Redox chemistry, acid reactivity, and hydrogenation reactions of two-electron mixed valence diiridium and dirhodium complexes

Article abstract of DOI:10.1021/ic2005248

The syntheses and reaction chemistry of two electron mixed-valence diphosphazane- bridged dirhodium and diiridium complexes M² _0,II(tfepma)₂(CN^tBu)₂Cl₂ [M = Rh (1), Ir (2); tfepma = MeN[P(OCH₂CF₃) ₂]₂, CN^tBu = tert-butyl isocyanide] are described. 1 and 2 undergo addition and two-electron oxidation and reduction chemistries. In the presence of CN^tBu, the addition product with the stoichiometry M²_0,II(tfepma)_2- (CN ^tBu)₃Cl₂ [M = Rh (3), Ir (3)] is generated; in the presence of 1 equiv of CN^tBu and 2 equiv of bis(pentamethyl- cyclopentadienyl)cobalt(II), 1 and 2 are reduced to furnish M₂ ^0,0(tfepma)₂(CN^tBu)₃ [M=Rh (5), Ir (6)], which feature both four- and five-coordinate M⁰ centers. Complexes 1, 2, 5, and 6 all possess coordinatively unsaturated square planarM⁰ centers that are reactive: (1) 2 reacts with PhICl ₂ to produce Ir₂^II,^II(tfepma) ₂(CN^tBu)₂Cl₄ (7); (2) protonation of 2 with HX yields Ir₂^II,^II(tfepma) ₂(CN^tBu)₂Cl₂HX [X = Cl^- (8), OTs^- (9)]; (3) protonation of 5 with HOTs produces [Rh ₂ ^I,I(tfepma)₂(CN^tBu) ₃(μ- H)](OTs); and (4) the reversible hydrogenation of 2 proceeds smoothly, furnishing the cisdihydride complex Ir₂^II, ^II(tfepma)₂(CN^tBu)₂(H) ₂Cl₂ (11). Substitution of tfepma in 2 with bis(diphenylphsophino)methane (dppm) yields the orthometalated complex Ir ₂^II,^II(dppm)(PPh(o-C₆H ₄)CH₂PPh₂)(CN^tBu)₂Cl ₂H (12). The X-ray crystal structures of 11 compounds are presented and discussed, and spectroscopic characterization by multinuclear and variable temperature NMR provides details about solution structures and in some cases the formation of isomeric products. The electronic spectra of the new complexes are also described briefly, with absorption and emission features derived from the bimetallic core.

Full text of DOI:10.1021/ic2005248

ARTICLE
pubs.acs.org/IC
Redox Chemistry, Acid Reactivity, and Hydrogenation Reactions of
Two-Electron Mixed Valence Diiridium and Dirhodium Complexes
Thomas S. Teets, Timothy R. Cook, Brian D. McCarthy, and Daniel G. Nocera*
Department of Chemistry, 6-335, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge,
Massachusetts 02139-4307, United States
S Supporting Information
b
ABSTRACT: The syntheses and reaction chemistry of two electron mixed-valence dipho-
sphazane-bridged dirhodium and diiridium complexes M20,II(tfepma)2(CNtBu)2Cl2 [M =
Rh (1), Ir (2); tfepma = MeN[P(OCH2CF3)2]2, CNtBu = tert-butyl isocyanide] are
described. 1 and 2 undergo addition and two-electron oxidation and reduction chemistries.
In the presence of CNtBu, the addition product with the stoichiometry M20,II(tfepma)2-
(CNtBu)3Cl2 [M = Rh (3), Ir (3)] is generated; in the presence of 1 equiv of CNtBu and 2
equiv of bis(pentamethyl-cyclopentadienyl)cobalt(II), 1 and 2 are reduced to furnish
M20,0(tfepma)2(CNtBu)3 [M = Rh (5), Ir (6)], which feature both four- and five-coordinate
M0 centers. Complexes 1, 2, 5, and 6 all possess coordinatively unsaturated square planar M0
centers that are reactive: (1) 2 reacts with PhICl2 to produce Ir2II,II(tfepma)2(CNtBu)2Cl4
(7); (2) protonation of 2 with HX yields Ir2II,II(tfepma)2(CNtBu)2Cl2HX [X = Clꢀ (8),
OTsꢀ (9)]; (3) protonation of 5 with HOTs produces [Rh2I,I(tfepma)2(CNtBu)3(μ-
H)](OTs); and (4) the reversible hydrogenation of 2 proceeds smoothly, furnishing the cis-
dihydride complex Ir2II,II(tfepma)2(CNtBu)2(H)2Cl2 (11). Substitution of tfepma in 2 with bis(diphenylphsophino)methane
(dppm) yields the orthometalated complex Ir2II,II(dppm)(PPh(o-C6H4)CH2PPh2)(CNtBu)2Cl2H (12). The X-ray crystal
structures of 11 compounds are presented and discussed, and spectroscopic characterization by multinuclear and variable
temperature NMR provides details about solution structures and in some cases the formation of isomeric products. The electronic
spectra of the new complexes are also described briefly, with absorption and emission features derived from the bimetallic core.
’ INTRODUCTION
reactive four-coordinate center.3 This necessity to photoactivate
the metal prior to HX addition translates to hydride-containing
intermediates present in only minor quantities, and often these
types of intermediates are unstable under the conditions used to
generate them. Accordingly, these limitations obviate the struc-
tural and spectral characterization of key hydride intermediates
of the catalytic cycle.
By avoiding high coordination metal centers, key hydride
intermediates may be generated by thermal addition of HX and
H2 to group 9 two-electron mixed valence complexes that feature
coordinatively unsaturated M0 centers. In addition to allowing
the thorough characterization of ground and excited state
reactivity as it pertains to HX splitting, the late transition metal
hydride complexes are of potential interest to those studying
hydrogenation,12 O2 reduction,13 aerobicoxidation,14ꢀ17 and CO2
reduction chemistries,18 among others.19 Indeed, we have shown
that by using only two bridging diphosphazane ligands and a
sufficiently bulky monodentate ligand, tert-butyl isocyanide, the
complex Rh20,II(tfepma)2(CNtBu)2Cl2 (1) could be accessed, and
it reacts rapidly with HCl to generate a hydrido-chloride complex
which shows interesting O2-reduction reactivity.20 Here, we
expand the chemistry of 1 as well as the rich reaction chemistry
Solar energy storage is arguably one of the most important
factors governing the widespread deployment of solar energy
resources.1 The splitting of acid halides (HX, X = Cl, Br) into H2
and X2 is an energetically uphill reaction, which, when executed
photocatalytically, can store quantities of solar energy compar-
able to that of the much heralded water-splitting reaction.2
Bimetallic complexes are especially adept at catalyzing the two-
electron, two-proton HX splitting photoreaction. Early success was
realized with diphosphazane-bridged dirhodium complexes of the
type Rh20,0(dfpma)3L2 (dfpma = bis(difluorophosphino)methyl-
amine, L = CO, PR3), which photocatalyze hydrogen production
from HX3 via dihydride-dihalide Rh2II,II intermediates.4
The two half-reactions of HX splitting, formally the reduction
of protons to H2 and the oxidation of halides to X2, each present
their own unique set of challenges, which must be simultaneously
overcome to design more efficient photocatalysts. Our interroga-
tion of the dirhodium catalysts affirmed that the sluggish halogen
photoelimination limited the overall catalytic efficiency,5,6 moti-
vating the study of more efficient halogen elimination from
bimetallic and monometallic centers.7ꢀ11 The reductive half-
reaction of HX splitting proceeds through hydrido-halide inter-
mediates, generated by oxidative addition of HX. In previous
work, HX addition was found to be preceded by photochemical
CO liberation from a five-coordinate Rh0 center, generating a
Received: March 14, 2011
Published: May 10, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ic2005248 Inorg. Chem. 2011, 50, 5223–5233
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Inorganic Chemistry
ARTICLE
associated with the diiridium congener Ir20,II(tfepma)2(CNtBu)2-
Cl2 (2) by describing two electron reduction and oxidation trans-
formations. Bimetallic hydride complexes are obtained by proton-
ation and hydrogenation of 2. The results described herein provide
access to a new suite of dirhodium and diiridium hydride com-
plexes, which are promising targets to further our understanding of
both H2 production and O2 reduction mediated by group 9
bimetallic complexes.
chilled overnight at ꢀ20 °C, at which time the supernatant was decanted
and the light orange product dried in vacuo. Yield: 643 mg (64.3%). 1H
NMR (500 MHz, CD2Cl2) δ/ppm: 4.96ꢀ5.13 (m, 4H), 4.77ꢀ4.88 (m,
2H), 4.47ꢀ4.64 (m, 4H), 4.29ꢀ4.46 (m, 6H), 2.87 (pseudoquintet, 6H),
1.38 (s, 9H), 1.33 (s, 9H). 31P{1H} NMR (121.5 MHz, CD2Cl2) δ/ppm:
132.3 (m, 2P), 85.9 (m, 2P). UVꢀvis (CH3CN): λ/nm (ε/Mꢀ1 cmꢀ1
)
271 (15000), 315 (6300), 338 (sh) (4100), 392 (5600), 449 (1600), 504
(860). IR (Nujol):~νCtN = 2071 (sh), 2132 (sh), 2100 cmꢀ1. Anal. Calcd.
for C28H40Cl2F24N4O8P4Ir2: C, 21.07; H, 2.53; N, 3.51. Found: C, 20.97;
H, 2.49; N, 3.27.
