Synthesis and Structural Characterization of Manganese(I) Complexes Ligated by 2-Azabutadienes (ArS)₂C=C(H)-N=CPh₂

Abstract

The thioether-functionalized 2-azabutadienes (ArS)₂C=C(H)-N=CPh₂ (L1 Ar = Ph, L2 Ar = p-Tol) ligate to [Mn(CO)₅Br] to form the octahedral five-membered S, N-chelate complexes fac-[MnBr(CO)₃{(ArS)₂C=C(H)-N=CPh₂] (1 Ar = Ph; 2 Ar = p-Tol), whose crystal structures have been solved by X-ray diffraction. Complex 1 crystallizes in the non-centrosymmetric orthorhombic space group P2₁2₁2₁, whereas 2 crystallizes in the triclinic space group P$\overline{1}$. The secondary interactions occurring in the packing have also been assessed by an Atoms in Molecules (AIM) topological analysis.

1. Introduction

2-Azabutadienes R₂C=C(H)-N=CR′₂ (R, R′ = Hal, alkyl, aryl) represent an interesting class of unsaturated compounds since they bear a π-conjugated C=C and C=N array in their scaffold, conferring them a rich potential both in organic chemistry and as ligands in coordination chemistry [1,2,3,4,5,6]. The simplest compound, H₂C=C(H)-N=CH₂, has been the object of several theoretical studies and its occurrence has been spectroscopically evidenced in the atmosphere of Titan [7]. Our research groups are notably working on the design and the reactivity of Cl₂C=C(H)-N=CAr₂ via 1,3-dipolar cycloaddition [8] and have demonstrated that the chlorine atoms therein can be readily substituted by thiolates NaSR, producing thioether-functionalized 2-azabutadienes (RS)₂C=C(H)-N=CAr₂ [9,10,11]. These latter compounds are valuable chelating ligands in coordination chemistry, since they can be chelated as S,N donors to a variety of transition metals and have been recently crystallographically characterized for [CdI₂{(ⁱPrS)₂C=C(H)-N=CPh₂}], [HgI₂{(ⁱPrS)₂C=C(H)-N=CPh₂}], and the dinuclear Cu(I) complex [LCu(μ₂-I₂)CuL] (L = ⁱPrS)₂C=C(H)-N=CPh₂) [12]. In the context of our interest in the photophysical properties of transition metal complexes, we have recently prepared an extended series of octahedral fac-[(OC)₃ReX{(RS)₂C=C(H)-N=CAr₂}] (X = Cl, Br, I, triflate) complexes [13]. In continuation of this project, we have prepared a series of related Mn(I) complexes fac-[(OC)₃MnBr{(RS)₂C=C(H)-N=CPh₂}] and present in this communication their synthesis and crystal structures. Note that in contrast to the numerous [(OC)₃MnX(N∩N)] complexes bearing chelating N,N donor ligands, notably of the α-diimine type [14], only three entries are found in the CSD data base (CSD release on 1 January 2025) [15] for related [(OC)₃MnBr(S∩N)] complexes, namely bromo-tricarbonyl-(2-(phenylsulfanyl)-N-[2-(phenylsulfanyl)ethyl]ethan-1-amine)manganese A (CSD refcode DIXLEE), tricarbonyl-bromo-(2-(phenylthiomethyl)pyridine-N,S)manganese B (CSD refcode IHIMIV) and bromotris(carbonyl)-{2-(ethylsulfanyl)-N-[(quinolin-4-yl)methyl]ethan-1-amine}manganese C (CSD refcode DOKXAG) (Scheme 1). Furthermore, there are two hits for dicarbonyl complexes of the type [(OC)₂MnX{(S∩N∩N)}] such as bromo-dicarbonyl-(N-(2-(methylsulfanyl)phenyl)-1-(pyridin-2-yl) ethanimine)-manganese D (CSD refcode RODKAY) and bromo-dicarbonyl-(1-(6-(4-fluorophenyl)pyridin-2-yl)-N-(2-(methylsulfanyl)phenyl)methanimine)-manganese E (CSD refcode RODKIG), depicted in Scheme 1 [16,17,18,19]. Very recently, the crystal structure of the chelate complex fac-[Mn(CO)₃Br{C₅H₄N-2-NH(P(=S)Ph₂)}] F with a diphenylphosphinesulfide function as the donor has also been reported [20]. Note that there are no entries for any chloro- or iodo manganese S,N-complexes. Interestingly, compounds A and C are also biologically active. For related crystallographically characterized [(OC)₃ReBr{(S∩N)] chelates, see references [21,22].

