Bypassing a highly unstable frustrated Lewis pair: Dihydrogen cleavage by a thermally robust silylium-phosphine adduct

Article abstract of DOI:10.1039/c4cc05905k

The thermally robust silylium complex [iPr₃Si-PtBu₃]⁺[B(C₆F₅)₄]^- (1) activates H₂/D₂ at 90 °C (PhCl); no evidence for dissociation into the separated Lewis pair is found. DFT calculations show H₂ cleavage proceeds via Si-P bond elongation to form an encounter complex directly from the adduct, thus avoiding the non-isolable iPr₃Si⁺-PtBu₃ FLP. This journal is

Full text of DOI:10.1039/c4cc05905k

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Bypassing a highly unstable frustrated Lewis pair:
dihydrogen cleavage by a thermally robust
silylium–phosphine adduct†
Cite this: Chem. Commun., 2014,
50, 12753
Received 29th July 2014,
Thomas J. Herrington, Bryan J. Ward, Laurence R. Doyle, Joe McDermott,
Andrew J. P. White, Patricia A. Hunt* and Andrew E. Ashley*
Accepted 26th August 2014
DOI: 10.1039/c4cc05905k
www.rsc.org/chemcomm
The thermally robust silylium complex [iPr3Si–PtBu3]+[B(C6F5)4]ꢀ (1) acti- in which decomposition of the robust [B(C6F5)4]ꢀ WCA was observed
vates H2/D2 at 90 8C (PhCl); no evidence for dissociation into the separated when heated in the presence of Et3Si+,9 presumably via [C6F5]ꢀ
Lewis pair is found. DFT calculations show H2 cleavage proceeds via Si–P abstraction or C–F activation.10 Such indiscriminate reactivity may
bond elongation to form an encounter complex directly from the adduct, be suppressed via incorporation of intramolecular donors which
thus avoiding the non-isolable iPr3Si+–PtBu3 FLP.
moderate the potent electrophilicity while preserving sufficient
reactivity for catalysis. In this respect, Oestreich and co-workers used
Since their discovery in 2006, ‘frustrated Lewis pairs’ (FLPs) the ferrocene-stabilised silylium ion [(C5H5)Fe(C5H4SitBuMe)]+ to
have continued to provide a fresh and novel approach to the selectively and catalytically hydrosilylate various ketones to the
field of bond activation chemistry.1 FLPs can be defined as corresponding alkyl/silyl ethers (RR0CH–O–SiR3; R/R0 = alkyl);11 by
combinations of a Lewis acid and base which, by dint of steric contrast the fully reduced alkanes R–CH2–R012 can be isolated when
hindrance, are unable to datively bind in the classical manner, the ‘untamed’ ‘[R3Si]+’ is used as catalyst. Steric protection of the Si
leading to unquenched reactivity capable of activating small centre has also been adopted to prevent undesired silylation of
molecules. One of the most exciting properties of FLPs is their aromatic solvents, as documented by Mu¨ller et al. Here, bulky
ability to induce heterolytic cleavage of H2 into protic and triarylsilylium compounds (Ar3Si+; Ar = C6H6ꢀxMex, x = 3–5), in
hydridic components (H+/Hꢀ),2 which can subsequently be conjunction with phosphine (R3P; R = alkyl, aryl) or silylene bases,13
transferred to reducible substrates either stoichiometrically are shown to generate FLPs that engage in H2 cleavage. Nonetheless,
(e.g. carbonyls, CO2)3 or catalytically (e.g. imines, silyl enol eventual solution-phase decomposition of these FLPs under ambient
ethers, alkenes).4 By far the most commonly used Lewis acids conditions was attributed to the long-term instability of the Ar3Si+
are organometallics of the Group 13 elements (e.g. R3E; E = B or ions, which leads to protonated arene products.14
Al, R = electron-deficient organyl), among which B(C6F5)3
Herein we report the synthesis and characterisation of a
remains prevalent. Silylium ions (R3Si+) are both isoelectronic classical donor–acceptor complex between tBu3P and the less
and isolobal with this class of compounds and, because of their sterically encumbered, highly reactive, silylium ion iPr3Si+. We
potent electrophilicity,5 can demonstrate similar FLP reactivity show that the use of a Lewis adduct considerably stabilises the
when employed as the Lewis acid partner. However, there are R3Si+ moiety, in comparison with the previously studied Ar3Si+
few reports of stable R3Si+ cations, which is attributed to the species. Furthermore, this species is shown to heterolytically
difficulties in stabilising the diffuse 3p Si valence orbital either cleave H2, the mechanism of which avoids the formation of a
through hyper- or p-conjugation.6 This property explains, in presumably highly unstable iPr3Si+–PtBu3 separated Lewis pair.
