Controlling the addition of metal centers to a bis(pyrazolyl)methane starburst ligand: Direct routes to mono-, bi-, and trimetallic rhenium(I) complexes

Article abstract of DOI:10.1021/om0508684

The bis(pyrazolyl)methane starburst ligand 1,3,5-tris[bis(1-pyrazolyl) methyl]benzene (1,3,5-[CH(pz)₂]3C₆H₃) was prepared by the cobalt-catalyzed condensation reaction between 1,3,5-triformylbenzene and thionyldipyrazole. The addition of Re(CO) ₅Br to an excess of 1,3,5-[CH(pz)₂₃C ₆H₃ resulted in the selective formation of the monometallic complex {1,3,5-[CH(pz)₂]C₆H ₃}Re(CO)₃Br (1). When 2 equiv of Re(CO)5Br was reacted with 1 equiv of 1,3,5-[CH(pz)₂₃C₆H₃, the bimetallic complex {μ1-1,3,5-{CH(pz)₂]₃C ₆H₃}[Re(CO)₃Br]₂ (2) was produced as the major product. The trimetallic complex {μ-1,3,5[CH(pz)_{2 3}C₆H₃}C₆H₃}[Re(CO) ₃Br]₃ (3) was prepared by the reaction of an excess of Re(CO)₃Br with 1,3,5[CH(pz)₂]₃C ₆H₃. Recrystallization of the complexes from acetone or acetonitrile resulted in the isolation of the crystalline compounds 1, 1.(CH₃)₂CO, 2.(CH₃)₂CO, 3-7CH ₃CN, 3.3(CH₃)₂CO, and 3.4.5(CH ₃)₂ CO. The supramolecular structures of all the complexes are influenced by the 3-fold symmetric, fixed geometry of 1,3,5-[CH(pz) ₂]₃C₆H₃, and each structure contains intricate networks of molecules, in some cases including alternating layers of complex and solvent molecules, organized through π..., CH...π, and other hydrogen-bonding interactions.

Full text of DOI:10.1021/om0508684

Organometallics 2006, 25, 743-755
743
Controlling the Addition of Metal Centers to a
Bis(pyrazolyl)methane Starburst Ligand: Direct Routes to Mono-,
Bi-, and Trimetallic Rhenium(I) Complexes
Daniel L. Reger,* Russell P. Watson, Mark D. Smith, and Perry J. Pellechia
Department of Chemistry and Biochemistry, UniVersity of South Carolina,
Columbia, South Carolina 29208
ReceiVed October 7, 2005
The bis(pyrazolyl)methane starburst ligand 1,3,5-tris[bis(1-pyrazolyl)methyl]benzene (1,3,5-[CH-
(pz)2]3C6H3) was prepared by the cobalt-catalyzed condensation reaction between 1,3,5-triformylbenzene
and thionyldipyrazole. The addition of Re(CO)5Br to an excess of 1,3,5-[CH(pz)2]3C6H3 resulted in the
selective formation of the monometallic complex {1,3,5-[CH(pz)2]C6H3}Re(CO)3Br (1). When 2 equiv
of Re(CO)5Br was reacted with 1 equiv of 1,3,5-[CH(pz)2]3C6H3, the bimetallic complex {µ-1,3,5-[CH-
(pz)2]3C6H3}[Re(CO)3Br]2 (2) was produced as the major product. The trimetallic complex {µ-1,3,5-
[CH(pz)2]3C6H3}[Re(CO)3Br]3 (3) was prepared by the reaction of an excess of Re(CO)5Br with 1,3,5-
[CH(pz)2]3C6H3. Recrystallization of the complexes from acetone or acetonitrile resulted in the isolation
of the crystalline compounds 1, 1‚(CH3)2CO, 2‚(CH3)2CO, 3‚7CH3CN, 3‚3(CH3)2CO, and 3‚4.5(CH3)2-
CO. The supramolecular structures of all the complexes are influenced by the 3-fold symmetric, fixed
geometry of 1,3,5-[CH(pz)2]3C6H3, and each structure contains intricate networks of molecules, in some
cases including alternating layers of complex and solvent molecules, organized through π‚‚‚π, CH‚‚‚π,
and other hydrogen-bonding interactions.
CH‚‚‚π interactions.4 Using flexible alkylidene spacers, for
example, we investigated the influence of counterion and spacer
size on the supramolecular structures of silver(I) complexes in
the series of ligands CH(pz)2(CH2)nCH(pz)2 (pz ) 1-pyrazolyl;
n ) 1, 2, 3).5 Semirigid tris(pyrazolyl)methane ligands, such
as C6H4[CH2OCH2C(pz)3]2 (ortho, meta, and para),6 1,3,5-
(CH3)3C6[CH2OCH2C(pz)3]3,7 and 1,2,4,5-C6H2[CH2OCH2C-
(pz)3]46a as well as the ferrocene-linked bis(pyrazolyl)methane
Introduction
The coordination chemistry of poly(pyrazolyl)borate and
-methane ligands has revealed an impressive number of
compounds with interesting structural, catalytic, and electronic
properties.1 The chemistry of poly(pyrazolyl)methane ligands
is less extensive than that of their borate analogues due to the
fact that convenient syntheses of functionalized neutral methane
species have only recently been developed.2 By functionalizing
the pyrazolyl groups in the original poly(pyrazolyl)borate and
-methane compounds, a multitude of “second-generation”
ligands have been prepared,1 and more recently, functionaliza-
tion of the borate or methane backbone has yielded a variety of
“third-generation” ligands.3 Our group has been exploring the
coordination chemistry of third-generation bis- and tris(pyra-
zolyl)methane compounds where two or more of the methane
units are linked through organic spacers of varying degrees of
flexibility, resulting in multitopic ligands. In addition to the
organization imparted by the shape and flexibility of the organic
spacer, these ligands are designed to take part in noncovalent
interactions that lead to supramolecular structures dominated
by π‚‚‚π interactions and weak hydrogen bonds, including
(3) (a) Reger, D. L.; Gardinier, J. R.; Gemmill, W. R.; Smith, M. D.;
Shahin, A. M.; Long, G. J.; Rebbouh, L.; Grandjean, F. J. Am. Chem. Soc.
2005, 127, 2303. (b) Reger, D. L.; Gardinier, J. R.; Bakbak, S.; Semeniuc,
R. F.; Bunz, U. H. F.; Smith, M. D. New J. Chem. 2005, 29, 1035. (c)
White, D.; Faller, J. W. J. Am. Chem. Soc. 1982, 104, 1548. (d) Brock, C.
P.; Das, M. K.; Minton, R. P.; Niedenzu, K. J. Am. Chem. Soc. 1988, 110,
817. (e) Janiak, C.; Braun, L.; Girgsdies, F. J. Chem. Soc., Dalton Trans.
1999, 3133. (f) Kisko, J. L.; Hascall, T.; Kimblin, C.; Parkin, G. J. Chem.
Soc., Dalton Trans. 1999, 1929. (g) Hardin, N. C.; Jeffrey, J. C.; McCleverty,
J. A.; Rees, L. H.; Ward, M. A. New J. Chem. 1998, 22, 661. (h) Niedenzu,
K.; Trofimenko, S. Inorg. Chem. 1985, 24, 4222. (i) Ja¨kle, F.; Polborn, K.;
Wagner, M. Chem. Ber. 1996, 129, 603. (j) Fabrizi de Biani, F.; Ja¨kle, F.;
Spiegler, M.; Wagner, M.; Zanello, P. Inorg. Chem. 1997, 36, 2103. (k)
Herdtweck, E.; Peters, F.; Scherer, W.; Wagner, M. Polyhedron 1998, 17,
1149. (l) Guo, S. L.; Peters, F.; Fabrizi de Biani, F.; Bats, J. W.; Herdtweck,
E.; Zanello, P.; Wagner, M. Inorg. Chem. 2001, 40, 4928. (m) Guo, S. L.;
Bats, J. W.; Bolte, M.; Wagner, M. J. Chem. Soc., Dalton Trans. 2001,
3572. (n) Bieller, S.; Zhang, F.; Bolte, M.; Bats, J. W.; Lerner, H.-W.;
Wagner, M. Organometallics 2004, 23, 2107.
(4) (a) Desiraju, G. R. Acc. Chem. Res. 2002, 35, 565. (b) Steiner, T.
Angew. Chem., Int. Ed. 2002, 41, 48. (c) Reger, D. L.; Semeniuc, R. F.;
Rassolov, V.; Smith, M. D. Inorg. Chem. 2004, 43, 537, and references
therein. (d) Rowland, R. S.; Taylor, R. J. Phys. Chem. 1996, 100, 7384.
(5) (a) Reger, D. L.; Watson, R. P.; Gardinier, J. R.; Smith, M. D. Inorg.
Chem. 2004, 43, 6609. (b) Reger, D. L.; Gardinier, J. R.; Semeniuc, R. F.;
Smith, M. D. J. Chem. Soc., Dalton Trans. 2003, 1712. (c) Reger, D. L.;
Gardinier, J. R.; Grattan, T. C.; Smith, M. R.; Smith, M. D. New J. Chem.
2003, 27, 1670.
* To whom correspondence should be addressed. E-mail: reger@
mail.chem.sc.edu.
(1) (a) Trofimenko, S. Scorpionates: The Coordination Chemistry of
Polypyrazolylborate Ligands; Imperial College: London, 1999. (b) Trofi-
menko, S. Chem. ReV. 1993, 93, 943. (c) Pettinari, C.; Santini, C. Compr.
Coord. Chem. II 2004, 1, 159. (d) Reger, D. L. Comments Inorg. Chem.
1999, 21, 1. (d) Bigmore, H. R.; Lawrence, S. C.; Mountford, P.; Tredget,
C. S. Dalton Trans. 2005, 635. (e) Long, G. J.; Grandjean, F.; Reger, D. L.
Top. Curr. Chem. 2004, 233, 91.
(2) (a) Pettinari, C.; Pettinari, R. Coord. Chem. ReV. 2005, 249, 525. (b)
Pettinari, C.; Pettinari, R. Coord. Chem. ReV. 2005, 249, 663. (c) Reger,
D. L.; Grattan, T. C.; Brown, K. J.; Little, C. A.; Lamba, J. J. S.; Rheingold,
A. L.; Sommer, R. D. J. Organomet. Chem. 2000, 607, 120. (d) Julia, S.;
del Mazo, J. M.; Avila, L.; Elguero, J. Org. Prep. Proc. Int. 1984, 16, 299.
(6) (a) Reger, D. L.; Semeniuc, R. F.; Smith, M. D. J. Organomet. Chem.
2003, 666, 87. (b) Reger, D. L.; Brown, K. J.; Smith, M. D. J. Organomet.
Chem. 2002, 658, 50. (c) Reger, D. L.; Wright, T. D.; Semeniuc, R. F.;
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10.1021/om0508684 CCC: $33.50 © 2006 American Chemical Society
Publication on Web 12/21/2005

