Study of Binuclear Silicon Complexes of Diketopiperazine at SN2

Jan 11, 2011 - †Department of Chemistry and Analytical Sciences, The Open University, Walton ... University of Southampton, Southampton, SO17 1BJ, U...
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Organometallics 2011, 30, 564–571 DOI: 10.1021/om1009318

Study of Binuclear Silicon Complexes of Diketopiperazine at SN2 Reaction Profile Sohail Muhammad,*,† Alan R. Bassindale,† Peter G. Taylor,† 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., and ‡EPSRC National Crystallography Service, School of Chemistry, University of Southampton, Southampton, SO17 1BJ, U.K. Received September 28, 2010

A series of binuclear pentacoordinate silicon complexes of diketopiperazine have been synthesized and substituent (leaving group) effects on the Si-O bond coordination have been studied by comparison of the five differently substituted analogous structures (X = F, Cl, OTf, Br, and I). Variable-temperature NMR spectroscopy supported by X-ray crystallography shows, for the first time in binuclear pentacoordinate silicon complexes, a complex equilibrium with both nonionic (O-Si) and ionic (Si-X) dissociation of the axial bonds in the silicon-centered trigonal bipyramids. The two dissociation pathways are consistent with a model for nucleophilic substitution in a binuclear pentacoordinate silicon compound at the silicon atom. Introduction X-ray crystallographic1-7 and NMR spectroscopic8-10 studies have been used to examine the molecular pathways of a variety of reactions. Such molecular structure correlations involve studying a range of structurally similar compounds and then sequencing them on the basis of key spectroscopic or structural data. A picture of the gradual molecular deformation on going from reactants to products is built up as each individual structure represents a snapshot of the reaction at a particular point on the modeled reaction profile. This methodology has been applied to nucleophilic substitution at silicon by a number of groups.1-6,8-10 By judicious choice of nucleophile and nucleofuge groups the trigonal-bipyramid (TBP) structure and species with geometries between TBP and tetrahedral may be stabilized as observable species. The general picture that has emerged involves formation of the nucleophile-silicon bond accompanied by lengthening of the silicon-leaving group (nucleofuge) *To whom correspondence should be addressed. E-mail: m.sohail@ open.ac.uk. (1) Barrow, M.; Ebsworth, E.; Harding, M. J. Chem. Soc., Dalton Trans. 1980, 1838–1844. (2) Sidorkin, V. F.; Vladimirov, V. V.; Voronkov, M. G.; Pestunovich, V. A. J. Mol. Struct. (THEOCHEM) 1991, 228, 1–9. (3) Ovchinnikov, Y.; Macharashvili, A.; Struchkov, Y.; Shipov, A.; Baukov, Y. J. Struct. Chem. 1994, 35, 91–100. (4) Britton, D.; Dunitz, J. D. J. Am. Chem. Soc. 1981, 103, 2971–2979. (5) Buergi, H. B.; Dunitz, J. D. Acc. Chem. Res. 1983, 16, 153–161. (6) Macharashvili, A. A.; Shklover, V. E.; Struchkov, Y. T.; Oleneva, G. I.; Kramarova, E. P.; Shipov, A. G.; Baukov, Y. I. J. Chem. Soc., Chem. Commun. 1988, 683. (7) Bassindale, A. R.; Sohail, M.; Taylor, P. G.; Korlyukov, A. A.; Arkhipov, D. E. Chem. Commun. 2010, 46, 3274–3276. (8) Bassindale, A. R.; Parker, D. J.; Taylor, P. G.; Auner, N.; Herrschaft, B. J. Organomet. Chem. 2003, 667, 66–72. (9) Bassindale, A. R.; Borbaruah, M.; Glynn, S. J.; Parker, D. J.; Taylor, P. G. J. Chem. Soc., Perkin Trans. 2 1999, 2099–2109. (10) Bassindale, A. R.; Borbaruah, M. J. Chem. Soc., Chem. Commun. 1991, 1991, 1499–1501. pubs.acs.org/Organometallics

Published on Web 01/11/2011

bond. The tetrahedral reactants are converted into an intermediate trigonal-bipyramidal structure with the nonparticipating groups in equatorial positions followed by reversion to a tetrahedral structure. The resulting trajectory for nucleophilic substitution at silicon exhibits a hyperbolic relationship between the Si-O and Si-X bond lengths and a regular variation of the position of the silicon atom relative to the plane of the three equatorial carbon atoms. Following on from a study of the pentacoordinate silicon pyridones,8-10 the diketopiperazines (DKP) 1 were studied. One reason for such a study was the possibility of forming binuclear silicon-containing piperazines in which there would be two silicon atoms each of which are suitably positioned to coordinate to the adjacent carbonyl oxygen atoms,11-15 Scheme 1. There are a number of possible bonding arrangements for such a system: (1) One of the silicon atoms is coordinated to the oxygen atom and becomes pentacoordinate, the other silicon atom remaining tetracoordinate, and no exchange takes place (A). (2) An oxygen atom is only coordinated to one silicon atom at any one time, but fast exchange of oxygen coordination between the two silicon atoms takes place (ATB), to give an averaged signal in the 29Si NMR spectrum. With sufficient cooling this exchange could become sufficiently slow such that the pentacoordinate and tetracoordinate silicon atoms give separate characteristic 29Si NMR signals, the same situation as in A. (3) The oxygen atom is simultaneously coordinated to (11) Breliere, C.; Carre, F.; Corriu, R. J. P.; Royo, G.; Wong Chi Man, M.; Lapasset, J. Organometallics 1994, 13, 307–314. (12) Loy, D. A.; Small, J. H.; Shea, K. J. Organometallics 1993, 12, 1484–1488. (13) Theis, B.; Burschka, C.; Tacke, R. Chem.;Eur. J. 2008, 14, 4618–4630. (14) Jimenez-Perez, V. M.; Camacho-Camacho, C.; Ramos-Organillo,  Ramı´ rez-Trejo, R.; Pe~ A.; na-Hueso, A.; Contreras, R.; Flores-Parra, A. J. Organomet. Chem. 2007, 692, 5549–5554. (15) Kalikhman, I.; Kingston, V.; Gostevskii, B.; Pestunovich, V.; Stalke, D.; Walfort, B.; Kost, D. Organometallics 2002, 21, 4468–4474. r 2011 American Chemical Society