part, their voracious affinity for nucleophiles; even traditionally
Treatment of iPr3SiH with [Ph3C][B(C6F5)4] (Bartlett–Condon–
‘inert’ arene solvents7 and weakly coordinating anions (WCAs)8 Schneider hydride transfer)15 in chlorobenzene afforded solutions
can demonstrate interactions with the silicon centre. The work of [iPr3SiꢁClPh]+, as previously described.16 Subsequent in situ
of Ozerov et al. reflects this extreme appetite for s and p donors, reaction with tBu3P furnished [iPr3Si–PtBu3]+[B(C6F5)4]ꢀ (1), upon
precipitation with hexanes and recrystallisation from PhCl, in
1
Department of Chemistry, Imperial College London, Exhibition Road,
London SW7 2AZ, U.K. E-mail: a.ashley@imperial.ac.uk;
excellent yield (Scheme 1). 1 has been characterised by H, 13C,
29Si and 31P NMR spectroscopy, high resolution MS (ES+), and
elemental analysis (see ESI†).
Web: http://www3.imperial.ac.uk/people/a.ashley
† Electronic supplementary information (ESI) available: Experimental section
including computational details, characterisation data and copies of the NMR
spectra compounds and reactions. CCDC 970728. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c4cc05905k
Slow cooling of a PhF solution of 1 to ꢀ25 1C produced large
colourless plates which were suitable for single crystal X-ray
diffraction,‡ and the solid state structure is shown in Fig. 1.
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However, the coupling constant is noticeably smaller than
those reported for [(C6Me5)3Si–PEt3]+[B(C6F5)4]ꢀ or zwitterionic
[Mes2(SiPh2H)P+CH2CH2BꢀH(C6F5)2] (1JSiP = 41.2 and 48.5 Hz,
respectively),14,18 and the 31P NMR resonance (d = 57.3 ppm,
298 K) is close to that observed for free tBu3P (d = 62.0 ppm,
298 K); collectively these data would suggest a weak Si–P bond.
In contrast to the results obtained using the silylium FLPs,
(C6Me5)3Si+/PR3,14 admission of H2 to 1 (4 bar, C6D5Cl solvent) at
room temperature led to no discernible reaction. However, heating
these solutions to 90 1C led to complete consumption of the
adduct (8 hours; Scheme 1), concomitant with formation of
iPr3Si-H (1H NMR, d = 3.43 ppm) and phosphonium borate
Scheme 1 Synthesis of 1 and subsequent reactivity with H2/D2 (C6D5Cl or
C6H5Cl solution, 0.096 M; H2 and D2 experiments respectively, 4 bar).
Reagents and conditions: (a) (i) [Ph3C][B(C6F5)4], PhCl, RT, –Ph3CH; (b)
tBu3P added in situ.
1
[tBu3P-H]+[B(C6F5)4]ꢀ (1H NMR, d = 4.17 ppm, JHP = 430 Hz; 31P
{1H} NMR, d = 60.3 ppm) in high conversion (90–94%, four runs).19
Conducting these experiments under D2 (C6H5Cl solvent) gave the
2
deuterated products iPr3Si-D and [tBu3P-D]+, as shown by H, 31P
and 29Si NMR spectroscopy (see ESI† for details);20 this conclu-
sively shows that H2/D2 is the source of H/D atoms in the formally
hydridic silane, and protic phosphonium ion.21 Upon reaction
completion, 19F and 11B NMR spectra showed only resonances
corresponding to the [B(C6F5)4]ꢀ counterion, demonstrating that
silylium-mediated decomposition of the anion had not occurred.
