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Trishomobarrelenedicarboxylate dianion as a bridge between dimolybdenum units: Comparison to similar compounds
Polyhedron 25 (2006) 219–223 www.elsevier.com/locate/poly
Trishomobarrelenedicarboxylate dianion as a bridge between dimolybdenum units: Comparison to similar compounds F. Albert Cotton a,*, Armin de Meijere b,*, Carlos A. Murillo Karsten Rauch b, Rongmin Yu a
a,*
,
a
b
Department of Chemistry and Laboratory for Molecular Structure and Bonding, P.O. Box 30012, Texas A&M University, College Station, TX 77842-3012, USA Institut fu¨r Organische und Biomolekulare Chemie der Georg-August-Universita¨t Go¨ttingen, Tammannstr. 2, 37077 Go¨ttingen, Germany Received 25 February 2005; accepted 23 May 2005 Available online 11 August 2005 Dedicated to Prof. Malcolm H. Chisholm for his contributions to the chemistry of metal–metal bonded compounds.
Abstract A new molecular pair composed of two quadruply bonded Mo2 ðDAniFÞ3 þ units (DAniF = (p-MeOC6H4)NC(H)N(p-MeOC6H4)) linked by a trishomobarrelene-1,5-dicarboxylate dianion has been prepared. This compound, [Mo2(DAniF)3]2(O2CC11H12CO2) (1), has been characterized by X-ray crystallography and by 1H NMR and UV–Vis spectroscopy. There is only weak electronic coupling between the two Mo2 units according to electrochemical measurements. A comparison with other dicarboxylate-linked molecular pairs is presented. 2005 Elsevier Ltd. All rights reserved. Keywords: Quadruple bond; Dimolybdenum compounds; Trishomobarrelene-1,5-dicarboxylate; Electronic coupling
1. Introduction It has been known that metal–metal bonded Mo2 nþ and Rh2 4þ units may be linked into pairs, loops, triangles, squares and a few other types of assembly by employing the dianions of dicarboxylic acids [1–3]. Although products containing more than 25 such dianions have already been reported, the work described here was prompted by several questions that were still not fully answered by the earlier work. With regard to compounds of the type (DAniF)3Mo2O2CXCO2Mo2(DAniF)3, the use of the bridge with trishomobarrelene as X, namely, the title dianion (Scheme 1) provides a *
Corresponding authors. Tel.: +1 9798454432; fax: +1 9798459351. E-mail addresses: [email protected] (F.A. Cotton), ameijer1@ gwdg.de (A.de Meijere), [email protected] (C.A. Murillo). 0277-5387/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2005.05.027
compound in which the bridge has very little flexibility and consists entirely of C–C single bonds. This means that the distance between the Mo2 units as determined in the crystal structure may be assumed with confidence to pertain to the molecule in solution. Given the ‘‘insulating’’ character of the trishomobarrelene unit and the rather large separation between the two Mo2 units electronic coupling between these Mo2 units should be negligible. Therefore, the difference between the potentials for the first and second oxidations should be of essentially electrostatic origin and should be small. We report here the preparation of (DAniF)3Mo2(O2CC11H12CO2)Mo2(DAniF)3 (1) where C11H12 is the trishomobarrelene diradical. Also reported are the structure of 1 and some of its spectroscopic properties.
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F.A. Cotton et al. / Polyhedron 25 (2006) 219–223
2– O O
O
O
Scheme 1. Trishomobarrelenedicarboxylate dianion.
2. Experimental 2.1. Materials and methods All manipulations and procedures were conducted under N2 using either an N2 drybox or standard Schlenk line techniques. Solvents were distilled and degassed prior to use: MeCN was twice distilled under N2, first from activated molecular sieves and then from CaH2. Dichloromethane was dried and distilled from P2O5, and ethyl ether was dried and distilled from Na/K-benzophenone. The compound Mo2(DAniF)3Cl2 was prepared by the literature method [4]. The salt (Bu4N)2(O2CC11H12CO2) was prepared and isolated as a colorless solid by neutralizing the corresponding diacid with 2 equiv. of Bu4NOH. It was then carefully dried in vacuum. All other reagents were purchased from commercial sources and used as received. Elemental analysis was performed by Canadian Microanalytical Service, Delta, British Columbia. The 1 H NMR spectrum was recorded on a Mercury 300 System NMR spectrometer, with chemical shifts (d) referenced to CD2Cl2. The differential pulse voltammogram and cyclic voltammogram were recorded on a CH Instruments electrochemical analyzer in 0.1 M Bu4NPF6 solution (CH2Cl2) with Pt working and auxiliary electrodes, an Ag/AgCl reference electrode. The scan rate for the CV was 100 mV/s. The oxidation potentials are referenced to the Ag/AgCl electrode, and under the present experimental conditions, the E1/2(Fc+/Fc) was measured at 440 mV. 2.2. Preparation of trishomobarrelene-1,5-dicarboxylic acid This linker was prepared from 1,5-dichlorotrishomobarrelene according to a modified protocol from a previously published one [5]: to a solution of 1,5-dichlorotrishomobarrelene (214 mg, 1 mmol) in anhydrous dichloromethane (2 ml) cooled to 78 C (dry ice/acetone) was added under an inert atmosphere (N2) within 10 min a 1 M solution of antimony pentachloride in anhydrous dichloromethane (Aldrich, 4 ml, 4 mmol). After stirring for an additional 1 h at the same temper-
