Structural, Non-Covalent Interaction, and Natural Bond Orbital Studies on Bromido-Tricarbonyl Rhenium(I) Complexes Bearing Alkyl-Substituted 1,4-Diazabutadiene (DAB) Ligands

: The synthesis, characterization, structural and computational studies of Re(I) tricarbonyl bromo complexes bearing alkyl-substituted 1,4-diazabutadiene ligands, [Re(CO) 3 (1,4-DAB)Br], where 1,4-DAB = N,N -bis(2,4-dimethylbenzene)-1,4-diazabutadiene, 2,4-Me2 DAB ( 1 ); N,N -bis(2,4-dimethylbenzene)-2,3-dimethyl-1,4-diazabutadiene, 2,4-Me2 DAB Me ( 2 ); N,N -bis(2,4,6-trimethylbenzene)-1,4-diazabutadiene, 2,4,6-Me3 DAB ( 3 ); and N , N -bis(2,6-diisopropylbenzene)-1,4-diazabutadiene, 2,6-ipr 2 DAB ( 4 ) are reported. The complexes were characterized by different spectroscopic methods such as FT-IR, 1 H-NMR, 13 C-NMR, and elemental analyses and their solid-state structures were confirmed by X-ray diffraction. In each complex, the Re(I) centre shows a distorted octahedral shape with a facial geometry of carbonyl groups. The gas phase geometry of the complexes was identified by density functional theory. Interesting intermolecular n … π* interactions of complexes 1 and 3 were investigated by non-covalent interaction index (NCI), and natural bond orbital (NBO) analyses. The intramolecular n … σ*, σ … π*, π … σ* interactions were also studied in complexes 3 and 4 .


Introduction
Transition metal carbonyl complexes have become one of the most important classes of coordination compounds in inorganic chemistry. These complexes are not only a subject of interest for basic synthesis and study in academic research but are also very important as homo-and heterogeneous catalysts in industry. The chemical bonding in transition metal carbonyl complexes themselves, or in metal carbonyl bearing diimine ligands, is based on the classical concept of synergistic σ-donation and π-back donation between the ligand (carbonyl or diimine) and the metal, introduced by Dewar-Chatt-Duncanson in 1951. An understanding of such properties of transition metal carbonyl complexes helps produce required knowledge of the properties of the molecular orbitals, spectra, and appropriate excited states [1]. Among different metal carbonyl complexes, Re(I)tricarbonyl complexes with diimine ligands of the type [Re(CO)3(α-diimine)(X)] 0/+ , in which X is a halide, bridging ligand, organic donor/acceptor, nitrogen donor or some other monodentate or ambidentate ligands, have been the subject of much attention, mainly because of their photophysical and photochemical properties [2][3][4] and their use in the photoreduction and electroreduction of CO2 to CO [5,6], a key process in the conversion and storage of solar energy as a model in natural photosynthesis, and in supramolecular chemistry and catalysis [7][8][9][10]. The photo-behaviour of these complexes may be interpreted in terms of three types of excited states: metal-to-ligand charge transfer (MLCT) states, ligand-to-ligand charge transfer (LLCT) states, and intra-ligand (IL) states [11][12][13][14][15][16]. On the other hand, the spectroscopic properties of the Re(I)-tricarbonyl complexes are ligand-dependent and can be tuned by changing the chelated diimine and/or axial ligands. As such, the quantitative description of the electronic properties of diimine ligands based on their σ-donation and π-back donation nature can clarify such properties. By changing the electronic and steric effects in the DAB ligands in their Ni(II) and Pd(II) complexes, they can be utilized as efficient catalysts in alkene polymerization [17,18]. The tuning of such electronic and steric effects has been examined before in some iminophenol complexes [19,20]. Besides all the aforementioned properties, one of the interesting features of metal carbonyl complexes is the presence of intra-and/or intermolecular n … π* interactions, which were not the subject of much attention experimentally and theoretically until their importance was noticed by Echeverría [21]. Since its introduction by Burgi-Dunitz, the so-called Burgi-Dunitz trajectory in the geometrical reaction coordinates in the nucleophilic addition to carbonyl group, most of the studies were focused on organic and biological systems [22][23][24][25][26][27][28][29][30][31][32].
It has been demonstrated that M-CO(lone pair)⋯π interactions are relevant in the structures of a number of transition metal carbonyl complexes, and they have important effects on their internal geometry in the related complexes and supramolecular interactions of metal carbonyl complexes [33,34]. In spite of their inherently weak nature, M-CO(lone pair)⋯π* interactions stabilize precise molecular conformations that maximize the overlap between the involved donor and acceptor orbitals in the interaction and can also provide a measure of stability to their crystal structures and lead to supramolecular architectures [35].