2. Results and Discussion

Series of complexes fac-[MnBr(CO)₃{(ArS)₂C=C(H)-N=CPh₂}] 1 and 2 was obtained via treatment of [Mn(CO)₅Br] with an equimolar amount of the corresponding azabutadiene ligand L in refluxing chloroform solution, as shown in Scheme 2. Upon cooling, small red crystals of air-stable 1 and 2 were isolated.

The IR spectra of the isolated products recorded in CH₂Cl₂, shown in Figure S1, are very distinctive in the carbonyl region. The infrared spectrum of 1 reveals three intense carbonyl vibrations at 2031 (vs), 1959 (s) and 1923 (s) cm⁻¹, a characteristic pattern for a fac-arrangement of the CO ligands. The ¹H NMR spectrum of 1 is not informative, since the resonance of the vinylic C=C(H), which appears quite distinct in the region at about δ 6.9–7.1 for the free ligands, is superposed by the aromatic signals. In the ¹H NMR spectrum of 2 recorded in CDCl₃, two distinct resonances attributed to methyl groups of the coordinated and uncoordinated p-tolyl substituents are found at δ 2.37 and 2.32 ppm (see Supporting Material).

In addition to the spectroscopic characterization of 1 in solution, the complexes were also unambiguously analyzed using X-ray diffraction performed at 115 K. Overall, the structures are very reminiscent of those of their Re(I) counterparts fac-[(OC)₃ReBr{(ArS)₂C=C(H)-N=CArPh₂}] [13]. Complex 1 crystallizes in the orthorhombic space group P2₁2₁2₁, containing a molecule of dichloromethane, whereas complex 2 crystallizes in the triclinic space group P$\overline{1}$.

Both complexes studied here exhibit a fac-octahedral geometry, in which the facial carbonyl groups, one bidentate S,N–azabutadiene chelate and one bromide form the coordination sphere around the central Mn (I) atom. The second SAr substituent is dangling and does not participate in bonding. The molecular structures are presented in Figure 1 and Figure 2 for 1 and 2, respectively. In order to allow a direct comparison of the molecular geometries, the enantiomer S is shown for 2. Complexation of L2 on Mn(I) does not alter the C=N and C=C bond lengths much with respect to those reported for the free ligand (1.321(3) vs. 1.2959(17) and 1.333(3) vs. 1.3515(18) Å) [10]. The mean Mn-Br bond length of 1 and 2 (2.5301(4) Å) fits with those reported for complexes A (2.5288(4) Å) and B (2.5382(5) Å) shown in Scheme 1. Also, the Mn-SPh distance of 1 (2.3454(7) Å) matches with those reported for complexes A and B (2.3748(6) and 2.3449(6) Å) [16,17].

The most relevant bond lengths around the manganese atom and within the azabutadiene linkage together with selected bond angles observed for the two complexes are presented in

Table 1. Selected bond lengths (Å) and angles (°) in complexes 1 and 2.

	1	2
Mn–Br	2.5326(4)	2.5289(4)
Mn–S	2.3454(7)	2.3227(6)
Mn–N	2.114(2)	2.1565(17)
Mn-CBr	1.801(3)	1.793(2)
Mn–CS	1.816(3)	1.828(2)
Mn-CN	1.808(3)	1.793(2)
N–C1	1.312(3)	1.321(3)
N–C2	1.407(3)	1.408(3)
C2-C3	1.342(4)	1.333(3)
S–Mn–N	83.68(6)	83.43(5)
Br–Mn–CBr	178.11(9)	178.06(7)

C^Br is the carbon atom trans to Br; C^S is the carbon atom trans to S; C^N is the carbon atom trans to N.

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We state that these metric parameters vary very little from one complex to another, indicating that the change in the ligand L does not significantly influence the molecular dimensions. On the other hand, the noncovalent interactions in the packing are different. They are discussed below.

Supramolecular Features

The system of noncovalent interactions giving rise to a supramolecular network is very rich in the crystal structure of 1.