’ EXPERIMENTAL SECTION
Preparation of [Rh20,II(tfepma)2(CNtBu)3Cl]Cl (3). To a solu-
tion of 1 (100 mg, 0.0706 mmol, 1.00 equiv) in 2 mL of THF was added
a solution of CNtBu (6.2 mg, 0.074 mmol, 1.1 equiv) in 0.5 mL of THF,
causing the color to darken slightly. The reaction was stirred at room
temperature for 2.5 h, following which the solvent was removed in vacuo.
The residue was washed with 4 mL of Et2O and 2 mL of hexane, leaving a
yellow solid which was dried in vacuo. The product was dissolved in
1.5 mL of THF and layered with 15 mL of hexane to recrystallize. After
about 48 h the supernatant was decanted, and the product dried in vacuo.
Yield: 63 mg (59%). Repeated attempts to obtain analytically pure
material by various recrystallization procedures were unsuccessful, likely
General Considerations. All reactions involving air-sensitive
materials were executed in a nitrogen-filled glovebox or on a high-vacuum
manifold using solvents previously dried by passage through an alumina
column under argon. The starting materials [IrI(COD)Cl]2 (COD = 1,5-
cyclooctadiene) and bis(diphenylphosphino)methane (dppm) were ob-
tained from Strem Chemicals. 2,6-lutidine, HCl (1 M in Et2O), bis-
(pentamethylcyclopentadienyl)cobalt(II) (CoCp*2), p-toluenesulfonic
acid hydrate (HOTs H2O), and tert-butylisocyanide (CNtBu) were obta-
3
ined from Sigma-Aldrich, whereas H2 was purchased from Airgas. Anhy-
drous 2,6-lutidinium hydrochloride (LutHþClꢀ) was prepared by adding
anhydrous 2,6-lutidine to 1 M HCl/Et2O under nitrogen. The ligand bis-
1
because of the reversible nature of the reaction. H NMR (500 MHz,
(bis(trifluoroethoxy)phosphino)methylamine (tfepma),21 PhICl2 and
22
CD3CN) δ/ppm: 4.15ꢀ5.15 (m, 16H), 2.95 (br, s, 6H), 1.47 (br, s,
27H). 31P{1H} NMR (121.5 MHz, CD3CN) δ/ppm: 144.0 (br, m, 2P),
125.7 (br, m, 2P). IR (Nujol): ν~CtN = 2131, 2160 cmꢀ1. Anal. Calcd. for
C33H49Cl2F24N5O8P4Rh2: C, 26.42; H, 3.29; N, 4.67. Found: C, 25.04;
H, 3.17; N, 4.49.
Rh20,II(tfepma)2(CNtBu)2Cl220 were prepared as described in the litera-
ture. Elemental analyses were performed by Midwest Microlab LLC or
Quantitative Technologies Incorporated.
Physical Methods. NMR spectra were recorded at the MIT
Department of Chemistry Instrumentation Facility on a Varian Mercury
300 NMR Spectrometer or a Varian Inova-500 NMR Spectrometer.
31P{1H} NMR spectra were referenced to an external standard of 85%
D3PO4, and 1H spectra were referenced to the residual proteo solvent
resonances. UVꢀvis samples were prepared in a nitrogen-filled glove-
box, and spectra were recorded at 293 K in tetrahydrofuran (THF)
solutions in quartz cuvettes on a Varian Cary 5000 UVꢀvisꢀNIR
spectrophotometer. Extinction coefficients were determined over a con-
centration range of ∼10ꢀ6ꢀ10ꢀ4 M, for which all compounds obeyed
Beer’s Law. IR spectra were recorded on a PerkinElmer Spectrum 400 FT-
IR/FT-FIR Spectrometer outfitted with a Pike Technologies GladiATR
attenuated total reflectance accessory with a monolithic diamond crystal
stage and pressure clamp. Samples were suspended in Nujolmineraloilfor
all IR measurements. Steady state emission spectra were recorded on an
automated Photon Technology International (PTI) QM 4 fluorimeter
equipped with a 150-W Xe arc lamp and a Hamamatsu R928 photo-
multiplier tube. Excitation light was excluded with appropriate glass filters.
Samples were housed in custom quartz EPR tubes with a ground-glass
joint and Teflon plug. Solution samples were prepared in 2-methylte-
trahydrofuran and freeze pump thaw degassed (4 cycles, 1 ꢁ 10ꢀ5 Torr).
The sample tubes were immersed in liquid N2 in a quartz finger dewar
prior to measurement.
Preparation of Ir20,II(tfepma)2(CNtBu)3Cl2 (4). A sample of 2
(100 mg, 0.0627 mmol, 1.00 equiv) was dissolved in 1.5 mL of THF. A
solution of CNtBu (5.7 mg, 0.068 mmol, 1.1 equiv) in 0.5 mL of THF
was added, and the yellow-orange solution was stirred at room tem-
perature for 24 h. The solution was concentrated in vacuo to give a
yellow-orange residue, which was suspended in 1 mL of CH2Cl2/15 mL
of pentane and chilled at ꢀ20 °C for 24 h. The supernatant was
decanted, and the resulting yellow product dried in vacuo. Yield: 73
mg (70%). Analytically pure material was obtained in much lower yields
(<50%) by recrystallization from a concentrated CH2Cl2 solution
layered with pentane. 1H NMR (500 MHz, CD3CN) δ/ppm:
4.86ꢀ4.97 (m, 4H), 4.44ꢀ4.69 (m, 12H), 2.94 (pseudoquintet, 6H),
1.45 (s, 9H), 1.43 (s, 18H). 31P{1H} NMR (121.5 MHz, CD3CN) δ/
ppm: 132.3 (m, 2P), 79.8 (m, 2P). UVꢀvis (CH3CN): λ/nm (ε/
M
ꢀ1 cmꢀ1) 271 (16000), 307 (5300), 325 (sh) (3700), 382 (6400), 444
(1800), 496 (960). IR (Nujol): ν~CtN = 2116, 2159, 2181 cmꢀ1. Anal.
Calcd. for C33H49Cl2F24N5O8P4Ir2: C, 23.61; H, 2.94; N, 4.17. Found:
C, 22.95; H, 2.85; N, 4.05.
Preparation of Rh20,0(tfepma)2(CNtBu)3 (5). A mixture of 1
(475 mg, 0.335 mmol, 1.00 equiv) and CNtBu (27.9 mg, 0.336 mmol,
1.00 equiv) were dissolved in 6 mL of THF, giving a red-orange solution
and a small amount of yellow precipitate. The mixture was added via
pipet to a stirred solution of CoCp*2 (221 mg, 0.671 mmol, 2.00 equiv)
in 2 mL of THF. Yellow [CoCp*2]Cl formed immediately, and the
mixture was allowed to stir at room temperature for 3 h. A yellow-green
solid was removed by filtration through a plug of glass wool, and the
volatiles were removed in vacuo to afford a dark red residue. The product
was dissolved in a mixture of 2 mL of Et2O/2 mL of hexanes, and upon
concentrating in vacuo an orange solid was obtained. The solid was
suspended in 8 mL of hexanes at ꢀ20 °C overnight. The supernatant
was decanted, and the solid was dried in vacuo. Yield: 432 mg (90.2%).
1H NMR (500 MHz, C6D6) δ/ppm: 4.13ꢀ4.40 (br, m, 16H), 2.80 (br,
pseudoquintet, 6H), 1.04 (br, s, 27H). 31P{1H} NMR (121.5 MHz,
0,II
Preparation of Ir2 (tfepma)2(CNtBu)2Cl2 (2). [IrI(COD)Cl]2
(421 mg, 0.627 mmol, 1.00 equiv) was suspended in 3 mL of Et2O in a 20-
mL scintillation vial. A solution of tfepma (611 mg, 1.25 mmol, 2.00
equiv), dissolved in 2 mL of Et2O, was added at a fast dropwise rate to give
a dark yellow/brown solution. A solution of CNtBu (104 mg, 1.25 mmol,
2.00 equiv) in 2 mL of Et2O was added immediately, giving a dark red
solution. The reaction mixture was stirred at room temperature for 22 h,
during which time some orange solid had formed. The mixture was
concentrated to dryness, leaving an orange solid and a red residue. The
product was resuspended in 2 mL of Et2O, and 16 mL of pentane was
added to the stirred mixture, liberating a light orange solid. The super-
natant was decanted, and the solid briefly dried in vacuo. The product was
dissolved in 10 mL of CH2Cl2 and filtered, removing a small amount of a
brown impurity. The solvent was removed invacuo, and theresultingsolid
was resuspended in 1 mL of CH2Cl2/15 mL of pentane. The mixture was
C6D6) δ/ppm: 156.0 (m, 4P). UVꢀvis (THF): λ/nm (ε/Mꢀ1 cmꢀ1
)
285 (19000), 379 (6200). IR (Nujol): ν~CtN = 2042 (sh), 2074 (sh),
2098 cmꢀ1. Anal. Calcd. for C33H49F24N5O8P4Rh2: C, 27.73; H, 3.46;
N, 4.90. Found: C, 27.80; H, 3.37; N, 4.80.
