The first ring-expanded NHC-copper(i) phosphides as catalysts in the highly selective hydrophosphination of isocyanates

Article abstract of DOI:10.1039/d0cc05694d

A range of N-heterocyclic carbene-supported copper diphenylphosphides (NHC = IPr, 6-Dipp, SIMes and 6-Mes) were synthesised. These include the first reports of ring-expanded NHC-copper(i) phosphides. The compounds were characterised by NMR spectroscopy and X-ray crystallography. Reaction of (6-Dipp)CuPPh2 with isocyanates, isothiocyanates and carbon disulfide results in the insertion of the heterocumulene into the Cu-P bond. The NHC-copper phosphides were found to be the most selective catalysts yet reported for the hydrophosphination of isocyanates. They provide access to a broad range of phosphinocarboxamides in excellent conversion and good yield. This journal is

Full text of DOI:10.1039/d0cc05694d

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Downie, J. W. Hall, T. P. Collier Finn, D. J. Liptrot, J. P. Lowe, M. F. Mahon, C. L. McMullin and M. K.
Whittlesey, Chem. Commun., 2020, DOI: 10.1039/D0CC05694D.
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The First Ring-Expanded NHC-Copper(I) Phosphides as Catalysts in
the Highly Selective Hydrophosphination of Isocyanates
Thomas M. Horsley Downie,a Jonathan W. Hall,a Thomas P. Collier Finn,a David J. Liptrot,*,a John P.
Received 00th January 20xx,
Accepted 00th January 20xx
Lowe,a Mary F. Mahon,*,b Claire L. McMullina and Michael K. Whittleseya
DOI: 10.1039/x0xx00000x
A
range of N-heterocyclic carbene-supported copper supported by sterically demanding ligands are far less common,
diphenylphosphides (NHC = IPr, 6-Dipp, SIMes and 6-Mes) as evidenced by the small number of structurally characterised
were synthesised. These include the first reports of ring- (NHC)CuPR2 (NHC = N-heterocyclic carbene) complexes hitherto
expanded NHC-copper(I) phosphides. The compounds were reported.18 Despite their relative rarity, such species hint at the
characterised by NMR spectroscopy and X-ray crystallography. potential of low-molecularity copper(I) phosphides. For
Reaction of (6-Dipp)CuPPh2 with isocyanates, isothiocyanates example, [(NHC)CuPPh2]3 complexes (NHC
=
IiPr, 1,3-
and carbon disulfide results in the insertion of the bis (iso propyl)-imidazol-2-ylidene; ItBu, 1,3-bis (tert-butyl)-
heterocumulene into the Cu-P bond. The NHC-copper imidazol-2-ylidene), mediate the hydrophosphination of
phosphides were found to be the most selective catalysts yet terminal alkynes at elevated temperature.19
reported for the hydrophosphination of isocyanates. They In contrast to the formation of alkyl and alkenyl phosphines
provide access to a broad range of phosphinocarboxamides in from alkenes and alkynes, hydrophosphination of isocyanates
excellent conversion and good yield.
