Rh{(η4-cod) or (PPh3)2}-Schiff base complexes with a Z′ = 2 structure: Syntheses, spectroscopy, thermalanalyses and DFT/TDDFT

Article abstract of DOI:10.1016/j.molstruc.2016.05.093

Reaction of Schiff bases with [Rh(η⁴-cod)(acetate)]₂ (cod = 1,5-cyclooctadiene) provides [Rh(η⁴-cod){N-(C₆H₅)-2-oxo-1-naphthaldiminato-κ²N,O}] (1) and [Rh(η⁴-cod){(R)-N-(p-BrC₆H₄)ethyl-2-oxo-1-naphthaldiminato-κ²N,O}] (2), respectively. Compound 2 reacts with triphenylphosphine (PPh₃) to give [Rh(PPh₃)₂{(R)-N-(p-BrC₆H₄)ethyl-2-oxo-1-naphthaldiminato-κ²N,O}] (3). Molecular structure determination for 1 reveals that the 2-oxo-1-naphthaldiminate ligand is coordinated to the Rh(η⁴-cod)-fragment with six-membered N,O-chelation in distorted square planar geometry. The structure shows two symmetry-independent molecules in the asymmetric unit to give a Z′ = 2 structure. Optimized structures correspond well to the experimental results. Electronic spectra by TDDFT show combined metal-metal and metal-ligand transition bands at 411 (1) or 402 (2) nm with the highest MO contributions of ca. 81% for HOMO-1 to LUMO, comparable to the experimental bands. Electronic spectra in different solvents show a negative solvatochromic trend with increasing dielectric constants and acceptor number of solvents, respectively. ¹H NMR spectra show two multiplets for exo- and endo-methylene protons, and two doublets for olefinic protons of cod-ligand, respectively. ³¹P NMR spectra show two sets of doublets at δ 38.59 and 39.55 ppm due to two asymmetric P-Atoms coordinated to the rhodium trans to N and O atoms, respectively in 3. DSC analyses demonstrate an irreversible phase transformation from crystalline to isotropic liquid phase.

Full text of DOI:10.1016/j.molstruc.2016.05.093

Accepted Manuscript
4
Rh{(η -cod) or (PPh ) }-Schiff base complexes with a Z′ = 2 structure: Syntheses,
3 2
spectroscopy, thermalanalyses and DFT/TDDFT
Mohammed Enamullah, Mohammad Ariful Islam, Christoph Janiak
PII:
S0022-2860(16)30562-2
10.1016/j.molstruc.2016.05.093
MOLSTR 22607
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 4 March 2016
Revised Date: 22 April 2016
Accepted Date: 27 May 2016
4
Please cite this article as: M. Enamullah, M.A. Islam, C. Janiak, Rh{(η -cod) or (PPh ) }-Schiff base
3 2
complexes with a Z′ = 2 structure: Syntheses, spectroscopy, thermalanalyses and DFT/TDDFT, Journal
of Molecular Structure (2016), doi: 10.1016/j.molstruc.2016.05.093.
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ACCEPTED MANUSCRIPT
Graphical Abstract
Rh{(η4-cod) or (PPh3)2}-Schiff base complexes with a Z' = 2 structure: syntheses,
spectroscopy, thermalanalyses and DFT/TDDFT
Mohammed Enamullaha,*, Mohammad Ariful Islam, Christoph Janiakb
a Department of Chemistry, Jahangirnagar University, Dhaka-1342, Bangladesh.
Email: enamullah@juniv.edu
b Institute for Inorganic and Structure Chemistry, Heinrich-Heine-University,
University Str. 1, D-40225 Düsseldorf, Germany. Email: janiak@uni-
duesseldorf.de
X
X
1/2[Rh(η4-cod)
(acetate)]2
(1)
N
N
Rh(η4-cod)
C6H6/MeOH
rt, 5-6 h
O-H
O
{X = C6H5 (1) or (R)-(p-
BrC6H4)ethyl (2)}
N
X
OH
CHCl3
rt, 4 d
2PPh3
X
N
PPh3
PPh3
Rh
(3)
O
(A)
(B)

ACCEPTED MANUSCRIPT
Rh{(η4-cod) or (PPh3)2}-Schiff base complexes with a Z' = 2 structure: syntheses,
spectroscopy, thermalanalyses and DFT/TDDFT
Mohammed Enamullaha,*, Mohammad Ariful Islam, Christoph Janiak b
a Department of Chemistry, Jahangirnagar University, Dhaka-1342, Bangladesh.
Email: enamullah@juniv.edu
b Institute for Inorganic and Structure Chemistry, Heinrich-Heine-University,
University Str. 1, D-40225 Düsseldorf, Germany. Email: janiak@uni-duesseldorf.de
Abstract
Reaction of Schiff bases with [Rh(η4-cod)(acetate)]2 (cod = 1,5-cyclooctadiene)
provides [Rh(η4-cod){N-(C6H5)-2-oxo-1-naphthaldiminato-κ2N,O}] (1) and [Rh(η4-
cod){(R)-N-(p-BrC6H4)ethyl-2-oxo-1-naphthaldiminato-κ2N,O}]
(2),
respectively.
Compound 2 reacts with triphenylphosphine (PPh3) to give [Rh(PPh3)2{(R)-N-(p-
BrC6H4)ethyl-2-oxo-1-naphthaldiminato-κ2N,O}] (3). Molecular structure determination
for 1 reveals that 2-oxo-1-naphthaldiminate ligand is coordinated to the Rh(η4-cod)-
fragment with six-membered N^O-chelation in distorted square planar geometry. The
structure shows two symmetry-independent molecules in the asymmetric unit to give a Z' =
2 structure. Optimized structures correspond well to the experimental results. Electronic
spectra by TDDFT show a combined metal-metal and metal-ligand transitions band at 411
(1) or 402 (2) nm with the highest MOs contributions of ca. 81% for HOMO-1 to LUMO,
comparable to the experimental bands. Electronic spectra in different solvents show a
negative solvatochromic trend with increasing dielectric constants and acceptor number of
1
solvents, respectively. H NMR spectra show two multiplets for exo- and endo-methylene
31
protons, and two boublets for olefin protons of cod-ligand, respectively. P NMR spectra
show two sets of doublets at δ 38.59 and 39.55 ppm due to two assymetric P-atoms
coordinated to the rhodium trans to N and O atoms, respectively in 3. DSC analyses
demonstrate an irriversible phase transformation from crystalline to isotropic liquid phase.
Keywords: Rh(η4-cod)-fragment; Chiral/achiral Schiff base, Triphenylphosphine; X-ray
structure; DFT/TDDFT calculations
1

