Unsupported Metal Chain Complex: Synthesis, Characterization, and EHMO Study Involving the Tetraosmium Complex [Os2(CO)5(thd)2]2

(1)

Unsupported Metal Chain Complex: Synthesis,

Characterization, and EHMO Study Involving the

Tetraosmium Complex [Os

2

(CO)

5

(thd)

2

]

2

Ming-Der Su,

†

_{Hsin-Yi Liao,}

‡

_{San-Yan Chu,}

‡

_{Yun Chi,*}

,‡

_{Chao-Shiuan Liu,*}

,‡

Feng-Jen Lee,

‡

_{Shie-Ming Peng,}

§

_{and Gene-Hsiang Lee}

§

School of Chemistry, Kaohsiung Medical University, Kaohsiung 80708,

Department of Chemistry, National Tsing Hua University, Hsinchu 30013, and Department of Chemistry and Instrumentation Center, National Taiwan University,

Taipei 10764, Taiwan, Republic of China Received June 23, 2000

Metal carbonyl complex Os3(CO)12 reacts with a hexane solution of (thd)H, thd )

2,2-dimethyl-3,5-heptanedionate, in a sealed autoclave at 190 °C to afford the Os4metal chain

complex [Os2(CO)5(thd)2]2(1), isolated from repeated recrystallization in air. Treatment of 1 with CO at room temperature gives the Os2 metal complex [Os2(CO)6(thd)2] (2) through

reversible cleavage of the central Os-Os bond. EHMO calculation has been carried out to explain the involvement of a short-long-short Os-Os‚‚‚Os-Os array.

There has been considerable interest in the genera-tion of the metal chain compounds as their properties may shed light on the reaction patterns and bonding interactions between metal atoms on the molecular scale.1_{Building such metal complexes normally requires}

the use of certain bridging carbene and carbyne ligands2

or polyamine molecules3_{that can serve as multidentate}

bridging ligands to link the metal atoms. However, the corresponding metal chain complexes held together by unsupported metal-metal bonding are rare. Such classes of complexes are best represented by triosmium com-plexes with the formula Os3(CO)12X2, X ) H, Me, and

halides,4_{the analogous derivative complexes that}

con-tain heterometallic terminus such as AuPPh3 and

SiCl3,5and the trimetallic complexes involving a

donor-acceptor metal chain in tandem,6_{on which the}

stabili-zation is provided by the relatively stronger metal-metal bonding involving the third row transition-metal-metal elements.7_{To elaborate the possibility for forming the}

higher nuclearity analogue, we now report the success-ful synthesis of a compound with a nearly linear chain

involving four metal atoms and with the bonding pattern Os-Os‚‚‚Os-Os, linked together without the bridging ligands.

Experimental Procedure

General Information and Materials. Infrared spectra were recorded on a Perkin-Elmer 2000 FT-IR spectrometer.

1_{H and}13_{C NMR spectra were recorded on Bruker AMX-300}

and AMX-600 instruments; chemical shifts are quoted with respect to internal standard tetramethylsilane (1_{H and} 13_C

NMR). All reactions were performed under a nitrogen atmo-sphere using deoxygenated solvents dried with an appropriate reagent. Elemental analyses were carried out at the NSC Regional Instrumentation Center at National Cheng Kung University, Tainan, Taiwan.

Reaction of Os3(CO)12with (thd)H. (thd)H (7 equiv, 0.72 mL, 3.88 mmol), 0.50 g of Os3(CO)12(0.55 mmol), and 50 mL

of hexane were added into a 100 mL stainless steel autoclave. The reactor was sealed under nitrogen and was then slowly heated to 190 °C. After 30 h, the hexane solution was transferred out of the reactor, affording a yellow orange solution which gradually turned dark-purple upon exposure to air. The hexane was evaporated under vacuum, and the residue was dissolved in CH2Cl2. Then, the insoluble

light-yellow powder, which is unreacted Os3(CO)12, was removed by

filtration and discarded. The filtrate was concentrated to dryness, and the oily residue was purified by repeated recrys-tallization using a mixture of CH2Cl2and MeOH, affording

140 mg of the dark-red crystalline solid [Os2(CO)5(thd)2]2(1,

0.079 mmol, 20%).

