Testing the Effectiveness of the Isoelectronic Substitution Principle through the Transformation of Aromatic Osmathiophene Derivatives into their Inorganic Analogues

Alejandro Vásquez-Espinal, Jordi Poater, Miquel Solà, William Tiznado, and Rafael Islas Doctorado en Fisicoquímica Molecular, Facultad de Ciencias Exactas, Universidad Andres Bello, República 275, Santiago, Chile Departament de Química Inorgànica i Orgànica & Institut de Química Teòrica i Computacional (IQTCUB), Universitat de Barcelona, Martí i Franquès 1-11, 08028 Barcelona, Catalonia, Spain ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain Institut de Química Computacional i Catàlisi (IQCC) and Departament de Química, Universitat de Girona, Campus Montilivi s/n, 17003Girona, Catalonia, Spain Departamento de Ciencias Químicas, Facultad de Ciencias Exactas, Universidad Andres Bello, República 275, Santiago, Chile


Introduction
The isoelectronic substitution (IS) principle, an important concept in chemistry, could be useful as a guide to design new molecules.2] For instance, Olson and Boldyrev proposed boron-hydrogen analogues of saturated hydrocarbons; showing that one CH unit can be changed by one BH -anion (both with five valence electrons) to form stable compounds.4] On the other hand, CH can also be replaced by other isoelectronic species, such as N.If one replaces all CH units of benzene by N, the N6 (D6h) structure is obtained.Based on a theoretical research, Roberts suggested in 1961 that this all-nitrogen benzene analogue, called hexazine, would present the same -delocalization pattern, provided the unshared electron pairs are regarded as strictly localized. 5 Additionally, Gimarc proposed that the instability is generated by the lone pairs repulsion. 8reover, if the valence isoelectronic fragment has an approximate shape and energy (in their respective frontier orbitals) to the fragment being replaced, the IS coincides with the isolobal principle, proposed by Hoffmann in 1982. 94] Interestingly, these authors demonstrated the existence of a relation between planar aromatic annulenes and tridimensional aromatic closo boron hydride clusters. 15These results seem to indicate that the IS principle applies effectively when designing boron analogues of aromatic organic compounds.However, in some cases the IS could drastically change the chemical bonding patterns in the designed compounds.7] For instance, borazine 18 (B3N3H6), also known as "inorganic benzol", 19 has structural parameters such as planarity and bond equalization and number of π electrons similar to benzene.[22][23][24][25][26] The aim of the present work is to evaluate, in an extreme situation, the validity of the concepts mentioned in the preceding paragraphs, in order to design new stable molecules.The chosen system to achieve this goal is [OsCl2(SC3H3)(PH3)2] + ,which presents some characteristics that make it suitable for this purpose: 1) it is a simplified model of the osmabicycles (aromatic compounds where one Os atom is present in five-membered metallacycles), studied both experimentally and theoretically by Esteruelas et al., 27 2) this system was classified as clearly aromatic, 28 with participation of Os-d-orbital (especially dxz and dyz orbitals) closing the circuit of πorbitals and allowing electronic delocalization on the planar OsC3S fragment. 29Additionally, the presence of an Os-S bond is another interesting aspect, even though S has almost the same electronegativity that C, it is nearly double the size of its covalent radius and it has an extra lone pair of electrons (this is expected to play an important role in electronic delocalization after performing the IS procedure).Moreover, the Os atom has two axial ligands, which are expected to directly affect the π delocalization.The question is how all these particular characteristics will evolve as the system is transformed to preserve, or not, the aromaticity as a key stabilizing factor.
The aromaticity in metallacyclopentadienes, like those analyzed in the present work, has been less studied than metallabenzenes.For the latter, in 2015, Fernández et.al, published a review discussing the role of the d orbitals of the metallic atoms in the electronic delocalization and aromaticity of these species. 30e current study involves the sequential transformation of [OsCl2(SC3H3)(PH3)2] + into its completely inorganic analogues OsCl2(SB2NH3)(PH3)2 and [OsCl2(SBN2H3)(PH3)2] 2+ .This has been performed starting from the [OsCl2(SC3H3)(PH3)2] + complex by all possible single substitutions of C atoms by N + or B -, double substitutions of CC by BN units, and triple substitutions of CCC by B2N - or BN2 + moieties.To evaluate the consequence of these substitutions, structural parameters and electronic delocalization have been thoroughly analyzed.

