 2.1. General considerations All the reagents used in this study were purchased from SigmaeAldrich and used without further purification. The 1H NMR and 13C NMR spectra were recorded on a Bruker AV400 II spectrometer at 400 MHz and 100 MHz respectively in DMSO-d6 or

S.P. Netalkar et al. / Journal of Molecular Structure 1108 (2016) 458e466

CDCl3 at room temperature using TMS as internal reference. IR spectra were recorded in a KBr disc matrix using an Impact-410 Nicolet (USA) FTIR spectrometer over the range of 4000e400 cm
1. All the compounds were analyzed for carbon, hydrogen and nitrogen using a Thermo quest elemental analyzer. The X-ray diffraction data were obtained at 293 K on a Bruker SMART APEX2 CCD area-detector diffractometer using a graphite monochromated Mo-Ka (k ¼ 0.71073 Å) radiation source. The frames were integrated with the Bruker SAINT Software package using a narrow-frame algorithm. In the absence of significant anomalous scattering, Friedel pairs were merged. The H atoms were all located in a difference map, but those attached to carbon atoms were repositioned geometrically. The H atoms were initially refined with soft restraints on the bond lengths and angles to regularize their geometry, after which the positions were refined with riding constraints [12]. All non-hydrogen atoms were refined anisotropically. Structure solution and refinement were performed using Crystals [13]. Molecular graphics was generated using Cameron [14]; Structure figures were generated with ORTEP3v2 [15]. 2.2. Synthesis of precursor, 1-chloro-2-(methoxyimino)-2phenylethane, 1 Methoxylamine hydrochloride (4.2 g, 50 mmol) and triethylamine (5 g, 50 mmol) were stirred in ether for 1 h in THF and the white precipitate of triethylaminehydrochloride obtained was filtered and separated. The solvent was removed under vacuum. To this, was added phenacyl chloride (3.72 g, 0.024 mol) in diethylether (50 mL) and the solution was cooled to 0e5  C in an ice bath. To this solution, 1.0 M tetrahydrofuran solution of TiCl4 (20 mL, 0.02 mol) was added dropwise under vigorous stirring. The mixture was stirred for 30 min at 0e5  C and then for 4 h at room temperature. An aqueous 1.0 M NaOH (100 mL) solution was added to the mixture and stirred for 10 min. The aqueous phase was extracted with ether (3  50 mL). The combined extracts were dried with magnesium sulphate. The solvent was removed under vacuum, to give colorless oil. The compound was purified by column chromatography using only hexane as eluent. Schematic representation for the synthesis of 1-chloro-2-(methoxyimino)-2phenylethane is given in Scheme 1. Yield: (48.6%). Anal. Calc. for C9H10ClNO %: C, 58.86; H, 5.49; Cl, 19.31; N, 7.63. Found: C, 57.66; H, 5.69; Cl, 19.51; N, 7.68. FTIR cm
1; 1595, (C]N). 1H NMR (DMSO-d6, 400 MHz, ppm); 7.69 (dd, 2H, Ph), 7.43 (m, 3H, Ph), 4.69 (s, 3H, OCH3), 3.99 (s, 2H, CH2). 13C NMR (DMSO-d6, 100 MHz, ppm; 152.5(C]N), 132.6(ipso-Ph), 129.71(pPh), 128.5(o-Ph), 126.0(m-Ph), 32.3(CH2Cl). MS (EI): m/z 183 [Mþ], 152 [Mþ-OCH3]. 119[(Mþ-OCH3)-CH2Cl]. 2.3. Synthesis of imidazolium chloride salts, 2e4 A round-bottom flask was charged with precursor, a-chloroimine (1.84 g, 10 mmol), and 1-methylimidazole (1.64 g, 20 mmol)/1-ethylimidazole (1.92 g, 20 mmol)/1-butylimidazole (2.58 g, 20 mmol) in 1,4-dioxane. The mixture was stirred for 24 h at 80  C. On cooling to room temperature, the oily layer formed

