Imido-hydrido complexes of Mo(IV): Catalysis and mechanistic aspects of hydroboration reactions

Article abstract of DOI:10.1039/c5dt02945g

Imido-hydrido complexes (ArN)Mo(H)(Cl)(PMe₃)₃ (1) and (ArN)Mo(H)₂(PMe₃)₃ (2) (Ar = 2,6-diisopropylphenyl) catalyse a variety of hydroboration reactions, including the rare examples of addition of HBCat to nitriles to form bis(borylated) amines RCH₂N(BCat)₂. Stoichiometric reactivity of complexes 1 and 2 with nitriles and HBCat suggest that catalytic reactions proceed via a series of agostic borylamido and borylamino complexes. For complex 1, catalysis starts with addition of nitriles across the Mo-H bond to give (ArN)Mo(Cl)(NCHR)(PMe₃)₂; whereas for complex 2 stoichiometric reactions suggest initial addition of HBCat to form the agostic complex Mo(H)₂(PMe₃)₃(η³-NAr-HBcat) (16).

Full text of DOI:10.1039/c5dt02945g

Dalton
Transactions
View Article Online
View Journal | View Issue
PAPER
Imido–hydrido complexes of Mo(IV): catalysis and
mechanistic aspects of hydroboration reactions†
Andrey Y. Khalimon,*a,b Philip M. Farhaa and Georgii I. Nikonov*a
Cite this: Dalton Trans., 2015, 44,
18945
Imido–hydrido complexes (ArN)Mo(H)(Cl)(PMe3)3 (1) and (ArN)Mo(H)2(PMe3)3 (2) (Ar = 2,6-diisopropyl-
phenyl) catalyse a variety of hydroboration reactions, including the rare examples of addition of HBCat to
nitriles to form bis(borylated) amines RCH2N(BCat)2. Stoichiometric reactivity of complexes 1 and 2 with
nitriles and HBCat suggest that catalytic reactions proceed via a series of agostic borylamido and boryl-
amino complexes. For complex 1, catalysis starts with addition of nitriles across the Mo–H bond to give
(ArN)Mo(Cl)(NvCHR)(PMe3)2; whereas for complex 2 stoichiometric reactions suggest initial addition of
HBCat to form the agostic complex Mo(H)2(PMe3)3(η3-NAr-HBcat) (16).
Received 30th July 2015,
Accepted 2nd October 2015
DOI: 10.1039/c5dt02945g
www.rsc.org/dalton
much less is known about the application of early transition
metal systems in hydroboration of carbonyl compounds,4d,8,9
Introduction
Reduction of unsaturated C–C, C–E, and E–E (where E – imines10 and nitriles.4d,11
heteroatom) bonds is one of the most fundamental reactions
We have recently reported catalytic and mechanistic studies
in synthetic organic chemistry.1,2 Historically, the hydrobora- on the hydrosilylation reactions mediated by the Mo(IV) imido–
tion reaction plays a major role in the reduction of unsaturated hydrido complex (ArN)Mo(H)(Cl)(PMe3)3 (1; Ar = 2,6-iPr2C6H3).12
organic molecules. While addition of aryl and alkyl hydrobor- Taking into account the more hydridic nature of the H–B bond
anes to multiple C–C, C–O, and C–N bonds is known to proceed in hydroboranes such as HBCat and HBPin, vs. hydrosilanes
quite easily,1,3 the use of deactivated hydroboranes such as RnSiH4−n, coupled with the increased Lewis acidity of the boron
HBCat and HBPin (Cat = catehol, Pin = pinacol), requires tran- centre vs. the silicon centre, we anticipated that the reactivity of
sition metal catalysis2b due to the significantly decreased Lewis 1 with hydroboranes would be enhanced compared to hydro-
acidity of the boron centre. Such reactions are mainly restricted silanes. And indeed, 1 has been found to catalyse a variety of
to late transition systems, such as Rh, Ir, etc.2,4
H–B addition reactions, including the first examples of hydro-
Due to the skyrocketing prices of heavy late transition borations of nitriles.13 Moreover, our preliminary mechanistic
metals and their recognised toxicity,5 the demand for cheaper studies suggested that these reactions proceed via agostic
and more environmentally benign surrogates has emerged. In B–H⋯M intermediates. Here we present further details of
this regard, early transition metals offer an appealing alterna- stoichiometric and catalytic reactions of imido–hydrido
tive as they are generally much cheaper and exhibit low toxi- complexes of Mo(IV), offering additional insights into the
city. Nevertheless, compared to the late metal systems, possible mechanism(s) of these reactions.
application of early transition complexes in hydroboration cat-
alysis is somewhat less studied.4d The reported examples of
catalytic hydroboration of unsaturated hydrocarbons by early
Results and discussion
transition metal complexes are mostly limited to Zr and Ti
systems.6 Stoichiometric hydroboration of olefins with HBCat
Complex 1 was prepared from the dichloride precursor (ArN)-
was also shown for Ta and Nb metallocene complexes.7 And
14
MoCl2(PMe3)3
according to the previously published
procedure and its spectroscopic and structural features have
been reported previously.12 Addition of an equivalent of
L-selectride to 1 selectively affords the imido–dihydrido deriva-
tive (ArN)MoH2(PMe3)3 (2, Scheme 1). 2 can be prepared from
(ArN)MoCl2(PMe3)3 directly via treatment with two equivalents
of L-selectride.
Complex 2 is unstable at room temperature, showing slow
decomposition into a mixture of unidentified compounds
even in the solid state over the course of several days, and
aDepartment of Chemistry, Brock University, 500 Glenridge Ave., St. Catharines, ON
L2S 3A1, Canada. E-mail: gnikonov@brocku.ca; Tel: +1 905 688 5550, ext 3350
bDepartment of Chemistry, School of Science and Technology, Nazarbayev University,
53 Kabanbay Batyr Ave., Astana 010000, Kazakhstan.
E-mail: andrey.khalimon@nu.edu.kz; Tel: +7 7172 709102
†Electronic supplementary information (ESI) available: Additional experimental
and spectroscopy details, general procedures for catalytic hydroboration reac-
tions. See DOI: 10.1039/c5dt02945g
This journal is © The Royal Society of Chemistry 2015
Dalton Trans., 2015, 44, 18945–18956 | 18945

View Article Online
Paper
Dalton Transactions
1620 cm−1 in the IR spectrum of 2. Based on these spectro-
scopic features and on the analogy with the mono(hydride)
derivative 1,12,13 we suggest an octahedral structure for the
dihydride 2, with one of the hydride ligands occupying the
apical position trans to the imido ligand. The second Mo-bound
hydride of 2 lies in the equatorial position, being co-planar with
all three PMe3 ligands, as depicted in Scheme 1.
Scheme 1 Synthesis of (ArN)MoH2(PMe3)3 (2).
Catalytic reactivity of (ArN)Mo(H)(Cl)(PMe3)3 (1) and
(ArN)MoH2(PMe3)3 (2)
therefore defies isolation in the analytically pure state. Despite
the instability, the dihydride 2 can be cleanly generated by the
above procedure and the freshly prepared samples can
Catalysis by (ArN)Mo(H)(Cl)(PMe3)3 (1). The imido–hydrido
be reliably characterised by multinuclear NMR and 1 has been found to catalyse a variety of hydroboration reac-
IR spectroscopy. Complex 2 is highly fluxional in solution and tions (Table 1).13 Thus, the addition of HBCat to ketones
its room temperature NMR spectra exhibit only broad feature- (iPr2C(O), Ph2C(O), cyclohexanone, PhC(O)Me,15 4-nitroaceto-
less resonances. However, the 31P{1H} NMR spectrum of phenone) in the presence of 5 mol% of 1 affords the corres-
2 recorded at −26 °C shows two mutually coupled resonances ponding boronic esters in high yields (91–100%; Table 1,
for two sets of non-equivalent phosphine groups at δ 14.8 ppm entries 1–5). Similarly, the ester MeC(O)OEt was converted into
2
(doublet) and 13.1 ppm (triplet) with JP–P = 19.4 Hz. At the ethoxy group of EtOBCat (entry 6). Alkenes (styrene,
1
−29 °C, the H NMR spectrum of 2 reveals two non-equivalent 1-hexene)6b–g,7 and alkynes (3-hexyne, phenylacetylene)6a,h,i
Mo-bound hydrides, which give rise to two mutually coupled can be reduced to the boryl-substituted alkanes and alkenes,
(2JH–H = 7.2 Hz) resonances at δ −5.31 ppm (dtd) and 2.08 ppm respectively (Table 1, entries 7–10). Interestingly, the 1-cata-
(multiplet, overlapping with the resonance of toluene-d8, lysed reaction with styrene also gives trans-PhCHvCHBCat
found by the 1H–1H COSY and 1H–31P HSQC NMR). In the and ethylbenzene in addition to the expected product
1H–31P HSQC NMR spectra, these hydride signals are coupled PhCH2CH2BCat. The former product likely forms as a result of
2
2
to the 31P signals with JH–P = 46.2 and 60.6 Hz and JH–P
=
dehydrogenative addition of HBCat to styrene. In contrast, a
43.2 Hz, respectively. The presence of hydride ligands is also reduced or no catalytic activity of the hydride 1 was observed
confirmed by the observation of the Mo–H stretch at in the hydroboration of 1-hexene, cyclohexene, α-methyl-
Table 1 Hydroboration of organic substrates with HBCat mediated by 1 a
Entry
Substrate
Conversionb, %
Product(s)
t
Yieldc, %
TONd
1
2
3
4
5
6
7
iPr2C(O)
Ph2C(O)
PhC(O)Me
4-Nitroacetophenone
Cyclohexanone
MeC(O)OEt
PhCHvCH2
91
100
99
100
100
100
100
iPr2CHOBCat
Ph2CH(OBCat)
PhCH(OBCat)Me
4-NO2-C6H4-CH(OBCat)Me
CyOBCat
EtOBCat
PhCH2CH2BCat
trans-PhCHvCHBCat
PhCH2CH3
20 h
30 min
15 min
1 h
1 h
2 h
91
100
99
100
100
100
32
18
20
20
20
20
20
20
20 h
53
15
8
1-Hexene
70
HexBCat
2-Hexene
60h
55
8
14
3-Hexene
7
94
99
100
100
50
92
67
33
55
10
100
32
68
67
9
3-Hexyne
PhCuCH
MeCN
PhCN
5-Hexynenitrile
94
99
100
100
50
92
100
EtCHvC(Et)BCat
trans-PhCHvCHBCat
EtN(BCat)2
21 h
20 h
12 h
12 h
20 h
48 h
12 h
19
20
20
20
10
18
20
10
11
12
13
PhCH2N(BCat)2
trans-NC(CH2)3CHvCHBCat
trans-NC(CH2)3CHvCHBCat
4-CN-C6H4-CH(OBCat)Me
4-(CatB)2NCH2-C6H4-CH(OBCat)Me
CatBCH2CH2CN
CatBCH2(CH2)2N(BCat)2
2-NC(CH2)2-C6H10-OBCat
2-NC(CH2)2-C6H10-OBCat
2-(CatB)2N(CH2)3-C6H10-OBCat
Ph2CH(OBCat)
14
15
16
4-Acetylbenzonitrile
Acrylonitrile
65
48 h
13
3-(2-Oxocyclohexyl)propanenitrile
100
100
5 min
48 h
20
20
17
Ph2C(O)/PhCN (1/1)
100
12 h
20
Ph2CH2N(BCat)2
33
a 5 mol% of 1, 22 °C, C6D6, substrate/HBCat = 1/1 (1/2 for entries 4, 9, 10, 12, and 15). b Conversion of organic substrate, except entries 12, 15 and
16 where the conversion of HBCat was calculated. c 1H NMR yields based on internal standard (tetramethylsilane). d Turnover numbers were
calculated at the maximum conversion.
18946 | Dalton Trans., 2015, 44, 18945–18956
This journal is © The Royal Society of Chemistry 2015

