Cationic hydrogen complexes of rhenium. 2. Synthesis, reactivity, and competition studies

Article abstract of DOI:10.1021/ja9643228

Cationic rhenium dihydrogen complexes, [Re(H₂)(PR₃)₂(CO)₃]B(Ar')4 (PR₃ = PCy₃, PiPr₃, PiPrPh₂, PPh₃; Ar' = 3,5-(CF₃)₂C₆H₃), have been prepared by the protonation of ReX(PR₃)₂(CO)₃ (X = H, CH₃) with [H(Et₂O)₂]B(Ar')₄ under a hydrogen atmosphere. Deuterium is incorporated into the H₂ ligand when placed under a D₂ atmosphere and large JHD values (30-33 Hz) are consistent with a dihydrogen formulation. Relaxation data indicate very short T(1 min) for these complexes. These complexes are susceptible to heterolytic cleavage of dihydrogen, and the reactivity with several bases has been investigated. Under vacuum or argon atmosphere the complexes readily lose hydrogen to form 16-electron complexes. In the solid state, [Re(PCy₃)₂(CO)₃]B(Ar')₄ exhibits an agostic interaction to a β C- H bond of the phosphine ligand. Variable-temperature ³¹P{¹H} NMR spectra of [Re(PCy₃)₂(CO)₃]B(Ar')₄ indicate a dynamic process involving hindered rotation about the Re-P bond. Competition studies have been conducted, and the hydrogen binding affinity is higher for [Re(H₂)(PCy₃)₂(CNtBu)₃]B(Ar')₄ (5) than for [Re(H₂)(PCy₃)₂(CO)₃]B(At')₄ (2a). Similar experiments also find that 5 also binds hydrogen preferentially over W(H₂)(PCy₃)₂(CO)₃.

Full text of DOI:10.1021/ja9643228

4172
J. Am. Chem. Soc. 1997, 119, 4172-4181
Cationic Hydrogen Complexes of Rhenium. 2. Synthesis,
Reactivity, and Competition Studies
D. M. Heinekey,* Catherine E. Radzewich, Mark H. Voges, and Beth M. Schomber
Contribution from the Department of Chemistry, Box 351700, UniVersity of Washington,
Seattle, Washington 98195-1700
ReceiVed December 16, 1996X
Abstract: Cationic rhenium dihydrogen complexes, [Re(H2)(PR3)2(CO)3]B(Ar′)4 (PR3 ) PCy3, PiPr3, PiPrPh2, PPh3;
Ar′ ) 3,5-(CF3)2C6H3), have been prepared by the protonation of ReX(PR3)2(CO)3 (X ) H, CH3) with [H(Et2O)2]B-
(Ar′)4 under a hydrogen atmosphere. Deuterium is incorporated into the H2 ligand when placed under a D2 atmosphere
and large JHD values (30-33 Hz) are consistent with a dihydrogen formulation. Relaxation data indicate very short
T1min for these complexes. These complexes are susceptible to heterolytic cleavage of dihydrogen, and the reactivity
with several bases has been investigated. Under vacuum or argon atmosphere the complexes readily lose hydrogen
to form 16-electron complexes. In the solid state, [Re(PCy3)2(CO)3]B(Ar′)4 exhibits an agostic interaction to a â
C-H bond of the phosphine ligand. Variable-temperature 31P{1H} NMR spectra of [Re(PCy3)2(CO)3]B(Ar′)4 indicate
a dynamic process involving hindered rotation about the Re-P bond. Competition studies have been conducted,
and the hydrogen binding affinity is higher for [Re(H2)(PCy3)2(CNtBu)3]B(Ar′)4 (5) than for [Re(H2)(PCy3)2(CO)3]B-
(Ar′)4 (2a). Similar experiments also find that 5 also binds hydrogen preferentially over W(H2)(PCy3)2(CO)3.
Introduction
the requirement for polar solvents, which can act as ligands to
electron-deficient metal centers. Anions can also act as potential
ligands, especially if the hydrogen ligand is labile.
Since their initial discovery by Kubas,1 H2 complexes of
transition metals have been the subject of intensive study by
several research groups.2 Cationic dihydrogen complexes
formed by protonation of neutral hydrides or by other routes
are numerous. Recently, there have been several interesting
reports on the synthesis and reactivity of dicationic dihydrogen
complexes.3 The effect that a net positive charge has upon the
stability of dihydrogen complexes is of interest in their design
and predicted reactivity. While there are reported comparisons
of dihydrogen complexes within the same transition metal
triad,4,5 there are few comparisons of cationic systems to closely
related neutral analogs.6-9 Cationic systems require extra
considerations. Study of cationic complexes is complicated by
X Abstract published in AdVance ACS Abstracts, April 15, 1997.
(1) Kubas, G. J.; Ryan, R. R.; Swanson, B. I.; Vergamini, P. J.;
Wasserman, H. J. J. Am. Chem. Soc. 1984, 106, 451-452.
(2) Reviews on dihydrogen complexes: (a) Heinekey, D. M.; Oldham,
W. J., Jr. Chem. ReV. 1993, 93, 913-926. (b) Morris, R. H.; Jessop, P. G.
Coord. Chem. ReV. 1992, 121, 155-289. (c) Crabtree, R. H. Acc. Chem.
Res. 1990, 23, 95-101. (d) Kubas, G. J. Acc. Chem. Res. 1988, 21, 120-
128.
(3) (a) Heinekey, D. M.; Luther, T. A. Inorg. Chem. 1996, 35, 4396-
4399. (b) Smith, K.-T.; Tilset, M.; Kuhlman, R.; Caulton, K. G. J. Am.
Chem. Soc. 1995, 117, 9473-9480. (c) Li, Z.-W.; Taube, H. J. Am. Chem.
Soc. 1991, 113, 8946-8947. (d) Harman, W. D.; Taube, H. J. Am. Chem.
Soc. 1990, 112, 2261-2263. (e) Schlaf, M.; Lough, A. J.; Maltby, P. A.;
Morris, R. H. Organometallics 1996, 15, 2270-2278.
(4) (a) Albertin, G.; Antoniutti, S.; Bordignon, E. J. Am. Chem. Soc.
1989, 111, 2072-2077. (b) Stefano, A.; Albertin, G.; Amendola, P.;
Bordignon, E. J. Chem. Soc., Chem. Commun. 1989, 229-230. (c)
Amendola, P.; Antoniutti, S.; Albertin, G.; Bordignon, E. Inorg. Chem. 1990,
29, 318-324.
(5) (a) Earl, K. A.; Jia, G.; Maltby, P. A.; Morris, R. H. J. Am. Chem.
Soc. 1991, 113, 3027-3039. (b) Bautista, M. T.; Earl, K. A.; Morris, R. H.
Inorg. Chem. 1988, 27, 1126-1128. (c) Earl, K. A.; Morris, R. H.; Sawyer,
J. F. Acta Crystallogr. 1989, C45, 1137-1138. (d) Ricci, J. S.; Koetzle, T.
F.; Bautista, M. T.; Hofstede, T. M.; Morris, R. H.; Sawyer, J. F. J. Am.
Chem. Soc. 1989, 111, 8823-8827. (e) Jia, G.; Lough, A. J.; Morris, R. H.
Organometallics 1992, 11, 161-171. (f) Bautista, M. T.; Cappellani, E.
P.; Drouin, S. D.; Morris, R. H.; Schweitzer, C. T.; Sella, A.; Zubkowski,
J. J. Am. Chem. Soc. 1991, 113, 4876-4887. (g) Bautista, M.; Earl, K. A.;
Morris, R. H.; Sella, A. J. Am. Chem. Soc. 1987, 109, 2780-3782.
(6) [Mn(H2)(CO)(dppe)2]B(Ar′)4 (dppe ) Ph2PC2H4PPh2): King, W. A.;
Luo, X.-L.; Scott, B. L.; Kubas, G. J.; Zilm, K. W. J. Am. Chem. Soc.
1996, 118, 6782-6783.
Since there have been numerous studies that describe the
synthesis, characterization, and reactivity of M(H2)(PCy3)2(CO)3
(M ) W, Mo, Cr),1,10 we have chosen to investigate the group
7 cationic analogs. A previous report of [Re(H2)(PMe3)2(CO)3]-
BF411 describes the low-temperature 1H NMR spectrum of this
species, but the thermal instability precluded isolation of the
complex. This instability is consistent with the previous
12
observation in our group that [Re(H2)(PCy3)2(CO)3]BF4 also
decomposes at low temperatures. We now report that the use
of more weakly coordinating anions13,14 has enabled us to isolate
and characterize a wide range of cationic complexes of rhenium.
This report details the synthesis and characterization of several
cationic rhenium dihydrogen complexes of the general formula
[Re(H2)(PR3)2(CO)3]+. The lability of the hydrogen ligand in
these complexes has also led us to investigate the formally 16-
electron complexes that result from H2 loss. A brief account
of this chemistry has been previously communicated.15 We have
recently reported on a closely related series of dihydrogen
complexes of the form, [Re(H2)(PR3)2(CNtBu)3]+, in which the
carbonyl groups have been substituted by isonitrile coligands.16
[ReL(PR3)2(CO)3]+ and [ReL(PR3)2(CNtBu)3]+ (L ) H2, ago-
(7) Mo(H2)(CO)(R2PC2H4PR2)2: (a) Kubas, G. J.; Ryan, R. R.; Wrob-
leski, D. A. J. Am. Chem. Soc. 1986, 108, 1339-1341. (b) Kubas, G. J.;
Ryan, R. R.; Unkefer, C. J. J. Am. Chem. Soc. 1987, 109, 8113-8115. (c)
Kubas, G. J.; Burns, C. J.; Eckert, J.; Johnson, S. W.; Larson, A. C.;
Vergamini, P. J.; Unkefer, C. J.; Khalsa, G. R. K.; Jackson, S. A.; Eisenstein,
O. J. Am. Chem. Soc. 1993, 115, 569-581. (d) Zilm, K. W.; Merrill, R.
A.; Kummer, M. W.; Kubas, G. J. J. Am. Chem. Soc. 1986, 108, 7837-
7839.
(8) MnH(H2)(R2PC2H4PR2)2 (R ) Me, Et): Perthuisot, C.; Fan, M.;
Jones, W. D. Organometallics 1992, 11, 3622-3629.
(9) [MH(H2)(R2PC2H4PR2)2]+ (M ) Fe, Ru): (a) Morris, R. H.; Sawyer,
J. F.; Shiralian, M.; Zubkowski, J. D. J. Am. Chem. Soc. 1985, 107, 5581-
5582. (b) Bautista, M.; Earl, K. A.; Morris, R. H.; Sella, A. J. Am. Chem.
Soc. 1987, 109, 3780-3782. (c) Bautista, M. T.; Earl, K. A.; Morris, R. H.
Inorg. Chem. 1988, 27, 1124-1126. (d) Ricci, J. S.; Koetzle, T. F.; Bautista,
M. T.; Hofstede, T. M.; Morris, R. H.; Sawyer, J. F. J. Am. Chem. Soc.
1989, 111, 8823-8827. (e) Bautista, M. T.; Cappellani, E. P.; Drouin, S.
D.; Morris, R. H.; Schweitzer, C. T.; Sella, A.; Zubkowski, J. J. Am. Chem.
Soc. 1991, 113, 4876-4887.
S0002-7863(96)04322-3 CCC: $14.00 © 1997 American Chemical Society

Cationic Hydrogen Complexes of Rhenium
J. Am. Chem. Soc., Vol. 119, No. 18, 1997 4173
Scheme 1
course of the thermolysis. Complexes 1e and 1f are synthesized
from ReH(CO)5 and PR3 (PR3 ) PiPrPh2, PPh3) under similar
conditions as used for their methyl analogs and are isolated in
high yields. Complexes 1e and 1f slowly convert to the
corresponding neutral chlorides over the course of several days
in CD2Cl2 or CDCl3. Alternate syntheses for complex 1f have
been previously reported.19
The progress of the phosphine-substitution reaction can be
conveniently monitored by 1H NMR spectroscopy. The mono-
substituted phosphine compounds are typically observed as
intermediates in the preparation of the disubstituted compounds.
The products are isolated by removing the toluene in Vacuo to
give a sticky yellow residue. White to yellow solids are
obtained by rinsing the products with several portions of pentane
or by recrystallization from heptane. Table 1 lists selected 1H
stic) react with a variety of small molecules, and the details of
these experiments will be reported separately.17
Results
Synthesis of Re(X)(PR3)2(CO)3 (X ) H, CH3). In the
course of this work, several neutral methyl and hydride
complexes of rhenium have been prepared. Compounds 1a-f
are generated by heating a toluene solution of Re(H)(CO)5 or
Re(CH3)(CO)5 with excess phosphine in a thick-walled glass
vessel (Scheme 1). The solutions are heated at 110-130 °C
for 40-140 h and degassed every 12 h. If the CO gas is not
removed periodically, the reaction forms unidentified products.
The reaction conditions are dependent upon the cone angle of
the phosphine. For the largest phosphine, PCy3, the highest
temperatures (130 °C) and longest reaction times (140 h) were
required. The yields for the formation of Re(CH3)(PCy3)2(CO)3
(1a) are low (50%) and can be contaminated with up to 50% of
ReH(PCy3)2(CO)3.18 Although ReH(PCy3)2(CO)3 is slightly
more soluble in heptane, the desired product, Re(CH3)(PCy3)2-
(CO)3, can only be purified by drastically sacrificing yield. By
contrast, the synthesis of Re(CH3)(PPh3)2(CO)3 (90% yield)
requires lower reaction temperatures and shorter reaction times,
and no formation of ReH(PPh3)2(CO)3 is observed during the
1
NMR and IR data for complexes 1a-f. The H NMR spectra
of the isolated complexes show a triplet for the rhenium methyl
or hydride ligand due to coupling to two equivalent phosphines.
The 31P{1H} NMR spectra show a single phosphorus signal for
each complex. A typical 13C{1H} NMR spectrum of the
carbonyl region shows two triplets from coupling to equivalent
phosphine ligands. The usual pattern of the carbonyl region in
the IR spectra are three bands with weak, strong, and medium
intensities as the frequency decreases. All of these data are
consistent with the carbonyls arranged in a meridinal configu-
ration and trans phosphine ligands. The stereochemistry of the
product was confirmed in one case by a crystal structure of Re-
(CH3)(PPh3)2(CO)3 (1d).20 Although the methyl and carbonyls
were disordered, the P-Re-P angle was found to be 177.5°.
The facial isomer of 1d has been previously synthesized and
characterized by Bergman and Simpson.21
Synthesis and Characterization of [Re(H2)(PR3)2(CO)3]B-
(Ar′)4 (PR3 ) PCy3 (2a), PiPr3 (2b), PiPrPh2 (2c), PPh3 (2d)).
Protonation of complexes 1a-f with [H(Et2O)2]B(Ar′)4 under
a hydrogen atmosphere in CH2Cl2 affords the corresponding
dihydrogen complex (eq 1). A broad resonance between -3
(10) (a) Kubas, G. J.; Ryan, R. R. Polyhedron 1986, 5, 473-485. (b)
Kubas, G. J.; Ryan, R. R.; Wrobleski, D. A. J. Am. Chem. Soc. 1986, 108,
1339-1341. (c) Wasserman, H. J.; Kubas, G. J.; Ryan, R. R. J. Am. Chem.
Soc. 1986, 108, 2294-2301. (d) Kubas, G. J.; Unkefer, C. J.; Swanson, B.
I.; Fukushima, E. J. Am. Chem. Soc. 1986, 108, 7000-7009. (e) Zilm, K.
W.; Merrill, R. A.; Kummer, M. W.; Kubas, G. J. J. Am. Chem. Soc. 1986,
108, 7837-7839. (f) Eckert, J.; Kubas, G. J.; Dianoux, A. J. J. Chem. Phys.
1988, 88, 466-468. (g) Gonzalez, A. A.; Zhang, K.; Nolan S. P.; Lopez
de la Vega, R.; Mukerjee, S. L.; Hoff, C. D.; Kubas, G. J. Organometallics,
1988, 7, 2429-2435. (h) Zhang, K.; Gonzalez, A. A.; Hoff, C. D. J. Am.
Chem. Soc. 1989, 111, 3627-3632. (i) Gonzalez, A. A.; Hoff, C. D. Inorg.
Chem. 1989, 28, 4295-4297. (j) Zhang, K.; Gonzalez, A. A.; Mukerjee, S.
L.; Chou, S.-J.; Hoff, C. D.; Kubat-Martin, K. A.; Barnhart, D.; Kubas, G.
J. J. Am. Chem. Soc. 1991, 113, 9170-9176. (k) Kubas, G. J.; Burns, C.
J.; Khalsa, G. R. K.; Van Der Sluys, L. S.; Kiss, G.; Hoff, C. D.
Organometallics 1992, 11, 3390-3404. (l) Eckert, J.; Kubas, G. J.; Hall,
J. H.; Hay, P. J.; Boyle, C. M. J. Am. Chem. Soc. 1990, 112, 2324-2332.
(m) Van Der Sluys, L. S.; Miller, M. M.; Kubas, G. J.; Caulton, K. G. J.
Am. Chem. Soc. 1991, 113, 2513-2520. (n) Lang, R. F.; Ju. T. D.; Kiss,
G.; Hoff, C. D.; Bryan, J. C.; Kubas, G. J. J. Am. Chem. Soc. 1994, 116,
7917-7918. (o) Eckert, J.; Kubas, G. J.; White, R. P. Inorg. Chem. 1992,
31, 1550-1551. (p) Khalsa, G. R. K.; Kubas, G. J.; Unkefer, C. J.; Van
Der Sluys, L. S.; Kubat-Martin, K. A. J. Am. Chem. Soc. 1990, 112, 3855-
3860. (q) Eckert, J.; Kubas, G. J. J. Phys. Chem. 1993, 97, 2378-2384. (r)
Kubas, G. J. Inorg. Synth. 1990, 27, 1-8. (s) Kubas, G. J.; Nelson, J. E.;
Bryan, J. C.; Eckert, J.; Wisniewski, L.; Zilm, K. Inorg. Chem. 1994, 33,
2954-2960.
1
and -5 ppm is observed for each compound in the H NMR
spectra, with no observable phosphorus coupling. The dihy-
drogen resonance is typically observed at 1.5 ppm lower field
than the corresponding neutral hydride in CD2Cl2. Table 2 lists
selected 1H NMR data for the hydride region. The complexes
are thermally robust and are stable indefinitely in halogenated
solvents (CD2Cl2, CDCl3, and 1,2-difluorobenzene) under a
partial H2 atmosphere. Complexes 2a and 2b have been isolated
as pale yellow solids in approximately 90% yield and completely
characterized by spectroscopic methods. For 2c and 2d, the
complexes are extremely reactive toward water and could not
be cleanly isolated, therefore, we have only characterized them
by 1H and 31P{1H} NMR spectroscopy. No change is observed
in the 1H or 31P{1H} NMR spectra from 190 to 300 K, indicating
that there is no substantial equilibrium with a dihydride structure,
(11) Gusev, D. G.; Nietlspach, D.; Eremenko, I. L.; Berke, H. Inorg.
Chem. 1993, 32, 3628-3636.
(12) Heinekey, D. M.; Crocker, L. S. Unpublished observations, 1987.
(13) Brookhart, M.; Grant, B.; Volpe, A. F., Jr. Organometallics 1992,
11, 3920-3922.
(14) Strauss, S. H. Chem. ReV. 1993, 93, 927-942.
(15) Heinekey, D. M.; Schomber, B. M.; Radzewich, C. E. J. Am. Chem.
Soc. 1994, 116, 4515-4516.
(16) Heinekey, D. M.; Voges, M. H.; Barnhart, D. M. J. Am. Chem.
Soc. 1996, 118, 10792-10802.
(19) (a) Luo, X.-L.; Liu, H.; Crabtree, R. H. Inorg. Chem. 1991, 30,
4740-4742. (b) Jones, W. D.; Maquire, J. A. Organometallics 1987, 6,
1728-1737.
(20) Heinekey, D. M.; Voges, M. H. Unpublished reults, 1992.
(21) Simpson, R. D.; Bergman, R. G. Organometallics 1993, 12, 781-
796.
(17) Heinekey, D. M.; Radzewich, C. E.; Voges, M. H.; Schomber, B.
M. To be submitted for publication.
(18) Walker, H. W.; Rattinger, G. B.; Belford, R. L.; Brown, T. L.
Organometallics 1983, 2, 775-776.

4174 J. Am. Chem. Soc., Vol. 119, No. 18, 1997
Heinekey et al.
