The first examples of platinum(II)-amine complexes containing 1,2-dicarba-closo-dodecaborane(12)

Article abstract of DOI:10.1016/j.inoche.2003.11.027

Treatment of the labile precursor complexes cis- or trans-[PtCl(dmf) (NH₃)₂]OTf (dmf=N,N-dimethylformamide; OTf=trifluoromethanesulfonate) with the monodentate 1-aminoalkyl-1,2-carborane ligands NH₂(CH₂)_nC₂B ₁₀H₁₁ (n=1,3) afforded the moderately-stable species cis- or trans-[PtCl(NH₃)₂{NH₂(CH₂) _nC₂B₁₀H₁₁}]OTf, respectively. The novel complexes are the first examples of platinum(II)-amine derivatives containing a 1,2-dicarba-closo-dodecaborane(12) (closo-1,2-carborane) ligand.

Full text of DOI:10.1016/j.inoche.2003.11.027

Inorganic Chemistry Communications 7 (2004) 289–291
www.elsevier.com/locate/inoche
The first examples of platinum(II)-amine complexes containing
1,2-dicarba-closo-dodecaborane(12) q
a
Jean A. Todd , Louis M. Rendina
b,
*
a
School of Chemistry and Physics, The University of Adelaide, Adelaide, South Australia 5005, Australia
b
School of Chemistry, The University of Sydney, Sydney, NSW 2006, Australia
Received 13 November 2003; accepted 24 November 2003
Published online: 19 December 2003
Abstract
Treatment of the labile precursor complexes cis- or trans-[PtCl(dmf)(NH3)2]OTf (dmf ¼ N,N-dimethylformamide; OTf ¼ triflu-
oromethanesulfonate) with the monodentate 1-aminoalkyl-1,2-carborane ligands NH2(CH2)nC2B10H11 (n ¼ 1; 3) afforded the
moderately-stable species cis- or trans-[PtCl(NH3)2{NH2(CH2)nC2B10H11}]OTf, respectively. The novel complexes are the first
examples of platinum(II)-amine derivatives containing a 1,2-dicarba-closo-dodecaborane(12) (closo-1,2-carborane) ligand.
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Platinum(II) complex; closo-1,2-Carborane; NMR spectroscopy; Electrospray ionisation mass spectrometry
The use of carborane ligands in late transition metal
chemistry is well-established (for reviews, see [1]), with
the vast majority of complexes falling into the metalla-
carborane category in which one or more metal atoms
(or their fragments) are incorporated within a polyhe-
dral carborane cage structure. Alternatively, carboranes
such as 1,2-, 1,7- and 1,12-dicarba-closo-dodecabo-
rane(12) may be functionalised at one or both of the
carbon atoms to give organophosphorus, thiol, and silyl
ligands that readily form a variety of coordination
complexes (for recent reviews, see [2]). Recently, we re-
ported the first examples of dinuclear platinum(II)-
amine complexes containing a closo-carborane moiety, a
new class of DNA-binding agents for potential use in
boron neutron capture therapy (BNCT) [3]. Notably,
only the closo-1,7-carborane complexes exhibited excel-
lent stability both in solution and in the solid state, but
the corresponding 1,2-isomers were unstable with re-
spect to extensive degradation of the boron cluster and
concomitant reduction of the platinum(II) centre.
Herein we report the preparation and characterisa-
tion of moderately-stable mononuclear triaminechloro-
platinum(II) complexes containing 1-aminoalkyl-1,
2-carborane ligands, the first examples of platinum(II)-
amine derivatives of closo-1,2-carborane. Despite their
inherent reactivity, the complexes are sufficiently robust
to be fully characterised by multinuclear (1H, 13C, 11B,
15N, and 195Pt) NMR spectroscopy and electrospray
ionisation mass spectrometry (ESI-MS). We also report
key observations in the attempted preparation of anal-
ogous cis-dichloroplatinum(II) complexes in order to
obtain a clearer understanding of the decomposition
process described above.
The target complexes were readily prepared from cis-
[PtCl2(NH3)2] (for 3 and 4) or the corresponding trans
isomer (for 5 and 6). In general, cis- or trans-
[PtCl2(NH3)2] was treated with one equivalent of AgOTf
in anhydrous DMF solution in order to remove selec-
tively one of the chloro ligands. The 1-aminoalkyl-1,2-
carborane ligand 1 or 2 was then added to the resulting
labile species cis- or trans-[PtCl(NH3)2(dmf)]þ (dmf ¼
N,N-dimethylformamide) to afford the target complexes
3–6 (Scheme 1) as off-white solids that exhibited
q Supplementary data associated with this article can be found, in the
online version, at doi:10.1016/j.inoche.2003.11.027.
* Corresponding author. Tel.: +61-2-9351-4781; fax: +61-2-9351-
3329.
E-mail address: rendina@chem.usyd.edu.au (L.M. Rendina).
1387-7003/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2003.11.027

