Inorganica Chimica Acta 424 (2015) 91–102

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Review

Synthesis, characterization and dynamic behavior of some iridiumcarbonyl cluster complexes derived from Ir4(CO)12 with N-, P- andC-donor ligands: A survey

Augusto Tassan initiated its research activity in the Chemistry Department of the Venice University. He then moved to the University of PIndustrial Chemistry Institute, under the supervision of Prof. R. Ros and R.A. Michelin, and collaborating with the Prof. R. Roulet of EPFL in LauHis research focuses on the synthesis of new organometallic platinum and iridium clusters, with particular interest on NMRcharacterization

Mirto Mozzon took a degree of Industrial Chemistry at the University of Padova with full marks. He became then CNR researcher in the groupby Prof. U. Belluco, and finally researcher at the University of Padova, under the supervision of Prof. R. A. Michelin. He is now Associate Profesmaintained a research cooperation whit Prof. A.J.L. Pombeiro at Instituto Superior Tecnico in Lisbon. He coauthored about 80 papers publispeerreviewed international journals in Inorganic and Organometallic Chemistry. He also filled 4 International Patents and written 2 student

http://dx.doi.org/10.1016/j.ica.2014.09.0140020-1693/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +39 498275518.E-mail address: augusto.tassan@unipd.it (A. Tassan).

Augusto Tassan a,⇑, Mirto Mozzon a, Giacomo Facchin b, Alessandro Dolmella c, Serena Detti d

a Dipartimento di Ingegneria Industriale, via Marzolo 9, 35131 Padova, Italyb Istituto per l’Energetica e le Interfasi IENI-CNR, via Marzolo 9, 35131 Padova, Italyc Dipartimento di Scienze del Farmaco, via Marzolo 5, 35131 Padova, Italyd Institute of Ecosystem Study ISE-CNR, via Moruzzi 1, 56122 Pisa, Italy

a r t i c l e i n f o

Article history:Received 4 August 2014Received in revised form 9 September 2014Accepted 14 September 2014Available online 28 September 2014

Keywords:Iridium clustersCarbonylIntramolecular dynamic

a b s t r a c t

The synthesis of iridium dodecacarbonyl cluster derivatives Ir4(CO)12 with donor ligand such as amine,phosphites, hydrido and cyclic mono and dioxycarbene, NMR and X-ray characterization and fluxionalbehavior study in solution at variable temperature is briefly reviewed.

� 2014 Elsevier B.V. All rights reserved.

adova,sanne.

headedsor. Hehed on

’s book.

Giacomo Facchin studied chemistry at the Università degli Studi di Padova (Italy) and completed his PhD in 1979. After a post doc term with Prof. R.J.Angelici at the Iowa State University he joined the Italian National Research Council (CNR) where is currently Senior Researcher at the Istituto perl’Energetica e le Interfasi (IENI). His scientific activity mainly focuses on organometallic and coordination chemistry, nanostructurated materials andmaterials containing metallic nanoparticles.

Alessandro Dolmella entered the Department of Pharmaceutical and Pharmacological Sciences in 1990 to work in the research group headed byProf. M. Nicolini, studying radiopharmacy and computational chemistry. He is presently interested in bioinorganic and coordination chemistry, with aspecial emphasis on transition metals complexes.

Serena Detti graduated in chemistry at the University of Pisa in 1996, under the supervision of Prof. F. Calderazzo and G. Pampaloni. She received herPhD in Chemistry at the Swiss Federal Institute of Technology of Lausanne in 2002, working in the group of Prof. R. Roulet in the field of metalscarbonyl clusters. She worked at the Italian forensic science service and later she devoted to research on nanotechnology, studying potentialinteractions of metal nanostructures and the environment.

92 A. Tassan et al. / Inorganica Chimica Acta 424 (2015) 91–102

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 922. 13CO-enrichment of tetrairidium carbonyl clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 933. Diamine derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 934. Monoamine derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 945. Hydrido derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 946. Cyclic mono and dioxycarbene derivatives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 967. Phosphites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 988. Intramolecular dynamics of [Ir4(CO)12] derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

1. Introduction

The chemistry of iridium carbonyls, notably, the chemistry ofclusters derived from Ir4(CO)12, were developed along variousresearch lines. Among the investigated topics, we can mentionthe stereochemistry of the ligands [1], the fluxional processesoccurring in solution [2], the studies on the kinetics of carbonylsubstitution reactions [3], the modellisation of metal surfaces forabsorption reactions of unsaturated substrates [4], the use ofsuch materials as catalysts or precursors in the hydrogenationprocesses of hydroformylation of unsaturated organic molecules[5].

Along with these perspectives, Garlaschelli and co-workers [6]prepared the starting complex Ir4(CO)12 from IrCl3�nH2O in

ethylene glycol monomethylether medium under a CO gas flowwith more than 80% yield.

The IR spectrum of the obtained mixture shows the typicalbands of terminal carbonyls in the range 2114–2000 cm�1.

A few years later Pruchnik et al. [7] reported an even more effi-cient method (ca. 95% yield) to obtain Ir4(CO)12 by reacting IrCl3�3H2O with formic acid in autoclave at 100 �C for 12 h.

Tri- and tetra-substituted derivatives of Ir4(CO)12 can beobtained in good yield by means of the direct reaction of thetetrairidium complex with different ligands (L). Further studies ofsubstitution reactions have identified as a process made of threeconsecutive steps:

Ir4ðCOÞ12 þ L !�COIr4ðCOÞ11Lþ L !�CO

Ir4ðCOÞ10Lþ L !�COIr4ðCOÞ9L

Scheme 2. Alternative synthesis of 13CO-enriched tetrairidium dodecacarbonyl.

A. Tassan et al. / Inorganica Chimica Acta 424 (2015) 91–102 93

In the first step, the substitution reaction follows a second-order kinetics law. It is characterized by an associative mechanism,and three of the (previously) all terminal CO undergo a structuralrearrangement that transforms them into bridging CO units onthe basal iridium atoms. The following two steps have an associa-tive-to-dissociative substitution mechanism. Factors as basicityand the bulkness of the incoming ligand do not vary the processkinetics [3].

