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The synthesis of a ferrocenyl Schiff base pyridine borane complex using NaBH4
www.elsevier.nl/locate/ica Inorganica Chimica Acta 303 (2000) 128 – 131
Note
The synthesis of a ferrocenyl Schiff base pyridine borane complex using NaBH4 Eric W. Ainscough*1, Andrew M. Brodie*2, Andrew D. Lowe, Joyce M. Waters Chemistry Institute of Fundamental Sciences, Massey Uni6ersity, Pri6ate Bag 11 222, Palmerston North, New Zealand Received 16 July 1999; accepted 22 October 1999
Abstract The reaction of excess NaBH4 with N-(o-hydrazonylpyridine)ferrocenylimine (L) in a THF/water mixture yields the borane adduct, LBH3, which has been characterised by IR and NMR spectroscopy. The single-crystal X-ray determination shows that the pyridine nitrogen of L is coordinated to the BH3 unit with a B – N length of 1.596(3) A, . © 2000 Elsevier Science S.A. All rights reserved. Keywords: Crystal structures; Borane complexes; Ferrocenyl– imine complexes
1. Introduction A range of synthetic methods is available for the synthesis of amine and phosphine complexes of borane (BH3). These include the reaction of the appropriate donors with the tetrahydrofuran (THF) or dimethylsulfide adducts of borane itself [1], or their reaction with NaBH4 in the presence of iodine [2] or CeCl3 [3]. In this paper, we report that in an attempt to reduce the imine bond in the ferrocene Schiff base derivative, N-(o-hydrazonylpyridine)ferrocenylimine (L), using excess NaBH4 in a THF/water mixture, we obtained, surprisingly, the borane complex, LBH3.
2. Experimental
2.1. General procedures These have been reported previously [4]. Reactions were carried out under argon. Mass spectra were recorded by the liquid secondary ion mass spectroscopy 1
*Corresponding author. *Corresponding author. Tel.: + 64-6-350 5102; fax: + 64-6-350 5682; e-mail: [email protected] 2
technique (LSIMS) using a Varian VG70-250S instrument from samples in a m-nitrobenzyl alcohol matrix. The boron-11 NMR spectrum is referenced with respect to external Et2O·BF3.
2.2. Synthesis of N-(o-hydrazonylpyridine) ferrocenylimine (L) To ferrocene carboxaldehyde (1.50 g, 7.00 mmol) dissolved in 150 ml benzene was added 2-hydrazinopyridine (0.76 g, 6.96 mmol) dissolved in 30 ml of the same solvent. After stirring under reflux for 6 h in a Dean Stark apparatus, the resulting solution was reduced in volume using a rotary evaporator until a brown solid formed. This was chromatographed on a silica gel column (70–400 mm mesh) using 1:9 (v/v) methanol/dichloromethane as eluant. The first band to be eluted was ferrocene carboxaldehyde and the second band, after crystallisation, yielded the product. Yield 1.06 g (50%). M.p. 164–166°C. Anal. Calc. for C16H15FeN3: C, 63.0; H, 5.0; N, 13.8. Found: C, 62.6; H, 4.7; N, 13.8%. MS m/z [M(56Fe)]+ 305 (100%). 1H NMR (CDCl3): 4.19 (s, 5H, C5H5), 4.34 (t, J=1.7 Hz, 2H, C5H4), 4.59 (t, J=1.7 Hz, 2H, C5H4), 6.74 (t, J= 5.5 Hz, 1H, C5H4N), 7.26 (d, J= 4.2 Hz, 1H,
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C5H4N), 7.58 (t, J = 5.6 Hz, 1H, C5H4N), 8.11 (d, J= 4.0 Hz, 1H, C5H4N), 8.45 (s, 1H, NH), 7.60 (s, 1H, CHN). 13C NMR (CDCl3): 69.0 (C5H5), 67.0, 69.5, 80.3 (C5H4), 107.3, 115.1, 138.0, 147.4, 156.7 (C5H4N), 139.2 (CN).
