Heterobinuclear Methyl Complexes of Rhodium/Iridium: Reactivity with Nucleophiles and Subsequent Ligand Rearrangements

Article abstract of DOI:10.1021/om980947y

Reaction of [RhIr(CH₃)(CO)₃(dppm)₂][CF₃SO ₃] (1) (dppm = Ph₂PCH₂PPh₂) with several phosphines and phosphites yields the carbonyl-substitution products [RhIr(CH₃)(PR₃)(CO)₂-(dppm) ₂][CF₃SO₃] (4), in which the added phosphine and the methyl ligands are coordinated to Ir. At -80°C an intermediate in this reaction, [RhIr(CH₃)(PR₃)(CO)₃(dppm)₂][CF ₃SO₃] (3), is observed in which the monodentate PR₃ group is bound to Rh. Similar reactions with the dicarbonyl compound [RhIr(CH₃)(CO)₂(dppm)₂][CF₃SO ₃] (2) suggests that attack occurs directly at Ir in this case. The structure of 2 is subtly different in solution and in the solid state, having the carbonyls both terminally bound (one to each metal) in solution, but having one bridging in the solid. Addition of cyanide and hydride ligands to 2 yields the neutral products [RhIr(CH₃)X(CO)₂(dppm)₂] (X = CN, H), in which both anionic ligands (X and CH₃) are bound to Ir. The hydrido species is unstable, eliminating methane at ambient temperature. Addition of iodide ion to 2 yields the related neutral species, which has the methyl group on Rh and the iodide ligand on Ir. The X-ray structures of compounds 2 and 4a (PR₃ = PMe₃) are reported. Compound 2 has a terminal carbonyl on Rh, essentially opposite the Rh-Ir bond, the methyl group in a similar position on Ir, and a semibridging carbonyl, which is more tightly bound to Ir. The geometry of 4a has a square-planar Rh center in which Ir occupies one of the coordination sites opposite a carbonyl, and an octahedral Ir center in which the PMe₃ group is opposite the Ir-Rh bond, and the methyl group is opposite the second carbonyl. The crowding at Ir upon addition of the PMe₃ group shows up in a number of distortions within the molecule.

Full text of DOI:10.1021/om980947y

Organometallics 1999, 18, 1629-1640
1629
Heter obin u clea r Meth yl Com p lexes of Rh od iu m /Ir id iu m :
Rea ctivity w ith Nu cleop h iles a n d Su bsequ en t Liga n d
Rea r r a n gem en ts
Okemona Oke, Robert McDonald,† and Martin Cowie*
Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
Received November 24, 1998
Reaction of [RhIr(CH3)(CO)3(dppm)2][CF3SO3] (1) (dppm ) Ph2PCH2PPh2) with several
phosphines and phosphites yields the carbonyl-substitution products [RhIr(CH3)(PR3)(CO)2-
(dppm)2][CF3SO3] (4), in which the added phosphine and the methyl ligands are coordinated
to Ir. At -80 °C an intermediate in this reaction, [RhIr(CH3)(PR3)(CO)3(dppm)2][CF3SO3]
(3), is observed in which the monodentate PR3 group is bound to Rh. Similar reactions with
the dicarbonyl compound [RhIr(CH3)(CO)2(dppm)2][CF3SO3] (2) suggests that attack occurs
directly at Ir in this case. The structure of 2 is subtly different in solution and in the solid
state, having the carbonyls both terminally bound (one to each metal) in solution, but having
one bridging in the solid. Addition of cyanide and hydride ligands to 2 yields the neutral
products [RhIr(CH3)X(CO)2(dppm)2] (X ) CN, H), in which both anionic ligands (X and CH3)
are bound to Ir. The hydrido species is unstable, eliminating methane at ambient
temperature. Addition of iodide ion to 2 yields the related neutral species, which has the
methyl group on Rh and the iodide ligand on Ir. The X-ray structures of compounds 2 and
4a (PR3 ) PMe3) are reported. Compound 2 has a terminal carbonyl on Rh, essentially
opposite the Rh-Ir bond, the methyl group in a similar position on Ir, and a semibridging
carbonyl, which is more tightly bound to Ir. The geometry of 4a has a square-planar Rh
center in which Ir occupies one of the coordination sites opposite a carbonyl, and an octahedral
Ir center in which the PMe3 group is opposite the Ir-Rh bond, and the methyl group is
opposite the second carbonyl. The crowding at Ir upon addition of the PMe3 group shows up
in a number of distortions within the molecule.
In tr od u ction
the Rh/Ir compounds. In attempts to learn more about
the factors influencing the reactivity of methyl groups
in the presence of adjacent metal centers, we have
extended our studies on the reactivity of the mixed Rh/
Ir methyl complex 1, to include the dicarbonyl complex
[RhIr(CH3)(CO)2(dppm)2][CF3SO3] (2). We were inter-
ested in determining the sites of ligand attack in these
two species, the subsequent ligand rearrangements that
occur, and the roles of the different metals in the
chemistry.
The reaction of the diiridium, dicarbonyl methyl
complex [Ir2(CH3)(CO)2(dppm)2][CF3SO3] with a variety
of small molecules results in the unusually facile C-H
bond cleavage of the methyl group by the adjacent metal
to give the methylene-bridged products [Ir2H(CO)2L(µ-
CH2)(dppm)2][SO3CF3] (L ) CO, SO2, CNR, PR3).1
Related studies with the mixed Rh/Ir tricarbonyl com-
plex [RhIr(CH3)(CO)3(dppm)2][CF3SO3] (1) have yielded
very different results.2 Whereas reaction of 1 with CO
gave rise to a number of uncharacterized products,
reaction with SO2 gave an acyl product in which the
methyl group has migrated to a carbonyl and the added
SO2 molecule bridges the metals, and reaction with
tBuNC gave an iminoacyl-bridged species resulting from
loss of a carbonyl and migratory insertion of the methyl
and isocyanide groups. The products of these reactions
were often the result of carbonyl substitution by the
added ligand, to give complexes having stoichiometries
analogous to the methylene-bridged diiridium products,
but no example of C-H bond cleavage was observed for
Although methyl-bridged species were implicated
throughout the studies of the diiridium complexes, they
were only proposed as transient intermediates and were
never observed. It was reasoned that the lower tendency
of Rh to oxidatively add across a C-H bond3 might allow
the isolation (or at least observation) of such methyl-
bridged species. We have therefore undertaken a study
of the reactivity of complexes 1 and 2 with a number of
phosphines and phosphites and with a few anionic
nucleophiles in attempts to learn more about the bind-
ing and subsequent activation of methyl groups under
a range of electronic and steric environments.
* Fax: (780)492-8231. E-mail: martin.cowie@ualberta.ca.
† Faculty Service Officer, Structure Determination Laboratory.
(1) (a) Antwi-Nsiah, F.; Cowie, M. Organometallics 1992, 11, 3157.
(b) Torkelson, J . R.; Antwi-Nsiah, F. H.; McDonald, R.; Cowie, M.;
Pruis, J . G.; J alkanen, K. J .; DeKock, R. L. J . Am. Chem. Soc., in press.
(2) (a) Antwi-Nsiah, F. H.; Oke, O.; Cowie, M. Organometallics 1996,
15, 506. (b) Antwi-Nsiah, F. H.; Oke, O.; Cowie, M. Organometallics
1996, 15, 1042.
(3) Collman, J . P.; Hegedus, L. S.; Norton, J . R.; Finke, R. G.
Principles and Applications of Organotransition Metal Chemistry;
University Science Books: Mill Valley, CA, 1987; p 239.
10.1021/om980947y CCC: $18.00 © 1999 American Chemical Society
Publication on Web 04/08/1999

