Role of N-Heterocyclic Carbenes as Ligands in ... - ACS Publications

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Role of N‑Heterocyclic Carbenes as Ligands in Iridium Carbonyl Clusters Shengjie Zhang,† Sawyer D. Foyle,† Alexander Okrut,‡ Andrew Solovyov,‡ Alexander Katz,*,‡ Bruce C. Gates,§ and David A. Dixon*,† †

Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487, United States Department of Chemical and Biomolecular Engineering, University of California at Berkeley, Berkeley, California 94720, United States § Department of Chemical Engineering, University of California at Davis, Davis, California 95616, United States ‡

S Supporting Information *

ABSTRACT: The low-energy isomers of Irx(CO)y(NHC)z (x = 1, 2, 4) are investigated with density functional theory (DFT) and correlated molecular orbital theory at the coupled cluster CCSD(T) level. The structures, relative energies, ligand dissociation energies, and natural charges are calculated. The energies of tetrairidium cluster are predicted at the CAM-B3LYP level that best fit the CCSD(T) results compared with the other four functionals in the benchmark calculations. The NHC’s behave as stronger σ donors compared with CO’s and have higher ligand dissociation energies (LDEs). For smaller isomers, the increase in the LDEs of the CO’s and the decrease in the LDEs of the NHC’s as more NHC’s are substituted for CO’s are due to π-back-bonding and electron repulsion, whereas the trend of how the LDEs change for larger isomers is not obvious. We demonstrate a μ3-CO resulting from the high electron density of the metal centers in these complexes, as the bridging CO’s and the μ3-CO’s can carry more negative charge and stabilize the isomers. Comparison of calculations for a mixed tetrairidum cluster consisting of two calixarene-phosphine ligands and a single calixarene-NHC ligand in the basal plane demonstrated good agreement in terms of both the ligand substitution symmetry (C3v derived), as well as the infrared spectra. Similar comparisons were also performed between calculations and experiment for novel monosubstituted calixarene-NHC tetrairidium clusters.

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INTRODUCTION There is substantial interest in tetrairidium clusters because the Ir4 framework offers a rich chemistry, being relatively stable and having been shown to provide the core of catalytic species that are active for a number of reactions in solution and when supported.1−4 Experimental results (e.g., vibrational spectra,5 crystal structures,6 and NMR spectra7) confirm that the parent cluster Ir4(CO)12 adopts a Td structure with 12 terminal carbonyl ligands, in contrast to the C3v symmetry of Co4(CO)12 and Rh4(CO)12.5,8 Braga et al.9 investigated the structure of M4(CO)12 and M4(CO)11L clusters (M = Co, Rh, and Ir) by using extended Hückel methodology to address the challenge of symmetry assignment. High-level electronic structure CCSD(T) calculations for Ir4(CO)12 showed that the Td structure is 4.0 kcal/mol more stable than the C3v structure.10 The catalytic activities of small iridium clusters extend to numerous reactions,11 including oxidation, hydrogenation, C− H bond activation, cycloaddition, cycloisomerization,12,13 and ring-opening.14 Mononuclear iridium complexes have also been widely used as catalysts in oxidation (e.g., of alcohols,15−17 phenols,18−20 and amines21,22), C−H bond activation,23−25 hydrogenation,26,27 cycloaddition (e.g., [2 + 2 + 2],28−30 [2 + 2 + 1],31,32 and [4 + 2]33,34), cycloisomerization,12,13 and ringopening reactions.35 Ir(CO)n (n = 1−4) and Ir2(CO)8 clusters © XXXX American Chemical Society

have been reported to be the products of the reaction of Ir with CO.36−38 Ir2(CO)8 clusters generated in matrices at 10−50 K dimerized into Ir4(CO)12 at ∼200 K.36,37 Recently, we reported a theoretical investigation of iridium carbonyl phosphine complexes Irx(PH3)y(CO)z (x = 1, 2, 4)39 after earlier work showed that the tetrairidium clusters could have more than one kind of active site, in which the reactivity at the apical Ir site was controlled by three calixarene-phosphine ligands bonded to the basal plane.40 We have now extended this work to include N-heterocyclic carbenes (NHCs or stable singlet nucleophilic carbenes) instead of the phosphines as ligand because of the importance of NHC’s in catalysis.41 NHCs were first successfully isolated by Arduengo in 1991.42 Since then, the catalytic properties of NHC-metal complexes have been extensively exploited.41,43−46 and were part of the work leading to recognition with the 2005 Nobel Prize in Chemistry. Although there has been prior work on NHC-Ir complexes,47−49 research on tetrairidium clusters with carbene ligands is rare.50,51 Received: May 2, 2017 Revised: June 9, 2017 Published: June 9, 2017 A

DOI: 10.1021/acs.jpca.7b04161 J. Phys. Chem. A XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry A Scheme 1. Synthesis of Tetrairidium Carbene Cluster and Structure Labeling

substituted iridium carbonyl clusters,39 we optimized the geometries of the Irx(CO)y(NHC)z clusters by using DFT with the SVWN5 functional. Second-derivative calculations were used to confirm that the optimized structures were minima and to provide vibrational frequencies. The SVWN5optimized geometries were used in single-point DFT calculations with various functionals, second-order Møller− Plesset (MP2) calculations,61,62 and coupled cluster CCSD(T) calculations The open-shell calculations were performed at the restricted ROMP2 level63 and R/UCCSD(T) levels.64−66 The following exchange-correlation functionals were used in the single-point DFT calculations: B3LYP,67,68 CAM-B3LYP, M06,69 PW91,70−72 and ωB97X-D. PW91 is a pure generalized gradient approximation (GGA) functional; B3LYP and M06 are hybrid functionals; CAM-B3LYP is a hybrid functional with long-range corrections; and ωB97X-D is a hybrid functional with long-range corrections and dispersion corrections. For all elements except Ir, the correlation-consistent doubleζ (cc-pVDZ) basis set73,74 was used in all calculations. For the Ir atom, the calculations were performed with a relativistic pseudopotential with the cc-pVDZ-PP basis set75 including 60 electrons in the pseudopotential (1s22s22p63s23p63d104s24p64d104f14) and 17 active electrons in the self-consistent field calculations (5s25p66s25d7). The 5s25p6 electrons on Ir and the 1s2 electrons on C and O are in the doubly occupied core in the valence-only CCSD(T) and MP2 calculations. We use “D” to denote such basis set combinations in the following discussion. All of the DFT calculations were performed using the Gaussian09 suite of programs,76 and the MP2 and CCSD(T) calculations were carried out using the MOLPRO2010 suite of programs.77,78 Experimental Section. All compounds were handled under dry argon atmosphere. Solvents were dry and distilled by standard methods. Sodium hydride 95% powder, tbutylcalix[4]arene, 1,3-dibromopropane, and 1-phenyl-imidazol

In the current study, we report the structures and bond dissociation energies of Irx(CO)y(NHC)z (x = 1, 2, 4), for which there are no available data, by using density functional theory (DFT)52 and coupled cluster theory (CCSD(T)).53−56 The geometries, charges, and ligand dissociation energies (LDEs) of these clusters were investigated to provide insights into the bonding of NHC-Ir complexes. We used the parent NHC, c-C3N2H4, as our carbene model. A range of DFT exchange-correlation functionals was used to provide a guide for their use with larger substituted NHCs. We compare the results with those reported for the phosphine ligands. To test the predictions of the computations, we report the synthesis and structures of new NHC-calixarene ligands Ca and Cb bound to the tetrairidium cluster, which lead to the monosubstituted clusters, D and E. The symmetry of the ligand substitution pattern on these clusters was compared between calculations and experimental data, which are based on infrared spectroscopic measurements in solution, as well as the structure of D, as determined via single-crystal X-ray diffraction. In addition, a trisubstituted tetrairidium with two phosphine (axial and equatorial) and a single NHC was synthesized and characterized by IR and NMR spectroscopies for comparison with the calculations to determine the basic symmetry of the substitution pattern.

