Synthesis and molecular structures and oxidation catalysis of mixed alkyl, fluoroalkyl pyrazolylborate metal complexes

www.elsevier.nl/locate/ica Inorganica Chimica Acta 297 (2000) 383 – 388

Synthesis and molecular structures and oxidation catalysis of mixed alkyl, fluoroalkyl pyrazolylborate metal complexes Sergiu M. Gorun a,*, Zhengbo Hu a, Robert T. Stibrany b, Gene Carpenter a b

a Department of Chemistry, Brown Uni6ersity, Pro6idence, RI 02912, USA Corporate Research, Exxon Research and Engineering Co., Annandale, NJ 08801, USA

Received 8 June 1999; accepted 6 October 1999

Abstract First row transition metal complexes of partly fluorinated tris- and bispyrazolylborate complexes are synthesized and structurally characterized. Both the red cobalt complex, [{h3-HB(3-CF3-5-CH3pz)3}Co(h2-NO3)(NCCH3)] (1a) and the colorless manganese analogue, [{h3-HB(3-CF3-5-CH3pz)3}Mn(h2-NO3)(NCCH3)] (1b), exhibit a N4O2 donor set which defines a distorted octahedral geometry. For the blue–purple complex Cu[h2-H2B(3-CF3-5-CH3pz)2]2 (2), the N4 donor set defines a symmetry-imposed tetragonal geometry. The ligand(s) in 1 and 2 define relatively open and closed fluorine-lined cavities around their metal centers, respectively. The cavity of 1a and 1b allows additional facial coordination by small ions and neutral molecules such as NO3 − and MeCN. In contrast, the metal in 2 is in a more saturated, quasi-octahedral environment, defined by two non-bonding axial Cu···H contacts and the N4 donor set. Catalytic oxidation of cyclohexane using 1a and cumene hydroperoxide as oxygen donor yields a 1:1 mixture of cyclohexanol and cyclohexanone, and occurs 50% faster compared with the non-fluorinated analogous complex. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Fluorinated ligands; Oxidation; Pyrazolylborate complexes; Copper complexes; Manganese complexes; Cobalt complexes

1. Introduction Transition metal complexes of heterocyclic nitrogendonor ligands have been used extensively in organometallic, inorganic and bioinorganic chemistry. Bis (Bp)- and trispyrazolyl borates (Tp) have been the focus of numerous synthetic efforts [1 – 4] since they are isoelectronic with acetylacetonates and cyclopentadienyl ligands, respectively. They also mimic the histidine (imidazole) binding sites in metalloenzymes. A number of complexes with perfluoro alkyl groups in both 3- and 5-positions have been reported in open [5 – 17] and patent literature [18]. A 3-trifluoromethyl-5-(2-thienyl) Tl complex is also known [19]. Less is known about first-raw complexes of Tp ligands containing mixed alkyl (electron-releasing) and fluoro alkyls (electronwithdrawing) groups [11,20 – 25]. * Corresponding author. Tel.: + 1-401-863 2738; fax: + 1-401-863 2594. E-mail address: sergiu – [email protected] (S.M. Gorun)

Several organometallic Rh and Ir complexes have been reported, but only recently have their first-raw transition metal complexes been disclosed [17,22,23]. No metal complex of the analogous Bp ligand seems to have been reported to date. Besides a reduction in charge density at the metal center, one of the attractive key features of fluoroalkyl substitution is the creation of a fluorine lined cavity around the metal center, an enhanced stability of the cavity immediately surrounding a metal center being expected due to the replacement of CH by CF bonds. This is particularly important since intramolecular CH bond activation has been early recognized to occur in 3-alkyl substituted pyrazolyl borate complexes [26,27]. As part of a bioinspired catalytic effort aimed, inter alia, at homogeneous oxidations, we prepared representative classical coordination complexes, [{h3-HB(3-CF35-CH3pz)3}Co(h2-NO3)(NCCH3)] (1a), [{h3-HB(3-CF35-CH3pz)3}Mn(h2-NO3)(NCCH3)] (1b) and Cu[h2H2B(3-CF3-5-CH3pz)2]2 (2), of partly fluorinated pyrazolyl borates. The ligands in 1 and 2 are abbreviated TpCF3,CH3 and BpCF3,CH3, respectively.

