Intramolecular C-H oxidative addition to iridium(I) in complexes containing a N,N′-diphosphanosilanediamine ligand

Article abstract of DOI:10.1021/ic4024458

The iridium(I) complexes of formula Ir(cod)(SiNP)⁺ (1 ⁺) and IrCl(cod)(SiNP) (2) are easily obtained from the reaction of SiMe₂{N(4-C₆H₄CH₃)PPh ₂}₂ (SiNP) with [Ir(cod)(CH₃CN) ₂]⁺ or [IrCl(cod)]₂, respectively. The carbonylation of [1][PF₆] affords the cationic pentacoordinated complex [Ir(CO)(cod)(SiNP)]⁺ (3⁺), while the treatment 2 with CO gives the cation 3⁺ as an intermediate, finally affording an equilibrium mixture of IrCl(CO)(SiNP) (4) and the hydride derivative of formula IrHCl(CO)(SiNP-H) (5) resulting from the intramolecular oxidative addition of the C-H bond of the SiCH₃ moiety to the iridium(I) center. Furthermore, the prolonged exposure of [3]Cl or 2 to CO resulted in the formation of the iridium(I) pentacoordinated complex Ir(SiNP-H)(CO)₂ (6). The unprecedented κ³C,P,P′ coordination mode of the [SiNP-H] ligand observed in 5 and 6 has been fully characterized in solution by NMR spectroscopy. In addition, the single-crystal X-ray structure of 6 is reported.

Full text of DOI:10.1021/ic4024458

Article
pubs.acs.org/IC
Intramolecular C−H Oxidative Addition to Iridium(I) in Complexes
Containing a N,N′‑Diphosphanosilanediamine Ligand
Vincenzo Passarelli,*,† Jesus J. Perez-Torrente,‡ and Luis A. Oro‡
́
́
†Centro Universitario de la Defensa, Ctra. Huesca s/n, ES−50090 Zaragoza, Spain
‡Departamento de Química Inorgan
Zaragoza-CSIC, Facultad de Ciencias, C/Pedro Cerbuna, 12, ES−50009 Zaragoza, Spain
́
ica, Instituto de Síntesis Química y Catalisis Homogenea-ISQCH, Universidad de
́
́
S
* Supporting Information
ABSTRACT: The iridium(I) complexes of formula Ir(cod)-
(SiNP)+ (1+) and IrCl(cod)(SiNP) (2) are easily obtained
from the reaction of SiMe2{N(4-C6H4CH3)PPh2}2 (SiNP)
with [Ir(cod)(CH3CN)2]+ or [IrCl(cod)]2, respectively. The
carbonylation of [1][PF6] affords the cationic pentacoordi-
nated complex [Ir(CO)(cod)(SiNP)]+ (3+), while the treat-
ment 2 with CO gives the cation 3+ as an intermediate, finally
affording an equilibrium mixture of IrCl(CO)(SiNP) (4) and
the hydride derivative of formula IrHCl(CO)(SiNP−H) (5)
resulting from the intramolecular oxidative addition of the C−
H bond of the SiCH3 moiety to the iridium(I) center.
Furthermore, the prolonged exposure of [3]Cl or 2 to CO
resulted in the formation of the iridium(I) pentacoordinated
complex Ir(SiNP−H)(CO)2 (6). The unprecedented κ3C,P,P′ coordination mode of the [SiNP−H] ligand observed in 5 and 6
has been fully characterized in solution by NMR spectroscopy. In addition, the single-crystal X-ray structure of 6 is reported.
respect to aliphatic diphosphanes like 1,2-bis-
(diphenylphosphano)ethane and 1,3-bis(diphenylphoshano)-
propane, both ligands always exhibit the expected κ2P,P′
coordination to the metal center (Scheme 1).
As far as SiNP in combination with rhodium is concerned,
Scheme 2 shows selected examples of complexes featuring the
κ2P,P′ coordination mode of SiNP.3
INTRODUCTION
■
The design of phosphano ligands containing P−N bonds has
attracted a lot of interest, due to the easy functionalization and/
or modification of the ligand backbone and to the consequent
great versatility of these ligands both in coordination chemistry
and in the study of stoichiometric and catalytic reactions.1
Relevant to this paper, Scheme 1 shows selected diphosphano
On this background, herein we describe the preparation and
characterization of novel iridium(I) complexes with SiNP and
the study of their reactivity. On one hand, it has been
confirmed the predictable ability of SiNP to act as a bidentate
κ2P,P′ ligand, and, on the other, it has been observed the
unprecedented easy intramolecular oxidative addition of the
SiCH2−H bond to the iridium(I) center with the consequent
formation of the [SiNP−H] ligand featuring a κ3C,P,P′
coordination mode.
Scheme 1
RESULTS AND DISCUSSION
■
Synthesis of Iridium(I) Complexes. The iridium(I)
complexes [Ir(cod)(SiNP)]+ (1+) and IrCl(cod)(SiNP) (2)
(Scheme 3, cod = 1,5-cyclooctadiene) have been obtained by
reaction of SiNP with [Ir(cod)(CH3CN)2][PF6] or [IrCl-
(cod)]2, respectively (Scheme 3).4
ligands containing the P−N functionality and different
backbones and/or linkers. Interestingly, the coordination ability
of SiMe2[(C5H4N-2)NPPh2]2 (SiNpyP)2 toward several tran-
sition metals has been described, and we have reported the
synthesis of SiMe2{N(4-C6H4CH3)NPPh2}2 (SiNP)3 and its
reactivity toward rhodium. Despite the differences in the
electronic and steric features of both SiNpyP and SiNP with
In agreement with a square planar geometry at the metal
center of [Ir(cod)(SiNP)]+ (1+), the 31P{1H} NMR spectrum
Received: September 26, 2013
Published: January 7, 2014
© 2014 American Chemical Society
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Scheme 2
Scheme 3
Figure 1. Definition of the left and right and the up and down
semispaces at the coordinated cod (A) and the coordinated SiNP (B)
ligands.
center has been observed at room temperature. Indeed, similar
to 1+, one singlet at 49.4 ppm is observed in its 31P{1H} NMR
1
spectrum, and the H NMR spectrum indicates equivalent
olefinic protons of the cod ligand and equivalent tolyl groups
(see Experimental Section). Further, at room temperature, 2
1
shows a sharp H singlet at 0.39 ppm for the SiCH3 protons,
1
and, even at 183 K, the H and 31P{1H} NMR spectra are
similar to those at room temperature, the only difference being
slightly broader signals. Thus, the up and down and the left and
right semispaces (Figure 1) both at the SiNP and at the cod
ligands should be equivalent or averaged by a fluxional process.