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both silicon atoms to give some degree of pentacoordinate character to both (C). The 29Si NMR spectrum should be similar to that for (2) at room temperature except that it would not split into two signals on cooling. (4) None of the oxygen atoms are coordinated, and both Si atoms are tetracoordinate (D).

Results and Discussion Synthesis of Pentacoordinate Silicon Complexes. Treatment of piperazine-2,5-dione 1 with two equivalents of N, O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) gave the corresponding trimethylsilyl derivative 2 in 90-95% yield (eq 1). Reaction of 2 with two equivalents of chloro(chloromethyl)dimethylsilane or (chloromethyl)dichloromethylsilane gave the chloro derivatives 3b and 4b, respectively (eq 2). The fluoro derivatives 3a and 4a were prepared in >90% yield by treating the chloro derivatives 3b and 4b with antimony trifluoride in excess (eq 3), and the products were purified by extraction into DCM to give white crystalline solids. The triflato (3c), bromo (3d), and iodo (3e) derivatives were prepared in 95%, 81%, and 80% yield, respectively, by treating 3b with two equivalents of the appropriate trimethylsilyl derivative (eq 4). The compounds 2-4 have been characterized by their 1H, 13 C, and 29Si NMR spectra and elemental analysis. Most of the compounds were further subjected to X-ray crystallographic analysis

Figure 1. ORTEP representation of 2a,b. Ellipsoids are drawn at the 50% probability level. Selected bond distances (A˚) and bond angles (deg) are given in Table 1. The N-silylated DKP in a unit cell is found to be present as two conformationally distinct forms, a planar (2a) and a puckered (2b) hexagon. Hydrogen atoms are omitted for clarity. Scheme 1. Some Possible Arrangements for Two Silicon Atoms That Are Suitably Positioned to Coordinate Intramolecularly to the Amido Carbonyl Oxygen Atoms

Table 1. Selected Bond Lengths and Bond Angles for 1 and Two Conformers, 2a,b 116

2a [planar]

2b [puckered]

Bond Lengths, A˚ CdO C-N OC-CH2 N-CH2

1.25 1.33 1.47 1.41

1.228(6) 1.343(6) 1.506(7) 1.482(6)

1.243(6), 1.231(6) 1.361(6), 1.354(6) 1.503(7), 1.496(7) 1.481(6), 1.471(6)

Bond Angles [deg] cyclic Δa a

120 ( 3 0.015 A˚

120 ( 2 0.008 A˚

115 ( 4 0.328 A˚

Deviation from the hexagon plane.

grown from acetonitrile solution, and its crystal structure was determined (Figure 1). Selected bond lengths and angles are listed in Table 1. The N-silylated DKP 2 in the unit cell is present as two conformers, a planar (2a) and a puckered (2b) hexagonal form. The bond length differences in the amide moiety of the two conformers are particularly noticeable. Generally the bond distances for CdO and C-N of an amide resonance are about 1.25 and 1.33 A˚, respectively.16 The differences in the N-CH2 and CH2-CO bond distances Diketopiperazine as a Ligand. The silylated DKP complex 2 was prepared as shown in eq 1. A single crystal of 2 was

(16) Corey, R. B. J. Am. Chem. Soc. 1938, 60, 1598–1604.

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Table 2. 1H, 13C, and 29Si NMR Chemical Shifts in ppm of 3a-e in CDCl3/CD3OD at Various Temperatures δ13C (ppm)

δ 1H (ppm)

temp (°C)

CdO

C-CO

NCH2

SiMe2

SiMe2

CH2-CO

NCH2

δ29Si (ppm)

3a (X = F), 20 3b (X = Cl), 20 -60 3c (X = OTF), 20 -60 3d (X = Br), 20 3e (X = I), 20

164.99 165.30 166.20 165.70 167.00 165.00br 164.00

51.20 50.60 51.30, 49.90 50.00 49.50, 49.00 49.84br 51.20

38.36d 38.30br 37.10 37.80 36.20 37.50br 38.00

0.75 -0.66 0.70, -0.60 -0.80 -0.40, 0.25

0.30d 0.36 0.37, 0.22 0.33

4.00 4.21 4.20, 4.34 4.25

2.53 2.90 2.9.0br 2.88

0.34, 0.24 -0.05

4.29 3.81

2.93 2.60

þ3.00, JSi-F = 263.20 -3.17 -16.0 -7.4 -25.0 -6.80, þ5.00 þ7.6

-0.25

Figure 2. 29Si NMR spectra featuring formation of the Si-O bond accompanied by lengthening of the Si-X bond. The tetrahedral (X = F) is transformed into pentacoodinate TBP (X = OTf) followed by reversion to tetrahedral (X = I) moving from 3a to 3e at room temperature in CDCl3/CD3OD.

