Synthetic, structural, and thermochemical studies of N-heterocyclic carbene (NHC) and tertiary phosphine ligands in the [(L)2Ni(CO)2] (L = PR3, NHC) system

Article abstract of DOI:10.1021/om8001125

Two new dicarbonyl N-heterocyclic carbene nickel(0) complexes of the type (NHC)₂Ni(CO)₂ (NHC = ICy, [N,N′- bis(cyclohexylimidazol)-2-ylidene (2), IMes [N,N′-bis(2,4,6- trimethylphenyl)imidazol)-2-ylidene] (3)) have been prepared by a substitution reaction of (NHC)Ni(CO)₂ (NHC = I^tBu [N,N′-bis(tert- butylimidazol)-2-ylidene], IAd [N,N′-bis(1-adamantylimidazol)-2-ylidene]) and 2 equivalents of ICy or IMes. Single-crystal X-ray analyses confirmed the monomeric 18-electron compositions of [(ICy)₂Ni(CO)₂] (2) and [(IMeS)₂Ni(CO)₂] (3). The greater electron-donating properties of the NHC ligands compared to tertiary phosphines are also demonstrated through calorimetric studies and enabled the determination of average bond dissociation enthalpies for Ni-L (L = NHC and tertiary phosphine).

Full text of DOI:10.1021/om8001125

Organometallics 2008, 27, 3181–3186
3181
Synthetic, Structural, and Thermochemical Studies of
N-Heterocyclic Carbene (NHC) and Tertiary Phosphine Ligands in
the [(L)2Ni(CO)2] (L ) PR3, NHC) System
Natalie M. Scott,‡ Herve´ Clavier,§ Parisa Mahjoor,‡ Edwin D. Stevens,‡ and
Steven P. Nolan*,‡,§
Department of Chemistry, UniVersity of New Orleans, New Orleans, Louisiana 70148, and Institute of
Chemical Research of Catalonia (ICIQ), AVenida Pa¨ısos Catalans 16, 43007 Tarragona, Spain
ReceiVed February 7, 2008
Two new dicarbonyl N-heterocyclic carbene nickel(0) complexes of the type (NHC)2Ni(CO)2 (NHC
) ICy, [N,N′-bis(cyclohexylimidazol)-2-ylidene (2), IMes [N,N′-bis(2,4,6-trimethylphenyl)imidazol)-2-
ylidene] (3)) have been prepared by a substitution reaction of (NHC)Ni(CO)2 (NHC ) ItBu [N,N′-bis(tert-
butylimidazol)-2-ylidene], IAd [N,N′-bis(1-adamantylimidazol)-2-ylidene]) and 2 equivalents of ICy or
IMes. Single-crystal X-ray analyses confirmed the monomeric 18-electron compositions of [(ICy)2Ni(CO)2]
(2) and [(IMes)2Ni(CO)2] (3). The greater electron-donating properties of the NHC ligands compared to
tertiary phosphines are also demonstrated through calorimetric studies and enabled the determination of
average bond dissociation enthalpies for Ni-L (L ) NHC and tertiary phosphine).
The importance of quantifying steric and electronic effects
of ligands has been recognized in pioneering studies by Tolman
on tertiary phosphines10 and have had a major impact on the
development of new and improved phosphine ligands for
catalysis.11 Although some spectroscopic studies have revealed
the close relationship between NHCs and phosphine ligands,12
a more detailed understanding of electronic and steric parameters
of this ligand class represents a key step for the development
of even more catalytic systems.13 Recent studies on the reacti-
vity of NHC ligands toward complexes containing low-valent
transition metals have shown them to be very useful models to
understand the strength of metal-ligand bonds.8c,d Nevertheless
Introduction
The use of N-heterocyclic carbenes (NHC) as ancillary
ligands in organometallic chemistry and homogeneous catalysis
is an area of intense activity.1 On the basis of the accepted
analogy between NHCs and trialkylphosphines (strong σ-donors
and weak π-accepting ability),2 many NHC metal complexes
are now well recognized as catalysts in a plethora of reactions,
including the Suzuki-Miyaura reaction,3 hydrogenation,4 and
ruthenium-based olefin metathesis.5 Whereas nickel was used
early on to isolate TM-NHC complexes6 and proved to be useful
in catalysis using mostly in situ generated catalysts,7 only a few
well-defined NHC Ni(0) complexes have been reported to date.
This includes mainly mono-8 or bis-NHC adducts6a,9 of nickel
carbonyls used for determination of electronic parameters of
NHC ligands.
(3) (a) Herrmann, W. A.; Cornils, B. E. Applied Homogeneous Catalysis
with Organometallic Compounds; VCH: Weinheim, Germany 1996; Vols.
¨
1-2. (b) Herrmann, W. A.; Ofele, K.; Preysing, D. V.; Schneider, S. K. J.
Organomet. Chem. 2003, 687, 229–248. (c) Navarro, O.; Kelly, R. A., III.;
Nolan, S. P. J. Am. Chem. Soc. 2003, 125, 16194–16195. (d) Marion, N.;
Ecarnot, E. C.; Navarro, O.; Amoroso, D.; Bell, A.; Nolan, S. P. J. Org.
Chem. 2006, 71, 3816–3821. (e) Marion, N.; Navarro, O.; Mei, J.; Stevens,
E. D.; Scott, N. M.; Nolan, S. P. J. Am. Chem. Soc. 2006, 128, 4101–4111.
(f) Navarro, O.; Marion, N.; Mei, J.; Nolan, S. P. Chem.-Eur. J. 2006, 12,
5142–5148.
* To whom correspondence should be addressed. E-mail: snolan@iciq.es.
Fax: (+34) 977-920-244.
‡ University of New Orleans.
