The Detection and Reactivity of Silanols and Silanes Using Hyperpolarized 29Si Nuclear Magnetic Resonance

Abstract Silanols and silanes are key precursors and intermediates for the synthesis of silicon‐based materials. While their characterization and quantification by 29Si NMR spectroscopy has received significant attention, it is a technique that is limited by the low natural abundance of 29Si and its low sensitivity. Here, we describe a method using p‐H2 to hyperpolarize 29Si. The observed signal enhancements, approaching 3000‐fold at 11.7 T, would take many days of measurement for comparable results under Boltzmann conditions. The resulting signals were exploited to monitor the rapid reaction of tris(tert‐butoxy)silanol with triflic anhydride in a T 1‐corrected process that allows for rapid quantification. These results demonstrate a novel route to quantify dynamic processes and intermediates in the synthesis of silicon materials.

Silicon is one of the most abundant elements known to man.
Silicon-based materials find use in aw ide range of applications,from the synthesis of bulk materials through to roles in molecular transformations.O fi ts three naturally occurring isotopes,o nly 29 Si has an on-zero magnetic moment, and its nuclear magnetic resonance (NMR) detection is of wide interest. Its wide chemical shift dispersion makes it highly diagnostic for characterization purposes and allows the examination of dynamic processes in solution and the solid state. [1] However, 29 Si NMR spectroscopy has limitations that reduce its potential use, [2] primarily due to its low sensitivity that is dependent upon the small population differences that exist between nuclear spin energy levels within am agnetic field. These populations are governed by the Boltzmann distribution, and therefore,a 29 Si NMR signal reflects just 1in 125 500 of these nuclei at 11.7 T. Its 4.7 %natural abundance and typically long T 1 values also hinder detection. [1] Furthermore,asitisfound in materials that make up an NMR probe, and an NMR tube,t he broad background signal can impede the detection of low-concentration species.
Hyperpolarization is aroute to overcome this insensitivity and refers to as ituation whereby the spin energy level populations are perturbed from Boltzmann equilibrium conditions.T hese techniques have been utilized in both medical and analytical science,a nd with respect to 29 Si, anumber of hyperpolarized magnetic resonance applications using dynamic nuclear polarization (DNP) have been reported. [3] Thep arahydrogen (p-H 2 ) [4] based method, signal amplification of reversible exchange (SABRE), [5] has been applied to the polarization of 1 Ha nd an umber of heteronuclei, including 29 Si. [6] However,these studies were limited to N-heterocyclic functionalized silanes because of the need to bind the substrate to the transition-metal polarization transfer catalyst.
TheS ABRE-Relay technique [7] overcomes this barrier, and consequently,the scope of p-H 2 -based hyperpolarization has been greatly expanded. [8] Now,acarrier agent, such as an amine,becomes hyperpolarized through the initial formation of an active polarization transfer catalyst of the type [Ir(H) 2 -(NHC)(RNH 2 ) 3 ]Cl as depicted in Scheme 1. Thehyperpolarized carrier amine enhances the target substrate via proton transfer. Here,w ed evelop the SABRE-Relay technique for the hyperpolarization of silanols by p-H 2 and direct silane polarization by SABRE.
TheS tçber process [9] is the method of choice for the synthesis of silica-based materials.I ti sasol-gel process [10] that begins with the hydrolysis of atetra orthosilicate to give am ixture of silanols,w hich are subsequently condensed to form the material. Then ature of the silanol intermediates determines the physical properties of these materials, [11] and therefore,methods for their rapid and cost-efficient detection and characterization are desirable.O ur starting point was to take tris(tert-butoxy)silanol (T t BOS;i nset, Scheme 1) as am odel substrate to develop the p-H 2 -derived hyperpolarization of silanols.A5mmNMR tube fitted with aJ .Y oungs tap containing as olution of [IrCl(COD)(IMes)] (1,5m m), benzylamine-d 7 (BnNH 2 -d 7 ,5 0mm), and T t BOS (50 mm)i n CD 2 Cl 2 (0.6 mL) was placed under a3bar atmosphere of H 2 . After 1h at 298 K, the resulting 1 HNMR spectrum showed clean conversion into [Ir(H) 2 (IMes)(BnNH 2 -d 7 ) 3 ]Cl. [7,8] No evidence for T t BOS binding to this iridium center was observed by NMR or MS methods.A fter shaking this sample under 3bar p-H 2 for 10 sa t298 Ka t70 G, it was rapidly transferred into an 11.7 Ts pectrometer for interrogation by 29 Si NMR spectroscopy.A 29 Si signal that was 82 AE 5-fold more intense than that in the corresponding thermally equilibrated control spectrum was detected. Thet ransfer of hyperpolarization from the silanol 1 Ht ot he 29 Si is likely to occur through both low-field thermal mixing [12] and nuclear Overhauser enhancement. [13] Consequently,w es ought to improve the 29 Si NMR signal gain to enable in situ reaction monitoring.
