(2,2′-Bipyridine-κ2 N,N′)[2-tert-butylanilinato(2−)]dichloridooxidomolybdenum(VI) dichloromethane hemisolvate

The MoVI atom in the title structure, [Mo(C10H13N)Cl2O(C10H8N2)]·0.5CH2Cl2, has a distorted octahedral coordination sphere with cis-orientated oxide and imide ligands, trans-chloride ligands and the 2,2′-bipyridine (bipy) ligand N atoms lying trans to the oxide and imide ligands. An imide-ligand tert-butyl-methyl-group H atom makes a close approach with the oxide ligand (distance = 2.53 Å) and the imide-ligand N atom (distance = 2.41 Å). Another imide-ligand tert-butyl-methyl-group H atom makes a close approach to a chloride ligand (distance = 2.82 Å). One bipy-ligand α-H atom makes a close approach to the oxide ligand (distance = 2.4 Å) and the other α-H atom makes a close approach to the imide-ligand phenyl-ring ortho-H atom (distance = 2.52 Å). These close approaches suggest the presence of weak intramolecular hydrogen bonds. The solvent molecule has been modelled under consideration of half-occupancy.


Comment
Complexes of molybdenum containing oxo and imido functions in the same molecule are still fairly rare. However they are relatively easy to prepare by a conproportionation reaction between bis-imido complexes of the form [Mo(NR) 2 Cl 2 (dme)] (dme =1,2-dimethoxyethane) and the bis-oxo complexes [Mo(O) 2 Cl 2 (dme)] (Bell et al., 1994). During attempts to prepare bis-imido complexes in which the imido ligand carried substituents on the aryl ring possessing potentially sterically hindering ortho-substituents, we reacted Na 2 MoO 4 with two equivalents of 2-tert-butylaniline in the presence of 8 equivalents of SiMe 3 Cl and 4 equivalents of NEt 3 in dme as solvent which is the normal protocol for producing [Mo(NR) 2 Cl 2 (dme)] complexes in good yield. A red solid was obtained which had the characteristic features of the complex. This complex was then reacted with 2,2'-bipyridine (bipy) to produce the complex [Mo(NC 6 H 4 CMe 3 -2) 2 Cl 2 (bipy)]. This type of complex was of interest as our studies of bis-imido tungsten complexes of the form [W(NC 6 H 5 ) 2 Cl 2 (bipy)] had shown there was a steric interaction between the bipy α-H atoms and the ipso-carbon of the phenyl ring (Bradley et al., 1987). Whereas the imido ligand nitrogen atoms were bent away from each other as expected, the ipso-carbon atoms of the phenyl were bent inwards towards each other [W-N-C bond angles 165.6 (12) and 164.4 (12)°] apparently to reduce contact with the bipy α-H atoms.
The imido ligand phenyl groups were also apparently rotated to reduce contact of the ortho-H atoms with the bipy α-H atoms.
The product obtained from the reaction with bipy crystallized nicely but did not give particularly good C, H and N analytical data. The NMR spectra suggested the bulk sample was indeed [Mo(NC 6 H 4 CMe 3 -2) 2 Cl 2 (bipy)] but the spectra also indicated a small amount of a second species was present. A crystal picked out from the mass and subjected to an X-ray analysis was found not to be the bis-imido complex but instead the oxo-imido complex [Mo (NC 6 H 4 CMe 3 -2)(O)Cl 2 (bipy)](1).
The oxo-imido function could have arisen as a by-product during the preparation of [Mo(NC 6 H 4 CMe 3 2) 2 Cl 2 (dme)] by incomplete oxo-imido exchange or by hydrolysis of one of the imido functions during the exchange of the dme ligand for the bipy ligand as this ligand was not dried after obtaining it from commercial sources.
The structure of (1) consists of a distorted-octahedral array about the molybdenum atom with a cis-orientation of the organoimido and oxo ligands, trans-chloro ligands and the nitrogen atoms of the bipyridyl ligand lying trans to the organoimido and oxo ligands (Fig. 1) (Nugent & Mayer, 1988). However this may not be a trans -influence effect as there are close approaches of the two α-hydrogen atoms of the bipy ligand with other parts of the molecule. to that found in the bis-imido complexes [Mo(NC 6 H 3 Cl 2 -2,6)(S 2 CNEt 2 ) 2 ][162.2 (7) and 162.9 (7)/%] ( Barrie et al., 1999) and [WCl 2 (NPh) 2 (bipy)] [165.6 (12) and 164.4 (11)°] (Bradley et al., 1987). However in these complexes the M-N-C bond angles are such that the phenyl or alkyl group bends in towards the adjacent oxo or imido ligand whereas in the present complex the bend is away from the oxo ligand. This appears to be caused by the tert-butyl substituent which in the crystal prefers to orientate over Cl(1) and O(1) rather than Cl(2) and O(1) which appears to be another possible orientation (Fig 1).
The rotation of the organoimido phenyl ring is such that the plane of the ring deviates from the plane made by the bipy rings by 46.5 (1)° (Fig 1). The orientation of the phenyl ring and the orientation of the 2-tert-butyl substituent has some interesting consequences for intramolecular contacts. For the 2-tert-butyl substituent the rotation about C(16) and C(17) is such that one of the equatorially positioned methyl groups [C(18)] lies directly above the oxo ligand and H(18a) makes a contact of 2.53 Å with it. This distance lies within the range of distances considered to involve weak hydrogen bonding to oxygen atoms (Desiraju, 1996)). There is an even shorter separation of 2.41 Å between H(18) and the imido nitrogen atom, N(1) and the separation is well within the range of values considered to involve weak hydrogen bonding to nitrogen [2.65%A (Demers et al., 2005)]. It is interesting to note that even though the nitrogen lone pair will be mostly involved in donation to molybdenum to make the imido ligand multiple bond, the bend made by the Mo-N(1)-C(11) system [165.8 (2)°] is such that any remaining lone pair is pointing in the direction of H(18a). The other equatorially positioned methyl group of the 2-tert-butyl substituent lies above Cl(1) with the H(19c) to Cl(1) separation of 2.82 Å. This distance is also within the range of values suggested as weak hydrogen bonding to chlorine (Aakeroy et al. 1999). The separation between H(19c) and the imido nitrogen N(1) is 2.39Å which suggests potential weak hydrogen bonding may also be involved. There is a similar approach of H(18a) to N1 (2.41 Å). However it should be realised that these close approaches are forced on the system by the molecular geometry of the tert-butyl group which may or may not imply the existence of attractive H-bonding. The remaining methyl group of the 2-tert-butyl substituent, which lies in an axial position, is rotated to give a gearing effect which removes any interaction of the H atoms with the nearest neighbour H atoms. Thus H(20a) is positioned so as to supplementary materials sup-3 bisect the C(18)-C(17)-C(19) angle and this allows H(20b) and H(20c) to lie in front of, but to either side of, the bipy α-hydrogen H(15). As a result of the positioning of the 2-tert-butyl substituent, H(12), which lies in the other ortho-position of the aromatic ring, makes a close contact with Cl(2) with the distance of 3.06 Å being just outside the limit of the H and Cl van der Waals radii (3.0 Å) but still representing a weak hydrogen bond (Aakeroy, 1999). H(12) also makes a close contact of 2.52 Å with the α-hydrogen of the nearby bipy ring which is just outside the van der waals radii of 2.4 Å (Aakeroy, 1999). On the other side of the molecule there is a close approach of the weak hydrogen bonding type, for H(1) which is the other α-hydrogen of the bipy ring, with the terminal oxo ligand oxygen [O(1)]. This arises since the bipy rings are essentially co-planar with the Mo-O multiple bond. The atomic separation is 2.44 Å which is even shorter than the distance the 2-tert-butyl substituent H(18a) atom makes with O(1) (2.53 Å). The separation for this type of interaction in [WCl 2 (NCMe 3 )(O)(bipy)] is 2.42 Å (Clegg, et al., 1993). There are no other significant intramolecular contacts in the structure. The disordered partial CH 2 Cl 2 molecule lies across a centre of symmetry in the crystal lattice but its H atoms make no significant approaches to the chlorine complex.

