Redetermination of the O-O bond length in the dioxygen-adduct of Vaska's complex

Article abstract of DOI:10.1016/j.jorganchem.2008.05.025

Solid state structural studies were performed with (Ph₃P)₂IrCl(CO)(O₂) and the O-O length redetermined to be 1.47(1) ? in contrast to results reported earlier.

Full text of DOI:10.1016/j.jorganchem.2008.05.025

Journal of Organometallic Chemistry 693 (2008) 2645–2648
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Communication
Redetermination of the O–O bond length in the dioxygen-adduct of Vaska’s complex
*
Hélène Lebel , Chehla Ladjel, Francine Bélanger-Gariépy, Frank Schaper
Département de chimie, Université de Montréal, 2900, Boul Edouard Montpetit, Montréal, Québec, Canada H3T 1J4
a r t i c l e i n f o
a b s t r a c t
Article history:
Solid state structural studies were performed with (Ph3P)2IrCl(CO)(O2) and the O–O length redetermined
to be 1.47(1) Å in contrast to results reported earlier.
Received 26 March 2008
Received in revised form 15 May 2008
Accepted 16 May 2008
Ó 2008 Elsevier B.V. All rights reserved.
Available online 24 May 2008
Keywords:
Iridium
Vaska’s complex
O–O Bond length
Dioxygen complex
1. Introduction
which featured a unprecendented O–O bond distance for this com-
plex. This note describes the details of this study.
Transition metal-catalyzed methylenation reactions with tri-
methylsilyldiazomethane have been previously reported by us
(Eq. (1)) [1]. As part of this work, we were interested in the mech-
anism of activation of trimethylsilyldiazomethane with metal
complexes. We have hypothesized the formation of a N-coordi-
nated intermediate, rather than a metal carbene with a ruthenium
complex [2]. More recently we found that Vaska’s complex was
also capable of catalyzing the methylenation reaction with
trimethylsilyldiazomethane [3].
2. Results and discussion
2.1. Initial solid state structural studies
As part of our studies of reactions between iridium complexes
and diazo compounds, we undertook an X-ray diffraction study
of a crystal obtained from a mixture of Vaska’s complex and
trimethylsilyldiazomethane. The obtained structure was initially
assigned as a nitrogen adduct of Vaska’s complex, i.e., (Ph3P)2IrCl-
(CO)(N2), which was however in contradiction with the elemental
analysis showing no nitrogen content and the coordination mode
of the ‘‘N2”-ligand. The crystal structure showed a trigonal bipyra-
midal coordination of the Ir with the supposed dinitrogen ligand in
N2
R2
R2
,
i-PrOH
TMS
H
(1)
R1
O
R1
PPh3, catalyst / THF
the equatorial plane, in a non-bridging
g
2-coordination. While this
catalyst: RhCl(PPh3)3 (2.5 mol%)
RuCl(NO)(PPh3)2 ( 5 mol%)
NHCCuCl (5 mol%)
IrCl(CO)(PPh3)2 (2.5 mol%)
coordination mode is unprecedented for dinitrogen [5], it is very
common for dioxygen [6]. Refinement of the structure as a dioxy-
gen ligand yielded indeed better agreement factors and we identi-
fied (vide infra) the compound as the known dioxygen adduct
(Ph3P)2IrCl(CO)(O2), 1 with an O–O bond length of 1.465(4) Å [7].
The molecular structure of 1 has been previously determined by
an X-ray single crystal diffraction study at room temperature and
reported in 1964 [8]. The authors reported an O–O bond length
of 1.30 0.03 Å, which is intermediate between that of O2
(1.21 Å) and O22ꢀ anion (1.49 Å), but agrees well with that of O2ꢀ
(1.33 Å). The dioxygen ligand should then be a superoxo with a for-
mal oxidation state of II at the iridium center. However, iridium
To probe our mechanistic proposal, we then performed a num-
ber of crystallization experiments with trimethylsilyldiazome-
thane and iridium complexes, as these are known to lead to
nitrogen-containing products with diazo compounds [4]. In the
course of these solid state structural studies, a crystal structure
of dioxygen-Vaska’s complex (Ph3P)2IrCl(CO)(O2) was obtained
complex 1 was diamagnetic [8b] and showed an
m(O–O) stretching
* Corresponding author. Tel.: +1 514 343 5826.
E-mail address: helene.lebel@umontreal.ca (H. Lebel).
frequency at 855 cmꢀ1 [9], and the authors consequently assigned
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.05.025

