Site-differentiated hexanuclear rhenium(III) cyanide clusters [Re6Se8(PEt3)n (CN)6-n]n-4 (n = 4, 5) and kinetics of solvate ligand exchange on the cubic [Re6Se8]2+ core

Article abstract of DOI:10.1021/ic020214c

The site-differentiated, cyanide-substituted hexanuclear rhenium(III) selenide clusters cis- and trans-[Re₆Se₈(PEt₃)₄- (CN)₂] and [Re₆Se₈(PEt₃)₅(CN)]⁺ have been prepared from heterogeneous reactions of the corresponding iodo clusters with AgCN in refluxing chloroform. Isolated yields are 68%, 46%, and 64% for cis-[Re₆Se₈(PEt₃)₄ (CN)₂], trans-[Re₆Se₈(PEt₃)₄ (CN)₂], and [Re₆Se₈(PEt₃)₅(CN)]⁺, respectively. The new compounds are air- and water-stable and are characterized by X-ray diffraction crystallography, ³¹P NMR and IR spectroscopies, and FAB mass spectrometry. In related work, the solvent exchange rates of two site-differentiated monosolvate clusters, [Re₆Se₈-(PEt₃)₅(MeCN)] (SbF₆)₂ and [Re₆Se₈(PEt₃)₅ (Me₂SO)](SbF₆)₂, in neat solvents were measured by ¹H NMR. These clusters are substitutionally inert; k ≈ 10^-5-10^-6 s^-1 at 318 K. Activation parameters indicate a dissociative ligand exchange mechanism; ΔH^? values obtained from least-squares fitting of temperature-dependent kinetics data exceed RT by a factor of ca. 50 over the temperature range studied. These results demonstrate that the substitutional lability encountered in a previous study of cluster photophysics (Gray, T. G.; Rudzinski, C. M.; Nocera, D. G.; Holm, R. H. Inorg. Chem. 1999, 38, 5932) cannot result from ground-state thermal reactions.

Full text of DOI:10.1021/ic020214c

Inorg. Chem. 2002, 41, 4211−4216
Site-Differentiated Hexanuclear Rhenium(III) Cyanide Clusters
[Re6Se8(PEt3)n(CN)6-n]n-4 (n ) 4, 5) and Kinetics of Solvate Ligand
Exchange on the Cubic [Re6Se8]2+ Core
Thomas G. Gray and R. H. Holm*
Department of Chemistry and Chemical Biology, HarVard UniVersity,
Cambridge, Massachusetts 02138
Received March 19, 2002
The site-differentiated, cyanide-substituted hexanuclear rhenium(III) selenide clusters cis- and trans-[Re6Se8(PEt3)4-
(CN)2] and [Re6Se8(PEt3)5(CN)]+ have been prepared from heterogeneous reactions of the corresponding iodo
clusters with AgCN in refluxing chloroform. Isolated yields are 68%, 46%, and 64% for cis-[Re6Se8(PEt3)4(CN)2],
trans-[Re6Se8(PEt3)4(CN)2], and [Re6Se8(PEt3)5(CN)]+, respectively. The new compounds are air- and water-stable
and are characterized by X-ray diffraction crystallography, 31P NMR and IR spectroscopies, and FAB mass
spectrometry. In related work, the solvent exchange rates of two site-differentiated monosolvate clusters, [Re6Se8-
(PEt3)5(MeCN)](SbF ) and [Re6Se8(PEt3)5(Me2SO)](SbF ) , in neat solvents were measured by 1H NMR. These
6 2
6 2
clusters are substitutionally inert; k ≈ 10-5−10-6 s-1 at 318 K. Activation parameters indicate a dissociative ligand
q
exchange mechanism; ∆H values obtained from least-squares fitting of temperature-dependent kinetics data exceed
RT by a factor of ca. 50 over the temperature range studied. These results demonstrate that the substitutional
lability encountered in a previous study of cluster photophysics (Gray, T. G.; Rudzinski, C. M.; Nocera, D. G.;
Holm, R. H. Inorg. Chem. 1999, 38, 5932) cannot result from ground-state thermal reactions.