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Inorganic Chemistry
ARTICLE
Preparation of Ir20,0(tfepma)2(CNtBu)3 (6). A suspension of 2
(500 mg, 0.313 mmol, 1.00 equiv) in 6 mL of Et2O was combined with a
solution of CNtBu (26.0 mg, 0.313 mmol, 1.00 equiv) in 2 mL of Et2O.
Most of the solid was drawn into solution, with only a small amount of
undissolved orange solid remaining. The mixture was added to CoCp*2
(206 mg, 0.627 mmol, 2.00 equiv) in 2 mL of Et2O. A yellow solid
formed immediately, and the mixture was stirred for 3 h, at which time
the precipitate was removed by filtration through glass wool and the
solution concentrated to dryness. The resulting orange solid was sus-
pended in 6 mL of pentane at ꢀ20 °C overnight. The supernatant was
decanted, and the bright orange solid dried in vacuo. Yield: 410 mg
(81.3%). 1H NMR (500 MHz, C6D6) δ/ppm: 4.15ꢀ4.39 (br, m, 16H),
2.74 (pseudoquintet, 6H), 1.03 (s, 27H). 31P{1H} NMR (202.5 MHz,
1H NMR(500 MHz, CD3CN) δ/ppm: 7.60 (d, 3JHꢀH = 8.5 Hz, 2H), 7.16
(d, 3JHꢀH = 8.5 Hz, 2H), 5.23 (m, 2H), 4.60ꢀ5.00 (m, 10H), 4.37ꢀ4.45
(m, 4H), 2.92 (pseudoquintet, 6H), 2.33 (s, 3H), 1.55 (s, 9H), 1.43 (s,
2
9H), ꢀ20.00 (t, JHꢀP = 16.0 Hz, 1H). 31P{1H} NMR (121.5 MHz,
CD3CN) δ/ppm: 91.1ꢀ93.9 (m, 2P), 77.7ꢀ80.5 (m, 2P) (AA0BB0, 18
lines resolved, δavg = 85.8 ppm). Minor isomer (11%): 1H NMR (500
MHz, CD3CN) δ/ppm: 7.60 (d, 3JHꢀH = 8.5 Hz, 2H) (coincident with
3
major isomer), 7.16 (d, JHꢀH = 8.5 Hz, 2H) (coincident with major
isomer), 4.28ꢀ5.12 (m, 16H) (overlap with peaks from major isomer),
2.98 (pseudoquintet, 6H), 2.33 (s, 3H) (coincident with major isomer),
1.56 (s, 9H), 1.44 (s, 9H), ꢀ20.71 (t, 2JHꢀP = 16.0 Hz, 1H) 31P{1H}
NMR (121.5 MHz, CD3CN) δ/ppm: ∼92.7 (m, 2P), ∼78.9 (m, 2P).
(peaks are completely overlapped with those of major isomer) UVꢀvis
(CH3CN) (mixture of isomers): λ/nm (ε/Mꢀ1 cmꢀ1) 270 (17000),
320 (5300), 348 (3500). IR (Nujol) (mixture of isomers): ν~CtN = 2171,
2181 (sh), 2189 (sh) cmꢀ1. Anal. Calcd. for C35H48Cl2F24N4O11P4SIr2:
C, 23.78; H, 2.74; N, 3.17. Found: C, 24.15; H, 2.42; N, 3.05.
C6D6) δ/ppm: 126.1 (s, 4P). UVꢀvis (THF): λ/nm (ε/Mꢀ1 cmꢀ1
)
266 (sh) (16000), 383 (4300). IR (Nujol): ν~CtN = 2038 (sh), 2088
(sh), 2113 cmꢀ1. Anal. Calcd. for C33H49F24N5O8P4Ir2: C, 24.65; H,
3.07; N, 4.36. Found: C, 24.72; H, 3.14; N, 4.27.
Preparation of [Rh2I,I(tfepma)2(CNtBu)3(μ-H)](OTs) (10).
Separate solutions of 5 (100 mg, 0.0700 mmol, 1.00 equiv) in 2 mL of
Preparation of Ir2II,II(tfepma)2(CNtBu)2Cl4 (7). In a 20-mL
scintillation vial, 2 (100 mg, 0.0627 mmol, 1.00 equiv) was dissolved in
2 mL of CH2Cl2. In a separate vial, PhICl2 (19.0 mg, 0.0691 mmol, 1.10
equiv) was dissolved in 2 mL of CH2Cl2. Both solutions were frozen in
the glovebox coldwell. They were removed, and upon thawing the
PhICl2 solution was added dropwise to the solution of 1. The color
immediately lightened to canary yellow, and the reaction was allowed to
warm to room temperature and stirred for 1 h. The resulting solution
was concentrated in vacuo and triturated with 4 mL of hexane to yield a
bright yellow solid. The product was washed with 2 mL of hexane and
dried in vacuo. Yield: 90 mg (86%). 1H NMR (500 MHz, CD3CN) δ/
ppm: 5.34ꢀ5.45 (m, 2H), 5.00ꢀ5.11 (m, 2H), 4.55ꢀ4.93 (m, 12H),
2.90 (pseudoquintet, 6H), 1.61 (s, 9H), 1.38 (s, 9H). 31P{1H} NMR
(121.5 MHz, CD3CN) δ/ppm: 76.1ꢀ77.6 (m, 2P), 70.3ꢀ71.8 (m, 2P)
(AA0BB0, 12 lines resolved, δavg = 73.9 ppm) . UVꢀvis (CH3CN): λ/nm
(ε/Mꢀ1 cmꢀ1) 282 (18000), 333 (6200), 364 (sh) (2500). IR (Nujol):
ν~CtN = 2186 cmꢀ1. Anal. Calcd. for C28H40Cl4F24N4O8P4Ir2: C, 20.18;
H, 2.42; N, 3.36. Found: C, 20.37; H, 2.41; N, 3.35.
CH3CN and HOTs H2O (13.3 mg, 0.699 mmol, 0.999 equiv) in 1 mL
3
of CH3CN were prepared and frozen in the glovebox coldwell. The
solutions were removed, and upon thawing the HOTs solution was
added dropwise to the stirred solution of 5, giving a red-orange solution,
which was allowed to warm to room temperature and stirred for 1 h. The
solution was concentrated to dryness in vacuo, yielding a red residue.
Trituration with 2 mL of Et2O afforded a yellow solid, and the resulting
suspension was placed in the freezer at ꢀ20 °C overnight. The product
was isolated by decantation and dried in vacuo. Yield: 92 mg (82%). 1H
NMR (500 MHz, CD3CN) δ/ppm: 7.59 (d, 2JHꢀH = 8.0 Hz, 2H), 7.14
(d, 2JHꢀH = 8.0 Hz, 2H), 4.28ꢀ4.50 (br, m, 16H), 2.80 (pseudoquintet,
6H), 2.32 (s, 3H), 1.45 (s, 27H), ꢀ12.63 (tquint, 1JHꢀRh = 14.8 Hz, 2JHꢀP
= 8.5 Hz, 1H). 31P{1H} NMR (121.5 MHz, CD3CN) δ/ppm: 150.7 (m,
4P). UVꢀvis (CH3CN): λ/nm (ε/Mꢀ1 cmꢀ1) 270 (13000), 283 (sh)
(12000), 339 (sh) (4400), 376 (sh) (3900), 443 (sh) (1400). IR (Nujol):
~νCtN = 2135, 2164 cmꢀ1. Anal. Calcd. for C40H57F24N5O11P4SRh2: C,
30.00; H, 3.59; N, 4.37. Found: C, 29.65; H, 3.31; N, 4.27.
Preparation of Ir2II,II(tfepma)2(CNtBu)2Cl3H (8). In a 20-mL
scintillation vial, 2 (100 mg, 0.0627 mmol, 1.00 equiv) was dissolved in
2 mL of acetonitrile. A solution of LutHþClꢀ (11.2 mg, 0.0780 mmol,
1.24 equiv) in 1 mL of acetonitrile was added, causing the color to fade to
pale yellow. The solution was stirred at room temperature for 30 min,
and then the solvent was removed in vacuo to give a cream-colored solid.
The solid was taken up in 8 mL of Et2O and filtered through a plug of
glass wool to remove unreacted LutHþClꢀ. To the resulting solution
was added 2 mL of hexane, and concentration in vacuo left a light yellow
solid, which was suspended in hexane and stored at ꢀ20 °C overnight.
The supernatant was decanted, and the product dried in vacuo. Yield: 98
Preparation of Ir2II,II(tfepma)2(CNtBu)2Cl2(H)2 (11).
A
J. Young NMR tube was charged with 2 (25 mg, 0.016 mmol) dissolved
in 0.7 mL of THF-d8. The solution was freezeꢀpumpꢀthaw degassed
three times on a high vacuum manifold. The tube was pressurized with
about 1.5 atm of H2 and upon shaking the tube a nearly colorless
solution was obtained. The NMR spectra were immediately recorded.
