Unravelling the Dramatic Electrostructural Differences Between N

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Unravelling the Dramatic Electro-Structural Differences Between NHC- and CAAC-Stabilized Low-Valent Main Group Species Eileen Welz, Julian Böhnke, Rian D. Dewhurst, Holger Braunschweig, and Bernd Engels J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 05 Sep 2018 Downloaded from http://pubs.acs.org on September 5, 2018

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Journal of the American Chemical Society

Unravelling the Dramatic Electro-Structural Differences Between NHC- and CAACStabilized Low-Valent Main Group Species AUTHORS Eileen Welz,† Julian Böhnke,‡,§ Rian D. Dewhurst,‡,§ Holger Braunschweig,‡,§ Bernd Engels‡,§,*

AFFILIATIONS †

Institute for Physical and Theoretical Chemistry, Julius-Maximilians-Universität Würzburg,

Am Hubland, 97074 Würzburg, Germany ‡

Institute for Inorganic Chemistry, Julius-Maximilians-Universität Würzburg, Am Hubland,

97074 Würzburg, Germany §

Institute for Sustainable Chemistry & Catalysis with Boron, Julius-Maximilians-Universität

Würzburg, Am Hubland, 97074 Würzburg, Germany

ABSTRACT: Cyclic (alkyl)(amino)carbenes (CAACs) and N-heterocyclic carbenes (NHCs) are widely used as stabilizing ligands in transition metal and main group element chemistry. Variations in their stabilizing properties have been cursorily explained in the literature by the greater -donating and -accepting properties of CAACs relative to NHCs and their differing steric demands, however, a more precise understanding, in particular a disentanglement of steric and electronic effects, is lacking. The recently reported compounds (E)(L)BB(L)(E) (L = NHC (I) / CAAC (II) and E = SPh) present an unique opportunity to investigate the differences between NHC and CAAC donors, as both forms are stable but differ considerably in their geometrical and electronic properties. The NHC systems possess a singlet ground state with a planar central SBBS unit while their CAAC counterparts show a triplet ground state with a twisted SBBS unit. Steric effects were found to be important in this case, however, it remained unclear how the different forms of twisting in I and II depend on the interplay of steric and electronic effects. In the present work we disentangle both effects. Our investigations explain all of these effects by MO considerations and show that for this kind of system the size of the singlet-triplet gaps are the key determinants of the differences. The different sizes of the S-T gaps result from variations in the antibonding effects within the HOMOs and LUMOs. Our explanation seems to contradict the general scientific consensus about variations in the HOMO and LUMO of these two classes of cyclic carbenes, however, comparisons to the Kekulé biradicaloids recently presented by Bertrand and coworkers indicate the generality of our approach. 1 ACS Paragon Plus Environment

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INTRODUCTION Undeniably one of the most exciting new classes of ligand to emerge over the last few decades has been the cyclic (alkyl)(amino)carbenes (CAACs).1-4 First prepared in 2005 by Bertrand and coworkers, these singlet carbenes are stable and isolable, allowing their direct application in reactions with elements that encompass an astonishingly wide swathe of the periodic table. Their unique combination of strong -donation, strong -acceptance and large, tunable and rigid steric bulk have rendered CAACs extremely useful for closed-shell applications in coordination chemistry, low-valent main-group chemistry and catalysis.1-4 Beyond this, a formidable body of work from Bertrand and others over the last few years has shown CAACs to have outstanding abilities to stabilize radical species where the unpaired electron resides at or near the p-orbital of the carbene center.5-13 Similarly, Bertrand, Roesky and others have used CAACs to stabilize a series of truly fascinating metal complexes described variously as monoradicals, as well as singlet and triplet biradicals.1-2, 14-15 In a qualitative sense, the ability of CAACs to stabilize such radical species can be attributed to the so-called “captodative effect”, the essence of which is that an unpaired electron at (in the case of CAACs) a sp2-hybridized carbon atom can be stabilized in a push-pull manner by addition of neighboring -donor (i.e. the N atom of CAACs) and -acceptor units (an empty p orbital on an atom bound to the CCAAC atom, e.g. tricoordinate boron).16 The captodative effect of such an all-sp2-hybridized N-C-B unit is now being used as a lead strategy in the engineering of molecules that undergo singlet fission for light harvesting and other molecular device applications.17-18 Our own work over the last few years has uncovered a number of new stable boryl radicals of the form [B(•)(CAAC)(R)(X)],19-21 however, our recent synthesis of a family of diradicals of the form [(CAAC)(R)(•)B–B(•)(R)(CAAC)] (II, Figure 1)22 – effectively dimers of boryl radicals – was an unexpected and intriguing outcome given that the alternative isomer of such a compound is the known diborene20,

23

geometry with a planar B=B bond

[(CAAC)(R)B=B(R)(CAAC)]. Indeed, we had previously reported CAAC-bound diborenes with the latter structure, namely the dihydro (R = H)24-25 and dicyano (R = CN) derivatives,26 attesting to the plausibility of such species. Moreover, the analogues of II with conventional (i.e. diamino) N-heterocyclic carbenes (NHCs) adopted planar, B=B-bonded diborene structures in the solid state. Differences in the properties of NHCs and CAACs, e.g. the tendencies of CAACs to generate stable radical and diradical species, are explained in the literature by the greater -donating and -accepting properties of CAACs in comparison to NHCs, however, the arguments often remain quite vague.5-15,19-21 Only Frenking and co-workers have examined the differences of 2 ACS Paragon Plus Environment

