Complexes of the tripodal phosphine ligands PhSi(XPPh2)3 (X = CH2, O): Synthesis, structure and catalytic activity in the hydroboration of CO2

Article abstract of DOI:10.1039/c6dt02135b

The coordination chemistry of Fe²⁺, Co²⁺ and Cu⁺ ions was explored with the triphosphine and triphosphinite ligands PhSi{CH₂PPh₂}₃ (1) and PhSi{OPPh₂}₃ (2), so as to evaluate the impact of the electronic properties of the tripodal phosphorus ligands on the structure and reactivity of the corresponding complexes. The synthesis and characterization of the complexes [Fe(κ³-PhSi{CH₂PPh₂}₃)(MeCN)₃][OTf]₂ (3) (OTf = O₃SCF₃), [Fe(κ³-PhSi{OPPh₂}₃)(MeCN)₃][OTf]₂ (3′), [Co(κ²-PhSi{CH₂PPh₂}₃)Cl₂] (4), [Co(κ³-PhSi{OPPh₂}₃)Cl₂] (4′), [Cu(κ³-PhSi{CH₂PPh₂}₃)Br] (5) and [Cu(κ³-PhSi{OPPh₂}₃)I] (5′) were carried out. The crystal structures of 3, 3′, 4, 4′, and of the solvates 5·3THF and 5′·THF are reported. Complexes 3-5′ were shown to promote the catalytic hydroboration of CO₂ with (9-BBN)₂ (9-BBN = 9-borabicyclo[3.3.1]nonane). While the iron and cobalt complexes of the triphosphine 1 are more active than the analogous complexes with 2, the opposite trend is observed with the copper catalysts. Overall, the copper catalysts 5 and 5′ are both more reactive and more selective than the Fe and Co catalysts, enabling the formation of the acetal H₂C(OBBN)₂ with a high molar ratio of H₂C(OBBN)₂ : CH₃OBBN up to 92 : 8.

Full text of DOI:10.1039/c6dt02135b

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DOI: 10.1039/C6DT02135B
Complexes of the Tripodal Phosphine Ligands PhSi(XPPh2)3 (X =
CH2, O): Synthesis, Structure and Catalytic Activity in the
Hydroboration of CO2†
Alicia
Aloisi,
JeanꢀClaude
Berthet,*
Caroline
Genre,
Pierre
Thuéry
and Thibault Cantat*
NIMBE, CEA, CNRS, Université ParisꢀSaclay, CEA Saclay 91191 GifꢀsurꢀYvette,
France.
Keywords: Tripodal ligands, Phosphorus ligands, Iron, Cobalt, Copper, Carbon
dioxide reduction, Hydroboration.
Abstract: The coordination chemistry of the Fe2+, Co2+ and Cu+ ions was explored
with the triphosphine and triphosphinite ligands PhSi{CH2PPh2}3 (1) and
PhSi{OPPh2}3 (2), so as to evaluate the impact of the electronic properties of the
tripodal phosphorus ligands on the structure and reactivity of the corresponding
complexes. The synthesis and characterization of the complexes [Fe(κ3ꢀ
PhSi{CH2PPh2}3)(MeCN)3][OTf]2
(3)
(OTf
=
O3SCF3),
[Fe(κ3ꢀ
[Co(κ3ꢀ
PhSi{OPPh2}3)(MeCN)3][OTf]2 (3’),
[Co(κ2ꢀPhSi{CH2PPh2}3)Cl2]
(4),
PhSi{OPPh2}3)Cl2] (4’), [Cu(κ3ꢀPhSi{CH2PPh2}3)Br] (5) and [Cu(κ3ꢀPhSi{OPPh2}3)I]
(5’) were carried out. The crystal structures of 3, 3’, 4, 4’, and of the solvates 5·3THF
and 5’ꢁTHF are reported. Complexes 3−5’ were shown to promote the catalytic
hydroboration of CO2 with (9ꢀBBN)2 (9ꢀBBN = 9ꢀBorabicyclo[3.3.1]nonane). While the
iron and cobalt complexes of the triphosphine 1 are more active than the analogous
complexes with 2, the opposite trend is observed with the copper catalysts. Overall,
the copper catalysts 5 and 5’ are both more reactive and more selective than the Fe
and Co catalysts, enabling the formation of the acetal H2C(OBBN)2 with a high molar
ratio of H2C(OBBN)2:CH3OBBN up to 92:8.
†
CCDC reference numbers 1481874–1481881. For crystallographic data in CIF or
other electronic format see DOI:
1

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Introduction
Tripodal ligands play a major role in transition metal chemistry, as they increase the
stability of their complexes and enable a fine control of the reactivity at the metal
center through their steric and electronic properties and their specific coordination
geometry. Within tripodal ligands, a few triphosphines have been utilized so far in
coordination chemistry and catalysis and, among them,1 triphos (MeC(CH2PPh2)3)
has attracted considerable attention over the last years, being a valuable ligand for a
number of late transition metals (with ca 700 crystal structures). The ability of triphos
to stabilize low metal oxidation states and to adopt both κ2ꢀ and κ3ꢀcoordination
modes has led to widespread and successful applications in catalysis, including CO2
hydrogenation, disproportionation of formic acid and reduction of amides.2,3,4,5,6,7 The
efficiency of catalytic reactions is generally strongly influenced by the nature of the
ancillary ligand. Because small changes within a ligand series, such as the pendant
alkyl chains or the nature and/or the position of the linking groups, can induce
different outcomes in a catalytic reaction, variations on the triphos backbone have
been reported. Tripodal molecules such as PP3 (PP3 = P{CH2PR2}3), N{CH2PR2}3,
R’Si{CH2PR2}3, etc. have been synthesized along with their coordination
complexes.8,9,10 In particular, R’Si(CH2PR2)3 ligands, featuring an organosilane linker,
have been investigated and a number of their complexes exist with Ti,11,12,13,14 V,
13,15,16,17,
1,19,20,21,22,23
Cr,18 Fe,
Cu,24 Nb,2,16,25 Mo,26,27 Ru,1,24,28,29,30,31 Rh,24 Pd,24
Ta,2,16,32 W,33 Os1 and Pt.24 Surprisingly, oxygen analogues of formula RSi(OPR’2)3
have remained elusive so far and, to our knowledge, only a single transition metal
complex,
eg
[Mo(MeSi{OPOMe2}3)(CO)3],
has
been
crystallographically
characterized.34
In this contribution we report the use of PhSi(CH2PPh2)3 (1) and its phosphinite
derivative PhSi(OPPh2)3 (2) in the coordination chemistry of firstꢀrow transition
metals, namely iron(II), cobalt(II) and copper(I) ions. The syntheses, NMR
characterizations and crystal structures of pairs of complexes with the ligands 1 and
2 are detailed. To establish the influence of replacing the phosphines groups in 1 with
phosphinites, the catalytic activity of the complexes was explored in the
hydroboration of CO2 with (9ꢀBBN)2. Because CO2 is a stable chemical waste, its
2

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selective multiꢀelectron reduction under mildꢀconditions remains a challenge35,36 and
an ideal platform to establish structure/activity relationships for analogous catalysts.
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Results and discussion
Synthesis of ligands 1 and 2
Scheme 1: Synthesis of the PhSi(XPPh2)3 ligands (X = O, CH2).