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COMPUTATIONAL AND EXPERIMENTAL METHODS Computational. In our previous work,10 we showed that the SVWN5 functional (Slater exchange57 plus the VWN version 5 fit of the electron gas for correlation58) gave excellent predictions of the geometries of iridium carbonyl clusters. The local SVWN5 functional did not provide good results characterizing the relative energies and LDEs, and we found that the CAM-B3LYP59 and ωB97X-D60 functionals performed best among the functionals that were tested. On the basis of our previous work on iridium carbonyl clusters10 and phosphineB


Article

The Journal of Physical Chemistry A were purchased from Aldrich. t-Butyl-tripropoxycalix[4]arene A,79 t-butyl-(3-bromopropoxy)-tripropoxy-calix[4]arene B,80 and tert-butyl-calix[4]-arene(OPr)3(OCH2PPh2) LP were synthesized81 using published procedures. 1H NMR spectra were recorded either on a Bruker DRX-500 or a AVQ-400 instrument at the UC Berkeley NMR facility. The 1H NMR data are referenced to residual solvent resonance. Analytical thin-layer chromatography was performed on precoated silica gel plates (Selecto), and silica gel (Selecto 60) was used for column chromatography. Details of the experimental characterization are given in the Supporting Information. Synthesis of Calixarene-Imidazolium Bromides Ca and Cb. The starting bromocalixarene B was kept under high vacuum for 3h. The residue was flashed two times with dry argon. A mixture of bromocalixarene B (0.8 mmol) and 1-Rimidazole (R = Ph Ca, nC4H9 Cb) (8.0 mmol) was heated at 110 °C in a minimal amount (3−5 mL) of toluene, which was freshly distilled over Na. The reaction was monitored by TLC. (Scheme 1) After disappearance of a high R f spot corresponding to bromocalixarene on the TLC plate (hexane/chloroform mixture as a eluente) after 18 h, the reaction mixture was evaporated and dried under high vacuum for 24h. The residue was treated with n-hexane (for reactions with 1-n-butylimidazole) or n-hexane-benzene mixture (10:0.5) (for reaction with phenylimidazole). After 24h, the resulting organic solution was separated and the white solid formed was subsequently crystallized from toluene. Monocarbene Ir4-Carbonyl-Complexes D and E. In a Schlenk flask under argon atmosphere, the corresponding calixarene imidazolium bromide (Ca or Cb) (264 mg/0.254 mmol (Ca), 371 mg/0.36 mmol (Cb) was mixed with Ag2O (29.4/0.127 mmol (Ca), 42 mg/0.181 mmol (Cb) in dichloromethane (10 mL for Ca, 20 mL for Cb). The flask was wrapped with aluminum foil to prevent light exposure, and the reaction mixture was stirred at room temperature for 16 h. The reaction mixture was filtered through Celite and into a solution of [Bu4N][Ir4(CO)11Br] (355 mg/0.254 mmol for Ca and 504 mg/0.361 mmol for Cb in dichloromethane (10 mL for Ca, 40 mL for Cb). The resulting mixture was stirred for 16h. The dichloromethane was evaporated, and the residue was extracted with heptane. The product was purified via column chromatography (90 mg; yield = 17% of D, 438 mg; yield = 60% of E). Synthesis of Mixed Carbene/Phosphine Complex F. In a Schlenk flask under argon atmosphere, monocarbene complex E (391 mg, 0.193 mmol) was added to a solution of tert-butylcalix[4]-arene(OPr)3(OCH2PPh2) [3] (380 mg, 0.390 mmol) in 40 mL toluene. The resulting mixture was stirred for 40 h at 70 °C, and the solvent was evaporated. The solid residue was purified via column chromatography to yield analytically pure F (480 mg, yield 64%). Single Crystal X-ray Diffraction of D. A yellow needle 0.08 × 0.06 × 0.04 mm in size was mounted on a Cryoloop with Paratone oil. Data were collected in a nitrogen gas stream at 100(2) K using phi and omega scans. Crystal-to-detector distance was 60 mm and exposure time was 10 s per frame using a scan width of 0.5°. Data collection was 99.9% complete to 25.00° in θ. A total of 175936 reflections were collected covering the indices, −15 ≤ h ≤ 15, −43 ≤ k ≤ 43, −22 ≤ l ≤ 21. 14542 reflections were found to be symmetry independent, with an Rint of 0.0283. Indexing and unit cell refinement indicated a primitive, monoclinic lattice. The space group was found to be P2(1)/c (No. 14). The data were integrated using

the Bruker SAINT software program and scaled using the SADABS software program. Solution by direct methods (SIR2008) produced a complete heavy-atom phasing model consistent with the proposed structure. All non-hydrogen atoms were refined anisotropically by full-matrix least-squares (SHELXL-97). All hydrogen atoms were placed using a riding model. Their positions were constrained relative to their parent atom using the appropriate HFIX command in SHELXL-97.

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RESULTS AND DISCUSSION The electronic structure of the stable NHC’s have been extensively studied.82−86 A simple schematic of the bonding in an NHC is shown in Scheme 2. The simplest NHC is best Scheme 2. Lewis Structure for the Simplest NHC

described as a singlet carbene with a CC double bond, two planar nitrogens with lone pairs perpendicular to the plane and the carbene carbon with a lone pair in the plane. NHC is a poor π acceptor because the out-of-plane (nominally empty) orbital on the carbene C atom is blocked from bonding as a consequence of its interactions with the lone pairs on the adjacent N atoms. This also prevents backbonding from the metal to the “empty” p orbital perpendicular to the plane at the carbene center. The corresponding triplet state for the carbene is 86.4 kcal/mol higher in energy so there is no radical character. Ir(CO)y(NHC)z Structures, Relative Energies, and Ligand Dissociation Energies. Di-, tri-, and tetra-coordinated mono-Ir complexes were optimized, and their relative energies are listed in Table 1. For each structure, the doublet state isomers are significantly lower in energy than the quartet states, and so only doublet state isomers are shown in Figure 1 and discussed further. Both Ir(CO) (NHC) and Ir(NHC)2 molecules have two minimum energy structures, one bent and one approximately linear, with the bent structures (∠C−Ir−C ≈ 110°) lower in energy by 13.6 and 15.4 kcal/mol, respectively, at the CCSD(T) level. For the tricoordinated Ir complex, a T-shaped planar geometry with one equatorial (eq = equatorial) and two axial (ax = axial) positions is the lowestenergy isomer. The substitution of an NHC for CO can take place on either ax or eq site. The ax NHC-substituted isomers of Ir(CO)3 forming 1e, and 1e forming 1g, are lower in energy by 6.4 and 2.6 kcal/mol, respectively, with respect to the corresponding eq NHC-substituted isomers 1f and 1h. The tetra-coordinated isomers have distorted D2d symmetry structures with an increase in the ∠C(ax)−C(eq)−Ir−C(ax) dihedral angles as more NHC ligands are substituted for CO ligands (∼155°, 160°, 175°, and 180° for 1j, 1k, 1m, and 1n). Isomer 1k with two ax-substituted NHCs is only 0.6 kcal/mol lower in energy than isomer 1l with one ax and one eq NHC ligand. We first consider the CO LDEs of the Ir(CO)y(NHC)z isomers listed in Table 2, at the CCSD(T) level. LDEs were calculated from eq 1. All LDEs are electronic C


Article

The Journal of Physical Chemistry A Table 1. Relative Energies of Ir(CO)y(NHC)z Isomers in kcal/mola isomer

ΔH(0K) B3LYP

ΔH(0K) CAMB3LYP

ΔH(0K) M06

ΔH(0K) PW91

ΔH(0K) ϖB97X-D

ΔH(0K) MP2

ΔH(0K) CCSD(T)

Ir(CO)2 linear Ir(CO)2 bent 4 Ir(CO)2 bent 4 Ir(CO)2 linear 2 IrCO(NHC) linear 2 IrCO(NHC) bent 4 IrCO(NHC) bent 4 IrCO(NHC) linear 2 Ir(NHC)2 linear 2 Ir(NHC)2 bent 4 Ir(NHC)2 linear 4 Ir(NHC)2 bent 2 Ir(CO)3 4 Ir(CO)3 2 Ir(CO)2(NHC) ax 2 Ir(CO)2(NHC) eq 4 Ir(CO)2(NHC) eq 4 Ir(CO)2(NHC) ax 2 IrCO(NHC)2 axax 2 IrCO(NHC)2 axeq 4 IrCO(NHC)2 axeq 4 IrCO(NHC)2 axax 2 Ir(NHC)3 4 Ir(NHC)3 2 Ir(CO)4 4 Ir(CO)4 2 Ir(CO)3(NHC) 4 Ir(CO)3(NHC) 2 Ir(CO)2(NHC)2 axax 2 Ir(CO)2(NHC)2 axeq 4 Ir(CO)2(NHC)2 axax 4 Ir(CO)2(NHC)2 axeq 2 IrCO(NHC)3 4 IrCO(NHC)3 2 Ir(NHC)4 4 Ir(NHC)4