has been far less well-studied. Often, significant competing
side-reactivity6 or poor substrate scope20 hamper access to, and
The exploitation of metal phosphides is a widespread route to thus exploitation of, phosphinocarboxamide products. These
organophosphorus compounds.1-4 Beyond their stoichiometric products are potential synthetic intermediates and, particularly,
application, catalytic intermediates of the form LnMPR2 have ligands owing to their polydentate nature comprising both hard
been integral to a green revolution in organophosphorus and soft donor atoms.21-23
chemistry, especially when the metals concerned are Earth- Ring expanded N-heterocyclic carbenes (RE-NHCs) can offer
abundant first row transition metals (i.e. Fe, Co). These mid-d- increased stability and reduced molecularity to reactive metal
block systems, however, are prone to redox-switching and species through their significant steric demand.24-26 RE-NHC-
single-electron transfer reactions which, coupled with the copper(I) complexes such as copper(I) halides and copper(I)
stability of phosphorus centred radicals, often result in alkoxides have been established as having enhanced catalytic
undesirable side reactions.5-7
activity over their five-membered ring counterparts.27-29
The propensity of copper(I) towards the well-defined isohypsic We thus set out to synthesise and characterise a range of low-
reaction steps of σ-bond metathesis, and insertion of coordinate copper phosphide species supported by both 5- and
unsaturated substrates should allow copper(I) phosphides to 6-membered NHCs in order to evaluate their potential as
overcome such issues. Despite this appeal, copper phosphides catalytic intermediates. Herein we report the rational synthesis
often form only as complex oligomers, with a concomitant of four (NHC)CuPPh2 compounds (NHC = IPr, 6-Dipp, SIMes and
reduction in reactivity.8-17 Low-coordinate copper phosphides 6-Mes), including the first examples of RE-NHC copper
phosphides. These compounds were fully characterised by NMR
spectroscopy and X-ray crystallography and investigated for
aDepartment of Chemistry, University of Bath, Claverton Down, Bath, BA2 7AY,
UK. E-mail: dl260@bath.ac.uk
bX-Ray Crystallography Suite, University of Bath, Claverton Down, Bath, BA2 7AY
UK
Email: m.f.mahon@bath.ac.uk
†
Electronic Supplementary Information (ESI) available: General synthetic
experimental details, NMR spectra, X-ray analysis of compounds (CCDC
1-7
numbers: 2014370-2014376), details of DOSY experiments, details of the
computational analysis and atomic coordinates of the DFT computed structures. For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/x0xx00000x
Scheme 1 Synthesis of (NHC)CuPPh2 complexes.
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their onward reactivity towards heterocumulenes. This access characterised by the presence of a 31P{1H} NMR resonance at δ
View Article Online
DOI: 10.1039/D0CC05694D
to well-defined putative reaction intermediates allows us to -26.1 ppm, consistent with the data previously reported by
report the most selective catalytic hydrophosphination of Nolan and co-workers.30 Application of these conditions to
isocyanates found to date.
(NHC)CuOtBu (NHC = 6-Dipp, SIMes and 6-Mes) yielded (6-
Dipp)CuPPh2,
Addition of 1 equivalent of Ph2PSiMe3 to a colourless benzene and (6-Mes)CuPPh2,
solution of (IPr)CuOtBu resulted in the instantaneous each exhibit a single 31P{1H} NMR resonance at δ -
and
appearance of a yellow colouration and the emergence of new 23.8, -32.0 and -23.5 ppm respectively. The presence of peaks
peaks in the 1H and 31P NMR spectra. (IPr)CuPPh2, , was attributed to tBuOSiMe3 in the 1H NMR spectra of each of these
isolated from this solution (Scheme 1) in 81% yield, and reactions was interpreted as an indication that the formation of
compounds occurs via a Cu-O/P-Si σ-bond metathesis.
Crystals suitable for single crystal X-ray diffraction analysis of
were grown from benzene/hexane solutions at room
temperature. Compounds (Figure S1) and (Figure 1a),
2
, as a yellow solid, as well as (SIMes)CuPPh2,
3,
4
, as orange solids (Scheme 1). Compounds
2
,
3
4
1
1-4
1
-
4
1
2
featuring bulky diisopropylphenyl groups on the flank of the
NHC, exhibit monomeric structures with a near-linear geometry
at the copper centre (
1
: C1-Cu1-P1 177.09(6)°;
2
: C1-Cu1-P1
177.51(6)°). Compounds
3
(Figure 1b) and
4 (Figure S2),
containing less sterically-encumbering mesityl groups, adopt
dimeric crystal structures in which the copper centres are three-
coordinate, bonding to two bridging phosphorus atoms to give
approximately rhomboid four-membered rings (
88.712(16)°, Cu1-P1-Cu1i 91.286(16)°;
: P1-Cu1-P2 85.61(3)°,
3
: P1-Cu1-P1i
4
Cu1-P2-Cu2 94.66(3)°). Previous reports of dimeric NHC-
copper(I) complexes supported by less bulky NHCs feature short
Cu···Cu distances of 2.23-2.31 Å, which were interpreted as
indicating the presence of copper-copper interactions. The
longer Cu···Cu interatomic distances of compounds
3 (3.3502(7)
Å) and (3.4852(5) Å) suggest there is no significant interaction
4
between the metal centres in these two cases. Comparatively,
the only other structurally characterised (NHC)CuPPh2,
[(ItBu)CuPPh2]3, adopts a trimeric structure in the solid state.19
The Cu-CNHC bond distances of compounds
1.90-1.97 Å. The monomeric complexes, (1.9037(17) Å) and
(1.9272(18) Å), possess shorter bonds than in (1.9234(15) Å)
and (1.969(3), 1.965(3) Å), an observation attributed to the
1-4 range between
1
2
3
4
increased steric demand provided by two bridging phosphides.
