Coordinating Ability of the Iminophosphorane Group in ortho-Carborane Derivatives

Article abstract of DOI:10.1002/ejic.201700487

The coordinating ability of the nitrogen donor atom of C-carboranyl iminophosphoranes was studied in different ligand systems. The analysis of organotin derivatives of the unsubstituted iminophosphorane I1 (SnMe₃I1 and SnClMe₂I1) and

Full text of DOI:10.1002/ejic.201700487

A Journal of
Accepted Article
Title: Coordinating ability of the iminophosphorane group in ortho-
carborane derivatives
Authors: Pablo Crujeiras, José Luis Rodríguez-Rey, and Antonio
Sousa-Pedrares
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European Journal of Inorganic Chemistry
10.1002/ejic.201700487
Coordinating ability of the iminophosphorane group in ortho-
carborane derivatives.
[a]
Pablo Crujeiras, José Luis Rodríguez-Rey, Antonio Sousa-Pedrares*
[a]
Departamento de Química Inorgánica, Facultad de Química, Universidade de
Santiago de Compostela, 15782 Santiago de Compostela, Spain. E-mail:
antonio.sousa.pedrares@usc.es
Abstract: The coordinating ability of the nitrogen donor atom of C-carboranyl
iminophosphoranes was studied in different ligand systems. The analysis of organotin
derivatives of the unsubstituted iminophosphorane I1 (SnMe
3 2
I1 and SnClMe I1) and
the comparison with the non-carboranyl analog (SnClMe I3), reveals the reduced
2
donor capacity of the nitrogen atom by effect of the closo-carborane group. In order
to promote the coordination of this weak donor atom, new phosphine-
2
iminophosphorane ligands (IP1 and IP2), with an extra -PPh group on the other
cage carbon atom, were synthesized and structurally characterized. The analysis of
the products formed by reaction of these ligands and the metal precursor cis-
2 2
[PdCl (PhCN) ] reveals that the phosphine group promotes the coordination of the
iminophosphorane nitrogen atom, yielding (P, N) chelates. However, this
coordination mode activates the closo-carborane ligands towards deboronation, and
the complexes evolve to the nido derivatives by effect of polar solvents. The
activation is increased by the presence of extra donor groups, like in the tridentate
ligand IP2, with an extra thioether group. The deboronation by coordination of (P, N)
bidentate carboranyl ligands has never been reported, although it has been studied for
other ligands, especially for chelating carboranyl diphosphines.
1
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European Journal of Inorganic Chemistry
10.1002/ejic.201700487
Introduction
Dicarbaboranes are cluster compounds with ten boron and two carbon atoms in an
icosahedral arrangement. These bulky molecules have a close shell structure that
[
1]
gives them a remarkable thermal and chemical stability. The thermal stability
depends on the relative position of the carbon atoms, as they isomerize from ortho- to
meta- and then to para-carborane on heating. This structural stability has been one of
the most exploited properties of carboranes, and since the beginning of the
development of carborane chemistry these molecules have been incorporated in other
[2]
[3]
materials producing, for example, very robust polymers, hybrid materials,
[
4]
dendrimers
and, more recently, thermally stable metal-organic frameworks
[
5]
(
MOFs). Apart from their stability and bulkiness, the carborane clusters possess
some particular electronic properties, like a σ-aromatic character and an irregular
charge distribution, that have also found application in the design of compounds with
[
6]-[7]
interesting electronic properties.
The irregular charge distribution is responsible
for the different inductive effect of substituted carboranes. The more electronegative
carbon atoms make the C-carboranyl substituents electron-withdrawing groups, and
due to their adjacent position in the ortho-carborane derivatives, the effect is more
pronounced for these isomers. On the contrary, B-substituted carboranes show an
electron-donating inductive effect, especially those substituted at the antipodal boron
atoms. This is reflected in the chemical behavior of the o-carborane group, like in the
[
1]
acidity of the hydrogen atoms at the cage carbon atoms (Cc-H) or in the different
[
7(a)]
a
pK values of C- and B-carboranyl-thiols.
The structural and electron-withdrawing electronic properties of o-carboranes
[8]
make them very attractive for ligand design. The accessible functionalization of the
[
1]
[9]
o-carborane carbon or boron atoms with functional groups bearing donor atoms
2
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European Journal of Inorganic Chemistry
10.1002/ejic.201700487
has produced different o-carborane ligands that have been used for the production of
[
1a,10]
exo-metallated carborane complexes with interesting properties and applications.
Some exo-metallaboranes have found application in material science, for the
[
11]
production of luminescent materials
and as building blocks for polymers and
[
12]
[13]
macrocycles. Other exo-metallaboranes have been used in medicinal chemistry
mainly for their hydrophobicity and/or high boron content (for Boron Neutron
Capture Therapy). However, the main application for exo-metallated carborane
[
10, 14]
derivatives is in the field of catalysis.
gets used because of its stability and bulky character,
effects of the o-carborane moiety have also been used to modulate the coordinating
Although the o-carborane moiety usually
[
8,15]
the different inductive
[
16]
[17]
ability of vicinal sulfur
and phosphorous
donor atoms in coordination
compounds. Thus, a thioether group shows reduced donor ability when connected to a
C-m-carboranyl moiety, but presents enhanced donor strength when connected to the
[
16]
antipodal boron atoms of o- and m-carborane groups. It has also been shown that
C-carboranyl phosphines present electron-poor character, while B9-meta-carboranyl
[
17]
phosphines are extremely electron-rich.
These studies have established the
[
8]
dichotomous character of carborane ligands. Following this trend, we have recently
studied the possibility of deactivating the donor ability of the nitrogen atom of an
iminophosphorane group by attaching it to the cage carbon atom of an o-carborane
[
18]
group.
These studies contributed to the little explored chemistry of carborane-
[
19,20]
iminophosphorane derivatives,
and revealed that the thiolate L1 [Figure 1(b)]
derived from the parent carborane-iminophosphorane I1 [Figure 1(a)] fails to promote
the expected (S, N) chelation to palladium(II) or platinum(II) but, instead, evolves to
different final products depending on the metal (Pd…H-B pseudo-agostic interaction
3
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European Journal of Inorganic Chemistry
10.1002/ejic.201700487
or Pt-B metalation). Thus, compounds with M-N bonds involving carborane-
iminophosphorane ligands have not yet been described.
As a continuation of our studies on the coordination ability of carboranyl-
iminophosphorane ligands, we now report some organotin compounds of the parent
iminophosphorane I1 [Figure 1(a)], which allow the study of the donor ability of the
iminophosphorane nitrogen atom when is not affected by the presence of extra donor
groups. To complete these studies, we have also designed the neutral phosphine-
iminophosphoranes IP1 and IP2 [Figure 1(c)] that present an extra
diphenylphosphino group (-PPh
2
) on the other cage carbon atom, and should promote
(P, N) chelation. In the case of the neutral ligand IP2, the extra thioether group should
promote (P, N, S) tridentate coordination by chelate effect. Although these carboranyl
derivatives are not known, non-carboranyl phosphine-iminophosphoranes are well
[
21]
known ligands that form stable (P, N) chelates with different metals. Some of these
hybrid ligands can reversibly change the coordination mode from (P, N) chelate to P-
[
22]
terminal (hemilabile behavior), which explains the variety of applications that these
[
21]
ligands have found in homogeneous catalysis.