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CAACs and NHCs for L2:SiCl2 compounds (L = NHC, CAAC) more closely.27-28 By means of NBO, EDA and other quantum chemical calculations they demonstrated that the CAAC2SiCl2 species is supposed to have an electron-sharing C-Si bond rather than the donor-acceptor C→Si bond, detected at the NHC-substituted equivalent. Furthermore, the difference in the donation / -acceptance and the singlet-triplet gaps (ST gaps) of CAACs and NHCs, and their respective influence on the relevant C-Si bond, have been investigated.27-31 To determine the impact of these carbenes in boron compounds, systems I and II (Figure 1) comprise a unique set of molecules in which both the NHC and CAAC analogues are stable and adopt vastly different geometries and electronic states. Hence, the differences between I and II allow a thorough exploration of the differences between NHC and CAAC donors and the consequences these have on their compounds. Thus, herein we present the computational study of model complexes of I and II wherein the steric effects are removed, allowing the underlying electronic effects to come to the fore. By computation of bond-rotated isomers of the molecules in their singlet and triplet states, we establish that in the absence of steric effects the central B-B unit – as well as the carbene substituents – prefer to form an all-coplanar system. By considering the energies, orbitals and bond orders of the relevant conformers, we also determine the reasons why, when steric effects are present, the two systems adopt their respective solid-state and electronic structures. This work thus represents a comprehensive study of the electronic differences between CAACs and NHCs and their respective effects on the open- and closed-shell structures of their derivatives, establishing the ingredients required for each bonding motif (B=C / B=B multiply bound) and each geometric (planar / twisted) and spin (singlet / triplet) state. Our computations indicate that the significantly different singlettriplet gaps are the key determinants of the different behavior of the CAAC- and NHCsubstituted systems. The larger HOMO-LUMO gaps of NHCs relative to those of CAACs is a general consensus in the literature. However, our simulations indicate that the larger HOMOLUMO gaps of NHCs are not a result of the stabilization of the HOMO and destabilization of the LUMO but that both HOMO and LUMO are destabilized, the LUMO to a greater extent. Encouragingly, comparison with the Kekulé biradicaloids presented recently by Bertrand and coworkers6-7 indicates that our approach can be transferred to other systems.

RESULTS AND DISCUSSION

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Figure 1. Connectivity of the investigated NHC- (I) and CAAC-bound (II) diboron molecules and the atom numbering and notation systems used in this work. Computational details Single-reference approaches are often sufficiently accurate.32-33 However, for biradicals with small singlet-triplet gaps, multi-reference approaches are needed to obtain accurate potential energy surfaces (PESs),34 electronically excited states35-36 or even properties.37-38 For the present investigation, the conformer with a twisted SBBS unit turned out to be such a multireference case. Hence, to obtain reliable results for all conformers on an equal footing we had to employ appropriate approaches for relative energies and geometries. The geometries were optimized using the local meta-NGA functional MN12L39 in conjugation with the 6-311G(d,p)4044

Pople basis set,45 followed by an optimization with CASSCF46,47,48-50 and a def2-SVP51-52

basis. For the CASSCF calculations we used a (2,4) space, where two electrons are distributed within the HOMO and the three lowest lying unoccupied orbitals. The justification for the CAS space stems from considering the -orbitals that are primarily responsible for the electronic effects leading to the distortion of the parent compounds I and II (see below). To obtain the relative energies, single-point calculations with the NEVPT253-55 / def2-TZVP51-52 basis set were performed. For these computations we used the same active space as used in the CASSCF computations. The DFT and CASSCF calculations were performed with the newest version of Gaussian1656 while the NEVPT2 calculations were performed with the ORCA program package.57 The NBO analyses were performed using the relevant NBO program.58

Explanation of the models and computational strategies used. Our recent study showed that the doubly NHC-stabilized diboron compound I (Figure 1) possesses a singlet ground state while the analogous doubly CAAC-stabilized species II shows a triplet ground state (Figure 4 ACS Paragon Plus Environment

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1).22 This fundamental electronic difference is accompanied by massive structural differences in the molecules. Compound I has a planar, doubly-bound (NHC)(R)B=B(R)(NHC) unit (Figure 1), with the cyclic NHC units arranged orthogonally to this plane, precluding the possibility of double bonding between the boron atom and the NHC carbene atom (CNHC). In contrast, the triplet system II has a SBBS dihedral angle of ca. 90°, ruling out the presence of B-B double bonding. However, as the CAAC rings lie in the corresponding SBC planes, a certain amount of -bonding between the boron atom and the adjacent CAAC carbene carbon atom (CCAAC) is apparent. The dramatic differences between I and II could conceivably be due to steric effects alone. If the bulky NHC moiety cannot rotate into the SBBS plane and thus form a B-CNHC double bond, the system will likely form a BB double bond instead. The smaller CAAC unit might allow rotation into the SBC plane so that B-CCAAC delocalization becomes possible. However, because the bulky groups of the CAAC do not allow all three central bonds (C-B, BB and B-C) to be coplanar, the system relaxes by adopting a SBBS dihedral angle of 90°. In other words, the system sacrifices the favorable formation of a BB double bond, as the stabilization gained from forming B-CCAAC multiple bonding is stronger. The importance of steric effects is indeed underlined by the structure of an unsymmetrical diborene of the form (CAAC)(Br)B=B(Br)(NHC) recently presented Kinjo and co-workers.59 In the structure the fivemembered ring of the CAAC is nearly coplanar with the B2Br2 moiety, whereas the NHC ring is significantly twisted with respect to the B2Br2 cores. Similar structures were also obtained for a mixed (CAAC)B=B(PMe3)2 system.60 In this hypothesis, steric effects predominate, and electronic properties of CAAC and NHC are of minor importance. However, the opposite situation is also possible. It is generally accepted that the replacement of an electronegative and -donating amino substituent of diaminocarbenes (NHCs) by a -donating (but not -donating) alkyl group (as in CAACs) increases both the nucleophilicity (-donation) and the electrophilicity (-acceptance) of the resulting carbene center. This effect may switch the relative bond strengths of the BC and BB double bonds in the systems I and II. In this case the NHC system would sacrifice the BC double bonds for the stronger BB double bond in order to release strain. Both explanations are in agreement with the fact that the steric effects are too strong to form completely planar systems in which -bonding is present across the central C-B, B-B and B-C bonds. The two possibilities both imply that the structure and electronic character of compounds I and II is determined by the interplay of the strengths of the double bonding in the B-B, and both B-C, bonds. However, in this second explanation, the differences in the geometrical and electronic structures of both systems are induced by the high strain in completely planar systems, but the electronic properties of the CAAC and NHC donors manifest the different ways in which this strain is released. 5 ACS Paragon Plus Environment