Cl O

+

NH2OCH3

459

was separated by repeated washing with fresh dioxane (3  10 mL) and ether (3  10 mL) and dried in vacuum. The compounds were isolated as brownish oil. Schematic representation for the synthesis of ligands, 2e4 is given in Scheme 2. 2.4. [3-Methyl-1-{2-(methoxyimino)-2-phenylethyl}- imidazolium] chloride, 2 Yield: 1.312 g (80.6%). Anal. Calc. for C13H17ClN3O %: C, 58.53; H, 6.42; Cl, 13.29; N, 15.75; O, 6.00; Found: C, 58.31; H, 6.81; N, 15.06. FTIR (KBr disc) cm
1; 1633, (C]N). 1H NMR (DMSO-d6, 400 MHz, ppm); 9.41 (s, 1H, NCHN), 7.76 (d, 1H, NCHCHN, near imine), 7.74 (d, 1H, NCHCHN, near NeMe), 7.72 (d, 2H, Ar), 7.41 (m, 3H, Ar), 5.61 (s, 2H, CH2), 4.02 (s, 3H, OMe), 3.83 (s, 3H, NMe), 13C NMR (DMSO-d6, 100 MHz, ppm); 151.4 (C]N), 137.0 (NCHN), 132.5 (ipso-Ph), 130.4 (p-Ph), 129.2 (o-Ph), 127.0 (m-Ph), 124.2 (NCCN, near imine), 123.2 (NCCN, near NeMe), 43.3 (NCH2), 63.1 (OCH3), 36.3 (NCH3). 2.5. [3-Ethyl-1-{2-(methoxyimino)-2-phenylethyl}- imidazolium] chloride, 3 Yield: 1.312 g (76.6%). Anal. Calc. for C14H19ClN3O %: C, 59.89; H, 6.82; Cl, 12.63; N, 14.97; O, 5.70; Found: C, 59.31; H, 6.81; N, 15.06. FTIR (KBr Disc) cm
1; 1633, (C]N). 1H NMR (DMSO-d6, 400 MHz, ppm); 9.52 (s, 1H, NCHN), 7.81 (d, 1H, NCHCHN, near imine), 7.72 (d, 1H, NCHCHN, near NeMe), 7.70 (d, 2H, Ar), 7.41 (m, 3H, Ar), 5.61 (s, 2H, CH2), 4.18 (s, 2H, NCH2), 4.01 (s, 3H, OMe), 1.328 (s, 3H, NCH2Me), 13C NMR (DMSO-d6, 100 MHz, ppm); 151.1 (C]N), 136.6 (NCHN), 132.0 (ipso-Ph), 129.9 (p-Ph), 128.6 (o-Ph), 126.6 (m-Ph), 122.9 (NCCN, near imine), 122.0 (NCCN, near NeMe), 44.2 (NCH2), 62.5 (OCH3), 42.89 (NCH2-), 15.20 (NCH2Me). 2.6. [3-Butyl-1-{2-(methoxyimino)-2-phenylethyl}- imidazolium] chloride, 4 Yield: 1.312 g (76.6%). Anal. Calc. for C16H23ClN3O %: C, 62.23; H, 7.51; Cl, 11.48; N, 13.61; O, 5.18 . Found: C, 62.31; H, 7.81; N, 13.06. FTIR (KBr Disc) cm
1; 1634, (C]N). 1H NMR (DMSO-d6, 400 MHz, ppm); 9.57 (s, 1H, NCHN), 7.72 (d, 1H, NCHCHN, near imine), 7.72 (d, 1H, NCHCHN, near NeMe), 7.70 (d, 2H, Ar), 7.42 (m, 3H, Ar), 5.52 (s, 2H, CH2), 4.12 (s, 2H, NCH2), 4.01 (s, 3H, OMe), 1.67 (quint, 2H, NCH2CH2), 1.08 (sext, 2H, NCH2CH2CH2), 0.832 (t, 3H, Butyl- Me). 13 C NMR (DMSO-d6, 100 MHz, ppm); 151.1 (C]N), 136.7 (NCHN), 132.1 (ipso-Ph), 129.9 (p-Ph), 128.6 (o-Ph), 126.5 (m-Ph), 123.1 (NCCN, near imine), 122.4 (NCCN, near NeMe), 48.6 (NCH2), 62.5 (OCH3), 43.18 (NCH2-), 31.1 (NCH2CH2-), 18.4 (NCH2CH2CH2), 13.0 (-CH2Me). 2.7. Synthesis of imidazolium hexafluorophosphate salts, 2a-4a To a solution of 2.65 g (10 mmol) of 2/2.8 g (10 mmol) of 3/3.08 g (10 mmol) of 4 in methanol was added 1.84 g (10 mmol) of KPF6 in methanol and stirred for 2 h and left to stand overnight. The white solid formed was filtered, washed with distilled water (3  5 mL) to remove unreacted KPF6, and air dried. The compounds were isolated as off white powder. Schematic representation for the synthesis of ligands, 2a-4a is given in Scheme 3. 2.8. [3-Methyl-1-{2-(methoxyimino)-2-phenylethyl}- imidazolium] hexaflurophosphate, 2a

TiCl4, THF Cl

0-5°C, Strr, 4h O

N

Scheme 1. Synthetic route for the preparation of 1-chloro-2-(methoxyimino)-2phenylethane.

