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Syntheses and characterizations of Co(II) and Ni(II) complexes of dihydrobis(thioxotetrazolyl)borato with Co[S4] and Ni[S4H2] cores
Polyhedron 24 (2005) 585–591 www.elsevier.com/locate/poly
Syntheses and characterizations of Co(II) and Ni(II) complexes of dihydrobis(thioxotetrazolyl)borato with Co[S4] and Ni[S4H2] cores Yu-Ling Wang, Rong Cao *, Wen-Hua Bi State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Graduate School of the Chinese Academy of Sciences, Yangqiao West Road 155, Fuzhou, Fujian 350002, Peoples Republic of China Received 23 November 2004; accepted 17 December 2004
Abstract The sodium salt of the dihydrobis(1-methyl-5-thiotetrazolyl)borato anion [BttMe] has been synthesized by the reaction of NaBH4 with 1-methyl-5-thiotetrazole. Treatment of Na[BttMe] with anhydrous CoCl2 and NiCl2 Æ 6H2O, respectively, gives the complexes Co(BttMe)2 (1) and Ni(BttMe)2 (2), which have been characterized by IR, 1H NMR, ESI-MS, UV–Vis spectroscopy as well as X-ray crystallography. Compound 1 contains a central [CoS4] core and exhibits a slightly distorted trigonal-bipyramidal coordination geometry with a very weak apical Co H–B interaction, whereas 2 features a central [NiS4H2] core in a distorted octahedral coordination geometry and exhibits relatively strong Ni H–B interactions. These results indicate that the ligand can act in the bidentate (k2-S,S) or tridentate (k3-S,S,BH) mode. The electrochemical behavior of 1 and 2 has also been investigated by cyclic voltammetry. 2005 Elsevier Ltd. All rights reserved. Keywords: Dihydrobis(thioxotetrazolyl)borato; Cobalt(II) complex; Nickel(II) complex; Co[S4] core; Ni[S4H2] core; Electrochemistry
1. Introduction The soft tripodal ligands Tm and Tt (Tm = hydrotris(thioxoimidazolyl)borato, Tt = hydrotris(thioxotrizolyl)borato) play an important role in coordination chemistry, and their complexes with transition metals have been extensively investigated in recent years as sulfur-rich metalloenzyme models in bioinorganic chemistry [1,2]. Recently, a great deal of attention has focused on the bis-substitution products Bm and Bt (Bm = dihydrobis(thioxoimidazolyl)borato; Bt = dihydrobis(thioxotriazolyl)borato) (Scheme 1) and their metal complexes [3,4]. The soft donor Bm or Bt ligands have also been employed in bioinorganic chemistry to study the structure and reactivity of the active sites of metalloenzymes such as liver alcohol dehydrogenase (LADH) *
Corresponding author. Tel./fax: +86 591 8379 6710. E-mail address: [email protected] (R. Cao).
0277-5387/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2004.12.019
and [NiFe] hydrogenases [3] and in biomedical fields for labelling biomolecules with 99mTc or 186/188Re [4]. To date, the notable examples of this family are Ni(BmMe)2, possessing a central [NiS4H2] core, M(BmMe)2(CO)3 (M = Re or Tc), containing a fac-[M(CO)3]+ moiety, and M(BmR)2 (M = Zn, Cd, Hg), with a central [MS4] core [3,4]. As previously reported, Bm and Bt can formally act as bidentate ligands through the S atoms. Furthermore, they display agonistic M H–B interactions toward most transition-metal ions, and this interaction may be influenced by many factors such as the electronic configuration of the metal ions, the substituents of the ligands, the flexibility and conformation of the chelating rings and the nature of the ligating atoms (N or S) [5]. In contrast to the extensively studied Bm and Bt ligands, there is no information on the Btt (Btt = dihydrobis(thioxotetrazolyl)borato) ligand (Scheme 1) and its related metal complexes. In this paper, we report the
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Y.-L. Wang et al. / Polyhedron 24 (2005) 585–591 R
Me
R
N
N
N N N
S
H
S
H
N
S
B
H
S
N
H
N
S
N N
N
N
R
R BmR
S
H
B N
2.2. Synthesis of the sodium salt of dihydrobis(1-methyl5-thiotetrazolyl)borato
N N
B H
N
BtR
N
N Me
BttMe
Scheme 1.
syntheses, crystal structures and properties of two complexes, 1 and 2, of the novel BttMe (BttMe = dihydrobis(1-methyl-5-thiotetrazolyl)borato) ligand, which may render models for studying various metalloenzymes derivatives such as Co-LADH [6].
Under a nitrogen atmosphere 2.32 g of 1-methyl-5thiotetrazole (20 mmol) and 0.378 g of NaBH4 (10 mmol) were mixed in 30 ml degassed THF solution. The solution was refluxed overnight and the resulting cloudy solution was allowed to cool to room temperature and filtered. The filtrate was concentrated under reduced pressure to 3 ml and the white precipitate formed was washed with CH2Cl2 and ether, and then dried in vacuo for 2 h. Yield: 75%. IR data (cm1): 3070 (w), 2954 (w), 2497 (m), 2480 (m), 1645 (m), 1618 (m), 1510 (m), 1458 (m), 1439 (m), 1392 (s), 1336 (s), 1275 (s), 1196 (s), 1151 (m), 1105 (s), 999 (w), 972 (w), 860 (m), 758 (m), 742 (m), 710 (m), 617 (m), 465 (w). 1H NMR data: d 3.72 (s, CH3). Anal. Calc. for Na(BttMe): C, 17.98; H, 3.00; N, 41.95. Found: C, 17.45; H, 2.36; N, 41.69%. UV–Vis (k/nm e/mol1 cm1): 246 nm (73 900). MS (m/z, I%): 244, 100 [BttMe]. 2.3. Synthesis of Co(BttMe)2 (1)
2. Experimental 2.1. Materials and physical measurements All reagents were commercial reagent grade and used without further purification unless otherwise noted. The solvents CH3CN and THF were purified and degassed by standard procedures. The synthesis of Na(BttMe) was carried out under N2 and other reactions were performed in air. The IR spectra were recorded on a Magna750 FT-IR spectrophotometer using KBr pellets from 4000 to 400 cm1. 1H NMR spectra were recorded on a Varian UNITY-500 spectrometer (500 MHz). The UV–Vis spectra in CH3CN solution were measured on a Perkin–Elmer Lambda 900 UV–Vis spectrometer. The electrospray mass spectra (ESI-MS) were run on a Finngan LCQ mass spectrometer using acetonitrile as the mobile phase. Electrochemical experiments were carried out on an Epsilon electrochemical analyzer. Cyclic voltametry (CV) was performed using a conventional three-electrode single compartment cell supplied by BAS Inc. The working electrode was a glassy carbon disc (GCE). The auxiliary electrode was a Pt wire. The reference electrode comprised an Ag wire immersed in acetonitrile solution containing 0.1 M (Bu4N)PF6 as supporting electrolyte. At the beginning of each experiment a cyclic voltammogram of the solution containing only the supporting electrolyte was measured under nitrogen atmosphere. Then the complexes were added and the resulting complex concentration was 1 mM. The CV was recorded at a scan rate of 200 mV/s. Formal redox potentials are given vs. the reference system Cp2Fe/Cp2Fe+ in volts.
