Chlorido­[(1,2,5,6-η)-cyclo­octa-1,5-diene](1-ethyl-4-isobutyl-1,2,4-triazol-5-yl­idene)rhodium(I)

Timothy G Lerch; Michael Gau; Daniel R Albert; Edward Rajaseelan

doi:10.1107/S2414314624007041

. 2024 Jul 23;9(Pt 7):x240704. doi: 10.1107/S2414314624007041

Chlorido[(1,2,5,6-η)-cycloocta-1,5-diene](1-ethyl-4-isobutyl-1,2,4-triazol-5-ylidene)rhodium(I)

Timothy G Lerch ^a, Michael Gau ^b, Daniel R Albert ^a, Edward Rajaseelan ^a,^*

Editor: M Bolte^c

PMCID: PMC11299652 PMID: 39108937

A new neutral triazole-based N-heterocyclic carbene rhodium(I) complex [RhCl(C₈H₁₂)(C₈H₁₅N₃)], has been synthesized and structurally characterized. The complex crystallizes with two molecules in the asymmetric unit. The rhodium center has a distorted square-planar conformation, formed by a cycloocta-1,5-diene (COD) ligand, an N-heterocyclic carbene (NHC) ligand, and a chloride ligand.

Keywords: crystal structure, rhodium, N-heterocyclic carbenes, neutral transition-metal complexes

Abstract

A new neutral triazole-based N-heterocyclic carbene rhodium(I) complex [RhCl(C₈H₁₂)(C₈H₁₅N₃)], has been synthesized and structurally characterized. The complex crystallizes with two molecules in the asymmetric unit. The central rhodium(I) atom has a distorted square-planar coordination environment, formed by a cycloocta-1,5-diene (COD) ligand, an N-heterocyclic carbene (NHC) ligand, and a chlorido ligand. The bond lengths are unexceptional. A weak intermolecular non-standard hydrogen-bonding interaction exists between the chlorido and NHC ligands.

Structure description

Numerous and ever-increasing applications of N-heterocyclic carbenes (NHCs) as supporting ligands in late transition-metal catalysis have been reported (Diez-González et al., 2009 ▸; Cazin, 2013 ▸; Rovis & Nolan, 2013 ▸; Ruff et al., 2016 ▸; Zuo et al., 2014 ▸). Their catalytic activity in the transfer hydrogenation of ketones and imines has also been studied and reported (Albrecht et al., 2002 ▸; Gnanamgari et al., 2007 ▸). The NHC ligands can be tuned sterically and electronically by having different substituents on the nitrogen atoms (Diez-González & Nolan, 2007 ▸; Gusev, 2009 ▸). Though many imidazole- and triazole-based NHC rhodium and iridium complexes have been synthesized and structurally characterized (Herrmann et al., 2006 ▸; Wang & Lin, 1998 ▸; Chianese et al., 2004 ▸; Nichol et al., 2009 ▸, 2010 ▸, 2011 ▸, 2012 ▸; Idrees et al., 2017a ▸,b ▸; Rood et al., 2021 ▸; Rushlow et al., 2021 ▸; Newman et al., 2021 ▸; Castaldi et al., 2021 ▸; Maynard et al., 2023 ▸; Lerch et al., 2024 ▸), new complexes with different substituents (‘wing tips’) on NHC ligands are being synthesized to study their effects in the catalytic properties of these complexes.

