Unexpected formation of a weak metal-metal bond: Synthesis, electronic properties, and second-order NLO responses of push-pull late-early heteronuclear bimetallic complexes with W(CO)3(1,10-phenanthroline) acting as a donor ligand

Article abstract of DOI:10.1021/om030247f

In attempts to bridge the complex [W(CO)₃(phen)(pyz)] (phen = 1,10-phenanthroline; pyz = pyrazine) to acceptor centers, either soft centers such as cis-M(CO)₂CL (M = Rh(I), Ir(I)) and fac-M(CO)₃Cl₂ (M = Ru(II), Os(II)) or hard centers such as BF₃, the pyrazine ligand is lost, while the fragment W(CO)₃(phen) behaves as a σ-donor base with the unexpected formation of heteronuclear early-late bimetallic compounds with a weak metal-metal bond, as confirmed by the easy substitution of W(CO)₃(phen) by soft ligands (PPh₃, CO, pyridine). The X-ray structures of [(CO)₃(phen)W-cis-Ir(CO)₂Cl] and [(CO)₃(phen)W-fac-Os(CO)₃Cl₂] confirm a single metal-metal bond with an halogen bridging asymmetrically the two metallic moieties and with the tungsten atom achieving a distorted (6 + 1) octahedral coordination. All the heteronuclear bimetallic complexes investigated show in their electronic spectra a new solvatochromic absorption band at around 385-450 nm in addition to the MLCT (W→π*_phen) absorption band typical of [W(CO)₃(phen)L] complexes (L = CO, pyz, CH₃CN) and an increased, in comparison to [W(CO)₄(phen)], negative nonlinear (NLO) second-order emission working with the EFISH technique with an incident wavelength of 1.907 μm. The increase is due to an additional negative contribution of the new absorption band at around 385-450 nm, as shown by a solvatochromic investigation.

Full text of DOI:10.1021/om030247f

Organometallics 2003, 22, 4001-4011
4001
Un exp ected F or m a tion of a Wea k Meta l-Meta l Bon d :
Syn th esis, Electr on ic P r op er ties, a n d Secon d -Or d er NLO
Resp on ses of P u sh -P u ll La te-Ea r ly Heter on u clea r
Bim eta llic Com p lexes w ith W(CO)3(1,10-p h en a n th r olin e)
Actin g a s a Don or Liga n d
Maddalena Pizzotti,* Renato Ugo, Claudia Dragonetti, and Elisabetta Annoni
Dipartimento di Chimica Inorganica, Metallorganica e Analitica dell’Universita` di Milano,
Unita` di ricerca dell’INSTM dell’Universita` di Milano e Istituto di Scienze e Tecnologie
Molecolari del CNR (ISTM), Via Venezian 21, 20133 Milano, Italy
Francesco Demartin
Dipartimento di Chimica Strutturale e Stereochimica Inorganica, Via Venezian 21,
20133 Milano, Italy
Patrizia Mussini
Dipartimento di Chimica Fisica ed Elettrochimica, Via Venezian 21, 20133 Milano, Italy
Received April 3, 2003
In attempts to bridge the complex [W(CO)3(phen)(pyz)] (phen ) 1,10-phenanthroline; pyz
) pyrazine) to acceptor centers, either soft centers such as cis-M(CO)2Cl (M ) Rh(I), Ir(I))
and fac-M(CO)3Cl2 (M ) Ru(II), Os(II)) or hard centers such as BF3, the pyrazine ligand is
lost, while the fragment W(CO)3(phen) behaves as a σ-donor base with the unexpected
formation of heteronuclear early-late bimetallic compounds with a weak metal-metal bond,
as confirmed by the easy substitution of W(CO)3(phen) by soft ligands (PPh3, CO, pyridine).
The X-ray structures of [(CO)3(phen)W-cis-Ir(CO)2Cl] and [(CO)3(phen)W-fac-Os(CO)3Cl2]
confirm a single metal-metal bond with an halogen bridging asymmetrically the two metallic
moieties and with the tungsten atom achieving a distorted (6 + 1) octahedral coordination.
All the heteronuclear bimetallic complexes investigated show in their electronic spectra a
new solvatochromic absorption band at around 385-450 nm in addition to the MLCT
(Wfπ*phen) absorption band typical of [W(CO)3(phen)L] complexes (L ) CO, pyz, CH3CN)
and an increased, in comparison to [W(CO)4(phen)], negative nonlinear (NLO) second-order
emission working with the EFISH technique with an incident wavelength of 1.907 µm. The
increase is due to an additional negative contribution of the new absorption band at around
385-450 nm, as shown by a solvatochromic investigation.
In tr od u ction
with pyrazine (pyz) and trans-1,2-bis(4-pyridyl)ethylene
(BPE) as polarizable linkers (L) between the W(CO)5
moiety acting, in the excitation process, as donor group5
and cis-Rh(CO)2Cl, cis-Re(CO)4Cl, or BF3 acting as the
In the past few years organometallic compounds,
studied as molecular systems with potential second-
order NLO properties, have shown a wide range of
advantages1 due to the control and tuning of the
electronic properties of the metal and consequently of
the highly polar metal to ligand or ligand to metal
charge-transfer processes. Heteronuclear bimetallic com-
plexes, in which the electron-accepting and -donating
properties of two organometallic fragments are com-
bined in order to generate a new kind of push-pull
system, are quite interesting NLO chromophores.2-4
Recently we reported3 the synthesis, electronic prop-
erties, and second-order NLO response of new push-
pull asymmetrical bimetallic heteronuclear complexes
(2) (a) Houbrechts, S.; Clays, K.; Persoons, A.; Cadierno, V.; Pilar
Gamasa, M.; Gimeno, J . Organometallics 1996, 15, 5266. (b) Mata, J .;
Uriel, S.; Peris, E.; Llusar, R.; Houbrechts, S.; Persoons, A. J .
Organomet. Chem. 1998, 562, 197. (c) Heck, J .; Dabek, S.; Meyer-
Friedrichsen, T.; Wong, H. Coord. Chem. Rev. 1999, 190-192, 1217
and references therein. (d) Lee, I. S.; Seo, H.; Chung, Y. K. Organo-
metallics 1999, 18, 1091. (e) Tamm, M.; Bannenberg, T.; Baum, K.;
Frohlich, R.; Steiner, T.; Meyer-Friedrichsen, T.; Heck, J . Eur. J . Inorg.
Chem. 2000, 1161. (f) Tranchier, J . P.; Chavignon, R.; Prim, D.;
Auffrant, A.; Planas, J . G.; Rose-Munch, F.; Rose, E.; Stephenson, G.
R. Tetrahedron Lett. 2001, 42, 3311. (g) Meyer-Friedrichsen, T.;
Mecker, C.; Prosenc, M. H.; Heck, J . Eur. J . Inorg. Chem. 2002, 239.
(3) Pizzotti, M.; Ugo, R.; Roberto, D.; Bruni, S.; Fantucci, P.; Rovizzi,
C. Organometallics 2002, 21, 5830 and references therein.
(4) (a) Coe, B. J .; J ones, C. J .; McCleverty, J . A.; Bloor, D.; Kolinsky,
P. V.; J ones, R. J . Polyhedron 1994, 13, 2107. (b) Coe, B. J .; J ones, C.
J .; McCleverty, J . A.; Bloor, D.; Kolinski, P. V. J . Chem. Soc., Chem.
Commun. 1989, 1485.
(1) (a) Long, N. J . Angew. Chem., Int. Ed. Engl. 1995, 34, 21. (b)
Whittall, I. R.; McDonagh, A. M.; Humphrey, M. G.; Samoc, M. Adv.
Organomet. Chem. 1998, 42, 291. (c) Di Bella, S. Chem. Soc. Rev. 2001,
30, 355.
(5) Kanis, D. R.; Lacroix, P. G.; Ratner, M. A.; Marks, T. J . J . Am.
Chem. Soc. 1994, 116, 10089.
10.1021/om030247f CCC: $25.00 © 2003 American Chemical Society
Publication on Web 09/04/2003

4002 Organometallics, Vol. 22, No. 20, 2003
Pizzotti et al.
acceptor group.6-8 In these push-pull systems the
MLCT (metal to ligand charge transfer) process (W f
π*L), the origin of the second-order NLO response,
remains localized on the part of the linker L close to
the push W(CO)5 group.3
moments (µ) were obtained in CHCl3 solution, using a WTW-
DM01 dipole meter (dielectric constant) coupled with a Pulfrich
Zeiss PR2 refractometer (refractive index), according to the
Guggenheim method.16 Elemental analyses were carried out
at the Dipartimento di Chimica Inorganica, Metallorganica e
Analitica of the Milan University and repeated, particularly
when not enough satisfactory and reproducible, by an external
microanalytical laboratory (REDOX Microanalytical Labora-
tory-Cologno Monzese, Milano, Italy).
To increase the second-order NLO response of these
kinds of bridged heteronuclear bimetallic systems, we
investigated the substitution of W(CO)5 as push group
with W(CO)3(phen), which should act as a better donor
due to a more facile MLCT transition (W f π*L). In fact,
[W(CO)4(phen)] (phen ) 1,10-phenanthroline) is re-
ported to have, in its visibile spectrum, an MLCT band
at lower energies (between 470 and 500 nm) together
with a higher dipole moment (between 7 and 8 D) with
respect to [W(CO)5(pyz)] (MLCT at about 400 nm and
dipole moment of about 4 D).9 In attempts to bridge the
complex [W(CO)3(phen)(pyz)] to an organometallic ac-
ceptor fragment such as cis-Ir(CO)2Cl, the pyrazine
bridge is lost, while in parallel the fragment W(CO)3-
(phen) behaves as a σ-donor base with the unexpected
formation of a new compound with an early-late (W-
Ir) heteronuclear metal-metal bond. The aim of this
work was thus to investigate in detail the synthesis,
electronic properties, and second-order NLO responses
of a series of these new asymmetrical early-late hetero-
bimetallic complexes with a W-M metal-metal bond
(M ) Ir(I), Rh(I), Os(II), Ru(II), or even BF3) and to
study their reactivity toward basic ligands of different
softness (CO, PPh3, pyridine) in order to evaluate the
strength of the W-M interaction, in comparison to
classical early-late heteronuclear metal-metal bonds.