View Article Online
Dalton Transactions
Paper
presence of the second hydride ligand in 2 often leads to
drastic changes in the product distribution, selectivity, and the
rate of the catalytic reactions. For example, the hydroboration
of styrene with HBCat in the presence of 5 mol% of 2 affords a
large amount of ethylbenzene (51% vs. 15% for the 1-catalysed
reaction; Table 1, entry 5 and Table 2, entry 8). Hydroboration
of 1-hexene with HBCat catalysed by 2 (5 mol%) is more
selective towards the hydroboration product with a higher
turn-over number and frequency than the reaction catalysed by
the hydrido–chloride 1. For example, for complex 3 the TON of
19 with 93% conversion of 1-hexene to a mixture of HexBCat
(85%), 2-hexene (3%) and 3-hexene (5%) was found (Table 2
entry 7) vs. the TON of 14 with 70% conversion of 1-hexene to
a mixture of HexBCat (55%), 2-hexene (8%) and 3-hexene (7%)
for complex 1 (Table 1, entry 7)).
Scheme 2 Transformation of nitriles to imines.
styrene, 1-octyne, and PhCuCMe. Additionally, 1 was found to
be active in the hydroboration of organic nitriles (MeCN and
PhCN) leading to the products of double addition of HBCat
across the CuN bond, i.e. RCH2N(BCat)2 (R = Me, Ph; Table 1,
entries 11 and 12).13 Moreover, PhCH2N(BCat)2 was found to
react easily with benzaldehyde to give the corresponding imine
PhCH2NvCHPh with the release of (CatB)2O. Taken together,
these novel hydroboration and coupling reactions present a useful
synthetic transformation of nitriles to imines (Scheme 2).16 Note-
worthy, the formation of imines from bis(boryl) amines proceeds
only with aldehydes but not with ketones.
Lastly, 1-catalysed addition of HBCat to polyfunctional com-
pounds (4-acetylbenzonitrile, acrylonitrile, and 3-(2-oxocyclo-
hexyl)propanenitrile) were shown to be not chemoselective,
affording mixtures of hydroboration products (Table 1, entries
14–16). In a similar vein, the hydroboration of an equimolar
mixture of Ph2CvO and PhCN afforded both Ph2CH(OBCat)
and Ph2CH2N(BCat)2 (entry 17). In contrast, the hydroboration
of 5-hexynenitrile takes place selectively on the alkyne moiety
to give trans-NC(CH2)3CHvCHBCat, leaving the nitrile group
unreacted (Table 1, entry 13).
A similar trend was also observed in the hydroboration of
ketones by 2 (Table 2). Thus, switching from 1 to 2 in the
hydroboration of 4-nitroacetophenone, di(isopropyl) ketone,
benzophenone, and cyclohexanone with HBCat leads to a
significant increase in TOF (4-nitroacetophenone: TOF 20 →
i
80; Pr2C(O): TOF 0.9 → 48; benzophenone: TOF 40 → 76;
cyclohexanone: TOF 20 → 46; compare Tables 1 and 2). In
contrast, the turnover frequency for the hydroboration of ethyl
acetate with complexes 1 and 2 decreases from 10 to 1.1,
respectively (Tables 1 and 2). At the moment, we have no
rationale for the latter observation.
On the other hand, the hydroboration of alkynes with the
dihydride 2 is slower than with the hydrido–chloride 1. Thus,
the reaction of phenyl acetylene with 1 equiv. of HBCat in the
presence of 5 mol% of 2 gives trans-PhCHvCHBCat with the
TOF of 0.3 vs. the TOF of 1.0 for the 1-catalysed reaction (entry
9 in Table 2 vs. entry 10 in Table 1). Interestingly, 2 was found
to be inactive in the hydroboration of 3-hexyne, whereas 1
Catalysis by (ArN)MoH2(PMe3)3 (2). Compared to 1, the new
imido–dihydrido complex
2 showed somewhat improved
catalytic activity in hydroboration reactions. The results are
summarised in Table 2. The scope of substrates for 2-catalysed
transformations is the same as for complex 1. However, the
Table 2 Hydroboration of organic substrates with HBCat mediated by 2 a
Entry
Substrate
Conversionb, %
Product(s)
t
Yieldc, %
TONd
1
2
3
4
5
6
7
PhC(O)Me
100
100
95
100
94
99
93
PhCH(OBCat)Me
4-NO2-C6H4-CH(OBCat)Me
Ph2CH(OBCat)
iPr2CH(OBCat)
CyOBCat
EtOBCat
HexBCat
15 min
15 min
15 min
25 min
25 min
19 h
>99
>99
95
>99
94
99
85
3
20
20
19
20
19
20
19
4-Nitroacetophenone
Ph2C(O)
iPr2C(O)
Cyclohexanone
MeC(O)OEt
1-Hexene
24 h
2-Hexene
3-Hexene
5
8
PhCHvCH2
100
PhCH2CH2BCat
trans-PhCHvCHBCat
PhCH2CH3
trans-PhCHvCHBCat
EtN(BCat)2
PhCH2N(BCat)2
tBuCH2N(BCat)2
CH3CH(BCat)CN
trans-NC(CH2)3CHvCHBCat
60 h
25
24
51
>99
74
99
76
92
42
20
9
PhCuCH
MeCN
100
74
99
76
92
42
48 h
24 h
24 h
24 h
20 h
48 h
20
15
20
15
18
8
10
11
12
13
14
PhCN
tBuCN
Acrylonitrile
5-Hexynenitrile
a 5 mol% of 2, 22 °C, C6D6, substrate/HBCat = 1/1 (1/2 for entries 6, 10–12). b Conversion of organic substrate, except entry 4 where the conversion
of HBCat was calculated. c 1H-NMR yields based on internal standard (tetramethylsilane). d Turnover numbers were calculated at maximum
conversion.
This journal is © The Royal Society of Chemistry 2015
Dalton Trans., 2015, 44, 18945–18956 | 18947

