Boron Carbonyl Analogues of Hydrocarbons: An Infrared

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Boron Carbonyl Analogues of Hydrocarbons: An Infrared Photodissociation Spectroscopic Study of B(CO) (n=4-6) 3

n+

Jiaye Jin, Guanjun Wang, and Mingfei Zhou J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b00440 • Publication Date (Web): 21 Feb 2018 Downloaded from http://pubs.acs.org on February 22, 2018

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The Journal of Physical Chemistry

Boron Carbonyl Analogues of Hydrocarbons: An Infrared Photodissociation Spectroscopic Study of B3(CO)n+ (n=4-6) Jiaye Jin, Guanjun Wang and Mingfei Zhou* Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysts and Innovative Materials, Fudan University, Shanghai 200433(China). E-mail: [email protected]

Abstract: The boron carbonyl cluster cations in the form of B3(CO)n+ (n=4-6) are produced and studied by infrared photodissociation spectroscopy in the carbonyl stretching frequency region in the gas phase. Their geometric structures are determined with the aid of density functional theory calculations. The B3(CO)4+ cation is characterized to have a D2d (OC)2B=B=B(CO)2 structure and 1A1 electronic ground state with a linear boron skeleton. The B3(CO)5+ cation is determined to have a chain boron framework with C2v symmetry. The B3(CO)6+ cation is a weakly bound CO-tagged complex involving a B3(CO)5+ ion core. Bonding analysis reveals that B3(CO)4+ has a chemical bonding pattern similar to allene, while bonding in B3(CO)5+ is similar to that in allyl anion.

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Introduction It is well known that carbon monoxide binds to transition metal centers in forming diverse kinds of transition metal carbonyl complexes.1-5 Carbon monoxide can also coordinate with s and p block main group elements in forming main group carbonyl complexes, most of which are not stable at ambient conditions.6,7 Genuine main-group carbonyl complexes that could be isolated so far are restricted to boron carbonyl complexes. Several carbonyl borane H3BCO and derivatives have been synthesized where the electron deficient boron binds the CO ligand dominantly via σ donation.8-12 Most recently, a rare example of a boron dicarbonyl complex [(RB)(CO)2] (R being a bulky aryl group) with two terminal carbonyl ligands which is stable under ambient conditions has been synthesized.13 Its chemical behavior shows typical features of carbonyl complexes which are known from transition metal carbonyls.13,14 Stable carbonyl complexes of other borylenes have also been synthesized.15-17 Homoleptic boron carbonyl complexes are unstable species at ambient conditions, and they have only been observed in lowtemperature matrices or in the gas phase.18-27 Some of these homoleptic boron carbonyls show intriguing structure and bonding properties. The OCBBCO molecule is characterized to be a boron-boron triple bonded species.20 Both BBCO and B4(CO)2 are - diradicals.22,23 The neutral boron tricarbonyl complex B(CO)3 possess a tilted η1(μ1-CO)-bonded carbonyl ligand, which serves as an unprecedented one-electron donor ligand.25 The B3(CO)3+ cation complex is determined to have a planar D3h structure featuring the smallest π-aromatic cyclic-B3+ moiety.26 The isolobal analogy widely used in organometallic chemistry to relate the structures of organic and inorganic molecular fragments can be applied to bridge the boron carbonyl complexes and the hydrocarbon molecules. The 4Σ– ground state for boron monocarbonyl BCO 2


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with three spins predominantly located on the boron atom is akin to the 4Σ– excited state for the CH group.23 On the basis of this isolobal relationship, the carbonyl borane H3BCO is isolobal to methane. The photoproducts of phenylborylene and CO, PhBCO with a triplet ground state, is isoelectronic with the carbene center in PhCH.17 The BB triple bonded OCBBCO molecule is the counterpart to acetylene. The counterpart of B4(CO)2, B2(CH)2 also has planar D2h symmetry involving a rhombic B2C2 ring.28 The isolobal relationship between BCO and CH has been applied to predict many other interesting new boron carbonyl compounds.29,30 Large boron carbonyl clusters (BCO)n+/0 (n = 3 - 24) isolobal with the Hückel aromatic species or polyhedral hydrocarbons have been predicted by theory.29,30 Here we report a combined infrared photodissociation spectroscopic and theoretical study on the boron carbonyl cluster cations B3(CO)n+(n=4-6) in the gas phase. The results show that B3(CO)4+ cation has an allene-like D2d structure, while the B3(CO)5+ cation has a chain boron framework analogous to the allyl anion. The results point to an isolobal relationship between CO and H- in bridging the boron carbonyl complexes and the hydrocarbon molecules. Experimental and computational methods The infrared photodissociation spectra of the boron carbonyl cation complexes were measured using a home-made collinear tandem time-of-flight mass spectrometer (TOFMS), as described in detail previously.31,32 The cation complexes were prepared by a pulsed laser vaporization supersonic expansion ion source using the second harmonic of a pulsed Nd:YAG laser (Spectra-Physics GCR-150, 10 Hz repetition rate). Both