These compare with the even longer Cu-CNHC bonds of 1.990-
1.997 Å observed in trimeric [(ItBu)CuPPh2]3.19 Complexes
4
2
and
, featuring ring-expanded NHCs, have noticeably longer Cu-
CNHC bonds than and respectively, likely due to the enhanced
1
3
steric demand of the flanking aryl groups resulting from the
increased N-C-N bond angle of the NHC. These trends are
conserved in the Cu-P bond distances which are longer for the
RE-NHC containing compounds,
respective 5-membered counterparts,
versus : 2.2113(5) Å; 2.3298(5) Å, versus
2.3908(8) Å) and wherein the dimeric species
2
and 4, compared to their
and , ( : 2.2079(5) Å,
2.3803(9),
and show
1
3 1
2
3
4
3
4
Fig. 1 ORTEP representations (30% probability ellipsoids) of (a) compound 2 and (b)
compound 3. Hydrogen atoms except those on the NHC backbone of 3 are omitted for
clarity. Selected bond lengths (Å) and angles (°): (2) Cu1-P1 2.2113(5), Cu1-C1 1.9272(18),
P1-C29 1.835(2), P1-C35 (1.836(2), C1-Cu1-P1 177.51(6), Cu1-P1-C29 107.31(6), Cu1-P1-
C35 102.92(6), C29-P1-C35 105.12(9); (3) Cu1-P1 2.3298(5), Cu-P1i 2.3558(5), Cu1-C1
1.9234(15), P1-C22 1.824(2), P1-C28 1.8208(18), C1-Cu1-P1 137.59(5), C1-Cu1-P1i
133.14(4) P1-Cu1-P1i 88.712(16) Cu1-P1-Cu1i 91.286(16), Cu1-P1-C22 110.55(5), Cu1-P1-
C28 121.30(6).
Scheme 2 Reactivty of (6-Dipp)CuPPh2, 2, with iPrNCO, PhNCS and CS2.
2 | J. Name., 2012, 00, 1-3
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stand at room temperature (Figure 2). In contrast VtioewpArreticvleioOunslinlye
reported phosphaureates which cDoOmI:p1r0is.1e039/D0CκC20-5(6N9,4OD)
coordination mode and delocalised π-bonding,21-23
features
a
5
κ1-coordination of the nitrogen to the copper atom. This results
in a relatively short Cu-N bond length (1.9053(11) Å), with C-N
(1.3314(19) Å) and C=O (1.2383(17) Å) bond lengths consistent
with localised single and double bonds respectively. Evidence
for the persistence of this bonding mode in solution can be
inferred from the 31P{1H} NMR signal at 2.2 ppm. This value is
significantly downfield of known Zr(IV) and Y(III) compounds
which feature a delocalised κ2-(N,O) bonding mode in the solid
state and solution 31P{1H} NMR spectra containing resonances
at δ -12.1 and -20.1 ppm respectively.21, 22
The facile insertion of an isocyanate into the Cu-P bond of
2
prompted a study of other electrophiles. Reactions with
excesses of phenyl isothiocyanate and carbon disulfide resulted
in insertion of the respective heterocumulene into the Cu-P
bond to give new compounds,
diffraction analysis performed on colourless crystals of
S3) and red crystals of (Figure S4) reveal that the copper
centre is bonded to the sulfur atom in both cases. In contrast to
the phosphaureate , the phosphorus atoms in compounds
and also coordinate to the metal centre resulting in CuSCP
four-membered rings in both cases. The 31P{1H} NMR spectra of
and each reveal a single major resonance, at δ 28.8 and 45.2
6
and
7
(Scheme 2). X-ray
Fig. 2 ORTEP representations (30% probability ellipsoids) of compound 5. Hydrogen
atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Cu1-N3
1.9053(11), Cu1-C1 1.9283(13), N3-C29 1.4871(18), N3-C32 1.3314(19), O1-C32
1.2383(17), P1-C32 1.8826(14), C1-Cu1-N3 173.90(5), Cu1-N3-C29 124.19(9), Cu1-N3-
C32 120.92(9), N3-C32-O1 128.34(13), N3-C32-P1 109.84(9), O1-C32-P1 121.76(11).