The compound is unstable in the absence of H2, losing H2 and reverting
back to 2. This instability precluded isolation of solid material for
microanalysis. Crystals for X-ray diffraction and IR analysis were ob-
tained by conducting an analogous preparation in CD3CN and chill-
ing the solution to ꢀ20 °C overnight in the presence of H2. For the
UVꢀvis measurement, a solution of 2 (63 μM in THF) was prepared in
a septum-sealed cuvette, and 2 mL of H2 was injected via gastight
1
mg (96%). H NMR (500 MHz, C6D6) δ/ppm: 5.93 (m, 2H), 5.62
(m, 2H), 5.44 (m, 2H), 4.96 (m, 2H), 4.80 (m, 2H), 4.64 (m, 2H), 3.93
(m, 4H), 2.54 (pseudoquintet, 6H), 1.08 (s, 9H), 1.03 (s, 9H), ꢀ20.69
(t, 2JHꢀP = 16.0 Hz, 1H). 31P{1H} NMR (121.5 MHz, C6D6) δ/ppm:
89.0ꢀ91.5 (m, 2P), 77.8ꢀ80.3 (m, 2P) (AA0BB0, 20 lines resolved,
δavg = 84.7 ppm). UVꢀvis (THF): λ/nm (ε/Mꢀ1 cmꢀ1) 285 (12000), 322
(8300), 360 (sh) (2400). IR (Nujol): ν~CtN = 2170 cmꢀ1. Anal. Calcd.
for C28H41Cl3F24N4O8P4Ir2: C, 20.60; H, 2.53; N, 3.43. Found: C,
21.29; H, 2.31; N, 3.28.
1
syringe. Major isomer (cis-H2, cis-P2): Yield (from NMR): 95%. H
NMR (500 MHz, THF-d8) δ/ppm: 5.60 (m, 2H), 4.84 (m, 2H), 4.68
(m, 4H), 4.42ꢀ4.56 (m, 4H), 4.29 (m, 2H), 4.13 (m, 2H), 2.82 (m, 6H),
1.48 (s, 9H), 1.46 (s, 9H), ꢀ12.63 to ꢀ12.03 (m, 2H). 31P{1H} NMR
(121.5 MHz, THF-d8) δ/ppm: 93.4ꢀ97.8 (m, 2P), 88.2ꢀ90.4 (m, 2P).
IR (Nujol): ν~IrꢀH = 2036, 2065 cmꢀ1, ~νCtN = 2161 cmꢀ1. Minor
isomer (cis-H2, trans-P2): Yield (from NMR): 5%. 1H NMR (500 MHz,
THF-d8) δ/ppm: 5.26 (m, 2H), 5.13 (m, 4H), about 4.84 (m, 2H)
(overlap with major isomer), about 4.68 (m, 4H) (overlap with major
isomer), 4.39 (m, 4H), 2.86 (pseudoquintet, 6H), 1.44 (s, 9H), 1.41
Preparation of Ir2II,II(tfepma)2(CNtBu)2Cl2H(OTs) (9). In the
glovebox, 2 (100 mg, 0.0627 mmol, 1.00 equiv) was dissolved in 2 mL of
THF. A solution of HOTs H2O (12.5 mg, 0.0657 mmol, 1.05 equiv) in
3
1 mL of THF was added, giving a nearly colorless solution. After 48 h,
the solvent was removed in vacuo. The product was redissolved in 4 mL
of hexane/6 mL of Et2O, and the resulting solution was concentrated in
vacuo to leave a white solid, which was treated with 4 mL of hexane and
stored at ꢀ20 °C overnight. The supernatant was decanted, and the
product dried in vacuo. Yield: 104 mg (93.7%). Major isomer (89%):
2
2
(s, 9H), ꢀ14.80 (td, JHꢀP = 17.9 Hz, JHꢀH = 4.6 Hz, 1H) ꢀ15.50
(td, 2JHꢀP = 13.8 Hz, 2JHꢀH = 4.6 Hz, 1H). 31P{1H} NMR (121.5 MHz,
THF-d8) δ/ppm: 109.6 (m, 2P), about 95.0 (m, 2P), (overlap with major
isomer). UVꢀvis (THF) (both isomers present): λ/nm (ε/Mꢀ1 cmꢀ1
)
267 (16000), 313 (4800), 342 (3300).
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Preparation of Ir2II,II(dppm)(PPh(o-C6H4)CH2PPh2)(CNtBu)2-
Cl2H (12). [IrI(COD)Cl]2 (242 mg, 0.360 mmol, 1.00 equiv) was
dissolved in 8 mL of THF to give an orange solution. A solution of dppm
(276 mg, 0.718 mmol, 1.99 equiv) in 6 mL of THF was added, followed
by CNtBu (60 mg, 0.72 mmol, 2.0 equiv) in 6 mL of THF. The resulting
dark green solution gradually faded to pale yellow, and over the course of
72 h stirring at room temperature a white solid precipitated from solution.
The solid was collected by vacuum filtration, washed with 10 mL of THF
and 10 mL of Et2O and dried in vacuo. Yield: 374 mg (74.0%). 1H NMR
(500 MHz, CD2Cl2) δ/ppm: 8.02 (m, 2H), 7.89 (m, 4H), 7.64ꢀ7.71 (m,
3H), 7.48ꢀ7.58 (m, 3H), 7.34ꢀ7.44 (m, 3H), 7.15ꢀ7.25 (m, 7H),
6.93ꢀ7.12 (m, 10H), 6.72ꢀ6.81 (m, 4H), 6.59 (m, 1H), 6.51 (m, 2H),
4.69 (m, 1H), 4.47 (m, 1H), 4.39 (m, 1H), 3.86 (m, 1H), 1.09 (s, 9H),
0.73 (s, 9H), ꢀ10.69 (dd, 2JHꢀP = 163 Hz, 19.1 Hz, 1H). 31P{1H} NMR
(121.5 MHz, CD2Cl2) δ/ppm: ꢀ9.8 to 1.5 (m, 2P), ꢀ20.7 to ꢀ19.6 (m,
1P), ꢀ33.4 (m, 1P). IR (Nujol): ~νIrꢀH = 2097 cmꢀ1, ~νCtN = 2134,
2162 cmꢀ1. Anal. Calcd. for C60H62Cl2N2P4Ir2: C, 51.83; H, 4.49; N,
2.01. Found: C, 51.76; H, 4.25; N, 2.02.
Scheme 1
X-ray Crystallographic Details. Single crystals of 2, 3, and 4 were
obtained by layering a CH2Cl2 solution with pentane, crystals of 5 and 6
were obtained from a toluene/pentane solution at ꢀ20 °C, crystals of 7
and 9 deposited from CH2Cl2/hexane at ꢀ20 °C, crystals of 8 were
grown in Et2O/pentane at ꢀ20 °C, crystals of 10 were obtained from
THF/hexane at ꢀ20 °C, crystals of 11 were grown from a CD3CN
NMR sample at ꢀ20 °C, and crystals of 12 deposited from THF at room
temperature. The crystals were mounted on a Bruker three circle
goniometer platform equipped with an APEX detector. A graphite
monochromator was employed for wavelength selection of the Mo
KR radiation (λ = 0.71073 Å). The data were processed and refined
using the program SAINT supplied by Siemens Industrial Automation.
Structures were solved by Patterson methods in SHELXS and refined by
standard difference Fourier techniques in the SHELXTL program suite
(6.10 v., Sheldrick G. M., and Siemens Industrial Automation, 2000).
Hydrogen atoms bonded to carbon were placed in calculated positions
using the standard riding model and refined isotropically; all non-
hydrogen atoms were refined anisotropically. The metal-bound hydro-
gen atoms in8, 9, 11, and 12 were locatedin the differencemap, restrained
to a distance of 1.60 Å from the metal, and refined isotropically. The
bridging hydride in 10 was restrained to be equidistant from the two
rhodium nuclei. The crystal of 3 was a nonmerohedral twin; two unit cell
domains were located using the program cell_now and used for integra-
tion in SAINT. The absorption correction was performed with TWI-
NABS, and the structure was refined against the major twin component.
The structures of 4, 10, and 12 all had one or more disordered
ꢀOCH2CF3 groups, and the structure of 7 had a disordered tert-butyl
group. In addition, the structures of 4 and 9 also contained a disordered
dichloromethane. The (1,2) and (1,3) distances of all disordered parts
were restrained to be similar using the SADI command; the rigid-bond
restraints SIMU and DELU were also used on disordered parts. Unit cell
parameters, morphology, and solution statistics for the structures of 1ꢀ3
are summarized in Supporting Information, Tables S1ꢀS3. All thermal
ellipsoid plots are drawn with carbon-bound hydrogen atoms, solvent
molecules and noncoordinating counterions omitted for clarity.
Figure 1. Thermal ellipsoid plot for 2. Ellipsoids are shown at the 50%
probability level with ꢀCH2CF3 groups and hydrogen atoms omitted
for clarity. Data were collected at 100 ( 2 K.
