Photo-isomerisation of alkenyl complexes of platinum(ii): Structural, spectroscopic, kinetic and computational investigations

Article abstract of DOI:10.1039/c3dt32354d

In this work UVA and blue light have been used to study photo-isomerisation about the CC double bond in complexes of the type [PtCl(-CHCHAr)(tmeda)] [Ar = C₆H₅, (E)-2a; 4-CH₃O-C₆H ₄, (E)-2b; 3-NO₂-C₆H₄, (E)-2c; and 3-CH₃O-C₆H₄, (E)-2d]. The progress of the reaction has been monitored by NMR spectroscopy following irradiation of the NMR sample. The NMR data have been complemented with X-ray diffractometric analysis of compounds (E)-2a-c and (Z)-2a. The kinetic data clearly indicate that a monomolecular mechanism is operating with the energy of the irradiating light influencing the rate of isomerisation but not the equilibrium composition, which is only slightly in favour of the Z isomer. DFT and TD-DFT theoretical investigations have been carried out to elucidate the nature of the main electronic transitions in the UV-Vis region and the mechanism of the photo-isomerisation reaction appears to proceed through a CC bond twist process similar to that involved in purely organic molecules such as stilbene. In the Z isomer, one ortho proton of the phenyl group can come close to platinum (Pt...H_ortho distance of 2.632 A in (Z)-2a). In the case of 2c, the difference in chemical shift between the two ortho protons varies from 3.30 ppm in the Z isomer, where interaction with Pt is possible, to 0.60 ppm in the E isomer, where such interaction cannot take place. The analysis of the DFT orbitals indicates that the most shifted H_ortho is that with a greater positive charge, pointing to an H-bond type of interaction. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3dt32354d

Dalton
Transactions
View Article Online
View Journal | View Issue
PAPER
Photo-isomerisation of alkenyl complexes of platinum(II):
structural, spectroscopic, kinetic and computational
investigations†
Cite this: Dalton Trans., 2013, 42, 6840
Carmen R. Barone,a Cecilia Coletti,b Ruth J. McQuitty,c Nicola J. Farrer,c
Giuseppe Lorusso,a Luciana Maresca,*a Alessandro Marrone,b Giovanni Natile,a
Concetta Pacifico,a Simon Parsons,d Nazzareno Re,b Peter J. Sadlerc and
Fraser J. Whited
In this work UVA and blue light have been used to study photo-isomerisation about the CvC double
bond in complexes of the type [PtCl(–CHvCHAr)(tmeda)] [Ar = C6H5, (E)-2a; 4-CH3O–C6H4, (E)-2b;
3-NO2–C6H4, (E)-2c; and 3-CH3O–C6H4, (E)-2d]. The progress of the reaction has been monitored by NMR
spectroscopy following irradiation of the NMR sample. The NMR data have been complemented with
X-ray diffractometric analysis of compounds (E)-2a–c and (Z)-2a. The kinetic data clearly indicate that a
monomolecular mechanism is operating with the energy of the irradiating light influencing the rate of
isomerisation but not the equilibrium composition, which is only slightly in favour of the Z isomer. DFT
and TD-DFT theoretical investigations have been carried out to elucidate the nature of the main elec-
tronic transitions in the UV-Vis region and the mechanism of the photo-isomerisation reaction appears to
proceed through a CvC bond twist process similar to that involved in purely organic molecules such as
stilbene. In the Z isomer, one ortho proton of the phenyl group can come close to platinum (Pt⋯Hortho
distance of 2.632 Å in (Z)-2a). In the case of 2c, the difference in chemical shift between the two ortho
protons varies from 3.30 ppm in the Z isomer, where interaction with Pt is possible, to 0.60 ppm in the
E isomer, where such interaction cannot take place. The analysis of the DFT orbitals indicates that the
most shifted Hortho is that with a greater positive charge, pointing to an H-bond type of interaction.
Received 5th October 2012,
Accepted 20th February 2013
DOI: 10.1039/c3dt32354d
www.rsc.org/dalton
into a metal–hydrogen,4–10 metal–carbon,11,12 or even metal–
Introduction
sulphur bond,13 (iii) oxidative addition to low valent metal
Metal alkenyl complexes of late transition elements have been species.14–17 In the formation of metal alkenyl complexes, the
known for several decades and their importance is associated presence of substituents on the unsaturated carbons can lead
with metal-mediated reactions involving unsaturated hydro- to different isomers whose yield depends upon their kinetic
carbons.1 With reference to species having the metal directly preference
and
subsequent
possible
isomerisation
bound to a vinyl carbon (also called metal–vinyl compounds), process.2,6,8,11 An interesting example of how the regio-selecti-
they are generally obtained as a result of: (i) nucleophilic vity can be governed by steric effects has been reported.18
addition to a coordinated alkyne or a metal vinylidene
Some years ago we found that alkenyl platinum(II) species
complex,2,3 (ii) insertion of an alkyne, or sometimes an allene, could rapidly be formed by deprotonation of a coordinated
styrene in cationic complexes of the type [PtCl(η2-CH2vCHAr)-
(tmeda)]+, 1, (tmeda
= N,N,N′,N′-tetramethyl-1,2-diamino-
aDipartimento di Chimica, Università degli Studi “Aldo Moro” di Bari, Via
E. Orabona 4, I-70125 Bari, Italy. E-mail: luciana.maresca@uniba.it
bDipartimento di Farmacia, Università degli Studi “Gabriele d’Annunzio”, Via dei
Vestini 31, I-06100 Chieti, Italy
cDepartment of Chemistry, University of Warwick, Coventry CV4 7AL, UK
dSchool of Chemistry, The University of Edinburgh, King’s Building, West Mains
ethane). The reaction of proton abstraction, promoted by
poorly-coordinating bases (such as tertiary aliphatic amines or
inorganic carbonate), was completely regio-selective and led to
the unique formation of the E isomer [PtCl(E-CHvCHAr)-
(tmeda)] [Ar = C6H5, (E)-2a; 4-CH3O–C6H4, (E)-2b; and 3-NO2–
C6H4, (E)-2c]. The observed selectivity appears to have a kinetic
origin since in the cationic precursor the abstracted proton (the
one trans to the aromatic ring) is the most exposed to the solvent
and, hence, the most easily accessed by the attacking base.19
Road, Scotland, EH9 3JJ, UK
†Electronic supplementary information (ESI) available: Pdf file: Fig. S1–S8 and
Tables S1–S5. CCDC 904374 [(E)-2a], 904375 [(Z)-2a], 904377 [(E)-2b] and 904378
[(E)-2c]. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c3dt32354d
6840 | Dalton Trans., 2013, 42, 6840–6851
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
In chlorinated solvents, the E isomer partially isomerises interaction between the platinum centre and the phenyl ortho
into the corresponding Z form. Thus we decided to investigate protons in the Z isomer.
the mechanism of the isomerisation process to see if the pres-
ence of a metal atom (platinum in the present case) directly
bound to an olefinic carbon would influence a transformation
very much alike to the classic cis–trans isomerisation of a stil-
Results
bene molecule. In our system the isomerisation is extremely The complexes
slow in the dark but is catalyzed by visible light. In the case of
(E)-2a and (E)-2b, some unidentified decomposition products
were formed in solution over prolonged period of time, the for-
mation of which was less evident in the photo-activated
The complexes [PtCl(E-CHvCHAr)(tmeda)] (Ar = C6H5, (E)-2a;
4-CH3O–C6H4, (E)-2b; 3-NO2–C6H4, (E)-2c; and 3-CH3O–C6H4,
(E)-2d, see Chart 1) were prepared by the reaction of triethyl-
amine with the corresponding parent cationic complex as pre-
process. We, therefore, decided to study the E–Z isomerisation
viously described for compounds (E)-2a–c.19 Complex (E)-2d is
process using a broad band light source.
new and we decided to also include in our series the complex
containing 3-CH3O-styryl since a meta-methoxy substituent on
the phenyl ring would behave as a pure, though weak, elec-
UV and visible radiation has been widely used in chemistry
to initiate chemical reactions or to carry out photochemical
processes. Particularly challenging is the possibility of indu-
cing specific reactions by using excitation radiation of a given
tron-withdrawing group since its +M effect does not affect the
ipso carbon atom.22 Thus 2d can be placed somewhat in
wavelength and, indeed, lasers and other light sources have
between 2a and 2c, as far as the electronic characteristics are
been used not only in conventional photochemistry, but also
concerned.
as a mean to induce specific effects and produce selective
Complexes (E)-2c and (E)-2d (both containing an electron-
withdrawing group on the phenyl ring) appear to be more
stable in chloroform solution than (E)-2a and (E)-2b and, after
exposure to sunlight for several hours, a E/Z ratio close to
photochemical reactions.20
In this work UVA and blue light have been used to study a
conventional process: the photo-isomerisation about a CvC
double bond. Thus, the progress of the reaction has been
50 : 50 was reached without any detectable formation of
monitored by NMR spectroscopy following irradiation of the
decomposition products.
