Kinetic versus thermodynamic isomers of the deltahedral dicobaltadicarbaboranes having nine to 12 vertices Alexandru Lupan a , R. Bruce King b,⇑ a Faculty of Chemistry and Chemical Engineering, Babes ß-Bolyai University, Cluj-Napoca, Romania b Department of Chemistry, University of Georgia, Athens, GA 30602, United States article info Article history: Received 13 August 2011 Accepted 22 November 2011 Available online 29 November 2011 Keywords: Cobalt Metallaboranes Density functional theory abstract The lowest energy structures for the dicobaltadicarbaboranes Cp 2 Co 2 C 2 B n4 H n2 (n = 9, 10, and 11) are found by density functional theory to be the most spherical borane deltahedra with the carbon atoms at degree 4 vertices and the cobalt atoms at degree 5 or 6 vertices. For the icosahedral 12-vertex dicobaltadi- carbaboranes Cp 2 Co 2 C 2 B 8 H 10 with only degree 5 vertices, the lowest energy structures are those without Co–Co or C–C edges. These theoretical predictions agree well with experimental data on the numerous known Cp 2 Co 2 C 2 B n4 H n2 (n = 9, 10, 11, and 12) derivatives. Thus for the nine-vertex Cp 2 Co 2 C 2 B 5 H 7 system only the two lowest energy isomers are found experimentally. For the 10-vertex Cp 2 Co 2 C 2 B 6 H 8 system three of the six lowest energy isomers have been synthesized. The 11- and 12-vertex Cp 2 Co 2 C 2 B n4 H n2 systems provide examples of stable high energy isomers with direct Co–Co or C–C bonds arising from the synthetic methods used. Thus one of the experimentally known 11-vertex Cp 2 Co 2 C 2 B 7 H 9 isomers is a high-energy structure with adjacent carbon atoms lying 26 kcal/mol above the global minimum. In addition to this high-energy isomer seven of the 12 predicted Cp 2 Co 2 C 2 B 7 H 9 isomers within 6 kcal/mol of the global minimum have been synthesized. Similarly, for the icosahedral Cp 2 Co 2 C 2 B 8 H 10 derivatives, high-energy isomers with a Co–Co bond lying 21.8 kcal/mol above the global minimum and with a C–C bond lying 19.4 kcal/mol above the global minimum have both been synthesized as stable compounds. Pyrolysis of these high energy Cp 2 Co 2 C 2 B 8 H 10 isomers at temperatures up to 340 °C gives a lower energy isomer with neither a Co–Co nor a C–C bond. Further pyrolysis of this Cp 2 Co 2 C 2 B 8 H 10 isomer at the incred- ibly high temperature of 650 °C for an organometallic reaction gives a complicated mixture of seven of the 12 possible lowest energy isomers, namely those with neither Co–Co nor C–C bonds. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction The basic building blocks of metal-free polyhedral boranes and isoelectronic carboranes have long been recognized to be the most spherical deltahedra, also known as closo deltahedra (Fig. 1) [1,2]. The prototypical examples of such closo deltahedral boranes are the dianions B n H n 2(6 6 n 6 12) as well as the isoelectronic carbo- rane monoanions CB n1 H n and particularly the neutral dicarbabor- anes C 2 B n2 H n . The vertices in these most spherical deltahedra are as nearly similar as possible. For the experimentally accessible deltahedral boranes with 6–12 vertices, this means a deltahedron having only degree 4 or 5 vertices except for the presence of a single degree 6 vertex in the 11-vertex closo deltahedron. A major breakthrough in both borane and organometallic chem- istry was the discovery by Hawthorne and co-workers [3–5] that the vertex atoms in the deltahedral dicarbaborane C 2 B n2 H n structures can be replaced with isolobal transition metal units leading to very stable metalladicarbaboranes and dimetalladicarbaboranes. Com- pounds having CpCo (Cp = g 5 -C 5 H 5 ) vertices in place of BH vertices in carborane structures are of particular interest since CpCo vertices, as formal donors of two skeletal electrons, are isolobal with BH ver- tices. Extensive series of very stable neutral CpCoC 2 B n3 H n1 and Cp 2 Co 2 C 2 B n4 H n2 isomers are known derived from the n-vertex deltahedral dicarbaboranes C 2 B n2 H n (n = 9, 10, 11, 12) by replace- ment of one or two BH vertices with CpCo vertices. Such com- pounds, along with the isoelectronic metal-free derivatives B n H n 2, CB n1 H n , and C 2 B n2 H n , derivatives are all 2n + 2 skeletal electron systems predicted by the Wade–Mingos rules [6–9] to ex- hibit the most spherical deltahedral structures (e.g., Fig. 1 for the systems having 9–12 vertices). The deltahedral dicarbaboranes as well as the corresponding metalladicarbaboranes and dimetalladicarbaboranes are interest- ing examples of molecules for which stable isomers can be synthe- sized which lie in energy far above the corresponding global minima. For example, the original source of the two carbon atoms in the syntheses of most 11- and 12-vertex metalladicarbaboranes and dimetalladicarbaboranes is an alkyne. The transition metal 0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2011.11.042 ⇑ Corresponding author. Tel.: +1 706 542 1901; fax: +1 706 542 9454. E-mail address: [email protected](R. Bruce King). Polyhedron 33 (2012) 319–326 Contents lists available at SciVerse ScienceDirect Polyhedron journal homepage: www.elsevier.com/locate/poly