The syntheses of triphosphine PhSi(CH2PPh)3 (1) and its phosphinite analogue
PhSi(OPPh)3 (2) are described in Scheme 1. Ligand 1 was obtained in a good 75 %
yield, from a published protocol,1,37,38 by reacting a toluene solution of
Ph2PCH2Li(TMEDA) with PhSiCl3 in a 3:1 molar ratio. The new species 2 was
prepared by analogy with the synthetic procedures reported for the phosphite esters
RSi(OP{OR’})3 (R =Ph, Me; R’ = OMe, OEt), by mixing PhSiCl3 with 3 equivalents of
Ph2P(=O)H. Ph2P(=O)H is involved in an equilibrium with the phosphorus(III)
derivatives Ph2P–OH and it readily adds to the chlorosilane, in the presence of NEt3
as a base, in toluene.34, 39ꢀ40 After 5 h at room temperature, the solution was filtered
to remove the HNEt3Cl deposit and 2 was obtained pure as a colorless oil, after
1
vacuum drying (88 % yield). Compound 2 was characterized by its H, 13C and
31P{1H} NMR spectra in d8ꢀTHF. 2 exhibits a singlet at δP 100.93 for the P atoms
which is shifted downfield in comparison to the chemical shift of −20.21 ppm in THF
(−23.59 in benzene) for 1 (vs –27.3 ppm for triphos (toluene, −40°C)).1,41 These δP
values for 1 and 2 fall in the range of classical phosphine and phosphinite groups,
respectively. Importantly, the thermal stabilities of 1 and 2 strongly differ. Whereas a
refluxing solution of 1 in benzene shows no sign of decomposition after 24 h, 2
rapidly degraded under the same conditions to a large number of unidentified
phosphorus species.
4

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Scheme 2: Synthesis of the Fe2+, Co2+ and Cu+ complexes with the PhꢀSi(YPPh2)3 ligands (Y = O,
CH2).
Formation of iron complexes 3 and 3’
Treatment of FeCl2 with RSi(CH2PPh2)3 (R = Ph, i-Pr)1 or the less sterically
demanding ligands R’Si(CH2PMe2)3 (R’ = tBu, Me)20, 22 has been previously reported,
both in THF and CH2Cl2. In these weakly polar solvents, the colorless paramagnetic
complexes [Fe(κ2ꢀRSi{CH2PPh2}3)Cl2] (R = Ph, i-Pr)1 or the purple, diamagnetic
cationic [(Fe(κ3ꢀRSi{CH2PMe2}3))2(ꢀꢀCl)3][Cl]20 complexes were formed. The pink
iron(II) monocationic derivatives [Fe(κ3ꢀRSi{CH2PMe2}3)(ꢀꢀCl)(Me2P(CH2)nPMe2)][Cl]
(n= 1,2)22,23 were also obtained from the dinuclear species by addition of dmpe or
dmpm (dmpe = dimethylphosphinoethane; dmpm = dimethylphosphinomethane).22
These results provide a trend where the tridentate coordination mode of the
RSi{CH2PR’2}3 ligands always affords diamagnetic and colored iron complexes,
whereas colorless and paramagnetic derivatives are obtained with κ2ꢀcoordinated
ligands.
The polarity of the solvent and the nature of the X ligand in the FeX2 salt were found
to influence the coordination mode (bidentate vs tridentate) of the RSi{CH2PR’2}3
triphosphine ligands. In addition, these parameters govern the crystallization of the
complexes, which are obtained either in their neutral or cationic form. We thus
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considered the reaction of 1 and 2 with Fe(OTf)2 and acetonitrile (ε = 37.5 vs 7.5 for
THF), aiming at preparing discrete cationic species with octahedral geometry.
Reaction of 1 and 2 with Fe(OTf)2 in acetonitrile afforded orange (Y = O) and red (Y =
CH2) solids, which crystallized readily in acetonitrile as 3 and 3’, respectively, in
moderate yields (55 and 52%, respectively). 3 and 3’ underwent partial desolvation of
1
the acetonitrile ligands during vacuum drying, and elemental analyses and H NMR
data collected for the resulting solids suggested a coordination of 2 to 3 acetonitrile
molecules per iron ion. The iron complexes are soluble in THF yet insoluble in nonꢀ
polar solvents. 3 and 3’ were characterized by 1H, 13C and 31P{1H} NMR
spectroscopy, from their acetonitrile solutions. Characteristic 31P{1H} resonance for 3
is a unique singlet at 30.15 ppm, shifted downfield with respect to the free ligand 1
(−20.21 ppm). A similar trend is noted for 3’, which 31P{1H} spectrum features a
singlet at δP 147.9, shifted +47 ppm downfield from the free ligand 2. The NMR data
of 3 are similar to those observed for [Fe(PhCH2C{CH2PPh2}3)(NCMe)3][BF4]2
(δP 33.2) and [Fe(triphos)(NCMe)3][BF4]2 (δP 32.3).42,43
Large red and orange crystalline platelets of 3 and 3’ were obtained by layering
diethyl ether on an acetonitrile solution of the compounds. Both complexes
crystallized as discrete cation–anion pairs and views of the cations of 3 and 3’ are
shown in Figure 1, respectively, while selected bond lengths and angles are gathered
in Table 1. 3 and 3’ are isostructural with the iron ion lying in a distorted octahedral
environment, formed by three acetonitrile molecules and the κ3ꢀcoordinated tripodal
ligand (1 or 2). The structures are reminiscent to that of the related [Fe(κ3ꢀ
MeSi{CH2PPh2}3)(NCMe)3][BF4]2 complex, reported during the course of the present
study,21 and to that of [Fe(κ3ꢀ RC{CH2PPh2}3)(NCMe)3][BF4]2 (R = Me, PhCH2).42 The
mean Fe−P and Fe−N distances in 3 [2.308(11) Å and 1.958(4) Å] compare well with
those reported in [Fe(κ3ꢀMeSi{CH2PPh2}3)(NCMe)3][BF4]2 [2.317(7) and 1.966(2) Å]21
or [Fe(κ3ꢀ triphos)(NCMe)3][BF4]2 [2.27(1) and 1.953(6) Å].44 While the mean Fe−N
bond lengths are similar in 3 and 3’ [1.962(4) Å for 3’], the Fe−P distances in 3’ are
notably shorter averaging 2.255(8) Å. This contraction is in agreement with the
greater 3s character in the phosphorus lone pair of a phosphinite ligand, compared to
a phosphine. The P−Fe−P and N−Fe−N bite angles average 92.7(8)° and 85.3(15)°
in 3, and 91.1(18)° and 86.2(14)° in 3’, whereas these values reach 88(2)° and 85(1)°
6

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in [Fe(κ3ꢀMeC{CH2PPh2}3)(NCMe)3][BF4]2 and 92(2) and 84.4(7) in [Fe(κ3ꢀ
MeSi{CH2PPh2}3)(NCMe)3][BF4]2.
Formation of the cobalt complexes 4 and 4’
Cobalt(II) chloride reacted readily with 1 in THF to afford a blue solution from which
complex 4 was isolated as a solid (49 % yield), after several washings with pentane.
Treating a THF solution of CoCl2 with 2 yielded a red solution from which 4’ was
isolated as a red solid (36 % yield). Paramagnetism of the 3d7 Co2+ ion impeded any
NMR characterization and the compounds were characterized by Xꢀray diffraction
and elemental analyses. Blue crystals of 4 and dark red crystals of 4’ were obtained
by slow diffusion of pentane into a THF and a dichloromethane solution of the
complexes, respectively. These distinct colors suggest different coordination
geometries around the Co2+ ion in 4 and 4’. It is well established that tetrahedral Co2+
complexes are generally blue, while they are pinkꢀred when the metal ion adopts an
octahedral configuration.45 In fact, within the triphos series, [Co(triphos)Cl2] provided
blue solutions in agreement with a four coordinate [Co(κ2ꢀtriphos)Cl2] structure.
