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Hypoelectronicity and Chirality in Dimetallaboranes of Group 9 Metals Szabolcs Jákó,† Alexandru Lupan,*,‡ Attila-Zsolt Kun,*,† and R. Bruce King*,§ †

Department of Chemistry and Chemical Engineering, Hungarian Line of Study, Faculty of Chemistry and Chemical Engineering, Babeş-Bolyai University, Cluj-Napoca 400084, Romania ‡ Department of Chemistry, Faculty of Chemistry and Chemical Engineering, Babeş-Bolyai University, Cluj-Napoca 400084, Romania § Department of Chemistry and Center for Computational Quantum Chemistry, The University of Georgia, Athens, Georgia 30602, United States S Supporting Information *

ABSTRACT: The structures and energetics of the dimetallaboranes Cp2M2Bn−2Hn−2 (n = 8, 9, 10, 11, 12; M = Co, Rh, Ir; Cp = η5-C5H5) were studied using density functional theory. The lowest energy Cp2M2B6H6 and Cp2M2B7H7 structures are chiral C2 structures based on the corresponding closo deltahedra, namely the bisdisphenoid and the tricapped trigonal prism. The permethylated iridaboranes Cp*2Ir2B6H6 and Cp*2Ir2B7H7 (Cp* = η5-Me5C5) were synthesized by Ghosh and co-workers. However, they were found by Xray crystallography to have nondeltahedral structures containing a quadrilateral face, namely a bicapped trigonal prism and a capped square antiprism for the 8- and 9-vertex systems, respectively. These structures correspond to a mean of the two opposite enantiomers and can also represent the “square” intermediate in the interconversion of the two enantiomers. The lowest energy structures for the 10-vertex Cp2M2B8H8 systems are two isocloso deltahedra with one metal atom at a degree 6 vertex and the other metal atom at a degree 5 vertex. Both isomers have been realized experimentally for Cp2Ir2B8H8. The lowest energy structures for the 11-vertex Cp2M2B9H9 systems have central closo/isocloso deltahedra with one metal atom at a degree 6 vertex and the other metal atom at a nonadjacent degree 5 vertex. This structure type has been found experimentally in both the rhodaboranes and iridaboranes Cp*2M2B9H9 (M = Rh, Ir). The lowest energy structures for the 12vertex systems Cp2M2B10H10 (M = Co, Rh, Ir) are deltahedra with two adjacent degree 6 vertices for the metal atoms. This type of structure is found experimentally in the rhodium complexes Cp*2Rh2B10H10−n(OH)n (n = 1, 2).

1. INTRODUCTION The pioneering work of Hawthorne and co-workers1 on metalladicarbaboranes included not only the synthesis of the 2n + 2 skeletal electron system Cp2Co2C2B6H8 but also the hypoelectronic 2n skeletal electron system Cp2Fe2C2B6H8.2 The structure of Cp2Co2C2B6H8 was found to be the expected 10-vertex closo deltahedron, namely the bicapped tetragonal antiprism by the Wade-Mingos rules.3−5 However, the structure of the hypoelectronic Cp2Fe2C2B6H8 was found to have an additional Fe−Fe edge. Subsequent work by Kennedy and coworkers6−9 on related metallaboranes of the second and third row transition metals led to the identification of a new series of deltahedra, called either isocloso10 or hypercloso11−13 deltahedra. Such deltahedra provide a degree 6 vertex for the metal atom and differ from the closo deltahedra for the 9- and 10-vertex systems (Figure 1). The diferradicarbaborane Cp2Fe2C2B6H8 discovered in 1975 by Hawthorne and co-workers2 represented the first example of a 10-vertex isocloso structure although it was not recognized as such at the time of its discovery. The 11-vertex closo deltahedron already has a degree 6 vertex for a metal atom and thus can serve as an isocloso as well as a closo deltahedron (Figure 1). A 12-vertex deltahedron with only one degree 6 vertex is topologically not possible. However, a 12-vertex deltahedron with two degree 6 vertices and thus two degree 4 vertices can serve as an isocloso deltahedron for a 12© XXXX American Chemical Society

Figure 1. Isocloso deltahedra and related deltahedra with degree 6 vertices.

vertex system. For the 8-vertex system, a capped pentagonal bipyramid is the closest equivalent to an isocloso deltahedron. It provides a degree 6 vertex for a metal atom and the central Received: September 20, 2016

A

DOI: 10.1021/acs.inorgchem.6b02281 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. Cp*2Ir2Bn−2Hn−2 (n = 8, 9, 10) diiridaboranes synthesized by Ghosh and co-workers17,18 shown as diamond-square-diamond processes for the interconversion of the C2 enantiomers in the 8- and 9-vertex systems. Unlabeled vertices are boron atoms. External hydrogen atoms and Cp* rings are omitted for clarity. Experimentally known structures are enclosed in boxes.

