Efficient Reduction of Carbon Dioxide to Methanol Equivalents Catalyzed by Two-Coordinate Amido-Germanium(II) and -Tin(II) Hydride Complexes

Article abstract of DOI:10.1021/acscatal.6b03306

The bulky amido-germanium(II) and -tin(II) hydride complexes, L^?EH [E = Ge or Sn; L^? = -N(Ar^?) (SiPrⁱ₃); Ar^? = C₆H₂Prⁱ{C(H)Ph₂}₂-4,2,6

Full text of DOI:10.1021/acscatal.6b03306

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Article
Efficient Reduction of Carbon Dioxide to Methanol Equivalents Catalyzed
by Two-Coordinate Amido-Germanium(II) and Tin(II) Hydride Complexes
Terrance John Hadlington, Christos E. Kefalidis, Laurent Maron, and Cameron Jones
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03306 • Publication Date (Web): 24 Jan 2017
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Efficient Reduction of Carbon Dioxide to Methanol Equivalents Catalyzed by Two-
Coordinate Amido-Germanium(II) and Tin(II) Hydride Complexes
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Terrance J. Hadlington,† Christos E. Kefalidis,‡ Laurent Maron*,‡ and Cameron Jones*,†
†Monash Centre for Catalysis, School of Chemistry, Monash University, P.O. Box 23, Melbourne,
Victoria, 3800, Australia
‡Université de Toulouse et CNRS, INSA, UPS, UMR 5215, LPCNO, 135 Avenue de Rangueil, F-
31077 Toulouse, France
Keywords: CO2, reduction, catalysis, germanium, tin, lowꢀcoordinate
Abstract
The bulky amidoꢀgermanium(II) and tin(II) hydride complexes, L†EH (E = Ge or Sn; L† = ꢀ
N(Ar†)(SiPri3); Ar† = C6H2Pri{C(H)Ph2}2ꢀ4,2,6), which are twoꢀcoordinate in solution, are shown
to be extremely efficient and highly selective "singleꢀsite" catalysts for the reduction of CO2 to
methanol equivalents (MeOBR2), using HBpin or HBcat as the hydrogen source. L†SnH is the
most active nonꢀtransition metal catalyst yet reported for such reductions, yielding TOFs of up to
1188 hꢀ1 at room temperature. Computational studies have identified two thermodynamically and
kinetically viable catalytic pathways by which these reductions may operate. Spectroscopic
investigations have identified several reaction intermediates, which leads to the conclusion that
one of these reaction pathways predominates in the experimental situation. Stoichiometric
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reactivity studies have shed further light on the reaction mechanisms in operation, and indicate
that involvement of the second reaction pathway cannot be ruled out. This study highlights the
potential of relatively cheap, low toxicity main group complexes as viable alternatives to
transition metal based systems in the catalytic transformation of small molecules.
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Introduction
The rising levels of carbon dioxide in Earth's atmosphere, and the climatic problems
associated with this,1 have led to a rapid escalation of research activities aimed at the catalytic
conversion of CO2 into value added chemicals and fuels.2 By and large, these efforts have been
directed at utilizing CO2 as a C1 feedstock for the synthesis of, for example, methane, formic
acid, formaldehyde, CO and methanol.2,3 While heterogeneously and electrochemically catalyzed
processes are of considerable importance in this realm,4 excellent progress has been made using
transition metal complexes as homogeneous catalysts for such transformations over the last
decade.5ꢀ7 In contrast, main group metal and nonꢀmetal compounds had been little used as
homogeneous catalysts for the reduction of CO2 until 2010.6,8 In this respect, and of most
relevance to this study, are reductions of CO2 to methanol equivalents, MeOBR2, using boranes,
HBR2, as hydrogen sources. To the best of our knowledge, there are only two reports of such
reductions (using HBpin, pin = pinacolato) being catalysed at a single site in main group metal
complexes, viz. the βꢀdiketiminato sꢀ and pꢀblock compounds, (DipNacnac)Ga(H)(But)9 and
(DipNacnac)M(THF)n(ꢀꢀH)B(C6F5)3 (M = Mg, n = 0; M = Ca, n = 1).10 However, in those cases,
high catalyst loadings (10 mol%) and elevated temperatures (60 °C) were required to achieve low
turnover frequencies (2.5, 0.07 and 0.1 hꢀ1 respectively). In addition, a handful of dual site
Frustrated Lewis Pair (FLP) catalysts and related compounds (both nonꢀmetal and main group
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metal based) have been employed as catalysts for the borane reduction of CO2 to methanol
equivalents.6,11 These reactions typically also require elevated temperatures and/or CO2 pressures
to proceed efficiently. The most active of these is the dual site nonꢀmetal system,
