Carbonyl complexes of rhodium with N-donor ligands: Factors determining the formation of terminal versus bridging carbonyls

Article abstract of DOI:10.1021/om901023n

Cationic rhodium carbonyl complexes supported by a series of different N₃- and N₄-donor ligands were prepared, and their ability to form carbonyl-bridged species was evaluated. Complex [Rh(- ³-bpa)(cod)]⁺ (1⁺) (bpa = bis(2-picolyl)amine, cod = cis,cis-1,5-cyclooctadiene) reacts with 1 bar of CO to form a tris-carbonyl-bridged species [Rh₂(-³-bpa) ₂(μ-CO)₃]²⁺ (2²⁺), which in solution slowly decomposes to the terminal monocarbonyl complex [Rh(- ³-bpa)(CO)]⁺ (3⁺). Similar conditions lead to direct formation of a terminal monocarbonyl species, [Rh(-³-Bu-bpa) (CO)]⁺ (5⁺), from [Rh(-³-Bu-bpa)(cod)] ⁺ (4⁺) (Bu-bpa = N-butylbis(2-picolyl)amine). Treatment of 4⁺ with 50 bar of CO leads to only partial conversion (-15%) to the tris-carbonyl-bridged species [Rh₂(-³-Bu-bpa) ₂(μ-CO)₃]²⁺ (6²⁺). Stabilization of tris-carbonyl bridges can be achieved by cooperative binding. Tethering two bpa moieties with a propylene linker allows cooperative CO binding to [(CO)Rh(μ-(bis--³)tppn)Rh(CO)]²⁺, producing the tetranuclear complex [Rh₄(μ-(bis--³)tppn) ₂((μ-CO)₃)₂]⁴⁺ (13)⁴⁺ at 50 bar of CO (tppn = tppn = N¹,N¹,N²,N ²-tetrakis(pyridin-2-ylmethyl)propane-1,2-diamine). Tetranuclear complex 13⁴⁺ is stable at room temperature in the absence of CO (in contrast to binuclear Rh(μ₂-CO)₃Rh-bridged complex 6²⁺). In solution, the cationic rhodium carbonyl complex [Rh(- ³-tpa)(CO)]⁺ (14⁺) (containing the N ₄-donor ligand tpa = tris(2-picolyl)amine)) exists in dynamic equilibrium with the dinuclear bis-carbonyl-bridged species [Rh(- ⁴-tpa)(μ-CO)]₂²⁺ (15²⁺). Remarkably, the bis-carbonyl-bridged Rh(μ₂-CO)₂Rh motive in 15²⁺ is not supported by a Rh-Rh bond or other bridging ligands. The thermodynamic parameters for dimerization of 14⁺ to 15²⁺ in acetone were measured (δH° = -28.4 ± 1.7 kJ-mol^-1 and δS° = -134 ± 7 J-mol-K^-1). Formation of bis-carbonyl-bridged species was not observed with the weaker Me₃tpa ligand. The stability of the bis- and tris-carbonyl-bridged structures clearly depends on a delicate balance between the favorable enthalpy (enhanced with stronger --donor ligands) and unfavorable entropy (that can be reduced by multivalent binding) associated with their formation. In the solid state complex 14⁺ reacts selectively with dioxygen to form a carbonato complex, [Rh(-⁴-tpa)(CO₃)]⁺ (16 ⁺).

Full text of DOI:10.1021/om901023n

Organometallics 2010, 29, 1629–1641 1629
DOI: 10.1021/om901023n
Carbonyl Complexes of Rhodium with N-Donor Ligands: Factors
Determining the Formation of Terminal versus Bridging Carbonyls
†
‡
‡
‡
ꢀ
Wojciech I. Dzik, Charlotte Creusen, Rene de Gelder, Theo P. J. Peters,
Jan M. M. Smits,‡ and Bas de Bruin*,†
†Department of Homogeneous and Supramolecular Catalysis, Van’t Hoff Institute for Molecular Sciences
(HIMS), University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands, and
‡Institute for Molecules and Materials, Radboud Universiteit Nijmegen, Heyendaalseweg 135,
6525 AJ Nijmegen, The Netherlands
Received November 25, 2009
Cationic rhodium carbonyl complexes supported by a series of different N3- and N4-donor ligands were
prepared, and their ability to form carbonyl-bridged species was evaluated. Complex [Rh(κ3-bpa)(cod)]þ
(1þ) (bpa = bis(2-picolyl)amine, cod = cis,cis-1,5-cyclooctadiene) reacts with 1 bar of CO to form a tris-
carbonyl-bridged species [Rh2(κ3-bpa)2(μ-CO)3]2þ (22þ), which in solution slowly decomposes to the
terminal monocarbonyl complex [Rh(κ3-bpa)(CO)]þ (3þ). Similar conditions lead to direct formation of a
terminal monocarbonyl species, [Rh(κ3-Bu-bpa)(CO)]þ (5þ), from [Rh(κ3-Bu-bpa)(cod)]þ (4þ) (Bu-bpa=
N-butylbis(2-picolyl)amine). Treatment of 4þ with 50 bar of CO leads to only partial conversion (∼15%) to
the tris-carbonyl-bridged species [Rh2(κ3-Bu-bpa)2(μ-CO)3]2þ (62þ). Stabilization of tris-carbonyl bridges
can be achieved by cooperative binding. Tethering two bpa moieties with a propylene linker allows
cooperative CO binding to [(CO)Rh(μ-(bis-κ3)tppn)Rh(CO)]2þ, producing the tetranuclear complex
[Rh4(μ-(bis-κ3)tppn)2((μ-CO)3)2]4þ (13)4þ at 50 bar of CO (tppn=tppn=N1,N1,N2,N2-tetrakis(pyridin-
2-ylmethyl)propane-1,2-diamine). Tetranuclear complex 134þ is stable at room temperature in the absence
of CO (in contrast to binuclear Rh(μ2-CO)3Rh-bridged complex 62þ). In solution, the cationic rhodium
carbonyl complex [Rh(κ3-tpa)(CO)]þ (14þ) (containing the N4-donor ligand tpa=tris(2-picolyl)amine))
exists in dynamic equilibrium with the dinuclear bis-carbonyl-bridged species [Rh(κ4-tpa)(μ-CO)]2
2þ
(152þ). Remarkably, the bis-carbonyl-bridged Rh(μ2-CO)2Rh motive in 152þ is not supported by a
Rh-Rh bond or other bridging ligands. The thermodynamic parameters for dimerization of 14þ to 152þ in
acetone were measured (ΔH°=-28.4 (1.7 kJ mol-1 and ΔS°=-134 (7J mol K-1). Formation of bis-
carbonyl-bridged species was not observed with the weaker Me3tpa ligand. The stability of the bis- and tris-
carbonyl-bridged structures clearly depends on a delicate balance between the favorable enthalpy
(enhanced with stronger σ-donor ligands) and unfavorable entropy (that can be reduced by multivalent
binding) associated with their formation. In the solid state complex 14þ reacts selectively with dioxygen to
form a carbonato complex, [Rh(κ4-tpa)(CO3)]þ (16þ).
3
3
3
Introduction
tris(3,5-dimethylpyrazolyl)borate) photochemically activate
C-H bonds of alkanes,2 show a high degree of fluxionality in
solution,3 and reveal interesting one-electron redox proper-
ties in the presence of a supporting phosphorus ligand.4
The arrangement of the pyrazolyl moieties of the
Tp-type ligands imposes a fac-coordination mode.1
Furthermore, RhI complexes with Tp-type ligands reveal
a hemilabile coordination mode with the κ2-κ3 equili-
brium producing diverse structures spanning from mo-
nonuclear square-planar and trigonal-bypiramidal or
square-pyramidal bis-carbonyls,5 to octahedral dinuclear
Carbonyl complexes of rhodium with the tridentate nitro-
gen-donor “scorpionato” trispyrazolyl (Tp) type of ligands
have received much attention over the past two decades.1 It
was shown that complexes of the type Rh(Tp*)(CO)2 (Tp* =
*Corresponding author. E-mail: b.debruin@uva.nl.
(1) For a review on trispyrazolyl borate rhodium complexes see:
´
Slugovc, C.; Padilla-Martınez, I.; Sirol, S.; Carmona, E. Coord. Chem.
Rev. 2001, 213, 129–157.
(2) (a) Ghosh, C. K.; Graham, W. A. G. J. Am. Chem. Soc. 1987, 109,
4726–4727. (b) Bromberg, S. E.; Yang, H.; Asplund, M. C.; Lian, T.;
McNamara, B. K.; Kotz, K. T.; Yeston, J. S.; Wilkens, M.; Frei, H.; Bergman,
R. G.; Harris, C. B. Science 1997, 278, 260–263.
(3) Webster, C. E.; Hall, M. B. Inorg. Chim. Acta, 2002, 330,
268-282, and references therein.
(4) For examples see: (a) Connelly, N. G.; Emslie, D. J. H.; Geiger,
W. E.; Hayward, O. D.; Linehan, E. B.; Orpen, A. G.; Quayle, M. J.;
Rieger, P. H. J. Chem. Soc., Dalton Trans. 2001, 670–683. (b) Geiger, W.
E.; Camire Ohrenberg, N.; Yeomans, B.; Connelly, N. G.; Emslie, D. J. H.
J. Am. Chem. Soc. 2003, 125, 8680–8688.
(5) Compounds characterized by X-ray diffraction: (a) Rheingold,
A. L.; Liable-Sands, L. M.; Incarvito, C. L.; Trofimenko, S. J. Chem.
Soc., Dalton. Trans. 2002, 2297–2301. (b) Adams, C. J.; Connelly, N. G.;
Emslie, D. J.; Hayward, O. D.; Manson, T.; Orpen, A. G.; Rieger, P. H.
Dalton. Trans. 2003, 2835–2845. (c) Rheingold, A. L.; Liable-Sands, L. M.;
Golan, J. A..; Trofimenko, S. Eur. J. Inorg. Chem. 2003, 2767–2773. (d)
Blake, A. J.; George, M. W.; Hall, M. B.; McMaster, J.; Portius, P.; Sun, X. Z.;
ꢀ
Towrie, M.; Webster, C. E.; Wilson, C.; Zaric, S. D. Organometallics 2008,
27, 189–201.
r
2010 American Chemical Society
Published on Web 03/10/2010
pubs.acs.org/Organometallics

