Nickel(II) and palladium(II) complexes of azobenzene-containing ligands as dichroic dyes

9136 Inorg. Chem. 2010, 49, 9136–9150

DOI: 10.1021/ic1000842

Nickel(II) and Palladium(II) Complexes of Azobenzene-Containing Ligands as

Dichroic Dyes

Octavia A. Blackburn,^†Benjamin J. Coe,*^,†John Fielden,^†Madeleine Helliwell,^†Joseph J. W. McDouall,^†and

Michael G. Hutchings^‡

^†School of Chemistry and ^‡DyStar UK Ltd, c/o School of Chemistry, University of Manchester, Oxford Road,

Manchester M13 9PL, U.K.

Received January 15, 2010

A large series of complexes has been synthesized with two chelating, Schiff base azobenzene derivatives connected linearly

by coordination to a central nickel(II) or palladium(II) ion. These compounds have the general formulas M^II(OC₆H₃-2-CHNR-4-

NdNC₆H₄-4-CO₂Et)₂[M = Ni; R = n-Bu (3c), n-C₆H₁₃(3d), n-C₈H₁₇(3e), n-C₁₂H₂₅(3f), Ph (3g), OH (3h), C₆H₄-4-CO₂Et

(3i). M = Pd; R = i-Pr (4a), t-Bu (4b), n-Bu (4c), n-C₆H₁₃(4d), n-C₈H₁₇(4e), n-C₁₂H₂₅(4f), Ph (4g)], M^II[OC₆H₃-2-CHN(n-

C₈H₁₇)-4-NdNC₆H₄-4-CO₂(n-C₈H₁₇)]₂[M = Ni (9), Pd (10)], M^II[OC₆H₃-2-CHN(n-C₈H₁₇)-4-NdNC₆H₄-4-C₆H₄-4-O(n-

C₇H₁₅)]₂[M = Ni (14), Pd (15)], and M^II[OC₆H₃-2-CHN(CMe₂)-4-NdNC₆H₄-4-CO₂Et]₂[M = Ni (17), Pd (18); the CMe₂

groups are connected]. These compounds have been characterized by using various physical techniques including ¹H NMR

spectroscopy and matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. Single-crystal X-ray structures

have been obtained for two pro-ligands and five complexes (3e, 4e, 14, 15, and 17). The latter always show a strictly square

planar arrangement about the metal center, except for the Ni^IIcomplex of a salen-like ligand (17). In solution, broadened ¹H

NMR signals indicate distortions from square planar geometry for the bis-chelate Ni^IIcomplexes. Electronic absorption

spectroscopy and ZINDO_S (Zerner’s intermediate neglect of differential overlap) and TD-DFT (time-dependent density

functional theory) calculations show that the lowest energy transition has metal-to-ligand charge-transfer character. The λ_maxof

this band lies in the range of 409-434 nm in dichloromethane, and replacing Ni^IIwith Pd^IIcauses small blue-shifts. Dichroic ratios

measured in various liquid crystal hosts show complexation-induced increases with Ni^II, but using Pd^IIhas a detrimental effect.

Introduction

defined as the ratio of the absorbances when illuminated

by light polarized parallel (A_||) and perpendicular (A_^) to

the LC director. To achieve a sufficiently large DR, the

transition dipole moment (μ₁₂) of the color-inducing

electronic transition must be aligned effectively with the

long molecular axis of the dye. The strength and nature of

the interactions between the dye molecules and their LC

host can also influence the DR. In addition, the absorp-

tion intensity, solubility in the LC medium, operating

temperature, and stability are all important considera-

tions when studying potential dyes for such purposes. The

order parameter (S) of the dye, defined by eq 1 and related

to DR by eq 2, is also often used to quantify dichroic

properties.

Reflective liquid crystal (LC) displays are well suited for

applications in emerging technologies such as electronic

paper,¹owing to their ultralow power consumptions achieved

through bistability.²In this context, much attention has been

directed at guest-host LC displays (GH-LCDs) that comprise

a dichroic dye dissolved in a LC host. GH-LCDs can achieve

excellent brightness combined with wide viewing angles, both

critical aspects for device applications. The absorption inten-

sity of a dichroic dye depends on its orientation with respect to

the incident light direction. Therefore, when such a dye is

ordered within a LC medium, an applied electric field can be

used to manipulate the orientation of the molecules and thus

change the appearance of the display.³

To be suitable for potential GH-LCD applications, a dye

must have an especially large dichroic ratio (DR). The DR is

A_||- A_^

A_||þ 2A_^

S ¼

ð1Þ

*To whom correspondence should be addressed. E-mail: b.coe@

manchester.ac.uk.

(1) (a) Graham-Rowe, D. Nat. Photonics 2007, 1, 248. (b) Sobel, A. Nat.

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DR - 1

DR þ 2

S ¼

ð2Þ

The two major classes of compound that have attracted the

most extensive interest in this field are the classical colorants,

r

pubs.acs.org/IC

Published on Web 09/14/2010

2010 American Chemical Society

Article

Inorganic Chemistry, Vol. 49, No. 20, 2010 9137

azobenzene and anthraquinone dyes.⁴Previous studies have

established that the S (and hence also DR) value generally

increases with the length of the dye molecules, but is inversely

related to molecular breadth.⁵Azobenzenes therefore have

obvious appeal because of their long, slender shapes which

can be expected to lead to good alignment within an LC

host.⁶Furthermore, the direction of the intense, low energy

π f π* transition in such dyes is directed approximately

along the long molecular axis.

To date, only a handful of studies have specifically ad-

dressed the dichroic properties of metal-containing dyes. For

example, transition metal dithiolene complexes have found

use as modulators of laser light because of their pronounced

dichroism in the near-IR region.⁷In other studies, Bruce et al.

investigated a series of metal dithiobenzoates and found

excellent DR values, but poor stability in solution.⁸The

objective of this study is to explore the use of square planar

coordinated transition metal ions to link two azobenzene

chromophores. Given that such complexes are approxi-

mately twice the length of the uncoordinated dyes, significant

enhancements in DR are expected to arise from this ap-

proach. Complexes of this general type with nickel have been

known for many years,⁹but analogous palladium species are

apparently unknown. In addition, previous investigations

have involved various closely related complexes of Ni^II, Cu^II,

and other first row d-block metal ions,¹⁰often as thermo-

tropic liquid crystalline materials,^{10b,c,e,g,h,j}but their dichroic

behavior has not to our knowledge been examined.

literature method.¹²Dichloromethane was dried by distillation

over CaH₂under nitrogen where necessary. All other chemicals

were used as supplied by Sigma Aldrich or Alfa Aesar, and the

LCs used for dichroism measurements were supplied by Merck

and Synthon. Products were dried at room temperature over-

night in a vacuum desiccator (CaSO₄) or by direct attachment to

a high vacuum line for several hours prior to characterization.

Representative synthetic and characterization details are re-

ported below, while all the others are included in the Supporting

Information.

General Physical Measurements. ¹H NMR spectra were

recorded on Bruker UltraShield 500, AV 400, or DPX 300

spectrometers, and all shifts are quoted with respect to tetra-

methylsilane (SiMe₄). The fine splitting of phenyl ring AA⁰BB⁰

patterns is ignored, and the signals are reported as simple

doublets, with J values (in Hz) referring to the two most intense

peaks. For the assignments of the protons labeled a-d, see the

Discussion section. Electrospray mass spectrometry was per-

formed on a Micromass Platform II at the University of

Manchester, while matrix-assisted laser desorption/ionization

(MALDI) mass spectra were recorded on a Micromass Tof Spec

2e or an Applied Biosystems Voyager instrument at the EPSRC

National Mass Spectrometry Service Centre (U.K.). In cases

where clusters of peaks are observed because of the presence of

various isotopes, the most intense peak is always quoted.

Elemental analyses were performed by the Microanalytical

Laboratory, University of Manchester. UV-vis spectra were

obtained by using a Shimadzu UV-2401 PC spectrometer, and

IR spectra were recorded with pure solids on a Varian Bio-Rad

Excalibur series spectrometer. Dichroism measurements were

performed on a custom-built Hewlett-Packard machine sup-

plied by HP Research (U.K.). Abbreviations: ald = aldehyde;

est = ester.; st = strong; m = medium; vbr = very broad; s =

singlet; d = doublet; dd = doublet of doublets; t = triplet; q =

quartet; qnt = quintet; spt = septet.

Experimental Section

Materials and Procedures. The compound 4-(n-heptoxy)-

phenylboronic acid was synthesized according to a published

procedure, by using 1-bromo-n-heptane in place of 1-bromo-

n-octane.¹¹2,3-Diamino-2,3-dimethylbutane was synthesized

by reduction of 2,3-dinitro-2,3-dimethylbutane according to a

Synthesis of 4-(3-Formyl-4-hydroxyphenylazo)benzoic Acid

Ethyl Ester (1). Ethyl-4-aminobenzoate (10.0 g, 60.5 mmol)

and hydrochloric acid (37%, 15.2 mL, 0.182 mol) were cooled

in an ice bath to below 5 ꢀC. A solution of NaNO₂(4.59 g, 66.5

mmol) in water (30 mL) was added dropwise to the stirred

solution, which was then stirred for a further 2 h with the

temperature maintained between 0 and 5 ꢀC. This yellow

diazonium salt solution was then added slowly to a cooled

solution of salicylaldehyde (salal, 6.34 mL, 59.5 mmol) in

aqueous NaOH (2 M, 69.5 mL, 0.139 mol), kept below 5 ꢀC

for the duration of the addition. The red/orange suspension was

stirred with cooling for 1 h, then allowed to warm to room

temperature and stirred for 18 h. The orange/brown precipitate

was filtered off, washed with water, and recrystallized from

ethanol to give an orange solid. Yield 6.46 g (36%). δ_H(400

MHz, CDCl₃) 11.39 (1 H, s, OH), 10.04 (1 H, s, CHO), 8.25 (1 H,

d, J = 2.4, a), 8.22-8.18 (3 H, d þ dd, c þ C₆H₄), 7.93 (2 H, d,

J = 8.7, C₆H₄), 7.14 (1 H, d, J = 8.9, b), 4.42 (2 H, q, J = 7.1,

CH₂), 1.43 (3 H, t, J = 7.1, Me). ν(CdO)_est1705st, ν(CdO)_ald

1654st cm^-1. Anal. Calcd (%) for C₁₆H₁₄N₂O₄: C, 64.42; H,

4.73; N, 9.39. Found: C, 64.39; H, 4.66; N, 9.35. m/z (-ES):

297.3 [M - H]^-.

Synthesis of 4-[4-Hydroxy-3-(i-propyliminomethyl)phenyla-

zo]benzoic Acid Ethyl Ester (2aH). Compound 1 (200 mg, 0.670

mmol) and i-propylamine (40 mg, 0.677 mmol) were combined in

ethanol (40 mL), and a few drops of glacial acetic acid were added.

The mixture was heated under reflux for 2 h. Upon cooling, the

orange solution was evaporated under reduced pressure, and the

resulting solid was triturated with a mixture of diethyl ether and

n-hexane. The orange solid was filtered off and washed with n-hexane.

Yield 124 mg (55%). δ_H(500 MHz, CDCl₃) 8.41 (1 H, s, CHN),

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Ed.; World Scientific: Singapore, 1992; Vol. 3, pp 65-208. (b) Ivashchenko, A. V.

Dichroic Dyes for Liquid Crystal Displays; CRC Press: Boca Raton, FL, 1994.

(c) Kelly, S. M. Flat Panel Displays: Advanced Organic Materials; RSC

Publishing: Cambridge, U.K., 2000.

(5) (a) Bahadur, B.; Sarna, R. K.; Bhide, V. G. Mol. Cryst. Liq. Cryst.

1981, 75, 121. (b) Constant, J.; Raynes, E. P.; Shanks, I. A.; Coates, D.; Gray,

G. W.; McDonnell, D. G. J. Phys. D: Appl. Phys. 1978, 11, 479. (c) Seki, H.;

Uchida, T.; Shibata, Y. Mol. Cryst. Liq. Cryst. 1986, 138, 349.

(6) See for examples: (a) Cognard, J.; Phan, T. H. Mol. Cryst. Liq. Cryst.

1981, 68, 207. (b) Lackner, A. M.; Sherman, E.; Wong, S.-Y.; Wong, S.-M. Liq.

Cryst. 1993, 14, 763. (c) Matsui, M.; Tanaka, N.; Andoh, N.; Funabiki, K.;

Shibata, K.; Muramatsu, H.; Ishigure, Y.; Kohyama, E.; Abe, Y.; Kaneko, M.

