Communication Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Introducing N‑Heterocyclic Borylenes: Theoretical Prediction of Stable, Neutral, Monomeric Boron(I) Carbenoids Priyam Bharadwaz and Ashwini K. Phukan* Department of Chemical Sciences, Tezpur University, Napaam, Assam 784028, India
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S Supporting Information *
such as the synthesis of systems featuring B−B double and triple bonds.6,7 While the use of group 14 bases is ubiquitous in coordination and main-group chemistry, similar studies on group 13 systems are far less well-explored, which may be attributed to the difficulties associated with the stabilization of molecules in which the central group 13 element exists in the formal oxidation state 1+. In 2000, Roesky and co-workers succeeded in synthesizing the first stable neutral monomeric aluminum(I) compound [{HC(CMeNAr)2}Al] (B; Ar = 2,6-iPr2C6H3) supported by a sterically encumbered βdiketiminate (known as NacNac) ligand framework.8a In the same year, its gallium analogue (C) was also isolated and structurally characterized by Power and co-workers.8b The indium (D) and thallium (E) analogues were also isolated.9 All of these six-membered compounds possess a nonbonding σsymmetric lone-pair orbital and a formally vacant valence p orbital at the group 13 center, thereby making them isolobal to NHC. On the other hand, in contrast to the amenability of heavier group 13 carbenoids toward isolation, isolation of a neutral, stable, cyclic boron(I) carbenoid is yet to be achieved experimentally. In an effort to synthesize a boron analogue of NHC, Nozaki and co-workers performed experimental studies and isolated the first nucleophilic N-heterocyclic boryl anion (F).10 Stabilization of a boron(I) carbenoid similar to Roesky’s aluminum(I) base is particularly challenging, which may be attributed to its low singlet−triplet energy gap (3.5 kcal mol−1) originating from the smaller valence s and p orbital energy separation in boron.11−13 Aldridge and co-workers successfully stabilized the elusive boron(I) heterocycle (G) within the coordination sphere of a transition-metal fragment in the complex [Cp*Fe(CO)2{B(NMesCMe)2CH}][BArf4] through a metal-mediated approach. Importantly, this complex features sufficient steric protection around the central boron center with appreciable boron → iron σ donation.14 It has been shown that the installation of ylidic moieties near a carbene carbon center leads to not only enhanced nucleophilicity but also substantial stabilization of the resulting singlet carbene.15−18 We envisage that the similar incorporation of ylidic moieties may be rewarding in efforts to stabilize neutral cyclic boron(I) systems. In this context, it is worth mentioning the seminal discoveries of a couple of highly stable cyclic four-π-electron, four-membered ylidic rings by Bertrand and co-workers (H and I, Chart 1).19−23 The remarkable stability of these cyclic ylides is attributed not only to their
ABSTRACT: Quantum-chemical calculations predict that synthetically accessible cyclic four-membered, fourπ-electron ylides could be used as building blocks for the realization of hitherto unknown N-heterocyclic boron(I) carbenoids. The boron(I) carbenoids proposed in this work possess the largest computed singlet−triplet separations known to date, which are comparable to those of the corresponding aluminum(I) analogue computed at the same level of theory. Furthermore, they owe their stability not only to the substantial transfer of electron density from nitrogen to boron atoms but also to the presence of thermodynamically robust ylidic bonds. On the basis of their computed proton affinity and carbonyl stretching frequencies, they may be considered as promising ligands for transition-metal complexes.
T
he isolation and stabilization of low-valent main-group compounds are of active research interest because their bifunctional behavior not only can stabilize several reactive species but also could facilitate various chemical transformations.1−4 In this aspect, N-heterocyclic carbene (NHC; A, Chart 1), the signature example of the divalent carbon family isolated by Arduengo and co-workers,5 has been extensively used in the stabilization of several unusual species Chart 1
Received: March 13, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b00731 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
the dramatic enhancement of singlet−triplet separations (Table 1). Indeed, the ΔES−T values of 3 and 5 (30 kcal mol−1) are comparable to that of the experimentally characterized aluminum(I) carbenoid B (33.0 kcal mol−1 calculated at the same level of theory). In order to check the reliability of the computed singlet−triplet gap, we reoptimized these molecules at different levels of theory and obtained comparable values (Table S2). The increased stability of these ylide-decorated borylenes may be attributed to the increased charge donation from the α-nitrogen atoms to the formally vacant 2p orbital of boron, which is further corroborated from molecular orbital (MO) analysis, which shows gradual stabilization of the π-symmetric occupied MO upon installation of the ylidic groups (Figure 1 and Table 1). It may be emphasized that both 3 and 5 owe their stability to not only the substantial transfer of electron density from the αnitrogen to boron atoms but