1630 Organometallics, Vol. 29, No. 7, 2010
Dzik et al.
Scheme 1. Different Coordination Behavior of Trispyrazolyl Ligands X = BH, CH, GaMe
triscarbonyl-bridged6 species (Scheme 1). Similar structures
(although κ2-κ3 isomerism was not reported) were found for
cyclic fac-coordinating tri-7 and hexa-amines,8 with the latter
ones forming supramolecular, tris-carbonyl-bridged tetra-
nuclear assemblies.
Scheme 2. mer- and fac-Coordination of a N3 Ligand toward
Rhodium Carbonyl
While these strictly fac-coordinating N-donor ligands
have been thoroughly studied, surprisingly little attention
has been given to rhodium carbonyl complexes with more
flexible podal N-donor ligands that can adopt both fac- and
mer-coordination modes.
Scheme 3. Ligands Used in This Study
Mathieu and Ros reported that bis[(3,5-dimethyl-1-pyra-
zolyl)methyl]ethylamine can bind in both fashions to a
cationic rhodium carbonyl center, resulting in formation of
either monocarbonyl square-planar or bis-carbonyl square-
pyramidal complexes (Scheme 2).9 The factors that deter-
mine the coordination mode of the N-donor ligand and as a
result the number of carbonyl ligands per metal atom were
not investigated.
The above structural diversity of N-donor ligand Rh-
carbonyl complexes is intriguing and could well result in
different reactivities. For this reason we became interested in
the factors that determine the coordination mode of both
the carbonyl ligands and the N-donor ligands in cationic
[{Rh(CO)x(N-donor ligand)}y]yþ complexes with flexible
podal N-donor ligands.
Given the rich chemistry of rhodium olefin complexes with
bispicolylamine (bpa)-type ligands10,11 and dual fac- and
mer-coordination behavior of bpa, we were especially inter-
ested in the coordination modes of the flexible ligands shown
in Scheme 3 and understanding the factors that drive the
formation of mononuclear complexes with terminal carbo-
nyl ligands versus binuclear carbonyl-bridged complexes.
Bridging carbonyl complexes are frequently “resting
state” or “dead end” species in several catalytic carbonyla-
tion reactions (e.g., hydroformylation),12,13 and therefore a
better understanding of the factors that determine their
formation might well be of synthetic relevance.
(6) Compound characterized by X-ray diffraction: (a) Methyl-
(trispyrazol-1-yl)gallate: Louie, B. M.; Rettig, S. J.; Storr, A.; Trotter,
J. Can. J. Chem. 1984, 62, 633–637. Other examples: (b) Tris-(2-pyr-
idy1)phosphineoxide ligand: Cesares, J. A.; Espinet, P.; Martín-Alvarez,
ꢀ
J. M.; Espino, G.; Perez-Manrique, M.; Vattier, F. Eur. J. Inorg. Chem. 2001,
289–296. (c) Tris(pyrazol-1-yl)methane: Estruelas, M. A.; Oro, L. A.;
ꢀ
Claramunt, R. M.; Lopez, C.; Lavandera, J. L.; Elguero, J. J. Organomet.
Chem. 1989, 366, 245–255. (d) Tris(pyrazol-1-yl)borate: O'Sullivan, D. J.;
Lalor, F. J. J. Organomet. Chem. 1974, 65, C47–49. (e) Cocivera, M.;
Desmond, T. J.; Ferguson, G.; Kaitner, B.; Lalor, F. J.; O'Sullivan, D. J.
Organometallics 1982, 1, 1125. (f) Tris(pyrazol-1-yl)methanesulfonate:
€
Klaui, W.; Schramm, D.; Peters, W.; Rheinwald, G.; Lang, H. Eur. J. Inorg.
Chem. 2001, 1415–1424.
(7) Compound characterized by X-ray diffraction: de Bruin, B.;
Donners, J. J. J. M.; de Gelder, R.; Smits, J. M. M.; Gal, A. W. Eur. J.
Inorg. Chem. 1998, 401–406.
(8) Compounds characterized by X-ray diffraction: (a) Macrocyclic
hexamines: Lecomte, J.-P.; Lehn, J.-M.; Parker, D.; Guilheim, J.;
Pascard, C. J. Chem. Soc., Chem. Commun. 1983, 296–298. (b) Johnson,
J. M.; Bulkowski, J. E.; Rheingold, A. L.; Gates, B. C. Inorg. Chem. 1987,
26, 2644–2646. Other representative example: (c) Parker, D. J. Chem. Soc.,
Chem. Commun. 1985, 1129–1131.
Results and Discussion
For the tridentate bpa type of ligands we compared the Bu-
bpa ligand having a butyl substituent on the central amine
donor with the nonfunctionalized bpa ligand to investigate
(9) Mathieu, R.; Esquius, G.; Lugan, N.; Pons, J.; Ros, J. Eur. J.
Inorg. Chem. 2001, 2683–2688.
´
(10) (a) Tejel, C.; Ciriano, M. A.; del Rıo, M. P.; van den Bruele, F. J.;
(12) van Leeuwen, P. W. N. M., Claver C., Eds. Rhodium Catalyzed
Hydroformylation; Kluwer Acedemic Publishers: Dordrecht, 2000.
(13) Formation of μ2-carbonyl-bridged rhodium dimers with term-
inal carbonyls was reported for some hydroformylation catalysts: (a)
Wilkinson’s catalyst: Evans, D.; Yagupsky, G.; Wilkinson, G. J. Chem.
Soc. A 1968, 2660–2665. (b) Chan, A. S. C.; Shieh, H. S.; Hill, J. R. J. Chem.
Soc., Chem. Commun. 1983, 688–689. (c) Diphosphines: James, B. R.;
Mahajan, S. J.; Rettig, G. M. Williams Organometallics 1983, 2, 1452–1458.
Hetterscheid, D. G. H.; Tsichlis i Spithas, N.; de Bruin, B. J. Am. Chem.
Soc. 2008, 130, 5844–5845. (b) Hetterscheid, D. G. H.; Klop, M.; Kicken,
R. J. N. A. M.; Smits, J. M. M.; Reijerse, E. J.; de Bruin, B. Chem.;Eur. J.
2007, 13, 3386–3405. (c) de Bruin, B.; Verhagen, J. A. W.; Schouten, C. H. J.;
Gal, A. W.; Feichtinger, D.; Plattner, D. A. Chem.;Eur. J. 2001, 7, 416–
422.
(11) de Bruin, B.; Brands, J. A.; Donners, J. J. J. M.; Donners, M. P.
J.; de Gelder, R.; Smits, J. M. M.; Gal, A. W.; Spek, A. L. Chem.-Eur. J.
1999, 5, 2921–2936.
ꢀ
ꢀ
(d) Castellanos-Paez, A.; Castillon, S.; Claver, C.; van Leeuwen, P. W. N. M.;
de Lange, W. G. J. Organometallics 1998, 17, 2543–2552. (e) Diphosphites:
Buisman, G. J. H.; van der Veen, L. A.; Kamer, P. C. J.; van Leeuwen, P. W. N.
M. Organometallics 1997, 16, 5681–5687.

Article
Organometallics, Vol. 29, No. 7, 2010 1631
Scheme 4. Synthesis of [Rh2(K3-bpa)2(μ-CO)3]2þ (22þ) and [Rh(κ3-bpa)(CO)]þ (3þ)
the influence of the electronic effects (stronger bpa donor
vs weaker Bu-bpa)14 on the formation of mononuclear
terminal versus bridging carbonyl complexes (Scheme 3).
We also prepared ditopic alkyl-bpa-bridged complexes with
an alkyl tether between two bpa units in an attempt to lower
the unfavorable entropic factors associated with the forma-
tion of bridged carbonyl complexes. For the tetradentate
tpa-type ligands we compared the stronger nonfunctiona-
lized tpa donor with the Me3tpa donor bearing methyl
substituents on the pyridine-6 positions (Scheme 3). These
substituents have some steric influence on the metal coordi-
nation, which results in the pyridine units being weaker
donors to the metal compared with nonsubstituted pyri-
dines.11
Rhodium Carbonyl Complexes with Bis(picolyl)amine and
N-Butyl(bis(picolyl)amine). Reaction of [Rh(κ3-bpa)(η4-cod)]-
PF6 ([1]PF6) (cod = cis,cis-1,5-cyclooctadiene) with CO at a
pressure of 1 bar in dichloromethane results in formation of
the tris-carbonyl-bridged binuclear complex [Rh2(κ3-bpa)2-
(μ-CO)3](PF6)2 ([2](PF6)2) (Scheme 4). The complex has C2
symmetry and is not fluxional in solution on the NMR time
scale, which results in separate NMR signals of the inequi-
valent pyridine and methylene groups of the bpa ligand. The
inequivalence is a result of the coordination mode in which
two CO ligands are trans to picolyl and amine moieties,
whereas the third CO is trans to the two picolyl groups of the
different bpa ligands. This coordination mode is similar to
that previously reported for [Rh2(μ-CO)3Cl2(Py)2]15 and was
confirmed by single-crystal X-ray diffraction. The crystal
was grown by top layering an acetone solution of [2](PF6)2
with hexanes at -20 °C. Although the quality of the crystal
data is hampered by severe PF6 disorder, the molecular
structure of the dication 22þ is unambiguously revealed by
the X-ray data. See Figure 1.
Figure 1. X-ray structure of [Rh2(κ3-bpa)2(μ-CO)3]2þ (22þ).
Thermal ellipsoids are drawn with 50% probability. Hydro-
gen atoms bound to the carbon atoms, the PF6- counterions,
and acetone molecules are omitted for clarity. Selected bond
lengths [A] and angles [deg]: Rh(1)-C(1) 2.004(14); Rh(1)-C-
(1i) 2.019(12); Rh(1)-C(2) 2.019(18); Rh(1)-N(1) 2.165(11);
Rh(1)-N(2) 2.176(12); Rh(1)-N(3) 2.184(12); Rh(1)-
Rh(1i) 2.5710(18); C(1)-O(1) 1.140(16); C(2)-O(2) 1.14(2);
C(1)-Rh(1)-C(1i) 82.1(6); C(1)-Rh(1)-C(2) 84.6(5); C-
(1i)-Rh(1)-C(2) 84.2(5); C(1)-Rh(1)-N(1) 177.4(5); C-
(1i)-Rh(1)-N(1) 100.5(5); C(2)-Rh(1)-N(1) 96.0(4); C(1)-
Rh(1)-N(2) 95.2(5); C(1i)-Rh(1)-N(2) 99.3(5); C(2)-Rh-
(1)-N(2) 176.5(5); N(1)-Rh(1)-N(2) 84.1(4); C(1)-Rh-
(1)-N(3) 98.6(5); C(1i)-Rh(1)-N(3) 177.5(5); C(2)-Rh(1)-
N(3) 98.3(5); N(1)-Rh(1)-N(3) 78.9(4); N(2)-Rh(1)-N(3)
78.2(5); Rh(1)-C(1)-Rh(1i) 79.5(4); Rh(1i)-C(2)-Rh(1)
79.1(8).
Scheme 5. Synthesis of [Rh(K3-Bu-bpa)(CO)]þ ([5]þ)
The IR spectrum of the carbonyl region reveals two over-
lapping bands at 1835 and 1828 cm-1, and 13C NMR reveals
two triplets of intensity 2:1 at 215.8 and 210.4 ppm, with
Rh-C coupling constants of 28.4 and 27.9 Hz, respectively.
This complex is stable in the solid state for at least 2 months.
In solution it slowly converts to the monocarbonyl complex
[Rh(κ3-bpa)(η1-CO)]PF6 ([3]PF6) (approximately 30% con-
version in acetone after 10 days, heating under reflux in
acetone for 2 h leads to quantitative conversion) (Scheme 4).
Complex 3þ reveals a single CO stretching band at 1989 cm-1
and 13C NMR resonance at 190.3 ppm with a Rh-C
coupling constant of 77.5 Hz in MeCN.
CO does not lead to a binuclear, tris-carbonyl-bridged
species analogous to 22þ. Instead, treatment of [4]PF6 with
1 bar of CO in CH3CN or CH2Cl2 results directly in a clean
and facile substitution of cod by CO with formation of only
the square-planar carbonyl complex [(κ3-Bu-bpa)Rh-
(CO)]PF6 ([5]PF6) (Scheme 5). In the conversion of the η4-
cod to the monocarbonyl complexes the coordination mode
of the Bu-bpa ligand changes from fac to mer, as evidenced
In marked contrast to its bpa analogue, reaction of the
[(κ3-Bu-bpa)RhI(η4-cod)]PF6 complex ([4]PF6) with 1 bar of
(14) The weaker donor strength of alkyl-bpa derivatives is caused by
steric effects. See ref 11.
(15) Heaton, B. T.; Jacob, C.; Sampanthar, J. T. J. Chem. Soc.,
Dalton Trans. 1998, 1403–1410.