None of the experimental Cp*2Ir2Bn‑2Hn−2 (Cp* = η5Me5C5; n = 8, 9) structures has the capped pentagonal bipyramid (for n = 8) or 9-vertex isocloso structures (Figure 2) that would be expected for dimetallaboranes having 2n skeletal electrons. However, both isomers of the 10-vertex Cp*2Ir2B8H8 system that were synthesized by Ghosh and co-workers18 have the expected isocloso 10-vertex structure for a 2n (= 20 for n = 10) skeletal electron systems (Figure 2). Both Cp*2Ir2B8H8 isomers have one iridium atom at the unique degree 6 vertex and the other iridium atom at one of the degree 5 vertices. The 11-vertex Cp*2M2B9H9 (M = Rh, Ir) structures were synthesized by Ghosh and co-workers18 and found by X-ray crystallography to have a central 11-vertex closo/isocloso deltahedron, as expected for a 2n skeletal electron system. The 12-vertex Cp2M2B10H10 (M = Co, Rh, Ir) systems do not appear to have been synthesized. However, traces of the related yellow hydroxydirhodaboranes Cp*2Rh2B10H9(OH) and Cp*2Rh2B10H8(OH)2 have been isolated along with larger amounts of 15- and 16-vertex rhodaboranes from the pyrolysis of a Cp*2Rh2Cl4/LiBH4·thf/BH3·thf mixture.20 These 12-vertex dirhodaboranes have been shown by X-ray crystallography to have the central 12-vertex deltahedron depicted in Figure 2 with the rhodium atoms at the adjacent degree 6 vertices. In view of these interesting diiridaborane structures recently synthesized by Ghosh and co-workers,17,18 we undertook a comprehensive density functional theory study of the complete series of dimetallaboranes Cp2M2Bn−2Hn−2 of the group 9 metals (M = Co, Rh, Ir). As was previously found for the isoelectronic Cp2Fe2C2Bn−4Hn−2 systems,15 only the 10-vertex Cp2M2B8H8 systems favored isocloso structures. The 12-vertex Cp2M2B10H10 systems were found to favor structures having two adjacent degree 6 vertices for the metal atoms rather than structures based on the regular icosahedron with exclusively degree 5 vertices.

pentagonal bipyramid is the 7-vertex closo deltahedron requiring 16 skeletal electrons (= 2n for n = 8) A topological model for the bonding in isocloso metallaboranes rationalized the 2n skeletal electron count in such systems in terms of three-center two-electron bonds in n faces of the isocloso deltahedron, including three of the faces meeting at the degree 6 metal vertex.14 A theoretical study on the 2n skeletal electron Cp2Fe2C2Bn−4Hn−2 systems indicated that isocloso structures were particularly favorable with 10-vertex systems.15 Furthermore, a theoretical study on the 2n skeletal electron CpMC2Bn−3Hn−1 (M = Mn, Re) systems suggested that isocloso structures are energetically more favorable for the third row transition metal rhenium than for the first row transition metal manganese.16 Experimental work on the Cp2M2Bn−2Hn−2 (M = Co, Rh, Ir) dimetallaboranes with 2n skeletal electrons was rather limited until the recent serendipitous syntheses of the diiridaboranes Cp*2Ir2Bn−2Hn−2 (Cp* = η5-Me5C5; n = 8, 9, 10) by Ghosh and co-workers (Figure 2).17,18 Two isomers of the 8-vertex Cp*2Ir2B6H6 system were synthesized. The yellow isomer was found to have a bisdisphenoidal structure corresponding to the 8-vertex closo deltahedron. The red Cp*2Ir2B6H6 isomer was found to have a bicapped trigonal prismatic structure with an open rectangular face. This structure of the red Cp*2Ir2B6H6 isomer corresponds to the square intermediate in the diamondsquare-diamond racemization of enantiomers of the yellow bisdisphenoidal Cp*2Ir2B6H6. The 9-vertex Cp*2Ir2B7H7 was also red and found to have a capped tetragonal antiprismatic structure, also with a nearly square open face. This latter diiridaborane structure contrasts with that of the related dicobaltamethoxyborane Cp*2Co2B7H6OMe, which Ghosh and co-workers19 found earlier by X-ray crystallography to have a central Co2B7 closo tricapped trigonal prism with the cobalt atoms as widely separated as possible at degree 5 vertices. B


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Inorganic Chemistry

Figure 3. Three lowest-energy Cp2M2B6H6 (M = Co, Rh, Ir) structures.

Table 1. Relative Energies (ΔH in kcal/mol), M−M Distances (Å) and WBIs, and Polyhedra in the Lowest Energy Cp2M2Bn−2Hn−2 (n = 8, 9) Structuresa M vertex structure

a

Cp2Ir2BnHn ΔE

degrees

Ir−Ir

B6M2−1 (C2) B6M2−2 (C2v) B6M2−3 (C1)

5,5 5,5 6,5

0.0(0.0) 0.3(0.5) 12.9

2.723 3.369 2.732

B7M2−1 (C2) B7M2−2 (Cs) B7M2−3 (C2v) B7M2−4 (Cs) B7M2−5 (Cs)

5,5 5,5 5,5 6,5 6,5

0.0(0.0) 0.5(5.0) 3.6(3.5) 5.5(3.8) 10.9(9.2)

3.555 2.675 2.703 2.750 3.361

Cp2Rh2BnHn WBI

ΔE

Rh−Rh

8-vertex structures 0.37 0.0 2.660 0.12 3.2 3.338 0.34 8.7 2.679 9-vertex structures 0.11 1.4 3.535 0.41 0.0 2.625 0.36 3.5 2.655 0.34 4.7 2.698 0.10 12.9 3.323

Cp2Co2BnHn WBI

ΔE

Co−Co

WBI

polyhedron

0.33 0.12 0.30

1.4 0.0 12.6

2.470 3.081 2.451

0.30 0.18 0.30

closo closo isocloso

0.11 0.35 0.31 0.30 0.10

0.0 3.4 4.8 11.4 14.9

3.294 2.435 2.473 2.505 3.065

0.16 0.33 0.30 0.27 0.15

closo closo closo isocloso isocloso

Relative energies of the permethylated Cp*2Ir2Bn−2Hn−2 structures are given in parentheses.

