Spacer length dependent architectural diversity in bis-dipyrrin copper(II) complexes

Article abstract of DOI:10.1039/c7dt00107j

A series of copper(ii) complexes (1-9 and 3′) derived from bis-dipyrrin ligands (L1-L9 and L3′) with diverse spacer lengths [-(CH₂)_n-] have been described. Structural diversities in these complexes have been explicitly established by spectral and structural studies on these and a closely related nickel(ii) complex (3′′). All the ligands and complexes have been thoroughly characterized by spectroscopic studies (ESI-MS, IR, ¹H, ¹³C NMR, UV/vis) and structures of 2, 3′, 3′′, 6, 8 and 9 were determined by X-ray single crystal analyses. It has been unambiguously established that ligands with n ≤ 6 gave heteroleptic binuclear (1-5), while those with n ≥ 7 yielded homoleptic mononuclear (6-9) bis-dipyrrinato complexes. Spectral and structural studies revealed distorted square planar (1-5) and distorted tetrahedral geometries (6-9) about the copper(ii) centre in these complexes which has further been evidenced by EPR and electrochemical studies. Structural differences based on the odd and even number of methylene spacers in these complexes have been supported by DFT studies. A line between the syn- and anti-conformations of the complexes has been drawn on the basis of a limiting spacer length.

Full text of DOI:10.1039/c7dt00107j

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Page 1 of 26
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Spacer length dependent architectural diversity in bis-dipyrrin copper(II)
complexes
Rajendra Prasad Paitandi, Roop Shikha Singh, Sujay Mukhopadhyay, Ashish Kumar and Daya
Shankar Pandey
Department of Chemistry, Institute of Science, Banaras Hindu University, Varanasi - 221 005,
India
---------------------------------------------------------------------------------------------------------------------
A series of copper(II) complexes (19 and 3) derived from bis-dipyrrin ligands (L1L9 and
L3′) with diverse spacer lengths [(CH2)nhave been described. Structural diversity in these
complexes have been explicitly established by spectral and structural studies on these and a
closely related nickel(II) complex (3ʺ). All the ligands and complexes have been thoroughly
1
13
characterized by spectroscopic studies (ESI-MS, IR, H, C NMR, UVvis) and structures of 2,
3, 3ʺ, 6, 8 and 9 determined by X-ray single crystal analyses. It has been unambiguously
established that ligands with n < 6 gave heteroleptic binuclear (15), while those with n > 7
yielded homoleptic mononuclear (69) bis-dipyrrinato complexes. Spectral and structural studies
revealed distorted square planer (15) and distorted tetrahedral geometry (69) about the
copper(II) centre in these complexes which has further been evidenced by EPR and
electrochemical studies. Structural differences based on odd and even number of methylene
spacers in these complexes has been supported by DFT studies. A line between syn and anti
conformations for the complexes has been drawn on the basis of limiting spacer length.
---------------------------------------------------------------------------------------------------------------------
*Author to whom all the correspondence should be addressed; E-mail ID: dspbhu@bhu.ac.in

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Introduction
Rational design and synthesis of supramolecular architectures with exciting properties via
complexation employing appropriate combination of the metal ion and ligands has been a theme
of enormous interest in contemporary chemical research.13 Among various types of ligands
being used for this purpose, dipyrrins also known as dipyrromethenes have proved to be
indispensable.47 Substantial current interest has also arisen towards development of multi-
nuclear metal complexes involving oligodipyrrins or the ligands having multiple dipyrrin units
linked by a variety of spacers capable of forming well defined architectures upon coordination
with the metal ions via self-assembly processes.816 Further, considering the importance of
copper(II) complexes in magnetochemistry and materials science a variety of dipyrrin ligands
and their copper(II) complexes including supramolecular systems have been developed and
extensively studied.1723 In addition, polynuclear copper(II) complexes have attracted the
attention of scientific community due to their usage in extending our knowledge on molecular
features influencing the nature of exchange coupling between neighbouring metal centres via
involvement of the bridging ligands, their efficacy towards DNA cleavage, and relevance in
metalloproteins.24 As well, bis-dipyrrin metal complexes with double helical/triangular structures
and conformationally flexible linkers have also been explored.13,2532
A number of reports dealing with bis-dipyrrins along with their copper and zinc complexes
bearing rigid and flexible spacers ranging from esters, amides, methylene, alkynyl, particularly at
/ position of the pyrrole are available in the literature.5,2541 However, those involving
rotatable meso- position as locus for the spacers with a well defined strategy for achieving bis-
dipyrrin based mono- and binuclear complexes by varying spacer lengths have not been reported.
Moreover, porphyrins also considered as cyclic bis-dipyrromethenes are highly attractive and
have been employed in the development of numerous supramolecular systems.42 It has been
categorically shown that expansion or contraction of the porphyrin core leads to variation of the
spin state of the metal centre.4344 This observation and the proven flexibility of the methylene
(CH2)nspacers have prompted us to explore this prospect in bis-dipyrrin based complexes.
Apparently, nuclearity in these complexes can be controlled by varying the chain length between
meso- substituted phenyl ring at ortho relative to meta or para position due to folding of the
ortho substituted ligands under steric crowding. Further, spacer length dependent folding of the