1
Table 1. Selective H NMR and IR Data for Compounds 1a-f
b
compd
δ(M-CH3)a
δ(M-H)a
JPH (Hz)a
ν )
CO (cm-1
Re(CH3)(PCy3)2(CO)3 (1a)
Re(CH3)(PiPr3)2(CO)3 (1b)
Re(CH3)(PiPrPh2)2(CO)3 (1c)
Re(CH3)(PPh3)2(CO)3 (1d)
Re(H)(PiPrPh2)2(CO)3 (1e)
Re(H)(PPh3)2(CO)3 (1f)
-0.05
-0.12
-1.12
-1.06
6.0
5.9
6.6
6.7
18.8
17.8
N/A
2004 (w), 1902 (s), 1866 (m)
2018 (w), 1920 (s), 1871 (m)
2018 (w), 1915 (s), 1878 (m)
2020 (w), 1919 (s)
-5.94
-5.20
2020 (w), 1925 (s)
a Recorded at 298 K in C6D6 (1a, 1b, 1e, 1f) and CDCl3 (1c, 1d). b Methylene chloride.
Table 2. 1H NMR Data of the Hydride Region for Compounds 2a-da
c
e,g
e,k
compd
δ(HH)b
δ(HD)b
JHD
∆δd,e
T1min
∆ν1/2
[Re(H2)(PCy3)2(CO)3]+ (2a)
[Re(H2)(PiPr3)2(CO)3]+ (2b)
[Re(H2)(PiPrPh2)2(CO)3]+ (2c)
[Re(H2)(PPh3)2(CO)3]+ (2d)
-4.78
-4.97f
-4.26
-3.86
-4.81
-5.01
-4.29
-3.89
32
33
30
32
34
37
30
30
9.3h
49
23
52
54
9.6i
10.5j
10.3j
a All measurements recorded at 298 K in CD2Cl2 except where noted. b Chemical shift in ppm. c Coupling constant in Hz. d Isotope shift (∆δ )
g
δ(HH) - δ(HD)) in ppb. e 500 MHz. f 200 MHz. T1min in ms. h 248 K. i 232 K. j 254 K. k Half-height width in Hz.
1
Figure 1. 200-MHz H NMR spectrum (hydride region) of 2b-d1 in
CD2Cl2. A large coupling to deuterium (1JHD ) 33 Hz) is observed as
well as a small phosphorus coupling (2JHP ) 2 Hz).
[Re(H)2(PR3)2(CO)3]B(Ar′)4. We have also observed that 2a
is formed from the addition of NaB(Ar′)4 to a solution of ReCl-
1
(PCy3)2(CO)3 in methylene chloride under an H2 atmosphere.
This reaction is quite slow and requires 24 h at room temperature
to reach completion.
Figure 2. Partial H NMR spectra of a solution of [Re(H2)(PCy3)2-
(CO)3]B(Ar′)4 (2a) and [Re(H2)(PCy3)2(CNtBu)3]B(Ar′)4 (5a) as a
function of H2 concentration (FPT cycles).
When [Re(H2)(PR3)2(CO)3]B(Ar′)4 (2a-d) is placed under
a D2 atmosphere, loss of the H2 ligand and formation of the D2
isotopomer is observed. For 2a and 2b, isotope exchange is
rapid and a distinct 1:1:1 triplet is observed with large JHD values
after 12 h. The line width for 2b-d1 is sufficiently narrow that
a 2 Hz coupling from the phosphine ligands is observed. The
dihydrogen complexes (Table 2). In contrast, the T1min of the
neutral complex ReH(PiPrPh2)2(CO)3 (1e) was observed to be
325 ms.
Exposure of solid 2a-d to vacuum or argon atmosphere leads
to H2 loss and a corresponding color change from pale yellow
or white to orange (Vide infra). This process is completely
reversible. Similar reversible H2 loss can be effected in solution
by removal of the H2 atmosphere and is accelerated by gentle
heating. Protonation of ReH(PR3)2(CO)3, 1e and 1f, under
vacuum only leads to a mixture of the dihydrogen complexes,
2c and 2d, and the hydrogen loss products.
Reactivity of [Re(H2)(PCy3)2(CO)3]B(Ar′)4 (2a) with Bases.
The dihydrogen ligand is readily deprotonated by several bases.
Addition of 1,8-bis(dimethylamino)naphthalene (Proton-Sponge)
or 2,6-di-tert-butyl-4-methylpyridine to a CD2Cl2 solution of
the dihydrogen complex leads to immediate formation of the
neutral hydride, ReH(PCy3)2(CO)3.18 The strong base KOtBu
will also deprotonate the dihydrogen complex, but only over
the course of several days due to its low solubility in CD2Cl2.
1
observed H NMR resonance of 2a-d1 and 2b-d1 is slightly
upfield of the H2 isotopomers and has a chemical shift difference
of approximately 35 ppb. Isotope shifts are also observed in
the 31P{1H} NMR spectrum, and all three isotopomers can be
observed as the resonances are observed to shift to higher field
as the deuterium in the H2 ligand is increased. Isotope exchange
for 2c and 2d is less facile and signals due to 2c-d1 and 2d-d1
were partially obscured by the presence of the residual H2
resonance. An inversion recovery experiment (180°-τ-90°)
with τ set to null the signal of the H2 ligand reveals the
resonance due to the HD ligand.
Relaxation data for 2a-d were collected at 500 MHz in CD2-
Cl2 by standard inversion recovery NMR methods. Very short
minimum T1 values of 10 ( 1 ms were observed for all four

Cationic Hydrogen Complexes of Rhenium
J. Am. Chem. Soc., Vol. 119, No. 18, 1997 4175
of the cyclohexyl rings.15 Complexes 3c and 3d are extremely
reactive toward water and could only be characterized in solution
by NMR spectroscopy. The 31P{1H} NMR spectra exhibit a
single resonance that does not shift significantly upon changing
the solvent (CD2Cl2, CDCl3, and 1,2-difluorobenzene) or upon
the removal of Et2O. A low-temperature 13C NMR spectrum
in CH2Cl2 does not show any resonances that can be attributed
to a solvent adduct. There is no evidence that the anion,
B(Ar′)4-, interacts with any of the cationic complexes since the
1H and 13C{1H} NMR spectrum of the anion is invariant for all
of the compounds we have studied.
Hydrogen Binding Competition Studies. The affinity of
hydrogen for [Re(PCy3)2(CNtBu)3]B(Ar′)4 (4) relative to the
carbonyl analog, [Re(PCy3)2(CO)3]B(Ar′)4 (3a), was determined
by a direct competition study in CD2Cl2 (eq 3). Equimolar
Figure 3. 31P{1H} spectra of a solution of [Re(H2)(PCy3)2(CO)3]B-
(Ar′)4 (2a) and [Re(H2)(PCy3)2(CNtBu)3]B(Ar′)4 (5) as a function of
H2 concentration (FPT cycles). Formation of 7 is indicated by the
asterisk.
quantities of complexes 4 and 3a were placed in a J. Young
NMR tube with a Teflon valve. The solids were exposed to
H2 (760 torr) until the solid became yellow. After excess H2
was removed at 77 K in Vacuo, CD2Cl2 was vacuum transferred
into the tube. The NMR tube was back-filled with argon (760
1
Figure 4. 81-MHz 31P{1H} NMR spectrum of [Re(PCy3)2(CO)3]B(Ar′)4
(3a) at 240 K (CD2Cl2).
Torr) at 298 K. Figures 2 and 3 show the H NMR spectra
and 31P{1H} NMR spectra, respectively, of the solution at
various hydrogen concentrations. The initial solution shows 1H
and 31P{1H} NMR resonances corresponding to 5, 2a, and 3a.
Bound hydrogen was gradually removed by repeated freeze-
pump-thaw cycles. The resonances corresponding to [Re(H2)-
(PCy3)2(CO)3]B(Ar′)4 completely disappeared before any reso-
nances corresponding to [Re(PCy3)2(CNtBu)3]B(Ar′)4 were
observed.
If the base is not sterically hindered, as is the case with ammonia
and aniline, displacement of the H2 ligand and coordination of
the base is observed.22 Two nitrogen bases that do not react
with 2a are pentafluoropyridine and 2,6-diisopropylaniline.
Synthesis of [Re(PR3)2(CO)3]B(Ar′)4 (PR3 ) PCy3 (3a),
PiPr3 (3b), PiPrPh2 (3c), PPh3 (3d)). When Re(CH3)(PR3)2-
(CO)3, 1a-d, is reacted with [H(Et2O)2]B(Ar′)4 under vacuum,
an orange solution results. Rapid methane evolution was
observed. The formally ligand deficient product is also
produced upon removal of H2 from 2a-d in the solid state or
solution (eq 2). The compounds are thermally stable in the solid
The bound hydrogen was removed from both 2a and 5 by
removing the solvent in Vacuo and exposing the sample to
dynamic vacuum for 15 h. Methylene chloride-d2 was rein-
troduced to the NMR tube and the hydrogen concentration was
slowly increased in the sample. Monitoring the progress of the
1
reaction by H and 31P{1H} NMR spectroscopy shows that 5
grows in completely before any 2a is detected.
A similar experiment, conducted in THF-d8, determined the
affinity of H2 for 4 relative to the tungsten carbonyl THF
complex, W(THF)(PCy3)2(CO)3 (eq 4). Equimolar quantities
of complex 4 and W(PCy3)2(CO)3 were placed in a J. Young
NMR tube with a Teflon valve. The solids were exposed to
H2 (760 Torr) until the purple color of the five-coordinate
complexes was discharged. The H2 gas was removed at 77 K
and THF-d8 was vacuum transferred to the tube. The NMR
tube was back-filled with argon (760 Torr) at 298 K. The initial
1H and 31P{1H} NMR spectra show resonances for 5, W(H2)-
(PCy3)2(CO)3, and W(THF)(PCy3)2(CO)3. Bound hydrogen was
state and in solution. 3a and 3b have been isolated in good
yield and completely characterized by spectroscopic and analyti-
cal methods. The solid state structure of 3a has been determined
and indicates an agostic interaction with a â C-H bond of one
(22) The products of H2 displacement are reported separately.17

4176 J. Am. Chem. Soc., Vol. 119, No. 18, 1997
Heinekey et al.
Table 3. H-H Bond Lengths from T1min Data in the Limit of
Slow and Fast Rotation
H-H (Å)
compd
slow rotation
fast rotation
[Re(H2)(PCy3)2(CO)3]+ (2a)
[Re(H2)(PiPr3)2(CO)3]+ (2b)
[Re(H2)(PiPrPh2)2(CO)3]+ (2c)a,b
[Re(H2)(PPh3)2(CO)3]+ (2d)a,c
0.95
0.95
0.97
0.97
0.75
0.76
0.77
0.77
a Corrected for neutral hydride relaxation. b Measurement of the T1min
of Re(H)(PiPrPh2)2(CO)3 was 325 ms at 217 K in a 500-MHz field
(CD2Cl2) solution). c Measurement of the T1min of Re(H)(PPh3)2(CO)3
was taken from ref 19a and corrected to a 500-MHz field.
and 1f. The long T1min values of 1e (325 ms) and 1f (354 ms)19a
are in the expected range of hydride complexes. This observa-
tion indicated the rapid relaxation in 2a-d is dominated by the
dipole-dipole interaction of the η2-H2 ligand.
gradually removed by several freeze-pump-thaw cycles. The
resonances corresponding to W(H2)(PCy3)2(CO)3 completely
disappeared before any resonances corresponding to 4 were
observed to grow in.
