Template synthesis of donor-functionalized NX-carbenes (X = P, Si)

Article abstract of DOI:10.1021/om900725e

Donor-functionalized (phosphino)(amino)- and (silyl)(amind)carbenes are generated via nucleophilic attack at the carbon atom of a coordinated isocy ande on a piano-stool iron(II) complex. The template synthesis methodology involves the formation of yliden

Full text of DOI:10.1021/om900725e

6370 Organometallics 2009, 28, 6370–6373
DOI: 10.1021/om900725e
Template Synthesis of Donor-Functionalized NX-Carbenes (X = P, Si)
Insun Yu, Christopher J. Wallis, Brian O. Patrick, and Parisa Mehrkhodavandi*
Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver,
British Columbia, Canada
Received August 18, 2009
Summary: Donor-functionalized (phosphino)(amino)- and
a judicious choice of alkyl substituents, can lead to efficient
catalysis.3b
(silyl)(amino)carbenes are generated via nucleophilic attack
at the carbon atom of a coordinated isocyande on a piano-stool
iron(II) complex. The template synthesis methodology involves
the formation of ylidene complexes, which are reduced to yield
the desired carbene complexes. The electronic properties of the
resulting carbene complexes are similar and indicate that in the
(phosphino)(amino)carbene the phosphine lone pair is not
interacting with the carbene carbon and is available for further
reactivity.
P- or Si-functionalized carbenes remain largely unex-
plored as ligands,2a,d,4 although rare examples were shown
to be excellent ligands for rhodium.5,6 Challenges in the
synthesis of the free carbenes and in the formation of
complexes via ligand substitution pathways have severely
limited the utility of these species as ligands. Donor-functio-
nalized analogues of these carbenes are hitherto unknown.
Herein we report donor-functionalized (phosphino)-
(amino)- and (silyl)(amino)carbenes formed using template
synthesis.7,8 Recently, template synthesis via nucleophilic
attack at the carbon atom of a coordinated isocyanide9,10
has been used to form M-NHC complexes with an H sub-
stituent at the N-position. These complexes were further
utilized to form the first NHC-containing macrocycles,11 but
to our knowledge this strategy has not been used for P- or Si-
substituted carbenes. Our general methodology involves
nucleophilic attack on a chelating isocyanide-triphenylpho-
sphine ligand coordinated to a CpFe(CO) fragment, complex
1, to form ylidene complexes followed by protonation of the
resulting imine to yield the desired carbene complexes
(Scheme 1).12,13
Changing the carbon-bound substituents of stable singlet
carbenes can have a profound effect on their steric and
electronic properties as ligands.1,2 A large collection of N-
heterocyclic carbenes (NHCs) with varied N-bound substit-
uents and their donor-functionalized analogues have been
reported.1 Whereas NHCs use two C-bound donors to
stabilize the carbene (push-push carbenes), early work on
acyclic carbenes2 and recent expansion of the field to stable
cyclic (alkyl)(amino)carbenes (CAACs)3a have demon-
strated that “push-spectator” carbenes (NX-carbenes;
X = C, P, Si) can be stronger donors than NHCs and, with
The reaction of the iron isocyanide complex 112 with 1
equiv of KPPh2 or KSiPh3 in THF at -78 °C affords the
corresponding iron ylidene complexes 2 and 3, respectively.14
*Corresponding author. E-mail: mehr@chem.ubc.ca.
(1) For recent reviews on NHCs and donor-functionalized analogues
ꢀ
see: (a) de Fremonta, P.; Marion, N.; Nolan, S. P. Coord. Chem. Rev.
2009, 253, 862–892. (b) Jacobsen, H.; Correa, A.; Poater, A.; Costabile, C.;
Cavallo, L. Coord. Chem. Rev. 2009, 253, 687–703. (c) Hahn, F. E.; Jahnke,
M. C. Angew. Chem., Int. Ed. 2008, 47, 3122–3172. (d) Herrmann, W. A.
Angew. Chem., Int. Ed. 2002, 41, 1290–1309.
