Yuji Sumimoto

Kodai Iijima

Dongho Yoo

Tadashi Kawamoto

Yann Le Gal

Dominique Lorcy

Takehiko Mori

Structures and transistor properties of extended and unsymmetrical birhodanines †

Keywords: devices fabrication, AFM images, and transistor characteristics

Birhodanines exhibit n-channel transistor properties with air stability. In this work, birhodanines with extended skeletons are investigated, in which a phenylene or quinoidal moiety is inserted in the central C=C part. We have also prepared N-phenyl and unsymmetrically N-substituted derivatives, including an unsubstituted N-H part. These compounds show n-channel transistor properties. In contrast to the herringbone structure of the parent compounds, the phenylene and phenyl compounds have stacking structures. The phenylene substitution decreases the acceptor ability, whereas quinoidal substitution improves the acceptor ability and air stability of the transistors. The N-H derivative has a Z-form, suggesting contribution of the enolic form, and unsymmetrically N-substituted derivatives have double-layer structures with ordered alkyl chains.

Introduction

Considerable attention has been paid to organic semiconductors used in organic transistors. 1 Although various high-performance materials have been proposed, 2 there still remains a lot of room in nchannel materials showing air-stable operation. 1 Previously, we have reported air-stable n-channel transistor properties in 5,5'bithiazolidinylidene-2,4,2',4'-tetrathiones (Scheme 1, SS-R). 3,4 These materials are obtained in the course of developing singlecomponent organic metals, 5 but show strong acceptor ability with the lowest unoccupied molecular orbitals (LUMO) around -4.2 eV. 4 The R = ethyl and propyl compounds (SS-Et and SS-Pr) exhibit excellently air-stable n-channel transistor characteristics. 3,4 Materials with longer alkyl chains show reduced performance, 6 though single-crystal transistors of the pentyl compound exhibit a mobility of 1.1 cm 2 V -1 s -1 . 7 In addition to these tetrathione derivatives, the 2,2'-dione-4,4'-dithione derivatives (OS-R) are regarded as a dimer of rhodanine, which is a well-known electron-deficient unit. 8 Birhodanine is easily prepared by reacting alkyl amine, carbon disulfide, and dimethyl acetylene dicarboxylate for several minutes at room temperature. 4 The LUMO level of OS-R is located around -3.8 eV, 4 which is a weaker electron acceptor than SS-R. SS-R has a stacking structure, whereas OS-R has a herringbone structure. OS-R shows similar transistor properties, though the performance and air stability are slightly reduced.

As an element of organic conductors, various π-extended molecules have been investigated. 9 In particular, introduction of a quinoidal unit is a versatile tool to improve the donor/acceptor ability. 10 In addition, even superconductivity has been found in vinylogous bis-fused tetrathiafulvalene. 11 Inspired by these findings, we have prepared π-extended birhodanines (Scheme 1). In 1, a phenylene unit is inserted in the central C=C part, 12 whereas in 2 a quinoidal part is introduced. 13 We have also developed a synthetic route to N-phenyl substituted compound, OS-Ph, and unsymmetrically N-substituted derivatives, OS-RR' containing different alky chains as well as an unsubstituted N-H part.

Results and discussion

Preparation n-Propyl and phenyl rhodanines 3-R were obtained from the dithiocarbamate and chloroacetic acid. 14 Similarly to the previous reactions, 12,15 the rhodanines were reacted with terephthalaldehyde in the presence of triethylamine to afford 1-R (Scheme 2). In order to prepare 2-R, rhodanines 3-R (R = Pr, Bu, and Ph) were reacted with 1,4-cyclohexanedione in the presence of ammonia. 13 The resulting 4-R was reacted with 2,3-dichloro-5,6-dicyano-p- a The LUMO levels were estimated from the first reduction potentials by assuming the reference energy level of ferrocene/ferrocenium to be 4.8 eV from the vacuum level. 17 The HOMO levels were obtained by subtracting the optical gaps from the LUMO levels. The values in the parentheses were calculated by using the ADF software with the B3LYP* functional and TZP basis set. 18 b The values in the parentheses were from absorption of the thin films.

A c c e p t e d M a n u s c r i p t benzoquinone (DDQ) to give 2-R. OS-Ph was not obtained by the direct reaction of aniline with carbon disulfide and dimethyl acetylene dicarboxylate. 4 Instead, rhodanine 3-Ph was condensed with isatine 5-Ph to afford OS-Ph. 16 The same reaction was applied to prepare unsymmetrically substituted OS-RR' (RR' = EtMe, PrMe, BuMe, and PrEt). Using this route, we can also obtain Nunsubstituted compounds (RR' = EtH and BuH). Among these compounds, 1-Pr, 2-Pr, and unsymmetrical compounds are new compounds.

Electrochemical properties

Cyclic voltammetry of these compounds shows clear reduction waves (Fig. 1), from which the LUMO levels are estimated as listed in Table 1. Compound 1 is a weaker acceptor than the parent OS-R due to the insertion of a phenyl ring. By contrast, 2 is a stronger acceptor than OS-R in agreement with the quinoidal structure, which is as strong as SS-R. 4 Optical gaps of 1 are slightly larger than OS-R, whereas those of 2 are significantly smaller than OS-R. As a result, the LUMO levels largely depend on the π-extension manner, though the highest occupied molecular orbital (HOMO) levels of these compounds are approximately the same. OS-Ph and OS-RH are slightly stronger acceptors than OS-Et, indicating the alkyl groups are weakening the acceptor ability. The reduction of OS-RH is, however, somewhat irreversible. 2.

1-Pr is a planar molecule except for the propyl chains (Fig. 2). A molecule is located on an inversion center, and a half molecule is crystallographically independent. The molecules are uniformly stacked along the a axis (Fig. 2(a)) with a large tilt angle (45 o ) of the molecular long axis from the stacking (a) axis. The molecular long axis does not correspond to neither the b or c axis (Fig. 2(b)). In the stack, the molecules are largely displaced both along the molecular long and short axes (Fig. 2(c)), in which a thiazol sulfur is located approximately on the top of the central phenylene ring. The molecules are stacked with a short axis displacement similar to SS-R crystals (Fig. 2(d)). 4 There are short S-S contacts, 3.64 and 3.94 Å along the stack. The intrachain transfer is comparatively small, but the interchain transfers are even smaller (Fig. 2). Transfer integrals of the LUMO-LUMO interactions are a = 140.9 and p = 10.2 meV.

