Elucidation of heterocumulene activation by a nucleophilic-at-metal iridium(I) carbene

Article abstract of DOI:10.1021/om8009766

A carbene complex supported by the (PNP)lr framework is shown to facilitate sulfur-atom transfer from CS₂ and PhNCS by an unusual multiple-bond metathesis pathway, and kinetically trapped intermediates are provided to support the proposed metal

Full text of DOI:10.1021/om8009766

Organometallics 2009, 28, 161–166
161
Elucidation of Heterocumulene Activation by a
Nucleophilic-at-Metal Iridium(I) Carbene
Matthew T. Whited and Robert H. Grubbs*
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, DiVision of Chemistry and
Chemical Engineering, California Institute of Technology, Pasadena, California 91125
ReceiVed October 9, 2008
A carbene complex supported by the (PNP)Ir framework is shown to facilitate sulfur-atom transfer
from CS2 and PhNCS by an unusual multiple-bond metathesis pathway, and kinetically trapped
intermediates are provided to support the proposed metal-initiated mechanism for heterocumulene
activation. Experimental and theoretical studies on a series of (PNP)Ir-L complexes suggest that a high-
2
lying, nucleophilic Ir(dz ) orbital mediates this unique reactivity. The combination of evidence indicates
that square-planar Ir(I) carbenes of this type are best formulated as nucleophilic-at-metal carbenes,
exhibiting reactivity initiated by a nucleophilic metal center rather than a nucleophilic or electrophilic
carbene and providing products that are complementary to those typically observed for high-valent
alkylidenes.
Scheme 1. Decarbonylation of Heterocumulenes
by Carbene 1
Introduction
The reactivity of transition metal-carbon multiple bonds has
been the subject of intense investigation for many years,
allowing the development of a host of transformations based
on reactivity of the carbene unit.1 In general, these transforma-
tions are dominated by carbon-based frontier orbitals.2 Thus,
metal-carbon multiple bonds are conventionally divided into
two classes, where Fischer carbenes (coordinated singlet car-
benes) are electrophilic at carbon and Schrock carbenes
(coordinated triplet carbenes) are nucleophilic at carbon.
We have been investigating the reactivity of iridium species
supported by Ozerov’s PNP ligand (PNP ) [N(2-PiPr2-4-Me-
C6H3)2]-)3 and recently reported an unusual transformation of
carbon dioxide, carbonyl sulfide, and phenyl isocyanate, where
atom and group transfer to a Fischer carbene at square-planar
iridium(I) was effected by CdE bond cleavage (E ) O, S,
NPh).4 We proposed that these unique metathesis reactions were
made possible by a nucleophilic iridium center, which could
initiate atom and group transfer by attack of an electrophilic
heterocumulene, cyclization to metallacycle 2, and elimination
of formate, thioformate, or formimidate to afford (PNP)Ir-CO
(3, Scheme 1).
This formulation implies that the high d-electron count and
coordinatively unsaturated nature of carbene 1 confer a degree
of nucleophilicity to the iridium center that controls reactivity
of the complex, contrasting previous reports of heterocumulene
reactivity where the metal-bound carbenes act as nucleophiles
and C-C bonds are formed.5,6 Such behavior would not be
entirely unexpected in light of Roper’s previous observation,
which we have confirmed for our system,4 that heteroatom-
substituted carbenes rarely exhibit electrophilic character when
attached to electron-rich, late transition metals.2f Additionally,
Stone has reported that a square-planar carbene of iridium(I)
performs the oxidative addition of dihydrogen, affording an
* Corresponding author. E-mail: rhg@caltech.edu.
(1) For leading references, see: (a) Fischer, E. O. Pure Appl. Chem.
1970, 24, 407. (b) Fischer, E. O. Pure Appl. Chem. 1978, 50, 857. (c) Cardin,
D. J.; Cetinkaya, B.; Lappert, M. F. Chem. ReV. 1972, 72, 545. (d) Cotton,
F. A.; Lukehart, C. M. Prog. Inorg. Chem. 1972, 16, 487. (e) Schrock,
R. R. Acc. Chem. Res. 1979, 12, 99. (f) Schrock, R. R. J. Chem. Soc.,
Dalton Trans. 2001, 18, 2541. (g) Schrock, R. R. Chem. ReV. 2002, 102,
145. (h) Doyle, M. P. Chem. ReV. 1986, 86, 919. (i) Brookhart, M.;
Studabaker, W. B. Chem. ReV. 1987, 87, 411. (j) Doetz, K. H.; Minatti, A.
Fischer Type Carbene Complexes. In Transition Metals for Organic
Synthesis; Beller, M., Bolm, C., Eds.; Wiley-VCH: Weinheim, 2004; Vol.
2, pp 397-425. (k) Handbook of Metathesis; Grubbs, R. H., Ed.; Wiley-
VCH: Weinheim, 2003.