(7) For cyclic C-amino phosphorus ylides as a source of bidentate
phosphine/aminocarbene ligands for transition metals see: Vignolle, J.;
Donnadieu, B.; Bourissou, D.; le Soleilhavoup, M.; Bertrand, G. J. Am.
Chem. Soc. 2006, 128, 14810–14811.
(2) For representative reviews on non-NHCs see: (a) Vignolle, J.;
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Cattoen, X.; Bourissou, D. Chem. Rev. 2009, 109, 3333-3384.
(b) Arnold, P. L; Pearson, S. Coord. Chem. Rev. 2007, 251, 596–609.
(c) Jones, N. D.; Cavell, R. G. J. Organomet. Chem. 2005, 690, 5485–5496.
(d) Canac, Y.; Soleilhavoup, M.; Conejero, S.; Bertrand, G. J. Organomet.
Chem. 2004, 689, 3857–3865. (e) Bourissou, D.; Guerret, O.; Gabbai, F. P.;
Bertrand, G. Chem. Rev. 2000, 100, 39–91.
(8) For intramolecular stabilization of (amino)(silyl)carbene com-
plexes via interaction of the metal center with the silicon substituent see:
Schwarz, M.; Kickelbick, G.; Schubert, U. Eur. J. Inorg. Chem. 2000,
1811–1817.
(9) (a) Tamm, M.; Hahn, F. E. Coord. Chem. Rev. 1999, 182, 175–209.
(b) Basato, M.; Michelin, R. A.; Mozzon, M.; Sgarbossa, P.; Tassan, A.
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G. Angew. Chem., Int. Ed. 2005, 44, 5705–5709. (b) Anderson, D. R.;
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(13) Liu et al. reported a diaminocarbene-phosphine complex via the
well-known nucleophilic addition of n-propylamine to a coordinated
isocyanide complex, 1 (ref 12a). We have found analogous reactions with
the less nucleophilic HPPh2 and HSiPh3 to be impossible.
(14) Complex 3 was obtained in 28% isolated yield along with an
unidentified byproduct (10% isolated yield).
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r
pubs.acs.org/Organometallics
Published on Web 09/23/2009
2009 American Chemical Society

Note
Organometallics, Vol. 28, No. 21, 2009 6371
Scheme 1. Methodology for the Template Synthesis of Donor-
Functionalized NX-Carbenes (X = P, Si)
Scheme 2. Conversion of Carbene Complex 4 to 1 and HPPh2
by phosphine elimination.6d This indicates that, unlike the
amino functionality, there is no π contribution from the
phosphino moiety; it retains its lone pair and is able to
deprotonate the amine. A similar reaction is not observed
with the analogous diaminocarbene complex.12a,13
The carbonyl stretching frequencies of complexes 2-5
were used to gauge the electronic differences between the
ligands. The ylidene complexes 2 and 3 had νCO stretches at
1912 and 1915 cm-1, respectively, while the carbene com-
plexes 4 and 5 had stretches at 1956 and 1959 cm-1. Although
the ylidene complexes 2 and 3 are neutral while the carbene
complexes 4 and 5 are cationic and thus a direct comparison
between them is not meaningful, the progressive increase in
the stretching frequencies of the latter suggests a decreased
electron density on the metal and corresponds to the decrease
in π electron contribution to the carbene center as substitu-
ents are changed from N (νCO 1949 cm-1) to P to Si.
In this work we have described the first examples of donor-
functionalized NX-carbenes (X = P, Si), which were gener-
ated via a two-step template synthesis on a piano-stool
iron(II) complex. Spectroscopic evaluation of these com-
plexes indicates that although they are both “push-spectator”
carbenes, they are nevertheless electronically distinct. This
work opens up avenues for template synthesis of currently
inaccessible acyclic and cyclic carbenes. We are conducting
computational studies to elucidate the electronic properties
of these systems and will expand this methodology to a
series of acyclic and cyclic carbene complexes on Ru and
Rh centers.