1-Ph has a planar core, though the terminal phenyl groups are tilted by 71.9 o from the core part (Fig. 3). A molecule is located on an inversion center, and a half molecule is crystallographically independent. The molecules are uniformly stacked along the b axis (Fig. 3(a)), where the molecular long axes are 26 o tilted from the stacking (b) axis. Owing to the monoclinic symmetry, molecules in the adjacent stacks are tilted in the opposite directions (Fig. 3(b)), where the interchain contacts run along the a + c axis (Fig. 3(c)). In the stack, the molecules are more largely displaced both along the molecular long and short axes (Fig. 3(d)). There are short S-S contacts, 3.63 and 4.04 Å, and significant intra-and inter-chain transfer integrals (Fig. 3).

OS-Ph has a planar core, though the terminal phenyl groups are tilted by 78.4 o from the core part (Fig. 4). A half molecule is crystallographically independent, and a molecule is located on an inversion center. Then, all molecules are parallel to each other, and stacked along the a axis (Fig. 4(a)). Accordingly, the packing pattern resembles SS-R rather than the herringbone OS-R. There are S-S contacts, 3.82, 4.01 and 4.02 Å along the stack, and a considerable intrachain transfer integral (Fig. 4). As shown in Fig. 5, unsymmetrically substituted OS-BuH has a planar core (Figs. 5(a) and (b)), but the sulfur atoms are located on the same side (Z-form). Since the O•••O distance is as small as 2.748 Å, we have to suspect hydrogen bonding, where the nitrogen H moves to O (Fig. S2). Although the ketone form is preferable to the enolic form in the crystal structure analysis, the contribution of the enolic form may be related to the Z-configuration.

The molecules are uniformly stacked along the b axis (Fig. 5(c)). The molecular long axes are parallel to the a axis (Fig. 5(b)), and the interchain interactions are along the c axis. In the stack, the molecules are largely displaced both along the molecular long and short axes (Fig. 5(a)); this overlap mode is very similar to SS-R. 4 The tilt angle of the molecular long axis from the stacking (b) axis is 33 o (Fig. 5 3.73, 3.78, 3.84, and 3.98 Å between these parallel molecules because the sulfur atoms are aligned in the same side. These are shorter than the intrastack S-S contacts of 3.94 and 4.06 Å. Transfer in this intercolumnar direction (p) is considerably large, and a two-dimensional network is formed.

OS-EtMe, OS-BuMe, and OS-PrEt have herringbone structures with the same space group (Fig. 6(a)), but are not entirely isostructural. OS-EtMe has a single layer structure along the a axis, whereas OS-BuMe and OS-PrEt have a double layer structure (Fig. 6(b)). This is because the methyl and ethyl groups in OS-EtMe are disordered, but other alkyl groups are ordered. In the symmetrical OS-R, the layers are alternately tilted in even R, but entirely parallel in odd R. In OS-BuMe and OS-PrEt, all layers are parallel, but still make a double layer structure because the longer alkyl chains are oriented to the same interlayer spacing.

Since the dihedral angle tends to increase with increasing the alkyl chain, 4 the observed dihedral angles (Table 3) are in the similar level to the symmetrical molecules. In contrast to the symmetrical OS-R, t c is larger than the diagonal transfer integrals, t p and t q , but forms a balanced two-dimensional network.

Thin film properties

Evaporated films of 1-Pr and 1-Ph show X-ray diffraction (XRD) peaks (Fig. 7(a)). However, d = 19.8 Å of 1-Pr does not agree with the crystal lattice (Table 2). Since the molecular length is estimated to be l = 20.8 Å from the crystal structure, the tilt angle θ in the thin film is expected to be θ = 19.8 o by using l = d/cosθ. The thin film structure is potentially different from the single-crystal structure. For 1-Ph, d = 14.2 Å is not far from a + b = 13.8 Å, corresponding to the molecular long axis (Fig. 3(c)). Then, the thin film structure is not largely different from the crystal structure. XRD of 2-R (R = Me, Et, Bu, and Ph) exhibit peaks at d = 12. 7, 13.0, 17.8, and 19.2 Å, respectively (Fig. 7(b)); these d values approximately correspond to the molecular lengths. Although crystal structures of these compounds have not been determined due to the comparatively poor crystallinity, these compounds construct ordinary layer structures.

The unsymmetrical OS-RR' show sharp XRD peaks with many higher-order peaks (Figs. 11.3, 12.1, 13.3, 13.6 Å, in agreement with the crystal a axis (Table 2). Then, the molecules are tilted from the substrate normal by an angle of 17~19 o (Table 3), which are in the same order as 18~19 o in the symmetrical molecules. Atomic force microscopic (AFM) images show highly crystalline morphology (Fig. 8). 1-Pr shows comparatively loose packing of large crystals (Fig. 8(a)). OS-RR' shows large crystals, which are consistent with the sharp XRD peaks, but large inter-grain space may be related to the transistor performance.

Transistor properties

These compounds show n-channel transistor properties as exemplified by Fig. 9. From the characteristics, the transistor parameters are extracted as listed in Table 4. 2-Et shows maximum mobility of 0.041 cm 2 V -1 s -1 , which is not largely different from 0.15 cm 2 V -1 s -1 in OS-Et. 4 This transistor shows reasonable air stability. Although the off current increases considerably, the drop of the on current is not significant. As a result, the on/off ratio drops sometimes more than one order, but the mobility does not drop to half. Since mobilities of OS-R transistors drop typically four times in air, 4 meaningful improvement of air stability is observed. In addition, although OS-R transistors show comparatively large threshold voltages, 2-Et exhibits a small threshold voltage. This is attributed to the drop of the LUMO levels from -3.8 eV to -4.2 eV due to the introduction of the quinoid structure (Table 1). The resulting LUMO level is as low as SS-R, although the air stability is not comparable to SS-R. The mobility of OS-Ph is not high, but the drop in air is comparatively small. This seems again to be associated with the relatively low LUMO level (-3.95 eV) as well as the same type of molecular packing as SS-R (Fig. 4). 1-Ph shows transistor properties, but 1-Pr does not show transistor properties. This is ascribed to the very small intermolecular transfers (Fig. 2). However, the performance of the 1-Ph transistor decreases two orders of magnitude in air. This is reasonable in view of the high LUMO level (-3.6 eV). We have also prepared OS-Ph, but the transistor properties are not significant. This may be related to the stacking structure with extremely onedimensional interaction (Fig. 4).