10

B-enriched and

10

B-depleted

targets were used. The cation complexes were produced from the laser vaporization process in expansions of helium seeded with 2-6 % carbon monoxide at a backing pressure of 0.4−1.2 3



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MPa using a pulsed valve (General Valve Series 9). After expansion and cooling, the cation complexes were skimmed to the second chamber, where they were pulse-pushed into a Wiley−McLaren time-of-flight mass spectrometer. The cations were mass-selected by their flight times and decelerated into the extraction region of a second collinear TOFMS, where they were dissociated by a tunable IR laser beam. The IR laser beam was generated by a KTP/KTA//AgGaSe2 optical parametric oscillator/amplifier system (OPO/OPA, LaserVision) pumped by a Continuum Surelite EX Nd: YAG laser, producing about 1.5-3.0 mJ/pulse tunable infrared light in the range of 1800−2300 cm-1. Resonant absorption of one or more IR photons leads to the dissociation of cation complexes. The fragment ions together with the undissociated parent ions were reaccelerated, and mass analyzed by the second collinear TOFMS. The infrared photodissociation spectrum was obtained by monitoring the yield of the fragment ion (normalizes to the parent ion signal) as a function of the dissociation IR laser wavelength. All of the spectra were recorded by scanning the dissociation laser in steps of 2 cm-1 and averaging over 500 laser shots at each step. Quantum chemical calculations were performed to optimize the equilibrium geometries, simulate the vibrational frequencies, and analyze the chemical bonding properties of the cation complexes. Unbiased global-minimum searches were carried out for the B3(CO)4+ and B3(CO)5+ cations using the stochastic surface walking method (SSW)33 at the PBE level.34 Low-lying structures were then re-optimized using the hybrid B3LYP functional35,36 with the aug-cc-pVTZ basis sets37 using the GAUSSIAN 09 program.38 The harmonic vibrational frequencies were simulated with analytic second derivatives. Chemical bonding analyses were performed by the adaptive natural density partitioning (AdNDP) method,39 which achieves seamless description 4


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of systems featuring both localized and delocalized bonding without invoking the concept of resonance. The AdNDP analyses were performed using the Multiwfn program.40 Results and discussion The mass spectra in the m/z range of 60-210 from laser evaporation of 10B-enriched target (a) and 10B-depleted target (b) in expansions of helium gas seeded with 5% CO are shown in Figure 1. The most intense peaks in the mass spectra correspond to B(CO)3+ (m/z=94 for 10B and m/z=95 for 11B) and B2(CO)4+ (m/z=122 for 10B and m/z=124 for 11B). An intense peak at m/z=196 shows no shift upon boron isotopic substitution is due to the Fe(CO)5+ ion, which is most likely formed in the ion source that is made of stainless steel. In addition, a series of mass peaks at m/z=142, 170 and 198 are observed in the spectrum with the 10B-enriched target. These peaks shift to m/z =145, 173 and 201 when the 10B-depleted target is used. These peaks can thus be assigned to the B3(CO)n+ (n= 4-6) cluster cations. Both the B3(CO)4+ and B3(CO)5+ cation complexes are found to be able to dissociate via the loss of one neutral CO ligand when the IR laser is on resonance with the fundamental CO stretching vibrations of the cation complexes. Both cation complexes fragment only under focused IR laser irradiation with low dissociation efficiency, suggesting that multiphoton absorption is necessary for photodissociation. In contrast, the B3(CO)6+ cation complex dissociates quite efficiently. It can loss two carbonyl ligands, suggesting that the sixth carbonyl ligand is weakly bound. The infrared photodissociation spectra of B3(CO)n+ (n= 4 -6) in the carbonyl stretching vibrational frequency region are shown in Figure 2. The spectra (a), (c) and (e) are from the 10

