Reaction of nitromethane with an iridium pincer complex. Multiple binding modes of the nitromethanate anion

Article abstract of DOI:10.1021/om050659j

The reaction of nitromethane with (PCP)Ir (PCP = κ³-2,6- (^tBu₂PCH₂)₂C₆H ₃) yields the bidentate O,O-ligated nitromethanate complex (PCP)Ir(H)(κ²-O,O-NO₂CH₂) (1). Reaction of 1 with CO affords a CO adduct with a mono-oxygen-ligated nitromethanate, 2, which represents the first characterized transition metal mono-oxygen-ligated nitromethanate complex. At elevated temperature, complex 2 isomerizes to give the carbon-bound nitromethyl complex 3. Complex 1 also undergoes addition of cyclohexylisocyanide (analogous to the reaction with CO) to form the mono-oxygen-ligated nitromethanate complex 4, which also isomerizes to form the corresponding nitromethyl complex, 5. The (PCP)Ir-(CH₃NO₂) system is the first species known to display three binding modes with a nitromethanate anion. Results from density functional calculations illustrate the structures and energies of the minima and transition states on the potential energy surfaces. The calculations suggest that 1 is the thermodynamic product of (PCP)Ir reacting with nitromethane; a kinetic product, formed via oxidative addition of a nitromethane C-H bond, should readily rearrange to form 1.

Full text of DOI:10.1021/om050659j

Organometallics 2006, 25, 1303-1309
1303
Reaction of Nitromethane with an Iridium Pincer Complex.
Multiple Binding Modes of the Nitromethanate Anion
Xiawei Zhang, Thomas J. Emge, Rajshekhar Ghosh, Karsten Krogh-Jespersen,* and
Alan S. Goldman*
Department of Chemistry and Chemical Biology, Rutgers, The State UniVersity of New Jersey,
New Brunswick, New Jersey 08903
ReceiVed July 30, 2005
The reaction of nitromethane with (PCP)Ir (PCP ) κ3-2,6-(tBu2PCH2)2C6H3) yields the bidentate O,O-
ligated nitromethanate complex (PCP)Ir(H)(κ2-O,O-NO2CH2) (1). Reaction of 1 with CO affords a CO
adduct with a mono-oxygen-ligated nitromethanate, 2, which represents the first characterized transition
metal mono-oxygen-ligated nitromethanate complex. At elevated temperature, complex 2 isomerizes to
give the carbon-bound nitromethyl complex 3. Complex 1 also undergoes addition of cyclohexylisocyanide
(analogous to the reaction with CO) to form the mono-oxygen-ligated nitromethanate complex 4, which
also isomerizes to form the corresponding nitromethyl complex, 5. The (PCP)Ir-(CH3NO2) system is the
first species known to display three binding modes with a nitromethanate anion. Results from density
functional calculations illustrate the structures and energies of the minima and transition states on the
potential energy surfaces. The calculations suggest that 1 is the thermodynamic product of (PCP)Ir reacting
with nitromethane; a kinetic product, formed via oxidative addition of a nitromethane C-H bond, should
readily rearrange to form 1.
carbon atom17-23 have been isolated and characterized with
different transition metal systems. In most such cases, base was
used to deprotonate the nitroalkane and to promote the
coordination. The structure proposed for the key nitromethanate
intermediates in the transition metal-catalyzed nitroaldol reac-
tions is monodentate oxygen-bound.4-6 However, no mono-
dentate oxygen-bound complexes have been previously isolated
and characterized.
Pincer-ligated transition metal complexes are currently the
subject of intense study.24-26 The 14-electron species (PCP)Ir
(PCP ) κ3-2,6-(tBu2PCH2)2C6H3), generated from (PCP)IrH2,
can readily cleave hydrocarbon C-H bonds,27 activate the O-H
bond in water,28 and oxidatively add N-H bonds of anilines.29
In this context, we have been investigating the reactivity of
Introduction
The inorganic and organometallic chemistry of nitroalkanes
has long been of interest. In recent years, metal-mediated
reactions of nitromethane have seen increasing applications in
organic synthesis. Aldol-type reactions have made accessible a
large number of new nitro alcohols and nitro glycols, which in
turn can be transformed into valuable building blocks.1-3 In
the past several years, the development of catalysts for asym-
metric nitroaldol (Henry) reactions has been a particularly active
area.4-6
Coordinated nitroalkanate ions, most commonly nitrometha-
nate, are the putative active species in reactions such as the
metal-catalyzed nitroaldol and related reactions. The nitrometha-
nate anion can bind in several different coordination modes.
Nitromethanate complexes with bidentate coordination through
the two oxygen atoms7-16 or monodentate coordination of one
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* To whom correspondence should be addressed. E-mail: agoldman@
rutchem.rutgers.edu; krogh@rutchem.rutgers.edu.
(1) Luzio, F. A. Tetrahedron 2001, 57, 915.
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(16) Yamamoto, T.; Kubota, M.; Miyashita, A.; Yamamoto, A. Bull.
Chem. Soc. Jpn. 1978, 51, 1835.
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metallics 1991, 10, 2956.
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Eur. J. Inorg. Chem. 1999, 2085.
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Inorg. Chem. 1981, 20, 2722.
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21, 253.
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1977, 135, C33.
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1981, 20, 2722.
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3781, and references therein.
Chim. Acta 1982, 65, 1101.
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43, 5442-5444.
(5) (a) Trost, B. M.; Yeh, V. S. C. Angew. Chem., Int. Ed. 2002, 41,
861-863. (b) Trost, B. M.; Yeh, V. S. C.; Ito, H.; Bremeyer, N. Org. Lett.
2002, 4, 2621-2623.
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Downey, C. W. J. Am. Chem. Soc. 2003, 125, 12692-12693.
(7) Balogh-Hergovich, EÄ .; Gre´czi, Z.; Kaizer, J.; Speier, G.; Re´glier, M.;
Giorgi, M.; Pa´rka´nyi, L. Eur. J. Inorg. Chem. 2002, 1687.
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Chem. 1998, 37, 6535.
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1792.
