Controlled tin catalyzed hydrolysis of 3-acryloxypropyltrimethoxysilane with mono- and multi-functional mercaptans

Journal of Organometallic Chemistry 724 (2013) 213e224

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Controlled tin catalyzed hydrolysis of 3-acryloxypropyltrimethoxysilane with mono- and multi-functional mercaptans R.S. Burkhalter a, C.L. Hogue a, D.L. Smith b, S.M. Sonner a, M.J. Winningham a, *, R.E. Youngman a a b

Science & Technology Division, Corning Incorporated, Corning, NY 14831, USA The University of Maine, Forest Bioproducts Research Institute, Orono, ME 04469, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 October 2012 Received in revised form 19 November 2012 Accepted 19 November 2012

Mechanistic investigations were initiated to further our understanding of the reaction pathways for hydrolysis and condensation of organofunctional alkoxysilanes in organic media. Previous work has shown that addition of mercaptans to solutions that contain dibutyltin dilaurate (DBTDL), which functions as a catalyst, and 3-acryloxypropyltrimethoxysilane can greatly reduce the rate of silane hydrolysis and condensation. Gas chromatography/mass spectrometry (GC/MS) hydrolysis studies were carried out to understand how mercaptan structure and concentration affected the relative hydrolysis rate of the acrylate silane in solution. Detailed nuclear magnetic resonance spectroscopy (NMR) and electrospray ionization Fourier transform ion cyclotron resonance (ESI-FTICR) mass spectrometry (MS) were carried out on simple mixtures of DBTDL and select mercaptans, and with varying mercaptan concentrations, to elucidate which significant tin complexes can form. These studies showed that mercaptans readily undergo ligand exchange reactions with the labile carboxylate groups of DBTDL and that an excess of mercaptans reduced the amount of proposed active catalyst species. It was found that an excess of mercaptan functional group was necessary to prevent significant and rapid hydrolysis of the organofunctional silane in organic media. A minimum of 1:4 DBTDL carboxylate group:mercaptan functional group was required for most mercaptans examined. A tetramercaptan, pentaerythritol tetrakis(3mercaptopropionate), was shown to be effective in deactivating the DBTDL catalyst, even at relatively low concentrations of tetramercaptan. The findings in this work provide further support for the proposed mechanism for catalyzed hydrolysis and condensation. The tin metal was determined to be predominately 4-fold coordinated in the various tin mercaptide compounds studied. Evidence for transient tin hydroxide reactive intermediates was detected using mass spectrometry, as upon the addition of water, oligomeric condensation products were observed. However, the lifetime of these reactive intermediates precluded their direct observation by either mass spectrometry or NMR. Oligomeric and cyclic tin mercaptide species were shown to form when multifunctional mercaptans were combined with DBTDL in solution. Ó 2012 Elsevier B.V. All rights reserved.

Keywords: Dibutyltin dilaurate Mercaptan Multifunctional mercaptan 119 Sn NMR Mass spectrometry Silane hydrolysis

1. Introduction Organofunctional silanes are typically incorporated into prepolymerized liquid coating compositions as additives to promote adhesion to siliceous and other inorganic surfaces [1,2]. Premature organofunctional silane hydrolysis and condensation in liquid coating compositions dramatically reduces the effectiveness of the coating additive in promoting adhesion, yet upon application of the coating to a substrate rapid chemical bonding is desirable. Therefore, understanding and controlling these chemical reactions in the liquid coating is of prime importance. * Corresponding author. Tel.: þ1 607 974 2965; fax: þ1 607 974 2383. E-mail address: [email protected] (M.J. Winningham). 0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jorganchem.2012.11.025

Organofunctional alkoxysilanes readily undergo hydrolysis in the presence of water and a catalyst. Catalysts include the same catalysts used to affect esterification and transesterification reactions, including acids, bases, titanates, and organometallic compounds [3]. Organofunctional tin compounds are used to make siloxane polymers from alkoxysilanes, where the tin compounds catalyze the hydrolysis and condensation reactions [3]. Tin catalyzed hydrolysis of silanes has been previously studied using 1H NMR spectroscopy, where hydrolysis reaction rates of vinyltrimethoxysilane and vinyltriethoxysilane were calculated in solvents that differed in polarity [4]. In systems such as these, where the catalyst is dibutyltin dilaurate (DBTDL), a dibutyltin laurate hydroxide intermediate, formed either by partial hydrolysis of a laurate ligand or via ligand exchange with water, has been

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proposed as the true active catalyst [5,6]. A mechanistic scheme proposed by van der Weij is shown in Fig. 1. DBTDL, 1, is hydrolyzed by water present in the coating system yielding the dibutyltin laurate hydroxide, 2, and lauric acid. The nucleophilic tin hydroxide intermediate, 2, may self-condense to form the distannoxane intermediate, 3, or react with the organofunctional silane, 4, releasing a molecule of methanol. Additional hydrolysis and condensation reactions of 3 would lead to organotin oligomers. The stannasiloxane, 5, can react with water to form a silanol product, such as 6, or react with a silanol to form condensation products, such as 7. While quite useful toward the synthesis of polysiloxanes and polysilsesquioxanes, if left unabated, these reactions can quickly lead to premature hydrolysis and condensation of organofunctional alkoxysilane in coating formulations prior to substrate application and polymerization [4e7]. Controlled inhibition of this catalytic pathway in liquid coating formulations is therefore essential. Carboxylic acids and mercaptans have been shown to inhibit organotin catalyst activity. Carboxylic acids are known to inhibit catalyst activity by shifting the equilibrium condition between 1 and 2, as shown in Fig. 1, thus reducing the concentration of the active tin hydroxide, 2 [5,6,8]. Attempts toward modification of catalyst activity of dialkyltin diesters, such as DBTDL, through ligand exchange with mercaptans was also previously examined, where it was indicated that an excess of alkylmercaptan added to DBTDL yielded an octahedrally coordinated tin complex with four mercaptide and two alkyl ligands [9]. Ligand exchange chemistry and structural studies of organotin mercaptides have suggested that 4-fold coordination predominates, yet higher coordinated complexes may occur with mercapto ester ligands, where these ligands have been reported to form bidentate bonding interactions with the tin metal center [10,11]. Houghton and Mulvaney showed