290
J.A. Todd, L.M. Rendina / Inorganic Chemistry Communications 7 (2004) 289–291
+
H
NH3
–OTf
(CH2)nNH2 Pt NH3
cis-[PtCl(dmf)(NH3)2]OTf
Cl
H
(3) n = 1
(4) n = 3
(CH2)nNH2
BH
C
H
+
NH3
–OTf
(1) n = 1
(2) n = 3
(CH2)nNH2 Pt Cl
NH3
trans-[PtCl(dmf)(NH3)2]OTf
(5) n = 1
(6) n = 3
Scheme 1.
moderate stability in the absence of water. In general,
the isolated yields of 3–6 were fairly modest, with the
highest yields recorded for those complexes containing
ligand 2 (52–64%). Indeed, such species could be stored
under vacuum for several weeks at room temperature
without any evidence of decomposition. The greater
stability of the propylene-tethered complexes 4 and 6
compared to the methylene-linked analogues 3 and 5 is
consistent with an intra- rather than inter-molecular
degradation process in which the relative proximity of
the carborane moiety to the platinum(II) centre appears
to play a crucial role in the overall stability of the
complexes.
Complexes 3–6 displayed a characteristic singlet at
approximately d )2400 in the 195Pt{1H} NMR spec-
trum, consistent with a PtN3Cl core [4]. 15N{1H} NMR
spectroscopy of selected complexes in which both am-
mine ligands were 15N-labelled confirmed the assigned
product stereochemistry. For example, 6 exhibited a
singlet resonance at d )60.1 (1JPtN ¼ 280 Hz) consistent
with mutually trans ammine ligands. Complexes 3–6
were further characterised by ESI-MS. As demonstrated
in Fig. 1, the correlation between experimental and
theoretical spectra was excellent in all cases. Further-
more, each of the lines in the ESI-MS was separated by
one mass unit, thus confirming that the molecular ions
bore a single positive charge. In some cases, MS/MS
spectra were also obtained. For example, the molecular
ion peak located at m=z 437 in the spectrum of 5 gave
rise to secondary fragmentation patterns at m=z 402 and
384 indicating a sequential loss of the chloro ligand and
one of the ammine ligands. The MS/MS recorded for the
molecular ion of the corresponding cis complex 3 gave
rise to identical fragmentation patterns. Indeed, there
was no observed difference in the MS behaviour of any
of the cis and trans isomers under our experimental
conditions.
Fig. 1. Experimental (top) and theoretical (bottom) isotopic distribu-
tion pattern for the molecular ion [6-OTf]þ.
process that was accompanied by extensive degradation
of the carborane cage particularly in the presence of
water. The decomposition process appears to follow a
two-step pathway involving deboronation of the closo-
1,2-carborane cage followed by reduction of the plati-
num(II) centre accompanied by further degradation of
the carborane. 195Pt{1H} NMR studies confirmed the
initial formation of the expected cis-dichloro species by
the presence of a singlet at approximately d )2100,
consistent with a PtCl2N2 core [4]. However, 11B{1H}
NMR studies indicated a gradual increase in the for-
mation of a nido-7,8-carborane species due to an
expulsion of the B-3 atom from the corresponding
All attempts at the preparation of cis-dichloroplati-
num(II) complexes of the type cis-[PtCl2(NH3)L] (L ¼ 1
or 2) by the reaction of 1 or 2 with M[PtCl3(NH3)] in
dry DMF (M ¼ K) or CH2Cl2 (M ¼ PPh4) solution
ultimately led to the formation of platinum metal, a