also to the presence of thermodynamically robust ylidic bonds. The presence of ylidic groups makes the α-nitrogen atoms highly electron-rich, thereby enabling enhanced charge transfer to the boron atom compared to that in 1. This is evident from an increase and a decrease in the values of natural charges at the αnitrogen and boron atoms, respectively, as one proceeds from 1 → 2 → 3 as well as from 1 → 4 → 5 (Table 1). In agreement with this charge-transfer process, the natural valence population of boron witnessed a steady increase in the order as given above (Table 2). The increased stability of 3 and 5 is also manifested by planarization of the central six-membered ring in their singlet geometries. In addition to having increased stability, the boron centers of 3 and 5 further benefit from the significant steric protection provided by the bulky substituents (−NiPr2 groups) present at the phosphorus atoms, thereby preventing possible dimerization. This is made clear from the appraisal of the optimized geometries of 1−3 in terms of van der Waals spheres of the atoms (Figure 2). Further, analysis of the Mulliken spindensity calculations obtained from NBO analysis indicates that the triplet states of these borylenes possess biradical character (Table S1). A detailed electronic structure analysis shows that, in the central ring of 3, the negative charge is largely delocalized in the pentadienyl fragment while the boron center carries a positive charge (Table 1 and Figure S1). On the other hand, in the two four-membered ylidic heterocycles, all four exocyclic P−N bonds are found to be slightly shorter than the endocyclic ones, indicating delocalization of the positive charge in the P(NiPr2)2 fragment, as is evident from the perfectly planar arrangement of the two exocyclic nitrogen atoms (Figure S2). Further, all of the endocyclic N−C and C− C bonds in the ylidic ring are found to be between the single and double bonds (as is evident from their calculated WBI
nonantiaromatic nature but also to the higher thermodynamic energy of the constituent phosphorus fragments. Herein, we present the results of a computational study24 toward the stabilization of cyclic boron(I) compounds by employing a variety of zwitterionic four-membered ylidic heterocyclic rings to stabilize the central borylene center (Scheme 1). Our choice of the six-membered β-diketiminate-stabilized boron(I) compounds is prompted by the experimental existence of similar group 13 compounds.8,9 Scheme 1. Schematic Representation of the Range of Borylenes Considered for This Study
Table 1 shows the important geometrical parameters of 1−5, and the full set of geometrical data are listed in Table S1 and Figure S1. A comparison of the central N−B−N bond angle between 1−3 and 1, 4, and 5 indicates that ylide-anchored borylenes possess more acute N−B−N bond angles, and this effect is found to be additive in nature (e.g., 1 > 2 > 3 and 1 > 4 > 5). Consequently, their B−N bond lengths and Wiberg bond index (WBI) values are found to be comparatively longer and lower, respectively, than those of the parent molecule (1). The decrease in the N−B−N bond angles and increase in the B−N bond lengths may be attributed to a change of hybridization of the B−N bonds, as is evident from natural bond orbital (NBO) analysis. The boron atom acquires more p character as one goes from 1 to 3 and 5 (Table 2). The lengthening of the B−N bonds may also be attributed to the presence of a boron(I) center in these molecules. Indeed, calculation at the same level of theory on B(NMe2)3, which has a Lewis acidic boron(III) center, shows that it possesses much shorter (1.437 Å) B−N bond lengths. In order to investigate the thermodynamic stability of these boron(I) heterocycles, their respective singlet−triplet energy separations (ΔES−T) were evaluated (Table 1). In agreement with previous reports, our calculated ΔES−T value for the parent borylene 1 is found to be only 1.7 kcal mol−1, thereby ruling out its possible experimental realization. On the other hand, successive annulations of the β-diketiminate framework of 1 with strongly electron-donating cyclic four-π-electron ylidic moieties lead to a significant stabilization of the singlet state. Compared to monoannulation, bisannulation results in
Table 1. M06/Def2-TZVP-Calculated B−N Bond Lengths (Å), N−B−N Bond Angles (deg), Natural Charges at Boron and αNitrogen (N1 and N2) Atoms, and Singlet−Triplet Energy Separations (ΔES−T, in kcal mol−1) of 1−5a molecule 1 2 3 4 5
r1 1.421, 1.459, 1.473, 1.465, 1.478,
0.955 0.789 0.736 0.765 0.730
r2 1.421, 1.513, 1.464, 1.516, 1.471,
0.955 0.696 0.744 0.696 0.735
N1−B−N2
q(N1)
q(B)
q(N2)
ΔES−T
119.9 105.3 102.7 103.7 100.7
−0.665 −1.074 −1.112 −1.074 −1.111
0.627 0.266 0.295 0.245 0.248
−0.665 −0.693 −1.122 −0.679 −1.118
1.7 9.4 30.2 8.7 30.0
a
The WBI values are given in italics. B
DOI: 10.1021/acs.inorgchem.9b00731 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
Table 2. Hybridization of the B−N Bonds and Natural Valence Population of Boron Atoms (NPB) in 1, 3, and 5 Computed at the M06/Def2-TZVP Level of Theory molecule
occupancy
%B
% s(B)
% p(B)
%N
% s(N)
% p(N)
NPB
1 3 5
1.926 1.971 1.969
21.9 16.9 17.1
47.5 21.8 21.3
52.4 78.0 78.5
78.1 83.1 82.8
27.8 49.6 51.3
72.2 50.3 48.6
2.35 2.71 2.72
Figure 1. Pictorial depiction of the stabilization of π-symmetric occupied MOs of 1−5.