Unsubstituted compounds, OS-EtH and OS-BuH, do not seem preferable to transistor performance. OS-BuH exhibits moderate transistor performance (Table 4), but the performance drops in air, reflecting the LUMO level of -4.0 eV. In general, OS-PrMe, OS-BuMe and OS-PrEt show mobilities around 0.02 cm 2 V -1 s -1 . This is not as large as 0.15 cm 2 V -1 s -1 in OS-Et, but comparable to 0.05 cm 2 V -1 s -1 in OS-Pr with similar R length. 4 This is also contrasting to the reduced performance of disordered OS-EtMe. Although the double layer structure does not significantly improve the mobility as unsymmetrical benzothienobenzothiophene (BTBT), 19 it is important to maintain the ordered structure. Large threshold voltage is, however, unavoidable in OS-RR'.

In this context, 2-Et shows a significant improvement.

Conclusions

We have prepared birhodanine molecules with extended πskeletons. Compounds 2 with quinoid skeleton realize as strong acceptor ability as SS-R, and the air stability of the transistor performance and the threshold voltage are improved. Since the preparation of OS-R is easier than SS-R in general, this is a versatile way to obtain an air-stable n-type material. In contrast, 1-R shows decreased acceptor ability and (β"-type) stacking structures with short-axis displacement. Together with OS-Ph and OS-BuH, this stacking structure resembles SS-R rather than the herringbone OS-R.

We have developed a method to prepare unsymmetrical OS-RR', including R = H and Ph, which are not accessible in the conventional method. N-H and N-Ph compounds show a slight improvement of the acceptor ability. The Z-form of OS-BuH is probably related to the contribution of hydrogen bond of the enolic form, and the enhanced S-S interaction is not too disadvantageous to the carrier transport despite the lowered symmetry. Basically, unsymmetrical OS-RR' has doublelayered herringbone structures, 19 and the ordered structure achieves comparable transistor performance to the symmetrical OS-R.

  Scheme 1. Molecular structures of SS-R, OS-R, and the extended derivatives.

  Scheme 2. Preparation of 1-R, 2-R, and OS-RR'.

Fig. 1 .

 1 Fig. 1. Cyclic voltammograms of (a) 1-R, (b) 2-R, (c) OS-RH, and (d) OS-RR', measured in n-Bu4N•PF6/CH2Cl2 (50 mV s -1 ). Normalized absorption spectra of (e) 1-R, (f) 2-R, (g) OS-RH, and (h) OS-RR', measured in DMF.

Fig. 2 .

 2 Fig. 2. Crystal structure of 1-Pr, (a) viewed along the molecular short axis, (b) viewed perpendicular to molecular plane, (c) molecular overlap mode, and (d) viewed along the molecular long axis. Transfer integrals of the LUMO-LUMO interactions are a = 2.3 and p = 0.1 meV.

  single crystal structure analyses were carried out for 1-Pr, 1-Ph, OS-Ph, OS-BuH, OS-EtMe, OS-BuMe, and OS-PrEt. The crystal data are listed in Table

Fig. 3 .

 3 Fig. 3. Crystal structure of 1-Ph, (a) viewed along the molecular short axis, (b) viewed along the molecular long axis, (c) viewed along the b axis, and (d) molecular overlap mode. Transfer integrals of the LUMO-LUMO interactions are b = 51.3, p = 0.4, and q = 12.9 meV.

Fig. 4 .

 4 Fig. 4. Crystal structure of OS-Ph, (a) viewed along the molecular long axis, and (b) molecular overlap mode.Transfer integrals of the LUMO-LUMO interactions are a = 140.9 and p = 10.2 meV.

  Fig. 5. Crystal structure of OS-BuH, (a) molecular overlap mode, (b) viewed along the b axis, (c) viewed along the molecular short axis, and (d) viewed along the molecular long axis. Transfer integrals of the LUMO-LUMO interactions are b = 49.9, and p = 19.1, q= 5.3, and r = 12.7 meV.

  (c)), but every two columns are tilted in the opposite directions (Fig.5(d)). Accordingly, molecules in the central two columns in Figs. 5(b) and (d) are parallel to each other; this part has the same (β"like) structure as SS-R. There are many S-S contacts in this

Fig. 6 .

 6 Fig. 6. (a) Crystal structure of OS-EtMe viewed along the molecular long axis. (b) Crystal structure of OS-BuMe viewed along the molecular short axis.

  7(c) and (d)). The peaks around 4 o in OS-EtH and OS-BuH correspond to d = 20.1 and 23.5 Å, respectively, indicating the double layer structures. The peaks around 8 o in OS-EtMe, OS-PrMe, OS-BuMe, and OS-PrEt correspond to

  4 OS-BuMe shows a clear peak at 4 o , indicating the double layer structure. The low-angle peak is less obvious in other OS-RR'.

  Fig. 7. X-ray diffraction pattern of (a) 1-R, (b) 2-R, (c) OS-RH, and (d) OS-RR'.

Fig. 8 .

 8 Fig. 8. Atomic force microscopic (AFM) images of (a) 1-Pr (b) 2-Et (c) OS-BuH, (d) OS-EtMe, (e) OS-BuMe, and (f) OS-PrEt.

Fig. 9 .

 9 Fig. 9. (a) Transfer and (b) output characteristics of 2-Et thin-film transistors measured under vacuum (blue) and in air (red). Transfer characteristics of (c) OS-BuH, and (d) OS-PrEt.

Table 1 .

 1 Reduction potentials, energy levels, and optical gaps.

		Ered (V)	ELUMO (eV) a	λedge (nm) b	Optical gap (eV) b	EHOMO (eV) a
	OS-Et 4	-1.00	-3.80 (-3.80)	482 (438)	2.57 (2.8)	-6.37 (-6.44)
	1-Pr	-1.17	-3.63 (-3.63)	472 (508)	2.63 (2.4)	-6.26 (-6.25)
	1-Ph	-1.24	-3.56 (-3.56)	472	2.63	-6.19 (-6.28)
	2-Me	-0.41	-4.21	624	1.99	-6.20
	2-Et	-0.39	-4.23	621	2.00	-6.23
	2-Pr	-0.40	-4.20	607	2.04	-6.24
	2-Bu	-0.40	-4.20	617	2.00	-6.20
	2-Ph	-0.39	-4.20	623	1.99	-6.19
	OS-Ph	-0.68	-3.95 (-3.80)	508	2.53	-6.48 (-6.34)
	OS-EtH	-0.64	-3.98	508	2.44	-6.42
	OS-BuH	-0.62	-4.00 (-3.88)	514	2.41	-6.41 (-6.72)
	OS-EtMe	-0.96	-3.84 (-3.61)	493	2.51	-6.35 (-6.39)
	OS-PrMe	-0.96	-3.84	493	2.51	-6.35
	OS-BuMe	-0.96	-3.84 (-3.81)	487	2.54	-6.38 (-6.48)
	OS-PrEt	-0.97	-3.83 (-3.80)	489	2.53	-6.36 (-6.49)

Table 2 .

