Synthesis and Hydrolysis–Condensation Study of Water-Soluble Self

Article pubs.acs.org/Organometallics

Synthesis and Hydrolysis−Condensation Study of Water-Soluble SelfAssembled Pentacoordinate Polysilylamides Muhammad Sohail,*,† Alan R. Bassindale,† Peter G. Taylor,*,† Alexander A. Korlyukov,‡ Dmitry E. Arkhipov,‡ Louise Male,§ Simon J. Coles,§ and Michael B. Hursthouse§ †

Department of Chemistry and Analytical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, U.K. A.N. Nesmeyanov Institute of Organoelement Compounds (INEOS), Russian Academy of Sciences, 28 Vavilov Street, B-334, Moscow 119991, Russia § EPSRC National Crystallography Service, School of Chemistry, University of Southampton, Southampton, SO17 1BJ U.K. ‡

S Supporting Information *

ABSTRACT: Polysilylamides (n = 1−8) with a Si−Cl functionality containing pentacoordinate silicon in the backbone were produced in high yield by transsilylation of bis(chloromethyl)methylchlorosilane and the trimethylsilyl derivative of diketopiperazine. Pentacoordinate polysilylamides were highly soluble in water as a result of silicon water coordination (Si←OH2) from hydrolysis of the Si−Cl group in each repeat unit. Interestingly, the water silicon coordination in polysilanolamides was stable toward self-condensation and found to contain pentacoordinate silicon even in water, thus avoiding siloxane (Si−O−Si) bond formation. In the gas phase the polysilanolamides underwent intramolecular stepwise hydrolysis−condensation possibly as a result of CC double-bond formation at each monomer unit, as observed by MALDI-TOF MS. Low-intensity peaks of macrocyclic polysilanolamides (n = 2−5) were also observed that contain water molecules. For a better understanding of the hydrolysis−condensation process of the polysilylamide, new model compounds of pentacoordinated silicon derivatives of pyridones were synthesized, characterized, and compared with the polysilanolamides using NMR and X-ray crystallography. X-ray analysis of the model compounds revealed insight into the silicon water coordination in each repeat unit and the mode of packing within the polymers that contain these monomer units. It is found that the partial hydrolysis of the model pentacoordinate chlorosilanes gives water-coordinated pentacoordinate silicon species that resemble an intermediate in the aqueous hydrolysis of pentacoordinate polysilylamides. pentacoordinate silicon atoms in a single molecule.18−23 Shea and co-workers24−27 reported the first examples of linear and cyclic polymers 3 with pentacoordinate siliconate groups, using catechol-type ligands. Lambert and co-workers28 have prepared and characterized pentacoordinate silicon macrocycles 4 based on Tacke ligands,29−31 containing protonated amine groups. Silicon-containing species have been known to be characterized by mass spectrometry, NMR, and X-ray crystallography. Recently soft ionization techniques such as matrix-assisted laser desorption/ionization (MALDI)32−34 have enabled a better understanding of the structure and composition of polymeric organosilicon materials.35,36 However, the characterization, even with soft ionization techniques, can be very challenging due to the fragmentation of activated molecular ions.37 In solution, NMR is a very useful technique to characterize silicon-containing materials together with their dynamic behavior, especially by using 29Si chemical shifts to determine the coordination number

1. INTRODUCTION Monoorganosilanes in common use have one organic nonhydrolyzable group and three hydrolyzable substituents, RSi(OR)3 or R-SiCl3.1−8 The hydrolysis of alkoxy silanes into silanols (Si−OH) and polycondensation into siloxanes (Si−O− Si) containing a nonhydrolyzable Si−R group leads to a variety of silsesquioxanes with various ladder network and cage-like structures.7,9 Fully condensed silsesquioxanes have been synthesized and characterized as cage structures including a few examples of host−guest species 1.10 Incompletely condensed structures containing Si−OH groups are also known.9,11 These species have one or more −OH groups per structural unit and can form ladder-type, open, and linear structures.5,12,13 Chlorosilane compounds have also been utilized in condensation reactions, in some cases to replace alkoxysilane compounds, due to the relatively high reactivity of the Si−Cl bond toward Si−O− Si bond formation.14−17 In contrast to carbon, silicon displays the ability to form hydrolyzable hypercoordinate silicon species with five, six, and seven substituents. Some such compounds 2 contain two © 2013 American Chemical Society

Received: December 1, 2012 Published: February 26, 2013 1721

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For a better understanding of the hydrolysis−condensation process, monochelates 10a,b and bischelates 11a,b were prepared as model compounds by the transsilylation of silylated pyridones 9. Monochelates were hydrolyzed into tetracoordinate disiloxanes 12 (Scheme 1). Scheme 1. Synthesis of Monochelates 10a,ba and Bischelates 11a,b As Model Compounds by the Transsilylation of Silylated Pyridones 9

of silicon.38−40 X-ray crystallography is also extensively used for modeling pentacoordinate silicon species as intermediates in the SN223,40 pathway and Berry pseudorotation process.41 In this contribution, the synthesis, characterization, and mechanistic insight based on a polymer containing pentacoordinate silicon in the backbone and the corresponding model compounds have been investigated by MALDI-TOF, NMR, and X-ray crystallography.

a

2. RESULTS AND DISCUSSION We report here the first example in which chlorosilanes with organic substrates self-assemble into a polymer capable of pentacoordination at silicon. The silylated diketopiperazine (DKP) 5 was treated with di(chloromethyl)methylchlorosilane 6 in toluene to give an oligomeric mixture of 7 (linear) and 8 (cyclic) with one Si−Cl functionality per monomer unit (eq 1). In spite of the variety of solvents used (polar to nonpolar), all the reactions tested afforded the same results.