1630 Organometallics, Vol. 18, No. 9, 1999
Oke et al.
removed in vacuo. Recrystallization from CH2Cl2/ether yielded
a yellow microcrystalline solid (65 mg, 68% yield). Anal. Calcd
for 4a , IrRhSP5F3O5C57H56: C, 50.29; H, 4.11. Found: C, 50.34;
H, 3.91. In the preparation of 4b-d , the procedure is the same
as described for 4a except that the reactions were carried out
using 1 equiv each of P(OPh)3, PPhMe2 and PPh(OMe)2,
respectively. Anal. Calcd for 4b, IrRhSP5F3O8C72H62: C, 54.24;
H, 3.92. Found: C, 54.19; H, 3.96 (85 mg, 70% yield). Anal.
Calcd for 4c, IrRhSP5F3O5C62H58: C, 52.31; H, 4.08. Found: C,
52.23; H, 3.70 (83 mg, 77% yield). Anal. Calcd for 4d ,
IrRhSP5F3O7C62H58: C, 51.16; H, 3.99. Found: C, 50.74; H, 4.13.
(d ) [Rh Ir (CH3)(P R2R′)(µ-CO)(d p p m )2][CF 3SO3] (R ) R′
) Me (5a ), R ) Me, R′ ) P h (5c)). Compound 4a (50 mg,
0.037 mmol) and a large excess of Me3NO (14 mg, 0.187 mmol)
were charged into a 50 mL flask and dissolved in 10 mL of
CH2Cl2, and the resulting mixture was stirred under a slow
N2 flow for 6 h, during which time the yellow color of the
starting material turned deep orange. The solution was taken
to dryness in vacuo and redissolved in ca. 1 mL of CH2Cl2. A
dark brown-orange solid was precipitated upon addition of 5
mL of ether. The product was separated by filtration, washed
twice with 5 mL of ether, and dried under N2 flow and in vacuo
overnight. The procedure for preparing 5c was similar except
that the reaction time was 2 h. Microanalyses for 5a and 5c
consistently gave variable results due to difficulties in separat-
ing the excess Me3NO used in these reactions.
Exp er im en ta l Section
Gen er a l Com m en ts. All solvents were appropriately dried
and distilled prior to use and were stored under dinitrogen.
Deuterated solvents used for NMR experiments were degassed
and stored under dinitrogen over molecular sieves. Reactions
were carried out routinely at room temperature (unless
otherwise stated) and under standard Schlenk conditions;
compounds that were isolated as solids were purified by
recrystallization. Hydrated rhodium trichloride was purchased
from Engelhard Scientific, whereas all the phosphines and
phosphites, silver trifluoromethanesulfonate (AgOTf), Super-
Hydride (lithium triethylborohydride (1.0 M solution in THF)),
tetrabutylammonium cyanide (96%), and trimethylamine N-
oxide were obtained from Aldrich and were used as received.
Potassium iodide was purchased from BDH Chemicals, while
13CO was supplied by Isotec Inc. All gases were used as
received. The compound [RhIr(CH3)(CO)3(dppm)2][CF3SO3]2b
(1) was prepared as previously reported, while the compounds
[RhIr(CH3)(CO)2(PR3)(dppm)2][CF3SO3] (4a , R ) Me; 4b, R )
OPh)4 were previously prepared but incompletely character-
ized.
All routine NMR experiments were conducted on a Bruker
AM-400 spectrometer, whereas the 13C{31P} experiments were
conducted on a Bruker AM-200 spectrometer operating at
50.32 MHz. H-13C HMQC experiments were carried out on
1
a Varian Unity 500 MHz spectrometer. The solid-state 13C-
{1H} NMR spectrum was recorded on a Bruker AMR-300
spectrometer (with MAS accessory) operating at 75.5 MHz.
Infrared spectra were obtained on a Nicolet 7199 Fourier
transform or a Perkin-Elmer 883 IR spectrometer, either as
Nujol mulls on KBr plates or as solutions in KCl cells with
0.5 mm window path lengths. Elemental analyses were
performed by the microanalytical service within our depart-
ment. Spectroscopic data for all compounds are given in Table
1.
P r ep a r a t ion of Com p ou n d s. (a ) [R h Ir (CH 3)(CO)2-
(d p p m )2][CF 3SO3] (2). Compound 1 (60 mg, 0.046 mmol) was
dissolved in 20 mL of CH2Cl2 and was gently refluxed under
a slow dinitrogen purge (ca. 0.2 mL s-1) for 1 h, during which
time the orange color of the solution deepened slightly.
Removal of the solvent under vacuum and recrystallization
from CH2Cl2/ether gave a bright orange solid (53 mg, 90%
yield). Anal. Calcd for IrRhSP4F3O5C54H47: C, 50.51; H, 3.66.
Found: C, 50.23; H, 3.45.
(b) Low -Tem p er a tu r e Rea ction of Com p ou n d s 1 a n d
2 w ith P h osp h in es a n d P h osp h ites. In an NMR tube, 30
mg (0.023 mmol) of compound 1 was dissolved in 0.6 mL of
CD2Cl2 under N2. The solution was then cooled to ca. -80 °C
by immersion in a dry ice/acetone bath. PMe3 (46 µL of 1 M
THF solution) was added via a syringe, and the mixture was
taken immediately for NMR analysis. The 31P{1H} and 1H
NMR data were collected at -60 °C, while 13C{1H} NMR
analysis was carried out at -40 °C. The NMR spectra showed
the presence of [RhIr(CH3)(CO)3(PMe3)(dppm)2][CF3SO3] (3a ).
Upon gradually warming the sample to 0 °C, 3a was replaced
by [RhIr(CH3)(CO)2(PMe3)(dppm)2][CF3SO3] (4a ). A similar
procedure was performed for P(OPh)3, PPh(OMe)2, and PPhMe2
for which the species 3b-d , respectively, were observed in
solution. For the reaction of compound 2 and PMe3, the
procedure was as described above except that 1 equiv of PMe3
was used for the reaction, and NMR data were collected at
-100 °C. Only compound 4a was observed in solution at this
temperature.
(e) [Rh Ir (CH3)(CN)(CO)2(d p p m )2] (6). A 30 mg (0.023
mmol) sample of compound 2 and 7 mg (0.026 mmol) of
tetrabutylammonium cyanide were charged into an NMR tube
and dissolved in 0.5 mL of THF-d8, causing an immediate color
change from orange to pale yellow. The sample was im-
mediately taken for NMR analysis. Attempts to isolate the
product resulted in formation of an intractable oil.
(f) Rea ction of Com p ou n d 2 w ith Su p er -Hyd r id e. A 30
mg (0.023 mmol) sample of 2 was dissloved in 0.5 mL of CD2-
Cl2 in an NMR tube at -80 °C. Approximately 1 equiv of
Super-Hydride (LiH(BEt3)3, (23 µL of 1 M THF solution, 0.023
mmol) was syringed into the solution. NMR data were collected
immediately. The compound [RhIr(CH3)(H)(CO)2(dppm)2][CF3-
SO3] (7) was shown to be the major 31P-containing species at
this temperature. The sample was then gradually warmed to
room temperature over a 0.5 h period causing the appearance
of [RhIr(CO)2(µ-H)(µ2-η3-(o-C6H4)P(Ph)CH2PPh2)(dppm)] (8),
the tricarbonyl species [RhIr(CO)3(dppm)2], and several uni-
dentified decomposition products. Compound 8 was never
successfully separated from the tricarbonyl and other species,
so was only characterized by NMR spectroscopy.
(g) [Rh Ir (CH3)(I)(CO)2(d p p m )2] (9). A 60 mg (0.046 mmol)
sample of 2 was dissolved in 10 mL of acetone. Slightly more
than 1 equiv of KI (8 mg, 0.048 mmol), dissolved in 5 mL of
acetone, was added via an addition funnel to a stirring solution
of 2 over a 0.5 h period, causing a slow color change from
orange to yellow. The mixture was stirred for 1 h, and the
solvent volume was then removed, to give an orange-yellow
residue. The product was extracted with 3 mL of CH2Cl2,
filtered through a frit, after which the solvent was removed
in vacuo, and then redissolved in 1 mL of acetone. A yellow
solid precipitated upon addition of 5 mL of pentane and which
was washed once with 5 mL of pentane, dried under N2 stream
and then in vacuo. Microanalysis consistently gave variable
results due to contamination with [RhIr(I)2(CO)2(dppm)2],
which was identified in the sample and has been previously
characterized.5
X-r a y Da ta Collection for [Rh Ir (CH3)(CO)2(d p p m )2]-
[CF 3SO3] (2). Orange crystals of 2 were obtained by slow
diffusion of ether into a concentrated solution of the compound.
(c) [Rh Ir (CH3)(CO)2(P R2R′)(d p p m )2][CF 3SO3] (RdR′ )
Me (4a ); RdR′ ) OP h (4b); R ) Me, R′ ) P h (4c); R ) OMe,
R′ ) P h (4d )). A solution of 1 equiv of neat PMe3 (97%, 15.7
µL, 0.076 mmol) was syringed into a 100 mL flask containing
100 mg (0.076 mmol) of 1 dissolved in 10 mL of CH2Cl2, causing
the dark orange solution to turn yellow immediately. The
mixture was stirred for 2 h, after which the solvent was
(4) Antwi-Nsiah, F. H. Ph.D. Thesis, University of Alberta, 1994;
Chapter 2.
(5) Vaartstra, B. A.; Xiao, J .; J enkins, J . A.; Verhagen, R.; Cowie,
M. Organometallics 1991, 10, 2708.

Heterobinuclear Methyl Complexes of Rh/ Ir
Organometallics, Vol. 18, No. 9, 1999 1631

1632 Organometallics, Vol. 18, No. 9, 1999
Ta ble 2. Cr ysta llogr a p h ic Exp er im en ta l Deta ils for Com p ou n d s 2 a n d 4a
Oke et al.
2
4a ‚2THF
A. Crystal Data
formula
fw
C54H47F3IrO5P4RhS
1283.97
C65H72F3IrO7P5RhS
1504.25
cryst dimers (mm)
cryst syst
space group
unit cell params
a (Å)
0.62 × 0.42 × 0.24
0.36 × 0.21 × 0.08
monoclinic
P21/n
monoclinic
P21/n (a nonstandard setting of P21/c [No. 14])
13.858(1)
15.637(1)
23.671(1)
96.525(6)
5096.1(7)
4
16.431(2)
23.117(3)
17.851(2)
93.41(1)
6768(1)
4
b (Å)
c (Å)
â (deg)
V (Å3)
Z
Fcalcd (g cm-3
)
1.673
9.661
1.476
2.414
µ (mm-1
)
B. Data Collection and Refinement Conditions
Siemens P4/RAa Enraf-Nonius CAD4d
diffractometer
radiation [l (Å)]
temp (°C)
scan type
data collcn 2θ limit (deg)
total no. of data collcd
no. of ind reflns
graphite-monochromated Cu KR (1.541 78)
graphite-monochromated Mo KR (0.710 73)
-60
-50
θ-2θ
115.0
θ-2θ
50.0
7133 (0 e h e 15, 0 e k e 17, -23 e l e 23)
12 274 (-19 e h e 19, 0 e k e 27, 0 e l e 21)
6817
11867
2
2
no. of obs (NO)
6119 (Fo g 2σ(Fo2))
4211 (Fo g 2σ(Fo2))
structure solution method
refinement method
abs corr method
direct methods/fragment search (DIRDIF-96)
full-matrix least-squares on F2 (SHELXL-93)
semiempirical (ψ scans)
direct methods (SHELXS-86)
full-matrix least-squares on F2 (SHELXL-93)
DIFABSe
range transm factors
no. of data/restraints/params
goodness-of-fit (S)b
final R indicesc
0.9595-0.1741
1.217-0.619
6815 [Fo g -3σ(Fo2)]/0/630
11858 [Fo g -3σ(Fo2)]/39f/647
2
2
2
2
1.087 [Fo g -3σ(Fo2)]
0.998 [Fo g -3σ(Fo2)]
2
R1 [Fo g 2σ(Fo2)]
0.0726
0.1980
2.474 and -3.664
0.0915
0.3249
1.761 and -2.362
2
wR2 [Fo g -3σ(Fo2)]
largest diff peak and hole (e Å)-3
a
b
Programs for diffractometer operation, data collection, data reduction, and absorption correction were those supplied by Siemens.
S
,
) [∑w(Fo - Fc2)2/(n - p)]1/2 (n ) number of data; p ) number of parameters varied; w )[σ2(Fo2) + (aoP)2 + a1P]-1 where P ) [max(Fo
2
2
0) + 2Fc2]/3).6 c R1 ) ∑(|Fo| - |Fc|)/∑|Fo|; wR2 ) ∑w(Fo - Fc2)2/∑w(Fo4)]1/2
.
Programs for diffractometer operation and data collection
2
d
were those supplied by Enraf-Nonius. e See ref 7. f Restraints were applied to fix the geometries of the triflate group (S-C91 ) 1.80 Å;
S-O91 ) S-O92 ) S-O93 ) 1.45 Å, F91-C91 ) F92-C91 ) F93-C91 ) 1.35 Å; F91-F92 ) F91-F93 ) F92-F93 ) 2.20 Å; O91-O92
) O91-O93 ) O92-O93 ) 2.20 Å; F91-O91 ) F91-O92 ) F92-O91 ) F92-O93 ) F93-O92 ) F93-O93 ) 3.04 Å) and the THF rings
(all bond distances 1.50 Å; all 1,3 distances [e.g. C101-C103] 2.45 Å).
2
Several crystals were mounted and flame-sealed in glass
capillaries under solvent vapor to minimize decomposition or
deterioration due to solvent loss. Unit cell parameters (at -60
°C) were obtained from a least-squares refinement of 33
reflections in the approximate range 57.0° < 2θ < 58.0°. The
cell parameters and systematic absences (h0l, h+l ) odd, 0k0,
k ) odd) defined the space group P21/n. Data were collected
at -60 °C on a Siemens P4RA diffractometer using graphite-
monochromated Cu KR radiation, employing a θ/2θ scan
technique to a maximum 2θ ) 115.0°. Backgrounds were
scanned for 25% of peak width on either side of the peak scan.
Of 7133 unique reflections, 6119 were considered to be
were considered to be observed [Fo g 3σ(Fo2)] and were used
in subsequent calculations. Absorption corrections were ap-
plied to the data using the method of Walker and Stuart.7,8
Crystal parameters and details of data collection are given in
Table 2.
Str u ctu r e Solu tion a n d Refin em en t for Com p ou n d 2.
The positions of Rh, Ir, and phosphorus atoms were located
using direct methods/fragment search program DIRDIF-96;9
the remaining atoms were found using a succession of least-
squares and Fourier maps. Refinement of the structure
proceeded using the program SHELXL-93.10 The hydrogen
positions were calculated by assuming idealized sp2 or sp3
geometries about their attached carbon atoms as appropriate
and were given thermal parameters 120% of the equivalent
isotropic displacement parameters of the attached carbons.
Initial refinements indicated unusually high thermal param-
eter for the atoms of the terminal and bridging carbonyls and
the methyl carbon atoms. In addition, the isotropic thermal
2
observed [Fo g 3σ(Fo2)] and were used in subsequent calcula-
tions. Absorption correction to 2 was by semiempirical method
using ψ scans. Crystal parameters and details of data collec-
tion are given in Table 2.
X-r a y Da t a Collect ion for [R h Ir (CH 3)(CO)2(P Me3)-
(d p p m )2][CF 3SO3] (4a )‚2THF . Diffusion of ether into a
concentrated THF solution of 4a yielded yellow crystals of the
complex, several of which were mounted and flame-sealed in
glass capillaries under solvent vapor to minimize decomposi-
tion and (or) solvent loss. Data were collected at -50 °C on an
Enraf-Nonius CAD4 diffractometer using graphite-monochro-
mated Mo KR radiation. Unit cell parameters were obtained
from a least-squares refinement of the setting angles of 24
reflections in the range 20.0° < 2θ < 23.9°. The monoclinic
diffraction symmetry and systematic absences (h0l, h+l ) odd,
0k0, k ) odd) were consistent with the space group P21/n.
Intensity data were collected using the θ/2θ scan technique to
a maximum 2θ ) 50.0°. Of 11 867 unique reflections, 4211
(6) The coefficients ao and a1 in the weighting scheme are suggested
by the least-squares refinement program: for 2, ao ) 0.1404 and a1
)
22.2339; for 4a ‚2THF, ao ) 0.1435 and a1 ) 0.
(7) Walker, N.; Stuart, D. Acta Crystallogr., Sect A: Found Crys-
tallogr. 1983, A39, 158.
(8) Programs used were those of the Enraf-Nonius Structure De-
termination Package by B. A. Frenz, in addition to local Programs by
R. G. Ball.
(9) Beurskens, P. T.; Beurskens, G.; Bosman, W. P.; de Gelder, R.;
Garcia Granda, S.; Gould, R. O.; Israel, R.; Smith, J . M. M. The
DIRDIF-96 program system; Crystallography Laboratory, University
of Nijmegen: The Netherlands, 1996.

