Molybdocalix[4]arene N,O-Schiff-base ligands*

Abstract The ‘one-pot’ reaction of 2-hydroxy-3,5-di-tert-butylsalicylaldehyde with in situ generated ‘amino calixarene’ (from {Mo[(2-NC6H4)2CH2CH2]Cl2(DME)}, KOtBu and p-tert-butylcalix[4]areneH4 LH4) afforded, after work-up, the heterobimetallic salt K(NCMe)2[Mo(NCMe)(OEt)(2-C6H4CH2CH2C6H4NH2-2)L] (1∙2MeCN). By constrast, use of the ‘amino calixarene’ [Mo(NCMe)(2-C6H4CH2CH2C6H4NH2-2)L] afforded the potassium/ethoxide-free complex [Mo(NCMe)(2-C6H4CH2CH2C6H4N-2-CHC6H2-2′-(OH)-3′,5′-tBu)L] (2∙2MeCN). Reaction of the ‘amino calixarene’ (two equivalents) with the dialdehyde [1,3-(CHO)2-5-MeC6H3OH-2] led, following work-up, to the isolation of the cation-anion pair [C46H42N4O2]2+[Mo6O19]2−·C2H3N (3∙MeCN). The molecular structures of 1·2MeCN, 2·2MeCN and 3·MeCN are reported, for which it was necessary to use synchrotron radiation for data collection. Complex 1 contains an eliptical calixarene conformation as a result of π-interactions between the K+ and the arene groups and a distorted O5N octahedral geometry at the molybdenum centre, whereas in ‘K+/ethoxide-free’ 2, the calixarene retains the cone conformation and the metal possesses a distorted octahedral O4N2 coordination environment. In 3, a protonated macrocyclic cation, formed via hydrolysis, has weak intermolecular interactions with the polyoxomolybdate anion.


Introduction
In recent years, there has been considerable interest in the use of calix[n]arenes in a variety of catalytic/polymerisation processes (1). In our previous work, we have focussed on the direct metallation of the calixarene lower (phenolic)-rim to afford new pre-catalysts for either α-olefin polymerisation (2,3) or for the ring opening of ε-caprolactone (4). In other work, we and others have had success of K + in the product and binding of ethoxide at the molybdenum centre. To obtain the K + /ethoxide-free Schiff-base, it proved necessary to first isolate the 'amino calixarene'see Scheme 1. Furthermore, in an attempt to target a hydroxyl-bis(imine) species, we isolated a salt containing a Schiff-base macrocycle as cation and a polyoxometallate as anion. Such macrocycles are of interest for their ability to coordinate multiple catalytically active metal centres (11). The molecular structures of these three complexes herein have been determined using synchrotron radiation due to small crystal size and thin morphology (12,13).

Results and discussion
The reaction of 2-hydroxy-3,5-di-tert-butylsalicylaldehyde with in situ generated 'amino calixarene' (from {Mo[(2-NC 6 H 4 ) 2 CH 2 CH 2 ]Cl 2 (DME)}, KOtBu and p-tert-butylcalix [4] areneH 4 LH 4 ) in refluxing ethanol afforded an orange crystalline solid isolated in moderate yield (ca. 35%). The IR spectrum contains a relatively strong band in the v(C=N) region at 1614 cm −1 . Small orange prisms suitable for an X-ray determination using synchrotron radiation were grown from a saturated solution of acetonitrile at ambient temperature. The diffraction study revealed the complex to be the heterobimetallic salt K(NCMe) 2 [Mo(NCMe)(OEt) (2-C 6 H 4 CH 2 CH 2 C 6 H 4 NH 2 -2)L] (1•2MeCN), and the molecular structure is shown in Figure 1 with selected bond lengths and angles given in Table 1. Crystallographic data are presented in Table 2.
In 1, the molybdenum centre adopts a distorted octahedral environment comprising the calix [4]arene phenoxide oxygens, the nitrogen of the imido group and The calixarene is distorted such that two opposite phenoxide rings are drawn together by π-interaction involving K(1) [K + to centroid of O(2) ring = 3.0652(17) Å; K + to centroid of O(4) ring = 3.2054(16) Å], which also binds to the oxygens O(1) and O(3) of the other two phenoxide groups as well as two molecules of acetonitrile. We have noted similar π interactions between potassium ions and calix [4]arenes in other systems, which also result in a distortion/'pinching' of the cone (14,15). The highly functionalized imido/ salicylaldimine group in 1 is acting as a four electron donor In order to avoid K + incorporation in the product, the bis(tert-butoxide) compound {Mo[(2-NC 6 H 4 ) 2 CH 2 CH 2 ] (OtBu) 2 } was first isolated and then subsequently reacted with p-tert-butylcalix [4]areneH 4 (LH 4 ) to afford the 'amino calixarene' [Mo(NCMe)(2-C 6 H 4 CH 2 CH 2 C 6 H 4 NH 2 -2)L], following which 2-hydroxy-3,5-di-tert-butylsalicylaldehyde was added to afford 2. In the case of 2, the room temperature 1 H NMR spectrum was consistent with a C 4v -symmetric cone conformation. The IR spectrum contained a strong band at 1614 cm −1 assigned as the v(C=N) stretch. As for 1, it proved possible to grow small (yellow) crystals suitable for X-ray diffraction using synchrotron radiation. The molecular structure was indeed found to be the potassium/ ethoxide-free complex [Mo(NCMe)(2-C 6 H 4 CH 2 CH 2 C 6 H 4 N-2-CHC 6 H 2 -2′-(OH)-3′,5′-tBu)L] (2•2MeCN), and the structure is displayed in Figure 2. Selected bond lengths and angles are given in Table 1 for comparison with those of 1. The molybdenum centre possesses a pseudo octahedral geometry with the molybdenum atom displaced from the mean plane (O 4 ) towards the imido/salicylaldimine nitrogen N(1) by 0.2489(6) Å. As in 1, the imido/salicylaldimine group is acting as a four electron donor [Mo(1)-N(1) = 1.7268 (15)  Reaction of the isolated 'amino metallocalix [4]arene' [p-tert-butylcalix [4] (3•MeCN) in ca. 35% yield. Small crystals suitable for X-ray diffraction using synchrotron radiation were obtained from a saturated acetonitrile solution on prolonged standing at ambient temperature. There is one acetonitrile of crystallization in the asymmetric unit. The geometrical 1, this group is trans to an acetonitrile ligand residing in the calixarene cavity. Again, there is internal H-bonding involving N(2) and O (5). Pairs of molecules are related by a centre of symmetry between the rings C(53)-C(58) and C(53′)-C(58′), which π⋯π stack; closest π⋯π stacking distances are C(55)-C(57′) = 3.511 Å and C(56)-C(58′) = 3.573 Å.  (19) 128.90(11) mo(1)-o(2)-c (12) 120.17 (19) 126.58 (11)    systems, the presence of alkali metals leads to an eliptical calixarene conformation as a result of π-interactions. The presence of water in such systems can lead to hydrolysis of the imido-molybdenum bond and, as evidenced in the reaction with 2-hydroxy-5-methyl-1,3-benzenedicarboxaldehyde, can lead to the formation of polyoxomolybdates. The potential of these molybdocalix [4]arene Schiff-bases as ligands for catalytically active metals is currently under investigation in our laboratory.

