A Spinning Umbrella: Carbon Monoxide and Dinitrogen Bound MB12

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A Spinning Umbrella: Carbon Monoxide and Dinitrogen Bound MB - Clusters (M = Co, Rh, Ir) 12

Ranajit Saha, Susmita Kar, Sudip Pan, Gerardo MartínezGuajardo, Gabriel Merino, and Pratim Kumar Chattaraj J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 31 Mar 2017 Downloaded from http://pubs.acs.org on March 31, 2017

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The Journal of Physical Chemistry

A Spinning Umbrella: Carbon Monoxide and Dinitrogen bound MB12- Clusters (M = Co, Rh, Ir) Ranajit Saha,1 Susmita Kar,1 Sudip Pan,2* Gerardo Martínez-Guajardo,3 Gabriel Merino,2* and Pratim K. Chattaraj1*

1

Department of Chemistry and Center for Theoretical Studies, Indian Institute of Technology Kharagpur, Kharagpur - 721302, India

2

Departamento de Física Aplicada, Centro de Investigación y de Estudios Avanzados, Unidad

Mérida. km 6 Antigua carretera a Progreso. Apdo. Postal 73, Cordemex, 97310, Mérida, Yuc., México 3

Unidad Académica de Ciencias Químicas, Área de Ciencias de la Salud, Universidad Autónoma de Zacatecas, Km. 6 carretera ZacatecasGuadalajara s/n, Ejido La Escondida C. P. 98160, Zacatecas, Zac., México

*Corresponding

authors:

[email protected]

(SP);

[email protected]

(GM);

[email protected] (PKC)

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Abstract Strong binding of carbon monoxide (CO) and dinitrogen (N2) by MB12- (M = Co, Rh, Ir) clusters results in a spinning umbrella-like structure. For OCMB12- and NNMB12- complexes, the bond dissociation energy values range within 50.3-67.7 kcal/mol and 25.9-35.7 kcal/mol, respectively, with the maximum value obtained in Ir followed by that in Co and Rh analogues. COMB12complex is significantly less stable than the corresponding C-side bonded isomer. The associated dissociation processes for OCMB12- and NNMB12- into CO or N2 and MB12- are highly endergonic in nature at 298 K, implying their high thermochemical stability with respect to dissociation. In OCMB12- and NNMB12- complexes, the C-O and N-N bonds are found to be elongated by 0.022-0.035 Å along with a large red-shift in the corresponding stretching frequencies, highlighting the occurrence of bond activation therein towards further reactivity due to complexation. The obtained red-shift is explained by the dominance of L←M π-back-donation (L = CO, OC, NN) over L→M σ-donation. The binding of L enhances the energy barrier for the rotation of the inner B3 unit within the outer B9 ring by 0.4-1.8 kcal/mol, which can be explained by a reduction in the distance of the longest bond between inner B3 and outer B9 rings upon complexation. A good correlation is found between the change in rotational barrier relative to that in MB12- and the energy associated with the L→M σ-donation. Born-Oppenheimer molecular dynamics simulations further support that the M-L bonds in the studied systems are kinetically stable enough to retain the original forms during the internal rotation of inner B3 unit.


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Introduction Since the discovery of Mond process in 1890, in which nickel was extracted and purified from its ores via the formation of volatile Ni(CO)4,1,2 several transition metal carbonyl complexes (TMCs) have been synthesized.3 -5 TMCs have impact in several fields like 4

coordination chemistry, organometallic chemistry, bioinorganic chemistry, biological chemistry, etc.6 -8 In contrast, despite being isoelectronic with CO, N2 is quite inert to react because of its 7

very high ionization potential, negative electron affinity, and large energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). It makes the artificial capture of N2 in transition metal complexes a very exciting field of research as the dual play of electron-donation and back-donation induces dinitrogen activation towards further reactivity by weakening the N-N bond, which has many important applications in industry.9 - 13 However, N2 is a poor σ-donor as well as a poor π-acceptor, making the activation a 10

1112

difficult task. Both TMCs and dinitrogen complexes have been characterized by the shift in the C-O stretching frequency, ν(CO), and N-N stretching frequency, ν(NN), in their corresponding infrared (IR) spectra,14,15 respectively. Different carbonyl and dinitrogen complexes of transition metals of group 9 at the lower or negative oxidation states are known in the literature.16 - 23 171819