2.3. Synthesis of LBH3 To a solution of L (0.47 g, 1.50 mmol) in 100 ml THF were added 40 ml deionised water. The solution was heated to 50–60°C and solid NaBH4 (1.50 g, 39.0 mmol) was added slowly with stirring (Caution hydrogen is evolved). When evolution of hydrogen ceased (after about 1 h), the orange organic phase was separated from the aqueous phase and washed three times with 5:5 (v/v) water/diethyl ether. The solvent was removed using a rotary evaporator and the resulting yellow solid dissolved in chloroform, filtered and chromatographed on silica gel (70 – 500 mm mesh) using 1:9 (v/v) ethanol/dichloromethane as eluant. Yield 0.43 g (85%). M.p. 155°C. Anal. Calc. for C16H18BFeN3: C, 60.2; H, 5.7; N, 13.2. Found: C, 60.1; H, 5.5; N, 13.2%. MS m/z [(M −2H)(11B56Fe)]+ 317 (70%), [M−BH3]+ 305 (100%). 1H NMR (CDCl3): 2.3 (br, 3H, BH3), 4.22 (s, 5H, C5H5), 4.41 (t, J =1.8 Hz, 2H, C5H4), 4.63 (t, J = 1.8 Hz, 2H, C5H4), 6.71 (t, J =6.8 Hz, 1H, C5H4N), 7.52 (d, J =8.8 Hz, 1H, C5H4N), 7.68 (t, J = 6.9 Hz, 1H, C5H4N), 8.15 (d, J =8.8 Hz, 1H, C5H4N), 7.88 (s, 1H, CHN), 9.32 (s, 1H, NH). 13C Table 1 Selected bond lengths (A, ) and angles (°) for LBH3 Bond lengths N(1)C(11) N(1)N(2) N(2)C(17) C(10)C(11) N(3)C(17) N(3)C(13) B(1)N(3) C(13)C(14) C(14)C(15) C(15)C(16) C(16)C(17)
1.264(2) 1.387(2) 1.350(2) 1.459(2) 1.352(2) 1.355(3) 1.596(3) 1.360(3) 1.382(3) 1.365(3) 1.401(3)
Bond angles C(11)N(1)N(2) C(17)N(2)N(1) N(1)C(11)C(10) C(17)N(3)C(13) C(17)N(3)B(1) C(13)N(3)B(1) N(3)C(13)C(14) C(13)C(14)C(15) C(16)C(15)C(14) C(15)C(16)C(17) N(2)C(17)N(3) N(2)C(17)C(16) N(3)C(17)C(16)
116.2(2) 117.8(2) 120.7(2) 118.3(2) 121.8(2) 120.0(2) 123.2(2) 118.5(2) 119.7(2) 119.5(2) 117.4(2) 122.0(2) 120.6(2)
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Table 2 Summary of crystal, intensity collection and refinement data for LBH3 Formula Formula weight Temperature (°C) Wavelength (A, ) Crystal system Space group a (A, ) b (A, ) c (A, ) b (°) V (A, 3) Z Dcalc (mg m−3) m (mm−1) F(000) Crystal size (mm) u Range for data collection (°) Index ranges Reflections collected Independent reflections Absorption corrections max, min Refinement method Data/restraints/parameters Goodness-of-fit on F 2 Final R indices [I\2s(I)] a R indices (all data) Largest difference peak and hole (e A, −3)
C16H18BFeN3 318.99 20(2) 0.710 73 monoclinic P21/n 7.512(2) 10.928(2) 18.801(4) 100.14(3) 1519.3(6) 4 1.395 0.987 664 0.40×0.30×0.20 2.16–27.97 05h59, 05k514, −245l524 3928 3658 [Rint =0.0141] 0.9990, 0.9822 full-matrix least-squares on F2 3658/0/190 1.087 R1 =0.0318, wR2 =0.0782 R1 =0.0447, wR2 =0.0850 0.382 and −0.198
a R1 = Fo − Fc / Fo , wR2 =[{w(F o2−F c2)2}/(F o2)2]1/2, where w−1 =[s 2(F o2)+(aP)2+bP], a = 0.0412; b =0.3886), and P= (F o2−2F c2)/3.