Heterobinuclear Methyl Complexes of Rh/ Ir
Organometallics, Vol. 18, No. 9, 1999 1633
unprimed atoms shown in Figure 1 have 55% occupancy
(except for O(1)) and the primed ones have 45% occupancy.
The solid bonds in C connect atoms of the major occupant,
while the open bonds connect atoms of the minor occupant.
Due to the disorder there will be some degree of uncertainty
about the bond distances involving the disordered atoms;
however the gross structure is clearly established. In addition,
the bridging mode of the carbonyl group in the solid state was
established by solid-state 13C NMR spectral results (vide infra).
A similar disorder has been observed in the X-ray structure
analyses for [RhIr(CH3)(CO)3(dppm)2][CF3SO3],2b [Ir2(CH3)-
(CO)2(dppm)2][[CF3SO3],1b [RhMn(CO)3(µ-CO3)(dppm)2],11 and
[RhM(CO)4(dppm)2] (M ) Mn, Re).12 Location of the dppm
atoms in 2 proceeded well with no evidence of disorder.
Str u ctu r e Solu tion a n d Refin em en t for com p ou n d 4a .
The positions of the Rh, Ir, and phosphorus atoms were
obtained through use of the direct-methods program SHELXS-
86.13 The remaining non-hydrogen atoms were located using
successive least-squares and difference Fourier maps. All
hydrogen atoms with the exception of the THF hydrogens were
included as fixed contributions but not refined. Their idealized
positions were calculated from the geometries about the
attached carbon atoms, using a C-H bond length of 0.95 Å,
and they were assigned thermal parameters 20% greater than
the equivalent isotropic U’s of their attached carbon atoms.
Refinement was completed using the program SHELXL-93.10
All non-hydrogen atoms of the complex cation and anion were
located. In addition, two molecules of THF per formula unit
of complex were also located.
Resu lts a n d Com p ou n d Ch a r a cter iza tion
The heterobinuclear methyl complex [RhIr(CH3)(CO)3-
(dppm)2][CF3SO3] (1) has previously been characterized2
and has been shown to have the geometry depicted in
Scheme 1, in which the methyl group is terminally
bound to iridium in the site opposite the Rh-Ir bond.
This species is static on the NMR time scale at ambient
temperature and shows no evidence of carbonyl ex-
change between the metals or transfer of the methyl
group from metal to metal as occurs readily in the
F igu r e 1. Illustration of the disorder within the “RhIr-
(CH3)(CO)(µ-CO)” unit of compound 2. Drawing A shows
the orientation of the molecule having 55% occupancy, B
shows the 45% occupancy molecule, and C shows the two
superimposed molecules. Unprimed atoms have 55% oc-
cupancy (except for O(1), which has 100% occupancy), and
primed atoms have 45% occupancy, so the metal position
Ir/Rh′ was refined with a mixed occupancy of 55% iridium
and 45% rhodium.
14
dicarbonyl analogues of Rh2 and Ir2.1 The exact
analogue of these homobimetallic, dicarbonyl complexes
can be generated through CO loss from 1 by refluxing
in CH2Cl2 to give [RhIr(CH3)(CO)2(dppm)2][CF3SO3] (2).
Based on previous work, two structures seemed possible
for 2 in the solid state, having the methyl group either
terminally bound to one metal, as in the solid-state
structures of [M2(CH3)(CO)(µ-CO)(dppm)2][CF3SO3] (M
) Rh,14 Ir1), or bridging the metals as in the dmpm
analogue [Ir2(CO)2(µ-CH3)(dmpm)2][CF3SO3] (dmpm )
Me2PCH2PMe2).15 The IR spectrum of 2 in the solid
indicates that one carbonyl is terminal (1997 cm-1) and
one is bridging (1858 cm-1). The CP-MAS 13C NMR
spectra of 13CO- and 13CH3-enriched samples of 2 were
undertaken in order to help establish its solid-state
structure. A broad (54 Hz at peak half-height) overlap-
ping pair of doublets, each displaying coupling to Rh of
80 Hz, appear at δ 177.0 and δ 176.0 for the Rh-bound
carbonyl. The observation of two resonances is consis-
tent with the disorder of 2 in the solid state (see
parameter for Ir was unusually high while that of Rh was
unusually low. This suggested a rotational disorder in which
there are two superimposed orientations for the complex as
depicted in Figure 1 (dppm ligands omitted for clarity). The
disorder of the terminal carbonyl group attached to Rh was
suggested by the extreme elongation of the thermal ellipsoids
of C(3) and O(3) in a direction perpendicular to the Rh-C(3)
bond axis. Subsequent difference Fourier maps indicated that
the disordered methyl and carbonyl carbons were not exactly
superimposed (with the exception of the oxygen atom O(1) of
the bridging carbonyl, which was shared between the two
disordered molecules and was refined at full occupancy) but
were slightly offset from each other. This together with the
anomalous thermal parameters for the metals led to the model
shown in which the two orientations A and B are disordered
in approximately a 55:45 ratio (determined from the relative
peak intensities of the ordered and disordered atoms), to give
the average structure C, having the weightings in which the
(10) Sheldrick, G. M. SHELXL-93. Program for crystal structure
determination; University of Gottingen, Gottengen, Germany, 1993.
2
2
Refinement on Fo for all reflections (all of those having Fo > -3σ-
(11) Antonelli, D. M.; Cowie, M. Organometallics 1991, 10, 2173.
(12) Antonelli, D. M.; Cowie, M. Organometallics 1990, 9, 1818.
(13) Sheldrich, G. M. Acta Crystallogr. 1990, A46, 467.
(14) Shafiq, F.; Kramarz, K. W.; Eisenberg, R. Inorg. Chim. Acta
1993, 213, 111.
(15) Reinking, M. K.; Fanwick, P. E.; Kubiak, C. P. Angew. Chem.,
Int. Ed. Engl. 1989, 20, 1377.
2
(Fo2). Weighted R factors wR2 and all goodness of fit are based on Fo
;
conventional R factors R1 are based on Fo, with Fo set to zero for
2
negative Fo’s. The observed criterion of Fo > -2σ(Fo2) is used only for
calculating R1 and is not relevant to the choice of reflections for
refinement. R factors based on Fo2 are statistically about twice as large
as those based on Fo, and R factors based on ALL data be even larger.

1634 Organometallics, Vol. 18, No. 9, 1999
Oke et al.
Sch em e 1
F igu r e 2. Perspective view of the [RhIr(CH3)(CO)(µ-CO)-
(dppm)2]+ complex ion of 2, showing the atom-labeling
scheme (only the major contributor to the disorder is
shown). Non-hydrogen atoms are shown with 20% thermal
ellipsoids; hydrogen atoms are shown with arbitrarily small
thermal parameters apart for those on the phenyl groups,
which are not shown.
from the X-ray results. However, the structure shown
in Figure 2 is consistent with all spectroscopic data in
the solid state (vide supra), and together these data
leave little doubt about the structure. Clearly one
carbonyl bridges the metals, and despite the disorder,
its semibridging nature, being more closely associated
with Ir than Rh, is established. The asymmetry in the
binding of the bridging carbonyl is obvious from the
parameters shown in Table 3, in which the Ir-C(1) bond
(1.99(2), 1.88(3) Å for the two disordered units) is
substantially shorter than the Rh-C(1) distance (2.28-
(2), 2.34(3) Å). This carbonyl is correspondingly more
linear with respect to Ir (Ir-C(1)-O(1) 153(2)°, 164(3)°;
Rh-C(1)-O(1) 124(2)°, 112(2)°). The asymmetry in this
carbonyl is in contrast to the essentially symmetrically
bridged carbonyl in the Rh2 and Ir2 analogues, and the
stronger binding of this bridging carbonyl to Ir is not
unexpected. This asymmetry is also reflected in the
carbonyl stretch (1858 cm-1), which is higher than both
the Rh2 (1851 cm-1) and Ir2 (1832 cm-1) analogues,
reflecting a slight tendency toward a terminal coordina-
tion in the mixed-metal species. All other parameters
for compound 2 are in close agreement with those for
the Rh2 and Ir2 analogues, and in particular the
terminal carbonyl is almost along the metal-metal axis
in all structures (M-M-CO angles: M2 ) Rh2, av 176°;
Ir2, av 177°; RhIr, av 175°), while the methyl group is
significantly off this axis (M-M-CH3 angles: M2 ) Rh2,
av 161°; Ir2, av 164°; RhIr, av 162°).
Experimental Section) in which there are two orienta-
tions of the complex and therefore two environments
for each carbonyl. The second carbonyl resonance ap-
pears at δ 193.0, in a downfield position consistent with
a bridging arrangement; however, the breadth of this
resonance (140 Hz) does not allow us to determine the
magnitude of the coupling to Rh. Presumably the
breadth of this resonance also masks the two environ-
ments anticipated for this carbonyl owing to the disor-
der. The resonance for the methyl group, at δ 14.8, is
far too broad (ca. 450 Hz) to display any useful coupling
information. The location of the terminal carbonyl on
Rh, rather than on Ir, is further supported by its
stretching frequency, which is 23 cm-1 higher than in
the Ir2 analogue,1 but close to the value of 2007 cm-1
observed in the Rh2 species.14 The possibility of the
triflate anion being coordinated to either metal is ruled
out by its IR spectrum, which shows stretches at 1226,
1257, and 1281 cm-1, consistent with the uncomplexed
ion.16 As such, the structure of 2, diagrammed in
Scheme 1, resembles the two homobinuclear, dppm-
bridged analogues, having a terminal methyl group and
a bridging carbonyl.
In solution the structure of 2 is somewhat different
than in the solid state. This is most evident from the
solution IR spectrum, which shows no bridging carbonyl
stretch and exhibits only one broad carbonyl band at
1954 cm-1; this band presumably is a combination of
the stretches for the terminal carbonyls on Rh and Ir.
The 13C{1H} NMR spectrum of 2 in solution shows one
carbonyl resonance at δ 175.4 with coupling of 73 Hz to
Rh and smaller coupling to the Rh-bound phosphines,
An X-ray structure determination of 2 was under-
taken in order to confirm the above bonding proposal
and to determine the asymmetry in the binding of the
bridging carbonyl. Unfortunately, owing to a 55:45
disorder, as described in the Experimental Section, a
detailed bonding picture cannot be established solely
(16) Stang, P. J .; Cao, D. Organometallics 1993, 12, 996.

