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Effect of Transition Metal Fragments on the Reverse Fritsch-Buttenberg-Wiechell Type Ring Contraction Reaction of Metallabenzynes to Metal-Carbene Complexes Chakkittakandiyil Anusha, Susmita De, and Pattiyil Parameswaran J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b10335 • Publication Date (Web): 27 Jan 2018 Downloaded from http://pubs.acs.org on January 28, 2018

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The Journal of Physical Chemistry

Effect of Transition Metal Fragments on the Reverse Fritsch-Buttenberg-Wiechell Type Ring Contraction Reaction of Metallabenzynes to Metal-carbene Complexes Chakkittakandiyil Anusha1, Susmita De1,2* and Pattiyil Parameswaran1*

1

Department of Chemistry, National Institute of Technology Calicut, Kozhikode, Kerala 673 601, India. E-mail: [email protected]; Fax: +91-495-228-7250; Tel: +91-495-228-5304 2

Department of Applied Chemistry, Cochin University of Science and Technology, Thrikakara, Kochi – 682022, Kerala, India. E-mail: [email protected], [email protected]; Fax: +91 484 2575804; Tel: +91 484 2862420

Metallabenzynes, benzyne, cyclopentadienylidene carbene and metal-carbenes ABSTRACT: Metallabenzynes (1M), contrary to their organic analogues, benzyne, undergo ring-contraction to metal-carbene complexes (2M) via. a reverse 'Fritsch-Buttenberg-Wiechell' (FBW)-type rearrangement. A detailed computational quantum mechanical study has been carried out to understand the effect of different third row transition metal fragments (ML 2L'2; M = W, Re, Os, Ir, Pt; L/L' = PH3, Cl, CO) on the stability of metallabenzynes and its reactivity towards reverse FBW type rearrangement. Our results indicate that the late transition metal fragments Ir(PH3)Cl3 and PtCl4 prefer 16 VE metal-carbene complex (2M), while the middle transition metal fragments W(PH3)4, Re(PH3)3Cl and Os(PH3)2Cl2 prefer metallabenzyne (1M). This can be attributed to the reduced overlap between the transition metal fragment ML2L'2 and organic fragment C5H4 in metallabenzyne 1M when M changes from W to Pt. Furthermore, the presence of π-accepting ligand CO on the metal fragment makes the conversion of 1M to 2M more feasible.

Introduction The stabilization of metallacycles, whose organic cyclic analogues are elusive, has been a fascinating research area. The stability of the metallacycles as well as their reactivities depend mainly on the metal fragment present in them.1-15 In 2001, Jia and co-workers reported the isolation of the first stable 18 valence electron (VE) osmabenzyne by strategically replacing a carbyne carbon atom in 3-methyl-2,4 trimethylsilylbenzyne with an isolobal 14 VE fragment Os(PPh3)2Cl2.16, 17 Note that the corresponding organic analogue benzyne has not yet been isolated. The same group also isolated rhenabenzyne in 2011.18, 19 The Re(PMe2Ph)3Cl fragment in rhenabenzyne is also a 14 VE transition metal fragment. The metallabenzynes of other transition metals have not yet been reported. Thus, understanding the role of transition metals and its ligand environments suitable for the stabilization of metallabenzynes would help to facilitate the research in this area. The 18 VE osmabenzyne undergoes reverse FritschButtenberg-Wiechell-type (FBW) rearrangement to the corresponding 16 VE osmium carbene complex. 20-25 The osmium carbene complex is a rare osmium analogue of the Grubbs catalyst.26 We have explored the effect of group VIII transition metal fragments for the possible ring contraction of metallabenzynes to metal-carbene complexes by reverse FBW rearrangement.25 Our study revealed that the first-row transition metal, iron shows kinetic and thermodynamic preference for the metal-carbene complex and the preference for the carbene complex reduces down the group. However, such an migratory insertion reaction is not reported for rhenabenzynes, which indicates that the transition metal fragment has a deterministic role in the stability of metallabenzynes as well as their ring contraction to metalcarbene complexes. The focus of the current study is to understand the role of isolobal third row 14 VE transition metal fragments (ML2L'2; M = W, Re, Os, Ir, Pt; L/L' = PH3, Cl) for the stability of metallabenzynes, 1M and their reactivity towards the reverse FBW type rearrangement to metal-carbene complexes, 2M (Scheme 1). We have also studied the effect of -accepting ligand on the FBW type rearrangement by re-

placing the strong -donating PH3 ligand with the strong donating and -accepting CO ligand in metallabenzynes (1MCO, M = W, Re, Os, Ir) and metal-carbene complexes (2MCO).