Table 2. Relative Energies (ΔH in kcal/mol), M−M Distances (Å) and WBIs, and Polyhedra in the Lowest Energy Cp2M2Bn−2Hn−2 (n = 10, 11, 12) Structuresa

a

structure

M vertex

symmetry

degrees

Cp2Ir2BnHn ΔE

Ir−Ir

B8M2−1 (Cs) B8M2−2 (Cs)

6,5 6,5

B9M2−1 (Cs) B9M2−2 (C1) B9M2−3 (Cs) B9M2−4 (C2)

6,5 6,5 6,5 6,6

0.0 3.8 8.2 14.9

3.585 2.726 3.795 3.467

B10M2−1 (C2v) B10M2−2 (C1)

6,6 6,5

0.0 23.7

2.745 3.851

0.0(0.0) 6.4(4.8)

2.740 3.525

Cp2Rh2BnHn WBI

ΔE

10-vertex structures 0.34 0.0(0.0) 0.08 9.0(6.0) 11-vertex structures 0.09 0.0 0.35 1.3 0.07 7.1 0.07 15.3 12-vertex structures 0.33 0.0 0.07 22.9

Cp2Co2BnHn

Rh−Rh

WBI

ΔE

Co−Co

WBI

polyhedron

2.692 3.499

0.30 0.08

0.0 4.2

2.512 3.249

0.27 0.11

isocloso isocloso

3.562 2.676 3.763 3.438

0.09 0.31 0.07 0.08

0.0 5.0 6.2 17.6

3.330 2.504 3.514 3.200

0.13 0.28 0.11 0.09

closo closo closo 2v6

2.699 3.596

0.29 0.08

0.0 10.3

2.569 3.379

0.25 0.11

2v6 2v6

Relative energies of the permethylated Cp*2Ir2B8H8 structures are given in parentheses. points after optimization were checked by calculations of the harmonic vibrational frequencies. If significant imaginary frequencies were found, the optimization was continued by following the normal modes corresponding to imaginary frequencies to ensure that genuine minima were obtained. Normally this resulted in reduction of the molecular symmetry. To obtain the lowest energy optimized structures of the pentamethylcyclopentadienyl derivatives, all of the hydrogen atoms in the Cp rings of the lowest-energy optimized Cp2M2 Bn−2Hn−2 structures were replaced by methyl groups. The resulting Cp*2M2Bn−2Hn−2 structures were then reoptimized using the same method. The low-energy Cp2M2Bn−2Hn−2 and Cp*2M2Bn−2Hn−2 structures were found to have substantial HOMO−LUMO gaps, typically ranging from 2 to 3 eV. All calculations were performed using the Gaussian 09 package27 with the default settings for the SCF cycles and geometry optimization, namely the fine grid (75 302) for numerically evaluating the integrals, 10−8 hartree for the self-consistent field convergence, maximum force

2. THEORETICAL METHODS The initial Cp2M2Bn−2Hn−2 (M = Co, Rh, Ir) structures were constructed by systematic substitution of two boron vertices in BnHn2− with metal atoms in various n-vertex polyhedra. Thus, 50 structures of the 8-vertex clusters Cp2M2B6H6, 52 structures of the 9-vertex clusters Cp2M2B7H7, 95 structures of the 10-vertex clusters Cp2M2B8H8, 70 structures of the 11-vertex clusters Cp2M2B9H9, and 26 structures of the 12-vertex Cp2M2B10H10 clusters were chosen as starting points for the optimizations (see the Supporting Information). Full geometry optimizations were carried out on these Cp2M2Bn−2Hn−2 (M = Co, Rh, Ir) starting structures at the B3LYP/ 6-31G(d)21−24 level for all atoms except for rhodium and iridium, for which the SDD (Stuttgart−Dresden ECP plus DZ) basis set25 was chosen. The lowest energy structures were then reoptimized at a higher level, i.e., M06L/6-311G(d,p)/SDD, and these are the structures presented in the manuscript.26 The natures of the stationary C


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Inorganic Chemistry

Figure 4. Five lowest energy Cp2M2B7H7 (M = Co, Rh, Ir) structures. of 0.000450 hartree/bohr, RMS force of 0.000300 hartree/bohr, maximum displacement of 0.001800 bohr, and RMS displacement of 0.001200 bohr. Wiberg bond indices (WBIs) for the M−M interactions in the optimized Cp2M2B6H10 structures determined using NBO analysis28 were used because they are well-established as means for evaluating M−M interactions in polyhedral dimetallaboranes29 as well as other binuclear and trinuclear transition-metal complexes.30 The structures, total and relative energies (M06L/6-311G(d,p)/ SDD, including zero-point corrections), and relevant interatomic distances for all optimized structures are given in the Supporting Information. These structures are numbered as Bn−2M2−x, where n is the number of vertices and x is the relative order of the structure on the energy scale. The corresponding Cp* structures are designated by asterisks. The Cp2Ir2Bn−2Hn−2 structures are those depicted in the figures. Only the lowest energy and thus potentially chemically significant structures (Figures 3−7 and Tables 1 and 2) are considered in detail in this paper. However, more comprehensive lists of structures, including higher energy structures, are given in the Supporting Information.

