Heterocyclic Substituted Methanides as Promising Alternatives to the Ubiquitous Nacnac Ligand † ‡

A series of group 13 complexes containing deprotonated bisheterocyclomethanes have been prepared and structurally as well as spectroscopically characterised. In the case of the parent neutral homo-disub-stituted bisheterocyclomethanes bis-(benzoxazol-2-yl)-methane (abbreviated as (NCOC 6 H 4) 2 CH 2) (1) and bis-(benzothiazol-2-yl)-methane (abbreviated as (NCSC 6 H 4) 2 CH 2) (2), two interesting ligand systems were investigated with respect to their coordination abilities toward group 13 metals. Upon deprotonation of the acidic methylene bridge in each case a β-diketiminate-like structure is formed, where the negative charge becomes delocalised about the whole ligand framework. There is a mesomeric stabilisation between the carbanionic and the amidic resonance structure. The ligands mentioned above were treated with AlMe 3 , AlMe 2 Cl and GaMe 3 to achieve deprotonation of the backbone and coincidental metal complexation. Therally characterized in the solid state by X-ray single crystal diffraction and in solution by different (hetero-nuclear) 1D/2D-NMR spectroscopic techniques. Each of those molecules shows a nearly planar arrangement with the group 13 metal cation coordinated by the two ring nitrogen atoms of the conjugated heterocycles. Furthermore, in this work (NCOC 6 H 4) 3 C(C 3 H 7) (9) could be isolated, which might turn out to be a promising analogue of the omnipresent trisoxazoline ligands in asymmetric catalysis.


Introduction
Referring to the monoanionic β-diketiminate ligand, the so called nacnac ligand, enabling stabilisation of main group elements in 25 low oxidation state, new ligand systems should be exploited, mimicking those electronic and steric properties. In the case of the nacnac substituted systems it is known, that the scaffold tends to deviate from a planar arrangement within metal coordination by means of twisting or bending, merely the backbone remains in 30 plane. This deviation is caused by the flexible but bulky organic substituents at the imine moieties. Although this feature of the twisting seems to be problematic, it also offers an efficient shielding of the coordinated (low-valent) metal cations, which are sensitive towards reactions with electrophiles or dimerization. 1 35 Scheme 1. Similarities between the nacnac ligand (left) and the bisheterocyclomethanides (right). Therefore, the newly designed ligand systems shown in Scheme 1 should maintain more rigidity and lead to a more planar 40 alignment in corresponding metal complexes. Due to this planarity within the six-membered metalla heterocycle C 3 N 2 M the orbital overlap between the donor atoms and the chelated metal atom should be increased, so that a more efficient conjugation and hence delocalisation can be anticipated. 45 In previous publications we investigated related compounds containing pyridyl substituents instead of benzoxazole resp. benzothiazole. 2 Several complexes of this bis-(pyrid-2-yl)-methanide with group 1 and 13 metals could be synthesized and structurally characterized by X-ray diffraction. Representing group 1 50 complexes [( [12]crown-4) 2 Li][Li{(2-NC 5 H 4 ) 2 CH}] can be highlighted as a solvent separated ion pair or [(thf) 2 Li{(2-NC 5 H 4 ) 2 CH}] as a monomeric lithiated contact ion pair both solely yielding Li-N contacts. 3,4 Additionally, some group 13 complexes like [Me 2 Al{(2-NC 5 H 4 ) 2 CH}] and [Me 2 Ga{(2- 55 NC 5 H 4 ) 2 CH}], where formally the (thf) 2 Li unit is replaced by a Me 2 Al or Me 2 Ga moiety, were prepared. 5, 6 Moreover, this kind of ligand system was altered by replacing the bridging CH unit isoelectronically by N, 7,8 P 9,10 or As. 11,12 Corresponding main group metal complexes were investigated. 60 In the past decade many main group metal containing β-diketiminate structures were synthesized and fully characterized. 13 -25 The presented aluminium(III) and gallium(III) complexes 3 − 8, which are described in the following, can be compared with many analogue compounds of the Dipp aluminium(III), gallium(III) and indium(III) compounds and in the case of Al and Ga also the reduced species in the oxidation state +I. 26,27,28 The former ones range from the alkyl and hydride to the halide substituted metal centres. The latter ones, the so called metallylenes, are rarely known and offer interesting 5 prospects as ligands in catalysis, where they might replace Nheterocyclic carbenes or phosphanes. 29,30 Promisingly the dialkylaluminium β-diketiminates already show catalytic activities towards ring-opening polymerisation. 31 The bisheterocyclomethanides show three optional donor sites: 10 the deprotonated bridging carbanionic centre, the ring-chalcogene (oxygen resp. sulfur) and -nitrogen atoms. Because O and S do not share the same good Lewis-donor abilities of the ringnitrogen atoms, each of the metallated species shows a nacnaclike coordination, wherein the metal gets chelated by the two 15 heteroaromatic nitrogen atom donors. It should be noted, that the presented ligand systems show also an analogy to the popular bis-(oxazolin-2-yl)-methanes, which are used in asymmetric catalysis. In contrast to 1 and 2 those ligands consist of saturated not benzannulated heterocycles with chiral centres, which can 20 transfer chiral information in catalysis. 32