Scheme 1: Schematic representation of metallabenzyne 1M and metal-carbene complex 2M.

Computational Methodology The geometries of all the molecules were optimized at the non-local DFT level of theory using the exchange functional of Becke in conjunction with the correlation functional of Perdew (BP86).27-29 The basis sets have double -quality augmented by one set of polarization functions (def2SVP).30,31 The calculations were carried out with the Gaussian 09 program package.32 Stationary points were characterized as minima by calculating the Hessian matrix analytically at this level.33,34 All transition states were confirmed by intrinsic reaction coordinate (IRC) calculations.35,36 The reaction energies are further verified using meta-GGA exchange-correlation functional M0637 with def2-TZVPP basis set.30,31 The electronic energies at the M06/def2-TZVPP level were corrected by adding the zero-point energies from the BP86/def2-SVP level of theory. The Gibbs free energy was calculated by adding electronic energy at the M06/def2-TZVPP to the thermal correction and free energy at the BP86/def2-SVP level of theory at 298.15 K and 1 atm. We have compared the relative energies of 1M, 2M, and 3M at the BP86, B3LYP27,38,39 and M06 level of theories using def2-SVP and def2-TZVPP basis sets (Table S1 in ESI). The conversion of 1Os to 2Os is experimentally reported in benzene solvent. The effect of benzene

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solvent on the relative energies was also calculated using polarized continuum model at the M06/def2-TZVPP level of theory.40 The dependence of exchange-correlation functionals, basis sets and solvent in the relative energies is marginal. The relative energies of 1Os, 2Os, and 3Os at the BP86 and B3LYP level of theory using def2-SVP and def2-TZVPP basis sets was further verified by incorporating the D3 version of Grimme’s dispersion with Becke-Johnson damping (GD3BJ).41 The contribution from the dispersion interaction to the relative energies is minimal (Table S2 in ESI). Hence, we have discussed the relative energies at the M06/def2TZVPP//BP86/def2-SVP level unless otherwise specified. The atomic partial charges by natural population analysis (NPA)42,43 and nuclear independent chemical shift (NICS)44-46 were also calculated at this level. Since the triplet geometries are either higher in energy or highly unstable, we have considered only singlet geometries.

Results and Discussion The optimized geometries of third row metallabenzynes (1M, M = W, Re, Os, Ir, Pt; L/L' = PH3, Cl), the ring contracted metal-carbene complexes (2M) and the transition states (3M) for the interconversion of 1M to 2M at the M06/def2-TZVPP//BP86/def2-SVP level of theory are given in Figure 1. The tetra-coordinated 14 VE metal fragments in 1M and 2M are W(PH3)4, Re(PH3)3Cl, Os(PH3)2Cl2, IrCl3(PH3) and PtCl4, respectively. The optimized geometrical parameters of 1Re and 1Os are close to the experimentally reported geometrical parameters of osmabenzyne16, 17 and rhenabenzyne.19 The calculated M–C1 (1.817 Å – 1.790 Å) and M–C5 (2.163 Å – 2.041 Å) bond lengths in 1M are within the range of metal-carbon triple and single bond lengths, respectively.16, 17,47-57 The six-membered C5H4M ring in 1M lies in the plane of L'-M-L'. The axial ligands in 1W and 1Pt are almost linear with L-M-L bond angle 176.8° and 175.8°, respectively. The axial ligands in 1Re, 1Os, and 1Ir are bent away from the six-membered ring (Figure 1). However, no distortion in the six-membered ring is observed due to the bending of the axial ligands. The metal-carbene complex 2W adopts a distorted trigonal bipyramidal (TBP) geometry, where the ML2L'2 fragment is square pyramidal. On the other hand, all other metalcarbene complexes 2M adopt trigonal bipyramidal geometry (Figure 1). The five-membered ring C5H4 lies in the pseudo equatorial L'-M-L' plane in 2W, whereas the five-membered ring C5H4 lies almost parallel to the axial L-M-L plane in other 2M (M = Re, Os, Ir, and Pt). The C5‒W (2.338 Å) and C5H‒W (2.257 Å) bond lengths are shorter than the nonbonding distances indicating a weak bonding interaction between C5-H -bond and W. This interaction is further supported by the longer C5‒H bond length (1.116 Å) as compared to the other C‒H bond lengths (1.099 Å) in 2W. The plane of the five-membered ring in 2Os and 2Ir is parallel to the axial L-M-L plane (Laxial–M–C1–C5 dihedral angle, θ is 1.5° and 0.0°, respectively), while that of 2Re and 2Pt lies in between the axial (L-M-L) and the equatorial (L'-M-L') planes of TBP geometry (L–M–C1–C5 dihedral angle, θ is 29.6° and 14.6°, respectively). The calculated M–C1 bond lengths in 2M ranges from 1.970 Å to 1.885 Å, which are within the experimentally reported M–C double bond lengths9,47-60 and are in between the M–C1 (1.871 Å - 1.790 Å) single and M– C5 triple bond lengths (2.163 Å - 2.041 Å) of 1M. The other