These were further hydrolyzed into tetracoordinate disiloxanes 12.

The model compounds, their hydrolyzed intermediates, and products were isolated and subjected to NMR and X-ray analysis. These model compounds not only provide accurate molecular parameters for the water silicon coordinated (Si←OH2) fragment, for use in the structural analysis of the polysilanolamide, but also give insight into the nature of the polymer−water interaction. One may argue that the aromatic pyridone derivatives are not perfect models for the saturated monomer unit in the pentacoordinate polysilanolamide. However, as has been suggested in our previous work, the nature of the nucleophilic carbonyl species is not a key factor in determining the degree of pentacoordination at the silicon atom.23,39,40

3. MALDI-TOF MS STUDY OF THE HYDROLYSIS−CONDENSATION PRODUCTS Recently, MALDI-TOF mass spectrometry has become a very powerful tool for the investigation of siloxanes,42 silsesquioxanes,36,43 and organic molecules on silicon44 and silica surfaces.45 Hence this technique was used to determine the molecular weights, molecular weight distributions, and structural details of the polysilylamides 7 and 8. 3.1. Linear Polysilylamide 7. Peaks from the polysilylamides 7 were detected from 497 m/z to about 1650 m/z, corresponding to oligomeric products (n = 2−8) in which each major cluster is separated by 200 g/mol (Figure 1). The nominal separation between these major clusters is equal to the repeat unit (−RCH2Si(OH)MeCH2−, theoretical value = 200.27g/ mol, where R = diketopiperazine ring). The silanol is observed as a result of hydrolysis in the matrix (Scheme 2). The polysilanolamides have one unreacted −SiOH group per repeat unit. If the silanols condensed to give siloxanes, we might expect to see cross-linked polymers and branching; however this was not observed in the MALDI-TOF MS. The regular repeat unit spacing of 200 g/mol indicates that a linear structure is obtained, as shown in Figure 1. Although peaks at higher m/z values are not observed in this spectrum, the formation of higher molecular weight species cannot be ruled out, since the MALDI-TOF

A preliminary characterization of the spectroscopic properties of the precipitate containing 7 and 8 was puzzling. For example, in the 1H NMR, the characteristic resonance of the terminal −NH groups was not clear enough to decide whether the polymer was cyclic or linear. Although it was clear from the 29Si NMR chemical shifts that the silicon atoms in the polymeric material were pentacoordinated. The first clues to the identity of 7 and 8 came from the matrix-assisted laser desorption/ ionization (MALDI-TOF) mass spectrometry in positive ion mode. MALDI-TOF MS analysis enabled us to characterize 7 and 8 as polysilylamides, providing information on the molecular mass and the specific number of pentacoordinate silicon groups per repeat unit. It was found that all the Si−Cl bonds of the polymers 7 and 8 were hydrolyzed into water silicon coordinated fragments, Si←OH2, within the matrix. However surprisingly it is evident from the NMR data that even in water these fragments did not self-condense into siloxane (Si−O−Si) to give silicone polymers. 1722

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Figure 1. MALDI-TOF MS in the 300−1650 m/z range showing the characteristic shape of an oligomeric mixture (where n = 2−8), in which each major cluster is separated by a nominal spacing of 200 g/mol.

Closer examination around the region of each oligomer shows that the spectrum is made up of clusters of more than one peak separated by 18 g/mol, which indicates the loss of water molecules (Figure 2). The major group at 497.1, 697.2 m/z and the next highest oligomer at 897.2 m/z correspond to an oligomer consisting of two (n = 2), three (n = 3), and four (n = 4) monomers, respectively, and result from the protonation and loss of a molecule of water. Each peak within the major cluster consists of a set of isotopically resolved individual peaks. To the left of the higher intensity cluster at 697.2 m/z (n = 3), there is a less intense minor cluster at 679.2 m/z, 18 g/mol lower in mass, corresponding to a polymer that has lost a water molecule. Further to the left at 661.2 m/z, an even lower intensity minor cluster was observed, corresponding to the loss of a further water molecule. This lowest minor cluster corresponds to the complete loss of water for this size of oligomer. With the tetramer the process continues for a fourth time to give a peak at 843.4 m/z, indicating that the oligomer has released all four water molecules. The loss of water could occur when one −SiOH group reacts intramolecularly with the hydrogen of a CH2 to form a CC double bond, 7e (Scheme 2).37,48 The number of CC double bonds is proportional to the number of silanol Si−OH groups in an oligomer (n).

Scheme 2. Proposed Hydrolysis−Condensation Mechanism of Pentacoordinate Polysilylamides 7a−e, As Observed in the MALDI-TOF MS

technique tends to show considerable molecular weight discrimination for polydisperse samples.46,47

Figure 2. Details of the MALDI-TOF mass spectrum of the polysilylamide 7 showing a spacing of 18 g/mol within a major cluster of dimers (n = 2, d = 1, 497; d = 2, 479 m/z), trimers (n = 3, d = 1, 697; d = 2, 679; d = 3, 661 m/z), and tetramers (n = 4, d = 1, 897; d = 2, 879; d = 3, 861; d = 4, 843 m/z). The 18 g/mol spacing in these single major clusters is indicative of a loss of water. 1723

