Cyclometalated Ir ( III ) complexes of deprotonated N-methylbipyridinium ligands : e ff ects of quaternised N centre position on luminescence †

(C^N = cyclometalating ligand; N^N = α-diimine) have been isolated and characterised as their PF6 and Cl salts. Four of the PF6 − salts have been studied by X-ray crystallography, and structures have been obtained also for two complex salts containing MeCN and Cl or two Cl ligands instead of N^N. The influence of the position of the quaternised N atom in C^N and the substituents on N^N on the electronic/optical properties are compared with those of the analogous complexes where C^N derives from 1-methyl-3-(2’-pyridyl)pyridinium (B. J. Coe, et al., Dalton Trans., 2015, 44, 15420). Voltammetric studies reveal one irreversible oxidation and multiple reduction processes which are mostly reversible. The new complexes show intramolecular charge-transfer absorptions between 350 and 450 nm, and exhibit bright green luminescence, with λmax values in the range 508–530 nm in both aqueous and acetonitrile solutions. In order to gain insights into the factors that govern the emission properties, density functional theory (DFT) and time-dependent DFT calculations have been carried out. The results confirm that the emission arises largely from triplet excited states of the C^N ligand (LC), with some triplet metal-toligand charge-transfer (MLCT) contributions.

In the context of luminescence, Ir III complexes with C^N ligands derived from pyridinium species were apparently unknown until we reported those of deprotonated 1-methyl-3-(2′-pyridyl)pyridinium (3,2′-C^N). 45The only previous account of structurally related species concerns catalytic studies with hydride complexes that are unsuitable for luminescence. 46 number of reports of cyclometalated complexes of N-methylbipyridinium species with Pd/Pt 47 or Ru 48 have appeared.The bright blue or blue-green emission and aqueous solubility of our Ir III 3,2′-C^N species 45 suggests potential uses in highly efficient OLEDs and/or bio-sensing/imaging.Since the use of Ir III complexes in the latter context [16][17][18][19][20]49 is often restricted by poor water solubility, 19 increasing their charge from the usual +1 to +3 is beneficial. Ths structural novelty opens doors for designing further complexes of C^N ligands based on quaternised bipyridinium units with attractive electronic and optical properties.In the previously published complexes [Ir III (C^N) 2 (N^N)] 3+ (N^N = α-diimine), both the nature and energy of the emission are highly influenced by substituents on the ancillary N^N ligand.45 Density functional theory (DFT) calculations show that the blue emission observed when N^N = 2,2′-bipyridyl (bpy) or 4,4′-( t Bu) 2 bpy is mainly triplet ligand-centred ( 3 LC) with some triplet metal-to-ligand chargetransfer ( 3 MLCT) character from C^N. Onthe other hand, the blue-green emission observed when N^N = 4,4′-(CF 3 ) 2 bpy has 3 L′C with some 3 ML′CT nature, due to efficient inter-ligand energy transfer to N^N (L′).
In addition to changing the substituents on N^N, it is of interest to study the effects on the emission properties of varying the location of the quaternised N centre in the C^N ligand.Here, we present a series of new Ir III complexes related to those described recently, but with the C^N ligands derived instead from 1-methyl-2-(2′-pyridyl)pyridinium of 1-methyl-4-(2′-pyridyl)pyridinium.Using as the ancillary ligand bpy, 4,4′-( t Bu) 2 bpy or 4,4′-(CF 3 ) 2 bpy allows detailed comparisons and reveals the importance of the position of the quaternised centre in achieving blue emission.

Materials and procedures
The compound 1-methyl-3-(2′-pyridyl)pyridinium hexafluorophosphate and the complex salts 4P-6P and 4Cl-6Cl were synthesised as described previously. 45All other reagents were obtained commercially and used as supplied.Products were dried overnight in a vacuum desiccator (silica gel) prior to characterisation.In each case, the bold number refers to the complex cation, while the counter-anions are denoted by P for PF 6  − or C for Cl − .Edinburgh Instruments FP920 Phosphorescence Lifetime Spectrometer equipped with a 5 W microsecond pulsed xenon flashlamp.Lifetime data were recorded following excitation with an EPL 375 picosecond pulsed diode laser (Edinburgh Instruments), using time-correlated single photon counting (PCS900 plug-in PC card for fast photon counting).Lifetimes were obtained by tail fitting on the data obtained, or by a reconvolution fit using a solution of Ludox® in the scatterer, and the quality of fit judged by minimisation of reduced chi-squared and residuals squared.Quantum yields were measured upon excitation at 420 nm by using a SM4 Integrating Sphere mounted on an Edinburgh Instruments FP920 Phosphorescence Lifetime Spectrometer.

