Rhenium(I) terpyridine π-bases: Reversible η2-coordination of ketones, aldehydes, and olefins in the terpyridine plane

Article abstract of DOI:10.1021/om980500p

The complex trans-[Re(terpy)(Br)(PPh₃)₂][OTf] (2) (terpy = 2,2′:6′,2″-terpyridine) is a convenient precursor to the electron-rich π-basic fragment {(terpy)(L)₂Re}⁺ (L = ^tBuNC or PMe₃). Reduction of 2 with activated magnesium in the presence of unsaturated organic molecules and an excess of either ^tBuNC or PMe₃ yields complexes of the type trans-[(terpy)-(L)₂Re(η²-π)][OTf] (L = ^tBuNC or PMe₃; π = olefin, aldehyde, or ketone). The dihapto-coordinated organic moieties show a preference for binding in the plane of the terpy ligand. Reaction of trans-[Re(terpy)(^tBuNC)₂(η²-acetone)][OTf] (9) with MeOTf yields an observable η²-ketonium complex. The electronic environment of these complexes has been probed by cyclic voltammetry, and the details of ligand exchange for the η²-ketone complexes are presented. Geometric features determined from X-ray crystal structure analyses of trans-[(terpy)(^tBuNC)₂Re(η ²-cyclopentene)][OTf] (4) and trans-[(terpy)(_tBuNC)₂Re(η ²-acetophenone)]-[OTf] (11) are reported.

Full text of DOI:10.1021/om980500p

Organometallics 1999, 18, 573-581
573
Rh en iu m (I) Ter p yr id in e π-Ba ses: Rever sible
η2-Coor d in a tion of Keton es, Ald eh yd es, a n d Olefin s in
th e Ter p yr id in e P la n e
Lisa E. Helberg, T. Brent Gunnoe, Benjamin C. Brooks, Michal Sabat, and
W. Dean Harman*
Department of Chemistry, The University of Virginia, Charlottesville, Virginia 22901
Received J une 17, 1998
The complex trans-[Re(terpy)(Br)(PPh3)2][OTf] (2) (terpy ) 2,2′:6′,2′′-terpyridine) is a
t
convenient precursor to the electron-rich π-basic fragment {(terpy)(L)2Re}+ (L ) BuNC or
PMe3). Reduction of 2 with activated magnesium in the presence of unsaturated organic
t
molecules and an excess of either BuNC or PMe3 yields complexes of the type trans-[(terpy)-
t
(L)2Re(η2-π)][OTf] (L ) BuNC or PMe3; π ) olefin, aldehyde, or ketone). The dihapto-
coordinated organic moieties show a preference for binding in the plane of the terpy ligand.
Reaction of trans-[Re(terpy)(tBuNC)2(η2-acetone)][OTf] (9) with MeOTf yields an observable
η2-ketonium complex. The electronic environment of these complexes has been probed by
cyclic voltammetry, and the details of ligand exchange for the η2-ketone complexes are
presented. Geometric features determined from X-ray crystal structure analyses of trans-
[(terpy)(tBuNC)2Re(η2-cyclopentene)][OTf] (4) and trans-[(terpy)(tBuNC)2Re(η2-acetophenone)]-
[OTf] (11) are reported.
Sch em e 1. η1 a n d η2 Bon d in g Mod es for Ald eh yd es
In tr od u ction
a n d Keton es
Aldehydes and ketones have played an important role
in the field of organic chemistry, and much is known
about the reactivity of these carbonyl compounds.1 One
of the great utilities of transition metal chemistry is the
ability to enhance the reactivity of organic compounds
upon binding them to a metal fragment.2 Although
metal-bound carbonyl compounds were known prior to
1970,3 research in this field has gained intensity only
recently. A large variety of aldehyde and ketone com-
plexes have been isolated and characterized, and the
reactivity of metal-bound carbonyl compounds has been
described.4-10
center on σ/π-bonding modes for group VI aldehyde
complexes has also been probed.6l
Whereas a number of aldehyde complexes have been
reported, and both coordination modes are common,5,6
well-characterized examples of η2-ketone complexes are
rare. Several π-coordinated ketone complexes have been
observed;8-10 however, the majority of these ketones
possess electron-withdrawing substituents.8 Systematic
studies of η2-ketone complexes bearing simple alkyl
substituents are uncommon.9a-e The scarcity of η2-
ketone complexes compared to their aldehyde analogues
has been attributed to both electronic and steric factors.
The π* orbital that participates in metal-to-ligand back-
bonding lies higher in energy for ketones compared to
the π* orbital of aldehydes, and the disubstituted
carbonyl of ketones is more sterically imposing than the
singly substituted carbonyl of aldehydes.4a
Two bonding modes exist for aldehyde and ketone
complexes: η1 (σ-complex) bound through oxygen and
η2 (π-complex) bound through both carbon and oxygen
(Scheme 1).11 Gladysz et al. have performed an elegant
series of studies concerning the relative reactivity and
equilibria of σ/π-bonding modes for aldehyde and ketone
complexes of rhenium,6i,8i and the η1 to η2 interconver-
sion has been studied in detailfor [(NH3)5Ru(acetone)]3+/2+
9f and [(NH3)5Os(acetone)]3+/2+ 9h
The effect of the metal
.
(1) Smith, M. B. Organic Synthesis; McGraw-Hill: New York, 1994.
(2) (a) Hegedus, L. E. Transition Metals in the Synthesis of Complex
Organic Molecules; University Science Books: Mill Valley, CA, 1994.
(b) Crabtree, R. H. The Organic Chemistry of the Transition Metals;
Wiley-Interscience: New York, 1994. (c) Collman, J . P.; Hegedus, L.
S.; Norton, J . R.; Finke, R. G. Principles and Applications of Organ-
otransition Metal Chemistry; University Science Books: Mill Valley,
CA, 1987.
(3) See, for example: Clarke, B.; Green, M.; Osborn, R. B. L.; Stone,
F. G. A. J . Chem. Soc. A 1968, 167.
(4) For reviews of transition metal aldehyde and ketone complexes,
see: (a) Huang, Y.-H.; Gladysz, J . A. J . Chem. Educ. 1988, 65, 298.
(b) Shambayati, S.; Schreiber, S. L. In Comprehensive Organic
Synthesis; Trost, B. M., Fleming, I., Schreiber, S. L., Eds.; Pergamon:
New York, 1991; Vol. 1, Chapter 1.10.
We recently reported a synthesis of unusually electron-
rich ReI coordination complexes from the reduction of
trans-[Re(terpy)(Cl)(PPh3)2]+ (terpy ) 2,2′:6′,2′′-terpy-
ridine).12 Although this material served as a useful
precursor to activated olefin complexes of rhenium(I),
reduction of trans-[Re(terpy)(Cl)(PPh3)2]+ in the pres-
ence of other unsaturated ligands (e.g., aldehydes,
ketones, and arenes) produced only intractable mixtures
of products. Hoping that replacement of the chloride
10.1021/om980500p CCC: $18.00 © 1999 American Chemical Society
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574 Organometallics, Vol. 18, No. 4, 1999
Helberg et al.
Sch em e 2. Syn th esis of tr a n s-[Re(ter p y)L2(π)]+
(w h er e π ) Olefin , Keton e, or Ald eh yd e)
ligand by a better leaving group would enhance the
utility of this approach, we prepared the complex trans-
[Re(terpy)(Br)(PPh3)2][OTf] (2). Unlike its chloride pre-
decessor, this material has proven to be a versatile
precursor to ReI terpy complexes of the form trans-
t
[(terpy)(L)2Re(η2-π)]+ (where L ) PMe3, BuNC; π )
cyclopentene, benzaldehyde, acetone, and acetophe-
(5) For examples of η2-formaldehyde complexes, see: (a) Brown, K.
L.; Clark, G. R.; Headford, C. E. L.; Marsden, K.; Roper, W. R. J . Am.
Chem. Soc. 1979, 101, 503. (b) Clark, G. R.; Headford, C. E. L.;
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Headford, C. E. L.; Roper, W. R. J . Organomet. Chem. 1983, 244, C53.
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Organometallics 1986, 5, 2425. (h) Head, R. A. J . Chem. Soc., Dalton
Trans. 1982, 1637. (i) Buhro, W. E.; Patton, A. T.; Strouse, C. E.;
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105, 1056. (j) Green, M. L. H.; Parkin, G.; Moynihan, K. J .; Prout, K.