(2) (a) Nugent, W. A.; Mayer, J. M. Metal-Ligand Multiple Bonds;
Wiley: New York, 1988. (b) Schrock, R. R. J. Am. Chem. Soc. 1975, 97,
6577. (c) Nugent, W. A.; McKinney, R. J.; Kasowski, R. V.; Van-Catledge,
F. A. Inorg. Chim. Acta 1982, 65, L91. (d) Nakatsuji, H.; Ushio, J.; Han,
S.; Yonezawa, T. J. Am. Chem. Soc. 1983, 105, 426. (e) Ushio, J.; Nakatsuji,
H.; Yonezawa, T. J. Am. Chem. Soc. 1984, 106, 5892. (f) Block, T. F.;
Fenske, R. F.; Casey, C. P. J. Am. Chem. Soc. 1976, 98, 441. (g) Gallop,
M. A.; Roper, W. R. AdV. Organomet. Chem. 1986, 25, 121. (h) Taylor,
T. E.; Hall, M. B. J. Am. Chem. Soc. 1984, 106, 1576. (i) Cundari, T. R.;
Gordon, M. S. J. Am. Chem. Soc. 1991, 113, 5231.
(5) Examples of more commonly observed nucleophilic reactivity
between metal carbenes and heterocumulenes: (a) Schrock, R. R. J. Am.
Chem. Soc. 1976, 98, 5399. (b) Klein, D. P.; Bergman, R. G. J. Am. Chem.
Soc. 1989, 111, 3079. (c) Lee, S. K.; Cooper, N. J. J. Am. Chem. Soc.
1990, 112, 9419. (d) Mindiola, D. J.; Hillhouse, G. L. J. Am. Chem. Soc.
2002, 124, 9976.
(6) This behavior is more closely related to what is observed for
Sadighi’s structurally and electronically similar Cu(I)-boryl complexes: (a)
Laitar, D. S.; Mu¨ller, P.; Sadighi, J. P. J. Am. Chem. Soc. 2005, 127, 17196.
(b) Laitar, D. S.; Tsui, E. Y.; Sadighi, J. P. J. Am. Chem. Soc. 2006, 128,
11036.
(3) (a) Fan, L.; Foxman, B. M.; Ozerov, O. V. Organometallics 2004,
23, 326. (b) Ozerov, O. V.; Guo, C.; Papkov, V. A.; Foxman, B. M. J. Am.
Chem. Soc. 2004, 126, 4792.
(4) Whited, M. T.; Grubbs, R. H. J. Am. Chem. Soc. 2008, 130, 5874.
10.1021/om8009766 CCC: $40.75
2009 American Chemical Society
Publication on Web 11/24/2008

162 Organometallics, Vol. 28, No. 1, 2009
Scheme 2. Reaction of Carbene 1 with Carbon Disulfide
Whited and Grubbs
iridium(III) dihydrido carbene.7 By analogy, the electron-rich
metal center in our system might be expected to play a central
role in substrate activation.
of the plane of the iridium-phosphorus bonds.11 The carbene
is observed by 13C NMR at δ 266.7 ppm, shifted significantly
downfield from the parent tert-butoxymethylidene (δ 210.0
ppm).12 As a result of the change in metal oxidation state, the
IrdC bond of 4 is slightly elongated (1.96 Å) relative to
compound 1 (1.88 Å), and the C(27)-O(1) bond is contracted
by approximately the same amount (from 1.35 Å to 1.28 Å),
indicating a decrease in metal-carbon multiple-bond character
and concomitant increase in the effective carbon-oxygen bond
order.
In this contribution, we report that reaction of 1 with CS2
and PhNCS, which are isoelectronic to the heterocumulenes
previously examined, allows observation and isolation of
kinetically trapped intermediate species, supporting a mechanism
of atom transfer that clearly requires a distinct mode of carbene
reactivity. Additionally, we present experimental and theoretical
2
studies suggesting that a high-lying, nucleophilic Ir(dz ) orbital
plays a critical role in initiating these unusual transformations.
Taken together, our findings suggest a new strategy for the
activation of heterocumulenes and other electrophiles across
metal-ligand multiple bonds at late transition metals. Interest-
ingly, these reactivity patterns are complementary to those
observed for the nucleophilic alkylidenes of the high-valent early
metals.2a,5a,8
Heating a benzene solution of 4 (70 °C, 16 h) caused
quantitative degradation to (PNP)Ir–CS (5) with concomitant
expulsion of tert-butyl thioformate and CS2 (Scheme 2),
indicating the potential intermediacy of CS2 dimer 4 in the
sulfur-atom transfer process (and suggesting CO2 dimers as
possible intermediates in previously reported oxygen-atom
transfer reactions).10 However, previous studies of the kinetics
of CO2 deoxygenation by 1 had shown a first-order rate
dependence on CO2 concentration, indicating that such a
mechanism was not operative.4 Additionally, the connectivity
of 4 is more consistent with intermediates typically invoked in
heterocumulene disproportionation reactions.13 Thus, we were
interested to see whether 4 was instead a kinetic product formed
from the trapping of an intermediate species formed by reaction
of 1 with CS2.