Bu Sn Bu

O2CC11H23

H2O

O2CC11H23

C11H23CO2H

that the reactivity of dialkyltin diesters and dialkyltin dimercaptides toward catalyzing urethane bond formation was substantially the same and that the equilibrium constant in the ligand exchange reaction was approximately one [12]. These researchers also showed that deactivation of the tin catalyst through incorporation of excess mercaptans was not due to higher coordination of the tin, as suggested earlier. Instead, the excess mercaptans shift the equilibrium away from the active form of the catalyst, which was proposed to be a tin alkoxide intermediate in the case of tin catalyzed urethane formation. A modified form of van der Weij’s mechanism of catalyzed silane hydrolysis and condensation shown in Fig. 1, which incorporates the work of Houghton and Mulvaney, can be found in Fig. 2 [5,12]. DBTDL undergoes rapid ligand exchange in the presence of a mercaptan, 8, to yield the dibutyltin dimercaptide, 9, with release of two equivalents of lauric acid. Alternately, an octahedral tin mercaptide, 10, could be formed [9]. Reaction of 9 with water in the system yields the tin mercaptide hydroxide species, 11, which may catalyze the hydrolysis and condensation of organofunctional alkoxysilanes, such as 4. Note that in the Houghton and Mulvaney reference the active species is a tin mercaptide alkoxide or a tin carboxylate alkoxide, whereas in Fig. 2, a tin mercaptide hydroxide, 11, or the aforementioned tin carboxylate hydroxide, 2, would be the active species [12]. An excess of mercaptan will shift the equilibrium conditions toward the dimercaptide species with release of lauric acid, which should simultaneously affect the equilibrium condition of 1 and 2, mentioned previously. Alkylmercaptans and alkylmercapto esters are typically added to radical polymerizable mixtures to adjust polymer molecular weight through a chain transfer mechanism. It was anticipated that these components could be used to stabilize organofunctional trimethoxysilanes in liquid coating compositions by deactivating

O2CC11H23

Bu Sn

OH

Bu

DBTDL, 1

-H2O

Bu

H2O

Bu

O2CC11H23

Sn

O

Bu Sn

2

C11H23CO2

Bu

3 R

Si(OCH3)3

- CH3OH

4

Bu Sn Bu

O2CC11H23 Organotin oligomers

R

O Si H3CO

H2O

5

OCH3 6

H3CO HO

R + 2

Si H3CO

R

O Si

OCH3 H3CO 6

OCH3 Si R

+ 2

OCH3 7

Fig. 1. Proposed mechanism for DBTDL catalyzed hydrolysis and condensation of organofunctional trialkoxysilanes according to van der Weij [5].

R.S. Burkhalter et al. / Journal of Organometallic Chemistry 724 (2013) 213e224

Bu Sn Bu

O2CC 11H23

H 2O

Bu

O2CC 11H23

- C11H23CO 2H

Bu

Sn

O2CC 11H23

2

R'-SH, 8

XS R'-SH, 8 R

- 2 C11H23CO2 H

- 2 C 11H23CO 2H

SR'

Bu

SR'

Bu

Sn

O2CC11H23 O

Bu

R Si

SR' SR'

9

- CH3OH

4

SR' Sn

Sn

Si(OCH 3 ) 3

Bu

SR' Bu

Organotin condensation products

OH

1

Bu

215

H 3CO

10

5

OCH 3

H2O - R'-SH

Bu

SR'

Si(OCH3 ) 3 4

R

Sn Bu

OH

Bu

SR' Sn

Bu

- CH3OH

11

O

Organosilane condensation products

R Si

H3CO

OCH 3

12 Fig. 2. Reaction pathways for mercaptide ligand exchange of DBTDL, 1 [5,12].

dialkyltin diesters, which are known to catalyze the hydrolysis and condensation of alkoxysilanes in liquid coating formulations. It was also proposed that multifunctional mercaptans would be particularly effective in deactivating DBTDL toward silane hydrolysis and condensation. Through careful mechanistic studies it was found that mercaptans readily undergo ligand exchange reactions with the labile carboxylate groups of DBTDL and that an excess of mercaptans reduced the amount of proposed active catalyst species. An excess of mercaptan functional group was necessary to prevent significant and rapid hydrolysis of the organofunctional silane in organic media.