Starting from the ideal pentacoordinated configurations for
IrCl(cod)(SiNP) shown in Figure 3, optimized structures of 2
were calculated by means of standard computational methods
(see DFT Geometry Optimization for details), and the
minimum free energy structure was found to be that derived
from the SPY-5-13 configuration,5 that is, a square pyramid
containing the chlorido ligand at the apical position (Figure 4).
Interestingly, this arrangement was previously observed in the
solid-state structure of IrCl(cod)(dppb)6 and IrCl(cod)-
(dppp).7
of 1+ shows one singlet at 53.5 ppm for the SiNP ligand, and
both equivalent tolyl groups and equivalent olefinic protons are
1
observed (1H, 13C). Additionally, the H and 13C{1H} NMR
spectra show equivalent SiCH3 moieties both at room
temperature and at 183 K. On this basis, both the left and
right and the up and down semispaces at the cod and the SiNP
ligands (Figure 1) should be equivalent or eventually averaged
by a fluxional process.
In agreement with the above-mentioned proposal, the
optimized structures 1a+ and 1b+ of the cation [Ir(cod)-
(SiNP)]+ (1+) (see DFT Geometry Optimization for details)
show a slightly distorted square planar arrangement at the metal
center with either a boat or a chair conformation of the
SiN2P2Ir ring, respectively (Figure 2). The free energy
difference between the two structures is 9.4 kJ mol−1, the
boat conformation being more stable than the chair
conformation. Thus, the equivalence both of the olefinic cod
protons and of the SiCH3 methyls in 1+ should be reasonably
the consequence of the fast reversible interconversion 1a+ ⇆
1b+.
The optimized SPY-5-13 structure of 28 features non-
equivalent up and down semispaces both at the SiNP and at the
cod ligands, but, as mentioned before, equivalent up and down
semispaces (both at cod and at SiNP) were observed for 2 even
at 183 K; thus a fluxional process exchanging the above-
mentioned semispaces should be operative. In this relationship,
it is noteworthy that the other optimized structures of 2, that is,
those derived from the SPY-5-32, SPY-5-23, TBPY-5-23,
TBPY-5-12, and TBPY-5-13 configurations, Figure 3, were
For the pentacoordinated iridium compound IrCl(cod)-
(SiNP) (2), an apparent symmetric environment at the metal
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Figure 2. Optimized structures of [Ir(cod)(SiNP)]+ (1+) featuring a
boat (1a+) and a chair (1b+) conformation at the SiN2P2Ir six-
membered ring. Hydrogen atoms are omitted, and only ipso carbon
atoms of the aromatic rings are shown for clarity (see the Supporting
Information for atomic coordinates). Selected bond distances of the
boat (normal type) and the chair (italics type) conformers (Å) are in
order: Ir(14)−C(78), 2.248, 2.343; Ir(14)−C(80) 2.236, 2.226;
Ir(14)−C(82) 2.235, 2.285; Ir(14)−C(84) 2.279, 2.225; Ir(14)−
P(3) 2.413, 2.379; Ir(14)−P(4), 2.380, 2.356; C(78)−C(80) 1.405,
1.394; C(82)−C(84) 1.398, 1.403.
Figure 4. Optimized SPY-5-13 and SP-4-3 structures of 2 and the
reaction profile for the equilibrium SP-5-13 ⇆ SP-4-3. Hydrogen
atoms are omitted, and only ipso carbon atoms of the aromatic rings
are shown for clarity (see the Supporting Information for atomic
coordinates). Selected bond distances of 2 with the SPY-5-13 (normal
type) and SP-4-3 (italics type) configurations (Å) are in order:
Ir(18)−P(12), 2.393, 2.371; Ir(18)−P(14), 2.407, n.d.; Ir(18)−C(87),
2.164, 2.234; Ir(18)−C(88), 2.185, 2.188; Ir(18)−C(99), 2.181, 2.166;
Ir(18)−C(102). 2.225, 2.132; Ir(18)−Cl(107), 2.567, 2.382; C(87)−
C(88), 1.435, 1.402; C(99)−C(102), 1.420, 1.428.
3) shown in Figure 4 and the SPY-5-13 structure and the
affordable activation barrier calculated for the equilibrium SPY-
5-13 ⇆ SP-4-3 (ΔH⧧ = 68.8 kJ mol−1; ΔS⧧ = 15.4 J mol−1 K−1)
suggest that the reversible dissociation of one phosphorus atom
from the metal center should be the process averaging the
semispaces both at the cod and at the SiNP ligands.
Figure 3. Pentacoordinated ideal configurations for IrCl(cod)(SiNP)
(2).
found to be significantly less stable than the SPY-5-13
conformation with relative free energies higher than 65 kJ
mol−1. Thus, the up-down exchange via the reversible
interconversion between different pentacoordinated structures
can be ruled out. Furthermore, the gas-phase ΔG0 values for the
formation of a square planar complex via the dissociation from
the metal either of the chlorido ligand (ΔG0 = 459 kJ mol−1) or
of one double bond of the cod ligand (ΔG0 = 122 kJ mol−1)
clearly indicate that none of the above-mentioned dissociation
equilibria can be proposed to justify the observed exchange. On
the other hand, the small free energy difference (+8.0 kJ mol−1)
between the optimized tetracoordinate square complex (SP-4-
CO-Promoted C−H Activation. The irreversible carbon-
ylation of [Ir(cod)(SiNP)]+ (1+) and IrCl(cod)(SiNP) (2) is
fast and complete under mild conditions (1 atm of CO, room
temperature, approx. 10 min) resulting in the formation of the
pentacoordinated cation of formula Ir(CO)(cod)(SiNP)+ (3+)4
isolated as both [3]Cl and [3][PF6], depending on the starting
complex used in the synthesis (Scheme 3).