Na(BttMe) 0.11 g (0.4 mmol) and anhydrous CoCl2 0.026 g (0.2 mmol) were stirred in CH3OH overnight. The resulting blue precipitate was dissolved in CH2Cl2, and hexane was carefully layered over the CH2Cl2 solution. After one week blue crystals suitable for X-ray diffraction were obtained. Yield: 53%. IR data (cm1): 2951 (w), 2472 (m), 2378 (w), 2318 (w), 2291 (w), 1645 (w), 1470 (s), 1425 (s), 1398 (s), 1367 (m), 1364 (s), 1275 (s), 1198 (s), 1147 (m), 1092(m), 1080(m), 1005 (w), 968 (w), 854 (w), 758 (m), 744 (m), 521 (w), 465 (m). Anal. Calc. for C8H16B2CoN16S4: C, 17.61; H, 2.93; N, 41.09. Found: C, 17.35; H, 2.52; N, 40.71%. 1 H NMR data: d 3.708 (s, CH3). UV–Vis (k/nm e/mol1 cm1): 240 (66 750), 376 (11 120), 565 (750). MS (m/z, I%): 380, 10, [Co2(L)Na]4+; 545, 12, [Co(L)2]+; 720, 100, [Co3(L)2(CH3OH)2]4+; 847, 68, [Co2(L)3]+. 2.4. Synthesis of Ni(BtMe)2 (2) Na(BttMe) 0.11 g (0.4 mmol) and NiCl2 Æ 6H2O 0.047 g (0.2 mmol) were dissolved in methanol and stirred overnight, and a yellow precipitate was obtained. The product was dissolved in CH3CN and the resulting solution slowly evaporated in air to give yellow crystals. Yield: 47%. IR data (cm1): 2951 (w), 2503 (s), 2384 (m), 2276 (s), 2245 (s), 2198 (m), 2017 (w), 1618 (w), 1471 (s), 1419 (s), 1362 (s), 1348 (s), 1277 (s), 1130 (m), 1093 (m), 1068 (m), 1001 (m), 943 (m), 845 (m), 756 (s), 615 (w), 530 (w), 486 (w). Anal. Calc. for C8H16B2N16NiS4: 17.62; H, 2.94; N, 41.10. Found: C, 17.38; H, 2.63; N, 40.83%. 1H NMR data: d 3.715 (s, CH3). UV–Vis
Y.-L. Wang et al. / Polyhedron 24 (2005) 585–591
(k/nm e/mol1 cm1): 246 (60 950); 403 (2490). MS (m/z, I%): 380, 94, [Ni2(L)Na]4+; 545, 30, [Ni(L)2]+; 567, 58, [Ni(L)2Na]+; 719, 100, [Ni3(L)2(CH3OH)2]4+; 845, 65, [Ni2(L)3]+.
2.5. X-ray structure determination X-ray diffraction data were collected on a Bruker P4 diffractometer with a Mercury CCD area detector (Mo ˚ ) for 1 and on a Siemens Smart CCD Ka; k = 0.71073 A ˚ ) for area-detector diffractometer (MoKa; k = 0.71073 A 2. Empirical absorption correction was applied using the program SADABS [7]. The structures were solved using direct methods and refined by full-matrix least-squares on F2 with SHELXTL (version 5.10) [8]. Non-hydrogen atoms were refined anisotropically. The hydrogen atoms were placed at their calculated positions except for hydrogen atoms on boron, which were located and refined isotropically for both complexes. The absolute structure of 1 was determined with a Flack parameter of 0.018(10) [9]. Additional information about crystal data collection and refinement parameters for 1 and 2 is presented in Table 1, while selected bond distances and angles are listed in Tables 2 and 3, respectively.
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Table 2 ˚ ) and angles () for Co(BttMe)2 Selected bond lengths (A Molecule I Bond lengths Co1–S1 Co1–S2 Co1–S3 Co1–S4 Co1–H1A Co1–H2A S1–C1 S2–C3 S3–C5 S4–C7 B1–H1A B1–H1B B2–H2A B2–H2B Bond angles S1–Co1–S4 S1–Co1–S3 S4–Co1–S3 S1–Co1–S2 S4–Co1–S2 S3–Co1–S2 H1A–B1–H1B H2B–B2–H2A
Molecule II 2.3175(10) 2.3773(9) 2.3570(9) 2.3315(9) 2.57(4) 2.32(4) 1.697(3) 1.702(3) 1.707(3) 1.699(3) 1.12(4) 1.20(4) 1.06(3) 1.16(4) 140.20(4) 100.94(4) 110.09(3) 107.36(4) 96.22(3) 91.72(3) 112(2) 112(3)
Chemical formula Formula weight Colour, habit Crystal size (mm) Crystal system Space group ˚) a (A ˚) b (A ˚) c (A a () b () c () ˚ 3) V (A Z F(0 0 0) Dcalc (Mg m3) ˚ )) Radiation (k(A T (K) l (mm1) Maximum and minimum transmission h range for data collection () Measured reflections Independent reflections Goodness-of-fit on F2 R1 wR2 Largest difference ˚ 3) peak/hole (e A
Co(BttMe)2
Ni(BttMe)2
C8H16B2CoN16S4 545.16 blue 0.28 · 0.25 · 0.20 orthorhombic P212121 15.451(4) 16.422(4) 17.474(4) 90 90 90 4433.7(18) 8 2216 1.633 0.71073 293(2) 1.185 0.728 and 0.789
C8H16B2N16NiS4 544.94 yellow 0.34 · 0.30 · 0.10 triclinic P 1 12.5583(3) 12.5692(3) 16.2972(3) 69.8120(10) 70.5490(10) 89.9820(10) 2257.47(9) 4 1112 1.603 0.71073 293(2) 1.263 0.644 and 0.881