The molecular structures of 2 and 3 were determined by
single-crystal X-ray crystallography and show octahedral
piano-stool iron centers similar to that of the reported
isocyanide complex 1.12 The metal-bound carbon atoms in
2 and 3 are planar. The Fe-Cylidene distances in 2 and 3 of
2.005(3) and 1.997(3) A are significantly longer than the
Fe-Cisocyanide distance reported for 1 (1.77 A), indicating
that the latter has a greater bond order than the Fe-Cylidene
bonds. In contrast to the sole reported Rh-NPC complex6a,b
the ylidene-bound phosphorus atom in 2 is not coordinated
to the metal. In this case η2 coordination is not expected
because complex 2 features a coordinatively saturated iron
center.15
The reaction of the ylidene complexes 2 and 3 with an
equimolar amount of HBF4 in CHCl3 at -35 °C forms the
corresponding carbene complexes 4 and 5 (Scheme 1). The
(silyl)(amino)carbene complex 5 was obtained in 97% iso-
lated yield; however, although the analogous (phosphino)-
(amino)carbene complex 4 is formed quantitatively, it de-
composes in solution and more slowly in the solid state (see
1
below). The characteristic downfield N-H H NMR reso-
nances at 9.64 and 10.62 ppm confirm the formation of the
carbene complexes 4 and 5, respectively; the resonance for
the related diaminocarbene iron complex appears at 7.97
ppm.12a,13 The 13C{1H} NMR spectra for 4 and 5 show
significantly deshielded Fe-Ccarbene signals at 276.4 and
299.4 ppm. These downfield signals are indicative of push-
spectator carbenes;2a the 13C{1H} NMR signals for reported
complexes of push-push carbenes are significantly more
upfield.6a,b The (silyl)(amino)carbene fragment does not
behave as a push-pull carbene and instead is analogous to
aryl- or alkylamino carbenes.6c
A THF solution of the (phosphino)(amino)carbene com-
plex 4 reverts to the isocyanide complex 1 and diphenylpho-
sphine in a few hours, although traces of 1 are evident after
only a few minutes at room temperature (Scheme 2).16 The
31P{1H} NMR spectrum of 4 shows two sets of doublets at
38.1 and 51.0 ppm for the Fe- and carbene-bound phos-
phines, respectively (3JP-P = 12 Hz). Over time, these signals
disappear and new signals for Fe-P in 1 (54.4 ppm) and
diphenylphosphine (-40 ppm) appear. The reaction likely
occurs via intramolecular deprotonation of the cyclic
amine by the carbene-bound diphenylphosphine, followed
Experimental Section
General Methods. Unless otherwise specified all procedures
were carried out using standard Schlenk techniques or in an
MBraun glovebox. A Bruker Avance 300 MHz spectrometer
and Bruker Avance 400dir MHz spectrometer were used to
record the 1H NMR, 13C{1H} NMR, and 31P{1H} NMR
spectra. 1H NMR chemical shifts are given in ppm versus
residual protons in deuterated solvents as follows: δ 5.32 for
CD2Cl2 and δ 7.27 for CDCl3. 13C{1H} NMR chemical shifts
are given in ppm versus residual 13C in solvents as follows:
δ 54.00 for CD2Cl2 and δ 77.23 for CDCl3. 31P{1H} NMR
chemical shifts are given in ppm versus 85% H3PO4 setat0.00ppm.
A Waters/Micromass LCT mass spectrometer equipped with
an electrospray (ESI) ion source and a Kratos-50 mass
spectrometer equipped with an electron impact ionization (EI)
source were used to record low-resolution and high-resolution
spectra. IR spectra were obtained on a Thermo Scientific FT-IR
spectrometer (Nicolet 4700). Diffraction measurements for
X-ray crystallography were made on a Bruker X8 APEX II
diffractometer with graphite-monochromated Mo KR radia-
tion. The structure was solved by direct methods and refined by
full-matrix least-squares using the SHELXTL crystallographic
software of Bruker-AXS. Unless specified, all non-hydrogens
were refined with anisotropic displacement parameters, and all
hydrogen atoms were constrained to geometrically calculated
positions but were not refined. CHN analysis was performed
using a Carlo Erba EA1108 elemental analyzer. The elemental
composition of unknown samples was determined by using a
(15) Despagnet, E.; Miqueu, K.; Gornitzka, H.; Dyer, P. W.;
Bourissou, D.; Bertrand, G. J. Am. Chem. Soc. 2002, 124, 11834–11835.