Chem. Mater. 1998, 10, 1921.

(7) Marshall, K. L.; Painter, G.; Lotito, K.; Noto, A. G.; Chang, P. Mol.

Cryst. Liq. Cryst. 2006, 454, 47.

(8) Bruce, D. W.; Dunmur, D. A.; Hunt, S. E.; Maitlis, P. M.; Orr, R.

J. Mater. Chem. 1991, 1, 857.

(9) Early examples: (a) LaLancette, E. A.; Eaton, D. R.; Benson, R. E.;

Phillips, W. D. J. Am. Chem. Soc. 1962, 84, 3968. (b) Holm, R. H.; Chakravorty,

A.; Dudek, G. O. J. Am. Chem. Soc. 1964, 86, 379.

(10) (a) Tanaka, M. Bull. Chem. Soc. Jpn. 1964, 37, 1210. (b) Aiello, I.;

Ghedini, M.; Neve, F.; Pucci, D. Chem. Mater. 1997, 9, 2107. (c) Aiello, I.;

Ghedini, M.; La Deda, M.; Pucci, D.; Francescangeli, O. Eur. J. Inorg. Chem.

1999, 1367. (d) Khandar, A. A.; Rezvani, Z.; Nejati, K.; Yanovsky, A. I.;

Martinez, J. M. Acta Chim. Slov. 2002, 49, 733. (e) Nejati, K.; Rezvani, Z.

New J. Chem. 2003, 27, 1665. (f) Rezvani, Z.; Ahar, L. R.; Nejati, K.;

Seyedahmadian, S. M. Acta Chim. Slov. 2004, 51, 675. (g) Rezvani, Z.; Abbasi,

A. R.; Nejati, K.; Seyedahmadian, M. Polyhedron 2005, 24, 1461. (h) Rezvani,

Z.; Divband, B.; Abbasi, A. R.; Nejati, K. Polyhedron 2006, 25, 1915. (i) Liu,

J.-N.; Wu, B.-W.; Zhang, B.; Liu, Y.-C. Turk. J. Chem. 2006, 30, 41. (j) Rezvani,

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1995, 5, 2175.

ꢀ

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Chem.-Eur. J. 2001, 7, 2007.

9138 Inorganic Chemistry, Vol. 49, No. 20, 2010

Blackburn et al.

8.18 (2 H, d, J = 8.4, C₆H₄), 8.01 (1 H, dd, J_bc= 9.4, J_ac= 2.3, c),

7.92 (1 H, d, J = 2.5, a), 7.89 (2 H, d, J = 8.4, C₆H₄), 7.03 (1 H, d,

J = 9.0, b), 4.41 (2 H, q, J = 7.1, OCH₂), 3.69 (1 H, spt, J = 6.4,

CHMe₂), 1.43 (3 H, t, J = 6.9, CH₂Me), 1.37 (6 H, d, J = 6.4,

CHMe₂). ν(CdO) 1703st, ν(CdN) 1622st, ν(CdC) 1603st and

1578st cm^-1. Anal. Calcd (%) for C₁₉H₂₁N₃O₃: C, 67.24; H, 6.24; N,

12.38. Found: C, 66.98; H, 6.21; N, 12.19. m/z(-ES): 338.2 [M - H]^-.

Synthesis of 4-Nitro-n-octylbenzoate (5). 4-Nitrobenzoyl-

chloride (10.0 g, 0.054 mol) was dissolved in dry dichloro-

methane (70 mL) under argon and cooled to 0 ꢀC. A solution of

n-octanol (8.5 mL, 0.054 mol) and pyridine (38.4 g, 0.485 mol) in

dry dichloromethane (20 mL) was added dropwise, and the

yellow solution was then left to stir for 16 h. The resulting sus-

pension was filtered, and the filtrate was washed with aqueous

NaOH (1 M, 2 ꢀ 20 mL), followed by aqueous HCl (1 M, 6 ꢀ

20 mL) and then water (3 ꢀ 20 mL). The organic layer was dried

over MgSO₄and evaporated to give a yellow oil. Yield 14.77 g

(98%). δ_H(400 MHz, CDCl₃) 8.28 (2 H, d, J = 9.1, C₆H₄), 8.20

(2 H, d, J = 9.1, C₆H₄), 4.36 (2 H, t, J = 6.7, OCH₂), 1.78 (2 H,

qnt, J = 6.7, CH₂), 1.45-1.25 (10 H, 5CH₂), 0.87 (3 H, t, J =

6.9, Me). ν(CdO) 1724st, ν(NO₂) 1528st and 1269st cm^-1. Anal.

Calcd (%) for C₁₅H₂₁NO₄: C, 64.50; H, 7.58; N, 5.01. Found: C,

64.50; H, 7.84; N, 5.01. m/z (-ES): 279.3 [M]^-.

Synthesis of 4-Amino-n-octylbenzoate (6). To a solution of 5

(4.56 g, 0.016 mol) in methanol (100 mL) were added NH₄Cl

(1.75 g, 0.033 mol) and zinc powder (7.47 g, 0.114 mol). The

gray/brown mixture was heated under reflux for 1 h, then cooled

to room temperature and filtered. The filtrate was evaporated to

dryness, dissolved in ethyl acetate (100 mL), and washed with

water (3 ꢀ 70 mL). The ethyl acetate was evaporated to give a

solid which was triturated with n-hexane to give a pale peach-

colored solid. Yield 3.20 g (75%). δ_H(400 MHz, CDCl₃) 7.85

(2 H, d, J = 8.8, C₆H₄), 6.64 (2 H, d, J = 8.8, C₆H₄), 4.25 (2 H, t,

J = 6.7, OCH₂), 4.03 (2 H, br, NH₂), 1.73 (2 H, qnt, J = 6.7,

CH₂), 1.45-1.25 (10 H, 5CH₂), 0.88 (3 H, t, J = 7.1, Me). m/z

(-ES): 248.3 [M - H]^-. ν(CdO) 1682st cm^-1. Note: this

material was found by ¹H NMR spectroscopy to contain

about 5% of the hydroxylamine intermediate; since this could

not be removed readily and is not expected to interfere with the

reaction to form 7 (see below), the product was used without

purification.

N, 8.51. Found: C, 73.12; H, 8.99; N, 8.52. m/z (-ES): 492.2

[M - H]^-.

Synthesis of 4-Amino-4⁰-(n-heptoxy)-1,1⁰-biphenyl (11). 4-

Bromoaniline (1.42 g, 8.25 mmol), 4-(n-heptoxy)phenylboronic

acid (2.15 g, 9.11 mmol), K₂CO₃(2.86 g, 20.7 mmol), acetone

(20 mL), and water (25 mL) were combined in a Schlenk tube,

and the mixture was degassed by two freeze-pump-thaw

cycles. The addition of a solution of Pd^II(OAc)₂(3.7 mg, 0.016

mmol) in acetone (5 mL) via a syringe was followed by three

further freeze-pump-thaw cycles. The mixture was heated

under argon at 65 ꢀC for 4 h, then cooled to room temperature

and extracted with diethyl ether (3 ꢀ 150 mL). The combined

extracts were washed with water (2 ꢀ 150 mL), dried over

MgSO₄and evaporated to give a gray solid. Repeated tritura-

tion with ethanol gave a white solid. Yield 1.65 g (64%). δ_H(400

MHz, CDCl₃) 7.44 (2 H, d, J = 8.9, C₆H₄), 7.36 (2 H, d, J = 8.7,

C₆H₄), 6.93 (2 H, d, J = 8.9, C₆H₄), 6.74 (2 H, d, J = 8.7, C₆H₄),

3.98 (2 H, t, J = 6.6, OCH₂), 3.68 (2 H, br, NH₂), 1.80 (2 H, qnt,

J = 6.7, CH₂) 1.51-1.25 (8 H, 4CH₂), 0.90 (3 H, t, J = 7.0, Me).

Anal. Calcd (%) for C₁₉H₂₅NO: C, 80.52; H, 8.89; N, 4.94.

Found: C, 80.17; H, 9.02; N, 4.79. m/z (þES): 284.3 [M þ H]^þ.

Synthesis of 4-(3-Formyl-4-hydroxyphenylazo)-4⁰-(n-heptoxy)-

1,1⁰-biphenyl (12). This compound was prepared in a manner

identical to 1. The addition of NaNO₂(0.40 g, 5.80 mmol) to 11

(1.50 g, 5.29 mmol) in aqueous HCl (37%, 1.6 mL, 0.16 mol) and

water (10 mL) gave a yellow suspension. Coupling with salal

(0.65 g, 5.32 mmol) in aqueous NaOH (2 M, 6.1 mL) produced a

green/brown precipitate. Purification was achieved by tritura-

tion with diethyl ether followed by column chromatography on

silica gel eluting with dichloromethane. The major yellow frac-

tion was collected and evaporated to dryness under vacuum

to afford a yellow solid. Yield 1.39 g (63%). δ_H(400 MHz,

(CD₃)₂SO) 10.23 (1 H, s, CHO), 7.98 (1 H, d, J = 2.8, a), 7.85

(1 H, dd, J_bc= 9.3, J_ac= 2.8, c), 7.80 (2 H, d, J = 8.8, C₆H₄),

7.75 (2 H, d, J = 8.8, C₆H₄), 7.67 (2 H, d, J = 8.8, C₆H₄), 7.02

(2 H, d, J = 8.8, C₆H₄), 6.64 (1 H, d, J = 9.3, b), 4.01 (2 H, t, J =

6.4, OCH₂), 1.73 (2 H, qnt, J = 6.7, CH₂), 1.46-1.22 (8 H,

4CH₂), 0.87 (3 H, t, J = 7.0, Me). ν(CdO) 1655st cm^-1. Anal.

Calcd (%) for C₂₆H₂₈N₂O₃: C, 74.97; H, 6.78; N, 6.73. Found:

C, 74.62; H, 7.22; N, 6.66. m/z (-ES): 415.3 [M - H]^-.

Synthesis of 4-[4-Hydroxy-3-(n-octyliminomethyl)phenylazo]-

4⁰-(n-heptoxy)-1,1⁰-biphenyl (13H). This compound was pre-

pared in a manner identical to 2aH by using 12 (500 mg, 1.20

mmol) and n-octylamine (155 mg, 1.20 mmol) to produce an

orange precipitate which was filtered off and washed with

ethanol. Yield 485 mg (76%). δ_H(400 MHz, CDCl₃) 14.43

Synthesis of 4-(3-Formyl-4-hydroxyphenylazo)benzoic Acid

Octyl Ester (7). This compound was prepared in a manner

identical to 1. Addition of NaNO₂(2.52 g, 0.037 mol) to 6

(8.28 g, 0.031 mol) in aqueous HCl (37%, 9.83 mL, 0.10 mol)

and water (20 mL) gave a peach-colored solution. The reaction

with salal (4.06 g, 0.033 mol) in aqueous NaOH (2 M, 38.0 mL)

produced an orange precipitate. The crude product was recrys-

tallized from ethyl acetate and washed with n-hexane to give an

orange powder. Yield 4.65 g (39%). δ_H(400 MHz, CDCl₃) 11.39

(1 H, s, OH), 10.05 (1 H, s, CHO), 8.25 (1 H, d, J = 2.4, a),

8.18-8.22 (3 H, d þ dd, c þ C₆H₄), 7.93 (2 H, d, J = 8.8, C₆H₄),

7.14 (1 H, d, J = 8.9, b), 4.35 (2 H, t, J = 6.7, OCH₂), 1.80 (2 H,

qnt, J = 6.7, CH₂), 1.50-1.25 (10 H, 5CH₂), 0.89 (3 H, t, J =

6.9, Me). ν(CdO)_est1709st, ν(CdO)_ald1666st cm^-1. Anal.

Calcd (%) for C₂₂H₂₆N₂O₄: C, 69.09; H, 6.85; N, 7.32. Found:

C, 68.90; H, 6.90; N, 7.28. m/z (-ES): 381.2 [M - H]^-.