Syntheses
The here discussed bisheterocyclomethanes 1 and 2 mimic the 25 related β-diketiminate ligand. Therefore, formally the imine substituent of nacnac is fixed to the backbone of the ligand by building up an additional five-membered heterocycle containing either oxygen or sulfur atoms as additional donor sites. As depicted in Scheme 2 both ligand systems were synthesized in 30 a cyclocondensation reaction of a suitable linker derived from malonic acid and two equivalents of the corresponding orthosubstituted anilines. The reaction pathway a) in Scheme 2 shows, that the cyclocondensation reaction of the phenol derivative is more complex than in the case of thiophenol b). Therefore, the former malononitrile has to be further activated before it undergoes a reaction 40 with the phenolic nucleophile. This can be achieved by generating the derived ethylbisimidate dihydrochloride, wherein ammonia as well as ethanol act as leaving groups. 33 The syntheses of closely related [Me 2 Al{Dipp 2 nacnac}] and [Me 2 Ga-{Dipp 2 nacnac}] can be achieved by facile reaction of nacnacH 45 with AlMe 3 resp. GaMe 3 . 13,14,31 Referring to this also LH 1 and 2 can be deprotonated at the acidic methylene bridge by release of methane and concerted coordination of the group 13 metal (Scheme 3). In this reaction protocol many different metallated compounds are accessible and can structurally be compared. In this context the effect of the diverging metal coordination (M = Al, Ga) and moreover the related substituents at these metal cations (R = R' = Me; R = Me, R' = Cl) on the bonding situation 55 within the different ligand systems is investigated.

Structural Data
In the following part a structural comparison of the compounds 1 -8, which could be obtained by the abovementioned reaction 60 protocols, will be discussed. For a better commensurability also the molecular solid state structures of the parent ligand systems 1 and 2 have been determined by X-ray single crystal diffraction and are shown in Figs. 1 and 2.  (1). Anisotropic displacement parameters are depicted at the 50% probability level. Ring hydrogen atoms are omitted for clarity. Structural data are given in Table  1 and Table 2.  Tables 1 and 2. 20 The most interesting feature within the series of the obtained structures is the deviation of the metal atom from the extended plane, which is made up from the N1−C2−C1−C9−N2 array. With this Mplane distance the folding angle between both heteroaromatic substituents is associated, which can be seen as a 25 reason for the different deviations. Various data concerning the Mplane distances, the torsion and the folding angles are listed in Table 2.  30 To highlight this special aspect the molecular structures of 3 and 8 are shown in Figures 3 and 6. Both species exhibit the greatest metal distance from the C 3 N 2 plane (29.57(26) and 20.61(26) °, resp.) and also the greatest folding angle of the heteroaromatic 35 rings (9.12(8) and 8.90(6) °, resp.). As a representative of the AlMeCl containing species the molecular structure of 6 is shown in Fig. 4. Concerning the series of the synthesized group 13 metal complexes of the bis-(benzothiazol-2-yl)-and bis-(benzoxazol-2- 40 yl)-methanides one has to emphasize that in the case of the oxygen containing ligand on average the bite angle is significantly smaller than in the sulfur analogues (89.2(2)-94.33(11) ° vs. 92.99(8)−96.84(9) °) and the angle around the bridging carbon atom in the backbone is also smaller (119.5(2)-45 120.2(3) ° vs. 123.54(14)−124.1(2) °). The aluminium species show a correlation between two important structural features: the narrower the N−M−N angle, the greater the metal distance from the C 3 N 2 plane and also the greater the folding angle of the heteroaromatic residues. However, in the case of the gallium 50 complexes this tendency is the opposite way round. Because of these observations there are reasons to believe that the ligand derived from 1 coordinates more strongly gallium than aluminium and vice versa from 2.
20.61(26) -8.90 (6) By comparing the structural features of the parent ligands with those of the metallated species given in Table 2 it can be considered that there is a change in hybridisation of the central bridging carbon atom upon metallation from sp 3 to sp 2 . This is 60 emphazised by the angular sum at the bridging carbon atom ranging from 359. 9(2) to 360.1(2) ° which displays nearly ideal trigonal planar coordination geometry for C bridge as expected for a sp 2 -hybridised carbon atom. In both cases the benzothiazole as well as the benzoxazole containing ligand a shortening of the 65 C ipso −C bridge bond (starting from 1.5106(15) and 1.4891(15) Å to approx. 1.39 Å) and a widening of the C ipso −C bridge −C ipso angle (from 109.59 (14) and 111.23 (9)    This kind of tripodal, tridentate ligand features interesting 65 coordination abilities because of the presence of soft as well as hard donor atoms within the scaffold and can be regarded as an analogue of trispyrazolylmethanes or even more of trisoxazoline ligands. The latter ones are known to be valuable auxiliary ligands in asymmetric catalysis. Furthermore, 9 shows structural similarities to the class of scorpionates, which are also tridentate ligands and form facial coordination geometries upon building metal complexes. 36 As common popular examples the mono-5 anionic trispyrazolylborates and the neutral isoelectronic trispyrazolylmethanes 37 have to be mentioned, which are used in bioinorganic chemistry to mimic the active sites in metallo proteins in model complexes. 38