Spectral data of 1. IR (C6H12): ν (CO), 2076 (s), 1994 (vs,

br), 1925 (m).1_{H NMR (CDCl}

3, 294K): δ 6.24 (s, 2H, CH), 5.67

(s, 2H, CH), 1.18 (s, 36H, CH3), 1.10 (s, 36H, CH3).13C NMR

(CDCl3, 294 K): δ 199.1 (4C, CO), 195.5 (4C, CO), 184.1 (4C,

CO), 178.9 (2C, CO), 176.8 (4C, CO), 95.7, (2C, CH), 94.2 (2C, CH), 41.6 (4C, CMe3), 41.4 (4C, CMe3), 28.4 (12C, CH3), 28.3

(12C, CH3). Anal. Calcd for C54H76O18Os4: C, 36.56; H, 4.32.

Found: C, 36.26; H, 4.08.

Treatment of [Os2(CO)5(thd)2]2with CO. A purple solu-tion of [Os2(CO)5(thd)2]2(25 mg) in 0.6 mL of CDCl3was first

placed in a 5 mm NMR tube. The air in the NMR tube was

†_{Kaohsiung Medical University.} ‡_{National Tsing Hua University.} §_{National Taiwan University.}

(1) (a) Cotton, F. A.; Daniels, L. M.; Murillo, C. A.; Pascual, I. J.

Am. Chem. Soc. 1997, 119, 10223. (b) Cotton, F. A.; Daniels, L. M.;

Jordan, G. T., IV; Murillo, C. A. J. Am. Chem. Soc. 1997, 119, 10377. (2) (a) Davies, S. J.; Howard, J. A. K.; Mosgrove, R. J.; Stone, F. G. A. Angew. Chem., Int. Ed. Engl. 1989, 28, 264. (b) Hart, I. J.; Hill, A. F.; Stone, F. G. A. J. Chem. Soc., Dalton Trans. 1989, 2261.

(3) (a) Shieh, S. J.; Chou, C. C.; Lee, G. H.; Wang, C. C.; Peng, S. M. Angew. Chem., Int. Ed. Engl. 1997, 36, 56. (b) Lai, S. Y.; Lin, T. W.; Chen, Y. H.; Wang, C. C.; Lee, G. H.; Yang, M. H.; Leung, M. K.; Peng, S. M. J. Am. Chem. Soc. 1999, 121, 250.

(4) (a) Moss, J. R.; Graham, W. A. G. Inorg. Chem. 1977, 16, 75. (b) Cook, N.; Smart, L.; Woodward, P. J. Chem. Soc., Dalton Trans. 1988, 1744.

(5) (a) Willis, A. C.; van Buuren, G. N.; Pomeroy, R. K.; Einstein, F. W. B. Inorg. Chem. 1983, 22, 1162. (b) Firfiray, D. B.; Irving, A.; Moss, J. R. J. Chem. Soc., Chem. Commun. 1990, 317. (c) Lewis, J.; Moss, J. R. Can. J. Chem. 1995, 73, 1236.

(6) (a) Batchelor, R. J.; Davis, H. B.; Einstein, F. W. B.; Pomeroy, R. K. J. Am. Chem. Soc. 1990, 112, 2036. (b) Liu, Y.; Leong, W. K.; Pomeroy, R. K. Organometallics 1998, 17, 3387.

(7) Deeming, A. J. Adv. Organomet. Chem. 1986, 26, 1.

