Rhenium dihydrogen complexes with isonitrile coligands: Novel displacement of chloride by hydrogen

Article abstract of DOI:10.1021/ja961994p

The syntheses, properties, and characterization of several new complexes, Re(CNtBu)₃(PCy₃)₂H (1a), Re(CNtBu)₃(PR₃)₂Cl [R = Cy, Ph (2a, 2b)], the 17-electron [Re(CNtBu)₃(PCy₃)₂Cl]⁺ (3a), the dihydrogen complex, [Re(CNtBu)₃(PR₃)₂(H₂)]⁺ [R = Cy, Fh (4a, 4b)], the coordinatively unsaturated [Re(CNtBu)₃(PCy₃)₂]⁺ (5a), and [Re(CNtBu)₄(PCy₃)₂]⁺ (6a) are reported. In addition, spectroscopic evidence for the dinitrogen complex [Re(CNtBu)₃(PCy₃)₂(N₂)]⁺ and the dihydrogen complex [Re(CNtBu)₅(H₂)]⁺ is presented. Thermodynamic parameters have been obtained for the equilibrium system, Re(CNtBu)₃(PCy₃)₂Cl (2a) + H₂ ? [Re(CNtBu)₃(PCy₃)₂(H₂)]Cl (4a). ¹H and ³¹P{¹H} NMR studies (CD₂Cl₂) over the temperature range 286-316 K afford values of ΔH° = - 18.0 ± 0.7 kcal/mol, ΔS° = -44 ± 2 eu, and ΔG°₂₉₈ = -4.8 ± 1.3 kcal/mol for this equilibrium. The complexes 4a and 4b are characterized as dihydrogen complexes. Their J(HD) R = Cy, 30.3 Hz; R = Ph, 30.9 Hz) and T(1(min)) values of 8 ms (300 MHz) are consistent with H-H distances of ca. 0.80 A?. [Re(CNtBu)₅(HD)]⁺ is also characterized as a dihydrogen complex based on a J(HD) of 33.4 Hz. This complex could not be isolated at room temperature, even with noncoordinating counteranions, due to the lability of the H₂ ligand. [Re(RMe₃)₅HD]⁺ shows no H-D couplhig and is characterized as a dihydride complex. The unsaturated complex [Re(CNtBu)₃(PCy₃)₂]⁺ (5a) is found to undergo a dynamic isonitrile rearrangement on the NMR time scale. ¹H NMR spin saturation transfer studies over a temperature range of 254-297 K afford the activation parameters ΔH(≠) = 11.4 ± 0.8 kcal/mol, ΔS(≠) = -17.6 ± 1.6 eu, and ΔG₂₉₈(+2=) = 16.6 ± 1.2 kcal/mol. The molecular structures of 2a and 3a have been determined by X-ray crystallography. 2a shows an unusually long Re-Cl bond distance of 2.596(2) A?.

Full text of DOI:10.1021/ja961994p

10792
J. Am. Chem. Soc. 1996, 118, 10792-10802
Rhenium Dihydrogen Complexes with Isonitrile Coligands:
Novel Displacement of Chloride by Hydrogen
D. M. Heinekey,* Mark H. Voges, and David M. Barnhart
Contribution from the Department of Chemistry, UniVersity of Washington, Box 351700,
Seattle, Washington 98195
ReceiVed June 12, 1996X
Abstract: The syntheses, properties, and characterization of several new complexes, Re(CNtBu)3(PCy3)2H (1a),
Re(CNtBu)3(PR3)2Cl [R ) Cy, Ph (2a, 2b)], the 17-electron [Re(CNtBu)3(PCy3)2Cl]+ (3a), the dihydrogen complex,
[Re(CNtBu)3(PR3)2(H2)]+ [R ) Cy, Ph (4a, 4b)], the coordinatively unsaturated [Re(CNtBu)3(PCy3)2]+ (5a), and
[Re(CNtBu)4(PCy3)2]+ (6a) are reported. In addition, spectroscopic evidence for the dinitrogen complex [Re(CNtBu)3-
(PCy3)2(N2)]+ and the dihydrogen complex [Re(CNtBu)5(H2)]+ is presented. Thermodynamic parameters have been
obtained for the equilibrium system, Re(CNtBu)3(PCy3)2Cl (2a) + H2 h [Re(CNtBu)3(PCy3)2(H2)]Cl (4a). 1H and
31P{1H} NMR studies (CD2Cl2) over the temperature range 286-316 K afford values of ∆H° ) -18.0 ( 0.7 kcal/
mol, ∆S° ) -44 ( 2 eu, and ∆G°298 ) -4.8 ( 1.3 kcal/mol for this equilibrium. The complexes 4a and 4b are
characterized as dihydrogen complexes. Their JHD (R ) Cy, 30.3 Hz; R ) Ph, 30.9 Hz) and T1(min) values of 8 ms
(300 MHz) are consistent with H-H distances of ca. 0.80 Å. [Re(CNtBu)5(HD)]+ is also characterized as a dihydrogen
complex based on a JHD of 33.4 Hz. This complex could not be isolated at room temperature, even with non-
coordinating counteranions, due to the lability of the H2 ligand. [Re(PMe3)5HD]+ shows no H-D coupling and is
characterized as a dihydride complex. The unsaturated complex [Re(CNtBu)3(PCy3)2]+ (5a) is found to undergo a
dynamic isonitrile rearrangement on the NMR time scale. 1H NMR spin saturation transfer studies over a temperature
range of 254-297 K afford the activation parameters ∆Hq ) 11.4 ( 0.8 kcal/mol, ∆Sq ) -17.6 ( 1.6 eu, and
∆Gq298 ) 16.6 ( 1.2 kcal/mol. The molecular structures of 2a and 3a have been determined by X-ray crystallography.
2a shows an unusually long Re-Cl bond distance of 2.596(2) Å.
Introduction
dihydrogen complex.6 Kubas and co-workers have shown that
H2 is also competitive with N2 and H2O for binding to M(CO)3-
(PR3)2 (M ) Mo, W; R ) Cy, iPr).7,8 Bianchini has reported
that [Re(CO)2(triphos)]+ [triphos ) MeC(CH2PPh2)3] shows the
following preference for ligands: CO > CH3CN > HCtCR
> H2 > C-H (agostic) > N2.9 Similarly, Milstein and co-
workers have recently reported the ligand preferences of the
sterically congested T-shaped Rh complex, Rh[HC(CH2CH2P-
(tBu)2)2], to be H2 > N2 > C2H4 > CO2.10
To the best of our knowledge, no binding studies have found
H2 to show binding affinity comparable to that of an anionic
ligand. Cationic dihydrogen compounds are described as either
stable to a given counterion or unstable. For example, the
dihydrogen complexes, [Re(CO)3(PR3)2(H2)]+ (R ) Cy, iPr, Ph),
have been recently reported.11 These cationic analogs of the
The ability of dihydrogen to form complexes with transition
metals was first recognized by Kubas and co-workers.1 While
the first isolable tungsten dihydrogen compounds were quite
labile with respect to H2 loss, subsequent work has demonstrated
that quite strong binding of H2 is possible.2 Particularly robust
H2 complexes are frequently formed when the metal carries a
positive charge. Isolation of such complexes requires a careful
choice of counterion and solvent. Anions of low nucleophilicity
are preferred and considerable effort has been expended on the
development of truly noncoordinating anions such as BArf- [Arf
) (3,5-(CF3)2C6H3)4].3,4 Solvent choice is equally important,
especially when polar, coordinating solvents are used. Even
methylene chloride, traditionally considered to be noncoordi-
nating, has demonstrated ligating ability.5
Kubas complexes could only be prepared with the noncoordi-
Several studies have compared the binding strength of
dihydrogen to that of other neutral ligands. In many cases, the
binding of H2 was found to be competitive with N2 binding. In
fact, Morris has shown that the ν(NtN) of dinitrogen complexes
can be used to predict the stability of the corresponding
-
nating anion, BArf-. Other anions, such as triflate (OSO2CF3
)
and chloride, were found to displace hydrogen and bind
irreversibly.
Typically, molecular hydrogen has been considered a weaker
ligand than chloride, especially when coordinated to cationic
metal fragments. Several metal dihydrogen complexes have
been made from metal chloride starting materials. These
examples, however, have all required the driving force of salt
X Abstract published in AdVance ACS Abstracts, October 15, 1996.
(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) For recent reviews on dihydrogen complexes, see: (a) Heinekey, D.
M.; Oldham, W. J., Jr. Chem. ReV. 1993, 93, 913-926. (b) Morris, B. H.;
Jessop, P. G. Coord. Chem. ReV. 1992, 121, 155-289.
(3) Brookhart, M.; Grant, B.; Volpe, A. F., Jr. Organometallics 1992,
11, 3920-3922.
(4) Strauss, S. H. Chem. ReV. 1993, 93, 927-942.
(5) CH2Cl2 complexes that have been characterized include the follow-
ing: [CpMo(CO)3(CH2Cl2)]+ (Beck, W.; Schloter, K. Z. Naturforsch. 1978,
33B, 1214), [Cp*Re(NO)(PPh3)(CH2Cl2)]BF4 (Winter, C. H.; Gladysz, J.
A. J. Organomet. Chem. 1988, 354, C33-C36), Ag2(CH2Cl2)4Pd(OTeF5)4
(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),
and [Cp*Ir(PMe3)(CH3)(ClCH2Cl)]+ (Arndtsen, B. A.; Bergman, R. G.
Science 1995, 270, 1970-1973).
(6) Morris, R. H.; Earl, K. A.; Luck, R. L.; Lazarowych, N. J.; Sella, A.
Inorg. Chem. 1987, 26, 2674-2683.
(7) Gonzales, A. A.; Zhang, K.; Nolan, S. P.; de la Vega, R. L.; Mukerjee,
S. L.; Hoff, Carl, D.; Kubas, G. J. Organometallics 1988, 7, 2429-2435.
(8) Kubas, G. J.; Burns, C. J.; Khalsa, G. R. K.; Van Der Sluys, L. S.;
Kiss, G.; Hoff, C. D. Organometallics 1992, 10, 3390-3404.
(9) Bianchini, C.; Marchi, A.; Marvelli, L.; Peruzzini, M.; Romerosa,
A.; Rossi, R.; Vacca, A. Organometallics 1995, 14, 3203-3215.
(10) Vigalok, A.; Ben-David, Y.; Milstein, D. Organometallics 1996,
15, 1839-1844.
(11) Heinekey, D. M.; Schomber, B. M.; Radzewich, C. E. J. Am. Chem.
Soc. 1994, 116, 4515-4516.
S0002-7863(96)01994-4 CCC: $12.00 © 1996 American Chemical Society

Rhenium Dihydrogen Complexes with Isonitrile Coligands
J. Am. Chem. Soc., Vol. 118, No. 44, 1996 10793
formation (eq 1).12
MCl + NaBF4 + H2 f [M(H2)]BF4 + NaCl
(1)
In this paper, we report the first unaided displacement of
chloride by H2 to form the novel cationic dihydrogen complex,
[Re(CNtBu)3(PCy3)2(H2)]Cl. This is only the second reported
dihydrogen complex with isonitrile coligands.13 By varying the
ratio of phosphines and isonitriles on Re, we have further
explored the suitability of isonitriles as ligands for stabilizing
dihydrogen complexes.