Heterobinuclear Methyl Complexes of Rh/ Ir
Organometallics, Vol. 18, No. 9, 1999 1635
of mixed RhM (M ) Ru,17 Os,18 Ir,19 Mn,20 Re,21 Mo,22
W22) complexes in which Rh coupling was observed in
a carbonyl bound to the adjacent metal, unless involved
in some form of bridging interaction with Rh. Further-
more, the 13C{1H} chemical shift of this primarily Ir-
bound carbonyl is at lower field than that for the
carbonyl on Rh, opposite to what is expected,23 suggest-
ing a weak semibridging interaction.24 In further sup-
port of this formulation, DFT calculations on the Ir2
analogue [Ir2(CH3)(CO)2(H2PCH2PH2)2]+ (phenyl groups
on dppm substituted by hydrogens to facilitate calcula-
tions) independently arrived at the same structure and
also indicated that the carbonyl trans to CH3 was tipped
in slightly toward the adjacent metal,1b presumably
reflecting a weak interaction with it. It should be
emphasized that the NMR spectra of 2 in the solid and
in solution are only subtly different, reflecting the subtle
structural change in which the semibridging carbonyl
moves slightly away from Rh, accompanied by move-
ment of the methyl group off the Rh-Ir axis.
Ta ble 3. Selected Bon d Len gth s a n d An gles for
[Rh Ir (CH3)(CO)(µ-CO)(d p p m )2][CF 3SO3] (2)a
Selected Interatomic Distances (Å)
Ir-Rh
2.8290(7)
2.317(2)
2.319(2)
1.99(2)
2.14(3)
1.88(3)
2.03(3)
2.311(2)
2.310(3)
2.28(2)
1.84(2)
Rh′-C(1′)
Rh′-C(3′)
P(1)-C(4)
P(2)-C(4)
P(3)-C(5)
P(4)-C(5)
O(1)-C(1)
O(1)-C(1′)
O(3)-C(3)
O(3′)-C(3′)
2.34(3)
1.79(2)
1.837(9)
1.826(9)
1.820(10)
1.830(9)
1.07(2)
1.16(3)
1.07(3)
1.13(3)
Ir/Rh′-P(1)
Ir/Rh′-P(3)
Ir-C(1)
Ir-C(2)
Ir′-C(1′)
Ir′-C(2′)
Rh/Ir′-P(2)
Rh/Ir′-P(4)
Rh-C(1)
Rh-C(3)
Selected Interatomic Angles (deg)
Rh/Ir′-Ir/Rh′-P(1)
Rh/Ir′-Ir/Rh′-P(3)
Rh-Ir-C(1)
91.15(6) P(4)-Rh-C(1)
97.8(4)
87.3(6)
138.1(9)
41.3(8)
172.7(9)
91.5(7)
88.1(8)
94.8(7)
87.0(8)
146.0(12)
93.15(6) P(4)-Rh-C(3)
53.1(6)
161.0(7)
173.57(8) Ir′-Rh′-C(3′)
C(1)-Rh-C(3)
Ir′-Rh′-C(1′)
Rh-Ir-C(2)
P(1)-Ir/Rh′-P(3)
P(1)-Ir-C(1)
90.4(5)
85.7(7)
96.0(5)
88.7(7)
145.5(9)
55.4(9)
162.4(8)
91.1(8)
87.5(7)
97.9(8)
86.4(7)
P(1)-Rh′-C(1′)
P(1)-Rh′-C(3′)
P(3)-Rh′-C(1′)
P(3)-Rh′-C(3′)
C(1′)-Rh′-C(3′)
Ir/Rh′-P(1)-C(4) 113.7(3)
Rh/Ir′-P(2)-C(4) 113.2(3)
Ir/Rh′-P(3)-C(5) 113.0(3)
Rh/Ir′-P(4)-C(5) 113.1(3)
Ir-C(1)-Rh
Ir-C(1)-O(1)
P(1)-Ir-C(2)
P(3)-Ir-C(1)
P(3)-Ir-C(2)
Unlike the Rh2 and Ir2 analogues, which are fluxional
in solution, with carbonyl and methyl groups migrating
readily from metal to metal, compound 2, like its
tricarbonyl precursor (1), is static at ambient temper-
ature. Presumably the stronger Ir-CO and Ir-CH3
bonds25 inhibit migration to Rh.
C(1)-Ir-C(2)
Rh′-Ir′-C(1′)
Rh′-Ir′-C(2′)
P(2)-Ir′-C(1′)
P(2)-Ir′-C(2′)
P(4)-Ir′-C(1′)
P(4)-Ir′-C(2′)
C(1′)-Ir′-C(2′)
Ir/Rh′-Rh/Ir′-P(2)
Ir/Rh′-Rh/Ir′-P(4)
Ir-Rh-C(1)
82.7(6)
153.2(16)
123.9(15)
83.4(10)
164.3(26)
112.3(22)
178.9(19)
176.3(30)
109.7(5)
110.3(5)
Compounds 1 and 2 have been reacted with a number
of nucleophiles in order to determine the sites of attack
in both compounds, the rearrangements of added sub-
strate and the methyl group that may occur, and
whether a bridging methyl group could be observed that
would model the C-H bond-activation process in the
Ir2 analogues. We first looked at the reactivity of 1 and
142.2(13) Rh-C(1)-O(1)
92.92(6) Ir′-C(1′)-Rh′
90.76(6) Ir′-C(1′)-O(1)
44.2(5)
177.2(7)
170.77(8) Rh′-C(3′)-O(3′)
90.7(4)
88.7(6)
Rh′-C(1′)-O(1)
Rh-C(3)-O(3)
Ir-Rh-C(3)
P(2)-Rh/Ir′-P(4)
P(2)-Rh-C(1)
P(2)-Rh-C(3)
P(1)-C(4)-P(2)
P(3)-C(5)-P(4)
t
a
2 with phosphines; the reactions of 1 with BuNC, SO2,
Primed and unprimed atoms are related by a 55:45 disorder.
See Figure 1 for an explanation of labeling.
and ethylene have already been reported.2b Compound
1 reacts with the phosphines (PMe3, PMe2Ph, P(OMe)2Ph,
and P(OPh)3) at ambient temperature with accompany-
ing CO loss to yield the products [RhIr(CH3)(PR2R′)-
(CO)2(dppm)2][CF3SO3] (4), having the added phosphine
and the methyl group bound to Ir, as shown in Scheme
1. Although this reaction proceeded smoothly with the
phosphines listed, no reaction was observed with PPh3,
even after prolonged reaction times, presumably due to
restricted approach of this bulky substrate by interac-
tions with the phenyl groups of the dppm ligands. At
ambient temperature the 31P{1H} NMR spectra of all
species display three broad unresolved resonances in a
2:2:1 intensity ratio, whereas at -40 °C these spectra
transform to complicated patterns of five resonances
and another at 184.9 as a broad resonance. Resolution
enhancement of this latter resonance gives a doublet
with coupling to Rh of 7 Hz. The 13C{1H} NMR spectrum
of a 13CH3-enriched sample shows the methyl resonance
as a triplet at δ 17.8, and the 1H NMR (unenriched
sample) shows a triplet at δ 0.59, with only 31P coupling
observed in both cases. The absence of Rh coupling
clearly establishes that the methyl group is bound to
Ir. These spectroscopic data mirror the spectral changes
between solid and solution that were observed for the
diiridium analogue. We therefore propose a structure
for 2 in solution as shown below (labeled 2a ), much like
that proposed in the Ir2 complex.1b
(17) Sterenberg, B. T. Ph.D. Thesis, University of Alberta, 1997,
Chapter 5.
(18) (a) Sterenberg, B. T.; Hilts, R. W.; Moro, G.; McDonald, R.;
Cowie, M. J . Am. Chem. Soc. 1995, 117, 245. (b) Sterenberg, B. T.;
McDonald, R.; Cowie, M. Organometallics 1997, 16, 2297.
(19) (a) Vaartstra, B. A.; Cowie, M. Inorg. Chem. 1989, 28, 3138.
(b) George, D. S. A.; McDonald, R.; Cowie, M. Can. J . Chem. 1996, 74,
2289. (c) George, D. S. A.; McDonald, R.; Cowie, M. Organometallics
1998, 17, 2553.
(20) (a) Wang, L.-S.; McDonald, R.; Cowie, M. Inorg. Chem. 1994,
33, 3735. (b) Wang, L.-S., Cowie, M. Can. J . Chem. 1995, 73, 1058. (c)
Wang, L.-S., Cowie, M. Organometallics 1995, 14, 2374.
(21) (a) Antonelli, D. M.; Cowie, M. Organometallics 1991, 10, 2550.
(b) Antonelli, D. M.; Cowie, M. Inorg. Chem. 1990, 29, 4039.
(22) Graham, T. W.; Van Gastel, F.; McDonald, R.; Cowie, M.
Organometallics, in press.
In 2a the slight (7 Hz) coupling to Rh either could
result from two-bond coupling through the Rh-Ir bond
or could arise from a very weak semibridging carbonyl
interaction. We favor the latter formulation since we
have seen no other examples in our extensive chemistry
(23) Mann, B. E.; Taylor, B. F. 13C NMR Data for Organometallic
Compounds; Academic Press: New York, 1981; p 14.
(24) Adams, H.; Chen, X.; Mann, B. E. J . Chem. Soc., Dalton Trans.
1996, 2159.

