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Communication

An Inverted-Sandwich Dichromium(I) Complex Stabilized by Guanidinate Ligands

1
Department of Chemistry, College of Science, King Faisal University, Al-Hassa 31982, Saudi Arabia
2
Department of Basic Sciences, Preparatory Year, King Faisal University, Al-Hassa 31982, Saudi Arabia
3
Lehrstuhl Anorganische Chemie II—Katalysatordesign, Sustainable Chemistry Centre, Universität Bayreuth, 95440 Bayreuth, Germany
4
Department of Life Sciences, College of Science, King Faisal University, Al-Hassa 31982, Saudi Arabia
*
Author to whom correspondence should be addressed.
Molbank 2024, 2024(4), M1901; https://doi.org/10.3390/M1901
Submission received: 4 October 2024 / Revised: 11 October 2024 / Accepted: 14 October 2024 / Published: 16 October 2024
(This article belongs to the Section Structure Determination)

Abstract

:
Reduction of the four-coordinate “ate” species [(RC(NAr)2Cr(µ-Cl)2Li(THF)2] (1) (R = diisopropylamine; Ar = 2,6-diisopropylphenyl) with potassium graphite (KC8) in toluene leads to the formation of a toluene-bridged inverted-sandwich dichromium(I) complex, (µ-η6:η6-C7H8)[Cr{RC(NAr)2}]2 (2). X-ray analysis confirms the dinuclear nature of (2). The compound [C69H104Cr2N6], crystallized in the monoclinic space group, P21/c, has the following cell parameters: a  =  15.108(3) b  =  29.155(6) c  =  17.486(4) Å, β  =  101.19(3)°, V  =  7555(3) A3, and Z  =  4.

1. Introduction

Stabilization and isolation of low-valent and low-coordinate transition metal complexes has recently attracted more attention and is of particular importance to us not only from a structural point of view but also due to their highly reactive nature towards small molecules [1,2]. The rarity of these metal complexes could be attributed to their electronic unsaturation and thus they need the employment of sterically crowded ligands. Recently, guanidinates have received considerable attention as ancillary ligands for the stabilization of not only a wide variety of metal centers from throughout the periodic table but also for such coordinatively unsaturated metal complexes due to their easy syntheses and the possibility of steric and electronic modification [3,4,5,6,7,8]. We have previously shown that slight steric variation in the –NR2 group present on the back of the NCN moiety plays a crucial role in determining the oxidation state of the reduced metal centers [7]. Our interest in guanidinate complexes focuses on the synthesis and reactivity of Cr(I) complexes. Reduction of the divalent chromium precursor, [(RC(NAr)2Cr(µ-Cl)2Li(THF)2] (R = diisopropylamine; Ar = 2,6-diisopropylphenyl) with KC8 in tetrahydrofuran (THF) has previously led to a monomeric Cr(0) complex [7]. Since we were interested in the isolation of CrI species, we replaced THF with toluene as the reaction medium and herein describe the synthesis and characterization of the rare example of dichromium inverted-sandwich complexes, which is also the first example of such species being stabilized by guanidinate ligands. Other known examples have been previously reported for closely related β-diketiminate and amidinate ligands. The synthesis of these inverted-sandwich-type dinuclear complexes has been achieved either by potassium graphite or magnesium reduction of the corresponding metal chlorides in aromatic solvents or due to the cleavage of the Cr-Cr quintuple bond upon reaction with terminal alkynes [9,10,11,12].

2. Results and Discussion

Reduction of a divalent Cr–ate complex, [(RC(NAr)2Cr(µ-Cl)2Li(THF)2] (1) (R = diisopropylamide; Ar = 2,6-diisopropylphenyl), with two equivalents of freshly prepared potassium graphite (KC8) in toluene and after workup in hexane affords an inverted-sandwich compound (µ-η6:η6-C7H8)[Cr{RC(NAr)2}]2 (2), in which a toluene molecule bridges two Cr centers in a µ-η6:η6 fashion (Scheme 1). Compound (2) was obtained in 89% yield as a red crystalline material. The 1H NMR spectrum of (2) exhibits broad signals typical for a paramagnetic compound (Figure S1) and a magnetic moment of 6.58 µB that is comparable to other CrI inverted-sandwich complexes (ranging between 6.45 and 7.41 µB) [9,10,11].
Since the compound is paramagnetic and the NMR studies were not very conclusive, crystals suitable for X-ray analysis were grown from hexane solution. Single-crystal X-ray analysis confirmed the dinuclear nature of the compound (Figure 1). The two N–C–N–Cr four-member rings are arranged in an orientation with an N1–Cr1–Cr2–N5 torsion angle of 95.75°. The N–Cr–N bond angles and Cr–N bond distances [64.23(15) and 64.90(14)° and 2.059(4), 2.054(4), 2.046(4) and 2.049(4) Å, respectively] in (2) are comparable to those in (1) [65.28(10)° and 2.0453(19) Å, respectively] [7]. The nearly identical C–N bond lengths [N1–C8 1.366(6), N2–C8 1.355(5) and N3–C8 1.386(6) Å of one ligand and N4–C39 1.352(5), N5–C39 1.357(5) and N6–C39 1.394(5) Å of the other ligand] and the sum of the N–C–N bond angles around C8 and C39 were approximately 360°, confirming the presence of sp2-hybridized nitrogen and carbon atoms. This may be attributed to the role of the lone pair of non-coordinating N-atoms in the π system of the ligand that can result in an increased electron density at the Cr centers and thus may lead to stronger binding of the ligand. The bridging arene ring is not planar, as indicated by inequivalent distances from the two Cr atoms, with metal-to-centroid distances of 1.736 and 1.752 Å. The Cr1–C distances to the toluene ring range from 2.194(5) to 2.339(5) Å, with Cr1–C3 being the shortest and Cr1–C5 the longest distance being to one of the methyl-group-substituted carbon atoms of the ring. The Cr2–C distances to the toluene ring range from 2.199(5) [Cr2–C2] to 2.329(5) Å [Cr2–C6]. This shows that both chromium atoms lean toward the same edge, C2–C3, and as a result the whole molecule is distorted. The average C–C distance for the bridging toluene molecule was determined to be 1.434(6) Å. These bond lengths are comparable with the reported values found for the μ-η6:η6-arene ligands in inverted-sandwich dichromium compounds [9,10,11,12], but slightly longer than those reported for free benzene (1.397(1) Å) [13] and toluene (1.39(1) Å) [14].

