[{ReH2(PMePH2)2}2(μ-H) 3]-: The first member of a new class of anionic polyhydride dimers [Re2H7L4]-

Article abstract of DOI:10.1021/ic010041b

[K(Q)][{ReH₂(PMePh₂)₂}₂(μ-H) ₃] is prepared by treatment of {ReH₂(PMePh₂)₂}₂)μ-H) ₄ with potassium hydride and a crown ether (Q = 18-crown-6 or 1,10-diaza-18-crown-6). The [K(1,10 diaza-18-crown-6)]⁺ salt contains NH···HRe protonic - hydridic bonds and has been characterized by ³¹P NMR, variable temperature ¹H NMR, IR, and X-ray crystallography. The pK_α^THF = 33 of {ReH₂(PMePh₂)₂}₂(μ-H) ₄ was determined on the basis of its reaction with [K(2,2,2-crypt)][PO(OEt)₂NPh.

Full text of DOI:10.1021/ic010041b

2480
Inorg. Chem. 2001, 40, 2480-2481
[{ReH2(PMePh2)2}2(µ-H)3]-: The First Member of a New Class of Anionic Polyhydride Dimers [Re2H7L4]-
Justin G. Hinman, Kamaluddin Abdur-Rashid, Alan J. Lough, and Robert H. Morris*
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario M5S 3H6, Canada
ReceiVed January 10, 2001
A variety of monomeric, anionic rhenium hydride salts are
known, and they have been conveniently synthesized by the
deprotonation of neutral polyhydride complexes by use of a strong
base, such as KH.1-4 The anionic complexes are of interest as
they have been shown to display a wide range of basicities4 and,
in the presence of a hydrogen bond donor source, they have also
been shown to form extended structures held together by
protonic-hydridic bonds5,6 (also referred to as dihydrogen
bonds7). To date, the only examples of anionic dirhenium
polyhydride complexes (greater than two hydrides8) reported are
[(H)6Re(µ-H)3Re(CH3C(CH2P(C6H5)2)3)]- 9,10 and [(CO)3Re(µ-
H)3Re(CO)3]-.11 The former unsymmetrical dimer was prepared
by the reaction of ReH92- with triphos and was shown to contain
three bridging hydrides. A few heteronuclear dimers are also
known: [(H)3(PPh3)2Re(µ-H)3M(CO)3)]-, M ) Cr, Mo, W12,13
and [(H)3(PPh3)2Re(µ-H)3U(C5Me5)2Cl)]-.14 Here we report the
synthesis and characterization of [K(Q)][{Re2H2(PMePh2)2}2(µ-
H)3] (Q ) 18-crown-6 or 1,10-diaza-18-crown-6), the first
members of a novel series of anionic dirhenium polyhydride salts
of the form [{Re2H2(L)2}2(µ-H)3]-.15 These have been prepared
by a one step deprotonation reaction of the neutral conjugate acid
{ReH2(L)2}2(µ-H)4, using KH in the presence of the appropriate
crown ether (Scheme 1).
Scheme 1
PEtPh2, and PMe2Ph) complexes.18,19 In this study, {ReH2(PMe-
Ph2)2}2(µ-H)4 is conveniently prepared by a two step process.20
The first step is the addition of hydride to ReCl4(PMePh2)2,21 by
use of LiAlH4, to form ReH7(PMePh2)2. The second step is similar
to the one reported by Chatt and Coffey,16 and it involves heating
ReH7(PMePh2)2 to 68 °C under Ar in an EtOH/THF (3:1) solution
to convert it to the {ReH2(PMePh2)2}2(µ-H)4 complex.
The neutral dimer reacts with 1 equiv of KH in the presence
of a crown-ether in THF to yield [K(Q)][{ReH2(PMePh2)2}2(µ-
H)3] (2) as a red, air-sensitive, crystalline salt.22 The H NMR
1
spectrum of the salt at room temperature shows a characteristically
broad singlet at -8.42 ppm due to a rapid exchange of terminal
and bridging hydrides. A singlet in the 31P{1H} NMR spectrum
at 19.03 ppm is consistent with four equivalent phosphines. The
1
variable temperature H NMR spectra in the hydride region of
The neutral rhenium polyhydride dimeric precursors (1) are
well known and can be conveniently prepared by elimination of
H2 from the monomeric heptahydride complexes, ReH7(L)2 (L
) PPh3, PMePh2, PEtPh2).16,17 An alternative preparation involves
hydride addition to Re2Cl4(L)4 (L ) PMe3, PEt3, PnPr3, PMePh2,
[K(1,10-diaza-18-crown-6)][{ReH2(PMePh2}2(µ-H)3] in THF-d8
were recorded. At -20 °C the peak decoalesces, and at -64 °C
two hydride resonances are detected in a 3:4 intensity ratio,
indicative of four terminal hydrides (δ ) -9.42, t, J ) 7.5 Hz,
4H) and three bridging hydrides (δ ) -7.46, br, 3H). The solid-
state infrared spectrum shows two terminal ReH vibrational
modes, at 1935 and 1892 cm-1
.
