Axial interaction of the [Ru2(CO)4]2+ core with the aryl C-H bond: Route to cyclometalated compounds involving a metal-metal-bonded diruthenium unit

Article abstract of DOI:10.1021/om060774+

Room-temperature activation of the aromatic C-H bond by the [Ru ₂(CO)₄]²⁺ core has been achieved. The reactions of 2-phenyl-1,8-naphthyridine (phNP) and 2-(2,5-dimethyl-3-furyl)-1,8- naphthyridine (Me₂-fuNP) with [Ru₂(CO)₄(MeCN) ₆][BF₄]₂ in dichloromethane provide the agostic-cyclometalated compounds [Ru₂(phNP)(C₆H ₄-NP)(CO)₄][BF₄] (1) and [Ru ₂(Me₂fuNP)(C₄OMe₂-NP)(CO) ₄][BF₄] (2), respectively. In both compounds, one of the ligands is ortho-metalated, while the second ligand is engaged in an agostic interaction. The ortho metalation is preferred over the potential S coordination for 2-(2-thienyl)-1,8-naphthyridine (thNP), yielding [Ru₂(thNP) (C₄H₂S-NP)(CO)₄][BF₄] (3). In acetonitrile, the compound [Ru₂(thNP)₂(CO) ₄][BF₄]₂ (4) is obtained exclusively. The donation of a C-H bonding electron pair to the Ru-Ru σ* LUMO and back-donation from the filled Ru-Ru π* orbital to the C-H σ* orbital cause facile C-H bond cleavage. In contrast, the isoelectronic [Rh₂]⁴⁺ provides the agostic compounds [Rh₂-(OAc)₃(phNP)Cl] (5) and [Rh₂(L) (η¹-L)(OAc)₂(CH₃CN)₂][BF ₄]₂ (L = phNP, nplNP (2-(2-naphthyl)-1,8-naphthyridine) for compounds 6 and 7, respectively). The molecular structures of compounds 1-3, 5, and 7 have been established by X-ray crystallography.

Full text of DOI:10.1021/om060774+

6054
Organometallics 2006, 25, 6054-6060
Axial Interaction of the [Ru2(CO)4]2+ Core with the Aryl C-H
Bond: Route to Cyclometalated Compounds Involving a
Metal-Metal-Bonded Diruthenium Unit
Sanjib K. Patra and Jitendra K. Bera*
Department of Chemistry, Indian Institute of Technology, Kanpur, 208016 India
ReceiVed August 24, 2006
Room-temperature activation of the aromatic C-H bond by the [Ru2(CO)4]2+ core has been achieved.
The reactions of 2-phenyl-1,8-naphthyridine (phNP) and 2-(2,5-dimethyl-3-furyl)-1,8-naphthyridine (Me2-
fuNP) with [Ru2(CO)4(MeCN)6][BF4]2 in dichloromethane provide the agostic-cyclometalated compounds
[Ru2(phNP)(C6H4-NP)(CO)4][BF4] (1) and [Ru2(Me2fuNP)(C4OMe2-NP)(CO)4][BF4] (2), respectively.
In both compounds, one of the ligands is ortho-metalated, while the second ligand is engaged in an
agostic interaction. The ortho metalation is preferred over the potential S coordination for 2-(2-thienyl)-
1,8-naphthyridine (thNP), yielding [Ru2(thNP)(C4H2S-NP)(CO)4][BF4] (3). In acetonitrile, the compound
[Ru2(thNP)2(CO)4][BF4]2 (4) is obtained exclusively. The donation of a C-H bonding electron pair to
the Ru-Ru σ* LUMO and back-donation from the filled Ru-Ru π* orbital to the C-H σ* orbital cause
facile C-H bond cleavage. In contrast, the isoelectronic [Rh2]4+ provides the agostic compounds [Rh2-
(OAc)3(phNP)Cl] (5) and [Rh2(L)(η1-L)(OAc)2(CH3CN)2][BF4]2 (L ) phNP, nplNP (2-(2-naphthyl)-1,8-
naphthyridine) for compounds 6 and 7, respectively). The molecular structures of compounds 1-3, 5,
and 7 have been established by X-ray crystallography.
Scheme 1
Introduction
The cyclometalation of aromatic hydrocarbons continues to
attract considerable interest, providing valuable insight into C-H
bond activation processes.1 Mononuclear Pd, Pt, Rh, Ir, and Ru
complexes are employed frequently for cyclometalation.2 This
work examines the use of a metal-metal-bonded dimetal unit
in the activation of the C-H bond. Cyclometalation reactions
of phosphine-based ligands at equatorial sites of the dirhodium
unit have been reported at elevated temperature.3 The axial
interaction of the [Ru-Ru]2+ unit with the aryl C-H bond has
been investigated in the synthesis of cyclometalated compounds,
as shown in Scheme 1.
sponding to a formal Ru-Ru single bond, has been employed
for this study. The ligands used include 2-phenyl-1,8-naphthy-
ridine (phNP), 2-(2,5-dimethyl-3-furyl)-1,8-naphthyridine (Me2-
fuNP), and 2-(2-thienyl)-1,8-naphthyridine (thNP) (Chart 1).
Room-temperature activation of the C-H bond and subsequent
formation of cyclometalated compounds containing the diru-
thenium unit are presented herein. Similar reactions with the
isoelectronic [Rh2]4+ unit are also reported.
The diruthenium complex [Ru2(CO)4(CH3CN)6][BF4]2 of
metal-metal valence configuration4 σ2π4δ2δ*2π*4σ*0, corre-
* To whom correspondence should be addressed. E-mail: jbera@
iitk.ac.in.
Results and Discussion
(1) (a) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals, 3rd ed.; Wiley: New York, 2001. (b) Crabtree, R. H. Dalton Trans.
2001, 2437. (c) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507. (d)
Shilov, A. E.; Shul’pin, G. B. Chem. ReV. 1997, 97, 2879. (e) Arndtsen, B.
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(e) Omae, I. Coord. Chem. ReV. 1980, 32, 235. (f) Omae, I. Chem. ReV.
