Stabilization of Boron–Boron Triple Bonds by Mesoionic Carbenes

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Article Cite This: ACS Omega 2018, 3, 13720−13730

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Stabilization of Boron−Boron Triple Bonds by Mesoionic Carbenes Ranajit Saha,† Sudip Pan,*,†,‡ and Pratim K. Chattaraj*,†,§ †

Department of Chemistry and Centre for Theoretical Studies, Indian Institute of Technology Kharagpur, Kharagpur 721302, India Institute of Advanced Synthesis, School of Chemistry and Molecular Engineering, Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing Tech University, Nanjing 210028, China § Department of Chemistry, Indian Institute of Technology Bombay, Mumbai 400076, India ‡

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S Supporting Information *

ABSTRACT: Density functional theory-based computations are carried out to analyze the electronic structure and stability of B2(MIC)2 complexes, where MIC is a mesoionic carbene, viz., imidazolin-4-ylidenes, pyrazolin-4-ylidene, 1,2,3-triazol-5ylidene, tetrazol-5-ylidene, and isoxazol-4-ylidene. The structure, stability, and the nature of bonding of these complexes are further compared to those of the previously reported B2(NHC)2 and B2(cAAC)2. A thorough bonding analysis via natural bond order, molecular orbital, and energy decomposition analyses (EDA) in combination with natural orbital for chemical valence (NOCV) reveals that MICs are suitable ligands to stabilize B2 species in its (3)1∑g+ excited state, resulting in an effective B−B bond order of 3. Their high dissociation energy and endergonicity at 298 K for the dissociations L−BB−L → 2 B−L and L−BB−L → BB + 2 L (L = Ligand) indicate their viability at ambient condition. The donor property of MICs is comparable to that of NHCMe. The orbital interaction plays a greater role than the coulombic interaction in forming the B−L bonds. The EDA-NOCV results show that the sum of the orbital energies associated with the (+, +) and (+, −) L→[B2]←L σ-donations is far larger than that of L←[B2]→L π-back donation. It also reveals that cAACMe possesses the largest σ-donation and π-back donation abilities among the studied ligands, and the σdonation and π-back donation abilities of MICs are comparable to those of NHCMe. Therefore, the present study shows that MICs would also be an excellent choice as ligands to experimentally realize new compounds having a strong B−B triple bond.

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INTRODUCTION In the first row of the periodic table, carbon and nitrogen are the only two candidates which were thought to form homoatomic triple bonds (e.g., RCCR, NN) until 2002 when Zhou et al.1 detected OC−BB−CO possessing a very short B−B bond with a bond distance of 1.453 Å and a high B−B bond dissociation energy (143.5 kcal/mol) via low-temperature matrix isolation technique. The B−B bond in OCBBCO is found to be much stronger and shorter than the B−B double bond in H−BB−H (1.507 Å and 113.0 kcal/mol, respectively).1,2 These observations led the authors to conclude about the presence of some degree of triple-bond character in OCBBCO. Subsequently, some theoretical works showed that there is remarkable B−B triple bond character in L→BB←L type of complexes, where, L = monoatomic noble gases (Ar, Kr), diatomic molecules (CO, CS, N2, BO−), and phosphine derivatives (PCl3, PMe3).3−6 In 2011, Frenking and co-workers reported in silico that the B2 molecule can be stabilized in a linear fashion in NHC→B2←NHC complex (NHC = N-heterocyclic carbene).7 In this complex, the B2 fragment is in third excited (3) 1 ∑ g+ state having valence electronic configuration (2σg)2(1πu)4, making the formal bond order of 3 (see Scheme 1). Next year, Braunschweig synthesized bottolable NHCDipp→ B2←NHCDipp (Dipp = 2,6-diisopropyl-phenyl) complex.8 The B−B bond length is found to be 1.449(3) Å by X-ray © 2018 American Chemical Society