NMR sample.21 This type of isomerisation generally causes
only minor changes in the UV-Vis spectrum; in contrast, large
Photo-isomerisation of (E)-2a and (E)-2b
differences can be observed in the chemical shifts of some
NMR signals belonging to the two isomers, allowing the ratio Compounds 2 were obtained as unique reaction products
between isomers to be precisely evaluated by recording the (1H NMR and elemental analyses); however, ESI-MS spectra,
NMR spectrum (1D 1H NMR in the present case).
recorded under low cone voltage, of samples of (E)-2a and
A second focus of the present work was a better under- (E)-2b, prepared according to the reported procedures (see
standing of the type of interaction occurring between the plati- ref. 19), did show the presence of trace amounts (undetectable
1
num centre and the phenyl ortho protons in the Z isomer in H NMR and eluded by elemental analysis) of [PtCl{η1-CH-
which, as in the case of compound (Z)-2c, can cause a down- (Ar)CH2NEt3}(tmeda)]+ and/or isomeric [PtCl{η1-CH2CH(Ar)-
field shift greater than 3 ppm and ten times greater for one NEt3}(tmeda)]+, which had been formed by amine (NEt3)
ortho proton over the other. The NMR and kinetic data have addition to the η2 olefin in the parent cationic complex [PtCl
been complemented with the X-ray diffractometric analysis of {η2-CH2vCH(Ar)}(tmeda)]+. To avoid the possibility that these
compounds (E)-2a–c and (Z)-2a and with DFT and TD-DFT impurities, during the irradiation process, could affect product
(time-dependent density functional theory) theoretical investi- formation, (E)-2a and (E)-2b were purified by conventional
gations which allowed us to shed light on the mechanism of column chromatography which eliminated the cationic species
the photo-isomerisation process as well as on the nature of the as confirmed by ESI-MS runs (always under low cone voltage).
Chart 1 Sketches of isomers E and Z with identification of alkenyl protons and numbering of the complexes.
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 6840–6851 | 6841

View Article Online
Paper
Absorption (UV-Vis) spectra in CDCl3 (Fig. S1†) reveal sig-
Dalton Transactions
nificant absorption bands for (E)-2a and (E)-2b at wavelengths
<380 nm, with an absorption maximum close to 280 nm which
has been assigned by the TD-DFT calculations to an essentially
π–π* transition (see below). Moreover, the absorption spectra
in CDCl3 do not show significant changes upon E–Z isomerisa-
tion. However, the E and Z isomers of a given alkenyl have well
1
separated H NMR signals which allow an accurate evaluation
of their relative concentrations, in particular the signal of the
high-field vinylic proton in the E isomers (HβE) and that of the
low field ortho-phenyl proton in the Z form (HoZ) (see Chart 1
and Fig. 1 for alkenyl proton identification scheme). Therefore
we decided to follow the isomerisation process by placing the
samples in NMR tubes and irradiating with continuous wave
(CW) light of wave length 351 or 420 nm. These wavelengths
fall in the tail of the UV absorption band, ensuring a low
absorption and a sufficient penetration of the radiation into
the NMR sample. For the same reason (penetration of the radi-
ation into the sample), the complex concentration was also
kept rather low (8–13 mM).
For both compounds, (E)-2a and (E)-2b, an E/Z ratio of ca.
38/62 was reached at equilibrium. The energy of the irradiating
light did not affect the equilibrium composition, but influ-
enced the rate of isomerisation. The photostationary state
(PSS), defined as the state for which the forward and the back
reactions have equal rates, was reached after ca. 30 and
120 min of irradiation at 351 and 420 nm, respectively. On the
basis of a first order transformation of one isomer into the
other, the integrated rate law for the PSS model is given by eqn
(1) which represents the rate law for the reaction:
Fig. 2 Photo-isomerisation of (E)-2b (λ = 351 nm). Top: plot of percentage of
Z isomer (Z%) versus time of irradiation (s). Bottom: plot of −ln{([Z]PSS − [Z]t)/
[Z]PSS} against time (s).
The experimental data show a good fit for eqn (1) and ln{[Z]PSS
− [Z]t)/[Z]PSS} plotted against t yields a straight line of slope −k.
Individual values of k1 and k−1 could then be calculated taking
into account eqn (2).
k1
½ZꢀPSS
½EꢀPSS
k1ðhν; ΔÞ
¼
ð2Þ
ꢁꢁꢁꢁꢁꢁ*
)ꢁꢁꢁꢁꢁꢁ
½Eꢀ
½Zꢀ
k
ꢁ1
k
ꢁ1ðhν; ΔÞ
ð1Þ
½Zꢀt ¼ ½ZꢀPSS ꢁ ½ZꢀPSSexpðꢁktÞ and
lnfð½ZꢀPSS ꢁ ½ZꢀtÞ=½ZꢀPSSg ¼ ꢁkt
The equilibrium constant, K = k1/k−1, was 1.66 0.08 and the
kinetic constants were close to 10−3 s−1, at λ = 351 nm ((E)-2a: k1 =
1.12( 0.06) × 10−3 s−1 and k−1 = 0.68( 0.03) × 10−3 s−1; (E)-2b:
where k ¼ k1 þ k
ꢁ1
k1 = 1.37( 0.07) × 10−3 s−1 and k−1 = 0.83( 0.04) × 10−3 s−1
)
(Fig. 2 and S2†), and nearly one order of magnitude lower at
λ = 420 nm.
X-ray structure
Crystals suitable for X-ray analysis were obtained for (E)-2a–c
(Fig. S3–S5 and Table S1†) and (Z)-2a (Fig. 3). Relevant bond
distances and angles are reported in Table 1, where calculated
values for E and Z isomers of 2a–d have been included (see
Quantum chemical calculations, later on in this section). In all
molecules, platinum(II) has the usual square-planar coordi-
nation geometry, when the alkenyl carbon, the chloride, and
the tmeda nitrogen atoms are taken into account. As expected,
the chelated diamine penta-atomic ring is puckered to accom-
modate the tetrahedral geometry of the nitrogen and carbon
atoms. The distances between the metal centre and the two
tmeda nitrogen atoms are significantly different, owing to the
stronger trans-influence of the σ carbon with respect to the
chlorido ligand (the Pt–N(2) bond trans to the alkenyl carbon
Fig. 1 Aromatic region of the 1H NMR spectrum of 2b in CDCl3 (13 mM). Top:
prior to irradiation (pure E isomer). Bottom: after irradiation to the PSS (λ =
351 nm, 35 min); apart from resonances of the Z isomer, residual signals of the
E form are also indicated (•). The final spectrum suggests that there may be a
small amount of photo-decomposition (*).
6842 | Dalton Trans., 2013, 42, 6840–6851
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
iminoether complexes of platinum.23 An oxygen atom adjacent
to a double bond often contributes to the π system.