 2 Crystallographic data.

		1-Pr	1-Ph	OS-Ph	OS-BuH	OS-EtMe	OS-BuMe	OS-PrEt
	Formula	C20H20N2O2S4	C22H18N2O2S4	C18H10N2O2S4	C10H10N2O2S4	C9H8N2O2S4	C11H12N2O2S4	C11H12N2O2S4
	Formula weight	448.63	516.66	414.53	318.44	304.41	332.47	332.47
	Crystal System	Triclinic	Monoclinic	Triclinic	Monoclinic	Monoclinic	Monoclinic	Monoclinic
	Space Group	P-1	P21/n	P-1	P21/n	P21/c	P21/c	P21/c
	Shape	Orange prism	Orange needle	Orange plate	Orange needle	Orange plate	Orange plate	Orange plate
	a (Å)	5.868(4)	12.6138(4)	4.8107(4)	14.3245(9)	11.429(4)	26.578(7)	25.445(6)
	b (Å)	9.531(6)	5.52905(16)	9.0732(6)	4.9698(3)	7.888(3)	7.813(3)	8.045(2)
	c (Å)	10.618(6)	16.8453(5)	11.0382(6)	19.9041(12)	7.254(3)	7.2475(12)	7.101(2)
	α (deg.)	107.56(5)	90	108.240(3)	90	90	90	90
	β (deg.)	93.87(6)	100.868(2)	98.143(4)	106.593(3)	99.20(3)	93.773(17)	93.45(2)
	γ (deg.)	106.20(6)	90	96.693(4)	90	90	90	90
	V (Å 3 )	536.2(7)	1153.76(6)	446.30(5)	1357.94(14)	645.6(7)	1501.7(7)	1451.0(9)
	Z-value	1	2	1	4	2	4	4
	T (K)	295	275	275	274	297	297	297
	Dcalc (g cm -3 )	1.389	1.487	1.542	1.557	1.566	1.470	1.522
	Total reflns.	3667	12276	4892	13966	1461	3359	3177
	Unique reflns.	3131	2098	1611	2393	1144	2727	2572
	(Rint)	(0.0563)	(0.0890)	(0.1151)	(0.1548)	(0.0950)	(0.0196)	(0.0664)
	R1 [F 2 > 2σ(F 2 )]	0.0447	0.1024	0.0796	0.0740	0.0563	0.0431	0.0447
	wR2 [All reflns.]	0.1240	0.3367	0.2371	0.2123	0.1713	0.1402	0.1402
	GOF	0.986	1.235	0.972	0.876	1.034	1.018	1.014
	Interplanar spacing (Å)	3.587	3.410	3.550	3.404	3.157	3.120	3.273

Table 3 .

 3 Dihedral angles and transfer integrals (meV) in OS

-RR'.

		Dihedral angle	t c	t p	t q	Tilt angle
	OS-MeEt	62 o	31.4	17.1	18.0	17 o
	OS-MeBu	61 o	37.8	20.1	22.3	17 o
	OS-EtPr	66 o	49.1	9.18	11.4	19 o

Table 4 .

 4 Characteristics of thin-film transistors.

			μmax (cm 2 V -1 s -1 )	μave (cm 2 V -1 s -1 )	Vth (V)	On/off ratio
	1-Ph	under vacuum	0.019	0.013	76	7 x 10 5
		in air	1.4 x 10 -4	9.8 x 10 -5	153	3 x 10 4
	2-Me	under vacuum	5.0 x 10 -5	3.4 x 10 -5	-4	6 x 10 2
	2-Et	under vacuum	0.041	0.038	4	2 x 10 4
		in air	0.028	0.026	12	70
	2-Bu	under vacuum	9.3 x 10 -5	5.8 x 10 -5	38	3 x 10 4
		in air	3.7 x 10 -5	2.2 x 10 -5	35	2 x 10 3
	OS-Ph	under vacuum	8.3 x 10 -5	6.5 x 10 -5	40	8 x 10 4
		in air	5.0 x 10 -5	4.0 x 10 -5	65	1 x 10 3
	OS-EtH	under vacuum	5.5 x 10 -4	3.9 x 10 -4	63	5 x 10 5
		in air	2.7 x 10 -5	2.7 x 10 -5	86	3 x 10 2
	OS-BuH	under vacuum	3.6 x 10 -3	2.7 x 10 -3	55	2 x 10 6
		in air	7.3 x 10 -4	6.0 x 10 -4	68	4 x 10 4
	OS-EtMe	under vacuum	3.4 x 10 -5	2.1 x 10 -5	59	5 x 10 2
	OS-PrMe	under vacuum	0.019	0.015	65	6 x 10 6
		in air	4.3 x 10 -3	2.4 x 10 -3	81	5 x 10 5
	OS-BuMe	under vacuum	0.024	9.6 x 10 -3	55	2 x 10 5
		in air	1.3 x 10 -3	8.7 x 10 -4	94	2 x 10 4
	OS-PrEt	under vacuum	0.013	0.010	45	2 x 10 6
		in air	7.1 x 10 -3	4.3 x 10 -3	78	5 x 10 5

Acknowledgements

This work was partly supported by the JSPJ KAKENHI Grant Number 18H02044, and the Takahashi Industrial and Economic Research Foundation (08-003-022). The authors are grateful to Tokyo Institute of Technology Center for Advanced Materials Analysis for XRD measurement.

Conflicts of interest

There are no conflicts to declare.