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bis-dipyrrins may bring the coordinated metal centres at mutually interacting distance which, in
turn, could facilitate exchange coupling between them.
Although, numerous dipyrrinato complexes have been synthesized and extensively studied,
those based on dipyrrins imparting two chelating units separated by methylene spacers
((CH2)n have not been reported. With an intent to investigate the architectural and
conformational limitations in spacer length directed fabrication of mono- and binuclear bis-
dipyrrin complexes some new dipyrromethanes (L1L9, L3′) with two N,N- donor bis-chelating
units separated by ortho substituted (CH2)n spacers have been synthesized. Effect of the ortho
substitution has also been realized by synthesizing bis-dipyrrins with para substituted (CH2)n
spacers (L3′). Through this work, we describe synthesis and characterization of L1L9, L3′
having two meso-2/4-phenoxy substituted dipyrrin units linked by (CH2)n spacers and their
reactivity with Cu(acac)2, Cu(OAc)2 and Ni(dtc)2 under varying conditions affording binuclear
copper(II) (15, 3′), nickel(II) (3′′) and mononuclear copper(II) complexes (69). Prominent role
of the spacer length directed architectural distinction on photophysical and electrochemical
behavior of the mono- and binuclear copper(II) complexes have also been evaluated by
theoretical studies.
Experimental section
Reagents
All the synthetic manipulations have been performed in deaerated solvents under nitrogen
atmosphere. Solvents were purified by standard literature procedures prior to their use.45 2-
hydroxybenzaldehyde, 1,2-dibromo derivatives (1,2-dibromoethane/-propane/-butane/-pentane/-
hexane-/-heptane/-octane/-nonane/-decane), 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ),
pyrrole, tetrabutyl ammonium perchlorate [(n-Bu)4N]ClO4, Cu(acac)2 and anhydrous Cu(OAc)2
were procured from the Sigma Aldrich Chemical Co. Pvt. Ltd., India and used as received.
General methods
Infrared and electronic absorption spectra were acquired on a Varian 3300 FT-IR and
Shimadzu UV-1601 spectrophotometers, respectively. 1H (300/500 MHz) and 13C (75/125 MHz)
NMR spectra at room temperature were obtained on a JEOL AL300/500 FT-NMR spectrometer
using tetramethylsilane [Si(CH3)4] as an internal reference. X-band EPR spectra were acquired

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on a JEOL JES FA200 spectrometer calibrated with diphenylpicrylhydrazyl (DPPH; g = 2.0037).
Electrospray ionization mass spectral (ESI-MS) data was acquired on a Bruker Daltonics-
Amazon SL ion trap mass spectrometer. Electrochemical measurements were made on a CHI-
620c electrochemical analyzer. The measurements were performed in a one compartment cell
equipped with glassy carbon working, platinum wire counter, and Ag/Ag+ reference electrode
under nitrogen atmosphere using tetrabutylammonium perchlorate [(n-Bu)4N]ClO4 as a
supporting electrolyte.
Synthesis of the aldehydes (A1-A9). In a typical reaction, a solution of salicylaldehyde
(0.244 g, 2.0 mmol) was treated with respective dibromo derivatives (1.0 mmol) and K2CO3
(0.276 g, 2.0 mmol) in DMF (20 ml) and the resulting reaction mixture heated overnight under
reflux at 100 ˚C. It was subsequently poured into 200 mL of water and the precipitate thus
obtained was filtered, thoroughly washed with water and dried in air.46
Synthesis of bis-dipyrromethanes (L1L9 and L3). Aldehyde (10.0 mmol) [A1 (2.70 g);
A2 (2.84 g); A3 (2.98 g); A4 (3.12 g); A5 (3.26 g); A6 (3.40 g); A7 (3.54 g); A8 (3.68 g), A9
(3.82 g) and A3 (2.98 g), respectively] and catalytic amounts of trifluoroacetic acid (15.4 μL,
0.20 mmol) was added to an excess of degassed pyrrole (13.8 mL, 200.0 mmol) and the reaction
mixture stirred overnight under nitrogen atmosphere.47 After completion of reaction the contents
of the flask were concentrated under reduced pressure and the obtained crude product was
subjected to column chromatography over silica gel (SiO2; hexane:ethylacetate, 8:2) to afford
corresponding dipyrromethanes as a white solid (characterization data given in the
Supplementary information).†
Synthesis of complexes (1-9 and 3). In a typical reaction, DDQ (0.227 g, 1 mmol)
dissolved in benzene (30 ml) was added drop wise with stirring to an ice cold solution of
respective dipyrromethanes (L1L9 and L3) (0.50 mmol) in CH2Cl2 (50 mL) over 1 h. It was
stirred for an additional 1 h and after completion of the reaction (monitored by TLC) solvent was
removed under reduced pressure. Resulting residue was re-dissolved in CH2Cl2/MeOH (70 mL;
1:1 v/v) and filtered. This filtrate was successively treated with triethylamine (1.0 mL) and solid
Cu(acac)2 (0.262 g, 1 mmol for L1L5, L3 and 0.131 g, 0.5 mmol for L6L9) and the reaction
mixture further stirred for ~4 h at room temperature. Ensuing reaction mixture was concentrated
under reduced pressure and the residue purified by column chromatography (SiO2, Hexane:

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CH2Cl2, 1:1) to afford the complexes 1-9 and 3 as red solid. (Yield and characterization data for
complexes have been given in the Supplementary information).†
Synthesis of complex 3ʺ (NiII analogue of 3). It was prepared following the above
procedure using Ni(dtc)2 (0.364 g, 1 mmol) in place of Cu(acac)2. Yield 42% (0.196 g). ESI-MS.
1
(Calcd, found, m/z) 961.1483, 961.1282 [M + 2H + Na]+. H NMR (CDCl3, 300 MHz, δ ppm):
1.23 (s, 6H), 1.91 (s, 4H), 3. 75 (s, 4H), 5.49 (s, 4H), 6.14 (d, J = 2.7 Hz, 4H), 6.44 (d, J = 3.3
Hz, 4H), 6.92 (d, J = 8.4 Hz, 4H), 6.99 (d, J = 8.1 Hz, 4H), 7.167.38 (m, 14H). 13C NMR
(CDCl3, 75 MHz, δ ppm): 25.2, 25.4, 29.6, 68.0, 101.5, 112.7, 116.4, 119.4, 126.6, 129.7, 131.8,
134.9, 141.7, 147.4, 156.8, 186.8. IR (KBr pellets, cm-1): 721, 988, 1030, 1506, 1530, 1586,
2854, 2923.
X-ray structure determinations
Crystals suitable for single crystal X-ray diffraction analyses were obtained by slow diffusion
of hexane over dichloromethane (DCM) solution of the respective complexes. Single crystal X-
ray diffraction data for 3 and 3ʺ were collected on ‘Bruker APEX-II CCD’ and for 2, 6, 8 and 9
on OXFORD DIFFRACTION X CALIBUR-S. Structures were solved by direct methods
(SHELXL-2014/7) and refined by full-matrix least square on F2 (SHELXL-2014/7).48 Crystal
structure of 2, 6 and 9 was refined by using restraints. Owing to the prolate ellipsoid in crystal
structure of 2, phenyl ring C atoms were refined without locating the H atoms on them. All non-
H atoms were treated anisotropically. H-atoms attached to the carbon were included as fixed
contribution and geometrically calculated and refined with the SHELX riding model. Computer
program PLATON was used for analyzing the interaction and stacking distances.4950 CCDC
deposition Nos. 1536918, (2); 1505321, (3); 1505322, (3ʺ); 1505319, (6); 1505320, (7);
1505370, (9) contain supplementary crystallographic data for this paper.
Results and discussion
Dipyrromethanes employed in this study 1,2-bis(2-phenoxy)-ethane-bis-dipyrromethane
(L1), 1,3-bis(2-phenoxy)-propane-bis-dipyrromethane (L2), 1,4-bis(2-phenoxy)-butane-bis-
dipyrromethane (L3), 1,4-bis(4-phenoxy)-butane-bis-dipyrromethane (L3′), 1,5-bis(2-phenoxy)-
pentane-bis-dipyrromethane (L4), 1,6-bis(2-phenoxy)-hexane-bis-dipyrromethane (L5), 1,7-bis-
(2-phenoxy)-heptane-bis-dipyrromethane (L6), 1,8-bis(2-phenoxy)-octane-bis-dipyrromethane
(L7), 1,9-bis(2-phenoxy)-nonane-bis-dipyrromethane (L8) and 1,10-bis(2-phenoxy)-decane-bis-

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dipyrromethane (L9) have been synthesized by treatment of the respective aldehydes (A1-A9
and A3′) with an excess of pyrrole in presence of catalytic amounts of trifluoroacetic acid.47
Simple synthetic strategy adopted for preparation of the dipyrromethanes L1L9 is shown in
Scheme 1a and L3′ in Scheme 1b. These have further been oxidized to respective dipyrrins using
an equimolar amount of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in benzene which
in-situ reacted with appropriate metal precursors/salts to afford the complexes in reasonably
good yields (Scheme 2).
Scheme 1. Showing synthesis of dipyrromethanes.

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Scheme 2. Showing synthesis of the complexes 19 and 3ʺ.
A series of flexible bis-dipyrrin ligands which may acquire diverse conformations in
presence of metal ions have been chosen for the present study. It is presumed that depending
upon spacer length one may obtain complexes wherein two metal centres are held at mutually
interacting distances. Notably, bis-dipyrrins containing upto six (CH2)n spacers (n < 6) reacted
with Cu(acac)2 and afforded heteroleptic binuclear bis-dipyrrinato complexes (15). Conversely,
those with relatively longer spacer (n > 7) afforded mononuclear complexes (69). It indicated
that dipyrrins are somehow attuned with the spacer length i.e. with long spacers these underwent
folding while shorter ones with considerable steric hindrance kept away from folding. This genre
of spacer length dependent folding in bis-dipyrrinato complexes has been observed for the first
time. On reaction with Cu(acac)2 bis-dipyrrins based on L1L5 and L3′ gave heteroleptic
binuclear complexes 15 and 3′ however L6L9, afforded mononuclear complexes (69)
wherein both the binding units of bis-dipyrrin chelate coordinate with the same metal centre. On
the other hand, binuclear heteroleptic structure for complexes with spacer lengths n < 6 has been
authenticated by single crystal X-ray analysis of the complex 2; however crystals for 3, 4 and 5
could not be obtained. In this direction, a NiII-analogue of complex 3 (3′′) was synthesized by
using Ni(dtc)2 and authenticated its heteroleptic binuclear structure by single crystal X-ray
analysis. Further, taking advantage of the analogous binding behavior of copper/nickel
precursors with dipyrrins, binding modes in 15 has also been deduced using 3′′ as a
prototype.23,5152
The complexes under investigation are crystalline, non hygroscopic solids, stable at room
temperature and soluble in common organic solvents such as chloroform, dichloromethane
(DCM), acetonitrile (CH3CN) and acetone (CH3)2CO. These have been thoroughly characterized
by spectral (ESI-MS, IR, UV/vis, EPR), and electrochemical (CV) studies and structures of 2, 3,
3ʺ, 6, 8 and 9 have been unequivocally determined by X-ray single crystal analyses.
IR spectra of the complexes displayed characteristic bands due to dipyrrin moiety at ~782,
937, 1020, 1275, 1355, 1415, 1531, 1552, 2923 cm-1 and diagnostic νC=O vibrations of the
coordinated acac [1577, 1; 1578, 2; 1578, 3; 1579, 4; 1580, 5 and 1578 cm-1, Cu(acac)2].
Oxidation of L1L9 to the respective dipyrrins and subsequent coordination with the metal ions
has further been supported by loss of the vibrations due to NH stretching in the IR spectra of