0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S 0 0 2 0 - 1 6 9 3 ( 9 9 ) 0 0 4 3 3 - 8

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2. Experimental

2.1. Materials and methods Solvents and reagents were obtained from commercial sources and used without purification. The ligands, potassium hydrotris(3-trifluoromethyl-5-methylpyrazol-2-yl)borate, KTpCF3,CH3, and potassium dihydrobis(3 - trifluoromethyl - 5 - methylpyrazol - 2 - yl)borate, KBpCF3,CH3 were synthesized following literature procedures [20,21] from 3-trifluoromethyl-5-methylpyrazole and potassium tetrahydroborate, except for the last purification step. The crude material was dissolved in chloroform, filtered, the solvent removed, and the resulting solid further purified by trituration with hexanes. The 1H, 19F, 13C and 11B NMR spectra were identical with those reported previously [20,21]. Caution: on one occasion sublimation of the unreacted pyrazole resulted in explosive gas evolution. Oxidations were carried out in acetonitrile. In a typical experiment 0.074 mmol of a metal complex was dissolved in 2 ml of solvent. This solution was added under N2 to a mixture of 2 ml of 80% cumene hydroperoxide, 5 ml of cyclohexane and 40 mg of solid NaHCO3 while stirring vigorously. The reaction progress was monitored by GC.

2.2. Preparation of metal complexes and their X-ray quality crystals [TpCF3,CH3Co(NO3)(NCCH3)] (1a), was prepared by adding 60 mg (0.12 mol) of solid KTpCF3,CH3 to a 10 ml solution of 40 mg Co(NO3)2·6H2O (0.14 mol) in acetonitrile, stirring for 10 min, filtering and evaporating the filtrate. Red crystals of 1a formed overnight (31 mg, 42% yield). 1H NMR (25°C CD3CN, TMS): 86 (1H, BH), 48.4 (3H, 4-CH), 37.5 (9H, 5-CH3), − 31 (3H, coordinated MeCN). 19F NMR (25°C, CD3CN, hexafluorobenzene): − 39.48. The pyrazolyl rings seem to be magnetically equivalent. Anal. Calc. for C17H16BN8O3F9Co: C, 32.87; H, 2.60; N, 18.04. Found: C, 33.01; H, 2.52; N, 18.00%. MS (DC1): 580.04, 1a less CH3CN. IR (KBr, cm − 1): n(BH) =2574, n(CN) = 2291. [TpCF3,CH3Mn(NO3)(NCCH3)] (1b), was prepared via a similar procedure, using the corresponding Mn salt. Anal. Calc. for C17H16BN8O3F9 Mn: C, 33.08; H, 2.61; N, 18.16. Found: C, 32.96; H, 2.73; N, 18.04%. IR (KBr, cm − 1): n(BH)= 2575, n(CN) =2286. In both cases X-ray quality crystals were obtained by the slow evaporation of their acetonitrile solutions in air. [Cu(BpCF3,CH3)2] (2), was obtained by adding 1 g (2.86 mmol) of solid KBpCF3,CH3 to a solution of 5 ml methyl formate and 30 ml of acetone containing 0.526 g (2.86 mol) of Cu(ClO4)2·6H2O. (Caution: perchlorate

salts are potentially explosive). After 20 min of stirring, 5 ml of toluene was added and the solution was filtered yielding, via slow evaporation of the solvents, 0.311 g (16% yield) of blue–purple X-ray quality crystals. Anal. Calc. for C20H20B2N8F12Cu: C, 35.04; H, 2.94; N, 16.35. Found: C, 34.29; H, 2.89; N, 15.94%. MS (EI, high-resolution). Calc.: 685.110. Found: 685.1083. IR (KBr, cm − 1): n(BH)= 2502, 2417, 2342. [TpCH3,CH3Co(NO3)] (3) the non-fluorinated version of 1a, but lacking the coordinated acetonitrile in solid state was also prepared1.

