Designing novel Sn-Bi, Si-C and Ge-C nanostructures, using simple theoretical chemical similarities
© Zdetsis; licensee Springer. 2011
Received: 24 December 2010
Accepted: 27 April 2011
Published: 27 April 2011
A framework of simple, transparent and powerful concepts is presented which is based on isoelectronic (or isovalent) principles, analogies, regularities and similarities. These analogies could be considered as conceptual extensions of the periodical table of the elements, assuming that two atoms or molecules having the same number of valence electrons would be expected to have similar or homologous properties. In addition, such similar moieties should be able, in principle, to replace each other in more complex structures and nanocomposites. This is only partly true and only occurs under certain conditions which are investigated and reviewed here. When successful, these concepts are very powerful and transparent, leading to a large variety of nanomaterials based on Si and other group 14 elements, similar to well known and well studied analogous materials based on boron and carbon. Such nanomaterias designed in silico include, among many others, Si-C, Sn-Bi, Si-C and Ge-C clusters, rings, nanowheels, nanorodes, nanocages and multidecker sandwiches, as well as silicon planar rings and fullerenes similar to the analogous sp2 bonding carbon structures. It is shown that this pedagogically simple and transparent framework can lead to an endless variety of novel and functional nanomaterials with important potential applications in nanotechnology, nanomedicine and nanobiology. Some of the so called predicted structures have been already synthesized, not necessarily with the same rational and motivation. Finally, it is anticipated that such powerful and transparent rules and analogies, in addition to their predictive power, could also lead to far-reaching interpretations and a deeper understanding of already known results and information.
Similarly, there is another global diagonal relationship between elements (composed mainly of the third and second periods) which demands that diagonally adjacent elements (primarily of the second and third periods) have similar properties (in particular those related to size and electronegativity) [2, 3]. The size of the atoms decreases when moving across a period of the periodic table and increases when moving down a group. Likewise, the electronegativity increases when moving across a period and the elements become progressively more covalent and more electronegative. On moving down a group, the elements become more ionic and less electronegative. Thus, crossing and descending the periodic table have opposite effects on these properties, cancelling out for diagonal pairs of elements such as B and Si (for instance, boron and silicon are both semiconductors). Similarly, and in addition to the diagonal relationship there is an obvious vertical relation between elements of the same column of the periodical table (groups) which have the same number of valence electrons and similar (more or less) chemical properties. Yet, as we descend down a group, there are systematic and regular differences, described by the well known inert pair effect [2–5] - that is, the increasing stability of oxidation states that are two less than the group valency, for heavier elements of the group. It can be also described as the resistance to losing the s-electron in the valence shell (for the group 13, 14 and 15 elements). This is a relativistic effect which is due to the better penetration of the s-valence electrons to the nucleus compared to p-electrons [2–5].
The obvious questions are how could one know beforehand which of the many conceivable similarities (or equivalence relations) are valid (or working) and what is the criterion or criteria?
A natural, but incomplete, answer should certainly depend on chemical (and physical) intuition and plain common sense, although sometimes in science common sense has lead to wrong conclusions. This approach suggests that not only the number of valence electrons but also the type of bonding (hybridization, relative magnitude) should be important for such similarities. For instance, carbon in diamond and crystalline silicon have similar properties because they are both sp3 bonded through strong covalent bonds. In such a case the replacement Si→C is valid but not in the case of graphite (which is sp2 bonded) or benzene (which is aromatically bonded). In this case we can derive the rule CH→Si1- (8), by noting that the Si6 6- multianion has a planar hexagonal structure similar to benzene (C6H6) [17, 18],from which it follows that Si6 6-→C6H6 or CH→Si1- (8). Both of these relations are isovalent (involve the same number of valence electrons and can be realized [17, 18] by placing lithium counter-ions in suitable positions. This has been known as the Si6Li6→C6H6 (9) replacement rule of thumb. Such bonding properties are reflected in the structure, symmetry and occupation of the frontier orbitals through which these simple ideas can be strictly formalized, classified and theoretically implemented. Analogous ideas have been applied for the silicon and carbon fullerenes [6–9] which are similar because they are similarly sp3 bonded (not sp2 as in the real carbon fullerenes). Such a formulation has lead to the isolobal principle (analogy) . The isolobal analogy was introduced about 30 years ago by Hoffman  in order to allow the correlation of seemingly very different chemical species, such as organic hydrocarbon fragments, with transition metal ligands. Two fragments are isolobal if the number, symmetry properties, approximate energy and shape of the frontier orbitals, as well as the number of electrons in them, are similar (but not identical). Although it seems strange, CH, Co(CO)3 or NH3, Co(CO)4 can be considered as classical examples of pairs of isolobal fragments. Closer examination reveals that all these fragments need the same number of missing electrons in order to reach a stable electronic configuration. For example, CH has five valence electrons and, therefore, it needs three more to reach the stable configuration of eight. Likewise, Co(CO)3 which has 15 valence electrons misses the stable configuration of 18 by three electrons. These fragments or building blocks can replace each other in more complex structures and can combine to form ordinary bonds.