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1. Introduction
Rh(η4-cod)-achiral/chiral Schiff base complexes, derived from the reaction between
Schiff bases and dinuclear [Rh(η4-cod)(acetate)]2 (cod = 1,5-cyclooctadiene) are of
continued interests because of their uses for asymmetric reduction of ketone and ketone
derivatives with substantial (%) ee values [1-4]. In recent years, we have given attentions
to synthesize the Rh(η4-cod)-complexes starting from chiral amino acids/amino alcohols
and [Rh(η4-cod)(acetate)]2 [5-10]. In this connection, we have reported the syntheses,
spectroscopy and crystal structures of mononuclear [Rh(η4-cod)(XY)] (XY = chiral amino
carboxxylato) and [Rh(η4-cod)(AA)](acetate) (AA = chiral amino alchohol) [5-9]. Crystal
structures reveal a five-membered N,O-chelation of the amino-carboxylate or amino-
alcohol to the Rh(η4-cod)-fragment with distorted tetrahedral geometry. In addition, the
di/tri-phosphine ligands (diphos/triphos) readily react with these complexes to yield the
cationic [Rh(diphos)2](XY) or neutral [Rh(diphos/triphos)(XY)] complexes [10].
We have further reported the syntheses and spectroscopic characterization of Rh(η4-
cod)-chiral Schiff base complexes including [Rh(η4-cod)(R/S-SB)] {SB = N-(Ar)ethyl-
salicylaldiminato/naphthaldiminato} [8,11-12], [Rh(η4-cod)(R-HL)](acetate) {HL = 2-(X-
benzaldimine)-2-phenylethanol} [13], and [Rh(η4-cod)(R/S-HL1 or (rac)HL2)] {HL1 =
N-2-(salicylaldiminate)-2-phenylethanol, HL2 = N-2-(salicylaldiminate)-1-phenylethanol}
[14]. Molecular structure ditermination demonstrates that a six-membered N,O-chelation of
the deprotonated Schiff base ligands to the Rh(η4-cod)-fragment with distorted square
planar geometry.
The present paper, in continuation, reports the results of syntheses, spectroscopy and
thermal analyses on [Rh(η4-cod){N-(C6H5)-2-oxo-1-naphthaldiminato-κ2N,O}] (1),
[Rh(η4-cod){(R)-N-(p-BrC6H4)ethyl-2-oxo-1-naphthaldiminato-κ2N,O}]
(2),
and
[Rh(PPh3)2{(R)-N-(p-BrC6H4)ethyl-2-oxo-1-naphthaldiminato-κ2N,O}] (3), respectively.
Single crystal structure for 1 has been dertermined. The optimized geometry and electronic
spectra are studied by DFT/TDDFT, which are compared with the experimental results.
2. Experimental
Materials and physical measurements
All reactions were carried out under an atmosphere of dry nitrogen using standard Schlenk
tube techniques. Solvents used were dried and deoxygenated under nitrogen prior to use. FT-
2

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IR (KBr disc) and ATR spectra were recorded on a Thermo Scientific spectrometer (Nicolet
iS10) at ambient temperature. UV-Vis. spectra were obtained with a Shimadzu UV 3150
spectrophotometer in different solvents at 20 oC. Elemental analyses were done on a Vario EL
instrument from Elementar Analysensysteme, GmbH. Differential scanning calorimeter (DSC)
o
analyses were performed on a Shimadzu DSC-60 at the range of 30-260 C (before
1
decomposition temperature) with a rate of 10 K min–1. H NMR spectra were run on Bruker
Avance DPX 400 spectrometer operating at 400 MHz (1H), respectively at 20 oC. EI/ESI-MS:
Thermo-Finnigan TSQ 700 .79/81Br isotopic distribution patterns are clearly visible following
the peaks at m/z 602, 586, 376 and 183 in 2, and at 994, 756/716, 557, 367 in 3. The starting
dinuclear [Rh(η4-cod)(acetate)]2 was synthesized from [Rh(η4-cod)Cl]2 according to our
previous literature [5-8]. Syntheses of the Schiff base ligands, N-(C6H5)-2-hydroxy-1-
naphthaldimine (HSB1) and (R)-N-(p-BrC6H4)ethyl-2-hydroxy-1-naphthaldimine (HSB2)
were described in our previous literature [7,11-12].
2.1. Synthesis of compounds 1 and 2
Two equivalents of N-(C6H5)-2-oxo-1-naphthaldimine (HSB1) (0.40 mmol) and one
equivalent of [Rh(η4-cod)(acetate)]2 (0.20 mmol) were dissolved in 10 mL of C6H6/MeOH
(5:1, v/v). The solution was stirred for about 6 h at room temperature, and color changed
from red-orange to bright-yellow. The solvent of reaction mixture was evaporated in vacuo
and obtained the yellow product. The product was then dissolved again in 10 mL of
C6H6/MeOH (5:1, v/v) and stirred the solution for another 30 min. Evaporated the solvent
again in vacuo and this procedure was repeated two times. The product was finally dried in
vacuo (0.1-0.2 mbar) at 40 oC and obtained bright yellow compound 1. The same procedure
was followed for synthesis of compound 2.
2.1.1. [Rh(η4-cod){N-(C6H5)-2-oxo-1-naphthaldiminato-κ2N,O}], [Rh(η4-cod)(SB1)] (1):
Yield: 130 mg (71%); C25H24NORh (457.38): calcd. C 65.65, H 5.29, N 3.06 %, found: C
65.64, H 5.93, N 2.46 %; FT-IR (KBr): ν = 3042, 3007, 2958, 2922 w (C-H), 1615, 1602
vs (C=N), and 1575, 1534 vs (C=C) cm–1; EI-MS (70 eV): m/z (%) = 457 (100) ([M]+),
349 (15) ([Rh(SB1)]+), 321 (20) ([Rh(SB1)-CO]+), 246 (10) ([HSB1-H]+), and 218 (35)
([HSB1-CHO]+); 1H NMR (200 MHz, CDCl3):
δ = 1.77 (m, 4H, CH2codexo), 2.42 (m, 4H,
CH2codendo), 3.30 (d, J = 2.4 Hz, 1H, CHcod), 4.61 (d, J = 2.4 Hz, 2H, CHcod), 7.10 (d, J
= 7.8 Hz, 2H, H-Ar), 7.24 (dt, J = 7.8, 1.8, 2H, H-Ar), 7.36 (m, 3H, H-Ar), 7.55 (m, 2H, H-
3

