Unsymmetrical bidentate ligands as a basis for construction of ambidentate ligands for functional materials: Properties of 4,4-dimethyl-1-phenylpentane-1,3-dionate

Article abstract of DOI:10.1016/j.poly.2017.08.022

To establish a background to the possible use of its ambidentate derivatives as stimulus-responsive ligands in functional complexes, the basic coordination chemistry of the anion derived from 4,4-dimethyl-1-phenylpentane-1,3-dione (benzoylpivaloylmethane,

Full text of DOI:10.1016/j.poly.2017.08.022

Accepted Manuscript
Unsymmetrical Bidentate Ligands as a Basis for Construction of Ambidentate
Ligands for Functional Materials: Properties of 4,4-dimethyl-1-phenylpen-
tane-1,3-dionate
Michał Kołodziejski, Anna Jenczak, Zbigniew Hnatejko, Jack Harrowfield,
Artur R. Stefankiewicz
PII:
S0277-5387(17)30553-3
DOI:
http://dx.doi.org/10.1016/j.poly.2017.08.022
POLY 12786
Reference:
Polyhedron
To appear in:
Received Date:
Revised Date:
Accepted Date:
26 June 2017
15 August 2017
17 August 2017
Please cite this article as: M. Kołodziejski, A. Jenczak, Z. Hnatejko, J. Harrowfield, A.R. Stefankiewicz,
Unsymmetrical Bidentate Ligands as a Basis for Construction of Ambidentate Ligands for Functional Materials:
Properties of 4,4-dimethyl-1-phenylpentane-1,3-dionate, Polyhedron (2017), doi: http://dx.doi.org/10.1016/j.poly.
2017.08.022
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Unsymmetrical Bidentate Ligands as a Basis for Construction of
Ambidentate Ligands for Functional Materials: Properties of 4,4-dimethyl-
1-phenylpentane-1,3-dionate
Michał Kołodziejski1,2, Anna Jenczak1,2, Zbigniew Hnatejko1, Jack Harrowfield3 and
Artur R. Stefankiewicz1,2,*
1
Faculty of Chemistry, Adam Mickiewicz University, Umultowska 89b, 61-614 Poznań,
Poland;
E-mails:
mic.kolodziejski1@gmail.com
(M.K.),
anna.jenczak@op.pl
(A.J.),
zbychuh@amu.edu.pl (Z.H.), harrowfield@unistra.fr (J.H.)
2 Centre for Advanced Technologies, Adam Mickiewicz University, Umultowska 89c, 61-614
Poznań, Poland
3
Institut de Science et d’Ingénierie Supramoléculaires, Université de Strasbourg, 8 allée
Gaspard Monge, 67083 Strasbourg Cedex, France
Abstract
To establish a background to the possible use of its ambidentate derivatives as stimulus-
responsive ligands in functional complexes, the basic coordination chemistry of the anion
derived from 4,4-dimethyl-1-phenylpentane-1,3-dione (benzoylpivaloylmethane, bpmH) has
been re-examined with selected main group and transition metal M(II) and M(III) species.
There is evidence that the steric bulk of the 1,3-dione substituents is not sufficient to prevent
oligomerisation of the Ni(II) complex and equilibrium mixtures of geometric isomers can be
detected in solution for at least the Al(III), Co(III) and Pd(II) complexes. Crystal structure
determinations on both [Cu(bpm)2] and [Pd(bpm)2] show, however, that a single isomer can
be isolated in the solid state.
Keywords: β-diketonate, structure, supramolecular, isomerism