744 Organometallics, Vol. 25, No. 3, 2006
Reger et al.
ligand Fe[C5H4CH(pz)2]2,8 have also been prepared by our group
and studied with metal systems that include cadmium,6c,7
silver,4,7,8 manganese,2c,6a and rhenium.6b Finally, we have
studied rigid (fixed), phenylene-linked bis(pyrazolyl)methane
ligands including the bitopic, tridentate heteroscorpionate
m-C6H4[C(pz)2(2-py)]2 (py ) pyridyl)9 and the bidentate series
(pyrazolyl)methane ligand had already been prepared,7 but based
on earlier studies,9 we reasoned that the utility of a rigid, tritopic
tris(pyrazolyl)methane ligand would be limited by low solubility.
We report here the preparation of the predictably more soluble
bis(pyrazolyl)methane compound 1,3,5-[CH(pz)2]3C6H3 (L), a
rigid, tritopic starburst14 ligand. The topicity and 3-fold sym-
metry of this ligand provide the potential for a supramolecular
chemistry distinct from the rigid bitopic ligands described
earlier.9,10 We have been able to add one, two, and three [Re-
(CO)3Br] units efficiently and selectively to this new ligand,
yielding the following homometallic tricarbonylrhenium(I)
compounds: the monometallic {1,3,5-[CH(pz)2]3C6H3}Re(CO)3-
Br (1), the bimetallic {µ-1,3,5-[CH(pz)2]3C6H3}[Re(CO)3Br]2
(2), and the trimetallic {µ-1,3,5-[CH(pz)2]3C6H3}[Re(CO)3Br]3
(3). The crystalline forms of these complexes include pseudopoly-
morphic species that show remarkable ordering of solvated
molecules and intricate supramolecular structures.
m- and p-C6H4[CH(pz)2]2 and p-C6H4[CH(pz4Bn)2]2 (pz4Bn
)
4-benzyl-1-pyrazolyl).10
One goal of our studies is to find efficient routes to
heterometallic complexes with homotopic poly(pyrazolyl)-
methane ligands. Such complexes offer the potential for unique
properties compared with their homometallic analogues.11 It is
often the case that incorporation of metal fragments into
multitopic ligands cannot be controlled to the extent that a single
species is produced in which free ligating sites are available
for coordination to a second metal center in a subsequent
reaction.12 For example, with homobitopic ligands, it is fre-
quently difficult to prepare a monometallic intermediate [Ligand]-
[M1] for use in a subsequent reaction with a different metal
system; in many cases, even in reactions that limit the amount
of the metal precursor, the bimetallic [Ligand][M1]2 complex
is produced. It is often too tedious or impossible to obtain a
practical yield of the desired monometallic complex. For this
reason, the construction of heterometallic complexes from
homotopic ligands is frequently impractical or unfeasible, and
alternative methods are employed.13
We have recently reported that by using specific reaction
conditions the addition of only one metal center to the rigid
homobitopic, phenylene-linked bis(pyrazolyl)methane ligand
m-C6H4[CH(pz)2]2 is indeed successful.10 The monometallic
complex {m-[CH(pz)2]2C6H4}Re(CO)3Br can be made in good
yield, and we have demonstrated its usefulness in producing
heterobimetallic complexes by incorporating a platinum(II)
center, yielding {µ-m-[CH(pz)2]2C6H4}[Re(CO)3Br][Pt(p-tolyl)2].
This heterometallic complex is the first we have been able to
isolate with the symmetrical multitopic ligands we have studied.
All previous attempts with other ligands yielded only homo-
metallic complexes.
Experimental Section
General Considerations. Air-sensitive materials were handled
under a nitrogen atmosphere using standard Schlenk techniques or
in a Vacuum Atmospheres HE-493 drybox. All solvents were dried
by conventional methods prior to use. The compounds triethyl 1,3,5-
benzenetricarboxylate,15 1,3,5-tris(hydroxymethyl)benzene,16 1,3,5-
triformylbenzene,17 and Re(CO)5Br18 were prepared following
reported procedures. 1,3,5-Triformylbenzene was also prepared as
described below using o-iodoxybenzoic acid (IBX).19 All other
chemicals were purchased from Aldrich or Fisher Scientific.
Pyrazole was purified by sublimation prior to use. Thionyl chloride
was distilled prior to use. All other purchased chemicals were used
as received. Reported melting points are uncorrected. IR spectra
1
were obtained on a Nicolet 5DXBO FTIR spectrometer. H and
13C NMR spectra were recorded on a Mercury/VX 300 or INOVA
500 spectrometer. All chemical shifts are in ppm and are secondary-
referenced using the signals from residual undeuterated solvents.
Mass spectrometric measurements were obtained on a MicroMass
QTOF spectrometer or on a VG 70S instrument. Elemental analyses
were performed on vacuum-dried samples by Robertson Microlit
Laboratories (Madison, NJ).
The successful isolation of {m-[CH(pz)2]2C6H4}Re(CO)3Br
prompted us to investigate the series of multimetallic complexes
that could potentially be made using a symmetric, tritopic ligand
linked by a rigid aromatic group. A similar semirigid tris-
1,3,5-Triformylbenzene. o-Iodoxybenzoic acid (7.49 g, 26.7
mmol) was dissolved in 30 mL of dimethyl sulfoxide (DMSO) at
room temperature with vigorous stirring over several minutes. To
this solution was added a 15 mL DMSO solution of 1,3,5-tris-
(hydroxymethyl)benzene (1.00 g, 5.95 mmol). The resulting solution
was stirred at room temperature overnight, during which time the
solution became cloudy. Saturated aqueous Na2S2O3 (30 mL) was
added to the system followed by 30 mL of saturated aqueous
NaHCO3. The resulting solution was extracted with CH2Cl2 (3 ×
30 mL), and the combined organic extracts were washed with water
(20 mL) and saturated aqueous NaCl (30 mL). After drying over
MgSO4, the solvent was removed by rotary evaporation, yielding
(7) Reger, D. L.; Semeniuc, R. F.; Smith, M. D. Inorg. Chem. 2003, 42,
8137.
(8) Reger, D. L.; Brown, K. J.; Gardinier, J. R.; Smith, M. D.
Organometallics 2003, 22, 4973.
(9) Reger, D. L.; Gardinier, J. R.; Smith, M. D. Polyhedron 2004, 23,
291.
(10) Reger, D. L.; Watson, R. P.; Smith, M. D.; Pellechia, P. J.
Organometallics 2005, 24, 1544.
(11) (a) Ceccon, A.; Santi, S.; Orian, L.; Bisello, A. Coord. Chem. ReV.
2004, 248, 683. (b) Fulton, J. R.; Hanna, T. A.; Bergman, R. G.
Organometallics 2000, 19, 602. (c) Fukuoka, A.; Fukagawa, S.; Hirano,
M.; Koga, N.; Komiya, S. Organometallics 2001, 20, 2065. (d) Fabre, S.;
Findeis, B.; Tro¨sch, D. J. M.; Gade, L. H.; Scowen, I. J.; McPartlin, M.
Chem. Commun. 1999, 577. (e) Schubart, M.; Mitchell, G.; Gade, L. H.;
Kottke, T.; Scowen, I. J.; McPartlin, M. Chem. Commun. 1999, 233. (f)
Scheider, A.; Gade, L. H.; Breuning, M.; Bringman, G.; Scowen, I. J.;
McPartlin, M. Organometallics 1998, 17, 1642. (g) Catalysis by Di- and
Polynuclear Metal Cluster Complexes; Adams, R. D., Cotton, F. A., Eds.;
Wiley-VCH: New York, 1998, and references therein.
(12) (a) Balzani, V.; Juris, A.; Venturi, M.; Campagna, S.; Serroni, S.
Chem. ReV. 1996, 96, 759. (b) Chong, S. H.-F.; Lam, S. C.-F.; Yam, V.
W.-W.; Zhu, N.; Cheung, K.-K.; Fathallah, S.; Costuas, K.; Halet, J.-F.
Organometallics 2004, 23, 4924. (c) Masllorens, J.; Roglans, A.; Moreno-
Man˜as, M.; Parella, T. Organometallics 2004, 23, 2533.
(14) For examples of starburst ligands and their applications, see: (a)
Seward, C.; Wang, S. Comments Inorg. Chem. 2005, 26, 103. (b) Seward,
C.; Jia, W.-L.; Wang, R.-Y.; Wang, S. Inorg. Chem. 2004, 43, 978. (c)
Sumby, C. J.; Steel, P. J. Organometallics 2003, 22, 2358. (d) Wu, Q.;
Hook, A.; Wang, S. Angew. Chem., Int. Ed. 2000, 39, 3933.
(15) Nielsen, A. T.; Christian, S. L.; Moore, D. W.; Gilardi, R. D.;
George, C. F. J. Org. Chem. 1987, 52, 1656.
(16) Cochrane, W. P.; Pauson, P. L.; Stevens, T. S. J. Chem. Soc. C
1968, 630.
(17) Fourmigue´, M. Johannsen, I.; Boubekeur, K.; Nelson, C.; Batail,
P. J. Am. Chem. Soc. 1993, 115, 3752.
(18) Schmidt, S. P.; Trogler, W. C.; Basolo, F. Inorg. Synth. 1990, 28,
160.
(13) For example: (a) Harriman, A.; Hissler, M.; Khatyr, A.; Ziessel,
R. Eur. J. Inorg. Chem. 2003, 955. (b) Bo¨rje, A.; Ko¨the, O.; Juris, A. J.
Chem. Soc., Dalton Trans. 2002, 843. (c) Lam, S. C.-F.; Yam, V. W.-W.;
Wong, K. M.-C.; Cheng, E. C.-C.; Zhu, N. Organometallics 2005, 24, 4298.
(19) (a) Frigerio, M.; Santagostino, M.; Sputore, S. J. Org. Chem. 1999,
64, 4537. (b) Frigerio, M.; Santagostino, M.; Sputore, S.; Palmisano, G. J.
Org. Chem. 1995, 60, 7272. (c) Nicolaou, K. C.; Mathison, C. J. N.;
Montagnon, T. J. Am. Chem. Soc. 2004, 126, 5192.

