224 Organometallics, Vol. 18, No. 2, 1999
Sterenberg et al.
complex 4 or 5 is at the coordinatively unsaturated
platinum atom, resulting in displacement of Cl- or CO
by [Ir(CO)4]- followed by loss of carbonyl ligands and
insertion of iridium into a Pt-P bond. This naturally
leads to 6a and not 6c. Formation of 6c would require
attack by [Ir(CO)4]- at the coordinatively saturated
iridium center in 4 or 5, followed by insertion into an
Ir-P bond.
Step 3: Ad d ition of th e Th ir d d p p m Liga n d . The
final step in the cluster formation is the substitution of
the third dppm ligand to bridge the second Pt-Ir bond
by displacement of carbonyl ligands from these metal
atoms. Both 6a and 6b were shown to react easily with
dppm to form the cluster 3. In the case of 6a , the out-
of-plane dppm ligand moves to the in-plane position
during the substitution. Again it is likely that a phos-
phorus atom of the incoming dppm molecule first adds
to the coordinatively unsaturated platinum center of 6,
followed by CO loss and coordination of the second
phosphorus atom to iridium.
with only two µ-dppm ligands, which are still thermally
robust. Complexes with the more common M3(µ-dppm)3
unit can only react at positions perpendicular to the
M3 triangle, but complexes such as 6 having the
M3(µ-dppm)2 unit also have two labile carbonyl ligands
in the plane and are less sterically hindered. They may
therefore exhibit higher reactivity and prove to be
particularly versatile models for supported platinum
catalysts. The binuclear PtIr clusters described here are
of interest because few such complexes having Pt-Ir
bonds are known11 and, in view of the interesting
chemistry of both diplatinum and diiridium complexes,12
the heterobinuclear Pt-Ir-bonded complexes can be
expected to have interesting properties.
Exp er im en ta l Section
Infared spectra were recorded as Nujol mulls by using a
1
Perkin-Elmer 2000 FTIR spectrometer. The H, 31P{1H}, and
13C{1H} NMR spectra were recorded using a Varian Gemini
300 NMR spectrometer. The compound [PPN][Ir(CO)4]5a was
prepared according to literature methods, and [Pt(dppm)2]Cl2
was prepared by reaction of [PtCl2(SMe2)2]13 with 2 equiv of
dppm in CH2Cl2. All manipulations were carried out using
standard Schlenk techniques under an atmosphere of prepu-
rified argon.
Con clu sion s
The cluster complex [PtIr2(CO)2(µ-CO)(µ-dppm)3] (3)
is readily synthesized by the reaction of [Pt(dppm)2]Cl2,
[Ir(CO)4]-, and dppm. The cluster formation proceeds
by way of dppm-bridged heterobinuclear Pt-Ir-bonded
complexes, which can also be isolated under appropriate
experimental conditions. The mechanism defined by
Scheme 2 is clearly capable of extension. For example,
addition of a new carbonyl anion to 4 or 5 might give
clusters containing three different metal atoms, or
addition of [Ir(CO)4]- to one of many known coordina-
tively unsaturated binuclear dppm-bridged complexes
might lead to new heteronuclear clusters containing
iridium.
Syn th esis of Com p ou n d s. [P tIr 2(CO)2(µ-CO)(µ-d p p m )3]
(3). The compounds PPN[Ir(CO)4] (100 mg, 0.119 mmol), [Pt-
(dppm)2]Cl2 (61 mg, 0.059 mmol), and dppm (23 mg, 0.059
mmol) were dissolved in CH2Cl2 (15 mL), resulting in the
formation of a yellow solution. The solution was stirred for 24
h, at which point the color had changed to deep orange. The
solvent was removed in vacuo, and the residue was extracted
into THF (3 mL) and filtered through Celite. The solvent was
removed from the filtrate, and the residue was recrystallized
from CH2Cl2/pentane. Yield: 70 mg, 65%. Anal. Calcd for
C
78H66Ir2O3PtP6: C, 51.56; H, 3.66. Found: C, 51.53; H, 3.49.
