Probing the C-H activation of linear and cyclic ethers at (PNP)Ir

Article abstract of DOI:10.1021/om900395f

Interaction of the amido/bis(phosphine)-supported (PNP)Ir fragment with a series of linear and cyclic ethers is shown to afford, depending on substrate, products of α,α-dehydrogenation (carbenes), α,β- dehydrogenation (vinyl ethers), or decarbonylation. W

Full text of DOI:10.1021/om900395f

4560 Organometallics 2009, 28, 4560–4570
DOI: 10.1021/om900395f
Probing the C-H Activation of Linear and Cyclic Ethers at (PNP)Ir
Matthew T. Whited,† Yanjun Zhu,‡,§ Samuel D. Timpa,§ Chun-Hsing Chen,§
Bruce M. Foxman,§ Oleg V. Ozerov,*,‡,§ 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, ‡Department of Chemistry,
Texas A&M University, College Station, Texas 77843, and Department of Chemistry, Brandeis University,
§
415 South Street, Waltham, Massachusetts 02454
Received May 14, 2009
Interaction of the amido/bis(phosphine)-supported (PNP)Ir fragment with a series of linear and
cyclic ethers is shown to afford, depending on substrate, products of R,R-dehydrogenation
(carbenes), R,β-dehydrogenation (vinyl ethers), or decarbonylation. While carbenes are exclusively
obtained from tert-amyl methyl ether, sec-butyl methyl ether (SBME), n-butyl methyl ether (NBME),
and tetrahydrofuran (THF), vinyl ethers or their adducts are observed upon reaction with diethyl
ether and 1,4-dioxane. Decarbonylation occurs upon interaction of (PNP)Ir with benzyl methyl
ether, and a mechanism is proposed for this unusual transformation, which occurs via a series of
C-H, C-O, and C-C bond cleavage events. The intermediates characterized for several of these
reactions as well as the R,R-dehydrogenation of tert-butyl methyl ether (MTBE) are used to outline a
reaction pathway for the generation of PNP-supported iridium(I) carbene complexes, and it is shown
that the long-lived, observable intermediates are substrate-dependent and differ for the related cases
of MTBE and THF. Taken together, these findings highlight the variety of pathways utilized by the
electron-rich, unsaturated (PNP)Ir fragment to stabilize itself by transferring electron density to
ethereal substrates through oxidative addition and/or the formation of π-acidic ligands.
Introduction
tion. Inspired by the wealth of reaction chemistry available
to pincer-supported rhodium and iridium centers,2 our
research groups have specifically focused on the use of
an amidophosphine pincer-type PNP ligand (PNP=[N(2-
PiPr2-4-Me-C6H3)2]-)3 to support C-H and C-X activation
and functionalization at rhodium and iridium.4,5
Most traditional approaches to Csp3-H functionalization
rely on elaboration of M-Csp3 species generated by a single
C-H cleavage. An alternative approach that has gained
traction more recently involves the direct generation of
MdCsp2 complexes by multiple C-H activations. Several
examples of carbene formation by double C-H activation
have been reported at late transition metals,6 although these
have yet to be utilized in a catalytic context. However, such
reactivity had not been demonstrated at square-planar,
pincer-supported iridium centers until recently.5b
The design of transition-metal catalysts capable of selec-
tive activation and functionalization of carbon-hydrogen
bonds remains an important goal of modern synthetic
chemistry.1 One key to the development of new methodolo-
gies is understanding the nature of the C-H activation steps
and the factors governing selectivity and product distribu-
*Corresponding authors. E-mail: ozerov@mail.chem.tamu.edu;
rhg@caltech.edu.
(1) For leading references, see: (a) Bergman, R. G. Nature 2007, 446,
391. (b) Arndtsen, B. A.; Bergman, R. G.; Mobley, T. A.; Peterson, T. H. Acc.
Chem. Res. 1995, 28, 154. (c) Crabtree, R. H. Chem. Rev. 1985, 85, 245. (d)
Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507. (e) Activation and
Functionalization of C-H Bonds; Goldberg, K. I., Goldman, A. S., Eds.;
ACS Symposium Series 885; American Chemical Society: Washington, DC,
2004. (f) Periana, R. A.; Bhalla, G.; Tenn, W. J.; Young, K. J. H.; Liu, X. Y.;
Mironov, O.; Jones, C. J.; Ziatdinov, V. R. J. Mol. Catal. A: Chem. 2004,
220, 7. (g) Dick, A. R.; Sanford, M. S. Tetrahedron 2006, 62, 2439.
(2) (a) van der Boom, M. E.; Milstein, D. Chem. Rev. 2003, 103, 1759.
(b) Goldman, A. S.; Renkema, K. B.; Czerw, M.; Krogh-Jespersen, K. Alkane
Transfer-Dehydrogenation Catalyzed by a Pincer-Ligated Iridium Complex.
In Activation and Functionalization of C-H Bonds; Goldberg, K. I.,
Goldman, A. S., Eds.; ACS Symposium Series 885; American Chemical
Society: Washington, DC, 2004; pp 198-215.
(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) (a) Fan, L.; Yang, L.; Guo, C.; Foxman, B. M.; Ozerov, O. V.
Organometallics 2004, 23, 4778. (b) Weng, W.; Guo, C.; Moura, C.; Yang,
L.; Foxman, B. M.; Ozerov, O. V. Organometallics 2005, 24, 3487. (c) Fan,
L.; Parkin, S.; Ozerov, O. V. J. Am. Chem. Soc. 2005, 127, 16772. (d) Gatard,
S.; Guo, C.; Foxman, B. M.; Ozerov, O. V. Organometallics 2007, 26, 6066.
(e) Zhu, Y.; Fan, L.; Chen, C.-H.; Finnell, S. R.; Foxman, B. M.; Ozerov, O. V.
Organometallics 2007, 26, 6701.
(5) (a) Whited, M. T.; Grubbs, R. H. J. Am. Chem. Soc. 2008, 130,
5874. (b) Romero, P. E.; Whited, M. T.; Grubbs, R. H. Organometallics
2008, 27, 3422. (c) Whited, M. T.; Grubbs, R. H. Organometallics 2008, 27,
5737. (d) Whited, M. T.; Grubbs, R. H. J. Am. Chem. Soc. 2008, 130, 16476.
(e) Whited, M. T.; Grubbs, R. H. Organometallics 2009, 28, 161.
ꢀ
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(6) (a) Boutry, O.; Gutierrez, E.; Monge, A.; Nicasio, M. C.; Perez,
P. J.; Carmona, E. J. Am. Chem. Soc. 1992, 114, 7288. (b) Luecke, H. F.;
Arndtsen, B. A.; Burger, P.; Bergman, R. G. J. Am. Chem. Soc. 1996, 118,
2517. (c) Holtcamp, M. W.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc.
1997, 119, 848. (d) Slugovc, C.; Mereiter, K.; Trofimenko, S.; Carmona, E.
Angew. Chem., Int. Ed. 2000, 39, 2158. (e) Carmona, E.; Paneque, M.;
Santos, L. L.; Salazar, V. Coord. Chem. Rev. 2005, 249, 1729. (f) Lee, D.-H.;
Chen, J.; Faller, J. W.; Crabtree, R. H. Chem. Commun. 2001, 213. (g)
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r
pubs.acs.org/Organometallics
Published on Web 07/10/2009
2009 American Chemical Society

Article
Organometallics, Vol. 28, No. 15, 2009 4561
Scheme 1. Formation of Carbenes from Methyl Ethers at (PNP)Ir
We have previously reported that (PNP)Ir, generated by
are amenable to the study of competitive R- and β-processes
at late metals have only recently been reported,11 it is clearly
necessary to develop an understanding of the factors con-
trolling these steps if catalytic reactions involving multiple
C-H activations and metal carbene generation are to be
realized.
interaction of (PNP)IrH2 (1) with norbornene (NBE), reacts
with tert-butyl methyl ether (MTBE) to perform R,R-dehy-
drogenation, affording a square-planar iridium(I) carbene
complex (2) upon loss of hydrogen (eq 1).5a,7 The complex is
related electronically and structurally to Fryzuk’s amido-
phosphine-supported iridium methylidene,8 but the unusual
reactivity of 2 is dominated by the nucleophilic nature of the
metal center and has been utilized to initiate atom and group
transfers from electrophilic heterocumulenes and diazo re-
agents by a multiple-bond metathesis process.5a,d,e In light
of the potential applications of these reactions in new
catalytic C-H functionalization protocols,9 we examined
the scope of ether C-H activation at (PNP)Ir in order to
investigate the potential utility of such catalytic schemes for
C-H functionalization via metal carbene generation and
shed light on the factors governing the R,R-dehydrogenation
process.
In this context, two salient examples of selective carbene
formation at late-metal centers merit attention. Bercaw and
co-workers have demonstrated that dehydrogenation of
diethyl ether at TMEDA-supported Pt(II) occurs via both
R,R- and R,β-modes, initially affording a mixture of carbene
and vinyl ether adducts of Pt(II) (TMEDA=N,N,N0,N0-
tetramethylethylenediamine).6c,12 However, slow isomeriza-
tion of the vinyl ether complex leads to isolation of the
diethyl ether-derived carbene as the thermodynamic pro-
duct. On the other hand, Carmona and co-workers have
reported that a relatively more encumbered tris(pyrazolyl)-
borate iridium(III) system reacts with diethyl ether and a
number of cyclic ethers to afford Fischer-type carbenes as the
sole products, and in these instances vinyl ethers are not
observed.6a,d,e
In this contribution, we report on the C-H activation of a
series of linear and cyclic ethers by (PNP)Ir. Depending on
the substrate, this process affords the products of R,R-
dehydrogenation (carbenes), R,β-dehydrogenation (vinyl
ethers), or decarbonylation. Furthermore, examination of
the intermediates in these reactions has led to the character-
ization of a number of species along the reaction pathway for
carbene formation.
