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formation of 8–11 from the reduction with the borane
reagent.[12] Particularly diagnostic for the C2 compound 11
are the 1H and 13C chemical shifts indicative of -O13CHO and
-O13CH2O- units (dH = 7.42 ppm, 1JHC = 228.9 Hz and dH =
of 4 displays a band at higher frequency (1621 cmÀ1), as
expected for a k1 coordination mode and authenticated by
labeling studies.[15,16] Previous studies have shown that 1
reversibly coordinates HBpin to produce the complex [RuH-
{(m-H)2Bpin}(h2-HBpin)(PCy3)2], with HBpin incorporated in
two different coordination modes, s-borane and dihydrobo-
rate.[17] When complexes 2 or 3 were used as catalyst
precursors (10 mol%), both complexes reacted with HBpin
to afford, before the introduction of CO2, the new dihydro-
borate complex [RuH{(m-H)2Bpin}(CO)(PCy3)2] (5), which
was independently isolated by addition of HBpin to the
carbonyl complex [RuH2(H2)(CO)(PCy3)2]. Complex 5 is
characterized in the 1H NMR spectrum by three broad signals
at d = À6.68, À8.67, and À10.04 ppm for the three different
hydrides.[18] The X-ray diffraction analysis shows that the
ruthenium center also displays two PCy3 ligands in trans
positions, the equatorial plane being occupied by a hydride, a
carbonyl, and a k2-coordinated dihydridopinacolborate ligand
1
5.39 ppm, JHC = 170.4 Hz; dC = 159.2 and 82.5 ppm, respec-
3
tively) and the observed coupling constants JHC (4.0 and
2
4.4 Hz, respectively) and JCC (2.3 Hz), which support the
connectivity between the two units.[13] Interestingly, after 22
days in the NMR tube at room temperature or five hours at
708C, the products 9–11 are cleanly transformed into 7 and
8.[12]
Because complexes 1 and 2 were never detected at the end
of the catalysis, we sought mechanistic information. Three
new complexes 3–5 and the known dicarbonyl complex
[RuH2(CO)2(PCy3)2] (6;[14] Scheme 3) were found in various
proportions depending on the catalyst loading (see below).
Complexes 3–5 were synthesized independently and fully
À
with a Ru B bond distance of 2.231(3) ꢀ (Figure 3).
Scheme 3. Complexes 3–6.
characterized by X-ray diffraction and IR and multinuclear
NMR spectroscopy.[12] The signals of the hydride ligands of 3
and 4 are detected at d = À17.76 and À4.68 ppm, respectively,
and the formate protons appear at d = 8.35 and 8.58 ppm,
respectively. The X-ray diffraction structures of 3 and 4 show
an octahedral environment with the two PCy3 ligands in trans
positions (Figure 2). In complex 3, the equatorial plane is
Figure 3. X-ray structure of complex 5 (ellipsoids at 30% probability).
Selected bond lengths [ꢁ] and angles [8]: Ru–Hy1 1.65(2), Ru–Hy2
1.64(3), Ru–Hy3 1.55 (3), B–Hy1 1.37(2), B–Hy2 1.42(3), Ru–C1
1.866(3), C1–O1 1.155(3); P1-Ru-P2 166.45(2).
As shown in Table 1 (entry 1), when 10 mol% of complex
1 were used, complex 3 (see also Figure 1) along with a very
small amount of complex 6 (< 5%) were detected after
30 min and with total consumption of HBpin. In contrast,
when 1 mol% of 1 was used, complexes 3, 5, and 6 were
observed in roughly a 1:4:1 ratio after 30 min (Table 1,
entry 2). After a longer reaction time (10 h), only complexes 4
and 6 were detected and HBpin conversion stopped at 89%
(Table 1, entry 3). Complexes 3–6 feature 13C-labeled car-
Figure 2. X-ray structures of complex 3 (left) and 4 (right; ellipsoids at
30% probability). Selected bond lengths [ꢁ] and angles [8]: (3) Ru–O1
2.2256(16), Ru–O2 2.3409(17), Ru–C2 1.811(2), Ru–Hy1 1.499(19),
C1–O1 1.266(2), C1–O2 1.258(2); P1-Ru-P2 165.41(17), O1-Ru-O2
57.71(5), O1-C1-O2 121.91(17). (4) Ru–C2 1.959(4), Ru–Hy1 1.42(3);
P1-Ru-P2 166.32(3).
Table 1: Ruthenium species resulting from the use of 1 as the catalyst
precursor under 1 atm of CO2.
Entry
1:HBpin ratio
Relative amount of formed complex [%]
3
4
5
6
occupied by a k2-formate, a carbonyl, and a hydride ligand.
The k2 coordination of the formate ligand is also evidenced by
1
2
3
1:10[a]
95
17
0
0
0
0
66
0
5
17
60
1:100[a]
1:100[b]
a band in the IR spectrum at 1559 cmÀ1 [15]
In contrast,
.
40
complex 4 exhibits a second carbonyl ligand, the formate
group now adopting a k1 coordination mode. The IR spectrum
[a] After 30 min. [b] After ten hours.
1672
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1671 –1674