Once most of the bound hydrogen was removed from both
complexes, 5 and W(H2)(PCy3)2(CO)3, the experiment was
reversed. The hydrogen concentration was slowly increased.
The 1H and 31P{1H} NMR spectra show that 5 grows in
completely, before any W(H2)(PCy3)2(CO)3 is detected.
The T1min values for the dihydrogen ligand can be used to
calculate the bond distance within the limits of fast and slow
rotation (Table 3).25 An average slow rotation bond length of
0.96 Å is calculated versus the fast rotation bond length of 0.76
Å. Gusev and co-workers have recently discussed the utility
of T1min data in determining the H-H bond distance for
dihydrogen complexes.26 Specifically, they have addressed the
difficulties in assigning “fast” or “slow” rotation corrections to
calculate the H-H bond distances. They have compared several
known dihydrogen complexes that have JHD values greater than
25 Hz as well as reported T1min data. The calculated H-H bond
lengths for this series as well as complexes 2a-d are best
approximated by the slow-spinning data. By correcting for a
fast-spinning H2 ligand, the H-H bond lengths are on average
0.2 Å shorter, and several approach the unreasonable distance
of 0.74 Å for free H2.27
Discussion
Synthesis of Re(X)(PR3)2(CO)3 (X ) H, CH3). The
thermolytic decarbonylation of ReX(CO)5 (X ) H, CH3, halides)
in the presence of phosphines is an often used synthetic method
for generating mixed phosphine-carbonyl complexes of rhe-
nium.23 These include mono-, bis-, or tris-substituted products
in various stereochemical configurations depending upon condi-
tions, phosphine, and X group. We were interested in generating
several bis-phosphine, tri-carbonyl complexes with bulky phos-
phines in a trans-mer configuration with methyl or hydride
ligands. The synthetic route allows for generation of complexes
1a-f in good yield and purity, although the more demanding
conditions required for the bulkier phosphines, PiPr3 and PCy3,
leads to lower yields than for PiPrPh2 and PPh3. These
complexes are convenient precursors to rhenium dihydrogen and
agostic complexes.
Recent reports from the groups of Heinekey3a and Morris28
have correlated the H-H bond distances determined by solid-
state NMR or neutron diffraction techniques to JHD values. The
JHD values of complexes 2a-d correspond to H-H bond
distances of approximately 0.89 Å. This value is intermediate
between those determined for slow and fast rotation from the
T1min data. Surprisingly, the H-H distance determined for
W(H2)(PiPr3)2(CO)3 by solid-state NMR is 0.89Å.10e,29 A recent
report by Kubas and co-workers has also indicated similar H-H
distances for isostructural cationic and neutral dihydrogen
complexes. Solid-state NMR was used to calculate an H-H
Synthesis and Characterization of [Re(H2)(PR3)2(CO)3]B-
(Ar′)4. Protonation of complexes 1a-f with [H(Et2O)2]B(Ar′)4
under a H2 atmosphere leads to a series of cationic rhenium
dihydrogen complexes. A broad peak in the hydride region of
the 1H NMR spectrum is consistent with a dihydrogen structure,
and partial substitution with deuterium leads to large HD
6
distance of 0.89 Å for [Mn(H2)(CO)(dppe)2]B(Ar′)4 and 0.88
29
Å for the analogous neutral complex Mo(H2)(CO)(dppe)2
(dppe ) Ph2PC2H4PPh2).
Stability of Dihydrogen Complexes: H2 Loss, Homolytic
Cleavage, Halogenated Solvents, Anions, and Heterolytic
Cleavage. Protonation of neutral hydrides is a common
synthetic route to cationic dihydrogen complexes.2 It has been
1
coupling values of 30-33 Hz. The H NMR chemical shifts
of the bound HD are slightly upfield of the corresponding H2
resonances (30 to 37 ppb), consistent with typical intrinsic
isotope shifts.24
(24) (a) Hamilton, D. G.; Luo, X.-L.; Crabtree, R. H. Inorg. Chem. 1989,
28, 3198-3203. (b) Luo, X.-L.; Crabtree, R. H. J. Am. Chem. Soc. 1990,
112, 4813-4821. (c) Luo, X.-L.; Michos, D.; Crabtree, R. H. Organome-
tallics 1992, 11, 237-241. (d) Baird, G. J.; Davies, S. G.; Moon, S. D.;
Simpson, S. J.; Jones, R. H. J. Chem. Soc., Dalton Trans. 1985, 1479-
1486. (e) Casey, C. P.; Tanke, R. S.; Hazin, P. N.; Kemnitz, C. R.;
McMahon, R. J. Inorg. Chem. 1992, 31, 5474-5479.
(25) Desrosiers, P. J.; Cai, L.; Lin, Z.; Richards, R.; Halpern, J. J. Am.
Chem. Soc. 1991, 113, 4173-4184.
(26) Gusev, D. G.; Kuhlman, R. L.; Renkema, K. B.; Eisenstein, O.;
Caulton, K. G. Inorg. Chem. 1996, 35, 6775-6783.
The T1min values of 2a-d listed in Table 2 are very short
and remarkably similar (9-10 ms). In order to gauge the
relaxation contribution of the rhenium metal center as well as
the phosphine ligands we have also investigated the T1min of 1e
(23) (a) Hoffman, N. W.; Prokopuk, N.; Robbins, M. J.; Jones, C. M.;
Doherty, N. M. Inorg. Chem. 1991, 30, 4177-4181. (b) Abel, E. W.;
Wilkinson, G. J. Chem. Soc. 1959, 1501-1505. (c) Abel, E. W.; Tyfield,
S. P. Can. J. Chem. 1969, 47, 4627-4633. (d) Chatt, J.; Dilworth, J. R.;
Gunz, H. P.; Leigh, G. J. J. Organomet. Chem. 1974, 64, 245-254. (e)
Bond, A. M.; Colton, R.; McDonald, M. E. Inorg. Chem. 1978, 17, 2842-
2847. (f) Reiman, R. H.; Singleton, E. J. Organomet. Chem. 1973, 59, 309-
315. (g) Jolly, P. W.; Stone, F. G. A. J. Chem. Soc. 1965, 5259-5261. (h)
Leins, A. E.; Coville, N. J. J. Organomet. Chem. 1991, 407, 359-367. (i)
Reimann, R. H.; Singleton, E. J. Chem. Soc., Dalton Trans. 1973, 841-
845.
(27) (a) Nageswara Rao, B. D.; Anders, L. R. Phys. ReV. 1965, 140,
A112. (b) Bloyce, P. E.; Rest, A. J.; Whitwell, I.; Graham, W. A. G.;
Holmes-Smith, R. J. Chem. Soc., Chem. Commun. 1988, 846-848.
(28) Klooster, W. T.; Koetzle, T. F.; Jia, G.; Fong, T. P.; Morris, R. H.;
Albinati, A. J. Am. Chem. Soc. 1994, 116, 7677-7681.
(29) Zilm, K. W.; Millar, J. M. AdV. Magn. Opt. Reson. 1990, 15, 163-
200.

Cationic Hydrogen Complexes of Rhenium
J. Am. Chem. Soc., Vol. 119, No. 18, 1997 4177
Table 4. IR Bond Stretching Frequencies of Related Carbonyl
Complexes
The electron density at the metal center is not only a factor
in the stability of these complexes toward anions and oxidation,
but also upon the reactivity of the dihydrogen ligand. Table 5
outlines the trends of dihydrogen lability, homolytic cleavage,
and heterolytic cleavage for several closely related complexes.
Greater backbonding from a more electron rich metal center
contributes both to a greater tendency to homolytically cleave
the H2 bond and to tighter binding to the metal center. The
neutral W complexes have been observed to have an equilibrium
between bound dihydrogen and dihydride, and yet the hydrogen
in this system is quite labile. Contrary to this, the cationic
isonitrile complex of rhenium, 5, binds hydrogen more strongly,
relative to the agostic CH bond, as determined by direct
competition studies, yet there has been no evidence that
oxidative addition to the dihydride complex occurs.
Certainly, there are other factors that will also affect the
lability and homolytic cleavage of a dihydrogen molecule. Since
these systems have been observed to interact with the phosphine
through a pendant C-H bond upon loss of the dihydrogen
ligand, the strength of the agostic interaction must also be
considered. The strength of the agostic bond can only be
measured indirectly and is presumed to be approximately 10 (
6 kcal/mol for W(PCy3)2(CO)3.10g Presumably the strength of
the agostic bond for 3a and 4 would not be significantly different
from this. Another factor that will affect the oxidative addition
of the dihydrogen ligand to a dihydride structure is a significant
rearrangement of the metal center. This transformation results
in a formal oxidation of the metal center by two electrons and
a structural change from a six-coordinate, octahedral complex
to a seven-coordinate complex. How these factors will affect
the observed reactivity of the dihydrogen ligand is difficult to
ascertain, but it is clear that there is more involved than a simple
measure of electron density at the metal center.
The acidity of the dihydrogen ligand appears to fall much
more within the expected trends based on electron density at
the metal center. Both W(H2)(PCy3)2(CO)3 and 5 are only
weakly acidic, and strong bases such as a copper alkoxide10m
and KOtBu,16 respectively, are used to deprotonate the dihy-
drogen complexes. As expected, the dihydride complex [ReH2-
(PMePh2)4(CO)]+ is also weakly acidic and could only be
deprotonated with KOH.30b Complexes with lower phosphine/
carbonyl ratios are easily deprotonated by NEt3 or other nitrogen
bases.30 For the reactivity of [Re(H2)(PCy3)2(CO)3]+ (2a) with
bases, we have observed three different possibilities. Sterically
protected nitrogen complexes that are relatively strong bases
will easily deprotonate 2a. Less sterically demanding nitrogen
bases, such as NH3 and aniline, have been observed to displace
hydrogen from 2a to form new adducts.17 Weaker bases, such
as pentafluoropyridine and 2,6-diisopropylaniline, show no
reactivity with 2a.
Hydrogen-Binding Competition Studies. We have been
interested for some time in the influence of the charge of a
transition metal complex upon the binding of dihydrogen. Both
[Re(H2)(PCy3)2(CO)3]B(Ar′)4 (2a) and W(H2)(PCy3)2(CO)3 have
been demonstrated to be labile toward H2 dissociation. Also
in both instances a stable, formally unsaturated complex is
generated that will re-add H2 to form the dihydrogen complex.