crystallography, which agrees well with the B−B triple bond length 1.46 Å as predicted by Pyykkö.9 In 2014, cAACDipp→ B2←cAACDipp complex (cAAC = cyclic alkyl amino carbenes) was synthesized with a slightly larger BB bond distance of 1.489 Å.10 The saturated NHCDipp-stabilized diboryne was also reported with a BB bond with a length of 1.465 Å.11 A previous study demonstrated that inclusion of metallocene, Zr(η5-Cp)2, into a borocycle is an excellent way to stabilize boron−boron triple bond in a cyclic environment.12 The Zr(η5Cp)2 fragment donates one pair of electrons to the in-plane πbonding orbital of BB triple bond. The stabilization originated from such interaction compensates the angle strain from cyclization.12 Thus, the bonding situation in this Zr(η5-Cp)2stabilized cyclic diboryne is different from that in the B2(L)2 complexes. The multidentate carbenes are also found to stabilize the diborynes and they can be used to form stable polymers.13,14 The above-discussed carbenes are known as “normal” carbenes, which were also reported to stabilize different transient species in a complex of the form L→En←L, where, En = C1,15 Si1,16,17 Ge1,18,19 B2,8,10 C2,20,21 Si2,22 Ge2,23 P2,24 As2,25 P4,26 P12,27 etc. Received: September 7, 2018 Accepted: October 9, 2018 Published: October 19, 2018 13720

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Scheme 1. Orbital Correlation Diagrams for (a) the X3∑−g Ground State, (b) the (3)1∑+g Excited State, and (c) Shape of the Molecular Orbitals (MO) of B2

Scheme 2. Schematic Representation of the Carbenes Considered for the Study

There is another type of carbene known as “abnormal” or “nonclassical” carbene.28−30 They are also called mesoionic carbenes (MICs) as no canonical resonance form can be drawn for these carbenes without the introduction of formal charges on the carbene carbon (Ccarb) center. In MICs, one or both the heteroatoms adjacent to the Ccarb may be removed. This removal of heteroatoms increases the donor ability of abnormal carbenes compared to the normal one, but at the same time, it reduces the stability of the carbene.31,32 There was always a question raised about the true carbenic properties of MICs. As the concept of carbenes starts with the divalency of Ccarb, it is also supposed to be valid for both of the normal and abnormal carbenes. But the Ccarb of MICs is tetravalent. Another flaw is that MICs do not dimerize following the Wanzlick equilibrium as found in the normal carbenes.29,31,32 Moreover, it is believed that a true carbene must have six valence electrons at Ccarb-center. But, only CH2 is found to follow this “sextet” rule at Ccarb-center. The normal carbenes are also represented with a Lewis structure

having six valence electrons at the Ccarb-center. But it receives π electron density from the adjacent heteroatoms and, thus, the Ccarb-center can be represented as 8e resonance structure. On the other hand, there is no possibility for MICs to attain a 6e structure because all probable resonance forms correspond to an 8e count at the Ccarb-center. Despite such differences in the free state, both normal carbenes and MICs show similar kind of electronic structure, bonding pattern, and chemical reactivity upon binding with metals.29,31 As soon as carbenes bind with metal, it loses its divalency and 6e count at the Ccarb center and octet character at this center is grown. Thus, the distinction between them is lost upon binding. The whole carbene family has proven its versatile behavior in metalcatalysis,33−36 organocatalysis,37−39 main group synthesis,15−27 activation of small molecules,40,41 and many more processes. In this paper, we have studied the possibility of attaining BB triple bond supported by MICs via density functional theory (DFT)-based computations, and their stability and bonding 13721

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Figure 1. Optimized geometries of the carbenes calculated at the B3LYP/6-311G(d,p) level. Bond lengths are in angstrom unit, and angles are in degree unit.

center and adjacent C atom. The Ccarb−C bond length in cAACMe is 1.525 Å, which is the largest and almost equal to a C− C single bond distance. But in the MICs, Ccarb−C bond lengths fall in the range of 1.340−1.416 Å, attaining some double-bond character due to conjugation. Therefore, the sharp differences in geometrical parameters between cAACMe and MIC1 indicate that the replacement of a single carbon atom of the ring by a nitrogen atom causes some significant structural alteration. The singlet−triplet energy gap (ΔES−T), which is used to describe the stability of carbene, is computed and presented in Table 1. A carbene with higher ΔES−T value would have larger

situation are compared to those reported for NHC- and cAACsupported BB triple bond compounds. We have considered five MICs, viz., imidazolin-4-ylidenes (MIC1)-, pyrazolin-4ylidene (MIC2)-, 1,2,3-triazol-5-ylidene (MIC3)-, tetrazol-5ylidene (MIC4)-, and isoxazol-4-ylidene (MIC5)-based carbenes (see Scheme 2).42−46 The electronic structure calculations and the bonding analysis via energy decomposition analysis (EDA) in conjunction with natural orbital for chemical valence (NOCV) suggest that MICs can stabilize the diboryne and the triple-bond character between the two B atoms is almost the same as in the NHCMe- and cAACMe-stabilized B2(L)2 systems.