5400 Organometallics 2000, 19, 5400-5403

Downloaded by NATIONAL TAIWAN UNIV on August 18, 2009

(2)

removed by purging with CO gas, during which time a yellow orange solution was immediately obtained. The1_{H and} 13_C

NMR analysis indicated the quantitative conversion to the diosmium complex [Os2(CO)6(thd)2] (2). The solution was then

concentrated by passing a relatively fast flow of CO into the NMR tube. Addition of MeOH over the top of the concentrated solution resulted in formation of a yellow orange crystalline complex of 2 and a small amount of dark blue noncrystalline precipitate over a period of 5 h.

Spectral data of 2. IR (C6H12): ν (CO), 2049 (s), 1984 (vs). 1_{H NMR (CDCl} 3, 294K): δ 5.75 (s, 2H, CH), 1.14 (s, 36H, CH3). 13_{C NMR (CDCl} 3, 294 K): δ 200.1 (4C, CO), 180.1 (4C, CO), 176.3 (2C, CO), 94.0, (2C, CH), 41.4 (4C, CMe3), 28.2 (12C, CH3).

X-ray Crystallography. Single-crystal X-ray diffraction data were measured on a Bruker SMART CCD diffractometer using λ(Mo KR) radiation, 0.7107 Å. The data collection was executed using the SMART program. Cell refinement and data reduction were made by the SAINT program. The structure was solved using the SHELXTL/PC program. Anisotropic displacement parameters were used for all non-hydrogen atoms, while the hydrogen atoms were given fixed isotropic displacement parameters. Crystallographic refinement pa-rameters of complexes 1 and 2 are summarized in Table 1.

Extended Hu1 ckel MO Calculation. Extended Hu¨ckel calculations were carried out on model complexes 3, 4, and 5 within the extended Hu¨ ckel formalism.8 _{Standard atomic}

distances were taken. The exponents (ξ) and the valence shell ionization potentials (Hiiin eV) were respectively the

follow-ing: 1.300, -13.60 for H 1s; 1.710, -21.40 for C 2s; 1.625, -11.40 for C 2p; 2.575, -28.20 for O 2s; 2.275, -12.40 for O 2p; 1.891, -6.550 for Os 6s; 1.280, -4.000 for Os 6p. The Hii

value for Os 5d was at -13.74. A linear combination of two Slater-type orbitals with exponents ξ1) 3.857 and ξ2) 1.797

with the weighting coefficients c1 ) 0.7598 and c2 ) 0.3899 was used to represent the Os 5d atomic orbitals.

Results and Discussion

The required Os4metal chain complex was prepared

from direct treatment of Os3(CO)12 and

2,2,6,6-tetra-methyl-3,5-heptanedione ((thd)H) in a stainless steel autoclave. As described in the Experimental Section, after the reaction was stopped and the hexane solution transferred out of the reactor, the color of the solution was found to change from orange-yellow to dark-purple during workup, indicating a spontaneous conversion to a secondary product. Thin-layer chromatography was not utilized for product separation as rapid decomposi-tion was noted upon applying the mixture to silica gel. However, a dark brown crystalline product which pos-sesses the formula [Os2(CO)5(thd)2]2(1) was isolated by

filtration to remove the insoluble material, followed by repeated crystallization using a mixture of CH2Cl2and

MeOH. Complex 1 shows a very dark color by the naked eye, but the color changed to red-brown under a microscope in bright illumination. Dissolution of 1 in CH2Cl2afforded a dark-purple solution, confirming that

it is the secondary product of the original mixture. This complex was then characterized using IR, 1_{H and}13_C

NMR, and single-crystal X-ray diffraction.

As indicated in Figure 1, the structure of 1 consists of a slightly bent arrangement of Os atoms where the unique Os-Os-Os bond angle is 160.6(1)°. Each Os unit adopts an octahedral coordination environment involv-ing one thd and at least two CO ligands. The thd ligands are positioned in the anti-anti-anti type of arrangement about the Os4vector. The ligands of the inner Os atom

are arranged in an eclipsed geometry with respect that of the outer Os atoms, while the respective CO and thd ligands located between the inner Os atoms showed a very different, staggered arrangement. Moreover, the outer Os-Os distance (2.7683(4) Å) is shorter than the average Os-Os distance (2.877(3) Å) observed in the (8) Hoffmann, R. J. Chem. Phys. 1963, 39, 1397.

