Six-, Five-, and Four-Coordinate Ruthenium(II) hydride complexes supported by N-Heterocyclic Carbene Ligands: Synthesis, characterization, fundamental reactivity, and catalytic hydrogenation of olefins, Aldehydes, and ketones

Article abstract of DOI:10.1021/om801111c

The Ru(II) hydride complex (IMes)₂Ru(Cl)(H)(CO) (1) {IMes = l,3-bis-(2,4,6-trimethylphenyl)imi-dazol-2-ylidene} was synthesized from [Ru(CO)₂Cl_2n and free IMes. Complex 1 rapidly reacts with CO to produce the cis-dicarbony

Full text of DOI:10.1021/om801111c

1758
Organometallics 2009, 28, 1758–1775
Six-, Five-, and Four-Coordinate Ruthenium(II) Hydride Complexes
Supported by N-Heterocyclic Carbene Ligands: Synthesis,
Characterization, Fundamental Reactivity, and Catalytic
Hydrogenation of Olefins, Aldehydes, and Ketones
John P. Lee,† Zhuofeng Ke,‡,§ Magaly A. Ram´ırez,† T. Brent Gunnoe,*,|
Thomas R. Cundari,*,‡ Paul D. Boyle,† and Jeffrey L. Petersen
Department of Chemistry, UniVersity of Virginia, CharlottesVille, Virginia 22904, Department of
Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695-8204, Center for AdVanced
Scientific Computing and Modeling (CASCaM), Department of Chemistry, UniVersity of North Texas,
Box 305070, Denton, Texas 76203-5070, C. Eugene Bennett Department of Chemistry, West Virginia
UniVersity, Morgantown, West Virginia 26506-6045, and School of Chemistry & Chemical Engineering,
Sun Yat-sen UniVersity, Guangzhou 510275, P.R. China
ReceiVed NoVember 21, 2008
The Ru(II) hydride complex (IMes)2Ru(Cl)(H)(CO) (1) {IMes ) 1,3-bis-(2,4,6-trimethylphenyl)imi-
dazol-2-ylidene} was synthesized from [Ru(CO)2Cl2]n and free IMes. Complex 1 rapidly reacts with CO
to produce the cis-dicarbonyl Ru(II) complex (IMes)2Ru(Cl)(H)(CO)2 (2). The reaction of 1 with NaBAr′4
{Ar′ ) 3,5-(CF3)C6H3} produces the four-coordinate Ru(II) cationic complex [(IMes)2Ru(H)(CO)][BAr′4]
(4), which can be trapped by two equivalents of tert-butylisonitrile to produce [(IMes)2Ru(H)-
(CO)(CNtBu)2][BAr′4] (5). Experimental and computational studies suggest that complex 4 is a diamagnetic
system that adopts a sawhorse structure. The hydride ligand of complex 2 is readily displaced as dihydrogen
upon reaction with HCl to produce (IMes)2Ru(Cl)2(CO)2 (3). Both complex 1 and 4 were found to react
with D2 (30 psi) at room temperature to produce the isotopomers (IMes)2Ru(Cl)(D)(CO) (1-d1) and
[(IMes)2Ru(D)(CO)][BAr′4] (4-d1), respectively, with the rate of formation of 4-d1 at least 28 times faster
than the conversion of 1/D2 to 1-d1. In the presence of excess D2 complex 4 reversibly incorporates
deuterium into the ortho methyl groups of the IMes ligands, whereas complex 1 does not show evidence
of H/D exchange with the IMes ligands. Both 1 and 4 were found to catalyze the hydrogenation of
olefins, ketones, and aldehydes.
(PPh3)3RhCl and related systems commonly adopt what has been
referred to as the dihydride mechanism for catalytic hydrogena-
tion, where coordination and subsequent oxidative addition of
H2 occur along the catalytic pathway.7,10 Interest in transition
metal-mediated coordination and activation of dihydrogen also
emanates from the close relevance to the coordination/activation
of other nonpolar X-H bonds (e.g., X ) C or Si).11-17
Significant effort has been directed toward the isolation of metal
complexes with σ-coordinated H-H, C-H, Si-H, and related
bonds as well as understanding the factors that control the
equilibrium between the σ-adduct and the oxidative addition
product.18-32
Introduction
Since the initial discovery of a transition metal dihydrogen
complex in 1983 by Kubas et al.,1-4 there has been significant
interest in the coordination chemistry of dihydrogen.5,6 It has
been shown that coordination of dihydrogen can precede H-H
bond cleavage to form two hydride ligands (i.e., oxidative
addition, Scheme 1), which is a key step in many catalytic
hydrogenation reactions.7-9 For example, Wilkinson’s catalyst
* To whom correspondence should be addressed. E-mail: tbg7h@
virginia.edu.
† North Carolina State University.
‡ University of North Texas.
§ Sun Yat-sen University.
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10.1021/om801111c CCC: $40.75
2009 American Chemical Society
Publication on Web 02/20/2009

Ruthenium(II) Hydride Complexes
Organometallics, Vol. 28, No. 6, 2009 1759
Scheme 1. Coordination of Dihydrogen and Subsequent
Oxidative Addition
(Cl)(H)(CO) (via the production of [HPCy3][BF4]), which was
found to be a more active catalyst than the 16-electron precursor
(IMes)(PCy3)Ru(Cl)(H)(CO).38 Also, a series of trans-(NH-
C)RuII mono and dihydride complexes have been prepared,43
and though these complexes were not studied for hydrogenation,
they show interesting reactivity toward intra- and intermolecular
X-H (X ) C, N, or O)44-47 and C-X (X ) C or N)47,48 bond
activation.
Studies have shown that NHC’s may possess similar elec-
tronic properties to trisalkylphosphines.39-42,49,50 However, with
“fan-shaped” geometries the steric properties of NHCs are
distinct from phosphines. In some cases, NHC ligands have been
found to simultaneously enhance catalyst stability and activity
(compared to their phosphine analogs) including applications
in olefin metathesis,51-56 hydrogenation,38,57-59 and hydrosi-
lylation reactions.60 Herein, we describe the synthesis, charac-
terization, and initial reactivity studies of five- and four-
coordinate Ru(II) hydride complexes supported by two IMes
ligands.
The d6 metals Ru(II) and Os(II) have been commonly used
to form σ-complexes with H2, and in many cases such species
also serve as effective hydrogenation catalysts.33-35 Ruthenium
monohydride systems often serve as precursors to hydrogenation
catalysts and/or dihydrogen complexes.34,36-38 Most germane
to the work presented herein are the monohydride Ru(II)
complexes supported by N-heterocyclic carbenes (NHC) and/
or phosphines. For example, the five-coordinate Ru(II) complex
(PCy3)2Ru(Cl)(H)(CO), where the Ru possesses square pyra-
midal geometry with the hydride ligand in the apical position,
has been reported to catalytically hydrogenate olefins at room
temperature under 1 atm of H2 with high turnover frequencies.37
NHC ligands have been employed by many research groups as
stable and strongly electron-donating alternatives to phosphine
ligands.39-42 Nolan et al. have replaced a single phosphine
ligand of (PCy3)2Ru(Cl)(H)(CO) with an NHC to produce
(IMes)(PCy3)Ru(Cl)(H)(CO) {IMes ) 1,3-bis-(2,4,6-trimeth-
ylphenyl)imidazol-2-ylidene}.38 The mixed NHC/phosphine
complex is less active as a hydrogenation catalyst (at room
temperature) than the bis-phosphine complex (PCy3)2-
Ru(Cl)(H)(CO) but exhibits similar activity at elevated tem-
peratures. The decreased activity for the mixed NHC/phosphine
complex at ambient temperature is proposed to be due to a
combination of stronger coordination of IMes (versus PCy3) and
enhanced steric congestion at the metal center. The addition of
HBF4 to (IMes)(PCy3)Ru(Cl)(H)(CO) is proposed to trap the
putative four-coordinate, 14-electron intermediate (IMes)Ru-
Results and Discussion
Synthesis, Characterization and Reactivity of (IMes)2-
Ru(Cl)(H)(CO) (1). Heating the previously reported [Ru-
(CO)2Cl2]n with excess IMes in toluene followed by the addition
of methanol forms the five-coordinate Ru(II) complex
(IMes)2Ru(Cl)(H)(CO) (1) in 32% isolated yield (eq 1). During
the course of our studies, the synthesis of (IMes)2Ru(Cl)(H)(CO)
(1) from Ru(AsPh3)3(CO)H2 or Ru(IMes)2(CO)(OH)H was
reported by Whittlesey et al.61 Our spectroscopic data for
complex 1 are consistent with the previously reported data. For
1
example, the H NMR spectrum of 1 shows a singlet assigned
as the Ru-H at -25.4 ppm (C6D6) while the IR spectrum (thin
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1760 Organometallics, Vol. 28, No. 6, 2009
Scheme 2. Proposed Fluxional Bond Rotations of 1 in Solutiona
Lee et al.
a
IMes backbone and aryl hydrogens have been labeled. RT ) room temperature.
film on KBr) of 1 reveals νCO at 1886 cm-1. In addition, we
used variable temperature 1H NMR spectroscopy to further study
the structure and bonding of complex 1 (see below).
1
Figure 1. Eyring plot from variable temperature H NMR spec-
troscopy of 1 (R2 ) 0.97). The plot relates rate constants for
Ru-CNHC bond rotation as a function of temperature.
The source of the hydride ligand in 1 is unclear. Use of
CD3OD in place of CH3OH or toluene-d8 in place of perprotio-
toluene does not result in deuterium incorporation into the final
product (1H NMR spectroscopy reveals no change for syntheses
using these deuterated reagents). Heating [Ru(CO)2Cl2]n with
two equivalents of free IMes in toluene-d8 (prior to methanol
addition) results in a mixture of products (by NMR and IR
spectroscopy) including a single Ru-H resonance at -4.5 ppm
without evidence of complex 1. Reflux of this mixture (after
heating in toluene-d8) in either CH3OH or CD3OD produces
complex 1 without evidence of deuterium incorporation into
the hydride position.
Also at -60 °C, the aryl hydrogens resonate as four singlets
that integrate for 2H each, and the methyl groups are observed
as five singlets (likely six symmetry unique groups with two
overlapping resonances). The NMR spectra at low temperatures
are consistent with hindered C-Nmesityl and hindered Ru-CNHC
bond rotation (Scheme 2). At room temperature, rotation about
the Ru-CNHC bonds is rapid relative to the NMR time scale.
Using CD2Cl2 as the solvent, kinetic data for Ru-CNHC bond
rotation have been derived at various temperatures. In CD2Cl2,
the slow exchange regime for Ru-CNHC bond rotation was
reached at -65 °C (based on decoalescence of an IMes-Me
broad singlet into two singlets), and these data were used to
calculate activation parameters for rotation about the Ru-CNHC
bond (Figure 1). The process is enthalpically dominated {∆Hq
) 13.7(1) kcal/mol} with relatively negligible entropy contribu-
tions {∆Sq ) 2.7(6) eu}, which is consistent with an intramo-
lecular fluxional process.
At room temperature, the 1H NMR spectrum of 1 is consistent
with Cs molecular symmetry (with a mirror plane that contains
Ru, H, Cl, and CO, if the square pyramidal geometry is assumed,
see above) and rapid rotation about the Ru-CNHC bonds, and
hindered Ncarbene-Cmesityl rotation (Scheme 2). For example, the
methyl groups of the mesityl rings resonate as three equivalent
broad singlets, one singlet is observed for the olefin backbone
hydrogen atoms and two singlets appear due to inequivalent
aryl hydrogens of the IMes ligands (see Scheme 2). Heating a
toluene-d8 solution of 1 to 95 °C does not result in any changes
Complex 1 is formulated as a 16-electron complex with an
available coordination site. However, the presence of two
sterically bulky IMes ligands might be expected to inhibit
coordination of a sixth ligand. To explore whether or not ligand
association is accessible, we reacted 1 with neutral 2-electron
donating substrates. Heating complex 1 up to 100 °C in the
presence of acetonitrile or in C6D6 with an excess PMe3 results
in no observed reaction by 1H NMR spectroscopy (Scheme 3).
1
in the H NMR spectrum. However, at -20 °C, two of the
methyl groups begin to broaden and decoalesce, and at -40 °C
resonances due to aryl hydrogen atoms begin to decoalesce. At
-60 °C, the resonance due to the olefinic IMes hydrogen atoms
decoalesces into two broad singlets that integrate for 2H each.