In our previous studies of ether C-H activation, MTBE
was utilized as a substrate in order to preclude the possibility
of β-hydrogen elimination, thereby favoring an R,R-dehy-
drogenative process to afford a carbene adduct. However, we
were interested in whether such a process might be observed
for other ethereal substrates, especially those containing
β-hydrogen atoms. Several reports have demonstrated that
R-hydrogen elimination can be kinetically favored at steri-
cally congested early metal centers.10 Although systems that
Results and Discussion
I. r,r-Dehydrogenation of Ethers at (PNP)Ir. In light of
our previous success generating tert-butoxymethylidene 2 by
double C-H activation of MTBE, several other methyl
ethers were initially targeted as potential carbene precursors.
As expected, reaction of (PNP)IrH2 (1) with norbornene in
tert-amyl methyl ether as solvent led to facile generation of
the corresponding carbene complex 3 with loss of hydrogen
(Scheme 1). As with 2, immediate formation of a long-lived
intermediate was observed by 31P NMR spectroscopy (δ 45
ppm), followed by slow appearance of 3.5a,13 Like carbene 2,
complex 3 is unstable to thermolysis, decomposing by a
similar mechanism to afford trans-(PNP)Ir(H)2(CO) upon
heating for several hours at 70 °C.5b The spectroscopic
(7) Fan, L. Ph.D. Thesis, Brandeis University, 2006.
(8) (a) Fryzuk, M. D.; Macneil, P. A.; Rettig, S. J. J. Am. Chem. Soc.
1985, 107, 6708. (b) Fryzuk, M. D.; Gao, X.; Joshi, K.; Macneil, P. A. J. Am.
Chem. Soc. 1993, 115, 10581.
(9) Whited, M. T.; Grubbs, R. H. Acc. Chem. Res., in press.
(10) (a) Parkin, G.; Burger, B. J.; Trimmer, M. S.; Asselt, A. V.;
Bercaw, J. E. J. Mol. Catal. 1987, 41, 21. (b) Schrock, R. R.; Seidel, S. W.;
€
Mosch-Zanetti, N. C.; Shih, K.-Y.; O'Donoghue, M. B.; Davies, W. M.; Reiff,
W. M. J. Am. Chem. Soc. 1997, 119, 11876.
(11) (a) Carmona, E.; Paneque, M.; Poveda, M. L. Dalton Trans.
(12) Holtcamp, M. W.; Henling, L. M.; Day, M. W.; Labinger, J. A.;
Bercaw, J. E. Inorg. Chim. Acta 1998, 270, 467.
(13) For a discussion on the nature of these intermediates, see
Section IV.
2003, 4022. (b) Paneque, M.; Poveda, M. L.; Santos, L. L.; Carmona, E.;
ꢀ
Lledos, A.; Ujaque, G.; Mereiter, K. Angew. Chem., Int. Ed. 2004, 43, 3708.

4562 Organometallics, Vol. 28, No. 15, 2009
Whited et al.
parameters of 3 are remarkably similar to those of 2, parti-
1
3
cularly the distinctive carbene H (δ 13.8 ppm, t, JPH
=
7.5 Hz) and 13C (δ 210 ppm) chemical shifts observed by
NMR spectroscopy.
Encouraged by this result, we turned our attention to sec-
butyl methyl ether (SBME) and n-butyl methyl ether
(NBME) as linear ethers with multiple protons R to oxygen
that might serve as sites for an initial C-H cleavage event. In
the case of SBME, methyl activation would be expected to
lead to carbene formation, as with MTBE, whereas methine
C-H activation should afford only vinyl ethers since double
C-H activation at this site is not possible. For NBME,
carbene formation should likewise be favored after an initial
methyl C-H activation event, whereas methylene C-H
activation could yield either the corresponding vinyl ether
or carbene. In light of the many possible products that could
be formed upon interaction of (PNP)Ir with these linear
ethers, we were pleased to find that both SBME and NBME
were dehydrogenated exclusively to (PNP)IrdC(H)OsecBu (4)
and (PNP)IrdC(H)OnBu (5), respectively, upon addition of
excess norbornene (3 equiv) to a solution of dihydride 1 in the
appropriate solvent, followed by brief heating (Scheme 1).
Like compound 3, the carbene complexes 4 and 5 are similar
spectroscopically to tert-butoxymethylidene 2, exhibiting di-
agnostic 13C NMR signals that are slightly downfield from
that of 2 (δ 216.4 ppm for 4, 216.3 ppm for 5).
Several systems have been reported for which methyl C-H
activation occurs with high selectivity in the presence of
methylene and/or methine protons, although this is not an
entirely general observation.14 In this case, the observation
of carbenes as the sole products from C-H activation of
SBME and NBME shows either that (PNP)Ir exhibits a high
affinity for the less encumbered H-CH2O bonds or, if
methylene or methine C-H activation is accomplished, such
a pathway is unproductive. Since the complete conversion of
1 to 4 and 5 occurs only in the presence of an excess of both
norbornene and substrate, it is possible that some equiva-
lents of NBE are consumed in a competing transfer dehy-
drogenation process such as those previously reported for
heteroatom-containing substrates at (PCP)Ir15 and observed
for our (PNP)Ir system with diethyl ether (vide infra). The
possibility of competing transfer dehydrogenation and the
factors dictating selectivity in these reactions are currently
the subject of theoretical studies.16 In any case, these results
point to the possible use of SBME and NBME as well as
more highly functionalized methyl ethers in catalytic oxida-
tion schemes involving multiple C-H activations.
Figure 1. Displacement ellipsoid (35%, including displacement
spheres for atoms O1, C15, C16, and C17) representation of
(PNP)IrdC(C3H6O) (6) with hydrogen atoms omitted for
clarity. Selected bond lengths (A) and angles (deg): Ir1-N1,
2.073(3); Ir1-P1, 2.287(1); Ir1-C14, 1.921(4); N1-Ir1-C14,
180.0; N1-Ir1-P1, 81.56(2); P1-Ir-P1*, 163.12(3).
complete conversion to the second species, which was iden-
tified as the carbene complex derived from R,R-dehydro-
genation of THF (6, eq 2). As expected, this complex is quite
thermally stable since any decarbonylation pathways, such
as those observed for 2 and 3, would necessarily invoke an
unfavorable carbon-carbon bond cleavage, and this ther-
mal stability may have important implications on catalytic
reactions involving cyclic carbenes such as 6.
Carbene 6 was analyzed by single-crystal X-ray diffraction
(XRD), revealing a structure remarkably similar to that of
the MTBE-derived carbene 2 (Figure 1).5b As previously
noted for 2, the carbene cants slightly (ca. 20° out of the
square plane) to maximize the push-pull interaction be-
tween the filled pπ amido and empty pπ carbene orbitals.
The Ir1-C14 bond is only slightly longer than that observed
for complex 2 (1.92 A versus 1.88 A). The structure of 6
suffers from disorder due to a crystallographically imposed
C2 axis containing the C14, Ir1, and N1 atoms, resulting in
positional disorder between the O1 and C15 atoms of the
cyclic carbene. Nevertheless, this disorder was satisfactorily
modeled with partially occupied C15 and O1 atomic posi-
tions, as described in the Experimental Section.
The intermediate species in the reaction leading to 6 was
also accessible via an alternate synthesis, allowing its full
solution characterization and identification. Addition of
excess 2,3-dihydrofuran (3 equiv) to a solution of dihydride
1 in benzene-d6 led to quantitative generation of the same
intermediate observed in the formation of carbene 6, which
was identified as the trans-dihydrido carbene complex 6-H2
(eq 2), a structural and electronic relative of the trans-
dihydrido aminocarbenes previously reported.5c 6-H2 exhi-
bits C2v symmetry in solution, indicating that rotation about
the iridium-carbon bond is fast on the NMR time scale, as
expected on the basis of the similar behavior of 2 and related
aminocarbenes.5b,c The 1H NMR spectrum of 6-H2 shows a
Encouraged by these results with methyl ethers and in-
spired by previous reports of carbene formation from tetra-
hydrofuran (THF),6a,b,h,12 dehydrogenation of THF by
(PNP)Ir was examined. Generation of the (PNP)Ir fragment
via reaction of (PNP)IrH2 (1) with norbornene (2 equiv) in
THF resulted in the immediate appearance of a new species,
which was observed by 31P NMR (δ 46.7 ppm). This complex
could not be cleanly isolated due to slow decomposition to
another species, observed at δ 43.1 ppm by 31P NMR.
Thermolysis of the reaction mixture (60 °C, 7 h) resulted in
(14) (a) Jones, W. D.; Hessel, E. T. J. Am. Chem. Soc. 1993, 115, 554.
(b) Periana, R. A.; Bergman, R. G. Organometallics 1984, 3, 508.
(15) (a) Gupta, M.; Kaska, W. C.; Jensen, C. M. Chem. Commun.
1997, 461. (b) Zhang, X.; Fried, A.; Knapp, S.; Goldman, A. S. Chem.
Commun. 2003, 2060.
(16) Brookes, N. J.; Whited, M. T.; Ariafard, A.; Stranger, R.;
Grubbs, R. H.; Yates, B. F., manuscript in preparation.

Article
Organometallics, Vol. 28, No. 15, 2009 4563
triplet hydride resonance characteristic of such a complex
(δ -7.86 ppm, t, 2JPH = 15 Hz, 2H), as well as a distinctive
carbene resonance in its 13C NMR spectrum (δ 261.2 ppm).