between 2b and 116 are also surprising. It is possible that those in 1 are to be attributed, in part at least, to the effect of electric charge. The cyclic bond angles of 1 and 2a are 120 ( 3° and 120 ( 2°, and deviation from the plane is as little as 0.015 and 0.008 A˚, respectively. In the case of 2b (puckered), the angles between all bonds are 115 ( 4° and deviation from the plane is 0.328 A˚. In contrast with X-ray structural data, the solution NMR spectra of compound 2 showed only one set of resonances compatible with fast exchange between planar and puckered conformers. Pentacoordinate Binuclear Silicon Complexes. An NMR spectroscopic study of the binuclear pentacoordinate silicon compounds 3b (X = Cl) and c (X = OTf) derived from diketopiperazine examined in CDCl3/CD3OD at room temperature shows broadening in the 1H, 13C, and 29Si NMR spectra, Table 2. Methanol is an unusual solvent for NMR studies of reactive silanes. Although halosilanes and triflatosilanes react rapidly and irreversibly with methanol in the presence of base, we have observed that in a sealed tube in the absence of a strong proton acceptor the equilibrium lies essentially fully toward the halo- or triflatosilane.10 These broadenings are suggestive of a dynamic equilibrium involving coordination-decoordination between the two possible coordination sites in A and B (Scheme 1). The rate of exchange for 3b (X = Cl) and 3c (X = OTF) was found to be intermediate on the NMR time scale at room temperature in CDCl3/MeOH. As a result, only one averaged 29Si broad peak was observed in the spectrum for each (3b,c) of the complexes (Figure 2 Table 2), at a chemical shift

appropriate for a fully pentacoordinate silicon on one site and a tetracoordinate silicon at the other site undergoing exchange at an intermediate rate on the NMR time scale.9,17 Generally the 29Si NMR chemical shift in related pentacoordinate silicon compounds is found at about -30 to -40 ppm, while the tetracoordinate precursors have resonances at þ25-40 ppm.1,7-10 This again is consistent with the resonances for 3b and 3c being approximately halfway between the two limiting structure resonances. It is also possible that each of the silicon centers are equally partially pentacoordinate, but the very broad peaks strongly suggest exchange. Substitution of chloro by fluoro (eq 3) ligands resulted in more weakly coordinated binuclear complex, 3a, as suggested from its sharp 29Si resonance at þ3.0 ppm (JSi-F = 263.2), Figure 2. In contrast with 3a-c, the two 29Si chemical shifts of equal intensity for 3d (X = Br) were observed at room temperature, Figure 2. It is evident that one of the silicon atoms is tetracoordinate at þ5.0 ppm, while the second silicon atom is clearly pentacoordinate at a chemical shift of -6.8 ppm. Neither peak appears at the chemical shift expected for full penta- or completely tetracoordinated silicon. The first two possibilities consistent with the data are that one silicon is more pentacoordinated than the other, but neither is fully coordinated and they are undergoing slow exchange, or if each is fully tetra- or pentacoordinate, then the peaks have shifted closer together through exchange at a rate close to the coalescence rate,17 Table 2. A third and possibly unprecedented alternative explanation is that there is, as one of the dynamic processes, dissociation of the Si-Br bond to form a five-membered ring with a simple covalent Si-O bond. The dissociation of a halogen from a hexacoordinate silicon species has been recently observed.18 Further evidence of the Si-X dissociation is obtained from the 29Si spectrum of 3e (X = I). The 29Si chemical shift of 3e at þ7.6 ppm suggests that the equilibrium could be shifted completely to the tetracoordinated cyclized compound as a result of Si-I bond ionization (Table 2). Attempts to shift the equilibrium toward the five-coordination side, by decreasing or increasing the temperature, did not result in any significant spectral changes. The presence of two almost independent coordination sites in a single molecule seems to have magnified the effects of the nucleofuge, X groups to the extent that the starting material (uncoordinated), intermediate (pentacoordinated), and product (cyclized species with loss of X-) are all observed in the same closely related series. As the crystal structures of the (17) Kertsnus-Banchik, E.; Gostevskii, B.; Botoshansky, M.; Kalikhman, I.; Kost, D. Organometallics 2010, DOI: 10.1021/om100461b. (18) Kalikhman, I.; Gostevskii, B.; Girshberg, O.; Sivaramakrishna, A.; Kocher, N.; Stalke, D.; Kost, D. J. Organomet. Chem. 2003, 686, 202–214.

