A cationic rhodium(I) N-heterocyclic carbene complex isolated as an aqua adduct

Ashley L Huttenstine; Edward Rajaseelan; Allen G Oliver; Jeffrey A Rood

doi:10.1107/S1600536811033125

. 2011 Aug 27;67(Pt 9):m1274–m1275. doi: 10.1107/S1600536811033125

A cationic rhodium(I) N-heterocyclic carbene complex isolated as an aqua adduct

Ashley L Huttenstine ^a, Edward Rajaseelan ^b, Allen G Oliver ^c, Jeffrey A Rood ^a,^*

PMCID: PMC3200919 PMID: 22058874

Abstract

The title complex, aqua[1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene](η⁴-cycloocta-1,5-diene)rhodium(I) tetrafluoridoborate, [Rh(C₈H₁₂)(C₂₇H₃₆N₂)(H₂O)]BF₄, exihibits a square-planar geometry around the Rh(I) atom, formed by a bidentate cycloocta-1,5-diene (cod) ligand, an N-heterocylcic carbene and an aqua ligand. The complex is cationic and a BF₄ ⁻ anion balances the charge. The structure exists as a hydrogen-bonded dimer in the solid state, formed via interactions between the aqua ligand H atoms and the BF₄ ⁻ F atoms.

Related literature

For the use of N-heterocyclic carbenes (NHCs) in transfer hydrogenation reactions, see: Gnanamgari et al. (2006 ▶); Nichol et al. (2009 ▶); Hillier et al. (2001 ▶). For aqua adducts, see: Feng et al. (2010 ▶). For an example of intramolecular H—F bonding, see: Hobbs et al. (2010 ▶). For other NHCs, see: Bappert & Helmchen (2004 ▶); Herrmann et al. (2006 ▶); Nichol et al. (2010 ▶). For the synthesis, see: Yu et al. (2006 ▶). For discussion of complexes with four-coordinate metal atoms, see: Yang et al. (2007 ▶).

Experimental

Crystal data

[Rh(C₈H₁₂)(C₂₇H₃₆N₂)(H₂O)]BF₄
M _r = 704.49
Triclinic,
a = 11.4351 (4) Å
b = 12.2267 (4) Å
c = 12.6198 (4) Å
α = 94.103 (2)°
β = 94.081 (2)°
γ = 97.591 (2)°
V = 1738.66 (10) Å³
Z = 2
Mo Kα radiation
μ = 0.54 mm⁻¹
T = 150 K
0.28 × 0.25 × 0.18 mm

Data collection

Bruker X8 APEXII CCD diffractometer
Absorption correction: multi-scan (SADABS; Bruker, 2008 ▶) T _min = 0.859, T _max = 0.907
26258 measured reflections
7097 independent reflections
5888 reflections with I > 2σ(I)
R _int = 0.038

Refinement

R[F ² > 2σ(F ²)] = 0.037
wR(F ²) = 0.067
S = 1.01
7097 reflections
413 parameters
H atoms treated by a mixture of independent and constrained refinement
Δρ_max = 0.49 e Å⁻³
Δρ_min = −0.61 e Å⁻³

Data collection: APEX2 (Bruker, 2008 ▶); cell refinement: SAINT (Bruker, 2008 ▶); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ▶); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ▶); molecular graphics: CrystalMaker (Palmer, 2009 ▶); software used to prepare material for publication: enCIFer (Allen et al., 2004 ▶) and publCIF (Westrip, 2010 ▶).

Supplementary Material

Crystal structure: contains datablock(s) I, global. DOI: 10.1107/S1600536811033125/om2457sup1.cif

e-67-m1274-sup1.cif^{(37.9KB, cif)}

Structure factors: contains datablock(s) I. DOI: 10.1107/S1600536811033125/om2457Isup2.hkl

e-67-m1274-Isup2.hkl^{(347.2KB, hkl)}

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

Table 1. Selected geometric parameters (Å, °).

Rh1—C1	2.046 (2)
Rh1—C33	2.074 (3)
Rh1—C32	2.086 (2)
Rh1—O1	2.117 (2)
Rh1—C29	2.178 (3)
Rh1—C28	2.208 (3)

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C1—Rh1—C33	92.21 (10)
C1—Rh1—C32	94.10 (9)
C33—Rh1—C32	39.31 (11)
C1—Rh1—O1	89.49 (9)
C33—Rh1—O1	159.09 (10)
C32—Rh1—O1	161.19 (10)
C1—Rh1—C29	159.12 (12)
C33—Rh1—C29	97.84 (11)
C32—Rh1—C29	82.49 (11)
O1—Rh1—C29	87.60 (10)
C1—Rh1—C28	164.61 (12)
C33—Rh1—C28	81.68 (11)
C32—Rh1—C28	90.08 (11)
O1—Rh1—C28	91.31 (10)
C29—Rh1—C28	36.23 (12)

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Table 2. Hydrogen-bond geometry (Å, °).