§ ICIQ.
(1) For reviews, see: (a) Regitz, M. Angew. Chem., Int. Ed. Engl. 1996,
35, 725–728. (b) Hermmann, W. A.; Ko¨cher, C. Angew. Chem., Int. Ed.
Engl. 1997, 36, 2163–2187. (c) Arduengo, A. J., III; Krafczyk, R. Chem.
Z. 1998, 32, 6–14. (d) Weskamp, T.; Bo¨cher, V. P. W.; Hermmann, W. A.
J. Organomet. Chem. 2000, 600, 12–22. (e) Jafarpour, L.; Nolan, S. P. AdV.
Organomet. Chem. 2001, 46, 181–222. (f) Herrmann, W. A. Angew. Chem.,
Int. Ed. 2002, 41, 1290–1309. (g) Scott, N. M.; Nolan, S. P. Eur. J. Org.
Chem. 2005, 1815–1828. (h) N-heterocyclic Carbenes in Synthesis; Nolan,
S. P., Ed.; Wiley-VCH: Weinheim, Germany, 2006. (i) N-Heterocyclic
Carbenes in Transition Metal Catalysis; Glorius, F., Ed.; Springer-Verlag:
Berlin, Germany, 2007. (j) Clavier, H.; Nolan, S. P. Annu. Rep. Prog. Chem.,
Sect B: Org. Chem. 2007, 103, 193–222.
(4) (a) Lee, H. M.; Jiang, T.; Stevens, E. D.; Nolan, S. P. Organome-
tallics 2001, 20, 1255–1258. (b) Hillier, A. C.; Lee, H. M.; Stevens, E. D.;
Nolan, S. P. Organometallics 2001, 20, 4246–4252. (c) Vasquez-Serrano,
L. D.; Owens, B. T.; Buriak, J. M. Chem. Commun. 2002, 2518–2519. (d)
Perry, M. C.; Cui, X.; Powell, M. T.; Hou, D.-R.; Reibenspies, H. J.;
Burgess, K. J. Am. Chem. Soc. 2003, 125, 113–123.
(5) (a) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18–29.
(b) Handbook of Metathesis; Grubbs, R. H., Ed.; Wiley-VCH: Weinheim,
Germany, 2003, Vols. 1-3. (c) Blechert, S.; Connon, S. J. Angew. Chem.,
Int. Ed. 2003, 42, 1900–1923. (d) Schrock, R. R.; Hoveyda, A. H. Angew.
Chem., Int. Ed. 2003, 42, 4592–4633.
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(2) (a) Pignolet, L. H., Ed. Homogeneous Catalysis with Metal Phosphine
Complexes; Plenum: New York, 1983. (b) Collman, J. P.; Hegedus, L. S.;
Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition
Metal Chemistry; University Science: Mill Valley, CA, 1987. (c) Parshall,
G. W.; Ittel, S. Homogeneous Catalysis; J. Wiley and Sons: New York,
1992. (d) Green, J. C.; Scur, R. G.; Arnold, P. L.; Cloke, G. N. Chem.
Commun. 1997, 1963–1964. (e) Huang, J.; Schanz, H.-J.; Stevens, E. D.;
Nolan, S. P. Organometallics 1999, 18, 2370–2375. (f) Huang, J.; Jafarpour,
L.; Hillier, A. C.; Stevens, E. D.; Nolan, S. P. Organometallics 2001, 20,
2878–2882. (g) Jafarpour, L.; Nolan, S. P. J. Organomet. Chem. 2001, 17,
617–618. (h) Denk, K.; Sirsch, P.; Herrmann, W. A. J. Organomet. Chem.
2002, 649, 219–224.
(6) (a) Ofele, K.; Herrmann, W. A.; Mihalios, D.; Elison, M.; Herdtweck,
E.; Scherer, W.; Mink, J. J. Organomet. Chem. 1993, 459, 177–184. (b)
Arduengo, A. J.; Gamper, S.; Calabrese, J. C.; Davidson, F. J. Am. Chem.
Soc. 1994, 116, 4391–4394.
(7) For recent examples, see:(a) Kelly, R. A.; Scott, N. M.; D´ıez-
Gonza´lez, S.; Stevens, E. D.; Nolan, S. P. Organometallics 2005, 24, 3442–
3447. (b) Lui, L.; Montgomery, J. J. Am. Chem. Soc. 2006, 128, 5348–
5349. (c) Schaub, T.; Backes, M.; Radius, U. J. Am. Chem. Soc. 2006,
128, 15964–15965. (d) Ho, C.-Y.; Jamison, T. F. Angew. Chem., Int. Ed.
2007, 46, 782–785. (e) Matsubara, K.; Ueno, K.; Koga, Y.; Hara, K. J.
Org. Chem. 2007, 72, 5069–5076. (f) Schleicher, K. D.; Jamison, T. F.
Org. Lett. 2007, 9, 875–878.
10.1021/om8001125 CCC: $40.75
2008 American Chemical Society
Publication on Web 06/05/2008

3182 Organometallics, Vol. 27, No. 13, 2008
Scott et al.
nickel complexes of the type [(NHC)2Ni(CO)2]. The synthesis,
structural characterization, and thermodynamic behavior of
complexes of the type [(L)2Ni(CO)2] (L ) PR3, NHC) are
reported, which enable the determination of Ni-NHC (NHC
) ICy, IMes) and Ni-P (P ) PCy3, PPh3, P(p-Tol)3, P(m-Tol)3)
bond dissociation energies.