202122

On the other hand, B19- with a pentagonal fragment of six B atoms enclosed by a ring of thirteen B atoms was detected by Huang et al.24 This cluster was reported to mimic the Wankel motor, an internal combustion engine, used in automobiles, motorcycles and aircrafts.25,26 Further research reveals that this type of internal rotation is found not only in B19- cluster, but also in other boron clusters like B11-, B13+, B182-, B20-, B40 etc.27

- 35 28293031

323334

Recently, the groups of Boldyrev

and Wang36 detected MB12- (M = Co, Rh) cluster and via computations they found a bowl-like structure in which the inner B3 ring is slightly out-of-plane compared to the outer B9 ring, and metal atom is located in the concave side of the bowl. They also noted that the activation energy barrier for the rotation of inner B3 ring within the outer ring reduces significantly due to the complexation with the metals. Very recently, this study has been extended to Ir metal and realized that the rotational barrier is smaller in Ir complex than those in Co and Rh analogues.37 Prediction of viable boron clusters is very useful in cluster and material science not only because of their utility as the building blocks for clusters-assembled nanomaterials but also due to their efficacy to act as ligands. The prospect of the latter one is very exciting in the future of 3 ACS Paragon Plus Environment


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condensed phase coordination chemistry. MB12- clusters are just examples in which B12 acts as a ligand to coordinate with M. Herein, we have shown that an appropriate M center, supported by a boron cluster, can further be employed as an active center to form strong bonds with small molecules, and in fact, the binding can cause bond activation within the bound molecules which would certainly have further implications in various ways. The CO and N2 binding ability of MB12- (M = Co, Rh and Ir) clusters, and their structure, stability and nature of bonding are investigated thoroughly via density functional theory (DFT) computations. The resulting complexes, LMB12- (L = OC, CO and NN) possess a beautiful umbrella-like structure. To reveal the stability of LMB12- clusters with respect to the dissociation into L and MB12-, the dissociation energy (D0), change in enthalpy (∆H) and Gibbs’ free energy (∆G) at room temperature are calculated. The bonding situation in between M and L is analyzed by means of natural bond orbital (NBO) and energy decomposition analysis (EDA) in conjunction with natural orbital for chemical valence (NOCV) method. The shift in C-O and N-N stretching frequency in these complexes, are compared to that in free cases, and the possible bond activation are evaluated and explained by the relative contributions of L→M σ-donation and L←M π-back-donation as obtained from EDA-NOCV. Apart from that, the influence of further perturbation into MB12- cluster on the rotational barrier of the inner B3 ring is also explored. Finally, Born-Oppenheimer molecular dynamics (BO-MD) simulations support that the M-L bonds in these systems are kinetically stable enough to remain in bound forms during the internal rotation of inner B3 unit.

Computational details Geometry optimization of the studied complexes is carried out at the PBE38/def2TZVPPD39 level, where the effective core potentials (ECPs) is used for 28 and 60 core electrons of Rh and Ir atoms, respectively, for the computational efficiency as well as to take care of the relativistic effect. As, in general, the relativistic effects are negligible for 3d elements and allelectron computations are not computationally expensive that much, ECP is not available for Co atom in def2-TZVPPD basis set. The harmonic vibrational frequency computations are also performed at the same level to ensure that the stationary state is a minimum or a transition state on their respective potential energy surfaces. We have chosen this level as it almost reproduces 4 ACS Paragon Plus Environment

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the corresponding experimental ν(CO) (2143 cm-1)40 and ν(NN) (2359 cm-1)41 values in the free CO and N2 molecules (see Table S1 in supporting information). Since such complexes are generally characterized by the IR spectra, proper description of stretching frequency is necessary. The standard counterpoise method of Boys and Bernardi is used to evaluate the correction from the basis set superposition error (BSSE).42 The ZPE- and BSSE-corrected dissociation energy values (D0) are reported for the dissociation of the minimum energy structures of LMB12complexes into L and MB12-, and the corresponding dissociation enthalpy change (∆H) and dissociation free energy change (∆G) are computed at room temperature and one atmospheric pressure. The natural population analysis (NPA)43 and the Wiberg bond index (WBI)44 computations are also performed at the same level to evaluate the charge on each atom and the bond order, respectively, using the NBO scheme.45 All of the above mentioned computations are done using Gaussian 09 program package.46 The dynamical behavior of OCIrB12- and NNIrB12- is studied through BO-MD simulations at the PBE/DZVP level by using the deMon2K software.47 A quasi-relativistic effective core potential48,49 is used for Ir to take care of the relativistic correction therein. The simulations are launched from the equilibrium geometry of the complexes with random velocities assigned to the atoms, employing a Hoover thermal bath, for a trajectory time of 25 ps with 1.0 fs time steps. Further, EDA-NOCV is performed using ADF 2013.01 program package50 at the revPBE-D351/TZ2P//PBE/def2-TZVPPD level to shed light on the nature of bonding. Instead of the frozen core approximation, an all-electron basis set is used. Scalar relativistic effects in the heavy elements are included using the zeroth-order regular approximation.52 In EDA53 calculation, the total interaction energy (∆Eint) between two fragments is decomposed into four energy terms, viz., the electrostatic interaction energy (∆Velstat), the Pauli repulsion (∆EPauli), the orbital interaction energy (∆Eorb), and the dispersion interaction energy (∆Edisp). Therefore, ∆Eint can be written as: ∆Eint = ∆EPauli + ∆Velstat + ∆Eorb + ∆Edisp