Transcript
Polyhedron 33 (2012) 319–326
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier .com/locate /poly
Kinetic versus thermodynamic isomers of the deltahedraldicobaltadicarbaboranes having nine to 12 vertices
Alexandru Lupan a, R. Bruce King b,⇑a Faculty of Chemistry and Chemical Engineering, Babes�-Bolyai University, Cluj-Napoca, Romaniab Department of Chemistry, University of Georgia, Athens, GA 30602, United States
a r t i c l e i n f o a b s t r a c t
Article history:Received 13 August 2011Accepted 22 November 2011Available online 29 November 2011
Keywords:CobaltMetallaboranesDensity functional theory
0277-5387/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.poly.2011.11.042
⇑ Corresponding author. Tel.: +1 706 542 1901; faxE-mail address: [email protected] (R. Bruce Ki
The lowest energy structures for the dicobaltadicarbaboranes Cp2Co2C2Bn�4Hn�2 (n = 9, 10, and 11) arefound by density functional theory to be the most spherical borane deltahedra with the carbon atoms atdegree 4 vertices and the cobalt atoms at degree 5 or 6 vertices. For the icosahedral 12-vertex dicobaltadi-carbaboranes Cp2Co2C2B8H10 with only degree 5 vertices, the lowest energy structures are those withoutCo–Co or C–C edges. These theoretical predictions agree well with experimental data on the numerousknown Cp2Co2C2Bn�4Hn�2 (n = 9, 10, 11, and 12) derivatives. Thus for the nine-vertex Cp2Co2C2B5H7 systemonly the two lowest energy isomers are found experimentally. For the 10-vertex Cp2Co2C2B6H8 systemthree of the six lowest energy isomers have been synthesized. The 11- and 12-vertex Cp2Co2C2Bn�4Hn�2
systems provide examples of stable high energy isomers with direct Co–Co or C–C bonds arising fromthe synthetic methods used. Thus one of the experimentally known 11-vertex Cp2Co2C2B7H9 isomers isa high-energy structure with adjacent carbon atoms lying �26 kcal/mol above the global minimum. Inaddition to this high-energy isomer seven of the 12 predicted Cp2Co2C2B7H9 isomers within 6 kcal/molof the global minimum have been synthesized. Similarly, for the icosahedral Cp2Co2C2B8H10 derivatives,high-energy isomers with a Co–Co bond lying 21.8 kcal/mol above the global minimum and with a C–Cbond lying 19.4 kcal/mol above the global minimum have both been synthesized as stable compounds.Pyrolysis of these high energy Cp2Co2C2B8H10 isomers at temperatures up to 340 �C gives a lower energyisomer with neither a Co–Co nor a C–C bond. Further pyrolysis of this Cp2Co2C2B8H10 isomer at the incred-ibly high temperature of 650 �C for an organometallic reaction gives a complicated mixture of seven of the12 possible lowest energy isomers, namely those with neither Co–Co nor C–C bonds.
� 2011 Elsevier Ltd. All rights reserved.
1. Introduction
The basic building blocks of metal-free polyhedral boranes andisoelectronic carboranes have long been recognized to be the mostspherical deltahedra, also known as closo deltahedra (Fig. 1) [1,2].The prototypical examples of such closo deltahedral boranes arethe dianions BnHn
2� (6 6 n 6 12) as well as the isoelectronic carbo-rane monoanions CBn�1Hn
� and particularly the neutral dicarbabor-anes C2Bn�2Hn. The vertices in these most spherical deltahedra areas nearly similar as possible. For the experimentally accessibledeltahedral boranes with 6–12 vertices, this means a deltahedronhaving only degree 4 or 5 vertices except for the presence of a singledegree 6 vertex in the 11-vertex closo deltahedron.
A major breakthrough in both borane and organometallic chem-istry was the discovery by Hawthorne and co-workers [3–5] that thevertex atoms in the deltahedral dicarbaborane C2Bn�2Hn structurescan be replaced with isolobal transition metal units leading to very
ll rights reserved.
: +1 706 542 9454.ng).
stable metalladicarbaboranes and dimetalladicarbaboranes. Com-pounds having CpCo (Cp = g5-C5H5) vertices in place of BH verticesin carborane structures are of particular interest since CpCo vertices,as formal donors of two skeletal electrons, are isolobal with BH ver-tices. Extensive series of very stable neutral CpCoC2Bn�3Hn�1 andCp2Co2C2Bn�4Hn�2 isomers are known derived from the n-vertexdeltahedral dicarbaboranes C2Bn�2Hn (n = 9, 10, 11, 12) by replace-ment of one or two BH vertices with CpCo vertices. Such com-pounds, along with the isoelectronic metal-free derivativesBnHn
2�, CBn�1Hn�, and C2Bn�2Hn, derivatives are all 2n + 2 skeletal
electron systems predicted by the Wade–Mingos rules [6–9] to ex-hibit the most spherical deltahedral structures (e.g., Fig. 1 for thesystems having 9–12 vertices).
