Synthesis and structure of two isomers of a molybdenum(II) 2-butyne complex stabilized by bioinspired S,N-bidentate ligands.

The synthesis and structural determination of two isomers of the molybdenum(II) complex (η2-but-2-yne)carbonylbis[2-(4,4-dimethyl-4,5-dihydro-1,3-oxazol-2-yl)benzenethiolato-κ2N,S]molybdenum(II), [Mo(C11H12NOS)2(C4H6)(CO)] or Mo(CO)(C2Me2)(S-Phoz)2, are presented. The N,N-cis-S,S-trans isomer 1 shows quite different bond lengths to the metal atom [Mo-N = 2.4715 (10) versus 2.3404 (11) Å; Mo-S = 2.4673 (3) versus 2.3665 (3) Å]. In the N,N-trans-S,S-cis isomer 2, which is isotypic with the corresponding W complex, the Mo-N bond lengths [2.236 (2) and 2.203 (2) Å], as well as the Mo-S bond lengths [2.5254 (8) and 2.5297 (8) Å], are almost the same. open access.

Introduction

In order to explore the inter­action of Mo and W centres with acetyl­ene (C2H2), which is accepted as a substrate by the tungstoenzyme acetyl­ene hydratase (Schink, 1985 ▸; Rosner & Schink, 1995 ▸), our group has focused on the synthesis of WII and MoII com­plexes containing bioinspired S,N-bidentate ligands and their subsequent oxidation to the respective WIV and MoIV com­plexes. Although N-donor ligands are not the closest structural mimics of the di­thiol­ene ligands in the active site of acetyl­ene hydratase (Seiffert et al., 2007 ▸) and other members of the dimethyl sulfoxide (DMSO) reductase enzyme family (Seelmann et al., 2020 ▸), the use of these ligands has resulted in the discovery of new reactivities at W centres (Vidovič et al., 2019 ▸; Ehweiner et al., 2021c ▸), the isolation of a so-far-elusive MoIV C2H2 com­plex (Ehweiner et al., 2021a ▸) and a detailed com­parison of W and Mo com­plexes with a variety of coordinated alkynes (Ehweiner et al., 2021b ▸). One of the early publications of our group in this research field focused on the reversible activation of C2H2 at a WIV centre coordin­ated by two 2-(4,4-di­methyl­oxazolin-2-yl)thio­pheno­late (S-Phoz) ligands (Peschel et al., 2015a ▸). Thereafter, the re­ver­sible binding of C2Me2 and C2Ph2 (Peschel et al., 2019 ▸) was investigated, with a particular focus on the flexibility of the S-Phoz ligand. The latter has also found application in Ni, Pd and Pt com­pounds (Peschel et al., 2015b ▸; Holzer et al., 2018 ▸), as well as in Zn (Mugesh et al., 1999 ▸) and Fe (Bottini et al., 2010 ▸) com­plexes.

Herein we report an improved synthetic procedure for Mo(CO)2(S-Phoz)2 and the preparation and structural characterization of carbon­yl(η2-1,2-di­methyl­ethyne)[2-(4,4-di­methyl­oxazolin-2-yl)benzene­thiol­ato-κ2 N,S]molydbenum(II), Mo(CO)(C2Me2)(S-Phoz)2, which forms two isomers (1 and 2) in solution, as well as in the solid state (see Scheme 1). This behaviour is different from that observed for the W variant which crystallized solely as the N,N-trans isomer and showed the presence of a second isomer in solution only to a minor extent.

