Group-transfer reactions of nickel-carbene and -nitrene complexes with organoazides and nitrous oxide that form new C=N, C=O, and N=N bonds

Article abstract of DOI:10.1021/ja904370h

(Chemical Equation Presented) 1-Adamantyl- and mesitylazide react with (dtbpe)Ni=CPh₂ (1; dtbpe = 1,2-bis(di-tertbutylphosphino) ethane) at ambient temperature to give the ketimines RN=CPh₂ (2a, R = Mes; 2b, R = Ad) in high yield. Ki

Full text of DOI:10.1021/ja904370h

Published on Web 08/19/2009
Group-Transfer Reactions of Nickel-Carbene and -Nitrene Complexes with
Organoazides and Nitrous Oxide that Form New CdN, CdO, and NdN Bonds
Nicole D. Harrold,† Rory Waterman,†,§ Gregory L. Hillhouse,*,† and Thomas R. Cundari*,‡
Gordon Center for IntegratiVe Science, Department of Chemistry, The UniVersity of Chicago, 929 East 57th Street,
Chicago, Illinois 60637, and Center for AdVanced Scientific Computing and Modeling (CASCaM), Department of
Chemistry, UniVersity of North Texas, Denton, Texas 76203
Received May 29, 2009; E-mail: g-hillhouse@uchicago.edu; t@unt.edu
Scheme 1
Nitrous oxide (N2O) and organoazides (N3R′) are energy-rich
molecules that undergo a variety of reactions with group 10
transition metal complexes resulting in transfer of an “O” or “NR”
moiety to the metal with loss of dinitrogen. Complexes of the type
L2NiR2 (L ) PMe3; L2 ) bipyridine, phenanthroline; R ) alkyl,
aryl) undergo quite general insertion reactions with nitrous oxide
to afford alkoxide or aryloxide products L2NiR(OR)1 and with
organoazides to yield amide derivatives L2NiR(NR′R) upon N2
extrusion.2 Organoazides have also been shown to react directly
with unsaturated group 10 metal fragments to give imido complexes
like (Ph2MeP)2MdNCF2CFHCF3 (M ) Pd, Pt),3 [Me2C3H{(2,6-
Me2C6H3)N}2]NidNAd,4 and (dtbpe)NidNMes (dtbpe ) 1,2-
bis(di-tert-butylphosphino)ethane).5 In the course of investigating
group-transfer reactions of ligands involved in multiple-bonding
to three-coordinate nickel,6 we encountered facile reactions of
nitrous oxide and organoazides with Ni-carbene and -imido
complexes that generate new CdN, CdO, and NdN double bonds.
Herein we report the results of these studies, including experimental
first-order in both [1] and [N3Ad] with a second-order rate constant
of 1.7 × 10-3 ((0.2) M-1 s-1 at 35 °C. No intermediates were
and computational data that give insight into the mechanism of
these unusual heteroatom-coupling reactions.
1
observed when the reaction was followed by 31P or H NMR. An
Reaction of cold diethylether solutions of the green diphenyl-
carbene complex (dtbpe)NidCPh2 (1)7 with 2 equiv of mesitylazide
(N3Mes) or 1-adamantylazide (N3Ad) results in elimination of N2
and formal “carbene-nitrene” coupling to give the corresponding
ketimines RNdCPh2 (2a, R ) Mes; 2b, R ) Ad) in 76% and 72%
isolated yields, respectively (Scheme 1). The resulting “(dtbpe)Ni0”
fragment is trapped by a second equivalent of N3R to give the
known organoazide complexes (dtbpe)Ni(η2-N3R) (3a, R ) Mes;
3b, R ) Ad) as kinetic products that undergo subsequent thermal
N2 elimination to afford the Ni(II) imido complexes (dtbpe)NidNR
(4a, R ) Mes; 4b, R ) Ad) as the ultimate nickel-containing
products.5 Such transition metal mediated carbene-nitrene coupling
reactions are unusual, but not unprecedented. Fischer has shown
that (CO)5MdCPh2 reacts with N3R to give (CO)5M(κ1-NRdCPh2)
(M ) Cr, W),8 and Grubbs has reported that N3R reacts with (N(2-
P-i-Pr2-4-Me-C6H3)2)IrdCH(O-t-Bu) to afford RNdCH(O-t-Bu)
and (N(2-P-i-Pr2-4-Me-C6H3)2)Ir(N2).9 A novel intramolecular
imido-carbene coupling reaction has been observed by Meyer in
a (trisNHC)CodNR complex (NHC ) N-heterocyclic carbene).10
A kinetic study of the reaction of 1 with N3Ad was carried out
to gain mechanistic insight into ketimine (2b) formation. (The
reaction of N3Mes with 1 was too fast to conveniently be monitored
by NMR, even at low temperature.) Disappearance of 1 and
Eyring analysis of the kinetic data reveals the reaction to have a
small activation enthalpy (∆H‡ ) 8 ((1) kcal/mol) and a large
activation entropy (∆S‡ ) -44 ((3) cal/(mol ·K)), suggestive of a
highly ordered transition state.