In this context it is worth noting the case of compound (Z)-
2a (Fig. 3) for which the dihedral angle of the phenyl ring with
respect to the olefin plane is very small (φ2 = 5°) while the
olefin and the coordination planes are almost orthogonal (φ1 =
87°). In such a situation the phenyl ring is also practically per-
pendicular to the metal coordination plane (making an angle
of 89°). This arrangement brings one ortho-proton of the aro-
matic ring in close contact with the platinum atom: the
Pt⋯Hortho distance being 2.632 Å (Pt⋯Cortho = 3.4012(4) Å and
Pt⋯Hortho–Cortho angle = 138°). Moreover, in (Z)-2a the plati-
num atom has a pyramidal distortion and is displaced by
0.0343(1) Å from the coordination plane towards the interact-
ing hydrogen. In all other complexes investigated here the
metal atom either exhibits a tetrahedral distortion [(E)-2a,b], or
it lies approximately on the coordination plane [displaced by
only 0.0034(4) Å in (E)-2c].
Fig. 3 Drawing of complex [PtCl{Z-CHvCHC6H5}(tmeda)], (Z)-2a, showing the
position of the ortho hydrogen apical on the Pt coordination plane (Pt⋯H2ortho
distance = 2.632 Å and Pt⋯Hortho–Cortho (ψ) angle = 138°). The ellipsoids enclose
30% probability.
NMR spectroscopy
being, on average, 0.1 Å longer than that trans to the chlorido
ligand). The Cα–Cβ and Cβ–Cipso bond lengths are typical of a
double and a single bond, respectively; indicating, though,
that there is little conjugation between the olefin double bond
and the π system of the phenyl ring.
Significant NMR parameters for both E and Z isomers of 2a–d
are summarized in Table 3. In the H NMR spectra of all com-
pounds, the methyl groups on the tmeda nitrogen atoms give
1
rise to two sharp singlets with significant differences in their
3
chemical shifts and JPt,H coupling constants. These differ-
There are three independent molecules in the asymmetric
unit of (E)-2a (Fig. S3†); the only differences among them are
in the values of the dihedral angle between the coordination
and the olefin plane (φ1) and between the olefin plane and the
phenyl ring (φ2) (the coordination plane is the mean plane
defined by the four donor atoms and the olefin plane is that
containing the Pt–Cα–Cβ–Cipso alkenyl back-bone). The values
of φ1 and φ2, for all structurally characterized molecules, are
reported in Table 2, where also the calculated values of the two
angles for E and Z isomers of 2a–d have been included.
The angle φ1 in most cases lies above 80° and therefore
close to the value of 90° which should minimize the steric
interactions between the alkenyl and the cis ligands. In two
cases, presumably depending upon crystal packing effects, it
exhibits a somewhat lower value, being ∼70° in (E)-2c and
∼50° in one of the molecules of (E)-2a.
The angle φ2, which gives a measure of the co-planarity of
the alkenyl moiety and the phenyl ring is usually small (<26°)
and in two cases is not higher than 6°. However, there is no
significant evidence of conjugation between the olefin double
bond and the aromatic π system judging from the values of the
Cα–Cβ and Cβ–Cipso bond lengths.
In (E)-2b, however, there is conjugation between the oxygen
lone pair of the methoxy substituent on C4 and the phenyl π
system, which is illustrated by the short value of the Me–O and
O–Ph bonds (1.437(3) and 1.385(4) Å, respectively) and the
large value of the Me–O–Ph angle (116.6(3)°). Seemingly the
oxygen atom has sp2 hybridization and the O–Ph bond some
double-bond character. A similar situation was found in the
parent cationic complex [PtCl{η2-CH2vCH(C6H4-4-OMe)}-
(tmeda)]+,22 and it was also previously described for some
ences are caused by the different trans influences of the two
ligands opposite to the two nitrogens. Thus, the two methyls
on each nitrogen are magnetically equivalent implying fast
rotation around the Pt–Cα bond. This feature is fully supported
by the variability of the φ1 angles found in the solid state struc-
ture of (E)-2a, described in the preceding section.
On going from the E to the Z isomers, the most significant
differences observed in the 1H NMR spectra are: (i) the smaller
3JH,H between the two vinyl protons which in the Z isomer are
cis disposed with respect to the olefinic double bond, and (ii)
the chemical shifts of the two ortho protons of the phenyl ring
which undergo a considerable downfield shift. When the two
ortho protons are equivalent (this is the case of compounds 2a
and 2b where the substituent on the phenyl ring is in the para
position), the shift, in the Z isomer, is ∼1.3 ppm downfield
with respect to the corresponding resonance in the E form.
When the two ortho protons are not equivalent (this is the case
of compounds 2c and 2d where the substituent on the phenyl
ring is in the meta position), the downfield shifts are 1.61 and
1.02 ppm for compound (Z)-2d and 3.0 and 0.30 ppm for com-
pound (Z)-2c (in both cases the more shifted proton is that
close to the substituent). Therefore, the difference in chemical
shift between the two ortho protons is much more marked in
(Z)-2c (nitro substituent) than in (Z)-2d (methoxy substituent)
with a downfield shift which is 10 times greater for one ortho
proton than for the other. It is possible to conclude that in the
case of (Z)-2c the residence time in the proximity of the metal
atom (which causes the downfield shift) is much greater for
one proton than for the other. This is most likely a conse-
quence of the electron-withdrawing properties of the nitro
group which increases the partial positive charge on the
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 6840–6851 | 6843

View Article Online
Paper
Dalton Transactions
Table 1 Selected bond lengths (Å) and angles (°) for the given alkenyl complexes. Calculated values are shown in square brackets. Numbering of atoms as in Fig. 3
(E)-2a
(Z)-2a
(E)-2b
(Z)-2b
(E)-2c
(Z)-2c
(E)-2d
(Z)-2d
Pt1–N1
2.068(4)
2.074(4)
2.072(4)
[2.107]
2.074(7)
2.069(3)
2.071(7)
[2.106]
[2.107]
[2.108]
[2.223]
[2.284]
[1.975]
[1.339]
[1.467]
[83.95]
[89.89]
[91.87]
[94.26]
[127.72]
[126.73]
[2.107]
[2.109]
[2.285]
[2.281]
[1.971]
[1.340]
[1.464]
[84.00]
[90.91]
[91.80]
[93.24]
[129.78]
[128.11]
[2.107]
[2.222]
[2.283]
[1.973]
[1.339]
[1.467]
[84.04]
[90.44]
[91.85]
[93.67]
[127.20]
[127.06]
[2.106]
[2.221]
[2.287]
[1.974]
[1.341]
[1.467]
[84.00]
[90.52]
[91.66]
[93.77]
[131.90]
[129.85]
Pt1–N2
2.164(4)
2.185(4)
2.180(4)
[2.222]
2.298(1)
2.305(1)
2.302(1)
[2.284]
1.984(4)
1.983(5)
1.977(4)
[1.974]
1.315(7)
1.322(7)
1.330(6)
[1.339]
1.479(6)
1.481(7)
1.475(6)
[1.467]
84.8(2)
84.4(2)
84.5(2)
[84.03]
90.4(1)
92.2(1)
91.7(1)
[90.17]
92.5(2)
91.8(1)
93.0(1)
[91.85]
2.161(7)
2.179(3)
2.160(7)
[2.223]
2.305(2)
[2.224]
2.3021(8)
[2.282]
2.304(2)
Pt1–Cl1
[2.288]
1.977(9)
[2.283]
1.992(3)
[2.282]
1.990(9)
Pt1–C8
[1.975]
1.34(1)
[1.975]
1.312(5)
[1.971]
1.33(1)
C7–C8
[1.341]
1.48(1)
[1.339]
1.479(4)
[1.339]
1.46(1)
C1–C7
[1.467]
84.5(3)
[1.467]
84.05(1)
[1.464]
83.8(3)
N1–Pt1–N2
Cl1–Pt1–C8
N2–Pt1–Cl1
C8–Pt1–N1
C7–C8–Pt1
C8–C7–C1
[83.98]
89.8(3)
[84.04]
89.9(1)
[84.04]
89.3(3)
[90.45]
91.9(2)
[90.29]
91.8(1)
[90.71]
92.5(2)
[91.76]
93.9(3)
[91.85]
93.8(1)
[91.80]
94.3(3)
92.4(2)
91.9(2)
90.8(2)
[93.96]
127.4(4)
131.7(4)
128.1(4)
[127.13]
127.5(5)
124.4(5)
127.1(4)
[127.06]
[93.78]
132.9(7)
[93.83]
132.7(3)
[93.43]
129.6(7)
[131.5]
129.5(8)
[127.03]
125.9(3)
[126.98]
127.0(9)
[129.0]
[127.43]
[126.66]
Table 2 Experimental and calculated dihedral angles (°) between coordination
and alkenyl planes (φ1) and between alkenyl and phenyl planes (φ2). Calculated
values are in square brackets
Complex
(E)-2a
φ1
φ2
Table 3 Chemical shifts (ppm) of phenyl protons in compounds 2 (CDCl3,
291 K)
49.7–86.1–89.8
[78.2]
83.9
25.8–5.8–23.2
[10.2]
19.9
Complex