Mono-, Bi-, and Trimetallic Re(I) Complexes
Organometallics, Vol. 25, No. 3, 2006 745
0.78 g (81%) of the desired aldehyde. This aldehyde could be used
without further purification, but flushing through a plug of silica
gel using CH2Cl2 as eluent yielded analytically pure product. The
aldehyde obtained by this method melted at 154-159 °C (lit.17
155-159 °C), and its spectral characterization matched that of
previous reports.16,17
white precipitate formed during the addition. After the addition was
complete, the system was stirred at reflux for an additional 5 h
and then allowed to cool to room temperature and stir overnight.
The white precipitate was isolated by filtration, washed with 5 mL
of Et2O, and dried in vacuo to yield 1.05 g of a solid consisting
mainly of 2 contaminated with 1 and 3. Layering Et2O onto an
acetone solution of 0.36 g of the crude product yielded 0.27 g of
X-ray quality crystals of analytically pure 2‚(CH3)2CO. X-ray
quality crystals could also be grown by the vapor diffusion of Et2O
into acetone solutions of the crude solid. Mp: 290 °C (dec). Anal.
Calcd for C36H30Br2N12O7Re2: C, 33.92; H, 2.37; N, 13.18.
Found: C, 33.87; H, 2.23; N, 13.34. IR, νCO (KBr, cm-1): 2026,
1,3,5-Tris[bis(1-pyrazolyl)methyl]benzene, 1,3,5-[CH(pz)2]3C6H3
(L). Sodium hydride (1.78 g, 74.2 mmol) was suspended in 250
mL of THF and cooled in an ice-water bath. Pyrazole (5.04 g,
74.0 mmol) was added, and the resulting solution was allowed to
stir at 0 °C for 30 min, after which time, thionyl chloride (2.7 mL,
37.0 mmol) was added dropwise. Once the addition was complete,
the ice-water bath was removed, and the resulting suspension was
allowed to warm to room temperature with stirring over 40 min.
1,3,5-Triformylbenzene (1.00 g, 6.17 mmol) and CoCl2 (0.24 g,
1.85 mmol) were then added at once, and the system was heated at
reflux for 20 h. After cooling to room temperature, water (150 mL)
was added, and the resulting solution was stirred at room temper-
ature for 1 h. Methylene chloride (100 mL) was added, the layers
were separated, and the aqueous phase was extracted with CH2Cl2
(2 × 50 mL). The combined organic extracts were washed with
water (150 mL) and dried over MgSO4. Removal of the solvent
left an off-white solid that was recrystallized from approximately
150 mL of hot absolute ethanol to yield 2.14 g (67%) of a white
solid. Mp: 209-210 °C. Anal. Calcd for C27H24N12: C, 62.78; H,
4.68; N, 32.54. Found: C, 62.48; H, 4.40; N, 32.32. IR (KBr, cm-1):
3117, 2954, 1516, 1454, 1430, 1388, 1311, 1238, 1197, 1037, 789,
750, 694. 1H NMR (300 MHz, acetone-d6): δ 7.90 (s, 3 H,
CH(pz)2), 7.71 (dd, J ) 2.4, 0.6 Hz, 6 H, 5-H pz), 7.48 (dd, J )
2.0, 0.6 Hz, 6 H, 3-H pz), 6.78 (s, 3 H, C6H3), 6.27 (dd, J ) 2.4,
1.8 Hz, 6 H, 4-H pz). 13C NMR (75.5 MHz, acetone-d6): δ 141.1,
139.3, 130.9, 127.4, 107.2, 77.5. MS ESI(+) m/z (rel int %) [assgn]:
539 (25) [M + Na]+, 517 (50) [M + H]+, 449 (100) [M - pz]+,
381 (10) [M - pz -Hpz]+.
1
1899. H NMR (300 MHz, acetone-d6): δ 8.53 (s, 2 H, CH(pz)2-
[Re]), 8.30 (dd, J ) 0.6, 2.7 Hz, 4 H, 5-H pz[Re]), 8.08 (d, J )
2.7 Hz, 4 H, 3-H pz[Re]), 7.81 (s, 1 H, CH(pz)2), 7.68 (d, J ) 2.4
Hz, 2 H, 5-H pz), 7.49 (d, J ) 2.1 Hz, 2 H, 3-H pz), 6.68 (t, J )
2.4, 4 H, 4-pz[Re]), 6.29 (dd, J ) 1.7, 2.6 Hz, 2 H, 4-H pz), 5.99
(s, 2 H, 4,6-H C6H3), 5.11 (s, 1 H, 2-H C6H3). MS ESI(+) m/z (rel
int %) [assgn]: 1239 (20) [M + Na]+, 1217 (100) [M + H]+, 1137
(75) [M - Br]+, 867 (35) [M + H - Re(CO)3Br]+, 787 (10)
[M - Re(CO)3Br - Br]+. HRMS: Direct probe (m/z) calcd for
C33H24Br2N12O6Re2, 1216.9464; found 1216.9423.
{µ-1,3,5-[CH(pz)2]3C6H3}[Re(CO)3Br]3 (3). Re(CO)5Br (0.790
g, 1.95 mmol) and 1,3,5-[CH(pz)2]3C6H3 (0.250 g, 0.484 mmol)
were dissolved together in acetone and heated at reflux for 3 days.
After removal of the solvent, the crude residue was washed with a
1:1 solution of acetone-hexanes, leaving 0.20 g (26%) of pure 3.
A small amount of impure 3 was also detected in the filtrate, but
further purification was not attempted. Mp: 280 °C dec. Anal. Calcd
for C36H24Br3N12O9Re3: C, 27.59; H, 1.54; N, 10.73. Found: C,
1
27.69; H, 1.70; N, 10.30. IR, νCO (KBr, cm-1): 2028, 1883. H
NMR (300 MHz, acetone-d6): δ 8.55 (s, 3 H, CH(pz)2), 8.26 (d,
J ) 2.7 Hz, 6 H, 5-H pz), 8.11 (d, J ) 2.7 Hz, 6 H, 3-H pz), 6.74
(t, J ) 2.4 Hz, 6 H, 4-H pz), 5.83 (s, 3 H, C6H3). MS ESI(+) m/z
(rel int %) [assgn]: 1607 (70) [M + K]+, 1487 (100) [M - Br]+.
Crystals suitable for X-ray analysis were grown by the vapor
diffusion of Et2O into 1 mL acetone or acetonitrile solutions of the
solid.
{1,3,5-[CH(pz)2]3C6H3}Re(CO)3Br (1). A suspension of Re-
(CO)5Br (0.410 g, 1.01 mmol) in 30 mL of toluene was heated
with stirring to ca. 70 °C until the solid was completely dissolved.
The hot solution, with the temperature being maintained at 70 °C,
was then added by cannula over 8 min to a refluxing 100 mL
toluene solution of an excess of 1,3,5-[CH(pz)2]3C6H3 (1.30 g, 2.52
mmol). A white precipitate was soon noticeable. After the addition
was complete, the system was stirred at reflux for an additional 5
h and then allowed to cool to room temperature and stir overnight.
The white precipitate was isolated by filtration and washed
repeatedly with hot toluene until NMR spectroscopy indicated that
all the excess ligand had been removed. The yield was 0.45 g (52%).
Mp: 260-262 °C (dec). Anal. Calcd for C30H24BrN12O3Re: C,
41.57; H, 2.79; N, 19.39. Found: C, 41.25; H, 2.70; N, 18.87. IR,
Crystal Structure Determinations. X-ray intensity data from
a colorless bar of 1‚(CH3)2CO, from colorless plates of 1 and 3‚4.5-
(CH3)2CO, and from colorless prisms of 2‚(CH3)2CO, 3‚7CH3CN,
and 3‚3(CH3)2CO were measured at 150(1) K on a Bruker SMART
APEX CCD-based diffractometer (Mo KR radiation, λ ) 0.71073
Å).20 Raw data frame integration and Lp corrections were performed
with SAINT+.20 Final unit cell parameters were determined by
least-squares refinement of 7707, 8335, 8850, 9965, 8198, and 8621
reflections from the data sets of 1, 1‚(CH3)2CO, 2‚(CH3)2CO,
3‚7CH3CN, 3‚3(CH3)2CO, and 3‚4.5(CH3)2CO, respectively, each
with I > 5σ(I). Analysis of the data showed negligible crystal decay
during data collection. All data sets were corrected for absorption
effects with SADABS.20 Direct methods structure solution, differ-
ence Fourier calculations, and full-matrix least-squares refinement
against F2 were performed with SHELXTL.21 All non-hydrogen
atoms were refined with anisotropic displacement parameters unless
otherwise noted. Hydrogen atoms were placed in geometrically
idealized positions and included as riding atoms. Details of the data
collections are given in Table 1, while further notes regarding the
solution and refinement for all six structures follow.
1
νCO (KBr, cm-1): 2027, 1900. H NMR (300 MHz, acetone-d6):
δ 8.50 (s, 1 H, CH(pz)2[Re]), 8.33 (dd, J ) 0.9, 2.7 Hz, 2 H, 5-H
pz[Re]), 8.05 (d, J ) 2.1 Hz, 2 H, 3-H pz[Re]), 7.84 (s, 2 H,
CH(pz)2), 7.68 (d, J ) 2.4 Hz, 4 H, 5-H pz), 7.48 (d, J ) 1.8 Hz,
4 H, 3-H pz), 6.75 (br s, 1 H, 4-H C6H3), 6.63 (t, J ) 2.6 Hz, 2 H,
4-pz[Re]), 6.28 (dd, J ) 1.7, 2.6 Hz, 4 H, 4-H pz), 5.78 (s, 2 H,
2,6-H C6H3). MS ESI(+) m/z (rel int %) [assgn]: 889 (15) [M +
Na]+, 867 (100) [M + H]+, 787 (30) [M - Br]+, 719 (10) [M -
Br - Hpz]+, 517 (5) [M + H - Re(CO)3Br]+, 449 (10) [M -
Re(CO)3Br - pz]+. HRMS: Direct probe (m/z) calcd for C30H24-
BrN12O3Re, 865.0886; found 865.0886. Crystals suitable for X-ray
analysis were grown by the vapor diffusion of Et2O into 1 mL
acetone solutions of the solid.
Unsolvated 1 crystallizes in the space group P21/n as determined
uniquely by the pattern of systematic absences in the intensity data.
The asymmetric unit consists of one complete molecule.
The compound 1‚(CH3)2CO crystallizes in the space group P21/n
as determined uniquely by the pattern of systematic absences in
{µ-1,3,5-[CH(pz)2]3C6H3}[Re(CO)3Br]2 (2). A suspension of
Re(CO)5Br (0.79 g, 1.9 mmol) in 50 mL of toluene was heated
with stirring to ca. 80 °C until the solid was completely dissolved.
The hot solution, with the temperature being maintained at 80 °C,
was then added by cannula over 15 min to a refluxing 75 mL
toluene solution of 1,3,5-[CH(pz)2]3C6H3 (0.50 g, 0.97 mmol). A
(20) SMART Version 5.625, SAINT+ Version 6.22, and SADABS Version
2.05; Bruker Analytical X-ray Systems, Inc.: Madison, WI, 2001.
(21) Sheldrick, G. M. SHELXTL Version 6.1; Bruker Analytical X-ray
Systems, Inc.: Madison, WI, 2000.