IR: ν(CO) 1944 s, 1901 ms, 1610 mb cm-1. NMR in CD2Cl2:
δ(1H) 3.67 [bm, 1H, dppm CH2], 4.60 [b, 2H, dppm CH2], 4.64
[bm, 1H, dppm CH2], 5.59 [b, 2H, dppm CH2]; δ(13C) 196 [2C,
terminal CO], 272 [1C, bridging CO]; δ(31P) -16.3 [dm, 3J (PaPb)
Cluster 3 is the third in a series of analogous tri-
metallic clusters, ranging from the dicationic, 42-elec-
tron cluster [Pt3(µ3-CO)(µ-dppm)3]2+ (1)6 to the cationic,
44-electron cluster [Pt2Ir(CO)(µ3-CO)(µ-dppm)3]+ (2)1b to
the neutral 46-electron cluster [PtIr2(CO)2(µ-CO)(µ-
dppm)2] (3). The series illustrates two effects. The most
obvious is the trend in electron count and structure,
arising from the tendency of platinum and iridium to
have 16- and 18-electron configurations, respectively.
However, a second trend is that the presence of iridium
atoms reduces the reactivity of the platinum centers
toward further ligand addition. Thus, cluster 1 revers-
ibly adds CO to give 44-electron [Pt3(µ3-CO)(CO)(µ-
) 132 Hz, J (PaPt) ) 3350 Hz, Pa], -28.6 [dm, J (PaPb) ) 132
1
3
Hz, Pb], -45.7 [m, Pc].
[P tIr Cl(CO)2(µ-d p p m )2] (4). The compounds [Pt(dppm)2]-
Cl2 (276 mg, 0.048 mmol) and PPN[Ir(CO)4] (224 mg, 0.048
mmol) were dissolved in acetone (25 mL) which had been
degassed with two freeze-pump-thaw cycles. The white solids
dissolved gradually to form a yellow solution, which slowly
changed color to golden yellow. After 3 h of stirring, the solvent
was removed in vacuo, and the residue was extracted into
THF/Et2O (3 mL/6 mL) and the extract filtered through Celite.
The solvent was removed in vacuo, and the residue was
recrystallized from CH2Cl2/pentane. Yield: 255 mg, 76%. Anal.
Calcd for C52H44ClIrO2PtP4: C, 50.06; H, 3.55; Cl, 2.84.
Found: C, 49.86; H, 3.59; Cl, 2.78. IR(Nujol): ν(CO) 1977 (mb),
dppm)3]2+ and 46-electron [Pt3(µ-CO)(CO)2(µ-dppm)3]2+
,
which are isostructural with 2 and 3, respectively, but
neither 2 nor 3 reacts with CO. It is particularly
noteworthy that 2 fails to add CO to give a 46-electron
cluster; this could just be a charge effect (2 is a
monocation, 1 is a dication), or it could be due to
differences in the nature of the metal-metal bonding.
In terms of the relationship to bimetallic Pt-Ir cata-
lysts, these results suggest that iridium at the bimetallic
surface might become coordinatively saturated and so
reduce the effective size of active platinum clusters or
that the presence of neighboring iridium might reduce
the reactivity of neighboring platinum atoms by Pt-Ir
bonding effects.
3
1886 (sb) cm-1. NMR in acetone-d6: δ(1H) 4.56 [m, J (PtH) )
2
2
58 Hz, dppm CH2]; δ(13C) 181.0 [t, J (PC) ) 14 Hz, J (PtC) )
51 Hz, IrCO]; δ(31P) 8.68 [m, 1J (PtP) ) 2975 Hz, PtP], -19.94
2
[m, J (PtP) ) 245 Hz, IrP].
[P tIr (CO)3(µ-dppm )2][P F6] (5). The compounds [Pt(dppm)2]-
[PF6]2 (74 mg, 0.059 mmol) and PPN[Ir(CO)4] (50 mg, 0.059
(11) (a) McEwan, D. M.; Markham, D. P.; Pringle, P. G.; Shaw, B.
L. J . Chem. Soc., Dalton Trans. 1986, 1809. (b) Markham, D. P.; Shaw,
B. L.; Thornton-Pett, M. J . Chem. Soc., Chem. Commun. 1987, 1005.
(c) Carr, S. W.; Pringle, P. G.; Shaw, B. L. J . Organomet. Chem. 1988,
341, 543.
(12) (a) Chaudret, B.; Delavaux, B.; Poilblanc, R. Coord. Chem. Rev.
1988, 86, 191. (b) Puddephatt, R. J . Chem. Soc. Rev. 1983, 12, 99.
(13) Hill, G. S.; Irwin, M. J .; Levy, C. J .; Rendina, L. M.; Puddephatt,
R. J . Inorg. Synth. 1998, 32, 149.
The clusters 6a and 6b are unusual for late-transi-
tion-metal clusters in having a triangle of metal atoms