In order to lose hydrogen and generate 6, complex 6-H2 must
isomerize to place the hydride ligands in a cis configuration
to allow for reductive elimination of hydrogen, but a buildup
of the cis-dihydrido isomer of 6-H2 is not observed.17 The
loss of hydrogen is likely a thermodynamically uphill pro-
cess, as previously indicated in computational studies of a
related system by Yates and co-workers.18 Thus, the rate of
formation of 6 is probably governed both by the isomeriza-
tion of 6-H2 and by a hydrogen-scavenging process that is
similar to that outlined by Yates in the dehydrogenation of
MTBE,18 and the role of these processes is discussed in more
detail in Section IV.
II. r,β-Dehydrogenation of Ethers at (PNP)Ir. The find-
ings described above demonstrate that THF exhibits a strong
preference for R- over β-elimination at (PNP)Ir. This result is
in contrast with previous reports regarding the transfer
dehydrogenation of THF to 2,3-dihydrofuran and furan
at pincer-supported iridium centers,15 although there are
a number of other systems that preferentially generate
cyclic carbenes via C-H activation of THF.6a,b,h Thus, we
endeavored to determine whether such a preference for
carbene formation in the presence of β-hydrogen atoms
would be general, as with Carmona’s tris(pyrazolyl)borate
complexes,6a,d,e or limited to THF, as was observed for
Bergman’s Cp*(PMe3)Ir system.6b Reaction of (PNP)IrH2
(1) with norbornene in diethyl ether leads to the immediate
formation of two products observed by 31P NMR spectros-
copy at 47.5 ppm (sharp singlet) and 29.5 ppm (broad
singlet). Over a period of 16 h, these convert cleanly to a
third complex, the ethyl vinyl ether adduct of iridium(I),
(PNP)Ir(H2CdC(H)OEt) (7) (eq 3). This divergent behavior
with respect to THF and diethyl ether resembles that ob-
served for the Cp*(PMe3)Ir system of Bergman, which
effects R,R-dehydrogenation and carbene formation from
dimethyl ether, MTBE, and THF, but affords an ethyl vinyl
ether adduct by R,β-dehydrogenation of diethyl ether.6b
Figure 2. Displacement ellipsoid (35%) representation of
(PNP)Ir(H2CdC(H)OEt) (7) with hydrogen atoms (except
those attached to C27 and C28) omitted for clarity. Selected
bond lengths (A) and angles (deg): Ir1-N1, 2.061(3); Ir1-P1,
2.2828(11); Ir1-P2, 2.2992(11); Ir1-C27, 2.130(4); Ir1-C28,
2.158(5); C27-C28, 1.449(6); C28-O1, 1.307(6); N1-Ir1-P1,
82.13(9); N1-Ir1-P2, 81.06(9); P1-Ir1-P2, 162.53(3); N1-
Ir1-C27, 165.30(14); N1-Ir1-C28, 155.20(15).
amide donor, maximizing through-metal interaction of the
filled amido pπ orbital and the empty π* orbital of the ethyl
vinyl ether. The consequences of this interaction as well as
strong π-donation from the electron-rich iridium(I) center
are manifested in the C27-C28 bond,19 which at 1.45 A is
significantly elongated relative to crystallographically char-
acterized free vinyl ethers (typically ca. 1.30 A)20 and other
metal-bound vinyl ethers (e.g., 1.36 A in a Pd(II) vinyl ether
complex).21 The complex also exhibits C1 symmetry by
NMR spectroscopy due to slow rotation of the vinyl ether
ligand.
In order to probe whether preference for carbene versus
olefin formation is related to whether the ether is cyclic, the
C-H activation of 1,4-dioxane and of tetrahydropyran
(THP) by (PNP)Ir was explored. Reaction of (PNP)IrH2
(1) with norbornene in the presence of THP inevitably leads
to regeneration of 1 as the major product, even when an
excess of NBE is utilized. Unfortunately, this reaction does
not proceed cleanly and the products have not been thor-
oughly characterized. Nonetheless, it is likely that THP
participates in a facile transfer dehydrogenation process.
Reaction of (PNP)IrH2 (1) with norbornene in 1,4-diox-
ane resulted in the immediate formation of a new product
exhibiting two doublet resonances in its 31P NMR spectrum
Vinyl ether adduct 7 was characterized by single-crystal
XRD analysis, and a molecular representation of 7 is pro-
vided in Figure 2. Of particular note is the orientation of the
vinyl ether fragment, which is canted slightly (ca. 20° out of
the square plane) to orient it exactly perpendicular to the
2
(δ 46.5 and 43.7 ppm, JPP=322 Hz). However, direct
isolation of this species proved elusive, as complete removal
of solvent or thermolysis led to regeneration of starting
material 1 with expulsion of 2,3-dihydro-1,4-dioxine.
(17) An alternative possibility is that hydrogen is lost directly from an
R-oxoalkyl hydride intermediate via 1,2-elimination, although the pre-
vious observation of cis-dihydrido aminocarbenes for the (PNP)Ir
system (ref 5c) and computational studies on the MTBE reaction
(ref 18) seem to discount this mechanism.
(18) Brookes, N. J.; Ariafard, A.; Stranger, R.; Yates, B. F. J. Am.
Chem. Soc. 2009, 131, 5800.
(19) The short C28-O1 bond length (1.31 A) suggests that an
alternative explanation for an elongated C27-C28 bond, namely,
polarization of the vinyl ether, may be operative. This type of distortion
serves to increase the σ-donating ability of heteroatom-substituted olefin
ligands while reducing their π-acidity. However, the relatively similar
Ir1-C27 and Ir1-C28 bond lengths as well as the expected low electro-
philicity of the (PNP)Ir fragment indicate that the contribution from this
sort of polarization is minimal. For leading references on polarization of
bound vinyl ethers, see: (a) Chang, T. C. T.; Foxman, B. M.; Rosen-
blum, M.; Stockman, C. J. Am. Chem. Soc. 1981, 103, 7361. (b) Watson,
L. A.; Franzman, B.; Bollinger, J. C.; Caulton, K. G. New J. Chem. 2003, 27,
1769.
The product from reaction of (PNP)Ir with 1,4-dioxane
was ultimately characterized by a combination of methods.
Partial removal of solvent and reconstitution of the mix-
ture in benzene-d6 allowed characterization of the inter-
1
mediate by H NMR, revealing a distinct triplet hydride
signal at -34.2 ppm (2JPH=13 Hz). The position of this
resonance is consistent with a Y geometry, a highly distorted
(20) See, for example: (a) Gagnon, M. K. J.; St. Germain, T. R.;
Vogels, C. M.; McNamara, R. A.; Taylor, N. J.; Westcott, S. A. Aust. J.
Chem. 2000, 53, 693. (b) Alickmann, D.; Frohlich, R.; Maulitz, A. H.;
Wurthwein, E.-U. Eur. J. Org. Chem. 2002, 1523.
(21) Nilsson, P.; Larhed, M.; Hallberg, A. J. Am. Chem. Soc. 2001,
123, 8217.

4564 Organometallics, Vol. 28, No. 15, 2009
Whited et al.
Scheme 2. C-H Activation of 1,4-Dioxane at (PNP)Ir
Figure 3. Displacement ellipsoid (35%) representation of
(PNP)Ir(Ph)(CO)(H) (9). Selected bond lengths (A) and angles
(deg): Ir1-N1, 2.1083(16); Ir1-P1, 2.3204(5); Ir1-P2,
2.3223(5); Ir1-C27, 1.847(2); Ir1-C28, 2.1595(19); C27-O1,
1.148(3); N1-Ir1-C27, 176.86(9); N1-Ir1-P1, 82.55(4); N1-
Ir1-C28, 90.13(7); P1-Ir1-P2, 161.194(19); P1-Ir1-C27,
98.52(7); P1-Ir1-C28, 91.62(5); C27-Ir1-C28, 92.79(9).
trigonal-bipyramidal structure with an acute H-Ir-X an-
gle.22 This orientation might be expected for a hydrido alkyl
complex of (PNP)Ir based on the presence of a π-basic amido
donor as well as comparison to (PNP)Ir(Ph)(H)4c and similar
Y-shaped dialkyl complexes reported by Fryzuk and co-
workers.23 Thus, on the basis of comparison to similar
complexes as well as the symmetry of the species, it seemed
most likely that this intermediate was the product of a single
C-H oxidative addition of 1,4-dioxane to (PNP)Ir, afford-
ing (PNP)Ir(H)(1,4-dioxan-2-yl) (8) (Scheme 2).
Goldman has shown that the five-coordinate (PCP)Ir-
(H)(Ph), which easily undergoes reductive elimination of
benzene, may be trapped by addition of CO to afford a stable
six-coordinate iridium(III) adduct.24 Such a route to stabiliza-
tion of a five-coordinate species is consistent with previous
studies by Goldberg and others, indicating that reductive
elimination from d6 transition metals preferentially occurs
from five- rather than six-coordinate intermediates,25 a find-
ing that has previously been confirmed for complexes of
(PNP)Ir.4e Thus, we found that trapping of 8 with carbon
monoxide (1 atm) allowed isolation and full spectroscopic
characterization of (PNP)Ir(H)(1,4-dioxan-2-yl)(CO) (8-CO,
Scheme 2), further supporting the assignment of8 as the single
C-H oxidative addition product of 1,4-dioxane. This com-
plex is structurally related to a previously reported carbonyl
adduct of a (PNP)-supported aminoalkyl hydride complex of
iridium(III) and exhibits a diagnostic carbonyl stretch in its
infrared absorbance spectrum (νCO =1980 cm-1).5c
piperidine into heteroatom-substituted carbenes but not
their six-membered counterparts.6h Thus, although six-
membered rings have been reported to form metal carbenes
by multiple C-H activations,6a our findings point to the
possibility that five-membered ring systems are especially
predisposed to R,R-dehydrogenation and carbene complex
formation.