Nevertheless, it was isolated as a red five coordinate [Co(κ3ꢀ triphos)Cl2], in the solid
state.45 The increased stability of 4 with respect to [Co(κ2ꢀtriphos)Cl2] evidences the
influence of the Si versus the C atom at the apex position of tripodal ligands. The
control of the coordination behavior is likely due to small interplays of steric tension at
the C−E−C (E = Si, C) angles. This finding is in agreement with the recent results
obtained by Apfel et al. on the coordination behavior of the homologous
RSi{CH2PPh2}3 (R = Me, Ph) and triphos ligands with FeCl2. The authors indeed
concluded, from titration experiments, that MeSi{CH2PPh2}3 binds FeCl2 more
strongly than triphos in acetonitrile.21, 45
Views of complexes 4 and 4’ are provided in Figure 2, respectively, and selected
bond lengths and angles are listed in Table 1. Xꢀray diffraction confirmed a
tetrahedral environment for the Co2+ ion in 4 and a distorted square pyramidal
coordination geometry in 4’. In the latter complex, the base is defined by the two
halides, P2 and P3 (rms deviation 0.018 Å) while P1 is at the vertex of the pyramid.
The metal ion lies inside the pyramid, 0.4372(5) Å above the base and the sum of the
ligandꢀCoꢀligand angles in the mean base is 351.14°. The distinct coordination
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environments in 4 and 4’ are in line with the different electron donating properties of
ligands 1 and 2. Because phosphinite ligands P(OR)R2 are less electron donating
than phosphines, CoCl2 preferentially accommodates ligand 2 in a tridentate manner
while κ2ꢀcoordination with the more electron rich triphosphine 1 is favored, resulting
in the formation of the fourꢀcoordinate species 4.
Within each complex, the two Co–Cl bond lengths differ by less than 0.03 Å, thus
confirming the neutral nature of these species. The Co–Cl distances are shorter in 4
than in 4’, which reflects the lower coordination number at the Co2+ ion in 4. In
addition, Co–P distances in 4 are ca 0.12 Å longer than in 4’, with average values of
2.3483(6) and 2.22(4) Å in 4 and 4’, respectively. These structural features are
consistent with a chelating effect resulting in a stronger metalꢀligand interaction in 4’
and the increased 3s character of the P lone pairs in 2. The Cl–Co–Cl angle is
strongly related to the steric environment of the metal ion. A decreasing coordination
number allows the Cl–Co–Cl angle to expand from 88.69(3)° in the fiveꢀcoordinate
complex 4’ to 121.73(4)° in the fourꢀcoordinate species 4. The P–M–P angles in the
tridentate MeSi(YPPh2)3 (Y=CH2, O) ligands are unexceptional and quite similar in
the iron and cobalt complexes.
Formation of the copper complexes 5, 5’ and 5”
Addition of CuBr to a THF solution of 1 afforded [Cu(κ3ꢀPhSi{CH2PPh2}3)Br] (5), after
24 h reaction at room temperature and washings with pentane. Repeating the same
procedure with 2 and CuX (X= I, Br), [Cu(κ3ꢀPhSi{OPPh2}3)I] (5’) and [Cu(κ3ꢀ
PhSi{OPPh2}3)Br] (5”) could be isolated in good 88 and 82 % yields, respectively.
1
The H, 13C and 31P{1H} NMR spectra recorded at 21°C are consistent with
complexes having a C3 symmetry for the three complexes, which was confirmed by
Xꢀray diffraction analyses. Elemental analyses were carried out on the bromide
complexes and, because [Cu(κ3ꢀPhSi{OPPh2}3)Br] could not be crystallized, its iodide
counterpart 5’ was further considered in this work. Crystals of the adducts 5ꢁ3THF
and 5’ꢁTHF were grown by diffusion of pentane into THF solutions of 5 and 5’.
The 31P{1H} NMR spectra of 5 and 5’ in dichloromethane at 21°C (Figure 3) exhibit a
quartet centered at −33 and + 80 ppm, respectively. A JP–Cu coupling constant of
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860 Hz is measured for 5’ while the 31P{1H} NMR spectrum of 5 exhibits two nearly
superimposed quartets related to the phosphorus coupling with each copper isotope
(JPꢀ63Cu ~ 860 Hz, JPꢀ65Cu ~ 900 Hz). Classically, the 31P{1H} NMR spectra of copperꢀ
phosphine complexes present broad signals and the coupling between the 31P and
the two 63Cu and 65Cu isotopes is rarely observed in solution because the isotopes of
copper are both quadrupolar nuclei.46 In fact, the relaxation rate of quadrupolar nuclei
is higher in solution than in the solid state and fluxional behaviors such as ligands
exchanges might occur in solution, thereby precluding the observation of fine NMR
structures. The observation of P–Cu coupling for 5 and 5’ might point to a strong
coordination of the tripodal ligands to the Cu+ ion and a highly symmetric structure
that decreases the electric field gradient at the tetrahedral center, thus reducing the
quadrupolar relaxation rate 47. On the contrary, the 31P{1H} NMR spectrum of 5’’ in
THF at 21°C shows a very broad signal at +81 ppm (Fig. S13), indicative of a
stronger electric field gradient induced by the combination of a bromide and
phosphinite ligand.
Figure 3: 31P{1H} NMR spectra of 5 (left) and 5’ (right) in d2-CH2Cl2 at 21°C.
Although decomposition of the free ligand 2 was noted at 60°C, no product could be
identified from crude THF solutions. It is noteworthy that heating for several hours a
suspension of CuI or CuCl2 and 2 in refluxing THF or MeCN also led to the
decomposition of the triphosphinite framework. Layering the crude MeCN or THF
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solution with diethyl ether or pentane, respectively, resulted in the deposition of
colorless crystals of the complexes [Cu2(Ph2POPPh2)I2(NCMe)2]ꢁ(MeCN) (6ꢁMeCN)
or [Cu(Ph3Si3O3{OPPh2}3)Cl] (7). Complex 6 is a dimeric copper(I) complex featuring
a
Ph2POPPh2 diphosphoxane ligand (Scheme 3). Metal complexes of
diphosphoxane ligands R2PꢀOꢀPR2 are scarce in coordination chemistry and only a
fistful of compounds have been crystallographically characterized , such as
M(CO)4(PPh2POPPh2)
(M
=
Mo,
Cr)48,49
[RuCl2(Ph2PO2PPh2)(PPh3)({Ph2PO2}2C2H4)],50 or [Pt3(PPh2POPPh2)(PPh2)(C6F5)2].51
These examples were often obtained inadvertently, with the P−O−P fragments
resulting from the evolution of R2POH and/or R2(O=)P−PR2 molecules due to
isomerisation of P=O bonding in the presence of M2+ ions (Cu, Coꢂ).52ꢀ54 Recently
Cu(I) and Cu(II) salts were also found to selectively catalyse the dehydrogenative
coupling of R2P(=O)H molecules to R2(O=)P−O−R2P(=O) derivatives, under aerobic
conditions.55 Here, the Ph2POPPh2 ligand in 6 might derive from degradation of 2 due
to reaction with adventitious traces of water, after prolonged heating. They would
regenerate the phosphinous acid Ph2POH and its evolution toward
Ph2POPPh2.55,49,56
Complex 7 presents the coordination of a novel type of triphosphinite ligand in which
the PPh2 moieties are linked through a [PhSiO]3 unit, similar to a silsesquioxane
arrangement (Scheme 3). To our knowledge, such an arrangement is unknown and
reminiscent of mesitylene based tripodal phosphorus ligands used to form mono and
oligomeric assemblages with a variety of
d and f
metal ions.57ꢀ64
Although ligand 2 is sensitive to temperature, it is notable that its complexes (e.g. 3’,
4’, 5’ and 5”) are more robust and solutions of the complexes in THF show no sign of
decomposition after 24 h at 60°C.