Experimental
Na 2 MoO 4 .2H 2 O (2.0 g, 8.3 mmol) was dried under vacuum by heating at 100°C for 1 h to yield anhydrous Na 2 MoO 4 (1.7 g, 8.3 mmol). 2-tert-butylaniline (2.46 g, 16.5 mmol) was added followed by 1,2-dimethoxyethane (50 cm 3 ) and then triethylamine (4.6 cm 3 33.0 mmol) and the mixture was stirred rapidly while chlorotrimethylsilane (8.4 cm 3 66.0 mmol) was added dropwise. The mixture was stirred for 16 h, refluxed for 8 h and then filtered while the mixture was still hot and the solvent removed to give a deep red crystalline solid (4.43 g). 0.76 g of this material was added to 2,2'-bipyridine (0.214 g, 1.4 mmol), CH 2 Cl 2 (30 cm 3 ) was added and the mixture stirred for 5 h. The solution was filtered, the solvent removed and the deep-red crystalline solid washed with petroleum spirit. The solid was dissolved in CH 2 Cl 2 (20 cm 3 ) the volume reduced to ca one-half and the solution allowed to stand at room temperature yielding a red-brown crystalline solid (0.55 g). A crystal was chosen from the mass and the X-ray crystal structure obtained.

Refinement
H atoms were placed in calculated positions riding on the atoms to which they are attached. The CH 2 Cl 2 solvent was located close to a centre of symmetry requiring that it be no more than half-weighted. No attempt was made to refine its site occupancy factor. Fig. 1. ORTEP