ature, dry carbon monoxide was bubbled through the suspension at 78 C. The suspension was then transferred under N2 via a cannula to a cold solution (78 C) of sodium methoxide (5 g, 217 mmol) in methanol (65 ml). The mixture was warmed up to ambient temperature, water (20 ml) was added, and the resulting mixture stirred for 6 h at 70 C. The precipitate was filtered off, the filtrate was extracted with pentane (4 · 40 ml). The aqueous phase was evaporated to dryness, and the residue dissolved in a minimum of water so that the resulting solution was completely clear. Under cooling in an ice-water bath, conc. hydrochloric acid was added to pH 1–2. After extraction with ether (6 · 50 ml) and evaporation to dryness, the residue weighed 164 mg and was crude trishomobarrelenedicarboxylic acid containing a little sodium chloride. Pure diacid was obtained by dissolving the crude material in saturated aqueous sodium hydrogencarbonate (5 ml), extracting the solution with ether (5 ml) and acidifying the aqueous phase with conc. hydrochloric acid under cooling in an ice-water bath to pH 1–2. The resulting colorless precipitate was filtered off and dissolved in ether (150 ml). The ether solution was filtered through a 2 cm pad of Celite, and the Celite was washed with an additional 100 ml of ether. The solvent was evaporated and the crystalline residue dried in vacuum to yield 106 mg (45%) of pure trishomobarrelene-1,5-dicarboxylic acid (m.p. 230 C, dec). 1H NMR (600 MHz, [d6]DMSO): d = 0.15 (dt, 3J = 7.5 Hz, 2J = 5.5 Hz, 3H), 0.78 (dd, 3J = 7.5 Hz, 3J = 4.0 Hz, 6H), 0.87 (dt, 2J = 5.5 Hz, 3J = 4.0 Hz, 3H), 12.33 ppm (s, 2H, COOH). 13 C NMR (62.9 MHz, [d6]-DMSO, additional DEPT): d = 0.6 (, 3C, CH2), 12.7 (+, 6C, CH), 38.4 (quat, 2C), 177.8 ppm (quat, 2C, COOH). MS (EI, 70 eV), m/z (5): 234.1 (52) [M+], 189.1 (46) [M+ COOH], 171.1 (47), 143.1 (99), 128.1 (100), 115.1 (77), 111.1 (56), 105.1 (29), 91.1 (87), 77.0 (76), 65.0 (38), 51.0 (30), 41.0 (35). 2.3. Preparation of [Mo2(DAniF)3]2(O2CC11H12CO2) (1) A solution of Mo2(DAniF)3Cl2 (0.450 g, 0.441 mmol) in 50 ml of MeCN was treated with Zn dust (4.5 g). The resulting suspension was stirred vigorously for 1.5 h. The yellow filtrate was transferred to a flask containing (Bu4N)2(O2CC11H12CO2) (0.142 g, 0.200 mmol). Within a few minutes of stirring, a yellow precipitate appeared. Stirring was continued for 2 h. The mixture was concentrated to about 5 ml, and 30 ml of methanol was added. The precipitate was isolated by filtration, washed with methanol (2 · 20 ml) and ether (1 · 30 ml), dried in vacuum, and then extracted with 20 ml of dichloromethane. The extract was layered with 40 ml of ether, and allowed to stand undisturbed for two days. A yellow crystalline product was isolated and dried for 24 h in vacuum.
F.A. Cotton et al. / Polyhedron 25 (2006) 219–223
Yield, 210 mg (49%). Anal. Calc. for (1), C103H102Mo4N12 O16 Æ 0.5CH2Cl2: C, 56.55; H, 4.73; N, 7.65. Found: C, 56.34; H, 4.85; N, 7.47%. 1H NMR d (ppm in CD2Cl2): 8.509 (s, 2H, –NCHN–), 8.385 (s, 4H, –NCHN–), 6.617 (m, 32H, aromatic), 6.471 (d, 8H, aromatic), 6.254 (d, 8H, aromatic), 3.692 (s, 24H, –OCH3), 3.650(s, 12H, –OCH3), 1.261 (s, 9H,), 0.395 (q, 3H). 2.4. X-ray structure determination Single crystals suitable for X-ray diffraction analysis were grown by diffusion of ether into a CH2Cl2 solution of the compound. The data were collected on a Bruker SMART 100 CCD area detector system. Cell parameters were determined using the program SMART [6]. Data reduction and integration were performed with the software package SAINT [7], while absorption corrections were applied by using the program SADABS [8]. The positions of the Mo atoms were found by direct methods using the program package SHELXTL [9]. Subsequent cycles of least-squares refinement followed by difference Fourier syntheses revealed the positions of the remaining non-hydrogen atoms. Hydrogen atoms were added in idealized positions. Non-hydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atoms were included in the calculation of the structure factors. The bridging dicarboxylate ligand has two rotational orientations in a ratio of 0.6–0.4 in the crystallographic structure. Some methoxyl groups of DAniF ligands are disordered in two positions. Disorder in the interstitial solvent molecules was also found. Cell parameters and refinement results are summarized in Table 1. Selected bond distances and angles are listed in Table 2. Table 1 Crystallographic data and refinement for 1 Æ 2CH2Cl2 Chemical formula C105H106Cl4Mo4N12O16 Molecular weight 2317.58 Crystal system monoclinic Space group P21/n ˚) a (A 21.342(1) ˚) b (A 19.239(1) ˚) c (A 25.513(2) b () 91.764(1) ˚ 3) V (A 10 471(1) Z 4 Dcalc. (Mg m3) 1.470 ˚) k (A 0.71073 l (mm1) 0.640 Crystal size (mm) 0.40 · 0.33 · 0.23 T (K) 213(2) R1, wR2 [I > 2r(I)] 0.0545, 0.1234 hP i1=2 hP h i P P wðF o F c Þ2 = wF 2o ; wR2 ¼ wðF 2o F 2c Þ2 = w R1 ¼ h i where P ¼ maxðF 2o ; 0Þ þ ðF 2o Þ2 1=2 ; w ¼ 1= r2 ðF 2o Þ þ ðaP Þ2 þ bP ; 2ðF 2c Þ=3.