Experimental

Synthetic manipulations were performed under a nitro­gen atmosphere using standard Schlenk and glove-box techniques. Solvents were purified via a Pure Solv Solvent Purification System. Chemicals were purchased from commercial sources and used without further purification. The precursor MoI2(CO)3(NCMe)2 was synthesized according to a literature procedure (Baker et al., 1986 ▸). For the synthesis of Mo(CO)2(S-Phoz)2, a slight modification of a published procedure was used (Peschel et al., 2013 ▸). 1H NMR spectra were recorded on a Bruker Avance III 300 MHz spectrometer at ambient tem­per­ature and are referenced to residual protons in the solvent. The multiplicity of peaks is denoted as singlet (s), doublet (d), doublet of doublets (dd) or multiplet (m). NMR solvents were stored over mol­ecular sieves. Solid-state IR spectra were measured on a Bruker ALPHA ATR–FT–IR spectrometer at a resolution of 2 cm−1. The relative intensity of signals is declared as strong (s), medium (m) and weak (w). Electron impact mass spectroscopy (EI–MS) measurements were performed with an Agilent 5973 MSD mass spectrometer with a push rod.

Synthesis and crystallization

Preparation of Mo(CO)2(S-Phoz)2

A solution of Li(S-Phoz) (853 mg, 4.00 mmol) in MeCN (8 ml) was added dropwise to a solution of MoI2(CO)3(NCMe)2 (1.03 g, 2.00 mmol) in MeCN (8 ml). The resulting blood-red solution was stirred for 2 h at 35 °C, whereupon the solvent was removed by evaporation. The residue was suspended in toluene (20 ml) and the resulting suspension was filtered through Celite. The blood-red filtrate was then evaporated to dryness. After repeated recrystallization from CH2Cl2/heptane at −25 °C, Mo(CO)2(S-Phoz)2 (yield 790 mg, 70%) was ob­tained as dark red crystals. NMR and IR data are in agreement with previously published results (Peschel et al., 2013 ▸).

Preparation of Mo(CO)(C2Me2)(S-Phoz)2

Mo(CO)2(S-Phoz)2 (339 mg, 0.60 mmol) was dissolved in CH2Cl2 (20 ml), whereupon 2-butyne (0.38 ml, 4.80 mmol) was added to the solution at 0 °C under stirring. The cooling bath was removed and the solution was heated under reflux for 24 h. Evaporation of the solvent gave a dark brown powder. Single crystals suitable for X-ray diffraction were obtained from CH2Cl2/heptane solutions at −35 °C. Crystals of both isomers (green plates of 1 and yellow needles of 2) were obtained from the same batch. The product is very sensitive to air and should be stored in a glove-box.

Analytical data

1H NMR for 1 (CD2Cl2, 300 MHz, S,S-trans isomer, 34%): δ 8.07 (dd, J = 8.1, 1.1 Hz, 1H, PhH), 7.78–7.72 (m, 3H, PhH), 7.35 (dd, J = 7.8, 1.1 Hz, 1H, PhH), 7.32–7.27 (m, 2H, PhH), 7.21–7.01 (m, 1H, PhH), 4.46 (d, J = 8.2 Hz, 1H, CH2), 4.18 (d, J = 8.1 Hz, 1H, CH2), 4.11 (d, J = 8.3 Hz, 1H, CH2), 3.78 (d, J = 8.2 Hz, 1H, CH2), 2.70 (s, 3H, C≡CCH3), 2.55 (s, 3H, C≡CCH3), 1.89 (s, 3H, CH3), 1.81 (s, 3H, CH3), 1.57 (s, 3H, CH3), 1.44 (s, 3H, CH3); 1H NMR for 2 (CD2Cl2, 300 MHz, N,N-trans isomer, 66%): δ 7.67–7.62 (m, 2H, PhH), 7.43 (dd, J = 8.1, 1.4 Hz, 1H, PhH), 7.21–7.01 (m, 4H, PhH), 6.90–6.84 (m, 1H, PhH), 4.11 (d, J = 8.3 Hz, 1H, CH2), 3.93–3.90 (m, 3H, CH2), 2.90 (s, 3H, C≡CCH3), 2.46 (s, 3H, C≡CCH3), 1.63 (s, 3H, CH3), 1.34 (s, 3H, CH3), 0.77 (s, 3H, CH3), 0.58 (s, 3H, CH3). IR (cm−1): 2995 (w), 2962 (w), 2928 (w), 2916 (w), 2894 (w), 1898 (s, C≡O), 1856 (m, C≡O), 1590 (s), 1572 (s), 1539 (m, C=N), 1455 (m), 1357 (m), 1326 (m), 1280 (m), 1246 (m), 1208 (m), 1160 (m), 1139 (m), 1053 (s), 966 (m), 818 (m), 776 (m), 741 (s), 695 (m), 653 (m). EI–MS (70 eV) m/z: [M – 2CO + O]+ 526.1.

