Chinese Journal of Chemical Physics  2016, Vol. 29 Issue (1): 47-52

The article information

JinJia-ye, WangGuan-jun, ZhouMing-fei
Infrared Photodisssociation Spectroscopy of Boron Carbonyl Cation Complexes
Chinese Journal of Chemical Physics, 2016, 29(1): 47-52
http://dx.doi.org/10.1063/1674-0068/29/cjcp1512238

Article history

Received: November 25, 2015
Accepted: December 8, 2015
Infrared Photodisssociation Spectroscopy of Boron Carbonyl Cation Complexes
Jia-ye Jin, Guan-jun Wang, Ming-fei Zhou     
Dated: Received on November 25, 2015; Accepted on December 8, 2015
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
Author: Ming-fei Zhou,E-mail: mfzhou@fudan.edu.cn
Abstract: The boron carbonyl cation complexes B(CO)3+, B(CO)4+ and B2(CO)4+ are studied by infrared photodissociation spectroscopy and theoretical calculations. The B(CO)4+ ions are characterized to be very weakly bound complexes involving a B(CO)3+ core ion, which is predicted to have a planar DD3h structure with the central boron retaining the most favorable 8-electron configuration. The B2(CO)4+ cation is determined to have a planar D2h structure involving a B-B one and half bond. The analysis of the B-CO interactions with the EDA-NOCV method indicates that the OC→B σ donation is stronger than the B→CO π back donation in both ions.
Key words: Boron carbonyl    Donor-acceptor bonding    Infrared photodissociation spectroscopy    Theoretical calculations    
Ⅰ. INTRODUCTION

Carbon monoxide is one of the most important ligand in inorganic and organometallic chemistry [1]. It can bind to a host of neutral and charged transition metal centers in forming diverse metal carbonyl complexes,in which carbon monoxide serves either as a two-electron donor in an end-on coordinated fashion or as a four-electron or even six-electron donor in the bridge bonded modes [2, 3, 4, 5, 6, 7, 8, 9, 10, 11]. Carbon monoxide can also coordinate with some main-group elements in forming main group carbonyl complexes [12, 13]. Homoleptic mono- and dicarbonyl complexes of main group elements with end-on bonded carbonyl ligands have been prepared and spectroscopically characterized either in low-temperature noble gas matrices or in the gas phase [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]. These carbonyl complexes are usually not stable at ambient conditions with the exception of [C(CO)2] and [N(CO)2]+ which have been crystallographically characterized [36, 37, 38].

In the case of boron,the electron deficient boron species are able to coordinate one CO ligand in forming the closed-shell carbonyl borane H3BCO and related derivatives,which are well-known stable carbonyl compounds [39, 40, 41, 42, 43]. Boron carbonyl species such as BCO,B(CO)2,B(CO)2-,OCBBCO,BBCO and B4(CO)2 were identified as products from the reactions of thermal- or laser-evaporated boron atoms with CO in solid argon [44, 45, 46, 47, 48, 49, 50]. The OCBBCO molecule was characterized to be a boron-boron triple bonded species [48]. Both BBCO and B4(CO)2 are $\sigma-\pi$ diradicals [49, 50]. Bonding analysis suggests that the linear closed-shell B(CO)2- anion should be considered as a donor-acceptor bonding complex rather than a cumulene O=C=B(-)=C=O molecule with electron sharing bonding [47]. Very 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 reported [51]. The chemical behavior shows typical features of carbonyl complexes which are known from transition metal carbonyls. Boron carbonyl cation complexes are not known so far. Here we report a combined infrared photodissociation spectroscopy and theoretical study on boron carbonyl cation complexes B(CO)$_3^ + $ and B2(CO)$_4^ + $ in the gas phase.