Methodology
All geometry optimizations, vibrational frequency, wave function stabilities, and relative energy calculations were performed with Gaussian 09 program 31 using the B3PW91 functional [32][33] and the def2-TZVP basis set, 34 including pseudo-potentials for Os atom. 35emical bonding analysis was performed with the adaptive natural density partitioning (AdNDP) method 36 at the B3PW91/def2-TZVP level.The AdNDP method analyzes the first-order reduced density matrix and it represents the electronic structure in terms of n-center-two-electron (nc-2e) bonds.This is done in order to recover both Lewis bonding elements (1c-2e or 2c-2e, i.e., lone pairs or two-center two-electron bonds) and delocalized bonding elements, which are associated with the concepts of aromaticity.[41][42] AdNDP analyses were performed with the Multiwfn program. 43e magnetic shielding tensors were computed with PBE0 [44][45] functional and def2-TZVP basis set.The induced magnetic field was calculated following the formula 7] In all cases, five-membered rings (5-MRs) were placed in the xy plane with the geometrical center coinciding at the origin of the Cartesian coordinates.The units of the B ind are ppm considering | ext | = 1 T. The negative of the zz (or 33) magnetic shielding tensor component, equivalent to B ind z (and NICSzz), is a good descriptor for the magnetic response of (anti)aromatic compounds 48 that has been employed successfully in previous 5-MR's heterometallacycles work. 28is methodology has been fruitfully employed in other different kind of systems such as silicon star-shaped molecules, 10, 49-50 boron clusters, [51][52][53][54][55] metallic clusters, [56][57] compounds with planar tetra-, [58][59][60] penta-, [61][62] or even hexacoordinated carbon atoms. 63 The electron delocalization multicenter index (MCI) was used as an electronic index of aromaticity.MCI stemmed from the Iring index which was defined by Giambiagi in 2000 70 as: where Sij (Ak) is the overlap between molecular orbitals i and j within the domain of atom k.In this formula, it is considered that the ring is formed by atoms in the string {A} = {A1, A2, …An}.An extension of this Iring index, by Bultinck and coworkers, 71 resulted in the so-called MCI index: where P(A) stands for the n! permutations of the elements in the string {A}.The MCI index has been successfully applied to a broad number of situations, from simple organic compounds to complex all-metal clusters with multiple aromaticity. 72The numerical integrations over the atomic domains were carried out within the "fuzzy atom" framework, 73 using the Becke- partitioning scheme 74 with the APOST-3D program. 75The Iring and MCI indexes were obtained with the ESI-3D program [76][77][78] at the B3PW91/def2-TZVP level.