III. Decarbonylation of Ethers at (PNP)Ir. The R,β-dehy-
drogenation process that was observed for diethyl ether and
1,4-dioxane suggested that other substrates should be exam-
ined for which such a reaction would not be possible, or at
least would not be facile. In this context, benzyl methyl ether
was an attractive candidate since no β-elimination would be
possible, and if a benzyloxymethylidene were to be formed, it
would be incapable of decomposing by the decarbonylative
pathways observed for carbene complexes 2 and 3 (vide
supra).5b Thus, we were surprised to find that exposure of
(PNP)IrH2 (1) to norbornene in benzyl methyl ether resulted
in the immediate formation of a new iridium complex con-
taining hydride and carbonyl ligands, as judged by NMR
and IR spectroscopy (νCO = 1995 cm-1). Single crystals of
the complex were analyzed by XRD, allowing definitive
identification of the product as (PNP)Ir(Ph)(CO)(H) (9)
(Figure 3).
This decarbonylation is quite remarkable, in that it in-
volves the cleavage of Csp3-H, Csp3-O, and Csp3-Csp2
bonds. The carbon-carbon bond cleavage is particularly
unusual. Although scission of Csp3-Csp2 bonds at group 9
metals has been reported,26 these processes typically occur
only in systems where the bond is tied into a polydentate
pincer-type ligand that preorganizes the complex for C-C
activation.27,28 In this case, the structural similarities
between complex 9 and the previously characterized
trans-(PNP)Ir(H)2(CO), which was shown to be formed
These findings regarding the reactivity of (PNP)Ir with
1,4-dioxane and THP indicate that the preference for car-
bene versus olefin formation in this system cannot be
ascribed simply to whether the ether is linear or cyclic. The
observed product selectivity is due to a subtle combination of
factors, and thus it is possible that the preference of THF for
carbene formation in the presence of β-hydrogen atoms is
substrate-specific. In related work, Caulton and co-workers
have reported that (iPr3P)2Ru(H)(Cl) converts THF and
(22) For leading references on the structural preferences of five-
coordinate, d6 metal complexes, see: (a) Lam, W. H.; Shimada, S.;
Batsanov, A. S.; Lin, Z.; Marder, T. B.; Cowan, J. A.; Howard, J. A. K.;
Mason, S. A.; McIntyre, G. J. Organometallics 2003, 22, 4557.
(b) Rachidi, I. E.-I.; Eisenstein, O.; Jean, Y. New J. Chem. 1990, 14, 671.
(c) Riehl, J.-F.; Jean, Y.; Eisenstein, O.; Pelissier, M. Organometallics 1992,
11, 729. (d) Olivan, M.; Eisenstein, O.; Caulton, K. G. Organometallics
1997, 16, 2227.
(23) (a) Fryzuk, M. D.; MacNeil, P. A.; Ball, R. G. J. Am. Chem. Soc.
1986, 108, 6414. (b) Fryzuk, M. D.; MacNeil, P. A.; Massey, R. L.; Ball, R. G.
J. Organomet. Chem. 1989, 328, 231.
(24) Kanzelberger, M.; Singh, B.; Czerw, M.; Krogh-Jesperson, K.;
Goldman, A. S. J. Am. Chem. Soc. 2000, 122, 11017.
(26) (a) Gozin, M.; Weisman, A.; Ben-David, Y.; Milstein, D. Nature
1993, 364, 699. (b) Liou, S.-Y.; Gozin, M.; Milstein, D. J. Am. Chem. Soc.
1995, 117, 9774. (c) Rybtchinski, B.; Vigalok, A.; Ben-David, Y.; Milstein,
D. J. Am. Chem. Soc. 1996, 118, 12406.
(27) For related studies of N-Csp3 cleavage in a pincer-type ligand,
see refs 3b and 4b.
(28) For an unusual case of C-C cleavage by β-phenyl elimination
ꢀ
that does not involve a preorganized system, see: Campora, J.; Gu-
(25) For leading references, see: Procelewska, J.; Zahl, A.; Liehr, G.;
van Eldik, R.; Smythe, N. A.; Williams, B. S.; Goldberg, K. I. Inorg.
Chem. 2005, 44, 7732.
ꢀ
ꢀ
tierrez-Puebla, E.; Lopez, J. A.; Monge, A.; Palma, P.; del Rı
´
o, D.;
Carmona, E. Angew. Chem., Int. Ed. 2001, 40, 3641.

Article
Organometallics, Vol. 28, No. 15, 2009 4565
Scheme 3. Proposed Mechanisms of Benzyl Methyl Ether Decarbonylation at (PNP)Ir
stereoselectively from a hydrido formyl complex of iridium-
(III), suggested that a hydrido benzoyl iridium(III) complex
might be the immediate precursor of 9. 1,2-Alkyl migration
at late metal acyls is a step with considerable precedent29
and is important in the catalytic decarbonylation of alde-
hydes.30
Thus, we propose that the unusual decarbonylation of
benzyl methyl ether is in this case made possible by a series of
bond activations and rearrangements that lead to formation
of benzaldehyde (Scheme 3). The reaction is likely initiated
by oxidative addition of a benzylic C-H bond to the (PNP)Ir
fragment. Subsequent β-methyl elimination, with C-O bond
cleavage, leads to loss of benzaldehyde and generation of an
iridium(III) methyl hydride,31 which is expected to reduc-
tively eliminate methane readily, regenerating the (PNP)Ir
fragment. The reactive fragment should oxidatively add the
acyl C-H bond easily, and stereoselective 1,2-phenyl migra-
tion at the resulting benzoyl hydride iridium(III) complex
affords the observed complex 9.32
An alternative mechanism that avoids the unusual possi-
bly thermodynamically uphill release of benzaldehyde in-
volves formation of a dihydrido carbene complex by double
benzylic C-H activation. Subsequent nucleophilic attack by
the bound hydride at the methoxy position would lead to
release of methane with formation of a (PNP)Ir-supported
benzoyl hydride complex, which could undergo 1,2-phenyl
migration to afford the observed complex 9. The SN2-type
nucleophilic attack of hydride required by this mechanism
appears to be disfavored by Baldwin’s rules,33 but the
distinct ring size and structure imparted by the octahedral
iridium center as well as the less directional metal-hydride
orbitals (compared with second-period nucleophiles in Bald-
win’s original paper) may be expected to render the requisite
transition state more favorable. That other related dihydrido
carbene species reported previously and in this paper do not
undergo similar decomposition mechanisms appears to ar-
gue against this mechanism as well.5c However, the other
dihydrido carbenes have a cyclic carbene structure (e.g., 6-
H2) that prevents the requisite conformation for attack by
hydride on the oxoalkyl, possess alkyl substituents on the
oxygen that are secondary or tertiary and thus less suscep-
tible to nucleophilic attack, or have an alkyl attached to
nitrogen (a much poorer leaving group).
Both mechanistic proposals are supported by the observa-
tion that (PNP)IrH2 (1) reacts immediately with benzalde-
hyde to afford complex 9 upon loss of H2. They are also
substantiated by the detection of methane (1H NMR spectro-
scopy) upon vacuum transfer of volatile components from the
decarbonylation reaction onto frozen C6D6 in a J. Young
tube. At this point, however, we have not been able to
distinguishbetweenthesetwomechanistic possibilities. None-
theless, the stereochemistry of the product, with the trans-
disposition of the phenyl and hydride ligands, strongly sug-
gests the intermediacy of the benzoyl hydride complex (shown
in Scheme 3) that is common to both proposed mechanisms.
The overall course of this reaction is quite similar to the
decarbonylation of alcohols with C-C cleavage observed by
Kubas and Caulton34 and Tilley and Bergman.35 It also
resembles the conversion of secondary amines to isocyanides
investigated by Goldman.36 However, this reaction is distinct
in that oxidation occurs via Csp3-O cleavage rather than the
typically much more facile O-H or N-H cleavage, and it
may point to new mechanistic possibilities for carbon func-
tionalization via C-C activation.37
(29) See, for example: (a) Alaimo, P. J.; Arndtsen, B. A.; Bergman, R.
G. J. Am. Chem. Soc. 1997, 119, 5269. (b) Alaimo, P. J.; Arndtsen, B. A.;
Bergman, R. G. Organometallics 2000, 19, 2130.
(30) (a) Ohno, K.; Tsuji, J. J. Am. Chem. Soc. 1968, 90, 99. (b) Abu-
Hasanayn, F.; Goldman, M. E.; Goldman, A. S. J. Am. Chem. Soc. 1992,
114, 2520. (c) Beck, C. M.; Rathmill, S. E.; Park, Y. J.; Chen, J.; Crabtree, R.
H.; Liable-Sands, L. M.; Rheingold, A. L. Organometallics 1999, 18, 5311.
(d) Fristrup, P.; Kreis, M.; Palmelund, A.; Norrby, P.-O.; Madsen, R. J. Am.
Chem. Soc. 2008, 130, 5206.
The C-H activation of 1,3,5-trioxane was also investigated
with the motivation of blocking β-hydrogen elimination and
(31) This step bears some resemblance to the β-hydrogen elimination
and loss of olefin that occurs in transfer dehydrogenation schemes at
pincer-supported iridium centers. For a discussion of the mechanism of
these types of reactions, see: Renkema, K. B.; Kissin, Y. V.; Goldman,
A. S. J. Am. Chem. Soc. 2003, 125, 7770.
(32) Such a stereoselective 1,2-migration is similar to what we and
others have previously observed in the decarbonylation of formaldehyde
(ref 5b).
(34) Van der Sluys, L. S.; Kubas, G. J.; Caulton, K. G. Organo-
metallics 1991, 10, 1033.
(35) Klei, S. R.; Golden, J. T.; Tilley, T. D.; Bergman, R. G. J. Am.
Chem. Soc. 2002, 124, 2092.
(36) Zhang, X.; Emge, T. J.; Ghosh, R.; Goldman, A. S. J. Am. Chem.
Soc. 2005, 127, 8250.
(37) For leading references on C-C activation, see: Rybtchinski, B.;
Milstein, D. Angew. Chem., Int. Ed. 1999, 38, 870.