Refinement

Crystal data, data collection, and structure refinement details are summarized in Table 1 ▸. The H atoms of the CH2 groups were placed at positions with approximately tetra­hedral angles and C—H distances of 0.99 Å, and common isotropic displacement parameters were refined for the H atoms of the same group. The H atoms of the arene rings were placed at the external bisectors of the C—C—C angles at C—H distances of 0.95 Å, and common isotropic displacement parameters were refined for the H atoms of the same ring. The H atoms of the methyl groups were refined with common isotropic displacement parameters for the H atoms of the same group and idealized geometries with tetra­hedral angles, enabling rotations around the C—C bonds, and with C—H distances of 0.98 Å.

Table 1

Experimental details

For both structures: [Mo(C11H12NOS)2(C4H6)(CO)], M r = 590.59, Z = 4. Experiments were carried out at 100 K with Mo Kα radiation using a Bruker APEXII CCD diffractometer. Absorption was corrected for by multi-scan methods (SADABS; Bruker, 2013 ▸). Refinement was on 332 parameters. Only H-atom displacement parameters were refined.

	(1)	(2)
Crystal data
Crystal system, space group	Monoclinic, P21/n	Monoclinic, P21/c
a, b, c (Å)	10.6159 (5), 8.9300 (4), 27.3801 (12)	9.1512 (4), 21.3515 (12), 13.1781 (7)
β (°)	96.189 (2)	98.483 (3)
V (Å3)	2580.5 (2)	2546.7 (2)
μ (mm−1)	0.70	0.71
Crystal size (mm)	0.18 × 0.18 × 0.10	0.23 × 0.07 × 0.07
Data collection
T min, T max	0.884, 1.000	0.776, 1.000
No. of measured, independent and observed [I > 2σ(I)] reflections	30042, 11363, 9549	22009, 7415, 5339
R int	0.029	0.068
(sin θ/λ)max (Å−1)	0.807	0.703
Refinement
R[F 2 > 2σ(F 2)], wR(F 2), S	0.028, 0.071, 1.04	0.043, 0.087, 1.01
No. of reflections	11363	7415
Δρmax, Δρmin (e Å−3)	0.72, −0.64	0.52, −0.83

Computer programs: APEX2 (Bruker, 2013 ▸), SAINT (Bruker, 2013 ▸), SHELX97 (Sheldrick, 2008 ▸), SHELXL2014 (Sheldrick, 2015 ▸) and modified ORTEP (Johnson, 1965 ▸).