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B3(CO)n+ cation complexes, while the spectra (b) and (d) are due to the

B3(CO)4+ and 10B3(CO)5+ cation complexes. The band positions are listed in Table 1. 5



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The infrared spectrum of 10B3(CO)4+ exhibits two partially resolved bands at 2182 and 2202 cm-1. No obvious shifts are observed in the spectrum of

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B3(CO)4+, suggesting that the

observed infrared bands are pure carbonyl stretching vibrations with negligible involvement of boron. Three bands centered at 2190, 2210 and 2232 cm-1 are observed in the spectrum of the 10

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B3(CO)5+ cation. These bands shifted to 2188, 2208 and 2230 cm-1 when 10B is replaced by

B. The infrared spectrum of 11B3(CO)6+ is essentially the same as that of 11B3(CO)5+, indicating

that the B3(CO)6+ cation is a weakly bound CO-tagged complex involving a B3(CO)5+ ion core. Quantum chemical calculations have been performed to determine the geometric structures of the observed cation complexes. Considering that the B3(CO)4+ and B3(CO)5+ cations may have many stable structural isomers, we have utilized the recently developed global optimization method, namely, the stochastic surface walking method to search for the most stable isomers with the large-scale DFT calculations at the PBE level. The search starts from a few guessed structures and stops when no further stable structure appears after extensive runs. For each ion, four stable isomers lying within 30 kcal/mol in energy were found, as shown in Figure 3. The relative energies after zero-point energy correction as well as the CO dissociation energies calculated at the B3LYP/aug-cc-pVTZ level are shown in Table 2. The global minimum structure of the B3(CO)4+ cation has a D2d symmetry involving a linear B-B-B skeleton with the two terminal boron centers each coordinated by two carbonyl ligands (Figure 3, structure I). The second lowest-lying isomer involves a cyclic B3 ring with C2v symmetry. One boron center is coordinated by two carbonyl ligands, while the other two boron centers each coordinated by one CO ligand (Structure II). This isomer was predicted to be 8.8 kcal/mol higher in energy than the global minimum structure at the B3LYP/aug-cc-pVTZ level 6


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of theory. The third isomer is a very weakly bound complex involving a cyclic B3(CO)3+ core ion. The fourth CO is weakly tagged to the core ion with a very long OC-B distance (3.563Å). The dissociation energy of this CO ligand was predicted to be only 1.6 kcal/mol. The last isomer involves a four-membered B3C ring with a bridged carbonyl ligand. The third and fourth isomers were predicted to lie 23.6 and 25.8 kcal/mol, respectively above the global minimum structure. The simulated infrared spectra of these four isomers are compared to the experimental spectrum in Figure 4. The spectrum of the lowest energy isomer provides the best match to the experimental spectrum, confirming that the experimentally observed B3(CO)4+ cation possesses the lowest energy D2d structure. The experimentally observed 2182 and 2202 cm-1 bands are attributed to the antisymmetric and symmetric CO stretching modes which were predicted at 2196 and 2234 cm-1 at the B3LYP level. The B-CO bond dissociation energy was calculated to be 58.8 kcal/mol, significantly larger than the IR photon energy. This is in accord with the very low dissociation efficiency (about 1 % at 2202 cm-1 with an IR laser energy of 2.2 mJ/pulse). The bands are quite broad, which is characteristic of multiphoton process due to power broadening. The most stable structure of the B3(CO)5+ cation complex also has a B-B-B chain skeleton with planar C2v symmetry (Figure 3, structure V). The two terminal boron centers each are coordinated by two carbonyl ligands while the central boron atom is coordinated only by one CO ligand. The second isomer (structure VI) involves a cyclic B3 ring with Cs symmetry, which can be regarded as being formed via adding a bridged carbonyl ligand to the second structure (II) of the B3(CO)4+ cation complex. The third isomer (structure VII) also involves a B3 ring with all of the carbonyl ligands terminally bound. This structure can be regarded as being 7