Introduction
We hold that rational means of site differentiation in
transition element clusters are imperative to nonserendipitous,
systematic advancement of metallocluster chemistry. Two
strategies for site-differentiating metal aggregates are obvi-
ous: (i) ligand-derived and (ii) cluster-enforced site-dif-
ferentiation. Cubane-type [Fe4S4] and [LnMFe3S4]2,9 and
cuboidal [Fe3S4]10 clusters exemplify strategy i. Such clusters,
being composed of first-row transition element ions, are
substitutionally labile and can become moreso upon reduction
of the cluster core.11 In these species, site-differentiation relies
upon the cavitand concept12 of ligand design. This concept
relies on elaborated organic ligands, specially designed to
capture the Fe3 face of an [Fe4S4] or [MFe3S4] cuboid. The
ligand conformation in the bound cluster approximates the
The concept of the site-differentiated cluster is long
appreciated by bioinorganic investigators,1,2 but remains less
familiar in more broadly oriented metallocluster research.3,4
By “site-differentiation” is meant the controlled and steadfast
occupancy of one or more of the exopolyhedral ligand-
binding sites of a cluster, such that all further reactions of
the remaining few ligands occur regioselectiVely. Site
differentiation presupposes that substitutionally inert nucleo-
philes satisfy the ligation requirements of several metal atoms
simultaneously; the site(s) left over bind more labile ligands.1
Cluster-centered ligand substitution chemistry is thus simpli-
fied, and new synthetic5-7 and spectroscopic8 opportunities
thereby emerge.
* Author to whom correspondence should be addressed. E-mail:
holm@chemistry.harvard.edu.
(1) Beinert, H.; Kennedy, M. C.; Stout, C. D. Chem. ReV. 1996, 96, 2335.
(2) Holm, R. H.; Ciurli, S.; Weigel, J. A. Prog. Inorg. Chem. 1990, 38,
1.
(3) (a) Catalysis by Di- and Polynuclear Metal Cluster Complexes; Adams,
R. D., Cotton, F. A., Eds.; Wiley-VCH: New York, 1998. (b)
Gonza´lez-Moraga, G. Cluster Chemistry; Springer-Verlag: New York,
1993.
(5) Zheng, Z.; Holm, R. H. Inorg. Chem. 1997, 36, 5173.
(6) Zheng, Z.; Gray, T. G.; Holm, R. H. Inorg. Chem. 1999, 38, 4888.
(7) Decker, A.; Simon, F.; Boubekeur, K.; Fenske, D.; Batail, P. Z. Anorg.
Allg. Chem. 2000, 626, 309.
(8) Gray, T. G.; Rudzinski, C. M.; Nocera, D. G.; Holm, R. H. Inorg.
Chem. 1999, 38, 5932.
(9) Holm, R. H. AdV. Inorg. Chem. 1992, 38, 1.
(10) Zhou, J.; Hu, Z.; Mu¨nck, E.; Holm, R. H. J. Am. Chem. Soc. 1996,
118, 1966.
(4) Zheng, Z.; Long, J. R.; Holm, R. H. J. Am. Chem. Soc. 1997, 119,
2163.
(11) Goh, C.; Segal, B. M.; Huang, J.; Long, J. R.; Holm, R. H. J. Am.
Chem. Soc. 1996, 118, 11844.
10.1021/ic020214c CCC: $22.00 © 2002 American Chemical Society
Published on Web 06/29/2002
Inorganic Chemistry, Vol. 41, No. 16, 2002 4211

Gray and Holm
lowest-energy conformation of the free ligand. The cavitand
concept has not yet been applied to single clusters of
nuclearity exceeding four, but certainly pertains to larger
entities. In this laboratory, the most successful such ligand
has been 1,3,5-tris((4,6-dimethyl-3-mercaptophenyl)thio)-
No other cluster system is as synthetically versatile in
producing type ii site-differentiated species as is [Re6Q8]2+
(Q ) S, Se). The reduced, group 6 halide clusters21
[Mo6X14]2-22 and [W6X14]2- 23,24 are isoelectronic with
[Re6Q8X6]2- clusters (X ) Cl, Br, I), but their substitution
chemistry is nonsystematic and is dominated by persubsti-
tution of the six apical halide ligands,25,26 obviating site-
differentiation. A similar situation prevails for the edge-
bridged niobium and tantalum halide clusters,26 and intersti-
tially stabilized Zr6 species.27 Recently, tungsten sulfide
clusters, also site-differentiated with phosphine ligands, have
appeared.28 Further, no polynuclear metal carbonyl cluster
exhibits a profusion of site-differentiated derivatives com-
parable to that of [Re6Q8]2+.
The purpose of this work is 2-fold. We seek first to extend
the range of site-differentiation of Re6Q8 clusters to encom-
pass cyanide-bound species. This development complements
the range of compounds prepared from the hexacyano
clusters of Fedorov,29 Ibers,30 Long,31 Kim,32 and their
respective co-workers. Second, we quantify the inertness of
the [Re6Se8]2+ core in well-defined solvent exchange reac-
tions, and therefrom draw conclusions concerning the mech-
anism of ligand substitution in such clusters. Resultant
kinetics parameters also resolve a standing issue concerning
certain lability observed in hexasolvate clusters upon pho-
toexcitation. New cyanide-ligated clusters and solvate clusters
are designated as in Chart 1.