Since neither [H–B(C6F5)3]ꢀ nor B(C6F5)3 could be observed in
solution, the possibility that H2 cleavage involves the known tBu3P/
B(C6F5)3 FLP pathway17 may be discounted.
In order to investigate the possibility that 1 may dissociate in
PhCl solution to generate tBu3P and solvated silylium ion, [iPr3Siꢁ
ClPh]+, a variable temperature (VT) 31P NMR experiment was
conducted (Ph3P internal capillary reference; see Fig. S7 in ESI†).
At low temperature the spectrum shows a very broad resonance
(d = 54.5 ppm, ꢀ40 1C) which moves progressively downfield
(d = 60.6 ppm, 100 1C), and markedly sharpens. However, it should
be noted that the 31P NMR resonance for tBu3P also moves by ca.
Fig. 1 ORTEP plot of the X-ray structure of 1. C atoms blue, P atom
orange, B atom pink, F atoms green and Si atom light brown. H atoms have
been removed for clarity, and thermal ellipsoids are shown at 50% prob-
ability; Si1–P1 = 2.4843(5) Å.
The [iPr3Si–PtBu3]+ fragment in 1 is derived from a donor– Dd = 6 ppm over the same temperature range and the behaviour is
acceptor interaction between the iPr3Si+ moiety and tBu3P, as likely due to a temperature-induced shift for both species. Further-
exemplified by the pyrimidalisation about the Si atom more, addition of tBu3P (1–10 eq., PhCl, 100 1C) to 1 produced
(0.5765(11) Å deviation from the plane of the three C atoms in 31P NMR spectra consisting only of their separate respective reso-
the C3Si unit). The C–Si–C bond angles, which range between nances; neither a discernible change in the lineshape nor chemical
110.51(8) and 111.86(9)1, are much closer to the idealised tetra- shift position of the adduct was observed. If rapid exchange between
hedral angle (109.51) than those found in iPr3Si(CHB11H5Cl6) 1 and appreciable concentrations of dissociated products were
(117.31),8 which possesses significant silylium character and indeed occurring, introduction of extraneous tBu3P would be
hence approaches a trigonal geometry. The [B(C6F5)4]ꢀ anions expected to lead to a significant perturbation of the 31P NMR
are well separated from the cations, with no close Si to F contacts, resonance of 1. Finally, we synthesised [iPr3SiꢁClPh]+[B(C6F5)4]ꢀ in
and hence are non-coordinating. However, the Si–P bond distance order to investigate its reactivity under the conditions of H2 activa-
is rather long (2.4843(5) Å; within the top 2% of those reported in tion, in the absence of added phosphine: at 90 1C this species
the CSD), and may be compared to [PhMe2Si–PtBu3]+[HB(C6F5)3]ꢀ degraded (40 min) via decomposition of the anion, producing
1
(Si–P = 2.376(2) Å), which has been prepared from reaction of B(C6F5)3, iPr3SiF (19F NMR d = ꢀ185.0 ppm; JFSi = 298 Hz) and
PhMe2SiH and the FLP system tBu3P/B(C6F5)3.17 This increased other unidentified products. This implies that, if dissociation of
distance may be attributed to the higher degree of steric strain tBu3P from 1 were to occur and generate iPr3Si+ or (more likely)
due to crowding between the organic groups along the Si–P axis. [iPr3SiꢁClPh]+, the rate at which this cation reacts with [B(C6F5)4]ꢀ
PhCl solutions of 1 proved to be stable for at least several would greatly exceed the observed rate of H2 activation.