Results and Discussion
Synthesis and Structure Determination of [(ICy)2Ni(CO)2]
(2) and [(IMes)2Ni(CO)2] (3). The substitution reaction involv-
ing [(ItBu)Ni(CO)2] (1) and 2 equivalents of smaller NHC
ligands, such as IMes and ICy, gave rise to the expected
saturated complexes of general composition [(NHC)2Ni(CO)2]
(eqs 1 and 2) in good isolated yields. Of note, the substitution
reactions can also be performed with [(IAd)Ni(CO)2] as starting
material to yield 2 and 3 in similar high yields. This synthetic
route is original, since [(NHC)2Ni(CO)2] complexes reported
to date have been synthesized by double substitution reaction
from [Ni(CO)4] using the very small NHC IMe6a or by reac-
tion with carbon monoxide from the corresponding
[(NHC)2Ni(COD)] complex (NHC ) IMes, InPr, IiPr,...; COD
) 1,5-cyclooctadiene).9
Figure 1. Common saturated and unsaturated NHCs.
some factors governing the high catalytic activity of many
transition metal-NHC complexes remain unclear. In the case
of some NHC ligands depicted in Figure 1, surprising differences
in catalytic activities of the corresponding metal catalysts have
already been observed, but could not be rationalized or explained
experimentally.3d,14
Recently, we reported the synthesis of stable three-coordinate
[(NHC)Ni(CO)2] (NHC ) ItBu, IAd) and four-coordinate
[(NHC)Ni(CO)3] (NHC ) ICy, IPr, SIPr, IMes, SIMes)
complexes in which the steric and electronic factors governing
NHC ligands were investigated in the series of substitution
reactions with [Ni(CO)4].8c,d It was observed that the formation
of three- or four-coordinate complexes was a result of NHC
steric bulk and resulting steric congestion around the metal
center. While for most NHC ligands one carbonyl group was
displaced, generating the saturated [(NHC)Ni(CO)3] species,
ItBu and IAd gave rise to unsaturated [(NHC)Ni(CO)2] com-
pounds, indicating that both ligands are substantially more bulky
than P(tBu)3.
The 1H NMR spectra of complexes 2 and 3 show a
characteristic resonance at low field for the two imidazole
protons, as well as signals attributable to their corresponding
side-chain R groups. The 13C NMR spectra display characteristic
low-field resonances for the carbenic carbon at around 190 ppm
in addition to one low-field signal for the two carbonyl ligands,
indicating that the CO molecules are equivalent. Infrared
spectroscopy revealed two νCO stretching frequencies with the
more basic NHC ligand ICy (A1νCO for [(ICy)2Ni(CO)2] )
1948.5 cm-1) exhibiting a better σ-donating capability than IMes
(A1νCO for [(IMes)2Ni(CO)2] ) 2050.5 cm-1). Elemental
analyses also confirmed the compositions of 2 and 3. To
unambiguously establish the structure of complexes 2 and 3,
single-crystal diffraction studies were performed on colorless
crystals obtained from cooled (-50 °C), concentrated pentane
solutions of 2 and 3.
ORTEP representations at 50% probability with selected bond
distances and angles are shown in Figures 2 (2) and 3 (2).
Complexes 2 and 3 show the expected tetrahedral geometry
around the metal center for Ni(0) 18-electron complexes15 with
bond angles between 100° and 115°. A 2-fold rotation axis is
present in 3, with both 2 and 3 having the NHC and carbonyl
ligands disposed cis to each other. In general, the Ni-C(NHC)
distances in 2 and 3 suggest single-bond character and are in
good accordance with their almost exclusive σ-donor charac-
We now employ the dicarbonyl nickel complexes as reagents
in reactions involving less bulky NHC ligands, resulting in
(8) For mono NHC adducts, see:(a) Lappert, M. F.; Pye, P. L. Dalton
Trans. Inorg. Chem. 1977, 21, 72–80. (b) Hermann, W. A.; Goossen, L. J.;
Artus, G. R.; Ko¨cher, J. Organometallics 1997, 16, 2472–2477. (c) Dorta,
R.; Stevens, E. D.; Hoff, C. D.; Nolan, S. P. J. Am. Chem. Soc. 2003, 125,
10490–10491. (d) Dorta, R.; Stevens, E. D.; Scott, N. M.; Costabile, C.;
Cavallo, L.; Hoff, C. D.; Nolan, S. P. J. Am. Chem. Soc. 2005, 127, 2485–
2495.
(9) Schaub, T.; Radius, U. Chem.-Eur. J. 2005, 11, 5024–5030. Schaub,
T.; Backes, M.; Radius, U. Organometallics 2006, 25, 4196–4206.
(10) Tolman, C. A. Chem. ReV. 1977, 77, 313–324.
(11) (a) Zapf, A.; Ehrentraut, A.; Beller, M. Angew. Chem., Int. Ed.
2000, 39, 4153–4155. (b) Littke, A. F.; Dai, C. Y.; Fu, G. C. J. Am. Chem.
Soc. 2000, 122, 4020–4028. (c) Shigeru, S.; Masaaki, Y. Curr. Org. Chem.
2007, 11, 17–31.
(12) (a) Chianese, A. R.; Li, X.; Janzen, M. C.; Faller, J. W.; Crabree,
R. H. Organometallics 2003, 22, 1663–1667. (b) Kelly, R. A.; Clavier, H.;
Giudice, S.; Scott, N. M.; Stevens, E. D.; Bordner, J.; Samardjiev, I.; Hoff,
C. D.; Cavallo, L.; Nolan, S. P. Organometallics 2008, 27, 202–210.
(13) For reviews on stereoelectronic parameters of NHCs, see:(a)
Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem. ReV. 2000,
100, 39–92. (b) D´ıez-Gonza´lez, S.; Nolan, S. P. Coord. Chem. ReV. 2007,
251, 874–883.