(16) The decomposition occurs at room temperature as well as at
-35 °C over several hours. The phosphine release from 6 and formation
of 1 was monitored by 31P{1H} NMR spectroscopy (see SI). The direct
reaction of isocyanide complex 1 with HPPh2 does not result in the
formation of carbene complex 7.

6372 Organometallics, Vol. 28, No. 21, 2009
Yu et al.
Figure 1. Molecular structures of 2 (left) and 3 (right) (thermal ellipsoids at 35% probability). Selected bond lengths (A) and angles
(deg): 2 C1-Fe1 1.732(5), C2-Fe1 1.997(3), P1-Fe1 2.173(5), C1-O1 1.158(2), C2-N1 1.262(2), C2-P2 1.885(2), P2-C2-Fe1
111.7(9). 3 C1-Fe1 1.741(3), C2-Fe1 2.005(3), P1-Fe1 2.186(9), C1-O1 1.163(3), C2-N1 1.289(4), C2-Si1 1.913(3), Si1-C2-Fe1
122.6(8).
calibration factor. The calibration factor was determined by
analyzing a suitable certified organic standard (OAS) of a
known elemental composition.
Synthesis of Complex 3. Triphenylsilylpotassium (KSiPh3)
was obtained as a yellow precipitate by reacting a solution of
hexaphenyldisilane (0.40 g, 0.76 mmol) in Et2O (10 mL) with
potassium metal (0.12 g, 3.04 mg/atom) at 35 °C under static
vacuum for 48 h. The solvent was removed in vacuo, and the
yellow residue was dissolved in THF (20 mL). This solution was
filtered and added dropwise to a suspension of the iron(II)
isocyanide-phosphine complex (1) (0.79 g, 1.52 mmol) in THF
(50 mL) at -78 °C, and the reaction mixture was warmed to
room temperature with stirring overnight. The solvent was
removed in vacuo, the residue redissolved in benzene, and the
solution filtered through Celite to remove KI. The solvent was
again removed in vacuo to collect a dark yellow precipitate. In
order to purify the product, the precipitate was left as a suspen-
sion in Et2O (20 mL) at room temperature overnight and then
filtered. The filtrate, containing a major amount of byproduct
and a small amount of complex 3, was removed as a brown
solution. These steps were repeated four times until complex 3
was isolated as a yellow solid (0.282 g, 28%). X-ray quality
orange prisms of complex 3 were grown in CDCl3 at -35 °C: IR
(CDCl3) 1915.17 cm-1 (νCO); 1H NMR (400 Hz, CDCl3)
δ 1.75-2.09 (m, 2 H), 2.57-2.80 (m, 2 H), 4.03 (t, J = 10.28
Hz, 1 H), 4.22 (s, 5H), 5.02-5.14 (m, 1 H), 7.11-7.23 (m, 4 H),
7.29-7.40 (m, 13 H), 7.41-7.55 (m, 2 H), 7.67-7.79 (m, 5H);
13C{1H} NMR (300 MHz, CDCl3) δ 22.64 (s, 1C), 34.52 (d, J =
20.99 Hz, 1C), 59.57 (d, J = 5.65 Hz, 1C), 83.40 (s, 5C),
127.22-140.44 (m, 30C), 219.49 (d, J = 36.33 Hz, -CO),
234.38 (d, J = 16.15 Hz, -CdN-); 31P{1H} NMR (CDCl3)
δ 58.37 (s, 1P); MS (ESI, m/z) calc mass 662.1731, obsd mass
All solvents were degassed and dried using 3 A molecular
sieves in an MBraun solvent purification system. THF, Et2O,
and C6H6 (C6D6) were further dried over Na/benzophenone and
distilled under N2. CH3CN (CD3CN), CH2Cl2 (CD2Cl2), and
CHCl3 (CDCl3) were dried over CaH2 and vacuum-transferred
to a Strauss flask and then degassed through a series of freeze-
pump-thaw cycles. Deuterium-labeled NMR solvents were
purchased from Cambridge Isotope Laboratory. Other chemi-
cals and solvents were purchased from Aldrich, Fisher, Alfa
Aesar, or Strem and were used without further purification.