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A third possible explanation is based on the known ability of CAAC substituents to stabilize radical centers and the fact that the triplet states of CAACs lie considerably lower than those of NHCs. Thus, in compound II each CAAC substituent stabilizes a radical boron center, a possibility that is unavailable to the NHC-substituted system I. As in the last explanation, the high strain inherent to the (hypothetical) completely planar system induces the difference, but again variations in the electronic properties are crucial. Potential experimental efforts to investigate the underlying reasons for the differences in the electronic structure of I and II by reducing the steric bulk of the substituents are hampered by the fact that this steric bulk is necessary to control the reactivity and ensure the stability of the products and their precursors. However, recent studies of the chemistry of related low-valent diboron

systems

have

confirmed

that

CAAC-stabilized

diborenes

of

the

form

(CAAC)(R)B=B(R)(CAAC) with distinct B=B double bonding and planar B=B units are indeed stable and accessible with sterically unimposing boron-bound substituents, namely the dihydro (R = H)24-25 and dicyano (R = CN) derivatives.26 However, as the -H, -CN and -SPh groups differ both in their electronic effects and their bulk, it is difficult to determine the reason for the relative stability of the diborenes with R = H, CN. Thus, many questions about the influence of the electronic structure of these low-valent diboron species remain. To determine the influence of the electronic effects on the stark differences in the structures of compounds I and II, in this work we have suppressed the steric effects as much as possible by replacing all bulky groups by hydrogen atoms and characterizing the resulting model systems (I’ and II’) by high-level quantum chemical approaches. Indeed, all resulting model systems are predicted to possess completely planar ground-state structures (C-B, B-B and B-C bonds all planar), i.e. if steric effects could be negated, no rotation of the B-B or B-C bonds would take place. This finding indicates that the steric effects indeed induce the different forms of twisting observed in both I and II, however, the question as to why both systems twist in different ways remains. To investigate the influence of both twisting patterns on the electronic structures of the systems we computed all of the conformers resulting from twisting the central (C-B, B-B and B-C) bonds of I’ and II’ and compared the variations in their energies and bonding situations. Our approach is an extension of a comparable ansatz of Lin and Yamashita et al.61 who computed the dependency of the lowest unoccupied molecular orbital (LUMO) and the electronic energy as a function of the CBBC torsion angle to gain insight into the electronic structure of tetra(o-tolyl)diborane(4). Tetra(o-tolyl)diborane(4) has a singlet ground state in combination with a twisted CBBC structure.62 This results from the combination of an empty BB -orbital and the steric demands of the o-tolyl groups.


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Influence of CAAC substituents. Computation of the experimentally-authenticated compound II reveals a triplet ground state. In the global minimum the angle 𝑆𝐵𝐵𝑆 is 90°, thus the CBS planes are mutually orthogonal. Consequently, a B-B -bond is not present in the molecule. In contrast, -bonds between the boron centers and the adjacent carbene carbon centers can form as the 𝑁𝐶𝐵𝐵 torsion angles are either 0° or 180°. For model system II’, in which the bulky substituents 2,6-C6H3iPr2 and Ph are replaced by hydrogens, the lowestenergy conformers have singlet ground states wherein the CCAAC-B and B-B bonds are nearly planar (see Table 1). Table 1. Energies of B, B-coplanar conformers of II’ relative to the lowest-energy conformer CAAC 1. The numbering of the atoms is as shown in Figure 1. The NEVPT2 [2,4] / def2-TZVP level of theory was employed for all computations. All values in kcal/mol. Conformer

𝐶𝑁1 𝐶1 𝐵1

𝑁1 𝐶1 𝐵1 𝐵2

𝑆1 𝐵1 𝐵2 𝑆2

𝐵1 𝐵2 𝐶2 𝑁2

𝐵2 𝐶2 𝑁2 𝐶

E(S)

E(T)

179

180

180

0

179

0.0

12.7

-179

0

180

0

-179

2.1

11.2

175

180

0

180

175

2.8

13.5

173

180

0

0

170

4.4

12.8

173

0

0

0

-166

5.7

10.8

CAAC 1

CAAC 2

CAAC 3

CAAC 4

CAAC 5



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This is shown in Table 1 which lists the five different planar conformers. The orientation of the SH moiety varies throughout the structures. In conformer CAAC 1 one S-bound hydrogen points away from the SBBS plane, while the other lies in this plane. This may result from weak NH⋯S hydrogen bonds, as suggested by the distances between the sulfur and neighboring NH moieties. Such interactions, combined with small residual steric repulsions, are responsible for the small energy differences between the various conformers (< 5 kcal/mol). An exception to this is conformer CAAC 5, whose higher energy results from steric repulsion between the NH moieties. In the following discussion, we focus on the CAAC 2 conformer, which is only 2 kcal/mol higher in energy than CAAC 1. We chose this conformer instead of CAAC 1 because its E configuration of the CAAC nitrogen centers is also found experimentally for a planar 1,2dihydrodiborene.24-25 The energetically lower-lying CAAC 1 possesses a Z configuration. This variation may be a result of the bulkier groups in the synthesized 1,2-dihydrodiborene. Furthermore, the effects discussed in the following for CAAC 2 are also found for CAAC 1, as shown in the SI.

Table 2. Geometrical parameters for the planar conformers CAAC 1 and CAAC 2 of II’. 𝐵2 𝐵1 𝑆1 𝐻

𝐵1 𝐵2 𝑆2 𝐻

𝑅𝑁1 𝐶1

𝑅𝐶1 𝐵1

𝑅𝐵1 𝑆1

𝑅𝐵1 𝐵2

𝑅𝐵2 𝑆2

𝑅𝐵2 𝐶2

𝑅𝐶2 𝑁2

CAAC 1

-26

-82

1.32

1.56

1.89

1.62

1.95

1.52

1.34

CAAC 2

-134

-59

1.33

1.54

1.92

1.62

1.92

1.54

1.33

Conformer

Figure 2. HOMO-1, HOMO and LUMO of selected conformers of compound II’ with one broken CCAAC-B or B-B -bond. The corresponding orbital energies (kcal/mol) are given below each orbital. Conformer

HOMO-1

HOMO

LUMO

CAAC 2

-119.9

-85.3

-55.3


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CAAC 2.2

-113.0

-76.1

-41.5

CAAC 2.4

-136.1

-67.5

-66.8

Table 3. Selected Wiberg bond indices illustrating variations in the bonding situation of the conformer CAAC 2 by twisting different combinations of the central (C-B, B-B and B-C) bonds of CAAC 2. (S) indicates the values obtained for the singlet state while (T) denotes the values for the corresponding triplet state. Conformer

N1-C1

C1-B1

B1-B2

B1-S1

B2-S2

B2-C2

C2-N2

CAAC 2 (S)