221
Table 2 ˚ ) and bond angles () for 1 Æ 2CH2Cl2 Selected bond distances (A Mo(1)–Mo(2) Mo(3)–Mo(4) Mo(1)–N(1) Mo(1)–N(3) Mo(1)–N(5) Mo(2)–N(2) Mo(2)–N(4) Mo(2)–N(6) Mo(3)–N(7) Mo(3)–N(9) Mo(3)–N(11) Mo(4)–N(8) Mo(4)–N(10) Mo(4)–N(12) Mo(1)–O(13) Mo(2)–O(14) Mo(3)–O(15) Mo(4)–O(16) N(1)–Mo(1)–N(3) N(3)–Mo(1)–N(5) N(5)–Mo(1)–O(13) O(13)–Mo(1)–N(1) N(2)–Mo(2)–N(4) N(4)–Mo(2)–N(6) N(6)–Mo(2)– O(14) O(14)–Mo(2)–N(2) N(9)–Mo(3)–N(7) N(9)–Mo(3)–N(11) N(11)–Mo(3)–O(15) O(15)–Mo(3)–N(7) N(8)–Mo(4)–N(10) N(10)–Mo(4)–N(12) N(12)–Mo(4)–O(16) O(16)–Mo(4)– N(8)
2.0916(7) 2.0920(6) 2.158(4) 2.109(4) 2.155(4) 2.132(4) 2.142(5) 2.143(4) 2.172(4) 2.135(4) 2.144(4) 2.130(4) 2.129(4) 2.145(4) 2.153(4) 2.143(4) 2.142(3) 2.147(3) 91.1(2) 92.7(2) 90.2(2) 85.7(2) 91.4(2) 94.2(2) 89.5(2) 84.5(2) 96.7(2) 92.1(2) 85.1(2) 85.7(2) 94.4(2) 93.4(2) 85.3(2) 86.4(2)
3. Results 3.1. Structure The structure of [Mo2(DAniF)3]2(O2CC11H12CO2) is presented in Fig. 1, where the formamidinate ligands are shown as N–C–N chains and hydrogen atoms are omitted for clarity. In this compound, two [Mo2(DAniF)3]+ units are linked by the trishomobarrelene-1,5dicarboxylate dianion. The twist angle between Mo2 axes is 22.8. The dimensions of the [Mo2] units retain the characteristics of quadruply bonded dimolybdenum
Fig. 1. Core structure of 1 in 1 Æ 2CH2Cl2 drawn with 50% probability ellipsoids. All p-anisyl groups and hydrogen atoms have been omitted for clarity.
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˚ for compounds. The Mo–Mo distances are 2.0916(7) A ˚ Mo(1)–Mo(2) and 2.0920(6) A for Mo(3)–Mo(4). The Mo–O and Mo–N distances fall in narrow ranges, ˚ for Mo–O and 2.109(4)–2.172(4) A ˚ 2.142(3)–2.153(4) A for Mo–N, respectively. The distance between the cen˚ . The structure of ters of the two Mo2 units is 11.06 A the dicarboxylate ligand is first studied here even though the trishomobarrelene core has been known for decades [10]. Within the error limits all structural parameters of this dicarboxylate ligand are in agreement with the ones for the parent hydrocarbon [11]. The single C–C bond ˚ . The distances in the cyclodistances are close to 1.52 A propyl rings do not significantly differ from others. However, the C–C–C angles vary considerably. The bicyclo[2.2.2]octane moiety in the trishomobarrelene1,5-dicarboxylate ligand is similar to that of the bicyclo[2.2.2]octane-1,4-dicarboxylic acid with bond angles close to 109.5 [12]. In contrast, the angles inside the cyclopropyl groups are very close to 60. The dihedral angles between the planes of the cyclopropyl groups and the plane defined by the axis of the propeller and two atoms from the cyclopropyl group are in the range of 62.0–66.6. A more detailed listing of bond angles and distances is available as supplementary material. 3.2. Electrochemistry The CV and DPV of [Mo2(DAniF)3]2(O2CC11H12CO2) in CH2Cl2 have the shape and current of a single step charge transfer of 2 electrons. This is not surprising because the coupling capability between the two Mo2 dimers is weak due to the large distances between them and the properties of the linker. The DE1/2 between the two successive one-electron oxidations was calculated to be 66 mV from the differential pulse voltammogram by the method of Richardson and Taube [13].