2,4,6-tris(p-tolylthio)benzene(3-), abbreviated LS3.12,13
A
number of other 1:3 site-differentiating ligands have also
been described.14-17
A new approach to site-differentiation in synthetic [Fe4S4]
clusters involves de-novo-designed peptides. Recent work
from this laboratory describes a helical, 63-mer peptide
designed at once to site-differentiate an [Fe4S4] cluster and
to induce bridging, through a cysteinate residue, to a nearby
nickel(II) center.18 The resulting supramolecular assembly
was prepared as an analogue of the spectroscopic A site of
the enzyme carbon monoxide dehydrogenase. In this entity,
as in LS3 complexes, site-differentiation is ligand-imposed.
Site-differentiation in cubic [Re6Q8] clusters (Q ) S, Se)19
represents an extreme example of strategy ii, where cluster
inertness to ligand substitution maintains stereochemical
integrity of the ligand sphere. As such, site-differentiation
relies on the intrinsic properties of the cluster itself, not on
its ligands. In this laboratory, site-differentiation is impelled
on the [Re6Q8]2+ core by thermal reactions with triethylphos-
phine in DMF. These reactions, which require prolonged
heating, produce mixtures of clusters, site-differentiated with
tightly bound phosphine ligands. Product distributions are
controllable through the loading of triethylphosphine and the
reaction duration. For example, reaction of (Bu4N)3[Re6S8-
Br6] with 50 equiv of PEt3 for 8 h, in refluxing DMF, yields
mer- and fac-(Bu4N)[Re6S8(PEt3)3Br3] in 33% and 5% yields,
respectively; refluxing for 36 h yields cis- and trans-
[Re6S8(PEt3)4Br2], in 43% and 19% respective yields; 60 h
of refluxing, under the same conditions, produces [Re6S8-
Chart 1. Designation of Clusters
cis-[Re6Se8(PEt3)4(CN)2]
trans-[Re6Se8(PEt3)4(CN)2]
[Re6Se8(PEt3)5(CN)](BPh4)
[Re6Se8(PEt3)5(solv)](SbF6)2
1
2
3
4,a 5b
a solv ) MeCN.5 b solv ) Me2SO.
Results and Discussion
(PEt3)5Br]Br in 58% yield.20 A parallel series of reactions
(n-4)+ 6
Cyanide-Terminated Clusters. The heterogeneous reac-
tion of cis- or trans-[Re6Se8(PEt3)4I2] with AgCN in refluxing
chloroform affords the corresponding cyano-terminated
clusters 1-3 in moderate isolated yields. Triethylphosphine
ligands preserve cluster stereochemistry. X-ray diffraction
quality crystals of 1 and 2 are obtained overnight by layering
diethyl ether upon concentrated dichloromethane solutions
of the cluster compounds at room temperature; crystals of 3
form upon standing for 4 h in chloroform solution after anion
metathesis with NaBPh4.
exists for selenium-iodine analogues [Re6Se8(PEt3)nI6-n
]
.
Notably, no circumstance is observed where the Re-PEt3
linkage ruptures, either thermally or photolytically, in fluid
solution. In solid samples, heating to 500 °C under vacuum
is required to decomplex PEt3 from the [Re6Se8]2+ core.5 So
tenaciously do [Re6Q8]2+ clusters retain triethylphosphine that
mixed-ligand entities are air- and water-stable, and the as-
prepared mixtures of these clusters are straightforwardly
separated by silica gel flash-column chromatography.
Figures 1, 2, and 3 depict the crystal structures of 1-3,
respectively. Salient crystallographic data are collected in
Table 1. Cluster metrical features of 1-3 are closely similar
(12) Stack, T. D. P.; Weigel, J. A.; Holm, R. H. Inorg. Chem. 1991, 29,
3745.
(13) Stack, T. D. P.; Holm, R. H. J. Am. Chem. Soc. 1987, 109, 2546;
1988, 110, 2484.
(14) Whitener, M. A.; Peng, G.; Holm, R. H. Inorg. Chem. 1991, 30, 2411.
(15) Evans, D. J.; Garcia, G.; Leigh, G. J.; Newton, M. S.; Santana, M. D.
J. Chem. Soc., Dalton Trans. 1992, 3229.
(21) Prokopuk, N.; Shriver, D. F. AdV. Inorg. Chem. 1998, 46, 1.
(22) Sheldon, J. C. J. Chem. Soc. 1960, 1007.
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den Schoor, R. C. G. M.; Feiters, M. C.; van der Linden, J. G. M.;
Steggerda, J. J.; Nolte, R. J. M. J. Chem. Soc., Dalton Trans. 1997,
449.
(17) Walsdorff, C.; Saak, W.; Pohl, S. J. Chem. Soc., Dalton Trans. 1997,
1857.