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G. J . Chem. Soc., Chem. Commun. 1986, 90. (l) Herberich, G. E.;
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Brunner, H.; Wachter, J .; Bernal, I.; Creswick, M. Angew. Chem., Int.
Ed. Engl. 1979, 18, 861. (b) Gauntlett, J . T.; Taylor, B. F.; Winter, M.
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Polyhedron 1985, 4, 1419. (f) Walther, D. J . Organomet. Chem. 1980,
190, 393. (g) Ferna´ndez, J . M.; Emerson, K.; Carsen, R. H.; Gladysz,
J . A. J . Am. Chem. Soc. 1986, 108, 8268. (h) Garner, C. M.; Me´ndez,
N. Q.; Kowalczyk, J . J .; Ferna´ndez, J . M.; Emerson, K.; Larsen, R. D.;
Gladysz, J . A. J . Am. Chem. Soc. 1990, 112, 5146. (i) Me´ndez, N. Q.;
Seyler, J . W.; Arif, A. M.; Gladysz, J . A. J . Am. Chem. Soc. 1993, 115,
2323. (j) Boone, B. J .; Klein, D. P.; Seyler, J . W.; Me´ndez, N. Q.; Arif,
A. M.; Gladysz, J . A. J . Am. Chem. Soc. 1996, 118, 2411. (k) Wang, Y.;
Agbossou, F.; Dalton, D. M.; Liu, Y.; Arif, A. M.; Gladysz, J . A.
Organometallics 1993, 12, 2699. (l) Schuster, D. M.; White, P. S.;
Templeton, J . L. Organometallics 1996, 15, 5467. (m) Rabinovich, D.;
Parkin, G. J . Am. Chem. Soc. 1991, 113, 5904. (n) Adams, H.; Bailey,
N. A.; Gauntlett, J . T.; Winter, M. J .; Woodward, S. J . Chem. Soc.,
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tallics 1995, 14, 4792. (s) Faller, J . W.; Ma, Y.; Smart, C. J .; DiVerdi,
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L. E.; White, P. S.; Templeton, J . L. J . Am. Chem. Soc. 1994, 116, 2878.
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none). The coordination details, electrochemistry, solid-
state geometrical features, and reactivity for this set of
complexes are reported below.
Resu lts
Reaction of the chelate complex (Br)2(PPh3)2Re(dN-
NdC(Ph)-O-) (1) with excess terpy in a benzene/MeOH
(1:1 v/v) mixture, followed by counterion metathesis
upon treatment with AgOTf, produces trans-[Re(terpy)-
(Br)(PPh3)2][OTf] (2). Complex 2 is characterized by
combustion analysis and electrochemistry, as compared
with the previously reported complex trans-[Re(terpy)-
(Cl)(PPh3)2][OTf].12
Rea ction w ith Cyclop en ten e. Reduction of 2 in the
presence of cyclopentene yields a new compound as-
signed as (terpy)Re(Br)(PPh3)(η2-cyclopentene) (3)
(Scheme 2). Complex 3 is not isolable but can be
observed by cyclic voltammetry. The positive shift in the
(7) For examples of η1-ketone complexes, see: (a) Foxman, B. M.;
Klemarczyk, P. T.; Liptrot, R. E.; Rosenblum, M. J . Organomet. Chem.
1980, 187, 253. (b) Boudjouk, P.; Woell, J . B.; Radonovich, L. J .; Eyring,
M. W. Organometallics 1982, 1, 582. (c) Crabtree, R. H.; Hlatky, G.
G.; Parnell, C. P.; Segmu¨ller, B. E.; Uriarte, R. J . Inorg. Chem. 1984,
23, 354. (d) Courtot, P.; Pichon, R.; Salau¨n, J . Y. J . Organomet. Chem.
1985, 286, C17. (e) Auffret, J .; Courtot, P.; Pichon, R.; Salau¨n, J . Y. J .
Chem. Soc., Dalton Trans. 1987, 1687. (f) Dalton, D. M.; Ferna´ndez,
J . M.; Emerson, K.; Larsen, R. D.; Arif, A. M.; Gladysz, J . A. J . Am.
Chem. Soc. 1990, 112, 9198. (g) Dalton, D. M.; Gladysz, J . A. J . Chem.
Soc., Dalton Trans. 1991, 2741. (h) Su¨nkel, K.; Urban, G.; Beck, W. J .
Organomet. Chem. 1985, 290, 231. (i) Bachand, B.; Wuest, J . D.
Organometallics 1991, 10, 2015. (j) Gambarotta, S.; Pasquali, M.;
Floriani, C.; Chiesi-Villa, A.; Guastini, C. Inorg. Chem. 1981, 20, 1173.
(k) See also refs 5m, 5r, 5u, and 5v.
(9) For examples of η2-ketones with alkyl or aryl substituents, see:
(a) Williams, D. S.; Schofield, M. H.; Anhaus, J . T.; Schrock, R. R. J .
Am. Chem. Soc. 1990, 112, 6728. (b) Barry, J . T.; Chacon, S. T.;
Chisholm, M. H.; Huffmann, J . C.; Streib, W. E. J . Am. Chem. Soc.
1995, 117, 1974. (c) Harman, W. D.; Fairlie, D. P.; Taube, H. J . Am.
Chem. Soc. 1986, 108, 8223. (d) Burkey, D. J .; Debad, J . D.; Legzdins,
P. J . Am. Chem. Soc. 1997, 119, 1139. (e) Bryan, J . C.; Mayer, J . M. J .
Am. Chem. Soc. 1990, 112, 2298. (f) Powell, D. W.; Lay, P. A. Inorg.
Chem. 1992, 31, 3542. (g) Tsou, T. T.; Huffman, J . C.; Kochi, J . K.
Inorg. Chem. 1979, 18, 2311. (h) Harman, W. D.; Sekine, M.; Taube,
H. J . Am. Chem. Soc. 1988, 110, 2439. (i) Harman, W. D.; Dobson, J .
C.; Taube, H. J . Am. Chem. Soc. 1989, 111, 3061. (j) Hill, J . E.;
Fanwick, P. E.; Rothwell, I. P. Organometallics 1992, 11, 1771. (k)
Okuda, J .; Herberich, G. E. Organometallics 1987, 6, 2331. (l) Chish-
olm, M. H.; Folting, K.; Klang, J . A. Organometallics 1990, 9, 607.
(10) For examples of η2-ketones (with alkyl or aryl substituents)
formed from carbonylation reactions, see: (a) Erker, G.; Dorf, U.;
Czisch, P.; Petersen, J . L. Organometallics 1986, 5, 668. (b) Wood, C.
D.; Schrock, R. R. J . Am. Chem. Soc. 1979, 101, 5421. (c) Mayer, J .
M.; Bercaw, J . E. J . Am. Chem. Soc. 1982, 104, 2157.
(8) For examples of η2-ketones with halogen-containing electron-
withdrawing groups, see: (a) Clemens, J .; Green, M.; Stone, F. G. A.
J . Chem. Soc., Dalton Trans. 1973, 375. (b) Countryman, R.; Penfold,
B. R. J . Chem. Soc., Chem. Commun. 1971, 1598. (c) Ittel, S. D. J .
Organomet. Chem. 1977, 137, 223. (d) Ittel, S. D. Inorg. Chem. 1977,
16, 2589. (e) Green, M.; Howard, J . A. K.; Laguna, A.; Smart, L. E.;
Spencer, J . L.; Stone, F. G. A. J . Chem. Soc., Dalton Trans. 1977, 278.
(f) Burgess, J .; Chambers, J . G.; Clarke, D. A.; Kemmitt, R. D. W. J .
Chem. Soc., Dalton Trans. 1977, 1906. (g) Clarke, D. A.; Hunt, M. M.;
Kemmitt, R. D. W. J . Organomet. Chem. 1979, 175, 303. (h) Grassi,
A.; Longo, P.; Musco, A.; Porzio, W.; Scrivanti, A. J . Organomet. Chem.
1985, 289, 439. (i) Klein, D. P.; Dalton, D. M.; Me´ndez, N. Q.; Arif, A.
M.; Gladysz, J . A. J . Organomet. Chem. 1991, 412, C7.
(11) Within the context of η2 bonding, some bound η2-ketones can
be described as metalloxiranes. For a theoretical discussion of η2 versus
η1 bonding, see: Delbecq, F.; Sautet, P. J . Am. Chem. Soc. 1992, 114,
2446.