Synthesis of 4-[4-Hydroxy-3-(n-octyliminomethyl)phenyla-

zo]benzoic Acid Octyl Ester (8H). This compound was prepared

in a manner identical to 2aH by using 7 (600 mg, 1.57 mmol) and

n-octylamine (200 mg, 1.55 mmol) to produce an orange pre-

cipitate which was filtered off and washed with ethanol. Yield 700

mg (90%). δ_H(400 MHz, CDCl₃) 8.38 (1 H, s, CHN), 8.17 (2 H, d,

J = 8.7, C₆H₄), 8.01 (1 H, dd, J_bc= 11.5, J_ac= 2.5, c), 7.92 (1 H,

d, J = 2.5, a), 7.89 (2 H, d, J = 8.7, C₆H₄), 7.03 (1 H, d, J = 9.0,

b), 4.34 (2 H, t, J = 6.7, OCH₂), 3.65 (2 H, t, J = 6.5, NCH₂),

1.83-1.70 (4 H, 2CH₂), 1.50-1.20 (20 H, 10CH₂), 0.91-0.85 (6

H, 2Me). ν(CdO) 1701st, ν(CdN) 1631st, ν(CdC) 1605st and

1582st cm^-1. Anal. Calcd (%) for C₃₀H₄₃N₃O₃: C, 72.99; H, 8.78;

(1 H, br, OH), 8.40 (1 H, s, CHN), 7.99 (1 H, dd, J_bc= 8.9, J_ac

=

2.5, c), 7.95-7.89 (3 H, C₆H₄þ a), 7.69 (2 H, d, J = 8.8, C₆H₄),

7.59 (2 H, d, J = 8.9, C₆H₄), 7.04 (1 H, d, J = 8.9, b), 6.99 (2 H, d,

J = 8.9, C₆H₄), 4.01 (2 H, t, J = 6.6, OCH₂), 3.64 (2 H, t, J = 6.7,

NCH₂), 1.86-1.69 (4 H, 2CH₂), 1.52-1.22 (18 H, 9CH₂), 0.94-

0.85 (6 H, 2Me). ν(CdN) 1636st, ν(CdC) 1604st and 1582st cm^-1

.

Anal. Calcd (%) for C₃₄H₄₅N₃O₂: C, 77.38; H, 8.59; N, 7.96. Found:

C, 77.53; H, 8.45; N, 7.69. m/z (-ES): 526.4 [M - H]^-.

Synthesis of 1,1,2,2-Tetramethyl-N,N⁰-Di{2-hydroxy-5-[4-

(ethoxycarbonyl)phenylazo]benzylidine}ethane-1,2-diamine (16H₂).

This compound was prepared in a manner similar to 2aH by

using 1 (206 mg, 0.691 mmol) and 2,3-diamino-2,3-dimethylbu-

tane (40 mg, 0.344 mmol) and ethanol (40 mL). The mixture was

heated under reflux for 5 h. After cooling, the precipitate was

filtered off to give an orange solid. Yield 105 mg (45%). δ_H(400

MHz, CDCl₃) 14.68 (2 H, s, 2OH), 8.47 (2 H, s, 2CHN), 8.17

(4 H, d, J = 8.7, 2C₆H₄), 8.01 (2 H, dd, J_bc= 9.0, J_ac= 2.4, 2c),

7.96 (2 H, d, J = 2.4, 2a), 7.87 (4 H, d, J = 8.7, 2C₆H₄), 7.05

(2 H, d, J = 9.0, 2b), 4.41 (4 H, q, J = 7.1, 2OCH₂), 1.49 (12 H, s,

4Me), 1.43 (6 H, t, J = 7.1, 2Me_est). ν(CdO) 1740st, ν(CdN)

1622st, ν(CdC) 1603st and 1589st cm^-1. Anal. Calcd (%) for

C₃₈H₄₀N₆O₆: C, 67.44; H, 5.96; N, 12.42. Found: C, 67.40; H,

5.94; N, 12.33. m/z (-ES): 675.7 [M - H]^-.

Article

Inorganic Chemistry, Vol. 49, No. 20, 2010 9139

Synthesis of Ni^II(2c)₂(3c). The pro-ligand 2cH (70 mg, 0.198

d, J = 8.7, 2C₆H₄), 6.63 (2 H, d, J = 9.1, 2b), 4.01 (4 H, t, J =

6.7, 2OCH₂), 3.89 (4 H, br, 2NCH₂), 1.94 (4 H, br, 2CH₂), 1.81

(4 H, qnt, J = 8.0, 2CH₂), 1.53-1.23 (36 H, 18CH₂), 0.93-0.84

(12 H, 4Me). ν(CdN)/ν(CdC) 1609st cm^-1. Anal. Calcd (%) for

C₆₈H₈₈N₆NiO₄: C, 73.44; H, 7.98; N, 7.56. Found: C, 73.83; H,

8.23; N, 7.45. m/z (þMALDI): 1112 [M]^þ. Diffraction-quality

crystals were obtained by slow evaporation of a dichloro-

methane solution at room temperature.

mmol) and Ni^II(OAc)₂4H₂O (25 mg, 0.100 mmol) were com-

3

bined in ethanol (20 mL), and the mixture was heated under

reflux for 2 h. Upon cooling, the resulting precipitate was

filtered off and washed with ethanol, then purified by recrys-

tallization from dichloromethane to give a brown solid. Yield

37 mg (49%). δ_H(500 MHz, CDCl₃) 8.91 (2 H, br, 2CHN), 8.16

(4 H, d, J = 8.6, 2C₆H₄), 7.89 (2 H, dd, J_bc= 8.8, 2c), 7.88 (2 H,

d, J = 2.4, 2a), 7.84 (4 H, d, J = 8.5, 2C₆H₄), 6.65 (2 H, d, J =

8.9, 2b), 4.41 (4 H, q, J = 7.4, 2OCH₂), 3.74 (4 H, br, 2NCH₂),

1.93 (4 H, qnt, J = 7.3, 2CH₂), 1.53 (4 H, qnt, J = 7.5, 2CH₂),

1.43 (6 H, t, J = 7.1, 2Me_est), 1.03 (6 H, t, J = 7.4, 2Me). ν(CdO)

1703st, ν(CdN) 1612st, ν(CdC) 1599st cm^-1. Anal. Calcd (%)

for C₄₀H₄₄N₆NiO₆: C, 62.92; H, 5.81; N, 11.01. Found: C, 62.50;

H, 5.53; N, 10.82. m/z (þES): 785.5 [M þ Na]^þ.

Synthesis of Pd^II(13)₂(15). This compound was prepared and

purified in a manner identical to 14 by using Pd^II(OAc)₂(17 mg,

0.076 mmol) in place of Ni^II(OAc)₂4H₂O and a reflux time of 24 h,

3

giving a red/brown solid. Yield 47 mg (54%). δ_H(400MHz, CDCl₃)

7.98 (2 H, dd, J_bc= 9.2, J_ac= 2.8, 2c), 7.93-7.87 (6 H, 2C₆H₄þ

2a), 7.76 (2 H, s, 2CHN), 7.67 (4 H, d, J=8.7, 2C₆H₄), 7.59(4H, d,

J = 8.8, 2C₆H₄), 6.99 (4 H, d, J = 8.8, 2C₆H₄), 6.95 (2 H, d, J =

9.1, 2b), 4.01 (4 H, t, J = 6.7, 2OCH₂), 3.76 (4 H, t,

J = 7.3, 2NCH₂), 1.88-1.77 (8 H, 4CH₂), 1.53-1.21 (36 H,

18CH₂), 0.93-0.85 (12 H, 4Me). ν(CdN) 1618st, ν(CdC) 1603st

cm^-1. Anal. Calcd (%) for C₆₈H₈₈N₆O₄Pd: C, 70.42; H, 7.65; N,

7.25. Found: C, 70.23; H, 7.81; N, 7.06. m/z (þMALDI): 1160 [M

þ H]^þ. Diffraction-quality crystals were obtained by layering of

ethanol on top of a chloroform solution at room temperature.

Synthesis of Ni^II(16)₂(17). The pro-ligand 16H₂(30 mg, 0.044

Synthesis of Pd^II(2a)₂(4a). The pro-ligand 2aH (53 mg, 0.156

mmol) and Pd^II(OAc)₂(21 mg, 0.094 mmol) were combined in

ethanol (15 mL) and heated under reflux for 24 h. After cooling to

room temperature, the resulting precipitate was filtered off and

washed with ethanol to give a brown solid which was purified by

reprecipitation from a dichloromethane solution with n-hexane

followed by filtration of a dichloromethane solution through

Celite. Removal of the solvent under vacuum gave an orange solid.

Yield 20 mg (32%). δ_H(500 MHz, CDCl₃) 8.17 (4 H, d, J = 8.5,

mmol) and Ni^II(OAc)₂4H₂O (11 mg, 0.044 mmol) were com-

3

bined in ethanol (15 mL) and heated under reflux for 2 h. The

resulting red precipitate was filtered off and washed with

ethanol. Yield 17 mg (53%). δ_H(400 MHz, CDCl₃) 8.16 (4 H,

d, J = 8.8, 2C₆H₄), 7.97 (2 H, dd, J_bc= 9.2, J_ac= 2.5, 2c), 7.92

(2 H, d, J = 2.5, 2a), 7.85 (4 H, d, J = 8.8, 2C₆H₄), 7.67 (2 H, s,

2CHN), 7.14 (2 H, d, J = 9.2, 2b), 4.41 (4 H, q, J = 7.1, 2OCH₂),

1.54 (12 H, s, 4Me), 1.42 (6 H, t, J = 7.1, 2Me_est). ν(CdO)

1740st, ν(CdN) 1606st, ν(CdC) 1597st cm^-1. Anal. Calcd (%)

for C₃₈H₃₈N₆NiO₆: C, 62.23; H, 5.22; N, 11.46. Found: C, 62.05;

H, 5.48; N, 10.83. m/z (þES): 732.6 [M]^þ, 755.6 [M þ Na]^þ.

Diffraction-quality crystals were obtained by layering of etha-

nol on top of a chloroform solution at room temperature.

Synthesis of Pd^II(16)₂(18). The pro-ligand 16H₂(40 mg, 0.059

mmol) and Pd^II(OAc)₂(14 mg, 0.062 mmol) were combined in

ethanol (20 mL) and heated under reflux for 24 h. The resultant

brown precipitate was filtered off, washed with ethanol and

purified by reprecipitation from a dichloromethane solution

with n-hexane followed by filtration of a dichloromethane

solution through Celite. Removal of the solvent under vacuum

gave an orange solid. Yield 10 mg (22%). δ_H(400 MHz, CDCl₃)

8.17 (4 H, d, J = 8.7, 2C₆H₄), 8.08 (2 H, dd, J_bc= 9.2, J_ac= 2.5,

2c), 8.04 (2 H, d, J = 2.5, 2a), 7.96 (2 H, s, 2CHN), 7.86 (4 H, d,

J = 8.6, 2C₆H₄), 7.28 (2 H, d, J = 9.2, 2b), 4.41 (4 H, q, J = 7.1,

2OCH₂), 1.59 (12 H, s, 4Me), 1.42 (6 H, t, J = 7.1, 2Me_est).

ν(CdO) 1720st, ν(CdN)/ν(CdC) 1597st cm^-1. Anal. Calcd (%)

for C₃₈H₃₈N₆O₆Pd: C, 58.43; H, 4.90; N, 10.76. Found: C, 58.32;

H, 4.94; N, 10.54. m/z (-ES): 779.2 [M - H]^-.

2C₆H₄), 8.00 (2 H, d, J = 2.2, 2a), 7.97 (2 H, dd, J_bc= 9.2, J_ac

=

2.4, 2c), 7.87 (4 H, d, J = 8.5, 2C₆H₄), 7.80 (2 H, s, 2CHN), 6.98 (2

H, d, J = 9.0, 2b), 4.83 (2 H, spt, J = 6.6, 2CHMe₂), 4.41 (4 H, q,

J = 7.1, 2OCH₂), 1.47 (12 H, d, J = 6.6, 2CHMe₂), 1.43 (6 H, t,

J = 7.1, 2Me_est). ν(CdO) 1711st, ν(CdN) 1626st, ν(CdC) 1599st

cm^-1. Anal. Calcd (%) for C₃₈H₄₀N₆O₆Pd 0.1CH₂Cl₂: C, 57.80;

3

H, 5.12; N, 10.62. Found: C, 57.39; H, 4.90; N, 10.39. m/z

(þMALDI): 781 [M - 2H]^þ.

Synthesis of Ni^II(8)₂(9). The pro-ligand 8H (100 mg, 0.203

mmol) and Ni^II(OAc)₂4H₂O (25 mg, 0.100 mmol) were com-

3

bined in ethanol (20 mL) and heated under reflux for 2 h. The

resulting light brown precipitate was filtered off, washed with

ethanol, and recrystallized from dichloromethane/methanol.