The most frequently encountered mode of noncovalent interactions in 1 is hydrogen bonding. It occurs between the C-H···Br, C-H···O and C-H···Cl (from solvent CH₂Cl₂ molecule) donor···acceptor couples and is partially depicted in Figure 3. The bromine atom is involved in three H-bonds with one solvent molecule and two other adjacent complex molecules. Furthermore, the oxygen atoms of three carbonyl groups are bound to hydrogen atoms of three different complex molecules. The solvent molecule behaves as a donor in the interaction with bromine and as an acceptor in the interaction with a CH group making part of a phenyl ring. These features evidently lead to the formation of a 3D supramolecular network. The lengths of the noncovalent interactions are gathered in Table S1 in the Supplementary Material.

There is also a loose contact of 2.86 Å between the aromatic hydrogen atom H23 of a SPh group and the C2 atom of the azabutadiene linkage, 0.04 Å shorter than the sum of VdW radii. The corresponding CH···π interactions are depicted in Figure 4.

Another type of interaction involving the π systems is also evidenced between a phenyl cycle of the SPh group (atom C14) bound to the metal with one phenyl group of the imine part (atoms C34 and C35) of the neighbor molecule as shown in Figure 5.

In order to verify the presence of π···π interactions, a topological analysis of electron density in the concerned region of the crystal was carried out on the model of 1, in which its innocent part for Ph···Ph π···π interaction containing the BrMn(CO)₃ fragment was neglected. The Quantum Theory of Atoms in Molecules (AIM) was applied for this analysis [23,24]. The molecular graph resulting from this analysis is presented in Figure 6. It reveals the presence of only one bond-critical point corresponding to a C···C interaction at the distance of 3.321 Å. The topological analysis also revealed the presence of S···HC hydrogen bonds and of CH···π interaction involving the C22 ortho carbon atom of the phenyl ring. The CH···S hydrogen bonds are very weak as revealed by the long S···H distances, longer than the sum of van der Waals radii and small electron densities at the bond critical points.

As secondary contacts, CH···Br, CH···O and also CH···S hydrogen bonds are present in the structure of 2. Instead of the π···π interaction detected in 1, a CH-π one is observed in 2. They are shown in Figure 7 and Figure 8.

3. Materials and Methods

[Mn(CO)₅Br] was synthesized as previously reported [25]. The reactions were performed under an argon atmosphere. The ¹H spectra were recorded on a Bruker AC 300 (Bruker, Wissembourg, France) at 300 MHz, using CDCl₃ as the solvent. Infrared spectra were recorded on a Vertex 70 spectrometer (Bruker, Wissembourg, France) using a Platinum ATR accessory equipped with a diamond crystal or on a Bruker Alpha II IRTF (Bruker, Wissembourg, France) spectrometer in solution.

Synthesis of 1 and 2: To a degassed solution of MnBr(CO)₅ (155 mg, 0.5 mmol) in CHCl₃ (5 mL), we added ligands L1 or L2 (0.55 mmol). The resulting mixture was gently refluxed for 36 h and the reaction was followed by IR spectroscopy. The volume of red solution was reduced to ca. 2 mL under reduced pressure. After several days, layering with hexane at 5 °C yielded red needles. X-ray-suitable crystals of 1 were obtained by a second recrystallisation from CH₂Cl₂/heptane. Yield: 69%. Anal. Calc. for C₃₀H₂₁BrMnNO₃S₂ × CH₂Cl₂ (M.W. = 727.40 g.mol⁻¹): C, 51.19; H, 3.19; N, 1.93; S, 8.82%. Found: C, 50.99; H, 3.10; N, 1.80, S, 8.68%. ¹H-NMR (CDCl₃) at 298 K: δ 7.12–7.94 (m, CH Ar and vinylic H) ppm. 2: ¹H-NMR (CDCl₃) at 298 K: δ 7.02–7.83 (m, CH Ar and vinylic H), 2.37 (tol-Me), 2.32 (tol-Me) ppm. Yield: 66%. Anal. Calc. for C₃₂H₂₅BrMnNO₃S₂ (M.W. = 670.53 g.mol⁻¹): C, 57.32; H, 3.76; N, 2.09; S, 9.56%. Found: C, 57.49; H, 3.96; N, 1.90; S, 9.68%.