Scheme 3: Complexes 6 and 7 obtained from degradation of ligand 2
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Views of the complexes 5, 5’, 6 and 7 are given in Figures 4ꢀ6, respectively, and
selected bond lengths and angles are listed in Table 2. Complexes 5 and 5’ comprise
a fourꢀcoordinate Cu+ ion, coordinated to three phosphorus atoms and one terminal
halide ion, so that the copper environment can be viewed as distorted trigonal
pyramidal, with the P atoms defining the base. The Cu–X (X = Br, I) and mean Cu–P
bond lengths [2.4137(12) and 2.285(4) Å for 5; 2.5491(3) and 2.276(2) Å for 5’] are
unexceptional and fall in the range of the fourꢀcoordinate complexes Cu(κ3ꢀtriphos)Br
[2.3983(15) Å and 2.286(6) Å ],65 Cu(κ3ꢀN{CH2PPh2}3)Br [2.4965(13) Å and 2.304(4)
Å],65 Cu(PMePh2)3Br [2.507(2) Å and 2.282(9) Å ]66 and [Cu(κ2ꢀdppe)( κ1ꢀdppe)I]
ꢀꢀ
[2.623(1) and 2.30(2) Å ].67 The averages for the P–Cu–P and P–Cu–X angles,
respectively 100.6(12) and 117(2)° in 5 and 96.55(9) and 120.5(18)° in 5’, are quite
distinct and reflect noticeable deviation from an ideal tetrahedral geometry. These
angles are similar to those found in Cu(κ2ꢀtriphos)Br.18 In all the complexes 3, 3’, 4,
4’, 5 and 5’, the Si–O and Si–CH2 distances are insensitive to the nature of the metal
ions and the coordination mode of the ligand.
Complex 6, [Cu(ꢀ2ꢀI)2(NCMe)]2(ꢀ2ꢀ(Ph2POPPh2)), crystallizes as a solvate with a
single acetonitrile molecule in the cell and it presents a twoꢀfold axis of symmetry,
defined by the O1 atom and the center of the CuꢀꢀꢀCu fragment. The Cu2I2 core
adopts a butterfly shape with the I− atoms at the wingtips. The two iodide ions and
the Ph2POPPh2 ligands are bridging and one acetonitrile molecule completes the
coordination sphere of the metal ion which is in a pseudoꢀtetrahedral environment
with a mean ligand−Cu−ligand angle of 109(7)°, close to the ideal value. The average
Cu−I bond length of 2.701(7) Å is close to mean values in the 4ꢀcoordinate phosphite
and phosphine derivatives [Cu(ꢀ2−I)2(P{OPh}3)(NCMe)]2 [2.65(2) Å]68 and
[Cu(ꢀ2−I)(NCMe)](ꢀ2ꢀ(C20H12O2SP}2NPh) [2.68(4) Å].69 The CuꢀꢀꢀCu distance of
2.8024(5) Å is classical and equals the sum of the Van der Waals radii (2.80 Å).70,71 It
is similar to that found in the cationic dimers [Cu(ꢀ2−dcpm)(NCMe)]2 [BF4]2 [2.809 Å]
(dcpm = bis(dicyclohexylphosphino)methane
)72 or [Cu(ꢀ2ꢀPh2P(CH2)2PPh2)(NCMe)2]2
b
[PF6]2 [2.873 Å].73 The corresponding distance in the twofold bridged dinuclear
74
complex [Cu(ꢀ2−X)(NCMe)]2(ꢀ2ꢀ(R2P)2(NꢀC6H4ꢀN)2(R2P)2)[Cu(ꢀ2−X)(NCMe)]2
are
notably shorter [X = Cl, Br, I ; 2.6893(4), 2.7177(8) and 2.6906(8) Å, respectively].
Such Cu(I)ꢀꢀꢀCu(I) distances vary considerably for ligandꢀsupported CuꢀꢀꢀCu bonding
interactions and contact distances as short as 2.35 Å have been reported.72
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Complex 7 crystallizes in the cubic space group Pa−3. The environment of the Cu+
atom is best described as distorted tetrahedral with bonding to three phosphorus
atoms of the novel Ph3Si3O3{OPPh2}3 ligand and one chloride anion. The line passing
by the centroid of the cyclic Si3O3 fragment and the Cu and Cl atoms is a threeꢀfold
axis of symmetry and the Cu atom is at a distance of 3.64 Å from the centroid of the
Si3O3 ring. The Cu1−Cl1 and Cu1−P1 bond lengths of 2.3022(15) and 2.2550(8) Å,
respectively, can be compared to those found in the other 4ꢀcoordinate complexes
[Cu(κ3ꢀtriphos)Cl] [2.260 and 2.292(7) Å]75 and [Cu(PPh3)3Cl] [2.347 and 2.318(1) Å];
76 the Cu−P bond length is also similar to those measured in 5 and 5’.
Catalytic hydroboration of CO2 with 3, 3’, 4, 4’, 5 and 5’
Having in hand a series of first row transition metal complexes, namely 3, 3’, 4, 4’, 5
and 5’, their catalytic properties were explored in the field of CO2 transformation. CO2
conversion is currently dominated by nonꢀredox processes that enable the
functionalization of CO2 to carbonates, carbamates and urethanes.35,36, 77ꢀ7835, 79,80
Recent progresses have enabled the coupling of CO2 and epoxides to either cyclic or
polymeric carbonates, with a high regioꢀ and stereoselectivity and using earth
abundant metal catalysts.81ꢀ86 These methods are progressively turning into efficient
technologies and some of them are currently under pilot development.80, 87 In contrast
to functionalization strategies, the reduction of CO2 offers an access to more
energetic C1 products such as formic acid, carbon monoxide or methanol, with
carbon atoms at the +II and –II oxidation states.36, 77ꢀ78,64a,68,88 So far, the electroꢀ
reduction of CO2 is mostly limited to two electrons processes with the formation of
HCOOH and CO, whereas the production of methanol by electrolysis still suffers from
low Faraday yields.89ꢀ93 Hydrogenation methodologies nicely supplement electrolysis
and, for example, CO2 hydrogenation to methanol can be promoted by solid copperꢀ
based catalysts, on the industrial scale.80, 87ꢀ88 In solution, several molecular catalysts
have been proposed for the hydrogenation of CO2 to formic acid or methanol.6, 94ꢀ97
So far, only a single molecular catalyst has been reported for the reduction of CO2 to
methanol with H2, based on ruthenium(II) complex supported by the triphos ligand.96
Replacing H2 with hydrosilanes (R3SiH) or hydroboranes (R2BH) enables the use of
extremely mild reaction conditions for the reduction of CO2, because the Si–H and B–
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H functionalities are polarized and more reactive.98ꢀ110 While hydrogenation of CO2
imposes the use of metal catalysts working at high temperature (>120 °C) and high
pressure (>80 bars), hydrosilylation and hydroboration reactions can be performed
under ambient conditions (25 °C, <1 bar CO2) with first row transition metal or metal
free catalysts. These advantages have been exemplified with the recent development
of novel diagonal reactions79 leading to the reductive functionalization of CO2 to
formamides, formamidines, methylamines, etc.111 Although hydrosilanes and
hydroboranes will not be suitable for large scale applications in the short run –
because of cost and atom efficiency – they offer an increased chemoselectivity in the
reduction of CO2. In particular, C0 derivatives such as acetals and aminals have been
successfully obtained from CO2 and hydrosilanes or hydroboranes, while the
corresponding formaldehyde state has been elusive so far when H2 is utilized in the
reduction of CO2.112,113
Ligands 1 and 2 are synthetic derivatives of the wellꢀknown tripodal triphos and PP3
(tris[2ꢀ(diphenylphosphino)ethyl]phosphine) ligands, which d transition metal (Ru, Fe,
Cu, etc.) complexes have been successfully utilized in hydrogen transfer catalysis,6,
114 specifically for the reduction of carbonyl compounds and CO2 or the generation of
H2 from formic acid.115,116,117 The catalytic activity of 3, 3’, 4, 4’, 5 and 5’ was thus
evaluated in the hydroboration of CO2 so as to determine the influence of the metal
ion and the electronic nature of the ligand in this model reaction.