(18) Laplaza, C. E.; Holm, R. H. J. Am. Chem. Soc. 2001, 123, 10255.
(19) Saito, T. J. Chem. Soc., Dalton Trans. 1999, 97.
(20) Willer, M. W.; Long, J. R.; McLauchlan, C. C. Inorg. Chem. 1998,
37, 328.
(23) Hogue, R. D.; McCarley, J. D. Inorg. Chem. 1970, 9, 1354.
(24) Franolic, J. D.; Long, J. R.; Holm, R. H. J. Am. Chem. Soc. 1995,
117, 8139.
(25) Johnston, D. H.; Gaswick, D. C.; Lonegran, M. C.; Stern, C. L.;
Shriver, D. F. Inorg. Chem. 1992, 31, 1869.
(26) Prokopuk, N.; Shriver, D. F. Inorg. Chem. 1997, 36, 5609.
(27) Cotton, F. A.; Hughbanks, T.; Runyan, C. E., Jr.; Wojtzczak, W. A.
In Early Transition Metal Clusters with π-Donor Ligands; Chisholm,
M., Ed.; VCH: New York, 1995; Chapter 1.
(28) Jin, S.; Venkataraman, D.; DiSalvo, F. J. Inorg. Chem. 2000, 39, 2747.
4212 Inorganic Chemistry, Vol. 41, No. 16, 2002

Site-Differentiated Hexanuclear Re(III) Cyanide Clusters
Figure 3. Structure of [Re6Se8(PEt3)5(CN)]+ (3) showing 50% probability
ellipsoids. For clarity, triethylphosphine carbon atoms are depicted as spheres
of arbitrary radii. A mirror plane bisects the cluster through four Re atoms
and the CN ligand. A partial atom-labeling scheme is shown.
Figure 1. Structure of cis-[Re6Se8(PEt3)4(CN)2] (1) showing 50% prob-
ability ellipsoids. For clarity, triethylphosphine carbon atoms are depicted
as spheres of arbitrary radii. This cluster has imposed centrosymmetry. A
partial atom-labeling scheme is shown.
31P NMR spectra of 1-3 are consistent with crystallo-
graphically determined stereochemistries. The cis cluster 1
exhibits equally intense resonances at δ -30.2 and -31.0
ppm; the trans isomer 2 exhibits a singlet at δ -30.5 ppm,
whereas monocyano cluster 3 shows two 31P resonances, in
a 4:1 ratio, at δ -30.5 and -31.0 ppm, respectively.30
Infrared spectra exhibit νCN stretches at 2121-2123 cm-1
for the three cyano clusters. These values exceed 2080 cm-1
for cyanide ion in aqueous solution and indicate that back-
donation to the CN π* orbitals, if any, is negligible.23
Extended Hu¨ckel calculations on 1-3 accord with this view;
they show that the highest occupied molecular orbitals of
each cluster are dominated by d-orbital amplitude on Re.
These orbitals, which are nondegenerate, derive from the eg
HOMOs of [Re6Q8X6]4- clusters in Oh symmetry.34,35 Figure
4 depicts a plot of these orbitals for the hypothetical model
(29) (a) Naumov, N. G.; Virovets, A. V.; Sokolov, M. N.; Artemkina, S.
B.; Fedorov, V. E. Angew. Chem., Int. Ed. 1998, 37, 1943. (b) Imoto,
H.; Naumov, N. G.; Virovets, A. V.; Saito, T.; Fedorov, V. E. J. Struct.
Chem. 1998, 39, 720. (c) Naumov, N. G.; Virovets, A. V.; Moronov,
Yu. I.; Artemkina, S. B.; Fedorov, V. E. Ukr. Chem. J. 1999, 65, 17.
(d) Naumov, N. G.; Artemkina, S. B.; Virovets, A. V.; Fedorov, V.
E. J. Solid State Chem. 2000, 153, 195. (e) Naumov, N. G.; Soldatov,
D. V.; Ripmeester, J. A.; Artemkina, S. B.; Fedorov, V. E. Chem.
Commun. 2001, 571. (f) Artemkina, S. B.; Naumov, N. G.; Virovets,
A. V.; Gromilov, S. A.; Fenske, D.; Fedorov, V. E. Inorg. Chem.
Commun. 2001, 4, 423.
(30) (a) Mironov, Y. V.; Cody, J. A.; Albrecht-Schmitt, T. E.; Ibers, J. A.
J. Am. Chem. Soc. 1997, 119, 493. (b) Mironov, Y. V.; Fedorov, V.
E.; McLauchlan, C. C.; Ibers, J. A. Inorg. Chem. 2000, 39, 1809. (c)
Mironov, Y. V.; Fedorov, V. E.; Ijjaali, I.; Ibers, J. A. Inorg. Chem.
