974 Organometallics, Vol. 29, No. 4, 2010
Mutseneck et al.
decomposes in solution but can be stabilized by exchanging
-
1.26 (s, 15H; CH3), 6.24 (vt, 2H; pzH-4), 6.44 (t, 1H, 3JHH = 2.4
Hz; μ-pzH-4), 7.13-7.17 (m, 4H; PhH), 7.24-7.28 (m, 8H;
the Cl- anion for PF6
.
3
Attempts to remove the Cl- ions by addition of 4 equiv of
TlPF6 already at the initial stage of the reaction did not lead
to [(p-cym)Ru(L1)]PF6, but resulted in a mixture of other
products. The most abundant constituents of this mixture
were the dinuclear complex [(p-cym)Ru(μ-Cl)(μ-pz)2Ru-
(p-cym)]PF6 and the mononuclear species [(p-cym)Ru-
(L2)] ([L2]2- = [Ph(pz)B(μ-O)(μ-OB(Ph)O)B(pz)Ph]2-).
[(p-cym)Ru(L2)] contains a promising new [N,O,N] (pyra-
zol-1-yl)borate ligand with a phenylboroxine backbone. In a
subsequent targeted synthesis, we prepared the lithium salt
Li[Li(thf)L2] from Ph3B3O3 and 2 equiv of Lipz and success-
fully transformed it into [(p-cym)Ru(L2)] by treatment with
[(p-cym)RuCl2]2.
PhH/pzH-3 or 5), 7.51 (d, 2H, JHH = 2.4 Hz; μ-pzH-3,5),
7.61 (br, 2H; pzH-3 or 5). 11B{1H} NMR (96.3 MHz, CDCl3): δ
5.7. 13C{1H} NMR (100.6 MHz, CDCl3): δ 9.9 (CH3), 73.3
(CCH3), 106.2 (pzC-4), 109.7 (μ-pzC-4), 127.1 (PhC), 127.3
(PhC-p), 130.2 (br, pzC-3 or 5), 130.4 (μ-pzC-3,5), 132.8
(PhC), 139.2 (br, pzC-3 or 5). Anal. Calcd for C31H34B2N6ORu
[629.33]: C, 59.16; H, 5.45; N, 13.35. Found: C, 59.11; H, 5.53;
N, 13.21.
Synthesis of [Cp*Ru(L1)]PF6. A mixture of [Cp*Ru(L1)]
(82 mg, 0.13 mmol) and [Cp2Fe]PF6 (36 mg, 0.11 mmol) was
stirred in CH2Cl2 (10 mL) overnight. The solvent was evapo-
rated under vacuum to give a dark red solid, which was extracted
with pentane to remove ferrocene and excess [Cp*Ru(L1)]. The
crude product was redissolved in CH2Cl2 (3 mL) and layered
with pentane (15 mL), whereupon dark red crystals grew near
the interface of the two liquids. The crystals were suitable for
X-ray analysis. Yield: 75 mg (88%). Anal. Calcd for C31H34-
B2F6N6OPRu [774.30]: C, 48.09; H, 4.43; N, 10.85. Found: C,
48.35; H, 4.48; N, 10.99.
Our preliminary experience with [L1]- and [L2]2- indicates
the latter ligand to be less prone to hydrolysis and more
tolerant to strongly Lewis acidic transition metal complex
fragments. Since the substitution pattern of the boroxine
backbone and/or the pyrazolyl rings can easily be varied, the
straightforward synthesis procedure developed for [L2]2-
should also make a broad selection of custom-tailored
derivatives readily accessible. Moreover, [L1]- and [L2]2-
provide very similar coordination environments but differ
in their electronic charge, thereby offering another set screw
for the gradual adjustment of the properties of a chelated
metal center (cf. the electrode potential required for the
partly reversible reduction of [(p-cym)Ru(L2)], which is more
cathodic by -0.31 V than that of [(p-cym)Ru(L1)]PF6).
In summary, we suggest L1- and L2-type ligands as viable
alternatives to more conventional mixed-donor bis(pyrazol-
1-yl)borates [RB(OR0)pz2]-, because they are comparatively
easy to prepare and show a higher conformational flexibility
together with a lower tendency to substituent scrambling.
Synthesis of [(p-cym)Ru(L1)]PF6. A mixture of [Li(thf)L1] (154
mg, 0.33 mmol) and [(p-cym)RuCl2]2 (100 mg, 0.16 mmol) was
stirred in THF (5 mL) for 2 h. A yellow precipitate formed,
which was isolated by filtration, washed with Et2O, redissolved
in CH2Cl2 (5 mL), and treated with NH4PF6 (53 mg, 0.33
mmol). After 2 h, the mixture was filtered and the filtrate
concentrated under reduced pressure to a volume of 2 mL.
