A cyanamido-bridged diiridium complex: A reactive building block for polynuclear cyanamido complexes

Article abstract of DOI:10.1021/om048978i

[Cp*IrCl₂]₂ reacts with 2 equiv of Na ₂NCN to afford the NCN-bridged diiridium complex [Cp*Ir- (μ₂-NCN-N,N)]₂ (5), which undergoes further reactions with donor molecules such as CO and phosphines. Complex 5 works as an excellent building block for the synthesis of the NCN-capped heterotrinuclear complexes [(Cp*Ir)₂(ML)(μ₃-NCN-N,N,N)₂] ⁺ (ML = Rh(cod), CpRu, Pd(η³-C₃H ₅)) on reactions with cationic group 8-10 metal complexes such as [Rh(cod)(acetone)_n]⁺, [CpRu-(MeCN)₃] ⁺, and [η³-C₃H₅)(acetone) _n]⁺, while the dimerization of 5 leads to the cubane-type tetrairidium complex [Cp*Ir(μ₃-NCN-N,N,N)₄.

Full text of DOI:10.1021/om048978i

Organometallics 2005, 24, 2251-2254
2251
A Cyanamido-Bridged Diiridium Complex: A Reactive
Building Block for Polynuclear Cyanamido Complexes
Hidenobu Kajitani,† Yoshiaki Tanabe,‡ Shigeki Kuwata,§
Masakazu Iwasaki,| and Youichi Ishii*,‡
Institute of Industrial Science, The University of Tokyo, Komaba, Meguro-ku,
Tokyo 153-8505, Japan, Department of Applied Chemistry, Faculty of Science and
Engineering, Chuo University, Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan, Department of
Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of
Technology, O-okayama, Meguro-ku, Tokyo 152-8552, Japan, and Department of Applied
Chemistry, Faculty of Engineering, Saitama Institute of Technology, Okabe,
Saitama 369-0293, Japan
Received December 25, 2004
Chart 1
Summary: [Cp*IrCl2]2 reacts with 2 equiv of Na2NCN
to afford the NCN-bridged diiridium complex [Cp*Ir-
(µ2-NCN-N,N)]2 (5), which undergoes further reactions
with donor molecules such as CO and phosphines.
Complex 5 works as an excellent building block for the
synthesis of the NCN-capped heterotrinuclear complexes
[(Cp*Ir)2(ML)(µ3-NCN-N,N,N)2]+ (ML ) Rh(cod), CpRu,
Pd(η3-C3H5)) on reactions with cationic group 8-10
metal complexes such as [Rh(cod)(acetone)n]+, [CpRu-
(MeCN)3]+, and [Pd(η3-C3H5)(acetone)n]+, while the dimer-
ization of 5 leads to the cubane-type tetrairidium complex
[Cp*Ir(µ3-NCN-N,N,N)]4.
the dimerization reaction to form 4 as well as the
heterotrinuclear complex formation through the incor-
poration of a heterometal fragment.