2001, 40, 6320.
(31) (a) Beauvais, L. G.; Shores, M. P.; Long, J. R. Chem. Mater. 1998,
10, 3783. (b) Shores, M. P.; Beauvais, L. G.; Long, J. R. Inorg. Chem.
1999, 38, 1648. (c) Shores, M. P.; Beauvais, L. G.; Long, J. R. J. Am.
Chem. Soc. 1999, 121, 775. (d) Beauvais, L. G.; Shores, M. P.; Long,
J. R. J. Am. Chem. Soc. 2000, 122, 2763. (e) Bennett, M. V.; Shores,
M. P.; Beauvais, L. G.; Long, J. R. J. Am. Chem. Soc. 2000, 122,
6664. (f) Bennett, M. V.; Beauvais, L. G.; Shores, M. P.; Long, J. R.
J. Am. Chem. Soc. 2001, 123, 8022.
Figure 2. Structure of trans-[Re6Se8(PEt3)4(CN)2] (2) showing 50%
probability ellipsoids. For clarity, triethylphosphine carbon atoms are
depicted as spheres of arbitrary radii. This cluster has imposed centrosym-
metry. A partial atom-labeling scheme is shown.
to those of related [Re6Se8]2+ species.6 In all cases, the [Re6-
Se8] core structures closely approach Oh symmetry; any
symmetry-breaking deviations in the structures reported here
and those described previously are asystematic. Mean values
of terminal ligand bond distances are summarized in Table
2. The Re-P bond lengths range from 2.47 to 2.49 Å; when
previous data5-7 are included, the interval is 2.44-2.51 Å.
Measured Re-C bond lengths and corresponding bond
angles are unexceptional.29-32
(32) Kim, Y.; Park, S.-M.; Nam, W.; Kim, S.-J. Chem. Commun. 2001,
1470.
Inorganic Chemistry, Vol. 41, No. 16, 2002 4213

Gray and Holm
Table 1. Crystal Dataa and Structure Refinement for Site-Differentiated Cyano Clusters
1
2
[3](BPh4)
formula
fw
C26H60N2Re6Se8P4
2273.59
C26H60N2Re6Se8P4
2273.59
C55H95NBRe6Se8P5
2684.96
space group
Z
Ibam
16
P1h
1
P21/n
4
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
18.5879(7)
35.3143(13)
18.0726(6)
10.5369(9)
11.6919(10)
12.8561(11)
71.208(2)
82.033(2)
70.620(2)
1413.5(2)
2.671
21.2879(8)
13.3830(5)
26.6159(10)
111.383(2)
11863.2(7)
2.657
17.324
1.15-22.50
0.0508, 0.1441
7060.8(5)
2.526
14.517
1.06-28.47
0.0852, 0.2142
d
calc, g/cm3
µ, mm-1
18.075
1.93-27.67
0.0304, 0.0881
θ range, deg
c
R1,b wR2
a Obtained with graphite-monochromatized Mo KR (λ ) 0.71073 Å) radiation at 213 K. b R1 ) ∑||Fo| - |Fc||/∑|Fo|. c wR2 ) [∑w(|Fo| - |Fc|)2/∑w|Fo|]1/2
Table 2. Interatomic Distances (Å) for Clusters 1-3
.
rate ) k[Re6Se8(PEt3)5(solv)]2+
(2)
Re-Re
mean
Re-Se
mean
2.622(13)-2.650(4)
2.637(7)
2.497(9)-2.530(7)
2.513(8)
Re-P
mean
Re-C
mean
2.457(9)-2.490(7)
2.474(9)
2.08(3)-2.11(3)
2.11(2)
progress was monitored under pseudo-first-order conditions
in [solv]. Plots of ln[Re6Se8(PEt3)5(solv)]2+, as measured by
the integrated area of the H NMR resonance of the bound
1
cluster trans-[Re6Se8(PH3)4(CN)2], which represents 2. A
Mulliken gross overlap population analysis indicates that the
cyanide valence orbitals are uninvolved (0% contribution)
in these high-energy occupied orbitals; however, their
exclusion is not symmetry-imposed.
acetonitrile, vs time, are linear to at least 3 half-lives; Figure
5 shows a representative example for the cluster [Re6Se8-
(PEt3)5(MeCN)]2+ at 328 K. Rate constants as a function of
temperature are summarized in Table 3. Activation param-
eters were determined by linear least-squares fitting to the
Eyring eq 336 as shown in Figure 6. We find that ∆Sq )
Kinetics of Thermal Solvent Exchange in [Re6Se8-
(PEt3)5(solv)]2+ (solv ) MeCN, Me2SO). We have observed
that certain clusters, including solvated species, expel ligands
under photolysis conditions at ambient temperature.8 Precise
data concerning thermal ligand elimination, which necessarily
competes with photosubstitution, are required to distinguish
ground-state from excited-state reactivity. We select the
following ligand exchange reactions (1) of the site-differenti-
ln(k/T) ) ln(kB/h) + ∆Sq - ∆Hq/RT
(3)
14.8 ( 1.0 cal K-1 mol-1 and ∆Hq ) 30.6 ( 1.4 kcal mol-1.