1632 Organometallics, Vol. 29, No. 7, 2010
Dzik et al.
Scheme 6. Reaction of [Rh(K3-Bu-bpa)(CO)]þ (5þ) with 50 bar of CO to Form [Rh2(κ3-Bu-bpa)2(μ-CO)3]2þ (62þ) and [Rh(κ3-Bu-
bpa)(CO)2]þ (7þ)
Table 1. IR and 13C NMR Spectroscopy Data of the Carbonyl
Ligands
by NOESY measurements.16 Cation 5þ is also obtained by
reaction of Bu-bpa and [{(CO)2Rh(μ-Cl)}2] in a 2:1 molar
ratio in MeOH followed by addition of NH4PF6, which
produces [5]PF6 as a yellow precipitate.
ν(CꢀO)
solution
ν
(CꢀO) solid 1J(Rh,C)
complex
[cm-1
]
state [cm-1
]
[Hz]
The substantially different stabilities of the Rh(μ-CO)3Rh-
bridged complexes with bpa versus Bu-bpa are remarkable,
considering the structural resemblance of these ligands. We
thus wondered if we could enforce the formation of Rh(μ-
CO)3Rh with the Bu-bpa ligand at higher CO pressures.
Treatment of [5]PF6 with 50 bar of CO for approximately
7 days results in partial conversion (approximately 15%) to a
mixture of the dinuclear, dicationic complex [Rh2(κ3-Bu-
bpa)2(μ-CO)3](PF6)2 ([6](PF6)2) and mononuclear, monoca-
tionic complex Rh[(κ3-Bu-bpa)(CO)2]PF6 ([7](PF6)), con-
taining two terminal CO ligands, as was observed by mass
spectrometry and IR spectroscopy (Scheme 6).17
[Rh2(bpa)2(μ-CO)3]2þ (22þ
)
1835
(shoulder
at 1828)
1989
1994
1836,
1815
28.4, 27.9
[Rh(bpa)(CO)]þ (3þ)
77.5
79.1
[Rh(Bu-bpa)(CO)]þ (5þ)
[Rh2(Bu-bpa)2(μ-CO)3]2þ
1832
(62þ
)
[Rh(Bu-bpa)(CO)2]þ (7þ)
[Rh2(tpen)(CO)2]2þ (112þ
1992,
2042
)
)
1999
1994
1838
(shoulder
at 1828)
67.5
79.0
29.1,
29.1
[Rh2(tppn)(CO)2]2þ (122þ
[Rh2(tppn)(μ-CO)3]24þ (134þ
)
FAB-MS (m/z = 945, 917) and FT-IR spectra (ν(CO)
=
[Rh2(tpbn)(CO)2]2þ
[Rh2(tpbn)(CO)2]n
1992
1843
1985
1740
1832 cm-1 (KBr)) of 62þ are in accordance with the tris-μ-
carbonyl-bridged structure, while m/z 772 and ν(CO)= 1992
and 2042 cm-1 confirm the presence of the bis-carbonyl
complex 7þ. Attempts to prepare [6](PF6)2 in high yield were
unsuccessful, which might indicate an equilibrium between
62þ and 5þ in the presence of CO. In accordance with this,
[6](PF6)2 is not stable in the solid state at room temperature
in the absence of CO and converts to [5]PF6 within days.
Heating of the isolated mixture of [5]PF6 and [6](PF6)2 in
acetonitrile to 50 °C for 2 h results in full conversion of the
contained [6](PF6)2 to [5]PF6.
2þ
[Rh(tpa)(CO)]þ (14þ)
1991
1749
79.5
19.6
[Rh(tpa)(μ-CO)]22þ (152þ
)
which should result in relative stabilization of the (μ-CO)3
bridge in 22þ compared to 62þ
.
The CO stretch frequencies of the square-planar com-
plexes 3þ and 5þ (1989 vs 1994 cm-1 for bpa and Bu-bpa,
respectively) confirm that Bu-bpa is indeed a weaker donor
than bpa (Table 1). Although the difference in stretch
frequencies (5 cm-1) does not seem to be large, it is signifi-
cant and might explain the different relative stability of the
It is difficult to imagine that substitution of a proton for a
butyl group will have any significant steric influence on the
formation of the Rh(μ-CO)3Rh bridge (tethered alkyl-sub-
stituted bpa complexes do form carbonyl-bridged species,
vide infra). Therefore, the markedly different stability of the
dirhodium tris-carbonyl species 22þ and 62þ is most likely
caused by the decrease in donor capacity of the ligand on
going from bpa to Bu-bpa. The more electron-rich rhodium
atom of bpa complex 22þ is expected to bind CO more
bridged carbonyl complexes 22þ and 62þ
.
Synthesis and Structure of Cyclooctadiene and Carbonyl
Rhodium Complexes with Tethered bpa Ligands. Concluding
that the Bu-bpa complex 62þ is thermodynamically unstable,
we decided to investigate if the tris-carbonyl-bridged species
could be stabilized by cooperative binding of binuclear
rhodium bpa species tethered with an alkyl chain. We
expected that the reduction of the entropy of binding by
tethering of the rhodium carbonyl complexes could lead to
structures similar to the reported hexamine macrocylic tris-
carbonyl-bridged rhodium complexes.8 For that reason we
synthesized corresponding cod and carbonyl complexes with
ligands that have bpa moieties tethered by a chain of two,
three, and four carbon atoms and investigated their reacti-
vity with CO.
strongly than the Bu-bpa-ligated metal in the complex 62þ
,
(16) In the 1H-NOESY spectrum of [(Bu-bpa)RhI(CO)]þ ([2]þ), clear
NOE contacts are observed between the R-CH2-group of the butylamine
fragment (N-CH2-Pr) and the equatorial N-CH2-Py protons (one of the
two AB-type doublets). The latter also show NOE contacts with Py-H3.
The axial N-CH2-Py protons (other AB-type doublet) show no NOE
contacts with either N-CH2-Pr or Py-H3. The above NOE pattern is
characteristic for the mer-coordination mode of a pyridine-amine-
pyridine ligand, PyCH2-N(CH2R)-CH2Py, clearly different from the
NOE pattern characteristic for the fac-coordþination mode (see ref 19).
(17) Due to the low yield of 62þ and 7 , it was not possible to
distinguish betweenþtheir NMR signals that were generally overlapping
with the signals of 5 . For that reason the structure of 62þ with two CO
ligands being trans to two picolyl donors and one trans to two amine
donors is tentative, and it cannot be ruled out that the actual structure is
similar to the structure of 22þ, with one CO trans to two picolyl groups
and two CO trans to one amine and one picolyl moiety.
[Rh2((μ-(bis-κ3)tpen)2(η4-cod)2](PF6)2 ([8](PF6)2), [Rh2((μ-
(bis-κ3)tppn)(η4-cod)2](PF6)2 ([9](PF6)2), and the previously
11
reported [Rh2((μ-(bis-κ3)tpbn)(η4-cod)2](PF6)2 ([10](PF6)2)
(tpen=N1,N1,N2,N2-tetrakis(pyridin-2-ylmethyl)ethane-1,2-
diamine, tppn = N1,N1,N2,N2-tetrakis(pyridin-2-ylmethyl)-
propane-1,2-diamine, tpbn = N1,N1,N2,N2-tetrakis(pyridin-
2-ylmethyl)butane-1,2-diamine) were synthesized by reacting
[Rh(μ-Cl)(η4-cod)]2 with the corresponding N3-donor ligands

Article
Organometallics, Vol. 29, No. 7, 2010 1633
Dicationic complexes [Rh2((μ-(bis-κ3)tpen)(CO)2]2þ (112þ
and [Rh2((μ-(bis-κ3)tppn)(CO)2]2þ (122þ), dinuclear ana-
logues of 5þ, were prepared by reaction of [{(CO)2Rh(μ-Cl)}2]
with the corresponding ligand in MeOH in a 1:1 molar ratio.
Addition of NH4PF6 led to the precipitation of 112þ and 122þ
)
-
as PF6 salts (Scheme 7). These complexes could also be
prepared by treatment of 82þ or 92þ with 1 bar of CO.
In contrast to the reaction of [5]PF6, the reaction of [12]-
(PF6)2 in CH3CN with 50 bar of CO is nearly quantitative with-
in 4 days. The tetranuclear complex [Rh4((μ-CO)3)2((μ-(bis-
κ3)tppn)2]4þ, [13]4þ, is formed, withtwo tris-μ-carbonyl bridges
(Scheme 8). The ν(CO) bands at 1838 and 1828 cm-1 (CH3CN)
and 13C NMR triplets at δ = 215 (1JC-Rh= 29.1 Hz) and δ =
213 (1JC-Rh= 29.1 Hz) in approximate intensity ratio of 2:1
1
are indicative for the tris-μ-carbonyl bridges (Table 1). H
NMR and 13C NMR signals for the tppn ligand show that
[13]4þ has effective D2h symmetry in solution (8 equivalent
pyridyl fragments). The tppn ligand in [13]4þ is fac-coordi-
Figure 2. Coordination geometry of the rhodium atom in the com-
plex [Rh2((μ-(bis-κ3)tpen)2(η4-cod)2]2þ (82þ). Hydrogen atoms, the
PF6- counterion, and the acetone molecule are omitted for clarity.
Thermal ellipsoids are drawn with 50% probability. Selected
bond lengths [A] and angles [deg]: Rh1-C5 2.073(10); Rh1-C6
2.102(10); Rh1-C1 2.114(8); Rh1-N2 2.117(6); Rh1-C2
2.124(9); Rh1-N1 2.158(7); Rh1-N3 2.411(7); C1-C2
1.378(12); C5-C6 1.446(16); C5-Rh1-C6 40.5(4); C5-Rh1-C1
96.0(4); C6-Rh1-C1 79.9(4); C5-Rh1-N2 93.4(4); C6-Rh1-
N2 90.0(3); C1-Rh1-N2 151.2(3); C5-Rh1-C2 81.1(5); C6-
Rh1-C2 90.7(4); C1-Rh1-C2 38.0(3); N2-Rh1-C2 170.6(3);
C5-Rh1-N1 175.5(4); C6-Rh1-N1 139.0(4); C1-Rh1-N1
88.0(3); N2-Rh1-N1 82.1(2); C2-Rh1-N1 103.4(4); C5-Rh1-
N3 103.2(4); C6-Rh1-N3 141.1(4); C1-Rh1-N3 127.2(3);
N2-Rh1-N3 76.3(2); C2-Rh1-N3 97.3(3); N1-Rh1-N3
75.7(3).
1
nated, as indicated by H-NOESY NMR.19 For the DFT-
optimized structure of [13]4þ see Supporting Information
(Figure S1).
In contrast to 62þ, 134þ is stable at room temperature both
in the solid state and in solution. Heating 134þ in CH3CN to
80 °C results in only approximately 20% conversion to 122þ
in 2 h.
The stability of the tetranuclear complex 134þ is most likely a
result of the cooperative binding of the four rhodium atoms
through carbonyl bridges. Formation of the first Rh(μ-
CO)3Rh bridge (that is not thermodynamically stable for the
binuclear analogue of 134þ, the Bu-bpa complex 62þ) enhances
the effective local concentration of rhodium and entropically
favors the formation of the second Rh(μ-CO)3Rh bridge. This
effect is similar to the well-known chelate effect that results in
stronger binding of multidentate versus monodentate ligands.
Consequently, multivalent binding (involving the interaction
of four Rh atoms)20 stabilizes 134þ from fragmentation into
terminal Rh-CO complexes and results in higher stability of
in methanol and precipitation as a PF6- salt. The N3 ligand is
coordinated in a fac-mode, and the cyclooctadiene is fluxional
on the NMR time scale, which results in equivalent signals for
both picolyl moieties and equivalent signals for the four
olefinic protons of the cod fragment.
Crystals of [8](PF6)2 suitable for X-ray diffraction were grown
from acetone. The structure of 82þ is shown in Figures 2 and 3.
Significantly different bond distances between the rho-
dium atom and picolyl donors suggest a rather high distor-
tion from the square-pyramidal geometry, with the weakest
amine donor N3 being coordinated on the apical position.
The bond distances are not significantly different from the
bond distances of [Rh2(κ3-tpbn)(η4-cod)2](PF6)2 (102þ), par-
tially caused by larger errors induced by some disorder in the
134þ compared to 62þ
.
Under similar carbonylation conditions the tpen carbonyl
complex 112þ (nor the tpen cod complex 8þ) did not form
any bridged carbonyl species, and only 112þ could be ob-
served in the solution. Although formation of an intramole-
cular CO bridge is not possible because of the “one hand up,
one hand down’” conformation of the molecule, one could
expect that intermolecular CO bridges could be formed,
leading to polymeric structures. Formation of the CO-
bridged species from the Bu-bpa complex 5þ (for which the
partial conversion to the bridged species 62þ was observed)
and tpen complex 122þ would have approximately the same
crystal packing of the measured crystal of 82þ
Analysis of the X-ray structures of [Rh2((μ-(bis-κ3)tpen)2-
(η4-cod)2]2þ (82þ) and[Rh2((μ-(bis-κ3)tpbn)(η4-cod)2]2þ (102þ
.
)
and DFT-optimized structure of [Rh2((μ-(bis-κ3)tppn)-
(η4-cod)2]2þ18 (92þ) indicates that the length of the methylene
tether connecting bis(picolyl)amine moieties in tpbn, tppn,
and tpen influences the structure of the binuclear complex. In
the complexes with two or four carbon atoms in the tether, the
two metal centers are found to be on the opposite sides of the
molecule, in a “one hand up, one hand down” configuration.
In complex 92þ, having a C3 linker, the metal centers are on
the same side, in a “hands up” configuration (see Figure 3).
These structural features of the tethered bpa-alkyl ligands
proved to have an impact on the structure of corresponding
rhodium carbonyl complexes (see below).
(19) In [(tppn)Rh2(μ2-CO)3]þ ([5]4þ) both the axial and the equatorial
N-CH2-Py protons (both AB-type doublets) show NOE contacts with
Py-H3 and the β-CH2 group of the propylene-diamine tether (N-CH2-
CH2-CH2-N). The R-CH2 groups of the tether (N-CH2-CH2-CH2-N)
show only NOE contacts with the equatorial N-CH2-Py protons (one of
the two AB-type doublets) and not with the axial N-CH2-Py protons.
The above NOE pattern is characteristic for the fac-coordination mode
of a pyridine-amine-pyridine ligand, PyCH2-N(CH2R)-CH2Py, clearly
different from the NOE pattern characteristic for the mer-coordination
mode (see ref 16). The β-CH2 group of the n-butylamine group in
[(BuBPA)Rh(cod)]þ, [1]þ, also shows NOE contacts with both the axial
and the equatorial N-CH2-Py protons. However, the observed exchange
correlation (EXSY contact) between both N-CH2-Py signals in [1]þ
troubles further assignment of NOE contacts.
(18) X-ray structure of [Rh2(κ3-tppn)(η4-cod)2](PF6)2 has shown such
connectivity; however very large disorder made the structure unsuitable
for publication.
(20) (a) Mulder, A.; Huskens, J.; Reinhoudt, D. N. Org. Biomol.
Chem. 2004, 2, 3409–3424. (b) Hunter, C. A.; Anderson, H. L. Angew.
Chem., Int. Ed. 2009, 48, 7488–7499.