Yield 89.02%. Anal. Calc. for C13H17F6N3OP %: C, 41.50; H, 4.55; F, 30.30; N, 11.17; O, 4.25; P, 8.23; Found: C, 41.31; H, 4.81; N, 11.06. FTIR (KBr disc) cm
1; 1616, (C]N), 1577, (C ¼ Nimida). 1H NMR (DMSO-d6, 400 MHz, ppm); 9.09 (s, 1H, NCHN), 7.71 (d, 1H,

460

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1,4-Dioxane Cl O

+

N

N R

Reflux, 24h

N

N

N R

Cl

N O

R = Me Et n-butyl

Scheme 2. Synthetic route for the preparation of imidazolium chloride salts.

N N O

N R

Cl

KPF6 Methanol

N

N R

PF6

N O

R = Me Et n-butyl

Scheme 3. Synthetic route for the preparation of imidazolium hexafluorophosphates.

NCHCHN, near imine), 7.70 (d, 1H, NCHCHN, near NeMe), 7.64 (d, 2H, Ar), 7.45 (m, 3H, Ar), 5.51 (s, 2H, CH2), 4.02 (s, 3H, OMe), 3.80 (s, 3H, NMe). 13C NMR (DMSO-d6, 100 MHz, ppm); 150.7 (C]N), 137.0 (NCHN), 132.1 (ipso-Ph), 130.1 (p-Ph), 128.7 (o-Ph), 126.4 (m-Ph), 123.7 (NCCN, near imine), 122.8 (NCCN, near NeMe), 42.5 (NCH2), 62.6 (OCH3), 35.8 (NCH3). 2.9. [3-Ethyl-1-{2-(methoxyimino)-2-phenylethyl}-imidazolium] hexaflurophosphate, 3a Yield 90.0%. Anal. Calc. for C14H19F6N3OP %: C, 43.08; H, 4.91; F, 29.21; N, 10.77; O, 4.10; P, 7.94. Found: C, 43.31; H, 4.81; N, 10.06. FTIR (KBr Disc) cm
1; 1603, (C]N), 1571, (C ¼ Nimida). 1H NMR (DMSO-d6, 400 MHz, ppm); 9.08 (s, 1H, NCHN), 7.72 (d, 1H, NCHCHN, near imine), 7.71 (d, 1H, NCHCHN, near NeMe), 7.68 (d, 2H, Ar), 7.43 (m, 3H, Ar), 5.62 (s, 2H, CH2), 4.16 (s, 2H, NCH2), 4.05 (s, 3H, OMe), 1.32 (t, 3H, NCH2Me). 13C NMR (DMSO-d6, 100 MHz, ppm); 150.1 (C]N), 136.9 (NCHN), 132.0 (ipso-Ph), 130.2 (p-Ph), 128.6 (o-Ph), 126.5 (m-Ph), 123.0 (NCCN, near imine), 122.6 (NCCN, near NeMe), 43.2 (NCH2), 62.5 (OCH3), 42.92 (NCH2-), 15.22 (NCH2Me). 2.10. [3-Butyl-1-{2-(methoxyimino)-2-phenylethyl}-imidazolium] hexaflurophosphate, 4a Yield 87.02%. Anal. Calc. for C16H23F6N3OP %: C, 45.94; H, 5.54; F, 27.25; N, 10.04; O, 3.82; P, 7.40; Found: C, 45.31; H, 5.81; N, 10.06. FT-IR (KBr Disc) cm
1; 1615, (C]N), 1570, (C ¼ Nimida). 1H NMR (DMSO-d6, 400 MHz, ppm); 9.17 (s, 1H, NCHN), 7.72 (d, 1H, NCHCHN, near imine), 7.72 (d, 1H, NCHCHN, near NeMe), 7.71 (d, 2H, Ar), 7.42 (m, 3H, Ar), 5.53 (s, 2H, CH2), 4.12 (s, 2H, NCH2), 4.01 (s, 3H, OMe), 1.67 (quint, 2H, NCH2CH2), 1.08 (sext, 2H, NCH2CH2CH2), 0.832 (t, 3H, Butyl- Me). 13C NMR (DMSO-d6, 100 MHz, ppm); 151.1 (C]N), 136.7 (NCHN), 132.1 (ipso-Ph), 129.9 (p-Ph), 128.6 (o-Ph), 126.5 (m-Ph), 123.1 (NCCN, near imine), 122.4 (NCCN, near NeMe), 48.6 (NCH2), 62.5 (OCH3), 43.18 (NCH2-), 31.1 (NCH2CH2-), 18.4 (NCH2CH2CH2), 13.0 (-CH2Me). 2.11. Synthesis of [(iminoimidazolin-2-yliden)AgPF6]n (5e7): general procedure 1 equivalent of iminoimidazolium hexafluorophosphate (5e7), in acetonitrile was added to slurry of excess of Ag2O in acetonitrile. The mixture was stirred under exclusion of light for 36 h at ambient temperature before filtration through a pad of celite to remove unreacted Ag2O. The solvent was concentrated under vacuum and ether was added to precipitate out the product, dried and recrystallized from acetonitrile-ether. The products were isolated as light