Results and Discussion
I. Reactivity with Carbon Disulfide. In light of the high
driving force for the formation of carbonyl complex 3, reaction
of carbene 1 with heterocumulenes lacking oxygen was exam-
ined with the goal of stabilizing an intermediate species
analogous to metallacycle 2. However, in contrast to the
reactivity observed with oxygen-containing heterocumulenes,4
dissolution of (PNP)IrdC(H)OtBu (1) in neat CS2 did not afford
(PNP)Ir–CS but instead yielded the unusual CS2 head-to-tail
dimer complex 4, in which the iridium carbene unit remained
intact (Scheme 2),9,10 as confirmed by X-ray diffraction analysis
Slow addition of 1 equiv of CS2 to a benzene solution of 1
resulted in the immediate and quantitative formation of thio-
carbonyl complex 5 at ambient temperature, ruling out the
intermediacy of 4 in the atom transfer reaction. Thus, as in our
original assessment of the mechanism, we propose that reaction
begins with nucleophilic attack by iridium at an electrophilic
heterocumulene carbon. Depending on the concentration of CS2,
two pathways are available for this intermediate, and it can either
be trapped by a second molecule of CS2, affording 4, or cyclize
to the irida(dithio)lactone and decompose to give 5 and tert-
1
(Figure 1). The carbene proton retains a distinctive H NMR
shift (δ 15.5 ppm), consistent with an iridium-carbon multiple
bond, but collapses from a triplet (in complex 1) to a singlet
(in complex 4) due to the shifting of the C-H bond vector out
(7) Fraser, P. J.; Roper, W. R.; Stone, F. G. A. J. Chem. Soc., Dalton
Trans. 1974, 760.
(8) (a) Tebbe, F. N.; Parshall, G. W.; Reddy, G. S. J. Am. Chem. Soc.
1978, 100, 3611. (b) Pine, S. H.; Zahler, R.; Evans, D. A.; Grubbs, R. H.
J. Am. Chem. Soc. 1980, 102, 3270.
(11) Similar behavior has been observed for phosphine-supported
ruthenium carbenes: (a) Nguyen, S. T.; Johnson, L. K.; Grubbs, R. H.; Ziller,
J. W. J. Am. Chem. Soc. 1992, 113, 3974. (b) Dias, E. L.; Nguyen, S. T.;
Grubbs, R. H. J. Am. Chem. Soc. 1997, 119, 3887. (c) Schwab, P.; Grubbs,
R. H.; Ziller, J. W. J. Am. Chem. Soc. 1996, 118, 100.
(12) Romero, P. E.; Whited, M. T.; Grubbs, R. H. Organometallics 2008,
27, 3422.
(13) (a) Chatt, J.; Chatt Kubota, M.; Leigh, G. J.; March, F. C.; Mason,
R.; Yarrow, D. J. J. Chem. Soc., Chem. Commun. 1974, 1033. (b) Fachinetti,
G.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. J. Am. Chem. Soc. 1979,
101, 1767. (c) Pasquali, M.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. Inorg.
Chem. 1980, 19, 3847. (d) Thewissen, D. H. M. W.; Van Gaal, H. L. M.
J. Organomet. Chem. 1979, 172, 69. (e) Gibson, J. A. E.; Cowie, M.
Organometallics 1984, 3, 984.
(9) This binding motif for CS2 has been observed previously: (a) Werner,
H.; Kolb, O.; Feser, R.; Schubert, U. J. Organomet. Chem. 1980, 191, 283.
(b) Cowie, M.; Dwight, S. K. J. Organomet. Chem. 1980, 198, C20. (c)
Cowie, M.; Dwight, S. K. J. Organomet. Chem. 1981, 214, 233. (d) Mason,
G.; Swepston, P. N.; Ibers, J. A. Inorg. Chem. 1983, 22, 411. (e) Carmona,
E.; Galindo, A.; Monge, A.; Mun˜oz, M. A.; Poveda, M. L.; Ruiz, C. Inorg.
Chem. 1990, 29, 5074. (f) George, D. S. A.; Hilts, R. W.; McDonald, R.;
Cowie, M. Inorg. Chim. Acta 2000, 300, 353.