1.000–28.28 36799 10982 1.048 0.0385 0.0799 0.372 and 0.319
0.987–25.02 11725 7844 1.037 0.0713 0.1559 0.472 and 0.536
S6–Co2–S7 S6–Co2–S5 S7–Co2–S5 S6–Co2–S8 S7–Co2–S8 S5–Co2–S8 H3B–B3–H3A H4B–B4–H4A
2.3503(9) 2.3208(9) 2.3204(9) 2.4186(9) 2.13(3) 2.99(4) 1.704(3) 1.703(3) 1.708(3) 1.712(3) 1.10(3) 1.09(3) 1.11(4) 1.19(4) 130.41(4) 115.35(3) 108.03(3) 95.39(3) 106.54(3) 91.58(3) 120(3) 112(3)
Table 3 ˚ ) and Angles () for Ni(BttMe)2 Selected Bond Lengths (A Molecule I
Table 1 Crystallographic data for Co(BttMe)2 and Ni(BttMe)2
Co2–S5 Co2–S6 Co2–S7 Co2–S8 Co2–H3A Co2–H4A S5–C9 S6–C11 S7–C13 S8–C15 B3–H3A B3–H3B B4–H4A B4–H4B
Bond lengths Ni1–S1 Ni1–S2 Ni1–S3 Ni1–S4 Ni1–H1A Ni1–H2A S1–C1 S2–C3 S3–C5 S4–C7 B1–H1A B1–H1B B2–H2A B2–H2B Bond angles S4–Ni1–S1 S4–Ni1–S3 S1–Ni1–S3 S4–Ni1–S2 S1–Ni1–S2 S3–Ni1–S2 S4–Ni1–H1A S1–Ni1–H1A S3–Ni1–H1A S2–Ni1–H1A S4–Ni1–H2A S1–Ni1–H2A S3–Ni1–H2A S2–Ni1–H2A H1A–Ni1–H2A H1A–B1–H1B H2A–B2–H2B
Molecule II 2.379(3) 2.399(3) 2.397(3) 2.374(3) 1.98(10) 1.89(10) 1.707(9) 1.691(9) 1.674(9) 1.705(9) 1.09(10) 1.13(10) 1.14(10) 1.11(10) 93.19(9) 94.80(10) 91.62(9) 91.62(10) 95.41(10) 170.20(11) 173(3) 94(3) 84(3) 89(3) 93(3) 174(3) 88(3) 84(3) 80(4) 113(7) 113(7)
Ni2–S5 Ni2–S6 Ni2–S7 Ni2–S8 Ni2–H3A Ni2–H4A S5–C9 S6–C11 S7–C13 S8–C15 B3–H3A B3–H3B B4–H4A B4–H4B S5–Ni2–S8 S5–Ni2–S6 S8–Ni2–S6 S5–Ni2–S7 S8–Ni2–S7 S6–Ni2–S7 S5–Ni2–H3A S8–Ni2–H3A S6–Ni2–H3A S7–Ni2–H3A S5–Ni2–H4A S8–Ni2–H4A S6–Ni2–H4A S7–Ni2–H4A H3A–Ni2–H4A H3A–B3–H3B H4A–B4–H4B
2.375(3) 2.394(3) 2.404(3) 2.377(3) 1.87(9) 1.93(6) 1.699(9) 1.677(9) 1.695(9) 1.699(9) 1.20(9) 1.22(9) 1.10(10) 1.06(10) 93.28(10) 95.51(10) 91.72(9) 91.77(10) 94.69(10) 170.00(11) 94(3) 172(3) 89(3) 84(3) 174(2) 92(2) 84(3) 88(3) 80(4) 111(6) 117(7)
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Y.-L. Wang et al. / Polyhedron 24 (2005) 585–591
3. Results and discussion The procedure of preparing the sodium salt of the dihydrobis(1-methyl-5-thiotetrazolyl)borato anion [BttMe] is similar to that of preparing Na(BmMe) (BmMe = dihydrobis(thioxoimidazolyl)borato) (Scheme 2) [3a]. Reaction of Na(BttMe) with anhydrous CoCl2 and Ni(NO3)2 Æ 6H2O (molar ratio 2:1) affords two novel mononuclear complexes Co(BttMe)2 (1) and Ni(BttMe)2 (2), respectively. Both complexes are soluble in dimethylsulfoxide, acetonitrile and dichloromethane, but insoluble in methanol.
3.1. Crystal structures X-ray diffraction shows the molecular structures in Figs. 1 and 2. Interestingly, two crystallographically independent molecules are found in the asymmetric unit of 1 and 2, which are similar in their configuration. The geometrical parameters of the two independent molecules in the asymmetric unit of 1 are somewhat different from each other (Table 2). The Co–S bond ˚ (Co1) lengths vary from 2.3175(10) to 2.3773(9) A ˚ ˚ (av. 2.3458 A) and 2.3204(9) to 2.4186(9) A (Co2) ˚ ), and the S–Co–S bond angles range (av. 2.3525 A from 91.72(3) to 140.20(4) (Co1) and 95.39(3) to 130.41(4) (Co2). It is noteworthy that 1 has a larger range of S–Co–S bond angles compared with that in previously reported M(Bm)2 complexes (M = Zn, Cd, Hg) (85–124), whose coordination geometry around the metal center are slightly distorted tetrahedral [3a]. At a first glance, 1 exhibits a severely distorted tetrahedral geometry in the solid state with a central [CoS4] core in which the cobalt atom is bound by two BttMe ligands through four S atoms. The Co1– ˚ ) is much longer compared S2 bond length (2.3773(9) A with the other Co–S distances, and the bond angles S2–Co1–S3, S2–Co1–S4 and S2–Co1–S1 are 91.72(3), 96.22(3) and 107.36(4), respectively, which are close to 90. In addition, the S2–Co1–H2A angle
Fig. 1. The ORTEP drawing of molecule I of Co(BttMe)2 with 60% thermal ellipsoids.
Fig. 2. The ORTEP drawing of molecule I of Ni(BttMe)2 with 50% thermal ellipsoids.
Me N Me N NaBH4
+
2
N
N S
N
N H
N
N THF/N2
S
H Na
B
rflux for overnight H
S
N N N Scheme 2.