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19. Bands associated with both coordinated acac and dipyrrin moiety (15), and those only
from dipyrrin (69) in the spectra of respective complexes strongly supported formation of the
homo-/ heteroleptic dipyrrinato complexes.811,2023
Compounds L1L9 (Fig. S1S9†) and 19 (Fig. S10S18†) have been characterized by ESI-
mass spectral studies. ESI-mass spectra of 15 displayed peaks due to [M + Na]+, besides these
also displayed a regular increase of 14 in molecular ion peaks attributable to each –CH2– unit
which clearly suggested their respective formulations (Fig. S10S14†, Table 1). In a similar
manner, 69 displayed molecular ion peaks due to [M + H]+ along with an analogous increase in
molecular ion peaks by 14 (CH2 units) (Fig. S15S18†). The ESI-mass of model complexes 3
and 3ʺ showed peaks due to [M + H]+ and [M + Na]+, respectively (Fig. S19S20†).
Table 1: Molecular ion peak for the complexes 19, 3 and 3ʺ.
Complex
Calcd./ Found
Molecular Complex
Calcd./ Found
Molecular
ion species
ion species
845.1281/845.1301
859.1438/859.1412
873.1594/873.1474
[M + Na]+
[M + Na]+
[M + Na]+
630.2156/630.2048 [M + H]+
644.2213/644.2214 [M + H]+
658.2369/658.2349 [M + H]+
694.2345/694.2331 [M + Na]+
873.1594/873.1443 [M + H]+
961.1483/961.1282 [M + Na]+
1
2
3
4
5
6
7
8
763.1407/763.5030 [M  acac]+
901.1907/901.1893
[M + Na]+
9
3
3ʺ
To affirm the formula, purity and integrity of ligands and nickel(II) complex (3ʺ), 1H and 13C
NMR spectra have been acquired in CDCl3 at room temperature (Fig. S21S29†). Notably,
1
similar H and 13C NMR spectral features are expected for L1L9, therefore spectra of the
1
representative compounds L3 and 3ʺ (NiII analogue of 3) are described here (Fig. S30†). H
NMR spectra of L3 displayed signals due to aliphatic (1.80 and 3.94 ppm), meso ( 5.42 ppm),
pyrrolic [ 5.96, 6.14, 6.68 and 7.87 ppm (br, NH)] and aromatic protons ( 6.82, 7.18 ppm). On
the other hand, 3ʺ showed small downfield shift for the phenyl, pyrrolic and aliphatic protons
with respect to L3 and loss of the meso- (–CH) and NH protons. Aliphatic protons for 3ʺ
exhibited an up-field shift relative to precursor complex Ni(dtc)2. It clearly suggested oxidation

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of L3 to respective dipyrrin and its subsequent complexation with the metal centre to afford 3ʺ.
1H NMR spectra of L1L9 displayed an analogous pattern where meso (–CH protons resonated
at ~  5.42 and –NH at ~  7.91 ppm (br) indicating formation of the respective compounds.
Associated aliphatic and aromatic protons resonated at their usual positions. Furthermore, 13C
NMR spectra of ligands are consistent with the earlier reports and supported their formation with
proposed formulations.37,47
Electronic absorption spectroscopy
Electronic absorption spectra of L1L9 and the complexes 19 have been acquired in
CH2Cl2 (Fig. 1). Spectra for L1L9 displayed weak bands due to ππ* transitions at ~275 nm
(Fig. S31†). Complexes 19 too exhibited similar spectral features and displayed strong
absorptions in the low energy region (493, 1; 492, 2; 492, 3; 491, 4; 493, 5; 468, 6; 469, 7; 468, 8
and 469 nm, 9) and two high energy bands (348, 298, 1; 349, 300, 2; 347, 299, 3; 348, 298, 4 and
349, 298 nm, 5; 365, 269, 6; 368, 271, 7; 365, 273, 8 and 364, 274 nm, 9). The low energy bands
have been assigned to ππ* transitions associated with dipyrrin chromophores while high energy
bands to ligand-centreed transitions (Fig. 1a).811,2023 Further investigations indicated that ππ*
transitions associated with the complexes 15 and 69, occurred at slightly different
wavelengths (~492 and ~470 nm), as well, 69 showed a hump at ~502 nm. From the available
data it may be concluded that in mononuclear complexes 69 the ππ* transitions due to dipyrrin
moiety splitted into two and displayed a band in the low energy region ~470 nm and red shifted
hump at ~502 nm. Most likely it occurred due to excitation coupling phenomenon known as
“Davydov Splitting” (Fig. 1b).53-54 This type of splitting is usually observed in bis-dipyrrin
tetrahedral complexes. Thus, significant differences in absorption profiles for 15 and 69 may
be attributed to noticeable structural disparity. Further, Davydov Splitting in 69 clearly
suggested tetrahedral geometry owing to formation of the mononuclear complexes. 811,2023,53-54
Furthermore, dipyrrin units in the mononuclear complexes assume two different planes due to
distorted tetrahedral structure whereas in binuclear complexes it adopted distorted square planer
geometry having two units in the same plane which has further been supported by
electrochemical studies (vide-infra).