Ⅱ. EXPERIMENTAL AND THEORETICAL METHODS

The boron carbonyl cation complexes are generated in the gas phase using a pulsed laser vaporization/supersonic expansion ion source as described previously [52, 53]. Bulk targets compressed from isotopically-enriched 10B and 11B powders were used. The ions are produced from the laser vaporization process in expansions of helium seeded with 2%-5% CO using a pulsed valve (General Valve,Series 9) at 0.5-1.0 MPa backing pressure. After free expansion and cooling,the cations are skimmed into a second chamber where they are pulse-extracted into a Wiley-McLaren type time-of-flight mass spectrometer. The cations of interest are mass selected and decelerated into the extraction region of a second collinear time-of-flight mass spectrometer,where they are dissociated by a tunable IR laser. The tunable IR laser used is generated by a KTP/KTA//AgGaSe2 optical parametric oscillator/amplifier system (OPO/OPA,Laser Vision) pumped by a Continuum Surelite EX Nd:YAG laser,producing about 1.0-2.5 mJ/pulse in the range of 1800-2400 cm-1. The ion density is too low for infrared absorption spectroscopy,thus,the infrared photodissociation spectroscopy is employed to record the vibrational spectra. Resonant absorption leads to fragmentation of the ion complex. The infrared photodissociation spectrum is obtained by monitoring the yield of the fragment ion as a function of the dissociation IR laser wavelength and normalizing to parent ion signal.

The geometry optimizations have been carried out without symmetry constraints at the B3LYP level using the aug-cc-pVTZ basis sets [54, 55, 56]. The harmonic vibrational frequencies were calculated with analytic second derivatives. These calculations were performed using the Gaussian 09 program [57]. The gradient corrected BP86 functional in conjunction with uncontracted Slater-type orbitals (STOs) as basis functions was used for the bonding analyses [58, 59, 60]. The latter basis sets for all elements have triple-$\xi$ quality augmented by one set of polarization functions (ADF-basis set TZP). An auxiliary set of s,p,d,f,and g STOs was used to fit the molecular densities and to represent the Coulomb and exchange potentials accurately in each SCF cycle. The BP86/TZP calculations were performed using the B3LYP/aug-cc-pVTZ optimized geometries with the program package ADF2014.10 [61].

Ⅲ. RESULTS AND DISCUSSION

The mass spectrum of boron carbonyl cation complexes in the $m/z$ range of 60-150 from the laser evaporation of a 10B-enriched target in expansions of helium gas seeded with 5% CO is shown in Fig. 1(a). Although the mass spectra depend strongly on the parameters of the ion source such as vaporization laser power,He and CO stagnation pressures and timing,the peaks corresponding to 10B(CO)$_3^ + $ ($m/z$=94) and 10B2(CO)$_4^ + $ ($m/z$=132) are always the most intense peaks,suggesting that these cations are formed preferentially with high stability. The mass spectrum from the experiments with the 11B-enriched target is shown in Fig. 1(b). The most intense peaks shifted to $m/z$=95 and 134,corresponding to the 11B(CO)$_3^ + $ and 11B2(CO)$_4^ + $ ions,respectively.

Fig. 1 Mass spectra of the boron carbonyl cation complexes formed by pulsed laser vaporization of a target in an expansion of helium doped with carbon monoxide. (a) 10B and (b) 11B.
Fig. 2 The experimental infrared photodissociation spectra of the (a) 10B(CO)4+ and (b) 11B(CO)4+ cation complexes in the CO stretching frequency region.