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isomers of 2M in which the five-membered ring is perpendicular to the axial ligands (2M', M' = Re, Os, Ir) are higher order saddle points (Figure 2). The rearrangement of 1M to 2M is a single step migratory ring contraction process via. the transition state 3M (Figure 1), which corresponds to the incipient formation of C1‒C5 bond and breaking of M‒C5 bond. The rearrangement of 1W to 2W involves ring contraction from a six-membered ring to a five-membered ring as well as the geometrical distortion of the metal fragment from the C2v point group to the C4v point group. On the other hand, the rearrangement of 1M (M = Re, Os, Ir, Pt) to 2M involves ring contraction from a sixmembered ring to a five-membered ring as well as the ring plane rotation. The C1–C5 lengths in the transition state 3M is in the range 1.713 Å - 1.944 Å, whereas the corresponding C1‒C5 lengths in 1M is in the range 2.502 Å - 2.558 Å. The C5H4 fragment is planar in transition state 3W, while it is slightly distorted in other transition states.

Scheme 2: Schematic representation of (a) the metal fragment (ML2L'2) orbitals in C2V and C4V point group symmetry, (b) the C5H4 fragment orbitals in metallabenzynes and (c) the C5H4 fragment orbitals in metal-carbene complexes.61-64 To understand the relative stability of metallabenzynes and metal-carbene complexes, we have analyzed the bonding in 1M and 2M. The d6-metal fragment ML2L'2 in C2v point group symmetry possesses three metal based orbitals (1a1, b1 and a2) and two hybrid orbitals, which are pointing away from the ligands (2a1 and b2, Scheme 2a).25,61-64 The bonding in metallabenzynes can be explained by considering two electrons in a2 symmetry orbitals and one electron each in all the remaining fragment molecular orbitals. The in-plane 2a1 and b2-orbitals of ML2L'2 have right symmetry to interact with the bonding and antibonding combinations of sp2-hybrid orbitals of the terminal carbon atoms in C5H4 fragment resulting M–C1 and M–C5 σ-bonds (Scheme 2a, b). The in-plane metal based orbital 1a1 can overlap with the in-plane p-orbital on C1 in C5H4 fragment to form the M– C1 in-plane π-bond. The interaction of the metal orbital b1 with the perpendicular p-orbital of C5H4 fragment results in aromatic delocalization. This is supported by the negative NICS


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Figure 1: Optimized geometries (BP86/def2-SVP) and important geometrical parameters of metallabenzynes 1M (M = W, Re, Os, Ir, Pt), metal-carbene complexes 2M and transition states 3M. Distances are given in angstroms (Å) and angles are given in degrees (°). Relative energies (E), Gibbs free energies (G, given in parenthesis) in kcal/mol at the M06/def2-TZVPP//BP86/def2-SVP level of theory and Laxial-M-C1-C5 dihedral angle () are also given.