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(a)
(b)
Fig. 1 UV/vis spectra of the complexes 15 (a) and 69 (b) in DCM (c, 10 M) at room
temperature.
Crystal structures
Diffraction quality crystals for 2, 6, 8, 9, 3′ and 3ʺ have been obtained by slow diffusion of
hexane in dichloromethane solution of respective complexes (Fig. 23). Crystallographic
refinement parameters and selected geometrical parameters are gathered in Tables 25. Complex
2 and 8 crystallize in monoclinic system with 'P21/n' and 'C1c1' space groups while 6 and 9
crystallize in orthorhombic system with 'Pbca' and 'P212121' space groups, respectively.
The metal centre in complex 2 is found to be coordinated two nitrogen and oxygen atoms
from dipyrrin and acetylacetonato moieties, respectively adopting a distorted square planar
geometry with dihedral angles for the planes, N(1)–Cu(1)–N(2) and O(1)–Cu(1)–O(2) being
4.88˚. The CuN and CuO bond lengths [Cu1N1, 1.954; Cu1N2, 1.955; Cu1O1, 1.939;
Cu1O2, 1.938 Å] and angles N1Cu1N2 (91.40˚), O1Cu1O2 (90.61˚), N2Cu1O1
(89.34˚) and N1Cu1O2 (88.79˚) are comparable to other related systems.811,2023
On the other hand, complexes 6, 8 and 9 adopt distorted tetrahedral geometry around the
CuII-metal centre (Fig. 2) with all the four coordination sites occupied by pyrrolic nitrogens (N1,
N2, N3 and N4) from the respective bis-dipyrrin ligands. Dihedral angles between the planes
N(1)–Cu(1)–N(2) and N(3)–Cu(1)–N(4) are 51.13˚ (6), 47.04˚ (8) and 47.26˚ (9) indicating the
ease of steric crowding in 69. CuII complexes are known to favor square planar geometry but
due to steric hindrance between the ortho substituted (CH2)n linker and dipyrrinato ligand
distortion must have occurred and the complexes would not have favored planar structures but
adopted a tetrahedral geometry. The CuN(dpm) bond distances in these complexes [CuN1,

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1.938; CuN2, 1.969; CuN3, 1.939; and CuN4, 1.964 Å (6); CuN1, 1.954; CuN2, 1.943;
CuN3, 1.957; CuN4, 1.955 Å (8); and CuN1, 1.954; CuN2, 1.945; CuN3, 1.931 and
CuN4, 1.939 Å (9)] are comparable to those with other closely related systems.811,5557
Similarly, bond angles around CuII [N1Cu1N2, 92.5; N1Cu1N4, 151.6; N2Cu1N3,
137.0 and N3Cu1N4, 92.6˚ (6); N1Cu1N2, 92.2; N1Cu1N4, 148.0; N2Cu1N3, 145.2
and N3Cu1N4, 91.8˚ (8); and N1Cu1N2, 91.4; N1Cu1N4, 144.1; N2Cu1N3, 149.0
and N3Cu1N4, 92.5˚ are comparable to those with other distorted tetrahedral
complexes.811,5557
Fig. 2 Crystal structures of 2 (a) 6 (b), 8 (c) and 9 (d).
Further, it has been observed that with an increase in the chain length (CH2)n between two
dipyrrin units in the complexes 6, 8 and 9, dihedral angles for the planes N(1)–Cu(1)–N(2) and
N(3)–Cu(1)–N(4) decreases due to expansion of the ring size causing lower steric repulsion and
thereby degree of distortion is minimized.

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Model complex 3 represents a simple binuclear complex crystallizes in monoclinic system
with 'P21/n' space group. The metal centre in this complex is coordinated by two each of nitrogen
and oxygen atoms from the dipyrrin unit and acetylacetonato moieties, respectively adopting a
square planar geometry with dihedral angles for the planes, N(1)–Cu(1)–N(2) and O(1)–Cu(1)–
O(2) being 2.67˚. The CuN and CuO bond lengths [Cu1N1, 1.965(3); Cu1N2, 1.958(3);
Cu1O1, 1.935(3); Cu1O2, 1.938(3) Å] and angles N1Cu1N2 (91.37˚), O1 Cu1O2
(90.81˚), N2Cu1O1 (88.78˚) and N1Cu1O2 (89.07˚) are comparable to other related
systems.811,2023 The meso-phenyl substituent is twisted by an angle of 62˚ with respect to the
dipyrrin moiety.
(a)
(b)
Fig. 3 Crystal structure of the model complexes 3 (a) and 3ʺ (b).
The NiII analogue of 3 i.e. model complex 3ʺ crystallizes in triclinic system with P1 space
group in which NiII adopts a distorted square planar geometry with two nitrogen from each
dipyrrin and two sulphur atoms from the dithiocarbamate units with the dihedral angle of 3.80˚
between the N1–Ni1–N2 and S1–Ni1–S2 planes. Distortion from the square planar geometry
around the NiII-centre may be attributed to small bite angles (N1NiN2, 93.13 and S1NiS2,
77.41˚). The NiN bond distances [Ni1N1, 1.903 (6); Ni1N2, 1.882 (8) Å], NiS [NiS1,
2.208 (3); Ni1S2, 2.182 (3) Å] and bond angles [N1Ni1N2 (93.13˚), S1Ni1S2 (77.41˚),

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N1Ni1S1 (95.49˚) and N2Ni1S2 (94.07˚)] in the complex are comparable to the earlier
reports.5152 In this complex too, the meso-phenyl substituent is twisted by 65˚ from the dipyrrin
core to greater extent relative to 3 (vide-supra). It evidently proved that molecular folding are
more pronounced in ortho-substituted dipyrrinato complexes than the para- ones; however, it
becomes effective only if sufficient spacer length is involved in the complex i.e. from 6 (n=7).
Density functional theory calculations
To gain a deep insight into the spacer dependent architectural disparity in the complexes
under investigation, DFT optimizations have been performed. Model structures for 15 and 7
have been designed by ChemBioDraw Ultra software and 3D views optimized by minimizing
energy in MM2 mode of the same software. On the other hand, models for 6, 8 and 9 have been
directly taken from their single crystal X-ray data and subjected to optimization. Geometry
optimization for the complexes under study has been carried out using GAUSSIAN 09 by
B3LYP methods.58
Fig. 4 DFT optimized structures of complex 4 (a) and complex 5 (b).
The metal centre CuII has been described by LANL2DZ basis set while non-metal atoms by
6-31G** and resulting coordinates optimized energetically.17 Calculated geometrical parameters
such as bond lengths, bond angles and bond energies are consistent with the single crystal X-ray
data (Table S1S2†).