1.4

1.1

1.4

0.8

0.8

1.1

1.4

CAAC 2 (T)

1.2

1.4

1.0

1.1

1.0

1.3

1.2

CAAC 2.2 (S)

1.2

1.4

1.3

0.9

1.1

0.9

1.5

CAAC 2.4 (S)

1.2

1.3

1.1

1.1

1.1

1.3

1.2

CAAC 2.4 (T)

1.2

1.3

0.9

1.2

1.1

1.4

1.2

CAAC 2.5 (S)

1.2

1.3

0.8

0.9

1.4

1.0

1.1

CAAC 2.7 (S)

1.0

1.4

0.8

1.1

1.3

1.2

1.1

The planar structure of CAAC 2 indicates that -delocalization over the whole molecule becomes favorable if steric effects are sufficiently small. An analysis of the highest occupied molecular orbitals of CAAC 2 indeed reveals (Figure 2) a delocalized -system. This is also reflected in the geometrical parameters (Table 2) and the Wiberg indices (Table 3), which predict multiple bonding character for the B-B (1.4) and C-N (1.4) bonds. The latter stands in contrast to the distinct antibonding CN character of the HOMO (Figure 2). It is interesting to note that this antibonding character is also found in the HOMO and LUMO of the Kekulé diradicaloids recently synthesized by Bertrand and coworkers,6-7 as well as a number of diborene compounds26, 63 and even a diradical diborahydrazine21 reported by Braunschweig 9 ACS Paragon Plus Environment


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and coworkers. Hence, this seems to be a general feature of CAAC compounds. However, to the best of our knowledge its influence on the electronic properties of CAACs vs. NHCs has not been considered. The B-S bond has single bond character (0.8). The Wiberg indices of the B-C bonds (1.1) also predict roughly single bond character which to some extent contradicts the bonding character observed in the HOMO, but may result from the HOMO being mainly concentrated on the BB moiety.

Table 4. Summary of the relative energies of the conformers obtained from the B,Bcoplanar conformer CAAC 2 ( 𝑵𝟏𝑪𝟏𝑩𝟏𝑩𝟐 = 𝟎°, 𝑺𝟏𝑩𝟏𝑩𝟐𝑺𝟐 = 180°, and 𝑩𝟏 𝑩𝟐𝑪𝟐𝑵𝟐 = 0°) by rotating the dihedral angles 𝑵𝟏𝑪𝟏𝑩𝟏𝑩𝟐 , 𝑩𝟏𝑩𝟐 𝑪𝟐𝑵𝟐 , and 𝑺𝟏𝑩𝟏𝑩𝟐𝑺𝟐 , by 90°. All relative energies (kcal/mol) are given with respect to the planar structure CAAC 2. The NEVPT2 [2,4] / def2-TZVP level of theory was employed for all computations.

Confor mer CAAC 2

CAAC 2.1

CAAC 2.2

CAAC 2.3

CAAC 2.4 CAAC 2.5

S-T

𝑁1 𝐶1 𝐵1 𝐵2

𝑆1 𝐵1 𝐵2 𝑆2

𝐵1 𝐵2 𝐶2 𝑁2

E(S)

E(T)

0

180

0

0.0

9.1

9.1

90

180

0

10.8

27.8

17.0

0

180

90

10.8

27.8

17.0

90

180

90

28.8

47.9

19.0

0

90

0

9.2

6.5

-2.7

90

90

0

12.3

24.9

12.7


gap

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CAAC 2.6

CAAC 2.7

0

90

90

12.3

24.9

12.7

90

90

90

46.7

39.1

-7.6

Due to the high repulsion between the bulky substituents, neither system I nor II remain planar. However, the two systems differ in how they relax to release this steric strain. To release the tension in system I the two NHC substituents twist out of the B=B plane (𝑁𝐶𝐵𝐵 = 90°) while the SBBS system remains planar (𝑆𝐵𝐵𝑆 = 0°). In system II the opposite occurs. The SBBS dihedral angle is twisted (𝑆𝐵𝐵𝑆 = 90°) but each NCB unit remains in one plane (𝑁𝐶𝐵𝐵 = 0°, 180°). These differences may result from the fact that CAAC and NHC influence the -bonding in different ways. It should be noted that -bonding between B and S is negligible in both systems. Consequently, the thiol substituents play little electronic part in determining the structures. In turn, the CN bonds can only indirectly contribute to the structural variations as their orientations relative to the neighboring carbene carbon centers do not change upon twisting. Additionally, cooperative effects might influence the energetics. For example, the strength of the C-B or C-N -bonds might increase if the B-B -bond cannot be formed, and vice versa. To characterize how the two different twisting motions (𝑆𝐵𝐵𝑆 = 0° → 90° or 𝑁𝐶𝐵𝐵 = 0° → 90°) change the electronic structure we computed the various conformers and compared their energies and electronic properties. Table 4 summarizes the corresponding energies of the conformers generated by rotating around the central bonds of CAAC 2 (𝑁1 𝐶1 𝐵1 𝐵2 = 0°; 𝑆1 𝐵1 𝐵2 𝑆2 = 180°; 𝐵1 𝐵2 𝐶2 𝑁2 = 0°). The energies summarized in Table 4 are obtained by freezing the dihedral angles 𝑁1 𝐶1 𝐵1 𝐵2 , 𝑆1 𝐵1 𝐵2 𝑆2 , and 𝐵1 𝐵2 𝐶2 𝑁2 to the values listed and optimizing all other geometrical parameters. This approach minimizes remaining steric effects and decouples electronic and steric phenomena as much as possible. Starting from the planar structure CAAC 2 and rotating the right or left CAAC substituent by 90° (providing conformers CAAC 2.1 and CAAC 2.2), the energy increases by 11 kcal/mol. A previously published variable-temperature NMR spectroscopic study of the CAAC-stabilized diborene [(CAACMe)(NC)B=B(CN)(CAAC)] (CAACMe = 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) allowed an estimate of the rotation barrier around the B-CCAAC bond (13.7 kcal/mol).26 However, unlike the model system II’, this CAAC bears the sterically-imposing 2,6-diisopropylphenyl substituent at its nitrogen atom, which will likely increase the rotation barrier. Given the calculated rotation 11 ACS Paragon Plus Environment