View Article Online
Paper
Dalton Transactions
showed conversion of 3-hexyne to EtCHvC(Et)BCat with the
TOF of 0.9 (Table 1, entry 9).
Lastly, hydroboration of nitriles with HBCat in the presence
of 5 mol% of 2 is slightly less efficient when compared to
catalyst 1. Thus, a decrease in TOF values was observed for the
hydroboration of CH3CN, PhCN, and 5-hexynenitrile upon
switching the catalyst from 1 to 2 (TOF 1.7 → 0.6, 1.7 → 0.8,
0.5 → 0.2, respectively; Tables 1 and 2). Similar to the 1-cata-
lysed reaction, the hydroboration of 5-hexynenitrile with 2
proceeds via selective addition of HBCat across the triple
carbon–carbon bond to give trans-NC(CH2)3CHvCHBCat
(Tables 1 and 2). Analogous reactivity was observed in the case
of 2-catalysed hydroboration of acrylonitrile, which gives rise
to the product of HBCat addition across the CvC double
bond, i.e. CH3CH(Bcat)CN (92%, Table 2, entry 13), whereas
the 1-catalysed reaction gives a mixture of CatB(CH2)2CN and
CatB(CH2)3N(Bcat)2 (55% and 10%, respectively; Table 1, entry
15). The decrease in TON and TOF values for the hydrobora-
tion of nitriles with HBCat upon switching the catalyst from
1 to 2 can be attributed to the partial degradation of HBCat to
BH3 and B2Cat3 when complex 2 is used as the catalyst.17 Such
a decomposition of HBCat was, for example, observed by NMR
in the hydroboration of 5-hexinenitrile in the presence of 2.
We tentatively assign this behaviour to the higher lability of
complex 2 that presumably has less tightly bound phosphines,
which favours the formation of the PMe3·BH3 adduct.17
Scheme 3 Catalytic transformation of nitriles to imines.
Stoichiometric reactivity of 1 and 2 and mechanistic insights
into hydroboration catalysis
Fig. 1 Methylenamide complexes 7–9 and products of the reactions of
1 with acrylonitrile and 4-formylbenzonitrile, 10 and 11, respectively.
Stoichiometric reactivity of (ArN)Mo(H)(Cl)(PMe3)3 (1). To
shed light on the mechanism of catalysis, stoichiometric reac-
tions between complex 1 and unsaturated organic molecules
and HBCat were studied.18 In our preliminary communi-
cation,13 we suggested a possible mechanism of the hydrobora-
tion of benzonitrile (Scheme 3) on the basis of studying
individual steps under stoichiometric conditions. It is
important to mention that the hydrido–chloride complex
1 reacts with HBCat very sluggishly: after 24 h at room
temperature only ca. 20% conversion of 1 to a mixture of (ArN)-
diffraction analysis described previously in a preliminary com-
munication.13 Compounds 3, 7–9 give rise to the diagnostic
imine proton (δ 7.09–7.43 ppm) and carbon signals
1
(δ 145.4–153.5 ppm; coupled in the H–13C HSQC NMR to the
corresponding 1H resonances) in their 1H and 13C NMR
spectra, respectively. It is important to mention that the Mo–
NvC bond angle in 3 is almost linear (172.3(4)°) suggesting
that the [NvCHPh] fragment acts as a 4e donor.19 This coordi-
nation mode leads to the stable 18e valence shell, assuming
that the linear imide ligand ArN2− (MovN–C 175.0(3)°)13
14
MoCl2(PMe3)3 and (ArN)Mo(H)2(PMe3)3 (2) was observed by
NMR. No products of oxidative addition of borane to Mo, akin
to (ArN)Mo(Cl)(H)2(BCat)(PMe3)2 or its derivative (ArN)Mo(Cl)-
(BCat)(PMe3)x (x = 2 or 3), were detected. On the other hand,
complex 1 readily reacts with PhCN (full conversion of 1 was
achieved in 50 min at room temperature) to give the vinylede-
ˉ
donates 6e.
To our surprise, we found that complex 7 derived from
acetonitrile reacts slowly (24 h at RT) with PhCN to form the
benzylideneamide complex 3 via the release MeCN. This
unusual reaction indicates the possibility of α-CH bond acti-
vation in the methyleneamide ligand (Scheme 4). We have
recently reported a related reversible insertion of carbonyls
into the M–H bond.20 But to the best of our knowledge, the
reversible insertion of nitriles into an early metal–hydride
bond has been previously observed only for Cp2*ScðNCHRÞ.19c
A similar reactivity pattern was also observed upon the treat-
ment of benzylideneamide 3 with benzaldehyde, which in the
presence of PMe3 leads to the exclusive formation of the
benzoxy complex (ArN)Mo(Cl)(OBn)(PMe3)3 (12; Scheme 4)12
neamide
derivative
(ArN)Mo(Cl)(NvCHPh)(PMe3)2
(3;
Scheme 3). A similar reactivity of complex 1 to yield products
of nitrile insertion into the Mo–H bond was observed with
MeCN, pent-3-enenitrile, and 4-acetylbenzonitrile to give com-
plexes 7–9, respectively (Fig. 1). In contrast, reactions of 1 with
acrylonitrile and 4-formylbenzonitrile afforded the olefin
complex 10 and the product of aldehyde insertion into the
M–H bond (11), respectively (Fig. 1).
Complexes 3 and 7–11 were characterised by spectroscopic
methods (IR, NMR) and the formulation of complex 3 as a
methylenamide species was further substantiated by X-ray
18948 | Dalton Trans., 2015, 44, 18945–18956
This journal is © The Royal Society of Chemistry 2015

View Article Online
Dalton Transactions
Paper
coupled doublets at δ −1.4 ppm and δ −13.0 ppm in the
31P NMR spectrum. The large value of 2JP–P = 212.0 Hz suggests
the trans arrangement of the phosphine ligands in 6. The
11B NMR spectrum of 5 revealed the presence of a broad signal
at δ 10.2 ppm, which is more downfield shifted in comparison
with 4, presumable because of weaker B–H interaction. All
together these features allow us to suggest the formation of a
κ1-(N-boryl)imine complex (ArN)Mo(H)(Cl){κ1-N(BCat)vCHPh}-
(PMe3)2 (5, Scheme 3) having non-equivalent PMe3 groups
because of restricted rotation about the Mo–N bond of the
κ1-(N-boryl)imine ligand at −50 °C.
Scheme 4 H-transfer reactions in complexes 7 and 3.
along with the release of PhCN. In contrast, no transfer
hydrogenation was observed in reactions of 7 with ketones
(acetone and acetophenone) even upon heating up to 60 °C.
This difference in reactivity of methyleneamide complexes
towards aldehydes and ketones is reflected in the
chemoselectivity of stoichiometric reactions of 1 with 4-acetyl-
benzonitrile and 4-formylbenzonitrile to give 4-acetylbenzylide-
neamide complex 9 and (4-cyanophenyl)methoxy derivative 11,
respectively (Fig. 1). Based on our observations that nitriles
react faster with 1 than aldehydes (2 h for PhCN vs. 5–6 h for
PhC(O)H12), one can assume that for both keto- and aldo-
nitriles the reactions proceed via initial insertion of the nitrile
group into the Mo–H bond. For 4-formylbenzonitrile, this
insertion can be followed by intermolecular hydrogen transfer
to the formyl moiety to form the more thermodynamically
stable complex 11.
Increasing the temperature up to 25 °C leads to
disappearance of complexes 4 and 5 and exclusive formation
of (ArN)Mo(H)(Cl){η2-CatBNvCHPh}(PMe3)2 (6; Scheme 3), the
structure of which was suggested on the basis of multinuclear
NMR spectroscopy. Thus, the 31P NMR spectrum of 6 shows
two non-equivalent PMe3 ligands, which give rise to two
mutually coupled doublets at δ −5.2 ppm and 2.4 ppm with
2
the JP–P = 88.5 Hz. The 1H NMR spectrum of 6 revealed an
3
upfield imine proton resonance at δ 5.00 ppm (dd, JH–P = 3.1
Hz), diagnostic for the η2-coordinated CatBNvCHPh, coupled
in 1H–13C HSQC NMR to an upfield shifted imine carbon reso-
nance at δ 62.9 ppm. Similar to the parent hydrido–chloride
complex 1, the Mo–H signal of 6 is shifted downfield to
2
δ 7.06 ppm (found by 1H–31P HSQC NMR; JH–P = 45.0 and
The reaction of complex 3 with HBCat was followed by
NMR spectroscopy at low temperature. At −30 °C, the
formation of a mixture of two bis(phosphine) compounds
(4 and 5 in Scheme 3) was observed. One of the products was
tentatively formulated as the agostic amido-borane
50.9 Hz), suggesting the cis disposition of the hydride and
imido ligands. The 11B NMR spectrum of 6 shows a downfield
signal at δ 15.3 ppm.
No further intermediates were observed upon addition of
another equivalent of HBCat to complex 6. Only the release of
adduct
(ArN)Mo(Cl){κ3-N(vCHPh)(CatB-H⋯)}(PMe3)2
(4;
PhCH2N(BCat)2 and formation of a mixture of the hydrido–
Scheme 3)21,22 on the basis of the following spectroscopic fea-
tures: (i) the complex has a Cs symmetric NMR-averaged struc-
ture with two equivalent PMe3 ligands giving rise to a singlet
14
chloride 1 and (ArN)MoCl2(PMe3)3
together with a small
amount of unknown decomposition products were detected.
The mechanism of addition of the second equivalent of HBCat
to the boryl imine moiety of 6 to form bis(boryl) amine
remains unknown. However, we suggest that this last step of a
possible catalytic cycle (depicted in Scheme 3) is assisted by
HBCat. One can assume that the reaction proceeds via inser-
tion of the boryl imine into the Mo–H bond to form a boryl
amide derivative, which can further react with HBCat in a
manner similar to methylenamide complexes (Scheme 3).
A similar mechanism can be also suggested for the hydro-
boration of carbonyl compounds as we found that the treat-
ment of either (ArN)Mo(Cl)(OBn)(PMe3)3 (12; generated from
1 and PhC(O)H),12 or (ArN)Mo(Cl)(OCH(Me)Ph)(PMe3)3 (13;
at δ −0.9 ppm in 31P NMR; (ii) H NMR revealed a downfield
1
imine signal at δ 8.91 ppm, coupled in H–13C HSQC NMR to
1
the 13C NMR resonance at δ 172.0 ppm; (iii) 11B NMR showed
the presence of a four-coordinate boron centre exhibiting a
doublet at δ 2.2 ppm (compare to δ 29 ppm for HBCat) with a
1
1
reduced JB–H of ca. 55 Hz (compared to the JB–H(terminal) =
135.0 Hz and 1JB–H(bridging) = 46.0 Hz for B2H6).
The second, fluxional product (5 in Scheme 3) was pro-
duced from 4 upon a gentle increase of temperature. However,
all attempts to find a temperature regime for the complete con-
1
version of 4 into 5 were unsuccessful. At −50 °C, the H NMR
spectrum of 5 reveals a broad upfield hydride signal at
δ
generated from
1
and PhC(O)Me) or (ArN)Mo(Cl)(OEt)-
−2.94 ppm and a downfield imine 1H resonance at
12,23
(PMe3)3
(14; generated from 1 and ethyl acetate) with
δ 8.31 ppm, coupled in 1H–13C HSQC NMR to the 13C NMR
signal at δ 154.2 ppm. A series of 1H, 1H{31P} and 1H{11B} NMR
experiments (Fig. S9 in ESI†) suggest some coordination of the
Mo-bound hydride to the boron centre of the borylimine
ligand of 5. This conclusion is supported by the significant
sharpening of the hydride signal upon decoupling from the
11B nucleus. Unfortunately, we could not measure the B–H
coupling constant from the very broad signal in 11B NMR. The
two non-equivalent PMe3 ligands give rise to two mutually
HBCat immediately regenerates the complex 1 and gives the
corresponding hydroboration products, PhCH2OBCat, PhCH-
(OBCat)Me and EtOBCat, respectively.24 A small amount of
(ArN)MoCl2(PMe3)3 14 was also observed by NMR, which can be
indicative of a possible catalyst deactivation pathway. All
attempts to elucidate any further intermediates were unsuc-
cessful due to the high reactivity of complexes 12, 13 and 14
with HBCat even at −40 °C. Based on our previous study of
hydrosilylation of carbonyl compounds and alcoholysis of
This journal is © The Royal Society of Chemistry 2015
Dalton Trans., 2015, 44, 18945–18956 | 18949