Hortho
Hmeta
Hpara
(E)-2b
[78.2]
72.3
[88.7]
[79.6]
87.3
[83.7]
[81.7]
[83.8]
[80.2]
[2.8]
15.1
[7.3]
[1.9]
(E)-2c
(E)-2a
(Z)-2a
(E)-2b
(Z)-2b
(E)-2c
(Z)-2c
(E)-2d
(Z)-2d
7.27
8.50
7.18
8.43
8.10, 7.50
11.10, 7.80
6.85, 6.88
8.46, 7.90
7.18
7.24
6.74
6.80
7.30
7.35
7.10
7.13
7.01
7.10
—
(E)-2d
(Z)-2a
5.1
—
[26.9]
[25.4]
[18.1]
[8.1]
7.85
7.95
6.59
6.70
(Z)-2b
(Z)-2c
(Z)-2d
6844 | Dalton Trans., 2013, 42, 6840–6851
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
adjacent ortho proton. As a consequence, this proton can form the Z isomer is particularly high for the H atom next to the
a hydrogen bond type interaction with the metal centre much electron-withdrawing –NO2 substituent in (Z)-2c. Parallel to
greater than that of the other ortho proton which is much this high charge increase, we observe a shorter Pt⋯Hortho dis-
further from the nitro group. It is also to be noted that the tance, 2.461 Å in (Z)-2c, to be compared with 2.738 Å in (Z)-2a;
mean downfield shift of the ortho protons in the Z isomer, both effects being diagnostic of a strong Pt⋯Hortho hydrogen
with respect to the E isomer, is in the range 1.23–1.31 ppm in bond interaction. The presence of such an interaction is
three out of the four complexes examined and becomes further supported by the comparison of the relative energies of
1.65 ppm in the nitro-derivative, further evidence of a strong the two possible conformers of the Z isomers (with either of
Pt⋯H interaction in the last case.
the two non-equivalent ortho hydrogens interacting with the
platinum centre). In (Z)-2c the conformer A, where the metal
atom interacts with the Hortho closer to the electron-withdraw-
ing –NO2 substituent, is significantly lower in energy (by 2.9 kJ
mol−1; 7.0 kJ mol−1 in gas phase) than conformer B; while in
(Z)-2d the two conformers are almost isoenergetic (they differ
by only 0.2 kJ mol−1; 0.7 kJ mol−1 in the gas phase). The larger
stability of conformer A in (Z)-2c also explains why the differ-
ence in chemical shift between the two ortho protons is so
marked (Δδ of 3.30 in (Z)-2c to be compared with a Δδ of
0.56 ppm in (Z)-2d, Table 3) with the proton closer to the sub-
stituent shifting at lower field. Indeed, in the case of (Z)-2c
conformer A, where the platinum atom interacts with the most
positively-charged ortho hydrogen, is much more populated
than conformer B; in contrast, in the case of (Z)-2d the two
conformers are almost equally populated.
The nature of the Pt⋯Hortho interaction in the Z isomers
was also investigated by performing a natural bond orbital
(NBO) analysis which, through a perturbation theory approach
in terms of bond orbitals, allows unravelling of specific donor–
acceptor interactions involving metal centred orbitals (empty
or filled) and two-centred C–H orbitals (bonding or antibond-
ing) (Table S4†). The analysis shows that the most energetically
important contribution to the Pt⋯Hortho interaction is a
donation from the filled dz2 orbital of the metal to the empty
σ* orbital of C–Hortho. Such an interaction has already been
proposed as the main theoretical feature of M⋯H–C hydrogen
bond-like interactions.24 An additional but minor donation
can come from a filled π(Pt–Cl) orbital (comprising an impor-
tant metal dxz component) and a chlorine lone pair to the
empty σ*(C–Hortho) antibonding orbital. Moreover, the metal
Quantum chemical calculations
Ground-state geometries. Geometry optimizations were
carried out for the ground-state electronic structures of all the
considered E and Z isomers of 2a–d and the main geometrical
parameters are reported in Table 1. All calculations have been
performed with the PBE0 exchange-correlation functional,
which gave the best geometrical and energetic results in a pre-
liminary study assessing several functionals, including B3LYP,
BH-HLYP, PBE and PBE0 (see Table S2† and Computational
methods in the Experimental section). The predicted ground-
state geometries are in good agreement with those derived
from the single-crystal X-ray diffraction study, with bond
lengths within 0.05 Å and bond angles within 5°, as indicated
by the data in Table 1. Slightly lower agreement is observed for
the dihedral angles φ1 and φ2 but these angles are determined
by crystal packing effects (Table 2).
Interaction between platinum and the phenyl ortho hydro-
gens in the Z isomer. To better understand the types of inter-
actions occurring between the platinum centre and the phenyl
ortho hydrogens in the Z isomer, we compared the Mulliken
atomic charges, on the Pt and the ortho H atoms of the phenyl
ring, and the Pt⋯Hortho distances in the Z isomers (Table S3†).
Two different charges are reported for compounds 2c and 2d,
where the substituent on the phenyl ring is in the meta pos-
ition and the two hydrogens are not equivalent (one being
ortho and the other para to the substituent). Correspondingly,
two different Pt⋯Hortho distances are reported for (Z)-2c and
(Z)-2d, one for each of the two possible conformations with
either of the two non equivalent ortho hydrogens interacting
with the platinum centre (A and B in Chart 2).
d
z2/dxz to σ*(C–Hortho) donation increases on going from (Z)-2a,
(Z)-2b or (Z)-2d to (Z)-2c and is significantly higher for confor-
mer A, where the most positively-charged Hortho is closer to the
Pt atom, further confirming the hydrogen-bond nature of the
Pt⋯Hortho interaction.
A marked increase of the charge on the Hortho is observed
on going from the E to the Z isomer. This charge increase in
E/Z photo-isomerisation pathway. The time-dependent
density functional theory (TD-DFT) method was used to esti-
mate the nature and energies of the spectroscopically accessi-
ble excited states. The lowest 20 singlet states for each
compound were probed using this method. A full list of elec-
tronic transitions, transition energies, and oscillator strengths
is presented in Table S5.† The calculated transitions for com-
pound (E)2-a are superimposed on the UV-Vis spectrum in
Fig. S6.† Two main allowed electronic transitions contribute to
the lowest energy absorption band centred at ca. 280 nm, one
falling at 260 nm (S6) and the other at 277 nm (S4). The most
intense transition at 277 nm has a predominantly intra-ligand
Chart 2 Sketches of the two possible conformers of (Z)-2c and (Z)-2d.
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 6840–6851 | 6845

View Article Online
Paper
Dalton Transactions
π → π* alkenyl character, as shown by the involved molecular not the most acidic one which, in free styrene, is that on the
orbitals reported in Fig. S7,† and corresponds to the HOMO → carbon bearing the phenyl group.28 The pivotal role of
LUMO transition. The second most intense transition at the steric bulk in this type of reaction is also supported by the
260 nm also has an intra-ligand character, but with a mainly observation that, in cationic complexes of type 1, the addition
π(CvC) → π*(phenyl) contribution. Three much weaker tran- of a tertiary aminic function to the olefin has been observed
sitions of lower energy, falling at 286 (S3), 290 (S2) and 309 (S1) exclusively in the case of un-substituted ethene29 and that, in
nm, with mainly π(CvC) → d or d → d character, contribute to the case of NHEt2, the product of addition to the cationic com-
the absorption shoulder around 300 nm. Note that the two plexes 1a and 1b sees, at equilibrium, a large preference for
most intense transitions of (E)-2a, S4 and S6, correspond to the the anti-Markovnikov isomer.27
two lowest excited states of stilbene and have mainly the same
character, respectively π(CvC) → π*(CvC) and π(CvC) → π*
Solid-state and solution structure of the alkenyl complexes
(phenyl), and similar excitation energies.