1. Introduction
Supramolecular chemistry is based upon the use of labile intermolecular interactions
to assemble complex structures with specific functionality [1]. While the nature and energy of
these labile interactions are both extremely varied, a particularly useful type of interaction is
that involving coordinate bonding to metal ions. A coordinate bond is not necessarily labile
but this is commonly the case and one feature of the coordinate bond that greatly enhances its
utility is that it can be converted from a labile to an inert state by the appropriate choice of
donor atoms and/or ligand structure. This can even be done selectively within the
coordination sphere of a single metal ion by the introduction of multidentate chelate or,
particularly, macrocyclic ligands [2]. An aspect of multidentate ligand design which has been
little explored to date in relation to the multifunctionality of complexes is the deliberate
introduction of ambidentate character. In principle, this should allow the complex of an
appropriate ligand to switch, under a particular stimulus [3], between forms with various
functions such as different degrees of catalytic activity. Thus, we have begun a programme of
research concerned with divergent multitopic ligands and the properties of their isomeric
complexes.
An especially versatile and long-studied group of chelate ligands is that based on 1,3-
diketones [4-14], acetylacetone (acacH) or pentane-2,4-dione being the best-known of such
molecules. While the neutral species can act as O,O’-chelates towards metal ions, provided
the carbon atom linking the carbonyl groups has at least one H-substituent, enabling
tautomerism with an enol form, these molecules are weak acids and the coordination
chemistry of their conjugate-base, delocalised diketonate anion derivatives is far more
extensive and includes examples of C-donor bonding as well as O,O’ chelation [4-14]. The
ease of synthesis of 1,3-diketones, not necessarily directed solely towards metal ion
complexation [15], means that an exceptional range of such species is accessible and
relatively recent work has provided examples of pyridyl derivatives which could indeed
function as ambidentate species [16]. Particularly useful as a means of introducing extra
functionality is a phenyl substituent and a variety of derivatives of benzoylpivaloylmethane
(bpmH) are known which illustrate this approach [17-19], although the substituents known in
this case, with one exception [19], do not provide metal-binding sites competitive with the
diketonate unit. A phenyl group or larger aromatic unit can also be employed in the
generation of poly-1,3-diketonate ligands, such species having been the basis of some
remarkable recent extensions of diketonate coordination chemistry [20-23]. In our focus on
the possibility of generating ambidentate ligands from a phenyl-substituted 1,3-diketone, we

were confronted with two issues potentially complicating their application: aggregation and
isomerism of the complexes. The diketone benzoylpivaloylmethane (bpmH) being a readily
available "parent" species for ambidentate ligand construction, we sought to make specific
expansions of the rather limited knowledge [17-19], [24-26] of the coordination chemistry of
the derived diketonate in relation to these potential complications using transition metal M(II)
and M(III) species, plus Al(III) as a main group representative (Figure 1).
Figure 1. Complexes obtained in reactions of 4,4-dimethyl-1-phenylpentane-1,3-dione
(bpmH) with M(II) and M(III) ions.
2. Experimental
2.1 Reagents and instrumentation
All reagents were purchased from commercial sources (Sigma, Fluorochem, AlfaAesar). High
purity solvents were purchased from POCH and anhydrous THF was dried over molecular
sieves under argon. NMR spectra were measured on a Bruker Ultrashield ™ 300 + 300
Fourier-Transform spectrometer using deuterated solvents (CDCl3, CD3CN). TOF-MS spectra
were measured using chloroform solutions on a BRUKER Impact HD ESI-Q-TOF mass
spectrometer. Solution electronic absorption and luminescence spectra were recorded in 1 cm