Rim Boubakri

Christian Jelsch

Christine Lucas

Frédéric Lefebvre

Cherif Werner Kaminsky

Ben Nasr

Kamel Kaabi
email: kamel_kaabi@yahoo.fr

Cherif Ben Nasr

A new 1D Zn

Keywords: Zn(II) complex, X-ray diffraction, Hirshfeld surface, CP-MAS-NMR spectroscopy, Hydrogen bonds

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Recently, metal-organic complexes have gained more interest due to their potential applications in several fields such as material [1] and medicinal chemistry [2], not only because of their great interesting structural diversity, but also due to their extensive exploitable properties such as in gas or liquid adsorption, ion exchange, catalysis, separation, magnetism, sensing [3-8], radioimmunotherapy and magnetic resonance imaging [9,10]. Among these complexes, the Zn complexes have interesting physical properties especially their photoluminescence allowing them to be used as sensors, biological imaging probes and electrochemical devices [11][12][13][14][15][16][17].

Furthermore, the external electronic configuration d 10 of Zn (II) gives a stabilization energy of the zero crystal field which allows this cation to adopt various geometries, in particular the tetrahedron, the octahedron and the dodecahedron [18]. As part of our continued involvement in the investigation of metal complexes, we report here the synthesis and characterization of a new Zn(II) complex with the 4-amino-6-methoxypyrimidine ligand. We chose a pyrimidine derivative as an organic ligand because the resulting complexes show interesting structural, chemical, and physical properties significant for photoluminescence, magnetism, ferroelectricity, conductivity, and non-linear optics [9][10][11][12]. The Hirshfeld surface analysis was performed to completely characterize the intermolecular interactions and explain the crystalline architecture. Moreover, the vibrational characteristics and NMR properties were also studied and DFT calculations were used for the interpretation of the results.

Materials and methods

Chemical preparation

A solution of ZnCl2 (54.52 mg, 0.4 mmol, Sigma-Aldrich) in water (10 mL) was added drop wise to a solution of 2-amino-4,6-dimethoxypyrimidine (62.1 mg, 0.4 mmol, Sigma-Aldrich) in ethanol (10 mL). After stirring for 45 min, the mixture was filtered and the resultant solution was allowed to evaporate at room temperature. Transparent prismatic single crystals of the title compound, which remained stable under normal conditions of temperature and humidity, were isolated after several days and subjected to X-ray diffraction analysis (yield 68%). Anal. Calc.

for [Zn2Cl4(C6H9N3O2)2]: C, 24.71 %; H, 3.08 %; N, 14.41 %. Found: C, 24.92 %; H, 3.51 %; N, 14.78 %.

Investigation techniques

X-ray data collection and structure solution

A colorless prism, measuring 0.13 x 0.12 x 0.09 mm 3 was mounted on a loop with oil. Data were collected at 21 o C on a Nonius Kappa CCD FR590 single crystal X-ray diffractometer using the Mo-radiation. Crystal-to-detector distance was 30 mm and exposure time was 90 seconds per degree for all sets. The scan width was 1.5 o . Data collection was 99.5% complete to 25 o in . 9529 merged reflections were collected covering the indices, -10h10, -22k25, -19l19. 5366 reflections were symmetry independent and the Rint = 0.0437 factor indicated that the diffraction data were of good quality. Indexing and unit cell refinement indicated the monoclinic lattice. The space group was found to be P21/n (N°. 14). The data were integrated and scaled using hkl-SCALEPACK [START_REF] Otwinowsky | Methods in Enzymology[END_REF]. This program applies a multiplicative correction factor (S) to the observed intensities (I) and has the following form: Solution by direct methods (SHELXS, SIR97 [24]) produced a complete heavy atom phasing model consistent with the proposed structure. The structure was completed by difference Fourier synthesis with SHELXL97 [START_REF] Sheldrick | SHELXL-97[END_REF][START_REF] Mackay | [END_REF]. Scattering factors were from Waasmair and Kirfel [START_REF] Waasmaier | [END_REF]. Hydrogen atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms with C---H distances in the range 0.95-1.00 Å. Isotropic thermal parameters Uiso were fixed such that they were 1.2Ueq of their parent atom for CH and NH2 groups and 1.5Ueq of their parent atom for methyl groups. All non-hydrogen atoms were refined anisotropically by full-matrix least-squares. The drawings were made with Diamond [START_REF] Brandenburg | Diamond Version 2.0 Impact GbR[END_REF].

Crystal data and experimental parameters used for the intensity data collection are summarized in Table 1.

Infrared spectroscopy

The Fourier Transform Infrared (FTIR) spectrum of a powder sample was obtained using a Nicolet 5700 spectrophotometer with potassium bromide (KBr) pellets (2 mg of sample in 200 mg of KBr). The number of scans was 32. The scanning range was 4000-400 cm -1 and the resolution 2 cm -1 .

DFT calculations

DFT calculations were performed with the Gaussian 09 Revision A-02 software [START_REF] Frisch | Gaussian 09, Revision B.01[END_REF]. The B3LYP functional was used with the 6-31+G* base set for all atoms, including zinc. Two studies were made, one on a species containing one ZnCl2 group and the two organic ligands and one where the effect of the two surrounding ZnCl2 groups was included, the coordination of Zn being completed by a NH3 group. The positions of all non-hydrogen atoms were those determined by the X-ray diffraction study. The positions of hydrogens were optimized at the B3LYP/6-31+G* level, starting from the positions of the X-ray diffraction study. The spectroscopic parameters (NMR and infrared spectra) were then calculated on this semioptimized geometry.

Results and discussion

Structure description

The main geometrical features of zinc(II) coordination polymer of formula [Zn2Cl4(C6H9N3O2)2]n are reported in Tables 2 and3 2). These values are similar to those reported for other tetrahedral Zn(II) complexes such as 2-[(2-aminoethyl)amino]ethanaminium dichlorozincate [START_REF] Nbili | [END_REF] and 4-(3,5diphenyl-1H-pyrazol-1-yl)-6-(piperidin-1-yl)pyrimidine dichlorozincate complexes [31]. The geometry of four-coordinated metal complex my conveniently be measured by the Yang 4parameter [32], 4 = [360 -(β + α)]/141 with β and α being the two largest angles, that is zero for a perfect square planar geometry and one for a perfect tetrahedral geometry. 3) to form layers parallel to the (a, c) plane which cross the unit cell at y = ½ (Figs. 3 and4).

These layers are; in turn, connected to each other via C6-H

•••Cl2, C8-H•••Cl3 and C11- H•••Cl3
hydrogen bonds to form a three-dimensional supramolecular network (Fig. 4).

