A capped trigonal pyramidal molybdenum hydrido complex and an unusually mild sulfur-carbon bond cleavage reaction

Article abstract of DOI:10.1039/c7cc06372e

DFT calculations reveal that a reported molybdenum hydride complex has an unprecedented geometry with the molybdenum in the base of trigonal pyramid defined by three thiolate ligands with a phosphine ligand on the apex and a hydride capping a PS₂ face. This complex reacts with methanol to produce a sulfido complex by a new reaction: the protonation of an ipso carbon of a thiolate ligand by coordinated methanol.

Full text of DOI:10.1039/c7cc06372e

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
first methanofullerene derivatives of Sc N@D5h-C80
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A capped trigonal pyramidal molybdenum hydrido complex and
an unusually mild sulfur-carbon bond cleavage reaction
Robert H. Morrisa
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
DFT calculations reveal that a reported molybdenum hydride
complex has an unprecedented geometry with the molybdenum
in the base of trigonal pyramid defined by three thiolate ligands
with a phosphine ligand on the apex and a hydride capping a PS2
face. This complex reacts with methanol to produce a sulfido
complex by a new reaction: the protonation of an ipso carbon of a
thiolate ligand by coordinated methanol.
Hydrodesulfurization (HDS) is an important step in the
upgrading of petroleum to clean fuels, usually using catalysts
based on molybdenum disulfide.1
There is theoretical
evidence for the formation of unstable molybdenum hydrides
(MoH) and molybdenum hydrosulfide (MoSH) groups on the
3
surface of the heterogeneous catalyst.2, The MoSH groups
are thought to be more stable under HDS conditions.3 The
mechanism of carbon-sulfur bond cleavage is still under
investigation. Inorganic chemists are studying the reactions of
well-defined metal complexes to establish the mechanisms of
carbon-sulfur bond cleavage.4 For example, low valent
molybdenum complexes have been shown to cleave C-H and S-
C bonds.5 The mechanisms of H2 activation at sulfido-bridged
dimolybdenum cyclopentadienyl complexes have been
investigated computationally and found to occur either
homolytically between sulfur ligands6a or heterolytically across
a Mo-S bond6b depending on the complex.
Figure 1. The cleavage of sulfur-carbon bonds under mild conditions by reaction
of MeO2H with MoH(PMePh2)(SAr)3, Ar = 2,4,6-Me3C6H2. The structure of the
dimer was determined by single crystal X-ray diffraction7, 8 and a similar PMe3
dimer (Mo2) by DFT calculation (this work). The hydrogens and all carbons except
those attached to the donor atoms have been removed for clarity. The level of
theory for this calculation and the others shown in this article is M11-L with basis
set SDD on Mo and 6-31G(d) on the other atoms with THF solvation applied using
the PCM model unless otherwise specified.
Some of us were involved in the study of homogenous,
diamagnetic molybdenum hydridoarylthiolato complexes
MoH(SAr)3(PRPh2), R = Me, Et, Ar = 2,4,6-iPr3C6H2 and 2,4,6-
Me3C6H2 that underwent a spontaneous S-C cleavage event at
(Mo2) with two bridging sulfido ligands (Figure 1).8 The Mo2
product was characterized by single crystal X-ray diffraction
but we were unable to obtain suitable crystals of the starting
hydride complex.
a sulfur-aryl bond when their THF solutions were exposed to
methanol.7,
8
When deuterated methanol (MeO2H) was
employed, monodeuterated arene was detected along with
non-deuterated free thiol and a dimeric molybdenum complex
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value agrees better with the range from experiment (79 to 87
Hz, sign not determined).
This CTP geometry explains why the complex
±
MoH(PMePh2)(SAr)3, Ar = 2,4,6-iPr3C6H2 readily reacts with
pyridine to give the complex MoH(PMePh2)(py)(SAr)3, Ar =
2,4,6-iPr3C6H2,7 now known to have
a capped trigonal
bipyramidal (CTB) geometry9 with the pyridine across from the
hydride and the phosphorus at a small angle to the hydride,
producing a 2JHP of 87 Hz (eq 1).
Figure 2. The structure of MoH(PMe3)(SAr)3 with all of the hydrogens on the ligands
and all of the carbons apart from the ipso carbons removed. Selected bond distances
(Å) and angles (°): Mo(1)-H(12) 1.65, Mo(1)-P(6) 2.37, Mo(1)-S(2) 2.34, Mo(1)-S(4) 2.36,
Mo(1)-S(8) 2.33; H(12)-Mo(1)-P(6) 54.1, H(12)-Mo(1)-S(2) 73.5, H(12)-Mo(1)-S(8) 84.6,
P(6)-Mo(1)-S(2) 115.0, P(6)-Mo(1)-S(4) 84.6, P(6)-Mo(1)-S(8) 91.3, S(2)-Mo(1)-S(4)
112.9, S(4)-Mo(1)-S(8) 121.1, S(8)-Mo(1)-S(2) 121.6.