ACCEPTED MANUSCRIPT
1
Ar), 7.68 (m, 2H, H-Ar), and 8.88 (d, J = 2.0 Hz, 1H, CHN) ppm. H NMR (200 MHz,
DMSO-d6): = 1.84 (m, 4H, CH2codexo), 2.37 (m, 4H, CH2codendo), 3.40 (d, J = 4.6 Hz,
δ
2H, CHcod), 4.20 (d, J = 5.6 Hz, 2H, CHcod), 7.02 (d, J = 9.0 Hz, 1H, H-Ar), 7.22 (d, J =
7.7 Hz, 1H, H-Ar), 7.33 (t, J = 6.4, 1H, H-Ar), 7.45 (m, 3H, H-Ar), 7.77 (m, 3H, H-Ar),
7.91 (d, J = 9.3 Hz, 1H, H-Ar), 8.04 (d, J = 8.8 Hz, 1H, H-Ar), and 8.94 (s, 1H, CHN)
ppm.
2.1.2. [Rh(
cod)(SB2)] (2)
η κ -
4-cod){(R)-N-(p-BrC6H4)ethyl-2-oxo-1-naphthaldiminato- 2N,O}], [Rh(η4
Yield: 155 mg (69%); C27H27BrNORh (564.33): calcd. C 57.47, H 4.82, N 2.48 %, found:
C 57.21, H 5.07, N 2.55 %; IR (KBr): ν = 3055, 3010, 2950 w (νH-C), 1611vs, 1602vs
(νC=N), and 1584, 1533vs (νC=C) cm–1; ESI(+)-MS (in methanol): m/z (%) = 602 (4)
([M+K]+),
586
(8)
([M+Na]+),
376
(6)
([HSB2+Na]+),
262
(100)
([{C6H5(CH3)CNH)}2+Na]+), and 183 (38) ([CH3CHC6H4Br]+) (79/81Br isotopic
1
distribution is clearly visible following the peaks at 602, 586, 376, 183, respectively). H
NMR (400 MHz, CDCl3):
δ = 1.74 (d, J = 6.8 Hz, 3H, CH3), 2.01 (m, 4H, CH2codexo), 2.56
(m, 4H, CH2codendo), 3.87 (d, J = 7.0 Hz, 2H, CHcod), 4.46 (q, J = 6.6Hz, 1H, CH), 4.67
(d, J = 7.5 Hz, 2H, CHcod), 7.04 (d, J = 9.1 Hz, 1H, H-Ar), 7.17 (t, J = 7.4 Hz, 1H, H-Ar),
7.31 (m, 3H, H-Ar), 7.46 (d, J = 8.4 Hz, 1H, H-Ar), 7.55 (d, J = 7.8 Hz, 2H, H-Ar), 7.60
(d, J = 7.8 Hz, 1H, H-Ar), 7.68 (d, J = 9.2 Hz, 1H, H-Ar), and 8.87 (s, 1H, CHN) ppm.
2.2. Synthesis of compound 3
A mixture of compound 2 with little excess of PPh3 in CHCl3 was stirred for 3-4 d at room
temperature under nitrogen atmosphere. Color changed from bright orange to red-orange. The
progress of reaction was monitored by taking 31P NMR spectra after 4h, 24h, and finally, after
4d. The results indicate the formation of [Rh(η4-cod)(PPh3)2]+, [Rh(PPh3)4]+ and PPh3(O)
species in addition to the expected product during the reaction process, evidenced by 31P NMR
o
signals. The product was filtered off, washed with diethylether, and dried in vacuo at 40 C,
for several hours and obtained the red-orange product of 3. The compound is very sensitive to
air and immediately formed the oxidative adducts.
2.2.1. [Rh(PPh3)2{(R)-N-(p-BrC6H4)ethyl-2-oxo-1-naphthaldiminato-κ2N,O}],
[Rh(PPh3)2(SB2)] (3)
4