746 Organometallics, Vol. 25, No. 3, 2006
Table 1. Crystal Data and Refinement Details for {1,3,5-[CH(pz)2]3C6H3}Re(CO)3Br (1),
Reger et al.
{1,3,5-[CH(pz)2]3C6H3}Re(CO)3Br‚(CH3)2CO (1‚(CH3)2CO), {µ-1,3,5-[CH(pz)2]3C6H3}[Re(CO)3Br]2‚(CH3)2CO (2‚(CH3)2CO),
{µ-1,3,5-[CH(pz)2]3C6H3}[Re(CO)3Br]3‚7CH3CN (3‚7CH3CN), {µ-1,3,5-[CH(pz)2]3C6H3}[Re(CO)3Br]3‚3(CH3)2CO (3‚3(CH3)2CO),
and {µ-1,3,5-[CH(pz)2]3C6H3}[Re(CO)3Br]3‚4.5(CH3)2CO (3‚4.5(CH3)2CO)
1
1‚(CH3)2CO
2‚(CH3)2CO
C36H30Br2-
N12O7Re2
1274.94
3‚7CH3CN
3‚3(CH3)2CO
C45H42Br3-
N12O12Re3
1741.24
3‚4.5(CH3)2CO
empirical formula
C30H24Br-
N12O3Re
866.72
C33H30BrN12-
O4Re
924.80
C50H45Br3N19-
O9Re3
1854.38
150(1)
C
49.50H51Br3-
N12O13.50Re3
1828.35
150(1)
fw
T (K)
150(1)
150(1)
150(1)
150(1)
cryst syst
space group
a, Å
b, Å
monoclinic
P21/n
15.0739(9)
11.7336(7)
18.0498(11)
90
92.7780(10)
90
3188.7(3)
4
monoclinic
P21/n
22.5279(12)
12.8446(7)
25.4675(14)
90
103.5980(10)
90
7162.8(7)
8
triclinic
P1h
orthorhombic
P212121
14.1204(8)
18.5671(11)
24.4197(14)
90
90
90
6402.2(6)
4
1.924
monoclinic
P21/c
10.6215(5)
14.1987(7)
36.1523(17)
90
92.7560(10)
90
5445.9(5)
4
triclinic
P1h
8.8952(5)
14.7907(8)
16.1834(9)
89.6620(10)
86.3940(10)
73.9530(10)
2042.0(2)
2
13.2164(8)
14.1142(9)
19.1630(12)
95.9450(10)
96.2050(10)
117.6460(10)
3099.6(3)
2
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
D(calc), Mg‚m-3
abs coeff, mm-1
cryst size, mm3
final R indices [I > 2σ(I)]
R1
1.805
5.117
1.715
4.564
2.074
7.943
2.124
8.922
1.959
7.844
7.595
0.44 × 0.26 × 0.04 0.40 × 0.20 × 0.16 0.24 × 0.20 × 0.12 0.48 × 0.34 × 0.30 0.26 × 0.20 × 0.12 0.40 × 0.30 × 0.12
0.0253
0.0625
0.0243
0.0562
0.0410
0.0991
0.0234
0.0551
0.0382
0.0858
0.0389
0.0996
wR2
Scheme 1
the intensity data. The asymmetric unit consists of two crystallo-
graphically independent rhenium complexes and two crystallo-
graphically independent acetone molecules of crystallization. Atoms
in the rhenium complexes were numbered identically except for
the suffix “A” or “B.”
The compound 3‚4.5(CH3)2CO crystallizes in the triclinic system.
The space group P1h was confirmed by the successful solution and
refinement of the data. There are one Re complex and five
independent acetone molecules of crystallization present in the
asymmetric unit. One of the acetones (O85) is disordered about an
inversion center and was refined as half-occupied. A common
isotropic displacement parameter was used for the C and O atoms
of this molecule. Acetone O84 was also refined with isotropic
displacement parameters. Nine geometric restraints were used to
maintain reasonable geometries for these two acetone molecules.
All other non-hydrogen atoms were refined with anisotropic
displacement parameters.
The compound 2‚(CH3)2CO crystallizes in the triclinic system.
The space group P1h was assumed and confirmed by the successful
solution and refinement of the structure. The asymmetric unit
consists of one dirhenium complex and one acetone molecule of
crystallization. The acetone molecule was refined using a common
isotropic displacement parameter. This molecule likely suffers from
minor positional disorder; three distance restraints were used to
maintain a reasonable chemical geometry for this species.
The compound 3‚7CH3CN crystallizes in the orthorhombic space
group P212121 as determined uniquely by the pattern of systematic
absences in the intensity data. There is one trirhenium complex
and seven crystallographically independent, well-ordered CH3CN
molecules of crystallization present in the asymmetric unit. The
final absolute structure (Flack) parameter refined to -0.011(5),
indicating the correct absolute structure and the absence of inversion
twinning.
Results and Discussion
Syntheses and Characterization. The bis(pyrazolyl)methane
starburst ligand 1,3,5-[CH(pz)2]3C6H3 (L) was prepared in four
steps from trimesic acid (1,3,5-benzenetricarboxylic acid), as
shown in Scheme 1. Fischer esterification of the acid in absolute
ethanol yielded the triethyl ester,15 which was then reduced to
the triol 1,3,5-tris(hydroxymethyl)benzene.16 At room temper-
ature, this triol was soluble only in water and DMSO, among
the common solvents, and its oxidation to 1,3,5-triformylbenzene
while co-suspended with pyridinium chlorochromate (PCC) in
CH2Cl217 proceeded in modest yield. We found, however, that
higher yields resulted from using the DMSO-soluble oxidant
o-iodoxybenzoic acid (IBX).19 This route was preferred not only
The compound 3‚3(CH3)2CO crystallizes in the space group P21/c
as determined by the pattern of systematic absences in the intensity
data. One Re complex and three independent acetone molecules of
crystallization are present in the asymmetric unit. All non-hydrogen
atoms were refined with anisotropic displacement parameters except
one solvent carbon (C87, isotropic).

Mono-, Bi-, and Trimetallic Re(I) Complexes
Organometallics, Vol. 25, No. 3, 2006 747
Scheme 2
because of the higher yields but also because it included a
workup less tedious and hazardous than that for the chromium-
(VI) reagent PCC (see Experimental Section). Once the aldehyde
was obtained, Peterson rearrangement22 involving the aldehyde
and thionyldipyrazole yielded L as a white solid freely soluble
in halogenated solvents and acetone but with limited solubility
in diethyl ether and acetonitrile.
of the crude mixture failed, but washing with a minimum
amount of a 1:1 acetone-hexanes solution afforded the pure
trimetallic complex, albeit in significantly reduced yield due to
loss of some of the desired product in the washing step.
Coordination of L to [Re(CO)3Br] can result in stereoisomers
that differ in the relative orientation of the methine hydrogen
and bromine ligand. NMR and IR spectra, however, indicated
that of the two, three, and four possible stereoisomers of the
mono-, bi-, and trimetallic complexes, respectively, only one
was formed for each complex. The carbonyl stretching frequen-
cies in the infrared indicate local C3V symmetry about rhenium
The monometallic complex {1,3,5-[CH(pz)2]3C6H3}Re(CO)3-
Br (1) was prepared by the dropwise addition of a hot toluene
solution of Re(CO)5Br to a refluxing solution of an excess of
L (Scheme 2). The precipitated complex could be obtained
analytically pure by washing away the leftover L with hot
toluene. Similarly, the gradual addition of 2 equiv of Re(CO)5Br
in hot toluene to a refluxing toluene solution of 1 equiv of L
resulted in the formation of the bimetallic complex {µ-1,3,5-
[CH(pz)2]3C6H3}[Re(CO)3Br]2 (2) as the major product (Scheme
2) along with the monometallic (1) and trimetallic (3) complexes
1
in each complex. The H NMR spectra of 1 and 2 are similar
in that for each compound there are two sets of bis(pyrazolyl)-
methane signals reflecting the two distinct chemical environ-
ments of the coordinated (bound) and uncoordinated (free) sites.
Both complexes possess a plane of symmetry in solution
rendering the two free sites of 1 chemically identical and the
two bound sites of 2 identical. In 1, therefore, the ratio of
integrations of the free to bound signals is 2:1, and in 2 the
same ratio is 1:2. Table 2 compares the chemical shifts of the
bis(pyrazolyl)methane units, uncoordinated and coordinated, for
the ligand and all three complexes. The respective free sites in
both 1 and 2 resonate at approximately the same frequencies
as those in the free ligand. The respective coordinated sites in
all three complexes also resonate at roughly the same frequencies
and are all shifted downfield relative to the free sites.
The three protons on the central arene rings of 1 and 2 are
also in two different environments. In 1 there is one proton
positioned between two free sites, and it resonates at the same
frequency as the arene protons in the free ligand: 6.75 ppm in
1 vs 6.78 ppm in L. The other two arene protons in 1 are
between both a free site and the coordinated one. These protons
1
as minor products. Judging from H NMR spectroscopy, the
amounts of 1 and 3 produced as side products varied, but the
combined yield of the two impurities could be as high as 33%
of the crude product. Recrystallization of this mixture from
acetone-Et2O yielded pure samples of the solvated form
2‚(CH3)2CO (see Solid State Structures). The trirhenium com-
plex of L, {µ-1,3,5-[CH(pz)2]3C6H3}[Re(CO)5Br]3 (3), was best
prepared in acetone according to Scheme 2. Heating an acetone
mixture of L and a slight excess of Re(CO)5Br at reflux for 3
days produced 3 in high yield but contaminated with traces of
the mono- and dirhenium species. Chromatographic separation
(22) (a) The´, K. I.; Peterson, L. K. Can. J. Chem. 1973, 51, 422. (b)
The´, K. I.; Peterson, L. K.; Kiehlman, E. Can. J. Chem. 1973, 51, 2448.
(c) Peterson, L. K.; Kiehlman, E.; Sanger, A. R.; The´, K. I. Can. J. Chem.
1974, 52, 2367.