Table 1. Crystallographic Refinement Parameters compounds

1 2

formula C54H76O18Os4 C28H38O10Os2

mol wt 1774 914.98

temp, K 295 150

crystal system monoclinic monoclinic

space group P21/n C2/c a (Å) 16.392(1) 15.4880(4) b (Å) 10.170(2) 10.2643(3) c (Å) 19.584(2) 20.8398(5) β (deg) 99.476(7) 101.763(1) volume (Å3₎ _3220.0(7) _3243.4(2) Z 2 4 Dc(g/cm3) 1.830 1.874 F(000) 1696 1752 h k l ranges -19 19, 0 12, 0 23 -19 20, -13 13, -27 13 crystal size, mm 0.65_{× 0.22 × 0.16 0.30 × 0.12 × 0.04} µ(Mo KR), mm-1 7.929 7.877 transmission: max, min 0.365, 0.205 0.492, 0.305 no. of data in refnmt 5645 3638 no. of parameters 344 180 R1, wR2with I > 2σ(I) 0.033, 0.074 0.036, 0.077 ext. coefficient 0.00073(4) 0.00021(4) D map, max/min, e/Å-3 1.234/-1.115 2.278/-1.932

Figure 1. Molecular structure of [Os2(CO)5(thd)2]2(1); the methyl groups of the thd ligands were omitted for clarity. Bond distances (Å): Os(1) ) 2.9778(6), Os(1)-Os(2) ) 2.7683(4), Os(1)-C(1) ) 1.831(8), Os(1)-C(2) ) 1.844(8), Os(1)-O(6) ) 2.065(5), Os(1)-O(7) ) 2.073(5), Os(2)-C(3) ) 1.892(9), Os(2)-C(4) ) 1.875(9), Os(2)-C(5) ) 1.925(9), Os(2)-O(8) ) 2.069(5), and Os(1)-O(9) ) 2.090(5).

Tetraosmium Complex [Os2(CO)5(thd)2]2 Organometallics, Vol. 19, No. 25, 2000 5401

(3)

parent complex Os3(CO)12.9In contrast, the inner

Os-Os distance (2.9778(6) Å) is significantly longer, showing the presence of a weakened metal-metal interaction. Stirring a CH2Cl2 solution of 1 under CO at room

temperature leads to a rapid color change from purple to yellow orange; the latter is identical to the color of the original solution obtained from direct treatment of Os3(CO)12 and (thd)H, and suggests formation of the

fragmentation product [Os2(CO)6(thd)2] (2) induced by

CO addition. A13_{C NMR study showed the occurrence}

of only one set of thd carbonyl signal at δ 200.1 and two additional signals at δ 180.1 and 176.3, assigned to the equatorial and the axial CO ligands, respectively. The formation of a discrete diosmium unit was further confirmed using an X-ray diffraction study on a single crystal, obtained from a mixed solution of CH2Cl2and

MeOH under a CO atmosphere.

The ORTEP diagram of 2 is depicted in Figure 2, showing both the staggered arrangement of ligands in the equatorial planes and the occurrence of a similar short Os-Os separation of 2.8006(5) Å. As all these data are consistent with the characteristics of the diosmium [Os2(CO)5(thd)2] unit in 1, we can confidently concluded

that complex 2 is the reaction intermediate that gives the generation of metal chain complex 1 during the initial reaction between Os3(CO)12and (thd)H:

To gain further insight into the electronic structure of the bimetallic and tetrametallic species (i.e., com-plexes 2 and 1), we have performed extended Hu¨ckel (EHMO) calculations for the model complexes 4 and 5 with geometries analogous to those observed in the solid state.