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Prieto, J.; Sanau´, M. Organometallics 2006, 25, 5113. (b) Lloret, J.; Estevan,
F.; Lahuerta, P.; Hirva, P.; Pe´rez-Prieto, J.; Sanau´, M. Organometallics 2006,
25, 3156. (c) Bieger, K.; Estevan, F.; Lahuerta, P.; Lloret, J.; Pe´rez-Prieto,
J.; Sanau´, M.; Siguero, N.; Stiriba, S. E. Organometallics 2003, 22, 1799.
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A.; Tocher, D. A.; Tocher, J. H. Organometallics 1985, 4, 8.
Axial Interaction of the [Ru2(CO)4]2+ Unit with the Aryl
C-H Bond. The aromatic C-H bond is placed at a site trans
to the Ru-Ru single bond by utilizing a 1,8-naphthyridine (NP)
ligand having the appropriate group covalently attached at the
2-position.5 Reactions of phNP, Me2fuNP, and thNP with
[Ru2(CO)4(CH3CN)6][BF4]2 in dichloromethane at room tem-
perature resulted in the formation of the red cyclometalated
compounds [Ru2(phNP)(C6H4-NP)(CO)4][BF4] (1), [Ru2(Me2-
fuNP)(C4OMe2-NP)(CO)4][BF4] (2), and [Ru2(thNP)(C4H2S-
NP)(CO)4][BF4] (3), respectively. The molecular structures of
1-3 have been established by X-ray crystallography. The
important metrical parameters are compared in Table 1.
(4) (a) Cotton, F. A.; Murillo, C. A.; Walton. R. A. Multiple Bonds
Between Metal Atoms, 3rd ed.; Springer: New York, 2005. (b) Cotton, F.
A.; Walton. R. A. Multiple Bonds Between Metal Atoms, 2nd ed.; Clarendon
Press: Oxford, U.K., 1993.
(5) Patra, S. K.; Sadhukhan, N.; Bera, J. K. Inorg. Chem. 2006, 45, 4007.
10.1021/om060774+ CCC: $33.50 © 2006 American Chemical Society
Publication on Web 11/18/2006

Axial Interaction of the [Ru2(CO)4]2+ Core
Organometallics, Vol. 25, No. 26, 2006 6055
Chart 1
The structure of 1 reveals that the “Ru2(CO)4” unit is spanned
by two naphthyridine ligands. One of the phNP ligands is
cyclometalated, involving Ru2 bonded to the ortho carbon atom
C20 of the phenyl fragment and N2 of the naphthyridine unit
(Figure 1). The distal N1 atom is bonded to the other ruthenium.
The Ru1-Ru2 and Ru2-C20 distances are 2.692(1) and 2.074-
(5) Å, respectively, and the Ru1-Ru2-C20 angle is 160.52-
(14)°. The second phNP ligand bridges the diruthenium unit at
equatorial sites, and the ortho hydrogen of the phenyl appendage
is located in the vicinity of Ru1. The metrical parameters of
Ru1-H40 ) 2.398 Å (calculated), Ru1-C40 ) 2.786(5) Å,
and Ru1-H40-C40 ) 104.0(3)° are indicative of an agostic
interaction.6 The torsion angle (N4-C38-C39-C40) of 50.2-
(7)° between the phenyl and NP planes is the manifestation of
the agostic configuration7 of the C-H bond with the diruthenium
unit. The corresponding value for the ortho-metalated phNP is
7.8(7)°.
Figure 1. ORTEP diagram (50% probability thermal ellipsoids)
of the cationic unit [Ru2(phNP)(C6H4-NP)(CO)4]+ in compound 1
with important atoms labeled. Hydrogen atoms, except for the
agostic hydrogen, are omitted for the sake of clarity.
1
The H NMR of compound 1 displays two distinct sets of
aromatic signals for the two ligands. The most unusual signal
appears as a doublet at δ 9.29 ppm, representing a significant
downfield shift as compared to the signal for the free ligand.
The shift is in the opposite direction of what is normally
observed for the hydrogen strongly bonded to a metal. Thummel
et al.8 reported a similar downfield shift of the hydrogen lying
at the axial site of the isoelectronic [Rh2]4+ unit and offered
several possible rationales. We assign the signal to the agostic
hydrogen and attribute the downfield shift to the withdrawal of
electron density from the C-H bond to the vacant Ru-Ru σ*
orbital. The 13C{1H} NMR exhibits a characteristic signal for
the carbon atom directly bonded to the ruthenium at δ 181.2
ppm. All other aromatic carbons appear in the region of δ 121-
161 ppm, whereas the carbons of CO exhibit signals in the
region δ 200-207 ppm.
With Me2fuNP, a similar agostic-cyclometalated compound
was isolated, [Ru2(Me2fuNP)(C4OMe2-NP)(CO)4][BF4] (2) (Fig-
ure 2), the key structural parameters of which are comparable
to those of 1 with Ru1-Ru2 ) 2.6875(6) Å and Ru2-C20 )
2.066(5) Å. The Ru1-H40 (calculated) and Ru1-C40 distances
of 2.358 and 2.741(2) Å and Ru1-H40-C40 angle of 103.6-
Figure 2. ORTEP diagram (50% probability thermal ellipsoids)
of the cationic unit [Ru2(Me2fuNP)(C4OMe2-NP)(CO)4]+ in com-
pound 2 with important atoms labeled. Hydrogen atoms, except
for the agostic hydrogen, are omitted for the sake of clarity.
(1)° are marginally shorter than those observed in compound
1. The N3-C38-C39-C40 torsion angle of 37.93(1)° is
1
significantly shorter than the corresponding angle in 1. In H
NMR, the agostic hydrogen appears as a doublet at δ 9.35 ppm,
in addition to two sets of aromatic signals for the two ligands.
