Efficient synthesis of N-aryl nickel(II) carbenes by protonation of nickel(0) isocyanide complexes

Article abstract of DOI:10.1021/om030108y

The new nickel isocyanide complex [Ni(tri-phos)(CN-xylyl)] (1) has been prepared by the reaction of Ni(COD)₂, triphos (triphos = bis(2-diphenylphosphi-noethyDphenylphosphine), and CN-xylyl (xylyl = 2,6-Me₂C₆H₃). Complex 1 adds 2 equiv of HBF₄ to afford a stable dicationic nickel carbene complex, [Ni(triphos)C(H)N(H)xylyl]²⁺(BF₄)₂^- (2), in high yield. Nickel carbene 2 exhibits transfer hydrogenation of ketones.

Full text of DOI:10.1021/om030108y

Organometallics 2003, 22, 2817-2819
2817
Efficien t Syn th esis of N-Ar yl Nick el(II) Ca r ben es by
P r oton a tion of Nick el(0) Isocya n id e Com p lexes
Hongyi Hou, Peter K. Gantzel,† and Clifford P. Kubiak*
Department of Chemistry and Biochemistry, University of California at San Diego,
9500 Gilman Drive, La J olla, California 92093-0358
Received February 19, 2003
Summary: The new nickel isocyanide complex [Ni(tri-
phos)(CN-xylyl)] (1) has been prepared by the reaction
of Ni(COD)2, triphos (triphos ) bis(2-diphenylphosphi-
noethyl)phenylphosphine), and CN-xylyl (xylyl ) 2,6-
Me2C6H3). Complex 1 adds 2 equiv of HBF4 to afford a
stable dicationic nickel carbene complex, [Ni(triphos)-
C(H)N(H)xylyl]2+(BF4)2 (2), in high yield. Nickel car-
-
bene 2 exhibits transfer hydrogenation of ketones.
Transition metal carbene complexes are useful cata-
lysts and important intermediates in the synthesis of
organic molecules. Carbene complexes play key roles in
olefin metathesis, cyclopropanation, furan synthesis,
and C-C coupling reactions.1 Although palladium and
platinum carbene complexes have been widely used in
Heck and Suzuki type reactions,2 nickel carbenes have
been less thoroughly studied.3 Among the nickel carbene
complexes, heteroatom-stabilized (Fischer) carbenes
represent the vast majority.4 The typical route to
methoxy(amino) or bis(amino) carbene complexes is
nucleophilic attack of alcohols or amines on coordinated
isocyanides.5 We now report the preparation of a new
nickel(0) isocyanide complex, [Ni(triphos)(CN-xylyl)] (1),
and its straightforward conversion to nickel carbene
complexes, by protonation.
F igu r e 1. ORTEP drawing of [Ni(triphos)(CN-xylyl)] (1)
with 50% probability ellipsoids. Phenyl rings on the
phosphorus are not included. Selected bond distances (Å)
and angles (deg): Ni(1)-C(43), 1.787(3); Ni(1)-P(1), 2.1405-
(8); Ni(1)-P(3), 2.1455(8); Ni(1)-P(2), 2.1493(8); N(1)-
C(43), 1.172(3); N(1)-C(31), 1.381(3); C(43)-Ni(1)-P(1),
121.99(8); C(43)-Ni(1)-P(3), 117.71(8); P(1)-Ni(1)-P(3),
90.49(3); C(43)-Ni(1)-P(2), 116.97(8); P(1)-Ni(1)-P(2),
89.80(3); P(3)-Ni(1)-P(2), 114.20(3); C(43)-N(1)-C(31),
168.7(3).
The Ni(0) isocyanide complex [Ni(triphos)(CN-xylyl)]
(1) was prepared by addition of 1 equiv of triphos
(triphos ) bis(2-diphenylphosphinoethyl)phenylphos-
phine) and 1 equiv of CN-xylyl (xylyl ) 2,6-Me2C6H3)
6
to 1 equiv of Ni(COD)2 (COD ) 1,5-cyclooctadiene) at
room temperature in THF.7
* To whom correspondence should be addressed. E-mail: ckubiak@
ucsd.edu.