(33) Baldwin, J. E. J. Chem. Soc., Chem. Commun. 1976, 734.

4566 Organometallics, Vol. 28, No. 15, 2009
Scheme 4. Possible Mechanism for Decarbonylation of 1,3,5-Trioxane at (PNP)Ir
Whited et al.
favoring carbene generation, since such a substrate would
provide a rough estimate for the difficulty in realizing R-
elimination from 1,4-dioxane, which is also a six-membered
cyclic ether. These reactions were made slightly more difficult
by the fact that 1,3,5-trioxane is a solid at ambient tempera-
ture, so they were carried out in mesitylene. Addition of a
mesitylene solution of norbornene and 1,3,5-trioxane to solid
(PNP)IrH2 (1) leads to the generation of three products at
ambient temperature: the previously characterized (PNP)-
Ir(H)(mes) (60%),4e trans-(PNP)Ir(H)2(CO) (10, 15%),5b
and a third unknown species (25%). However, at elevated
temperatures, all three complexes cleanly convert to trans-
(PNP)Ir(H)2(CO) (10, eq 4), which was previously character-
ized as the product generated upon thermolytic decomposi-
tion of tert-butoxymethylidene 2.5b
Scheme 5. Proposed Mechanism of Formation for Carbene 6
for 6-H2. In the case of the aminocarbenes, H2 loss is not
observed because the more electron-releasing aminocarbene
ligand confers proportionally greater basicity to the iridium
center and precludes the reductive elimination of hydrogen.
In contrast, comparison to the results of Yates suggests that
loss of hydrogen from cis-6-H2 is favorable, but the conver-
sion of 6-H2 to 6+H2 is uphill.18 Thus, the consumption of
H2 (probably by hydrogenation of norbornene) provides an
additional, potentially rate-limiting, barrier for this reaction
(vide infra).
Computational studies by Yates and co-workers showed
that the hydrido/oxoalkyl and dihydride/carbene isomers
derived from MTBE are nearly isoergic and are separated by
a trivial barrier of <2 kcal/mol.18 It seems safe to infer that
the observed preference of THF to form a dihydrido carbene
compared with the formation of R-oxoalkyl hydride isomers
from 1,4-dioxane and MTBE (vide infra) is thermodynamic
in origin. That rather similar compounds (dialkyl ethers) give
rise to different isomers supports the notion that the thermo-
dynamic preferences are modest. Furthermore, the predilec-
tion exhibited by THF for a carbene formulation at (PNP)Ir
may be general, especially since it is in accord with the
findings of Bergman for Cp*(PMe3)Ir6b and Caulton for
(iPr3P)2Ru(H)(Cl).6h
These findings concerning the formation of 6 prompted us
to reexamine the formation of tert-butoxymethylidene 2
from MTBE. We wondered whether the long-lived inter-
mediate observed during the preparation of 2 was a dihy-
drido carbene such as 6-H2, an R-oxoalkyl hydride such as 8,
or a simple adduct of MTBE coordinated to iridium through
oxygen as we had originally postulated on the basis of similar
intermediates observed by Bercaw and co-workers at catio-
nic Pt(II) centers.12 The identity of the intermediate was
ultimately determined by two related experiments. First,
upon addition of norbornene and a small amount of MTBE
(ca. 50 μL) to a solution of dihydride 1 in cyclohexane-d12,
31P NMR spectroscopy revealed generation of the inter-
mediate species (δ 45 ppm), and 1H NMR spectroscopy
showed a hydride signal (δ -34.7 ppm, 2JPH=13 Hz) nearly
identical to that observed for the R-oxoalkyl hydride
complex 8. Additionally, trapping of the intermediate with
This result is not unexpected when considering that 1,3,5-
trioxane is a formal trimer of formaldehyde, with which
(PNP)IrH2 (1) reacts to generate complex 10. Previous
reports have shown that the depolymerization of 1,3,5-
trioxane may be promoted by strongly acidic catalysts.38
In contrast, it seems most likely in this case that a com-
pound derived from C-H activation of 1,3,5-trioxane
readily disproportionates to release formaldehyde. One pos-
sible mechanism for this transformation is depicted in
Scheme 4.
IV. Mechanism of Carbene Formation at (PNP)Ir. The
results described in the preceding sections begin to provide a
framework for understanding and predicting the ultimate
course that C-H activation reactions at (PNP)Ir will take
with a variety of cyclic ethers. Perhaps more interesting is
that these reactions, especially those involving THF and 1,4-
dioxane, have allowed characterization of species believed to
be important intermediates in the R,R- and R,β-dehydro-
genation processes and have prompted reexamination of the
previously reported dehydrogenation of MTBE to afford
carbene 2.
As noted above, the dehydrogenation of THF proceeds by
immediate generation of the carbene 6-H2, which contains
hydride ligands in a trans orientation and gradually converts
to the thermodynamic product 6 with loss of hydrogen.
Thus, the formation of 6 proceeds by a pathway, as depicted
in Scheme 5, where C-H oxidative addition at (PNP)Ir and
R-hydrogen elimination to form the kinetically favored 6-H2
occur quickly. The R-elimination step is reversible, and the
rate of H2 loss to generate 6 is limited by isomerization from
6-H2 to cis-6-H2. The trans-to-cis isomerization of related
dihydrido aminocarbenes at (PNP)Ir was previously shown
to require elevated temperatures,5c and the same may be true
(38) Walker, J. F.; Chadwick, A. F. Ind. Eng. Chem. 1947, 39, 974.

Article
Organometallics, Vol. 28, No. 15, 2009 4567
Scheme 6. Proposed Mechanisms of Formation for Carbene 2
and Trapped Intermediate 11-CO
(2 and 6) would involve different observed intermediates,
this finding seems to be related to the observation that five-
membered heterocycles often form carbenes much more
readily than their six-membered and acyclic counterparts.
The studies described in this paper do not allow definitive
assignments of rate-limiting steps for each reaction, but they
certainly underscore the importance of numerous subtle
factors in the thermodynamic stabilization of one isomer
relative to another for related substrates at (PNP)Ir.
Conclusions
In conclusion, we have presented a detailed synthetic study
of the reactivity of the (PNP)Ir fragment with a series of
linear and cyclic ethers, providing reaction mechanisms that
rationalize a number of distinct but related R,R-dehydro-
genation, R,β-dehydrogenation, and decarbonylation path-
ways. In general, these findings support the notion that the
(PNP)Ir fragment is extremely unsaturated and electron rich,
participating in a variety of reactions that allow the transfer
of electron density to various substrates, through either
oxidative addition or conversion of the substrates to π-acidic
ligands. In Scheme 7, a number of relevant isomers of the
elemental content [(PNP)Ir+ether] are depicted. For those
isomers that contain cis-coordinated hydride ligands, there is
also the possibility of hydrogen loss and consumption by
iridium-mediated transfer to a sacrificial olefin, which is
often a thermodynamically downhill process.
Given this range of possible options, the fate of a parti-
cular ether upon interaction with (PNP)Ir depends intri-
cately on its structure and cannot easily be rationalized
by simple linear versus cyclic arguments, giving rise to a
fascinating complexity of outcomes within a rather narrow
substrate scope. The development of catalytic protocols
involving C-H activation and functionalization of ethers
depends on understanding the subtle reasons underlying this
complexity, some of which we have elucidated herein. Thus,
further study promises to illuminate more effectively the
factors controlling R- versus β-elimination processes, allow-
ing the development of new catalytic reactions relying on the
selective use of one or the other.
carbon monoxide resulted in the formation of (PNP)Ir-
(H)(CH2OtBu)(CO) (11-CO, Scheme 6). These results
strongly indicate that the observable intermediate in the
dehydrogenation of MTBE is the product of the first oxida-
tive addition (11), which is followed by a second C-H
activation step and loss of hydrogen to afford the isolated
carbene complex 2 (Scheme 6). These findings are consistent
with recent theoretical studies suggesting that the formation
of 11 should be facile and that 11 is likely the dominant
species in solution before formation of the observed carbene
product 2.18
Another important consideration, which has previously
been discussed in greater detail by Yates,18 is that the THF
and MTBE reactions necessarily involve the loss of hydro-
gen. Simple considerations would indicate that, aside from a
small entropic benefit, this reaction should be thermodyna-
mically uphill. In fact, the necessity of scavenging the hydro-
gen evolved in such reactions using a suitable sink has been
noted previously for related chemistry.6h,39 This rationalizes
the need for at least 2 equiv of norbornene in all of the
carbene reactions described and suggests that hydrogen
scavenging may provide a further limitation on the rate of
carbene formation. Yates and co-workers have provided
theoretical evidence that interaction of hydrogen with com-
plex 11 to regenerate dihydride 1 is facile, although this step
is limited by the low concentration of hydrogen since con-
version of 11 to 2+H2 is uphill.18 However, in the formation
of 6, simple considerations indicate that isomerization of
6-H2 must occur prior to interaction with free hydrogen since
6-H2 is an 18-electron complex without a vacant site for
binding H2, and this probably imposes an additional barrier
that is not present in the formation of 2. At the present time,
we have no direct evidence regarding the consumption of
evolved hydrogen by intermediates in the formation of 6, and
we simply note that this sort of process clearly plays an
important role for all of the carbene formation reactions
described.
Experimental Section
General Considerations. All operations were performed under
inert atmosphere in nitrogen- or argon-filled gloveboxes or by
using standard Schlenk techniques unless otherwise specified.