Our new studies on carboranyl phosphine-iminophosphoranes also contribute
to increase the knowledge on (P, N) carborane hybrid ligands. Although the (P, N)
[
23]
ligands are the most abundant types of non-carboranyl hybrid ligands, in the field
of carboranes only three examples of (P, N) donor ligands are known. The phosphine-
2 2 2 2 10
amino ligand closo-1-PPh -2-CH NMe -1,2-C B H10 was used to obtain rhodium
and iridium complexes that were isolated and used in catalytic hydrogenation
[24]
i
2 2 10
reactions; the ligands 1-PR -2-(4- Pr-oxazolin-2-yl)-1,2-C B H10, R= Ph, Cy, were
[25]
also used in rhodium-catalyzed asymmetric hydrogenation processes, although the
metal complexes were not isolated; and finally, very recently, a (P, N) bis(phosphane)
4
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European Journal of Inorganic Chemistry
10.1002/ejic.201700487
[
26]
derivative of carborane was used to obtain copper(I) and silver(I) complexes.
Besides, these few examples, the literature also describes some phosphine-imine and
phosphite-imine carborane compounds that could be used as (P, N) chelating ligands,
[
27]
although no attempts have yet been made to obtain their metal complexes.
It is
interesting to note that none of the literature examples presents both (P, N) donor
atoms directly attached to the cage carbon atoms of the o-carborane moiety, so the
new ligands IP1 and IP2 should provide an interesting comparison.
Results and Discussion
Synthesis and characterization of the carborane ligands
The carboranyl-iminophosphorane ligands I1 and I2 were obtained as described in
[
18]
the literature.
These compounds were used to obtain organotin derivatives (see
below) and also as starting materials for the production of potentially bidentate
phosphine-iminophosphorane ligands. Thus, the disubstituted carboranyl derivatives
IP1 and IP2 were obtained by reaction of the carboranyl-lithium derivatives of the
iminophosphorane precursors, I1 and I2, with chlorodiphenylphosphine. The lithiated
derivatives were prepared in situ by reaction of the iminophosphorane precursors and
n-butyllithium (see Scheme 1). The new phosphine-iminophosphorane compounds,
IP1 and IP2, were obtained in very good yields (77% and 84%, respectively) and
characterized by elemental analysis, standard spectroscopic techniques and X-ray
diffraction analysis.
The disubstituted carboranes IP1 and IP2 are less stable compounds than the
[
18]
parent iminophosphoranes I1 and I2. The introduction of the phosphine group on
the other cage carbon atom does not alter the stability of the iminophosphorane
group, P=N, which is stable towards hydrolysis by dilute acids like HCl, or the
stability of the closo-carborane cage, which does not show signs of deboronation
5
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European Journal of Inorganic Chemistry
10.1002/ejic.201700487
after reflux in common organic solvents, including polar alcohols. However, the
weak point of the phosphine-iminophosphorane derivatives is the tendency towards
aerial oxidation of the phosphine group. The thioether derivative IP2 seems to be
more sensitive and was isolated with a small amount of oxide impurity (after column
chromatography). This is surprising, as o-carboranyl-phosphines, in contrast to
organic phosphines, are usually stable towards oxidation in air or in presence of mild
oxidizing agents, due to the electron-withdrawing character of the o-carborane
[
28]
group.
3
1
The oxide impurity in IP2 is revealed, for example, by P NMR
spectroscopy, which is the most informative characterization technique for these
3
1
1
compounds in solution (see spectra in Supporting Information). The P{ H} NMR
spectrum of the disubstituted thioether derivative IP2 shows two major signals, due
to the iminophosphorane group (3.5 ppm) and to the phosphine group (-0.5 ppm) of
the non-oxidized compound IP2, along with two minor signals due to the oxidized
3
1
1
impurity (7.6 ppm for P=N and 18.7 ppm for P=O). However, the P{ H} NMR
spectrum of IP1 only shows two peaks at 5.6 ppm (P=N) and at -0.7 ppm
(phosphine), although it was subject of a similar purification by column
chromatography. It is interesting to note that the signals of the iminophosphorane
phosphorous atoms of IP1 and IP2 are shifted downfield respect to those of the
[
18]
parent iminophosphoranes I1 (3.1 ppm) and I2 (0.6 ppm).
This trend was also
observed for the previously published sulfur derivatives of I1 and I2, although in
[
18]
those cases the downfield shift was more pronounced. It is also interesting to note
that the values found for the diphenylphosphine groups (-PPh ) are very shifted
upfield respect to the value found for 1-diphenylphosphino-carborane (25.2 ppm),
due to the presence of the iminophosphorane group on the other cage carbon atom.
2
[
29]
6
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European Journal of Inorganic Chemistry
10.1002/ejic.201700487
The value found for the phosphine oxide group in IP2, 18.7 ppm, is only slightly
3
1
lower than the values found in the P NMR spectra of other carboranyl-
2 2
diphenylphosphine oxide derivatives described in the literature, like 1,2-(OPPh ) -
[
28,30]
closo-C
2
10
B H10 (23.67 ppm)
2 2 10
or 1-OPPh -2-R-closo-C B H10 [R= Me (19.28
[
28]
ppm), Ph (19.65 ppm)]
.
1
The H NMR spectrum of the IP1 shows signals only in the aromatic region,
due to the protons of the triphenyliminophosphorane and diphenylphosphine groups.
The spectrum of the thioether derivative IP2 is similar, showing also the singlet
1
signal of the thioether group (2.09 ppm). The H NMR spectrum of IP2 also shows
1
1
1
minor peaks due to the oxide impurity present in the sample. The B{ H} NMR
spectra of IP1 and IP2 are very similar, which indicates that the carborane cage is not
affected to a great extent by the substitution by the -SMe group on a phenyl ring of
the iminophosphorane moiety. They both display six broad resonances in the range (-
2
.9)-(-12.2) ppm, which are typical values for closo-carborane derivatives.
The analysis of solid samples of IP1 and IP2 by IR spectroscopy proves the
presence in both compounds of the closo-carborane cage and the iminophosphorane
-
group. Both compounds exhibit a very intense ν(B-H) multiple band around 2580 cm
1
typical of closo-carborane derivatives, almost in the same position as the parent
[18]
-1
compounds I1 and I2. The presence in both cases of a strong band at 1346 cm
-1
(
IP1) and 1332 cm (IP2), attributable to the ν(PN) stretching frequency, proves the
integrity of the iminophosphorane group in the disubstituted compounds. The shift to
-1
lower wavenumbers compared to the parent iminophosphoranes I1 (1393 cm ) and
-1 [18]
I2 (1384 cm )
accounts for a slight weakening of the P=N bond after
functionalization with the diphenylphosphine group.
7
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European Journal of Inorganic Chemistry
10.1002/ejic.201700487
Elemental analysis and mass spectrometry (electronic impact) also confirmed
the stoichiometries of the ligands IP1 and IP2 (see the Experimental Section). The
crystal structures of these two compounds were studied by single crystal X-ray
analysis.