The B(CO)$_3^ + $ cations are mass-selected and subjected to infrared photodissociation. It is found that the B(CO)$_3^ + $ cations dissociate via losing a CO ligand when excited with infrared light in the 2140-2150 cm-1 frequency region but the dissociation efficiency (less than 0.5%) is too low to achieve an effective spectrum. This suggests that the B(CO)$_3^ + $ cations are very stable species with quite high CO binding energy as expected,as it satisfies the octet rule. In contrast to B(CO)$_3^ + $,the B(CO)$_4^ + $ cation complexes are able to dissociate via loss of a CO ligand with very high efficiency ($>$50%),indicating that B(CO)$_4^ + $ is a very weakly bound complex. This confirms our expectation that B(CO)$_3^ + $ is a fully coordinated ion and the fourth CO in B(CO)$_4^ + $ is a weakly bound external carbonyl ligand. Therefore,the B(CO)$_4^ + $ cation can be regarded as a CO "tagged" cation complex involving a B(CO)$_3^ + $ core ion. The infrared photodissociation spectrum of B(CO)$_4^ + $ represents the spectrum of the B(CO)$_3^ + $ core ion that is weakly perturbed by the tagged CO ligand. The tagging effect is expected to change the position of the B(CO)$_3^ + $ band only slightly as discussed previously [62, 63, 64]. The infrared photodissociation spectra of 11B(CO)$_4^ + $ and 10B(CO)$_4^ + $ in the C-O stretching frequency region are shown in Fig. 2. The spectrum of 10B(CO)$_4^ + $ (Fig. 2(a)) exhibits a very strong band centered at 2145 cm-1 along with a weak band at 2178 cm-1. The 2145 cm-1 band is just 2 cm-1 blue-shifted from the frequency of gas phase carbon monoxide (2143 cm-1). This band can be attributed to the antisymmetric CO stretching vibrations of the 10B(CO)$_3^ + $ core ion. The much weak band at 2178 cm-1 is assigned to the CO stretching vibration of the weakly tagged CO ligand,consistent with previous observations for other weakly bound metal ion carbonyls [64, 65, 66]. The same bands were also observed in the spectrum of 11B(CO)$_4^ + $ as shown in Fig. 2(b). The band positions are essentially the same indicating that the corresponding vibrational modes are pure CO stretching vibrations with negligible boron involvement. Besides 2145 and 2178 cm-1 bands,additional weak bands at 2214 and 2259 cm-1 were observed in the spectrum of 11B(CO)$_4^ + $. Both bands are located in the frequency range suitable for the symmetric CO stretching vibrations. We tentatively assign the 2259 cm-1 band to the symmetric stretching vibration of the 11B(CO)$_3^ + $ core ion and the 2214 cm-1 band to a combination or an overtone level that is in Fermi resonance with the symmetric CO stretching fundamental. When 11B is substituted by 10B,the two levels are no long close enough to show Fermi resonance,therefore,both bands were not observed in the spectrum of 10B(CO)$_4^ + $.

The B2(CO)$_4^ + $ cation is able to dissociate via loss of a CO ligand under focused IR laser irradiation. The parent ions can be depleted by about 5% at the laser pulse energy of 1.4 mJ/pulse at 2108 cm-1. The infrared photodissociation spectrum of 10B2(CO)$_4^ + $ is shown in Fig. 3. The spectrum exhibits two bands centered at 2108 and 2152 cm-1.

Fig. 3 Experimental and simulated IR spectra of 10B2(CO)$_4^ + $ in the CO stretching frequency region. The experimental spectrum (a) was measured by monitoring the CO fragmentation channel leading to the formation of B2(CO)$_3^ + $. The simulated spectrum (b) was obtained from scaled (0.971) harmonic frequencies and intensities calculated at the B3LYP/aug-cc-pVTZ level.

We carried out high-level calculations in order to validate the identify of the cations and to analyze their electronic structures. The theoretically predicted geometries of B(CO)$_3^ + $ and B(CO)$_4^ + $ are shown in Fig. 4. The B(CO)$_3^ + $ cation has a $^1$A$_{1}'$ ground state with planar D$_{3\textrm{h}}$ symmetry. The calculations indicate that the CO bond distances in B(CO)$_3^ + $ (1.127 Å) is slightly shorter than that of free CO calculated at the same level (1.128 Å). As shown in Fig. 5,the highest doubly occupied molecular orbital (HOMO,a$_2"$) is primarily a central B2p orbital,which comprises significant B2p to CO 2$\pi^*$ back bonding. Therefore,the $^1$A$_{1}'$ ground state B(CO)$_3^ + $ cation correlates to an electronic excited singlet state B+ with the associated valence electronic configuration of 2s0,2p($\sigma$)0,2p($\pi$)2,2p($\pi'$)0. The 11B(CO)$_3^ + $ cation with D$_{3\textrm{h}}$ symmetry has only one IR active antisymmetric CO stretching mode calculated at 2223 cm-1,which is doubly degenerate. This mode for 10B(CO)$_3^ + $ is predicted at 2224 cm-1. The symmetric stretching mode is predicted at 2317 cm-1 which is IR inactive. Consistent with the experimental observations,the B(CO)$_4^ + $ cation was predicted to be a weakly bound complex as the predicted B-CO distance of the fourth CO is quite large with the geometry of the B(CO)$_3^ + $ core ion being essentially the same as the free cation. Due to symmetry reduction by CO coordination,the double degeneracy of the antisymmetric CO stretching mode of B(CO)$_3^ + $ is lifted,and the E mode splits into two distinct modes. Calculations at the B3LYP level show very small mode split of 6 cm-1,which cannot be well-resolved experimentally.