NMR (CDCl3): 69.2 (C5H5), 67.6, 70.3, 78.6 (C5H4), 109.0, 113.3, 139.6, 145.6, 152.7 (C5H4N), 145.6 (CN).
2.4. X-ray data collection, solution and refinement Data were collected on a CAD4 diffractometer, with a graphite monochromator using the v–2u scan technique. Crystal, collection and structure refinement data are given in Table 2. Absorption corrections were determined by the C-scan method. The structure was solved by the heavy atom method and refined by a full-matrix least-squares technique [5]. Anisotropic thermal parameters were assumed for all non-hydrogen atoms. Hydrogen atoms for the ligand were inserted in calculated positions with fixed bond lengths [CH = 0.96 A, ] and isotropic thermal parameters.
3. Results and discussion The borane adduct, LBH3, was isolated, after column chromatography, as an air stable yellow crystalline solid in good yield. While the most prominent peak in
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the mass spectrum is due to the [M − BH3]+ ion, an intense peak is observed for the [M− 2H]+ (relative intensity 70%) ion. The IR spectrum shows bands at 2290 and 2250 cm − 1, which can be assigned to terminal n(BH) vibrations and n(NH) and n(CN) imine stretches at 3260 and 1620 cm − 1. These latter bands are at higher wave numbers than observed for L itself (by ca. 80 and 30 cm − 1), which is consistent with the BH3 moiety not being bound to the imine nitrogen. The single-crystal X-ray determination (Fig. 1 and Table 1) confirms that the pyridine nitrogen is bound to the BH3 unit with a B(1)N(3) length of 1.596(3) A, . This distance is comparable to the distance of 1.592(3) A, found in the pyridine adduct, (C5H5N)BCl3 [6] and well within the range (1.56 – 1.66 A, ) found for a variety of nitrogen s-donors bound to BH3 [7]. Significant p character would be expected to shorten the BN distance to nearer 1.4 A, [8]. The boron atom is close to lying in the pyridine ring plane being displaced from it by only 0.036(4) A, . The plane through the organic tether, C(11), N(1), N(2), lies at an angle of 16.7(3)° to that of the plane of the C(6)C(10) ring and the pyridine ring is twisted a further 7.0(3)° from the C(11)N(2) plane. As a result, the cyclopentadienyl and pyridine rings lie at an angle of 22.8(1)° to one another pointing to minimal electron delocalisation between the rings. The carboncarbon and nitrogennitrogen bond distances along the tether show the expected bond alternation pattern [C(10)C(11) 1.459(2), N(1)C(11) 1.264(2), N(1)N2) 1.387(2), N(2)C(17) 1.350(2) A, ] and the imine bond still to be intact. The two cyclopentadienyl rings have an essentially eclipsed configuration, the twist angle being 8°. The NMR data are consistent with the solid state structure. The 11B NMR spectrum displays a quartet centered at −17.6 ppm (JBH =90 Hz), which is in the region ( − 10 to − 20 ppm) found for substituted pyridine and other amine adducts of borane [2a,7a–d]. The 13C and 1H NMR spectra of the free L and its borane adduct are very similar (see Section 2). The influence of pyridineBH3 coordination is most marked
for the resonances associated with the imine bond in the tether with the signals for C(11) and N(2)H observed at 139.2 and 8.45 ppm in L moving to higher frequency (D = +6.4 and + 0.89 ppm, respectively) for the adduct and a smaller shift in the same direction (D= + 0.29 ppm) being seen for the imine proton, C(11)H. The formation of the borane adduct, LBH3, from NaBH4 in excess without activation by iodine or CeCl3 is somewhat surprising. However, from mechanistic studies on the aqueous hydrolysis of the borohydride ion, it has been postulated that the complex, H2BH3, is formed as an intermediate (Eq. (1)) [9], although the species has been detected only in an argon matrix [10]. H2O+ [BH4] − H2BH3 + OH −
(1)
We suggest that in the reducing atmosphere provided by the liberated hydrogen this intermediate could be swept into the THF layer for reaction with L, either directly or via the adduct, (THF)BH3. We have found that other conventional reducing agents do not reduce the imine bond and the reason for this may be stereochemical in nature. Our preliminary studies on the binding of L to divalent transition metal ions in THF also indicate coordination at the pyridine nitrogen, but not at the imine centre. This reluctance for L to act as a chelate may similarly be stereochemical in origin.