2.3. X-ray structural analyses 1a. A red, rectangular parallelepiped crystal, 0.6× 0.6× 0.7 mm, was sealed inside a thin-walled glass capillary and Mo Ka radiation was employed to collect a total of 5679 independent reflections having 2uB 55.0. Space group at 209 1°C: monoclinic P21/n [an alternate setting of P21/c (No. 14)]: a=15.703(3), b= 8.730(2), c= 19.792(4) A, , b= 114.46(2)°; V=2470(1) A, 3; Dcalc = 1.670 g cm − 3 and Z= 4. The structure was solved using the direct methods techniques of the SHELXTL PLUS package (Siemens) and refined using counter-weighted full-matrix least-squares techniques and a structural model, which incorporated anisotropic thermal parameters for all non-hydrogen atoms and isotropic thermal parameters for all hydrogen atoms. The four methyl groups were included in the structural model as rigid rotors with sp3-hybridized geometry and a CH bond length of 0.96 A, . The refined positions for the rigid rotor methyl groups gave CCH angles, which ranged from 104 to 116°. All three trifluoromethyl groups showed displacements roughly normal to the CF bonds. One of these groups was refined as two parts with site occupancy factors of 0.68 and 0.32. The BH proton was located from a difference Fourier map and refined isotropically. The remaining hydrogen atoms were included in the structure factor calculations as idealized atoms (assuming sp2-hybridization of the carbon atoms and a CH bond length of 0.96 A, ) ‘riding’ on their respective carbon atoms. The resulting structural parameters have been refined to convergence to R1 (based on F)=0.040 for 3456 independent reflections with I\ 3s(I).

1 Complex 3 was prepared by a method similar to that employed for 1a. Elemental analysis for TpCH3,CH3Co(NO3), C15H22BN7O3Co. Calc: C, 43.08; H, 5.30; N, 23.45. Found: C, 42.76; H, 5.19; N, 23.28%. Solutions of 3 are red and exhibit NMR peaks (25°C, CD3CN, TMS) at −61.0 (9H, 3-CH3), 56.4 (3H, 4-CH), 39.2 (9H, 5-CH3) consistent with 1 and TpH,iPrCo(NO3) [28]. The spectrum is quite distinct from that of the ‘bis – tris’ complex [Co(TpiPr,iPr)2 [29– 32] which isomerizes in solution [29], and from that of Co(TpCH3,CH3)2 which does not [30 – 32]. These data suggest similar coordination for 1a and 3 in solution.

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1b. A colorless prism (0.12 × 0.33 × 0.34 mm) was mounted on a quartz fiber and examined using a Siemens P4 diffractometer equipped with a CCD area detector. Space group at 2091°C; P21/c, a= 15.9615(2), b=8.8590(2), c= 19.7575(3) A, , b= 112.92(2)°, V=2573.29(5) A, 3 and Z =4. Data reduction and correction were performed using the SAINT and SADABS programs, respectively. The structure was solved by direct methods using the SHELXTL 2 PC package. The refinement was performed on F and it proceeded similarly to that of 1a, with the exception of the site occupancy factors of the disordered trifluoromethyl group, which were 0.78 and 0.22. The resulting structural parameters have been refined to convergence to R1 =0.056 for 3691 independent reflections with I\ 3s(I). 2. A blue–purple, rectangular parallelepiped crystal (0.34 ×0.50×0.60 mm) was sealed inside a thin-walled glass capillary. Mo Ka radiation was employed to collect a total of 3305 independent reflections having 2u B 55.0°. Space group at 20 9 1°C: P21/n with a= 8.395(2), b=16.016(4), c= 11.110(3) A, , b = 105.01(2)°, V = 1442.9(6) A, 3, and Z = 2. The structure was solved and refined using the SHELXTL PLUS package (Siemens), as described for 1a. All non-hydrogen atoms were refined anisotropically. The BH protons were located from a difference Fourier map and refined isotropically. The remaining hydrogen atoms were treated as those of 1a. The structure converged to R1 (based on F)=0.039 for 1926 independent reflections having I \3s(I).