748 Organometallics, Vol. 25, No. 3, 2006
Reger et al.
1
Figure 1. Mirror symmetry of {µ-1,3,5-[CH(pz)2]3C6H3}[Re(CO)5Br]3 (3) in solution (left, [Re] ) [Re(CO)3Br]); H NMR spectrum of
3 (500 MHz, acetone-d6) at 25 °C (top right) and -90 °C (bottom right).
Table 2. Comparison of the Chemical Shifts of Protons
Associated with Uncoordinated and Coordinated Ligating
Sites in the Ligand (L) and in the Mono- (1), Di- (2), and
Trirhenium (3) Complexes of La
the central arene ring with the third on the opposite side (vide
infra). The 1H NMR spectrum of 3 at 25 °C shows a single set
of resonances for the complexed ligand that indicates chemical
equivalence of all three -CH(pz)2[Re(CO)3Br] units as well as
of the three protons on the central arene ring (Figure 1). At
-90 °C, however, the spectrum shows the complexity expected
of 3 if it were to adopt a “two up-one down” conformation as
observed in its crystal structure. The rapid equilibration on the
NMR time scale that occurs at 25 °C begins to slow appreciably
near -10 °C, as indicated by a noticeable broadening of the
signals. Using line shape analysis to determine the rate of
exchange at varying temperatures, an Arrhenius plot was
generated (see Supporting Information), from which the free
energy of rotation of the -CH(pz)2[Re(CO3Br] groups about
the central arene ring was calculated to be 31 kJ/mol.
Uncoordinated Sites
methine
5-pz
3-pz
4-pz
L
1
2
3
7.90
7.84
7.81
7.71
7.68
7.68
7.48
7.48
7.49
6.27
6.28
6.29
Coordinated Sites
5-pz
methine
3-pz
4-pz
L
1
2
3
8.50
8.53
8.55
8.33
8.30
8.26
8.05
8.08
8.11
6.63
6.68
6.74
Solid-State Structures. When {1,3,5-[CH(pz)2]3C6H3}Re-
(CO)3Br (1) is recrystallized from acetone-diethyl ether, two
pseudopolymorphic species are isolated from the same mother
liquor. The more abundant crystals are colorless plates of the
unsolvated complex 1. Less abundant are colorless needles of
the solvated complex {1,3,5-[CH(pz)2]3C6H3}Re(CO)3Br‚(CH3)2-
CO (1‚(CH3)2CO). Both species crystallize in the monoclinic
space group P21/n. Figure 2 shows ORTEP representations of
the metal complexes in each form. The solvated species contains
two crystallographically independent complex molecules, labeled
here as 1a‚(CH3)2CO and 1b‚(CH3)2CO. Selected bond distances
and angles for each species are given in Table 3. The most
salient difference between the two crystalline species is the
positions of the pyrazolyl rings of the free bis(pyrazolyl)methane
units in reference to the plane defined by the central arene ring.
In both forms the coordinated bis(pyrazolyl)methane unit lies
almost entirely on one side of the ring. In the nonsolvated
crystal, for each of the free bis(pyrazolyl)methane units one of
the pyrazolyl rings is on one side of the arene plane and the
other is on the opposite side (Figure 3, top). The solvated form,
however, adopts a conformation in which all four of the
pyrazolyl rings of the free bis(pyrazolyl)methane units are
completely or almost completely on the same side of the central
ring as the coordinated bis(pyrazolyl)methane unit (Figure 3,
bottom). This conformation is not observed in any of the bi- or
trimetallic species discussed below and has only been observed
a Resonances are in δ units and were recorded at 300 MHz in acetone-
d6 at room temperature.
are shifted upfield of those in the free ligand to 5.78 ppm. In 2
there are also two arene protons between both free and bound
sites, and they resonate together at 5.99 ppm. The proton
positioned between two bound sites resonates at 5.11 ppm. The
three equivalent arene protons in 3, which resonate at 5.83 ppm,
are each between a coordinated site. Unlike the consistent
downfield shift of the coordinated pyrazolyl and methine proton
signals observed for each complex, there appears to be no trend
in the chemical shift of the arene protons that are between a
free and a bound site (as in 1 and 2) or between two bound
sites (as in 2 and 3) other than the fact that the signals of all
these protons are shifted upfield of the arene protons in L. This
upfield shift suggests that the π clouds associated with
coordinated pyrazolyl groups provide stronger shielding envi-
ronments that do the π clouds of uncoordinated pyrazolyl
groups. Analyses of the solid-state structures described below
support the conclusion that the pyrazolyl rings are responsible
for the relative shielding of the arene protons in each complex.
The solution dynamics of the trimetallic complex 3 were
further studied using information garnered from its solid-state
structure. The geometry observed in crystalline 3 consists of
two of the -CH(pz)2[Re(CO)3Br] units oriented on one side of

Mono-, Bi-, and Trimetallic Re(I) Complexes
Organometallics, Vol. 25, No. 3, 2006 749
Figure 2. ORTEP drawings of the monometallic molecules of {1,3,5-[CH(pz)2]3C6H3}Re(CO)3Br (1) and {1,3,5-[CH(pz)2]3C6H3}Re(CO)3-
Br‚(CH3)2CO (1‚(CH3)2CO). The two crystallographically independent molecules of 1‚(CH3)2CO are labeled 1a‚(CH3)2CO and 1b‚(CH3)2-
CO. Ellipsoids are drawn at the 50% probability level.
Table 3. Selected Bond Distances (Å) and Angles (deg) for 1
side of the linking arene group appears to be stable. A driving
force for the arrangement may be noncovalent interactions
involving the exposed face of the central ring, as discussed
below. Even in the unsolvated crystal, the presence of two of
the free pyrazolyl rings on the same side as the coordinated
site suggests minimal steric conflict among the noncoordinated
bis(pyrazolyl)methane sites.
and 1‚(CH3)2COa
1
1a‚(CH3)2CO
1b‚(CH3)2CO
Re(1)-N(11)
Re(1)-N(21)
Re(1)-C(71)
Re(1)-C(72)
Re(1)-C(73)
Re(1)-Br(1)
2.175(3)
2.178(2)
1.915(4)
1.914(3)
1.901(3)
2.6440(3)
2.181(2)
2.176(2)
1.907(3)
1.909(3)
1.909(3)
2.6271(3)
2.181(2)
2.173(2)
1.905(3)
1.914(3)
1.915(3)
2.6183(3)
As observed in earlier studies of phenylene-linked bis-
(pyrazolyl)methane systems,10 the [Re(CO)3Br] unit of both 1
and 1‚(CH3)2CO is coordinated such that the attached bromine
atom is cis to the methine hydrogen atom in the boat conforma-
tion of the six-membered metallocycle formed by coordination
to rhenium. This stereoisomer is the only one formed for 1, as
N(11)-Re(1)-N(21)
N(11)-Re(1)-Br(1)
N(21)-Re(1)-Br(1)
C(71)-Re(1)-Br(1)
C(71)-Re(1)-N(11)
C(71)-Re(1)-C(72)
83.96(9)
86.54(6)
83.93(7)
90.60(9)
176.26(10)
91.17(13)
84.72(8)
83.49(6)
82.86(6)
91.36(8)
174.27(10)
89.67(12)
85.86(8)
81.27(6)
83.64(6)
92.85(10)
175.10(11)
88.58(13)
1
indicated by H NMR and infrared spectra of the bulk sample
a The two crystallographically independent molecules of 1‚(CH3)2CO
are denoted as 1a‚(CH3)2CO and 1b‚(CH3)2CO.
produced in the reaction. We have previously offered an
explanation for this isomeric purity based on transition state
theory involving a hydrogen-bromine interaction as well as π
bonding between the carbonyl ligands and the central arene
ring.10,23 It is reasonable to assume that such a mechanism is at
work in the formation of 1 as well. Also consistent with the
earlier dirhenium complexes is the fact that the [Re(CO)3Br]
unit is oriented in space such that a carbonyl ligand is positioned
over the central arene ring and is participating in intramolecular
πCO‚‚‚πAr interactions. Details of these interactions, observed
for all the complexes in this work, are presented in Table 4.
Both the centroid‚‚‚centroid distances and the centroid‚‚‚ring-
plane perpendicular distances are given. As expected, each
centroid‚‚‚centroid distance is longer than the corresponding
perpendicular distance from the arene ring plane to the centroid
of the carbonyl group because none of the carbonyl groups lie
directly over the center of the arene ring. The perpendicular
distance for each carbonyl group is within accepted values for
π‚‚‚π interactions.24
The supramolecular structure of unsolvated 1 is dominated
by multiple hydrogen bonds and one strong πpz‚‚‚πpz stacking
interaction that combined arrange the molecules into one-
dimensional chains of dimers along the b direction (Figure 4).
No significant noncovalent forces exist between these chains.
The repeating unit of these chains can be viewed as dimers
formed by two molecules participating in reciprocal 2.96 Å
CH‚‚‚πAr bonds between a 5-pyrazolyl hydrogen atom of a
ligated site on one molecule and the central arene ring of the
Figure 3. Comparison of the relative conformations of 1 (top) and
1‚(CH3)2CO (bottom) as viewed in the plane of the connecting arene
rings. The two crystallographically independent molecules of
1‚(CH3)2CO are shown. Hydrogen atoms are omitted.
among our linked complexes in the related bitopic monometallic
compound {m-[CH(pz)2]2C6H4}Re(CO)3Br,10 which displays an
analogous conformation in which all four pyrazolyl rings are
on the same side of the central ring. In the absence of other
sterically demanding, coordinated metal-ligand systems, this
surprising conformation with all pyrazolyl rings on the same
(23) Reger, D. L.; Gardinier, J. R.; Pellechia, P. J.; Smith, M. D.; Brown,
K. J. Inorg. Chem. 2003, 42, 7635.
(24) Janiak, C. J. Chem. Soc., Dalton Trans. 2000, 3885.