(the distance between centers of Mo2 units) shown in Fig. 2. Points 1–4 shown by circles are for compounds with ‘‘insulating’’ bridges in which the connector X is a rigid unit that does not provide a p-orbital pathway, except point 1 which is for oxalate where there is no X. The lack of a p-orbital pathway is obvious for 3 and 4, but it is also true of 2, which has p electrons but the adjacent double bonds are orthogonal to each other. We first note that compound 1, as represented by point 4, provides a significant extension to the line through points 1, 2 and 3. However, Fig. 2 also shows that there is a dearth of data for compounds with ‘‘insulating’’ bridges. There are none for distances between 7 ˚ , and none beyond about 11 A ˚ . While there is litand 9 A ˚ tle likelihood of filling the 7–9 A gap, one or more com˚ should be affordable. pounds in the region beyond 11 A For example, if the dicarboxylic acid with two trishomobarrelenes instead of only one could be made, a ˚ would be possible. point at about 15 A Also shown in Fig. 2 are points for compounds in which X is one of five (CH@CH)n (n = 1–4) groups as well as C„C. For details see the figure caption. Apart from some scatter among points 5, 6 and 7, there is a smooth (though not linear) downward trend in DE1/2 values. This presumably means that the presence of a p bond, or an extended sequence of conjugated p bonds, allows electronic coupling in addition to electrostatic coupling. In connection with Fig. 2, it must be noted that regardless of the nature of the coupling mechanism, as d ! 1, the value of DE1/2 should drop to 35.6 mV,
3.3. Electronic spectrum The only absorption band observed between 380 and 600 nm was a shoulder on the absorption rising into the shorter wave length region. This had a maximum at 430 ± 10 nm and a molar extinction of about 800 mol1 cm1. It is very similar to a band found in (DAniF)3Mo2O2CCH3 [14] at 430 nm and e = 1000, and assignment to the d ! d* transition is indicated.
4. Discussion The mere existence of compound 1 is not particularly interesting, but several features of interest arise when it is compared with other compounds of the type (DAniF)3Mo2O2CXCO2Mo2(DAniF)3 [15,16]. These features are evident in the data plot of DE1/2 versus d
Fig. 2. The DE1/2 values for two classes of (DAniF)3Mo2O2CXCO2Mo2(DAniF)3 compounds, plotted against the distance between the two Mo2 units. Points 2–4 are for those in which there is little or no electronic coupling between them. Points 5–11 are for those in which p electrons are present. The X for point 1 is simply the C–C bond. For the others the X groups are as follows: 2 – C@C@C; 3 – C(l–CH2)3C; 4 – trishomobarrelene; 5 – cis-CH@CH; 6 – trans-CH@CH; 7 – C„C; 8 – cis,cis-CH@CH–CH@CH; 9 – trans,trans-CH@CH–CH@CH; 10 – all trans-(CH@CH)3; 11 – all trans-(CH@CH)4.
F.A. Cotton et al. / Polyhedron 25 (2006) 219–223
which corresponds to the purely statistical value for KC (the comproportionation constant) of 4. On this basis, the linear trend in the points for compounds with ‘‘insulating’’ connection cannot continue; instead, the DE1/2 values for longer distances (d) must asymptotically approach 35.6 mV. Thus, data for one or two more com˚ would be very pounds in the range above d = 11 A desirable. It is also interesting that the points for the compounds with conjugated polyene connectors appear to approach an asymptote of about 60 mV instead of 35.6 mV. This observation makes it even more interesting to extend the range of d for compounds with ‘‘insulating’’ connectors, and we shall seek to do so. It is known that DE1/2 values may have a ‘‘significant – and sometimes unpredictable – dependence’’ on the solvents and counter ions [17].
5. Supplementary material Crystallographic data (excluding structure factors) have been deposited with the Cambridge Crystallographic Data Center as supplementary publication No. CCDC 259665. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK, fax: +44 1223 336 033, e-mail: [email protected], www.ccdc. cam.ac.uk/data_request/cif.
Acknowledgments Support for this work was provided by the Welch Foundation and National Science Foundation at
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TAMU and the State of Niedersachsen at GAU, Go¨ttingen.