Yield 70 mg (67%). δ_H(400 MHz, CDCl₃) about 9.4 (2 H, vbr,

2CHN), 8.15 (4 H, d, J = 8.6, 2C₆H₄), 7.97 (2 H, br, 2c), 7.89 (2

H, br, 2a), 7.83 (4 H, d, J = 8.4, 2C₆H₄), 6.61 (2 H, d, J = 9.1,

2b), 4.34 (4 H, t, J = 6.7, 2OCH₂), 4.02 (4 H, br, 2NCH₂), 1.93 (4

H, br, 2CH₂), 1.79 (4 H, qnt, J = 7.6, 2CH₂), 1.52-1.22 (40 H,

20CH₂), 0.93-0.83 (12 H, 4Me). ν(CdO) 1709st, ν(CdN)

1612st, ν(CdC) 1597st cm^-1. Anal. Calcd (%) for C₆₀H₈₄N₆-

NiO₆: C, 69.02; H, 8.11; N, 8.05. Found: C, 68.68; H, 8.19; N,

7.93. m/z (þMALDI): 1043.4 [M - H]^þ.

Synthesis of Pd^II(8)₂(10). The pro-ligand 8H (100 mg, 0.203

mmol) and Pd^II(OAc)₂(27 mg, 0.120 mmol) were combined in

ethanol (20 mL) and heated under reflux for 24 h. The resultant

brown precipitate was filtered off, washed with ethanol, and

purified by column chromatography on silica gel eluting with

dichloromethane. The major orange fraction was collected and

evaporated to dryness under vacuum to give an orange solid.

Yield 60 mg (54%). δ_H(400 MHz, CDCl₃) 8.16 (4 H, d, J = 8.5,

2C₆H₄), 7.94-8.01 (4 H, 2a þ 2c), 7.87 (4 H, d, J = 8.5, 2C₆H₄),

7.78 (2 H, s, 2CHN), 6.95 (2 H, d, J = 8.7, 2b), 4.84 (4 H, t, J =

6.7, 2OCH₂), 3.77 (4 H, t, J = 7.5, 2NCH₂), 1.74-1.88 (8 H,

2qnt, 4CH₂), 1.17-1.51 (40 H, 20CH₂), 0.93-0.83 (12 H, 4Me).

ν(CdO) 1705st, ν(CdN) 1622st, ν(CdC) 1599st cm^-1. Anal.

Calcd (%) for C₆₀H₈₄N₆O₆Pd: C, 66.01; H, 7.76; N, 7.70.

Found: C, 65.01; H, 7.99; N, 7.43. m/z (þMALDI): 1091.5 [M]^þ.

Synthesis of Ni^II(13)₂(14). The pro-ligand 13H (80 mg, 0.152

X-ray Crystallography. Crystal structures of the pro-ligands

2eH and 2gH and of the complexes 3e, 4e, 14, 15, and 17 have

been obtained. Data were collected on Oxford Diffraction

XCalibur 2 or Bruker APEX II CCD X-ray diffractometers by

˚

using MoK_Rradiation (λ = 0.71073 A), and the data were

processed by using the Oxford Diffraction CrysAlis RED¹³or

Bruker SAINT¹⁴software packages with an absorption correc-

tion for 4e carried out with SADABS.¹⁵The structures were

solved by direct methods by using SIR-92¹⁶via WinGX,¹⁷or

SHELXS-97,¹⁸and refined by full-matrix least-squares on all

mmol) and Ni^II(OAc)₂4H₂O (19 mg, 0.076 mmol) were com-

(13) CrysAlis RED, Version 1.171.32.4; Oxford Diffraction Ltd.: Yarnton,

Oxfordshire, U.K., 2006.

3

bined in ethanol (20 mL) and heated under reflux for 2 h. The

resulting dark green precipitate was filtered off, washed with

ethanol and recrystallized from dichloromethane. Yield 58 mg

(69%). δ_H(400 MHz, CDCl₃) about 9.3 (2 H, vbr, 2CHN),

7.93 (2 H, d, J = 9.4, 2c), 7.89-7.82 (6 H, 2C₆H₄þ 2a), 7.67

(4 H, d, J = 8.6, 2C₆H₄), 7.59 (4 H, d, J = 8.7, 2C₆H₄), 6.99 (4 H,

(14) SAINT, Version 6.45; Bruker AXS Inc.: Madison: WI, 2003.

(15) SADABS, Version 2.10; Bruker AXS Inc.: Madison: WI, 2003.

(16) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla,

M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435.

(17) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.

(18) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467.

9140 Inorganic Chemistry, Vol. 49, No. 20, 2010

Blackburn et al.

Table 1. Crystallographic Data and Refinement Details for the Pro-Ligands 2eH and 2gH and the Complexes 3e, 4e, 14, 15, and 17

2eH

2gH

3e

4e

14

15

17

empirical formula C₂₄H₃₁N₃O₃

409.52

C₂₂H₁₉N₃O₃

373.40

triclinic

P1

6.4080(12)

11.809(2)

12.790(2)

109.641(4)

91.511(4)

94.302(4)

907.5(3)

2

C₄₈H₆₀N₆NiO₆C₄₈H₆₀N₆O₆Pd C₆₈H₈₈N₆NiO₄

C₆₈H₈₈N₆O₄Pd C₃₈H₃₈N₆NiO₆

fw

cryst syst

space group

875.73

triclinic

P1

923.42

triclinic

P1

1112.15

triclinic

P1

1159.84

triclinic

P1

733.45

monoclinic

P2₁/c

triclinic

P1

˚

a/A

5.6096(17)

8.297(3)

24.364(7)

90.544(5)

90.773(5)

105.117(5)

1094.5(6)

2

8.463(16)

9.997(17)

14.83(3)

76.12(3)

84.69(4)

69.92(3)

1144(3)

1

8.856(9)

8.927(10)

15.326(16)

87.95(2)

74.088(16)

83.86(2)

1159(2)

1

12.090(1)

16.281(2)

16.502(2)

111.935(11)

92.051(9)

94.684(9)

2995.3(6)

2

10.0155(16)

10.6712(16)

15.734(3)

93.604(3)

106.048(4)

108.118(4)

1515.4(4)

1

18.447(2)

12.3599(15)

15.3860(18)

˚

b/A

˚

c/A

R/deg

β/deg

γ/deg

U/A

Z

T/K

105.989(2)

3

˚

3372.4(7)

4

100(2)

0.083

100(2)

0.093

230(2)

0.478

100(2)

0.454

100(2)

0.378

100(2)

0.359

μ/mm^-1

cryst size/mm

0.634

0.18 ꢀ 0.18

0.50 ꢀ 0.50

0.65 ꢀ 0.10

0.45 ꢀ 0.35

0.60 ꢀ 0.10

0.40 ꢀ 0.20

0.40 ꢀ 0.08

ꢀ 0.06

ꢀ 0.10

ꢀ 0.02

ꢀ 0.05

appearance

reflns collected

independent

orange plate

7832

3827 (0.0607)

orange needle orange plate

4782

3155 (0.0651)

orange plate

5567

2416 (0.1184)

yellow rhombic plate orange needle

15493

8552 (0.0638)

orange plate

20453

4847 (0.0777)

5819

3973 (0.0943)

7646

5264 (0.0827)

reflns (R_int

reflns with

I > 2σ(I)

)

2621

1589

2592

1810

3912

3588

3943

GOF on F²

final R1,

0.970

0.0477, 0.0997 0.0559, 0.1028 0.0539, 0.1137

0.794

0.875

1.058

0.0785, 0.1659

0.856

0.0486, 0.1223

0.907

0.0661, 0.1155

1.212

0.0713, 0.1028

wR2 [I > 2σ(I)]

(all data)

peak and

0.0757, 0.1099 0.1075, 0.1165 0.0790, 0.1228

0.260, -0.174 0.266, -0.218 0.369, -0.344

0.1089, 0.1938

1.080, -0.866

0.1171, 0.1561

0.318, -0.505

0.0994, 0.1276

0.834, -0.488

0.0971, 0.1106

0.489, -0.487

-3

˚

hole/eA

2

F_odata using SHELXL-97.¹⁹All other calculations were

carried out by using the SHELXTL package.²⁰All non-hydro-

gen atoms were refined anisotropically, and hydrogen atoms

were included in idealized 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. For

to give antiparallel alignment, were filled by capillary action

above the clearing temperature of the LC and allowed to cool.

The absorption of the solution was measured parallel and

perpendicular to the direction of rubbing, and the DR (A_^/A_||)

was calculated at the absorption maximum. A cell containing

only the LC was used as a reference. Quoted values are averages

of at least two separate measurements. The error on each value is

approximated to be a maximum of (10%, as determined from

repeated measurements with a standard black dye (Merck).

Theoretical Calculations. ZINDO (Zerner’s intermediate ne-

glect of differential overlap) calculations were carried out on the

complete molecules 2eH and 3e by using the Gaussian 03 suite of

programs,²¹with the crystallographically determined geometries.

The first 50 excited states were obtained, covering wavelength

ranges of approximately 167-495 nm (2eH) and 211-616 nm (3e).

ZINDO calculations were also performed with structures opti-

mized at the semiempirical ZNDDO_2 level, as implemented in the

ORCA programs.²²Structure optimization (BP86/Def2-SVP)^23,24

and time-dependent density functional calculations were also

carried out on model systems by using Gaussian 03.²¹The first

60 excited states were obtained using B3LYP²⁵and MPW1PW91²⁶

exchange-correlation functionals and Def2-SVP basis sets, cover-

ing wavelength ranges of approximately 159-511 nm (pro-ligand)

and 243-728 nm (Ni^IIcomplex). The UV-vis absorption spectra

were simulated by using the GaussSum program.²⁷

˚

14 and 17, the data were cut at 0.9 A resolution, and for 4e, they

˚

were cut at 1 A, because the crystals diffracted rather weakly at

high angles. The complexes 3e, 4e, and 15 all show crystal-

lographic inversion centers at the metal atom. Crystallographic

data and refinement details are presented in Table 1.

Dichroic Ratio Measurements. DR values were measured by

using custom equipment and software courtesy of Hewlett-

Packard Ltd. LC cells were purchased from Linkam Scientific,

and syringe filters were purchased from Fisher Scientific U.K.

Ltd. A dye (approximately 1% by weight) was dissolved in the

LC by stirring above the clearing temperature for approximately

16 h. Slow cooling was followed by filtration through a PTFE

syringe filter of pore size 0.2 μm to remove any undissolved dye.

Glass cells, with a 5 μm wide gap and rubbed polyimide surface

(19) Sheldrick, G. M. SHELXL 97, Program for Crystal Structure

€

Analysis, Release 97-2; University of Gottingen: Gottingen, Germany, 1997.

(20) SHELXTL, Version 6.10; Bruker AXS Inc.: Madison: WI, 2000.

(21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,

M. A.; Cheeseman, V. G.; Montgomery, J. A., Vreven, Jr. T.; Kudin, K. N.;

Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;

Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakat-

suji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;

Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,

X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;

Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;

Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;

Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels,

A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Ragha-

vachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.;

Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;

Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,

C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.;

Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision

C.02; Gaussian, Inc.: Wallingford, CT, 2004.

Results and Discussion

Synthesis. The chemistry used to access the ethyl ester-

functionalized pro-ligands 2aH-iH and their corres-

ponding Ni^IIand Pd^IIcomplexes is shown in Scheme 1.

(22) Neese, F. ORCA - an ab initio, Density Functional and Semiempirical

€

Program Package, v. 2.6-35; Universitat Bonn: Bonn, Germany, 2007.

(23) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (b) Perdew, J. P. Phys.

Rev. B 1986, 33, 8822.

(24) Weigend, F.; Ahlrichs, R. Phys. Chem. Chem. Phys. 2005, 7, 3297.

(25) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.

(26) Adamo, C.; Barone, V. J. Chem. Phys. 1998, 108, 664.

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2008, 29, 839.

Article

Inorganic Chemistry, Vol. 49, No. 20, 2010 9141

Scheme 1. Syntheses of the New Ethyl Ester-Functionalized Pro-Ligands 2aH-iH and Their Ni/Pd Complexes

Scheme 2. Syntheses of the New n-Octyl Ester-Functionalized Pro-Ligand 8H and Its Ni/Pd Complexes

The precursor 1 was prepared via a diazonium coupling

reaction with salicylaldehyde (salal) and isolated in a

moderate yield. Schiff base condensations with the ap-

propriate amines give the pro-ligands in yields ranging

from 55% (2aH) to 90% (2bH).

pro-ligands 2hH and 2iH were not prepared in view of the

very poor solubility properties of their Ni^IIcounterparts.