Ruthenium(II) Hydride Complexes
Organometallics, Vol. 28, No. 6, 2009 1761
1.57(2) Å},38 CpRu(dppe)H {Ru-H 1.60(2) Å}, and CpRu(d-
pbz)H {Ru-H 1.56(5) Å}.62 The heterocyclic rings of the trans
IMes ligands of 2 approach coplanarity with the imidazole rings
only slightly twisted {twist angle (θt) ) 0.5[N3-C24-C3-N1
+ N4-C24-C3-N2]}43 by 17.2° relative to one another.
Twisting of IMes ligands has been observed in other trans-
IMes2Ru complexes.43 The C24-Ru-C3 bond angle deviates
from linearity to 168.69(5)° with a slight bending of the two
carbene ligands away from the CO ligand (C1) toward the
hydride ligand (H1). This may be a result of the slight steric
bulk of CO compared with the hydride ligand; however,
electronic effects cannot be eliminated as a root cause of the
C24-Ru-C3 bending. For example, Eisenstein et al. have
discussed electronic factors that contribute to P-Os-P angular
distortions (167.5° and 141.4°) in six-coordinate Os(II) systems
(PR3)2Os(Cl)(CO)(X)(carbene) (X ) H or Cl).63 The Os system
resembles complex 2 with two strong σ-donors (PR3 for the Os
complex and IMes for 2), two strong π-acids (carbene and CO
for the Os systems and 2 CO ligands for complex 2) and Cl/H
ligands. Thus, bending of the IMes ligands away from the π-acid
CO may enhance Ru-to-CO dπ-back-bonding.63
Scheme 3. Complex 1 Does Not React with Acetonitrile or
Trimethylphosphine
In contrast to NCMe and PMe3, the reaction of 1 with CO (1
atm) at room temperature rapidly produces the cis-dicarbonyl
Ru(II) complex (IMes)2Ru(Cl)(H)(CO)2 (2) (eq 2). The syn-
thesis, characterization, and crystal structure of (IMes)2-
Ru(Cl)(H)(CO)2 (2) have been previously reported;61 however,
the Ru-H IR stretch of 2 was not disclosed. Consistent with
1
the reported data for 2, the H NMR spectrum reveals the
At room temperature, 2 and excess HCl produce the 6-co-
ordinate Ru(II) complex (IMes)2Ru(Cl)2(CO)2 (3), which is
resonance due to the hydride ligand at -4.20 ppm (C6D6). The
IR spectrum (thin film on KBr) shows a low intensity absorption
at 1941 cm-1 assigned as the νRu-H stretch as well as symmetric
and asymmetric νCO stretches at 2035 and 1905 cm-1, respec-
tively. The assignment of the absorption at 1941 cm-1 (IR) as
νRu-H is supported by the synthesis of the isotopomer
(IMes)2Ru(Cl)(D)(CO)2 (2-d1) from 2 and D2 (30 psi) (eq 3).
In the IR spectrum of 2-d1, the band at 1941 cm-1 is absent.
The calculated νRu-D of 2-d1 (based on difference in reduced
mass) is approximately 1384 cm-1; however, the IR spectrum
of 2 between 1485 cm-1 and 1380 cm-1 contains multiple
absorptions, and we were unable to identify the Ru-D stretch
in the IR spectrum of 2-d1. The mechanism for the conversion
of 2 and D2 to 2-d1 has not been probed, although it is reasonable
to assume that it involves an initial ligand dissociation.
1
isolated as a white solid in 82% yield (eq 4). The H NMR
spectrum of 3 shows no resonance upfield of 0 ppm, which is
consistent with the absence of a hydride ligand. The IMes
1
ligands of 3 approach coplanarity as determined by H NMR
spectroscopy. For example, two resonances are observed in a
2:1 ratio due to the ortho and para methyl groups. Variable
temperature NMR experiments have not been performed to
differentiate between a static structure or transient access to a
symmetric geometry. The IR spectrum (thin film on KBr) shows
the symmetric and asymmetric CO absorption at 2025 and 1955
cm-1. Complex 3 is likely produced via protonation of the
hydride ligand followed by H2/Cl- ligand exchange. Monitoring
the reaction by 1H NMR spectroscopy did not allow the
observation of [(IMes)2Ru(Cl)(η2-H2)(CO)2][Cl] (no reaction
intermediates were observed at room temperature); however,
free H2 was observed at 4.42 ppm (1H NMR). Since the hydride
ligand of complex 2 was readily replaced with a chloride ligand
via reaction with HCl, we attempted the same reaction with
complex 1. However, neither reaction with HCl nor other
sources of chlorine (e.g., N-chlorosuccinimide or CCl4) resulted
in the clean formation of (IMes)2Ru(Cl)2(CO) from
(IMes)2Ru(Cl)(H)(CO) (1) (see Supporting Information).
Synthesis and Characterization of [(IMes)2Ru(H)(CO)]-
[BAr′4] (4). The reaction of 1 with NaBAr′4 {Ar′ ) 3,5-
(CF3)C6H3} at room temperature produces a new species
assigned as four-coordinate [(IMes)2Ru(H)(CO)][BAr′4] (4) (eq
5), which is isolated in 70% yield as a brick-red powder. The
1H NMR spectrum of 4 shows the Ru-H as a broad resonance
at -26.4 ppm (C6D6), which integrates to 1H, and the IR
A single crystal of 2 suitable for an X-ray diffraction study
was grown, and the resulting structure is shown in Figure 2
with crystallographic data and collection parameters given in
Table 1. The data for 2 from our study are similar to those
previously reported.61 The structure contains a CO/Cl site
disorder with the positions of the disordered atoms obtained
from a difference Fourier map (see Supporting Information).
The hydride position of 2 was recovered from a difference map
with a Ru-H bond distance of 1.54(2) Å, which is in agreement
with other reported RuII-H bond distances from X-ray diffrac-
tion studies including (IMes)(PCy3)Ru(Cl)(H)(CO) {Ru-H
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498.

1762 Organometallics, Vol. 28, No. 6, 2009
Lee et al.
Figure 2. ORTEP of (IMes)2Ru(Cl)(H)(CO)2 (2) (30% probability with most hydrogen atoms omitted). Selected bond lengths (Å): Ru-C3,
2.115(1); Ru-C24, 2.110(1); Ru-Cl, 1.986(2); Ru-H1, 1.54(2); C1-O1, 1.136(2). Selected bond angles (deg): C3-Ru-C24, 168.69(5);
C1-Ru-H1, 179.3(7); C24-Ru-C1, 96.13(5); C3-Ru-H1, 85.4(7); C24-Ru-H1, 83.2(7). (Inset) Complex 2 viewed down the
C3-Ru-C24 bond (only the imidazolyl rings and Ru are shown) to highlight the degree to which the imidazolyl rings are twisted.
Table 1. Selected Crystallographic Data and Collection Parameters
geometries in which C-H agostic interactions complete the “six-
coordinate” nature are adopted for these systems.69-72 For
for (IMes)2Ru(Cl)(H)(CO)2 (2) and
[(IMes)2Ru(H)(CNtBu)2(CO)][BAr′4] (5)
example, the complex [{P(tBu2Me)}2Ru(Ph)(CO)][BAr′4] has
2
5
been reported with an IR spectrum that is consistent with C-H
agostic interactions (νCH ) 2722 and 2672 cm-1).70 The Ru(II)
complex [{P(tBu2Me)}2Ru(H)(CO)][BAr′4], which is closely
related to complex 4 with two strongly donating ligands
(alkylphosphines) and hydride/CO ligands, has been reported
to possess two C-H agostic interactions as determined by X-ray
crystallography.71 Studies by Nolan et al. of low coordinate Rh
and Ir systems with N-heterocyclic carbene ligands are also
relevant here.73 Two interesting Ru(II) systems that are four-
coordinate without evidence of stabilizing agostic interactions
are (PNPtBu)Ru(Cl) {PNPtBu ) [(tBu)2PCH2SiMe2)2N} and κ2-
P,O-(tBu2PCH2SiMe2O)2Ru reported by Caulton et al.74-76 The
triplet ground-state of these d6 complexes has been attributed
to the square planar geometry and π-donating N- or O-based
empirical formula
fw
crystal system
space group
a (Å)
C44H49ClN4O2Ru
802.39
monoclinic
P21/c
21.109(8)
10.832(4)
18.474(2)
108.41(2)
4008.1(3)
4
0.22 × 0.16 × 0.16
0.375, 0.0772
1.038
C85H79BF24N6ORu
1768.42
monoclinic
P21/n
16.535(1)
24.432(2)
21.548(1)
97.57(1)
8629.2(9)
4
b (Å)
c (Å)
ꢀ (deg)
V (Å3)
Z
crystal size (mm)
R1, wR2 {l > 2σ(l)}
GOF
0.12 × 0.14 × 0.40
0.0548, 0.143
1.011
spectrum shows a νCO stretch at 1933 cm-1 (C6H6). Thus,
conversion of 1 to 4 results in a 47 cm-1 increase in the CO
absorption energy, which is consistent with the decrease in
electron density and metal dπ-back-bonding to CO that is
expected upon removal of chloride to generate a cationic
complex. Similar to 1, the Ru-H stretching frequency was not
identified in the IR spectrum of 4. In agreement with our
assignment of 4 as the product of chloride abstraction, the
reaction of 4 with [Bu4N][Cl] quantitatively produces 1 (by 1H
NMR spectroscopy, C6D6) at room temperature (eq 5).
ligands.74 By IR and H NMR spectroscopy (see below), we
1
see no evidence of C-H agostic bonding for 4. For example,
there are no absorption bands between 2750 and 2000 cm-1 in
the IR spectrum (see Supporting Information). In addition,
variable temperature 1H NMR spectroscopy in toluene-d8 under
an argon atmosphere reveals no evidence of fluxionality down
to -40 °C. Below -40 °C, solubility issues complicate the
NMR analysis. Furthermore, IR spectra of 4 in toluene,
methylene chloride and hexafluorobenzene reveal no significant
(66) Watson, L. A.; Pink, M.; Caulton, K. G. J. Mol. Catal. A: Chem.
2004, 224, 51–59.
(67) Ingleson, M. J.; Pink, M.; Huffman, J. C.; Fan, H.; Caulton, K. G.
Organometallics 2006, 25, 1112–1119.
(68) Walstrom, A.; Pink, M.; Yang, X.; Tomaszewski, J.; Baik, M.;
Caulton, K. G. J. Am. Chem. Soc. 2005, 127, 5330–5331.
(69) Huang, D.; Huffman, J. C.; Bollinger, J. C.; Eisenstein, O.; Caulton,
K. G. J. Am. Chem. Soc. 1997, 119, 7398–7399.
(70) Huang, D.; Streib, W. E.; Bollinger, J. C.; Caulton, K. G.; Winter,
R. F.; Scheiring, T. J. Am. Chem. Soc. 1999, 121, 8087–8097.
(71) Huang, D.; Bollinger, J. C.; Streib, W. E.; Folting, K.; Young, J.,
V.; Eisenstein, O.; Caulton, K. G. Organometallics 2000, 19, 2281–2290.
(72) Baratta, W.; Herdtweck, E.; Rigo, P. Angew. Chem., Int. Ed. 1999,
39, 1629–1631.
(73) Scott, N. M.; Dorta, R.; Stevens, E. D.; Correa, A.; Cavallo, L.;
Nolan, S. P. J. Am. Chem. Soc. 2005, 127, 3516–3526.
(74) Watson, L. A.; Ozerov, O. V.; Pink, M.; Caulton, K. G. J. Am.
Chem. Soc. 2003, 125, 8426–8427.
On the basis of characterization data (see below), complex 4
appears to be a four-coordinate Ru(II) complex. Although scant,
recent examples of formally four-coordinate 14-electron Ru(II)
systems have been reported, and as anticipated, such systems
are highly reactive.64-68 With limited exceptions, octahedral
(64) Ingleson, M. J.; Yang, X.; Pink, M.; Caulton, K. G. J. Am. Chem.
Soc. 2005, 127, 10846–10847.
(65) Walstrom, A. N.; Watson, L. A.; Pink, M.; Caulton, K. G.
Organometallics 2004, 23, 4814–4816.
(75) Walstrom, A. N.; Pink, M.; Caulton, K. G. Inorg. Chem. 2006, 45,
5617–5620.
(76) Yang, X.; Walstrom, A.; Nikolay, T.; Maren, P.; Caulton, K. G.
Inorg. Chem. 2007, 46, 4612–4616.