Theoretical studies
DFT and time-dependent DFT (TD-DFT) calculations were undertaken on the complex cations 1-3 and 7 by using Gaussian 09. 53Geometry optimisations of the singlet ground (S 0 ) and first triplet excited (T 1 ) states and subsequent TD-DFT calculations were carried out by using the M06 functional 54 with the Def2-QZVP 55,56 basis set and pseudopotential on Ir and Def2-SVP 57 on all other atoms.MeCN was used as CPCM solvent model. 58,59Using these parameters, the first 100 excited singlet states were calculated and simulated UV-vis absorption spectra were convoluted with Gaussian curves of fwhm 3000 cm −1 by using GaussSum. 60his This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.

Synthesis and characterisation
The new complexes 1-3 and 7-9 (Fig. 1) were synthesised by following the procedure used previously for 4-6. 45The cyclometalated chloride-bridged dimeric intermediates were isolated in crude form only, but identified by 1 H NMR spectroscopy.Reacting these dimers with the appropriate N^N ligand in the presence of AgPF 6 affords the hexafluorophosphate salts 1P-3P and 7P-9P which were isolated after purification by column chromatography on Sephadex SP C-25.
Yields are in the range ca.30-60%.The chloride salts 1C-3C and 7C-9C were prepared in near quantitative yields by anion metathesis with [N n Bu 4 ]Cl in acetone.The identities and purities of the new complex salts are confirmed by 1 H NMR spectroscopy, elemental analyses and +electrospray mass spectrometry, and in several cases also by X-ray crystallography (see below).Portions of representative 1 H NMR spectra for the bpycontaining complex salts 1P, 4P and 7P are depicted in Fig. 2, showing large changes as the position of the quaternised N atom varies.
In order to further characterise the system, 13 C NMR experiments were carried out for selected compounds 1P, 4P and 7P in CD 3 CN (see the ESI, Table S1 †).The signals were assigned via HMQC and HMBC spectroscopy.As expected, the 13 C NMR signals arising from the cyclometalating ring show the most variability as the quaternised N atom moves around.Notably, the cyclometalated carbon atom in 4P (175.08 ppm) is deshielded when compared with 1P (157.05ppm) and 7P (161.66 ppm).This difference is attributed to the electron-withdrawing effect of the quaternised N atom located in the paraposition in 4P.The observed low field chemical shift is similar to those found for the carbenic carbon atom in related Ir III complexes of N-heterocyclic carbene ligands, 49b,61-64 and gives an indication of the carbene-like character of the cyclometalated C atom in complexes 4-6.

Structural studies
Single crystals were obtained for solvated forms of 1P-3P and 7P, and also unexpectedly for the chloride complexes in 10P and 11P.Crystallographic data and refinement details are summarised in Table 1.Representions of the molecular structures of the complex cations are shown in Fig. 3, and selected distances and angles are presented in Table 2.The structures of solvates of 4P and 6P have been reported previously, 45 and data are included here for comparison purposes.
All of the tris-chelate complexes in 1P-4P, 6P and 7P exhibit a pseudooctahedral geometry around the Ir centre with two cyclometalating bipyridinium ligands and one bidentate bpy, 4,4′-( t Bu) 2 bpy or 4,4′-(CF 3 ) 2 bpy ligand.The strong trans effects of the C-donor fragments affect the structures in two important ways.First, these units adopt a cis geometry, so the pyridyl rings of the C^N ligands are trans disposed.Second, the Ir-N distances to the N^N ligand are extended by ca.0.07-0.09Å when compared with those to the C^N ligand in all cases, except for one of the independent complexes in 4P.The Ir-N distances to the N^N ligand are not affected significantly by varying the R substituents.The bite angle of N^N is essentially constant at ca. 77°, while that of the C^N ligand ranges from ca. 82-89°.Inspection of the bond distances within the cyclometalating rings does not reveal any clear evidence for quinoidal character in 4P or 6P that might be expected due to their carbene-like nature indicated by other physical measurements.
The structures broadly resemble those reported for related monocationic Ir III complexes, although the C^N bite angles are a little larger than those observed typically (ca.80-81°). 34,36,39,65he fortuitously obtained structure of 11P is relatively unusual.Various chloride-bridged dimeric Ir III complexes with C^N derived from 2-phenylpyridine ( ppy) or its derivatives have been characterised crystallographically. 66 However, apparently the only reported structure of a related monometallic dichloride complex is that of cyclometalated 2-(2,4-difluorophenyl)pyridine, crystallised as a monoanion with the cation [Ir III (C^N) 2 (bpym)] + (bpym = 2,2′-bipyrimidine) containing the same ligand. 67The average Ir-Cl distance in this known compound of 2.493(4) Å is slightly longer than the corresponding distance in 11P (2.464(1) Å, Table 2).Several neutral complexes Ir III (C^N) 2 Cl(MeCN) have been reported previously, 66a,68 Fig. 1 Structures of the studied tris-chelate complex salts and the bis-chelates characterised by X-ray crystallography only.