ACCEPTED MANUSCRIPT
Yield: 235 mg (60%); IR (KBr): ν = 3060, 3030, 3006, 2940 w (νH-C), 1614vs, 1606vs
(νC=N), and 1588, 1537vs (νC=C) cm–1. ESI (+)-MS (in acetonitrile): m/z (%) 716 (22) ([M-
PPh3]+),
557
(30)
([(PPh3O)2+H]+),
495
(8)
([Rh(SB2)+K]+),
367
(70)
([{CH(CH3)(C6H4Br)}2+H]+), and 279 (100) ([(PPh3O)+H]+). ESI (+)-MS (in ethanol): m/z
(%) 994 (1.0) ([M(O)]+), 857 (60) ([(PPh3O)3+K]+), 756 (18) ([M-PPh3+K]+), 579 (100)
([(PPh3O)2+Na]+), 557 (95) ([(PPh3O)2+H]+), 478 (20) ([Rh(SB2)+Na]+), 367 (80)
([{(C6H4Br)CH(CH3)}2+H]+), and 279 (85) ([(PPh3O)+H]+) (79/81Br isotopic distribution
patterns are clearly visible following the peaks at 994, 756 or 716, 367, respectively). 1H NMR
(300 MHz, CDCl3): δ = 1.85 (m, 3H, CH3), 4.35 (m, 1H, CH), 7.08 (m, 30H, H-PPh3), 7.12
(m, 2H, H-Ar), 7.40 (m, 4H, H-Ar), 7.50 (m, 1H, H-Ar), 7.69 (m, 3H, H-Ar), and 8.45 (m, 1H,
CHN) ppm.
2.3. X-ray crystallography
Data Collection: Bruker APEX-II with CCD area-detector, temperature 172(2) K, Mo-K
α
radiation ( = 0.71073 Å), graphite monochromator, -scans, data collection and cell
λ
ω
refinement with SMART [15] data reduction with SAINT [15], experimental absorption
correction with SADABS [16]. Structure Analysis and Refinement: The structure was solved
by direct methods (SHELXS-97) [17,]; refinement was done by full-matrix least squares on
F2 using the SHELXL-97 program suite [17]. All non-hydrogen positions were found and
refined with anisotropic temperature factors. Hydrogen atoms on C (CH, CH, and CH2) were
calculated with appropriate riding models (AFIX 3, 13, and 23, respectively) and Ueq(H) =
1.2 Ueq(C). Details of X-ray structure determination and data refinement are provided in
Table 1. Graphics were drawn with DIAMOND (Version 3.0e) [18].
Table 1. Crystal data and structure refinement for 1.
Empirical formula
C25H24NORh
M (g mol−1)
457.36
Crystal size (mm)
Temperature (K)
θ range (°)
0.28 x 0.19 x 0.13
172(2)
0.97 to 25.00
–15, 27; –11, 9; ±23
Monoclinic
h; k; l range
Crystal system
Space group
a, b, c (Å)
P21/c
23.1368(17), 9.3168(7), 20.1129(15)
115.3990(10)
3916.5(5)
β
(deg.)
V (Å3)
Z/D (calc/g cm−3)
F(000)/µ (mm−1)
8/1.551
1872/0.887
5

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Max/min transmission
Reflections collected
0.8934/0.7892
19242
Independent reflections (Rint)
Data / restraints / parameters
Largest diff. peak and hole (∆ρ /e Å−3)
6878 (0.0375)
6878/0/490
4.061/−1.406
0.0763/0.1833
0.0827/0.1866
1.245
R1/wR2 [I > 2σ
(I)] a
R1/wR2 (all reflect.) a
Goodness-of-fit on F2 b
Absolute structure parameter [19]
−0.04(2)
2
2 2
−
a R1 = [Σ(║Fo│−│Fc║) / Σ │Fo│]; wR2 = [Σ [w(Fo −Fc ) ]/Σ[w(Fo 2)2]]1/2. − b Goodness-of-fit = [Σ [w(Fo
2−Fc2)2] /(n p)]1/2.
−
2.4. Computational method
For computation, the initial geometries were generated from the X-ray structures of 1 and 2
[12], respectively. All calculations were performed with the Gaussian 09 software package
[20]. The gas phase equilibrium geometries of both compounds were optimized and then the
vibrational frequencies were calculated with density functional theory (DFT) using B3LYP
functional with SDD basis sets. The absence of any imaginary frequencies confirmed that the
stationary points correspond to minima on the Potential Energy Surface. To rationalize the
experimental electronic spectra, the excited state properties were studied by time-dependent
density functional theory (TDDFT) with B3LYP/SDD, incorporating PCM (Polarization
Continuum Model) using chloroform as a solvent [9,21]. In TDDFT calculations, 72
excitation states were considered (Table S1-S2). After computation, the molecular orbital
(MOs) calculations were conducted using same level of theory.
3. Results and discussion
3.1. Syntheses of the compounds
Reaction of Schiff base ligand N-(C6H5)-2-hydroxy-1-naphthaldimine or (R)-N-(p-
BrC6H4)ethyl-2-hydroxy-1-naphthaldimine with dinuclear [Rh(η4-cod)(acetate)]2 gives the
mononuclear [Rh(η4-cod){N-(C6H5)-2-oxo-1-naphthaldiminate}] (1) or [Rh(η4-cod){(R)-
N-(p-BrC6H4)ethyl-2-oxo-1-naphthaldiminate}] (2) in C6H6/MeOH (5:1, v/v) (Scheme 1).
The triphenylphosphine (PPh3) reacts with 2 to provide the [Rh(PPh3)2{(R)-N-(p-
BrC6H4)ethyl-2-oxo-1-naphthaldiminate}] (3).
6

ACCEPTED MANUSCRIPT
X
N
X
X
N
1/2[Rh(η4-cod)
(µ-O2CMe)]2
N
2PPh3
PPh3
PPh3
Rh(η4-cod)
X
N
Rh
C6H6/MeOH
rt, 5-6 h
- AcOH
CHCl3
rt, 4 d
O-H
O
O
(3)
{X = C6H5 (1) or (R)-
(p-BrC6H4)ethyl (2)}
OH
Scheme 1. Synthetic route to the formulation of the compounds 1 - 3.
3.2. Mass spectra
Mass spectra of the compounds (Figure 1, S1) show the molecular ion peaks at m/z 457
[M]+ (1), 586/602 [M+Na/K]+ (2), and 994 [M(O)]+ (3), confirm the formation of the
compounds as depicted in the reaction Scheme 1. The spectra further show several ion
peaks for [Rh(SB)]+ (SB = deprotonated Schiff base), [HSB]+, and fragmented Schiff base
ligands. Compound 3 is very sensitive to air and forms the oxidative adduct of [M(O)]+
while running the spectra. The spectrum further shows several ion peaks for [M−PPh3]+,
[M−PPh3+K]+, [Rh(SB2)+Na/K]+ and oxidized triphenylphosphine [(PPh3O)n+H/Na/K]+
(n = 1, 2, 3) (Figure 1), which are further supported by 31P NMR spectra discussed below.
D :\icis \jaacd56b
10/01/07 11:57:00 AM
EN -20 EI-direkt s 95 ,
Source:150',70eV,500uA ,m /z 41-600
jaacd56b #1 R T: 3.39 AV:
T: + c EI Full m s [ 41.00-600.00]
1
N L: 3.77E6
457 [M]+
457.1
3600000
3400000
3200000
3000000
2800000
2600000
2400000
2200000
2000000
1800000
1600000
1400000
1200000
1000000
800000
+
217.9
218 [HSB1-CHO]
321 [Rh(SB1)-CO]+
458.1
321.0
349 [Rh(SB1)]+
349.0
246 [HSB1-H]+
600000
455.1
246.0
400000
207.9
205.9
181.9
319.0
247.1
76.9
102.8
78.9
100
114.9
149.0
322.0
323.0
200000
0
228.5
459.1
292.0
350.0
365.0
248.0
270.9
167.9
57.0
427.1
403.1
400
50
150
200
250
300
350
450
500
550
m /z
7