(12) Helberg, L. E.; Barrera, J .; Sabat, M.; Harman, W. D. Inorg.
Chem. 1995, 34, 2033.

Rhenium(I) Terpyridine π-Bases
Organometallics, Vol. 18, No. 4, 1999 575
Sch em e 3. NOE Mea su r em en ts in Su p p or t of th e
Assign ed Dom in a n t Solu tion Rotom er of 4
Magnesium reduction of 2 in neat acetone yields
(terpy)(Br)(PPh3)Re(η2-acetone) (8; Scheme 2). Rotation
of the ketone is hindered on the time scale of the NMR
experiment as indicated by two resonances in the 1H
NMR spectrum for the methyl substituents of the
acetone ligand. While sterics may contribute to this
hindered rotation, it is likely that the dπ orbital dis-
crepancy caused by back-bonding with the terpy and
phosphine ligands contributes significantly to the rota-
tional barrier. The ketone ligand of 8 is labile. An
acetone-d6 solution of 8 slowly exchanges the protioac-
etone ligand with the deuterated solvent. Thus, over
time the resonances for the bound ligand disappear, and
incipient resonances for free acetone are observed.
Resonances for partially deuterated species due to
proton exchange were not observed.
ReII/ReI reduction potential for this complex (Ep,a ) 0.1
V vs NHE) compared to that of 2 is consistent with
replacement of a phosphine in the coordination set of 2
t
t
with the olefin. In situ reaction of 3 with excess BuNC
In situ reaction of 8 with BuNC or PMe3 results in
or PMe3 yields trans-[(terpy)(L)2Re(η2-cyclopentene)]-
the displacement of PPh3 and bromide (Scheme 2). In
neither case is the displacement of acetone observed.
Spectral features for trans-[(terpy)(tBuNC)2Re(η2-aceto-
ne)][OTf] (9) and trans-[(terpy)(PMe3)2Re(η2-acetone)]-
[OTf] (10) are similar to those of 8. The 13C NMR spectra
for 9 and 10 indicate resonances for the bound carbonyl
carbons at 89.6 and 88.7 ppm, respectively. A notable
difference from complex 8 is the equivalence of the
methyl groups of the acetone ligand in the 1H NMR
spectra of 9 and 10. This equivalence is consistent with
coordination of acetone in the plane of the terpy ligand.
NOE enhancements are observed between the methyl
resonances of the ketone and one of the terpy protons
(21% for 9; 16% for 10). For complex 10, interactions
are noted between the methyl groups of the PMe3 ligand
and the acetone ligand (5%) and between the phosphine
ligands and terpy protons (4%).
t
[OTf] [L ) BuNC (4); PMe3 (5)] (Scheme 2).
Proton and carbon NMR and electrochemical data, as
well as a solid-state X-ray structural study of 4 (vide
infra), confirm the identities of 4 and 5. Upfield chemical
shifts for the bound olefinic protons (3.8-4.7 ppm) in
1
the H NMR spectrum and 13C NMR resonances for the
bound carbon atoms (64-66 ppm) are consistent with
t
an η2-bound olefin. The inequivalence of the BuNC and
PMe3 ligands in 4 and 5, respectively, is consistent with
1
either trans or cis geometries. H NMR NOE experi-
ments suggest that the cyclopentene ligand is bound in
the plane of the terpy moiety with a trans configuration
for the isonitrile or phosphine ligands. For complex 4,
a large NOE interaction is observed between the ortho
proton on the terpy ligand and the olefinic protons (13%)
as well as an enhancement (5%) between the same terpy
proton and one of the diastereotopic cyclopentene pro-
tons (Scheme 3). Both interactions are concordant with
the geometry of 4 in the solid state (vide infra), and
similar enhancements (12 and 9%) are observed for the
PMe3 complex 5.
The dihapto-coordinated acetophenone complexes
trans-[(terpy)(L)2Re(η2-π)](OTf) (L ) tBuNC or PMe3; π
) acetophenone (11, 12)) can be prepared by reaction
sequences similar to the preparation of 9 and 10
(Scheme 2). The (terpy)(Br)(PPh3)Re(η2-acetophenone)
intermediate can be observed with electrochemical
experiments; however, unlike complex 8, this putative
complex eluded isolation. The trans-{(terpy)(L)2Re}+
fragments have also been observed spectroscopically (1H
NMR) to form stable complexes with benzophenone and
2-butanone, although detailed characterization of these
complexes was frustrated by the presence of impurities.
Rea ction w ith Ben za ld eh yd e. Analogous to the
reaction of trans-[Re(terpy)(Br)(PPh3)2][OTf] (2) with
cyclopentene, the reduction of 2 with Zn/Hg in the
presence of benzaldehyde produces a new product that
exhibits a cyclic voltammogram consistent with the
formation of (terpy)Re(Br)(PPh3)(η2-benzaldehyde). Again,
isolation of a new product was only possible after in situ
t
The observation of (terpy)Re(Br)(PPh3)(η2-cyclopen-
tene) (3) and (terpy)Re(Br)(PPh3)(η2-acetone) (8) and
their facile conversion to trans-[(terpy)(L)2Re(π)][OTf]
addition of BuNC or PMe3. Isolation of trans-[(terpy)-
t
(L)2Re(η2-benzaldehyde)][OTf] {L ) BuNC (6); PMe3
(7)} was achieved in 80 and 93% yield, respectively
t
(Scheme 2). Upfield chemical shifts for the aldehydic
(π ) cyclopentene or acetone; L ) BuNC, 4, 9; L )
1
proton and the carbonyl carbon in the H and 13C NMR
PMe3, 5, 10) provides direct evidence for the mechanism
of formation of the dihapto-coordinated complexes
(Scheme 4). A single electron reduction of trans-[Re-
(terpy)(Br)(PPh3)2][OTf] (2) with Mg results in the loss
of a single phosphine ligand and coordination of the
unsaturated organic ligand (π) to yield (terpy)Re(Br)-
(PPh3)(η2-π). It is difficult to discern between initial loss
of bromide followed by coordination of π and displace-
ment of a phosphine ligand by bromide (path A) and
the other possible route, simple loss of phosphine,
rearrangement, and coordination of π (path B). In situ
spectra are diagnostic for dihapto-coordinated aldehyde
ligands.4a Resonances at 6.65 and 6.75 ppm (1H NMR
spectrum) are assigned to the aldehyde protons of 6 and
7, respectively, and the 13C NMR spectra show upfield
chemical shifts for the carbonyl carbons (6, 86.1 ppm;
7, 83.6 ppm).
Rea ction s w ith Keton es. The ability of the electron-
rich trans-{(terpy)(L)2Re}+ fragment to bind olefins and
benzaldehyde in an η2 fashion prompted our exploration
into the coordination of ketones. Specifically, we sought
to probe the π-basicity of the rhenium metal center by
comparing the proclivity for π versus σ binding of alkyl
and aryl ketones.
t
reaction of (terpy)Re(Br)(PPh3)(π) with excess BuNC or
PMe3 yields trans-[(terpy)(L)2Re(η2-π)][OTf] (L ) tBuNC,
PMe3). The nature of the halide ligand seemingly plays

576 Organometallics, Vol. 18, No. 4, 1999
Helberg et al.
Sch em e 4. P ossible Mech a n ism s for th e
Rep la cem en t of P P h 3 by π (w h er e π ) Olefin ,
t
Keton e, or Ald eh yd e; L ) P Me3, Bu NC)
F igu r e 1. ORTEP diagram for the complex trans-[Re-
(terpy)(tBuNC)2(η2-cyclopentene)][OTf] (4).
a vital role in these reactions. If bromide is replaced
with chloride, access to trans-[(terpy)(L)2Re(η2-π)][OTf]
complexes is lost.12
Solid -Sta te Str u ctu r a l Stu d ies. Single-crystal X-
ray diffraction studies have provided geometric details
of the solid-state structures of trans-[(terpy)(tBuNC)2Re-
(η2-cyclopentene)][OTf] (4) and trans-[(terpy)(tBuNC)2Re-
(η2-acetophenone)][OTf] (11). ORTEP diagrams for 4
and 11 are depicted in Figures 1 and 2, and relevant
crystallographic data and collection parameters and
selected bond lengths and angles are presented in
Tables 1 and 2.