values at the center and 1 Å above the center of the sixmembered ring (Table 1) of 1M. The electron donating groups such as good -donor PH3 as ligands on the transition metal increases the electron density on the metals which, in turn, favors the bonding interaction with the organic fragment. On the other hand, the electron withdrawing ligands such as weak -donor Cl decreases the electron density on the metal which, in turn, reduces the overlap of metal d-orbitals with the organic fragment. The reduction in the orbital overlap between the transition metal fragments and organic fragments is reflected in the Wiberg bond indices of M-C1 and M-C5 bonds, which reduces when metal fragment changes from W(PH3)4 to PtCl4. The reduction in Wiberg bond indices is quite significant for M-C1 bond, which reduces from 2.02 for 1W, 2.08 for 1Re, 1.78 for 1Os, 1.51 for 1Ir to 1.12 for 1Pt. On the other hand, the reduction in Wiberg bond indices for M-C5 bond is less prominent (1.0 for 1W, 0.92 for 1Re, 0.90 for 1Os, 0.80 for 1Ir and 0.66 for

1Pt). Note that the negative NBO partial charge (Table 1) on metal in 1M decreases from -1.39 e (1W) to 0.28 e (1Pt) as well as the positive charge on C1 atom increases from 0.14 e (1W) to 0.47 e (1Pt). The gradual reduction in the orbital overlap between the in-plane p-orbital of C1 and 1a1 type metal fragment orbital is reflected in the energy level of the inplane -MO (LUMO+1, Figure 3). The stability of LUMO+1 increases as the number of Cl ligands increases (0.92 eV in 1W to -4.84 eV in 1Pt), which in turn reduces the stability of the metallabenzyne (1W to 1Pt). Similarly, the overlap between the perpendicular p-orbital on C1 and b1 type orbital of the metal fragment also reduces from 1W to 1Pt. This is reflected in the reduction of the delocalization over the six-membered ring in metallabenzyne as indicated by the NICS values (Table 1). The d6-metal fragment ML2L'2 in 2W is square pyramidal with approximately C4v point group symmetry, whereas the metal fragment ML2L'2 in all other metal-carbene com



Figure 2: Optimized geometries (BP86/def2-SVP) and important geometrical parameters of metal-carbene complexes 2M' (M = Re, Os, and Ir). Distances are given in angstroms (Å) and angles are given in degrees (°). Relative energies (E), Gibbs free energies (G in parenthesis) with reference to 1M in kcal/mol M06/def2-TZVPP//BP86/def2-SVP level of theory, Laxial-M-C1-C5 dihedral angle () and the number of imaginary frequencies (NImg) are also given. plexes 2M has C2v point group symmetry. The square pyramidal (C4v) geometry is considered as the transition state for Berry pseudo rotation of trigonal bipyramidal geometry. 57,58 As the geometry changes from that of a C2v to a C4v point group, the energies of a2 and 1a1 orbitals do not vary much (Scheme 2a). The hybrid 2al is slightly destabilized due to the antibonding interaction. The b1 and b2 orbitals are converged to an e set. The bonding between the C4v metal fragment and carbene C5H4 can be explained as follows. The a2 type orbital and one of the orbitals of e set (b 2 type orbital in C2v point group symmetry) are doubly occupied, while the 2a1 and one of the orbitals of e set (b1 type orbital in C2v point group symmetry) are singly occupied. Hence, the in-plane 1a1 fragment orbital would be empty. The 2a1 type orbital of transition metal fragment has right symmetry to interact with the -type orbital of cyclopentadienyl carbene (C5H4) resulting M‒C1 bond, whereas singly occupied orbital of the e-set (b1-type in C2v point group symmetry) overlaps with the -type orbital of C5H4 resulting M‒C1 -bond (Scheme 2a, c). The highly diffused empty 1a1-type d-orbital on 2W can accept electrons from the C-H bond of C5H4 resulting in a -agostic complex.60 In all other metal-carbene complexes 2M, the trigonal bipyramidal geometry at the metal center is retained. 25 Thus, the bonding can be explained by considering two electrons in a2 and b1 type orbitals as well as one electron each in 2a1 and b2 type orbitals. Accordingly, the in-plane 1a1 type orbital would be vacant. The 2a1 type orbital can overlap with the -type orbital of cyclopentadienyl carbene (C5H4) resulting in the M‒C1 -bond, whereas the b2-type orbital overlaps with the -type orbital of C5H4 resulting the M‒C1 -bond (Scheme 2c). Hence, the organic fragment should undergo a ring plane rotation to achieve the proper overlap with the b2 symmetric πtype metal fragment orbital. The electron donating groups such as good -donor PH3 as ligands on the transition metal stabilizes the vacant 1a1 orbital, which in turn destabilizes metal-carbene complexes. On the other hand, the electron withdrawing groups such as weak -donor Cl as ligands on the transition metal decreases the electron density of the metal, which in turn stabilizes the metal-carbene complexes. Note