Mono- or di-substituted derivatives of Ir4(CO)12 cannot insteadbe isolated by direct synthesis from the tetrairidium complex,because of its insolubility in common organic solvents and alsobecause the reaction requires high temperatures (80–120 �C).Accordingly, alternative routes had to be sought. An opportunity,although with low yields, is given by the reduction of Ir(CO)2(H2-

NC6H4Me-p)Cl by zinc metal in the presence of carbon monoxideand the ligands [8]; another one is offered by the reaction of Ir4-

H(CO)11 with the appropriate ligands [9].Still another possibility was explored by Chini et al., who pre-

pared the anionic complexes of the type [Ir4(CO)11X]� (X = Br orI) [10]. Since the latter are soluble and more reactive than iridiumdodecacarbonyl, they can undergo replacement reactions of a COwith several ligands, including PF3 and SO2 [11], olefin [12], phos-phines [13], arsines [14], yielding various complexes.

In the following sections, we provide a brief account on the syn-thesis, characterization and dynamic behavior of new tetrairidiumclusters with H-, N-, P-, and C-donor ligands derived from Ir4(CO)12.

2. 13CO-enrichment of tetrairidium carbonyl clusters

The molecular structure and the stereochemistry of metal car-bonyl derivatives might be investigated by means of 13C NMR anal-ysis. However, a problem arises because the exchange reactions donot always occur with free 13CO; while for some metal carbonylclusters, such as Co4(CO)12 and Rh4(CO)12, this exchange is easy,the reaction involving Os3(CO)12 and Ir4(CO)12 occur only withmore difficulty. The main obstacle is the low solubility of the clus-ters, which forces the exchange reactions to take place in heteroge-neous phase and makes them extremely slow, even at highpressures and high temperatures.

Tassan and co-workers have reported [15] two simple proce-dures to make the 13CO exchange process easier. The first oneinvolves the use of anionic clusters; the second one requires theuse of the well-known decarbonylating agent trimethylamine-N-oxide, Me3NO [16–18], in the presence of free 13CO.

The first method (a two-step process) is illustrated in Scheme 1.At the beginning, as reported by Chini et al. [10], the reaction ofIr4(CO)12 with NEt4I in THF at 70 �C leads to the formation of theanionic iridium tetracarbonyl [Ir4(CO)11I]�. The second step, thedisplacement of iodide by 13CO, occurs in THF at room temperatureand affords the enriched Ir4(⁄CO)12 cluster.

Both reactions occur with more than 90% yield. The degree ofthe first enrichment A1 is given by the molar fraction of coordi-nated 13CO and can be calculated from Eq. (1) below:

A1 ¼bþ 11A0

12¼ 0:09266 ð1Þ

Scheme 1. Reaction path for the synthesis of 13CO-enriched tetrairidium dodeca-carbonyl. ⁄Mixture of 12CO and 13CO.

where A0 and b are, respectively, the molar fractions for naturalabundance 13CO (0.01108) and for used 13CO (0.99). A secondenrichment step can then be carried out, again, in THF at room tem-perature as outlined in Eq. (2):

NEt4½Ir4ð�COÞ11� þ 13CO! Ir4ð�COÞ12 # þNEt4I ð2Þ

i.e., by repeating the reaction path described in Scheme 1, this timeusing the enriched Ir4(⁄CO)12 as starting material. The molar frac-tion of 13CO can be calculated from Eq. (3) below:

A2 ¼bþ 11A1

12¼ 0:16743 ð3Þ

Further A3, A4, . . .Ax values can then be calculated from the fol-lowing Eq. (4):

Ax ¼bþ 11Ak�1

12ðk ¼ 1;2;3; . . .Þ ð4Þ

Eq. (4) has basically the form:

Ak ¼ f ðAk�1Þ ð5Þ

where f is a linear function that converges to the point (b = 0.99), avalue which verifies the equation t = f(t).

As mentioned above, the alternative direct method uses Me3NOas decarbonylating reagent, according to the following scheme:

The first reaction of the Scheme 2 is carried out at �30 �C inTHF, with a Me3NO/cluster stoichiometric ratio and a slight excessof 13CO, affording the anionic [Ir4(⁄CO)11I]� complex. The subse-quent reaction is performed at room temperature and yields thetetrairidium carbonyl complex in more than 90% yield. In general,this kind of reaction allows the preparation of a large number of13CO-enriched iridium carbonyl cluster complexes with differentligands, including monodentate phosphines, diphosphines orarsines. The fluxional behavior of all these species can be readilyanalyzed by 13C NMR (eq. (6)).

Ir4ðCOÞ12�nLn þ13 COþMe3NO! Ir4ð�COÞ12�nLn þ CO2

þ NMe3 ð6Þ

The exchange rate 12CO–13CO was determined by following,through IR spectrometry, the enrichment reaction of the complexIr4(CO)11(PPh3) in CH2Cl2. As expected, as long as 13CO increasesthere is a lowering of the CO stretching frequencies. Changing from99% 12CO to 98% 13CO, the IR spectrum presents the same overallshape and the same number of bands, however, with a shift of47.0–48.5 cm�1 for terminal carbonyls, and of 40 cm�1 foredge-bridging CO. These frequency shifts are in agreement withthe values shown by the 12CO and 13CO ligands, whose vibrationsare related only to their reduced masses and not influenced bythe coupled cluster moiety.

m 12COð Þ � m 13COð Þm 13COð Þ

¼ 1�l 13COð Þl 12COð Þ

!" #1=2

ð7Þ

3. Diamine derivatives

The first diamine derivatives of tetrairidium dodecacarbonylhave been synthesized by Tassan and co-workers [19]. Initially(Scheme 3), [Ir4(CO)11X]� (X = Br or I) reacts with a large excessof aromatic ligand in presence of Ag+ (one equivalent) in dichloro-

[NEt4][Ir4(CO)11X]+ (N-N) -AgX

-30°C[Ir4(CO)11{η1-(N-N)}]-

1/2 Ir4(CO)10{η2-(N-N)} + 1/2 Ir4(CO)12 + 1/2 (N-N)

R.T.