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Optimization on a series of model complexes for 19 clearly showed that homoleptic
mononuclear complexes are quite achievable from n = 6 (complex 5) onwards. It affirmed that
steric factors are not strong enough to resist folding/bending of the molecules to form homoleptic
mononuclear complexes. Though, in line with other studies like UV/vis where complexes 15
and 69 exhibited different absorption profiles from one another, it can reasonably be stated that
from complex 6 onwards (spacer length n ≥ 7) homoleptic mononuclear complexes are created
which is consistent with the experimental observations, too from single crystal X-ray data for 6,
8 and 9. So, it has to be assumed that spacer lengths n < 7 are not sufficient to withstand folded
mononuclear homoleptic architectures. Herein, this factual assumption may be varified by DFT
optimization studies to reveal the probable factor for this change in coordination modes on going
from n = 6 to 7. It is an inherent tendency for 6 and 7 membered sp3-alkyl chains to acquire anti-
and syn configurations, respectively. The syn configuration (n = 7, complex 6) naturally tends to
form homoleptic mononuclear while anti- (n = 6, complex 5) prefers to form heteroleptic
binuclear complex due to their spatial orientations. However, it is logical only for the threshold
value for ‘n’ (6 – 7) because the complexes with n < 6 and > 7 unequivocally attain heteroleptic
and homoleptic coordination modes, respectively. It is supported by DFT optimization of
complexes with n = 2 to 6 where all acquire only heteroleptic binuclear mode and confirmed by
single crystal X-ray study for complex 2 which attains a heteroleptic binuclear structure
irrespective of its syn configuration of alkyl chain (n = 3) (Fig. 4 and S32S33†).
The syn configuration thus supports the folding of the molecule to form mononuclear
complex and apparently navigates the central metal to coordinate with both terminal dipyrrin
cores simultaneously, to attain the mononuclearity. Similarly, for complexes with n ≥ 7 syn-anti
configuration factor also become meaningless most probably due to adequate flexibility in the
molecule to acquire folded mononuclear architecture. Moreover, it is observed that after a certain
specific chain length mononuclear homoleptic structure (6-9) are more stable relative to
heteroleptic binulear complexes (1-5) which may be due to chelate effect of the ligand. After a
certain chain length (n > 6), chelate effect predominate over steric hindrance which favors
formation of mononuclear metallamacrocyclic structures. Therefore, one can have a clear-cut
idea about the formation of complex depending on spacer length that mononuclear homoleptic
complex begins to form with large spacer groups (n ≥ 7) and the most probable factors
responsible for transition from bi- to mononuclear complex formation are steric and syn-

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conformation effects. The crystal structures of 8 and 9 with spacer lengths n = 9 and 10
respectively, also confirmed that these are engendered by reduced steric crowding in the systems
relative to the dipyrrin complexes with n < 6.
EPR spectral studies
EPR spectrum of the copper complexes in solution the provides useful information about
environment around the metal centre. Data for complexes under investigation are presented in
Table (S3†) and resulting spectra depicted in Fig. 5 and S3435.† The X-Band EPR spectra of
copper(II) complexes 19 were recorded in r.t CH2Cl2 solvent as well as in frozen solution at
liquid nitrogen temperature (Fig. 2 and Table 1). The g║ > g┴ > 2.003 observed for the complexes
are typical for d9 copper(II) and a ground state doublet indicating presence of the unpaired
2
2
electron in a dx y orbitals.17,2324 Room temperature X-band EPR for 15 displayed three line
axial spectra due to merger of the third and fourth band. Solution state spectra in the complexes
69 show only one signal without any hyperfine splitting and a broadening of g⊥, thus indicating
severely distorted tetrahedral geometry, comparable to other related systems.17,2324 On the other
hand, spectra for 19 in frozen solution at liquid nitrogen temperature showed typical four line
pattern expected from Cu(II) complexes. From the Fig. 5 and S34 we can see that the shape and
the parameters of the EPR spectra of 1-5 and 6-9 are very similar, which indicate their similar
coordination environments. The values of hyperfine coupling constant A|| for 69 are quite low in
comparison to the complexes 15. The factor g||/A|| offers a convenient empirical index for
tetrahedral distortion in Cu(II) complexes, higher value of this factor for complexes 69 relative
to 15 indicated greater distortion of the tetragonal site which has also been confirmed by the
single crystal XRD study.