C6H4(PPh2)(Bcat)ꢀ1,2 (cat = catecholato), which catalyzes (at 0.33 mol%) the reduction of CO2
by BH3(SMe2) at 60 °C with a TOF of 853 hꢀ1.11(a) For sake of comparison, the most active of any
homogeneous catalyst used for the reduction of CO2 to a methanol equivalent is a palladium
thiolate system, which was very recently reported to yield TOFs of up to 1780 hꢀ1 when HBcat
was used as the hydrogen source.7(a)
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It would be of significant merit to develop new, more efficient main group element based
catalysts for the reductive hydroboration of CO2, which could be used as cheaper and less toxic
alternatives to transition metal catalysts in this area.12 Moreover, expanding the ranks of such
catalysts may lead to a greater understanding of the mechanisms by which they operate. This is
important as, at present, there is sparse experimental mechanistic information available for main
group compounds that catalyze CO2 reductions. One compound type that we believed might
prove effective in this regard, are the germanium(II) and tin(II) hydride complexes, 1 and 2
(Figure 1), which we have shown to be twoꢀcoordinate in solution, due to the their extremely
bulky amide ligands,13,14 and to possess an empty pꢀorbital at the metal center. This leads to the
electrophilic compounds being powerful and selective reagents for the hydrometallation of a
range of unsaturated substrates (e.g. alkenes,15 aldehydes and ketones16). Furthermore, the lowꢀ
coordinate nature of both compounds has led to them finding use as extremely active catalysts for
the hydroboration of unactivated ketones and aldehydes.16 Because of this, it seemed reasonable
that they could also catalyze the reduction of CO2 to methanol equivalents, using boranes. Further
impetus to investigate this possibility came from the fact that experimental stoichiometric,17 and
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computationally predicted catalytic,18 reductions of CO2 to methanol, mediated by higher
coordinate germanium(II) hydrides (e.g. (DipNacnac)GeH), have recently been reported. Here we
show that 1 and 2 are efficient, and highly selective catalysts for the reductive hydroboration of
CO2 to MeOBR2 compounds, and that 2 is by far the most active nonꢀtransition metal catalyst yet
described for such reductions. The mechanisms of these reactions are investigated in detail using
spectroscopic and computational analyses, in addition to stoichiometric reactivity studies.
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Figure 1. Catalysts used in this study
Results and Discussion
Catalytic Studies. The reduction of CO2 to methanol equivalents, using either HBpin or the more
reactive HBcat as hydrogen sources, proceeded according to the overall reaction depicted in
equation 1. In the case of HBpin, and as found previously for the catalytic hydroboration of
aldehydes and ketones,16 the germanium(II) hydride 1 is a less active catalyst than its more
electrophilic tin(II) counterpart, 2. At a 10 mol% loading of 1, the CO2 reduction reaction is
complete after 24 h at 20 °C, yielding a TOF of 0.4 hꢀ1 (see Table 1). Lowering the catalyst
loading marginally incresed the TOF, but the reaction was slower and the yield was reduced. In
contrast, when 2 was employed as the catalyst at 10 mol% loading, the TOF at reaction
completion was more than an order of magnitude higher at 8 hꢀ1. Lowering the loading to 1 mol%
yielded an overall TOF of 14.5 hꢀ1, which is significantly greater than that achieved by the
previously most active singleꢀsite main group metal catalyst, (DipNacnac)Ga(H)(But).9
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Importantly, the reactions catalyzed by both 1 and 2 were typically extremely clean,
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yielding MeOBpin and (pinB)2O in greater than 99 % yield upon completion, with no other
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identifiable boron containing products. However, following the reactions by H and 11B{1H}
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NMR spectroscopy did shed light on implied intermediates in the reaction. In both cases, and at
various stages of the reactions, the mixtures contained differing amounts of the likely initial CO2
insertion products, viz. the metal formates L†E(κ2ꢀO,O'ꢀO2CH) (E = Ge 3, Sn 4; see below),
(pinBO)2CH2 5,19 and the final products (pinB)2O and MeOBpin. During the early stages of the
reactions the amount of (pinBO)2CH2 present increased at the expense of the formate, while as
the reaction progressed (pinBO)2CH2 was lost as more (pinB)2O and MeOBpin were formed.