Crystal structures of the iminophosphorane-phosphine derivatives IP1 and IP2
Both crystals were obtained by slow evaporation of solutions of the compounds in a
mixture of dichloromethane and hexane. The crystallographic data can be found in
Table 1. Selected bond lengths and angles for these compounds are collected in Table
2
. The crystal structure of IP2 contains two molecules of ligand per asymmetric unit,
so the structural parameters appear twice. The x-ray analysis show that both
compounds present distorted icosahedral carborane moieties substituted on the cage
carbon atoms with a diphenylphosphino group and a triphenyliminophosphorane
group (IP1, Figure 2) or a (2-SMe-phenyl)diphenyliminophosphorane group (IP2,
Figure 3). In the case of compound IP2, the phosphorous atoms of the -PPh
2
groups
of both molecules in the asymmetric unit are oxidized to a different extent, 15% and
5
0% (see Experimental Section).
The structural parameters that involve the cage carbon atoms are very
interesting, as they reflect the interaction with the directly attached donor atoms. It is
known that donor atoms on a cage carbon atom of an o-carborane group can engage
in exo Cc-X π-bonding (X: donor atom), which leads to the lengthening of the Cc-Cc
bond and to the shortening of the Cc-X bond, compared to unsubstituted
[
31]
carboranes.
.828(3) (P=O 15%) Å and 1.776(3) (P=O 50%) Å, are all longer than the Cc-Cc
distance reported in the literature for the unsubstituted iminophosphorane I2,
Thus, the Cc-Cc values found for IP1, 1.785(2) Å, and for IP2,
1
[32]
1
.688(3) Å, while the Cc-N distances [1.369(2) Å for IP1 and 1.355(3) (P=O 15%)
8
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European Journal of Inorganic Chemistry
10.1002/ejic.201700487
and 1.368(3) (P=O 50%) Å for IP2] are all shorter than the value reported for the
[18]
unsubstituted precursors I1, 1.373(3) Å, and I2, 1.368(2) Å. The Cc-Cc and Cc-N
distances found for IP2 seem to be affected by the extent of oxidation of the
phosphine phosphorous atom, as the phosphine oxide does not engage in exo π-
bonding with the cage carbon atom, which results in shorter Cc-Cc bonds and longer
Cc-N bonds. The literature also provides examples of shorter Cc-Cc distances for
carboranyl phosphine oxide derivatives compared to analogous carboranyl
phosphines. Thus, the Cc-Cc distance found for the literature compound 1-OPPh
2
-2-
[
28]
Me-closo-C
analog 1-PPh
2 10
B H10, 1.677(2) Å, is shorter than the one found for the phosphine
[
33]
2
2 10
-2-Me-closo-C B H10, 1.702(6) Å, as expected.
The Cc-Cc values found for IP1 and IP2 are shorter than the literature values
for the sulfur derivatives of carboranyl-iminophosphoranes (thiol, disulfide and
[
18]
trisulfide), in the range 1.832(5)-1.889(4) Å, which indicates that the sulfur donor
groups contribute more than the diphenylphosphino group to the lengthening of the
Cc-Cc distance. This result had already been suggested by Teixidor et al through the
i
structural analysis of a (P, S) carboranyl hybrid ligand, 1-PPh
2
-2-S Pr-1,2-
[
34]
C
2
B
10
H
10
.
The diphenylphosphino group of the literature ligand also shows the
same pattern of P-C distances, i.e. longer for P-Cc bonds [range 1.858(2)-1.8758(17)
Å for IP1 and IP2] than for P-CPh bonds [range 1.817(2)-1.8331(18) Å for IP1 and
IP2].
The P=N bond distances of the iminophosphorane group [1.5800(15) Å for
IP1 and 1.5815(18) and 1.5812(18) Å for IP2] are very similar to the values found in
[
18,20]
the literature for other non-coordinated carboranyl iminophosphoranes,
and also
similar to the values found for other free non-carboranyl triphenyliminophosphorane
9
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European Journal of Inorganic Chemistry
10.1002/ejic.201700487
derivatives [mean value of 1.586 Å, 170 structures, Cambridge Structural Database
Version 5.38, November 2016)].
(
Synthesis of metal complexes
Organotin derivatives of I1
The parent iminophosphorane I1 presents an unsubstituted C-carboranyl group
connected to the iminophosphorane nitrogen atom, with no extra donor atoms. The
acidic Cc-H position can be used to obtain organometallic compounds, following a
similar procedure as used for the preparation of the disubstituted compounds IP1 and
IP2. In the final products, the metal atom will be forced to be in the vicinity of the
potentially donating nitrogen atom, which can result in an M-N interaction. We
decided to obtain organotin derivatives of I1, as the final compounds were expected
to be stable molecular compounds with good solubility in common organic solvents.
These properties enable not only a structural characterization of the products in solid
state by x-ray crystallography, but also a detailed characterization of the products in
solution by NMR spectroscopy. The NMR studies are especially interesting for
organotin derivatives, as they can be related to the coordination geometry around the
metal atom (see below). Thus, these studies should give some insight on the
coordinating ability of the iminophosphorane nitrogen atom when is not affected by
the existence of extra donor atoms in the structure of the carboranyl ligand. Although
the coordination of the iminophosphorane nitrogen atom to the tin atom is not
geometrically favored in the final products, as it leads to the formation of a four-
membered chelate ring, the availability of different tin precursors with different
electrophilicities enables the study of the Sn…N interaction under different forcing
conditions. Thus, we have synthesized derivatives of I1 with -SnMe
3 2
and -SnClMe
organotin centers. The electronegative chlorine substituent on the tin atom enhances
10
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European Journal of Inorganic Chemistry
10.1002/ejic.201700487
its electrophilicity and promotes the coordination of the nitrogen atom. This strategy
has already been described in the literature to promote the (Cc, N) coordination of an
[35]
o-carboranylamino ligand to a tin metal center.
The organotin derivatives SnMe I1 and SnClMe2I1 were easily prepared by
reaction of the tin precursors, SnMe Cl or SnMe Cl , and the lithiated derivative of
3
3
2
2
I1, obtained in situ by reaction of I1 and n-butyllithium in diethyl ether at -10ºC (see
Scheme 1). After purification, the final tin compounds were obtained in good yields
1
11
31
(
62% and 50%, respectively) and characterized by IR spectroscopy, H, B, P and
1
19
Sn NMR spectroscopy, mass spectrometry, elemental analysis and X-ray
diffraction analysis. Elemental analysis and mass spectrometry confirmed the
expected stoichiometries of the tin compounds (see the Experimental Section). The
rest of the characterization techniques, not only confirmed the formation of the
expected organotin compounds, but also gave information about the coordination
environment of the tin atom, both in solution and solid state.
Although the literature reports several organotin(IV) compounds with
nitrogen donor atoms, the information about iminophosphorane-tin(IV) compounds is
very scarce. Due to the lack of appropriate bibliographic data for comparison, we
decided to obtain the non-carboranyl analog of SnClMe
2
I1. The organic analog,
Cl with the lithiated
SnClMe I3, was obtained by reaction of the tin precursor SnMe
2
2
2
derivative of the precursor N-(2-bromophenyl)-triphenyliminophosphorane (I3-Br),
as depicted in Scheme 2 (experimental details in Supporting Information). This new
tin compound was characterized by elemental analysis and standard spectroscopic
techniques, and it will be used to trace the effect of the carboranyl group on the
coordination ability of the nitrogen donor atom.