(10) Though not observed for our system, an iridium-supported head-
to-tail dimer of CO2 has also been reported: Herskovitz, T.; Guggenberger,
L. J. J. Am. Chem. Soc. 1976, 98, 1615.

Nucleophilic-at-Metal Iridium(I) Carbene
Organometallics, Vol. 28, No. 1, 2009 163
i
Figure 1. Displacement ellipsoid (30%) representation of complexes 4 (left) and 5 (right) with Pr phosphine substituents omitted from 4
for clarity. Relevant bond lengths (Å) and angles (deg), for 4: Ir1-C27 1.960(4), Ir1-S1 2.397(1), Ir1-N1 2.119(4), Ir1-C33 1.998(5),
Ir1-P1 2.350(1), Ir1-P2 2.357(1), C27-O1 1.281(5), S1-Ir1-C27 179.1(1), N1-Ir1-C33 178.5(2), P1-Ir1-P2 159.52(4). For 5: Ir1-C27
1.82(2), Ir1-N1 2.04(1), Ir1-P1 2.298(3), Ir1-P2 2.290(3), N1-Ir1-C27 170.0(7), P1-Ir1-P2 164.2(1).
i
Figure 2. Displacement ellipsoid (30%) representation of complexes 6 (left) and 8 (right) with Pr phosphine substituents omitted from 8
for clarity. Relevant bond lengths (Å) and angles (deg), for 6: Ir1-C27 1.852(1), Ir1-N1 2.071(9), Ir1-P1 2.2811(3), Ir-P2 2.2877(3),
C27-N2 1.197(1), N1-Ir1-C27 177.09(4), P1-Ir1-P2 162.16(1). For 8: Ir1-C27 1.937(2), Ir1-S1 2.3648(5), Ir1-N1 2.155(2), Ir1-C34
2.049(2), Ir1-P1 2.3392(6), Ir1-P2 2.3533(6), C27-Ir-S1 177.47(6), N1-Ir1-C34 177.42(6), P1-Ir1-P2 160.36(2).
butylthioformate as thermodynamic products (Scheme 2).14
Therefore, compound 4 can be viewed as a protected form of
an intermediate species in the atom transfer reaction, supporting
the view that atom and group transfers to carbene complex 1
are initiated by a nucleophilic metal rather than an electrophilic
carbene ligand.15,16 This interpretation is also consistent with
the observation that reductive condensations of CS2 are generally
promoted by nucleophilic, electron-rich metal centers.9a,e
II. Reaction with Phenyl Isothiocyanate. The reactivity of
1 with isothiocyanates is considerably more complex, and
depending on reaction conditions, several species were observed
to form. In analogy to reaction with CS2, slow addition of 1
equiv of PhNCS to a dilute solution of Fischer carbene 1 resulted
in quantitative sulfur-atom transfer to generate (PNP)Ir–CNPh
(6, Figure 2) and tert-butyl thioformate (Scheme 3). Although
metal-mediated desulfurization of isothiocyanates has been
reported,16b,17 this process typically occurs by sulfur-atom
transfer to a phosphine ligand and has not been observed for
metal-bound carbenes.
(14) It is possible that the CS2 adduct and dithiolactone shown in Scheme
2 are in equilibrium, but we currently have no evidence supporting this
view.
(15) The reactivity of metal-ligand multiple bonds with carbon disulfide
is typically initiated by nucleophilic ligands: (a) Mayr, A.; Lee, T.-Y. Angew.
Chem., Int. Ed. 1993, 32, 1726. (b) Zuckerman, R. L.; Bergman, R. G.
Organometallics 2000, 19, 4795.
(16) The nucleophilicity of d8 metal complexes is well-known and has
been extensively investigated: (a) Shriver, D. F. Acc. Chem. Res. 1970, 3,
231. (b) Werner, H. Pure Appl. Chem. 1982, 54, 177. (c) Pearson, R. G.;
Figdore, P. E. J. Am. Chem. Soc. 1980, 102, 1541.
(17) (a) Manuel, T. A. Inorg. Chem. 1964, 3, 1703. (b) Werner, H.;
Juthani, B. J. Organomet. Chem. 1981, 209, 211.

164 Organometallics, Vol. 28, No. 1, 2009
Scheme 3. Reaction of Carbene 1 with Phenyl Isothiocyanate
Whited and Grubbs
Scheme 4. Nucleophilic Reactivity of Isocyanide Complex 6
nide complex 6, and application of vacuum causes quantitative
regeneration of 6 (Scheme 4). Nevertheless, the analogies
between the nucleophilic reactivity observed for (PNP)Ir-CNPh
(6) and (PNP)IrdC(H)OtBu (1) highlight the isoelectronic
relationship between these molecules and suggest that compu-
tational studies can illuminate the factors controlling reactivity
of these types of molecules.
III. Computational Examination of (PNP)Ir-L Com-
plexes. Having obtained experimental support for our proposed
mechanism of nucleophilic attack by iridium to initiate atom
and group transfer reactions from heterocumulenes, we turned
our attention to computational methods to help elucidate the
ground-state configuration of the carbene shown to effect these
unusual transformations. Using the atomic coordinates deter-
mined from the crystal structure of 1, density functional
calculations were performed at the B3LYP/LACVP** level of
theory. The molecular surfaces of the frontier orbitals produced
by these calculations are represented in Figure 4 (HOMO-1,
HOMO, and LUMO).