The deltahedral dicarbaboranes as well as the correspondingmetalladicarbaboranes and dimetalladicarbaboranes are interest-ing examples of molecules for which stable isomers can be synthe-sized which lie in energy far above the corresponding globalminima. For example, the original source of the two carbon atomsin the syntheses of most 11- and 12-vertex metalladicarbaboranesand dimetalladicarbaboranes is an alkyne. The transition metal
Fig. 1. The BnHn2� deltahedra (9 6 n 6 12). Degree 4 and 6 vertices are designated by j and , respectively. Degree 5 vertices are unlabeled.
B
Co
CB
B
Co
B
BB
C
B
C
FeB
B
Fe
B
BB
C
closo-Cp2Co2C2B6H8 isocloso-Cp2Fe2C2B6H8
Fig. 3. A comparison of the closo structure for Cp2Co2C2B6H8 and the isoclosostructure for Cp2Fe2C2B6H8. External hydrogen atoms and Cp rings are omitted forclarity.
320 A. Lupan, R. Bruce King / Polyhedron 33 (2012) 319–326
vertices typically are introduced by capping open faces of an inter-mediate dicarbaborane polyhedron. Since this open face in thepolyhedral intermediate normally contains both carbon atoms,the initially produced metalladicarbaborane or dimetalladicarbab-orane may have the two carbon atoms in adjacent (ortho) positionsas well as the maximum number of M–C edges consistent with thedeltahedral geometry and the preference of carbon atoms fordegree 4 rather than degree 5 vertices. Pyrolysis of the initially pro-duced metallacarborane or dimetallacarborane gives more ther-modynamically stable isomers analogous to the formation of themeta and para isomers of the icosahedral C2B10H12 carborane bypyrolysis of the initially formed ortho C2B10H12 (Fig. 2). Previoustheoretical studies [10,11] show that the same is true for thecobaltadicarbaboranes CpCoC2Bn�3Hn�1. Thus the initially synthe-sized icosahedral cobaltadicarbaborane CpCoC2B9H11 is a high-en-ergy structure having adjacent carbon atoms and two Co–C edges,which is shown theoretically to lie �19 kcal/mol above the globalminimum. The global minimum CpCoC2B9H11 structure is still anicosahedral structure but has the carbon atoms in antipodal posi-tions (like para-carborane in Fig. 2).
The dimetalladicarboranes present an additional feature ofinterest since their skeletal electron count can be varied in evenelectron increments in homometallic derivatives. The classicalexample of this is the pair of 10-vertex derivatives Cp2M2C2B6H8
(Fig. 3: M = Co, Fe). The dicobalt derivative has 22 skeletal electrons(=2n + 2 for n = 10) and thus conforms to the Wade–Mingos rules[6–9] for a most spherical closo deltahedral structure, namely theD4d bicapped square antiprism with two degree 4 vertices andeight degree 5 vertices [13,14]. Both of the cobalt atoms in thisCp2Co2C2B6H8 structure are located at degree 5 vertices. The diironderivative Cp2Fe2C2B6H8, with only 20 skeletal electrons (=2n forn = 10), has a different so-called isocloso (or hypercloso) deltahedralstructure, namely a C3v deltahedron with a unique degree 6 vertex,six degree 5 vertices, and three degree 4 vertices. One of the ironatoms is located at the unique degree 6 vertex and the other ironatom is located at an adjacent degree 5 vertex. Even more unusualskeletal electron counts are found in the carbon-free dirhenabor-anes Cp�2Re2Bn�2Hn�2 (Cp⁄ = g5-Me5C5; 8 6 n 6 12) [15–17] where
Fig. 2. The ortho, meta, and para isomers of the icosahedral carborane C2B10H12. Hydroisomers given in the figure are those obtained from the calculations of Schleyer and Na
application of the Wade–Mingos skeletal electron counting proce-dure leads to 2n � 4 apparent skeletal electrons. Such dirhenabor-anes exhibit unusual non-spherical ‘‘flattened’’ oblate deltahedralstructures with the rhenium atoms at degree 6 or 7 vertices andmany of the boron atoms correspondingly at degree 4 rather thandegree 5 vertices [18].