Addition of Et2O (20 mL) resulted in the precipitation of a
yellow solid, which was collected on a frit, washed with Et2O,
and dried under vacuum. Yield: 171 mg (69%). X-ray-quality
crystals were obtained by slow diffusion of hexane into a CH2Cl2
1
solution of the complex. H NMR (300 MHz, CDCl3): δ 0.72
(d, 6H, 3JHH = 6.9 Hz; (CH3)2CH), 1.63 (s, 3H; CH3), 1.83 (sept,
3
1H, JHH = 6.9 Hz; (CH3)2CH), 5.31, 5.54 (2 ꢀ d, 2 ꢀ 2H,
3JHH = 6.0 Hz; p-cymH), 6.31 (vt, 2H; pzH-4), 6.58 (t, 1H,
3JHH = 2.4 Hz; μ-pzH-4), 7.27-7.29, 7.39-7.41 (2 ꢀ m, 4H, 6H;
3
PhH), 7.44 (d, 2H, JHH = 2.4 Hz; pzH-3 or 5), 7.62 (d, 2H,
3JHH = 2.4 Hz; μ-pzH-3,5), 8.07 (d, 2H, 3JHH = 2.1 Hz; pzH-3
or 5). 11B{1H} NMR (96.3 MHz, CDCl3): δ 5.9. 13C{1H} NMR
(100.6 MHz, CDCl3): δ 18.2 (CH3), 21.8 ((CH3)2CH), 30.6
((CH3)2CH), 81.3, 81.9, 101.1, 106.1 (p-cymC), 109.2 (pzC-4),
111.2 (μ-pzC-4), 128.1 (PhC), 129.0 (PhC-p), 131.8 (PhC), 132.1,
132.3 (μ-pzC-3,5/pzC-3 or 5), 143.6 (pzC-3 or 5). Anal. Calcd for
C31H33B2F6N6OPRu [773.29]: C, 48.15; H, 4.30; N, 10.87.
Found: C, 47.89; H, 4.30; N, 10.63.
Experimental Section
General Considerations. All reactions were carried out under
nitrogen using standard Schlenk techniques. All solvents were
dried and distilled prior to use. Starting materials [Cp*RuCl]4,54
PhB(NMe2)2,51 2,4,6-Ph3B3O3,52 and [Li(thf)L1]42 were pre-
pared as published in the literature. [(p-cym)Ru(μ-Cl)3-
(p-cym)]PF6 was synthesized by adapting a synthesis protocol
previously reported for the synthesis of the analogous benzene
derivative.68 Compounds [Cp2Fe]PF6 and [(p-cym)RuCl2]2 were
Synthesis of [(p-cym)Ru(μ-Cl)3Ru(p-cym)]PF6. A mixture of
[(p-cym)RuCl2]2 (100 mg, 0.16 mmol) and TlPF6 (230 mg, 0.65
mmol) in THF (15 mL) was stirred for 2 h. TlCl was removed by
filtration, the filtrate was evaporated to dryness under reduced
pressure, and the orange solid residue was carefully dried. Yield:
1
obtained from commercial sources. H, 13C{1H}, and 11B{1H}
NMR spectra were recorded with Bruker AMX 300 or Avance
400 spectrometers at room temperature. Abbreviations: s =
singlet, d = doublet, t = triplet, sept = septet, vt = virtual
triplet, dd = doublet of doublets, m = multiplet, br = broad;
pz = pyrazolide, Tos = tosylate; Tfl = triflate; p-cym =
p-cymene. Cyclic voltammograms were recorded using an
EG&G Princeton Applied Research 263A potentiostat. UV/
vis spectra were recorded on a Varian Cary 50 UV/vis spectro-
photometer. For spectroelectrochemical measurements, the
spectrometer was equipped with a Hellma 661.500 quartz
immersion probe. Elemental analyses were performed by the
microanalytical laboratory of the Goethe University Frankfurt.
Synthesis of [Cp*Ru(L1)]. A mixture of [Li(thf)L1] (96 mg,
0.20 mmol) and [Cp*RuCl]4 (55 mg, 0.05 mmol) was stirred in
THF (10 mL) for 12 h. The solvent was removed under vacuum
and the orange residue extracted into hexane (20 mL). The
extract was concentrated to a volume of 3 mL under reduced
pressure and kept at -30 °C overnight, whereupon yellow-
orange crystals precipitated that were suitable for an X-ray
1
111 mg (95%). H NMR (250.0 MHz, CDCl3): 1.31 (d, 6H,
3JHH = 7.0 Hz; (CH3)2CH), 2.24 (s, 3H; CH3), 2.78 (sept, 1H,
3JHH = 7.0 Hz; (CH3)2CH), 5.48, 5.65 (2 ꢀ d, 2 ꢀ 2H, 3JHH
=
6.3 Hz; p-cymH).
Synthesis of Li(thf)([12]-c-4)[Li(thf)L2]. A mixture of Lipz
(47 mg, 0.64 mmol), Ph3B3O3 (100 mg, 0.32 mmol), and [12]-
crown-4 (113 mg, 0.64 mmol) was stirred in THF (5 mL) for
30 min. Hexane (3 mL) was added, and the solution was cooled
to -5 °C overnight. Colorless X-ray-quality crystals formed,
which were isolated on a frit, rinsed with hexane (2 ꢀ 5 mL), and
briefly dried under reduced pressure. Yield: 260 mg (95%). Since
the crystals tend to lose THF, most of the crop was kept under
vacuum for several hours in order to obtain a well-defined
sample for elemental analysis. 1H NMR (300.0 MHz, THF-
d8): δ 3.58 (s, 16H; [12]-c-4), 5.96 (vt, 2H; pzH-4), 6.80-6.87,
7.07-7.10, 7.25-7.27 (3 ꢀ m, 6H, 4H, 3H; PhH), 7.28, 7.60 (2 ꢀ
d, 2 ꢀ 2H, 3JHH = 1.8 Hz; pzH-3,5), 7.97-8.00 (m, 2H; PhH).
11B{1H} NMR (96.3 MHz, THF-d8): δ 5.5 (s, 2B), ca. 30 (h1/2
=
1
analysis. Yield: 81 mg (64%). H NMR (300 MHz, CDCl3): δ
720 Hz, 1B). 13C{1H} NMR (100.6 MHz, CDCl3): δ 71.6