Moreover, the cohesion of the structure of this compound is reinforced by strong hydrogen bonds of type N-H…Cl whose N...Cl distances vary between 3.184 (3) and 3.268 (3) Å.

Table 3. Hydrogen-bond geometry (Å, º) in the title complex. ). This can be attributed to the donor mesomeric effect of the methoxy group [33][34][35]. Furthermore, the C-N bond distances of the NH2 groups ([C4-N3]= 1.327 (4)

D-H•••A D-H H•••A D•••A D-H•••A C2-H2•••Cl2 i 0.
Å and [C10-N6]= 1.336 (4) Å) are short for C-N single bonds, but still not quite as contracted as one would expect for a fully established C=N double bond. These bond length features are consistent with an imino resonance form as it is commonly found for a C-N single bond involving sp 2 hybridized C and N atoms [36,37].

Hirshfeld contacts analysis

Analysis of intermolecular interactions using the Hirshfeld surface represents a tool to gain insight into the crystal packing. The Hirshfeld surface was computed with the MoProViewer software [38]. The fingerprint plots (Fig. 5) were obtained from software CrystalExplorer3.1 [39]. The contact enrichment ratio Exy between chemical species X and Y is obtained by comparing the actual contacts Cxy in the crystal with those computed as if all types of contacts had the same probability to form. The chemical nature of contacts and their enrichment in the crystal structure are shown in Table 4. To obtain an integral Hirshfeld surface around each moiety (Zn(II), chloride anion and organic ligands), a set of entities not in contact with each other were selected in the crystal packing (Fig. 6).

The major contact is constituted by the weak C-H…Cl -; it is moderately enriched (Exy=1.54) as

Hc and Cl -are the most abundant atoms on the Hirshfeld surface (Cxy=21.4%). The strong hydrogen bond Cl -…H-N is the second most abundant contact and is quite over-represented at Exy=2.33. The zinc cation is interacting with Cl -, N and O atoms and all these contacts are enriched, the Zn(II)…Cl -ionic bridge being the most abundant and the Zn(II)…N the most enriched at E=3.7. The Cl -anions are mostly interacting with the zinc cations and hydrogen atoms. The zinc cation is not coordinated by the methoxy oxygen atoms but there are four long distance attractive electrostatic interactions (in the 2.76-2.84 Å range) between these weakly polar oxygen atoms and the Zn(II) atom.

The self-contacts between charged species (Zn(II), N -, O -and Hn + ) are all strongly avoided with Exx values equal or close to zero, as they represent electrostatic repulsion. The contact proportions Cxy of the two independent but identical organic ligands are very similar as they are 94.3% correlated.

40% of the surface is occupied by the hydrophobic Hc and C atoms and the C…C contacts are the most enriched at Exy = 2.8. There is no parallel aromatic stacking occurring, rather the two cycles form contacts with the angle between the two planes being 78.6°. Slightly enriched C-H… weak hydrogen bonds occur with the methyl groups. Globally the contacts between hydrophobic atoms are slightly enriched at Exy = 1.17, those between polar atoms are normally represented (Exy=0.99) while polar…hydrophobic interactions are only slightly impoverished (E=0.88) due to the presence of CH3…Cl -weak hydrogen bonds.

The electrostatic complementarity of molecules in a crystal packing can be mapped on the Hirshfeld surfaces [40]. The electrostatic potential (ESP) values on the Hirshfeld surface around the two independent organic ligands are shown in Fig. 7. The ESP was computed using software MoPro [41] using a multipolar atom model transferred from the ELMAM2 electron density database [42]. Electroneutrality constraint was applied after transfer to the asymmetric unit by adjusting the charge of Zn cation to +1.2e, while the transferred charge of Cl -was -0.56e.

NMR results

The 13 C CP-MAS NMR spectrum of the coordination polymer [Zn2Cl4(C6H9N3O2)2]n is shown in Fig. 8. It exhibits in the resonance domain of the methoxy groups, between 57.9 and 60.6 ppm, three sharp resonances with one of the peaks, at 57.9 ppm, having about double the integrated intensity of the other two and corresponding to the four crystallographically independent methoxy carbon atoms, . The spectrum can thus be interpreted as featuring four resonances in this region, with two of them in close proximity with substantial overlap. This is in agreement with two organic molecules being present in the asymmetric unit cell as revealed by X-ray structure determination. The peak at 78.6 ppm corresponds to the C5 carbon atom since it is located between two carbon atoms of the pyrimidine ring, so it is the most shielded (see below for the calculation of the NMR chemical shifts). The carbon atoms C4 and C6, which are linked to oxygen, have practically the same chemical environment and therefore they are the less shielded and resonate at 172.8 ppm.

The 15 N CP-MAS NMR spectrum of the title compound, presented in Fig. 9, exhibits two well-resolved peaks. The first one at -287.5 ppm is assigned to aliphatic nitrogen atoms of the NH2 groups, while the second one, at -211.7 ppm, corresponds to the aromatic nitrogen atoms.

Theoretical calculations were undertaken in order to assign the NMR resonances to the different crystallographic non-equivalent carbon and nitrogen atoms of the unit cell. All calculations were carried out with Gaussian 09, the B3LYP method and the 6-31+G * base for all atoms. In a first study, the calculations were made on a system containing only one zinc complex with its two surrounding organic ligands but as one nitrogen of the organic molecule is also linked to another metallic atom, these calculations cannot allow a good determination of the chemical displacements in NMR and therefore another calculations were carried out by adding two ZnCl2 groups and completing the coordination around these two zinc atoms with an NH3 group. In both cases the positions of the protons were optimized before doing these calculations. The atoms are labeled as depicted below:

The results are listed in Table 5. Clearly, there is a very good agreement between the experimental and theoretical values calculated after optimization of the position of the protons, allowing unambiguously the attribution of the different NMR signals.

Quantum mechanical study

DFT calculations were made (by use of the B3LYP/6-31+G* method for all atoms) on the geometry obtained after optimization of the protons. The energies of the frontier orbitals in the two calculation models are listed in Table 6. It follows from these calculations that the HOMO is localized essentially on the chlorine atoms, whatever the chosen model, while the LUMO is localized on the organic groups (Figs. 10a and10b). The difference in HOMO / LUMO energy is 5.46 eV in the case of the complex alone and 4.34 eV if the ZnCl2NH3 groups are added, values proving the stability of this compound. The energy distribution of the different orbitals for both cases (Fig. 11) shows a great analogy between the two distributions.