11
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European Journal of Inorganic Chemistry
10.1002/ejic.201700487
The IR spectra of the carboranyl-organotin complexes SnMe
SnClMe I1 show the typical strong bands associated with closo-carborane cages
ν(B-H): 2613-2546 cm (SnMe
bands appear as multiple bands, like in the IR spectrum of the free ligand I1, and
3
I1 and
2
-1
-1
[
3 2
I1) and 2608-2561 cm (SnClMe I1)]. These
[
18]
almost at the same position.
with the P=N stretching frequencies at 1327 cm (SnMe
SnClMe I1). These values are both of them smaller than the value found for the free
ligand (1393 cm ),
The spectra also show the intense band associated
-1
-1
3
I1) and at 1276 cm
(
2
-1
[18]
[36]
although to a different extent. The protonation
or the
[
37]
coordination to metal atoms of the iminophosphorane nitrogen atom weakens the
P-N bond, which shifts the ν(PN) stretching frequency to lower wavenumbers.
-1
However, the shift for the trimethyltin derivative (Δν= -66 cm ) is smaller than for
-1
SnClMe
for the more electrophilic -SnClMe
found in the solid state for SnClMe
below). In the case of the organic analogue SnClMe
2
I1 (Δν= -117 cm ), which indicates that the Sn…N interaction is stronger
group, as expected. The stronger interaction
I1 was confirmed by x-ray crystallography (see
I3, the value of the ν(PN)
stretching frequency, 1205 cm , is more shifted to lower wavenumbers from the
2
2
2
-1
-1
-1
value found for the organic precursor I3-Br (1336 cm ), Δν= -131 cm . These
results indicate stronger Sn…N interactions for the non-carboranyl derivatives in the
solid state, which support the idea that a C-carboranyl group deactivates the donor
ability of the iminophosphorane nitrogen atom. However, none of these ν(PN) values
correspond to covalent Sn-N bonds, as a further shift is expected. Typical ν(PN)
values reflecting a M-N covalent bond are provided by the palladium complexes
-1
described later on, in the range 1162-1181 cm (see below).
The characterization in solution using different NMR spectroscopy techniques
also provides interesting information about the coordination sphere of the tin centers.
12
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European Journal of Inorganic Chemistry
10.1002/ejic.201700487
1
1
1
The B{ H} NMR spectra of the carboranyl organotin derivatives are similar to the
[
18]
spectrum of the free ligand I1, displaying broad resonances in a similar range (-
.7)-(-12.3) ppm, typical of closo-derivatives. However, the spectrum of the
3
trimethyltin derivative presents four broad signals and the chlorodimethyltin
derivative presents five signals, indicating lower symmetry, perhaps due to the
3
1
1
stronger Sn…N interaction. The P{ H} NMR spectra of the organotin derivatives
present one signal due to the iminophosphorane group. Each spectrum presents a
3 2
singlet signal, at 4.6 ppm (SnMe I1) and at 11.8 ppm (SnClMe I1), shifted
[
18]
downfield respect to the free ligand I1 (3.1 ppm) . The small shift found, accounts
for a weak Sn…N interaction, especially in the case of the compound with the less
electrophilic -SnMe
3
group. A comparison with the value found for the non-
I3 (17.5 ppm), more shifted respect to the organic
carboranyl analog SnClMe
2
precursor I3-Br (0.68 ppm), indicates again that the carboranyl derivatives show
weaker Sn…N interactions. In any case, the shifts found for all the organotin
compounds imply only weak Sn…N intramolecular interactions of different strength,
as a proper M-N bond causes higher shifts, from 30 to 60 ppm, depending on the
metal and its coordination geometry, as described in the literature for other
[
38]
cyclometalated iminophosphorane complexes.
1
The H NMR spectra of SnMe
3
I1 and SnClMe
2
I1 confirmed the formation of
the desired organotin complexes by the disappearance of the Cc-H band found in the
[
18]
spectrum of the free ligand I1 (3.82 ppm),
signal due to the methyl groups on the tin atom (at 0.34 ppm for SnMe
ppm for SnClMe I1). The comparison non-carboranyl derivative also show the
methyl groups as singlet signals at 0.08 ppm (SnClMe I3), slightly shifted upfield
respect to the carboranyl analogs due to the higher electron-withdrawing character of
and by the appearance of a singlet
3
I1 and at 0.77
2
2
13
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European Journal of Inorganic Chemistry
10.1002/ejic.201700487
1
the carboranyl group compared to the phenyl one. The H NMR spectra of
methyltin(IV) compounds are structurally very informative, as the J coupling
1
19
1
119
1
constants between the methyl groups and the Sn nucleus, J( Sn, H), can be
[
39]
related to the coordination number of the tin center.
carboranyl compounds, 54 and 64 Hz, respectively, indicate that both compounds
present tetrahedral (Td) geometry, although SnClMe I1 is in transition to trigonal
The values found for the
2
bipyramidal (TBP) geometry. The values found in the literature for other carboranyl-
methyltin derivatives show coupling constants in the range 54.0-58.5 Hz for Td
[
35,40]
[35,41,42]
compounds
value found for the non-carboranyl analog SnClMe
taking into account the rest of the characterization data. The main evidence for the
and in the range 67.5-101.1 Hz for TBP compounds.
The
2
I3 (58.1 Hz) is abnormally low,
1
19
coordination of the tin atoms in solution was provided by
Sn NMR
spectroscopy. The values found for the carboranyl-tin compounds (23.3 ppm for
SnMe I1 and 36.5 ppm for SnClMe I1) indicate Td geometry, as the normal range
for this geometry is from 200 to -60 ppm.
non-carboranyl analog SnClMe I3, -21.5 ppm, also indicates Td geometry, although
[
43]
3
2
[
43]
The value found for the comparison
2
the negative value indicates a higher distortion towards TBP geometry.
In conclusion, the different characterization techniques, in solution and in
solid state, indicate that the carboranyl organotin derivatives SnMe
SnClMe I1 only show a weak Sn…N interaction, that is stronger for the more
forcing –SnClMe derivative. The comparison with the non-carboranyl analog
SnClMe I3 shows that the donor ability of the iminophosphorane nitrogen atom is
slightly reduced by the effect of the C-carboranyl group.
Crystal structures of the tin complexes, SnMe I1 and SnClMe
3
I1 and
2
2
2
3
2
I1
14
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European Journal of Inorganic Chemistry
10.1002/ejic.201700487
3 2
The solid state structures of the tin derivatives SnMe I1 and SnClMe I1 were
studied by single crystal X-ray analysis. Both crystals were obtained by slow
evaporation of a solution of the compound in a mixture of diethyl ether and hexane.
The crystallographic data can be found in Table 1. Selected bond lengths and angles
are collected in Tables 3 and 4, respectively.
3 2
The organotin compounds SnMe I1 and SnClMe I1 present the C-
carboranyl-iminophosphorane derivative I1 functionalized on the other cage carbon
atom with trimethyltin or chlorodimethyltin groups, respectively (see Figures 4 and
5
). The analysis of the structural parameters (see below), indicates that the geometry
around the tin centers should be described as distorted Td, with a weak Sn…N
intramolecular interaction involving the iminophosphorane nitrogen atom. As
expected, the distortion towards a five-coordinate TBP structure is more pronounced
in the case of the chlorodimethyl derivative, SnClMe
The strength of the Sn…N interaction can be estimated through the values of
the Sn…N distances: 3.3458(14) Å for SnMe I1 and 2.9240(13) Å for SnClMe I1.
2
I1.