The IR spectrum of 3+ (CH2Cl2 solutions) shows a strong
νCO absorption at 1994 cm−1 in agreement with the presence of
the coordinated CO ligand. At room temperature, the 1H NMR
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spectrum of 3+ shows nonequivalent SiCH3 groups, thus
indicating that nonequivalent up and down semispaces at SiNP
exist (Figure 1). Further, equivalent phosphorus nuclei (31P)
and tolyl groups (1H and 13C) are observed at room
temperature, while at 183 K the 31P{1H} NMR spectrum of
3+ shows two signals (δP = 41.2, 37.3 ppm, see Supporting
Information Figure E1) and accordingly two methyl 1H
resonances at 2.20 and 2.04 ppm for the tolyl moieties, as
well. Thus, besides the up and down nonequivalence at the
SiNP ligand observed at room temperature, also nonequivalent
left and right semispaces should exist, and therefore a fluxional
process should exchange them at room temperature (vide
infra).
the boat conformation of the SiN2P2Ir ring was found to be
more stable than the chair conformation (+16.6 kJ mol−1).
As far as the possible fluxional processes affecting both the
1
cod and the SiNP ligands are concerned, the nature of the H
NMR spectra (see Supporting Information Figure E3)
1
prevented both the H line shape analysis and the quantitative
1
analysis of H−1H EXSY spectra at different temperatures.
Nevertheless, the line shape analysis of the 31P{1H} resonances
was easily carried out in the range 183−223 K affording the
kinetic constants for the exchange of the left and right
semispaces at the SiNP ligand. The activation parameters
obtained from the Eyring plot (ΔH⧧ = 50.1 0.5 kJ mol−1;
ΔS⧧ = 44.3
2.6 J mol−1 K−1; see Supporting Information
Table E1 and Figure E2) and the fact that the exchange of the
methyls of the Si(CH3)2 moiety is not observed reasonably
indicate that the mechanism for the left−right exchange at the
SiNP ligand is nondissociative. In this respect, it is worth
mentioning that besides the TBPY-5-12 configuration, the
TBPY-5-23 and the SPY-5-32 structures shown in Figure 6A
As far as the cod ligand is concerned, at room temperature its
1
olefinic protons are totally equivalent, only one H resonance
1
being observed at 3.86 ppm. Nevertheless, at 183 K, the H
NMR spectrum of 3+ shows four olefinic resonances (δH
=
4.25, 3.20, 1.80, and 1.30 ppm; see Supporting Information
Figure E3 for the assignment), indicating that both the up and
down and the left and right semispaces at the cod ligand of 3+
are nonequivalent at that temperature, and that the observed
equivalence at room temperature results from a fluxional
process averaging the above-mentioned semispaces. The ideal
configurations matching the NMR spectroscopic data are SPY-
5-12, SPY-5-23, and TBPY-5-12 (Figure 5). All of them were
optimized by standard computational methods (see DFT
Geometry Optimization for details), and the minimum free
energy structure was found to be the TBPY-5-12 configuration
shown in Figure 5,9 the others featuring relative free energies
higher than 70 kJ mol−1 with respect to TBPY-5-12.
Noteworthy, similar to 1+ and 2, in the TBPY-5-12 structure,
Figure 6. (A) Optimized SPY-5-32 and TBPY-5-23 structures of
[Ir(CO)(cod)(SiNP)]+ (3+) with the relative free energies given with
respect to TBPY-5-12. (B) Representation of the TBPY-5-12 ⇆ SPY-
5-32 and TBPY-5-12 ⇆ TBPY-5-23 equilibria emphasizing the
changes in the coordination sphere of the metal center.
were found to be local minima in the potential free energy
surface, being located at +25.7 and +27.5 kJ mol−1, respectively,
with respect to the TBPY-5-12 structure. Thus, the equilibria
TBPY-5-12 ⇆ SPY-5-32 and/or TBPY-5-12 ⇆ TBPY-5-23
(Figure 6B) should be operative in solution and could be
responsible for the left−right exchange at the SiNP ligand.
Additionally, it is worth mentioning that these equilibria
exchange the up and down (TBPY-5-12 ⇆ TBPY-5-23) and
the left and right (TBPY-5-12 ⇆ SPY-5-32) semispaces at the
cod ligand as well; thus reasonably they account also for the
Figure 5. (A) Selected ideal configurations for [Ir(CO)(cod)(SiNP)]+
(3+). (B) Optimized TBPY-5-12 structure of 3+. Hydrogen atoms are
omitted, and only ipso carbon atoms of the aromatic rings are shown
for clarity (see the Supporting Information for atomic coordinates).
Selected bond distances (Å) are in order: Ir(14)−C(87), 1.922;
Ir(14)−C(89), 2.361; Ir(14)−C(91), 2.345; Ir(14)−C(93), 2.177;
Ir(14)−C(95), 2.196; Ir(14)−P(12), 2.518; Ir(14)−P(13), 2.428;
C(87)−C(88), 1.155.
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1
dependence on the temperature of the H pattern of the cod
ligand.
arrangement of the donor atoms at the metal center of 4, the
tolyl groups are not equivalent (see Experimental Section). As a
confirmation, the optimized structure of 4 features a square
planar arrangement of the donor atoms at the metal center,
and, similar to 1+, 2, and 3+, the boat conformation of the
SiN2P2Ir ring is calculated to be slightly more stable (6.4 kJ
mol−1) than the chair one (Figure 7). In this connection, the
interconversion between the two conformers should be
responsible for the observed equivalence between the SiCH3
methyls.
Dichloromethane solutions of [3]Cl are thermally unstable,10
and the complete conversion of [3]Cl is observed in
approximately 6 h at room temperature affording a mixture
of IrCl(CO)(SiNP) (4)4 and IrHCl(CO)(SiNP−H) (5)4 with
a molar ratio of 4:1, regardless of the concentration of [3]Cl.
The H and 31P{1H} NMR measurements indicate that at the
1
beginning of the transformation of [3]Cl, the cod ligand is
released and 4 is cleanly formed, no intermediate being
detected even at 253 K. In addition, after approximately 15 min
from the formation of 4, measurable quantities of 5 are
observed in the 1H and 31P{1H} NMR spectra as the
consequence of the activation of one C−H bond of the
SiCH3 group (Scheme 4).