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Fig. 1. EI and ESI mass spectra of compounds 1 (top) and 3 (bottom), respectively.
3.3. Solid state X-ray structure
Complex 1 has the 2-oxo-1-naphthaldiminato ligand coordinated with six-membered
Rh-N,O-chelate ring formation and the rhodium-bound η4-cod fragment (Figure 2) in
distorted square planar geometry. The bond lengths and angles are listed in Table 2,
comparable as seen before in the analogous Rh{N,O}(η4-cod) complexes with {N,O} =
salicylaldiminato/naphthaldiminato Schiff base ligands [8,11,13]. From the aromatic rings
in the complexes there are no detectable π-π interactions [22], but intermolecular C-H···π
contacts [23] are evident in the packing; a detailed analysis is reported in the Supporting
Information.
8

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Fig. 2. Thermal ellipsoid plot (50% level) for compound 1; bond lengths and angles are
listed in Table 1.
Compound 1
Compound 2
X-ray (Molecule A)
X-ray (Molecule B)
Opt.
X-ray a
Opt.
9

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Table 2
Selected bond lengths [Å] and angles [°] in X-ray and optimized structures for compounds
1 and 2.
Compound 1
Compound 2
X-ray (Molecule A)
X-ray (Molecule B)
Opt. Str.
Rh–O
X-ray a
2.028(2)
2.073(3)
2.146(3)
2.130(3)
2.111(3)
2.111(3)
1.898(4)
88.87
Opt. Str.
2.034
2.085
2.202
2.180
2.169
2.156
1.964
88.74
87.10
85.34
162.26
156.37
164.21
156.80
99.09
96.99
Rh1–O1
Rh1–N1
Rh1–C18
Rh1–C19
Rh1–C22
Rh1–C23
2.029(5)
2.075(7)
2.094(8)
2.126(8)
2.108(8)
2.121(8)
Rh2–O2
Rh2–N2
Rh2–C43
Rh2–C44
Rh2–C47
Rh2–C48
2.042(5)
2.048(7)
2.109(10)
2.091(9)
2.129(8)
2.108(8)
89.0(2)
2.0396
2.0856
2.1737
2.1948
2.1520
2.1784
88.40
Rh–O
Rh–N
Rh–N
Rh–C18
Rh–C20
Rh–C21
Rh–C24
Rh–C25
C-Br
Rh–C19
Rh–C22
Rh–C23
O1–Rh1–N1 89.1(2) O2–Rh2–N2
O–Rh–N
O–Rh–C18
O–Rh–C19
O1–Rh1–C18 88.0(3) O2–Rh2–C43 90.4(3)
O1–Rh1–C19 88.5(3) O2–Rh2–C44 88.3(3)
85.93
O–Rh–N
87.57
O–Rh–C20 87.86(12)
O–Rh–C21 86.47(15)
O–Rh–C24 162.12(12)
O–Rh–C25 157.44(12)
N–Rh–C20 166.14(13)
N–Rh–C21 155.05(11)
N–Rh–C24 96.42(12)
N–Rh–C25 96.49(12)
O1–Rh1–C22 155.5(3) O2–Rh2–C47 161.6(3) O–Rh–C22
O1–Rh1–C23 164.3(3) O2–Rh2–C48 158.8(3) O–Rh–C23
N1–Rh1–C18 156.5(3) N2–Rh2–C43 168.7(3) N–Rh–C18
N1–Rh1–C19 169.7(4) N2–Rh2–C44 161.8(3) N–Rh–C19
156.30
162.97
158.02
162.93
96.06
N1–Rh1–C22 95.7(3) N2–Rh2–C47 96.2(3)
N1–Rh1–C23 95.5(3) N2–Rh2–C48 94.5(3)
N–Rh–C22
N–Rh–C23
99.24
a from ref [12]; Opt. Str. = optimized structure.
There are two symmetry-independent molecules in the asymmetric unit, that is,
molecule A with Rh1 and molecule B with Rh2 centre. The structures of the analogous
Ni/Cu/Zn(II)-complexes with R/S-N-1-(Ar)ethyl-2-oxo-1-naphthaldiminate (Ar = C6H5 or
p-C6H4OMe) also contained two symmetry-independent molecules [24,25,26]. Two
symmetry-independent molecules, that is, two identical chemical formula units in the
structural asymmetric unit [27] give a Z' = 2 structure. The definition of Z' refers to the
number of formula units in the unit cell (here 8) divided by the number of independent
general positions (here 4) [28]. Such Z' > 1 structures [29] can derive from a metastable
structure, that is, a crystal "on the way", [27-29, 30] or from strong and special
supramolecular (e.g. hydrogen bonding) interactions between the two (or more) symmetry-
independent units [31]. A molecule with different equi-energetic conformations also gives
a high Z' such that these conformations co-exist in the crystal [32]. Here, in the Rh
structure 1 the conformational difference between the symmetry-independent molecules is
nearly insignificant. The two symmetry-independent molecules are quite superimposable
(Figure 3). These very small deviations do not appear to justify the presence of two
symmetry-independent molecules.
10