Ruthenium(II) Hydride Complexes
Organometallics, Vol. 28, No. 6, 2009 1763
Figure 3. ORTEP of [(IMes)2Ru(H)(CNtBu)2(CO)]+ (5) (30% probability; BAr′4 anion and most hydrogen atoms omitted). Selected bond
lengths (Å): Ru-C1, 2.11(2); Ru-C22, 2.12(2); Ru-C53, 1.87(2); Ru-C48, 2.05(2); Ru-H1, 1.68; C53-O1, 1.15(3); C48-N6, 1.15(2);
C43-N5, 1.16(3). Selected bond angles (deg): C1-Ru-C22, 167.5(7); C1-Ru-H1, 82.1; C43-Ru-C48, 86.8(8); C53-Ru-H1, 88.2;
C43-Ru-H1, 172.2; C53-Ru-C48, 168.2(8); Ru1-C53-O1, 171(2); Ru1-C48-N6, 168(2); Ru1-C43-N5, 171(2). (Inset) Complex
5 viewed down the C22-Ru-C1 axis (only the imidazoly1 rings and Ru are shown) to highlight the degree to which the imidazoly1 rings
are twisted.
change within the deviation of the instrument in the CO
stretching frequency, which suggests that solvent does not
coordinate to complex 4.
Cooling a solution of 4 in toluene-d8 in the NMR probe results
in the appearance of resonances due to a new complex with a
simultaneous decrease in the intensity of the resonances due to
complex 4. At -60 °C, an approximate 2:1 molar ratio of the
resonances due to complex 4 and the new complex is observed
(based on integration of resonances due to IMes olefinic groups).
excess CNtBu rapidly produce the cis-isonitrile Ru(II) complex
Upon warming back to room temperature, the original 1H NMR
[(IMes)2Ru(H)(CNtBu)2(CO)][BAr′4] (5) (eq 7). The H NMR
spectrum is acquired. These data suggest a temperature depend-
spectrum of 5 shows the Ru-H resonance at -7.99 ppm (C6D6),
ent and reversible equilibrium, and we propose that an equi-
and the IR spectrum (thin film on KBr) shows an absorption at
librium between complex 4 and the N2 adduct [(IMes)2-
1975 cm-1 assigned as νCO as well as symmetric and asymmetric
Ru(H)(CO)(N2)][BAr′4] (4-N2) is likely (eq 6). To further
νCN stretches at 2150 and 2120 cm-1, respectively. A small
confirm coordination of dinitrogen by 4 at low temperatures, a
shoulder is observed for the CO absorption at 1975 cm-1, which
toluene-d8 solution of 4 was transferred to a J-Young NMR tube
is likely due to an overlap of a low intensity Ru-H stretch with
and was thoroughly degassed via multiple freeze
the CO absorption.
1
-pump-thaw cycles and back-filled with argon (1 atm).
Cooling this solution in the NMR probe from 20 to -40 °C
results in no observable changes. Hence, in the absence of
dinitrogen, at low temperatures the second complex assigned
as the nitrogen adduct 4-N2 is not observed. In addition,
pressurizing a J-Young NMR tube containing a solution of 4
with 80 psi of dinitrogen results in the observation of a mixture
of 4 and 4-N2 at room temperature (by 1H NMR spectroscopy).
Furthermore, subsequent degassing of this reaction tube with
argon (via multiple freeze-pump-thaw cycles) results in the
disappearance of 4-N2 and quantitative conversion back to
complex 4. Unfortunately, attempts to acquire low temperature
IR spectra (in order to observe the coordinated dinitrogen ligand)
were thwarted by decomposition of 4 (see the Supporting
Information for details). Several attempts were made to grow
crystals of 4 suitable for an X-ray diffraction study. In all
attempts, complex 4 apparently crystallizes as [(IMes)2-
Ru(H)(CO)(N2)][BAr′4] (4-N2); however, the X-ray structure
analysis is complicated by disorder. Details of a representative
data set are provided in the Supporting Information. Finally,
recrystallization of complex 4 under an argon atmosphere with
a variety of solvent combinations did not produce X-ray quality
crystals.
A single crystal of 5 suitable for an X-ray diffraction study
was grown, and the resulting structure is shown in Figure 3
with crystallographic data collection and parameters given in
Table 1. The hydride ligand of 5 was located with a Ru-H
bond distance of 1.68 Å. In contrast to 2, the trans IMes ligands
of 5 are approximately perpendicular to each other with a twist
angle θt {0.5[N1-C1-C22-N4 + N2-C1-C22-N3]}43 of
74.4°. Both of the isonitrile ligands were found to be slightly
bent into an open area between the mesityl rings of an IMes
ligand. These open “pockets” are a result of twisting of the IMes
ligands. For example, the Ru1-C48-N6 bond angle is 168(2)°
with the isonitrile bent away from a mesityl group of the
C1-NHC into an opening between the mesityl rings of the
C22-NHC ligand. Likewise, the Ru1-C43-N5 bond angle is
172(2)° with the isonitrile bent away from the mesityl ring of
Additional evidence for the identity of complex 4 comes from
reaction with tert-butylisonitrile. At room temperature 4 and

1764 Organometallics, Vol. 28, No. 6, 2009
Lee et al.
Figure 5. d-block orbitals for four-coordinate transition metal
complexes in the tetrahedron, square planar, and sawhorse coor-
dination geometries.
Figure 4. Qualitative d-orbital splitting diagram for [(IMes)2-
Ru(H)(CO)][BAr′4] (4) and (PNPtBu)Ru(Cl).
qualitative correlation diagram linking the d-block orbitals for
these three geometries is shown in Figure 5. [(IMes)2-
Ru(H)(CO)]+ (4) has a Ru(II) metal center with a d6 configu-
ration. This d6 complex is less likely to adopt a tetrahedral
geometry, due to the large energy splitting between the
the C22-NHC ligand into an open pocket. The bending of the
isonitrile ligands are within the range observed for other
isonitrile ligands, which readily distort as a consequence of
packing interactions.77,78 Yu et al. have recently discussed the
softness of the bending modes of the tert-butyl isonitrile ligand.79
Hence, it is plausible to surmise that these ligands will readily
distort as a consequence of packing interactions.
2
2
2
nonbonding pair (dz , dx -y ) and the antibonding triad (dxy, dyz,
and dxz; Figure 5). Additionally, steric repulsion is anticipated
to promote a trans orientation of the two bulky IMes ligands.
In fact, no examples of a tetrahedral structure for related four-
coordinate 14-electron Ru(II) systems have been reported. These
Ru(II) systems adopt either a sawhorse structure,69-72 in which
the open sites of the metal center form C-H · · · Ru agostic
interactions to complete the “six-coordinate” nature, or a square
planarstructurewithoutformationofC-Hagosticinteractions.74,75
Our spectral analysis shows no evidence of C-H agostic
interactions in complex 4. It is also very interesting that 4 is
characterized to be diamagnetic, which contrasts the reported
square planar PNP-Ru-X systems that are suggested to be
paramagnetic triplet species.74,75 Density functional theory
(DFT) calculations were thus carried out to interpret the
geometry and electronic structure of the four-coordinate complex
4.
When adopting a square planar structure without C-H agostic
interactions, a d6 complex has the possibility to be a singlet or
triplet depending on the magnitude of d-orbital splitting (Figure
5). A π-donor ligand may raise the energy of one or more of
the nonbonding d-orbitals closer to that of the dz2 orbital, leading
to a triplet state. However, a π-acidic ligand, such as CO, might
stabilize the nonbonding d-orbitals and increase the d-splitting
energy, resulting in a singlet state (Figure 4). To evaluate the
spin multiplicity of 4 in an assumed square planar geometry
and the π-donor and π-acceptor effects of the ligand on the
ground-state spin multiplicity, DFT calculations were employed
for [(IMes)2Ru(H)(CO)]+ (4) and a hypothetical (IMes)2Ru-
(H)(NH2) (6) complex in both singlet and triplet spin states.
The optimized structures are shown in Figure 6. The optimiza-
tion of singlet 4 from a square planar starting geometry leads
to two minima. One (4SP-S-1) is square planar, while the other
minimum (4SP-S-2) has two trans-oriented C-H agostic interac-
tions to complete a “six-coordinate” octahedral geometry, as
shown in Figure 6. In 4SP-S-2, the Ru · · · CH3 distances are
calculated to be ∼2.7 Å, which is quite similar to those of the
complex [{P(tBu2Me)}2Ru(Ph)(CO)][BAr′4] (with two short
Ru-CH3 distances: 2.87 and 2.88 Å).70 The Ru · · · H distances
for the agostic interaction in 4SP-S-2 are calculated to be short at
Although we have not been able to obtain a crystal of 4
suitable for X-ray diffraction, spectroscopic data, reactivity, and
computational chemistry studies (see below) are consistent with
the formulation of 4 as a monomeric four-coordinate Ru(II)
complex in solution at room temperature. Furthermore, NMR
spectra of 4 are consistent with a diamagnetic system, and
utilization of the Evans NMR method to detect paramagnetic
systems resulted in no difference in chemical shift of internal
and external standards. It is interesting that 4 is apparently
diamagnetic while related four-coordinate Ru(II) systems re-
ported by Caulton et al. (see above) are paramagnetic triplet
species. The triplet ground states for the two complexes reported
by Caulton et al. have been attributed in part to the π-donating
ability of the amido and alkoxo moieties.74,75 Figure 4 depicts
a qualitative molecular orbital diagram for the (PNP)Ru(Cl)
systems reported by Caulton et al. that provides a rationalization
for their high spin electron configuration. The increased energy
of dxz, which takes on antibonding character due to π-interaction
with the amido moiety, results in a small energy gap between
orbitals that possess dz2 and dxz character. In contrast, complex
4 does not possess a π-donor ligand but has a strongly π-acidic
CO ligand. Hence, the Ru-CO back-bonding interaction is
anticipated to lower the energy of dxz and dxy relative to the
PNP-Ru system and thus increase the HOMO-LUMO gap of
4 relative to the Caulton system, which might account for the
low spin ground state. To explore the structure of 4, we
employed DFT calculations.
Computational Studies for [(IMes)2Ru(H)(CO)]+ (4). Four-
coordinate transition metal complexes generally adopt three ideal
geometries: tetrahedron, square planar and sawhorse.80
A
(77) Leach, P. A.; Geib, S. J.; Cooper, N. J. Organometallics 1992, 11,
4367–4370.
(78) Leach, P. A.; Geib, S. J.; Corella II, J. A.; Warnock, G. F.; Cooper,
N. J. J. Am. Chem. Soc. 1994, 116, 8566–8574.
(79) Yu, Y.; Sadique, A. R.; Smith, J. M.; Dugan, T. R.; Cowley, R. E.;
Brennessel, W. W.; Flaschenriem, C. J.; Bill, E.; Cundari, T. R.; Holland,
P. L. J. Am. Chem. Soc. 2008, 130, 6624–6638.
(80) Cirera, J.; Ruiz, E.; Alvarez, S. Inorg. Chem. 2008, 47, 2871–2889.