(14) For copper-catalyzed hydrosilylation, see:(a) D´ıez-Gonza´lez, S.;
Kaur, H.; Kauer Zinn, F.; Stevens, E. D.; Nolan, S. P. J. Org. 2005, 70,
4784–4798. For metathesis transformations, see: (b) Ritter, T.; Hejl, A.;
Wenzel, A. G.; Funk, T. W.; Grubbs, R. H. Organometallics 2006, 25,
5740–5745.
(15) Hou, H.; Grantzel, P. K.; Kubiak, C. P. Organometallics 2003, 22,
2817–2819.

NHC and Tertiary Phosphine Ligands
Organometallics, Vol. 27, No. 13, 2008 3183
Table 1. Crystallographic Data for Compounds [(NHC)2Ni(CO)2]
2
3
emperical formula
fw
space group
C32H48N4NiO2
579.45
P21/c
C44H48N4NiO2
723.57
Pccn
cryst syst
a, Å
b, Å
c, Å
monoclinic
11.1412(18)
18.133(3)
16.174(3)
90
103.358
90
0.643
orthorhombic
10.7392(6)
18.4417(10)
20.2836(11)
90
90
90
0.523
4
R, deg
ꢀ, deg
γ, deg
µ(Mo), mm-1
Z
4
R
Rw
0.0355
0.0739
542
10 319
4019
0.0316
0.0607
323
38 689
1876
no. of refined params
no. of data collected
no. of unique data, l > 2σ
Rmerge
0.0709
0.0471
Figure 2. ORTEP diagram of [(ICy)2Ni(CO)2] (2). Thermal
ellipsoids are drawn at 50% probability. Hydrogen atoms have been
omitted for clarity. Selected bond lengths (Å) and angles (deg):
Ni(1)-C(1) 2.021(6), Ni(1)-C(16) 1.997(6), Ni(1)-C(40) 1.796(7),
Ni(1)-C(41) 1.755(7), C(40)-O(40) 1.153(6), C(41)-O(41)
1.176(6); C(40)-Ni(1)-C(41) 113.5(3), C(1)-Ni(1)-C(16) 100.9(3),
C(1)-Ni(1)-C(40) 115.3(3), C(1)-Ni(1)-C(41) 104.5(2), C(40)-
Ni(1)-C(16) 107.3(2), C(41)-Ni(1)-C(16) 114.9(3).
Table 2. Selected Bond Lengths (Å) in NHC-Containing Carbonyl
Nickel Complexes
entry
complex
[Ni(CO)4]17
Ni-CNHC
Ni-Cco
C-O
1
2
1.817(3)
1.7535(13)
1.7489(13)
1.761(2)
1.759(2)
1.757(5)
1.758(5)
1.775(5)
1.127(3)
[(ItBu)Ni(CO)2]8c
[(IAd)Ni(CO)2]8c
[(IMe)2Ni(CO)2]8a
[(ICy)2Ni(CO)2], 2
1.957(1)
1.953(2)
1.1396(16)
1.1384(16)
1.147(2)
3
4
5
1.971(4)
1.982(4)
2.021(4)
1.997(3)
1.983(3)
2.002(2)
1.977(4)
1.986(3)
1.972(2)
1.971(3)
1.962(4)
1.961(3)
1.142(5)
1.152(5)
1.164(5)
6
7
8
9
10
11
12
13
[(IMes)2Ni(CO)2], 3
[(iPr)2Ni(CO)2]9a
1.791(3)
1.758(2)
1.758(5)
1.794(4)
1.790(3)
1.793(4)
1.789(4)
1.780(3)
1.161(3)
1.155(2)
1.147(5)
1.134(5)
1.145(5)
1.144(4)
1.143(4)
1.148(3)
[(NHC)2Ni(CO)2]8b,a
[(NHC)Ni(CO)3]8b,a
[(IPr)Ni(CO)3]8d
[(IMes)Ni(CO)3]8d
[(SIPr)Ni(CO)3]8d
[(SIMes)Ni(CO)3]8d
Figure 3. ORTEP diagram of [(IMes)2Ni(CO)2] (3). Thermal
ellipsoids are drawn at 50% probability. Hydrogen atoms have been
omitted for clarity. Selected bond lengths (Å) and angles (deg):
Ni(1)-C(1) 1.983(3), Ni(1)-C(40) 1.761(3), C(40)-O(40) 1.161(3);
C(1)-Ni(1)-C(1) 124.22(17), C(40)-Ni(1)-C(40) 105.60(18),
C(1)-Ni(1)-C(40) 110.72(12), C(40)-Ni(1)-C(1) 102.23(12).
teristics.16 While the Ni-C(NHC) bond length (1.983(3) Å) in
3 is comparable to analogous distances in related Ni(0) carbene
complexes (Table 2),6a,8b–d,17 2 possesses one slightly shorter
(1.997 Å) and one longer (2.021 Å) Ni-C(NHC) bond length.
Since the IR data and enthalpy of reaction measurements (see
discussion below) on 2 support that ICy is more strongly bound
than IMes, the elongation of the Ni-C(NHC) bond length in 2
may arise simply from crystal packing effects. Furthermore, no
significant decrease in the Ni-CO bond length in 2 was
observed compared to analogous distances in 3, and it is similar
to other [(L)Ni(CO)3] systems (Table 2).