sensitive white powders. Schematic representation for the synthesis of complexes, 5-7is given in Scheme 4.

2.12. [(3-Methyl-1-{2-(methoxyimino)-2-phenylethyl}imidazolinn-2-ylidene)AgPF6]n, 5 Yield: 60%. FT-IR (KBr disc) cm
1; 1630, (C]N) 1568, (C ¼ Nimida). 1H NMR (DMSO-d6, 400 MHz, ppm); 7.63 (d, 1H, NCHCHN, near imine), 7.61 (d, 1H, NCHCHN, near NeMe), 7.38 (d, 2H, Ar), 7.32 (m, 3H, Ar), 5.41 (s, 2H, CH2), 3.88 (s, 3H, OMe), 3.67 (s, 3H, NMe). 13C NMR (DMSO-d6, 100 MHz, ppm); 180.8 (NCN), 153.6 (C]N), 133.5 (ipso-Ph), 130.1 (p-Ph), 129.0 (o-Ph), 127.2 (m-Ph), 123.4 (NCCN, near imine), 123.1 (NCCN, near NeMe), 62.6 (NCH2), 44.8 (OCH3), 38.7(NCH3). ESI-MS: m/z 568.3 (M - PF6) 20%, 337.0 (M-L22) 12%, 229.9(M-L22-Ag) 100%. (calc. 230).

2.13. [(3-Ethyl-1-{2-(methoxyimino)-2-phenylethyl}- imidazolinn2-ylidene)AgPF6]n, 6 Yield 52%. FT-IR (KBr Disc) cm
1; 1624, (C]N), 1568, (C ¼ Nimida). 1H NMR (DMSO-d6, 400 MHz, ppm); 7.60 (d, 1H, NCHCHN, near imine), 7.58 (d, 1H, NCHCHN, near NeMe), 7.58 (d, 2H, Ar), 7.32 (m, 3H, Ar), 5.43 (s, 2H, CH2), 3.95 (s, 2H, NCH2), 3.87 (s, 3H, OMe), 1.20 (t, 3H, NCH2Me). 13C NMR (DMSO-d6, 100 MHz, ppm); 180.1 (NCN), 153.4 (C]N), 133.6 (ipso-Ph), 130.2 (p-Ph), 128.1 (o-Ph), 126.7 (m-Ph), 123.3 (NCCN, near imine), 122.9 (NCCN, near NeMe), 62.4 (NCH2), 45.2 (OCH3), 44.92 (NCH2-), 15.42 (NCH2Me).

2.14. [(3-Butyl-1-{2-(methoxyimino)-2-phenylethyl}- imidazolinn2-ylidene)AgPF6]n, 7 Yield 55%. FT-IR (KBr Disc) cm
1; 1628, (C]N), 1565, (C ¼ Nimida). 1H NMR (DMSO-d6, 400 MHz, ppm); 7.54 (d, 1H, NCHCHN, near imine), 7.52 (d, 1H, NCHCHN, near NeMe), 7.38 (d, 2H, Ar), 7.31 (m, 3H, Ar), 5.44 (s, 2H, CH2), 3.97 (t, 2H, NCH2), 3.88 (s, 3H, OMe), 1.60 (quint, 2H, NCH2CH2), 1.03 (sext, 2H, NCH2CH2CH2), 0.77 (t, 3H, Butyl-Me). 13C NMR (DMSO-d6, 100 MHz, ppm); 179.5 (NCN), 153.7 (C]N), 132.8 (ipso-Ph), 129.5 (p-Ph), 128.3 (o-Ph), 126.8 (m-Ph), 123.1 (NCCN, near imine), 121.5 (NCCN, near NeMe), 50.7 (NCH2), 62.1 (OCH3), 44.9 (NCH2-), 32.8 (NCH2CH2-), 18.8 (NCH2CH2CH2), 13.3 (-CH2Me). ESI-MS: m/z 650.24 (M -PF6) 20%, 378.07 (M-L24) 15%, 272.17 (M-L24-Ag) 88%.