Deter m in a tion of th e Secon d -Or d er NLO Resp on se by
t h e E F ISH Tech n iq u e a n d b y t h e Solva t och r om ic
Meth od . EFISH (electric field second harmonic generation)
measurements17 of âλ, the projection of the quadratic hyper-
polarizability tensor â along the dipole moment axis working
with an incident wavelength λ, were carried out at the
Dipartimento di Chimica Inorganica, Metallorganica e Anal-
itica of Milan University on CHCl3 solutions of different
concentrations (10-3-10-4 M), working at a nonresonant
fundamental incident wavelength of 1.907 µm, obtained using
a Q-switched, mode-locked Nd3+:YAG laser connected to a
Raman shift with pulse durations of 15-90 ns at a 10 Hz
repetition rate. All experimental EFISH â1.907 values are
defined according to the “phenomenological” convention.18
The quadratic hyperpolarizability along the charge-transfer
direction, âCT, was determined by the solvatochromic method,
taking into account the solvatochromic shift of the two major
absorption bands in various solvents (toluene, ethyl ether,
chloroform, ethyl acetate, THF, dichloromethane, 1,2-dichlo-
roethane, acetone, and dimethylformamide; see Table 4). While
the heteronuclear bimetallic compounds investigated show a
fluorescent emission (Table 3), the solvatochromic shifts of this
emission do not give the linear relationships expected accord-
ing to the most applied theoretical approach.19 Therefore, the
cavity radius (a) was not indirectly evaluated by a parallel use
of both absorption and emission solvatochromism but was
evaluated using the traditional molecular weight approxima-
tion.19 The acceptability of this latter choice was based on the
evidence that the calculated values are within the range of
the values of a determined from structural data. For instance,
for pseudolinear molecules the radius a has been generally
determined either by adding 0.5 Å to the half length of the
molecule or as 0.7 times the length of the molecule (see for
instance ref 7 reported in ref 19). By these two approaches
the calculated values of a are 5.97 and 7.66 Å, respectively,
for 1 (to be compared to 7.12 Å calculated from the molecular
weight) and 6.07 and 7.80 Å, respectively, for 4 (to be compared
to 7.30 Å calculated from the molecular weight). In conclusion,
for the complexes investigated in this work, we have an
acceptable convergence of values of a calculated either from
molecular weight or from structural data.
Exp er im en ta l Section
Gen er a l Com m en ts. W(CO)6, RhCl3‚3H2O, RuCl3‚3H2O,
OsCl3‚3H2O, 1,10-phenanthroline, and BF3‚OEt2 were pur-
chased from Sigma-Aldrich and were used without further
purification. The dimers [cis-Rh(CO)2Cl]2 and [fac-M(CO)3Cl2]2
(M ) Ru, Os) were prepared according to well-known synthe-
ses;10 [Re(CO)4Cl]2,11 [W(CO)4(phen)],12 [fac-W(CO)3(phen)(CH3-
CN)],13 and [cis-Ir(CO)2Cl(pyz)]14 were prepared as described.
[cis-Re(CO)4Cl(THF)] was prepared in situ from the dimer [Re-
(CO)4Cl]2 dissolved in THF. [W(CO)5(THF)] was prepared by
photochemical activation of [W(CO)6] in THF.15
All reactions and workups of the reaction mixtures were
carried out under a nitrogen atmosphere, and all solvents were
dried over molecular sieves (4 Å) prior to use. 1H NMR spectra
were obtained on a Bruker Advance DRX 300 instrument,
electronic spectra on a J ASCO V-530 spectrophotometer,
fluorescence spectra on a J ASCO FP-777 spectrofluorimeter,
and IR spectra on a J ASCO FT-IR 420 instrument. Dipole
The quadratic hyperpolarizability tensor âCT along the
charge-transfer axis of the MLCT (metal to ligand charge
transfer) transitions controlling the NLO response was calcu-
lated according to the Oudar two-level equation20
νa reg2∆ µeg
(6) Zulu, M. M.; Lees, A. J . Organometallics 1989, 8, 955.
(7) Lesley, M. J . G.; Woodward, A.; Taylor, N. J .; Marder, T. B.;
Cazenobe, I.; Ledoux, I.; Zyss, J .; Thornton, A.; Bruce, D. W.; Kakkar,
A. K. Chem. Mater. 1998, 10, 1355.
2
3
âCT
)
(1)
2
2
2
2h2c2 (νa - νL2)(νa - 4νL
)
(8) (a) Roberto, D.; Ugo, R.; Bruni, S.; Cariati, E.; Cariati, F.;
Fantucci, P.; Invernizzi, I.; Ledoux, I.; Zyss, J . Organometallics 2000,
19, 1775. (b) Roberto, D.; Ugo, R.; Tessore, F.; Lucenti, E.; Quici, S.;
Vezza, S.; Fantucci, P.; Invernizzi, I.; Bruni, S.; Ledoux-Rak, I.; Zyss,
J . Organometallics 2002, 21, 161.
(9) Cheng, L. T.; Tam, W.; Eaton, D. F. Organometallics 1990, 9,
2856.
(10) Roberto, D.; Psaro, R.; Ugo, R. Organometallics 1993, 12, 2292.
(11) Dolcetti, G.; Norton, J . R. Inorg. Synth. 1976, 16, 35.
(12) (a) Stiddard, M. H. B. J . Chem. Soc. 1962, 4712. (b) Saito, H.;
Fujita, J .; Saito, K. Bull. Chem. Soc. J pn. 1968, 41, 359.
(13) Wrighton, M. S.; Morse, D. L. J . Organomet. Chem. 1975, 97,
405.
where reg is the transition dipole moment related to the
(16) (a) Guggenheim, E. A. Trans. Faraday Soc. 1949, 45, 203. (b)
Smith, W. Electric Dipole Moments; Butterworth Scientific: London,
1965. (c) Thompson, B. J . Chem. Educ. 1966, 43, 66.
(17) (a) Levine, B. F.; Bethea, C. G. Appl. Phys. Lett. 1974, 24, 445.
(b) Singer, K. D.; Garito, A. F. J . Chem. Phys. 1981, 75, 3572. (c)
Ledoux, I.; Zyss, J . Chem. Phys. 1982, 73, 203.
(18) Willetts, A.; Rice, J . E.; Burland, D. M.; Shelton, D. P. J . Chem.
Phys. 1992, 97, 7590.
(19) Bruni, S.; Cariati, F.; Cariati, E.; Porta, F. A.; Quici, S.; Roberto,
D. Spectrochim. Acta, Part A 2001, 57, 1417.
(14) Dragonetti, C.; Pizzotti, M.; Roberto, D.; Galli, S. Inorg. Chim.
Acta 2002, 330, 128.
(15) Kolodziej, R. M., Lees, A. J . Organometallics 1986, 5, 450.
(20) (a) Oudar, J . L.; Chela, D. S. J . Chem. Phys. 1977, 66, 2664.
(b) Oudar, J . J . Chem. Phys. 1977, 67, 446.

Late-Early Heteronuclear Bimetallic Complexes
Organometallics, Vol. 22, No. 20, 2003 4003
oscillator strength f of the absorption band, νa is the frequency
of the MLCT absorption band, νL is the frequency of the
fundamental incident radiation (in our case 1.907 µm), and
∆µeg is the variation of the dipole moment between excited and
ground states.19
of the solvent. The final suspension of the brown powder was
filtered under a nitrogen atmosphere and washed with n-
hexane (yield 25%). Compound 2 must be stored under
nitrogen in the dark. IR (CH2Cl2; (cm-1)): νCO 2054 (s), 1980
1
(s), 1938 (m), 1840 (br). H NMR (CDCl3; δ (ppm)): 9.50 (dd,
Syn th esis of th e Com p lexes. The numbering used in the
attribution of the H NMR signals of the 1,10-phenanthroline
(phen) and pyrazine (pyz) complexes is
2H, H1H1′), 8.60 (dd, 2H, H3H3′), 8.09 (s, 2H, H4H4′), 7.93-7.97
(m, 2H, H2H2′). Visible spectrum (CHCl3; λmax (nm) (log ꢀ)): 385
(3.74), 503 (3.59). Anal. Found (calcd) for C17H8N2O5ClWIr: C,
28.0 (27.9); H, 1.11 (1.09); N, 3.80 (3.83).
1
[(CO)3(p h en )W-fa c-Ru (CO)3Cl2] 3. To a solution of [Ru-
(CO)3Cl2]2 (109 mg, 0.213 mmol) in THF (10 mL) at 0-5 °C
(ice bath) was added dropwise a blue solution of [fac-W(CO)3-
(phen)(CH3CN)] (209 mg, 0.429 mmol) in THF (45 mL). After
1 h the temperature was raised to room temperature, and after
5 h the suspension was filtered off. Then the solvent was
partially removed under reduced pressure and compound 3
precipitated, by addition of n-hexane, as a brown powder
together with some amount of [(CO)3(phen)W-fac-Ru(CO)3Cl2]‚
THF with one molecule of THF coordinated, as evidenced by
the high number of carbonyl bands in the IR spectrum of the
[fa c-W(CO)3(p h en )(p yz)]. To a solution of pyrazine (216
mg, 2.7 mmol) in THF (5 mL) at 0-5 °C (ice bath) was added
dropwise a blue solution of [fac-W(CO)3(phen)(CH3CN)] (140
mg, 0.27 mmol) in THF (20 mL) over 5 min. The reaction
mixture changed color immediately from blue to violet and was
stirred for 15 min. Then small portions of n-hexane were
added, alternating with slow evaporation of the solvent. Finally
the violet powder was filtered off, washed with n-hexane, and
dried in vacuo (yield 36%). This compound must be stored
under nitrogen in the dark. IR (CH2Cl2; cm-1): νCO 1898 (s),
1786 (br). 1H NMR (CD2Cl2; δ (ppm)): 9.68 (dd, 2H, H1H1′),
8.49 (dd, 2H, H3H3′), 8.15 and 8.22 (2m, 4H, H5H6H7H8), 7.99
(s, 2H, H4H4′), 7.84-7.88 (m, 2H, H2H2′). Visible spectrum (CH2-
Cl2; λmax (nm) (log ꢀ)): 356 (2.87), 511 (3.09), 578 (3.28). Anal.