(1).

The ZPE- and BSSE-uncorrected bond dissociation energy, De can be written in terms of the ∆Eint and ∆Eprep as: 5 ACS Paragon Plus Environment


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- De = ∆Eint + ∆Eprep

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(2)

where ∆Eprep is the preparation energy, which is necessary to bring the ground state fragments to the distorted geometries and electronic state that they have in the complex. Furthermore, EDA-NOCV54 decomposes the total deformation density (∆ρ(r)) into its individual differential densities (∆ρi(r)), which can be expressed over the pairs of NOCV. Thus, ∆ρ(r) = ∑ ∆ρi(r)

(3).

These computations help in identifying those fragment orbitals, which are contributing the most towards the chemical bond formation. It also provides the information regarding the direction of the charge flow in between those fragments. On a similar manner, the total ∆Eorb is also decomposed into ∆Eiorb corresponding to each charge transfer channel as ∆Eorb = ∑ ∆Eiorb

(4).

Structures and energetics The global minimum energy structures of MB12- clusters and their CO and N2 bound complexes along with the corresponding transition states for the rotation of the inner B3 unit are depicted in Figure 1. Similar to the bare structures, CO and N2 bound complexes also correspond to a C3v point group and 1A1 electronic state. The resulting structures have an umbrella-like shape where M-L acts like the stick of the umbrella. CO can bind with M center of MB12- cluster both via the C- and O-ends. The significantly higher D0 values in the C-side bonded isomer, OCCoB12-, than that in O-side bonded isomer, COMB12- reveal that the former is more stable than the latter by 41.0 kcal/mol (see Table 1), whereas for the Rh and Ir complexes such relative energies are larger (44.4 and 59.2 kcal/mol, respectively). For OCMB12- and NNMB12- complexes, the metal-ligand bond is the strongest in Ir complexes, followed by that in the Co and Rh analogues. Note that for a given M, the D0 values in OCMB12- are considerably larger by 20.6-32.0 kcal/mol than those in NNMB12- complexes. The computed ∆H and ∆G for the dissociation show the same trend as that found in the D0 values. The computed ∆H values are found to be positive, which reveal that the 6 ACS Paragon Plus Environment