View Article Online
Paper
Dalton Transactions
Scheme 5 Proposed mechanism for reactions of 12, 13 and 14 with
HBCat.
silanes mediated by complex 1,12 we tentatively suggest that
the alkoxy derivatives 12, 13 and 14 react with HBCat to
furnish the corresponding hydroboration products via hetero-
lytic splitting of the B–H bond (Scheme 5).
Scheme 6 Reactions of complex 2 with HBCat and HBPin.
For nitriles having a carbonyl functionality (for example,
4-formylbenzonitrile, 4-acetylbenzonitrile) and for mixtures of
nitriles with carbonyl compounds, the insertion of the CvO
and CuN moieties into the Mo–H bond of 1 becomes competi-
tive in the presence of a large excess of HBCat. This suggestion
stems from our previous observation that addition of alde-
hydes to 1 starts with the formation of the adduct trans-(ArN)-
Mo(H)(Cl)(η2-OvCRH)(PMe3)2, which in the presence of a
large excess of RHC(O) rearranges slowly (≥5 h) into an alkoxy
complex via dissociation of PMe3.12 Addition of excess borane
could significantly accelerate this process through the for-
mation of a PMe3–borane adduct, making it competitive with
(or even faster than) the formation of methylenamide deriva-
tives (∼2 h). This could be a possible explanation for the loss
of chemoselectivity in the hydroboration reactions under cata-
lytic conditions.
ment of 2 with the less Lewis acidic borane HBPin results in
the formation of another product, different in structure from
1
the agostic complex 15. The H NMR spectrum of this species
2
shows two hydride resonances at δ −6.89 ppm (q, 1H, JH–P
=
63 Hz) and −4.10 ppm (broad, 2H). The latter hydride signal
1
appears as a sharp quartet (2JH–P = 27 Hz) in the H{11B} NMR
spectrum. The 11B NMR spectrum of this species showed the
presence of a four-coordinate boron centre with a signal at δ
−3.62 ppm. These spectroscopic features indicate a boro-
hydride structure tentatively formulated as (ArN)Mo(η2-
H2BPin)(H)(PMe3)3 (16, Scheme 6).
Both complexes 15 and 16 are not stable in solution and
decompose after several hours at room temperature to the
tetrahydride complex MoH4(PMe3)4 (17, Scheme 6) previously
reported by Brookhart et al.27 Despite the instability, the
agostic compound 15 can be generated on preparative scale in
82% yield by the reaction of the dihydride 2 with one equi-
valent of HBCat at −30 °C and can be stored at this tempera-
ture under inert atmosphere for several days.
On the other hand, treatment of the dihydride complex 2
with ketones resulted in complex reaction mixtures. In the
reaction mixture obtained upon mixing 2 with cyclohexanone,
we identified a species tentatively assigned the structure (ArN)-
Stoichiometric reactivity of (ArN)MoH2(PMe3)3 (2). To
further identify the potential catalytically active species in the
hydroboration reactions, stoichiometric reactions between
complex 2, unsaturated organic molecules and HBR (R = Cat,
Pin) were studied. At ambient temperature, NMR scale reaction
of 2 with HBCat results, within minutes, in the formation of a
new tris(phosphine) complex, which was tentatively formu-
lated as the agostic borane complex Mo(H)2(PMe3)3(η3-NAr-
HBcat) (15, Scheme 6).25 This suggestion was made on the
basis of the following spectroscopic features. Complex 15 is
Mo(H)(OCy)(PMe3)3 (18) on the basis of analogy of its spectral
12b
features with the compound (ArN)Mo(Cl)(OCy)(PMe3)3
and
the correlation of its 31P signals in the 1H–31P HSQC NMR
with the Mo–H signal at δ 4.00 ppm. In contrast, a reaction of
complex 2 with 2 equiv. of acetonitrile results in a double
addition of MeCN and selective formation of bis(vinylidene-
amide) (ArN)Mo(NvCHMe)2(PMe3)2 (19), obtained as an equi-
librium mixture of three isomers in the ratio 1 : 0.7 : 0.1
(19A, 19B and 19C, respectively; Scheme 7).
1
fluxional at room temperature; however, at −30 °C its H NMR
spectrum revealed three hydride resonances, at δ −6.03 ppm
(dt, 2JH–P = 39.6 Hz, 2JH–P = 88.2 Hz), −2.79 ppm (t, 2JH–P = 59.9
Hz) and −1.82 ppm (broad singlet). Upon decoupling from
11B nucleus, the hydride signal at δ −1.82 ppm resolves into a
broad doublet, presumably, due to coupling to the trans-PMe3
ligand (2JH–P = 15.2 Hz). Also, 11B NMR showed the presence of
The activation parameters for the exchange between the
a
four-coordinate boron species exhibiting a signal at
1
δ −0.59 ppm (vs. 29 ppm for HBCat). The 31P{1H} NMR spec-
trum of 15 displays two mutually coupled resonances with 1 : 2
intensities: a triplet at δ 2.54 ppm (2JP–P = 17.0 Hz) and a
doublet at δ 23.9 ppm, coupled to two hydride signals at
major isomers 19A and 19B were found through 1D H EXSY
NMR (ΔS≠ = −12.0 2.8 kcal mol−1, ΔH≠ = 12.3 1.1 kcal
mol−1). The negative entropy of activation suggests an intra-
molecular exchange mechanism.
δ −6.03 ppm and −2.79 ppm in the H–31P HSQC NMR spec-
trum. The agostic borane structure of 15 is further supported
by the observation of a red-shifted B–H band at 2357 cm−1 in
the IR spectrum (vs. ν(B–H) ≈ 2670 cm−1 for HBCat).26 Treat-
1
All three isomers of 19 were characterised by NMR spec-
troscopy and their 1H NMR spectra revealed the presence of
downfield imine resonances at δ 8.13 ppm (for 19A), 8.12 and
7.70 ppm (for 19B) and 7.66 ppm (for 19C). These imine
18950 | Dalton Trans., 2015, 44, 18945–18956
This journal is © The Royal Society of Chemistry 2015

View Article Online
Dalton Transactions
Paper
proton signals were found to be coupled in 1H–13C HSQC
NMR to the imine 13C resonances at δ 145.7 ppm (for 19A and
19B) and 145.8 ppm (for 19C) and to the 31P resonances at δ
1.31 ppm, 2.42 ppm and 2.41 ppm (for 19A, 19B and 19C,
respectively)28 in 1H–31P HSQC NMR.
Since all four Me groups of the ArN2− ligand are equivalent
1
and give rise to a sharp doublet in the H NMR spectra of 19,
we suggest that the MovN–Ar fragment is almost linear and
ˉ
the imido ligand acts as a 6e donor. If this assumption is true,
ˉ
complex 19 can be thought of as a stable 18e species, with
Scheme 7 Reaction of complex 2 with acetonitrile.
each vinylideneamide ligand donating three electrons to moly-
bdenum due to delocalisation of the nitrogen lone pair. As
such, the interconversion of isomers is thought to occur by the
transformation of the formal double MvNC bond (σ + dative
π) into a single Mo–NvC bond (pure σ), followed by rotation
(Scheme 8).
Addition of two equivalents of HBCat to 19 results in
immediate formation of the hydroboration product, EtN-
(BCat)2. However, no recovery of the dihydride 2 was observed.
NMR analysis of the reaction mixture showed the formation of
a mixture of unknown products. Thus, in order to probe an
alternative pathway for the hydroboration of acetonitrile, we
studied the reaction of the agostic amido borane complex 15
with CH3CN. At −15 °C, 15 reacts with 2 equiv. of CH3CN to
give (ArN)Mo(–N(Bcat)vCHMe)(η2-NuCCH3)(PMe3)2 (20) as a
mixture of two isomers (Scheme 9; ≈3 : 1 ratio by NMR). For-
mation of 20 is accompanied with the release of a molecule of
Scheme 8 Proposed mechanism for the exchange between isomers
of 19.
1
PMe3 and an equivalent of H2. In the H NMR spectrum, both
isomers of 20 give rise to characteristic imine proton reson-
ances at δ 7.38 ppm (for 20A) and 8.12 ppm (for 20B), coupled
1
in H–13C HSQC NMR to the corresponding imine 13C reson-
ances at δ 151.8 ppm and 145.7 ppm, respectively. An η2-
coordination mode of acetonitrile is suggested based on the
diagnostic 13C NMR signal at δ 203.1 ppm for the nitrile
carbon nucleus.29 The 11B NMR spectrum of 20 showed the
presence of a downfield signal at δ 13.3 ppm. While we could
Scheme 9 Reaction of complex 15 with CH3CN.
Table 3 Hydroboration of organic substrates with HBCat mediated by 15 a
Entry
Substrate
Conversion, %b
Product(s)
t
Yield, %c
TONd
1
2
3
4
5
6
PhC(O)Me
100
95
99
90
99
82
PhCH(OBCat)Me
4-O2N-C6H4-CH(OBCat)Me
Ph2CH(OBCat)
10 min
10 min
10 min
25 min
10 min
24 h
>99
95
99
90
9
79
3
4
77
6
20
19
20
18
20
16
4-Nitroacetophenone
Ph2C(O)
iPr2C(O)
iPr2CH(OBCat)
Cyclohexanone
1-Hexene
CyOBCat
HexBCat
2-Hexene + 3-hexene
PhCH2CH2BCat
trans-PhCHvCHBCat
PhCH2CH3
7
PhCHvCH2
87
24 h
17
8
9
10
11
12
PhCuCH
MeCN
99
99
99
90
95
trans-PhCHvCHBCat
EtN(BCat)2
PhCH2N(BCat)2
tBuCH2N(BCat)2
trans-NC(CH2)3CHvCHBCat
24 h
12 h
12 h
9 h
99
99
99
90
95
20
20
20
18
19
PhCN
tBuCN
5-Hexynenitrile
36 h
a 5 mol% of 15, 22 °C, C6D6, substrate/HBCat = 1/1 (1/2 for entries 9–11). b Conversion of organic substrate. c 1H NMR yields based on internal
standard (tetramethylsilane). d Turnover numbers were calculated at maximum conversion.
This journal is © The Royal Society of Chemistry 2015
Dalton Trans., 2015, 44, 18945–18956 | 18951