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(27) Kanzelberger, M.; Singh, B.; Czerw, M.; Krogh-Jespersen, K.;
Goldman, A. S. J. Am. Chem. Soc. 2000, 122, 11017-11018.
(12) Kubota, M.; Yamamoto, A. Bull. Chem. Soc. Jpn. 1978, 51, 2909.
10.1021/om050659j CCC: $33.50 © 2006 American Chemical Society
Publication on Web 01/25/2006

1304 Organometallics, Vol. 25, No. 5, 2006
Zhang et al.
Scheme 1. Binding Modes in Metal Nitromethanate
Complexes
Scheme 2. Species Isolated from the Reaction of (PCP)Ir
with Nitromethane
Figure 1. X-ray structure of complex 1.
Selected bond distances and bond angles are listed in Table 1.31
The nitromethanate ligand and the iridium atom are coplanar.
The C-N distance is 1.294(2) Å, about typical for that of an
oxime.32 The two N-O bond distances are nearly identical
(1.3235(18) and 1.3288(19) Å); a comparison with the N-O
bond distances in oximes (1.36-1.42 Å) and nitro compounds
(1.22-1.25 Å)32 indicates that the N-O bonds of 1 are of
predominantly, but not exclusively, single-bond character. Three
resonance forms may be readily considered for the nitrometha-
nate anion (Scheme 3). On the basis of considerations noted
above, the bond lengths and angles of the nitromethanate ligand
in complex 1, as well as the spectral properties, correlate best
with resonance structure a; the short N-O distances suggest
some contribution of structure b.
pincer-iridium complexes toward various classes of organic
molecules. Herein, we report the formation of three iridium
pincer complexes from the reaction of (PCP)Ir with ni-
tromethane; each complex formed features a different binding
mode of the nitromethanate ligand. Reaction mechanisms are
proposed on the basis of results from DFT calculations.
Experimental Results
Formation of (PCP)Ir(H)(K2-O,O-NO2CH2) (1). The reac-
tion of the 16-electron species (PCP)IrH2 with norbornene in
p-xylene solution is known to generate a precursor of the highly
active 14-electron species (PCP)Ir(I).30 When nitromethane (2
equiv) is added to a p-xylene solution of (PCP)IrH2 in the
presence of excess norbornene, the red solution turns a dark
blue. The same initial color change is observed upon addition
of nitrobenzene or 2-methyl-2-nitropropane. In accord with
electronic structure calculations discussed in the next section
of this paper, we believe that these dark blue species are simple
O-bound nitroalkane (or nitroarene) complexes.
Reaction of 1 with CO. Complex 1 reacted readily with CO
(800 Torr); a bright yellow p-xylene solution rapidly turned pale
yellow. Evaporation of solvent yielded a pale yellow solid, 2
(98% yield), which was characterized by NMR and IR spec-
troscopy and X-ray crystallography. In the selectively decoupled
31P NMR spectrum, complex 2 resonates at δ 58.22 ppm as a
1
Within 10 s, the dark blue solution turns yellow and NMR
reveals formation of a new complex, 1, in quantitative yield
(31P{1H} NMR: δ 59.16 ppm, d, JPH ) 12.8 Hz). The NMR
data for 1 are consistent with its formulation as a six-coordinate
(PCP)Ir(III) complex with a hydride trans to a low-field ligand
(e.g., a coordinating oxygen atom; δ -29.32 ppm, t, JPH ) 13.5
Hz). At room temperature, the 1H NMR spectrum shows a broad
singlet at δ 5.01 ppm with an integrated peak area of 2 H.
However, at lower temperature (< ca. 0 °C) an AB pattern is
observed, consistent with the presence of two inequivalent,
coupled, methylene protons. The IR spectrum of the pentane
doublet (JPH ) 13.9 Hz). In the H NMR spectrum, a hydride
signal appears as a triplet at δ -7.43 ppm (JPH ) 13.5 Hz),
within the usual range for hydrides trans to a CO ligand in six-
coordinate (PCP)Ir(III) complexes (approximate range -7 to
-12 ppm). A broad singlet at δ 6.01 ppm with an integral of 2
H sharpens to an AB pattern below room temperature in a
manner similar to the behavior of complex 1, suggesting that
the nitromethanate structure is preserved in the CO adduct 2.
A signal at δ 93.4 ppm (s) in the 13C NMR spectrum is assigned
to the nitromethanate ligand; this is 7 ppm upfield from the
corresponding signal in 1. A strong absorption at 2004 cm-1 in
the IR spectrum is clearly attributable to the CO ligand. A
second strong absorption at 1547 cm-1 is probably due to the
stretching mode of a weakened C-N bond (cf. 1575 cm-1 for
1). Thus, the spectroscopic data are consistent with the formula-
tion of 2 as a six-coordinate complex with CO and a κ1-O-
nitromethanate ligand coordinated to iridium.
solution of 1 shows an intense absorption at 1575 cm-1
,
consistent with ν(CdN) values of previously reported κ2-nitroal-
kanate complexes7,9,10,16 A signal at δ 100.1 ppm (s) in the 13
C
NMR spectrum is attributable to the nitromethanate ligand.
Complex 1 was crystallized from pentane solution. A single-
crystal X-ray structure determination of 1 (Figure 1) reveals a
distorted octahedral geometry around the iridium atom with a
bidentate O,O-bound nitromethanate ligand; a hydride ligand
was located trans to one of the oxygen atoms (see Scheme 2).
A single-crystal X-ray structure determination of 2 (Figure
2) confirms the structure of the complex inferred from IR and
NMR spectroscopy. Selected bond distances and bond angles
are given in Table 1.31 Complex 2 has an approximately
octahedral geometry around the iridium atom with a monoden-
(28) Morales-Morales, D.; Lee, D. W.; Wang, Z.; Jensen, C. M.
Organometallics 2001, 20, 1144-1147.
(29) Kanzelberger, M.; Zhang, X.; Emge, T. J.; Goldman, A. S.; Zhao,
J.; Incarvito, C.; Hartwig, J. F. J. Am. Chem. Soc. 2003, 125, 13644-13645.