6
(Figure
7
5
6
7
greater Cu-P distances than the monomeric compounds,
1 and
6
7
2
.
ppm respectively.
Addition of 1 equivalent of diphenylphosphine to a colourless
In order to investigate the persistence of the observed solid-
state structures in solution, the compounds were interrogated
by diffusion ordered 1H NMR spectroscopy (DOSY). Compounds
solution of compound
5 resulted immediately in yellow
colouration, and inspection of the 31P{1H} NMR spectrum
suggested that Ph2PC(O)NH(iPr) had been liberated with the
1
and
corresponded to their crystallographically derived monomeric
structures ( : rDOSY = 5.40 Å, rcomp = 5.31 Å; : rDOSY = 5.27 Å, rcomp
= 5.52 Å). The hydrodynamic radius observed by DOSY for
compound showed a close correlation with that calculated for
its dimeric structure seen in the solid state ( : rDOSY = 6.03 Å,
rcomp = 6.50 Å). This interpretation is corroborated by the low-
temperature (235 K) 13C{1H} NMR spectrum of
which shows a
broad triplet for the carbenic carbon from a 2J13 -31 coupling to
2 were found to have hydrodynamic radii that
regeneration of complex
could be assembled into a catalytic manifold (Figure 3) to
hydrophosphinate range of isocyanates with
2. These stoichiometric reactions
1
2
a
3
diphenylphosphine in C6D6 in the presence of 1 mol% of
(Table 1).
1-4
3
Whilst catalyst-free hydrophosphination of isocyanates can be
conducted under strictly neat conditions, the reactivity was
limited to a range of small aromatic isocyanates;20 cyclohexyl
isocyanate exhibited no reactivity under such conditions. In
3
C
P
the two bridging but equivalent phosphorus atoms. Compound
, however, was found to have a hydrodynamic radius that was
more consistent with it existing in solution as a monomer, not
the dimer observed in the solid state ( : rDOSY = 5.29 Å, rcomp
5.21 Å for monomer, rcomp = 6.55 Å for dimer).
Addition of a slight excess of isopropyl isocyanate to a yellow
C6D6 solution of
resulted in a loss of colour. Analysis by 31P{1H}
NMR spectroscopy revealed the attenuation of the resonance
assigned to , and the appearance of a single major resonance
4
contrast, 1 mol% of complexes
3 and 4 brought about full
conversion of both cyclohexyl and isopropyl isocyanate within
30 minutes, and even the very sterically-congested tertbutyl
isocyanate reached excellent conversions (>90 %) within 2
4
=
hours. 2 was found to catalyse the reaction more slowly than 1,
2
2
at δ 2.2 ppm. Comparison to the 31P{1H} NMR signal of δ -4.0
ppm observed for the free phosphaurea Ph2PC(O)NH(iPr) was
interpreted as implying that insertion of the isocyanate into the
Cu-P bond had occurred (Scheme 2).6 The corresponding 1H
NMR spectrum exhibited a new set of peaks consistent with a
complex incorporating the 6-Dipp ligand and the isopropyl
isocyanate.
The formation of (6-Dipp)Cu[N(iPr)C(O)PPh2],
5, was confirmed
by X-ray diffraction analysis, performed on colourless single
crystals which grew from the reaction solution when left to
Fig. 3 Proposed mechanism for the copper-catalysed hydrophosphination of isocyanates.
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J. Name., 2013, 00, 1-3 | 3
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Table 1 Catalytic hydrophosphination of isocyanates (0.20 mmol) with Ph2PH (35 μL,0.20
mmol) and 1 mol% 1-4 in C6D6 (0.5 mL). aConversions based on relative 1H and 31P NMR
spectroscopic integrations of starting material and product. bIsolated yields of I from a
preparative scale reaction in toluene, see SI.
Notes and references
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Entry
R
iPr
Catalyst
Time (h)
0.5
5
0.5
0.5
2
4
2
2
0.5
0.5
0.5
0.5
0.5
0.5
Conversion (%)a
I:II
1
2
3
4
5
6
7
8
9
1
2
3
4
1
2
3
4
1
2
3
4
3
4
90
96
>99 (70)b
>99
94
81
91 (67)b
96
>99
48
>99 (77)b
>99
>99 (79)b
>99
100:0
100:0
100:0
100:0
100:0
100:0
100:0
100:0
100:0
92:8
8
9
tBu
Ph
Cy
10 T. A. Annan, R. Kumar and D. G. Tuck, J. Chem. Soc., Chem.
Commun., 1988, 446-448.