To achieve more extended structures, an n-octyl ester

analogue of 1 (7) was prepared via a diazonium coupling

between 4-amino-n-octylbenzoate (6) and salal. Because 6

is not commercial, direct esterification of 4-aminobenzoic

acid via the acid chloride was attempted, but this ap-

proach gave none of the desired product. Therefore, 6

was synthesized by the method shown in Scheme 2, using

the activated precursor 4-nitrobenzoylchloride. 4-Nitro-

n-octylbenzoate (5) is known,²⁸but was prepared by using

an adapted procedure that involves reacting n-octanol

with 4-nitrobenzoylchloride in the presence of pyridine as

a base.²⁹This reaction proceeds almost quantitatively

when using fresh 4-nitrobenzoylchloride, but hydrolysis

of the acid chloride over time can limit the yield. 5 was

reduced to 6 by using zinc and ammonium chloride in

methanol.³⁰The product contained about 5% of an

impurity which could not be removed by column chro-

matography. This byproduct is likely to be a hydroxyla-

mine intermediate, based on its ¹H NMR spectrum. Since

the literature indicates that a hydroxylamine may not

interfere with a diazonium coupling, such a reaction was

The ethyl ester-functionalized complexes were obtained

by reacting approximately 2 equiv of the appropriate pro-

ligand with Ni^IIor Pd^IIacetate in ethanol under reflux. The

reaction times were 2 h with Ni^II(OAc)₂4H₂O, but 24 h

3

with Pd^II(OAc)₂, to allow for the decreased lability toward

ligand substitutions of Pd^II. The products form as precipi-

tates and were isolated in yields of 47-93% for M = Ni and

13-74% for M = Pd. However, not all of the Ni^IIcom-

plexes were isolated. The complex of the i-propyl-substi-

tuted pro-ligand 2aH was detected, but not isolated in pure

form because of decomposition via ligand dissociation. The

t-butyl-substituted pro-ligand 2bH gave no trace of the

desired complex from reactions in several different solvents,

including attempts with added triethylamine base. These

findings suggest that steric interactions of the bulkier Schiff

base ligands about the Ni^IIcenter may prevent the forma-

tion of stable complexes. The fact that this effect was ob-

served only for nickel is consistent with the importance of

steric effects since Ni^IIis significantly smaller than Pd^II. The

wide range of isolated yields for the Pd^IIcomplexes may be

due in part to solubility considerations. The low yield (13%)

for complex 4b was achieved after column chromatographic

purification, while 4f was isolated in a much higher yield and

shows especially low solubility. The Pd^IIcomplexes of the

(28) See for example: (a) Armstrong, M. D.; Copenhaver, J. E. J. Am.

Chem. Soc. 1943, 65, 2252. (b) Barry, J.; Bram, G.; Decodts, G.; Loupy, A.;

Orange, C.; Petit, A.; Sansoulet, J. Synthesis 1985, 40. (c) Loupy, A.; Pigeon, P.;

Ramdani, M. Tetrahedron 1996, 52, 6705.

(29) Ohgiya, T.; Nakamura, K.; Nishiyama, S. Bull. Chem. Soc. Jpn. 2005,

78, 1549.

(30) Tsukinoki, T.; Tsuzuki, H. Green Chem. 2001, 3, 37.

9142 Inorganic Chemistry, Vol. 49, No. 20, 2010

Blackburn et al.

Scheme 3. Syntheses of the New Biphenyl-Based Pro-Ligand 13H and Its Ni/Pd Complexes

Scheme 4. Syntheses of the New Salen-Like Pro-Ligand 16H₂and Its Ni/Pd Complexes

performed without further purification of 6, producing 7

in a yield similar to that obtained for 1. 7 was converted

into its n-octyl Schiff base derivative 8H which was then

reacted with the metal acetates to produce complexes 9

and 10 (Scheme 2).

The biphenyl unit in the precursor compound 11 was

formed via a Suzuki-Miyaura cross-coupling reaction

(Scheme 3). 11 has been reported previously,³¹but was

prepared via reduction of its nitro precursor. The required

boronic acid was synthesized by slightly adapting a litera-

ture procedure which involves alkylation of 4-bromophenol

followed by lithiation and reaction with tri-(i-propyl)-

borate.¹¹Coupling this boronic acid and 4-bromoaniline

with Pd⁰(PPh₃)₄as a catalyst gives 4-(n-heptoxy)biphenyl as

the major product after column chromatography. This

undesired product is attributable to a side reaction involving

exchange of Pd and P-bound aryl groups,³²especially pre-

valent with electron-rich aryl halides,³³but avoidable by

using “ligand-less” Pd catalysts.³⁴Hence, using Pd^II(OAc)₂

markedly improves the selectivity of the reaction and in-

creases the yield of 11 by almost 3-fold. The latter was then

diazotized and coupled with salal to afford 12. The solubility

of 12 is lower than that of its ester-substituted analogues, but

improves on conversion into 13H. Complexation of 13H

gives 14 and 15 (Scheme 3).

Complexes related to those described above but with

the two azobenzenes cis-orientated are of interest. Previous

studies have involved derivatives of N,N⁰-ethylenebis(sali-

cylaldiminate) (salen) and closely related tetradentate spe-

cies.^10a-c,e,iWe have carried out preliminary studies with

pro-ligands prepared by condensing 2 equiv of 1 with 1,2-

ethylenediamine and 1,2-phenylenediamine, but their Ni

complexes are very poorly soluble in common organic sol-

vents. However, the addition of methyl substituents increases

solubility while not greatly expanding the molecular width,

so we have also prepared the pro-ligand 16H₂via condensa-

tion of 1 with 2,3-diamino-2,3-dimethylbutane (Scheme 4).

The latter was prepared from commercial 2,3-dinitro-2,3-

dimethylbutane via tin/HCl reduction according to a litera-

ture method.¹²16H₂leads to the complexes 17 and 18 which

are more soluble than their unmethylated analogues.

(31) Kaharu, T.; Takahashi, S. Mol. Cryst. Liq. Cryst. 1994, 257, 35.

(32) (a) Kong, K.-C.; Cheng, C.-H. J. Am. Chem. Soc. 1991, 113, 6313.

(b) O'Keefe, D. F.; Dannock, M. C.; Marcuccio, S. M. Tetrahedron Lett. 1992,

33, 6679.

(33) Goodson, F. E.; Wallow, T. I.; Novak, B. M. J. Am. Chem. Soc. 1997,

119, 12441.

(34) See for example: (a) Wallow, T. I.; Novak, B. M. J. Org. Chem. 1994,

~

59, 5034. (b) Moreno-Manas, M.; Pajuelo, F.; Pleixats, R. J. Org. Chem. 1995,

60, 2396. (c) Badone, D.; Baroni, M.; Cardamone, R.; Ielmini, A.; Guzzi, U.

J. Org. Chem. 1997, 62, 7170.

Article

Inorganic Chemistry, Vol. 49, No. 20, 2010 9143

1

In addition to H NMR spectroscopy (see below), the

new compounds were characterized by satisfactory CHN

elemental analyses, IR spectroscopy, and mass spectro-

metry. The Pd^IIcomplexes 4a and 4b retain small amounts

of dichloromethane of crystallization which was not re-

moved by heating under vacuum. This residual solvent was

detected in the ¹H NMR spectra by peaks at about 5.2 ppm

integrating for the quantities indicated by the elemental

analyses. For 4g and 10, the ¹H NMR spectra do not reveal

any residual solvents, but the C analyses are lower than

those expected, by about 3% for 4g and 1% for 10. Despite

numerous repeatanalyses, the data quotedare the bestthat

could be obtained, and options for purification are espe-

cially limited for 4g because of its very low solubility.

The imine group is expected to give a stretching band at

about 1610-1630 cm^-1, and previous reports have noted

low energy shifts of 10-30 cm^-1on metal complexa-

tion of such groups, implying decreased CdN bond

order.^10d-h,35Such shifts are consistent with π-back-

bonding from the electron-rich metal to the imine unit.

A similar situation applies to the new Ni^IIcomplexes and

also their Pd^IIcounterparts, excepting 4a and 4c. The

ν(CdN) band energy is a little higher for the Pd^IIcom-

plexes when compared with their Ni^IIanalogues, except

for 4g and 18 (the latter gives a broad band because of

overlapped ν(CdN) and ν(CdC) absorptions). It is prob-

ably not appropriate to draw inferences concerning varia-

tions in bonding between the Ni^IIand Pd^IIcomplexes

because IR absorptions can be influenced by intermolecu-

lar as well as intramolecular factors, especially in the solid-

state. The pro-ligands and precursors give excellent mass

spectra when using electrospray ionization. Most of the

complexes do not give adequate spectra via electrospray,

but MALDI gives good results, even with the insoluble

phenyl derivatives, with excellent agreement between observed

and predicted isotope patterns.

As noted above, solubility is an important considera-

tion for potentially useful dichroic dyes. The pro-ligands

2aH-iH, 8H, 13H, and 16H₂show good solubility in

common organic solvents; most are soluble in ethanol

and all in chloroform. The complexes all precipitated out

of ethanol during their syntheses, but are generally solu-

ble in chloroform. However, the phenyl-substituted com-

plexes 3g and 4g and the Ni^IIcomplexes where R = OH

(3h) or C₆H₄-4-CO₂Et (3i) show very low solubility and

are only truly soluble in pyridine, attributable to axial

ligation of this solvent to produce octahedrally coordi-

nated complexes.³⁶Such poor solubility hinders charac-

terization and rules out any potential use as dichroic dyes.

The presence of N-alkyl substituents hence appears to

enhance the solubility of these complexes. However, the

solubilities of the n-octyl ester-substituted complexes 9

and 10 are lower than those of their ethyl ester counter-

parts 3e and 4e (9 and 10 precipitate from chloroform

solutions on standing).

1

Figure 1. Aromatic regions of the H NMR spectra of the pro-ligand

2fH and its Ni^II(3f) and Pd^II(4f) complexes recorded at 400 MHz in

CDCl₃at 293 K.

¹H NMR Spectroscopy. Except for the poorly soluble

complexes 3g-i and 4g, all of the new compounds give

well-defined ¹H NMR spectra (in CDCl₃, except for 2hH

and 12), consistent with their expected structures. Figure 1

shows representative spectra of the pro-ligand 2fH and its

Ni^II(3f) and Pd^II(4f) complexes, including the labeling

scheme used for the C₆H₃protons.

On conversion of the aldehydes 1, 7, and 12 into their

respective Schiff bases 2aH-fH, 2hH, 8H, 13H, and

16H₂, the methine proton singlet signal shifts upfield by

about 1.6-1.8 ppm, reflecting the replacement of oxygen

by the less electronegative nitrogen (note however that the

spectra for 2hH and 12 were recorded in (CD₃)₂SO). The

corresponding shifts of about 1.25 ppm observed on

moving from 1 to 2gH or 2iH are smaller than those for

the other related imines because the electron-withdrawing

phenyl substituents are deshielding while the alkyl groups

are electron donors. The multiplet signals for the phenyl

ring protons a, b, and c are readily assigned via their

coupling constants; all of these also shift upfield on imine

formation, but by only about 0.3 ppm or less. The OH

singlet signal is found at 11.39 ppm for 1 and 7. However,

this signal is not observed for 12 in (CD₃)₂SO, or for many

of the Schiff base pro-ligands, but gives a broad resonance

(35) Other examples: (a) Albert, J.; Granell, J.; Sales, J. J. Organomet.

Chem. 1984, 273, 393. (b) Lee, M.; Yoo, Y. S.; Choi, M. G. Macromolecules

1999, 32, 2777.

(36) See for example: (a) Romano, V.; Maggio, F.; Pizzino, T. J. Inorg.

Nucl. Chem. 1971, 33, 2611. (b) Warrier, K. G. K.; Pavithran, C.; Das, P. N. M.;

Joseph, P. T. Indian J. Chem. 1978, 16A, 628. (c) Ma, Y.; Zhang, W.; Ou-Yang,

Y.; Yoshimura, K.; Liao, D.-Z.; Jiang, Z.-H.; Yan, S.-P. J. Mol. Struct. 2007,

833, 98.

9144 Inorganic Chemistry, Vol. 49, No. 20, 2010

Blackburn et al.

in the region of about 14-14.8 ppm for 2eH-gH, 13H

and 16H₂. Sharper phenoxy OH singlets are observed at

13.60 ppm for 2iH and at 11.53 ppm for 2hH; in the latter,

the singlet at slightly higher field (11.03 ppm) is likely to

be attributable to the hydroxyl group attached to the

more shielding imine unit. The OH signal is readily

distinguished from the aldehyde signal in 1 and 7 by its

slightly broadened appearance and higher chemical shift.