1634 Organometallics, Vol. 29, No. 7, 2010
Dzik et al.
Figure 3. Molecular structures of [Rh2((μ-(bis-κ3)tpen)2(η4-cod)2]2þ (82þ), [Rh2((μ-(bis-κ3)tppn)(η4-cod)2]2þ (92þ), and the previously
reported [Rh2((μ-(bis-κ3)tpbn)(η4-cod)2]2þ (102þ).
Scheme 7. Synthesis of Binuclear Rhodium Carbonyl Complexes [Rh2((μ-(bis-K3)tpen)(CO)2]2þ ([11]2þ) and [Rh((μ-(bis-κ3)tppn)-
(CO)2]2þ ([12]2þ
)
Scheme 8. Formation of Tetranuclear Rhodium Carbonyl Complex [Rh4((μ-CO)3)2(tppn)2]4þ, 134þ
unfavorable entropy contribution, but the enthalpy of for-
mation of the bridged species is likely even lower for tpen
because of electronic effects. The tpen ligand imposes a lower
(tpa = tris(picolyl)amine) ligand, which can be regarded as a
bpa ligand functionalized with an additional picolyl group.
This ligand can be compared with our previously reported
Me3tpa complexes in its coordinating behavior toward Rh-
carbonyl species.21
1
electron density on the metal (ν(CO)=1999 cm-1, JC-Rh
=
=
67.5 Hz) compared to the Bu-bpa or tppn ligands (ν(CO)
1994 cm-1, 1JC-Rh= 79 Hz) (Table 1), which most probably
does not allow for coordination of any extra CO molecule.
The lower donor capacity of the tpen ligand compared to Bu-
bpa can be rationalized by the fact that the amine nitrogens
of tpen are competing for the electrons from a very short C2
Reaction of the in situ generated [Rh(μ-Cl)(CO)2]2 with tpa
in methanol yields [Rh(κ3-tpa)(CO)]PF6 ([14]PF6) as a yellow
powder after precipitation with KPF6. Complexes 14þ with
-
PF6-, B(m-xylyl)4-, and B(m-tolyl)4 counterions (A-) were
also prepared by reaction of the Rh(κ3-tpa)(ethene)þ(A-) com-
plexes with CO in the solid state. Solution IR in low concentra-
tion shows only one CO absorption band at 1991 cm-1, indicat-
ing that the complex has a 16 VE square-planar geometry with
the tpa ligand being in a κ3-coordination mode. In polar sol-
vents such as acetone or acetonitrile, the mononuclear species
14þ is in equilibrium with the dinuclear bis-μ-CO-bridged
species [Rh(κ4-tpa)(μ-CO)]22þ (152þ) (Scheme 9), and at higher
concentrations a weak IR absorption band at 1749 cm-1 re-
veals the formation of bridging ketonic carbonyls. After eva-
poration of the solvent a dark purple solid is obtained, showing
an IR (solid state) absorption band of the bridging carbonyls of
the dinuclear complex 152þ at νCO = 1740 cm-1 and a terminal
carbonyl band at 1985 cm-1 (Table 1). Formation of 152þ is
reversible, and dissolution of the solid purple mixture of 14þ
•
alkyl bridge, which effectively acts as an electron-poor “CH2
radical” substituent of the bpa moiety.
Treatment of the [Rh2((μ-(bis-κ3)tpbn)(η4-cod)2]2þ (102þ
)
complex with 10 bar of CO at 50 °C for 10 h in acetonitrile
results in formation of a yellow precipitate. IR measurements
(KBr) showed the presence of bridging carbonyls (ν(CO)
=
=
1843 cm-1) and a minor amount of terminal carbonyls (ν(CO)
1993 cm-1) in the precipitate, while the filtrate after evapora-
tion of the solvent gave signals at 1838 and 1992 cm-1. This
could indicate formation of polymeric Rh(μ-CO)3Rh-bridged
species, which should be driven by precipitation.
Rhodium Carbonyl Complexes with Tris(picolyl)amine. The
above results suggest that the formation of the Rh(μ-CO)3-
Rh-bridged species is driven by use of stronger σ-donating
N3 ligands and can be further stabilized by entropic factors in
the case of homoditopic ligands. To further investigate the
effect of stronger σ-donors, we studied the tetradentate tpa
(21) Dzik, W. I.; Smits, J. M. M.; Reek, J. N. H.; de Bruin, B.
Organometallics 2009, 28, 1631–1643.

Article
Organometallics, Vol. 29, No. 7, 2010 1635
Figure 4. Van’t Hoff plot of the dimerization of 14þ to 152þ
:
ΔH=-28.4 ( 1.7 kJ mol-1; ΔS=-134 ( 7 J mol K-1 (r2=
3
3
3
0.9985). The equilibrium constant is defined as K=[152þ]/[14þ]2.
Scheme 9. Dynamic Equilibrium between the Monouclear
[Rh(K3-tpa)(CO)]þ (14þ) and Dinuclear [Rh(κ4-tpa)(μ-CO)]2
2þ
(152þ
)
1
Figure 5. Variable-temperature H NMR spectrum showing di-
and 152þ leads to disappearance of the bridging carbonyl band.
Such a monomer-dimer equilibrium is remarkable and has not
been observed for the related [Rh(κ3-Me3tpa)(CO)]þ com-
plex.21 This different behavior should be ascribed to the
stronger donor capacity of tpa versus Me3tpa.
merization of 14þ and the fluxional behavior of 14þ and 152þ in
acetone-d6. Legend: red points = 14þ; black points = 152þ
.
to the metal, forming a transient 18 VE κ4-complex followed
by dissociation of another picolyl moiety to re-form the 16
VE square-planar κ3-complex.21
The equilibrium between the mononuclear and binuclear
complexes could be studied by NMR in polar solvents such
as acetonitrile or acetone, whereas in dichloromethane only
the mononuclear form could be detected (see Figure S2,
Supporting Information). Both the monomer and the dimer
are fluxional on the NMR time scale, involving exchange of
the axial and equatorial picolyl moieties. VT-NMR measure-
ments further allowed us to calculate the thermodynamics
for dimerization of 14þ in acetone through a van’t Hoff plot
in the range from 283 to 225 K (Figure 4).
The signals of the dinuclear species 152þ start to appear
upon cooling the solution close to room temperature. The
signals of both the mononuclear species 14þ and dinuclear
species 152þ are substantially broadened at 308 K. Sharpen-
ing of the signals of both compounds appears in the same
temperature range. This behavior indicates that the broad-
ening is due to coalescence associated with the reversible
dimerization of 14þ to 152þ. The signals of all three picolyl
arms of the dinuclear compound 152þ are equivalent down to
218 K. Clearly the dinuclear complex remains fluxional in the
entire measured temperature range. At temperatures below
253 K the signals of the dimer are becoming broader and the
signal of Py-H6 completely disappears at 218 K while
approaching coalescence.22
Formation of binuclear complex 152þ from 14þ is enthal-
pically favorable by -28 kJ mol-1, but entropically disfa-
3
vored. The large negative entropy factor of -134 J mol K-1
agrees well with a dimerization process and dominates at
room temperature. The overall process at 298 K is slightly
3
3
endergonic (ΔGo298K = þ11.5 kJ mol-1, K298K ≈ 9.5 ꢁ 10-3).
Since the fluxional behavior of 152þ down to 218 K is not
caused by the dimer-monomer equilibrium, it has to origi-
nate from exchange of the picolyl moieties on the octahedral
18 VE rhodium center that is fast on the NMR time scale.
Noticeably, the octahedral 18 VE complex 22þ does not show
a similar fluxional behavior in solution. An important
difference between 22þ and 152þ is the presence of a Rh-Rh
bond in the former, which is absent in the latter (see below).
3
The 1H NMR spectra recorded in the temperature range
from 330 to 218 K are presented in Figure 5. At 330 K only
the mononuclear form 14þ is visible (K330K ≈ 3.2 ꢁ 10-3). All
the picolyl arms of the ligand are equivalent, which indicates
that at this temperature the molecule is fluxional on the
NMR time scale. This process is frozen at 263 K, where the
protons of the coordinated and dangling picolyl groups show
different signals. We expect that the mechanism of fluxion-
ality of 14þ is the same as for the recently reported [Rh-
(Me3tpa)CO]þ; that is, the dangling picolyl arm coordinates
(22) Unfortunately, we were not able to record spectra below 218 K
because of the NMR spectrometer restrictions.