Results and discussion

Crystal structure analysis

Isomers 1 and 2 crystallize without any solvent mol­ecules in the monoclinic space groups P21/n and P21/c, respectively, and both have one metal com­plex in the asymmetric unit. In N,N-cis isomer 1 (Fig. 1 ▸), the Mo—N distance of the oxazole ring trans to the butyne ligand [Mo1—N13 = 2.4715 (10) Å] is much longer than that trans to the carbonyl ligand [Mo1—N33 = 2.3404 (11) Å]. In N,N-trans isomer 2 (Fig. 2 ▸), these distances [Mo1—N13 = 2.236 (2) Å and Mo1—N33 = 2.203 (2) Å] are com­parable to those observed in the dicarbonyl derivative [2.2333 (9) Å; Peschel et al., 2013 ▸] or in the isotypic W com­pound [W1—N13 = 2.2153 (16) Å and W1—N33 = 2.1862 (16) Å; Peschel et al., 2019 ▸]. In contrast to this, the Mo—S distances of the benzene­thiol­ate residues in isomer 1 are significantly different, although they are trans to one another, and both are clearly shorter [Mo1—S1 = 2.4673 (3) Å and Mo1—S2 = 2.3665 (3) Å] than in isomer 2 [Mo1—S1 = 2.5254 (8) Å and Mo1—S2 = 2.5297 (8) Å] or in the W com­pound [W—S = 2.5232 (4)–2.5243 (4) Å]. On the other hand, in both isomers, the distances are almost the same between the central atom and the butyne ligands [2.0310 (12)–2.0664 (12) versus 2.024 (3)–2.059 (3) Å] and to the carbonyl ligands [1.9417 (13) versus 1.953 (3) Å], although both are arranged in trans positions with respect to the N atoms of the oxazole rings in 1, and trans to the S atoms of the benzene­thiol­ate groups in 2. In both isomers, the CO ligands [C3—O3 = 1.1555 (16) and 1.157 (3) Å] lie roughly in the best planes through the butyne ligands [C1—C2 = 1.2965 (18) and 1.314 (4) Å] and the Mo atoms.

Figure 1

The mol­ecular structure of isomer 1. Displacement ellipsoids are drawn at the 50% probability level and H atoms have been omitted for clarity. The rather long Mo—N distance [Mo1—N13 = 2.4715 (10) Å] is indicated by a dashed line.

Figure 2

The mol­ecular structure of isomer 2. Displacement ellipsoids are drawn at the 50% probability level and H atoms have been omitted for clarity.

Comparing all known structures of M(CO)(C2 R 2)(S-Phoz)2 com­plexes (Table 2 ▸), the following conclusions can be made: whereas N,N-trans conformations for R = H and CH3, and S,S-trans conformations for R = Ph were observed (Peschel et al., 2015a ▸, 2019 ▸) for the W com­plexes, both conformations were found in the first two crystal structures of the analogous Mo com­plexes with R = CH3. In general, the Mo—N distances are clearly longer in the S,S-trans conformers, and slightly longer for the S-Phoz ligands trans to the alkyne ligands than those trans to the carbonyl ligand (e.g. M—N13 is larger than M—N33). In isomer 1, the Mo—N distance of the S-Phoz ligand trans to the butyne ligand is exceptionally large due to the wide C1—Mo1—N13 angle of 173.53 (4)° and large C—M—S1 angles. The Mo—S distances are the same in the N,N-trans conformers, but in the S,S-trans conformers, M—S1 is distinctly longer than M—S2. Therefore, the S-Phoz ligands whose oxazole rings are trans to the alkyne ligands are more weakly bound to the metal centre than the others. In all six com­plexes (Table 2 ▸), the M—C1 distance is significantly shorter than M—C2, presumably due to the carbonyl ligand near atom C2.

Table 2

Selected geometric parameters (Å, °) for M(CO)(C2 R 2)(S-Phoz)2 com­plexes

The labels C1 and C2 of the alkyne ligand were choosen such that the torsion angle C2—C1—M—C3 is approximately 0°. The selected ligand containing atoms S1 and N13 was that in which one of these atoms is trans to the alkyne ligand.