* To whom correspondence should be addressed. E-mail:
rmorris@chem.utoronto.ca.
(1) Bruno, J. W.; Huffman, J. C.; Green, M. A.; Caulton, K. G. J. Am. Chem.
Soc. 1984, 106, 8310.
(2) Baudry, D.; Boydell, P.; Ephritikhine, M.; Felkin, H.; Guilhem, J.;
Pascard, C.; Tran Huu Dan, E. J. Chem. Soc., Chem. Commun. 1985,
670.
Complex 1 in THF at 20 °C was shown to be in equilibrium
with the acid OP(OEt)2NHPh (4), with pKRTHF ) 32 ( 44 (Scheme
THF
2) and with an equilibrium constant of Keq ) 0.079. The pKR
(3) Alvarez, D.; Lundquist, E. G.; Ziller, J. W.; Evans, W. J.; Caulton, K.
G. J. Am. Chem. Soc. 1989, 111, 8392.
(4) Abdur-Rashid, K.; Fong, T. P.; Greaves, B.; Gusev, D. G.; Hinman, J.
G.; Landau, S. E.; Morris, R. H. J. Am. Chem. Soc. 2000, 122, 9155.
(5) Abdur-Rashid, K.; Lough, A. J.; Morris, R. H. Can. J. Chem. 2001, in
press.
(6) Abdur-Rashid, K.; Gusev, D. G.; Landau, S. E.; Lough, A. J.; Morris,
R. H. J. Am. Chem. Soc. 1998, 120, 11826.
(7) Crabtree, R. H.; Siegbahn, P. E. M.; Eisenstein, O.; Rheingold, A. L.
Acc. Chem. Res. 1996, 29, 348.
(8) Hlatky, G. G.; Crabtree, R. H. Coord. Chem. ReV. 1985, 65, 1.
(9) Ginsberg, A. P.; Abrahams, S. C.; Marsh, P.; Ataka, K.; Sprinkle, C. R.
J. Chem. Soc., Chem. Commun. 1984, 1321.
(10) Abrahams, S. C.; Ginsberg, A. P.; Koetzle, T. F.; Marsh, P.; Sprinkle,
C. R. Inorg. Chem. 1986, 25, 2500.
(11) Ginsberg, A. P.; Hawkes, M. J. J. Am. Chem. Soc. 1968, 90, 5930.
(12) Freeman, J. W.; Arif, A. M.; Ernst, R. D. Inorg. Chim. Acta 1995, 240,
33.
(13) Drabnis, M. H.; Bau, R.; Mason, S. A.; Freeman, J. W.; Ernst, R. D.
Eur. J. Inorg. Chem. 1998, 851.
(18) Fanwick, P. E.; Root, D. R.; Walton, R. A. Inorg. Chem. 1989, 28, 3203.
(19) Brant, P.; Walton, R. A. Inorg. Chem. 1978, 17, 2074.
(20) Diethyl ether (15 mL) was added to a mixture of ReCl4(PMePh2)2 (0.950
g, 1.30 mmol) and LiAlH4 (0.30 g, 7.9 mmol) under Ar and the reaction
mixture was stirred for 3 h. It was then was filtered, and ethanol (15
mL) was added dropwise to the filtrate over a period of 2.5 h. The solvent
was then removed under vacuum, and the resulting brown powder was
shown by 1H NMR to be ReH7(PMePh2)2 . This was dissolved in 20
mL of THF/ethanol (1:3) and refluxed for 4 h under Ar. {ReH2-
(PMePh2)2}2(µ-H)4 precipitated as a bright red solid and was washed
with EtOH (3 × 8 mL). Yield: 0.354 g, 46.0%. 1H NMR (benzene-d6):
7.68-7.57 (m, 16 H, C6H5), 6.98-6.86 (m, 24 H, C6H5), 1.80 (d, 12H,
2J(PH) ) 8.7 Hz, PCH3), -5.85 (quin, 8H, 2J(PH) ) 9.0 Hz, ReH). 31
NMR(benzene-d6): 14.97 (s).
