Microwave-Assisted Synthesis and Crystal Structure of Oxo(diperoxo)(4,4'-di-tert-butyl-2,2'-bipyridine)-molybdenum(VI)

The oxodiperoxo complex MoO(O2)2(tbbpy) (tbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine) was isolated from the reaction of MoO2Cl2(tbbpy) in water under microwave-assisted heating at 120 °C for 4 h. The structure of the oxodiperoxo complex wasdetermined by single crystal X-ray diffraction. The MoVI centre is seven-coordinated with a geometry which strongly resembles a highly distorted bipyramid. Individual MoO(O2)2(tbbpy) complexes are interdigitated along the [010] direction to form a column. The crystal structure is formed by the close packing of the columnar-stacked complexes. Interactions between neighbouring columns are essentially of van der Waals type mediated by the need to effectively fill the available space.

Some of us recently became interested in the microwave-assisted synthesis (MAS) of metal-organic compounds [19]. Early studies on the use of MAS for metal carbonyl complexes of the type [M(CO) 4 (L) n ] (M = Cr, Mo, W) indicated that the reaction times could be reduced and the yields increased when compared to conventional methods which use, for example, oil bath heating [20][21][22]. Despite these promising results, the potential use of microwave-accelerated syntheses of metal-organic compounds has hardly been explored, especially when compared with work in organic synthesis using this method [23]. During studies aimed at using MoO 2 Cl 2 (L-L) complexes as precursors to polymeric inorganic/organic hybrid materials, we unexpectedly isolated the oxodiperoxo complex MoO(O 2 ) 2 (tbbpy) (tbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine) as a by-product of the reaction of MoO 2 Cl 2 (tbbpy) with water under microwave-assisted heating. A detailed description of the synthesis of this compound and its crystal structure are given in this paper. To the best of our knowledge, this is the first report describing the isolation of this type of complex in the absence of an oxidant such as H 2 O 2 or tert-butyl hydroperoxide, that is, using only air as the oxygen source.

Synthesis and spectroscopic characterisation
Treatment of MoO 2 Cl 2 (tbbpy) with water in a microwave oven (dynamic power, 120 ºC, 4 h) gave a suspension. After removal of the white solid by filtration in air, acetone was added to the filtrate, and yellow needles of the complex MoO(O 2 ) 2 (tbbpy) (1) were obtained during slow diffusion of diethyl ether vapour into the water-acetone solution. When the same reaction was carried out under conventional reflux by using oil bath heating instead of microwave-assisted heating, no oxodiperoxo complex was formed. The only product obtained was the white solid, which, on the basis of preliminary analyses, is a polymeric inorganic/organic hybrid compound. Full characterization of this material is still ongoing in our laboratories.
The complex was characterised by FT-IR and Raman spectroscopy, and 1 H NMR. The FT-IR spectrum of 1 shows two characteristic strong absorption bands at 939 (Mo = O) cm −1 and 860 (O-O) cm −1 , a weak shoulder at ca. 870 (O-O) cm −1 , and a medium intensity band at 1,614 (C = N) cm −1 . The latter band is indicative of the bidentate coordination mode of the tbbpy ligand to the Mo VI centre [24]. For comparison, the Raman spectrum shows strong bands at 874 cm −1 , 939 cm −1 and 1,610 cm −1 . The ν(M(O 2 ) 2 ) modes for 1 are located between 500 cm −1 and 660 cm −1 , as found for related complexes [9,15,25]. The Raman spectrum shows one strong absorption at 533 cm −1 , a medium one at 589 cm −1 , and a weaker band at 658 cm −1 . The corresponding bands appear with strong intensity at 530 cm −1 , 588 cm −1 and 657 cm −1 in the FT-IR spectrum. The 1 H-NMR spectrum of 1 shows a set of signals corresponding to the tbbpy moiety at 9.13 (H 6,6' ) ppm, 8.64 (H 3,3' ) ppm, 7.81 (H 5,5' ) ppm and 1.44 (CH 3 ) ppm. When compared to the spectrum of the free ligand (doublets at 8.61 ppm and 8.40 ppm, doublet of doublet at 7.49 ppm, and singlet at 1.35 ppm), the set of signals for 1 is shifted to lower field and the resonances are less well defined (such that the H 3,3' and H 5,5' signals appear as a singlet and a doublet, respectively, i.e., further splitting of the signals due to coupling between the two hydrogens on a given pyridyl ring is not resolved). Taking into account the molecular structure described below, the apparent broadening of the lines in the 1 H-NMR spectrum of 1 is probably related with the fact that the two peroxo groups and the oxo group are, respectively, directed towards the two different rings, making the protons of each ring slightly non-equivalent.
In the MoO(O 2 ) 2 (tbbpy) complex the coordinated tbbpy molecule and the oxo group are structurally located on the mirror plane present in space group Pnma; the two peroxo groups are crystallographically equivalent and generated by mirror symmetry (Figure 1). These O-containing coordinating groups were found to be structurally disordered over two distinct positions, for which the partial occupancies were modelled to 78.1(7)% and 21.9(7)%. Nevertheless, by rotating the two individual complexes by 180º it is evident that the two complexes are almost identical as shown in Figure 2. Indeed, this disorder in the crystal structure seems to arise from erroneous close packing along the [010] direction: individual MoO(O 2 ) 2 (tbbpy) complexes are interdigitated along this direction to form a column as depicted in Figure 3; the ABAB·alternation (i.e., adjacent complexes mutually rotated by 180º) seems to minimise steric repulsion between coordinated groups and, thus, appears as the spatial distribution more frequently observed in the crystal structure. Despite the almost perfect alignment of aromatic rings along this direction, the inter-planar distance of ca.4.21 Å is too long to consider the presence of supramolecular π-π stacking interactions.
The crystallographically independent Mo VI centre is coordinated to two peroxo groups, one oxo group and one N,N-chelated tbbpy molecule in a seven-coordinated fashion, {MoN 2 O 5 }, with a geometry strongly resembling a highly distorted bipyramid (if one considers the centres of gravity of the peroxo groups as vertices-see Figure 1). The Mo-O peroxo distances lie in the 1.854(14) -1.939(4) Å range, which agrees well with those reported for related compounds (62 hits in the CSD; 1.80 -1.99 Å range with median of 1.93 Å).  The steric hindrance directly imposed on the peroxo groups can be inferred from the inter-centroid angle, i.e., we may assume that for close-packed structures the steric pressure on these groups will bring them into closer proximity, resulting in a smaller angle. A search in the CSD reveals that this angle usually varies between 120.2º and 139.9º (median of 129.4º). In 1 the observed value is ca. 135.5º, which is a clear indication that the combination of the columnar arrangement along the [010] direction and the magnitude of the disorder do not imply a considerable steric pressure on the complexes. Taking the centroids of the peroxo groups as the vertices of the coordination polyhedron, it is clear that the resultant bipyramid is considerably distorted: while the equatorial angles vary between ca. 108.0º and 135.5º, those with the apical positions (occupied by the oxo group and one nitrogen atom) are in the 70.4º -104.6º range.
The crystal structure of 1 (Figure 4) is formed by the close packing of individual columnar-stacked MoO(O 2 ) 2 (tbbpy) complexes (as shown in Figure 3). Interactions between neighbouring columns are essentially of van der Waals type mediated by the need to effectively fill the available space.