Very detailed studies have been conducted by Kubas, Hoff, and
co-workers to determine the relative energetics of H2 binding
to M(PR3)2(CO)3 (M ) W, Mo, Cr; R ) Cy, iPr).10g-l A direct
comparison between 2a and W(H2)(PCy3)2(CO)3 has been
hindered by the lack of a common solvent system that will allow
compd
ν
CO (cm-1, Nujol)
[Re(CO)4(PPh3)2]+ (6)
[Re(CO)4(PCy3)2]+ (7)
[Re(CO)(PCy3)2(CNtBu)3]+ (8)
W(CO)4(PCy3)2
2002
1983
1900
1870
observed in the [Re(H2)(PR3)2(CO)3]+ system that the choice
of acid to form the dihydrogen complexes is crucial to their
stability. The labile dihydrogen ligands in [Re(H2)(PR3)2-
(CO)3]+ (2) can be easily displaced by anionic ligands such as
chloride and triflate.17 Even BF4-, which is often considered
an unreactive anion, led to the decomposition of 2a at low
temperature.12 The use of the less reactive anion, B(Ar′)4-, has
allowed the isolation of a stable series of dihydrogen complexes,
2a-d.
The nature of the ligands as well as the charge on the metal
complex can be used to predict the expected backbonding to
the H2 ligand. Isonitrile ligands are expected to be better donors
than carbonyls, and the donation from the alkyl phosphines
should be greater than that from aryl phosphines. Neutral
complexes should also be more electron rich than cationic
complexes. Table 4 lists the carbonyl stretching frequencies
of a series of compounds in order to demonstrate these trends.
There is now a large series of closely related cationic rhenium
dihydrogen complexes which have been studied by several
different research groups.30 We have attempted to draw
comparisons based on the reactivity of these complexes as
summarized in Table 5. The stability toward various anions is
apparently a function of the electron density at the metal center.
When the carbonyl ligands of 2a are replaced by the less π
acidic isonitrile ligands, the reactivity is dramatically different.
Not only is [Re(H2)(PCy3)2(CNtBu)3]+ stable with a wider
variety of anions, but the cationic dihydrogen complex can also
be formed by chloride displacement from ReCl(PCy3)2(CNtBu)3
by H2.16 Complexes with a higher phosphine-to-carbonyl ratio
such as [(triphos)Re(H2)(CO)2]+ 30c and [ReH2(PR3)4(CO)]+ 30a,b
-
are also stable toward more nucleophillic anions such as BF4
and CF3COO-. Three of the complexes listed in Table 5 are
reported to be thermally unstable and have only been investi-
1
gated by low-temperature H NMR spectroscopy. It is likely
that the anions used in these complexes could contribute to their
thermal instability.
The series of rhenium carbonyl complexes [ReL(PR3)2-
(CO)3]+ are also surprisingly stable in halogenated solvents such
as methylene chloride, chloroform, and Freon-21 (CHFCl2). The
analogous group 6 complexes, ML(PR3)2(CO)3 (M ) W, Mo,
Cr), must be studied in toluene, although it has been shown
that W(H2)(PiPr3)2(CO)3 can briefly withstand dissolution in
CD2Cl2 at low temperature (<-20 °C).26 Even the rhenium
isonitrile complexes [ReL(PCy3)2(CNtBu)3]+ are only moder-
ately stable in halogenated solvents.16 Both of these systems
are more electron rich than the rhenium carbonyl complexes
and are presumably more susceptible to oxidation. This is
evidenced by the isolation and characterization of a stable Re-
(II) chloride complex, [ReCl(PCy3)2(CNtBu)3]+, which forms
by decomposition of [Re(PCy3)2(CNtBu)3]+ in chlorinated
solvents (CD2Cl2, CDCl3, CDFCl2).16 Similarly, W(PCy3)2-
(CO)3 can also be oxidized by one electron to form a neutral
17 electron halide complex, WI(PCy3)2(CO)3.10n
(30) (a) Gusev, D. G.; Nietlispach, D.; Eremenko, I. L.; Berke, H. Inorg.
Chem. 1993, 32, 3628-3636. (b) Luo, X.-L.; Michos, D.; Crabtree, R. H.
Organometallics 1992, 11, 237-241. (c) Bianchini, C.; Marchi, A.; Marvelli,
L.; Peruzzini, M.; Romerosa, A.; Rossi, R.; Vacca, A. Organometallics 1995,
14, 3203-3215.
(31) 2a is insoluble in nonpolar solvents such as benzene, toluene, and
heptane. W(H2)(PCy3)2(CO)3 and W(PCy3)2(CO)3 are generally reactive with
halogenated solvents except solvents such as C7F8, C6F6, and 1,2-C6F2H4
in which they are not soluble.

4178 J. Am. Chem. Soc., Vol. 119, No. 18, 1997
Table 5. Reactivity of Related Dihydrogen Complexes
Heinekey et al.
compd
H2 lability
labile
labile
slowly labile
med. labile
thermally unstable
thermally unstable
thermally unstable
no H2 loss
halogentated solvent
dihydride formation
equilibrium
none
none
none
anions
a
W(H2)(PCy3)2(CO)3
stable < -20 °C
stable
stable for a few days
stable
-
[Re(H2)(PCy3)2(CO)3]+ (2a)
[Re(H2)(PCy3)2(CNtBu)3]+ (5)
[triphosRe(H2)(CO)2]+ b
[Re(H2)(PMe3)2(CO)3]+ c
[Re(H2)(PMe3)3(CO)2]+ c
[Re(H2)(PMePh2)3(CO)2]+ d
[ReH2(PMePh2)4(CO)]+ d
[ReH2(PMe3)4(CO)+ c
B(Ar′)4
Cl-, OTf-, BF4-, B(Ar′)4
-
-
BF4
none
none
CF3COO-
CF3COO-
-
equil/dihydride at higher T
dihydride
equil/dihydride at RT
BF4
BF4
-
stable
stable
no H2 loss
CF3COO-
a References 1 and 10. b Reference 30c. c Reference 30a. d Reference 30b.
both solubility and stability to the complexes.31 However,
comparison studies to a related complex, [Re(H2)(PCy3)2-
(CNtBu)3]+ (5), have become possible.
solid state. All of the 18-electron complexes studied thus far
have been observed to be pale yellow or colorless in solution
and in the solid state. Similar observations were made for [Re-
(PR3)2(CNtBu)3]+ and W(PR3)2(CO)3: both are dark purple in
solution and as solids, and the 18-electron complexes are yellow
or colorless.
Since the acid that we have used in synthesizing all of the
cationic rhenium complexes is an etherate salt, [H(Et2O)2]B-
(Ar′)4, we were interested in showing that the complex is not
an ether adduct, considering the moderate to strong binding
observed with H2O and THF.17 When analytically pure [Re-
(PCy3)2(CO)3]B(Ar′)4 is placed in CD2Cl2 there are no reso-
nances that can be associated with diethyl ether and no change
to the other proton resonances is observed. The 31P{1H} NMR
chemical shift is likewise invariant in the presence or absence
of ether.
The results comparing 4 and 3a (eq 3) are unambiguous, since
there is no indication that either of the agostic complexes are
solvated by methylene chloride. Molecular hydrogen clearly
binds preferentially to the isonitrile complex (4). Complexes
4 and 2a are never detected in solution at the same time.
1
Assuming a minimum detection limit of 1% for the H and
31P{1H} NMR spectra, a lower limit for the equilibrium constant
can be calculated for the reaction shown in eq 3. The Keq is
calculated to be g9800, which corresponds to ∆G°298 e -5.4
kcal/mol. We therefore are able to conclude that, at 298 K, H2
prefers [Re(PCy3)2(CNtBu)3]+ (4) over [Re(PCy3)2(CO)3]+ (3a)
by at least 5.4 kcal/mol.
The obvious difference between the complexes is the π-acid-
ity of their respective coligands. CO is a much better π-acid
than CNtBu. Since the H2 ligand is known to act not just as a
σ-donor by also as a π-acceptor, presumably it is the difference
of π-bond strengths in 5 and 2a that is accounting for these
observations. In the presence of the less π-acidic CNtBu
coligands, there is much more accessible π-electron density for
donation into the σ* orbital of the H2 ligand. This can only be
expected to strengthen its overall binding to the metal center.
Competition studies between 5 and W(H2)(PCy3)2(CO)3 were
limited to THF, the only common solvent in which all species
are soluble and stable. Complex 4 has been observed to not
have any significant interaction with THF17 and W(PCy3)2(CO)3
has been shown to weakly bind THF,10k allowing an indirect
comparison of the H2 binding ability of 4 and W(PCy3)2(CO)3.
We have observed that molecular hydrogen binds preferentially
to 4, in THF solution, and that 4 and W(H2)(PCy3)2(CO)3 are
never detected in solution at the same time. There is a
qualitative increase in H2 binding to 5 as compared to W(H2)-
(PCy3)2(CO)3.
Synthesis and Characterization of [Re(PR3)2(CO)3]B(Ar′)4.
In the solid state, [Re(PCy3)2(CO)3]B(Ar′)4 exhibits a distorted
octahedral structure in which a â C-H bond from one of the
cyclohexyl rings is interacting with the electron-deficient metal
center.15 This structure is very similar to the reported structure
of the neutral tungsten analog, W(PCy3)2(CO)3, and is consistent
with the observed agostic interaction of W(PiPr3)2(CO)3.10c We
have synthesized several other similar compounds of the general
formula Re(PR3)2(CO)3+, where PR3 ) PiPr3, PiPrPh2, and
PPh3, that have not been structurally characterized. We were
interested in investigating the nature of this agostic complex in
solution by NMR spectroscopy methods. Based on our reactiv-
ity studies, this complex has been observed to react with and
coordinate a wide variety of small molecules in varying
strengths.17 Reaction of the agostic species with small molecules
to give 18-electron complexes is accompanied by a dramatic
color change. The agostic complex in methylene chloride
solution is bright orange and forms dark orange crystals in the
We have also searched for evidence of any interaction with
the solvent, which in most of our studies is methylene chloride.