In both compounds 1 and 2, metal insertion into the C-H
bond occurs at one of the ligands, while the second ligand is
engaged in an agostic interaction. Prolonged heating in refluxing
Table 1. Relevant Metrical Parameters of Compounds 1-3
1
2
3
Bond Distances (Å)
2.6875(6)
2.066(5)
Ru-Ru
2.6924(9)
2.074(5)
2.6938(7)
2.100(6)
2.5864(17)
2.175(5), 2.170(5),
2.161(5), 2.188(5)
1.848(7)-1.879(7)
Ru-C(ax)
Ru-S(ax)
Ru-N(eq)
2.163(4), 2.161(4),
2.143(4), 2.218(4)
1.854(6)-1.864(5)
2.162(4), 2.177(4),
2.184(4), 2.185(4)
1.854(6)-1.857(5)
Ru-C(CO)
Bond Angles (deg)
Ru-Ru-C(ax)
Ru-Ru-S(ax)
Ru-Ru-N(eq)
160.52(14)
159.48(14)
158.7(2)
163.74(4)
84.61(11), 85.92(11),
83.62(11), 82.15(11)
87.18(15), 88.11(15)
79.21(18), 88.10(17)
91.4(2), 95.1(2)
86.35(11), 84.75(10),
82.26(11), 82.57(11)
83.07(15), 86.29(15)
79.03(17), 87.85(17)
89.3(2), 95.9(2)
82.20(13), 83.10(13),
84.15(13), 85.38(13)
86.62(19), 84.43(19)
78.8(2), 86.4(2)
93.1(3), 97.7(3)
N-Ru-N
C-Ru-N
C(ax)-Ru-C(CO)
Torsion Angles (deg)
3.5(6)
N-C-C-Ca
7.8(7)
5.3(10)
a Angle between the planes of NP and the cyclometalated aromatic ring.

6056 Organometallics, Vol. 25, No. 26, 2006
Patra and Bera
Scheme 2
dichloroethane does not lead to a second metalation, and only
the monometalated product was identified from the NMR and
absorption spectra (vide infra).
evidence for the agostic configuration of the C-H bond to the
dirhodium unit (Rh2-H20 ) 2.462 Å (calculated), Rh2-C20
) 2.822(3) Å, Rh2-H20-C20 ) 102.4(2)° and N2-C18-
C19-C20 ) 47.0(5)°). The Rh1-Rh2 and the Rh1-Cl1
distances are 2.3947(4) and 2.4593(9) Å, respectively (Table
2).
The use of [Rh2(OAc)2(MeCN)6][BF4]2, involving the labile
ligands MeCN, did not result in the expected product [Rh2-
(phNP)2(OAc)2][BF4]2. The isolated product is [Rh2(phNP)(η1-
The thienyl appendage in thNP offers two possibilities: axial
S coordination to the metal and metal insertion into the ortho
C-H bond. In dichloromethane, the red cyclometalated com-
pound [Ru2(thNP)(C4H2S-NP)(CO)4][BF4] (3) was isolated as
the major product, in addition to a small amount of the yellow
[Ru2(thNP)2(CO)4][BF4]2 (4). In acetonitrile, however, com-
pound 4 was obtained exclusively, with no trace of 3 (Scheme
2). This result shows that noncoordinating solvents are preferred
over coordinating solvents for the C-H bond activation. The
ortho metalation of one thNP over the potential S coordination
was unambiguously confirmed from the X-ray structure of 3
(Figure 3).
The second thNP prefers axial S ligation and not C-H
coordination to ruthenium. The relevant metrical parameters are
Ru1-Ru2 ) 2.694(1) Å, Ru2-C20 ) 2.100(6) Å, and Ru1-
Ru2-C20 ) 158.7(2)° (Table 1). The torsion angle (N3-C38-
C39-S2) between the S-ligated thienyl and NP planes is
35.2(8)°. The corresponding value for the ortho-metalated thNP
is 5.3(10)°. The 1H NMR of compound 3 displays two distinct
sets of aromatic signals for the two ligands. The identical
environments of the two ligands around the dimetal unit in
compound 4 are reflected in the 1H NMR spectrum, exhibiting
one set of aromatic signals. The NMR and UV-vis absorption
data suggest a structure similar to that of [Ru2(fuNP)2(CO)4]-
[BF4]2 (fuNP ) 2-(2-furyl)-1,8-naphthyridine), reported earlier.5
Both of the thNP ligands in compound 4 are engaged axially
through S coordination (Scheme 2).
Figure 3. ORTEP diagram (30% probability thermal ellipsoids)
of the cationic unit [Ru2(thNP)(C4H2S-NP)(CO)4]+ in compound
3 with important atoms labeled. Hydrogen atoms are omitted for
the sake of clarity.
Agostic Compounds with the [Rh2]4+ Unit. The successful
cleaving of the C-H bond by the [Ru2(CO)4]2+ core prompted
us to use the isoelectronic [Rh2]4+ precursor. The weakening
of the C-H bond at axial site of the [Rh2]4+ unit was suggested
from the NMR data.8 Reaction of phNP with Rh2(OAc)4 in
refluxing 1,2-dichloroethane and subsequent crystallization in
dichloromethane resulted in the formation of [Rh2(OAc)3-
(phNP)Cl] (5), incorporating only one phNP in the dirhodium
core. The molecular structure of 5, depicted in Figure 4, provides
(6) (a) Tenorio, M. J.; Mereiter, K.; Puerta, M. C.; Valerga, P. J. Am.
Chem. Soc. 2000, 122, 11230. (b) Baratta, W.; Herdtweck, E.; Rigo, P.
Angew. Chem., Int. Ed. 1999, 38, 1629. (c) Huang, D.; Streib, W. E.;
Bollinger, J. C.; Caulton, K. G.; Winter, R. F.; Scheiring, T. J. Am. Chem.
Soc. 1999, 121, 8087. (d) Crabtree, R. H. Angew. Chem., Int. Ed. Engl.
1993, 32, 789. (e) Brookhart, M.; Green, M. L. H.; Wong, L. L. Prog.
Inorg. Chem. 1988, 36, 1.
(7) Toner, A. J.; Gru¨ndemann, S.; Clot, E.; Limbach, H.-H.; Donnadieu,
B.; Sabo-Etienne, S.; Chaudret, B. J. Am. Chem. Soc. 2000, 122, 6777.