J.A. Todd, L.M. Rendina / Inorganic Chemistry Communications 7 (2004) 289–291
291
closo-1,2-carborane structure [5]. The deboronation of
closo-1,2-carborane ligands has previously been ob-
served in the preparation of palladium(II) complexes
containing 1,2-(PR2)2-1,2-C2B10H10 (R ¼ aryl, alkyl,
alkoxy) [6] and copper(I) and gold(I) derivatives of 1-
PPh2-2-SR-1,2-C2B10H10 (R ¼ Et, Bz) [7] in the pres-
ence of nucleophilic solvents such as EtOH. Despite all
attempts in avoiding the use of such solvents in the
preparation of cis-dichloroplatinum(II) complexes,
deboronation was still observed (although considerably
retarded in solvents such as CH2Cl2) indicating that
the amine functionality appears to play a key role in
the degradation process by intra- or inter-molecular
reaction with the closo-1,2-carborane cage prior to
complexation. Indeed, the use of labile precursor
complexes such as trans-[PtCl(NH3)2(dmf)]þ instead of
[PtCl3(NH3)]ꢀ in the reaction with 1 or 2 ensures that
the free amine has minimal opportunity to attack the
closo-1,2-carborane cage prior to complexation, and
the isolation of 3–6 becomes feasible under anhydrous
conditions (vide supra).
the platinum(II) centre than the corresponding ami-
nomethylene analogues. We are currently evaluating the
use of N-donor ligands related to 1 and 2, including
their nido-derivatives, in late transition metal chemistry
and the results of this work will be presented in due
course.
Supplementary material
Experimental details of the synthesis and characteri-
sation (NMR and ESI-MS) of 3–6.
Acknowledgements
We are grateful to Dr. S.M. Pyke (University of
Adelaide) for valuable advice regarding some of the 2D
NMR spectra. We also thank Johnson Matthey for the
generous loan of K2[PtCl4]. This work was supported by
the ARC and Anti-Cancer Foundation of SA.
It is noteworthy that cis-dichloroplatinum(II) deriv-
atives of nido-7,8-carborane could not be isolated owing
to their inherent instability. Once formed, the carborane
cage was oxidised by the platinum(II) centre to afford
colloidal platinum and a number of degraded cage
species such as derivatives of 4,5-dicarba-nido-nonabo-
rane(11) and 5,6-dicarba-nido-decaborane(12) contain-
ing 7 and 8 BH units, respectively, that gave rise to very
complicated 11B{1H} NMR spectra. This type of cage
degradation phenomenon is highly reminiscent of the
oxidation of the nido-7,8-carborane anion by aqueous
FeCl3 solution [8]. Clearly, the presence of only one
chloro ligand in the coordination sphere sufficiently
stabilises the platinum(II) centre toward reduction and
the isolation of 3–6 becomes feasible. Ligands such as
thiolate and 2,20:60,200-terpyridine (or bidentate diimines
such as 2,20-bipyridine) are also known to inhibit the
reduction process [9].
References
[1] (a) R.N. Grimes, Coord. Chem. Rev. 200–202 (2000) 773;
(b) I.T. Chizhevsky, A.I. Yanovsky, Y.T. Struchkov, J. Organo-
met. Chem. 536–537 (1997) 51;
(c) R.N. Grimes, Appl. Organomet. Chem. 10 (1996) 209;
(d) R.N. Grimes, Coord. Chem. Rev. 143 (1995) 71;
(e) P.A. Jelliss, F.G.A. Stone, J. Organomet. Chem. 500 (1995) 307;
(f) N.S. Hosmane, J.A. Maguire, J. Cluster Sci. 4 (1993) 297;
(g) A.K. Saxena, N.S. Hosmane, Chem. Rev. 93 (1993) 1081.
[2] (a) S.O. Kang, J. Ko, Adv. Organomet. Chem. 47 (2001) 61;
(b) O. Crespo, M.C. Gimeno, A. Laguna, Appl. Organomet. Chem.
14 (2000) 644.
[3] S.L. Woodhouse, L.M. Rendina, Chem. Commun. (2001) 2464.
[4] (a) N. Farrell, Y. Qu, Inorg. Chem. 28 (1989) 3416;
(b) Y. Qu, N. Farrell, Inorg. Chem. 31 (1992) 930.
[5] J. Yoo, J.-W. Hwang, Y. Do, Inorg. Chem. 40 (2001) 568.
~
J. Organomet. Chem. 555 (1998) 17.
€€
[6] C. Vinas, M. Mar Abad, F. Teixidor, R. Sillanpaa, R. Kivekas,
€
In conclusion, 3–6 are the first examples of plati-
num(II)-amine complexes containing a 1,2-carborane
ligand. The geometry of the complexes appears to play
no significant role in their overall stability, but the
presence of more than one chloro ligand leads to facile
decomposition particularly in the presence of water. The
stability of the complexes is also related to the length of
the alkyl tether linking the platinum(II) centre to the
carborane moiety, with the aminopropylene complexes
exhibiting a much greater stability toward reduction of
~
[7] F. Teixidor, R. Benakki, C. Vinas, R. Kivekas, R. Sillanpaa, Inorg.
€
€€
Chem. 38 (1999) 5916.
[8] (a) J. Plesek, S. Hermanek, Coll. Czech. Chem. Commun. 39 (1974)
821;
(b) H.M. Colquhoun, T.J. Greenhough, M.G.H. Wallbridge,
S. Hermanek, J. Plesek, J. Chem. Soc., Dalton Trans. (1978)
944.
[9] (a) K. Base, M.W. Grinstaff, Inorg. Chem. 37 (1998) 1432;
(b) J.A. Todd, L.M. Rendina, Inorg. Chem. 41 (2002) 3331;
(c) J.A. Todd, D. Caiazza, E.R.T. Tiekink, L.M. Rendina, Inorg.
Chim. Acta 352 (2003) 208.