2. Experimental 2.1. Materials DBTDL and the various mercaptans of interest (M1-M3, M5, M6, Table 1), were obtained from Aldrich Chemical Company and used without further purification. Table 1 also includes nomenclature and thiol functionality used in this study. Cyclohexylphenylketone (internal GC/MS standard), deuterated NMR solvents (dichloromethane-d2, tetrahydrofuran-d8 and dimethylsulfoxide-d6), tetrahydrofuran (THF), acetonitrile and methylene chloride were

Table 1 Nomenclature and structures of mercaptan chemistry used in this study. Mercaptan ID

Full name

Functionality

Structure

M1

1-Pentanemercaptan

1

SH

M2

1-Octadecanemercaptan

1

M3

Methyl-3-mercaptopropionate

1

SH

H

18

O

SH O

M4

3-Mercaptopropyltrimethoxysilane

1

OCH3 H3CO Si H3CO

M5

1,2-Bis(2-mercaptoethoxy)ethane

2

HS

O

SH

SH

O

O

M6

Pentaerythritol tetrakis(3-mercaptopropionate)

O

HS

O

O

SH

HS

O

O

SH

4

O

O

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obtained from Aldrich Chemical Company or Fisher Scientific and used without further purification. Di-n-butyltin bis(2ethylhexylmercaptoacetate), di-n-butyl bis(2-dodecylthio) tin (biscat), 3-mercaptopropyltrimethoxysilane (M4) and 3acryloxypropyltrimethoxysilane (acrylate silane) were obtained from Gelest Incorporated and used without further purification. A portion of biscat was further purified via vacuum distillation for one of the hydrolysis studies. 2.2. Characterization NMR samples were prepared based on the ratio of reactive groups. The mercaptans were added to the starting catalyst material, DBTDL, in ratios of 1:1, 1:2, 1:4 and 1:8 of carboxylate groups of DBTDL:mercaptan functional group. The samples were all diluted to a final volume of 1 mL with dichloromethane-d2 or tetrahydrofuran-d8 and mixed with a vortex mixer. All of the liquid components were measured using positive displacement pipettes. This sample preparation procedure was modified for several of the investigations. The presence of water was expected to affect hydrolysis of the laurate groups and thus enable substitution of the mercaptan compound. Samples with M4 were prepared with and without addition of 15 mL of water to investigate the differences. Samples were analyzed with 119Sn NMR to determine the number and the chemical shift of organotin species. Samples were analyzed neat in a 5 mm NMR tube. An insert containing a solution of 5% Me4Sn in dichloromethane-d2 was added to the tube to provide an internal 119Sn chemical shift reference at 0 ppm, as well as a deuterium lock signal. NMR experiments were conducted on a Varian UnityInova spectrometer, in conjunction with a 7.0 T superconducting magnet and Varian NMR narrow band probes. The NMR probe was tuned to 119Sn (111.8 MHz), and proton decoupling was used. Pulse width (p/2) and T1 relaxation time for 119Sn were determined using DBTDL and found to be 4.2 ms and 0.7 s, respectively. The pulse sequence recycle time was set to 15 s to accommodate this and longer relaxation times. NMR measurements were typically conducted at room temperature, except for several experiments run at 60  C in an effort to isolate transient intermediate species. All samples for ESI-MS analysis were dissolved in THF prior to analysis. Neat samples and reaction mixtures diluted in 50:50 (v/v) of THF:acetonitrile were analyzed with an IonSpec Ultima II 7T Fourier transform ion cyclotron resonance mass spectrometer operating in the positive mode of electrospray ionization. The capillary voltage was maintained at 3.4 kV, the end plate voltage at 3.1 kV, and the flow rate was 1 mL/min throughout each of the experiments. Other experimental factors related to external accumulation/gated trapping conditions were unique to each experiment. The DBTDL hydrolysis experiments were carried out on a Thermo Finnigan LCQ Deca XP ion trap mass spectrometer. Just prior to analysis, a 10% relative volume of Millipore water was added to the DBTDL in THF solution. The iSpray voltage was maintained at 4.2 kV and the flow rate was maintained at 5 mL/min. An initial model study was conducted to test the theory that mercaptan additives will inhibit the catalytic degradation of the acrylate silane. Therefore GC/MS samples were prepared at a concentration of 1% for the acrylate silane and internal standard components, and 0.1% for the catalyst component. Three equivalents of water were added per one equivalent of acrylate silane. The mercaptans were added at a concentration of 0.3%. The addition of mercaptans was varied to observe the impact of the mercaptan stabilizers on the acrylate silane in the presence and absence of the catalyst. The samples were all diluted to a final volume of 1.5 mL. Test solutions were prepared by first adding THF to the autosampler