Found (calcd) for C19H12N4O3W: C, 42.9 (43.2); H, 2.29 (2.27);
N, 9.96 (10.1).
1
mixture (Table 1) and by its H NMR spectum.21 However, in
CHCl3 solution the mixture slowly rearranged to compound 3
(about 24 h), which was recovered by evaporation to dryness
(yield 80%). IR (CHCl3; (cm-1)): νCO 2107 (s), 2030 (s), 2018
1
(s), 2009 (m), 1952 (m), 1871 (s), 1827 (s). H NMR (CDCl3; δ
(ppm)): 10.0 (m, 2H, H1H1′), 8.57 (d, 2H, H3H3′), 8.02 (s, 2H,
H4H4′), 7.87 (dd, 2H, H2H2′). Visible spectrum (CHCl3; λmax (nm)
(log ꢀ)): 412 (3.63), 500 (3.43). Anal. Found (calcd) for
C
18H8N2O6Cl2WRu: C, 30.8 (30.7); H, 1.18 (1.15); N, 4.08
(4.00).
[(CO)3(p h en )W-fa c-Os(CO)3Cl2] 4. To a solution of [Os-
(CO)3Cl2]2 (93 mg, 0.135 mmol) in THF (5 mL) at 0-5 °C (ice
bath) was added dropwise [fac-W(CO)3(phen)(CH3CN)] (132
mg, 0.270 mmol) in THF (30 mL). After 1 h the temperature
was raised to room temperature, and after 4 h the dark brown
suspension was filtered off. The solution was evaporated to
two-thirds of the original volume, and then, by adding n-
hexane, compound 4 precipitated as a brown powder together
with some amount of [(CO)3(phen)W-fac-Os(CO)3Cl2]‚THF with
one molecule of THF coordinated, as evidenced by the high
number of carbonyl bands in the IR spectrum of the mixture
[(CO)3(p h en )W(p yz)W(CO)3(p h en )]. This compound was
prepared by following the above procedure for the synthesis
of the monomer [fac-W(CO)3(phen)(pyz)] but using a molar
ratio of 1:1. After 10 min a violet dimer precipitated from the
solution. It was filtered off and washed with xylene (yield 47%).
This compound must be stored in the dark. IR (Nujol; cm-1):
1
νCO 1877 (vs), 1792 (vs), 1758 (s). H NMR (CD2Cl2; δ (ppm)):
9.47 (dd, 2H, H1H1′), 8.59 (s, 4H, H5H6H7H8), 8.49 (dd, 2H,
H3H3′), 8.0 (s, 2H, H4H4′), 7.79-7.86 (m, 2H, H2H2′). Anal.
Found (calcd) for C34H20N6O6W2: C, 41.6 (41.8); H, 2.09 (2.05);
N, 8.61 (8.61).
1
(Table 1) and by its H NMR spectum.21 In CHCl3 solution the
mixture slowly rearranged to compound 4 (about 24 h), which
was recovered by evaporation to dryness (yield 74%). IR
(CHCl3; (cm-1)): νCO 2107 (s), 2029 (vs), 2021 (s), 1951 (m),
[(CO)3(p h en )W-cis-Rh (CO)2Cl] 1. To a solution of [Rh-
(CO)2Cl]2 (50 mg, 12.8 mmol) in THF (5 mL) at 0-5 °C (ice
bath) was added dropwise a blue solution of [fac-W(CO)3(phen)-
(CH3CN)] (125 mg, 25.6 mmol) in THF (30 mL) over 5 min.
After 15 min small portions (3 mL) of n-hexane were added to
the brown solution, alternating with slow evaporation of the
solvent. Finally the suspension of the brown powder was
filtered off under a nitrogen atmosphere, washed with n-
hexane, and dried in vacuo (yield 40%). Compound 1 must be
stored under nitrogen in the dark. IR (CHCl3; (cm-1)): νCO 2063
1
1869 (m), 1827 (m). H NMR (CDCl3; δ (ppm)): 9.99 (m, 2H,
H1H1′), 8.56 (d, 2H, H3H3′), 8.02 (s, 2H, H4H4′), 7.87 (dd, 2H,
H2H2′), Visible spectrum (CHCl3; λmax (nm) (log ꢀ)): 406 (3.66),
495 (3.39). Anal. Found (calcd) for C18H8N2O6Cl2WOs: C, 27.5
(27.2); H, 1.06 (1.02); N, 3.56 (3.53).
Rea ctivity of 1 a n d 2 w ith P P h 3. To a solution of [(CO)3-
(phen)W-cis-M(CO)2Cl] (M ) Rh(I) 1, Ir(I) 2; 0.08 mmol) in
THF (10 mL) was added a solution of PPh3 (43 mg, 0.16 mmol)
in THF (10 mL) under a nitrogen atmosphere. The reaction is
instantaneous, as confirmed by the IR spectrum registered
after the addition of PPh3, which shows the parallel formation
of [W(CO)4(phen)]9 and [trans-M(CO)Cl(PPh3)2] (M ) Rh(I),22b
Ir(I)22a) in quantitative yields.
1
(s), 1997 (s), 1929 (s), 1834 (s); νRhCl (Nujol) 287 (w). H NMR
(CDCl3; δ (ppm)): 9.51 (dd, 2H, H1H1′), 8.59 (dd, 2H, H3H3′),
8.07 (s, 2H, H4H4′), 7.90-7.95 (m, 2H, H2H2′). Visible spectrum
by deconvolution (CHCl3; λmax (nm) (log ꢀ)): 392 (3.86), 444
(3.65), 517 (3.85). Anal. Found (calcd) for C17H8N2O5ClWRh:
C, 32.1 (31.8); H, 1.29 (1.25); N, 4.32 (4.36).
Compound 1 was also prepared by starting from [Rh(CO)2Cl-
(pyz)] instead of [Rh(CO)2Cl]2 and [fac-W(CO)3(phen)(CH3CN)],
following the same procedure (yield 37.5%).
[(CO)3(p h en )W-cis-Ir (CO)2Cl] 2. To a yellow solution of
[cis-Ir(CO)2Cl(pyz)] (93.1 mg, 0.256 mmol) in THF (5 mL) at
0-5 °C (ice bath) was added dropwise a deep blue solution of
[fac-W(CO)3(phen)(CH3CN)] (125 mg, 0.256 mmol) in THF (30
mL) over 5 min. After 10 min the dark brown solution was
evaporated to dryness and the violet-black residue was dis-
solved in 8 mL of THF; after filtration of a small insoluble
residue, a brown powder was precipitated by adding succes-
sively small portions of n-hexane followed by slow evaporation
Rea ctivity of 3 a n d 4 w ith P P h 3. To a solution of [(CO)3-
(phen)W-fac-M(CO)3Cl2] (M ) Ru(II) 3, Os(II) 4; 0.07 mmol)
(21) For [(CO)3(phen)W-fac-Ru(CO)3Cl2]‚THF: 1H NMR (CDCl3; δ
(ppm)) 9.62 (d, 2H, H1H1′), 8.48 (d, 2H, H3H3′), 8.01 (s, 2H, H4H4′), 7.79
(dd, 2H, H2H2′), 4.37 and 2.07 (two multiplets due to coordinated THF).
For [(CO)3(phen)W-fac-Os(CO)3Cl2]‚THF: 1H NMR (CDCl3; δ (ppm))
9.63 (d, 2H, H1H1′), 8.48 (d, 2H, H3H3′), 8.01 (s, 2H, H4H4′), 7.79
(dd, 2H, H2H2′), 4.37 and 2.07 (two multiplets due to coordinated
THF).
(22) (a) Heilweil, E. J .; Cavanagh, R. R.; Stephenson, J . C. Chem.
Phys. Lett. 1987, 134, 181. (b) Williams, A. F.; Bhaduri, S.; Maddock,
A. G. J . Chem. Soc., Dalton Trans. 1975, 1958. (c) Chatt, J .; Melville,
D. P.; Richards, R. L. J . Chem. Soc. A 1969, 18, 2841. (d) J ohnson B.
F. G.; J ohnston, R. D.; Lewis, J . J . Chem. Soc. A 1969, 5, 792. (e)
Bradford, C. W.; Nyholm, R. S. Chem. Commun. (London) 1967, 8, 384.

4004 Organometallics, Vol. 22, No. 20, 2003
Pizzotti et al.
in THF (10 mL) was added a solution of PPh3 (37 mg, 0.14
mmol) in THF (10 mL) under a nitrogen atmosphere. The color
turned slowly from brown to violet. When the reaction was
followed by IR spectroscopy, after 5 h the complete transfor-
mation of compounds 3 and 4 into [W(CO)3(phen)(PPh3)]22c and
[fac-M(CO)3Cl2(PPh3)] (M ) Ru(II),22d Os(II)22e) was observed.
The solution was evaporated to dryness, and the mixture was
recrystallized from CHCl3/n-hexane in order to separate the
two products, the violet [W(CO)3(phen)(PPh3)] being insoluble
in n-hexane and the white [fac-M(CO)3Cl2(PPh3)] (M ) Ru, Os)
being soluble in n-hexane.
Rea ctivity of 1 w ith P yr id in e. To a solution of compound
1 (50 mg, 0.078 mmol) in THF (20 mL), pyridine (6.5 mg, 0.080
mmol, 6.6 µL) was added under nitrogen atmosphere. Follow-
ing the reaction by IR spectrum, after 2 h we observed the
complete transformation of compound 1 into [W(CO)4(phen)]9
and [cis-Rh(CO)2Cl(py)].23
Rea ctivity of 3 a n d 4 w ith CO. Portions of compounds 3
and 4 (50 mg) were dissolved in THF (10 mL), and CO was
bubbled through the solution. Following the reaction by IR
spectroscopy, after 4 h we observed the complete conversion
of compounds 3 and 4 into [W(CO)4(phen)]9 and [fac-M(CO)3Cl2-
(THF)] (M ) Ru(II), Os(II)).24
Attem p ted Syn th esis of [(CO)3(p h en )WBF 3] 5. In a 50
mL flask equipped with a condenser, 43 mg of [fac-W(CO)3-
(phen)(pyz)] were dissolved in 20 mL of CH2Cl2. To this violet
solution was added dropwise BF3‚OEt2 (10 µL). Immediately
a green powder precipitated. It was filtered under nitrogen,
washed with dry CH2Cl2 in order to remove a small amount
of the adduct [pyzBF3], and dried with a flux of dry nitrogen.