The 31P{1H} NMR spectrum of 1 in THF consists of a
triplet and a doublet centered at 63.7 and 48.2 ppm with
J ) 44.3 Hz and a ratio of 1:2. The IR showed an intense
ν(CN) stretching band at 1992 cm-1. In the solid state,
1 is stable indefinitely under an inert atmosphere and
is stable for several hours in air. The complex is also
stable in organic solvents in the absence of oxygen. The
† Address correspondence pertaining to crystallographic studies to
this author. E-mail: pgantzel@ucsd.edu.
(1) (a) Ku¨cu¨kbay, H.; Cetinkaya, B.; Salaheddine, G.; Dixneuf, P.
H. Organometallics 1996, 15, 2434. (b) Trnka, T. M.; Grubbs, R. H.
Acc. Chem. Res. 2001, 34, 18. (c) McGuinness, D. S.; Cavell, K. J .
Organometallics 2000, 19, 741. (d) Peris, E.; Loch, J . A.; Mata, J .;
Crabtree, R. H. J . Chem. Soc., Chem. Commun. 2001, 201. (e) Schwarz,
J .; Bo¨hm, V. P. W.; Gardiner, M. G.; Grosche, M.; Hermann, W. A.;
Hieringer, W.; Raudaschl-Sieber, G. Chem. Eur. J . 2000, 6, 1773.
(2) (a)Herrmann, W. A.; Cornils, B. E. Applied Homogeneous
Catalysis with Organometallic Compounds; VCH: Weinheim, Ger-
many, 1996; Vol. 1; Vol. 2. (b) Herrmann, W. A.; Reisinger, C. P.;
Spiegler, M. J . Organomet. Chem. 1998, 557, 93. (c) Enders, D.; Gielen,
H.; Raabe, G.; Runsink, J .; Teles, J . H. Chem. Ber. 1996, 129, 1483.
(3) (a) Arduengo, A. J ., III; Gamper, S. F.; Calabrese, J . C.; Davidson,
F. J . Am. Chem. Soc. 1994, 116, 4391. (b) Mindiola, D. J .; Hillhouse,
G. L. J . Am. Chem. Soc. 2002, 124, 9976
(4) (a) McGuinness, D. S.; Cavell, K. J .; Skelton, B. W.; White, A.
H. Organometallics 1999, 18, 1596. (b) Herrmann, W. A.; Schwarz, J .;
Gardiner, M. G.; Spiegler, M. J . Organomet. Chem. 1999, 575, 80. (c)
Xu, D.; Miki, K.; Tanaka, M.; Kasai, N.; Yasuoka, N.; Wada, M. J .
Organomet. Chem. 1989, 371, 267.
(5) Oguro, K.; Wada, M.; Okawara, R. J . Chem. Soc., Chem.
Commun. 1975, 97, 899.
(6) Schunn, R. A. Inorg. Synth. 1974, 15, 5.
(7) Ni(COD)2 (275 mg, 1 mmol), triphos (534 mg, 1 mmol), and CN-
xylyl (131 mg, 1 mmol) were added to a 50 mL round-bottom flask.
THF (25 mL) was then added to the flask. The reaction was allowed
to stir for 12 h at 25 °C. The yellow solution was concentrated under
vacuum, and diethyl ether was added to the residue to precipitate 1
as a bright yellow powder. The solid was collected by filtration and
washed with diethyl ether three times and then dried under vacuum
to obtain 1 in yields of 90%. Anal. Calcd for NiP3NC43H42: P, 12.83; C,
71.29; N, 1.93; H, 5.84. Found: P, 12.79; C, 70.95; N, 1.70; H, 5.88. IR
showed intensive ν(CN) at 1992 cm-1 31P{1H} NMR (THF): δ 63.7(t)
.
and 48.2(d) ppm. 1H NMR (d8-THF): δ 2.3 (s, 6H, -CH3 on xylyl), 1.6,
2.1 (m, 8H, PCH2CH2P). Crystals of 1 were grown from THF/ether by
slow diffusion of ether.