The isolobal analogy allows us to relate and compare organic, inorganic and/or organometallic compounds on a uniform basis. Inversely, if we can find molecular fragments or building blocks that can replace each other in complex structures, we can conclude that they are isolobal. Therefore, the best way to examine and validate any (local) chemical similarity, discovered by chemical intuition and/or general concepts (based on isoelectronic or isovalent analogies) is the scheme demanded by the isolobal principle. The replacement BH→Si (1) and most of the analogies (1)-(7) involving (directly or indirectly) boron are special cases of a more general isolobal analogy which the author, scoptically and synoptically, has termed 'the boron connection'. This analogy provides a mechanism to predict new, hopefully stable, molecules and to compare molecular fragments with each other and with familiar species from organic chemistry and also offers clues about reactivity and reaction mechanisms. The boron connection is an attempt to map the structural chemistry of boranes and carboranes [26–31] to the structural chemistry of silicon  and silicon-carbon clusters, respectively. Boranes and carboranes constitute a very rich and well-established branch of chemistry with well-known structural and stability rules and powerful concepts [26–30]. Since boranes and borane-based molecular fragments and nano composites - including carboranes, bisboranes and metallaboranes - and metallacarboranes are very well studied [26–31, 33–35] and are very well known species with many technological, chemical and biomedical [33, 34] applications, their silicon, silicon-carbon (and other group 14, and 15) analogues [35, 36] are expected to be very important and very promising for analogous applications.
It should be emphasized that not all of the similarities and results obtained were initially conceived as such but they have later been unified, classified and generalized within the framework and the scope of the present work. Clearly, not all the results presented here are reviews of earlier work. Several totally new results and predictions have been obtained here.
Results and discussion
The boron connection for Si and Si-C structures
The boron connection for Sn, Bi and Sn-Bi structures
(CH)→(SiH) or (CH4)→(SiH4) substitution: Si fullerenes
The CH→Si1- or CH→Si1-Li1+ rule of thumb: planar aromatic silicon rings
Other possible applications and implications in nano-medicine and biology
In this work we have been mainly concerned with total substitutions of the form BH→Si, Si→C, Si→Sn, BH1-→CH, and CH→Bi and others. However, partial substitutions could be as (or even more) useful as total substitutions. Such partial substitutions, which have not so far been investigated or tested (and, thus, constitute new global predictions of the current paper) could be very useful in medical and pharmaceutical applications, such as drug delivery, radioimaging and radiolabelling, among others. The building blocks of such functionalized nanosystems are borane ions, carboranes, metallacarboranes, metallaboranes and other heteroatom derivatives or combinations. For example, mixed BHC and SiC nanorods, obtained by partial (not total) BH→Si substation in the model systems discussed above, should be able to be used for radiolabelling by iodination or tritiation, respectively, as described by Hawthhome and Maderna [33, 34]. Such a possibility has recently been successfully explored  with very promising preliminary results. In all these analogous material symmetry is very important and should be taken into account, provided there are additional tests to verify that no imaginary frequency modes occur, leading unavoidable to distortions (as in the case of Si12 2-).
Finally, the recent (well after this work was finalized) discovery of A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus by Wolfe-Simon et al., indicates that such rather simple and 'innocent' isovalent (or isolobal) substations used throughout this work, could have very far-reaching implications in many branches of science and technology.
It has been illustrated that chemical intuition supported by rather simple and transparent, but very powerful techniques, can form a general scheme or framework leading to molecular engineering and theoretical design of very important and functional nanomaterials and nanostructures based on silicon, carbon and other group14 and group 15 elements. It has been also illustrated that the framework presented here, in addition to the older and newer predictions (some of which have been already tested and/or materialized), could lead to very far-reaching implications and applications in many branches of science and technology, including nanomedicine and molecular biology.
Yet, its implementation is not always unique or straightforward, due to many alternative misleading routes. Chemical (and physical) intuition is always very important. Apparently, the same scheme, based on observed and well-defined properties of one category of materials should be able to predict and design analogous materials, with similar chemical properties for a (seemingly) different category of structures. We have restricted our attention to group 14 (and group 15) elements and structures (based on well-defined properties of group 13 elements and, in particular, boron) due to the high technological importance of silicon and carbon (and the other connected elements and fragments). Even within this limited range of the periodical table (illustrated in Figure 2), the possibilities are endless and the possible routes unlimited. Hopefully, the scheme presented here should be able to guide the search, the theoretical design and the validation (through the isolobal criteria) of the (theoretical) results. Obviously (and unavoidably), experiment and technological synthesis is the final test of any theoretical molecular engineering design.
Dedication: This work is dedicated to the memory of my late wife, Elpiniki Zdetsis.
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