Synthesis, structure, and reactions of a three-coordinate nickel-carbene complex, {1,2-bis(di-tert-butylphosphino)ethane}Ni=CPh2

Article abstract of DOI:10.1021/ja0269183

The three-coordinate nickel-carbene complex (dtbpe)NiCPh₂ (3) was prepared from the thermolysis of the diphenyldiazoalkane complex (dtbpe)Ni(N,N-:η²-N₂CPh₂) (2) (dtbpe = 1,2-bis(di-tert-butylphosphino)ethane). Complex 3 was structurally characterized by single-crystal X-ray diffraction methods (Ni-C = 1.836(2) A). Complex 3 reacts with 2 equiv of CO₂ to afford (dtbpe)Ni{OC(O)CPh₂C(O)O} (4), with diphenylketene to give (dtbpe)Ni{OC(CPh₂)CPh₂} (5), with excess CO to transfer the carbene fragment and generate diphenylketene and (dtbpe)Ni(CO)₂ (6), with sulfur dioxide to give the metallasulfone (dtbpe)Ni{C,S:η²-S(O)₂CPh₂} (7), and with the Bronsted acid [HNMe₂Ph][B(C₆F₅)₄] to give the alkyl cation [(dtbpe)Ni(CHPh₂)][B(C₆F₅)4] (8). Complexes 4, 5, and 7 have also been characterized by single-crystal X-ray diffraction methods. Copyright