750 Organometallics, Vol. 25, No. 3, 2006
Table 4. Distances for Intramolecular πCO‚‚‚πAr (Å) Interactions in Rhenium Complexes
Reger et al.
perpendicular distance
(ring plane‚‚‚CO centroid)
ring centroid‚‚‚CO
centroid distance
compound
carbonyl group
1
C(73)O(73)
C(73A)O(73A)
C(73B)O(73B)
C(73)O(73)
C(76)O(76)
C(73)O(73)
C(76)O(76)
C(79)O(79)
C(73)O(73)
C(76)O(76)
C(79)O(79)
C(73)O(73)
C(76)O(76)
C(79)O(79)
2.970(3)
3.182(3)
3.250(3)
3.017(6)
3.007(6)
3.283(5)
2.717(6)
3.122(5)
3.083(6)
3.060(6)
2.795(9)
3.010(8)
3.045(7)
3.489(8)
2.978(3)
3.221(3)
3.278(3)
3.043(6)
3.046(6)
3.320(5)
3.980(6)
3.187(5)
3.269(6)
3.123(6)
4.127(9)
3.652(8)
3.389(7)
4.089(8)
1‚(CH3)2CO
2‚(CH3)2CO
3‚7CH3CN
3‚3(CH3)2CO
3‚4.5(CH3)2CO
other molecule (Figure 4, blue dashed lines). Each dimer is
connected to the two adjacent dimers along the b-axis through
a pair of πpz‚‚‚πpz interactions between uncoordinated pyrazolyl
groups (Figure 4, red dashed lines). This interaction is fairly
strong judging from its relatively short perpendicular (3.59 Å)
and centroid‚‚‚centroid (3.69 Å) distances. Alternatively, the
dimers created by this πpz‚‚‚πpz interaction can be taken as the
base units linked by the CH‚‚‚πAr interactions. Combining these
two views results in an interlocking arrangement of molecules.
Further hydrogen bonding re-enforces this overall supramo-
lecular structure. For instance, two CH‚‚‚N interactions, at 2.51
and 2.59 Å, exist between molecules reciprocally bound through
the πpz‚‚‚πpz interaction. The hydrogen donors in these interac-
tions are from a 5-pyrazolyl position on a ligated site and from
the central arene ring, respectively. A third CH‚‚‚N hydrogen
bond, involving a methine hydrogen atom at 2.61 Å, re-enforces
the CH‚‚‚πAr interaction that creates the original dimeric units.
supported by a variety of intermolecular interactions. Figure 6
shows a view of the metal complex molecules in the ab-plane
of 1‚(CH3)2CO. The two crystallographically distinct molecules,
types A and B, each form separate parallel chains running along
the b direction and alternating with each other along the a
direction. The molecules of type B are linked together by
CH‚‚‚πpz interactions (2.94 Å; Figure 6, red dashed line), thus
forming chains, but no such interactions exist within type A
The supramolecular structure of solvated 1‚(CH3)2CO consists
of sheets of complex molecules in the ab-plane that alternate
with parallel layers of acetone molecules stacked along the
c-axis, as shown in Figure 5. Bridging the complex and solvent
layers is a network of hydrogen bonds created by two distinct
CH‚‚‚Br interactions (2.84 and 2.93 Å), with acetone molecules
as hydrogen donors and bromine atoms from the metal
complexes as acceptors. These CH‚‚‚Br hydrogen bonds work
in concert with three types of CH‚‚‚O interactions (2.53, 2.57,
and 2.44 Å), in which the hydrogen donors are the pyrazolyl
rings of the complex molecules and the acceptors are the acetone
molecules.
Figure 5. View of the layered structure of 1‚(CH3)2CO down the
b-axis. Acetone molecules are drawn as space-filling models.
No noncovalent interactions exist between molecules in the
solvent layer. As a result, the layers of solvent molecules must
be held together by the layers of metal complexes, which are
Figure 6. Section of a two-dimensional sheet in the ab-plane of
1‚(CH3)2CO. A CH‚‚‚πpz interaction between type B molecules is
shown by a red dashed line. CH‚‚‚πpz and CH‚‚‚N interactions
between the two chains are shown as blue dashed lines. πCO‚‚‚πAr
interactions within each chain are shown as green dashed lines.
Figure 4. View of the chain of dimeric units along the b-axis in
unsolvated 1. CH‚‚‚πAr interactions are shown as blue dashed lines
and πpz‚‚‚πpz interactions as red dashed lines.

Mono-, Bi-, and Trimetallic Re(I) Complexes
Organometallics, Vol. 25, No. 3, 2006 751
Figure 8. Section of a chain of complex molecules in 2‚(CH3)2-
CO as viewed down the a-axis. Noncovalent interactions are shown
by red (πpz‚‚‚πpz), blue (CH‚‚‚πpz), and green (CH‚‚‚N) dashed lines.
Table 5. Selected Bond Distances (Å) and Angles (deg) for
2‚(CH3)2CO
Re(1)-N(11)
Re(1)-N(21)
Re(1)-C(71)
Re(1)-C(72)
Re(1)-C(73)
Re(1)-Br(1)
Re(2)-N(31)
Re(2)-N(41)
Re(2)-C(74)
Re(2)-C(75)
Re(2)-C(76)
Re(2)-Br(2)
2.184(6)
2.180(6)
1.919(8)
1.902(8)
1.917(7)
2.6313(8)
2.183(6)
2.185(6)
1.895(8)
1.908(7)
1.922(7)
2.6202(8)
Figure 7. ORTEP drawing of the bimetallic molecule of {µ-1,3,5-
[CH(pz)2]3C6H3}[Re(CO)5Br]2‚(CH3)2CO (2‚(CH3)2CO). Ellipsoids
are drawn at the 50% probability level.
chains. Instead, the type A chains are assembled through
intermolecular 3.64 Å πCO‚‚‚πAr interactions between carbonyl
ligands on the rhenium center of one type A molecule and the
exposed face of the central arene ring of an adjacent type A
molecule. The type B chains are re-enforced by such πCO‚‚‚πAr
interactions as well (3.65 Å). In Figure 6, both sets of πCO‚‚‚πAr
interactions are shown by green dashed lines. The chains are
bound together into sheets by CH‚‚‚πpz interactions between
type A molecules in one chain and the type B molecules in an
adjacent chain (Figure 6, blue dashed lines). These 2.99 Å
interactions are likely weaker than the CH‚‚‚N hydrogen bonds
(2.41 Å) that are also binding the chains together. These CH‚‚‚N
bonds involve a type A methine hydrogen atom and an
uncoordinated type B pyrazolyl nitrogen atom. Additional
CH‚‚‚N hydrogen bonds present re-enforce the structure de-
scribed and are not pictured.
N(11)-Re(1)-N(21)
N(11)-Re(1)-Br(1)
N(21)-Re(1)-Br(1)
C(71)-Re(1)-Br(1)
C(71)-Re(1)-N(11)
C(71)-Re(1)-C(72)
N(31)-Re(2)-N(41)
N(31)-Re(2)-Br(2)
N(41)-Re(2)-Br(2)
C(74)-Re(2)-Br(2)
C(74)-Re(2)-N(31)
C(74)-Re(2)-C(75)
84.4(2)
83.09(15)
82.95(15)
94.6(2)
177.6(3)
87.0(3)
82.6(2)
86.29(14)
84.47(14)
89.9(2)
176.1(3)
86.2(3)
which the bromine atom is cis to the methine hydrogen atom
in the metallocycle formed upon coordination to rhenium.
Noncovalent interactions in 2‚(CH3)2CO organize the metal
complex molecules into one-dimensional chains parallel to the
c-axis (Figure 8), and these chains are further and weakly joined
into two-dimensional sheets in the ac-plane. A strong πpz‚‚‚πpz
interaction, shown by the red dashed lines in Figure 8, exists
between coordinated pyrazolyl rings that are nearly perfectly
parallel (dihedral angle between planes ) 0.02°; centroid‚‚‚
centroid distance ) 3.75 Å; shortest perpendicular distance )
3.37 Å). Supporting these πpz‚‚‚πpz interactions are reciprocal
CH‚‚‚Br hydrogen bonds (not pictured) between Br(2) and the
5-pz hydrogen atom on the nearby pyrazolyl ring of an adjacent
molecule. The hydrogen donors are associated with the same
ligating groups undergoing the πpz‚‚‚πpz stack. The dimers
formed by the two noncovalent associations just described are
further linked with adjacent dimers by a combination of
CH‚‚‚πpz and CH‚‚‚N hydrogen bonds. The blue dashed lines
of Figure 8 outline the CH‚‚‚πpz interactions between a 4-pz
hydrogen donor of a coordinated group with an uncoordinated
pyrazolyl ring acceptor (CH‚‚‚centroid distance and angle are
2.76 Å and 123°). In support of this interaction is the 2.33 Å
CH‚‚‚N hydrogen bond at a 169° angle (green dashed lines)
between H(3), the methine hydrogen atom from an uncoordi-
nated ligating group, and a nitrogen atom from the uncoordinated
Among the arene-linked bi- and tritopic bis(pyrazolyl)methane
complexes we have studied, 1‚(CH3)2CO presents the first
example of intermolecular πCO‚‚‚πAr interactions. This feature
is largely due to the fact that in the majority of the complexes
the arene ring is partially blocked by a coordinated metal-ligand
system and therefore lacks intermolecular access to adjacent
molecules. The only other complex investigated in which an
exposed arene ring face is present in the crystal structure is
{m-[CH(pz)2]2C6H4}Re(CO)3Br, but no intermolecular πCO‚‚‚πAr
interactions are observed for this compound.
The dirhenium complex 2 crystallizes from acetone-diethyl
ether in a single form as the solvate {µ-1,3,5-[CH(pz)2]3C6H3}-
[Re(CO)5Br]2‚(CH3)2CO (2‚(CH3)2CO; space group P1h). Figure
7 shows an ORTEP representation of the metal complex, and
selected bond distances and angles are given in Table 5. As
observed in previously studied dirhenium complexes of arene-
linked ligands,10 the [Re(CO)3Br] units in 2‚(CH3)2CO are
positioned on opposite sides of the central arene ring. As in
both of the structures of 1, each [Re(CO)3Br] unit is oriented
such that a carbonyl ligand is participating in intramolecular
πCO‚‚‚πAr interactions with the central ring (see Table 4 for
details), and the only stereoisomer produced was the one in