The analysis is commenced from hypothetical Os2

complex 4, of which the theoretical work on complexes with the structural formula M2L10 has already been

investigated by a number of approaches.10_{It is}

instruc-tive to look first at the frontier orbitals (FMO) of the basic units 3 from which complex 4 is built up. On the left of the interaction diagram (Figure 3), we find three “t2g” like orbitals, labeled δ (dxy) and π (dxz, dyz). Above

the “t2g” orbitals, there are two d orbitals descendent

from the “eg” set, i.e., the occupied σ (dz2) which is higher

in energy than the unoccupied δ with considerable dx2-y2

character. As a result, the Os2complex 4 exhibits a set

of eight fragment MOs which span σ* and σ (out-of-phase and in-(out-of-phase combinations of the hybrid FMO σ), δ* and δ (out-of-phase and in-phase combinations of the t2g-type FMO δ), and π* and π (out-of-phase and

in-phase combinations of the t2g-type FMO π), for which

the ordering of valence MOs is π < π* < δ < δ* < σ <

σ*. Since the complex 4 belongs to a d7_-d7_{system, this}

leads to an electron configuration of (π)4_(π*)4_(δ)2_(δ*)2

-(σ)2_{and affords a formal metal-metal bond order of 1.}11

For complex 5, it should be emphasized that on the basis of our EH calculations the HOMO (σ2) is well

separated from the LUMO (1.77 eV), suggesting that 5 should be stable against spontaneous dissociation. Moreover, the mixing of the dimer FMO σ, resulting in σ2(out-of-phase) and σ1(in-phase) orbitals, is of interest.

(9) Churchill, M. R.; DeBoer, B. G. Inorg. Chem. 1977, 16, 878.

(10) (a) Cotton, F. A.; Walton, R. A. Multiple Bonds between Metal

Atoms; John Wiley: New York, 1982. (b) Heijsen, W.; Baerends, E. J.;

Ros. P. Discuss. Faraday Soc. (Symp.) 1980, 14, 211. (c) Shaik, S.; Hoffmann, R.; Fisel, C. R.; Summerville, R. H. J. Am. Chem. Soc. 1980,

102, 4555. (d) Nakatsuji, H.; Hada, M.; Kawashima, A. Inorg. Chem.

1992, 31, 1740. (e) Ma˜rquez, A.; Sanz, J. F.; Gelize´, M.; Dargelos, A.

J. Organomet. Chem. 1992, 434, 235.

(11) The electron distribution (σ)2_(π)4_(δ)2_(δ*)2_(π*)4_{, characteristic of}

a M-M single bond, has already been reported, see: Cotton, F. A.; DeBoer, B. G.; LaPrade, M. D.; Ripal, J. R.; Ucko, D. A. J. Am. Chem.

Soc. 1970, 92, 2926.

Figure 2. Molecular structure of [Os2(CO)6(thd)2] (2). Bond distances (Å): Os(1)-Os(1) ) 2.8006(5), Os(1)-C(1) ) 1.883(6), Os(1)-C(2) ) 1.884(6), Os(1)-C(3) ) 1.972(7), Os(1)-O(4) ) 2.102(3), and Os(1)-O(5) ) 2.108(3).

Figure 3. Orbital interaction diagram for the hypothetical

dimerization and tetramerization processes.

2Os3(CO)12+ 6(thd)H f

3[Os₂(CO)₆(thd)₂] (2) + 3H₂+ 6CO 2(2) a [Os2(CO)5(thd)2]2(1) + 2CO

5402 Organometallics, Vol. 19, No. 25, 2000 Su et al.

(4)

According to our EHMO calculations, there exists a large percentage contribution from the dz2orbitals of the

inner Os atoms to the σ1and σ2orbitals of 5 (38% inner

Os and 11% outer Os, and 51% inner Os and 16% outer Os in character for σ2and σ1, respectively). The HOMO

(σ2) is therefore delocalized over the entire metal chain,

which suggests that all reactions involving gain or loss of electrons will be exclusively metal-centered. It is therefore conceivable that the σ1and σ2orbitals should

play a significant role in determining the final structure. Moreover, since this HOMO is antibonding between the inner Os atoms, but bonding between the outer and inner Os atoms, it suggests that filling this orbital would result in lengthening of the inner Os-Os bond and shortening of the outer Os-Os bonds. In other words, the electronic configuration of 5 is ‚‚‚‚‚‚(δ2*)2(σ1)2(π2*)4

-(σ2)2and therefore implies the observation of a

strength-ened single σ bond in the outer Os-Os atoms and a nonbonding interaction in the inner Os-Os atoms, which is qualitatively consistent with the measured Os-Os distances observed in 1.