General Methods
All chemicals used were analytical reagent grade. All solvents purchased from Merck were reagent grade and purified by standard techniques where required. CH3CN was distilled over P2O5 for synthesis. Commercially available Re(CO)5Br from Aldrich was used as received. The 1 H-NMR and 13 C-NMR (125 MHz) spectra were recorded using a BRUKER AVANCE 500 MHz spectrometer in CDCl3. IR spectra in the region of 4000-400 cm −1 were recorded in KBr pellets with a Shimadzu IRPrestige-21 FTIR spectrophotometer. Electronic absorption spectra were measured with a Rayleh 5E spectrophotometer in dichloromethane solutions. The elemental analyses were done using a LECO CHNS instrument. The preparation of all Re(I) complexes (Scheme 1) was achieved using the previously reported procedure based on phenanthroline-type ligands, except that DAB ligands were used [41]. The FTIR, 1 H-NMR, and 13 C{ 1 H}-NMR spectra of the complexes are listed in the supplementary materials. The DAB ligands, L1-L4, were prepared by condensation of glyoxal or diacetyl with the appropriate primary amine, according to literature procedures (see Supporting Information) [42]. The analytical data on L1-L4 are shown in Figures S1-S8.  (1). A mixture of Re(CO)5Br (203 mg, 0.5 mmol) and 2,4-Me 2DAB (132 mg, 0.5 mmol) in a mixture of CH2Cl2 (10 mL) and toluene (30 mL) was heated at reflux for 4 h to give a dark-brown solution. The volume of the solution was reduced to 10 ml and by addition of cold nhexane the complex was precipitated. The crude material recrystallized from CH2Cl2/hexane to give the complex as a pure dark-brown microcrystalline powder.

X-Ray Crystallography
Single crystals of 1-4, suitable for X-ray diffraction analysis, were grown by slow vapor diffusion of n-hexane into a dichloromethane solution of the complexes. X-ray intensity data were collected using the full-sphere routine by φ and ω scans strategy on the Agilent SuperNova dual wavelength EoS S2 diffractometer with mirror monochromated Mo Kα radiation (λ = 0.71073 Ǻ) for 1, 2, and 4 and with Cu Kα radiation for 3. For all data collections, the crystals were cooled to 150(2) K using an Oxford diffraction Cryojet low-temperature attachment. The data reduction, including an empirical absorption correction using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm, was performed using the CrysAlisPro software package [43]. The crystal structures of 1-4 were solved by direct methods using the online version of AutoChem 2.0 in conjunction with the OLEX2 suite of programs [44] implemented in the CrysAlis software, and then refined by full-matrix least-squares (SHELXL-2018) on F 2 [45]. The non-hydrogen atoms were refined anisotropically. All of the hydrogen atoms were positioned geometrically in idealized positions and refined with the riding model approximation, with Uiso(H) = 1.2 or 1.5 Ueq(C). For the molecular graphics, the program SHELXTL was used [45]. All geometric calculations were carried out using the PLATON software [46]. The full crystal data, bond lengths and angles are listed in the supplementary materials.

Computational Details
Density functional theory (DFT) calculations have been performed using the Gaussian09 package [47] to perform geometry optimizations for vibrational frequencies and electronic structures of complexes 1-4. The structures of all complexes were optimized using the computational model (PBE1PBE) by combining the Perdew-Burke-Erzenrhof with the quasi-relativistic Stuttgart-Dresden (SDD) effective core pseudopotential (ECP) and corresponding set of basic functions for the Re atom and 6-31G* (five pure d functions) for C, H, N, O, and 6-311+G* for Br [48,49].

Synthesis and Characterization
The new [(NN)Re(CO)3X] (NN = diazabutadiene) complexes were synthesized via substitution reaction from the DAB ligands and Re(CO)5Br in a mixture of CH2Cl2 and toluene under reflux condition. Complexes 1-4 were recrystallized from a mixture of CH2Cl2/n-hexane, and the purity of all complexes was confirmed by elemental analyses. The characteristic feature of the complexes incorporating fac-[Re(CO)3] + by losing two carbonyl groups in cis positions is the appearance of three intense carbonyl bands at about 2050-1880 cm -1 , including a sharp intense band at about 2030-2050 cm −1 and two closely spaced lower energy bands consistent with the A ′ (cis), A ′′ (cis), and A ′ (trans) modes expected in Cs symmetry, with energy ordering A ′ (cis) >A ′′ (cis) > A ′ (trans) as reported by Cotton-Karihanzel based on force field calculation [50][51][52]. As a normal trend, the position of the absorption bands is influenced by the electronic nature of the axial ligand. Normally, with the weakly donating ligands in the axial position, the ν(CO) is further increased. The electronic nature of the axial ligand (X) in fac-Re(CO)3(NN)(X) complexes influences merging or splitting of the lower energy bands of the carbonyl groups. A stronger π-acceptor ligand in the basal plane shifts the CO stretching bands to higher frequencies. For complexes 2 and 4, by merging two lower energy bands, there seems to be an approximate C3v spectroscopic symmetry, leading to a virtual "E" band. The FT-IR spectra of complexes 1-4 are shown in Figures S9-S12. The stretching frequencies of the complexes are listed in the experimental section. The 1 H-NMR and 13 C{ 1 H}-NMR spectra of complexes 1-4 are shown in Figures S13-S20.