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Scheme 2. SN2 Profile Where the Carbonyl O Is the Nucleophile and X is the Nucleofuge

Figure 3. ORTEP representation of 3a (X = F) and 3e (X = I). Ellipsoids are drawn at the 80% probability level. Selected bonding distances (A˚) and bond angles (deg) are given in Table 3. Hydrogen atoms are omitted for clarity. Table 3. Geometry of the Silicon Center and Key Bond Lengths (A˚) and Bond Angles [deg] of 3a,b,d,e and 4a in the X-ray Crystal Structurea X (complex)

O-Si Si-X C2-O1 N1-C1-Si O1-Si-C1 X-Si-C1 O1-Si-X P C-Si-C ΔSib references

F (3a)

Cl (3b)

OTf (3d)

I (3e)

F2 (4a)

2.2124(12) 1.6605(10) 1.2553(19) 113.89(10) 79.18(5) 93.08(6) 172.26(5) 355.70 þ0.226 this work

2.0436(13) 2.2796(6) 1.260(2) 110.48(12) 82.08(7) 88.49(6) 170.54(4) 359.11 þ0.102 19

1.8446(18) 2.1899(19) 1.283(3) 105.99(16) 86.44(9) 85.13(9) 170.50(8) 359.2 -0.098 19

1.831(2) 3.150(2) 1.287(3) 104.84(17) 87.43(10) 78.35 164.56 356.89 -0.19 this work

2.1069(19) 1.6383(16) 1.254(3) 112.02(16) 81.16(9) 94.32(10) 172.31(8) 355.96 0.207 this work

a For numbering scheme see Figure 3. b The perpendicular distance of the silicon center from the least-squares plane C1-C4-C5. A positive distance indicates a displacement toward the leaving group.

series of coordinated compounds generally show the same trends in degree of coordination as is inferred from solution spectra,8-10 we obtained the X-ray crystal structures for these compounds. We were unsuccessful in growing suitable crystals for the bromo derivative 3c. The X-ray crystal structures of 3a (X=F, Figure 3), 3b19 (X = Cl), and 3d19 (X = OTF) all show two well-defined pentacoordinate silicon atoms with slightly distorted TBP geometries and a structure with a center of inversion. In 3a (X = F), the O-Si bond distance of 2.21 A˚ represents only a weak coordination of the carbonyl with a Si-F bond length of 1.66 A˚, and the O-Si-F bond angle is 172.26°. Since fluoride is the poorest leaving group of the series 3a-e, 3a models the earliest point in the substitution process with the longest O-Si distance. The silicon atom is situated 0.22(1) A˚ from the TBP plane closer to the fluorine atom, Table 3. It is likely that crystal-packing effects are responsible for the equal coordination of each silicon center. In the case of 3e (X = I), it is evident that both of the silicon atoms are tetracoordinate and essentially tetrahedral (Figure 3). The Si-I distance, 3.1 A˚, represents only a very weak coordination of the anion, leaving the silicon atom clearly fourcoordinate with an O-Si-I angle of 164.56° (Table 3). Strong O-Si coordination was found as shown by a bond (19) Mozzhukhin, A. O.; Yu, M. Y. A.; Struchkov, T.; Shipov, A. G.; Kramerova, E. P.; Y, I. B. Metalloorg. Khim. (Russ.) (Organomet. Chem. (USSR)) 1992, 5, 906.

length of 1.83 A˚, which is close to that of a normal Si-O bond length. The silicon atom has passed through the TBP plane and is near the center of the reverse tetrahedron, 0.19 A˚ from the basal plane toward the oxygen atom, and the substitution has been completed.8 The close correlation between the structural information obtained by NMR spectroscopy and X-ray crystallography in binuclear pentacoordinate silicon complexes confirms that the variation of the leaving groups (F, Cl, OTf, Br, and I) at both silicon atoms leads to a series of pentacoordinate silicon compounds in equilibrium in both solid and liquid phases whose structures can be proposed, as shown in Scheme 2. The proposed Scheme 2 explains the observation of 29Si chemical shifts in binuclear pentacoordinate silicon complexes (3a-e), resulting from both a nonionic equilibrium dissociation of the O-Si dative bond exchanging between neutral penta- and tetracoordinate silicon complexes and an ionic equilibrium dissociation of the Si-X dative bond exchanging between penta- and tetracoordinate silicon complexes.20 The two dissociation patterns are consistent with the model for nucleophilic displacement at a tetracoordinate silicon atom.21 Variable-Temperature Confirmation of Neutral Si-O and Ionic Si-X Bond Dissociation Patterns. At low temperature the 29Si NMR spectrum clearly features two signals for 3b (X = Cl) and 3c (X = OTf), Figure 4, one typical of pentacoordination (δ 29Si -16.0 and -25.0 ppm at -60 °C) and the other of tetracoordination (δ 29Si þ5.1 and þ5.0 ppm at 20 °C), respectively. The intensity ratio of these two signals is not equal but is temperature dependent in a fully reversible fashion and is assigned to the ionic Si-X bond dissociation equilibrium 300 T3000 , Scheme 2. As the temperature decreases, the relative proportion of the tetracoordinate site (3000 ) increases as a result of ionic Si-X bond dissociation. It is evident that the 29Si NMR signals at room temperature are not an average of the two separate signals that appeared for 3b (Figure 4A) and 3d (Figure 4B) at the lower temperature and that the changes are a result of Si-X bond breaking.22-25 If Si-O bond breaking was predominant, we would have observed an average signal of the two separate signals in the 29Si NMR spectra at lower temperature. (20) Kost, D.; Kalikhman, I. Acc. Chem. Res. 2009, 42, 303–314. (21) Gostevskii, B.; Silbert, G.; Adear, K.; Sivaramakrishna, A.; Stalke, D.; Deuerlein, S.; Kocher, N.; Voronkov, M. G.; Kalikhman, I.; Kost, D. Organometallics 2005, 24, 2913–2920. (22) Probst, R.; Leis, C.; Gamper, S.; Herdtweck, E.; Zybill, C.; Auner, N. Angew. Chem., Int. Ed. Engl. 1991, 30, 1132–1135. (23) Kost, D.; Kingston, V.; Gostevskii, B.; Ellern, A.; Stalke, D.; Walfort, B.; Kalikhman, I. Organometallics 2002, 21, 2293–2305. (24) Kost, D.; Kalikhman, I.; Kingston, V.; Gostevskii, B. J. Phys. Org. Chem. 2002, 15, 831–834. (25) Pogozhikh, S. A.; Ovchinnikov, Y. E.; Kramarova, E. P.; Negrebetskii, V. V.; Shipov, A. G.; Albanov, A. I.; Voronkov, M. G.; Pestunovich, V. A.; Baukov, Y. I. Russ. J. Gen. Chem. 2004, 74, 1501– 1507.