Scheme 3. Synthesis of diamine derivatives of tetrairidium dodecacarbonyl. (N-N)= 1,10-phenantroline (1); 4,7-dimetylphenantroline (2); 5,6-dimetylphenantroline(3); 3,4,7,8-tetrametylphenantroline (4); 2,20-dipiridine (5); 4,40-dimetyl-2,20-dipir-idine (6).

a

a b

c

d

c

ee g

d

L

L

Fig. 1. Proposed structure for the diamine derivatives. (L-L = N-N).

94 A. Tassan et al. / Inorganica Chimica Acta 424 (2015) 91–102

methane at low temperature (�30 �C); the diamines are thenobtained after a disproportion reaction at room temperature.

The configuration of the complexes (Fig. 1) has been assignedon the basis of IR and 13C{1H} NMR spectroscopies at lowtemperature (180 K). The complexes show six resonances: forexample in the case of N-N = 1,10-phenantroline the 13C{1H}NMR carbonyls a at 226.6 (relative intensity 2), b at 200.1 (r.i. 1),d (r.i. 2), c (r.i. 2), e (r.i. 2) and g (r.i. 1). The IR spectroscopic datashow the presence of terminal carbonyls (2070s, 2043vs, 1995s)and bridged carbonyls (1830m and 1797m).

The structure proposed in Fig. 1 has been confirmed by singlecrystal X-ray diffraction analysis (Fig. 2). The complex presents atetrahedral cluster of iridium atoms and a distribution of COligands similar to that found in many other derivatives of Ir4(CO)12

[20]. The Ir–C bond distances become shorter in presence of thediamino groups. The diamino ligands chelate the cluster througha basal iridium via the lone-pairs of nitrogen atoms lying in axialand radial positions. The Ir2–Ir3 and Ir2–Ir4 distances involvingthe iridium atom (Ir2) which is chelated the diamine ligand arecomparable to the unbridged Ir–Ir bond distances [20].

Fig. 2. X-ray structure of Ir4(CO)11(1,10-phentroline) 1.

4. Monoamine derivatives

The work on diamino ligands was extended by preparing com-plexes with monoamine [21], notably, with pyridine (7), 4-methyl-pyridine (8), 4-ter-buthylpyridine (9), 3,5-dimethylpyridine (10)and 3,4-dimethylpyridine (11). In this case, [Ir4(CO)11Br]� reactsreadily with an excess of aromatic monoamine and one equivalentof AgBF4 in CH2Cl2 at �25 �C. The products are obtained with60–71% yield, after recrystallization from a CH2Cl2/MeOH mixture.

NEt4½Ir4ðCOÞ11Br� þ AgBF4 þ L! Ir4ðCOÞ11Lþ AgBrðwhere L ¼monoamine ligandsÞ ð8Þ

The IR spectra of these compounds, in CH2Cl2 solution, showeither the presence of the characteristic terminal CO bands(2100–1950 cm�1) and also two adsorptions in the region of bridg-ing CO. However, by replacing the CH2Cl2 solvent with cyclohex-ane, the bands of bridging CO disappear. This can be explained(Scheme 4) by assuming the presence of at least two species insolution: an isomer with all terminals ligands (A), an isomer withbridging CO and the amine in axial position (B) and another onewith the amine group in equatorial position (C).

Likewise, the 13C NMR analysis of compound (8) enriched with13CO ca. 20% reveals two sets of signals in 36/73 ratio. The first setcan be attributed to the 8B isomer, with the monoamine ligand inaxial position, the second and more abundant one to isomer 8A.The same analysis for compound 9 shows the presence of threesets of signals originated by the three isomers (A, B, C) in 42/55/3 ratio, respectively.

The X-ray crystal structure of Ir4(CO)11(4-methylpyridine)(Fig. 3) shows that the molecule contains a nearly tetrahedral Ir4

core and all terminal ligands, as resulting also from the analysisof the 13C NMR spectra. The Ir1–Ir4 distance, in a pseudo-trans posi-tion with respect to the amine ligand, is lower (2.659(6) Å) thanthe average value found for the remaining metal–metal distances(2.687(17) Å); this may be due to the weaker sigma-trans influencewith respect to the carbonyl ligand.

5. Hydrido derivatives

Known hydrido derivative of tetrairidium dodecacarbonylare [H2Ir4(CO)10]2� [10], [HIr4(CO)11]2� [22], and the neutralorthometalled compound [HIr4(CO)7(Ph2PCH@CHPPh2)(PhC6H4

PCH@CHPPh2)] [18] reported by Albano et al. that shows a bridginghydride between two iridium atoms with IrAH bond lengths of1.71 and 1.76 Å. With respect to similar derivatives, it is worthnoting that the deprotonated form of the dppm diphosphineligand, bis(diphenylphosphino)methanide [(Ph2P)2CH]�, has beenused for its ability to behave as a two-, four- or six electrons donor[23]. In fact, simple deprotonation of dppm ligand with a baseaffords the preparation of new hydride iridium cluster derivatives[24]. The reaction underlined below (Eq. (9)) is carried out with anexcess of KOH dry powder in dichloromethane at �20 �C and givesthe product with 76% yield:

Ir4ðCOÞ10ðl-dppmÞ þ 2KOHþ PPNCl

! ½PPN�½Ir4ðCOÞ9ðl3-ðPh2PÞ2CHÞ� þ KClþ KHCO3 ð9Þ

Since in the IR spectrum there are no bands due to bridging car-bonyls, this complex shows in solution and solid state a symmetrywith only terminals CO ligands. At 173 K the 13C NMR shows thatthe apicals CO are already slowly exchanging and at 203 K the onlyfluxional mechanism observed arises from the rotation of apicalcarbonyls.