N Me
Y.-L. Wang et al. / Polyhedron 24 (2005) 585–591
(172.0) is close to the theoretical value of 180 for a trigonal bipyramid. These parameters indicate that the overall geometry around the Co1 atom should be best considered as a distorted trigonal bipyramid, if the very weak Co1 H2A–B2 interaction is taken into account. Interestingly, if the other weaker Co1 H1A– B1 interaction is further considered, the coordination geometry around Co1 can also be described as highly distorted octahedral. The coordination geometry around the Co2 atom is somewhat similar to that around the Co1 atom, however, the long distance of ˚ ) indicates no interaction between Co2–H4A (2.99(4) A Co2 and H4A. Hence, only a trigonal-bipyramidal coordination geometry of Co2 should be considered, with the axial bond angle S8–Co2–H3A being 173.6, and the bond angles S8–Co2–S5, S8–Co2–S6 and S8–Co2–S7 being 91.58(3), 95.39(3) and 106.54(3), respectively. The Co–H distances are 2.57(4) (Co1–H1), 2.32(4) (Co1–H2), 2.13(3) (Co2– ˚ , respectively, which are H3) and 2.99(4) (Co2–H4) A much longer than that found in other borohydrido Co derivatives such as Co(TmPh)2 [Co– Me ˚ ], Co(pzBm )2 [(Co–H) = 1.95 A ˚ ] and H = 2.11(3) A ˚ ] (Bp = dihydrobis(pyrCo(BpBut,Pri)2 [(Co–H) = 1.95 A azoly)borato) [10–12]. In the crystal structure of complex 2, the geometrical parameters of the two independent molecules in the asymmetric unit are almost identical within experimental error. The most notable feature of 2 is that it contains a central [NiS4H2] core, and the coordination geometry around the Ni atom is described as distorted octahedral, surrounded by four S atoms and two H atoms from two BttMe ligands with two relatively strong Ni H–B interactions in which the two borohydrido groups are in a cis arrangement. From this viewpoint, the BttMe ligand acts as an S, S, H tripod. The structure of 2 is similar to that of the closely related nickel complexes Ni(Bm)2 and Ni(Bt)2 [3b,5]. The Ni–H distances are 1.98(10) (Ni1–H1), 1.89(10) (Ni1–H2), 1.87(9) ˚ , which are somewhat (Ni2–H3) and 1.93(6) (Ni2–H4) A ˚) longer than that found in Ni(Bm)2 (Ni–H = 1.863 A ˚ ) [3b,5]. The Ni–S bond and Ni(Bt)2 (av. Ni–H = 1.86 A lengths are slightly different from each other, with those trans to the borohydrido groups being slightly shorter than the axial ones. These different Ni H and Co H interactions result that the BttMe ligands in 1 adopt a boat-like eightmembered ring configuration and in 2 the BttMe ligands adopt a twist eight-membered ring configuration. 3.2. Infrared spectroscopy In the IR spectra of the ligand, two medium absorptions at 2497 and 2480 cm1 show the typical B–H stretching vibrations. For complex 1, the medium absorption at 2472 cm1 can be assigned to a terminal
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B–H stretching vibration and two weak absorptions at 2378 and 2318 cm1 are indicative of a weak Co H– B interaction. In the infrared spectrum of 2, the corresponding bands are at 2550–2350 and 2300–2200 cm1, respectively. Compared with 1, the absorption intensity of the B–H bond at low frequency is much greater than that of the corresponding one in 1, and the result is attributable to the relatively strong Ni H–B interaction. These are in good agreement with the crystal analytical results. 3.3. Electra spectra In the UV–Vis spectra of 1 and 2 in CH3CN solution, the absorptions [k/nm e/mol1 cm1] at 240 (66 750) for 1 and 246 (60 950) for 2 display p–p* transitions for the coordinated BttMe ligands, based on the absorption at 246 (73 900) in the spectra of the uncoordinated ligands. The absorption observed at 376 (11 120) in the UV–Vis spectrum of 1 can be assigned to the charge transfer from the ligands to the Co(II) center [6] and the absorption at 565 (750) to a d–d transition [13]. The position of the 376 nm absorption is higher in energy than the band of similar intensity found in Co(PhTt)2 (380 (7400)) (PhTt = phenyltris(methylthio)methylborate) [14]. For 2, the absorption at 403 (2490) may be associated with the d–d transition [3A2g(F) ! 3T1g(P)] in an octahedral field for a d8 ion [5], with an energy which is comparable with that observed in the related complex Ni(Bt)2 (417 (1120)) [5]. 3.4. Electrochemistry Electrochemical data of Na(BttMe) and complexes 1 and 2 are summarized in Table 4. The cyclic voltammogram of Na(BttMe) in acetonitrile revealed one irreversible oxidation peak at +0.47 V and one irreversible reduction peak at –1.83 V (vs. Cp2Fe/Cp2Fe+). Figs. 3 and 4 display the cyclic voltammograms of 1 and 2, respectively. As shown in Fig. 3, a scan initiated in the negative direction reveals an irreversible reduction at
Table 4 Electrochemical data Ef (V)
Complex Me
Na(Btt
)
Co(BttMe)2
Me
Ni(Btt
)2
DEp (mV)
Ip,ox/Ip,red
+ 0.47 1.83
irrev irrev
1.56 0.23 +0.45 0.78
1.1
Irrev irrev irrev quasi-rev
0.73
irrev irrev quasi-rev
1.11 +0.45 +0.75
142
131
590
Y.-L. Wang et al. / Polyhedron 24 (2005) 585–591
4. Supplementary materials Crystallographic data for the structures are available from the Cambridge Crystallographic Data Centre (CCDC Nos. 248456 and 248278 for compounds Co(BttMe)2 and Ni(BttMe)2, respectively). These data can be obtained free of charge via www.ccdc.cam. ac.uk/data_request/cif, by emailing data_request@ccdc. cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Acknowledgements Fig. 3. Cyclic voltammogram of Co(BttMe)2 in 0.1 M TBAP-CH3CN at a glassy-carbon electrode with a scan rate of 200 mV/s.
We give thanks for the financial support from the State Key Basic Project (001CB108906) and the National Natural Science Foundation of China (20325106), the Natural Science Foundation of Fujian Province (B982003), and the ‘‘one-hundred Talent’’ Project from CAS.
References
Fig. 4. Cyclic voltammogram of Ni(BttMe)2 in 0.1 M TBAP-CH3CN at a glassy-carbon electrode with a scan rate of 200 mV/s.
1.56 V of 1. Then, an irreversible oxidation at 0.23 V of the reductive product of 1 is observed. It is noted that the irreversible oxidation at 0.45 V should be assigned to the oxidation of the coordinated BttMe ligands, for the oxidation potentials in 1 and the free ligand are nearly identical. Finally, a quasi-reversible redox process at +0.78 V is observed, which should be contributed to a metal-based quasi-reversible redox process. Compared with 1, complex 2 displays similar cyclic electrochemical behavior. An irreversible reduction of 2 at 1.11 V is observed. The irreversible oxidation of the coordinated ligands at +0.48 V is also indicative of the oxidation of the coordinated BttMe ligands. A metal-based quasi-reversible redox process at +0.75 V is also observed. However, the related complex Ni(Bm)2 exhibits irreversible oxidation [2b].
[1] (a) C. Kimblin, B.M. Bridgewater, D.G. Churchill, G. Parkin, Chem. Commun. (1999) 2301; (b) B.M. Bridgewater, T. Fillebeen, R.A. Friesner, G. Parkin, J. Chem. Soc., Dalton Trans. (2000) 4494; (c) J. Reglinski, M. Garner, I.D. Cassidy, P.A. Slavin, M.D. Spicer, D.R. Armstrong, J. Chem. Soc., Dalton Trans. (1999) 2119; (d) C. Kimblin, T. Hascall, G. Parkin, Inorg. Chem. (1997) 2580; (e) M. Tesmer, M.H. Shu, H. Vahrenkamp, Inorg. Chem. (2001) 4022; (f) I. Cassidy, M. Garger, A.R. Kennedy, G.B.S. Potts, J. Reglinski, P.A. Slavin, M.D. Spicer, Eur. J. Inorg. Chem. (2002) 1235; (g) S. Bakbak, C.D. Incarvito, A.L. Rheingold, D. Rabinovich, Inorg. Chem. 41 (2002) 998. [2] (a) M. Careri, L. Elviri, M. Lanfranchi, L. Marchio`, M.A. Pellinghelli, Inorg. Chem. 42 (2003) 2109; (b) M. Lanfranchi, L. Marchio`, C. Mora, M.A. Pellinghelli, Inorg. Chimi. Acta 357 (2004) 367. [3] (a) H.M. Alvarez, T.B. Tran, M.A. Richter, D.M. Alyounes, D. Rabinovich, Inorg. Chem. 42 (2003) 2149; (b) H.M. Alvarez, M. Krawiec, B.T. Donovan-merkert, M. Fouzi, D. Rabinovich, Inorg. Chem. 40 (2001) 5736. ˜ ngela Domingos, I. Santos, R. Alberto, [4] (a) R. Garcia, A. Paulo, N J. Am. Chem. Soc. 132 (2000) 11240; ˜ ngela Domingos, A. Paulo, I. Santo, R. Alberto, (b) R. Garcia, N Inorg. Chem. 41 (2002) 2422; ˜ ngela Domingos, I. Santos, J. (c) R. Garcia, A. Paulo, N Organomet. Chem. 632 (2001) 41. [5] R. Cammi, M. Lanfranchi, L. Marchio`, C. Mora, C. Paiola, M.A. Pellinghelli, Inorg. Chem. 42 (2003) 1769. [6] D. Swenson, N.C. Baenziger, D. Coucouvanis, J. Am. Chem. Soc. 110 (1978) 1932. [7] Area-Detector Absorption correction; Siemens Industrial Automation, Inc., Madison, WI, 1996.