Ir2B6 polyhedron in the experimental red Cp*2Ir2B6H6 isomer is a bicapped trigonal prism. In fact, the experimental red Cp*2Ir2B6H6 structure corresponds to the square intermediate in the conversion of B6Ir2−1* to its mirror image (Figure 3). The diagonals in the “square” face of the experimental red Cp*2Ir2B6H6 structure of 2.369 and 2.584 Å average ∼2.48 Å. The edge broken in the diamond-square transformation of B6Ir2−1* to the bicapped trigonal prismatic structure is 1.913 Å, and the two vertices of B6Ir2−1* coming together to form a new edge start from a distance of 2.888 Å. The average value of these distances is ∼2.40 Å. The experimental red Cp*2Ir2B6H6 isomer is thus essentially the average of the two enantiomers of B6Ir2−1*. A bicapped trigonal prismatic Cp* 2 Ir 2 B 6 H 6 structure corresponding to the experimental structure has imaginary vibrational frequencies. Following the corresponding normal modes leads to B6Ir2−1*. The yellow Cp*2Ir2B6H6 isomer synthesized by Ghosh and co-workers corresponds to B6Ir2−2 with the iridium atoms at nonadjacent degree 5 vertices (Table 1 and Figure 3). The predicted Ir···Ir nonbonding distance in the permethylated analogue B6Ir2−2* of 3.391 Å is close to the experimental value of 3.349 Å.17 Each iridium atom in B6Ir2−2* forms three edges of length 2.10 Å to degree 4 vertices and two edges of length 2.30 Å to degree 5 vertices. These values are close to the corresponding average experimental Ir−B edge lengths of 2.06 and 2.28 Å, respectively. The closeness in energy of B6Ir2−1* and B6Ir2−2* is consistent with the isolation of both isomers from the reaction of Cp*2Ir2Cl4 with BH3·thf used for the syntheses of these diiridaboranes. The third Cp*2M2B6H6 structure B6M2−3, lying 12.6, 8.7, and 12.9 kcal in energy above B6M2−1 for M = Co, Rh, and Ir, respectively, has the central capped pentagonal bipyramidal M2B6 unit (Figure 1) expected by the Wade-Mingos rules3−5 for an 8-vertex system having 16 (= 2n for n = 8) skeletal electrons. The metal atoms occupy the unique degree 6 vertex and an adjacent degree 5 vertex with WBIs for the M−M interaction of ∼0.3, corresponding to a metal−metal surface single bond. Five low-energy structures were found for the 9-vertex Cp2M2B7H7 (M = Co, Rh, Ir) systems (Figure 4 and Table 1). The three lowest energy structures have the central M2B7 closo

3. RESULTS 3.1. 8- and 9-Vertex Cp2M2Bn−2Hn−2 (n = 8, 9; M = Co, Rh, Ir) Structures. The 8- and 9-vertex Cp2M2Bn−2Hn−2 (n = 8, 9; M = Co, Rh, Ir) structures (Table 1) are discussed first in view of the interesting permethylated diiridaboranes Cp*2Ir2Bn−2Hn−2 (n = 8, 9) recently reported by Ghosh and co-workers (Figure 2).17,18 The two lowest energy Cp2M2B6H6 (M = Co, Rh, Ir) structures are the two possible structures with a central M2B6 bisdisphenoid having both metal atoms at degree 5 vertices (Table 1). For the experimentally observed Cp*2Ir2B6H6 system, these two structures are essentially energetically degenerate within 0.5 kcal/mol. The lower energy of these structures B6M2−1 is a chiral C2 structure having adjacent metal atoms with M−M WBIs of 0.3 to 0.4 consistent with a metal−metal surface single bond. Structure B6Ir2−1 is closely related to the experimentally observed red Cp*2Ir2B6H6 isomer.17,18 The experimental Ir−Ir distance of 2.762 Å for this isomer, determined by X-ray crystallography, is very close to the predicted Ir−Ir distance in B6Ir2−1* of 2.780 Å. However, the central Ir2B6 polyhedron in the predicted structures B6Ir2−1 and its permethylated analogue B6Ir2−1* is a bisdisphenoid, whereas the central D