Results
Preparation of Re(CNtBu)3(PR3)2H (1a, 1b). Re(CNtBu)3-
(PCy3)2H (1a) (Cy ) cyclohexyl) was prepared in 78% yield
by heating Re(PCy3)2H7 and CNtBu in toluene at 45 °C for 2
days. NMR data indicate that 1a possesses two sets of isonitrile
ligands in a 2:1 ratio and one hydride ligand showing coupling
to two equivalent phosphines (JHP ) 20.4). While both fac,cis
and mer,trans geometry are consistent with the spectroscopic
data, we favor the mer,trans structure based on the steric bulk
of the phosphine ligands and the structures of 2a and 3a reported
below.
The PPh3 analog of 1a, Re(CNtBu)3(PPh3)2H (1b), has been
previously reported by Jones and Maguire.14 They prepared
1b by reacting Re(PPh3)2H7 with cyclopentadiene to make Re-
(PPh3)2(η4-C5H6)H3, followed by ligand displacement with
CNtBu to give 1b. We find that CNtBu reacts directly with
Re(PPh3)2H7 to afford 1b in 70% isolated yield.
Figure 1. ORTEP projection for compound 2a.
1. Bond distances and angles are given in Table 2 and 3,
respectively.
Reaction of Re(CNtBu)3(PCy3)2Cl (2a) with Chlorocar-
bons. Further reaction of yellow 2a with CH2Cl2, CHFCl2,
CHCl3, or CCl4 affords a pink solution. The qualitative rates
vary from CCl4, which reacts instantly, to CH2Cl2, which requires
several days before the reaction is apparent. Broad, temperature-
dependent resonances suggesting a paramagnetic species are
observed to grow in when the reaction is monitored by 1H NMR.
Pink solid, shown by elemental analysis to be consistent with
the 17-electron [Re(CNtBu)3(PCy3)2Cl]Cl‚3CH2Cl2 (3a-Cl),
could be isolated from the reaction of 2a with CH2Cl2. The
same paramagnetic species was isolated as the triflate salt (3a-
OTf) from the slow reaction of CD2Cl2 with the unsaturated
complex [Re(CNtBu)3(PCy3)2]OTf (5a-OTf) (Vide infra).
Structure of [Re(CNtBu)3(PCy3)2Cl]OTf (3a-OTf). Dif-
fraction quality crystals were grown by layering a CD2Cl2
solution of 3a-OTf with diethyl ether and cooling to -20 °C.
Crystallographic analysis reveals that complex 3a-OTf has a
quasioctahedral structure with a Re-Cl bond distance of
2.412(5) Å (Figure 2). Information pertinent to the data
collection and refinement of the structure is given in Table 1.
Bond distances and angles are given in Tables 2 and 3,
respectively.
Reaction of Re(CNtBu)3(PR3)2H with Chlorocarbons.
Both 1a and 1b were found to react with methylene chloride-
d2 at 25 °C to give complete conversion to Re(CNtBu)3(PR3)2-
Cl [R ) Cy (2a), Ph (2b)]. The reaction of 1a (6 days) is slower
than that of 1b (4 h). Spectroscopic characterization by NMR
indicated that 2a and 2b each possess two equivalent phosphine
ligands and two sets of isonitrile ligands in a 2:1 ratio. When
1
the reaction was conducted in CD2Cl2, and monitored by H
NMR, formation of CHD2Cl was detected.
Analytically pure chloride complex 2a was prepared and
isolated by treating the hydride 1a with excess CH3Cl in
methylene chloride at 25 °C for 2 h. When the reaction was
conducted in CD2Cl2 and monitored by 1H NMR, formation of
CH4 (as opposed to CHD2Cl) was detected. Two other
chlorocarbons, CDCl3 and CDFCl2, were also found to effect
this transformation. In each case, formation of the product
carbon species, containing one less chlorine atom and one
additional hydrogen (CHDCl2 and CHDFCl, respectively), was
detected. CH3Cl reacts more rapidly than CH2Cl2, CHFCl2, or
CHCl3. In contrast to these other chlorocarbons, CH3Cl does
not oxidize the chloride product 2a (Vide infra).
Reaction of Re(CNtBu)3(PR3)2Cl (2a) with H2. 2a revers-
ibly reacts with H2 to form the novel cationic dihydrogen
complex [Re(CNtBu)3(PCy3)2(H2)]Cl (4a-Cl) (eq 2).
Structure of Re(CNtBu)3(PCy3)2Cl (2a). The identity of
2a was further confirmed by X-ray structural determination of
crystals formed at -25 °C in methylene chloride. An ORTEP
diagram of 2a (Figure 1) shows an octahedral disposition of
the ligands in a mer,trans geometry. Both phosphines are bent
away from the chloride and one of the trans isonitriles, i.e. into
the quadrant defined by the two other isonitriles. The P-Re-P
angle is 173.2(1)°. Both trans isonitriles are bent toward the
chloride. The most remarkable structural feature is the long
Re-Cl bond distance, 2.596(2) Å. Information pertinent to the
data collection and refinement of the structure is given in Table
This reaction is rapid in chlorinated solvents (CH2Cl2, CHCl3,
and CHFCl2). Within seconds, the yellow solution becomes
colorless. 1H NMR data for 4a-Cl show two peaks for the tert-
butyl groups on the isonitrile ligands in a 2:1 ratio, and a broad
resonance in the hydride region (δ -6.30 ppm), which integrates
to two hydrogens. The 31P{1H} NMR spectrum shows one
resonance, indicating equivalent phosphines.
Characterization of 4a-Cl as a dihydrogen complex is based
on the large H-D coupling observed (JHD ) 30.3 Hz) for the
partially deuterated derivative (Figure 3). In addition, the
measured T1 for the H2 ligand at the maximum rate of relaxation
(T1(min)) is 8 ms (225 K, 300 MHz), consistent with an H-H
(12) Cf.: Cappellani, E. P.; Maltby, P. A.; Morris, R. H.; Schweitzer,
C. T.; Steele, M. R. Inorg. Chem. 1989, 28, 4437-4438.
(13) Conroy-Lewis, F. M.; Simpson, S. J. J. Chem. Soc., Chem. Commun.
1986, 506-507.
(14) Jones, W. D.; Maguire, J. A. Organometallics 1987, 6, 1728-1737.

10794 J. Am. Chem. Soc., Vol. 118, No. 44, 1996
Heinekey et al.
Table 1. Crystal Data and Parameters for 2a and 3a
compound
2a
3a
molecular formula
empirical formula
color; habit
crystal size, mm
crystal system
space group
unit cell dimensions
a, Å
Re(CNtBu)3(PCy3)2Cl‚2CH2Cl2
C53H97Cl5N3P2Re
clear yellow blocks
0.34 × 0.36 × 0.38
monoclinic
[Re(CNtBu)3(PCy3)2Cl]SO3CF3‚2CH2Cl2
C56H97Cl5F3N3O3P2ReS
clear pink plate
0.5 × 0.1 × 0.15
monoclinic
P21/c
P21/m
14.270(3)
22.041(4)
18.827(4)
91.15(3)
5935(3)
4
11.440(2)
17.229(3)
16.009(3)
99.67(3)
3110.5(16)
2
b, Å
c, Å
â, deg
volume, Å3
Z
formula wt
1197.8
1.341
2.361
1374.8
1.468
2.306
density(calc) Mg/m3
abs coeff, mm-1
F(000)
2476
1422
radiation
temp, K
MoKR (λ ) 0.71073 Å)
183
monochromator
2θ range, deg
scan type
graphite
2-45
2-50
ω
scan speed
variable; 1.5 to 5.5 deg/min in ω
0.80 + 0.35(tan θ)
scan range (ω), deg
reflcn collected
independent reflcns
obsd reflctns
no. of parameters refined
R(observed data), %
Rw, %
8344
3224
7703 (Rint ) 4.22%)
5756 (F > 4.0σ(F))
560
4.08
5.84
1.16
3026 (Rint ) 3.17%)
2120 (F > 4.0σ(F))
337
4.36
5.46
0.99
goodness-of-fita
transmission factor
0.891, 0.973
0.736, 0.792
a GOF ) [∑(weight‚del2)/(M - N)]1/2; M ) observed reflections; N ) parameters refined; del ) |F(obs) - F(calc)}.
Table 2. Selected Bond Distances (Å) for 2a and 3a
Table 3. Selected Bond Angles (deg) for 2a and 3a
2a
3a
2a
3a
Re-Cl
2.596(2)
2.449(2)
2.450(2)
1.974(8)
1.927(8)
2.003(8)
1.19(1)
1.18(1)
1.17(1)
1.46(1)
1.43(1)
1.44(1)
2.412(5)
2.486(3)
P1-Re-Cl
92.4(1)
93.0(1)
177.5(2)
82.7(2)
83.2(2)
173.2(1)
88.3(2)
86.4(2)
92.5(2)
88.4(2)
88.3(2)
92.1(2)
165.9(3)
173.5(7)
177.9(7)
175.4(6)
153.6(7)
172.6(8)
175.5(8)
95.6(1)
Re-P1
Re-P2
Re-C1
Re-C6
Re-C11
C1-N1
C6-N2
C11-N3
N1-C2
N1-C7
N1-C12
P2-Re-Cl
Cl-Re-C(6)
178.6(6)
82.3(5)
83.5(6)
2.08(3)
2.03(3)
2.04(3)
1.14(3)
1.13(3)
1.16(3)
1.47(3)
1.43(2)
1.42(3)
Cl-Re-C(1)
Cl-Re-C(11)
P1-Re-P2
P1-Re-C(1)
P1-Re-C(6)
P1-Re-C(11)
P2-Re-C(1)
P2-Re-C(6)
P2-Re-C(11)
C(1)-Re-C(11)
Re-C(1)-N(1)
Re-C(6)-N(2)
Re-C(11)-N(3)
C(1)-N(1)-C(2)
C(6)-N(2)-C(7)
C(11)-N(3)-C(12)
88.5(1)
84.5(1)
92.8(1)
165.8(8)
173.1(17)
178.8(11)
173.0(18)
160.1(18)
179.4(15)
180.0(2.4)
distance of ca. 0.80 Å (fast rotation).15 1H NMR spectroscopy
provided no evidence for the presence of a dihydride tautomer
at any temperature examined (180-300 K).
The reversibility of the reaction (eq 2) at room temperature
was probed in CD2Cl2 by removing the dissolved hydrogen
using standard freeze-pump-thaw (FPT) technique. Twenty-
four FPT cycles were required to shift the ratio of 4a:2a from
10:1 to 1:1. The relative concentrations of the dihydrogen
product, free H2 (corrected for 25% NMR-silent para-hydro-
gen),16 and the neutral chloride complex were determined by
1H and 31P{1H} NMR (Table 4). Apparent Keq values were
calculated using two models for the equilibrium.
