Reactivity of phosphonodithioato-dppt NiII mixed ligand complexes with halogens: First example of a metal-coordinating tribromide anion

Article abstract of DOI:10.1039/c2dt30540b

The first example of a metal complex containing a tribromide anion is presented and characterised by X-ray diffraction. Hybrid DFT calculations were used to investigate the nature of the bond in coordinating trihalides and the differences with the corresponding mono-halide complexes.

Full text of DOI:10.1039/c2dt30540b

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COMMUNICATION
Reactivity of phosphonodithioato-dppt NiII mixed ligand complexes with
halogens: first example of a metal-coordinating tribromide anion†
M. Carla Aragoni,*a Massimiliano Arca,a Susanne L. Coles (neé Huth),b Francesco A. Devillanova,a
Michael B. Hursthouse,b,c Francesco Isaiaa and Vito Lippolisa
Received 6th March 2012, Accepted 13th April 2012
DOI: 10.1039/c2dt30540b
The first example of a metal complex containing a tribromide
anion is presented and characterised by X-ray diffraction.
Hybrid DFT calculations were used to investigate the nature
of the bond in coordinating trihalides and the differences
with the corresponding mono-halide complexes.
−
Trihalides X3 (X = Cl, Br, I) are usually found as counterions
in compounds containing a variety of inorganic or organic
cations, or as building blocks of polyhalides exhibiting topolo-
gies largely determined by the size, shape, and charge of the
templating cation.1 In spite of such a variety, only few examples
−
of metal complexes containing I3 as ligand,2 and one with the
− 3
mixed trihalide BrI2
, have been prepared and structurally
characterised so far, whilst no examples of complexes featuring
−
Br3 directly bound to the metal have been reported previously.
While exploring the reactivity of trans-[Ni(MeOpdt)2·(dppt)2]
− 4
[MeOpdt: (4-CH3O–C6H4)(CH3O)PS2
]
(see ESI†) towards
elemental Br2 in CH2Cl2, a small amount of crystals of the new
compound cis-[Ni(dppt)2(Br3)2] [1; dppt: bis(5,6-diphenyl-3-(2-
pyridyl)-1,2,4-triazine)] was isolated and established by single
crystal X-ray diffraction as the first coordination compound con-
−
taining Br3 acting as a ligand towards a metal ion (Fig. 1; see
ESI†). The analogous reaction with I2, carried out in a 1 : 1 v/v
CH3OH–CHCl3 mixture, yielded trans-[Ni(dppt)2(I)2] (2;
Fig. 1). The molecular structures of 1 and 2 show the replace-
−
ment of the two phosphonodithioates MeOpdt by two Br3 and
I− ligands, cis and trans disposed around the nickel ion in 1 and
2, respectively. In the crystal structure of compound 1 the central
aDipartimento di Scienze Chimiche e Geologiche, Università degli Studi
di Cagliari, Cittadella Universitaria Monserrato, 09042 Cagliari, Italy.
E-mail: aragoni@unica.it; Fax: +39 070 6754456;
Fig. 1 Molecular view of 1 (top; ′ = 1 − x, 2 − y, +z) and 2 (bottom;
′ = 1.5 − x, 1.5 − y, 1 − z). For clarity hydrogen atoms have been
omitted. Displacement ellipsoids are drawn at 50% probability level.
Tel: +39 070 6754491
bSchool of Chemistry, University of Southampton, Highfield,
Southampton, SO17 1BJ, UK
nickel(II) ion is positioned on a 2-fold rotation axis, so that only
half of the complex molecule is present in the asymmetric unit.
The overall coordination geometry of the metal ion is pseudo-
octahedral and the two identical linear tribromide ligands [Br1–
Br2–Br3 178.07(4)°] coordinate the nickel ion with Ni–Br1–Br2
angles of 106.83(4)°. The Ni–Br1 distance [2.5772(12) Å] falls
in the range found for octahedral nickel complexes containing a
simple bromide ligand.‡ The halogen–halogen bond lengths are
different, with that involving the coordinating Br1 atom longer
than the other one [Br1–Br2 2.6760(12), Br2–Br3 2.4406(14) Å].