Activation parameters for the related cluster [Re6Se8-
(PEt3)5(Me2SO)]2+ are similar (Table 3). These results
indicate a dissociative ligand exchange of substantial activa-
tion barrier. From the structures of cis- and trans-[Re6Se8-
(PEt3)4(OSMe2)2]2+,6,7 the sulfoxide is O-bonded. Since, at
298 K, RT ) 0.59 kcal mol-1, we conclude that thermal
ligand exchange at room temperature cannot account for the
ligand lability which we have documented to occur in the
presence of exciting radiation.8
1
ated clusters 4 and 5 for H NMR study.
[Re6Se8(PEt3)5(solv)](SbF6)2 + solv* f
[Re6Se8(PEt3)5(solv*)](SbF6)2 + solv (1)
solv ) MeCN, Me2SO; solv* )
MeCN-d3, Me2SO-d6, respectively
Decay of the bound solv 1H resonance was monitored with
time at several temperatures. The reaction proceeds cleanly
at all temperatures investigated. The bound acetonitrile H
Summary
The following are the primary results and conclusions of
this work.
1
resonance (δ 2.75) decreases monotonically with time in CD3-
CN; other resonances are unaffected; the Me2SO-ligated
cluster (δ 2.59) behaves analogously in Me2SO-d6. Chemical
(1) Cyanide can be regioselectively substituted for iodide
in cis- and trans-[Re6Se8(PEt3)4I2] and in [Re6Se8(PEt3)5I]I
in heterogeneous reactions with AgCN. The resulting clusters
are air- and water-stable. These clusters are also phospho-
rescent, and luminescence properties are detailed elsewhere.37
1
shifts of all H resonances are independent of temperature.
The ligand exchange product, [Re6Se8(PEt3)5(solv*)](SbF6)2,
can be recrystallized quantitatively by layering diethyl ether
onto the reaction mixture at 4 °C or room temperature.
Solvent exchange is first order overall (eq 2). Reaction
(2) Infrared spectra of clusters 1, 2, and 3 indicate
negligible back-bonding from the [Re6Se8]2+ core into
cyanide π* orbitals.
(33) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coor-
dination Compounds, 4th ed.; Wiley-Interscience: New York, 1986;
p 272.
(34) Hughbanks, T.; Hoffmann, R. J. Am. Chem. Soc. 1983, 105, 1150.
(35) Mealli, C.; Lo´pez, J. A.; Sun, Y.; Calhorda, M. J. Inorg. Chim. Acta
1993, 213, 199.
(36) Espenson, J. H. Chemical Kinetics and Reaction Mechanisms, 2nd
ed.; McGraw-Hill: New York, 1995.
(37) Gray, T. G.; Rudzinski, C. M.; Meyer, E.; Holm, R. H.; Nocera, D.
G. To be submitted for publication.
4214 Inorganic Chemistry, Vol. 41, No. 16, 2002

Site-Differentiated Hexanuclear Re(III) Cyanide Clusters
Figure 4. Extended Hu¨ckel HOMO and second HOMO of trans-[Re6Se8(PH3)4(CN)2], which serves as a model for 2. A partial atom-labeling scheme is
shown.
Figure 5. Graph of the logarithm of the integrated area of the bound MeCN
1H NMR resonance of [Re6Se8(PEt3)5(MeCN)](SbF6)2 at δ 2.75 ppm,
recorded at 55 °C in MeCN-d3 at 10 min intervals.
Table 3. Kinetics Data for Solvate Exchange in [Re6Se8(PEt3)5(solv)]2+
solvent T (K)
k (s-1
)
∆Hq (kcal mol-1
30.6 ( 1.4
)
∆Sq (cal K-1 mol-1
14.8 ( 1.4
)
MeCN
318
323
328
333
338
343
313
318
323
328
333
338
343
9.0 × 10-6
1.9 × 10-5
4.7 × 10-5
7.9 × 10-5
2.2 × 10-5
5.8 × 10-4
7.5 × 10-7
1.6 × 10-6
3.3 × 10-6
6.7 × 10-6
1.3 × 10-5
2.6 × 10-5
4.9 × 10-5
DMSO
29.2 ( 0.8
6.7 ( 0.2
Figure 6. Eyring plots for solvent exchange in [Re6Se8(PEt3)5(solv)]2+
(a) solv ) MeCN; (b) solv ) Me2SO.