Electrochemistry
The results of cyclic and differential pulse voltammetric measurements on the PF 6 − salts 1P-9P recorded in acetonitrile are shown in Table 3. Cyclic voltammograms of 1P-3P are shown in Fig. 4. All potentials are quoted in V with respect to the Ag-AgCl reference electrode.
Each compound shows an irreversible oxidation process in the region ca.2.2-2.5 V, which might be formally assigned to an Ir IV/III couple.The relatively high potentials when compared with related complexes of ppy 6,[11][12][13][33][34][35][36][37][38][39]42 are attributable to the presence of the electron-deficient pyridinium units. DFT calcuations (see below) show that the C^N ligands contribute to the HOMO significantly.With a given bipyridinium isomer, the E pa value increases in the order N^N = 4,4′-( t Bu) 2 bpy < bpy < 4,4′-(CF 3 ) 2 bpy, showing a modest influence (130-190 mV) of the R substituents.As N^N is kept constant, the E pa value increases in the order C^N = 2,2′-≈ 4,2′-< 3,2′-, revealing that the complexes in which the quaternised N atom is located para to the Ir centre are the most difficult to oxidise (by 180-240 mV).
The reductive regions include multiple processes (Fig. 4).For the 2,2′-C^N complexes in 1P-3P, four reversible waves are observed, corresponding with sequential one-electron reductions to give a monoanionic final product.The E 1/2 values are similar for the complexes of bpy (1) and 4,4′-( t Bu) 2 bpy (3), while those for the complex of 4,4′-(CF 3 ) 2 bpy (2) are lower by 60-420 mV, because the electron-withdrawing influence of the -CF 3 groups facilitates reduction.Similar behaviour is shown by the 4,2′-C^N complexes in 7P-9P, except that 7 and 8 display only three waves instead of four.For both 2,2′-and 4,2′-C^N series, the 1+/0 E 1/2 value shows the largest dependence on R, suggesting that this third reduction is localised on the N^N ligand.Therefore, the first and second reductions may be assigned to the C^N ligands.In contrast, the reductive behaviour of the 3,2′-C^N complexes in 4P-6P is much less well-defined, with most of the processes being irreversible.Also, sharp anodic return peaks are observed for 1 and 3, indicative of adsorption onto the electrode surface.The nature of these data preclude the discernment of any further trends.

Electronic absorption spectroscopy
The absorption spectral data for the PF 6 − salts of the new complexes 1-3 and 7-9 recorded in acetonitrile are shown in Table 4, together with those for 4-6. 45Corresponding data for the Cl − salts in water are collected in Table S2, † and spectra of 1P-4P and 7P are shown in Fig. 5.All of complexes 1-9 show intense bands below 320 nm which are assigned to π → π* and high energy MLCT transitions involving both the C^N and N^N ligands.Weaker bands are observed also, with tails extending up to ca. 480 nm in some cases.The lowest energy (LE) band is clearly blueshifted in the 3,2′-C^N complexes 4-6 (λ max for shoulders ≈ 350-360 nm) when compared with the new complexes 1-3 and 7-9 (λ max for shoulders ≈ 425-445 nm) (Fig. 5a).DFT shows that this band in 4-6 has 1 MLCT character with some 1 ML′CT and also 1 LL′CT contributions for 4 and 5 (L = C^N; L′ = N^N). 45The slight shifts (ca.0.1 eV) when R changes (Fig. 5b) are consistent with the largely 1 MLCT assignment.The observed blue-shifts when varying the structure of the C^N ligands are attributable to destabilisation of their π* orbitals because the para-position of the quaternised N increases its neutral carbene character.Therefore, the energy gap of the MLCT transition increases in 4-6 with respect to their isomeric counterparts 1-3 and 7-9.The absorption spectra are almost unaffected by changing the counter-anions and solvent (Tables 4 and S2 †).