vial followed by H2O, internal standard, catalyst and mercaptan at the appropriate concentrations. The solutions were thoroughly mixed by inverting several times and allowed to sit for a few seconds prior to adding the acrylate silane, which began the experiment. The control sample contained only the acrylate silane (without catalyst and mercaptan additive). A model loading study was also conducted to test the functionality and concentration effects on the stabilization of the mercaptans relative to the degradation of the acrylate silane. Samples for this study included a control containing only the acrylate silane component. The mono and dimercaptans (M1-M3 and M5), tetramercaptan (M6), and mercapto silane (M4), were added to each sample in mole ratios of 1:1, 1:2, 1:4 and 1:8 of carboxylate groups of DBTDL:mercaptan functional group. The samples were all diluted to a final volume of 1.5 mL with water and THF. All of the liquid components were measured using positive displacement pipettes. A Varian 3800 gas chromatograph equipped with a Saturn 2000 Ion Trap Mass Spectrometer system was used to analyze all solutions in this study. The mass spectrometer operated over a 10e 650 m/z mass range collected from 1.5 to 25 min. The column used for the analysis was a J&W DB-5MS, 30 m  0.25 mm ID  0.25 mm film thickness with a carrier gas of helium at a constant flow of 1.0 mL/min. The column oven temperature cycle began at 75  C for 3 min followed by a ramp to 250  C at 10  C/min with a hold at 250  C for 7 min, followed by a second ramp to 325  C at 10  C/min with a final hold at 325  C for 20 min. The second temperature ramp and hold cycle was implemented to eliminate catalyst carryover effects in subsequent analyses. Sample solutions were injected using a Varian Model 8200 Auto sampler and a Varian Model 1079 Split/Splitless Injector operating at 300  C in the split mode. The samples were injected using 1 mL neat injections at a rate of 5.0 mL/s. All GC/MS responses were calculated as a ratio of peak areas of the acrylate silane to internal standard. 3. Results & discussion 3.1. GC/MS hydrolysis studies Solution studies were conducted to monitor the reactivity of the acrylate silane, in the presence of the DBTDL catalyst and mercaptans, toward hydrolysis. Acrylate silane was mixed in various solutions with and without the DBTDL catalyst and mercaptans. The concentration of acrylate silane was monitored over a four-day period using GC/MS. Results in Fig. 3 clearly show that the acrylate silane is fully reacted in only a few hours when mixed with the catalyst alone, while the same reactions are inhibited in solutions containing the catalyst plus excess mercaptans. Studies were conducted to determine the amount of additive required to effectively inhibit acrylate silane hydrolysis. Both concentration and mercaptan functionality were investigated. Ratios of 1:1, 1:2, 1:4, and 1:8 equivalents of tin metal carboxylate groups to mercaptan functional group were examined. Mercaptans included in this study were monofunctional (M1-M4), difunctional (M5), and tetrafunctional (M6). Again, the concentration of parent acrylate silane in each solution was measured over a four-day period using GC/MS. The results provided in Fig. 4 show that M5 and M6 were more effective than M1-M4 in inhibiting acrylate silane hydrolysis. The GC/MS results clearly show that only M6 completely inhibits acrylate silane hydrolysis at a 1:1 equivalent ratio of reactive groups. Solutions containing M1-M5 showed rapid disappearance of acrylate silane at the 1:1 equivalent ratio of reactive groups. Both M6 and M5 inhibit acrylate silane hydrolysis at a 1:2 equivalent ratio. At the 1:4 equivalent ratio, all mercaptans demonstrated inhibition of

R.S. Burkhalter et al. / Journal of Organometallic Chemistry 724 (2013) 213e224

catalyzed hydrolysis of the silane. No significant differences were observed at the 1:8 equivalent ratio (data not shown). It is interesting to note that the monofunctional mercaptans did not perform the same at lower concentrations. Results indicate M4 is a more effective catalyst inhibitor at the lower concentrations as degradation of the acrylate silane was not as rapid as when compared to solutions containing the M2, yet both monofunctional mercaptans performed essentially the same when in stoichiometric excess. The situation for M4 is further complicated by the fact that it contains reactive silane functionality which could be in competition with the acrylate silane in these hydrolysis studies, i.e. there are additional reaction sites where hydrolysis can occur. Hydrolysis studies with the biscat tin species confirmed that both organotin esters and organotin mercaptides are active catalysts toward hydrolysis of 3-acryloxypropyltrimethoxysilane. Rapid degradation of the acrylate silane was observed in the presence of the biscat, as shown in Fig. 5. Samples containing M4 and biscat do not show any significant degradation of the acrylate silane component. Our results seem to correlate with literature findings that tin dimercaptides have similar catalyst activity as tin dicarboxylates [12]. These results further support the chemical mechanism depicted in Fig. 2, where an excess of ligand significantly reduces the level of active tin catalyst.

GCMS Peak Ratio (AS/IS)

1.4 1.2 1.0 0.8

AS + DBDTL AS DBTDL + M4 DBTDL + M6 AS + M4 AS + M6

0.6 0.4 0.2 0.0 0

1

2

3

4

5

Time (Days) Fig. 3. GC/MS response for 1% acrylate silane (AS) relative to 1% internal standard (IS) with and without 0.1% DBTDL catalyst and either 0.3% M4 or 0.3% M6 additives.

a

b

1.6

1.6 1.4

GCMS Peak Area Ratio (AS/IS)

GCMS Peak Area Ratio (AS/IS)

1.4 1.2 1.0 0.8 M4 M2 M5 M6 M1 M3

0.6 0.4 0.2 0.0 -0.5

217

1.2 1.0 0.8 0.6 0.4 0.2 0.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

-0.5

M4 M2 M5 M6 M1 M3

0.0

0.5

1.0

Time (days)

c

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Time (days)

1.6

GCMS Peak Area Ratio (AS/IS)

1.4 1.2 1.0 0.8 0.6 0.4 0.2

M4 M2 M5 M6 M1 M3

0.0 -0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Time (days) Fig. 4. Acrylate silane stability in the presence of the DBTDL catalyst and various concentrations of the mercaptan stabilizers under investigation: (a) 1:1 DBTDL:mercaptan, (b) 1:2 DBTDL:mercaptan, (c) 1:4 DBTDL:mercaptan. GC/MS response for 1% acrylate silane relative to 1% internal standard. Ratios denote DBTDL carboxylate group to mercaptan functional group.

218

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differences in solvent polarity, the differences in polarity of THF (m ¼ 1.63 D) and methylene chloride (m ¼ 1.60 D) is minor [13]. However, the solubility of water in these two solvents is quite different, where water is freely soluble in THF but poorly soluble in methylene chloride and thus it is likely that the THF studies are significantly affected by higher water content in solution.

1.4

GCMS Peak Area Ratio

1.2 1.0 0.8

3.2. Structural characterization 0.6 AS+IS AS+IS+DBTDL AS+IS+purified Biscat AS+IS+purified Biscat+1:5 M4 AS+IS+purified Biscat+1:20 M4

0.4 0.2 0.0 -0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

Time (days) Fig. 5. Acrylate silane stability in the presence of the biscat catalyst, with and without a stabilizer. GC/MS response for 1% acrylate silane relative to 1% internal standard. 1:5 and 1:20 ratios designate ratio of DBTDL carboxylate group to M4 functional group.