We began by varying the identity of the catalysts N-heterocyclic carbene (NHC) ligand as it has been shown to affect the observed polarization level. Thes elective 2 H labeling of the NHC can result in increased levels of polarization because of reduced spin dilution and longer T 1 relaxation times. [14] Therefore,a na nalogous SABRE-Relay sample was prepared using d 22 -1.After SABRE-Relay transfer, an improved silanol signal gain of 92 AE 4-fold was observed ( Figure 1B). Modification of the steric and electronic properties of the NHC ligand has also been shown to modulate the ligand dissociation rates and thereby lead to better SABRE enhancement. [15] Ther ate of equatorial BnNH 2 dissociation from [Ir(H) 2 (IMes)(BnNH 2 -d 7 ) 3 ]Cl is 3.33 s À1 and therefore lower than predicted to be optimal. [16] Bulky catalyst 2,f urnished with tert-butyl groups,i ncreases the signal gain to 128 AE 11-fold whereas its isotopologue, d 34 -2, gave a1 50 AE 9-fold gain. Catalyst 3,w hich bears the even more bulky SIMes ligand, also improved the 29 Si signal gain compared to 1.C ontinuing with this trend, we were able to further increase the steric effects of the NHC through the use of catalysts 4 (IPr) and 5 (SIPr), which gave signal enhancements of 157 AE 15 and 310 AE 22, respectively.Wewould expect these signal enhancements to be further improved by the use of the deuterated isotopologues and are currently exploring routes to their synthesis.
In order to rationalize these differences in polarization level, we calculated the DG°2 98 value of ligand dissociation for equatorially bound BnNH 2 and H 2 loss for the active complexes.T hese values were calculated as described in the Supporting Information. Thebarrier to H 2 loss is consistently higher than that of BnNH 2 dissociation for each catalyst, which is consistent with the expected dissociative mechanism ( Figure 1C). Thec atalyst derived from 1,[ Ir(H) 2 (IMes)-(BnNH 2 ) 3 ]Cl, gave DG°2 98 values of 66.79 and 68.81 kJ mol À1 for BnNH 2 and H 2 loss,r espectively.T hese DG°2 98 values decrease when the steric bulk of the NHC ligand is increased across the series of catalysts 1-5.For catalyst 2,they are 66. 16 and 68.56 kJ mol À1 ,r espectively.C atalyst 5,w hich gave the largest 29 Si signal enhancements after 10 spolarization transfer, has the lowest values of DG°2 98 (62.54 and 63.5 kJ mol À1 for BnH 2 and H 2 loss,r espectively). These data confirm that lower barriers to ligand loss promote more effective SABRE-Relay transfer.T his effect is likely to be attenuated by relaxation of the NH protons of BnNH 2 -d 7 in the presence of [Ir(H) 2 (SIPr)(BnNH 2 -d 7 ) 3 ]Cl whose T 1 was now just 0.8 sa t 11.7 T. Thus,r apid ligand exchange allows for the effective replenishment of the polarized transfer agent;however, if this exchange is too fast, rapid relaxation will limit hyperpolarization buildup whilst depleting the p-H 2 . Thec orresponding hyperpolarized 29 Si signal lifetime of T t BOS was 138.4 sasmeasured by avariable flip angle pulse sequence (see the Supporting Information). As the decay of the created 29 Si magnetization is slow,wepostulated that the NMR signal gains could be increased if the SABRE-Relay time was extended beyond 10 s. Consequently,anincrease in signal gain to 625 AE 34-fold was obtained using 5 when the polarization time was extended to 50 s( Figure 1D). Extending this time further decreased the signal intensity because of the finite amount of p-H 2 becoming limiting during the SABRE-Relay process.Upon repeating this experiment with d 34 -2,asignal gain of 767 AE 38-fold was reached after 70 s exposure to p-H 2 .W ec onclude that while 5 leads to am ore rapid buildup of polarization, its higher rate of ligand exchange consumes the p-H 2 in the sample.C omplex d 34 -2 yields higher signal gains with extended polarization transfer times and the same finite volume of p-H 2 as relaxation effects are reduced by slower exchange.I ns upport of this,t he relaxation times of the NH protons of BnNH 2 -d 7 in the presence of d 34 -2 were measured to be 3.2 sa t1 1.7 T.