Luminescence properties
The emission spectral data for the PF 6 − and Cl − salts of the new complexes 1-3 and 7-9 recorded in deoxygenated and oxygenated acetonitrile or water are shown in Table 5, together with those for 4-6. 45Spectra of 1P, 4P and 7P are shown in Fig. 6, while those of the other compounds are in the ESI (Fig. S1 and S2 †).Changing the counter-anion and solvent has only slight effects on the excitation profiles that remain constant in all cases while monitoring at all the emission maxima.All of the spectra are structured, especially for complexes 4-6, 45 indicating significant ligand contributions to the luminescence.The profiles and emission energies of 1-3 and 7-9 are similar and red-shifted when compared to 4-6 (Fig. 6, S1 and S2 †).The importance of the position of the quaternised  N is evident; when it is located meta to the cyclometalating C atom, green emission is observed (λ max = 508-530 nm), but when it is positioned para to the C, blue or blue-green emission arises (λ max = 468-494 nm).This difference is attributable to the almost carbene-like character and consequent relative orbital energies in 4-6.Various other types of Ir complexes emit in the green region, e.g.[Ir III (ppy-PBu 3 ) 3 ][PF 6 ] 2 [ppy-PBu 3 = cyclometalated 2-(5-tri-n-butylphosphoniumphenyl)pyridine] 69 and monocationic complexes of 4,4′-( t Bu) 2 bpy with -SF 5 substituents on the C^N ligands. 70he influence of the R substituents in the 2,2′-C^N (1-3) and 4,2′-C^N (7-9) complexes is small.The emission energies follow the expected trend R = t Bu < H < CF 3 , but with a difference of only ca.0.05 eV between the extremes (Table 5).This effect can be attributed to the stabilisation of the metal orbitals caused by placing electron-withdrawing groups on the ancillary ligand.These results suggest that in all of these new complexes the emission has a mainly 3 LC character involving the C^N ligand with some 3 MLCT contribution.By contrast, for the 3,2′-C^N complexes, such a situation pertains for 4 and 6, but 5 behaves differently and gives N^N-based emission. 45herefore, in that series the emission energy is decreased (and the bands red-shifted) for the -CF 3 derivative with respect to the other complexes.
When keeping N^N constant, the quantum yields (Φ) of 1-3 and 7-9 are generally similar to, or a little larger than those of   4-6 (Table 5), showing that moving the quaternised N to a meta-position with respect to the cyclometalating C does not facilitate non-radiative decay.In contrast to the almost invariant emission energies, τ always increases and Φ increases in most instances (sometimes markedly) on moving from a PF 6 − salt in acetonitrile to its Cl − analogue in water.In all cases, both τ and Φ increase substantially on deoxygenation, consistent with emission originating from triplet excited states.