These experimental observations imply that 3 or 4, and 5, are intermediates in the CO2 reduction
reactions. It is of note that these compounds are similar to previously proposed intermediates in
related reductions mediated by main group systems.10,17
Table 1. Secondary borane reduction of CO2, catalysed by 1 or 2
Loading
(mol%)
Yield
(%)c
TOF
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Time
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BR2
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a Catalyst 2 can also be generated in situ using the preꢀcatalyst,
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L†SnOBut. All reactions carried out in d6ꢀbenzene at 20 °C.
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under ~ 2 bar CO2 using 1 equiv of HBpin. Obtained by
integration of product signals in 1H NMR spectra vs signals for
d
HBR2 and tetramethylsilane internal standard. Turn over
frequency ꢀ average value for complete reaction.
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In order to increase the rate of the reduction reactions, the more potent borane, HBcat,
was used in place of HBpin. In the case of the reduction catalysed by 1, the product mixture was
not as clean as that obtained using HBpin, and contained a number of unidentifiable species, in
addition to (catB)2O and MeOBcat. However, when a 1 mol% loading of catalyst 2 was used, the
reaction was extremely clean, and was complete within 5 mins, giving rise to a remarkable, and
reproducible, TOF of 1188 hꢀ1. This far exceeds any reported value for the hydroboration of CO2
yielding methanol equivalents, catalyzed by nonꢀtransition metal species.6 Furthermore, this
includes situations where more reactive boranes (e.g. BH3, 9ꢀBBN) and higher temperatures are
employed.6 In the current case, the reactions are so rapid, that possible intermediates could not be
observed spectroscopically.
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Computational Studies. So as to gain an understanding of the mechanisms of the reductions
mentioned above, computational studies (ωB97xD) were employed to probe the profile of the
reaction between CO2 and HBpin, catalyzed by 1, as a test case. It should be mentioned that a
variety of pathways were identified for this reaction, though the majority were found to lead to
reaction deadꢀends, or encountered one or more transition states en route to the final products,
(pinB)2O and MeOBpin, that were considered too high in energy to be viable (see ESI). With that
said, two routes were uncovered that had similar energy barriers to their progression, and which
were deemed viable. Schematic representations of the catalytic cycles are depicted in Scheme 1,
while the energies of the reaction profiles can be found in Figures 2 and 3.
The first thing to mention about the calculated catalytic cycles is that reaction pathways
very similar to both of those described here have previously been proposed (but not
computationally assessed) for main group metal hydride catalyzed reductions of CO2 to methanol
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equivalents.10 Secondly, the first two steps, leading to germaꢀ/boraꢀacetal intermediate 6, are
common in both cycles. In both, the initial reaction is the hydrogermylation of CO2, giving
intermediate int-B (Figure 2), which upon isomerization affords the energetically more stable and
experimentally observed (see below) germanium formate, 3 (ꢁH = ꢀ24.2 kcal/mol), with a low
kinetic barrier (ꢁH≠ = 13.5 kcal/mol). As has been shown previously, calculated CO2 reduction
mechanisms typically involve an initial nucleophilic attack of catalyst at the Cꢀcenter of CO2, or
dual site Lewis acid/Lewis base activation of CO2.6 Both possibilities were considered here, with
the first one calculated as being operative. Regarding the second case, we explored the reaction of
CO2 with two molecules of ambiphilic 1, i.e. one molecule acting as a Lewis acid through its
empty pꢀorbital, and one as a Lewis base through its Ge lone pair. In addition, the
hydrogermylation of CO2 by the GeꢀGe bonded dimer of 1, viz. L†(H)Ge=Ge(H)L† (i.e. as the
compound exists in the solid state), was investigated, but no realistic pathway could be found in
the last two cases.20
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Scheme 1. Two cycles calculated for the reduction of CO2 to MeOBpin, catalyzed by 1.