4. Supplementary material Crystallographic data for LBH3 have been deposited with the Cambridge Crystallographic Data Centre (deposition no. CCDC 127832). Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (Fax: +44-1223-336-033; e-mail: [email protected]. ac.uk or www: http://www.ccdc.cam.ac.uk).
Acknowledgements We thank the Massey University Research Fund for financial support.
References
Fig. 1. ORTEP diagram of LBH3 with ellipsoids drawn at the 50% probability level.
[1] J. Emri, B. Gyo¨ri, in: G. Wilkinson, R.D. Gillard, J.A. McCleverty (Eds.), Comprehensive Coordination Chemistry, vol. 3, Pergamon, Oxford, UK, 1987, p. 83 and Refs. therein. [2] (a) K.C. Nainan, G.E. Ryschkewitsch, Inorg. Chem. 8 (1969) 2671. (b) K.C. Nainan, G.E. Ryschkewitsch, Inorg. Synth. 15 (1974) 122. [3] T. Imamoto, T. Kusumoto, N. Suzuki, J. Am. Chem. Soc. 107 (1985) 5301.
E.W. Ainscough et al. / Inorganica Chimica Acta 303 (2000) 128–131 [4] E.W. Ainscough, A.M. Brodie, R.K. Coll, A.J.A. Mair, J.M. Waters, J. Organomet. Chem. 509 (1996) 259. [5] G.M. Sheldrick, SHELXL-93, Program for the Refinement of Crystal Structures, University of Go¨ttingen, Germany, 1993. [6] K. To¨pel, K. Hensen, M. Tro¨mel, Acta Crystallogr., Sect. B 37 (1981) 969. [7] (a) J.-M. Dupart, A. Grand, S. Pace, J.G. Riess, Inorg. Chem. 23 (1984) 3776. (b) D. Grec, L.G. Hubert-Pfalzgraf, A. Grand, J.G. Riess, Inorg. Chem. 24 (1985) 4642. (c) N. Kuhn, M. Schulten, E. Zauder, N. Augart, R. Boese, Chem. Ber. 122 (1989) 1891. (d) H. No¨th, S. Thomas, M. Schmidt, Chem. Ber. 129 (1996) 451. (e) J.L. Flippen-Anderson, R. Gilardi, C. George, Acta Crystallogr.,
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Sect. C 42 (1986) 1185. (f) W.K. Holley, G.E. Ryschkewitsch, A.E. Koziol, G.J. Palenik, Inorg. Chim. Acta 239 (1995) 171. (g) H.M. Colquhoun, G. Jones, J.M. Maud, J.F. Stoddart, D.J. Williams, J. Chem. Soc., Dalton Trans. (1984) 63. (h) C.B. Knobler, E.F. Maverick, K.N. Trueblood, S.P. Ho, Acta Crystallogr., Sect. C 53 (1997) 1827. [8] P.P. Power, Angew. Chem., Int. Ed. Engl. 29 (1990) 449. [9] (a) M.M. Kreevoy, J.E. Hutchins, J. Am. Chem. Soc. 94 (1972) 6371. (b) J.F. Stanton, W.N. Lipscomb, R.J. Bartlett, J. Am. Chem. Soc. 111 (1989) 5173 and Refs. therein. [10] T.J. Tague, L. Andrews, J. Am. Chem. Soc. 116 (1994) 4970.
. .