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Fig. 3. Overlay of the two symmetry independent molecules A and B in 1. The Rh1
molecule is shown in green, while Rh2 molecule in red. Three atoms RhNO were specified
to orient the overlay which was managed with the "Molecule overlay" option in Mercury
3.8 (copyright CCDC 2001-2016, http://www.ccdc.cam.ac.uk/mercury/).
Thus, the rationalization of the Z' = 2 structure must be sought for in the packing
arrangement. Each independent molecule in 1 is arranged in layers composed of molecules
of the same kind (Figure 4). These layers lie parallel to the bc plane and stack alternately
for the independent molecules along the a direction. Even if the CH-π interactions between
the Rh1 and Rh2 molecules appear rather weak supramolecular interactions (see listing of
CH-π interactions and Figure S1 in Supporting Information).
11

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Fig. 4. Packing diagrams for 1 with projection on two different planes. The Rh1 molecules
are shown in green, and Rh2 molecules in red.
3.4. Optimized structures and vibrational spectra
The optimized structures by DFT, calculated at B3LYP/SDD, based on the X-ray data for
compounds 1 and 2 are shown in Figure 5. The experimental and calculated bond lengths and
angles are comparable (Table 2). Experimental vibrational spectra show several characteristics
bands mainly for the coordinated Schiff base ligand at the range of 3055-2922, 1615 - 1602
and 1584 - 1533 cm-1, associated with the νC−H, νC=N and νC=C, respectively (Figure S2
and Table 3) [10-14,15]. The computed vibrational spectra show these bands at compareable
range (Figure S3 and Table 3). The spectra taken as KBr disc show a strong broad band at
3449 cm-1 due to νH2O from KBr, which is obviously absent in the ATR spectra (Figure S2).
12

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Fig. 5. Optimized structures, calculated at B3LYP/SDD, for compounds 1 and 2.
Table 3.
Experimental and calculated (at B3LYP/SDD) vibrational spectra of 1 and 2 at ambient
temperature.
νC−Ha
νC=Na
νC=Ca
Entity
1b
2c
3b
3042, 3007, 2958, 2922w
(3220, 3180, 3100, 3060, 3004s)
3055, 3010, 2950w
1615, 1602vs (1668, 1575, 1534vs (1572,
1612vs) 1516vs)
1611, 1602vs (1668, 1584, 1533vs (1580,
(3220, 3140, 3100, 3060, 3004s)
1612vs)
1516vs)
3060, 3030, 3006, 2940w
1614, 1606vs
1588, 1537vs
a Values in parentheses correspond to the computed spectra; b spectra taken as KBr dics; c
ATR spectra.
3.5. 1H NMR spectra
In 1H NMR spectra (Figure 6 and Table 4), the rhodium coordinated 1,5-cyclooctadiene
shows different multiplets for the exo- and endo-methylene protons {i.e., CH2(cod)exo and
CH2(cod)endo} and olefin protons {CH(cod)} [5-11,33,34]. The exo- and endo-methylene
protons show a multiplet in each at δ 1.77 and 2.42 ppm in 1 and δ 2.01 and 2.56 ppm in 2,
respectively. The four olefin protons show two doublets at δ 3.30 (J = 2.6 Hz) and 4.61
ppm (J = 2.4 Hz) in 1 and δ 3.87 (J = 7.0 Hz) and 4.67 (J = 7.5 Hz) ppm in 2. The
presence of two boublets for olefin protons is explained by the trans effects of the
coordinated N,O-chelate ligand on the proton resonances [5-11,33]. It has been reported
that two kinds of olefin protons resonances (i.e., giving two boublets) arise from two sets
13

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of two equivalent protons trans to N (downfielded peak) and trans to O atom (upfielded
peak), respectively. This result reflects the asymmetric nature of the olefin protons.
However, the rhodium coordinated Schiff base ligand shows a doublet at δ 1.74 ppm (J =
6.8 Hz) and a quartet at δ 4.46 ppm (J = 6.6Hz) for the methyl and methine protons,
respectively in 2. The imine proton (CHN) shows a doublet at δ 8.88 ppm (J = 2.0 Hz) in 1
and a singlet at δ 8.87 ppm in 2, the doublet is due to 103Rh−1H coupling [1-3,8,10-12].
CH2cod(exo)
H-Ar
CH2cod(endo)
*
CHN
CHcod
CHcod
*
Fig. 6. 1H NMR (400 MHz, δ/ppm) spectra of compound 1 in CDCl3 at 20 oC (asterisked
are due to solvent).
Table 4
1H NMR data (400 MHz, δ/ppm) of compounds 1-3 in CDCl3 at 20 oC a.
Compound CH2(cod)exo
CH2(cod)endo CH(cod)
CH
CH3
CHN
1*
1.77 (m, 4H) 2.42 (m, 4H) 3.30
2H),
(m,
-
-
8.88 (d, J =
2.0Hz, 1H)
4.61
(m, 2H)
(m, 3.85
2.63 2H),
(m, 2H)
1.93
2H),
(m, 2H)
-
(m, 2.45
(m, 4.46 (q, J = 1.74 (d, J = 8.87 (d, J =
2
2.01 2H),
4.64 6.6Hz, 1H)
6.7 Hz, 3H)
2.0Hz, 1H)
(m, 2H)
-
-
4.3 (m, 1H)
1.85 (m, 3H) 8.45 (s, 1H)
3
a
*
Aromatic protons peaks are listed in the experimental section; spectra run on 200 MHz
Spectrometer.
3.6. 31P NMR spectra
31
The reaction progress was monitored by taking P NMR spectra (Figure 7, Table 5)
after 4 h, 24 h and 4 days, respectively. The spectra show the formation of a mixture
products of [Rh(PPh3)4]+, [Rh(η4-cod)(PPh3)2]+, and PPh3(O) even after 24 h of reaction.
The former two compounds show two doublets at δ 12.94 ppm (JP-Rh = 77 Hz) and 30.83
14