Y.-L. Wang et al. / Polyhedron 24 (2005) 585–591 [8] G.M. Sheldrick, SHELXTL. Version 5.1, Bruker AXS Inc., Madison, WI, USA. [9] H.D. Flack, Acta Crystallogr., Sect. A 39 (1983) 876. [10] C. Kimbin, D.G. Churchill, B.M. Bridgewater, J.N. Girard, D.A. Quarless, G. Parkin, Polyhedron 20 (2001) 1891. [11] C. Kimbin, B.M. Bridgewater, D.G. Churchill, T. Hascall, G. Parkin, Inorg. Chem. 39 (2000) 4240.
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[12] P. Ghosh, J.B. Bonanno, G.J. Parkin, J. Chem. Soc., Dalton Trans. (1998) 2779. [13] A. Bernalte-Garcı´a, F.J. Garcı´a-Barros, F.J. Higes-Rolando, F. Luna-Giles, Polyhedron 24 (2001) 3315. [14] C. Ohrenberg, P. Ge, P. Schebler, C.G. Riordan, G.P.A. Yap, A.L. Rheingold, Inorg. Chem. 35 (1996) 749.
Syntheses and characterizations of Co(II) and Ni(II) complexes of dihydrobis(thioxotetrazolyl)borato with Co[S4] and Ni[S4H2] cores Yu-Ling Wang, Rong Cao *, Wen-Hua Bi State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Graduate School of the Chinese Academy of Sciences, Yangqiao West Road 155, Fuzhou, Fujian 350002, Peoples Republic of China Received 23 November 2004; accepted 17 December 2004
Abstract The sodium salt of the dihydrobis(1-methyl-5-thiotetrazolyl)borato anion [BttMe] has been synthesized by the reaction of NaBH4 with 1-methyl-5-thiotetrazole. Treatment of Na[BttMe] with anhydrous CoCl2 and NiCl2 Æ 6H2O, respectively, gives the complexes Co(BttMe)2 (1) and Ni(BttMe)2 (2), which have been characterized by IR, 1H NMR, ESI-MS, UV–Vis spectroscopy as well as X-ray crystallography. Compound 1 contains a central [CoS4] core and exhibits a slightly distorted trigonal-bipyramidal coordination geometry with a very weak apical Co H–B interaction, whereas 2 features a central [NiS4H2] core in a distorted octahedral coordination geometry and exhibits relatively strong Ni H–B interactions. These results indicate that the ligand can act in the bidentate (k2-S,S) or tridentate (k3-S,S,BH) mode. The electrochemical behavior of 1 and 2 has also been investigated by cyclic voltammetry. 2005 Elsevier Ltd. All rights reserved. Keywords: Dihydrobis(thioxotetrazolyl)borato; Cobalt(II) complex; Nickel(II) complex; Co[S4] core; Ni[S4H2] core; Electrochemistry
1. Introduction The soft tripodal ligands Tm and Tt (Tm = hydrotris(thioxoimidazolyl)borato, Tt = hydrotris(thioxotrizolyl)borato) play an important role in coordination chemistry, and their complexes with transition metals have been extensively investigated in recent years as sulfur-rich metalloenzyme models in bioinorganic chemistry [1,2]. Recently, a great deal of attention has focused on the bis-substitution products Bm and Bt (Bm = dihydrobis(thioxoimidazolyl)borato; Bt = dihydrobis(thioxotriazolyl)borato) (Scheme 1) and their metal complexes [3,4]. The soft donor Bm or Bt ligands have also been employed in bioinorganic chemistry to study the structure and reactivity of the active sites of metalloenzymes such as liver alcohol dehydrogenase (LADH) *
Corresponding author. Tel./fax: +86 591 8379 6710. E-mail address: [email protected] (R. Cao).
0277-5387/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2004.12.019
and [NiFe] hydrogenases [3] and in biomedical fields for labelling biomolecules with 99mTc or 186/188Re [4]. To date, the notable examples of this family are Ni(BmMe)2, possessing a central [NiS4H2] core, M(BmMe)2(CO)3 (M = Re or Tc), containing a fac-[M(CO)3]+ moiety, and M(BmR)2 (M = Zn, Cd, Hg), with a central [MS4] core [3,4]. As previously reported, Bm and Bt can formally act as bidentate ligands through the S atoms. Furthermore, they display agonistic M H–B interactions toward most transition-metal ions, and this interaction may be influenced by many factors such as the electronic configuration of the metal ions, the substituents of the ligands, the flexibility and conformation of the chelating rings and the nature of the ligating atoms (N or S) [5]. In contrast to the extensively studied Bm and Bt ligands, there is no information on the Btt (Btt = dihydrobis(thioxotetrazolyl)borato) ligand (Scheme 1) and its related metal complexes. In this paper, we report the
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Y.-L. Wang et al. / Polyhedron 24 (2005) 585–591 R
Me
R
N
N
N N N
S
H
S
H
N
S
B
H
S
N
H
N
S
N N
N
N
R
R BmR
S
H
B N
2.2. Synthesis of the sodium salt of dihydrobis(1-methyl5-thiotetrazolyl)borato
N N
B H
N
BtR
N
N Me
BttMe
Scheme 1.
syntheses, crystal structures and properties of two complexes, 1 and 2, of the novel BttMe (BttMe = dihydrobis(1-methyl-5-thiotetrazolyl)borato) ligand, which may render models for studying various metalloenzymes derivatives such as Co-LADH [6].
