Mono-vs . bi-metallic assembly on a bulky bis ( imino ) terpyridine framework : a combined experimental and theoretical study

The bis(imino)terpyridine ligands, 6,6′′-{(2,6-i-Pr2C6H3)N=CR}2-2,2′:6′,2′′-C15H9N3 (R = H L1, Me L2), have been prepared in high yield from the condensation reaction of the corresponding carbonyl compound with two equivalents of 2,6-diisopropylaniline. The molecular structure of L2 reveals a transoid relationship between the imino and pyridyl nitrogen groups throughout the ligand framework. Treatment of aldimine-containing L1 with one equivalent or an excess of MX2 in n-BuOH at 110 ◦C gives the mononuclear five-coordinate complexes, [(L1)MX2] (M = Fe, X = Cl 1a; M = Ni, X = Br 1b; M = Zn, X = Cl 1c), in which the metal centre occupies the terpyridine cavity and the imino groups pendant. Conversely, reaction of ketimine-containing L2 with excess MX2 in n-BuOH at 110 ◦C affords the binuclear complexes, [(L2)M2X4] (M = Fe, X = Cl 3a; M = Ni, X = Br 3b; M = Zn, X = Cl 3c), in which one metal centre occupies a bidentate pyridylimine cavity while the other a tridentate bipyridylimine cavity. H NMR studies on diamagnetic 3c suggests a fluxional process is operational at ambient temperature in which the central pyridine ring undergoes an exchange between metal coordination. Under less forcing conditions (room temperature in dichloromethane), the monometallic counterpart of 1b [(L2)NiBr2] (2b) has been isolated which can be converted to 3b by addition of one equivalent of (DME)NiBr2 (DME = 1,2-dimethoxyethane) in n-BuOH at 110 ◦C. Quantum mechanical calculations (DFT) have been performed on [(L1)ZnCl2] and [(L2)ZnCl2] for different monometallic conformations and show that 1a is the energetically preferred structure for L1 while there is evidence for dynamic behaviour in L2-containing species leading to bimetallic formation. Single-crystal X-ray diffraction studies have been performed on 1a, 1b, 1c, 2b, 3a, 3b(H2O) and 3c.