Fig. 4 Optimized structures of the B(CO)$_3^ + $, B(CO)$_4^ + $ and B2(CO)$_4^ + $ cation complexes.
Fig. 5 Molecular orbital pictures of the highest doubly occupied (HOMO) of B(CO)$_3^ + $ and the singly occupied (SOMO) and highest doubly occupied orbitals (HOMO and HOMO-1) of B2(CO)$_4^ + $.
Table Ⅰ Experimental and simulated IR spectra of $^{10}$B$_2$(CO)$_4^ + $ in the CO stretching frequency region. The experimental spectrum (a) was measured by monitoring the CO fragmentation channel leading to the formation of B2(CO)$_3^ + $. The simulated spectrum (b) was obtained from scaled (0.971) harmonic frequencies and intensities calculated at the B3LYP/aug-cc-pVTZ level.

The B2(CO)$_4^ + $ cation was predicted to have a doublet ground state (2B$_{2\textrm{g}}$) with planar D$_{2\textrm{h}}$ symmetry. The B-B bond length is predicted to be 1.638 Å,which is intermediate between typical B-B single bond and double bond [67]. The cation with planar D$_{2\textrm{h}}$ structure has four CO stretching modes with only two of them are IR active,which are calculated to be 2175 and 2208 cm-1 at the B3LYP/aug-cc-pVTZ level. The molecular orbital pictures shown in Fig. 5 clearly indicate that the singly occupied (SOMO) b$_{2\textrm{g}}$ MO is a B-B $\pi$ antibonding orbital. The highest doubly occupied b$_{3\textrm{u}}$ orbital (HOMO) is a B-B $\pi$ bonding orbital. Both the b$_{2\textrm{g}}$ and b$_{3\textrm{u}}$ orbitals comprise substantial B2+ to CO $\pi^*$ back-donation. Both orbitals are highly delocalized involving two B and four C centers. The doubly occupied a$_\textrm{g}$ molecular orbital (HOMO-1) is a B-B $\sigma$ bonding orbital. The ground state B2(CO)$_4^ + $ cation complex can thus be viewed as the interaction of a 2$\Pi_\textrm{g}$ excited-state B2+ and four CO's,which involves one B-B $\sigma$ bond and a half B-B $\pi$ bond.

The bond dissociation energies of B(CO)$_3^ + $,B(CO)$_4^ + $ and B2(CO)$_4^ + $ are calculated. At the B3LYP level,the BDE of B(CO)$_3^ + $ is 79.5 kcal/mol with respect to the dissociation limit B(CO)2+($^1\Delta_\textrm{g}$)+CO or 60.7 kcal/mol with respect to the ground state reactants: B(CO)2+($^3\Sigma_\textrm{g}$)+CO. The dissociation energy of the tagged CO in B(CO)$_4^ + $ is calculated to be only 2.9 kcal/mol. The binding energy of B2(CO)$_4^ + $ is 60.4 kcal/mol with respect to the dissociation limit B2(CO)$_3^ + $(2B1)+CO or 39.2 kcal/mol for the dissociation into the ground state B2(CO)$_3^ + $(2B2) and CO.