CNtBu furnishes the two-electron mixed valence complex 2, which
was isolated in 64% yield after a 24 h reaction time. Spectroscopic
characterization of 2 supports the formulation of a two-electron
mixed valence core. In the 1H NMR spectrum, two distinct tert-butyl
resonances are observed at 1.38 and 1.33 ppm, suggesting chemical
inequivalency of the two CNtBu ligands owing to the mixed valence
character of the bimetallic core. Furthermore, the 31P{1H} NMR
spectrum (Supporting Information, Figure S1) shows two sym-
metric multiplets at 85.9 and 132.3 ppm, with characteristic multi-
plicities arising from the AA0XX0 spin system. In total, 16 of the
possible 20 lines of the AA0XX0 splitting pattern are observed. Fi-
nally, the IR spectrum shows two distinct CtN stretching frequen-
cies at 2100 and 2132 cmꢀ1, once again indicative of an asymmetric
structure for 2.
X-ray crystallography confirms the two-electron mixed valence
structure of diiridium complex 2. As shown in Figure 1, distinct
coordination environments are observed for the square planar, d9
Ir0center and the octahedral, d7 IrII center. The Ir(1)ꢀIr(2) inter-
nuclear distance of 2.7112(3) Å is consistent with a metalꢀmetal
bond, as expected for a d7ꢀd9 complex. The bimetallic core is
bridged by both tfepma ligands, as is the case for all complexes
discussed here; there are no instances of chelating tfepma.
’ RESULTS
Synthesis and Characterization of Ir20,II(tfepma)2(CNtBu)2-
Cl2 (2). We have previously reported the two-electron mixed
valence complex Rh20,II(tfepma)2(CNtBu)2Cl2 (1, tfepma =
Reaction Chemistry. A reaction chemistry originating from 1
and 2 is summarized in Scheme 2. The chemical structures shown
in Scheme 2 represent crystallographically confirmed geometries.
AdditionofCNtButo1and2. As1and 2 feature coordinatively
unsaturated M0 centers, we sought to explore the possibility of
occupying the vacant coordination site with an additional equivalent
[P(OCH2CF3)2]2NMe), which contains an unsaturated four-
0,II
coordinate Rh0 center.20 The diiridium counterpart, Ir2
-
(tfepma)2(CNtBu)2Cl2 (2), is prepared in nearly identical fashion,
as described in Scheme 1. Treatment of [IrI(COD)Cl]2 (COD =
1,5-cyclooctadiene) with stoichiometric amounts of tfepma and
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Scheme 2
Figure 2. Thermal ellipsoid plots for 3 and 4. Ellipsoids are shown at the
50% probability level with ꢀCH2CF3 groups, CNtBu methyl groups,
outer-sphere anions, and hydrogen atoms omitted for clarity. Data were
collected at 100 ( 2 K.
center is now five-coordinate, distorted trigonal bipyramidal, and at the
octahedral IrII center the two phosphorus atoms are cis to one another.
The metalꢀmetal distance has lengthened by 0.08 Å to 2.7920(3) Å,
though this distance suggests that the metalꢀmetal bond is intact.
When crystals of 4 are redissolved in acetonitrile, the NMR and
UVꢀvis spectra suggest isomerization to a considerably more
symmetric structure. The 31P{1H} NMR spectrum (Supporting
Information, Figure S3) shows only two resonances, with an
AA0XX0 splitting pattern. The spectrum is similar to that of parent
Ir20,II complex 2 (Supporting Information, Figure S1), though the
upfield resonance shifts by about 6 ppm and the multiplicity is
distinct. The 1H NMR spectrum features two closely spaced, sharp
tert-butyl resonances with a 2:1 integration ratio, suggesting that all
three CNtBu ligands remain bound. Furthermore, the UVꢀvis
spectra of 2 (Supporting Information, Figure S13) and 4
(Supporting Information, Figure S14) are decidedly similar, with
only slight 5ꢀ15 nm hypsochromic shifts of most peaks in the
spectrum of 4 relative to that of 2. These spectral data suggest that
in acetonitrile solution, chloride dissociation from 4 leads to the
ionic complex [Ir20,II(tfepma)2(CNtBu)3Cl]Cl, whose structure is
analogous to 3. The octahedral IrII and square planar Ir0 sites pos-
sess similar coordination environments to the parent Ir20,II com-
plex 2, giving rise to only slightly differing spectral features.
of CNtBu. Treatment of Rh20,II complex 1 with 1 equiv of CNtBu
leads to reversible association of the added ligand, as judged by
NMR spectra. The 31P{1H} spectrum, shown in Supporting
Information, Figure S2, features broad resonances whose chemical
shifts are nearly identical to parent complex 1. The 1H spectrum is
similarly broadened and only shows a single tert-butyl resonance,
offering little insight to the structure. UVꢀvis spectra of the isolated
product, recorded in the range of 3ꢀ50 μM, are identical to that of
complex 2, suggesting complete dissociation of the additional
CNtBu ligand at these low concentrations. X-ray crystallography
reveals the structure of the addition product to be [Rh20,II(tfepma)2-
(CNtBu)3Cl]Cl (3), shown in Figure 2. The incoming CNtBu
ligand substitutes for one of the chloride ligands at the octahedral
RhII center whereas the Rh0 center is unperturbed. The intermetallic
distances, 2.6847(10) Å and 2.6857(10) Å for the two crystal-
lographically independent molecules, verify that the metalꢀmetal
bond and two-electron mixed valency are preserved upon ligand
addition.
0,II
Chemical Reduction of 1 and 2. Whereas treatment of M2
complexes 1 and 2 with bis(cyclopentadienyl)cobalt(II) (CoCp2,
E°0 = ꢀ1.33 V) leads to incomplete reactions and mixtures of
products, treatment with the stronger reducing agent bis(penta-
methylcyclopenta-dienyl)cobalt(II) (CoCp2*, E°0 = ꢀ1.94 V)23
0,II
completely consumes the M2 starting material. Integration of
1
the crude H NMR spectra showed the major product of each
reaction to contain three bound CNtBu ligands, and indeed opti-
mized yields and purities of these major products, M20,0(tfepma)2-
(CNtBu)3 [M = Rh (5), Ir (6)], are obtained when complexes 1
and 2 are treated with both 2 equiv of CoCp*2 and 1 equiv of
CNtBu, as described in Scheme 2. The solid state structures of
compounds 5 and 6 are shown in Figure 3. An asymmetric struc-
ture is observed, with a four-coordinate, square planar environ-
ment about one M0 center, and a five-coordinate, trigonal
bipyramidal structure about the other. Metalꢀmetal distances of
2.7099(5) Å (5) and 2.7631(4) Å (6) are observed for these
formally d9ꢀd9 complexes.
Treatment of Ir20,II complex 2 with 1 equiv of CNtBu in THF
(Scheme 2) leads to a distinct outcome as compared to the
addition chemistry of Rh20,II complex 1 with CNtBu. After 21 h,
the 31P{1H} NMR spectrum of the crude reaction mixture shows
a major product with three resonances—a multiplet at 102.6
ppm integrating to two phosphorus nuclei, and multiplets at 80.6
and 69.6 ppm each integrating to one phosphorus. These upfield
resonances are attributed to the phosphorus nuclei coordinated to IrII,
and their inequivalency suggests a cis arrangement of the phosphorus
ligands at IrII. The solid state structure of Ir20,II(tfepma)2(CNtBu)3Cl2
(4), shown in Figure 2, is consistent with this expectation. The Ir0
The room temperature NMR spectra of 5 and 6 suggest
conformational fluxionality in solution. Although the solid state
structureof bothcomplexes would predict an asymmetric31P{1H}
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Figure 3. Thermal ellipsoid plots for 5 and 6. Ellipsoids are shown at the 50% probability level with ꢀCH2CF3 groups and hydrogen atoms omitted for
clarity. Data were collected at 100 ( 2 K.
NMR spectrum, it (Supporting Information, Figures S4 and S5)
shows a single resonance, which appears as a symmetric multiplet
in 5 and a sharp singlet in 6; the splitting in 5 arises from JPꢀRh
coupling. In addition, the room temperature 1H NMR spectra of 5
and 6 show a single tert-butyl resonance integrating to 27 protons,
indicating that all three CNtBu ligands remain bound in solution,
1
albeit in a fluxional manner. Variable temperature H NMR,
depicted in Figure 4 for the tert-butyl region of complex 6, shows
the evolution of two distinct tert-butyl resonances at low tempera-
ture, with a 2:1 (upfield:downfield) integration ratio. This behav-
ior indicates that the four- and five-coordinate Ir0 sites exchange
slowly at low temperature, though even as low as ꢀ80 °C the two
CNtBu ligands at the five-coordinate site rapidly exchange. The
variable temperature 1H NMR behavior for 5 (Supporting Informa-
Figure 4. Variable temperature 1H NMR spectra for complex 6,
showing the tert-butyl resonances. Spectra were recorded in toluene-
d8 at 500 MHz.
tion, Figure S22) is qualitatively the same, and for both complexes
1
the remaining H resonances are minimally perturbed as the
temperature is altered. Variable temperature 31P{1H} NMR spectra,
shown in Supporting Information, Figures S23 and S24, demon-
strate considerable broadening as the temperature is lowered, but a
single resonance is observed over the entire temperature range.