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Table 1. Adiabatic Singlet−Triplet Energy Gap (ΔES−T, kcal/ mol), HOMO−LUMO Energy Gap (ΔEH−L, eV), and NPA Charge at the Carbenic Center (q(Ccarb), |e|) in Carbenes at the B3LYP/6-311G(d,p) Level

RESULTS AND DISCUSSION Structures and Stability. First of all, we validate our chosen level of theory by comparing the computed geometrical parameters to those of the previously synthesized B2(NHCDipp)2 and B2(cAACDipp)2 (Dipp = 2,6-diisopropylphenyl) complexes (see Figure S1, Supporting Information). Our computed results show good agreement with the experimentally determined parameters, and the differences are within the range of solidstate effects. We then replace the bulky −Dipp group with −Me group for computational economy and maintain similar side group in all of the considered ligands. The optimized geometries of all of the seven carbenes under study are presented in Figure 1 along with their structural parameters. The ∠X−Ccarb−X (X = C, N) angle varies between 99.4 and 106.5°. The maximum ∠X−Ccarb−X angle is 106.5° in cAACMe. The ∠X−Ccarb−X angle in all MICs is slightly lower than or comparable to the ∠N−Ccarb−N angle in NHCMe. The Ccarb−N bond lengths vary in the range of 1.309 Å (cAACMe) to 1.407 Å (MIC1), which is between those of C−N single (1.421 Å) and CN double-bond lengths (1.293 Å),26 suggesting some degree of double-bond character due to the conjugation. A similar kind of situation can be noted in the bond between Ccarb-

NHCMe cAACMe MIC1 MIC2 MIC3 MIC4 MIC5

ΔES−T

ΔEH−L

q(Ccarb)

−85.5 −48.9 −58.0 −37.8 −59.0 −56.0 −40.1

6.40 5.29 4.85 3.74 4.66 4.56 3.79

0.01 0.08 −0.19 −0.43 −0.17 0.05 −0.45

stability in its singlet state. The adibatic ΔES−T values for NHCMe and cAACMe are −85.5 and −48.9 kcal/mol, respectively, where a negative sign indicates higher stability of singlet state than the triplet one. The absence of one nitrogen center in cAACMe (one adjacent nitrogen center) diminishes the ΔES−T value by 36.6 kcal/mol compared to that in NHCMe (two adjacent nitrogen centers). We note that the ΔES−T value of a ligand plays a very crucial role in the electronic structure of the ligand-stabilized species. Recently reported (E)(L)BB(L)(E) (L 13722