quartz cells at ambient temperature in MeCN on Shimadzu UV-2401 PC and Hitachi F-7000
Fluorescence instruments, respectively. Luminescence spectra were also recorded on the solid
complexes.
X-Ray diffraction data were obtained on a 4-circle Xcalibur EosS2 diffractometer
(Agilent Technologies) equipped with a CCD detector.
2.2 Synthesis
2.2.1 4,4-dimethyl-1-phenylpentane-1,3-dione (benzoylpivaloylmethane), bpmH
The ligand was prepared according to a literature procedure [15], although it is also available
commercially. Pinacolone (5.54 g; 0.055 mol) was added to a suspension of NaH (6.65 g;
0.166 mol) in dry THF (50 mL). After 30 min. of stirring, methyl benzoate (3.5 mL; 0.277
mol) was added to the grey-yellow suspension. The mixture was heated at 47 °C for 24 h.
Reaction was quenched by the addition of water, followed by aqueous HCl to reduce the pH
to ~4. The product was extracted into ethylacetate and the extract evaporated down after
drying over Na2SO4. The crude product was purified by addition of CuCl2•2H2O, isolation of
the solid complex formed and then dissociating the ligand from this by treatment with hot
aqueous H2SO4. Extraction into DCM, drying and evaporation of the extract yielded the
1
desired product as a yellow oil (2.81 g; 50.0 % yield). H NMR (300 MHz, CDCl3) δ 16.53 (s,
1H, enol-form -OH, H6); 7.90, 7.89 (d, 2H, H3); 7.54, 7.52, 7.50 (t, 1H, H5); 7.47, 7.45, 7.43
(t, 2H, H4), 7.26 (CDCl3), 6.31 (s, 1H, enol-form –CH2–, H2), 4.19 (s, 2H, keto-form -CH2-,
H2) , 1.26 (s, 9H, enol-form –CH3, H1), 1.23 (s, 9H, keto-form –CH3, H1) (Signals assigned
13
according to the figure S8). C NMR (75 MHz, CDCl3) δ 202.97 (C9), 184.65 (C8), 135.59
(C7), 132.20 (C6), 128.65 (C5), 127.05 (C4), 92.17 (C3), 77.58, 77.16, 76.74 (Solvent, CDCl3),
39.93 (C2), 27.47 (C1) (Signals assigned according to the figure S9). Absorption spectrum
(CH3CN): λmax= 312 nm, εmax = 2.07x102 M-1 cm-1. Microanalysis: calculated (%) for
C13H16O2: C 76.44, H 7.90, O 15.66; found: C 76.31, H 8.11, O 15.58.
2.2.2 Tris(4,4-dimethyl-1-phenylpentane-1,3-dionato)aluminium(III), [Al(bpm)3]
As for the free diketone, the syntheses of the Al(III) diketonate and of the other complexes
described herein were based on literature procedures [17], [18], [24], [25]. To a yellow
solution of Al(NO3)3•9H2O (65.2 mg; 0.173 mmol) and bpmH (106.5 mg; 0.521 mmol) in
MeOH/H2O (1:1, 3 mL), NaHCO3 (51.1 mg; 0.608 mmol) was added. The resulting
suspension was stirred for 30 min at room temperature. After removal of the solvents under
reduced pressure, the residue was extracted with CHCl3 (2 x 15 mL) and washed with water
(2 x 15 mL). The combined organic extracts were dried with Na2SO4 after which the solvent

was removed to give an oily residue. Addition of EtOAc / hexane (1:1, v/v, 5 mL) resulted in
the precipitation of a cream solid, which was filtered off and dried in vacuo to give a white
powder which was recrystallized from chloroform, yielded white, hairy small crystals (37.1
1
mg; 33.4 % yield). H NMR (300 MHz, CDCl3) δ 7.97, 7.94, 7.92, 7.90, 7.88, 7.86 (sext., 6H,
H3); 7.44, 7.43, 7.41, 7.39, 7.36, 7.35, 7.32 (sept., 9H, H5, H4); 7.26 (CDCl3), 6.37, 6.34, 6.32,
6.26 (q, 3H, H2); 1.23, 1.20, 1.19, 1.17 (q, 27H, H1) (Signals assigned according to the figure
1). TOF-MS: m/z = 659.2974 - calculated for C39H45AlNaO6 (Na[Al(bpm)3]+); found: m/z
+
659.2929; m/z = 433.1993 - calculated for C26H30AlO4 ([Al(bpm)2]+); found: m/z =
433.1959. Absorption spectrum (CH3CN): λmax = 325 nm, εmax = 1.76x103 M-1 cm-1.
Microanalysis: caluclated (%) for C39H45AlO6: C 73.56, H 7.12, O 15.08; found: C 73.72, H
6.98, O 15.23.
2.2.3 Tris(4,4-dimethyl-1-phenylpentane-1,3-dionato)cobalt(III), [Co(bpm)3]
To a solution of bpmH (203 mg; 0.996 mmol) in MeOH (2 ml), TEA (143 µl; 1.03 mmol)
was added. After 5 min. of stirring at room temperature, a MeOH solution (2 ml) of
Co(ClO4)2•6H2O (121.48 mg; 0.332 mmol) with H2O2 (11 µl; 0.365 mmol) was added. The
resulting mixture was stirred for 30 min. and then H2O was added to finish precipitation. The
voluminous dark green precipitate was filtered off, washed with water, dried and
recrystallized from chloroform, yielded dark green flake-shaped crystals (365 mg; 55% yield).
1H NMR (300 MHz, CDCl3) δ 7.91, 7.88, 7.86, 7.84, 7.81, 7.78 (sext., 6H, H3); 7.43, 7.41,
7.39, 7.37, 7.34, 7.32, 7.30 (sept., 9H, H5, H4); 7.26 (CDCl3), 6.34, 6.30, 6.28, 6.20 (q, 3H,
H2); 1.26, 1.23, 1.21, 1.19 (q, 27H, H1) (Signals assigned according to the figure S10). TOF-
MS: m/z = 669.2626 - calculated for C39H46CoO6 (H[Co(bpm)3]+); found: m/z = 669.2626.
Absorption spectrum (CH3CN): λmax = 595 nm, εmax = 183.6 M-1 cm-1 (the visible region
absorption due to d-d transitions). Microanalysis: calculated (%) for C39H45CoO6: C 70.05,
H 6.78, O 14.36; found: C 70.19, H 6.63, O 14.27; calculated (%) for C78H90Co3O12: C 67.09,
H 6.50, O 13.75; found: C 66.54, H 6.96, O 13,48 (green paramagnetic species, after
dehydration by heating).
2.2.4 Diaqua-bis(4,4-dimethyl-1-phenylpentane-1,3-dionato)nickel(II), [Ni(bpm)2(OH2)2]
To a solution of bpmH (124 mg; 0.608 mmol) in MeOH (2 ml), TEA (61 µl; 0.608 mmol)
was added. After 5 min. of stirring at room temperature, a MeOH solution (2 ml) of
Ni(CH3COO)2•4H2O (76 mg; 0.304 mmol) was added. The resulting mixture was stirred for
30 min. and then H2O was added to finish precipitation. The voluminous light green