When dealing with 5, the spectroscopic data clearly indicate
the presence of a hydride moiety and of the [SiNP−H] ligand
featuring a κ3C,P,P′ coordination mode, both resulting from the
intramolecular SiCH2−H oxidative addition to the iridium(I)
center of 4. The 31P{1H} and 13C{1H} NMR spectra (see
Supporting Information Figures E4 and E5) point unambigu-
ously at the presence of two mutually cis phosphorus nuclei (δP
Scheme 4. CO-Promoted C−H Activation in 3+ (298 K)
2
= 48.1, 43.0 ppm, JPP = 9.0 Hz) with the carbon of the CO
ligand cis to both of them (13C{1H} triplet at 172.48 ppm, 2JPC
2
= 4.5 Hz). On the other hand, the coupling constants JHP of
the hydride ligand (δH = −9.11 ppm) indicate that it occupies a
cis position to one phosphorus (2JHP = 18.0 Hz) and the trans
one to the other (2JHP = 177.9 Hz) (see Supporting
Information Figure E6). Finally, the methylene carbon atom
of the activated SiCH3 group occupies a cis position with
respect to both phosphorus atoms, and the coordination sphere
of iridium should be completed by one chlorido ligand. In
agreement with the proposed structure, nonequivalent tolyl
groups (see Experimental Section) and nonequivalent methyl-
enic hydrogen atoms are observed (δH = 0.79, Ha; 1.40 ppm,
1
Hb). Further, the H resonances for Ha and Hb show a pattern
clearly indicating that Hb should feature a smaller dihedral angle
with respect to the hydride moiety than Ha (Scheme 4, see
Supporting Information Figure E6). Indeed, the scalar coupling
constant of Hb with the hydride is 3.0 Hz, while no scalar
coupling was observed between Ha and the hydrogen of the Ir−
H moiety (see Supporting Information Figure E6). Because the
Karplus curve for a H−X−Y−H system generally features a
local maximum at the dihedral angle H−X−Y−H of 0° and a
local minimum near 90°, the dihedral angle Ha−C−Ir−H
should be close to 90°.11
As a confirmation, the proposed structure of 5 was optimized
by standard computational methods (see DFT Geometry
Optimization for details) and was found to feature a distorted
octahedral arrangement of the donor atoms at the metal center
with Hx−C−Ir−H dihedral angles of 100° (x = a) and 16° (x =
Interestingly, the CH2Cl2 solutions of [3][PF6] are thermally
stable, and the addition of equimolar amounts of [PPN]Cl
resulted in the formation of the mixture 4+5 (4:1 molar ratio),
as well. When the reaction was carried out varying the molar
ratio Cl−/3+ (from 0.5 to 2), the same products, that is, 4 and
5, and the same molar ratio between them, that is, 4:1, were
observed.
The IR spectrum of a CH2Cl2 solution of the mixture 4+5
shows absorptions at 2007 (4) and 2026 cm−1 (5), indicating
the presence of two species each containing coordinated carbon
monoxide. The NMR spectrum of 4 suggests a square planar
coordination of the metal center. Indeed, two nonequivalent
phosphorus atoms are observed with the expected coupling
constants for a cis arrangement (2JPP = 30.5 Hz), and a 13C{1H}
doublet-of-doublets is observed at 181.3 ppm (2JPC = 124.3,
11.1 Hz), confirming that the CO ligand occupies the trans
position with respect to one phosphorus and a cis one with
respect to the other one. In agreement with the proposed
3
b) in agreement with the above-mentioned JHH coupling
constants (Figure 7). Additionally, it is worth mentioning that
the calculated νCO wave numbers (4, 2017; 5, 2029 cm−1) are
found close to the experimental ones (4, 2007; 5, 2026 cm−1).
Finally, the transition state structure for the transformation 4
⇆ 5 was calculated12 showing that the boat conformer of 4
allows the C−H bond to approach the metal-centered HOMO
shown in Figure 7. Consequently, the synchronous cleavage of
the SiCH2−H bond (1.535 Å) and formation of the Ir−CH2Si
(2.398 Å) and Ir−H (1.641 Å) bonds take place along with
shift of the CO ligand from the equatorial plane to the axial
position.
The treatment of a solution of 4+5 with CO (1 atm, 12 h)
yields the formal elimination of HCl and the quantitative
formation of the pentacoordinated iridium(I) dicarbonyl
complex 6 still featuring the metalated [SiNP−H] ligand
(Scheme 4).13 The solid-state structure of 6 has been
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Figure 7. Optimized structures of IrCl(CO)(SiNP) (4), IrHCl(CO)(SiNP−H) (5), and the transition state for the transformation 4 ⇆ 5 along with
the calculated HOMO of the boat conformer of 4. Selected bond distances (Å) are given in the table, and selected angles (deg) are in order: C(65)−
Ir(86)−C(88), 175.1 (5), 152.0 (TS); C(65)−Ir(86)−Cl(87), 89.5 (5), 92.2 (TS). Hydrogen atoms are omitted, and only ipso carbon atoms of the
aromatic rings are shown for clarity (for atomic coordinates, see the Supporting Information).
determined by single-crystal X-ray diffraction, and its molecular
structure is shown in Figure 8. Table 1 contains selected bond
distances and angles.
Table 1. Selected Bond Angles (deg) and Distances (Å) of
Ir(SiNP−H)(CO)2 (6)
Ir(1)−C(81) 1.885(5)
Ir(1)−C(91) 1.885(5)
Ir(1)−C(71) 2.185(4)
Ir(1)−P(1) 2.290(2)
Ir(1)−P(2) 2.3100(19)
P(1)−N(1) 1.691(3)
P(1)−C(11) 1.811(4)
P(1)−C(21) 1.815(4)
P(2)−N(2) 1.701(3)
P(2)−C(41) 1.809(4)
P(2)−C(31) 1.823(4)
Si(1)−N(2) 1.756(3)
Si(1)−N(1) 1.766(3)
Si(1)−C(71) 1.830(4)
Si(1)−C(72) 1.842(4)
N(1)−C(51) 1.418(4)
N(2)−C(61) 1.441(4)
C(81)−O(82) 1.135(5)
C(91)−O(92) 1.145(5)
C(71)−Si(1)−C(72) 120.59(17)
C(81)−Ir(1)−C(91) 100.09(19)
C(81)−Ir(1)−C(71) 173.53(16)
C(91)−Ir(1)−C(71) 85.91(16)
C(81)−Ir(1)−P(1) 95.34(13)
C(91)−Ir(1)−P(1) 120.10(13)
C(71)−Ir(1)−P(1) 79.45(11)
C(81)−Ir(1)−P(2) 95.29(13)
C(91)−Ir(1)−P(2) 128.67(14)
C(71)−Ir(1)−P(2) 82.65(10)
P(1)−Ir(1)−P(2) 106.61(7)
Figure 8. ORTEP views of Ir(SiNP−H)(CO)2 (6) with the
numbering scheme adopted and the thermal ellipsoids at 50%
probability.