Calorimetric Results. With the isolation of three- and four-
coordinate nickel carbonyl complexes of the type [(NHC-
)Ni(CO)2] (NHC ) ItBu, IAd) and [(NHC)Ni(CO)3] (NHC )
ICy, IMes) the equilibrium associated with [Ni(CO)4] and NHC
was found to be crucial in the measurement of Ni-NHC bond
dissociation energies.8b,c Consequently, we successfully evalu-
ated the bond dissociation energy of Ni with the sterically
demanding ItBu and IAd ligands (BDE for Ni-ItBu ) 39 ( 3
kcal/mol, BDE for Ni-IAd ) 43 ( 3 kcal/mol) based on the
equilibrium established between three-coordinate [(NHC)Ni-
(CO)2] and [Ni(CO)4]. However, we were not successful in
establishing an equilibrium between the saturated [(NHC-
)Ni(CO)3] (NHC ) ICy, IMes) and [Ni(CO)4]. In the present
work, we are delighted to report the dissociation energy of the
Ni-L bond from the reaction of [(NHC)Ni(CO)2] (NHC ) ItBu,
IAd) with smaller NHC ligands, ICy and IMes, and the
substitution reactions involving a variety of tertiary phosphines
(PCy3, PPh3, P(p-Tol)3, and P(m-Tol)3) with [(NHC)Ni(CO)2].
These data help increase our understanding of the fundamental
steric and electronic factors influencing NHC ligands and afford
a direct comparison with a large array of tertiary phosphine
data.10 The thermochemistry of [(PR3)2Ni(CO)2] complexes was
(16) Herrmann, W. A.; Elison, M.; Fischer, J.; Kocher, C.; Artus, G. R.
Angew. Chem., Int. Ed. 1995, 34, 2371–2374. McGuinness, D. S.; Cavell,
K. J.; Skelton, B. W.; White, A. H. Organometallics 1999, 18, 1596–1605.
(17) Braga, D.; Grepioni, F.; Orpen, A. G. Organometallics 1993, 12,
1481–1483.

3184 Organometallics, Vol. 27, No. 13, 2008
Scott et al.
Table 3. Enthalpies of Ligand Substitution (kcal/mol) from
[(ItBu)Ni(CO)2]a
Table 5. Bond Dissociation Energies of Ni-NHC and Ni-Pa
entry
L
BDEItBu
BDEIAd
BDEaV
[(ItBu)Ni(CO)2]soln + 2 Lsoln f [(L)2Ni(CO)2]soln
1
2
3
4
5
6
ICy
IMes
PCy3
P(p-Tol)3
P(m-Tol)3
PPh3
29.9(3.0)
26.9(3.0)
25.8(3.0)
25.4(3.0)
25.2(3.0)
24.8(3.0)
30.2(3.0)
28.2(3.0)
27.5(3.0)
27.4(3.0)
27.0(3.0)
26.6(3.0)
30.1(3.0)
27.5(3.0)
26.6(3.0)
26.4(3.0)
26.1(3.0)
25.7(3.0)
entry
L
-∆Hrxn (kcal/mol)b
1
2
3
4
5
6
ICy
IMes
PCy3
P(p-Tol)3
P(m-Tol)3
PPh3
20.9(0.2)
14.8(0.6)
12.6(0.1)
11.8(0.3)
11.5(0.3)
10.7(0.2)
a BDEItBu: BDE calculated based on [(ItBu)Ni(CO)2] substitution
reactions. BDEIAd: BDE calculated based on [(IAd)Ni(CO)2] substitution
reactions. BDEaV: average of BDEItBu and BDEIAd
.
a Enthalpy of solution is included. b Enthalpy values are reported with
95% confidence limits.
( 6°16 and 200°,18 respectively) not undergoing substitution
reactions, while the smaller P(p-Tol)3 (θ ) 143°), P(m-Tol)3
(165°), PCy3 (170°), and PPh3 (145°) ligands exhibiting rapid
substitiution.16,19
As shown in Table 4, the enthalpy values for the reaction of
[(IAd)Ni(CO)2] show a similar pattern as for [(ItBu)Ni(CO)2];
however, the magnitude of enthalpy of the reaction is marginally
smaller. This is a result of the stronger Ni-IAd bond (43 ( 3
kcal/mol) in comparison to the Ni-ItBu bond (39 ( 3 kcal/
mol).
Table 4. Enthalpies of Ligand Substitution (kcal/mol) from
[(IAd)Ni(CO)2]a
[(Ad)Ni(CO)2]soln + 2 Lsoln f [(L)2Ni(CO)2]soln
entry
L
-∆Hrxn (kcal/mol)b
1
2
3
4
5
6
ICy
IMes
PCy3
P(p-Tol)3
P(m-Tol)3
PPh3
17.5(0.3)
13.4(0.2)
12.0(0.2)
11.9(0.3)
11.0(0.3)
10.2(0.2)
Absolute Ni-NHC and Ni-P Bond Enthalpies. The
enthalpies of the substitution reactions of the type shown in eq
3 can be used to evaluate the Ni-NHC bond dissociation
energies as well as Ni-P bond dissociation energies. Using the
enthalpies of reaction for the substitution of [(ItBu)Ni(CO)2]
with ICy (-20.9 ( 0.2 kcal/mol) or IMes (-14.8 ( 0.6 kcal/
mol) and the previously reported bond dissociation energy of
Ni-ItBu (39 ( 3 kcal/mol), these data were used to calculate
the bond dissociation energy for Ni-ICy (BDEItBu for Ni-ICy,
29.9 ( 3.0 kcal/mol) and Ni-IMes (BDEItBu for Ni-IMes, 26.9
( 3.0 kcal/mol) (Table 5, entries 1 and 2).20 Substitution
reactions involving [(IAd)Ni(CO)2] and ICy or IMes give bond
dissociation values (BDEIAd) of 30.2 ( 3.0 kcal/mol for Ni-ICy
and 28.2 ( 3.0 kcal/mol for Ni-IMes. As shown in Table 5,
there is a reasonable agreement between the two values BDEItBu
and BDEIAd obtained for Ni-ICy and Ni-IMes BDEs from
respectively [(ItBu)Ni(CO)2] and [(IAd)Ni(CO)2] substitution
reactions, which yield an average value (BDEaV) of 30.1 ( 3.0
kcal/mol for Ni-ICy and 27.5 ( 3.0 kcal/mol for Ni-IMes
bond dissociation energies. The bond strength for Ni-PR3 was
calculated in a similar manner, and the bond dissociation
energies are tabulated in Table 5.