The original proposal for the sulfur-carbon bond cleavage of
Figure 1 was that the hydride would attack the carbon;8
however this would involve isotope exchange with the hydride
The molybdenum in MoH(SAr)3(PRPh2) was proposed to be
in a trigonal bipyramidal geometry with the hydride trans to
the phosphine ligand on the basis of the observation in the
2
NMR spectra of a large JHP coupling constant ranging from 79
to 87 Hz between the hydride, detected at 3.3 ppm for one of
the compounds, and the phosphorus nuclei.7, 8 However DFT
calculations
using
a
slightly
simplified
structure
(Mo(PMe3)(H)(SAr)3, Ar = 2,6-Me2C6H3) now provide strong
evidence that the most stable isomer of these complexes has
molybdenum in an unprecedented distorted capped trigonal
pyramidal (CTP) geometry (Figure 2).
In this diamagnetic, five coordinate complex the
molybdenum sits 0.3 Å above the middle of a distorted triangle
defined by the three sulfur atoms (Figure 2). The phosphorus
is at the vertex of the distorted S3P trigonal pyramid with the
short Mo-P bond (2.37 Å) at a 78° angle to the S3 basal plane.
The hydride ligand caps one PS2 face of this CTP with a small
2
HMoP angle of 54.1°. Thus the large JHP is associated with this
small angle between these nuclei. There are no distortions of
the PMe3 ligand that would indicate bonding favouring 5-
coordinate phosphorus. The HOMO is a lone pair with
approximately (dxz)2(Mo) character, out of phase with a (pz)2
lone pair on S8 (see the SI) where the x direction is along the
Mo-S8 bond and the z direction is between the Mo-H and Mo-
P
bonds. None of the 620 five-coordinate complexes
containing a terminal hydride found in the Cambridge
Structural Database has such a CTP geometry. Two isomers
with the hydride approximately trans to the phosphorus in a
distorted trigonal bipyramidal (TBP) geometry were located as
stable minima at 10 and 13 kcal/mol higher in energy and a
distorted trigonal bipyramidal complex in the triplet state was
located at 5 kcal/mol higher in energy. Even the model
complex Mo(PMe3)(H)(SMe)3 with much smaller substituents
that was not observed and our DFT studies could not find a
suitable transition state for the hydride attack. The methanol
that is needed to trigger this cleavage reaction (Figure 1) can
easily coordinate to give a formally Mo(IV) methanol complex
as shown in eq 1. From this experimentally unobserved
methanol adduct, the C-S bond is readily cleaved in an
on the sulfurs adopts this CTP geometry.
Apparently
molybdenum is large enough to accommodate ligands cis to
each other, thus avoiding trans H-Mo-P and H-Mo-S bond
2
weakening. The JHP (unscaled) were calculated using the
Figure 3. (a) The methanol adduct MoH(PR3)(OHMe)(SAr)3 (Mo(1)-H(22) 1.66, Mo(1)-
P(13) 2.39, Mo(1)-O(15) 2.21, Mo(1)-S(2) 2.36, Mo(1)-S(4) 2.33, Mo(1)-S(17) 2.36,
O(15)-H(16) 0.97, S(4)-C(5) 1.78 Å; H(22)-Mo(1)-P(13) 52.1, H(22)-Mo(1)-O(15) 148.5,
P(13)-Mo(1)-O(15) 158.7, Mo(1)-S(4)-C(5) 113.0°); (b) the hydrogen transfer transition
state (OH…CSMo) (1635i cm-1; Mo(1)-H(22) 1.66, Mo(1)-P(13) 2.45, Mo(1)-O(15) 2.07,
Mo(1)-S(2) 2.37, Mo(1)-S(4) 2.20, Mo(1)-S(17) 2.38, O(15)-H(16) 1.21, S(4)-C(5) 2.14,
gauge-independent atomic orbital method as -70 Hz for the
CTP geometry and -31 Hz for the TBP geometry. The former
H(16)-C(5) 1.33 Å; H(22)-Mo(1)-P(13) 54.3, H(22)-Mo(1)-O(15) 146.7, P(13)-Mo(1)-O(15)
157.7, Mo(1)-S(4)-C(5) 107.1°); (c) the sulfido product MoH(S)(PR3)(OMe)(SAr)2.HAr
This journal is © The Royal Society of Chemistry 20xx
2 | J. Name., 2012, 00, 1-3
(Mo(1)-H(22) 1.65, Mo(1)-P(13) 2.56, Mo(1)-O(15) 1.93, Mo(1)-S(2) 2.40, Mo(1)-S(4)
2.14, Mo(1)-S(17) 2.42 Å; H(22)-Mo(1)-P(13) 54.8, H(22)-Mo(1)-O(15) 145.6, P(13)-
Mo(1)-O(15) 155.7° plus HAr (C(5)-H(16) 1.10 Å).
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.