0.0 9.9 53.1 51.3 0.0 15.0 55.1 55.2 0.0 17.2 52.6 52.4 0.0 48.0 0.0 4.8 48.9 51.2 0.0 2.1 50.1 50.2 0.0 44.3 0.0 68.7 0.0 66.8 0.0 1.8 56.4 63.2 0.0 55.2 0.0 50.9

0.0 12.0 51.0 48.9 0.0 18.0 53.1 52.9 0.0 20.8 51.6 51.5 0.0 45.5 0.0 5.7 46.3 48.4 0.0 2.4 47.2 47.3 0.0 42.2 0.0 69.4 0.0 69.7 0.0 1.4 54.3 62.6 0.0 53.8 0.0 49.8

0.0 7.1 60.8 57.4 0.0 12.1 62.0 61.1 0.0 14.1 55.4 55.3 0.0 56.0 0.0 4.0 55.0 59.2 0.0 0.4 56.2 57.2 0.0 52.2 0.0 74.7 0.0 70.5 0.0 1.0 63.7 69.8 0.0 61.9 0.0 64.3

0.0 1.7 49.2 51.7 0.0 7.4 51.8 55.8 0.0 9.7 50.5 50.5 0.0 50.1 0.0 3.4 50.6 53.3 0.0 0.8 51.8 51.8 0.0 43.9 0.0 68.1 0.0 62.8 0.0 2.1 56.1 62.9 0.0 53.2 0.0 47.6

0.0 10.0 55.4 53.8 0.0 16.3 57.9 58.3 0.0 19.1 54.5 54.3 0.0 50.2 0.0 5.3 50.7 53.6 0.0 1.4 51.5 53.2 0.0 46.7 0.0 72.9 0.0 73.9 0.0 0.8 59.1 65.8 0.0 58.3 0.0 11.5

0.0 1.1 51.2 56.5 0.0 8.5 54.7 61.4 0.0 9.7 72.3 72.5 0.0 56.8 0.0 6.6 60.2 61.1 0.0 2.1 70.2 107.2 0.0 47.6 0.0 84.8 0.0 78.1 0.0 1.0 71.0 78.8 0.0 72.1 0.0 58.8

0.0 7.9 46.9 50.8 0.0 13.6 49.0 53.5 0.0 15.4 66.6 66.7 0.0 48.7 0.0 6.4 50.9 51.4 0.0 2.6 59.4 94.0 0.0 41.3 0.0 71.7 0.0 71.8 0.0 0.6 57.4 64.9 0.0 57.3 0.0 50.7

2 2

a

All energies are single-point energies at the SVWN5 optimized geometries.

LDE = E(product) + E(ligand) − E(parent cluster)

higher LDEs of the NHC ligands suggest that NHC is a stronger σ-donor and a better Lewis base than CO (the proton affinity of the simplest NHC is 250 kcal/mol and is much higher than the value of 141 kcal/mol for CO82,90). When a CO ligand is replaced by an NHC ligand on an iridium carbonyl cluster, the 5d orbitals of Ir are occupied by more electrons, which results in more π-back-bonding from the metal core to the CO π* orbitals. The increase in the Ir-CO LDEs as more NHC ligands are substituted is the result of additional πback-bonding resulting from the availability of more electrons on the Ir from the added NHC ligands. As a consequence of the lack of readily available antibonding orbitals, the NHC is poor at accepting π electrons from the Ir. Therefore, instead of strengthening the M−L bond through π-back-bonding, the additional electrons on the metal core increase the electron repulsion between the σ electrons donated by each NHC ligand. The decrease of the LDEs of NHC ligands is thus a consequence of stronger electron repulsion as more NHC ligands are substituted for CO ligands. Consistent with this

(1)

energies with a zero-point energy correction included, giving ΔH(0 K). As more NHC ligands are substituted for CO ligands, the LDEs of the eq CO ligands increase by 6 kcal/mol from 1e to 1g, and by 4 kcal/mol from 1j to 1k and an additional 10 kcal/mol to 1m. The LDEs of the ax CO ligands have the same trend, increasing by 5 kcal/mol from 1e to 1h. However, the LDEs of the NHC ligands decrease by almost 30 kcal/mol as more CO ligands are replaced, from a high of 64 kcal/mol for 1e to 36 kcal/mol for 1n. All LDEs of CO ligands are lower than those of NHC ligands. The LDEs of trisubstituted isomers (∼33 to 64 kcal/mol) are higher than those of the tetra-substituted ones (∼22 to 47 kcal/mol). The basic Dewar−Chatt−Duncanson model87,88 provides a picture of σ-electron donation from the ligand to the metal, and π-back-bonding from metal d-orbitals to the ligand-unoccupied orbitals can be used to analyze the LDEs. When this model is expanded89 to include additional orbitals, one can explain the trends for the LDEs of the Ir(CO)y(NHC)z complexes. The D


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The Journal of Physical Chemistry A

Figure 1. Optimized geometries of low-lying 2Ir(CO)y(NHC)z isomers and CCSD(T) energies, in kcal/mol.

statement, we find an increase in the C−O bond lengths and a decrease in the Ir-CO bond lengths with a concomitant decrease in the CO stretching frequencies as more NHC ligands are substituted for CO ligands, further confirming the existence of π-back-bonding (Table 3). Moreover, the natural population analysis (NPA) charges (Table 3) from the natural bond orbitals (NBOs)91−96 are consistent with this analysis as the negative charges on the CO ligand increase as a result of π-back-bonding, and the positive charges on the NHC decrease as a result of electron repulsion. Ir2(CO)y(NHC)z Structures, Relative Energies, and Ligand Dissociation Energies. We previously predicted three different low-energy structures for Ir2(CO)8.10 After one CO is replaced, 2g and 2i have structures similar to those of 2a (C2v) and 2c (D3d), whereas 2h has a new bridging CO ligand (Figure 2). The dissociation of a terminal (ter = terminal) CO from 2g and 2h results in structure 2k, which has two bridging CO ligands. This comparison suggests that, in contrast to Ir2(CO)8 and Ir2(PH3)y(CO)z,39 Ir2(CO)y(NHC)z does not have low-energy isomers close to D2d or C2 symmetry. For Ir2(CO)5(NHC)2, the reaction from 2n to 2p shows that a D3dbased isomer is no longer formed. Relative energies of the Ir2(CO)y(NHC)z isomers and the corresponding LDE’s are given in Tables 4 and 5, respectively. Among the coordinatively saturated clusters (Ir2(CO)8, Ir2(CO)7(NHC), and Ir 2(CO)6(NHC)2), the D3d-based isomers (2c, 2i, and 2n) consistently have the lowest relative energies, followed by the D2d-based isomers (2b). In contrast, among the unsaturated clusters (Ir2(CO)7, Ir2(CO)6(NHC) and Ir2(CO)5(NHC)2), the D3d-based isomers (2f and 2l) have the highest relative energies. These results can be explained by the high LDEs of the ligands on the Ir−Ir axis of D3d structures, which are 20−30 kcal/mol higher than the LDEs of the other