(30) Zhang, X.; Kanzelberger, M.; Emge, T. J.; Goldman, A. S. J. Am.
Chem. Soc. 2004, 126, 13192-13193.
(31) See Supporting Information for experimental details and procedures,
and spectroscopic and crystallographic data.
(32) See ref 11 and refs 30 and 31 therein.

Multiple Binding Modes of the Nitromethanate Anion
Organometallics, Vol. 25, No. 5, 2006 1305
Table 1. Selected Bond Distances (Å) and Angles (deg) for Nitromethanate Complexes 1-3 and 531
1 (κ2-O,O-bound)
2 (κ1-O-bound)
3 (κ1-C-bound)
5 (κ1-C-bound)
Ir(1)-C(1)(PCP)
Ir(1)-O(1)
2.0283(17)
2.1952(12)
2.2540(12)
2.3117(5)
2.3185(5)
1.518(16)
2.042(6)
2.148(5)
2.091(3)
2.090(2)
Ir(1)-O(2)
Ir(1)-P(1)
2.3325(16)
2.3349(16)
1.78(2)
2.3447(9)
2.3499(9)
1.531(18)
2.194(4)
1.448(5)
1.225(5)
1.229(5)
1.911(4)
1.144(4)
2.3374(5)
2.3399(5)
1.574(16)
2.209(2)
1.445(3)
1.229(3)
1.233(3)
1.997(2)
Ir(1)-P(2)
Ir(1)-H(1)
Ir(1)-C(25)(CH2NO2)
N(1)-C(25)
1.294(2)
1.3235(18)
1.3288(19)
1.307(9)
1.261(8)
1.361(7)
1.914(6)
1.154(8)
169.3(2)
N(1)-O(2)
N(1)-O(1)
Ir(1)-C(26)(CO or CNR)
O(3)-C(26)
C(1)-Ir(1)-O(1)
C(1)-Ir(1)-C(25)
C(1)-Ir(1)-H(1)
O(1)-Ir(1)-H(1)
C(26)-Ir(1)-C(1)
O(2)-Ir(1)-H(1)
P(1)-Ir(1)-P(2)
O(2)-N(1)-O(1)
C(25)-N(1)-O(1)
O(2)-N(1)-C(25)
C(25)-N(1)-O(1)-Ir(1)
O(2)-N(1)-O(1)-Ir(1)
O(1)-N(1)-C(25)-Ir(1)
175.20(6)
168.77(14)
84.6(14)
170.21(8)
86.1(10)
78.6(10)
106.1(10)
87(3)
84(3)
90.9(3)
92.06(14)
95.86(8)
165.6(10)
162.218(16)
112.72(13)
123.41(16)
123.88(16)
176.90(16)
-2.81(14)
158.78(6)
119.2(5)
115.6(6)
125.2(6)
-153.9(6)
27.8(9)
158.37(3)
121.7(4)
118.7(4)
119.5(4)
158.004(19)
121.5(2)
119.1(2)
119.3(2)
-95.5(4)
80.0(3)
Scheme 3. Resonance Forms of the Nitromethanate Anion
complex 1, the nitromethanate ligand in complex 2 appears to
have slightly more character of resonance structure b (although
still predominantly a in character) with the negatively charged
oxygen bound to iridium (Scheme 3). The slightly upfield 13C
NMR resonance of the nitromethanate carbon, relative to 1, also
seems consistent with this resonance interpretation. An analo-
gous conversion of (PCP)Ir-coordinated bicarbonate from bi-
dentate oxygen coordination to mono-oxygen coordination has
been reported by Jensen and co-workers.33
To the best of our knowledge, complex 2 is the first
characterized transition metal complex with a mono-oxygen-
coordinated nitroalkanate ligand. The closest reported precedents
are mono-oxygen-coordinated phenylnitromethanate alkali metal
complexes.34 The apparent contribution of resonance form b
may be important in the context of aldol-type reactions, where
a putative mono-oxygen-coordinated nitroalkanate acts as a
carbon nucleophile.5,6
tate oxygen-coordinated nitromethanate ligand and a CO ligand
trans to a hydride. Relative to complex 1, one coordinating
oxygen atom of the CH2NO2- ligand has been displaced by the
CO molecule. The remaining oxygen binds to iridium with a
bond shorter (2.148(5) Å) than those observed in 1 (2.1952-
(12) and 2.2540(12) Å, respectively). The nitromethanate ligand
remains coplanar. The C-N distance (1.307(9) Å) in 2 is slightly
longer than that in complex 1 (1.294(2) Å), while the two N-O
bonds are distinctly inequivalent (N-O(1) ) 1.361(7) Å,
N-O(2) ) 1.261(8) Å) and on average slightly shorter than
those in 1. The N-O(1) (oxygen bound to iridium) bond is
slightly longer than the N-O bond in complex 1 (ca. 1.326 Å),
indicating that the N-O(1) bond in complex 2 is essentially a
single bond, while the shorter N-O(2) bond distance in 2 is
close to that typical of N-O double bonds. Thus, relative to
Upon heating under CO atmosphere in p-xylene solution at
90 °C for 24 h, complex 2 was converted to complex 3 in 75%
yield with some additional formation of four-coordinate (PCP)-
Ir(CO). A doublet at δ 50.08 ppm appears in the selectively
1
decoupled 31P NMR spectrum of complex 3. In the H NMR,
a signal at δ -9.61 ppm (t, JPH ) 15.8 Hz) is indicative of a
hydride ligand bound to six-coordinate (PCP)Ir(III). A triplet
at δ 5.61 ppm (JPH ) 5.0 Hz) with an integral of 2 H may be
assigned to a C-bound nitromethyl group. The 13C NMR
spectrum of 3 reveals a triplet at δ 32.9 ppm (JCP ) 4.5 Hz),
attributable to the nitromethyl group; this is upfield by over 60
ppm from the nitromethanate signals (singlets) in 1 and 2. This
upfield value is in excellent agreement with the only other 13
C
NMR data reported, to our knowledge, for a (nitromethyl)iridium
complex: δ 36.7 ppm for the chemical shift of the nitromethyl
carbon of Cp*Ir(CH2NO2)[(R,R)-Tscydn].35 The IR spectrum
of the benzene solution of 3 shows intense absorptions at 1498
(33) Lee, D. W.; Jensen, C. M.; Morales-Morales, D. Organometallics
2003, 22, 4744-4749.