The green powder was stored under nitrogen; after 2 h it
became blue and insoluble in all common solvents (yield 70%).
The freshly prepared material shows the following IR and
analytical data. IR (Nujol; (cm-1)): νCO 2054 (vw), 2010 (s),
1924 (s); νBF 1087 (br). Anal. Found (calcd) for C15H8N2O3-
WBF3‚2CH2Cl2: C, 28.9 (29.8); H, 2.05 (1.75); N, 4.40 (4.08)
These data suggest the loss of the pyrazine bridge and the
presence of the W(CO)3(phen) and BF3 moieties.
Attem p ted Syn th esis of [W(CO)3(p h en )-cis-Re(CO)4Cl].
A 77 mg (0.115 mmol) portion of [Re(CO)4Cl]2 was dissolved
in THF (10 mL) under a nitrogen atmosphere in order to obtain
[cis-Re(CO)4Cl(THF)]. To this solution, cooled in a ice bath,
was slowly added a solution of [fac-W(CO)3(phen)(CH3CN)]
(112 mg, 0.229 mmol) in THF (30 mL). The mixture turned
immediately black and then brown. At the end of the reaction
we observed by IR spectroscopy the presence in solution of
[W(CO)4(phen)] and some [Re(CO)4Cl]2. This behavior sug-
gests the probable formation of the very unstable bimetallic
compound [W(CO)3(phen)-cis-Re(CO)4Cl], which rapidly de-
composes to the very stable [W(CO)4(phen)] and some [Re-
(CO)4Cl]2.
Attem p ted Syn th esis of [W(CO)3(p h en )W(CO)5]. To a
solution of [fac-W(CO)3(phen)(CH3CN)] (145 mg, 0.296 mmol)
in THF (20 mL), cooled by an ice bath, was added dropwise a
freshly prepared solution of [W(CO)5(THF)]15 (117 mg, 0.296
mmol) in THF (16 mL) under a nitrogen atmosphere. After 4
h the brown solution was evaporated to a small volume and
n-hexane was added. The precipitated brown powder was
recognized as impure [W(CO)4(phen)], while in the mother
liquor the infrared spectrum suggested the presence of some
[W(CO)6] and [W(CO)5(THF)].
) 17.050(9) Å, c ) 6.905(8) Å, Z ) 4, dcalcd ) 2.381 Mg m-3, µ
) 123.0 cm-1, F(000) ) 1328, R ) 0.048, Rw ) 0.120 for 1578
observed reflections (I > 2σ(I)). Compound 4: C18H8Cl2N2O6-
OsW, fw 793.23, triclinic, space group P1h (No. 2), a ) 7.3963-
(7) Å, b ) 11.3377(11) Å, c ) 12.3266(8) Å, R ) 83.58(1)°. â )
88.50(1)°, γ ) 82.00(1)°, Z ) 2, dcalcd ) 2.590 Mg m-3, µ )
121.92 cm-1, F(000) ) 724, R ) 0.031, Rw ) 0.075 for 3785
observed reflections (I > 2σ(I)). Intensity data collection was
performed with graphite-monochromated Mo KR radiation (λ
) 0.71073 Å) on an Enraf-Nonius CAD4 diffractometer for
compound 2 and on a Siemens SMART CCD diffractometer
for compound 4. Data sets were corrected for Lorentz-
polarization effects, decay (only for 2; maximum decay 6% at
the end of the data collection), and absorption.25 Both struc-
tures were solved by direct methods using SHELXS 86;26 full-
matrix least-squares refinements on F2 were performed using
SHELXL 93.27 All non-H atoms were refined anisotropically,
and H atoms were seen in difference Fourier maps, placed,
and thereafter allowed to ride on their parent atoms. The
significant structural parameters are reported in Table 2.
Crystallographic data (excluding structure factors) for struc-
tures of compounds 2 and 4 have been deposited with the
Cambridge Crystallographic Data Centre as Supplementary
Publication Nos. CCDC-213211 and 213212. Copies of the data
can be obtained free of charge on application to The Director,
CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (fax, int.
code +(1223) 336-033; e-mail, teched@chemcrys.cam.ac.uk).
Volta m m etr ic In vestiga tion . The cyclic voltammetric
investigation was carried out on carefully deareated CH2Cl2
solutions (typically 0.0008-0.001 M) with an Autolab PGSTAT
30 potentiostat/galvanostat run by a PC with GPES software.
The solutions were made up with HPLC-grade CH2Cl2 and
tetrabutylammonium perchlorate TBAP (>99% Fluka) as the
0.1 M supporting electrolyte. Each compound was studied in
the interval -0.5 to +2.2 V (SCE), at different potential scan
rates (v) ranging from 0.02 to 0.5 V s-1, on an AMEL glassy-
carbon-disk electrode (GC, ∼2.5 mm diameter), using a
platinum counter electrode and a saturated calomel electrode
(SCE) as a reference. Since the reaction products appeared to
strongly film the working electrode, the latter was carefully
cleaned before each first cycle and the reproducibility of the
resulting CV results carefully checked. The electrochemical
reversibility and number of electrons transferred in the first
oxidation peak for each compound were checked by classical
tests such as Ep vs log v, Ep - Ep/2, and Ip vs v1/2 analysis, plus
deconvolution of the original CV characteristics followed by
analysis of the resulting “stationary”, steplike waves in order
to obtain the corresponding half-wave potentials E1/2 and n
(or Rn) values.28
Resu lts a n d Discu ssion
Syn th esis of Bim eta llic Com p lexes. The complex
[fac-W(CO)3(phen)(pyz)] was synthesized by reaction in
THF at 0-5 °C of the known complex [fac-W(CO)3-
(phen)(CH3CN)]13 with a large excess of pyrazine (pyz)
(1:10), in order to avoid the formation of the symmetrical
dimer [(CO)3(phen)W(pyz)W(phen)(CO)3] which precipi-
tates. The monomeric pyrazine compound which, ac-
cording to its infrared spectrum in the carbonyl stretch-
ing region (Table 1), has a fac arrangement of the three
X-r a y Str u ctu r e Deter m in a tion of [(CO)3(p h en )W-cis-
Ir (CO)2Cl] 2 a n d [(CO)3(p h en )W-fa c-Os(CO)3Cl2] 4. Suit-
able crystals of both compounds 2 and 4 were obtained by slow
crystallization in CH2Cl2/n-hexane.
Cr ysta l Da ta . Compound 2: C17H8ClIrN2O5W, fw 731.75,
orthorhombic, space group P21212 (No. 18), a ) 17.337(4) Å, b
(25) North, A. C. T.; Phillips, D. C.; Mathews, F. S. Acta Crystallogr.,
Sect. A 1968, 24, 351. SADABS Area-Detector Absorption Correction
Program, Bruker AXS, Inc. Madison, WI.
(26) Sheldrick, G. M. SHELXS 86. Acta Crystallogr., Sect. A 1990,
46, 467.
(27) Sheldrick, G. M. SHELXS 93; University of Go¨ttingen, Go¨ttin-
gen, Germany, 1993.
(28) Bard, A. J .; Faulkner, L. R. Electrochemical Methods: Funda-
mentals and Applications; Wiley: New York, 2002; pp 231-239, 247-
252.
(23) Heaton, B. T.; J acob, C.; Sampanthar, J . T. J . Chem. Soc.,
Dalton Trans. 1998, 8, 1403.
(24) For Ru(CO)3Cl2(THF): νCO (THF) 2131 (s), 2056 (vs) cm-1. For
Os(CO)3Cl2(THF): νCO (THF) 2122 (s), 2034 (vs), 2025 (vs) cm-1
.

Late-Early Heteronuclear Bimetallic Complexes
Organometallics, Vol. 22, No. 20, 2003 4005
carbonyl ligands as in the starting complex [fac-W(CO)3-
(phen)(CH3CN)], is slightly labile in solution toward
1
pyrazine dissociation. In its H NMR spectrum at room
temperature in CDCl3 or CD2Cl2 the slow formation of
a certain amount of the symmetrical dimer [(CO)3-
(phen)W(pyz)W(phen)(CO)3] is detected. The latter sepa-
rates slowly as an insoluble compound (see Experimen-
tal Section). The heteronuclear bimetallic complex 1 was
first casually prepared in an attempt to synthesize
heteronuclear bimetallic complexes, with pyrazine as
bridging linker, by reaction of [fac-W(CO)3(phen)(pyz)]
with [Rh(CO)2Cl]2 in THF. The fragment W(CO)3(phen)
was still present in the reaction product, as suggested
1
by IR and H NMR spectra; however, both the elemental
1
analysis and the H NMR spectrum did not confirm the
presence of the pyrazine bridge, suggesting the forma-
tion of a heteronuclear metal-metal bond, as confirmed
later by the X-ray structural determination of the
related iridium compound 2 (see later), with a W-Ir
heteronuclear early-late metal-metal bond. (Figure 1).
By reaction of [fac-W(CO)3(phen)(pyz)] with BF3(OEt2)
in CH2Cl2, a green powder separated 5, very insoluble
in all common solvents and unstable even under a
nitrogen atmosphere (in 2 h the color turns from green
to blue). Due to its instability it was not possible to
characterize 5 in a satisfactory way by elemental
analyses. However, both νCO and νBF stretchings of its
solid-state IR spectrum (Table 1) and the order of
magnitude of the elemental analyses suggest the loss
of the pyrazine bridge and the presence of both fac-
W(CO)3(phen) and BF3 moieties.
The tendency of the W(CO)3(phen) fragment to behave
as a σ-donor base toward various “soft” organometallic
acceptors was successfully confirmed, using as starting
material the more easily available complex [fac-W(CO)3-
(phen)(CH3CN)] according to the reactions
[fac-W(CO)3(phen)(CH3CN)] + 1/2[Rh(CO)2Cl]2 THF8
[(CO)3(phen)W-cis-Rh(CO)2Cl] + CH3CN (2)
1
[fac-W(CO)3(phen)(CH3CN)] +
[cis-Ir(CO)2Cl(pyz)] THF8
F igu r e 1. Two perspective views of [(CO)3(phen)W-cis-Ir-
(CO)2Cl] 2.