These bond values can be compared with those observed in the
cDepartment of Chemistry, Faculty of Science, King Abdulaziz
University, Jeddah 21588, Saudi Arabia
†Electronic supplementary information (ESI) available: Experimental
section; theoretical calculations; discussion. Figures: molecular view of
Ni(MeOpdt)2, 1(S) (S1); packing views of 1(S) (S2, S3); packing views
of 1 (S4, S5) and 2 (S6); scatter plot of Br1–Br2 versus Br2–Br3 dis-
tances for linear tribromides from a search on CSD (S7); Kohn–Sham
frontier MO’s of complexes 1(S), 2, 6–8 (S8, S9). Tables: crystal data
collections and refinements (S1) and bond lengths and angles (S2) for
1(S), 1 and 2. CCDC reference numbers 869711 and 869712. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c2dt30540b
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Scheme 1
Fig. 2 RP model scheme for hypervalent systems (left) and drawings
of the symmetric (center) and anti-symmetric (right) combinations of the
alpha KS–MO’s calculated for 1: Ψb, (a) HOMO − 33, (b) HOMO −
30; Ψnb, (c) HOMO − 9, (d) HOMO; Ψab, (e) LUMO + 3, (f) LUMO +
6; contour value 0.05 e.
discrete tribromide systems characterised structurally found in the
CSD. The halogen–halogen bond distances of 1 fall within the
range expected, thus confirming that no significant structural
changes occurred in the tribromide anions upon metal coordi-
nation (see ESI†). In the crystal lattice the tribromide anions inter-
act with phenyl hydrogen atoms of neighboring molecules, but no
soft–soft intermolecular Br⋯Br interactions are found (see ESI†).
With the aim of better understanding the different nature of
the complexes 1 and 2 obtained starting from the two different
halogens Br2 and I2, hybrid DFT calculations were performed on
the cis and trans isomers of both [Ni(dppt)2(Br3)2] (1 and 3), the
hypothetical analogues [Ni(dppt)2(I3)2] (4 and 5), along with the
cis and trans isomers of the simpler complexes [Ni(dppt)2(I)2] (6
and 2) and [Ni(dppt)2(Br)2] (7 and 8), featuring halides in place
of the trihalides (Scheme 1). The optimised bond distances and
degenerate MO’s having bonding, nonbonding, and antibonding
character. It is worth noting that the symmetric combination
leading to the non-bonding MO (Fig. 2c) shows a significant inter-
action between the two tribromide anions deriving from the
overlap between the in-phase pz-orbitals of the bromine atoms
coordinating the metal. This interaction is supported by the crystal-
lographic data since the observed distance between the two
involved bromine atoms Br1 and Br1′ is smaller than the sum of
the van der Waals radii (dBr1⋯Br1′ 3.429, 3.591, 3.70 Å, for the
experimental, calculated, and van der Waals distances).6 The elec-
tronic structure of the analogous cis-[Ni(dppt)2(I3)2] (4) closely
resembles that described above for 1. More detailed information on
the nature of the bonding between the central metal ion and the
ligands in complexes featuring coordinating monohalides (2, 6–8)
or trihalides (1, 3–5) comes from a Natural Bond Orbital (NBO)
analysis.7 A comparison between the natural charges in the two
sets of compounds shows that, although Ni–X bonds in all the
examined complexes are calculated to retain a remarkably covalent
character, they show a larger polarisation in complexes 1/3–5 than
in 2/6–8 (Table 1). Moreover, Ni–X bonds in the corresponding
complexes reflect the electronegativity of the X halogen species,
those featuring X = Br (1, 3, 7, 8) being more polarised than those
with X = I (4, 5, 6, 2, respectively). Wiberg Bond Indexes
(WBI’s)8 between the Ni and the coordinating halogens X1 follow
the same trend, indicating for complexes 1, 3–5 bond orders
smaller than for 2, 6–8 (Table 2). Finally, in agreement with the
asymmetry found structurally within the Br3− ions in 1, WBI’s cal-
culated between the central and the terminal halogen atoms X2 and
X3 are sensibly higher than those calculated for the bond X2–X1
involving the coordinating halogen atom; the sum of WBI’s relative
to each of the interhalogen systems is very close to unity (Table 2).
The different nature of the bonds only slightly reflects on the whole
molecule, and similar NBO charges on the dppt N-atoms and
WBI’s between the nickel ion and the residual ligands are found.