View Article Online
Paper
Dalton Transactions
not find any literature data for the free N-borylimine purchased from Sigma-Aldrich and used without further
CatB-NvCHR, this chemical shift falls between the signals for purification. HBCat was additionally purified by distillation
the boron amides CatB-NR2 (about 26 ppm) and the four- before use. (ArN)Mo(H)(Cl)(PMe3)3 (1)12 and (ArN)Mo(Cl)-
coordinate catecholborane-imine adducts CatBH*N(R)vCR2 (NvCHPh)(PMe3)2 (3)13 were prepared according to the pre-
(−19.2 ppm).10 Although not being clear-cut evidence, we viously published procedures. Compounds (ArN)Mo-
believe that the boron chemical shift of 13.3 ppm is better con- (H)2(PMe3)3 (2) and Mo(H)2(PMe3)3(η3-NAr-HBCat) (15) are
sistent with the three-coordinate boron structure of 20 as unstable at room temperature whereas compounds (ArN)Mo-
depicted in Scheme 9. The 31P-NMR spectra for both isomers (H)2(PMe3)3 (2) and (ArN)Mo(Cl)(NvCHMe)(PMe3)2 (7) were
of 20 show broad signals for the PMe3 groups at δ 1.31 ppm isolated as viscous oils, which did not allow for elemental ana-
and 2.42 ppm (for 20A and 20B, respectively), suggesting the lysis to be performed. All catalytic, NMR scale reactions and
equivalency of phosphine ligands and, thus, the orientation of kinetic experiments were done under nitrogen atmosphere
the η2-NuCCH3 ligand coplanar with the MovN–Ar moiety.
using NMR tubes equipped with Teflon valves. The structures
Addition of two equiv. of HBCat to a freshly generated and yields of all hydroboration products were determined by
complex 20 results in immediate formation of the hydrobora- NMR using tetramethylsilane as an internal standard.
tion product, EtNB(Cat)2, and, in contrast to the reaction of 19
NMR scale reaction of (ArN)Mo(Cl)(NvCHPh)(PMe3)2 (3) with
HBCat
with HBCat, complete recovery of the starting dihydride 15
(Scheme 9). Moreover, when complex 15 was subjected to
hydroboration reactions under catalytic conditions, it proved A: Room temperature reaction. A solution of HBCat (3.5 μl,
to be the most catalytically potent, when compared to com- 0.033 mmol) in 0.6 ml of C6D6 was added in one portion at
plexes 1 and 2 (see Tables 1–3). Most importantly, complex 15 room temperature to solid 3 (18.5 mg, 0.033 mmol). The
was found to be much more efficient in the hydroboration of mixture was immediately transferred into an NMR tube and
nitriles (Table 3, entries 9–11). This result suggest that hydro- left at room temperature for 10 min. During this time the
boration of acetonitrile with HBcat more likely proceeds with colour of the mixture turned red. NMR analysis after 10 min
the intermediacy of the agostic amido borane 15, followed by showed formation of a difficult-to-separate mixture of the start-
the formation of complex 20 rather than through the bis(vinyli- ing material and (ArN)Mo(H)(Cl)(η2-N(BCat)vCHPh)(PMe3)2
deneamide) derivative 19.
(6) (1 : 1). Addition of another equivalent of HBCat to the
reaction mixture leads to full conversion of 3 into 6. All
attempts to isolate complex 6 were unsuccessful due to its
instability. A 31P–31P EXSY NMR spectrum of complex 6
revealed an intramolecular exchange of the PMe3 ligands;
Conclusions
In conclusion, imido–hydrido complexes 1 and 2 were found however, addition of PhCN to a solution of 6 in C6D6 does not
to catalyse a variety of hydroboration reactions with HBCat, afford CatBNvCHPh. On the other hand, addition of a stoi-
including a rare example of hydroboration of nitriles to give chiometric mixture of PhCN and HBCat to a solution of 6 in
bis(boryl) amines. Mechanistic studies of hydroboration of C6D6 leads to slow (2 days) conversion of 6 into 3 and
nitriles suggest that reactions proceed via a series of agostic formation of PhCH2N(BCat)2.
borylamido and borylamino complexes. No evident oxidative B: Low temperature VT reaction. HBCat (3.6 μl,
addition of HBCat to the Mo(IV) centre to form boryl complexes 0.033 mmol) was added in one portion at room temperature to
was observed.
a frozen in liq. N2 solution of 3 (23.5 mg, 0.042 mmol) in
0.6 ml of C6D6 in an NMR tube. The mixture was warmed up
to −30 °C and placed into an NMR spectrometer pre-cooled to
−30 °C. The temperature was dropped down to −50 °C and the
sample was warmed gradually and monitored by NMR spectro-
scopy. At −50 °C, NMR analysis revealed the presence of a
Experimental
General methods and instrumentation
All manipulations were carried out under nitrogen atmos- mixture of the starting material 3, (ArN)Mo(Cl){η3-N(vCHPh)-
phere, using either conventional Schlenk techniques or an (CatB-H⋯}(PMe3)2 (4), and (ArN)Mo(H)(Cl)(η1-N(BCat)vCHPh)-
inert atmosphere MBraun glovebox. Dry solvents (THF, ether, (PMe3)2 (5). Compound 4 is slowly transferred to 5 upon
hexane, dichloromethane, toluene, and acetonitrile) were increase of the temperature of the reaction mixture. Warming
obtained using Innovative Technologies Pure Solv. purification the sample up to 25 °C leads to the formation of (ArN)Mo(H)-
system. DME was dried over sodium/benzophenone; ethyl (Cl){η2-CatBNvCHPh}(PMe3)2 (6). Addition of another equi-
acetate was dried over CaH2. Benzene-d6 and toluene-d8 were valent of HBCat affords PhCH2N(BCat)2 and a mixture of 1,
dried by distillation over K/Na alloy. NMR spectra were (ArN)MoCl2(PMe3)3 14 and unknown decomposition products.
obtained with Bruker DPX-300 (1H: 300 MHz; 13C: 75.5 MHz;
31P: 121.5 MHz; 11B: 96.3 MHz) and Bruker DPX-600 (1H:
(ArNv)Mo(Cl){η3-N(vCHPh)(CatB-H⋯}(PMe3)2 (4). 1H NMR
600 MHz; 2D: 92.1 MHz; 13C: 151 MHz; 31P: 243 MHz; 11B: (600 MHz; 225 K; PhMe-d8; δ, ppm): 8.91 (br s, 1H, CHPh);
3
192.6 MHz) spectrometers. IR spectra were measured on 8.08 (d, JH–H = 7.4 Hz, 2H, o-H, CHPh); 6.63–7.37 (m, overlap-
an ATI Mattson FTIR spectrometer. Organic substrates were ping aromatic signals of HBCat, 2, 4, and 5); 5.06 (br s, 1H,
18952 | Dalton Trans., 2015, 44, 18945–18956
This journal is © The Royal Society of Chemistry 2015