[(CO)3(phen)W-cis-Ir(CO)2Cl] + CH3CN + pyz (3)
adducts [(CO)3(phen)W-fac-M(CO)3Cl2]‚THF (M ) Ru,
Os); however, the presence of coordinated THF was
evidenced by the 1H NMR spectra in CDCl3 with the
THF signal shifted downfield by about 0.5 ppm with
respect to THF as such. In CHCl3 solution these
mixtures slowly rearrange only to compounds 3 and 4,
respectively. An attempt to synthesize the complex
[(CO)3(phen)W-cis-Re(CO)4Cl] by dropping a solution of
[fac-W(CO)3(phen)(CH3CN)] in THF into a solution of
[cis-Re(CO)4Cl(THF)] in THF at 0-5 °C failed. The
solution turned immediately dark brown with the
formation of an unstable species, probably the expected
bimetallic complex, which quickly decomposes into
[W(CO)4(phen)], easily separated and purified, and a
brown precipitate, probably an insoluble carbonyl clus-
ter of rhenium which we were unable to characterize.
A similar reaction occurred when we tried to synthesize
the complex [(CO)3(phen)W-W(CO)5] by reaction in
THF of [fac-W(CO)3(phen)(CH3CN)] at 0-5 °C with
[W(CO)5(THF)]. A brown solution, probably the expected
2
[ fac-W(CO)3(phen)(CH3CN)] +
1/2[fac-M(CO)3Cl2]2 THF8
+ CH CN (4)
[(CO)3(phen)W-fac-M(CO)3Cl2]
M ) Ru 3, Os 4
3
Infrared spectra (Table 1) suggest for compounds 1
and 2 a cis geometry of the two carbonyl ligands and
for compounds 3 and 4 three carbonyl ligands with a
fac geometry. These suggestions were confirmed by the
X-ray structural determinations of 2 and 4 (Figures 1
and 2, respectively). Although compounds 3 and 4 were
the major components of the reaction mixtures, small
amounts of the fac isomer with one THF molecule
coordinated to one of the two metal atoms were obtained
1
in parallel, as evidenced by the IR and H NMR spectra
of the reaction mixtures (see Experimental Section). We
were unable to isolate with an acceptable purity the

4006 Organometallics, Vol. 22, No. 20, 2003
Pizzotti et al.
Ta ble 1. IR Ca r bon yl Str etch in gs
νCO (cm-1
compd
)
solvent
[W(CO)3(phen)(CH3CN)]12
[W(CO)3(phen)(pyz)]
1898 (vs), 1783 (s)
1898 (s), 1786 (br)
1883 (s), 1785 (s), 1758 (vs)
2092 (s), 2019 (s)
2086 (s), 2012 (s)
2081 (s), 2001 (s)
2081 (s), 2008 (s)
2139 (s), 2079 (s), 2055 (s)
2131 (s), 2057 (s), 2029 (s)
2063 (s), 1997 (s), 1929 (s), 1834 (s)
2054 (s), 1980 (s), 1938 (m), 1840 (br)
2107 (s), 2030 (s), 2018 (s), 1952 (m), 1871 (s), 1827 (s)
2124 (m), 2024 (vs), 2005 (s), 1950 (s), 1888 (vs), 1838 (s)
2107 (s), 2029 (vs), 2021 (s), 1951 (s), 1869 (m), 1827 (m)
2125 (m), 2025 (vs), 2006 (s), 1949 (s), 1888 (s), 1838 (s)
2054 (vw), 2010 (s), 1924 (s)
CH3CN
CH2Cl2
Nujol
CH2Cl2
THF
CH3CN
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
CH2Cl2
CHCl3
THF
[cis-Rh(CO)2Cl(pyz)]13
[cis-Ir(CO)2Cl(pyz)]13
[fac-Ru(CO)3Cl2(pyz)]13
[fac-Os(CO)3Cl2(pyz)]13
[(CO)3(phen)W-cis-Rh(CO)2Cl] 1
[(CO)3(phen)W-cis-Ir(CO)2Cl] 2
[(CO)3(phen)W-fac-Ru(CO)3Cl2] 3
[(CO)3(phen)W-fac-Ru(CO)3Cl2]‚THF
[(CO)3(phen)W-fac-Os(CO)3Cl2] 4
[(CO)3(phen)W-fac-Os(CO)3Cl2]‚THF
[(CO)3(phen)WBF3] 5
CHCl3
THF
Nujol
ties such as W(0) and Re(I) (both 5d6) or two W(0)
species, an easy breakup of the metal-metal bond takes
place with the shift of a carbonyl ligand.
IR a n d 1H NMR Sp ectr a . In the IR spectra (Table
1) of complexes 1-4 the stretchings of the carbonyls of
the fragment W(CO)3(phen) are at higher frequencies
(about 50 cm-1) with respect to [fac-W(CO)3(phen)(CH3-
CN)] or [fac-W(CO)3(phen)(pyz)], as expected for a W
atom acting as a σ-donor base. At the same time the
stretchings of the carbonyls of the Rh(I), Ir(I), Ru(II),
and Os(II) acceptor fragments are shifted to slightly
lower frequencies with respect to Rh(I) or Ir(I) cis or
Ru(II) or Os(II) fac carbonyl complexes with a rather
weak σ-donor base such as pyrazine14 (Table 1). These
observations suggest a not irrelevant σ donation from
the W atom. The IR spectrum in Nujol of 5 shows the
three stretching carbonyl bands expected for the W(CO)3-
(phen) fragment but at much higher frequencies (∆ν ≈
150 cm-1) with respect to the complex [fac-W(CO)3-
(phen)(pyz)] together with the band between 1000 and
1100 cm-1 attributable to the stretching νB-F of a BF3
group (Table 1).
The IR spectra in CHCl3 of complexes 3 and 4 show
six carbonyl bands, of which three bands, at lower
frequency, are attributable to νCO stretching of a fac-
W(CO)3(phen) fragment; the other three, at high fre-
quency, are due to the νCO stretching of the fragment
fac-M(CO)3Cl2 (M ) Ru, Os). If a molecule of THF is
coordinated to compounds 3 and 4, the stretchings of
some carbonyl bands are slightly shifted, about 10-20
cm-1 to higher frequency (Table 1).
1
The H NMR spectra of complexes 1-4 in CDCl3 are
very similar. They are characterized by a slight down-
field shift, with respect to free phenanthroline, of the
protons in the positions R to the coordinated nitrogen
atoms. This shift is due to the electron transfer from
1
the phenanthroline ligand to W.29 In addition, the H
NMR spectra suggest a perfect symmetry of the phenan-
throline ligand along the axis W-M, as later confirmed
by the X-ray structure of compounds 2 (Figure 1) and 4
(Figure 2). The 1H NMR spectra also show that the
acceptor properties of the fragments fac-M(CO)3Cl2 (M
) Ru,Os) are slightly higher than that of cis-M(CO)2Cl
(M ) Rh, Ir), since the protons of the phenanthroline
ligand in the positions R to the nitrogen atoms are more
F igu r e 2. Two perspective views of [(CO)3(phen)W-fac-
Os(CO)3Cl2] 4.
bimetallic compound, was suddenly formed, which was
very unstable, decomposing quickly with formation of
[W(CO)4(phen)] and some [W(CO)6]. It appears thus that
when the two fragments have similar electronic proper-
(29) Roberto, D.; Ugo, R.; Tessore, F.; Lucenti, E.; Quici, S.; Vezza,
S.; Fantucci, P.; Invernizzi, I.; Bruni, S.; Ledoux-Rak, I.; Zyss, J .
Organometallics 2002, 21, 161.

Late-Early Heteronuclear Bimetallic Complexes
Organometallics, Vol. 22, No. 20, 2003 4007
Ta ble 2. Selected In ter a tom ic Dista n ces (Å) a n d
An gles (d eg) for [(CO)3(p h en )W-cis-Ir (CO)2Cl] 2
a n d [(CO)3(p h en )W-fa c-Os(CO)3Cl2] 4
shifted to low field in complexes containing Ru(II) and
Os(II) (∆δ about 0.5 ppm) than in those containing Rh-
(I) and Ir(I) (∆δ about 0.3 ppm). Finally, when THF is
coordinated to compounds 3 and 4, this shift to low field
is intermediate (∆δ about 0.4 ppm), with a parallel shift
downfield of about 0.5 ppm of the THF signal, so that
M ) Ir 2
M ) Os 4
M-W
2.728(2)
2.368(7)
2.958(1)
2.399(2)
2.418(2)
1.895(8)
1.896(7)
1.927(8)
2.540(2)
2.242(5)
2.221(5)
1.956(7)
1.942(7)
1.967(7)
1.149(9)
1.153(8)
1.159(9)
1.118(11)
1.115(9)
1.119(10)
1.382(8)
1.329(8)
1.353(10)
1.376(9)
1.401(10)
1.418(9)
1.395(11)
1.338(15)
1.408(13)
1.444(15)
1.319(16)
1.427(17)
1.382(14)
1.406(10)
1.325(18)
1.366(12)
73.6(2)
M-Cl(1)
1
M-Cl(2)
in the H NMR spectrum the simultaneous presence of
M-C(16)
1.76(3)
1.86(3)
some [(CO)3(phen)W-fac-M(CO)3Cl2]‚THF (M ) Ru, Os)
M-C(17)
adducts is easily identified.21
M-C(18)
X-r a y Str u ctu r es of [(CO)3(p h en )W-cis-Ir (CO)2Cl]
an d [(CO)3(ph en )W-fa c-Os(CO)3Cl2]. Perspective views
of the structure of [(CO)3(phen)W-cis-Ir(CO)2Cl] 2 and
[(CO)3(phen)W-fac-Os(CO)3Cl2] 4 are shown in Figures
1 and 2, respectively; selected interatomic distances and
angles are reported in Table 2. Both structures confirm
the coordination of a W(CO)3(phen) moiety to a cis-Ir-
(CO)2Cl and a fac-Os(CO)3Cl2 fragment formally by a
kind of acid-base interaction, with the W atom acting
as a σ-donor base. In both complexes this interaction
gives rise to a W-M bond asymmetrically bridged by a
chlorine atom; the W-M bond length (2.728(2) Å for M
) Ir and 2.958(1) Å for M ) Os) falls in the range of
values usually reported for single bonds.30 The sym-
metry of the X-ray structures of 2 and 4 is what is
expected on the basis of the 1H NMR spectra. The
tungsten atom achieves a distorted (6 + 1) octahedral
coordination, with the two nitrogen atoms of the phenan-
throline ligand and two carbonyls defining the equato-
rial plane of the octahedron, whereas a chlorine ligand
and the remaining carbonyl are located in apical posi-
tions. The arrangement of the carbonyl ligands in the
W(CO)3(phen) fragment is in agreement with the fac
geometry suggested by the IR spectra. Significant
deviations from the idealized octahedral geometry are
evident. For instance, the departure of the N(1)-W-
N(2) angle from the ideal 90° value (73.6(7) and 73.6-
(2)°, in 2 and 4, respectively) is determined by the bite
of the phenanthroline ligand. The C(13)-W-C(14) and
C(13)-W-C(15) angles are also quite lower than 90°,
but the C(14)-W-C(15) angle is significantly enlarged.