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Figure 2. Calculated (DFT: ωB97xD/6ꢀ311G(d,p)//6ꢀ31G(d,p)) reaction profile for the formation
of intermediate 6.
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Figure 3. Two calculated (DFT: ωB97xD/6ꢀ311G(d,p)//6ꢀ31G(d,p)) reaction profiles for the
formation of MeOBpin and (pinB)2O from intermediate 6 ([Ge] = L†Ge). Route A (top) and
Route B (bottom). The areas encircled in red are common to both pathways.
Once formate 3 is formed, it undergoes a cycloꢀaddition reaction with HBpin across one
GeO bond, to give sixꢀmembered transition state, TSC-D. This then leads to int-E, which can been
described as an adduct of pinBC(=O)(H) with complex 1. After a low energy hydrogen migration
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to the electrophilic carbon of the formate fragment, complex 6 is formed.21 The possibility that
the reaction between 3 and HBpin instead involved a formal σꢀbond metathesis reaction, yielding
free 1 and pinBC(=O)(H), which then react together to give 6, was also examined. Although
similar reactivity has been suggested for main group catalyzed CO2 reductions in the past,10
calculations here show it to be a reaction deadꢀend.22 Once formed, intermediate 6 can proceed to
the final products via two similar pathways. In route A (Scheme 1) it undergoes a σꢀbond
metathesis reaction with HBpin to regenerate 1 and give (pinBO)2CH2 5. Given the near
equivalent enthalpies of the products and reactants of this stage of the reaction, they could be in
equilibrium with each other. Compounds 1 and (pinBO)2CH2 then react in the rate determining
step (ꢁH‡ = 36.0 kcal/mol), yielding MeOBpin and the experimentally observed (see below)
germanium(II) borate ester, 7. The latter readily reacts with the final equivalent of HBpin to give
catalyst, 1, and the diboroxane product, (pinB)2O. This seems to be the most dominant pathway
that is operational in the experimental situation, as 3, 5, and products, MeOBpin and (pinB)2O,
were all spectroscopically observed, and their concentrations shown to be interdependent, during
the course of the reaction. The rather large kinetic barrier to the formation of (pinBO)2CH2 5 is
consistent with the slow rate of the overall reaction (TOF = 0.4 hꢀ1).
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The alternate catalytic cycle (route B, Scheme 1) involves elimination of formaldehyde
from 6, to give 7, as the rate determining step. The barrier to this process (ꢁH‡ = 36.7 kcal/mol)
is only slightly higher than that for route A. Intermediate 7 in route B undergoes a σꢀbond
metathesis reaction with HBpin to give 1 and (pinB)2O, the former of which hydrogermylates
formaldehyde to give the germanium methoxide, 8. Finally, this reacts with HBpin to give the
methanol equivalent, MeOBpin, and catalyst 1. Since this is as kinetically viable as route A, the
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two routes may operate simultaneously, despite the experimental observation of significant
quantities of (pinBO)2CH2 (derived from Route A) in the catalytic reaction.
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Although computational limitations did not allow for calculation of the fully optimized
reaction profiles (see ESI) for the reduction of CO2 catalyzed by 2, using either HBpin or HBcat
as hydrogen sources, it was ascertained that these profiles were similar to those involving 1,
including the rate determining process that led to (pinBO)2CH2. The approximate barriers to this
process were ꢁH‡ = 34 kcal/mol (HBpin) and 19 kcal/mol (HBcat), which are consistent with the
significantly enhanced experimental reaction rates (TOF = 14.5 hꢀ1 and 1188 hꢀ1 respectively),
relative to that achieved using 1 as the catalyst.