View Article Online
Dalton Transactions
Paper
H-B); 4.33 (br m, 2H, 2CH, ArN, overlapping with CH (ArN) of (CH, ArN); 26.6 (CH, ArN); 24.9 (CH3, ArN); 24.8 (CH3, ArN);
3); 1.39 (br m, 6H, 2CH3, ArN); 1.29 (bm, 6H, 2CH3, ArN); 1.24 17.5 (PMe3); 17.1 (PMe3).
(br m, 18H, 2 PMe3). 31P{1H} NMR (243 MHz; 225 K; PhMe-d8;
Preparation of (ArN)Mo(H)2(PMe3)3 (2)
δ, ppm): −0.9 (br s, PMe3). 11B{1H} NMR (193 MHz; 253 K;
14
PhMe-d8; δ, ppm): 2.3 (br s, HBCat). 11B NMR (193 MHz; (ArN)Mo(Cl)2(PMe3)3
(0.50 g, 0.9 mmol) was dissolved in
283 K; PhMe-d8; δ, ppm): 2.2 (br d, JB–H ≈ 55 Hz; HBCat). 13C ∼25 mL of toluene and cooled −30 °C. THF solution of L-selec-
1
{1H} NMR (151 MHz; 253 K; PhMe-d8; δ, ppm): 172.0 (s, tride (C = 1 M, 1.8 mL, 1.8 mmol) was added dropwise to the
NvCHPh, found by H–13C HSQC NMR); 129.7 (s, o-C, CHPh); mixture. The colour of the solution almost immediately
1
27.5 (s, CH, ArN); 23.8 (s, CH3, ArN); 23.6 (s, CH3, ArN); 14.3 changed to a darker brown upon addition of L-selectride. The
(vt, 1JC–P = 24 Hz, 2 trans-PMe3).
reaction was allowed to proceed for ∼30 min until the bath
NMR was defrosted, and then allowed to react further at room tem-
(ArN)Mo(H)(Cl)(η1-N(BCat)vCHPh)(PMe3)2
(5). 1H
(600 MHz; 225 K; PhMe-d8; δ, ppm): 8.31 (br s, 1H, NvCHPh); perature for 1 hour. The contents were filtered and extracted
6.63–7.37 (m, overlapping aromatic signals of HBCat, 2, 4, and with toluene. All volatiles were removed under vacuum, result-
5); 4.12 (br s, 1H, CH, ArN); 3.92 (br s, 1H, CH, ArN); 1.32 ing in dark brown viscous oil (0.41 g, 88% yield). The product
2
(br m, 6H, 2CH3, ArN); 1.29 (d, JH–P = 8.4 Hz, 9H, PMe3); 1.21 is highly fluxional and unstable at room temperature (slowly
(br m, 6H, 2CH3, ArN); 0.87 (d, 2JH–P = 8.1 Hz, 9H, PMe3); −2.94 decomposes into a mixture of unidentified products). Without
(br m, 1H, MoH). 31P{1H} NMR (243 MHz; 225 K; PhMe-d8; decomposition, complex 2 can be stored at −30 °C under inert
2
2
δ, ppm): −1.4 (d, JP–P = 212.0 Hz, PMe3); −13.0 (d, JP–P
=
atmosphere. 1H NMR (600 MHz; toluene-d8; 244 K; δ, ppm):
212.0 Hz, PMe3). 11B NMR (193 MHz; 253 K; PhMe-d8; δ, ppm): −5.31 (dtd, 2JH–P = 60.6 Hz, 2JH–P = 46.2 Hz, 2JH–H = 7.2 Hz, 1H,
10.1 (br s, BCat). 11B NMR (193 MHz; 283 K; PhMe-d8; δ, ppm): MoH (trans- to PMe3)); 1.35 (m, 21H, 4 CH3 of NAr and PMe3);
10.2 (br s, BCat). 13C{1H} NMR (151 MHz; 253 K; PhMe-d8; 1.49 (br m, 18H, 2 PMe3); 2.08 (m, overlapping with residual
δ, ppm): 154.2 (s, NvCHPh, found by 1H–13C HSQC NMR); toluene-d8 signal, JH–P = 43.2 Hz, JH–H = 7.2 Hz, 1H, MoH
2
2
27.2 (s, CH, ArN); 26.9 (s, CH, ArN); 25.7 (bs, CH3, ArN); 25.2 (s, (trans- to NAr), found by 1H–1H COSY and 1H–31P HSQC NMR);
1
1
3
CH3, ArN); 13.1 (d, JC–P = 22.8 Hz, PMe3); 12.7 (d, JC–P
21.7 Hz, PMe3).
=
4.43 (sept, JH–H = 6.6 Hz, 2H, 2 CH, NAr); 6.91–7.32 (m, over-
lapping with residual toluene-d8 resonances, 3H, m-H and p-H
ArN)Mo(H)(Cl){η2-CatBNvCHPh}(PMe3)2
(6). 1H
NMR of NAr). 1H{31P} NMR (600 MHz; toluene-d8; 243 K; selected
3
2
(600 MHz; 295 K; PhMe-d8; δ, ppm): 7.65 (d, JH–H = 7.7 Hz, resonances; δ, ppm): −5.31 (d, JH–H = 7.2 Hz, 1H, MoH); 1.38
2H, o-H, CHPh, major isomer); 7.45 (d, 3JH–H = 7.7 Hz, 2H, o-H, (s, 9H, PMe3); 1.50 (s, 18H, 2 PMe3); 2.09 (d, 2JH–H = 7.2 Hz, 1H,
CHPh, minor isomer); 7.06 (m, MoH of major isomer, MoH). 31P{1H} NMR (243 MHz; toluene-d8; 247 K; δ, ppm): 14.8
obscured by aromatic signals, found by 1H–31P HSQC NMR); (d, JP–P = 19.4 Hz, 2P, 2 PMe3); 13.1 (t, JP–P = 19.4 Hz, 1P,
6.9–7.3 (m, overlapping aromatic signals of CPh and NAr of 2, PMe3). 1H–13C HSQC NMR ( f1: 600 MHz; f2: 151 MHz;
5, 6, and (ArN)MoCl2(PMe3)3); 6.91 (m, 2H, BCat, major toluene-d8; 243 K; 13C projection; selected resonances;
isomer); 6.78 (m, 2H, BCat, major isomer); 6.73 (br m, MoH of δ, ppm): 124.9, 122.0, 121.9, (m-C and p-C, NAr); 26.3 (CH,
2
2
minor isomer, found by H–31P HSQC NMR); 5.00 (dd, JH–P
=
NAr); 26.2 (CH3, NAr); 25.9 (PMe3), 25.0 (CH3, NAr); 24.2
1
3
3.0 Hz; 1H, NvCHPh, major isomer); 4.39 (br s, 1H, (PMe3). IR (nujol): 1620 cm−1 (broad, medium, Mo–H).
3
NvCHPh, minor isomer); 4.22 (sept, JH–H = 6.7 Hz, 1H, CH,
ArN, major isomer); 4.13 (sept, JH–H = 6.7 Hz, 1H, CH, ArN,
3
Preparation of Mo(H)2(PMe3)3(η3-NAr-HBCat) (15)
major isomer); 3.98 (br m, 1H, CH, ArN, minor isomer); 3.84 2 (0.388 g, 0.724 mmol) was dissolved in 30 mL of diethyl
(sept, 3JH–H = 6.7 Hz, 1H, CH, ArN, minor isomer); 1.80 (d, 2JH–P ether and cooled to −30 °C. HBCat (77.2 μL, 0.724 mmol) was
2
= 10.1 Hz, 9H, PMe3, major isomer); 1.72 (d, JH–P = 10.1 Hz, added to the solution via syringe. Immediate formation of a
9H, PMe3, minor isomer); 1.35 (bm, 9H, PMe3, minor isomer); blue precipitate was observed. The latter was washed with
3
1.24 (d, JH–H = 6.7 Hz, 6H, 2CH3, ArN, major isomer); 1.20 (d, small portions of cold ether and dried under vacuum (0.371 g,
3
2JH–P = 9.9 Hz, 9H, PMe3, major isomer); 1.18 (d, JH–H = 6.7 82% yield). The product is fluxional at room temperature and
Hz, 6H, 2CH3, ArN, major isomer). 31P{1H} NMR (243 MHz; decomposes after several hours. 1H NMR (600 MHz; toluene-
2
2
2
295 K; PhMe-d8; δ, ppm): 2.4 (d, JP–P = 88.5 Hz, PMe3, both d8; 244 K; δ, ppm): −6.03 (dt, JH–P = 39.6 Hz, JH–P = 88.2 Hz,
2
2
isomers); −5.2 (d, JP–P = 88.5 Hz, PMe3, both isomers). 1H, MoH); −2.79 (t, JH–P = 59.9 Hz, 1H, MoH); −1.82 (bs, 1H,
31P NMR (selectively decoupled from methyl groups at ArN–B–H–Mo agostic); 0.86 (d, JH–P = 7.3 Hz, 9H, PMe3); 1.33
2
1.20 ppm in the 1H NMR spectrum; 243 MHz; 295 K; PhMe-d8; (bs,6H, 2 iPr-CH3 of ArN); 1.35 (d, 2JH–P = 9.0 Hz, 18H, 2 PMe3);
2
2
2
i
δ, ppm): 2.4 (dd, JP–P = 88.5 Hz, JP–H = 50.9 Hz, PMe3); −5.2 1.55 (d, JH–P = 6.2 Hz, 6H, 2 Pr-CH3 of ArN); 3.45 (sept, 2H,
(br m, PMe3). 31P NMR (selectively decoupled from methyl 2 Pr-CH of ArN); 6.97–6.8 (br m, 4H, BCat). 1H{31P} NMR
i
groups at 1.80 ppm in the 1H NMR spectrum; 243 MHz; 295 K; (243 MHz, toluene-d8, 243 K; δ, ppm): −6.03 (bs, MoH); −2.79
PhMe-d8; δ, ppm): 2.4 (br m, PMe3); −5.2 (dd, JP–P = 88.5 Hz, (bs, MoH). 31P{1H} NMR (243 MHz, toluene-d8, 243 K; δ, ppm):
2
2JP–H = 45.0 Hz, PMe3). 11B NMR (193 MHz; 295 K; PhMe-d8; 2.54 (t, JP–P = 17.0 Hz, 1P, PMe3); 23.9 (d, JP–P = 17.0 Hz, 2P,
δ, ppm): 15.3 (br s, BCat). 1H–13C HSQC NMR ( f1: 300 MHz; f2: 2 PMe3). 1H–13C HSQC NMR ( f1: 600 MHz; f2: 151 MHz;
75.5 MHz; J = 145 Hz; 296 K; PhMe-d8; 13C projection for toluene-d8, 243 K; 13C projection; selected resonances;
major isomer; δ, ppm): 128.9 (o-C, CPh); 62.9 (NvCHPh); 27.2 δ, ppm): 125.0, 123.0, 121.9 (m-C and p-C, NAr); 112.0, 122.3
2
2
This journal is © The Royal Society of Chemistry 2015
Dalton Trans., 2015, 44, 18945–18956 | 18953