Note
The synthesis of a ferrocenyl Schiff base pyridine borane complex using NaBH4 Eric W. Ainscough*1, Andrew M. Brodie*2, Andrew D. Lowe, Joyce M. Waters Chemistry Institute of Fundamental Sciences, Massey Uni6ersity, Pri6ate Bag 11 222, Palmerston North, New Zealand Received 16 July 1999; accepted 22 October 1999
Abstract The reaction of excess NaBH4 with N-(o-hydrazonylpyridine)ferrocenylimine (L) in a THF/water mixture yields the borane adduct, LBH3, which has been characterised by IR and NMR spectroscopy. The single-crystal X-ray determination shows that the pyridine nitrogen of L is coordinated to the BH3 unit with a B – N length of 1.596(3) A, . © 2000 Elsevier Science S.A. All rights reserved. Keywords: Crystal structures; Borane complexes; Ferrocenyl– imine complexes
1. Introduction A range of synthetic methods is available for the synthesis of amine and phosphine complexes of borane (BH3). These include the reaction of the appropriate donors with the tetrahydrofuran (THF) or dimethylsulfide adducts of borane itself [1], or their reaction with NaBH4 in the presence of iodine [2] or CeCl3 [3]. In this paper, we report that in an attempt to reduce the imine bond in the ferrocene Schiff base derivative, N-(o-hydrazonylpyridine)ferrocenylimine (L), using excess NaBH4 in a THF/water mixture, we obtained, surprisingly, the borane complex, LBH3.
2. Experimental
2.1. General procedures These have been reported previously [4]. Reactions were carried out under argon. Mass spectra were recorded by the liquid secondary ion mass spectroscopy 1
*Corresponding author. *Corresponding author. Tel.: + 64-6-350 5102; fax: + 64-6-350 5682; e-mail: [email protected] 2
technique (LSIMS) using a Varian VG70-250S instrument from samples in a m-nitrobenzyl alcohol matrix. The boron-11 NMR spectrum is referenced with respect to external Et2O·BF3.
2.2. Synthesis of N-(o-hydrazonylpyridine) ferrocenylimine (L) To ferrocene carboxaldehyde (1.50 g, 7.00 mmol) dissolved in 150 ml benzene was added 2-hydrazinopyridine (0.76 g, 6.96 mmol) dissolved in 30 ml of the same solvent. After stirring under reflux for 6 h in a Dean Stark apparatus, the resulting solution was reduced in volume using a rotary evaporator until a brown solid formed. This was chromatographed on a silica gel column (70–400 mm mesh) using 1:9 (v/v) methanol/dichloromethane as eluant. The first band to be eluted was ferrocene carboxaldehyde and the second band, after crystallisation, yielded the product. Yield 1.06 g (50%). M.p. 164–166°C. Anal. Calc. for C16H15FeN3: C, 63.0; H, 5.0; N, 13.8. Found: C, 62.6; H, 4.7; N, 13.8%. MS m/z [M(56Fe)]+ 305 (100%). 1H NMR (CDCl3): 4.19 (s, 5H, C5H5), 4.34 (t, J=1.7 Hz, 2H, C5H4), 4.59 (t, J=1.7 Hz, 2H, C5H4), 6.74 (t, J= 5.5 Hz, 1H, C5H4N), 7.26 (d, J= 4.2 Hz, 1H,
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C5H4N), 7.58 (t, J = 5.6 Hz, 1H, C5H4N), 8.11 (d, J= 4.0 Hz, 1H, C5H4N), 8.45 (s, 1H, NH), 7.60 (s, 1H, CHN). 13C NMR (CDCl3): 69.0 (C5H5), 67.0, 69.5, 80.3 (C5H4), 107.3, 115.1, 138.0, 147.4, 156.7 (C5H4N), 139.2 (CN).