2646
H. Lebel et al. / Journal of Organometallic Chemistry 693 (2008) 2645–2648
the ligand as a peroxo with reversible binding properties. Subse-
quently, there were some doubts about the value of 1.30 Å for the
O–O bond length [10], as other closely related compounds showed
values between ꢁ1.4 and 1.5 Å [7]. To establish the correct value
of the O–O bond length, we then decided to re-investigate in details
the crystal structure of 1.
geometrical data between the previous study and ours is presented
in Table 1.
The obtained geometrical data is identical between the two
studies (in the limit of their accuracy), with the notable exception
of the O3–Ir–O4 bond angle and the O3–O4 bond distance of
1.465(4) Å, which is significantly longer than the 1.30 0.03 Å re-
ported previously. The longer bond distance found here agrees
more closely with a description of the dioxygen ligand as O22ꢀ, in
agreement with the obtained IR data. The O–O distance of the closely
related complex (EtPh2P)2Ir(O2)Cl(CO) was reported to be 1.469 Å
[16], also in good agreement with the value found here. An attempt
to simulate the conditions of the earlier study (restriction to reflec-
2.2. Complex synthesis
Complex 1 was prepared from a reaction between Vaska’s com-
plex and oxygen in THF and was isolated as light orange, air-stable
crystals. In the FT-IR spectrum, an m(O–O) stretching frequency was
observed as a band of medium intensity at 856 cmꢀ1, in good
agreement with values previously reported for this complex
(855 cmꢀ1 and 854 cmꢀ1, respectively) [9,11] and comparable to
tions with h < 18° and I > 2r(I), isotropic refinement) did not influ-
ence the O–O bond distance. While the better agreement with
comparable compounds and spectroscopic results, as well as the
higher amount and the better quality of the collected data, make
us confident in the structural values determined, we were curious
about the reasons for the mismatch with the older study, which
seemed to be of sufficient quality.
those observed for (Ph3P)2Ir(O2)I(CO) (862 cmꢀ1) [12] and (
l-
dppp)2{IrCl(CO)}2(O2) (843 cmꢀ1
) [13]. The difference in the
m
(CO) stretching frequency between Vaska’s complex and 1 is
47 cmꢀ1 (1949 and 1996 cmꢀ1, respectively), in good agreement
with the literature [14]. Finally, the 31P NMR spectrum shows
2 equiv. PPh3 ligands, which give rise to a singlet resonance at d
5.34.
We have considered several systematical errors in X-ray diffrac-
tion studies, which might cause a shortening of observed bond
lengths, notably the effects of libration, disorder and decomposi-
tion. Libration effects have been reported previously to be respon-
sible for artificially short O–O bond lengths [17]. Room
temperature diffraction studies of 1, however, both with Mo radi-
ation (as used in the original study) and with Cu radiation, did not
yield structural data different from our low temperature results
(Table 1, entries 3 and 4). Insufficiently described disorder in a
molecule was another explanation offered for incorrect O–O dis-
tances [18]. Although an influence of the Cl/CO disorder on the
positioning of the dioxygen ligand in 1 is imaginable, careful
inspection of the thermal displacement parameters obtained in
the low temperature study tends to disprove this theory. Size
and orientation of the thermal displacement parameters of the
O3 and O4 atoms (Ueq = 0.034 and 0.044 Å2, respectively) do not
indicate any pronounced disorder of these atoms, when compared
to the rest of the molecule (cf. 0.03–0.05 Å2 for CPh, 0.02 Å2 for P).
Nolte et al. found that decomposition of a single crystal of
[(dppe)2Ir(O2)][PF6] caused an apparent change in the O–O bond
length; in their case an elongation from 1.52 to 1.74 Å [10]. While
we did not conduct any decomposition studies, we reinvestigated
the same crystal after several days of storage at ambient tempera-
tures (entry 5). The data was of general lower quality, and the O–O
bond length increased to 1.76 Å. While decomposition has thus
clearly an influence on the O–O bond length, it does not provide
an explanation for the shortened bond length observed in the pre-
vious study. There is no indication that the longer bond length ob-
served here might be due to crystal decomposition.
2.3. Crystal structure determinations
As described previously [8] 1 crystallized in the triclinic space
ꢀ
group P1. An X-ray diffraction study at 150 K showed a trigonal
bipyramidal coordination of the iridium atom (Fig. 1). As observed
for Vaska’s complex [15] and in agreement with the previous
study, the chloride and carbon monoxide ligands were found to
be disordered [8]. The Ir–C–O bond angles of 173° and 174° are
comparable to those observed in Vaska’s complex (175°), where a
similar CO/Cl disorder was observed. A comparison of selected
2.4. Conclusion
While the discrepancies between the two studies cannot be ex-
plained without access to the original crystals, the low tempera-
ture used in this study, the higher number and the better quality
of the obtained reflection data, its improved agreement values
and internal consistency, the identical data obtained from three
independent crystals (not provided in Table 1) and the better
agreement with literature data of comparable compounds, all ar-
gue that the data presented here is a better description of the
structural reality. With the correction of the values for the O–O dis-
tance in 1 (1.30 Å corrected to 1.47 Å) and in [(dppe)2Ir(O2)][PF6]
(1.74 Å corrected to 1.52 Å) [10], O–O distances in Ir(g
2-O2) com-
plexes now range from 1.43 to 1.53 Å (based on 12 structures with
an accuracy better than 0.05 Å), which confirms the statement
made by Vaska in 1976 that ‘‘coordinated O2 shows, or at least
Fig. 1. Molecular structure of complex 1. Hydrogen atoms have been omitted for
clarity. Displacement ellipsoids are drawn at the 50% probability level.