On deprotonation of the alcohols 2cH-fH, 8H, and

13H and complexation to Ni^II, peaks corresponding to

protons close to the metal center become broadened. The

imine signal is the most affected, with the triplet signal for

the CH₂protons adjacent to the imine group and the

signals for protons a-c also showing some broadening.

These effects are attributable to a degree of paramagnetic

character due to slight distortions toward a tetrahedral

coordination geometry. Previous studies with related

complexes provide ample precedents for such behavior.³⁷

Notably, the i-propyl-substituted complex derived from

2aH which decomposed readily (see above) showed a set

of sharp NMR signals corresponding to the ethyl group

and the phenylene ring protons, with several broad

signals. These observations indicate a more severe distor-

tion toward a tetrahedral geometry in solution, attribu-

table to steric considerations.

imine proton signal shifts upfield by 0.51 ppm, while the

signals for protons a, b, and c all shift downfield, by 0.08,

0.23, and 0.07 ppm, respectively.

The upfield shifting of the imine signals for all of the

Pd^IIcomplexes and the Ni^IIcomplex 17 is evidence for

π-back-bonding from the metal centers. The large deshield-

ing effects evident for the imine signal in all of the Ni^IIbis-

chelates appear to contradict the corresponding shielding

observed in their Pd^IIanalogues. However, the likely pre-

sence of paramagnetic shifts precludes conclusions regard-

ing π-back-bonding in the Ni^IIbis-chelates. Without com-

plicating paramagnetism, comparison of the shifts observed

for the imine signals of complexes 17 and 18 is valid. The

respective peak positions of 7.67 and 7.96 ppm suggest that

π-back-bonding is stronger in the Ni^IIcomplex than its Pd^II

counterpart, since the upfield shift of about 0.3 ppm on

moving from 18 to 17 indicates greater shielding.

Finally, it is worth noting that the ¹H NMR spectra of

both the new pro-ligands and complexes show some

evidence for trans-to-cis photoisomerization of the azo-

benzene units. The observed minor signals shifted to

higher field with respect to the main peaks are character-

istic of cis-azobenzenes,³⁸and these are present in levels as

high as about 16%. When the samples are placed in

amberized NMR tubes which block light of 300-800

nm, the minor signals disappear, indicating thermal

relaxation back to the trans isomers.

Electronic Spectroscopy Studies. The UV-vis absorp-

tion spectra of the new pro-ligands and complexes have

been measured in dichloromethane, and the results are

shown in Table 2. Representative spectra of 2cH, 3c, 4c

and 13H, 14, and 15 are shown in Figure 2.

The pro-ligands show intense absorptions with λ_maxin the

range 353-379 nm, characteristic of the π f π* intramo-

lecular charge-transfer (ICT) transitions of azobenzene. In

each case, a pronounced shoulder on the low energy side of

the ICT absorption is attributable to the n f π* transition

which appears at about 450 nm for trans-azobenzene.³⁹

Each compound also displays a second, less intense band to

higher energy, with λ_maxin the range 246-278 nm.

On complexation, λ_maxof the dominant absorption

band increases (Figure 2), indicating that the energy gap

between the highest occupied molecular orbital (HOMO)

and the lowest unoccupied molecular orbital (LUMO)

decreases. It is likely that the ICT process acquires some

degree of metal-to-ligand charge-transfer (MLCT) char-

acter, with the HOMO energy being raised by deprotona-

tion and complexation of the hydroxyl group. The red

shifts are almost constant at about 0.5 eV on moving from

the pro-ligands 2cH-2fH, 8H and 16H₂to their Ni^II

complexes, but a smaller shift of 0.3 eV is observed for the

biphenyl derivatives 13H and 14. Generally smaller re-

lative red shifts occur on coordination of the pro-ligands

to Pd^II; about 0.4 eV for 2aH, 2cH-2eH, 8H, and 16H₂,

but 0.25 eV for 2bH and 13H. The main absorptions of the

Pd^IIcomplexes are hence blue-shifted (by 0.05-0.13 eV)

with respect to those of their Ni^IIanalogues (Figure 2).

These blue shifts are consistent with greater stabilization of

Significant peak shifts are also observed on complexa-

tion of the pro-ligands to Ni^II. The imine proton signal

shifts downfield by about 0.5-1.1 ppm, and the doublet

signal for proton b shifts upfield by about 0.4 ppm. The

NCH₂signals are also shifted downfield by up to 0.4 ppm.

The doublet of doublets for proton c shifts upfield, but by

only about 0.1 ppm or less, while even smaller shifts are

detected for the doublet for proton a. These shifts are

probably associated significantly with the paramagnet-

ism discussed above. Notably, for the salen-like pro-

ligand 16H₂, a quite different pattern of shifts is observed

on moving to complex 17. An absence of signal broad-

ening implies a perfectly square planar geometry in solu-

tion imposed by the rigidity of the tetradentate ligand.

The a and c proton signals still shift slightly upfield, but

the b proton signal shifts downfield by 0.09 ppm, and the

imine proton signal moves upfield by 0.8 ppm. Note

however that slight distortion is nonetheless observed in

the solid-state structure of 17 (see below).

1

The H NMR spectra of the Pd^IIcomplexes show no

broadening, consistent with their expected perfectly

square planar structures. Pd^IIcomplexation of the pro-

ligands 2aH, 2cH-fH, 8H, and 13H shifts the imine

proton signal upfield by about 0.6 ppm (Figure 1), show-

ing increased shielding. Only slight shifts are observed for

the C₆H₃signals; the a proton signal shifts downfield by

0.04-0.08 ppm, while the b and c proton signals shift

upfield. Notably, the Pd^IIcomplexation-induced shifts of

the signals for the t-butyl derivative 2bH differ greatly

from those observed for the other closely related species.

The imine proton signal shows a larger upfield shift of

0.87 ppm in 4b, while the signals for protons a-c all shift

downfield slightly. On Pd^IIcomplexation of 16H₂, the

(37) See for example: (a) Sacconi, L.; Paoletti, P.; Ciampolini, M. J. Am.

Chem. Soc. 1963, 85, 411. (b) Holm, R. H.; Swaminathan, K. Inorg. Chem. 1963,

2, 181. (c) Sacconi, L.; Ciampolini, M. J. Am. Chem. Soc. 1963, 85, 1750.

(d) Sacconi, L.; Ciampolini, M.; Nardi, N. J. Am. Chem. Soc. 1964, 86, 819.

(38) Tait, K. M.; Parkinson, J. A.; Bates, S. P.; Ebenezer, W. J.; Jones,

A. C. J. Photochem. Photobiol. A 2003, 154, 179.

(39) Haberfield, P.; Lux, M. S.; Rosen, D. J. Am. Chem. Soc. 1977, 99,

6828.

Article

Inorganic Chemistry, Vol. 49, No. 20, 2010 9145

Table 2. UV-vis Absorption Data for the New Pro-Ligands and Their Ni^II/Pd^IIComplexes in Dichloromethane^a

λ

_max, nm

(ε, 10³M^-1cm^-1

/E_max, eV

λ

_max, nm

(ε, 10³M^-1cm^-1

/E_max, eV

λ_max, nm

)

(ε, 10³M^-1cm^-1

/E_max, eV

)

pro-ligand

Ni^IIcomplex

Pd^IIcomplex

2aH

370 (21.3)/3.35

277 (14.7)/4.48

378 (20.9)/3.28

276 (14.0)/4.49

370 (21.0)/3.35

274 (15.6)/4.53

4a

418 (52.5)/2.97

278 (27.9)/4.46

408 (43.4)/3.04

259 (40.2)/4.79

418 (74.5)/2.97

280 (36.9)/4.43

2bH

2cH

4b

4c

3c

3d

3e

3f

433 (47.3)/2.86

344 (22.5)/3.60

269 (33.3)/4.61

434 (59.3)/2.86

339 (26.6)/3.66

269 (38.7)/4.61

429 (63.2)/2.89

347 (30.1)/3.57

270 (47.8)/4.59

433 (63.9)/2.86

338 (29.0)/3.67

269 (42.1)/4.61

b

2dH

2eH

2fH

371 (22.2)/3.34

277 (15.4)/4.48

4d

4e

4f

419 (69.3)/2.96

280 (34.3)/4.43

372 (23.7)/3.33

278 (14.9)/4.46

418 (61.9)/2.97

280 (35.5)/4.43

371 (27.7)/3.34

277 (17.1)/4.48

418 (67.9)/2.97

280 (34.3)/4.43

2gH

8H

353 (34.0)/3.51

266 (17.1)/4.66

371 (23.9)/3.34

276 (15.2)/4.49

b

9

434 (56.3)/2.86

339 (25.7)/3.66

269 (37.3)/4.61

418 (66.3)/2.97

252 (41.4)/4.92

433 (51.9)/2.86

257 (44.7)/4.82

10

418 (73.7)/2.97

279 (36.1)/4.44

13H

379 (36.4)/3.27

246 (22.6)/5.04

370 (51.9)/3.35

276 (34.0)/4.49

14

17

15

18

411 (79.8)/3.02

241 (50.9)/5.15

418 (71.1)/2.97

271 (35.2)/4.58

16H₂

^aSolutions about 4 ꢀ 10^-6-6 ꢀ 10^-5M. The ε values are the averages from measurements made at four different concentrations (with ε showing no

significant concentration dependence). ^bNot measured because of insufficient solubility.

the HOMO for Pd^IIwhen compared with Ni^II, because of

the increase in effective nuclear charge when moving from a

first to second row transition metal.⁴⁰In all cases except for

16H₂, the ε values increase about 2-3-fold on complexa-

tion, since two azobenzene chromophores are combined

into one molecule. For 5 out of 7 comparable pairs, the band

intensities are also considerably larger for the Pd^IIcom-

plexes than for their Ni^IIanalogues. For 16H₂, ε remains

unchanged on complexation to Ni^II(17), but increases by

about 40% on moving to the Pd^IIcomplex 18. Most of the

Ni^IIcomplexes also display a shoulder on the high energy

side of their ICT band, but this is not the case for their Pd^II

counterparts. The ICT bands of the biphenyl-containing

complexes 14 and 15 are slightly blue-shifted when com-

pared with those of the other related species because of the

replacement of an electron-accepting ester with an 4-(n-

heptoxy)phenyl electron donor.

Crystallographic Studies. Single crystal X-ray structures

have been obtained for the pro-ligands 2eH and 2gH and the

complexes 3e, 4e, 14, 15, and 17. Representations of the

molecular structures are shown in Figures 3-9, and selected

geometric parameters in Table 3.

The structure of 2eH (Figure 3) shows a hydrogen bond

between the hydroxyl proton and the nearby imine nitrogen

˚

with a N H distance of 1.84 A. The two phenyl rings of the

azobenzene unit are almost coplanar, with a dihedral angle

3 3 3

of 5.5(1)ꢀ. In the closely related 2gH (Figure 4), the hydro-

gen-bond distance is 1.86 A, and the dihedral angle within

˚

the azobenzene group is 7.4(1)ꢀ. A much larger twist of

41.3(1)ꢀ is observed between the N-phenyl substituent and

the nearby phenolic ring.

Figure 2. UV-vis absorption spectra in dichloromethane at 293 K: (a)

the pro-ligand 2cH (blue), its Ni^IIcomplex 3c (red), and its Pd^IIcomplex

4c (green); (b) the pro-ligand 13H (blue), its Ni^IIcomplex 14 (red) and its

Pd^IIcomplex 15 (green).

(40) (a) Kirgan, R. A.; Rillema, D. P. J. Phys. Chem. A 2007, 111, 13157.

(b) Shimazaki, Y.; Stack, T. D. P.; Storr, T. Inorg. Chem. 2009, 48, 8383.

9146 Inorganic Chemistry, Vol. 49, No. 20, 2010

Blackburn et al.

Figure 3. Representation of the molecular structure of 2eH, with the

disorder in the n-octyl chain removed (50% probability ellipsoids).

Figure 6. Representation of the molecular structure of 4e, with the H

atoms omitted for clarity (50% probability ellipsoids).

Figure 4. Representation of the molecular structure of 2gH (50%

probability ellipsoids).

Figure 7. Representation of the molecular structure of 14, with the H

atoms omitted for clarity (50% probability ellipsoids).

Figure 5. Representation of the molecular structure of 3e, with the

disorder in the n-octyl chains removed and H atoms omitted for clarity

(50% probability ellipsoids).