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Table 1. Assignment of MALDI-TOF MS Peaks of Polysilanolamides 7 in the Range 460−845 m/za

a

m/z (exptl)

species

structures

m/z (theoretical)

not observed 497.1 479.1 not observed 697.2 679.2 661.2 not observed 897.2 879.2 861.2 843.2

n = 2, d = 0 n = 2, d = 1 n = 2, d = 2 n = 3, d = 0 n = 3, d = 1 n = 3, d = 2 n = 3, d = 3 n = 4, d = 0 n = 4, d = 1 n = 4, d = 2 n = 4, d = 3 n = 4, d = 4

C18H31N6O8Si2+=poly(OH)2 C18H29N6O7Si2+=poly(OH)-H2O C18H27N6O6Si2+=poly-2H2O C25H43N8O11Si3=poly(OH)3 C25H41N8O10Si3+=poly(OH)2-H2O C25H39N8O9Si3+=poly(OH)-2H2O C25H37N8O8Si3+=poly-3H2O C32H55N10O14Si4+=poly(OH)4 C32H53N10O13Si4+=poly(OH)3-H2O C32H51N10O12Si4+=poly(OH)2-2H2O C32H49N10O11Si4+=poly(OH)-3H2O C32H47N10O10Si4=poly-4H2O

515.17 (+H+) 497.16 (+H+) 479.15 (+H+) 715.24 (+H+) 697.22 (+H+) 679.21 (+H+) 661.20 (+H+) 915.30 (+H+) 897.29 (+H+) 879.28 (+H+) 861.27 (+H+) 843.25 (+H+)

See Supporting Information for detailed structures.

quite rapidly. This means that species with more OH groups are more likely to lose water; hence the intensity of signals corresponding to d = 2 increases relative to that of d = 1 with the increasing number of silicon atoms (Figures 1 and 2). Indeed for n = 4 the major peak is not d = 1 (897 m/z) but d = 2 (879 m/ z). 3.2. Cyclic Polysilanolamides-Macrocycles 8. Closer examination of the MALDI-TOF spectra shows not only highintensity peaks of the linear oligomers but also lower intensity peaks attributable to cyclic oligomers 8 (n = 2−5) (Figure 4). The lower intensity regions of the spectrum represent five oligomers of masses n = 2−5. As with the linear polysilylamides 7, each peak in the cyclic polysilylamides 8 consists of groups of more than one peak with nominal mass separation between the two major clusters of 200 m/z, which is equal to the isotopic averaged mass of the repeat unit [−R(MeSiOH)CH2−, where R is DKP] and one −SiOH group per repeat unit. In contrast to the linear polysilaneamides, the peaks of each cluster of macrocycle are above the parent ion and separated by 18 m/z probably as a result of hydration instead of condensation. This could be a result of water molecules residing on the cyclic polysilanolamides by strong hydrogen bonding. To the right of the cluster at 601.1 m/z (n = 3), there is a more intense cluster at 619.1 m/z, which is 18 g/mol higher in mass, as a result of one hydrogen-bonded water molecule residing on the cyclic polysilanoamides. The lower intensity peak at 637.1 m/z further to the right, higher in mass by another 18 g/mol, is evidence of the contribution of a second water molecule (Figure 4). Similarly, peaks of macrocyclic oligomers with different degrees of hydration were observed at 419 m/z (n = 2), 801.2 m/ z (n = 4), and 1002.2 m/z (n = 5) (Table 2). For the hydration products, the mass m of the cyclic products is given by the equation

The peaks arising from the intramolecular condensation are shown for every oligomer of the polysilylamide 7 in Table 1.35,36 For the condensation products, the mass m of the linear products having n repeat groups is given by the equation m = (114 + 200n) + p − (18d)

(2)

where n is the number of repeat units (which is equal to the number of Si atoms per oligomer), p is the mass of proton (1g/ mol), 114 g/mol is the molecular mass of the DKP terminal group, and d is equal to the number of double bonds in the molecule or degree of intramolecular condensation. The MALDI-TOF mass spectrum gives information on not only the primary distribution of species based on the number of silicon atoms (n) in their structures but also a secondary distribution of the species with different values of d for a constant value of n. The major peaks observed at 497.1 and 697.2 m/z are assigned to the species having n = 2, d = 1, n = 3, d = 1, respectively, ionized with H+ (Figure 3b). The calculated isotopic

Figure 3. Simulated (a) and experimental (b) isotopic patterns of MALDI-TOF MS signals at 497.1 m/z (n = 2) and 697.2 m/z (n = 3).

m = (200n) + p + (18d)

distributions corresponding to these two major structures consist of a set of isotopically resolved individual peaks (Figure 3a). The experimentally observed patterns are in excellent agreement with the predicted molar mass values. However, experimental values at n = 2−8 and d = 0 were not observed in the spectra. The absence of these species suggests that once hydrolysis has occurred in the matrix (a → b → c, Scheme 2) and the intramolecular condensation has occurred in the gas phase (c → e, Scheme 2), further loss of water can occur

(3)

where n is the number of repeat units (which is equal to the number of silicon atoms in the macrocycle), p is the mass of a proton (1 g/mol), and d is equal to the water of hydration in the cavity of the macrocycles. These oligomers are highly soluble in water, and their 1H and 29 Si NMR spectra show no evidence of aggregation or micelle formation with time. 1724

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Figure 4. Low-intensity signals showing the characteristic features of the cyclic oligomeric polysilylamide 8. The major cluster to the left is for n = 2, whereas that on the right is for n = 5. The peaks of each cluster are above the parent ion and separated by 18 m/z as a result of hydration.