Molecular Electrostatic Potential analysis (MEP)

The Molecular Electrostatic Potential (MEP) surface is depicted on Fig. 12. The MEP is used to determine the nuclear and electronic charge distribution of a given molecule. The maps were obtained at the B3LYP/6-31+G * level of theory. As it can be seen, the electrostatic potential maps are color-coded and are subdivided into many regions where those various colors are used to identify different potentials. Blue and red colors indicate the positive and negative potentials, respectively. In both cases, the most negative zones are on the aromatic ring, while the most positive ones are located at the protons bonded to the nitrogen atom.

IR spectroscopy

FT-IR spectroscopy was used to verify the functional groups present in the crystal, and to investigate their vibrational behavior in the solid state. The IR spectrum of [Zn2Cl4(C6H9N3O2)2]n is shown in Fig. 13a. The assignments of the observed bands are based on comparisons with data previously reported for similar materials [43][44][45]. The bands in the high-wavenumber region, between 3500 and 3200 cm -1 , correspond to the stretching vibrations of the N-H and O-H groups interconnected by a system of hydrogen bonds in the crystal [44]. The weak band at 2942 cm -1 corresponds to the C-H stretching modes. The bands at 1667 and 1607 cm -1 are assigned to the stretching vibrations of C=C and C=N groups and to the N-H bending vibration modes [46,47].

The bands of medium intensities observed at 1470 and 1385 cm -1 are associated to the C-H asymmetric and symmetric bending vibrations, respectively. The characteristic absorption peak at 1223 cm -1 can be associated to Ar-O-C stretching vibration. The observed bands in the 1200 -1000 cm -1 region correspond to the valence vibrations of C-C and C-N groups [48,49]. The bands in the range 1000-600 cm -1 can be attributed to the out of plane bending modes of C-H, N-H, C-C and C-N groups [50]. DFT calculations of the frequencies were made by use of the B3LYP/6-31+G* method on the geometry obtained after optimization of the protons. The resulting IR spectrum between 4000 and 400 cm -1 is shown on Fig. 13-b and is very similar to the experimental one. A close agreement between the experimental and theoretical wave numbers (R 2 = 0.9932) is mostly achieved in the finger print region as shown in Fig. 14. Thus, the precision is well-sufficient to assign the experimental frequencies and to confirm the attributions proposed above.

S

  is calculated from the scale factor and the B factor which are determined for each frame and is then applied to I to give the corrected intensity (Icorr).

  . The zinc polymeric complex crystallizes in the monoclinic space group P21/n. The asymmetric unit of the title compound consists of two Zn(II) cations, two neutral bidentate 2-amino-4,6-dimethoxypyrimidine ligands and four chloride ions Cl -(Fig.1). The asymmetric unit is in fact a [Zn2Cl4(C6H9N3O2)2] unit which leads to a 1-D chain with molecular formula [Zn2Cl4(C6H9N3O2)2]n running along the b-axis direction (Fig.2). The coordination geometry around each Zn(II) atom can be described as a distorted tetrahedral ZnCl2N2 moiety. Each Zn(II) center is tetracoordinated by two nitrogen from the pyrimidine ring of the organic ligand and two chloride anions (N1, N4, Cl1 and Cl2 for Zn(1); N2, N5, Cl3 and Cl4 for Zn(2)). In the atomic arrangement of this compound, the Zn-N and Zn-Cl bond lengths vary respectively in the range 2.082(3)-2.2401(9) and 2.077(3)-2.2381(10) Å and the bond angles around the Zn atom vary between 97.24(10) and 115.30(8)° for the Zn(1)Cl2N2 tetrahedron and between 107.96(9) and 112.02(4)° for the Zn(2)Cl2N2 tetrahedron (Table

  Symmetry codes: (i) -x-½, y+½, -z+½; (ii) -x-½, y-½, -z+½.The calculated 4 values of the title compound are4(Zn1) = 0.93 for the Zn(1)Cl2N2 tetrahedron (β = 115.30 (8)° and α = 113.07 (8)°) and 4(Zn2) = 0.96 for the Zn(2)Cl2N2 tetrahedron (β = 112.02 (4)° and α = 111.89 (8)°), indicating a quite small distortion from the regular tetrahedral and that the first tetrahedron is slightly more distorted than the second one. The chains [Zn2Cl4(C6H9N3O2)2]n, located at z = (2n+1)/4, are interconnected via C12-H•••Cl1 (C…Cl = 3.776 (4) Å) and C5-H•••Cl2 (C…Cl = 3.542 (3) Å) weak hydrogen bonds (Table

  The inner and outer ESP values are anticorrelated with r=-0.57 and -0.65 for the ligand #1 and #2, respectively. In both cases, the outer potential shows mostly electropositive values on the surface while the inner potential shows more balanced positive and negative values. This is due to the fact that the ligand was modeled with a global negative charge of -0.08e while the "external" ZnCl2 moiety bears a charge of +0.08e. The two ligands have in common the strong N -…Zn(II) coordination contacts with strong negative/inner and positive/outer ESP values.The strong Hn…Cl -hydrogen bonds are also shared by the two ligands with highly positive inner ESP values. The negative outer ESP around the chloride anions is attenuated by the nearby presence of Zn(II) cation. Significant Hc…Cl -weak H-bonds are also visible for both ligands in Fig.7a,b in regions of weaker inner ESP. The O…Zn contacts occur for the two ligands and involves the most negative inner ESP values. Globally, the two scatterplots show very similar trends for the two independent molecules.

Fig. 1

 1 Fig.1

Fig. 2

 2 Fig. 2

Fig. 4 A

 4 Fig. 4

Fig. 5 Fingerprint

 5 Fig. 5

Fig. 7

 7 Fig. 7

Fig. 8

 8 Fig. 8 13 C CP-MAS NMR spectrum of the polymeric complex [Zn2Cl4(C6H9N3O2)2]n

Fig. 11

 11 Fig. 11 Energy distribution of the different orbitals for the polymeric complex [Zn2Cl4(C6H9N3O2)2]n. With only a zinc complex (left). With adding two ZnCl2 and two NH3 groups (right).

29

 29

Fig. 12 MEP

 12 Fig. 12

Fig. 13

 13 Fig. 13

( a )

 a Experimental and (b) Calculated infrared absorption spectrum of the polymeric complex [Zn2Cl4(C6H9N3O2)2]n.