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that, the decrease of the negative NBO charge on transition metal from -1.33 e for 2W to 0.47 e for 2Pt can be attributed to the increasing numbers of electron withdrawing groups as the carbene complex changes from 2W to 2Pt. This in turn decreases the diffuseness of metal fragment orbitals and increases the energy gap between b1 and b2 type orbitals. The ML2L'2 fragment in 2W has higher diffused frontier orbitals and less energy gap between b1 and b2 type orbitals. The ML2L'2 fragment in 2Pt has less diffused frontier orbitals and the larger energy gap between b1 and b2 type orbitals. This, in turn, stabilizes low spin configuration and hence favors metalcarbene complexes. The relative energy difference between metallabenzynes (1M), metal-carbene complexes (2M) and the energy barrier for their interconversion are shown in Figure 1. The relative Gibbs free energies (G) are close to the relative electronic energies (E), and hence the results are discussed in terms of electronic energy (E). The metalcarbene complexes 2Ir and 2Pt are thermodynamically more stable (E = -16.7 and -34.2 kcal/mol, respectively) than the corresponding metallabenzynes 1Ir and 1Pt, whereas the metal-carbene complexes 2Re is thermodynamically less stable (E = 10.5 kcal/mol) than the corresponding metallabenzynes 1Re. On the other hand, the metal-carbene complexes 2W and 2Os have the comparable energy (E = 1.6 and 0.1 kcal/mol, respectively) as those of the corresponding metallabenzynes 1W and 1Os. The energy barrier for the conversion of 1W to 2W (12.2 kcal/mol) as well as for the conversion of 1Pt to 2Pt (12.7 kcal/mol) are low. However, the corresponding barrier for Re (24.8 kcal/mol) and Ir (20.7 kcal/mol) complexes are higher. The highest energy barrier is for osmium complex (34.2 kcal/mol). The earlier theoretical studies by Jia and coworkers suggested similar energy barriers (38.1 - 40.5 kcal/mol) for the conversion of substituted osmabenzynes to osmium carbene complexes.8

Figure 3: The in-plane*-MO (LUMO+1) of 1M and 1MCO (ML2'L2, M = W, Re, Os, Ir, Pt; L/L' = PH3, Cl, CO). Eigen values are given in eV. Thus, the relative stability of metallabenzynes and metal-carbene complexes as well as the energy barrier for their interconversion can be tuned by the transition metal fragment. Hence, the late transition metals (Pt, Ir) of the third row show the kinetic and thermodynamic preference for the carbene complexes, whereas the middle transition metals of the third row (W, Re, and Os) prefer metallabenzyne geometry. It is also noteworthy that osmabenzynes and rhenabenzynes are the only known metallabenzynes till now.16-19


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Table 1: Nuclear independent chemical shift (NICS) and charge distribution are given by the natural population analysis for 1M and 2M (M = W, Re, Os, Ir, Pt) at the M06/def2-TZVPP//BP86/def2-SVP level of theory. NICS(0)a NICS(1)a q(C1)b q(M)b q(C5)b q(ML2L'2)b (NICS(0)zz) (NICS(1)zz) -7.54 -7.14 0.14 -1.39 -0.26 0.25 1W (-14.55) (-17.53) -5.89 -5.76 0.22 -1.06 -0.21 0.03 1Re (-10.92) (-14.29) -6.62 -6.18 0.31 -0.49 -0.15 -0.16 1Os (-9.69) (-14.30) -6.33 -5.12 0.43 -0.13 -0.07 -0.48 1Ir (-4.84) (-10.71) -5.34 -3.96 0.47 0.28 0.00 -0.68 1Pt (1.33) (5.42) -7.40 -9.84 -0.13 -1.33 0.33 2W (1.48) (-24.87) 4.80 -1.23 -0.03 -0.84 0.20 2Re (21.82) (-1.56) 4.00 -0.99 0.02 -0.23 0.11 2Os (22.7) (0.84) 13.91 6.64 0.18 0.08 -0.21 2Ir (48.82) (24.37) 30.83 20.67 0.32 0.47 -0.50 2Pt (98.22) (67.31) a NICS(0) and NICS(1) represent NICS values at the center and 1 Å above of the ring. The dissected NICS(0) zz and NICS(1)zz are given in the parentheses. bq represents the charge of atom or group of atoms.

