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B.L. Tran et al. / Inorganica Chimica Acta 447 (2016) 45–51
1.2
-
-
S
S
N
1.0
0.8
0.6
0.4
0.2
0.0
S
N
N
N
N
N
N
N
N
N
B
N
B
N
H
H
-
(TmMe
)
(Tp*)-
N
N
N
N
340
360
380
400
420
440
460
480
500
N
O
S
N
Wavelength (nm)
N
N
2.0
1.5
1.0
0.5
0.0
H
H
(L10O)-
Scheme 1. Ligands referred to in this study.
(L3S)-
[12,14,16,18]. Finally, we demonstrated a metal ion dependence
between Mo(VI) and W(VI) on both isomerization kinetics and
OAT reactivity [15,26]. Herein, utilizing complexes of [(L10O)
MoO2Cl] and [(L3S)MoO2Cl], we explore the effect of solvent on
OAT reactivity along with the detection of [(L10O)MoOCl(OPPh3)]
and [(L3S)MoOCl(OPPh3)] intermediates by mass spectrometry
and 31P NMR spectroscopy. Moreover, utilizing 31P NMR spec-
troscopy, we have monitored the ability of these complexes to
mediate catalytic OAT between DMSO and triphenylphosphine.
340
360
380
400
420
440
460
480
500
Wavelength (nm)
Fig. 1. Plot shows rapid formation of trans-[(L10O)MoOCl(OPPh3)] indicated by the
increase at 400 nm in acetonitrile (top). After several minutes, decomposition of the
intermediate occurs with isosbestic point at 370 nm.
2. Results
2.1. trans-[(L10O)MoO2Cl]
was confirmed by ESI-MS recorded in positive mode in acetonitrile
which revealed a cluster centered at 812.8 m/z, (Fig. 2) consistent
with the simulated isotope pattern expected for the bridging inter-
mediate (Fig. S1). Furthermore, monitoring the OAT reaction in
benzene by 31P NMR revealed three peaks, assignable to PPh3
(ꢁ4 ppm), OPPh3 (26 ppm), and the [(L10O)MoOCl(OPPh3)] at
51 ppm. The subsequent exponential decay of the 400 nm band
represents the dissociation of the OPPh3 product from the primary
coordination sphere. This is consistent with the solvent assisted
dissociative interchange (Id) mechanism proposed for a related sys-
tem [25].
Isolation of the pure trans isomer of [(L10O)MoO2Cl] remains
difficult due to its rapid conversion to the thermodynamically
more stable cis isomer under a variety of conditions. The mecha-
nism of isomerization at the metal center is proposed to occur
via a trigonal twist mechanism as the bromide–chloride exchange
was not observed by mass spectrometry [16]. Optimal synthetic
efforts have yielded a product with an 11:1 trans/cis ratio based
on proton NMR [15].
Examination of the OAT reactivity of trans-[(L10O)MoO2Cl] in
acetonitrile, DMF, benzene, and pyridine under pseudo-first order
conditions with 50-fold excess of phosphine was monitored by
UV–Vis spectroscopy. On mixing of acetonitrile solution of trans-
[(L10O)MoO2Cl] and phosphine, the optical spectra depicted in
Fig. 1 (top) showed a rapid increase of the band at 400 nm and
simultaneous appearance of the weak, broad, d–d transition band
in the 600–800 nm region (data not shown) as the direct result
of the two electron reduction of Mo(VI) to Mo(IV). This reaction
proceeded rapidly leading to a bright green solution. After several
minutes in acetonitrile and as shown in Fig. 1 (bottom), the band at
400 nm decreased exponentially with concomitant growth of a
new peak at 360 nm with an isosbestic point at 370 nm. The d–d
band at 600–800 nm also decreased slightly in intensity. These
UV–Vis spectral features are identical to those reported during
the OAT chemistry of the related, [TmMeMoO2Cl]. The rapid growth
of the 400 nm peak corresponds to the formation of the intermedi-
ate [(L10O)MoOCl(OPPh3)]. The assignment of 400 nm peak
observed in the UV–Vis as being due to the phosphine adduct
The effect of pyridine on the reaction run in DMF or acetonitrile
was probed through UV–Vis spectroscopy by allowing steady-state
formation of the phosphine adduct to age for ꢂ10 min followed by
addition of an aliquot of pyridine. Introduction of pyridine resulted
in the more rapid decay of the 400 nm shoulder with concomitant
growth of bands at 500 nm and in the 600–800 nm region. The
final spectrum is identical to the previously isolated and character-
ized trans-[(L10O)MoOCl(py)]. Probing the same sample via ESI
mass spectrometry also detected [(L10O)MoOCl(py)] at 613 m/z
(Fig. 3) confirmed by the simulated isotope pattern (data not
shown). When the OAT was performed in pure pyridine, rapid for-
mation of the [(L10O)MoOCl(OPPh3)] intermediate was again wit-
nessed at 400 nm along with the rise of the broad Mo(IV) d–d
transition band at 600–800 nm (Fig. 4). The stability of phosphine
bound intermediate in pyridine is much less than that in DMF, ace-
tonitrile, and benzene however as indicated by the near simultane-
ous appearance of
a shoulder at 500 nm corresponding to
formation of both the trans-[(L10O)MoOCl(py)] complex and the