precipitate was filtered off, washed with water, dried and recrystallized from chloroform,
yielded small light green cubes (163 mg; 58% yield). TOF-MS: m/z = 465.1575 - calculated
for C26H31NiO4 (H[Ni(bpm)2]+); found: m/z = 465.1563. Absorption spectrum (CH3CN): λmax
= 602 nm, εmax = 255.8 M-1 cm-1 (the visible region absorption due to d-d transitions).
Microanalysis: calculated (%) for: C26H34NiO6: C 62.30, H 6.94, O 19.15; found: C 62.08, H
7.12, O 18.99.
2.2.5 Bis(4,4-dimethyl-1-phenylpentane-1,3-dionato)copper(II), [Cu(bpm)2]
To a solution of bpmH (199 mg; 0.975 mmol) in THF (5 ml), Na2CO3 (207 mg; 1.95 mmol)
was added. After 5 min. of stirring at room temperature, a THF solution (3 ml) of
Cu(BF4)2•6H2O (77.0 mg; 0.325 mmol) was added. The resulting green mixture was stirred
for 24 h at room temperature. A small amount of flocculent material was filtered off and
green filtrate taken to dryness in vacuo. The solid obtained was dissolved in DCM / EtOH
mixture and left to crystallize by slow solvent evaporation (112 mg; 49 % yield). TOF-MS:
m/z = 470.1553 - calculated for C26H31CuO4 (H[Cu(bpm)2]+); found: m/z = 470.1505.
Absorption spectrum (CH3CN): λmaxI = 663 nm, εmaxI = 52.6 M-1 cm-1; λmaxII = 538 nm, εmaxII
=
47 M-1 cm-1 (the visible region absorption due to d-d transitions). Microanalysis:
calculated (%) for C26H30CuO4: C 66.43, H 6.43, O 13.61; found: C 66.68, H 6.29, O 13.49.
2.2.6 Hydrated bis(4,4-dimethyl-1-phenylpentane-1,3-dionato)zinc(II), [Zn(bpm)2(OH2)n]
To a solution of bpmH (119 mg; 0.583 mmol) in MeOH (2 mL), NaOH (23.4 mg; 0.583
mmol) was added. After stirring the mixture 5 min, Zn(CH3COO)2•2H2O (42.7 mg; 0.194
mmol) was added to give cloudy, yellow solution, which was then stirred for a further 24 h at
room temperature. The suspension was filtered and the filtrate taken to dryness to give a
yellow solid. This was dissolved in the minimum amount of MeCN and addition of Et2O/n-
hexane (1:1, v/v, 10 ml) used to precipitate the desired product as yellow solid, recrystallized
from dichloromethane / diethyl ether mixture, yielded hairy small white crystals (42.3 mg, 32
1
% yield). H NMR (300 MHz, CDCl3) δ 7.87, 7.85 (d, 4H, H3), 7.57, 7.55, 7.53 (t, 2H, H5),
7.50, 7.48, 7.46 (t, 4H, H4), 7.26 (CDCl3), 6.30, 6.28 (d, 2H, H2), 1.26 (s, 18H, H1) (Signals
assigned according to the figure 4). TOF-MS: m/z = 471.1514 – calculated for C26H31O4Zn
(H[Zn(bpm)2]+); found: m/z = 471.1508. Absorption spectrum (CH3CN): λmax = 309 nm, εmax
= 2.18x103 M-1 cm-1. Microanalysis: calculated (%) for C26H34ZnO6: C 61.48, H 6.75, O
18.90; found: C 61.31, H 6.96, O 18.68.