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Fig. 5 EPR spectra of Complex 5 (a) and 6 (b) in CH2Cl2 solvent at liquid nitrogen temperature.
Electrochemical studies
Electrochemical behavior of 19 have been investigated by cyclic voltammetry (CV) in
CH3CN (c, 100 M) using 0.1 M tetrabutylammonium perchlorate [(n-Bu)4N]ClO4 as a
supporting electrolyte. Measurements have been made in the range of +1.5 to 2.0 V at a scan
rate of 100 mVs−1, and redox potentials referenced to Fc/Fc+ couple (0.1 V). Cyclic
voltammograms of 15 exhibited one quasi-reversible oxidation wave (Epa 0.680, 1; 0.640, 2;
0.630, 3; 0.630, 4; 0.620 V, 5) assignable to oxidation of the dipyrrin moiety alongwith one
irreversible (Epc1 0.55, 1; 0.53, 2; 0.50, 3; 0.51, 4; 0.51 V, 5) and one reversible reduction
wave (Epc1 1.26, 1; 1.24, 2; 1.23, 3; 1.20, 4; 1.18 V, 5). On the other hand, 69 displayed
two electron oxidation wave (Epa 0.63, 0.77, 6; 0.61, 0.75, 7; 0.640, 0.818, 8; 0.640, 0.818 V, 9)
assignable to oxidation of the dipyrrin moieties as well as one each of irreversible (Epc1 0.49, 6;
0.47, 7; 0.48, 8; 0.48 V, 9) and reversible reduction wave (Epc1 1.22, 6; 1.20, 7; 1.21, 8;
1.20, V 9).17,22,5556,59 Weak irreversible reduction waves in 19 are characteristic of reduction
of copper(II) to copper(I) while reversible reduction wave arises due to reduction of the dipyrrin
moiety (Fig. 6 and S3536, †).
Fig. 6 Cyclic voltammogram of 5 (a) and 6 (b) in CH3CN (c, 100 M) at room temperature.
To affirm number of electrons involved in the redox process, chronocoulometric experiments
have been performed on 5 and 6 as a representative of bi- and mononuclear complexes by
determining their oxidation and reduction wave potentials (Fig. S3738†). It gave 1:1 ratio for

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the number of electrons involved in oxidation and reduction processes. Therefore, it is concluded
that oxidation and reduction of 5 and 6 is a two-electron process. The same conclusion could be
drawn for other binuclear (14) and mononuclear copper complexes (79) as the shape of the
voltammograms are similar in nature. In general, structural variations associated with a redox
reaction induces a change in the redox potential. In square planer complexes 15, presence of
two dipyrrin moieties in the same plane resulted coalescence of the first and second oxidation
wave. On the other hand, as discussed earlier mononuclear complexes 69 adopted enforced
tetrahedral coordination geometry and two dipyrrin moieties assume different planes, hence two
distinct oxidation waves due to dipyrrin moieties. The above results corroborated well with the
findings from single crystal X-ray diffraction analyses. This architectural dissimilarity further
attested different spectral patterns in 15 as compared to 69.
Above studies clearly supported the formation of binuclear and mononuclear complexes in
presence of ligands having identical dipyrrin terminals with variable number (n) of adjoining
spacer units (-CH2-). It has been demonstrated that complex 15 attain a binuclear heteroleptic
architecture pertaining to the steric effects due to small chain lengths where ‘n’ varies from 2
from 6. Conversely, from complex 6 with n = 7, long chain spacers bring enough flexibility in
the complexes so that steric effects become ineffective leading to formation of folded mono-
nuclear homoleptic architecture. The folding at n = 7 has also been facilitated by the syn
conformation of the complex which is attainable only with odd number of spacers as confirmed
by DFT studies as well. The formation of bi-/mononuclear complexes with given number of
spacers has further been supported by electrochemical and EPR studies. Thus, it may be
concluded that such dipyrrin complexes may attain stable mononuclear architecture only if
sufficient spacer chain lengths are provided.
Conclusion
In summary, through this work homo-/heteroleptic bis-dipyrrinato copper complexes have
been synthesized by varying alkyl spacer length and their structural, optical, and electrochemical
properties examined by various techniques. It has been noted that number of (CH2)n units in
alkyl spacer directly influences the geometry and construct of resulting complexes. From single
crystal X-ray and DFT studies it has been categorically shown that dipyrrins with spacer length n
< 6 created a binuclear heteroleptic complex while those with n > 7 gave mononuclear homo-

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leptic complexes. DFT studies additionally revealed that odd number of (CH2)n units stabilizes
syn conformation while even stabilizes anti conformation of the bis-dipyrrin complexes. The
present work clearly demonstrated that position and length of the spacers between two dipyrrin
units strongly influence the coordination behavior and supramolecular assembly of the
dipyrrinato complexes. These observations can be further escalated in controlled designing of the
supramolecular assemblies based on dipyrrin complexes.
Acknowledgements
Authors gratefully acknowledge financial support from the Department of Science and
Technology (DST), New Delhi, India for providing financial assistance through the Scheme
[SR/S1/IC-25/2011] and also to CSIR, New Delhi, India for the award of a senior Research
Fellowship to RPP (No. 09/013(0514)/2013-EMR-I). We are also thankful to the Head,
Departments of Chemistry, Institute of Science, Banaras Hindu University, Varanasi (U.P.)
India, for extending laboratory facilities.
Supporting Information
1H, 13C NMR, ESI-MS, and Tables are provided. CCDC No. 1536918, (2); 1505319, 6; 1505320,
8; 1505321, (3); 1505322, (3); 1505370, (9); contains the crystallographic data for this paper. It
can be had free of charge via https://summary.ccdc.cam.ac. uk/structure-summary-form (or from
the CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K.; Fax: +44−1223−336033; Email:
deposit@ccdc.cam.ac.uk).