Introduction
The application of 2,6-linked oligopyridine ligands, C 5 H 4 N-(C 5 H 3 N) n C 5 H 4 N (n = 2-7), in coordination chemistry is now well established with mono-, double-or triple-stranded helicates being a feature of the structural types. 1 For example quinquepyridine (n = 3) can readily form homobimetallic salts of the type [(quinquepyridine) 2 M 2 ] n+ with a variety of 3d metal ions including cobalt,2 nickel3 and copper. 4e have been interested recently in developing oligopyridine ligands featuring sterically demanding imino end-groups, ArN=CR(C 5 H 3 N) n CR=NAr (n = 3, 4, 5; Ar = 2,6-substituted aryl group; R = H or hydrocarbyl), and have termed this family of ligands as oligopyridylimines (Fig. 1). 5 It was envisaged that the steric attributes of these ligands would to some extent inhibit multi-stranded helicate formation and allow access to well-defined polymetallic complexes of the form [(oligopyridylimine)M x X y ] (X = anionic monodentate ligand) that could be amenable to further functionalisation at the metal centres and/or for catalytic applications.
Herein we report our efforts at developing the coordination chemistry of the potentially pentadentate (n = 3) member of Scheme 1 Reagents and conditions: 2 eq.2,6-i-Pr 2 C 6 H 3 NH 2 , EtOH, cat.H + , heat.
Table 1 Selected bond distances (A ˚) and angles ( • ) for L2 C(13)-N(1) 1.284( 6) C( 18)-C(19) 1.506( 6) C(29)-N (5)  1.255( 6) C(23)-C(24) 1.458 (7)   C(29)-N(5)-C(30) 119.8 (4)   the reaction in neat 2,6-diisopropylaniline at elevated temperature over short time periods. 5Both ligands have been characterised by IR, 1 H and 13 C NMR spectroscopy along with ES mass spectrometry (see Experimental section).L2 has also been the subject of a single-crystal X-ray diffraction study.Pale yellow crystals of L2 suitable for the X-ray determination were grown by the slow evaporation of a dichloromethane solution containing the compound.A view of L2 is shown in Fig. 2; selected bond distances and angles are listed in Table 1.The structure of L2 consists of three 2,6-linked pyridine rings with the imino groups occupying the ends of the oligopyridylimine chain.The nitrogen atoms of the pyridine groups adopt a mutually transoid conformation which is also extended to the terminal imine groups in a manner reminiscent of that found in 2,6-oligopyridines.] from planarity is displayed throughout the imine-pyridine backbone as has been observed for L1. 5 The 2,6-diisopropyl substituents on the terminal aryl groups sit above and below the planes of the nearest pyridine units within the chain.The N(1)-C(13) and N(5)-C(29) bond lengths of 1.284(6) and 1.255(6)A ˚are consistent with their being double bond character.The IR spectra of L1 and L2 confirm the presence of the imine with m(C=N) bands at ca. 1645 cm −1 which is supported by the 13  No evidence for bimetallic products was apparent on addition of two equivalents or greater of MX 2 or after stirring for longer periods.All products have been characterised by microanalysis, FAB mass spectrometry, IR spectroscopy and, in the cases of 1a and 1b, by magnetic measurements and 1c by 1 H NMR spectroscopy (see Table 2 and Experimental section).In addition, crystals of 1a, 1b and 1c have been subjected to singlecrystal X-ray diffraction studies.
Crystals of 1a, 1b and 1c suitable for the X-ray determinations were grown by slow cooling of hot acetonitrile solutions containing the complexes.The structures of 1a, 1b and 1c are similar and only 1a will be discussed in any detail.A view of 1a is depicted in Fig. 3; selected bond distances and angles for 1a, 1b and 1c are listed in Table 3.The molecular structure of 1a depicts a single iron centre bound by L1 and two terminal chloride ligands.The metal centre occupies the tridentate terpyridine cavity with the two imine groups pendant and exo to the coordinated terpyridine moiety.The geometry at the metal centre can be best described as distorted trigonal bipyramidal with N(2) and N(2A) defining the axial sites The FAB mass spectra for 1a-c show fragmentation peaks corresponding to the loss of one halide group from the corresponding molecular ion peak.The IR spectra for 1a-c display m(C=N) bands at ca. 1635 cm −1 and in a similar region to that for free L1, supporting the pendant nature of the imino groups.Complexes 1a and 1b are paramagnetic and display magnetic moments of 5.3 and 2.8 l B (Evans Balance at ambient temperature) which are consistent with high spin configurations possessing four and two unpaired electrons, respectively.In contrast, complex 1c is  (ii) Reaction of MX 2 with L2.The reaction of L2 with two equivalents of MX 2 [MX 2 = FeCl 2 , (DME)NiBr 2 , ZnCl 2 ] in nbutanol at elevated temperatures gave complexes [(L2)M 2 X 4 ] (M = Fe, X = Cl 3a; M = Ni, X = Br 3b; M = Zn, X = Cl 3c) in high yield (Scheme 2).No evidence for monometallic products was apparent under these experimental conditions nor when one equivalent of MX 2 was employed.All products have been characterised by FAB mass spectrometry, IR spectroscopy and, in the cases of 3a and 3b,  by magnetic measurements and for 3c by 1 H NMR spectroscopy (see Table 2 and Experimental section).In addition, crystals of 3a and 3c have been the subject of single-crystal X-ray diffraction studies.Crystals of 3a and 3c were grown by slow cooling of hot acetonitrile solutions of the corresponding complexes.The structures of 3a and 3c are essentially the same and will be discussed together.A view of 3c is depicted in Fig. 4; selected bond distances and angles are listed for both 3a and 3c in Table 4.The molecular structures reveal bimetallic complexes in which the two metal centres are supported on the same L2 ligand frame and each bound terminally by two chloride ligands.One of the metal centres [M(2)] occupies a bidentate pyridylimine pocket while the other [M(1)] a tridentate dipyridylimine cavity so as to generate a tetrahedral geometry at M(2) and a distorted square pyramidal geometry at M(1).Within the ligand frame the pyridylimine and dipyridylimine moieties are both nearly planar with each of the planes being disposed almost orthogonally to one another [torsion angles: N(3)-C(18)-C(19)-N(4) 91.5 • (3a), 90.6 Attempted crystallisation of 3b using the conditions applied for 3a and 3c proved unsuccessful.However, by prolonged standing in chloroform adventitious reaction with water occurs to give [(L2)Ni 2 Br 4 (OH 2 )] [3b(H 2 O)] as orange crystals (Scheme 2).The dataset obtained for the single-crystal X-ray determination was, unfortunately, of poor quality.Nevertheless, the structure was unequivocal in revealing a bimetallic species displaying structural features similar to that observed in 3a and 3c (vide supra) but with one molecule of water additionally bound to the nickel centre [Ni(2)] occupying the bidentate cavity in L2.For comparison, Table 4 also contains the corresponding bond lengths and angles for 3b(H 2 O).
The FAB mass spectra for 3a-c show fragmentation peaks corresponding to the loss of one or two halide groups from the corresponding molecular ion peak.In the IR spectra for 3a-c, the m(C=N) bands are seen at ca. 1590 cm −1 (in both solid and solution state) and shifted to a lower wavenumber by ca.52 cm −1 in comparison with the free ligand L2; supporting coordination of both imine groups.Complexes 3a and 3b are paramagnetic and display magnetic moments of 7.0 l B and 4.3 l B (Evans Balance at ambient temperature) which are consistent with non-spin coupled Fe(II) (S = 2)-Fe(II) (S = 2) and Ni(II) (S = 1)-Ni(II) (S = 1) systems (using l 2 = l i 2 , where l i is the magnetic moment of the individual metal centres). 9he 1 H NMR spectrum of 3c in CDCl 3 (at ambient temperature) indicates that the molecule has average C 2 symmetry in solution with only five resonances for the pyridyl hydrogen atoms occurring at d 8.08, 8.24, 8.31, 8.45 and 8.68.In addition, there is only one resonance for the methyl protons on the imine groups.Attempts to lower the symmetry by recording the spectrum at lower   5) temperature were, however, hampered by solubility problems.Scheme 3 indicates a possible fluxional process that could be operating at room temperature in solution in which the central pyridine group could be flipping between metal centre coordination.An alternative explanation may be that the central pyridine nitrogen donor is not coordinated in solution.
With the intent of synthesising monometallic complexes bound by L2, the reaction with MX 2 was attempted under milder conditions.Thus, when a cooled dichloromethane solution of (DME)NiBr 2 is treated with one equivalent of L2 and left to stir overnight at room temperature, the yellow mononuclear complex [(L2)NiBr 2 ] (2b) can be obtained in good yield.Attempted Scheme 3 Possible dynamic process operating for 3.
isolation of the mononuclear zinc analogue of 2b, [(L2)ZnCl 2 ] (2c), under the same reaction conditions as employed above, proved problematic with 3c and L2 being the main species identifiable in the 1 H NMR spectrum.Complex 2b was characterised by FAB mass spectrometry, IR spectroscopy and by magnetic measurements (see Table 2 and Experimental section).In addition, a crystal of 2b has been the subject of a single-crystal X-ray diffraction study.
Crystals of 2b suitable for the X-ray determination were grown by prolonged standing in chloroform.A view of 2b is shown in Fig. 5; selected bond distances and angles are listed in Table 5.The molecular structure consists of a single nickel atom bound by both L2 and two terminal bromide ligands.As with its L1-containing counterpart, 1b, the metal centre occupies the terpyridine cavity of the bis(imino)terpyridine ligand.In this case, however, only one of the imino groups is exo while the other is endo and forms a partial interaction with the nickel centre [Ni(1) The effect of the interaction in comparison with 1b is two-fold.Firstly, an increase in the Br-Ni-Br angle from 129.51(6) • in 1b to 172.84(2) • in 2b occurs so that the nickel assumes a more octahedral geometry in 3b.Secondly, the occupation of terpyridine cavity is more uneven with the nickel centre more disposed towards The FAB mass spectrum for 2b shows fragmentation peaks corresponding to the loss of one or two bromide groups from the molecular ion peak.In the IR spectra for 2b, m(C=N) bands at 1594 and 1630 cm −1 support the presence of both bound and free imino groups, respectively.As with 1b, 2b is paramagnetic exhibiting a magnetic moment of 2.4 l B (Evans Balance at ambient temperature) which is consistent with presence of two unpaired electrons.
Conversion of yellow 2b to orange 3b can be achieved in good yield by treating 2b with one equivalent of (DME)NiBr 2 in n-butanol at 110 • C for 30 min (Scheme 2).Similarly, 2b could be converted to 3b on refluxing 2b with one equivalent of (DME)NiBr 2 in dichloromethane for two days, these conditions also proving successful for the preparation of 3b directly from L2.