2.2.7 Bis(4,4-dimethyl-1-phenylpentane-1,3-dionato)palladium(II), [Pd(bpm)2]
To a solution of bpmH (0.156 g; 0.763 mmol) in MeOH (3 mL), NaOH (38.1 mg; 0.953
mmol) was added and the mixture stirred until (1 h) a clear, yellow solution had formed. A
solution of PdCl2 (64.4 mg; 0.363 mmol) in MeOH (5 mL) was added, rapidly producing a
green precipitate, and the suspension was stirred for 24 h at r.t. The product was isolated by
filtration, washing on the filter with with EtOH and Et2O and recrystallization from
chloroform / diethyl ether system, yielded small yellow-green, needle-shaped crystals (109
1
mg, 55.2 % yield). H NMR (300 MHz, CDCl3) δ 7.86, 7.83 (d, 4H, H3); 7.50, 7.47, 7.45 (t,
2H, H5); 7.42, 7.39, 7.37 (t, 4H, H4); 7.26 (CDCl3); 6.24 (s, 2H, H2); 1.26 (s, 18H, H1)
(Signals assigned according to the figure 3). TOF-MS: mz/ = 513.1311 - calculated for
C26H31O4Pd (H[Pd(bpm)2]+); found: m/z = 513.1257. Absorption spectrum (CH3CN): λmax
=
589 nm, εmax = 90.2 M-1 cm-1 (the visible region absorption due to d-d transitions).
Microanalysis: calculated (%) for C26H30PdO4: C 60.88, H 5.90, O 12.48; found: C 60.75, H
5.74, O 12.69.
2.3 Crystallography
Crystals of Cu(bpm)2 were obtained by slow evaporation of a solution of the complex in a 1:1
dichloromethane/ethanol mixture. Crystals of Pd(bpm)2 were obtained by diffusion in a two-
phase system of the complex in chloroform as the bottom layer and diethyl ether as the top
layer in a closed vial. Diffraction data were collected at room temperature using graphite-
monochromated MoKα radiation (λ = 0.71073 Å) with the ω-scan technique. For data
reduction, UB-matrix determination and absorption correction, CrysAlisPro [27] software was
used.
Using Olex2 [28], the structures were solved by direct methods using SHELXL [29]
and refined by full-matrix least-squares against F2 utilizing SHELXL. All non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were calculated at ideal positions by
molecular geometry and refined as rigid groups. Uiso of hydrogen atoms were set as 1.2 (for
C-carriers) times Ueq of the corresponding carrier atom. Crystal and refinement data are
summarised in Table 1 and selected structural parameters are reported in Tables S1-S4. The
data have been deposited in the form of cifs with the Cambridge Crystallographic Data Centre
(CCDC), deposition numbers CCDC 1558066 (for [Cu(bpm)2]) and CCDC 1558067 (for
[Pd(bpm)2]). These can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif,
or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge

Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223
336033.
Table 1 Crystal and refinement data
Complex
[Cu(bpm)2]
C26H30CuO4
470.04
[Pd(bpm)2]
C26H30O4Pd
512.90
Empirical formula
Formula weight
Temperature/K
Crystal system
Space group
293(2)
293(2)
triclinic
monoclinic
P21/n
P-1
6.4306(7)
9.5411(12)
10.3037(9)
82.414(9)
86.448(8)
72.626(10)
597.89(12)
1
a/Å
10.6894(2)
12.0326(2)
18.8931(4)
90
b/Å
c/Å
α/°
β/°
97.597(2)
90
γ/°
Volume/Å3
2408.72(8)
4
Z
ρcalcg/cm3
1.305
1.414
μ/mm-1
0.941
0.798
247.0
F(000)
1056.0
Crystal size/mm3
Radiation
0.25 × 0.2 × 0.02
MoKα (λ = 0.71073)
6.64 to 48.798
0.2 × 0.15 × 0.1
MoKα (λ = 0.71073)
4.158 to 56.69
2Θ range for data collection/°
Index ranges
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
Final R indexes [I>=2σ (I)]
Final R indexes [all data]
Largest diff. peak/hole / e Å-3
-14 ≤ h ≤ 14, -15 ≤ k ≤ 15, -24 ≤ l ≤
-7 ≤ h ≤ 7, -11 ≤ k ≤ 10, -11 ≤ l ≤ 11
5575
25
117908
5768 [Rint = 0.0690, Rsigma = 0.0309]
5768/0/289
1873 [Rint = 0.0595, Rsigma = 0.0702]
1873/0/145
1.032
1.182
R1 = 0.0448, wR2 = 0.0943
R1 = 0.0665, wR2 = 0.1027
0.22/-0.21
R1 = 0.0454, wR2 = 0.0819
R1 = 0.0823, wR2 = 0.0895
0.39/-0.36

3. Results and discussion
3.1 Synthesis: Both the diketone and diketonate complex syntheses were based on very
familiar literature procedures for the same or closely related species and provided no
surprising results. Methanol proved to be a convenient solvent for all the complex ion
syntheses, although various other solvents such as ethanol, acetonitrile or tetrahydrofuran
could be used with equal success.
3.2 Composition, structure and stereochemistry of the complexes: Although the syntheses
were designed to produce just the neutral, homoleptic species M(bpm)n (possibly as solvates)
and were successful in this regard, it is of course possible that such species could oligomerise
[8-10]. Mass spectra of the complexes (see experimental) provided no evidence for the
presence of oligomers but the necessary use of ion association to obtain charged species
appeared in several cases to result in partial breakdown by dissociation of bpm-, so that
depolymerisation of initially weakly associated species cannot be excluded. However, it is
known [8-10], [30] that oligomerisation is inhibited by bulky substituents on the diketonate
unit and complexes of Ni(II) [30] are of particular significance in this regard, as the colour
(i.e. the absorption spectrum) of diamagnetic square-planar mononuclear species is quite
different to that of paramagnetic octahedral oligomers such as formed, for example, by
dehydration of octahedral trans-[Ni(acac)2(OH2)2]. While the square- planar form of the
Ni(II) complex of the symmetrical dipivaloylmethanate (dpm-) ligand was established long
ago by an X-ray structure determination [31] and shown to be maintained in solution [32],
there is, however, evidence [17], [24] largely based on solution molecular weight estimates,
that benzoylpivaloylmethanate is a less restrictive ligand in regard to oligomerisation. This
conclusion is supported by the present observations on the Ni(II) complex, which was isolated
as the green, octahedral [Ni(bpm)2(OH2)2] complex that, on heating, underwent dehydration
to give a green, paramagnetic species analogous to [Ni3(acac)6] (see experimental).
An alternative index of ligand-ligand interactions within the coordination sphere of a
complex can be provided in the case of an unsymmetrical ligand such as bpm- in the
distribution of isomeric forms of bis- or tris-ligand complexes. In complexes of sufficient
stability and stereochemical rigidity, this isomer distribution can be monitored by NMR
1
spectroscopy. Thus, in the present work the H spectra of the diamagnetic complexes
[Al(bpm)3], [Co(bpm)3], [Zn(bpm)2(OH2)n] and [Pd(bpm)2] provided clear evidence for the
presence of two species in each case. For the expected octahedral form of the Al(III) (Figure
2) and Co(III) (Figure S10) complexes, the C3-symmetric fac isomer should give rise to a
single peak for the methine and methyl group protons, while the C1-symmetric mer isomer