4a, exists as one species, a solvent-separated ion pair. Values
of Keq calculated this way remained constant (mean value )
2.21 × 103 M-1; (4%) as successive amounts of H2 are
removed from solution. The second model (eq 4) assumes that
4a exists as free ions. Values of Keq calculated using this model
steadily decrease as the concentration of H2 is reduced. We
have therefore adopted the solvent-separated ion pair model as
the correct expression for the equilibrium constant.
The temperature dependence of Keq for eq 2 was determined
over the temperature interval 286-316 K. The observed
equilibrium constants varied from 13.6 × 103 M-1 at 286 K to
[4a]
Keq
Keq
)
)
(3)
(4)
[2a][H2]
[4a]2
(15) Desrosiers, P. J.; Cai, L.; Lin, Z.; Richards, R.; Halpern, J. J. Am.
Chem. Soc. 1991, 113, 4173-4184.
(16) Abragam, A. Principles of Nuclear Magnetism; The International
Series of Monographs on Physics 32; Clarendon: Oxford, 1961; p 226.
[2a][H2]
The first model (eq 3) assumes that the dihydrogen complex,

Rhenium Dihydrogen Complexes with Isonitrile Coligands
J. Am. Chem. Soc., Vol. 118, No. 44, 1996 10795
Figure 4. Plot of ln Keq Vs 1/T for the equilibrium Re(CNtBu)3(PCy3)2-
Cl + H2 h Re(CNtBu)3(PCy3)2(H2)]Cl.
Figure 2. ORTEP projection for compound 3a.
(500 torr), the ratio of neutral chloride to dinitrogen complex
is approximately 20:1.
Characterization by NMR spectroscopy indicates that [Re-
(CNtBu)3(PCy3)2(N2)]Cl possesses two equivalent phosphine
ligands and two sets of isonitrile ligands in a 2:1 ratio. The
binding mode of the N2 ligand could not be determined. The
expected region for the end-on bound NtN stretch (2100 cm-1
)
overlaps with the strongly-absorbing, broad isonitrile reso-
nances. It should be noted that ν(CtNR) for all these isonitrile
complexes were consistently very broad. Similar broad and
uninformative IR absorptions were reported by Maguire and
Jones for the complexes Re(CNR)3(PPh3)2H (R ) Me, Et, iPr,
Cy, CH2C(CH3)3, 2,6-Me2C6H3, and tert-butyl).14
Protonation of Re(CNtBu)3(PR3)2H. The dihydrogen com-
plexes, [Re(CNtBu)3(PCy3)2(H2)]+ (4a) and [Re(CNtBu)3(PPh3)2-
(H2)]+ (4b), were readily obtained by protonation of the
corresponding neutral hydride complexes with a wide variety
of acids (eq 6): HOTf (OTf- ) OSO2CF3-), HBF4‚OEt2, and
HBArf‚(OEt2)2.3 4b exhibited similar properties to 4a (see
above), and was also characterized as a dihydrogen complex.
For 4b, the T1(min) was found to be 8 ms (207 K, 300 MHz).
The H-D coupling of the partially deuterated complex was
measured to be 30.9 Hz, slightly greater than for 4a.
Figure 3. Partial 1H NMR spectrum (hydride region) of a mixture of
4a and 4a-d1 (300 MHz, methylene chloride-d2, 298 K). An upfield
shift of 47 pbb is observed for 4a-d1. 4a-d1 shows a large coupling to
deuterium (1JHD ) 30.3 Hz) and a much smaller coupling to the two
equivalent phosphines (2JHP ≈ 2 Hz).
Table 4. Data for the Removal of H2 from a Solution of
[Re(CNtBu)3(PCy3)2(H2)]Cl (4a-Cl)
Keq(ion pair),
[2a], M
[H2], M
[4a], M
Keq(free ion)
M-1
0.39 × 10-2 2.45 × 10-3 2.11 × 10-2
0.66 × 10-2 1.23 × 10-3 1.84 × 10-2
1.02 × 10-2 0.68 × 10-3 1.48 × 10-2
1.23 × 10-2 0.46 × 10-3 1.27 × 10-2
1.45 × 10-2 0.33 × 10-3 1.05 × 10-2
46.6
41.7
31.6
28.5
23.0
2.21 × 103
2.27 × 103
2.13 × 103
2.24 × 103
2.19 × 103
4.9 × 102 M-1 at 316 K. A Van’t Hoff plot is shown in Figure
4. The thermodynamic parameters for the formation of [Re-
(CNtBu)3(PCy3)2(H2)]Cl from Re(CNtBu)3(PCy3)2Cl and H2 are
as follows: ∆H° ) -18.0 ( 0.7 kcal/mol, ∆S° ) -44 ( 2 eu,
and ∆G°298 ) -4.8 ( 1.3 kcal/mol.
The reaction between 2a and H2 was found to be highly
sensitive to the nature of the solvent and phosphine coligands.
The reaction is favorable in chlorinated solvents, but does not
occur in THF. Also, chloride displacement by hydrogen does
not occur in the PPh3 complex, Re(CNtBu)3(PPh3)2Cl (2b).
In contrast to the complete reaction with H2, 2a only partially
reacts with N2 in CD2Cl2 to form a small equilibrium concentra-
tion of [Re(CNtBu)3(PCy3)2(N2)]Cl (eq 5). Under N2 pressure
In situ NMR tube preparations of 4a or 4b resulted in clean
and complete conversion by 1H and 31P{1H} NMR spectroscopy.
This was observed to be the case even in the absence of an H2
atmosphere.
Preparation of 4a-OTf and 4a-BF4 was readily achieved by
adding HOTf or HBF4‚OEt2 to a solution of 1a in diethyl ether.
A slight deficiency of acid was used to avoid overprotonation.
Both 4a and 4b react with strong acid, but the products have

10796 J. Am. Chem. Soc., Vol. 118, No. 44, 1996
Heinekey et al.
not been identified. Filtration and drying under a stream of H2
gives 4a in 70-90% yield.
Attempts to isolate pure 4b-BF4 or 4b-BArf by similar
technique failed to yield product in greater than 80% purity as
determined by NMR. The 31P{1H} NMR spectrum (CD2Cl2)
shows two minor impurities at 16.4 and 6.81 ppm. The latter
is identified as the previously reported compound [Re(CNtBu)4-
(PPh3)2]+.17
The dihydrogen complex, [Re(CNtBu)3(PCy3)2(H2)]Cl (4a-
Cl), can also be generated by the reaction of 1a with HCl.
Attempts to isolate 4a-Cl free from solvent failed because the
white precipitate that is formed loses hydrogen irreversibly in
the solid state and yields the yellow chloride, 2a.
The basicity of the neutral hydride, Re(CNtBu)3(PCy3)2H
(1a), was further probed by reacting this complex with weaker
acids. Reaction of 1a with [HNEt3]Cl in methylene chloride
gave complete protonation. In agreement with this observation,
simple amines, even in 10-fold excess, failed to remove the
proton from the dihydrogen product. Other nitrogen bases such
as Proton Sponge® (1,8-bis(dimethylamino)naphthalene) also
failed to deprotonate 4a. Complete deprotonation was ultimately
achieved using excess NaOMe or KOtBu. The high basicity
of 1a is further demonstrated by the observation that the addition
of 5 equiv of methanol to a methylene chloride solution of 1a
leads to 80% protonation.
Figure 5. Plot of ln (k/T) Vs 1/T for the isonitrile isomerization in
[Re(CNtBu)3(PCy3)2]+ (5a).
Scheme 1
Removal of Bound H2: Formation of Ligand Deficient
Complex. When 4a is prepared with a noncoordinating
counteranion (OTf-, BF4-, BArf-), it reversibly loses H2 in the
solid state to form the “coordinatively unsaturated”18 complex
[Re(CNtBu)3(PCy3)2]+ (5a). Hydrogen loss is easily monitored
calculated as ∆Hq ) 11.4 ( 0.8 kcal/mol and ∆Sq ) -17.6 (
1.6 cal/(mol‚K). This corresponds to ∆Gq298 ) 16.6 ( 1.2 kcal/
mol.
Synthesis of [Re(CNtBu)4(PCy3)2]BF4 (6a). Complex 6a
and the previously reported [Re(CNtBu)4(PPh3)2]BF4 (6b) were
observed by 1H and 31P{1H} NMR spectroscopy as minor
impurities (<10%) in many of the above reactions. The identity
of the previously unreported 6a was confirmed by independent
preparation. Our synthesis of 6a follows that reported by
Walton et al. for 6b.19 Reaction of Re(CNtBu)3(PCy3)2H (1a)
with excess CNtBu in NaBF4/methanol solution gives 6a in 82%
yield after 12 h. Elemental analysis confirms the empirical
formula. 1H and 31P{1H} NMR spectra respectively show a
single resonance for the isonitriles and the phosphines, indicating
that members of each group are equivalent.
1
by inspection: 4a is white, while 5a is dark purple. The H
NMR spectrum (298 K) of 5a shows two sets of exchange-
broadened isonitrile ligands in a 2:1 ratio. 31P{1H} NMR
spectroscopy indicates equivalent phosphines. 1H and 31P{1H}
NMR spectra of 5a-OTf and 5a-BF4 were identical, confirming
the lack of coordination by the counteranion. Significant solvent
coordination by CD2Cl2 or THF-d8 also appears to be absent.
Elemental analysis clearly demonstrates this for 5a in the solid
state; recrystalization of 5a-BF4 from THF yields analytically
pure compound. In solution, the lack of significant solvent
coordination was ruled out by the very minor 31P chemical shift
difference (∆δ ) 2.5 ppm) observed for 5a dissolved in the
solvents CD2Cl2 and THF-d8. This contrasts with a much larger
chemical shift change for 5a dissolved in the coordinating
solvent CD3CN. In this solvent, the acetonitrile adduct [Re-
(CNtBu)3(PCy3)2(NCCD3)]+ forms as evidenced by the 31P
chemical shift having moved 15.8 ppm upfield of 5a dissolved
in CD2Cl2. It is worth noting that this same species is formed
when the dihydrogen complex, 4a, is dissolved in CD3CN,
indicating that bound H2 is readily displaced by acetonitrile.
Dynamics of [Re(CNtBu)3(PCy3)2]+ (5a). The fluxional
behavior of the isonitrile ligands was investigated by 1H NMR
spin saturation transfer techniques. Saturation of either isonitrile
resonance showed spin saturation transfer to the other resonance.
Rate data for this process (Scheme 1) were measured over a 43
degree temperature range. The empty box in Scheme 1
represents a vacant coordination site, which is either always
present or transiently formed by the dissociation of an agostic
C-H bond or a weakly coordinating solvent molecule (see
Discussion). Figure 5 shows an Eyring plot for the rate
constants. The activation parameters for this process were
[Re(CNtBu)5(H2)]BArf. The reaction of Re(CNtBu)5Cl20
with NaBArf under H2 affords solutions of the very labile H2
1
complex [Re(CNtBu)5(H2)]BArf. The H NMR spectrum in
CD2Cl2 exhibits a broad resonance at δ -5.81 ppm. Upon
exposure to D2, a well-resolved 1:1:1 triplet was observed with
JHD ) 33.4 Hz. The same material results from protonation of
20
Re(CNtBu)5CH3 with HBArf‚2Et2O under an atmosphere of
hydrogen. This complex could not be isolated as a solid, and
decomposed over several hours in methylene chloride to afford
products with no hydride ligands.