There have been several recent reports on coordination of
methylene chloride as well as complexes that oxidatively add
CH2Cl2 through a C-Cl bond to give chloromethyl chloride
complexes. There are three reports of isolable methylene
chloride complexes that have been structurally characterized,32
as well as others that have been identified only in solution.33
Since the structure of [Re(PCy3)2(CO)3]B(Ar′)4 which was
crystallized from CH2Cl2/pentane did not contain any solvent
of crystallization much less a coordinated CH2Cl2, we were
interested in looking for evidence of a solvent interaction in
solution. Gladysz and co-workers have shown that an errant
peak in the 13C{1H} NMR spectrum of [Cp*Re(NO)(PPh3)(ClCH2-
Cl)]BF4 is associated with a bound CH2Cl2 and can be
distinguished from the resonance for free CH2Cl2.33a We
investigated the low temperature 13C{1H} NMR spectra of [Re-
(PR3)2(CO)3]B(Ar′)4 (PR3 ) PCy3 and PiPrPh2) in CH2Cl2 and
did not observe any peaks that could be attributed to bound
solvent. The signals due to the CO ligands are identical with
those reported for the room temperature spectrum. We also
1
find that the H and 13C NMR resonances due to the B(Ar′)4
anion are entirely independent of the nature of the cation present.
The solid-state structure of 3a indicates an interaction between
the metal center and a pendant C-H bond of a single phosphine
ligand. If the inequivalent phosphine ligands could be observed
by solution 31P{1H} NMR spectroscopy, a spectrum with two
different doublet resonances with a large JPP should result. At
room temperature only one phosphorus resonance for [Re(PR3)2-
(CO)3]B(Ar′)4 (3a-d) was observed. It is likely that the agostic
complexes are quite fluxional and exchange occurs between the
(32) (a) Newbound, T. D.; Colsmann, M. R.; Miller, M. M.; Wulfsberg,
G. P.; Anderson, O. P.; Strauss, S. H. J. Am. Chem. Soc. 1989, 111, 3762-
3764. (b) Arndtsen, B. A.; Bergman, R. G. Science 1995, 270, 1970-1973.
(c) Butts, M. D.; Scott, B. L.; Kubas, G. J. J. Am. Chem. Soc. 1996, 118,
11831-11843.
(33) (a) Winter, C. H.; Gladysz, J. A. J. Organomet. Chem. 1988, 354,
C33-C36. (b) Beck, W.; Schloter, K. Z. Naturforsch. 1978, 33B, 1214.

Cationic Hydrogen Complexes of Rhenium
J. Am. Chem. Soc., Vol. 119, No. 18, 1997 4179
two phosphine ligands. If the limiting chemical shifts for the
agostic proton and the adjacent carbon could be frozen out, the
resonances would be expected to appear at higher field in the
1H and 13C{1H} NMR than the other protons and carbons of
the alkyl and aryl groups. The agostic hydrogen would have
an intermediate chemical shift between the alkane CH bond and
Chart 1
1
a metal hydride. No such resonances were detected in the H
and 13C{1H} NMR spectra at the lowest accessible temperatures.
The 31P{1H} NMR spectra of 3b-d did not show any change
when cooled to the freezing point of methylene chloride.
In contrast, the 31P{1H} NMR spectrum for [Re(PCy3)2-
(CO)3]B(Ar′)4 shows a dramatic change upon cooling. At room
temperature, [Re(PCy3)2(CO)3]B(Ar′)4 shows a single broad
resonance at 27.6 ppm in CD2Cl2. The spectrum decoalesces
at 240 K into an AB pattern and a broad resonance. The AB
spectrum (δ ) 42.2 and 32.1 ppm) shows a large coupling of
93 Hz, consistent with a species with inequivalent trans
phosphines. The broad resonance that is observed at 24 ppm
integrates as 50% of the spectrum. Our investigations of the
analogous isonitrile chemistry led to similar observations for
[Re(PCy3)2(CNtBu)3]+. The 31P{1H} NMR resonance at 26.4
ppm broadens when the temperature is lowered and decoalesces
at 220 K into an AB pattern with a broad resonance. The AB
resonances at 32.6 and 24.6 ppm show a large equivalent
coupling (Jpp ) 144 Hz) and the broad resonance at 25.5 ppm
accounts for 50% of the spectrum. Decoalesence has not been
observed in the 31P{1H} NMR spectra for [Re(PR3)2(CO)3]B-
(Ar′)4 (PR3 ) PiPr3, PiPrPh2, PPh3) or [Re(PPh3)2(CNtBu)3]+.
Recent reports by Caulton and co-workers have also docu-
mented very similar observations in 5- and 6-coordinate Ru and
Ir systems.34 These complexes contain trans PtBu2Me ligands
which have been observed to decoalesce into similar patterns
at low temperature. Significantly, they have explained the
appearance of an AB pattern as a rotational conformer in which
the phosphines are still trans yet are inequivalent and another
resonance as a conformer in which the phosphines are related
by a mirror plane of symmetry. These conformers have been
formed as a result of freezing out the rotation along the M-P
single bond. Newman projections can show the two conformers
that may account for the 31P NMR spectrum in our systems. In
the Ru and Ir systems the alkyl groups of the phosphine are
not all the same; therefore, various rotamers are presented in
which the R groups of the phosphines are eclipsed to one
another. The rotamer in which the R groups are related by a
plane of symmetry possesses equivalent phosphines, while the
non-mirror-symmetric rotamer accounts for the inequivalent
phosphines. With tricylcohexylphosphine it is necessary to
consider a staggered conformation to explain the inequivalent
phosphines.
structures. For the structures of the five-coordinate agostic
complexes, [Re(PCy3)2(CO)3]B(Ar′)4,15 W(PCy3)2(CO)3,10c and
W(PiPr3)2(CO)3,10c the phosphines are arranged in a staggered
formation (structure A). For the structurally characterized six-
coordinate species in this series; ReCl(PCy3)2(CNtBu)3,16 [ReCl-
(PCy3)2(CNtBu)3]OTf,16 W(H2)(PiPr3)2(CO)3,1 and W(OH2)-
(PiPr3)2(CO)3,10k the alkyl groups of the phosphines are arranged
in an eclipsed formation (structure B).
Several other complexes, [ReL(PCy3)2(CNtBu)3]+ (L ) H2,
NH3, C2H4) and ReX(PCy3)2(CNtBu)3 (X ) H, Cl, I),16,17 have
been investigated by low-temperature 31P{1H} NMR spectros-
copy and shown to decoalesce into similar spectra. Interestingly,
similar observations have not been made in the carbonyl system
for [ReL(PCy3)2(CO)3]B(Ar′)4 (L ) N2, NH3, H2, C2H4) and
ReCl(PCy3)2(CO)3.17 Caulton and co-workers have shown that
the size of the meridinal groups is a factor in the freezing out
of the conformational isomers in addition to the bulkiness of
the phosphines. The isonitrile ligands are considerably bulkier
than the carbonyl ligands, which could explain why complexes
with isonitrile coligands tend to decoalesce at higher temper-
atures. Also it is quite interesting that the decoalescence of
the [Re(PCy3)2(CO)3]B(Ar′)4 would occur at reasonable tem-
peratures while it is unobserved for the 18-electron carbonyl
complexes, [ReL(PCy3)2(CO)3]+ and ReX(PCy3)2(CO)3, we
have studied thus far in this system. This would indicate that
the agostic interaction has some bearing upon increasing the
barrier to rotation of the M-P bond. The reported ∆G‡240 (10.4
kcal/mol) for the decoalescence of the 31P{1H} NMR spectrum
of [Re(PCy3)2(CO)3]B(Ar′)4 does not directly reflect the strength
of the agostic bond, as previously suggested.15
Conclusions
Several dihydrogen complexes have been synthesized by
protonation of neutral methyl and hydride complexes of rhenium
under H2. The complexes have short H-H bonds as evidenced
by the low T1min values and large HD coupling constants. The
thermal stability of the dihydrogen complexes as well as the
A staggered orientation of the cyclohexyl groups of the
phosphines would render them inequivalent as viewed in
structure A. This would lead to the observation of two chemical
shifts and large coupling between the inequivalent, mutually
trans phosphines (AB pattern). Structure B, which shows an
eclipsed pattern for the cyclohexyl groups, would have equiva-
lent phosphines and corresponds to the broad resonance observed
in the spectrum. These observations are also quite consistent
with those of Kubas and co-workers for the low-temperature
31P{1H} NMR spectra of M(PCy3)2(CO)3 (M ) Mo, W).35
Structures A and B have also been observed in X-ray crystal
agostic complexes that result from H2 loss is attributed to the
-
noncoordinating anion, B(Ar′)4
.
Although W(H2)(PCy3)2(CO)3 is in equilibrium with a dihy-
dride structure, the cationic analog [Re(H2)(PCy3)2(CO)3]+
exclusively adopts the dihydrogen structure, in spite of a similar
binding strength of H2. The cationic complex is expected to
backbond less to the σ* of the dihydrogen ligand and compen-
sates by a greater electrophillic interaction with the σ bond.
Direct hydrogen binding studies between [Re(H2)(PCy3)2-
(CNtBu)3]+ and W(H2)(PCy3)2(CO)3 as well as [Re(H2)(PCy3)2-
(CO)3]+ show that the less π acidic CNtBu ligands increase
the H2 binding strength by at least 4-5 kcal/mol. This effect
is mainly attributed to a greater backbonding to the σ* of the
H2 ligand.
(34) (a) Notheis, J. U.; Heyn, R. H.; Caulton, K. G. Inorg. Chim. Acta
1995, 229, 187-193. (b) Poulton, J. T.; Sigala, M. P.; Eisenstein, O.;
Caulton, K. G. Inorg. Chem. 1993, 32, 5490-5501. (c) Poulton, J. T.;
Folting, K.; Streib, W. E.; Caulton, K. G. Inorg. Chem. 1992, 31, 3190-
3191.
Tricyclohexylphosphine complexes such as [Re(L)(PCy3)2-
(35) Kubas, G. J. Personal communication, 1993.
(CNtBu)3]+ and [Re(L)(PCy3)2(CO)3]+ (L ) agostic or other

4180 J. Am. Chem. Soc., Vol. 119, No. 18, 1997
Heinekey et al.
ligands) exhibit complex 31P{1H} NMR spectra at low temper-
ature as a result of hindered rotation about the Re-P bond. The
bulkier isonitrile ligands contribute to a greater barrier to rotation
compared to the carbonyl complexes. Similar complexes with
PPh3, PiPrPh2, and PiPr3 ligands do not show hindered rotation
at accessible temperatures.
°C. Further recrystallizations from pentane gave “analytically pure”
solid that was free from ReH(PiPr3)2(CO)3.
trans-mer-Re(CH3)(PCy3)2(CO)3 (1a): Yield 53%. 1H NMR
(C6D6) δ -0.05 (t, 3JPH ) 6.0 Hz, 3 H, Re-CH3); 0.9 to 2.5 (m, 66 H,
PCy3). 31P{1H} NMR (C6D6) δ 10.2 (s).
trans-mer-Re(CH3)(PiPr3)2(CO)3 (1b): Yield 65%. 1H NMR
(C6D6) δ -0.12 (t, 3JPH ) 5.9 Hz, 3H, ReCH3); 1.17 (dd, 3JPH ) 12.9
Hz, 3JHH ) 7.0 Hz, 36H, P(CH(CH3)2)3); 2.31 (m, 6H, P(CH(CH3)2)3).