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formed via adding a terminal carbonyl ligand to one of the mono-coordinated boron center of the second structure (II) of the B3(CO)4+ cation complex. The fourth isomer is a weakly bound CO-tagged complex involving a cyclic B3(CO)4+ cation core (structure II). The last three isomers were predicted to be 22.7, 27.8 and 30.2 kcal/mol higher in energy than the global minimum structure at the B3LYP level. The simulated infrared spectra of these isomers together with the experimental spectrum are shown in Figure 5. Apparently, only the simulated spectrum of the most stable structure matches the experimental one. The experimentally observed bands at 2188 and 2208 cm-1 can roughly be assigned to the antisymmetric stretching vibrations of the four CO ligands coordinated on the terminal boron centers, while the band at 2230 cm-1 is largely due to the stretching vibration of the carbonyl ligand of the central boron atom. The other two carbonyl stretching modes were predicted to have very low IR intensities (Table 2) and were not observed experimentally. The three experimentally observed modes were calculated to show very small boron isotope shifts (12 cm-1), in agreement with the experimental observations (Table 1). The B-CO bond dissociation energy of the most stable B3(CO)5+ structure is 22.4 kcal/mol, lower than that of B3(CO)4+. Experimentally, the dissociation efficiency of B3(CO)5+ (about 5%) is about five-fold higher than that of B3(CO)4+. The quite good agreement between the observed infrared photodissociation spectra and the simulated spectra provides considerable credence for assigning the observed cation complexes to the most stable D2d structure for B3(CO)4+ and the C2v structure for B3(CO)5+. The bonding interactions between carbon monoxide and the metal centers are well-known, which involves synergic  donation from the carbon lone-pair 5 orbital to the metal and  back-donation of electron density from the metal center into the 2* antibonding orbitals of CO. The π back8


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donation tends to weaken the CO bond causing a red-shift of the CO stretching frequency with respect to free CO, while the σ donation has a much weaker impact on the carbonyl stretching frequency than π back-donation.41 When CO is coordinated to a positively charged metal center, the carbonyl complex exhibits a blue-shifted CO stretching frequency,2-4 which has little metal to CO  backdonation and the bonding interactions come mainly from CO to metal  donation.42,43 The carbonyl stretching frequencies measured for the B3(CO)4+ and B3(CO)5+ cation complexes in the present study are blue-shifted with respect to that of free CO (2143 cm1

), implying that the B-CO interactions come mainly from CO to boron  donation with little

boron to CO  backdonation. The isolobal analogy concept has been applied to understand the structure and bonding of boron carbonyl complexes and to predict new boron carbonyl complexes analogies to stable hydrocarbons.23,29,30 An isolobal relationship between BCO and CH groups has been suggested.23 However, this isolobal relationship cannot apply to higher carbonyl complexes, such as B(CO)2, B(CO)3+ as well as the presently reported B3(CO)4+ and B3(CO)5+ cation complexes. According to the above bonding analyses, the carbonyl ligands in these boron carbonyl cation complexes serve as a two-electron donor. Thus, the carbonyl ligand can be regarded as “isolobal” with the H- anion in electron counting. Following this isolobal relationship and the B-/C analogy, the boron mono- and dicarbonyl complexes BCO and B(CO)2 are isoelectronic with the methylidyne CH and methylene anion CH2-, respectively, while the B(CO)3+ cation is isolobal to the methyl anion CH3-. The B3(CO)4+ cation complex can be regarded as isolobal to the allene molecule (H2CCCH2), and the B3(CO)5+ cation complex is the counterpart of the allyl anion C3H5-. These isolobal relationships are further supported by 9



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chemical bonding analyses using the adaptive natural density partitioning (AdNDP) method. As shown in Figure 6, the B3(CO)4+ cation involves two 2c-2e σ bonds between the skeletal boron atoms, four 2c-2e σ bonds between boron and carbonyl ligand, and two 6c-2e delocalized π bonds. Correspondingly, the allene molecule possesses two 2c-2e C-C σ bonds, four 2c-2e CH σ bonds and two 2c-2e C-C π bonds. The only difference lies in the fact that the two C-C π bonds in allene are localized, whereas the two B-B π bonds of B3(CO)4+ are delocalized that comprise significant B2 CO π back bonding. The two B-B bonds were calculated to have a bond distance of 1.573 Å, slightly longer than the value for a standard B=B double bond (1.56 Å).44 Similar B3 chain connected by two B-B double bonds is recently found in the Triaminotriborane molecule, whose B-B bond length is reported to be 1.56 Å.45 The bonding pattern of B3(CO)5+ involves two 2c-2e sigma bonds between skeletal boron atoms, five 2c-2e bonds between boron and the carbonyl ligand, and two delocalized π bonds. This chemical bonding pattern is very similar to that of the allyl anion, which contains two C-C σ bonds, five C-H σ bonds and two 3c-2e C-C π bonds (See Figure 6). There are some discussions on the nature of the 3c-4e resonance of delocalized C-C π bonds in allyl anion.46-48 We choose the model with two 3c-2e bonds. The structures with an open-chain boron skeleton for the B3(CO)4+ and B3(CO)5+ cation complexes are starkly different from that of B3(CO)3+, which was determined to have D3h symmetry involving a cyclic B3+ ring.26 The isolobal analogy concept provide an intuitive rationale for understanding the relative stability and structural difference of these B3(CO)n+ (n=3-5) cation complexes. The B3(CO)3+ cation complex is isolobal to the cyclopropylene cation, which is the most stable structure for species with C3H3+ stoichiometry. For the B3(CO)4+ 10