Density functional theory calculations
The capacity of Lx, depending on the nature of the imino-carbon substituent, to have a selectivity for the number of metal(II) halide units it binds was unexpected.Although it could be argued that differences in electronic properties within the two ligand manifolds (e.g., donor capability of the nitrogen donors within Lx is expected to follow the order: pyridine > ketimine > aldimine 10 ) should be influential, such a dramatic change in binding affinity was nevertheless surprising.Therefore, a theoretical study on the relative stability of mononuclear [(L2)MX 2 ] and [(L1)MX 2 ] was carried out in order to investigate in more detail the energetic properties of these systems.Specifically, density functional calculations have been performed on ZnCl 2 -containing complexes and is likely, given the similarity of the experimental results, to be representative of the other metal halide systems employed in this work.
For both L1 and L2, B3LYP calculations have been undertaken on mono-zinc dichloride species in conformations 1 Lx and 2 Lx (Fig. 6) which are based on the conformations adopted in structurally determined 1c and 2b.In addition, hypothetical species 4 Lx derived from the removal of one zinc dichloride unit from the bidentate pocket in bimetallic 3c are also studied.As 1c is the only monometallic ZnCl 2 structure characterised by X-ray diffraction, it was used to test our theoretical approach.For this configuration, theoretical (1 L1 ) and experimental (1c) structures are in good agreement with the discrepancies found being less than 0.07 A ˚for the bond lengths and 10 • for the angles (see Tables 3 and 6).The coordinated terpyridine portion of the ligand remains planar with an out of plane displacement of 0.03 and 0.05 A ˚for the theoretical and experimental structures, respectively [Fig.7(ii)].It is likely that any discrepancies can be attributed to the vacuum conditions of the calculations.Therefore, it was viewed that this initial study demonstrated that using the B3LYP functional with our basis set is viable for studying such large systems.To extend the investigation, we have applied this theoretical approach to mono-zinc complexes that have not been characterised crystallographically.   6).This observation is likely linked to the steric constraints imposed by the methyl units of the imino-carbon groups which are in close proximity in this conformation [Fig.7 Secondly, a theoretical study of mono-zinc dichloride complexes bound by respectively L2 (2 L2 ) or L1 (2 L1 ) in conformations adopted in the structurally characterised nickel bromide complex 2b [Fig.7(iv)], was undertaken.As a first observation, it is noted that the optimised structure of 2 L2 does not show any noticeable changes when compared with 2b, apart from the expected variation in metal-nitrogen distances [Fig.7(v) vs. (iv) and Tables 5 and 6].Indeed, 2 L2 remains in a distorted octahedral geometry with its four nitrogen donor ligands from L2 occupying the equatorial sites and the halide ligands axial.Two of the Zn-N distances are elongated [Zn(1)-N(1) 2.55 A ˚and Zn(1)-N(4) 2.54 A ˚] indicative of a weaker coordination to the metal centre.In addition, the planarity of the pyridine rings within L2 is reduced when compared with structurally determined 2b with an increase in the out of plane displacement evident  described as more five-coordinate than six-coordinate.Therefore, on comparison of the hypothetical zinc complexes 2 L2 and 2 L1 , it would seem that the presence of an endo-oriented imino group stabilises an octahedral-type conformation in the L2 system while it leads to a significant structural rearrangement for the L1 system.
Thirdly, to examine the possibility for equilibration of monometallic species in solution (Scheme 4, vide supra) and to probe potential intermediates in the formation of bimetallic species (e.g., 3c), a hypothetical mono-zinc species has been generated by removing one ZnCl 2 molecule from structurally characterised 3c.Specifically, we have focused on a conformation (4 L2 ), in which the single ZnCl 2 unit is bound in the tridentate iminobipyridine pocket within L2 [N(1), N(2) and N(3)]; the corresponding 4 L1 has also been studied [Fig.7