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Inorganic Chemistry deltahedron, namely the tricapped trigonal prism (Figure 2), and lie within ∼5 kcal/mol of energy of each other. These structures represent the three nonequivalent ways of placing two metal atoms at the degree 5 vertices of the tricapped trigonal prism. The lowest energy structures B7M2−1 are chiral C2 structures with the metal atoms at a pair of nonadjacent degree 5 vertices. The WBIs for the metal− metal interactions in these structures are 0.16 (M = Co) and 0.11 (M = Rh and Ir). The next group of Cp2M2B7H7 structures B7M2−2 has the metal atoms along a “horizontal” edge of the underlying trigonal prism, whereas the B7M2−3 structures have the metal atoms along a “vertical” edge of the trigonal prism. The M−M WBIs in B7M2−2 and B7M2−3 range from 0.3 to 0.4 with the WBIs for the horizontal edge M−M interactions being 0.03 to 0.05 larger than those for vertical edge M−M interactions. This is consistent with horizontal M−M distances being ∼0.03 Å shorter than vertical M−M distances in the underlying trigonal prisms. The experimental Cp*2Ir2B7H7 structure corresponds to the capped tetragonal antiprism square intermediate in the interconversion of the mirror image enantiomers of B7M2−1 (Figure 2).17 The diagonals in the square face of the experimental red Cp*2Ir2B7H7 structure of 2.373 and 2.498 Å average ∼2.43 Å. The length of the edge broken in the diamond-square transformation of B7Ir2−1* to the bicapped trigonal prismatic structure is 2.015 Å, and the two vertices of B7Ir2−1* coming together to form a new edge start from a distance of 2.852 Å. The average value of these distances is ∼2.43 Å. Thus, the experimental Cp*2Ir2B7H7 structure is essentially the average of the two enantiomers of B7Ir2−1*. The experimental nonbonding Ir···Ir distance in Cp*2Ir2B7H7 of 3.526 Å is close to the calculated Ir···Ir distance of 3.555 Å for B7Ir2−1*. An attempt to optimize the experimental capped square antiprismatic Cp*2Ir2B7H7 led instead to the tricapped trigonal prismatic structure B7Ir2−1*. Higher energy Cp2M2B7H7 (M = Co, Rh, Ir) structures are found having the central M2B7 9-vertex isocloso deltahedron consistent with their 2n (= 18 for n = 9) skeletal electrons by the Wade-Mingos rules (Figure 2).3−5 For Cp2Co2B7H7, these isocloso structures lie more than 11 kcal/mol above the lowest energy closo structure B7Co2−1 (Figure 4 and Table 1). However, for Cp2M2B7H7 (M = Rh, Ir), the isocloso structures B7M2−4 (M = Rh, Ir) lie only ∼5 kcal/mol in energy above the lowest energy closo structures B7M2−1. Both of the isocloso Cp2M2B7H7 structures B7M2−4 and B7M2−5 have one metal atom at the unique degree 6 vertex and the other metal atom at a degree 5 vertex. In B7M2−4, the metal atoms are at adjacent vertices with M−M WBIs of ∼0.3. In the higher energy Cp2M2B7H7 structures B7M2−5, the metal atoms are located at nonadjacent vertices with M···M WBIs of 0.15 (for M = Co) and 0.10 (for M = Rh, Ir). 3.2. 10- to 12-Vertex Cp2M2Bn−2Hn−2 (n = 10, 11, 12; M = Co, Rh, Ir) Structures. Two low-energy structures are found for each of the 10-vertex Cp2M2B8H8 systems (M = Co, Rh, Ir). Both structures are based on the isocloso 10-vertex deltahedron (Figure 1) with one metal atom located at the unique degree 6 vertex (Figure 5 and Table 2). In the lower energy Cp2M2B8H8 structures B8M2−1, the second metal atom is located at a degree 5 vertex adjacent to the degree 6 vertex. The metal− metal distances in B8M2−1 are bonding distances with the expected WBIs of ∼0.3 for a formal metal−metal single bond on the deltahedral surface. In the higher energy Cp2M2B8H8 structures B8M2−2, lying 6.4 (Ir), 9.0 (Rh), and 4.2 (Co)

Figure 5. Two lowest energy Cp2M2B8H8 (M = Co, Rh, Ir) structures.

kcal/mol in energy above the corresponding B8M2−1 structures, the second metal atom is located at a degree 5 vertex remote from the degree 6 vertex. For these structures, the nonadjacent M···M interactions ranging from ∼3.2 Å for cobalt to ∼3.5 Å for iridium have much lower WBIs of ∼0.1. Pyrolysis of a mixture of Cp*2Ir2Cl4 with BH3·THF in xylene gives a complicated reaction mixture from which two isomers of Cp*2Ir2B8H8 were isolated, corresponding to B8Ir2−1* and B8Ir2−2* (Figure 5 and Table 2).18 The experimental bonding Ir−Ir distance of 2.802 Å in the isomer corresponding to B8Ir2−1*, as determined by X-ray crystallography, is nearly identical to the distance of 2.811 Å calculated for B8Ir2−1*. The experimental nonbonding Ir···Ir distance of 3.500 Å in the isomer corresponding to B8Ir2−2* is close to the 3.540 Å distance calculated for B8Ir2−2*. The degree 6 vertex in B8Ir2−2* is bonded to three degree 4 boron vertices and three degree 5 boron vertices with calculated average lengths of ∼2.14 and ∼2.31 Å, respectively. These are close to the experimental values of ∼2.13 and ∼2.31 Å found by X-ray crystallography. The three lowest energy structures for each of the 11-vertex Cp2M2B9H9 systems are based on the 11-vertex deltahedron, namely the “edge-coalesced icosahedron”, that can be either a closo or isocloso deltahedron (Figure 1). All three of these structures have one of the metal atoms located at the unique degree 6 vertex and the second metal atom located at a degree 5 vertex (Figure 6 and Table 2). The lowest energy Cp2M2B9H9 structures B9M2−1 as well as the higher energy structures B9M2−3 lying 6 to 8 kcal/mol in energy above B9M2−1 have the second metal atom located at a vertex nonadjacent to the unique degree 6 vertex with WBIs of ∼0.1 for the metal−metal interactions. The nonadjacent M···M distances in the B9M2−3 structures are ∼0.2 Å longer than those in the B9M2−1 structures, corresponding to WBIs lower by ∼0.02. The B9M2−2 structures, intermediate in energy between B9M2− 1 and B9M2−3, have the second metal atom on a degree 5 vertex adjacent to the degree 6 vertex metal atom with the expected WBIs of ∼0.3 for the metal−metal interactions. The permethylated Cp*2M2B9H9 (M = Rh, Ir) derivatives were synthesized and shown by X-ray crystallography to have the lowest energy structures B9M2−1*.18 The experimental structures are found to be very close to the corresponding optimized B9M2−1* structures (Table 3). The nonadjacent M···M distances are ∼3.6 Å. Furthermore, the distances from the unique degree 6 metal atom to the adjacent degree 4 and 5 boron vertices are ∼2.1 and ∼2.3 Å, respectively. The next Cp2M2B9H9 structures B9M2−4, lying 14.9 (Ir), 15.3 (Rh), and 17.6 (Co) kcal/mol in energy above B9M2−1, E


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Inorganic Chemistry

Figure 7. Two lowest energy Cp2M2B10H10 (M = Co, Rh, Ir) structures.