Table 2. Assignment of MALDI-TOF MS Peaks of the Cyclic Polysilanolamide 8 in the Range 400−1050 m/z m/z (exptl)

species

structures

m/z (theor)

401.2 419.1 601.1 619.1 637.1 801.2 819.2 1001.2

n = 2, d = 0 n = 2, d = 1 n = 3, d = 0 n = 3, d = 1 n = 3, d = 2 n = 4, d = 0 n = 4, d = 1 n = 5, d = 0

C14H25N6O4Si2=poly(OH)2 C14H27N6O5Si2=poly(OH)2+ H2O C21H37N6O9Si3=poly(OH)3 C21H39N6O10Si3=poly(OH)3+ H2O C21H41N6O11Si3=poly(OH)3+ 2H2O C28H49N8O12Si4=poly(OH)4 C28H51N8O13Si4=poly(OH)4 + H2O C35H61N10O15Si5=poly(OH)5

401.13 (+H+) 419.1 (+H+) 601.19 (+H+) 619.2 (+H+) 637.21 (+H+) 801.25 (+H+) 819.2 (+H+) 1001.32 (+H+)

Table 3. Comparison of Selected NMR Chemical Shifts from 1H, 13C, and 29Si NMR for 7 and the Model Complexes of 10 and 11a 7 (in MeOD) 1

H

a

13

C

29

Si

SiMe NCH2 SiCH2 CO NCH2

0.2−0.3, three peaks 2.5−2.8, three peaks 3.8−4.3(CH2CO), three peaks 164, 171, 170 ∼37, three peaks −9, −40, −41, −42 −37.5, −38, −38.2 (D2O)

10a (6-Me)

10b (6-Cl)

11a (6-Cl)

11b (5-Me)

0.75 3.6 (2H, s, NCH2) 3.2 (2H, s, ClCH2)

0.7 3.7 (2H, d, JAB = 17.04, NCH2) 3.2 (2H, s, ClCH2)

0.5 3.8 (4H, q, JAB = 16.83) 6.69−7.9 (3H, m, arom)

0.47 3.8 (4H, q, JAB = 17.04, NCH2) 6.8−8.0 (3H, m, arom)

164.6 39 −50.2

164.7 40 −50

163.8 37.89 −47.5

161.3 38.5 −47.3

For numbering scheme, see Figure 5A.

4. NMR STUDIES OF POLYSILYLAMIDE AND MODEL COMPOUNDS The mechanism of hydrolysis of chlorosilanes via nucleophilic substitution is one of the key research areas of organosilicon chemistry.39 Many pentacoordinate silicon species with Si−X (F, Cl, Br, I, OTf) have been prepared as models along the SN2 reaction profile.40,49 1H, 13C, and 29Si NMR spectroscopic studies of 6−12 provide useful information for the characterization of silicon species; in particular the 29Si chemical shift is sensitive to the coordination number at silicon. Generally, 29Si NMR chemical shifts move upfield on intramolecular coordination between the amide carbonyl and the silicon atom (O→Si) to form a five-membered ring and a pentacoordinate silicon atom.39 In CD3OD, four upfield sharp signals in the 29Si NMR spectra were observed for polysilylamides as evidence of pentacoordination at silicon, one at −9.3 ppm and a triplet of two lower intensity peaks at −40 and −42 ppm and one higher intensity peak at −41 ppm (Table 3). These multiple signals might be a result of intra- or intermolecular interactions that stabilize different conformers or could arise from different solvates of the same silicon species. The 1H NMR spectrum of 7 in methanol

shows a methanol methyl singlet at 4.4 ppm, which is unusually downfield (one would expect it at 3.3−3.4 ppm) and could be a result of silicon methanol coordination. The 29Si signal at −9.3 ppm disappeared, and the remaining three resonances are shifted downfield by 3 ppm upon dissolution in D2O. This is possibly due to complete hydrolysis of all the Si−Cl groups into Si←OH2 groups. However they are still sufficiently upfield, at −37.5, −38, and −38.2 ppm, to suggest retention of pentacoordination at the silicon atoms. The most likely explanation is extra coordination at silicon by water (Si←OH2), as has been observed previously.50 Interestingly, aqueous solutions of the polysilylamide did not undergo any further transformation at room temperature over a period of six months. No evidence was found of the formation of Si−O−Si bonds through condensation. In contrast to polysilylamides, model compounds of 10a,b undergo hydrolysis−condensation into tetracoordinate disiloxanes even in anhydrous solvents (Scheme 3).51 The 29Si NMR chemical shifts of mono (10a,b) and bis (11a,b) pentacoordinate compounds were in the range −47 to −50 ppm, typical of pentacoordinate complexes and do not change with temperature (−70 to +140 °C) (Table 3). 1725

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in the NMR time scale at room temperature (to be discussed in a subsequent paper).