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Figure 4. Variable-temperature 29Si NMR spectra of 3b (X = Cl) (A) and d (X = OTF) (B) featuring ionic Si-X and covalent O-Si bond dissociation from the equilibrium 30 T300 T3000 (Scheme 2) in CDCl3/MeOH. Scheme 3. Proposed Ion Pairing in Which the X- (Cl, Br, I) Ion Migrates between the Silicon and the Amide Nitrogen, Inter- (A) or Intramolecularly (B)

At 20 °C the signal due to 3000 has completely disappeared as a result of rapid exchange between the small residual signal intensity of 3000 with that of the predominant 300 . We believed that at higher temperature there is a lengthening of the Si-O bond distance in 3b (X = Cl, Figure 4A) and 3c (X = OTf, Figure 4 B), which is typical for hypervalent silicon compounds. The 29Si NMR resonance signals of compounds 3a-c are shifted to lower field as the temperature increases (δ 29Si = -16.0 at -60 °C to -3.1 ppm at 20 °C for X = Cl; -25.0 at -60 °C to -7.0 ppm at 20 °C for X = OTf, Table 2), which is consistent with weakening of the coordinative OfSi bonding (or neutral dissociation of the OfSi bond).26 Ion Pairing. In principle there should be a resistance to the ionization of both Si-X bonds in 3a-e as a result of the buildup of two positive charges on each of the amide nitrogen atoms and the consequent repulsion between them. Attempts to shift the equilibrium toward the four-coordinate side by changing the temperature for 3a-c results in ionization of only one of the Si-X bonds out of two, leading to one positive charge on either of the amide nitrogens. With better leaving group, ionization of both Si-X bonds is observed, leading to a positive charge on at least one of the amide nitrogen atoms. This is possible by a novel type of proposed ion pairing in which a halide ion (except fluoride) migrates between the silicon and the positively charged amide moiety either inter-17 (Scheme 3A) or intramolecularly27 (Scheme 3B), moving from covalent bonding in the pentacoordinate silicon complex to ion pairing in the tetracoordinate complex. (26) Kalikhman, I.; Kertsnus-Banchik, E.; Gostevskii, B.; Kocher, N.; Stalke, D.; Kost, D. Organometallics 2009, 28, 512–516. (27) Kalikhman, I.; Girshberg, O.; Lameyer, L.; Stalke, D.; Kost, D. J. Am. Chem. Soc. 2001, 123, 4709–4716.

Evidence for the double Si-X ionization in 3b is presented in Figures 5 and 6: the low-temperature 1H and 13C NMR spectra of the 3b feature two signals for the ring CH2, one typical of pentacoordination (δ 13C 51.2 ppm, δ 1H 4.34 ppm; assigned to the CH2 adjacent to a pentacoordinate silicon) and the other of tetracoordination (δ 13C 49 ppm, δ 1H 4.20 ppm; assigned to the CH2 adjacent to the tetracoordinated silicon). Similar temperature dependence of the 1H and 13C NMR chemical shifts was also observed for the SiMe2 group. The intensity ratio of these two signals is temperature dependent in a fully reversible fashion, such that at lower temperatures the relative population of the tetracoordinate species (3000 ) increases. If this were a result of Si-O bond dissociation or siloxane hydrolysis, there would be two CdO chemical shifts in 13C NMR spectra at lower temperature. The X-ray crystal packing of 3e (X = I) shows the iodide ion siting between the silicon and the positively charged amide nitrogen with a I-N distance of 3.5 A˚, Figure 7. The interaction of the iodide group with the silicon is best described as an electrostatic interaction with an O-Si-I bond angle of 164.56°. The Si-I bond distance of 3.1 A˚ represents only a weak coordination of the anion, leaving the silicon atom clearly four-coordinate. The crystal structure of 3e suggests helical packing of the chains with weak intermolecular Si---I----N bridges, stabilized by the adjacent positive and negative charges, Figure 8. The disubstituted halo complexes 4a,b were prepared by analogy with the monosubstituted complexes (3a,b, eqs 2, 3). In these compounds the oxygen to silicon coordination is stronger featuring 29Si NMR chemical shifts of -75 to -47.1 ppm, respectively. This is the first time that a difluoro binuclear pentacoordinate silicon complex has been prepared from the corresponding dichloro complex (eq 3). As with the other complexes of this series, the crystal structural analysis of 4a (Figure 9, Table 3) shows both the silicon atoms are pentacoordinate with a Si-O bond length of 2.10 A˚ and an O-Si-F1 bond angle of 172.31°; the center of the molecule lies on the inversion center.

Conclusion In conclusion, Scheme 2 can be viewed as a model for the nucleophilic displacement reaction at a tetracoordinate silicon in which the X substituent is replaced by a carbonyl oxygen to obtain an ionic structure. The sequence of models represents attack by the neutral carbonyl nucleophile on a tetracoordinate silicon to form the neutral pentacoordinate intermediate. This is followed by departure of the X leaving group, forming the ionic siliconium salt. The variable-temperature study

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Figure 5. Variable-temperature 1H NMR spectra for 3b (X = Cl) featuring ionic Si-Cl bond dissociation of the equilibrium mixture 300 T 3000 (Scheme 2) in MeOD/CD2Cl2.

Figure 6.

13

C variable-temperature spectra of 3b (X = Cl) featuring ionic Si-Cl bond dissociation in MeOD/CD2Cl2.

Figure 7. X-ray crystal packing of 3e (X = I) showing a chain formation by a weak intermolecular Si---I---N bridge. Hydrogen atoms are omitted for clarity.

describes the first observation of the temperature dependence of the 29Si NMR chemical shifts in binuclear pentacoordinate silicon complexes, resulting from both a nonionic equilibrium dissociation of the O-Si dative bond, exchanging between neutral penta- and tetracoordinate silicon complexes, and an ionic equilibrium dissociation of the Si-X dative bond, exchanging between penta- and tetracoordinate silicon complexes. The two dissociation patterns offer a model for nucleophilic displacement at a pentacoordinate silicon atom. Thus a complete nucleophilic substitution coordinate is demonstrated.

Experimental Section The reactions were carried out under nitrogen using Schlenk techniques. Solvents were dried and purified by

standard methods. NMR spectra were recorded on a JEOLEX 400 FT NMR or a JEOL-LA 300 FT NMR spectrometer fitted with a multinuclear probe, for 1H, 13C, and 29Si spectra. Spectra are reported in δ (ppm) relative to TMS, as an internal standard. Silicon-containing materials were obtained from Sigma-Aldrich. Melting points were measured 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 analyses. The X-ray crystallography analyses were performed at a temperature of 120(2) K and wavelength of 0.71073 A˚ by the EPRSC X-ray crystallography services by using a Bruker-Nonius APEX II CCD camera (f scans and w scans to fill the asymmetric unit). Crystallographic data have been deposited with the Cambridge Crystallographic data Centre (CCDC). The CCDC numbers are listed in Table 4.

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Figure 8. Ellipsoid and space-filling representations of 3e showing helical-like crystal packing when viewed along the c-axis. Hydrogen atoms are omitted for clarity.

Figure 9. ORTEP representation of 4a. Ellipsoids are drawn at the 70% probability level. Selected bonding distances (A˚) and bond angles (deg) are given in Table 3. Hydrogen atoms are omitted for clarity. 1,4-Bis(trimethylsilyl)-2,5-piperazinedione (2) (ref 28). Diketopiperazine 1 (2.5 g, 21.9 mmol) was suspended in dry acetonitrile (10 mL), and N,O-bis(trimethylsilyl)trifluoroacetamide (11.28 g, 43.8 mmol) was added. The mixture was refluxed overnight, and the solvent was removed under vacuum to gives crystals of 5.0 g (90%) of 1,4-bis(trimethylsilyl)-2,5-piperazinedione (2). Alternatively half of the solvent was removed and the remaining mixture was left overnight to form larger crystals. The solvent was removed by decanting, and the product was used without further purification. NMR spectroscopic data: δH (300 MHz)/CD3CN 0.21(d, 18H, Si (CH3)3), 3.7 (s, 4H, CH2 CO); δC (300 MHz)/CD3CN 1.5, 49.6, 174.4 δSi (400 MHz), CD3CN: 4.4. Mp: 36-38 °C. 1,4-Bis(dimethylchlorosilylmethyl)-2,5-piperazinedione (3b) (ref 28). Chloromethyldimethylchlorosilane (1.2 g, 8.4 mmol) was added slowly to a stirred solution of 1,4-bis(trimethylsilyl)2,5-piperazinedione (2) (1.0 g, 3.8 mmol) in dry toluene under nitrogen. The reaction mixture was stirred for 12 h. The solid was filtered or washed with dry ether under nitrogen and dried under vacuum to give 1.20 g of 3b in 95% yield. Mp: 116-117 °C. NMR spectroscopic data: δH (300 MHz, CDCl3/CD3OD, SiMe4) 0.36 (s, 12H, Si(CH3)3), 2.9 (s, 4H, NCH2), 4.21 (s, 4H, CH2CO); δC (300 MHz, CD3OD, SiMe4) -0.66 (C4, 5), 38.3 (C1), 50.6 (C3), 165.3 (C2); δSi (400 MHz, CDCl3/CD3OD, SiMe4) -3.17. Anal. Calcd: C, 36.69; H, 6.16; Cl, 21.66; N, 8.56. Found: C, 36.72; H, 6.21; Cl, 21.16; N, 8.60. X-ray crystals: the compound was suspended in a dry mixture of acetonitrile and toluene (1:1) and dissolved by dropwise addition of dry methanol (28) Shipov, A. G.; Artamkina, O. B.; Kramarova, E. P.; Oleneva, G. I.; Baukov, Y. I. J. Gen. Chem. USSR 1991, 61, 1770–1771.

