Analysis of the Aromaticity of Osmabicycles Analogous to the

The electron π-delocalization through the bicycle has a marked influence on the ..... 0.97 mmol) were added to a suspension of OsH2Cl2(PiPr3)2 (520 m...
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Analysis of the Aromaticity of Osmabicycles Analogous to the Benzimidazolium Cation Miguel Baya,* Miguel A. Esteruelas,* and Enrique O~nate Departamento de Química Inorganica, Instituto de Síntesis Química y Catalisis Homogenea (ISQCH), Universidad de Zaragoza, CSIC, 50009 Zaragoza, Spain

bS Supporting Information ABSTRACT: Complex OsH2Cl2(PiPr3)2 (1) reacts with 1,2-phenylenediamine in the presence of triethylamine to give OsH2(k-N,N-oHNC6H4NH)(PiPr3)2 (2), containing an osmabenzimidazolium core. The planarity and length equalization of the bicycle, along with the negative NICS values calculated for both rings and the aromatic MO delocalization, suggest that, as the organic counterpart, the osmabenzimidazolium moiety of 2 is aromatic. The electron π-delocalization through the bicycle has a marked influence on the chemical behavior of the metal center. Thus, in contrast to 1, complex 2 is stable in acetonitrile and reacts with [FeCp2]PF6 to give a 1:1 mixture of [OsH(k-N,N-o-HNC6H4NH)(NCCH3)(PiPr3)2]PF6 (3) and [OsH3(k-N,N-o-HNC6H4NH)(PiPr3)2]PF6 (4), containing bicycles that also appear to be aromatic.

’ INTRODUCTION Aromatic metallacycles are transition-metal-containing ring systems that exhibit aromatic properties.1 The basis of the metalloaromaticity is the isolobal analogy. An inorganic fragment is isolobal with an organic moiety when the number, symmetry, properties, approximate energy, and shape of the frontier orbitals and the number of electrons in them are similar.2 Thus, the substitution of a ring atom in an aromatic organic molecule by an isolobal metal fragment preserves the aromaticity in the new compound.3 The difference between the aromatic metallacycles and the regular aromatic compounds is that the π-bonding in the former involves the metal d orbitals.4 The nucleus-independent chemical shift5 (NICS) is widely used to characterize aromaticity and antiaromaticity. Its popularity as a quantitative aromaticity index is due to its simplicity and efficiency. Data for a large number of organic molecules have been reported.6 The NICS(0) value is calculated at the center of the ring, whereas NICS(1) refers to the location 1.0 Å above the center. Negative values are found for aromatic systems, while those positive indicate antiaromatic compounds. A few metallacycles have also been investigated.7 The NICS values of metallabenzenes such as (C5H5Ir)(PH3)3, (C5H5Pt)Cp, and [(C5H5Pt)(PH3)3]+ compare well with those of benzene, suggesting that they may be aromatic.8 The ruthenium-di(o-benzoquinonediimine) complex RuF2(k-N,N-o-HNC6H4NH)2 also appears to be aromatic with negative values for both the five-membered and the benzene rings. On the other hand the five-membered ring of the osmium counterpart OsF2(k-N,N-o-HNC6H4NH)2 has positive values of +8.56 and +2.90.9 We now show that the [CH]+ group of the cation benzimidazolium can be replaced by the metal fragments OsH2(PiPr3)2, [OsH(CH3CN)(PiPr3)2]+, and [OsH3(PiPr3)2]+. Thus, their coordination to o-benzoquinonediimine affords metalloaromatic r 2011 American Chemical Society

species. This research arose in the context of our interest on the chemical behavior of both MH2X2(PiPr3)2 (M = Ru, Os) complexes10 and metalloaromatic species.11

’ RESULTS AND DISCUSSION We reasoned that the chloride ligands of complex OsH2Cl2(PiPr3)2 (1) could be easily replaced by the chelate nitrogen donor ligand and, in fact, the treatment at room temperature of dichloromethane suspensions of 1 with 1.2 equiv of 1,2-phenylenediamine in the presence of 4.0 equiv of triethylamine leads to OsH2(k-N,N-o-HNC6H4NH)(PiPr3)2 (2), which is isolated in 73% yield, according to eq 1. The presence of an o-benzoquinonediimine ligand in 2 was initially confirmed12 by the 1H NMR spectrum of the obtained orange solid in dichloromethane-d2, at room temperature. The spectrum contains a NH resonance at 8.01 ppm. The hydride ligands display at 13.85 ppm a triplet with a H P coupling constant of 38.4 Hz. The 31P{1H} NMR spectrum shows a singlet at 41.5 ppm.