Chlorine Oxidation of 2. We have previously reported the
synthesis and structure of Rh2II,II(tfepma)2(CNtBu)2Cl4, pre-
pared by treatment of 1 with PhICl2 in toluene.20 In both solution
and solid state, this Rh2II,II complex (excluding ꢀOCH2CF3 groups)
adopts a centrosymmetric structure where both RhII sites are
equivalent. In contrast, treatment of diiridium complex 2with PhICl2
leads to nearly exclusive formation of an asymmetric oxidation pro-
duct. The 31P{1H} NMR spectrum of Ir2II,II(tfepma)2(CNtBu)2Cl4
(7), shown in Supporting Information, Figure S6, reveals two closely
spaced multiplets, which are related by a mirror plane and suggestive
of an AA0BB0 spin system for which 12 of the possible 24 lines24
are resolved. A very minor product (<5%), present in both the crude
reaction mixture and the isolated material, appears as a singlet in
the 31P{1H} spectrum, consistent with a small amount of the
Figure 5. Thermal ellipsoid plot for 7. Ellipsoids are shown at the 50%
probability level with ꢀCH2CF3 groups and hydrogen atoms omitted
for clarity. Data were collected at 183 ( 2 K.
1
centrosymmetric isomer being formed. The H NMR spectrum
also reveals asymmetry—two distinct tert-butyl peaks, each integrat-
ing to nine protons, are evident.
Protonation of 2 with HX (X = Clꢀ, OTsꢀ). Reversible
protonation of dirhodium complex 1 with HCl has been detailed
previously,20 and we reasoned that analogous complexes could
be accessed from Ir20,II complex 2. By treating 2 with either HCl
The solid state X-ray structure of 7 presented in Figure 5
confirms the asymmetric structure predicted by NMR. Two near-
ly octahedral IrII centers separated by an intermetallic distance of
2.7774(3) Å are observed. The chloride ligands about Ir(1) main-
tain the cis arrangement found in Ir20,II complex 2 (see Figure 1),
whereasonIr(2) the chlorideligandsare trans tooneanother, with
an axial CNtBu ligand. The complex clearly lacks the inversion
II,
or lutidinium hydrochloride (LutHþClꢀ), a single isomer of Ir2
II(tfepma)2(CNtBu)2Cl3H (8) is formed rapidly and quantita-
tively. The 31P{1H} NMR spectrum (Supporting Information,
Figure S7) again shows two multiplets attributed to an AA0BB0
spin system; in this case 20 of the 24 possible lines are resolved.
The most definitive evidence for the presence of an iridium-bound
II,II
center found in the analogous Rh2 version, giving rise to the
asymmetry observed in the NMR spectra.
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Figure 6. Thermal ellipsoid plots for 8 and 9. Ellipsoids are shown at the 50% (8) or 30% (9) probability level withꢀCH2CF3 groups, solvent atoms and
carbon-bound hydrogen atoms omitted for clarity. Data was collected at 100 ( 2 K (8) and 175 ( 2 K (9).
hydride comes from the 1H NMR, which shows a 1:2:1 triplet at
ꢀ20.69 ppm. This hydride peak integrates to one proton, and the
observed 2JHꢀP of 16.0 Hz indicates that the hydride is positioned
cis to two equivalent phosphorus atoms. The solid state structure
of 8, shown in Figure 6, is analogous to that of 7, with a hydride
replacing one of the chlorides. Both IrII centers are again octahe-
dral, and hydride ligand H(1) is arranged trans to Cl(3) and syn
with respect to the CNtBu ligand on the adjacent IrII center. The
metalꢀmetal distance is 2.7821(3) Å, and the strong trans
influence of the hydride ligand is readily apparent. The Ir(1)ꢀ
Cl(3) bond, situated trans to the hydride, has a distance of
2.5021(11) Å, which is substantially longer than the other
equatorial chloride bond distance [Ir(2)ꢀCl(2)] (d
=
2.4002(10) Å).
Figure 7. Thermal ellipsoid plot for the cation of 10. Ellipsoids are
shown at the 50% probability level with ꢀCH2CF3 groups, the outer-
sphere tosylate anion and carbon-bound hydrogen atoms omitted for
clarity. Data were collected at 100 ( 2 K.
Addition of HOTs to 2 proceeds readily, and when carried out
in acetonitrile, the H NMR reveals two hydride-containing
1
products in about a 1:1 ratio. By changing the reaction solvent
to THF, a 9:1 ratio of the two products is obtained for the
isolated product. The major isomer of Ir2II,II(tfepma)2-
(CNtBu)2Cl2H(OTs) can be crystallized. Its structure, which is
shown in Figure 6, bears many similarities to its HCl-addition
analogue, namely, an axial CNtBu ligand and a trans arrangement
of the H(1) and OTsꢀ ligands at the site of protonation. The
major difference between the structures of 8 and 9 is the position
of the hydride relative to the CNtBu ligand on the adjacent
iridium; for 9, the hydride is in an anti conformation relative to
the vicinal CNtBu. Additionally, the metalꢀmetal distance in 9 at
2.7573(5) Å is slightly contracted relative to 8. The 31P NMR
spectrum of tosylate complex 9 (Supporting Information, Figure
S8) is again characteristic of an AA0BB0 spin system, and its 1H
NMR spectrum shows an IrꢀH resonance at ꢀ20.00 ppm, only
marginally upfield relative to the hydride resonance of 8. The
observed 2JHꢀP coupling constant in 9 is identical to that of 8 at
16.0 Hz. As alluded to above, when 9 is isolated about 10% of a
minor isomer is present. The minor product shows a hydride
resonance at ꢀ20.71 ppm in the 1H spectrum, and yet again the
two-bond proton-phosphorus coupling constant is 16.0 Hz,
placing the hydride cis to its neighboring phosphorus nuclei.
The remaining 31P and 1H NMR features of the minor isomer are
virtually identical to those of the major product, suggesting an
identical structure but with the hydride flipped to the other side
of the metalꢀmetal axis to position it syn to the vicinal CNtBu.
This isomerization will produce the same structure as that of
hydrido-chloride complex 8, but with the chloride trans to the
hydride substituted for OTsꢀ. We cannot rule out other possi-
bilities such as intermetallic exchange of Clꢀ and OTsꢀ, though
the simple isomerization described above seems most plausible
for the structure of the minor product.
Protonation of 5 with HOTs. Reactions involving M20,0 com-
plexes 5 and 6 were explored with a variety of acids, though in most
cases, complex and intractable mixtures of products were formed.
The lone exception was the exclusive production of [Rh2I,I(tfep-
ma)2(CNtBu)3(μ-H)](OTs) (10), obtained from the treatment of
Rh20,0 complex 5 with 1 equiv of HOTs in thawing acetonitrile. The
crystal structure of 10 is depicted in Figure 7. The position of the
hydride ligand could not be located precisely, and the refinement
statistics were indistinguishable if the hydride was modeled either as
bridging or as a terminal hydride on Rh(1). However, on the basis of
the NMR data (vide infra), complex 10 is assigned as a RhI RhI
3 3 3
μ-hydride species, as shown in Figure 7. All three CNtBu ligands
remain bound, and the intermetallic distance of 2.7449(11) Å
compares favorably with other hydride-bridged dirhodium com-
plexes spanned by two diphosphines.25ꢀ27 The OTsꢀ counterion is
outer-sphere and does not interact with the cationic complex in the
crystal structure.
1
Both the 31P{1H} and the H NMR spectra support the
assignment of a valence symmetric structure for 10. The 31P
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Figure 9. Low-frequency region of the 1H NMR spectrum of 11,
showing the IrꢀH resonances for the major and minor isomers. The
structure of the major isomer and a likely structure of the minor isomer
are shown near the peaks they are assigned to.
1
Figure 8. 1H (a) and H{31P} (b) NMR spectra of 10, showing the
RhꢀH resonance. Spectra were recorded in CD3CN at 500 MHz
and 293 K.
NMR spectrum (Supporting Information, Figure S9) shows a
single, symmetric multiplet centered at 150.7 ppm with a splitting
0,0
pattern reminiscent of Rh2 precursor 6. This contrasts the
expected four-spin, second-order system expected for a two-
electron mixed-valence state (vide supra). In the room tempera-
1
ture H NMR spectrum, a single, sharp tert-butyl resonance is
observed, which splits into two distinct peaks at low temperature,
as shown in Supporting Information, Figure S25. The most
definitive evidence for a bridging hydride ligand comes from the
1
low-frequency region of the H NMR spectrum. As shown
Figure 8a, in the 1H NMR spectrum the rhodium-hydride
resonance appears as a complex, symmetric multiplet, with 9
lines clearly resolved and an additional 4 lines appearing as
shoulders. In the 1H{31P} spectrum (Figure 8b), only coupling
to 103Rh is observed, and a triplet splitting pattern results. This
triplet multiplicity indicates equal coupling to both rhodium
Figure 10. Thermal ellipsoid plot of 11. Ellipsoids are shown at the 50%
probability level with ꢀCH2CF3 groups and carbon-bound hydrogen
atoms omitted for clarity. Data were collected at 100 ( 2 K.