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Å, which is even smaller than that in B2(NHCMe)2. We note that direct correlation between bond length and the strength of the donor−acceptor interaction cannot be drawn as observed previously. Upon complexation, the bond lengths between the Ccarb-center and the adjacent atom (hetero or homo) and the ∠X−Ccarb−X angles in carbenes increase compared to those in the free carbenes in all cases. It is shown that the B−B bond stretching frequencies (∼1700 cm−1) in B2(NHCDipp)2, B2(NHCDep)2, and B2(cAACDipp)2 fit well with the trend known for triple bonds in dinitrogen (N N) and alkynes (CC).50 The corresponding frequencies of the B−B stretching mode (ν(BB)) in B2(L)2 complexes are presented in Table 2. Here, we have only considered 11B/11B pair of B-atoms for our study. The simulated ν(BB) is 1687 cm−1 for B2(NHCDipp)2 and 1645 cm−1 for B2(cAACDipp)2, which are in good agreement with the experimental frequency values.50 The computed values for the −Me variants are slightly larger than the former ones (1724 and 1667 cm−1 for B2(NHCMe)2 and B2(cAACMe)2 complexes, respectively). For MICs-supported complexes, the ν(BB) values range from 1724 (MIC4) to 1680 (MIC2) cm−1. Therefore, all of them have larger ν(BB) values than those in cAACMe. In fact, MIC4 equalizes the value with NHCMe. Therefore, very comparable ν(BB) values to those in experimentally reported and theoretically well-established systems indicate that all of these B−B bonds can be considered as triple bonds. Next, we have checked the bonding indices. The WBIs for the B−B and B−Ccarb bonds and natural charges on the B and C atoms in these complexes are provided in Table 2. The WBI(BB) values are computed to be 2.06 and 1.71 for the B−B bonds in B2(NHCMe)2 and B2(cAACMe)2 complexes, respectively. We note that although the WBI(BB) values are much lower than 3 and often much lowered value than the actual bond order is noted for delocalized systems, we may use these values as a parameter to compare with other B2(MICs)2. For these systems, the WBI(BB) values vary from 1.98 (MIC4) to 2.16 (MIC2), indicating almost equal bond order as that in NHCMe. The natural charges on the B2 in B2(L)2 complexes are negative except for the B2(cAACMe)2 complex. The B−L bonds in the B2(L)2 complexes are described as donor−acceptor interaction (vide infra). The total natural charges on the B2 and carbenes indicate about the relative size of electron donation and back donation. The net negative charge on the B2 moiety indicates the acceptance of more electron density through the L→[B2]←L σdonation than the L←[B2]→L π-back donation. We have analyzed two dissociation processes that include B− B and B−Ccarb bond dissociations in the complexes: (i) L−BB− L (1A)→2 B−L (4A) and (ii) L−BB−L (1A)→BB (X3∑g+) + 2 L (1A). The first dissociation channel determines the strength of the BB bonds, and the second one determines the B−Ccarb bond strength in B2(L)2 complexes (see Table 3). The zeropoint energy (ZPE)-corrected dissociation energies (D0) and free-energy changes (ΔG) at 298 K representing BB bond (first dissociation channel) and B−Ccarb bond (second dissociation channel) are computed to have very large values, reflecting very strong bond formation. The high endergonicity of such dissociations is also confirmed from the corresponding ΔG values. In all cases, the sum of two B−L bonds is somewhat stronger than the BB bond. Irrespective of B−Ccarb or BB, the B2(cAACMe)2 complex has the highest D0 value among others. The rBB is maximum for the B2(cAACMe)2 complex; thus, a low D0 value for the BB bond dissociation is expected. But one should remember that the correlation between the bond

= NHC, cAAC; E = SPh) is a classic example of that where a triplet ground state with a twisted SBBS unit in cAAC-stabilized complex and a singlet ground state with a planar central SBBS in NHC bound analogue can be properly explained from their ΔES−T values.47 On the other hand, the ΔES−T values for MIC1, MIC3, and MIC4 are larger than those in cAACMe, but in the cases of MIC2 and MIC5 because of the absence of any Ccarb−N conjugation, the ΔES−T values eventually become lower than those in the latter one. Therefore, Ccarb−N interaction has an important role to gain large ΔES−T. Further, the computed energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), ΔEH−L, which is also a stability and reactivity indicator, is well correlated with the ΔES−T, except for cAACMe. Similar to ΔES−T, the ΔEH−L value is maximum for NHCMe (6.40 eV) and minimum for MIC2 (3.74 eV). All MICs have somewhat lower ΔEH−L value than cAACMe, indicating that MICs would show higher reactivity than the latter one. The partial natural charges at the Ccarb-center are also listed in Table 2, which shows that the Ccarb-center of NHCMe and cAACMe are Table 2. B−B Stretching Frequency (ν(BB), cm−1), Wiberg Bond Indices for B−B (WBI(BB)) and B−Ccarb (WBI(BCcarb)) Bonds, and Charges on the B (q(B), |e|) and Ccarb (q(Ccarb), |e|) Atoms in B2(L)2 Complexes Calculated at the B3LYP/6-311G(d,p) Level B2(NHCMe)2 B2(cAACMe)2 B2(MIC1)2 B2(MIC2)2 B2(MIC3)2 B2(MIC4)2 B2(MIC5)2

ν(BB)

WBI(BB)

WBI(BCcarb)

q(B2)

q(Ccarb)