2.3. Synthesis of LBH3 To a solution of L (0.47 g, 1.50 mmol) in 100 ml THF were added 40 ml deionised water. The solution was heated to 50–60°C and solid NaBH4 (1.50 g, 39.0 mmol) was added slowly with stirring (Caution hydrogen is evolved). When evolution of hydrogen ceased (after about 1 h), the orange organic phase was separated from the aqueous phase and washed three times with 5:5 (v/v) water/diethyl ether. The solvent was removed using a rotary evaporator and the resulting yellow solid dissolved in chloroform, filtered and chromatographed on silica gel (70 – 500 mm mesh) using 1:9 (v/v) ethanol/dichloromethane as eluant. Yield 0.43 g (85%). M.p. 155°C. Anal. Calc. for C16H18BFeN3: C, 60.2; H, 5.7; N, 13.2. Found: C, 60.1; H, 5.5; N, 13.2%. MS m/z [(M −2H)(11B56Fe)]+ 317 (70%), [M−BH3]+ 305 (100%). 1H NMR (CDCl3): 2.3 (br, 3H, BH3), 4.22 (s, 5H, C5H5), 4.41 (t, J =1.8 Hz, 2H, C5H4), 4.63 (t, J = 1.8 Hz, 2H, C5H4), 6.71 (t, J =6.8 Hz, 1H, C5H4N), 7.52 (d, J =8.8 Hz, 1H, C5H4N), 7.68 (t, J = 6.9 Hz, 1H, C5H4N), 8.15 (d, J =8.8 Hz, 1H, C5H4N), 7.88 (s, 1H, CHN), 9.32 (s, 1H, NH). 13C Table 1 Selected bond lengths (A, ) and angles (°) for LBH3 Bond lengths N(1)C(11) N(1)N(2) N(2)C(17) C(10)C(11) N(3)C(17) N(3)C(13) B(1)N(3) C(13)C(14) C(14)C(15) C(15)C(16) C(16)C(17)
1.264(2) 1.387(2) 1.350(2) 1.459(2) 1.352(2) 1.355(3) 1.596(3) 1.360(3) 1.382(3) 1.365(3) 1.401(3)
Bond angles C(11)N(1)N(2) C(17)N(2)N(1) N(1)C(11)C(10) C(17)N(3)C(13) C(17)N(3)B(1) C(13)N(3)B(1) N(3)C(13)C(14) C(13)C(14)C(15) C(16)C(15)C(14) C(15)C(16)C(17) N(2)C(17)N(3) N(2)C(17)C(16) N(3)C(17)C(16)
116.2(2) 117.8(2) 120.7(2) 118.3(2) 121.8(2) 120.0(2) 123.2(2) 118.5(2) 119.7(2) 119.5(2) 117.4(2) 122.0(2) 120.6(2)
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Table 2 Summary of crystal, intensity collection and refinement data for LBH3 Formula Formula weight Temperature (°C) Wavelength (A, ) Crystal system Space group a (A, ) b (A, ) c (A, ) b (°) V (A, 3) Z Dcalc (mg m−3) m (mm−1) F(000) Crystal size (mm) u Range for data collection (°) Index ranges Reflections collected Independent reflections Absorption corrections max, min Refinement method Data/restraints/parameters Goodness-of-fit on F 2 Final R indices [I\2s(I)] a R indices (all data) Largest difference peak and hole (e A, −3)
C16H18BFeN3 318.99 20(2) 0.710 73 monoclinic P21/n 7.512(2) 10.928(2) 18.801(4) 100.14(3) 1519.3(6) 4 1.395 0.987 664 0.40×0.30×0.20 2.16–27.97 05h59, 05k514, −245l524 3928 3658 [Rint =0.0141] 0.9990, 0.9822 full-matrix least-squares on F2 3658/0/190 1.087 R1 =0.0318, wR2 =0.0782 R1 =0.0447, wR2 =0.0850 0.382 and −0.198
a R1 = Fo − Fc / Fo , wR2 =[{w(F o2−F c2)2}/(F o2)2]1/2, where w−1 =[s 2(F o2)+(aP)2+bP], a = 0.0412; b =0.3886), and P= (F o2−2F c2)/3.
NMR (CDCl3): 69.2 (C5H5), 67.6, 70.3, 78.6 (C5H4), 109.0, 113.3, 139.6, 145.6, 152.7 (C5H4N), 145.6 (CN).
2.4. X-ray data collection, solution and refinement Data were collected on a CAD4 diffractometer, with a graphite monochromator using the v–2u scan technique. Crystal, collection and structure refinement data are given in Table 2. Absorption corrections were determined by the C-scan method. The structure was solved by the heavy atom method and refined by a full-matrix least-squares technique [5]. Anisotropic thermal parameters were assumed for all non-hydrogen atoms. Hydrogen atoms for the ligand were inserted in calculated positions with fixed bond lengths [CH = 0.96 A, ] and isotropic thermal parameters.