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ppm (JP-Rh = 150 Hz), respectively. While the latter one shows a singlet at δ 29.56 ppm. In
[Rh(PPh3)4]+, four P-atoms coordinate to the rhodium atom in an uniform environment,
resulting the formation of a doublet. Similarly, two P-atoms coordinate to the rhodium
atom which is further bonded to the four equvelent olefin carbon atoms in [Rh(η4-
cod)(PPh3)2]+ and results in a doublet. However, the compound [Rh(PPh3)2(SB2)] (3),
isolated after 4 days, shows two doublets at δ 38.59 (JP-Rh = 124Hz) and 39.55 ppm (JP-Rh
=
153Hz) (Figure 7), assigned for two P-atoms coordinated to the rhodium trans to N and
trans to O atoms, respectively [10]. The spectrum further shows peaks for PPh3(O) and
[Rh(PPh3)4]+. As discussed above, compound 3 is very sensitive to air and easily
decomposes to form the oxidized triphenylphosphine while preparing the sample for
running the spectra. In addition, very little amount of [Rh(PPh3)4]+ is yet present in the
sample though the product is repeatedly washed.
[Rh(η4-cod)(PPh3)2]+
Mixture products
after 4 h
PPh3(O)
[Rh(PPh3)4]+
30.0
25.0
20.0
15.0
[Rh(η4-cod)(PPh3)2]+
Mixture products
after 24 h
PPh3(O)
30.0
25.0
20.0
15.0
15

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Product collected
after 4 h
PPh3(O)
(3)
[Rh(SB2)(PPh3)2]+
40.0
35.0
30.0
25.0
20.0
15.0
Fig. 7. 31P NMR spectra of mixture products in course of reaction, and the compound 3 in
CDCl3 at 20 oC.
Table 5.
31P NMR data (121.5 MHz, δ/ppm) of compound 3 in CDCl3 at 20 oC.
Prod. [Rh(PPh3)2(SB2)] (3) [Rh(PPh3)4]+
PPh3(O)
29.56 (s)
29.55 (s)
29.49 (s)
[Rh(η4-cod)(PPh3)2]+
4h a
12.94 (d,
12.94 (d,
12.96 (d,
J
J
J
P-Rh = 77Hz)
P-Rh = 77Hz)
P-Rh = 77Hz)
30.83 (d,
J
J
P-Rh = 150 Hz)
P-Rh = 150 Hz)
-
24h a
4d b
30.83 (d,
-
-
38.59 (d,
39.55 (d,
J
J
P-RhN = 124 Hz)
P-RhO = 153 Hz)
a Mixture products in course of reaction, b Product collected after 4 days.
3.7. Thermally induced structural phase transformation
Thermally induced structural phase transformation has been reported for the transition
metal-N,O-chelate complexes which accompanying a change from low temperature
crystalline phase at distorted square planar/tetrahedral to high temperature isotropic liquid
phase at regular square planar/tetrahedral geometry [9,21,24-26,35,36]. Differential
Scanning Calorimetry (DSC) has successfully been used to study the phenomenon. DSC
o
heating curve (Figure 8) shows a peak at ca. 237 (∆H = − 17.14 kJ/mol for 1) and 200 C
(∆H = − 24.17 kJ/mol for 2), associated to the structural phase transformation. On the other
hand, cooling curve shows no corresponding peak on the reverse direction. The result
indicates an irreversible phase transformation from crystalline to isotropic liquid phase.
16

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T/oC
200 oC
237 oC
Fig. 8. DSC analyses curves for compounds 1 and 2.
3.8. Electronic spectra and TDDFT calculations
Electronic spectra of the compounds in chloroform (Figure 9) feature several strong
bands/shoulders below 300 nm, associated with the intra-ligand π→π /n→π transitions
(LLCT). The spectra further show a medium broad band at 300 - 350 nm (λmax = 324 nm),
and a weak broad band at 350 - 460 nm (λmax = 418 for 1, and 414 nm for 2), assigned to
the metal-ligand charge transfer (MLCT) transitions based on the formation of Rh(η4-
cod)+ and Rh(naphthaldiminate) species, respectively [5-12,33]. Computed spectra by
TDDFT, calculated at B3LYP/SDD, are comparable to the experimental spectra (Figure 9
and Table 6). A straightforward assignment, however, on computed spectra is hampered
by the complexity of the system and the presence of a large number of excitations at a
single wave length [9,21,25-27]. Thus, we report herein only the selected and simplified
transitions with their assignments relevant to the experimental data (i.e., those transitions
contributing most to the spectra) (Table 6). The assignments reveal that the several metal-
centered transitions occur deeply in the UV region, where the ligand-centered transitions
usually occur. Although the d-d transitions are not allowed in Rh(I), the distortion from
square-planar geometry and coupling of the σ-type ligand orbitals with the metal d-orbitals
exhibit the metal-centered d-d excitation [9,21]. Hence a combined metal-metal (M-M)
and metal-ligand (M-L) transitions bands appear with λmax at 411 (1) and 402 nm (2) with
the highest molecular orbitals (MOs) contributions of ca. 81% for HOMO-1 to LUMO
transitions in each. Similarly, a combined M-M, M-L and ligand-ligand (L-L) bands
17