General
Elemental analysis was performed at the University of Aveiro. FT-IR spectra of undiluted (neat) samples were recorded on a FTIR Mattson-7000 infrared spectrophotometer using a Golden Gate single reflection diamond ATR system with KRS-5 lenses (Specac). Raman spectra were recorded on a Bruker RFS100/S FT instrument (Nd:YAG laser, 1064 nm excitation, InGaAs detector). 1 H-NMR spectra were measured in solution using a Bruker CXP 300 spectrometer. Chemical shifts are quoted in parts per million from tetramethylsilane. The microwave-assisted synthesis was carried out in a Discover S-Class (CEM Corporation, USA) microwave oven using a glass vessel with a capacity of 35 mL. A dynamic method was used, in which the power was automatically controlled based on the temperature feedback measured using a vertical focused IR sensor. MoO 2 Cl 2 (tbbpy) was prepared according to published procedures [27].

Single crystal X-ray diffraction
Single-crystals of MoO(O 2 ) 2 (tbbpy) (1) were quickly transferred from the mother liquor to a glass vial containing highly viscous FOMBLIN Y perfluoropolyether vacuum oil (LVAC 140/13) purchased from Sigma-Aldrich. A suitable crystal was mounted on a Hampton Research CryoLoop [28] with the help of a Stemi 2000 stereomicroscope equipped with Carl Zeiss lenses. X-ray data were collected on a Bruker X8 Kappa APEX II CCD area-detector diffractometer (Mo K α graphite-monochromated radiation, λ = 0.71073 Å) controlled by the APEX2 software package [29], and equipped with an Oxford Cryosystems Series 700 cryostream monitored remotely using the software interface Cryopad [30]. Images were processed using the software package SAINT+ [31], and data were corrected for absorption by the multi-scan semi-empirical method implemented in SADABS [32]. The structure was solved using the Patterson synthesis algorithm implemented in SHELXS-97 [33,34], which allowed the immediate location of the metal centre. All remaining non-hydrogen atoms were located from difference Fourier maps calculated from successive full-matrix least squares refinement cycles on F 2 using SHELXL-97 [33,35].
The oxo and peroxo groups coordinated to the metal centre were found to be structurally disordered over two distinct positions. This disorder was modelled into the final structure by considering refineable occupancy rates for each set of coordinated groups [O1 and O2 = O3; O1' and O2' = O3'] which ultimately converged to 78.1(7) and 21.9(7)%, respectively. Figures 1, 3 and 4 show only the most probable location for these atoms. Nevertheless, geometrical parameters of the distorted bipyramidal {MoN 2 O 5 } coordination environment concerning the two crystallographic locations for these coordinated groups are given in Table 1. With the exception of O2', all non-hydrogen atoms were successfully refined using anisotropic displacement parameters.
Hydrogen atoms bound to carbon were located at their idealised positions using appropriate HFIX instructions in SHELXL (43 for the aromatic and 33 for the terminal -CH 3 methyl groups) and included in subsequent refinement cycles in riding-motion approximation with isotropic thermal displacements parameters (U iso ) fixed, respectively, at 1.2 or 1.5 times U eq of the carbon atom to which they are attached. The last difference Fourier map synthesis showed the highest peak (0.984 eÅ −3 ) and deepest hole (−1.715 eÅ −3 ) located at 1.33 Å and 1.56 Å from H12A and H16B, respectively.
Information concerning crystallographic data collection and structure refinement details is summarised in Table 2.

Conclusions
The isolation of the oxodiperoxo complex MoO(O 2 ) 2 (tbbpy) in significant amounts from the reaction of MoO 2 Cl 2 (tbbpy) with water under air is surprising, since all known standard procedures for the synthesis of this type of complex involve a peroxide source such as H 2 O 2 or tert-butyl hydroperoxide (TBHP). For example, we recently found that complex 1 is formed upon treatment of the thiocyanate complex MoO 2 (NCS) 2 (tbbpy) with an excess of TBHP [25]. The use of microwaveassisted heating seems to play a role in the formation of complex 1 from MoO 2 Cl 2 (tbbpy) and water, although further investigation is required to fully delineate its influence compared with conventional heating methods.