View Article Online
Paper
Dalton Transactions
(CatB); 26.8 (iPr-CH, NAr); 25.2 (2 PMe3); 24.8 (PMe3); 23.7
Two equiv. of HBCat (5.76 μL, 0.054 mmol) were added to a
(CH3, NAr); 23.5 (CH3, NAr); 11B NMR (192.6 MHz; toluene-d8, solution of 19 resulting in the formation of hydroboration
243 K; δ, ppm): −0.59 (br s, ArN-BCat).
product EtN(BCat)2 and a mixture of unidentified, Mo-contain-
ing decomposition products.
NMR reaction of (ArN)Mo(H)2(PMe3)3 (2) with HBPin
NMR reaction of Mo(H)2(PMe3)3(η3-NAr-HBCat) (15) with
MeCN
HBPin (5.2 μL; 0.036 mmol) was added in one portion to a
solution of 2 (18.5 mg; 0.036 mmol) in 0.6 mL of toluene-d8 in
an NMR tube. The reaction was monitored by NMR. The for-
mation of a fluxional borohydride complex, (ArN)Mo(η2-
H2BPin)(H)(PMe3)3 (16), was observed. 1H NMR (600 MHz;
MeCN (2.48 μL; 0.048 mmol) was added in one portion to a
solution of complex 15 (14.8 mg; 0.024 mmol) in 0.6 mL of
toluene-d8 in an NMR tube. The reaction was monitored by
NMR and complete conversion was observed after several
minutes at room temperature. Since the obtained product is
fluxional at room temperature, NMR analysis was performed at
−15 °C to show the presence of (ArN)Mo{N(BCat)vCHMe}(η2-
CH3CN)(PMe3)2 (20) as a mixture of two isomers (≈3 : 1 ratio
by NMR). Evolution of H2 gas was also observed. Major
isomer: 1H NMR (600 MHz; toluene-d8; 270 K; δ, ppm): 7.38 (q,
3JH–H = 4.3 Hz, 1H, N(BCat)vCHCH3); 6.83 (m, 2H, CatB); 6.74
(m, 2H, CatB); 4.02 (sept, 3JH–H = 6.7 Hz, 1H, CH, NAr); 3.08 (br
2
toluene-d8; 244 K; δ, ppm): −6.89 (q, JH–P = 63 Hz, 1H, MoH);
2
−4.13 (br s, 2H, η2-PinBH2); 1.10 (d, JH–P = 7.3 Hz, 18H, 2
2
PMe3); 1.20 (d, JH–P = 8 Hz, 9H, PMe3); 1.27 (s, 12 H, Me of
i
3
Bpin); 1.36 (br s, 12H, Pr-CH3 of NAr); 3.62 (sept, JH–H = 6.2
Hz, 2H, Pr-CH of NAr). 31P{1H} NMR (243 MHz; toluene-d8;
i
243 K; δ, ppm): 50.0 (br s, 2P, 2 PMe3); 12.7 (s, 1P, PMe3). 11B
NMR (192.6 MHz; toluene-d8; 243 K; δ, ppm): −3.6 (bs,
H2BPin).
3
s, 3H, NCCH3); 2.72 (sept, JH–H = 6.7 Hz, 1H, CH, NAr); 1.65
NMR reaction of (ArN)Mo(H)2(PMe3)3 (2) with MeCN
3
3
(br d, JH–H = 4.3 Hz, 3H, N(BCat)vCHCH3); 1.57 (d, JH–H
=
3
MeCN (2.86 μL; 0.055 mmol) was added in one portion to a
solution of 2 (14.5 mg; 0.027 mmol) in 0.6 mL of toluene-d8 in
an NMR tube. The reaction was monitored by NMR and com-
plete conversion was observed after several minutes at room
temperature. The solution turned dark red. The obtained
product is fluxional at room temperature, and therefore NMR
analysis was performed at −15 °C to show the formation of
(ArN)Mo(NvCHMe)2(PMe3)2 (19) as a mixture of three isomers
6.7 Hz, 3H, CH3, NAr); 1.53 (d, JH–H = 6.8 Hz, 3H, CH3, NAr);
1.32 (d, JP–H = 7.7 Hz, 9H, PMe3); 1.18 (d, JH–H = 6.8 Hz, 3H,
CH3, NAr); 0.93 (d, JP–H = 7.7 Hz, 9H, PMe3); 0.72 (d, JH–H
2
3
2
3
=
6.7 Hz, 3H, CH3, NAr). 1H–13C HSQC NMR ( f1: 600 MHz; f2:
151 MHz; toluene-d8; 270 K; 13C projection; selected reson-
ances; δ, ppm): 151.8 (N(BCat)vCHCH3); 121.2 (CatB); 111.2
(CatB); 27.9 (CH, Ar); 26.3 (CH, Ar); 25.2 (CH3, Ar); 23.4 (CH3,
Ar); 23.5 (CH3, Ar); 21.7 (CH3CHvN); 20.8 (CH3CN); 18.2
(PMe3), 14.6 (PMe3). 1H–13C HMBC NMR ( f1: 600 MHz; f2:
151 MHz; toluene-d8; 270 K; 13C projection; selected reson-
ances; δ, ppm): 203.8 (CH3CN). 31P{1H} NMR (243 MHz,
1
1
in the ratio of 1 : 0.7 : 0.1 by H NMR. Major isomer: H NMR
(600 MHz; toluene-d8; 258 K; δ, ppm): 8.13 (q, 2H,
3
2 NvCHCH3); 4.67 (sept, JH–H = 6.9 Hz, 2H, 2CH, NAr); 2.24
3
2
(br s, 6H, 2 NvCHCH3); 1.39 (d, JH–H = 6.8 Hz, 12H, 4CH3,
toluene-d8, 270 K; δ, ppm): 10.5 (d, JP–P = 177.6 Hz, PMe3),
NAr); 1.11 (m, 18H, 2 PMe3). 1H–13C HSQC NMR ( f1: 600 MHz;
f2: 151 MHz; toluene-d8; 258 K; 13C projection; selected reson-
ances; δ, ppm): 145.7 (CH3CHvN); 26.9 (iPr CH, NAr);
24.5 (CH3CHvN); 23.9 (iPr CH3, NAr); 13.8 (PMe3). 31P{1H}
NMR (243 MHz; toluene-d8; 258 K; δ, ppm): 1.31 (br s, 2 PMe3).
Second major isomer: 1H NMR (600 MHz; toluene-d8; 258 K;
δ, ppm): 8.12 (q, 1H, NvCHCH3); 7.70 (br q, 1H, NvCHCH3);
2
−1.86 (d, JP–P = 177.6 Hz, PMe3). 11B{1H} NMR (192.6 MHz,
1
toluene-d8, 270 K; δ, ppm): 13.3 (vb s). Minor isomer: H NMR
(600 MHz; toluene-d8; 270 K; δ, ppm): 8.56 (br s, 1H, N(BCat)v
3
CHCH3); 3.42 (sept, JH–H = 6.9 Hz, 2H, 2CH, NAr); 2.88 (br s,
3
3H, NCCH3); 2.62 (br s, 3H, NvCHCH3); 1.50 (d, JH–H = 6.9
2
Hz, 6H, CH3, NAr); 1.44 (d, JP–H = 7.0 Hz, 9H, PMe3), 1.29 (d,
3JH–H = 6.9 Hz, 6H, CH3, NAr), 0.92 (obscured by the major
isomer, PMe3). 1H–13C HSQC NMR ( f1: 600 MHz; f2: 151 MHz;
toluene-d8; 270 K; 13C projection; selected resonances; δ,
ppm): 26.9 (CH3, NAr), 20.5 (CH3CN), 21.3 (CH3CHvN), 18.1
(PMe3), 15.6 (CH3, NAr). 31P{1H} NMR (243 MHz, toluene-d8,
3
4.52 (sept, JH–H = 6.9 Hz, 2H, 2CH, NAr); 2.13 (br s, 6H,
3
2 NvCHCH3); 1.42 (d, JH–H = 6.8 Hz, 12H, 4CH3, NAr); 1.12
(m, 18H, 2 PMe3). 1H–13C HSQC NMR ( f1: 600 MHz; f2:
151 MHz; toluene-d8, 258 K; 13C projection; selected reson-
ances; δ, ppm): 145.7 (CH3CHvN); 27.2 (iPr CH, NAr);
26.1 (CH3CHvN); 23.3 (iPr CH3, NAr); 13.9 (PMe3). 31P{1H}
NMR (243 MHz; toluene-d8; 258 K; δ, ppm): 2.42 (br s, 2 PMe3).
258 K; δ, ppm): 9.0 (d, 2JP–P = 167.2 Hz, PMe3), −4.46 (d, 2JP–P
=
167.2 Hz, PMe3). Two equivalents of HBCat (5.12 μL,
0.048 mmol) were added to a solution of complex 20, resulting
in the formation of hydroboration product EtN(BCat)2 and
regeneration of the starting complex 15.
1
Minor isomer: H NMR (600 MHz; toluene-d8; 258 K; δ, ppm):
3
7.66 (q, 2H, 2 NvCHCH3); 4.40 (sept, JH–H = 6.9 Hz, 2H, 2CH,
3
NAr); 2.10 (br s, 6H, 2 NvCHCH3); 1.49 (d, JH–H = 6.8 Hz,
12H, 4CH3, NAr); 1.11 (m, 18H, 2 PMe3). 1H–13C HSQC NMR
( f1: 600 MHz; f2: 151 MHz; toluene-d8; 13C projection; selected
resonances; 258 K; δ, ppm): 145.8 (CH3CHvN); 27.1 (iPr CH,
Acknowledgements
NAr); 24.2 (CH3CHvN); 24.7 (iPr CH3, NAr); 13.8 (PMe3). This work was supported by NSERC (DG grant to GIN). GIN
31P{1H} NMR (243 MHz; toluene-d8; 258 K; δ, ppm): 2.41 (br s, further thanks the CFI/OIT for a generous equipment grant.
2 PMe3).
AYK thanks the OGS scholarship.
18954 | Dalton Trans., 2015, 44, 18945–18956
This journal is © The Royal Society of Chemistry 2015