3
2
These distances are too long to be considered covalent bonds but are shorter than the
sum of van der Waals radii (3.81 Å), so they should be considered as intramolecular
interactions. The distances suggest a stronger interaction for the chlorodimethyl
derivative, as expected. Examples of covalent Sn-N bonds can be found in the
literature for three trigonal bipyramidal carboranyl-tin compounds with extra
[
35]
[41]
amine
.503-2.701 Å.
The transition from Td to TBP geometry, by effect of the Sn…N interaction,
or oxazoline
nitrogen donor groups, with Sn-N distances in the range:
2
produces other structural changes that reflect the degree of the interaction. The
iminophosphorane group bends towards the tin atom, closing the Cc-Cc-N angle
15
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European Journal of Inorganic Chemistry
10.1002/ejic.201700487
from the theoretical value of 120º to 113.38(13)º for SnMe
SnClMe I1, and the tin atom also approximates the nitrogen atom, closing the Cc-
Cc-Sn angle from the theoretical value of 120º to 118.01(10)º for SnMe I1 and to
07.87(9)º for SnClMe I1.
In the case of the chloro-derivative SnClMe
3
I1 and to 110.34(12)º for
2
3
1
2
2
I1, the hypothetical Sn-N
coordination would produce a distorted trigonal bipyramid with the nitrogen and
chlorine atoms at the apical positions and the three carbon donor atoms (cage carbon
and the two methyl groups) on the equatorial plane (see Figure 5). In the case of
SnMe
Figure 4). The higher distortion towards five coordination of compound SnClMe
compared to SnMe I1 is reflected in equatorial C-Sn-C angles closer to 120º than to
07.5º (see Table 4) and in a smaller deviation of the tin atom from the equatorial
plane (C2, C21, C22); 0.6499(12) Å for SnMe I1 and 0.3541(10) Å for SnClMe I1.
The literature reports the crystal structure of one carboranyl-trimethyltin derivative,
3
I1, the methyl group labeled C23 would occupy the other axial position (see
2
I1
3
1
3
2
[40]
[SnMe
3
(1-pyridine-carborane)],
and
several
carboranyl-halodimethyltin
[
35,41,42]
derivatives (halogen: Cl, Br),
all of them bearing additional nitrogen
[
35]
[41]
[42]
[41]
(
amine , oxazoline ), phosphorous (phosphine)
or oxygen (ether)
donor
groups on the carboranyl moiety. All these halodimethyl derivatives present the
halogen atom and the extra donor atom on the axial positions of the trigonal
bipyramid, as described for SnClMe
2
I1.
The Sn-Cl bond distance found in SnClMe
2
I1, 2.3884(4) Å, provides an
indirect evidence of the weakness of the Sn…N interaction is trans position, as it is
shorter than the value of 2.458 Å of the Sn-Cl bond distance found in the only
carborane derivative with a -SnClMe
2
group described in the literature, the five-
[
35]
coordinate compound [SnClMe (1-Me NCH -1,2-C B H10)].
2
2 2 2 10
16
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European Journal of Inorganic Chemistry
10.1002/ejic.201700487
Finally, the different degree of Sn…N interactions of SnMe
3
I1 and
SnClMe I1 is also shown in some structural parameters of the iminophosphorane-
2
carborane ligand. As mentioned before, the engagement of the donor atom at a cage
carbon atom in exo π-bonding, leads to the shortening of the Cc-X bond and to the
[
31]
lengthening of the Cc-Cc bond.
The coordination of the donor atom to a metal
center competes with the exo π-bonding, producing the opposite effect; a lengthening
of the Cc-X bond and a shortening of the Cc-Cc bond, compared to the free ligand.
Thus, the Cc-Cc and Cc-N distances found in SnMe
3
I1, 1.701(2) and 1.387(2) Å
respectively, suggest no Sn…N interaction, in comparison to the free ligands [Cc-Cc:
[
18,32]
1
.688(3) Å for I2 and Cc-N: 1.373(3) Å (I1) and 1.368(2) Å (I2)].
However, the
data found for SnClMe
2
I1 [Cc-Cc: 1.677(2) Å and Cc-N: 1.4020(19) Å] suggests a
weak interaction. Another structural parameter from the ligand that also gets affected
by the strength of the Sn…N interaction is the value of the P-N distance of the
iminophosphorane group, that increases from the value found in the free ligand I1,
[
18]
1
.5711(18) Å, to 1.5755(15) Å for SnMe
3 2
I1 and to 1.5918(14) Å for SnClMe I1.
The Sn-C distances follow the same trend found for the other methyltin
[
35,40-42]
derivatives of carborane.
Thus, the Sn-Cc distances [2.1893(16) for SnMe
3
I1
and 2.1629(15) Å for SnClMe
2
I1] are longer than the Sn-C(methyl) distances, in the
range 2.1134(16)-2.1342(19) Å. All the Sn-C distances are slightly shorter for the
chlorodimethyl derivative due to the presence of the more electronegative chlorine
atom.
Palladium complexes of IP1 and IP2
The new disubstituted carborane derivatives IP1 and IP2 were used to obtained
palladium complexes, in order to investigate the donor ability of the phosphine-
iminophosphorane system. These ligands present a diphenylphosphino group on the
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European Journal of Inorganic Chemistry
10.1002/ejic.201700487
cage carbon atom adjacent to the iminophosphorane group, and could be used as (P,
N) bidentate neutral ligands. Closo-carboranyl-phosphines are known to be good
ligands towards late transition metals, and several metal complexes have been
structurally characterized with both monodentate and bidentate closo-carboranyl-
[44]
phosphines.
Thus, the diphenylphosphino group should function as a good first
coordinating site for the soft palladium atom and promote the coordination of the
iminophosphorane nitrogen atom by the formation of a five-membered ring (chelate
effect). In the case of the ligand IP2 the extra thioether group provides a third
coordinating site, enabling it to act as a (P, N, S) tridentate ligand.
The closo-disubstituted derivatives IP1 and IP2 were treated under inert gas
[
45]
with the palladium precursor cis-[PdCl
2
(PhCN)
2
]
in a 1:1 molar ratio in dry
acetonitrile at room temperature (see Scheme 3). Although both reactions resulted in
the formation of yellow solids, the analysis of the samples revealed their different
nature. The characterization of the product precipitated from the reaction of ligand
IP1 (PdclosoIP1) by elemental analysis and standard spectroscopic techniques
showed that it was the expected palladium complex depicted in Scheme 3, i.e. the
2
PdCl adduct of the chelating neutral closo-derivative IP1. The characterization by
-1
IR spectroscopy is very informative. The strong band at 2609-2565 cm (multiple
band), that can be assigned to the ν(B-H) stretching frequencies, is typical of closo-
carborane derivatives, and almost identical to the value found for the free ligand IP1
-1
(
2600-2569 cm ). The strong band associated with the stretching frequency of the
-1
P=N bond, 1181 cm , is very shifted to lower wavenumbers respect to the ν(PN)
-1
signal found in the free ligand (1346 cm ), indicating that the iminophosphorane
nitrogen atom is coordinated to the palladium metal. These two very important
features shown by IR spectroscopy could be confirmed by other techniques. The
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European Journal of Inorganic Chemistry
10.1002/ejic.201700487
1
1
1
B{ H} NMR spectrum of PdclosoIP1 shows a group of five broad signals in the
range (-1.4)-(-14.8) ppm, which confirms the presence of the closo-carborane cage.