Thepolarization times were extended further by evacuating and refilling the NMR tubes containing the SABRE-Relay solutions with p-H 2 periodically until 300 sw as reached. For 5,t he time between fills was 40 s, and for d 34 -2, it was 60 s. Forb oth catalysts,alinear increase in 29 Si polarization level was observed over time but d 34 -2 led to the highest signal gain of 2142 AE 180-fold after 300 s. For 5,t he signal gain was 1580 AE 120-fold. This behavior is reflected in the lower magnetization buildup slope illustrated in Figure 1D.I ncreasing the pressure of p-H 2 from 3bar to 5bar yielded afurther 10 %increase in signal gain to 2313-fold with d 34 -2 due to increased p-H 2 availability.A na utomated polarizer, which introduces ac onstant flow of p-H 2 into the solution, gave as imilar linear increase in signal gain; however, the signal gains only reach am aximum of 100-fold (see the Supporting Information). We suggest that this is due to inefficient mixing of p-H 2 in solution.
After optimization, we concluded that the best conditions for the polarization T t BOS are d 34 -2 (5 mm), BnNH 2 -d 7 (50 mm), and T t BOS (30 mm)i nC D 2 Cl 2 (0.6 mL) and exposure to p-H 2 at 5bar for 300 sa t7 0G with refreshing the p-H 2 atmosphere every 60 s. This yields atotal signal gain of 2852 AE 112-fold (2.3 %) in the 29 Si NMR spectrum ( Figure 2). These conditions were then applied to the polarization of an umber of other silanols as summarized in Figure 2. As their T 1 relaxation times are shorter than that of T t BOS,t he maximum signal gains are achieved with shorter total polarization transfer times. DNP has previously been used for quantitative reaction rate determination using 1 Ha nd 13 Cn uclei. [17] Theh yperpolarized 29 Si NMR signals and the long T 1 relaxation time of T t BOS were exploited to monitor amolecular transformation. Thenucleophilic substitution of triflic anhydride (Tf 2 O) with T t BOS was chosen as it has been reported as am ethod for functionalization of silica surfaces. [18] Ar elaxation-corrected variable flip angle sequence was used to overcome the loss of magnetization due to T 1 relaxation during the reaction and to give an immediate concentration profile as detailed in the Supporting Information. [19] TheS ABRE-Relay polarization of T t BOS was conducted using our previously optimized conditions,p rior to the introduction of as olution of Tf 2 O (10 equiv) and pyridine (10 equiv) in CD 2 Cl 2 (0.1 mL). Subsequent rapid sample insertion into an 11.7 Tm agnetic field prior to acquisition of a 29 Si spectrum every 5s for ad uration of 60 s ( Figure 3). When Tf 2 Ow as present in excess,w eo bserved conversion of T t BOS (d Si = À90.8 ppm) into its triflate derivative (d Si = À102.7 ppm). Thei dentity of these signals was unequivocally confirmed by independent synthesis (see the Supporting Information). After an induction period of 10 s, which we attribute to diffusion of the Tf 2 O into the NMR detection region, the expected pseudo-firstorder consumption of the starting silanol and the corresponding production of its triflate product was observed. Ther ate constant for this was determined to be 0.070 AE 0.001 s À1 and is not affected by changing the concentration of the SABRE catalyst, which confirms that the catalyst does not participate in the nucleophilic reaction. This data would not be possible  to collect using 29 Si NMR spectroscopy under Boltzmann conditions because of the requirement for signal averaging and long T 1 values;t he speed of this reaction means that it would be complete before the first measurement could be made.W hent he reaction was repeated with substoichiometric quantities of Tf 2 O, anew resonance at d Si = À93.2 ppm was observed, which we attribute to the product of dimerization (see the Supporting Information). It is formed by reaction of T t BOS with its triflate intermediate in atwo-step process.The same signal is present in the 29 Si NMR spectrum when the reaction was repeated with tris(tert-butoxy)silyl chloride; however, the reaction is now too rapid to derive any kinetic data. As the oligomerization of silanols is ak ey step in the synthesis of silica materials,t he result demonstrates that it may be possible to detect and quantify intermediates in the sol-gel process.