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Stoichiometric Reactivity Studies. With a view to add weight to the validity of the catalytic
cycles discussed above, a series of stoichiometric reactivity studies involving 1, 2, CO2 and/or
HBpin were carried out, with some interesting results. Firstly, it was confirmed that 1 and 2 react
with CO2, in the absence of HBpin, to give the corresponding formates, 3 and 4 (Scheme 2).
These compounds could alternatively be prepared in good yield via reaction of amidoꢀgroup 14
halides, L†EX (E = Ge, X = Cl; E = Sn, X = Br) with potassium formate, KOC(H)O. Both 3 and
4 were crystallographically characterized as monomers with the formate ligands κ2ꢀO,O'ꢀ
chelating their metal centers (see Figure 4 for the molecular structure of 4). This situation
contrasts with related complexes incorporating bulky bidentate amide ligands, in which the
formate acts as a monodentate ligand to the metal centre, e.g. (DipNacnac)EOC(H)=O (E = Ge or
Sn).23 Also, this thermodynamic stability, favoring the chelated over monodentate isomers, is
reflected in the estimated enthalpy energy difference (ꢁH= 3.5 kcal/mol) as depicted in Figure 2
1
(int-B vs. 3). The H NMR spectra of the pure compounds are identical to those of the reaction
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components predicted to be metal formates in the catalytic studies described above. Moreover,
when samples of 3 or 4 were added to large excesses of HBpin in C6D6 solutions under CO2
atmospheres, the formation of MeOBpin and (pinB)2O proceeded in a similar fashion as when the
metal hydrides, 1 or 2, were used as catalysts. This gives good evidence that 3 and 4 are, indeed,
intermediates in the catalytic cycle(s) in operation.
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Interestingly, while the reaction of 2 with excess CO2 rapidly, and essentially
quantitatively, led to 4, the reaction of 1 with CO2 was slower. In addition to 3, several other
products were spectroscopically observed in the reaction mixture (Scheme 2). Two of these, the
unprecedented digermaꢀacetal, 9, and the bis(germylene)ether, 10, were crystallized from the
1
reaction mixture and structurally authenticated (Figure 4). Their H NMR spectra were identical
to those of two of the components in the reaction mixture resulting from the treatment of 1 with
CO2. It seems likely that 9 is formed because the reaction of 1 with CO2 is slow enough that a CO
bond of formed 3 can undergo a 1,2ꢀaddition reaction with 1. This was confirmed by reacting a
1:1 mixture of 1 and 3, in the absence of CO2 or HBpin, which gave 9 in a moderate isolated
yield.
Compound 10 could result from elimination of formaldehyde from thermally unstable 9.
1
This possibility was tested by heating a solution of pure 9 in C6D6 for 2 hrs at 60 °C. The H
NMR spectrum of the mixture confirmed that 9 had largely decomposed to a mixture of 10, 1, 3
and another product, believed to be the germanium methoxide, 8. Compounds 1 and 3 could
result from a βꢀhydride elimination process involving the bridging methylene unit of 9 (cf.
ambient temperature reversible βꢀhydride eliminations from L†Ge(alkyl) compounds15), while 8
could be generated by hydrogermylation of formaldehyde by 1. Unfortunately efforts to rationally
synthesize 8 to confirm its identity in the reaction mixture were not successful. On the other
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hand, 10 was alternatively synthesized in good yield by treating a solution of the amidoꢀ
digermyne, L†GeGeL†, with N2O, as we have previously reported for closely related amidoꢀ
digermynes.24 Although this reactivity is interesting, compound 10 is unreactive towards HBpin.
Therefore the series of reactions leading to it are not likely to be integral to, or even occurring in,
the experimental reduction of CO2 by HBpin, in the presence of catalyst 1.
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Figure 4. Thermal ellipsoid plots (25% probability surface) of (a) compound 4, (b) compound 7,
(c) compound 9, and (d) compound 10.
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Scheme 2. Synthesis of compounds 3 and 4, and proposed pathways to 8ꢀ10 from 3.