The crystal structure of [PPN][Ir4(CO)10(l3-(Ph2P)2CH)] showsCs symmetry, with all terminals CO and with the ligand [(Ph2P)2-

L

a a

b

cc

dd

ee

g

fa a

b

cc

dd

ee

g

hL

6 6

5

L

112 2

3 3

44

AB C

Scheme 4. Possible arrangements for amine ligands.

Fig. 3. ORTEP view of the complex Ir4(CO)11(4-methylpyridine) 8.

A. Tassan et al. / Inorganica Chimica Acta 424 (2015) 91–102 95

CH]� face bridging with the plane P(2)ACAP(3) eclipsed withrespect to the plane Ir(3)AIr(2)AIr(4). The IrAIr distance (meanvalue = 2.700(1) Å) is shorter than the one found for Ir4(CO)7

(l-CO)3-(l-(Ph2P)2CHMe) (2.729(1) Å). This finding has been sup-ported by other structural data, confirming that the CO-unbridgedmetal bonds are shorter than those bridged [25]. Likewise, thehydrido-cluster [PPN][HIr4(CO)9(l-dppm)] is obtained in 85% yieldaccording to the following reaction, (Eq. (10)) by using a largeexcess of 1,8-diazobicylo[5,4,0]undec-7-ene (DBU) as base underCO atmosphere in wet CH2Cl2 at �20 �C and [Ir4(CO)10(l-dppm)]as starting material [24]:

Ir4ðCOÞ10ðl-dppmÞ þ DBUþH2Oþ PPNCl

! ½PPN�½HIr4ðCOÞ9ðl-ðdppmÞÞ� þ CO2 þ ½DBUH�Cl ð10Þ

Also for this hydrido compound the geometry in solution and inthe solid state have been found to be the same, with three bridgingCO, the remaining CO terminal and the hydride ligand coordinatedin axial position. Interestingly, in the 1H NMR spectrum the meth-ylene CH2 protons signals of dppm originate an ABX2 spin systemdue to the inequivalence of the two protons. While the proton HB

shows a chemical shift of 2.69 ppm, the HA is observed at6.03 ppm, indicating a strongly deshielded nucleus. This cluster isfluxional in solution. The first CO scrambling process takes placeat 200 K and involves the basal carbonyls a, b, d and f; a secondprocess takes place at 220 K and involves the rotation of apicalcarbonyls e and g.

The crystal structure of the hydrido-cluster [PPN][HIr4(CO)9

(l-dppm)] shows Cs symmetry, with three bridging CO on the

basal plane and the bidentate diphosphine ligand taking two axialpositions. The mean value of the IrAIr distance (2.769(3) Å) islonger than that reported for the [PPN][Ir4(CO)10((Ph2P)2CH)],while the IrAH distance (2.08(6) Å) is longer than those found formonometallic complexes [24]. The exact location of the hydrideligand in the complex could not be successfully defined byconventional X-ray diffraction analysis. Consequently, a neutrondiffraction experiment was performed at the Institute LaueLangevin in Grenoble [26]. Fig. 4 illustrates the outcome of thisexperiment. The IrAH distance found is 1.618(14) Å and it is thefirst experimental determination of an iridium cluster. Comparisonwith the value above reported of 2.08(6) Å proves the latter to beincorrect and, confirms the predictive power of ab initiocalculations [26], and, at the same time, highlights once more thelimits of conventional X-ray diffraction analysis in defining theposition of light atoms in the proximity of heavy ones.

As described above, Ir4(CO)10(l-dppm) quickly reacts with anexcess of KOH to give [Ir4(CO)10(l-(PPh2P)2CH)]�, which in turn con-verts into the decarbonylated anion [PPN][Ir4(CO)9(l3-(PPh2P)2CH)],whereas the hydrido-derivative Ir4H(CO)9(l-dppm) is obtained ifthe same reaction is carried out with an excess of DBU. Detti andco-workers [27] further studied this reaction in dichloromethanewith an excess of DBU and PPNCl, using different phosphinesand arsines, such as: bis(diphenylphosphino)methane, 1,1-bis(diphenylphosphino)ethane, 1,2-bis(diphenylphosphino)ethane, 1,3-bis(diphenylphosphino)propane and bis(diphenylarsino)meth-ane. All the corresponding hydrido-complexes (12, 13, 14, 15, 16,respectively) were obtained with more than 75% yield. Theproposed mechanism requires the nucleophilic attack of OH� onthe metal carbonyl, as in Scheme 5:

On the contrary, the reaction of Ir4(CO)10(l-dppmMe) carriedout in presence of an excess of DBU, but without PPNCl, the hydr-ido-compound (13a) (75% yield, see Scheme 6) with [DBUH]+ ascounterion was obtained together with a secondary derivative(13b, 1%) with [DBUMe]+. Using Ir4(CO)10(L) (L = dppm, dppe, dppp,dpam) as starting materials and the same reaction conditions usedfor Ir4(CO)10(l-dppmMe) does not lead to the formation of theanalogous hydrides-anions. The explanation appears to be thatthe nucleophilic attack by such a strong base as DBU on a diphos-phinic chain produces a lack of a site and this is related with theweak acidity of the methyl group [28]. The compounds (12–16)show the characteristic IR bands in the bridging CO region. The31P {1H} NMR spectra have only one signal for the diphosphine, likethe starting complexes. The hydrido ligand is located in axial posi-tion and presents a single 1H NMR signal at low field (�15 ppm).Finally, the low-temperature 13C{1H} NMR spectra obtained fromenriched compounds show the typical pattern of carbonyls.

The molecular structures of compounds (13) with counterion[DBUH]+ and [DBUMe]+ respectively, and (14, 15) with [PPN]+ areillustrated in Fig. 5. All structures show the diphosphinic andhydride ligands in axial position with respect to the Ir1AIr2AIr3

plane that also accommodates three bridging CO units.

Fig. 4. Left: ORTEP drawing of the [HIr4(CO)9(l-dppm)]� (12) obtained from X-rays diffraction. Right: Structure obtained from neutron diffraction.