We analyze the nature of the donor-acceptor interactions in B(CO)$_3^ + $ and B2(CO)$_4^ + $ with the EDA (energy decomposition analysis) in conjunction with the NOCV (natural orbitals for chemical valence) method [68],which gives a detailed insight into the bonding situation. The numerical results of the OC-B interactions in B(CO)$_3^ + $ and B2(CO)$_4^ + $ at the BP86/TZP level are listed in table Ⅱ. The data show that both species have very similar orbital interaction energy ($\Delta {E_{{\rm{orb}}}}$). Further inspection of the orbital components of $\Delta {E_{{\rm{orb}}}}$ reveals that both species have very similar CO$\rightarrow$B $\sigma$ donation interaction,which is much stronger than the B$\rightarrow$CO $\pi$ back donation interactions. There is a large difference in the strength of the B$\rightarrow$CO $\pi$ back donation between the two complexes. The $\pi$ contribution of $\Delta {E_{{\rm{orb}}}}$($\pi_\perp$)}+ $\Delta E_{\textrm{orb}}$($\pi_\parallel$) in B2(CO)$_4^ + $ is much larger than that in B(CO)$_3^ + $. Figure 6 displays the deformation densities $\Delta\rho$($\sigma$) and $\Delta\rho$($\pi$) which are connected to the $\sigma$ donation and $\pi$ backdonation in B(CO)$_3^ + $ and B2(CO)$_4^ + $. The direction of the charge flow is indicated by the colors red$\rightarrow$blue. The shape of $\Delta\rho$($\sigma$) clearly indicates that the charge flow comes mainly from the lone-pair electrons at carbon to the boron atom. While the $\pi$ backdonation leads to charge accumulation mainly at the carbon atom of CO.

Table Ⅱ EDA-NOCV results of the chemical bonding in OC-B(CO)$_2^ + $ and OC-B2(CO)$_3^ + $ at BP86/TZP. Energy values are given in kcal/mol.
Fig. 6 Plot of deformation densities $\Delta \rho$ of the pairwise orbital interactions and the associated interaction energies $\Delta E_{\textrm{orb}}$ (in kcal/mol) between CO and B(CO)$_2^ + $ or B2(CO)$_3^ + $ in B(CO)$_3^ + $ and B$_2$(CO)$_4^ + $. The direction of charge flow is red$\rightarrow$blue.
Ⅳ. Conclusion

Boron carbonyl cation complexes are produced via a laser vaporization supersonic ion source in the gas phase. The cations of interest are each mass-selected and their infrared spectra are measured via infrared photodissociation spectroscopy in the carbonyl stretching frequency region. Density functional calculations have been performed and the calculated vibrational spectra are compared to the experimental data to identify the gas-phase structures of the ions. The B(CO)$_3^ + $ and B2(CO)$_4^ + $ cations are the most intense peaks in the mass spectrum. The B(CO)$_3^ + $ ion is too strongly bound to achieve an effective IR spectrum. In contrast,the B(CO)$_4^ + $ ion dissociates very efficiently under IR irradiation. It is characterized to be a very weakly bound complex involving a B(CO)$_3^ + $ core ion,which is predicted to have a planar D$_{3\textrm{h}}$ structure with the central boron retaining the most favorable 8-electron configuration. The B2(CO)$_4^ + $ cation is determined to have a planar D$_{2\textrm{h}}$ structure involving a B-B bond. Both the B(CO)$_3^ + $ and B2(CO)$_4^ + $ ions have slightly red- or blue-shifted CO stretching frequencies with respect to free CO. The analysis of the B-CO interactions with the EDA-NOCV method indicates that the OC$\rightarrow$ B $\sigma$ donation is stronger than the B$\rightarrow$CO $\pi$ back donation.

Ⅴ. Acknowledgments

The work was supported by the Ministry of Science and Technology of China (No.2013CB834603) and the National Natural Science Foundation of China (No.21173053 and No.21433005).

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