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fragmentation process is endothermic in nature. The quite high positive ∆G values for OCMB12and NNMB12- complexes highlight that the dissociations are endergonic at room temperature and accordingly these complexes are thermochemically stable with respect to dissociation. In contrast, the dissociation for the O-side bound complexes are non-spontaneous at room temperature for Co and Ir analogues with ∆G values of 4.0 and 0.5 kcal/mol, respectively, whereas for Rh complex the same turns out to be exergonic with a ∆G value of -1.7 kcal/mol at 298 K. This indicates that slightly lower temperature would be preferential for these O-side bound isomers to be viable. The C-O and N-N bond lengths in free CO and N2 are calculated as 1.137 and 1.103 Å, respectively, at the PBE/def2-TZVPPD level (experimentally determined value is 1.128 Å for CO and 1.098 Å for N2).55,56 The comparison of the C-O and N-N bond distances in the complexes with those in the free states shows that irrespective of C-side or O-side binding of the ligand, the C-O and N-N bonds get elongated by 0.013-0.035 Å due to complexation, which suggests the occurrence of at least weak bond activation therein, being the maximum for C-side bound complexes and minimum for O-side bound analogues. Now, let us check for a given L which M causes the maximum elongation of the bond (∆r) among the present complexes. The ∆r value in the C-O bond of OCMB12- complexes is the largest in case of Ir complex, where the C-O bond length is 0.035 Å larger than that in free CO, followed by OCRhB12- (0.029 Å) and OCCoB12- (0.027 Å). For COMB12- complexes, the trend is different. Here, ∆r value is the largest in COIrB12- (0.019 Å) followed by that in COCoB12- (0.017 Å) and CORhB12- (0.013 Å). Most importantly, the bond of the inert N2 molecule gets elongated upon binding with the MB12-, which is the maximum in NNIrB12- (0.028 Å) among others. The ∆r value for the N-N bond is equal in the cases of NNCoB12- and NNRhB12- (0.023 Å). The bond activations within L are clearly reflected in the corresponding WBI values, where in case of CO the WBI value gets reduced to 2.01-2.10 in the bound complexes from 2.30 in the free state, whereas in case of N2 the same becomes 2.59 (Ir) and 2.70 (Co, Rh) from 3.03 in free N2. Obviously, the effect of this bond elongation would be reflected in their corresponding stretching frequency. The computed C-O and N-N stretching frequencies in the LMB12complexes are significantly red-shifted (by 160-257 cm-1) with respect to the corresponding values in free cases. For a given L, the red-shift (∆ν) in the C-O and N-N stretching frequencies 7 ACS Paragon Plus Environment


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follows the same trend as that of the ∆r value with variation in M. For example, in a given type of complex the red-shift is the maximum in Ir complex, whereas it is minimum in Co complex for OCMB12- and in Rh complex for COMB12-. On the other hand, similar to the corresponding ∆r values, in case of NNMB12-, both Co and Rh analogues have almost similar ∆ν(NN) values (only 6 cm-1 deviation). In TMCs, the bonding between M and CO is generally described by the synergistic effect of CO, where it acts simultaneously as a σ-donor (OC→M) and as a π-acceptor (OC←M). The HOMO of CO is used for σ-donation to a vacant orbital on the M center, whereas in OC←M πback-donation, the electron density goes from filled orbital of M to the LUMO of CO, which is a π* antibonding orbital. Depending on the dominating nature of σ-donation and π-acceptance over each other, TMCs can be classified as ‘classical’ and ‘non-classical’ or ‘abnormal’ carbonyls. In ‘classical’ carbonyls, due to dominant π-back-bonding, there occurs a red-shift in ν(CO) in IR spectra, while in ‘abnormal’ carbonyls, relatively weaker π-back-donation and strong σ-donation results in a blue-shift. The oxidation state of M center plays a decisive role in determining the nature of shift in ν(CO) as it has an inverse relationship with the OC←M π-back-donation, i.e., higher the oxidation number on M, lower is the OC←M π-back-donation and vice versa.57 - 61 58

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Due to the low oxidation state of M, the present complexes behave like ‘classical’ carbonyls. Bonding In bare MB12- complexes, M centers possess very small charge, being small negative charge in Rh-analogue and small positive charge in both the Co and Ir cases. Upon binding with L, the charge at M centers becomes more negative (-0.20 to -0.65 |e|), except for COIrB12- and NNIrB12- complexes, where Ir still remains positively charged, although lower than that in the bare cluster (see Table 2). Note that the gain in charge at the M center is significantly larger than the net electron-transfer from L (q(L) values in Table 2). In fact, in few cases q(L) is either zero or negative, which means that L→M electron transfer is compensated or even overcompensated by the L←M back-electron transfer. Therefore, the excess charge at the M centers is, of course, transferred from the B12 moiety to M, which is confirmed by the reduction of negative charge in B12 moiety upon complexation (q(B12) values in Table 2). For a particular M with CO ligand, the O-atom bears a negative charge and the C-atom bears a positive charge, both for the C- and O8 ACS Paragon Plus Environment

