Crystallographic coincidence of two bridging species in a dinuclear CoIII ethynyl­benzene complex

Wesley A Hoffert; Matthew P Shores

doi:10.1107/S1600536811019969

. 2011 Jun 4;67(Pt 7):m853–m854. doi: 10.1107/S1600536811019969

Crystallographic coincidence of two bridging species in a dinuclear Co^III ethynylbenzene complex

Wesley A Hoffert ^a, Matthew P Shores ^a,^*

PMCID: PMC3151980 PMID: 21836854

Abstract

In the title compound, trans,trans-[μ-(m-phenylene)bis(ethyne-1,2-diyl)]bis[chlorido(1,4,8,11-tetraazacyclotetradecane)cobalt(III)]–trans,trans-[μ-(5-bromo-m-phenylene)bis(ethyne-1,2-diyl)]bis[chlorido(1,4,8,11-tetraazacyclotetradecane)cobalt(III)]–tetraphenylborate–acetone (0.88/0.12/2/4), [Co₂(C₁₂H₄)Cl₂(C₁₀H₂₄N₄)₂]_0.88[Co₂(C₁₀H₃Br)Cl₂(C₁₀H₂₄N₄)₂]_0.12(C₂₄H₂₀B)₂·4C₃H₆O, with the exception of the acetylene and bromine groups, all atomic postitions are the same in the two compounds and are modeled at full occupancy. The Co^III ions are six-coordinate with acetylide and chloride ligands bound to the axial sites and the N atoms from the cyclam rings coordinated at the equatorial positions. N—H⋯O and N—H⋯Cl hydrogen-bonding interactions help to consolidate the crystal packing.

Related literature

Metallodendrimers are of interest for their unique catalytic and optical properties, see: Mery & Astruc (2006 ▶); Onitsuka & Takahashi (2003 ▶). For Pt(II)- and Ru(II)-containing dendrimers based on a 1,3,5-triethynylbenzene (H₃TEB) linkage, see: Onitsuka et al. (2004 ▶); McDonagh et al. (2003 ▶). For a discussion of the structural similarity between halogen and ethynyl substituents, see: Robinson et al. (1998 ▶). For related metal–acetylide structures, see: Weyland et al. (1998 ▶); Onitsuka et al. (2004 ▶). For the structure of [(cyclam)CoCl₂]Cl, see: Ivaniková et al. (2006 ▶). For the preparation of trans-[(cyclam)CoCl₂]Cl, see: Bosnich et al. (1965 ▶). General Sonogashira conditions were used to prepare a mixture of 1,3,5-triethynylbenzene and 1-bromo-3,5-diethynylbenzene (Weber et al., 1988 ▶).

Experimental

Crystal data

[Co₂(C₁₂H₄)Cl₂(C₁₀H₂₄N₄)₂]_0.88[Co₂(C₁₀H₃Br)Cl₂(C₁₀H₂₄N₄)₂]_0.12·(C₂₄H₂₀B)₂·4C₃H₆O
M _r = 1614.61
Triclinic,
a = 10.1434 (4) Å
b = 17.1412 (7) Å
c = 25.5250 (11) Å
α = 92.609 (1)°
β = 96.864 (1)°
γ = 104.323 (1)°
V = 4256.2 (3) Å³
Z = 2
Mo Kα radiation
μ = 0.56 mm⁻¹
T = 120 K
0.60 × 0.30 × 0.30 mm

Data collection

Bruker APEXII CCD area-detector diffractometer
Absorption correction: multi-scan (SADABS; Bruker, 2009 ▶) T _min = 0.729, T _max = 0.850
40567 measured reflections
25801 independent reflections
20379 reflections with I > 2σ(I)
R _int = 0.016

Refinement

R[F ² > 2σ(F ²)] = 0.037
wR(F ²) = 0.097
S = 1.02
25801 reflections
1001 parameters
H-atom parameters constrained
Δρ_max = 0.52 e Å⁻³
Δρ_min = −0.42 e Å⁻³

Data collection: APEX2 (Bruker, 2009 ▶); cell refinement: SAINT (Bruker, 2009 ▶); data reduction: SAINT; program(s) used to solve structure: SHELXTL (Sheldrick, 2008 ▶); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL.