Ir

CO

H2O -H+

Ir CO

OHIr H + CO2

Scheme 5. The formation of hydride-derivatives by nucleophilic attack of OH� onmetal carbonyl.

96 A. Tassan et al. / Inorganica Chimica Acta 424 (2015) 91–102

The investigation of structural data highlights that the hydrideligand has a stronger trans-influence compared to the CO group.As for the bond distances, the Ir3AIr4 distance is longer than theremaining IrAIr bonds and also longer than those found in thestarting compounds. In contrast, the IrAP distances are shorterthan those observed in Ir4(CO)10(l-dppmMe) and Ir4(CO)10(l-dppp). As expected, the determination of hydride bond lengthproved difficult [26]. The IrAH bond length is 1.32(5) Å and it isshorter than that found for [HIr4(CO)9(l-dppm)]� (1.618(14) Å)by neutron diffraction.

6. Cyclic mono and dioxycarbene derivatives

Tassan and co-workers [29] have reported the synthesis and theinvestigation of the fluxional dynamics and the X-ray molecularstructures of a series of new dioxycarbene compounds obtainedfrom Ir4(CO)11(L) (L = PtBu3 (17), PPh3 (18,19) and Ir4(CO)10(LAL)(LAL = Ph2PCH2PPh2 (20), norbornadiene (21) and 1,5-cyclooctadi-

C

H

CH3

P

P

Ir

Ir

+ dbu

P

P

Ir

Ir

C

P

P

Ir

Ir

C

-dbuH+

-dbuCH3+

Scheme 6. The formation of hydride-derivatives w

ene (22,23)). The starting phosphine derivative Ir4(CO)11(PtBu3)was obtained by reacting Ir4(CO)11(norborn-2ene) [15] with a stoi-chiometric amount of tri(ter-butyl)phosphine (PtBu3) in dichloro-methane. The 31P{1H} NMR spectrum shows a single resonancefor phosphine at 65.9 ppm. The values of the coordination chemicalshift (Dd = dcoord. � dfree phosphine) [13] of 2.6 ppm suggests that thephosphine lies in axial position. This idea is supported by the pres-ence in the 13C{1H} NMR spectrum of two bands in the radial COdomain, one of which, f, Scheme 7, shows a coupling of 8.1 Hz withthe phosphorus atom, and, in the apical ligands field, 27.1 Hzpseudo-trans-coupling of CO g with the same atom.

The reaction of Ir4(CO)11(PtBu3) with oxirane 2-bromoethanoland sodium bromide as catalyst leads to the formation of themonocyclic dioxocarbene derivative Ir4ðCOÞ10ðP

tBu3ÞðCOCH2CH2OÞ. The IR spectrum of this complex shows the presenceof three bands (at 1862, 1819 and 1795 cm�1) due to bridging COthat are typical of complexes having a ground state C3v symmetry.The 31P{1H} NMR at 230 K exhibits two resonances, d = 62.29 and64.56 ppm, due to two different isomers, A and B (ratio = 28:72;17; see Scheme 7); the latter may be separated by TLC.

From values of the calculated coordination chemical shifts,Dd = 1.3 and �1.0 ppm, it is possible to infer that in isomer A thephosphine and carbene ligands are both in axial position, whilein isomer B the ter-butyl is in axial and the carbene in radialposition. Likewise, the 13C{1H} NMR spectrum in CD2Cl2 at 230 Kpresent two sets of signals. Those relating to major isomer B are

CH3 CH(CH3)

P

P

Ir

Ir

H CH2

P

P

Ir

Ir

+H+

+H+

75 %

1 %

ith two possible types of direct attack of dbu.

Fig. 5. (A,B) Molecular structures of [HIr4(CO)9(dppmMe)] 13a with [DBUH+] and [DBUMe+] 13b as counterion, respectively; (C,D) molecular structures of [HIr4(CO)9(dppe)]14 and [HIr4(CO)9(dppp)] 15, respectively, both with [PPN]+ as counterion.

L

b a

b'

c

d'd

eeg

f *C

c'

b a

b'

c

d'd

e'eg

L *C

L

b a

b'

c

*Cd

e'eg

f c'

17A18A

18C 17B18B

Scheme 7. Possible arrangements of ligands. L = PtBu3 (17A–17B), PPh3 (18A–18B),

*C = COCH2CH2O.

A. Tassan et al. / Inorganica Chimica Acta 424 (2015) 91–102 97

identified by the carbene chemical shift at 212.04 ppm (COO groupin radial position); the 198.91 ppm value of the minority (28%)isomer indicates that in this case the carbene occupies an axialposition. The molecular structure of 17A shows that the fouriridium atoms define a regular tetrahedron and the phosphineand carbene ligands are axially bonded to two vicinal Ir atoms ofthe basal plane. The values of the dihedral angles between the tet-rahedron base on the plane Ir1AIr2ACO12, Ir1AIr2ACO13 andIr2AIr3ACO23 [7.7(5)�, 0.7(7)� and 2.4(6)�, respectively] suggest

an asymmetrical bridging of the CO units. The reaction of Ir4

(CO)11(PPh3) [13] with a large excess of oxirane, NaBr and 2-bro-moethanol gives compounds 18 and 19 with 37 and 40% yield,respectively. The three bands at �9.68, �10.10 and 20.36 ppm(42:39:19 ratios) in the 31P{1H} spectrum of 18 at 183 K identifythree possible isomers, 18A–18C. The resonances at �9.68 and�10.10 ppm were assigned to PPh3 in axial position (18A and18B) because they look like the starting complex (dax = �11.08for Ir4(CO)11(PPh3)); the two isomers differ for carbene position,as 17A and 17B above. The resonance at 20.36 ppm is coherentwith a radial coordination of the PPh3 moiety and belongs toisomer 18C (see Scheme 7). When the 31P{1H} NMR spectrum iscollected at 310 K, the above three resonances coalesce into abroad signal, an indication that the isomers undergo structuralrearrangement according to ‘‘merry-go-round’’ and ‘‘change ofbasal face’’ of CO. A further confirmation of the existence of thethree isomers 18A–18C is given by the 13C{1H} spectrum. The lattershows three separate sets of resonances with 42:39:19 ratios, eachone with eleven resonances in the areas typical of bridging and ter-minal CO.