should show three peaks of equal intensity for each type of proton, enabling ready
identification of the separate isomers and therefore a direct establishment of the fac:mer ratio
(equilibrium constant). For the Al(III) and Co(III) complexes, this ratio was essentially
identical at 1.65, a value little different from that expected for the statistical distribution (2:1
fac:mer).
1
Figure 2. Comparison of the H NMR (300 MHz) spectra of bpmH (bottom, black) and the
complex [Al(bpm)3] (top, blue) in CDCl3 at room temperature.
For the expected square-planar form of [Pd(bpm)2], cis and trans isomers are possible. On
1
recording the H NMR spectrum of [Pd(bpm)2] immediately after dissolving crystals of the
bulk recrystallized complex, only one methine signal was observed and thus assigned to the
trans form, given that this is the species found in the crystal lattice. For the same sample after
an hour, signals consistent with the presence of a mixture of cis and trans isomers were
observed indicating a cis:trans ratio of 1:1.66 (Figure 3), in this case differing more
significantly from the statistical value (of 1:1). In any case, both these spectroscopic results
and the nature of the Ni(II) complex indicate that ligand-ligand interactions in complexes of
bpm- are not strong, making this ligand somewhat closer to acac- than to dpm-
(dipivaloylmethanate) in its coordinative properties.

1
Figure 3. Comparison of the H NMR (300 MHz) spectra of bpmH (bottom, black), the
complex [Pd(bpm)2] immediately after preparing the solution (middle, blue) and the complex
[Pd(bpm)2] after one hour in solution (top, green) in CDCl3 at room temperature.
Elemental analyses for the Zn(II) complex indicated the presence of two water
molecules per Zn and thus were consistent with different formulations such as
[Zn(bpm)2(OH2)]•H2O or [Zn(bpm)2(OH2)n]. The former is consistent with the square-
pyramidal structure known for the acetylacetonate analogue [33], but in either case cis and
1
trans isomeric forms would be possible. At room temperature, the H NMR spectrum of this
complex shows two methine peaks with an intensity ratio of 1:1.5 (Figure 4) but in this case
their assignment to particular isomers is not possible, although the fact that a single methine
peak is apparent for each species does not exclude the possibility that any [Zn(bpm)2(OH2)n]
species could be a cis isomer. The nature of the Zn(II) complex in chloroform solution,
however, is perhaps, more complicated than expected, as its spectrum shows a significant
temperature dependence (Figure 5) in which the higher-field methine resonance shifts upfield
and broadens considerably as the temperature is lowered. This possibly indicative of an
equilibrium between five- and six-coordinate species rather than or as well as cis/trans
isomerism.

1
Figure 4. Comparison of the H NMR (300 MHz) spectra of bpmH (bottom, black) and the
complex [Zn(bpm)2(OH2)n] immediately after preparing the solution (top, blue) in CDCl3 at
room temperature.

Figure 5. Variable Temperature NMR (600 MHz) spectra for [Zn(bpm)2(OH2)n] recorded in
CDCl3.
While electronic absorption spectra in the visible region of the coloured complexes show
d-d absorption bands at positions consistent with O-donor environments (and essentially
identical to those in their acetylacetonate analogues [8-10]), these bands are broad and not
readily resolved into separate isomer contributions. The much stronger UV absorptions seen
in all complexes are also broad and not deconvolutable but they do illustrate the dramatic
spectral changes that result from replacement of the acidic ligand proton by a metal ion
(Figure S1).
Given that at least under the conditions of synthesis and crystallisation presently applied,
all the metal ions used could be considered to be labile, slow crystallisation was considered as
a potential means of isolating a single species from a solution mixture. The paramagnetic
[Cu(bpm)2] crystals (reported before by S.V.Borisov et al. [34]) were obtained by slow
evaporation of solvent from solution of complex in a 1:1 dichloromethane/ethanol mixture.
Crystals of diamagnetic [Pd(bpm)2] were obtained by diffusion in two-phase system of the
complex in chloroform as the bottom layer and diethyl ether as the top layer in a closed vial.
The X-Ray structure of the [Cu(bpm)2] complex was the sole means of establishing its
1
stereochemistry because its paramagnetism excluded the use of H NMR spectroscopy. Both