10.1021/om030108y CCC: $25.00 © 2003 American Chemical Society
Publication on Web 06/03/2003

2818 Organometallics, Vol. 22, No. 14, 2003
Communications
Sch em e 1
ment of 1 with 2 equiv of HBF4 affords the stable
cationic nickel aminocarbene complex [Ni(triphos)C-
(H)N(H)xylyl]2+(BF4)2- (2) in 85% yield, eq 1.9 The 31P-
{1H} NMR spectrum of 2 in CD2Cl2 consists of a triplet
and a doublet centered at 104.2 and 53.1 ppm with J )
34.4 Hz and a ratio of 1:2.
F igu r e 2. ORTEP drawing of [Ni(triphos)C(H)N(H)-
xylyl]2+(BF4)2- (2) with 50% probability ellipsoids. Phenyl
rings on the phosphorus are not included. Selected bond
distances (Å) and angles (deg): Ni(1)-C(5), 1.860(4); Ni-
(1)-P(3), 2.1871(10); Ni(1)-P(2), 2.1927(9); Ni(1)-P(1),
2.1943(10); N(1)-C(5), 1.268(5); N(1)-C(1F), 1.427(4);
C(5)-Ni(1)-P(3), 176.62(11); C(5)-Ni(1)-P(2), 93.55(10);
P(3)-Ni(1)-P(2), 84.40(4); P(3)-Ni(1)-P(1), 85.58(4); C(5)-
Ni(1)-P(1), 96.21(10); P(2)-Ni(1)-P(1), 168.68(4); C(5)-
N(1)-C(1F), 126.7(3); N(1)-C(5)-Ni(1), 131.5(3).
Protonation of 1 to afford 2 results in oxidation of
Ni(0) to Ni(II). This is evident in the change of coordina-
tion geometry from tetrahedral in 1 to square planar
in 2. The C(5)-N(1) bond distance (1.268(5) Å) is shorter
than a typical C-N single bond (1.472(5) Å),8 but longer
than the C-N triple bond of isocyanides(1.157(5) Å). The
three substituents on the carbene carbon C5 are copla-
nar, consistent with the sp2 character of the carbon
atom. The nickel-carbon bond distance of 2, d(Ni1-
C5) ) 1.860(4) Å, is 0.07 Å longer than the Ni-C
distance of 1. Overall, the metrical parameters fall into
a range that reflects essentially no π-carbene (a ) char-
acter and significant iminium formyl character (c)
(Scheme 1).
Our results show that protonation of 1 is a facile
means to prepare stable cationic nickel carbene com-
plexes in good yield. A proposed mechanism is based
on initial metal protonation,10 which gives a nickel
F igu r e 3. ORTEP drawing of [Ni(triphos)C(CH3)d
Nxylyl]+(I)- (3) with 50% probability ellipsoids. Phenyl
rings on the phosphorus are not included. Selected bond
distances (Å) and angles (deg): Ni(1)-C(5), 1.926(3); Ni-
(1)-P(3), 2.2088(9); Ni(1)-P(2), 2.1807(9); Ni(1)-P(1),
2.1804(9); N(1)-C(5), 1.273(4); N(1)-C(1F), 1.434(3); C(5)-
Ni(1)-P(3), 177.83(10); C(5)-Ni(1)-P(2), 95.94(9); P(3)-
Ni(1)-P(2), 83.27(3); P(3)-Ni(1)-P(1), 84.67(3); C(5)-
Ni(1)-P(1), 96.23(9); P(2)-Ni(1)-P(1), 167.52(4); C(5)-
N(1)-C(1F), 121.7(3); N(1)-C(5)-Ni(1), 119. 2(3).