(34) (a) Boche, G.; Marsch, M.; Massa, W.; Baum, G.; Klebe, G.; Boehn,
K. H.; Harms, K.; Sheldrick, G. M. Stud. Org. Chem. (Amsterdam) 1987,
31, 149. (b) Klebe, G.; Boehn, K. H.; Marsch, M.; Boche, G. Angew. Chem.
1987, 99, 62.
(35) Murata, K.; Konishi, H.; Ito, M.; Ikariya, T. Organometallics 2002,
21, 253-255.
Figure 2. X-ray structure of complex 2.

1306 Organometallics, Vol. 25, No. 5, 2006
Zhang et al.
Figure 4. X-ray structure of complex 5.
Figure 3. X-ray structure of complex 3.
ingly less likely pathway might involve O-H activation from
nitronic acid, the tautomer of nitromethane.
and 1355 cm-1, attributable to asymmetric and symmetric
stretches of the NO2 group, respectively. A strong IR absorption
band at 1988 cm-1 is assigned to the CO group.
A single-crystal X-ray structure determination of complex
3, crystallized from benzene solution, reveals a distorted
octahedral geometry around the iridium atom with a C-bound
nitromethyl ligand located trans to the PCP phenyl carbon
(Figure 3 and Table 1). The C-N bond distance, 1.448(5) Å,
is much longer than that of 1 or 2 (ca. 1.30 ( 0.01 Å) and
indicative of a single bond (1.47 Å). The two N-O bonds
(1.225(5) and 1.229(5) Å) are typical of an uncoordinated nitro
group (1.22-1.25 Å).
The computed free energy difference between nitromethane
and nitronic acid is 13.8 kcal/mol in favor of nitromethane,
implying an equilibrium constant in the gas phase of Keq ≈ 10-10
(T ) 300 K). The computed free energy barrier for the
intramolecular 1,3-H shift is 53.4 kcal/mol. Thus, in accord with
conclusions derived from previous calculations,36 the presence
of (or reactions involving) nitronic acid does not need to be
considered further given the reaction conditions employed in
this study.
Reaction of 1 with Isocyanide. Complex 1 displayed
reactivity with isocyanide analogous to that observed with CO.
When a small excess of cyclohexylisocyanide was added to a
solution of 1, the yellow solution quickly turned pale yellow.
Complex 4 was formed and was characterized by 31P{1H} and
1H NMR spectroscopy. In the selectively decoupled 31P NMR
spectrum, complex 4 resonates at δ 57.40 ppm as a doublet. In
Nitromethane Complexes. Considering the unsaturated
nature of the reactive metal complex, the formation of precursor
complexes between the reactants, (pincer)Ir and CH3NO2, should
be quite favorable. Indeed, a four-coordinate complex, (PCP′)-
Ir(O2NCH3) (6a′), in which both oxygen atoms point toward
the Ir atom but only one is coordinating (Ir-O1 ) 2.091 Å,
Ir-O2 ) 3.368 Å, N-O1 ) 1.298 Å, N-O2 ) 1.259 Å, C-N
) 1.478 Å, Ir-O1-N-O2 ) 5.2°) was readily located with a
free energy 14.5 kcal/mol below that of the isolated reactants
(∆Hbinding ) 24.7 kcal/mol). A second complex (6b′), also
featuring κ1-O binding (Ir-O1 ) 2.150 Å, Ir-O2 ) 4.293 Å,
Ir-C ) 3.410 Å, N-O1 ) 1.288 Å, N-O2 ) 1.249 Å, C-N
) 1.488 Å, Ir-O1-N-O2 ) 170.8°) and possibly a very weak
C-H R-agostic interaction (C-HR ) 1.106 Å, remaining C-H
≈ 1.101 Å; Ir-HR ) 2.785 Å), was located 6.3 kcal/mol above
this complex (∆Gbinding ) 8.2 kcal/mol, ∆Hbinding ) 18.4 kcal/
mol). We were not successful in locating a (PCP)Ir-nitromethane
complex with κ2-O,O binding clearly involving both oxygen
atoms; all trial structures with bidentate O,O interactions
rearranged upon geometry optimization to a structure showing
only κ1-O binding (6a′or 6b′). In 6b′, the nitromethane unit is
lying in the “equatorial plane” (the plane bisecting the P-Ir-P
axis) of the pincer-iridium complex, whereas in 6a′ the
nitromethane species is oriented approximately in the “vertical
plane” (the plane containing the P-Ir-P axis and, approxi-
mately, the PCP aryl group). The actual (PCP)Ir complex (with
PtBu2 rather than PMe2 groups) offers a cleft for small molecules
oriented approximately in the equatorial plane, and the presence
1
the H NMR spectrum, there is a hydride triplet at δ -9.35
ppm (t, JPH ) 15.9 Hz). The methylene protons of the
nitromethyl ligand resonate at δ 5.96 ppm as a broad singlet.
All the spectroscopic data are consistent with a structure
analogous to the CO complex 2 in which the nitromethyl group
coordinates to iridium with one oxygen atom, while the
isocyanide ligand occupies the position trans to the hydride.
Crystallization of complex 4 was not successful.
At room temperature in p-xylene solution, complex 4 isomer-
ized to 5 over a period of several hours. Complex 5 was
characterized by NMR spectroscopy and a single-crystal X-ray
structure determination. The NMR spectral characteristics of 5
are very similar to those of complex 3. The X-ray crystal
structure of 5 (Figure 4 and Table 1)31 confirms a structure
analogous to that of the C-bound nitromethyl complex 3.
Computational Results and Discussion
Electronic structure calculations at the DFT level (see
Computational Details) were carried out to further illustrate the
minima and transition states on the potential energy surfaces
for species 1-5. To computationally model the pincer ligand,
the bis(dimethylphosphino) analogue of PCP ((Me2PCH2)2C6H3,
PCP′) was used for all calculations in this work.