:
loss, and are qualitatively consistent with the very slow (∼7
days) conversion of [Re6S8(OH2)6]2+ to [Re6S8Cl6]4- in 1 M
HCl.38 For approximate comparison with ligand exchange
reactions of trivalent ions, we calculate from eq 3 k ) 3.9
× 10-7 s-1 (4) and k ) 7.5 × 10-8 s-1 (5) at 298 K. These
data are in same regime as rate constants (s-1) for [CrL6]3+
(L ) H2O, 2.4 × 10-6; DMF, 3.3 × 10-7; Me2SO, 3.1 ×
10-8) and [Ru(OH2)6]3+ (3.5 × 10-6),39 but substantially less
than for [Rh(OH2)6]3+ (2.2 × 10-9) and [Ir(OH2)6]3+ (1.1 ×
10-10).40 A further assessment of the kinetics characteristics
(3) The first terminal ligand substitution rates have been
determined for [Re6Q8]2+ clusters. Solvate ligand exchange
of 4 and 5 in neat solvents is extremely slow (k ≈ 10-5-
10-6 s-1at 318 K) and is dissociative, with a substantial
barrier to Re-terminal ligand bond scission. Observed values
of ∆Hq exceed RT by a factor of ca. 50 at ambient
temperature, conclusively demonstrating that the lability
previously encountered upon photoexcitation cannot be
thermal in origin.
The results described here broaden the range of site-
differentiated clusters to include soluble, metal-metal
bonded polynuclear cyanide complexes. Also, the first
quantitative kinetics data on [Re6Se8]2+ ligand exchange are
presented. These data indicate extreme inertness to ligand
(38) Fedin, V. P.; Virovets, A. A.; Sykes, A. G. Inorg. Chim. Acta 1998,
271, 228.
(39) Lincoln, S. F.; Merbach, A. E. AdV. Inorg. Chem. 1995, 42, 1.
(40) Cusanelli, A.; Frey, U.; Richens, D. T.; Merbach, A. E. J. Am. Chem.
Soc. 1996, 118, 5265.
Inorganic Chemistry, Vol. 41, No. 16, 2002 4215

Gray and Holm
of [Re6Q8]2+ cores would be greatly aided by determination
of the rate constant for aquo ligand exchange of [Re6Q8-
(OH2)6]2+ in water. Substitutional inertness also concurs with
older observations that network solids built upon the
[Re6Q8]2+ core (Q ) S, Se) defy cluster excision.41 Ligand
substitution of [Re6Q8]2+ cores is dissociative, and the loss
of intercluster bonding in extended solids is energetically
prohibitive. The kinetic stability of such solids, and also of
newfangled, expanded Prussian blue-type materials, is now
understandable in terms of the inherent propensities of the
constituent clusters. In particular, steric impenetrability of
extended cluster solids need not be the only factor promoting
their stability.42
trans-[Re6Se8(PEt3)4(CN)2]. To a 50 mL Schlenk flask equipped
with a stir bar was added 0.1169 g of trans-[Re6Se8(PEt3)4I2]5 (47.22
µmol); the cluster was dissolved in dichloromethane to give an
orange solution. Silver cyanide (0.10593 g, 0.7912 mmol, 4.03
equiv) was added, and the resulting orange suspension was refluxed
in chloroform under nitrogen purge for 12 h. The green suspension
was filtered through Celite to give an air-stable orange solution.
The filtrate was evaporated to near-dryness, and the residue was
dissolved in 4 mL of dichloromethane. Evaporation of the solution
overnight gave gold-colored crystals, which were washed with
diethyl ether (3 × 30 mL). Yield: 0.0496 g (46%). 1H NMR
(CDCl3): δ 1.12 (t, Me), 2.22 (q, CH2), 2.71 (s, MeCN). 31P
NMR: δ -30.4. FAB MS: 2274.4 (M+). IR (KBr): νCN 2123
cm-1
.
[Re6Se8(PEt3)5(CN)](BPh4). [Re6Se8(PEt3)5I]I5 (0.0615 g, 23.73
µmol) was dissolved in 20 mL of chloroform. AgCN (0.06154 g,
0.4596 mmol, 19 equiv) was added, and the resulting orange
suspension was refluxed for 21 h under dinitrogen. The green
suspension was filtered through Celite to give an air-stable orange
solution. Solvent was removed, and the orange solid was dissolved
in acetonitrile. An excess of sodium tetraphenylborate (0.9554 g,
2.792 mmol) was added, with immediate dissolution. The solution
was stirred for 15 min in air and filtered through Celite to give an
orange solution. Solvent was removed under vacuum, and the
compound was redissolved in acetonitrile-d3. Yield: 0.041 g (64%).