References [1] F.A. Cotton, C. Lin, C.A. Murillo, Acc. Chem. Res. 34 (2001) 759. [2] F.A. Cotton, C. Lin, C.A. Murillo, Proc. Natl. Acad. Sci. USA 99 (2002) 4810. [3] F.A. Cotton, C.A. Murillo, R.A. Walton (Eds.), Multiple Bonds between Metal Atoms, third ed., Springer Science and Media, Inc., New York, 2005. [4] F.A. Cotton, L.M. Daniels, G.T. Jordan IV, C. Lin, C.A. Murillo, J. Am. Chem. Soc. 120 (1998) 3398. [5] A. de Meijere, O. Schallner, G.D. Mateescu, P. Go¨litz, P. Bischof, Helv. Chim. Acta 68 (1985) 1114. [6] SMART for Windows NT, Version 5.618, Bruker Advanced X-ray Solutions, Inc., Madison, WI, 2001. [7] SAINT. Data Reduction Software. Version 6.36A, Bruker Advanced X-ray Solutions, Inc., Madison, WI, 2001. [8] SADABS. Area Detector Absorption and other Corrections Software, Version 2.05, Bruker Advanced X-ray Solutions, Inc., Madison, WI, 2000. [9] G.M. Sheldrick, SHELXTL, Version 6.12, Advanced X-ray Solutions, Inc., Madison, WI, 2002. [10] A. de Meijere, C. Weitemeyer, Angew. Chem., Int. Ed. Engl. 9 (1970) 376. [11] B. Anderson, O. Schallner, A. de Meijere, J. Am. Chem. Soc. 97 (1975) 3351. [12] O. Ermer, J.D. Dunitz, J. Chem. Soc., Chem. Commun. (1968) 567. [13] D.E. Richardson, H. Taube, Inorg. Chem. 20 (1981) 1278. [14] F.A. Cotton, C.Y. Liu, C.A. Murillo, Inorg. Chem. 43 (2004) 2267. [15] F.A. Cotton, J.P. Donahue, C. Lin, C.A. Murillo, Inorg. Chem. 40 (2001) 1234. [16] F.A. Cotton, J.P. Donahue, C.A. Murillo, J. Am. Chem. Soc. 125 (2003) 5436. [17] D.M. DAlessandro, F.R. Keene, Dalton Trans. (2004) 3950.
Trishomobarrelenedicarboxylate dianion as a bridge between dimolybdenum units: Comparison to similar compounds F. Albert Cotton a,*, Armin de Meijere b,*, Carlos A. Murillo Karsten Rauch b, Rongmin Yu a
a,*
,
a
b
Department of Chemistry and Laboratory for Molecular Structure and Bonding, P.O. Box 30012, Texas A&M University, College Station, TX 77842-3012, USA Institut fu¨r Organische und Biomolekulare Chemie der Georg-August-Universita¨t Go¨ttingen, Tammannstr. 2, 37077 Go¨ttingen, Germany Received 25 February 2005; accepted 23 May 2005 Available online 11 August 2005 Dedicated to Prof. Malcolm H. Chisholm for his contributions to the chemistry of metal–metal bonded compounds.
Abstract A new molecular pair composed of two quadruply bonded Mo2 ðDAniFÞ3 þ units (DAniF = (p-MeOC6H4)NC(H)N(p-MeOC6H4)) linked by a trishomobarrelene-1,5-dicarboxylate dianion has been prepared. This compound, [Mo2(DAniF)3]2(O2CC11H12CO2) (1), has been characterized by X-ray crystallography and by 1H NMR and UV–Vis spectroscopy. There is only weak electronic coupling between the two Mo2 units according to electrochemical measurements. A comparison with other dicarboxylate-linked molecular pairs is presented. 2005 Elsevier Ltd. All rights reserved. Keywords: Quadruple bond; Dimolybdenum compounds; Trishomobarrelene-1,5-dicarboxylate; Electronic coupling
1. Introduction It has been known that metal–metal bonded Mo2 nþ and Rh2 4þ units may be linked into pairs, loops, triangles, squares and a few other types of assembly by employing the dianions of dicarboxylic acids [1–3]. Although products containing more than 25 such dianions have already been reported, the work described here was prompted by several questions that were still not fully answered by the earlier work. With regard to compounds of the type (DAniF)3Mo2O2CXCO2Mo2(DAniF)3, the use of the bridge with trishomobarrelene as X, namely, the title dianion (Scheme 1) provides a *
Corresponding authors. Tel.: +1 9798454432; fax: +1 9798459351. E-mail addresses: [email protected] (F.A. Cotton), ameijer1@ gwdg.de (A.de Meijere), [email protected] (C.A. Murillo). 0277-5387/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2005.05.027
compound in which the bridge has very little flexibility and consists entirely of C–C single bonds. This means that the distance between the Mo2 units as determined in the crystal structure may be assumed with confidence to pertain to the molecule in solution. Given the ‘‘insulating’’ character of the trishomobarrelene unit and the rather large separation between the two Mo2 units electronic coupling between these Mo2 units should be negligible. Therefore, the difference between the potentials for the first and second oxidations should be of essentially electrostatic origin and should be small. We report here the preparation of (DAniF)3Mo2(O2CC11H12CO2)Mo2(DAniF)3 (1) where C11H12 is the trishomobarrelene diradical. Also reported are the structure of 1 and some of its spectroscopic properties.
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F.A. Cotton et al. / Polyhedron 25 (2006) 219–223
2– O O
O
O
Scheme 1. Trishomobarrelenedicarboxylate dianion.