Tautomerization Prior to Addition. A priori, plausible
pathways for the formation of 1 include C-H addition to (PCP)-
Ir, followed by ligand rearrangement, or O-coordination of
nitromethane followed by proton transfer to iridium. A seem-
(36) Lammertsma, K.; Prasad, B. V. J. Am. Chem. Soc. 1993, 115, 2348-
51.

Multiple Binding Modes of the Nitromethanate Anion
Organometallics, Vol. 25, No. 5, 2006 1307
mol (only 2.3 kcal/mol above the total free energy of (PCP)Ir
and CH3NO2). Intrinsic reaction coordinate (IRC) calculations
verified that this transition state (TS1, Scheme 4) connects 6b′
to C-bound 7′. The TS geometry shows that the Ir-H bond is
almost fully formed and the C-H bond fully broken while Ir-
O1 bonding is mostly maintained (Ir-H ) 1.633 Å, C-H )
1.737 Å, Ir-O1 ) 2.259 Å, Ir-C ) 3.040 Å, C-N ) 1.388
Å, Ir-O2 ) 4.022 Å, N-O1 ) 1.330 Å, N-O2 ) 1.255 Å,
Ir-C-N-O1 ) -27.8°). A second TS (TS2, Scheme 4), also
leading to 7′ as the product, was located only 1.2 kcal/mol above
the isolated (PCP)Ir and CH3NO2 reactants. In this TS, C-H
activation occurs in a nitromethane molecule oriented “face-
on” to the Ir atom with the C-N bond lying in the horizontal
plane and the oxygen atoms pointing toward the phosphino alkyl
groups (Ir-H ) 1.733 Å, C-H ) 1.259 Å, Ir-C ) 2.464 Å,
C-N ) 1.489 Å, Ir-O1 ) 3.469 Å, Ir-O2 ) 3.500 Å, N-O1
) 1.252 Å, N-O2 ) 1.259 Å, Ir-C-N-O1 ) 90.1°). Thus,
nitromethane C-H addition is calculated to be facile either with
or without concomitant binding to an O atom, although the
presence of tert-butyl groups on phosphorus would be expected
to selectively raise the energy of the second TS.
Figure 5. Schematic illustration of bond lengths in selected
calculated complexes.
We have not found a direct route to the formation of 1′ from
a nitromethane precursor in our calculations, and the κ2-O,O-
coordinated species 1 is most likely formed from a C-bound
species analogous to 7′. The computed activation energy for
the intramolecular migration of the nitromethanate ligand (7′
f 1′) is only ∆Gq ) 13.9 kcal/mol (TS3, Scheme 4). IRC
calculations and visual examination of the reaction coordinate
at TS3 shows that the nitromethanate species pivots around the
Ir-O1 linkage, which is maintained throughout the migration.
The Ir-O1 bond length is very short in TS3 (Ir-O1 ) 2.171
Å versus 2.408 Å in 7′ and 2.258 Å in 1′; N-O1 ) 1.372 Å,
Ir-O1-N-C ) -45.6°), and there appears to be no particular
metal interaction with the C atom (Ir-C ) 3.120 Å) or with
the other O atom (Ir-O2 ) 3.995 Å), which is essentially
doubly bonded to the N atom, cf. resonance form b in Scheme
3 (N-O2 ) 1.258 Å, C-N ) 1.343 Å). On the basis of these
calculations, we thus suggest that the observed species 1 is the
thermodynamic product of (PCP)Ir reacting with nitromethane,
while a precursor with a C-bound nitromethanate ligand
rearranges to 1 too rapidly to be isolated.
of tert-butyl groups would therefore be expected to significantly
raise the energy of 6a′ relative to 6b′.
Experimentally, as noted above in the first section, the
reaction of “(PCP)Ir” with nitromethane, or with nitrobenzene
or 2-methyl-2-nitropropane, results in the appearance of a very
short-lived dark blue color. It appears very plausible that these
dark blue species are nitro-O-bound adducts, analogous to
calculated models 6a′ and/or 6 b′.
Nitromethanate Complexes. Two (PCP′)Ir-hydride-ni-
tromethanate complexes were located computationally: the κ2-
O,O-coordinated species, 1′ (Ir-O1 ) 2.258 Å, Ir-O2 ) 2.295
Å, C-N ) 1.324 Å; the analogue of the isolated complex 1)
and a species coordinating primarily through the C atom with
some additional binding via one O atom (7′; Ir-C ) 2.225 Å,
Ir-O1 ) 2.408 Å, C-N ) 1.454 Å) (see Scheme 4). The free
energy of species 1′ is 6.1 kcal/mol less than that of species 7′
and 8.7 kcal/mol less than the free energy of the most stable
precursor complex (6a′). The computed C-N stretching fre-
quency of 1′ is 1573.5 cm-1, exceedingly close to the frequency
of 1575 cm-1 observed in 1.
Starting with κ1-O bound 6b′ as the reactant, a transition state
for C-H bond breaking was located with ∆Gq ) 10.5 kcal/
The two CO-containing complexes, 2′ and 3′, are computed
to be separated by ∆G ) 10.1 kcal/mol, in favor of C-bound
Scheme 4. Calculated Free-Energy Values (kcal/mol) for Various Nitromethane and Nitromethanate Complexes and Transition
States ([Ir] ) (PCP)Ir)

1308 Organometallics, Vol. 25, No. 5, 2006
Zhang et al.
species 3′. The formation reaction for 2′ from CO and 1′ is
computed to be moderately exothermic (∆H ) -25.6 kcal/mol,
∆G ) -16.2 kcal/mol). In 2′, the calculations predict the
(unscaled) C-N and C-O stretch frequencies at 1524 and 1956
cm-1, respectively, in good agreement with the observed
frequencies (1547 and 2004 cm-1). In 3′, the calculations predict
the N-O symmetric and asymmetric stretch frequencies at 1344
and 1524 cm-1, respectively, and the C-O stretch frequency
at 1959 cm-1, again in good agreement with the observed
frequencies (1355, 1498, and 1988 cm-1). The transition state
for intramolecular rotation of nitromethanate, leading to inter-
conversion of 2′ and 3′, is positioned 24.8 kcal/mol above 2′.