3. Results and discussion

3.1. Synthesis and characterization of KTp CF3 ,CH3 and KBp CF3 ,CH3 ligands The bioinorganic chemistry of mixed alkyl, fluoroalkyl ligands is largely unexplored despite the fact that they have been known for more than 10 years [20]. These ligands are expected to be stable, even when compared with the non-fluorinated ones, as suggested by the relative stability of the Rh complex of TpCF3,CH3 versus its TpCH3,CH3 counterpart [20,21]. However, perfluorination may not be beneficial. For example,

Scheme 1.

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TpCF3,CF3 does not react with either RhCl3·3H2O or RhCl3·3(NCCH3), while TpCF3,CH3 does react with the latter [17]. Owing to their stability, the potassium salts of the ligands can indeed be easily purified in air via fractional crystallization. 1H, 19F and 13C NMR data confirm the formation of NB bonds and the integrity of the substituted pyrazole rings. We find, however, that 11B NMR is the best tool for distinguishing the bisfrom the tris-fluorinated pyrazolyl borates and both from residual borates. The B signal is easily observable using a quartz NMR tube and its splitting pattern reveals directly the degree of substitution of the boron center. Thus, for the tris salt, KTpCF3,CH3, the 11B NMR signal (in CD3CN, versus B(OH)3 at 19.6 ppm) is a doublet at 2.66 and 3.79 ppm (1JBH = 108 Hz) while for the bis analogue, KBpCF3,CH3, the 11B NMR signal is a triplet centered at 6.9 ppm (1JBH = 96 Hz). The H signal of the BH protons, on the other hand, is less informative being usually broad due to quadrupolar relaxation effect of boron.

3.2. Synthesis and characterization of [Tp CF3 ,CH3 (NO3)(NCCH3)], M= Co(II), Mn(II), and [Cu(Bp CF3 ,CH3)2] complexes The syntheses of 1 and 2 metal complexes via metathesis, as shown above, is straightforward and can be easily extended to other metals. Purification via crystallization usually yields crystals of X-ray quality. The solid state ligand-induced structural properties of metal complexes are of interest. In principle, mononuclear h3-and tetragonal, h2-bis type complexes, could adapt topologies shown schematically in Scheme 1 (CF3 groups are depicted as spheres, M is Co or Mn). Only the a- and c-type coordination shown below are expected for mononuclear TpCF3,CH3 and square planar BpCF3, CH3 complexes, respectively, in the absence of additional bulky ligands. The b-type square planar coordination is less likely to occur due to severe H-(B) repulsions. The single crystal X-ray structure of 1a and 1b (Fig. 1) shows that both Mn and Co adopt the ‘a’ topology in solid state. The metal coordination geometry is distorted octahedral. The ligand is facially, h3-coordinated (one apical N), the other three coordination sites being occupied by a bidentate nitrate and an acetonitrile molecule, thus yielding a metal–N4O2 chromophore. The departure from a regular octahedron geometry in 1a and 1b is due primarily to the small, 5992° bite angle of the nitrate ion, which leads to trans oxygen–metal–nitrogen angles as low as 158 and 156° for M= Co and Mn, respectively. Tables listing full bond lengths and angles are given in the supplementary material. The solid state h3-coordination of TpCF3,CH3 is, so far, uncommon for transition metals [24,25,33], probably due to lack of complexes