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side bonding, while with N2 ligand, the metal-bound N-atom possesses a negative charge and the rear N-atom has a positive charge, except in the Ir-analogue, where both the N-atoms are negatively charged. It indicates that the interaction of L with MB12- induces different degree of polarization within L moiety. The charge separation within L (∆q(L)) represents the degree of polarization in the C-O or N-N bonds. Now, larger the ∆q(L) value, larger would be the ionic character in the bond resulting in an elongated bond with red-shift in stretching frequency. In Cside bonded analogues, the C-O bond becomes more polarized by 0.10-0.25 |e| than that in free CO corroborating the observed red-shift in ν(CO). In the same line, the red-shift in ν(NN) can also be explained by the increase in polarization by 0.04-0.13 |e|. However, in case of O-side binding, although red-shift occurs in ν(CO), the polarization is found to decrease compared to that in free CO. Therefore, this cannot be explained by the change in polarization. On the other hand, in OCMB12- complexes, the WBI values for the M-C bond (1.05-1.18) are higher than the WBI values for M-O bond (0.28-0.34) in the COMB12- complexes, which implies that, while a single covalent bond is formed between M and C centers in OCMB12-, the M-O interaction is less covalent in nature. In the cases of NNMB12- complexes, the WBI values range within 0.72-0.76 implying the formation of almost a single bond between M and N centers. To get a better insight into the nature of the M-L bond, the EDA-NOCV computations are performed on the LMB12- complexes, taking L as one fragment and MB12- as another (Table 3). The corresponding ∆Eprep value involved in each fragment is provided in Table S2. While total ∆Eprep values lie within the range of 2.6-7.9 kcal/mol for OCMB12- and NNMB12-, it is considerably smaller (0.6-1.3 kcal/mol) for COMB12-. The contributions from the ∆Velstat (ca. 46.5-49.9 %) and ∆Eorb (ca. 45.4-49.6 %) towards the total attraction energy are almost equal, except for the OCMB12- complexes, where the ∆Velstat (ca. 54.2-56.2 %) has a higher contribution as compared to the ∆Eorb (ca. 42.2-43.6 %). Therefore, the M-L bonds in these complexes exhibit both the ionic and covalent characters almost in equal proportion where the former one is slightly more dominant over the latter in M-C bonds of OCMB12-. Note that the higher WBI values in MC bonds than those in M-O bonds can be inferred from the significantly larger magnitude of ∆Eorb term in the former than that in the latter. In other words, small WBI values in M-O bonds originate because of the small orbital interaction therein. The contribution from ∆Edisp is the least in the formation of these M-L bonds. 9 ACS Paragon Plus Environment


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Further, ∆Eorb is decomposed into its σ and π contributions. The pictorial depictions of the corresponding deformation densities (∆ρ(r)) for the pair-wise orbital interactions in these LMB12- complexes are presented in Figure 2 for Co, and Figures S1 and S2 for Rh and Ir analogues, respectively, where the depletion in the electron density (∆ρ(r) < 0) is shown in red color and the accumulation in the electron density (∆ρ(r) > 0) is shown in blue color. So, the direction of the flow of the electron density is from red to blue. In ∆ρ(σ1) plot, the electron density is shifted from L to M center, which is further shifted to the adjacent boron centers. Next two ∆ρ(π1) and ∆ρ(π2) plots represent the electron density shift from d orbitals of M to L via πback bonding. An electron transfer is also noted from some boron centers of B12 moiety to L. The polarization within L is described by the ∆ρ(σ2) plot. The L→M σ-donation is responsible for only 25-36% of total ∆Eorb value, whereas the amount of L←M π-back-donation (45-62% of total ∆Eorb value) is much higher than the σ-donation. So, the effect of the σ-donation is over

balanced by the π-back-donation and as a result, the bond in L gets elongated and a red-shift in the corresponding stretching frequency is observed. In general, a good correlation between the degree of π-back-donation and ∆ν is expected for the same type of M-L bonds as ∆ν depends on both L→M σ-donation and L←M π-back-donation but in opposite direction where a little variation in latter was reported to have significantly more impact on ν than the former in case of CO. Previously, an excellent correlation was noted between ∆ν(CO) and % of π-back-donation (∆Eπ) in a series of CO-Be complexes.62 Here, we have found that for a given L, larger the total magnitude of ∆Eπ, larger is the red-shift. The detailed pictures about the interacting pairs in the OCCoB12- complex are given in Figure 3 (see Figures S3 and S4 for COCoB12- and NNCoB12- complexes, respectively). Clearly, the σ-donation occurs due to the electron donation from the HOMO of CO to the LUMO on the MB12- fragment. The in-plane and out-of-plane π-back-donations occur due to the electron donation from the degenerate HOMOs of the MB12- fragment to the degenerate π* LUMOs of the CO. For OCCoB12-, the HOMO-19 in Figure 3 represents the corresponding molecular orbital of the σ-donation and the HOMO pictures are associated with the π-back-donation. Rotational barrier of the inner B3 ring