[Re(PMe3)5H2]OTf. In contrast to the relatively electron
deficient pentaisonitrile complex, we find that protonation of
the electron rich Re(PMe3)5H21 with HOTf affords the stable
cationic dihydride [Re(PMe3)5H2]+, which has been previously
reported by Green and co-workers.22 At room temperature, the
1H NMR spectrum (CD2Cl2) of [Re(PMe3)5H2]+ in the hydride
region exhibits a sextet at δ ) -7.55 ppm with JHP ) 28.6 Hz.
(19) Allison, J. D.; Cameron, C. J.; Wild, R. E.; Walton, R. A. J.
Organomet. Chem. 1981, 218, C62-C66.
(20) Chiu, K. W.; Howard, C. G.; Wilkinson, G.; Galas, A. M. R.;
Hursthouse, M. B. Polyhedron 1982, 1, 803-808.
(21) Chiu, K. W.; Howard, C. G.; Rzepa, H. S.; Sheppard, R. N.;
Wilkinson, G.; Galas, A. M. R.; Hursthouse, M. B. Polyhedron 1982, 1,
441-451.
(17) Allison, J. D.; Wood, T. E.; Wild, R. E.; Walton, R. A. Inorg. Chem.
1982, 21, 3540-3546.
(18) 5a most likely contains an agostic interaction to a pendant C-H
bond of one of the cyclohexyl rings.
(22) Allen, D. L.; Green, M. L. H.; Bandy, J. A. J. Chem. Soc., Dalton
Trans. 1990, 541-549.

Rhenium Dihydrogen Complexes with Isonitrile Coligands
J. Am. Chem. Soc., Vol. 118, No. 44, 1996 10797
making it less useful for the preparation of 2a. Extended
reaction times lead to the ReII decomposition product, [Re-
(CNtBu)3(PCy3)2Cl]Cl (3a).
The reaction of 1a with CH3Cl is the exception to the above
reactivity trend. Even though it contains one less chlorine than
CH2Cl2, it reacts more rapidly. In fact, 1a reacts preferentially
with CH3Cl in methylene chloride. This suggests that this
reaction may proceed by a different (non-radical chain) mech-
anism. Consistently rapid reaction times (t1/2 ≈ 10 min) make
this reaction preparatively the most useful.
Reaction of Re(CNtBu)3(PCy3)2Cl (2a) with H2. It is
surprising that H2, which binds to a metal Via its bonding
electron pair, can displace chloride which offers both a non-
bonding electron pair as well as a negative charge complemen-
tary to that of the cationic metal center. It is necessary to
consider both the factors which make the M-H2 interaction
stronger than expected and those which weaken the M-Cl
interaction. One clear distinction is that dihydrogen acts as a
π-acid, while chloride behaves as a π-base. On a d6 metal
center, with all π-symmetry orbitals filled, binding of a π-acid
ligand would clearly be favored. Indeed, the ability of the empty
H2 σ* orbital to accept electron density from a filled metal π
orbital is well supported by theoretical calculations2 and studies
of the rotational barrier of bound H2.25 These include recent
studies of d2-complexes in which it has been possible to block
the rotation of bound H2 on the NMR time scale.26 Our studies
(in CD2Cl2) of the relative binding strengths of H2 and Cl-
indicate that H2 binding is strongly enthalpically favored: ∆H°
) -18.0 ( 0.7 kcal/mol. However, an unfavorable entropic
term (∆S° ) -44 ( 2 eu) disfavors the reaction. At higher
temperatures (79 °C and above), the free energy of reaction is
predicted to become positive.
Figure 6. 81-MHz 31P{1H} NMR spectra of 4a, 4a-d1, and 4a-d2 as a
function of time for the reaction of 4a with roughly 1 equiv of deuterium
gas at 298 K.
A
31P NMR spectrum with the methyl protons selectively
decoupled shows a triplet, consistent with the presence of two
hydride ligands. Exchange with D2 gas does not occur. A
partially deuterated sample was prepared by addition of D+ to
the neutral hydride. No H-D coupling was observed. The
single 31P{1H} NMR resonance observed at 298 K suggests a
fluxional compound. At lower temperatures the 31P{1H} NMR
spectrum decoalesced into three resonances which integrated
2:2:1. The spectrum is consistent with a static seven-coordinate
pentagonal bipyramidal structure with the two hydrides occupy-
ing equivalent positions in the equatorial plane.
It is useful to compare these thermodynamic parameters with
those determined by Hoff and co-workers for the binding of
H2 to the agostic complex, W(CO)3(PCy3)2 (eq 7).7
H2/D2 Exchange and the Formation of HD. All three
dihydrogen complexes, [Re(CNtBu)3(PCy3)2(H2)]+ (4a), [Re-
(CNtBu)3(PPh3)2(H2)]+ (4b), and [Re(CNtBu)5(H2)]+, were
observed to incorporate D2 into the dihydrogen site when placed
under D2 gas. They also rapidly scramble the isotopes to form
both free and bound HD. Qualitatively, the rate of isotope
scrambling was observed to be most rapid for [Re(CNtBu)5-
(H2)]+.
In the case of 4a and 4b, it was possible to simultaneously
observe all three isotopomers (4, 4-d1, and 4-d2) by 31P{1H}
NMR and monitor the reaction of 4a with D2 (ca. 1 eq) in CD2-
Cl2 over time, as shown in Figure 6. Significantly, the peak
corresponding to 4a-d2 was observed to grow in before the peak
for 4a-d1.
In toluene, ∆H° was reported to be -9.9 kcal/mol, and ∆S°
was estimated to be -35 eu. It should be pointed out that the
relatively low ∆H° includes displacement of the agostic
interaction which is assumed to contribute ca. 10 kcal/mol. Thus
the total estimated binding strength of H2 to the coordinatively
unsaturated fragment, W(CO)3(PCy3)2, is ca. 20 kcal/mol,
roughly the same magnitude as ∆H° for the reaction of Re-
(CNtBu)3(PCy3)2Cl with H2 (eq 2). This result implies that H2
binding is much stronger in [Re(CNtBu)3(PCy3)2(H2)]+ (4a) than
in W(CO)3(PCy3)2H2, or that part of the magnitude of ∆H° can
be accounted for by differential solvation of the ionic product,
4a, Versus the uncharged reactants, 2a and H2, or both. The
first possibility, that [Re(CNtBu)3(PCy3)2]+ (5a) binds H2 much
more strongly than does W(CO)3(PCy3)2, is unlikely since the
corresponding dihydrogen complexes both show facile exchange
of H2 for D2 in solution.27 The other possibility seems more
Discussion
Reaction of Re(CNtBu)3(PCy3)2H (1a) with Chlorocar-
bons. The reaction of Re(CNtBu)3(PCy3)2H (1a) with CCl4,
CHCl3, or CH2Cl2 to form Re(CNtBu)3(PCy3)2Cl (2a) is a well
precedented reaction for metal hydrides. The increasing reactiv-
ity as the number of Cl atoms increases also follows the
previously observed trend.23 Limited mechanistic data for these
types of reactions suggest that they proceed by free radical chain
mechanisms.24 Consistent with a radical chain mechanism, the
reaction of 1a with CH2Cl2 shows highly variable reaction times,
(24) Kinney, R. J.; Jones, W. D.; Bergman, R. G. J. Am. Chem. Soc.
1978, 100, 7902-7915.
(25) (a) Eckert, J.; Kubas, G. J. J. Phys. Chem. 1993, 97, 2378-2384.
(b) Eckert, J.; Kubas, G. J.; Hall, J. H.; Hay, P. J.; Boyle, C. M. J. Am.
Chem. Soc. 1990, 112, 2324-2332.
(26) (a) Jalo´n, F. A.; Otero, A.; Manzano, B. R.; Villasen˜or, E.; Chaudret,
B. J. Am. Chem. Soc. 1995, 117, 10123-10124. (b) Sabo-Etienne, S.;
Chaudret, B.; el Makarim, H. A.; Barthelat, J.-C.; Daudey, J.-P.; Ulrich,
S.; Limbach, H.-H.; Mo¨ıse, C. J. Am. Chem. Soc. 1995, 117, 11602-11603.
(27) Kubas, G. J.; Unkefer, C. J.; Swanson, B. I.; Fukushima, E. J. Am.
Chem. Soc. 1986, 108, 7000-7009.
(23) (a) Geoffroy, G. L.; Lehman, J. R. AdV. Inorg. Chem. Radiochem.
1977, 20, 189-290. (b) Moore, D. S.; Robinson, S. D. Chem. Soc. ReV.
1983, 12, 415-452.

10798 J. Am. Chem. Soc., Vol. 118, No. 44, 1996
Heinekey et al.
Table 5. Isonitrile/Phosphine Vs Carbonyl/Phosphine Ratios and the Nature of H2 Binding
no. of
PR3
isonitrile/phosphine
complexes
carbonyl/phosphine
complexes
type of binding
ref
type of binding
dihydrogen
ref
0
2
[Re(CNtBu)5(H2)]+
[Re(CNtBu)3(PR3)2(H2)]+
R ) Cy, Ph
dihydrogen
dihydrogen
a
a
[Re(CO)3(PR3)2(H2)}+
R ) Cy, iPr, Ph, Me
11,37
)]+
3
[Re(CNtBu)2(PPh3)3H2]+
dihydride
33
[Re(CO)2(triphos)(H2
[Re(CO)2(PMe3)3(H2)]+
[Re(CO)2(PMe2Ph)3H2]+
[Re(CO)(PMe3)4H2]+
[Re(CO)(PMe2Ph)4H2]+
dihydrogen
dihydrogen
dihydride/dihydrogen
dihydride
9
38
39
38
39
4
5
[Re(CNR)(dppe)2H2]+
R ) Me, tBu, 2,6-Me2C6H3
[Re(PMe3)5H2]+
dihydride
dihydride
33
22
dihydride
likely; in fact the exquisite solvent sensitivity of the reaction
Re(CNtBu)3(PCy3)2Cl with H2 (eq 2) supports this claim. While
the reaction of Re(CNtBu)3(PCy3)2Cl with H2 is favorable in
methylene chloride (∆G°298 ) -4.8 ( 1.4 kcal/mol), it does
not proceed to any detectable extent in THF at 298 K. Clearly
differential solvation of the product, 4a, and the starting
complex, 2a, must make a significant contribution to the
thermodynamics of this reaction. Consistent with this hypoth-
esis, the observed ∆S° (-44 ( 2 eu) is larger than the values
observed for the reaction of M(CO)3(PCy3)2 with H2 (M ) Mo,
∆S° ) -23.8 ( 2.1 eu; M ) Cr, ∆S° ) -25.6 ( 1.7 eu).28
This may be due to solvent ordering around the charged species.
increase in the Re-Cl distance (0.18 Å) is much greater in this
case, while the decrease in the average Re-P distance (0.04
Å) is smaller. The differences between the two sets of
complexes can be attributed to several factors including the
differences in the oxidation states being compared (ReII, ReI Vs
ReIII, ReII), the differences in coligands, and the differences in
sterics. For instance, the greater overall steric crowding present
in complexes 2a and 3a Versus Re(dppe)2Cl2 and [Re(dppe)2-
Cl2]+ may both exacerbate the lengthening of the Re-Cl bond
and impede the shortening of the Re-P bond upon the one-
electron reduction of 3a.