S.P. Netalkar et al. / Journal of Molecular Structure 1108 (2016) 458e466

N N

N R

PF6

+ Ag2O

N O

N

Under dark, MeCN Stirr, 36h

461

R

Ag

R = Me Et n-butyl

N O

N O

N PF6

N R

n

Scheme 4. Synthetic route for the preparation of polymeric AgeNHC hexafluorophosphate, 5e7.

3. Results and discussion 3.1. Syntheses and characterization The general synthetic protocol is outlined in Schemes 1e4. The precursor, a-chloroimine was synthesized by stirring a solution methoxyamine and phenacyl chloride at 0e5  C in the presence of TiCl4 according to the procedure given by De Kimpe et al. [16] with minor modifications. The methoxyamine hydrochloride was neutralized with triethylamine before reaction with phenacyl chloride. Treatment of the a-chloroimine with 1-substituted imidazoles in refluxing 1,4-dioxane yielded the highly moisture sensitive oily/sticky imidazolium chloride (2e4) salts (Scheme 2) and therefore were converted to their more stable PF6 counterparts (2a4a) using salt metathesis by KPF6 in methanol by stirring for 1 h (Scheme 3). All the compounds were obtained in good yields (80e90%). The prepared chloride salts were highly hygroscopic and readily soluble in common organic solvents like methanol, ethanol, acetone, dichloromethane, DMSO and DMF but their PF6 counterparts were partially soluble in methanol and ethanol and completely soluble in acetone, acetonitrile, DMSO and DMF. The new imidazolium salts 2e4 and 2a-4a were all spectroscopically characterized (see Experimental section for details), in addition to the solid state structures of 2a and 3a being established by X-ray analysis. The imidazolium salts are more frequently used as precursors for metal N-heterocyclic carbene complexes with the deprotonation of acidic CeH, brought about by use of suitable base or the use of basic silver salts which has been found advantageous [17]. Interestingly, Ag(I)eNHC complexes display fascinating structural variety, including monomeric, dimeric, and polymeric solid-state

Fig. 1. ORTEP projection of 2a showing 50% probability ellipsoids.

structures [18e20]. Reaction of the imidazolium hexaflurophosphate salts 2a-4a with a slurry of Ag2O in acetonitrile at room temperature for 40 h led to the corresponding Ag(I) iminocarbene complexes 5e7 with [Ag(carbene)PF6]n formulation (Scheme 4). The solid state structure determination of 4 crystallized from acetonitrileediethyl ether mixture unambiguously showed the formation of a coordination polymer (Fig. 3). The resulting Ag(I) carbene complexes act as carbene transfer agents to a variety of other metals, in a sense, a catalyst for easy synthesis of other metal carbene complexes which has not been discussed in this present contribution. These new Ag(I)eNHC complexes are nonhygroscopic and stable to air and moisture at room temperature but sensitive to light. These complexes are readily soluble in common organic solvents such as methanol, ethanol, acetonitrile, chloroform, dichloromethane, DMF and DMSO.

3.2. Infrared spectral studies FTIR spectra of all the imidazolium salts and their silverecarbene complexes were recorded over the scan range 4000e400 cm
1 and are reported in detail in the experimental section. The chloride (2e4) and hexaflurophosphate salts (2a-4a) evidenced two distinct sharp bands of high intensity around 16301615 cm
1 and 1560-1575 cm
1, the higher energy band being attributed to free C]N stretching and the lower one to the

Fig. 2. ORTEP projection of 3a showing 50% probability ellipsoids.

462

S.P. Netalkar et al. / Journal of Molecular Structure 1108 (2016) 458e466

Fig. 3. ORTEP projection (top) of 5 showing 50% probability ellipsoids, hexaflurophosphate anion omitted for clearity and a different perspective of polymeric chain (bottom). Hydrogen atoms are omitted for the sake of clearity.

imidazole ring C]N stretching [21]. The bands observed at around 1050 and 1150 cm
1 are ascribed to the substitutions at 1- and 3positions. The FTIR spectra of AgeNHC complexes, 5e7 also show two prominent bands corresponding to azomethine group, nC]N absorptions at ca. 1625 and 1565 cm
1. Both these absorptions are essentially shifted to higher frequency by an amount of 10e15 cm
1 in complexes (5e7) compared to those seen for the corresponding imidazolium precursors 2a-4a. In addition to this, the FTIR spectra also showed vibrational bands for CeH fragments in the range 2860e3150 cm
1 which in spectra of complexes remained unaltered. The IR data coupled with NMR data suggests that the Ag is bounded by both the carbene and the imine group in these iminocarbenes-Ag complexes. Since Ag(I) are known to form linear structures, this can only be possible if one bond to Ag comes from carbene carbon and other through imine nitrogen of another molecule of iminocarbene ligand in polydente fashion.