The calculated relative energies for 2′ and 3′, as well as the
magnitude of the barrier separating the two species (Scheme
4), are consistent with the observation that the O-bound carbonyl
adduct is sufficiently long-lived to be observable, but is
thermodynamically unstable. The considerably larger barrier for
2′ f 3′ intramolecular nitromethanate rotation (24.8 kcal/mol),
compared to that for 7′ f 1′ (∆Gq ) 13.9 kcal/mol), presumably
reflects the coordinative saturation of the CO adducts.
Experimental Section
General Methods. Unless otherwise noted, all reactions, re-
crystallizations, and routine manipulations were performed at
ambient temperature in an argon-filled glovebox or by using
standard Schlenk techniques. Benzene and p-xylene were distilled
from sodium/benzophenone under argon. Deuterated solvents for
use in NMR experiments were dried as their protiated analogues,
but were vacuum transferred from the drying agent. (PCP)IrH2 was
prepared according to published methods.38 All other chemicals
were used as received from commercial suppliers.
1H and 31P{1H} NMR spectra were obtained on a 300 MHz,
1
Varian Mercury 300 spectrometer. H NMR chemical shifts are
reported in ppm downfield from tetramethylsilane and were
referenced to residual protiated (1H) solvents. 31P NMR chemical
shifts are reported in ppm downfield from 85% H3PO4 and were
referenced to external (capillary) PMe3 in C6D6. Infrared (IR) spectra
were recorded on an ATI Mattson Genesis Series FTIR in pentane
or benzene solutions.
(PCP)Ir(H)(K2-O,O-NO2CH2) (1). (PCP)IrH2 (20 mg, 0.034
mmol) was dissolved in 2 mL of p-xylene solution containing 5.0
mg of norbornene (0.053 mmol) at room temperature. To the
resulting solution was added 2.8 µL of nitromethane (0.052 mmol);
after stirring for ca. 1 min, the red solution turned orange-yellow
quickly. Solvent was evacuated and the resulting solid was
redissolved in and recrystallized from pentane; yellow crystals were
obtained (98% yield). 31P NMR (121.4 MHz, benzene-d6): δ 59.16
(d, JPH ) 12.8 Hz). 1H NMR (300 MHz, benzene-d6): 6.85-6.92
(m, 3H, aromatic), 5.008 (s, 2H, CH2), 23.233 (d of vt, JPH ) 3.3
Complexes 2′ and 3′ are both 18-e species with a monodentate
H2CNO2 anion (O- and C-bound, respectively). 3′ is the
thermodynamically more stable isomer by about 10 kcal/mol.
This can be rationalized/predicted on the basis of the general
rule that M-X and H-X bonds have similar relative bond
energies,37 and nitromethane is more stable than its nitronic acid
tautomer by about 14 kcal/mol. The greater electronegativity
of O versus C might be expected to favor the Ir-O-bonded
isomer by more than 4 kcal/mol; however, this factor might be
offset by Ir(dπ)-O(pπ) repulsive interactions present in 3′.
Isomerization from the isocyanide bound, mono-oxygen-
coordinated nitromethanate complex, 4, to the carbon-coordi-
nated nitromethyl complex 5 is analogous to the isomerization
of complex 2 to 3. Perhaps because of steric bulk of the
cyclohexylisocyanide ligand, complex 4 is more labile than
complex 2. Electronic structure calculations, using methyliso-
cyanide as a ligand rather than cyclohexylisocyanide, indicate
that the O- to C-bound conversion is exothermic by ap-
proximately 10.0 kcal/mol (∆G ) 9.4 kcal/mol), only about 1
kcal/mol less than the computed value in the CO case
(complexes 2′ and 3′). The calculated enthalpy of activation is
22.8 kcal/mol (∆Gq ) 23.5 kcal/mol), also about 1 kcal/mol
less than what was found in the CO case.
Hz, JHH ) 16.5 Hz, 2H, CH2), 2.948 (d of vt, JPH ) 4.2 Hz, JHH
)
16.5 Hz, 2H, CH2), 1.433 (t, JPH ) 6.3 Hz, 18H, C(CH3)3), 1.296
(t, JPH ) 6.3 Hz, 18 H, C(CH3)3), -29.315 (t, JPH ) 13.5 Hz, 1H,
Ir-H). 13C NMR (100.6 MHz, benzene-d6): δ 148.9 (t, JCP ) 8.2
Hz, Ar C-Ir), 137.1 (s, Ar o-C), 137.0(s, Ar o-C), 121.5 (s, Ar
p-CH), 120.5 (vt, JCP ) 7.6 Hz, Ar m-CH), 100.1 (s, NO2CH2),
35.9 (vt, JCP ) 9.2 Hz, PC(CH3)3), 35.0 (vt, JCP ) 9.2 Hz,
PC(CH3)3), 34.9 (vt, JCP ) 14.0 Hz, CH2P), 29.5 (vt, JCP ) 2.5
Hz, PC(CH3)3), 29.3 (vt, JCP ) 2.5 Hz, PC(CH3)3). IR (pentane):
ν(CdN) ) 1575 cm-1, νas(NO2) ) 1181 cm-1, 1119 cm-1, νs(NO2)
)
948 cm-1. Anal. Calcd for C25H46IrNO2P2: C, 46.40; H, 7.17; N,
2.17. Found: C, 46.32; H, 7.00; N, 1.95.
(PCP)Ir(H)(CO)(K1-O-ONOCH2) (2). 1 (15 mg, 0.023 mmol)
was dissolved in 2 mL of p-xylene The solution was freeze-pump-
thawed, and 800 Torr of CO was immediately added. The orange
solution quickly turned yellow. Solvent was evaporated under
reduced vacuum, and a pale yellow solid was obtained (95% yield).
31P NMR (121.4 MHz, benzene-d6): δ 58.22 (d, JPH ) 13.9 Hz).