Figure 8. Representation of the molecular structure of 15, with the

disorder in the n-octyl chains removed and H atoms omitted for clarity

(50% probability ellipsoids).

The structure of complex 3e (Figure 5) is in most respects

reminiscent of that reported for the analogous complex

having instead n-pentyl groups on the imine N atoms and

n-butyl substituents in place of the ester units.^10dAlthough

many similar structures of molecules containing the central

N₂O₂-coordinated Ni^IIcore have been reported, there ap-

pear to be no others with azobenzene substituents. The

mostly noteworthy point relating to 3e is that the coordina-

tion geometry around the Ni^IIion is strictly square planar. It

is hence apparent that the slight distortions toward tetra-

hedral geometry inferred in solution from ¹H NMR spectra

(see above) are absent in the solid state. Nevertheless, it is

quite conceivable that some departure from planarity may

occur when such complexes are dissolved in a LC medium.

Furthermore, the previously reported related structure does

show a significant distortion, with a dihedral angle of 13.2ꢀ

between the two fully planar six-membered chelate rings.^10d

As expected, the two chelating ligands in 3e adopt a trans

arrangement with respect to the planar core, preventing

steric clashes between the n-octyl chains. When compared

with the pro-ligand 2eH, 3e shows a much larger dihedral

angle of 24.2(1)ꢀ between the two rings of the azobenzene

units. The CdN distance in the imine group is significantly

longer in 3e when compared with 2eH, while the phenolic

C-O and the C-C distance between the imine unit and

phenyl ring both shorten somewhat on Ni^IIcomplexation

Figure 9. Representation of the molecular structure of 17, with the H

atoms omitted for clarity (50% probability ellipsoids).

(Table 3). The phenyl C-C distance that is also part of the

six-membered chelate ring does not change significantly on

moving from 2eH to 3e.

The structure of the analogous Pd^IIcomplex 4e(Figure6)

also shows a strictly square planar geometry, as expected.

Once again, various related structures have been reported

(although not as many as for Ni^II, and lacking azobenzene

groups).⁴¹In 4e, the azobenzene dihedral angle is less than

(41) See for example: (a) Lanfredi, A. M. M.; Ugozzoli, F.; Ghedini, M.;

Licoccia, S. Acta Crystallogr., Sect. C 1985, 41, 192. (b) Ghedini, M.;

Pellegrino, C.; Armentano, S.; de Munno, G.; Bruno, G. Inorg. Chim. Acta

1986, 122, 193. (c) Lai, Y.-C.; Chen, H.-Y.; Hung, W.-C.; Lin, C.-C.; Hong, F.-E.

Tetrahedron 2005, 61, 9484.

Article

Inorganic Chemistry, Vol. 49, No. 20, 2010 9147

˚

Table 3. Selected Interatomic Distances (A) and Angles (deg) for the Pro-Ligands 2eH and 2gH and for the Complexes 3e, 4e, 14, 15, and 17

2eH

2gH

3e

4e

14

15

17

CdN

1.279(2)

1.454(2)

1.407(2)

1.349(2)

1.280(3)

1.459(4)

1.404(4)

1.347(3)

1.304(4)

1.428(4)

1.396(5)

1.317(4)

1.833(3)

1.917(3)

1.353(12)

1.446(13)

1.425(14)

1.332(11)

2.008(6)

2.027(8)

1.291(6), 1.286(6)

1.418(7), 1.431(7)

1.431(7), 1.422(7)

1.310(6), 1.309(6)

1.826(3), 1.832(3)

1.932(4), 1.931(4)

1.290(5)

1.436(6)

1.421(6)

1.309(5)

1.978(3)

2.025(4)

1.293(5), 1.294(5)

1.438(6), 1.431(6)

1.412(6), 1.420(6)

1.302(5), 1.300(5)

1.844(3), 1.847(3)

1.853(3), 1.846(3)

C(im)^a-C(Ph)

C(Ph)-C(Ph)^b

C(Ph)-O

M-O

M-N

C-N-C^c

N-C-C^d

N-M-O^e

N-M-O

N-M-N

O-M-O

119.4(6)

121.90(17)

120.1(3)

121.6(3)

114.5(3)

127.0(3)

93.04(16)

86.96(16)

114.4(8)

126.9(10)

91.4(3)

114.0(4), 113.5(4)

127.9(5), 127.6(5)

92.80(17), 92.32(17)

87.18(18), 87.72(17)

116.1(4)

129.2(5)

92.16(14)

87.84(14)

120.0(4), 120.0(4)

125.2(4), 125.6(4)

94.98(14), 95.13(14)

173.46(15), 175.14(14)

86.75(16)

88.6(3)

83.65(13)

^aim = imine. ^bFor the bond within the chelating unit. ^cAngle subtended at the imine N atom. ^dAngle subtended at the imine C atom. ^eWithin the six-

membered chelate rings.

half of that in 3e (11.4(3)ꢀ), although this difference is likely

due to crystal packing forces rather than intramolecular

Table 4. Dichroic Ratios Measured in LC Hosts

compound

ZLI-2293

ZLI-1132

K15

electronic effects. As expected, the metal-ligand distances

in 4e are somewhat larger than those in 3e. On moving from

this Ni^IIcomplex to its Pd^IIanalogue, the M-O distance

2aH

2bH

2cH

2dH

2eH

2fH

8H

13H

14

15

5.2

5.4

5.5

5.0

6.0

5.3

8.4

12.1

5.8

5.5

9.7

5.4

3.6

3.2

3.7

3.3

3.4

3.9

2.8

6.3

˚

increases by about 0.18 A, while the corresponding increase

˚

for the M-N distance is about 0.11 A. The N-M-O

chelating angle is slightly larger in 3e when compared with

4e, while the imine CdN distance is slightly longer in 4e

than in 3e (Table 3).

8.1

10.6

5.8

The structure of the Ni^IIcomplex 14 (Figure 7) resembles

that of 3e, with a fully square planar geometry and indis-

tinguishable bond distances and angles around the metal

center. The length of the molecule from alkoxy oxygen to its

16H₂

17

18

3.7

7.0

3.7

˚

symmetry-related partner is 34.9 A, and the structure is quite

presence of a linkage between the N atoms, and means that

the Ni-O and Ni-N distances in 17 are not significantly

different. The O-M-N angle in 17 is slightly larger (by

ca. 2ꢀ) when compared with those in 3e and 14. In contrast to

3e and 14, but as for the more closely related structures in

the literature,⁴²17 shows a sight distortion away from per-

fect square planar coordination geometry, with an angle of

7.54(0.12)ꢀ between the planes defined by O1-Ni1-N1 and

O4-Ni1-N4. The molecule is distinctly bowed when viewed

sideways, with one azobenzene unit only slightly twisted

(dihedral angle =9.35(0.12)ꢀ), but the other much more so

(dihedral angle =30.13(0.14)ꢀ). The torsion angle of the

ethylene bridging unit (N1-C17-C20-N4) is 43.3(4)ꢀ.

Dichroic Ratio Measurements. The pro-ligands are

highly soluble in a range of LCs and their DRs measured

in both ZLI-2293⁴³and K15 (4-pentyl-4⁰-cyano-

biphenyl) are shown in Table 4. It should be noted that

a lower wavelength limit of 380 nm is imposed by the spec-

trometer used, so the DRs quoted are not necessarily

calculated at the absorption maximum. The values in K15

are lower than those in ZLI-2293 since the lower clearing

temperature of the LC corresponds to a lower S at room

temperature. A correlation between the S of a host LC

and DR values of dissolved dyes is established.^5,44The

twisted with respective dihedral angles of 28.2(2)/31.2(2) and

17.6(2)/22.3(2)ꢀ within the biphenyl and azobenzene units.

Except for some disorder at the ends of the alkyl chains, the

analogous Pd^IIcomplex 15 (Figure 8) is essentially isostruc-

tural with 14, with corresponding dihedral angles of 30.4(2)

and 17.8(1)ꢀ. Again, the bond distances and angles around

the metal center are very similar for 15 and 4e. On moving

from 14 to 15, the average M-O distance increases by about

˚

0.15 A, while the corresponding increase for the M-N dis-

˚

tance is about 0.09 A. The imine CdN distances in 14 and 15

are effectively identical, so it is not appropriate to propose

electronic changes as causing the apparent small difference in

these distances observed between 4e and 3e (see above). For

each of 3e, 4e, 14, and 15, the M-N distances are larger than

the M-O distances, but the differences are noticeably great-

II

˚

er for the Ni complexes (ca. 0.08 and 0.10 A for 3e and 14,

respectively) than for their Pd^IIcounterparts (ca. 0.02 and

0.05 A for 4e and 15, respectively).

˚

The structure of the Ni^IIcomplex 17 (Figure 9) resembles

those of related species containing a similar chelating motif,⁴²

although no structures of similar azobenzene-substituted

complexes have been reported to our knowledge. The in-

evitable cis disposition of the N atoms in 17 does not greatly

influence the Ni-O distances, but the Ni-N distances are

˚

(43) The precise compositions and chemical identities of the commercial

LC mixtures ZLI-1132 and ZLI-2293 are unknown to us. However,

comparisons of their ¹H NMR spectra obtained in CDCl₃with that of the

reference compound 4-(trans-4-n-pentylcyclohexyl)bromobenzene indicate

that both of these substances contain primarily 4,4⁰-disubstituted cyclohexyl-

benzene derivatives with long alkyl chain(s).

shortened by about 0.07-0.08 A when compared with those

in 3e and 14. This difference is probably attributable to the

(42) (a) de C. T. Carrondo, M. A. A. F.; de Castro, B.; Coelho, A. M.;

Domingues, D.; Freire, C.; Morais, J. Inorg. Chim. Acta 1993, 205, 157. (b)

€

Fritzsche, S.; Lonnecke, P.; Hocher, T.; Hey-Hawkins, E. Z. Anorg. Allg. Chem.

2006, 632, 2256.

(44) Ichinose, H.; Sato, H.; Naemura, S. Mol. Cryst. Liq. Cryst. 2004, 409,

401.

9148 Inorganic Chemistry, Vol. 49, No. 20, 2010

Blackburn et al.

The complexes 17 and 18 are more soluble than the bis-

bidentate species, so were studied in K15 as well as ZLI-

2293; the results are presented in Table 4. Once again, an

increase in DR is observed on complexation of the pro-ligand

to Ni^II(in 17). The flexibly linked chromophores in 16H₂

become one long, rigid dye on complexation. The DR

increases in this case are apparently larger than those for

the biphenyl complexes, being almost 2-fold in K15. This

more pronounced effect of complexation may be because the

l/d ratio increases more dramatically when compared with

the bis-bidentate compounds. Notably, the DRs of the Pd^II

complex 18 areessentiallythesameasfor16H₂. Thefactor(s)

causing a decline in dichroism on moving from Ni to Pd is/

are apparently less effective in the tetradentate system.

Theoretical Studies. To rationalize the electronic struc-

tures and optical properties of the representative pro-

ligand 2eH and its Ni^IIcomplex 3e, ZINDO_S calculations

were carried out by using Gaussian 03,²¹with ORCA²²for

structural optimization. The UV-vis spectra were simulated

(GaussSum)²⁷by using both the optimized and the X-ray

crystallographic (see above) geometries. Using the latter struc-

tures gives reasonable agreement with the experimental data

for 2eH, but a lesser degree of correlation is found for the

complex (Supporting Information, Figure S1). The predicted

transition energies are presented in Supporting Information,

Table S1, and the principal orbitals involved in the ICT tran-

sitions are shown in Supporting Information, Figures S2-S5.

Since time-dependent density functional theory (TD-

DFT) has become the preferred computational approach

recently, such calculations were performed on model systems

by using Gaussian 03.²¹The models were obtained from the

crystal structures of 2eH and 3e by replacing the octyl and

ethyl chains with methyl groups (the alkyl chains are not

expected to affect signficantly the electronic structures).

Geometries optimized at the BP86/Def2-SVP level^23,24were

used in TD-DFT calculations of the electronic spectra

(Figure 11 and Supporting Information, Figure S6), with

the B3LYP²⁵and MPW1PW91²⁶functionals. It is known

that B3LYP can give poor predictions of ICT transitions and

MPW1PW91 is apparently better at describing such pro-

cesses. The predicted transition energies are shown in Table 5,

and the principal orbitals involved in the main transitions

are depicted in Figures 12 and 13.