P
(21) Hahn, F. E.; Imhof, L.; Lu¨gger, T. Inorg. Chim. Acta 1997, 261, 109.
(22) Typical procedure conducted under Ar: THF (5 mL) was added to a
mixture of Re2H8(PMePh2)4 (72 mg, 0.061 mmol), 1,10-diaza-18-crown-6
(16 mg, 0.061 mmol), and KH (8 mg, 0.20 mmol), and was stirred for
8 h. Excess KH was filtered from the solution and washed with THF (3
mL). The combined filtrate was evaporated to dryness to yield a dark
red crystalline powder which was recrystallized from THF/diethyl ether.
Yield: 50 mg (55%). 1H NMR (THF-d8): 7.76-7.07 (m, 40 H, C6H5),
3.63 (m, 16 H, CH2), 2.74 (m, 8 H, CH2), 1.65 (qnt, 2 H, NH), 1.89 (d,
2J(PH) ) 7.5 Hz, PCH3), -8.42 (br, 7H, ReH). 31P{1H} NMR (THF-
d8): 19.03 (s). IR (neat):νRe-H 1935 (s), 1892 (s); νNH 3292 (w), 3231
(14) Cendrowski-Guillaume, S. M.; Ephritikhine, M. J. Chem. Soc., Dalton
Trans. 1996, 1487.
(15) Hinman, J. G.; Roesche, A.; Morris, R. H., manuscript in preparation.
(16) Chatt, J.; Coffey, R. S. J. Chem. Soc. A 1969, 1963.
(17) Bau, R.; Carroll, W. E.; Teller, R. G.; Koetzle, T. F. J. Am. Chem. Soc.
1977, 99, 3872.
(br) cm-1
.
10.1021/ic010041b CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/25/2001

Communications
Inorganic Chemistry, Vol. 40, No. 11, 2001 2481
Figure 1. The structure of {ReH2(PMePh2)2}2(µ-H)4 in molecule A of
two independent molecules. Selected bond lengths (Å) and angles (deg):
Re(1A)-Re(1A)#1 2.5338(2), Re(1A)-H(4A) 1.55(4), Re(1A)-H(3A)
1.57(4), Re(1A)-H(2A) 1.68(4), Re(1A)-H(2A)#1 1.93 (4), Re(1A)-
H(1A) 1.84 (4), Re(1A)-H(1A)#1 1.77(4), Re(1A)-P(1A) 2.3351(8),
Re(1A)-P(2A) 2.3503(9). H(4A)-Re(1A)-H(3A) 119(2), H(4A)-Re-
(1A)-H(2A) 78(2), H(3A)-Re(1A)-H(2A) 147(2), H(4A)-Re(1A)-
H(1A)#1 86(2), H(3A)-Re(1A)-H(1A)#1 137(2), H(2A)-Re(1A)-
H(1A)#1 66(2).
Figure 2. The structure of [K(1,10-diaza-18-crown-6)][{ReH2(PMePh2)2}2-
(µ-H)3]. Selected bond lengths (Å) and angles (deg): Re(1)-H(1Re)
1.51(3), Re(1)-H(2Re) 1.54(4), Re(1)-P(2) 2.3131(8), Re(1)-P(1)
2.3137(7), Re(1)-Re(1)#1 2.5958(2). H(1Re)-Re(1)-H(2Re) 137(2),
H(1Re)-Re(1)-P(2) 72(1), H(2Re)-Re(1)-P(2) 77(1), H(1Re)-Re(1)-
P(1) 82(1), H(2Re)-Re(1)-Re(1)#1 109(1), H(2Re)-Re(1)-Re(1)#1
113(1).
ously by single-crystal X-ray and neutron diffraction.26 [K(1,10-
diaza-18-crown-6)]+[{ReH2(PMePh2)2}2(µ-H)3]- crystallizes with
the phosphines eclipsed across a Re-Re bond distance of
2.5958(2) Å. This Re-Re bond distance is significantly longer
than that determined for the neutral conjugate acid, but is similar
to the 2.594(1) Å Re-Re bond distance for (µ-H)3 bridged
complex [NEt4][Re2(µ-H)3H6(CH3C(CH2P(C6H5)2)3)].10 The ex-
tended structure of [K(1,10-diaza-18-crown-6)][{ReH2(PMePh2)2}2-
(µ-H)3], as determined by X-ray diffraction, consists of chains
held together by protonic hydridic bonds, with NH‚‚‚HRe
distances of about 2.2 Å between the terminal ReH and the NH.