This paper describes our density functional theory (DFT) studieson the dicobaltadicarbaboranes Cp2Co2C2Bn�4Hn�2. The dicobaltderivatives were chosen for this initial study in order to have the sys-tems with 2n + 2 skeletal electrons and thus are expected by theWade–Mingos rules [6–9] to exhibit closo deltahedral structures.In addition, numerous compounds of this type have been synthe-sized as very stable compounds so extensive comparison of our pre-dicted structures with the experimental structures is possible.However, most of the experimental work dates back to the 1960sand 1970s when large amounts of decaborane were available fromUS government stockpiles as a starting material for the synthesesof these dimetalladicarbaboranes. At that time X-ray crystallogra-phy was not as readily available and routine as it is now. Thus noneof the original papers involving synthesis of the dicobaltadicarb-aboranes Cp2Co2C2Bn�4Hn�2 contain any structures determined byX-ray crystallography. Instead most of the structures of the dicobalt-adicarbaboranes were established only by indirect methods such as11B NMR spectra, which provide information only on the following:(1) the symmetry of the dicobaltadicarbaborane cage, normally
gen atoms on the boron atoms are omitted for clarity. The relative energies of thejafian [12].
Table 1A summary of the Cp2Co2B5C2H7 structures within 20 kcal/mol of the global minimum. Experimentally known structures are starred and direct Co–Co bonding distances areindicated in bold type.
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whether it has no symmetry (C1) or has a two-fold symmetry ele-ment (C2, Cs, or Ci); (2) the presence of degree 4 boron vertices adja-cent to the cobalt vertex as indicated by an unusually low field 11Bresonance. The limited number of dicobaltadicarbaborane struc-tures that have been determined by X-ray diffraction include twonine-vertex Cp2Co2C2B5H7 structures [19], one 10-vertexCp2Co2C2B6H8 structure [14], and one 12-vertex Cp2Co2C2B8H10
structure [20]. By the time that X-ray structure determination be-came routine and relatively rapid, the large stockpiles of the decab-orane raw material had been depleted and destroyed so thatdecaborane became much more expensive as a starting material.Since relatively few structures of the relevant dicobaltadicarboraneshave been definitively established by X-ray diffraction, DFT methodsbecome particularly important to determine the lowest energystructures consistent with the experimental observations.
Fig. 4. The 10 Cp2Co2C2B5H7 structures within 20 kcal/mol of the global minimum.Experimentally known structures are enclosed in boxes.
2. Theoretical methods
Full geometry optimizations have been carried on theCp2Co2C2Bn�4Hn�2 systems at the B3LYP/6-31G(d) [21–24] level oftheory. The initial structures were chosen by systematic substitutionof two boron atoms from BnHn
2� by two carbon atoms followed byfurther substitution of all possible different pairs of boron atoms inthe resulting dicarbaborane by two CpCo units. This led to a largenumber of different starting structures. Thus 279 structures of thenine-vertex clusters Cp2Co2C2B5H7, 346 structures of the 10-vertexclusters Cp2Co2C2B6H8, 774 structures of the 11-vertex clustersCp2Co2C2B7H9, and 273 structures of the 12-vertex clustersCp2Co2C2B8H10 were chosen as starting points for the optimizations(see Supporting information). The natures of the stationary pointsafter optimization were checked by calculations of the harmonicvibrational frequencies. If significant imaginary frequencies werefound, the optimization was continued by following the normalmode corresponding to the largest imaginary frequency to insurethat genuine minima were obtained. Structures with imaginary fre-quencies smaller than 50i cm�1 were considered as minima on thepotential energy surface [25].
The structures, total and relative energies, and relevant inter-atomic distances for all calculated systems are given in Supportinginformation. Structures are numbered as B(n � 4)C2Co2 � x wheren is the number of vertices and x is the relative order of the struc-ture on the energy scale. Only the lowest energy and thus poten-tially chemically significant structures are considered in detail inthis paper, although more comprehensive lists of structures,including higher energy structures, are given in the SupportingInformation. The energy cutoff points chosen depend upon the dis-tribution of isomer structures and their energies and are indicatedin the figure captions. The chosen energy cutoff points led to sets of
7–12 structures for a given cluster size discussed in detail in thispaper.
All calculations were performed using the Gaussian 98 package[26] with the default settings for the SCF cycles and geometryoptimization, namely the fine grid (75302) for numerically evaluat-ing the integrals, 10�8 hartree for the self-consistent field conver-gence, maximum force of 0.000450 hartree/bohr, RMS force of 0.000300 hartree/bohr, maximum displacement of 0.001800 bohr, andRMS displacement of 0.001200 bohr.
3. Results and discussion
3.1. The nine-vertex Cp2Co2C2B5H7 isomers
A total of 25 isomers of the nine-vertex Cp2Co2C2B5H7 systemwere found with energies up to 34.8 kcal/mol above the globalminimum (see Supporting information). The 10 such structureswithin 20 kcal/mol of the global minimum B5C2Co2-1 are listedin Table 1 and depicted in Fig. 4. Seven of these 10 structures havegeometries based on the most spherical nine-vertex deltahedron,namely the tricapped trigonal prism (Fig. 1). In all of these low-energy structures both carbon atoms are located at degree 4vertices and both cobalt atoms are located at degree 5 vertices.Of these seven structures four of them, namely B5C2Co2-2,
Fig. 5. The nine Cp2Co2C2B6H8 structures within 20 kcal/mol of the globalminimum. Experimentally known structures are enclosed in boxes.