Application of the isoelectronic substitution principle
Complex [OsCl2(SC3H3)(PH3)2] + (labeled as 1 in this work) was chosen to test the effectiveness of the IS principle.In a previous work, it was proven that 1 has the most diatropic response among the systems with the general formula: M(XC3H3)(PH3)2, where M = OsH3, OsCl3, OsCl2, RuCl2, RhCl2 or IrCl2,and X = NH, O, S, CH − , or CH + . 28Therefore, any change in the structure and in the electronic delocalization after the IS should be noticeable.The IS procedure consisted in replacing carbon atoms in 1 by other 4-valence electron species: the anion B -and the cation N + .In the first step, only one carbon atom was replaced by one B -anion, yielding the Bseries, conformed by three neutral isomers (2-4 in Scheme 1).In system 2, the boron atom is bonded to the transition metal; in system 3, to two carbon atoms, and in system 4, to the sulfur atom.In the second step, the same procedure was performed, but this time with the nitrogen cation, obtaining the series N-series with three isomers (5-7 in Scheme 1).In system 5, the nitrogen atom is bonded to the transition metal, in system 6, to two carbon atoms, and in system 7, to the sulfur atom.In the third step, starting again from 1, a double substitution was carried out.This systematic substitution consisted of one C replaced by one N + , and another C by one B -.With these substitutions, a series (N,B-series) with six isomers (8-13 in Scheme 1) was obtained.These included those where a C-C unit is replaced by a B-N one (systems 8-11); whereas in systems 12 and 13, the boron and the nitrogen atoms are bonded through a carbon atom.In the last step, all carbon atoms were replaced, in order to obtain two "inorganic" analogues of 1 (see systems 14 and 15 in Scheme 1).Finally, with the aim of studying the role of sulfur in the stability and aromaticity in these series, the S 2+ unit in system 15 was replaced by a B -one, to retrieve system 16.
All the obtained systems after IS procedure were optimized and the stability of the wave functions was analyzed.The B-series is conformed by 2, 3 and 4 systems (see Scheme 1).Among them, 2 is the only stable system with similar geometry (planarity) and electronic structure to the original system 1, since system 3 is not planar and 4 presents singlet instability for restricted wave function.System 4 in its triplet ground state is 59.8 kcal mol -1 more stable than in its lowest-lying singlet closed-shell state and system 3 is 26.5 kcal mol -1 less stable than system 2.Meanwhile, the N-series is conformed by three stable isomers (5, 6, and 7) without instabilities in their respective wave functions, and besides they present comparable structural parameters as planar 5-MR rings.
The wave function of the six B,N-series isomers were also analyzed, and it was found UHF instabilities in the systems 10, 11, and 13.Additionally, system 9 is not a planar 5-MR ring.Only

Structural parameters
The geometries reported in Table 1 show the bond lengths (only for cycles) and internal angles of the fifteen compounds proposed in this work (see Scheme 1).When compared with system 1, deviations in Os-S bond length are not longer than 0.172 Å.The shortest bond length deviation is at system 12 (0.003 Å) and the longest one is at system 9 (0.172 Å).For Os-X bonds (where X = C, B -or N + in the 1-15 systems), the longest deviation is at system 3 (0.129 Å) and the shortest one is at system 10 (0.008 Å).The internal angles are also affected; the most dramatic change is at system 3, where the  and  angles change by more than 10° each.Internal angles and in system 14 are also affected by the IS, with 12° and 15°, respectively.As shown in Table 2 (where the dihedral angles of some selected systems are depicted), the IS not only affects the bond lengths and the internal angles, but the planarity of the rings is also affected; this is more evident in systems 3 and 9 (the most distorted rings).The equality of bond lengths and planarity are the most characteristic structural parameters of aromatic compounds; therefore, the results presented above suggest dramatic changes in aromaticity after the IS.This aspect will be further discussed in the next section, using different theoretical approximations.