This distance was calculated by correcting the observed N-H
distance, 0.77(4) Å, to the 1.0 Å NH distance that is typically
determined by neutron diffraction methods, thus shortening the
observed H‚‚‚H distance of 2.35 Å to approximately 2.2 Å. The
terminal hydrides were located and refined isotropically, with
Re-H distances of 1.51(3) and 1.54(4) Å, however, the bridging
hydrides were not located. Attempts to obtain X-ray quality
crystals of the 18-crown-6 analogue without the protonic hydridic
interaction have not been successful to date, thus illustrating the
utility of using azacrowns and protonic-hydridic bonding in
crystallization.
Scheme 2
(the approximate free-ion pKa)23 for {ReH2(PMePh2)2}2(µ-H)4 was
calculated to be 33 ( 4 by using eq 1 and an estimate of the
difference in ion pair dissociation constants ∆pKd ) 0.4.24
pKRTHF(1) ) pKRTHF(4) - pKeq + ∆pKd
(1)
The structures of {ReH2(PMePh2)2}2(µ-H)4 and [K(1,10-diaza-
18-crown-6)][{ReH2(PMePh2)2}2(µ-H)3], as determined by single-
crystal X-ray crystallography, are shown in Figures 1 and 2,
respectively.25 Both structures have a center of inversion midway
between the two rhenium atoms. The Re-Re distance in {ReH2-
(PMePh2)2}2(µ-H)4 is 2.5338(2) Å, which is similar to those of
the {ReH2(L)2}2(µ-H)4 complexes that have been studied previ-
(23) pKRTHF is an approximation to the absolute free ion pKaTHF value, which
is obtained by crudely correcting the observed K for 1:1 ion pairing
effects by use of the Fuoss equation (see ref 4).
(24) The correction for ion pairing ∆pKd (see ref 4 for an explanation of this
correction) was calculated using the ionic radius of [K(2,2,2-crypt)]+ (5
Å), of [OP(OEt)2NPh]- (3 Å), and of [{ReH2(PMePh2)2}2(µ-H3)]- (4.3
Å) (determined from the distance between K and the midpoint of the
Re-Re bond).
(25) Crystals of [K(1,10-diaza-18-crown-6)][{ReH2(PMePh)2}2(µ-H3)] suitable
for X-ray analysis were grown by slow diffusion of hexane into a THF
solution of the salt. Crystallographic data for C64H86KN2O6P4Re2: a )
11.0535(2), b ) 12.157(1), c ) 13.2904(3) Å, R ) 66.782(1)°, â )
74.983(1)°, γ ) 83.151(1)°, with Z ) 1 in space group P1h. V )
1584.91(5) Å3, T ) 150(1) K, Dcalc ) 1.553 mg/m3, R1(F) [I > 2σ(I)]
) 0.0260, wR2 [all data] ) 0.0624, and data/restraints/parameters )
7250/0/364. Crystals of {ReH2(PMePh2)2}(µ-H)4 suitable for X-ray
analysis were grown by slow diffusion of hexane into a THF solution
of the compound. Crystallographic data for C52H60P4Re2: a ) 9.2683(1),
b ) 13.3189(2), c ) 19.4136(4) Å, R ) 89.148(1)°, â ) 84.436(1)°, γ
) 87.011°, with Z ) 2 in space group P1h. V ) 2381.83(7) Å3, T )
150(1) K, Dcalc ) 1.647 mg/m3, R1(F) [I > 2σ(I)] ) 0.0301, wR2 [all
data] ) 0.0720, and data/restraints/parameters ) 13768/0/560.
Further work will examine the effect of the ligand L in the
dimers [Re2H7L4]- on the hydrogen bond accepting ability and
the basicity of these anions, as measured by the pKRTHF value of
the conjugate acid Re2H8L4. Initial studies show that analogous
anionic dimers, with other phosphine ligands such as P(p-C6H4F)3
and PMe3, can be prepared.15
Acknowledgment. We thank NSERC for an operating grant
to R.H.M. and the University of Toronto for an open scholarship
to J.G.H.
Supporting Information Available: X-ray crystallographic files, in
CIF and PDF format, for C52H60P4Re2 and C64H86KN2O6P4Re2. This
material is available free of charge via the Internet at http://pubs.acs.org.
IC010041B
(26) Cotton, F. A.; Luck, R. L. Inorg. Chem. 1989, 28, 4522.