Full Paper
HMBC NMR spectrum. The resonance at d=185 ppm is strong-
ly enhanced when 13CO2 is used (Figure 5, inset A), suggesting
incorporation of the CO2 carbon atom. Moreover, it indicates
that the resonance at d=185 ppm is not a PMI ligand reso-
1
nance. Interestingly, the H,13C HMBC NMR spectrum revealed
that the methyl protons are also coupled to a 13C NMR reso-
nance at d=75 ppm, which is an uncommon shift for a PMI
ligand.
This 13C NMR resonance is split into a doublet with a cou-
pling constant of 55 Hz in the 13C labeled compound (Figure 5,
inset B), which is consistent with coupling to the carbon atom
1
in 13CO2 and is in the range of JC,C coupling constants for sub-
stituted carbonic acid/ester (RO(O=)CR’).[22] Furthermore the
1H NMR resonance at d=1.14 ppm is split into a doublet with
a coupling constant of 3 Hz in the labeled complex (see the
Supporting Information, Figure S29), which is not uncommon
for a 3-bond C,H coupling.[22]
Figure 6. ORTEP representation of the molecular structure of
“[(iPr PhPMI)Mo(CO)3(CO2)]K”, with ellipsoids shown at the 50% probability
2
level. Hydrogen atoms and the co-crystallized, non-coordinated THF mole-
cule are omitted for clarity. Selected bond length () and angles (8): Mo1ÀN1
2.365(3), Mo1ÀN2 2.272(3), Mo1ÀC20 1.948(4), Mo1ÀC21 1.910(4), Mo1ÀC22
1.933(4), Mo1ÀO4 2.283(2), C20ÀO1 1.173(5), C21ÀO2 1.181(4), C22ÀO3
1.178(5), C23ÀO4 1.273(4), C23ÀO5 1.230(4), O4ÀK1 2.834(2), O5ÀK1 2.748(3),
N1ÀC2 1.527(5), C2ÀC3 1.523(5), C2ÀC23 1.572(5); N1-Mo1-C21 164.17(13),
N2-Mo1-C20 173.64(13), N1-Mo1-N2 73.76(10), N2-Mo1-C21 101.81(13), N1-
Mo1-C20 100.23(14), C20-Mo1-C21 84.54(16), C22-Mo1-O4 175.56(13), N1-
Mo1-C22 110.25(12), C21-Mo1-C22 85.17(15), O4-Mo1-C21 96.60(13), N1-
Mo1-O4 67.77(9), Mo1-C21-O2 177.5(3), Mo1-C20-O1 176.0(4), Mo1-C22-O3
176.1(3), Mo1-O4-C23 116.9(2), O4-C23-O5 125.8(3), C2-C23-O4 114.8(3), C2-
C23-O5 119.4(3), C1-C2-N1 114.4(3), C1-C2-C3 114.0(3), C1-C2-C23 111.3(3),
N1-C2-C3 109.3(3), N1-C2-C23 104.1(3), C3-C2-C23 102.5(3), C8-N1-C2
115.3(3), Mo1-N1-C8 137.2(2), Mo1-N1-C2 101.51(19), O4-K1-O5 47.02(7).
Taken together, the findings from the NMR spectroscopic
investigation of the product obtained by treating
[(iPr PhPMI)Mo(CO)3]2À with CO2 are consistent with the forma-
2
tion of a CO2 adduct in which the CO2 carbon atom binds to
the imine carbon atom of the reduced complex and is thus for-
mulated as [(iPr PhPMI)Mo(CO)3(CO2)]2À (Scheme 1).
2
spectroscopy either underwent an oxidative process to form
a [(iPr PhPMI)Mo(CO)3(CO2)]À 19 eÀ radical monoanion or, more
2
likely, that the charge was compensated by a proton instead of
a potassium cation. The most likely protonation site would be
the former imine nitrogen atom. Accordingly, refining a hydro-
gen atom on the former imine nitrogen atom N1 resulted in
a stable refinement with almost equal R-values and bonding
parameters than without the H atom. Since the presence of
a hydrogen atom is not provable by X-ray diffraction alone,
the data without the hydrogen atom are presented below and
briefly discussed, since the structural data provide additional
proof for the CÀC coupling that was proposed based on the
NMR data.[27] A comparison of both solutions is provided in the
Supporting Information. The central molybdenum ion exhibits
a strongly distorted octahedral coordination geometry. Note-
worthy is the long Mo1ÀN1 bond length of 2.365(3) . The mo-
lybdenum metal is also bound to one of the former CO2
oxygen atoms O4. The bond length Mo1-O4 of 2.238(2) is
also rather long and consistent with the slow exchange of the
three remaining CO ligands observed by 13C NMR spectroscopy.
The carbon atom (C23) of the former CO2 molecule is bound
to the former imine carbon atom (C2) of the PMI ligand and
a bond length of 1.572(5) is in the order of a CÀC single
bond.
Scheme 1. Proposed reaction path for the reaction of the dianion
[(iPr PhPMI)Mo(CO)3]2À with CO2.
2
Although unusual, this type of ligand-centered CO2 binding
mode has been observed before. For example, Braunstein et al.
in 1981 obtained a similar result to ours by activation of CO2 at
a square planar Pd diphenylphosphinoacetate complex.[23] Sim-
ilar binding patterns of carbon dioxide have recently been re-
ported for PNN–Ru[24] and PNP–Ru/Re[25] pincer complexes, as
well as b-diketiminate Sc[26] complexes.
Keeping the CO2 adduct in [D8]THF under an atmosphere of
CO2 causes decomposition with a half-life of about one day.
Multiple, so far unidentified products were detected following
1
this process by H NMR spectroscopy. Removal of the solvent
and the CO2 atmosphere, followed by dissolution in fresh
[D8]THF enhanced the stability of the CO2 adduct. This sample
showed significantly less decomposition when monitored by
1H NMR spectroscopy over the course of several days. Howev-
er, attempts to crystallize or isolate [(iPr PhPMI)Mo(CO)3(CO2)]2À
2
led to decomposition. Crystals that grew from a THF solution
at À358C provided strong evidence for the CÀC bond forma-
tion between the former imine carbon atom and the CO2
carbon atom (Figure 6). Only one potassium atom could be lo-
cated in the Fourier density maps, indicating that the (diamag-
The CO2 molecule is bent and, with a sum of angles of 3608,
the carbon atom C23 is sp2 hybridized. Consequentially, the CÀ
O bond lengths C23ÀO4 (1.273(4) ) and C23ÀO5 (1.230(4) )
are consistent with a delocalized C=O double bond and
netic) CO2 adduct [(iPr PhPMI)Mo(CO)3(CO2)]2À observed by NMR
2
Chem. Eur. J. 2015, 21, 8497 – 8503
8501
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim