Amino versus imino-transition metallic complexes in functionalized mesoporous silica: a lone pair degenerated into a singlet electron system

Abstract

Imine terminated mesoporous silica nanoparticles (IMSNP) appear to be an effective mesoporous media with potential for multifarious applications such as catalytic support, effluent treatment, and sensors. With this goal, several transition metal (TM = Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺) ions have been complexed with IMSNP and investigated. In this report, insights generated using microscopic probes (PL, ESR) reveal the inter-conversion of a covalent lone pair of electrons in the amine-functionalised mesoporous system (AMSNP), manifesting into a singlet electron state in the imine functionalised mesoporous system (IMSNP).

---

In view of fascinating fundamental results with the potential for numerous applications, mesoporous solids constitute an important branch of materials research and chemical syntheses. In particular, mesoporous silica with different modifications, in terms of crosslinking of chemical bonds, having a wide, size tuneable range (2–50 nm), might facilitate a variety of applications, viz., effluent treatments¹ sensors,² catalytic supports,³ and protection of metallic structures⁴. Fascinated by the application scope of the mesoporous silica, we endeavoured to chemically modify the chemical network (amine functionalized) of this mesoporous system, through a sol–gel route comprising a reaction with acetone leading to the formation of an imine system, which in turn has been complexed with several transition metal precursors. Moreover, owing to the intense photochemical activity of mesoporous silica, we consider that complexing with optically active transition metal ions might merit practical significance.^5–8 Basically, structural modifications following complexing (also cross-linking) are spatially localized, centering electron transfer and electronic transitions. This vital point serves as the motivation for us to employ reliable local probes based on diffuse reflectance spectra (DRS), photoluminescence (PL) and electron spin resonance (ESR) spectroscopies, yielding reliable information in the appropriate energy range corresponding to the system under investigation (∼a few eV).

Amine terminated silica gels were prepared by quick mixing of TEOS and APTES in DMF in vial A and by subsequently adding water in DMF in vial B at room temperature.⁷ These silica based precursors essentially comprise silane and amino networks, which upon reaction with DMF results in a colourless amine-terminated silica gel (Fig. 1), constituting the base for generating imino organometallics. The dangling amine present in the APTES acts as a catalyst in the gelation of the Si–O–Si network.

Fig. 1 Chemical reaction sequence for IMSNP complexed with TM ions from AMSNP with images of sol–gel product(s) in respective stages. Images (in the last row) of TM ion incorporated IMSNP gels indicate change in body colour of different TM-IMSNP samples.

The resulting amine terminated mesoporous silica nanoparticle (AMSNP) gel was dipped in acetone, resulting in the formation of imine terminated mesoporous silica nanoparticles (IMSNP) with the AMSNP gel's body colour turning to yellow from colourless (Fig. 1). The unique amine to imine conversion in this process underlies a more facile condensation of an amine and a carbonyl compound, in contrast to a more tedious method employing a Schiff base.^9–12

To synthesize transition metal ion complexed imino organometallics, the imine terminated silica gels were dipped in 3 mmol of the respective transition metal chloride solution (Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺) resulting in a variety of organometallics. The process of complexing with transition metal ions appears to be complete in 5 min, as can be observed from the stabilization indicated by a colour change of the IMSNP, and henceforth be referred to as the transition metal ion-IMSNP (TM-IMSNP). However, during the course of this synthesis, there is a clear possibility of unreacted precursor sol seeping into the pores of the gels. Therefore, for removing these unreacted precursors, the gels were washed thrice with DMF followed by acetone washes and dried under supercritical (sc) conditions using scCO₂, as schematized in Fig. S1 in the ESI.† The scCO₂ drying process continued until the complete removal of residual sol (∼3–4 h).

Obviously, the IMSNP sample looking cream in colour in contrast to AMSNP (colourless) readily suggests selective photochemical absorption. Furthermore, complexing with different transition metal ions leads to the display of colours as determined by the coordinated TM-ion, as can be observed from the change in body colour of the TM-IMSNP samples, viz., dark brown for Fe²⁺, yellow for Cu²⁺, dark green for Co²⁺ and deep yellowish green for Ni²⁺.

The chemical structure of the organometallic complexes thus synthesized can be proposed (as in Fig. 1) by following the standard Schiffs base convention.¹³

Detailed spectral analysis through diffused reflectance studies (Fig. S2 in ESI†) show convincing spectral changes in the absorption maxima of the TM-IMSNP samples (with respect to the position of the respective TM chloride precursors), which is not observed in the case of the AMSNP system. This clearly establishes the effective complexing ability of the IMSNP with TM ions, unlike in the case of AMSNP. This observation is quite consistent with the stronger donor ability of imine over the amine system.¹⁴

The photochemical reaction, observed as a change in body colour of the organometallic, underlies the incorporation of some colour/defect center through some photo oxidation–reduction process. We hypothesize that the complexion of electronic states undergoes a drastic delocalization, meriting investigation using local probes addressing the electronic centers/states. However, on closer scrutiny of the particle morphology and crystal structure using transmission electron microscopic images and electron diffraction patterns, we could not find any substantial change in particulate morphology or crystal structure-character, which essentially remain amorphous in all cases, notwithstanding marginal improvements in the definition of particulate morphology for the imine system.

As can be observed from the FT-IR spectra (Fig. 2), the amine to imine conversion is quite obvious from the absorption at 1694 cm⁻¹, corresponding to the new stretching vibration of C=N.¹⁵ This absorption band is of moderate intensity and is quite characteristic of the imine system. A precise control of the amine¹⁶ to imine conversion becomes more obvious with the stretching vibration of the Si–O band at 1067 cm⁻¹ remaining intact even after the conversion.

Fig. 2 FTIR spectra of AMSNP and IMSNP.

Furthermore, information generated using the photoluminescence spectra concerning the photo-excitation process, usually confined to the first coordination sphere around electron centre(s), can obtain a precise image of the electronic state(s). Upon long-wave UV excitation (λ_ex = 365 nm), the AMSNP and IMSNP samples showed bright blue and green glows, respectively. Moreover, IMSNP samples when complexed with different transition metal ions did not show any significant change in the green glow, notwithstanding the difference in their body colour appearance under normal conditions.

Corresponding photoluminescence spectra showed the emission maxima of AMSNP and IMSNP being located at 412 and 523 nm, respectively, with little change in the λ_emission even when complexed with TM ions (Fig. 3). Moreover, it can be observed that the stokes shift (ΔE) for the excited electronic states in the case of AMSNP works out to be ΔE_amine ∼ 4900 cm⁻¹, which in turn suggests a stronger ion lattice coupling strength of S ∼ 1.5. While in the case of IMSNP, the stokes shift works out to be ΔE_imine ∼ 2830 cm⁻¹, with this significant difference suggesting a decrease in covalence, as can be inferred by a lower ion lattice coupling strength.¹⁷

Fig. 3 Photoluminescence excitation, emission spectra (T = 300 K) of AMSNP, IMSNP and IMSNP complexed with different TM ions. Stoke's shift (ΔE_S) = ΔE_amine/imine.

A higher ion lattice coupling strength might suggest an increase in covalency between the electrons (photo-excited) and that of the lattice/network, observed in the case of AMSNP, as schematized in Fig. 4. It is quite likely that the lone pair of electrons in the dangling amine system might undergo a configurational change during the course of the photo excitation process, eventually transforming into a more covalent complex as a photo product, manifesting in the luminescence band in a localized-confined way, as can be inferred from the narrow, single Gaussian band.

Fig. 4 Schematics illustrating relaxation of photo excited electrons in the strongly coupled AMSNP vs. the weakly coupled IMSNP.

On the other hand, the luminescence band in IMSNP acquires a totally new complexion, with the emission maximum shifting to lower energy with broad, diffused spectral bands. Moreover, the observation that the ion-lattice coupling strength is significantly reduced (halved) in the case of the IMSNP leads to a decreased propensity for the electron to be attached to the dangling amine. It is quite possible that during the amine to imine conversion, the lone pair of electrons degenerating into a singlet electron state might result in totally new electron centre/state. Thus, the liberated electron might manifest into a new center/state, eventually facilitating complexation with transition metal ions. This may very well account for why the saturated AMSNP turning into an unsaturated IMSNP exhibits a tendency to complex with TM ions with enhanced stability and reclaimability.^18–20 However, we observe that the photoluminescence spectra of the IMSNP samples, when complexed with different TM ions, do not show any appreciable spectral change or shift. To explain this, we should consider two possibilities, namely, (i) from the Pauling's electronegativity scale we note that these TM ions have almost the same electronegativity values, i.e., (1.8–1.9). This may be rationalized in terms of not much of a change in the electronegativity values eventually determining ionic-character versus complexing ability. (ii) A relaxation process of the photo-excited electronic state undergoing a large deformation will eventually render it non radiative. On the other hand, it is quite possible that the green luminescence band observed in the case of IMSNP (Fig. 3) can have its origin stemming from the amine to imine transformation centering (–NH₂ → –C=N) condensation. The hypothesis of a lone pair to singlet electron state conversion process is corroborated by the observation of the signature of an electron state/center in the X-band ESR spectrum at g = 2.55, only in the case of the IMSNP involving unsaturated imine groups. It should be noted that this signature cannot be observed in the case of the AMSNP system (Fig. 5).

Fig. 5 X-band ESR spectra (T = 300 K) of silica, AMSNP and IMSNP. * denotes the occurrence of an electronic state arising from the double bond in IMSNP.

A singlet structure at a higher g value (g > 2) provides clear experimental evidence for the generation of new electron states in the unsaturated double bonded imine system. In this context, it is pertinent to take note of significant contributions, pertaining to the ESR, upon complexing the IMNSP network with transition metal ions, viz., complexing with cobalt yielding a g ∼ 2.6,²¹ while complexing with Cu²⁺ yields a g ∼ 2.23,²² thus highlighting the pronounced complexing ability of the degenerated singlet electron state. Moreover, it should be borne in mind that the AMSNP containing a lone pair of electrons exhibits no band.