1
nuclei, with a JHꢀRh of 14.8 Hz, and accordingly places the
For the minor product, the two hydride ligands are chemically
inequivalent, with a straightforward triplet of doublets splitting
pattern for each resonance. The two peaks have slightly different
2JHꢀP coupling constants: the downfield resonance (ꢀ14.80
ppm) has a value of 17.9 Hz whereas the upfield resonance
(ꢀ15.50 ppm) is coupled to phosphorus with a 13.8 Hz coupling
constant. These two values indicate that each of the inequivalent
hydrides is arranged cis to the two adjacent phosphorus nuclei.
hydride in a bridging position. The remainder of the multiplet in
the phosphorus-coupled 1H spectrum is generated by consider-
ing equal coupling of the hydride nucleus to all four phosphorus
2
nuclei, with an observed JHꢀP of 8.5 Hz. This value is con-
siderably smaller than the 2JHꢀP coupling constants in bimetallic
complexes of similar architectures featuring terminal hydride
complexes.11,20 This arrangement of coupling constants would
give a maximum of 15 lines in a triplet of quintets pattern, 13 of
which are resolved in the phosphorus-coupled spectrum.
2
The doublet splitting results from JHꢀH coupling of the two
hydride ligands, and a coupling constant of 4.6 Hz indicates a cis
arrangement of the two hydrides as observed for numerous other
cis dihydride complexes of iridium.28ꢀ32 One of the two reso-
nances for this minor isomer is clearly visible in the 31P{1H}
NMR spectrum (Supporting Information, Figure S10), and its
appearance as an AA0XX0 multiplet suggests a trans arrangement
of the two phosphorus nuclei at each iridium. A likely structure
for this minor product is included in Figure 9, though a structure
in which the equatorial hydride is switched with the CNtBu (L)
trans to it would also be consistent with the spectral data.
Solutions of 11 are unstable with respect to H2 loss. The orange
Reaction of 2 with H2. When a solution of 2 in THF-d8 is
introduced to about 1.5 atm of H2, the bright orange color rapidly
fades to give a nearly colorless solution. NMR spectroscopy shows
clean conversion to Ir2II,II(tfepma)2(CNtBu)2Cl2(H)2 (11). The
31P{1H} NMR spectrum (Supporting Information, Figure S10)
1
and the NꢀCH3 and tert-butyl regions of the H spectrum
(Supporting Information, Figure S11) clearly show two isomers,
formed in about a 19:1 ratio. The low-frequency region of the 1H
NMR spectrum, reproduced in Figure 9, gives some insight into
the structural differences between the two isomers. The major
product shows a complex, asymmetric multiplet integrating to two
protons centered at about ꢀ12.3 ppm. The large 187 Hz splitting
between the two most intense peaks suggests the presence of trans
2JHꢀP coupling. Though the precise structure of the major product
was not elucidated from NMR spectra, it has been structurally
characterized by X-ray crystallography as detailed below.
0,II
color and NMR features of Ir2 complex 2 are restored by
repeated freezeꢀpumpꢀthaw cycling of a solution of 11 or by
removing all volatiles and drying in vacuo.
The major isomer of 11 was crystallized from CD3CN under
an H2 atmosphere, and its structure has been determined
unequivocally by single crystal X-ray diffraction, as shown in
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ARTICLE
frequencies, suggestive of the very rigid binding mode of the
bridging ligands.
UVꢀvis Absorption Spectra. Excepting complexes 3 and 12,
which are unstable in dilute solution, the UVꢀvis absorption
features of all new compounds are collected in Supporting
Information, Figures S13ꢀS21. The electronic spectrum of
Ir20,II complex 2 (Supporting Information, Figure S13) is decid-
edly more complex than that of dirhodium analogue 1,20 with 6
closely spaced, overlapping but resolved bands. This complexity
is mirrored in Ir20,II complex 3 (Supporting Information, Figure
S14), which is prepared by CNtBu addition to 2 and presumed to
have a similar solution structure in acetonitrile, giving rise to
similar absorption features as discussed above. The absorption
0,0
spectra of M2 complexes 5 and 6 (Supporting Information,
Figure S15 and S16) also share similarities with one another,
showing a maximum at 379 nm in dirhodium complex 5 and
383 nm in diiridium complex 6, along with a shoulder toward the
visible region and intense UV features. Complexes 7ꢀ9 and 11
II,II
all feature a d7ꢀd7 Ir2
core and have largely analogous
Figure 11. Thermal ellipsoid plot of 12. Ellipsoids are shown at the 50%
probability level with the solvent molecule and carbon-bound hydrogen
atoms omitted for clarity. Data were collected at 100 ( 2 K.
absorption profiles. All show two distinct maxima and a shoulder
(Supporting Information, Figures S17ꢀS19, S21) whose posi-
tions are marginally responsive to the ligand environment. In all
cases the highest energy peak is the wavelength of maximum
absorption, which ranges between 267 nm (11) and 285 nm (8).
The electronic spectrum of dirhodium μ-hydride 10, shown in
Supporting Information, Figure S20, shows a series of five closely
spaced overlapping absorption bands. In addition to the electron-
Figure 10. Both iridium centers are six-coordinate, with a cis-P2,
cis-H2 geometry at Ir(1). The geometry about Ir(1) is substan-
tially distorted from octahedral, marked by a P(2)ꢀIr(2)ꢀP(4)
angle of 118.46(5)° and a H(1)ꢀIr(2)ꢀH(2) angle of 79(3)°.
The H(1)ꢀH(2) distance is 2.04(7) Å, with an Ir(1)ꢀIr(2)
bond distance of 2.7487(3) Å similar to the other metalꢀmetal
bonded Ir2II,II complexes described herein.
0,II
II,II
ic absorption features described above, the Ir2 and Ir2
complexes 2, 3, and 7ꢀ9 are emissive at 77 K in 2-methylte-
trahydrofuran solution. The emission bands are about 600 nm,
with maxima at 613 nm (2), 598 nm (3), 624 nm (7), 641 nm
(8), and 582 nm (9). For the Ir2II,II complexes 7ꢀ9, the relative
energies of the emission maxima track well with the energy of the
absorption maxima, and in all cases the large Stokes shifts suggest
emission from a triplet excited state.
Bimetallic Iridium Hydride Complex Formation by Cyclo-
metalation. In the course of preparing M20,II complexes 1 and 2,
we sought similar complexes bearing other bridging phosphine
ligands. Reaction of [IrI(COD)Cl]2 with 2 equiv of dppm and 2
equiv of CNtBu in THF leads to immediate formation of a dark
green solution, followed by the gradual deposition of a cream-
colored solid, identified spectroscopically and crystallographi-
cally as Ir2II,II(dppm)(PPh(o-C6H4)CH2PPh2)(CNtBu)2Cl2H
(12), which bears the expected stoichiometry but involves an
intermetallic cyclometalation of one dppm ligand. The solid state
structure is depicted in Figure 11. The most noteworthy feature is
the orthometalation of a phenyl ring bonded to P(4) onto the
opposite iridium site, Ir(1). The phenyl and hydride ligands are
arranged cis to one another, and again the geometry is distorted
from octahedral. The P(1)ꢀIr(1)ꢀP(3) angle is 109.83(3)°
whereas the C(82)ꢀIr(1)ꢀH(1) angle is 81.4(13)°, giving a
coordination geometry similar to that of 11. The Ir(1)ꢀIr(2)
distance is 2.7928(4) Å.
Complex 12 suffers from limited solution stability, decompos-
ing to an intractable mixture of products within hours. However,
the NMR spectra of 12 do suggest that prior to decomposition,
the solid-state structure is maintained in solution, and that the
CꢀH activation is irreversible at room temperature. The 31P-
{1H} NMR spectrum (Supporting Information, Figure S12)
shows three distinct chemical environments for the phosphorus
nuclei, with complex multiplicity in all of the peaks. In addition to
the expected 1H NMR resonances arising from dppm and CNtBu
protons, a doublet of doublets integrating to one proton is seen at
ꢀ10.69 ppm for the iridium-hydride. The 2JHꢀP values of 163 and
19.1 Hz are indicative of coupling of the hydride nucleus to a
trans and cis phosphorus nucleus, respectively. Furthermore, the
dppm ꢀCH2ꢀ resonances are each split into two distinct
’ DISCUSSION
Two-electron mixed valence dirhodium and diiridium com-
plexes can serve as platforms for a diverse range of chemistries,
including photocatalytic H2 production,3,4 multielectron
photochemistry,5,6 reversible H2 addition,33 CꢀH activation,34
and other organometallic chemistry.35 In all of these previous
examples, the M20,II complexes contain M0 centers that are five-
coordinate, precluding reactivity at this reduced metal site. We
reasoned that addition reactions could be facilitated by preparing
two-electron mixed valence group 9 bimetallic complexes with
four-coordinate M0 sites. Complexes 1 and 2, which meet this
criterion, assemble readily when [MI(COD)Cl]2 (M = Rh, Ir) is
treated with 2 equiv of tfepma and 2 equiv of CNtBu. The net
result is valence disproportionation of the M2I,I precursor to yield
M20,II(tfepma)2(CNtBu)2Cl2 [M = Rh (1), Ir (2)]. The coordi-
native unsaturation at the M0 site in 1 and 2 drives the binding of
donor ligands such as CNtBu to furnish complex 4. In the solid
state, 4 contains three bound CNtBu molecules, indicating that
the open coordination site at the Ir0 center in 2 can accept an
additional ligand. However, the solution behavior of 4, and the
disparate reactivity of Rh20,II complex 1 toward CNtBu revealed
some unexpected results concerning ligand addition and sub-
stitution. When 1 is treated with CNtBu, the broadened NMR
features suggest reversible binding to form 3. Crystals of 3 revea-
led the unexpected structure shown in Figure 2, where the
reaction results in ligand substitution instead of addition.