(8) Thummel, R. P.; Lefoulon, F.; Williamson, D.; Chavan, M. Inorg.
Chem. 1986, 25, 1675.
Figure 4. ORTEP diagram (50% probability thermal ellipsoids)
of the molecular structure of [Rh2(phNP)(OAc)3Cl] (5) with
important atoms labeled. Hydrogen atoms, except for the agostic
hydrogen, are omitted for the sake of clarity.

Axial Interaction of the [Ru2(CO)4]2+ Core
Organometallics, Vol. 25, No. 26, 2006 6057
Table 2. Relevant Metrical Parameters of Compounds 5 and
7
Table 3. Absorption and Emission Spectral Dataa for
Compounds 1-7 at Room Temperature
5
7
emission
compd
absorption λmax, nm (log ꢀb)
λ
max, nm
Bond Distances (Å)
Rh-Rh
2.3947(4)
2.4615(9)
2.199(6)
1
2
3
4c
5
6
7
248 (4.55) 324 (4.52) 434 (3.78)
252 (4.34) 338 (4.12) 420 (3.79) 525 (3.26)
272 (4.67) 342 (4.58) 362 (4.59) 505 (3.46) 406, 472
264 (4.43) 342 (4.36) 354 (4.36)
231 (4.37) 284 (4.46) 314 (4.06) 484 (3.57)
230 (4.74) 264 (4.71) 310 (4.66) 342 (4.65)
232 (4.87) 320 (4.72) 392 (sh)
415
435
Rh-N(ax)
Rh-Cl(ax)
Rh-N(eq)
2.4593(9)
2.040(3), 2.010(3)
2.044(5), 1.975(6),
2.002(6), 1.985(6)
2.014(4)-2.042(5)
410
378
392
385
Rh-O(eq)
2.033(2)-2.049(2)
Bond Angles (deg)
a Absorption and emission studies were done in dichloromethane unless
Rh-Rh-N(ax)
Rh-Rh-Cl(ax)
Rh-Rh-N(eq)
169.51(15)
otherwise noted. b ꢀ is given in units of cm3 mol-1 cm-1 c Absorption and
.
176.81(2)
88.14(8), 89.69(8)
93.89(17), 88.63(16),
100.45(18), 89.3(2)
90.4(2), 91.3(2)
88.7(2)-91.2(2),
173.9(2)-178.7(2)
emission studies were done in acetonitrile.
N-Rh-N
O-Rh-N(eq)
effects of the aromatic ring current of the NP ligand.9 A careful
examination of the X-ray structure reveals the nonequivalent
interactions of the methyl protons with the NP rings. Similar
behavior is also noted for compound 6.
The agostic protons in dirhodium compounds exhibit larger
downfield shifts compared to the [Ru2]2+ compounds reported
herein. Metalation, however, was not accomplished, even after
prolonged heating of compounds 5-7 in refluxing 1,2-dichlo-
roethane.
Absorption and Emission Spectra. The reactions described
above provide access to new types of cyclometalated compounds
that contain a metal-metal-bonded system. The absorption data
of the compounds are compiled in Table 3. The absorption
spectra of the compounds 1-4 display strong bands in the far-
UV region at 248, 252, 272, and 264 nm, respectively, assigned
to dimetal-based π* f σ* transitions.10 The lower energy
transitions in the region 320-420 nm are likely to be from
[Ru2]2+ bond orbitals to empty π* orbitals largely based on the
ligands. These values are comparable to those observed in [Ru2-
(CO)4L2]2+, where L denotes NP-based ligands with furyl,
thiazolyl, and pyridyl appendages at the 2-position.5 The most
salient feature for the ortho-metalated compounds 1-3 is the
appearance of weak and broad absorption peaks at 434, 525,
and 505 nm, respectively, which are apparently absent in similar
compounds but do not contain an Ru-C bond.11 These are
assigned to MLCT transitions typical of the cyclometalated
diruthenium compounds. The lack of a similar low-energy band
in 4 confirms that the thienyl units are not ortho-metalated but,
rather, coordinated axially through the S centers.
89.63(11), 92.09(10)
phNP)(OAc)2(CH3CN)2][BF4]2 (6). The 1H NMR of compound
6 exhibits two sets of signals for the bridging and axially
coordinated phNP and shows no C-H activation but a sub-
stantial weakening of the aryl C-H bond. The doublet at δ 10.18
ppm indicates an agostic hydrogen lying close to the dirhodium
internuclear axis.8 An analogous agostic compound, [Rh2-
(nplNP)(η1-nplNP)(OAc)2(CH3CN)2][BF4]2 (7), has been syn-
thesized from the reaction of [Rh2(OAc)2(MeCN)6][BF4]2 and
nplNP. The structure has been determined by X-ray crystal-
lography (Figure 5), and the important metrical parameters are
given in Table 2. In this compound, the dirhodium unit possesses
one nplNP ligand bridging the dimetal core, with a second nplNP
ligand acting as a monodentate axial ligand involving one of
the N atoms of the NP unit. The ortho “H” atom of the naphthyl
appendage of the bridging NP is in the vicinity of Rh2. The
relevant structural parameters are Rh1-Rh2 ) 2.4615(9) Å,
Rh2-H20 ) 2.499 Å (calculated), Rh2-C20 ) 2.654(9) Å,
and Rh2-H20-C20 ) 88.7(1)°. The N2-C18-C19-C20
torsional angle of 48.9(11)° reveals an agostic configuration of
the naphthyl C-H bond with the [Rh2]4+ core. The correspond-
ing angle of the axially coordinating nplNP is 1.9(10)°. The
methyl protons of the two coordinated acetonitriles display four
resonances with intensities 2:1:1:2. Three of the resonances are
shifted upfield, and this shift is attributed in large part to the
The absorption spectra of compounds 5-7 exhibit multiple
strong absorptions in the range of 230-400 nm. These are
assigned as metal-based π* f σ* and MLCT transitions
associated with the NP ligands. Compound 5, which bears an
axial chloride, exhibits an additional low-energy absorption at
484 nm. The UV-vis absorptions of [Rh2]4+ complexes are
known to vary widely with the axial donors.10,12
The emission spectra of cyclometalated compounds contain-
ing C∧N-coordinating ligands are of considerable interest.13 All
of the compounds are luminescent in degassed solution at room
temperature, and the emission data are provided in Table 3.