The M atom (M ) Ir, Os) can also be envisioned as
almost capping a triangular face of the octahedron,
defined by the atoms Cl(1), C(14), and C(15). Short
contacts with carbonyls 14 and 15 are observed for
complex 2 (Ir‚‚‚C(14) ) 2.62(3) Å, Ir‚‚‚C(15) ) 2.69(3)
Å), which however are ca. 0.5 Å larger than typical Ir-
CO (bridging or semibridging) distances. In complex 4
the Os atom, upon formation of a single W-Os bond,
achieves a distorted-octahedral coordination. Here again
a relatively short contact of 2.747(7) Å between Os and
carbonyl CO(15) is observed. These weak interactions
observed in either 2 or 4 do not produce large deviations
from linearity of the W-C-O system involved, but only
a pushing of the CO(14) and CO(15) ligands out of the
equatorial plane of the octahedron toward CO(13), and
thus account for the distortions observed. The Ir-Cl(1)
distance found in 2 (2.368(7) Å) is much closer to typical
Ir-Cl(terminal) distances rather than to Ir-Cl(bridg-
ing) distances.31 The same holds for the Os-Cl(1)
W-Cl(1)
2.567(8)
2.22(2)
2.20(2)
1.98(3)
1.97(2)
1.97(3)
1.16(4)
1.16(3)
1.14(3)
1.12(4)
1.12(4)
W-N(1)
W-N(2)
W-C(13)
W-C(14)
W-C(15)
C(13)-O(13)
C(14)-O(14)
C(15)-O(15)
C(16)-O(16)
C(17)-O(17)
C(18)-O(18)
N(1)-C(1)
1.38(3)
1.32(3)
1.33(4)
1.37(3)
1.43 (4)
1.37 (3)
1.49 (5)
1.32 (5)
1.45(4)
1.47(4)
1.31 (4)
1.43 (4)
1.45 (4)
1.43(3)
1.34 (5)
1.48(4)
73.6(7)
94.0(9)
91.0(9)
162.9(9)
167.5(9)
86.2(6)
83.1(6)
94(1)
98(1)
80(1)
81(1)
101(1)
53.0(2)
179(1)
99.6(8)
98.6(8)
60.0(2)
96(1)
N(1)-C(2)
N(2)-C(11)
N(2)-C(12)
C(1)-C(5)
C(1)-C(12)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(5)-C(6)
C(6)-C(7)
C(7)-C(8)
C(8)-C(9)
C(8)-C(12)
C(9)-C(10)
C(10)-C(11)
N(1)-W-N(2)
N(1)-W-C(15)
N(2)-W-C(14)
N(1)-W-C(14)
N(2)-W-C(15)
N(1)-W-Cl(1)
N(2)-W-Cl(1)
N(1)-W-C(13)
N(2)-W-C(13)
C(13)-W- C(14)
C(13)-W- C(15)
C(14)-W- C(15)
Cl(1)-W-M
Cl(1)-W-C(13)
Cl(1)-W-C(14)
Cl(1)-W-C(15)
Cl(1) -M-W
Cl(1)-M-C(17)
C(16)-M-C(17)
C(16)-M-W
Cl(1)-M-C(16)
W-M-C(17)
M-Cl(1)-W
Cl(1)-M-C(17)
Cl(1)-M-C(18)
C(17)-M-C(18)
C(18)-M-W
Cl(2)-M-C(16)
Cl(2)-M-C(17)
Cl(2)-M-C(18)
Cl(2)-M-W
Cl(2)-M-Cl(1)
C(16)-M-C(18)
93.9(2)
94.6(3)
167.9(2)
160.1(2)
82.8(1)
83.8(2)
96.8(2)
89.4(2)
79.6(3)
76.5(3)
96.5(3)
51.05(4)
173.0(2)
99.4(2)
110.5(2)
55.42(4)
169.9(2)
91.3(3)
91.0(2)
90.3(2)
114.6(2)
73.53(5)
169.9(2)
97.9(2)
92(1)
113(1)
172(1)
155(1)
66.9(2)
92.0(3)
153.1(2)
176.7(2)
90.0(2)
84.2(3)
91.17(5)
88.93(7)
92.8(3)
(30) (a) Einstein, F. W. B.; Pomeroy, R. K.; Rushman, P.; Willis, A.
C. Organometallics 1985, 4, 250. (b) Baumann, F. E.; Howard, J . A.
K.; Musgrove, R. J .; Sherwood, P.; Ruiz, M. A.; Stone, F. G. A. J . Chem.
Soc., Chem. Commun. 1987, 1881. (c) Rosenberg, S.; Whittle, R. C.;
Geoffroy, G. L. J . Am. Chem. Soc. 1984, 106, 5934.
(31) Dickson, R. S. In Organometallic Chemistry of Rhodium and
Iridium; Academic Press: London, 1983.
distance in 4, which is even slightly shorter than a
typical terminal Os-Cl distance.
Rea ctivity tow a r d P P h 3, P yr id in e, a n d CO a n d
a n In vestiga tion of Oxid a tive Ad d ition s on Rh (I)
a n d Ir (I) Cen ter s. The direct linking of two electroni-

4008 Organometallics, Vol. 22, No. 20, 2003
Pizzotti et al.
Ta ble 3. Absor p tion (λa ) a n d Em ission (λe)
Electr on ic Sp ectr a in CHCl3
cally different transition-metal atoms usually produces
a heteronuclear complex with a polar metal-metal
bond. The polarity is influenced not only by the nature
of the metals involved but also by the set of ligands
coordinated to the metals. The greater the disparity of
the electronic properties of the linked metallorganic
fragments, the greater the polarity of the metal-metal
bond. This concept is realized in its extreme form in the
chemistry of early-late heteronuclear bimetallic com-
plexes, whose study was initially hampered by the
apparent instability of most compounds synthesized.
However, the early-late heteronuclear bimetallic com-
plexes 1-4 synthesized in this work, which cannot be
considered classical in the sense of Gade’s definition,32
are very stable in the solid state and in solution,
suggesting a heteronuclear metal-metal bond relatively
inert with respect to an electron-transfer process such
as homolytic or heterolytic cleavage. This stability
suggests thus that the W-M bond in compounds 1-4
cannot be described by a total sharing of bonding
electrons,32 but better by a reversible σ donation from
the W atom, without a real change of the oxidation state
of the donor and acceptor centers. The formation of
heteronuclear bimetallic complexes where the basic
metal may act as a donor to donate electrons to another
metal with formation of a donor-acceptor bond has been
well described.33 To confirm this picture, we investigated
the σ-donor properties of the W(CO)3(phen) fragment
by ligand displacement reactions with relatively weak
σ-donor ligands with increasing π-acceptor properties
such as CO > PPh3 > pyridine.
λe
compd
λa (nm)
(nm)a
[W(CO)4(phen)]8,12
388 (3.41), 499 (3.79)b 568c
[W(CO)3(phen)(CH3CN)]12
[W(CO)3(phen)(pyz)]
350 (sh), 526 (3.4)b,d
356 (2.87), 511 (3.09),f
578 (3.28)b,c
e
e
[(CO)3(phen)W-cis-Rh(CO)2Cl]g 1 392 (3.86), 444 (3.65),h 506,
517 (3.85)b
595
[(CO)3(phen)W-cis-Ir(CO)2Cl] 2
385 (3.74),h 503 (3.59)b 622
[(CO)3(phen)W-fac-Ru(CO)3Cl2] 3 412 (3.63),h 500 (3.43)b 552
[(CO)3(phen)W-fac-Os(CO)3Cl2] 4 406 (3.66),h 495 (3.39)b 567
a
b
Exciting wavelength λ ) λmax of MLCT. Attributed to a
d
MLCT (W f π*phen). c In CH2Cl2. In CH3CN. e Not fluorescent.
g
f Attributed to MLCT (Wfπ*pyz). The assignment of the bands
h
was determined by deconvolution. Attributed to an additional
MLCT transition (M f π*phen).
A similar substitution takes place by reaction in THF
of the complex [(CO)3(phen)W-cis-Rh(CO)2Cl] 1 with the
less soft pyridine (py) ligand. The reaction is complete
in 2 h with formation of [W(CO)4(phen)] and [cis-Rh-
(CO)2Cl(py)].23 Therefore, also a less soft, better σ-donor
and lower π-acceptor with respect to PPh3 such as
pyridine still behaves as a softer ligand than W(CO)3-
(phen). Thus, it was not surprising that by reaction of
the much softer ligand CO with complexes 3 and 4, an
easy substitution reaction takes place according to
[(CO)3(phen)W-fac-M(CO)3Cl2] + CO THF8
[W(CO)4(phen)] + [fac-M(CO)3Cl2(THF)] (7)
Reactions with PPh3 were carried out in THF by
adding a solution of PPh3 to a solution of the bimetallic
complex in the molar ratio 2:1. The replacement of
W(CO)3(phen) is immediate for complexes 1 and 2, with
breaking of the metal-metal bond and the simultaneous
shift of a carbonyl ligand according to
M ) Ru(II),24 Os(II)24
The relatively poor soft properties of W(CO)3(phen)
as a ligand were confirmed by the complete inertness
of the bimetallic Rh(I) and Ir(I) complexes 1 and 2 to-
ward oxidative reactions not only with H2 but also with
the rather reactive CH3I, which reacts easily with Rh(I)
and Ir(I) complexes with a very soft ligand environment
such as [trans-Ir(PPh3)2(CO)Cl] or [Rh(CO)2I2]-.34
When the interaction occurs with a hard σ-δonor
ligand such as an excess of THF, complexes 1 and 2 are
very stable; only for complexes 3 and 4 do we have
evidence of a coordinative interaction of one THF
molecule, but without breaking of the metal-metal
bond. This interaction may involve the breaking of the
weak chlorine bond with the W center due to the
bridging chlorine by a weak interaction with a THF
molecule, which therefore is probably located in the
coordination sphere of the W atom.