322 A. Lupan, R. Bruce King / Polyhedron 33 (2012) 319–326
B5C2Co2-3, B5C2Co2-7, and B5C2Co2-9 have Co–Co bonds alongan edge of the tricapped trigonal prism. In B5C2Co2-2 andB5C2Co2-9 the Co–Co edge is a ‘‘vertical’’ edge of the underlyingtrigonal prism shared by two rectangular faces. However, inB5C2Co2-3 and B5C2Co2-7 the Co–Co edge is a ‘‘horizontal’’ edgeof the underlying trigonal prism shared by a triangular and a rect-angular face.
Three of the 10 lowest energy Cp2Co2B5H7 structures are excep-tional. Structures B5C2Co2-6 and B5C2Co2-10 are not deltahedrasince they have a single quadrilateral face as well as 12 triangularfaces. Structure B5C2Co2-8 is derived from the nine-vertex isoclosodeltahedron having a unique degree 6 vertex flanked by four de-gree 4 vertices with four more remote degree 5 vertices. In thisstructure one of the cobalt atoms is located at the unique degree6 vertex and both carbon atoms at degree 4 vertices.
Two Cp2Co2C2B5H7 isomers have been isolated from reactionsof the seven-vertex dicarbaborane C2B5H7 involving the insertionof two cobalt atoms using either CpCo(CO)2 in the gas phase [27]or reaction with sodium naphthalide in solution followed by reac-tion with CoCl2/NaC5H5 [28]. X-ray crystallography studies [19] onthese two Cp2Co2C2B5H7 isomers support the predictions by DFTsummarized in Table 1 and Fig. 4. Thus the dark red Cp2Co2C2B5H7
isomer is shown to have the global minimum structure B5C2Co2-1whereas the dark green Cp2Co2C2B5H7 isomer, shown to havestructure B5C2Co2-2, is found to lie only 2.4 kcal/mol above thisglobal minimum. The dramatic color difference has been relatedto the presence of a direct Co–Co bonded edge in structureB5C2Co2-2 but the lack of such a direct Co–Co bonded edge inB5C2Co2-1. Similar color differences have been related to the pres-ence or absence of a Co–Co bonded edge in other dicobaltdicarbab-orane structures when less definitive experimental structuralinformation is available. Furthermore, the two Cp2Co2C2B5H7 iso-mers B5C2Co2-1 and B5C2Co2-2 have been shown to undergo areversible thermal interconversion in the vapor phase at tempera-tures around 340 �C in accord with their small energy difference.Also there is an predicted energy gap of more than �6 kcal/molbetween structure B5C2Co2-2 and the next higher energy struc-ture, namely B5C2Co2-3.
3.2. The 10-vertex Cp2Co2C2B6H8 isomers
A total of 32 isomers of the 10-vertex Cp2Co2C2B6H8 systemwere found within 40 kcal/mol of the global minimum. The ninesuch structures within 20 kcal/mol of the global minimumB6C2Co2-1 are listed in Table 2 and depicted in Fig. 5. All of thesestructures are based on the most spherical 10-vertex deltahedron,namely the D4d bicapped square antiprism. In all nine of thesestructures both cobalt atoms are located at degree 5 vertices. Inthe three lowest energy Cp2Co2C2B6H8 structures lying within3.5 kcal/mol of energy, both carbon atoms are located at degree 4
Table 2A summary of the Cp2Co2B6C2H8 structures within 20 kcal/mol of the globalminimum. The DE values are given in kcal/mol and distances in Å. Experimentallyknown structures are starred and direct Co–Co bonds are indicated in bold type.
vertices. These three structures exhaust the possibilities ofCp2Co2C2B6H8 structures with both cobalt atoms at degree 5vertices and both carbon atoms at degree 4 vertices. There is thena �7 kcal/mol energy gap between the highest energy of the threeCp2Co2C2B6H8 structures with two degree 5 cobalt vertices and twodegree 4 carbon vertices, namely B6C2Co2-3, and the next struc-ture, namely B6C2Co2-4, in which one of the two carbon atomsis located at a less favorable degree 5 vertex.
The polyhedral expansion of 1,7-C2B6H8 by reduction with so-dium naphthalide followed by addition of CoCl2/NaC5H5 gives a darkgreen Cp2Co2C2B6H8 isomer, shown by X-ray crystallography to beB6C2Co2-6. This relatively high-energy structure, at 13.5 kcal/molabove the global minimum B6C2Co2-1, contains a direct Co–Cobond and each cobalt atom is directly bonded to a single carbonatom. Pyrolysis of B6C2Co2-6 in boiling hexadecane (�290 �C)[29] results in rearrangement to the lowest energy Cp2Co2C2B6H8
isomer, namely the global minimum B6C2Co2-1 (Fig. 6). This globalminimum is red rather than green consistent with the absence of aCo–Co edge in the polyhedron. The rearrangement of B6C2Co2-6 toB6C2Co2-1 involves the migration of one of the cobalt atoms butretains the carbon atoms at the antipodal pair of degree 4 vertices.