Interestingly, two other reported monomeric ReI complexes
with long Re-Cl bonds also contain mixtures of phosphine and
isonitrile coligands. They are Re(CNtBu)2(PMe3)3Cl (Re-Cl
) 2.570 Å)21 and Re(CNMe)(dppe)2Cl (Re-Cl ) 2.607 Å).32
The reaction of these complexes with H2 has not been reported,
however, Re-Cl bond length arguments lead us to predict that
these compounds will readily react with hydrogen. The
products, however, would not likely be dihydrogen complexes.
Walton and co-workers33 have prepared both [Re(CNR)-
(dppe)2H2]+ (R ) Me, tBu, 2,6-Me2C6H3) and [Re(CNtBu)2-
(PPh3)3H2]+ with the BF4- counterion (see Table 5). The large
JHP coupling constants of the hydrides strongly imply that these
species are dihydrides and not dihydrogen complexes.34
Unlike H2, N2 does not displace chloride very well. Under
N2 pressure (500 Torr), less than 5% of Re(CNtBu)3(PCy3)2Cl
is converted to [Re(CNtBu)3(PCy3)2(N2)]Cl. The relative bind-
ing of H2 and N2 in this system therefore differs from that of
the M(CO)3(PCy3)2 (M ) W, Mo, Cr) system which favors N2.28
In this regard, [Re(CNtBu)3(PCy3)2]+ is similar to other ReI
species, including [Re(CO)3(PCy3)2]+,11 [Re(CO)2(triphos)]+,9
and the T-shaped RhI complex, Rh[HC(CH2CH2P(tBu)2)2].10 All
of these complexes bind H2 in preference to N2.
Structural Comparisons of Re(CNtBu)3(PCy3)2Cl (2a) and
[Re(CNtBu)3(PCy3)2Cl]+ (3a). The unique reactivity of 2a is
consistent with its unusual structure. The Re-Cl bond length,
2.596(2) Å, is one of the longest reported for a non-bridging
ReI-Cl bond.29 It exceeds the Re-Cl bond length reported by
Warner and Lippard for an analogous complex, Re(CNMe)3-
(PMePh2)2Cl (Re-Cl ) 2.47(2) Å), by more than 0.1 Å.30 This
dramatic increase in bond length, resulting from minor changes
in the ligand set, is consistent with the different reactivities
observed for 2a and 2b. The PPh3 analog, 2b, shows no
reactivity with H2. No structural information exists for 2b;
however, it is reasonable to predict that its Re-Cl bond length
would be close to the 2.47-Å distance observed for Re(CNMe)3-
(PMePh2)2Cl.
[ReL5H2]+ Complexes (L ) CNtBu, PMe3). [Re(CNtBu)5-
(H2)]+ is one of the few known dihydrogen complexes not
containing a phosphine ligand. It may be relatively unstable
in solution due to the lack of a pendant C-H bond to form an
agostic interaction in the formally 16-electron cation which
results from H2 loss. [Re(CNtBu)5(H2)]+ nicely fills a gap
between analogous group 6 and group 8 compounds. There
are d6-dihydrogen complexes in both of these groups in which
all the coligands are identical: M(CO)5(H2) (M ) Cr, Mo, W)35
(28) Gonzalez, A. A.; Zhang, K.; Mukerjee, S. L.; Hoff, C. D.; Khalsa,
G. R. K.; Kubas, G. J. In Bonding Energetics of Organometallic Compounds;
Marks, T. J., Ed.; ACS Symp. Ser. 428; American Chemical Society:
Washington, DC, 1990; pp 133-147.
(29) The average terminal Re-Cl distance for all structures collected
through 1989 is 2.389 Å. Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard,
O.; Watson, D. G.; Taylor, R. J. Chem. Soc., Dalton Trans. 1989, S1-
S84.
The structures of the 17 electron cation, [Re(CNtBu)3(PCy3)2-
Cl]+ (3a), and the 18-electron neutral chloride, Re(CNtBu)3-
(PCy3)2Cl (2a), provide a useful comparison between two
complexes that differ only in d-electron count. Few such
comparisons have been made in the literature of organometallic
chemistry. One particularly relevant example is the structural
comparison made by Salih et al. for trans-[Re(dppe)2Cl2]X (X-
) Cl-, BF4-) (dppe ) Ph2PCH2CH2PPh2) with trans-Re(dppe)2-
Cl2.31 In this case, a 16-electron, d4, ReIII complex is compared
with a 17-electron, d5, ReII complex. They found that the Re-
Cl distances increase by 0.1 Å and the average M-P distance
decreases by 0.08 Å upon reduction of ReIII to ReII. Similar
overall trends are observed in the complexes [Re(CNtBu)3-
(PCy3)2Cl]+ (3a) and Re(CNtBu)3(PCy3)2Cl (2a). The addition
of an electron to 3a (17-electron, d5, ReII) to form 2a
(18-electron, d6, ReI) also results in an increase in the Re-Cl
distance and a decrease in the Re-P distances. However, the
(30) Warner, S.; Lippard, S. J. Organometallics 1989, 8, 228-236.
(31) Salih, T. A.; Duarte, M. T.; Frausto da Silva, J. J. R.; Galva˜o, A.
M.; Guedes da Silva, M. F. C.; Hitchcock, P. B.; Hughes, D. L.; Pickett,
C. J.; Pombeiro, A. J. L.; Richards, R. L. J. Chem. Soc., Dalton Trans.
1993, 3015-3023.
(32) Carvalho, M. F. N. N.; Duarte, M. T.; Galva˜o, A. M.; Pombiero,
A. J. L. J. Organomet. Chem. 1994, 469, 79-87.
(33) Moehring, G. A.; Walton, R. A. J. Chem. Soc., Dalton Trans. 1987,
715-720.
(34) Heinekey, D. M.; Liegeois, A.; van Roon, M. J. Am. Chem. Soc.
1994, 116, 8388-8389.
(35) (a) Upmacis, R. K.; Gadd, G. E.; Poliakoff, M.; Simpson, M. B.;
Turner, J. J.; Whyman, R.; Simpson, A. F. J. Chem. Soc., Chem. Commun.
1985, 27-30. (b) Upmacis, R. K.; Gadd, G. E.; Poliakoff, M.; Turner, J. J.
J. Am. Chem. Soc. 1986, 108, 3645-3651.

Rhenium Dihydrogen Complexes with Isonitrile Coligands
and [Os(NH3)5(H2)]+2 36 [Re(CNtBu)5(H2)]+ now provides a
J. Am. Chem. Soc., Vol. 118, No. 44, 1996 10799
.
However, the majority of the reported complexes with a
carbonyl/phosphine ratios of 2:3, [Re(CO)2(PMe3)3(H2)]+ and
[Re(CO)2(triphos)(H2)]+,37,9 also have the dihydrogen structure.
The exception is [Re(CO)2(PMe2Ph)3H2]+,39 which exists as a
mixture of the dihydrogen and dihydride tautomers. The
unexpected observation that PMe3, the most basic of the
phosphines, stabilizes the non-classical tautomer for complexes
group 7 example of this type of complex. It is interesting to
note that the stability of these complexes increases from group
6 to group 8. M(CO)5(H2) complexes are very unstable and
could only be studied at low temperature in argon or hydro-
carbon matrices. [Re(CNtBu)5(H2)]+ is more stable and could
be observed in solution by NMR. Of all these, the group 8
dication, [Os(NH3)5(H2)]+2, is the most stable and could be
readily isolated.
of the type [Re(CO)2(PR3)3(H2)]+ has already been noted.37
A
satisfactory explanation remains elusive, especially since Kubas
and co-workers established the opposite trend for the Mo
complexes of the form Mo(CO)(R2PCH2CH2PR′2)2(H2). Here,
R and R′ were varied in a stepwise fashion between aliphatic
and aromatic groups.40 As is normally expected, aromatic R
groups were found to stabilize the dihydrogen tautomer.
As the metal center becomes more electron rich and less
Lewis acidic, σ- and π-donating ligands generally bind less
strongly. In a few cases, it has been shown that ligands such
as chloride can be labilized to the point where dissociation
occurs even in the absence of a competing ligand. Wilkinson
has shown this to be the case for Re(PMe3)5Cl.20 This
compound behaves as a covalent complex in nonpolar solvents,
but ionizes in ethanol to give the chloride-dissociated complex
[Re(PMe3)5]Cl. The ionized form reacts with N2 to give the
dinitrogen complex [Re(PMe3)5N2]Cl. Surprisingly, [Re(PMe3)5]-
Cl was reported to react with neither H2 nor even CO. The
anticipated product from the reaction of [Re(PMe3)5]Cl with
hydrogen, [Re(PMe3)5H2]+, has been prepared by Green and
co-workers Via protonation of the corresponding neutral hy-
dride.22 We have prepared the partially deuterated analog
[Re(PMe3)5HD]+, which shows no H-D coupling. A fluxional
seven coordinate dihydride structure is consistent with all of
the spectoscopic observations. The observation that this
complex is a dihydride and not a dihydrogen complex comes
as no surprise. Not only does it fit the trends established in
Table 5, but the ν(NtN) for [Re(PMe3)5N2]Cl (2030 cm-1) falls
into the range where the corresponding dihydrogen analog is
predicted to oxidatively add to form a dihydride.6 Our results
are also consistent with early work of Muetterties and co-
workers for the less basic phosphite analog, [Re(P(OMe)3)5H2]+.37
This complex is reported to exhibit a large hydrogen-
phosphorus coupling (JHP ) 20 Hz), which is very suggestive
of a dihydride structure.33 The use of the more electron donating
PMe3 ligands can only be expected to further stabilize the
dihydride structure. In fact, the coupling constant does increase
significantly (JHP ) 28.6 Hz).
Dihydrogen Ws Dihydride Structures. As reported for the
complexes [Re(CO)3(PR3)2(H2)]+ (R ) Cy, iPr, Ph;11 Me38 ),
low-temperature NMR (180 K) of [Re(CNtBu)3(PR3)2(H2)]+ (4a
and 4b) and [Re(CNtBu)5(H2)]+ provided no evidence for the
presence of a dihydride tautomer. These observations are in
contrast to the report of Walton and Moehring, who studied
closely related complexes with two isonitrile and three phos-
phine ligands, as well as complexes with one isonitrile and four
phosphines.33 Complexes such as [Re(CNtBu)2(PPh3)3H2]+ and
[Re(CNtBu)(dppe)2H2]+ were found to be dihydrides. This
empirical evidence clearly shows that ReI cationic complexes
with an isonitrile/phosphine ratio greater than or equal to 3:2
have a dihydrogen structure, while those with a smaller ratio
adopt the dihydride structure (see Table 5). Our observation
that [Re(PMe3)5H2]+, a complex with no isonitrile coligands,
is a dihydride also fits this trend.