The structure of 4 is consistent with the assigned
stereochemistry (vide supra), including coordination of
cyclopentene in the plane of the terpy ligand with trans
tBuNC ligands (Figure 1). The olefinic C-C bond length
of 1.46 Å is elongated from that of unbound cyclopentene
(1.39 Å). The increase in bond length is indicative of
metal back-bonding into the π* orbital of the olefinic
ligand. The cyclopentene ligand is complexed in an
approximately symmetrical fashion. The rhenium-
carbon bond distances of 2.21 and 2.22 Å are typical of
rhenium-olefin complexes.13 The bond distances from
rhenium to the terpy nitrogen atoms cis to cyclopentene
are longer than the coordinated nitrogen juxtaposed
trans to the olefin ligand (2.166(9) and 2.148(9) Å vs
2.04(1) Å).
F igu r e 2. ORTEP diagram for the complex trans-[Re-
(terpy)(tBuNC)2(η2-acetophenone)][OTf] (11).
of 1.37 Å is increased from unbound acetophenone (1.26
Å).14 As with complex 4, the coordination of the unsat-
urated organic molecule occurs in the plane of the terpy
ligand, leaving the isonitrile ligands in a trans config-
uration. The Re-C(1) bond length is 2.17 Å, while the
Re-O1 distance is 2.02 Å. Direct comparison of the
structural features of 11 with other rhenium-ketone
complexes is difficult due to a lack of such examples.
However, the rhenium-carbon and rhenium-oxygen
bond lengths are similar to those reported for a series
of rhenium η2-aldehyde complexes.6j As is observed with
The structure of 11 confirms the designated π bonding
of the ketone ligand (Figure 2). The C-O bond length
(13) See, for example: (a) Bodner, G. S.; Ferna´ndez, J . M.; Arif, A.
M.; Gladysz, J . A. J . Am. Chem. Soc. 1988, 110, 4082. (b) Bodner, G.
S.; Peng, T.; Arif, A. M.; Gladysz, J . A. Organometallics 1990, 9, 1191.
(14) The C-O bond length is similar to other reported distances for
η2-ketones. See, for example, refs 9b, 9c, 9e, 9j, and 9l.

Rhenium(I) Terpyridine π-Bases
Organometallics, Vol. 18, No. 4, 1999 577
Ta ble 1. Selected Cr ysta llogr a p h ic Da ta for
[Re(ter p y)(tBu NC)2(η2-cyclop en ten e)][OTf] (4) a n d
[Re(ter p y)(tBu NC)2(η2-a cetop h en on e)][OTf] (11)
Sch em e 5. Molecu la r Or bita l Dia gr a m for
Com p lexes of th e F or m [Re(ter p y)(L)2(π)]+ (w h er e
t
π ) Olefin , Keton e, or Ald eh yd e; L ) P Me3, Bu NC)
[Re(terpy)(tBuNC)2
(η2-cyclopentene)]
[OTf]‚Et2O (4)
[Re(terpy)(tBuNC)2
(η2-acetophenone)]
[OTf] (11)
formula
mol wt
cryst syst
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
C
35H47N5F3O4SRe
C34H37N5F3O4SRe
854.96
triclinic
P1h (No. 2)
11.534(5)
16.597(7)
9.805(4)
103.03(3)
94.90(3)
877.05
monoclinic
P21/n (No. 14)
14.266(3)
15.801(5)
17.482(5)
90
107.76(2)
90
3758(3)
4
103.13(3)
1762(3)
2
Z
D
calcd, g cm-3
1.55
1.61
cryst dimens,
mm
0.38 × 0.32 × 0.11
0.32 × 0.23 × 0.46 mm
temp, °C
radiation
(λ, Å)
2θ max, deg
µ(Mo KR),
cm-1
-120
0.710 69
-120
0.710 69
46
33.85
46
36.08
Ta ble 3. Cyclic Volta m m ogr a m Da ta for Selected
Com p lexes
R
Rw
0.047
0.060
0.056
0.072
complex
Re(II/I)a Re(I/0)a
[Re(terpy)(tBuNC)2(η2-cyclopentene)][OTf] (4)b
[Re(terpy)(PMe3)2(η2-cyclopentene)][OTf] (5)b
[Re(terpy)(PMe3)2(η2-benzaldehyde)][OTf] (7)c
(terpy)Re(Br)(PPh3)(η2-acetone) (8)c
0.52
0.22
0.28
-1.38
-1.42
-1.44
-1.38
-1.42
-1.46
-1.32
-1.42
Ta ble 2. Selected Bon d Dista n ces a n d An gles for
[Re(ter p y)(tBu NC)2(η2-cyclop en ten e)][OTf] (4) a n d
[Re(ter p y)(tBu NC)2(η2-a cetop h en on e)][OTf] (11)
-0.04d
0.46d
0.08d
0.34
[Re(terpy)(tBuNC)2(η2-acetone)][OTf] (9)b
[Re(terpy)(PMe3)2(η2-acetone)][OTf] (10)c
[Re(terpy)(tBuNC)2(η2-acetophenone)][OTf] (11)c
[Re(terpy)(PMe3)2(η2-acetophenone)][OTf] (12)c
4
11
Bond Distances (Å)
0.22d
Re-C1
Re-C2
Re-N1
Re-N2
Re-N3
Re-C21
Re-C26
C1-C2
2.22(1)
2.21(1)
2.166(9)
2.04(1)
2.148(9)
2.06(1)
2.05(1)
1.46(2)
Re-C1
Re-O1
Re-N1
Re-N2
Re-N3
Re-C24
Re-C29
C1-O1
2.17(1)
2.024(7)
2.13(1)
2.03(1)
2.152(9)
2.09(1)
2.09(1)
1.37(1)
a
b
d
Volts vs NHE. In DMA. c In acetone. Reported for Ep,a
.
preference for binding in the plane of the terpy ligand.
All of the canonical dπ orbitals will be filled in these
Re(I) d6 complexes. If the terpy ligand is assigned to
reside in the xy plane, both dyz and dxz orbitals will
participate in back-bonding with the polypyridyl ligand
(Scheme 5). Back-bonding with the isonitrile and phos-
phine ligands will further stabilize the dxz and dyz
orbitals. The HOMO of the {(terpy)Re(L2)}+ fragment
would then be the dxy orbital. Thus, binding of π in the
xy plane would allow maximum dπ metal-to-ligand
bonding.
Electr och em istr y. Electrochemical experiments pro-
vide a convenient analytical technique for characteriza-
tion of these complexes. Table 3 provides a list of the
(II/I) and (I/0) reduction potentials for complexes 4-5
and 7-12. The (I/0) reduction potentials are relatively
invariant over the series of complexes. Thus, we at-
tribute these couples to ligand-centered (terpy) reduc-
tions. Given the poor donating abilities of ketones, the
metal-to-ligand π back-bonding interaction is vital to the
stability of the η2-ketone complexes. Therefore, it is not
surprising that a single-electron oxidation of the ReI
ketone complexes results in a destabilization of the
dihapto-coordinated ketone. In accord with this, the (II/
I) oxidation couples for complexes 8-10 and 12 are
irreversible on the time scale of the cyclic voltammo-
gram experiment (ν ) 50-200 mV/s). In contrast to the
ketone complexes, the cyclopentene complexes 4 and 5
exhibit reversible (II/I) oxidation waves. The better
donating ability of the cyclopentene ligand compared to
the ketone ligands and the absence of lone electron pairs
capable of η1 binding likely contribute to the increased
Bond Angles (deg)
C1-Re-C21
C2-Re-C21
C1-Re-C26
C2-Re-C26
C1-Re-N1
C2-Re-N1
C1-Re-N2
C2-Re-N2
C1-Re-N3
C2-Re-N3
N1-Re-N3
N1-Re-N2
N2-Re-N3
N1-Re-C21
N1-Re-C26
N2-Re-C21
N2-Re-C26
N3-Re-C21
N3-Re-C26
Re-C1-C5
Re-C2-C3
83.4(5)
85.5(5)
95.6(5)
90.7(4)
86.3(4)
124.9(4)
160.0(4)
160.3(4)
123.9(4)
85.4(4)
149.7(4)
74.1(4)
75.7(4)
90.3(4)
94.6(4)
100.6(5)
82.0(4)
92.5(4)
84.0(4)
120.8(8)
119.2(8)
C1-Re-C24
O1-Re-C24
C1-Re-C29
O1-Re-C29
C1-Re-N1
C1-Re-N2
C1-Re-N3
O1-Re-N1
O1-Re-N2
O1-Re-N3
N1-Re-N3
N1-Re-N2
N2-Re-N3
N1-Re-C24
N1-Re-C29
N2-Re-C24
N2-Re-C29
N3-Re-C24
N3-Re-C29
Re-C1-C8
Re-C1-C2
90.3(4)
93.6(4)
89.9(4)
94.5(4)
92.0(4)
165.6(4)
117.9(4)
129.8(3)
154.0(4)
80.3(3)
149.6(3)
75.2(4)
75.7(4)
82.8(4)
84.3(4)
81.7(4)
95.2(4)
101.8(4)
89.5(5)
118.5(8)
121.4(8)
the cyclopentene complex 4, the rhenium-nitrogen bond
distance trans to the unsaturated organic ligand is
shorter than the distance to the cis nitrogen atoms (2.13-
(1) and 2.152(9) Å vs 2.03(1) Å).