3
1
1
The P{ H} NMR spectrum presents two singlet signals at 34.1 ppm (P=N) and 49.1
ppm (phosphine), both of them very shifted downfield by effect of the coordination
of both donor groups. This technique confirms that the closo-IP1 ligand is
1
coordinated to the palladium metal as a (P, N) chelating ligand. The H NMR
spectrum of this compound is less informative, as it only shows signals in the
aromatic region, due to the different phenyl rings of the ligand IP1. Elemental
analysis and mass spectrometry (MALDI-TOF) further confirmed the stoichiometry
of the complex (see Experimental Section).
The crystal structure of PdclosoIP1 could not be studied, although its
crystallization was tried from different solvents. All attempts to recrystallize the
sample resulted in the formation of very fine yellow needles that did not give
detectable diffraction of x-ray radiation. During the attempts to crystallize it from a
mixture of methanol and dichloromethane, a different compound was obtained,
PdnidoIP1. The analysis by x-ray diffraction of the orange crystals obtained (see
below) revealed that the original complex had been deboronated, giving rise to the
2
palladium dimer with nido-IP1 ligands, [Pd(μ-Cl)(nido-IP1-κ P,N)]
2
, as depicted in
Scheme 3. This product, PdnidoIP1, was also quantitatively obtained by refluxing
the closo-derivative PdclosoIP1 in a 1:1 mixture of methanol and dichloromethane
(see Experimental Section).
The characterization of the solid precipitated from the reaction of the
potentially tridentate ligand IP2, including x-ray diffraction studies (see below),
showed that in this case the final product was the monomer complex [PdCl(nido-IP2-
3
κ P,N,S)] (PdnidoIP2), depicted in Scheme 3. This product, very similar to the just
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European Journal of Inorganic Chemistry
10.1002/ejic.201700487
presented compound PdnidoIP1, is the deboronated product of the expected adduct.
Apart from x-ray crystallography, both nido-derivatives PdnidoIP1 and PdnidoIP2
were also characterized by elemental analysis and standard spectroscopic techniques.
-
1
The IR spectra of these compounds show strong ν(B-H) bands at 2536 cm
-1
(
PdnidoIP1) and at 2527 cm (PdnidoIP2), shifted to lower wavenumbers respect to
the closo precursors IP1 and IP2 or to the closo palladium complex PdclosoIP1.
These bands indicate that the carborane cages in these two compounds are in the nido
form. The IR spectra of these nido complexes also show the strong band associated
-1
with the ν(PN) stretching frequencies at 1162 and 1173 cm , respectively. These
-
values are slightly lower than the value found for the complex PdclosoIP1 (1181 cm
1
)
and are also indicative of the coordination of the nitrogen atom to the palladium
atom in both nido complexes. Unfortunately, due to the low solubility of the two nido
complexes in common solvents (CDCl , CD Cl , DMSO-d ), it was not possible to
confirm the presence of the nido-carborane cage in the complexes using B NMR. It
3
2
2
6
11
1
31
was only possible to register the H and P NMR spectra, with a bad signal to noise
1
ratio (see Supplementary Material). The H NMR spectra of these compounds show a
complicated aromatic region and a broad absorption at -2.13 ppm (PdnidoIP1) and -
2
.85 ppm (PdnidoIP2), which implies a B-H-B interaction on the C
2 3
B open face of
the carborane cage, proving the presence of the nido-derivatives in the complexes.
3
1
The
P NMR spectra show two singlet signals that correspond to the
iminophosphorane group [31.9 ppm for PdnidoIP1 and 37.5 ppm for PdnidoIP2]
and to the phosphine group [74.7 ppm for PdnidoIP1 and 76.8 ppm for PdnidoIP2].
The degradation of the cage to nido-carborane shifts the signals of the phosphine
phosphorous atoms downfield respect to the position in PdclosoIP1 (49.1 ppm). The
signals due to the iminophosphorane phosphorous atom are not affected by the
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European Journal of Inorganic Chemistry
10.1002/ejic.201700487
degradation and appear at a similar position as in PdclosoIP1 (34.1 ppm). In any
case, both signals appear shifted downfield respect to the signals of the free ligands,
proving the coordination of the phosphorous and nitrogen donor atoms in both nido-
[
46]
complexes.
The literature presents several palladium adducts with the neutral
31
diphosphine 1,2-(PPh
2
)
2
-closo-carborane, which present signals in P NMR at
[
47,48]
[49]
around 80 ppm,
which are shifted to lower field in the nido derivatives. Thus,
the values found for the phosphine phosphorous atom of the nido derivatives
PdnidoIP1 and PdnidoIP2 could be considered normal, but the value found for the
3
1
closo derivative PdclosoIP1 suggests a weaker Pd-P bond. According to the
P
NMR data, the change from closo to nido results in an increase of the donor capacity
of the phosphine group, although the iminophosphorane group seems less affected.
The dimeric nature of the complex PdnidoIP1, shown by x-ray diffraction
(see below), could not be proved by mass spectrometry. The MALDI mass spectrum
of PdnidoIP1 only shows the peak associated with half the dimer, form by breakage
of the chloro bridges.
The dimeric compound PdnidoIP1 has resulted from the degradation of the
adduct PdclosoIP1 in methanol. The deboronation of the neutral closo-derivative IP1
gives rise to the anionic nido-derivative, which results in the loss of a chloro ligand
from the palladium adduct, and thus, in the dimerization of the complex by formation
of the di-μ-chloro bridge, which is a very common motif for palladium(II)
[
50]
complexes. The deboronation of coordinated closo-carborane ligands by the action
of polar alcohols (methanol, ethanol) has already been described for other chelating
closo-carboranyl ligands. This mild method of deboronation was firstly observed by
[
51]
Teixidor et al. in 1993 for closo-carboranyl-diphosphines,
and since then it has
been extensively studied for different metal complexes with carboranyl-
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European Journal of Inorganic Chemistry
10.1002/ejic.201700487
[
52,53]
diphosphines.
2 2 3
The use of low nucleophilic solvents (CH Cl , CHCl , toluene)
can lead to the isolation of the closo-adducts, while more nucleophilic ones, like
alcohols, promote the degradation to nido-derivatives. A pertinent literature example
i
from palladium chemistry is the reaction of the diphosphine 1,2-(P Pr
2
)
2
-C
2
B
10
H
10
with [PdCl
2
(COD)], COD: 1.5-cyclooctadiene, in dichloromethane, which produces
i
the palladium adduct [PdCl
2
(1,2-(P Pr
2
)
2
-C
2
B
10
H10)], which evolves to the dimeric
i
[47]
nido compound [Pd(μ-Cl)(1,2-(P Pr
2
)
2
-C
2
B
10
H
10)]
2
by reflux in acetonitrile.
Although the degradation of closo-carboranyl ligands to nido-derivatives by
complexation to metal atoms has mainly been observed for carboranyl-diphosphines,
it has also been described for other bidentate carboranyl ligands with two neutral
donor groups on the cage carbon atoms. Thus, a related (O, O) bis(phosphine-oxide)
[
54,55]
[56]
ligand
and an (S, S) 1,2-bis(thioether)-carboranyl ligand
also evolve to the
nido derivatives during complexation experiments in ethanol. Other known examples
are two related (P, S) phosphine-thioether carboranyl ligands, that evolve to the nido-
[57]
derivatives in ethanol by complexation to gold(I) and copper(I) metal centers, or to
[
58]
rhodium atoms.