When comparing the TD-DFT predictions with the

experimental absorption spectra for 2eH and 3e (Figures 11

and Supporting Information, Figure S6), both func-

tionals give quite similar results and reproduce the qua-

litative details well, with B3LYP generally giving transi-

tion energies slightly lower than MPW1PW91 (Table 5).

For the pro-ligand, the ICT energy is slightly better pre-

dicted by using MPW1PW91, while B3LYP gives a closer

match with the experimental profile for the nickel com-

plex. The relative superiority of TD-DFT over the ZIN-

DO_S method is most readily apparent for the complex,

with the latter predicting a substantial increase of about

0.3 eV in the ICT E_max(Figures 11b, Supporting Informa-

tion, Figures S1b and S6b).

For the methylated pro-ligand model, BP86/Def2-SVP

geometry optimization yields an almost completely pla-

nar structure that resembles that observed for 2eH via

X-ray crystallography (see above). Both the ZINDO_S

calculations with the X-ray structure and the TD-DFT

studies confirm the π f π* nature of the major excitation

Figure 10. Absorption profiles measured parallel (purple) and perpen-

dicular (green) to the LC director for the Ni^IIcomplex 14 in ZLI-1132.

DRs for all of the pro-ligands, except 13H, are the same

within experimental error. Notably, neither the presence

of a longer terminal alkyl chain (in 8H) nor the connec-

tion of two chromophores via an ethylene bridge (in

16H₂) improves the dichroism when compared with the

bidentate ethyl ester-substituted pro-ligands. Different R

groups (Scheme 1) also do not affect the DR value. The

biphenyl-based 13H shows a significantly higher DR, pro-

bably as a result of better ordering within the LC because

of its mesogenic subunit.

Unfortunately, most of the complexes show poor solubi-

lity in the widely used single-component LC K15 and insuffi-

cient absorption precludes meaningful measurements. Their

solubility in commercial LC mixtures (ZLI-2293, etc.) is far

superior to that in K15, although in most cases a 5 μm path

length does not provide sufficient absorption intensity.

The biphenyl-based complexes 14 and 15 are the most

soluble of the bis-bidentate species. Using two adjacent

cells to give sufficient absorption, the DRs for these

compounds determined in ZLI-2293 and ZLI-1132 are

shown in Table 4. A representative absorption spectrum

is shown in Figure 10. The Ni^IIcomplex 14 has a signi-

ficantly higher DR when compared with its pro-ligand

13H in ZLI-2293. However, in ZLI-1132, the observed

increase may not be significant. The increases in DR are

certainly less than 2-fold, although the length of the dye is

at least doubled. Several factors may prevent a doubling

of the DR. An accompanying increase in the width of the

dye because of the staggered nature of the coordination

sphere, implies that the length-to-diameter ratio (l/d) is

less than double that of the pro-ligand.^5cNevertheless,

the DR of 14 is sufficiently high for potential electronic

device applications. Interestingly, the DRs of the Pd^II

complex 15 are smaller than those of the pro-ligand, des-

pite being essentially isostructural with 14. The origin of

these decreases is unclear, although possible explanations

include dimerization via Pd-Pd interactions. Such effects

have been invoked to explain an absence of LC behavior

for Pd^IIbis-dithiolene complexes when compared with

their Ni^II(and also Pt^II) analogues,⁴⁵but do seem rather

unlikely here, given the low concentrations involved. More

generally, intermolecular interactions between dye and LC

play an important role in influencing DRs, and these may

be affected by the differing sizes of the metal ions and

charge distributions in the complexes.

(45) Mueller-Westerhoff, U. T.; Nazzal, A.; Cox, R. J.; Giroud, A. M.

Mol. Cryst. Liq. Cryst. 1980, 56, 249.

Article

Inorganic Chemistry, Vol. 49, No. 20, 2010 9149

Table 5. Data from TD-DFT Calculations Based on BP86/Def2-SVP Geometries

of the Pro-ligand and Ni^IIComplex Models

functional

B3LYP

ΔE (eV)

f_os

major contributions

μ₁₂(D)

Pro-Ligand

3.21

4.13

1.10 HOMO f LUMO

0.12 HOMO-3 f LUMO

HOMO-4 f LUMO

0.12 HOMO-4 f LUMO

HOMO-3 f LUMO

9.52

2.76

4.31

2.73

HOMO f LUMOþ2

4.84

5.19

0.17 HOMO-4 f LUMOþ1

HOMO f LUMOþ2

3.07

3.25

0.21 HOMO-7 f LUMO

HOMO-4 f LUMOþ1

HOMO-3 f LUMOþ1

HOMO f LUMOþ4

5.50

3.30

4.30

0.16 HOMO f LUMOþ4

1.14 HOMO f LUMO

0.14 HOMO-4 f LUMO

HOMO-3 f LUMO

2.77

9.53

2.98

MPW1PW91

4.47

5.00

0.10 HOMO-4 f LUMO

HOMO-3 f LUMO

2.44

3.03

HOMO f LUMOþ2

0.17 HOMO-4 f LUMOþ1

HOMO-3 f LUMOþ1

HOMO f LUMOþ2

5.36

5.68

0.24 HOMO-4 f LUMOþ1

HOMO f LUMOþ3

3.44

2.67

0.15 HOMO f LUMOþ4

Ni Complex

B3LYP

2.79

3.18

4.00

1.62 HOMO f LUMO

1.26 HOMO-1 f LUMOþ1

0.51 HOMO-6 f LUMO

HOMO-5 f LUMOþ2

12.39

10.24

5.78

Figure 11. TD-DFT-calculated (blue) UV-vis spectra of (a) the pro-

ligand model system (MPW1PW91) and (b) the Ni^IIcomplex model

system (B3LYP); and experimental (green) UV-vis spectra of (a) 2eH

and (b) 3e. 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).

MPW1PW91

2.97

3.36

4.26

2.29 HOMO f LUMO

0.77 HOMO-1 f LUMOþ1

0.49 HOMO-6 f LUMO

HOMO-4 f LUMOþ2

14.27

7.77

5.49

(Figure 12 and Supporting Information, Figure S7),

showing that the π-bonding and antibonding orbitals

involved span both rings, their substituents and the azo

unit. This picture is consistent with previous studies on

azobenzenes.⁴⁶The ZINDO_S calculations also predict a

very weak excitation to low energy of the ICT band

(Supporting Information, Table S1), possibly attributa-

ble to an n f π* transition. Interestingly, optimization of

the molecular structure using the ZNDDO_2 procedure,

as implemented in ORCA, results in a large dihedral angle

(83.7ꢀ) between the two rings which contrasts strongly

with the crystal structure (see above). ZINDO_S calcula-

tions using this highly twisted geometry produce a similar

electronic picture, but the ester-substituted phenyl ring no

longer contributes to the frontier orbitals (Supporting

Information, Figure S4).

BP86/Def2-SVP geometry optimization of the methy-

lated model for Ni complex 3e again produces an almost per-

fectly planar structure. TD-DFT calculations with B3LYP

show that two transitions lead to the broad ICT profile

observed experimentally (Figure 11b). The MPW1PW91

functional affords a similar picture, but with slightly higher

transition energies and a less equal weighting (Supporting

Information, Figure S6). When using either functional, the

Figure 12. TD-DFT-derived depictions of the main orbitals involved in

the ICT transition in the pro-ligand model system obtained by using the

MPW1PW91 functional (isosurface value 0.03 au).

(46) See for example: (a) Forber, C. L.; Kelusky, E. C.; Bunce, N. J.;

Zerner, M. C. J. Am. Chem. Soc. 1985, 107, 5884. (b) Bhat, K. L.; Trachtman,

lowest energy intense ICT transition is HOMO f LUMO in

nature, with the next lowest energy transition being HOMO-

1 f LUMOþ1 (Figure 13 and Supporting Information,

ꢀ

M.; Bock, C. W. Dyes Pigments 2001, 48, 197. (c) Zalesny, R.; Matczyszyn, K.;

Kaczmarek, A.; Bartkowiak, W.; Cysewski, P. J. Mol. Model. 2007, 13, 785.

9150 Inorganic Chemistry, Vol. 49, No. 20, 2010

Blackburn et al.

Figure 13. TD-DFT-derived depictions of the main orbitals involved in the ICT transition in the Ni^IIcomplex model system obtained by using the B3LYP

functional (isosurface value 0.03 au).

Conclusions

We have synthesized a family of N₂O₂-chelated Ni^IIor Pd^II

complexes containing azobenzene chromophores, with various

substituents close to and remote from the metal center. Single

crystal X-ray structures show a trans-square planar arrange-

ment for all of the bis-bidentate chelates, although a slight

distortion of the Ni^IIcomplexes toward a tetrahedral geometry

in solution is evidenced by broadened ¹H NMR spectra. Salen-

like complexes give sharp ¹H NMR spectra, but X-ray crystal-

lography of the Ni^IIcomplex reveals a slight distortion in the

solid-state. The characteristic azobenzene π f π* electronic

transition gains some degree of MLCT character on complexa-

Figure 14. Calculated μ₁₂vectors (scaled to show direction) for the first

excited state of the model pro-ligand (MPW1PW91) with ΔE = 3.30 eV

(pink) and the first two excited states of the model Ni complex (B3LYP)

with ΔE = 2.79 eV (orange) and ΔE = 3.18 eV (pink).

tion of the pro-ligands. Bathochromic shifts of 0.25-0.5 eV

are produced which are always larger for the Ni^IIcomplexes

than for their Pd^IIanalogues, while the extinction coefficients

approximately double on complexation. The contribution of

the metal to the HOMO in these Ni^IIcomplexes is confirmed by

ZINDO_S and TD-DFT calculations. These theoretical meth-

ods also indicate a good alignment of μ₁₂with the long mole-

cular axis. DR measurements on pro-ligands show that a

biphenyl unit confers favorable ordering within the LC. While

complexation to Ni^IIincreases the DR, Pd^IIcomplexation of a

biphenyl-containing pro-ligand is detrimental. The dichroism

of one of the Ni^IIcomplexes may be sufficient for potential

applications in guest-host displays.

Figure S8). The ICT occurs from orbitals based on the metal

center and adjacent rings to the π* orbitals on the azo unit,

with small contributions from other aromatic rings and ester

groups. Involvement of the metal in the HOMO is thus

supported by these calculations. ZINDO_S calculations using

either an optimized geometry or the X-ray crystal structure

afford a qualitatively similar picture (Supporting Informa-

tion, Figures S3 and S5). It is notable that, as with 2eH, the

lowest energy conformation from ZNDDO_2 optimization

shows a much greater twist angle between the azobenzene

rings (71.7ꢀ) than is observed in the solid-state structure (see

above). This optimized geometry produces spatially similar

high-lying occupied orbitals on ZINDO_S treatment, but the

LUMO and LUMOþ1 show much greater contribution

from orbitals on the ester-substituted ring (Supporting In-

formation, Figure S5).

We have also used TD-DFT calculations to approximate

the directions of the transition dipole moments (μ₁₂) for the

ICT transitions occurring in the pro-ligand model (using

MPW1PW91) and the Ni model complex (using B3LYP).

These predictions indicate that the μ₁₂vectors are reasonably

well aligned with the long molecular axes in both cases

(Figure 14). For the Ni complex, the two transitions com-

posing the visible absorption band give very similar μ₁₂align-

ment. ZINDO_S calculations with the crystallographically

observed structures afford results similar to the TD-DFT

predictions (Supporting Information, Figure S9).

Acknowledgment. We thank the University of Manche-

ster for support and also the EPSRC National Mass

Spectrometry Service Centre, Swansea, U.K. We also

wish to thank Hewlett-Packard Ltd for the use of the

dichrometer and generous donations of LCs.

Supporting Information Available: Crystallographic informa-

tion in CIF format; further synthetic and characterization details;

additional molecular orbital pictures, spectra, and data from TD-

DFT calculations on the model compounds and from ZINDO_S

studies on the pro-ligand 2eH and its Ni^IIcomplex 3e; Cartesian

coordinates of theoretically optimized geometries; ¹H NMR spectra

for all new and sufficiently soluble compounds (PDF). This material

is available free of charge via the Internet at http://pubs.acs.org.

Note Added after ASAP Publication. This paper was

published on the Web on September 14, 2010. Clarifica-

tion to abbreviations s and st have been made, and the

corrected version was reposted on September 20, 2010.

Article Doi

Nickel(II) and palladium(II) complexes of azobenzene-containing ligands as dichroic dyes

DOI: 10.1021/ic1000842

Source and publish data:

Authors:

Article abstract of DOI:10.1021/ic1000842

Full text of DOI:10.1021/ic1000842

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