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The equatorial Clꢀ ligand at the RhII site is displaced by the incoming
CNtBu, maintaining the square planar Rh0 and octahedral RhII
geometries that are found in 1. This suggests that the electro-
philicity of the RhII center dictates the reactivity, as opposed to
the vacant coordination site at Rh0. The NMR and UVꢀvis spec-
tra of diiridium complex 4 indicate that a structure analogous to
dirhodium analogue 3 results when 4 is dissolved in acetonitrile.
These results reveal that in the two-electron mixed valence state a
four-coordinate M0 center is quite favorable, even when the
possibility of forming a five-coordinate center exists.
Scheme 3
complexes. Hydrogenation of a face-to-face diiridium(I) complex
yields a vicinal dihydride product, where the two ter-
minal hydride ligands are syn with respect to each other.39 When
diiridium A-frame complexes are treated with H2 the thermo-
dynamically stable species is once again a vicinal dihydride, though
in this case the two Hꢀ ligands occupy axial positions;39,40 it has
since been shown that a geminal cis-dihydride forms initially which
rearranges to this final product.41,42 In our earlier work on
two-electron mixed-valence complexes,33 hydrogenation of the
iridiumꢀiridium bond was observed, producinga vicinal dihydride
where the two electron mixed valency was preserved in a formally
IrIfIrIII core. The addition of H2 to 2, where the coordinative
unsaturation resides at the four-coordinate Ir0 center, produces a
result distinct from these previously characterized reactions. In the
same vein as the HX addition reactions, H2 adds to a single metal
center, resulting in an Ir2 geminal cis-dihydride product 11,
whose structure is depicted in Figure 10, as well as a minor product
which also features a cis-dihydride arrangement. On the basis of
X-ray crystallography and H NMR spectroscopy we see no
evidence for rearrangement to vicinal dihydride isomers. The
distance between the two hydrogen nuclei (d = 2.04(7) Å)
indicates that the HꢀH bond is cleaved completely and that the
description of 11 as a dihydride complex is valid.43 The cis
arrangement of the hydride ligands in 11, compared to the trans
HX complexes described above, suggests the possibility of a
different mechanism for H2 and HX oxidative additions. H2
addition is clearly reversible, and removal of the H2 headspace
from a solution of 11 regenerates complex 2 cleanly.
We have demonstrated that multielectron photocatalytic pro-
cesses are enabled when the bimetallic catalyst can span multiple
oxidation states differing by two electrons each.3,4 Thus, the
ability to prepare a homologous series of d7ꢀd7, d7ꢀd9, and
d9ꢀd9 complexes motivate future studies of multielectron cata-
0,II
lysis supported on the complexes described here. Indeed, M2
complexes 1 and 2 may be both reduced and oxidized by two
electrons to prepare the full suite of oxidation states. Our initial
goal was to reduce 1 and 2 to complexes of the stoichiometry
M20,0(tfepma)2(CNtBu)2, which would feature two four-coordi-
nate M0 sites. However, such species are not observed, and
isostructural complexes M20,0(tfepma)2(CNtBu)3 [M = Rh (5),
Ir (6)], are instead obtained where an additional equivalent of
CNtBu is required to stabilize the fully reduced core. Never-
theless, the structures of 5 and 6 (Figure 3) each reveal a four-
coordinate M0 center, motivating further study of demanding
addition reactions on these highly reducing species. Oxidation of
2 with PhICl2 produces Ir2II,II(tfepma)2(CNtBu)2Cl4 (7), which
is structurally distinct from its dirhodium analogue.20
Having established a redox chemistry of 1 and 2, routes for the
preparation of hydride-containing complexes are available. Proton-
ation reactions of isoelectronic valence-symmetric dirhodium A-frame
complexes have been shown to give products featuring a bridging
hydride, where the net reaction is the protonation of the metalꢀmetal
bond.26 The reactivity of our first-generation Ir20,II complexes33 arises
from a vacant axial site on the MII center. Reagents such as HX and H2
result in oxidative addition across the metalꢀmetal bond, with two-
electron mixed valency preserved in the products. In contrast, addition
of HX (X = Clꢀ, OTsꢀ) and H2 to 2 furnishes products suggestive of
addition to the Ir0 site, and the IrII center unperturbed. As seen in the
crystal structures of Ir2II,II(tfepma)2(CNtBu)2Cl2HX [X = Clꢀ (8),
OTsꢀ (9)], the hydride and Xꢀ ligand situate in a trans configuration
on the same metal center, and a valence-symmetric Ir2II,II core ensues.
Only terminal hydride products are observed for these acid addition
reactions. Ir20,II complex 2 reacts much more readily with HX than its
dirhodium analogue 1. HCl addition to 1 is decidedly reversible20
whereas in the case of 2 a single equivalent of HCl begets quantitative
formation of hydride-containing 8. In addition, Ir2II,IICl3H com-
plex 8 is conveniently prepared by protonation of 2 with solid HCl
surrogate LutHþClꢀ, though dirhodium complex 1 is completely
unreactive to this reagent. This difference in reactivity is likely tied
to the stronger iridium-hydride bond strengths as compared to
rhodium.36 Attempted protonation reactions of M20,0 complexes 5
and 6 were generally not fruitful, though Rh20,0 complex 5 does
react readily with HOTs to furnish μ-hydride complex 10
(Figure 7). This outcome is largely reminiscent of earlier examples
of protonation reactions involving other group 9 M20,0 complexes,
wherethe addition of acid alsogivesa hydride-bridgedproduct and
an outer-sphere conjugate base anion.25,37,38
II,II
1
A route to a diiridium hydride complex is also achieved from
intramolecular cyclometalation. With the bridging ligand dppm
and reaction conditions otherwise identical to the synthesis of 2,
the complex Ir2II,II(dppm)(PPh(o-C6H4)CH2PPh2)(CNtBu)2-
Cl2H (12) is formed as the final product. At early reaction time a
dark green solution is observed, which is qualitatively indicative
of an initial IrI IrI product prior to the cyclometalation. The
3 3 3
final product possesses an IrII,II core oxidation state with the
expected stoichiometry (Figure 11). The cyclometalated binding
mode of dppm seen in the structure of 12 has been observed in a
Cp*-ligated diiridium complex,44 where an ortho CꢀH bond was
activated in a base-assisted fashion. Although the mechanism of
cyclometalation of 11 has not been probed in detail, a plausible
route is shown in Scheme 3. The initially formed IrI IrI com-
3 3 3
0,II
plex rearranges to an Ir2 state, and cyclometalation at the
reactive Ir0 center yields 11. Direct cyclometalation of IrI IrI
3 3 3
cannot be explicitly ruled out, though a more complex mecha-
nism involving chloride-bridged intermediates and/or phos-
phine dissociation would be required to avoid hypercoordinate
intermediate species.
To summarize, we have demonstrated a multifaceted reaction
chemistry centered around coordinatively unsaturated two-
electron mixed valence dirhodium and diiridium complexes. We have
shown that stable complexes with d9ꢀ d9, d7ꢀ d9, and d7ꢀ d7
electron counts can all be prepared with the ligand framework
The hydrogenation reaction of 2 is also distinct from that of
previous diiridium constructs. Several products have been identi-
fied from hydrogenation reactions of valence-symmetric diiridium
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Inorganic Chemistry
ARTICLE
employed here. Three distinct routes to access hydride-contain-
ing products are outlined—addition of HX (X = Clꢀ, OTsꢀ),
addition of H2, and intramolecular cyclometalation. On account
of the coordinative unsaturation at Ir0, both HX and H2 oxidative
addition furnish hydride-containing products with novel struc-
tures; these complexes are candidates for future HX splitting
schemes. In addition, hydrido-halide complex 8 is a structural
analogue to a dirhodium analogue which mediates O2 reduction
to water.20 The O2 reactivity of the diiridium hydride complexes
shown here is under active investigation with the objective of
providing insight into the mechanism of O2 reduction. Our
interest in bimetallic hydride-containing complexes is ongoing, as
we continue to explore their reactivity and unveil new routes for
their facile preparation.
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’ ASSOCIATED CONTENT
Supporting Information. 31P{1H} NMR spectra for all
S
b
complexes, partial 1H NMR spectrum of 11, electronic absorp-
tion and emission spectra for 2 and 4ꢀ11, variable temperature
NMR spectra of 5, 6, and 10, X-ray crystallographic data table,
and the crystallographic information file (CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: nocera@mit.edu.
(32) Nishihara, Y.; Takemura, M.; Osakada, K. Inorg. Chim. Acta
2009, 362, 2951–2956.
’ ACKNOWLEDGMENT
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This work was supported by NSF Grant CHE-0750239.
Grants from the NSF also supported the MIT Department of
Chemistry Instrumentation Facility (CHE-9808061 and DBI-
9729592). T.S.T. acknowledges the Fannie and John Hertz
Foundation for a graduate research fellowship.
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