3f (5.76%), and 3g (5.74%). On the other hand, no contributions come from the structures 3a and 3b, suggesting that the resulting electronic structure is best described by the endocyclic charge-delocalized structure 3h, which is in agreement with computed natural charges. Figure 3 depicts the MOs that contribute to the charge-delocalized structure 3h. Similar resonance structures can also be envisaged for 5 (Figure S3, and important MOs are given in Figure S4). Analysis of the nature and energies of the key frontier MOs demonstrates that the highest occupied molecular orbital (HOMO) is clearly σ-symmetric for all of the borylenes (1−5) and is localized at the central boron(I) center with energies (Eσ) ranging from −3.12 to −4.49 eV, indicating their strongly basic nature (Table 3). A comparison of the ligand properties of 3 and 5 with aluminum (B) and gallium (C) carbenoids as well as NHC and cyclic (alkyl)(amino)carbene (CAAC)25 shows that the former compounds possess significantly higher electron-donating ability than the latter compounds (Table 3). However, the electron-accepting ability of 3 and 5 is computed to be comparable to that of NHC but lower than that of B, C, and CAAC. On the basis of their better electron-donating abilities coupled with π-accepting abilities comparable to those of conventional NHCs, the borylenes 3 and 5 can be envisioned as a promising class of ligands and could be used
Figure 2. Optimized geometries of 1−3. The spheres represent the van der Waals spheres of the atoms.
values), indicating that the N−C−C fragment of the fourmembered ylidic ring can be best described as an anionic propenyl system.23 The electronic structure of bis(ylide)annulated 3 can be visualized in terms of the resonance structures 3a−3h, as shown in Scheme 2. Calculations involving the natural resonance theory show that the dominant contributions to the electronic structure of 3 come from resonance structures 3c (30.45%), 3d (17.75%), 3e (16.8%), Scheme 2. Resonance Structures of 3
C
DOI: 10.1021/acs.inorgchem.9b00731 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 3. Pictorial depiction of the important MOs of 3.
Table 3. M06/Def2-TZVP-Calculated Energies of σSymmetric Lone-Pair Orbitals (Eσ in eV), π-Symmetric Unoccupied MOs (Eπ* in eV) Concentrated at the Boron Center, Proton Affinity (PA in kcal mol−1), and Their Carbonyl Stretching Frequencies (νCO in cm−1) for 1−5, NHC, CAAC, B, and C molecule
Eσ
Eπ*
PA
νCO
1 2 3 4 5 NHC CAAC B C
−4.49 −3.19 −3.12 −3.40 −3.46 −6.10 −5.62 −4.73 −5.74
0.40 1.03 1.10 0.63 1.03 1.19 −0.45 −0.76 −0.95
309.8 327.0 329.2 320.5 321.3 257.5 266.7 262.2 237.6
2105 2098 2091 2105 2093 2142 2141 2081 2124
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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/acs.inorgchem.9b00731.
in transition-metal systems in tandem with other Lewis basic ligands. In order to further probe the nucleophilicilty of the proposed stable borylenes 3 and 5, we have evaluated their proton affinities, which are found to be markedly higher (>320 kcal mol−1) than those for B, C, NHC, and CAAC (Table 3) and correlate well with their respective Eσ values. The basicities of 3 and 5 were further tested by evaluating the carbonyl stretching frequencies (νCO) of their complexes with Rh(CO)2Cl. Gratifyingly, the computed νCO values of 3 and 5 are considerably lower than those of NHC and CAAC but comparable to that of B (Table 3), implying the presence of a significantly electron-rich boron center in 3 and 5. To the best of our knowledge, to date, no boron(I) carbenoid with such a large singlet−triplet separation as our proposed compounds 3 and 5 is known, and their comparable stability to known aluminum(I) carbenoids8 makes them interesting potential targets for synthetic chemists. A hint toward the possible isolation of 3 and 5 employing the cyclic ylides came from the exergonicity associated with the formation of these boron(I) carbenoids [−15.9 and −8.3 kcal mol−1 for 3 and 5, respectively, reaction (1)].
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Computational details, optimized geometries, calculated values of the geometrical parameters, tables containing different calculated parameters, and Cartesian coordinates of all of the molecules (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Ashwini K. Phukan: 0000-0001-9286-1198 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Prof. Gernot Frenking and Prof. Holger Braunschweig for kind reading of the manuscript and helpful suggestions. A.K.P. thanks Dr. Rian D. Dewhurst for kind reading of the manuscript and helpful comments. We thank DST-SERB, New Delhi, India, for providing financial assistance in the form of a research project (Project EMR/ 2016/005294). P.B. thanks CSIR for a Senior Research Fellowship. The computational facilities provided by CDAC Pune at Tezpur University are also gratefully acknowledged. D
DOI: 10.1021/acs.inorgchem.9b00731 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
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DOI: 10.1021/acs.inorgchem.9b00731 Inorg. Chem. XXXX, XXX, XXX−XXX