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Previous studies36, 37 showed that the rotational energy barrier (∆E≠) of inner B3 ring in the MB12- moiety decreases in the order as Co > Rh > Ir. Here, although the attachment of the ligands like CO and N2 causes an increase in the ∆E≠ values (by 0.4-1.8 kcal/mol) as compared to that in the bare MB12-, the corresponding ∆E≠ values are not too large to force them to behave like rigid clusters (see Table 4). In fact, BO-MD simulations for OCIrB12- and NNIrB12complexes at 800 K show the internal rotation of B3 unit within peripheral boron ring, while the M-L bond remains intact (see the movies in electronic supporting information). Therefore, the present complexes would behave like a spinning umbrella. This internal rotational motion is akin to the pseudorotation in cyclopentane64,65 which in turn may be topologically mapped to the rotational motion of a soliton (kink/ twist) around a ring.66 For a given L, the ∆E≠ value is found to be the minimum for the Ir followed by Rh and Co, which is the same as the reported trend in the bare clusters. Among other geometrical parameters, the B1-B3 bond length (the longest bond between inner B3 and outer B9 rings, see Figure 1) was reported to be a good parameter to explain the diminished ∆E≠ value in MB12- compared to that in bare B12 cluster where the B1-B3 bond length gets increased in MB12- in comparison to that in B12, accounting for the lower ∆E≠ value in former cases than that in the latter.37 In LMB12- complexes, it is noted that the B1-B3 bond lengths get decreased as compared to those in MB12- clusters which can be attributed to the higher ∆E≠ values in these complexes albeit the bonds in LMB12- complexes are longer than those reported in B12 clusters so that it does not cause any impediment towards its fluxional behavior. For a given L, the increase (∆∆E≠) in the energy barrier for the B3 rotation in LMB12complexes with respect to that in bare MB12- moiety is the maximum in Ir complex followed by Co and Rh complexes. Clearly, stronger the L binding energy, higher is the increase in ∆E≠ value. In Figure 4, scatter correlation plots between ∆∆E≠ and the energy associated with the L→M σ-donation (∆Eσ) for a given M is provided. Here, the plot has negative slope (as ∆Eσ < 0) with linear correlation coefficients (R2) of 0.999, 0.888 and 0.934 for Co, Rh and Ir, respectively, implying a good correlation between ∆∆E≠ and ∆Eσ values. So, it may be stated that larger is the σ-donation, more will be the increase in the rotational barrier.



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Summary and Outlooks CO and N2 can form a molecular umbrella when coupled with MB12- clusters and these complexes are viable at room temperature, but for COMB12- (particularly CORhB12-). The dissociation energy values reveal that for OCMB12- and NNMB12-, the Ir-analogues are most stable followed by Co and Rh, whereas for COMB12-, Co complex is slightly more stable than the Ir analogue. For this type of complexes, the C-O and N-N bonds get elongated resulting in red-shifts in their corresponding stretching frequencies in IR spectra. For a given ligand, the degree of red-shift is the maximum in the Ir-complex. The bonding between M and L is mainly dominated by the orbital and electrostatic interactions, almost in equal contributions. A detailed analysis of the EDA-NOCV results reveals that the total L←M π-back-donation dominates over the L⟶M σ-donation, resulting in the red-shift in stretching frequency. The attachment of ligand to the bare MB12- moiety increases the barrier height of the internal rotation of the inner B3 ring. For a particular ligand, Ir complexes posses the least amount of barrier height among the complexes of the three metals considered here. The lower interaction energies of the Ir complexes, compared to their lighter analogues, are responsible for lower barrier heights of internal rotation in Ir-analogues. The B1-B3 bond length (the longest bond between inner B3 and outer B9 rings) and the energy associated with the L→M σ-donation, are found to be nicely correlated with the obtained rotational barrier in LMB12- complexes. Therefore, the present study shows that an appropriate metal center supported by a boron cluster can further be employed to bind with small molecules resulting in the bond activation within the bound molecule. This feature would certainly be important in various processes. Here, we have tested only MB12clusters and CO and N2 as a case study, but the other reported transition metal bound boron clusters can also be employed to bind various small molecules associated with stronger bond activation.