Solvents were deoxygenated and dried by thorough sparging
with Ar gas followed by passage through an activated alumina
column.40 (PNP)IrH2 (1) was prepared according to a published
procedure.4c tert-Amyl methyl ether (Aldrich) was distilled from
CaH2 and degassed prior to use. sec-Butyl methyl ether and n-
butyl methyl ether (Aldrich) were dried by passage through
activated alumina and degassed prior to use. THF and 1,4-
dioxane were dried over NaK/Ph2CO/18-crown-6, distilled, and
stored over molecular sieves in an Ar-filled glovebox. All other
reagents were purchased from commercial vendors and used
without further purification. NMR spectra were obtained at
ambient temperature on Mercury-300, Mercury-500, or Varian
iNova-400 spectrometers. 1H and 13C spectra were referenced to
residual solvent, and 31P NMR chemical shifts are reported
relative to an external standard of 85% H3PO4. Infrared spectra
were recorded on a Perkin-Elmer Paragon 1000 FT-IR
Finally, although it is somewhat counterintuitive that the
generation of two otherwise similar carbene complexes
(39) Ozerov, O. V.; Pink, M.; Watson, L. A.; Caulton, K. G. J. Am.
Chem. Soc. 2004, 126, 2105.
(40) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518.

4568 Organometallics, Vol. 28, No. 15, 2009
Scheme 7. Possible Isomers of the Elemental Content [(PNP)Ir+ether] ([Ir]=(PNP)Ir)
Whited et al.
spectrometer. HRMS were acquired using an Agilent 6200
Series TOF with an Agilent G1978A Multimode source in
electrospray ionization (ESI), atmospheric pressure chemical
ionization (APCI), or mixed (MM) ionization mode. Elemental
analyses were carried out at Desert Analytics (Tucson, AZ) and
CALI, Inc. (Parsippany, NJ).
restraint for C14-O1-C17 of 110.0(5)o and distance restraints
of C15 to C16, C17 to O1, C14 to C15, C16 to C17, and C14 to
O1 of 1.500, 1.490, 1.400, 1.480, and 1.350 (with esd’s of 0.005),
respectively.
Synthesis of (PNP)IrdC(H)OtAm (3). (PNP)IrH2 (1) (42.1
mg, 0.0676 mmol) and norbornene (13.8 mg, 0.147 mmol) were
combined in tert-amyl methyl ether (700 μL) and transferred to
an NMR tube. The sample was allowed to react at ambient
temperature for 2 h and heated for 20 min until complete
conversion to 3 was observed by 31P NMR with concomitant
color change from brown to purple. Volatile components were
removed in vacuo, and the residues were redissolved in pentane
(5 mL), filtered, and dried to afford carbene 3 as a viscous purple
oil (42.2 mg, 87%). 1H NMR (C6D6): δ 13.80 (t, 3JPH =7.5 Hz,
-C(H)OtAm), 7.83 (d, J=8 Hz, 2H, Ar-H), 7.22 (s, 2H, Ar-
H), 6.86 (d, J = 8 Hz, 2H, Ar-H), 2.68 (m, 4H, -CH(CH3)2),
2.29 (s, 6H, Ar-CH3), 1.47 (q, 3JHH = 7.5 Hz, 2H, and -OC-
(CH3)2(CH2CH3)), 1.37 (dvt, 12H, -CH(CH3)2), 1.26 (dvt,
X-ray Crystallography Procedures. X-ray quality crystals
were grown as indicated in the experimental procedures for
each complex. For complexes 7 and 9, the crystals were mounted
on a glass fiber with Paratone-N oil. Operations were performed
on a Bruker KAPPA APEX II diffractometer using graphite-
monochromated Mo KR radiation, and structures were deter-
mined using direct methods with standard Fourier techniques
using the Bruker AXS software package. In some cases, Patter-
son maps were used in place of the direct methods procedure.
For complex 6, all operations were performed on a Bruker-
Nonius Kappa Apex2 diffractometer, using graphite-mono-
chromated Mo KR radiation. All diffractometer manipulations,
including data collection, integration, scaling, and absorption
corrections, were carried out using the Bruker Apex2 software.41
Preliminary cell constants were obtained from three sets of 12
frames. Data collection was carried out at 120 K, using a frame
time of 10 s and a detector distance of 60 mm. The optimized
strategy used for data collection consisted of three phi and nine
omega scan sets, with 0.5° steps in phi or omega; completeness
was 99.9%. A total of 3173 frames were collected. Final cell
constants were obtained from the xyz centroids of 9725 reflec-
tions after integration.
From the systematic absences, the observed metric constants
and intensity statistics, space group Pbca was chosen initially;
subsequent solution and refinement confirmed the correctness
of this choice. The structure was solved using SIR-9242 and
refined (full-matrix least-squares) using the Oxford University
Crystals for Windows program.43 All ordered non-hydrogen
atoms were refined using anisotropic displacement parameters;
hydrogen atoms were fixed at calculated geometric positions
and allowed to ride on the corresponding carbon atoms.
The carbene ring was placed such that the O atom was in the
2-position (as established from spectroscopic evidence). The
carbene ring lies on a crystallographic 2-fold axis containing
the Ir and C14 atoms, and thus the O and C atoms must be
disordered in the 2- and 5-positions. Beyond this expected
disorder, it was noted that the oxygen atom (O1) and the carbon
atom in the 5-position (C15) are not strictly related by the
crystallographic 2 axis; further, the atoms in the 3- and 4-
positions (C17, C16) are also not related by the 2-fold axis. In
the final model the occupancies of atoms O1, C15, C16, and C17
were fixed at 0.5 and refined by using isotropic displacement
parameters. In addition, both angular and distance restraints
were made to various atoms on the ring, with an angular
3
12H, -CH(CH3)2), 0.81 (t, JHH = 7.5 Hz, 3H, -OC(CH3)2-
(CH2CH3)), 0.12 (s, 6H, -OC(CH3)2(CH2CH3)). 13C{1H}
NMR (C6D6): δ 210.1 (IrdC(H)OtAm), 164.5 (t, J=10.5 Hz),
133.0, 131.6, 126.7 (t, J=21 Hz), 127.7, 116.8, 83.8, 33.8, 26.5,
26.4, 26.3, 20.9, 20.3, 19.2, 8.8. 31P{1H} NMR (C6D6): δ 40.7 (s).
HRMS (MM: ESI-APCI): calcd for C32H52IrNOP2 [(M+
Na)+] 744.3047, found 744.3065.
Synthesis of (PNP)IrdC(H)OsecBu (4). A solution of norbor-
nene (17.9 mg, 0.190 mmol) in n-butyl methyl ether (1 mL) was
added to a vial containing (PNP)IrH2 (1) (39.5 mg, 0.0634
mmol), and an immediate color change from red to brown
was observed. The solution was transferred to an NMR tube
and heated at 90 °C for 2 h, resulting in a color change to dark
red. Volatile components were removed in vacuo, and the
residues were redissolved in pentane (5 mL), filtered, and dried
1
to afford 4 as a viscous purple oil (38.5 mg, 86%). H NMR
(C6D6): δ 13.46 (t, 3JPH =7.4 Hz, -C(H)OsecBu), 7.84 (dt, J1=
8.4 Hz, J2=2.1 Hz, 2H, Ar-H), 7.22 (d, J=1.8 Hz, 2H, Ar-H),
6.87 (dd, J1 = 8.4 Hz, J2 = 1.8 Hz, 2H, Ar-H), 3.41 (sextet,
3JHH = 6.0 Hz, 1H, -OCH(CH3)(CH2CH3)), 2.68 (m, 4H,
-CH(CH3)2), 2.29 (s, 6H, Ar-CH3), 1.55-1.13 (m, 26H, -CH-
3
(CH3)2 and -OCH(CH3)(CH2CH3)), 1.07 (d, JHH = 6.6 Hz,
3
3H, -OCH(CH3)(CH2CH3)), 0.80 (t, JHH =7.5 Hz, -OCH-
(CH3)(CH2CH3)). 13C{1H} NMR (C6D6): δ 216.4 (IrdC(H)-
OsecBu), 164.5, 133.1, 131.6, 126.6 (t, J = 25 Hz), 125.8, 116.8,
89.0, 30.3, 26.5 (t, J = 11 Hz), 21.0, 20.9, 20.3, 20.2, 19.2, 19.1,
10.6, 10.5. 31P{1H} NMR (C6D6): δ 40.9 (s). HRMS (MM: ESI-
APCI): calcd for C31H50IrNOP2 [(M+H)+] 708.3071, found
708.3050.
Synthesis of (PNP)IrdC(H)OnBu (5). (PNP)IrH2 (1) (72.1
mg, 0.116 mmol) and norbornene (32.7 mg, 0.347 mmol) were
combined in n-butyl methyl ether (2 mL), causing an immediate
color change from red to brown. The solution was heated at
90 °C for 2.5 h in a sealed vial, resulting in a color change to purple.
Volatile components were removed in vacuo, and the residues
were redissolved in pentane, filtered, and dried to afford 5 as a
viscous purple oil (72.3 mg, 88%). 1H NMR (C6D6): δ 13.49 (t,
3JPH =7.2 Hz, 1H, -C(H)OnBu), 7.85 (dt, J1 =8.4 Hz, J2 =2.1
Hz, 2H, Ar-H), 7.23 (m, 2H, Ar-H), 6.88 (d, J = 8.4 Hz, 2H,
(41) Apex2, Version 2 User Manual, M86-E01078; Bruker Analytical
X-ray Systems: Madison, WI, 2006.
(42) Altomare, A.; Cascarano, G.; Giacovazzo, G.; Guagliardi, A.;
Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27,
435.
(43) Betteridge, P. W.; Carruthers, J. R.; Cooper, R. I.; Prout, K.;
Watkin, D. J. J. Appl. Crystallogr. 2003, 36, 1487.

Article
Organometallics, Vol. 28, No. 15, 2009 4569
3
Ar-H), 3.48 (t, JHH = 6.5 Hz, 2H, -OCH2(CH2)2CH3), 2.68
(m, 4H, -CH(CH3)2), 2.30(s, 6H, Ar-CH3), 1.50-0.95 (m, 28H,
20.5, 19.6, 18.6, 17.9, 17.4, 16.7, 15.3, 9.6. 31P{1H} NMR
(C6D6): δ 36.6 (d, JPP = 340 Hz), 30.4 (d, JPP = 340 Hz).