(viii) and (ix)].
A structural reorganisation is observed on inspection of the theoretical (4 Lx ) and experimental structures (3c) and consists of a change of the relative positions of the chloride ligands on the metal centre.This structural rearrangement is linked to a permutation in and out of the medium plane of the N(2)-and N(3)-containing pyridine rings.These changes are most likely due to the absence of a second metal centre and the isolated condition of the complex in the theoretical calculations.The most important feature though, is that both theoretical structures do not produce any noticeable structural discrepancies (see Tables 4 and 6) with a difference in bond lengths of 0.04 A ˚and in angles of 4.2 • .
Several points emerge from inspection of the relative energies of the optimised structures established for the mono-zinc dichloride species, 1 L1 (1c), 1 L2 , 2 L2 , 2 L1 , 4 L2 and 4 L1 (Table 7).  ) suggests that an equilibrium is likely in the gas phase but with a slight preference towards 4 L2 .It is worthy of note that for both ligands the type-2 conformation is the least stable of all although, given the accuracy of the theoretical method employed, the significance of this inference should be treated with some caution.However, the isolation and structural characterisation only in the case NiBr 2 derivative 2b suggests that the nature of the halide group may have a stabilising influence for this particular conformation.This will be the subject of further study elsewhere.In summary, the density functional study carried out on the monometallic ZnCl 2 species with L1 and L2 as the ligand frame supports the experimental observations and indicates that the nature of the substituent on the imino-carbon moiety has a dramatic effect on the stability of the monometallic species.Several points emerge from the study.Firstly, it is apparent that the difference in the stability of species adopting conformation 1 Lx is mainly steric in origin which in turn directly influences the chelating properties around the metal.In the case of L1 steric interactions are minimised while good coordination within the terpyridine moiety is maximised.This stability allied with the substantial reduction of the solvent accessibility to the metal centre in this conformation would appear to suggest why 1 L1 (1c) does not react with a further molecule of ZnCl 2 to form a bimetallic complex.Secondly, for L2, all three conformations 1 L2 , 2 L2 and 4 L2 are distorted and can be considered as isoenergetic.These two points together indicate that an equilibrium could be operational in solution for L2 but not for L1 (Scheme 4).It would be expected, therefore, that further addition of a molecule of ZnCl 2 to [(L2)ZnCl 2 ] would drive the equilibrium towards the right hand side and allow occupation of the bidentate pocket in 4 to furnish bimetallic complex 3.