However, the other metal atom in B10M2−2 is located at a degree 5 vertex not adjacent to the degree 6 metal vertices. In B10M2−2, the nonbonding M···M distances range from ∼3.38 Å for cobalt to ∼3.85 Å for iridium with corresponding WBIs from 0.11 (Co) to 0.07 (Ir). Structure B10Co2−2 lies 10.4 kcal/mol in energy above B10Co2−1. However, the energy difference between B10M2−2 and B10M2−1 is much larger for rhodium and iridium at ∼23 kcal/mol. This difference in energy separation may be a consequence of the more diffuse orbitals of the second and third row transition metals rhodium and iridium having a greater preference for degree 6 vertices over degree 5 vertices relative to the first row transition metal cobalt.31

Figure 6. Four lowest energy Cp2M2B9H9 (M = Co, Rh, Ir) structures.

Table 3. Comparison of the Predicted B9M2-1 Geometries with the Experimental Geometries for the Cp*2M2B9H9 (M = Rh, Ir) Derivatives M···M (Å)

compound Cp*2Rh2B9H9

B9Rh2−1

3.573 3.552

Cp*2Ir2B9H9

exptl (Xray) B9Ir2−1 exptl (Xray)

3.596 3.540

degree 6 metal−boron distances (Å)a 2.114a, 2.278, 2.258, 2.258, 2.278 2.085a, 2.283, 2.252, 2.252, 2.288 2.122a, 2.295, 2.271, 2.270, 2.293 2.088a, 2.307, 2.266, 2.270, 2.291

4. DISCUSSION The lowest energy structures for the 8- and 9-vertex dimetallaboranes Cp2M2B6H6 and Cp2M2B7H7 (M = Co, Rh, Ir) with 2n skeletal electrons are not the isocloso deltahedra providing a degree 6 vertex for one of the metal atoms (Figure 1). Instead, they are the same as the most spherical closo deltahedra expected for deltahedral boranes with 2n + 2 skeletal electrons, namely the bisdisphenoid and tricapped trigonal prism for n = 8 and 9, respectively (Figure 3). This situation is analogous to the related binary boron chlorides BnCln (n = 8 and 9), which are also 2n skeletal electron systems.32,33 The formation of closo rather than isocloso structures for 8- and 9vertex systems has been attributed to the nondegeneracy of the frontier orbitals (HOMO and LUMO) in the closo deltahedra.34 This suggests that 8- and 9-vertex closo structures for 2n and 2n + 4 skeletal electron systems are nearly as favorable as for the 2n + 2 skeletal electron systems implied by the Wade-Mingos rules.3−5 However, for the 9-vertex Cp2M2B7H7 systems, isocloso structures (B7M2−4 and B7M2−5 in Figure 4 and Table 1) lie only ∼5−11 kcal/mol in energy above the lowest energy closo structure. Similarly, for the 8-vertex Cp2M2B6H6 systems, isocloso-like capped pentagonal bipyramid structures with a degree 6 vertex for a metal atom (B6M2−3 in Figure 3) lie ∼8 to ∼13 kcal/mol in energy above the lowest energy closo structure. The lowest energy Cp2M2B6H6 and Cp2M2B7H7 structures B6M2−1 and B7M2−1, respectively, are chiral deltahedral structures having only C2 symmetry (Figures 3 and 4). The experimental structures determined by X-ray crystallography for the red Cp2M2B6H6 and Cp2M2B7H7 isomers are not these deltahedral structures but are instead a bicapped trigonal prism for the 8-vertex structure and a capped square antiprism for the 9-vertex structure.17 These structures represent the average of

2.128a, 2.117a, 2.136a, 2.132a,

a

Distances between the degree 6 metal atom and the degree 4 boron vertices.

have a central M2B9 deltahedron with two degree 6 vertices where the metal atoms are located (Figure 6 and Table 2). The degree 6 vertices in B9M2−4 are nonadjacent with nonbonding metal−metal distances ranging from 3.200 Å for cobalt to 3.467 Å for iridium, corresponding to the expected small WBIs around 0.08. The lowest energy structure B10M2−1 for the 12-vertex Cp2M2B10H10 (M = Co, Rh, Ir) systems are based on a C2v deltahedron with adjacent degree 6 vertices where the metal atoms are located (Figure 7 and Table 2). The metal−metal distances ranging from 2.569 Å for cobalt to 2.745 Å for iridium correspond to surface single bonds with the expected WBIs of ∼0.3. The two degree 4 boron vertices in these B10M2−1 deltahedra bridge the M−M edge. This deltahedron is found experimentally in the hydroxydirhodaboranes Cp*2Rh2B10H9(OH) and Cp*2Rh2B10H8(OH)2, which have been structurally characterized by X-ray crystallography.20 The less symmetrical higher energy Cp2M2B10H10 structures B9M2−2 are also based on the same 12-vertex deltahedron with two adjacent degree 6 vertices bridged by the two degree 4 vertices (Figure 7 and Table 2). In the B10M2−2 structures, one metal atom is located at one of the degree 6 vertices. F