1H NMR (300 MHz, benzene-d6): 6.957 (t, JHH ) 7.8 Hz, 1H,
aromatic), 6.852 (d, JHH ) 7.8 Hz, 2H, aromatic), 6.006 (s, 2H,
CH2), 2.996 (d of vt, JPH ) 4.2 Hz, JHH ) 16.5 Hz, 2H, CH2),
2.924 (d of vt, JPH ) 4.2 Hz, JHH ) 16.5 Hz, 2H, CH2), 1.379 (t,
JPH ) 7.05 Hz, 18H, C(CH3)3), 1.212 (t, JPH ) 6.6 Hz, 18 H,
C(CH3)3), -7.425 (t, JPH ) 15.2 Hz, 1H, Ir-H). 13C NMR (100.6
MHz, benzene-d6): δ 184.8 (t, JCP ) 6.8 Hz, Ir-CO), 146.7 (t, JCP
) 6.0 Hz, Ar C-Ir), 133.9 (s, Ar o-C), 123.3 (s, Ar p-CH), 121.2
Conclusions
Pincer iridium nitromethanate complexes of three different
binding modes were synthesized and characterized, within a
single system. Reaction of (PCP)Ir with nitromethane forms the
bidentatenitromethanate complex (PCP)Ir(H)(κ2-O,O-NO2CH2)
(1). Reaction of 1 with CO affords the mono-oxygen-ligated
nitromethanate CO adduct 2, which is the first characterized
transition metal mono-oxygen-ligated nitroalkanate complex. An
analogous reaction occurs with cyclohexylisocyanide to give
mono-oxygen-ligated nitromethanate complex 4. Upon heating,
complexes 2 and 4 isomerize to give the carbon-ligated
nitromethyl complexes 3 and 5, respectively.
(vt, JCP ) 7.3 Hz, Ar m-CH), 93.4 (s, NO2CH2), 36.6 (vt, JCP
)
14.1 Hz, CH2P), 35.7 (vt, JCP ) 12.7 Hz, PC(CH3)3), 35.3 (vt, JCP
) 9.8 Hz, PC(CH3)3), 29.3 (vt, JCP ) 2.2 Hz, PC(CH3)3), 28.7 (vt,
JCP ) 1.7 Hz, PC(CH3)3). IR: ν(CO) ) 2004 cm-1, ν(C-N) ) 1547
cm-1. Anal. Calcd for C26H46IrNO3P2: C, 46.25; H, 6.87; N, 2.08.
Found: C, 46.23; H, 6.87; N, 1.89.
Computational studies indicate that the formation of complex
1 occurs via C-H oxidative addition to give a C-bound
nitromethyl iridium hydride with weak Ir-O bonding. The
(formally anionic) nitromethyl ligand can then rotate to give
the κ2-O,O′-bound nitromethanate present in 1. This appears to
be the first reported example of oxidative addition of a C-H
bond leading to a nitroalkanate ligand.
(PCP)Ir(H)(CO)(CH2NO2) (3). 2 (15 mg, 0.023 mmol) was
dissolved in 2 mL of p-xylene; the solution was then refluxed under
(37) Bryndza, H. E.; Fong, L. K.; Paciello, R. A.; Tam, W.; Bercaw, J.
E. J. Am. Chem. Soc. 1987, 109, 1444-56.
(38) Zhu, K.; Achord, P. D.; Zhang, X.; Krogh-Jespersen, K.; Goldman,
A. S. J. Am. Chem. Soc. 2004, 126, 13044-13053.

Multiple Binding Modes of the Nitromethanate Anion
Organometallics, Vol. 25, No. 5, 2006 1309
argon for 5 h. Solvent was evacuated, and the resulting solid was
redissolved in and recrystallized from pentane; yellow crystals were
obtained (90% yield). 31P NMR (121.4 MHz, benzene-d6): δ 50.08
and correlation functionals. The relativistic, small-core ECP and
corresponding basis sets (split valence triple-ú) of Dolg et al. were
used for the Ir atom (SDD model).41 This 60-electron ECP releases
the penultimate valence electrons (5s25p6 for Ir) for explicit coverage
by basis functions along with the valence electrons (6s25d7). We
used all-electron, full double-ú plus polarization function basis sets
for the second- and third-row elements C, N, O, and P (D95(d)).42
The hydrogen atom which formally becomes a hydride in the
product complexes was described by the triple-ú plus polarization
311G(p) basis set;43 regular hydrogen atoms carried a double-ú
quality 21G basis set.44
Reactant, transition state, and product geometries were fully
optimized and the stationary points were characterized further by
normal-mode analysis. The (unscaled) vibrational frequencies
formed the basis for the calculation of vibrational zero-point energy
(ZPE) corrections. Standard thermodynamic corrections were made
to convert from purely electronic reaction or activation energies
(∆E, ∆Eq; no ∆ZPE) to enthalpies and free energies (∆H, ∆Hq;
∆G, ∆Gq; ∆ZPE included, T ) 298 K, P ) 1 atm).45 To substantiate
the nature of a particular transition state located, minimum energy
paths were traced using the intrinsic reaction coordinate approach.46
All calculations were executed using the GAUSSIAN 03 series of
computer programs.47
1
(d, JPH ) 6.0 Hz). H NMR (300 MHz, benzene-d6): 6.96-7.06
(m, 3H, aromatic), 5.612 (t, JPH ) 4.95 Hz, 2H, CH2), 3.104 (t,
JPH ) 7.5 Hz, 4H, CH2), 1.294 (t, JPH ) 6.9 Hz, 18H, C(CH3)3),
1.063 (t, JPH ) 6.6 Hz, 18 H, C(CH3)3), -9.608 (t, JPH ) 15.8 Hz,
1H, Ir-H). 13C NMR (100.6 MHz, benzene-d6): δ 181.5 (t, JCP
)
4.0 Hz, Ir-CO), 147.3 (t, JCP ) 5.6 Hz, Ar C-Ir), 145.4 (s, Ar o-C),
123.8 (s, Ar p-CH), 120.5 (vt, JCP ) 7.3 Hz, Ar m-CH), 40.7 (vt,
JCP ) 14.5 Hz, CH2P), 36.9 (vt, JCP ) 10.0 Hz, PC(CH3)3), 36.2
(vt, JCP ) 12.9 Hz, PC(CH3)3), 32.9(t, JCP ) 4.5 Hz, NO2CH2),
29.3 (s, PC(CH3)3), 28.6 (s, PC(CH3)3). IR: ν(CO) ) 1988 cm-1
,
νas(NO2) ) 1498 cm-1, νs(NO2) ) 1355 cm-1. Anal. Calcd for C26H46-
IrNO3P2: C, 46.25; H, 6.87; N, 2.08. Found: C, 46.32; H, 6.78;
N, 1.74.