barrier of ca. 11 kcal/mol, the rotation barrier observed in the NMR spectroscopic experiment is likely to be predominantly due to the unfavorable breaking of the B=C -interaction. If both substituents are rotated (conformer CAAC 2.3), the energy rises to about 29 kcal/mol, which is somewhat higher than the sum of the destabilization obtained if only one CAAC substituent is twisted. The corresponding triplet states are always higher in energy. For the planar structure we obtain a singlet-triplet gap of about 9 kcal/mol. This gap increases to ca. 17-19 kcal/mol if one or both CAAC substituents are twisted. It should be noted that all singlet states possess single-reference character, in which the HOMO is doubly occupied. An accurate characterization of the singlet state of conformer CAAC 2.4, in which the molecule is twisted around the B-B bond (𝑆1 𝐵1 𝐵2 𝑆2 = 90°), turned out to be very difficult. The two leading configurations of the [2,4] NEVPT2 wavefunction have coefficients of 0.50 (HOMO doubly occupied) and 0.49 (LUMO doubly occupied). Due to this high multi-reference character, even geometry optimizations for the singlet state employing the open-shell single reference MN12L approach failed because they led to unsymmetrical structures. Hence, we used a CASSCF [2,4] approach to determine the geometry of the singlet state of CAAC 2.4. Test calculations of all other geometries showed that this problem only exists in the case of CAAC 2.4. The computed energy values of the singlet and triplet states underline the exceptional electronic character of the conformer CAAC 2.4. The singlet state lies about 9 kcal/mol higher than that of the planar structure while the triplet state is roughly 3 kcal/mol lower in energy, i.e. the triplet represents the ground state of conformer CAAC 2.4. In the cases where the B-B bond and one of the B-C bonds are orthogonal (CAAC 2.5 and CAAC 2.6) the energy of the singlet increases to ca. 12 kcal/mol while the corresponding triplet states are again much higher in energy (ca. 25 kcal/mol).


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Figure 3. Variations of the MOs of the CBBC moiety of the planar system (left) upon twisting diboron species I and II around one NCBB dihedral angle (center: one CB double bond is broken) and around the SBBS dihedral angle (right: the BB double bond is broken).

Frenking and co-workers rationalized the electronic structure of CAAC and NHC complexes of the type L2E2 (E = group 14 elements) by orbitals that were obtained by combining the electronic structure of the E2 unit with the frontier orbitals of CAAC or NHC.64-65 To explain the trends in Table 4, we focus on the simplified frontier orbitals depicted in Figure 3, which illustrate the variations in simplified MOs of the CBBC moiety upon twisting one of the CAAC substituents or the central SBBS unit. We use these simplified MOs to explain the trends, as the corresponding atomic contributions are obscured within the complicated shapes of the computed MOs (see below and Figure 2). The orbitals shown are occupied with only two electrons in total as the remaining electrons of the boron and the carbene carbon centers are employed to form the -bonds. This consideration was the basis for the choice of the active space of the CASSCF and the NEVPT2 computations. In the active space we distributed two electrons within the HOMO and the three lowest-lying unoccupied orbitals. For a planar molecule the MOs are delocalized over the whole CBBC moiety, thus we obtain the -MOs of a four-atom unit of the form ABBA. For the planar singlet state (Figure 3, left) the two electrons occupy the lowest orbital, while for the triplet the two lowest orbitals have to be occupied. Thus, from an energetic standpoint, the singlet state is the ground state while the singlet-triplet gap correlates with the HOMO-LUMO energy difference. Additionally, both states should be described competently by single-reference approaches. If one of the CAAC substituents is twisted (Figure 3, center), the corresponding p-orbital of one carbene carbon center is orthogonal to the remaining three p-orbitals so that the available -MOs are only delocalized over three centers. The singlet state is thus again lower in energy, as both electrons occupy the bonding MO, and a single-reference approach should be sufficiently accurate. The situation where only the B-B bond is twisted (i.e. CAAC 2.4) is shown in Figure 3, right. The situation can either be described by two localized CB -bonds (not shown) or by their positive and negative linear combinations, as shown in Figure 3. Due to the degeneracy of both MOs the singlet wavefunction needs at least two configurations. Furthermore, the triplet state is expected to be lower in energy. On the simplest level the energy difference between the two states can be assumed to be equal to twice the exchange integral of both degenerate MOs. Our reduced MO model only includes the -MOs resulting from the linear combinations of the corresponding AOs of the spacer between both CAAC substituents plus the corresponding AO of the carbene carbons of the CAAC moieties, i.e. it neglects effects resulting from the nitrogen 13 ACS Paragon Plus Environment


center of CAAC, the influence of the sulfur substituent and of lower lying orbitals. Nevertheless, a comparison with the diradicaloids recently synthesized by Bertrand and coworkers 6-7 indicate that this reduced model can be generalized to other CAAC-substituted central moieties consisting of main group elements. In the diradicaloid systems presented by Bertrand and coworkers, the CAAC units are neighbored by a CC triple bond, being part of a larger diyne spacer. The corresponding HOMO possesses a nodal plane between the carbon centers of the triple bond but bonding character between the carbene carbon and the neighboring carbon center of the triple bond, i.e. it reflects the structure of the LUMO depicted in Figure 3 (left). The correspondence between the LUMO of our system and the HOMO of the diradicaloid systems is a result of the latter possessing two additional electrons due to the presence of carbon in the place of boron. The variations in the orbitals upon going from conformer CAAC 2 to CAAC 2.4 also resemble the variations in the LUMO of tetra(o-tolyl)diborane(4) that were described by Lin and Yamashita et al. upon twisting the CBBC angle.61 The LUMO of the planar tetra(o-tolyl)diborane(4) corresponds to the HOMO of CAAC 2 because it possesses two fewer electrons. To obtain more detailed insight into the bonding of our system, we can use the Wiberg bond indices (WBIs) of the key bonds (Table 3). We employed Hartree-Fock methods in combination with def2-SVP basis sets for their computation because they do not depend on the employed method.21, 66-69 Hence, they should be sufficiently accurate for qualitative considerations. For the singlet state of the planar structure, a double bond character was only found for the BB bond (1.4) and both CN bonds (1.4), while the CB (1.1) and BS bonds (0.8) are predicted to be essentially single bonds. For the triplet state of the planar structure (CAAC 2), the Wiberg bond indices reveal cooperative effects (Table 3). As expected, the B-B double bond is broken (WBI of 1.0), but surprisingly, the double bond character of the CN bond also decreases (WBI decreases from 1.4 to 1.2). In contrast, the double bond characters of the CB bonds increase from 1.1 to 1.4. Thus, it is clear that if the BB -bond is broken, electron density is shifted into the CB -bond. The variations found between the singlet and triplet state of the planar structure resemble those between the singlet states of the planar structure and B-B-twisted conformer CAAC 2.4. Going from the singlet of the planar structure to that of CAAC 2.4 the BB -bond is nearly broken (1.1). Additionally, the double bond characters of the CB bonds increase (1.1 to 1.3) while those of the CN bonds slightly decrease (1.4 to 1.2). For conformer CAAC 2.4 the bonding characters of the singlet and triplet states are nearly identical as expected from the doubly degenerate MOs depicted in Figure 3. The similarities between the electronic structures of the triplet state of the planar geometry and singlet and triplet states of conformer CAAC 2.4 can be rationalized by the MOs depicted in Figure 3. For the triplet state of the planar structure (Figure 3, left) the HOMO and LUMO are singly occupied. In the singlet and triplet state of conformer CAAC 2.4 (right hand side in Figure 3) both degenerate lower lying orbitals are 14 ACS Paragon Plus Environment