2. Experimental 2.1. Materials and methods All manipulations and procedures were conducted under N2 using either an N2 drybox or standard Schlenk line techniques. Solvents were distilled and degassed prior to use: MeCN was twice distilled under N2, first from activated molecular sieves and then from CaH2. Dichloromethane was dried and distilled from P2O5, and ethyl ether was dried and distilled from Na/K-benzophenone. The compound Mo2(DAniF)3Cl2 was prepared by the literature method [4]. The salt (Bu4N)2(O2CC11H12CO2) was prepared and isolated as a colorless solid by neutralizing the corresponding diacid with 2 equiv. of Bu4NOH. It was then carefully dried in vacuum. All other reagents were purchased from commercial sources and used as received. Elemental analysis was performed by Canadian Microanalytical Service, Delta, British Columbia. The 1 H NMR spectrum was recorded on a Mercury 300 System NMR spectrometer, with chemical shifts (d) referenced to CD2Cl2. The differential pulse voltammogram and cyclic voltammogram were recorded on a CH Instruments electrochemical analyzer in 0.1 M Bu4NPF6 solution (CH2Cl2) with Pt working and auxiliary electrodes, an Ag/AgCl reference electrode. The scan rate for the CV was 100 mV/s. The oxidation potentials are referenced to the Ag/AgCl electrode, and under the present experimental conditions, the E1/2(Fc+/Fc) was measured at 440 mV. 2.2. Preparation of trishomobarrelene-1,5-dicarboxylic acid This linker was prepared from 1,5-dichlorotrishomobarrelene according to a modified protocol from a previously published one [5]: to a solution of 1,5-dichlorotrishomobarrelene (214 mg, 1 mmol) in anhydrous dichloromethane (2 ml) cooled to 78 C (dry ice/acetone) was added under an inert atmosphere (N2) within 10 min a 1 M solution of antimony pentachloride in anhydrous dichloromethane (Aldrich, 4 ml, 4 mmol). After stirring for an additional 1 h at the same temper-
ature, dry carbon monoxide was bubbled through the suspension at 78 C. The suspension was then transferred under N2 via a cannula to a cold solution (78 C) of sodium methoxide (5 g, 217 mmol) in methanol (65 ml). The mixture was warmed up to ambient temperature, water (20 ml) was added, and the resulting mixture stirred for 6 h at 70 C. The precipitate was filtered off, the filtrate was extracted with pentane (4 · 40 ml). The aqueous phase was evaporated to dryness, and the residue dissolved in a minimum of water so that the resulting solution was completely clear. Under cooling in an ice-water bath, conc. hydrochloric acid was added to pH 1–2. After extraction with ether (6 · 50 ml) and evaporation to dryness, the residue weighed 164 mg and was crude trishomobarrelenedicarboxylic acid containing a little sodium chloride. Pure diacid was obtained by dissolving the crude material in saturated aqueous sodium hydrogencarbonate (5 ml), extracting the solution with ether (5 ml) and acidifying the aqueous phase with conc. hydrochloric acid under cooling in an ice-water bath to pH 1–2. The resulting colorless precipitate was filtered off and dissolved in ether (150 ml). The ether solution was filtered through a 2 cm pad of Celite, and the Celite was washed with an additional 100 ml of ether. The solvent was evaporated and the crystalline residue dried in vacuum to yield 106 mg (45%) of pure trishomobarrelene-1,5-dicarboxylic acid (m.p. 230 C, dec). 1H NMR (600 MHz, [d6]DMSO): d = 0.15 (dt, 3J = 7.5 Hz, 2J = 5.5 Hz, 3H), 0.78 (dd, 3J = 7.5 Hz, 3J = 4.0 Hz, 6H), 0.87 (dt, 2J = 5.5 Hz, 3J = 4.0 Hz, 3H), 12.33 ppm (s, 2H, COOH). 13 C NMR (62.9 MHz, [d6]-DMSO, additional DEPT): d = 0.6 (, 3C, CH2), 12.7 (+, 6C, CH), 38.4 (quat, 2C), 177.8 ppm (quat, 2C, COOH). MS (EI, 70 eV), m/z (5): 234.1 (52) [M+], 189.1 (46) [M+ COOH], 171.1 (47), 143.1 (99), 128.1 (100), 115.1 (77), 111.1 (56), 105.1 (29), 91.1 (87), 77.0 (76), 65.0 (38), 51.0 (30), 41.0 (35). 2.3. Preparation of [Mo2(DAniF)3]2(O2CC11H12CO2) (1) A solution of Mo2(DAniF)3Cl2 (0.450 g, 0.441 mmol) in 50 ml of MeCN was treated with Zn dust (4.5 g). The resulting suspension was stirred vigorously for 1.5 h. The yellow filtrate was transferred to a flask containing (Bu4N)2(O2CC11H12CO2) (0.142 g, 0.200 mmol). Within a few minutes of stirring, a yellow precipitate appeared. Stirring was continued for 2 h. The mixture was concentrated to about 5 ml, and 30 ml of methanol was added. The precipitate was isolated by filtration, washed with methanol (2 · 20 ml) and ether (1 · 30 ml), dried in vacuum, and then extracted with 20 ml of dichloromethane. The extract was layered with 40 ml of ether, and allowed to stand undisturbed for two days. A yellow crystalline product was isolated and dried for 24 h in vacuum.