Despite the increasing interest in polynuclear transi-
tion-metal complexes with nitrogen-based bridging
ligands as advanced materials,1 use of cyanamide anions
(NCN2- and NCNH-) as bridging ligands for the con-
struction of polynuclear systems remains in the early
stages of development.2 However, because of their soft,
sterically small, and potentially polydentate nature,
these anions are expected to provide a new entry for
the chemistry of polynuclear complexes. In this context
we have shown that the diiridium complex [Cp*IrCl2]2
(1; Cp* ) η5-C5Me5) reacts with NaNCNH to afford the
macrocyclic complex [Cp*IrCl(µ2-NCNH-N,N′)]4 (2), which
is further converted into the C3-elongated cubane-like
complex [Cp*Ir(µ3-NCN-N,N,N′)3(IrCp*)3(µ3-NCN-N,N,N)]
(3) (Chart 1) and the regular cubane-type complex
[Cp*Ir(µ3-NCN-N,N,N)]4 (4).3 Now we have synthesized
the NCN-bridged diiridium complex [Cp*Ir(µ2-NCN-
N,N)]2 (5) and revealed its novel reactivities, including
Switching the cyanamide source to react with 1 from
NaNCNH to Na2NCN resulted in a striking change of
the reaction product. Thus, when 1 was treated with 2
equiv of Na2NCN at room temperature, the NCN-
bridged diiridium complex 5, instead of 2, was obtained
as dark red crystals in 47% yield (Scheme 1).5 Complex
5 shows one strong IR absorption at 2093 cm-1 assign-
able to the stretching vibration of the NCN moiety, and
the 1H NMR spectrum exhibits only one signal at δ 1.87
assignable to the Cp* protons. The molecular structure
of 5 was determined by X-ray crystallography.5 The
molecule has a crystallographic Cs symmetry. As de-
picted in Figure 1, each NCN ligand bridges the iridium
centers with a µ2-κN,κN coordination mode to form the
Ir2N2 core, which is strongly puckered with a N(1)-
Ir(1)-Ir(2)-N(1*) torsion angle of 118.0(7)°. It should
be pointed out that complexes with µ2-NCN-N,N ligands
have rarely been reported in the literature.6 The bond
angles around the N(1) atom (sum 332°) as well as the
* To whom correspondence should be addressed. E-mail: ishii@
chem.chuo-u.ac.jp.
† The University of Tokyo.
‡ Chuo University.
(4) Complex 1 (1.0425 g, 1.31 mmol) and Na2NCN (229.5 mg, 2.67
mmol) were dissolved in methanol (30 mL), and the mixture was stirred
for 15 min at room temperature. The resulting dark red solution was
dried in vacuo to give a dark red solid, which was dissolved in CH2Cl2-
methanol (30:1, 1.0 mL) and the solution passed through a column
packed with Al2O3 (Merck; eluent 30:1 CH2Cl2-methanol). The first
yellow band contains [Cp*Ir(µ3-NCN-N,N,N′)3(IrCp*)3(µ3-NCN-N,N,N)]
(3),3 while the second dark red band eluted was collected and
evaporated to dryness. Recrystallization from CH2Cl2-diethyl ether
gave dark red needles of [Cp*Ir(µ3-NCN-N,N)]2 (5) (451.6 mg, 47%
yield).
§ Tokyo Institute of Technology.
| Saitama Institute of Technology.
(1) (a) Dunbar, K. R.; Heintz, R. A. Prog. Inorg. Chem. 1997, 45,
283-391. (b) Ribas, J.; Escuer, A.; Monfort, M.; Vicente, R.; Corte´s,
R.; Lezama, L.; Rojo, T. Coord. Chem. Rev. 1999, 193-195, 1027-1068.
(c) Verdaguer, M.; Bleuzen, A.; Marvaud, V.; Vaissermann, J.; Seule-
iman, M.; Desplanches, C.; Scuiller, A.; Train, C.; Garde, R.; Gelly,
G.; Lomenech, C.; Rosenman, I.; Veillet, P.; Cartier, C.; Villain, F.
Coord. Chem. Rev. 1999, 190-192, 1023-1047.
(2) For recent examples, see: (a) Schneider, W.; Angermaier, K.;
Schmidbaur, H. Z. Naturforsch. 1996, B51, 801-805. (b) Mindiola, D.
J.; Tsai, Y.-C.; Hara, R.; Chen, Q.; Meyer, K.; Cummins, C. C. Chem.
Commun. 2001, 125-126. (c) Cao, R.; Tatsumi, K. Chem. Commun.
2002, 2144-2145.
(5) Crystallographic data for 5: C22H30Ir2N4, FW ) 734.95, ortho-
rhombic, space group Pnma, a ) 21.365(6) Å, b ) 15.428(5) Å, c )
6.697(5) Å, V ) 2207(2) Å3, Z ) 4, T ) 21 °C, Fcalcd ) 2.211 g cm-3
,
µ(Mo KR) ) 120.93 cm-1, 2519 unique reflections, R (Rw) ) 0.052
(0.054) for 156 variables and 1731 reflections (I > 3σ(I)), GOF ) 1.001.