A third Cp2Co2C2B6H8 isomer has been obtained, albeit in lowyield, by using a different synthetic approach. Thus reaction of
Fig. 6. The pyrolysis of the green Cp2Co2C2B6H8 isomer B6C2Co2-6 to give the redCp2Co2C2B6H8 isomer B6C2Co2-1.
Table 3A summary of the Cp2Co2B7C2H9 structures within 10 kcal/mol of the globalminimum as well as two higher energy structures of interest. The DE values aregiven in kcal/mol and distances in Å. Experimentally known structures are starred anddirect Co–Co and C–C bonds are indicated in bold type.
a The assignments to the Cp2Co2B8C2H8 isomers III through VII are those reportedby Evans et al. [29] using their designations.
b The Cp2Co2C2B7H9 structure B7C2Co2-19 is unusual since it has a C2Co2
trapezoidal face.
A. Lupan, R. Bruce King / Polyhedron 33 (2012) 319–326 323
the seven-vertex closo pentagonal bipyramidal dicarbaborane withsodium naphthalide followed by addition of CoCl2/NaC5H5 gave acomplicated mixture of cobaltacarboranes, which were separatedby chromatography [28]. One of the products was the nine-vertexglobal minimum Cp2Co2C2B5H7 structure B5C2Co2-1 (Fig. 4 andTable 1). However, another product was a red-brown Cp2Co2C2
B6H8 isomer clearly different from either B6C2Co2-1 or B6C2-Co2-6 discussed above. This Cp2Co2C2B6H8 isomer has not beencharacterized by X-ray diffraction. However, the boron-11 NMRspectrum indicates a 2:2:1:1 (2212) pattern of equivalent boronatoms as well as a single boron atom bonded to both cobalt atoms.This coupled with the red-brown color suggesting the absence of aCo–Co edge and the assumption of no C–C edge leads uniquely tostructure B6C2Co2-4 lying 10.4 kcal/mol above the global mini-mum B6C2Co2-1 for this structure.
3.3. The 11-vertex Cp2Co2C2B7H9 isomers
The 11-vertex Cp2Co2C2B7H9 system is considerably more com-plicated than the 9- and 10-vertex systems discussed above notonly because of the larger number of vertices but also because ofthe lower symmetry (C2v) of the most spherical 11-vertex deltahe-dron compared with the symmetries of the most spherical 9- and10-vertex deltahedra, namely D3h and D4d, respectively. Further-more the most spherical 11-vertex deltahedron has a unique de-gree 6 vertex as well as two degree 4 vertices flanking the degree6 vertex and eight degree 5 vertices. As a result a total of 66 struc-tures of the 11-vertex Cp2Co2C2B7H9 system were found within32 kcal/mol of the global minimum B7C2Co2-1. Even using a cutoffenergy of 20 kcal/mol above the global minimum still leaves 33structures. Fig. 7 and Table 3 use 10 kcal/mol as the energy cutoffabove the global minimum. This leaves seven structures, four ofwhich are known experimentally. In addition, two higher energystructures of particular interest, namely B7C2Co2-19 at 14.8 kcal/mol with an unusual Co2C2 trapezoidal face and the experimentallyknown B7C2Co2-44 are also shown in Fig. 7 and Table 3.
All seven of the Cp2Co2C2B7H9 structures within 10 kcal/mol arebased on the most spherical 11-vertex deltahedron with a unique
Fig. 7. The seven Cp2Co2C2B7H9 structures within 10 kcal/mol of the globalminimum as well as two higher energy structures of particular interest. Structuresknown experimentally are enclosed in boxes.
degree 6 vertex and two degree 4 vertices as well as eight degree 5vertices. In all seven of these Cp2Co2C2B7H9 structures both carbonatoms are located at the degree 4 vertices. In the lowest energyCp2Co2C2B7H9 structure B7C2Co2-1 one cobalt atom is located atthe unique degree 6 vertex and the other cobalt atom at a remotedegree 5 vertex. The lowest energy Cp2Co2C2B7H9 structure with adirect Co–Co edge is B7C2Co2-6 with a Co–Co distance of 2.61 Åand lying 8.0 kcal/mol above the global minimum B7C2Co2-1 (Table3). The lowest energy Cp2Co2C2B7H9 structure with a direct C–C edgeis B7C2Co2-19 with a C–C distance of 1.47 Å and lying 14.8 kcal/molabove B7C2Co2-1. Structure B7C2Co2-19 is also unusual since it isnot an 11-vertex deltahedron with 18 triangular faces. Instead struc-ture B7C2Co2-19 has one trapezoidal face as well as 16 triangularfaces. The unique trapezoidal face is distinctive since it is a Co2C2
face without any boron atoms. Locating the two carbon atoms at this‘‘hole,’’ i.e., non-triangular face, of this unusual 11-vertex polyhe-dron allows each carbon atom to maintain its favorable vertexdegree of 4.