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Yield, 210 mg (49%). Anal. Calc. for (1), C103H102Mo4N12 O16 Æ 0.5CH2Cl2: C, 56.55; H, 4.73; N, 7.65. Found: C, 56.34; H, 4.85; N, 7.47%. 1H NMR d (ppm in CD2Cl2): 8.509 (s, 2H, –NCHN–), 8.385 (s, 4H, –NCHN–), 6.617 (m, 32H, aromatic), 6.471 (d, 8H, aromatic), 6.254 (d, 8H, aromatic), 3.692 (s, 24H, –OCH3), 3.650(s, 12H, –OCH3), 1.261 (s, 9H,), 0.395 (q, 3H). 2.4. X-ray structure determination Single crystals suitable for X-ray diffraction analysis were grown by diffusion of ether into a CH2Cl2 solution of the compound. The data were collected on a Bruker SMART 100 CCD area detector system. Cell parameters were determined using the program SMART [6]. Data reduction and integration were performed with the software package SAINT [7], while absorption corrections were applied by using the program SADABS [8]. The positions of the Mo atoms were found by direct methods using the program package SHELXTL [9]. Subsequent cycles of least-squares refinement followed by difference Fourier syntheses revealed the positions of the remaining non-hydrogen atoms. Hydrogen atoms were added in idealized positions. Non-hydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atoms were included in the calculation of the structure factors. The bridging dicarboxylate ligand has two rotational orientations in a ratio of 0.6–0.4 in the crystallographic structure. Some methoxyl groups of DAniF ligands are disordered in two positions. Disorder in the interstitial solvent molecules was also found. Cell parameters and refinement results are summarized in Table 1. Selected bond distances and angles are listed in Table 2. Table 1 Crystallographic data and refinement for 1 Æ 2CH2Cl2 Chemical formula C105H106Cl4Mo4N12O16 Molecular weight 2317.58 Crystal system monoclinic Space group P21/n ˚) a (A 21.342(1) ˚) b (A 19.239(1) ˚) c (A 25.513(2) b () 91.764(1) ˚ 3) V (A 10 471(1) Z 4 Dcalc. (Mg m3) 1.470 ˚) k (A 0.71073 l (mm1) 0.640 Crystal size (mm) 0.40 · 0.33 · 0.23 T (K) 213(2) R1, wR2 [I > 2r(I)] 0.0545, 0.1234 hP i1=2 hP h i P P wðF o F c Þ2 = wF 2o ; wR2 ¼ wðF 2o F 2c Þ2 = w R1 ¼ h i where P ¼ maxðF 2o ; 0Þ þ ðF 2o Þ2 1=2 ; w ¼ 1= r2 ðF 2o Þ þ ðaP Þ2 þ bP ; 2ðF 2c Þ=3.
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Table 2 ˚ ) and bond angles () for 1 Æ 2CH2Cl2 Selected bond distances (A Mo(1)–Mo(2) Mo(3)–Mo(4) Mo(1)–N(1) Mo(1)–N(3) Mo(1)–N(5) Mo(2)–N(2) Mo(2)–N(4) Mo(2)–N(6) Mo(3)–N(7) Mo(3)–N(9) Mo(3)–N(11) Mo(4)–N(8) Mo(4)–N(10) Mo(4)–N(12) Mo(1)–O(13) Mo(2)–O(14) Mo(3)–O(15) Mo(4)–O(16) N(1)–Mo(1)–N(3) N(3)–Mo(1)–N(5) N(5)–Mo(1)–O(13) O(13)–Mo(1)–N(1) N(2)–Mo(2)–N(4) N(4)–Mo(2)–N(6) N(6)–Mo(2)– O(14) O(14)–Mo(2)–N(2) N(9)–Mo(3)–N(7) N(9)–Mo(3)–N(11) N(11)–Mo(3)–O(15) O(15)–Mo(3)–N(7) N(8)–Mo(4)–N(10) N(10)–Mo(4)–N(12) N(12)–Mo(4)–O(16) O(16)–Mo(4)– N(8)
2.0916(7) 2.0920(6) 2.158(4) 2.109(4) 2.155(4) 2.132(4) 2.142(5) 2.143(4) 2.172(4) 2.135(4) 2.144(4) 2.130(4) 2.129(4) 2.145(4) 2.153(4) 2.143(4) 2.142(3) 2.147(3) 91.1(2) 92.7(2) 90.2(2) 85.7(2) 91.4(2) 94.2(2) 89.5(2) 84.5(2) 96.7(2) 92.1(2) 85.1(2) 85.7(2) 94.4(2) 93.4(2) 85.3(2) 86.4(2)
3. Results 3.1. Structure The structure of [Mo2(DAniF)3]2(O2CC11H12CO2) is presented in Fig. 1, where the formamidinate ligands are shown as N–C–N chains and hydrogen atoms are omitted for clarity. In this compound, two [Mo2(DAniF)3]+ units are linked by the trishomobarrelene-1,5dicarboxylate dianion. The twist angle between Mo2 axes is 22.8. The dimensions of the [Mo2] units retain the characteristics of quadruply bonded dimolybdenum
Fig. 1. Core structure of 1 in 1 Æ 2CH2Cl2 drawn with 50% probability ellipsoids. All p-anisyl groups and hydrogen atoms have been omitted for clarity.