The interaction of M‒C5 -bonding electrons with the antibonding M‒C1 in-plane *-MO of 1M results in the migratory ring contraction reaction leading to the formation of 2M. Therefore, the proximity of the C1 and C5 atoms as well as the energy level of M‒C1 in-plane π*-MO becomes crucial in the ring contraction of 1M to 2M. The C1‒C5 distance decreases from 1W to 1Ir (2.558 Å - 2.502 Å). Moreover, the low lying in plane *‒MO of metallabenzyne of the late transition metal having electron withdrawing Cl‒ ligands favors the ring contraction reaction. This agrees with the reaction energetics, which indicates thermodynamic and kinetic stability for the metal-carbene complexes in late transition metals and a reverse trend in middle transition metals of the third row. We have also studied the effect of the -accepting ligand on the FBW rearrangement by replacing the strong donating PH3 ligand with the strong -donating and strong accepting CO ligand in metallabenzynes (1MCO, M = W, Re, Os, Ir) and metal-carbene complexes (2MCO). The optimized geometries of 1MCO, 2MCO, and the transition state 3MCO are given in Figure 4. The M‒C1 bonds (1.922 Å - 1.816 Å), M‒C5 bonds (2.267 Å - 2.067 Å) and C1‒C5 bonds (2.553 Å 2.529 Å) in 1MCO are comparatively longer than that in 1M. This is attributed to the -back donation from metal d-orbitals to the C‒O π*-MO (Scheme 3).63,65 In contrast to 2M, all 2MCO have trigonal bipyramidal geometry around the metal center. The five-membered ring undergoes a ring plane rotation with the ring contraction from 1MCO to 2MCO (Figure 4). The Laxial–M–C1–C5 dihedral angle varies from 41.9° in 2WCO to ° in 2IrCO. The M‒C1 bonds (1.994 Å - 1.898 Å) in 2MCO are longer than that in 2M (1.970 Å to 1.885 Å). The

conversion of 1MCO to 2MCO is a single step ring contraction reaction through the transition state 3MCO, which is a fused bicyclic compound similar to 3M. The reverse FBW type rearrangement becomes more exothermic when PH3 is replaced by CO (Figure 4). This could be attributed to the stabilizing effect of the π-acceptor CO ligand to the metal-carbene complex 2MCO. The corresponding energy barriers are also less as compared to 1M to 2M conversion. Hence, replacing the σ-donating PH3 ligand with the π-accepting CO ligand increases both the thermodynamic and kinetic preference for the metal-carbene complex. This is in accordance with our previous discussion that the energy barrier can be correlated with the tendency of breaking of the M-C5 bond by the donation of M‒C1 σ-bond to the M‒C5 π*-MO. The 1a1 and b1 type orbitals of ML2L'2 are responsible for -bonding with the CH4 fragment in 1MCO. These fragment orbitals have right symmetry to interact with CO *-MO as well. Since CO *-MO accepts electrons from 1a1 and b1 type orbitals of ML2L'2, the bonding interaction of metal fragment with the CH4 fragment in 1MCO is significantly reduced (Scheme 3). It is evident from the low NICS values (Table 2) and high positive NBO partial charge on C1 in 1MCO in comparison to 1M. This is consistent with the more stabilized in-plane M-C1 π*-MOs of 1MCO (LUMO+1, Figure 3) as compared to that of 1M, which is responsible for lower kinetic energy barrier for the formation of 2MCO from 1MCO. From the geometrical and MO analysis it is clear that apart from the C5‒C1 distance, size and coordination number of the metal atom, the isolobal fragments and the ligand environment will influence the kinetic and thermodynamic preference of the migratory ring contraction reactions in metallabenzynes.