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O1—H1W⋯F4	0.80 (3)	1.97 (3)	2.768 (3)	173 (3)
O1—H2W⋯F2ⁱ	0.79 (3)	1.86 (3)	2.644 (3)	175 (3)

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Symmetry code: (i) Inline graphic .

Acknowledgments

ALH and JAR gratefully acknowledge Elizabethtown College and the Department of Chemistry and Biochemistry for start-up funds and support, and the National Science Foundation (grant CHE-0958425) for instrument support.

supplementary crystallographic information

Comment

N-heterocyclic carbenes (NHCs) have received tremendous interest in recent times as ligands in catalytic transition metal complexes. An alternative to commonly used phosphines, NHCs provide numerous ways to tune the sterics and electronics of the complex (Herrmann, et al., 2006). Here, we report the crystal structure of the title compound, 1, as a stable aqua adduct that also exhibits H—F bonding interactions in the solid state.

The asymmetric unit of 1 contains a full molecule with the Rh(I) ion in a square planar geometry (Figure 1). The coordination sphere of the Rh(I) is completed through bonds to cycooctadiene, the carbene, and an aqua ligand, creating a complex cation. Charge balance is achieved with a non-coordinating tetrafluoroborate anion.The existence of nearly idealized square planar geometry can be supported using a recently reported metric, τ₄, for determining molecular shape in four coordinate complexes (Yang, et al., 2007). Here, a τ₄ value near zero is determined for square planar complexes; however, as the value approaches one, tetrahedral geometry is observed. By evaluation of the bond angles around the central rhodium atom, the τ₄ parameter was determined to be 0.028 for the title compound.

The bond distances and angles observed in 1 are within the usual ranges for rhodium-carbene [2.046 (2) Å] and rhodium-aqua [2.117 (2) Å] contacts. The O—H distances of the aqua ligand are similar in length [O1—H1W 0.80 (3) Å; O1—H2W 0.79 (4) Å]. The two diisopropylphenyl rings of the carbene ligand are approximately perpendicular to the carbene plane. The carbene atom, C1, deviates from an idealized sp² hybridization in that the N1—C1—N2 bond angle is 103.77 (19)°.

Some related cationic rhodium - imidazol-2-ylidene carbene complexes with tetrafluroborate counteranions have been reported (Nichol, et al., 2009, 2010; Bappert, et al., 2004) albeit, with neutral donors other than H₂O. In the case of 1, presumably due to the steric of the cod and carbene ligands, H₂O was found to be the only neutral ligand of appropriate size to occupy the fourth coordination site.

The presence of the aqua ligand and tetrafluroborate anion support the formation of a hydrogen-bonded dimer through a center of inversion in the solid state (Figure 2). O—H···F interactions of intermediate strength exist between the aqua ligand and the tetrafluroborate fluorine atoms [H1W—F4 1.968 (4) Å; H2W—F2 1.857 (4) Å]. Interestingly, although an aqua adduct, 1 is not soluble in water likely due to the hydrophobic periphery created by the carbene and cod ligands in the dimer.

In summary, we have reported the crystal structure of a cationic rhodium carbene complex containing an aqua ligand. The structure exists as a hydrogen-bonded dimer in the solid state. Future work aims to investigate the reactivity of this and other similar complexes for various organic transformations.

Experimental

All chemicals were purchased commercially, except for the neutral rhodium (I) carbene complex, [(cod)Rh(NHC)Cl], which was prepared according to the procedure of Yu, et al. (2006). The following manipulations were carried out under an inert nitrogen atmosphere. The cationic compound, [(cod)Rh(NHC)H₂O]BF₄, was synthesized by mixing the neutral rhodium compound (0.170 mmol) with water (10 drops) and silver tetrafluoroborate (0.170 mmol) in dichloromethane (20 ml). A yellow solution was obtained after stirring at room temperature for four days along with the formation of a white precipitate. The solution was gravity filtered over Celite and the filtrate was dried in vacuo to give a dark orange product (63.33%). X-ray quality crystals were obtained by dissolving the product in a small amount of dichloromethane and layering with pentane. ¹H NMR (400 MHz, CDCl₃): δ = 1.087 (d, 12H, CH₃-ⁱPr-NHC), 1.399 (d, 12H, CH₃-ⁱPr-NHC), 1.223–1.468 (br, 8H, CH₂ -cod), 1.617 (br, H₂O), 1.890 (br, 2H, CH(CH₃)₂-ⁱPr-NHC), 2.576 (sp, 2H, CH(CH₃)₂-ⁱPr-NHC), 3.162 (s, 2 H, CH -cod), 4.335 (s, 2 H, CH-cod), 7.099 (s, 2H, NCH), 7.120–7.607(m, 6H, Ar-H). ¹⁹F NMR (376.18 MHz, CDCl₃): δ = -152.105 p.p.m..