Activation of CO2 by phosphinoamide hafnium complexes

Article abstract of DOI:10.1039/c3cc38286a

Hf-phosphinoamide cation complexes behave as metal-based frustrated Lewis pairs and bind one or two equivalent of CO₂ and in as well can activate CO₂ in a bimetallic fashion to give a pseudo-tetrahedral P₂CO₂ fragment linking two Hf centres. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3cc38286a

Chemical Communications
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Volume 49 | Number 26 | 4 April 2013 | Pages 2581–2720
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COMMUNICATION
Michael J. Sgro and Douglas W. Stephan
Activation of CO2 by phosphinoamide hafnium complexes

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Activation of CO2 by phosphinoamide hafnium
complexes†
Cite this: Chem. Commun., 2013,
49, 2610
Michael J. Sgro and Douglas W. Stephan*
Received 17th November 2012,
Accepted 27th January 2013
DOI: 10.1039/c3cc38286a
www.rsc.org/chemcomm
Hf–phosphinoamide cation complexes behave as metal-based and Milstein groups have recently used ruthenium catalysts
frustrated Lewis pairs and bind one or two equivalent of CO2 containing the tridentate ligands (C5H3N)(CH2PtBu2)(C5H4N)
and in as well can activate CO2 in a bimetallic fashion to give a and (C5H3N)(CH2PtBu2)(CH2NEt2) for the catalytic reduction of CO2
pseudo-tetrahedral P2CO2 fragment linking two Hf centres.
to MeOH,12,13 exploiting systems capable of the cooperative activa-
tion of CO2 by the ligand and the metal center.29,30 Very recently we
Global warming and climate change are issues of considerable have been exploring complexes with phosphinoamine ligands,31,32
importance that are undeniably linked to the increase in the and reported a tris–aminophosphine ruthenium complex that can
atmospheric concentration of CO2.1 It is this reason that has trap CO2 in an FLP-like fashion.33 In this case, addition of boranes
prompted the flourishing interest in the reactivity of CO2, with results in the catalytic reduction of CO2 to MeOBR2 and R2BOBR2.
extensive efforts on-going to develop strategies to effect its capture In seeking to extend the range of complexes capable of CO2 capture,
and storage. For example, materials such as zeolites, aluminas, we were prompted to explore early metal phosphinoamide
activated carbons2 and metal organic frameworks (MOFs)3–5 have complexes. Herein we report the synthesis of Hf complexes
been explored in this regard. An alternative approach to address of such ligands and demonstrate that the weakly interacting
the increasing atmospheric CO2 level is to employ CO2 as a C1 phosphine donors afford FLP-like reactivity to capture CO2. Such
feedstock.6–11 This latter approach has generated interest in the derivatives are shown to either capture up to two equivalents of
development of catalysts for the reduction of CO2, with Ru,10–14 CO2 or afford the unprecedented FLP-like double activation of
Ir15 and Fe16,17 receiving particular attention.
CO2 by metal-based Lewis acids and phosphine donors.
The preparation of phosphinoamide complexes was under-
As an alternative to the more conventional organometallic
approach to the activation of small molecules, the past few years taken via the reaction of the phosphinoamines S(CH2CH2NHPR2)2
has seen the concept of ‘‘frustrated Lewis pairs’’ (FLPs) emerge as (R = Ph 1, iPr 2) with Hf(CH2Ph)4 in THF. This generated
an effective strategy.18–20 In the case of CO2, since the initial report complexes S(CH2CH2NPR2)2Hf(CH2Ph)2 (R = Ph 3, iPr 4) in 78%
of CO2 capture by FLPs, the chemistry has indeed been expended and 72% yield, respectively (Scheme 1). The 31P{1H} NMR spectra
to effect reduction. In this context, Ashley et al.21 demonstrated of these products display singlet resonances at 4.0 ppm and
the conversion of CO2 to MeOH under forcing conditions. Sub- 11.8 ppm, respectively, and the in situ 1H NMR spectra confirm
sequently we reported the stoichiometric conversion of CO2 to the loss of the NH functionalities and the formation of toluene.
methanol22 or CO23 using Al/P based FLPs. In related work, Piers In addition, triplet resonances were observed at 2.62 ppm (3)
and coworkers described the catalytic deoxygenative hydrosilation and 2.42 ppm (4) in the 1H NMR spectra and these signals
of CO2, generating methane.24
correlate with 13C{1H} resonances at 73.2 and 73.8 ppm, respec-
More recently, transition metal systems capable of FLP-like tively. Interestingly these resonances exhibit coupling to ligand
reactivity have been described.25–28 Wass et al. have demonstrated
the ability of metallocenium phosphinoaryloxide complexes to
capture CO2 in a FLP like fashion between the Lewis acidic metal
center and the pendant phosphine.26 In a related sense, the Sanford