Both NHC ligands ICy and IMes possess slightly stronger
bond dissociation energies, suggesting that NHC ligands are
among the most strongly bound ligands to Ni(0).21,22 The
relative BDE values for Ni-ICy (BDE ) 30.1 ( 3.0 kcal/mol)
and Ni-IMes (BDE ) 27.5 ( 3.0 kcal/mol) are in accordance
with the infrared data that indicated that ICy (νCO for
[(ICy)2Ni(CO)2] ) 1948.6 cm-1) is a better donor than IMes
(νCO for [(IMes)2Ni(CO)2] ) 2050.5 cm-1). As expected, the
strongest nickel-phosphine bond dissociation energy was
obtained for the most basic phosphine, PCy3 (BDE ) 26.6 (
a Heat of solution is included. b Enthalpy values are reported with
95% confidence limits.
made possible by the rapid and quantitative reaction of
[(NHC)nNi(CO)2] (n ) 1, 2) with stoichiometric amounts of
tertiary phosphine ligands (eq 3).
[(NHC)Ni(CO)2] + 2PR3
8
toluene, 30 ° C
(3)
[(PR3)2Ni(CO)2] + NHC
NHC ) ItBu, IAd
PR3 ) PPh3, PCy3, P(p-Tol)3, P(m-Tol)3
The enthalpy values were determined by anaerobic solution
calorimetry in toluene at 30 °C by reacting 2 equiv of either
NHC or tertiary phosphine ligands with 1 equiv of [(NHC)-
Ni(CO)2] (NHC ) ItBu, IAd). Importantly, the final product of
each calorimetric experiment was characterized by NMR
spectroscopy in order to exclude other side reactions such as
the formation of [Ni(CO)4] and free NHC. Experimental
enthaply data are tabulated in Tables 3 and 4 respectively. Not
surprisingly, the magnitude of the enthalpy of reaction is affected
by both the steric and electronic nature of the ligands. The NHC
ligands ICy (entry 1) and IMes (entry 2) have more exothermic
reaction enthalpy than even the very basic tertiary phosphine
PCy3 (entry 3), indicating the greater donor property of these
NHC ligands. Interestingly, ICy binds the nickel atom more
strongly than IMes, but the gap is generally more important.2e
Steric factors around the metallic center explain this difference.
The tetrahedral geometry of these Ni complexes allows for steric
interactions between crowded ligands, and it has been proved
that IMes is significantly larger than ICy.8d,12b Furthermore, the
choice of the phosphine has a significant effect on the reaction
outcome. No reaction was observed when reacting the ortho-
substituted phosphine ligands, tri-o-tolylphosphine and tri-o-
methoxyphenylphosphine, with [(NHC)Ni(CO)2] (NHC ) ItBu,
IAd). However, the reaction readily took place using P(p-Tol)3,
P(m-Tol)3, PCy3, and PPh3. The difference in reactivity can be
explained by sterics, with the larger tri-o-tolylphosphine and
tri-o-methoxyphenylphosphine ligands (cone angle (θ) ) 194
(18) (a) Grim, O. S.; Yankowsky, A. W. J. Org. Chem. 1977, 42, 1236–
1239. (b) Pruchnik, F. P.; Smolenski, P.; Wajda-Hermanowicz, K. J.
Organomet. Chem. 1998, 570, 63–69.
(19) Eriks, K.; Giering, W. P.; Lui, H. Y.; Prok, A. Inorg. Chem. 1989,
28, 1759–1763.
(20) The formula of the calculations is the following: ∆Hrxn ) [-2
BDE(Ni-L) + BDE(Ni-ItBu)]; so BDE(Ni-L) ) 1/2 [-∆Hrxn
+
BDE(Ni-ItBu)].
(21) (a) Nolan, S. P.; Hoff, C. D.; Landrum, J. T. J. Organomet. Chem.
1985, 282, 357–362. (b) Nolan, S. P.; Lopez de la Vega, R.; Hoff, C. D.
Inorg. Chem. 1986, 25, 4446–4448.
(22) Tolman, C. A.; Reutter, D. W.; Seidel, W. C. J. Organomet. Chem.
1976, 117, C30-C33.

NHC and Tertiary Phosphine Ligands
Organometallics, Vol. 27, No. 13, 2008 3185
C-80), which was periodically calibrated using the TRIS8b reaction or
the enthalpy of solution of KCl in water.21 Experimental enthalpy data
are reported with 95% confidence limit. Elemental analysis was
performed at Desert Analysis, Tucson, AZ. The compounds [(ItBu-
)Ni(CO)2] (1) and [(IAd)Ni(CO)2] were synthesized according to the
literature procedure.8c The NHC ligands were synthesized following
literature procedures.24 Triphenylphosphine (PPh3), tricyclohexylphos-
phine (PCy3), tri-p-tolylphosphine (P(p-Tol)3), tri-m-tolylphosphine
(P(m-Tol)3), tri-o-tolylphosphine (P(o-Tol)3), and tri-o-methoxyphe-
nylphosphine (P(o-MeOPh)3) were used as received (Strem). The
identities of nickel phosphine complexes [(PPh3)2Ni(CO)2],25
[(PCy3)2Ni(CO)2],25 [(P(p-Tol)3)2Ni(CO)2],26 and [(P(m-Tol)3)2-
Ni(CO)2]26 were confirmed by comparison with literature spectroscopic
data.