1636 Organometallics, Vol. 29, No. 7, 2010
Dzik et al.
Scheme 10. Reaction of [Rh(K3-tpa)(CO)]þ (14þ) with Dioxygen
to Form the Carbonato Complex [Rh(κ4-tpa)(CO)3]þ (16þ)
The Rh-Rh distance of 3.0585(7) indicates little or no
bonding interaction between the metal atoms,23 in agree-
ment with the saturated 18 VE configuration of the two
metals in the absence of a Rh-Rh bond. The Rh-C-Rh
angle (100.9(2)°) is considerably larger than the usual 80-90°
bond of the “classical” bridging rhodium carbonyls.24
Although the angle is somewhat more acute than most of
the reported M-C-M angles for ketonic carbonyls (having
more sp2 character of the carbon atom), which are in the
range 107-120°,25 the CO stretching frequency of 1749 cm-1
(MeCN) is in full agreement with a ketonic character of the
carbonyl group.25d,e To our best knowledge, complex 152þ is
the first example of a binuclear bis-CO-bridged rhodium
species not supported by any other ancillary bridging li-
gands, nor a Rh-Rh bond.26 This seems to be also the first
example of a dynamic equilibrium between a mononuclear
terminal carbonyl RhI species and binuclear Rh(μ-CO)xRh-
bridged species measured in solution.
Figure 6. Proposed mechanism of exchange of the pyridyl
groups of 152þ leading to observed fluxionality of the complex.
In the solid state 14þ reacts with air to form carbonato
complex [Rh(κ4-tpa)(CO3)]þ (16þ), similar to the oxygena-
tion reaction recently reported for [Rh(κ3-Me3tpa)(CO)]þ21
(Scheme 10). Reaction with air in dichloromethane or ace-
tone/water led to a mixture of unidentified products. Top
layering a dichloromethane solution of [16]B(m-xylyl)4 with
hexanes yielded crystals of [16]B(m-xylyl)4 suitable for X-ray
diffraction (Figure 8).
Figure 7. X-ray structure of [Rh(κ4-tpa)(μ-CO)]22þ, 152þ. Ther-
mal ellipsoids are drawn with 50% probability. Hydrogen atoms,
the B(p-tolyl)4- counterions, and two dichloromethane molecules
are omitted for clarity. Selected bond lengths [A] and angles [deg]:
Rh1-C1 1.972(4), Rh1-C1a 1.993(4), Rh1-N2 2.088(3), Rh1-
N1 2.111(3), Rh1-N4 2.201(3), Rh1-N3 2.242(3), C1-O1
1.205(5), N1-Rh1-N4 80.14(12), N1-Rh1-N3 81.07(12), N2-
Rh1-N4 79.13(12), N2-Rh1-N3 93.62(12), C1-Rh1-C1a
79.05(18).
As expected, the Rh-N bond distances of 16þ are consider-
ably shorter than in the analogous complex [Rh(Me3tpa)-
(CO3)]þ, in which the ligand tpa is functionalized with methyl
groups on the 6 position of the pyridyl ring.21
The structure of 16þ is similar to the structure of
[Co(tpa)(CO3)]þ,27 albeit with longer metal to ligand bonds.
We therefore propose that the exchange process of 152þ follows
the sequence shown in Figure 6. Decoordination of one of the
picolyl arms (PyC trans to PyA) leads to a transient 16 VE square-
pyramidal species that can rearrange the coordinated picolyl
groups via a Berry-preudorotation mechanism (via a trigonal-
bypiramidal structure). Recoordination of the free picolyl group
leads to re-formation of the octahedral species with a different
order of the picolyl groups. DFT calculations suggest that the
detachment of the picolyl moiety of 152þ is facilitated by forma-
tion of a weak σ-type metal-metal interaction in the “unsatu-
rated” tbpy intermediate (see Supporting Information, Figure S3).
Compound 22þ already has a σ-type Rh-Rh bond and cannot
easily increase its bond order, and hence the pentacoordinate
intermediate required for exchange should be less easily accessible.
Formation of the dinuclear species 152þ was further
confirmed by single-crystal X-ray diffraction. Top layering
a dichloromethane solution of [14](B(m-tolyl)4)2 with hexa-
nes yielded purple crystals of [15](B(m-tolyl)4)2. The mole-
cular structure of 152þ is shown in Figure 7.
(23) The longest reported unsupported Rh-Rh bonds do not exceed
2.93-2.94 A: Chifotides, H. T. Dunbar, K. R. In Multiple Bonds
between Metal Atoms, 3rd ed.; Cotton, F. A.; Murillo, C. A.; Walton,
R. A., Eds.; Springer: New York, 2005; pp 465-589, and references therein.
(24) Colton, R.; McCormick, M. J. Coord. Chem. Rev. 1980, 31, 1–52.
(25) For examples see: (a) Cowie, M.; Southern, T. G. Inorg. Chem.
1982, 21, 246–253. (b) Cowie, M.; Vasapollo, G.; Sutherland, B. R.; Ennett, J. P.
Inorg. Chem. 1986, 25, 2648–2653. (c) Carmona, D.; Ferrer, J.; Lahoz, F. J.; Oro,
L. A.; Reyes, J.; Esteban, M. J. Chem. Soc., Dalton Trans. 1991, 2811–2820.
(d) Connelly, N. G.; Einig, T.; Herbosa, G. G.; Hopkins, P. M.; Mealli, C.; Orpen,
A. G.; Rosair, G. M.; Viguri, F. J. Chem. Soc., Dalton Trans. 1994, 2025–2039.
(e) Tejel, C.; Bordonaba, M.; Ciriano, M. A.; Edwards, A. J.; Clegg, W.; Lahoz,
F. J.; Oro, L. A. Inorg. Chem. 1999, 38, 1108–1117.
(26) Crystal structures of bis-μ-CO-bridged rhodium species sup-
ported by a Rh-Rh bond: (a) Allevi, C.; Golding, M.; Heaton, B.;
Ghilardi, C. A.; Midollini, S.; Orlandini, A. J. Organomet. Chem. 1987,
326, C19–C22. (b) Enders, M.; Kohl, G.; Pritzkow, H. J. Organomet. Chem.
2004, 689, 3024–3030. Similar tetranuclear species were also reported: (c)
Lahoz, F. J.; Martin, A.; Estruelas, M. A.; Sola, E.; Serrano, J. L.; Oro, L. A.
Organometallics 1991, 10, 1794–1799. (d) Cotton, F. A.; Dikarev, E. V.;
Petrukhina, M. A. J. Chem. Soc., Dalton Trans. 2000, 4241–4243.
(27) Cheyne, S. E.; McClintock, L. F.; Blackman, A. G. Inorg. Chem.
2006, 45, 2610–2618.

Article
Organometallics, Vol. 29, No. 7, 2010 1637
logues. The presented results clearly show that the stability
of the bis- and tris-carbonyl-bridged structures depends on a
delicate balance between favorable enthalpic factors (enhanced
with stronger σ-donor ligands) and unfavorable entropic fac-
tors (that can be reduced by multinuclear binding using ditopic
ligands).
Experimental Section
General Methods. All procedures were performed under N2
using standard Schlenk techniques. Solvents (p.a.) were deoxyge-
nated by bubbling through a stream of N2 or by the freeze-
pump-thaw method. The temperature indication rt corresponds
to ca. 20 °C. [{(CO)2Rh(μ-Cl)}2],28 tppn,29 [Rh(κ3-bpa)(η4-cod)]-
PF6 ([1]PF6), [Rh(κ3-Bu-bpa)(η4-cod)]PF6 ([4]PF6), and [Rh2((μ-
(bis-κ3)tpbn)(η4-cod)2](PF6)2 ([10](PF6)2)11 were prepared accord-
ing to literature procedures. All other chemicals are commercially
available and were used without further purification.
NMR experiments were carried out on a Bruker DPX200 (200
and 50 MHz for 1H and 13C, respectively), Bruker AC300 or
DRX300 (300 and 75 MHz for 1H and 13C, respectively), Bruker
Figure 8. X-ray structure of [Rh(κ4-tpa)(CO)3]þ H2O (16þ
1
WM400 (400 and 100 MHz for H and 13C, respectively), and
3
3
Varian Inova500 (500 and 125 MHz for 1H and 13C, respec-
H2O). Thermal ellipsoids are drawn with 50% probability. Hydro-
gen atoms of the tpa ligand and the B(m-xylyl)4- counterion were
omitted for clarity. Selected bond lengths [A] and angles [deg]:
Rh1-N3 2.007(3), Rh1-N2 2.012(3), Rh1-O2 2.021(2), Rh1-
N4 2.030(3), Rh1-N1 2.033(3), Rh1-O1 2.036(2), Rh1-C1
2.462(4), C1-O3 1.224(4), C1-O1 1.318(4), C1-O2 1.334(4),
H81a-O1 1.94(4); O2-Rh1-N4 106.15(10), O2-Rh1-N2
88.95(11), N4-Rh1-N2 82.77(11), O2-Rh1-O1 65.11(9), N4-
Rh1-O1 171.23(10), N2-Rh1-O1 95.97(10), O2-Rh1-N3
171.01(10), N4-Rh1-N3 82.45(10), N2-Rh1-N3 94.85(11),
O1-Rh1-N3 106.32(10), O2-Rh1-N1 91.49(10), N4-Rh1-
N1 84.34(11), N2-Rh1-N1 166.69(11), O1-Rh1-N1 96.28(11),
N3-Rh1-N1 86.68(11).
1
tively). Solvent shift reference for H NMR: [D6]-acetone δH
=
2.05, CD3CN δH=1.98. For 13C NMR: [D6]-acetone δC=29.50,
CD3CN δC=1.28. Abbreviations used are s=singlet, d = doublet,
dd = doublet of doublets, t=triplet, p=pentet, m = multiplet, and
br=broad. The couplings between the protons in the pyridine ring
are not fully resolved, and hence we use a simplified assignment of
the multiplicities of the signals as doublets, triplets, and double
triplets.
Elemental analyses (C, H, N) were carried out on a Carlo
Erba NCSO-analyzer. Mass spectra (FAB) were recorded on a
VG 7070 mass spectrometer or on a JEOL JMS SX/SX102A
four-sector mass spectrometer. Solution and KBr FT-IR spectra
were recorded on a Perkin-Elmer 1720X spectrometer or on a
Bruker Vertex 70 FTIR spectrometer. Solid-state IR measure-
ments were performed on a Shimadzu FTIR 8400S spectrometer
equipped with a Specac MKII Golden Gate Single Reflection
ATR system.
X-ray Diffraction. The structures are shown in Figures 1, 2, 7,
and 8,30 which include selected bond distances and angles. The
crystal data are shown in Table 2. Crystals were mounted on glass
needles. The intensity data of [2](PF6)2, [15](B(p-tolyl)4)2, and
[16]B(m-xylyl)4 were collected on a Nonius Kappa CCD single-
crystal diffractometer, using Mo KR radiation and applying φ and
ω scan modes. The intensity data of [8](PF6)2 were collected on an
Enraf-Nonius CAD4 single-crystal diffractometer using Mo KR
radiation and applying the ω scan mode. The intensity data were
corrected for Lorentz and polarization effects. A semiempirical
multiscan absorption correction was applied (SADABS)31 on [2],
[15], and [16], while [8] was corrected for absorption semiempiri-
cally from ψ scans. The structures were solved by the PATTY
option32 of the DIRDIF program system.33 All non-hydrogen
Although the picolyl moiety is a stronger donor than amine,
the Rh-O1 bond trans to the amine N4 is slightly longer
compared with the Rh-O2 bond trans to the picolyl N3. This
is most probably caused by the hydrogen bonding between
O1 and a water molecule present in the unit cell.
Conclusions
Synthesis of rhodium carbonyl complexes ligated with a
series of different bis(2-picolyl)amine derivatives allowed us
to study the factors that determine the relative stabilities of
terminal monocarbonyl and tris-carbonyl bridges. Whereas
for the relatively strong electron-donating bpa ligand the
formation of the Rh(μ-CO)3Rh bridge is thermodynamically
favorable at 1 bar of CO, weaker donors such as Bu-bpa or
tpen do not allow formation of such species under such
conditions. The unfavorable entropy of formation of the
carbonyl bridges can be reduced by tethering the bpa moi-
eties with a propylene linker. This allows for cooperative
binding of four rhodium centers to assemble a stable tetra-
nuclear compound with two tris-carbonyl bridges.
(28) McCleverty, J. A.; Wilkinson, G.; Lipson, L. G.; Maddox, M. L.;
Kaesz, H. D. Inorg. Synth. 1990, 28, 84.
(29) tppn has been prepared by a method similar to that reported for
tpen: Toftlund, H.; Yde-Andersen, S. Acta Chem. Scand., Ser. A 1981,
35, 575–585.
(30) (a) Ortep-3 for Windows: Farrugia, L. J. J. Appl. Crystallogr.
1997, 30, 565. (b) Rendering was made with POV-Ray 3.6, Persistence of
Vision Pty. Ltd. (2004), Persistence of Vision Raytracer (Version 3.6),
retrieved from http://www.povray.org/download/.
Similarly, the stronger tris(2-picolyl)amine (tpa) N4 donor
allows formation of ketonic bis-carbonyl-bridged species 152þ
,
which exists in dynamic equilibrium with the mononuclear
monocarbonyl species 14þ in solution, whereas Rh-carbonyl
species with the weaker N4-donor Me3tpa (tris(2,6-lutidyl)-
amine) exist only in the mononuclear terminal carbonyl form.
We thus showed that subtle changes in the ligand structure
have a major impact on the stability of the carbonyl-bridged
compounds compared to their terminal monocarbonyl ana-
€
(31) Sheldrick, G. M. SADABS; University of Gottingen: Germany,
1996.
(32) Beurskens, P. T.; Beurskens, G.; Strumpel, M.; Nordman, C. E.
In Patterson and Pattersons; Clarendon: Oxford, 1987; p 356.
(33) Beurskens, P. T.; Beurskens, G.; Bosman, W. P.; de Gelder, R.;
€
Garcıa-Granda, S.; Gould, R. O.; Israel, R.; Smits, J. M. M. DIRDIF
´
program system; University of Nijmegen: The Netherlands, 1996.