3.3.

1

H and

13

C NMR spectral studies

The NMR spectra of all compounds were carried out in DMSO-d6 over scan range of 0e16 ppm for 1H and 0e200 ppm for 13C NMR. The 1H NMR spectra of the prepared imidazolium salts 2e4 and 2a4a showed the expected signals anticipated for the imidazolium salts. The imidazolium proton (NCHN) resonated in the range 9.4e9.6 ppm, in case of chloride salts and slightly upfield, in the range 9.09e9.18 ppm, in case of PF6 salts, as a singlet in both salts. The imidazolium H40 /H50 protons appeared in the range 7.72e7.82 ppm as distinct doublets and the aromatic signals were observed in the expected range of d 7.4e7.7 ppm in all the salts. The methylene protons resonated in the range 5.4e5.6 ppm as a sharp singlet in all the salts. The signals of methyl, ethyl and n-butyl

group of imidazolium salts appeared as anticipated. In the 13C NMR, the signal for C20 carbon was observed in the range of 135.7e137.0 ppm and that of imine carbon (C]N) was observed in the range of 150.0e151.2 ppm. The successful formation of Ag(I)eNHC complexes, 5e7 was established by 1H and 13C NMR spectra. The absence of the characteristic imidazolium H2' signals in the respective 1H NMR spectra of the AgeNHC complexes evidently are attributed to the deprotonation of H2' protons [23e25]. The Ag-complexes also displayed a sharp singlet peak for the methylene protons connecting the imine part to the imidazolium ring in the range d 5.40e5.45, showing a slight upfield shift of 0.10 ppm, compared to free imidazolium hexaflurophosphate salts, due to the conformational flexibility in the solution [26]. The signals of the aromatic protons in the arene ring appeared in various shapes in the range of 7.26e7.42 ppm. Similarly, the imidazolium H4' and H5' signals appeared in various shapes in the range of 7.52e7.63 ppm. Both the Aromatic signals and H4' and H5' signals are upfield shifted. In the 13C NMR spectra of the Ag-complexes the C2' signal of the carbene carbon appeared at 179e180 ppm [22] and the imine (C]N) carbene signal showed a slight downfield shift from ca. d 151 observed in free salts to d 153 in the Ag-complexes. Additionally, elemental analysis results are consistent with the assigned structures of the imidazolium salts and silverecarbene complexes.

3.4. ESI-mass spectral studies Only the spectral data of NMR, IR, and elemental analysis cannot ascertain unambiguously whether the structures of 5e7 are monomeric, oligomeric or polymeric. Very useful information was derived from Electrospray mass spectrometry analysis of silver(I) carbene complexes, 5e7 (Figs. 4 and 5). Due to their structural

S.P. Netalkar et al. / Journal of Molecular Structure 1108 (2016) 458e466

463

Fig. 4. ESI-Mass spectrum of 5.

Fig. 5. ESI-Mass spectrum of 6.

similarities, all the NHCeAg(I) complexes have similar fragmentation pattern in their ESI-Mass spectrum. The ESI-Mass spectra of all the Ag(I)eNHC complexes indicated the presence of a [NHCarbene / Ag ) NHCarbene]þ fragment at m/z 567, 594 and

650 respectively for 5, 6 and 7 strengthening the earlier observation derived from NMR, IR and elemental analysis. This fragment subsequently decomposes to a fragment corresponding to [NHCarbene / Ag]þ indicated at m/z 336, 351 and 378 respectively

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S.P. Netalkar et al. / Journal of Molecular Structure 1108 (2016) 458e466

N N N

R Ag

N

PF6 N

O

_

N

PF6 N

O

N N R

R

O

Ag

O

N

N

_L

N

N

n

N

R _ Ag

N

+ Ag(I)

R

N N

O

O

N R

L =

R N

N

N O

R = Me, Et, n-Bu Scheme 5. ESI-MassFragmentation pattern in complexes, 5e7.

for 5, 6 and 7, which finally loses the Ag(I) ion yielding [NHCarbene]þ fragment. The fragmentation pattern is detailed in Scheme 5. The solid-state structure determination of 5 by XRD techniques (vide infra) contrasted the ESI-MS findings in establishing that the solid-state structure is indeed polymeric.