Electr on ic Sp ectr a a n d Volta m m etr ic In vestiga -
tion . The visibile spectrum in CHCl3 solution of [W(CO)4-
(phen)] (Table 3) shows a metal to ligand transition
MLCT (W f π*phen) at 499 nm, together with ligand
field LF (d f d) transitions at higher energy.9,13
Electron-acceptor substituents, such as the nitro group,
on the phenanthroline ligand lower the energy of this
MLCT transition.9,13 A significant shift to lower energy
is shown also by substitution of a CO ligand by a better
σ-donor ligand, as in [fac-W(CO)3(phen)(CH3CN)] (526
nm in CH3CN), if compared to [W(CO)4(phen)] (451 nm
[(CO)3(phen)W-cis-M(CO)2Cl] + 2PPh3 f
[trans-M(CO)Cl(PPh3)2] + [W(CO)4(phen)] (5)
M ) Rh(I),22a Ir(I)22b
while with compounds 3 and 4 the same reaction is
slower (5 h) so that it can be followed easily by the
disappearance of the band at 1950 cm-1 in the IR
spectrum (Table 1), characteristic of the presence of the
metal-metal bond. Two compounds are formed, in
accordance with reaction (6), easily separated by crys-
tallization (CHCl3/n-hexane), the violet W22c compound
being insoluble in n-hexane and the white Ru and Os
compounds22d,e being soluble in n-hexane:
[(CO)3(phen)W-fac-M(CO)3Cl2] + 2PPh3 f
[fac-W(CO) (phen)(PPh )] [fac-M(CO) Cl (PPh )]
+
3
3
3
2
3
violet
white
(6)
M ) Ru(II),22d Os(II)22e
This reactivity suggests that PPh3 behaves as a ligand
softer than W(CO)3(phen) toward M(I) (M ) Rh, Ir) and
M(II) (M ) Ru, Os) soft carbonyl complexes.
(32) Gade, L. H. Angew. Chem., Int. Ed. 2000, 39, 2658.
(33) Zhang, Z. Z.; Cheng, H.; Kuang, S. M.; Zhou, Y. Q.; Liu, Z. X.;
Zhang, J . K.; Wang, H. G. J . Organomet. Chem. 1996, 516, 1.
(34) (a) Adamson, G. W.; Daly, J . J .; Forster, D. J . Organomet. Chem.
1974, 71, C17. (b) Murphy, M. A.; Smith, B. L.; Torrence, G. P.; Aguilo,
A. Inorg. Chim. Acta 1985, 101, L47.

Late-Early Heteronuclear Bimetallic Complexes
Organometallics, Vol. 22, No. 20, 2003 4009
Ta ble 4. Dep en d en ce of th e λm a x (n m ) Va lu es of th e MLCT Tr a n sition s on th e Solven t P ola r ity
λmax
ethyl
dichloro-
ethane
compd
transition toluene CHCl3 acetate THF CH2Cl2 CH3CN
acetone DMF
[(CO)3(phen)W-cis-Rh(CO)2Cl] 1
a
448
528
444
517
432
500
434
503
433
491
424
469
MLCT
[(CO)3(phen)W-cis-Ir(CO)2Cl] 2
[(CO)3(phen)W-fac-Ru(CO)3Cl2] 3
a
384
515
385
503
380
489
380
488
371
461
MLCT
a
414
514
412
500
398
488
401
491
396
484
385
458
397
482
389
469
388
462
MLCT
[(CO)3(phen)W-fac-Os(CO)3Cl2] 4
a
415
508
406
495
402
484
399
475
387
465
MLCT
a
Attributed to an additional MLCT (Mfπ*phen).
Ta ble 5. CV P ea k P oten tia ls, Ep (I,ox), a n d Ha lf-Wa ve P oten tia ls (a fter Decon volu tion ), E1/2, for th e F ir st
Oxid a tion Step of Differ en t Meta l Com p lexesa
Ep(I ,ox)/
E1/2/
compd
[W(CO)4(phen)]
[W(CO)4(phen)(CH3CN)]
[(CO)3(phen)W-cis-Rh(CO)2Cl] 1
[(CO)3(phen)W-fac-Os(CO)3Cl2] 4
[fac-Os(CO)3Cl(pyz)]
V (SCE)
V (SCE)
remarks
0.765
0.219
0.785
0.921
0.733
0.196
0.715
0.893
1e; approaching EC reversibility at low scan rates; no return peak
1e; approaching EC reversibility at low scan rates; return peak
1e; irreversible peak; second oxidation peak at 1.2 V
1e; EC reversible peak with return peak
no peaks in the investigated range
[cis-Rh(CO)2Cl(pyz)]
1.46
flat, shoulder-like signal
a
Conditions: CH2Cl2 + 0.1 M TBAP medium; glassy-carbon electrode, scan rate 0.2 V s-1
.
in CH3CN), or as in [fac-W(CO)3(phen)(pyz)] (578 nm
in CH2Cl2), if compared to [W(CO)4(phen)] (482 nm in
CH2Cl2). These two kinds of shifts have different origins.
While substitution on the phenanthroline ligand by a
nitro group induces a stabilization of the acceptor π*phen
orbitals, the substitution of one π-acceptor carbonyl
ligand by a better σ-donor ligand such as CH3CN or
pyrazine produces probably a shift to slightly higher
energy of the occupied d orbitals of the W(0) atom. As
we will see later, this hypothesis has been confirmed
by a voltammetric investigation.
The visible spectra in CHCl3 solution of the hetero-
nuclear bimetallic complexes 1-4 show two bands in
the region 385-517 nm, both solvatochromic and both
of comparable intensity (log ꢀ about 3.5) (Table 3). The
band at lower energy can be attributed to the well-
described MLCT transition (W f π*phen) of the W(CO)3-
(phen) fragment, only slightly red-shifted with respect
to [W(CO)4(phen)]. The attribution of the second band
at higher energy is not straightforward. Its significant
solvatochromism (Table 4) excludes an assignment to
the poorly solvatochromic π f π* transitions of the
phenanthroline ligand of the W(CO)3(phen) moiety, and
its absence in the electronic spectra of the monomeric
complexes [W(CO)4(phen)], [fac-W(CO)3(phen)(CH3CN)],
and [fac-W(CO)3(phen)(pyz)] (Table 3) confirms that its
origin is due to the presence of the metal-metal bond.
Its significant solvatochromism would suggest either an
additional W f π*L MLCT transition or a metal to metal
charge transfer (MMCT).
clearly affect the energies of the d orbitals of the W(0)
atom. Therefore, in complexes 1-4 the MLCT transi-
tions W f π*phen involving the second π*phen level could
be shifted to lower energy at about 400 nm. This
assignment is supported by the significant blue-shifted
solvatochromism (corresponding to a decrease of the
dipole moment in the excited state typical of a W f
π*phen transition) and by the intensity of the band at
about 400 nm (Table 3). The attribution to a MMCT
would require in the excitation process an electron
transfer from the oxidized metal centers (M ) Rh(I),
Ir(I), Ru(II), Os(II)) to the W(0) atom in order to justify
the blue-shifted solvatochromism. MMCT transitions
were reported for complexes with a heteronuclear
metal-metal bond, with the Fe(0) atom of [trans-Fe-
(CO)3L2] (L is a particular tertiary phosphine) acting
as a σ donor toward a series of Lewis acids.33 However,
the attribution of absorption bands of quite variable
intensity to a MMCT process was based only on the
agreement of their energy with J ørgensen’s theory of
charge-transfer transitions, and in any case the electron-
transfer process was proposed to take place from the
Fe(0) atom to the second acceptor metal atom.
The voltammetric behavior of [W(CO)4(phen)], [fac-
W(CO)3(phen)(CH3CN)], and complexes 1 and 4, when
compared to that of [cis-Rh(CO)2Cl(pyz)] and [fac-Os-
(CO)3Cl2(pyz)] (Table 5), shows that the W(0) atom is
always easily oxidized, the oxidability decreasing on
going from [fac-W(CO)3(phen)(CH3CN)] to [W(CO)4-
(phen)] and finally to 1 and 4, respectively, as expected
for a decrease of the electron density on the W(0) atom
due to an increased electron donation. However, the Os-
(II) atom is not oxidized either in [fac-Os(CO)3Cl2(pyz)]
or in 4, while the Rh(I) atom is oxidized, but much less
easily than the W(0) atom, either in [cis-Rh(CO)2Cl(pyz)]
or in 1 (Table 5). This trend would suggest that the d
electron density located on the oxidized acceptor centers
(either Rh(I) or Os(II)) is not easily available for an
electron-transfer process, differently from the d electron
It is reported that the complex [W(CO)4(phen)] should
have, in addition to the three allowed MLCT transitions,
within the absorption band at 499 nm involving the first
π*phen level, two allowed MLCT transitions at higher
energies involving the second π*phen level.35 Not only will
the formation of the bimetallic species alter the overall
symmetry properties but also the metal-metal bond will
(35) Servaas, P. C.; van Dijk, H. K.; Snoeck, T. L.; Stufkens, D. J .;
Oskam, A. Inorg. Chem. 1985, 24, 4494.

4010 Organometallics, Vol. 22, No. 20, 2003
Pizzotti et al.