1H NMR (CDCl3): δ 1.05 (q, 3), 1.12 (q, 12), 2.08 (q, 2), 2.18 (q,
8), 2.91 (1). 31P NMR: δ -30.1 (4), -31.0 (1). IR (KBr): νCN
Excited-state lability, previously documented,8 offers relief
from the ground-state inertness of [Re6Q8]2+ species. We note
that binding to rhenium(III) is expected to render solvate
ligands relatively electropositive. Any consequent reactions
with nucleophiles, combined with photodecomplexation, may
potentially become catalytic. Ensuing photocatalytic schemes
are likely manipulable through site-differentiation with
photoinert triethylphosphine. The chemistry of site-differenti-
ated clusters then affords, not for the first time, new
experimental prospects.
Experimental Section
2121 cm-1
.
Preparation of Compounds. Standard Schlenk and vacuum line
techniques were employed for all manipulations of dioxygen- and/
or moisture-sensitive compounds. Solvents were distilled from
appropriate drying agents and degassed prior to use. Reagents were
of commercial origin and were used as received. 1H and 31P NMR
spectra of all compounds were determined in CD3CN solution.
Because of the small scale of the preparations, compounds were
not analyzed. Product identities were established by a combination
of NMR and mass spectrometric results and X-ray structure
determinations. All compounds were >95% pure by an NMR
criterion. The compound [Re6Se8(PEt3)5(MeCN)](BF4)2 was pre-
pared as described;5 [Re6Se8(PEt3)5(OSMe2)](SbF6)2 was obtained
Other Physical Measurements. NMR spectra were recorded
on a Bruker AM 500 spectrometer. Chemical shifts of 31P{1H}
NMR spectra are referenced to external 85% H3PO4 (negative values
upfield). FAB mass spectra were obtained with a JEOL SX-102
instrument using 3-nitrobenzyl alcohol as matrix. Electrospray mass
spectra were recorded using a Platform 2 mass spectrometer
(Micromass Instruments, Danvers, MA). X-ray diffraction data for
compounds 1-3 were collected on a Siemens (Bruker) SMART
CCD-based diffractometer. Structures were solved and refined using
the procedures for other [Re6Se8]2+ clusters that are detailed
elsewhere.7 Crystallographic data and agreement factors are con-
tained in Table 1.
7
by the method for trans-[Re6Se8(PEt3)4(OSMe2)2](SbF6)2 but
starting with [Re6Se8(PEt3)5I]I.5
Calculations. Extended Hu¨ckel calculations were performed
within the program Cacao43 using default parameters.44
cis-[Re6Se8(PEt3)4(CN)2]. To a 50 mL Schlenk flask equipped
with a stir bar was added 0.5751 g of cis-[Re6Se8(PEt3)4I2] (23.23
µmol); the cluster was dissolved in dichloromethane to give a red
solution. Silver cyanide (0.196 g, 0.1464 mmol, 6.30 equiv) was
added, and the resulting orange suspension was refluxed in
chloroform under dinitrogen purge for 12 h. The green suspension
was filtered through Celite to give an air-stable orange solution.
The filtrate was evaporated to near-dryness, the residue was
dissolved in 1 mL of chloroform, and the solution was filtered
through Celite. Diethyl ether was introduced by vapor diffusion at
4 °C over 48 h into the concentrated filtrate to produce orange-
Acknowledgment. This research was supported by
National Science Foundation Grant CHE 98-36452. T.G.G.
was a National Science Foundation Predoctoral Fellow
(1997-1999). We thank Dr. M. W. Willer for useful
discussions.
Supporting Information Available: X-ray crystallographic data
in CIF format for the three compounds in Table 1. This material is
available free of charge via the Internet at http://pubs.acs.org.
1
brown crystals. Yield: 0.358 g (68%). H NMR (CDCl3): δ 1.09
(q, 3), 1.13 (q, 3), 2.11 (q, 2), 2.22 (q, 2), 2.88 (1). 31P NMR: δ
-30.2, -31.0 ppm. IR (KBr): νCN 2121 cm-1. FAB MS: 2274.4
(M+).
IC020214C
(43) Mealli, C.; Proserpio, D. M. J. Chem. Educ. 1990, 67, 399.
(44) Alvarez, S. Tables of Parameters for Extended Hu¨ckel Calculations;
Departmento de Qu`ımica Inorga`nica, Universitat de Barcelona:
Barcelona, 1989.
(41) Lee, S. C.; Holm, R. H. Angew. Chem., Int. Ed. Engl. 1990, 29, 840.
(42) Tulsky, E. G.; Long, J. R. Chem. Mater. 2001, 13, 1149.
4216 Inorganic Chemistry, Vol. 41, No. 16, 2002