Supplementary Material

Crystal structure: contains datablock(s) I, global. DOI: 10.1107/S1600536811019969/qm2008sup1.cif

e-67-0m853-sup1.cif^{(62KB, cif)}

Structure factors: contains datablock(s) I. DOI: 10.1107/S1600536811019969/qm2008Isup2.hkl

e-67-0m853-Isup2.hkl^{(1.2MB, hkl)}

Supplementary material file. DOI: 10.1107/S1600536811019969/qm2008Isup3.mol

Additional supplementary materials: crystallographic information; 3D view; checkCIF report

Enhanced figure: interactive version of Fig. 1

Table 1. Hydrogen-bond geometry (Å, °).

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
N5—H5⋯Cl2ⁱ	0.93	2.48	3.2377 (12)	139
N6—H6⋯O3	0.93	2.15	2.9440 (19)	143
N7—H7⋯O2	0.93	2.11	2.9894 (17)	157
N8—H8⋯O1	0.93	2.03	2.8730 (17)	149

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Symmetry code: (i) Inline graphic .

Acknowledgments

The authors wish to thank Colorado State University and the ACS Petroleum Research Fund for financial support.

supplementary crystallographic information

Comment

From a technological standpoint, metallodendrimers are of interest for their unique catalytic and optical properties (Mery & Astruc, 2006; Onitsuka & Takahashi, 2003). A particular subset of metallodendrimers based on ethynylbenzene have been pursued because of their structural rigidity and topological anisotropy. Although a variety of Pt(II)- and Ru(II)-containing dendrimers based on a 1,3,5-triethynylbenzene (H₃TEB) linkage have been reported (Onitsuka et al., 2004; McDonagh et al., 2003), we are interested in the properties of first row transition metal TEB complexes for potential applications in molecular magnetism (Weyland et al., 1998). For elaboration to higher nuclearity species, the inclusion of an axially coordinated anionic ligand that is poized for substitution is vital.

The synthesis of these macromolecules can be accomplished by divergent or convergent pathways; regardlesss, each strategy hinges upon the isolation of structurally characterized "building blocks" prior to dendrimer assembly. The preparation of complexes that contain first-row metals is synthetically challenging because of their high kinetic lability relative to their second and third row counterparts. In that respect, our initial synthetic targets contain Co^III because of its relative inertness.

The combination of H₃TEB with two equivalents of trans-[(cyclam)CoCl₂]Cl produces the dinuclear Co^III arylacetylide-bridged complex 1 in good yield (Figure 1). Initial refinement attempts on high quality X-ray data did not converge satisfactorily, as the third aromatic substituent showed apparent disorder of the alkynyl group. However, structure refinement proceeds smoothly if compositional disorder is invoked. The accepted method for the preparation of 1,3,5-triethynylbenzene involves Sonogashira coupling between 1,3,5-tribromobenzene and trimethylsilylacetylene (Weber et al., 1988). On one occasion, following the protocol resulted in a batch of TEB containing a sizeable amount of 1-bromo-3,5-diethynylbenzene (Robinson et al., 1998), indicating incomplete substitution. The impurity was carried through several purification steps, eventually affording a mixture of the ethynyl- (1) and bromo- (2) substituted complexes. The crystal structure revealed that both components of the ligand mixture were incorporated into metal complexes and the atomic sites were superimposed. During structure refinement, the compositional disorder at the aromatic 1 position was modeled with a free variable. Final site occupancy factors indicate that the two ligand components are present in an 88:12 1:2 ratio. This compares favorably with subsequent ¹H NMR analysis of the batch of "H₃TEB" ligand, which shows resonance integrations in an 87:13 H₃TEB:H₂BrTEB ratio.