For the dioxycarbene (19) the 31P{1H} NMR spectrum at 183 Khas three signals at �7.08, �9.81 and 19.71 ppm with 55:34:11ratios, thus indicating the existence of three isomeric forms19A–19B–19C (see Scheme 8). The Dd (�0.2 and 2.9 ppm) suggestan axial coordination of PPh3 (19A, 19B) and the value of 26.6 ppm

ca' a

b *Ch

e'e

g

fca' a

b

*C

h*C

e'e

g

fc

h

a' a

bd

e'e

g

*C

19B19A 19C

C*

PPh3

PPh3

PPh3

*C

Scheme 8. Arrangements of ligands in the dicarbene derivatives.

Fig. 6. ORTEP plot (30% of probability) of compound 20.

98 A. Tassan et al. / Inorganica Chimica Acta 424 (2015) 91–102

a radial-coordinated of PPh3 (19C). The low-temperature 13C{1H}spectrum is similar to that obtained previously (for compound18), indicating the presence of three edge-bridging, two radialand three apical CO units. On the other hand, the two signalsrelated to COOA show that in isomer 19A they hold one radialand one axial position on two separate Ir atoms; in isomer 19Bthey also are placed in radial and axial positions, but on the samebasal iridium atom; finally, in the minority isomer 19C the phos-phine is radial, and both COOA groups take axial positions.

The compound 20, where the phosphine is dppm, can be pre-pared in a similar manner to that reported for PtBu3. The 31P spec-troscopic data show two signals at �51.14 and �57.25 ppm with23/77 ratio, compared with d = �52.2 ppm for the starting cluster.These data are consistent with the diphosphine being coordinatedin axial–axial positions [13]; hence, the two isomers differ only bythe position of the COOA group. Scheme 9 and Fig. 6 show the X-ray crystal structure, confirming the results of the spectroscopicanalysis. The cluster has Cs symmetry, with the four iridium atomsdefining a regular tetrahedron, three bridging CO on the basal faceIr1AIr2AIr3 and the carbene and diphosphine ligands in axial posi-tions. The mean IrAP distance of 2.300(3) Å is in agreement withknown data [13,18,30].

Also the carbene derivatives obtained with olefinic ligands (nor-bornadiene, 21, and 1,5-cyclooctadiene 22, 23) can be reacting[Ir4(CO)11Br]� with suitable olefin and the complexes have beencharacterized by means of IR and 13C NMR spectroscopy. Cluster21 presents two isomers with 89:11 ratios, where the carbeneligand binds to an axial and to a radial position, respectively. Com-pounds 22 and 23 are obtained with 50% and 23% yield, respec-tively. The IR spectra of both clusters show bands of bridging andterminal CO. The 13C NMR spectrum of compound 22 at 200 Kshows two sets of resonances (10 signals, relative intensities18:82). Compound 23 shows three sets of signals. The first is givenby two carbenes holding a radial and an axial position on twoseparate Ir atoms; the second refers to a couple of carbenes againplaced in radial and axial positions, but on the same Ir atom;finally, the third one indicates two axially-bonded carbenes ontwo separate basal iridium atoms.

Ph2P

PPh2

b b

b hf

geg

f *C cPh2P

PPh2

b b

bf

geg

f

*C

20A 20B

Scheme 9. Structure of Ir4(CO)9(dppm)(COCH2CH2O)

7. Phosphites

The reaction of anionic clusters [Ir4(CO)11Br]� with phosphiteligands such as phenyl-dimethoxyphosphine, diphenyl-methoxy-phosphine and diphenyl-phenoxyphosphine have been investi-gated by Detti et al. [30,31]. The bromide is displaced by oneequivalent of phosphite at room temperature, giving the monosub-stituted products [Ir4(CO)11{L}] [L = PPh(OMe)2 24; PPh2(OMe) 25and PPh2(OPh) 26]. An excess of ligand affords the disubstitutedcompounds [Ir4(CO)10{L2}] [L = PPh(OMe)2 27; PPh2(OMe) 28 andPPh2(OPh) 29]. The monosubstituted complexes 24–26 can beobtained with 35–60% yield. The IR spectra collected in dichloro-methane solution show two m(CO) stretching bands below1900 cm�1, indicating the presence of bridging CO ligands. The31P{1H} spectra obtained at 195 K in CD2Cl2 solution show onlyone resonance, suggesting the presence of a single isomer. Besides,the 13CO-enriched (ca. 30%) 13C NMR spectra of all compoundspoint to the presence of two axial, three bridging, three radialand three apical carbonyl groups, indicating that the phosphitecoordinates through an axial position.

The crystal structure of 26 and the selected labeling scheme areshown in Fig. 7. The molecule contains a nearly tetrahedral Ir4 core,with three CO units bridging to the basal face and with the phos-phite ligands in axial position. The presence of a good r�donorsuch as the diphenyl-phenoxyphosphine makes the Ir4AIr2, Ir4AIr3

distances (mean 2.755 Å) longer than the Ir1AIr2, Ir1AIr3 and

Fig. 7. ORTEP view of the molecular structure of [Ir4(CO)11{PPh2(OPh)}] 26. Thermalellipsoids at 50% probability.

Table 1Activation parameters at 298 K; h = Tolman’s cone angle [4].

L h(deg) DG– (kJ mol�1)

PPh3 145 45.6 ± 0.4PPh2(OPh) 139 44.3 ± 0.8PPh2(OMe) 132 42.8 ± 0.8P(OMe)3 107 37.5 ± 0.4

A. Tassan et al. / Inorganica Chimica Acta 424 (2015) 91–102 99

Ir2AIr3 ones (mean 2.707 Å). The IrAC distances are comparablewith those of the other iridium clusters [32]. The IrAP distanceof 2.301(2) Å is greater than that found in Ir4(CO)11{P(OMe)3}(2.258 Å), but shorter than those observed for phosphines (thatis, 2.311 Å for PMe3 [32], 2.335 Å for PPh3 [33]). As expected, thedifferences of the bond distances between phosphites andphosphines derive from the high p-withdrawing character of thephosphites with respect to the phosphines.