Scheme 3. Plausible Hydrolysis−Condensation Mechanism for Chlorosilanes 10a51,52

a

5. HYDROLYSIS−CONDENSATION STUDIES BY X-RAY CRYSTALLOGRAPHY Further to the synthesis, MALDI-TOF, and NMR study, we present X-ray crystallography studies of some model compounds that give insight into the pentacoordination at the silicon atom, interaction with water, mode of packing, and inter- or intramolecular contacts in these systems. There are very few examples in which hydrolyzed pentacoordinate intermediates have been isolated.49,50 We are unaware of any case where the Si−Cl and Si←OH2 compounds of a polymer precursor have been reported. In support of the proposed Schemes 2 and 3, pentacoordinate chlorosilanes 10a,b and their hydrolyzed intermediate products (ii−iv, Scheme 3) were isolated and subjected to X-ray crystallography. Careful hydrolysis of the Si− Cl bond followed by slow evaporation of the solvent led to crystals of the intermediate species (Si←OH2, 10a.H2O and 10b.H2O) and a further condensed disiloxane (Si−O−Si, 12.HCl), Figure 5. 5.1. Structural Characterization of the Pentacoordinate Chlorosilane (Si−Cl). In the X-ray single-crystal structure of 10b, the Si−Cl bond distance is 2.2911(9) Å and the intramolecular coordination of the silicon oxygen atom of the amide carbonyl (O→Si) has a bond distance of 1.913(2) Å (Table 4). The geometry around the pentacoordinate silicon is trigonal bipyramidal (TBP), where two CH2 and one methyl group occupy the equatorial position and the axial positions are shared between the oxygen and chlorine atom with a O−Si−Cl bond angle of 170.51(6)°, Figure 5A. Although the silicon atom deviates from the TBP plane toward the chlorine atom by 0.049 Å, the sum of the equatorial C−Si−C bond angles is 359.79°, which is very close to that of an ideal TBP. Similar geometrical parameters around the silicon would be expected at each monomer unit of the polymer. Generally, chlorosilanes are very sensitive to moisture and undergo a hydrolysis−condensation to

R = 6-Me, 6-Cl, and 5-Me.

The 13C NMR spectrum for polysilylamides shows three carbonyl signals at 164, 171, and 170 ppm, which probably correspond to coordinated, noncoordinated, and terminal carbonyl groups, respectively. In contrast to polysilylamides, only one carbonyl shift in the range 161−164 ppm was observed for both carbonyl groups in each of the bis pentacoordinate compounds 11b (5-Me), which indicates that both carbonyls are coordinated to the silicon atom as a result of Si−Cl ionization. Similar results were observed for 11a (R = 6-Cl), where an electron-withdrawing group was introduced into the ring to reduce the Si−O coordination by suppressing the Si−Cl ionization (Table 3). As in the 13C NMR, three broad signals were found in the 1H NMR spectrum of the polysilylamides, corresponding to the SiCH2 around 2.6 ppm and the cyclic CH2 around 4.0 ppm as a result of coordinating, noncoordinating, and terminal sites of the ligand. The signals of the NCH2 protons at 3.8 ppm appear as multiplets of an AB system (2JHH = 16−17 Hz) as a result of the presence of the chiral silicon atoms in the pentacoordinate derivatives 10a,b and 11a,b. This indicates that the Δ (righthand) and Λ (left-hand) enantiomers are configurationally stable

Figure 5. Schematic crystallographic structures showing the hydrolysis−condensation mechanism. ORTEP drawing of 10b (A), 10b.H2O (B), 10a.H2O (C), and 12.HCl (R = H, D) with the thermal ellipsoids shown at the 50% probability level. Carbon-bonded hydrogen atoms are omitted for clarity. 1726

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Table 4. Comparison of Selected Bond Lengths [Å] and Bond Angles [deg] for the X-ray Single-Crystal Structures of 2 and 10−12 2 (X = Cl) Si−Oamide Si−Owater Si−Cl O−Si−O/Cl ΔSia ∑Ceq−Si−Ceq refs a

10b (R = 6-Cl)

10b.H2O

10a.H2O

11b (R = 5-Me)

11a (R = 6-Cl)

12.HCl

2.04(1)

1.913(2)

2.280(1) 170.54(4) 0.102 359.11 23

2.291(1) 170.51(6) 0.049 359.79 this work

1.893(2) 1.877(2) 4.357(1) 170.43(8) 0.026 359.77 this work

1.900(1) 1.856(1) 4.315(1) 175.20(6) 0.020 359.94 this work

1.874(2) 1.870(2) 4.167(1) 169.64(9) 0.018 359.97 this work

1.843(1) 1.874(1) 4.104(1) 168.98(5) 0.001 360 41

3.570(2) 1.614(1) 4.054(1) 167.03(4) 0.657 325.30 this work

ΔSi is the deviation from the equatorial plane.

Figure 6. X-ray crystal packing and arrangement of hydrogen bonding within a crystal of 10a.H2O (R = 6-Me) along the a-axis. Carbon-bonded hydrogen atoms are omitted for clarity.

Figure 7. X-ray crystal packing and the hydrogen-bonding arrangement within a crystal of 10b.H2O (R = 6-Cl). Carbon-bonded hydrogen atoms are omitted for clarity.

Extensive intermolecular hydrogen bonding throughout the crystal packing of 10a.H2O (R = 6-Me) and 10b.H2O (R = 6-Cl) was observed, which provides insight into how this type of monomer may organize within the polymer. Each of the chlorine atoms in 10a.H2O is hydrogen-bonded with three hydrogen atoms, two of which are associated with the free water molecules, and the third hydrogen atom is associated with the water molecule coordinated to the silicon atom, Figure 6. Effectively there are chains of hydrogen bonding running through the structures. These chains are made of puckered, fused six- and 10-membered rings with hydrogen atoms in alternate positions. The chains extend indefinitely along the b- and c-axes, repeating every 8.825 and 13.186 Å, respectively. Along the aaxis, the chains stack one above the other, repeating every 11.806 Å. In 10b.H2O (R = 6-Cl), no additional water molecules are present in the unit cell and the chloride ion is positioned as a bridge between the two water molecules coordinated to a silicon atom (Figure 7). There are no rings in the chains but a zigzag parallel pattern along the a-axis, repeating every 7.839 Å. Along