X-ray parameters of (E)-2a–c and (Z)-2a are in agreement with
To shed some light on the first steps along the E/Z photo- literature data for platinum alkenyl complexes.30,31 In our
isomerisation pathway, a TD-DFT relaxed scan of the potential series of complexes the metal-to-carbon distances are just
energy surface (PES) for the excited S4 state of (E)-2a was per- below 2 Å and are in line with those reported for other plati-
formed as a function of the Pt–CαvCβ–Hβ torsion angle num-to-carbon σ bonds opposite a donor of similar trans influ-
around the alkenyl CvC double bond (ϕ varied from 180 to ence (an aminic nitrogen in our case).30 Similarly, the trans
105°). The results are reported in Fig. S8† and show how, on influence of an alkenyl ligand is found to be practically identi-
decreasing ϕ from 180° to 105°, there is a slow decrease of S4 cal to that of an alkyl group (Pt–N distances trans to Pt–Cα
which intersects the excited states S1–S3 and becomes the (Table 1) in the range 2.16–2.18 Å as found for the bond
lowest excited state for ϕ below ca. 150°. Angles below 105° between platinum and the tmeda nitrogen trans to
a
were not considered due to the well-known instability of the sp3-hybridized carbon atom).27 The lack of conjugation
TD-DFT approach near the conical intersection with the between the vinyl group and the π system of an aromatic sub-
ground state. This picture was confirmed by a full geometrical stituent appears to be quite general31 notwithstanding the
optimization of the lowest triplet state, not showing any stab- quasi co-planarity of the alkenyl and phenyl moieties observed
ility problem, which leads to the expected structure with ϕ = in several cases in the solid state. In contrast, there is
90° and a broken alkenyl CvC double bond.
conjugation between the oxygen lone-pair of a methoxy substi-
Thus the obtained results suggest, for the E/Z photo-isomer- tuent and the phenyl π-system fully confirming previous
isation process, the traditional CvC bond twist mechanism observations.22,23
characterized for the analogous purely organic stilbene mol-
Of particular interest is the very short Pt⋯Hortho distance
ecule.25 Upon photo-excitation, the excited state relaxes out of observed in compound (Z)-2a (2.632 Å), accompanied by a
the FC region toward partially twisted structures of the pyramidal distortion of the square-planar coordination and a
E-isomer on the S4 surface and reaches the S4/S0 conical inter- significant displacement of the platinum atom from the
section, thus isomerizing to the ground state Z-isomer. The coordination plane towards the phenyl ortho proton, while the
presence of three curve crossings with S1–S3 could slow down Pt⋯H–C angle (ψ) is ca. 140°. The values of M⋯H distance
the relaxation to the conical intersection with S0, thus allowing and ψ angle have been assumed as useful criteria to dis-
part of the excited state population to quickly decay back to tinguish between agostic and non-agostic M⋯H–X
the ground state via fluorescence. However the latter possi- interactions.24,32–35 In particular, agostic interactions (which
bility was not investigated since it cannot be adequately involve two electrons and three centres)32 are characterized by
approached with the TD-DFT method.
a relatively short M⋯H distance (1.8–2.3 Å) and a ψ angle value
in the range 90–140°. Unlike agostic interactions, hydrogen
bond interactions (which involve four electrons and three
a relatively longer M⋯H distance
(2.3–2.9 Å) and a larger ψ angle which can range from
160–180° for stronger bonds to 140–160° for weaker bonds,
centres)34,35 exhibit
Discussion
Deprotonation versus nucleophilic addition
The formation of alkenyl complexes by deprotonation of a η2- thus approaching the upper value of the range reported for
bonded olefin represents an interesting synthetic procedure, agostic interactions. However, the values of crystallographic
which, however, depends very much upon steric effects. The parameters alone might not be sufficient to evaluate the type
η2-bonded olefin in compounds of type 1 is highly electrophilic of interaction between a metal and a hydrogen atom lying in
and usually reacts with
a base to give nucleophilic its proximity, whereas solution NMR data are much more
addition.22,26,27 The steric bulk of triethylamine, used as a informative in this respect.
base, produces severe retardation of the nucleophilic addition
A most relevant feature resulting from the NMR investi-
(only a tiny amount of addition product is formed) allowing gation is the downfield shift experienced by the ortho phenyl
the alternative process of olefin deprotonation to become protons upon E to Z isomerisation. In principle this downfield
dominant. The deprotonation involves the hydrogen most shift could be ascribed to the magnetic anisotropy of the metal
exposed to the solvent (that lying trans to the phenyl ring) and centre. Large downfield shifts are typical of protons in a
6846 | Dalton Trans., 2013, 42, 6840–6851
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
pseudo-axial position above the platinum atom in a square- Z+ transformation appears to be faster than that occurring
planar complex.36 However, the difference in downfield shift between neutral molecules).37
between the two ortho phenyl protons in 2d and in 2c cannot
In our case the equilibrium composition was only slightly
be due to a generic effect of the magnetic anisotropy of the in favour of the Z isomer and the kinetic data clearly indicate
metal core, nor to an agostic interaction (which typically that a monomolecular mechanism is operating. As expected,
produce an upfield shift), but points to a type of hydrogen the energy of the irradiating light influences the rate of isomer-
bond interaction. The possibility for the platinum atom to act isation but not the equilibrium composition.
as H-bond acceptor, also in neutral complexes, is now well
As far as the mechanism of the isomerisation process is
documented.35 In the alkenyl complexes here described, we concerned, TD-DFT calculations indicate that it proceeds
are in the fortunate case of a phenyl group with the two ortho through the traditional CvC bond twist process operating in
protons having different partial positive charges depending purely organic molecules, like stilbene.
upon the electron-withdrawing strength of the adjacent substi-
tuent. The greater the positive charge accumulated on the
hydrogen atom, the greater is its downfield shift. In the case of
Experimental
(Z)-2c (nitro substituent) the downfield shift for the more posi-
Reagents and methods
tive ortho proton is ten times greater than for the other ortho
Reagents and solvents were commercially available and used
as received without further purification. Elemental analyses
were performed with a CHN Eurovector EA 3011.
proton, indicating that the greater the positive charge of the
hydrogen, the more exclusive is its interaction with the metal
centre; this is typical of the H-bond type interaction. There is
no detectable JPt,H coupling resulting from such a Pt⋯H inter-
action, which could be essentially electrostatic. It must be
pointed out that, even in compound (Z)-2c, where the
Pt⋯Hortho interaction is very strong, in solution the alkenyl
moiety can freely rotate about the Pt–Cα bond as indicated by
the equivalence of the methyls on each nitrogen of the tmeda
ligand.
The nature of the Pt⋯Hortho interaction is fully supported
by the results obtained with the performed calculations. In
particular the good agreement between experimental and cal-
culated value of the Pt⋯Hortho distance in (Z)-2a gives complete
significance to the calculated values of all other Z isomers
setting an important H-bond interaction between the metal
and the Hortho protons of the phenyl group. However, even in
the case of (Z)-2c; where there is a very strong H-bond inter-
action with one Hortho, the free rotation around the Cβ–Cipso
bond is not prevented in solution (as already pointed out in
the previous paragraph). Moreover, this picture is also sup-
ported by the calculated charges on the Hortho, which are con-
sistent with the experimental 1H NMR shifts. Indeed, the
analysis of the DFT orbitals indicates that the most shifted
1H and 13C NMR spectra were recorded on a 300 MHz
Mercury Varian, DPX-WB 300 and 500 Avance Bruker instru-
ments equipped with probes for inverse detection and with z
gradient for gradient-accelerated spectroscopy. 1H and 13C
NMR spectra were referenced to TMS; the residual proton
signal of the solvent was used as internal standard. 1H/13C
inversely detected gradient-sensitivity enhanced hetero-corre-
lated 2D NMR spectra for normal coupling (INVIEAGSSI) was
acquired using standard Bruker automation programs and
pulse sequences. Each block of data was preceded by eight
dummy scans. The data were processed in the phase-sensitive
mode.