Figure 1 shows an ORTEP drawing of 2. As expected for a MH2X2(PR3)2 (M = Os, Ru) species,13 the geometry around the metal is non-octahedral. It can be described as derived from a Received: June 7, 2011 Published: July 28, 2011 4404

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Figure 2. Molecular orbitals with b-symmetry of the π-system (contour value: 0.03) in [C7H7N2]+ (left) and 2t (right).

Figure 1. Molecular diagram of complex 2. Some of the hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Os N(1) 2.010(4), Os N(2) 2.011(4), N(1) C(1) 1.379(5), N(2) C(2) 1.375(5), C(1) C(2) 1.415(6), C(2) C(3) 1.403(6), C(3) C(4) 1.390(6), C(4) C(5) 1.389(6), C(5) C(6) 1.379(6), C(6) C(1) 1.408(6), H(01) Os H(02) 118(2), P(1) Os P(2) 114.38(4).

distorted square antiprism with two missing vertices. One of the square planes is made by the phosphorus atoms and the hydride ligands, which occupy alternate positions. The nitrogen atoms are located in the second plane, which is rotated by about 40° from the first one.14 The molecule has an idealized C2 axis bisecting the P(1) Os P(2) (114.38(4)°) and N(1) Os N(2) (76.35(15)°) angles. The metallabicycle core is planar (maximum deviation 0.028(3) Å for N(1)), and the Os N (2.010(4) and 2.011(4) Å), C N (1.379(5) and 1.375(5) Å), and C C (between 1.379(6) and 1.415(6) Å) bond lengths clearly lie between those expected for single and double bonds. The tendency toward bond length equalization and planarity on the metallabicycle core is consistent with an aromatic species. This conclusion is also supported by the NICS values (B3PW91/ 6-311+G*/lanl2dz) calculated for the five- and six-membered rings of the model compound OsH2(k-N,N-o-HNC6H4NH)(PMe3)2 (2t), 12.6 (NICS(0)) and 11.9 (NICS(1)) for the five-membered ring and 10.1 (NICS(0)) and 10.8 (NICS(1)) for the six-membered ring, which agree well with those obtained for the benzimidazolium cation, 12.9 (NICS(0)) and 9.3 (NICS(1)) for the five-membered ring and 10.9 (NICS(0)) and 11.8 (NICS(1)) for the six-membered ring. An overall inspection of the molecular orbitals of 2t highlights the participation of the metal center in the delocalization of the π-electrons. This is clearly shown by the orbitals of the metallabicycle π-system with b-symmetry (MO 63, 72, and 79), which present significant contribution of the metal-based dyz orbital (Figure 2). These orbitals compare well with those with b-symmetry of the π-system of the benzimidazolium cation. The participation of the osmium atom in the delocalization of the π-electrons of the metallabicycle has a marked influence on the chemical behavior of the metal center of 2. The nonaromatic complex 1 coordinates Lewis bases to afford octahedral elongated dihydrogen derivatives.15 In acetonitrile, the coordination of the solvent affords the trans-dichloro OsCl2(η2-H2)(CH3CN)(PiPr3)2, which evolves into its cis-dichloro isomer.16 In contrast to 1, complex 2 is stable in acetonitrile. A detailed analysis of

Figure 3. Frontier orbitals (contour value: 0.03) of the [OsH2(PMe3)2]2+ fragment.