ACCEPTED MANUSCRIPT
appear with λmax = 348 (1) and 340 nm (2) with high MOs contributions of ca. 56 and 49%
for HOMO-2 to LUMO transitions, respectively. These two bands including several other
computed bands are very close to the experimental bands/shoulders (Figure 9 and Table 6).
The HOMOs-1 and LUMOs for both compounds are presented in Figure 10.
Fig. 9. Experimtal and calculated (at B3LYP/SDD ) electronic spectra of 1 and 2 in
chloroform at 20 oC.
Table 6.
Selected excitation properties, calculated at B3LYP/SDD level of theory, for 1 and 2 in
chloroform.
MOs Contributions (%) a
Assignments b
#
Oscillator Strength (f)
Wavelength (nm)
Compound 1
411 (418)
367
0.0683
0.0468
M-M, M-L
M-M, M-L
H-1→L (81), H→L+2 (6)
H-3→L (57), H-2→L (36)
H-3→L (36), H-2→L (56)
H-1→L+1 (57)
348 (324)
310
0.2299
0.2517
0.0899
0.1561
0.1466
0.2433
M-M, M-L,L-L
M-M, ML, L-L
M-M, M-L, L-L
M-M, M-L, L-L
M-L, L-L
280 (~270sh)
248
H-1→L+3 (86)
H-3→L+4 (11), H-1→L+5 (18)
H-5→L+1 (15), H-2→L+5 (23)
H-4→L+5 (18), H-7→L+2 (11)
233 (~234sh)
215
ML, LM, L-L
Compound 2
402 (414)
0.0871
H-1→L (81), H→L+6 (11)
M-M, M-L
18

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370
340 (324)
308
0.0269
H→L+2 (19), H→L+6 (41)
H-3→L (41), H-2→L (49)
H-1→L+1 (76)
M-M, M-L
M-M, M-L, L-L
M-M M-L, L-L
M-M, M-L, L-L
M-M, M-L, L-L
M-M, M-L, L-L
M-L, L-M, L-L
L-M, L-L
0.1841
0.2199
0.1053
0.1051
0.1214
0.1391
0.1312
283 (~270sh)
247
H-1→L+4 (9), H-1→L+5 (73)
H-3→L+2 (10), H-3→L+3 (36)
H-9→L (19), H-3→L+3 (36)
H-6→L+2 (17), H-5→L+5 (13)
244 (~240sh)
220
217
H-7→L+3 (46), H-7→L+6 (16)
b
# Experimental values in parentheses; a H/L = HOMO/LUMO; M-M = metal d-d, M-L/L-M = metal-
ligand/ligand-metal, L-L= ligand-ligand transitions.
Compound 1
LUMO
HOMO-1
Compound 2
HOMO-1
LUMO
Fig. 10. HOMOs-1 and LUMOs, calculated at B3LYP/SDD level of theory, for
compounds 1 and 2 in chloroform.
Electronic spectra of 1 in different solvents (Figure S5 and Table 7) demonstrate that
both the absorption maxima (λmax) and band intensities (εmax) change with the solvents'
nature [9,37,38,39]. Thus a negative solvatochromic trend is envisioned with shifting λmax
to the higher energies (blue shift) in the solvents of increasing dielectric constants (ε) and
Gutmann’s Acceptor (AN) number [40], respectively (Figure 11).
19

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Table 7.
Electronic spectral data of compound 1 in different solvents at 20 oC.
Solvent a
Rh(SB): MLCT
π→π /n→π :
Rh(η4-cod)+: MLCT
b
b
b
(λmax/εmax
)
LLCT (λmax/εmax
)
(λmax/εmax)
1
MeOH (0.018)
EtOH (0.025)
AC (0.023)
C6H6 (0.025)
C6H14 (0.025)
DCM (0.025)
CHCl3 (0.025)
EtOAc (0.026)
2
<300 (226/23888)
<300 sh
-
300-350 (321/5778)
300-350 (322/8280)
-
360-500 (415/2306)
360-500 (417/3140)
360-500 (416/7196)
360-500 (420/8380)
360-500 (421/6900)
360-500 (417/7820)
360-500 (418/5940)
360-500 (417/9423)
<300 sh
<300 sh
<300 sh
<300 sh
<300 sh
300-350 (324/24940)
300-350 (322/21320)
300-350 (323/24100)
300-350 (324/18520)
300-350 (321/28288)
CHCl3 (0.026)
<300 sh
300-350 (324/22077)
330-430 (414/7212)
a
b
Concentration in parentheses in mM·L-1; λmax in nm, εmax in L·mM–1·dm–3; SB =
deprotonated Schiff base ligands.
Fig. 11. Changes of absorption maxima (λmax/nm) with (A) Dielectric constant (ε) and (B)
Acceptor number (AN) of the solvents for 1 at 20 °C, respectively.
4. Conclusion
Rh(η4-cod)-Schiff
base complexes are formed via coordination of 2-oxo-1-
naphthaldiminate ligand to the Rh(η4-cod)-fragment in distorted square planar geometry.
The structure consists of two symmetry-independent molecules (A and B) in the
asymmetric unit to give a Z' = 2 structure. The optimized geometry and excitation
20

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1
properties calculated by DFT/TDDFT are comparable to the experimental results. In H
NMR spectra, rhodium coordinated cod ligand shows two multiplets for the exo- and endo-
methylene protons, respectively. While the olefin protons show two doublets due to trans
31
effects of the coordinated N,O-chelate ligand on the proton resonances. P NMR spectra
demonstrate two sets of doublets for two assymetic P-atoms bound to the Rh(I) trans to N
and O atoms, respectively. An irrversibel phase transformation from crystalline to isotropic
liquid phase is revealed by DSC analyses. The present results will certainly be useful in
understanding the syntheses, reactivities and cataltic behaviors of Rh(I)-N,O-chelate Schiff
base complexes.
Acknowledgements
We acknowledge the“Wazed Miah Science Research Centre”at Jahangirnagar University,
Dhaka, Bangladesh for obtaining elemental data. Our sincere thank to Professor KS. Hagen,
Department of Chemistry, Emory University, for X-ray measurement of compound 1. For
computational resource, we thank to Professor ABP Lever, Department of Chemistry, York
University, Toronto, and the Shared Hierarchical Academic Research Computing Network
(SHARCNET: www.sharcnet.ca) Ontario, Canada.
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Highlights
* Rh{(η4-cod) or (PPh3)2}-Schiff base complexes.
* Two symmetry-independent molecules in the asymmetric unit to give a Z' = 2 structure.
31
* P NMR show two doublets due to two assymetric P-atoms bound to the Rh(I) trans to N
and O atoms, respectively.
* DFT/TDDFT calculations correspond well to the experimental results.