(6) Rajca, G.; Weidlein, J. Z. Anorg. Allg. Chem. 1986, 538, 36-44.
(3) Tanabe, Y.; Kuwata, S.; Ishii, Y. J. Am. Chem. Soc. 2002, 124,
6528-6529.
10.1021/om048978i CCC: $30.25 © 2005 American Chemical Society
Publication on Web 04/02/2005

2252 Organometallics, Vol. 24, No. 10, 2005
Communications
Scheme 1
short N(2)-C(1) bond distance (1.15(3) Å) indicate that
the NCN moiety is better described as a cyanamido(2-)
ligand rather than a carbodiimido(2-) ligand, and the
N(1) atom is sp3 hybridized. On the other hand, the Ir-
Ir distance at 2.8179(9) Å is slightly longer than those
of the structurally related imido complexes [Cp*Ir(µ2-
NR)]2 (R ) Ph,7 2.778(1) Å; R ) cyclopentyl,8 2.6133(5)
Å) but suggests the presence of a metal-metal bonding
interaction, which is reflected in the potentially unsat-
urated nature of the diiridium core.
As expected, complex 5 reacts smoothly with donor
molecules. Treatment of 5 with atmospheric CO at room
temperature followed by recrystallization under N2 led
to the CO-insertion product [Cp*Ir(CO)(µ2-NCN-N,N)(µ2-
OCNCN)IrCp*] (6) in 82% yield (Scheme 1).9 The IR
spectrum of 6, showing two strong ν(NCN) bands at
2186 and 2107 cm-1 as well as two ν(CO) bands at 2016
and 1615 cm-1, is in full agreement with the formula-
tion. The presence of two distinct iridium centers is also
confirmed by the two Cp* signals at δ 1.86 and 1.75 in
the 1H NMR spectrum. The molecular structure of 6 has
been unambiguously established by an X-ray analysis
(Figure 2).10 One of the NCN ligands in 5 is coupled with
CO11 to form the unprecedented cyanogen isocyanate
ligand (NCNCO), and the Ir(1) atom is further coordi-
nated by a CO ligand. The diiridacycle is nearly planar,
and the large Ir‚‚‚Ir separation (3.5687(3) Å) excludes
any metal-metal bonding interaction. The related
carbodiimido complex [Cp*Ir(CNXy)(µ2-NCN-N,N)(µ2-
XyNCNCN)IrCp*] (7) was also obtained from the reac-
tion of 5 with XyNC (Xy ) 2,6-dimethylphenyl) (Scheme
1).12
Figure 1. Molecular structure of 5. Thermal ellipsoids are
shown at the 50% probability level. Hydrogen atoms are
omitted for clarity.
(7) Dobbs, D. A.; Bergman, R. G. Organometallics 1994, 13, 4594-
4605.
(8) Danopoulos, A. A.; Wilkinson, G.; Sweet, T. K. N.; Hursthouse,
M. B. J. Chem. Soc., Dalton Trans. 1996, 3771-3778.
(9) The initial products formed under CO, which are tentatively
characterized as stereoisomers of [Cp*Ir(CO)(µ2-NCN-N,N)(µ2-OCNCN)-
Ir(CO)Cp*] (1H NMR (CDCl3): δ 1.88, 1.91, 1.92, 1.94), are not stable
under N2 and are quantitatively transformed into 6 through dissocia-
tion of one CO molecule.
(10) Crystallographic data for 6:
C24H30Ir2N4O2, FW ) 790.97,
triclinic, space group P1h, a ) 8.888(5) Å, b ) 11.704(4) Å, c ) 12.696(4)
Å, R ) 88.75(3)°, â ) 86.05(4)°, γ ) 76.30(4)°, V ) 1280.2(9) Å3, Z ) 2,
T ) 21 °C, Fcalcd ) 2.052 g cm-3, µ(Mo KR) ) 104.40 cm-1, 5779 unique
reflections, R (Rw) ) 0.026 (0.026) for 320 variables and 4755 reflections
(I > 3σ(I)), GOF ) 1.006.