The thermodynamic parameters for the isomerization A M Bhave been determined integrating the 31P{1H} NMR signalsrecorded at variable temperature (185–300 K) in toluene-d8 solu-tion, because in this solvent the two isomers are present in similarproportions. In the 185–210 K temperature range the exchangebetween the two populations is slow and it is possible to calculatethe rate constant Keq = [A]/[B] at different temperatures. The linearregression of logKeq versus 1/T allows to determine the differenceenergy between the two isomers. The calculated thermodynamicparameters are: DHeq = 2.132 ± 0.155 kJ mol�1, DSeq = 0.014 ±0.005 kJ K�1 mol�1, DGeq = 1.970 ± 0.155 kJ mol�1.

The variable-temperature (190–300 K) 13C{1H} NMR spectra inCD2Cl2 solution of compound A, the only isomer formed in this sol-vent (Scheme 10), was carried out to investigate its fluxionalbehavior. By analyzing the spectra obtained between 190 and230 K, it was possible to identify only ‘‘merry-go-round’’ processesof basal CO groups (bridging and radial, Scheme 10B). In the 230–300 K range, it was not possible to define the other two processes:face exchange and rotation of apical carbonyls, because the peakshinting at the two processes were overlapped (see Scheme 10A),besides, above 300 K the compound decomposed. A simulation ofthe NMR spectra by means of the Exchange program [34] allowedto calculate the activation energy of the process at severaltemperatures.

By using the Eyring linear regression equation we found for thisprocess: DG– = 44.3 ± 0.8 kJ mol�1 at 298 K; DH– = 37.3 + 0.8 -kJ mol�1; DS– = �23.6 ± 3.5 J K�1 mol�1. In Table 1, the values ofthe calculated activation energies for the ‘‘merry-go-round’’process are compared with those experimentally obtained forsimilar compounds. These data indicate the effect of the ligandbulk, that is, the effect of increasing the angle between the basalplane and the iridium-carbonyl bond.

The infrared spectra of compounds (27–29) collected indichloromethane solution show two m(CO) stretching bands below1900 cm�1, indicating the presence of bridging carbonyl ligands inall complexes. The 31P{1H} spectra have been carried out in CD2Cl2

solution at 195 K and reveal two resonances due of the radial andaxial phosphorous. In addition, the analysis of 13CO-enriched (ca.30%) 13C NMR spectra of all compounds point to the presence oftwo axial, three bridging, two radial and three apical carbonylgroups, indicating that two phosphite units coordinate throughan axial and a radial position.

The crystal structures of 27 and 29 and the selected labelingschemes are shown in Fig. 8. The two molecules contain a nearly

a

bb

eeg

PPh2(OPh)

dd

fc c

eeg

PPh2(OPh)

dd

fc c

d dA B

Scheme 10. Two-isomer equilibrium for compound 26.

tetrahedral Ir4 core with three CO units bridging to the basal faceand with the phosphite ligands in axial and radial positions. Theaverage IrAIr distance for 27 is 2.724 Å, a value consistent withthose found for related compounds such as Ir4(CO)10{P(OMe)3}2,2.728 Å, and Ir4(CO)10(PPh3)2, 2.739 Å [33], and also in the IrAIrdistance range of dodecacarbonyl derivatives, but longer than thatof Ir4(CO)12 (2.693 Å). The Ir2AIr3 bond (2.702(10) Å) (see Fig. 8) isconsiderably shorter than Ir2AIr4 (2.7399(7) Å) and Ir3AIr4(2.7419(6) Å). Moreover, as observed for Ir4(CO)10(PPh3)2, the dis-tances between the iridium atoms of the basal plan and the onein apical position (Ir1) are all different: 2.7367(6) Å (Ir2AIr1),2.7159(6) Å (Ir4AIr1) and 2.7089(6) Å (Ir3AIr1). The IrAP distancesfor P4 (radial) and P2 (axial), are 2.262(2) and 2.251(2) Å, respec-tively. They are shorter than those found in bis-diphenylphosphinoderivatives [33], because the two AOCH3 groups make the ligand agood p-accepter.

The metal–metal bond distances in the basal plane (Ir2AIr3;where the bound phosphorus atoms are located) for 29 are longerthan the other (2.770 versus 2.755 Ir3AIr4, and 2.762 Å Ir2AIr4),and the Ir1AIr4 are shorter than the other (2.735 versus 2.7678Ir1AIr3 and 2.7624 Å Ir1AIr2).

8. Intramolecular dynamics of [Ir4(CO)12] derivatives

Most studies on the fluxional behavior of the tetrahedral clusterof iridium covers the migration of carbon monoxide. This migra-tion has been described, in particular, with the models developedby Cotton [35,36] and by Johnson and Benfield [37,38]. The firstis named ‘‘merry-go-round’’ and describes the exchange of sitesaround the metal backbone; the second is called LPM, ‘‘LigandPolyhedral Model’’, and describes the exchange of the CO site asthe result of a rotation (or libration) of the metallic skeleton withinthe envelope of the ligands whose donor atoms form the vertices ofa polyhedron which can deform (icosahedral M anticubeoctahe-dral M icosahedral, for example).

The first experimental evidence, IR and NMR, of the ‘‘merry-go-round’’ process has been obtained by the Roulet’s group duringthe studies of [Ir4(CO)9(l3-1,3,5-trithiane], where the unbridgedisomer (A), which is in general the transition state of themerry-go-round, was found both in solid state and in solution(see Fig. 9) [39].