In light of the reactivity observed for CS2, we were hopeful
that exposure of 1 to an excess of PhNCS would allow the
isolation of an iridium-supported PhNCS dimer similar to 4.
Addition of an excess (2-10 equiv) of PhNCS to concentrated
solutions of 1 led to the formation of species 7, for which the
spectroscopic data proved nearly identical to those of CS2 dimer
1
4, particularly the distinctive shift of the carbene proton in H
NMR (δ 15.5 ppm). However, this apparent PhNCS dimer was
observed to decompose quickly (1 h) to isocyanide complex 6,
indicating the more facile reversibility of PhNCS versus CS2
condensation in these complexes.
As predicted from simple molecular orbital considerations,
2
In the presence of excess PhNCS and tert-butyl thioformate,
compound 6 converts to a new, structurally unusual species 8,
in which the Cs symmetry has been broken (Scheme 4). X-ray
diffraction analysis of single crystals of 8 revealed the structural
motif depicted in Figure 2, which results from the reductive
condensation of PhNCS and tert-butyl thioformate. Based on
the structure observed for complex 8 and the spectroscopic
similarities between complexes 7 and 4, we tentatively assign
the connectivity of the unstable PhNCS dimer 7 as shown in
Scheme 3. Although the isomeric metallacycle resulting from
C-S bond formation is the more commonly observed product
of isothiocyanate condensation (I in Figure 3),9e,13d metallacycles
such as 7, which result from C-N bond formation, have been
reported (II in Figure 3).13e,18
the frontier orbitals represent the nonbonding Ir(dz ), irid-
ium-carbene π, and iridium-carbene π* orbitals. In contrast
to the HOMO and LUMO, which are significantly delocalized
over the molecule, the HOMO-1 is localized entirely on
2
iridium, representing a prototypical dz orbital. Based on the
reactivity patterns observed previously and in this report, we
propose that this high-lying, localized orbital is primarily
responsible for the nucleophilic activation of heterocumulene
substrates, leading to group transfer or reductive condensation.
Since (PNP)IrdC(H)OtBu (1), (PNP)Ir-CO (3), (PNP)Ir-CS
(5), and (PNP)Ir-CNPh (6) are isoelectronic, DFT calculations
were extended to this entire series of molecules.19 In fact, the
gross ordering and shape of the orbitals remain constant
throughout the series. However, the energies of the high-lying
Isocyanide complex 6 was also shown to react with large
excesses of CS2, forming varying amounts of a species
2
orbitals, particularly dz (HOMO-1), vary across the series,
reflecting to some degree the relative reactivities of the
molecules.
2-
formulated as the C2S4 adduct of the isocyanide complex
1
analogous to CS2 dimer 4 (observed by H and 31P NMR).
As described above, carbene complex 1, for which the
HOMO-1 orbital is highest in energy (Table 1), forms the
dimeric adduct 4 upon reaction with excess CS2. Isocyanide
Unlike 4, which is stable in solution and the solid state at
ambient temperature, this adduct is in equilibrium with isocya-
2
complex 6, which possesses a dz orbital close in energy to that
calculated for 1, reacts with CS2 to generate the analogous
dimeric adduct (Scheme 4). The carbonyl and thiocarbonyl
complexes, which have significantly lower energy calculated
2
dz orbitals (Table 1), show no evidence for reactivity with neat
(18) Itoh, K.; Matsuda, I.; Ueda, F.; Ishii, Y.; Ibers, J. A. J. Am. Chem.
Soc. 1977, 99, 2118.
(19) Further details regarding calculations on 1, 3, 5, and 6 are provided
in the Supporting Information.
Figure 3. Common bonding motifs for isothiocyanate dimers.

Nucleophilic-at-Metal Iridium(I) Carbene
Organometallics, Vol. 28, No. 1, 2009 165
Figure 4. Molecular surfaces of frontier orbitals calculated for (PNP)IrdC(H)OtBu (1).
carbon of CO2,4 ultimately oxygenating the carbene ligand and
transferring a strongly π-acidic carbonyl to the electron-rich,
d8 iridium center, providing a complementary reactivity pathway
to the tantalum case (Scheme 5).