(11) Ye, C.; Sharp, P. R. Inorg. Chem. 1995, 34, 55-59.
(12) Spectral data for 7: 1H NMR (C6D6, δ) 1.42, 1.90 (both s, 15H
each, Cp*); IR (KBr, cm-1) 2159 (m, NCN), 2132 (s, NCN), 2090 (s,
CN), 1553 (s, CN).
Figure 2. Molecular structure of 6. Thermal ellipsoids are
shown at the 50% probability level. Hydrogen atoms are
omitted for clarity.

Communications
Organometallics, Vol. 24, No. 10, 2005 2253
Figure 3. Molecular structure of 8d. Thermal ellipsoids
are shown at the 50% probability level. Hydrogen atoms
are omitted for clarity.
The reaction of 5 with 1 equiv of PR3 resulted in the
formation of the mono(phosphine) complex [Cp*Ir(PR3)-
(µ2-NCN-N,N)2IrCp*] (8a, R ) Ph; 8b, R ) p-Tol; 8c, R
) p-C6H4F; 8d, R ) Me) in 34-83% yield (Scheme 1).13
No further reaction was observed, even in the presence
of an excess amount of PR3. In accordance with the
formulation, complexes 8 have one strong IR absorption
at nearly 2060 cm-1 assignable to the NCN stretching
Figure 4. Structure of the cationic part of 10aBPh4‚CH2Cl2.
Thermal ellipsoids are shown at the 50% probability level.
Hydrogen atoms are omitted for clarity.
ligands is eliminated as phosphine imide.7 We attribute
this difference to the soft character of the cyanamido
ligand to form more stable N-metal bonds with late-
transition-metal centers; namely, the π-electron conflict
between the filled metal dπ orbitals and the lone pair
electrons of the metal-bound nitrogen is greatly weak-
ened by the electron-withdrawing effect of the CN group.
On the other hand, the lone pair electrons of the sp3-
hybridized bridging nitrogen atoms in 5 are directed
outward and are expected to interact with a Lewis acidic
metal fragment. Indeed, complex 5 readily reacts with
[Rh(cod)(acetone)n](OTf) (cod ) 1,5-cyclooctadiene, OTf
) OSO2CF3) to form the heterotrinuclear complex
[(Cp*Ir)2{Rh(cod)}(µ3-NCN-N,N,N)2]+ (10a), which was
isolated as the BPh4 salt 10aBPh4‚CH2Cl2 in 87% yield
(Scheme 1).16 Figure 4 depicts the structure of 10a,
which clearly shows that it possesses a triangular Ir2Rh
core capped by two µ3-NCN ligands from both sides.17
The metric features within the M3 core are typical of a
1
vibration, and their H NMR spectra display two Cp*
signals at δ 1.55-1.61 and 1.40-1.46. The molecular
structure of 8d has been established by an X-ray
analysis (Figure 3).14 On coordination of the phosphine
ligand, the Ir2N2 core is considerably flattened, and the
Ir‚‚‚Ir interatomic distance is elongated (3.2985(5) Å).