complexes proved to be the expected bis(diketonate) species and to have a square-planar,
trans geometry in the solid state [35], [36]. X-ray structures of the complex units within their
lattices are shown below (Figure 6) and crystallographic data for both structures are
summarised in Table 1.
Figure 6. Views of (left) the unique molecular unit present in the lattice of [Cu(bpm)2] and
(right) the two inequivalent but very similar molecular units present in the lattice of
[Pd(bpm)2].
Unlike their acetylacetonate analogues [37], [38], [Cu(bpm)2] and [Pd(bpm)2] are not
isomorphous but their lattices are nonetheless quite similar, with stacked columns of
M(bpm)2 units lying parallel to a in both (Figures S2, S3). Consideration of the Hirshfeld
surfaces [39], [40] (Figures S4, S5) shows that interactions between the unique [Cu(bpm)2]
units do not exceed dispersion forces whereas interactions of the two, inequivalent
[Pd(bpm)2] units involve, in addition to dispersion, both aromatic-CH...O and aromatic-C...O
interactions that link Pd1 and Pd1' in a reciprocal manner (H5...O2' 2.670(3) ; C5...O1'
3.141(5) Å) along the a direction (Figure S6), and aliphatic-CH...C (H13E...C3 2.872(6) Å)
interactions, again reciprocal (Figure S6) that link the Pd1 units into a two-dimensional sheet
parallel to the (1 0 -1) plane (Figure S7). These differences between the Cu(II) and Pd(II)
complexes must reflect differences in the electronic influence of the two metal ions, although
the two Pd units are involved in quite different interactions, indicating that a delicate balance
of factors may be operative. An interesting feature of the slipped-stack column in the Cu(II)
complex lattice is that it places each Cu(II) symmetrically in between two phenyl rings,
indicating that there may be some axial interaction, even if it does not exceed dispersion, and
this could well explain the preference for the trans configuration of each complex unit. A
similar situation clearly does not apply in the Pd(II) complex lattice, where the preference for

the trans isomer is presumably to be associated with the weak interactions beyond dispersion
revealed in the Hirshfeld surface.
Conclusions
Despite the relatively bulky nature and different electronic character of the
substituents attached to the dionate unit of bpmH, there appears to be little preference in
regard to the isomeric form of its homoleptic complexes in solution, although individual
species can be isolated in the crystalline state. In the two cases structurally characterised, the
solid state interactions associated with the preferred isomeric form are quite different,
indicating that the bound metal ion has a significant and unique influence on the peripheral
properties of its complex. In relation to the possible use of the phenyl ring of bpm- as a site
for the introduction of an alternative metal ion binding site, it would seem that the evaluation
of the resultant complexes as heterogeneous catalysts containing a single isomeric species is
the most obvious starting point.
Appendix A. Supplementary data
CCDC 1558066 contains the supplementary crystallographic data for [Cu(bpm)2].
CCDC 1558067 contains the supplementary crystallographic data for [Pd(bpm)2].
These can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html or from
The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK;
fax: +44 1223 336033; or e-mail: deposit@ccdc.cam.ac.uk
Acknowledgments
We thank the National Centre for Research and Development for funding – LIDER
024/391/L-5/13/NCBR/2014.
Conflicts of interests
The authors declare no conflict of interests.
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4,4-dimethyl-1-phenylpentane-1,3-dione (benzoylpivaloylmethane, bpmH) has been re-
examined with selected main group and transition metal M(II) and M(III) ions. The
mixture of isomers of Al(III), Co(III), Pd(II), Zn(II), Ni(II) and Cu(II) is present in
solution. For Al(III), Co(II) and Pd(II) cis/trans equilibrium can be well described by
1NMR measurements in solution. Crystal structure determinations on Cu(II) and Pd(II)
complexes showed that only one isomer can be isolated from the mixture in solution in
the solid state. Zn-complex made us realize that water molecules also can lead to
competition between five- and six- coordinated species. Ni(II) complex is evidence that
the steric bulk of 1,3-dione substituents is not sufficient to prevent oligomerisation.