Full text of DOI:10.1021/ja0269183

Published on Web 08/01/2002
Synthesis, Structure, and Reactions of a Three-Coordinate Nickel-Carbene
Complex, {1,2-Bis(di-tert-butylphosphino)ethane}NidCPh2
Daniel J. Mindiola and Gregory L. Hillhouse*
Searle Chemistry Laboratory, Department of Chemistry, The UniVersity of Chicago, Chicago, Illinois 60637
Received May 15, 2002
Since the discovery of transition-metal carbenes, there has been
enormous interest in their preparation and in the development and
study of a variety of carbon-carbon bond-forming and carbene-
group-transfer reactions in which they participate.1 Hallmark studies
by Schrock2 and Grubbs3 have resulted in the design, through
“evolutionary” processes, of efficient metal-carbene catalysts for a
variety of alkene metathesis reactions. Terminal carbenes of the
group 10 metals are rare, limited to the electron-rich cycloprope-
nylidene complex cis-PdCl2{cyclo-C(CNMe2)2}(P-n-Bu3),4 the fluo-
rene carbene (R3P)2PddCC12H8,5 the family of N-heterocyclic
carbenes such as (CO)3Ni{cyclo-CN(R)CH2CH2NR} and Ni{cyclo-
CN(Mes)CHdCHNMes}2,6,7 and the heteroatom-stabilized carbene
hydride [(tmeda)Pt(H){dC(Me)OEt}][BAr4].8 Silylene,9 ger-
mylene,10 and stannylene11 complexes, heavier congeners of car-
benes, have also been reported for group 10 metals. Our recent
reports of three-coordinate nickel imido12 and phosphinidene13
complexes stimulated the pursuit of a related nickel-carbene
complex. Moreover, because the “L2Ni” fragment is isolobal with
“CH2”, a L2NidCR2 complex seemed to be a reasonable target by
analogy with simple olefins.14
Figure 1. A perspective view of the molecular structure of 3 (H atoms
have been omitted). Salient metrical parameters: Ni-C(7) ) 1.836(2),
Ni-P(1) ) 2.2003(5), Ni-P(2) ) 2.1812(5) Å; P(1)-Ni-P(2) )
91.03(2), P(1)-Ni-C(7) ) 142.02(6), P(2)-Ni-C(7) ) 126.94(6),
Ni-C(7)-C(81) ) 113.25(13), Ni-C(7)-C(91) ) 130.96(14), C(81)-
C(7)-C(91) ) 115.5(2)°.
Addition of 1 equiv of N2CPh2 to a suspension of (dtbpe)Ni-
(cod) (1; dtbpe ) 1,2-bis(di-tert-butylphosphino)-ethane; cod ) 1,5-
cyclooctadiene)15 in THF gives a homogeneous red-brown solution
from which the diphenyldiazomethane complex (dtbpe)Ni(N,N′:
η2-N2CPh2) (2) can be isolated as red crystals in 95% yield (Scheme
1).16 Related diazoalkane complexes of nickel have been reported.17
Complex 2 exhibits moderate thermal stability, but slowly decom-
poses upon thermolysis (C6D6, 80 °C, 48 h) to give a complex
mixture of products. However, heating solutions of 2 containing a
catalytic amount of anhydrous Sm(OTf)3 (7 mol %, C6H6, 75 °C)
results in N2 extrusion with formation of the diphenylcarbene
complex (dtbpe)NidCPh2 (3), isolated in 70% yield as green
crystals (Scheme 1).16,18 Complex 3 exhibits a diagnostic resonance
Scheme 1
Scheme 2
2
for the carbene carbon at δ 222 (t, JPC ) 51 Hz) in its 13C{1H}
NMR spectrum.19 The 1H, 13C, and 31P NMR spectra of 3 are
consistent with the proposed formulation and a solution structure
having C2V point symmetry. The solid-state structure of 3 was
crystallographically determined and shows a planar, three-coordinate
nickel with a diphenylcarbene ligand planar at C(7) and a short
Ni-C(7) distance (1.836(2) Å) consistent with a double bond.16,19
The {C(7), C(81), C(91)} plane of the carbene ligand is oriented
perpendicular to the Ni-coordination plane, allowing for π-overlap
between a carbon p-orbital and a metal orbital of b2 symmetry
(Figure 1). This is the geometry predicted by simple molecular
orbital analysis and is that observed for related (dtbpe)NidPR and
(dtbpe)NidNR complexes.12-14
of (dtbpe)Ni{OC(O)CPh2C(O)O} (4) as a bright-yellow precipitate
in 90% isolated yield (see Scheme 2).16 Complex 4 has been
1
characterized by infrared, H, 13C, and 31P NMR spectroscopies,
and by single-crystal X-ray diffraction analysis (Figure 2). The
incorporation of 2 equiv of carbon dioxide by 3 to give 4 possibly
proceeds by a formal [2 + 2] cycloaddition of CO2 across the
NidCPh2 double bond, followed by insertion of a second equivalent
of CO2 into the Ni-C bond of the resulting metallalactone
Treatment of a toluene solution of 3 with an excess of CO2 (5
equiv) results in a color change from green to orange with formation
* To whom correspondence should be addressed. E-mail: g-hillhouse@
uchicago.edu.
9
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J. AM. CHEM. SOC. 2002, 124, 9976-9977
10.1021/ja0269183 CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Figure 2. Perspective views of the molecular structures of 4 (left), 5 (center), and 7 (right). H-atoms have been omitted for clarity.
4, 5, and 7 (CIF). This material is available free of charge via the
Internet at http://pubs.acs.org.
intermediate. Diphenylketene reacts slowly to undergo just such a
cycloaddition at the carbene ligand of 3 to afford an O,C:η2-
oxametallacyclobutane complex, (dtbpe)Ni{OC(dCPh2)CPh2} (5),
as auburn blocks in 68% isolated yield (Scheme 2).16 NMR (1H,
13C, 31P) spectra for 5, broader than usual for square-planar Ni(II)
in this system (possibly due to a fluxional process), indicate
inequivalent phosphorus environments; the highly sterically con-
gested structure was confirmed by single-crystal X-ray diffraction
(Figure 2).16
Exposure of solutions of 3 to excess carbon monoxide (∼5 equiv)
results in rapid, quantitative conversion of 3 to (dtbpe)Ni(CO)2 (6)15
with concomitant formation of free diphenylketene, isolated in 94
and 87% yields, respectively (Scheme 2). The ketene complex
(dtbpe)Ni(η2-OCdCPh2) is a likely intermediate in this transforma-
tion, and in an independent experiment we have shown that it indeed
reacts rapidly with CO to give 6 and free diphenylketene.20 Similar
group transfer reactions have been observed for isolobal (dtbpe)-
NidNR and (dtbpe)NidPR complexes, which react with CO to
give the corresponding RNdCdO and RPdCdO products along
with 6.21,22
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Sulfur dioxide reacts with 3 via addition across the Ni-C bond
to give the metallasulfone (dtbpe)Ni{C,S:η2-S(O)2CPh2} (7) as
orange crystals in 72% yield (Scheme 2).16 Complex 7 has been
characterized spectroscopically and crystallographically, and a
perspective view of its structure is shown in Figure 2.
Finally, the Brønsted acid [HNMe2Ph][B(C6F5)4] protonates the
carbene ligand in 3 to give the alkylnickel(II) salt [(dtbpe)Ni-
(CHPh2)][B(C6F5)4] (8) in 55% yield (Scheme 2).16 We are currently
examining the structure and reactivity of 8 (and related cationic
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reaction to determine if R-deprotonation of cationic group 10 alkyls
is a viable route to the corresponding neutral carbene complexes.
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alkyls that yield stable alkylidene complexes.2 We are also exploring
the scope of Lewis acid-catalyzed N2-elimination from diazoalkane
complexes as a general route to other carbene complexes of group
10 metals.18
(12) Mindiola, D. J.; Hillhouse, G. L. J. Am. Chem. Soc. 2001, 123, 4623.
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(16) See the Supporting Information for complete experimental, spectral, and
crystallographic details.
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Casarrubios, L.; Perez, J. A.; Brookhart, M.; Templeton, J. L. J. Org.
Chem. 1996, 61, 8358. (b) Xie, W. H.; Fang, J. W.; Li, J.; Wang, P. G.
Tetrahedron 1999, 55, 12929.
(19) For comparison, bis(2,2′dimesitylimidazol-2-ylidene)nickel(0) exhibits
D(Ni-C) ) 1.827(6) and 1.830(6) Å and a 13C NMR shift at δ 193.2.7
(20) Ni(η2-OCdCPh2) was prepared by a procedure analogous to that for the
known complex (dtbpm)Ni(OCdCPh2). Hofmann, P.; Perez-Moya, L. A.;
Steigelmann, O.; Riede, J. Organometallics 1992, 11, 1167.
(21) Mindiola, D. J.; Hillhouse, G. L. J. Chem. Soc., Chem. Commun. 2002,
in press.
Acknowledgment. G.L.H. thanks the National Science Founda-
tion for financial support. D.J.M. acknowledges postdoctoral
fellowship support from the Ford Foundation and the National
Institutes of Health.
(22) Mindiola, D. J.; Melenkivitz, R.; Hillhouse, G. L., manuscript in
preparation.
Supporting Information Available: Experimental, spectroscopic,
and analytical details (PDF); complete crystallographic details for 3,
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