Conclusions
The bulky bis(imino)terpyridine ligand Lx has been used as an effective scaffold to support one or two metal(II) halide units.The nature of the imino-carbon substituent is found to have an effect on the number of metal halide units the ligand can bind with the aldimine ligand L1 showing a strong preference for a single unit while the ketimine L2 has the capacity to bind one or two metal centres.These experimental results have been further complemented by DFT calculations which reveal that a significant energy gap is apparent for L1-containing systems leading to a stabilised conformation of type-1 and also suggests that dynamic behaviour is likely for L2-supported species leading to bimetallic formation.

General
All reactions, unless otherwise stated, were carried out under an atmosphere of dry, oxygen-free nitrogen, using standard Schlenk techniques or in a nitrogen purged glovebox.Solvents were distilled under nitrogen from appropriate drying agents and degassed prior to use. 11The IR spectra were recorded on a Perkin-Elmer Spectrum One FT-IR spectrometer on solid samples.The ES and the FAB mass spectra were recorded using a micromass Quattra LC mass spectrometer and a Kratos Concept spectrometer with methanol or NBA as the matrix respectively.Accurate Mass FABMS were recorded on Kratos Concept spectrometer (xenon gas, 7 kV) with NBA as matrix. 1 H and 13 C NMR spectra were recorded on a Bruker ARX spectrometer (250 or 300 MHz); chemical shifts (d) are referred to the residual protic solvent peaks.Magnetic Susceptibility studies were performed using an Evans Balance (Johnson Matthey) at room temperature.The magnetic moments were calculated following standard methods 12 and corrections for underlying diamagnetism were applied to the data. 13Elemental analyses were performed at the Science Technical Support Unit, London Metropolitan University.