1636 Organometallics, Vol. 18, No. 9, 1999
Oke et al.
consistent with an ABCDMX spin system (X ) Rh) in
which each phosphorus nucleus is chemically distinct.
Taking the spectrum of 4a as an example, the dppm
31P resonances appear in the expected regions, with
those bound to Rh displaying the normal coupling to
this nucleus (between 123 and 157 Hz in compounds
4a -d ), while the PMe3 resonance appears as a multiplet
at δ -62.4. Appropriate decoupling experiments estab-
lish that this group displays coupling to Rh of 17 Hz
and to the two phosphorus nuclei on Rh of 15 Hz; no
coupling is observed between the PMe3 group and the
dppm phosphorus nuclei bound to Ir. Although the
observed 31P-31P coupling suggests that the PMe3 group
is bound to Rh, the coupling to this metal is far too small
1
to correspond to J Rh-P, so the PMe3 group is assumed
to be bound to Ir in the site opposite the Rh-Ir bond.
Strong coupling through a metal-metal bond has been
observed previously.5,26 Although we have not observed
situations in which a group is coupled to the dppm
phosphines on an adjacent metal and not to those on
the same metal, we have previously observed a related
example, [IrRu(CO)3(PMe3)(µ-CH2)(dppm)2][BF4],27 in
which the PMe3 group, shown to be terminally bound
to one metal, did display substantial coupling (ca. 9 Hz)
to the dppm phosphorus nuclei on the adjacent metal.
In this case, however, coupling between PMe3 and the
dppm phosphorus nuclei on the same metal was also
observed.
F igu r e 3. Perspective view of the [RhIr(CH3)(CO)2(PMe3)-
(dppm)2]+ complex ion of 4a showing the atom labeling.
Thermal parameters as for Figure 2.
Ta ble 4. Selected Bon d Len gth s a n d An gles for
[Rh Ir (CO)2(CH3)(P Me3)(d p p m )2][CF 3SO3]‚2C4H8O
(4a )
Selected Interatomic Distances (Å)
Ir-Rh
Ir-P1
Ir-P3
Ir-P5
Ir-C1
Ir-C2
Rh-P2
Rh-P4
Rh-C3
2.859(2)
2.331(7)
2.328(6)
2.414(7)
1.85(3)
P1-C7
P2-C7
P3-C8
P4-C8
P5-C4
P5-C5
P5-C6
O1-C1
O3-C3
1.81(2)
1.84(2)
1.87(2)
1.85(2)
1.77(2)
1.79(3)
1.71(3)
1.14(3)
1.13(3)
1
In the H NMR spectrum the methyl ligand appears
as a doublet of triplets at δ 0.33 while the PMe3 protons
appear as a doublet at δ 0.23. Selective 31P-decoupling
experiments indicate that all coupling to the methyl
ligand results from the Ir-bound phosphines, and this,
together with the lack of Rh coupling, indicates that it
is bound to Ir.
2.31(3)
2.320(7)
2.299(6)
1.83(3)
To confirm the geometry proposed for these phosphine
adducts, particularly in light of the unusual 31P{1H}
NMR results, the X-ray structure of 4a was determined.
A perspective view of the cation of 4a is shown in Figure
3, with relevant bond lengths and angles presented in
Table 4. This structure determination confirms the
formulation proposed in which the PMe3 group is
coordinated to Ir, in the site opposite the metal-metal
bond, and the methyl group is also bound to Ir, opposite
the carbonyl ligand. The geometry at Rh is square
planar, while that at Ir is distorted octahedral (consid-
ering the Rh-Ir bond as occupying one site in each
case). Distortions from idealized octahedral geometry
about Ir result from repulsions between the PMe3 group
and the adjacent ends of the dppm ligands, as shown
by the P(1)-Ir-P(3) angle (157.3(2)°), which deviates
substantially from the idealized 180°. Figure 3 shows
the close contacts between the PMe3 group and the
dppm phenyl groups 1 and 6, with the resulting geom-
etry at P(1) and P(3) being skewed, forcing rings 2 and
5 toward the Rh end of the molecule. The slightly longer
Selected Interatomic Angles (deg)
Rh-Ir-P1
Rh-Ir-P3
Rh-Ir-P5
Rh-Ir-C1
Rh-Ir-C2
P1-Ir-P3
P1-Ir-P5
P1-Ir-C1
P1-Ir-C2
P3-Ir-P5
P3-Ir-C1
P3-Ir-C2
P5-Ir-C1
P5-Ir-C2
C1-Ir-C2
Ir-Rh-P2
Ir-Rh-P4
Ir-Rh-C3
79.2(2)
78.2(2)
175.7(2)
94.1(12)
90.1(6)
157.3(2)
100.5(2)
88.9(11)
90.2(6)
102.2(2)
90.8(10)
91.9(6)
90.2(12)
85.5(6)
175.4(12)
91.2(2)
91.7(2)
173.6(10)
P2-Rh-C3
P2-Rh-P4
P4-Rh-C3
Ir-P1-C7
Rh-P2-C7
Ir-P3-C8
Rh-P4-C8
Ir-P5-C4
Ir-P5-C5
Ir-P5-C6
C4-P5-C5
C4-P5-C6
C5-P5-C6
Ir-C1-O1
Rh-C3-O3
P1-C7-P2
P3-C8-P4
87.3(8)
174.9(2)
90.3(8)
110.1(8)
113.7(7)
110.1(9)
114.1(8)
119.2(8)
112.2(10)
120.9(9)
102.3(14)
100.4(13)
98.5(16)
177.4(30)
174.9(32)
105.4(11)
103.8(12)
Ir-P (2.331(7), 2.328(6) Å) versus Rh-P (2.320(7),
2.299(6) Å) distances for the dppm groups are probably
a reflection of the greater crowding at Ir, whereas the
long Ir-PMe3 bond (2.414(7) Å) may result either from
the high trans influence of the metal-metal bond28 or
from repulsions with both dppm ligands bound to Ir.
An additional consequence of the crowding in the
complex is the twisting of the P(1)-Ir-P(3) unit with
respect to P(2)-Rh-P(4) by about 40°, as shown in
Figure 4, in which the cation of 4a is viewed ap-
proximately along the Rh-Ir bond. This twisting results
in different static environments for P(2) (adjacent to
C(2)H3) and P(4) (adjacent to C(1)O(1)). The inequiva-
(25) (a) Ziegler, T.; Tschinke, V. Bonding Energetics in Organome-
tallic Compounds; Marks, T. J ., Ed.; American Chemical Society:
Washington, DC, 1990; Chapter 19. (b) Ziegler, T.; Tschinke, V.;
Uresenbach, B. J . Am. Chem. Soc. 1987, 109, 4825. (c) Shen, J .-K.;
Tucker, D. S.; Basolo, F.; Hughes, R. P. J . Am. Chem. Soc. 1993, 115,
11312.
(26) (a) Antwi-Nsiah, F. H.; Torkelson, J . R.; Cowie, M. Inorg. Chem.
Acta 1997, 259, 213. (b) Mague, J . T. Organometallics 1986, 5, 918.
(c) Brown, M. P.; Fisher, J . R.; Hill, R. H.; Puddephatt, R. J .; Seddon,
R. R. Inorg. Chem. 1981, 20, 2516.

Heterobinuclear Methyl Complexes of Rh/ Ir
Organometallics, Vol. 18, No. 9, 1999 1637
semibridging arrangement, although remaining prima-
rily bound to Rh. Movement of a carbonyl from Ir to the
bridging site, in which a more crowded environment at
Rh results, is not expected based on steric arguments,
but probably occurs in order to alleviate some of the
buildup of electron density on Rh upon addition of the
phosphine. A similar situation has previously been
observed in a related diiridium system in which ligand
addition at one metal caused a carbonyl on an adjacent
metal to move to a bridging site.29
Upon warming 3a to -40 °C peaks due to the
dicarbonyl product 4a begin to appear, and by -20 °C
4a is the predominant species in solution. Rearrange-
ment accompanied by CO loss results in transfer of the
phosphine from Rh to Ir. This may occur either by direct
transfer via a phosphine-bridged intermediate or by
phosphine loss and subsequent recoordination at the
different metal. We have no evidence to support either
route. Although no PR3-bridged complexes are known,
phosphine transfer in binuclear systems, via a bridged
intermediate, has been suggested.30,31 In addition, a
related trialkylstibane-bridged analogue, [Rh2Cl2(µ-
SbiPr3)(µ-CPh2)] is known,32 and the possibility of
phosphine-bridged clusters has also been suggested.33
F igu r e 4. Alternate view of complex 4a , with all but the
ipso carbons of the dppm phenyl rings omitted.
lence of P(2) and P(4) renders P(1) and P(3) inequivalent
and accounts for the different chemical environments
of all dppm phosphorus nuclei as observed in the low-
temperature 31P{1H} NMR spectrum. At ambient tem-
perature, in solution, the molecule is presumably oscil-
lating about the Rh-Ir axis, giving rise to an averaged
environment for the 31P nuclei on a given metal,
whereas at low temperature the different environments
can be detected on the NMR time scale.
In the case of 2, in which both metals are coordina-
tively unsaturated, reaction with trimethylphosphine
again yields compound 4a . However in this case attack
appears to occur directly at Ir, since no other species is
observed at temperatures down to -100 °C.
One carbonyl can be removed from compounds 4a and
4c by reaction with Me3NO, yielding the monocarbonyl
products [RhIr(CH3)(PR3)(µ-CO)(dppm)2][CF3SO3] (5a
and 5c), as shown in Scheme 1. Carbonyl loss is
accompanied by migration of the monodentate phos-
phine from Ir back to Rh as seen by the magnitude of
the Rh coupling (1J RhP ) 138 Hz (spectral parameters
for 5a are discussed)). The methyl group remains bound
to Ir as seen by 1H NMR experiments in which coupling
to the Ir-bound dppm phosphorus nuclei (3J PH ) 6 Hz)
is observed. A slight, additional coupling to the PMe3
group (4J PH ) 2 Hz) is also observed and suggests an
alignment of PMe3 and CH3 ligands opposite the metal-
metal bond as in the related dicarbonyl monomethyl
species. In solution the carbonyl is shown both by IR
(ν(CO) ) 1746 cm-1) and by 13C{1H} NMR spectroscopy
to be bridging the metals. The 13CO resonance at δ 214.5
is at low field, typical of a bridging group and shows
coupling to Rh (36 Hz) and to all five phosphorus nuclei.
In the solid state the structure of 5 again differs from
that in solution, but now in the opposite way compared
to 2; the IR spectra of 5a and 5c in the solid show only
a terminal carbonyl band. Clearly packing effects in the
solid state induce subtle structural changes favoring a
We were interested in the initial site of phosphine
attack in compound 1, for which two scenarios seemed
possible if the reaction were associative. Either phos-
phine attack occurs at the unsaturated Rh center,
followed by CO loss and migration of the phosphine to
Ir, or transfer of a carbonyl from Ir to Rh could occur,
followed by phosphine attack directly at Ir and loss of
a carbonyl ligand. If the reaction of 1 with phosphines
is monitored at -60 °C or lower, the first species
observed is the tricarbonyl, phosphine adduct 3 as
shown in Scheme 1. Again the spectral parameters for
all phosphine adducts are similar, so only those of the
PMe3 adduct (3a ) are discussed. At -60 °C the 31P{1H}
NMR spectrum for 3a shows a resonance for the PMe3
group as a doublet of triplets at δ -35.4. The coupling
of 147 Hz to Rh clearly identifies this group as being
directly bound to this metal. Furthermore, it is note-
worthy that this resonance is ca. 20 ppm downfield from
the PMe3 resonance in 4a and parallels the trend in 31P
chemical shifts for the dppm resonances in which the
Rh end is generally downfield from the Ir end. The 13C-
{1H} NMR spectrum of 3a shows three equal intensity
peaks, one at δ 262.7 (1J RhC ) 25 Hz) corresponding to
a bridging carbonyl, a second at δ 202.4 (1J RhC ) 53 Hz)
for the carbonyl on Rh, and a third resonating at δ 185.1,
corresponding to a carbonyl which is terminally bound
to Ir. The slight downfield shift of the central resonance
and the magnitude of its coupling to Rh, which is less
than usually observed for a terminal carbonyl, suggests
that although primarily bound to Rh, it has a weak
semibridging interaction with Ir. The net result of PMe3
attack at Rh is that one of the two iridium-bound
carbonyls of 1 has transferred to the bridging position,
while the terminal carbonyl on Rh has adopted a weak
(27) Dell’Anna, M. M.; Cowie, M. Unpublished results.
(28) (a) Cowie M.; Dwight, S. K. Inorg. Chem. 1980, 19, 209. (b)
Cowie, M.; Gibson, J . A. E. Organometallics 1984, 3, 984. (c) Farr, J .
P.; Olmstead, M. M.; Balch, A. L. Inorg. Chem. 1983, 22, 1229. (d)
Sutherland, B. R.; Cowie, M. Organometallics 1984, 3, 1814.
(29) Cowie, M.; Vasapollo, G.; Sutherland, B. R.; Ennett, J . P. Inorg.
Chem. 1986, 25, 2648.
(30) Washington, J . S. Ph.D. Thesis, University of Alberta, 1994,
Chapter 3.
(31) J ean, Y.; Lledos, A. J . Chem. Soc., Chem. Commun. 1998, 1443.
(32) Schlab, P.; Mahr, N.; Wolf, J .; Werner, H. Angew. Chem., Int.
Ed. Engl. 1994, 33, 97.
(33) Bender, R.; Braustein, P.; Dedieu, A.; Dusausoy, Y. Angew.
Chem., Int. Ed. Engl. 1989, 28, 923.