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Fig. 1. The structure of 1 (ORTEP, 50% probability). One CF3 group is rotationally disordered over two orientations: the major one (solid CF bonds) is specified by F(21), F(22) and F(23); the minor one (open CF bonds) is specified by F(21)%, F(22)% and F(23)%. Selected bond distances (A, ) and angles (°) for complexes 1a (M= Co) and 1b (M= Mn), first and second values, respectively: MN(12)= 2.186(3), 2.300(3); MO(1)= 2.138(3), 2.223(3); MN(22)=2.112(3), 2.232(3); MO(2)=2.143(3), 2.270(3); MN(32)=2.111(3), 2.224(3); MN(1)= 2.142(3), 2.280(4); O(1)MO(2)= 60.1(1), 57.1(1); N(12)MN(22)= 86.8(1), 84.2(1); O(1)MN(12)=94.2(1), 98.2(1); N(12)M N(32)=85.0(1), 82.7(1); O(1)MN(32)=109.0(1), 113.4(1); N(22) MN(32)=92.6(1), 90.4(1); O(1)MN(1)=85.6(1), 86.2(1); N(22) MN(1)=96.0(1), 95.2(1); O(2)MN(12)=98.6(1), 100.3(1); N(32) MN(1)= 88.5(1), 87.69(12); O(2)MN(22)= 98.3(1), 99.2(1); O(1) MM(22)= 158.3(1), 156.2(1); O(2)MN(1)=87.3(1), 89.3(1); O(2) MN(32)= 168.6(1), 170.2(1); M(12)MN(1)= 173.1(1); 170.4(1).

rather than any special property of this ligand, although it should be noted that both chelating [24,33] and dinucleating [33] coordination modes are observed for Cu(I). In addition, a h2-coordination mode is observed for [TpCF3,CH3RhL] complexes (in solid state [21], L= (CO)2 and in solution [22], L= COD). For TpCF3,CH3Ir(CO)(C2H4) the h3-coordination mode appears to occur only in solution [20,21]. The TpCF3,CH3 ligand is ordered in both 1a and 1b, with the exception of one rotationally disordered CF3 group located between the nitrate and acetonitrile. The MN and MO bond distances (see Fig. 1) are normal, and comparable with those encountered in the only t other relevant pyrazolyl borate nitrate, (h3-Tp Bu)M(h2NO3), M=Co (five-coordinated) [34]. No other Mn/ Tp/NO3 complexes seem to have been structurally characterized thus far [36]. It is interesting to note that in the case of the tBu substituted ligand, the NO3 coordination is asymmetric for Co, but symmetric for the Cu and Ni analogues. The non-hindered TpH,HZn(NO3) complex, on the other hand, exhibits symmetric NO3 binding [35]. In the absence of solid-

state structural information for a possible Tp Bu Co(NO3)(NCCH3) complex, it is difficult to separate electronic from steric effects, but the relatively less sterically demanding CF3 group (compared with tBu) and, perhaps, its electron withdrawing effects seem to favor a symmetrical bidentate coordination mode of NO3 for both metals. The paramagnetically shifted 1H NMR spectrum of 1a suggests that the solid state geometry is largely maintained in solution. The 6.25 (4-H) and 2.25 (5CH3) ppm 1H resonances observed in the spectrum of KTpCF3,CH3 shift to 48.4 and 37.5 ppm, respectively, upon replacement of K by Co. Thus, in the Co and Mn cases, the presence of CF3 does not impose a severe steric hindrance on the metal complex sufficient to enforce a coordination number lower than six. Consequently, the fluorinated cavity created by the CF3 groups around the metal centers can accommodate additional small anionic or neutral substrates, both in solid state and in solution. The single crystal X-ray structure of complex 2, shown in Fig. 2, reveals an almost square–planar geometry around the Cu, which is located on a crystallographically imposed inversion center. A similar geometry has been observed [37] for the perfluoromethyl analogue of 2. The NCuN angles deviate by only 93° from 90° since the ligand ‘bite’ angle is 87°. Two CF3 groups exhibit a rotational disorder similar to that encountered in 1a and 1b. The symmet-

Fig. 2. The structure of 2 (ORTEP, 50% probability). One of the CF3 groups appears to have two alternate orientations in the lattice. The major (85%) orientation for the fluorine atoms is specified by atoms F(21), F(22) and F(23) (represented by solid CF bonds) while the minor (15%) one is specified by F(24), F(25) and F(26) (represented by open CF bonds). Selected bond distances (A, ) and angles (°): CuN(12)= 2.013(3); CuN(22)= 2.004(3); Cu···H(2b) =2.54(3); N(12)CuN(22)=87.0(1); N(22)CuN(12%)=93.0(1).