isomers. The dissociation of these ligands significantly increases the relative energies of the products. Except for those of the D3d-based isomers, the LDEs of the CO ligands range from 15.3 to 28.5 kcal/mol, and those of the NHC ligands range from 38 to 49 kcal/mol. There are no obvious trends in how these LDEs vary. As more NHC ligands are substituted for CO ligands, the trends in the NPA charges and C−O stretching frequencies of the Ir2(CO)y(NHC)z compounds become very similar to those of Ir(CO)y(NHC)z: (1) the positive charges of the NHC ligands decrease; (2) the negative charges of the CO ligands increase; and (3) the C−O stretching frequencies decrease (selected values are given in Table 6). The bridging CO ligands have more negative charge than do the terminal CO ligands, and the bridging C−O stretching frequencies are much lower (2a, 2g, and 2m). These differences between the bri and ter CO ligands show that the two Ir−CO bonds for the bri CO have more π-back-bonding even though they do not match as perfectly in terms of their orientation with the CO π* orbitals as do the ter CO ligands. As more NHC ligands are substituted for CO, the bri CO ligands accept more electrons, which leads to decreases in the Ir−CO(bri) bond distances from 2.09 to 2.05 Å and in the Ir−Ir distances from 2.78 to 2.75 Å; there is a corresponding increase in the Ir−Ir bond dissociation energies (BDEs), from 44 to 50 kcal/mol. In contrast, the isomers with all ter ligands (2c, 2i, and 2n) are not characterized by large changes in either the Ir−Ir distances or the BDEs. These results indicate that the stability of the Ir−Ir bonds may increase in the presence of bri CO ligands. DFT Benchmarks of the LDEs for Ir and Ir2 Clusters. The CCSD(T) dissociation energies of mononuclear complexes and di-iridium clusters were used in the above discussion. The dissociation of ligands leads to a coordinatively E


Article

The Journal of Physical Chemistry A Table 2. Predicted Ir(CO)y(NHC)z LDEs, in kcal/mol, as a Function of Electronic Structure Method LDE reaction

process

ΔH(0K) B3LYP

ΔH(0K) CAMB3LYP

ΔH(0K) M06

ΔH(0K) PW91

ΔH(0K) ϖB97X-D

ΔH(0K) MP2

ΔH(0K) CCSD(T)

Ir(CO)3 → Ir(CO)2 bent + CO Ir(CO)3 → Ir(CO)2 linear + CO Ir(CO)2(NHC) ax → Ir(CO)2 bent + NHC Ir(CO)2(NHC) ax → IrCO(NHC) bent + CO Ir(CO)2(NHC) ax → IrCO(NHC) linear + CO Ir(CO)2(NHC) eq → Ir(CO)2 linear + NHC Ir(CO)2(NHC) eq → IrCO(NHC) bent + CO IrCO(NHC)2 ax,ax → IrCO(NHC) bent + NHC IrCO(NHC)2 ax,ax → Ir(NHC)2 linear + CO IrCO(NHC)2 ax,eq → IrCO(NHC) bent + NHC IrCO(NHC)2 ax,eq → IrCO(NHC) linear + NHC IrCO(NHC)2 ax,eq → Ir(NHC)2 bent + CO Ir(NHC)3 → Ir(NHC)2 bent + NHC Ir(NHC)3 → Ir(NHC)2 linear + NHC Ir(CO)4 → Ir(CO)3 + CO Ir(CO)3(NHC) → Ir(CO)3 + NHC Ir(CO)3(NHC) → Ir(CO)2(NHC) ax + CO Ir(CO)3(NHC) → Ir(CO)2(NHC) eq + CO Ir(CO)2(NHC)2 ax,ax → Ir(CO)2(NHC) eq + NHC Ir(CO)2(NHC)2 ax,ax → IrCO(NHC)2 ax,ax + CO Ir(CO)2(NHC)2 ax,eq → Ir(CO)2(NHC) ax + NHC Ir(CO)2(NHC)2 ax,eq → IrCO(NHC)2 ax,eq + CO IrCO(NHC)3 → IrCO(NHC)2 ax,eq + NHC IrCO(NHC)3 → IrCO(NHC)2 ax,ax + NHC Ir(NHC)4 → Ir(NHC)3 + NHC

vertical adiabatic vertical

41.6 31.7 59.4

43.7 31.7 63.1

41.4 34.3 59.6

49.3 47.6 65.6

43.3 33.3 64.8

45.8 44.7 72.0

38.6 30.7 63.5

vertical

47.8

50.7

47.8

56.1

50.9

55.2

46.1

adiabatic

32.8

32.7

35.7

48.7

34.5

46.7

32.5

adiabatic

44.7

45.3

48.6

60.4

49.5

64.3

49.1

vertical

43.0

45.0

43.8

52.7

45.5

48.6

39.7

vertical

51.0

55.3

52.0

57.6

57.4

65.4

57.3

adiabatic

36.8

37.2

39.7

52.4

39.1

53.6

38.2

vertical

48.9

52.9

51.6

56.8

56.0

63.4

54.7

adiabatic

33.9

35.0

39.5

49.4

39.6

54.9

41.1

vertical

51.9

55.5

53.4

61.2

56.8

61.2

51.0

vertical adiabatic adiabatic adiabatic adiabatic

43.6 26.4 27.1 43.7 25.9

48.5 27.8 29.1 47.3 27.9

46.7 32.6 27.6 46.3 28.0

52.3 42.7 36.9 52.3 36.1

52.4 33.3 30.5 51.0 29.4

60.7 51.0 35.1 57.7 31.5

53.2 37.8 24.1 46.9 22.0

vertical

30.7

33.6

32.0

39.5

34.8

38.1

28.4

vertical

39.0

43.4

42.2

47.0

46.8

53.2

44.0

adiabatic

30.9

33.0

34.0

42.0

35.0

36.3

26.4

adiabatic

32.4

36.2

37.2

41.5

40.7

45.5

37.0

vertical

31.2

34.0

33.4

40.7

35.5

37.3

28.4

vertical

32.0

36.4

36.9

40.3

41.0

46.1

38.5

adiabatic

29.9

34.0

36.5

39.4

39.5

44.0

35.9

adiabatic

29.5

33.5

35.9

38.8

38.9

45.9

36.4

Table 3. Calculated NBO Charges of COs and NHCs, C−O Stretching Frequencies, and Bond Distances (r), for Compounds in the Family Ir(CO)y(NHC)z molecule Ir(CO)3 Ir(CO)2(NHC) ax Ir(CO)2(NHC) eq Ir(CO) (NHC)2 ax,ax Ir(CO) (NHC)2 ax,eq Ir(NHC)3 Ir(CO)4 Ir(CO)3(NHC) Ir(CO)2(NHC)2 ax,ax Ir(CO)2(NHC)2 ax,eq Ir(CO) (NHC)3 Ir(NHC)4 a

NBO charge (CO) −0.02a, 0.03b −0.07a, −0.05b −0.16a −0.12b −0.17a −0.05 −0.06, −0.16 −0.26 −0.15 −0.21

NBO charge (NHC) 0.18a 0.30b 0.13a 0.05a, 0.23b 0.02a, 0.15b 0.23 0.19 0.11 0.14, −0.05 0.12, −0.11

ν(C−O) (cm−1) 2033, 2113 2010, 2047 1957, 2026 1982 1951

r(C−O) (ax, Å)

r(C−O) (eq, Å)

r(Ir−CO) (ax, Å)

r(Ir−CO) (eq, Å)

1.156 1.165 1.170

1.159 1.168

1.904 1.873 1.877

1.829 1.817

1.176 1.180

2043(e), 2058, 2125 1970, 2008, 2063 1905, 1965 1953, 1995 1927

1.157 1.161

1.802 1.848

1.884 1.181

1.174

1.902 1.894

1.884 1.871

1.872 1.185

1.855

ax ligands. beq ligands.

unsaturated Ir atom, which is usually associated with increased catalytic activity in tetrairidium clusters.97 However, the size of

the tetrairidium clusters makes the CCSD(T) calculations computationally very expensive. We used various DFT F


Article


Figure 2. Optimized geometries of low-lying Ir2(CO)y(NHC)z isomers and CCSD(T) energies in kcal/mol.