ESI-MS spectra were recorded on a Platform II Mass spec-
trometer (Micromass, Manchester, U.K.).
UV-Vis electronic absorption spectra were obtained on a
Varian Cary 300 UV-Vis spectrophotometer equipped with a
temperature control block using 1 cm path-length cuvettes. All
spectra were baseline-corrected using pure solvent as a back-
ground. Data were processed using Microsoft Office Excel.
The complexes
Hortho is that with greater positive charge, pointing to an The alkenyl complexes [PtCl(η1-CαH–CβHAr)(tmeda)], 2, (Ar =
H-bond type of interaction.
C6H5, (E)-2a; 4-CH3O–C6H4, (E)-2b; 3-NO2–C6H4, (E)-2c; and
3-CH3O–C6H4, (E)-2d) were prepared according to the pro-
cedure described in ref. 19. Analytical data for the yet un-
reported 2d, isolated as pure E isomer, are hereafter listed.
Isomerisation around the double bond
The E/Z isomerisation around the CvC double bond, which Elemental analysis calcd (%) for C15H25ClN2OPt (479.9):
was another focus of this work, is a process which is seldom C 37.54, H 5.25, N 5.84; found: C 37.25, H 5.31, N 5.70. NMR
observed in alkenyl metal complexes, where the presence of (CDCl3, 294 K): δ(1H) = 2.76 and 2.92 (s, 12 H), CH3N (tmeda);
different forms generally stems from different reaction mech- 2.60 and 2.80 (m, 4 H), CH2N (tmeda); 3.78 (s, 3 H),
3
anisms rather than from a post-formation isomerisation –C6H4OCH3; 6.42 and 7.32 (d, 2H, JH–H cis = 15 Hz) –CαH–
process.6
CβH–; aromatic ring: 6.59 (dd, 1H) Hp; 6.85 (s, 1 H) and 6.88
Prior to light irradiation experiments (E)-2a and (E)-2b
There are cases, however, in which, starting from a kineti- (dd, 1 H) Ho; 7.10 (m, 1H) Hm.
cally-determined isomeric composition of the alkenyl, a new
composition, under thermodynamic control, has been samples were chromatographed on silica gel, using dichloro-
obtained by administering some energy as heat2,10 or light (as methane–ethyl acetate, 9 : 1 v/v, as eluent. All manipulations
in our case), or by electrochemical oxidative catalysis (the E+ → were carried out by shielding the complexes from light.
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 6840–6851 | 6847

View Article Online
Paper
Dalton Transactions
Combined irradiation-NMR experiments
experimental spectrum in Fig. S1,† also shows that the PBE0
functional can better reproduce the experimental features (e.g.
277 vs. 280 nm for the maximum with a shift of only 3 nm,
instead of about 10 nm for all other functionals). We therefore
employed the PBE0 functional for further calculations on the
ground state of all considered alkenyl Pt(II) complexes (E) and
(Z)-2a–d, and on the excited states of (E)-2a.
Reaction enthalpies and free energies for the E/Z isomerisa-
tion were estimated at this level of theory for all Pt alkenyl
complexes. For 2c and 2d species two conformations per metal
complex, differing by a 180° rotation of the phenyl ring around
the Cβ–Cipso bond (C7–C1 in Fig. 3), were considered. The
rotation is expected to be almost free at room temperature.
The reported enthalpies and free energies were computed by
using the lowest energy conformation for each isomer. The
free energy differences between these two conformations in
complexes 2-c and 2-d were used to perform a Boltzmann-
weighted average of chemical shifts (vide infra).
Solutions of compound (E)-2a and (E)-2b (600 μL, 13 mM in
CDCl3) in a 5 mm NMR tube were irradiated in a Luzchem
1CH2 photo-reactor (Hitachi FL8BL), fitted with B UVA (λmax
351 nm) or Luzchem LZC-420 (λmax = 420 nm) bulbs at 310 K.
The power of the light source was measured with an A.W.
Sperry SLM-110 power meter and an International Light Tech-
nologies ILT 1400-A photometer using a UVA#28880 or a
W#12664 filter in the case of lower and higher wavelength,
respectively. At regular intervals the samples were removed
and analysed by 1H NMR spectroscopy.
=
In both sets of experiments, the spectroscopic analyses took
approx. 15 min to be completed. When not being irradiated
the solutions were handled under low lighting conditions and
no change in speciation was observed on this timescale in the
1
absence of the light source. All H NMR spectra were acquired
with 32 scans on a Bruker DRX-500 NMR spectrometer
(frequency width was 20 000 Hz (20 ppm) pulse angle was
30°, the relaxation delay 1 s). The data were analysed using
Topspin (version 2.1). Integration of proton signals HoZ, HoE
HαZ and HαE was used to determine the ratios of the different
The structures obtained at the DFT and TD-DFT levels were
confirmed to be energy minima by calculating the harmonic
vibrational frequencies using numerical differences. The Karls-
ruhe triple-ζ basis sets augmented with one set of polarization
functions, designed for small core ECPs (def2-TZVP) were
used.50 All calculations have been done with TURBOMOLE.51
The effect of chloroform solvent was taken into account by
,
isomers.
X-ray investigations
Diffraction data were collected: for (E)-2a, (Z)-2a, and (E)-2b on
a Bruker Smart Apex-II diffractometer equipped with an
Oxford Cryosystems low-temperature device operating at 150 K;
and for (E)-2d on an Bruker AXS X8 APEX CCD system
equipped with a four-circle Kappa goniometer and a 4K CCD
detector (in this case data collection was performed at room
temperature). MoKα radiation was used in all cases. Correc-
tions for systematic errors were performed with SADABS
program.38 The structures were solved using DIRDIF,39
SHELXS,40 or SIR2004;41 and they were refined by full matrix
least squares against F2 and all data (SHELXL).40 All calcu-
lations and molecular graphics were carried out using
PARST97,42 WinGX,43 and Mercury CSD 2.0.44 All non-H atoms
were refined with anisotropic displacement parameters, and
H-atoms were included in ideal positions. Symmetry checks on
structure (E)-2a, which contains three molecules in the asym-
metric unit, do not indicate any missed translational
operation.
COSMO polarisable continuum model (ε
=
4.9), as
implemented in TURBOMOLE.51
Further calculations on the PBE0 optimized geometries of
alkenyl complexes were performed by using Gaussian 03.52
Natural bond orbitals (NBO) analyses, Mulliken atomic
charges and isotropic shielding constants were calculated by
using PBE0 (PBE1PBE in the Gaussian 03 implementation)
and double-ζ basis sets: the all electron 6-31G(d,p) basis set
was used for C, N, O, Cl and H atoms,53 whereas the LANL2DZ
basis set, comprising the LANL2 effective core potential for the
inclusion of relativistic effects, was used for the Pt atom.54
1H chemical shifts of (E) and (Z)-2a–d complexes were calcu-
lated by using the gauge-independent atomic orbital (GIAO)
method.55 The chemical shifts were obtained by subtracting
the reference isotropic shielding of TMS protons calculated at
the same PBE0 level of theory. Both GIAO and NBO calcu-
lations were carried out in the presence of a polarizable conti-
nuum simulating the chloroform solution (ε = 4.9) by means
of the CPCM method56 implemented in Gaussian 03.