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ric puckering in 2 seems to result in a relatively open coordination environment around the metal center (see Scheme 1(c)), compared with the more crowded one (on one side of the complex), depicted in Scheme 1(b). However, the six-membered copper metallacycle, boatlike, conformation facilitates two long, non-bonding Cu···H(B) interactions of 2.54(3) A, . These interactions manifest themselves in the low frequency n(BH) stretching values of 2417 and 2342 cm − 1 [38]. Thus, the Cu could be viewed as pseudo-octahedrally coordinated, with an axial H···Cu···H vector deviation of approximately 27° from the perpendicular to the CuN4 plane. In the absence of solution NMR data for paramagnetic 2, its coordination geometry and interesting purple color were quantified via electronic spectroscopy. Absorptions at 535 nm (o = 50) and 360 nm (o =880) observed in acetone solutions are tentatively assigned to the allowed d–d (2B1g “ 2B2g) and forbidden d–p* (2B1g “ 2A2g) transitions, respectively. These assignments suggest that the tetragonal geometry is preserved in solution, in contrast with the bis, Cu(II)(TpCF3,CH3)2 complex, which dissociates easily in acetonitrile [33].

3.3. Reacti6ity of [Tp CF3 ,CH3Co(NO3)(NCCH3)] (1a) The presence in complex 1a of an exchangeable acetonitrile (potentially open coordination site) prompted us to explore the possibility of using 1a as an oxidation catalyst. By using cumene hydroperoxide as oxygen donor we have observed the formation of cyclohexanone (K) and cyclohexanol (A) in about 1:1 ratio at about 0.2 turnovers per hour. A maximum of 0.5 turnovers per hour is reached after 24 h. The reaction is consistent with a Haber – Weiss mechanism, in which the metal-catalyzed decomposition of hydroperoxides yields oxygen-based free radicals. The hydroperoxide decomposition is likely to occur via an inner-sphere coordination complex, as demonstrated for other hydroperoxides [39,40]. For example, a structural determination [41] of a cyclohexane oxidizing complex, Co(BPI)OBz(OO-tBu), BPI= 1,3-bis(pyridylimino) isoindoline, along with structures of other cobalt alkylperoxy complexes [42 – 44], reveals the requirement for an open coordination site for catalytic activity. Importantly, we have observed no oxidation in the absence of the catalyst (both the ligand and cobalt omitted), when KTpCF3,CH3 was used (only cobalt was omitted) or by using cobaltous nitrate (only the ligand was omitted). Thus, catalysis occurs only in the presence of the metal complex. For comparison purposes, [TpCH3,CH3Co(NO3)] (3), was also used as a catalyst. Similar K/A ratios were observed but the turnover per hour for 3 was about 50% that of 1a. Thus, the modestly enhanced catalytic activity can be attributed to the replacement of CH3 by CF3 groups.

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4. Conclusions Representative metal complexes of mixed alkyl, fluoroalkyl bis- and trispyrazolylborate ligands exhibiting fluorine-lined cavities around metal centers have been prepared and spectroscopically and structurally characterized. Additional facial coordination of small anionic and neutral molecules is demonstrated for mononuclear trispyrazolylborate complexes, which exhibit a relatively open topology both in solid state and solution. Catalytic oxidation of alkanes using cumene hydroperoxide as an oxygen source is observed for the cobalt complex.

5. Supplementary material Supplementary material listing of atomic fractional coordinates thermal parameters, interatomic distances and angles for 1a, 1b and 2 (11 pages) and structure factors tables are available upon request from S.M.G.

Acknowledgements C. Day (Crystallytics Co.) and B. Liang (Exxon) are thanked for some of the X-ray data and the 11B NMR spectra, respectively. S.G. Roussis (Imperial Oil, Ltd.) is thanked for the mass spectrometric analysis.

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