Table 4. Relative Energies of Ir2(CO)y(NHC)z Isomers in kcal/mol Isomer

ΔH(0K) B3LYP

ΔH(0K) CAMB3LYP

ΔH(0K) M06

ΔH(0K) PW91

ΔH(0K) ϖB97X-D

ΔH(0K) MP2

ΔH(0K) CCSD(T)

Ir2(CO)8 (2c) Ir2(CO)8 (2b) Ir2(CO)8 (2a) Ir2(CO)7 (2e) Ir2(CO)7 (2d) Ir2(CO)7 (2f) Ir2(CO)7(NHC) (2i) Ir2(CO)7(NHC) (2h) Ir2(CO)7(NHC) (2g) Ir2(CO)6(NHC) (2k) Ir2(CO)6(NHC) (2j) Ir2(CO)6(NHC) (2l) Ir2(CO)6(NHC)2 (2n) Ir2(CO)6(NHC)2 (2m) Ir2(CO)5(NHC)2 (2o) Ir2(CO)5(NHC)2 (2p)

0.0 −1.2 4.7 0.0 10.8 29.6 0.0 2.1 3.2 0.0 3.3 17.8 0.0 0.7 0.0 6.1

0.0 0.1 6.1 0.0 12.6 30.0 0.0 2.1 3.1 0.0 3.0 18.4 0.0 −1.5 0.0 7.0

0.0 −0.9 5.1 0.0 11.0 30.2 0.0 1.9 2.9 0.0 7.0 18.4 0.0 −0.3 0.0 2.2

0.0 −1.9 0.8 0.0 4.6 28.8 0.0 −1.2 −1.1 0.0 3.4 24.1 0.0 −4.7 0.0 6.7

0.0 −0.2 6.5 0.0 12.4 29.7 0.0 2.5 3.6 0.0 2.9 17.8 0.0 −0.7 0.0 6.8

0.0 3.3 5.1 0.0 4.7 27.5 0.0 5.9 4.8 0.0 −6.6 21.0 0.0 3.5 0.0 19.7

0.0 1.7 7.4 0.0 11.3 27.4 0.0 4.8 5.4 0.0 1.3 16.0 0.0 2.2 0.0 8.8

functionals (Tables 2 and 5) to find a functional that provides reliable estimates of the LDEs of the tetrairidium clusters. The CAM-B3LYP functional predicted the most consistent reaction energies as compared with CCSD(T), with most energy differences being less than 4 kcal/mol.

The LDEs of the larger tetrairidium clusters were thus calculated at the CAM-B3LYP level. Ir4(CO)y(NHC)z Structures, Relative Energies, and LDEs. We systematically investigated the tetrairidium clusters in the family ranging from monosubstituted G


Article

The Journal of Physical Chemistry A Table 5. Ir2(CO)y(NHC)z LDEs in kcal/mol reaction number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

LDE reaction

ΔH(0K) B3LYP

ΔH(0K) CAMB3LYP

ΔH(0K) M06

ΔH(0K) PW91

ΔH(0K) ϖB97X-D

ΔH(0K) MP2

ΔH(0K) CCSD(T)

19.4 14.6 43.0 40.4

23.6 17.1 47.2 47.3

19.7 14.6 44.0 42.4

26.9 25.1 52.0 47.2

25.0 19.3 48.8 51.8

29.5 26.6 57.5 55.6

20.9 15.3 44.4 48.6

30.8

35.7

32.4

42.6

40.5

49.8

37.9

62.4

67.8

64.6

70.3

72.7

83.2

70.2

22.4

26.0

26.5

29.8

27.6

22.2

21.5

19.1

23.0

19.5

26.4

24.6

28.8

20.2

20.3

24.0

20.5

26.4

25.8

27.7

20.7

40.1

44.5

40.8

49.4

46.0

57.0

41.5

35.8

42.9

37.9

42.9

47.4

54.6

44.4

54.3

59.8

56.0

62.3

64.5

75.4

62.5

21.2

25.0

25.5

28.9

26.7

18.5

19.7

27.3

32.0

27.7

35.6

33.5

38.2

28.5

28.0

30.6

27.4

31.0

32.9

41.7

30.7

Ir2(CO)8 (2a C2v) → Ir2(CO)7 (2d) + CO Ir2(CO)8 (2b C2) → Ir2(CO)7 (2e) + CO Ir2(CO)8 (2c D3d) → Ir2(CO)7 (2f) + CO Ir2(CO)7(NHC) (2g) → Ir2(CO)7 (2d) + (NHC) Ir2(CO)7(NHC) (2h) → Ir2(CO)7 (2e) + (NHC) Ir2(CO)7(NHC) (2i) → Ir2(CO)7 (2f) + (NHC) Ir2(CO)7(NHC) (2g) → Ir2(CO)6(NHC) (2j) + CO Ir2(CO)7(NHC) (2g) → Ir2(CO)6(NHC) (2k) + CO Ir2(CO)7(NHC) (2h) → Ir2(CO)6(NHC) (2k) + CO Ir2(CO)7(NHC) (2i) → Ir2(CO)6(NHC) (2l) + CO Ir2(CO)6(NHC)2 (2m) → Ir2(CO)6(NHC) (2p) + (NHC) Ir2(CO)6(NHC)2 (2n) → Ir2(CO)6(NHC) (2l) + (NHC) Ir2(CO)6(NHC)2 (2m) → Ir2(CO)5(NHC)2 (2o) + CO Ir2(CO)6(NHC)2 (2m) → Ir2(CO)5(NHC)2 (2p) + CO Ir2(CO)6(NHC)2 (2n) → Ir2(CO)5(NHC)2 (2p) + CO

Table 6. Calculated NBO Charges of CO and NHC ligands, C−O Stretching Frequencies, Bond Distances (r), and Bond Dissociation Energies (BDE) for Ir2(CO)y(NHC)z molecule

a

NBO charge (CO)

NBO charge (NHC)

2a

0.01 , −0.28

2g

0.04a, −0.41b

0.28

2m

−0.03a, - 0.46b

0.25

2c 2i 2n

−0.10 −0.15 −0.20

0.35 0.30

a

b

ν(C−O) (cm−1)

r(Ir−CO) (bri, Å)

r(Ir−Ir) (Å)

BDE(Ir−Ir) (kcal/mol)

2067−2130a 1891−1899b 2037−2105a 1770−1878b 2020−2071a 1722−1738b 2043−2135 1990−2108 1970−2054

2.090

2.777

43.9

2.080

2.762

48.9

2.053

2.746

50.3

2.833 2.837 2.836

51.4 54.3 52.5

ter CO. bbri CO.

controls the exact structures and that stronger π-acceptors favor Td structures. On the basis of the above discussion regarding the di-iridium clusters, we suggest that the bri CO ligands that carry more negative charge lower the energies of the isomers derived from the C3v structure, and that the structures derived from the C3v structure become increasingly important as more electrons are inserted into the σ-framework by the NHC ligands. This model explains the increase in the energy difference between the C3vderived isomers and T d -derived ones, and why, for coordinatively saturated isomers, those with bri NHC ligands (4a, 6a, 8a, and 10a) have higher relative energies than other C3v-derived structures. The dissociation of CO leads to coordinatively unsaturated isomers. A transformation of a bri NHC into an eq NHC (reactions (4) and (35), Figure S7 in the Supporting Information) can occur during a dissociation of CO, analogous to the transformation of a bri PH3 into an eq PH3 considered previously.39 The NHC ligand that shares the Ir atom with an eq CO ligand moves to this eq position after dissociation of the

(Ir4(CO)11(NHC)) to tetra-substituted (Ir4(CO)8(NHC)4). For each structure, we considered various isomers according to the positions (apical, equatorial, bridging, and axial) of ligand substitution and where dissociation occurs. In total, 117 Ir4(CO)y(NHC)z isomers were optimized, including those derived from both C3v and Td structures, as shown in the Supporting Information, with isomer relative energies summarized in Table 7. For the parent Ir4(CO)12 clusters, the isomer with the Td structure has the lowest energy. We previously predicted39 that the Td-based structure is of lower energy when one CO ligand is substituted by a PH3, but that the C3v-based structure is lower in energy after a second CO is replaced by a PH3. The lowest-energy isomer of Ir4(CO)11(NHC) is derived from the C3v structure (4c), with the isomer derived from the Td structure (4e) being 2.5 kcal/mol higher in energy. The energy differences increase from to 7.7 to 13.9 to 16.3 kcal/mol when additional NHC ligands are substituted for CO ligands. As the NHC ligands are not π-acceptors, this result is consistent with the hypothesis that the π-acidity of the substituting ligands H