752 Organometallics, Vol. 25, No. 3, 2006
Reger et al.
Figure 9. ORTEP drawings of the trimetallic molecules of {µ-1,3,5-C6H3[CH(pz)2]3}[Re(CO)3Br]3‚7CH3CN (3‚7CH3CN), {µ-1,3,5-C6H3-
[CH(pz)2]3}[Re(CO)3Br]3‚3(CH3)2CO (3‚3(CH3)2CO), and {µ-1,3,5-C6H3[CH(pz)2]3}[Re(CO)3Br]3‚4.5(CH3)2CO (3‚4.5(CH3)2CO). Ellipsoids
are drawn at the 50% probability level.
Table 6. Selected Bond Distances (Å) and Angles (deg) for
ligating group of an adjacent molecule. Both of these interactions
are reciprocal. The two-dimensional supramolecular structure
of 2‚(CH3)2CO is completed by weak CH‚‚‚O hydrogen bonds
(not pictured) between a 5-pz hydrogen atom on a free pyrazolyl
group and an oxygen atom from a carbonyl ligand on an adjacent
chain along the a-axis, thereby forming sheets in the ac-plane.
The acetone molecules in 2‚(CH3)2CO are positioned in small
cavities between the sheets of metal complex molecules and
participate in little to no hydrogen-bonding interactions with
other solvent molecules or with metal complex molecules.
The recrystallization of {µ-1,3,5-C6H3[CH(pz)2]3}[Re(CO)3Br]3
(3) by the vapor diffusion of diethyl ether into separate
acetonitrile and acetone solutions yielded three distinct species.
From the acetonitrile solution crystallized a compound in the
chiral orthorhombic space group P212121 with seven ordered
acetonitrile molecules of solvation, {µ-1,3,5-C6H3[CH(pz)2]3}-
[Re(CO)3Br]3‚7CH3CN (3‚7CH3CN). From the same acetone
solution crystallized two pseudopolymorphs, one in the mono-
clinic space group P21/c with three acetone molecules of solva-
tion,{µ-1,3,5-C6H3[CH(pz)2]3}[Re(CO)3Br]3‚3(CH3)2CO(3‚3(CH3)2-
CO), and the other in the triclinic space group P1h with 4.5
acetone molecules of solvation per asymmetric unit, {µ-1,3,5-
C6H3[CH(pz)2]3}[Re(CO)3Br]3‚4.5(CH3)2CO (3‚4.5(CH3)2CO).
Figure 9 shows ORTEP representations of the trimetallic
molecules of all three crystalline species. Selected bond
distances and angles are compared in Table 6. The trimetallic
molecules in each crystalline species differ from one another
in the relative orientations of the -CH(pz)2[Re(CO)3Br] units
about the central arene ring, particularly the positions of the
carbonyl groups (see Table 4). These differences are possible
through the rotational freedom of the bis(pyrazolyl)methane
groups about the arene ring and may be caused by varying
degrees of noncovalent interactions in the supramolecular
structures (see below). In each structure, the “two up-one
down” conformation, invoked in interpreting the NMR data
above, is observed. Spectral data indicate that the amorphous
solid 3 is formed exclusively as one stereoisomer. The structures
of all three crystalline forms show that this isomer is the one in
which each bromine atom bound to rhenium is cis to its
respective methine hydrogen atom and that a carbonyl ligand
bound to rhenium participates in intramolecular πCO‚‚‚πAr
interactions (Table 4). The [Re(CO)3Br] units in 3, therefore,
have the same relative orientations as those in 1 and 2.
3‚7CH3CN, 3‚3(CH3)2CO, and 3‚4.5(CH3)2CO
3‚7CH3CN 3‚3(CH3)2CO 3‚4.5(CH3)2CO
Re(1)-N(11)
Re(1)-N(21)
Re(1)-C(71)
Re(1)-C(72)
Re(1)-C(73)
Re(1)-Br(1)
Re(2)-N(31)
Re(2)-N(41)
Re(2)-C(74)
Re(2)-C(75)
Re(2)-C(76)
Re(2)-Br(2)
Re(3)-N(51)
Re(3)-N(61)
Re(3)-C(77)
Re(3)-C(78)
Re(3)-C(79)
Re(3)-Br(3)
2.177(4)
2.177(4)
1.910(5)
1.919(5)
1.903(5)
2.6386(6)
2.184(4)
2.169(4)
1.917(5)
1.911(5)
1.910(5)
2.6245(6)
2.175(4)
2.168(4)
1.918(5)
1.904(5)
1.905(5)
2.6395(5)
2.190(5)
2.175(5)
1.920(7)
1.910(6)
1.902(8)
2.6355(8)
2.179(5)
2.183(5)
1.916(7)
1.919(7)
1.904(7)
2.6247(7)
2.184(6)
2.177(5)
1.921(8)
1.911(7)
1.901(7)
2.6242(7)
2.184(6)
2.172(6)
1.927(7)
1.918(8)
1.912(7)
2.6293(8)
2.176(5)
2.172(6)
1.906(7)
1.913(8)
1.911(7)
2.6280(8)
2.167(6)
2.173(6)
1.902(7)
1.920(9)
1.898(8)
2.6307(8)
N(11)-Re(1)-N(21)
N(11)-Re(1)-Br(1)
N(21)-Re(1)-Br(1)
C(71)-Re(1)-Br(1)
C(71)-Re(1)-N(11)
C(71)-Re(1)-C(72)
N(31)-Re(2)-N(41)
N(31)-Re(2)-Br(2)
N(41)-Re(2)-Br(2)
C(74)-Re(2)-Br(2)
C(74)-Re(2)-N(31)
C(74)-Re(2)-C(75)
N(51)-Re(3)-N(61)
N(51)-Re(3)-Br(3)
N(61)-Re(3)-Br(3)
C(77)-Re(3)-Br(3)
C(77)-Re(3)-N(51)
C(77)-Re(3)-C(78)
86.83(15)
83.96(10)
81.81(10)
90.97(18)
174.9(2)
88.7(2)
84.50(14)
83.55(10)
86.47(11)
90.96(17)
173.99(19)
88.6(2)
85.03(14)
84.15(11)
83.47(10)
94.45(16)
177.0(2)
89.8(2)
83.86(19)
87.35(14)
83.72(14)
91.7(2)
177.8(3)
87.2(3)
84.46(19)
85.14(14)
84.06(14)
92.8(2)
176.7(3)
88.6(3)
85.2(2)
83.23(14)
84.45(15)
91.2(2)
174.2(3)
88.4(3)
84.1(2)
84.74(15)
85.08(15)
90.2(2)
174.7(3)
89.5(3)
84.2(2)
84.7(2)
86.33(13)
82.61(13)
92.9(2)
177.2(2)
88.0(3)
83.89(15)
83.89(15)
91.4(2)
175.2(3)
88.8(3)
1‚(CH3)2CO. In 3‚7CH3CN, layers of metal complex molecules
alternate with layers of acetonitrile molecules in stacking along
the c-axis (Figure 10). Multiple hydrogen bonds bridge the
solvent and complex molecule layers. In the complexes, the
hydrogen donors include the methine groups of the bis-
(pyrazolyl)methane units and the 3- and 5-positions of the
pyrazolyl rings. The acceptors associated with these donors are
the acetonitrile molecules. The methyl groups of the solvent
molecules also act as donors to the rhenium-bound bromine
atoms and the oxygen atoms of the carbonyl ligands in the
The crystalline metal complex 3‚7CH3CN consists of a
layered structure analogous to the one discussed above for