Excitation wavelengths are selected on the basis of absorption
(9) Tikkanen, W. R.; Binamira-Soriaga, E.; Kaska, W. C.; Ford, P. C.
Inorg. Chem. 1984, 23, 141.
(10) Martin, D. S., Jr.; Webb, T. R.; Robbins, G. A.; Fanwick, P. E.
Inorg. Chem. 1979, 18, 475.
(11) Li. H.-C.; Chou, P.-T.; Hu, Y-H.; Cheng, Y.-M.; Liu, R.-S.
Organometallics 2005, 24, 1329.
(12) (a) Clark, R. J. H.; Hempleman, A. J. Inorg. Chem. 1989, 28, 746.
(b) Cotton, F. A.; Felthouse, T. R. Inorg. Chem. 1981, 20, 584.
(13) (a) Dixon, I. M.; Collin, J.-P.; Flamigni, L.; Encinas, S.; Barigelletti,
F. Chem. Soc., ReV. 2000, 29, 385. (b) Balzani, V.; Juris, A.; Venturi, M.;
Campagna, S.; Serroni, S. Chem. ReV. 1996, 96, 759.
Figure 5. ORTEP diagram (30% probability thermal ellipsoids)
of the cationic unit [Rh2(nplNP)(η1-nplNP)(OAc)2(CH3CN)2]2+ in
compound 7 with important atoms labeled. Hydrogen atoms, except
for the agostic hydrogen, are omitted for the sake of clarity.

6058 Organometallics, Vol. 25, No. 26, 2006
Patra and Bera
Scheme 3
orbital to accommodate electron density from the second C-H
σ bond once the first metalation is achieved.17 The inability of
the isoelectronic [Rh2(OAc)2]2+ core to cleave the C-H bond
is attributed to the higher positive charge on Rh, resulting in
weak back-bonding from the dirhodium unit to the C-H σ*
orbital.
Conclusion
spectra. The diruthenium cyclometalated compounds 1 and 2
display broad emissions centered at 415 and 435 nm, respec-
tively. The lowest energy emission of 3 appears at 472 nm, in
addition to a peak at 406 nm. The diruthenium compound 4
shows emission at 410 nm. It should be noted here that
compound 3 involves C-ligated and S-ligated thienyl units at
axial sites of the Ru-Ru core, whereas compound 4 contains
only S-ligated thienyls. The electronic transitions responsible
for luminescence in these complexes have been assigned to a
The present work endorses Chisholm’s assertion18 with regard
to the scope of the dimetal unit. The highlight of this work is
the room-temperature cleavage of the aromatic C-H bond by
the [Ru2(CO)4]2+ core in dichloromethane, leading to cyclo-
metalated compounds containing an Ru-Ru single bond. The
metal insertion into the C-H bond occurs at one of the aryl
units located at a site trans to the Ru-Ru bond. The second
aryl C-H is engaged in an agostic interaction. Noncoordinating
solvents are preferred over acetonitrile for C-H bond activation.
The [Rh2]4+ unit containing acetates as ancillary ligands provides
only the agostic compounds. The agostic interactions recognized
in the isolated compounds offer insight into the mechanism of
these reactions. The electronic basis for the advantage of the
dimetal unit over its monometal congener is suggested. It is
our expectation that this work will open up a new area of
organometallic chemistry that employs metal-metal-bonded
dimetal units.
3
mixture of metal-to-ligand charge-transfer (MLCT) and (π-
π*) ligand states.14 The dirhodium compounds exhibit emissions
at much higher energy in the region of 378-392 nm.
Possible Reaction Pathway. The agostic configuration of
the C-H bond at the axial site of the dimetal core, as reflected
in the X-ray structures of the compounds, strongly suggests that
the metalation proceeds through the intermediacy of the “agostic
complex”. The deprotonation of the agostic intermediate and
the elimination of HBF4 during metalation most likely occurs
via the electrophilic pathway recently proposed by Milstein.15
The coordinatively saturated Ru center does not support a
possible “oxidative addition/reductive elimination” mechanism.1f
We offer a theoretical framework based on orbital interactions
to explain the propensity of the [Ru2(CO)4]2+ core to activate
the C-H bond. The DFT calculation on the [Ru2(3-MeNP)2-
(CO)4]2+ unit (3-MeNP ) 3-methyl-1,8-naphthyridine) confirms
that the LUMO is a metal-based σ* orbital resulting from the
Experimental Section
General Methods. All reactions with metal complexes were
carried out under an atmosphere of purified nitrogen using standard
Schlenk-vessel and vacuum line techniques. Glasswares were flame-
dried under vacuum prior to use. Solvents were dried by conven-
tional methods, distilled over nitrogen, and deoxygenated prior to
use.19 RuCl3‚nH2O (39% Ru) was purchased from Arora Matthey,
Calcutta, India. The compound [Ru2(CO)4(CH3CN)6][BF4]2 was
synthesized by following a procedure similar to the synthesis of
2
antibonding interaction of the two dz orbitals and the HOMO
and HOMO-1 are closely spaced Ru-Ru π* orbitals originating
from out-of-phase dxz-dxz and dyz-dyz interactions.5 The C-H
bond at the site trans to the Ru-Ru bond donates the bonding
electron pair to the Ru-Ru σ* LUMO, and the back-donation
occurs from the filled Ru-Ru π* to the C-H σ* orbital, as
depicted in Scheme 3. The combination of these two interactions
results in C-H bond cleavage. The extent of back-donation from
the high-energy Ru-Ru dπ* orbital is anticipated to be higher
in comparison to the electron flow from the “nonbonding” dπ
orbital of the monometal species. This explains the advantage
of the [Ru-Ru]2+ unit over mononuclear ruthenium complexes,
which often require high thermal energy for cyclometalation.16
Notably, the subsequent metalation of the second ligand does
not occur, despite the presence of another Ru-Ru dπ* orbital
fully available for back-donation. The reason probably lies with
the fact that the Ru-Ru σ* orbital is no longer a good acceptor
21
[Ru2(CO)4(CH3CN)6][PF6]2.20 The compounds Rh2(OAc)4 and
22
[Rh2(OAc)2(CH3CN)6][BF4]2 were synthesized according to the
literature procedures. The ligands phNP, Me2fuNP, thNP, and nplNP
were prepared by the Friedlander condensation of 2-aminonicoti-
naldehyde with the corresponding acyl derivatives.23 Synthetic
procedures and NMR data for the ligands are provided as Sup-
porting Information. Infrared spectra were recorded in the range
4000-400 cm-1 on a Vertex 70 Bruker spectrophotometer on KBr
1
pellets. H NMR spectra were obtained on a JEOL JNM-LA 400
1
MHz spectrometer. H NMR chemical shifts were referenced to
the residual hydrogen signal of the deuterated solvents. Elemental
analyses were performed on a Thermoquest EA1110 CHNS/O
analyzer. Electronic absorptions were measured on a Perkin-Elmer
Lambda-20 spectrophotometer. Emission spectral data at room
temperature were obtained with a Perkin-Elmer LS 50B lumines-
(17) The destabilization of a metal-metal σ* orbital due to axial
coordination has been reported. See: Norman, J. G., Jr.; Kolari, H. J. J.