1724 1667 1707 1680 1711 1724 1687

2.06 1.71 2.15 2.16 2.08 1.98 2.12

1.17 1.33 1.10 1.00 1.12 1.19 0.98

−0.34 0.10 −0.42 −0.38 −0.32 −0.22 −0.32

0.12 −0.11 −0.15 −0.42 −0.16 0.03 −0.45

slightly electropositive in nature, whereas among MICs, except for MIC4, all carry significant negative charges on the Ccarbcenter. We note that among MICs, only MIC4 has two N-atoms at the adjacent position of the Ccarb, which are responsible for the positive charge on Ccarb. The minimum energy structures of all of the studied B2(L)2 structures are presented in Figure 2. While B2(NHCMe)2 corresponds to a D2 symmetry and B2(cAAC Me )2 and B2(MIC4)2 belong to C1 point group, other complexes have C2 symmetry in their singlet spin state. The B−B bond length in free X3∑g− B2 molecules is computed to be 1.616 Å at our computational level (experimental B−B bond length is 1.590 Å in B2 molecule).48,49 The B−B bond lengths are 1.455 and 1.487 Å in B2(NHCMe)2 and B2(cAACMe)2 complexes, respectively, which agree well with the experimental B−B bond lengths in B2(NHCDipp)2 (1.449 Å) and B2(cAACDipp)2 (1.489 Å) complexes. On the other hand, the B−B bond distance in B2(MICs)2 complexes ranges between 1.453 (MIC1) and 1.457 (MIC4) Å. Hence, it is clear that the B−B bond distances in B2(MICs)2 are similar to those in B2(NHCMe)2 and somewhat smaller than those in B2(cAACMe)2. This distance is also in good agreement with the predicted BB bond length of 1.46 Å.9 The B−Ccarb bond lengths in B2(NHCMe)2 and B2(cAACMe)2 are 1.482 and 1.454 Å, respectively. All of the B−Ccarb bond lengths in B2(MICs)2 complexes are larger than those in B2(cAACMe)2. Among MICs, the smallest B−Ccarb bond length is noted in B2(MIC4)2 as 1.476 13723

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Figure 2. Optimized geometries of the carbenes-stabilized diboryne B2(L)2 (L = NHCMe, cAACMe, MIC1, MIC2, MIC3, MIC4, and MIC5) complexes calculated at the B3LYP/6-311G(d,p) level. Bond lengths are in angstrom unit, and angles are in degree unit.

Figure 3. In the case of B2(NHCMe)2 complex, the HOMO and HOMO−1 correspond to the B−B π-bonding, and HOMO−6 and HOMO−14 correspond to the B−B σ-bonding. HOMO− 13 represents the B−B antibonding in the complex, thus giving a triple bond character in this B2(NHCMe)2 complex. HOMO−13 and HOMO−14 represent the electron σ-donation from NHC to the vacant 2σu* and 3σg orbitals of B2 in the third excited 1∑g+ state.54 A similar set of MOs in the B2(MIC)2 complexes is also presented. In all cases, HOMO and HOMO−1 represent the two π-MOs in the complexes. These MOs have p(π)-orbital coefficients from the central moiety as well as the donor atom in the MICs. The next three MOs depict the σ-bonding in the complexes as can be found in the NHCMe- and cAACMestabilized complexes. Energy Decomposition Analysis. Further important information about the bonding situation can be obtained from the EDA-NOCV results in which the orbital interaction is presented as a sum of pairwise contributions from specific pairs of orbitals detailing about their strength numerically. Further, this analysis also allows us to graphically visualize the impact of such pairwise orbital interaction on the electronic structure. Table 4 provides the numerical results of EDA-NOCV computations of B2(L)2 complexes taking (3)1∑g+ B2 species as one fragment and (L)2 as another.55 Because of the similar MOs reflecting similar electronic structure, for B2(MIC)2, this scheme is the most logical one. Similar to the D0 values, the intrinsic interaction energy (ΔEint) for the B−L bonds in the B2(cAACMe)2 complex is the largest (−351.8 kcal/mol),

Table 3. PE-Corrected Dissociation Energies (D0, kcal/mol) and Free-Energy Changes (ΔG, kcal/mol) at 298 K for the Dissociation Processes Calculated at the B3LYP/6311G(d,p) Level (i) L−BB−L → 2 B−L