Acknowledgements RS thanks UGC, New Delhi for a senior research fellowship and SK thanks IIT Kharagpur for a post doctoral fellowship. PKC thanks DST, New Delhi for the J. C. Bose


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National fellowship. GM and SP acknowledge Conacyt (Grant CB-2015-252356) for the funding. Supporting Information Supporting Information Available: Computed bond stretching frequencies of the CO and N2 at different computational levels, plots of deformation densities, ∆ρ(r), of LMB12- (L = OC, CO and NN; M = Rh, Ir) complexes, the pair-wise orbital interactions of LCoB12- (L = CO, NN), BO-MD simulation movies and the coordinates of the optimized geometries of studied systems. This material is available free of charge via the Internet at http://pubs.acs.org.

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Tables 18 ACS Paragon Plus Environment

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Table 1. ZPE- and BSSE-corrected dissociation energies (D0), dissociation enthalpy changes (∆H), Gibbs free energy changes (∆G) for the dissociation process: LMB12- → L + MB12- (L = OC, CO and NN; M = Co, Rh and Ir); Wiberg bond index of M-L bond (WBIM-L), Wiberg bond index for C-O and N-N bond (WBIL), change in C-O and N-N bond lengths in bound complexes compared to the free one (∆r(L), Å), C-O and N-N stretching frequency (ν(L)) and the change in the frequency compared to that in free one (∆ν(L)) in LMB12- complexes at the PBE/def2TZVPPD level. (All energies are in kcal/mol and frequencies are in cm-1.) System

D0

∆H

∆G

WBIM-L

WBIL

∆r(L)

ν(L)

CO

2.30

2128

N2

3.03

2356

∆ν ν(L)

OCCoB12-

52.6 54.0

43.3

1.05

2.10

0.027

1963

-165

COCoB12-

12.6 13.5

4.0

0.28

2.06

0.017

1946

-181

NNCoB12-

32.0 33.3

23.0

0.72

2.70

0.023

2142

-214

OCRhB12-

50.3 50.7

40.9

1.13

2.08

0.029

1947

-181

CORhB12-

7.3

7.5

-1.7

0.34

2.10

0.013

1968

-160

NNRhB12-

25.9 26.3

16.9

0.75

2.70

0.023

2136

-220

OCIrB12-

67.7 68.0

57.8

1.18

2.01

0.035

1935

-192

COIrB12-

9.9

10.0

0.5

0.34

2.02

0.019

1921

-207

NNIrB12-

35.7 36.1

26.4

0.76

2.59

0.028

2099

-257

C-O and N-N bond lengths in free CO and N2 are 1.137 and 1.103 Å, respectively, at the PBE/def2-TZVPPD level.



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Table 2. NPA charges at M, C, O and N centers (q, |e|), total net charge in L moiety (q(L), |e|) and the charge separation in L (∆q(L), |e|) of LMB12- complexes and the charge in B12 moiety (q(B12), |e|) at the PBE/def2-TZVPPD level. System

q(M)

q(C) or q(N) q(O) or q(N)

q(L)

∆q(L)

CO

0.46

-0.46

0.00

0.92

N2

0.00

0.00

0.00

0.00

q(B12)

CoB12-

0.03

OCCoB12-

-0.63

0.70

-0.47

0.22

1.17

-0.59

COCoB12-

-0.20

0.40

-0.41

-0.01

0.81

-0.79

NNCoB12-

-0.37

-0.05

0.08

0.03

0.13

-0.66

RhB12-

-0.05

OCRhB12-

-0.65

0.66

-0.47

0.19

1.14

-0.54

CORhB12-

-0.26

0.42

-0.42

0.00

0.84

-0.74

NNRhB12-

-0.41

-0.05

0.06

0.01

0.11

-0.61

IrB12-

0.19

OCIrB12-

-0.20

0.54

-0.48

0.07

1.02

-0.87

COIrB12-

0.09

0.40

-0.46

-0.06

0.86

-1.03

NNIrB12-

0.01

-0.06

-0.02

-0.08

0.04

-0.93

-1.03

-0.95

-1.19


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Table 3. EDA-NOCV results of LMB12- (L = OC, CO and NN; M = Co, Rh and Ir) considering L as one fragment and MB12- as another at the revPBE-D3/TZ2P//PBE/def2- TZVPPD level (All energies are in kcal/mol). ∆Edispa

∆E π1

b

∆E π2

b

System

∆EPauli

∆Eelstata

∆Eorba

OCCoB12-

132.3

-102.4 (54.2)

-82.4 (43.6)