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cation complex, the structures with a cyclic B3+ moiety are higher in energy than the global minimum chain structure. The lowest energy structure with a cyclic B3+ moiety of B3(CO)4+ (Structure II) is isolobal to cyclopropylene, which is well known to be less stable than the allene isomer due to strong ring strain.49,50 Conclusions The boron carbonyl cation complexes B3(CO)n+ (n=4~6) have been prepared in the gas phase via a pulsed laser vaporization supersonic expansion ion source. Their vibrational spectra in the carbonyl stretching frequency region are measured via mass-selected infrared photodissociation spectroscopy. Theoretical calculations are performed to find the minimum energy structures and to predict their infrared spectra to interpret the experimental data. The results reveal that B3(CO)4+ has a linear boron skeleton with D2d symmetry. The B3(CO)5+ cation is determined to have a chain boron framework with C2v symmetry. The B3(CO)6+ cation is a weakly bound CO-tagged complex involving a B3(CO)5+ ion core. According to bonding analyses, the carbonyl ligand can be regarded as “isolobal” with the H- anion in electron counting. Following this isolobal relationship and the B-/C analogy, the B3(CO)4+ cation complex is isolobal to the allene molecule, and the B3(CO)5+ cation complex is the counterpart of the allyl anion. This isolobal relationship is useful in bridging the boron carbonyl complexes and the hydrocarbon molecules. Acknowledgements We gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant No. 21688102). References

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(26) Jin, J. Y.; Wang, G. J.; Zhou, M. F.; Andrada, D. M.; Hermann, M.; Frenking, G. The B3(NN)3+ and B3(CO)3+ Complexes Featuring the Smallest -Aromatic Species B3+. Angew. Chem. Int. Ed. 2016, 55, 2078-2082. (27) Jin, J. Y.; Wang, G. J.; Zhou, M. F. Infrared Photodisssociation Spectroscopy of Boron Carbonyl Cation Complexes. Chin. J. Chem. Phys. 2016, 29, 47-52. (28) Jin, J.W.; Li, W.; Wu, X.; Zhou, M. F. Double C–H Bond Activation of Acetylene by Atomic Boron in Forming Aromatic Cyclic-HBC2BH in Solid Neon. Chem. Sci. 2017, 8, 4443–4449. (29) Wu, H. S.; Jiao, H. J.; Wang, Z. X.; Schleyer, P. v. R. Monocyclic Boron Carbonyls: Novel Aromatic Compounds. J. Am. Chem. Soc. 2003, 125, 4428-4429. (30) Wu, H. S.; Qin, X. F.; Xu, X. H.; Jiao, H. J.; Schleyer, P. v. R. Structures and Energies of Isolobal (BCO)n and (CH)n Cages. J. Am. Chem. Soc. 2005, 127, 2334-2338. (31) Wang, G. J.; Chi, C. X.; Xing, X. P.; Ding, C. F.; Zhou, M. F. A Collinear Tandem Timeof-Flight Mass Spectrometer for Infrared Photodissociation Spectroscopy of Mass-Selected Ions. Sci. China Chem. 2014, 57, 172-177. (32) Wang, G. J.; Chi, C. X.; Cui, J. M.; Xing, X. P.; Zhou, M. F. Infrared Photodissociation Spectroscopy of Mononuclear Iron Carbonyl Anions. J. Phys. Chem. A 2012, 116, 24842489. (33) Shang, C.; Liu, Z. P. Stochastic Surface Walking Method for Structure Prediction and Pathway Searching. J. Chem. Theory Comput. 2013, 9, 1838-1845. (34) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865-3868; 15