Four of the six lowest energy Cp2Co2C2B7H9 structures havebeen realized experimentally (Fig. 7 and Table 3) [29]. In additiona high energy Cp2Co2C2B7H9 structure with adjacent carbon atoms,namely B7C2Co2-44 at 25.8 kcal/mol above the global minimumB7C2Co2-1 is observed as a minor byproduct in the large scalepreparation of a CpCoC2B9H11 isomer with adjacent carbon atoms.The formation of a high energy Cp2Co2C2B7H9 isomer with adjacentcarbon atoms under such conditions can be traced back to thesource of the two carbon atoms from an acetylene in the startingortho-carborane (Fig. 2).
The thermal rearrangements of these five Cp2Co2C2B7H9 isomersare of interest (Fig. 8). The high energy Cp2Co2C2B7H9 isomer B7C2-Co2-44 with adjacent carbon atoms as well as the Cp2Co2C2B7H9
isomer B7C2Co2-6 undergo rearrangement under relatively mildconditions (�100 �C) to give B7C2Co2-5 in which neither the cobaltnor the carbon atoms are adjacent [29]. Continuing the pyrolysis at100 �C for a more extended time converts B7C2Co2-5 lying 5.7 kcal/mol above the global minimum to B7C2Co2-2 lying only 0.7 kcal/mol above the global minimum. However, increasing the pyrolysistemperature of B7C2Co2-2 to�200 �C leads to the global minimumCp2Co2C2B7H9 isomer B7C2Co2-1. Thus the course of these thermalrearrangements is completely consistent with our theoreticalresults.
3.4. The 12-vertex Cp2Co2C2B8H10 structures
A total of 37 Cp2Co2C2B8H10 structures were found within33 kcal/mol of the global minimum. All of these structures are based
Fig. 8. Thermal rearrangements of the Cp2Co2C2B7H9 isomers.
Table 4A summary of the Cp2Co2B8C2H10 isomers within 6 kcal/mol of the global minimumas well as two higher energy isomers found experimentally. The DE values are givenin kcal/mol and distances in Å. Experimentally known structures are starred.
a In this shorthand notation a sets of b equivalent boron atoms are designated asba.
b These structural designations of compounds observed experimentally refer tothose used in the paper Evans [29].
324 A. Lupan, R. Bruce King / Polyhedron 33 (2012) 319–326
on a central Co2C2B8 icosahedron. Since a regular icosahedron hasonly degree 5 vertices, the relatively large energy differencebetween structures with cobalt atoms at degree 5 and the lessfavorable 4 vertices and with carbon atoms at degree 4 and the lessfavorable degree 5 vertices are not found. For this reason a large per-centage of these Cp2Co2C2B8H10 structures, namely 27 out of the 37total structures, lie within 20 kcal/mol of the global minimumB8C2Co2-1. In accord with expectation, the 12 Cp2Co2C2B8H10
structures without either direct Co–Co or C–C bonds are the lowestenergy structures and lie within�6 kcal/mol of the global minimum(Fig. 9 and Table 4).
The Cp2Co2C2B8H10 isomers known experimentally include notonly seven of the lowest energy 12 structures, namely those with-out Co–Co and C–C edges (Table 4), but also two higher energy C2v
isomers, namely B8C2Co2-30 with a Co–Co edge lying 21.6 kcal/
Fig. 9. The 12 Cp2Co2C2B8H10 isomers within 10 kcal/mol of the global
mol above the global minimum B8C2Co2-1 and B8C2Co2-27 witha C–C edge lying 19.4 kcal/mol above B8C2Co2-1 (Fig. 10 and Table4). The C2v symmetry of these high energy known Cp2Co2C2B8H10
isomers B8C2Co2-27 and B8C2Co2-30 is suggested by their NMRspectra. In addition, structure B8C2Co2-30 has been verified byX-ray crystallography [20] and found experimentally to have aCo–Co distance of 2.387(2) Å, which is very close to our calculatedvalue of 2.40 Å (Table 4). Similarly, a mercurated derivative
minimum. Experimentally known structures are enclosed in boxes.
Fig. 10. Two higher energy Cp2Co2C2B8H10 isomers with four Co–C edges. Both ofthese structures are known experimentally.
A. Lupan, R. Bruce King / Polyhedron 33 (2012) 319–326 325
(Cp2Co2C2B8H9Hg)2(l-Cl)2 of structure B8C2Co2-27 has been char-acterized by X-ray crystallography. In both of the high energyexperimentally known Cp2Co2C2B8H10 isomers B8C2Co2-27 andB8C2Co2-30 four of the 30 edges of the Co2C2B8 icosahedron corre-spond to Co–C bonds. However, the lowest energy Cp2Co2C2B8H10
isomer with a direct Co–Co bond is not B8C2Co2-30 but insteada C2v isomer B8C2Co2-13 with only one Co–C edge lying12.7 kcal/mol above the global minimum B8C2Co2-1. Similarly,the lowest energy Cp2Co2C2B8H10 isomer with a direct C–C bondis not B8C2Co2-27 but instead a C2v isomer B8C2Co2-19 with noCo–C edges lying 16.0 kcal/mol above B8C2Co2-1. Thus the highestenergy Cp2Co2C2B8H10 isomers obtained experimentally, namelyB8C2Co2-27 and B8C2Co2-30, have not only direct C–C or Co–Cobonds, respectively, but also the maximum possible number ofCo–C edges, namely four. This reflects their generation by a CpCounit capping an open face of a nido CpCoC2B8H12 derivative con-taining both carbon atoms.