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˚ for compounds. The Mo–Mo distances are 2.0916(7) A ˚ Mo(1)–Mo(2) and 2.0920(6) A for Mo(3)–Mo(4). The Mo–O and Mo–N distances fall in narrow ranges, ˚ for Mo–O and 2.109(4)–2.172(4) A ˚ 2.142(3)–2.153(4) A for Mo–N, respectively. The distance between the cen˚ . The structure of ters of the two Mo2 units is 11.06 A the dicarboxylate ligand is first studied here even though the trishomobarrelene core has been known for decades [10]. Within the error limits all structural parameters of this dicarboxylate ligand are in agreement with the ones for the parent hydrocarbon [11]. The single C–C bond ˚ . The distances in the cyclodistances are close to 1.52 A propyl rings do not significantly differ from others. However, the C–C–C angles vary considerably. The bicyclo[2.2.2]octane moiety in the trishomobarrelene1,5-dicarboxylate ligand is similar to that of the bicyclo[2.2.2]octane-1,4-dicarboxylic acid with bond angles close to 109.5 [12]. In contrast, the angles inside the cyclopropyl groups are very close to 60. The dihedral angles between the planes of the cyclopropyl groups and the plane defined by the axis of the propeller and two atoms from the cyclopropyl group are in the range of 62.0–66.6. A more detailed listing of bond angles and distances is available as supplementary material. 3.2. Electrochemistry The CV and DPV of [Mo2(DAniF)3]2(O2CC11H12CO2) in CH2Cl2 have the shape and current of a single step charge transfer of 2 electrons. This is not surprising because the coupling capability between the two Mo2 dimers is weak due to the large distances between them and the properties of the linker. The DE1/2 between the two successive one-electron oxidations was calculated to be 66 mV from the differential pulse voltammogram by the method of Richardson and Taube [13].
(the distance between centers of Mo2 units) shown in Fig. 2. Points 1–4 shown by circles are for compounds with ‘‘insulating’’ bridges in which the connector X is a rigid unit that does not provide a p-orbital pathway, except point 1 which is for oxalate where there is no X. The lack of a p-orbital pathway is obvious for 3 and 4, but it is also true of 2, which has p electrons but the adjacent double bonds are orthogonal to each other. We first note that compound 1, as represented by point 4, provides a significant extension to the line through points 1, 2 and 3. However, Fig. 2 also shows that there is a dearth of data for compounds with ‘‘insulating’’ bridges. There are none for distances between 7 ˚ , and none beyond about 11 A ˚ . While there is litand 9 A ˚ tle likelihood of filling the 7–9 A gap, one or more com˚ should be affordable. pounds in the region beyond 11 A For example, if the dicarboxylic acid with two trishomobarrelenes instead of only one could be made, a ˚ would be possible. point at about 15 A Also shown in Fig. 2 are points for compounds in which X is one of five (CH@CH)n (n = 1–4) groups as well as C„C. For details see the figure caption. Apart from some scatter among points 5, 6 and 7, there is a smooth (though not linear) downward trend in DE1/2 values. This presumably means that the presence of a p bond, or an extended sequence of conjugated p bonds, allows electronic coupling in addition to electrostatic coupling. In connection with Fig. 2, it must be noted that regardless of the nature of the coupling mechanism, as d ! 1, the value of DE1/2 should drop to 35.6 mV,
3.3. Electronic spectrum The only absorption band observed between 380 and 600 nm was a shoulder on the absorption rising into the shorter wave length region. This had a maximum at 430 ± 10 nm and a molar extinction of about 800 mol1 cm1. It is very similar to a band found in (DAniF)3Mo2O2CCH3 [14] at 430 nm and e = 1000, and assignment to the d ! d* transition is indicated.
4. Discussion The mere existence of compound 1 is not particularly interesting, but several features of interest arise when it is compared with other compounds of the type (DAniF)3Mo2O2CXCO2Mo2(DAniF)3 [15,16]. These features are evident in the data plot of DE1/2 versus d
Fig. 2. The DE1/2 values for two classes of (DAniF)3Mo2O2CXCO2Mo2(DAniF)3 compounds, plotted against the distance between the two Mo2 units. Points 2–4 are for those in which there is little or no electronic coupling between them. Points 5–11 are for those in which p electrons are present. The X for point 1 is simply the C–C bond. For the others the X groups are as follows: 2 – C@C@C; 3 – C(l–CH2)3C; 4 – trishomobarrelene; 5 – cis-CH@CH; 6 – trans-CH@CH; 7 – C„C; 8 – cis,cis-CH@CH–CH@CH; 9 – trans,trans-CH@CH–CH@CH; 10 – all trans-(CH@CH)3; 11 – all trans-(CH@CH)4.
F.A. Cotton et al. / Polyhedron 25 (2006) 219–223
which corresponds to the purely statistical value for KC (the comproportionation constant) of 4. On this basis, the linear trend in the points for compounds with ‘‘insulating’’ connection cannot continue; instead, the DE1/2 values for longer distances (d) must asymptotically approach 35.6 mV. Thus, data for one or two more com˚ would be very pounds in the range above d = 11 A desirable. It is also interesting that the points for the compounds with conjugated polyene connectors appear to approach an asymptote of about 60 mV instead of 35.6 mV. This observation makes it even more interesting to extend the range of d for compounds with ‘‘insulating’’ connectors, and we shall seek to do so. It is known that DE1/2 values may have a ‘‘significant – and sometimes unpredictable – dependence’’ on the solvents and counter ions [17].
5. Supplementary material Crystallographic data (excluding structure factors) have been deposited with the Cambridge Crystallographic Data Center as supplementary publication No. CCDC 259665. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK, fax: +44 1223 336 033, e-mail: [email protected], www.ccdc. cam.ac.uk/data_request/cif.
Acknowledgments Support for this work was provided by the Welch Foundation and National Science Foundation at
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TAMU and the State of Niedersachsen at GAU, Go¨ttingen.
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