Am. Chem. Soc. 1978, 100, 791.
(18) Chisholm, M. H. Anything One Can Do, Two Can Do, Too - and
It’s More Interesting. ACS Symp. Ser. 1981, No. 155, 17.
(19) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of
Laboratory Chemicals, 2nd ed.; Pergamon Press: 1980.
(20) Klemperer, W. J.; Zhong, B. Inorg. Chem. 1993, 32, 5821.
(21) Rempel. G. A.; Smith, L. H.; Wilkinson, G. Inorg. Synth. 1972,
13, 90.
(22) Pimblett, G.; Garner, C. D.; Clegg, W. J. Chem. Soc., Dalton Trans.
1986, 1257.
(23) (a) Reddy, K. V.; Mogilaiah, K.; Sreenivasulu, B. J. Indian Chem.
Soc. 1986, 63, 443. (b) Thummel, R. P.; Lefoulon, F.; Cantu, D.;
Mahadevan, R. J. J. Org. Chem. 1984, 49, 2208. (c) Majewicz, T. C.;
Caluwe, P. J. Org. Chem. 1974, 39, 720. (d) Hawes, E. M.; Wibberley, D.
G. J. Chem. Soc. 1966, 315.
(14) (a) Wilkinson, A. J.; Goeta, A. E.; Foster, C. E.; Williams, J. A.
Inorg. Chem. 2001, 43, 6513. (b) Lamansky, S.; Djurovich, P.; Murphy,
D.; Abdel-Razzaq, F.; Lee, H.-E.; Adachi, C.; Burrows, P. E.; Forrest, S.
R.; Thompson, M. E. J. Am. Chem. Soc. 2001, 123, 4304. (c) Sprouse, S.;
King, K. A.; Spellane, P. J.; Watts, R. J. J. Am. Chem. Soc. 1984, 106,
6647.
(15) Vigalok, A.; Uzan, O.; Shimon, L. J. W.; Ben-David, Y.; Martin,
J. M. L; Milstein, D. J. Am. Chem. Soc. 1998, 120, 12539.
(16) (a) Gupta, P.; Dutta, S.; Basuli, F.; Peng, S.; Lee, G.; Bhattacharya,
S. Inorg. Chem. 2006, 45, 460. (b) Pearson, P.; Kepert, C. M.; Deacon, G.
B.; Spiccia, L.; Warden, A. C.; Skelton, B. W.; White, A. H. Inorg. Chem.
2004, 43, 683. (c) Hadadzadeh, H.; DeRosa, M. C.; Yap, G. P. A.; Rezvani,
A. R.; Crutchley, R. J. Inorg. Chem. 2002, 41, 6521. (d) Ryabov, A. D.;
Sukharev, V. S.; Alexandrova, L.; Lagadec, R. L.; Pfeffer, M. Inorg. Chem.
2001, 40, 6529. (e) Bonnefous, C.; Chouai, A.; Thummel, R. P. Inorg. Chem.
2001, 40, 5851.

Axial Interaction of the [Ru2(CO)4]2+ Core
Organometallics, Vol. 25, No. 26, 2006 6059
Table 4. Crystallographic Data and Refinement Parameters for Compounds 1-3, 5 and 7
1‚2CH2Cl2
2‚CH2Cl2
3
5‚CH2Cl2
7‚3CH2Cl2
empirical formula
C34H23BCl4F4-
N4O4Ru2
982.31
triclinic
P1h
8.7967(18)
13.448(3)
15.832(3)
86.44(3)
88.89(3)
71.43(3)
1771.9(6)
2
C33H25BCl2F4-
N4O6Ru2
933.42
orthorhombic
Pbca
11.6387(7)
20.9611(13)
28.5005(17)
C28H15BF4N4-
O4Ru2S2
824.51
monoclinic
P21/c
10.9954(14)
20.666(3)
15.687(2)
C21H19Cl3N2-
O6Rh2
707.55
monoclinic
P21/c
16.2584(8)
8.3000(4)
19.3585(10)
C47H42B2Cl6F8-
N6O4Rh2
1347.01
monoclinic
P21/c
11.6974(17)
22.563(3)
21.130(3)
formula wt
cryst syst
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
106.961(2)
109.4310(10)
95.951(3)
6953.0(7)
8
1.783
1.095
3696
3409.5(8)
4
1.606
1.067
1616
2463.5(2)
4
1.908
1.705
1392
5546.8(14)
4
1.613
0.958
2688
Z
F
calcd, g cm-3
1.841
1.221
968
µ, mm-1
F(000)
no. of rflns
collected
indep
obsd (I > 2σ(I))
no. of variables
goodness of fit
final R indices (I > 2σ(I))a
R1
10 218
7045
5354
478
38 819
7092
6130
468
18 812
6945
4689
401
13 893
5015
4598
304
36 413
13 579
8697
680
1.067
1.000
1.241
0.961
1.065
0.0496
0.1095
0.0597
0.1120
0.0610
0.1411
0.0355
0.0928
0.0866
0.1777
wR2
R indices (all data)a
R1
0.0724
0.1208
0.0729
0.1166
0.0924
0.1543
0.0386
0.0949
0.1353
0.2016
wR2
2
2
2
2
2 2
a R1 ) ∑||Fo| - |Fc||/∑|Fo| with Fo > 2σ(Fo ). wR2 ) [∑w(|Fo | - |Fc |)2/∑|Fo | ]1/2
.