Although it is sterically the least favorable position,
coordination in the plane of the terpy ligand is observed
for all of the dihapto-coordinated complexes. The fact
that the unsaturated organic compounds bind in this
sterically hindered position reflects a strong electronic

578 Organometallics, Vol. 18, No. 4, 1999
Helberg et al.
Sch em e 6. Meth yla tion of a n η2-Aceton e Liga n d by
stability of the Re(II) olefin complex compared to its
ketone analogues. Interestingly, while the II/I couple
for the PMe3 complex trans-[(terpy)(PMe3)2Re(η2-ac-
etophenone)][OTf] (12) is irreversible, trans-[(terpy)-
(tBuNC)2Re(η2-acetophenone)][OTf] (11) has a reversible
(II/I) oxidation couple. The electron-withdrawing aryl
group lowers the energy of the π* orbital and decreases
the dπ-π* energy gap. As a result, π back-bonding is
expected to be more efficient with aryl ketones due to
better orbital overlap. It is possible that π back-bonding
is operative even for the oxidation product of 11 and
that this factor is responsible for an apparent increase
in stability compared to acetone. Inconsistent with this
CH3OTf
Sch em e 7. Reson a n ce Str u ctu r es for th e Meth yl
Aceton iu m Com p lex 13
t
hypothesis is the observation that when the BuNC
ligands are replaced with a weaker π-acid (PMe3), the
II/I oxidation couple becomes irreversible. Apparently,
the increased steric bulk of the phosphine ligands
relative to isonitrile destabilizes the Re(II) complex of
12.
The effect of the relative π-acceptor abilities of tBuNC
versus PMe3 is seen by comparing the (II/I) oxidation
potentials for pairs of complexes that differ only in their
axial ligands. For example, trans-[Re(terpy)(tBuNC)2-
(η2-cyclopentene)][OTf] (4) shows a (II/I) reduction
potential 300 mV higher than trans-[Re(terpy)(PMe3)2-
(η2-cyclopentene)][OTf] (5) (0.52 V versus 0.22 V). This
difference in potential is consistent with the greater
π-acidity of the isonitrile ligand versus PMe3. Aldehydes
are generally considered to be superior π-acids when
compared to ketones (vide supra), and the positive shift
for Ep,a for the benzaldehyde complex 7 compared to
corresponding ketone complexes is consistent with this
notion.
Electr op h ilic Ad d ition . The electron-rich trans-
[(terpy)(L)2Re]+ (L ) tBuNC, PMe3) functions as a potent
π base, as evidenced by its ability to bind unsaturated
organic molecules in an η2 fashion. Electron donation
from a metal dπ orbital to dihapto-coordinated moieties
can serve to activate the η2 ligands toward electrophilic
attack. Reaction of trans-[(terpy)(tBuNC)2Re(η2-aceto-
ne)][OTf] (9) with MeOTf yields the η2-ketonium com-
plex trans-[(terpy)(tBuNC)2Re{η2-(CH3)2C(OMe)}][OTf]2
(13) (Scheme 6). A similar methylation has been ob-
served for aldehyde complexes of pentaammineosmium-
(II).15 Complex 13 could be structurally described by
several different forms. The actual structure is likely
somewhere along the continuum between the two bond-
ing extremes shown in Scheme 7. The methylated
acetone ligand in structure I remains bound in an η2
fashion, while structure II represents a formal oxidation
at the metal center {from Re(I) to Re(III)} and formation
of an η2-methoxymethyl complex. The most compelling
evidence that structure I dominates is a slight downfield
shift for the carbonyl carbon of 13 (92.1 ppm) compared
to that of the acetone complex 9 (89.6 ppm). A significant
contribution from structure II would increase sp3 char-
acter for the carbonyl carbon, resulting in an upfield
shift. The methyl group on the oxygen resonates at 4.30
Liga n d Exch a n ge. As mentioned above, an acetone-
d6 solution of (terpy)(Br)(PPh3)Re(η2-acetone) (8) un-
dergoes acetone ligand exchange. Partially deuterated
acetone (free or bound) was not observed. Thus, ligand
exchange is invoked to explain the disappearance of the
bound acetone signals rather than proton exchange. In
contrast to 8, even upon heating, the dihapto-acetone
complexes 9 and 10 do not undergo analogous exchange
reactions. The acetophenone ligand of complexes 11 and
12 is labile, as evidenced by the appearance of free
acetophenone and the formation of trans-[Re(terpy)(L)2-
(η2-acetone-d6][OTf] (L ) tBuNC or PMe3) in an acetone-
1
d6 solution. In a H NMR experiment, treatment of 11
or 12 with cyclopentene yields the corresponding η2-
cyclopentene product. In contrast, the benzaldehyde
complexes 6 and 7 show no signs of ligand exchange.
1
ppm in the H NMR spectrum and at 67.9 ppm in the
13C NMR spectrum. The equivalence of the ketone
Ligand exchange reactions can in some cases be
oxidatively catalyzed. For example, trans-[Re(terpy)-
(tBuNC)2(η2-acetone)][OTf] (9) shows no signs of ex-
change in an acetone-d6 solution after several days at
20 °C. But, addition of a catalytic amount of [Cp2Fe]-
[PF6] (Cp ) cyclopentadienyl) results in rapid incorpo-
ration of acetone-d6 into the coordination sphere (less
than 1 h to >95% completion). This exchange is likely
the result of a mechanism similar to that elucidated for
the {Os(NH3)5}2+ system.9h Here, the catalyst oxidizes
the d6 metal to its d5 configuration where an η2 to η1
linkage isomerization takes place. Once in its η1 form,
rapid ligand exchange may take place. Reversing this
sequence of reactions reforms the catalyst.
t
methyl groups and of the BuNC ligands in the NMR
spectra of 13 suggests that a fluxional process is
occurring that is rapid on the time scale of the NMR
experiments. Apparently the methyl group on the
oxygen undergoes a wiperlike inversion at oxygen that
is rapid relative to the NMR time scale.
Con clu sion s
The complex trans-[Re(terpy)(PPh3)2Br]OTf has proven
to be a versatile precursor to the fragments trans-
(15) For similar reactions with [(NH3)5Os(η2-aldehyde)]2+ complexes,
see: Spera, M. L.; Chen, H.; Moody, M. W.; Hill, M. M.; Harman, W.
D. J . Am. Chem. Soc. 1997, 119, 12772.

Rhenium(I) Terpyridine π-Bases
Organometallics, Vol. 18, No. 4, 1999 579
t
{(terpy)(L)2Re}+ (L ) BuNC or PMe3). These metal
orange solid. Solvent removal under reduced pressure yielded
a green solid (33.44 g, 73%).
systems show exceptional π-donor ability, as is demon-
strated by their propensity toward η2-coordination of
unsaturated organic molecules and by the reactivity of
one of these products, trans-[Re(terpy)(tBuNC)2(η2-ac-
etone)]OTf, with methyl triflate.
tr a n s-[Re(ter p y)(Br )(P P h 3)2][OTf] (2). Complex 1 (10.64
g, 10.60 mmol) was suspended in a benzene/MeOH (1:1 v/v,
∼400 mL) mixture. Terpy (2.671 g, 11.45 mmol) was added to
this suspension, and the reaction mixture was refluxed for 3
days. The solution color changed from green to black during
the period of reflux. The solution was cooled, and the solvent
volume was reduced to 35 mL under vacuum. Filtration
yielded a dark black solid. The solid was washed with benzene
(3 × 4 mL) and Et2O (3 × 5 mL) and dried in vacuo. The black
solid has been assigned as trans-[Re(terpy)(Br)(PPh3)2][Br]
(7.865 g collected, 67%; CV (DMA/TBAH/100 mV/s) E1/2 ) 0.52
V, E1/2 ) -0.92 V vs NHE). A portion of trans-[Re(terpy)(Br)-
(PPh3)2][Br] (3.009 g, 2.72 mmol) was dissolved in MeOH (200
mL). To this solution was added AgOTf (0.701 g, 2.73 mmol)
in MeOH (5 mL). A precipitate formed upon addition of the
AgOTf. The reaction was stirred for 2 h, and then the solvent
volume was brought to approximately 50 mL under reduced
pressure. A dark brown precipitate was collected by vacuum
filtration and washed with aliquots of MeOH until the wash-
ings were brown instead of green. Extraction of the remaining
solid with acetone (4 × 100 mL) was performed until an
insoluble gray solid remained. The solvent from the extraction
was removed under reduced pressure, and the dark brown
residue was collected (2.567 g, 82%). CV (DMA/TBAH/100 mV/
s): E1/2 ) 0.52 V, E1/2 ) -0.94 V vs NHE. Anal. Calcd for
ReC52H41N3BrF3O3P2S: C, 53.25; H, 3.52; N, 3.58. Found: C,
53.21; H, 3.83; N, 3.78.