The compound [RhCl(C₈H₁₂)(C₈H₁₅N₃)] (3), as illustrated in Fig. 1 ▸, crystallizes in the triclinic space group P Inline graphic with two molecules in the asymmetric unit. No solvent molecules were found in the structure. The coordination sphere around the Rh^I ion is formed by the bidentate COD, NHC, and chlorido ligands, resulting in a distorted square-planar shape. The carbene atom, C1, deviates from the expected sp² hybridization in that the N1—C1—N3 bond angle in the triazole-based carbene is 102.77 (17)° [N1′—C1′—N3′ is 102.45 (16)°]. Other selected bond lengths and angles in the structure are: Rh1—C1(NHC) = 2.020 (2) Å, Rh1′—C1′(NHC) = 2.012 (2) Å, Rh1—Cl1 = 2.3846 (5) Å, Rh1′—Cl1′ = 2.3887 (5) Å, C1—Rh1—Cl1 is 88.36 (5)°, and C1′—Rh1′—Cl1′is 88.57 (6)°. The two substituent ‘wing tips’ in the NHC (N1-ethyl and N3-isobutyl) are oriented in a syn arrangement with respect to one-another. The ethyl and isobutyl ‘wingtips’ are both oriented away from the COD ring as illustrated in Fig. 2 ▸. The packing, as illustrated in Fig. 3 ▸, is consolidated through weak non-standard hydrogen-bonding interaction between the NHC and chlorido ligands of adjacent molecules. The non-standard hydrogen-bonding interactions are summarized in Table 1 ▸ and shown as dotted green lines in Fig. 3 ▸.

Asymmetric unit of the title compound (3) showing the two molecular units. Displacement ellipsoids are drawn at the 50% probability level.

View of one molecule of the title compound (3) showing the ethyl and isobutyl wingtips oriented on the same side of the NHC ring and away from the COD ligand.

Crystal packing diagram of the title compound (3) viewed along the a axis. C—H⋯Cl non-standard hydrogen-bonding interactions are shown as dotted green lines.

Table 1. Hydrogen-bond geometry (Å, °).

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
C2′—H2′⋯Cl1ⁱ	0.95	2.62	3.502 (2)	155

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Symmetry code: (i) Inline graphic .

Synthesis and crystallization

1-Ethyl-1,2,4-triazole (1) was purchased from Matrix Scientific. All other compounds used in the syntheses, detailed in Fig. 4 ▸, were obtained from Sigma-Aldrich and Strem and used as received; all syntheses were performed under a nitrogen atmosphere. NMR spectra were recorded at room temperature in CDCl₃ on a 400 MHz (operating at 100 MHz for ¹³C and 162 MHz for ³¹P) Varian spectrometer and referenced to the residual solvent peak (δ in p.p.m.). The title compound (3) was crystallized by slow diffusion of pentane into a CH₂Cl₂ solution.

Reaction scheme for the synthesis of the title compound (3).

1-Ethyl-4-isobutyl-1,2,4-triazolium bromide (2): 1-Ethyl-1,2,4-triazole (1) (1.020 g, 10.50 mmol) and excess 1-bromo-2-methylpropane (5.436 g, 39.67 mmol) were added to toluene (15 ml), and the mixture was refluxed in the dark for 48 h. After the mixture was cooled, the white solid was filtered, washed with ether, and dried under vacuum. Yield: 0.625 g (25.4%). ¹H NMR: δ 11.71 (s, 1 H, N—C5H—N), 8.62 (s, 1 H, N—C3H—N), 4.90 (q, 2 H, N—CH₂ of ethyl), 4.38 (d, 2 H, N—CH₂ of isobutyl), 2.32 (m, 1 H, CH of isobutyl), 1.64 (t, 3H, CH₃ of ethyl), 1.03 (d, 6 H, CH₃ of isobutyl). ¹³C NMR: δ 143.49 (N—C5—N), 142.66 (N—C3—N), 55.46 (N—CH₂ of isobutyl), 48.50 (N—CH₂ of ethyl), 29.31 (CH of isobutyl), 19.49 (CH₃ of isobutyl), 14.18 (CH₃ of ethyl).