Under a nitrogen atmosphere 2.32 g of 1-methyl-5thiotetrazole (20 mmol) and 0.378 g of NaBH4 (10 mmol) were mixed in 30 ml degassed THF solution. The solution was refluxed overnight and the resulting cloudy solution was allowed to cool to room temperature and filtered. The filtrate was concentrated under reduced pressure to 3 ml and the white precipitate formed was washed with CH2Cl2 and ether, and then dried in vacuo for 2 h. Yield: 75%. IR data (cm1): 3070 (w), 2954 (w), 2497 (m), 2480 (m), 1645 (m), 1618 (m), 1510 (m), 1458 (m), 1439 (m), 1392 (s), 1336 (s), 1275 (s), 1196 (s), 1151 (m), 1105 (s), 999 (w), 972 (w), 860 (m), 758 (m), 742 (m), 710 (m), 617 (m), 465 (w). 1H NMR data: d 3.72 (s, CH3). Anal. Calc. for Na(BttMe): C, 17.98; H, 3.00; N, 41.95. Found: C, 17.45; H, 2.36; N, 41.69%. UV–Vis (k/nm e/mol1 cm1): 246 nm (73 900). MS (m/z, I%): 244, 100 [BttMe]. 2.3. Synthesis of Co(BttMe)2 (1)
2. Experimental 2.1. Materials and physical measurements All reagents were commercial reagent grade and used without further purification unless otherwise noted. The solvents CH3CN and THF were purified and degassed by standard procedures. The synthesis of Na(BttMe) was carried out under N2 and other reactions were performed in air. The IR spectra were recorded on a Magna750 FT-IR spectrophotometer using KBr pellets from 4000 to 400 cm1. 1H NMR spectra were recorded on a Varian UNITY-500 spectrometer (500 MHz). The UV–Vis spectra in CH3CN solution were measured on a Perkin–Elmer Lambda 900 UV–Vis spectrometer. The electrospray mass spectra (ESI-MS) were run on a Finngan LCQ mass spectrometer using acetonitrile as the mobile phase. Electrochemical experiments were carried out on an Epsilon electrochemical analyzer. Cyclic voltametry (CV) was performed using a conventional three-electrode single compartment cell supplied by BAS Inc. The working electrode was a glassy carbon disc (GCE). The auxiliary electrode was a Pt wire. The reference electrode comprised an Ag wire immersed in acetonitrile solution containing 0.1 M (Bu4N)PF6 as supporting electrolyte. At the beginning of each experiment a cyclic voltammogram of the solution containing only the supporting electrolyte was measured under nitrogen atmosphere. Then the complexes were added and the resulting complex concentration was 1 mM. The CV was recorded at a scan rate of 200 mV/s. Formal redox potentials are given vs. the reference system Cp2Fe/Cp2Fe+ in volts.
Na(BttMe) 0.11 g (0.4 mmol) and anhydrous CoCl2 0.026 g (0.2 mmol) were stirred in CH3OH overnight. The resulting blue precipitate was dissolved in CH2Cl2, and hexane was carefully layered over the CH2Cl2 solution. After one week blue crystals suitable for X-ray diffraction were obtained. Yield: 53%. IR data (cm1): 2951 (w), 2472 (m), 2378 (w), 2318 (w), 2291 (w), 1645 (w), 1470 (s), 1425 (s), 1398 (s), 1367 (m), 1364 (s), 1275 (s), 1198 (s), 1147 (m), 1092(m), 1080(m), 1005 (w), 968 (w), 854 (w), 758 (m), 744 (m), 521 (w), 465 (m). Anal. Calc. for C8H16B2CoN16S4: C, 17.61; H, 2.93; N, 41.09. Found: C, 17.35; H, 2.52; N, 40.71%. 1 H NMR data: d 3.708 (s, CH3). UV–Vis (k/nm e/mol1 cm1): 240 (66 750), 376 (11 120), 565 (750). MS (m/z, I%): 380, 10, [Co2(L)Na]4+; 545, 12, [Co(L)2]+; 720, 100, [Co3(L)2(CH3OH)2]4+; 847, 68, [Co2(L)3]+. 2.4. Synthesis of Ni(BtMe)2 (2) Na(BttMe) 0.11 g (0.4 mmol) and NiCl2 Æ 6H2O 0.047 g (0.2 mmol) were dissolved in methanol and stirred overnight, and a yellow precipitate was obtained. The product was dissolved in CH3CN and the resulting solution slowly evaporated in air to give yellow crystals. Yield: 47%. IR data (cm1): 2951 (w), 2503 (s), 2384 (m), 2276 (s), 2245 (s), 2198 (m), 2017 (w), 1618 (w), 1471 (s), 1419 (s), 1362 (s), 1348 (s), 1277 (s), 1130 (m), 1093 (m), 1068 (m), 1001 (m), 943 (m), 845 (m), 756 (s), 615 (w), 530 (w), 486 (w). Anal. Calc. for C8H16B2N16NiS4: 17.62; H, 2.94; N, 41.10. Found: C, 17.38; H, 2.63; N, 40.83%. 1H NMR data: d 3.715 (s, CH3). UV–Vis
Y.-L. Wang et al. / Polyhedron 24 (2005) 585–591
(k/nm e/mol1 cm1): 246 (60 950); 403 (2490). MS (m/z, I%): 380, 94, [Ni2(L)Na]4+; 545, 30, [Ni(L)2]+; 567, 58, [Ni(L)2Na]+; 719, 100, [Ni3(L)2(CH3OH)2]4+; 845, 65, [Ni2(L)3]+.
2.5. X-ray structure determination X-ray diffraction data were collected on a Bruker P4 diffractometer with a Mercury CCD area detector (Mo ˚ ) for 1 and on a Siemens Smart CCD Ka; k = 0.71073 A ˚ ) for area-detector diffractometer (MoKa; k = 0.71073 A 2. Empirical absorption correction was applied using the program SADABS [7]. The structures were solved using direct methods and refined by full-matrix least-squares on F2 with SHELXTL (version 5.10) [8]. Non-hydrogen atoms were refined anisotropically. The hydrogen atoms were placed at their calculated positions except for hydrogen atoms on boron, which were located and refined isotropically for both complexes. The absolute structure of 1 was determined with a Flack parameter of 0.018(10) [9]. Additional information about crystal data collection and refinement parameters for 1 and 2 is presented in Table 1, while selected bond distances and angles are listed in Tables 2 and 3, respectively.