The deboronation by complexation of a (P, N) bidentate carboranyl ligand has
not been previously described. The isolation of the closo adduct PdclosoIP1 and its
deboronation product PdnidoIP1 indicates that the process that has taken place is
analogous to that experienced by closo-diphosphines, i.e. the cage gets activated by
coordination to the metal center of both donor atoms at the cage carbon atoms. This
reasoning leads us to think that the complex PdnidoIP2 has been formed following a
similar procedure: the ligand closo-IP2 should form the expected (P, N) adduct in a
first step and then suffer deboronation by the solvent (acetonitrile), evolving to the
nido-derivative. The change from neutral closo to anionic nido produces the loss of a
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European Journal of Inorganic Chemistry
10.1002/ejic.201700487
chloro ligand, and its position is occupied by the extra thioether group, leading to the
monomeric complex PdnidoIP2. The presence of the extra thioether group seems to
further activate the sensitivity of the carborane cage to nucleophilic attack by the
solvent, so the closo-adduct is not isolated.
The presented results indicate that the closo ligands IP1 and IP2 indeed
coordinate the palladium atom as (P, N) chelating ligands in a first step. This means
that the combination of the phosphine group with the iminophosphorane group at
both carbon atoms of the carborane cage activates the donor ability of the
iminophosphorane nitrogen atom. The tin complexes presented in the first part of this
paper indicate that the iminophosphorane nitrogen atom, when is not affected by the
presence of other donor atoms, presents reduced donor ability compared to non-
carboranyl organic ligands, by effect of the electron-withdrawing C-carboranyl
group. Thus, the use of the extra phosphine group seems to be a good way for
promoting Pd-N bonds, although the (P, N) chelation of these ligands also promotes
the evolution to the nido-derivatives. It is interesting to remember that our previous
investigations on iminophosphorane-carborane ligands showed how the combination
of the iminophosphorane group with an anionic thiolate group on the other cage
carbon atom is not a good way for promoting Pd-N interactions, as those potential (S.
[18]
N) ligands do not promote chelation.
The characterization of the palladium complexes presented in this paper (see
before) indicates that closo-IP1 is less donating than the anionic nido-IP1 ligand.
However, the gain in donor strength is limited, and the final nido-ligands present
similar donating abilities as non-carboranyl analogs. Although this was suggested by
some characterization techniques, it is made clear by the x-ray analysis of the crystal
structures of the nido-derivatives PdnidoIP1 and PdnidoIP1, and the comparison
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European Journal of Inorganic Chemistry
10.1002/ejic.201700487
with the structural parameters of non-carboranyl palladium complexes (see below).
The increase of donor strength experienced by donor atoms at the cage carbon atoms
of deboronated o-carborane is an effect of the reduced electron-withdrawing
character of the nido-carborane cage, and has already been exploited for increasing
[
59]
the donor capacity of bis(thioether)-carboranyl ligands.
Crystal structures of the palladium complexes PdnidoIP1 and PdnidoIP2
Single crystals suitable for X-ray diffraction analysis were obtained by slow
evaporation of a solution of the compound in chloroform (PdnidoIP1) or in a mixture
of dichloromethane and hexane (PdnidoIP2). The crystallographic data can be found
in Table 1. Selected bond lengths and angles for these compounds are collected in
Tables 5 and 6, respectively.
Compound PdnidoIP1 (Figure 6) is a neutral dinuclear palladium(II) complex
with a di-μ-chloro bridge and two equivalent nido-IP1 ligands. This compound
presents a two-fold rotation axis that is perpendicular to both middle points of the
lines than join the two metal atoms and two chlorine atoms [symmetry operation #1: -
x+3/2, -y, z]. The equivalent anionic nido-carborane ligands chelate the metal atom
through their nitrogen and (phosphine) phosphorous donor atoms, forming five-
membered chelate rings. Each palladium atom completes its distorted square planar
geometry with two chlorine ligands that bridge the two metal atoms, forming a [(μ-
Cl)
form and angle of 47.62(4)º. Both (P, N) chelating ligands are disposed anti to each
other, around the central Pd Cl ring, as imposed by the two-rotation axis. The main
2 2 2
Pd ] four-membered ring. This central ring is not planar, as the two PdCl units
2
2
sources of distortion with respect to the ideal square planar geometry are the
formation of the rings: the (P, N) chelate ring with the nido-IP1 ligand [P-Pd-N:
#
1
7
2 2
9.15(12)º] and the central [(μ-Cl) Pd ] ring [Cl-Pd-Cl : 84.45(5)º], both of them
24
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European Journal of Inorganic Chemistry
10.1002/ejic.201700487
forcing cis angles lower than 90º. The simultaneous formation of the two opposed
rings affects the planarity of the coordination sphere of the palladium atom (rms:
#
1
0
.1521 Å), as the N-Pd-P plane is rotated 13.89(11)º respect to the Cl-Pd-Cl plane.
Compound PdnidoIP2 (Figure 7) is a neutral monomeric palladium(II)
complex. In this case, the palladium atom is coordinated to a terminal chlorine atom
and to the nitrogen, (phosphine) phosphorous and (thioether) sulfur donor atoms of
the nido derivative of the ligand IP2, that behaves as an anionic (P, N, S) tridentate
chelating ligand. The chlorine atom is located trans to the nitrogen donor atom. The
(P, N) coordination forms a five-membered ring while the (N, S) coordination forms
a six-membered ring. These rings are the main sources of distortion respect to the
ideal square planar geometry, forcing low cis angles [(P, N) chelate angle of
8
1.50(4)º and (N, S) chelate angle of 86.64(4)º]. However, the simultaneous
formation of the two adjacent chelate rings does not affect the planarity of the
coordination sphere of the palladium atom (rms: 0.0326 Å).
In both complexes, the five-membered (P, N) chelate rings have an envelope
disposition: the N-Cc-Cc-P carborane fragment is planar [rms: 0.0036 Å for
PdnidoIP1 and 0.0324 Å for PdnidoIP2] and forms and angle of 40.1(2)º
(
PdnidoIP1) and 32.53(7)º (PdnidoIP2) with the N-Pd-P plane.
The bond lengths that involve the metal atom are very interesting, as they
reflect the coordinating abilities of the donor atoms. The iminophosphorane nitrogen
atom is located in both palladium complexes trans to a chlorine atom, which
modulates the value of the Pd-N bond distance (trans influence). The values of the
Pd-Cl bond distances depend on the coordinating mode of the ligand (terminal or
bridging). In the case of the monomeric complex PdnidoIP2, the terminal Pd-Cl
bond [2.2834(5) Å] and the Pd-N bond [2.0447(13) Å] present values that are similar
25
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European Journal of Inorganic Chemistry
10.1002/ejic.201700487
to those found for other palladium complexes with a terminal Pd-Cl bond trans to a
non-carboranyl iminophosphorane ligand (Pd-N) deposited in the Cambridge
Structural Database (37 structures, CCDC Version 5.38, November 2016). In the
literature examples, the mean values found for the Pd-Cl bonds, 2.303 Å, and for the
Pd-N bonds, 2.062 Å, are only slightly longer than those found for PdnidoIP2.