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(35) Becke, A. D. Density-Functional Thermochemistry. 3. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648-5652. (36) Lee, C. T.; Yang, W. T.; Parr, R. G. Development of the Colle-Salvetti CorrelationEnergy Formula into a Functional of the Electron-Density. Phys. Rev. B 1988, 37, 785-789. (37) Woon, D. E.; Dunning Jr, T. H. Gaussian-Basis Sets for Use in Correlated Molecular Calculations .4. Calculation of Static Electrical Response Properties. J. Chem. Phys. 1994, 100, 2975-2988. (38) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; et al. Gaussian 09, Revision D.01, Gaussian, Inc., Wallingford CT, 2013. (39) Zubarev, D. Y.; Boldyrev, A. I. Developing Paradigms of Chemical Bonding: Adaptive Natural Density Partitioning. Phys. Chem. Chem. Phys. 2008, 10, 5207-5217. (40) Lu, T.; Chen, F. W. Multiwfn: A Multifunctional Wavefunction Analyzer. J. Comput. Chem. 2012, 33, 580-592. (41) Chen, M. H; Zhang, Q. N.; Zhou, M. F.; Andrada, D. M.; Frenking, G. Carbon Monoxide Bonding With BeO and BeCO3: Surprisingly High CO Stretching Frequency of OCBeCO3. Angew. Chem. Int. Ed. 2015, 54, 124-128. (42) Lupinetti, A. J.; Jonas, V.; Thiel, W.; Strauss, S. H.; Frenking, G. Trends in Molecular Geometries and Bond Strengths of the Homoleptic d10 Metal Carbonyl Cations [M(CO)n]x+ (Mx+=Cu+, Ag+, Au+, Zn2+, Cd2+, Hg2+; n =1–6): A Theoretical Study. Chem. Eur. J. 1999, 5, 2573-2583.

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(43) Diefenbach, A.; Bickelhaupt, F. M.; Frenking, G. The Nature of the Transition Metal−Carbonyl Bond and the Question about the Valence Orbitals of Transition Metals. A Bond-Energy Decomposition Analysis of TM(CO)6q (TMq = Hf2-, Ta-, W, Re+, Os2+, Ir3+)† J. Am. Chem. Soc. 2000, 122, 6449-6458. (44) Pyykkö, P.; Atsumi, M. Molecular Double-Bond Covalent Radii for Elements Li-E112. Chem. Eur. J. 2009, 15, 12770-12779. (45) Morisako, S.; Shang, R.; Yamamoto, Y.; Matsui, H.; Nakano, M. Triaminotriborane(3): A Homocatenated Boron Chain Connected by B-B Multiple Bonds. Angew. Chem. Int. Ed. 2017, 56, 15234-15240. (46) Gobbi, A.; Frenking, G. Resonance Stabilization in Allyl Cation, Radical, and Anion. J. Am. Chem. Soc. 1994, 116, 9275-9286. (47) Mo, Y. R.; Lin, Z. Y.; Wu, W.; Zhang, Q. N. Delocalization in Allyl Cation, Radical, and Anion. J. Phys. Chem. 1996, 100, 6469-6474. (48) Linares, M.; Humbel, S.; Braïda, B. The Nature of Resonance in Allyl Ions and Radical. J. Phys. Chem. A 2008, 112, 13249-13255. (49) Wiberg, K. B. The Concept of Strain in Organic Chemistry. Angew. Chem. Int. Ed. 1986, 25, 312-322. (50) Yoshimhe, M.; Pacansky, J.; Honjout, N. Ab Initio Studies of the C3H4 Surface. 3. Thermal Isomerization. J. Am. Chem. Soc. 1989, 111, 4198-4209.

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Page 18 of 25

Table 1. Experimental and Calculated (B3LYP/aug-cc-pVTZ, unscaled) Infrared Frequencies and Isotope Shifts (Δ, cm-1) of [B3(CO)4]+ and [B3(CO)5]+. Expt.

Calc.