Synthesis of [(ICy)2Ni(CO)2] (2). Dropwise addition of a
pentane solution (20 mL) of ICy (328 mg, 1.44 mmol) to a pentane
solution (5 mL) of [(ItBu)Ni(CO)2] (1; 200 mg, 0.677 mmol) with
magnetic stirring led to a rapid development of a yellow-colored
solution. The resulting solution was stirred at room temperature
for 1 h. The resulting solution was subsequently concentrated to
ca. 10 mL. Cooling the concentrated solution to -50 °C resulted
in the formation of a yellow precipitate. The solution was filtered,
and the filtrate was washed with cold pentane (3 × 5 mL) and
dried in Vacuo. Yield: 310 mg (79%). X-ray quality crystals were
grown by slow evaporation of a concentrated pentane solution of
complex 2. Anal. Calcd for C32H48N4NiO2 (MW ) 579.45): C,
66.32; H, 8.35; N, 9.66. Found: C, 66.22; H, 8.36; N, 9.65. 1H
NMR (C6D6, 400 MHz, δ): 6.10 (s, 4H, NCHdCHN), 5.00 (m,
4H, CHCy), 2.20 (m, 8H, CH2Cy), 1.62 (m, 8H, CH2Cy), 1.46 (m,
4H, CH2Cy), 1.40 (m, 8H, CH2Cy), 1.22 (m, 8H, CH2Cy), 0.90 (m,
4H, CH2Cy). 13C NMR (C6D6, 400 MHz, δ: 205.15 (N-C-N), 198.33
(CO), 115.77 (NCHdCHN), 58.55 (CHCy), 33.77 (CH2Cy), 25.80
(CH2Cy). IR νCO (toluene, cm-1): 1948.6 (s), 1877.9 (vs).
Synthesis of [(IMes)2Ni(CO)2] (3). A pentane solution (20 mL)
of IMes (453 mg, 1.49 mmol) was added dropwise to a pentane
solution (5 mL) of [(ItBu)Ni(CO)2] (1; 200 mg, 0.677 mmol). The
resulting solution was stirred at room temperature for 1 h, during
which time a red precipitate was formed. The solution was
subsequently concentrated to ca. 10 mL and then filtered. The solid
was washed with cold pentane (-50 °C, 2 × 5 mL), affording
complex 3, which was dried in Vacuo. Yield: 356 mg (73%). Anal.
Calcd for C44H48N4NiO2 (MW ) 723.57): C, 73.03; H, 6.68; N,
7.73. Found: C, 72.96, H, 6.68; N, 7.70. 1H NMR (C6D6, 400 MHz,
δ): 6.80 (s, 8 H, CHAr), 6.23 (s, 4 H, NCHdCHN), 2.11 (s, 12 H,
CH3Mes), 2.01 (s, 24 H, CH3Mes). 13C NMR (C6D6, 400 MHz): δ
198.71 (N-C-N), 194.25 (CO), 139.07 (CAr), 135.65 (CAr), 129.60
(CHAr), 121.96 (CAr), 21.19 (CH3Mes), 17.72 (CH3Mes). IR νCO
(toluene, cm-1): 2050.5 (s), 1886.8 (vs).
3.0 kcal/mol, entry 3), while the weakest bond strength (BDE
) 25.7 ( 3.0 kcal/mol, entry 6) was found for Ni-PPh3.
A calorimetric study of the reactions of various phosphorus
ligands (and tBuCN) with Ni(COD)2 by Tolman et al.22 showed
that both the extent of reaction and mean Ni-P bond strengths
tend to decrease with increasing ligand size, with P(OMe)3 (θ
) 107°)10,23 having the highest enthalpy value of -57 kcal/
mol in the formation of [NiL4] (L ) P(OMe)3). While the
reaction of bulky P(O-o-C6H4-iPr)3 (θ ) 148°)10 and [Ni(COD)2]
resulted in the formation of [(L)2Ni(COD)] (L ) P(O-o-C6H4-
iPr)3), it was found to have the lowest enthalpy value of -15
kcal/mol. However, enthalpy values (Table 3) in our system
show that BDE of Ni-P in the formation of [(PR3)2Ni(CO)2]
complexes is dependent on the electron donor-acceptor char-
acter of the phosphorus ligand. The decreasing exothermicity
in the formation of [(PR3)2Ni(CO)2] (PR3 ) PCy3, P(p-Tol)3,
P(m-Tol)3), PPh3), which results in a smaller BDE values (Table
5), corresponds closely to the decreasing electron-donating
ability of the phosphorus ligand. Nevertheless, only a small
difference was observed between the most basic phosphine,
PCy3 (entry 3), and the less donating phosphine, PPh3 (entry
6). PCy3 having a larger cone angle (θ ) 170°) compared to
PPh3 (θ ) 145°)6a indicates the overwhelming importance of
steric effects in defining the magnitude of the Ni-P bond
dissociation energy in this specific nickel system.
Conclusion
The synthesis and structural characterization of [(ICy)2-
Ni(CO)2] (2) and [(IMes)2Ni(CO)2] (3) is described. We have
evaluated the bond dissociation energy of Ni-L associated with
the very rapid and quantitative NHC double substitution
reactions involving [(NHC)Ni(CO)2] (NHC ) ItBu, IAd) with
smaller NHC ligands, ICy and IMes, in addition to reactions
involving a variety of tertiary phosphines (PCy3, PPh3, P(p-
Tol)3, P(m-Tol)3). The enthalpy values of the formation of
[(L)2Ni(CO)2] (L ) ICy, IMes, PCy3, PPh3, P(o-Tol)3, P(m-
Tol)3) were measured by solution calorimetry and used to
determine Ni-NHC and Ni-P bond dissociation energies. The
NHC ligands ICy and IMes were found to have greater electron-
donating properties than the most basic tertiary phosphine ligand.