Ruthenium(II) Hydride Complexes
Organometallics, Vol. 28, No. 6, 2009 1765
Figure 6. Optimized structures for singlet/triplet [(IMes)2Ru(H)(CO)]+ complex (4) and hypothetical (IMes)2Ru(H)(NH2) (6). Subscript SP
and SH denote square planar and sawhorse structures, respectively; subscript S and T indicate singlet and triplet states, respectively. Most
hydrogen atoms on the IMes ligands are omitted for clarity. Calculated free energies are shown in parentheses at 289.15 K and 1 atm (unit:
kcal/mol).
1.89 Å. Nevertheless, as compared with 4SP-S-1, 4SP-S-2 is 0.3
kcal/mol higher in free energy even though it has agostic
interactions. Excluding entropic effects, 4SP-S-2 is calculated to
be 5.4 kcal/mol lower in enthalpy than 4SP-S-1, which indicates
that the lack of observation of C-H agostic interactions for 4
may be a result of a substantial entropic penalty for restraining
the orientation of the IMes ligands. More importantly, DFT
calculations indicate a more stable triplet square planar structure
4
SP-T, which is 10.7 kcal/mol lower in free energy than the
singlet 4SP-S-1. As shown in Figure 5, for an ideal square planar
complex (σ-only ligands), the dz2 is expected to be close in
energy to the nonbonding dxy, dyz and dxz orbitals. Although the
dxy and dxz can be stabilized by the π-acceptor ligand CO, the
dyz orbital remains unperturbed, leading to a nearly unchanged
∆E between dyz and dz2 (dyz may also be stabilized by IMes
ligands, however, the rotation of IMes ligands can also result
in steric repulsion between CO and IMes). The calculated
d-splitting of 4SP-T is only 0.41 eV. Thus, assuming a square
planar geometry, [(IMes)2Ru(H)(CO)]+ is more likely to be a
triplet d6 complex (i.e., paramagnetic). The DFT calculations
suggest that the π-acceptor effect of CO cannot lead to a singlet
4 in a square planar structure, which is contrary to the
experimental observation that the cation [(IMes)2Ru(H)(CO)]+
is diamagnetic. Replacing CO with the π-donor ligand NH2,
we cannot locate a square planar stationary point for the singlet
(IMes)2Ru(H)(NH2) (6). Singlet 6 changed from a square planar
starting geometry to a sawhorse geometry upon full DFT
geometry optimization. For the square planar geometry, the
perturbation due to π-donor effects of NH2 raises the dxz slightly
higher than dz2 in energy, resulting in a small d-splitting, similar
to the case in Caulton’s system.74 As shown in Figure 7, the
d-splitting energy for 6SP-T is calculated to be only 0.15 eV.
Thus, in a square planar geometry, the IMes-Ru(II) system
tends to be paramagnetic with both π-acceptor or π-donor
ligands. This initial set of simulations suggests a possible
resolution to the apparent theory-experiment mismatch vis-a`-
vis complex 4, i.e., a sawhorse geometry.
Figure 7. Perturbed d-block of [(IMes)2Ru(H)(CO)]+ (4) and
hypothetical (IMes)2Ru(H)(NH2) (6) in a square planar geometry.
The d-splittings are obtained from single point RODFT calculations
with the unrestricted DFT optimized structures.
being lower in free energy by 35.5 kcal/mol. This singlet
sawhorse 4SH-S is also more stable than the square planar triplet
4SP-T, lower in free energy by 24.8 kcal/mol. However, the triplet
sawhorse 4SH-T is calculated to be 30.3 kcal/mol higher in free
81
energy than 4SH-S
.
Therefore, the singlet 4SH-S in sawhorse
geometry is calculated to be the most thermodynamically
stable structure for cation [(IMes)2Ru(H)(CO)]+. As depicted
in Figure 8, because of the stabilization of the d-orbitals by
π-acceptor ligands, IMes and CO, the HOMO/LUMO gap of
4
SH-S is calculated to be 3.92 eV, significantly higher than that
of the 4SP-S (2.05 eV). The H-Ru-CO angle of 4SH-S is
calculated to be 84.1°, while that of triplet 4SH-T is 117.0°. The
triplet 4SH-T is thus more akin to an equatorial-vacant trigonal
bipyramid. Typically, d6 diamagnetic complexes have strong
C-H agostic interactions due to the vacant sites on the metal
centers when adopting a sawhorse geometry.69-72 However, for
the cationic complex [(IMes)2Ru(H)(CO)]+, DFT calculations
A sawhorse structure can be viewed as the cis-divacant
fragment of an octahedral complex. The energy of dz2 is thus
expected to be much higher than that of the nonbonding dxy, dyz
and dxz orbitals,,as shown in Figure 5. Therefore, the d6 complex
may tend to be diamagnetic with doubly occupied nonbonding
dxy, dyz and dxz orbitals. The optimized sawhorse structures for
both singlet and triplet 4 are shown in Figure 6. The singlet
(81) The optimized 4SH-T has one small imaginary frequency, 2.85i cm-1
.
Because the free energy difference between 4SH-T and 4SH-S is large, this small
imaginary frequency will not affect our discussion. No further attempts were
made due to the formidable workload of reoptimization.
4SH-S is much more stable than the square planar singlet 4SP-S-1,

1766 Organometallics, Vol. 28, No. 6, 2009
Lee et al.
of the model complex [Ru(Ph)(CO)(PH3)2]+.82 Thus, it is
possible that the presence/absence of π-active (donor or accep-
tor) ligands dictates the ground-state geometry and that the
presence/absence of agostic interactions plays a minor role. But,
the calculations also suggest that (IMes)2Ru(H)(NH2) will adopt
a diamagnetic sawhorse ground state, which indicates that the
square planar paramagnetic Ru(II) complexes may result from
a constrained ligand geometry rather than the presence of a
π-donor ligand. However, experimental results to corroborate
the calculations are not yet available for (IMes)2Ru(H)(NH2).
Computational Studies for [(IMes)2Ru(H)(CO)(N2)]-
[BAr′4] (4-N2). DFT calculations were also performed to
evaluate the formation of the dinitrogen complex 4-N2. The
coordination of N2 to 4 leads to two isomers of 4-N2. As shown
in Scheme 4, one isomer positions N2 trans to the hydride ligand
(4-N2-1), while the second isomer has N2 trans to the carbonyl
(4-N2-2). 4-N2-2 is calculated to be more stable than 4-N2-1 by
3.7 kcal/mol in free energy. The Ru-N bond length in 4-N2-2
is calculated to be 2.089 Å, which is markedly shorter than that
in 4-N2-1 (2.185 Å). The calculated thermodynamic data indicate
a temperature dependent and reversible equilibrium between 4,
4-N2-1 and 4-N2-2. At 273.15 K, the coordination of N2 to 4 to
form 4-N2-2 is calculated to be exergonic by 2.8 kcal/mol;
however, the formation of 4-N2-2 is calculated to be endergonic
by 0.2 kcal/mol at 298.15 K. These calculations are consistent
with experimental observations in which the formation of 4-N2
is only observed at low temperature (at atmospheric pressure
of N2). Additionally, the more stable 4-N2-2 isomer is calculated
to be close in energy to the isomer 4-N2-1. Therefore, it is not
surprising that X-ray data for 4-N2 are complicated by disorder.
Figure 8. Perturbed d-block of the cation [(IMes)2Ru(H)(CO)]+
(4) and hypothetical (IMes)2Ru(H)(NH2) (6) in a sawhorse geometry.
did not locate a stationary point with C-H agostic interactions,
perhaps due to the bulky IMes ligands. As discussed above,
2
even with the more Lewis acidic nonbonding vacant dz in the
singlet state, 4SP-S-2 is less stable than the 4SP-S-1 by 0.3 kcal/
mol. Thus, the combined experimental and computational studies
indicate that the four-coordinate cation [(IMes)2Ru(H)(CO)]+
is a diamagnetic complex, with a sawhorse structure, and without
C-H agostic interactions.
H/D Exchange of Hydride with D2: (IMes)2Ru(Cl)-
(H)(CO) (1) Versus [(IMes)2Ru(H)(CO)][BAr′4] (4). Follow-
ing the reaction of (IMes)2Ru(Cl)(H)(CO) (1) with D2 (30 psi)
at room temperature by 1H NMR spectroscopy reveals the
production of (IMes)2Ru(Cl)(D)(CO) (1-d1) and free HD (1:
Additional support for the congruity between experiment and
theory for complex 4 is the calculated νCO for the singlet
sawhorse structure, which is 1936 cm-1 after scaling for
anharmonicity (compared with the experimental value of 1933
cm-1). The calculated CO absorption energies for the square
planar geometry (with and without agostic interactions) are
between 1982 and 1994 cm-1. Furthermore, the room temper-
1
1:1 triplet, JHD ) 43 Hz, 4.42 ppm) (Scheme 5), which
compares well to free HD (1JHD ) 43 Hz).3,83 At later reaction
times, the production of free H2 (singlet, 4.42 ppm) is also
observed, which likely results from the reaction of HD with 1
to produce 1-d1 and H2. Consistent with the incorporation of
deuterium into the hydride position of 1 to produce 1-d1, the
reaction of 1 with D2 results in the disappearance of the
resonance at -25.4 ppm in the 1H NMR spectrum. Furthermore,
the H/D exchange of the Ru-hydride has been confirmed with
1
ature H NMR spectrum of 4 is consistent with the sawhorse
geometry in which there is rapid rotation (relative to the NMR
time scale) around the Ru-CNHC bonds to render the four
mesityl rings time average equivalent as well as all four )CH
moieties of the NHC ligands. The three methyl groups on each
mesityl ring remain inequivalent, presumably due to hindered
N-C bond rotation.
a
2H NMR spectrum of 1-d1, which shows a singlet at
1
approximately -25 ppm (Figure 9). Consistent with the H
NMR spectrum from the reaction of 1 with D2, the 2H NMR of
1-d1 does not display deuterium incorporation at other ligand
positions.
Previous reports of four-coordinate Ru(II) complexes fall into
two categories: (1) square planar and paramagnetic stabilized
by a π-donor ligand and (2) diamagnetic sawhorse structure
with two C-H agostic interactions to complete an octahedral
coordination sphere. These prior results suggest that the presence
of a strongly π-donating ligand can drive the preferred geometry
from octahedral (or sawhorse if the C-H agostic interactions
are discounted) to square planar, which results in a triplet ground
state. In their report on (PNP)RuCl, Caulton et al. made the
acute observation, “The present structure shows that agostic
interactions are not inevitable in a 14-electron species, but that
a triplet state, with half-filling of two orbitals, is another way
to make the best outcome of an otherwise electron-deficient (14-
electron) situation.”73 The combined experimental and compu-
tational studies of [(IMes)2Ru(H)(CO)]+ (4) suggest that a
diamagnetic 14-electron species is also accessible in the absence
of agostic interactions, a result that was previously suggested
by Caulton et al. based on computational studies on the structure
Reaction of the formally 14-electron cation [(IMes)2-
Ru(H)(CO)][BAr′4] (4) with D2 (30 psi) at room temperature
produces the isotopomer [(IMes)2Ru(D)(CO)][BAr′4] (4-d1) by
1H NMR spectroscopy (Scheme 5). Neither free HD nor H2 is
observed in the 1H NMR at room temperature (see below), but
consistent with the H/D exchange the resonance at -26.4 ppm
disappears. In contrast to 1, in addition to H/D exchange at the
hydride ligand, the reaction of 4 with D2 incorporates deuterium
into the ortho methyl groups of the IMes ligands as observed
1
2
by H and H NMR spectroscopy. This result suggests that an
(82) Huang, D.; Streib, W. E.; Eisenstein, O.; Caulton, K. G. Angew.
Chem., Int. Ed. 1997, 36, 2004–2006.
(83) Maltby, P. A.; Schlaf, M.; Steinbeck, M.; Lough, A. J.; Morris,
R. H.; Klooster, W. T.; Koetzle, T. F.; Srinivas, R. C. J. Am. Chem. Soc.
1996, 118, 5396–5407.

Ruthenium(II) Hydride Complexes
Organometallics, Vol. 28, No. 6, 2009 1767
Scheme 4. Coordination of N2 to Singlet Sawhorse [(IMes)2Ru(H)(CO)]+ (4)a
a
Hydrogen atoms are omitted and the aryl groups of IMes ligands are shown in wire frame for clarity. Free energies are obtained using unscaled
vibrational frequencies in the gas phase at 1 atm and 213.15 K; data in parentheses are obtained at 298.15 K.
multiple uncharacterized (IMes)Ru products, and a decrease in
intensity of resonances due to complex 4.
Careful monitoring of the reaction of 4 with D2 (30 psi)
reveals production of the isotopomer 4-d1, [(IMes)2Ru(D)(CO)]+
after 5 min with negligible deuterium incorporation into the
ortho methyl groups. The system was then frozen and degassed
to remove all D2 and placed under N2 (1 atm). After 15 min at
room temperature, the hydride resonance (-26.4 ppm) returns
1
by H NMR spectroscopy (Scheme 6). These results suggest
that H/D exchange into the ortho methyl groups of the IMes
ligands upon reaction of 4 and D2 likely occurs by initial H/D
exchange at the Ru-H position followed by H/D exchange
between the methyl groups and Ru-D (Scheme 6), which may
occur via metalation of a methyl group of the mesityl ring.
Deuterium should be incorporated into the ortho methyl groups
1
of the IMes ligands; however, the anticipated multiplet by H
NMR spectroscopy of a CH2D group is not observed. This is
likely due to the H/D exchange occurring with all eight ortho
methyl groups of the IMes ligands (two resonances by 1H NMR
spectroscopy), which result in only 2.5% D at each methyl
ligand (20% total deuterium incorporation evenly spread over
8 methyl groups). The rate of H/D exchange into the ortho
methyl groups of the IMes ligands of 4 was determined at 40
°C (30 psi D2) to be approximately 1.4 × 10-4 s-1 (Supporting
Information). A short induction period (approximately 10 min)
was observed prior to significant change at the ortho position
of the IMes ligands, which is likely due to H/D exchange at
the Ru-H position prior to incorporation into the methyl groups.
We were unable to measure the rate accurately over three half-
lives due to a competitive decomposition pathway at prolonged
reaction times. This decomposition precluded attempts to
measure the rate of H/D exchange back to perprotio-
[(IMes)2Ru(H)(CO)][BAr′4] (4) when the solution was degassed
and pressurized with H2 (30 psi). Intramolecular C-H, C-N
and C-C bond activations of RuII bound IMes (and other NHCs)
ligands have been reported: C-C and C-N bond activations
were found to be stoichiometric and lead to new cyclometalated
IMes ligands (C-C activation)47 or new N-coordinated NHC
ligands (C-N activation).48 The intramolecular C-H bond
activation of IMes produces an IMes cyclometalated RuII
complex,47 and other NHCs that show intramolecular C-H
activation have been successfully incorporated into catalytic
C-C bond formation reactions.45,46
Figure 9. 2H NMR of (IMes)2Ru(Cl)(D)(CO) (1-d1) showing Ru-D
resonance at approximately -25 ppm.
Scheme 5. Hydride/Deuteride Ligand Exchange for
Complexes 1 and 4 upon Reaction with D2
1
Variable-temperature H NMR was used to further explore
the lack of observation of free H2/HD upon reaction of 4 with
D2 at room temperature (see above). A toluene-d8 solution of 4
in a J-Young NMR tube was pressurized with D2 (30 psi), and
placed in the NMR probe. At room temperature the spectrum
showed resonances consistent with complex 4-d1. Cooling the
probe from 20 to -60 °C did not result in any observable change
in the spectrum (i.e., no observation of free H2 or HD). The
probe was then warmed from 20 to 80 °C, which did not show
agostic-like species is energetically accessible, even though the
calculations suggest that it may not be a stable minimum. After
3 h at room temperature at 30 psi D2, 20% H/D exchange (by
integration of 1H NMR spectra) is observed at the IMes methyl
groups. Prolonged reaction with D2 results in the production of