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Table 2: Crystal data and structure refinement parameters for 2, 6, 8 and 9.
2
6
8
9
Empirical formula
C43H40N4Cu2O6
C37H34CuN4O2
C39H38CuN4O2
C40H40CuN4O2
Crystal system
Space group
a (Å)
monoclinic
'P 21/n'
orthorhombic
Pbca
monoclinic
C1c1
orthorhombic
P212121
14.1141(3)
16.4924(3)
17.4915(3)
90.00
14.9525 (3)
15.1818(3)
27.7558(5)
90.00
25.9984(2)
9.3703(2)
28.3965(8)
90.00
8.9995(2)
14.841(3)
26.069(5)
90.00
b (Å)
c (Å)
α (deg)
β (deg)
105.647(2)
90.00
90.00
105.34(3)
90.00
90.00
90.00
90.00
(deg)
V (Å3), Z
3920.70(13), 4
1.54184
6300.7(2), 27
0.71073
6671.2(4), 8
0.71073
3481.9(2), 4
0.71073
λ (Å)
Red, needle
293(2)
Green, block
293(2)
Green, block
293(2)
Green, block
293(2)
Color and habit
T (K)
reflns collected
25906
55068
26373
5658
refins/restraint/
params
7543/11/430
5539/9/397
5865/0/415
5658/5/401
Dcalcd (g cm-3)
µ (mm-1)
1.416
1.329
1.311
1.283
1.770
0.733
0.695
0.667
GOF on F2
1.083
1.030
1.039
1.072
final R indices I>2σ(I)
R1 = 0.1023
R1 = 0.0544
R1 = 0.0628
R1 = 0.1298
wR2 = 0.3137
R1 = 0.1312
wR2 = 0.1378 wR2 = 0.1247
R1 = 0.0741 R1 = 0.1219
wR2 = 0.1460
R1 = 0.3477
R indices (all data)
wR2 = 0.3495
wR2 = 0.1524 wR2 = 0.1611
wR2 = 0.1987

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Table 3: Crystal data and structure refinement parameters for 3 and 3ʺ.
3
3
Empirical formula
C44H42Cu2N4O6
C44H48N6Ni2O2S4
Crystal system
Space group
a (Å)
monoclinic
P21/n
triclinic
P -1
7.2758(2)
14.6438(6)
18.4718(8)
90.00
9.531(5)
16.488(9)
16.546(9)
61.42(2)
88.05(2)
79.41 (2)
b (Å)
c (Å)
α (deg)
β (deg)
91.52(3)
90.00
(deg)
V (Å3), Z
1967.4(2), 4
0.71073
2240.0(2), 2
0.71073
λ (Å)
Red, needle
273(2)
Red, needle
301(2)
Color and habit
T (K)
reflns collected
refins/restraint/
params
25113
4036
6593/0/ 264
6680/0/212
Dcalcd (g cm-3)
1.435
1.391
µ (mm-1)
1.135
1.070
GOF on F2
0.981
1.024
final R indices I>2σ(I)
R1 = 0.0631
R1 = 0.0619
wR2 = 0.1494
R1 = 0.1732
wR2 = 0.1189
R1 = 0.1436
R indices (all data)
wR2 = 0.2148
wR2 = 0.1493

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Table 4: Selected bond lengths (Å) and angles (˚) for 6, 8 and 9.
2
Bond
length (Å)
CuN1
CuN2
CuN3
CuN4
Bond length (Å)
6
8
9
1.954
1.955
1.939
1.938
CuN1
CuN2
CuO1
CuO2
1.938
1.969
1.939
1.964
1.954
1.943
1.957
1.955
1.954
1.945
1.931
1.939
Bond Angle (˚)
N1Cu1N2
O1Cu1O2
N2Cu1O1
N1Cu1O2
6
8
9
91.40˚
90.61
89.34
88.79
92.5
151.6
92.2
98.1
96.6
91.8
91.4
97.3
97.5
92.5
N1Cu1N2
N1Cu1N4
N2Cu1N3 137.0
N3Cu1N4 92.6
Table 5: Selected bond lengths (Å) and angles (˚) for 3 and 3.
Bond length (Å)
3
Bond length (Å)
3
CuN1
CuN2
1.965
1.958
1.935
1.938
NiN1
NiN2
Ni1S1
Ni1S2
1.903
1.882
2.208
2.182
Cu1O1
Cu1O2
Bond Angle (˚)
Bond Angle (˚)
3
3
91.37
90.81
88.78
89.07
93.13
77.41
95.49
94.07
N1Cu1N2
O1Cu1O2
N2Cu1O1
N1Cu1O2
N1Ni1N2
S1Ni1S2
N1Ni1S1
N2NiS2

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Spacer length dependent architectural diversity in bis-dipyrrin copper(II)
complexes
Rajendra Prasad Paitandi, Roop Shikha Singh, Sujay Mukhopadhyay, Ashish Kumar and Daya
Shankar Pandey*
Department of Chemistry, Institute of Science, Banaras Hindu University, Varanasi - 221 005,
India
---------------------------------------------------------------------------------------------------------------------
Copper(II) complexes (1-9 and 3) derived from bis-dipyrrin ligands (L1-L9 and L3′) with
diverse spacer lengths [-(CH2)n-have been described. It has been noted that number of
-(CH2)n- units in alkyl spacer directly influence the geometry and construct of resulting
complexes. Dipyrrins with spacer length n < 6 created a binuclear heteroleptic complex while
those with n > 7 gave mononuclear homoleptic complexes. A line between syn- and anti-
conformation of the complexes has been drawn and based on it a limiting spacer length (n > 7)
set for achieving mononuclear dipyrrin complexes.
-(CH2) -
n
n= 2-9
n ≤ 6
Heteroleptic-Binuclear (1-5)
n ≥ 7
N
O
Cu
Homoleptic-Mononuclear(6-9)