Over the years, a lot of monosubstituted tetrairidium deriva-tives with Cs symmetry of general formula [Ir4(CO)11L] (L = PEt3,PAr3, PMePh2, PHPh2, PH2Ph2, PPh3, P(OMe)3, P(OPh)3, etc.) havebeen investigated [40,2]. Roulet and co-workers further deepenedthe studies about the intramolecular dynamics of iridium carbonylclusters by analyzing the solution and the solid state behavior ofbidentate donor ligands, such as: 1,1-bis-(methylthio)ethane 30,ethylidenebis(diphenylphosphine) 31 and propane-1,3-diyl-bis(diphenylphosphine)] 32 [41]. The [Ir4(CO)10(l2-(MeS)2CHMe)](30) has a ground state geometry with only terminal CO units;on the contrary, compounds 31 and 32 show three edge-bridgingCO groups, both in solution and solid state.

The crystallographic analysis for compound 30 shows a tetrahe-dral metal core of Cs symmetry and only terminal CO ligands andthe S-atoms in axial–axial positions; it is one of the few Ir clusters

Fig. 8. ORTEP view of the molecular structure of 27 and 29. Thermal ellipsoids at 50% probability.

a a

a bb

ee

e

b

g g

g

cc

c c

cc

S S

S

SS

S

A B

Fig. 9. Ir4(CO)9(l3-1,3,5-trithiane in solution, unbridged (A) and bridged (B) isomer.

100 A. Tassan et al. / Inorganica Chimica Acta 424 (2015) 91–102

without any bridging CO. The CAO bond lengths are in the typicalrange for terminal CO groups. Interestingly, complex (30) can existin two conformations (A and B). Upon coordination to the axialpositions, the ligand forms a five-member ring, where C1 may stayapart from the Ir1AIr2AIr3 plane or lie beneath. Of course, eachone of these conformers may also have two isomers (a and b),

Ir4

Ir3

Ir1

X

Ir2

X

C1H

CH3

H

CH3

a b

Conformers A

Scheme 11. The two conformation A and B with the

depending on whether the Me group bound to C1 is also placedaway or under the Ir1AIr2AIr3 plane (Scheme 11).

As mentioned above, complexes 31 and 32 both have a struc-ture with three bridging CO on the basal triangle Ir1AIr2AIr3,and a diphosphine ligand bound to axial positions (see Fig. 10).Compound 31 has an Aa conformation, with the CAMe bondroughly parallel to the Ir1AIr2AIr3 triangular face.

Complexes 31 and 32 both have Cs symmetry, but while for 31the phenyl moieties P(1) and (P2) are not related by symmetry;complex 32 shows a mirror plane passing through Ir3AIr4 andIr1AIr2 bond. The reason of the difference between the two com-plexes both, must be ascribed to intramolecular steric effects anda different hydrogen bonding network, which has already beendescribed [42].

The 1H NMR spectrum of compound 30 shows one quartet andone doublet relative to HACAMe that indicate the conformation ofthe coordinated ligand; moreover, the presence of a singlet for thetwo SAMe shows the mirror symmetry of the complex. The13CO-enriched (30%) 13C{1H} NMR spectrum in CD2Cl2 at 177 K ofcompound 30 presents six resonances for terminal CO units at:167.7 (a), 164.9 (b), 164.1 (c), 158.8 (d), 157.2 (g) and154.3(e) ppm, with relative intensities 2:2:1:2:2:1. The 2D-EXSY

Ir4

Ir3

Ir1

X

Ir2

X

C1H3C

HCH3

H

a b

Conformers B

two possible isomers a and b (X = SCH3, PCH3).

Fig. 10. ORTEP-like view of molecular structures of 30, 31, 32.

Fig. 11. 2D-EXSY spectrum of 30 in CD2Cl2 at 215 K.

A. Tassan et al. / Inorganica Chimica Acta 424 (2015) 91–102 101

spectrum in CD2Cl2 at 215 K shows one intense cross-peak,between 164.9 and 158.8 ppm, indicating the dynamic connectiv-ity b M a M d (see Fig. 11), and one less intense at 164.1 and158.8 ppm indicating c M d exchange; a third g M e exchange pro-cess takes place at 270 K. The exchange b M a M d corresponds tothe ‘‘merry-go-round’’ of the six CO groups about the Ir1AIr2AIr3triangular face; the second and third exchanges involve only twosites, that is, those arising from the rotation of three CO residingon the mirror plane Ir3 and Ir4. The free activation enthalpiescalculated by Eyring linear regression equation at 298 K are:DG1

– = 42.6 ± 0.4, DG2– = 47.0 ± 0.4 and DG3

– = 58.0 ± 0.8 kJ mol�1.The cluster 31 has the same geometry both in solution and in

the solid state. The 13CO-enriched (30%) 13C{1H} NMR spectrumof compound 31 shows seven resonances at: 223.5 (a), 203.8 (b),179.5 (f), 171.3 (d), 163.8 (c), 162.2(e) and 157.7(g) ppm, withrelative intensities 1:2:2:1:1:1:2. The 2D-EXSY spectrum of 31 inTHF at 215 K is similar to that of 32 [18]. The lowest energyprocess, again, the ‘‘merry-go-round’’ one, involves with rateconstant k1 the a, b, f and d CO units. At 247 K, the signal for COc starts to broaden (rate constant k2), while at 287 K emerges theexchange between CO g and e. The free activation enthalpies, calcu-lated by Eyring linear regression equation are: DG1

– = 38.7 ± 0.4,DG2

– = 50.4 ± 0.4 and DG3– = 60.1 ± 0.6 kJ mol�1.

The unobserved intermediate of ‘‘merry-go-round’’ incomplexes 31 and 32 has a geometry with all CO in terminal posi-tions. It may be that the transition state for the ‘‘merry-go-round’’

process has a semi-bridged geometry, as already reported [43]. Theenergy barrier of ‘‘merry-go-round’’ process for compound 31 is

102 A. Tassan et al. / Inorganica Chimica Acta 424 (2015) 91–102

lower than that calculated for 32 (DG1– = 38.7 kJ mol�1 and

53.9 kJ mol�1 respectively).

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