Computational methods
The molecular structures of the ground states of the (E) and
(Z)-2a complexes were initially optimized at the DFT level using
the B3LYP45 BH-HLYP,46 PBE,47 and PBE048 functionals. The
Conclusions
energies of the 20 lowest lying singlet-singlet energy tran- The alkenyl complexes of platinum(II), examined in the present
sitions for (E)-2a were calculated at the optimized geometry investigation, can have E or Z configuration. The ground state
with the time dependent density functional theory (TD-DFT)49 energy of the two isomers is quite similar and, in chlorinated
employing the same exchange-correlation functionals. The solvents, the [Z]/[E] ratio at equilibrium is only slightly above
comparison of the optimized geometries with the experimen- 1. The E forms are kinetically favoured and rather stable at
tal X-ray geometry, see Table S2,† shows a better agreement for room temperature; however, by heating or by irradiation with
the PBE0 functional. The comparison of the calculated UV-Vis UVA and blue light, the thermodynamic equilibrium between
spectra (only that for the PBE0 is reported in Fig. S7†) with the E and Z isomers can be reached.
6848 | Dalton Trans., 2013, 42, 6840–6851
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
The isomerisation process most probably occurs through
the classical CvC bond twist mechanism operating in organic
molecules such as stilbenes.
H. Kurosawa and N. Kambe, Organometallics, 2008, 27,
4788–4802.
14 B. E. Mann, B. L. Shaw and N. I. Tucker, J. Chem. Soc. A,
1971, 2667–2673.
In the Z isomers an H-bond can be established between an
ortho hydrogen of the phenyl ring and the platinum centre 15 T. W. Bell, D. M. Haddleton, A. McCamley, M. G. Partridge,
whose strength parallels the Mulliken charge of the hydrogen
atom.
R. N. Perutz and H. Willner, J. Am. Chem. Soc., 1990, 112,
9212–9226.
16 K. J. Cavell and H. Jin, J. Chem. Soc., Dalton Trans., 1995,
4081–4089.
17 M. A. Bennett, L. Kwan, A. D. Rae, E. Wenger and
A. C. Willis, J. Chem. Soc., Dalton Trans., 2002,
226–233.
Acknowledgements
We are grateful to Prof. Ferdinando Scordari and Dr Ernesto
Mesto (University of Bari, Italy) for the X-ray diffraction data 18 A. Hamze, O. Provot, J.-D. Brion and M. Alami, J. Organo-
collection (compound 2c). We thank the EPSRC (grant no EP/ met. Chem., 2008, 693, 2789–2797.
G006792/1 and studentship for RJM); ERC (grant no 247450); 19 G. Lorusso, G. Boccaletti, N. G. Di Masi, F. P. Fanizzi,
AWM/ERDF/Science City; the University Consortium for
Research in Chemistry of Metal Ions in Biological Systems
(C.I.R.C.M.S.B.) for their support for this work.
L. Maresca and G. Natile, Eur. J. Inorg. Chem., 2004,
4751–4754.
20 S. Kimel and S. Speiser, Chem. Rev., 1977, 77, 437–472;
J. Svoboda and B. König, Chem. Rev., 2006, 106, 5413–5430;
M. Fagnoni, D. Dondi, D. Ravelli and A. Albini, Chem. Rev.,
2007, 107, 2725–2756; N. Hoffmann, Chem. Rev., 2008, 108,
1052–1103.
Notes and references
1 T. L. Gianetti, N. C. Tomson, J. Arnold and R. G. Bergman, 21 H. Kornweitz, M. Brith and H. Friedmann, J. Photochem.,
J. Am. Chem. Soc., 2011, 133, 14904–14907; D. Vasu, A. Das
and R.-S. Liu, Chem. Commun., 2010, 46, 4115–4117;
L.-B. Han, Y. Ono, Q. Xu and S. Shimada, Bull. Chem. Soc.
Jpn., 2010, 83, 1086–1099.
2 D. L. Reger and P. J. McElligot, J. Am. Chem. Soc., 1980, 102,
5293–5294.
3 M. J. Cowley, J. M. Lynam, R. S. Moneypenny,
A. C. Whitwood and A. J. Wilson, Dalton Trans., 2009,
9529–9542.
4 H. Werner, M. A. Esteruelas and H. Otto, Organometallics,
1986, 5, 2295–2299.
1980, 102, 5293–5294; K. M. Tait, J. A. Parkinson,
S. P. Bates, W. J. Ebenezer and A. C. Jones, J. Photochem.
Photobiol., A, 2003, 154, 179–188; S. W. Magennis,
F. S. Mckay, A. C. Jones, K. M. Tait and P. J. Sadler, Chem.
Mater., 2005, 17, 2059–2062.
22 C. R. Barone, R. Cini, E. Clot, O. Eisenstein, L. Maresca,
G. Natile and G. Tamasi, J. Organomet. Chem., 2008, 693,
2819–2827.
23 R. Cini, P. A. Caputo, F. P. Intini and G. Natile, Inorg.
Chem., 1995, 34, 1130–1137.
24 D. Braga, F. Grepioni and E. Tedesco, Organometallics,
1997, 16, 1846–1856; Y. Zhang, J. C. Lewis, R. G. Bergman,
J. A. Ellman and E. Oldfield, Organometallics, 2006, 25,
3515–3519.
5 M. R. Torres, A. Vegas, A. Santos and J. Ross, J. Organomet.
Chem., 1986, 309, 169–177.
6 H. C. Clark, G. Ferguson, A. B. Goel, E. G. Janzen,
H. Ruegger, P. Y. Siew and C. S. Wong, J. Am. Chem. Soc., 25 V. Molina, M. Merchàn and B. O. Roos, J. Phys. Chem. A,
1986, 108, 6961–6972.
7 C. Allevi, L. Garlaschelli and M. C. Malatesta, J. Chem. Soc.,
Dalton Trans., 1988, 1949–1955.
1997, 101, 3478–3487; M. J. Bearpark, F. Bernardi,
S. Clifford, M. Olivucci, M. A. Robb and T. Vreven, J. Phys.
Chem. A, 1997, 101, 3841–3847; R. Improta and F. Santoro,
J. Phys. Chem. A, 2005, 109, 10058–10067.
8 J. A. Cabeza, I. del Rio, S. Garcia-Granda, L. Martinez-
Mendez and E. Pérez-Carreno, Chem.–Eur. J., 2005, 29, 26 F. P. Fanizzi, L. Maresca, G. Natile and C. Pacifico, Gazz.
6040–6052.
Chim. Ital., 1994, 124, 137–142; L. Maresca and G. Natile,
Inorg. Chim. Acta, 1999, 285, 301–304.
9 J. A. Cabeza, I. del Rio, J. M. Fernandez-Colina, E. Pérez-
Carreno and D. Vazquez-Garcìa, Organometallics, 2010, 29, 27 G. Lorusso, N. G. Di Masi, L. Maresca, C. Pacifico and
4818–4828.
G. Natile, Inorg. Chem. Commun., 2006, 9, 500–503;
G. Lorusso, C. R. Barone, N. G. Di Masi, L. Maresca,
C. Pacifico and G. Natile, Eur. J. Inorg. Chem., 2007,
2144–2150.
10 S. L. Chatwin, M. F. Mahon, T. J. Prior and
M. K. Whittlesey, Inorg. Chim. Acta, 2010, 363, 625–632.
11 J. M. Huggins and R. G. Bergman, J. Am. Chem. Soc., 1979,
101, 4410–4411.
28 M. Meot-Ner and S. A. Kafafi, J. Am. Chem. Soc., 1988, 110,
6297–6303.
12 F. Balegroune, P. Braunstein, T. M. Gomes Carneiro,
D. Grandjean and D. Matt, J. Chem. Soc., Chem. Commun., 29 A. Tiripicchio, M. Tiripicchio-Camellini, L. Maresca,
1989, 582–584.
G. Natile and G. Rizzardi, Cryst. Struct. Commun., 1979, 8,
689–693; L. Maresca and G. Natile, J. Chem. Soc., Dalton
Trans., 1982, 1903–1906; G. Annibale, L. Maresca, G. Natile,
13 H. Kuniyasu, K. Takekawa, F. Yamashita, K. Miyafuji,
S. Asano, Y. Takai, A. Ohtaka, A. Tanaka, K. Sugoh,
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 6840–6851 | 6849

View Article Online
Paper
Dalton Transactions
A. Tiripicchio and M. Tiripicchio-Camellini, J. Chem. Soc., 42 M. Nardelli, PARST: a system of FORTRAN routines for cal-
Dalton Trans., 1982, 1587–1591.
culating molecular structure parameters from results of
crystal structure analyses, Comput. Chem., 1983, 7, 95–97;
M. Nardelli, PARST: a system of FORTRAN routines for cal-
culating molecular structureparameters from results of
crystal structure analyses, J. Appl. Crystallogr., 1995, 28,
659.