1638 Organometallics, Vol. 29, No. 7, 2010
Table 2. Crystallographic Data for [2](PF6)2, [8](PF6)2, [15](B(p-tolyl)4)2, and [16]B(m-xylyl)4
Dzik et al.
[Rh2(κ3-bpa)2(μ-
CO)3](PF6)2 2C3H6O
[Rh2((μ-(bis-κ3)tpen) (η4-
cod)2](PF6)2 C3H6O
[Rh(tpa)CO]2(B-
tolyl4)2 4CH2Cl2
[Rh(tpa)(CO3)](B-
xylyl4) H2O
3
3
3
3
cryst color
cryst shape
cryst size [mm]
empirical formula
fw
translucent yellow-green
rather regular rod
0.28 ꢁ 0.14 ꢁ 0.13
C33H38F9N6O5 P1.50Rh2
1021.97
translucent yellow
irregular chunk
0.35 ꢁ 0.19 ꢁ 0.10
C24H32F6N3OPRh
626.41
dark brown
rough rod
0.72 ꢁ 0.24 ꢁ 0.13
C98H100B2Cl8N8O2Rh2
1932.90
translucent light yellow
rough fragment
0.28 ꢁ 0.24 ꢁ 0.22
C51H56BN4O4Rh
902.72
temperature [K]
radiation
wavelength [A]
cryst syst
space group
a [A]
b [A]
c [A]
R [deg]
β [deg]
208(2)
Mo KR (graphite mon.)
0.71073
tetragonal
I42d
30.368(8)
30.368(8)
9.2873(4)
90
90
298(2)
Mo KR (graphite mon.)
0.71073
monoclinic
P21/n
9.3568(9)
208(2)
298(2)
Mo KR (graphite mon.)
0.71073
monoclinic
P21/n
9.9552(9)
24.1566(7)
21.377(2)
90
96.852(8)
90
5104.2(7)
Mo KR (graphite mon.)
0.71073
triclinic
P1
13.8374(10)
14.319(2)
14.5273(9)
115.639(7)
111.926(7)
94.289(10)
2309.1(4)
21.675(2)
13.0980(14)
90
92.54(2)
90
2653.7(5)
γ [deg]
volume [A]
90
8565(3)
Z
8
1.585
0.910
Nonius KappaCCD
with area detector
φ and ω scan
4092
4
1.568
0.768
Enraf-Nonius CAD4
1
1.390
0.642
Nonius KappaCCD
with area detector
φ and ω scan
996
4
1.175
0.378
Nonius KappaCCD
with area detector
φ and ω scan
1888
density [Mg m-3
]
abs coeff [mm-1
diffractometer
]
scan
F(000)
ω scan
1276
θ range [deg]
index ranges
2.12 to 25.00
-36 e h e 36
-36 e k e 36
-11 e l e 11
82 568/3781
1.82 to 24.97
0 e h e 11
-25 e k e 25
-15 e l e 15
9730/4648
2.14 to 25.00
-16 e h e 16
-17 e k e 17
-17 e l e 17
25 089/8074
3.00 to 27.50
-12 e h e 12
-31 e k e 31
-26 e l e 27
98 373/11 680
reflns collected/
unique
R(int)
reflns obsd [Io >
2σ(Io)]
0.1218
3738
0.1070
2478
0.0573
6527
0.0586
8901
data/restraints/
params
3781/0/261
1.231
4648/3/338
1.021
8074/0/545
1.052
11 680/2/564
1.107
goodness-
of-fit on F2
SHELXL-97
weight params
final R indices
[I > 2σ(I)]
R1
0.0732, 193.7360
0.0687, 0.6537
0.0379, 4.2236
0.0628, 3.9564
0.0877
0.2183
0.0688
0.1375
0.0485
0.1005
0.0580
0.1282
wR2
R indices (all data)
R1
wR2
0.0883
0.2187
0.1516
0.1683
0.0714
0.1101
0.0896
0.1393
largest diff peak
and hole [e A-3
1.681/-1.116
0.634/-0.543
0.692/-0.635
1.073/-0.329
]
3
atoms were refined with anisotropic temperature factors. The
hydrogen atoms were placed at calculated positions and refined
isotropically in riding mode, except for the hydrogen atoms on O81
in [16]B(m-xylyl)4, which were refined freely.
section below) requires the presence of 2 PF6 moieties per
complex; therefore we have to assume that the residual electron
density stands for 0.5 PF6 (4 PF6 moieties in the whole unit cell).
The SQUEEZE procedure shows four voids of 229 A3 each,
containing 52 electrons in the whole unit cell. These voids are
centered around positions with y = 0.25 and y = 0.75, so around
special positions with a multiplicity of 4, which is in accord with
the number of missing PF6 moieties. The number of electrons in
these voids reported by the SQUEEZE procedure (52/void) is
rather low, as a PF6 moiety has 69 electrons. However, the voids
are rather large and therefore merge to form channels through the
structure running parallel to the c-axis. Measurements on crystals
grown from acetone/diethyl ether did not result in improved data;
indeed they were worse. However, these data showed indications
of the missing PF6 moieties on a general position in these
channels, in which case an occupancy factor of 0.5 has to be
assumed, together with the presence of more, unidentified solvent
moiety (acetone or diethyl ether). Despite the problems encoun-
tered with this structure, we do believe that the structure and
geometry of the dicationic dinuclear Rh complex are correct and
reliable. The physical properties given here are based on the
presence of 2 PF6 and 2 acetone moieties in the structure.
[2](PF6)2. The crystal structure analysis was hampered by PF6
disorder and the poor quality of the data. Because of the limited
reliability of the higher order data, all data above 25° θ were not
used in the refinement. The least-squares refinement showed a
moderate racemic twinning (BASF = 0.113). The difference
Fourier map showed a substantial residual density, which could
not be parametrized, and therefore the SQUEEZE procedure34
was used to account for this electron density. Nevertheless the
final difference Fourier map showed rather large residual den-
sity, especially close to the rhodium atom. To a large extent this
is probably due to absorption effects.
The parameter set suggests a ratio of 1 dinuclear Rh com-
plex:1.5 PF6 moieties:2 acetone moieties. Obviously, the correct
charge balance of the dinuclear Rh complex (as confirmed by the
analytical and spectroscopic data of [2](PF6)2, see Synthesis
(34) Spek, A. L. PLATON, A Multipurpose Crystallographic Tool;
Utrecht University: Utrecht, The Netherlands, 2003.