3.5. Single crystal X-ray diffraction studies Single Crystals of ligands, 2a and 3a, suitable for X-ray diffraction, were obtained by slow evaporation of acetonitrile/methanol mixture at ambient temperature. The Ag(I) complex, 5 suitable for X-ray diffraction, was grown by slow diffusion of ether into acetonitrile solution of the complex. Summaries of the crystallographic data, bond lengths and bond angles and hydrogen-bond geometry of ligands, 2a, 3a and silver(I) complex, 5 are tabulated

in Tables 1e3. The hexafluorophosphate imidazolium salts 2a and 3a, crystallize in monoclinic and orthorhombic crystal systems with space groups P21/c and P212121, stabilized through three-dimensional network of hydrogen bonding. Asymmetric units of the salts 2a (Fig. 1) and 3a (Fig. 2), consists of an imidazolium cation and a hexafluorophosphate anion occupying an asymmetric unit. The bond distances of C2eN1 1.283(3) and 1.292(7)Å agrees well with the value for double bond character, confirming the formation of imine bonds. Planes of central imidazole and phenyl rings in the ligands 2 and 3 are perpendicular to each other as indicated by their dihedral angles of 86.67 and 88.70 respectively, thus forming a Vshaped structure. The internal ring angles of imidazoles (NeCeN) are 108.8(2) and 110.8(4) respectively well in agreement with the previous reports.

Table 1 Crystal data and structure refinement details of ligands and complex. Crystal data

2a

3a

5

Empirical formula Formula weight Crystal system space group Unit cell dimensions a (Å) b (Å) c (Å)
(o) a ^ (o) a ~ (o) a V (Å3) Z Density (calcd) Mg/m3 Abs coeff (mm
1) F(000) Crystal size (mm) Temperature (K) Limiting indices h k l  e Min, max (o) Reflections collected Unique reflections Number of parameters R1 wR2

C13H16F6N3OP 375.25 Monoclinic P21/c

C14H18F6N3OP 389.28 Orthorhombic P212121

C13H15AgF6N3OP 482.11 Monoclinic P21/n

10.525 (5) 9.550 (5) 16.306 (5) 90 99.873 (5) 90 1614.7 (12) 4 1.544 0.240 768 0.20  0.16  0.14 296

7.793 (5) 11.278 (5) 20.399 (5) 90 90 90 1792.9 (15) 4 1.442 0.219 800 0.20  0.18  0.14 296

11.4661 (2) 10.7562 (2) 13.8647 (2) 90 98.465 (1) 90 1691.33 (5) 4 1.893 1.355 952 0.16  0.14  0.12 296


17 / 16
14 / 14
25 / 25 2.0, 34.2 24037 6535 73 0.0730 0.2168


10 / 9
10 / 15
17 / 27 2.0, 28.7 9922 4427 76 0.0887 0.2182


18 / 18
15 / 10
21 / 21 2.2, 33.9 26212 6832 226 0.0483 0.1296

S.P. Netalkar et al. / Journal of Molecular Structure 1108 (2016) 458e466 Table 2 Selected bond lengths (Å) and angles (o) of ligands and complex. Bond lengths (A )

2a

C1eO1 N1eO1 N1eC2 N2eC9 N2eC10 C2eC3 C7eC8 N1eAg1 C10eAg1 Bond angles (o) O1eN1eC2 C9eN2eC10 C9eN2eC11 N1eO1eC1 N1eC2eC3 N2eC10eN3 O1eN1eAg1 C2eN1eAg1 N3eC10eAg1 N2eC10eAg1 N1eAg1eC10

1.412 1.410 1.284 1.465 1.335 1.478 1.401 e e

(3) (2) (2) (3) (3) (3) (3)

1.401 1.370 1.284 1.432 1.317 1.494 1.353 e e

(6) (5) (5) (6) (5) (6) (9)

1.404 1.392 1.273 1.455 1.342 1.488 1.390 2.149 2.071

(4) (3) (3) (3) (4) (3) (5) (2) (3)

110.72 125.43 126.50 109.1 116.28 108.82 e e e e e

(18) (19) (18) (2) (17) (19)

113.8 125.0 129.1 109.8 118.9 110.9 e e e e e

(3) (4) (3) (3) (3) (4)

113.0 124.6 124.3 110.9 116.1 104.5 127.97 118.96 124.9 130.58 176.86

(2) (2) (3) (3) (2) (2) (16) (18) (2) (18) (10)