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occupied. The respective (+ and –) linear combinations of the occupied orbitals lead to two localized, bonding -orbitals that extend either over the left or the right CB subunit. The fact that this results from occupying the HOMO and LUMO for the triplet state or from twisting the molecule around the B-B bond to form conformer CAAC 2.4 is not particularly important because in both cases the resulting two local CB subunits do not interact. The relative energies of the various conformers given in Table 4 also explain why the experimentally authenticated system II relaxes its steric strain by twisting the SBBS dihedral angle rather than by twisting both (or only one) of the CAAC moieties. The triplet state of conformer CAAC 2.4 represents the lowest-energy conformer aside from the singlet state of the planar conformer CAAC 2. It is interesting to note that even the singlet states of the conformers with only one twisted CAAC moiety (CAAC 2.1 and CAAC 2.2) possess higher energies than the triplet state of CAAC 2.4. This explanation is underlined by the fact that computation of truncated model compound II’ in the geometry of bulky compound II (i.e. B-Btwisted) also gives a triplet ground state. The frontier orbitals of the planar structure computed using the MN12L functional (Figure 2) are in line with the simplified frontier orbitals depicted in Figure 3. However, the shapes of the corresponding orbitals of the conformers CAAC 2.2 and CAAC 2.4 are rather complicated, making trends difficult to detect. This is best seen in the HOMO and LUMO of the conformer CAAC 2.4. Despite the twisted SBBS unit, the HOMO and LUMO reveal a non-negligible amount of -bonding character between the boron centers. The bond characters discussed above (localized CB -orbitals) can only be appreciated from the linear combinations of both MOs. The legitimacy of taking linear combinations in this case is shown by the NEVPT2 wavefunction, whose two leading configurations either occupy the HOMO or the LUMO. The orbitals of the conformers with more than one rotated CAAC moiety are so complicated that they provide little useful information. Natural orbitals from CASSCF or NEVPT2 computations are also too complicated to reveal clear trends. Influence of NHC substituents. As the second part of the study, we investigate the electronic effects influencing compound I. Experimental studies showed this compound to have a singlet ground state with a planar SBBS moiety but twisted NHC substituents. Again, we replace the bulky N-bound aryl groups by hydrogens – resulting in the model system I’ – in order to diminish steric effects such that the underlying electronic effects can be investigated. The global minimum of I’ has a planar singlet ground state (see Table 5) in which the central B-B bond has an E configuration. Model compound I’ has only one further conformer (Z conformation) because of the higher symmetry of the NHC substituent relative to that of the CAAC substituent. The energy difference between both conformers of I’ is ca. 15 kcal/mol, 15 ACS Paragon Plus Environment


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while a difference of only 6 kcal/mol was found for the corresponding conformers of compound II’. This difference indicates the higher steric demand of the NHC substituent relative to that of the CAAC substituent. A comparison of the geometries of I’ and II’ (Tables 5 and 2, respectively) indicates that the replacement of CAAC with NHC does not lead to strong changes in the electronic or geometrical structure of the singlet ground state. A striking difference, however, was found in the relative energies of the triplet states. The S-T gap values of the planar CAAC-bound compound II’ were ca. 9 kcal/mol (Table 4). For NHC substituents, this value increases to ca. 29 kcal/mol, i.e. the triplets lie roughly 20 kcal/mol higher in energy (Table 5). Table 5. Comparison of the B,B-coplanar conformers of I’. The numbering of the atoms is as shown in Figure 1. All values are in kcal/mol and were obtained at the NEVPT2[2,4] level of theory. Conformer

𝐶𝑁1 𝐶1 𝐵1

𝑁1 𝐶1 𝐵1 𝐵2

𝑆1 𝐵1 𝐵2 𝑆2

𝐵1 𝐵2 𝐶2 𝑁2

𝐵2 𝐶2 𝑁2 𝐶

E(S)

E(T)