Mono-, Bi-, and Trimetallic Re(I) Complexes
Organometallics, Vol. 25, No. 3, 2006 753
Figure 12. Alternating chain formed by the quadruple pyrazolyl
embrace (red dashed lines) in 3‚7CH3CN.
Figure 10. Layered structure of 3‚7CH3CN as viewed down the
b-axis. Acetonitrile molecules are shown as space-filling models.
Figure 13. Layered structure of 3‚3(CH3)2CO as viewed down
the c-axis. Acetone molecules are shown as space-filling models.
Figure 11. View in the ab-plane of a sheet of ordered acetonitrile
molecules in 3‚7CH3CN. The repeating unit of seven molecules is
highlighted in the lower right corner of the sheet.
the CH‚‚‚centroid distances, 3.38 and 3.33 Å, are at the upper
limit of typical values for these interactions.5b
The overall supramolecular motif of the acetone-solvated
pseudopolymorph 3‚3(CH3)2CO is similar to the structures of
1‚(CH3)2CO and 3‚7CH3CN in that 3‚3(CH3)2CO exhibits a
sandwich structure resulting from the alternate stacking of sheets
of complex molecules with layers of solvent molecules along
the a-axis (Figure 13). As in the other two layered structures,
hydrogen bonds in 3‚3(CH3)2CO bridge the metal complex and
solvent layers. The hydrogen donors in 3‚3(CH3)2CO are
methine groups and a 5-pyrazolyl position, and the oxygen
atoms of acetone serve as acceptors. There are no intermolecular
interactions within the acetone layers. The metal complex layers,
therefore, provide part of the cohesion for the solvent layers,
as was observed for the previous two layered structures. The
layers of metal complex molecules in 3‚3(CH3)2CO are not
composed of independent chains as in 3‚7CH3CN but rather
two-dimensional networks organized by pyrazolyl embrace
interactions (Figure 14). Thus, each bis(pyrazolyl)methane
ligating site is participating in noncovalent interactions with an
appropriate site from an adjacent molecule.
The second structure from acetone, 3‚4.5(CH3)2CO, displays
no extended structure among complex molecules beyond the
dimeric unit shown in Figure 15, and no layered structure is
observed. Solvent molecules are intercalated between trimetallic
molecules, and noncovalent interactions among complex mol-
ecules are notably weaker compared with the previous two
compounds, as evidenced by longer distances for the observed
CH‚‚‚πpz and πpz‚‚‚πpz interactions. For instance, CH‚‚‚πpz
interactions for the pyrazolyl embrace (Figure 15, red dashed
complexes. There is also a long (3.33 Å) CH‚‚‚πpz involving
an acetonitrile hydrogen donor. The supramolecular structure
of 3‚7CH3CN is particularly unusual in the large number (seven)
of ordered acetonitrile molecules present. The sheet of aceto-
nitrile molecules shown in Figure 11 lies in the ab-plane and
extends in both the a and b directions. A unique set of seven
solvent molecules is highlighted. There is little to no hydrogen
bonding between molecules in this layer.
Because of the lack of extensive intermolecular interactions
in the acetonitrile layer, the layers of metal complex molecules
largely organize these solvent layers. The supramolecular
structure of the metal complex layers in 3‚7CH3CN is dominated
by a set of cooperative noncovalent interactions that we have
previously termed the “quadruple pyrazolyl embrace”.5b In the
pyrazolyl embrace, four pyrazolyl rings from two nearby bis-
(pyrazolyl)methane sites are associated through concerted
CH‚‚‚πpz and πpz‚‚‚πpz interactions, forming an organized
supramolecular motif. The pyrazolyl embrace has been found
to be a common feature in poly(pyrazolyl)borate and -methane
compounds. As shown in Figure 12, the complex molecules of
3‚7CH3CN are linked by the pyrazolyl embrace into chains
propagating along the a direction while winding in the ab-plane.
The propagation of these chains imparts the homochiral feature
of the crystals. There are no important interactions between
adjacent chains. The CH‚‚‚πpz interactions of the embrace are
atypical, with one involving a 5-pz hydrogen atom (H42) rather
than the most commonly observed 4-pz hydrogen atom. Also,

754 Organometallics, Vol. 25, No. 3, 2006
Reger et al.
previous study with bimetallic complexes,10 but the extent to
which the carbonyl ligands have “slipped” off the central ring
may be influenced more by crystal packing (including nonco-
valent interactions, vide supra) and electronic effects than by
steric constraints.
Each of the complexes in this study displays intramolecular
CH‚‚‚πpz interactions in the solid state between an arene
hydrogen atom and an adjacent pyrazolyl ring. The distances
for these interactions are in the range of 2.6 to 2.9 Å, and each
interaction always involves at least one coordinated pyrazolyl
ring. The presence of interactions in the crystalline forms
supports the interpretation of the NMR data presented earlier,
in which for the complexes, arene proton signals adjacent to at
least one coordinated pyrazolyl ring are shifted upfield relative
to those in the free ligand.
Conclusions
Synthetic methods for the selective preparation and purifica-
tion of the mono-, di-, and trirhenium complexes {1,3,5-[CH-
(pz)2]3C6H3}Re(CO)3Br (1), {µ-1,3,5-[CH(pz)2]3C6H3}[Re(CO)5-
Br]2 (2), and {µ-1,3,5-[CH(pz)2]3C6H3}[Re(CO)5Br]3 (3) from
the reactions of Re(CO)5Br and the tritopic, third-generation
bis(pyrazolyl)methane ligand 1,3,5-[CH(pz)2]3C6H3 (L) have
been developed. The use of stoichiometric amounts of starting
reagents, whether attempting to prepare the mono-, bi-, or
trimetallic complex, always resulted in the isolation of a mixture
of all three compounds. This problem was obviated in making
the monorhenium complex by using a large excess of the ligand
such that 1 was produced exclusively. This solution, however,
was not available for the bimetallic species, and the crude
product 2 had to be recrystallized to separate it from the other
species. In contrast to the preparation of the monometallic
compound, the trimetallic species could not be obtained free of
the aforementioned contaminants regardless of the size of the
excess of Re(CO)5Br used, and extraction of the impurities was
necessary. The starburst ligand L is the second of the rigid,
arene-linked bis(pyrazolyl)methane ligands we have studied that
has proven useful for the preparation of metal complexes with
at least one open ligating site. Because a major goal of these
studies is to provide a convenient route to heterometallic
complexes of homomultitopic ligands, synthetic studies of the
potential of 1 and 2 to serve as intermediates to such complexes
are currently underway.
All of the tricarbonylrhenium(I) complexes we have prepared
with rigid, arene-linked ligands have shown themselves pre-
disposed to specific intramolecular and supramolecular features
in the crystalline form. Each complex contains intramolecular
CH‚‚‚πpz interactions between the central arene ring hydrogen
atoms and coordinated pyrazolyl rings (as well as uncoordinated
rings in some examples). Additionally, each complex exhibits
intramolecular πCO‚‚‚πAr interactions and, in one instance,
intermolecular πCO‚‚‚πAr when the crystalline conformation of
the complex allows intermolecular access to the central π region.
The quadruple pyrazolyl embrace5b becomes an important
organizing factor for the trimetallic complexes, but is not
dominant in the other compounds.
Figure 14. View of the bc-plane in 3‚3(CH3)2CO organized by
quadruple pyrazolyl embrace interactions (red dashed lines).
Figure 15. Dimeric association of complex molecules in 3‚4.5-
(CH3)2CO organized by quadruple pyrazolyl embrace interactions
(red dashed lines) and CH‚‚‚Br interaction (green dashed lines).
lines) in the structure of 3‚4.5(CH3)2CO are at 3.34 Å and
πpz‚‚‚πpz stacking occurs at a distance of 4.10 Å. Average values
for these interactions are 2.71 and 3.71 Å, respectively.5b The
elongation of these distances appears to be caused by a distortion
of the pyrazolyl embrace that allows a CH‚‚‚Br interaction with
a distance of 2.81 Å (Figure 15, green dashed lines). An
analogous distortion is noticeable in the structure of 2‚(CH3)2-
CO described above, where CH‚‚‚Br bonds also seem to have
disrupted the pyrazolyl embrace to such an extent that the
interaction cannot be confidently said to exist in that structure.
During our studies of the bitopic ligands m-[CH(pz)2]2C6H4,
p-[CH(pz)2]2C6H4, and p-[CH(4Bnpz)2]2C6H4 (4Bnpz ) 4-benzyl-
1-pyrazolyl), all of the bimetallic [Re(CO)3Br] species adopted
a conformation in which the two ligating sites were on opposite
sides of the ring.10 The compound reported here, {µ-1,3,5-[CH-
(pz)2]3C6H3}[Re(CO)5Br]3 (3), is therefore the first complex we
have observed in these rigidly linked systems with two metal
centers on the same side of the central arene ring. The steric
strain of having two octahedral tricarbonylrhenium centers in
such proximity is relieved in this system by rotation of the bis-
(pyrazolyl)methane groups about the central ring, thus decreas-
ing the π-orbital overlap between the central ring and a carbonyl
group bound to rhenium. The presence of two metallic units on
the same side of the ring prevents either of these proximate
carbonyl ligands in each molecule from being directly over the
center of the ring (see Table 4), a conformation observed in the
Of the five solvated crystal structures reported above, three
exhibit the structural motif of alternating layers of complex and
solvent molecules. Thus, 1‚(CH3)2CO, 3‚7CH3CN, and 3‚3(CH3)2-
CO contain two-dimensional sheets of complex molecules,
which are themselves held together through various noncovalent
interactions, that stack upon acetone or acetonitrile layers
through noncovalent attractions (typically hydrogen bonds). In
all three structures, there are no intrasheet interactions in the

Mono-, Bi-, and Trimetallic Re(I) Complexes
Organometallics, Vol. 25, No. 3, 2006 755
solvent layers; they are entirely organized by their interactions
with the sheets of complex molecules. This separation of
complex and solvent molecules is indicative of the stability of
the supramolecular assemblies of the complex molecules as
brought about by the potential for noncovalent interactions
intentionally designed into the ligands. Consistent supramo-
lecular properties such as the ones described above may be
useful in understanding and designing metal-ligand systems
with predictable intermolecular interactions.
for financial support. We also thank the Alfred P. Sloan
Foundation for support of R.P.W. The Bruker CCD single
crystal diffractometer was purchased using funds provided by
the NSF Instrumentation for Materials Research Program
through Grant DMR:9975623.
Supporting Information Available: X-ray crystallographic files
in CIF format for all structures reported. An Arrhenius plot and
free energy calculations in text format. This information is available
free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. We thank Radu Semeniuc for helpful
discussion and the National Science Foundation (CHE-0414239)
OM0508684