3a

5

465

(NeCeN) at the carbene center is 104.5(2)o for N2eC10eN3. This considerable decrease in the internal ring angles of imidazole rings is originated from the formation of new CeAg bond from carbene carbon atom. The transoid conformation of the iminocarbene ligand leads to an one-dimensional infinite chain where the ligand acts as a spacer between two Ag(I) atoms via the carbene and imine nitrogen donor groups of different molecule of ligand in a “head-totail” orientation. The PF6 ions sandwich between these parallel coordination polymers through intermolecular CeH/F hydrogen bonds interactions. The ligand stabilizes the metal centre through three types of intraionic interactions C/Ag, N/Ag and O/Ag, with bond distances of Ag1eC1 3.236(5) Å, Ag1eC2 2.981(2) Å, Ag1eC1 3.022(2) Å, Ag1eN1 2.144(3) Å and Ag1eO1 3.199(2) Å respectively. In conclusion, we present a novel fascinating building block for supramolecular assemblies, based on non-chelating ditopic iminocarbene ligand which forms unique coordination polymers with silver, the coordination to silver coming from carbene and imine nitrogen of different units of iminocarbene ligand thus forming a 1Dimensional polymer chain. Further studies include the transmetallation reactions of these Ag(I) coordination polymers and application of these in catalysis which are in progress. Acknowledgments

Table 3 Selected hydrogen-bond geometry (Å,o) for ligands and complexes. D-H/A interactionsa 2a C9eH91/F1#1 C8eH81/F4#1 C7eH71/F6#2 C13eH133/F2#3 ð / ð interactions C6/C10#4 3a C8eH81/F6#5 C9eH92/F5#6 C10eH101/F4#5 C10eH101/F3#5 C10eH101/F1#6 C12eH121/F1#7 C13eH132/F5 5 C1eH11/F2#8 C12eH121/F3#9 C9eH92/F4#10 C1eH12/F6

a

D-H

H/A

D/A

D-H/A

0.983 0.940 0.907 0.936 Cg/Cg 3.278(4)

2.407 2.567 2.611 2.587

3.333(3) 3.334(4) 3.496(4) 3.287(4)

156.8 139.0 165.3 132

0.919 0.966 0.935 0.935 0.935 0.932 0.967

2.647 2.629 2.431 2.495 2.495 2.495 2.47

3.30(1) 3.32(1) 3.127(8) 3.328(7) 3.090(8) 3.405(9) 3.40(1)

128.9 128.3 131.1 148.4 117.4 165.4 161.2

0.981 0.905 0.956 0.950

2.530 2.660 2.418 2.642

3.506(8) 3.431(6) 3.238(7) 3.511(5)

173.4 143.5 143.6 152.2

The authors thank University Science Instrumentation Center, Karnatak University, Dharwad, for providing various spectral facilities. One of the authors (Sandeep P. Netalkar) is thankful to Department of Science & Technology (DST) for providing financial assistance under INSPIRE fellowship program. Appendix A. Supplementary data

a Symmetry codes: #1: 1-x, -½þy, 1.5-z; #2:
1þx, y, z; #3:x, ½-y, ½þz; #4: 1-x, ½þy, 1.5-z; #5: 1 þ x, y, z;#6: 1-x, -½þy, ½-z; #7: -x, -½þy, ½-z; #8: 1-x, 1-y, -z; #9:x, y, 1 þ z;#10: ½þx, 1.5-y, ½þz.

The silver(I) carbene complex, 5 crystallizes in monoclinic space group P21/n. Asymmetric units of the 5 is given in Fig. 3. The asymmetric unit of the complex consists of a Ag(I) complex cation and a hexafluorophosphate anion, located at a crystallographic center of symmetry. Single-crystal analysis of Ag(I) complex revealed the existence of inversion center. The above Ag(I) complex represents an example of Ag-containing polymeric compound based on an organic ligand that can afford both carbene carbon and heteroatom (nitrogen) as coordinating sites. The Ag(I) ion is arranged symmetrically between a carbene and nitrogen donor derived from imidazole and imine nitrogen respectively, with a bond distance of 2.144(3) Å for N1eAg1 and 2.071(3) for C10eAg1 motifs respectively. The C10eAg1eN1 angle in Ag(I) complex deviates from a linear arrangement by only about 4 . The imidazole and phenyl ring adopt a boat conformer with bond angles of N2eC9eC2 (112.4(3) ) and C3eC2eC9 (122.0(3) ) respectively, on either side of inversion center. The internal ring angle of imidazole

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