176

0

180

0

177

0.0

28.8

175

180

0

180

175

14.8

29.6

NHC 1

NHC 2

Conformer

HOMO-1

HOMO

LUMO

-117.6

-73.8

-30.0

NHC 1


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NHC 1.2

-108.3

-66.9

-13.8

-129.0

-45.0

-42.6

NHC 1.4

Figure 4. HOMO-1, HOMO and LUMO of selected conformers obtained from conformer NHC 1. The orbital energies are given in kcal/mol for ease of comparison with those in Tables 4 and 6. This increase in S-T gap is nicely reflected in the orbital energies of the respective systems (CAAC: Figure 2; NHC: Figure 4). If the CAAC substituents are replaced by NHCs, the orbital energy of the HOMO of the planar E configuration (Figure 2: CAAC 2 vs. Figure 4: NHC 1) increases by about 11 kcal/mol from -85 kcal/mol (Figure 2) to -74 kcal/mol (Figure 4). This increase can be rationalized by considering the nodal planes between the carbene carbon atom and the nitrogen centers of the NHCs. Similar nodal planes are present in both CAACs and NHCs, however, the antibonding effect in NHCs is likely to be larger because they contain two nitrogen centers instead of one. The relative energy of the LUMO is distinctly higher (-55 kcal/mol to -30 kcal/mol) in the NHC case than the CAAC case, i.e. the increase of the HOMOLUMO gap is due from the larger change in the energy of the LUMO. The energetic variations in the HOMO and LUMO can be attributed to the shape of the orbitals. Going from CAAC (Figure 2) to NHC (Figure 4), the HOMO orbitals become more localized at the B-B unit. This is also reflected in the corresponding Wiberg indices, which increase from 1.4 for CAAC substituents (Table 3) to 1.6 for NHC substituents (Table 7). The localization towards the B-B unit may result from increased antibonding between the carbene carbon centers and the respective two nitrogen centers of the NHC. The LUMOs of the conformers CAAC 2 and NHC 1 mainly represent antibonding * interactions at the B-B unit. For NHCs, the density of the LUMO at the carbene carbon centers is clearly higher than in the HOMO. This transfer of density is partly reflected in the Wiberg bond indices of the CB bonds, which increase from 1.1 to 1.4 if the singlet and triplet state are compared. Due to the higher density at the two carbene carbon centers in the LUMO, their antibonding interactions with the adjacent nitrogen centers are greater than in the corresponding HOMO. Similar effects are seen in the CAAC-substituted 17 ACS Paragon Plus Environment


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examples, but the more intense antibonding in the NHC-substituted compounds (due to each having two neighboring nitrogen centers) explains why the orbital energy of the LUMO increases to a greater extent than the HOMO upon changing from CAAC to NHC. It should be noted that an increase of the HOMO-LUMO gap going from CAAC to NHC and the accompanied reduction of the S-T gap is generally accepted in the literature,14, 70-71 but the subjects of the discussions are different. Most discussions only refer to the - and -frontier orbitals of the CAAC or NHC moieties, while our model includes the interacting frontier orbitals of the bridge between both ligands. Focusing only on the frontier orbitals of the CAAC or NHC moieties, the gap increases because the energy of the -orbital (HOMO) of the ligands decreases while the energy of the p-orbital of the ligand (LUMO) increases. In our model, variations in the -orbital (HOMO) of the ligands are not taken into account because they vary little upon twisting either the ligand or the central SBBS unit. Our model instead focuses the interaction of the p-orbital of the ligand with the -type frontier orbitals of the bridge. In this model the S-T gap increases because the antibonding effects within the LUMO are stronger than in the HOMO, as explained above. Table 6. Summary of the relative energies of the conformers obtained from the B,Bcoplanar conformer NHC 1 ( 𝑵𝟏𝑪𝟏𝑩𝟏𝑩𝟐 = 𝟏𝟖𝟎°, 𝑺𝟏𝑩𝟏𝑩𝟐𝑺𝟐 = 0°,and 𝑩𝟏𝑩𝟐𝑪𝟐𝑵𝟐 = 0°) by rotating the dihedral angles 𝑵𝟏𝑪𝟏𝑩𝟏𝑩𝟐 , 𝑩𝟏𝑩𝟐 𝑪𝟐𝑵𝟐 , and 𝑺𝟏𝑩𝟏𝑩𝟐𝑺𝟐 , by 90°. All relative energies (kcal/mol) are given with respect to the planar structure 1. Confo rmer NHC 1

NHC 1.1

NHC 1.2

NHC 1.3

S-T

𝑁1 𝐶1 𝐵1 𝐵2

𝑆1 𝐵1 𝐵2 𝑆2

𝐵1 𝐵2 𝐶2 𝑁2

E(S1)

E(T1)

0

180

0

0.0

28.8

28.8

90

180

0

13.6

47.2

33.7

0

180

90

13.6

47.2

33.7

90

180

90

29.1

66.1

37.1


gap

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NHC 1.4

NHC 1.5

NHC 1.6

NHC 1.7

0

90

0

29.4

28.8

- 0.6

90

90

0

43.6

43.9

0.3

0

90

90

43.6

43.9

0.3

90

90

90

38.5

61.6

23.1

Table 7. Selected Wiberg bond indices illustrating variations in the bonding situation of the conformers obtained from NHC 1. (S) indicates the values obtained for the singlet state while (T) denotes the values for the corresponding triplet state.

Conformer

N1-C1

C1-B1

B1-B2

B1-S1

B2-S2

B2-C2

C2-N2

NHC 1

(S) 1.2

1.1

1.6

0.8

0.8

1.1

1.2

NHC 1

(T)

1.2

1.4

0.9

0.9

0.9

1.4

1.1

NHC 1.1 (S) 1.2

1.1

1.7

0.9

0.9

0.8

1.3

NHC 1.3 (S) 1.2

0.9

1.8

0.9

0.9

0.9

1.3

NHC 1.4 (S) 1.1

1.3

1.0

1.1

1.1

1.3

1.0

NHC 1.4 (T)

1.1

1.4

0.9

1.1

1.1

1.4

1.1

NHC 1.6 (S) 1.2

1.1

1.6

1.0

0.9

0.9

1.3

NHC 1.7 (S) 1.2

0.9

1.7

0.9

0.9

0.9

1.2

If we compare the energies of the various conformers obtained from NHC 1 (Table 6) with the corresponding values obtained for CAAC 2 (Table 4), the main differences again result from the strong increase in the S-T gap. The destabilization of the singlet state resulting from rotating one of the BC bonds in NHC 1.1, NHC 1.2 (Table 6) is ca. 14 kcal/mol, which is only ca. 3 kcal/mol higher than the values obtained for the corresponding CAAC conformers CAAC 19 ACS Paragon Plus Environment