Fig. 14 Comparison

 14 Fig. 14

Table 1

 1 Experimental details of coordination polymer [Zn2Cl4(C6H9N3O2)2]n

	Crystal data
	Chemical formula	C12H18Cl4N6O4Zn2
	Mr (g.mol -1 )	582.86
	Dx (Mg m -3 )	1.757
	F(000)	1168
	Crystal system, space group	Monoclinic, P21/n
	Temperature (K)	294
	a, b, c (Å)	7.5855 (3), 19.7127 (9), 14.8326 (6)
	β (°)	96.599 (2)
	V (Å 3 )	2203.23 (16)
	Z	4
	Radiation type	MoKα (λ = 0.71073 Å)
	µ (mm -1 )	2.69
	Crystal size (mm)	0.13 × 0.12 × 0.09
	Data collection
	Diffractometer	KappaCCD
	No. of measured, independent and	9529, 5366, 3165
	observed [I > 2σ(I)] reflections
	Rint	0.047
	sinθmax /λ (Å -1 )	0.667
	Refinement
	R[F 2 > 2σ(F 2 )], wR(F 2 ), S	0.041, 0.087, 0.96
	No. of reflections	5366
	No. of parameters	257
	H-atom treatment	H-atom parameters constrained
	Δρmax, Δρmin (e Å -3 )	0.38, -0.55

Table 2

 2 Selected bond distances and angles (Å, º) in the title complex

	N1-Zn1	2.092 (3)	N4-Zn1-N1	97.24 (10)
	N4-Zn1	2.082 (3)	N4-Zn1-Cl1	115.30 (8)
	Cl1-Zn1	2.2152 (10)	N1-Zn1-Cl1	112.74 (8)
	Cl2-Zn1	2.2401 (9)	N4-Zn1-Cl2	113.07 (8)
	N2-Zn2	2.084 (2)	N1-Zn1-Cl2	110.28 (8)
	N5-Zn2 i	2.077 (3)	Cl1-Zn1-Cl2	107.95 (4)
	Cl3-Zn2	2.2471 (10)	N5 ii -Zn2-N2	111.02 (10)
	Cl4-Zn2	2.2381 (10)	N5 ii -Zn2-Cl4	107.96 (9)
	C1-N1	1.347 (4)	N2-Zn2-Cl4	111.89 (8)
	C1-C2	1.375 (4)	N5 ii -Zn2-Cl3	109.06 (8)
	C2-C3	1.371 (4)	N2-Zn2-Cl3	104.88 (8)
	C3-N2	1.343 (4)	Cl4-Zn2-Cl3	112.02 (4)
	C4-N3	1.327 (4)	C10-N6	1.336 (4)
	C4-N2	1.344 (4)	C10-N4	1.342 (4)
	C4-N1	1.353 (4)	C10-N5	1.351 (4)
	C9-N4	1.344 (4)	C7-O3	1.330 (4)
	C1-O1	1.328 (4)	C11-O3	1.442 (4)
	C5-O1	1.430 (4)	C9-O4	1.324 (4)
	C3-O2	1.339 (4)	C12-O4	1.449 (4)
	C6-O2	1.443 (4)

Table 4 .

 4 Analysis of contacts on the Hirshfeld surface. Reciprocal contacts X…Y and Y…X are merged. The second line shows the chemical content on the surface. The % of contact types between chemical species is given followed by their enrichment ratio. The major contacts as well as the major enriched ones are highlighted in bold characters. The hydrophobic hydrogen atoms bound to carbon (Hc) were distinguished from the more polar one bound to nitrogen

	(Hn).
	atom	Zn	Cl	N	O	Hn	Hc	C
	%	11.0	28.3	7.7	5.6	8.1	26.1	13.2
	Zn	0.0
	Cl	13.7	2.1		%	surface
	N	8.1	0.3	0.0
	O	3.5	1.4	1.6	0.0
	Hn	1.7	10.2	0.0	0.1	0.2
	Hc	2.7	21.4	3.6	1.1	2.2	5.1
	C	0.9	6.8	0.4	1.9	0.8	6.6	3.5
	Zn	0
	Cl	1.55	0.25			enrichment
	N	3.7	0.08	0
	O	2.3	0.49	2.41	0
	Hn	0.72	2.3	0.03	0.12	0.35
	Hc	0.36	1.54	1.08	0.48	0.59	0.9
	C	0.23	0.96	0.21	1.66	0.41	1.14	2.4

Table 5 .

 5 Comparison of calculated and experimental chemical shift values (ppm) of carbonand nitrogen atoms in the title complex.

	Atoms	Complex		Complex + 2 ZnCl2(NH3)	Experiment
	C1	53.5	55.2	52.6	54.7	57.9
	C2	54.1	54.0	54.9	55.0
						59.6 and 60.5
	C3	154.2	154.5	152.0	152.7	159.6

Table 6 .

 6 Energy values of frontier orbitals (eV) in the title complex.

	Energy (eV) Zn complex	Zn complex +
	Orbitals	alone	2 ZnCl2NH3 groups
	HOMO-5		-7.13
	HOMO-4		-7.10
	HOMO-3		-6.90
	HOMO-2		-6.86
	HOMO-1		-6.66
	HOMO	-6.53	-6.65
	LUMO	-1.07	-2.31
	LUMO+1		-2.23
	LUMO+2		-1.50

Conclusions

A new Zn(II) polymeric complex with the 2-amino-4,6-dimethoxypyrimidine, [Zn2Cl4(C6H9N3O2)2]n, has been prepared at room temperature. In the title compound, the organic entity behaves as a neutral bidentate ligand and Zn(II) adopts a tetrahedral coordination.

The contacts enrichment ratios derived from the Hirshfeld surface analysis shows that besides formation of the Zn(II) metal coordination complex with N ligands and chloride anions, the crystal packing is mainly stabilized by N/C-H…O and N/C-H…Cl -hydrogen bonds. The MEP map defines the nucleophile and the electrophile sites. The band gap energy between HOMO and LUMO indicates that the title compound is kinetically stable. NMR signals are in full agreement with the crystallographic data. DFT calculations allow the attribution of the experimental NMR lines and of IR bands at low frequencies. As a solution study has not been done, we cannot know if this complex is stable in solution.

Supplementary data

Crystallographic data for the structural analysis have been deposited at the Cambridge Crystallographic Data Centre, CCDC No 1983914. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK: fax: (+44) 01223-336-033; e-mail: deposit@ccdc.cam.ac.