587
Table 2 ˚ ) and angles () for Co(BttMe)2 Selected bond lengths (A Molecule I Bond lengths Co1–S1 Co1–S2 Co1–S3 Co1–S4 Co1–H1A Co1–H2A S1–C1 S2–C3 S3–C5 S4–C7 B1–H1A B1–H1B B2–H2A B2–H2B Bond angles S1–Co1–S4 S1–Co1–S3 S4–Co1–S3 S1–Co1–S2 S4–Co1–S2 S3–Co1–S2 H1A–B1–H1B H2B–B2–H2A
Molecule II 2.3175(10) 2.3773(9) 2.3570(9) 2.3315(9) 2.57(4) 2.32(4) 1.697(3) 1.702(3) 1.707(3) 1.699(3) 1.12(4) 1.20(4) 1.06(3) 1.16(4) 140.20(4) 100.94(4) 110.09(3) 107.36(4) 96.22(3) 91.72(3) 112(2) 112(3)
Chemical formula Formula weight Colour, habit Crystal size (mm) Crystal system Space group ˚) a (A ˚) b (A ˚) c (A a () b () c () ˚ 3) V (A Z F(0 0 0) Dcalc (Mg m3) ˚ )) Radiation (k(A T (K) l (mm1) Maximum and minimum transmission h range for data collection () Measured reflections Independent reflections Goodness-of-fit on F2 R1 wR2 Largest difference ˚ 3) peak/hole (e A
Co(BttMe)2
Ni(BttMe)2
C8H16B2CoN16S4 545.16 blue 0.28 · 0.25 · 0.20 orthorhombic P212121 15.451(4) 16.422(4) 17.474(4) 90 90 90 4433.7(18) 8 2216 1.633 0.71073 293(2) 1.185 0.728 and 0.789
C8H16B2N16NiS4 544.94 yellow 0.34 · 0.30 · 0.10 triclinic P 1 12.5583(3) 12.5692(3) 16.2972(3) 69.8120(10) 70.5490(10) 89.9820(10) 2257.47(9) 4 1112 1.603 0.71073 293(2) 1.263 0.644 and 0.881
1.000–28.28 36799 10982 1.048 0.0385 0.0799 0.372 and 0.319
0.987–25.02 11725 7844 1.037 0.0713 0.1559 0.472 and 0.536
S6–Co2–S7 S6–Co2–S5 S7–Co2–S5 S6–Co2–S8 S7–Co2–S8 S5–Co2–S8 H3B–B3–H3A H4B–B4–H4A
2.3503(9) 2.3208(9) 2.3204(9) 2.4186(9) 2.13(3) 2.99(4) 1.704(3) 1.703(3) 1.708(3) 1.712(3) 1.10(3) 1.09(3) 1.11(4) 1.19(4) 130.41(4) 115.35(3) 108.03(3) 95.39(3) 106.54(3) 91.58(3) 120(3) 112(3)
Table 3 ˚ ) and Angles () for Ni(BttMe)2 Selected Bond Lengths (A Molecule I
Table 1 Crystallographic data for Co(BttMe)2 and Ni(BttMe)2
Co2–S5 Co2–S6 Co2–S7 Co2–S8 Co2–H3A Co2–H4A S5–C9 S6–C11 S7–C13 S8–C15 B3–H3A B3–H3B B4–H4A B4–H4B
Bond lengths Ni1–S1 Ni1–S2 Ni1–S3 Ni1–S4 Ni1–H1A Ni1–H2A S1–C1 S2–C3 S3–C5 S4–C7 B1–H1A B1–H1B B2–H2A B2–H2B Bond angles S4–Ni1–S1 S4–Ni1–S3 S1–Ni1–S3 S4–Ni1–S2 S1–Ni1–S2 S3–Ni1–S2 S4–Ni1–H1A S1–Ni1–H1A S3–Ni1–H1A S2–Ni1–H1A S4–Ni1–H2A S1–Ni1–H2A S3–Ni1–H2A S2–Ni1–H2A H1A–Ni1–H2A H1A–B1–H1B H2A–B2–H2B
Molecule II 2.379(3) 2.399(3) 2.397(3) 2.374(3) 1.98(10) 1.89(10) 1.707(9) 1.691(9) 1.674(9) 1.705(9) 1.09(10) 1.13(10) 1.14(10) 1.11(10) 93.19(9) 94.80(10) 91.62(9) 91.62(10) 95.41(10) 170.20(11) 173(3) 94(3) 84(3) 89(3) 93(3) 174(3) 88(3) 84(3) 80(4) 113(7) 113(7)
Ni2–S5 Ni2–S6 Ni2–S7 Ni2–S8 Ni2–H3A Ni2–H4A S5–C9 S6–C11 S7–C13 S8–C15 B3–H3A B3–H3B B4–H4A B4–H4B S5–Ni2–S8 S5–Ni2–S6 S8–Ni2–S6 S5–Ni2–S7 S8–Ni2–S7 S6–Ni2–S7 S5–Ni2–H3A S8–Ni2–H3A S6–Ni2–H3A S7–Ni2–H3A S5–Ni2–H4A S8–Ni2–H4A S6–Ni2–H4A S7–Ni2–H4A H3A–Ni2–H4A H3A–B3–H3B H4A–B4–H4B
2.375(3) 2.394(3) 2.404(3) 2.377(3) 1.87(9) 1.93(6) 1.699(9) 1.677(9) 1.695(9) 1.699(9) 1.20(9) 1.22(9) 1.10(10) 1.06(10) 93.28(10) 95.51(10) 91.72(9) 91.77(10) 94.69(10) 170.00(11) 94(3) 172(3) 89(3) 84(3) 174(2) 92(2) 84(3) 88(3) 80(4) 111(6) 117(7)
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3. Results and discussion The procedure of preparing the sodium salt of the dihydrobis(1-methyl-5-thiotetrazolyl)borato anion [BttMe] is similar to that of preparing Na(BmMe) (BmMe = dihydrobis(thioxoimidazolyl)borato) (Scheme 2) [3a]. Reaction of Na(BttMe) with anhydrous CoCl2 and Ni(NO3)2 Æ 6H2O (molar ratio 2:1) affords two novel mononuclear complexes Co(BttMe)2 (1) and Ni(BttMe)2 (2), respectively. Both complexes are soluble in dimethylsulfoxide, acetonitrile and dichloromethane, but insoluble in methanol.
3.1. Crystal structures X-ray diffraction shows the molecular structures in Figs. 1 and 2. Interestingly, two crystallographically independent molecules are found in the asymmetric unit of 1 and 2, which are similar in their configuration. The geometrical parameters of the two independent molecules in the asymmetric unit of 1 are somewhat different from each other (Table 2). The Co–S bond ˚ (Co1) lengths vary from 2.3175(10) to 2.3773(9) A ˚ ˚ (av. 2.3458 A) and 2.3204(9) to 2.4186(9) A (Co2) ˚ ), and the S–Co–S bond angles range (av. 2.3525 A from 91.72(3) to 140.20(4) (Co1) and 95.39(3) to 130.41(4) (Co2). It is noteworthy that 1 has a larger range of S–Co–S bond angles compared with that in previously reported M(Bm)2 complexes (M = Zn, Cd, Hg) (85–124), whose coordination geometry around the metal center are slightly distorted tetrahedral [3a]. At a first glance, 1 exhibits a severely distorted tetrahedral geometry in the solid state with a central [CoS4] core in which the cobalt atom is bound by two BttMe ligands through four S atoms. The Co1– ˚ ) is much longer compared S2 bond length (2.3773(9) A with the other Co–S distances, and the bond angles S2–Co1–S3, S2–Co1–S4 and S2–Co1–S1 are 91.72(3), 96.22(3) and 107.36(4), respectively, which are close to 90. In addition, the S2–Co1–H2A angle
Fig. 1. The ORTEP drawing of molecule I of Co(BttMe)2 with 60% thermal ellipsoids.
Fig. 2. The ORTEP drawing of molecule I of Ni(BttMe)2 with 50% thermal ellipsoids.
Me N Me N NaBH4
+
2
N
N S
N
N H
N
N THF/N2
S
H Na
B
rflux for overnight H
S
N N N Scheme 2.