Aromaticity
In Figure 1, the B ind z profiles along the z-axis are plotted for all the analyzed systems.When B ind z is calculated at R=0 (geometrical center of the ring placed in the xy plane) it is called B ind z(0).
The larger the B ind z(0) values, the more paratropic (antiaromatic) the system is, meanwhile the smaller the B ind z(0), the more diatropic (aromatic) the system is.Please note that the non-planar structures 3 and 9 were discarded here and in further analyses.Systems with UHF instabilities (4,   10, 11, 13 and 15) are studied in their lowest lying closed shell singlet excited state.The profiles of four of the studied compounds are highlighted by broader lines in the plot: the reference molecule 1, the systems with the largest and the smallest B ind z(0) values (14 and 7, respectively) and finally, the boroazometallocycle 16.For comparison purposes, benzene (the aromatic compound par excellence) and tiophene are included in the plot.It is highly striking to notice that some of the proposed systems are more aromatic than the starting point (system 1).The aromatic character increases (according to B ind z(0)) as follows: 1<6<15<5<12<tiophene≈7<benzene.It is also important to remark that nitrogen mono-substituted cycles are more diatropic than the boron monosubstituted ones, reaching B ind z(0) values similar to the one of benzene in system 7.Moreover, systems 7, 12, and 15 (where there is an S-N bond) present a higher diatropic response compared to other compounds.This is particularly interesting because of the relatively large difference in electronegativity of S and N atoms, and thus a ring current disturbance could be expected.The counterpart is found in the systems with the S-B bond, which present a significant increase of the B ind z values.Even some of them are predicted to be anti-aromatic compounds.The close relation between the presence of an S-B bond and the anti-aromatic character is evidenced when B replaces S in system 14 to produce system 15.Both systems are fully inorganic rings, but the first one has a B ind z(0) close to 8 ppm, compared with -5 ppm in the second one.Finally, the boroazometallocycle 16 is a non-aromatic system in spite of its structural parameters, such as its perfect planarity.From Table 1, it is clear that this system resembles the non-aromatic cyclopentadiene with two localized B=N double bonds (d2 and d4) and one B-N single bond (d3).

Figure 1.
Profile plots of the B ind z computed in all the heterometallocycles proposed.Benzene is included for a better comparison.R = 0 corresponds to the center of the ring placed in the xy plane.The shielding tensors were computed at PBE0/def2-TZVP.
At this point of the discussion, the question of the role of aromaticity on the stabilization of the proposed systems arises.In an attempt to answer this question, the aromaticity (according to the B ind z profiles) is compared with the relative stabilities among the isomers forming each proposed series (see Figure 2).
The B-series systems are not depicted in Figure 2 because only 2 is planar and described by a stable wave function.For the N-series, isomer 6 is only 1 kcal mol -1 less stable than 5 and both systems present a very similar magnetic response.On the other hand, the most diatropic compound 7 is 15 kcal mol -1 higher in energy than 5 (the most stable and least aromatic compound in this series).This behavior could be attributed to the presence of a weak S-N bond in complex 7.While this bond favors the electronic delocalization, its weakness leads to species that are relatively unstable.In - Complexes fact, the bonding energies (computed at B3PW91//def2-TZVP level) for the hemolytic dissociation in species HS-NH2, HS-BH2, H3C-NH2, H3C-BH2, and H2B-NH2, are -69.4,-115.5, -88.3, -106.9, and -145.2 kcal/mol, respectively. For the B,N-series (8, 9, and 12 systems), the B ind z profiles indicate that 12 is the most diatropic isomer even though it is 32 kcal mol -1 less stable than the most stable 8. Whereas the non-planar system, 9, is 35 kcal mol -1 less stable than 8. Again, the presence of S-N bonds in 9 and 12 justifies their lower stabilities as compared to 8.These results show, once again, that in this double substituted series thermodynamic stability is associated to the strength of their formed chemical bonds rather than to their electronic delocalization in the ring.The main objective of this work is to assess whether IS is a reliable tool for molecular design.One of the most sensitive changes is the electronic delocalization.As mentioned in the introduction of the current work, one of the differences between benzene and borazine is their aromatic character.9] To complement the analysis of the magnetic properties performed in this work, maps of isolines of B ind z computed for 1, 7, and 14 are plotted in Figure 3.These complexes were selected because 1 is the reference point, whereas7 and 14 are the most diatropic and paratropic compounds, respectively.The xz plane was chosen in such a way that it contains the Os atom and bisects the ring; whereas the yz plane contains the sulfur atom.The maps show that a paratropic region surrounds the osmium atom in the three systems.However, 1 and 7 can be considered as diatropic rings.The nitrogen atom in 7 is surrounded by diatropic regions, which could be contributing to negative values of the B ind z at the center of the ring.On the other hand, the isolines of B ind z, plotted in the xy plane, of the double boron substituted system 14 show two paratropic regions: one in the center of the ring and another one around the boron bonded to the Os atom.As a consequence, 14 is the best candidate to prove that IS can affect, in an unpredictable way, the electronic delocalization in rings with a transition metal; and thus, it is not an infallible methodology, at least when trying to tune the magnetic response.For xy plane the PH3 units were removed from the picture for a better appreciation of the inplane B ind z isolines.Grey, yellow, white, blue, dark blue, green, pink and orange, represent carbon, sulfur, hydrogen, nitrogen, osmium, chlorine, boron and phosphorus atoms, respectively.Units are ppm.