3. Experimental

All manipulations were performed with rigorous exclusion of oxygen and moisture in Schlenk-type glassware either on a dual-manifold Schlenk line or in a N2-filled glove box (mBraun 120-G) with a high-capacity recirculator (<0.1 ppm O2). Solvents were dried by distillation from sodium wire/benzophenone. CrCl2 was purchased from ABCR GmbH. Deuterated solvents were obtained from Cambridge Isotope Laboratories and were degassed, dried and distilled prior to use. The 1H NMR spectrum was recorded on a Varian 400 MHz at ambient temperature. The chemical shifts are reported in ppm relative to the internal TMS. Elemental analyses (CHN) were determined using a Vario EL III instrument. Compound (1) was synthesized following the literature procedures [7]. The effective magnetic moment was determined using a Sherwood Scientific Magnetic Susceptibility Balance. X-ray crystal structure analysis was performed with a STOE-IPDS II equipped with an Oxford Cryostream low-temperature unit. Structure solution and refinement was accomplished using SIR97 [15], SHELXL2018 [16,17], WinGX [18] and Olex2 [19]. One of the isopropyl groups shows disorder that leads to a relatively large displacement parameter. A toluene solvate molecule was so badly disordered that it could not be modeled with constraints and was squeezed using the squeeze command in Olex2. An Olex2 solvent mask was used to calculate the void space and the electron count. All parameters reported in the CIF take a toluene solvate molecule into account, leading to a couple of alerts in the checkCIF report. No absorption correction was applied to the data and some of the reflections at certain angles were omitted during the refinement. CCDC-2347002 contains the supplementary crystallographic data for this paper. The crystallographic data as collected by single-crystal analysis are presented in Table 1. These data can be obtained free of charge at https://www.ccdc.cam.ac.uk/ accessed on 4 October 2024 (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44-1223-336-033; e-mail: [email protected]).
Synthesis: [(RC(NAr)2Cr(µ-Cl)2Li(THF)2] (0.460 g, 0.62 mmol) in toluene (15 mL) was added to two equivalents of freshly prepared KC8 (0.169 g, 1.34 mmol) in toluene (10 mL) at −20 °C. After stirring for ca. 10 min, the solution was brought to room temperature and further stirred for 24 h. Toluene was evaporated under vacuum and the product was dissolved in hexane (20 mL). The solution was filtered and the filtrate was reduced in vacuum to ca. one-third to afford the product as a red crystalline material at −25 °C. Yield: 0.31 g (88.8%) C69H104Cr2N6 (1121.60): Calcd. C 73.89, N 7.49, H 9.35; found C 73.56, N 7.33 H 9.13; 1H-NMR (C6D6, 298 K): δ = 0.88 (s), 1.23 (s), 1.40 (s), 2.10 (s), 3.46 (s), 5.57 (br s), 6.48 (br s), 6.96 (br tr), 11.11 (br s), 12.05 (br s) ppm.

4. Conclusions

In conclusion, a rare example of an inverted-sandwich dichromium(I) complex stabilized by guanidinate ligands was isolated. X-ray analysis confirms the dimeric nature of the compound in which a µ-η6:η6-coordinated toluene molecule bridges the two chromium centers.

Supplementary Materials

Figure S1: 1H NMR spectrum of (2).

Author Contributions

Conceptualization, A.N.; methodology, A.N. and S.Q.; software, A.N. and A.D.; validation, A.N.; formal analysis, A.N., S.Q. and A.D.; investigation, A.N.; resources, A.N.; writing—original draft preparation, A.N.; writing—review and editing, A.N., S.Q., A.D. and M.E.O.; visualization, A.N. and M.E.O.; supervision, A.N.; project administration, A.N. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

All data generated or analyzed during this study are included in this published article. CCDC-2347002 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge at https://www.ccdc.cam.ac.uk/ accessed on 4 October 2024 (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: + 44-1223-336-033; e-mail: [email protected]).