N Me
Y.-L. Wang et al. / Polyhedron 24 (2005) 585–591
(172.0) is close to the theoretical value of 180 for a trigonal bipyramid. These parameters indicate that the overall geometry around the Co1 atom should be best considered as a distorted trigonal bipyramid, if the very weak Co1 H2A–B2 interaction is taken into account. Interestingly, if the other weaker Co1 H1A– B1 interaction is further considered, the coordination geometry around Co1 can also be described as highly distorted octahedral. The coordination geometry around the Co2 atom is somewhat similar to that around the Co1 atom, however, the long distance of ˚ ) indicates no interaction between Co2–H4A (2.99(4) A Co2 and H4A. Hence, only a trigonal-bipyramidal coordination geometry of Co2 should be considered, with the axial bond angle S8–Co2–H3A being 173.6, and the bond angles S8–Co2–S5, S8–Co2–S6 and S8–Co2–S7 being 91.58(3), 95.39(3) and 106.54(3), respectively. The Co–H distances are 2.57(4) (Co1–H1), 2.32(4) (Co1–H2), 2.13(3) (Co2– ˚ , respectively, which are H3) and 2.99(4) (Co2–H4) A much longer than that found in other borohydrido Co derivatives such as Co(TmPh)2 [Co– Me ˚ ], Co(pzBm )2 [(Co–H) = 1.95 A ˚ ] and H = 2.11(3) A ˚ ] (Bp = dihydrobis(pyrCo(BpBut,Pri)2 [(Co–H) = 1.95 A azoly)borato) [10–12]. In the crystal structure of complex 2, the geometrical parameters of the two independent molecules in the asymmetric unit are almost identical within experimental error. The most notable feature of 2 is that it contains a central [NiS4H2] core, and the coordination geometry around the Ni atom is described as distorted octahedral, surrounded by four S atoms and two H atoms from two BttMe ligands with two relatively strong Ni H–B interactions in which the two borohydrido groups are in a cis arrangement. From this viewpoint, the BttMe ligand acts as an S, S, H tripod. The structure of 2 is similar to that of the closely related nickel complexes Ni(Bm)2 and Ni(Bt)2 [3b,5]. The Ni–H distances are 1.98(10) (Ni1–H1), 1.89(10) (Ni1–H2), 1.87(9) ˚ , which are somewhat (Ni2–H3) and 1.93(6) (Ni2–H4) A ˚) longer than that found in Ni(Bm)2 (Ni–H = 1.863 A ˚ ) [3b,5]. The Ni–S bond and Ni(Bt)2 (av. Ni–H = 1.86 A lengths are slightly different from each other, with those trans to the borohydrido groups being slightly shorter than the axial ones. These different Ni H and Co H interactions result that the BttMe ligands in 1 adopt a boat-like eightmembered ring configuration and in 2 the BttMe ligands adopt a twist eight-membered ring configuration. 3.2. Infrared spectroscopy In the IR spectra of the ligand, two medium absorptions at 2497 and 2480 cm1 show the typical B–H stretching vibrations. For complex 1, the medium absorption at 2472 cm1 can be assigned to a terminal
589
B–H stretching vibration and two weak absorptions at 2378 and 2318 cm1 are indicative of a weak Co H– B interaction. In the infrared spectrum of 2, the corresponding bands are at 2550–2350 and 2300–2200 cm1, respectively. Compared with 1, the absorption intensity of the B–H bond at low frequency is much greater than that of the corresponding one in 1, and the result is attributable to the relatively strong Ni H–B interaction. These are in good agreement with the crystal analytical results. 3.3. Electra spectra In the UV–Vis spectra of 1 and 2 in CH3CN solution, the absorptions [k/nm e/mol1 cm1] at 240 (66 750) for 1 and 246 (60 950) for 2 display p–p* transitions for the coordinated BttMe ligands, based on the absorption at 246 (73 900) in the spectra of the uncoordinated ligands. The absorption observed at 376 (11 120) in the UV–Vis spectrum of 1 can be assigned to the charge transfer from the ligands to the Co(II) center [6] and the absorption at 565 (750) to a d–d transition [13]. The position of the 376 nm absorption is higher in energy than the band of similar intensity found in Co(PhTt)2 (380 (7400)) (PhTt = phenyltris(methylthio)methylborate) [14]. For 2, the absorption at 403 (2490) may be associated with the d–d transition [3A2g(F) ! 3T1g(P)] in an octahedral field for a d8 ion [5], with an energy which is comparable with that observed in the related complex Ni(Bt)2 (417 (1120)) [5]. 3.4. Electrochemistry Electrochemical data of Na(BttMe) and complexes 1 and 2 are summarized in Table 4. The cyclic voltammogram of Na(BttMe) in acetonitrile revealed one irreversible oxidation peak at +0.47 V and one irreversible reduction peak at –1.83 V (vs. Cp2Fe/Cp2Fe+). Figs. 3 and 4 display the cyclic voltammograms of 1 and 2, respectively. As shown in Fig. 3, a scan initiated in the negative direction reveals an irreversible reduction at
Table 4 Electrochemical data Ef (V)
Complex Me
Na(Btt
)
Co(BttMe)2
Me
Ni(Btt
)2
DEp (mV)
Ip,ox/Ip,red
+ 0.47 1.83
irrev irrev
1.56 0.23 +0.45 0.78
1.1
Irrev irrev irrev quasi-rev
0.73
irrev irrev quasi-rev
1.11 +0.45 +0.75
142
131
590
Y.-L. Wang et al. / Polyhedron 24 (2005) 585–591
4. Supplementary materials Crystallographic data for the structures are available from the Cambridge Crystallographic Data Centre (CCDC Nos. 248456 and 248278 for compounds Co(BttMe)2 and Ni(BttMe)2, respectively). These data can be obtained free of charge via www.ccdc.cam. ac.uk/data_request/cif, by emailing data_request@ccdc. cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Acknowledgements Fig. 3. Cyclic voltammogram of Co(BttMe)2 in 0.1 M TBAP-CH3CN at a glassy-carbon electrode with a scan rate of 200 mV/s.
We give thanks for the financial support from the State Key Basic Project (001CB108906) and the National Natural Science Foundation of China (20325106), the Natural Science Foundation of Fujian Province (B982003), and the ‘‘one-hundred Talent’’ Project from CAS.
References
Fig. 4. Cyclic voltammogram of Ni(BttMe)2 in 0.1 M TBAP-CH3CN at a glassy-carbon electrode with a scan rate of 200 mV/s.
1.56 V of 1. Then, an irreversible oxidation at 0.23 V of the reductive product of 1 is observed. It is noted that the irreversible oxidation at 0.45 V should be assigned to the oxidation of the coordinated BttMe ligands, for the oxidation potentials in 1 and the free ligand are nearly identical. Finally, a quasi-reversible redox process at +0.78 V is observed, which should be contributed to a metal-based quasi-reversible redox process. Compared with 1, complex 2 displays similar cyclic electrochemical behavior. An irreversible reduction of 2 at 1.11 V is observed. The irreversible oxidation of the coordinated ligands at +0.48 V is also indicative of the oxidation of the coordinated BttMe ligands. A metal-based quasi-reversible redox process at +0.75 V is also observed. However, the related complex Ni(Bm)2 exhibits irreversible oxidation [2b].
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