X-ray Crystal Structures
The solid-state structures of complexes 1-4 were determined by X-ray crystallography and are shown with their atom labelling scheme in Figure 1. Details of data collection and refinement parameters are given in Table 1. Selected bond lengths and angles for complexes 1-4 are listed in Table 2. The details of the hydrogen bonding interactions are listed in Table 3.   The geometry around the Re(I) is a distorted octahedron involving the carbonyl groups in facial arrangement, a DAB ligand and axial bromo group. In complex 2, the methyl group in the ortho position was disordered over two positions with a refined site-occupancy ratio of 0.81(2)/0.19 (2). The most significant angular distortion is associated with the bite angles of the DAB ligands N(1)-Re(1)-N(2) in the range of 73.3(4)-75.62(12)° which is due to the formation of the strained five-membered chelate ring. The trans angle C(1)-Re(1)-Br(1) in 1-4 falls in the range of 173.55(12)-177.7(4)°, respectively. The bond lengths, angles and coordination geometry of the crystal structures in 1-4 are similar to those structures reported previously [53,54]. There is only one report related to the crystal structure of 1,4-alkyldiazabutadiene, namely 1,4-di(tert-butyl) diazabutadiene, which shows significantly different Re-N [2.170 (15) (3), and 1.01(3) Å] bond lengths compared to 1,4-aryldiazabutadiene, but the Re-Br bond length is similar to complexes 1-4 [13]. The interesting feature of complexes 1 and 3 is the intermolecular n … π* interactions which connects neighbouring molecules into a 1-D extended chain along the a-and b-axis, respectively (Figures 2 and 3). As it is summarized in Table 3 (1) 99.45 (15)   Symmetry codes: (i) 1 -x, 1-y, 1 -z (ii) 1 + x, 1 + y, z (iii) ½ + x, ½ -y, -1/2 + z (iv) 1 -x, ½ + y, -1 -z.

Non-Covalent Interaction Index and Natural Bond Orbitals (NBOs)
Non-covalent interactions were evaluated using the non-covalent index (NCI) approach, which relies on the topological analysis of the electron density and its derivatives at low density regions based on the reduced density gradient (RDG) [55,56]. The NCI isosurface regions show both stabilizing and destabilizing weak interactions. These are distinguishable according to the total sign of the second eigenvalue of the Hessian (λ2) matrix, where the sign of the λ2 quantity can vary accordingly and are thus suggested as a useful descriptor to characterize such situations. Negative values of the product given by ρ*sign (λ2), denote stabilizing interactions. Values close to zero account for weak interactions (van der Waals forces), while positive values account for weak repulsive cases. One of the main reasons to include the Br ligand in the axial position of the complexes was to increase the possibility of intra-and intermolecular interactions through halogen … halogen and halogen … oxygen bonding, since Br has a greater tendency to act as halogen bond acceptor compared to Cl, because of the greater σ-hole [57]. Complexes 1, 3, and 4 showed some interesting intra-and intermolecular interactions which were investigated by non-covalent interaction index (NCI) and NBO calculations. The NCI analyses for single molecules were done based on the optimized structures, but in case of the dimer pairs they were calculated directly from the crystallographically generated dimers without optimization. The surface NCI analyses are depicted in Figures 5-8, revealing stabilizing weak non-covalent intramolecular interactions in single-molecule and intermolecular interactions between interacting pairs, respectively. On the other hand, to shed light on the nature and strength of the intra-and intermolecular interactions depicted by NCI calculations, NBO analysis of other suitable descriptors for bond analysis was used with more details. Natural bond orbital calculations were performed on 1, 3, and 4 with the same level of theory for molecular orbital calculations [58]. The NBO analysis shows that the attractive nature is associated with donoracceptor orbital interactions in single molecules and between the pairs. Figure 5 shows the intermolecular interactions in 1 with contributions from the lone pair of oxygen in the carbonyl group and the π* of the imine functional group, ns(O3) … π*(C10=N1) and π(C3≡O3) … π*(C10=N1), with 0.47 kcal·mol −1 according to the NBO analysis from second-order perturbation energy. The energy contribution of ns(O2) … π*(C8=C9) was negligible.   On the other hand, the NCI plot and NBO calculations of the interaction pairs of 3 were also analyzed and depicted in Figure 7. The total energy release of the n(O) … π* and n(O) … σ* was 0.68 kcal mol -1 , while the contribution from n(Br) … π* was 0.31 kcal mol -1 based on second-order perturbation energy calculations.
The interaction area is shown with black circles.

Conclusions
In this paper, we have described the synthesis, characterization, structural and full computational studies of four bromide-tricarbonyl Re(I) complexes 1-4, bearing substituted diazabutadiene ligands. The molecular structures of the complexes were established by single-crystal X-ray diffraction, feature the metal in a distorted octahedral environment with facial arrangement of the carbonyl groups in the complexes, which was also confirmed by FT-IR spectroscopy. The nature and energy of the intermolecular n(O) … π* interactions between carbonyl-bound metal and π* of benzene ring and imine segments in 1 and n(O) … π* and n(Br) … π* in 3 were investigated in detail. The presence of such interactions was also confirmed by NCI index based on colour codes. The complexes 3 and 4 showed interesting intramolecular np(Br) … σ* interactions, which stabilized the internal geometry of the coordinated ligand around the metal center.