the position trans to C(71) and the other in the equatorial
plane of the molecule, the overall coordination polyhedron
being a distorted trigonal bypiramid (TBPY-5-23). With this
respect, it is worth mentioning that Ir(1) lays approximately 0.3
Å out of the equatorial plane, slightly shifted toward the C(81)
atom. As far as bond distances are concerned, related
The [SiNP−H] ligand features a tridentate κ3C,P,P′
coordination mode involving both the two phosphorus atoms
and the carbon atom of the SiCH2 moiety. Interestingly, the
P(1), P(2), and C(71) atoms show a distorted fac arrangement
at the metal center with an expanded P(1)−Ir(1)−P(2) angle
(106.61°) and compressed P(1)−Ir(1)−C(71) and P(2)−
Ir(1)−C(71) angles (79.45°, 82.65°) with respect to the ideal
90° value, reasonably due to the presence of two fused five-
membered rings, Ir(1)−P(1)−N(1)−Si(1)−C(71) and Ir(1)−
P(2)−N(2)−Si(1)−C(71). The coordination sphere at the
metal center is completed by two CO ligands, one occupying
structurally characterized iridium(I) complexes, such as Ir-
14
{C6H3(CH2P(CF3)2)2}(CO)2
and Ir(CF2CF2H)-
(CO)2(PPh3)2,15 feature similar Ir−C−O, Ir−P, and Ir−C
bond distances, although with different coordination polyhedra
or ligands’ distribution around the metal center. On the other
hand, the Si(1)−C(71) and Si(1)−C(72) are very similar to
the silicon−carbon bond distances observed in RhCl2(η3−
C3H5)(SiNP) (1.854; 1.839 Å).3 Nevertheless, a severe
distortion of the C−Si−C angle is observed. Indeed, the C−
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Si−C angle in RhCl2(η3-C3H5)(SiNP) is 108.02°, whereas the
C(71)−Si(1)−C(72) angle in Ir(SiNP−H)(CO)2 is 120.59°,
thus suggesting that the coordination of the methylene moiety
to iridium entails a significant expansion of the C−Si−C angle.
For the sake of comparison, it should be mentioned that when
comparing angles and bond distances of the [IrP2(CH2)-
(COax)] moiety in the calculated structure of 5 with those in
the solid-state structure of 6, very small differences (≤3%) were
observed, major deviations being observed only for one of the
two Ir−P bond distances (that for the phosphorus trans to the
hydride ligand in 5) and the P−Ir−P angle (wider in 6 due to
the less congested coordination sphere at the metal center).
Finally, the solid-state structure of 6 is maintained in
solution. Indeed, two IR absorptions (CH2Cl2) were observed
at 1927 and 2001 cm−1 (νCO) in agreement with the presence
of a Ir(CO)2 moiety in an environment of Cs symmetry.
Further, the 1H, 13C{1H}, and 31P{1H} NMR spectra indicate a
fac symmetrically coordinated [SiNP−H] ligand; that is, both
the two N−P arms of the ligand and the protons of the IrCH2
moiety are equivalent (see Experimental Section).4
In relation to the C−H activation in the coordinated SiNP
ligand, it is worth mentioning that, to the best of our
knowledge, the intramolecular C−H addition of a methyl
moiety from a coordinated diphosphane ligand to a group 9
metal has previously been reported only in rhodium complexes
containing the ligands 1,3-(CH2PtBu2)2(C6H4) or 1,3-
(CH2PtBu2)-4,6-(CH3)2(C6H2).16 The ability of both diphos-
phano ligands to adopt a trans-spanning coordination mode
and their rigidity were found to be decisive in getting the CH3
moiety close to the metal center, and finally in promoting the
addition of the C−H bond to the metal center (Scheme 5).16
interconversion(s) between pentacoordinated isomers should
1
be responsible for the averaged H and 31P NMR spectra of 3+
at room temperature.
Starting from the carbonyl species [Ir(CO)(cod)(SiNP)]+
(3+), the C−H addition to the iridium(I) center easily takes
place at room temperature when the chloride ion is present in
solution, either as the counter-ion in [3]Cl or if added as
[PPN]Cl to [3][PF6]. Under these conditions, the preliminary
formation of the tetracoordinated carbonyl complex IrCl(CO)-
(SiNP) (4) has been observed along with the consequent
formation of the hydride complex IrHCl(CO)(SiNP−H) (5)
via the synchronous cleavage of the SiCH2−H bond, the
formation of the Ir−CH2Si and Ir−H bonds, and the shift of
the CO ligand from the equatorial plane in 4 to the axial
coordination site in 5.
Finally, the κ3C,P,P′ coordination mode of the [SiNP−H]
ligand has been fully characterized in the solid state in the
pentacoordinated dicarbonyl iridium(I) complex Ir(SiNP−
H)(CO)2 (6) showing that it easily fits three mutually cis
coordination sites and that a severe distortion of the C−Si−C
angle is necessary to accommodate the methylene moiety in the
coordination sphere of the metal center.
The influence of the ancillary ligands on the C−H bond
activation and the application of the resulting Ir[κ3C,P,P′-
(SiNP−H)] platform to both stoichiometic and catalytic
processes is being investigated, and results will be published
in due course.
EXPERIMENTAL SECTION
■
All of the operations were carried out using standard Schlenk-tube
techniques under an atmosphere of prepurified argon or in a Braun
glovebox under dinitrogen or argon. The solvents were dried and
purified according to standard procedures. [PPN]Cl (Aldrich) and CO
(Praxair) were commercially available and were used as received. The
compounds [IrCl(cod)]2,17 [Ir(cod)(CH3CN)2][PF6],18 and SiNP3
were prepared as previously described in the literature. NMR spectra
were measured with Bruker spectrometers (AV300, AV400, AV500)
and are referred to as SiMe4 (1H, 13C), H3PO4 (31P), and CFCl3 (19F).
1H and 13C data are given together in the following way: δH
(multiplicity, 1H-assignment, number of hydrogen, δC). Only the
quaternary carbon atoms (when observed) are given in the 13C{1H}
NMR subsection. The diffusion experiments were performed using the
stimulated echo pulse sequence19 without spinning, and the collected
data were treated as previously described.3 The hydrodynamic radius
(Rh) was calculated using the equation of Stokes−Einstein, and the
radius of gyration (Rg) was calculated according the literature, using
optimized molecular structures.20 Infrared spectra were recorded on a
ThermoNicolet Avatar 360 FT-IR spectrometer. Elemental analyses
were performed by using a Perkin-Elmer 2400 microanalyzer.