10
In summary, a variety of group 13 metal complexes containing bisheterocyclomethanides as ligand systems could successfully be synthesized and structurally evaluated. In each case the metal cation is coordinated in a distorted tetrahedral fashion by the ringnitrogen atoms acting as Lewis-bases and the ligand became 15 almost perfectly planar. Depending on the considered combination of the heterocycle (benzoxazole or benzothiazole) and the chelated metal (Al or Ga) some significant correlations between the folding angle of the scaffold and the metal distance from the C 3 N 2 plane were observed. The metal coordination in all 20 cases takes place exclusively by donation of nitrogen atoms within the heterocyclic moieties. The chalcogene atoms did not participate in coordination, neither in the solid state nor in solution as proven in the appropriate crystal structures and the 1 H, 15 N-HMBC NMR experiments. 25 These empirical results gave rise to the occurrence of preferred ligand-metal pairs in presumably most stabile complexes: Whereas 1 seems to suit gallium more and 2 fits aluminium most. Moreover, the negative charge arising upon deprotonation seems to be delocalised in the whole ligand backbone as this is indicated 30 by the observed N-C ipso resp. C ipso -C bridge bond lengths. In this context the carbanionic canonical formula can be neglected, because the linking carbon atoms show clear sp 2 -hybridisation and no involvement in metal coordination. Referring to the title of this work it is obvious that the new 35 bisheterocyclomethanide ligands can definitely compete with the popular β-diketiminate ligand and offer a promising opportunity for investigation and exploring a new interesting research area.

Experimental Section
General Procedures 40 All manipulations were carried out under an atmosphere of N 2 or Ar by using Schlenk techniques. All solvents used within metallation reactions were distilled from Na or K before use. The starting materials were purchased commercially und used as received. Ethylbisimidate dihydrochloride was prepared 45 according to literature methode. 33

55
Both ligand systems were synthesised by cyclocondensation reaction of a suitable linker derived from malonic acid and the corresponding ortho-substituted anilines. (NCOC 6 H 4 ) 2 CH 2 (1): 2-Aminophenol (1.99 g, 18.0 mmol, 2.0 eq.) and ethylbisimidate dihydrochloride (2.11 g, 9.1 mmol, 60 1.0 eq.) were dissolved in methanol (50 mL). Subsequently the reaction mixture was heated to reflux for 3 h and after cooling to rt stored at −32 °C in a refrigerator. The resulting crystalline material was filtered off, washed with sat. aq. NaHCO 3 solution (2 x 50 mL) and water (2 x 50 mL) and dried under reduced  95 To a solution of the corresponding ligand 1 or 2 (1.0 eq.) in toluene a slight excess of the pure organometallic reactant AlMe 3 , AlMe 2 Cl or GaMe 3 (1.1 eq.) was slowly added at 0 °C. The reaction mixture was stirred over night and allowed to warm to rt. In the case of 5 and 6 the resulting precipitate was filtered off, the

X-ray Crystallographic Studies
Single crystals were selected from a Schlenk flask under argon or nitrogen atmosphere and covered with perfluorated polyether oil 105 on a microscope slide, which was cooled with a nitrogen gas flow using the X-TEMP2 device. 39 An appropriate crystal was selected using a polarize microscope, mounted on the tip of a MiTeGen©MicroMount or glass fiber, fixed to a goniometer head and shock cooled by the crystal cooling device. 110 The data for 1−9 were collected from shock-cooled crystals at 100(2) K. The data of 1, 2, 5, 6, 8 and 9 were collected on a INCOATEC Mo Microsource 40 and compound 7 was collected on a INCOATEC Ag Microsource, each equipped with mirror optics and APEX II detector with a D8 goniometer. The data of 3 and 4 were 115 measured on a BRUKER TXS-Mo rotating anode with mirror optics and APEX II detector with a D8 goniometer. All diffractometers were equipped with a low-temperature device and used either MoK α radiation of λ = 71.073 pm or AgK α radiation of λ = 56.086 pm. The data were integrated with SAINT 41 and an empirical absorption correction (SADABS) 42 was applied. The 5 structures were solved by direct methods (SHELXS-97) and refined by full-matrix least-squares methods against F 2 (SHELXL-97). 34,43 All non-hydrogen-atoms were refined with anisotropic displacement parameters. The hydrogen atoms were refined isotropically on calculated positions using a riding model with 10 their U iso values constrained to equal to 1.5 times the U eq of their pivot atoms for terminal sp 3 carbon atoms and 1.2 times for all other carbon atoms. Disordered moieties were refined using bond length restraints and isotropic displacement parameter restraints. Crystallographic data for the structures reported in this paper 15 have been deposited with the Cambridge Crystallographic Data Centre. The CCDC numbers, crystal data and experimental details for the X-ray measurements are listed in the supporting information. Copies of the data can be obtained free of charge from The Cambridge Crystallographic Data Centre via 20 www.ccdc.cam.ac.uk/data_request/cif or from the corresponding author.