To study the impact of solvent on the rate of acrylate silane hydrolysis, two series of solutions were prepared in THF and methylene chloride (CH2Cl2). Samples were prepared at a 1:1 equivalent ratio of reactive carboxylate groups of DBTDL to mercaptan functional groups using both solvents. The results in Fig. 6 show a significant difference in the two solvent series. The samples prepared with THF show much more rapid degradation than the samples prepared with CH2Cl2, except for M6 solutions where solvent did not substantially affect the rate of acrylate silane hydrolysis. Two “blank” samples were prepared using only the acrylate silane component and internal standard in THF and CH2Cl2 to monitor the stability in the absence of tin catalyst and mercaptans, where only slight disappearance of acrylate silane was observed, confirming the impact of DBTDL as a catalyst in this reaction. While others have observed a strong solvent effect in DBTDL catalyzed silane hydrolysis [4], which has been attributed to

GCMS Peak Area Ratio (AS/IS)

1.2

1.0

0.8

0.6

AS/IS AS/IS M4 M4 M2 M2 M5 M5 M6 M6

0.4

0.2

0.0 0

1

2

3

4

5

6

7

8

NMR spectroscopy was used to characterize the structure of the starting catalyst material (DBDTL) and the products of its reaction with the mercaptan additives. DBTDL has a 119Sn chemical shift of 157.1 ppm, which is similar to that of other compounds with two oxygens and two alkyl groups bonded to a tin metal center [14,15]. Mixtures of DBTDL and the mercaptan compounds used in this study produce products with a positive 119Sn chemical shift of 113e127 ppm (Table 2). Published values indicate that a monomercaptan substituted product could have a 119Sn chemical shift of around 90 ppm and that the dimercaptan substituted product could have a 119Sn chemical shift of about 144 ppm [14,15]. The 119Sn NMR spectra in Fig. 7 indicate that only one product (or group of products), which has a chemical shift of 113e127 ppm in each case, is formed in these reactions. A sample of a commercially available tin dimercaptide, biscat, was also analyzed and had a 119Sn chemical shift of 124.7 ppm. This supporting information indicates that the major products formed as a result of the reaction of DBTDL and the mercaptans involved in this study are dimercaptans. In an effort to isolate any mono substituted product, a reaction sample was prepared by quickly adding M4 to an NMR tube of DBTDL in dichloromethane-d2 that had been cooled to 60  C in an NMR probe. The sample was then immediately analyzed with 119Sn NMR. Only one product peak formed, with a chemical shift of 125 ppm, consistent with the disubstituted species. Any monomercaptan monolaurate intermediate would have a more negative chemical shift than the disubstituted species but less negative than the DBTDL, i.e. an intermediate chemical shift between 126 and 158 ppm. Based on the chemical shifts observed in the 119Sn NMR spectra (Table 2), there are some apparent differences in the structure of the complexes formed. The M2 forms a single product peak at approximately 125 ppm. With an excess of mercaptan, i.e. 1:4 DBTDL carboxylate functional group:mercaptan functional group, reaction with M5 results in two major product peaks with 119Sn chemical shifts at 115.6 and 115.4 ppm, and an additional minor product peak at 127.0 ppm. The 119Sn NMR spectra for the M5 and M6 reactions are much more complex than the monomercaptans (Fig. 7), indicating formation of multiple species with subtle structural differences. Table 2 119 Sn chemical shifts for reaction mixtures of DBTDL and the mercaptans in Table 1, as well as other reference compounds as described in the text. Reaction mixture ratios are expressed in terms of DBTDL carboxylate group to mercaptan functional group.

9

10

Time (days) Fig. 6. Acrylate silane stability investigating the solvent effects of using THF (closed symbols) versus CH2Cl2 (open symbols) when preparing 1:1 sample solutions using the various mercaptans. GC/MS response for 1% acrylate silane relative to 1% internal standard.

Compound or reaction mixture

119

DBTDL Biscat Di-n-butyltin bis (2-ethylhexyl-mercaptoacetate) 1:4 DBTDL:M1 1:4 DBTDL:M2 1:4 DBTDL:M3 1:4 DBTDL:M4 1:4 DBTDL:M5 1:4 DBTDL:M6

157.1 124.7 67.1 125.0 124.9 119.5 125.4a 115.6, 115.4, 127.0 124.5, 124.8, 126.2

a 119

Sn NMR spectrum obtained at 60  C.

Sn resonances (ppm)

R.S. Burkhalter et al. / Journal of Organometallic Chemistry 724 (2013) 213e224

219

The unique isotopic distribution of tin proved instrumental throughout these experiments in identifying the presence and number of tin atoms in a given complex. Additionally, throughout the mass spectral analyses described, the addition of an internal standard resulted in mass deviations of the observed exact mass from the theoretical exact mass of the proposed structure of less than 1.5 ppm. All of the organotin mercaptan complexes analyzed for this study were found to ionize as their sodiated and potassiated species. The mass spectrum of the products of the reaction of DBTDL with M5 is shown in Fig. 8. The most abundant ion currents observed at 849 and 865 Da were found to correspond to the sodiated and potassiated adduct of two mixed species. The ion current observed at 849 Da is actually due to the sodium-bound dimer of the cyclic structure 13.