In summary, a TM ion complexed IMSNP system has been synthesised using a facile sol–gel synthesis method with precise control of the surface modification. The IMSNP system is effective at complexing with different transition metal ions as can be observed by the change in body colour and optical spectra (DRS) of the IMSNP samples. Investigations using local probes PL and ESR have clearly established the interconversion of a lone pair of electrons degenerating into a singlet electron state during the conversion of AMSNP to IMSNP.

Acknowledgements

Our sincere thanks to the learned editor and reviewers for the constructive comments. Authors thank CSIR for 12th five year project (Project No: CSC 0114) and the Department of Science and Technology for funding (GAP 10/14) under the fast track scheme.

References

1. (a) A. Sayari, S. Hamoudi and Y. Yang, Chem. Mater., 2005, 17, 212–216; (b) F. Wang, Y. Hu and C. Guo, Bioresour. Technol., 2012, 110, 120–124.
2. (a) I. I. Slowing, B. G. Trewyn, S. Giri and V. S. Y. Lin, Adv. Funct. Mater., 2007, 17, 1225–1236; (b) J. Lei, L. Wang and J. Zhang, Chem. Commun., 2010, 46, 8445–8447.
3. (a) J. Kim, J. E. Lee, J. Lee, J. H. Yu, B. C. Kim, K. An, Y. Hwang, C.-H. Shin, J.-G. Park, J. Kim and T. Hyeon, J. Am. Chem. Soc., 2006, 128, 688–689; (b) Y. Wang and F. Caruso, Chem. Mater., 2005, 17, 953–961; (c) C. M. Crudden, M. Satheesh and R. Lewis, J. Am. Chem. Soc., 2005, 127, 10045–10050.
4. (a) D. Borisovo, H. Mohwald and D. G. Shchukin, ACS Nano, 2011, 5, 1939–1946; (b) D. G. Shcuhukin, M. Zheludkevich, K. Yasakau, S. Lamaka, M. G. S. Ferreira and H. Mohwald, Adv. Mater., 2006, 18, 1672–1678.
5. Y. D. Glinka, S. H. Lin, L. P. Hwang and Y. T. Chen, Appl. Phys. Lett., 2000, 77, 3968–3971.
6. Y. D. Glinka, A. S. Zyubin, A. M. Mebel, S. H. Lin, L. P. Hwang and Y. T. Chen, Chem. Phys. Lett., 2002, 358, 180–186.
7. N. Chandrasekaran, S. Sivabalan, A. Prathap, S. Mohan and R. Jagannathan, RSC Adv., 2014, 4, 17879–17883.
8. L. Vaccaro, A. Morano, V. Radzig and M. Cannas, J. Phys. Chem. C, 2011, 115, 19476–19481.
9. M. G. Hosseini, M. Ehtesamzadeh and T. Shahrabi, Electrochim. Acta, 2007, 52, 3680–3685.
10. Z. Quan, S. Chen, Y. Li and X. Cui, Corros. Sci., 2002, 44, 703–715.
11. M. Lefort, G. Popa, E. Seyrek, R. Szamocki, O. Felix, J. Hemmerele, D. L. Vidal, J.-C. Voegel, F. Boulmedias, G. Decher and P. Schaff, Angew. Chem., 2010, 122, 10308–10311.
12. F. Zhao and Z.-Q. Liu, J. Phys. Org. Chem., 2009, 22, 791–798.
13. N. E. Borisova, M. D. Reshetova and Y. A. Ustynyuk, Chem. Rev., 2007, 107, 46–79.
14. G. A. Melson, Coordination chemistry of macrocylic compounds, Plenum press, Newyork, 1979.
15. J. C. Chisem, J. Rafelt, M. T. Shieh, J. Chisem, J. H. Clark, R. Jachuck, D. MacQuarrie, C. Ramshaw and K. Scott, Chem. Commun., 1998, 1949.
16. M. A. Meador, E. F. Fabrizio, F. llhan, A. Dass, G. Zhang, P. Vassilaras, J. C. Johnston and N. Leventis, Chem. Mater., 2005, 17, 1085–1098.
17. G. Blasse, Prog. Solid State Chem., 1988, 18, 79–171.
18. D. A. House and N. F. Curtis, J. Am. Chem. Soc., 1964, 86, 223–225.
19. J.-F. Ayme, J. E. Beves, D. A. Leigh, R. T. McBurney, K. Rissanen and D. Schultz, Nat. Chem., 2012, 4, 15.
20. C. D. Meyer, C. S. Joiner and J. F. Stoddart, Chem. Soc. Rev., 2007, 36, 1705.
21. R. J. P. Corriu, E. Lancelle-Beltran, A. Mehdi, C. Reye, S. Brandes and R. Guilard, Chem. Mater., 2003, 15, 3152–3160.
22. E. F. Murphy, L. Schmid, T. Burgi, M. Maciejewski and A. Baiker, Chem. Mater., 2001, 13, 1296–1304.

---

Footnote

† Electronic supplementary information (ESI) available: Experimental section, methods. See DOI: 10.1039/c6ra04697e

---