As a means to gaining further information on the role of formates 3 and 4 in a catalytic
context, toluene solutions of each were reacted with one equivalent of HBpin, in the absence of
CO2, at ambient temperature. These reactions led to one major product as determined by 1H NMR
spectroscopic analyses of the mixtures. Concentrating and cooling of the reaction solutions
afforded good yields of the germanium(II) and tin(II) borate esters, 7 and 11 (Scheme 3). Both of
these were crystallographically characterized (see Figure 4 for the molecular structure of 7), and
represent the first structurally authenticated examples of pꢀblock metalloꢀborate esters. Variable
temperature NMR spectroscopic analyses (ꢀ80 to 25 °C, d8ꢀtoluene solutions) of the 1:1 reactions
were enlisted to try and identify any intermediates. However, although signals for several species
were observed upon warming, and before ultimate conversion to 7 or 11, nothing definitive could
be concluded about the nature of the intermediates. With that said, the fact that reactions of the
formates 3 and 4 with one equivalent of HBpin gave high spectroscopic yields of the metalloꢀ
borate esters, 7 and 11 (presumably also eliminating formaldehyde), suggests that they proceed as
per route B (Scheme 1). If they were to proceed via the sequence in route A, two equivalents of
HBpin would be required for complete conversion to 7 and 11, and MeOBpin would be generated
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as an observable byꢀproduct. This result is, of course, in contrast to the catalytic studies
mentioned above, which were surmised to proceed via route A, on the basis of spectroscopic
analyses of the reaction mixtures. However, those were carried out in the presence of very large
excesses of HBpin which would certainly favour route A. Whatever the case, 7 and 11 are likely
intermediates in both calculated catalytic cycles. Strong evidence for this comes from the fact that
when toluene solutions of both were treated with one equivalent of HBpin, the reactions cleanly
and rapidly generated approximately 1:1 mixtures of the metal hydride catalysts, 1 or 2, and the
diboroxane, (pinB)2O.
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Scheme 3. Synthesis of compounds 7 and 11, and their reactivity towards HBpin.
Conclusions
In summary, we have shown that bulky amidoꢀgermanium(II) and tin(II) hydride
complexes, which are twoꢀcoordinate in solution, can be extremely efficient and highly selective
singleꢀsite catalysts for the reduction of CO2 to methanol equivalents, using secondary boranes as
the hydrogen source. Moreover, the amidoꢀtin(II) hydride is the most active nonꢀtransition metal
catalyst yet reported for such reductions, yielding TOF's as high as 1188 hꢀ1. Computational
studies have identified two thermodynamically and kinetically viable catalytic cycles by which
these reductions may operate. Following the course of the reactions using spectroscopic
techniques leads to the conclusion that one of these reaction pathways predominates in the
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experimental situation. A series of related stoichiometric reactions have shed further light on the
reaction mechanisms in operation, and indicate that involvement of the second reaction pathway
cannot be ruled out. As a whole, this study further indicates that relatively cheap and low toxicity
main group complexes can rival the practicality of transition metal based systems in catalytic
transformations of small molecules. We continue to explore and develop this area of chemistry.
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Supporting Information Available: Details of the synthesis and characterizing data for all new
compounds. Full details and references for the catalysis, crystallographic and computational
studies. Crystallographic data in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
Acknowledgement. This research was supported by the Australian Research Council (grant to
CJ) and the US Air Force Asian Office of Aerospace Research and Development (grant FA2386ꢀ
14ꢀ1ꢀ4043 to CJ). Part of this research was undertaken on the MX1 beamline at the Australian
Synchrotron, Victoria, Australia. LM acknowledges the Alexander von Humboldt Foundation for
a grant of experience researcher, the Chinese Academy of Science for an senior researcher grant,
and CAlMip for computational time.