Scheme 5
On the contrary, SiNP tends to coordinate the metal center
occupying two cis sites, and the boat conformation of the
SiN2P2Ir is the necessary requisite to allow the SiCH2−H group
to approach the metal center and finally add to the metal
center.
CONCLUSIONS
■
The N,N′-diphosphanosilanediamine ligand SiMe2{N(4-
C6H4CH3)PPh2}2 (SiNP) is able to coordinate iridium(I)
affording tetra- or pentacoordinated complexes (1+, 2, and 3+).
The solution structure of the pentacoordinated complexes of
formula [IrX(SiNP)(cod)]n+ (2, X = Cl, n = 0; 3+, X = CO, n =
1) depends on the nature of the ancillary ligand X. Indeed, the
chlorido ligand occupies the apical position of a square planar
pyramid, while the trigonal bypiramydal structure is the favored
one with the CO ligand, in both cases a boat conformation
being adopted by the SiN2P2Ir ring. Different fluxional
processes for 2 and 3+ are observed in solution as a
consequence of the different geometries at the metal center:
indeed, the reversible dissociation of one phosphorus atom of
the SiNP ligand in IrCl(cod)(SiNP) (2) was calculated to be
the process averaging the up and down semispaces both at the
cod and at the SiNP ligands. On the other hand, the
Synthesis of [Ir(cod)(SiNP)][PF6] ([1][PF6]). A solution of SiNP
(150 mg, 0.235 mmol, 638.79 g/mol) in CH2Cl2 (10 mL) was added
to [Ir(cod)(CH3CN)2][PF6] (124 mg, 0.235 mmol, 527.47 g/mol).
After 30 min of stirring, the resulting deep orange solution was
evaporated, and the resulting solid was washed with hexane (3 × 5
mL). The deep orange solid was dried under a vacuum and identified
as [Ir(cod)(SiNP)][PF6] ([1][PF6], 224 mg, 88% yield). Anal. Calcd
for C48H52F6IrN2P3Si (1084.15): C, 53.18; H, 4.83; N, 2.58. Found: C,
53.02; H, 5.00; N, 2.52. 1H NMR (CD2Cl2, 298 K): δ7.30−7.51 (20H,
PPh, δC = 128.2, m-PPh, 131.5, p-PPh, 133.7, o-PPh), 6.85 (d, 8.2 Hz,
4H, C3Htol, δC = 129.4), 6.68 (d, 8.2 Hz, 4H, C2Htol, δC = 129.6), 4.31
(br, 4H, Csp2Hcod, δC = 88.6), 2.28−2.15 (10H, Csp3Hcod + CH3tol, δC =
20.4, CH3tol), 2.07 (m, 4H, Csp3Hcod, δC = 30.8), −0.67 (s, 6H, SiCH3,
δC = 3.7). 19F NMR (CD2Cl2, 298 K): δ −73.5 (d, 710.5 Hz). 31P{1H}
NMR (CD2Cl2, 298 K): δ 53.5 (s, SiNP), −144.4 (sp, 710.5 Hz,
−
PF6 ). D (CDCl3, 298 K) = (5.94 0.03) × 10−6 cm2 s−1, Rh = 6.45
0.03 Å (Rg = 6.33 Å).
978
dx.doi.org/10.1021/ic4024458 | Inorg. Chem. 2014, 53, 972−980

Inorganic Chemistry
Article
Synthesis of IrCl(cod)(SiNP)(2). A suspension of SiNP (466 mg,
0.730 mmol, 638.79 g/mol) in CH2Cl2 (5 mL) was added to
[IrCl(cod)]2 (252 mg, 0.375 mmol, 671.70 g/mol). The mixture was
stirred for 1 h, and the resulting solid was filtered off, dried in vacuo,
and identified as IrCl(cod)(SiNP) (2, 498 mg, 70% yield). Anal. Calcd
for C48H52ClIrN2P2Si (974.64): C, 59.15; H, 5.38; N, 2.87. Found: C,
1
59.24; H, 5.33; N, 2.55. H NMR (CDCl3, 298 K): δ7.62 (m, 8H, o-
3
PPh, δC = 134.31), 7.31 (t, JHH = 7.1 Hz, 4H, p-PPh, δC = 129.84),
7.22 (t, 3JHH = 7.1 Hz, 8H, m-PPh, δC = 126.99), 6.69 (d, 7.4 Hz, 4H,
C3Htol, δC = 128.69), 6.57 (d, 7.4 Hz, 4H, C2Htol, δC = 130.9), 3.42
(br, 4H, Csp2Hcod, δC = 74.97), 2.25−2.06 (10H, CH3tol + Csp3H2cod, δC
= 20.74, CH3tol, 31.58, Csp3H2cod), 1.74 (m, 4H, Csp3H2cod), 0.39 (s,
6H, SiCH3, δC = 4.21). 31P NMR (CDCl3, 298 K): δ49.4 (s). D
(CD2Cl2, 298 K) = (8.87 0.21) × 10−6 cm2 s−1, Rh = 5.72 0.14 Å
(Rg = 5.97 Å).
Synthesis of [Ir(CO)(cod)(SiNP)][PF6] ([3][PF6]). A solution of
[Ir(cod)(SiNP)][PF6] (156 mg, 0.144 mmol, 1084.15 g/mol) in
CH2Cl2 (3 mL) was stirred under an atmosphere of CO (1 atm) for
20 min. A pale yellow solution was obtained, which was partially
evaporated and added to hexane (10 mL) affording a yellow solid
identified as [Ir(CO)(cod)(SiNP)][PF6] ([3][PF6], 147 mg, 92%
yield). Anal. Calcd for C49H52F6IrN2OP3Si (1112.16): C, 52.92; H,
4.71; N, 2.52. Found: C, 53.01; H, 4.92; N, 2.48. νCO (CH2Cl2), 1994
cm−1. 1H NMR (CD2Cl2, 298 K): δ 7.36−7.69 (PPh, 20H, δC
134.98, m-PPh; 132.48, m-PPh; 128.31, o-PPh; 128.11, o-PPh; 132.12,
p-PPh; 131.57 or 131.64, p-PPh), 6.97 (d, 8.2 Hz, 2H, C3Htol, δC
129.54), 6.92 (d, 8.2 Hz, 2H, C2Htol, δC = 131.57 or 131.64), 6.77 (d,
=
=
8.2 Hz, 2H, C3Htol, δC = 129.34), 6.18 (d, 8.2 Hz, 2H, C2Htol, δC
=
131.33), 3.86 (br, 4H, Csp2Hcod, δC = 82.90), 2.22 (s, CH3tol, 6H, δC =
20.52), 2.05 (m, 4H, Csp3Hcod, δC = 32.51), 1.93 (m, Csp3Hcod, 4H, δC
= 32.51), 0.56 (s, 3H, SiCH3, δC = 1.24), −0.29 (s, 3H, SiCH3, δC =
3.26). 13C{1H} NMR (CD2Cl2, 298 K): δ 179.4 (t, 4.6 Hz, CO).