1768 Organometallics, Vol. 28, No. 6, 2009
Scheme 6. Reversible H/D Scrambling into the Ortho IMes Methyl Groups of 4
Lee et al.
any observable change in the spectrum; however, the extent of
H/D exchange at the IMes ortho methyl positions was found to
increase (as expected) as the temperature was increased.
In a separate experiment, a J-Young NMR tube containing a
toluene-d8 solution of 4 was pressurized with D2 (30 psi) for
20 min. After the reaction time, complete H/D exchange at the
mechanism for the H/D exchange between 4/D2 to 4-d1 is shown
in Scheme 7, and a similar mechanism can be proposed for
conversion of 1/D2 to 1-d1 with the addition of IMes ligand
dissociation prior to D2 coordination and D-D cleavage. At-
tempts to measure the rate of H/D exchange of 1 with D2 (30
psi) and added free IMes were thwarted due to decomposition
of the reaction solution to uncharacterized species. However,
though only qualitative, the large enhancement of rate of H/D
exchange upon removal of a Cl- anion is consistent with a
dissociative pathway.
1
Ru-H position was observed by H NMR spectroscopy, and
the solution was degassed by freeze-pump-thaw cycles and
backfilled with H2 (30 psi). At room temperature the spectrum
showed resonances consistent with complex 4-d1, and a broad
resonance at 0.62 ppm. Cooling the probe from 20 to -60 °C
resulted in the observation of the resonance at 0.62 ppm
broadening into the baseline. The probe was then warmed from
20 to 80 °C resulted in the observation of the resonance at 0.62
ppm increasing in intensity. These data suggest that the
resonance at 0.62 ppm is a time average of free H2 and the
Ru-H resonances undergoing rapid exchange on the 1H NMR
time scale. Indeed, pressurizing a J-Young NMR tube with H2
(30 psi) containing a toluene-d8 solution of 4 resulted in the
same observations as 4-d1 under H2 pressure. Thus, we suggest
a time average resonance at 0.62 ppm for free H2 and the Ru-H
where we were unable to reach the decoalescence point at low
temperatures and at room and elevated temperatures the rate of
exchange is fast, which is consistent with the lack of observation
of free HD for reaction of 4 with D2.
Catalytic Hydrogenation. Since the hydride complexes 1
and 4 were found to activate D2, we attempted catalytic
hydrogenation reactions. Both 1 and 4 are active for the addition
of H-H bonds across CdC and CdO bonds. As depicted in
Table 2, the olefin substrate scope ranges from terminal to
internal olefins including mono- and disubstituted olefins.
Hydrogenation was also extended to CdO bonds of acetone
and benzaldehyde (Table 3). All hydrogenation reactions were
monitored by 1H NMR spectroscopy, and only complex 1 or 4
was observed during the course of the reaction, which suggests
that the monohydride systems are the catalyst resting states.
Using complex 4 as catalyst, the rates of hydrogenation for
hexenes follow the trend: trans-3-hexene > trans-2-hexene >1-
hexene (entries 2-4, Table 2). In fact, monitoring the hydro-
1
genation of 1-hexene by H NMR spectroscopy reveals initial
We were interested in determining the extent to which
removal of chloride from 1 impacts the rate of hydride/deuteride
ligand exchange. The reaction of 1 with D2 (30 psi) at room
and complete isomerization to trans-2-hexene prior to hydro-
genation (Scheme 8). Due to overlapping resonances, it cannot
be stated with certainty whether trans-2-hexene is further
isomerized to trans-3-hexene prior to hydrogenation, although
this seems likely given the rapid rate of isomerization compared
with rate of hydrogenation. The present results are in agreement
with other monohydride Ru systems that isomerize olefins.37
Attempts at the hydrogenation of benzene with 4 resulted in no
reaction (60 °C, 30 psi H2) or decomposition of 4 (120 °C, 60
psi H2). The hydrogenation of the disubstituted olefin 1,4-
cyclohexadiene occurs at approximately the same rate as the
monosubstituted olefin cyclohexene (entries 6 and 7, Table 2).
During the course of 1,4-cyclohexadiene hydrogenation, cyclo-
hexene is observed as a short-lived intermediate. The hydro-
genation of styrene using D2 in place of H2 results in the
incorporation of deuterium into the ethyl group of ethylbenzene
(eq 8). The 1H NMR spectrum of the reaction product of styrene
with D2 shows broad multiplets at 2.43 and 1.06 ppm for the
methylene and methyl groups of ethylbenzene, respectively.
1
temperature was followed by H NMR spectroscopy over the
course of three hours, at which time conversion to 1-d1 is
complete. A plot of ln[1] versus time gives a linear fit, and the
average kobs from multiple reactions is 1.70(2) × 10-4 s-1. The
reaction of 4 with D2 (30 psi) at room temperature results in
complete conversion to 4-d1 in less than 5 min as determined
by 1H NMR spectroscopy. Thus, at room temperature the half-
life for the conversion of 4/D2 to 4-d1 is less than 150 s
compared to approximately 70 min for the conversion of 1/D2
to 1-d1. Though the rate data for conversion of 4/D2 to 4-d1 is
not quantitative, the reaction is at least 28 times faster than
production of 1-d1 from 1/D2.
For H/D exchange with complex 1, we also probed the impact
of D2 pressure on the reaction rate. Increasing the D2 pressure
was found to not have an impact on the rate of H/D exchange
at the hydride position for complex 1. This suggests that the
rate-determining step may involve IMes ligand dissociation. A

Ruthenium(II) Hydride Complexes
Organometallics, Vol. 28, No. 6, 2009 1769
Scheme 7. Possible Pathways for the Reaction of 4 with D2 to Produce 4-d1
Table 2. Ru-Catalyzed Hydrogenation of Olefins (Unless Noted Otherwise, Reactions Performed in C6D6, at 60 °C, 30 psi H2 and 5 mol % of
Catalyst Relative to Olefin Concentration)
b
a Percent conversations were determined by 1H NMR spectroscopy. TON/sec (10-4). c No observation of aromatic hydrogenation. d Percent
conversation to cyclohexane. e Sixty psi H2 100 °C.
(IMes)(PCy3)Ru(Cl)(H)(CO) the TOF is 3000 h-1 at room
temperature and 4 atm H2.37,38 Complex 1 and 4 were found to
hydrogenate 1-hexene with a TOF of 0.8 and 3.3 h-1, respec-
tively (60 °C and 2 atm H2). At room temperature, 5 mol % 4
was found to hydrogenate/isomerize 1-hexene completely (by
1H NMR) to a mixture of trans-2-hexene (10%) and hexane
The cationic and coordinatively unsaturated complex 4 is a
more active catalyst for hydrogenation than complex 1. For
example, under the same conditions, the hydrogenation of
1-hexene is complete in only 6 h using 4 and after 27 h with
complex 1 as catalyst (entries 1 and 2, Table 2). Both 1 and 4
are significantly less active for the hydrogenation of 1-hexene
compared to the previously reported (PCy3)2Ru(Cl)(H)(CO)37
and (IMes)(PCy3)Ru(Cl)(H)(CO)38 systems. The catalytic hy-
drogenation of 1-hexene using (PCy3)2Ru(Cl)(H)(CO) has a TOF
of 12 000 h-1 at room temperature and 1 atm H2, and for
(90%), at which time significant catalyst decomposition had
occurred. The substantially decreased activity of the bis-IMes
complex 4 compared to the mixed IMes/phosphine, and to an
even greater extent the bis-phosphine complex, is most likely a
combination of steric factors and the strong binding nature of
IMes ligands, which prohibits ligand dissociation and may thus
inhibit olefin coordination. Over the course of catalytic hydro-
genation reactions using 4, there is no evidence of olefin
coordination to the 4-coordinate 14-electron Ru(II) cation, which
further supports the steric argument.

1770 Organometallics, Vol. 28, No. 6, 2009
Lee et al.
Table 3. Ru-Catalyzed Hydrogenation of Carbonyl Groups (All Reactions Performed in C6D6, at 60 °C, 30 psi H2 and 5 mol % of Catalyst
Relative to Carbonyl Concentration)
b
a Percent conversations were determined by 1H NMR spectroscopy. TON/sec (10-4). c Percent conversation to 3-ethyl-benzylacohol. d TOF for 2
equiv of H2.
Scheme 8. Isomerization and Subsequent Hydrogenation of
1-Hexene Mediated by 4
As noted above, 4 isomerizes 1-hexene to trans-2-hexene
prior to hydrogenation. Thus, we studied the reaction of 4 with
trans-2-hexene in the absence of H2 (eq 9). After 20 h, 27%
conversion of trans-2-hexene to trans-3-hexene is observed with
5 mol % 4 at 60 °C, longer times resulted in no further changes
by 1H NMR spectroscopy. Recently, Grotjahn et al. have
reported the remarkable isomerization of dotriacont-31-en-ol to
dotriacontan-2-one catalyzed by a cationic Ru(II) complex,
which moves the CdC double bond 30 C-C linkages.84 The
reaction of 9-decen-1-ol with 5 mol % 4 at 70 °C results in the
complete isomerization to an internal olefin, likely 8-decen-1-
ol, after 2 h (Scheme 9). Prolonged reaction time (24 h) results
in no further changes. In contrast, the Ru(II) catalyst 4, the
Grotjahn system, [CpRu{κ2-P,N-P(iPr)2(C8H13N2)}(NCMe)]-
[PF6] (5 mol %), was found to catalyze the conversion of
9-decen-1-ol (70 °C) to decanal in 84% yield after 4 h.84
Scheme 9. Isomerization of 9-Decen-1-ol Catalyzed by
Complex
4
Compared to the [CpRu{K2-P,N-P(iPr)2-
(C8H13N2)}(NCMe)][PF6] Catalyzed Isomerization
A possible mechanism for the hydrogenation (and isomer-
ization of olefins) is shown in Scheme 10. Assuming that
complex 4 is 4-coordinate Ru(II) with a 14-valence electron
count, olefin association is likely the initial step. If N2 is
coordinated to 4, it is likely very labile (at room temperature,
see above). Olefin insertion would give a 5-coordinate 16
electron complex that could be followed by insertion of the
olefin into the Ru-H bond. After insertion, ꢀ-hydride elimina-
tion/reinsertion/ꢀ-hydride elimination sequences followed by
olefin dissociation provides a pathway for olefin isomerization.
At this time, we do not have data that would allow for us to
discern between H2 oxidative addition and σ-bond metathesis.
The reduction of aldehydes and ketones using H2 in place of
NaBH4 or LiAlH4 (stoichiometric reducing agents) is a widely
employed reaction for both commodity and fine chemical
synthesis due to the atom economical nature of the reaction,
and the ability to produce enantiomerically pure alcohols from
prochiral ketones.85 Ru(II) complexes have been shown to
possess exceptional reactivity for the addition of H2 across CdO
bonds.85-87 Extension of hydrogenation to carbonyl groups by
1 and 4 was explored (Table 3). Complex 4 is less active for
the hydrogenation of carbonyl groups than for olefins, and
complex 1 essentially shows little reactivity for carbonyl
hydrogenation (entry 1, Table 3). Whittlesey et al. have reported
that complex 1 catalyzes the hydrogenation of the CdO group
of para-substituted acetophenones. Similar to our observations
with benzaldehyde, they report less than quantitative conversions
under forcing conditions (75 °C, 150 psi H2, 10 h).61
The hydrogenation of benzaldehyde by 4 to benzyl alcohol
occurs at a slightly slower rate (entry 2, Table 3) than the
hydrogenation of styrene (entry 5, Table 2). The hydrogenation
of acetone by 4 was found to be slowest of all substrates studied
(entry 3, Table 3). To explore the selectivity of hydrogenation
of CdC versus CdO substrates, we attempted the hydrogenation
(86) Sandoval, C. A.; Ohkuma, T.; Muiz, K.; Noyori, R. J. Am. Chem.
Soc. 2003, 125, 13490–13503.
(87) Noyori, R. Asymmetric Catalysis in Organic Synthesis; John Wiley
and Sons: New York, 1994.
(84) Grotjahn, D. B.; Larsen, C. R.; Gustafson, J. L.; Nair, R.; Sharma,
A. J. Am. Chem. Soc. 2007, 129, 9592–9593.
(85) Noyori, R. Angew. Chem., Int. Ed. 2002, 41, 2008–2022.