Net “carbene-oxygen” coupling with formation of the red
benzophenone adduct (dtbpe)Ni(η2-OCPh2) (5; 65% isolated yield)
occurs upon addition of a controlled excess (∼2 equiv) of nitrous
oxide to a -78 °C toluene solution of 1, followed by slow warming
to ambient temperature (Scheme 1). Complex 5 was characterized
by comparison to an authentic sample prepared by literature
methods.11 The reaction conditions were optimized for the produc-
tion of 5, and use of a larger excess of N2O in the reaction results
in oxidation of the dtbpe ligand, formation of free benzophenone,
and precipitation of Ni(0).
Treatment of ether solutions of the imido complex 4a with 2
equiv of N3Mes results in formal “nitrene-nitrene” coupling to
give 1,2-dimesityldiazene (6a, MesNdNMes; 78% yield) along with
4a. 4b reacts similarly with N3Ad to afford 1,2-bis(1-adamantyl)-
diazene (6b, AdNdNAd; 64% yield) and 4b (Scheme 2). 4a reacts
with N3Ad to give the mixed diazene MesNdNAd (6c), demon-
strating that the diazenes do not form by bimolecular decomposition
of 4a,b under these conditions. Azo-coupling is not catalytic with
excess N3R because the diazene products compete with N3R
as Ni(0) traps. Reaction of the Ni(0) benzene complex
{(dtbpe)Ni}2(C6H6)12 with 6a gives (dtbpe)Ni(MesNdNMes) (7)
in high yield. The diazene products 6a-c are unusual in that
1,4-tetrazenes are the usual organoazide coupling products with
transition metal imides,13 exemplified by the reactions of N3R
1
formation of 2b were followed by H NMR spectroscopy over a
temperature range of 35-65 °C (C7D8 solution). The reaction is
† The University of Chicago.
‡ University of North Texas.
§ Current address: Department of Chemistry, University of Vermont, Burlington,
VT 05405.
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12872 J. AM. CHEM. SOC. 2009, 131, 12872–12873
10.1021/ja904370h CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
Transition state geometries for [(dhpe)NidNMe + N3Me] and
[N2O + (dhpe)NidCH2] are planar at Ni, while that for [N3Me +
(dhpe)NidCH2] is tetrahedrally distorted. Calculated activation
parameters for the reaction of (dhpe)NidCH2 + N3Me are ∆H‡ )
+6.7 kcal/mol and ∆S‡ ) -40.7 cal/(mol·K), giving a ∆G‡ of
+18.8 kcal/mol (at 298.15 K). These values compare well with
the experimentally obtained values (∆H‡ ) 8 ((1) kcal/mol; ∆S‡
) -44 ((3) cal/(mol ·K)) for the reaction of 1 with N3Ad. No
evidence for a precursor adduct between (dtbpe)NidE and N2X
was found, so the calculated barriers are relative to separated
reactants. The transition state structures collapsed to stable Huisgen
intermediates, but these were considerably higher in energy than
the eventual products, which result after N2 elimination to give
(dtbpe)Ni(η2-EX). Experimentally, in the reaction of 1 with N2O
this is the observed product (i.e., 5), but the reactions of 1 and
4a,b with N3R give imido complexes in which a second equivalent
of N3R replaces the bulky EdX ligand and N2 is extruded.
In summary, we have shown that the nickel carbene complex 1
and the imido complexes 4a and 4b readily react with N2X
substrates via net “CR2”, “NR”, or “O” transfer to form multiply
bonded organic products. Experimental, kinetic, and computational
results all support a mechanism involving 1,3-dipolar cycloaddition
of N2X to (dtbpe)NidCPh2 or (dtbpe)NidNR to give five-
membered Huisgen-type intermediates. Product formation results
on elimination of N2.
with Cp2Zr(dN-t-Bu)(THF) to form Cp2Zr(κ2-RN4-t-Bu) or
(cymene)OsdN-t-Bu to yield (cymene)Os(κ2-RN4-t-Bu).14
B3LYP/6-311+G(d) computations were carried out to explore
the notion that the reactions of 1 and 4 with N3R and N2O, classic
1,3-dipolar reagents, proceeded by a cycloaddition pathway with
five-membered ring (i.e., Huisgen) intermediates. A summary of
the calculated results based on the model complexes (dhpe)NidE
(E ) CH2, NMe) with N2X (dhpe ) 1,2-bis(dihydridophosphino)-
ethane; X ) O, NMe) is shown in Figure 1 and provides a consistent
picture for all three reactions modeled. The reactions proceed via
initial 1,3-dipolar cycloaddition of (dhpe)NidE with N2X to give
a five-membered transition state (Figure 2), which is evocative of
transition states calculated by Houk et al. for organic 1,3-dipolar
cycloadditions.15 Enthalpic barriers for 1,3-dipolar cycloadditions
are reasonable and consistent with the exothermicity of the reactions
to form Huisgen intermediates.