(9) [Ni(triphos)(CN-xylyl)] (1) (181 mg, 0.25 mmol) was dissolved
in 10 mL of THF. A HBF4/ether solution (69 µL, wt % 54, d 1.18, 0.5
mmol) was then added to the solution. The reaction mixture was
allowed to stir for 3 h, during which time the pale yellow solid 2
precipitated. 2 was collected by filtration and washed with THF three
times and then dried under vacuum. Yield: 85%. Anal. Calcd for NiP3-
NC43H44B2F8: P, 10.32; C, 57.38; N, 1.56; H, 4.93. Found: P, 10.03; C,
57.00; N, 1.26; H, 4.95. IR showed a medium ν(CN) at 1558 cm-1 31P-
.
{1H} NMR (CH2Cl2): δ 104.2(t) and 53.1(d) ppm. 1H NMR (CD2Cl2):
δ 1.5 (s, 6H, -CH3 on xylyl), 3.6, 3.2, 2.7 (m, 8H, PCH2CH2P), 12.1 (m,
1H, -ΝΗ), 10.0 (ddt, 1H, : CΗ-Ν J P(trans)-H ) 19.6 Hz, J H(N)-H ) 6.8
Hz, J P(cis)-H ) 2.4 Hz). gHMQC technique showed the C(carbene) δ
246.5 ppm through the correlation with the proton attached on the
carbon at 10.0 ppm. Single crystals of 2 were obtained by slow diffusion
of diethyl ether into a methylene chloride solution of 2.
coordination geometry about Ni in 1 is a distorted
tetrahedron, as expected for an 18e- Ni(0) complex. The
C(43)-N(1) bond distance [1.172(3) Å] is slightly longer
than that for the free isocyanide (1.157(5) Å).8 Treat-
(10) (a) J ames, T. L.; Cai, L.; Muetterties, M. C.; Holm, R. H. Inorg.
Chem. 1996, 35, 4148. (b) Miedaner, A.; Dubois, D. L.; Curtis, C. J .
Organometallics 1993, 12, 299. (c) McEwen, G. K.; Rix, C. J .; Traynor,
M. F.; Verkade, J . G. Inorg. Chem. 1974, 13, 2800. (d) Schunn, R. A.
Inorg. Chem. 1970, 9, 394.
(8) Dean, J . A. Lange’s Handbook of Chemistry; McGraw-Hill
Handbooks: New York, 1999.

Communications
Organometallics, Vol. 22, No. 14, 2003 2819
Sch em e 2
hydride complex (d ). Subsequent insertion of the iso-
cyanide ligand to give a nickel imino formyl complex
(e) followed by N-protonation gives 2 (Scheme 2).
The corresponding MeI alkylation reaction adds some
support to this mechanism. Reaction of 1 equiv of MeI
with 1 stops at the imino acyl step to give [Ni(triphos)-
C(CH3)dNxylyl]+(I)- (3) as the final product, eq 2.11
F igu r e 4. ORTEP drawing of [Ni(triphos)CN-xylyl]2+
-
-
(BF4)2 (4) with 50% probability ellipsoids. Phenyl rings
on the phosphorus are not included. Selected bond dis-
tances (Å) and angles (deg): Ni(1)-C(5), 1.843(4); Ni(1)-
P(3), 2.1639(11); Ni(1)-P(2), 2.2172(11); Ni(1)-P(1), 2.2220-
(11); N(1)-C(5), 1.163(5); N(1)-C(1F), 1.402(5); C(5)-
Ni(1)-P(3), 173.06(13); C(5)-Ni(1)-P(2), 96.20(12); P(3)-
Ni(1)-P(2), 83.34(4); P(3)-Ni(1)-P(1), 84.52(4); C(5)-
Ni(1)-P(1), 95.85(12); P(2)-Ni(1)-P(1), 167.86(4); C(5)-
N(1)-C(1F), 172.5(4); N(1)-C(5)-Ni(1), 170.7(4).