give water silicon coordination (Si←OH2), which further condenses into siloxanes (Si−O−Si).5,13 5.2. Structural Characterization of the Pentacoordinate Water Silicon Coordination (Si←OH2). X-ray crystallography reveals that the Si−Cl bonds in the pentacoordinate chlorosilanes 10a (R = 6-Me) and 10b (R = 6-Cl) are partially hydrolyzed into Si←OH2 fragments (10a.H2O and 10b.H2O), which is analogous to a backbone unit of the polysilylamide. The silicon atom is essentially pentacoordinate, where one axial position is occupied by the oxygen atom of a water molecule, as proposed in Schemes 2 and 3, and Figure 5. One of the water molecules in 10a.H2O is hydrogen-bonded to another water molecule, which in turn is hydrogen-bonded to a chloride ion to form a 3D framework (Figures 5C, 6). In contrast, the water molecule in 10b.H2O is hydrogen-bonded directly to the Cl ion only (Figures 5B, 7). The crystal structures of both compounds show that the chloride ion in the crystal is not bonded directly to the silicon. The distance between the silicon and the chlorine atoms in 10a.H2O and 10b.H2O is 4.315−4.357 Å by comparison with a Si−Cl bond distance of 2.2911(9) Å in 10b (R = 6-Cl). 1727

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Figure 8. ORTEP drawing of 11b (R = 5-Me) with thermal ellipsoids shown at the 70% probability level. Water molecules in A and hydrogen atoms in B are omitted for clarity.

the b- and c-axes, the chains stack one above the other, repeating every 21.022 and 7.455 Å, respectively. The Si−Oamide bond length corresponding to intramolecular pentacoordination at silicon atoms by the oxygen atom of the amide carbonyl in the chlorosilane 10b is 1.913(2) Å, and those in the water-coordinated silicon species (10a.H2O and 10b.H2O) are 1.893(2)−1.900(1) Å, Table 4. The Si−Owater bond distances are similar to the Si−Oamide bond distances, and the Oamide−Si− Owater axial bond angle is 170.43(8)−175.20(6)°. In the absence of ring strain the structure should be perfectly trigonal bipyramidal. This is confirmed by the fact that the three equatorial substituents and the silicon atom effectively lie in the same plane (ΔSi = 0.02−0.049 Å). The sum of the three Ceq−Si− Ceq bond angles is 359−359.9°, very close to the ideal value of 360°. 5.3. Structural Characterization of the Disiloxane (Si− O−Si). Normally, chlorosilanes undergo hydrolysis−condensation to give hydrolyzed siloxane products. In the crystal structure of the hydrolyzed siloxane 12.HCl, two HCl molecules were hydrogen-bonded with an oxygen atom of each amide as shown in Figure 5D. As expected, the silicon atom in disiloxane is tetracoordinate with a tetrahedral geometry at the silicon. The Si−O bond length of the protonated condensed disiloxane (1.6 Å) is shorter than the Si−O1 bond length (1.877(2)−1.856(1) Å) of 10a.H2O and 10b.H2O. The sum of the C−Si−C bond angles in the protonated disiloxane is 325.30°, which is very different from a trigonal bipyramidal environment but is very close to an ideal tetrahedron. The O2 atom lies on an inversion center, and consequently the Si−O−Si linkage is linear with a bond angle of 180°. 5.4. Structural Characterization of the Hydrate of Bischelate Siliconium Cations (O−Si−O). In contrast to the polysilylamide 6, both of the carbonyl oxygen atoms of the bischelated amides 11a and 11b are coordinated to the silicon atom as a result of Si−Cl ionization, as confirmed by the X-ray crystal structure, Figure 8. The structures have propeller-like shapes with pairs of left-handed (Λ) and right-handed (Δ) enantiomers in a unit cell. The water molecules present in the crystal structure are hydrogen-bonded to the chloride ion. The Si−O bonds in the biscationic complexes 11a,b are significantly shorter than those in the monochelate neutral precursor 10b (Table 4). This indicates that the Si−O coordination in the former is stronger than that in the latter and may explain the more facile Si−Cl ionization of the bischelate complexes compared to the monochelate complex. The Si−O bond lengths

of 11a and 11b remain the same irrespective of the nature of the substituent on the ligand ring (Table 4). Each chloride ion forms a nearly linear fragment of Me−Si---Cl with a bond angle of 160− 175° and a long Si−Cl distance of 3.7−5.1 Å. The asymmetric unit of the crystal structure contains a fully ordered silicon-centered cation with one fully occupied chloride position, Cl1, a second chloride anion, Cl2, with 0.75 occupancy, and a three-site disordered mixture of 0.75 H3O+ and 0.25 H2O (Figure 8). Hydrogen atoms have not been located on the disordered sites, but the O···Cl2 distances indicate the presence of hydrogen-bonding interactions, in which only the anions are acceptors. Although some rings are necessarily parallel by symmetry, the distances between their centroids are too large for any significant stacking.

6. SIGNIFICANCE OF THE MODEL STRUCTURES A purpose of this work was to examine the relationships and trends in the structures of the model mononuclear, binuclear, and linear short-chain and cyclic polysilylamide along the hydrolysis−condensation reaction profile. The comparison of the structures studied in this work with those of the related compounds would permit the identification of structural trends in the polymer. This information might allow reasonable predictions to be made about the conformation and packing behavior of the polysilylamide. The structural parameters for the model compounds are listed in Table 3 in the Supporting Information. A comparison of the structural data of the mono- and binuclear pentcoordinate silicon amide complexes shows that, although some variations exist in the values of the Si−O bond lengths in each compound, the arrangements of atoms around the silicon atom have similar TBP geometry. The values of the Si−O bond lengths, 1.913(2), 1.893(2), and 1.900(1) Å, for 10b, 10b.H2O, and 10a.H2O are shorter than those in the binuclear complex 2 (X = Cl), 2.0436(13) Å. Thus the Si−O bond distance in the corresponding polymer is expected to be >2 Å, which is also confirmed by the 29Si NMR chemical shifts for the polymer of around −40 ppm as compared to the corresponding monomers with a chemical shift of around −50 ppm. Perhaps the most interesting results of this study are from the structures of 10b.H2O and 10a.H2O. The structures provide insight into the possible arrangements of relatively larger size polymer with the Si←OH2 group at the backbone, as well as the confirmation of the backbone itself. The most striking feature of these molecules is the coordination of a water molecule to the silicon atom, O−Si← 1728