Anal. Calcd for C30H48IrNOP2: C, 52.00; H, 6.98; N, 2.02.
Found: C, 51.87; H, 6.68; N, 1.97.
2
2
3
-CH(CH3)2 and -OCH2(CH2)2CH3), 0.79 (t, JHH = 7.2 Hz,
3H, -OCH2(CH2)2CH3). 13C{1H} NMR (C6D6): 216.3
(IrdC(H)OnBu), 164.5, 133.1, 131.6, 126.6 (t, J=2 0 H z ) , 1 2 5 . 8 ,
116.8, 89.0, 30.3, 26.4, 21.0, 20.9, 20.2, 19.2, 19.1, 10.5. 31P{1H}
NMR (C6D6): 41.1 (s). HRMS (MM: ESI-APCI): calcd for
C31H50IrNOP2 [(M+H)+] 708.3071, found 708.2962.
Solution Characterization of (PNP)Ir(H)(1,4-dioxan-2-yl) (8).
(PNP)IrH2 (1) (14.8 mg, 0.024 mmol) was combined with
norbornene (14 mg, 0.14 mmol) in a J. Young tube and dissolved
in 0.6 mL of 1,4-dioxane. 31P NMR of the sample immediately
displayed two doublets and an unidentified compound with a
singlet at 42.9 ppm (ca. 15% by 31P NMR). The volatiles were
partially removed and the residues redissolved in C6D6. 31P
NMR in C6D6 showed the same percentage of a singlet at
41.9 ppm (ca. 15%) and two doublets representing 8 together
Synthesis of (PNP)IrdC(OC3H6) (6). (PNP)Ir(H)2 (1) (76 mg,
0.12 mmol) and norbornene (83 mg, 0.86 mmol) were dissolved
in tetrahydrofuran (600 μL) and transferred to a J. Young tube.
An immediate color change to brown was accompanied by
the appearance of a singlet in the 31P{1H} NMR spectrum at
δ 46.7 ppm. Thermolysis of the solution (60 °C, 7 h) resulted in
a color change to dark red and ca. 97% conversion to another
product at δ 43.1 ppm. The solution was filtered through Celite,
and the filtrate was collected. Volatiles were removed in vacuo, and
6 was obtained as a pure solid by slow evaporation of pentane
1
with some amount of (PNP)IrH2 (1). H NMR(C6D6): δ 7.75
(m, 2H, Ar-H), 7.02 (d, J=6 Hz, 1H, Ar-H), 6.97 (d, J=6 Hz,
1H, Ar-H), 6.83 (d, J = 8 Hz, 2H, Ar-H), 5.65 (m, 1H,
C4H7O), 4.02 (m, 2H, C4H7O), 3.80 (m, 2H, C4H7O), 3.74 (d,
J=10 Hz, 1H), 3.52 (d, J=11 Hz, 1H), -34.2 (t, 3JPH=13 Hz,
1H). All other protons of 8 were obscured by an unknown
compound and traces of (PNP)IrH2. 31P{1H} NMR: δ 46.5
(d, J=322 Hz), 43.7 (d, J = 322 Hz). Thermolysis of 8 in 1,
4-dioxane leads to the transformation back to (PNP)IrH2.
Removal of solvent in vacuo also regenerates (PNP)IrH2.
Synthesis of (PNP)Ir(H)(1,4-dioxan-2-yl)(CO) (8-CO).
(PNP)IrH2 (1) (53 mg, 0.085 mmol) was combined with nor-
bornene (26 mg, 0.26 mmol) in a J. Young tube and dissolved in
0.6 mL of 1,4-dioxane. A 31P NMR spectrum of the sample
immediately revealed two doublets and an unidentified com-
pound with a singlet at 42.9 ppm (ca. 15% by 31P NMR). The
solution was frozen and the headspace evacuated and backfilled
with carbon monoxide (1 atm). As the solution thawed, a
gradual change in color from brown to yellow was observed,
and 31P NMR spectroscopy showed that the mixture contained
80% 8-CO. The volatiles were removed under vacuum, and the
residue was crystallized in toluene/pentane to obtain analyti-
cally pure 8-CO (26 mg, 43%). 1H NMR (C6D6): δ 7.64 (d, J=8
Hz, 2H, Ar-H), 6.95 (s, 1H, Ar-H), 6.82 (s, 1H, Ar-H), 6.71 (t,
J = 8 Hz, 2H, Ar-H), 5.27 (m, 1H, C4H7O), 3.94 (m, 2H,
C4H7O), 3.78 (m, 2H, C4H7O), 3.68 (d, 3JHH =8 Hz, 1H), 3.54
(d, 3JHH=9 Hz, 1H, C4H7O), 2.43 (m, 1H, CH(CH3)2), 2.37 (m,
2H, CH(CH3)2), 2.19 (s, 3H, Ar-CH3), 2.16 (s, 3H, Ar-CH3),
2.07 (m, 1H, CH(CH3)2), 1.36 (q, 3JHH=8 Hz, 3H, CH(CH3)2),
1.20 (q, 3JHH=8 Hz, 9H, CH(CH3)2), 1.12 (q, 3JHH=8 Hz, 9H,
1
from a concentrated solution at -35 °C (21.6 mg, 24%). H
NMR (C6D6): δ 7.87 (d, J = 8.4 Hz, 2H, Ar-H), 7.25 (s, 2H,
Ar-H), 6.86 (d, J=8.4 Hz, 2H, Ar-H), 3.50 (t, 3JHH=6.9 Hz,
2H, -C3H6O), 2.67 (m, 4H, CH(CH3)2), 2.29 (s, 6H, Ar-CH3),
3
1.39 (quintet, JHH =7.2 Hz, 2H, C3H6O), 1.30 (dvt, 12H,
CH(CH3)2), 1.24 (dvt, 12H, CH(CH3)2), 0.39 (t, 3JHH=7.5 Hz,
2H, C3H6O). 13C{1H} NMR (C6D6): δ 232.6 (t, 2JPC = 11 Hz,
IrdC), 164.1 (t, J =17 Hz, C-N), 132.5 (PNP-aryl C), 131.2
(PNP-aryl C), 125.3 (PNP-aryl C), 124.7 (PNP-aryl C), 116.4
(PNP-aryl C), 74.6, 60.3, 26.2, 24.2, 20.6, 19.8, 18.7. 31P{1H}
NMR: δ 42.9 (s). Anal. Calcd for C30H46NOP2Ir: C, 51.98; H,
6.66; N, 1.90. Found: C, 52.16; H, 6.71; N, 2.03.
Solution Characterization of trans-(PNP)Ir(H)2dC(C3H6O)
(6-H2). (PNP)IrH2 (1) (39 mg, 0.062 mmol) was dissolved in
0.6 mL of C6D6 in a J. Young tube followed by the addition of
2,3-dihydrofuran (14 μL, 0.19 mmol). 31P{1H} NMR spectroscopy
showed >97% conversion to 6-H2 (δ 46.7 ppm). 1H NMR
(C6D6): δ 7.78 (d, J = 8 Hz, 2H, Ar-H), 7.02 (s, 2H, Ar-H),
6.73 (d, J=8 Hz, 2H, Ar-H), 3.66 (t, 3JHH=6 Hz, 2H, C3H6O),
2.14-2.24 (m, 10H, Ar-CH3 and -CH(CH3)2), 1.02-1.22 (m,
28H, -CH(CH3)2 and C3H6O), -7.86 (t, 3JPH=15 Hz, 2H, Ir-
H). 13C{1H} NMR (C6D6): δ 261.2 (br, IrdC), 161.9 (t,
J = 9 Hz, C-N), 130.7 (PNP-aryl C), 124.7 (t, J = 25 Hz,
PNP-aryl C), 122.8 (t, J = 24 Hz, PNP-aryl C), 115.6 (t, J = 4
Hz, PNP-aryl C), 79.7, 65.4, 26.4 (t, J=16 Hz), 23.7, 20.6, 18.9,
18.6. 31P{1H} NMR: δ 46.7 (s). H NMR (CD2Cl2): δ 7.29 (d,
CH(CH3)2), 0.91 (q, JHH = 8 Hz, 3H, CH(CH3)2), -8.34
1
3
3
J=8 Hz, 2H, Ar-H), 6.89 (s, 2H, Ar-H), 6.65 (d, J=8 Hz, 2H,
Ar-H), 4.41 (t, J = 7 Hz, 2H, C3H6O), 2.62 (t, J = 8 Hz, 2H,
C3H6O), 2.33 (m, 4H, CH(CH3)2), 2.18 (s, 6H, Ar-CH3), 1.80
(m, 2H, C3H6O), 1.10 (m, 24H, CH(CH3)2), -8.58 (t, J=15 Hz,
2H, Ir-H). 31P{1H} NMR (CD2Cl2): δ 46.7 (s).
(t, JPH = 17 Hz, 1H, Ir-H). 13C{1H} NMR (C6D6): δ 179.7
(t, 2JPC=5 Hz), 161.1 (t, J=8 Hz, C-N), 160.9 (t, J=8 Hz, C-
N), 131.7 (PNP-aryl C), 131.5 (PNP-aryl C), 130.7 (PNP-aryl C),
130.6 (PNP-aryl C), 124.4 (t, J = 6 Hz, PNP-aryl C), 124.1
(t, J=3 Hz, PNP-aryl C), 124.0 (t, J=3 Hz, PNP-aryl C), 123.1
(t, J=6 Hz, PNP-aryl C), 115.8 (t, J=4 Hz, PNP-aryl C), 115.2
(t, J=4 Hz, PNP-aryl C), 81.6, 70.5, 67.1, 51.8 (t, J=2 Hz), 26.6
(t, J=16 Hz), 26.5, 25.9 (t, J=15 Hz), 25.8, 20.5, 20.4, 18.7, 18.4,
18.2, 18.0, 17.6, 17.5, 17.1, 17.0. 31P{1H} NMR: δ 28.6 (s). IR
(cm-1) ν(CO): 1980. Anal. Calcd for C31H48NO3P2Ir: C, 50.69;
H, 6.54; N, 1.85. Found: C, 50.53; H, 6.57; N, 1.90.