However, we did not obtain the expected linear relation-
ship of the solvatochromic shift of the emission bands
with the solvent polarity, required in order to use both
the absorption and the emissive solvatochromic data for
the determination of the solvent cavity radius a (Table
6).19
EF ISH a n d Solva toch r om ic In vestiga tion of th e
Secon d -Or d er NLO Resp on se of Heter on u clea r
Bim eta llic Com p lexes. The molecular quadratic hy-
perpolarizability âλ at the incident wavelength λ of
bimetallic complexes 1-4 was measured in CHCl3
solution using the EFISH tecnique,17 which provides
information through the equation
γEFISH ) (µâλ/5kT) + γ(-2ω; ω, ω, 0)
(8)
where µâλ/5kT is the dipolar orientational contribution
and γ(-2ω; ω, ω, 0), a third-order term at frequency ω
of the incident radiation, is the electronic contribution
always considered negligible in these kinds of metal
complexes.5,8 âλ (or â1.907), the vectorial projection of the
quadratic hyperpolarizability tensor along the dipole
moment direction, has been obtained by working with
a fundamental incident wavelength of 1.907 µm (Table
6). In this way the second harmonic (λ/2 ) 953 nm) is
quite far from any significant absorption band (Table
3) so that any resonance enhancement is minimized.36
In parallel, we determined the quadratic hyperpolariz-
ability âCT along the major charge-transfer transitions
by a solvatochromic investigation, as an additional way
to investigate the charge-transfer origin of the second-
order NLO response and consequently the contribution
of each significant electronic transition which can be
attributed to a charge-transfer process.19 Of course, â1.907
and âCT can be compared only when the charge-transfer
processes, involved in the second-order NLO response,
are close in direction to the dipole moment axis. As can
be inferred from Table 6, in all of the complexes 1-4
both â1.907 and âCT have negative signs and comparable
absolute values. This suggests that the major charge-
transfer transitions are almost parallel to the dipole
moment axis, although due to a small but significant
orthogonal contribution to the total dipole moment, for
instance, in complexes carrying the cis-M(CO)2Cl moi-
ety,3 the W-M axis (M ) Rh, Ir, Ru, Os), where the
charge transfer transitions are probably located, cannot
perfectly correspond to the dipole moment axis. How-
ever, along the W-M axis is located the major compo-
nent of the dipole moment. Complexes 1-4, if compared
to [W(CO)4(phen)], have slightly lower dipole moments
F igu r e 3. Absorption electronic spectra in CHCl3 of (a,
top) [(CO)3(phen)W-cis-Rh(CO)2Cl] 1 and (b, bottom) [(CO)3-
(phen)W-cis-Ir(CO)2Cl] 2 by deconvolution.
density located on the W(0) atom. As a consequence, the
attribution of the relatively strong absorption bands
around 400 nm to a MMCT process with electron
transfer from the oxidized metal centers to the W(0)
atom, in agreement with the blue-shifted solvato-
chromism, is not substantiated.
One major difference must be stressed between the
spectra of 1 and 2 in the region 370-450 nm (Figure
3). While 1 shows a solvatochromic band at 444 nm
together with a stronger, but much less solvatochromic,
band at 392 nm, 2 shows only a solvatochromic band at
385 nm (Table 3 and Figure 3). Attempts to detect for
the complex 2 by deconvolution an absorption band in
the region 420-450 nm covered by the strong absorption
at 385 nm failed.
but increased absolute values of both â1.907 and âCT
,
which maintain the negative sign as expected if the
phenanthroline ligand still behaves as a good σ donor
in the ground state and as a significant π acceptor in
the MLCT processes (Table 6). The very slight decrease
of the dipole moment suggests that the dipolar contribu-
tions of the metal-metal bond and of the “cis-M(CO)2Cl”
(M ) Rh, Ir) and “fac-M(CO)3Cl2” (M ) Ru, Os) frag-
ments are very small, providing additional evidence for
poorly polar W-M bonds. In [W(CO)4(phen)] the major
contribution to âCT is given by the MLCT absorption
band at lower energy (499 nm) characterized, as ex-
pected for a W f π*phen charge-transfer process,9 by a
Complexes 1-4 all show an emission when irradiated
at the lower energy MLCT wavelength (Table 3). The
emission bands are always solvatochromic, having a
blue shift with increasing polarity of the solvent.
(36) Orr, B. J .; Ward, J . F. Mol. Phys. 1971, 20, 513.

Late-Early Heteronuclear Bimetallic Complexes
Organometallics, Vol. 22, No. 20, 2003 4011
Ta ble 6. Sign ifica n t Electr on ic Sp ectr a , Dip ole Mom en ts, EF ISH â1.907 Va lu es in CHCl3, a n d
Solva toch r om ic âCT Va lu es (In cid en t Wa velen gth a t 1.907 µm )
compd
[W(CO)4(phen)]
λ
max (nm)
µ (D)
8.4c
8.0
â1.907 (10-30 esu)
âCT (10-30 esu)
∆µeg (D)
fa
a (Å)b
6.23
499 (MLCT)
444e
-13c,d
-28
-10.6
-4.5
0.0771
0.0377
0.0957
0.1672
0.0741
0.1174
0.0422
0.1260
0.0366
-3.78
-31.96
-8.96
[(CO)3(phen)W-cis-Rh(CO)2Cl] 1
[(CO)3(phen)W-cis-Ir(CO)2Cl] 2
[(CO)3(phen)W-fac-Ru(CO)3Cl2] 3
-31.7
-35.7
-36
6.84
}
}
517 (MLCT)
-8.3
385e
-4.0
9.1
6.9
7.5
-25.8
-76
7.12
7.03
503 (MLCT)
412e
-27.14
-24.8
-9.6
-20.2
-24.7
-18.6
-23.3
-61.1
-67.5
}
500 (MLCT)
-36.3
-38.7
-28.8
406e
[(CO)3(phen)W-fac-Os(CO)3Cl2] 4
-75.6
7.30
}
495 (MLCT)
a
b
f is the transition oscillator strength, obtained from the experimental integrated absorption coefficient in CHCl3.18 The cavity radius
a was calculated with the molecular weight method.18 c Reference 8. In CH2Cl2. e Attributed to an additional MLCT (Mfπ*phen).
d
negative ∆µeg value (difference between excited and
ground-state dipole moment), while the absorption band
at higher energy (388 nm) (Table 3) is not solvatochro-
mic, as expected for the low charge-transfer character
of a π f π* transition of the unsaturated aromatic
system.
In complexes 1-4, in addition to the above MLCT
absorption band, there is a new absorption band at
higher energy, which we tentatively have attributed to
an additional MLCT transition. This band is character-
ized by a significant negative solvatochromism (∆µeg<
0 being the electron-transfer opposite to the ground-
state transfer of the dipole moment), which adds to the
negative solvatochromism of the MLCT transition at
finally by the decreased oxidability of the W(0) atom
upon formation of the metal-metal bond. The metal-
metal interactions are in the range of single metal-
metal bonds of low polarity, as supported by the X-ray
structure and by the slight decrease of the dipole
moment on going from [W(CO)4(phen)] to the hetero-
nuclear dimeric complexes 1-4. These latter complexes
show in their electronic spectra an additional new band
at higher energy, generated by the presence of the
metal-metal bond, which, as suggested by the blue-
shifted solvatochromism and its significant intensity,
was tentatively attributed to a charge-transfer process,
such as an additional MLCT (W f π*phen) transition.
The EFISH and solvatochromic investigations on
complexes 1-4 have shown an increase of the absolute
value of the negative second-order NLO response. This
increase, which becomes higher on going from M(I) to
M(II) as acceptor centers, is not due to an higher
lower energy, thus increasing the absolute value of âCT
.
While the absolute value of the contribution to âCT of
the lower energy MLCT transition is quite constant in
all complexes 1-4, the absolute value of the contribution
to âCT of the higher energy MLCT transition increases,
as expected, in the series 1 < 2 < 3 < 4 (Table 6).
Interestingly, in compound 4 the contribution of this
latter band becomes more relevant than that of the
MLCT transition at lower energy.
The increase of the absolute value of the quadratic
hyperpolarizability â1.907 from -13 × 10-30 esu for
[W(CO)4(phen)] to about -26 × 10-30 or -76 × 10-30
esu for the heteronuclear bimetallic complexes 1-4
(Table 6) is much greater than that occurring by the
introduction of a strong withdrawing substituent such
as the nitro group on the phenanthroline ring (from -13
× 10-30 to -18 × 10-30 esu, as reported in ref 9). This
significant increase clearly originates, as our solvato-
chromic investigation has shown, from an electronic
effect due to the presence of a relatively weak W-M
bond (M ) Rh(I), Ir(I), Ru(II), Os(II)).
contribution of the lower energy MLCT (W f π*phen
)
transition, which remains substantially constant, but
to a significant contribution of the new higher energy
charge-transfer transition, which increases from com-
pound 1 to compound 4. In conclusion, an heteronuclear
metal-metal bond, even if slightly polar and weak, can
produce a significant effect on the second-order nonlin-
ear response.
Finally, our results provide further evidence that in
simple organometallic chromophores, molecular struc-
tures with modest ground-state dipole moments, as first
observed by L. T. Cheng et al.,9 lead to significant
nonlinear responses.
Ack n ow led gm en t. This work was supported by the
Ministero dell’Universita` e della Ricerca Scientifica e
Tecnologica (Programma di ricerca MURST di tipo
interuniversitario, nell’area delle scienze chimiche (ex
40%) year 2001; title: Nanotecnologie Molecolari per
Materiali Magnetici e per Ottica non Lineare) and by
the Consiglio Nazionale delle Ricerche. We thank Mr.
Pasquale Illiano and Mr. Americo Costantino for NMR
measurements.
Con clu sion s
Several interesting observations concerning the na-
ture of the new stable nonclassical heteronuclear early-
late metal-metal bonds, investigated in this work, have
emerged. The fundamentally acid-base nature of the
heteronuclear W-M (M ) Rh(I), Ir(I), Ru(II), Os(II))
bond is supported by the synthetic method, based on
the unexpected σ donor behavior of the “W(CO)3(phen)”
moiety, by the easy substitution of this latter moiety,
acting as a donor ligand, by more soft ligands, and
Su p p or tin g In for m a tion Ava ila ble: Crystallographic
data for compounds 2 and 4, in electronic files as CIF files.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OM030247F