Initial studies of the reactivity of carbene 2 reveal
unprecedented types of carbene reactivity including
transfer hydrogenation and hydrocarbation of alkenes.12
Treatment of 2 with excess acetone leads to the trans-
formation of 2 to [Ni(triphos)(CN-xylyl)]2+(BF4)2- (4) and
the transfer hydrogenation product 2-propanol, eq 3.13
Results of our studies of the hydrocarbation chemistry
of carbene 2 will be communicated separately.14
Ack n ow led gm en t. We gratefully acknowledge the
DOE (DE-FG03-99ER14992) for support.
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal-
lographic data collection and refinement parameters, positional
and thermal parameters, and bond distances and angles for
1, 2, 3, and 4 (PDF and CIF). This material is available free
of charge via the Internet at http://pubs.acs.org.
(11) [Ni(triphos)(CN-xylyl)] (1) (91 mg, 0.125 mmol) was dissolved
in 5 mL of toluene. MeI (7.8 µL, 0.125 mmol) was then added to the
solution. The reaction was allowed to stir for 12 h, during which time
the yellow solid 3 precipitated. 3 was collected by filtration and washed
with ether three times and then dried under vacuum. Yield: 83%. Anal.
Calcd for NiP3NC44H45I‚C7H8: C, 63.91; N, 1.46; H, 5.57. Found: C,
63.33; N, 1.05; H, 5.52. (Because of the low stability of 3, analysis was
carried out multiple times. A trace amount of toluene was found in
the NMR spectrum, so the analytical data showed a partial amount of
OM030108Y
-
(13) [Ni(triphos)C(H)N(H)xylyl]2+(BF4)2 (2) (200 mg, 0.22 mmol)
was dissolved in 10 mL of acetone. The reaction was allowed to stir at
45 °C for 1 day. 2-Propanol was detected by GC in the reaction mixture.
The yellow solution was concentrated under vacuum, and diethyl ether
was added to the residue to precipitate 4 as yellow powder. The solid
was collected by filtration and washed with diethyl ether three times
and then dried under vacuum to obtain 4 in yields of 80%. IR showed
an intense ν(CN) stretching band at 2174 cm-1. Anal. Calcd for NiP3-
NC43H42B2F8: C, 57.51; N, 1.56; H, 4.71. Found: C, 57.24; N, 1.61; H,
4.63. 31P{1H} NMR (acetone): δ 120.1(t) and 61.8(d) ppm. 1H NMR
(d6-acetone): δ 1.8 (s, 6H, -CH3 on xylyl). Crystals of 4 were grown
from acetone/ether by slow diffusion of ether.
toluene.) IR showed an intense ν(CN) at 1556 cm-1 31P{1H} NMR (CH2-
.
Cl2): δ 86.6 and 35.3 ppm. 1H NMR (CD2Cl2): δ 1.4 (s, 6H, -CH3 on
xylyl), 3.1, 2.6, 2.2 (m, 8H, PCH2CH2P), 1.7 (d, 3H, -CΗ3). gHMBC
technique showed the C(Ni-C) δ 190.2 ppm through the correlation
with the proton of the methyl group (1.7 ppm) attached on the carbon.
Single crystals of 3 were obtained by slow diffusion of diethyl ether
into a THF solution of 3.
(12) (a) Casey, C. P.; Fagan, P. J . J . Am. Chem. Soc. 1982, 104, 4950.
(b) Casey, C. P.; Meszaros, M. W.; Fagan, P. J .; Bly, R. K.; Marder, S.
R.; Austin, E. A. J . Am. Chem. Soc. 1986, 108, 4043.
(14) Hou, H.; Gantzel, P. K.; Kubiak, C. P. J . Am. Chem. Soc.,
submitted.