Figure 4: Optimized geometries (BP86/def2-SVP) and important geometrical parameters of metallabenzynes 1MCO (ML2'L2, M = W, Re, Os, Ir; L/L' = Cl, CO), metal-carbene complexes 2MCO and transition states 3MCO. Distances are given in angstroms (Å) and angles are given in degrees (°). Relative energies (E), Gibbs free energy (G in parenthesis) in kcal/mol M06/def2-TZVPP//BP86/def2-SVP level of theory and Laxial-M-C1-C5 dihedral angle () are also given. We have also calculated the variations in the reaction energies for the interconversion of metallabenzyne (1OsNHC) to metalcarbene complex (2OsNHC) when PH3 group is replaced by strong σ-donating and weak π-accepting N-heterocyclic carbene (NHC). The reaction energetics are similar to that for the interconversion of 1Os to 2Os (Figures S1 and S2 in ESI), which indicates that the effect of NHC as compared to PH3 is only marginal.

Conclusions A computational quantum mechanical study was carried out to explore the effect of the 14 valence electron transition metal fragments (ML2L'2; M = W, Re, Os, Ir, Pt; L, L' = PH3, Cl,

Scheme 3: Schematic representation of (a) b1 (b) a2 and (c) 1a1 type metal d orbitals interacting with C-O * orbitals in a ML2L'2fragment.63,65


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Table 2: Nuclear independent chemical shift (NICS) and charge distribution are given by the natural population analysis for 1MCO and 2MCO (M = W, Re, Os, Ir, Pt) at the M06/def2-TZVPP//BP86/def2-SVP level of theory. NICS(0)a (NICS(0)zz) -6.80 (-11.72)

NICS(1)a (NICS(1)zz) -5.99 (-14.28)

q(C1)b

q(M)b

q(C5)b

q(ML4)b

0.32

-1.39

-0.17

-0.15

1ReCO

-3.84 (-4.83)

-4.24 (-8.71)

0.35

-1.06

0.16

-0.25

1OsCO

-5.39 (-5.84)

-5.11 (-11.19)

0.37

-0.54

-0.09

-0.36

1IrCO

-5.12 (-1.77)

-4.38 (-8.17)

0.45

-0.15

-0.03

-0.59

2WCO

7.33 (30.49)

-0.16 (6.05)

0.03

-0.82

0.07

2ReCO

10.95 (41.32)

3.30 (14.63)

0.09

-0.64

-0.06

2OsCO

6.09 (30.87)

0.72 (7.85)

0.09

-0.23

-0.04

2IrCO

17.19 (60.22)

9.50 (33.84)

0.23

0.11

-0.31

1WCO

a

NICS(0) and NICS(1) represent NICS values at the center and 1 Å above of the ring. The dissected NICS(0) zz and NICS(1)zz are given in the parentheses. bq represents the charge of atom or group of atoms.

CO) on the stability of metallabenzynes (1M) and their conversion (reverse FBW rearrangement) to corresponding metal carbene complexes (2M). The energy barrier for the interconversion of 1M to 2M varies with the metal fragments. While the late transition metal fragments (M = Ir and Pt) prefer 16 VE metal-carbene complex (2M), the middle transition metal fragments (M = W, Re, and Os) show the preference for the metallabenzyne (1M). The replacement of the strong donating PH3 ligand in 1M with the strong -donating and the strong -accepting ligand CO makes the conversion of 1MCO to 2MCO more exothermic, and the energy barrier for this conversion also becomes low. Hence, the exothermicity of the FBW/reverse FBW rearrangement and their energy barriers in metallabenzynes can be tuned by changing the metal fragment present in it.

ASSOCIATED CONTENT Electronic supplementary information (ESI) available: Optimized Cartesian coordinates of all the calculated molecules at the BP86/def2-SVP level of theory and relative energies of IM, 2M, and 3M at different levels of theory.

AUTHOR INFORMATION Corresponding Authors 1

E-mail: [email protected] Tel: +91-495-228-5304

2

E-mail: [email protected], [email protected] Tel: +91 484 2862420 ORCID Pattiyil Parameswaran - 0000-0003-2065-2463

ACKNOWLEDGMENT CA thanks CSIR for the research fellowship and SD and PP thanks the financial support received from the Department of Science and Technology, India.

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Susmita De - 0000-0002-9326-3373



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