The thermal rearrangement of the Cp2Co2C2B8H10 isomers hasbeen studied in detail (Fig. 11) [28]. Initially the two highest energyavailable isomers, namely B8C2Co2-30 with a direct Co–Co bondand B8C2Co2-27 with a direct C–C bond are converted cleanlyupon pyrolysis to B8C2Co2-6, lying 3.4 kcal/mol above the globalminimum B8C2Co2-1. The required temperatures are 250 �C forB8C2Co2-30 and 340 �C for B8C2Co2-27. The other low energyCp2Co2C2B8H10 isomers are obtained as a complicated mixture byheating B8C2Co2-6 or B8C2Co2-27 to the very high temperatureof 650 �C. The seven individual low energy Cp2Co2C2B8H10 isomersB8C2Co2-1, B8C2Co2-3, B8C2Co2-4, B8C2Co2-6, B8C2Co2-7,B8C2Co2-8, and B8C2Co2-9 were separated by chromatography.
Fig. 11. The thermal rearrangement of the two highest energy Cp2Co2C2B8H10
isomers.
The extremely high pyrolysis temperature to generate the lowestenergy Cp2Co2C2B8H10 isomers is a testimony to the high kineticbarriers to their isomerizations as well as the extremely high ther-mal stability of these unusual organometallic compounds.
4. Summary
The lowest energy structures for the dicobaltadicarbaboranesCp2Co2C2Bn�4Hn�2 (n = 9, 10, and 11) are the most spherical boranedeltahedra with the carbon atoms at degree 4 vertices and the cobaltatoms at degree 5 or 6 vertices. For the icosahedral 12-vertex dico-baltadicarbaboranes Cp2Co2C2B8H10 with only degree 5 vertices, thelowest energy structures are those without Co–Co or C–C edges.
The theoretical predictions from this study agree well withexperimental data on the numerous known Cp2Co2C2Bn�4Hn�2
(n = 9, 10, 11, and 12) derivatives. Thus for the nine-vertexCp2Co2C2B5H7 system only the two lowest energy isomers arefound experimentally; these lie within 3 kcal/mol of each otherand �6 kcal/mol below the next lowest energy isomer (Table 1).For the 10-vertex Cp2Co2C2B6H8 system three of the six lowest en-ergy isomers are known (Table 2). The highest energy of thesethree isomer and the only one with a direct Co–C bond is convertedat 290 �C to the lowest energy Cp2Co2C2B6H8 isomer (Fig. 6).
The 11- and 12-vertex Cp2Co2C2Bn�4Hn�2 systems provide exam-ples of stable high energy isomers arising from the particularsynthetic method used. In particular, the ultimate source of thetwo carbon atoms in such dicobaltadicarbaboranes can be an alkyneleading ultimately to high energy isomers with adjacent carbonatoms. Thus one of the experimentally known 11-vertex Cp2Co2
C2B7H9 isomers B7C2Co2-44 (Table 3) is a high-energy isomer withadjacent carbon atoms lying 25.8 kcal/mol above the global mini-mum B7C2Co2-1 (Table 3). This high energy Cp2Co2C2B7H9 isomeris converted to lower energy isomer upon pyrolysis at 100 �C andultimately to the global minimum isomer B7C2Co2-1 at �200 �C.In addition to this high energy isomer B7C2Co2-44 seven of the12 predicted Cp2Co2C2B7H9 isomers within 6 kcal/mol of the globalminimum have been synthesized.
The methods used to synthesize icosahedral Cp2Co2C2B8H10
derivatives have provided two high-energy isomers as stable com-pounds, namely B8C2Co2-30 with a Co–Co bond lying 21.8 kcal/mol above the global minimum and B8C2Co2-27 with a C–C bondlying 19.4 kcal/mol above the global minimum (Table 4). Pyrolysisof these high energy Cp2Co2C2B8H10 isomers at temperatures up to340 �C gives an isomer B8C2Co2-6 with neither a Co–Co nor a C–Cbond lying only 3.4 kcal/mol above the global minimum (Fig. 11).Further pyrolysis of this Cp2Co2C2B8H10 isomer at the incrediblyhigh temperature of 650 �C for an organometallic reaction gives acomplicated mixture of seven of the 12 possible lowest energy iso-mers, namely those with neither Co–Co nor C–C bonds.
Acknowledgment
Funding from the Romanian Ministry of Education andResearch, Grant PCCE 140/2008 and the US National ScienceFoundation (Grant CHE 1057466)is gratefully acknowledged.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.poly.2011.11.042.