Figure 4. A reaction coordinate diagram for the C-S bond cleavage reaction shown in Figure 1. Calculations were done with M11L/6-31G(d)/SDD on Mo/THF solvation.
unprecedented mechanism: the attack by the hydrogen of the
methanol ligand on the ipso carbon of an aryl (Figure 3). The established. The reaction may proceed via a singly bridged
result of this transfer is the formation of a C-H bond in the sulfido complex such as Mo2( -S)(SH)(OMe)2(PMe3)2(SAr)3
arene product, consistent with the experimental observation (Figure 6) which might eliminate a further equivalent of thiol
that deuterium from MeO2H ends up on this carbon (Figure HSAr
to give the observed dimeric product
3b). (PMe3)(MeO)(ArS)Mo( -S)2Mo(SAr)(OMe)(PMe3) (Mo2) of
The details of the subsequent steps have not been
µ
µ
The hydrogen of the methanol ligand (Figure 3a) transfers Figure 1 found at the zero energy point of the reaction
to the aryl carbon 5 via a transition state with arenium coordinate diagram, Figure 4.
character. Indeed the Mulliken positive charges on the carbons
These unusual reactions and geometries highlight the rich
of the ring other than carbon 5 increase on going from the redox chemistry of molybdenum in a sulfur environment. The
methanol adduct to the transition state. Atoms with increased chemistry has possible relevance to HDS catalysis but also the
negative charges include carbon 5, sulfur 4, hydrogen 16 and action of molybdenum enzymes.15
hydrogens on the methoxide methyl. The reaction has This project was funded by an NSERC discovery grant, a
characteristics as described for other electrophilic aromatic Killam Research Fellowship to R. H. M. from the Canada
11
substitution reactions.10,
product of this reaction is a formally Mo(VI) CTB sulfido resources from Sharcnet (www.sharcnet.ca), Scinet
The immediate metal-containing Council for the Arts, and supported by computational
complex MoH(S)(PR3)(OMe)(SAr)2 with a short Mo-S bond of (www.scinethcp.ca)
2.14 Å (Figure 3c). The metal undergoes a formal two electron (www.computecanada.ca).
oxidation in this reaction, paradoxically indicating that the
hydrogen transferred as a hydride.
and
Compute
Canada
The (OH…CSMo) transition state (1635i cm-1) is the highest
Conflicts of interest
energy point in the overall reaction shown in Figure 1 with a
transition state energy of 21 kcal/mol above the energy of the
methanol adduct MoH(PR3)(OHMe)(SAr)3 (Figure 4); the same
TS energy was obtained for gas phase and liquid phase (THF)
calculations. The use of higher basis sets on the central atoms
(6-311++G(d,p)) resulted in an energy of 22 kcal/mol and the
use of the PBE0 functional with dispersion correction D3 gave
24 kcal/mol. Little is known about the mechanism of
protonolysis of carbon-sulfur bonds.12, 13
There are no conflicts to declare.
Another interesting step in the overall reaction involves the
transition state for the formal reductive elimination of the
Mo(VI) CTB hydridosulfido complex MoH(S)(PR3)(OMe)(SAr)2
to
produce
the
Mo(IV)
hydrosulfido
complex
Mo(SH)(PR3)(OMe)(SAr)2 (Figure 5). This step has a barrier of
15 kcal/mol and is uphill thermodynamically by 7 kcal/mol but
is likely driven by subsequent dimerization steps where HSAr is
eliminated, a strongly downhill reaction (Figure 4). A similar
transition state involving hydride migration to a bridging
sulfido ligand in a formal hydride to proton conversion has
been reported.6b Molybdenum hydrido sulfido complexes can
also readily eliminate dihydrogen.14
Figure 5. (a) Transition state structure MoH..S TS (959i cm-1) for the hydride reductive
elimination step (Mo(1)-H(27) 1.77, Mo(1)-P(9) 2.59, Mo(1)-O(11) 1.92, Mo(1)-S(2)
2.35, Mo(1)-S(8) 2.23, Mo(1)-S(15) 2.35, H(27)-S(8) 1.59 Å; H(27)-Mo(1)-P(9) 66.0,
Mo(1)-H(27)-S(8) 82.7, P(9)-Mo(1)-O(11) 161.3, S(2)-Mo(1)-S(15) 119.8°, S(2)-Mo(1)-
S(8) 122.3°) to give the (b) hydrosulfido complex Mo(SH)(PR3)(OMe)(SAr)2 (Mo(1)-P(9)
2.52, Mo(1)-O(11) 1.92, Mo(1)-S(2) 2.30, Mo(1)-S(8) 2.39, Mo(1)-S(15) 2.29, H(27)-S(8)
1.34 Å; P(9)-Mo(1)-O(11) 159.3, S(2)-Mo(1)-S(15) 117.1, S(2)-Mo(1)-S(8) 120.1, S(8)-
Mo(1)-S(15) 122.8°).
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Figure 6. Structure of a possible intermediate in the reaction shown in Figure 1, a
sulfido bridged dimer Mo2(µ-S)(SH)(OMe)2(PMe3)2(SAr)3.
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