Department of Chemistry, University of Toronto, Toronto, Ontario,
Canada M5S 3H6. E-mail: dstephan@chem.utoronto.ca
† Electronic supplementary information (ESI) available: Synthetic and spectro-
scopic data are deposited. CCDC 911093–911095. For ESI and crystallographic
Scheme 1 Synthesis of 3 and 4.
data in CIF or other electronic format see DOI: 10.1039/c3cc38286a
c
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Fig. 1 POV-ray depiction the molecular structure of 3. C: black, P: orange, N:
aquamarine, S: yellow, Hf: slate blue; all H-atoms are omitted for clarity.
Fig. 3 POV-ray depiction of the cation of the molecular structure of 5. C: black,
P: orange, N: aquamarine, O: red, S: yellow, Hf: slate blue; phenyl carbon from
PPh2 except Cipso omitted for clarity. All H-atoms are omitted for clarity.
plates of 5 in 62% yield. An X-ray diffraction study of 5
revealed the capture of two equivalents of CO2 by two
Hf complexes yielding the twofold symmetric bimetallic species
[S(CH2CH2NPPh2)2Hf(CH2Ph)(CO2)]2[B(C6F5)4]2 (Scheme 2). Two
pendant phosphines from the two complexes bind to each of the
Fig. 2 POV-ray depiction of the molecular structure of 4. C: black, P: orange, N:
aquamarine, S: yellow, Hf: slate blue; all H-atoms are omitted for clarity.
2
phosphorus atoms (3JHP = 5 Hz, JCP = 7 Hz). In each case, these C atoms of the two CO2 molecules while the oxygen atoms are
signals were attributed to the methylene groups of two remaining bound to the two Hf centers (Fig. 3). The P–C bond lengths of
benzyl groups on Hf. Single crystal X-ray diffraction studies of 3 and 1.878(2) and 1.880(2) Å are similar to those observed for the
4 reveal unique coordination geometries about Hf (Fig. 1 and 2) Zr–FLP system [Cp2Zr(OC6H4PtBu2CO2)]+ described by Wass and
in which the metal is coordinated to the N2S donors of the coworkers (1.892(4) Å). The C–O bonds of 1.383(3) and 1.369(3) Å
phosphinoamide ligand and two benzyl carbons. The Hf–N and are both significantly longer than those previously described in
Hf–S distances were found to be 2.052(2), 2.071(2) and 2.8939(8) Å, B/P and Al/P-based FLP–CO2 complexes consistent with the
respectively. The latter distance reflects the soft nature of the strong Lewis acidity of the Hf centers. Although Cummins and
thioether S donor. In addition, the P atoms also interact with the coworkers35 have reported a bimetallic species in which Ta
Hf center to some degree. For 3, the Hf–P distances are 2.6897(8) and centers are linked by a OCH2O2À fragment derived from CO2
3.0743(8) Å, while for 4, the two Hf–P distances were found to be insertion in the corresponding Ta-hydride species, to the best of
2.7212(14) and 2.8244(11) Å. The longer Hf–P distance in 3 suggests a our knowledge, 5 represents the only example in which CO2 is
very weak interaction consistent with lower basicity of the PPh2 doubly activated by phosphine donors that has been crystallo-
fragment in comparison to the PiPr2 units in 4. The shorter graphically characterized. In this regard, Fontaine and coworkers
distances in 4 are consistent with the greater basicity of the P have described the species (Me2PCH2AlMe2)2(CO2) in which two
centers. Previously reported Hf–P bonds in phosphine adducts P–C bonds are also formulated.36
of HfCl4 range from 2.808(2) to 2.825(5) Å (ref. 34) while
Complex 5 exhibited poor solubility in most organic solvents,
Wass et al. reported a P–Hf distance of 2.8209(6) Å in the cation but NMR data were obtained in d8-THF. Employing 13CO2, a
[Cp2HfOC6H4PtBu2]+.25 The differences in the N–Hf–P angles in 3 of doublet at 168.1 ppm with a JCP of 138.9 Hz is observed in the
1
31.05(7) and 38.13(6)o parallel the Hf–P distances and may arise 13C{1H} NMR spectrum. The corresponding 31P{1H} NMR spectrum
from the steric congestion about the metal. In the case of 4 the shows a doublet at 36.3 ppm (1JPC 138.9 Hz) and a singlet at
N–Hf–P angles are 35.83(9) and 37.3(1)o.
2.5 ppm. These data infer that 5 is monomeric in THF solution.
Addition of [Ph3C][B(C6F5)4] to 3 or 4 in C6D5Br generates the Variable temperature NMR studies to À80 1C showed no evidence
corresponding cationic species by benzyl abstraction. In the case of dimer formation.
of the reaction of 3, further slow addition of an atmosphere of
Subjecting a solution of 5 in THF to 1 atm of CO2 prompted
CO2 over 12 h, results in the subsequent isolation of colorless quantitative formation of a new product, 6, as evidenced by
c
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activation of other small molecules and developing new
approaches to catalysis.
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´
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´
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¨
¨
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