(PCP)Ir(H)(cyclohexylisocyanide)(K1-O-ONOCH2) (4). Cy-
clohexylisocyanide (3.5 µL, 0.032 mmol) was added to a solution
of 15 mg of 1 (0.023 mmol) in 2 mL of p-xylene, and the orange
solution quickly turned pale yellow. Solvent was evaporated
immediately under reduced vacuum, and a pale yellow solid was
obtained (90% yield). 31P NMR (121.4 MHz, benzene-d6): δ 57.401
1
(d, JPH ) 11.9 Hz). H NMR (300 MHz, benzene-d6): 7.059-
Our computational model for (PCP)Ir has methyl groups attached
to the phosphorus atoms (i.e., PR2 ) P(CH3)2), a compromise
between the use of hydrogen atoms and the bulky tert-butyl groups
actually employed in the experimental systems. Methyl groups
capture most of the electronic effects imparted by the tert-butyl
groups, but they obviously do not fully model the steric bulk.48,49
6.895 (m, 3H, PCP aromatic H), 5.964 (s, 2H, CH2), 3.092 (d of
vt, JPH ) 3.5 Hz, JHH ) 15.6 Hz, 2H, CH2), 3.035 (d of vt, JPH
)
3.9 Hz, JHH ) 15.6 Hz, 2H, CH2), 1.653-1.296(m, 11H, cyclo-
hexyl), 1.488 (t, JPH ) 6.5 Hz, 18H, C(CH3)3), 1.337 (t, JPH ) 6.5
Hz, 18 H, C(CH3)3), -9.345 (t, JPH ) 15.9 Hz, 1H, Ir-H). 13C NMR
(100.6 MHz, benzene-d6): δ 146.7 (t, JCP ) 6.6 Hz, Ar C-Ir), 141.1
(br, CN cyclohexyl), 138.3 (s, Ar o-C), 122.1 (s, Ar p-CH), 120.4
(vt, JCP ) 6.9 Hz, Ar m-CH), 92.8 (s, NO2CH2), 53.7 (s, CNCH
cyclohexyl), 37.1 (vt, JCP ) 13.5 Hz, CH2P), 35.5 (vt, JCP ) 9.0
Hz, PC(CH3)3), 35.4 (vt, JCP ) 12.6 Hz, PC(CH3)3), 32.3 (s,
Acknowledgment. Financial support by the National Science
Foundation (Grant CHE-0316575, and Grant CHE-0091872 for
purchase of the Bruker SMART APEX diffractometer) is
gratefully acknowledged.
cyclohexyl C), 29.9 (vt, JCP ) 2.2 Hz, PC(CH3)3), 29.2 (vt, JCP
)
1.7 Hz, PC(CH3)3), 24.7 (s, cyclohexyl C), 23.5 (s, cyclohexyl C).
Supporting Information Available: Crystallographic data for
complexes 1, 2, 3, and 5. This material is available free of charge
via the Internet at http://pubs.acs.org.
(PCP)Ir(H)(cyclohexylisocyanide)(CH2NO2) (5). Complex 4
isomerized to form complex 5 slowly. At room temperature, the
isomerization was completed in 24 h. At 80 °C, the isomerization
was completed within 2 h. Complex 4 (30 mg, 0.040 mmol) in
p-xylene was heated at 80 °C for 2 h, solvent was evacuated, and
the resulting solid was redissolved in and recrystallized from
benzene as pale yellow crystals (90% yield). 31P NMR (121.4 MHz,
OM050659J
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1
benzene-d6): δ 47.293 (d, JPH ) 7.29 Hz). H NMR (300 MHz,
benzene-d6): 7.058-6.997 (m, 3H, PCP aromatic H), 5.786 (t, JPH
) 4.95 Hz, 2H, CH2), 3.302 (d of vt, JPH ) 3.0 Hz, JHH ) 16.2
Hz, 2H, CH2), 3.191 (d of vt, JPH ) 4.4 Hz, JHH ) 16.2 Hz, 2H,
CH2), 1.643-1.296(m, 11H, cyclohexyl), 1.410 (t, JPH ) 6.5 Hz,
18H, C(CH3)3), 1.206 (t, JPH ) 6.2 Hz, 18 H, C(CH3)3), -11.729
(t, JPH ) 16.7 Hz, 1H, Ir-H). 13C NMR (100.6 MHz, benzene-d6):
δ 150.5(s, Ar o-C), 147.3 (t, JCP ) 6.4 Hz, Ar C-Ir), 137.4 (t, JCP
) 5.2 Hz, CN cyclohexyl), 122.6 (s, Ar p-CH), 119.7 (vt, JCP
7.2 Hz, Ar m-CH), 53.9 (s, CNCH cyclohexyl), 41.2 (vt, JCP
)
)
14.3 Hz, CH2P), 37.1 (vt, JCP ) 9.2 Hz, PC(CH3)3), 35.9 (vt, JCP
) 13.1 Hz, PC(CH3)3), 34.1 (t, JCP ) 3.9 Hz, NO2CH2), 32.8 (s,
cyclohexyl C), 29.8 (s, PC(CH3)3), 29.1 (s, PC(CH3)3), 24.5 (s,
cyclohexyl C), 23.7 (s, cyclohexyl C). Anal. Calcd for C32H57-
IrN2O2P2: C, 50.82; H, 7.60; N, 3.71. Found: C, 51.22; H, 7.73;
N, 3.49.
Computational Details. For the electronic structure calculations
we employed the DFT39 method along with the PBE40 exchange
(39) Parr, R. G.; Yang, W. Density-Functional Theory of Atoms and
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