the metal ligand interaction in 2t reveals that from the five frontier orbitals of the [OsH2(PMe3)2]2+ fragment17 (LUMO +2, LUMO+1, LUMO, HOMO, and HOMO 1 in Figure 3), the LUMO and LUMO+2 orbitals are engaged in σ-interactions, whereas LUMO+1 is engaged in the delocalization of the π-electron system of the metallabicycle. Thus, as a consequence of this, the osmium atom does not have an empty orbital to interact with the Lewis base. The addition of 1.0 equiv of [Fe(η5-C5H5)2]PF6 to the acetonitrile solutions of 2 gives rise to a 1:1 mixture of the monohydride [OsH(k-N,N-o-HNC6H4NH)(CH3CN)(PiPr3)2]PF6 (3) and the trihydride [OsH3(k-N,N-o-HNC6H4NH)(PiPr3)2]PF6 (4). The first of them is the result of the substitution of one of the hydride ligands by an acetonitrile molecule, whereas the second one is a consequence of the protonation of the metal center. In agreement with the latter, we have also observed that the addition of 1.9 equiv of HBF4 to diethyl ether solutions of 2 produces the instantaneous precipitation of 4, as a green solid in 78% yield (Scheme 1). In this context, it should be mentioned that the frontier HOMO 1 orbital of the [OsH2(PiPr3)2]2+ metal fragment remains nonbonding in 2t. On the other hand, the addition of H+ to the diethyl ether solutions of the nonaromatic complex 1 produces the protonation of one of the chloride ligands, which is released as HCl. The resulting [OsH2Cl(PiPr3)2]+ species is stabilized by the remaining starting compound to afford the dimer cation [{OsH2(PiPr3)2}2(μ-Cl)3]+.18 The formation of the mixture can be rationalized according to Scheme 2. The one-electron oxidation of 0.5 equiv of 2 with 0.5 equiv of ferrocenium should initially afford the Os(V) 4405

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Scheme 1

Scheme 2

Figure 4. Molecular diagram of cation 3. Some of the hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Os(1) N(1) 2.019(5), 2.016(5); Os N(2) 1.989(5), 2.000(5); Os N(3) 2.039(5), 2.036(5); N(1) C(1) 1.342(8), 1.359(8); N(2) C(6) 1.362(8), 1.363(8); C(1) C(2) 1.410(9), 1.418(9); C(2) C(3) 1.362(9), 1.370(9); C(3) C(4) 1.416(10), 1.417(10); C(4) C(5) 1.377(9), 1.361(9); C(5) C(6) 1.407(9), 1.404(9); C(1) C(6) 1.429(9), 1.412(9); N(1) Os(1) N(3) 165.5(2), 164.9(2); N(2) Os(1) H(01) 171(2), 168(2), P(1) Os(1) P(2) 131.96(6), 136.84(6).

dihydride intermediate a. The oxidation should increase the acidity of the hydride ligands.19 Thus, the protonation of the remaining 0.5 equiv of 2 with a could give 4 and the 15-valenceelectron Os(III) monohydride b. The coordination of the solvent to the metal center of the latter should afford c. Its subsequent oxidation with the remaining 0.5 equiv of ferrocenium should finally yield 3.

Figure 5. Molecular diagram of cation 4t. Selected bond lengths (Å) and angles (deg): Os N(1) 2.051, Os N(2) 2.051, N(1) C(1) 1.330, N(2) C(2) 1.330, C(1) C(2) 1.445, C(2) C(3) 1.424, C(3) C(4) 1.369, C(4) C(5) 1.431, C(5) C(6) 1.369, C(6) C(1) 1.424, H(1) H(2) 1.663, H(2) H(3) 1.663, P(1) Os P(2) 156.88.