Exposing an acetonitrile solution of 9ꢀBBN and 1.5 mol% 5 to an excess CO2 (1 bar)
resulted in the formation of the boryl acetal H2C(OBBN)2 and methoxyborane
CH3OBBN products in 32 and 13 % yield, after 24 h at 25 °C (Entry 3 in Table 4).
This result corresponds to a conversion of 45 % of the B–H functionalities and to an
H2C(OBBN)2/CH3OBBN molar ratio of 71:29. Interestingly, the triphosphinite complex
5’ exhibits a higher catalytic activity and a conversion of 69 % is reached after only
4 h at 25 °C, while full completion is noted after 17 h (Entries 4 and 5 in Table 4). 5’
is also more selective than 5 and provides a selectivity of H2C(OBBN)2/CH3OBBN=
80:20 at full conversion (92:8 after 4 h). Complexes 5 and 5’ are only the second
examples of copper complexes able to catalyze the hydroboration of CO2. In 2013,
Shintani and Nozaki indeed reported the use of copper(I) Nꢀheterocyclic carbene
(NHC) complexes able to reduce CO2 to formoxyboranes HCOOBR2 at low
temperature (35 °C).118 Importantly, whereas (NHC)CuX (X= OCHO, Ot-Bu) catalysts
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selectively yield formate derivatives, 5 and 5’ converge directly to the acetal product
showing that the hydroboration of the latter to the methoxyborane is kinetically
limiting. This selectivity is rather unusual and has been described, so far, only with
(dmpe)2FeH2, as reported recently by the group of SaboꢀEtienne and Bontemps.113
In contrast, the iron and cobalt complexes 3, 3’, 4 and 4’ show no catalytic activity in
the hydroboration of CO2 with 9ꢀBBN at room temperature and raising the reaction
temperature to 60 °C is needed to observe the reduction of CO2. Using the
triphosphine iron complex 3, 45 % of the hydroborane is converted after 24 h at
60 °C to a 19:81 mixture of H2C(OBBN)2 and CH3OBBN (Entry 10 in Table 4).
Interestingly, the triphosphinite analogue 3’ displays a lower catalytic activity and a
conversion of only 3 % under identical conditions (Entry 12). The cobalt complexes 4
and 4’ exhibit a slightly enhanced reactivity, with a conversion of 63 and 6 % with 4
and 4’, respectively, after 24 h at 60 °C. The iron and cobalt complexes thus differ
from copper catalysts 5 and 5’ as they show a lower activity in hydroboration for the
phosphinite derivatives (3’ and 4’) than the phosphine complexes (3 and 4). In
addition, different selectivities are observed between the two groups of catalysts.
Whereas iron and cobalt complexes 3, 3’, 4 and 4’ produce the methoxyborane as a
major product (with a H2C(OBBN)2/CH3OBBN molar ratio of ca. 20:80), even at low
conversions. The selectivity of 3 and 4 toward the methoxyborane is expectedly more
pronounced with an excess of (9ꢀBBN)2 (Entries 20ꢀ23), because CH3OBBN is the
thermodynamic product. The copper complexes 5 and 5’ favor the formation of the C0
product (H2C(OBBN)2/CH3OBBN ratio of ca. 80:20), which is only slowly converted to
the methoxide level. These trends reveal that different mechanisms are at play in the
metalꢀcatalyzed hydroboration of CO2. Presumably, the copper complexes act as
Lewis acids in this transformation and their catalytic activity is enhanced when the
Cu+ ion is coordinated to the less electron donating ligand 2. These different
mechanisms also translate into different reactivities of the catalysts in the presence of
a high concentration of hydroborane. Indeed, while catalysts 3 and 4 are more active
with an excess hydroborane, doubling the initial quantity of 9ꢀBBN poisons 5’ which
Turn Over Frequency (TOF) drops from 11.5 to 1.6 h–1 (Entries 19─23 in Table 4).
To the best of our knowledge, complexes 4 and 4’ are the first examples of cobalt
catalysts active in the hydroboration. Overall, complexes 3, 3’, 4, 4’, 5 and 5’
complete the small series of catalysts reported for this transformation, based on
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nickel, ruthenium, iron and copper complexes.107,108,113, 118,119 Although the most
reactive catalysts in the hydroboration of CO2 remain based on nickel pincer
structures and organic phosphorus bases, the present copper system has a catalytic
activity and a selectivity close to (dmpe)2FeH2.106, 109, 113
Conclusion
In conclusion, the synthesis and characterization (crystal structure, NMR
spectroscopy) of metal complexes of the first row Fe2+, Co2+, and Cu+ ions have been
described with the tripodal ligands PhSi(YPPh)3 (Y = CH2 (1), O (2)). Replacing the
CH2 group with an O atom affects the coordination properties of the molecule
favoring κ3ꢀPhSi(OPPh)3 versus κ2ꢀPhSi(CH2PPh)3 coordination onto CoCl2. All the
complexes are active in the catalytic hydroboration of CO2 with (9ꢀBBN)2. While the
PhSi(CH2PPh)3 (1) ligand favors the catalytic performances of Fe2+ and Co2+ ions
over its oxygen analogue PhSi(OPPh)3 (2), the reverse trend is observed with the
Cu+ complexes 5 and 5’ which proved the most active catalysts. The activity of the
triphosphinite complex 5’ is higher than that of 5. Both copper catalysts favor the
reduction of CO2 to the C0 acetal product, and the selective and quantitative
conversion of (9ꢀBBN)2 into a 80:20 mixture of H2C(OBBN)2 and CH3OBBN is
obtained at room temperature. The iron and cobalt catalysts are more sluggish and
require heating at 60 °C to perform the hydroboration of CO2. They also exhibit a
selectivity different from the copper complexes and facilitate the reduction of CO2 to
methoxyboranes, preferentially. The different mechanisms at play in these catalysts
classes are currently under scrutiny in our laboratory, to highlight the different active
forms of the metal catalysts.
Experimental
The complexes described herein are moisture sensitive. Syntheses and
manipulations of the compounds were conducted under ultraꢀhigh purity argon
atmosphere with rigorous exclusion of air and water (< 5 ppm oxygen or water), using
Schlenkꢀvessel and vacuumꢀline techniques or in recirculating mBraun LabMaster DP
gloveboxes.
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Glassware was dried overnight at 70 ºC before use. All NMR spectra were obtained
using a Bruker DPX 200 MHz spectrometer. Chemical shifts for 1H and 13C{1H} NMR
spectra were referenced to solvent impurities. The solvents toluene, pentane, diethyl
ether and tetrahydrofuran were dried over a mixture of sodiumꢀbenzophenone, and
pyridine over potassium and they were distilled immediately before use. Deuterated
solvents (Eurisotop), tetrahydrofuran (
d8ꢀTHF), benzene and toluene were dried over
a sodium/benzophenone mixture and acetonitrile (d3ꢀMeCN) over KH; they are
distilled and kept under argon in the gloves box. The commercially available
compounds trichlorophenylsilane, diphenylphosphine oxide were obtained from
Aldrich, used as received and stored in the gloves box. The amine NEt3 (Aldrich) was
dried over molecular sieve before use, then distilled under vacuum and kept on 4 Å
molecular sieve under inert atmosphere. The metal salts CoCl2, CuX (X = I, Br) and
Fe(OTf)2 (OTf = O3SCF3) were purchased from Strem or Aldrich, degassed and
dried under vaccum for 15h and stored under argon. Synthesis of PhSi(CH2PPh)3 (1)
(sticky beige solid) has already been reported1,25 and the new ligand PhSi(OPPh)3
was prepared from a derived procedure.34, 40
Synthetic procedures
Synthesis of PhSi(OPPh)3
PhSi(OPPh)3 (2) : A 10 mL roundꢀbottom flask was charged with diphenylphosphine
oxide (230 mg, 1.1 mmol), NEt3 (175 ꢃL, 1.2 mmol), PhSiCl3 (60 ꢃL, 0.37 mmol) and
freshly distilled toluene (5 mL) was introduced. After 5 h at room temperature the
[HNEt3]Cl salt which deposited from the colorless solution was filtered off on a frit in
the gloves box and further washed with 5 mL diethyl ether. The solvents of the
combined solutions were evaporated under vacuum affording pure 2, as a colorless
oil,
1H
in
good
yield
(228
mg,
(m,
88
35
%).