3. Results and discussion The borane adduct, LBH3, was isolated, after column chromatography, as an air stable yellow crystalline solid in good yield. While the most prominent peak in
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the mass spectrum is due to the [M − BH3]+ ion, an intense peak is observed for the [M− 2H]+ (relative intensity 70%) ion. The IR spectrum shows bands at 2290 and 2250 cm − 1, which can be assigned to terminal n(BH) vibrations and n(NH) and n(CN) imine stretches at 3260 and 1620 cm − 1. These latter bands are at higher wave numbers than observed for L itself (by ca. 80 and 30 cm − 1), which is consistent with the BH3 moiety not being bound to the imine nitrogen. The single-crystal X-ray determination (Fig. 1 and Table 1) confirms that the pyridine nitrogen is bound to the BH3 unit with a B(1)N(3) length of 1.596(3) A, . This distance is comparable to the distance of 1.592(3) A, found in the pyridine adduct, (C5H5N)BCl3 [6] and well within the range (1.56 – 1.66 A, ) found for a variety of nitrogen s-donors bound to BH3 [7]. Significant p character would be expected to shorten the BN distance to nearer 1.4 A, [8]. The boron atom is close to lying in the pyridine ring plane being displaced from it by only 0.036(4) A, . The plane through the organic tether, C(11), N(1), N(2), lies at an angle of 16.7(3)° to that of the plane of the C(6)C(10) ring and the pyridine ring is twisted a further 7.0(3)° from the C(11)N(2) plane. As a result, the cyclopentadienyl and pyridine rings lie at an angle of 22.8(1)° to one another pointing to minimal electron delocalisation between the rings. The carboncarbon and nitrogennitrogen bond distances along the tether show the expected bond alternation pattern [C(10)C(11) 1.459(2), N(1)C(11) 1.264(2), N(1)N2) 1.387(2), N(2)C(17) 1.350(2) A, ] and the imine bond still to be intact. The two cyclopentadienyl rings have an essentially eclipsed configuration, the twist angle being 8°. The NMR data are consistent with the solid state structure. The 11B NMR spectrum displays a quartet centered at −17.6 ppm (JBH =90 Hz), which is in the region ( − 10 to − 20 ppm) found for substituted pyridine and other amine adducts of borane [2a,7a–d]. The 13C and 1H NMR spectra of the free L and its borane adduct are very similar (see Section 2). The influence of pyridineBH3 coordination is most marked
for the resonances associated with the imine bond in the tether with the signals for C(11) and N(2)H observed at 139.2 and 8.45 ppm in L moving to higher frequency (D = +6.4 and + 0.89 ppm, respectively) for the adduct and a smaller shift in the same direction (D= + 0.29 ppm) being seen for the imine proton, C(11)H. The formation of the borane adduct, LBH3, from NaBH4 in excess without activation by iodine or CeCl3 is somewhat surprising. However, from mechanistic studies on the aqueous hydrolysis of the borohydride ion, it has been postulated that the complex, H2BH3, is formed as an intermediate (Eq. (1)) [9], although the species has been detected only in an argon matrix [10]. H2O+ [BH4] − H2BH3 + OH −
(1)
We suggest that in the reducing atmosphere provided by the liberated hydrogen this intermediate could be swept into the THF layer for reaction with L, either directly or via the adduct, (THF)BH3. We have found that other conventional reducing agents do not reduce the imine bond and the reason for this may be stereochemical in nature. Our preliminary studies on the binding of L to divalent transition metal ions in THF also indicate coordination at the pyridine nitrogen, but not at the imine centre. This reluctance for L to act as a chelate may similarly be stereochemical in origin.
4. Supplementary material Crystallographic data for LBH3 have been deposited with the Cambridge Crystallographic Data Centre (deposition no. CCDC 127832). Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (Fax: +44-1223-336-033; e-mail: [email protected]. ac.uk or www: http://www.ccdc.cam.ac.uk).
Acknowledgements We thank the Massey University Research Fund for financial support.
References
Fig. 1. ORTEP diagram of LBH3 with ellipsoids drawn at the 50% probability level.
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