The monohydride 3 was separated from the mixture by crystallization in dichloromethane:diethyl ether and characterized by X-ray diffraction analysis. The structure has two chemically equivalent but crystallographically independent cations in the asymmetric unit. Figure 4 shows a view of one of them. Complex 3 is not a member of the OsH2Cl2(PiPr3)2 family, and, consistent with this, its structure shows marked differences with that of 2. The acetonitrile molecule and the hydride ligand are almost coplanar with the bicycle and coordinate trans to the nitrogen atoms with N(1) Os(1) N(3) and N(2) Os(1) H(01) angles of 165.5(2)° and 164.9(2)° and 171(2)° and 168(2)°, respectively. The phosphine groups approach the hydride ligand, forming P Os P angles of 131.96(6)° and 136.84(6)°. It is well known that the octahedral geometry is not favorable for heavy metal d4, which prefers to be diamagnetic. These complexes therefore undergo a distortion that destabilizes one orbital from the t2g set and simultaneously stabilizes some occupied orbitals.20 The structure is consistent with the 1H and 31 1 P{ H} NMR spectra. In agreement with the presence of the hydride ligand, the 1H NMR spectrum contains at 8.42 ppm a triplet with a H P coupling constant of 36.3 Hz. The NH resonances are observed at 9.12 and 13.09 ppm. The 31P{1H} NMR spectrum shows a singlet at 46.9 ppm. The metallabicycle of 3 also appears to be aromatic. Its core is planar (maximum deviation 0.057(3) Å for Os), and the bond lengths tend toward equalization, with Os N, N C, and C C distances between 1.989(5) and 2.019(5) Å, 1.342(8) and 1.363(8) Å, and 1.361(9) and 1.429(9) Å, respectively. The NICS values for the model cation [OsH(k-N,N-o-HNC6H4NH)(CH3CN)(PiPr3)2]+ (3t) are also negative. Those obtained for the five-membered ring, 11.4 (NICS(0)) and 11.9 (NICS(1)), compare well with the NICS values of 2t, whereas the corresponding ones to the six-membered ring, 6.8 (NICS(0)) and 9.1 (NICS(1)), are slightly smaller than those calculated for 2t. The metallabicycle of the trihydride derivative 4 also appears to be aromatic. Figure 5 shows a view of the optimized (B3PW91) structure of the model cation [OsH3(k-N,N-oHNC6H4NH)(PMe3)2]+ (4t). Like in 2 and 3, the bicycle core is planar, with the Os N (2.051 Å), N C (1.330 Å), and C C (1.369 1.445 Å) bond lengths being intermediate between those expected for single and double bonds. The NICS parameters are also negative, with values of 10.8 (NICS(0)) and 11.7 (NICS(1)) for the five-membered ring and 3.9 4406

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Organometallics (NICS(0)) and 7.1 (NICS(1)) for the six-membered ring. The latter suggest a reduction of the aromaticity of this ring, as a consequence of the protonation of the metal center. The coordination polyhedron around the osmium atom is the typical distorted pentagonal bipyramid found in OsH3XL(PiPr3)2 trihydride species,21 with phosphine in apical positions (P Os P = 156°) and the hydrides separated by 1.66 Å. In agreement with the presence of the latter, the 1H NMR spectrum of 4 in dichloromethane-d2 at room temperature contains at 7.85 ppm a triplet with a H P coupling constant of 16.2 Hz, which is consistent with the operation of a thermally activated site exchange process between the central and corner positions. Lowering the sample temperature produces a broadening of the resonance. However decoalescence is not observed down to 170 K. The 300 MHz T1 study of the resonance, over the temperature range 260 170 K, affords a T1(min) value of 89 ( 1 ms at 189 K, which agrees well with that calculated from the structural parameters of 4t22 (94 ms). The NH resonance appears at 12.45 ppm. The 31P{1H} NMR spectrum shows a singlet at 49.1 ppm.

’ CONCLUSIONS Complex OsH2(k-N,N-o-HNC6H4NH)(PiPr3)2 shows planarity and bond length equalization for the bicycle core, negative NICS values similar to those of the benzimidazolium cation, aromatic MO delocalization, and metal reactivity influenced by the electron π-delocalization. The four criteria analyzed—geometry, magnetic properties, molecular orbitals, and reactivity—certainly point in the direction of the aromaticity.