Ruthenium(II) Hydride Complexes
Organometallics, Vol. 28, No. 6, 2009 1771
Scheme 10. Possible Pathway for Olefin Hydrogenation Mediated by Complex 4
Scheme 11. Stepwise Hydrogenation of 3-Vinyl-Ben-
zaldehyde Catalyzed by 4
a reactive 4-coordinate Ru(II) complex, [(IMes)2Ru(H)(CO)]-
[BAr′4] (4). Facile ligand coordination (e.g., tBuNC), reversible
intramolecular C-H activation and reversible N2 coordination
have been observed for 4. In addition, complex 4 was found to
be active for olefin, ketone, and aldehyde hydrogenation with
kinetic selectivity for CdC over CdO hydrogenation. DFT
calculations are consistent with the temperature dependent and
reversible coordination of N2 to 4.
Previous examples of four-coordinate Ru(II) complexes
include paramagnetic square planar systems that are stabilized
by a π-donor ligand and diamagnetic systems in which two
C-H agostic interactions complete an octahedral coordination
sphere. However, the combined experimental and computational
results that have been presented herein suggest that a diamag-
netic 14-electron species is accessible in the absence of C-H
agostic interactions. As anticipated, initial studies have revealed
an array of reactivity of the four-coordinate complex 4 including
catalytic hydrogenation, intramolecular C-H activation, coor-
dination of dinitrogen and facile coordination of traditional
Lewis bases.
of 3-vinyl-benzaldehyde. Complete hydrogenation of the vinyl
group to produce 3-ethyl-benzaldehyde is observed after 24 h
1
by H NMR spectroscopy (Scheme 11). At this time, there is
1
no evidence (by H NMR spectroscopy) for reduction of the
carbonyl group. The intermediate aldehyde is characterized by
1H NMR spectroscopy with a new aldehyde C-H resonance
(9.72 ppm) compared to the vinyl-aldehyde (9.64 ppm),
3
Experimental Section
resonances consistent with an ethyl group (2.29 ppm, q, JHH
) 8 Hz; 0.95 ppm, t, 3JHH ) 8 Hz) and complete disappearance
of the resonances due to the vinyl group. Prolonged reaction
time (84 total hours) results in the complete hydrogenation of
the carbonyl to produce 3-ethyl-benzylalcohol (entry 4, Table
3). Though complex 4 can hydrogenate both CdC and CdO
substrates, these results suggest that it is kinetically selective
for the CdC functionality. Attempted extension of catalytic
hydrogenation to carboxamides (N-methylacetamide) or esters
(methyl acetate) resulted in no reaction or decomposition of 4,
respectively.
General Methods. Unless otherwise noted, all reactions and
procedures were performed under anaerobic conditions in a nitrogen
filled glovebox or using standard Schlenk techniques. Glovebox
purity was maintained by periodic nitrogen purges and monitored
by an oxygen analyzer {O2(g) < 15 ppm for all reactions}. Hexanes
were purified by passage through two columns of activated alumina.
Acetonitrile and methanol were purified by distillation from calcium
hydride. Pentane was purified by distillation from sodium. Benzene,
tetrahydrofuran, and toluene were purified by distillation from
sodium/benzophenone. CD3CN, C6D6, CDCl3, CD3OD, CD2Cl2, and
C6D5CD3 were degassed via three freeze-pump-thaw cycles and
stored over 4 Å sieves. Photolysis experiments were performed
using a 450 W power supply, 450 W lamp, and a quartz cooling
jacket with flowing water. 1H and 13C NMR spectra were obtained
on either a Varian Mercury 300 MHz or Varian Mercury 400 MHz
Summary and Conclusions
The sterically encumbering and strongly donating IMes
ligands of 1 were utilized to synthesize, isolate and characterize

1772 Organometallics, Vol. 28, No. 6, 2009
Lee et al.
spectrometer (operating frequencies for 13C NMR are 75 and 100
MHz, respectively) and referenced against tetramethylsilane using
residual proton signals (1H NMR) or the 13C resonances of the
deuterated solvent (13C NMR). 19F NMR spectra were obtained on
a Varian 300 MHz spectrometer (operating frequency 282 MHz)
and referenced against an external standard of hexafluorobenzene
(δ ) -164.9). 31P NMR spectra were obtained on a Varian 400
MHz spectrometer (operating frequency 161 MHz) and referenced
137.7, 137.0, 136.7, 129.8, 129.7 (each a s, IMes-aryl), 123.4
(NCHdCHN), 21.6 (s, p-CH3), 19.2 (s, o-CH3), 19.1 (s, o-CH3).
(IMes)2Ru(Cl)2(CO)2 (3). HCl (4 M solution in 1,4-dioxane,
0.050 mL, 0.20 mmol) was added to a yellow solution of 2 (0.055
g, 0.068 mmol) in benzene (20 mL) in a 100 mL round-bottom
flask. The solution was stirred for 1 h at which time the volatiles
were reduced to ∼1 mL in vacuo. Hexanes were added to precipitate
a white solid, which was collected by vacuum filtration, washed
with hexanes, and dried in vacuo (0.047 g, 82% yield). IR (KBr):
νCO ) 2025 and 1955 cm-1. 1H NMR (C6D6, δ): 6.72 (8H total, br
s, IMes aryl), 5.95 (4H total, s, IMes HCdCH), 2.27, 2.14 (36H
total, 2:1 ratio, each a s, IMes -CH3). 13C NMR (CDCl3, δ): 194.7
(s, Ru-CO), 174.5 {s, Ru-C(IMes)}, 138.6, 138.5, 136.6 (each a s,
IMes-Aryl), 125.0 (NCHdCHN), 21.4 (s, p-CH3), 18.8 (s, o-CH3).
MS (EI) Calculated (Found): m/z 801.05 (801, σ ) 0.4 ppm)
{(IMes)2Ru(CO)2(Cl)}+.
2
against an external standard of H3PO4 (δ ) 0). H NMR spectra
were obtained on a Bruker 500 MHz spectrometer (operating
frequency 78 MHz) in C6H6 and referenced against residual C6D6
(δ ) 7.16). Unless otherwise noted NMR spectra were acquired at
room temperature. IR spectra were obtained on a Mattson Genesis
II spectrometer as either thin films on a KBr plate or in a solution
flow cell. Elemental analyses were performed by Atlantic Microlabs,
Inc. [Ru(CO)2Cl2]n, IMes, and NaBAr′4 were prepared according
to reported procedures.88-90 Cyclohexene was degassed via three
freeze-pump-thaw cycles and stored over 4 Å sieves. Styrene
was vacuum distilled from CaH2. Carbon monoxide (99.5%) and
H2 (99.9%) were obtained from MWSC High-Purity Gases and used
as received. D2 (99.8% D) was obtained from Cambridge Isotope
Laboratories and used as received. All other reagents were
purchased from commercial sources and used without further
purification.
[(IMes)2Ru(H)(CO)][BAr′4] (4). NaBAr′4 (0.085 g, 0.096 mmol)
was added to a yellow solution of 1 (0.094 g, 0.120 mmol) in
benzene (20 mL), and the mixture was stirred at room temperature
for 24 h. The resulting orange-yellow solution was filtered through
a plug of Celite. The filtrate was reduced to ∼1 mL in vacuo.
Hexanes were added to precipitate an orange-red solid. After storing
at -20 °C overnight to maximize precipitation, the brick-red solid
was collected by vacuum filtration, washed with hexanes, and dried
1
in vacuo (0.105 g, 54% yield). IR (C6H6): νCO ) 1933 cm-1. H
(IMes)2Ru(Cl)(H)(CO) (1). A yellow heterogeneous solution
of [Ru(CO)2Cl2]n (0.162 g, 0.711 mmol), IMes (0.443 g, 1.46 mmol)
and toluene (20 mL) in a sealed pressure tube was heated in an oil
bath to 100 °C for 24 h. After the heating period, the orange solution
was filtered, and the volatiles were removed from the filtrate in
vacuo to produce an orange residue. The residue was dissolved in
methanol (20 mL) and heated to reflux for 48 h. During the heating
period, an orange precipitate formed. The volatiles were reduced
to approximately 5 mL. After stirring overnight in methanol, the
orange solid was collected by vacuum filtration, washed with
hexanes, and dried in vacuo (0.148 g, 27% yield). IR (KBr): νCO
NMR (C6D6, δ): 8.39 (8H total, br s, o-Ar′), 7.69 (4H total, br s,
p-Ar′), 6.69 (8H total, br s, IMes aryl), 6.00 (4H total, br s, IMes
HCdCH), 2.21, 1.58, 1.50 (36H total, each a s, 1:1:1 ratio IMes
-CH3), -26.43 (1H total, br s, Ru-H) 13C NMR (C6D6, δ): 202.9
(br s, Ru-CO), 188.4 {br s, Ru-C(IMes)}, 163.1 (1:1:1:1 ratio, q,
1JBC ) 50 Hz, B-C), 139.7, 135.8, 135.6, 135.2, 135.0, 130.1, 130.0,
127.4, 123.8, 123.3, 120.2 (each a sharp s, IMes-aryl and BAr′4-
aryl), 131.0, 130.1, 129.6 (each a br s, IMes-aryl and BAr′4-aryl),
1
125.6 (q, JFC ) 271 Hz, BAr′4-CF3), 118.4 (NCHdCHN), 21.2
(s, o-CH3), 17,7 (s, p-CH3). 19F NMR (C6D6, δ): -60.8 (s).
[(IMes)2Ru(H)(CNtBu)2(CO)][BAr′4] (5). CNtBu (0.012 mL,
0.11 mmol) was added to a yellow solution of 4 (0.051 g, 0.032
mmol) in benzene (20 mL) in a 100 mL round-bottom flask. The
solution immediately turned colorless. After stirring for 1 h, the
volatiles were reduced to ∼1 mL in vacuo. Hexanes were added to
precipitate a white solid, which was collected by vacuum filtration,
washed with hexanes, and dried in vacuo (0.046 g, 82% yield).
Crystals suitable for an X-ray diffraction study were grown by
layering a benzene solution of 5 with pentane at room temperature
1
) 1886 cm-1. H NMR (room temperature C6D6, δ): 6.83, 6.79
(8H total, 1:1 ratio, each a br s, IMes aryl), 6.17 (4H total, s, IMes
HCdCH), 2.34, 2.19, 2.08 (36H total, 1:1:1 ratio, each a br s, IMes
-CH3), -25.30 (s, Ru-H). 13C NMR (room temperature C6D6, δ):
2
2
202.2 (d, JCH ) 10 Hz, Ru-CO,), 195.3 {d, JCH ) 10 Hz, Ru-
C(IMes)}, (The CO and C(IMes) resonances were found to be
coupled to the Ru-H; however, when a 13C{1H} was obtained with
only the Ru-H resonance decoupled the CO and C(IMes) resonances
collapsed to singlets.) 137.7, 137.3, 136.7, 136.4, 129.4 (each a s,
IMes-aryl, likely one overlap), 121.9 (NCH)CHN), 21.7 (s, p-CH3),
19.5 (s, o-CH3), 19.4 (s, o-CH3). MS (EI) Calculated (Found): m/z
738.6 (739, σ ) 0.4 ppm) {(IMes)2Ru(H)(CO)}+.
under a N2 atmosphere. IR (KBr): νCN ) 2150 cm-1, 2120 cm-1
,
νCO ) 1975 cm-1(also has a low energy shoulder, likely Ru-H
1
stretch). H NMR (C6D6, δ): 8.42 (8H total, s, o-Ar′), 7.72 (4H
(IMes)2Ru(Cl)(H)(CO)2 (2). Carbon monoxide was gently
bubbled through a yellow solution of 1 (0.078 g, 0.100 mmol) in
benzene (20 mL) in a 100 mL round-bottom Schlenk flask. The
solution immediately turned colorless. After stirring for 1 h at room
temperature, the CO purge was stopped, and the volatiles were
reduced to ∼1 mL in vacuo. Hexanes were added to precipitate a
white solid, which was collected by vacuum filtration, washed with
hexanes, and dried in vacuo (0.066 g, 82% yield). Crystals suitable
for an X-ray diffraction study were grown by layering a benzene
solution of 2 with pentane at room temperature under a N2
atmosphere. IR (KBr): νRu-H ) 1941 cm-1, νCO ) 2035 cm-1, 1905
cm-1. 1H NMR (C6D6, δ): 6.74, 6.73 (8H total, 1:1 ratio, each a br
s, IMes aryl), 6.07 (4H total, s, IMes HCdCH), 2.25, 2.19 (36H
total, 1:2 ratio, each a s, IMes -CH3), -4.22 (1H, s, Ru-H) 13C
NMR (C6D6, δ): 204.1 (s, Ru-CO), 185.5 {s, Ru-C(IMes)}, 139.5,
total, s, p-Ar′), 6.68, 6.67 (8H total, 1:1 ratio, each a br s, IMes
aryl), 5.87 (4H total, s, IMes HCdCH), 2.13, 1.76 (36H total, 1:2
ratio, each a s, IMes -CH3), 0.98, 0.77 {18H total,1:1 ratio, each a
s, -C(CH3)3}, -7.99 (1H total, s, Ru-H). 13C NMR (C6D6, δ): 200.8
(s, Ru-CO), 183.0 {s, Ru-C(IMes)}, 162.0 (1:1:1:1 ratio, q, 1JBC
)
50 Hz, B-C), 139.3, 138.4, 136.0, 135.8, 135.0, 129.6, 129.5, 126.6,
124.2, 123.0, 119.4, 117.6 (each a s, IMes-aryl, BAr′4-aryl, Ru-
CNtBu, NCHdCHN), 129.0 (q, 2JFC ) 28 Hz, meta-BAr′4), 124.8
1
(q, JFC ) 271 Hz, BAr′4-CF3), 57.5, 56.4 {each a s, -C(CH3)3}
30.2, 29.7, 21.2, 19.0, 18.8 {each a s, -C(CH3)3 o-CH3, p-CH3}.
19F NMR (C6D6, δ): -60.8 (s). Anal. Calcd For
C85H79B1F24N6O1Ru: C, 57.74; H, 4.47; N, 4.75. Found: C, 57.26;
H, 4.45; N, 4.76.
Reaction of [Ru(CO)2Cl2]n With IMes in Toluene-d8 and
CD3OD. A yellow heterogeneous solution of [Ru(CO)2Cl2]n (0.037
g, 0.16 mmol), IMes (0.110 g, 0.36 mmol) and toluene (5 mL) in
a sealed pressure tube was heated in an oil bath to 100 °C for 24 h.
(88) Grocott, S. C.; Wild, S. B. Inorg. Chem. 1982, 21, 3535–3540.
(89) Arduengo III, A. J.; Dias, H. V. R.; Harlow, R. L.; Kline, M. J. Am.
Chem. Soc. 1992, 114, 5530–5534.
(90) Yakelis, N. A.; Bergman, R. G. Organometallics 2005, 24, 3579–
3591.
1
After the heating period, an aliquot was analyzed by H NMR
spectroscopy, which showed a mixture of (IMes)Ru products (none
of which corresponded to 1) and a single resonance upfield of 0