31P{1H} NMR (CD2Cl2, 298 K): δ 39.4 (s, SiNP), −144.4 (sp, 710.5
c. Synthesis of Ir(SiNP−H)(CO)2 (6). A suspension of IrCl(cod)-
(SiNP) (185 mg, 0.190 mmol, 974.64 g/mol) in CH2Cl2 (5 mL) was
degassed, contacted with CO (1 atm), and stirred at room temperature
for 18 h affording a deep yellow solution. The solution was partially
evaporated (approx. 2 mL left) and layered with hexane. After 2 days,
bright yellow crystals precipitated out. The supernatant solution was
decanted, and the crystals were washed with hexane, dried in vacuo,
and identified as Ir(SiNP−H)(CO)2 (6, 138 mg, 82% yield). Anal.
Calcd for C42H39IrN2O2P2Si (886.02): C, 56.93; H, 4.44; N, 3.16.
Found: C, 56.02; H, 4.45; N, 3.28. νCO (CH2Cl2), 2001, 1927 cm−1.
1H NMR (CDCl3, 298 K): δ 7.57 (m, 4H, o-PPhendo, δC = 133.87),
7.42−7.29 (10H, o-PPhexo + m-PPhendo + p-PPhexo, δC = 131.97, o-PPh,
127.56, m-PPh, 129.83, p-PPh), 7.15 (m, 4H, m-PPhexo, δC = 127.41),
−
Hz, PF6 ). D (CD2Cl2, 298 K) = (7.89 0.06) × 10−6 cm2 s−1, Rh =
6.43
0.04 Å (Rg = 6.31 Å). Selected 1H NMR data at 183 K
(CH2Cl2): δ 4.25 (br, 1H, Csp2Hcod), 3.11 (br, 1H, Csp2Hcod), 2.22 (s,
3H, CH3tol), 2.01 (s, 3H, CH3tol), 1.82 (br, 1H, Csp2Hcod), 1.35 (br,
1H, Csp2Hcod), 0.20 (s, 3H, SiCH3), −0.35 (s, 3H, SiCH3).
Reaction of IrCl(cod)(SiNP) with CO. a. Synthesis of [Ir(CO)-
(cod)(SiNP)]Cl ([3]Cl). A suspension of IrCl(cod)(SiNP) (159 mg,
0.163 mmol, 974.64 g/mol) in CH2Cl2 (5 mL) was degassed and
contacted with CO (1 atm). As soon as the dissolution of the solid was
observed (approx. 30 min), all volatiles were removed in vacuo, and
the resulting solid was washed with hexane (3 × 5 mL), dried in vacuo,
and finally identified as [Ir(CO)(cod)(SiNP)]Cl ([3]Cl, 151 mg, 92%
yield). Anal. Calcd for C49H52ClIrN2OP2Si (1002.65): C, 58.70; H,
5.23; N, 2.79. Found: C, 59.00; H, 5.11; N, 2.68. νCO (CH2Cl2), 1994
cm−1. 1H and 31P NMR spectra of a freshly prepared solution of [3]Cl
in CD2Cl2 are very similar to those measured for [3][PF6].
b. Formation of [IrCl(CO)(SiNP)] (4) and IrHCl(CO)(SiNP−H) (5). A
suspension of IrCl(cod)(SiNP) (2, 220 mg, 0.226 mmol, 974.64 g/
mol) in CH2Cl2 (5 mL) was degassed and contacted with CO (1 atm).
As soon as the dissolution of the solid was observed (approx. 30 min),
CO gas was replaced by argon gas, and the solution was stirred for
about 12 h. Finally, all volatiles were removed in vacuo, and the
resulting solid was washed with hexane (3 × 5 mL), dried in vacuo,
and finally identified as a mixture of [IrCl(CO)(SiNP)] (4, 80%) and
IrHCl(CO)(SiNP−H) (5, 20%) (157 mg, 78% yield). Anal. Calcd for
C41H40ClIrN2OP2Si (894.47): C, 55.05; H, 4.51; N, 3.13. Found: C,
55.15; H, 4.68; N, 3.15.
3
7.25 (m, 2H, p-PPhendo, δC = 129.83), 6.84 (d, 4H, JHH = 8.1 Hz,
C3Htol, δC = 128.86), 6.55 (d, 4H, 3JHH = 8.1 Hz, C2Htol, δC = 129.37),
2.21 (s, 6H, CH3tol, δC = 20.76), 0.61 (t, 2H, 3JHP = 10.2 Hz, SiCH2Ir,
δC = 0.14, t, 2JCP = 8.1 Hz), −0.36 (s, 3H, SiCH3, δC = 26.7). 31P{1H}
NMR (CDCl3, 298 K): δ 49.6 (s).
X-ray Measurements and Structure Determination of
Ir(SiNP−H)(CO)2 (6). Single crystals of Ir(SiNP−H)(CO)2 (6) were
obtained by layering hexane over a CH2Cl2 solution of 6. Intensities
were collected at 100 K using a Bruker SMART APEX diffractometer
with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å)
following standard procedures. Intensities were integrated and
corrected for absorption effects using the SAINT+21 and SADABS22
programs, included in the APEX2 package. The structure was solved
by standard direct methods. All non-H atoms were located in the
subsequent Fourier maps. Refinement was carried out by full-matrix
IrCl(CO)(SiNP) (4). νCO (CH2Cl2), 2007 cm−1. 31P{1H} NMR
(CD2Cl2, 298 K): δ 63.4 (d, 2JPP = 30.5, P1), 41.2 (d, 2JPP = 30.5, P2).