1-Azido-3-chloropropane, N-(3-chloroprophyl)triphenylphos-
phinimine,
N-(3-(diphenylphosphino)propyl)triphenylphos-
phinimine, CpFe(CO)2I, complex 1,1 KPPh2,2 and KSiPh3
were prepared according to literature procedures.
3
Synthesis of Complex 2. A solution of KPPh2 was prepared by
adding a THF solution (10 mL) of diphenylphosphine (0.70 g,
3.78 mmol) to potassium metal (0.18 g, 4.62 mg/atom) in THF
(10 mL) dropwise at -78 °C, and the resulting solution was
stirred at -78 °C for 12 h. The solution of KPPh2 was filtered at
room temperature through Celite to remove the excess potas-
sium metal. The filtered solution was added dropwise to a
suspension of the iron(II) isocyanide-phosphine complex (1)
(2.00 g, 3.78 mmol) in THF (30 mL) at -78 °C, and the reaction
mixture was stirred at room temperature overnight. Half of the
solvent (ca. 15 mL) was removed in vacuo. The resulting yellow
suspension was filtered, and the collected precipitate was
washed twice with a minimal amount of THF. The yellow
precipitate was redissolved in benzene, and the solution was
filtered through Celite to remove KI. The solvent was removed
in vacuo to yield complex 2 as a yellow powder (1.78 g, 80%).
X-ray quality yellow needles were grown in CDCl3 at room
temperature: IR (CDCl3) 1912 cm-1 (νCO); 1H NMR (300
MHz, CDCl3) δ 1.52 (m, 1 H), 2.00-2.25 (m, 1 H), 2.73 (td,
J = 12.97, 2.88 Hz, 1 H), 3.02-3.17 (m, 1 H), 3.63 (t, J = 10.51
Hz, 1 H), 3.91 (d, J = 1.37 Hz, 5 H), 4.57 (dd, J = 10.43, 7.96 Hz,
1 H), 7.10-7.23 (m, 5 H), 7.32-7.43 (m, 6 H), 7.44-7.54 (m, 5H)
7.79 (m, 2H), 7.86-7.98 (m, 2H); 13C{1H} NMR (400 MHz,
CDCl3) δ 23.04 (s, 1 C), 36.97 (d, J = 20.08 Hz, 1 C), 56.05 (d,
J = 19.64, 1 C), 84.00 (s, 5C), 126.64-143.15 (m, 24 C), 214.30
(dd, J = 70.57, 23.96 Hz, -CdN-), 221.04 (dd, J = 16.91,
20.42 Hz, -CO); 31P{1H} NMR (300 MHz, CDCl3) δ 31.51 (d,
J = 23.76 Hz, -C-PPh2), 57.28 (d, J = 23.76 Hz, Fe-PPh2-);
MS (ESI, m/z) calcd mass 588.1309, obsd mass 588.1319 (Mþ).
(This compound slowly decomposes in the solid state at -35 °C;
EA was not possible.)
662.1736 (Mþ). Anal. Calcd for 3 ([C40H36FeNOPSi] 1/2-
3
[CHCl3]): N, 1.94; C, 67.44; H, 5.10. Found: N, 2.54; C, 67.34;
H, 5.29.