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obtained by slow diffusion of diethyl ether into a dichloromethane solution of the mixture. Spectroscopic data for 3: 1H NMR (CD3CN): δ 8.42 (t, 2JHP = 36.3, 1H, Os-H); 1.29 (dvt, N = 14.1, 3JHH = 6.9, 36H, PCHCH3); 2.47 (m, 6H, PCHCH3); 6.70 7.60 (4 H, C6H4 ), 9.12, 13.09 (both br, 1H each, NH). 31P{1H} NMR (CD3CN): δ 46.9 (s, PiPr3); 144.6 (sept, 2JFP = 717.6, PF6 ). Spectroscopic data for 4: see below.

Reaction of OsH2(j-N,N-o-HNC6H4NH)(PiPr3)2 with HBF4.

HBF4 (1:1 solution in diethyl ether, 46 μL, 0.34 mmol) was added to a solution of 2 (110 mg, 0.18 mmol) in diethyl ether (8 mL). An oily residue immediately appeared, which was decanted, washed with diethyl ether (3  5 mL), and dried in vacuo. A green solid was obtained, which was characterized as [OsH3(k-N,N-o-HNC6H4NH)(PiPr3)2]BF4 (4). Yield: 98 mg (0.14 mmol, 78%). Anal. Calcd for C24H51BF4N2OsP2: C, 40.79; H, 7.27; N, 3.96. Found: C, 40.60; H, 6.92; N, 4.18. IR (cm 1): 3302 (m, N-H); 2106 (w, Os-H); 1047, 1030 (vs, BF4). 1H NMR (CD2Cl2): δ 7.85 (t, 2JHP = 16.2, 3H, Os-H); 1.04 (dd, N = 14.7, 3JHH = 6.9, 36H, PCHCH3); 2.04 (m, 6H, PCHCH3); 7.10 7.60 (4H, C6H4 ), 12.45 (br, 2H, NH). 31P{1H} NMR (CD2Cl2): δ 49.1 (s).

’ ASSOCIATED CONTENT

bS

Supporting Information. Details of the X-ray analysis, crystal structure determinations, and a CIF file giving crystal data for 2 and 3. Computational details, orthogonal coordinates, and absolute energies of the optimized theoretical structures 2t, 3t, and 4t. This material is available free of charge via the Internet at http://pubs.acs.org.

’ EXPERIMENTAL SECTION

’ AUTHOR INFORMATION

General Methods and Instrumentation. All reactions were carried out under argon with exclusion of air using Schlenk-tube techniques. Solvents were dried by the usual procedures and distilled under argon prior to use. Complex 1 was prepared according to the published method.13a Reagents were obtained from commercial sources. NMR spectra were recorded on a Varian Gemini 2000 or a Bruker Avance 300 MHz instrument with resonating frequencies of 300 MHz (1H) and 121.5 MHz (31P). Chemical shifts (ppm) are referenced to residual solvent peaks (1H) or external H3PO4 (31P). Coupling constants, J and N, are given in hertz. IR spectra were run on a Perkin-Elmer Spectrum 100 FT-IR spectrometer. C, H, and N analyses were carried out in a Perkin-Elmer 2400 CHNS/O analyzer. Preparation of OsH2(j-N,N-o-HNC6H4NH)(PiPr3)2 (2). Triethylamine (0.5 mL, 3.6 mmol) and ortho-phenylenediamine (105 mg, 0.97 mmol) were added to a suspension of OsH2Cl2(PiPr3)2 (520 mg, 0.89 mmol) in dichloromethane (15 mL), and the mixture was stirred for 30 min. The resulting solution was vacuum-dried. The residue was extracted in toluene (30 mL), and the solution was vacuum-dried. The residue was washed with pentane (2  3 mL). An orange solid is obtained. Yield: 401 mg (0.65 mmol, 73%). Anal. Calcd for C24H50N2OsP2: C, 46.58; H, 8.14; N, 4.53. Found: C, 46.54; H, 8.02; N, 4.64. IR (cm 1): 3367, 3373 (vw, N-H); 2079, 2085 (s, Os-H). 1H NMR (CD2Cl2): δ 13.85 (t, 2JHP = 38.4, 2H, Os-H); 1.19 (dd, 3JHP = 12.6, 3 JHH = 7.2, 36H, PCHCH3); 2.22 (m, 6H, PCHCH3); 6.50 7.10 (4H, C6H4 ), 8.01 (br, 2H, NH). 31P{1H} NMR (CD2Cl2): δ 41.5 (s).