Experimental Section
2
13C{1H} NMR (C6D6) δ -30.0 (t, JPC ) 7.3 Hz, ReCH3); 19.7 (s,
General Considerations. Due to the extreme air and moisture
sensitivity of some of the organometallic products, manipulations were
conducted with rigorous exclusion of air and water. Solid samples
were handled and stored under argon in Vacuum Atmosphere or Braun
inert-atmosphere boxes. Solution samples were handled by using
standard vacuum line or Schlenk techniques. Chlorinated solvents were
predried by distillation from P2O5 or CaH2 under argon and stored under
vacuum over activated silica gel (activated by heating at 320 °C under
dynamic vacuum for 4 h) in glass vessels equipped with a Teflon needle
valve. Hydrocarbon solvents were predried by distillation from Na/K
alloy/benzophenone under argon and stored under vacuum over
activated silica gel in glass vessels equipped with a Teflon needle valve.
Deuterated solvents (Cambridge Isotope Labs) were dried and stored
in a manner similar to their protio analogs. All solvents were subjected
to 3 freeze-pump-thaw cycles and vacuum transferred immediately
prior to use.
P(CH(CH3)2)3); 26.7 (t, AXX′, JPC + JP′C ) 11.4 Hz, P(CH(CH3)2)3);
197.3 (t, JPC ) 7.2 Hz, CO); 203.8 (t, JPC ) 8.8 Hz, CO). 31P{1H}
NMR (C6D6) δ 19.9 (s). IR (cm-1, Nujol, νCO): 2004 (w), 1902 (s),
1866 (m). Anal. Calcd (found): C, 43.62 (43.19); H, 7.49 (7.21).
trans-mer-Re(CH3)(PiPrPh2)2(CO)3 (1c): Yield 53%. 1H NMR
(CDCl3) δ -1.12 (t, 3JPH ) 6.6 Hz, 3H, ReCH3); 1.11 (dd, 3JPH ) 14.9
Hz, 3JHH ) 7.0 Hz, 12H, P(CH(CH3)2)Ph2); 2.91 (sept, 3JHH ) 7.0 Hz,
2H, P(CH(CH3)2)Ph2); 7.38 (m, 12H, PiPrPh2 meta and para); 7.51 (m,
2
8H, PiPrPh2 ortho). 13C{1H} NMR (CDCl3) δ -25.7 (t, JPC ) 6.6
Hz, ReCH3); 19.5 (s, P(CH(CH3)2)Ph2); 28.9 (t, AXX′, JPC + JP′C
)
14.0 Hz, P(CH(CH3)2)Ph2); 129.0 (vt, JPC ) 4.3 Hz, PiPrPh2 ortho or
meta); 130.6 (PiPrPh2para); 134.4 (t, AXX′, JPC + JP′C ) 20.9 Hz,
PiPrPh2 ipso); 134.7 (vt, JPC ) 4.8 Hz, PiPrPh2 ortho or meta); 197.1
2
2
(t, JPC ) 6.7 Hz, CO trans to CH3); 200.4 (t, JPC ) 9.0 Hz, CO cis
to CH3). 31P{1H}NMR (CDCl3) δ 19.7 (s). IR (cm-1, CH2Cl2, νCO):
2023 (w), 1916 (s), 1872 (m).
trans-mer-Re(CH3)(PPh3)2(CO)3 (1d): Yield 90%. 1H NMR (CDCl3)
δ -1.06 (t, 3JPH ) 6.7 Hz, 3H, Re-CH3); 7.29 (m, 18H, PPh3 meta and
para); 7.49 (m, 12H, PPh3 ortho). 13C{1H} NMR (CDCl3) δ -22.3 (t,
2JPC ) 6.3 Hz, Re-CH3); 128.0 (vt, JPC ) 4.5 Hz, PPh3 ortho or meta);
129.6 (s, PPh3 para); 133.6 (vt, JPC ) 5.6 Hz, PPh3 ortho or meta);
135.3 (t, AXX′, JPC + JP′C ) 23.2 Hz, PPh3 ipso); 195.4 (br,CO trans
Reagent grade chemicals were used as received unless stated
otherwise. Re(CH3)(CO)5 and ReH(CO)5 were prepared from Re2-
(CO)10 (Strem) by using literature methods.36,37 The synthesis and
characterization of [Re(PCy3)2(CNtBu)3]+ (4), [Re(H2)(PCy3)2(CNtBu)3]+
(5), [Re(CO)4(PPh3)2]+ (6), [Re(CO)4(PCy3)2]+ (7), and [Re(CO)(PCy3)2-
(CNtBu)3]+ (8) have been reported separately.16,17 W(PCy3)2(CO)3 was
prepared by a published procedure.10r PCy3 (Strem) was recrystallized
from ethanol, PiPrPh2 and PPh3 (Aldrich) were used as received, and
PiPr3 (Strem) was degassed and stored under argon. [H(Et2O)2]B(Ar′)4
(Ar′ ) 3,5-(CF3)2C6H3) was prepared by the method of Brookhart.13
Hydrogen (Airco, 99.999%) and deuterium (Cambridge, 99.8%) were
used as received.
2
to CH3); 199.0 (t, JPC ) 9.3 Hz, CO cis to CH3). 31P{1H} NMR
(CDCl3) δ 16.8 (s). IR (cm-1, CH2Cl2, νCO): 2023 (w), 1916 (s), 1872
(m).
trans-mer-Re(H)(PiPrPh2)2(CO)3 (1e): Yield 75%. 1H NMR
2
3
(C6D6) δ -5.55 (t, JHP ) 18.8 Hz, Re-H); 1.08 (dd, JPH ) 16.2 Hz,
3JHH ) 6.9 Hz, P(CH(CH3)2)Ph2); 2.61 (m, P(CH(CH3)2Ph2); 6.94 (m,
PiPrPh2 meta and para); 7.74 (m, PiPrPh2 ortho). 13C{1H} NMR (CD2-
Cl2) δ 18.6 (s, P(CH(CH3)2)Ph2); 29.4 (t, AXX′, JPC + JP′C ) 15.6 Hz,
P(CH(CH3)2)Ph2); 128.2 (s, PiPrPh2 ortho or meta); 129.7 (s, PiPrPh2
1H, 13C{1H}, and 31P{1H} NMR spectra were recorded on Bruker
1
AC-200 (200.133 MHz H, 81.015 MHz 31P), AF-300 (300.117 MHz
1
1H, 75.465 MHz 13C) and WM-500 (500.136 MHz H) spectrometers
equipped with B-VT 1000 temperature controller modules with copper-
constantan thermocouples. Temperature calibration was accomplished
following the Van Geet methanol calibration method.38 1H and 13C
NMR chemical shifts (δ) are referenced to the internal residual proton
or natural abundance 13C resonances of the deuterated solvent relative
to TMS. 31P{1H} NMR chemical shifts (δ) are reported in parts per
million relative to 85% H3PO4 (external standard). All NMR tube
reactions were conducted in flame-sealed tubes or J. Young screw-cap
tubes. T1 measurements were performed on a Bruker WM-500
spectrometer, using a standard 180°-τ-90° inversion-recovery pulse
sequence.
para); 133.3 (s, PiPrPh2 ortho or meta); 137.0 (t, AXX′, JPC + JP′C
)
20.8 Hz, PiPrPh2 ipso); 177.8 (m, CO trans to H); 179.1 (m, CO cis to
H). 31P{1H} NMR (C6D6): 29.6 (s). IR (cm-1, CH2Cl2, νCO): 2020
(w), 1919 (s).
trans-mer-Re(H)(PPh3)2(CO)3 (1f): Yield 77%. 1H NMR (C6D6)
2
δ -4.45 (t, JPH ) 17.8 Hz, Re-H); 7.03 (m, PPh3 meta and para);
7.84 (m, PPh3 ortho). 31P{1H} NMR (C6D6) δ 22.8 (s). IR (cm-1
CH2Cl2, νCO): 2020 (w), 1925 (s).
,
[trans-mer-Re(H2)(PCy3)2(CO)3]B(Ar′)4 (2a). A small glass vessel
with an 8 mm Kontes tap was charged with Re(CH3)(PCy3)2(CO)3 (200
mg, 0.236 mmol) and [H(Et2O)2]B(Ar′)4 (239 mg, 0.236 mmol).
Methylene chloride (4 mL) was vacuum transferred to the vessel, and
the solution was warmed to room temperature with stirring under an
atmosphere of H2 for 1 h. The solvent volume was reduced by half,
and 8 mL of pentane was transferred to the solution. The solution
was warmed to room temperature under a H2 atmosphere, and a yellow
precipitate formed immediately. After being stirred for 45 min, the
solvent was removed via cannula under a H2 flow. The solid was briefly
placed under dynamic vacuum and then dried under a H2 atmosphere.
The dihydrogen complex was recovered in 73% yield (292 mg) and
stored in a sealed ampule under a slight H2 atmosphere. 1H NMR (CD2-
Cl2) δ -4.75 (br, 2H); 1.30-1.95 (br, 60H); 2.16 (br, 6H); 7.57 (s,
4H, B(Ar′)4 para); 7.73 (s, 8H, B(Ar′)4 ortho). 13C{1H} NMR (CD2-
Cl2) δ 26.3 (s, P-δ-C); 27.6 (t, JPC ) 4.8 Hz, P-â-C); 30.6 (s, P-γ-C);
The 1H and 13C{1H} resonances for B(Ar′)4- are identical with those
reported for complex 2a and have been omitted from the spectral
characterization of subsequent complexes.
Infrared spectra were recorded on a Perkin Elmer 1600 series FT
spectrophotometer as Nujol mulls or in solution. Elemental analyses
were performed by Galbraith Laboratories, Inc., Knoxville, TN, and
Canadian Microanalytical Service Ltd., Delta, BC.