Chlorido[(1,2,5,6-η)-cycloocta-1,5-diene](1-ethyl-4-isobutyl-1,2,4-triazol-5-ylidene)rhodium(I) (3): Triazolium bromide (2) (0.095 g, 0.406 mmol) and Ag₂O (0.047 g, 0.203 mmol) were stirred at room temperature in the dark for 1 h in CH₂Cl₂ (10 ml). The mixture was then filtered through Celite into [Rh(cod)Cl]₂ (0.100 g, 0.203 mmol), and stirred again in the dark for 1.5 h. The resulting solution was filtered through Celite and the solvent was removed under reduced pressure in a rotary evaporator. The yellow solid product (3) was dried under vacuum. Yield: 0.149 g (92%). ¹H NMR: δ 7.82 (s, 1 H, N—C3H—N), 4.74 (q, 2 H, N—CH₂ of ethyl), 4.66 (d, 2 H, N—CH₂ of isobutyl), 4.30 (m, 2 H, CH of COD), 4.20 (m, 2H, CH of COD), 3.37, 3.24 (m, 4 H, CH₂ of COD), 2.60, 2.46 (m, 4 H, CH₂ of COD), 2.32 (m, 1 H, CH of isobutyl), 1.59 (t, 3 H, CH₃ of ethyl), 1.08 (d, 6 H, CH₃ of isobutyl). ¹³C NMR: δ 184.95 (d, Rh—C, J_C—Rh = 50.9 Hz), 142.29 (N—C3H—N), 99.43,99.36, 99.13, 99.06 (CH of COD), 56.21 (N—CH₂ of isobutyl), 48.01 (N—CH₂ of ethyl), 47.91, 33.29,32.45,30.80,29.30 (CH₂ of COD), 29.13 (CH of isobutyl), 20.02 (CH₃ of isobutyl), 15.36 (CH₃ of ethyl).

Refinement

Crystal data, data collection and structure refinement details are summarized in Table 2 ▸.

Table 2. Experimental details.

Crystal data
Chemical formula	[RhCl(C₈H₁₂)(C₈H₁₅N₃)]
M _r	399.76
Crystal system, space group	Triclinic, P
Temperature (K)	100
a, b, c (Å)	9.6253 (1), 13.6771 (2), 13.7938 (2)
α, β, γ (°)	76.410 (1), 83.455 (1), 80.345 (1)
V (Å³)	1734.78 (4)
Z	4
Radiation type	Mo Kα
μ (mm⁻¹)	1.14
Crystal size (mm)	0.30 × 0.23 × 0.15

Data collection
Diffractometer	Rigaku XtaLAB Synergy-S
Absorption correction	Multi-scan (CrysAlis PRO; Rigaku OD, 2024 ▸)
T_min, T_max	0.770, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	52942, 8619, 7885
R _int	0.037
(sin θ/λ)_max (Å⁻¹)	0.667

Refinement
R[F² > 2σ(F²)], wR(F²), S	0.025, 0.066, 1.03
No. of reflections	8619
No. of parameters	385
H-atom treatment	H-atom parameters constrained
Δρ_max, Δρ_min (e Å⁻³)	1.02, −0.50

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Computer programs: CrysAlis PRO (Rigaku OD, 2024 ▸), SHELXT (Sheldrick, 2015a ▸), SHELXL2018/3 (Sheldrick, 2015b ▸), OLEX2 (Dolomanov et al., 2009 ▸) and publCIF (Westrip, 2010 ▸).

Supplementary Material

Crystal structure: contains datablock(s) I. DOI: 10.1107/S2414314624007041/bt4153sup1.cif

x-09-x240704-sup1.cif^{(1.7MB, cif)}

Structure factors: contains datablock(s) I. DOI: 10.1107/S2414314624007041/bt4153Isup2.hkl

x-09-x240704-Isup2.hkl^{(684.1KB, hkl)}

CCDC reference: 2371669

Additional supporting information: crystallographic information; 3D view; checkCIF report

Acknowledgments

TL was supported in this work by the Millersville University Neimeyer–Hodgson Research Grant and Student Research Grant

full crystallographic data