Prototropic and metallotropic migration of isolobal fragments on indol rings . Theoretical study and NBO analysis

Molecular structures, energies, NBO analysis and sigmatropic behaviour of 1-Indenyl(dihydro)borane (1) and 1Indenyl-threecarbonylcobalt(I) (2) were investigated using DFT and ab initio molecular orbital methods. In these compounds BH2 and Co(CO)3 fragments are isolobal. The Results of calculations using B3LYP, HF and MP2 methods [Basis set 6-311+G**] showed that -BH2 and -Co(CO)3 had similar behaviour in sigmatropic shifts. Prototropic shifts in compounds 1 and 2 have similar mechanisms too. Results showed that metallotrotropic shift is faster than Prototrpic shift in compounds 1 and 2. The activation energies (Ea) of Prototropic shift in compounds 1 and 2 are 18.83 and 17.38 kcal.mol -1 . These energies are higher than -BH2 shifts in compound 1 (10.11 kcal.mol -1 ) or migration of -Co(CO)3 fragment in compound 2 (12.39 kcal.mol -1 ). Lower amount of activation energy in borotropic shift and cobalt`s fragment shift show that rotation of boron and cobalt on the indol ring can happen in the ambient temperature. Calculation results revealed that migration of proton and Co(CO)3 was carried out via suprafacial[1,2]-sigmatropic mechanism while -BH2 shift took place via antrafacial [1,3]-rearangment.


Scheme 1: Preparation of 1-indenyl(diethyl)borane
NMR spectra of borane 3a was taken in the temperature interval 25-80 • C and three cross-peaks corresponding to the migrations of the diethylboryl group from position 1 to position 3 were found [7].

Scheme 2: sigmatropic migration of hydrogen and boron
Sigmatropic migration of many fragments on aromatic rings has been observed.These compounds are fluxional.Experimental [1] and theoretical [4] studies on barrier energies and rate of ()metallotropic and ()prototropic shift on Fluxional compounds were carried out.
The isolobal principle has been devised by Roald Hoffmannin in Organometallic Chemistry [16].The methyl radical is isolobal with the metal carbonyl species manganese pentacarbonyl Mn(CO)5 because both molecules have a single electron in a hybrid orbital pointing away from the plane of the molecule (though they are not isoelectronic).This resemblance is reflected in the chemistry of both molecules.As the methyl radical can dimerize to ethane, Mn(CO)5 can dimerize to (CO)5Mn-Mn(CO)5 and both radicals can form Mn(CO)5CH3. So, it can be inferred that -BH2 and -Co(CO)3 are isolobal groups.Those were showed their HOMO and LUMO diagram in fig 1 .As the above mentioned, fluxional behaviour of boron in indenyl(dihydro)boran observed [5], but mechanism of this behaviour was not investigated.On the other hand, any experimental or theoretical investigation about structural, fluxional behaviour and sigmatropic shifts mechanism of isolobal compound with indenyl(dihydro)boran not performed.
The main goal of this work is investigation of sigmatropic behaviour of indenyl(dihydro)boran.Also, presentation, investigation of structural and fluxional properties and sigmatropic shifts mechanism for inorganic isolobal compound with indenyl(dihydro)boran and comparison together.

CALCULATION DETAILS
Ab initio molecular orbital (MO) calculations were carried out using B3LYP/6-311+G** level of theory with the GAUSSIAN 2003 package of programs [21] on a personal computer.
Vibrational frequency amounts for all ground and transition state structures were calculated at B3LYP/6-311+G** level of theory for compound 1, but for compound 2, due to their large size, they were calculated at B3LYP/lanl2dz level of theory.
Energy-minimum molecular geometries were located by minimizing energy with respect to all geometrical coordinates without imposing any structurally symmetrical constraints and transition state structures were obtained by QST2 subroutine.[22,23] Natural Bond Orbital (NBO) analysis was then performed at the B3LYP/6-311+G** level by the NBO 3.1 program included in the GAUSSIAN 03 package of programs.