1638 Organometallics, Vol. 18, No. 9, 1999
Oke et al.
Sch em e 2
carbonyl-bridged species for 2 and a structure having a
terminal carbonyl group for 5. The carbonyl-bridged
structure observed in solution for 5 seems to be the
electronically favored one, in which this bridging car-
bonyl functions as a π acceptor for both electron-rich
metals.
bound to Ir. The 13C{1H} NMR spectrum displays a
triplet at 187.2 and a doublet of triplets (1J RhC ) 67 Hz)
at δ 183.8 for the Ir- and Rh-bound carbonyls, respec-
tively. In the IR spectrum these terminal carbonyls
appear at 1960 and 1932 cm-1, while the cyanide stretch
appears at 2084 cm-1, within the normal range for
terminally bound cyanide groups.35 Although, in the
absence of 13C NMR data for the CN ligand, its position
on Ir cannot be verified, the spectral parameters are
similar to those of related neutral dialkyl complexes
having this geometry,36 for which the coordination sites
of the alkyl groups can be clearly established based on
13C{31P} and 1H{31P} NMR experiments. The slight
downfield shift of the Ir-bound carbonyl of 6 again
suggests a weak semibridging interaction, as in the
precursor (2).
Compound 2 also reacts with hydride sources such
as Super-Hydride, although at room temperature sev-
eral products are obtained which are difficult to char-
acterize due to overlapping peaks. At -40 °C (and lower)
several species were again observed, but the major
product is identified as [RhIrH(CH3)(CO)2(dppm)2] (7).
Compound 7 shows the methyl resonance as a triplet
at δ -1.02 and a hydride resonance, also a triplet, at δ
-10.08 in the 1H NMR spectrum; the lack of Rh coupling
and selective 31P-decoupling experiments establish that
We had hoped that addition of another phosphine
ligand to 5 might induce C-H bond cleavage, owing to
the very basic metal centers that would result, or that
a methyl-bridged product might result with the methyl
group being forced toward Rh upon addition of the
phosphine at Ir. However, the reaction of 5a with
additional PMe3 leads to fragmentation of the “RhIr-
(dppm)2” framework, yielding a number of unidentified
decomposition products. We have previously observed
dppm displacement by PMe3 in a dirhodium complex.34
The reactivity of 2 with a number of anionic ligands
has been investigated in order to generate neutral
methyl complexes analogous to the cationic species such
as 4. With tetrabutylammonium cyanide, reaction of 2
proceeds smoothly to yield [RhIr(CH3)(CN)(CO)2(dppm)2]
(6) as shown in Scheme 2. As with the phosphines,
compound 6 apparently has both the methyl group and
the added CN- anion bound to Ir. The 1H NMR
spectrum shows the methyl protons as a triplet at δ
-0.74 with coupling to only the dppm phosphorus nuclei
(35) Ittel, S. D.; Tolman, C. A.; English, A. D.; J esson, J . P. J . Am.
Chem. Soc. 1978, 100, 7577, and references therein.
(36) Oke, O.; Cowie, M. Unpublished results.
(34) McKeer, I. R.; Sherlock, S. J .; Cowie, M. J . Organomet. Chem.
1988, 352, 205.