The Ir(2)-N(1) bond distance (1.996(7) Å) is signifi-
cantly shorter than the Ir(1)-N(1) bond distance (2.113(7)
Å), which indicates some π donation from the NCN
ligands to the Ir(2) atom. Unlike the mono(phosphines),
dppm (bis(diphenylphosphino)methane) works as a bi-
dentate ligand to form the coordinatively saturated bis-
(phosphine) complex [{Cp*Ir(µ2-NCN-N,N)}2(µ-dppm)]‚
1.5C6H6 (9‚1.5C6H6) in 76% yield (Scheme 1), whose
molecular structure has also been confirmed crystallo-
graphically (ORTEP drawing not shown).15
1
48e trinuclear cluster with three M-M bonds. The H
Interestingly, the above reaction with phosphines
forms a sharp contrast to that of the dimeric phenyl-
imido complex [Cp*Ir(µ2-NPh)]2, where one of the imido
NMR spectrum of 10aBPh4 exhibits signals attributable
to the Cp* (δ 2.03 (30H)) and cod protons (δ 3.90-3.96
(4H), 2.34-2.48 (8H)), and the IR spectrum displays one
strong NCN stretching band at 2159 cm-1. Similar
reactions of 5 with [CpRu(MeCN)3]+ (Cp ) η5-C5H5) and
(13) Spectral data for 8a: 1H NMR (CDCl3, δ) 1.40 (d, 15H, J ) 2.0
Hz, Cp*), 1.55 (s, 15H, Cp*); 31P{1H} NMR (CDCl3, δ) 18.7 (s); IR (KBr,
cm-1) 2064 (s, NCN). Spectral data for 8b: 1H NMR (CDCl3, δ) 1.40
(d, 15H, J ) 2.0 Hz, Cp*), 1.55 (s, 15H, Cp*); 31P{1H} NMR (CDCl3, δ)
17.0 (s); IR (KBr, cm-1) 2060 (s, NCN). Spectral data for 8c: 1H NMR
(CDCl3, δ) 1.42 (d, 15H, J ) 2.0 Hz, Cp*), 1.57 (s, 15H, Cp*); 31P{1H}
NMR (CDCl3, δ) 18.3 (s); IR (KBr, cm-1) 2065 (s, NCN). Spectral data
for 8d: 1H NMR (C6D6, δ) 1.46 (s, 15H, Cp*), 1.61 (s, 15H, Cp*); 31P{1H}
NMR (C6D6, δ) -23.2 (s); IR (KBr, cm-1) 2062 (s, NCN).
(16) To a CH2Cl2 solution (30 mL) of 5 (247.8 mg, 0.337 mmol) was
added an acetone solution of [Rh(cod)(acetone)n]+, which was prepared
in situ from [RhCl(cod)]2 (84.0 mg, 0.170 mmol) and AgOTf (87.0 mg,
0.339 mmol) in acetone (10 mL). The mixture was stirred for 12 h at
room temperature, and the resultant dark red solution was dried in
vacuo. The residue was dissolved in methanol (30 mL) containing
NaBPh4 (ca. 170 mg, 0.50 mmol). The mixture was stirred for 12 h at
room temperature to form a red precipitate, which was collected by
filtration, dried in vacuo, and extracted with CH2Cl2. Recrystallization
from CH2Cl2-methanol yielded dark red platelike crystals of [(Cp*Ir)2-
{Rh(cod)}(µ3-NCN-N,N,N)2](BPh4)‚CH2Cl2 (10aBPh4‚CH2Cl2), the CH2Cl2
molecule of which was lost by drying in vacuo to give a red powder of
10aBPh4 (369.3 mg, 0.292 mmol, 87% yield).
(17) Crystallographic data for 10aBPh4‚CH2Cl2: C55H64BCl2Ir2Rh,
FW ) 1350.20, orthorhombic, space group Pccn, a ) 12.680(2) Å, b )
12.734(2) Å, c ) 32.297(4) Å, V ) 5214(1) Å3, Z ) 4, T ) 21 °C, Fcalcd
) 1.720 g cm-3, µ(Mo KR) ) 55.59 cm-1, 5934 unique reflections, R
(Rw) ) 0.051 (0.052) for 329 variables and 3844 reflections (I > 3σ(I)),
GOF ) 1.009.
(14) Crystallographic data for 8d:
C25H39Ir2N4P, FW ) 811.02,
orthorhombic, space group Pnma, a ) 19.199(5) Å, b ) 13.714(5) Å, c
) 9.781(3) Å, V ) 2575(1) Å3, Z ) 4, T ) -165 °C, Fcalcd ) 2.092 g
cm-3, µ(Mo KR) ) 104.35 cm-1, 3028 unique reflections, R (Rw) ) 0.058
(0.068) for 184 variables and 2557 reflections (I > 3σ(I)), GOF ) 1.006.