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Organometallics

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X-ray crystallography services by using a Bruker-Nonius APEX II CCD camera (f scans and w scans to fill the asymmetric unit). Synthesis of Polysilylamides (7 and 8). Bis(chloromethyl)chloromethylsilane 6 (0.3 g, 2.5 mmol) was added slowly to a stirred solution of N,N-bis(trimethylsiloxy)diketopiperazine 523(2.5 mmol) in dry toluene under nitrogen. The solid formed on stirring was further refluxed for 3−6 h and filtered under nitrogen and dried under vacuum. Yield: 95%, mp 300−350 °C. NMR spectroscopic data: δ1H (300 MHz)/CD3OD 0.2−0.3 (s, 3H, SiCH3, three peaks), 2.6−2.8 (s, 4H, NCH2, three peaks), 3.8−4.3 (s, 4H, CH2CO, three peaks); δ13C (75 MHz, CD3OD) −0.78 (C4, 5), 37.35 (NCH2), 50.1 (C3), 165.5 (C2); δ29Si (79.5 MHz, CD3OD) −9, −40, −41, −42; (D2O) −37, −38, and −38.2. A similar product was observed when the reaction was repeated in hexane and acetonitrile. 1-[Chloro(chloromethyl)methylsilylmethyl]-6-methyl-2-pyridone (10a). Bis(chloromethyl)chloromethylsilane 5 (1.0 g, 6 mmol) was added slowly to a stirred solution of 6-methyl-2-trimethylsiloxypyridine53 (1 g, 5.5 mmol) in dry toluene under nitrogen. The reaction mixture was stirred for 3 h. The solid was filtered under nitrogen and dried under vacuum. Yield: 80%, mp 120 °C. NMR spectroscopic data: δH (300 MHz, CDCl3, Me4Si) 0.75 (3H, s, SiMe), 2.6 (6-Me), 3.2 (2H, s, ClCH2), 3.6 (2H, s, NCH2), and 6.7−7.7 (3H, m, arom); δC (75 MHz, CDCl3, Me4Si) 6.3 (SiMe), 20.6 (6-Me), 37 (ClCH2), 39 (NCH2), 112.69 (C5), 113.2 (C3), 140.5 (C4), 143.5 (C6), and 164.6 (C2); δSi (79.5 MHz, CDCl3, Me4Si) −50.2. Anal. Calcd: C, 43.20; H, 5.24; N, 5.60. Found: C, 42.90; H, 5.74; N, 5.20. 1-[Chloro(chloromethyl)methylsilylmethyl]-6-chloro-2-pyridone (10b). Bis(chloromethyl)chloromethylsilane 5 (1.0 g, 6 mmol) was added slowly to a stirred solution of 6-chloro-2-trimethylsiloxypyridine53 (1 g, 5.5 mmol) in dry toluene under nitrogen. The reaction mixture was stirred for 3 h. The solid was filtered under nitrogen and dried under vacuum. Yield: 96%, mp 121 °C. NMR spectroscopic data: δH (300 MHz, CDCl3, Me4Si) 0.77 (3H, s, SiMe), 3.2 (2H, s, ClCH2), 3.7 (2H, d, JAB = 17.04 Hz, NCH2), and 6.8−7.7 (3H, m, arom); δC (75 MHz, CDCl3, Me4Si) 6.4 (SiMe), 37 (ClCH2), 40 (NCH2), 112.69 (C5), 113.2 (C3), 140.5 (C4), 143.5 (C6), and 164.7 (C2); δSi (79.5 MHz, CDCl3, Me4Si) −50. Anal. Calcd: C, 35.51; H, 3.72; N, 5.18. Found: C, 35.41; H, 3.92; N, 5.08. [Bis(6-chloro-2-pyridonemethyl)]methylsiliconium Chloride (11a) (ref 41). Bis(chloromethyl)methylchlorosilane (0.23 g, 1.3 mmol, 1:2) was added slowly to a stirred solution of 6-chloro-2trimethylsiloxypyridine53 (0.5 g, 2.6 mmol) in dry toluene under nitrogen. The reaction mixture was stirred for 1 h and then heated with a hot gun. The solid was filtered under nitrogen and dried under vacuum to afford 0.9 g, 90% yield and crystallized for X-ray analysis from hot chloroform to get colorless crystals. Mp: 91−94 °C. NMR spectroscopic data: δH (300 MHz, CDCl3, Me4Si) 0.50 (3H, s, SiMe), 3.85 (4H, q, JAB = 30 Hz, 2NCH2), and 6.6−7.9 (6H, m, arom); δC (75 MHz, CDCl3, Me4Si) 1.4 (SiMe), 37.89 (NCH2), 112.35 (C5), 115.2 (C3), 142.1 (C6), 144.8 (C4), and 163.8 (C2); δSi (79.5 MHz, CDCl3, Me4Si) − 47.5. Anal. Calcd for C16H20Cl3N2O2Si: C, 47.24; H, 4.96 N, 6.89; Si, 6.90. Found: C, 47.50; H, 4.73; N, 7.18; Si, 6.37. [Bis(5-methyl-2-pyridonemethyl)]methylsiliconium Chloride (11b). Bis(chloromethyl)methylchlorosilane (0.48 g, 2.75 mmol) was added slowly to a stirred solution of 5-methyl-2-trimethylsiloxypyridine53 (1 g, 5.5 mmol) in dry toluene under nitrogen. The reaction mixture was stirred for 1 h and then heated with a hot gun. The solid was filtered under nitrogen, washed with diethyl ether, and dried under vacuum to afford 1.6g, 89% yield and crystallized for X-ray analysis from hot chloroform to get colorless crystals. Mp: 157 °C. NMR spectroscopic data: δH (300 MHz, CDCl3, Me4Si) 0.47 (3H, s, SiMe), 2.3 (5-Me), 3.8 (4H, q, JAB = 17.04 Hz, NCH2), and 6.8−8.2 (3H, m, arom); δC (75 MHz, CDCl3, Me4Si) 1.5 (SiMe), 17.3 (5-Me), 38.5 (NCH2), 114−146 (C3−C6), and 161.3(C2); δSi (79.5 MHz, CDCl3, Me4Si): −47.3. Anal. Calcd: C, 55.80; H, 10.98; N, 8.68; Si, 8.70. Found: C, 55.39; H, 10.47; N, 8.39; Si, 8.52.