During the course of our investigation into hyperpolarized 29 Si NMR spectroscopy,w ea lso discovered am ethod to hyperpolarize important silanes via SABRE. When asample containing d 22 -1 (5 mm), dimethylethoxysilane (50 mm), and BnNH 2 -d 7 (50 mm)inCD 2 Cl 2 (0.6 mL) was shaken at 70 Gfor 10 s, ah yperpolarized signal gain of 206 AE 24-fold was observed in the 29 Si NMR spectrum as an antiphase doublet (J Si-H = 205 Hz;F igure 4B). Antiphase character is typically seen for two inequivalent p-H 2 -derived hydride ligands; [4b,5c, 12] here,similarly complex polarization is spontaneously created, but now shared between a 29 Si and a 1 Hnucleus.The T 1 value for this signal was measured to be 38 AE 1.2 s. In the 1 HNMR spectrum, after SABRE polarization, the intensity of the Si À Hr esonance is 70 AE 5t imes greater than in at hermally equilibrated reference spectrum. Thed ominant hydridecontaining species in the 1 HNMR spectrum was [Ir(H) 2 (d 22 -IMes)(BnNH 2 ) 3 ]Cl, [8a] and no hydride ligands were seen that could be attributed to asilane complex. As SABRE transfer is seen when a7 0G field is employed, it demonstrates the existence of ap olarization transfer route involving p-H 2derived hydride ligands,a nd dihydride-h 2 -silane complexes have been exemplified elsewhere. [20] We searched spectroscopically for the presence of such as pecies at low temperature in our SABRE sample;however, no signals attributable to an intermediate h 2 -silane complex could be detected.
After SABRE catalysis at 70 G, a 1 H-29 Si INEPT based transfer sequence was utilized at 11.7 Tt oc reate the same antiphase signal as that of Figure 4B with an increased signal gain of 772 AE 56 ( Figure 4C). Theu se of alternative coligands,s uch as DMSO-d 6 and CD 3 CN,d id not increase the signal gains when compared to those with BnNH 2 -d 7 .A dditionally,w hen the sample is placed under aD 2 atmosphere (3 bar) at room temperature for 24 h, a60% reduction in the SiÀHs ignal was observed in the 1 HNMR spectrum, thus indicating that while H/D exchange occurs,itisslow.NoSi ÀH site exchange was observed by EXSY methods on the timescale of relaxation, which further suggests that the key intermediate is of an h 2 -silane type rather than an oxidative silyl hydride,w hich would undergo rapid Si À Hs crambling.
Changing the silane to pentamethyldisiloxaneg ave a 29 Si signal gain of 252 AE 22 in the 1 H-29 Si INEPT spectrum. However,anumber of other silanes,s uch as triethoxysilane and triphenylsilane,y ielded no SABRE catalysis and therefore highlight the sensitivity of this approach to the steric and electronic properties of the silane.W ork is now ongoing to further characterize the intermediates involved in this process in order to broaden scope and applicability.
In summary,wehave demonstrated the hyperpolarization of silanols and silanes using p-H 2 .D evelopment of the SABRE-Relay method [7] gave large 29 Si signal gains that approach 3000-fold. Theeffects of catalyst structure and their influence on ligand exchange processes were determined through calculation of DG°2 98 values and the influence on carrier amine T 1 relaxation rates.The large polarization levels attained and the long T 1 values were exploited using ar elaxation-corrected variable flip angle pulse sequence to measure kinetic data for the reaction of as ilanol and Tf 2 O through a 29 Si NMR response.F inally,s ilanes have been shown to be amenable to SABRE-based polarization transfer when BnNH 2 -d 7 was used as ac o-ligand to give 29 Si signal gains of 772 AE 56-fold. We propose that an iridium dihydrideh 2 -silane is the active catalyst in this process, [20] and full rationalization of this SABRE pathway is ongoing. We anticipate that the hyperpolarized methods exemplified here for 29 Si will extend more generally to other slow-relaxing heteronuclei such that intermediate characterization by NMR spectroscopy is widely supported.