months at room temperature, and these can be heated at 90 1C
In order to determine whether thermally-induced dissocia-
for 24 h without evidence of decomposition. H and 13C NMR tion of 1 (and hence a typical FLP-mediated mechanism) was
spectra (C6D5Cl; 298 K) of 1 are commensurate with the solid responsible for H2 heterolysis, we examined this system further
state structure, and the upfield 29Si resonance (d = 43.1 ppm, using DFT calculations. The results of our computational
1JSiP = 23 Hz) reveals the Si–P bond to be intact in solution. calculations (M06-2X/6-311+G(d,p) level of theory; see ESI†)
1
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which appear to hold the fragments in place, fulfilling a similar
role to those of the C–Hꢁ ꢁ ꢁF H-bonds reported for the frustrated
encounter complex [tBu3P]ꢁ ꢁ ꢁ[B(C6F5)3] (the precursor species to
H2 cleavage by that FLP system).23 It is notable that the Bꢁ ꢁ ꢁP
distance in the latter (4.20 Å) compares well with the Siꢁ ꢁ ꢁP
length in TSAB, yet both are appreciably shorter than that those
calculated for the ‘encounter complexes’ in Mu¨ller’s (C6Me5)3Si+/
PR3 systems (R = alkyl, aryl; range 4.45–5.73 Å); this likely reflects
the much greater steric bulk of the Ar3Si+ fragment in these ‘true’
FLPs.14 Heterolytic cleavage of H2 subsequently proceeds to give
[tBu3P-H]+ and H-SiiPr3 as a dihydrogen bonded intermediate
(B), after which dissociation to form the free products is strongly
entropically driven. The TS molecular orbitals exhibit H2 con-
tributions (Fig. S19, ESI†) while BCPs from H2 to both P and Si
are obtained (Table S5, ESI†). Moreover, the early TS exhibits
nascent NBO P - H2(s*) and H2(s) - Si electron transfer,
approximately equal to 8 and 34 kJ molꢀ1 respectively, which are
expected to increase as the reaction proceeds. Heterolysis of H2
in this manner is consistent with the electron transfer model,24
with no observable deviation in H–H distance between that in
TSAB and free H2, denoting an early transition state.
Scheme 2 Solvent-phase enthalpy (DH) and Gibbs free energy (DG)
profile for H2 activation mediated by 1 in PhCl solution, showing inter-
mediates and transition states along the reaction coordinate (T = 363 K).
P atom orange and Si atom pale green.
which took into account secondary (conductor-like polarizable
continuum model; C-PCM) solvent interaction, are presented in
Scheme 2 (and Table S1, ESI†). Various conformations of 1 were
identified (Table S2, ESI†), which are separated in free energy
by only ca. 13–30 kJ molꢀ1; this lends support to the solution-
phase VT 31P NMR data whereby the dynamic lineshape observed
for 1 at low temperatures can be explained through interconversion
between conformers. In total, two intermediates (A and B), and a
single transition state for their interconversion (TSAB) were located
along the reaction coordinate for H2 activation, starting from the
lowest energy conformer of 1.
On progressing to the transition state for H2 activation,
TSAB, the Siꢁ ꢁ ꢁP distance lengthens considerably (Table 1), to
an extent that is greater than the sum of the van der Waals radii
of the elements (3.90 Å).22 The incipient cavity accompanying
this bond elongation permits entry of a molecule of H2 while
attractive electrostatic Pꢁ ꢁ ꢁSi and secondary van der Waals
interactions between iPr3Si and tBu3P fragments lead to binding
beyond covalent distances, thus retarding tBu3P dissociation.
This factor explains the experimental observation that the rate of
H2 activation outcompetes decomposition, (which would be
anticipated to be the faster process if dissociation to a true
FLP were to occur at these elevated temperatures). A range of van
der Waals interactions give rise to QTAIM bond critical points
(BCP) between C–Hꢁ ꢁ ꢁH–C and C–Hꢁ ꢁ ꢁP (Tables S3 and S4, ESI†)
Formation of TSAB is both enthalpically unfavourable owing
to the weakening of the Si–P interaction, and entropically
unfavourable due to the increased ordering as a result of H2
coordination (Table S1, ESI†). The substantial energy barrier
(122.53 kJ molꢀ1; A-TSAB) associated with H2 activation is
testament to the strong Si–P dative bond in 1 and explains the
elevated temperatures required experimentally to achieve bond
elongation and access the encounter complex. Comparable
rate-determining steps have been observed elsewhere in the
literature; Lammertsma et al. report that the experimentally
observed insertion of CO2 into dimeric P/Al-based Lewis pairs
proceeds at room temperature, despite having computed a
significant energy barrier (ca. 140 kJ molꢀ1).25
Whilst a number of Lewis pairs have been documented that
exhibit classical/frustrated borderline reactivity with H2 or
alkynes,26 spectroscopic evidence for the existence of the dis-
sociated constituents has always been demonstrated, due to the
stability of the Lewis acid as an independent entity. In our
particular example, however, we have shown that the critical
development of an encounter complex prior to H2 activation
can be achieved directly from the Lewis pair adduct via simple
bond elongation and weakening, obviating the need for the
formation of a true FLP; this is especially important when the
Lewis acid partner (in our case iPr3Si+) is too reactive to isolate
in the free form.