Article

The Journal of Physical Chemistry A Table 7. Relative Energies of Ir4(CO)y(NHC)z Isomers at the CAM-B3LYP Level in kcal/mol

a b c d e f g h i j k l m n o p q r s t u v w x y z a

isomers 3a− 3e

isomers 4a− 4e

isomers 5a− 5x

isomers 6a− 6g

isomers 7a− 7x

isomers 8a− 8f

isomers 9a− 9z

isomers 10a− 10e

isomers 11a− 11o

2.5 14.7 4.4 1.9 0.0a

13.5b 2.1 0.0 3.5 2.5a

19.6b 11.9b 8.9 30.7b 22.3b 0.0 6.0 9.9 21.6 8.6 6.8 3.4 18.7 8.0 8.4 22.8 7.8 22.3 15.2 12.2 24.1 8.4 4.5a 6.3a

8.0b 4.4 2.0 5.5 0.0 7.7a 7.9a

10.7b 18.5b 19.0b 28.2b 5.4 0.0 22.8 5.1 6.7 20.1 6.8 2.5 9.9 12.5 25.3 5.1 21.5 3.6c 5.5 17.7 5.5 11.4a 9.5 10.2

10.6b 4.6 8.4 0.0 16.8a 13.9a

16.3b 17.8b,c 34.4b 18.3b 29.4c 19.1c 19.9b 11.3 11.9 10.3 28.0 27.9 0.0 2.1 7.5c 22.8c 27.9 13.7 27.9 5.1 13.7 22.1 19.0a 22.6a 12.1 15.1

17.6b 7.7 7.7 0.0 16.3a

23.2b 22.8b 9.1 21.1b 6.7 21.6c 17.8 0.7c 14.0 30.6 13.7 0.0 10.2 25.3 22.3

Isomers with Td-based structure. bIsomers with bridging NHCs. cIsomers with μ3-COs.

enough to form a μ3-CO ligand. Additional bri CO ligands between an apical Ir atom and a basal Ir atom are predicted to form in 9b, 9e, and 9f. The transformations of a bridging to an equatorial NHC ligand and the formation of μ3- and bridging carbonyl ligands stabilize these product isomers, leading to a decrease in the CO LDEs. The LDEs of the NHC and carbonyl ligands of the Ir4 clusters are summarized in Table 9, together with the adiabatic reaction energies. The LDEs of the NHC ligands on each site of the lowest-energy isomers gradually decrease (except for one in C3v bri and one in C3v ax) as more NHC ligands are substituted for CO ligands. This result is consistent with our model that increasing the number of NHC ligands increases the σ-electron repulsion and weakens the Ir−NHC bonds. Among the four sites in the C3v-based structure, the eq NHC ligands have the highest LDEs, ranging from ∼61 to ∼69 kcal/mol, followed by the ax NHC ligands, which range from ∼54 to ∼63 kcal/mol. The range of the LDEs of the api NHC ligands is from ∼35 to ∼55 kcal/mol, which is similar to those of ter NHC ligands in Td-based structures, which range from ∼33 to ∼54 kcal/mol. Consistent with the high relative energies of the isomers having bri NHC ligands, the LDEs of NHC on the bri site are the lowest, ranging from ∼25 to ∼45 kcal/mol. The Ir4 clusters have CO ligands with lower LDEs, ranging from ∼28 to ∼57 kcal/mol, and the range is comparable to that of the LDEs of the NHC ligands. The carbonyl ligands on the eq sites still have the highest LDEs, ranging from ∼49 to ∼57 kcal/mol. The trend in the LDEs of the NHC ligands differs from that of the bri CO ligands, which have the second- highest LDEs, ranging from ∼37 to ∼43 kcal/mol. The LDEs of the ax CO ligands, bri CO ligands, and ter CO ligands in the Td-based

CO. In reactions (82), (83), (108), and (113), carbonyls binding to 3 Ir atoms (μ3-CO) are observed in the products 9o, 9p, 11f, and 11h. The stretching frequencies and NBO charges of the ter, bri, and μ3-CO ligands of these four molecules are listed in Table 8. Among these CO ligands, the μ3-CO ligands Table 8. Stretching Frequencies and NBO Charges of ter, bri, and μ3-COs of 9o, 9p, 11f, and 11h molecule

ν(C−O) (cm−1)

NBO charge

9o ter CO 9o bri CO 9o μ3-CO 9p ter CO 9p bri CO 9p μ3-CO 11f ter CO 11f bri CO 11f μ3-CO 11h ter CO 11h bri CO 11h μ3-CO

1986−2062 1746−1772 1635 1917−2062 1755−1860 1659 1915−2019 1860 1645 1982−2023 1724−1758 1512

−0.07 −0.42 −0.60 −0.08 −0.34 −0.59 −0.13 −0.44 −0.55 −0.07 −0.22 −0.79

have the lowest frequencies and the highest negative charges, suggesting the strongest π-back-bonding. Considering the triand tetra-substituted Ir4 clusters that have high electron densities on the metal, the results show that the dissociation of one carbonyl ligand leads to an Ir4 core with a greater negative charge. Then, the formation of a μ3-CO ligand can effectively stabilize the structure. No μ3-CO ligands were found in the dissociation products of the mono- and disubstituted Ir4 clusters, suggesting that their electron densities are not high I


Article

The Journal of Physical Chemistry A Table 9. Ir4(CO)y(NHC)z LDEs at CAM-B3LYP Level in kcal/mola molecule

C3v eq

C3v ax

C3v api

C3v bri

Td

Ir4(CO)11(NHC) Ir4(CO)10(NHC)2

68.9(4b) 71.8(6a) 66.3(6b) 65.8(6c) 65.9(6d) 65.7(8a) 63.5(8b) 61.2(8c)

60.8(4c) 53.9(6c) 56.9(6e)

54.8(4d) 47.0(6d)

45.4(4a) 41.1(6a)

53.8(4e) 45.9(6f) 47.5(6g)

56.5(8a) 48.6(8b) 44.8(8c) 53.6(8d) 41.3(10a) 43.2(10b)

42.2(8c)

44.1(8a)

42.6(8e) 40.9(8f)

40.7(10a) 35.0(10c)

25.1(10a)

32.6(10e)

46.6(10c) 62.6(10d) 38.8(4a)

36.1(4a)

28.4(4a)

32.0(4e) 33.7(4e)

36.4(4b) 34.7(4b) 37.7(4c)

33.8(4b) 31.2 (4b) 37.9(4c)

27.8(4b) 37.7(4b) 38.4(4c)

34.9(4d)

41.7(4d)

38.7(4d)

42.1(6a)

33.8(6a)

41.6(6a)

49.5(6b)

31.9(6b)

26.7(6b)

32.2(6b)

49.3(6c) 50.9(6d) 48.9(6e) 38.2(8a)

36.0(6c) 30.8(6d) 36.6(6e) 39.0(8a)

35.9(6c) 38.2(6d) 36.7(6e) 48.4(8a)

52.9(8b)

36.4(8b)

49.2(8c) 51.8(8d) 23.2(10a) 54.7(10c) 57.0(10d)

49.2(8c)

31.7(6c) 35.5(6d) 34.8(6e) 35.4(8a) 36.8(8a) 37.0(8b) 25.1(8b) 28.8(8c) 34.8(8d) 37.3(10a) 30.7(10b) 24.8(10c) 31.7(10d)

Ir4(CO)9(NHC)3

Ir4(CO)8(NHC)4

Ir4(CO)11(NHC)

Ir4(CO)10(NHC)2

Ir4(CO)9(NHC)3

Ir4(CO)8(NHC)4

a

57.4(10a) 60.9(10b) 60.8(10b) 60.8(10c) 25.4(4a) 47.2(4a) 49.4(4b) 48.6(4c) 52.8(4c) 48.8 (4d) 50.6 (4d) 28.5(6a)

35.2(10a) 41.8(10b) 37.7(10c)

27.1(8b) 44.1(8c) 43.4(8d) 36.9(10a) 45.7(10b) 38.0(10c) 41.9(10d)

33.0(6f) 30.2(6f) 33.4(6g) 28.7(6g)

31.9(8e) 35.4(8e) 27.9(8f) 30.9(8f)

37.8(10e)

LDEs for the lowest energy isomers in italics. Isomers in parentheses are the coordinatively saturated reactants before dissociation.