-4.2 (2.2) -56.7 -21.1 (25.6) -25.5 (30.9) -25.5 (30.9)

-5.9 (7.2)

COCoB12-

49.3

-29.3 (46.5)

-30.5 (48.5)

-3.2 (5.0) -13.6

-8.0 (26.2)

-2.5 (8.1)

NNCoB12-

91.4

-60.4 (47.3)

-63.1 (49.4)

-4.2 (3.3) -36.2 -16.3 (25.9) -18.9 (30.0) -18.9 (30.0)

-4.6 (7.3)

OCRhB12-

170.2

-126.1 (56.2)

-94.8 (42.2)

-3.5 (1.6) -54.2 -24.5 (25.9) -27.9 (29.5) -27.9 (29.5) -10.1 (10.6)

CORhB12-

42.9

-25.1 (49.9)

-22.8 (45.4)

-2.4 (4.8)

-5.5 (24.3)

-2.3 (10.0)

NNRhB12-

104.3

-65.8 (49.1)

-64.1 (48.2)

-3.2 (1.9) -28.8 -17.9 (27.9) -18.1 (28.2) -18.1 (28.2)

-6.3 (9.7)

OCIrB12-

223.0

-167.2 (55.6) -129.8 (43.2) -3.6 (1.2) -77.6 -37.8 (29.1) -36.2 (27.9) -36.2 (27.9)

-12.6 (9.7)

COIrB12-

62.5

-35.7 (48.4)

-35.4 (48.0)

-2.6 (3.6) -11.2 -12.6 (35.6)

-8.1 (22.8)

-3.5 (9.8)

NNIrB12-

143.4

-90.2 (48.6)

-92.1 (49.6)

-3.3 (1.8) -42.3 -28.0 (30.3) -24.7 (26.8) -24.7 (26.8)

-8.7 (9.5)

∆Eint

-7.5

∆Eσb

-9.2 (30.1)

-7.5 (32.8)

-8.0 (26.2)

-5.5 (24.3)

-8.1 (22.8)

∆Epol b

a

The values within the parentheses are in percentage and show the contribution towards the total attractive interaction. b The values within parentheses are the percentage contribution towards ∆Eorb.



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Table 4. Rotational barrier heights (∆E≠), change in rotational barrier heights (∆∆E≠) of the inner B3 ring in MB12- and LMB12- and the B1-B3 bond distance (rB1-B3, Å). All energies are in kcal/mol. System ∆E≠ ∆∆E≠ rB1-B3 CoB12-

8.5

1.922

OCCoB12- 9.8

1.3 1.906

COCoB12- 9.3

0.8 1.911

NNCoB12- 9.6

1.1 1.905

RhB12-

8.1

1.927

OCRhB12- 9.1

1.0 1.911

CORhB12- 8.5

0.4 1.921

NNRhB12- 9.0

0.9 1.914

IrB12-

6.0

1.966

OCIrB12- 7.8

1.8 1.934

COIrB12- 6.9

0.9 1.950

NNIrB12- 7.5

1.6 1.938


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Figures

Figure 1. Minimum energy structures of MB12- and LMB12- (L = OC, CO and NN; M = Co, Rh and Ir) complexes obtained at the PBE/def2-TZVPPD level, and the corresponding transition states (TS) for the internal rotation of B3 unit. The important bond distances are also provided in Å.



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Figure 2. Plots of deformation densities, ∆ρ(r), of the pair-wise orbital interactions in of LCoB12- (L = OC, CO and NN) complexes at the revPBE-D3/TZ2P//PBE/def2-TZVPPD level. The associated orbital interaction energies are given in kcal/mol. The color code of the charge flow is red→blue.


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Figure 3. Plots of the deformation densities, ∆ρ(r), and shape of the interacting orbitals of the pair wise orbital interactions between CoB12- and CO in OCCoB12-. The associated orbital interaction energies are given in kcal/mol. The energy values, given in parentheses of energy level of molecular orbital, are in a.u. The color code of the charge flow is red→blue.



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Figure 4. Scatter plot of change in the rotational barrier height (∆∆E≠, kcal/mol) against σ-donation (∆Eσ, kcal/mol) for LCoB12- (L = OC, CO and NN; M = Co, Rh and Ir) complexes at the revPBED3/TZ2P//PBE/def2-TZVPPD level.


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TOC Graphic


A Spinning Umbrella: Carbon Monoxide and Dinitrogen Bound MB12

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