2
Table 1. Energies of Ir(dz ) Orbitals for (PNP)Ir-L Molecules
complex
energy of HOMO-1 (eV)
(PNP)IrdC(H)OtBu (1)
(PNP)Ir-CNPh (6)
(PNP)Ir-CO (3)
-4.62
-4.80
-4.99
-5.22
The carbene ligand of 1 ultimately acts as an electrophile in
the reported reactions, consistent with the classical behavior of
Fischer-type carbenes, but this only occurs after substrate
activation by the electron-rich iridium center renders the
heterocumulene more nucleophilic and the carbene more elec-
trophilic. Thus, the coordinatively unsaturated nature of the
carbene complex reveals a distinct pathway for the activation
of multiple bonds via a metal- rather than ligand-initiated
mechanism. These findings suggest that square-planar carbenes
of the late transition metals may generally exhibit a strong
propensity to activate electrophilic substrates toward atom and
group transfer across metal-carbon multiple bonds, and the
application of these types of transformations in catalytic
processes is the subject of ongoing investigation.
(PNP)Ir-CS (5)
CS2 or PhNCS. Thus, as outlined in Table 1, the propensity of
a (PNP)Ir-L complex to form a C2S4 adduct with CS2 is
correlated to the relative destabilization of the dz molecular
2-
2
orbital (HOMO-1), consistent with our proposed mechanism.
Conclusions
In conclusion, we have reported well-defined sulfur-atom
transfer reactions from CS2 and PhNCS to an iridium-supported
Fischer carbene. These reactions represent, to the best of our
knowledge, the first examples of sulfur-atom transfer from
heterocumulenes across an MdC bond and appear to be
facilitated by the nucleophilic character of the iridium center
of complex 1. Under suitable conditions, exposure of carbene
1 to these reagents allows observation of trapped kinetic adducts,
which clearly require a metal-initiated mechanism, in contrast
to the standard reactivity patterns observed for metal carbenes.2
DFT calculations were performed on a series of (PNP)Ir-L
molecules and highlight the importance of a high-lying, nu-
Experimental Section
General Considerations. All manipulations were carried out
using standard Schlenk or glovebox techniques under a dinitrogen
atmosphere. Unless otherwise noted, solvents were deoxygenated
and dried by thorough sparging with Ar gas followed by passage
through an activated alumina column.21 (PNP)IrdC(H)OtBu (1)
and (PNP)Ir-CO (3) were prepared according to literature proce-
dure.4 Phenyl isothiocyanate was purchased from Aldrich and
degassed in vacuo prior to use. Other reagents were purchased from
commercial vendors and used without further purification. Elemental
analyses were carried out at Desert Analytics, Tucson, AZ. NMR
spectra were recorded at ambient temperature on Varian Mercury
300 and 500 MHz spectrometers. 1H and 13C NMR chemical shifts
were referenced to residual solvent. 31P NMR chemical shifts are
reported relative to an external standard of 85% H3PO4. X-ray
diffraction studies were carried out in the Beckman Institute
CrystallographicFacilityonaBrukerKAPPAAPEXIIdiffractometer.
X-ray Crystallography Procedures. X-ray quality crystals were
grown as indicated in the experimental procedures for each complex.
The crystals were mounted on a glass fiber with Paratone-N oil.
Structures were determined using direct methods with standard
2
cleophilic Ir(dz ) orbital in initiating this unusual heterocumulene
reactivity.
Drawing from these experimental and theoretical results, we
propose that complex 1 is best formulated as a nucleophilic-
at-metal carbene. It is particularly beneficial to consider these
findings in relation to the known reactivity of high-valent
alkylidene complexes of the early metals. Schrock has reported
that a tris(neopentyl)neopentylidene tantalum(V) complex reacts
instantaneously with CO2 to produce di-tert-butylallene and a
polymeric tantalum oxide (Scheme 5)5a and has proposed that
this and related olefination reactions proceed by initial substrate
coordination to the coordinatively unsaturated, highly electro-
philic tantalum.20 Subsequent group transfer releases olefin and
attaches the strongly π-basic oxo ligand to the high-valent
tantalum center. In contrast, we have reported that the square-
planar iridium(I) carbene 1 attacks the electrophilic central
Scheme 5. Contrasting CO2 Reactivity of Schrock-Type and Nucleophilic-at-Metal Carbenes (Np ) neopentyl)

166 Organometallics, Vol. 28, No. 1, 2009
Whited and Grubbs
Fourier techniques using the Bruker AXS software package. In some
cases, Patterson maps were used in place of the direct methods
procedure.
(s). Anal. Calcd for C33H45IrN2P2: C, 54.75; H, 6.27; N, 3.87. Found:
C, 54.87; H, 6.20; N, 3.86.
Observation of Complex 7. Phenyl isothiocyanate (9.1 µL, 0.076
mmol) was added in one portion to an NMR tube containing a
solution of complex 1 (24.4 mg, 0.0345 mmol) in C6D6 (600 µL),
causing an immediate color change from dark purple to bright red.