cence spectrometer. Degassed dichloromethane or acetonitrile
solvent was used for the emission measurements in the concentra-
tion range of 10-5-10-6 M.
characterized as the cyclometalated complex [Ru2(thNP)(C4H2S-
NP)(CO)4][BF4] (3). Yield: 42 mg (68%). H NMR (CD2Cl2, δ):
1
9.07 (dd, 1H), 8.49 (m, 2H), 8.16 (dd, 1H), 8.03 (m, 3H), 7.90
(dd, 1H), 7.86 (d, 1H). 7.73 (m, 1H), 7.56 (dd, 1H), 7.53 (d, 1H),
7.57 (q, 1H), 7.18 (dd, 1H), 7.10 (q, 1H). IR (KBr, cm-1): ν(CO)
2028, 1956; ν(BF4-) 1060. Anal. Calcd for C28H15N4S2O4BF4Ru2:
C, 40.79; H, 1.83; N, 6.80; S, 7.78. Found: C, 41.10; H, 1.98; N,
6.94; S, 7.55.
The bright yellow product was characterized as [Ru2(thNP)2-
(CO)4][BF4]2 (4). Yield: 14 mg (20%). 1H NMR (CD3CN, δ): 8.58
(d, 2H), 8.52 (d, 2H), 8.14 (d, 2H), 8.06 (d, 2H), 7.88 (d, 2H),
7.77 (m, 2H), 7.54 (d, 2H), 7.28 (d, 2H). IR (KBr, cm-1): ν(CO)
2061, 1985; ν(BF4-) 1068. Anal. Calcd for C28H16N4S2O4B2F8-
Ru2: C, 36.86; H, 1.77; N, 6.14; S, 7.03. Found: C, 36.63; H,
1.87; N, 6.28; S, 6.95.
Synthesis of [Ru2(phNP)(C6H4-NP)(CO)4][BF4] (1). A dichlo-
romethane solution (10 mL) of phNP (19 mg, 0.092 mmol) was
added dropwise to a dichloromethane solution (15 mL) of [Ru2-
(CO)4(MeCN)6][BF4]2 (30 mg, 0.041 mmol), and the solution was
stirred for 4 h at room temperature. The red solution was
subsequently concentrated, and ether was added to induce precipita-
tion. The red solid was collected by filtration, washed with ether,
and dried under vacuum. Yield: 29 mg (87%). 1H NMR (CD2Cl2,
δ): 9.29 (dd, 1H), 8.45 (d, 1H), 8.42 (dd, 1H), 8.31-8.18 (m, 5H),
8.04 (t, 1H), 7.96 (d, 1H), 7.93 (m, 2H), 7.76 (m, 2H), 7.55 (td,
1H), 7.50 (q, 1H), 7.25 (m, 1H), 7.09 (m, 1H), 6.99 (d, 1H). 13C-
{1H} NMR (CD2Cl2, δ): 207.4, 203.8, 200.3, 181.2, 167, 161.2,
156, 134.2, 132.3, 130, 129, 125.1, 124, 122.7, 121.4. IR (KBr,
cm-1): ν(CO) 2025, 1945; ν(BF4-) 1058. Anal. Calcd for
C32H19N4O4BF4Ru2: C, 47.31; H, 2.36; N, 6.90. Found: C, 47.16;
H, 2.39; N, 7.04.
[Ru2(thNP)2(CO)4][BF4]2 (4) was isolated in 86% yield when
the reaction was carried out in acetonitrile by following a procedure
similar to that described for compound 3.
Synthesis of [Rh2(phNP)(OAc)3Cl] (5). A sample of [Rh2-
(OAc)4] (35 mg, 0.079 mmol) was treated with 36 mg (0.175 mmol)
of phNP in 15 mL of 1,2-dichloroethane and refluxed for 12 h.
The red solution was concentrated, and ether was added to induce
precipitation. The red residue obtained was washed with ether and
Synthesis of [Ru2(Me2fuNP)(C4OMe2-NP)(CO)4][BF4] (2). The
reaction of [Ru2(CO)4(MeCN)6][BF4]2 (28 mg, 0.038 mmol) and
Me2fuNP (20 mg, 0.089 mmol) was carried out by following a
procedure similar to that described in the synthesis of complex 1.
1
1
Yield: 30 mg (93%). H NMR (CD2Cl2, δ): 9.35 (d, 1H), 8.49
dried under vacuum. Yield: 42 mg (85%). H NMR (CDCl3, δ):
(m, 2H), 8.32 (1H), 8.14 (1H), 7.90 (s, 1H), 7.70 (m, 2H), 7.58
(m, 2H), 7.37 (t, 1H), 2.73 (s, 6H), 2.61 (s, 3H), 1.97 (s, 3H). IR
(KBr, cm-1): ν(CO) 2030, 1954; ν(BF4-) 1064. Anal. Calcd for
C32H23N4O6BF4Ru2: C, 45.30; H, 2.73; N, 6.60. Found: C, 45.02;
H, 2.79; N, 6.72.