Conversion of 2b to 3b
An oven-dried Schlenk flask equipped with a magnetic stir bar was evacuated and backfilled with nitrogen.The flask was charged with (DME)NiBr 2 (0.048 g, 0.157 mmol) and n-BuOH (10 ml) and the suspension heated to 110   where n is the number of reflections and p the number of parameters.

Density functional calculations
Quantum mechanical calculations have been carried out using the Gaussian 03 package of programs. 14The density functional theory (DFT) was applied, in particular the functional Becke's three-parameter hybrid exchange method combined with LYP correlation functional (B3LYP). 15The quasi-relativistic effective core potential (ECP) LANL2DZ was used for the metal atoms (Zn). 16 The valence double-f with polarisation 6-31 G(d)

Fig. 2
Fig.2Molecular structure of L2 including the atom numbering scheme.

Fig. 3
Fig. 3 Molecular structure of 1a; the atoms labelled with an additional A are generated by symmetry (1 − x, −y, z).All hydrogen atoms, apart from H13, have been omitted for clarity.

Fig. 4
Fig. 4 Molecular structure of 3c including the atom numbering scheme.All hydrogen atoms have been omitted for clarity.

Fig. 5 Table 5
Fig. 5 Molecular structure of 2b including the atom numbering scheme.All hydrogen atoms have been omitted for clarity.Table 5 Selected bond distances (A ˚) and angles ( • ) for 2b
(iii)].The geometry around the metal is also altered in the fact that both chlorine atoms while equivalent in 1 L1 are non-equivalent in 1 L2 leading to a distorted structure intermediate between trigonal-bipyramidal and squarebased pyramidal.The planarity of the tridentate portion of the ligand is significantly reduced in comparison with 1 L1 [out of plane displacement: 0.31 A ˚(1 L2 ) vs. 0.03 A ˚(1 L1 )].Overall, a monometallic zinc dichloride species supported by L2 exhibits a considerable deformation when it is forced to adopt a type-1 conformation.

Table 2 Characterisation
data for the new complexes 1-3 Compound Colour m(C=N) a /cm −1 l eff b /l B 1 H NMR c (d) F A B m a s s 1a Brown 1635 5.3 d 733 [M] + ,698 [M − Cl] + 1b Yellow 1640 2.8 d 746 [M − Br] + , 665 [M − 2Br] + 1c Yellow 1638 e 1.49 (d, 24H, 3 J HH 6.7 Hz, CH(CH a Recorded on a Perkin-Elmer Spectrum One FT-IR spectrometer on solid samples.b Recorded on an Evans Balance at room temperature.c Recorded in CDCl 3 solution at room temperature.d Broad paramagnetically shifted resonances.e Diamagnetic.

Table 3
Selected bond distances (A ˚) and angles ( •
• C until the nickel This journal is © The Royal Society of Chemistry 2006 Dalton Trans., 2006, 2350-2361 | 2359

Table 8
Crystallographic and data processing parameters for L2

, 1a-1c, 2b, 3a, 3b(H
19,18basis was used for N and Cl and the minimal basis STO-3G for C and H. Bruker APEX 2000 CCD diffractometer.Details of data collection, refinement and crystal data are listed in Table8.The data were corrected for Lorentz and polarisation effects and empirical absorption corrections applied.Structure solution by direct methods and structure refinement on F 2 employed SHELXTL version 6.10.19Hydrogen atomsere included in calculated positions (C-H = 0.96 A ˚) riding on the bonded atom with isotropic displacement parameters set to 1.5 U eq (C) for methyl H atoms and 1.2 U eq (C) for all other H atoms.With the exception of 3b(H 2 O) (only Ni and Br refined anisotropically) and 1c (all atoms apart from C21 and C9), all non-H atoms were refined with anisotropic displacement parameters.Disordered MeCN was omitted using the SQUEEZE option in PLATON for 1b and 2b.CCDC reference numbers 289324-289331.For crystallographic data in CIF or other electronic format see DOI: 10.1039/b516083a