After 15 min, analysis of the solution by NMR revealed a mixture
Computational Methods. Hybrid density functional theory
calculationswereperformedfor(PNP)IrdC(H)OtBu(1),(PNP)Ir-CO
(3), (PNP)Ir-CS (5), and (PNP)Ir-CNPh (6) using the Jaguar
package (version 7.5, release 110), employing the B3LYP functional
and 6-31G** basis set.22 Iridium was represented with the
LACVP** basis set.23,24 Atomic coordinates were imported from
relevant single-crystal X-ray structures and subjected to geometry
optimization.
1
of complexes 7 and 1 (70:30). Spectral data for complex 7: H
NMR (C6D6): δ 15.5 (s, 1H, IrdC(H)OtBu), 7.59 (d, J ) 8.4 Hz,
2H, Ar-H), 7.53-7.35 (m, 4H, Ar-H), 7.29-7.19 (m, 4H, Ar-H),
6.81-6.63 (m, 6H, Ar-H), 3.01 (m, 2H, -CH(CH3)2), 2.19 (m,
2H, -CH(CH3)2), 2.13 (s, 6H, Ar-CH3), 1.47-1.29 (m, 12H,
-CH(CH3)2), 1.19 (dvt, 6H, -CH(CH3)2), 1.00 (dvt, 6H,
-CH(CH3)2). 31P{1H} NMR (C6D6): δ 18.8 (br s).
Preparation of (PNP)Ir(dC(H)OtBu)(C2S4) (4). To a stirring
solution of (PNP)IrdC(H)OtBu (1) (50.0 mg, 0.0707 mmol) in
pentane (5 mL) was added excess carbon disulfide (ca. 200 µL),
causing the immediate precipitation of a brown-orange solid. The
solution was stirred for 15 min and the orange powder isolated by
filtration. The powder was dissolved in a minimal amount of THF,
and analytically pure red crystals of 4 were isolated by vapor
diffusion of pentane into the concentrated THF solution (28.0 mg,
Preparation of Complex 8. Phenyl isothiocyanate (8.2 µL, 0.069
mmol) was added dropwise as a solution in benzene (1 mL) over
1 min to a stirring purple solution of complex 1 (48.0 mg, 0.0679
mmol) in benzene (5 mL), causing a gradual color change to red-
orange over a period of 30 min. After 2 h, additional phenyl
isothiocyanate (ca. 100 µL) was added as a solution in benzene (1
mL) to the stirring reaction, causing further lightening to bright
orange. After 24 h, volatiles were removed in vacuo and the residues
crystallized by slow evaporation of pentane from a concentrated
solution to afford complex 8 as bright orange crystals (25.4 mg,
1
46%). H NMR (C6D6): δ 15.55 (s, 1H, -C(H)OtBu), 7.53 (dt, J1
) 8.4 Hz, J2 ) 2.4 Hz, 2H, Ar-H), 6.73 (dd, J1 ) 8.7 Hz, J2 ) 1.8
Hz, 2H, Ar-H), 6.58 (m, 2H, Ar-H), 2.74 (m, 2 H, -CH(CH3)2),
2.11 (s, 6H, Ar-CH3), 2.09 (m, 2H, -CH(CH3)2), 1.11 (m, 12H,
-CH(CH3)2), 0.91 (dvt, 6H, -CH(CH3)2), 0.77 (s, 9H, -C(CH3)3),
0.74 (dvt, 6H, -CH(CH3)2. 13C{1H} NMR (CD2Cl2): δ 269.7 (Ir-
CS2), 266.7 (Ir-C(H)OtBu), 236.8 (Ir-SCS2). 31P{1H} NMR (C6D6):
δ 20.5 (s). Anal. Calcd for C33H50IrNOP2S4: C, 46.13; H, 5.87; N,
1.63. Found: C, 45.98; H, 5.81; N, 1.57.
Preparation of (PNP)Ir–CS (5). Method A. A resealable NMR
tube was charged with compound 4 (24.0 mg, 0.0279 mmol) in
THF (ca. 1 mL) and heated at 70 °C for 16 h, causing a slight
lightening of the solution from brownish-red to pale red. NMR
spectroscopy confirmed the presence of 5 as the sole component.
Method B. Compound 1 (31.1 mg, 0.0440 mmol) was dissolved
in benzene (3 mL) and carbon disulfide (2.7 µL, 0.045 mmol) added
dropwise as a dilute solution in benzene (1 mL), causing a color
change from purple to pale red. After 30 min, volatile components
were removed in vacuo and the residues were crystallized by slow
evaporation of pentane from a concentrated solution to afford
brown-red crystals of analytically pure 5 (25.4 mg, 87%). 1H NMR
(C6D6): δ 7.58 (d, J ) 8.4 Hz, 2H, Ar-H), 6.96 (br s, 2H, Ar-H),
6.78 (d, J ) 8.7 Hz, 2H, Ar-H), 2.55 (m, 4H, -CH(CH3)2), 2.17
(s, 6H, Ar-CH3), 1.42 (dvt, 12H, -CH(CH3)2), 1.15 (dvt, 12H,
-CH(CH3)2). 13C{1H} NMR (C6D6): δ 262.0 (Ir-CS). 31P{1H} NMR
(C6D6): δ 53.2 (s). Anal. Calcd for C27H40IrNP2S: C, 48.78; H, 6.06;
N, 2.11. Found: C, 48.52; H, 6.06; N, 2.03.