Even among the two known complexes with a carbonyl/
phosphine ratio of 1:4, there is evidence for a dihydrogen
complex. [Re(CO)(PMe3)4(H2)]+ can be formed at -95 °C by
protonation of Re(CO)(PMe3)4H, however, it only exists as an
unstable intermediate, which oxidatively adds H2 irreversibly
at temperatures greater than -30 °C.37 The dihydride structure
is thermodynamically preferred for both [Re(CO)(PMe3)4H2]+
and [Re(CO)(PMe2Ph)4H2]+.39 Despite the unpredictable de-
pendence on the nature of PR3, it is apparent that substitution
of CNtBu with the more π-acidic CO ligands shifts the ligand
mix required for oxidative addition of H2. An isonitrile/
phosphine ratio of at least 3:2 is needed to favor the dihydrogen
structure, while a lower ratio usually suffices when the coligand
combination is carbonyl/phosphine. These trends are sum-
marized in Table 5.
Acidity of the Dihydrogen Complexes. The acidity of [Re-
(CNtBu)3(PCy3)2(H2)]+ (4a) is similar to its group 6 carbonyl
analogs. While simple amines fail to deprotonate either 4a or
W(CO)3(PCy3)2(H2), both can be deprotonated by stronger bases
such as alkoxides.41 However, when carbonyls are substituted
for isonitriles on Re, the complex becomes more acidic. For
instance, the direct carbonyl analog, [Re(CO)3(PCy3)2(H2)]+,
can be deprotonated by the mild base 2,6-di-tert-butylpyridine.42
Bianchini and co-workers report that [Re(CO)2(triphos)(H2)]+
(triphos ) MeC(CH2PPh2)3) is deprotonated by NEt3 in meth-
ylene chloride.9 Similarly, group 8 dicationic analogs are also
more acidic than 4a. Tilset and co-workers recently reported
that [Os(NCMe)3(PiPr3)2(H2)]2+ could be deprotonated with
piperidine.43 These observations are in agreement with the idea
that cationic dihydrogen complexes are more acidic than neutral
ones, and that complexes with the more electron withdrawing
set of coligands (carbonyl > isonitrile > acetonitrile) are also
more acidic.
Isotope Exchange Reactions. There has been much specu-
lation on the mechanism by which metal-(H2) complexes react
with D2 to form the metal-(HD) isotopomer.44 ,8 One of the
most plausible mechanisms is shown in Scheme 2.
This mechanism necessitates facile H2/D2 exchange, a het-
erolytically activated dihydrogen ligand, as well as an adventi-
(39) Luo, X.-L.; Crabtree, R. H. Organometallics 1992, 11, 237-219.
(40) (a) Kubas, G. J.; Ryan, R. R.; Wrobleski, 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.; Veranmini, P. J.; Unkefer, C. J.;
Khalsa, G. R. K.; Jackson, S. A.; Eisenstein, O. J. Am. Chem. Soc. 1993,
115, 569-581. (d) Luo, X.-L.; Kubas, G. J.; Burns, C. J.; Eckert, J. Inorg.
Chem. 1994, 33, 5219-5229.
(41) Van Der Sluys, L. S.; Miller, M. M.; Kubas, G. J.; Caulton, K. G.
J. Am. Chem. Soc. 1991, 113, 2513-2520.
(42) Heinekey, D. M.; Schomber, B. M.; Radzewich, C. E. Unpublished
results.
Somewhat different observations have been reported for the
mixed carbonyl/phosphine complexes (Table 5). Like the
isonitrile/phosphine complexes above, complexes with carbonyl/
phosphine ratios of 3:2, [Re(CO)3(PR3)2(H2)]+ (R ) Cy, iPr,
Ph;11 Me37), were also found to have the dihydrogen structure.
(36) Harman, W. D.; Taube, H. J. Am. Chem. Soc. 1990, 112, 2261-
2263.
(37) Choi, H. W.; Muetterties, E. L. J. Am. Chem. Soc. 1982, 104, 153-
(43) Smith, K. T.; Tilset, M.; Kuhlman, R.; Caulton, K. G. J. Am. Chem.
Soc. 1995, 117, 9473-9480.
(44) Albeniz, A. C.; Heinekey, D. M.; Crabtree, R. H. Inorg. Chem. 1991,
30, 3632-3635.
161.
(38) Gusev, D. G.; Nietlspach, D.; Eremenko, I. L.; Berke, H. Inorg.
Chem. 1993, 32, 3628-3636.

10800 J. Am. Chem. Soc., Vol. 118, No. 44, 1996
Heinekey et al.
Scheme 2
alkane bond in Mo(CO)5(heptane) to be 15 ( 1 kcal/mol.48 Since
the rate-limiting step (k1 or k2) for the proposed dissociative
mechanism is not known, the value of 11.4 ( 0.8 kcal/mol can
only serve as an upper limit to the strength of the agostic
interaction.
Comparison of [Re(CNtBu)3(PCy3)2]+ (5a) with W(CO)3-
(PCy3)2. It is useful to highlight the remarkable similarities
between 5a and W(CO)3(PCy3)2. Both complexes are purple
solids and the corresponding dihydrogen complexes are lightly
colored: [Re(CNtBu)3(PCy3)2(H2)]+ (4a) is white, and W(CO)3-
(PCy3)2(H2) is yellow. Both of these, in turn, show similar
qualitative rates of H2/D2 exchange and isotope scrambling.
Furthermore, as discussed above, they show similar acidities.
Finally, it is worth noting that one electron oxidation to give
stable 17-electron, d5, halide derivatives is possible for both
fragments; [Re(CNtBu)3(PCy3)2Cl]+ (3a) is analogous to the
neutral WI complex, W(CO)3(PiPr3)2I.49 It is indeed reasonable
to conclude that by exchanging the strongly π-accepting
carbonyl ligands for the more weakly π-accepting isonitriles it
is possible to maintain similar overall electronics by moving
one group to the right on the periodic table while forming a
cationic complex.
Scheme 3
Conclusion
The Lewis-acidic rhenium complex, [Re(CNtBu)3(PCy3)2]+,
strongly binds dihydrogen in preference to chloride. This result
is attributed, in part, to the weak Re-Cl bond in the neutral
chloride. The Re-Cl bond length in Re(CNtBu)3(PCy3)2Cl has
been determined and compared to the corresponding ReII cation
complex.
Complexes of the form [ReL5H2]+ were found to adopt either
the formally ReIII dihydride structure (L ) PMe3) or the ReI
dihydrogen structure (L ) CNtBu). More basic ligands
(phosphines) favor the dihydride structure, while π-acid ligands
such as CO and CNR favor the dihydrogen structure. At least
three isonitrile ligands must be present for formation of a
dihydrogen structure.
tious base. Complexes 4a and 4b provide a unique opportunity
to evaluate this mechanism, since it is possible to spectroscopi-
cally resolve all three isotopomers, 4, 4-d1, and 4-d2, by 31P-
{1H} NMR spectroscopy (Figure 6). As long as base catalysis
is slow, the mechanism predicts that when 4a reacts with D2,
the first newly observed species should be 4a-d2, not 4a-d1. This
prediction is in fact confirmed by experiment.
Structure and Dynamics of [Re(CNtBu)3(PCy3)2]+ (5a).
Since additional ligands can be added to [Re(CNtBu)3(PCy3)2]+
(5a), it fits the operational definition of coordinative unsaturation
presented by Strauss.45 The solution structure of 5a possesses
either weak solvent coordination, a vacant coordination site, or
an agostic interaction. Agostic interactions have been convinc-
ingly demonstrated for both W(CO)3(PCy3)2 and [Re(CO)3-
(PCy3)2]+ in the solid state.46 ,11 If an agostic interaction also
exists for 5a, then the activation parameters derived for the
isonitrile rearrangement (Scheme 1) provide some insight into
the strength of this interaction. In order for an isonitrile to
change its position, we can consider both dissociative and
associative mechanisms. A dissociative mechanism (Scheme
3) implies that the agostic bond must first break in order to
free the sixth coordination site. For the second step, orbital
arguments suggest that the most favorable mechanism is for
just a single isonitrile to move, so that an energetically
unfavorable D3h transition state can be avoided.47 If the pre-
equilibrium dissociation of the agostic bond (k1) is rate limiting,
then the activation enthalpy (∆Hq ) 11.4 ( 0.8 kcal/mol) for
this process approximates the Re‚‚‚H-C agostic bond strength.
An agostic bond strength of 11.4 ( 0.8 kcal/mol is in fact
reasonable. Hoff has estimated the strength of the agostic
interaction in W(CO)3(PCy3)2 to be 10 ( 6 kcal/mol.7 Fur-
thermore, results from photoacoustic calorimetry show the metal
The activation parameters of the isonitrile rearrangement
process were determined for the complex [Re(CNtBu)3-
(PCy3)2]+. A mechanistic hypothesis was advanced, which
relates ∆Hq for this rearrangement to the strength of the
Re‚‚‚H-C agostic interaction.
Experimental Section
General Procedures. All reactions were performed under vacuum
or argon atmosphere using standard Schlenk, drybox, and high-vacuum
techniques. The solvents pentane, heptane, tetrahydrofuran, toluene,
and diethyl ether were distilled from Na/K and benzophenone under
oxygen- and water-free N2 (deoxygenated over BASF R3-11 CuO
catalyst and dried by passing through a column of P2O5). Methylene
chloride was distilled from P2O5. All deuterated solvents were obtained
from Cambridge Isotope Laboratories. Tetrahydrofuran-d8, toluene-
d8, and benzene-d6 were dried over benzophenone ketyl in an evacuated
vessel. Acetonitrile-d3, chloroform-d1, and methylene chloride-d2 were
dried and stored over calcium hydride in an evacuated vessel. All other
reagents were obtained from commercial sources and used without
further purification.
Infrared spectra were obtained on a Perkin Elmer 1600 Series FTIR.
Mass spectra were obtained on a Kratos Profile sector instrument. NMR
spectra were obtained using a Bruker AC-200, AF-300, or WM-500
spectrometer. All NMR tube reactions were conducted in flame-sealed
1
tubes or J. Young® screw-cap tubes. Variable-temperature H NMR
experiments were conducted using a Bruker B-VT 1000 temperature
control module with a copper-constantan thermocouple. Proton T1
(45) Strauss, S. H. Chemtracts-Inorg. Chem. 1994, 6, 1-13.
(46) Wasserman, H. J.; Kubas, G. J.; Ryan, R. R. J. Am. Chem. Soc.
1986, 108, 2294-2301.
(47) Albright, T. A.; Burdett, J. K.; Whangbo, M.-H. Orbital Interactions
in Chemistry; Wiley: New York, 1985; p 319.
(48) Walsh, E. F.; Popov, V. K.; George, M. W.; Poliakoff, M. J. Phys.