*Author to whom correspondence should be addressed. Eꢀmail: cameron.jones@monash.edu,
laurent.maron@irsamc.upsꢀtlse.fr
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(a) IPCC, Climate Change 2007: Synthesis Report, Intergovernmental Panel on Climate
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Selected reviews: (a) Kortlever, R.; Shen, J.; Schouten, K. J. P.; CalleꢀVallejo, F.; Koper,
T. M. J. Phys. Chem. Lett. 2015, 6, 4073ꢀ4082. (b) Costentin, C.; Robert, M.; Saveant, J.
M. Chem. Soc. Rev. 2013, 42, 2423ꢀ2436. (c) Takeda, H.; Ishitani, O. Coord. Chem. Rev.
2010, 254, 346ꢀ354. (d) Benson, E. E.; Kubiak, C. P.; Sathrum, A. J.; Smieja, J. M. Chem.
Soc. Rev. 2009, 38, 89ꢀ99.
(5)
Review: Wang, W.ꢀH.; Himeda, Y.; Muckerman, J. T.; Manbeck, G. F.; Fujita, E. Chem.
Rev. 2015, 115, 12936ꢀ12973.
(6)
(7)
Review: Chong, C. C.; Kinjo, R. ACS Catal. 2015, 5, 3238ꢀ3259.
Examples of relevant transition metal catalyzed CO2 reductions that appeared after review
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Commun. 2016, 52, 14262ꢀ14265. (b) Lu, Z.; Williams, T. J. ACS Catal. 2016, 6, 6670ꢀ
6673.
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(9)
Abdalla, J. A. B.; Riddlestone, I. M.; Tirfoin, R.; Aldridge, S. Angew. Chem. Int. Ed.
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(c) Wang, T.; Stephan, D. W. Chem. Commun. 2014, 50, 7007ꢀ7010. (d) das Neves
Gomes, C.; Blondiaux, E.; Thuéry, P.; Cantat, T. Chem. Eur. J. 2014, 20, 7098ꢀ7106. (e)
Blondiaux, E.; Pouessel, J.; Cantat, T. Angew. Chem., Int. Ed. 2014, 53, 12186ꢀ12190. (f)
Mukherjee, D.; Shirase, S.; Spaniol, T. P.; Mashima, K.; Okuda, J. Chem. Commun. 2016,
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(12) Selected recent reviews: (a) Power, P. P. Nature 2010, 463, 171ꢀ177. (b) Power, P. P. Acc.
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(18) Takagi, N.; Sakaki, S. J. Am. Chem. Soc. 2013, 135, 8955ꢀ8965.
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(19) Bontemps, S.; Vendier, L.; SaboꢀEtienne, S. Angew. Chem. Int. Ed. 2012, 51, 1671ꢀ1674.
(20) N.B. It was also experimentally determined that L†GeH 1 does not react, or form an
adduct with, HBpin. Therefore, it is assumed that 1 is not activated towards reaction with
CO2 in the presence of HBpin.
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(21) N.B. Similar reactivity patterns have been proposed, computationally, for CO2/HBcat
reduction reactions, catlyzed by nickel pincer complexes. Huang, F.; Zhang, C.; Jiang, J.;
Wang, Z. X.; Guan, H. Inorg. Chem. 2011, 50, 3816ꢀ3825.
(22) N.B. The 1,2ꢀaddition reaction of HBpin across one CO bond of 3, to give 6, was also
examined computationally (ꢁH‡ = 29.2 kcal/mol), but found to be kinetically less
accessible (see Figure S4 in SI).
(23) (a) Jana, A.; Ghoshal, D.; Roesky, H. W.; Objartel, I.; Schwab, G.; Stalke, D. J. Am.
Chem. Soc. 2009, 131, 1288ꢀ1293. (b) Jana, A.; Roesky, H. W.; Schulzke, C.; Doering, A.
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Efficient Reduction of Carbon Dioxide to Methanol Equivalents Catalyzed by Two-
Coordinate Amido-Germanium(II) and Tin(II) Hydride Complexes
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Terrance J. Hadlington,† Christos E. Kefalidis,‡ Laurent Maron*,‡ and Cameron Jones*,†
†Monash Centre for Catalysis, School of Chemistry, Monash University, P.O. Box 23, Melbourne,
Victoria, 3800, Australia
‡Université de Toulouse et CNRS, INSA, UPS, UMR 5215, LPCNO, 135 Avenue de Rangueil, F-
31077 Toulouse, France
FOR TABLE OF CONTENTS ENTRY ONLY
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