2.1 and CAAC 2.2 (ca. 11 kcal/mol; Table 4). As for the planar structure, the S-T gap values of the conformers NHC 1.1 and NHC 1.2 are considerably larger (34 kcal/mol) than for the corresponding CAAC conformers (17 kcal/mol). This larger S-T gap value also influences the relative energy of conformer NHC 1.4, in which the SBBS moiety is twisted so that a B-B double bond cannot be formed. For conformer NHC 1.4 we compute a destabilization of ca. 30 kcal/mol with respect to the planar structure, while only ca. 7 kcal/mol were computed for the corresponding conformer CAAC 2.4. This trend also holds for the conformers NHC 1.5 and NHC 1.6, in which -bonds cannot form for the B-B and one B-C bond due to their geometries. These conformers are considerably higher in energy than conformers NHC 1.1 and NHC 1.2, in which only one C-B -bond is broken (ca. 43.6 kcal/mol vs. 13.6 kcal/mol). For the CAAC system the corresponding energy differences were only ca. 2 kcal/mol (Table 3). The trends found in Table 6 can again be rationalized with the schematic orbitals given in Figure 3, but in this case a considerably larger HOMO-LUMO gap has to be taken into account. Table 6 also indicates why compound I relaxes the steric effects by twisting the NHC substituents, while compound II twists around the B-B bond. In the CAAC-substituted system, the triplet state of conformer CAAC 2.4 is the lowest-energy non-planar conformer. This is not the case for conformer NHC 1.4. For this conformer the triplet state is again the ground state, but due to the large S-T gap found in the planar structure this conformer is so high in energy that for NHC substituents a twist around the BB bond is not the most energetically favorable option to release steric effects. For the triplet state of NHC 1.4, neither planarization of the BB unit (forming triplet NHC 1) nor conversion to singlet NHC 1.4 alone suffice to significantly stabilize the molecule; both planarization and conversion to a singlet (i.e. forming singlet NHC 1) must be performed.

CONCLUSIONS The remarkable differences between the geometries and the electronic structures of compound I and II undeniably result from the differences between the CAAC and NHC carbene substituents, however, the relative importance and interplay of steric and electronic effects in these molecules was unclear at the outset. Thus, to obtain insight into possible electronic effects we replaced the bulky substituents of I and II by hydrogen atoms and characterized the resulting compounds I’ and II’ by high-level multi-reference wavefunction-based approaches. Truncated model complexes I’ and II’ were designed to negate steric effects so that the underlying electronic effects would predominate. To reveal possible electronic effects leading to the twisted structures of I and II we computed the conformers resulting from rotating the


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substituents (𝑁𝐶𝐵𝐵 = 0 and 90°) or from twisting the molecule around the BB bond (𝑆𝐵𝐵𝑆 = 0 and 90°). Our computations predict the truncated models I’ and II’ to be completely planar – including the carbene rings – and the bulkier experimentally-realized compounds I and II are twisted (albeit in different ways). This indicates that: (a) the need for twisting is a result of steric pressure, but (b) the manner in which the two systems twist is determined by electronic effects. Due to the energetically accessible triplet level of CAAC-bound species II, this compound chooses to twist at the B-B bond, while the relatively unstable triplet state of I means this compound instead twists at the B-CNHC bonds. Interestingly, both NHCs are twisted in I, despite the significant electronic energy penalty that comes with twisting. In turn, an explanation of the electronic effects leading to the different twisting patterns of I and II can be derived from analyzing the planar structures I’ and II’. Our calculations underline the importance of electronic effects in the structural differences between I and II, and indicate that these electronic effects can be explained by differences in the corresponding singlet-triplet gaps of the planar structures (I’  30 kcal/mol vs. II’  10 kcal/mol). The key rationale here is that the electronic structures of the triplet planar species I’(T) and II’(T) (i.e. two singly-occupied orbitals, each representing one localized B-C  bond) are essentially equivalent to the electronic structures of both the singlet and triplet states of the conformers with a twisted B-B bond. This means that the S-T gaps of the planar structures (I’  30 kcal/mol vs. II’  10 kcal/mol) are essentially the same as the energetic cost of twisting the central B-B bond (I’  29 kcal/mol vs. II’  7 kcal/mol). The latter values in turn determine whether or not the B-B bond twists when the larger carbenes are present in the experimentally realized compounds (i.e. no for I, yes for II), and thus also the electronic structures of I (diamagnetic diborene) and II (diradical). Delving further into the root causes of the observed results, the differences in S-T gaps between the NHC- and CAAC-bound compounds can be explained by differences in the HOMOs and LUMOs of the planar structures I’ and II’, which reveal antibonding character between the nitrogen and carbene carbons of the cyclic carbene substituents. As this effect is stronger with NHC groups (which have two nitrogen centers) than with CAACs (only one nitrogen), the HOMO and LUMO orbitals of the NHC systems are both higher in energy in those of the CAAC systems, but the LUMO more so than the HOMO. Due to the additional nodal plane of the LUMO, its density is shifted towards the carbene carbon center, so that the antibonding CN character in this orbital is stronger than that in the HOMO. Consequently, the HOMO-LUMO gaps – and thus also S-T gaps – are larger with NHCs than CAACs.



By computationally considering all possible bond-twist conformers – and, importantly, both their singlet and triplet states – we have determined the reasons why: (a) the molecules I and II must twist out of planar to be stable, (b) why they twist at different points when NHCs or CAACs are used, and (c) why NHCs lead to larger S-T gaps than CAACs. The result is the establishment of a “recipe” for the future design and synthesis of low-valent, carbene-stabilized boron species with desired spin states, geometries, and bonding motifs.

ASSOCIATED CONTENT Supporting Information. Computational details and further DFT results. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors * [email protected] ORCID Eileen Welz: 0000-0002-6968-8400 Rian D. Dewhurst: 0000-0001-5978-811X Holger Braunschweig: 0000-0001-9264-1726 Bernd Engels: 0000-0003-3057-389X Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS H.B. and B.E. thank the Deutsche Forschungsgemeinschaft (DFG) for financial support through the Graduate Research School 2112 – Molecular Biradicals: Structure, Properties and Reactivity. REFERENCES 1. Melaimi, M.; Jazzar, R.; Soleilhavoup, M.; Bertrand, G., Cyclic (Alkyl)(amino)carbenes (CAACs): Recent Developments. Angew. Chem., Int. Ed. 2017, 56, 10046-10068. 2. Soleilhavoup, M.; Bertrand, G., Cyclic (Alkyl)(Amino)Carbenes (CAACs): Stable Carbenes on the Rise. Acc. Chem. Res. 2015, 48, 256-266. 3. Lavallo, V.; Canac, Y.; DeHope, A.; Donnadieu, B.; Bertrand, G., A Rigid Cyclic (Alkyl)(amino)carbene Ligand Leads to Isolation of Low-Coordinate Transition-Metal Complexes. Angew. Chem., Int. Ed. 2005, 44, 7236-7239.


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