General
All manipulations were carried out under an atmosphere of dry nitrogen using conventional Schlenk and cannula techniques or in a conventional nitrogen-filled glove box. Diethyl ether and tetrahydrofuran were refluxed over sodium and benzophenone. Toluene was refluxed over sodium. Dichloromethane and acetonitrile were refluxed over calcium hydride. All solvents were distiled and degassed prior to use. IR spectra (Nujol mulls, KBr windows) were recorded on a Nicolet Avatar 360 FT IR spectrometer. Elemental analyses were performed by the elemental analysis service at the London Metropolitan University and the University of Hull. The precursors 2,6-(CHO) 2 -4-R-C 6 H 2 OH and (2-NH 2 C 6 H 4 ) 2 O were prepared by the literature method (23,24). The complex {Mo[(2-NC 6 H 4 ) 2 CH 2 CH 2 ]Cl 2 (DME)} was prepared as reported previously (25).
parameters associated with the [Mo 6 O 19 ] 2− anion are similar to those reported previously (16). The macrocyclic cation adopts a twisted conformation (see Figure 3), and exhibits intramolecular H-bonding between each of the phenoxide oxygens and the two adjacent NH groups. The C=N bond lengths [1.284(7)-1.290(7) Å] are in the range of bond lengths previously reported for bis(imino)phenol-based macrocyclic Schiff-bases exhibiting intramolecular H-bonding (17)(18)(19)(20)(21). The cavity size for the cationic macrocycle is 5.6 × 3.9 Å, which compares favourably with that found in the neutral macrocycle [6.5 × 4.3 Å], but is far smaller than those reported [16.0 × 5.8 Å] for the bis(imino)phenol-based nickel complex reported by Na et al.; it should be noted that for the latter nickel complex however the macrocycle is near planar (5(1) o between the phenolic benzene plane and C 6 H 4 -benzene plane) (22). Complex 3 packs such that pairs of anions lie close together with weak intermolecular, centrosymmetric pairs of Mo⋯O interactions. There are also several weak C-H⋯O cation⋯anion interactions and C-H⋯N cation⋯acetonitrile contacts.

Conclusion
In conclusion, to synthesise alkali-metal-free Schiff-base complexes incorporating molybdocalix [4]arenes, the procedure is best carried out in a two-step synthesis which involves the initial isolation of the bis(tert-butoxide) complex prior to addition of calixarene. As seen in other MeCN (20 ml 1•2MeCN, 2•2MeCN, and  3

•MeCN
Intensity measurements were made on a Bruker SMART 1K or APEX II CCD area detector diffractometer using synchrotron radiation (λ = 0.6942 Å) at Daresbury SRS Station 9.8 (12,26), using silicon 111-monochromated radiation. Further details are given in Table 2. Data were corrected for Lp effects and for absorption based on repeated and symmetry equivalent measurements. The structures were solved by direct methods and refined on F 2 for all data, with anisotropic displacement parameters for all non-H atoms (27,28). H atoms attached to C were constrained, those in OH and NH groups had coordinates refined. In 1•2MeCN the tBu methyl groups at C(18) and C(70) were modelled as twofold disordered with major occupancy 51.0(10) and 62(5)% respectively. For 3•2MeCN the N-H distances were gently restrained to be similar.

Disclosure statement
No potential conflict of interest was reported by the authors.

Supplementary material
CCDC 1574780-1574782 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.