(15) Spectral data for 9: 1H NMR (CDCl3, δ) 1.45 (s, 30H, Cp*), 3.84
(t, 2H, J ) 11.4 Hz, CH2); 31P{1H} NMR (CDCl3, δ) 7.8 (s); IR (KBr,
cm-1) 2049 (s, NCN). Crystallographic data for 9‚1.5C6H6: C56H61
-
Ir2N4P2, FW ) 1236.51, triclinic, space group P1h, a ) 12.084(4) Å, b )
12.105(4) Å, c ) 19.525(7) Å, R ) 99.37(1)°, â ) 91.82(1)°, γ )
118.88(1)°, V ) 2447(1) Å3, Z ) 2, T ) 21 °C, Fcalcd ) 1.678 g cm-3
,
µ(Mo KR) ) 55.55 cm-1, 10 868 unique reflections, R (Rw) ) 0.031
(0.036) for 638 variables and 8788 reflections (I > 3σ(I)), GOF ) 1.004.

2254 Organometallics, Vol. 24, No. 10, 2005
Communications
[Pd(η3-C3H5)(acetone)n]+ result in the formation of
the heterotrinuclear complexes [(Cp*Ir)2(ML)(µ3-NCN-
N,N,N)2]+ (10b, ML ) RuCp; 10c, ML ) Pd(η3-C3H5))18
(Scheme 1). To the best of our knowledge, 10a-c provide
the first examples of NCN-bridged heteronuclear com-
plexes.
Finally, the dimerization reaction of 5 has been
investigated. In 1-propanol at 80 °C, 5 was selectively
converted into the cubane-type complex 4, which was
isolated in 75% yield. A preliminary kinetic study was
performed by following the reaction by means of 1H
NMR. The reaction obeys second-order kinetics, where
the rate constant k ) (1.19 ( 0.05) × 10-2 L mol-1 s-1
at 80 °C. It is interesting to note that the dinuclear
complex 5 is thermodynamically less stable than the
cubane-type complex 4 but kinetically stable at room
temperature. This is in contrast to the related dimer-
ization of the sulfido-bridged diiridium species [Cp*Ir-
(µ2-S)]2, which can be observed only at low tempera-
tures.19
In conclusion, the NCN-bridged dinuclear iridium
complex 5 has been synthesized and found to show rich
reactivities, including CO insertion, dimerization, and
heterotrinuclear complex formation. These results clearly
demonstrate that the NCN2- anion can act as an
effective and versatile bridging ligand with a variety of
coordination modes to form polynuclear complexes.
Acknowledgment. We thank the Ministry of Edu-
cation, Culture, Sports, Science and Technology of
Japan for a grant-in-aid.
Supporting Information Available: Text giving experi-
mental and spectroscopic details, a table of kinetic data and a
figure for the dimerization reaction of 5, and tables of crystal-
lographic data, positional and thermal parameters, and bond
distances and angles and thermal ellipsoid plots for complexes
5, 6, 8d, 9‚1.5C6H6, and 10aBPh4‚CH2Cl2; crystallographic data
are also available in a CIF file. This material is available free
of charge via the Internet at http://pubs.acs.org.
OM048978I
(19) (a) Tang, Z.; Nomura, Y.; Ishii, Y.; Mizobe, Y.; Hidai, M. Inorg.
Chim. Acta 1998, 267, 73-79. (b) Dobbs, D. A.; Bergman, R. G. Inorg.
Chem. 1994, 33, 5329-5336. (c) Herberhold, M.; Jin, G.-X.; Milius, W.
Chem. Ber. 1995, 128, 557-560. (d) Feng, Q.; Krautscheid, H.;
Rauchfuss, T. B.; Skaugset, A. E.; Venturelli, A. Organometallics 1995,
14, 297-304.
(18) Spectral data for 10bBPh4: 1H NMR (CD2Cl2, δ) 1.88 (s, 30H,
Cp*), 4.83 (s, 5H, Cp); IR (KBr, cm-1) 2170 (s, NCN). Spectral data for
10cOTf: 1H NMR (CDCl3, δ) 1.94 (s, 30H, Cp*), 3.26 (d, 2H, J ) 12.3
Hz, CH2), 4.19-4.35 (m, 2H, CH2), 5.85-5.95 (m, 1H, CH); IR (KBr,
cm-1) 2145 (s, NCN).