Muhammad et al. while heating gently until the entire compound was dissolved. On cooling to room temperature, colorless crystals, suitable for X-ray crystallography, were obtained. 1,4-Bis(dimethylfluorosilylmethyl)-2,5-piperazinedione (3a). (Chlorodimethylsilylmethyl)-2,5-piperazinedione (0.6 g, 1.8 mmol) was dissolved (or suspended) in 5 mL of dry toluene, antimony trifluoride (in excess) was added, and the reaction mixture was stirred for 3 h. The reaction mixture was diluted with water and extracted with chloroform (3  75 mL). The washed extract was dried over anhydrous magnesium sulfate, and 90% of the solvent was removed using a rotary evaporator to obtain colorless crystals (0.5 g, 92%), which were dried under vacuum and were suitable for X-ray crystallography. Mp: 156-158 °C. NMR spectroscopic data: δH (300 MHz, CD3OD, SiMe4) 0.30 (d, 12H, Si (CH3)3), 2.5 (s, 4H, NCH2), 4.07 (s, 4H, CH2CO); δC (300 MHz, CD3OD, SiMe4) 0.75 (C4, 5), 38.3 (C1), 51.2 (C3), 164.9 (C2); δSi (400 MHz, CD3OD, SiMe4) þ3.0. Anal. Calcd: C, 40.79; H, 6.85; F, 12.90; N, 9.51. Found: C, 41.26; H, 6.70; F, 12.71; N, 9.44. X-ray crystals were obtained from chloroform by slow evaporation under nitrogen. 1,4-Bis(dimethytrifluoromethylsulfonyloxysilylmethyl)-2,5-piperazinedione (3c) (ref 28). A trimethylsilyltriflate (1.4 g, 6.3 mmol) was added dropwise to a solution of compound 3b (1.0 g, 3.0 mmol) in 5 mL of dry acetonitrile and warmed to dissolve. On the following day crystals that formed were removed by filtration and dried under vacuum (2.0 g, 95%). Mp: 289-291 °C. NMR spectroscopic data: δH (300 MHz, CDCl3/CD3OD, SiMe4) 0.33 (s, 12H, Si(CH3)3), 2.88 (s, 4H, NCH2), 4.25 (s, 4H, CH2CO); δC (300 MHz, CDCl3/CD3OD, SiMe4) -0.80 (C4, 5), 37.80 (C1), 50.1 (C3), 165.7 (C2); δSi (400 MHz, CDCl3/CD3OD, SiMe4) -7.4. Anal. Calcd: C, 25.99; H, 3.63; S, 11.56; N, 5.05; F, 20.55. Found: C, 25.85; H, 3.38; S, 11.21; N, 5.57; F, 20.80. 1,4-Bis(dimethylbromosilylmethyl)-2,5-piperazinedione (3d). Method 1: A mixture of 1,4-bis(trimethylsilyl)-2,5-piperazinedione (1.0 g, 3.8 mmol) and bromomethyldimethylchlorosilane (1.6 g, 8.5 mmol) in 5 mL of dry toluene was stirred for 5 h. The crystalline compound that formed was filtered off and washed with dry ether to yield 1.3 g (81%) of the required compound. Method 2: A trimethylsilyl bromide (6.3 mmol) was added dropwise to a solution of compound 3b (1.0 g, 3.0 mmol) in 5 mL of dry acetonitrile and warmed to dissolve. On the following day crystals that formed were removed by filtration and dried under vacuum. Mp: 121-123 °C. NMR spectroscopic data: δH (300 MHz, CDCl3/CD3OD, SiMe4) 0.34, 0.24 (s, 12H, Si(CH3)3), 2.9 (s, 4H, NCH2), 4.29 (s, 4H, CH2CO); δC (300 MHz, CDCl3/CD3OD, SiMe4) -0.55, -1.0 (C4, 5), 37.5 (C1), 50.0 (C3), 165.0 (C2); δSi (400 MHz, CDCl3/CD3OD, SiMe4) -6.8, 5.4. Anal. Calcd: C, 36.69; H, 6.16; Cl, 21.66; N, 8.56. Found: C, 36.64; H, 6.17; Cl, 20.16; N, 8.54 1, 4-Bis(dimethyliodo)-2,5-piperazinedione (3e). Trimethylsilyl iodide (1.0 g, 5 mmol) was added dropwise to a solution of compound 3b (0.6 g, 1.8 mmol) in 5 mL of dry acetonitrile and warmed to dissolve. On the following day crystals that formed were filtered off and dried under vacuum (0.7 g, 80%). Mp: 145-147 °C. NMR spectroscopic data: δH (300 MHz, CDCl3/ CD3OD, SiMe4) -0.05 (s, 12H, Si(CH3)4), 2.6 (s, 4H, NCH2), 3.81 (s, 4H, CH2CO); δC (300 MHz, CDCl3/CD3OD, SiMe4) -0.25 (C4, 5), 38 (C1), 51.2 (C3), 164 (C2); δSi (400 MHz, CDCl3/ CD3OD, SiMe4) þ7.6. Anal. Calcd: C, 23.54; H, 3.95; I, 49.74; N, 5.49; O, 6.27. Found: C, 23.64; H, 3.98; I, 49.24; N, 5.49; O, 6.57 1,4-Bis(methyldichlorosilylmethyl)-2,5-piperazinedione (4a). Chloromethylmethyldichlorosilane (1.3 g, 8 mmol) was added slowly to a stirred solution of 2 (1.0 g, 3.8 mmol) in dry toluene under nitrogen. The reaction mixture was stirred for 5 h. The solid was filtered and washed with dry ether under nitrogen and dried under vacuum to give 1.0 g in 70% yield. This compound was used for synthesis of 4b without further purification. Mp: 93-95 °C. NMR spectroscopic data: δH (300 MHz, CDCl3/ CD3OD, SiMe4) 0.36 (s, 6H, Si(CH3)3), 2.9 (s, 4H, NCH2), 4.21