3
angles calculated for 1 and 2 in the ground state (3B and Ag,
respectively considering the molecules to belong to the C2 and
Ci point groups, respectively) are in very good agreement with
the experimental ones, and the geometric features of compounds
3–8 are on the whole in line with those expected. In particular,
the trihalides bound to the nickel ion are calculated to be linear
and asymmetric, with the interhalogenic bond distances invol-
ving the coordinating halogen (X1–X2) longer than the other
one (X2–X3), in agreement with the metric parameters found
structurally within the Br3− system in 1.
The total electronic energies, including Zero-Point Energy
(ZPE) corrections, calculated for the investigated compounds
show that complexes featuring coordinating trihalides are more
stable (by about 40 kcal mol−1, see ESI†) as compared to the
systems composed of the analogue halide complex plus two diha-
logen units. Moreover, cis and trans isomers for all compounds
only slightly differ in energy, indicating that the different isomer-
ism of the isolated compounds 1 and 2 cannot be explained in
terms of electronic energies only, so that packing interactions or
even the different solvent can be determining in the isolation of
one of the two isomers from the reaction mixture.
The composition of Kohn–Sham Molecular Orbitals (KS–
MO’s) calculated for 1 clearly confirms the hypervalent nature of
the coordinating tribromide ligands. In Fig. 2 the well-known
Rundle–Pimentel5 (RP) model scheme for electron-rich 3c–4e
systems and the isosurfaces of the corresponding KS–MO’s calcu-
lated for 1 are comparatively depicted. According to the RP model,
the three bromine pz-orbitals from each linear Br3− system combine
in a symmetric and anti-symmetric mode to give pairs of almost
Conclusions
cis-[Ni(dppt)2(Br3)2] represents the first case of a complex featur-
−
ing a Br3 anion coordinating a metal ion. The nature of the
6612 | Dalton Trans., 2012, 41, 6611–6613
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Table 1 Selected NBO charges (e) calculated for 1–8a
1
2
3
4
5
6
7
8
Ni
0.582
0.406
−0.494
0.597
0.494
0.509
0.414
−0.484
0.507
−0.536
0.493
−0.545
X1
X2
X3
N1
N2
N3
N4
−0.252
−0.078
−0.307
−0.296
−0.183
−0.469
−0.499
−0.284
−0.073
−0.291
−0.288
−0.199
−0.475
−0.499
−0.191
−0.089
−0.309
−0.300
−0.184
−0.470
−0.502
−0.227
−0.082
−0.286
−0.297
−0.202
−0.474
−0.505
−0.261
−0.196
−0.477
−0.479
−0.271
−0.180
−0.479
−0.485
−0.264
−0.178
−0.479
−0.480
−0.251
−0.196
−0.478
−0.470
a Numbering refers to Fig. 1. X = I (2, 4–6); Br (1, 3, 7, 8).
Table 2 Selected Wiberg bond indexes calculated for 1–8a
1
2
3
4
5
6
7
8
Ni–N1
Ni–N4
Ni–X1
X1–X2
X2–X3
0.274
0.271
0.470
0.362
0.618
0.256
0.257
0.608
0.267
0.268
0.466
0.345
0.644
0.271
0.268
0.249
0.259
0.617
0.247
0.261
0.572
0.254
0.256
0.572
0.267
0.525
0.365
0.609
0.266
0.517
0.346
0.642
a Numbering refers to Fig. 1. X = I (2, 4–6); Br (1, 3, 7, 8).
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experimental structural data and DFT calculated electronic fea-
tures, which evidence the enduring of the 3c–4e nature within the
tribromide involved in the coordination to the nickel ion. The
experimental and theoretical investigation indicates that the main
difference between the complexes containing a monohalide X−
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stable species and could be possibly isolated, and a determining
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product is suggested. Works are in progress in our laboratory to
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amounts in order to better understand their chemical and physical
properties and to explore their reactivity and the possibility to
adopt them as supramolecular building blocks.
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Notes and references
−
of compounds containing Br3 anions, and were assumed to direct
the assembly of tribromide ions and dibromine molecules into zigzag
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‡CSD (v. 5.33, 2012) search made on octahedral nickel complexes con-
taining a bromide ligand (R < 0.1): Ni–Br bond distances ranging
between 2.308 and 3.078 Å, mean value = 2.579 Å (calculated on 132
structures, 293 fragments).
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This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 6611–6613 | 6613