Exp er im en ta l Section
Gen er a l Meth od s. Unless otherwise noted, all reactions
were performed under a dry nitrogen atmosphere in a Vacuum
Atmospheres Co. glovebox. 1H and 13C NMR spectra were
recorded on a General Electric QE-300 or GN-300 spectrometer
at room temperature. All chemical shifts are reported in ppm
and are referenced to TMS using residual 1H signals and to
the 13C signals of the deuterated solvents. Coupling constants
are given in hertz. Electrochemical experiments were per-
formed under a nitrogen atmosphere using a PAR model 362
potentiostat driven by a PAR model 175 universal program-
mer. Cyclic voltammograms were recorded (Kipp and Zonen
BD90 XY recorder) in a standard three-electrode cell from
+1.50 to -2.20 V with a glassy carbon working electrode. All
potentials are reported vs NHE using ferrocene (E1/2 ) 0.55
V) or cobaltinium hexafluorophosphate (E1/2 ) -0.78 V) as
internal standard. The peak-to-peak separation (Ep,a - Ep,c
)
was between 70 and 100 mV for all reversible couples.
Elemental analyses were obtained on a Perkin-Elmer PE-2400
Series II CHN analyzer. All distillations were performed under
a nitrogen atmosphere. Methylene chloride was distilled from
P2O5; benzene, hexanes, and diethyl ether were distilled from
Na/benzophenone ketyl; methanol was distilled from Mg-
(OMe)2 (freshly prepared from Mg activated by elemental
iodine); acetonitrile and DMA were distilled from CaH2; and
DME was distilled from Na metal. All solvents were thor-
oughly purged with nitrogen prior to use. Deuterated solvents
were deoxygenated by repeated freeze-pump-thaw cycles.
tr a n s-[Re(ter p y)(tBu NC)2(η2-cyclop en ten e)][OTf] (4). A
flask was charged with a DMA suspension of trans-[Re(terpy)-
(PPh3)2Br]OTf (2) (0.100 g) and excess cyclopentene (10 equiv).
To the suspension was added excess Mg0 (1.3 g). The reaction
was stirred vigorously for 15 min, and a color change from
brown to green was observed. The Mg0 was filtered off and
washed with DME (10 mL). The DME wash was combined
with the DMA filtrate. The putative (terpy)Re(Br)(PPh3)(η2-
cyclopentene) (3) can be observed electrochemically (Ep,a ) 0.1
Re(O)(OEt)(Br)2(PPh3)2 was prepared according to a litera-
ture procedure.16 Magnesium powder (Aldrich, 50 mesh) was
activated by treatment with elemental iodine in DME, followed
by washing with DMA, acetone, and diethyl ether. All other
reagents were used as purchased from commercial sources.
Note: Peak assignments for 13C NMR spectra of complexes 6,
7, 11, and 13 were complicated by the large number of
resonances in the range of approximately 120-150 ppm.
Therefore, isochronous resonances resulted in only partial
assignment of these spectra.
Abbreviations: DME ) 1,2-dimethoxyethane; DMA ) N,N-
dimethylacetamide; NHE ) normal hydrogen electrode; TBAH
) tetrabutylammonium hexafluorophosphate; terpy ) 2, 2′:
6′,2′′-terpyridine; OTf ) trifluoromethanesulfonate (triflate);
tBuNC ) tert-butylisocyanide.
t
V) but not isolated. Excess BuNC (10 equiv) was added, and
the reaction was stirred at room temperature for 30 min
followed by heating at 60 ˚C for 15 min. After cooling to room
temperature, the reaction mixture was filtered. The filtrate
was slowly added to stirring Et2O (100 mL), and a precipitate
formed. The precipitate was collected by vacuum filtration and
washed with Et2O. The resulting solid was dried in vacuo (44%
yield). 1H NMR (acetone-d6, δ): 9.62 (d, 2H), 8.68 (d, 4H), 7.92
(t, 2H), 7.76 (t, 1H), 7.57 (t, 2H), 4.31 (d, 2H), 3.80 (m, 2H),
2.56 (m, 2H), 1.92 (m, 2H), 0.98 (s, 9H), 0.79 (s, 9H). 13C NMR
(acetone-d6, δ): 157.0 (2C), 154.9 (2CH), 152.6 (2C), 136.3
(2CH), 132.5 (CH), 127.6 (2CH), 123.7 (2CH), 121.3 (2CH), 66.1
(bound cyclopentene, 2CH), 56.7 ({CH3}3CNC), 56.4 ({CH3}3-
CNC), 36.5 (cyclopentene, 2CH2), 31.6 ({CH3}3CNC), 31.2
t
({CH3}3CNC), 25.2 (cyclopentene, CH2), bound BuNC carbons
Re(dN-NC(P h )O-)(P P h 3)2Br 2 (1). This synthesis is a
modification of a previously reported procedure.17 A suspension
of Re(O)(OEt)(PPh3)2Br3 (42.82 g, 45.96 mmol) and excess PPh3
(43.64 g, 166.00 mmol) in EtOH (2100 mL) was prepared. In
a separate flask, H2NN(H)C(O)Ph (18.62 g, 136.77 mmol) was
suspended in EtOH (100 mL). Concentrated HBr (48%, 23.05
g, 136.77 mmol) was slowly added to the EtOH suspension of
H2NN(H)C(O)Ph, causing total dissolution. Dropwise addition
of the H2NN(H)C(O)Ph/HBr/EtOH solution to the stirring
rhenium suspension was made. The reaction solution was
refluxed for 1.5 h and then cooled in an ice bath. The
precipitate that formed upon cooling was collected by
vacuum filtration and was washed with EtOH (2 × 10 mL)
and Et2O (2 × 25 mL). The dark green solid was extracted
with hot benzene (∼2 L) and filtered from the remaining
not well resolved even at large delay times (9s). CV (DMA/
TBAH/100 mV/s): E1/2 ) 0.52 V, E1/2 ) -1.38 V vs NHE. IR
(glaze): νCN ) 2029 cm-1. Anal. Calcd for ReC31H37N5F3O3S:
C, 46.36; H, 4.65; N, 8.72. Found: C, 45.69; H, 4.80; N, 8.91.
tr a n s-[Re(ter p y)(P Me3)2(η2-cyclop en ten e)][OTf] (5). The
procedure for complex 4 was followed except PMe3 was used
t
instead of BuNC (66% yield). 1H NMR (acetone-d6, δ): 9.83
(d, 2H), 8.55 (overlapping d, 4H), 7.70 (t, 2H), 7.39 (m, 3H),
4.63 (m, 2H), 3.89 (m, 2H), 2.77 (m, 2H), 2.03 (m, 1H), 1.98
(m, 1H), 0.24 (dd, 9H, J PH ) 1, 8 Hz), 0.12 (dd, 9H, J PH ) 1, 8
Hz). 13C NMR (acetone-d6, δ): 153.8 (2C), 152.9 (2CH), 149.9
(2C), 132.3 (2CH), 127.2 (CH), 126.0 (2CH), 122.1 (2CH), 117.6
(2CH), 65.3 (bound cyclopentene, 2CH), 34.6 (cyclopentene,
2CH2), 26.1 (cyclopentene, CH2), 9.7 ({CH3}3P, d, J PC ) 26 Hz),
5.7 ({CH3}3P, d, J PC ) 26 Hz). CV (DMA/TBAH/100 mV/s): E1/2
)
0.22 V, E1/2 ) -1.42 V vs NHE. Anal. Calcd for
(16) Chatt, J .; Rowe, G. A. J . Chem. Soc. 1962, 4019.
