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

Two isomers of the molybdenum(II) complex Mo(CO)(C2Me2)(S-Phoz)2 [S-Phoz is 2-(4,4-dimethyloxazolin-2-yl)thiophenolate] have been synthesized and characterized by X-ray diffraction at 100 K and by spectroscopy (NMR and IR). They show quite different Mo—N and Mo—S distances.


Introduction
In order to explore the interaction of Mo and W centres with acetylene (C 2 H 2 ), which is accepted as a substrate by the tungstoenzyme acetylene hydratase (Schink, 1985;Rosner & Schink, 1995), our group has focused on the synthesis of W II and Mo II complexes containing bioinspired S,N-bidentate ligands and their subsequent oxidation to the respective W IV and Mo IV 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 Ehweiner et al., 2021c), the isolation of a so-far-elusive Mo IV C 2 H 2 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 C 2 H 2 at a W IV centre coordinated by two 2-(4,4-dimethyloxazolin-2-yl)thiophenolate (S-Phoz) ligands . Thereafter, the reversible binding of C 2 Me 2 and C 2 Ph 2  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( 2 -1,2-dimethylethyne)[2-(4,4-dimethyloxazolin-2-yl)benzenethiolato-2 N,S]molydbenum(II), Mo-(CO)(C 2 Me 2 )(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 MoI 2 -(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). 1 H 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 À1 . 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.
crystals suitable for X-ray diffraction were obtained from CH 2 Cl 2 /heptane solutions at À35 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.

Refinement
Crystal data, data collection, and structure refinement details are summarized in Table 1. The H atoms of the CH 2 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 .

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.
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 ( Comparing all known structures of M(CO)(C 2 R 2 )(S-Phoz) 2 complexes (Table 2), the following conclusions can be made: whereas N,N-trans conformations for R = H and CH 3 , and S,Strans conformations for R = Ph were observed  for the W complexes, both conformations were found in the first two crystal structures of the analogous Mo complexes with R = CH 3 . 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.

NMR spectroscopy
1 H NMR spectra recorded in CD 2 Cl 2 and CD 3 CN show a 1:2 ratio of the two isomers of Mo(CO)(C 2 Me 2 )(S-Phoz) 2 , while a 1:1 ratio is observed in CDCl 3 . The NMR data of isomer 2, which presumably adopts the N,N-trans configuration, are almost identical with those of the W analogue , of which only the N,N-trans isomer was crystallized. In CD 2 Cl 2 solutions, the two isomers of W(CO)-(C 2 Me 2 )(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)-(C 2 Me 2 )(S-Phoz) 2 shows a very strong band at 1898 cm À1 which is attributed to the C O bond. Due to weaker -backbonding of the Mo centre, this bond is stronger by 18 cm À1 compared to that in the respective W compound , 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. Table 2 Selected geometric parameters (Å , ) for M(CO)(C 2 R 2 )(S-Phoz) 2 complexes.

N,N-cis-(η 2 -But-2-yne)carbonylbis[2-(4,4-dimethyl-\ 4,5-dihydro-1,3-oxazol-2yl)benzenethiolato]molybdenum(II) (1)
Crystal data  Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.
The non-hydrogen atoms were refined with anisotropic displacement parameters without any constraints.
The H atoms of the CH 2 groups were put at positions with approx. 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 (AFIX 23 of SHELXL). The H atoms of the phenyl rings were put 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 (AFIX 43 of SHELXL).
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 C-H distances of 0.98 Å (AFIX 137 of SHELXL).

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Mo1 0.89263 (2) 0.62618 (2)  0.0200 (7) 0.0277 (7) 0.0179 (6) 0.0063 (6) −0.0065 (5) −0.0030 (5)  C25 0.0229 (7) 0.0256 (7) 0.0177 (6)   Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger. The non-hydrogen atoms were refined with anisotropic displacement parameters without any constraints. The H atoms of the CH 2 groups were put at positions with approx. 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 (AFIX 23 of SHELXL). The H atoms of the phenyl rings were put 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 (AFIX 43 of SHELXL). 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 C-H distances of 0.98 Å (AFIX 137 of SHELXL).  (12)