Recently, Peng and co-workers have reported a struc-tural study of an isoelectronic rhodium complex, [Rh4

-(s-pqdi)2(pqdi)4(CO)4]2+.12In this distinctive system, the

Rh cations have aligned in a nearly linear chainlike arrangement with distances of 2.848 and 2.858 Å for the outer and inner Rh-Rh bonds, respectively. These experimental findings are in good agreement with our predictions. Furthermore, from the chemical bonding point of view, the metal σ-bonding interaction in 1 is equivalent to the well-known π-bonding of the butadiene molecule which possesses the configuration (π1)2(π2)2.13

The π1orbital is the in-phase combination of two local

ethylene π molecular orbitals and π2is the

correspond-ing out-of-phase combination. In consequence, oxidation of complex 1 (or butadiene) should shorten the inner Os-Os distance (or the respective inner C-C bond). As there are no relevant experimental data on such system, this prediction remains a pure speculation.

Conclusion

Treatment of Os3(CO)12 with (thd)H in a stainless

steel autoclave at 190 °C afforded the diosmium complex [Os2(CO)6(thd)2] (2), which can be easily converted to

the tetrametallic chain cluster [Os2(CO)5(thd)2]2(1) by

spontaneous elimination of CO during workup. This dimerization process is somewhat reversible (Scheme 1), which is clearly demonstrated by quantitative

re-generation of 2 upon addition of CO in solution at room temperature, as monitored by1_{H NMR spectroscopy.}

Moreover, complex 1 consists of a nearly linear array of four d7 _{Os atoms, and the bonding along the metal}

chain can be illustrated by the simple 18-electron concept. For the [Os2(CO)5(thd)2] fragments of 1, the

terminal Os has 18e and the inner Os possesses only 16e due to loss of a 2e donor ligand from 2. Two such fragments can be held together if the inner Os atom of one [Os2(CO)5(thd)2] unit donates its σ electrons to the

inner, electron-deficient Os atom of the second Os2unit.

Likewise, the second Os2 unit also uses the same σ

electron pair donated to the first one, giving a total of 18 electrons for both the inner metal atoms. However, as there are four valence electrons involved in forming sucha double donor-acceptor interaction and affording the electron configuration (σ + σ)2_{(σ - σ)}2 _{) (σ}

12σ22),

formation of the net nonbonding interaction between the inner Os atoms and the strengthened single bond interaction for other Os-Os bonds is expected. Concep-tually, this double dative interaction is related to the donor-acceptor interaction observed in several other osmium cluster complexes.6,14

Acknowledgment. We thank the National Science Council of the Republic of China for financial support (Grant NSC 88-2113-M-007-034).

Supporting Information Available: X-ray crystallo-graphic file (cif) for 1 and 2. This material is available free of charge via the Internet at http://pubs.acs.org.

OM000537L (12) Chern, S.-S.; Lee, G.-H.; Peng, S.-M. J. Chem. Soc., Chem.

Commun. 1994, 1645.

(13) Albright, T. A.; Burdett, J. K.; Whangbo, M.-H. Orbital

Interac-tions in Chemistry; John Wiley: New York, 1985; p 213. 1993, 12, 3079.(14) Wang, W.; Einstein, F. W. B.; Pomeroy, R. K. Organometallics

Scheme 1

Tetraosmium Complex [Os2(CO)5(thd)2]2 Organometallics, Vol. 19, No. 25, 2000 5403