D (CD2Cl2, 298 K) = (8.58 0.03) × 10−6 cm2 s−1, Rh = 5.91 0.02
Å (Rg = 5.92 Å). Selected NMR data (δH, bold type, and δC, italics
type) are shown in the following scheme.
2
least-square procedure (based on F0 ) using anisotropic temperature
factors for all non-hydrogen atoms. The H atoms were placed in
calculated positions with fixed isotropic thermal parameters (1.2Uequiv
)
of the parent carbon atom. Calculations were performed with the
SHELX-9723 program implemented in the WinGX24 package.
Crystal Data for 6. C42H39IrN2O2P2Si, M = 885.98 g mol−1; pale
IrHCl(CO)(SiNP−H) (5). νCO (CH2Cl2), 2026 cm−1. 31P{1H} NMR
(CD2Cl2, 298 K): δ 48.1 (d, 2JPP = 9.0, P1), 43.0 (d, 2JPP = 9.0, P2). D
(CD2Cl2, 298 K) = (8.45 0.11) × 10−6 cm2 s−1, Rh = 6.00 0.08 Å
(Rg = 6.04 Å). Selected NMR data (δH, bold type, and δC, italics type)
are shown in the following scheme.
yellow needle, 0.15 × 0.17 × 0.40 mm3; triclinic, P1; a = 12.007(12) Å,
̅
b = 12.770(12) Å, c = 12.878(12) Å, α = 102.208(11)°, β =
100.476(11)°, γ = 100.405(11)°; Z = 2; V = 1848(3) Å3; Dc = 1.592 g
cm−3; μ = 3.771 mm−1, minimum and maximum absorption correction
979
dx.doi.org/10.1021/ic4024458 | Inorg. Chem. 2014, 53, 972−980

Inorganic Chemistry
Article
factors 0.416 and 0.568; 2θmax = 57.22°; 21 576 collected reflections,
8627 unique (Rint = 0.0358); 8627/0/454 data/restraints/parameters;
final GOF = 1.059; R1 = 0.0318 (7665 reflections, I > 2σ(I)); wR2 =
0.0754 for all data. CCDC refcode 941530.
DFT Geometry Optimization. The molecular structures were
optimized at the ONIOM(BP3LYP/GenECP:UFF) level using the
Gaussian 09 program.25 The aromatic rings were included in the low
level layer and the remaining atoms in the high level layer. The
LanL2TZ(f) basis and pseudo potential were used for iridium and the
6-31G(d,p) basis set for the remaining atoms. Stationary points were
characterized by vibrational analysis (transition states featuring one
imaginary frequency, minimum energy molecular structures featuring
only positive frequencies).
M. Z.; Woolins, J. D. Phosphorus, Sulfur Silicon Relat. Elem. 2001, 168−
169, 329−332.
(3) Passarelli, V.; Benetollo, F. Inorg. Chem. 2011, 50, 9958−9967.
(4) The proposed mononuclear structure agrees with the measured
diffusion coefficient and with the calculated hydrodynamic radius (see
Experimental Section).
(5) Nomenclature of Inorganic Chemistry. IUPAC Recommendations
2005; Connely, N. G., Damhus, T., Eds.; Royal Society of Chemistry:
Cambridge, 2005.
(6) Makino, T.; Yamamoto, Y.; Itoh, K. Organometallics 2004, 23,
1730−1737.
(7) Shibata, T.; Yamashita, K.; Ishida, H.; Takagi, K. Org. Lett. 2001,
3, 1217−1219.
(8) As observed for [Ir(cod)(SiNP)]+ (1), the boat conformation of
the SiN2P2Ir ring in the SPY-5-13 optimized structure of IrCl(cod)
(SiNP) (2) is more stable than the chair conformation (+38 kJ mol−1).
(9) The calculated νCO wavenumber for the TBPY-5-12 structure is
1998 cm−1, and the experimental value for 3+ is 1994 cm−1.
(10) A similar behavior of [3]Cl was observed in THF at room
temperature.
ASSOCIATED CONTENT
■
S
* Supporting Information
Selected NMR and kinetic data, the CIF file for the structure of
6, and atomic coordinates of the optimized structures. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(11) Minch, M. J. Concepts Magn. Reson. 1994, 6, 41−56.
(12) The transition state for the reaction IrCl(CO)(SiNP) (4) ⇆
IrHCl(CO)(SiNP−H) (5) was located at 98.8 kJ mol−1 higher than 4.
(13) It is noteworthy that while [3]Cl readily reacts with excess of
CO affording 6 after 12 h of exposure to a CO atmosphere, [3][PF6]
do not react with CO even after prolonged exposure to a CO
atmosphere, thus indicating that the presence of the chloride ion is
essential to obtain 6.
(14) CCDC refcode ATOQUV, in: Adams, J. J.; Arulsamy, N.;
Roddick, D. M. Organometallics 2011, 30, 697−711.
(15) CCDC refcode JEPTIH, in: Burrell, A. K.; Roper, W. R.
Organometallics 1990, 9, 1905−1910.
(16) (a) Sundermann, A.; Uzan, O.; Milstein, D.; Martin, J. M. L J.
Am. Chem. Soc. 2000, 122, 7095−7104. (b) Rybtchinski, B.; Milstein,
D. J. Am. Chem. Soc. 1999, 121, 4528−4529. (c) van der Boom, M. E.;
Liou, S.-Y.; Ben−David, Y.; Gozin, M.; Milstein, D. J. Am. Chem. Soc.
1998, 120, 13415−13421.
AUTHOR INFORMATION
■
Corresponding Author
*Tel.: +34 976 739863. E-mail: passarel@unizar.es.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
́
Financial support from the Spanish “Ministerio de Economia y
Competitividad” (CTQ2010-15221) and “Diputacion
́
General
de Aragon
́
” (Group E07) is gratefully acknowledged. V.P. is
indebted to Dr. Pablo Sanz (ISQCH, Universidad de Zaragoza)
for insightful comments and useful discussions about the X-ray
determination of complex 6.
(17) Herde, J. L.; Lambert, J. C.; Senoff, C. V. Inorg. Synth. 1974, 15,
18−20.
(18) Day, V. W.; Klemperer, W. G.; Main, D. J. Inorg. Chem. 1990,
29, 2345−2355.
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