Article
[8](PF6)2. The PF6 moiety is rather disordered, as is the
Organometallics, Vol. 29, No. 7, 2010 1639
(500 MHz, CD3CN): δ 8.62 (d, 3J(H,H) = 4.5 Hz, 1H; Py-H6);
8.52 (d, 3J(H,H) = 4.5 Hz, 1H; Py-H60); 7.88 (dt, 4J(H,H) = 1.5
Hz, 3J(H,H) = 8.0 Hz, 2H; Py); 7.46 (m, 3H; Py); 7.38 (t, 3J(H,
H) = 6.0 Hz, 1H; Py); 5.72 (t, 3J(H,H) = 4.0 Hz, 1H; NH); 4.87
(dd[AB], 2J(H,H) = 17.0 Hz, 3J(H,H) = 7.0 Hz, 2H; N-CH2-
Py); 4.78 (dd[AB], 2J(H,H) = 17.0 Hz, 3J(H,H) = 7.0 Hz, 2H;
N-CH2-Py); 4.33 (m, 4H; N-CH2-Py). 13C NMR (125 MHz,
acetone moiety (C3H6O). Within the acetone moiety only atom
C43 could be split up during the refinement, but all C-C bond
distances had to be restrained. It was not possible to parametrize
the PF6 disorder. The same holds for the acetone. Moreover, it is
unlikely that the acetone parameters give an adequate descrip-
tion of the disorder: the geometry of at least one of the two
proposed positions is unsatisfactory, but based on the synthetic
route and packing considerations acetone seems to be the only
likely candidate for the electron density present in the Fourier
map. Geometrical calculations34 revealed neither unusual geo-
metric features nor unusual short intermolecular contacts. The
calculations revealed no higher symmetry and no (further)
solvent-accessible areas.
[15](B(p-tolyl)4)2. Only one suitable, rather fragile, single
crystal was available, and therefore it was not cut to a smaller
size. A collimator producing an X-ray beam of 0.8 mm was used
during the measurements. Geometrical calculations revealed
neither unusual geometric features nor unusual short intermo-
lecular contacts. The calculations revealed no higher symmetry
and no (further) solvent-accessible areas.
[16](B(m-xylyl)4). The hydrogen atoms, except those on O81,
were placed at calculated positions and refined isotropically in
riding mode. The hydrogen atoms on O81 were found from a
difference Fourier map, but the O-H bond distances had to be
restrained in order to prevent physically unacceptable results.
The structure contains some heavily disordered and probably
partially occupied solvent moieties. Attempts to parametrize
these moieties were not successful and resulted in physically
unacceptable results or unstable refinements.
1
CD3CN): δ 215.8 (t, J(C,Rh) = 28.4 Hz, 2xμ2-CO), 210.4 (t,
1J(C,Rh) = 27.9 Hz, μ2-CO); 161.1 (Py-C1); 160.9 (Py-C10);
149.5 (Py-C5); 149.3 (Py-C50); 140.8 (Py-C3); 140.7 (Py-C30);
125.8 (Py); 124.8 (Py); 60.0 (N-CH2-Py); 59.9 (N-CH2-Py0). IR
(MeCN): ν(CꢀO) = 1835 with a shoulder at 1828 cm-1, solid
state: ν(CꢀO) = 1836, 1815 cm-1. Anal. Calcd (C27H26F12N6-
O3P2Rh2 0.5CH2Cl2): C, 32.36; H, 2.67; N, 8.23. Found: C,
32.34; H, 2.97; N, 8.43.
3
[Rh(K3-bpa)(CO)]PF6 ([3]PF6). A 153 mg amount of [2](PF6)2
(0.156 mmol) was dissolved in 10 mL of MeOH and kept under
reflux for 3 h, allowing the evolved CO to escape from the vessel.
The unreacted [2](PF6)2 was filtered off, and the filtrate was
condensed to ca. 5 mL, causing precipitation of the product,
which was filtered off and dried in vacuo. Yield: 59 mg (0.124
mmol, 39.6%). 1H NMR (300 MHz, CD3CN): δ 8.39 (d, 3J(H,
H) = 5.4 Hz, 2H; Py); 7.92 (dt, 4J(H,H) = 1.5 Hz, 3J(H,H) =
3
7.8 Hz, 2H; Py); 7.34 (d, J(H,H) = 8.1 Hz, 2H; Py); 7.30 (t,
3J(H,H) = 6.9 Hz, 2H; Py); 5.57 (s, br, 1H, N-H); 4.60 (d[AB],
3
2J(H,H) = 15.9 Hz, J(H,H) = 9.6 Hz, 2H; N-CH2-Py); 4.46
(d[AB], 2J(H,H) = 15.9 Hz, 3J(H,H) = 6.0 Hz, 2H; N-CH2-Py).
13C NMR (75 MHz, CD3CN): δ 190.3 (d, 1J(Rh,C) = 77.5 Hz;
(CO)); 165.2 (Py); 154.6 (Py); 139.8 (Py); 125.7 (Py); 123.4 (Py);
57.8 (N-CH2-Py). IR (MeCN): ν(CꢀO) = 1989 cm-1. Anal.
Calcd (C13H13F6N3OPRh): C, 32.86; H, 2.76; N, 8.84. Found:
C, 32.93; H, 3.00; N, 8.72.
Therefore the SQUEEZE procedure34 was used to account
for the observed electron density. It detected two symmetry-
related voids of 367 A3 containing 11 electrons each. This
electron count is not very likely, and therefore we assume partial
occupancy. It is not possible to draw any reliable conclusions
about the nature of the moieties or the amount present, and
hence they could not be taken into account while calculating
physical properties. Geometrical calculations revealed neither
unusual geometric features nor unusual short intermolecular
contacts. The calculations revealed no higher symmetry and no
(further) solvent-accessible areas.
[Rh(K3-Bu-bpa)(CO)]PF6 ([5]PF6). From reaction of [1]PF6
with CO: 100 mg [1]PF6 was dissolved in 10 mL of CH3CN,
and a stream of CO was bubbled through the solution for 10
min. [2]PF6 was isolated in nearly quantitative yield by evapora-
tion of the solvent.
From reaction of Bu-bpa with [{(CO)2Rh(μ-Cl)}2]: 110 mg
(0.283 mmol) of [{(CO)2Rh(μ-Cl)}2] and 140 mg (0.548 mmol) of
Bu-bpa were dissolved in 20 mL of MeOH. The resulting orange
solution was stirred for 1 h at rt and filtered. A solution of 350 mg
of NH4PF6 in 5 mL of MeOH was added. Partial evaporation of
the solvent caused the precipitation of [3]PF6 as a yellow solid.
DFT Calculations. Geometry optimizations were carried out
with the Turbomole program package35 coupled to the PQS
Baker optimizer36 at the ri-DFT37 level using the BP8638 func-
tional and SV(P) basis set.39
1
Yield: 180 mg (0.339 mmol, 60.0%). H NMR (400.14 MHz,
CD3CN): δ 8.33 (d, 3J(H,H) = 6.7 Hz, 2H, Py-H6), 7.88-7.33
(m, 6H, Py-H3, Py-H4, Py-H5), 4.80 (d[AB], 3J(H,H) = 16.0 Hz,
N-CH2-Py), 4.21 (d[AB], 3J(H,H) = 16.0 Hz, N-CH2-Py), 2.76
(m, 2H, N-CH2-C3H7), 1.43 (m, 2H, N-CH2-CH2-C2H5), 1.14
(m, 2H, N-C2H4-CH2-CH3), 0.67 (t, 3J(H,H) = 8.6 Hz, 3H, N-
C3H6-CH3). 13C NMR(100.61 MHz, CD3CN, 298 K): δ 190.0 (d,
1J(Rh,C) = 79.1 Hz, CO), 164.2 (Py-C2), 154.6 (Py-C6), 140.2
(Py-C4), 126.0 (Py-C3), 124.1(Py-C5), 66.5(N-CH2-Py), 62.1(N-
CH2-C3H7), 26.7 (N-CH2-CH2-C2H5), 21.0 (N-C2H4-CH2-CH3),
14.0 (N-C3H6-CH3). FT-IR (CH3CN, cm-1): ν(CꢀO) = 1994
cm-1. FAB-MS: m/z 386 [M - PF6]þ, 917 [2M - PF6]þ. Anal.
Calcd (C17H21N3RhOPF6): C, 38.44; H, 3.98; N, 7.91. Found: C,
38.54; H, 4.09; N, 7.89.
Measurements of the Equilibrium between 14þ and 152þ
.
Measurements were performed on a 0.0036 M solution of 14þ
in acetone-d6 on a Bruker DRX300 NMR spectrometer. The
sample temperature was calibrated by measuring the chemical-
shift separation between the OH and CH3 resonances of metha-
nol. Relative concentrations of 14þ and 152þ were estimated by
integration of the -CH2- proton signals of the tpa ligand.
Synthesis. [Rh2(K3-bpa)2(μ-CO)3](PF6)2 ([2](PF6)2). A 290
mg portion of [Rh(bpa)(cod)]PF6 (0.522 mmol) was placed in a
100 mL Schlenk flask and dissolved in 15 mL of CH2Cl2. CO was
bubbled through the solution for 25 min, during which a yellow
precipitate formed. The solution was stirred for an additional 30
min under CO atmosphere, and the yellow solid was filtered and
[Rh2((μ-(bis-K3)tpen)(cod)2](PF6)2 ([8](PF6)2). A 271 mg sam-
ple of [Rh(cod)(μ-Cl)]2 (0.550 mmol) was suspended in 50 mL of
MeOH, and 221 mg (0.520 mmol) of tpen was added. The
solution was stirred for 1 h, after which the unreacted [Rh(cod)-
(μ-Cl)]2 was filtered off. Excess NH4PF6, dissolved in 8 mL of
MeOH, was added. The yellow precipitate was filtered, washed
with a small amount of methanol, and dried in vacuo. Yield: 504 mg
(0.443 mmol, 85.2%). 1H NMR (400.14 MHz, CD3CN): δ 9.03 (d,
3J(H,H) = 5.2 Hz, 4H, Py-H6), 7.70 (dt, 4H, 3J(H,H) = 7.7 Hz,
3J(H,H) = 1.3 Hz, Py-H4), 7.29 (t, 3J(H,H) = 6.1 Hz, 4H, Py-H5),
7.25 (d, 3J(H,H) = 7.4 Hz, 4H, Py-H3), 4.58 (d[AB], 2J(H,H) =
16.3 Hz, 4H, N-CH2-Py), 4.30 (d[AB], 2J(H,H) = 16.3 Hz, 4H, N-
CH2-Py), 4.57 (s, 4H, N-CH2-CH2-N), 3.77 (s, 8H, CHdCH);
1
dried in vacuo. Yield: 167 mg (0.170 mmol, 65.4%). H NMR
(35) Ahlrichs, R. Turbomole Version 5; Theoretical Chemistry Group,
University of Karlsruhe, 2002.
(36) PQS version 2.4; Parallel Quantum Solutions: Fayetteville, AR,
2001. The Baker optimizer is available separately from PQS upon request:
Baker, J. J. Comput. Chem. 1986, 7, 385-395.
(37) Sierka, M.; Hogekamp, A.; Ahlrichs, R. J. Chem. Phys. 2003,
118, 9136–9148.
(38) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098–3100. (b) Perdew,
J. P. Phys. Rev. B 1986, 33, 8822–8824.
(39) Schaefer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97,
2571–2577.