Theoretical calculations
DFT and TD-DFT calculations have been carried out on the complexes 1-3 and 7 by using the M06 functional with the Def2-QZVP basis set on Ir and Def2-SVP on all other atoms, as used for 4-6 previously. 45he optimised structures in the ground state agree well with the data obtained from X-ray crystallography (see the ESI, Table S3 †).Selected MOs are depicted in Fig. S3-S6, † and the orbital compositions are shown in Table S4.† The frontier orbitals HOMO−2 to LUMO+2 are essentially invariant within the cations 1-3 and 7.The HOMO−1 and HOMO−2 are mainly centered on the Ir atom (67-72%) with some contribution from the C^N ligand (16-22%), while the HOMO has almost the same contribution from both fragments (Ir 47-55%; C^N 43-51%).The LUMO and LUMO+1 are based on C^N almost completely (96-98%), while the LUMO+2 is located on N^N (95-97%).
The S 0 → S 1 transition energies calculated in acetonitrile and the corresponding major orbital contributions are presented in Table S5 (ESI †).The simulated absorption spectra for complexes 1-3 agree well with those measured (Fig. 7).The calculated wavelengths of the LE transitions 421 (1), 412 (2), 424 (3) and 416 nm ( 7) are slightly blue shifted with respect to the experimental λ max values (437 (1), 426 (2), 445 (3) and 435 nm (7); Table 4).In all complexes, this transition has almost pure HOMO → LUMO character (92-95%).Therefore, the LE bands can be assigned to a mixture of 1 LC and 1 MLCT (L = C^N).The observed modest dependence of the LE transition on the R substituents is reproduced by the calculations on 1-3, i.e.ΔE increases in the order R = t Bu < H < CF 3 .Comparisons with the data obtained for 4-6 45 show that the level of theory applied also predicts the red-shifts of the LE band observed on moving from the 3,2′-C^N complexes to their 2,2′-C^N counterparts 1-3 (and for 4 → 7); the calculated λ values are 347 (4), 342 (5) and 348 nm (6).
For 1-3 and 7, the first computed transition involving mainly the N^N ligand (i.e. a dominant component to LUMO+2; in all cases HOMO−2 → LUMO+2 1 MLCT) lies to markedly higher energy when compared with the LE transition.The electronic influence of the R substituents is manifested in the predicted energies.The transition is at 3.90 eV (318 nm) for 1, 3.99 eV (311 nm) for 3 and 3.87 eV (320 nm) for 7, but 3.69 eV (336 nm) for 2 due to the stabilisation of the LUMO+2 by the -CF 3 groups.
The nature of the luminescence was addressed by optimising the first triplet excited states T 1 of 1-3 and 7.The computed geometric parameters of these states are similar to those found for the corresponding ground states S 0 (Table S3, ESI †).The lowest energy emissions calculated in acetonitrile as ΔE(T 1 − S 0 ) at 531 (1), 527 (2), 538 (3) and 521 nm (7) are close to the experimental values (526 (1), 519 (2), 530 (3) and 520 nm (7); Table 5).Also, the calculations reproduce the experimental trend in the emission wavelengths (λ em 2 < 1 < 3).The spin densities for the T 1 state for all complexes (Fig. 8) are located mainly on one C^N ligand, together with the Ir atom to   a lesser extent.Therefore, the emissions can be assigned as 3 LC involving the cyclometalating ligand with some 3 MLCT contribution.These results are similar to those obtained for complexes 4 and 6, 45 showing that the only complex of the nine studied with a different nature of the emission is 5.In that case, the -CF 3 substituents stabilise the 3 L′C state, causing efficient inter-ligand energy transfer from the C^N to the emitting N^N fragment.In complex 2, the stabilisation due to the -CF 3 groups is insufficient to bring the energy of the 3 L′C below the 3 LC state.

Conclusions
We have synthesised and characterised a series of new Ir III complexes by using three different isomers of 1-methyl-(2′-pyridyl)pyridinium to generate cyclometalating ligands C^N.
Because the latter are charge-neutral, adding an α-diimine ancillary ligand N^N affords species with a 3+ charge and unusually high solubility in water when isolated as Cl − salts.Such enhanced aqueous solubility increases the prospects for applications in bioimaging.The complexes are characterised fully as both their PF 6 − and Cl − salts, and in four cases, their structures are confirmed by single-crystal X-ray crystallography.Electrochemistry reveals for each complex an irreversible oxidation of the {Ir III (C^N) 2 } 3+ unit and multiple ligand-based one-electron reductions.The reductive behaviour is much better defined for the new 2,2′-C^N and 4,2′-C^N complexes when compared with their 3,2′-C^N counterparts, with up to four reversible waves being observed.Within each series, the redox potentials are affected significantly by varying the R substituents on the N^N ligand.All of the complex salts appear yellow coloured and their UV-vis absorption spectra show low energy bands in the region ca.350-450 nm.DFT and TD-DFT calculations using the M06 functional confirm that these are attributable to 1 MLCT transitions with some 1 LC, 1 ML′CT and also 1   analogues are reproduced theoretically.In contrast to the 3,2′-C^N complexes that show intense blue or blue-green emission, the new isomeric species are all green emitters.The emission is shown by DFT to originate from 3 LC excited states involving the cyclometalating ligand with some 3 MLCT contribution.Therefore, changing the N^N ligand has only a minor influence on the luminescence.Neither the absorption nor emission spectra show more than slight solvent dependence.In terms of future prospects, there is great scope for modifying the optical properties of these highly charged complexes, for example by changing the substituent on the quaternised N atom and/or attaching other groups to either the C^N or N^N ligands.