Synthesis of Complex 4. A solution of complex 2 (0.61 g,
1.03 mmol) in CHCl3 (10 mL) was cooled to -35 °C, and an
equimolar solution of 6.2 M HBF4 (0.17 mL) in diethyl ether was
added dropwise using a microsyringe. The solution was
warmed to room temperature for 5 min, the solvent was
removed in vacuo, and complex 4 was collected as a yellow solid.
To separate complex 4 from decompostion products iron(II)
isocyanide-phosphine (1) and diphenylphosphine, the product
was suspended in THF overnight at room temperature. The
suspension was filtered through a frit to collect the product
1
4 (0.208 g, 30%): IR (CDCl3) 1955.87 cm-1 (νCO); H NMR
(CDCl3) δ 1.37 (br s., 1 H), 2.20-2.48 (m, 1 H), 3.10-3.41
(m, 2 H), 4.17 (s, 6 H), 4.29 (br s, 1 H), 7.22 (br s., 2 H),
7.31-7.39 (m, 4 H), 7.43-7.55 (m, 6 H), 7.60 (d, J = 6.04 Hz,
3 H), 7.70 (d, J = 9.06 Hz, 2 H), 7.87 (t, J = 7.00 Hz, 2 H), 9.64
(br s, 1 H); 13C{1H} NMR (CDCl3) δ 23.68 (br s, 1 C), 34.94

Note
Organometallics, Vol. 28, No. 21, 2009 6373
(d, J = 24.48 Hz, 1 C), 51.47 (br s, 1 C), 85.93 (s, 5C),
128.62-139.56 (m, 24 C), 217.17 (dd, J = 19.58, 8.97 Hz, -CO),
276.39 (dd, J = 61.20, 23.66 Hz, dC-NH-); 31P{1H} NMR
(CDCl3) δ 38.05 (d, J = 11.88 Hz, -C-PPh2), 50.98 (d, J = 11.88
Hz, Fe-PPh2-); MS (ESI, m/z) calcd mass 588.1309, obsd mass
588.1298 (Mþ). (This compound slowly decomposes in the solid
state at -35 °C and in solution; EA was not possible.)
(400 MHz, CDCl3) δ 23.09 (s, 1C), 34.99 (d, J = 22.61 Hz, 1C),
54.28 (br s, 1C), 85.80 (s, 5C), 127.86-138.89 (m, 30C), 216.45
(d, J = 32.29 Hz, -CO), 299.38 (br s, dC-NH-); 31P{1H}
NMR (300 MHz, CDCl3) δ 55.42; MS (ESI, m/z) calcd mass
662.1732, obsd mass 662.1747 (Mþ). Anal. Calcd for 5
(C40H37BF4FeNOPSi): N, 1.87; C, 64.11; H, 4.98. Found: N,
2.17; C, 64.24; H, 5.21.
Synthesis of Complex 5. A solution of HBF4 (6.2 M, 0.012 mL,
0.074 mmol) in Et2O was added to a solution of complex 3
(0.05 g, 0.08 mmol) in CHCl3 (5 mL) at -35 °C. The reactionmix-
ture was stirred at room temperature for 1 h. The solvent was
then removed in vacuo. Et2O was added to the residue to
precipitate complex 5 as a yellow solid (0.058 g, 97%): IR
(CDCl3) 1958.83 cm-1 (νCO); 1H NMR (400 MHz, CDCl3)
δ 1.46 (br s, 1 H), 2.23-2.53 (m, 1 H), 2.90 (t, J = 12.91, 1 H),
3.25-3.45 (m, 1 H), 4.40 (br s, 6 H), 4.50 (br s, 1 H), 6.95-7.14
(m, 3 H), 7.26-7.77 (m, 22 H), 10.62 (br s, 1 H); 13C{1H} NMR
Acknowledgment. The authors thank UBC, NSERC,
CFI, and BCKDF for funding, and Prof. Paula
Diaconescu for thought-provoking discussions.
Supporting Information Available: Experimental and crystallo-
graphic details as well as observation of the decomposition of
7 to 1 and HPPh2. This material is available free of charge via the
Internet at http://pubs.acs.org.