(ii) L−BB−L → BB + 2 L

complexes

D0

ΔG

D0

ΔG

B2(NHCMe)2 B2(cAACMe)2 B2(MIC1)2 B2(MIC2)2 B2(MIC3)2 B2(MIC4)2 B2(MIC5)2

167.6 178.5 163.3 158.0 164.2 168.0 158.4

153.6 165.7 149.8 144.5 149.0 153.7 143.3

180.5 213.3 183.5 179.6 182.7 177.2 175.5

160.0 191.3 161.1 157.6 160.8 155.1 149.7

length and bond strength does not hold good in all cases.51−53 The D0 value for the BB bond in B2(NHCMe)2 is computed to be 167.6 kcal/mol, whereas the same for MICs-supported complexes ranges from 155.2 (MIC5) to 164.2 (MIC3) kcal/ mol, reflecting comparable BB bond strength to that with the former one. Moreover, the B−Ccarb bond strength in B2(MICs)2 is also very similar to that in B2(NHCMe)2, which indicates the similarity in the donor−acceptor interactions in these systems. Molecular Orbitals. Further, the similarity in the electronic structure and bonding patterns between the MICs and NHCMeand cAACMe-stabilized B2(L)2 complexes can easily be understood from the associated molecular orbital (MO) pictures. The most informative MOs for these complexes are depicted in 13724

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Figure 3. Relevant frontier molecular orbitals and eigenvalues (in electronvolt) of the B2(L)2 (L = NHCMe, cAACMe, MIC1, MIC2, MIC3, MIC4, and MIC5) complexes generated at the B3LYP/6-311G(d,p) level.

whereas for the remaining complexes, the ΔEint values range from −302.3 (MIC4) to −309.5 (MIC2) kcal/mol and are comparable to those in the NHCMe-supported complex. These

high ΔEint values suggest strong interaction between the excited B2 fragment and donor centers and it can easily compensate the electronic excitation energy of B2. For all complexes, the ΔEorb 13725

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Table 4. EDA-NOCV Results for L−BB−L Complexes Taking (L)2 in Singlet Ground State and B2 in (3)1∑g+ Excited State Calculated at the B3LYP-D3(BJ)/TZ2P//B3LYP/6-311G(d,p) Levela complex

B2(NHCMe)2

B2(cAACMe)2

B2(MIC1)2

B2(MIC2)2

B2(MIC3)2

B2(MIC4)2

B2(MIC5)2

ΔEint ΔEPauli ΔEdispb ΔEelstatb ΔEorbb ΔEσ1c L → [B2] ← L (+,+) σ-donation ΔEσ2c L → [B2] ← L (+,−) σ-donation ΔEπ1c L ← [B2] → L π-back donation ΔEπ2c L ← [B2] → L π′-back donation ΔEorb(rest)c

−307.6 263.7 −8.9 (1.6) −258.9 (45.3) −303.5 (53.1) −111.0 (36.6) −85.0 (28.0) −42.1 (13.9) −44.5 (14.7) −20.9 (6.9)

−351.8 298.8 −10.8 (1.7) −282.2 (43.4) −357.6 (55.0) −120.2 (33.6) −89.7 (25.1) −64.0 (17.9) −63.8 (17.8) −19.9 (5.6)

−309.4 265.1 −8.7 (1.5) −265.8 (46.3) −300.0 (52.2) −112.5 (37.5) −84.5 (28.2) −40.9 (13.6) −37.3 (12.4) −24.8 (8.3)

−309.5 259.1 −8.7 (1.5) −261.3 (45.9) −298.7 (52.5) −98.3 (32.9) −90.0 (30.1) −50.3 (16.8) −30.9 (10.3) −29.2 (9.8)

−307.8 261.0 −8.4 (1.5) −258.3 (45.4) −302.1 (53.1) −112.2 (37.1) −85.9 (28.4) −41.7 (13.8) −38.9 (12.9) −23.4 (7.7)

−302.3 261.5 −7.1 (1.3) −248.6 (44.1) −308.1 (54.6) −110.1 (35.7) −83.4 (27.1) −48.0 (15.6) −47.1 (15.3) −19.5 (6.3)

−304.4 257.3 −8.4 (1.5) −256.5 (45.7) −296.9 (52.8) −114.5 (38.6) −79.1 (26.6) −35.1 (11.8) −36.9 (12.4) −31.3 (10.5)

a Energy values are in kcal/mol. bThe values in parentheses give the percentage contribution to the total attractive interactions (ΔEelstat + ΔEorb + ΔEdisp). cThe values in parentheses give the percentage contribution to the total orbital interactions ΔEorb.