1
39%). H NMR (C6D6): 7.68 (d, J ) 6.9 Hz, 1H, Ar-H), 7.60 (m,
1H, Ar-H), 7.52 (m, 1H, Ar-H), 7.40 (m, 1H, Ar-H), 7.31-7.16
(m, 4H, Ar-H), 7.15-6.94 (m, 3H, Ar-H), 6.92-6.69 (m, 3H, Ar-
H), 6.63 (m, 2H, Ar-H), 6.24 (s, 1H, -CH(OtBu)), 3.96 (m, 1H,
-CH(CH3)2), 3.45 (m, 1H, -CH(CH3)2), 2.67 (m, 1H, -CH(CH3)2),
2.48 (m, 1H, -CH(CH3)2), 2.18 (s, 3H, Ar-CH3), 2.16 (s, 3H, Ar-
CH3), 1.97 (m, 3H, -CH(CH3)2), 1.80-1.45 (m, 9H, -CH(CH3)2),
1.38-0.98 (m, 12H, -CH(CH3)2), 0.90 (s, 9H, -C(CH3)3). 31P
2
2
NMR (C6D6): δ 16.1 (d, JPP ) 361 Hz), 7.5 (d, JPP ) 361 Hz).
Anal. Calcd for C45H60IrN3OP2S2: C, 55.31; H, 6.19; N, 4.30. Found:
C, 55.12; H, 6.15; N, 4.06.
Acknowledgment. We gratefully acknowledge the Moore
Foundation (fellowship to M.T.W.) and BP (MC2 program)
for financial support. Larry Henling provided crystallographic
assistance, and Dr. Ian Stewart aided with DFT calculations.
Supporting Information Available: Calculated surfaces and
energies for frontier molecular orbitals of 1, 3, 5, and 6. Crystal-
lographic details for complexes 4, 5, 6, and 8 are provided in CIF
format. This material is available free of charge via the Internet at
http://pubs.acs.org.
OM8009766
(20) (a) The electrophilicity of the metal center in many alkylidene
complexes is so pronounced that the MdC-H bond angle is contracted to
allow an R-agostic interaction with the electron-deficient metal center: Schultz,
A. J.; Williams, J. M.; Schrock, R. R.; Rupprecht, G. A.; Fellmann, J. A.
J. Am. Chem. Soc. 1979, 101, 1593. (b) Schultz, A. J.; Brown, R. K.;
Williams, J. M.; Schrock, R. R. J. Am. Chem. Soc. 1981, 103, 169. (c)
Goddard, R. J.; Hoffmann, R.; Jemmis, E. D. J. Am. Chem. Soc. 1980,
102, 7667.
(21) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518.
(22) Jaguar 7.5; Schrodinger, LLC: New York, NY, 2008.
(23) (a) Becke, A. D. Phys. ReV. A 1988, 38, 3098. (b) Becke, A. D.
J. Chem. Phys. 1993, 98, 5648. (c) Lee, C. T.; Yang, W. T.; Parr, R. G.
Phys. ReV. B 1988, 37, 785. (d) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J.
Phys 1980, 58, 1200.
Preparation of (PNP)Ir-CNPh (6). Phenyl isothiocyanate (9.15
µL, 0.0765 mmol) was added dropwise as a solution in benzene (1
mL) over 1 min to a stirring purple solution of compound 1 in
benzene (5 mL), causing a gradual color change to red-orange over
a period of 30 min. Volatiles were removed in vacuo to afford an
orange powder, which was crystallized by slow evaporation of
pentane from a concentrated solution to afford analytically pure 6
1
as orange crystals (40.0 mg, 72%). H NMR (C6D6): δ 7.79 (d, J
) 8.7 Hz, 2H, Ar-H), 7.33 (d, J ) 7.5 Hz, 2H, Ar-H), 7.10-6.90
(m, 5H, Ar-H), 6.85 (d, J ) 8.1 Hz, 2H, Ar-H), 2.38 (septet, 3JHH
) 3.0 Hz, -CH(CH3)2), 2.22 (s, 6H, Ar-CH3), 1.35 (dvt, 12H,
-CH(CH3)2), 1.14 (dvt, 12H, -CH(CH3)2). 13C{1H} NMR (C6D6):
δ 176.8 (t, 3JPC ) 9.3 Hz, Ir-CNPh). 31P{1H} NMR (C6D6): δ 53.6
(24) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (b) Wadt,
W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284.