Ruthenium(II) Hydride Complexes
Organometallics, Vol. 28, No. 6, 2009 1773
ppm at -4.5 ppm. The mixture was filtered, and the orange filtrate
was equally divided between two 38 mL pressure tubes. CH3OH
and CD3OD (5 mL) were added separately to the reaction mixtures.
The tubes were sealed and heated in an oil bath to 80 °C for 24 h.
After the heating period, aliquots were removed from each, and
the volatiles were removed in vacuo. The residues from each
reaction were dissolved in C6D6. H NMR spectroscopy of both
samples (CH3OH and CD3OD reactions) revealed complex 1
without incorporation of deuterium.
benzene (20 mL) in a 100 mL round-bottom flask. The solution
was stirred for 3 h, after which the volatiles were reduced to ∼1
mL in vacuo. Hexanes were added to precipitate a cream-yellow
solid, which was collected by vacuum filtration, washed with
hexanes, and dried in Vacuo (0.057 g). IR (KBr): νCO ) 1939 cm-1
.
1
The H NMR spectrum (CDCl3) revealed two complexes with no
resonances upfield of 0 ppm; however, we were unable to separate
the complexes despite several attempts at chromatographic separa-
tion. (b) Reaction of (IMes)2Ru(Cl)(H)(CO) (1) with CCl4. A yellow
solution of 1 (0.010 g, 0.013 mmol) in CCl4 (20 mL) in a 100 mL
round-bottom flask was heated to reflux. After 3 h, analysis of the
reaction mixture by IR (thin film on KBr) showed no reaction. (c)
Reaction of (IMes)2Ru(Cl)(H)(CO) (1) with N-chlorosuccinimide.
N-Chlorosuccinimide (0.003 g, 0.023 mmol) was added to a yellow
solution of 1 (0.016 g, 0.021 mmol) in toluene (5 mL) in a 100
mL round-bottom flask. The solution was heated to reflux for 3 h.
1
1
Variable Temperature H NMR Spectroscopy of (IMes)2-
Ru(Cl)(H)(CO) (1). (a) An orange solution of complex 1 (0.006
g, 0.008 mmol) in toluene-d8 (0.6 mL) was transferred to a screw-
cap NMR tube. 1H NMR spectra were obtained at 10 °C intervals
from 25 to 95 °C, which resulted in only minor changes from the
room temperature 1H NMR spectrum. No observation of the methyl
resonances coalescing into a 2:1 ratio was made, which would be
indicative of rapid rotation of both the N-Cmesityl and Ru-CNHC
bonds on the NMR time scale. (b) The solution was then cooled in
the NMR probe. 1H NMR spectra were obtained at 20 °C intervals
ranging from 20 to -60 °C. At -20 °C, two resonances due to the
methyl resonances began to decoalesce. At -40 °C, the resonances
due to the aryl hydrogens began to decoalesce. At -60 °C, a near
idealized Cs molecular symmetry for 1 was observed where the
aryl hydrogens were 4 singlets, the backbone hydrogens of the
imidazole rings resonate as 2 singlets, and the methyl resonances
were observed as 5 singlets (possibly 6 resonances due to overlap
with the solvent). (c) An orange solution of complex 1 (0.006 g,
0.008 mmol) in methylene chloride-d2 (0.6 mL) was transferred to
1
Analysis of the reaction mixture by IR (thin film on KBr) and H
NMR spectroscopy showed the formation of multiple (IMes)Ru
systems. (d) Reaction of (IMes)2Ru(Cl)2(CO)2 (3) with Me3NO.
Me3NO (0.025 g, 0.333 mmol) was added to a yellow solution of
3 (0.012 g, 0.015 mmol) in toluene (10 mL) in a 100 mL round-
bottom flask. The solution was heated to reflux for 8 h. Analysis
of the reaction mixture by IR (thin film on KBr) showed no reaction.
1
Variable Temperature H NMR Spectroscopy of [(IMes)2-
Ru(H)(CO)][BAr′4] (4). (a) An orange solution of complex 4 (0.012
g, 0.007 mmol) in toluene-d8 (0.6 mL) was transferred to a screw-
cap NMR tube. 1H NMR spectra were obtained at 10 °C intervals
ranging from 25 to -80 °C. At 0 °C, two complexes (2:1 ratio)
were observed. At temperatures below 0 °C, no further changes
are observed until the temperature reaches -80 °C where all
resonances broadened into the baseline. Warming back to room
1
an NMR tube. H NMR spectra were obtained at 5 °C intervals
ranging from 0 to -70 °C. At -10 °C, a set of IMes methyl
resonance is at the coalescence temperature. Further cooling allowed
this resonance to decoalesce into two singlets. These resonances
were followed until no further change was observed in chemical
shift or peak width at half-height (-65 °C). These data were used
to calculate a rate constant for Ru-CIMes bond rotation using line
broadening and the coalescence temperature.
1
temperature resulted in the original spectrum. b) H NMR spectra
were then obtained at 20 °C intervals ranging from 20 to 80 °C.
As the temperature was increased only the single room temperature
complex was present, and the resonances sharpened slightly
including the Ru-H resonance.
Reactivity of (IMes)2RuCl(H)(CO) (1) with Acetonitrile
and Trimethylphosphine. (a) As a control experiment, an orange
solution of complex 1 (0.006 g, 0.008 mmol) in C6D6 (0.6 mL)
was transferred to a screw-cap NMR tube. The solution was heated
to 90 °C for 12 h, which resulted in no reaction by 1H NMR
spectroscopy. (b) An orange solution of complex 1 (0.006 g, 0.008
mmol) in C6D6 (0.6 mL) was transferred to a screw-cap NMR tube.
Acetonitrile (3 µL, 0.057 mmol) was added. The solution was heated
Reversible formation of [(IMes)2Ru(H)(CO)(N2)][BAr′4]
(4-N2) Under N2 Pressure. An orange solution of complex 4 (0.007
g, 0.004 mmol) in C6D5CD3 (0.6 mL) was transferred to a J-Young
NMR tube. The tube was evacuated by two freeze-pump-thaw
cycles and backfilled with N2 (80 psi) at room temperature. The
room temperature 1H NMR spectrum showed resonances consistent
with complex 3 and complex 4-N2 (as observed by low temperature
1H NMR spectroscopy) in an approximate 3:1 ratio favoring
complex 4. The tube was evacuated by five freeze-pump-thaw
cycles and back-filled with argon (40 psi). The room temperature
1H NMR spectrum showed complex 4 with no evidence of complex
4-N2.
1
to 90 °C for 12 h with no reaction by H NMR spectroscopy. (c)
An orange solution of complex 1 (0.010 g, 0.01 mmol) in C6D6
(0.6 mL) was transferred to a screw-cap NMR tube. Trimethylphos-
phine (7 µL, 0.079 mmol) was added. The solution was heated to
1
90 °C for 12 h with no reaction by H NMR spectroscopy.
1
Variable Temperature H NMR Spectroscopy of [(IMes)2-
NMR Scale Preparation of (IMes)2Ru(Cl)(D)(CO)2 (2-d1). An
orange solution of complex 1 (0.008 g, 0.009 mmol) in C6D6 (0.6
mL) was transferred to a J-Young NMR tube. The tube was
evacuated by two freeze-pump-thaw cycles, pressurized with D2
(30 psi) and heated to 60 °C. After 6 h, complete conversion to the
Ru(H)(CO)][BAr′4] (4) Under Argon. An orange solution of
complex 4 (0.013 g, 0.008 mmol) in C6D5CD3 (0.6 mL) was
transferred to a J-Young NMR tube. The tube was evacuated by
four freeze-pump-thaw cycles and backfilled with argon (1 atm)
1
at room temperature. The room temperature H NMR spectrum
1
Ru-D isotopomer (2-d1) was observed by H NMR spectroscopy.
showed resonances consistent with complex 4 including the Ru-H
and one unassigned resonance at 3.5 ppm. The NMR probe was
cooled from 20 to -40 °C, and spectra were obtained at 20 °C
intervals. At low temperature only one complex was present in
solution as compared to two complexes when the solution of 4 is
under a dinitrogen atmosphere.
NMR Scale Preparation of (IMes)2Ru(Cl)(D)(CO) (1-d1). An
orange solution of complex 1 (0.007 g, 0.009 mmol) in C6D6 (0.6
mL) was transferred to a J-Young NMR tube. The tube was
evacuated by two freeze-pump-thaw cycles, pressurized with D2
(30 psi) and heated to 60 °C. After 6 h, complete conversion to the
Ru-D isotopomer (1-d1) and free H2 and HD were observed by 1H
NMR spectroscopy. Incorporation of deuterium into the methyl
Incorporation of deuterium into the methyl groups of the IMes
ligands was not observed. IR analysis (thin film on KBr) showed
disappearance of the absorption at 1941 cm-1
.
Reaction of [(IMes)2Ru(H)(CO)][BAr′4] (4) with [Bu4N]-
[Cl]. An orange solution of complex 4 (0.006 g, 0.004 mmol) in
C6D6 (0.6 mL) was added to [Bu4N][Cl] (0.003 g, 0.011 mmol),
stirred for 5 min and transferred to an NMR tube. Quantitative
1
formation of complex 1 was observed by H NMR spectroscopy.
Attempts to produce (IMes)2Ru(Cl)2(CO) from (IMes)2-
Ru(Cl)(H)(CO) (1). (a) Reaction of (IMes)2Ru(Cl)(H)(CO) (1) with
HCl. HCl (4 M solution in 1,4-dioxane, 0.050 mL, 0.20 mmol)
was added to a yellow solution of 1 (0.062 g, 0.080 mmol) in