H. Lebel et al. / Journal of Organometallic Chemistry 693 (2008) 2645–2648
2647
Table 1
Selected bond lengths [Å] and angles [°] for 1 at different temperatures and radiations
Original study (#1)a
Low T (#2)
Room temp. (#3)
296
Room temp. (#4)
296
Aged crystal (#5)
296
Temperature (K)
Radiation
R1 (observed data)
O3–O4
Ir–O3
Ir–O4
O3–Ir–O4
Ir–P1
Ir–P2
296
150
MoK
7.1%
1.30(3)
2.04(3)
2.09(3)
36.7(9)
2.36(1)
2.38(1)
172.8(5)
84.1(9); 90.6(7)
85.4(7); 89.2(9)
0.02
a
CuK
3.1%
a
CuK
4.1%
a
MoK
4.0%
a
CuKa
9.9%
1.76(2)
1.465(4)
2.038(3)
2.064(3)
41.9(1)
2.359(1)
2.363(1)
173.9(1)
85.4(1); 90.1(1)
83.8(1); 90.7(1)
0.003
1.492(7)
2.010(4)
2.090(4)
42.6(2)
2.370(1)
2.373(1)
174.0(1)
85.1(1); 90.3(1)
83.9(1); 91.2(1)
0.003
1.470(5)
1.992(4)
2.106(3)
41.9(1)
2.379(1)
2.379(1)
174.0(1)
85.4(1); 90.1(1)
84.0(1); 91.0(1)
0.000
2.090(14)
2.115(11)
49.4(7)
2.378(4)
2.379(4)
175.3(1)
85.5(3); 92.1(4)
83.2(4); 91.7(3)
0.001
P1–Ir–P2
P1–Ir–O3/4
P2–Ir–O3/4
D
D
Ir–Pb (Å)
P–Cc (Å)
0.014
0.024
0.012
0.048
a
Data taken from Ref. [8]. It should be noted that the O–O distance in Ref. [8] is reported as 1.30(3) Å as well as 1.30 0.03 Å.
b
c
D
D
Ir–P = jd(Ir–P1) ꢀ d(Ir–P2)j.
P–C = d(P–C)max ꢀ d(P–C)min
.
strongly tends to show, a constant state in either class of compounds”
(Fig. 2) [7c].
6
5
4
3
2
1
incorrect value,
redetermined
3. Experimental
3.1. Synthesis of IrCl(O2)(CO)(PPh3)2
1
Dioxygen was bubbled through a solution of (Ph3P)2IrCl(CO)
(0.010 g, 0.028 mmol) in THF (5 mL) for 3 h at room temperature.
The solvent was evaporated and the resulting powder dissolved
in THF and crystallized by layering with hexane.
3.2. X-ray diffraction studies
Cell refinement and data reduction were done using APEX2 [19].
Structures were solved by direct methods using SHELXS97 and re-
fined on F2 by full-matrix least squares using SHELXL97 [20]. Non-
hydrogen atoms were refined anisotropically. The disordered CO
and Cl ligands were refined isotropic. Uiso was constrained to be
identical for the disordered C1, C2, O1 and O2, and Cl1 and Cl2,
respectively. Dataset #5 was of very low quality. Refinement was
possible only using additional geometric restraints. Hydrogen
1.30
1.35
1.40
1.45
1.50
1.55
1.60
d(O-O) [Å]
Fig. 2. O–O distances observed in several Ir(g