In conclusion, we have shown that a classical donor–acceptor
adduct incorporating a highly electrophilic trialkylsilylium ion
can activate H2; this reaction is thus not rigidly confined to truly
separated R3Si+/base FLP combinations. Indeed, the formation
of a stable adduct incorporating such species may provide a
general strategy towards the protection of highly reactive Lewis
acids/bases in Lewis pair systems, without excluding such
systems from participating in characteristic FLP-type chemistry.
We are currently exploring the small molecule reactivity of
other [R3Si–(base)]+ adducts, in addition to investigating their
Table 1 Pertinent interatomic distances corresponding to selected H2
activation intermediates and transition states (Å)
Siꢁ ꢁ ꢁP
Hꢁ ꢁ ꢁH
Siꢁ ꢁ ꢁH
Pꢁ ꢁ ꢁH
1
A
TSAB
B
2.50
2.50
4.10
4.63
0.74a
0.74
0.74
2.34
5.36
2.87
1.50
6.09
2.98
1.41
Products
—
—
1.50
1.40
a
Bond length corresponding to free H2.
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potential use in the catalytic hydrogenation of unsaturated
substrates.
The authors wish to acknowledge the Royal Society for a
University Research Fellowship (AEA), the EPSRC for student-
ship funding (TJH and BJW), and Dr Richard Matthews for his
assistance with the QTAIM analysis.
19 1 proved to be highly reactive to even trace amounts of H2O.
Accordingly, the synthesis and solution-phase studies required
meticulous drying of solvents, conducting reactions in Teflons
vials, and performing NMR experiments in Teflons inserts. Reac-
tions in dilute solution, such as those under H2, invariably become
exposed to trace amounts of H2O, leading to [tBu3PH]+[B(C6F5)4]ꢀ
and (iPr3Si)2O (2 : 1). The yield for H2 conversion is most reliably
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resonance against that of tBu3P-H (1 : 1 from H2). Accordingly,
hydrolysis produces twice the amount of phosphonium salt than
the reaction with H2, and accounts for the slightly sub-quantitative
yields (90–94%). In support of this conclusion, 2H NMR spectro-
scopy of the D2 cleavage reaction shows a 1 : 1 tBu3P-D : D-SiiPr3
ratio, since adventitious moisture is introduced as H2O, and not
D2O (see Fig. S8 in ESI†).
Notes and references
‡ Crystallographic data for 1: C45H48BF20PSi, M = 1038.70, triclinic, a =
11.6772(5) Å, b = 12.5736(4) Å, c = 17.0286(6) Å, a = 79.442(3)1, b =
3
%
75.102(3)1, g = 77.292(3)1, U = 2335.61(15) Å , T = 173 K, space group P1
(no. 2), Z = 2, rcalcd = 1.477 g cmꢀ3, m(CuKa) = 1.792 mmꢀ1, colourless
blocks, Agilent Xcalibur PX Ultra A diffractometer; 8923 independent
measured reflections (Rint = 0.0180), F2 refinement,27 R1(obs) = 0.0354,
wR2(all) = 0.0948, 7708 independent observed absorption-corrected
reflections [|Fo| 4 4s(|Fo|), 2ymax = 1471], 613 parameters. CCDC
970728.
20 A full kinetic study by 1H NMR was hampered by iPr3SiH interaction
with 1, which masks the actual amount of silane produced until
reaction completion (i.e. complete conversion of 1). This pheno-
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proton is rendered invisible to 1H NMR spectroscopy. It is plausible that
a similar bridging silane interaction is also operational here.
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