Inorganic Chemistry

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the two enantiomeric mirror image structures. Because these experimental structures each have one quadrilateral face, they may also be regarded as the square intermediate in the diamond-square-diamond process, converting one enantiomer into the opposite enantiomer. The previous study on the 10-vertex diferradicarbaboranes15 Cp2Fe2C2Bn−4Hn−2 (n = 9 to 12) with 2n skeletal electrons indicated that the isocloso structure (Figure 1) having a single degree 6 vertex for a metal atom was particularly favorable.15 This is also the case for the Cp2M2B8H8 systems (M = Co, Rh, Ir) also with 10 vertices. Two types of low-energy isocloso Cp2M2B8H8 structures are found (Figure 5). The lower energy structures B8M2−1 have one metal atom at the degree 6 vertex and the other metal atom at an adjacent degree 5 vertex. The higher energy structures B8M2−2 also have one metal atom at the degree 6 vertex and the other metal atom at a degree 5 vertex. However, the degree 5 vertex with the metal atom in B8M2−2 is not adjacent to the degree 6 vertex. Both types of Cp*2Ir2B8H8 structures have been synthesized and structurally characterized by X-ray crystallography.18 For the 11-vertex dimetallaboranes, the same deltahedron, namely the “edge-coalesced icosahedron”, is both the closo and isocloso deltahedron because it has a degree 6 vertex for the metal atom (Figure 1). It is therefore not surprising that the three lowest energy Cp2M2B9H9 structures (B9M2−1, B9M2− 2, and B9M2−3 in Figure 6) are based on this deltahedron. All three of these structures have one metal atom at the unique degree 6 vertex and the metal atom at one of the degree 5 vertices. Permethylated rhodium and iridium complexes B9M2−1* having the lowest energy of these structures have been synthesized and structurally characterized by X-ray crystallography.18 The previous theoretical study on the 12-vertex Cp2Fe2C2B8H10 system showed the six lowest energy structures not to be icosahedral structures but instead structures having degree 6 vertices for both iron atoms balanced by two degree 4 vertices bridging the iron atoms.15 The three lowest energy Cp2Fe2C2B8H10 structures are based on a 12-vertex deltahedron having two adjacent degree 6 vertices for the iron atoms. This deltahedron is derived from the regular icosahedron favored in most of boron chemistry by a single diamond-square-diamond rearrangement. The lowest energy Cp2M2B10H10 (M = Co, Rh, Ir) structures B10M2−1 by substantial margins ranging from ∼10 kcal/mol for cobalt to ∼24 kcal/mol for iridium also are of this type (Figure 7). The hydroxydirhodaboranes Cp*2Rh2B10H9(OH) and Cp*2Rh2B10H8(OH)2, which have been synthesized and structurally characterized by X-ray crystallography, have structure B10Rh2−1* with one or two of the external hydrogen atoms replaced by hydroxy groups.20

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Article

AUTHOR INFORMATION

Corresponding Authors

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

R. Bruce King: 0000-0001-9177-5220 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Funding from the Romanian Ministry of Education and Research (Grant PN-II-RU-TE-2014-4-1197) is gratefully acknowledged. Additional computational resources were provided by the high-performance computational facility MADECIP, POSCCE, COD SMIS 48801/1862 cofinanced by the European Regional Development Fund of the European Union.

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REFERENCES

(1) Callahan, K. P.; Hawthorne, M. F. Ten years of metallocarboranes. Adv. Organomet. Chem. 1976, 14, 145. (2) Callahan, K. P.; Evans, W. J.; Lo, F. Y.; Strouse, C. E.; Hawthorne, M. F. Structures of metallocarboranes. V. Synthesis and crystal and molecular structure of the closo 20-electron bimetallocarborane 1,6-bis(η-cyclopentadienyl)-1,6-diferra-2,3-dicarba-closodecaborane(8), 1,6-(η-C5H5)2−1,6,2,3-Fe2C2B6H8. J. Am. Chem. Soc. 1975, 97, 296. (3) Wade, K. The structural significance of the number of skeletal bonding electron-pairs in carboranes, the higher boranes and borane anions, and various transition-metal carbonyl cluster compounds. J. Chem. Soc. D 1971, 792. (4) Mingos, D. M. P. A general theory for cluster and ring compounds of the main group and transition elements. Nature, Phys. Sci. 1972, 236, 99. (5) Mingos, D. M. P. Polyhedral skeletal electron pair approach. Acc. Chem. Res. 1984, 17, 311. (6) Bould, J.; Kennedy, J. D.; Thornton-Pett, M. Ten-vertex metallaborane chemistry. Aspects of the iridadecaborane closo→ isonido→isocloso structural continuum. J. Chem. Soc., Dalton Trans. 1992, 563. (7) Kennedy, J. D.; Štibr, B. In Current Topics in the Chemistry of Boron; Kabalka, G. W., ed., Royal Society of Chemistry: Cambridge, 1994; pp 285−292. (8) Kennedy, J. D. In The Borane-Carborane-Carbocation Continuum; Casanova, J., ed., Wiley: New York, 1998; Chapter 3, pp 85−116. (9) Štibr, B.; Kennedy, J. D.; Drdáková, E.; Thornton-Pett, M. Ninevertex polyhedral iridamonocarbaborane chemistry. Products of thermolysis of [(CO)(PPh3)2IrCB7H8] and emerging alternative cluster-geometry patterns. J. Chem. Soc., Dalton Trans. 1994, 229. (10) Kennedy, J. D. Structure and bonding in recently isolated metallaboranes. Inorg. Chem. 1986, 25, 111. (11) Baker, R. T. Hyper-closo metallaboranes. Inorg. Chem. 1986, 25, 109. (12) Johnston, R. L.; Mingos, D. M. P. Molecular orbital calculations relevant to the hypercloso vs. iso-closo controversy in metallaboranes. Inorg. Chem. 1986, 25, 3321. (13) Johnston, R. L.; Mingos, D. M. P.; Sherwood, P. Bonding and electron counting in hyper closo metalloboranes and metallocarboranes. New J. Chem. 1991, 15, 831. (14) King, R. B. Topological aspects of the skeletal bonding in “isocloso” metallaboranes containing “anomalous” numbers of skeletal electrons. Inorg. Chem. 1999, 38, 5151. (15) Lupan, A.; King, R. B. Limited occurrence of isocloso deltahedra with 9 to 12 vertices in low-energy hypoelectronic diferradicarbaborane structures. Inorg. Chem. 2011, 50, 9571.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02281. Distance and energy ranking tables, initial structures, orbital energies and HOMO/LUMO gaps, complete Gaussian09 reference (ref 27) (PDF) Concatenated file containing the optimized structures (XYZ) G