Bu Fig. 7. 119Sn NMR spectra of (a) biscat, (b) 1:4 DBTDL:M2, (c) 1:4 DBTDL:M5 and (d) 1:4 DBTDL:M6. Reaction mixtures are denoted by the ratio of DBTDL carboxylate group to mercaptan functional group.

Electrospray ionization mass spectrometry was used in conjunction with NMR spectroscopy to more completely understand the structures of the final products. Since the discovery of electrospray ionization by Fenn in the early 1980’s, the technique has provided invaluable structural information in the analysis of biomolecules and biomolecular interactions [16]. The technique has gained increasing acceptance as an attractive alternative to matrixassisted laser desorption ionization in the analysis of synthetic polymers [17]. Here we demonstrate the effectiveness of electrospray ionization in the monitoring of elementary reactions of organotin complexes.

S

CH2CH2

O CH2

Sn Bu

S

CH2 CH2CH2

O

13 Bu

S

(C2H4O)2C2H4 S

Sn Bu

Bu Sn

S

(C2H4O)2C2H4 S

14

Fig. 8. The ESI-FTICR mass spectrum of the 1:8 DBTDL carboxylate to M5 functional group reaction products.

Bu

220

R.S. Burkhalter et al. / Journal of Organometallic Chemistry 724 (2013) 213e224

Cation-bound dimers are frequent artifacts of the electrospray process of organometallic complexes, due primarily to concentration effects and the conformational stability of the sandwich-like structure formed in the gas phase [13]. In addition, the ion current at 849 Da is also believed to be due to the intermolecular substitution where two dimercaptans bridge two tin metals, 14. Two other unique adducts, the trimer, 15, and sodium bound dimer:monomer, 16, were also detected at 1277 Da. Structures 13 and 14 are isobaric and were differentiated through a series of variable-energy sustained off-resonance irradiation collisionally activated dissociation experiments (SORI-CAD) [18]. Very low energy was required for the successful fragmentation of the gas-phase bound dimer into the sodiated monomer.

HS (CH2)2 CO2

CH2

CH2

O2C (CH2)2 SH

C Bu

S (CH2)2 CO2

CH2

CH2

O2C (CH2)2 SH

S (CH2)2 CO2

CH2

CH2

O2C (CH2)2 SH

CH2

O2C (CH2)2 SH

Sn Bu

C HS (CH2)2 CO2

CH2

18

However, a step change in energy was then required for further fragmentation of the remaining parent ion, the intermolecular substitution product 14, depicted above. Once again, exact mass values and isotopic distributions were in excellent agreement with the proposed structures. The mass spectrum of the products of the reaction of DBTDL with M6 is shown in Fig. 9. The electrospray mass spectrum readily identified the presence of organic impurities lacking a tin center, which for the purpose of this study were ignored. Several unique tin-containing reaction products were also identified. The ion currents observed at 743 and 759 Da correspond to the sodiated and potassiated adducts of 17.

O

O S (CH2)2

Bu

C OCH2

Sn Bu

CH2O C

(CH2)2 SH

CH2O C

(CH2)2 SH

C S (CH2)2

C OCH2

O

O

17 The ion currents observed at 1231 and 1247 Da are sodiated and potassiated adducts of 18, and the ion currents observed at 1462 and 1478 Da are due to the sodiated and potassiated adducts of 19,

HS (CH2)2 CO2

CH2

CH2

O2C (CH2)2 SH

CH2

O2C (CH2)2 S

C S (CH2)2 CO2

Bu

CH2

Sn Bu

Bu Sn

S (CH2)2 CO2

CH2

HS (CH2)2 CO2

CH2

CH2

O2C (CH2)2 S

CH2

O2C (CH2)2 SH

Bu

C

19

shown below, as well as the sodium and potassium bound dimer of 17. Other identified organoetin complexes at 655, 1143, and 1374 Da are due to the replacement of the M6 with the tetramercaptan impurity, a trimercaptan, thus forming analogous trimercaptan compounds to 17, 18, and 19. This demonstrates the fact that these chemistries are complicated by impurities in the various starting materials. This complication was possibly a factor in the 119 Sn NMR studies, where for M5 and M6 studies, multiple 119Sn resonances were detected (Fig. 7c and d). In the latter, the presence of a trimercaptan impurity likely accounts for some of the NMR spectral complexity. Ready formation of cyclic and oligomeric tin compounds such as 13-19 observed in the electrospray mass spectra may account for the increased effectiveness of M5 and M6 in deactivating DBTDL

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221

Fig. 9. The ESI-FTICR mass spectrum of the reaction products of DBTDL and M6.

toward acrylate silane hydrolysis. These oligomer and cyclic structures form readily and once formed may be somewhat less susceptible to ligand exchange. It was reported that for certain tin mercaptoesters the oxygen in a carbonyl can be in a spatial position to bond with the tin center and increase the coordination of tin from tetrahedral to trigonal bipyramid or octahedral [11]. In the 119Sn NMR spectra of tin mercaptide and tin mercapto ester complexes, the chemical shifts for the majority of the products formed were found to be very similar. Even though M6 has carbonyl oxygen atoms in spatial positions that could potentially complex with the tin center, the NMR data did not indicate such complexes were forming in that no additional shielding of tin was observed in the 119Sn NMR (discussed below). As shown in Table 2, the product peak when DBTDL is reacted with M1 is at 125 ppm. When DBTDL is reacted with M3, which also contains a carbonyl capable of bonding with the tin center, the main product peak is shifted only slightly when compared with M1, to 120 ppm. Another tin mercapto ester, di-n-butyltin bis (2-ethylhexylmercaptoacetate), was also examined using 119Sn NMR. The 119Sn resonance of this compound is shifted upfield from the previously examined species, with a chemical shift of 67 ppm, as shown in Table 2. It is believed that the difference in the 119Sn chemical shift of this compound compared to other tin dimercaptides studied is significant. Mass spectrometry was used to confirm the identity and purity of this purchased material. The chemical shift for tin in this species provides some evidence that the acetate groups are weakly coordinating with the tin center thus increasing the coordination number. Others have indicated that the change in 119Sn chemical shielding due to a change in coordination is substantial, with a change from 4 to 5 or 6-fold coordination causing an