11 A. H. Cowley, D. M. Giolando, R. A. Jones, C. M. Nunn and J.
M. Power, J. Chem. Soc., Chem. Commun., 1988, 208-209.
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Ed., 1993, 32, 242-245.
10
11
12
13
14
, and
95:5
100:0
100:0
100:0
15 C. Meyer, H. Grützmacher and H. Pritzkow, Angew. Chem.
Int. Ed., 1997, 36, 2471-2473.
3
4, attributable to the greater steric demands of the 6-
Dipp ligand.
16 S. Scholz, M. Bolte, M. Wagner and H.-W. Lerner, Z. Anorg.
Allg. Chem., 2007, 633, 1199-1204.
In contrast to the iron-catalysed hydrophosphination of
isocyanates,6 only very limited amounts of double insertion of 17 P. J. Harford, J. Haywood, M. R. Smith, B. N. Bhawal, P. R.
Raithby, M. Uchiyama and A. E. H. Wheatley, Dalton Trans.,
2012, 41, 6148-6154.
the N=C bond were observed for phenyl isocyanate when
catalysed by or , and none was observed when or were
the precatalyst. In all other cases, the reaction was specific for
single insertion, and in no case was the formation of Ph2PPPh2
observed.
2
3
1
4
18 B. Khalili Najafabadi and J. F. Corrigan, Dalton Trans., 2015,
44, 14235-14241.
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2010, 29, 2557-2565.
In conclusion, the synthesis of a series of NHC-copper(I)
phosphides was accomplished by reaction of alkoxide
precursors with Ph2PSiMe3. Complexes
reports of RE-NHC copper(I) phosphides (compounds
1
-
4
, including the first
and ),
2
4
were found to be either monomeric or dimeric in the solid- and 22 W. Yi, J. Zhang, L. Hong, Z. Chen and X. Zhou,
Organometallics, 2011, 30, 5809-5814.
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solution-state depending on the steric bulk of the NHC ligand.
Reaction of with isocyanates, isothiocyanates and carbon
disulfide resulted in the insertion of the heterocumulene into
the Cu-P bond to give compounds . Compounds were
2
5
-
7
1-4
found to be highly active and selective precatalysts for the 25 M. J. Page, W. Y. Lu, R. C. Poulten, E. Carter, A. G. Algarra, B.
M. Kariuki, S. A. Macgregor, M. F. Mahon, K. J. Cavell, D. M.
Murphy and M. K. Whittlesey, Chem.- Eur. J., 2013, 19, 2158-
2167.
hydrophosphination of isocyanates. We continue to investigate
the nature and scope of this catalytic reactivity, including its
application
in
the
hydrophosphination
of
other
26 R. C. Poulten, M. J. Page, A. G. Algarra, J. J. Le Roy, I. López, E.
Carter, A. Llobet, S. A. Macgregor, M. F. Mahon, D. M.
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heterocumulenes.
This research made use of the Balena High Performance
Computing (HPC) Service at the University of Bath. DJL thanks
the Royal Society for the support of a University Research
Fellowship. We acknowledge financial support from the EPSRC
Centre for Doctoral Training in Catalysis (EP/L016443/1) for
JWH.
28 J. W. Hall, D. M. L. Unson, P. Brunel, L. R. Collins, M. K.
Cybulski, M. F. Mahon and M. K. Whittlesey,
Organometallics, 2018, 37, 3102-3110.
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and S. Díez-González, ChemCatChem, 2018, 10, 2041-2045.
30 G. C. Fortman, A. M. Z. Slawin and S. P. Nolan,
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Conflicts of interest
There are no conflicts to declare.
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RNCO
DOI: 10.1039/D0CC05694D
O
R
Ph2PH
Ph2P
N
H
The first copper(I) phosphides supported by ring-expanded N-heterocyclic carbenes have been
synthesised and react readily with heterocumulenes. These copper(I) phosphides are highly active
and selective in the hydrophosphination of isocyanates.