The dimeric complex PdnidoIP1 presents an asymmetric chloro bridge,
which is a reflection of the higher trans influence of the phosphine group compared
to the iminophosphorane one. Thus, the Pd-Cl bond distance trans to the phosphine
phosphorous atom is longer, 2.4783(13) Å, and the Pd-Cl bond distance trans to the
iminophosphorane nitrogen atom is shorter, 2.3213(13) Å. The Pd-N bond distance is
2
.072(4) Å. The literature shows five related structures with non-carboranyl
[
60]
iminophosphorane ligands. All the literature examples are palladium dimers with a
central asymmetric [(μ-Cl) Pd ] ring and metallated iminophosphorane ligands that
2
2
act as (N, C) chelating ligands. In these structures, the mean values of the Pd-Cl and
Pd-N bond distances trans to each other (2.333 Å and 2.050 Å respectively) are very
similar to those discussed for the dimeric complex PdnidoIP1. In conclusion, the
analysis of the bond distances involving the palladium atom suggests that the
nitrogen donor atom of the iminophosphorane group attached to a nido-carborane
cage has similar donor strength as an organic, non-carboranyl, iminophosphorane
ligand.
The Cc-Cc distances of the nido-carborane cages [1.566(7) Å for PdnidoIP1
and 1.569(2) Å for PdnidoIP2] are much shorter than those found in the free closo
derivatives IP1 [1.785(2) Å] and IP2 [1.828(3) and 1.776(3) Å]. This is the result of
two Cc-Cc shortening effects: the evolution from closo to nido and the coordination
to the palladium metal. Although there are no examples of carborane derivatives with
26
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European Journal of Inorganic Chemistry
10.1002/ejic.201700487
phosphorous and nitrogen donor atoms at the cage carbon atoms, this is made clear
by comparison with published data for carboranyl-diphosphines. Thus, the free ligand
[61(a)]
[61(b)]
1
,2-(PPh
2
)
2
-1,2-C
2
B
10
H10, presents a Cc-Cc distance of 1.697(4)
-1.722(4)
[
55]
Å, that is reduced to 1.61(2) Å by evolution to the nido derivative.
The
coordination of the nido-derivative to palladium in the dimeric complex [Pd(μ-
Cl)(1,2-(PPh
2
)
2
-nido-carborane]
2
, similar to PdnidoIP1, further reduces the Cc-Cc
[62(a)]
[62(b)]
distance to 1.556(13)
-1.570(6)
Å. Another effect of the evolution from closo
to nido is the elongation of the Cc-N and the Cc-P distances.
As mentioned before, the coordination of the iminophosphorane nitrogen
[
37]
atom to the metal weakens the P-N bond,
which is reflected in longer P-N
distances [1.611(5) Å for PdnidoIP1 and 1.6100(14) Å for PdnidoIP2] compared to
the free ligands [range 1.5800(15)-1.5815(18) Å]. However, the P-N values in the
complexes are very similar to those found for PdCl
2
adducts of neutral non-
carboranyl phosphine-iminophosphoranes that form five-membered chelate rings
[
63]
with the palladium atom, in the range 1.601-1.627 Å, suggesting again a similar
strength of the Pd-N bond.
Conclusions
As a continuation of our investigations on carboranyl-iminophosphoranes, we have
isolated and characterized organotin derivatives of 1-triphenyliminophosphorane-o-
3 2
carborane I1 (SnMe I1 and SnClMe I1) in which the nitrogen atom directly attached
to the o-carborane cage, not affected by the presence of other donor atoms, is forced
to interact with the metal atom. These studies show that the nitrogen atom has a
tendency to interact with the tin atom, and that the Sn…N intramolecular interactions
can be strengthened by increasing the electrophilicity of the tin atom. The
2
comparison with the non-carboranyl analog SnClMe I3, with stronger Sn…N
27
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European Journal of Inorganic Chemistry
10.1002/ejic.201700487
interactions, indicates that the reduction of donor capacity is a genuine effect of the
electron-withdrawing character of the carborane cage.
In order to exploit the weak tendency of the carboranyl-iminophosphorane
group towards coordination to metal atoms, it was combined with a
2
diphenylphosphino group, -PPh , on the other cage carbon atom, producing new
phosphine-iminophosphorane compounds (IP1 and IP2) that are potential (P, N)
chelating ligands. In the case of IP2, the presence of an extra thioether group, -SMe,
on a phenyl ring of the iminophosphorane group makes it a potential (P, N, S)
2
chelating ligand. The isolation of the PdCl adduct of the neutral ligand closo-IP1,
PdclosoIP1, shows that the phosphine group can promote the coordination of the
nitrogen atom to the palladium atom, forming (P, N) chelates. The evolution of this
complex to the dimeric complex PdnidoIP1, in which the neutral ligand closo-IP1
has evolved to the anionic ligand nido-IP1 by effect of methanol, proves that the (P,
N) coordination of the phosphine-iminophosphorane ligand activates the carborane
cage towards deboronation. The deboronation of coordinated carboranyl ligands has
already been observed for other neutral donor groups, especially diphosphines, but
has never been observed for (P, N) carboranyl hybrid ligands. In the case of the
tridentate ligand IP2, the presence of the extra –SMe group further activates the
deboronation of the carborane cage, and the closo adduct is not isolated. However,
the complexes isolated with ligand IP1 suggest that the complex PdnidoIP2 should
2
be an evolution of the expected PdCl adduct with closo-IP2, by effect of the solvent.
It is also interesting to note that this ligand IP2 shows a tendency towards aerial
oxidation, which is not common for carboranyl-phosphines.
It is interesting to remember that the only studies found in the literature on the
coordination chemistry of carboranyl-iminophosphorane ligands, recently published
28
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European Journal of Inorganic Chemistry
10.1002/ejic.201700487
[14]
by our research group,
indicate that the combination of the iminophosphorane
group with a thiolate group on the other cage carbon atom does not produce (N, S)
chelating ligands, as the donor capacity of the iminophosphorane nitrogen atom is
completely deactivated. The new results show that the use of donor groups less
involved in exo π-bonding with the carborane cage, like a phosphine group, can
promote the coordination of the iminophosphorane nitrogen atom. The analysis of the
new palladium complexes reveals that the (P, N) coordination of the closo phosphine-
iminophosphorane ligand is rather weak, but gets reinforced by the evolution to the
nido derivative, which shows a similar strength in the interaction with the palladium
metal as other non-carboranyl phosphine-iminophosphorane analogs.
Experimental section
Instrumentation. Elemental analysis were performed using a Thermo Finnigan
3 2
Flash 1112 microanalyzer. IR spectra for IP1, SnMe I1 and SnClMe I1 were
recorded as KBr mulls with a BrukerIFS-66V spectrophotometer. ATR-IR spectra for
the rest of the compounds were recorded on a high-resolution spectrometer FT-IR
1
31
Perkin Elmer Spectrum Two. The H NMR and P NMR spectra were recorded on a
1
1
119
Varian Mercury 300 spectrometer. The B NMR and
Sn NMR spectra were
recorded on a Varian Inova 500. All NMR spectra were recorded in CDCl
3
solutions
(TMS), those
, those for B{ H} NMR were referenced to
and those for Sn NMR were referenced to SnMe . Chemical
1
at 25ºC. Chemical shift values for H NMR were referenced to SiMe
4
3
1
11
1
3 4
for P were referenced to 85% H PO
1
19
external BF
3
·OEt
2
4
shifts are reported in units of parts per million downfield from the reference, and all
coupling constants are reported in Hertz. Mass spectra of the organic derivatives were
determined on a Micromass Autospec instrument (positive electronic impact). Mass
spectra of the metal complexes were determined on a Bruker Microtof instrument in
29
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