Corresponding Author

Reaction of OsH2(j-N,N-o-HNC6H4NH)(PiPr3)2 with [Fe(η5C5H5)2]PF6. A suspension of [Fe(η5-C5H5)2]PF6 (63 mg, 0.19 mmol)

in acetonitrile (8 mL) was added via cannula to a solution of 2 (109 mg, 0.18 mmol) in acetonitrile (7 mL). After 10 min the solution was vacuum-dried and the residue was washed with diethyl ether (3  5 mL). A green solid was obtained, which was characterized as a 1:1 mixture of [OsH(k-N,N-o-HNC6H4NH)(CH3CN)(PiPr3)2]PF6 (3) and [OsH3(k-N,N-o-HNC6H4NH)(PiPr3)2]PF6 (4). Crystals of 3 were selectively

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

’ ACKNOWLEDGMENT Financial support from the MICINN of Spain (Projects CTQ2008-00810 and Consolider Ingenio 2010 CSD2007-00006) and Departamento de Ciencia, Tecnología y Universidad del Gobierno de Aragon (E35) and the European Social Fund is acknowledged. M.B. thanks the Spanish MICINN/Universidad de Zaragoza for funding through the “Ramon y Cajal” Program. ’ REFERENCES (1) (a) Masui, H. Coord. Chem. Rev. 2001, 219 221, 957. (b) Bleeke, J. R. Chem. Rev. 2001, 101, 1205. (c) Wright, L. J. Dalton Trans. 2006, 1821. (d) Landorf, C. W.; Haley, M. M. Angew. Chem., Int. Ed. 2006, 45, 3914. (e) Bleeke, J. R. Acc. Chem. Res. 2007, 40, 1035. (f) Hung, W. Y.; Zhu, J.; Wen, T. B.; Yu, K. P.; Sung, H. H. Y.; Williams, I. D.; Lin, Z.; Jia, G. J. Am. Chem. Soc. 2006, 128, 13742. (g) Zhang, H.; Lin, R.; Hong, G.; Wang, T.; Wen, T. B.; Xia, H. Chem.—Eur. J. 2010, 16, 6999. (h) Paneque, M.; Poveda, M. L.; Rendon, N. Eur. J. Inorg. Chem. 2011, 19. (2) Hoffmann, R. Angew. Chem., Int. Ed. 1982, 21, 711. (3) See for example: (a) Paneque, M.; Posadas, C. M.; Poveda, M. L.; Rend on, N.; Salazar, V.; O~ nate, E.; Mereiter, K. J. Am. Chem. Soc. 2003, 125, 9898. (b) Chin, C. S.; Lee, H. Chem.—Eur. J. 2004, 10, 4518. (c) Alvarez, E.; Paneque, M.; Poveda, M. L.; Rendon, N. Angew. Chem., Int. Ed. 2006, 45, 474. (d) Zhang, H.; Xia, H.; He, G.; Wen, T. B.; Gong, L.; Jia, G. Angew. Chem., Int. Ed. 2006, 45, 2920. (e) Zhang, H.; Feng, L.; Gong, L.; Wu, L.; He, G.; Wen, T.; Yang, F.; Xia, H. Organometallics 2007, 26, 2705. (f) He, G.; Zhu, J.; Hung, W. Y.; Wen, T. B.; Sung, H. H.-Y.; Williams, I. D.; Lin, Z.; Jia, G. Angew. Chem., Int. Ed. 2007, 46, 9065. (g) Jacob, V.; Landorf, C. W.; Zakharov, L. N.; Weakley, T. J. R.; Haley, M. M. Organometallics 2009, 28, 5183. (h) Clark, G. R.; O’Neale, T. R.; 4407

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