Scheme 4 : symmetry-allowed suprafacial [1,2] rearrangement of proton
BH2 shifts cannot occur through [1,2] superafacial rearrangement.This migration is a [1,3] antrafacial rearrangement (scheme 5), But Co(CO)3 shifts can occur through [1,2] superafacial rearrangement as illustrate in (scheme 6).In When BH2 is in vinylic position these compounds are not in ground state.Infra Red (IR) vibration frequencies (an imaginary frequency) and Aromatic Stabilization Energy (ASE = 3.62 Kcal.mol-1) as well as bond lengths of C-B that increased to 1.69 Angstrom results showed that this structure is a transition state one (fig.3 and table 1).Also IRC calculation confirmed that BH2 in vinylic position is a transition structure (fig.5).
Prototropic and metallotropic shift pathways of compounds 1 and 2 are shown in Figures 3 and 4, respectively.
Structural parameters, total electronic (Eel) energy, corrected zero point (ZPE c ) energies of compounds 1 and 2, calculated on the ab initio and DFT levels of theory are given in Tables 1-7.The migration mechanisms for the BH2, Co(CO)3 and hydrogen were also investigated and structure of ground states and transition states of all compounds were investigated.Calculated results revealed that Indol rings are planar in the ground state and non planar in transition state for compounds 1 and 2. δ1239 and δ2398 values are 0.0000 for ground state of compounds 1 and 2 while these values are 16.34 and -10.04 for compound 1 and -3.05 and 2.56 for compound 2, respectively( see table 1 and 4) Also, reaction pathways were shown in Figures 3 and 4. In metallotropic 1,2-shifts, the reaction pathways are symmetrical and in the transition state structure, the BH2, Co(CO)3 groups are equidistant from two carbon atoms of the C9H7 rings (see Tables 1 and 4 ).According to the results, comparing C-B and C-Co shows an increase in bond lengths while in the contrary, the dissociation of a C-B bond is easier than that of a C-Co bond which in turn, leads to the migration of the metal around the indol ring with a lower energy barrier.This result is in good agreement with the mentioned experimental data.
As it can be inferred from the Figures 3 and 4 the prototropic 1,2-shifts require higher energy barrier than metallotropic 1,2-shifts, consequently, the rate of this process is much slower than that of metal migration.Therefore, it was revealed that prototropic shifts for compound 1 have lower migration energy comparing to compound 2. It can be concluded that the prototropic 1,2-shifts occur only at higher temperatures.These results have a good agreement with structural and NBO results.
BH2 shift in compound 1 cannot occurred via [1,2] migration and performed via [1,3]  Barrier height of metalotropic shift is lower than prototrophic shift.Lower amount of activation energy in borotropic shift and cobalt`s fragment shift show that rotation of boron and cobalt on the indol ring can happen in the ambient temperature, but prototropic migration occurred at higher temperature.

Figure 1 :
Figure 1:Similarity of HOMO and LUMO diagrams for optimized structure of BH2 and [Co(CO)3]The nature of the stationary points for compounds has been determined by virtue of the number of imaginary frequencies.For minimum state structures, only real frequency values, and in the transition state only single imaginary frequency values (with a negative sign) are accepted.

Figure 2 :
Figure 2: HOMO diagram of optimized metallotropic transition state structural of compound 1, 2 Ab initio and DFT calculation results showed that the energy barrier of prototropic shift in compound 1 and 2 from C(1) to C(2) is equal to the prototropic shift from C(2) to C(3) ( see table 1).Calculation results revealed that BH2 [1,2] migration need 10.11 kcal.mol - that is a lower migration energy comparing to Co(CO)3 (12.39 kcal.mol-1) while bond lengths of C-B and C-Co are 1.562 and 2.082 Angstrom.On the other hand NBO calculation results showed that electron occupancies of C1 -B bond is 1.9 while occupancies of.C1 -Co bond is 1.74.The mentioned results concluded that C-Co bond has an irregular strength.The reason for straight of C-Co bond and the higher level of the barrier energy for Co(CO)3 migration is related to the existence of a π back bonding between Co and C. B3LYP/6-311+G** results showed sigmatropic shift because compound 1 has one node on C(2) atom in HOMO diagram while in compound 2 this migration occurred at [1,2]metallotropic shift because HOMO diagram of this compound showed suitable overlap of orbitals for [1,2]-sigmatropic shift of Co(CO)3 fragment.In a similar vein, the occurrence of [1,2]-prototrophic shift for the two compounds was easy.Bond legth of Co-C is longer than B-C and electronic occupancies of Co-C is lower than B-C while sigmatropic barrier energy for Co(CO)3 migration is higher than BH2 migration, because Co-C bond has a -back bonding which makes it stronger than B-C.

Table 4 . B3LYP/lanl2dz Calculated Structural Parameters and Energies of the Ground States and TransitionStates of -indenyl(tricarbonyl)cobalt..Bond Lengths are in Angstrom Units (Å) and Angles in Degrees (º)
a Corrected by multiplying by a scaling factor (0.9409).bRelative to the most stable structure