Synthesis of Neutral Methyl and Hydride Complexes (1a-f). A
thick-walled glass vessel, equipped with a Kontes valve, was charged
with 20 mL of toluene, Re(CH3)(CO)5 (500 mg, 1.46 mmol) or ReH-
(CO)5 (0.2 mL, 1.40 mmol), and 3 equiv of phosphine. The solution
was cooled to -78 °C, evacuated, and warmed to room temperature
and the procedure was repeated twice. The solution was heated at 130
°C for 50 to 140 h and evacuated every 12 h to remove CO gas
generated during the reaction. In most cases, the toluene was removed
in Vacuo and the resulting yellow residue was washed with pentane to
give a white solid. Complex 1b was considerably more soluble and
was carefully precipitated from a minimum amount of pentane at -40
37.8 (t, JPC ) 12.4 Hz, P-R-C); 117.9 (s, B(Ar′)4p-C); 125.0 (q, JCF
)
272 Hz, B(Ar′)4 CF3); 129.2 (q, JCF ) 32 Hz, B(Ar′)4m-C); 135.2 (s,
B(Ar′)4o-C); 162.1 (q, JBC ) 50 Hz, B(Ar′)4i-C); 189.2 (t, JPC ) 7.0
Hz, CO); 192.9 (t, JPC ) 5.5 Hz, CO). 31P{1H} NMR (CD2Cl2) δ 23.9
(s). IR (cm-1, CH2Cl2, νCO): 2069 (w), 1969 (s), 1944 (m).
[trans-mer-Re(H2)(PiPr3)2(CO)3]B(Ar′)4 (2b). A small glass vessel
with an 8-mm Kontes tap was charged with Re(CH3)(PiPr3)2(CO)3 (75
mg, 0.124 mmol) and [H(Et2O)2]B(Ar′)4 (125 mg, 0.123 mmol).
(36) Beck, W.; Raab, K. Inorg. Synth. 1989, 26, 107-108.
(37) Urbancic, M. A.; Shapley, J. R. Inorg. Synth. 1989, 26, 77-80.
(38) Van Geet, A. L. Anal. Chem. 1970, 42, 679-680.

Cationic Hydrogen Complexes of Rhenium
J. Am. Chem. Soc., Vol. 119, No. 18, 1997 4181
Methylene chloride (3 mL) was vacuum transferred to the vessel, and
the solution was warmed to room temperature with stirring under an
atmosphere of H2 for 10 min. The solvent volume was reduced slightly,
and 3 mL of pentane was transferred to the solution. The solution
was warmed to room temperature under a H2 atmosphere and the
pentane diffused slowly over an hour to form a white microcrystalline
solid. The dihydrogen complex was dried under a H2 flow and
recovered in 89% yield (159 mg). 1H NMR (CD2Cl2) δ -4.97 (br,
2H); 1.31 (m, JHH ) 7.0 Hz, 36H, P(CH(CH3)2)3); 2.49 (m, JHH ) 7.0
Hz, 6H, P(CH(CH3)2)3). 13C{1H} NMR (CD2Cl2) δ 19.9 (s, P(CH-
(CH3)2)3); 28.3 (t, JPC ) 13.3 Hz, P(CH(CH3)2)3); 188.7 (br, CO); 192.0
1H NMR (CD2Cl2) δ 1.30-1.75 (br, 60H); 2.53 (br, 6H). 13C{1H}
NMR (CD2Cl2) δ 26.1 (s, P-δ-C); 27.8 (br, P-â-C); 29.7 (s, P-γ-C);
39.1 (t, JPC ) 11.0 Hz, P-R-C); 190.0 (t, JPC ) 5.5 Hz, CO); 198.9 (t,
JPC ) 7.3 Hz, CO). 31P{1H} NMR (CD2Cl2) δ 27.2 (s). IR (cm-1
,
Nujol, νCO) 2061 (w), 1966 (s), 1939 (m). Anal. Calcd (found): C,
50.33 (50.38); H, 4.64 (4.79).
[trans-mer-Re(PiPr3)2(CO)3]B(Ar′)4 (3b). A 10-mL round bottom
flask was charged with Re(CH3)(PiPr3)2(CO)3 (71 mg, 0.117 mmol)
and [H(Et2O)2]B(Ar′)4 (132 mg, 0.131 mmol) and attached to a swivel
frit apparatus. The swivel frit was attached to a vacuum line and 5
mL of CH2Cl2 was vacuum transferred at -78 °C. The solution was
warmed to room temperature and stirred for 5 min. The solvent was
removed in Vacuo, and the solid was placed under dynamic vacuum
for 30 min. The addition and removal of CH2Cl2 was repeated followed
by drying under vacuum for 1 h. Methylene chloride (3 mL) was
vacuum transferred to the solid followed by 5 mL of pentane. Orange
needles were formed as the pentane diffused into the solution over 2 h
at room temperature. The solid was filtered and washed with 2 × 3
mL of cold pentane. The solid was dried under vacuum for 2 h and
(br, CO). 31P{1H} NMR (CD2Cl2) δ 32.5 (s). IR (cm-1, CH2Cl2, νCO
)
2073 (w), 1974 (s), 1950 (m).
[trans-mer-Re(H2)(PiPrPh2)2(CO)3]B(Ar′)4 (2c). A screw-cap NMR
tube was charged with Re(CH3)(PiPrPh2)(CO)3 (10 mg, 0.013 mmol)
and [H(Et2O)2]B(Ar′)4 (13 mg, 0.013 mmol). CD2Cl2 (0.5 mL) was
vacuum transferred to the tube. The solution was frozen, and the
1
headspace was evacuated and replaced with an atmosphere of H2. H
NMR (CD2Cl2) δ -4.26 (ReH2); 1.17 (m, P(CH(CH3)2)Ph2); 3.04 (m,
P(CH(CH3)2)Ph2); 7.55 and 7.73 (m, PiPrPh2 and B(Ar′)4). 31P{1H}
NMR (CD2Cl2) δ 14.7 (s).
[trans-mer-Re(H2)(PPh3)2(CO)3]B(Ar′)4 (2d). A screw-cap NMR
tube was charged with Re(CH3)(PPh3)2(CO)3 (10 mg, 0.013 mmol) and
[H(Et2O)2]B(Ar′)4 (13 mg, 0.013 mmol). CD2Cl2 (0.5 mL) was vacuum
transferred to the tube. The solution was frozen, and the headspace
was evacuated and replaced with an atmosphere of H2. 1H NMR (CD2-
Cl2) δ -3.8 (br s, ReH2); 6.7 to 8.0 (m, PPh3 and B(Ar′)4). 31P{1H}
NMR (CD2Cl2) δ 8.1 (s).
Reactivity of [Re(H2)(PCy3)2(CO)3]B(Ar′)4 (2a) with Base: For-
mation of ReH(PCy3)2(CO)3. In a typical reaction, 2a (10 mg, 0.006
mmol) with 2,6-di-tert-butyl-4-methylpyridine (2 mg, 0.010 mmol) was
added to a sealable NMR tube attached to a 4-mm Kontes valve.
Methylene chloride-d2 (0.5 mL) was vacuum transferred to the tube
and placed under H2 (400 Torr) before sealing. The solution im-
mediately turned colorless. 1H and 31P{1H} NMR indicate clean
formation of ReH(PCy3)2(CO)3 in addition to the appropriate resonances
due to protonated base. 1H NMR (CD2Cl2) δ -6.66 (t, JPH ) 20.5 Hz,
2H); 1.1-2.1 (br, 66H). 31P{1H} NMR (CD2Cl2) δ 30.6 (s).
[trans-mer-Re(PCy3)2(CO)3]B(Ar′)4 (3a). A 10-mL round bottom
flask was charged with Re(CH3)(PCy3)2(CO)3 (113 mg, 0.133 mmol)
and [H(Et2O)2]B(Ar′)4 (132 mg, 0.131 mmol) and attached to a swivel
frit apparatus. The swivel frit was attached to a vacuum line and 3
mL of CH2Cl2 was vacuum transferred at -78 °C. The solution was
warmed to room temperature and stirred for 15 min. The solvent was
removed in Vacuo, and the solid was placed under dynamic vacuum
for 1 h. The addition and removal of CH2Cl2 was repeated. Methylene
chloride (1 mL) was vacuum transferred to the solid followed by 3
mL of pentane. An orange crystalline solid, which formed upon mixing,
was filtered and washed with 5 mL of pentane. The solid was collected
in 85% yield (115 mg) and stored in a sealed ampule under vacuum.
isolated in 62% yield (106 mg). 1H NMR (CD2Cl2) δ 1.17 (m, JHH
)
7.0 Hz, 36H, P(CH(CH3)2)3); 2.76 (m, JHH ) 7.0 Hz, 6H, P(CH(CH3)2)3).
13C{1H} NMR (CD2Cl2) δ 18.6 (s, P(CH(CH3)2)3); 29.5 (t, JPC ) 11
Hz, P(CH(CH3)2)3); 189.8 (br, CO); 197.8 (t, JPC ) 8.0 Hz, CO).
31P{1H} NMR (CD2Cl2) δ 32.2 (s). IR (cm-1, Nujol, νCO) 2064 (w),
1969 (s), 1950 (m). Anal. Calcd (found): C, 43.62 (43.19); H, 7.49
(7.21).
[trans-mer-Re(PiPrPh2)2(CO)3]B(Ar′)4 (3c). A sealable NMR tube
was charged with Re(CH3)(PiPrPh2)2(CO)3 (15 mg, 0.020 mmol) and
[H(Et2O)2]B(Ar′)4 (20 mg, 0.020 mmol). A 1 mL portion of CH2Cl2
was vacuum transferred to the tube and removed in Vacuo to remove
excess ether from the solution. A 0.5-mL portion of CD2Cl2 was
vacuum transferred to the tube. The sample was degassed by 3 freeze-
pump-thaw cycles before sealing the tube. 1H NMR (CD2Cl2) δ 1.17
(m, P(CH(CH3)2)Ph2); 3.04 (m, P(CH(CH3)2)Ph2); 7.55 and 7.73 (m,
PiPrPh2 and B(Ar′)4). 31P{1H} NMR (CD2Cl2) δ 20.9 (s).
[trans-mer-Re(PPh3)2(CO)3]B(Ar′)4 (3d). A sealable NMR tube
was charged with Re(CH3)(PPh3)2(CO)3 (15 mg, 0.018 mmol) and
[H(Et2O)2]B(Ar′)4 (19 mg, 0.019 mmol). A 1-mL portion of CH2Cl2
was vacuum transferred to the tube and removed in Vacuo to remove
excess ether from the solution. A 0.5-mL portion of CD2Cl2 was
vacuum transferred to the tube. The sample was degassed by 3 freeze-
pump-thaw cycles before the tube was sealed. 1H NMR (CD2Cl2) δ
6.7 to 8.0 (m, PPh3 and B(Ar′)4). 31P{1H} NMR (CD2Cl2) δ 14.6 (s).
Acknowledgment. We thank the Department of Energy,
Office of Basic Energy Science, for support of this research.
We are grateful to Greg Kubas and Ken Caulton for helpful
discussions.
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