H).
NMR
δ(d
8ꢀTHF):
7.63−7.27
1
13C{1H}NMR
δ
(
d
8ꢀTHF): 144.04 (d, JPC = 23Hz , C
i
PPh), 135.17 (s, Cm,o SiPh),
131.45 (s, C
p
SiPh), 129.70 (s, Cp PPh), 128.46 (s, Cm,o SiPh), 130.67ꢀ128.46 (m,
C
m,o, PPh). 31P{1H} NMR
δ(
d8ꢀTHF): 100.93.
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Synthesis of the complexes
[Fe(PhSi{CH2PPh2}(MeCN)x][OTf]2
(x
=
2ꢀ3)
and
crystal
of
[Fe(PhSi{CH2PPh2}(MeCN)3][OTf]2 (3) : A 25 mL round bottom flask was charged
with Fe(OTf) (65 mg, 0.18 mmol) and a solution of 1 (140 mg, 0.20 mmol) in
acetonitrile (10 mL). The red solution was stirred for 24 h at room temperature and
the solvent evaporated off. The solid residue was washed with pentane (3 x 5 mL)
and dried under vacuum to give [Fe(PhSi{CH2PPh2}3)(MeCN)x][OTf]2 (x = 2ꢀ3) as a
red
solid
(117
mg
,55
%).
Dark
red
crystalline
needles
of
[Fe(PhSi{CH2PPh2}3)(MeCN)3][OTf]2 (3) were obtained by slow diffusion of diethyl
ether
1H NMR
δ(d
13C{1H}NMR
into
3ꢀacetonitrile): 7.80ꢀ7.54 (m, 35H), 2.06 (m, 6H), 1.94 (CH3CNꢀsignals);
8ꢀTHF): 134.53 (s), 134.30 (s), 134.06 (s), 133.81 (s), 131.99 (q,
2.85 Hz), 131.44 (s), 130.61 , 128.87 (q,
= 3.09 Hz), 128.60, 5.8 (m, CH2); 31P{1H}
NMR 3ꢀacetonitrile): 30.15 (s). Elemental analysis (%) calc. for
C53H50F6FeN3O6P3S2Si C, 53.95; H, 4.27; N, 3.56 and calc. for
C51H47F6FeN2O6P3S2Si : C, 53.78; H, 4.16; N, 2.46; found :C 53.60 ; H 4.41; N 2.93
an
acetonitrile
solution
of
the
isolated
product.
δ(
d
J =
J
δ(d
:
[Fe(PhSi{OPPh2}3)(MeCN)x][OTf]2 (x
=
2ꢀ3)
and
crystals
of
[Fe(PhSi{OPPh2}3)(MeCN)3][OTf]2 (3’) : A 25 mL round bottom flask was charged
with Fe(OTf)2 (100 mg, 0.28 mmol) and a solution of 2 (215 mg, 0.30 mmol) in
acetonitrile (10 mL). The orange solution was stirred for 24 h at room temperature
and the solvent evaporated off. The orange solid residue was successively washed
with diethyl ether (3 x 5 mL) and then pentane (3 x 5 mL) and dried under vacuum to
yield [Fe(PhSi{OPPh2}3)(MeCN)x][OTf]2 (x = 2ꢀ3) as an orange solid (172 mg, 52 %).
Orange crystals of [Fe(PhSi{OPPh2}3)(MeCN)3][OTf]2 (3’) were obtained by slow
diffusion of diethyl ether into an acetonitrile solution of the isolated product.
1H NMR
δ(
d3ꢀacetonitrile): 8.45 (m, 1H), 7.90 (m, 4H), 7.51 (m, 8H), 7.28 (m, 23H),
1.94
(CH3CNꢀsignals);
13C{1H}NMR
SiPh), 133.16 (m, C
δ
(d
3ꢀacetonitrile): 135.43 (s, Cm,o SiPh), 134.91 (s, C
p SiPh), 133.85 (s,
Ci
i
PPh), 131.09 (s, C PPh), 129.51 (d, JPC = 7.5Hz, Cm,o
p
,
PPh), 129.50 (s, Cm,o, SiPh), 129.43 (d, JPC = 7.5Hz, Cm,o, PPh), 128.70 (d, JPC
=
31
7Hz, Cm,o, PPh), 128.63 (d, JPC = 7Hz, Cm,o, PPh); P{1H} NMR δ(d3ꢀacetonitrile):
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147.93 (s). Elemental analysis (%) calc. for C50H44F6FeN3O9P3S2Si : C 50.64 ; H
3.74; calc. for C48H41F6FeN2O9P3S2Si : C 50.36 ; H 3.61; found C 50.78 ; H 3.54.
[Co(PhSi{CH2PPh2}3)Cl2] (4) : A 25 mL round bottom flask was charged with CoCl2
(15 mg, 0.12 mmol) and a solution of 1 (100 mg, 0.14 mmol) in 10 mL THF. The
resulting blue solution was stirred for 24 h at room temperature. The solvent was
evaporated off affording a blue powder which was washed with pentane (3 x 5mL)
and then dried under vacuum to yield pure 4 as a blue solid (56 mg, 49%).
Blue crystals of 4 were obtained by slow diffusion of pentane into a THF solution of
the
isolated
product.
1H NMR
δ(d2ꢀCH2Cl2): 18.15 (s, W1/2 = 36Hz, 1H), 17.13 (s, W1/2 = 48Hz, 1H), 13.91
(s, W1/2 = 63Hz, 18H), 8.37−7.63 (br.m,15H), 2.33 (s, W1/2 = 295 Hz, 6H); 13C and
31P{1H} NMR: no signals due to the paramagnetic nature of cobalt(II). Elemental
analysis (%) calc. for C45H41Cl2CoP3Si : C 64.91 ; H 4.96; found: C 64.03 ; H 4.68.
[Co(PhSi{OPPh2}3)Cl2] (4’) : A 25 mL round bottom flask was charged with blue
CoCl2 (25 mg, 0.20 mmol) and a solution of 2 (165 mg, 0.23 mmol) in 10 mL THF.
The resulting deep red solution was stirred for 24 h at room temperature. The solvent
was evaporated off affording a red solid which was washed successively with diethyl
ether (3 x 5mL) and pentane (3 x 5mL) and then dried under vacuum to give 4’ in
moderate
Dark red crystals of 4’ were obtained by slow diffusion of pentane into a
dichloromethane solution of the isolated product. 1H NMR
8ꢀTHF): 11.13 (s, W1/2
390Hz, 12H), 7.80−7.19 (m, 23H 23H). 13C and 31P{1H} NMR (
8ꢀTHF): no signals
yield,
as
a
red
solid
(60
mg,
36
%).
δ(
d
=
d
due to the paramagnetic nature of cobalt(II). Elemental analysis (%) calc. for
C42H35Cl2CoO3P3Si: C 60.16 ; H 4.21; found: C 60.12 ; H 4.33.