Synthesis of (PNP)Ir(H2CdC(H)OEt) (7). To a red solution
of (PNP)IrH2 (1) (63.3 mg, 0.102 mmol) in diethyl ether (5 mL)
was added norbornene (18.6 mg, 0.198 mmol) in diethyl ether
(3 mL). The color of the solution initially lightened to a golden
hue and turned dark red over a period of 12 h. To ensure
complete reaction, excess norbornene (10.0 mg, 0.106 mmol)
was added and the solution stirred for an additional 24 h.
Volatiles were removed in vacuo to give a red film, which was
redissolved in pentane (5 mL), filtered, and dried to give 7 as a
red-orange solid. An analytically pure, crystalline sample of 7
was obtained by slow evaporation of pentane from a concen-
trated solution (22.3 mg, 32%). 1H NMR (C6D6): δ 7.73 (d, J=
5.5 Hz, 2H, Ar-H), 6.94 (m, 2H, Ar-H), 6.84 (d, J = 8.0 Hz,
2H, Ar-H), 5.13 (m, 1H, -OC2H3), 4.01 (m, 1H, -OC2H3),
3.60 (m, 1H, -OC2H3), 2.56 (m, 2H, -OCH2), 2.52-2.27 (m,
3H, -CH(CH3)2), 2.22 (s, 6H, Ar-CH3), 2.11 (m, 1H, -CH-
(CH3)2), 1.40 (m, 6H, -CH(CH3)), 1.21 (t, 3JHH =5.0 Hz, 3H,
-OCH2CH3), 1.12 (m, 6H, -CH(CH3)), 1.02 (m, 6H, -CH-
(CH3)), 0.96 (m, 6H, -CH(CH3)). 13C{1H} NMR (C6D6): 164.6,
164.1, 132.2, 132.0, 131.7, 131.5, 125.9, 125.6, 124.6 (d, J = 39
Hz), 123.4 (d, J = 40 Hz), 115.6, 115.3, 77.3, 66.3, 24.2, 23.8,
Synthesis of (PNP)Ir(Ph)(CO)(H) (9). Method A. To a
solution of (PNP)IrH2 (1) (18.6 mg, 0.0299 mmol) in benzyl
methyl ether/pentane (6 mL, 1:5) was added norbornene (2.8 mg,
0.030 mmol) in benzyl methyl ether (2 mL). Over a period of
several hours, a color change from red to golden was observed.
After 4 h, the volatiles were removed by vacuum distillation at
60 °C. Analytically pure 9 was recovered as yellow crystals by slow
evaporation of pentane from a concentrated solution at ambient
temperature (18.0 mg, 83%). 1H NMR (C6D6): δ 8.32 (br, 1H,
-C6H5), 7.89 (d, J=8.4 Hz, 2H, PNP-aryl H), 7.60 (br, 1H, -
C6H5), 7.16-6.96 (m, 3H, PNP-aryl H and -C6H5), 6.78 (m, 1H
-C6H5), 6.75 (d, J = 8.4 Hz, 2H, PNP-aryl H), 2.25 (m, 2H,
-CH(CH3)2), 2.13 (s, 6H, Ar-CH3), 1.93 (m, 2H, -CH(CH3)2),
1.18 (dvt, 6H, -CH(CH3)2), 1.08 (dvt, 6H, -CH(CH3)2), 0.92
(dvt, 6H, -CH(CH3)2), 0.66 (dvt, 6H, -CH(CH3)2), -9.83

4570 Organometallics, Vol. 28, No. 15, 2009
Whited et al.
3
(t, JPH = 15 Hz, 1H, Ir-H). 13C{1H} NMR (C6D6): δ 175.6
generation of the previously observed intermediate species (δ 45
ppm).5a,b The solution was frozen and the headspace evacuated
and backfilled with carbon monoxide (1 atm). As the solution
thawed, an immediate color change from brown to yellow was
observed, and 31P NMR revealed a mixture containing a new
species (δ 30 ppm) and the previously characterized (PNP)Ir-
CO5a in a >9:1 ratio. Volatile components were removed in
vacuo and the yellow residues redissolved in C6D6, allowing
characterization of the new product, 11-CO. 1H NMR (C6D6): δ
7.70 (dt, J1=8.4 Hz, J2=2.1 Hz, 2H, Ar-H), 6.96 (m, 2H, Ar-
H), 6.75 (dd, J1 =8.4 Hz, J2 = 2.1 Hz), 4.45 (t, 3JPH = 4.2 Hz,
2H, -CH2OtBu), 2.55 (m, 2H, -CH(CH3)2), 2.21 (s, 6H, Ar-
CH3), 2.20 (m, 2H, -CH(CH3)2), 1.32 (dvt, 6H, -CH(CH3)2),
1.27-1.12 (m, 12H, -CH(CH3)2), 1.22 (s, 9H, -OC(CH3)3),
(t, 2JPC=7 Hz, Ir-CO), 161.5 (t, J=10 Hz), 132.4, 132.1, 128.9,
128.7, 127.8, 127.3, 125.9 (t, J=4 Hz), 123.3, 122.4 (t, J=25 Hz),
118.0 (t, J=5 Hz), 29.2 (t, J=18 Hz), 28.6 (t, J=16 Hz),
20.7, 20.3, 20.1, 19.7, 19.4, 19.0. 31P{1H} NMR (C6D6): δ 39.6
(s). IR (THF, KBr, cm-1) ν(CO): 1995. Anal. Calcd for
C33H46IrNOP2: C, 54.53; H, 6.38; N, 1.93. Found: C, 54.23;
H, 6.73; N, 1.84.
Method B. To a vial containing (PNP)IrH2 (1) (20.7 mg,
0.0332 mmol) was added an excess of benzaldehyde (ca. 2 mL).
As the red solids dissolved, the solution adopted a golden
hue. After 20 min, volatile components were removed by
vacuum distillation at 60 °C to leave a golden residue. Quanti-
tative generation of 9 was confirmed by 1H and 31P NMR
spectroscopy.
Synthesis of trans-(PNP)Ir(H)2(CO) (10) from 1,3,5-Triox-
ane. (PNP)IrH2 (1) (4.8) (16.5 mg, 0.026 mmol) was placed in a J.
Young tube followed by the addition of a mesitylene solution of
1,3,5-trioxane (33 mg, 0.36 mmol) and norbornene (5 mg, 0.053
mmol). 31P{1H} NMR spectroscopy showed instantaneous
generation of a reaction mixture containing 60% of the known
compound (PNP)Ir(H)(mes),4e 15% of the trans-(PNP)Ir(H)2-
(CO) (10),5b and 25% of an unidentified compound (42.1 ppm).
The reaction mixture was heated at 85 °C for 30 min, after which
time 31P and 1H NMR showed quantitative generation of
trans-(PNP)Ir(H)2(CO) from all three intermediates.
2
1.04 (dvt, 6H, -CH(CH3)2), -8.19 (t, JPH = 1H, 1H, Ir-H).
13C{1H} NMR (C6D6): δ 180.3 (Ir-CO), 161.7 (t, J =7.4 Hz),
131.8, 131.3, 125.1 (t, J=25 Hz), 124.3, 116.1, 73.7, 27.9, 27.2,
27.1, 27.0, 20.9, 19.1, 18.7, 18.5. 31P{1H} NMR (C6D6): δ 30.4
(s). IR (THF, KBr, cm-1) ν(CO): 1976 cm-1
.
Acknowledgment. We gratefully acknowledge BP
(MC2 Program), the Moore Foundation (Fellowship to
M.T.W.), the National Science Foundation (CHE-
0517798 and CHE-0809522), the Alfred P. Sloan Foun-
dation, and the Dreyfus Foundation for funding. S.D.T.
is a student of Centenary College of Louisiana who con-
tributed to this work in the summer of 2008 at Brandeis
University; his support through an NSF-REU program
(CHE-0552767) is acknowledged with gratitude. We also
thank the National Science Foundation for the partial
support of this work through grant CHE-0521047 for the
purchase of a new X-ray diffractometer at Brandeis
University and Larry Henling for crystallographic assis-
tance at Caltech.
Observation of (PNP)Ir(H)(CH2OtBu) (11). To a vial contain-
ing (PNP)IrH2 (1) (23.4 mg, 0.0376 mmol) and MTBE (ca. 50 μL)
was added a solution of norbornene (12.5 mg, 0.133 mmol)
in cyclohexane-d12 (ca. 700 μL), causing an immediate color
change from red to brown. 31P NMR spectroscopy revealed
generation of an intermediate species (δ 45 ppm), and 1H NMR
spectroscopy showed a distinct hydride signal at δ -34.7 ppm
(t, 2JPH=12.6 Hz), consistent with similar previously character-
ized complexes4c as well as the analogous R-oxoalkyl hydride
complex 8.
Solution Characterization of (PNP)Ir(H)(CH2OtBu)(CO)
(11-CO). (PNP)IrH2 (1) (25.0 mg, 0.0401 mmol) and norbor-
nene (8.1 mg, 0.086 mmol) were combined in MTBE (800 μL),
and the resulting brown solution was transferred to a J. Young
tube. 31P NMR spectroscopy after 10 min revealed quantitative
Supporting Information Available: Crystallographic details
for complexes 6, 7, and 9 are provided in CIF format. This
material is available free of charge via the Internet at http://
pubs.acs.org.