OH2. Effectively there is a hydrogen-bonding network running through the structures, and it is assumed there will be a similar network for the polysilylamides. The orientation of the aromatic rings in the bischelate siliconium cation complexes 11a and 11b is also interesting. Although stacking of the rings with the other molecules is not obvious, a tendency does exist for the chains to form collinear arrangements. This feature may serve to enhance the crystallinity of the materials. The X-ray crystal packing of 2 (X = I) shows the iodide ion sitting between the silicon and the positively charged amide nitrogen, leaving the silicon atom clearly four-coordinate. The structure suggests helical packing of the chains with weak intermolecular Si---I----N bridges, stabilized by an electrostatic force of attraction. The results of this study suggest that the skeletal conformation of the polysilylamides may be maintained by a variety of electrostatic interactions and a hydrogen-bonding network in which the water molecules are attached to the silicon atom in the backbone of the polymer.

7. CONCLUSIONS A novel water-soluble polysilylamide containing a pentacoordinated silicon backbone with a Si−Cl functionality was synthesized for the first time. This facile synthetic method can be utilized to prepare a variety of highly soluble bridged polysilylamides, and the Si−Cl functionality could be used for further modification of the polymer. The complete hydrolysis− condensation process of the Si−Cl functionality was demonstrated by MALDI-TOF MS, NMR, and X-ray crystallography. A comparison with model pentacoordinate compounds suggested that the Si−Cl bonds of the polysilylamides hydrolyzed to give relatively stable water-coordinated silicon groups (Si←OH2), which do not condense into siloxanes (Si−O−Si). Partial hydrolysis of model pentacoordinate chlorosilanes led to the water silicon coordinated pentacoordinate species, which resembled the intermediates in the aqueous hydrolysis of the polysilanolamides. These results also provide strong evidence that the pentacoordinated silanol hydrochlorides (Si←OHHCl) are primary products of hydrolysis of the pentacoordinated chlorosilanes in the absence of HCl acceptors. The final stage of the process is the formation of disiloxane as a result of condensation, which occurs only in the presence of suitable HCl acceptors. 8. EXPERIMENTAL SECTION The reactions were carried out under nitrogen using Schlenk techniques. Solvents were dried and purified by standard methods. The polyamide 7 was very soluble in water, and initial MALDI-TOF MS analysis was carried out on a Voyager DE-STR spectrometer (irradiated with a pulsed N2 laser, 337 nm, f = 3 or 20 Hz, 20 kV) by using a DHB (2,5dihydroxybenzoic acid) matrix in positive-reflector mode, where a simple oligomeric series was observed. To confirm these species, analysis was repeated with an α-CHCA (α-cyano-4-hydroxycinnamic acid) matrix where the species were confirmed, but several others were also observed. To rule out the possibility of sample degradation, analysis was repeated with a DHB matrix, and similar data were observed. NMR spectra were recorded on a JEOL-EX 400 FT NMR and JEOL-LA 300 FT NMR spectrometer fitted with multinuclear probe, for 1H (300 MHz), 13C (75 MHz), and 29Si (79.5 MHz) spectra. Spectra are reported in δ (ppm) relative to TMS as internal standard. Siliconcontaining materials were obtained from Sigma-Aldrich. Melting points were determined by using a Buchi melting point instrument and are uncorrected. Elemental analyses were conducted by MEDAC LTD, Brunel Science Center, Surry, UK. All results are based on the average of duplicate analysis. The X-ray crystallography analyses were performed at a temperature of 120(2) K and wavelength of 0.71073 Å by the EPRSC 1729

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ASSOCIATED CONTENT

S Supporting Information *

Experimental data, MALDI-TOF MS data with detail structures, crystallographic data (the structural parameters for the model compounds), and NMR data for the required compounds are available free of charges via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (M.S.); p.g. [email protected] (P.G.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Allen Bowden for the NMR spectra. We thank the EPSRC National Mass Spectroscopy Service Centre, Swansea, for MALDI-TOF MS data.

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REFERENCES

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dx.doi.org/10.1021/om301137b | Organometallics 2013, 32, 1721−1731