[Cu(PhSi{CH2PPh2}3)Br] (5) : A 25 mL round bottom flask was charged with CuBr
(17 mg, 0.12 mmol) and a solution of 1 (100 mg, 0.14 mmol) in 10 mL of THF. The
resulting colorless solution was stirred for 24 h at room temperature without change
of color. The solution was concentrated to 3 mL and the addition of 6 mL pentane
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induced precipitation of a white solid of 5 which was isolated pure after filtration and
drying under vacuum (77 mg, 76 %). Colorless crystals of the solvate 5·(THF)3 were
obtained by diffusion of pentane into a THF solution of the product.
1H
13C{1H}NMR
SiPh), 133.37 (s, Cm,o SiPh), 131.54 (d, JPC = 11Hz, Cm,o, PPh), 131.44 (d, JPC
11Hz, Cm,o, PPh), 130.40 (s, C SiPh), 129.30 (s, C PPh), 128.48 (s, Cm,o SiPh),
128.39 (d, JPC = 6.3Hz, Cm,o, PPh), 128.33 (d, JPC = 6.3Hz, Cm,o, PPh), 8.33 (s,
NMR
δ
(
d2ꢀCH2Cl2):
7.67−7.08
(m,
35H),
1.71
(s,
6H);
δ
(CH2Cl2ꢀ 2): 136.99 (br.s, W1/2 = 30Hz, C
d
i
PPh), 136.19 (d, JPC = 7Hz,
Ci
=
p
p
CH2); 31P{1H} NMR
δ
(d2ꢀCH2Cl2): −33 (m,
J
~ 840 Hz,
J
~ 900 Hz).
Pꢀ Cu
63
Pꢀ Cu
65
Elemental analysis (%) calc. for C45H41BrCuP3Si: C 63.87 ; H 4.88; found: C 64.03 ;
H 4.39.
[Cu(PhSi{OPPh2}3)Br] (5’’) : A 25 mL round bottom flask was charged with CuBr (20
mg, 0.14 mmol) and a solution of 2 (110 mg, 0.15 mmol) in 10 mL THF. The initially
colorless solution was stirred for 24 h at room temperature without change of color.
The solution was concentrated to 3 mL and the addition of 6 mL pentane induced
precipitation of a white solid of 5’’ which was isolated pure after filtration and drying
under vacuum (98 mg, 82 %). Colorless crystals of the solvate 5’·(THF) were
obtained by diffusion of pentane into a THF solution of the product.
1H NMR
31P{1H} NMR
δ(
d
8ꢀTHF): 8.05−8.02 (m, 2H), 7.69−7.57 (m, 17H), 7.13−7.02 (m, 16H);
δ(
d
8ꢀTHF): 81 (very broad singlet); (C6D6); 82.9 (very broad singlet);
δ
Elemental analysis (%) calc. for C42H35CuBrO3P3Si : C 59.20 ; H 4.14; found: C
60.67; H 4.79.
[Cu(PhSi{OPPh2}3)I] (5’) : A 25 mL round bottom flask was charged with CuI (30 mg,
0.16 mmol) and a solution of 2 (150 mg, 0.21 mmol) in 10 mL THF. The initially
colorless solution was stirred for 24 h at room temperature without change of color.
The solution was concentrated to 3 mL and the addition of 6 mL pentane induced
precipitation of a white solid of 5’ which was isolated pure after filtration and drying
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under vacuum (125 mg, 88 %). Colorless crystals of the solvate 5’·(THF) were
obtained by diffusion of pentane into a THF solution of the product.
1H NMR
13C{1H}NMR
SiPh), 128. 57 (s, Cm,o SiPh), 132.64 (s, C
JPC = 13Hz, Cm,o, PPh) et 129.20 (d, JPC = 13Hz, Cm,o, PPh), 128.27(d, JPC = 6.9Hz,
m,o, PPh) et 128.21 (d, JPC = 6.9Hz, Cm,o, PPh), 126.27 (s, C
SiPh); 31P{1H} NMR
2ꢀCH2Cl2): 80 (m, PꢀCu ~ 860 Hz).
δ
(d
2ꢀCH2Cl2): 8.02−7.99 (m, 2H), 7.70−7.49 (m, 17H), 7.19−7.03 (m, 16H);
2ꢀCH2Cl2): 137.72 (br.s, W1/2 = 33 Hz, C PPh), 134.42 (s, Cm,o
SiPh), 130.16 (s, C PPh), 129.34 (d,
δ(
d
i
p
p
C
i
δ(d
J
Crystals of the solvate compound [Cu2(Ph2POPPh2)I2(NCMe)2]·(MeCN) (6ꢁMeCN)
: In an NMR tube, CuI (10 mg, 0.05 mmol) and 2 (37mg, 0.05 mmol) were charged in
0.5 mL acetonitrile. The suspension, kept at room temperature for 24 h and heated at
80°C for 2 h, afforded a colorless solution. Colorless crystalline blocks of 6ꢁMeCN
were obtained by slow diffusion of diethyl ether into the solution.
Crystals of [Cu(Ph3Si3O3{OPPh2}3)Cl] (7) : In a NMR tube, CuCl2 (9.4 mg, 0.07
mmol) and 2 (271 ꢃL, 0.07 mmol) were charged in 0.5 mL of THF. The suspension
was heated at reflux for 15 h. Colorless crystals of the reduced complex 7 were
obtained by diffusion of pentane into the THF.
Catalytic experiments
Catalytic reactions have been performed in a JꢀYoung NMR tube equipped with
valve. In an NMR tube, the borane (9ꢀBBN)2 (ca 0.09 or 0.20mmole) and the catalyst
(1.5 mol%) were weighted and dissolved in 0.4 mL of
d3ꢀacetonitrile, then 7 ꢃL
(0.05 mmol) of mesitylene were added. The tube was then fitted onto a vacuum line,
degassed under vacuum after having freeze the solution and filled with 1 atmosphere
of pure CO2 (quality 4.5, ca 0.08 mmole).
Crystallography
The data were collected at 150(2) K on a Nonius KappaꢀCCD area detector
diffractometer120 using graphiteꢀmonochromated MoꢀK
α radiation (λ = 0.71073 Å).
The crystals were introduced into glass capillaries with a protective coating of
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ParatoneꢀN oil (Hampton Research). The unit cell parameters were determined from
ten frames, then refined on all data. The data (combinations of ꢀ and ꢀscans with a
ϕ
ω
minimum redundancy of at least 4 for 90% of the reflections) were processed with
HKL2000.121 Absorption effects were corrected empirically with the program
SCALEPACK.121 The structures were solved by intrinsic phasing with SHELXT122
expanded by subsequent difference Fourier synthesis and refined by fullꢀmatrix leastꢀ
squares on F2 with SHELXLꢀ2014.123ꢀ124 All nonꢀhydrogen atoms were refined with
anisotropic displacement parameters.
In compound 3, one of the trifluoromethanesulfonate counterꢀions is
disordered over two positions sharing two oxygen and two fluorine atoms, which were
refined with occupancy parameters constrained to sum to unity and restraints on
bond lengths and displacement parameters.
In compound 4, the displacement parameters of several carbon atoms in the
phenyl rings are highly anisotropic, which is probably indicative of unresolved
disorder.
In compound 4’, the phenyl ring bound to Si1 is rotationally disordered over
two positions sharing two carbon atoms, which were refined with occupancy
parameters constrained to sum to unity; one of these positions (A) was refined as an
idealized hexagon and the other (B) with restraints on bond lengths and angles.
Complex 5ꢁ3THF was refined as a twoꢀcomponent inversion twin, with a Flack
parameter of 0.479(13).