Furthermore, an increase in steric bulk (and cone angle for
tertiary phosphines) also appears to play a crucial role in the
formation of [(L)2Ni(CO)2] (L ) ICy, IMes, PCy3, PPh3, P(p-
Tol)3, P(m-Tol)3) complexes and consequently in their bond
dissociation energy. This study highlights the intimate relation
between electronic and steric parameters and the intrinsic
difficulty in separating these.
NMR Titrations. Prior to every set of calorimetric experiments, a
Wilman screw-capped NMR tube was charged with (0.1 mg of
organometallic complex and fitted with a septum, and toluene-d8 was
subsequently added. A solution of the ligand of interest was then
injected to the NMR tube through the septum with a microsyringe,
Experimental Section
1
General Considerations. All reactions were performed under an
inert atmosphere of argon or nitrogen using standard high-vacuum or
Schlenk techniques or in an MBraun glovebox containing less than 1
ppm oxygen and water. Solvents were distilled from appropriate drying
agents or were passed through an alumina column in an MBraun
solvent purification system; other anhydrous solvents were purchased
from Aldrich and degassed prior to use by purging with dry argon
and were kept over molecular sieves. Solvents for NMR spectroscopy
were degassed with argon and dried over molecular sieves. NMR
spectra were recorded on a 400 MHz Varian Gemini. Infrared spectra
were recorded on a PE 2000 FT-IR spectrometer. Calorimetric
measurements were performed using a Calvet calorimeter (Setaram
followed by vigorous shaking. The reactions were monitored by H
NMR and IR spectroscopy. The reactions were found to be suitable
for calorimetric experiments since they are all rapid, clean, and
quantitative under experimental calorimetric conditions.
Calorimetric Measurements for Reaction of [(ItBu)Ni(CO)2]
or [(IAd)Ni(CO)2] with Tertiary Phosphine and NHC Ligands.
The mixing vessels of the Setaram C-80 were cleaned, dried in an
oven maintained at 120 °C, and then taken into the glovebox. A
(24) Arduengo, A. J.; Krafczk, R.; Schmutzler, R.; Craig, H. A.;
Georlich, J. R.; Marshall, W. J.; Unverzagt, M. Tetrahedron 1999, 55,
14523–14534.
(25) Aresta, M.; Quaranta, E.; Tommasi, I. J. Chem. Soc., Chem.
Commun. 1988, 7, 450–452.
(23) Bunten, K. A.; Chen, L.; Fernandez, A. L.; Poe¨, A. J. Coord. Chem.
ReV. 2002, 233-23441-51.
(26) Ballester, L.; Gutierrez-Alonso, A.; Perez-Serrano, S.; Perpinan,
M. F. Inorg. Chem. 1991, 30, 2954–2955.

3186 Organometallics, Vol. 27, No. 13, 2008
Scott et al.
15-20 mg sample of [(ItBu)Ni(CO)2] or [(IAd)Ni(CO)2] was
weighed out accurately into the lower vessel; it was then closed
and sealed with 1.5 mL of mercury. Four milliliters of a solution
of the ligand of the interest was added. The remainder of the cell
was assembled, removed from the glovebox, and inserted in the
calorimeter. The reference vessel was loaded in an identical fashion
with the exception that no organometallic complex was added to
the lower vessel. After the calorimeter has reached thermal
equilibrium at 30 °C (approximately 3 h), the calorimeter was
inverted, thereby allowing the reactants to mix. After the reaction
had reached completion and the calorimeter had once again reached
thermal equilibrium (approximately 3 h), the vessels were removed
from the calorimeter, taken into the glovebox, opened, and analyzed
using NMR and IR spectroscopy. The enthalpy of ligand substitution
listed in Tables 2 and 3 represents the average of 3-5 individual
calorimetric determinations with all species in solution.
Enthalpy of Solution of [(ItBu)Ni(CO)2] and [(IAd)Ni(CO)2].
In order to consider all species in solution, the enthalpy of solution
of [(ItBu)Ni(CO)2] and [(IAd)Ni(CO)2] had to be directly measured.
This was performed by using a similar procedure to the one
described above, with the exception that no ligand was added to
the reaction cell. The enthalpies of solution of [(ItBu)Ni(CO)2], 4.2
( 0.2 kcal/mol, and [(IAd)Ni(CO)2], 2.6 ( 0.2 kcal/mol, represent
the average of 3-5 individual determinations.
The CIF files of crystal structures 2 and 3 have been deposited
with the CCDC, No. CCDC-246236 and -246237, respectively.
Copies of the data can be obtained free of charge on applications
to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, fax: +44
1223 336 033; http://www.ccdc.cam.ac.uk; e-mail: deposit@
ccdc.cam.ac.uk.
Acknowledgment. The National Science Foundation is
gratefully acknowledged for financial support of this work.
N.M.S. thanks the American Australian Education Fund for
financial support. S.P.N. (research grant) and H.C. (post-
doctoral fellowship) both acknowledge the Ministerio de
Educacio´n y Ciencia (Spain) as well as the ICIQ Foundation
for support of this work. S.P.N. is an ICREA Research
Professor. Dr. Kenneth Moloy and DuPont CR&D are
especially thanked for the gift of [Ni(CO)4], as is Eli Lilly
and Co. for the generous donation of amines. We are grateful
to Professor Carl D. Hoff for his constructive comments.
Supporting Information Available: The CIF files of crystal
structures 2 and 3 are available free of charge via the Internet at
http://pubs.acs.org.
OM8001125