30 M. K. Cooper, P. J. Guerny, H. J. Goodwin and
M. McPartlin, J. Chem. Soc., Dalton Trans., 1982, 757–764;
H. C. Clark, G. Ferguson, A. B. Goel and B. L. Rhul, Organo-
metallics, 1984, 3, 15–17.
31 C. J. Cardin, D. J. Cardin, H. E. Parge and A. C. Sullivan,
J. Chem. Soc., Dalton Trans., 1986, 2315–2320; A. R. Siedle, 43 L. J. Farrugia, WinGX suite for small-molecule single-
W. B. Gleason and R. A. Newmark, Organometallics, 1986,
5, 1969–1975.
crystal crystallography, J. Appl. Crystallogr., 1999, 32,
837–838.
32 M. Brookhart and M. L. H. Green, J. Organomet. Chem., 44 C. F. Macrae, I. J. Bruno, J. A. Chisholm, P. R. Edgington,
1983, 250, 395–408; M. Brookhart, M. L. H. Green and
G. Parkin, Proc. Natl. Acad. Sci. U. S. A., 2007, 104,
6908–6914.
P. McCabe, E. Pidcock, L. Rodriguez-Monge, R. Taylor,
J. van de Streek and P. A. Wood, J. Appl. Crystallogr., 2008,
41, 466–470.
33 D. M. Roe, P. M. Bailey, K. Moseley and P. M. Maitlis, 45 A. D. Becke, J. Chem. Phys., 1993, 98, 5648–5652; C. Lee,
J. Chem. Soc., Chem. Commun., 1972, 1273–1274; W. Yang and R. G. Parr, Phys. Rev. B, 1988, 37, 785–789.
A. Albinati, C. Arz and P. S. Pregosin, Inorg. Chem., 1987, 46 A. D. Becke, J. Chem. Phys., 1993, 98, 1372–1377.
26, 508–513; A. Albinati, P. S. Pregosin and F. Wombacher, 47 J. P. Perdew, K. Burke and M. Enzerhof, Phys. Rev. Lett.,
Inorg. Chem., 1990, 29, 1812–1817; W. Yao, O. Eisenstein
1996, 77, 3865–3868.
and R. H. Crabtree, Inorg. Chim. Acta, 1997, 254, 105–111; 48 C. Adamo and V. Barone, J. Chem. Phys., 1999, 110,
R. H. Crabtree, O. Eisenstein, G. Sini and E. Peris, J. Organo-
met. Chem., 1998, 567, 7–11.
6158–6170; M. Enzerhof and G. E. Scuseria, J. Chem. Phys.,
1999, 110, 5029–5036.
34 L. Brammer, J. M. Charnock, P. L. Goggin, R. J. Goodfellow, 49 R. Bauernschmitt and R. Ahlrichs, Chem. Phys. Lett., 1996,
A. Guy Orpen and T. Koetzle, J. Chem. Soc., Dalton Trans.,
1991, 1789–1798; S. G. Kazarian, P. A. Hamley and
256, 454–464; F. Furche and R. Ahlrichs, J. Chem. Phys.,
2002, 117, 7433–7447.
M. Poliakoff, J. Am. Chem. Soc., 1993, 115, 9069–9079; 50 A. Schäfer, C. Huber and R. Ahlrichs, J. Chem. Phys., 1994,
J. M. Casas, L. R. Falvello, J. Forniés and A. Martìn, Inorg.
Chem., 1996, 35, 6009–6014; L. Brammer, Dalton Trans.,
2003, 3145–3157.
100, 5829–5835; F. Weigend, M. Häser, H. Patzelt and
R. Ahlrichs, Chem. Phys. Lett., 1998, 294, 143–152.
51 TURBOMOLE V6.0 2009, a development of University of
Karlsruhe and Forschungszentrum Karlsruhe GmbH,
1989–2007, TURBOMOLE GmbH, since 2007, available
from http://www.turbomole.com
35 A. Albinati, F. Lianza, P. S. Pregosin and B. Müller, Inorg.
Chem., 1994, 33, 2522–2526; R. Cini, F. P. Fanizzi,
F. P. Intini and G. Natile, Inorg. Chim. Acta, 1996, 251,
111–118; A. Martìn, J. Chem. Educ., 1999, 76, 578–583; 52 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
S. Rizzato, J. Bergès, S. A. Mason, A. Albinati and
J. Kozelka, Angew. Chem., Int. Ed., 2010, 49, 7440–7443.
36 R. G. Miller, R. D. Stauffer, D. R. Fahey and D. R. Parnell,
J. Am. Chem. Soc., 1974, 92, 1511–1521; W. I. Sundquist,
D. P. Bancroft and S. J. Lippard, J. Am. Chem. Soc., 1990,
112, 1590–1596; L. Maresca, C. Pacifico, M. C. Pappadopoli
and G. Natile, Inorg. Chim. Acta, 2000, 304, 274–282; Y. Liu,
C. Pacifico, G. Natile and E. Sletten, Angew. Chem., Int. Ed.,
2001, 40, 1226–1228.
37 R. H. Philp Jr. and D. L. Reger, Organometallics, 1989, 8,
1714–1718.
38 G. M. Sheldrick, SADABS, Program for Empirical Absorption
Correction of Area Detector Data, University of Göttingen,
Germany, 1996.
39 P. T. Beurskens, G. Beurskens, W. P. Bosman, R. de Gelder,
S. Garcia-Granda, R. O. Gould, R. Israel and J. M. M. Smits,
The DIRDIF96 Program System, University of Nijmegen,
Nijmegen, The Netherlands, 1996.
40 G. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr.,
2008, 64, 112–122.
M. A. Robb, J. R. Cheeseman Jr., J. A. Montgomery,
T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam,
S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi,
G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji,
M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai,
M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross,
V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts,
R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi,
C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma,
G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski,
S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas,
D. K. Malick, A. D. Rabuck, K. Raghavachari,
J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford,
J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko,
P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith,
M. A. Al-Laham, C. Y. Peng, A. Nanayakkara,
M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen,
M. W. Wong, C. Gonzalez and J. A. Pople, GAUSSIAN 03
(Revision C.02), Gaussian, Inc., Wallingford, CT, 2004.
41 M. C. Burla, M. Camalli, B. Carrozzini, G. L. Cascarano, 53 P. Hariharan and J. A. Pople, Chem. Phys. Lett., 1972, 16, 217–
G. Giacovazzo, G. Polidori and R. Spagna, SIR2002: the
219; V. A. Rassolov, M. A. Ratner, J. A. Pople, P. C. Redfern
program, J. Appl. Crystallogr., 2003, 36, 1103.
and L. A. Curtiss, J. Comput. Chem., 2001, 22, 976–984.
6850 | Dalton Trans., 2013, 42, 6840–6851
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
54 P. J. Hay and W. R. Wadt, J. Chem. Phys., 1985, 82,
270–283; W. R. Wadt and P. J. Hay, J. Chem. Phys.,
1985, 82, 284–298; P. J. Hay and W. R. Wadt, J. Chem.
Phys., 1985, 82, 299–310.
1974, 27, 789–807; J. L. Dodds, R. McWeeny and
A. J. Sadlej, Mol. Phys., 1980, 41, 1419–1430; K. Wolinski,
J. F. Hilton and P. Pulay, J. Am. Chem. Soc., 1990, 112,
8251–8260.
55 F. London, J. Phys. Radium, 1937, 8, 397–409; R. McWeeny, 56 V. Barone and M. Cossi, J. Phys. Chem. A, 1998, 102,
Phys. Rev., 1962, 126, 1028–1034; R. Ditchfield, Mol. Phys.,
1995–2001.
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 6840–6851 | 6851