structures are similar, ranging from ∼28 to ∼38 kcal/mol. As more CO ligands are replaced by NHC ligands, only the LDEs of the eq CO ligands increase. There is no discernible trend in the LDE values of the remaining CO LDEs. These results suggest that any electron density changes were distributed over a larger number of sites so that individual electronic effects are of minor importance. For some isomers, a hydrogen bond is found between an O atom of CO and an H(N) atom of the NHC. Such intramolecular hydrogen bonds can stabilize these isomers as the number of hydrogen bonds increases. Comparison of Experimental and Computational Structures for a Monocarbene Complex. For a single phosphine ligand, we predict that the energy of the monosubstituted tetrairidium cluster with a ax PH3 in a C3vderived structure is higher in energy than the Td-derived structure by 1.1 kcal/mol at the ω-B97X-D level.39 The C3vderived structure for Ir4(CO)11PH3 with an ax PH3 is predicted to be lower in energy by 2.8 kcal/mol at the M06 level. A single

crystal X-ray crystallography study showed a C3v-derived structure with a phosphine-calixarene ligand in the axial position,81 as is generally found. Thus, the predicted energy difference between the C3v- and Td-derived structures is small and is functional dependent. The calculated C3v-based ax structure at the local SVWN DFT level has average bond lengths shorter for r(Ir−Ir) by 0.007 Å and longer by 0.003 Å for r(Ir-CO(bri)) as compared to experiment. The calculated value for r(Ir−P) is shorter than experiment by 0.032 Å. Two calixarene-carbene ligands consisting of Ca and Cb were synthesized, and these were attached to a tetrairidum cluster core in compounds D and E, respectively. According to FTIR spectroscopy (see Figure 3), which shows no bridging only terminal CO bands for both D and E, both of these clusters possess a Td-derived structure, in contrast to the C3v derived structure for the monosubstituted phosphine cluster.81 This Td-derived structure is further confirmed via single crystal X-ray crystallography of D (Figure 4). The DFT calculations predict the Ir4(CO)11NHC cluster with a much smaller carbene J


Article


CAM-B3LYP and M06 functionals predict respective larger energy differences favoring the C3v derived structure by 2.5 and 6.0 kcal/mol. The calculated Td derived structure (4e) has average bond lengths for r(Ir−Ir) longer by 0.011 Å and shorter by 0.029 Å for r(Ir-CO) than experimentally observed for D. The calculated r(Ir-NHC) bond distance is 0.049 Å shorter than experiment observed experimentally for D. The CO stretching frequencies of the calculated and experimental clusters (C3v for the phosphine and Td for the carbene) are listed in Table 10. There is good agreement between experiment and theory with the calculated values (corrected by 17 cm−1 from the difference between the calculated and experimental value98 for CO). Computed values are uniformly higher than experiment within 20 cm−1 for the terminal COs and within 40 cm−1 for the bridging COs. In addition, compound F, the mixed diphosphine/monocarbene complex was synthesized and consisted of a single axial calixarene-phosphine ligand, an equatorial calixarene-phosphineligand, and what is likely to be an equatorial calixarenecarbene ligand (the latter on the basis of steric considerations). This assignment is made on the basis of the 31P NMR spectrum (Supporting Information), which clearly differentiates equatorial and axial phosphines, and by analogy with the structure of the triphosphine tetrairidium complex, which consists of one axial and two equatorial phosphine ligands.99 On the basis of the infrared spectrum of the molecule (see Figure 5), it was assigned to a C3v-derived structure. In agreement with experiment, the calculated C3v-derived structure (Figure 6a) for Ir4(CO)9(PH3)2(NHC) is more stable than the Td-derived structure (Figure 6b) by 4.6, 5.2, and 9.3 kcal/mol at the wB97x-D, CAM-B3LYP, and M06 levels, respectively. This is consistent with the assigned structure for F. The calculated C3v derived structure for Ir4(CO)9(PH3)2(NHC) has the two PH3 and the NHC ligands in the equatorial position. The calculated CO stretching frequencies (including the 17 cm−1 correction) of the C3v-derived Ir4(CO)9(PH3)2(NHC) clusters are listed in Table 11 and compared to the experimental results for F. The

Figure 3. FTIR spectra of monocarbene complexes (a) D and (b) E.

(4c) to have the NHC in an ax position with a C3v-derived structure. At the ω-B97X-D level at the local DFT geometry, the C3v-derived is predicted to be lower in energy than the Td structure (4e) by 1.2 kcal/mol. This small energy difference on the order of 1 kcal/mol as compared to experiment is within the accuracy of the method, especially considering the differences in the carbene ligand from experiment. The

Figure 4. Crystal structure of monocarbene complex F. K


Article


Table 10. Calculated and Experimental CO Stretching Frequencies (cm−1) of Ir4(CO)11L Clusters with a Single Ligand ν(C−O) (cm−1)

calculated Ir4(PH3) (CO)11 (C3v, 4a)a

experimental ir4(PR3) (CO)11 (C3v)81

calculated Ir4(CO)11NHC (Td, 4e)a

experimental Ir4(CO)11NHC (Td, E)

terminal CO

2110(376)/2093 2082(1048)/2065 2081(1539)/2064 2058(1027)/2041 2045(686)/2028 1893(692)/1876 1877(774)/1860

2088

2115(387)/2098 2085(1734)/2068 2066(889)/2049 2022(350)/2005

2087 2047 2025 2003

bridging CO a

2055 2022 1848 1821

Values in parentheses are infrared intensities in km/mol. Values after the slash are corrected by 17 cm−1. See text.

terminal CO stretches, which are predicted to be in the range of 2059 to 1925 cm−1. The calculated values for the Td derived structure clearly cannot account for the experimentally observed spectra confirming the presence of the C3v-derived structure for F. Comparison of the LDEs for Ir4(CO)y(NHC)z and Ir4(PH3)y(CO)z. The LDEs for the isomers with the lowest energy of both the Ir4(CO)y(NHC)z and Ir4(PH3)y(CO)z clusters are listed in Table 12. Most LDEs of the NHC ligands Table 12. LDEs for the Ir4(CO)y(NHC)z and Ir4(PH3)y(CO)z Isomers with the Lowest Energies in kcal/ mola Ir4(CO)y(NHC)z

Figure 5. FTIR spectra of monocarbene complex F. monosubstitution

disubstitution

trisubstitution

Figure 6. Calculated Ir4(CO)9(PH3)2(NHC) structures at SVWN5/ cc-PVDZ (ECP). (a) C3v-derived structure. (b) Td-derived structure.

Table 11. Calculated and Experimental CO Stretching Frequencies (cm−1) of Ir4(CO)9L3 Clusters with Mixed Ligands ν(C−O) (cm−1) terminal CO

bridging CO

calculateda Ir4(CO)9(PH3)2(NHC) (C3v, Figure 6a)

experimental Ir4(CO)9L2C, F (C3v)

2057(954)/2040 2041(1164)/2024 2001(688)/1984 1991(794)/1974 1854(697)/1837 1852(692)/1835

2027 1992 1985 1784 1765

tetra-substitution

a

Ir4(PH3)y(CO)z

substitution site

CO

NHC

CO

PH3

eq ax api bri ter(Td) eq ax api bri ter(Td) eq ax api bri ter(Td) eq ax api bri ter(Td)

48.6 37.7 37.9 38.4 32.0 48.9 36.6 34.8 36.7 30.2 51.8 36.4 34.8 43.4 37.8 57.0 37.7 31.7 41.9 37.8

68.9 60.8 54.8 45.4 53.8 65.8 56.9 47.0 41.1 45.9 63.5 53.6 42.2 44.1 40.9 60.8 62.6 35.0 25.1 32.6

53.4 43.9 42.5 43.5 38.2 53.5 48.9 39.2 45.9 39.2 59.4 50.9 40.7 43.7 38.7 60.3

50.8 40.2 37.1 29.1 38.0 49.4 40.7 33.8 30.0 34.1 51.1 41.5 32.7 29.2 33.6 48.5 35.4 32.0 28.3 32.7

47.0 43.3

Ir4(PH3)y(CO)z values are from ref 39.