(17) Chatt, J .; Dilworth, J . R.; Leigh, G. J .; Gupta, V. D. J . Chem.
Soc. (A) 1971, 2631.
ReC27H37N3F3O3P2S: C, 41.11; H, 4.73; N, 5.33. Found: C,
40.43; H, 4.71; N, 4.99.

580 Organometallics, Vol. 18, No. 4, 1999
Helberg et al.
tr a n s-[R e(t er p y)(tBu NC)2(η2-b en za ld eh yd e)][OTf] (6).
Complex 2, trans-[Re(terpy)(Br)(PPh3)2][OTf] (0.100 g), was
suspended in DMA (1 mL) and MeOH (3 mL). Excess benzal-
dehyde (10 equiv) was added to the suspension, followed by
the addition of excess Zn0/Hg0 (1.5 g). The reaction was stirred
with an observed color change from brown to olive green. After
the cyclic voltammagram indicated the disappearance of all
of the starting complex (approximately 3 h), the Zn0/Hg0 was
removed by filtration and washed with MeOH (2 mL). The
MeOH washing was combined with the DMA/MeOH filtrate.
Excess tBuNC (10 equiv) was added to the filtrate. The reaction
was stirred at room temperature for 1 h and then heated
overnight at 60 °C. After cooling to room temperature, a
precipitate was removed by vacuum filtration and washed with
MeOH (3 mL). The combined filtrate was then reduced to an
oil. The oil was redissolved in a minimum amount of MeOH
(2 mL) and precipitated by slow addition to stirring Et2O (100
mL). The resulting solid was collected by filtration and dried
1H), 8.71 (d, 1H), 8.02 (t, 1H), 7.90 (t, 1H), 7.74 (t, 1H), 7.65
(t, 1H), 7.60 (t, 1H), 2.60 (s, 6H), 0.86 (s, 18H). 13C NMR
(acetone-d6, δ): 153.6 (C), 153.3 (C), 152.2 (CH), 148.3 (C),
148.2 (C), 148.1 (CH), 139.4 (d, C, J PC ) 13 Hz), 136.4 (CH),
134.1 (CH), 129.2 (CH), 125.8 (CH), 124.7 (CH), 122.9 (CH),
121.4 (CH), 120.3 (CH), 119.2 (CH), 89.6 (bound acetone, C),
55.5 ({CH3}3CNC), 30.2 (CH3), 29.4 (CH3). CV (DMA/TBAH/
100 mV/s): Ep,a ) 0.46 V, E1/2 ) -1.42 V vs NHE. IR (glaze):
νCN ) 2083 cm-1. Anal. Calcd for ReC29H35N5F3O4S: C, 43.93;
H, 4.45; N, 8.83. Found: C, 44.22; H, 4.52; N, 8.68.
tr a n s-[Re(ter p y)(P Me3)2(η2-a ceton e)][OTf] (10). The pro-
cedure for complex 9 was followed except PMe3 was used in
t
1
place of BuNC (61% yield). H NMR (acetone-d6, δ): 9.98 (d,
1H), 9.59 (d, 1H), 8.74 (overlapping d, 3H), 8.59 (d, 1H), 7.84
(t, 1H), 7.64 (m, 2H), 7.46 (t, 1H), 7.27 (t, 1H), 2.52 (s, 6H),
0.21 (t, 18H, J PH ) 3 Hz). 13C NMR (acetone-d6, δ): 154.3 (C),
152.2 (CH), 151.3 (C), 148.1 (CH and C overlapping), 145.7
(C), 136.3 (CH), 131.6 (CH), 126.3 (CH), 125.4 (CH), 125.1
(CH), 124.3 (CH), 122.0 (CH), 120.3 (CH), 118.4 (CH), 88.7
1
under vacuum (80% yield). H NMR (acetone-d6, δ): 9.98 (d,
1H), 9.78 (d, 1H), 8.76 (overlapping doublets, 4H), 8.08 (t, 1H),
7.88 (t, 1H), 7.80 (t, 1H), 7.64 (m, 4H), 7.31 (t, 2H), 7.04 (t,
1H), 6.65 (s, 1H), 1.03 (s, 9H), 0.57 (s, 9H). 13C NMR (acetone-
d6, δ): 153.7, 153.4, 153.1, 148.4, 147.7, 147.3, 136.8, 134.1,
130.1, 126.9, 125.5, 125.3, 125.1, 125.0, 122.9, 121.8, 120.3,
119.3 (aromatic C’s), 86.1 (bound benzaldehyde, CH), 56.0
({CH3}3CNC), 55.3 ({CH3}3CNC), 29.9 (CH3), 29.6 (CH3), 29.2
(bound acetone, C), 33.0 (acetone, CH3), 9.4 ({CH3}3P, t, J PC
)
2 Hz). CV (DMA/TBAH/100 mV/s): Ep,a ) 0.12 V, E1/2 ) -1.44
V vs NHE. CV (acetone/TBAH/100 mV/s): E1/2 ) 0.08 V, E1/2
) -1.46 V vs NHE. Anal. Calcd for ReC25H35N3F3O4P2S: C,
38.56; H, 4.53; N, 5.40. Found: C, 38.65; H, 4.90; N, 5.28.
tr a n s-[Re(ter p y)(tBu NC)2(η2-a cetop h en on e)][OTf] (11).
The procedure for complex 6 was followed, except acetophenone
was used in place of benzaldehyde (68.0% yield). 1H NMR
(acetone-d6, δ): 10.09 (d, 1H), 9.87 (d, 1H), 8.70 (overlapping
d, 4H), 8.06 (t, 2H), 7.84 (m, 2H), 7.70 (d, 2H), 7.68 (t, 1H),
7.30 (m, 2H), 7.03 (t, 1H), 2.80 (s, 3H), 0.94 (s, 9H), 0.50 (s,
9H). 13C NMR (CD3CN, δ): 153.5 (C), 152.8 (C), 151.9 (CH),
151.5 (C), 148.3 (C), 148.0 (CH), 147.7 (C), 136.6, 134.2, 129.9,
126.8, 125.6, 125.2, 125.0, 124.7, 122.7, 121.4, 120.0, 119.0
(CH), 88.5 (bound acetophenone, C), 55.9 ({CH3}3CNC), 55.1
({CH3}3CNC), 29.4 (CH3), 29.0 (CH3), 28.6 (CH3). CV (DMA/
TBAH/100 mV/s): Ep,a ) 0.42 V, E1/2 ) -1.32 V vs NHE. CV
(acetone/TBAH/100 mV/s): E1/2 ) 0.34 V, E1/2 ) -1.32 V vs
1
(CH3). The product is > 95% pure by H and 13C NMR.
tr a n s-[Re(ter py)(P Me3)2(η2-ben zaldeh yde)][OTf] (7). The
procedure for complex 6 was followed (PMe3 used in place of
tBuNC) (93% yield). 1H NMR (acetone-d6, δ): 10.17 (d, 1H),
9.90 (d, 1H), 8.75 (m, 3H), 8.66 (d, 1H), 7.95 (d, 2H), 7.90 (t,
1H), 7.68 (m, 3H), 7.52 (m, 1H), 7.43 (t, 2H), 7.36 (t, 1H), 6.75
(d, 1H), 0.24 (dd, 9H, J PH ) 1, 8 Hz), -0.16 (dd, 9H, J PH ) 1,
8 Hz). 13C NMR (acetone-d6, δ): 152.3 (C), 151.7 (CH), 149.6
(C), 147.5 (C), 147.1 (CH), 146.2 (C), 144.4 (C), 134.9 (CH),
130.5 (CH), 129.0 (CH), 127.6 (CH), 126.0 (CH), 125.6 (CH),
125.5 (CH), 124.3 (CH), 122.8 (CH), 121.2 (CH), 118.8 (CH),
117.0 (CH), 83.6 (bound benzaldehyde, CH), 8.4 ({CH3}3P, d,
J PC ) 25 Hz), 6.1 ({CH3}3P, d, J PC ) 25 Hz). CV (DMA/TBAH/
100 mV/s): E1/2 ) 0.28 V, E1/2 ) -1.46 V vs NHE. CV (acetone/
TBAH/100 mV/s): E1/2 ) 0.28 V, E1/2 ) -1.44 V vs NHE. The
NHE. IR (glaze): νCN
ReC34H37N5F3O4S: C, 47.77; H, 4.36; N, 8.19. Found: C, 47.21;
) . Anal. Calcd for
2083 cm-1
H, 4.40; N, 7.61.
tr a n s-[R e(t er p y)(P Me3)2(η2-a cet op h en on e)][OTf] (12).