2-O2) compounds.
Table 2
Details of X-ray diffraction studies of 1
Low T (#2)
Room temp. (#3)
Room temp. (#4)
Aged crystal (#5)
Formula; Mw (g/mol); F(000)
Crystal color, form & Size (mm)
C37H30ClIrO3P2; 812.20; 800
Pale yellow fragment;
0.04 ꢂ 0.07 ꢂ 0.12
ꢀ
Crystal system; space group; Z
Triclinic; P1; 2
Wavelength; T (K)
Dcalc (g/cm3)
Unit cell
a (Å)
1.54178; 150
1.655
1.54178; 296
1.612
0.71073; 296
1.615
1.54178; 296
1.612
9.7273(5)
9.8689(5)
9.8066(8)
9.9432(8)
9.802(2)
9.936(3)
9.8026(11)
9.9196(9)
b (Å)
c (Å)
17.0963(8)
95.763(3)
17.3223(14)
96.477(4)
17.319(4)
96.653(4)
17.4084(19)
97.018(5)
a
(°)
b (°)
90.847(2)
91.346(4)
91.375(4)
91.857(7)
c
(°)
93.367(2)
93.973(4)
1673.4(2)
94.168(4)
1670.1(7)
94.113(5)
1674.3(3)
V (Å3)
1629.74(14)
2.6–72.0; 0.96
19594; 6120; 0.041
9.898; multi-scan
3.1%; 7.9%
3.2%; 8.0%
1.046
h Range (°); completeness
Reflections: collected/independent; Rint
2.6–72.7; 0.97
39900; 6433; 0.063
9.640; multi-scan
4.1%; 10.9%
4.9%; 11.2%
1.071
1.2–31.4; 0.96
117094; 10581; 0.112
4.208; multi-scan
4.0%; 7.0%
9.9%; 7.6%
0.925
2.6–68.6; 0.86
23802; 5237; 0.086
9.635; multi-scan
9.9%; 24.4%
13.4%; 26.6%
1.096
l
(mmꢀ1); abs. corr.
R1 (F); wR (F2) [I > 2
r(I)]
R1 (F); wR (F2) (all data)
Goodness-of-fit (F2)
Residual electron density
1.16
1.08
1.24
3.14
Instrumentation: #2, #4: Bruker Platform, SMART4K, rotating anode; #3: Bruker Smart, APEX2, sealed tube; #5: Bruker Microstar, Platinum 135, rotating anode.

2648
H. Lebel et al. / Journal of Organometallic Chemistry 693 (2008) 2645–2648
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Acknowledgments
This research was supported by NSERC (Canada), the Canadian
Foundation for Innovation, the Canada Research Chair Program
and the Université de Montréal. Acknowledgement is also made
to the Donors of the American Chemical Society Petroleum
Research Fund for partial support of this research.
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Appendix A. Supplementary material
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Further details of the X-ray diffraction studies. Supplementary
data associated with this article can be found, in the online version,
at doi:10.1016/j.jorganchem.2008.05.025.
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