In compound 7, some voids in the lattice indicate the presence of unresolved
solvent molecules, but, the residual electronic density in these voids being quite low,
the SQUEEZE software (PLATON)125 did not improve the data significantly. These
solvent molecules may have at least partially left the lattice, as suggested by the
partly opaque aspect of all crystals.
In all compounds, the hydrogen atoms were introduced at calculated positions
and they were treated as riding atoms with an isotropic displacement parameter
equal to 1.2 times that of the parent atom (1.5 for CH3, with optimized geometry).
Crystal data and structure refinement parameters are given in Table 3. The molecular
plots were drawn with ORTEPꢀ3.126
21

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DOI: 10.1039/C6DT02135B
Acknowledgments
This work was supported financially by the CEA, CNRS, CHARMMMAT Laboratory of
Excellence, and the European Research Council (ERC starting grant agreement no.
336467). T.C. thanks the Foundation Louis D.ꢀInstitut de France for its support. The
authors thank Dr. Emmanuel Nicolas for fruitful discussions regarding NMR
spectroscopy.
22

Page 23 of 39
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DOI: 10.1039/C6DT02135B
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DOI: 10.1039/C6DT02135B
Captions to Figures
Scheme 1: Synthesis of the PhSi(XPPh2)3 ligands (X = O, CH2).
Scheme 2: Synthesis of the Fe2+, Co2+ and Cu+ complexes with the PhSi(YPPh2)3
ligands (Y = O, CH2).
Scheme 3: Complexes 6 and 7 obtained from degradation of ligand 2
Figure 1. Views of complexes 3 (left) and 3’ (right). Displacement ellipsoids are
drawn at the 30% probability level. The counterions and hydrogen atoms are omitted.
Figure 2. Views of complexes 4 (left) and 4’ (right). Displacement ellipsoids are
drawn respectively at the 30% and 40% probability level. The hydrogen atoms are
omitted. For 4’ only one position of the disordered aromatic ring is represented
Figure 3. 31P{1H} NMR spectra of 5 (left) and 5’ (right) in d2-CH2Cl2 at 21°C
Figure 4. Views of complexes 5 (left) and 5’ (right). Displacement ellipsoids are
drawn at the 30% probability level. The solvent molecules and hydrogen atoms are
omitted.
Figure 5. View of complex 6. Displacement ellipsoids are drawn at the 50%
probability level. The solvent molecule and hydrogen atoms are omitted. Symmetry
code: i = 1 – x, y, 3/2 – z.
Figure 6. View of complex 7. Displacement ellipsoids are drawn at the 30%
probability level. The hydrogen atoms are omitted. Symmetry codes: i = z, x, y; j = y,
z, x.

Dalton Transactions
Page 28 of 39
Table 1. Selected bond lengths (Å) and angles (°) in the complexes 3, 3’ and 4, 4’
3
3’
4
4’
Distances
Fe1–N1
Fe1–N2
Fe1–N3
Fe1–P1
Fe1–P2
Fe1–P3
<Fe–P>
<Fe–N>
<P–C>
1.963(4)
Fe1–N1
Fe1–N2
Fe1–N3
Fe1–P1
Fe1–P2
Fe1–P3
<Fe–P>
<Fe–N>
<P–O>
1.967(2)
1.958(2)
1.9624(18)
2.2452(6)
2.2532(6)
2.2656(6)
2.255(8)
1.962(4)
1.625(6)
1.6409(3)
1.816(2)
Co1–Cl1
Co1–Cl2
Co1–P1
Co1–P2
< Co1–P>
<P–C>
2.2318(10) Co1–Cl1
2.2061(10) Co1–Cl2
2.3476(10) Co1–P1
2.3489(10) Co1–P2
2.2600(8)
2.2418(8)
2.2755(8)
2.1893(8)
2.2070(8)
2.22(4)
1.953(4)
1.958(3)
2.3031(12)
2.3224(11)
2.2972(12)
2.308(11)
1.958(4)
2.3483(6)
1.827(13)
1.881(16)
1.866(4)
Co1–P3
< Co1–P>
<P–O>
<Si–C>
1.640(4)
Si1–C40
<Si–O>
Si1–C37
1.6402(2)
1.8244(19)
1.833(8)
<Si–C>
Si1–C40
Angles
1.871(13)
1.855(4)
<Si1–O>
Si1–C37
N2–Fe1–N3 83.22(14)
N2–Fe1–N1 85.75(14)
N3–Fe1–N1 86.84(13)
P3–Fe1–P2 93.07(4)
P1–Fe1–P2 91.61(4)
P3–Fe1–P1 93.53(4)
N2–Fe1–N3
N2–Fe1–N1
N3–Fe1–N1
P3–Fe1–P2
P1–Fe1–P3
P1–Fe1–P2
<P–Fe–P>
85.43(7)
88.19(8)
85.06(7)
93.47(2)
89.12(2)
90.67(2)
91(2)
Cl1–Co1–Cl2
P1–Co1–P2
121.73(4)
104.27(3)
Cl1–Co1–Cl2
P2–Co1–P3
P2–Co1–P1
P3–Co1–P1
<P–Co–P>
88.69(3)
92.84(3)
92.42(3)
94.56(3)
93.3(9)
<P–Fe–P>
92.7(8)

Page 29 of 39
Dalton Transactions
Table 2. Selected bond lengths (Å) and angles (°) in the copper complexes 5ꢀ3THF, 5’ꢀTHF, 6ꢀMeCN and 7
5ꢀ3THF
5’ꢀTHF
6ꢀMeCN
7
Distances
Cu1–Br1
Cu1–P1
Cu1–P2
Cu1–P3
<Cu–P>
<P–C>
2.4137(12)
Cu1–I1
Cu1–P2
Cu1–P1
Cu1–P3
<Cu–P>
<P–C>
2.5491(3)
Cu1–I1
Cu1–I1i
2.6937(3)
Cu1–Cl1
Cu1–P1
P1–O1
2.3022(15)
2.2550(8)
1.623(2)
2.287(2)
2.280(2)
2.289(2)
2.285(4)
1.832(7)
1.886(14)
1.863(8)
2.2777(7)
2.2783(6)
2.2728(7)
2.276(2)
1.809(2)
1.829(2)
2.7074(3)
2.2079(6)
1.973(2)
Cu1–P1
Cu1–N1
Cu1–Cu1i
P1–O1
P1–C13
Si1–O1
Si1–C1
<Si–O2>
1.819(3)
2.8024(5)
1.6498(12)
1.8165(15)
1.626(2)
1.833(3)
<Si–C>
Si1–C40
Angles
Si1–C37
<P1–C(Ph)>
1.6355(15)
P1–Cu1–P2 98.85(8)
P2–Cu1–P3 101.38(8)
P1–Cu1–P3 101.46(8)
P2–Cu1–Br1 114.58(6)
P1–Cu1–Br1 120.54(7)
P3–Cu1–Br1 116.84(6)
P3–Cu1–P2 96.64(2)
P3–Cu1–P1 96.43(2)
P2–Cu1–P1 96.57(2)
P3–Cu1–I1 122.43(2)
P2–Cu1–I1 120.858(19)
P1–Cu1–I1 118.109(18)
I1–Cu–I1i
109.697(10)
124.59(6)
106.81(6)
104.75(6)
106.114(18)
104.369(17)
100.77(9)
100.28(8)
116.33(7)
124.31(14)
P1–Cu1–Cl1
P1–Cu1–P1i
105.08(3)
113.48(2)
N1–Cu1–P1
N1–Cu1–I1
N1–Cu1–I1i
P1–Cu1–I1
P1–Cu1–I1i
O1–P1–C7
O1–P1–C1
O1–P1–Cu1
P1–O1–P1i
<P–Co–P>
100.6(12)
<P–Co–P>
96.55(9)