1774 Organometallics, Vol. 28, No. 6, 2009
Lee et al.
groups of the IMes ligands was not observed. The volatiles were
removed in vacuo, and the sample was dried overnight. The residue
was taken up in C6H6 (0.6 mL) and transferred to an NMR tube.
2H NMR spectroscopy showed a singlet at -25 ppm (referenced
temperature NMR spectroscopy was identical to that described
immediately above.
Rate of Incorporation of Deuterium into (IMes)2Ru-
(Cl)(H)(CO) (1) and [(IMes)2Ru(H)(CO)][BAr′4] (4). (a) A 0.013
M stock solution of 1 (0.031 g, 0.040 mmol) in C6D6 (3 mL) was
prepared. Aliquots (0.5 mL) of the stock solution were transferred
to three separate J-Young NMR tubes along with hexamethyldisi-
loxane (2 µL, 0.0094 mmol) as an internal standard. The NMR
tubes were degassed by two freeze-pump-thaw cycles and back-
filled with D2 (30 psi). The reactions were monitored by 1H NMR
spectroscopy. For all data acquisition, the spectral window was 8
ppm to -28 ppm with a 10 s pulse delay. The disappearance of
the hydride resonance was measured as a function of time. Each
sample a plot of ln[hydride resonance integration] versus time gave
a straight line, and a kobs was found by taking the average of the
three plots (see Supporting Information). (b) An orange solution
of complex 4 (0.014 g, 0.0087 mmol) in C6D5CD3 (0.6 mL) was
transferred to a J-Young NMR tube. The tube was evacuated by
two freeze-pump-thaw cycles and pressurized with D2 (30 psi)
at room temperature. After 5 min, the Ru-H resonance had
2
to residual C6D6). The H NMR spectrum was acquired overnight
using a 10 s pulse delay.
NMR Scale Preparation of [(IMes)2Ru(D)(CO)][BAr′4]
(4-d1). An orange solution of complex 4 (0.010 g, 0.0062 mmol)
in C6D6 (0.6 mL) was transferred to a J-Young NMR tube. The
tube was evacuated by two freeze-pump-thaw cycles and pres-
surized with D2 (30 psi) at room temperature. After 3 h, complete
conversion to the Ru-D isotopomer (4-d1) was observed by 1H NMR
spectroscopy. In addition, incorporation of deuterium into the methyl
groups of the IMes ligand(s) was observed (approximately 20% D
incorporation after 3 h). The volatiles were removed in vacuo, and
the sample was dried overnight. The residue was taken up in C6H6
2
(0.6 mL) and transferred to an NMR tube. H NMR spectroscopy
showed only a singlet at 1.7 ppm for D incorporation into the IMes
Me groups (referenced to residual C6D6) with no observable
resonance in the region of -20 to -30 ppm, which is consistent
1
1
completely disappeared by H NMR spectroscopy. Thus, t1/2 e 1
with the very broad Ru-H resonance for 4 observed by H NMR
spectroscopy. The 2H NMR spectrum was acquired overnight using
a 10 s pulse delay time.
min.
Rate Dependence on D2 for H/D Exchange at the Hydride
Position of (IMes)2Ru(Cl)(H)(CO) (1). A 0.013 M stock solution
of 1 (0.121 g, 0.156 mmol) in C6D6 (12 mL) was prepared. Aliquots
(0.5 mL) of the stock solution were transferred to three separate
J-Young NMR tubes along with hexamethyldisiloxane (2 µL,
0.0094 mmol) as an internal standard. The NMR tubes were
degassed by two freeze-pump-thaw cycles and back-filled with
D2. Each set of three reactions was done at 15 psi, 45 and 60 psi
1
Variable Temperature H NMR Spectroscopy of [(IMes)2-
Ru(H)(CO)][BAr′4] (4) Under D2 Pressure. An orange solution
of complex 4 (0.014 g, 0.0087 mmol) in C6D5CD3 (0.6 mL) was
transferred to a J-Young NMR tube. The tube was evacuated by
two freeze-pump-thaw cycles and pressurized with D2 (30 psi)
at room temperature. At room temperature, 1H NMR spectroscopy
showed complex 3 and no resonance for the Ru-H. The probe
was cooled from 20 to -60 °C, and spectra were obtained at 10
°C intervals. As the probe was cooled there was no observable
change in the 1H NMR spectrum. As the probe was warmed (20 to
1
D2. The reactions were monitored by H NMR spectroscopy. For
all data acquisition the spectral window was 8 ppm to -28 ppm,
and a 10 s pulse delay time was used. The disappearance of the
hydride resonance was measured as a function of time. For each
sample, a plot of ln[hydride resonance integration] versus time gave
a straight line, and a kobs was found by taking the average of the
three plots (see Supporting Information).
1
80 °C) there was no observable change in the H NMR spectrum
of 4.
1
Variable Temperature H NMR Spectroscopy of [(IMes)2-
Ru(D)(CO)][BAr′4] (4-d1) Under H2 Pressure. An orange solution
of complex 4 (0.017 g, 0.011 mmol) in C6D5CD3 (0.6 mL) was
transferred to a J-Young NMR tube. The tube was evacuated by
two freeze-pump-thaw cycles and pressurized with D2 (30 psi)
at room temperature. After 20 min, 1H NMR spectroscopy showed
complete disappearance of the Ru-H resonance of 4. The tube was
evacuated by two freeze-pump-thaw cycles and pressurized with
Sample Hydrogenation Reaction (Cyclohexene). NaBAr′4
(0.006 g, 0.007 mmol) was added to a yellow solution of 1 (0.005
g, 0.006 mmol) in C6D6 (0.6 mL) in a screw-cap vial. After stirring
for 5 min, the solution was transferred to a J-Young NMR tube
along with hexamethyldisiloxane (2 µL, 0.0094 mmol) as an internal
standard. The olefin (cyclohexene; 0.013 mL, 0.128 mmol) was
added to the NMR tube. The NMR tube was degassed by two
freeze-pump-thaw cycles and back-filled with H2 (30 psi) and
heated to 60 °C. The reactions were monitored periodically by 1H
NMR spectroscopy until either the reaction had reached completion
(complete consumption of olefin) or the reaction had stopped.
1
H2 (30 psi) at room temperature. At room temperature H NMR
spectroscopy showed complex 4, no resonance for the Ru-H, and
a broad singlet at 0.62 ppm assigned as a time average between
free dihydrogen and Ru-coordinated hydrogen species due to rapid
exchange. The probe was cooled from 20 to -60 °C with 1H NMR
spectra obtained at 10 °C intervals. As the probe was cooled the
broad resonance at 0.62 ppm broadened into the baseline, which is
consistent with the assignment of the resonance at 0.62 ppm as the
average of a rapid exchange between free dihydrogen and Ru-
coordinated hydrogen; however, at -60 °C the slow exchange
regime was not reached. Warming the sample solution (20 to 80
°C) resulted in sharpening of the resonance at 0.62 ppm (broad at
room temperature). These data are consistent with a time-averaged
resonance for free dihydrogen and coordinated hydrogen ligands
at 0.62 ppm and explain the inability to observe free HD from the
reaction of D2 with complex 4.
Hydrogenation of 1-Hexene at Room Temperature and 60
psi H2. NaBAr′4 (0.011 g, 0.012 mmol) was added to a yellow
solution of 1 (0.007 g, 0.009 mmol) in C6D6 (0.6 mL) in a screw-
cap vial. After stirring for 5 min, the solution was transferred to a
J-Young NMR tube along with hexamethyldisiloxane (2 µL, 0.0094
mmol) as an internal standard. The olefin (1-hexene; 0.025 mL,
0.200 mmol) was added to the NMR tube. The NMR tube was
degassed by two freeze-pump-thaw cycles and back-filled with
H2 (30 psi) and heated to 60 °C. The reactions were monitored
periodically by 1H NMR spectroscopy until the reaction had
stopped.
Isomerization of Trans-2-hexene. NaBAr′4 (0.010 g, 0.010
mmol) was added to a yellow solution of 1 (0.010 g, 0.011 mmol)
in C6D6 (0.6 mL) in a screw-cap vial. After stirring for 5 min, the
solution was transferred to a screw-cap NMR tube along with
hexamethyldisiloxane (2 µL, 0.0094 mmol) as an internal standard.
Trans-2-hexene (0.026 mL, 0.207 mmol) was added to the NMR
tube, and the solution was heated to 60 °C. The reaction was
monitored periodically by 1H NMR spectroscopy until equilibrium
1
Variable Temperature H NMR Spectroscopy of [(IMes)2-
Ru(H)(CO)][BAr′4] (4) Under H2 Pressure. An orange solution
of complex 4 (0.010 g, 0.0061 mmol) in C6D5CD3 (0.6 mL) was
transferred to a J-Young NMR tube. The tube was evacuated by
two freeze-pump-thaw cycles and pressurized with H2 (30 psi)
at room temperature. At room temperature, 1H NMR spectroscopy
showed complex 4 and a broad singlet at 0.62 ppm. Variable

Ruthenium(II) Hydride Complexes
Organometallics, Vol. 28, No. 6, 2009 1775
between trans-2-hexene and trans-3-hexene was reached (no other
organic products were observed by H NMR spectroscopy).
Stevens (SBK) valence basis sets triplet-zeta and effective core
potentials were employed for ruthenium atom. Standard 6-31G
basis set were used for the carbon and hydrogen atoms of the four
2,4,6-trimethylphenyl substituents in IMes ligands.96 Standard
6-311G** basis sets were used for all other atoms for the calculated
systems. The largest system is composed of 725 contracted Gaussian
basis functions contracted from primitive Gaussians. All systems
were fully optimized unrestricted Kohn-Sham formalism without
symmetry constraint. Analytic calculations of the energy Hessian
were performed to confirm species as minima and to obtain
enthalpies and free energies (using unscaled vibrational frequencies)
in the gas phase at 1 atm and 298.15 K/213.15 K.
1
Isomerization of 9-decen-1-ol. NaBAr′4 (0.008 g, 0.009 mmol)
was added to a yellow solution of 1 (0.006 g, 0.008 mmol) in C6D6
(0.6 mL) in a screw-cap vial. After stirring for 5 min, the solution
transferred to a screw-cap NMR tube along with hexamethyldisi-
loxane (2 µL, 0.0094 mmol) as an internal standard. 9-Decen-1-ol
(0.030 mL, 0.168 mmol) was added to the NMR tube, and the
solution was heated to 60 °C. The reaction was monitored
periodically by 1H NMR spectroscopy. After 2 h, 1H NMR showed
complete conversion to an isomer containing an internal CdC bond,
which is likely 8-decen-1-ol. Prolonged heating (24 h) resulted in
only minor additional changes.
Hydrogenation of Styrene With D2. NaBAr′4 (0.013 g, 0.014
mmol) was added to a yellow solution of 1 (0.011 g, 0.014 mmol)
in C6D6 (0.6 mL) in a screw-cap vial. After stirring for 5 min the
solution transferred to a J-Young NMR tubes along with hexam-
ethyldisiloxane (2 µL, 0.0094 mmol) as an internal standard. Styrene
(0.030 mL, 0.272 mmol) was added to the NMR tube. The NMR
tube was degassed by two freeze-pump-thaw cycles and back-
filled with D2 (30 psi) and heated to 60 °C. The reactions were
monitored periodically by 1H NMR spectroscopy until the reaction
had reached completion (complete consumption of olefin) to
produce PhCHDCH2D.
Computational Methods. All calculations were carried out using
the Gaussian03 package,91 with the B3LYP functional (Becke’s
three-parameter hybrid functional92 using the LYP correlation
functional containing both local and nonlocal terms of Lee, Yang,
and Parr)93 and VWN (Slater local exchange functional94 plus the
local correlation functional of Vosko, Wilk, and Nusair).95 The
Acknowledgment. T.B.G. acknowledges The Office of
Basic Energy Sciences, U.S. Department of Energy (DE-
FG02-03ER15490) for financial support. T.R.C. acknowl-
edges The Office of Basic Energy Sciences, U.S. Department
of Energy (DE-FG02-03ER15387) for financial support. Z.K.
thanks the China Scholarship Council (No. 2007102840) for
their support of his research at the University of North Texas.
We thank Professor Hanna Gracz (NCSU) for assistance with
2H NMR experiments, Ms. Joanna Webb for assistance with
experiments, Mr. Matthew M. Lyndon (NCSU), and Dr.
George Dubay (Duke University) for assistance with mass
spectrometry experiments and Professor Kenneth G. Caulton
(Indiana University) for valuable discussions.
Supporting Information Available: Complete data for X-ray
diffraction studies of complexes 2, 4-N2 and 5, sample kinetic plots,
1
and sample H NMR spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.
(91) Frisch, M. J.; et al. Gaussian 03, revision D.01; Gaussian, Inc.,
Wallingford, CT, 2004.
(92) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652.
(93) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. 1988, B37, 785–789.
(94) Kohn, W.; Sham, L. Phys. ReV. 1965, 140, 1133–1138.
(95) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Chem. 1980, 58, 1200–
1211.
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(96) Stevens, W. J.; Krauss, M.; Basch, H.; Jasien, P. G. Can. J. Chem.
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