Acknowledgments

Rhett Kempe from University of Bayreuth, Germany, is gratefully acknowledged for allowing access to the laboratory facilities.

Conflicts of Interest

There are no conflicts of interest to declare.

References

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Scheme 1. Synthesis of the dichromium complex.
Scheme 1. Synthesis of the dichromium complex.
Molbank 2024 m1901 sch001
Figure 1. Molecular structure of the Cr complex. Ellipsoids are set at 50% probability; selected bond lengths [Å] and angles [°]: Cr1–N2 2.059(4), Cr1–N1 2.054(4), Cr1–C5 2.339(5), Cr1–C3 2.194(5), Cr1–C2 2.280(5), Cr1–C1 2.273(5), Cr1–C4 2.270(5), Cr1–C6 2.216(5), Cr2–N5 2.046(4), Cr2–N4 2.049(4), Cr2–C5 2.232(4), Cr2–C3 2.294(5), Cr2–C2 2.199(5), Cr2–C1 2.217(6), Cr2–C4 2.226(5), Cr2–C6 2.329(5), N5–C39 1.357(5), N2–C8 1.355(5), N6–C39 1.394(5), N1–C8 1.366(6), N4–C39 1.352(5), N3–C8 1.386(6), C2–C1 1.431(7), C3–C2 1.442(7), C3–C4 1.422(7), C5–C4 1.437(6), C5–C6 1.451(6), C1–C6 1.418(7), C5–C7 1.501(6); N1–Cr1–N2 64.23(15), N5–Cr2–N4 64.90(14), N5–C39–N6 124.6(4), N4–C39–N5 108.4(4), N4–C39–N6 126.9(4), N2–C8–N1 106.9(4), N2–C8–N3 127.0(5) and N1–C8–N3 126.1(4).
Figure 1. Molecular structure of the Cr complex. Ellipsoids are set at 50% probability; selected bond lengths [Å] and angles [°]: Cr1–N2 2.059(4), Cr1–N1 2.054(4), Cr1–C5 2.339(5), Cr1–C3 2.194(5), Cr1–C2 2.280(5), Cr1–C1 2.273(5), Cr1–C4 2.270(5), Cr1–C6 2.216(5), Cr2–N5 2.046(4), Cr2–N4 2.049(4), Cr2–C5 2.232(4), Cr2–C3 2.294(5), Cr2–C2 2.199(5), Cr2–C1 2.217(6), Cr2–C4 2.226(5), Cr2–C6 2.329(5), N5–C39 1.357(5), N2–C8 1.355(5), N6–C39 1.394(5), N1–C8 1.366(6), N4–C39 1.352(5), N3–C8 1.386(6), C2–C1 1.431(7), C3–C2 1.442(7), C3–C4 1.422(7), C5–C4 1.437(6), C5–C6 1.451(6), C1–C6 1.418(7), C5–C7 1.501(6); N1–Cr1–N2 64.23(15), N5–Cr2–N4 64.90(14), N5–C39–N6 124.6(4), N4–C39–N5 108.4(4), N4–C39–N6 126.9(4), N2–C8–N1 106.9(4), N2–C8–N3 127.0(5) and N1–C8–N3 126.1(4).
Molbank 2024 m1901 g001
Table 1. Crystallographic data of the compound.
Table 1. Crystallographic data of the compound.
Empirical FormulaC76H112Cr2N6
Formula weight1213.71
crystal system monoclinic
space groupP21/c
a [Å]15.108(3)
b [Å]29.155(6)
c [Å]13.9971(8)
α [deg]90
β [deg]101.19(3)
γ [deg]90
V, [Å3]7555(3)
Z4
crystal size, [mm3] 0.33 × 0.28 × 0.09
ρcalcd, [g cm−3]1.067
µ, [mm−1] (Mo Kα)0.329
T, [K]133(2)
2θ range, [deg]2.74–49.39
no. of reflections unique14,233
no. of reflections obs. [I > 2σ (I)]5576
no. of parameters739
wR2 (all data) 0.1280
R value [I > 2σ (I)]0.0683
Goodness of fit0.786
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Noor, A.; Qayyum, S.; Dickert, A.; El Oirdi, M. An Inverted-Sandwich Dichromium(I) Complex Stabilized by Guanidinate Ligands. Molbank 2024, 2024, M1901. https://doi.org/10.3390/M1901

AMA Style

Noor A, Qayyum S, Dickert A, El Oirdi M. An Inverted-Sandwich Dichromium(I) Complex Stabilized by Guanidinate Ligands. Molbank. 2024; 2024(4):M1901. https://doi.org/10.3390/M1901

Chicago/Turabian Style

Noor, Awal, Sadaf Qayyum, Andre Dickert, and Mohamed El Oirdi. 2024. "An Inverted-Sandwich Dichromium(I) Complex Stabilized by Guanidinate Ligands" Molbank 2024, no. 4: M1901. https://doi.org/10.3390/M1901

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