Acknowledgment. This work was supported by the National
Science Foundation through Grant CHE-0615274 (to G.L.H.) and
a predoctoral GAANN Fellowship from the Department of Educa-
tion (to R.W.). T.R.C. acknowledges the NSF for equipment support
(CRIF, CHE-0741936) and a grant from Basic Energy Sciences,
Department of Energy (DEFG02-03ER15387). We thank Jack
Halpern for assistance with the kinetic analysis.
Supporting Information Available: Experimental and computa-
tional procedures with characterization data and kinetic data. This
material is available free of charge via the Internet at http://pubs.acs.org.
References
(1) (a) Matsunaga, P. T.; Hillhouse, G. L.; Rheingold, A. L. J. Am. Chem.
Soc. 1993, 115, 2075. (b) Koo, K.; Hillhouse, G. L.; Rheingold, A. L.
Organometallics 1995, 14, 456. (c) Matsunaga, P. T.; Mavropoulos, J. C.;
Hillhouse, G. L. Polyhedron 1995, 14, 175.
(2) (a) Matsunaga, P. T.; Hess, C. R.; Hillhouse, G. L. J. Am. Chem. Soc.
1994, 116, 3665. (b) Koo, K.; Hillhouse, G. L. Organometallics 1996, 15,
2669.
(3) McGlinchey, M. J.; Stone, F. G. A. J. Chem. Soc. D 1970, 1265a.
(4) Kogut, E.; Wiencko, H. L.; Zhang, L.; Cordeau, D. E.; Warren, T. H. J. Am.
Chem. Soc. 2005, 127, 11248.
Figure 1. B3LYP/6-311+G(d)-calculated enthalpies (1 atm, 298.15 K) for
model 1,3-dipolar cycloaddition mechanisms.
(5) Waterman, R.; Hillhouse, G. L. J. Am. Chem. Soc. 2008, 130, 12628.
(6) (a) Waterman, R.; Hillhouse, G. L. J. Am. Chem. Soc. 2003, 125, 13350.
(b) Mindiola, D. J.; Hillhouse, G. L. Chem. Commun. 2002, 1840.
(7) Mindiola, D. J.; Hillhouse, G. L. J. Am. Chem. Soc. 2002, 124, 9976.
(8) Fischer, H.; Zeuner, S. J. Organomet. Chem. 1985, 286, 201.
(9) Whited, M. T.; Grubbs, R. H. J. Am. Chem. Soc. 2008, 130, 16476.
(10) Hu, X.; Meyer, K. J. Am. Chem. Soc. 2004, 126, 16322.
(11) Mindiola, D. J.; Waterman, R.; Jenkins, D. M.; Hillhouse, G. L. Inorg.
Chim. Acta 2003, 345, 299.
(12) Bach, I.; Po¨rschke, K.-R.; Goddard, R.; Kopiske, C.; Kruger, C.; Rufinska,
A.; Seevogel, K. Organometallics 1996, 15, 4959.
(13) (a) Ashley-Smith, J.; Green, M.; Stone, F. G. A. J. Chem. Soc., Dalton
Trans. 1972, 1805. (b) Dubrawski, J.; Kriege-Simondsen, J. C.; Feltham,
R. D. J. Am. Chem. Soc. 1979, 102, 2091. (c) Trogler, W. C. Acc. Chem.
Res. 1990, 23, 426.
(14) (a) Meyer, K. E.; Walsh, P. J.; Bergman, R. G. J. Am. Chem. Soc. 1995,
117, 974. (b) Michelman, R. I.; Andersen, R. A.; Bergman, R. G.
Organometallics 1993, 12, 2741.
(15) (a) Ess, D. H.; Houk, K. N. J. Phys. Chem. A 2005, 109, 9542. (b) Jones,
G. O.; Ess, D. H.; Houk, K. N. HelV. Chim. Acta 2005, 88, 1702.
Figure 2. Calculated transition state for the 1,3-dipolar cycloaddition of
N3Me to (dhpe)NidNMe in forming a 1,2-tetrazene intermediate.
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