increase in tin shielding of several hundred ppm [14,15]. A 119Sn chemical shift of 245.5 ppm was reported for a six-coordinated tetraorganotin compound, a decrease of over 100 ppm over a similar 4-fold coordinated compound [19]. We obviously do not detect changes in 119Sn chemical shielding of this magnitude, but a change from nominally 125 ppme67 ppm for the di-n-butyltin bis (2-ethyl-hexyl-mercaptoacetate) is in the right direction (i.e. more shielding) and thus is consistent with a weak increase in Sn coordination. However, since we only detect 119Sn resonances around 113e127 ppm in our various ratio studies, it is confirmed that these mercaptans predominately form 4-fold coordinated tin species under all studied conditions.

3.3. Reaction of tin complexes with water The hydrolysis of DBTDL, and the role of water in general, was the target of additional NMR and mass spectrometric experiments. If water impacts the reaction of mercaptans with DBTDL, then one might expect to capture changes using 119Sn NMR. It was expected that the addition of water to the reaction mixtures in dichloromethane-d2 or tetrahydrofuran-d8 could enhance ligand exchange or cleavage of the carboxylic acid bonds of the laurate groups to form mono- and dihydroxy tin species. As proposed earlier, highly reactive hydroxy tin compounds can react with organofunctional alkoxysilanes to form stannosilane intermediates (Fig. 2). There was no evidence of hydroxy tin in the NMR studies, even when conducted at low temperature, showing that excess water to that already present in the NMR solvent does not affect the product distribution. Given the length of signal acquisition time (>30 s), transient hydroxyl tin species could not be completely ruled out.

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Fig. 10. The ESI-FTICR mass spectrum of DBTDL in neat THF.

Electrospray ionization mass spectrometry was utilized to further investigate the proposed hydrolysis/condensation mechanism depicted in Figs. 1 and 2. DBTDL was analyzed directly in neat THF. Following this analysis, the solution was modified with the addition of 18 MU water, yielding a 10% H2O/90% THF solution that was then analyzed by direct infusion. The results of these experiments confirmed DBTDL to form organotin oligomer condensation products upon reacting with water. The repeat unit molecular weight of these organotin oligomers is 248 Da, corresponding to the proposed repeat unit, 20, resulting in structure 3 (Fig. 1) and higher molecular weight condensation products analogous to structure 3.

Fig. 11. The ESI-FTICR mass spectrum of DBTDL in 10%H2O/90%THF.

(CH2)3CH3 Sn

O

n (CH2)3CH3

20

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223

Fig. 12. The negative ion electrospray mass spectrum of DBTDL depicting the most weakly bound ligands to be: laurate (199 Da), myristate (227 Da) and palmitate (255 Da).

The electrospray ionization mass spectrum of DBTDL in neat THF is shown in Fig. 10. The mass spectrum is somewhat complicated by the fact that: 1) the electrospray ionization of organotin complexes with weakly bound ligands, such as carboxylates, involves the cleavage of the most labile bond yielding a cationic tin-containing species [Ca]þ and the anionic component [An] [20], 2) the sample has already undergone some hydrolysis resulting in the most prominent ion current at 681.3 Da corresponding to the cationic component of the dimer, structure 3, as well as 3) the heterogeneity of the dilaurate, resulting in the stepwise increase of 28 Da (C2H4) due to myristate and palmitate analogs of the species described. However, the experiments were more than sufficient to demonstrate that the proposed hydrolysis mechanism was at work. In addition to the above, the second most abundant ion current, at 433 Da, was identified as the cationic component of DBTDL. Tandem collisionally induced dissociation (CID) experiments were performed to identify ligand attachment by traditional neutral loss values, in addition to the observed mass and isotopic distribution. The electrospray ionization mass spectrum of DBTDL in 10% H2O/90%THF is plotted in Fig. 11. Clearly evident is the increase of molecular species of higher molecular weight, a stepwise increase of 248 Da, once again corresponding to the cationic component of structure 3 (681 Da), and the cationic trimer (929 Da) and tetramer (1178 Da) condensation products analogous to structure 3, consistent with the expected chain elongation unit, 20, in the proposed oligomerization scheme. The positive identification of several extension products corresponding to structure 3, upon the addition of water, and higher molecular weight analogs of structure 3, is evidence that further hydrolysis and condensation reactions were at work. Furthermore, it was also noticed that upon the addition of a higher concentration of water to the solution, the solution became quite turbid, which again is consistent with the production of higher molecular weight condensation products. Unfortunately, these condensation products themselves were not