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Table 4. Crystallographic Data and Experimental Parameters for the Structure Analysis of 2, 3a,e, and 4a 2 CCDC code empirical formula fw temperature wavelength cryst syst space group unit cell dimens

volume Z density (calcd) absorp coeff F(000) crystal cryst size θ range for data collection reflns collected indep reflns completeness to q = 27.48° max. and min. transmn refinement method data/restraints/params goodness-of-fit on F2 final R indices [F2 > 2σ(F2)] R indices (all data) largest diff peak and hole

3a

797260 797257 C10H22N2O2Si2 C10H20F2N2O2Si2 258.48 294.46 120(2) K 120(2) K 0.71073 A˚ 0.71073 A˚ triclinic monoclinic P1 P21/c a = 6.6235(3) A˚, a = 10.5949(8) A˚, R = 90° R = 83.067(5)o b = 10.3618(3) A˚, b = 10.6983(9) A˚, o β = 94.955(2)° β = 64.209(5) c = 10.7661(7) A˚, c = 10.8213(5) A˚, γ = 88.796(5)o γ = 90° 739.91(5) A˚3 1090.04(14) A˚3 3 2 1.322 Mg/m3 1.181 Mg/m3 0.235 mm-1 0.258 mm-1 420 312 slab; colorless block; colorless 0.32  0.20  0.20 mm3 0.08  0.06  0.03 mm3 2.92-27.48° 3.66-27.48° 15 066 8715 1692 [Rint = 0.0389] 3830 [Rint = 0.0849] 99.50% 99.80% 0.9930 and 0.9815 0.9502 and 0.9220 full-matrix least-squares on F2 3830/0/226 1692/0/84 1.098 1.079 R1 = 0.0828, R1 = 0.0358, wR2 = 0.1541 wR2 = 0.0881 R1 = 0.1275, R1 = 0.0447, wR2 = 0.1782 wR2 = 0.0926 0.272 and 0.552 and -0.284 e A˚-3 0.389 e A˚3

(s, 4H, CH2CO); δC (300 MHz, CDCl3/CD3OD, SiMe4) -0.66 (C4, 5), 38.3 (C1), 50.6 (C3), 165.3 (C2); δSi (400 MHz, CDCl3/ CD3OD, SiMe4) -47. Anal. Calcd: C, 26.10; H, 3.83; Cl, 38.52; N, 7.61; O, 8.69. Found: C, 26.60; H, 3.93; Cl, 37.82; N, 7.51; O, 8.59. 1,4-Bis(difluoromethylsilylmethyl)-2,5-piperazinedione (4b). 4 (1.0 g, 2.7 mmol) was dissolved (or suspended) in 5 mL of dry toluene. Antimony trifluoride (in excess) was added, and the reaction mixture was stirred for 3 h. The reaction mixture was diluted with water and extracted with chloroform (3  75 mL). The washed extract was dried over anhydrous magnesium sulfate, and the solvent was removed using a rotary evaporator to obtain a colorless crystalline solid, which was dried under vacuum. Mp: 110-112 °C. NMR spectroscopic data: δH

3e

4a

797258 C10H20I2N2O2Si2 510.26 120(2) K 0.71073 A˚ monoclinic P21/c a = 7.3571(2) A˚, R = 90° b = 9.7865(3) A˚, β = 101.671(2)° c = 12.1388(5) A˚, γ = 90° 855.93(5) A˚3 2 1.980 Mg/m3 3.811 mm-1 488 block; orange 0.30  0.28  0.18 mm3 3.43-27.48° 9373 1955 [Rint = 0.0333] 99.90% 0.5470 and 0.3943

797259 C8H12F4N2O2Si2 300.38 120(2) K 0.71073 A˚ monoclinic P21/n a = 9.5058(5) A˚, R = 90° b = 6.2071(2) A˚, β = 91.070(2)o c = 10.5312(4) A˚, γ = 90° 621.27(5) A˚3 2 1.606 Mg/m3 0.331 mm-1 308 block; colorless 0.50  0.44  24 mm3 3.81-27.47° 6866 1420 [Rint = 0.0399] 99.70% 0.9247 and 0.8518

1955/0/84 1.12 R1 = 0.0235, wR2 = 0.0491 R1 = 0.0286, wR2 = 0.0507 0.852 and -0.757 e A˚-3

1420/0/83 1.069 R1 = 0.0453, wR2 = 0.1194 R1 = 0.0575, wR2 = 0.1264 0.795 and 0.876 e A˚3

(300 MHz, CDCl3, SiMe4) 0.28 (t, 6H, Si(CH3)2), 2.46 (s, 4H, NCH2), 4.15 (s, 4H, CH2CO); δC (300 MHz, CDCl3, SiMe4) -1.3 (C4), 32.9 (C3), 46.6 (C2), 164.5 (CdO); δSi (400 MHz, CDCl3, SiMe4) -47.2. Anal. Calcd: C, 31.78; H, 4.67; F, 25.13; N, 9.26; O, 10.58. Found: C, 31.87; H, 4.76; F, 25.73; N, 9.62; O, 11.12

Acknowledgment. We thank Dr. Allen Bowden for the variable-temperature NMR spectra. Supporting Information Available: Crystallographic data and variable-temperature NMR for the required compounds are available free of charges via the Internet at http://pubs.acs.org.