Synthesis of [Ru2(thNP)(C4H2S-NP)(CO)4][BF4] (3) and [Ru2-
(thNP)2(CO)4][BF4]2 (4). A dichloromethane solution (10 mL) of
thNP (36 mg, 0.169 mmol) was added dropwise to a dichlo-
romethane solution (15 mL) of [Ru2(CO)4(MeCN)6][BF4]2 (55 mg,
0.075 mmol), and the solution was stirred for 4 h at room
temperature. A small amount of bright yellow residue was formed,
which was separated by filtration through a filter-paper-stripped
cannula. The red filtrate was concentrated, and diethyl ether was
added to induce precipitation. The red residue was washed with
diethyl ether and dried under vacuum. The red product was
11.38 (d, 1H), 8.61 (d, 2H), 8.32 (q, 2H), 8.08 (2H), 7.92 (dd, 2H),
7.75 (t, 1H), 2.16 (s, 3H), 1.68 (s, 3H), 1.23 (s, 3H). IR (KBr,
cm-1): ν(OAc-) 1604, 1552, 1434. Anal. Calcd for C20H19N2O6-
ClRh2: C, 38.46; H, 3.07; N, 4.48. Found: C, 38.68; H, 3.12; N,
4.36.
Synthesis of [Rh2(phNP)(η1-phNP)(OAc)2(CH3CN)2][BF4]2
(6). A sample of [Rh2(OAc)2(MeCN)6][BF4]2 (38 mg, 0.051 mmol)
was treated with 25 mg (0.116 mmol) of phNP in 15 mL of 1,2-
dichloroethane and refluxed for 12 h. The red solution was
concentrated, and ether was added to induce precipitation. The red
residue obtained was washed with ether and dried under vacuum.
1
Yield: 42 mg (83%). H NMR (CDCl3, δ): 10.18 (d, 1H), 8.58
(dd, 2H), 8.72 (dd, 1H), 8.41 (m, 2H), 8.33 (td, H), 8.25 (d, 2H),
8.17 (d, H), 7.99 (m, 6H), 7.57 (d, H), 7.42 (m, 3H), 2.91 (s, 1H),
2.14 (t, 6H), 2.05 (s, 1H), 2.02 (s, 1H), 1.24 (br, 1H), 0.85 (m,

6060 Organometallics, Vol. 25, No. 26, 2006
Patra and Bera
2H). IR (KBr, cm-1): ν(OAc-) 1609, 1556, 1444; ν(BF4-) 1058.
Anal. Calcd for C36H32N6O4B2F8Rh2: C, 43.58; H, 3.25; N, 8.47.
Found: C, 43.36; H, 3.37; N, 8.61.
using the SHELX suite of programs,26 while additional crystal-
lographic calculations were performed by the program PLATON.27
Figures were drawn using ORTEP32.28 The hydrogen atoms were
included in geometrically calculated positions in the final stages
of the refinement and were refined according to the “riding model”.
For compounds 1-3, all non-hydrogen atoms were refined with
anisotropic thermal parameters, with the sole exception of C15 in
2. The “SQUEEZE” option in PLATON was used to remove a
disordered solvent molecule from the overall intensity data of
compounds 3 and 7. The disordered CH2Cl2 in 5 was modeled
satisfactorily. All non-hydrogen atoms, except the C and Cl atoms
of the disordered solvent molecule, were refined anisotropically.
Synthesis of [Rh2(nplNP)(η1-nplNP)(OAc)2(CH3CN)2][BF4]2
(7). The reaction of [Rh2(OAc)2(MeCN)6][BF4]2 (31 mg, 0.042
mmol) and 25 mg (0.097 mmol) of nplNP in 15 mL of 1,2-
dichloroethane was carried out by following a procedure similar
to that described in the synthesis of complex 6. Yield: 38 mg (84%).
1H NMR (CDCl3, δ): 10.21 (d, 1H), 8.74 (d, 1H), 8.33 (dd, 2H),
8.26 (d, 2H), 7.92 (q, 2H), 7.82 (td, 4H), 7.67 (d, 1H), 7.45 (m,
4H), 7.37 (m, 2H), 7.23 (m, 5H), 2.95 (s, 2H), 2.23 (s, 6H), 1.98
(s, 1H), 1.24 (br, 1H), 0.84 (m, 2H). IR (KBr, cm-1): ν(OAc-)
1610, 1557, 1442; ν(BF4-) 1058. Anal. Calcd for C44H36N6O4B2F8-
Rh2: C, 48.39; H, 3.32; N, 7.69. Found: C, 48.12; H, 3.49; N,
7.57.
Acknowledgment. We thank the DST India for financial
support of this work.
X-ray Data Collections and Refinement. The red single crystals
of 1-3, 5, and 7 were harvested by layering petroleum ether onto
the dichloromethane solutions of the compounds in 8 mm o.d. glass
tubes. Single-crystal X-ray structural studies were performed on a
CCD Bruker SMART APEX diffractometer equipped with an
Oxford Instruments low-temperature attachment. Data were col-
lected at 100(2) K using graphite-monochromated Mo KR radiation
(λR ) 0.710 73 Å). The frames were indexed, integrated, and scaled
using the SMART and SAINT software package,24 and the data
were corrected for absorption using the SADABS program.25
Pertinent crystallographic data for compounds 1-3, 5, and 7 are
summarized in Table 4. The structures were solved and refined
Supporting Information Available: Text and figures giving
details on the syntheses and H NMR data for the ligands, CIF
1
files giving X-ray crystallographic data for compounds 1-3, 5, and
7, and a figure showing the contour surface of the σ* (LUMO)
and π* (HOMO) in [Ru2(3-MeNP)2(CO)4]2+ (3-MeNP ) 3-methyl-
1,8-naphthyridine). This material is available free of charge via the
Internet at http://pubs.acs.org.
OM060774+
(26) (a) SHELXTL Package, version 6.10; Bruker AXS, Madison, WI,
2000. (b) Sheldrick, G. M. SHELXS-86 and SHELXL-97; University of
Go¨ttingen, Go¨ttingen, Germany, 1997.
(27) Spek, L. PLATON; University of Utrecht, Utrecht, The Netherlands,
2001.
(24) SAINT+ Software for CCD Diffractometers; Bruker AXS, Madison,
WI, 2000.
(25) Sheldrick, G. M. SADABS Program for Correction of Area Detector
Data; University of Go¨ttingen, Go¨ttingen, Germany, 1999.
(28) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565.