Chem. 1995, 99, 12016-12020.
(49) 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.

Rhenium Dihydrogen Complexes with Isonitrile Coligands
J. Am. Chem. Soc., Vol. 118, No. 44, 1996 10801
studies were performed using the standard inversion recovery 180°-
τ-90° pulse sequence.50 Temperature calibration was accomplished
following the Van Geet methanol calibration method.51 1H NMR
spectra were referenced at all temperatures to the internal residual
hydrogen signal of the deuterated solvent relative to TMS. 31P{1H}
NMR was referenced to an external standard of 85% H3PO4. Elemental
analyses were performed by Canadian Microanalytical Service Ltd.,
Delta, BC.
Re(PCy3)2H7,52 Re(PPh3)2H7,14 Re(PMe3)5H,14 Re(CNtBu)5Cl,20 Re-
(CNtBu)5Me,20 and HBArf‚2Et2O3 were prepared by reported proce-
dures.
mmol) was added via Teflon tubing attached to a gas-tight syringe. A
white precipitate immediately formed. The slurry was cannula-
transferred onto a glass frit and filtered. The collected product was
washed with diethyl ether (3 × 5 mL) and dried under a stream of
hydrogen. Yield: 222 mg (0.205 mmol, 83%). Complex 4a-OTf was
similarly prepared and isolated in comparable yield. Attempts to isolate
4a-Cl were not successful. Loss of H2 from 4a-Cl to form the neutral
chloride 2a was observed to be irreversible in the solid state (see Method
B for 2a, above.) 1H NMR and 31P NMR chemical shifts of 4a were
observed to be identical, regardless of counteranion. This observation
also applies to 4a-Cl and 4a-BArf, which were observed by NMR but
not isolated. 1H NMR (CD2Cl2) δ 2.2-1.2 (br, 66H), 1.41 (s, 9H),
1.37 (s, 18H), -6.30 (br, 2H); 31P{1H} NMR (CD2Cl2) δ 16.9 (s);
13C{1H} NMR (CD2Cl2) (obtained for 4a-BArf and 4a-BF4) δ 148.1
(t, JCP ) 10 Hz, CNR), 146.1 (t, JCP ) 7 Hz, CNR), 57.1 (s,
CNC(CH3)3), 56.7 (s, CNC(CH3)3), 37.3 (br, P-R-C), 31.3 (s, CNC-
(CH3)3), 30.6 (s, CNC(CH3)3), 30.2 (s, P-γ-C), 27.8 (t, JPC ) 4 Hz,
P-â-C), 27.0 (s, P-δ-C). T1(min) ) 8 ms (225 K, 300 MHz). Anal.
Calcd (found) for 4a-OTf: C, 54.42 (53.80); H, 8.34 (7.85); N, 3.66
(3.43).
Re(CNtBu)3(PCy3)2H (1a). Re(PCy3)2H7 (4.86 g, 6.44 mmol),
CNtBu (3.2 mL, 28 mmol), and toluene (200 mL) were stirred under
Ar atmosphere for 48 h at 45 °C. The solution turned deep dark red
and maintained this color throughout the reaction. The deep red color
is attributed to using less than completely pure Re(PCy3)2H7. Upon
addition of pentane and subsequent filtration, a rust colored impurity
was removed. The solvents were removed under vacuum. The
remaining oily solid was triturated and washed with acetone resulting
in a yellow slurry. Upon filtration, yellow microcrystals were collected
on a frit and washed with acetone (3 × 10 mL). Yield: 5.01 g (5.02
mmol, 78%). 1H NMR (CD2Cl2) δ 2.2-1.2 (br, 66H), 1.31 (s, 18H),
[Re(CNtBu)3(PPh3)2(η2-H2)]X (X- ) BF4- (4b-BF4), BArf-, (4b-
BArf)). For 4b-BArf, 1b (179 mg, 0.186 mmol) was dissolved in Et2O
(80 mL) under a H2 atmosphere. A clear yellow solution was observed
to form. HBArf‚(OEt2)2 (12.6 mg, 1.24 × 10-2 mmol) was separately
dissolved in Et2O (3 mL). The acid solution was transferred via Teflon
cannula to the first solution under vigorous stirring. The yellow solution
was observed to lose color. After reducing the solution to 4 mL under
vacuum, pentane (20 mL) was added causing a white precipitate to
form. The slurry was cannula-transferred onto a glass frit and filtered.
An off-white solid was collected. The product was washed with pentane
(3 × 10 mL) and dried under a stream of H2. Yield: 173 mg. 1H
NMR and 31P NMR show the solid to be approximately 80% 4b-BArf.
In contrast, NMR tube preparations of 4b-BArf and 4b-BF4 were clean
and quantitative. 1H NMR (cation portion) (CD2Cl2) δ 7.42 (br, 30H),
0.94 (s, 9H), 0.82 (s, 18H), -5.56 (br, 2H); 31P{1H} NMR (CD2Cl2) δ
18.7 (s). T1(min) ) 8 ms (207 K, 300 MHz).
2
1.28 (s, 9H), -7.49 (t, JPH ) 20.4 Hz, 1H); 31P{1H} NMR (CD2Cl2)
δ 28.5 (s); 13C{1H} NMR (toluene-d8) δ 173.9 (br t, CNR), 166.8 (t,
JCP ) 10 Hz, CNR), 54.1 (s, CNC(CH3)3), 53.6 (s, CNC(CH3)3), 38.1
(br, P-R-C), 32.0 (s, CNC(CH3)3), 31.9 (s, CNC(CH3)3), 30.3 (s, P-γ-
C), 28.6 (t, JPC ) 4 Hz, P-â-C), 27.9 (s, P-δ-C). Anal. Calcd
(found): C, 61.41 (61.07); H, 9.50 (9.13); N, 4.21 (4.12).
Re(CNtBu)3(PPh3)2H (1b). This preparation is more direct than
the one originally reported by Jones and Maguire.14 Re(PPh3)2H7 (0.30
g, 0.42 mmol), tert-butylisonitrile (0.57 mL, 5.0 mmol), and benzene
(5 mL) were stirred under Ar at 65 °C for 2 h. The product was
collected on a frit in the air and washed with pentane (2 × 10 mL).
Yield: 282 mg (0.29 mmol, 70%). 1H NMR (C6D6) agrees with that
reported by Jones and Maguire.14 31P{1H} NMR (C6D6) δ 32.7 (s).
Re(CNtBu)3(PCy3)2Cl (2a). Method A. Dry CH2Cl2 (5 mL) was
vacuum transferred into a 20 mL reaction flask containing 1a (107
mg, 0.107 mmol). The flask was pressurized with 530 Torr of CH3Cl.
The reactants were stirred at room temperature for 2 h. The initially
formed canary yellow solution was observed to darken to gold during
the course of the reaction. The solution was concentrated and cooled
to -78 °C. Yellow solid precipitated and was collected on a glass
frit. Yield 56 mg (0.054 mmol, 51%). Method B. Under an Ar
atmosphere, 1a (230 mg, 0.230 mmol) was stirred in Et2O (20 mL).
HCl in diethyl ether (1 M, 0.23 mL, 0.23 mmol) was added via syringe
through a rubber septum. The solution was observed to immediately
lose its yellow color and form a white precipitate. The slurry was
cannula transferred onto a glass frit and filtered. During the filtration,
the collected solid was observed to change color from white to yellow
and then redissolve in solution (accounting for the low yield).
Presumably, the white solid is 4a-Cl. As it loses H2, it is directly
converted to 2a, which is yellow. The collected product was washed
with pentane (3 × 5 mL). Yield: 50 mg (0.011 mmol, 21%). 1H
NMR (CD2Cl2) δ 2.2-1.2 (br, 66H), 1.44 (s, 18H), 1.25 (s, 9H);
31P{1H} NMR (CD2Cl2) δ 8.6 (s). Anal. Calcd (found): C, 59.36
(59.08); H, 9.08 (9.15); N, 4.07 (4.05).
-
[Re(CNtBu)3(PCy3)2]X (X- ) BF4 (5a-BF4), OTf- (5a-OTf)).
Coordinated H2 was removed from 4a-BF4 or 4a-OTf either by
exposing the white solids to dynamic vacuum (24 h, 23 °C) or by simply
leaving them under Ar atmosphere for 2 weeks. Removal of H2 was
monitored by observing the solids change color from white to dark
purple. This reaction was observed to be quantitative, and it was not
accompanied by any decomposition. Analytically pure compound was
obtained by recrystallization from THF. With exceptions made for
the different anions, 1H NMR, 13C NMR, and 31P NMR chemical shifts
of 5a-BF4 and 5a-OTf were observed to be identical. 1H NMR (CD2-
Cl2) δ 2.62 (broad, 6H), 2.0-1.2 (br, 60H), 1.59 (s, 18H), 1.20 (s,
9H); 31P{1H} NMR (CD2Cl2) δ 26.4 (s); 13C{1H} NMR (CD2Cl2) (for
5a-BF4) δ 160.1 (s, CNR), 144.0 (s, CNR), 57.4 (s, CNC(CH3)3), 39.2
(t, JPC ) 11 Hz P-R-C), 31.9 (s, CNC(CH3)3), 31.6 (s, CNC(CH3)3),
29.9 (s, P-γ-C), 27.9 (t, JPC ) 5 Hz, P-â-C), 26.6 (s, P-δ-C). Anal.
Calcd (found) for 5a-BF4: C, 56.54 (56.30); H, 8.65 (8.81), 3.88 (3.77).
[Re(CNtBu)4(PCy3)2]BF4 (6a-BF4). 1a (160 mg, 0.160 mmol) was
slurried overnight in a methanol solution (30 mL) containing excess
tert-butylisonitrile (60 µL, 0.53 mmol). Slow gas evolution was
observed during the first several hours. After 12 h, the solution had
become pale-yellow and homogeneous. A slurry of NaBF4 (220 mg)
in acetone (150 mL) was added via cannula, affording a homogeneous
solution upon stirring. The solvent was removed under vacuum, and
the product was extracted with CH2Cl2 (2 × 25 mL) resulting in a
yellow solution. Upon addition of diethyl ether (100 mL), a white
precipitate formed. The precipitate was collected by filtration and then
washed with diethyl ether (2 × 15 mL). Yield: 168 mg (0.144 mmol,
82%). 1H NMR (CD2Cl2) δ 2.28 (broad virtual triplet, JHP ) 12 Hz,
6H), 2.0-1.3 (br, 60H), 1.46 (s, 36H); 31P{1H} NMR (CD2Cl2) δ 5.6
(s). 13C{1H} NMR (CD2Cl2) δ 149.0 (t, JCP ) 8 Hz, CNR), 56.8 (s,
CNC(CH3)3), 37.2 (t, JPC ) 10 Hz, P-R-C), 31.2 (s, CNC(CH3)3), 29.8
(s, P-γ-C), 27.7 (t, JPC ) 4 Hz, P-â-C), 26.9 (s, P-δ-C). Anal. Calcd
(found): C, 57.66 (57.12); H, 8.82 (8.58); N, 4.80 (4.70).