View Article Online
Dalton Transactions
Paper
T. Frejd, Chem. – Eur. J., 2001, 7, 2158; (f) S.-G. Roh,
Y.-C. Park, T.-J. Kim and J. H. Jeong, Polyhedron, 2001, 20,
1961; (g) P. Hegarty, R. Lau and W. B. Motherwell, Tetrahe-
dron Lett., 2003, 44, 1851; (h) M. Locatelli and P. G. Cozzi,
Angew. Chem., Int. Ed., 2003, 42, 4928; (i) S. G. Roh,
J. U. Yoon and J. H. Jeong, Polyhedron, 2004, 23, 2063;
( j) L. Koren-Selfridge, H. N. Londino, J. K. Vellucci,
B. J. Simmons, C. P. Casey and T. B. Clark, Organometallics,
2009, 28, 2085; (k) A. A. Oluyadi, S. Ma and C. N. Muhoro,
Organometallics, 2013, 32, 70.
9 Esters: (a) S. C. Berk, K. A. Kreutzer and S. L. Buchwald,
J. Am. Chem. Soc., 1991, 113, 5093; (b) S. C. Berk and
S. L. Buchwald, J. Org. Chem., 1992, 57, 3751; (c) K. J. Barr,
S. C. Berk and S. L. Buchwald, J. Org. Chem., 1994, 59,
4323; (d) M. T. Reding and S. L. Buchwald, J. Org. Chem.,
1995, 60, 7884; (e) X. Verdaguer, M. C. Hansen, S. C. Berk
and S. L. Buchwald, J. Org. Chem., 1997, 62, 8522;
(f) L. Pehlivan, E. Métay, S. Laval, W. Dayoub,
D. Delbrayelle, G. Mignani and M. Lemaire, Eur. J. Org.
Chem., 2011, 7400; (g) Z. Mao, B. T. Gregg and A. R. Cutler,
J. Am. Chem. Soc., 1995, 117, 10139; (h) D. Bezier,
G. T. Venkana, L. C. Misal Castro, J. Zheng, T. Roisnel,
J.-B. Sortais and C. Darcel, Adv. Synth. Catal., 2012, 354,
1879; (i) K. Junge, B. Wendt, S. Zhou and M. Beller,
Eur. J. Org. Chem., 2013, 2061; ( j) A. J. Ruddy, C. M. Kelly,
S. M. Crawford, C. A. Wheaton, O. L. Sydora, B. L. Small,
M. Stradiotto and L. Turculet, Organometallics, 2013, 32,
5581.
Notes and references
1 H. C. Brown, Hydroboration, Wiley, NY, 1962.
2 Selected reviews: (a) L. Tonks and J. M. Williams, Contemp.
Org. Synth., 1997, 353; (b) I. Beletskaya and A. Pelter, Tetra-
hedron, 1997, 53, 4957 and references therein;
(c) T. Hayashi, in Comprehensive Asymmetric Catalysis, ed.
E. N. Jacobsen, A. Pfaltz and H. Yamamoto, Springer, NY,
1999, vol. 1, p. 349; (d) H. Braunschweig and M. Colling,
Coord. Chem. Rev., 2001, 223, 1; (e) C. M. Crudden and
D. Edwards, Eur. J. Org. Chem., 2003, 4695;
(f) W. Carruthers and I. Coldham, Modern Methods of
Organic Synthesis, Cambridge University Press, Cambridge,
4th edn, 2004, pp. 315–331; (g) C. Pubill-Ulldemolins,
A. Bonet, C. Bo, H. Gulyas and E. Fernandez, Org. Biomol.
Chem., 2010, 8, 2667.
3 (a) H. C. Brown, Organic Synthesis via Boranes, Wiley,
New York, 1975; (b) A. Pelter, K. Smith and H. C. Brown,
Borane Reagents, Academic Press, New York, 1988 and refer-
ences therein.
4 (a) D. Männig and H. Nöth, Angew. Chem., Int. Ed. Engl.,
1985, 24, 878; (b) K. Burgess and M. J. Ohlmeyer, Chem.
Rev., 1991, 91, 1179; (c) C. M. Vogels and S. A. Westcott,
Curr. Org. Chem., 2005, 9, 687; (d) C. C. Chong and
R. Kinjo, ACS Catal., 2015, 5, 3238 and references therein.
5 For data on the toxicity of transition metals, see: http://
www.ema.europa.eu/docs/en_GB/document_library/Scienti-
fic_guideline/2009/09/WC500003586.pdf.
6 For examples of alkene and alkyne hydroborations, see: 10 Imines: R. T. Baker, J. C. Calabrese and S. A. Westcott,
(a) D. A. Evans, A. R. Muci and R. Sturmer, J. Org. Chem., J. Organomet. Chem., 1995, 498, 109.
1993, 58, 5307; (b) K. Burgess and W. A. van der Donk, 11 For uncatalysed addition of alkyl boranes and BH3 to
Organometallics, 1994, 13, 3616; (c) K. Burgess and
W. A. van der Donk, J. Am. Chem. Soc., 1994, 116, 6561;
(d) E. A. Bijpost, R. Duchateau and J. H. Teuben, J. Mol.
Catal. A: Chem., 1995, 95, 121; (e) S. Pereira and M. Srebnik,
Organometallics, 1995, 14, 3127; (f) S. Pereira and
nitriles, see: (a) Y. Chujo, I. Tomita and T. Saegusa, Macro-
molecules, 1994, 27, 6714; (b) K. Wade, M. G. Davidson,
M. A. Fox, W. R. Gill, T. G. Hibbert and J. A. H. Maceride,
Phosphorus, Sulfur Silicon Relat. Elem., 1997, 124–125, 73;
(c) D. Jaganyi and A. Mzinyati, Polyhedron, 2006, 25, 2730.
M. Srebnik, Tetrahedron Lett., 1996, 37, 3283; (g) X. He and 12 (a) E. Peterson, A. Y. Khalimon, R. Simionescu,
J. F. Hartwig, J. Am. Chem. Soc., 1996, 118, 1696;
(h) J. F. Hartwig, C. N. Muhoro, X. He, O. Eisenstein,
R. Booque and F. Maseras, J. Am. Chem. Soc., 1996, 118,
10936; (i) D. H. Motry, A. G. Brazil and M. R. Smith III, J.
Am. Chem. Soc., 1997, 119, 2743; ( j) J. F. Hartwig and
L. G. Kuzmina, J. A. K. Howard and G. I. Nikonov, J. Am.
Chem. Soc., 2009, 131, 908; (b) A. Y. Khalimon,
O. G. Shirobokov, E. Peterson, R. Simionescu,
L. G. Kuzmina, J. A. K. Howard and G. I. Nikonov, Inorg.
Chem., 2012, 51, 4300.
C. N. Muhoro, Organometallics, 2000, 19, 30; (k) Y. D. Wang, 13 A. Y. Khalimon, P. Farha, L. G. Kuzmina and G. I. Nikonov,
G. Kimball, A. S. Prashad and Y. Wang, Tetrahedron Lett.,
2005, 46, 8777.
7 (a) D. R. Lantero, D. L. Ward and M. R. Smith III, J. Am.
Chem. Soc., 1997, 119, 9699; (b) D. R. Lantero, S. L. Miller,
Chem. Commun., 2012, 48, 455.
14 S. K. Ignatov, A. Y. Khalimon, N. H. Rees, A. G. Razuvaev,
P. Mountford and G. I. Nikonov, Inorg. Chem., 2009, 48,
9605.
J.-Y. Cho, D. L. Ward and M. R. Smith III, Organometallics, 15 Uncatalysed reaction for acetophenone gives only 50% con-
1999, 18, 235. version after 2 days.
8 Ketones: (a) D. A. Evans and A. H. Hoveyda, J. Org. Chem., 16 Similar transformations are known for bis(silyl) amines.
1990, 55, 519; (b) C. W. Lindsley and M. DiMare, Tetrahe-
dron Lett., 1994, 35, 5141; (c) G. Giffels, C. Dreisbach,
U. Kragl, M. Weiderding, H. Waldmann and C. Wandrey,
Angew. Chem., Int. Ed. Engl., 1995, 34, 2005; (d) A. J. Blake,
A. Cunningham, A. Ford, S. J. Teat and S. Woodward,
Chem. – Eur. J., 2000, 6, 3586; (e) I. Sarvary, F. Almqvist and
However, the reactions require either harsh conditions or
the presence of a catalyst: (a) N. Duffaut and J. P. Dupin,
Bull. Soc. Chim. Fr., 1966, 10, 3205; (b) R. J. P. Corriu,
V. Huynh, J. J. E. Moreau and M. Pataud-Sat, J. Organomet.
Chem., 1983, 225, 359; (c) T. Morimoto and M. Sekiya,
Chem. Lett., 1985, 1371.
This journal is © The Royal Society of Chemistry 2015
Dalton Trans., 2015, 44, 18945–18956 | 18955

View Article Online
Paper
Dalton Transactions
17 For previous discussion of degradation of HBcat under
catalytic conditions, see: S. A. Westcott, H. P. Blom,
T. B. Marder, R. T. Baker and J. C. Calabrese, Inorg. Chem.,
1993, 32, 2175.
K. Radacki, Angew. Chem., Int. Ed., 2008, 47, 5978;
(b) Y. Gloaguen, G. Alcaraz, A.-F. Pécharman, E. Clot,
L. Vendier and S. Sabo-Etienne, Angew. Chem., Int. Ed.,
2009, 48, 2964.
18 For the stoichiometric reactivity of 1 with carbonyls, see 23 (ArN) Mo(Cl)(OEt)(PMe3)3 (14) can be also prepared from
ref. 12. the hydrido–chloride 1 by reaction with EtOH. See ref. 12.
19 (a) H. M. M. Shearer and J. D. Sowerby, J. Chem. Soc., 24 For characterization of hydroboration products, see ESI.†
Dalton Trans., 1973, 2629; (b) G. Erker, W. Fromberg, 25 Similar to the reaction of complex 1 with HBCat, no
J. L. Atwood and W. E. Hunter, Angew. Chem., 1984, 96, 72;
(c) J. E. Bercaw, D. L. Davies and P. T. Wolczanski, Organo-
product of oxidative addition of borane to Mo akin to (ArN)-
Mo(H)(BCat)(PMe3)x (where x = 2 or 3) was detected.
metallics, 1986, 5, 443; (d) Y. Tanabe, H. Seino, Y. Ishii and 26 Data from Sigma-Aldrich.
M. Hidai, J. Am. Chem. Soc., 2000, 122, 1690; 27 M. Brookhart, K. Cox, F. G. N. Cloke, J. C. Green,
(e) M. F. C. Guedes da Silva, J. J. R. Frausto da Silva and
A. J. L. Pombeiro, Inorg. Chem., 2002, 41, 219.
M. L. H. Green, P. M. Hare, J. Bashkin, A. E. Derome and
P. D. Grebenik, J. Chem. Soc., Dalton Trans., 1985, 423.
20 A. Y. Khalimon, S. K. Ignatov, A. I. Okhapkin, 28 Since isomers of 19 are in fast exchange even at −15 °C, the
R. Simionescu, L. G. Kuzmina, Judith A. K. Howard and
G. I. Nikonov, Chem. – Eur. J., 2013, 19, 8573.
PMe3 resonances in the 31P NMR spectrum appear as
broad singlets.
21 For related agostic borylamides, see: (a) T. D. Forster, 29 (a) A. B. Jackson, C. K. Schauer, P. S. White and
H. M. Tuononen, M. Parvez and R. Roesler, J. Am. Chem. Soc.,
2009, 131, 6689; (b) D. Srimani, Y. Diskin-Posner, Y. Ben-David
and D. Milstein, Angew. Chem., Int. Ed., 2013, 52, 14131.
J. L. Templeton, J. Am. Chem. Soc., 2007, 129, 10628;
(b) A. Y. Khalimon, R. Simionescu, L. G. Kuzmina,
J. A. K. Howard and G. I. Nikonov, Angew. Chem., Int. Ed.,
2008, 47, 7701; (c) A. Y. Khalimon, R. Simionescu and
G. I. Nikonov, J. Am. Chem. Soc., 2011, 133, 7033.
22 For examples of agostic B–H complexes, see:
(a) H. Braunschweg, R. D. Dewhurst, T. Herbst and
18956 | Dalton Trans., 2015, 44, 18945–18956
This journal is © The Royal Society of Chemistry 2015