1640 Organometallics, Vol. 29, No. 7, 2010
Dzik et al.
2.49 (m, 8H; CH2), 1.80 (m, 8H; CH2). 13C NMR (100.61 MHz,
CD3CN, 298 K): δ 160.1 (Py-C2), 151.8 (Py-C6), 139.5 (Py-C4),
125.5 (Py-C3), 124.3 (Py-C5), 77.1 (d, 1J(Rh,C) = 13.3 Hz;
(CHdCH)), 63.7 (N-CH2-Py), 58.9 (N-CH2-CH2-N), 31.7
(CH2). Anal. Calcd (C42H52N6Rh2P2F12): C, 44.38; H, 4.61;
N, 7.39. Found: C, 44.29; H, 4.65; N, 7.33.
only traces of [4](PF6)2 and almost quantitative yield of [5]-
(PF6)4. 1H NMR (300.13 MHz, CD3CN): δ8.69 (d, 3J(H,H) =5.2
Hz, 8H, Py-H6), 7.99-7.40 (m, 24H, Py-H3, Py-H4, Py-H5), 4.80
(d[AB], 3J(H,H) = 16.0 Hz, 16H, N-CH2-Py), 4.18 (d[AB], 3J(H,
H) = 16.0 Hz, 16H, N-CH2-Py), 2.80 (t, 3J(H,H) = 7.9 Hz, 8H, N-
3
CH2-CH2-), 1.90 (p, J(H,H) = 7.9 Hz, 4H, N-CH2-CH2-). 13C
[Rh2((μ-(bis-K3)tppn)(cod)2](PF6)2 ([9](PF6)2).
A
147 mg
NMR (75.47 MHz, CD3CN, 298 K): δ 214.9 (t, 1J(C,Rh) = 29.1
Hz, μ2-CO), 213.1 (t, 1J(C,Rh) = 29.1 Hz, μ2-CO), 160.1 (Py-C2),
150.1 (Py-C6), 141.4 (Py-C4), 126.3 (Py-C3), 125.0 (Py-C5), 65.4
(N-CH2-Py), 63.5 (N-CH2-CH2-), 17.7 (N-CH2-CH2-). IR (CsI):
amount of [Rh(cod)(μ-Cl)]2 (0.298 mmol) was suspended in 50
mL of MeOH, and 119 mg (0.271 mmol) of tppn was added. The
solution was stirred for 1 h, after which the unreacted [Rh(cod)(μ-
Cl)]2 was filtered off. Excess NH4PF6, dissolved in 4 mL of MeOH
was added. The yellow precipitate was filtered, washed with a small
amount of methanol, and dried in vacuo. Yield: 226 mg (0.196
mmol, 72.3%). 1H NMR (400.14 MHz, CD3CN): δ 9.08 (d, 3J(H,
H) = 5.0 Hz, 4H, Py-H6), 7.65 (dt, 4H, 3J(H,H) = 7.2 Hz, 3J(H,
H) = 1.3 Hz, Py-H4), 7.23 (t, 3J(H,H) = 7.6 Hz, 4H, Py-H5), 7.15
(d, 3J(H,H) = 7.7 Hz, 4H, Py-H3), 4.51 (d[AB], 2J(H,H) = 16.0
Hz, 4H, N-CH2-Py), 4.16 (d[AB], 2J(H,H) = 16.0 Hz, 4H, N-CH2-
ν
(CꢀO) = 1838 (s), 1825 (sh) cm-1. IR (CH3CN): ν(CꢀO) = 1838
(s), 1828 (sh) cm-1. FAB-MS: m/z 1891 [M - PF6]þ, 1835 [M -
CO - PF6]þ. Calcd for [[5](PF6)3]þ (C60H60N12Rh4O6P3F18) m/z
1890.990439; found m/z 1891.004300 (Δ = -7.3).
[Rh(K3-tpa)(CO)]PF6 ([14]PF6). A 197 mg amount of [Rh(μ-
Cl)(coe)2]2 (0.275 mmol) was suspended in 10 mL of MeOH, and
167 mg of tpa (0.575 mmol) was added. CO was bubbled until all
the solid dissolved and the solution turned yellow-brown. The
solution was stirred for additional 20 min under CO atmo-
sphere, after which 143 mg of KPF6 (0.781 mmol) was added,
causing precipitation of a yellow solid, which was filtered off and
washed twice with 2 mL of MeOH. Yield: 213 mg (0.376 mmol).
Compound was recrystallized from 10 mL of hot MeOH to yield
140 mg (0.247 mmol, 45.0%) of analytically pure product.
After dissolution of 14þ in acetone, compound 14þ exists in equi-
librium with 152þ. Signals of 152þ were omitted for clarity. PyC =
2
Py), 4.15 (d, J(H,H) = 16.0 Hz 4H, N-CH2-), 3.79 (s, 8H,
CHdCH); 2.65 (m, 8H; CH2), 2.61 (m, 2H, N-CH2-CH2-CH2-
N2), 1.80 (m, 8H; CH2). 13C NMR (100.61 MHz, CD3CN, 298 K):
δ 160.1 (Py-C2), 152.0 (Py-C6), 139.3 (Py-C4), 125.2 (Py-C3),
124.1 (Py-C5), 76.7 (d, 1J(Rh,C) = 13.5 Hz; (CHdCH)), 63.3 (N-
CH2-Py), 61.6 (N-CH2-), 31.7 (CH2), 21.1 (N-CH2-CH2-CH2-N).
FAB-MS: m/z 1005 [M - PF6]þ, 1151 [M þ H]þ, 1173 [M þ Na]þ.
Anal. Calcd (C43H54N6Rh2P2F12 H2O): C, 44.19; H, 4.83; N, 7.19.
3
1
coordinated picolyl moiety, PyD = dangling picolyl moiety. H
Found: C, 44.04; H, 4.62; N, 7.37.
NMR (500 MHz, acetone-d6, -5 °C): δ 8.44 (d, 3J(H,H) = 5.0 Hz,
[Rh2((μ-(bis-K3)tpen)(CO)2](PF6)2 ([11](PF6)2). A 121 mg
amount of [Rh(CO)2(μ-Cl)]2 (0.313 mmol) was suspended in
50 mL of MeOH, 132 mg (0.311 mmol) of tpen was added, and
the dark bordeaux-red solution was stirred for 1 h, after which
the color changed to orange. Excess NH4PF6, dissolved in 5 mL
of MeOH, was added. The red-brown precipitate was filtered,
washed with a small amount of methanol, and dried in vacuo.
The color of the solid turned to black upon drying and gave an
orange solution upon dissolution in acetonitrile. Yield: 243 mg
(0.250 mmol, 80.4%). 1H NMR (400.14 MHz, CD3CN): δ 8.24
(d, 3J(H,H) = 5.5 Hz, 4H, Py-H6), 7.89 (t, 4H, 3J(H,H) = 7.8
3
2H; PyC-H6); 8.35 (d, J(H,H) = 3.5 Hz, 1H; PyD-H6); 8.02 (t,
3J(H,H) = 7.5 Hz, 2H; PyC-H4); 7.98 (d, 3J(H,H) = 7.5 Hz, 1H;
PyD-H3); 7.65 (t, 3J(H,H) = 7.5 Hz, 1H; PyD-H4); 7.61 (d, 3J(H,
H) = 8.0 Hz, 2H; PyC-H3); 7.40 (t, 3J(H,H) = 7.0 Hz, 2H; PyC-
H5); 7.14 (t, 3J(H,H) = 6.0 Hz, 1H; PyD-H5); 5.23 (d[AB], 2J(H,
H) = 15.5 Hz, 2H; N-CH2-PyC); 4.84 (d[AB], 2J(H,H) = 15.5 Hz,
2H; N-CH2-PyC); 4.38 (s, 2H; N-CH2-PyD). 13C NMR (125 MHz,
1
acetone, -27 °C): δ 190.8 (d, J(Rh,C) = 79.5 Hz; (CO)); 165.1
(PyC); 155.3 (PyC); 155.2 (PyD); 154.2 (Py); 151.1 (PyD); 150.9
(PyD); 140.7 (PyC); 138.0 (PyD), 129.5 (Py); 129.3 (py); 66.4 (N-
CH2-Py); 65.9 (N-CH2-Py). IR (MeCN): ν(CꢀO) = 1991 cm-1
.
3
Hz, Py-H4), 7.33 (t, J(H,H) = 6.5 Hz, 4H, Py-H5), 7.26 (d,
3J(H,H) = 7.8 Hz, 4H, Py-H3), 4.79 (d[AB], 2J(H,H) = 16.0 Hz,
4H, N-CH2-Py), 4.11 (d[AB], 2J(H,H) = 16.0 Hz, 4H, N-CH2-
Py), 3.13 (s, 4H, N-CH2-). 13C NMR (100.61 MHz, CD3CN, 298
K): δ 190.0 (d, 1J(Rh,C) = 67.5 Hz; (CO)), 163.0 (Py-C2), 154.9
(Py-C6), 140.4 (Py-C4), 126.4 (Py-C3), 124.4 (Py-C5), 66.8 (N-
Anal. Calcd (C19H18F6N4OPRh): C, 40.30; H, 3.20; N, 9.89. Found:
C, 40.63; H, 3.57; N, 9.83
[Rh(K3-tpa)(CO)]B(m-xylyl)4 ([14] B(m-xylyl)4). Complex 14þ
-
-
with B(m-xylyl)4 and B(m-tolyl)4 counterions could also be
obtained by reaction of corresponding ethene complexes40 with
CO in the solid state. 1H NMR (CD2Cl2, 200 MHz, v br): δ 8.14
(s, 2H); 7.65 (m, 2H); 7.42 (s, 2H); 7.16 (s, 8H); 6.97 (d, 6H);
6.54 (s, 4H); 3.61 (s, 2H); 3.12 (d, 4H); 2.09 (s, 24H). IR (KBr):
ν(CꢀO) = 1990 cm-1. Calcd mass for 14þ (C19H18N4ORh)
421.05357; accurate mass 421.05274 (Δ = 1.90).
CH2-Py), 59.2 (N-CH2-). FT-IR (CH3CN, cm-1): ν(CꢀO)
=
1999 cm-1. Anal. Calcd (C28H28N6O2Rh2P2F12): C, 35.45; H,
2.89; N, 8.61. Found: C, 35.17; H, 3.11; N, 8.87.
[Rh2((μ-(bis-K3)tppn)(CO)2](PF6)2 ([12]PF6)2). A 100 mg
amount (0.257 mmol) of [{(CO)2Rh(μ-Cl)}2] and 110 mg
(0.251 mmol) of tppn were dissolved in 20 mL of MeOH. The
resulting orange solution was stirred for 1 h at rt and filtered. A
solution of 350 mg of NH4PF6 in 5 mL of MeOH was added.
Partial evaporation of the solvent caused the precipitation of
[4](PF6)2 as a yellow solid. Yield: 183 mg (0.185 mmol, 74%). 1H
NMR (200.13 MHz, CD3CN): δ 8.20 (d, 3J(H,H) = 6.7 Hz, 4H,
Py-H6), 7.99-7.33 (m, 12H, Py-H3, Py-H4, Py-H5), 4.80
(d[AB], 3J(H,H)= 16.0 Hz, 4H, N-CH2-Py), 4.18 (d[AB],
[Rh(K4-tpa)(CO)]2(PF6)2 ([15](PF6)2). After dissolution of
14þ in acetone, compound 152þ exists in equilibrium with 14þ.
Signals of 14þ were omitted for clarity. 1H (500 MHz, acetone-
3
d6, -5 °C): δ 8.52 (d, J(H,H) = 5.0 Hz, 3H; Py-H6); 7.82 (d,
3J(H,H) = 7.5 Hz 4J(H,H) = 1.0 Hz, 3H; Py-H4); 7.46 (d, 3J(H,
H) = 7.5 Hz, 3H; Py-H3); 7.25 (t, 3J(H,H) = 6.5 Hz, 3H; Py-
H5); 5.08 (s, 6H; N-CH2-Py). 13C (125 MHz, acetone, -60 °C,
all peaks broad): δ 208.3 (t, 1J(Rh,C) = 19.6 Hz; (CO)); 162.5
(Py); 153.2 (Py); 140.4 (Py); 140.1 (Py); 126.4 (Py); 126.1 (Py);
3
3J(H,H) = 16.0 Hz, 4H, N-CH2-Py), 2.80 (t, J(H,H) = 6.6
125.2 (Py); 65.7 (N-CH2-Py). IR (MeCN): ν(CꢀO) = 1749 cm-1
.
Hz, 4H, N-CH2-CH2-), 1.90 (p, 3J(H,H) = 6.6 Hz, 2H, N-CH2-
CH2-). 13C NMR (100.61 MHz, CD3CN, 298 K): δ 190.0 (d,
1J(Rh,C) = 79.0 Hz, CO), 163.9 (Py-C2), 154.7 (Py-C6), 140.3
(Py-C4), 126.2 (Py-C3), 121.7 (Py-C5), 66.8 (N-CH2-Py), 60.5
(N-CH2-CH2-), 29.4 (N-CH2-CH2-). FT-IR (CH3CN, cm-1):
[Rh(K4-tpa)(CO3)]B(m-xylyl)4 ([16] B(m-xylyl)4). Rh(tpa)-
(CO)B(m-xylyl)4 (50 mg) was put in an atmosphere 50/50 O2/
N2 for 2 days. The product was obtained as an off-white
solid. Yield: >95%. Crystals suitable for X-ray diffraction
were obtained by diffusion of hexane into a solution of
dichloromethane. 1H NMR (CD2Cl2, 400 MHz): δ 8.82
ν
(CꢀO) = 1994 cm-1. FAB-MS: m/z 845 [M - PF6]þ, 700 [M -
2PF6]þ. Anal. Calcd (C29H30N6Rh2O2P2F12): C, 35.17; H, 3.05;
N, 8.49. Found: C, 34.90; H, 3.09; N, 8.39.
[Rh2((μ-(bis-K3)tppn)]{(μ-CO)3)2}(PF6)4 ([13](PF6)4). A solu-
tion of 100 mg of [4](PF6)2 in 5 mL of CH3CN was kept under 50
bar of CO for 4 days. Evaporation of the solvent yielded
(40) [Rh(tpa)(ethene)]B(m-xylyl)4 and [Rh(tpa)(ethene)]B(p-tolyl)4
were prepared using a method described for [Rh(tpa)(ethene)]BPh4:
de Bruin, B.; Boerakker, M. J.; Verhagen, J. A. W.; de Gelder, R.; Smits,
J. M. M.; Gal, A. W. Chem.-Eur. J. 2000, 6, 298–312.
1
[5](PF6)4 as a yellow solid. H NMR indicated the presence of

Article
Organometallics, Vol. 29, No. 7, 2010 1641
(dd, 3J(H,H) = 5.7 Hz, 4J(H,H) = 0.7 Hz, 1H; PyA-H6), 8.50 (d,
J = 5.6 Hz, 2H PyB-H6), 7.87 (dt, J = 7.8 Hz, J = 1.6 Hz, 2H),
7.63 (dt, J = 7.8 Hz, J = 1.5 Hz, 1H), 7.41 (dd, J = 8.1 Hz,
J = 6.5 Hz, 3H), 7.34(dd, J = 7.8 Hz, J = 5.8 Hz, 3H), 7.25 (d,
J = 7.9 Hz, 2H), 7.13 (s, 8H), 6.80 (d, J = 7.7 Hz; 1H), 6.52
(s, 4H), 4.51 (d[AB], J = 16.0 Hz, 2H), 3.93 (d[AB], J =
15.9 Hz, 2H), 3.85 (s, 2H), 2.09 (s, 24H). 13C NMR (CD2Cl2,
400 MHz): δ 166.2 (M-CO3), 164.4 (q, 1J(B,C) = 49 Hz, BAr-
C1), 161.4 (PyB-C2), 159.7 (PyA-C2), 150.1 (PyB-C6), 149.4
(PyB-C6), 140.7 (PyB-C4), 140.1 (PyA-C4), 134.2 (BAr-C2/3),
126.5 (PyB-C3/5), 126.1 (PyA-C3/5), 123.9 (PyB-C3/5), 123.8
(PyA-C3/5), 122.0 (BAr-C4), 69.6 (N-CH2-PyB), 67.9 (N-CH2-
PyA), 22.0 (BAr-Me). IR (KBr) (cm-1): νCdO =1690, 1661,
1631; νC-O = 1205 (C-O) (the observation of multiple CdO
bands is probably caused by different H-bonding moieties
with water). MS (ESI/acetone): 453 [M - B(m-xylyl)4]þ. Calcd
mass (C19H18N4O3Rh) 453.04339; accurate mass 453.04119
(Δ = 4.79).
Acknowledgment. We thank The Netherlands Organiza-
tion for Scientific Research-Chemical Sciences (NWO-CW
VIDI project 700.55.426), the European Research Counsel
(Grant Agreement 202886), the Radboud University of
Nijmegen, and the University of Amsterdam (UvA) for
financial support. We thank Jan Meine Ernsting for his
assistance with the low-temperature NMR experiments.
Ferry van Nisselroij and Caroline Schouten are acknow-
ledged for the synthesis of some of the described compounds.
Supporting Information Available: Crystallographic informa-
tion (cif file) containing data for complexes 2, 8, 15, and 16.
The crystal structures were also deposited with the CCDC
(deposition numbers CCDC767613-767616). DFT-optimized
geometries of compounds 9 and 13. VT-NMR spectrum of
species 14þ in dichloromethane. This material is available free
of charge via the Internet at http://pubs.acs.org.