Reaction of 1b with CD2Cl2: Formation of Re(CNtBu)3(PPh3)2Cl
(2b). Methylene chloride-d2 (0.4 mL) was vacuum transferred into a
sealable NMR tube containing 2b (4.1 mg, 4.3 × 10-3 mmol). The
tube was sealed under vacuum, and the reaction was monitored by 1H
NMR and 31P{1H} NMR. The conversion of 1b to 2b was observed
to go cleanly to completion over 2.5 h at 23 °C. Concomitant formation
of CD2HCl was also observed by 1H NMR. 1H NMR (CD2Cl2) δ 7.77
(br, 12H), 7.30 (br, 18H), 0.95 (s, 18H), 0.71 (s, 9H); 31P{1H} NMR
(CD2Cl2) δ 18.44 (s).
[Re(CNtBu)3(PCy3)2(η2-H2)]X (X- ) BF4-, OTf-) (4a-BF4, 4a-
OTf). For 4a-BF4, 1a (248 mg, 0.248 mmol) was dissolved in Et2O
(20 mL). Under a counter-flow of argon, HBF4‚OEt2 (42 µL, 0.29
(50) Hamilton, D. G.; Crabtree, R. H. J. Am. Chem. Soc. 1988, 110,
4126-4133.
(51) Van Geet, A. L. Anal. Chem. 1970, 42, 679-680.
(52) Zeiher, E. H. K.; DeWitt, D. G.; Caulton, K. G. J. Am. Chem. Soc.
1984, 106, 7006-7011.
Spectroscopic Observation of [Re(CNtBu)3(PCy3)2(N2)]Cl. Me-
thylene chloride-d2 (0.4 mL) was vacuum transferred into a sealable
NMR tube containing 2a (4.1 mg, 4.3 × 10-3 mmol). The tube was
pressurized with N2 (0.5 atm) and sealed. Partial conversion (5%) to

10802 J. Am. Chem. Soc., Vol. 118, No. 44, 1996
Heinekey et al.
[Re(CNtBu)3(PCy3)2(N2)]Cl was observed by NMR. 1H NMR (CD2-
Cl2) δ 2.2-1.2 (br, 66H), 1.50 (s, 18H), 1.37 (s, 9H); 31P{1H} NMR
(CD2Cl2) δ 6.9 (s).
An Eyring plot of ln (kobs/T) Vs 1/T was constructed. A best-fit line
drawn from a least-squares analysis of the data provided the enthalpy
and entropy of activation from the slope and the intercept, respectively.
Uncertainties in the activation enthalpy and entropy were calculated
from the uncertainties in the slope and intercept of the best-fit line.
[Re(CNtBu)3(PCy3)2Cl]X (X- ) Cl- (3a-Cl), OTf- (3a-OTf)).
These decomposition products were isolated in variable yields (10-
80% yield) from the reactions of 1a, 2a, 4a, 5a, or 7a and methylene
chloride. The following two examples are representative: The reaction
of 2a with methylene chloride was observed to be slow (t1/2 ≈ 1 week)
and produce a pink solution. 1H NMR spectra show very broad
temperature-dependent peaks. Upon addition of diethyl ether, a pink
solid was precipitated and isolated. Anal. Calcd (found) for
3a-Cl‚(CH2Cl2)3: C, 49.05 (49.20); H, 7.55 (7.68); N, 3.10 (3.18).
The reaction of 5a-OTf with methylene chloride was also observed to
be slow and produce a pink solution with identical NMR chemical shifts.
Layering the solution with diethyl ether resulted in the growth of small
pink diffraction quality crystals.
X-ray Structural Determination of Re(CNtBu)3(PCy3)2Cl (2a).
Single crystals of 2a were grown at -20 °C from methylene chloride
solutions. A crystal of suitable size (0.34 × 0.36 × 0.38 mm) coated
with paratone was mounted using epoxy glue. Diffraction measure-
ments were made at 183 K using the Enraf Nonius low-temperature
device. Twenty-five reflections in the range 32° to 38° in 2θ were
found and an orientation matrix was determined providing for a well
oriented monoclinic cell with a volume of 5935 Å3. The crystals had
a semiopaque appearance and were of good quality as evidenced by
the reasonably narrow peaks (hwfm ) 0.3° in 2θ) and agreement of
the equivalents. A high ø reflection was scanned to provide for an
absorption correction. The decay was negligible. The data to parameter
ratio was 10.3/1. Reduction of the data was carried out using XCAD4
and all further work was carried out using the Siemens version of
SHELX. The Laue merging R factor was 2% for 474 reflections with
a density of 1.33 for 4 molecules in the unit cell, based on the
assumption of a unique molecule.
Spectroscopic Observation of [Re(CNtBu)5(H2)]BArf. Method
A. Methylene chloride-d2 (0.4 mL) was vacuum transferred into a
screw-cap NMR tube containing Re(CNtBu)5Me (6 mg, 1 × 10-2
mmol) and excess HBArf‚2Et2O (6 mg, 6 × 10-3 mmol). Before
closing the cap, the headspace was pressurized with H2 (800 Torr).
The resulting orange solution turned brown when warmed to 298 K.
Method B. Methylene chloride-d2 (0.4 mL) was vacuum transferred
into a screw-cap NMR tube containing Re(CNtBu)5Cl (8.3 mg, 1.3 ×
10-2 mmol) and NaBArf (12 mg, 1.3 × 10-3 mmol). Before closing
the cap, the headspace was pressurized with H2 (800 Torr). The
resulting yellow solution turned brown when warmed to 298 K. In
The structure was solved by direct methods and determined from
the difference map thus obtained. Two methylene chloride solvent
molecules were located, although they suffered from disorder. The
solution contains a model of this disorder that provided for the best fit
of all data. Hydrogen atoms were introduced by calculation using a
C-H distance of 0.96 Å and fixed isotropic temperature factors. All
atoms except the hydrogens were refined anisotropically, and a final R
of 4.1% with a GOF of 1.16 was obtained. The weighting scheme
required a correction of 0.002.
1
both cases, H NMR (298 K) showed a broad resonance (δ ) -5.81
ppm). Several CNC(CH3)3 resonances were also observed in the region
between 1.3 and 1.5 ppm. These, however, could not be correlated
with the upfield resonance. For Method A, additional resonances due
to CH4 and Et2O could also be observed. When the H2 gas in the
headspace of either of these solutions (Method A or B) was removed
and replaced with D2, rapid H/D atom exchange resulted. A sharp
1:1:1 triplet was observed in the hydride region (δ ) -5.86 ppm; JHD
) 33.4 Hz).
X-ray Structural Determination of [Re(CNtBu)3(PCy3)2Cl]OTf
(3a). Well-formed pink crystals of 3a were grown at -20 °C by
layering a CH2Cl2 solution with diethyl ether. A crystal of small size
(0.05 × 0.10 × 0.15 mm) coated with paratone was mounted using
epoxy glue. Twenty-five reflections in the range 20° to 30° in 2θ were
found and an orientation matrix was determined. The monoclinic space
group was assigned as P21/m. A high ø reflection led to a good
absorption correction. The Laue merging R factor for 136 reflection
was 2.2%. The decay was less than 1%. Reduction of the data was
carried out using XCAD4 and all further work was performed using
the PC version of Siemen’s SHELX PLUS PC. The structure was
solved by locating the position of the rhenium atom using the Patterson
function. A final R of 4.3% with a GOF of 1 was obtained. The
weighting scheme required a correction of 0.002.
Method B was also carried out at lower temperatures (230 K) and
monitored by 1H NMR. The reaction did not proceed until a
temperature of 260 K was attained, at which point deomposition
products as well as [Re(CNtBu)5(H2)]BArf were observed to form
simultaneously.
Spectroscopic Observation of [Re(PMe3)5H2]OTf. Methylene
chloride-d2 (0.4 mL) was vacuum transferred into a screw-cap NMR
tube containing Re(PMe)5H (12.5 mg, 2.2 × 10-2 mmol). HOTf (1-2
µL, ca. 2 × 10-2 mmol) was added Via syringe under a blanket of
argon. The solution was observed to change from yellow/orange to
1
pale yellow. The conversion is clean and quantitative by H and 31P-
The tert-butyl groups were disordered in the mirror plane of the
cell and standard techniques were used to resolve as much as possible
the disorder. Carbons were introduced in the only mirror plane position
possible and site occupancies were determined by refinement. Due to
symmetry, it was not feasible to use distance fixing techniques and
attempts to approximate this failed. The methyl carbons were refined
with a common temperature factor. The anion was also disordered
and disordered atoms were introduced from a difference map and
occupancies determined. Two methylene chlorides were present and
badly disordered. Similar attempts to resolve this were attempted, and
the results are poor, probably due to the weak overall intensity ratio.
Hydrogen atoms were introduced by calculation using a C-H distance
of 0.96 Å and fixed isotropic temperature factors. A final R of 4.1%
with a GOF of 1.16 was obtained.
{1H} NMR. The product shows no signs of decomposition in solution.
1H NMR (CD2Cl2; 298 K) δ 1.66 (d, 45H, JHP ) 7.3 Hz), -7.55 (sextet,
2H, JHP ) 28.6 Hz); 31P{1H} NMR (CD2Cl2; 298 K) δ -45.5 (s);
31P{1H} NMR (CD2Cl2; 172 K) δ -31.5 (s, 1P), -46.1 (s, 2P), -48.3
(s, 2P). Reaction of the product solution with D2 (1280 Torr) was not
detected after 1 week.
Preparation of [Re(PMe3)5HD]OTf was achieved by the reaction of
Re(PMe)5H with DOTf following the above procedure. The initial
1
product ratio ([Re(PMe3)5H2]+:[Re(PMe3)5HD]+) observed by H and
31P{1H} NMR spectroscopy was 3.44:1. The hydride resonance of the
monodeuterated species was shifted downfield by 26 ppb relative to
the diprotio complex.
Spin Saturation Transfer Experiments on 5a-BF4. The sample
was prepared by combining 5a-BF4 (21 mg) and CD2Cl2 (0.4 mL) in
a flame-sealed 5-mm NMR tube. T1’s for the isonitrile resonances were
determined at each temperature (255-298 K) by the 180°-τ-90° pulse
sequence. Decoupler power was adjusted so that complete saturation
of the irradiated peak was observed after irradiating for at least five
T1’s. The extent of saturation transfer was ascertained by examining
difference spectra resulting from subtraction of data collected with and
without saturation at the upfield isonitrile resonance. Rates of exchange
were calculated as previously outlined for a two-site exchange process.53
Acknowledgment. We are grateful to the Department of
Energy, Office of Basic Energy Science, for support of this
research.
Supporting Information Available: Summary of X-ray
analysis of 2a and 3a, including data collection and refinement
procedures, tables of positional and thermal parameters, and
bond distances and angles (25 pages). See any current masthead
page for ordering and Internet access instructions.
(53) Faller, J. W. In Determination of Organic Structures by Physical
Methods; Nochod, F. C., Zuckerman, J. J., Eds.; Academic Press: New
York, 1973; Vol. 5, Chapter 2.
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