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CONCLUSIONS The present quantum chemical DFT calculations reveal that mesoionic carbenes (MICs), viz., imidazolin-4-ylidenes (MIC1), pyrazolin-4-ylidene (MIC2), 1,2,3-triazol-5-ylidene (MIC3), tetrazol-5-ylidene (MIC4), and isoxazol-4-ylidene (MIC5) are suitable candidates to stabilize B2 species in its (3)1∑g+ excited state resulting in an effective B−B bond order of 3. The B−B bond distance, Wiberg bond order, and molecular orbitals are very comparable to those of experimentally reported B2(NHCMe)2 and B2(cAACMe)2. Further, the inspection of the stability of these B2(L)2 complexes against L−BB−L → 2 B−L and L−BB−L → BB + 2 L indicates high stability of MICssupported complexes, which is also very comparable to that in B2(NHCMe)2. Finally, EDA-NOCV results confirm the similar bonding situation in B2(MICs)2 complexes and B2(NHCMe)2 and B2(cAACMe)2, where 3σg and 2σu* orbitals of (3)1∑g+ B2 moiety accept (+, +) and (+, −) σ-donations from (L)2, respectively, with the former interaction stronger than the latter one. L←[B2]→L π-back donation from the two π orbitals on B2 to L also plays a role in the B−L interaction, but its contribution is significantly smaller than the L→[B2]←L σ-donation. Among the considered ligands, cAACMe has the largest σ-donation and π-back donation ability, whereas the σ-donation and π-back donation abilities of MICs are comparable to those in NHCMe. In fact, the π-back donation ability of MICs is somewhat lower than that in NHCMe, except for MIC4. Therefore, the present results indicate that apart from NHC and cAAC, MICs are also very suitable ligands to produce compounds containing B−B triple bond, at ambient conditions. For this aspect, so far, these ligands have not been considered and hence they need proper attention toward experimental realization of new B2(L)2 complexes with BB bond. In future, we would investigate the difference in reactivity between B2(MICs)2 complexes and that of B2(NHCMe)2 and B2(cAACMe)2.

term plays the major role (contribution: ca. 52.2−55.0%) in the stabilization of the B−L bonds, whereas the coulombic attraction, ΔEelstat, is responsible for the 43.4−46.3% of the total attraction. The dispersion energy is least important here (ca. 1.3−1.7%). We note that both the enhanced orbital and coulomb interactions are responsible for the larger ΔEint value in the B2(cAACMe)2 complex than that in the others, although larger Pauli repulsion in the former nullifies some part of such attractive gain. The magnitudes of ΔEorb and ΔEelstat terms in MICs are quite comparable to those in NHCMe. The NOCV calculations further decompose the ΔEorb into pairwise contributions between occupied and vacant MOs of the interacting fragment terms, presenting us with the quantitative size of L→[B2]←L σ-donations and L←[B2]→L π-back donations. The associated electron density flow is visualized through the plots of four main deformation densities, Δρ1−Δρ4, as shown in Figure 4. Here, the charge is shifted from the red to blue region. Δρ(σ1) represents the in-phase (+, +) σ-donation from the two carbene ligands toward the vacant 3σg orbital (LUMO + 1) on (3)1∑g+ B2 moiety, whereas Δρ(σ2) represents the out-of-phase (+, −) σ-donation from the two carbene ligands toward the vacant 2σu* orbital (LUMO) of the B2 fragments. Because of the better overlap in the (+, +) combination than in the (+, −) combination, the associated ΔEσ1 is larger than ΔEσ2. The next two deformation channels, Δρ(π1) and Δρ(π2), are originated from the two L←[B2]→L π-back donations from the two π orbitals on B2 to L. Obviously, the contribution from the sum of two σ-donations (ca. 58.7−65.7% of ΔEorb) is significantly larger than that from the sum of two π-back donations (ca. 24.2−35.7% of ΔEorb). Since (+, +) and (+, −) σdonations populate bonding and antibonding σ-orbitals of B2, respectively, their impact on the bond order would almost nullify each other. On the other hand, L←[B2]→L π-back donations would weaken the BB bond. This would be the reason to get lower WBI value than the ideal 3. An inspection of each orbital term indicates that cAACMe has the largest σ-donation and πback donation ability. On the other hand, while the σ-donation ability of MICs is almost the same as in NHCMe, except for MIC2, which is only slightly smaller (by 7.7 kcal/mol), the πback donation ability of MICs is somewhat smaller (by 5.4−14.6 kcal/mol) than that in NHCMe, except for MIC4. This explains why we get slightly larger WBI(BB) values for MICs than NHCMe, except for MIC4. Therefore, all of these analyses clearly indicate that MICs could indeed stabilize B2 species in its (3)1∑g+ excited state and the stability is very comparable to that of NHCMe.