Chemical Bonding Analysis.
For a deeper understanding of the chemical bonding of these complexes, we performed an AdNDP analysis in the following systems: the reference (1), the most diatropic (7) and the most paratropic ( 14) systems.The figures corresponding to the σ bonding pattern of molecules 1,7, and 14, revealed by AdNDP, are available in the Supporting Information.As it can be seen, AdNDP showed eight lone pairs: three on each chlorine atom, one on the osmium atom, and another one on the sulfur atom.The σ bonding pattern of these three systems is similar: almost all bonds are classic 2c-2e, with the exception of the Os-S and the Os-C (only in system 1) bonds.All bonds involving the hydrogen atoms are found as classic 2c-2e σ bonds, except for 14 in which boron, hydrogen, and osmium atoms form up a small delocalized triangle, where AdNDP reveals that two electrons are delocalized in the three centers (3c-2e) (see Figure 4).The π-bonding pattern of molecules 1, 7, and 14 is shown in Figure 5.In all three systems, two 3c-2e π bonds, involving the sulfur atom, were found with a relatively high occupation number (1.87-2.00|e|),which suggest delocalization of these electrons.The third π-bonding element, which is centered on the osmium atom, was also found as a 3c-2e element with ON of 1.71, 1.57 and 1.70 |e|, for systems 1, 7 and 14, respectively.However, it is worth mentioning that only in systems 1 and 7 the last bond could be considered as π (having a nodal plane coinciding with the plane of the molecule).Then, according to AdNDP, systems 1 and 7 should be aromatic (six π-electrons) and system 14 should be antiaromatic (four π-electrons).Moreover, the π-bond that is centered on the Os, is distributed on the molecular ring of system 7 (which is the most aromatic, according to the magnetic criteria), whereas in system 1 it is delocalized to the phosphine axial ligands.Therefore, this result agrees with the analysis of the magnetic properties, discussed above.supporting the paratropic response computed with B ind z.The electronic delocalization of these compounds is not the key for their stability, other factors, such as the bond strength of the different bonds involved plays a more important role in determining the stability of these systems.

systems 8 Scheme 1 .
Scheme 1, where the UHF unstable systems are marked by a solid-line square, and the non-planar

Figure 2 .
Figure 2. B indz profiles of the a) nitrogen-, and b) double substituted rings.The relative energies of the isomers are in parentheses and their units are kcal mol -1 computed at B3PW91/def2-TZVP including zero point corrections.

Figure 3 .
Figure 3. Isolines of B ind z computed in 1,7, and 14.The isolines are plotted in the xy, xz, and yz planes.For xy plane the PH3 units were removed from the picture for a better appreciation of the inplane B ind z isolines.Grey, yellow, white, blue, dark blue, green, pink and orange, represent carbon, sulfur, hydrogen, nitrogen, osmium, chlorine, boron and phosphorus atoms, respectively.Units are ppm.

Figure 4 .
Figure 4. Delocalized B-H-Os bond in molecule 14.ON stands for occupation number in |e|.

Table 3 .
Calculated MCI of complexes1-16 under study.a Units are electrons.
a Non-planar 3 and 9 complexes not included