The procedure for complex 6 was followed (PMe3 used in place
of tBuNC) (46% yield). 1H NMR (acetone-d6, δ): 10.49 (d, 1H),
9.94 (d, 1H), 8.78 (d, 1H), 8.75 (d, 1H), 8.70 (d, 1H), 8.69 (d,
1H), 7.88 (t, 1H), 7.74 (t, 1H), 7.68 (m, 3H), 7.34 (m, 2H), 7.18
(m, 2H), 6.95 (m, 1H), 2.85 (s, 3H), 0.30 (d, 9H), -0.37 (d, 9H).
13C NMR (acetone-d6, δ): 153.9, 152.8, 151.9, 149.3, 147.1,
147.0, 144.8, 135.4, 131.7, 130.7, 129.0, 128.1, 127.6, 125.6,
125.3, 124.8, 124.1, 123.0, 120.9, 118.9, 117.0 (aromatic C),
86.8 (bound acetophenone, t, J PC ) 4 Hz), 31.6 (acetophenone
CH3), 8.5 ({CH3}3P, d, J PC ) 25 Hz), 8.0 ({CH3}3P, d, J PC ) 25
Hz). CV (acetone/TBAH/100 mV/s): Ep,a ) 0.22 V, E1/2 ) -1.42
V vs NHE. Anal. Calcd for ReC30H37N3F3O4P2S: C, 42.85; H,
4.44; N, 5.00. Found: C, 42.64; H, 4.37; N, 5.67.
1
product is > 95% pure by H and 13C NMR.
tr a n s-[Re(ter p y)(Br )(P P h 3)(η2-a ceton e)] (8). Excess Mg0
was added to an acetone (3 mL) suspension of trans-[Re(terpy)-
(PPh3)2Br][OTf] (2) (0.100 g). The reaction mixture was stirred
for 1 h, during which time the color changed from brown to
green. The Mg0 was filtered off and washed with acetone (3
mL). The solid was collected upon solvent removal from the
1
filtrate and dried in vacuo (66% yield). H NMR (acetone-d6,
δ): 9.82 (d, 1H), 9.48 (d, 1H), 8.58 (d, 1H), 8.28 (d, 1H), 8.12
(d, 1H), 7.71 (d, 1H), 7.39 (m, 2H), 7.29 (t, 1H), 7.18 (m, 6H),
7.11 (m, 1H), 6.95 (m, 6H), 6.62 (t, 1H), 6.55 (m, 3H), 2.61 (s,
3H), 2.41 (s, 3H). CV (acetone/TBAH/100 mV/s): Ep,a ) -0.04
1
V, E1/2 ) -1.38 V vs NHE. The product is > 95% pure by H
NMR.
tr a n s-[Re(ter p y)(tBu NC)2(η2-a ceton e(Me))][OTf]2 (13).
2,6-Di-tert-butylpyridine (0.018 g) was added to an acetonitrile
solution of trans-[(terpy)(tBuNC)2Re(η2-acetone)][OTf] (9) (0.022
g). The solution was cooled to -35 °C, and MeOTf (0.021 g)
was added. Upon warming to room temperature, a color change
from purple to orange was noted. Slow addition of Et2O (50
mL) yielded an orange precipitate. The orange solid was
collected by vacuum filtration, washed with Et2O, and dried
tr a n s-[Re(ter p y)(tBu NC)2(η2-a ceton e)][OTf] (9). The pro-
cedure for the preparation of complex 8 was followed. But,
rather than isolating 8 after the acetone wash, excess BuNC
t
was added to the filtrate. The reaction mixture was stirred at
room temperature for 1 h, then heated to 60 °C for an
additional hour. After cooling to room temperature, the reac-
tion mixture was filtered to remove any precipitate. Addition
of the filtrate to stirring Et2O (10 mL) resulted in precipitate
formation. The precipitate was discarded (the first precipitate
was mostly the N2 complex, trans-[Re(terpy)(PPh3)2(N2)][OTf].
The remaining filtrate was reduced in volume to 2 mL and
then precipitated by dropwise addition to Et2O (75 mL)
followed by the slow addition of hexanes (25 mL). The
precipitate was collected by vacuum filtration, washed with
Et2O, and dried in vacuo (66% yield). 1H NMR (acetone-d6, δ):
9.85 (d, 1H), 9.55 (d, 1H), 8.83 (d, 1H), 8.78 (d, 1H), 8.75 (d,
1
in vacuo (75% yield). H NMR (CD3CN, δ): 9.18 (d, 1H), 8.76
(d, 1H), 8.65 (d, 1H), 8.61 (d, 1H), 8.54 (d, 1H), 8.51 (d, 1H),
8.08 (t, 1H), 8.00 (t, 1H), 7.97 (t, 1H), 7.76 (m, 2H), 4.30 (s,
3H), 2.60 (s, 6H), 0.90 (s, 18H). 13C NMR (acetone-d6, δ): 153.3,
150.6, 149.0, 148.8, 139.5 (CH), 138.5 (CH), 137.6 (CH), 127.6
(CH), 127.3 (CH), 124.7 (CH), 124.1 (CH), 121.8 (CH), 92.1
(bound ketonium C), 67.9 (O-bound CH3), 56.8 ({CH3}3CNC),
29.0 (CH3), 28.9 (CH3).

Rhenium(I) Terpyridine π-Bases
Organometallics, Vol. 18, No. 4, 1999 581
Cr ysta l Str u ctu r e of tr a n s-[Re(ter p y)(tBu NC)2(η2-
cyclop en ten e)][OTf]‚Et2O (4). A dark purple plate of dimen-
sions 0.38 × 0.32 × 0.11 mm was grown by diffusion of diethyl
ether into an acetone solution of 4. The crystal was determined
to be of monoclinic symmetry (P21/n, No. 14) with the following
cell dimensions (λ(Mo) ) 0.710 69 Å): a ) 14.266(3) Å; b )
15.801(5) Å; c ) 17.482(5) Å; â ) 107.76(2)°; Z ) 4; V ) 3758-
(3) Å3; Fw ) 877.05 (C35H47N5O4F3SRe); Dcalc ) 1.55 g/cm3. Data
were collected at -120 °C on a Rigaku AFC6C diffractometer
as outlined in Table 1. A total of 5672 data were collected with
5455 unique reflections and 3323 reflections with I > 3σ(I).
The error fit was 1.79 and final R ) 0.047 (Rw ) 0.060).
Empirical absorption corrections were applied using the ψ scan
method with a range of transmission factors of 0.42-1.00.18
Cr ysta l Str u ctu r e of tr a n s-[Re(ter p y)(tBu NC)2(η2-a c-
etop h en on e)][OTf] (11). A black block (0.32 × 0.23 × 0.46
mm) was grown from diethyl ether diffusion into an acetone
solution of 11. The crystal was determined to be triclinic (P1,
No. 2) with the following cell dimensions (λ(Mo) ) 0.71069
Å): a ) 11.534(5) Å; b ) 16.597(7) Å; c ) 9.805(4) Å; R )
103.03(3)°; â ) 94.90(3)°; γ ) 103.13(3)°; Z ) 2; V ) 1762(3)
Å3; Fw ) 854.96 (C34H37N5O4F3SRe); Dcalc ) 1.61 g/cm3. Data
were collected on a Rigaku AFC6C diffractometer at -120 °C.
A total of 5200 reflections were measured with 4898 unique
reflections and 3938 reflections with I > 3σ(I). The error fit
was 2.27 and final R ) 0.056 (Rw ) 0.072). Empirical
absorption corrections were applied using the ψ scan method
with a range of transmission factors of 0.31-1.00.
Ack n ow led gm en t is made to the National Science
Foundation (NSF Young Investigator Program) and the
Alfred P. Sloan Foundation for their generous support
of this work.
Su p p or tin g In for m a tion Ava ila ble: Tables containing
complete positional and thermal parameters, bond lengths, and
bond angles for 4 and 11 (19 pages). Ordering information is
given on any current masthead page.
OM980500P