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COMPUTATIONAL DETAILS Geometries of all of the studied compounds were optimized without any constraints with B3LYP56−58 functional in conjunction with 6-311G(d,p)59 basis set. The resulting structures are then reoptimized at the same level by imposing the closest point group. The frequency calculations were also performed at the same level taking the optimized geometries of the complexes. All real frequencies confirm that the structures are at the minima on their respective potential energy surfaces. The partial charge on each atomic center and Wiberg bond indices (WBI) were computed using the natural bond orbital 13726

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Figure 4. Plots of deformation densities, Δρ(r), of the pairwise orbital interactions in of B2(L)2 complexes considering (3)1∑g+ B2 and (L)2 as the fragments generated at the B3LYP-D3(BJ)/TZ2P//B3LYP/6-311G(d,p) level. The associated orbital interaction energies are given in kcal/mol. The color code of the charge flow is red → blue.

analysis scheme.60−63 All of the above computations were performed with the help of the GAUSSIAN 09 program.64

The energy decomposition analysis in conjunction with natural orbitals of chemical valence (EDA-NOCV) computa13727

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ACS Omega tions65,66 was carried out at the B3LYP-D3(BJ)/TZ2P// B3LYP/6-311G(d,p) level using the ADF2017.01 program package.67,68 In the EDA scheme, the interaction energy (ΔEint) between two fragments under consideration is decomposed into four energy terms as ΔEint = ΔE Pauli + ΔEelstat + ΔEorb + ΔEdisp

(1)

(2)

N /2

∑ ΔEkorb = k

∑ νk[−F −TSk + FkTS] k=1

(3)

TS where −FTS −k and Fk are diagonal Kohn−Sham matrix elements corresponding to NOCVs with the eigenvalues −ν k and ν k, respectively. The ΔEorb k terms are assigned to a particular type of bond by visual inspection of the shape of the deformation density, Δρk. The EDA-NOCV scheme thus provides both qualitative (Δρorb) and quantitative (ΔEorb) information about the strength of orbital interactions in chemical bonds.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b02305.

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REFERENCES

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In EDA-NOCV, ΔEorb is given by the following equation ΔEorb =

ACKNOWLEDGMENTS

P.K.C. acknowledges DST, New Delhi, for the J.C. Bose National Fellowship, grant number SR/S2/JCB-09/2009. S.P. acknowledges Nanjing Tech University for the postdoctoral fellowship and the High Performance Computing Center of Nanjing Tech University for supporting the computational resources. R.S. acknowledges UGC, New Delhi, for his Senior Research Fellowship. The departmental computational facility “Kangsavati” at the Department of Chemistry, IIT Kharagpur, is also acknowledged.

The Pauli repulsion energy, ΔEPauli, accounts for the repulsion between the electrons of the same spin of the occupied orbitals, and it is calculated by employing Kohn−Sham determinant on the superimposed fragments to obey the Pauli principle by antisymmetrization and renormalization. The classical electrostatic interaction energy, ΔEelstat, is computed by taking the two fragments at their optimized coordinates (as in the parent complexes), but the charge distribution is considered to be unperturbed on each fragment by the other one. ΔEorb stands for the orbital interaction energy and it originates from the mixing of the orbitals, charge transfer, and polarization between the fragments. Finally, ΔEdisp represents the dispersion energy correction toward the total attraction energy. The EDA-NOCV calculation combines the energy and charge decomposition schemes and divide the deformation density, Δρ(r), associated with the bond formation, into different components (σ, π, δ) of a chemical bond. From the mathematical point of view, for each NOCV, ψi is defined as an eigenvector of the deformation density matrix in the basis of fragment orbitals. ΔPψi = viψi

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Optimized geometries of B 2 (NHC D i p p ) 2 and B2(cAACDipp)2 (Dipp = 2,6-diisopropylphenyl) complexes and the coordinates (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.P.). *E-mail: [email protected] (P.K.C.). ORCID

Sudip Pan: 0000-0003-3172-926X Pratim K. Chattaraj: 0000-0002-5650-7666 Notes

The authors declare no competing financial interest. 13728

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