addressable by the solution-based technique of electrospray ionization. Fig. 12 depicts the negative ion electrospray mass spectrum of DBTDL. Clearly present, in decreasing abundance, are laurate, myristate, palmitate, as well as other fully saturated even number fatty acids. This, in addition to the positive ion tandem mass spectral experiments, provides additional verification of the heterogeneity of the carboxylate ligands. Additional experiments with tin mercaptides, specifically di-nbutyl bis(2-dodecylthio) tin, and water, resulted in oligomerization and a complex distribution of products. 4. Conclusions Acrylate silane hydrolysis studies were carried out with acrylate silane in the presence of DBTDL and varying levels of mercaptans. It was found that an excess of mercaptan functional group was necessary to prevent significant and rapid hydrolysis of the organofunctional silane, in most cases. Both the acrylate silane hydrolysis studies and the structural studies confirmed that a minimum ratio of 1:4 carboxylate groups of DBTDL:mercaptan functional group is required for most mercaptans examined. Multifunctional mercaptans, such as M6, pentaerythritol tetrakis(3mercaptopropionate), were shown to be effective in deactivating the tin catalyst toward silane hydrolysis, even at 1:1 carboxylate groups of DBTDL:mercaptan functional group concentrations. Careful structural characterization of tin mercaptide complexes demonstrated that a variety of mercaptans undergo ligand exchange reactions with DBTDL, a common catalyst used in the synthesis of coating raw materials. NMR and MS experiments have shown that the exact nature of the product is dependent upon the mercaptan ligands, where the dimercaptan substituted product is favored almost exclusively. The rate of reaction is solvent dependent, likely due to dissolved water content. Four fold coordination of the tin species was preserved in ligand exchange reactions.

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Oligomeric and cyclic tin mercaptide species were shown to form when multifunctional mercaptans were combined with DBTDL in solution. MS analyses indicated that rapid oligomerization of tin species occurred when water was present and that tin hydroxide intermediates, such as 2 and 11, are likely short lived. Acknowledgments We would like to acknowledge the important contributions of Dr. Susan Gasper, Dr. Ching-Kee Chien and Mr. Kevin McCarthy (Corning Incorporated). References [1] E.P. Plueddemann, Silane Coupling Agents, second ed., Plenum, New York, 1991. [2] Z.W. Wicks, F.N. Jones, S.P. Pappas, Organic Coatings: Science and Technology, vol. 2, Wiley, New York, 1994. [3] M.A. Brook, Silicon in Organic, Organometallic and Polymer Chemistry, Wiley, New York, 2000. [4] Y.A. Dubitsky, A. Zaopo, G. Zannoni, L. Zetta, Tin catalyzed hydrolysis of vinyltrialkoxysilanes studied by NMR spectroscopy, J. Mater. Sci. Lett. 19 (2000) 627e629. [5] F.W. van der Weij, The action of tin compounds in condensation-type RTV silicone rubbers, Makromol. Chem. 181 (1980) 2541e2548. [6] J. Toynbee, Silane crosslinking of polyolefins: observations on the tin catalyst employed, Polymer 35 (1994) 438e440. [7] L. Matĕjka, O. Dukh, J. Brus, W.J. Simonsick Jr., B. Meissner, Cage-like structure formation during solegel polymerization of glycidyloxypropyltrimethoxysilane, J. Non Cryst. Solids 270 (2000) 34e47.

[8] A.C. Draye, T.J. Tondeur, Kinetic study of organotin-catalyzed alcoholeisocyanate reactions: part 1: inhibition by carboxylic acids in toluene, J. Mol. Catal. A Chem. 138 (1999) 135e144. [9] L.G. Dammann, G.M. Carlson, Tin or bismuth complex catalysts and trigger cure of coatings therewith, US Patent 4,788,083 (1988). [10] D. Sukhani, V.D. Gupta, R.C. Mehrotra, Thiolysis of alkyltin alkoxides and oxides, J. Organometal. Chem. 7 (1967) 85e90. [11] C.H. Stapfer, R.H. Herber, The structure of organotin mercaptoesters, J. Organometal. Chem. 66 (1974) 425e436. [12] R.P. Houghton, A.W. Mulvaney, Mechanism of tin(IV)-catalysed urethane formation, J. Organometal. Chem. 518 (1996) 21e27. [13] NIST database: http://cccbdb.nist.gov/. [14] R.K. Harris, B.E. Mann, NMR and the Periodic Table, Academic Press, London, 1978. [15] B. Wrackmeyer, NMR of tin compounds, in: A.G. Davies, M. Gielen, K.H. Pannell, E.R.T. Tiekink (Eds.), Tin Chemistry: Fundamentals, Frontiers, and Applications, Wiley, New York, 2008, pp. 17e52. [16] B.E. Winger, S.A. Hofstadler, J.E. Bruce, H.R. Udseth, R.D. Smith, High-resolution accurate mass measurements of biomolecules using a new electrospray ionization ion cyclotron resonance mass spectrometer, J. Am. Soc. Mass Spectrom. 4 (1993) 566e577. [17] P.B. O’Connor, F.W. McLafferty, Oligomer characterization of 4e23 kDa polymers by electrospray Fourier transform mass spectrometry, J. Am. Chem. Soc. 117 (1995) 12826e12831. [18] M.W. Senko, J.P. Speir, F.W. McLafferty, Collisional activation of large multiply charged ions using Fourier transform mass spectrometry, Anal. Chem. 66 (1994) 2801e2808. [19] V.G. Kumar Das, L.K. Mun, C. Wei, T.C.W. Mak, Synthesis, spectroscopic study, and X-ray crystal structure of bis[3-(2-pyridyl)-2-thienyl-C, N]diphenyltin(IV): the first example of a six-coordinate tetraorganotin compound, Organometallics 6 (1987) 10e14. [20] M. Hol capek, L. Kolárová, A. R u zi cka, R. Jambor, P. Jandera, Structural analysis of ionic organotin(IV) compounds using electrospray tandem mass spectrometry, Anal. Chem. 78 (2006) 4210e4218.