Article

Inorganic Chemistry (16) Lupan, A.; King, R. B. The prevalence of isocloso deltahedra in low-energy hypoelectronic metalladicarbaboranes with a single metal vertex: manganese and rhenium derivatives. Dalton Trans 2012, 41, 7073. (17) Borthakur, R.; Mondal, B.; Nandi, P.; Ghosh, S. Hypoelectronic isomeric diiridaboranes [(Cp*Ir)2B6H6]: the “Rule-Breakers”(Cp* = η5-C5Me5). Chem. Commun. 2016, 52, 3199. (18) Roy, D. K.; Borthakur, R.; Prakash, R.; Bhattacharya, S.; Jagan, R.; Ghosh, S. Hypoelectronic 8−11-vertex irida- and rhodaboranes. Inorg. Chem. 2016, 55, 4764. (19) Chakrahari, K. K.; Sharmila, D.; Barik, S. K.; Mondal, B.; Varghese, B.; Ghosh, S. Hypoelectronic metallaboranes: Synthesis, structural characterization, and electronic structures of metal-rich cobaltaboranes. J. Organomet. Chem. 2014, 749, 188. (20) Roy, S. K.; Mondal, B.; Shankhari, P.; Anju, R. S.; Geetharani, K.; Mobin, S. M.; Ghosh, S. Supraicosahedral polyhedra in metallaboranes: Synthesis and structural characterization of 12-, 15-, and 16-vertex rhodaboranes. Inorg. Chem. 2013, 52, 6705. (21) Vosko, S. H.; Wilk, L.; Nusair, M. Accurate spin-dependent electron liquid correlation energies for local spin density calculations: a critical analysis. Can. J. Phys. 1980, 58, 1200. (22) Becke, A. D. Density-functional thermochemistry. III. The role of exact exchange. J. Chem. Phys. 1993, 98, 5648. (23) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. Ab Initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields. J. Phys. Chem. 1994, 98, 11623. (24) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785. (25) Andrae, D.; Haussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Energy-adjusted ab initio pseudopotentials for the second and third row transition elements. Theor. Chim. Acta 1990, 77, 123. (26) Truhlar, D. G.; Zhao, Y. The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06-class functionals and 12 other functionals. Theor. Chem. Acc. 2008, 120, 215. (27) Gaussian 09, revision E.01; Gaussian, Inc.: Wallingford, CT, 2009. The complete reference is given in the Supporting Information. (28) Weinhold, F.; Landis, C. R. Valency and Bonding: A Natural Bond Order Donor-Acceptor Perspective; Cambridge University Press: Cambridge, England, U.K., 2005; pp 32−36. (29) Lupan, A.; King, R. B. Metal−metal interactions in deltahedral dirhoda- and diiridadicarbaboranes. Inorg. Chim. Acta 2013, 397, 83. (30) Wang, H.; Xie, Y.; King, R. B.; Schaefer, H. F. Remarkable aspects of unsaturation in trinuclear metal carbonyl clusters: the triiron species Fe3(CO)n (n = 12, 11, 10, 9). J. Am. Chem. Soc. 2006, 128, 11376. (31) Jemmis, E. D. Overlap control and stability of polyhedral molecules. closo-Carboranes. J. Am. Chem. Soc. 1982, 104, 7017−7020. (32) Morrison, J. A. Chemistry of the polyhedral boron halides and the diboron tetrahalides. Chem. Rev. 1991, 91, 35. (33) Corminboeuf, C.; Wodrich, M. W.; King, R. B.; Schleyer, P. v. R. Effects of halogen substitution on the properties of eight- and ninevertex closo-boranes. Dalton Trans 2008, 1745. (34) O’Neill, M. E.; Wade, K. Closo clusters with unusual electron numbers: The significance of frontier orbital degeneracies and bonding characteristics. J. Mol. Struct.: THEOCHEM 1983, 103, 259.

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