The X-ray data collections were performed on a Nonius Kappa CCD using the Collect program [26]. The data were reduced with Denzo software [27] without absorption correction. The missing absorption corrections were partially compensated by the data scaling procedure in the data reduction with the Scalpack program [28]. The structures were solved with SIR92 [27]. The refinements of the structures were performed with SHELXL [28,29,30]. All non-hydrogen atoms were refined with anisotropic temperature factors. The hydrogen atoms were placed in calculated positions and refined in an isotropic riding model. A disorder of one tolyl substituent, consisting of a slight twist of a six-membered C₆ ring between two sites, is observed in the structure of 2. The site occupancies were refined to 0.52 and 0.48. The intermolecular interactions were explored with the MERCURY program [31] and the molecular structures were plotted with ORTEP3 [31]. Crystal data and refinement details are gathered in

Table 2. Crystal data, data collection and structure refinement for 1 and 2.

Compound	1	2
Formula	C30H21BrMnNO3S2 × CH2Cl2	C32H25BrMnNO3S2
Formula weight	727.37	670.50
Temperature/K	115(2)	115(2)
Wavelength/Å	0.71073	0.71073
Crystal system	orthorhombic	triclinic
Space group	P212121	P$\overline{1}$
a/Å	11.7945(3)	10.7043(3)
b/Å	14.1405(4)	10.7338(4)
c/Å	18.6642(5)	13.4189(4)
a/°	90	80.9070(10)
β/°	90	77.926(2)
γ/°	90	85.699(2)
Volume/Å3	3112.82(14)	1487.34(8)
Z	4	2
ρ(calc.) g/cm3	1.552	1.497
μ/mm−1	2.047	1.961
F(000)	1464	680
Crystal size/mm	0.18 × 0.11 × 0.11	0.20 × 0.10 × 0.10
θ range for data collection/°	1.81 to 27.47	1.57 to 27.46
Index ranges	−15 ≤ h ≤ 15 −18 ≤ k ≤ 18 −24 ≤ l ≤ 24	−13 ≤ h ≤ 13 −13 ≤ k ≤ 13 −17 ≤ l ≤ 17
Reflections collected	6761	12,170
Independent reflections	6761	6742 [R(int) = 0.0200]
Refl. greater [I > 2σ(I)]	6523	6057
Absorption correction	none	none
Transmission max	0.4419	0.7980
Transmission min	0.3386	0.6494
Refinement method	Full-matrix least squares on F2	Full-matrix least squares on F2
Data/restraints/parameters	6761/0/372	6742/0/388
Goodness-of-fit on F2	1.138	1.101
Flack parameter	0.045(6)	-
Final R indexes [I > 2σ(I)]	R1 = 0.0277 wR2 = 0.0580	R1 = 0.0300 wR2 = 0.0612
R indexes (all data)	R1 = 0.0306 wR2 = 0.0603	R1 = 0.0370 wR2 = 0.653
Largest diff. peak and hole/e. Å−3	0.293 and −0.306	0.399 and−0.346

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Data were collected using graphite-monochromated MoK_α radiation λ = 0.71073 Å and were deposited at the Cambridge Crystallographic Data Centre as 2457097 (1) and 2457098 (2) (Supplementary Materials). The data can be obtained free of charge from the Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/getstructures (accessed on 26 June 2025).

The wave functions for AIM topological analyses were calculated with GW09-W package [32,33]. The 6-311G (d,p) standard basis set was applied for all, except for the manganese atom. Since the AIM theory should preferentially use the all-electron and not the ECP (effective core potential) basis sets, the double zeta polarized DZP-DKH basis sets of Jörge were chosen for the Mn atom [34]. Because we were interested in comparative interpretation of metric parameters found in the structures, these calculations were performed on the molecular geometries really present in the crystal structures of 1 and 2. Topological parameters were calculated using the AIMALL packages [35]. The calculations were performed either at the Centre de Calculs de Université de Bourgogne (DNUM) for AIMALL or on a local PC for wave functions.

4. Conclusions

We have demonstrated that the addition of the thioether-functionalized azabutadienes L1 and L2 affords in a carbonyl substitution reaction the structurally characterized octahedral S,N-chelated complexes 1 and 2, in which only one of the SAr groups is ligated to the metal. The propensity of the second non-coordinated SAr group for binding to other soft metal ions such as Cu(I), Ag(I) or Hg(II) may allow the assembly of heterometallic complexes in on-going studies.