1 H
NMR spectra were recorded on a Bruker UltraShield AV-400 spectrometer, with all shifts referenced to residual solvent signals and quoted with respect to TMS.Elemental analyses were performed by the Microanalytical Laboratory, University of Manchester, and UV-vis spectra were obtained by using a Shimadzu UV-2401 PC spectrophotometer.Mass spectra were recorded by using +electrospray on a Micromass Platform II spectrometer.Cyclic voltammetric measurements were performed by using an Ivium CompactStat.A single-compartment cell was used with a silver/silver chloride reference electrode (3 M NaCl, saturated AgCl) separated by a salt bridge from a 2 mm disc platinum working electrode and platinum wire auxiliary electrode.Acetonitrile was used as supplied from Sigma-Aldrich (HPLC grade), and [N n Bu 4 ]PF 6 (Fluka, electrochemical grade) was used as the supporting electrolyte.Solutions containing ca. 1.5 × 10 −4 M analyte (0.1 M [N n Bu 4 ]PF 6 ) were deaerated by purging with N 2 .All E 1/2 values were calculated from (E pa + E pc )/2 at a scan rate of 100 mV s −1 .Steadystate emission and excitation spectra were recorded on an 25Et 2 O) and hydrogen atoms were included in idealised positions by using the riding model, with thermal parameters 1.2 times those of aromatic parent carbon atoms, and 1.5 times those of methyl parent carbons.The asymmetric unit of 1P•1.5Me 2 CO•0.25Et 2 O contains the complex cation, three disordered PF 6 − anions, an ordered acetone molecule, a disordered acetone at 0.5 occupancy and a disordered diethyl ether molecule at 0.25 occupancy.All non-H atoms were refined anisotropically, except for the disordered diethyl ether; some restraints were applied for the disordered atoms.H atoms were included in calculated positions, except those of the disordered diethyl ether, which were omitted.The asymmetric unit of 2P•2C 4 H 8 O 2 contains the complex cation, three PF 6 − anions and two 1,4-dioxane molecules.The C-O distances in one 1,4-dioxane molecule had to be restrained to 1.4 Å and the displacement parameters for the six ring atoms were refined by using the RIGU and DELU commands.The asymmetric unit of 3P•4MeCN contains the complex cation, three PF 6 − anions, one of which is disordered, and four acetonitrile molecules, three of which are disordered.Restraints were applied to the F atoms of the disordered PF 6 − .Crystallographic data and refinement journal is © The Royal Society of Chemistry 2015 Dalton Trans., 2015, 44, 20392-20405 | 20395 Open Access Article.Published on 20 October 2015.Downloaded on 9/27/2018 1:56:05 AM.

Fig. 2
Fig. 2 Aromatic regions of the 1 H NMR spectra of the PF 6 − salts of complexes 1 (blue), 4 (red), and 7 (green) recorded at 400 MHz in CD 3 CN.The asterisks denote the signals attributed via COSY studies to the protons of the N^N ligand.

a
Solutions ca. 1 × 10 −5 -2 × 10 −4 M; ε values are the averages from measurements made at three or more different concentrations (with ε showing no significant variation).b Denotes the position with respect to the quaternised N atom of the N-coordinated 2′-pyridyl ring.c Data taken from ref. 45.
Fig. 7 M06/Def2-QZVP/Def2-SVP-calculated (blue) UV-vis spectra of (a) 1, (b) 2 and (c) 3, and the corresponding experimental data (green) for the PF 6 − salts in acetonitrile.The ε-axes refer to the experimental data only and the vertical axes of the calculated data are scaled to match the main experimental absorptions.The oscillator strength axes refer to the individual calculated transitions (red).

Fig. 8
Fig.8M06/Def2-QZVP-SVP-calculated spin density plots for the T 1 state of the complexes 1, 2, 3 and 7.In each case, the N^N ligand is pointing upwards.