Heterobinuclear Methyl Complexes of Rh/ Ir
Organometallics, Vol. 18, No. 9, 1999 1639
both ligands are bound to Ir. The 13C{1H} NMR spec-
trum shows one resonance as a doublet of triplets at δ
182.4 (1J RhC ) 83 Hz), corresponding to the rhodium-
bound carbonyl and a second, broad resonance at δ 190.4
for that bound to Ir. As with compounds 2 and 6, the
downfield 13C chemical shift for the Ir-bound carbonyl
suggests a weak semibridging interaction.
3.1 with a coupling of 22 Hz to Rh. In the IR spectrum,
the one-terminal and one-bridging carbonyl formulation
receives support from the carbonyl stretches at 1931 and
1761 cm-1. No additional species were observed upon
repeating this reaction at lower temperatures; clearly
the rearrangement that brings the methyl group from
Ir to Rh is facile. Compound 9 is the anticipated product
(or an isomer thereof) of oxidative addition of MeI to
[RhIr(CO)3(dppm)2], followed by CO loss. But surpris-
ingly, no reaction was observed between these reagents.
This is in contrast to the facile oxidative addition of
CH3I to the diiridium analogue [Ir2(CO)3(dppm)2] which
did give the anticipated [Ir2I(CH3)(CO)3(dppm)2].40
Upon warming to ambient temperature, compound 7
transforms into two major species, along with minor
unidentified products. The one product is the well-
known, tricarbonyl species [RhIr(CO)3(dppm)2],37 which
has resulted from methane loss from 7 and scavenging
of a carbonyl group, presumably from the decomposition
products. The other major product is [RhIr(CO)2(µ-H)-
(µ2-η3-(o-C6H4)P(Ph)CH2PPh2)(dppm)] (8), resulting from
methane loss and subsequent orthometalation of one of
the dppm phenyl groups. If a sample of 7 is allowed to
warm under an atmosphere of CO, the major product
obtained is the tricarbonyl complex. Compound 8 dis-
plays a complex four-resonance pattern in the 31P{1H}
NMR spectrum that is characteristic of an ABCDX spin
system, with the Rh-bound phosphorus nuclei at δ 6.7
and 5.6, while the two on Ir appear at δ -18.6 and
-33.7. Although the Ir-bound phosphines display 31P-
31P coupling of 367 Hz, characteristic of a trans align-
ment, no large 31P-31P coupling is observed for the other
resonances, suggesting a cis arrangement at Rh. The
large difference in the Ir-bound phosphines and the
high-field shift of one resonance, typical of a four-
membered ring,38 suggest that orthometalation has
occurred at the third-row metal. The hydride resonance
in the 1H NMR spectrum appears at δ -11.92 as a
complex multiplet, and 31P decoupling shows that this
hydride couples weakly to Rh (1J RhH ) 5 Hz), consistent
with a bridging arrangement in which the hydride is
more strongly bound to Ir. The 13C{1H} NMR spectrum
is characteristic for a species having a terminal carbonyl
on each metal. Although the spectral data available are
insufficient to unambiguously define the geometry, the
close similarity of the spectral parameters to those of
[IrOs(H)2(CO)3(µ2-η3-(o-C6H4)P(Ph)CH2PPh2)(dppm)]39
suggests a related geometry, and a tentative structural
proposal for 8 is shown in Scheme 2.
Compound 2 also reacts with KI to give the neutral
iodomethyl product [RhIr(I)(CH3)(CO)2(dppm)2] (9). How-
ever, this reaction does not proceed cleanly, and 9 is
often accompanied by traces of the previously character-
ized diiodo species [RhIr(I)2(CO)2(dppm)2],5 from which
it could not be separated. Unlike compounds 6 and 7,
in which the added nucleophile and the methyl group
are on Ir, compound 9 has the methyl group on Rh, as
seen by the 1H NMR spectrum in which the methyl
protons, at δ -0.63, show 1.5 Hz a coupling to Rh
together with coupling to the Rh-bound phosphines. In
the 13C{1H} NMR spectrum one carbonyl appears at δ
212.3 with coupling of 40 Hz to Rh, consistent with a
bridging group, and the other, on Ir, appears at δ 179.7.
The binding of the methyl group to Rh is conclusively
established by the 13C{1H} NMR spectrum of a 13CH3-
enriched sample, which shows the methyl carbon at δ
Discu ssion
One purpose of this study was to determine the initial
site of attack by substrate molecules in our heterobi-
nuclear Rh/Ir complexes. Compound 1, [RhIr(CH3)(CO)3-
(dppm)2][CF3SO3], having a 16e Rh and an 18e Ir center
behaves as anticipated with attack by phosphines
occurring at the unsaturated Rh center. It should be
emphasized, however, that a previous study failed to
identify a product of isocyanide attack at Rh, even at
low temperatures.2 So whether initial attack by other
substrates at Ir can occur or whether migration of these
smaller substrates from Rh to Ir is too facile to observe
at the temperatures investigated is not known. Attack
at the saturated metal in 1 could occur via prior transfer
of a carbonyl to Rh, generating the necessary unsat-
uration at Ir.
Attack at the 16e/16e complex [RhIr(CH3)(CO)(µ-CO)-
(dppm)2][CF3SO3] (2) appears to occur directly at Ir in
the reactions studied. Given the solid-state structure of
2, in which the geometry at each metal is rather similar,
it is difficult to envision that attack at Ir should be as
kinetically favored as implied by the lack of attack at
Rh observed at -80 °C. However it should be recalled
that the structure in solution (2a ) is quite different.
Although both metals in 2a have square-planar geom-
etries, that at Rh, defined by Ir, a carbonyl, and the Rh-
bound phosphorus atoms, is essentially perpendicular
to the square plane at Ir, defined by the methyl, the
carbonyl, and the phosphine ligands. With such a
geometry, attack at Rh requires the incoming group to
approach essentially perpendicular to the RhIrP4 core.
Such an approach should be inhibited by the four phenyl
rings on either side of the RhIrP4 plane and by the
carbonyl and methyl groups on Ir. Attack at Ir, on the
other hand, requires the ligand to approach from the
end of the complex, an approach that may be more
favorable, with fewer interactions with the dppm phenyl
groups, and no interference from the adjacent metal.
The structures of the analogous compounds, [RhIr-
(CH3)(CO)2(dppm)2][CF3SO3] (2) and [RhIr(CH3)(PR3)-
(CO)(dppm)2][CF3SO3] (5), present an interesting di-
chotomy; although both solution structures differ from
the structures in the solid state, both do so in opposite
ways. The solid-state structure of 2 is similar to the
solution structure of 5, and vice versa. In solution, the
dicarbonyl species 2 has both carbonyls terminally
bound, whereas one is bridging in the solid state. For
(37) McDonald, R.; Cowie, M. Inorg. Chem. 1990, 29, 1564.
(38) (a) Garrou, P. E. Inorg. Chem. 1975, 14, 1435. (b) Garrou, P.
E. Chem. Rev. 1981, 81, 229.
(39) Hilts, R. W.; Franchuk, R. A.; Cowie, M. Organometallics 1991,
10, 1297.
(40) Muritu, J .; Torkelson, J . R.; McDonald, R.; Cowie, M. Manu-
script in preparation.

1640 Organometallics, Vol. 18, No. 9, 1999
Oke et al.
5, the carbonyl ligand bridges in solution and is terminal
in the solid state. We view the solid-state structures as
the anomalies, with intermolecular packing forces hav-
ing a significant influence on the structure. For 2 the
solution structure, in which each low-valent metal has
one carbonyl attached, strikes us as the electronically
favorable situation, and certainly this is borne out by
DFT calculations on an Ir2 analogue. Compound 5, on
the other hand, with only one carbonyl ligand, is
expected to favor a bridging mode in which the ligand
can function as a π acid to both electron-rich metals.
Binuclear complexes, having the metals in close
proximity, are ideally suited to facile ligand migration
from metal to metal. Unlike the homobinuclear Rh2 and
Ir2 analogues, for which such migrations occur readily
in the absence of added substrate, compounds 1 and 2
are static at ambient temperature. Upon addition, or
in some cases removal of a ligand, rearrangements do
occur, with ligand transfer from one metal to the other.
One important factor inhibiting facile migration appears
to be the greater metal-ligand bond strengths of Ir over
Rh, favoring ligand binding to the former. This tendency
is most notable for the methyl ligand, which remains
on Ir in all but one example. The trend appears to be
that in the cationic compounds the anionic ligand (CH3)
is bound to Ir. In the reactions of 2 with the anionic
hydride and cyanide groups to yield the neutral methyl
complexes [RhIr(CH3)X(CO)2(dppm)2] (X ) H (7), CN
(6)), both anionic ligands in the products are bound to
Ir to yield a (formally) Rh(0)/Ir(+2) combination. The
exception occurs for iodide attack in which the methyl
group migrates to Rh to give the Rh(+1)/Ir(+1) com-
pound [RhIr(CH3)I(CO)2(dppm)2], having one anionic
group on each metal. We assume that methyl, rather
than iodide, migration to Rh occurs owing to a more
favorable Ir-I bond that is formed. Why the hydride
and cyanide compounds do not also adopt this Rh(+1)/
Ir(+1) arrangement is not clear. The structure shown
for 9 in Scheme 2 is based on that of the isoelectronic
diiodo analogue [RhIr(I)2(CO)(µ-CO)(dppm)2],5 and this
geometry is in keeping with iodide attack on 2a at the
site on Ir opposite the Rh-Ir bond (as in attack by H-
and CN-), assuming that a merry-go-round rearrange-
ment of ligands occurs with CH3 movement to Rh
accompanied by CO movement from Rh to the bridging
site.
would favor a methyl-bridged structure in order to
alleviate some of the crowding at Ir. Certainly the
structure of the PMe3 adduct has severe crowding at
Ir, as shown by the substantial distortions that have
occurred. However, this structure maintains a strong
metal-metal interaction, as shown by the alignment of
the two metal centers in Figure 3. Moving the methyl
group to a bridging position would substantially distort
the central core, reducing metal-metal overlap. Not
surprisingly, the bonding gain resulting from some type
of bridging methyl arrangement would presumably be
offset by the loss in metal-metal bonding.
In this study we strove to understand the roles of the
different metals in the reactions of the two mixed Rh/
Ir methyl complexes 1 and 2 with simple nucleophiles.
The initial sites of attack (at either Rh or Ir) can be
rationalized on the basis of either the site of unsatura-
tion (Rh) in 1 or the solution structure of 2, in which
approach of substrates appears to be favored at Ir. In
much of this chemistry, the rearrangements that occur
appear to be driven by the stronger bonds involving the
third-row metal. For the cationic species, the location
of the anionic methyl group on Ir, leaving the positive
charge on Rh, can be rationalized on the basis of the
higher electronegativity of Ir,41 making it more favorable
for this metal to have an anionic donor ligand than the
positive charge. Certainly however, a few questions
remain, particularly in the neutral complexes for which
structures having either Rh(0)/Ir(+2) (both anionic
ligands on Ir) or Rh(+1)/Ir(+1) (one anionic ligand on
each metal) are observed. It is not clear why one
structure type should be favored over the other, al-
though the tendency of the third-row congener to be in
the higher oxidation state is well-known.42
Ack n ow led gm en t. We thank the Natural Sciences
and Engineering Research Council of Canada (NSERC)
and the University of Alberta for financial support of
the research and NSERC for funding the P4/RA diffrac-
tometer.
Su p p or tin g In for m a tion Ava ila ble: Tables of X-ray
experimental details, atomic coordinates, interatomic distances
and angles, anisotropic thermal parameters, and hydrogen
parameters for compounds 2 and 4a . This material is available
free of charge via the Internet at http://pubs.acs.org.
OM980947Y
Our attempts to induce the terminal methyl group in
2 to adopt a bridging arrangement, through ligand
addition, failed. We had thought that phosphine ligands
(41) Allred, A. L.; Rochow, E. G. J . Inorg. Nucl. Chem. 1958, 5, 264.
(42) Lee, J . D. Concise Inorganic Chemistry; Chapman & Hall: New
York, 1996; Chapter 18.