of the Ir4(CO)y(NHC)z isomers are greater than the LDEs of the CO ligands. The differences between these two sets of LDEs decrease corresponding to the decrease of the LDEs of the NHC ligands as more CO ligands are replaced. The LDEs of the CO ligands of Ir4(CO)8(NHC)4 are even greater than the LDEs of the NHC ligands on the api, bri, and ter(Td) sites. The LDEs of the CO ligands of the Ir4(PH3)y(CO)z isomers are consistently higher than the LDE’s of the PH3 ligands. The general qualitative order for the LDEs is the following: NHC (25−69 kcal/mol) > CO in the Ir4(PH3)y(CO)z isomers (39− 59 kcal/mol) > CO in the Ir4(CO)y(NHC)z isomers (32−57

a

Values in parentheses are infrared intensities in km/mol. Values after the slash are corrected by 17 cm−1. See text.

calculated values are in good agreement with the experiment for the terminal COs, with deviations less than 11 cm−1. The calculated stretching frequencies for the bridging COs are higher than experiment by 53 and 70 cm−1, just as found for the monosubstituted ligand case. The Td-derived structure has only L


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bonding than for the ter CO ligands. The bri CO ligands also act to increase the Ir−Ir bond energy. The C3v-derived Ir4(CO)y(NHC)z isomers are of lower energy than the Td-derived ones, and the energy difference increases as more CO ligands are replaced. The isomers with bri NHC ligands are usually of higher energy. These results can be explained by the stronger π-back-bonding between the Ir atom and bri CO ligands that stabilizes the structure analogous to that involving the bri CO ligands in the Ir2(CO)y(NHC)z isomers. The products of a CO dissociation may undergo a geometric transformation whereby a bri NHC ligand becomes an eq NHC ligand if the NHC shares an Ir atom with the eq CO that is removed. A μ3-CO ligand that carries more negative charge than a bri CO ligand forms to further stabilize the structure after a CO ligand is dissociated from a tri- or tetraNHC-substituted Ir4 isomer. A comparison with data from two newly synthesized NHCmonosubstituted tetrairidium clusters D and E that employ novel sterically bulky NHC-calixarene ligands demonstrates both clusters to be stable in the Td derived form when dissolved in solution. This was the same form observed for D in the solid state, as ascertained via single-crystal X-ray diffraction, and is consistent with the small difference in the calculated energies between the Td and C3v derived forms. In addition, a mixed trisubstituted tetrairidium cluster consisting of two phosphinecalixarene ligands and a single NHC-calixarene ligand in the basal plane yielded C3v derived form in solution, which is in good agreement with calculations, along with its observed infrared spectrum. As more NHC ligands are substituted for the CO ligands in the tetramer, the LDEs of the NHC ligands in the isomers with the lowest energies decrease for most isomers, consistent with the trend for the PH3 ligands predicted previously.39 The LDEs of the NHC ligands decrease by a larger amount as compared to PH3 because of stronger electron repulsion and steric effects. There is, however, no discernible trend for how the LDEs of the CO ligands change in the Ir4(CO)y(NHC)z isomers, consistent with what we have found for the Ir4(PH3)y(CO)z clusters. A possible reason for the lack of a trend in the CO LDEs is that the additional electron density resulting from the substitution of the NHC ligands is distributed over a larger number of sites, and local effects at a specific Ir atom begin to dominate. The general order for the LDEs is the following: NHC > CO (Ir4(PH3)y(CO)z) > CO (Ir4(CO)y(NHC)z) > PH3. For both the NHC and PH3 ligands, the LDEs have the order eq > ax > api ≈ ter (Td) > bri. The eq CO ligands are consistently characterized by the highest LDEs, followed by the bri CO ligands in the Ir4(CO)y(NHC)z isomers, whereas the bri, ax, and api CO ligands in the Ir4(PH3)y(CO)z isomers are characterized by similar LDEs. This pattern might be attributed to the higher electron densities in the Ir4(CO)y(NHC)z isomers that increase the importance of the bri CO ligands.

kcal/mol) > PH3 (28−51 kcal/mol). The relatively low LDEs of the NHC ligands in the Ir4(CO)8(NHC)4 isomers could be an indication of steric effects associated with the four NHC ligands in the reactants. Although the LDEs of the PH3 ligands also exhibit a decreasing trend, the percentage decrease is smaller with respect to the changes in the LDEs of the NHC ligands because the PH3 is less bulky than the NHC, and the PH3 is not as strong a σ donor as the NHC, so that there is less electron repulsion between the electrons donated by the ligand to the iridium cluster. The LDEs of the NHC and the PH3 ligands on a given site all follow the order eq > ax > api ≈ ter (Td) > bri. The eq CO ligands consistently have the highest LDEs. The bri CO ligands have the second-highest LDEs among the Ir4(CO)y(NHC)z isomers, . The LDEs of the bri CO ligands of the Ir4(PH3)y(CO)z isomers are similar to those of the ax and the api CO ligands (39−51 kcal/mol). A possible reason for the difference in the order is that the NHC ligands, as stronger σ donors, donate more electron density to the iridium cluster than do the PH3 ligands. This difference in electron donation makes the bri CO ligands in the Ir4(CO)y(NHC)z isomers more important in stabilizing the structures through stronger πback-bonding.

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CONCLUSIONS The structures and bond energies of a family of compound Irx(CO)y(NHC)z (x = 1, 2, 4) were calculated. The LDEs for the Ir1 complexes and Ir2 clusters calculated at the correlated molecular orbital MP2 and CCSD(T) levels and at the DFT level with several exchange-correlation functionals. The benchmark calculations show that the LDEs at the CAM-B3LYP level provide the best fit with the results of the CCSD(T) calculations. Therefore, the energies of tetrairidium clusters were predicted at the CAM-B3LYP level. All of the reported LDEs account for the breaking of Lewis acid−base donor− acceptor type bonds leading to the colosed shell CO and NHC ligands. For the mononuclear iridium complexes 2Ir(CO)y(NHC)z, the NHC ligands have higher LDEs than the CO ligands, consistent with NHC’s being a stronger σ donor. The LDEs of the CO ligands increase as more CO ligands are substituted by NHC ligands, both for the ax and eq sites, whereas the LDEs of the NHC ligands decrease. The increase in the LDEs of CO ligands is attributed to the additional electron density donated by the NHC ligands to the σ framework that results in stronger π-back-bonding to the CO. The decrease in the CO stretching frequencies, the increase in the C−O bond lengths, the decrease in the Ir−CO bond lengths, and the increase in the negative charges on the CO ligands confirms the existence of the π-back-bonding. Because the NHC is a poor π acceptor, the additional electrons on the Ir atom lead to a stronger σ-electron repulsion and a decrease the LDEs of the NHC ligands. The D3d-based isomers, which are coordinatively saturated, with all ter ligands for the di-iridium clusters are more stable than the C2v-based isomers with two bri CO ligands. The M−L bonds on the Ir−Ir axis have strong bonds, and dissociation of these ligands significantly increases the relative energies. The trends for the changes in the C−O stretching frequencies and NBO charges as more NHC ligands are substituted for CO ligands are the same as those of the Ir(CO)y(NHC)z isomers. The increase in electron density and the lower C−O stretching frequencies of the bri CO ligands indicate stronger π-back-

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpca.7b04161. Complete citations for refs 76 and 77, experimental synthesis details, NMR spectra characterizing compounds, crystal data and structure refinement information, computational geometry information, CAM-B3LYP LDEs, low-lying Ir4(CO)y(NHC)z structures and reacM


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tions 1−122 for the tetramers, and optimized Cartesian (x,y,z) coordinates in Å (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Alexander Katz: 0000-0003-3487-7049 David A. Dixon: 0000-0002-9492-0056 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences, Catalysis Center Program and contract DE-SC0005822, for financial support. AK, AO, and AS are grateful to the Management and Transfer of Hydrogen via Catalysis Program funded by Chevron Corporation and the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (DE-FG0205ER15696) for additional support. Some of the computational work was performed at the Molecular Science Computing Facility, William R. Wiley Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is operated for DOE by Battelle Memorial Institute under Contract No. DE-AC06-76RLO-1830. We thank the Alabama Supercomputing Center for providing computational resources. D.A.D. thanks the Robert Ramsay Chair Fund of The University of Alabama for support.

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