M, R	W, H a	W, CH3 b	Mo, CH3 c	Mo, CH3 c	W, Ph b	W, Ph b
	N,N-trans	N,N-trans	N,N-trans	S,S-trans	S,S-trans	S,S-trans
M—C1	2.0268 (17)	2.0210 (17)	2.024 (3)	2.0310 (12)	2.0510 (19)	2.036 (4)
M—C2	2.0548 (18)	2.0565 (17)	2.059 (3)	2.0664 (12)	2.078 (2)	2.057 (4)
M—C3	1.9623 (18)	1.9535 (19)	1.953 (3)	1.9417 (13)	1.949 (2)	1.966 (4)
C3—O3	1.160 (2)	1.164 (2)	1.157 (3)	1.1555 (16)	1.155 (3)	1.154 (5)
M—N13	2.2120 (14)	2.2153 (16)	2.236 (2)	2.4715 (10)	2.3087 (18)	2.350 (3)
M—N33	2.1987 (14)	2.1862 (16)	2.203 (2)	2.3404 (11)	2.2975 (17)	2.304 (4)
M—S1	2.5050 (4)	2.5232 (4)	2.5254 (8)	2.4673 (3)	2.4620 (5)	2.4741 (12)
M—S2	2.5067 (4)	2.5243 (4)	2.5297 (8)	2.3665 (3)	2.3698 (5)	2.3773 (11)
C1—C2	1.327 (3)	1.314 (3)	1.314 (4)	1.2965 (18)	1.309 (3)	1.305 (6)
N13—M—N33	169.58 (5)	167.56 (6)	168.04 (8)	92.41 (3)	83.29 (6)	86.47 (13)
S1—M—S2	78.869 (14)	78.972 (15)	79.54 (3)	162.979 (11)	175.564 (18)	169.56 (4)
C1—M—N13	92.88 (6)	97.14 (7)	96.97 (9)	173.53 (4)	165.94 (7)	169.64 (15)
C2—M—N13	93.66 (7)	94.92 (6)	94.67 (9)	146.80 (4)	150.09 (7)	148.68 (15)
C3—M—N33	94.24 (6)	94.52 (7)	94.51 (9)	168.19 (4)	159.92 (8)	164.04 (15)
C1—M—S1	164.79 (6)	164.06 (5)	163.76 (8)	97.54 (3)	85.61 (5)	91.62 (13)
C2—M—S1	153.79 (6)	156.79 (5)	156.87 (8)	96.29 (3)	87.98 (5)	89.33 (12)
C3—M—S2	163.06 (5)	166.23 (5)	167.27 (9)	85.88 (4)	87.58 (6)	87.74 (14)

References: (a) Peschel et al. (2015a ▸); (b) Peschel et al. (2019 ▸); (c) this work.

NMR spectroscopy

1H NMR spectra recorded in CD2Cl2 and CD3CN show a 1:2 ratio of the two isomers of Mo(CO)(C2Me2)(S-Phoz)2, while a 1:1 ratio is observed in CDCl3. The NMR data of isomer 2, which presumably adopts the N,N-trans configuration, are almost identical with those of the W analogue (Peschel et al., 2019 ▸), of which only the N,N-trans isomer was crystallized. In CD2Cl2 solutions, the two isomers of W(CO)(C2Me2)(S-Phoz)2 exhibit a 95:5 ratio, with a clear preference for the N,N-trans configuration of isomer 2.

IR spectroscopy

The IR spectrum of an average sample of Mo(CO)(C2Me2)(S-Phoz)2 shows a very strong band at 1898 cm−1 which is attributed to the C≡O bond. Due to weaker π-backbonding of the Mo centre, this bond is stronger by 18 cm−1 com­pared to that in the respective W com­pound (Peschel et al., 2019 ▸), which is in accordance with previous observations on Mo and W carbonyl com­plexes (Ehweiner et al., 2021a ▸,b ▸,c ▸). Despite the existence of two isomers, only one C≡O bond is visible.

Crystal structure: contains datablock(s) 1, 2, global. DOI: 10.1107/S2053229622002029/wv3008sup1.cif

Structure factors: contains datablock(s) 1. DOI: 10.1107/S2053229622002029/wv30081sup2.hkl

Structure factors: contains datablock(s) 2. DOI: 10.1107/S2053229622002029/wv30082sup3.hkl

CCDC references: 2153636, 2153635