[11B3(CO)4]+

[10B3(CO)4]+

Δ

[11B3(CO)4]+

[10B3(CO)4]+

Δ

2182

2182

0

2196

2198

+2

2202

2202

0

2234

2234

0

[11B3(CO)5]+

[10B3(CO)5]+

Δ

[11B3(CO)5]+

[10B3(CO)5]+

Δ

2188

2190

+2

2165

2167

+2

2208

2210

+2

2182

2183

+1

2230

2232

+2

2207

2208

+1

Table 2. The Calculated Relative Energy (ΔE), CO Dissociation Energy (D0) and CO Stretching Vibrational Frequencies (cm-1, unscaled) of the [11B3(CO)n]+ (n = 4-6) Cation Complexes.a

11

11

11

B3(CO)4+

B3(CO)5+

B3(CO)6+

Struct.

ΔE

D0

Calc.

I

0.0

58.8

2196(10852), 2234(1552), 2261(0)

II

8.8

16.3

2205(1952), 2208(833), 2225(979), 2261(54)

III

23.6

1.58

2238(1394), 2239(497), 2244(928), 2273(11)

IV

25.8

57.5

1846(578), 2215(1434), 2232(802), 2266(441)

V

0.0

22.4

2153(39), 2165(877), 2182(2702), 2207(1502), 2257(2)

VI

22.7

25.7

1868(710), 2190(1581), 2216(490), 2234(877), 2273(351)

VII

27.8

3.4

2144(1640), 2170(27), 2182(1750), 2205(1142), 2250(60)

VIII

30.2

1.3

2204(2024), 2206(875), 2224(954), 2237(41), 2261(50)

1.3

2150(50), 2164(990), 2181(2610), 2210(1389), 2236(68), 2257(11)

a

The calculated intensities are listed in parentheses in km/mol. Energies are in kcal/mol. The structures are shown in Figure 3.

18


Page 19 of 25

11

+

B(CO)3

Fe(CO)5

2 11

11

+

B3(CO)5

1

+

+

B3(CO)4

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


11

+

B3(CO)6

(b)

10

+

B2(CO)4

10 10

+

B3(CO)5

+

B3(CO)4

10

+

B3(CO)6

(a)

0 60

80

100

120

140

160

180

200

m/z

Figure 1. Mass spectra of the boron carbonyl cation complexes in the m/z range of 60-210 from pulsed laser evaporation of (a)

10

B-enriched and (b)

expansion of helium seeded with 5% CO.

19


10

B-depleted boron targets in


20 (e)

15

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 25

(d)

10 (c)

5

(b) (a)

0 1800

1900

2000

2100

2200

2300

-1

Wavenumber (cm )

Figure 2. Infrared photodissociation spectra of the B3(CO)n+ (n= 4-6) cation complexes in the carbonyl stretching frequency region. (a) 11

11

B3(CO)4+, (b)

B3(CO)5+, (d) 10B3(CO)5+, and (e) 11B3(CO)6+.

20


10

B3(CO)4+, (c)

Page 21 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. Optimized structures of the B3(CO)n+ (n=4, 5) cation complexes. The

symmetry and electronic state are shown in the figure.

21



10

(e) 8

(d)

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 25

6

(c)

4

(b)

2

(a)

0 1800

1900

2000

2100

2200

2300

-1

Wavenumber (cm )

Figure 4. The experimental (a) and simulated (b-e) vibrational spectra of the 11B3(CO)4+ cation in the carbonyl stretching frequency region. The simulated spectra (b)-(e) were obtained from scaled harmonic frequencies and intensities for the four lowest-lying structural isomers (structure I-IV in Figure 3) calculated at the B3LYP/aug-cc-pVTZ level.

22


Page 23 of 25

(e)

4

(d)

Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(c)

2

(b)

(a)

0 1800

1900

2000

2100

2200

2300

-1

Wavenumber (cm )

Figure 5. The experimental (a) and simulated (b-e) vibrational spectra of the 11B3(CO)5+ cation in the carbonyl stretching frequency region. The simulated spectra (b)-(e) were obtained from scaled harmonic frequencies and intensities for the four lowest-lying structural isomers (structure V-VIII in Figure 3) calculated at the B3LYP/aug-cc-pVTZ level.

23



Figure 6. AdNDP bonding orbitals of the B3(CO)4+ and B3(CO)5+ cation complexes, and related C3H4 and C3H5- species. ON stands for occupation number. The orbitals for CO ligands are not shown in the figure.

24


Page 24 of 25

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TOC graphic

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Boron Carbonyl Analogues of Hydrocarbons: An Infrared

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