The molecular structure of the complex cations in 1 and 2 are shown in Figure 1. Each pseudo-octahedral Co^III center coordinates four nitrogen atoms from the cyclam rings at the equitorial positions with an average Co—N bond length of 1.9767 (11) Å, which is only slightly longer than the corresponding bond length from the reported structure of trans-[(cyclam)CoCl₂]Cl (1.9741 (12); Ivaniková et al., 2006). Anionic chloride and acetylide ligands occupy the axial Co^III coordination sites with average metal-ligand distances of 2.3076 (4) and 1.8770 (14) respectively. The former bond length is significantly longer than the average Co—Cl distance in trans-[(cyclam)CoCl₂]Cl, suggesting that the arylacetylide ligand imparts a stronger trans influence than chloride. The cationic charge is balanced by the presence of two tetraphenylborate anions, and the asymmetric unit includes four molecules of acetone.

Shown in Figure 2, the crystal packing in 1 and 2 is influenced by several weak hydrogen bonding interactions. Notably, the complex cations experience a dimeric interaction through pairwise N—H···Cl contacts with a complex in a neighboring unit cell. Furthermore, three of the four acetone molecules participate in hydrogen bonds through the cyclam N–H groups.

In summary, a mixture of H₃TEB and 1-bromo-3,5-diethynylbenzene combined with trans-[(cyclam)CoCl₂]Cl to yield a co-crystallized mixture of 1 and 2. The compounds are superimposed in the solid state with the exception of the 5-position acetylene and bromine groups. Using a free variable to model the compositional disorder, we conclude that the two compounds are present in a 88:12 ratio. The first coordination sphere for each Co^III ion includes an axially replaceable chloride ligand, which is a necessary condition for future metallodendrimer assembly. This result exemplifies the key role of crystallographic analysis in organometallic synthesis development.

Experimental

trans-[(cyclam)CoCl₂]Cl was prepared by a previously descibed method (Bosnich et al. 1965). General Sonogashira conditions were used to prepare a mixture of 1,3,5-triethynylbenzene and 1-bromo-3,5-diethynylbenzene (Weber et al., 1988). Triethylamine was purchased from Sigma-Aldrich and was distilled prior to use.

Elemental analysis was performed by Robertson Microlit in Madison, NJ.

Preparation of 1 and 2: Triethylamine (0.34 ml, 2.42 mmol) was added to a 100 ml round-bottomed flask containing a green methanolic (10 ml) solution of [(cyclam)CoCl₂]Cl (233 mg, 0.637 mmol) and freshly sublimed mixture (45.5 mg) of 1,3,5-triethynylbenzene (87% by ¹H NMR) and 1-bromo-3,5-diethynylbenzene (13% by ¹H NMR). The flask was fitted with a condenser tube and the solution was refluxed for 24 h, during which time the solution turned orange-brown. The solvent was removed by rotary evaporation, and the resulting red-brown residue was washed with 10 ml of absolute ethanol, causing an orange solid to precipitate. The solid was isolated by filtration, washed with ethanol (3 × 3 ml) and diethyl ether (3 × 3 ml) and dried in air to afford 92.1 mg of an orange solid. The orange solid was dissolved in methanol (10 ml) and a solution of excess sodium tetraphenylborate in methanol (5 ml) was added, causing a salmon-colored solid to precipitate. The solid was isolated by filtration, washed with methanol (3 × 3 ml) and diethyl ether (3 × 3 ml) and dried in air to afford 131 mg of product (0.094 mmol, 30% based on [(cyclam)CoCl₂]Cl). Anal. Calcd. for C_85.74H_103.87B₂Br_0.13Cl₂Co₂N₈O₂: C, 68.68; H, 6.98; N, 7.47. Found: C, 68.33; H, 7.02; N, 7.85. Single crystals suitable for X-ray analysis were grown by diffusing diethyl ether vapor into a concentrated solution of the compound in acetone for 2 days.